SATELLITE CELLS AND THE MUSCLE STEM CELL.pdf

SATELLITE CELLS AND THE MUSCLE STEM CELLNICHEHang Yin, Feodor Price, and Michael A. Rudnicki

Regenerative Medicine Program, Ottawa Hospital Research Institute, Ottawa, Ontario, Canada

LYin H, Price F, Rudnicki MA. Satellite Cells and the Muscle Stem Cell Niche. PhysiolRev 93: 23–67, 2013; doi:10.1152/physrev.00043.2011.—Adult skeletal musclein mammals is a stable tissue under normal circumstances but has remarkable abilityto repair after injury. Skeletal muscle regeneration is a highly orchestrated processinvolving the activation of various cellular and molecular responses. As skeletal muscle

stem cells, satellite cells play an indispensible role in this process. The self-renewing proliferation ofsatellite cells not only maintains the stem cell population but also provides numerous myogeniccells, which proliferate, differentiate, fuse, and lead to new myofiber formation and reconstitutionof a functional contractile apparatus. The complex behavior of satellite cells during skeletal muscleregeneration is tightly regulated through the dynamic interplay between intrinsic factors withinsatellite cells and extrinsic factors constituting the muscle stem cell niche/microenvironment. Forthe last half century, the advance of molecular biology, cell biology, and genetics has greatlyimproved our understanding of skeletal muscle biology. Here, we review some recent advances,with focuses on functions of satellite cells and their niche during the process of skeletal muscleregeneration.

I. INTRODUCTION: SATELLITE CELLS AS... 23II. FUNCTIONS OF SATELLITE CELLS IN... 30III. ANATOMIC AND FUNCTIONAL... 41IV. CONCLUDING REMARKS AND... 53

I. INTRODUCTION: SATELLITE CELLS ASADULT STEM CELLS IN MUSCLE

Skeletal muscle is a form of striated muscle tissue, account-ing for �40% of adult human body weight. Skeletal muscleis composed of multinucleated contractile muscle cells (alsocalled myofibers). During development, myofibers areformed by fusion of mesoderm progenitors called myo-blasts. In neonatal/juvenile stages, the number of myofibersremains constant, but each myofiber grows in size by fusionof satellite cells, a population of postnatal muscle stem cells.Adult mammalian skeletal muscle is stable under normalconditions, with only sporadic fusion of satellite cells tocompensate for muscle turnover caused by daily wear andtear. However, skeletal muscle has a remarkable ability toregenerate after injury. Responding to injury, skeletal mus-cle undergoes a highly orchestrated degeneration and regen-erative process that takes place at the tissue, cellular, andmolecular levels. This results in the reformation of inner-vated, vascularized contractile muscle apparatuses. This re-generation process greatly relies on the dynamic interplaybetween satellite cells and their environment (stem cellniche). During the last half century, advances in molecularbiology, cell biology, and genetics has greatly improved ourunderstanding of skeletal muscle regeneration. In particu-lar, extensive research on satellite cells and their niche has

elucidated many cellular and molecular mechanisms thatunderlie skeletal muscle regeneration. These studies havecontributed to the development of therapeutic strategies.These strategies serve to alleviate the physiological andpathological conditions associated with poor muscle regen-eration observed in sarcopenia and muscular dystrophy.

Here, we concentrate on the functions of satellite cells andthe regulation of their niche during the process of skeletalmuscle regeneration. We first describe the current under-standing of satellite cells with respect to their characteris-tics, heterogeneity, and embryonic origin. We then providean integrated view of the roles played by satellite cells dur-ing muscle regeneration and normal postnatal musclegrowth. We also discuss the contribution of several nonsat-ellite cell populations in muscle regeneration and their lin-eage relationships with satellite cells. Next, we focus on thesatellite cell niche with emphasis on the regulatory mecha-nisms associated with each niche component. We furtherreview the links between malfunction of satellite cells andtheir niche factors during aging. This review focuses onsatellite cells and their niche in mammalian models, payinglimited attention to the studies of satellite cell biology inother model organisms.

A. A Brief History of Satellite Cells

Half a century ago, Alexander Mauro observed a group ofmononucleated cells at the periphery of adult skeletal mus-cle myofibers by electron microscopy (329). These cellswere named satellite cells due to their sublaminar location

Physiol Rev 93: 23–67, 2013doi:10.1152/physrev.00043.2011

230031-9333/13 Copyright © 2013 the American Physiological Society

and intimate association with the plasma membrane ofmyofibers.

The direct juxtaposition of satellite cells and myofibers im-mediately raised a hypothesis that these cells may be in-volved in skeletal muscle growth and regeneration (329).Indeed, experiments by [3H]thymidine labeling and elec-tron microscopy demonstrated that satellite cells undergomitosis, assume a cytoplasm-enriched morphology, andcontribute to myofiber nuclei (355, 437). Later on, [3H]thy-midine tracing experiments indicated that satellite cells aremitotically quiescent in adult muscle but can quickly enterthe cell cycle following muscle injury (499). The same studyalso demonstrated that satellite cells give rise to proliferat-ing myoblasts (myogenic progenitors cells), which were pre-viously shown to form multinucleated myotubes in vitro(276, 499, 574). More definitive evidence came from invitro cultures of individually dissected myofibers, wherebythe behaviors of single myofibers and their resident satellitecells during regeneration can be tracked by phase-contrastmicroscopy (51, 277). It was observed that myofiber necro-sis is accompanied by satellite cell outgrowth, clonal expan-sion, and later fusion to form functional regenerated myo-tubes. These experiments support the notion that it is thesatellite cell, rather than the myonuclei, that contribute topostnatal muscle growth and repair.

The pivotal function of the satellite cell in muscle regener-ation stimulated research to determine their regulatorymechanisms. This started with the finding that quiescentsatellite cells on single myofibers were activated to enter thecell cycle by a mitogen originating from crushed skeletalmuscles (53). In search of this mitogen, it was found that theproliferation and differentiation of satellite cell-derivedmyoblasts cultured in vitro respond to various growth fac-tors in a dose-dependent manner (10, 11). Numerous studiesfurther revealed that the proliferation and differentiation ofsatellite cells during muscle regeneration is profoundly influ-enced by innervation, the vasculature, hormones, nutrition,and the extent of tissue injury (121, 224, 250, 332, 360, 410).Interestingly, satellite cells in contact with the plasma mem-brane of intact myofibers were found to have reduced sen-sitivity to mitogen stimulation (50). All of these findings ledto an intriguing notion that satellite cells reside in a specialmicroenvironment in vivo, which profoundly affects theirbehavior.

By definition, stem cells found in adult tissues can bothreplicate themselves (self-renew) and give rise to functionalprogeny (differentiate). Although the ability of satellite cellsto differentiate into myofibers was clearly proven, theirability to self-renew was once questioned. The first evidenceof satellite cell self-renewal came from a single myofibertransplantation assay. It was found that as few as sevensatellite cells, when transplanted into irradiated regenera-tion-insufficient mice along with their resident single myo-

fiber, can give rise to hundreds of satellite cells and thou-sands of myonuclei. Most importantly, transplanted satel-lite cells on a single myofiber were able to supportsubsequent rounds of muscle regeneration (105). These ob-servations demonstrate that satellite cells are bona fide mus-cle stem cells.

Asymmetric division is a common manner of stem cell self-renewal. Close examination of satellite cell divisions onsingle myofibers revealed that satellite cells can undergoboth asymmetric division and symmetric division (280).The fashion of symmetric versus asymmetric divisionlargely depends on the relative position of the daughter cellsin relation to the myofiber. This finding strongly argues thatsatellite cell self-renewal is governed by the structure andsignaling present in their immediate niche (280). Furtherinvestigation has revealed an important role for noncanoni-cal Wnt signaling in the regulation of satellite cell self-re-newal (293).

The aforementioned observations, together with those fromother studies, formed the basis of our current view of satel-lite cells as muscle stem cells whose functions are dictatedby their surrounding niche.

B. Identification and Isolationof Satellite Cells

Classically, satellite cells were identified based on a uniqueanatomical location: beneath the surrounding basal laminaand outside the myofiber plasma membrane. This anatom-ical location gives satellite cells a “wedged” appearancewhen viewd by electrictron microscropy (329). This tech-nique also revealed other morphological characteristics ofsatellite cells: large nuclear-to-cytoplasmic ratio, few organ-elles, small nucleus, and condensed interphase chromatin.This morphology is in harmony with the notion that mostsatellite cells, in healthy unstressed muscles, are mitoticallyquiescent (in G0 phase) and transcriptionally inactive (472).In addition to electron microscopy, satellite cells can beidentified by phase-contrast microscopy on single myofiberexplants (53). Based on the same principle, the behaviors ofsatellite cells on single myofiber explants can be recorded byutilizing live cell imaging techniques (490). The identifica-tion of satellite cells by fluorescence microscopy relies onspecific biomarkers (FIGURE 1). In adult skeletal muscle, allor most of satellite cells express the paired domain tran-scription factors Pax7 (478) and Pax3 (72), myogenic reg-ulatory factor Myf5 (116), homeobox transcription factorBarx2 (336), cell adhesion protein M-cadherin (244), ty-rosine receptor kinase c-Met (13), cell surface attachmentreceptor �7-intergin (73, 192), cluster of differentiationprotein CD34 (37), transmembrane heparan sulfate pro-teoglycans syndecan-3 and syndecan-4 (113), chemokinereceptor CXCR4 (429), caveolae-forming protein caveo-lin-1 (192, 552), calcitonin receptor (177), and nuclear en-

YIN, PRICE, AND RUDNICKI

24 Physiol Rev • VOL 93 • JANUARY 2013 • www.prv.org

velope proteins lamin A/C and emerin (192). Of these, Pax7is the canonical biomarker for satellite cells as it is specifi-cally expressed in all quiescent and proliferating satellitecells (478) across multiple species, including human (334),

monkey (334), mouse (478), pig (402), chick (216), sala-mander (353), frog (96), and zebrafish (219). Of note, someof the aforementioned satellite cell markers (e.g., �7-in-tergin, CD34) are also expressed on other cell types within

A

B

Synd3/4CalcR

cMet

Itga7Itgb1

Ncam1

MyoD

Myf5

Pax7 Pax3

MyoD

Myf5

Pax7

Pax3

Emerin

Mcad

Lamin A/C

Cav1

cMet

Cav1

Notch

Ncam1

Vcam1

CalcR

Itga7Itgb1

Synd3/4

CD34

CXCR4

Fzd7

Mcad

Lamin A/C

Emerin

Vcam1 CD34

CXCR4

Fzd7

Notch

Basal lamina

Transcription factors

Membrane proteins

Asymmetric division

Satellite Stem Cell

Satellite cell markers

Satellite Myogenic Cell

Sarcolemma

SarcoplasmMyonuclei

Symmetric division

Symmetric divisionSymmetric division

FIGURE 1. Characteristics of the satellite cell. A: numerous proteins are expressed in satellite cells and havebeen used as markers to distinguish between surrounding cell types within skeletal muscle. Due to heteroge-neity in satellite cell populations, it is unlikely that all of these markers are expressed in a given satellite cell ata specific time. Nevertheless, this panel summarizes the cellular location of markers used to identify satellitecells. B: the satellite cell population is heterogeneous and can be classified in a hierarchical manner based onfunction and gene expression. Evidence from lineage tracing experiments identified a subpopulation of satellitecells having never expressed the myogenic transcription factor Myf5 (satellite stem cells) are placed hierar-chically above satellite cells that have expressed Myf5 at some point during development (satellite myogeniccells). Satellite stem cells, upon asymmetric division (typically in a apical-basal orientation), will give rise to twodaughter cells, only one of which has activated Myf5. Functional differences in regenerative potential existbetween satellite stem cells and satellite myogenic cells. Following transplantation, satellite stem cells prefer-entially repopulate the satellite cell niche and contribute to long-term muscle regeneration. In contrast, satellitemyogenic cells preferentially differentiate upon transplantation in vivo.

SATELLITE CELLS AND THE MUSCLE STEM CELL NICHE

25Physiol Rev • VOL 93 • JANUARY 2013 • www.prv.org

skeletal muscle, and thus should not be utilized alone tounequivocally identify satellite cells. Satellite cells can beidentified by fluorescence microscopy via combined immu-nofluorescence labeling of laminin and M-cadherin, whichrespectively label the basal lamina and plasma membrane ofsatellite cells contacting the myofibers (517). In vivo, satel-lite cell populations can be visualized with the aid of newlydeveloped bioluminescence imaging techniques (454, 558).

Multiple methodologies have been developed to isolate sat-ellite cells from skeletal muscle. The choice of methodslargely depends on the isolation scale and the subsequentexperiment. In small scale, limited amounts of satellite cellscan be released from single myofiber explants by physicaltrituration (446) or enzymatic digestion (107). In largescale, satellite cells can be isolated from skeletal muscles byfluorescent-activated cell sorting (FACS). In the lattermethod, single cells are released from muscle tissue chunksby enzymatic digestion, followed by immunofluorescencelabeling of satellite cell-specific cell surface markers (posi-tive selection) and definitive cell surface markers for non-satellite cell populations (negative selection). As Pax7 isspecifically expressed in satellite cells within skeletal mus-cle, satellite cells, in both quiescent and proliferating stages,can be FACS-sorted by fluorescent protein expression intamoxifen-injected animals carrying both a Pax7CreER alleleand a fluorescence Cre reporter allele (e.g., R26R-EYFP). Inaddition, two transgenic mouse strains, which carry fluo-rescence proteins driven by Nestin- or Pax7-regulatory ele-ments, are also useful for isolating satellite cells by FACS(63, 127).

Protocols have been developed to isolate satellite cells inbulk utilizing their characteristic proliferation and adhesioncharacteristics under defined culturing conditions (144,364, 425). It is noteworthy that the satellite cell progenycultured on regular collagen-coated plastic dishes and un-der activation conditions, also called satellite cell-derivedprimary myoblasts, are molecularly and functionally dis-tinct from the satellite cells freshly isolated from muscles(160, 177, 350). In addition, the gene expression profile ofthese cultured primary myoblasts differs from that of acti-vated satellite cells in vivo (398). These essential differencesare possibly due to the lack of a satellite cell niche in in vitroculturing conditions. One challenge in future studies is tounderstand the components and role of the satellite cellniche to better control and manipulate their quiescence,activation, proliferation, and differentiation.

C. Heterogeneity of Satellite Cell Population

Satellite cells were considered to be a homogeneous popu-lation of committed muscle progenitors (52). However, re-cent evidence demonstrated that the satellite cell populationis heterogeneous and that satellite cells differ in their geneexpression signatures, myogenic differentiation propensity,

stemness, and lineage potential to assume nonmyogenicfates.

With regard to gene expression, it was first observed thatonly a subset of satellite cells expresses Pax3, a close para-log to Pax7 (350, 431). Although Pax3� satellite cells tendto be associated with skeletal muscle of certain anatomicallocations (e.g., diaphragm and truck muscles), the differ-ence of Pax3 expression does not seem to correlate withtheir embryonic origins, metabolic fiber types, or types ofinnervating motor neurons. Examination of the expressionof satellite cell markers CD34, M-cadherin, and Myf5 byimmunofluorescence staining revealed that a subpopulationof satellite cells do not express these markers (37). Similarly,human satellite cells also manifest heterogeneity in theirPax7, neuronal cell adhesion molecule (NCAM), c-Met,and Dlk1 expression (311). By RT-qPCR, two recent stud-ies showed that satellite cells from head or body muscles aredistinct in their molecular signatures (225, 456). Notably,the heterogeneity of gene expression in single satellite cellswas investigated in a recent study, wherein the expression ofPax7, Pax3, Myf5, and MyoD was interrogated by RT-qPCR for 40 individually FACS-isolated muscle stem cells(CD45�/CD11b�/CD31�/Sca-1�/�7-intergin�/CD34�)(454). All of these muscle stem cells express Pax7, indicat-ing they are satellite cells. In addition to the varied expres-sion of Pax3, it was also found that 25% of investigatedsatellite cells also express MyoD, a basic helix-loop-helixtranscription factor critical for myogenic commitment anddifferentiation. A recent study discovered a small subpopu-lation of satellite cells, characterized by their surface mark-ers Sca-1�/ABCG2� (as compared with Sca-1�/ABCG2�

for most satellite cells), is able to exclude Hoechst 33342dye and thus belongs to the skeletal muscle side population(satellite-SP cells) (518). After transplantation into dam-aged muscle, satellite-SP cells can both fuse to regeneratingmyofibers and return to quiescence in the satellite cell niche.However, by lineage tracing, ABCG2� cells (including sat-ellite-SP cells) seem to only have minor contribution tomyofiber regeneration (146).

Satellite cells are also heterogenic in their differentiationpotential. Satellite cells on single myofibers isolated fromvarious skeletal muscle sources were transplanted into irra-diated muscles of mdx/nude mice, which underwent re-peated regeneration and were depleted of endogenous sat-ellite cells (105). It was observed that the number of regen-erated myofibers contributed by tibialis anterior (TA)originated grafts was significantly less than that derivedfrom either extensor digitorum longus (EDL) or soleus mus-cle. Although the potential contribution from donor myo-fibers cannot be precisely evaluated, this observationstrongly suggests inherent differences in proliferation/dif-ferentiation potential of satellite cells and/or composition ofsatellite cell subpopulations from various muscles. In linewith this view, continuous BrdU labeling of satellite cells in



vivo revealed two satellite cell populations that are distinctin terms of their mitotic rates (471). It was found that themajority (�80%) of satellite cells readily enter the cell cycle(responsive population), whereas the remaining 20% of sat-ellite cells (reserve population) do this in a much slowermanner. It has been proposed that the reserve populationmaintains in the quiescent state at the beginning of musclegrowth/regeneration and only moves into this proliferativestate in response to the need for extensive muscle growth/regeneration (471). In line with this view, a recent studytraced satellite cell divisions by a fluorescent dye PKH26,which revealed the minority of PKH26high slow-dividingsatellite cells retained long-term self-renewal ability (388).Notably, a recent study thoroughly compared the gene ex-pression profiles and proliferation/differentiation potentialof satellite cells isolated from limb and facial muscles ofadult and aged mice (387). This study revealed broad satel-lite cell heterogeneity at both the population and single-celllevels. Despite their heterogeneous gene expression profiles,satellite cells isolated from limb (EDL) and facial (masseter)muscles are comparable in their ability to repair a limb (TA)muscle injury after transplantation. This finding suggeststhat although satellite cells from various muscles have dis-tinct gene expression and behaviors in vitro, their regener-ation potential in vivo might be largely determined by hoststem cell niche and microenvironment.

Most importantly, recent studies revealed satellite cell hetero-geneity in terms of their stemness and indicate that only a smallpercentage of satellite cells are true stem cells (109, 280, 489).By immunofluorescence staining of freshly isolated singlemyofibers from Myf5-nLacZ mice, our group demonstratedthat 13% of quiescent satellite cells on EDL muscles areLacZ�, making them distinct from the LacZ� satellite cells(280). As Myf5 is a myogenic regulatory factor, the absence ofMyf5 expression suggests a less committed cell fate for thoseLacZ� satellite cells. Moreover, by the Cre-LoxP based lineagetracing technique, our group further discovered that 10% ofsatellite cells in Myf5Cre;R26R-YFP mice do not express YFP(Pax7�/YFP� satellite cells), indicating that this small percentof satellite cells have never expressed Myf5 as did the majorityof satellite cells (Pax7�/YFP�). Remarkably, these two distinctsatellite cell subpopulations also differ in terms of their regen-erative potential: the Pax7�/YFP� satellite cells were able toreconstitute the stem cell niche and repaired muscles in a sus-tainable manner, whereas the Pax7�/YFP� satellite cells di-rectly underwent myogenic differentiation when transplantedin regenerating muscles of Pax7�/� mice. We further demon-strated that only Pax7�/YFP� satellite cells could undergoasymmetric cell divisions, giving rise to a Pax7�/YFP� satellitestem cell and a Pax7�/YFP� satellite committed progenitorcell. These findings indicate that a hierarchical lineage progres-sion from the Pax7�/YFP� (satellite stem cell) to the Pax7�/YFP� (satellite committed progenitors) exists within the totalsatellite cell population. Consistent with this notion, multiplestudies reported that only a subset of satellite cells undergo

asymmetric division in vivo or in vitro (108, 109, 489). Forexample, Numb, an inhibitor of Notch signaling and a cell-fate determinant, was found to asymmetrically distribute insome but not all satellite cell divisions (108, 489). By pulselabeling or tandem pulse labeling of growing/regenerat-ing muscles with halogenated thymidine analogs, it wasfurther discovered that all “older” chromosomes (thetemplate chromosome during DNA replication during Sphase) cosegregate into the more stemlike daughter cell,whereas “younger” chromosomes are inherited by themore differentiated daughter cell during asymmetric divi-sions of satellite cells both in vivo and in vitro (109, 489).According to the “immortal DNA strand” hypothesis (75),this preferential retention of “older” chromosomes protectsstem cells against the accumulation of mutations intro-duced during DNA replications (109, 489). Alternatively, ithas been also proposed that the nonrandom segregation ofsister chromatids, and hence their different epigeneticstates, is essential to the gene expression patterns and cel-lular fates of satellite stem cells and satellite progenitor cells(289). In summary, the aforementioned findings supportsatellite cell heterogeneity whereby the existence of a hier-archy delineates a small population of true stem cells (sat-ellite stem cells) from a more committed myogenic progen-itor population of satellite cells. The satellite stem cells are lesscommitted to the myogenic lineage and tend to retain oldertemplate DNA during division. Through asymmetric division,satellite stem cells self-renew to replenish the stem cell pooland produce more committed myogenic progenitors that par-ticipate in skeletal muscle growth and regeneration.

Satellite cells also exhibit heterogeneity in respect to their cellfate potential. Our group first revealed that satellite cells havean intrinsic potential to differentiate into multiple mesenchy-mal lineages (23). When cultured on solubilized basementmembrane matrix (matrigel), satellite cells from single myofi-bers spontaneously differentiate into myocytes, adipocytes,and osteocytes. This finding indicates that satellite cells func-tionally resemble bone marrow-derived mesenchymal stemcells. This notion is substantiated by another study whereinsatellite cells were found to assume the adipocyte lineage,which can be enhanced upon oxygen-rich culture conditions(120). By clonal analysis, it was found that myogenic andadipogenic satellite cells are two separate populations in thesatellite cell compartment, although both populations expressthe myogenic marker Pax7 as well as the adipogenic markersperoxisome proliferator-activated receptors � (PPAR�) andCCAAT/enhancer binding proteins (C/EBPs) (485). This sim-ilar molecular signature may reflect a common developmentalorigin of satellite cells and adipogenic progenitors in embry-onic somites (discussed in sect. IIE). Satellite cell heterogeneitywith regard to myogenic and nonmyogenic potential was thor-oughly investigated by in vitro differentiation and in vivotransplant assays (486). In this study, myofiber-associated sat-ellite cells were isolated from undamaged skeletal muscle by atwo-step enzymatic digestion and sorted by FACS based on



their differential expression of cell surface markers, CD45 andSca-1. It was found that the vast majority of myogenic satellitecells are within the CD45�/Sca-1� population and exhibitedno in vitro differentiation potential into fibroblasts or adi-pocytes. In contrast, the minor population of CD45�/Sca-1�

satellite cells can differentiate into both fibroblasts and adi-pocytes in culture, a similarity that is shared with CD45�/Sca-1� mesenchymal stem cells found in multiple tissues (316,401, 417, 441, 509, 519). Despite the plasticity of satellite cellsin vitro, it is important to note that myogenesis is the predom-inant fate of satellite cells in vivo as fibrosis or adipose infiltra-tion is not normally observed in young healthy muscle. Fur-thermore, recent studies indicate that intramuscular adi-pocytes and fibroblasts can also arise from fibrocyte/adipocyteprogenitors (FAPs), which reside in the muscle interstitium(252, 541; and discussed in sect. IIIB1). Indeed, lineage tracingexperiments indicate that adipocytes derived from myofibersisolated from MyoDiCre;R26R-EYFP mice have never tran-scribed MyoD (507). As the vast majority of adult satellite cellswere permanently labeled with EYFP in this experiment, thisfinding argues that most satellite cells from myofiber culturesdo not spontaneously differentiate into adipocytes. Furtherinvestigation with more definitive lineage tracing (e.g.,Pax7CreER;R26R-EYFP) methods would clarify the exact con-tribution of satellite cells to other nonmyogenic lineages bothin vitro and in vivo.

As discussed here, multiple lines of evidence demonstrate thatsatellite cells represent a heterogeneous population. However,our understanding of this heterogeneity is far from complete.First, although several markers can separate the total satellitecell population into functional subpopulations, it is still un-known whether these subpopulations are homogeneous intheir function and gene expression. Further studies to identifyadditional satellite markers will potentially help distinguishthe various satellite cell lineages. Moreover, future inves-tigations should attempt to identify the intrinsic differ-ences between satellite cell subpopulations at the molec-ular and functional levels during muscle regeneration.Such findings would elucidate regulatory mechanisms gov-erning the transition between different satellite cell subpopu-lations and potentially distinct roles of satellite cell subpopu-lations during muscle regeneration. In addition, it would be ofgreat importance to understand the dynamics of satellite cellheterogeneity in response to various environmental cues withregard to research in muscle regeneration and disease.

D. Variance of Satellite Cells Numberand Location

In addition to the heterogeneity of satellite cells, the quantity ofsatellite cells differs between muscles, myofiber types, develop-mental stages, and species. In general, satellite cells account for30–35% of the sublaminal nuclei on myofibers in early post-natal murine muscles, and this number declines to 2–7% inadult muscles (9, 227, 450, 469). In adults, the percentage of

satellite cells in soleus muscle is generally two- to fourfoldhigher than that in tibialis anterior muscle or EDL muscle(190, 466, 500). Within the same muscle, the number ofsatellite cells found on slow muscle myofibers (type I) isgenerally higher than those on fast myofibers (type IIa andtype IIb) (190, 324, 383). The biological meaning and thepotential regulatory mechanisms underlying these phenom-ena are poorly understood. However, it is conceivable thatthese variances may reflect intrinsic heterogeneity of satel-lite cells on different myofibers and implicate a potentialrole of myofibers as a niche factor in regulating the homeo-stasis of their resident satellite cells.

Along a myofiber, the distribution of satellite cells is notrandom. It has been reported that the density of satellitecells is higher at the ends of the myofibers, where the longi-tudinal growth of skeletal muscles happens (14). A higherincidence of satellite cells has been observed at perisynapticregions compared with that at extrasynaptic regions (190,269, 567). Moreover, satellite cells have been observed inclose proximity to capillaries (100, 466). In fact, 88% ofsatellite cells in adult human muscles were observed to belocated within a 21-�m distance of a capillary (100). Thistight association with capillaries seemed to be compromisedby denervation (130). These observations indicate thathoming of satellite cells is influenced by their niche, both bylocal motor neurons and blood vessels (discussed in sect.IIIB).

It is noteworthy that some reported variation in satellite cellnumbers may be partially due to techniques or statisticalanalysis employed in satellite cell counting. For example,satellite cell counting based on immunofluorescence label-ing of satellite cell specific markers relies on the comparableexpression levels of these markers on all satellite cells underinvestigation. As such, special caution should be taken intoaccount when interpreting data between independent ex-periments.

E. Origins of Adult Satellite Cells

1. Embryonic origins

By classic techniques of developmental biology, it has longbeen established that skeletal muscles within both the adulttrunk and limbs develop from embryonic somites (462).However, the exact origin of adult satellite cells was ob-scure until recently.

Early experiments using a quail-chick chimera techniquesuggested a somitic origin of satellite cells in amniotes (18).Embryonic somites are segments of paraxial mesodermformed on both sides of the body axis. In this experiment,somites from donor quail embryos were transplanted intohost chick embryos. After embryonic development, the con-tribution of quail cells to the chick satellite cell compart-



ment was determined using Feulgen staining, which distin-guishes quail-specific interphase nuclei from those of chick.It was found that donor cells from quail somites integratedinto the chick limb and contributed to both terminally dif-ferentiated muscle fibers and satellite cells. This finding in-dicated a common somitic origin for all myogenic cell lin-eages, including satellite cells. However, the progenitor celltypes at the origin and the developmental route remainunknown.

Advances in mouse genetics, particularly the generation ofPax3 and Pax7 knock-in reporter alleles, allows precisetracing of Pax3/Pax7-expressing myogenic progenitor cellsduring muscle development in a temporal and spatial man-ner (265, 326, 350, 432, 433). These reporters togetherwith labeling of cells by electroporation (40, 203), retrovi-rus (464), Cre-LoxP based lineage tracing (263, 300, 464),and traditional quail-chick transplantation (203, 464),jointly shed light on the embryonic origins of adult satellitecells. Accumulating evidence indicates that adult satellitecells originate from the dermomyotome (203, 265, 434,464), an epithelial structure formed on the dorsal part of thesomite. The dermomyotome contains multipotent pro-genitor cells, which eventually give rise to multiple adulttissues/cell types including dermal fibroblasts, endothe-lial cells, vascular smooth muscle, brown fat tissue, andall skeletal muscles of the trunk and limbs (71). The cellfate decisions largely depend on the relative position ofthese multipotent progenitor cells with respect to adja-cent tissues such as the notochord, neural tube, dorsalectoderm, and myotome (71).

Embryonic muscle development takes place in two succes-sive stages. During the first stage, a group of postmitoticmononucleated myocytes, expressing Myf5 and Mrf4, mi-grate out from the border regions of the dermomyotomeand form primitive muscles beneath the dermomyotome(204, 261). These primitive muscles constitute the primarymyotome and are the source of fetal and adult trunk mus-cles. During the second stage of muscle growth, the centralportion of the dermomyotome undergoes an epithelial-to-mesenchymal transition (EMT). During EMT, tightlypacked epithelial cells tease apart and turn in a loose mes-enchymal state prior to assuming different developmentalfates: cells in the medial dermomyotome will develop intobrown fat, dermis, and trunk muscle, while cells in thelateral dermomyotome will give rise to endothelia and limbmuscles. The different cell fates assumed by the same groupof progenitor cells are proposed to be due to asymmetric celldivision (40, 101). EMT is accompanied by the extensivecell migration (203, 265, 434, 464). With the breakdown ofdermomyotome, a group of proliferating progenitor cells,expressing both Pax3 and Pax7, migrate from the centralregion of the dermomyotome into the previously formedprimary myotome. Upon arrival, some progenitor cells con-tinue to proliferate and replenish the progenitor pool. These

cells, which were absent from the primary myotome at ear-lier stages, count for the majority of all proliferating cells inembryonic/fetal trunk muscles (203, 434). Some of the pro-liferating Pax3�/Pax7� progenitor cells persist into late fe-tal development stages and are enveloped beneath the basallamina of developing myofibers (203, 434). These cells,which reside in the satellite cell compartment, are presumedto subsequently become the postnatal satellite cells in trunkmuscles. Besides proliferation, progenitor cells also exit thecell cycle and begin differentiating into embryonic/fetaltruck muscles. Cell cycle withdrawal is concomitant withthe expression of the myogenic regulatory factors MyoDand Myf5 (453).

During EMT, another group of proliferating progenitorcells, expressing Pax3 (but not Pax7 in mouse), delaminatefrom the ventral-lateral border of the dermomyotome andmigrate to the limb bud mesenchyme (265, 392, 464). Theseprogenitor cells still maintain their multipotency as theygive rise to the limb vascular, lymphatic endothelia, andlimb muscles (226, 264). At E11.5 of mouse embryo devel-opment, some progenitor cells start to express Pax7 in theanterior limb buds (433). The expression of Pax7 specifiesthese cells to the myogenic lineage (242). Similar to themyotome-located progenitors, these Pax3�/Pax7� progen-itor cells in limb buds undergo proliferation/differentiationwhile a portion withdraw from cell cycle and become satel-lite cells.

All together, these observations indicate that Pax3�/Pax7�

embryonic progenitor cells are the major source of adultsatellite cells in truck and limb muscles; it is, however, note-worthy that the aforementioned observations from lineagetracing and immunofluorescence labeling experiments can-not exclude the possibility that some adult satellite cells mayoriginate from other sources during fetal and postnatalmuscle development. For example, embryonic dorsal aortaexplants, when cultured and disaggregated in vitro, canefficiently give rise to myogenic precursors (129). Thesemyogenic precursors are similar to satellite cells in theirgross morphology and expression of molecular markers.Given that adult satellite cells also express endothelialmarkers, it was proposed that some adult satellite cells orig-inated from the embryonic dorsal aorta (129). However,recent studies revealed that both skeletal muscles andsmooth muscles found in dorsal aorta are derived from thesame Pax3� cell population in the paraxial mesoderm(155). Thus it is also possible that the observed similaritiesare merely reminiscent of a common embryonic origin be-fore myogenic specification.

Distinct from trunk and limb muscles, head muscles havemultiple embryonic origins. The majority of head muscles,including branchiomeric muscles and most extraocularmuscles, arise from the cranial paraxial mesoderm (CPM)(225, 540). Posterior neck muscles and tongue develop



from occipital somites. Similar to progenitor cells migratingto limb buds, the progenitor cells for tongue delaminatefrom the ventral-lateral border of occipital somites. A smallfraction of extraocular muscles also arise from theprechordal mesoderm (PM). On the basis of observationsfrom lineage tracing experiments, it has been found thatadult satellite cells of the various head muscles originatefrom their corresponding embryonic muscles and expressdistinct combinations of signature genes (225). Unliketrunk and limb progenitors, most progenitor cells for headmuscles (except for tongue) express MesP1 rather thanPax3.

2. Alternative origins of adult satellite cells

Accumulating evidence indicates that some adult satellitecells may have alternative origins other than dermomyo-tome-derived Pax3�/Pax7� progenitor cells.

First, multiple studies have demonstrated that several typesof nonsatellite cells can reconstitute the satellite cell nicheand turn into bona fide satellite cells (Pax7-expressing myo-genic cells) after transplantation into regenerating skeletalmuscles (for details, see sect. IID). It remains, however,largely unknown to what extent these cells contribute to theadult satellite cell pool and muscle development underphysiological conditions. Notably, a recent study utilizingTN-APCreERT2 and VE-cadherinCreERT2 alleles showed thatalkaline phosphatase (ALPL) expressing pericytes, but notVE-cadherin-expressing endothelial cells, can develop intopostnatal satellite cells and participate in normal develop-ment of limb muscles (135).

It is noteworthy that some adult satellite cells in mammalsmay be derived from dedifferentiation of fetal/adult myofi-bers. It has long been established that the dedifferentiationof “terminally” differentiated multinucleated myofibers oc-curs in injured skeletal muscle of urodele amphibians, suchas the newt (69). Nevertheless, it remains controversial as towhether mammalian myotubes in vitro or myofibers in vivocan undergo a similar dedifferentiation process. Studies uti-lizing immortalized murine myoblast cell lines (e.g., C2C12or pmi28 cells) have shown that some myotubes formed bythese cells can dedifferentiate into mononucleated myo-genic cells in the presence of protein extract from regener-ating newt muscles (331), bioactive compounds like myos-everin or its derivatives (149, 443), ciliary neurotrophicfactor (CNTF) (95), or by genetic manipulations such asoverexpression of Msx1 (379), Twist1 (232), Barx2 (335),or knockdown of Rb1 in conjunction with a deficiency forCdkn2a (p16Ink4a) (395). By a novel fusion-dependent lin-eage tracing technique, two recent studies reported thatdifferentiated myotubes formed by satellite cell-derived pri-mary myoblasts (397) or muscle-derived cells (MDCs)(359) can dedifferentiate into Pax7-expressing mononu-cleated myogenic cells in vitro in response to an inhibitorcocktail (a tyrosine phosphatase inhibitor plus an apoptosis

inhibitor) or within regenerating muscle in vivo. Althoughstill not experimentally tested on myofibers formed duringdevelopment, these intriguing observations jointly suggestthat some adult satellite cells may be “recycled” from multi-nucleated myofibers in vivo during postnatal muscle growthor regeneration. Future studies may employ the fusion-de-pendent lineage tracing technique in chimeric mice to inves-tigate the physiological relevance of this alternative sourceof satellite cells.

II. FUNCTIONS OF SATELLITE CELLS INMUSCLE REGENERATION

Skeletal muscles consist of myofibers, neurons, vasculaturenetworks, and connective tissues, of which the structuraland functional element of skeletal muscle is the myofiber.Each myofiber is surrounded by the endomysium (alsocalled the basement membrane or basal lamina). Bundles ofmyofibers are surrounded by the perimysium, while the en-tire muscle is contained within the epimysium. Each myofi-ber is anchored at its extremities to tendons or tendon-likefascia at the myotendinous junctions (MTJs) (531). Myofi-bers are composed of actin and myosin myofibrils repeatedas a sarcomere, which is the basic functional unit of skeletalmuscle. Responding to the signals from motor neurons,myofibers depolarize and release calcium from the sarco-plasmic reticulum (SR). This drives the movement of actinand myosin filaments relative to one another and leads tosarcomere shortening and muscle contraction.

Based on their physiological properties, skeletal muscle fiberscan be grouped into a slow-contracting/fatigue-resistant typeand a fast-contracting/fatigue-susceptible type. Myofibers alsovary in terms of their myosin heavy chain (MyHC) isoforms(fast or slow) and metabolism types (oxidative or glycolytic).The choice of myosin gene expression is under the dynamicregulation of thyroid hormone and work load (reviewed inRef. 28). Recent studies demonstrated that the specification ofmyosin expression is also regulated by intronic microRNAswithin MyHC genes (545, 546).

Mammalian skeletal muscle during adulthood is a stable post-mitotic tissue with infrequent turnover of myonuclei (467).Minor lesions inflicted by day-to-day wear and tear can berepaired without causing cell death, inflammatory responses,or histological changes. For instance, local plasma membranedamage caused by spontaneous eccentric muscle contractionscan be efficiently repaired by recruiting intracellular vesicles topatch the damaged membrane (30, 508). This repair processinvolves dysferlin and caveolin-3 (30, 181), and mutations ofthese genes cause limb girdle muscular dystrophy 2B (LGMD-2B) (36, 314) and 1C (LGMD-1C) (344), respectively. In con-trast, severe muscle injuries due to either traumatic lesions(e.g., extensive physical activity such as resistance training, orexposure to myotoxin) or genetic defects (e.g., muscular dys-trophies) are accompanied by myofiber necrosis, inflamma-



tory responses, activation of satellite cells, proliferation, anddifferentiation of satellite cell-derived myoblasts (FIGURE 2).This process, starting from myofiber necrosis and ending withnew myofiber formation, is called muscle regeneration. Itshould be stressed that satellite cells play a pivotal role duringmuscle regeneration under either physiological conditions(e.g., extensive exercise) (400, 457) or pathological conditions(e.g., myotoxin induced injury) (301, 361, 457). This notion isclearly supported by the findings that ablation of the totalsatellite cell pool (all Pax7� cells) in adulthood completelyabolished muscle regeneration (301, 361, 457). It has beenreported that several types of nonsatellite cells can undergomyogenic differentiation and contribute to muscle regenera-tion after transplantation into regenerating muscle (24, 210,287, 345, 413). Nevertheless, the contribution of these cells toadult muscle regeneration seems to be negligible comparedwith satellite cells, implying the physiological relevance ofnonsatellite cell-based myogenesis might depend on Pax7 ex-pression and/or the existence of considerable numbers of sat-ellite cells.

In this section, we examine the extensive cellular and mo-lecular dynamics during muscle regeneration, with empha-sis on satellite cell. We also review the potential of nonsat-ellite cell lineages on muscle regeneration. At the end of thissection, we briefly describe the function of satellite cells innormal postnatal muscle development, and compare andcontrast this with muscle regeneration in adulthood.

A. An Introduction to Muscle Regeneration

Muscle regeneration occurs in three sequential but overlap-ping stages: 1) the inflammatory response; 2) the activation,differentiation, and fusion of satellite cells; and 3) the mat-uration and remodeling of newly formed myofibers.

Muscle degeneration begins with necrosis of damaged mus-cle fibers. This event is initiated by dissolution of the myo-fiber sarcolemma, which leads to increased myofiber per-meability. Disruption of myofiber integrity is reflected by

Pax7+ / Myf5– / MyoD– / MyoG–

Pax7+ / Myf5+ / MyoD– / MyoG–

Basal lamina

Differentiation

Return toquiescence

Return toquiescence

Activation Activation

Quiescence

Pax7+ / Myf5– / MyoD– / MyoG–

Pax7+ / Myf5– / MyoD+ / MyoG–

Proliferation

Pax7– / Myf5+ / MyoD+ / MyoG+

Pax7– / Myf5– / MyoD+ / MyoG+

Pax7+ / Myf5+ / MyoD+ / MyoG– Pax7– / Myf5+ / MyoD– / MyoG+

Pax7+ / Myf5+ / MyoD– / MyoG– Pax7– / Myf5– / MyoD– / MyoG+

Pax7– / Myf5– / MyoD– / MyoG–

Differentiation

FIGURE 2. Satellite cell activation, differentiation, and fusion. The myogenic program is orchestrated by keytranscription factors that dictate the progression from quiescence, activation, proliferation, and differentia-tion/self-renewal of satellite cells. This results in the transformation of individual satellite cells into a syncytialcontractile myofiber. Initially satellite cells are mitotically quiescent (G0 phase) and reside in a sublaminar niche.Quiescent satellite cells are characterized by their expression of Pax7 and Myf5 but not MyoD or Myogenin.Damage to the environment surrounding satellite cells results in the deterioration of the basal lamina and theirexit from the quiescent state (satellite cell activation). Proliferating satellite cells and their progeny are oftenreferred to as myogenic precursor cells (MPC) or adult myoblasts. Adult myoblasts express the myogenictranscription factors MyoD and Myf5. Following proliferation, adult myoblasts begin differentiation by down-regulating Pax7. The initiation of terminal differentiation and fusion begins with the expression of Myogenin,which in concert with MyoD will activate muscle specific structural and contractile genes. During regeneration,activated satellite cells have the capability to return to quiescence to maintain the satellite cell pool. This abilityis critical for long-term muscle integrity.



increased plasma levels of muscle proteins and microRNAs,such as creatine kinase (17) and miR-133a (292), which areusually restricted to the myofiber cytosol. Similarly, thecompromised sarcolemmal integrity also allows the uptakeof low-molecular-weight dyes, such as Evans blue or pro-cion orange, by the damaged myofiber, which is a reliableindication of sarcolemmal damage associated with exten-sive exercise and muscle degenerative diseases (218, 328,396). Moreover, myofiber necrosis is accompanied by in-creased calcium influx or calcium release from the SR,which in turn activates calcium-dependent proteolysis anddrives tissue degeneration (reviewed in Refs. 7, 19, 39). Inthis process, calpain, a calcium-activated protease, has beenshown to cleave myofibrillar and other cytoskeletal proteins(reviewed in Ref. 145). Myofiber necrosis also activates thecomplement cascade and induces inflammatory responses(389). Subsequent to inflammatory responses, chemotacticrecruitment of circulating leukocytes occurs at local sites ofdamage (reviewed in Ref. 530). Neutrophils are the firstinflammatory cells to infiltrate the damaged muscle, with asignificant increase in their number being observed as earlyas 1–6 h after myotoxin- or exercise-induced muscle dam-age (168). Following neutrophil infiltration, two distinctsubpopulations of macrophages sequentially invade the in-jured muscle and become the predominant inflammatorycells (91). The early invading macrophages, characterizedby the surface markers CD68�/CD163�, reach their highestconcentration in damaged muscle at �24 h after the onsetof injury and thereafter rapidly decline. These CD68�/CD163� macrophages secrete proinflammatory cytokines,such as tumor necrosis factor-� (TNF-�) and interleukin(IL)-1, and are responsible for the phagocytosis of cellulardebris. A second population of macrophages, characterizedby the surface markers CD68�/CD163�, reach their peakat 2–4 days after injury. These macrophages secrete anti-inflammatory cytokines, such as IL-10, and persist in dam-aged muscle until the termination of inflammation. Nota-bly, the CD68�/CD163� macrophages also reportedly fa-cilitate the proliferation and differentiation of satellite cells(80, 81, 303, 341, 503).

A highly orchestrated regeneration process follows mus-cle degeneration. A hallmark of this stage is extensive cellproliferation. Blocking cell proliferation by colchicinetreatment (411) or irradiation (423) drastically reducedmuscle regenerative capacity. Experiments by [3H]thymi-dine labeling have clearly demonstrated that the prolifer-ation of satellite cells and their progeny provide a suffi-cient source of new myonuclei for muscle repair (498,499, 554; reviewed in Ref. 79 and discussed in sect. IIB).It is commonly agreed that following proliferation, myo-genic cells differentiate and fuse to existing damaged fi-bers or fuse with one another to form myofibers de novo.This process, in many but not all respects, recapitulatesembryonic myogenesis.

Muscle regeneration can be characterized by a series ofmorphological characteristics based on histological and im-munofluorescence staining. On muscle cross-sections,newly formed myofibers can be readily distinguished bytheir small caliber and centrally located myonuclei. Thesemyofibers are often basophilic in the beginning of regener-ation due to protein synthesis and the expression of embry-onic/developmental forms of MyHC (217, 562). On musclelongitudinal sections and on isolated single myofibers, thecentrally localized myonuclei were observed in discrete seg-ments of regenerating myofibers or along the entire newmyofiber, which suggests that cell fusion during regenera-tion happens in a focal, rather than diffuse, manner (58).Occasionally, concentrated regenerative processes may ap-pear as local protrusions (also called budding) on myofi-bers. Muscle regeneration can often lead to architecturalchanges of the regenerated myofibers, which are presum-ably due to incomplete fusion of regenerating fibers withinthe same basal lamina (57, 58, 64, 465). Newly formedmyotubes may not fuse to each other, resulting in clusters ofsmall caliber myofibers within the same basal lamina. Al-ternatively, they may fuse only at one end, leading to theformation of forked (also called branching or splitting)myofibers. Myofiber branching was commonly observed inmuscles from patients suffering neuromuscular diseases, inhypertrophied muscles, and in aging muscles, suggestingthis phenotype may relate to abnormal muscle regenerativecapacity (59, 90). Small regenerating myofibers may alsoform outside the basal lamina in the interstitium, due tomigration of satellite cells or other types of myogenic cells.Finally, the reconstitution of myofiber integrity may be pre-vented by scar tissue that separates the two regenerativesites, leading to the formation of a new myotendinous junc-tion.

At the end of muscle regeneration, newly formed myofibersincrease in size, and myonuclei move to the periphery of themuscle fiber. Under normal conditions, the regeneratedmuscles are morphologically and functionally indistin-guishable from undamaged muscles.

B. Satellite Cell Activationand Differentiation

In intact muscle, satellite cells are sublaminal and mitoti-cally quiescent (G0 phase). Quiescent satellite cells are char-acterized by their expression of Pax7 but not MyoD orMyogenin (116). Examination of �-galactosidase activity inMyf5-LacZ mice indicated that the Myf5 locus is active in�90% of quiescent satellite cells, which suggests most sat-ellite cells are committed to the myogenic lineage (37).

Upon exposure to signals from a damaged environment,satellite cells exit their quiescent state and start to prolifer-ate (satellite cell activation). Proliferating satellite cells andtheir progeny are often referred to as myogenic precursor



cells (MPC) or adult myoblasts. Satellite cell activation isgoverned by multiple niche factors and signaling pathways(discussed in detail in sect. IIIA). Satellite cell activation isnot only restricted to the site of muscle damage. In fact,localized damage at one end of a muscle fiber leads to theactivation of all satellite cells along the same myofiber andmigration of these satellite cells to the regeneration site(473). Satellite cell activation is also accompanied by exten-sive cell mobility/migration. It has been observed that sat-ellite cells can migrate between myofibers and even musclesacross barriers of basal lamina and connective tissues dur-ing muscle development, growth, and regeneration (241,251, 557). Recently, sialomucin CD34, whose expression ishigh on quiescent satellite cells but dramatically reducedduring satellite cell activation, was demonstrated to act asan antiadhesive molecule to facilitate migration and pro-mote the proliferation of satellite cells at very early stages ofmuscle regeneration (8). In addition, dynamic regulation ofEph receptors and ephrin ligands in activated satellite cellsand regenerating myofibers have been shown to direct sat-ellite cell migration (506).

Unlike quiescent satellite cells, myogenic precursor cells arecharacterized by the rapid expression of myogenic tran-scription factors MyoD (111, 114, 116, 175, 207, 497, 572,585) and Myf5 (111, 116). Of note, the presence of MyoD,desmin, and Myogenin in satellite cells was observed asearly as 12 h after injury, which is before any noticeable signof satellite cell proliferation (426, 497). This early expres-sion of MyoD is proposed to be associated with a subpop-ulation of committed satellite cells, which are poised todifferentiate without proliferation (426). In contrast, themajority of satellite cells express either MyoD or Myf5 by24 h following injury (111, 116, 585) and subsequentlycoexpress both factors by 48 h (111, 116). The ability ofsatellite cells to upregulate either MyoD or Myf5 suggeststhese two transcription factors may have different functionsin adult myogenesis.

First, MyoD�/� mutant mice display markedly reducedmuscle mass (338). This atrophy phenotype is reportedlydue to delayed myogenic differentiation (564, 573). Simi-larly, muscle regeneration is also impaired in MyoD�/�

mice, resulting in an increased number of myoblasts withinthe damaged area (338). These MyoD�/� myoblasts persistfor extended periods of time, fail to differentiate, and do notfuse into myotubes. This is consistent with the notion thatMyoD�/� myoblasts, when cultured in myogenic differen-tiation conditions, continue to proliferate and eventuallygive rise to a decreased number of differentiated mononu-cleated myocytes (114, 452, 573). Intriguingly, trans-planted MyoD�/� myoblasts have been reported to surviveand engraft into MyoD�/� regenerating muscles with im-proved efficacy (compared with wild-type myoblasts). Thisphenotype is reportedly due to their increased stem cellcharacteristics and repressed apoptotic potential (22, 231).

After regeneration, these transplanted MyoD�/� myoblastsnot only give rise to myonuclei but also contribute to thesatellite cell pool (22). On the other hand, ectopic expres-sion of MyoD in NIH-3T3 and C3H10T1/2 fibroblasts issufficient to activate the complete myogenic program inthese cells (234). Taken together, these observations indi-cate that expression of MyoD is an important determinantof myogenic differentiation, and in the absence of MyoD,activated myoblasts have a propensity for proliferation andself-renewal (452). In contrast to the MyoD�/� mice,Myf5�/� mutant mice show a myofiber hypertrophy phe-notype (187), and the proliferation of Myf5�/� myoblasts iscompromised (187, 542). Together, these results implicatea distinct role for Myf5 in adult myoblast proliferation,while MyoD is essential for differentiation. Notably, thedisparate functions of Myf5 and MyoD in adult muscleregeneration parallel the proposed roles for these transcrip-tion factors throughout the development of distinct myo-genic lineages during embryogenesis (188, 213, 256–259;reviewed in Ref. 260). Together, the aforementioned obser-vations suggest a hypothesis that satellite cells enter differ-ent myogenic programs depending on whether Myf5 orMyoD expression predominates (450). Predominance ofMyoD expression would drive the program toward earlydifferentiation, as exemplified by the behavior of Myf5�/�

myoblasts (349). In contrast, predominance of Myf5 ex-pression would direct the program into enhanced prolifer-ation and delayed differentiation, as shown by the behaviorof MyoD�/� myoblasts (452). Finally, myoblasts coex-pressing both Myf5 and MyoD would exhibit the interme-diate growth and differentiation propensities as shown bymost of satellite cell-derived myoblasts. This hypothesis isconsistent with the observation that MyoD and Myf5 havedifferent expression profiles throughout the cell cycle.MyoD expression peaks in mid G1, whereas Myf5 expres-sion is maximal at the G0 and G2 phases of the cell cycle(273). Therefore, disruptions to the MyoD/Myf5 ratio maydetermine the choice of myogenic programs. This hypothe-sis also explains the spectrum of proliferation and differen-tiation potential observed in different primary myoblastclones cultured in vitro.

Several studies have revealed that MyoD expression in pro-liferating myoblasts is positively regulated by serum re-sponse factor (SRF), which binds to the serum responseelement (SRE) within the MyoD regulatory region (186,285). In proliferating myoblasts, SRF only drives low levelsof MyoD expression (286), whose activity is inhibited bycyclin D1 induced cyclin dependent kinase 4 (Cdk4) (587).However, the induction of MEF2 expression prior to differ-entiation enables MEF2 to out-compete SRF for the SREbinding site and leads to high levels of MyoD expressionand initiation of differentiation (286). This function ofMEF2 is further regulated by a member of the myocardinfamily of transcription factors, MASTR, whose expressionis upregulated in response to muscle injury (348).



Notably, our group recently revealed a pro-proliferationfunction of MyoD in myoblasts (191). We found that the �isoform of p38 kinase (p38�) phosphorylates MyoD, whichnegates the transcriptional activation potential of MyoDand leads to a repressive MyoD complex occupying theMyogenin promoter (191). This positive effect of p38� onmyoblast proliferation is also supported by the observationthat Myogenin is prematurely expressed in p38�-deficientmuscle, which displays markedly reduced myoblast prolif-eration (191). These results also support the notion that thefunctional state of MyoD depends on cofactors present inthe MyoD transcriptional complex.

Moreover, multiple studies demonstrated that MyoD ex-pression does not always warrant myogenic commitment.Monitoring satellite cell lineage progression revealed thatsome Pax7�/MyoD� proliferating myoblasts could retractback to a Pax7�/MyoD� state and eventually return toquiescence (127, 216, 584; discussed in sect. IIC). In addi-tion, the reciprocal inhibition of Pax7 with MRFs (MyoDand Myogenin) has been revealed in C3H10T1/2 fibro-blasts and MM14 myoblasts in vitro (385). It was foundthat Pax7 decreases MyoD transcription activity and stabil-ity, whereas Myogenin represses Pax7 transcription likelyvia the HMGB1-RAGE axis (385, 438). Based on theseobservations, it was proposed that the ratio of Pax7 andMyoD activities are critical for satellite cell fate determina-tion (385). A high ratio of Pax7 to MyoD (as seen in quies-cent satellite cells) keeps satellite cells in their quiescentstate. An intermediate ratio of Pax7 to MyoD allows satel-lite cells to proliferate, but not differentiate. Satellite cellswith a low Pax7-to-MyoD ratio begin to differentiate, andfurther reduction in Pax7 levels are observed following ac-tivation of Myogenin.

After limited rounds of proliferation, the majority of satel-lite cells enter the myogenic differentiation program andbegin to fuse to damaged myofibers or fuse to each other toform new myofibers. The initiation of terminal differentia-tion starts with the expression of Myogenin and Myf6 (alsocalled Mrf4) (114, 116, 207, 497, 572). The induction ofMyogenin expression primarily depends on MyoD and isproposed to enhance expression of a subset of genes previ-ously initiated by MyoD (83). Target genes of MyoD andMyogenin have been revealed by candidate approaches(405), ChIP-on-chip experiments (44, 56, 83), and morerecently by ChIP-Seq analysis (84). These investigationsjointly reveal a convoluted hierarchical gene expression cir-cuitry centered on MyoD and its immediate downstreamtargets: Myogenin and Mef2 transcription factors (Mef2s).Based on the temporal expression pattern of MyoD, Myo-genin and Mef2s, a feed-forward regulatory circuit is pro-posed. In this hypothesis, myogenic differentiation is anirreversible procedure and is driven by the sequential ex-pression of key transcription factors (master regulators),which are destined to transduce gene expression signals to

their target genes (45, 405). A large portion of target genesinduced by MyoD, Myogenin, and Mef2s are muscle-spe-cific structural and contractile genes, such as those encodingactins, myosins, and troponins. The expression of thesegenes is essential for the proper formation, morphology,and function of skeletal muscle and thus they are regulatedby multiple mechanisms (41).

First, the transcriptional activities of MyoD, Myogenin,and Mef2s are regulated by posttranscriptional modifica-tions (reviewed in Ref. 420). The � and � isoforms of p38kinase (p38-�/�) have an important role in the expression ofmuscle-specific genes (405) and in muscle terminal differen-tiation (571). The function of p38-�/� is at least partiallyresponsible for the phosphorylation of Mef2s as inhibitionof p38-�/� disrupts the transcriptional activities of Mef2s.In contrast, the expression of constitutively active forms ofp38-�/� promotes myogenesis (117, 221, 390, 420, 571,589). p38-�/� activity stimulates the binding of MyoD andMef2s to the promoters of muscle-specific genes, leading tothe recruitment of chromatin remodeling complexes, andultimately the RNA polymerase II holoenzyme (405, 424,491). Similarly, the transcriptional activity of Myogenin isregulated by protein kinase A (PKA) (308) and protein ki-nase C (PKC)(309). Protein inhibitors of MRFs also controlmyogenic gene expression. The transcriptional activity ofMRFs relies on heterodimerization with E proteins (ITF1,ITF2, E12, E47) (291, 362, 363). This heterodimerization isnegatively regulated by a group of inhibition of DNA bind-ing proteins (Ids: Id1, Id2, Id3, and Id4), which are alsohelix-loop-helix proteins but lack the basic DNA-bindingdomain (42, 43). Id proteins heterodimerize with E proteinsand prevent their association with MRFs, thus abrogatingmyogenic gene expression (43). Similar results occur whenMist1 directly interacts with MyoD and prevents MyoDfrom binding E-boxes (298). MyoD activity can be furtherinhibited by the sequestration of E proteins, by Twist (504).Finally, the transcriptional activity of MyoD is also deter-mined by specific cofactors present on the promoters ofmyogenic genes (reviewed in Ref. 450). In vitro, MyoDassociates with histone acetyltransferases (HATs) p300 andp300/CBP (CREB-binding protein)-associated factor(PCAF) on E-box motifs of its target genes (419). This as-sociation is presumed to induce histone acetylation andtranscriptional activation (45). MyoD also interacts withhistone deactylases (HDACs), which negatively regulate thetranscriptional activity of MyoD either directly (325) or ina Mef2-dependent manner (319).

Besides MRFs and their regulators, other factors have beenshown to be involved in myogenic differentiation. Micro-RNAs are 20–22nt noncoding small RNAs, which functionto repress translation and reduce the stability of their targetmRNAs. Recent studies demonstrated that MRFs, such asMyf5, MyoD, and Myogenin, activate the expression of acollection of myogenic microRNAs (e.g., miR-1, miR-133,



miR-206 together called MyomiRs). These myogenicmicroRNAs, in conjunction with other microRNAs, mod-ulate the expression levels of key myogenic transcriptionfactors and regulators, such as Pax3 (119, 196, 231), Pax7(94, 139), SRF (93), c-Met (526, 577), and Dek (98) duringsatellite cell activation/proliferation and differentiation(also reviewed in Refs. 374, 565). Furthermore, recent stud-ies have demonstrated the requirement for caspase-3 and itsactivation of CAD (caspase-activated DNase) in initiatingmyoblasts differentiation in vitro (164, 290). Activation ofCAD by caspase-3 induces double-stranded DNA breaks(DSBs) in the genome and the association of CAD withvarious target promoters. One such promoter is the cyclin-dependent kinase (CDK) inhibitor p21(Waf1/Cip1) whichfollowing binding by CAD is activated (290). MyoD alsoinduces the expression of p21(Waf1/Cip1) (209, 215) andsubsequent permanent cell cycle arrest. p21 is known todephosphorylate the retinoblastoma protein (Rb) and causethe inactivation of the Rb-associated E2F family of tran-scription factors known to activate S-phase genes (reviewedin Ref. 559). Thus, through this pathway, MyoD inducespermanent cell cycle withdrawal in myoblasts. Consis-tently, myoblasts in p21�/� mice are defective in cell cyclearrest and myotube formation, leading to increased apopto-sis (555, 588). Similarly, Rb-deficient myoblasts cannotcomplete cell cycle withdrawal and arrest during both S andG2 phases of the cell cycle (378).

After exiting the cell cycle, myogenic cells undergo cell-to-cell fusion to repair damaged myofibers or form nascentmultinucleated myofibers. The cellular events in this com-plex process have been extensively studied (274, 312, 418,428, 553). Akin to embryonic myogenesis, de novo forma-tion of myofibers during muscle regeneration happens intwo stages. In the first stage, individual differentiated myo-blasts fuse to one another and generate nascent myotubeswith few nuclei. In the second phase, additional myoblastsincorporate into the nascent myotubes, forming a maturemyofiber with increased size and expression of contractileproteins. In recent years, studies in myoblast fusion in vitrohave identified a cadre of cell surface, extracellular, andintracellular molecules, which are important for these twostages of myogenic cell fusion (reviewed in Ref. 238). Forexample, cell membrane proteins �1-integrin (474), VLA-4integrin (445), integrin receptor V-CAM (445), caveo-lin-3 (180), and transcription factor FKHR (Forkhead inhuman rhabdomyosarcoma, also called FOXO1a) havebeen shown to act in myoblast-to-myoblast fusion.Whereas the cytokine IL-4 (238) and calcium and cal-modulin activated NFATC2 (nuclear factor of activatedT cell isoforms C2) pathway (237) is critical for the fu-sion of myoblasts with nascent myotubes.

As discussed here, activation of satellite cells following mus-cle injury results in the expansion of the myogenic cell pooland leads to the initiation of the myogenic program. This

program, orchestrated by key transcription factors, dictatesthe balance between proliferation and differentiation anddrives the functional transformation from individual prolif-erating myogenic cells to a syncytial contractile myofiber.Although numerous studies have shed light on this complexprocess, there are still many interesting questions that re-main to be answered. For example, what are the intrinsicand extrinsic mechanisms that determine the scale of satel-lite cell proliferation in vivo, which essentially generate suf-ficient but not excessive numbers of myogenic cells for mus-cle regeneration? Similarly, what mechanisms regulate themagnitude of muscle regeneration in response to variablelevels of damage and eventually prevent muscle atrophy orhypertrophy? With the recent advances in high-throughputsequencing and system biology, is it possible to elucidate acore regulatory network of myogenic gene expression andemploy such knowledge on directing myogenic determina-tion and differentiation from multipotent cells? The an-swers to these questions will improve our understanding ofmuscle regeneration and facilitate future development oftherapeutic approaches to cure muscle diseases.

C. Satellite Cell Self-Renewal

A hallmark of stem cells is the ability to self-renew. Stemcells can divide and self-renew in two fashions: asymmet-ric cell division and symmetric cell division. In asymmet-ric cell division, one parental stem cell gives rise to twofunctionally different daughter cells: one daughter stemcell and another daughter cell destined for differentia-tion. In symmetric cell division, one parental stem cell di-vides into two daughter stem cells of equal stemness. Ineither fashion, the number of stem cells is maintained at aconstant level. Stem cell self-renewal by asymmetric celldivision is exemplified by neuronal stem cells (590). Stemcell self-renewal by symmetric cell division is frequentlyobserved in hematopoietic stem cells and mammalian malegermline stem cells (spermatogonia) (570).

The self-renewing capability of satellite cells is clearly dem-onstrated by their remarkable ability to sustain the capacityof muscle to regenerate. For example, in a single myofibertransplantation experiment (105), 7–22 satellite cells to-gether with their intact myofibers were transplanted intoirradiated muscles of immunodeficient dystrophic (scid-mdx) mice. It was found that one grafted myofiber can giverise to over 100 new myofibers, which contain �25,000–30,000 differentiated myonuclei. In addition, the graftedsatellite cells can undergo a 10-fold expansion via self-re-newal. The expanded satellite cells are functional as theycan be activated and support further rounds of muscle re-generation (105). Similarly, the self-renewing capability ofsatellite cells was further proven by single satellite cell trans-plantation experiments (454). In this study, single Lin�/�7-integrin�/CD34� cells freshly isolated by FACS were trans-planted into irradiated muscles of scid-mdx mice. It was



observed that progeny from these single cells not only gen-erated myofibers, but also migrated to the satellite cell nicheand persisted in host muscles (454).

An intriguing question is how satellite cells renew them-selves. By using the Cre-LoxP based permanent lineage trac-ing technique (Myf5Cre;R26R-loxP-stop-loxP-YFP), ourgroup first demonstrated that satellite cells can undergoboth asymmetric and symmetric divisions within their nat-ural niche environment in mildly damaged EDL muscle(280). The choice of asymmetric versus symmetric divisionis largely correlated to the mitotic spindle orientation rela-tive to the longitude axis of the myofiber. Asymmetric divi-sions are only observed for Pax7�/Myf5� satellite cells,which give rise to one satellite stem cell (Pax7�/Myf5�) andone satellite myogenic (Pax7�/Myf5�) cell. Asymmetric di-vision predominantly happens when the mitotic spindle isperpendicular to the myofiber axis (apical-basal division)with the satellite stem cell (Pax7�/Myf5�) in close contactwith the basal lamina (basal) and the Pax7�/Myf5� satellitemyogenic cell adjacent to the myofiber plasma membrane(apical). Occasionally, it was observed that Pax7 expressionis dampened in the apical satellite cell, suggesting a progres-sion towards terminal differentiation. Furthermore, theasymmetric nature of this kind of division is also under-scored by the observation that the apical Pax7�/Myf5�

cells express higher levels of the Notch ligand Delta-1,whereas the basal Pax7�/Myf5� satellite cells expresshigher levels of the Notch-3 receptor. Symmetric divisionswere observed for both Pax7�/Myf5� satellite cells andPax7�/Myf5� satellite cells and in either case the mitoticspindle was frequently parallel to the myofiber axis (planardivision) and both daughter cells were in contact with themyofiber plasma membrane and the basal lamina.

When both satellite stem cells and satellite myogenic cellsare freshly sorted and transplanted into Pax7�/� muscleslacking any functional endogenous satellite cell, both celltypes can colonize the host muscle. However, only Pax7�/Myf5� satellite cells can efficiently reconstitute the satellitecell compartment (�7-fold more efficient than Pax7�/Myf5�). These observations indicate that Pax7�/Myf5�

satellite cells represent a bona fide stem cell population,which can undergo self-renewal by both asymmetric divi-sion and symmetric division (280).

Although asymmetric self-renewal may be sufficient underphysiological conditions, satellite stem cells may favor sym-metric self-renewal divisions to replenish and expand theirpopulation in response to acute need for a large number ofsatellite cells after injury or disease state (354). Indeed, itwas observed that Pax7�/Myf5� satellite stem cells repre-sent �10% of total satellite cells in intact muscles, whereasthis percentage increases to �30% at 3 wk after injury(280). In search of intrinsic and extrinsic cues regulatingPax7�/Myf5� satellite stem cell self-renewal, our group fur-

ther revealed that symmetric self-renewal is promoted by aniche factor, Wnt7a, during muscle regeneration (293; dis-cussed in sect. IIIA1A). Therefore, the mode of satellite cellself-renewal is governed by the satellite cell niche.

With regards to activated satellite cells returning to quies-cence, observations on the dynamic expression of Pax7,MyoD, and Myogenin in ex vivo cultured single myofiberssuggest it is possible (127, 216, 584). After myofiber isola-tion (the isolation procedure per se is a form of injury toinitiate muscle regeneration), satellite cells are activatedand rapidly start to express MyoD (572, 584). It has beenobserved that almost all myoblasts express MyoD after�24 h of in vitro culture (584). While most of theseproliferating Pax7�/MyoD� myoblasts differentiate intoPax7�/MyoD�/Myogenin� cells after 72 h of culture, somePax7�/MyoD� myoblasts (also called reserve cells) none-theless have been observed to repress MyoD expression andmaintain Pax7 expression and eventually withdraw fromthe cell cycle (216, 584). These Pax7�/MyoD� satellite cellsacquired the quiescence marker Nestin, albeit at a muchlater (3–4 wk in culture) stage (127). These observationssuggest that quiescent satellite cells renew by lineage regres-sion from committed myogenic cells at least in vitro, andthis renewal may be promoted by Pax7 (584). In addition,the capability of activated satellite cells to generate reservecells may be an important mechanism to maintain the sizeof the satellite cell pool, as reduction of such capability wassuggested to lead to decreased numbers of satellite cells withage (128). Intriguingly, this lineage regression has also beenrecently observed in vivo (482). Sprouty-1, a negative reg-ulator of receptor tyrosine kinase (RTK) signaling, plays acritical role in satellite cell renewal (482). Sprouty-1 is ex-pressed in quiescent but not proliferating satellite cells (177,482). Upon injury, released fibroblast growth factor (FGF)and hepatocyte growth factor (HGF) act through the RTK/ERK pathway to stimulate satellite cell proliferation (482).Most satellite cells entered the cell cycle during regenerationas indicated by BrdU labeling. It was confirmed that somesatellite cell-derived myogenic progenitors withdrew fromthe cell cycle, returned to the satellite cell compartment, andregained Sprouty-1 expression, indicative of satellite cellrenewal in vivo. In the absence of Sprouty-1, it was ob-served that satellite cell-derived myogenic progenitors pro-liferate and differentiate normally, but markedly reducednumbers of satellite cells exist after regeneration. Furtherinvestigations revealed that Sprouty-1 is required only for adistinct subset of myogenic progenitors to return to theirquiescence state (482). These results indicate that Sprouty-1is essential for some satellite cells to regain quiescence fol-lowing regeneration. The function of Sprouty-1 in this pro-cess is most likely due to its inhibitory effect on the ERKpathway and subsequent enhanced cell cycle withdrawal. Itshould be stressed that this renewal of satellite cells at apopulation level and the aforementioned self-renewal ofsatellite cells at a single-cell level are not mutually exclu-



sive but rather represent two different perspectives of thesame regeneration process. Future studies are needed toidentify the essential differences between Sprouty-1-de-pendent and Sprouty-1-independent populations and un-derstand whether these differences reflect any other func-tional distinction. In addition, it would be interesting toinvestigate the regulatory mechanisms governing Sprouty-1expression, for example, whether the reappearance ofSprouty-1 is regulated by Pax7 or Wnt signaling.

D. Contributions and Therapeutic Potentialof Nonsatellite Cells in Trauma-InducedMuscle Regeneration

1. Bone marrow stem cells

The myogenic potential of bone marrow cells was demon-strated by either intravenous injection of bone marrow cellsinto subjects following muscle injury or directly via intra-muscular injection of whole bone marrow into injured mus-cles. Both of these methods led to the incorporation of bonemarrow-derived cells into newly formed myofibers withinregenerating muscles (54, 166, 210). Because of the ex-tremely low efficacy of incorporation, doubts persist re-garding the ability of bone marrow-derived cells to recon-stitute the satellite cell niche (210). Nevertheless, lineagetracing experiments demonstrated that bone marrow cells,when systematically injected into irradiated mice, were ableto reconstitute the satellite cell niche and expressed satellitecell markers, such as Myf5, c-Met, and �7-integrin (287).These bone marrow-derived satellite cells can undergomyogenic differentiation in vitro and give rise to myofiberswhen injected into damaged muscles (287). Adult bonemarrow contains hematopoietic stem cells (HSC) and stro-mal cells (also called mesenchymal stem cells). Due to a lackof a standardized definition, conflicting results exist regard-ing the exact bone marrow-derived progenitor cell type(s)that contribute to muscle regeneration. Bone marrow-resid-ing hematopoietic stem cells, isolated by cell surface mark-ers c-Kit�/Sca-1� or CD45�/Sca-1�, or CD45�/c-Kit�/Sca-1� are able to incorporate into newly forming myofi-bers (1, 78, 112, 581), although it was unclear whether theprogeny of these cells were able to occupy the satellite cellniche. It was also demonstrated that Mac-1low bone mar-row SP cells, particularly those c-Kit�/CD11b� immaturecells, were able to generate myofibers (147, 382). On theother hand, mesenchymal stem cells were able to incorpo-rate into myofibers in injured muscles (140, 488) and gaverise to Pax7� satellite cells, which can support multiplerounds of muscle regeneration (140). Interestingly, the highmyofiber turnover rate induced by muscle stress and injuryseems to facilitate bone marrow cell engraftment into regen-erating muscles (1, 68, 287, 460, 487). These results furtheremphasize the importance of the microenvironment on sat-ellite cell specification. In accordance with this, stromal cell-derived factor-1 (SDF-1)/chemokine receptor 4 (CXCR4)

signaling and chemokine receptor 2 (CCR2) have been im-plicated in guiding bone marrow cells towards muscle in-jury sites (429, 510). However, it has also been reportedthat SDF-1/CXCR4 signaling is dispensable for the homingof CD45� hematopoietic lineage cells to injured muscle,which are instead responsive to HGF/c-Met signaling (447).Overall, it is noteworthy that bone marrow transplantationhas not been reported to robustly or therapeutically ame-liorate any muscle disease. Future studies are required todefine the subpopulation(s) of bone marrow cells, whichhave long-term myogenic potential in vivo especially in thecontext of muscle diseases. Also, it would be interesting toinvestigate whether bone marrow-derived cells are involvedin postnatal muscle growth since satellite cells with similarmarkers have been identified in adult muscles.

2. Muscle side population cells

Similar to bone marrow, skeletal muscle contains a uniquecell population termed muscle side population (SP) cells,which can be isolated based on their ability to effluxHoechst 33342 dye (210, 245). The majority of muscle SPcells are characterized by CD45�/c-Kit�/Sca-1�/ABCG2�/Pax7�/Myf5�/Desmin� and reside in the skeletal muscleinterstitium juxtaposed to blood vessels, which make themdistinct from satellite cells and bone marrow-derived SPcells (24, 146, 210, 439, 464). Multiple lines of evidenceindicate that muscle SP cells are heterogenic (24, 464). Ini-tially, a minor population of CD45� muscle SP cells wasidentified (24, 464, 478). These CD45� muscle SP cellspersist in Pax7�/� germline null mice and can efficientlydifferentiate into hematopoietic cells in culture (24, 478). Inaddition, CD45� muscle SP cells also have myogenic poten-tial when isolated from both wild-type and Pax7�/� mice.They can be induced to differentiate into myogenic cellsfollowing coculture with primary myoblasts or through theforced expression of Pax7 or MyoD (24, 477). In addition,lineage tracing experiments have revealed that only themain population of CD45� muscle SP cells arise from em-bryonic Pax3� hypaxial somitic cells, suggesting the minorCD45� SP cells might have distinct origins, possibly fromendothelial, hematopoietic stem, or bone marrow cells(464). This is consistent with results from transcriptomeanalysis of muscle SP cells in resting and regenerating mus-cles, which indicate the increased expression of c-Kit andCD45 in the muscle SP population 5 days after injury (337).Furthermore, it was also demonstrated that somite-derivedCD45� SP cells have more myogenic potential than CD45�

SP cells when cultured in vitro, suggesting that the develop-mental origin of SP cells affect their intrinsic myogenic ca-pacity (464). Lastly, a recent study revealed that a subpop-ulation of satellite cells also have an SP phenotype (518).These SP cells that reside in the satellite cell compartment,termed satellite-SP cells, are characterized by CD45�/Sca-1�/ABCG2�/Pax7�/Syndecan-4�, which makes them dis-tinct from the majority of CD45� SP cells in muscle. Fol-lowing direct injection into regenerating muscles, the grafted



muscle SP cells were found in myofibers and the satellite cellniche, which suggests that these cells can contribute to long-term muscle regeneration (24). Importantly, when introducedintravenously into mdx mice, muscle SP cells from wild-typemice incorporate into host myofibers and restore their dys-trophin expression (27, 210), suggesting these cells are ableto migrate to regenerating muscles through the blood-stream. Indeed, compared with satellite cells and their de-rivatives, muscle SP cells are much more efficient at engraft-ing into mdx hosts after systemic delivery (368). Interest-ingly, the systemic delivery of wild-type SP cells into mdxmice can also repopulate the bone marrow of an irradiatedhost, although at lower efficacy compared with bone mar-row-derived SP cells (210).

Muscle SP cells constitute a distinct cell population basedon their unique properties. However, it is still unclearwhether the SP phenotype per se is of any importance to themuscle regeneration process. Although muscle SP cells out-perform the main population in muscle regeneration, futurestudies are needed to carefully compare muscle SP cells withother defined myogenic precursors, such as non-SP satellitecells, based on their myogenic potential.

3. PW1� interstitial cells

It has been reported that some interstitial cells, characterizedby their expression of PW1 (also called peg3), might be in-volved in perinatal skeletal muscle growth (345). Intriguingly,a recent study from the same group reported that adult stemcells/progenitors residing in multiple tissues/organs can be di-rectly identified and isolated from PW1-reporter mice (46).PW1 is encoded by a �8.5 kb long, intronless transcript,which is strongly transcribed in skeletal muscle in bothembryonic and postnatal stages (435). Within skeletal mus-cle, PW1 expression was detected in both satellite cells anda group of Sca-1�/CD34�/Pax7� interstitial cells (PIC)(351, 376, 476). In transgenic mice with a dominant nega-tive form of PW1 (�PW1) driven by a Myogenin promoter,embryonic and fetal muscle development is normal but post-natal muscle growth is severely impaired with a profound phe-notype, to some extent, reminiscent to that of Pax7�/� germ-line mutant mice (376, 478). Compared with wild-typemice, the number of quiescent satellite cells is markedlyreduced in postnatal muscle of �PW1 mice (376). In con-trast, the number of Pax7�/PW1� interstitial cells (PIC) issignificantly increased within postnatal muscles fromPax7�/� mice (345), which is accompanied by progressiveloss of Pax7� satellite cells (345, 380, 478). Interestingly,PICs have myogenic potential when cultured in vitro (345).This myogenic potential is enhanced by coculture with sat-ellite cells and is compromised in PICs isolated fromPax7�/� mice (345). Importantly, when transplanted intoinjured muscles, FACS isolated Sca-1�/CD34�/CD45�

PICs give rise to both interstitial cells and Pax7� satellitecells, whereas Sca-1�/CD34�/CD45� cells (enriched forsatellite cells) only derive into Pax7� satellite cells (345).

Although not directly assessed by lineage tracing, these ob-servations together support a hypothesis that PICs may con-tribute to the satellite cell population during postnatal mus-cle growth and during adult muscle regeneration (345). Inaddition, Pax7 expression is presumably essential for themyogenic specification of PICs as PICs isolated fromPax7�/� mice cannot give rise to myogenic cells in vitro(345). Of note, Pax3 lineage tracing experiments furtherindicated that PICs do not arise from embryonic Pax3�

myogenic progenitor cells, indicating PICs and satellite cellsare derived from distinct lineages (345).

These intriguing observations also raised several issues re-garding the identity and lineage progression of the Sca-1�/CD34�/CD45�/(�50% PW1�) interstitial cells. A recentstudy indicated that FAPs isolated from adult muscle arecharacterized by similar markers Sca-1�/CD34�/CD45�

(also CD31�) but only give rise to adipogenic and fibro-genic lineages in vitro (252 and discussed in sect. IIIB1A).Coculture with myogenic cells cannot promote the myo-genic potential of FAPs (252). Accordingly, FAPs also donot assume the myogenic lineage in vivo when transplantedinto either uninjured or injured muscles. These discrepantobservations on PICs and FAPs lead to two possibilities:1) FAPs are depleted of CD31� cells and thus the myogenicpotential resides in these CD31� cells; or more likely2) PICs and FAPs represent the same cell population but inyoung and adult stages, respectively. Notably, muscle SPcells (Sca-1�/ABCG2�) are also CD34� and thus could beoriginating from the same lineage as FAPs and PICs (226).Future studies may provide direct answers in terms of thelineage origin and hierarchy of these cell types. Future in-vestigations should also focus on the mechanisms that reg-ulate the myogenic potential of these interstitial cells.

4. Muscle-derived stem cells

A series of studies have demonstrated that a distinct cellpopulation, termed muscle-derived stem cells (MDSCs), canbe isolated from skeletal muscles based on their weak ad-hesion characteristics and long-term proliferation behav-iors in culture (247, 294, 421, 422). Further characteriza-tion of MDSCs indicated that these cells are CD45�/M-Cadherin�/CD34�/Flk-1�/Sca-1�/Desmin�, which makesthem distinct from satellite cells and hematopoietic cells butmore similar to muscle SP cells (294, 421). MDSCs andsatellite cell-derived myoblasts in culture appear to repre-sent two distinct populations as evidenced by the findingthat MDSCs can be isolated from Pax7�/� germline nullmice (318). Like muscle SP cells, the myogenic differentia-tion of MDSCs depends on Pax7 (318). A unique charac-teristic of MDSCs is their multipotency (294, 421, 536). Ithas been reported that MDSCs can differentiate into myo-genic, adipogenic, osteogenic, chondrogenic, and hemato-poietic lineages (reviewed in Ref. 404). After intra-arterialtransplantation, MDSCs contribute to regenerated myofi-bers, incorporate into the satellite cell niche, and also give



rise to endothelial and neural cells (421). Interestingly, likemuscle SP cells, MDSCs can also repopulate the hematopoi-etic lineage in irradiated murine hosts, and the reconstitutedbone marrow can in turn contribute to muscle regeneration(82). In addition, MDSCs are more efficient than myoblastsat forming dystrophin� myofibers when directly trans-planted into mdx mice (243). Due to the prolonged isola-tion procedure of MDSCs and the accompanied dynam-ics of gene expression, it is difficult to trace the origin ofMDSCs in vivo based on cell surface markers. Futureinvestigation by candidate lineage tracing and clonalanalysis may help to elucidate the lineage relationshipbetween MDSC and other myogenic cells.

5. Mesoangioblasts

Mesoangioblasts arise from the embryonic dorsal aorta andare characterized by CD34�/c-Kit�/Flk-1�/NKX2.5�/Myf5�/Oct4� expression (129, 343). Mesoangioblasts arehighly proliferative when cultured in vitro, and long-termcultured mesoangioblasts are multipotent, being able togive rise to multiple mesodermal lineages, such as bone,cartilage, smooth, cardiac, and skeletal muscle followingtransplantation (343). Although the contribution of me-soangioblasts to normal muscle development is controver-sial, multiple studies have demonstrated that mesoangio-blasts can be employed to facilitate muscle regeneration,particularly for dystrophic muscles. First, it was found that�-sarcoglycan�/� mesoangioblasts can be transduced invitro with �-sarcoglycan expressing lentiviral vector androbustly generate �-sarcoglycan� myofibers followingtransplantation into �-sarcoglycan�/� mice (a model forlimb girdle muscular dystrophy) via intra-artery injection(458). The high efficiency of normal myofiber reconstitu-tion observed in this study suggests that intrinsic propertiesof mesoangioblasts may help them travel to regeneratingmuscles via the bloodstream. In addition, mesoangioblastscan release immunosuppressive and tolerogenic molecules,which supposedly help mesoangioblasts incorporate intoallogeneic dystrophic hosts (211). The myogenic potentialof mesoangioblasts isolated from human patient biopsiessuffering from dermatomyositis (DM), polymyositis (PM)and inclusion-body myositis (IBM) were compared (352). Itwas observed that mesoangioblasts from the IBM patientscould not differentiate into myofibers when cultured invitro or xenotransplanted into injured mouse muscles. In-terestingly, this defect can be rescued by either transientoverexpressing of MyoD or knockdown of a MyoD inhib-itor protein, bHLH-B3, which suggests that the myogenicprogram of mesoangioblasts is MyoD-dependent (352).Several investigations demonstrated that factors involved innormal muscle regeneration also facilitate mesoangioblast-based muscle regeneration. For example, HMGB1, SDF-1,CXCR4, and �4-integrin have been shown to enhance themigration of mesoangioblasts from blood vessels towardinjured muscles (182, 399). Similarly, pretreatment of me-soangioblasts with nitric oxide before transplantation also

stimulated �-sarcoglycan expression in mesoangioblast-de-rived myofibers (475). Taken together, investigations onmesoangioblasts have revealed great therapeutic potentialfor the amelioration of human dystrophic diseases. Futurestudies may focus on identifying the adult counterparts ofthese cells and characterizing their myogenic potentials.

6. Pericytes

Pericytes (also called Rouget cells or Mural cells) are con-tractile connective tissue cells residing beneath the micro-vascular basement membrane. Pericytes originate from theembryonic sclerotome and are believed to regulate theblood flow in capillaries (415, 284, 406). As a multipotentstem cell population, pericytes can differentiate into adi-pocytes (161), chondrocytes (161), and osteoblasts (142) invitro. Two recent studies indicated that pericytes, which donot express Pax7, Myf5, or MyoD, can differentiate intoskeletal muscles both in vitro and in vivo (135, 136), sug-gesting pericyte-mediated myogenesis may follow a myo-genic differentiation program distinct from that of satellitecells. FACS sorted human pericytes display the followingcell surface marker combination: CD45�/CD34�/CD56�/CD144�/CD146�/PDGFR-�1�/NG2 proteoglycan�, whichmakes them distinct from embryonic mesoangioblasts, he-matopoietic cells, or satellite cells (136). Intriguingly, cellscarrying the same surface marks can be isolated fromvarious tissues, including pancreas, adipose, and pla-centa and differentiate into skeletal muscle cells whencultured in vitro, irrespective of their origins (118). Thefact that pericytes also display cell surface markers char-acteristic of mesenchymal stem cells (CD10�/CD13�/CD44�/CD73�/CD90�) raised the hypothesis that peri-cytes may represent a significant portion of mesenchymalstem cells in the adult (85, 118, 163). Xenographic trans-plantation of human pericytes into combined immune defi-cient-X-linked, mouse muscular dystrophy (scid-mdx) micethrough the femoral artery gave rise to numerous myofibersexpressing human dystrophin (136). Interestingly, a smallportion (3–5%) of transplanted pericytes incorporated be-neath the basal lamina of myofibers, indicating that peri-cytes are able to occupy the satellite cell niche in dystrophicmuscle. Importantly, pericyte transplantation significantlyimproved the physiological performance of treated dystro-phic muscles, indicating that pericyte-derived myofibers arefunctional. Similar promising results were achieved in aprototype experiment, wherein pericytes isolated fromDuchenne patients were transduced in vitro with a humanmini-dystrophin expressing lentiviral vector and trans-planted into scid-mdx mice (136). The myogenic potential,together with their abilities to be cultured in vitro and pene-trate the blood vessel wall, makes pericytes a promising can-didate for future cell-based therapies to treat muscular dystro-phy. Future research may investigate whether transplantedpericytes possess sufficient long-term myogenic capability indystrophic muscles.



7. CD133� cells

Recent studies demonstrated that a fraction of CD133�

(also called AC133) mononucleated cells in adult peripheralblood have myogenic potential (535). Freshly isolated hu-man CD133� cells can undergo myogenic differentiationwhen cocultured with myogenic cells or exposed to Wnt-producing cells in vitro (535). Importantly, when xeno-transplanted via intra-arterial or intramuscular injection,these CD133� cells can fuse with regenerating muscles inscid-mdx mice and improve the force generation of treatedmuscles. Moreover, donor human CD133� cells, express-ing satellite cell markers M-Cadherin and Myf5, were de-tected in the satellite cell compartment after transplanta-tion, indicating they can reconstitute the satellite cellniche (535). A small subpopulation of CD133�/ CD34�

double positive cells can be identified in the blood andskeletal muscle interstitium (233). Similar to blood-de-rived CD133� cells, human muscle-derived CD133� cellsmanifested remarkable myogenic differentiation capacity inregenerating muscle (372). As CD133� cells can be readilyisolated from the blood, manipulated in vitro and deliveredthrough the circulation, a couple of studies have exploredtheir application to treating Duchenne muscular dystrophy(DMD). In one of these studies, CD133� cells isolated fromhuman dystrophic muscles were genetically engineered exvivo to restore dystrophin expression and subsequently de-livered into scid-mdx mice (41). Treated dystrophic muscleswere improved in terms of muscle morphology, function,and dystrophin expression, although the long-term effect ofthis treatment has not been documented so far. The safetyof this CD133� cell therapy (without dystrophin correc-tion) was later confirmed in human DMD patients by au-tologous transplantation (534). Taken together, CD133�

cells are one of the most promising candidates for cell-basedtherapy of DMD. Future studies should focus on whetherthese cells can survive long term and support repeated boutsof regeneration in DMD patients.

8. Embryonic stem cells and induced pluripotentstem cells

Embryonic stem (ES) cells are pluripotent stem cells, whichcan differentiate into all three germ layers and hence havebeen extensively studied for cell-based therapies. In an earlystudy, mouse ES cells cocultured with muscle cells weretransplanted into irradiated mdx mice by intramuscular in-jection (47). Donor-derived myofibers were occasionallyfound on the surface of the host muscles. The low efficacy ofengraftment in this study suggested that selective markersmight be necessary to enrich myogenic precursors from EScell cultures. In a later study, a subpopulation of CD73�/N-CAM� human ES cells, which are enriched for mesen-chymal precursors, were injected into regenerating muscleof scid mice (31). It was observed that human donor cellsgave rise to myoblasts and established prolonged engraft-ment; however, the overall efficiency was not compared

with other myogenic cells, such as satellite cells. Recently,several strategies have been established to achieve persistentengraftment and avoid teratoma formation. In one study,Pax3 was transiently expressed to induce paraxial meso-derm formation during embryoid body differentiation, andPDGFR-�, a paraxial mesoderm marker, was subsequentlyused to isolate a homogeneous population of proliferatingmyogenic progenitors (124). The transplantation of thispopulation into mdx mice resulted in improved musclefunction, and teratoma formation was prevented. In an-other study, PDGFR-�� cells were directly isolated from EScells in an earlier stage without Pax3 induction (455). It wasfound that these early paraxial mesodermal cells, whentransplanted into injured muscles, could give rise to bonafide satellite cells that can further differentiate into func-tional myofibers. Similarly, transient expression of Pax7seems to facilitate the myogenic specification of ES cells(125). Recently, it was shown that SM/C-2.6� cells iso-lated from cultured embryoid bodies can differentiateinto skeletal muscle myofibers both in vitro and in vivo(88). When transplanted into injured muscles, these cellscan incorporate into the satellite cell compartment andexpress satellite cell markers, such as Pax7 and M-cad-herin. Importantly, these ES-derived satellite cells un-dergo extensive self-renewal during subsequent muscleinjury and can be further transplanted with high engraft-ment efficiency (88).

Over the last five years, various types of differentiated so-matic cells have been successfully induced into pluripotentstem cells (iPS cells) via a process involving transcriptionfactor based reprogramming (371, 384, 515, 516, 582).Given their adult somatic origin and pluripotent nature, iPScells hold tremendous potential for cell-based therapy. In anattempt to apply iPS cells to muscle regeneration, a recentstudy demonstrated that SM/C-2.6� cells isolated frommouse iPS cells have comparable regenerative potential tothose from mES cells (346). In addition, conditional expres-sion of Pax7 in human iPS cells was successful in inducinglarge quantities of myogenic progenitors, which were ableto efficiently engraft and produce abundant human-deriveddystrophin� myofibers in dystrophic mouse muscle (123).Notably, a new strategy of iPS cell-based therapy to treatDMD was reported recently (267). In this study, iPS cellswere derived from mdx mice or a human DMD patient, andthe genetic deficiency of dystrophin in these cells was cor-rected by transferring a human artificial chromosome(HAC) containing a complete genomic dystrophin sequencevia microcell-mediated chromosome transfer (MMCT).These genetically corrected iPS cells can differentiate intomuscle-like tissues and can be used to generate chimericmice, wherein tissue-specific expression of dystrophin wasobserved (267). These inspiring results warrant future in-vestigations on regulatory mechanisms governing the myo-genic specification of ES and iPS cells.



E. Satellite Cells in Perinatal/JuvenileMuscle Growth

In neonatal mouse muscle, satellite cells account for �30–35% of the sublaminal nuclei on myofibers (9, 227, 471).This proportion of satellite cells decreases while the numberof myonuclei increases over postnatal growth (150). Exper-iments by [3H]thymidine labeling indicate that satellite cellsare proliferative in growing muscle, give rise to myonuclei,and continue to fuse with myofibers (355, 470, 481). Thecell cycle time of juvenile satellite cells in growing rat mus-cles was determined to be �32 h, with 14 h within the Sphase (471). This high proliferation rate of juvenile satellitecells is likely due to a large requirement for muscle growthat this stage. Like their adult counterparts, juvenile satellitecells are also likely heterogeneous. By continuous BrdU la-beling and tandem BrdU/[3H]thymidine labeling, two sub-populations of satellite cells in growing muscle have beenidentified (471). A fast-cycling satellite cell population, ac-counting for �80% of the cells, was readily labeled over thefirst 5 days of continuous BrdU infusion. In contrast, slow-dividing satellite cells incorporated BrdU at a much slowerrate, and some of them were not labeled with BrdU evenafter an additional 9 days. Moreover, only a small portionof the satellite cells labeled with BrdU during the first 5 dayscould be labeled with [3H]thymidine during a second 5-daycontinuous infusion. This observation suggests that 1) themajority of fast-cycling satellite cells only undergo a limitednumber of mitotic divisions prior to fusion, and 2) the num-ber of fast-cycling satellite cells in growing muscle are main-tained by the slow-cycling satellite cells through asymmetricdivisions (471). Interestingly, the functional difference andlineage relationship between these two satellite cell popula-tions during postnatal muscle growth largely resemble thatof satellite cells and myoblasts during adult muscle regen-eration. This analogy may reflect a common scheme of myo-genesis in both postnatal muscle growth and adult muscleregeneration.

Despite their similarities, studies have revealed distinct ge-netic requirements for both juvenile and adult satellite cellpopulations. Pax7 is required for the maintenance of a func-tional myogenic cell population in the perinatal/juvenilestage, as muscle growth and regeneration are severely im-paired in Pax7 germline null mutants during this period(279, 393, 478). In contrast, Pax7 seems to be dispensablefor adult muscle regeneration in Pax7 conditional knockoutmutants, wherein Pax7 expression is ablated only in theadult stage (299). In conditional Pax7 knockout mice, sat-ellite cells, expressing M-cadherin but not Pax7, were ob-served in the satellite cell compartment in adult muscle afterone round of regeneration, and these cells can support asecond round of regeneration (299). The regenerative ca-pacity of Pax7� satellite cells was not due to a compensa-tory mechanism by Pax3, as muscle regeneration is notimpaired by simultaneous deletion of both Pax7 and Pax3in adulthood. It is well known that both juvenile and adult

satellite cells express Pax7 (478). The apparent distinct re-quirement for Pax7 in juvenile but not adult muscle regen-eration may suggest that Pax7 is only necessary for theinitial establishment of perinatal satellite cells and/or denovo myogenic specification of other nonsatellite cell lin-eages, which presumably contribute to the total satellite cellpool during postnatal growth. In contrast, the maintenanceof satellite cell establishment might be Pax7 independentand the rare occasion of de novo myogenic specificationfrom other nonsatellite cell lineages may be negligible inadulthood. In this sense, the sustained expression of Pax7 inadult satellite cells may be due to sustained interactionswith their niche, rather than a necessity for Pax7 function.In support of this hypothesis, genetically marked hemato-poietic stem cells (bone marrow derived CD45�/c-Kit�/Sca-1�/Lin� cells) have been shown to express Pax7 once theyare present in the satellite cell niche (148, 581).

III. ANATOMIC AND FUNCTIONALDIMENSIONS OF THE SATELLITECELL NICHE

Based on the stem cell niche concept, behaviors of tissue-specific stem cells are determined by structural and bio-chemical cues emanating from the surrounding microenvi-ronment (380). In recent years, adult stem cells and theircorresponding niches were identified in numerous tissuesincluding bone marrow (566), brain (322), testis (240), liver(528), intestine (55), heart (302), white fat (441, 519), andskin (174). Similar to these adult stem cells, satellite cells arealso present in a highly specified niche, which consists of theextracellular matrix (ECM), vascular and neural networks,different types of surrounding cells, and various diffusiblemolecules (FIGURE 3). Furthermore, satellite cells, as one ofthe niche components, also influence each other by means ofcell-cell interaction and autocrine or paracrine signals. Thedynamic interactions between satellite cells and their nichespecifically regulate satellite cell quiescence, self-renewal,proliferation, and differentiation. In this fashion, muscleatrophy or excessive growth is prevented, and the satellitecell pool is maintained during regeneration. Understandingthe interactions between satellite cells and their niche is ofparamount importance for the development of therapies totreat both age-related skeletal muscle atrophy (sarcopenia)and skeletal muscle diseases. In the following section, wewill define the scope of the satellite cell niche and discusshow these niche factors affect satellite cells during muscleregeneration.

A. The Immediate Niche

1. Satellite cell: regulatory signaling pathways

A) WNT SIGNALING. Wnt signaling controls diverse biologicalprocesses, such as cell proliferation, cell fate determination,



Myotube

Satellite cell

Muscle fascicle

Tibia

Androgens

Nitric Oxide–Epithelial cells–Endothelial cells–Fibroblasts–Macrophages–Myofibers

IL-6–Myofibers_Neutrophils–Satellite cells

Immune cells–Monocytes–Macrophages–Neutrophils

Fibula

Sarcolemma

Glycoproteins

Basal lamina

Collagen

Entactin

Laminin

Decorin

Proteoglycans

Perlecan

Fibronectin

Wnt7a

bFGF

SDF-1

HGFEGF

IGF

Fibroblast

Capillaries

Motorneurons(axons)

Connectivetissue

Perimysium

Adipocytes

Pericytes

ibfifi

Signaling within the Systemic Milieu

Signaling within the Local Milieu

Signaling within the Immediate Niche



cell adhesion, cell polarity, and morphology. Wnt signalingis activated by binding of extracellular Wnt family glyco-proteins with Frizzled receptors and low-density lipopro-tein receptor-related protein (LRP5, LRP6) pairs. The Friz-zled receptor can initiate two distinct signaling pathways:the Wnt/�-catenin and Wnt/PCP pathways. Wnt/�-cateninpathway is characterized by the regulation of �-catenin sta-bilization and its entry into the nucleus. The Wnt/PCP path-way is involved in planar cell polarization (PCP) and estab-lishment of polarized cellular structures (183).

In the Wnt/�-catenin pathway, the level and subcellularlocalization of �-catenin determine the ultimate output.Normal levels of �-catenin dimerize and associate withboth transmembrane cadherins and cytoskeleton-bound�-catenin, which promotes cell adhesion and controls cellshape. The normal levels of cytoplasmic �-catenin are main-tained by constitutive degradation of excess �-catenin byGSK3-�. In the cytoplasm, GSK3-� and �-catenin complexwith Axin and adenomatous polyposis coli (APC), leadingto �-catenin phosphorylation by GSK3-�. Phosphorylated�-catenin is bound by the �-TrCP component of the ubiq-uitin ligase complex and degraded by the proteosome.GSK3-� is constitutively active but can be inhibited by Di-shevelled (Dvl), which is activated by Wnts binding to aFrizzled and LRP receptor pair. Thus, when Wnt signalingis active, elevated levels of cytoplasmic �-catenin enter thenucleus and interact with Tcf/Lef transcription factors toregulate gene expression (373).

Accumulating evidence indicates that Wnt/�-catenin signal-ing is involved in satellite cell function during muscle regen-eration, although its defined roles remain controversial.First, Brack et al. (65) suggested that Wnt signaling pro-motes myogenic commitment and terminal differentiationin adult myogenesis. By examining the effects of exogenousWnt3a and sFRP3 as activator and inhibitor of Wnt signal-ing, respectively, it was found that regenerating muscles invivo and myoblasts derived from cultured single myofibersare responsive to Wnt signaling only at a late stage of dif-ferentiation. Activation of Wnt/�-catenin signaling in-creased both Desmin expression in myogenic cells in vitroand the size of newly formed myofibers in vivo, whereasinhibition of Wnt signaling caused the opposite effects. In-terestingly, Notch signaling seems to antagonize the effects

of Wnt signaling at early stages of myoblast proliferation.This counteracting effect is reflected by the levels of theactive form of GSK3-�, which are high in the early Notchactivating stage and low in later Wnt activating stage. It wasproposed that low activity of Wnt signaling in early prolif-eration allows the expansion of enough myoblasts by Notchsignaling for later differentiation (65). Notably, althoughthis study reported that the mRNA levels of Wnt ligandsand Frizzled receptors within isolated myofibers increase inlate culture, it is still possible that Wnt ligands may origi-nate from other sources in vivo, such as the ECM, and mayfunction through preexisting Frizzled receptors on satellitecells.

Perez-Ruiz et al. (407) proposed that �-catenin promotesself-renewal of satellite cells and prevents them from imme-diate myogenic differentiation. By retrovirus-based overex-pression and RNAi knockdown of proliferating Pax7�/MyoD� satellite cells on single myofibers, it was found thatconstitutive expression of stabilized �-catenin, or inhibitionof degradation by GSK3� leads to downregulation ofMyoD and Myogenin expression. In contrast, when�-catenin was downregulated by RNAi or when its targetgenes were repressed by the dominant-negative �-catenin-ERD, fewer Pax7�/MyoD� myoblasts were observed. As�-catenin levels determined the observed cellular changes, itwas proposed that the Wnt/�-catenin pathway is involved(407). It should be stressed that although MyoD expressioncan be regulated by directly changing �-catenin levels, thephysiological relevance of this phenomenon still largely de-pends on the availability of Wnt ligands and expression ofFrizzled receptors on satellite cells. In addition, a recentstudy revealed that �-catenin directly interacts with MyoDto enhance MyoD binding to E-box elements to initiate themyogenic program (271). However, in vivo studies are re-quired to further confirm and elaborate on the mechanismwhereby �-catenin interacts with MyoD.

Otto et al. (391) proposed that Wnt ligands are importantregulators of proliferation for satellite cells during adultmuscle regeneration. mRNAs for Wnt ligands (Wnt1,Wnt3a, Wnt5a, Wnt11) were expressed in single myofibers(together with satellite cells) upon 2 days of culture in vitro.These data along with reporter assays indicated that Wntsignaling is active at this point. It was found that exogenous

FIGURE 3. The satellite cell niche. A: satellite cells reside between the basal lamina and the sarcolemma of adult skeletal myofibers and as such areinfluenced by structural and biochemical cues emanating from this microenvironment. A complex set of diffusible molecules (e.g., Wnt, IGF, and FGF)are exchanged between the satellite cell and the myofiber to maintain quiescence or promote activation. In addition, numerous extracellular matrixcomponents and cellular receptors are present either on the surface of the sarcolemma, satellite cell, or contained within the basal lamina. Thesecomponents comprise the immediate niche of the satellite cell and dictate rapid changes in the satellite cell state. B: the muscle fascicle defines theextremities of the local milieu an environment that is more diverse compared with the immediate niche due to the heterogeneity of cell types andsignaling factors. The local milieu surrounding the satellite cell is made up of other myofibers in addition to interstitial cells, capillaries, andneuromuscular junctions. Each of these cell types influences the surrounding environment and thereby affects the state of the satellite cell.C: the systemic milieu contains the greatest diversity with which the satellite cell is influenced. Exposure of satellite cells to the host’s immunesystem, and circulating hormones along with the skeleton and surrounding skeletal muscles present the broadest environment for the satellitecell. Changes that occur in the systemic milieu affect the satellite cell gradually. Prolonged exposure of signals from the systemic milieu plays animportant role in the ability of satellite cells to participate in multiple rounds of regeneration.



Wnt1, Wnt3a, and Wnt5a induced satellite cell prolifera-tion, whereas exogenous Wnt4 and Wnt6, whose expres-sion is absent from regenerating muscle, inhibited the pro-liferation. This is consistent with the inhibitory effect ofECGC, a potent Wnt signaling inhibitor, on satellite cellproliferation (391). Importantly, the expression and subcel-lular location of the active form of �-catenin (Act-�-Cat)was carefully examined by immunofluorescence staining inthis study. Nuclear Act-�-Cat was observed only afterMyoD expression and was absent in Myogenin� cells,which suggests Wnt signaling is inactive either before satel-lite cell activation or after differentiation determination.This temporal progression of Wnt signaling was furtherconfirmed in mouse regenerating muscles and human dys-trophic muscle biopsies. Thus Otto et al. (391) concludedthat activating Wnt ligands function to promote satellitecell proliferation during muscle regeneration, whereas in-hibitory Wnt ligands antagonize the proliferation by seclud-ing �-catenin to the cell membrane in quiescent satellitecells. Perplexingly, the pro-proliferation function of Wnt3asignaling revealed by this study seems to oppose the pro-differentiation function reported by Brack et al. (65). Thismight be due to different sources of Wnt ligands, eitherfrom cultured Wnt-expressing cells or from recombinantprotein, which in turn affect Wnt concentration and effec-tive time.

In addition to �-catenin-dependent gene activation, recentstudies indicate that the binding of Wnt ligands with Frizzled/LRP receptors can also lead to a transduction cascade toestablish PCP, which is referred to as the Wnt/PCP pathway(reviewed in Ref. 492). In this signaling pathway, signalingfrom Wnt ligand/receptor binding activates Rho/Rac smallGTPase and Jun NH2-terminal kinase (JNK) via Dishev-elled. Dishevelled activates Rac, which further activatesJNK to assist in the subsequent regulation of cytoskeletonorganization and gene expression. The Wnt/PCP signalingpathway plays an important role in the development ofanterior-posterior extension of the neural tube and the es-tablishment of the aligned cellular structure in epithelium.

Emerging evidence reveals that satellite cells are under theregulation of Wnt/PCP signaling pathway. Our group dem-onstrated that quiescent satellite cells can be separated intotwo functional subpopulations based on their Myf5 expres-sion: Pax7�/Myf5� satellite stem cells and Pax7�/Myf5�

satellite progenitor cells (280). Of interest, we determinedthat mRNAs of the Frizzled7 receptor are preferentiallypresent in Pax7�/Myf5� quiescent satellite stem cells (293).However, upon injury, Frizzled7 is ubiquitously expressedin all satellite cells and their proliferating progeny. In afrozen injury model, Wnt1, Wnt2, Wnt5b, Wnt8b,Wnt10a, Wnt16a, Frizzled receptors, and sFRP inhibitorswere increased in muscles 3 days postinjury (acute phase ofregeneration). In contrast, an increase of Wnt7a andWnt10a was detected in muscles 6 days postinjury, when

satellite cells returned to the sublaminar position. It wasdetermined that Wnt7a acts through the Frizzled7 receptorto specifically activate the Wnt/PCP pathway and inducecell polarity both ex vivo and in vitro. Activation of theWnt/PCP pathway by recombinant Wnt7a simulated thesymmetrical division of satellite cells and hence expandedthe satellite stem cell population on single myofiber ex-plants. Knockdown of Wnt/PCP downstream component,Vangl2, by RNAi caused the opposite effects, confirmingthe involvement of Wnt/PCP pathway in this process. Invivo, Wnt7a injection into regenerating muscles remark-ably improved muscle regeneration as evidenced by in-creased muscle mass and myofiber size. We postulated thatthe activation of Wnt/PCP pathway promotes the planaralignment of two daughter cells along the myofiber duringsatellite cell division. This alignment and the induced cellpolarity may facilitate the attachment of both daughter cellsto the basal lamina and hence maintain their stem cell fate(293). Although it is still unknown, one possible source ofendogenous Wnt7a is the regenerative myofiber, given itsclose proximity to satellite cells and perfect alignment to thebasal lamina (65, 413). Intriguingly, a recent study revealedsynergistic functions of Wnt/�-catenin and Wnt/PCP path-ways in oriented elongation of myocytes during embryonicmuscle patterning (205). Thus it is enticing to further spec-ulate that the activated Wnt/�-catenin pathway in regener-ating myofibers may induce Wnt7a secretion and activationof the Wnt/PCP pathway in adjacent satellite stem cells,which in turn would direct their symmetric division andreplenish the satellite cell pool.

Wnt signaling is also implicated in satellite cell-relatedtransdifferentiation. Our group reported that muscle resid-ing CD45�/Sca1� cells increased in number and underwentmyogenic differentiation in injured muscles but not in intactmuscles (413). CD45�/Sca1� cells express Frizzled recep-tors and upregulated �-catenin during regeneration, sug-gesting the Wnt/�-catenin pathway is required for the myo-genic commitment of these cells. Indeed, the myogenic com-mitment of CD45�/Sca1� cells can be induced by lithiumtreatment, or coculture with myoblasts or Wnt secretingcells. Daily injections of Wnt signaling inhibitors, sFRPs,severely reduced the myogenic recruitment of CD45�/Sca1� cells during regeneration. These data further suggestthat Wnt signaling is required for muscle regeneration andmay increase the myogenic potential of CD45�/Sca1� cells(413). Similarly, it was reported that �-catenin is requiredand sufficient for myogenic commitment of P19 embryoniccarcinoma cells (408). Of note, a recent study showed thatWnt signaling is important for transdifferentiation of satel-lite cells into fibrogenic cells (66). Activated satellite cellsfrom old mice have higher Wnt signaling activity and agreater tendency to assume fibrogenic fate in vitro, com-pared with those from young mice. Exposure of activatedsatellite cells from old mice to young serum or pairing withyoung mice in a heterochronic parabiotic system reversed



the Wnt signaling activity and associated fibrogenic ten-dency, suggesting they are not intrinsic. Conversely, acti-vated satellite cells from young mice exposed to aged serumor linked parabiotically activated Wnt signaling and be-came fibrogenic. These observations indicated that the fi-brogenic transdifferentiation of satellite cells is influencedby an unknown circulating factor (66). As the effect of thisfactor can be neutralized by a recombinant secreted form ofFrizzled, it was believed that an increase in Wnt ligands inold mice may account for the fibrogenic transformationobserved (66). Notably, an alternative possibility is thatcertain Wnt inhibitors are decreased in old mice. This pos-sibility is supported by a recent finding that a Wnt bindingprotein, Klotho, is a circulating hormone and can affecttissue aging (281, 313). Wnt signaling also controls thebalance between myogenic and adipogenic potentials ofmyoblasts in vitro. First, downregulation of Wnt/�-cateninsignaling mediated by Wnt10b was implicated in the in-crease of adipogenic differentiation of aged myoblasts(527). Similarly, it was observed that in Wnt10b�/� mice,the adipogenic potential of myoblasts increased and exces-sive lipid accumulated in regenerating myofibers (549). Arecent mechanistic study indicated that Wnt10b activatedthe Wnt/�-catenin pathway and that SREBP-1c-mediatedlipogenesis and insulin resistance reciprocally counteracteach other and thus determine the myogenic and adipogenicfate of myoblasts (2). The above-mentioned experimentsserve to demonstrate the complexities involved in Wnt reg-ulation of myogenesis. Future studies focused on in vivoregulation of a myogenic to adipogenic switch will elabo-rate on the physiological relevance of the above experimen-tal evidence.

B) NOTCH SIGNALING. Notch signaling regulates cell prolifer-ation, differentiation, and cell fate determination (21). Theactivation of Notch signaling pathway is initiated by thebinding of Delta and Jagged family of ligands to Notchtransmembrane receptors, the result of which induces se-quential enzymatic cleavage of the Notch receptor and re-lease of an active truncated form of Notch, termed theNotch intracellular domain (NICD). Upon enzymatic cleav-age, the NICD translocates from the cytoplasm to the nu-cleus and activates the CSL (CBF1, Suppressor of Hairlessand Lag-1) family of transcription factors, leading to targetgene expression.

Notch signaling plays key roles in skeletal muscle regener-ation (reviewed in Ref. 320). Upon muscle injury, the Notchligand Delta is quickly upregulated in both activated satel-lite cells and in myofibers (108). The upregulation of Deltain satellite cells is accompanied by the appearance of theNICD, indicative of activated Notch signaling. Activationof Notch signaling stimulates the proliferation of satellitecells and their progeny and thus leads to the expansion ofproliferating myoblasts. Indeed, inhibition of Notch signal-ing abolishes satellite cell activation and impairs muscle

regeneration (108). Reversely, constitutive Notch activa-tion in satellite cells from Pax7-CreER;ROSANotch resultedin increased Pax7� satellite cells and impaired muscle re-generation (560). In aged muscles, the expression of Notchin satellite cells remains unchanged (107). However, theinduction of Delta in myofibers is no longer responsive tomuscle injury, which results in reduced satellite cell prolif-eration and impaired muscle regeneration (107).

At the onset of myogenic differentiation, however, the in-activation of Notch signaling in myoblasts is required fortheir fusion (108). Two mechanisms exist to downregulateNotch signaling. The first involves Numb, a Notch signal-ing inhibitor, which is asymmetrically partitioned duringsatellite cell/myoblast division (108, 489). The daughter cellinherits Numb and activates the expression of Desmin lead-ing to differentiation (108). Numb inhibits Notch signalingby inducing ubiquitination of the NICD, which abolishes itsactivity (21). Second, the activation of Wnt signaling indifferentiating myoblasts antagonizes Notch signaling andfacilitates terminal differentiation (65). This is supported byobservations that activation of Notch signaling is associ-ated with phosphorylation of tyrosine 216 of GSK-3b,whereas Notch inhibitor treatment or activation of Wntsignaling in myoblasts coincided with dephosphorylation ofthis residue (65). Although the precise mechanism remainsambiguous, the crosslink between Wnt signaling and Notchsignaling at Dishevelled and Presenilin has been reported(131). In addition, �-catenin has been reported to associatewith MyoD and facilitate the transcription activity ofMyoD (271). Thus it would be interesting to investigatewhether MyoD/�-catenin can drive the expression of Notchsignaling inhibitors, such as Numb, Itch, or the newly re-ported bHLH transcription factor Stra13 (511).

As both Notch receptors and their ligands are transmem-brane proteins, Notch signaling is an important signalingpathway mediating cell-cell communications. Our grouprecently demonstrated that Megf10, a multiple EGF repeat-containing membrane protein, may function similar toDelta and Jagged in Notch signaling activation (235).Notch-mediated cell-cell communications, presumablyconveyed between “bumping” satellite cells or between sat-ellite cells and the myofiber, may serve as a controllingmechanism for satellite cell quiescence and the number ofsatellite cells on a single myofiber under physiological con-ditions. In line with this view, double knockout mice ofHey1 and HeyL, two downstream target genes of Notchsignaling, showed remarkably reduced number of satellitecells, which is likely due to failure of satellite cells in keepingquiescence in adult muscle (176).

Three Notch receptors are abundantly expressed on satel-lite cells: Notch1, Notch2, and Notch3. A recent study sug-gests that Notch3 may act as a Notch1 repressor by activat-ing Nrarp, a negative-feedback regulator of Notch signaling



(272). In line with this view, Notch3-deficient mice showedmarkedly increased numbers of sublaminar quiescent satel-lite cells and muscle hyperplasia after repetitive muscle in-juries, which are phenotypically opposite to those fromNotch1 mutants.

In addition to its functions during muscle regeneration,Notch signaling is also involved in the maintenance of cellquiescence in resting health muscle. It was found that ge-netic abrogation of Rbpj, the main effector for canonicalNotch signaling, specifically in satellite cells results in theloss of their quiescent state (356). In Pax7-CT2;Rbpjflox/�

mice, Tamoxifen induction and Rbpj depletion in satellitecells gradually decreased the number of Pax7� satellitecells, which is accompanied with spontaneous myogenicdifferentiation and failure of muscle regeneration upon in-jury. Interestingly, the spontaneous myogenic differentia-tion of the majority of mutant satellite cells in this modelseems to start with G0 (quiescent) satellite cells withoutentry into S phase in vivo.

The above findings support the notion that Notch signalingis critical for satellite cell activation and proliferation. AsNotch signaling can be activated by cell-to-cell contact, it isvery intriguing to envision that communal satellite cells ex-isting on the same myofiber can signal each other. Thiscommunity niche effect may serve to determine the numberof quiescent satellite cells on a myofiber and/or control thesize of the satellite cell pool during regeneration. In thisrespect, recent studies have revealed a close relationshipbetween Notch signaling and Hippo (Salvador/Warts/Hippo) signaling pathways in determining cell proliferationand organ size in Drosophila (77, 339, 412). It would beinteresting to investigate the interactions of these two path-ways in satellite cell activation and proliferation.

C) SPHINGOLIPID SIGNALING. Sphingolipids are a large groupof naturally occurring glycolipids, characterized by theirsphingoid backbone. Although originally recognized asinert precursors and intermediate products in lipidmetabolism, sphingolipids, particularly ceramide, cer-amide-1-phosphate, sphingosine, and sphingosine-1-phosphate (S1P), have been recently revealed as importantbioactive signaling molecules in regulating cell prolifera-tion, migration, death, and senescence (reviewed in Ref.223). Sphingomyelin is the most abundant component inthe sphingolipid pathway and also serves as a precursor ofceramide, sphingosine, and S1P. It was found that sphingo-myelin is enriched in plasma membrane of quiescent satel-lite cells, yet it markedly diminishes after satellite cell acti-vation and reappears in some Pax7�/MyoD� satellite cellsreturning to the quiescent state (369). The dynamics ofsphingomyelin are connected with the biosynthesis of mito-genic S1P, which promotes satellite cells to enter the cellcycle on ex vivo cultured myofibers and improves muscleregeneration (370, 461). On the other hand, pharmacolog-

ical inhibition of sphingomyelin-to-S1P processing reducedthe number of activated satellite cells and perturbed muscleregeneration. This action of S1P on satellite cell activationand muscle regeneration is in line with the proliferative,proinflammatory (through COX-2) and antiapoptotic(through inhibition of caspase-3 and BAX) functions ofS1P-mediated pathways in general. Accumulating evidenceindicates that S1P, being one of the more soluble sphingo-lipids, acts through S1P receptors (S1PRs; a group of high-affinity G protein-coupled receptors) on satellite cells in anautocrine/paracrine fashion (76, 122). In addition, abnor-mal modulation of S1P activity is also involved in the pa-thology of muscular dystrophy (as exemplified in mdx mice)(317).

Our current understanding of the functions of bioactivesphingolipids on satellite cells is far from complete. Forexample, the role of ceramide, another central player of thesphingolipid signaling pathway, on satellite cell activation/proliferation/differentiation during muscle regeneration isvastly unknown. Moreover, whether S1P may bind to otherintracellular proteins and exert S1PR-independent actionsremains to be investigated.

2. The myofiber niche

The myofibers are the primary component of the satellitecell niche due to their direct contact with satellite cells. Theoverarching effect provided by the myofiber for satellitecells was revealed through selective killing of myofiberswith Marcaine while leaving the basal lamina intact. Thisresulted in greater numbers of proliferating satellite cellscompared with viable myofibers (50). This suggested thatthe myofiber emanates a “quiescent” signal either by itsphysical association or by releasing chemical compounds(50, 53). In accordance with this hypothesis, recent studieshave revealed numerous satellite cell regulatory factors thatare presented on myofibers or secreted by myofibers. First,myofibers secrete SDF-1, which serves as ligand to the cellsurface receptor CXCR4 on satellite cells (429, 486). SDF-1/CXCR4 signaling has been shown to stimulate satellitecell migration (429). In addition, it has been found that thetransmembrane Notch ligand Delta is upregulated on myo-fibers after injury, which can activate the Notch signalingcascade in satellite cells and hence induce their proliferation(107). Such an activation signal from myofibers is probablyamplified by the expression of both Notch receptors andDelta ligands on satellite cells, functioning in autocrine andjuxtacrine fashion (108, 280).

In aged muscle, both the caliber and the number of myofi-bers decline (202), although the direct effect of these mor-phological changes is not known. Interestingly, the presenceof Delta ligands is not sufficiently expressed on aged myo-fibers, an indication of diminished Notch signaling possiblycompromising satellite cell activation and muscle regenera-tion (107).



3. ECM and associated factors

The basal lamina of the myofiber is composed of a networkof ECM that directly contacts the satellite cell and separatesit from the muscle interstitium. The basal lamina of skeletalmuscle is composed of type IV collagen, laminin, entactin,fibronectin, perlecan, and decorin glycoproteins along withother proteoglycans (459). These ECM molecules aremainly synthesized and excreted by interstitial fibroblastsbut can also be produced and remodeled by myoblasts dur-ing muscle development and regeneration (278). Satellitecells reside between the basal lamina, composed primarilyof a type IV collagen network, and the apical sarcolemmacovered in laminin (568). The basal lamina presents a largenumber of binding sites for �7/�1-integrins; sites that an-chor the actin cytoskeleton of satellite cells to the ECM(501; reviewed in Ref. 330). This physical tethering is crit-ical for satellite cell activation as it allows the transductionof extracellular mechanical force into intracellular chemicalsignals (62; reviewed in Ref. 73). In addition, muscle spe-cific laminin-2 (�2/�1/�1 subunits) and laminin-4 (�2/�2/�1) are also associated with myofibers though �7/�1-integ-rins and dystroglycan on the surface of myofibers (208;reviewed in Ref. 459). Proteoglycans reside on the surfaceof satellite cells and function as receptors to bind a suite ofsecreted, yet inactive growth factor precursors. These pre-cursors, including HGF (521), basic fibroblast growth fac-tor (bFGF) (141), epidermal growth factor (EGF) (193),insulin-like growth factor isoforms (IGF-I, IGF-II) (323)and various Wnt glycoproteins (65, 293), originate fromeither satellite cells, myofibers, interstitial cells, or serum. Inresting muscle, the sequestering of inactive growth factorsexists as a local reservoir, to facilitate a rapid response tomuscle injury. These growth factors can be swiftly renderedactive by proteolytic enzymes present in serum or intersti-tium, such as thrombin, serine proteases, and matrix met-alloproteinases (MMPs). This highly coordinated processserves to stimulate satellite cell survival, activation, andproliferation upon muscle injury (249, 288, 386, 521, 576).

A) HGF/SCATTER FACTOR. HGF is a heterodimer of its � and �subunits, both of which are processed from a single-chaininactive precursor (pro-HGF). HGF is critical for cellgrowth, migration, and organ morphogenesis through itsmitogen, motogen, and morphogen activities (reviewed inRefs. 381, 586). In skeletal muscle, HGF functions in theearly phase of muscle regeneration and is one of the keyactivators of satellite cells (342, 521). In intact muscle, lowlevels of HGF mRNA can be detected in satellite cells butnot in fibroblasts, suggesting an autocrine function of HGF(13, 484, 521). However, the majority of active and inactiveforms of HGF are sequestered by heparan sulfate proteogly-cans (HSPGs) within the basal lamina (13, 484, 520, 521).Upon injury, the transcription of HGF is increased in pro-portion to the degree of injury, and an active form of HGFis released from the ECM without the need of proteolyticcleavage of pro-HGF (484, 520, 521, 525, 569). In vitro

and in vivo data have revealed that release of nitric oxidesynthase (NOS) from the basal lamina after myofiberstretch or damage leads to the production of nitric oxide(NO), which in turn activates MMPs and release HGF fromlocal HSPGs (522, 523, 576). Alternatively, the observedrapid upregulation of HGF following muscle injury is dueto release of HGF from intact organs, such as the spleen(513).

Released HGF and newly synthesized HGF act directly onsatellite cells and myoblasts through the cell surface recep-tor c-Met found on both quiescent and activated satellitecells (13, 116, 179, 521). Multiple studies have demon-strated that the activation of HGF/c-Met pathway stimu-lates quiescent satellite cells to enter the cell cycle, increasesmyoblasts proliferation, and inhibits myogenic differentia-tion (13, 179, 342, 521). Fittingly, HGF isolated fromcrushed muscle extracts induces quiescent satellite cell acti-vation, and an anti-HGF antibody abolished this activity(521). Furthermore, direct injection of HGF into injuredmuscle blocked the regeneration process but increased myo-blast numbers (342, 521). Forced expression of a constitu-tive active form of c-Met in C2C12 myoblasts resulted inthe inhibition of differentiation (15). HGF inhibits myo-blast differentiation by coordinately repressing transcrip-tional activity of MRF/E-protein complexes, increasingMyoD inhibitor Twist expression, as well as decreasingp27kip1 expression (179, 306). The dual function of HGF inpromoting proliferation and inhibiting differentiation in-volves sustained activation of MAPK/Erk signaling throughbinding of Grb2 to phosphorylated c-Met (214, 304, 305).Moreover, HGF promotes satellite cell migration to the siteof injury as demonstrated by the in vitro chemotactic activ-ity of this factor on satellite cells and C2C12 myoblasts (49,512, 551). The function of HGF in muscle regeneration isimportant during the early phase of repair because overtime levels of HGF decline and exogenous HGF does notaffect muscle regeneration when administrated at laterstages (342, 521). Therefore, these results demonstrate thatHGF is a satellite cell activator, playing a critical role in theearly stage of muscle regeneration. The activation of satel-lite cells by HGF is due to both transcription activation ofHGF in satellite cells and release of HGF from the ECM byMMPs, which coordinately function in an autocrine andparacrine fashion to induce the proliferation of satellitecells.

B) FIBROBLAST GROWTH FACTORS. Fibroblast growth factors(FGFs) are a large family of mitogens, involved in cellgrowth, survival, migration, and embryonic development.Numerous FGFs have been found in skeletal muscle in vivoand upon culture and differentiation of myoblasts in vitro(12, 199, 222, 266). Inhibition of endogenous FGF-1 bysiRNAs led to myogenic differentiation of myoblasts invitro, while exogenous FGF-1, -2, -4, and -6 robustly stim-ulate the proliferation of rat primary myoblasts in vitro



(266, 483). Of the particular FGF family members, FGF-6 isof particular interest given its muscle specific expressionand upregulation during muscle regeneration (134, 172). Astudy in FGF-6�/� mutant mice revealed severe muscle re-generation defects characterized by a reduction in MyoD�

and Myogenin-expressing cells along with increased colla-gen deposition and fibrosis (172). FGF-2 is also likely in-volved in myoblast proliferation during muscle regenera-tion as endogenous FGF-2 was found in the basal lamina(113). Moreover, injecting a neutralizing antibody ofFGF-2 at the time of injury (297) or injecting exogenousFGF-2 into mdx mice (296) caused a delay in myoblastproliferation and increased myoblast proliferation, respec-tively. In addition, FGFs also facilitate muscle regenerationthrough their well-known function in angiogenesis (295).

FGF signaling is mediated by virtue of four transmembranetyrosine kinase receptors known as FGFR1–4, which differin their specificity of ligand binding. Of these, FGFR1 andFGFR4 are the most prominent receptors in rat satellite cellsin vitro (172). In vitro, FGF-1 can induce the expression ofFGFR1, and this induction effect can be further augmentedby HGF (483). Stable overexpression of FGFR1 increasesmyoblast proliferation and delays their differentiation,whereas overexpression of a dominant negative form ofFGFR1 leads to the opposite effects (463). In skeletal mus-cle, dimerization of FGFs with FGFRs leads to tyrosinephosphorylation of FGFR and activates the downstreamRas/MAPK pathway. This is supported by the finding thatconstitutively expressed active RAS in myoblasts inducesproliferation and blocks differentiation even in the absenceof exogenous FGFs (162). In addition, inhibition of mito-gen-activated protein kinase kinase (MAPKK) activity com-pletely abolishes the FGF2-dependent inhibition of myo-genic differentiation in vitro (561). Therefore, activation ofthe Ras/MAPK signaling pathway seems to play a centralrole in FGF/FGFR-mediated pro-proliferation and anti-dif-ferentiation effects in skeletal muscle cells.

In addition to the canonical FGF receptors, HSPGs alsoserve as low-affinity FGF receptors (reviewed in Ref. 427).Although HSPGs are ubiquitously present on most mam-malian cells, both quiescent and activated satellite cells spe-cifically express syndecan-3 and syndecan-4 (113). Furtherinvestigations revealed that syndecan-3�/� and syndecan-4�/� mutant mice display distinct panels of phenotypes,implicating separate roles of these two HSPGs in satellitecell function (115). It is believed that HSPGs interact withboth FGFs and FGF receptors to promote ligand recogni-tion (236). In accordance with this, satellite cell activationand proliferation is impaired and MAPK signaling is com-pletely abolished in myoblasts derived from syndecan-4�/�

mutant mice (115). In contrast, hyperplasia of satellite cellsand myonuclei and overactivation of MAPK signaling wereobserved in syndecan-3-mutant mice, which suggests thatan inhibitory mechanism is missing (115). In light of this,

the downregulation of syndecan-4 mRNA was observed inturkey satellite cells in response to exogenous FGF-2 (548).

C) IGF. IGF-I and IGF-II play important roles in the regula-tion of satellite cell activity (reviewed in Ref. 409). IGF-I haspleiotropic functions including anti-inflammation, cell mi-gration, and stimulation of both proliferation and differen-tiation in satellite cells. In contrast, IGF-II is believed to onlypromote myogenic differentiation (11, 103, 151, 169, 170,547). Elevation of IGF-I signaling within muscle cells invitro results in muscle hypertrophy with increased DNAand protein content in muscle (6, 34, 87, 103, 365). Thehypertrophic effects of IGF-I are attributed to both the ac-tivation of satellite cell proliferation, which gives rise tomore myonuclei, and protein synthesis, which increases thecytoplasmic-to-DNA volume ratio (32, 35, 366). IGF-II ex-pression and secretion are elevated prior to myoblast differ-entiation (171), and knock-down of either IGF-I or IGF-IIin vitro results in impaired myogenic differentiated (158,171). Consistently, IGF2�/� mutant mice are �40% lightercompared with the weight of wild-type mice at birth, andthis defect persists postnatally (133).

IGF can function in endocrine, autocrine, and paracrinefashions to promote satellite cell proliferation and differen-tiation, but all of these are mediated by IGF-I binding to theIGF-I receptor (IGF1R), which is a ligand-activated recep-tor tyrosine kinase. In contrast to IGF-I-induced musclehypertrophy, IGF1R�/� mutant mice display a dystrophicphenotype and usually die at birth due to weak respiratorymuscles (315). Activation of IGF1R in satellite cells inducesthe expression of MRFs (170) and initiates intracellularsignaling cascades involving both mitogenic and myogenicresponses (110, 367). Two primary signaling pathways areactivated by IGF1R. In the first pathway, activated IGF1Rrecruits and phosphorylates insulin receptor substrate pro-teins (IRSs), leading to the activation of phosphatidylinosi-tol 3-kinase (PI3K). Activation of PI3K is involved in mul-tiple cellular processes including an anti-apoptotic effectmediated by activation of the AKT pathway (502), modu-lation of intracellular calcium levels by the inositol phos-phate cascade (156), and promoting protein translation (5).Although evidence supports that IGF-I-dependent satellitecell proliferation is mediated by the AKT/mTOR pathway(220), the activation of PI3K/AKT and p38-MAPK path-ways are also involved in myoblast differentiation (197,539). In the second pathway, IGF1R activates the Ras/Raf/extracellular response kinases (ERKs) cascade, which inturn activates other protein kinases and several transcrip-tion factors (110, 347). The activation of the Ras/Raf/ERKpathway is also required for satellite cell proliferation(110). In addition to the aforementioned pathways, accu-mulating evidence indicates that the calcium/calcineurinpathway is also involved in IGF-I signaling-dependent mus-cle differentiation and hypertrophy (137, 173, 366, 479).Specifically, it has been revealed that GATA-2-mediated



myofiber hypertrophy depends on IGF-I stimulation of thecalcineurin-mediated signaling pathway (366). Further evi-dence of the pleiotropic functions of IGF-I involves a rolein anti-inflammatation demonstrated by a reduction inchronic inflammation upon IGF-I� treatment during muscleregeneration (357).

Activated satellite cells and cultured myoblasts express in-sulin-like growth factor binding proteins (IGFBPs), whichare a family of secreted proteins that specifically bind IGFsand function as carriers during circulation and regulateIGF turnover, transport, and half-life (reviewed in Ref.253). Of the five IGFBPs, IGFBP-5 plays a critical role inmuscle growth and differentiation. IGFBP-5 expressionis induced during differentiation and precedes that ofIGF-II in cultured myoblasts (246, 436). It was foundthat IGFBP-5 functions to promote myogenic differenti-ation by switching on an IGF-II-mediated positive-feed-back loop (436). Knockdown of IGFBP-5 impairs myo-genesis and suppresses IGF-II expression (436).

Notably, recent studies demonstrated that alternative splic-ing of IGF-I pre-mRNA gives rise to a unique peptide,termed mechano-growth factor (MGF) (194). MGF pro-motes myoblast proliferation in vitro (26, 578). However,the existence of MGF and its functions in vivo remain con-troversial (discussed in Ref. 327).

D) MMPS. Matrix metalloproteinases (MMPs) are a family ofzinc-dependent enzymes that can degrade components ofthe extracellular matrix such as collagens, elastin, fibronec-tin, laminin, and proteoglycans. In skeletal muscle, MMPsplay an important role in satellite cell activation, migration,and differentiation during muscle regeneration (reviewed inRef. 86). Of more than 20 MMPs, MMP-2, -3, -7, and -9were found in skeletal muscle. In particular, MMP-2 andMMP-9, which can degrade denatured collagen types (gel-atins) and heparan sulfate proteoglycans, have critical func-tions on ECM remodeling in skeletal muscle regeneration.Upon injury, MMP-2 is secreted by satellite cells and re-generating myofibers, and its activity is elevated in bothdegenerative and regenerating stages (167, 178, 270). Incontrast, MMP-9 is generated by leukocytes and macro-phages, and its activity drastically decreases after thedegenerative stage (270). In the degeneration stage, acti-vated MMP-2 and MMP-9 degrade collagen IV in themuscle ECM, which allows satellite cells to migrateacross the basement membrane to the site of injury (377).In the regeneration stage, it is believed that MMP-2 isinvolved in remodeling and maintenance of the ECM.Recent studies indicate that NO upregulates enzymaticactivity and protein expression levels of MMP-2 (270)and MMP-9, and this activation of MMP-2 is requiredfor the release of HGF from the ECM upon satellite cellactivation (575, 576).

E) ECM STIFFNESS. Recent studies reveal a previously unappre-ciated role for the stiffness of the ECM in regulating satellitecell proliferation and differentiation. Resting healthyskeletal muscles and cultured myotubes possess a similarelastic stiffness (elastic modulus �12 kPa) (106, 152),whereas aged (184, 444) and dystrophic (152) skeletalmuscles are apparently stiffer (elastic modulus �418kPa). These changes of stiffness are presumably due toincreased extracellular matrix deposition, in particularcollagen deposition by fibroblasts, a result of repeated mus-cle regeneration. The significance of changes in stiffness wasevaluated by assessing the proliferation and differentiationability of myoblasts on various in vitro culturing surfaces.Primary myoblast proliferation on polyacrylamide gelscross-linked with entactin, type IV collagen, and laminin isoptimal at an elastic modulus equal to 21 kPa, while it isimpaired on softer (3 kPa) or stiffer (80 kPa) culturing sub-strates (61). Similarly, C2C12 myoblasts optimally differ-entiate on collagen-coated polyacrylamide gels that mimicthe physiological elasticity of skeletal muscle (12 kPa),whereas either softer (5 kPa) or stiffer (420 kPa) gels greatlycompromise their differentiation efficiency (152). Althoughthe stiffness of the ECM surrounding the satellite cell nichehas not been directly measured yet, these in vitro data drawattention to a causal relationship between biophysical stim-uli and satellite cell function in vivo.

B. The Microenvironment Beyond theImmediate Niche

1. Local milieu

In addition to the immediate niche surrounding satellitecells, local interstitial cells, motor neurons, blood vessels,and their associated secretable factors reside within skeletalmuscle and have the potential to regulate satellite cell func-tion and affect muscle regeneration.

A) INTERSTITIAL CELLS. Interstitial cells are the major compo-nent of the stromal tissue between the basal lamina andepimysial sheath surrounding skeletal muscle. Fibroblastscomprise a major component of the interstitial stromal cellpopulation (51). Fibroblasts secrete growth factors, such asFGFs, and also contribute to the skeletal muscle ECM bydepositing collagen, laminin, fibronectin, HSPGs, tenascin,and NCAM (185). Increased levels of fat and connectivetissue (fibrosis), evidence for increased numbers of adi-pocytes and fibroblasts, respectively, are well documentedfor several physiological and pathological conditions, suchas aging, obesity, and muscular dystrophy (38, 66, 195,198, 200). It is believed that satellite cells may account forthe increase in adipocytes and fibroblasts due to transdiffer-entiation along alternative mesenchymal lineages. This no-tion is supported by the observations that myoblasts canundergo adipogenic differentiation in vitro (239, 310, 485,579) and satellite cells on single myofibers can give rise to



both adipocytes and fibroblasts in vitro (23, 66). Recently,two studies revealed that these intramuscular adipocytesand fibrocytes can arise from muscle residing fibrocyte/adi-pocyte progenitors (FAPs) (252, 541).

In one study, a population of SM/C-2.6�/PDGFR-�� cellsis able to differentiate into adipocytes in culture (541). Invivo, these cells localize to the interstitial space betweenmyofibers and muscle bundles. After transplantation intoglycerol-injected fatty degenerating muscle, SM/C-2.6�/PDGFR-�� cells can differentiate into adipocytes, a resultthat is not observed in normal regenerating muscle (541). Inanother study, a Sca-1�/ CD34� or �7-integrin�/CD45�/CD31� cell population was isolated and found to sponta-neously form both adipocytes and ER-TR7� fibroblasts inculture (252). Although the origin of these cells was notdirectly investigated, it was further demonstrated that theseSca-1�/�7-integrin� cells also express PDGFR-�, and countfor the vast majority of PDGFR-�� cells in skeletal muscle(252). This suggests strong similarities exist between the cellpopulations used in both studies. Consequently, Sca-1�/�7-integrin� cells also only give rise to adipocytes in degener-ating muscle in vivo, in contrast to their limited adipogenicpotential in intact muscle and in injured regenerating mus-cle. Moreover, a single Sca-1�/�7-integrin� cell was able todifferentiate into both adipocytes and fibroblasts in vitro,indicating that these cells are bipotent and thus termedFAPs (252).

Both studies clearly demonstrate that FAPs do not havemyogenic potential either in vitro or in vivo, which makesthem distinct from satellite cells and other muscle-residingmyogenic cells (252, 541). The distinct adipogenic poten-tials of FAPs in degenerating and regenerating musclessuggest that the differentiation of FAPs is regulated bytheir local microenvironment. Indeed, both groups dem-onstrated that FAPs proliferated and transiently in-creased in number in regenerating muscle without differ-entiating into adipocytes or fibroblasts (252, 541). More-over, when adipogenic FAPs isolated from degeneratingmuscle were transplanted into regenerating muscle, theregenerative microenvironment was sufficient to inhibitadipogenic differentiation (541). These observations indi-cate that regenerating muscle provides a specialized mi-croenvironment, which favors myogenic differentiation andmaintains FAPs in their undifferentiated state. Importantly,the undifferentiated FAPs in such a microenvironment mayfacilitate the myogenic differentiation of satellite cells,which is supported by the observation that, in co-cultureconditions, FAPs increase the commitment of myoblasts toterminal differentiation (252). IL-6 signaling was impli-cated in this process as evidenced by induced IL-6 expres-sion in FAPs in response to muscle regeneration (252).

In summary, these intriguing results support the hypothesisthat skeletal muscle provides a balanced environment for

coexistence of both myogenic precursors (myoblasts) andadipogenic/fibrogenic precursors (FAPs) during muscle re-generation. Regeneration is supported by both satellite cellsand FAPs, whereas in degenerating muscle, this balance isdisrupted and FAPs differentiate into adipocytes (and pre-sumably also fibroblasts). These findings also raise severalinteresting questions including: How do satellite cells re-spond to increased adipocytes and fibroblasts in the afore-mentioned physiological and pathological conditions? Re-cent observations regarding the effect of fibrosis on thestiffness of the ECM raise the question as to whether in-creased depositions from fibroblasts or adipocytes can altermyofiber stiffness and therefore affect the physiologicalroles attributed to satellite cells (152; discussed in sect.IIIA). Finally, it would be interesting to investigate the em-bryonic origin and postnatal lineage progression of FAPs,particularly their lineage relationship to the CD45�/Sca-1�

mesenchymal stem cells found in multiple tissues (316, 401,417, 441, 509, 519). Further study is required to under-stand whether other regulatory factors are also involved inthe determination of FAP differentiation and whether thesefactors change in response to various physiological andpathological conditions.

In addition to FAPs, recent studies identified telocytes (TCs,formerly called interstitial Cajal-like cells or ICLCs) as anew type of cells within muscle interstitium, which are inclose vicinity of satellite cells, myofibers, nerve endings, andblood vessels (414). Unlike satellite cells and fibroblasts,skeletal muscle TCs express the cell surface marker c-kit.Transmission electron microscopy (TEM) examination ofTCs in muscle and studies on TCs from other tissues suggestTCs transmit intercellular signaling by shedding/intakingmicrovesicles (also called exosomes), which are enriched forproteins and RNAs (230, 414, 543). Although their exactrole in muscle regeneration is still unknown, the finding thatthese cells secrete VEGF suggests telocytes are an importantplayer in promoting satellite cells self-renewal, facilitatingvasculogenesis, and preventing fibrosis (132; further dis-cussed in sect. IIIB1C).

Another cellular component of skeletal muscle interstitiumis the SP, a population of cells characterized by their expres-sion of Abcg2 and hence the ability to efflux Hoechst dye. Arecent study indicated that endothelial cells and interstitialcells constitute the major fraction of skeletal muscle SP(146). Ablation of Abcg2 impairs skeletal muscle regenera-tion. In addition, a small percentage of these SP cells havemyogenic potential and may contribute to myogenic regen-eration in injured muscle (discussed in sect. IID).

B) MOTOR NEURONS. It is well known that denervation resultsin progressive skeletal muscle atrophy. During acute dener-vation of muscle, the percentage of satellite cells increasesduring the first week, indicating a proliferation phase simi-lar to muscle injury (468). However, long-term denervation



results in a drastic decline in satellite cell numbers (442,550), which is at least partially due to decreased mitoticcapability and increased apoptosis of satellite cells. Experi-ments illustrate that an absence of PCNA (proliferating cellnuclear antigen)-positive satellite cell can be found on myo-fibers denervated for more than 1 wk. This implies thatlong-term denervation may cause satellite cells to lose theircapability to enter the mitotic cell cycle (282). Second, sat-ellite cells from muscle denervated for 6 and 10 wk dis-played a twofold increase of apoptosis, compared with con-trol cells from normal innervated muscle (248). Althoughthe underlying mechanism of a neural influence on satellitecell behavior is still elusive, neurotrophins such as nervegrowth factor (NGF) and brain-derived neurotrophic factor(BDNF) are involved.

NGF is expressed in developing muscles (563), and its ex-pression is downregulated after birth (153). The expressionof NGF reappears in adult muscle under several pathologi-cal conditions such as muscular dystrophy (537) and amyo-trophic lateral sclerosis (283), wherein expression of NGFwas found in regenerating myofibers and connective tissuefibroblasts within the damaged muscles (99, 537), suggest-ing that it may be involved in muscle regeneration (340).Indeed, ectopic expression of NGF neutralizing antibodiesin vivo causes muscle dystrophy (448). Within skeletal mus-cle, NGF seems to bind to its low-affinity receptor p75NTR

(138), which is expressed in satellite cells (358) and humanprimary myoblasts and myotubes (33). The expression ofboth NGF and p75NTR is upregulated during primary myo-blast differentiation in vitro, and they function to stimulatemorphological changes involved in myoblast fusion (138).

Similar to NGF, developing muscles express high levels ofBDNF, whereas in adult myofibers it is not detectable (201,358). BDNF is present in most adult satellite cells, and itsexpression is correlated with Pax3 expression (358). In cul-tured primary myoblasts, the expression of BDNF is de-creased dramatically during myogenic differentiation; inconcordance, a reduction in endogenous BDNF leads toearly myogenic differentiation, while addition of exogenousBDNF can reverse this phenotype (358). In vivo, BDNF�/�

satellite cells are decreased and hence compromise muscleregeneration (102). These observations suggest that thefunction of BDNF in adult skeletal muscle is to maintainsatellite cells in their quiescent state and prevent their myo-genic differentiation (358). This is consistent with the ob-servation that neural electrical activity stimulates the acti-vation of satellite cells (reviewed in Ref. 470) and repressesthe expression of BDNF (358). Since the TrkB receptor ofBDNF was not detected in skeletal muscle, BDNF signalingis proposed to occur via the p75NTR in a similar fashion toNGF.

Taken together, these data indicate that neurotrophic fac-tors function in an autocrine fashion to regulate satellite cell

behavior and muscle regeneration. Thus it is conceivablethat systematic release of neurotrophic factors after dener-vation may also exert similar regulatory roles on satellitecell activation and differentiation. In addition, denervationcan also cause secondary effects on satellite cells by directlyinfluencing the physiological properties of myofibers (229).Moreover, it has been recently found that satellite cells maycontrol myofiber innervation during muscle regenerationby secreting semaphorin 3A, a well-known factor involvedin axon guidance and growth (524). This finding furtheremphasizes the notion that proper muscle regeneration re-lies on the dynamic interaction of satellite cells with theirniche.

In aged muscle, examination of neuromuscular junctions byEM revealed decreases in nerve terminal area, mitochondriaand synaptic vesicles, along with occasional denervatedpostsynaptic regions (159). These changes, in addition toprogressive myofiber atrophy, may potentially disturb sig-nals between myofibers and satellite cells.

C) VASCULATURE. Skeletal muscle is nourished by the micro-vascular network. The importance of this niche componentis reflected by the fact that most satellite cells are closelyassociated with capillaries in intact adult muscle (100, 465).It is also well known that angiogenesis and myogenesisproceed at the same time during muscle regeneration (321).Vascular endothelial growth factor (VEGF) plays an impor-tant role in satellite cell function and muscle regeneration(416). It has been reported that overexpression of VEGF byadenovirus can stimulate satellite cell proliferation and im-prove myofiber regeneration in vivo (20). A recent studyrevealed that in coculture, endothelial cells promote myo-blasts proliferation by secreting a panel of growth factors,such as IGF-I, HGF, FGF, platelet-derived growth fac-tor-BB (PDGF-BB), and VEGF (100). In addition, it wasfound that periendothelial cells (smooth muscle cells andendomysial fibroblasts) promote a subset of myoblasts (pre-sumably reserve cells) to return to the quiescent state (100).This procedure mimics the self-renewal of satellite cells dur-ing muscle regeneration. Indeed, recent studies demon-strated that the Tie-2 receptor is preferentially expressed byquiescent satellite cells and angiopoietin-1/Tie-2 signalingacts through ERK1/2 pathway to promote the reentry toquiescence and thus self-renewal of a subset of satellite cells(3, 100). Based on these observations, it was proposed thatduring muscle regeneration, while vessels are not stabilized,endothelial cells and myogenic precursors interact witheach other to promote both myogenesis and angiogenesis.Once homeostasis of muscle is achieved, the proximity ofsatellite cells and periendothelial cells permits angiopoi-etin-1, which is secreted by periendothelial cells, to bindTie-2 receptors on satellite cells to simultaneously stabilizevessels and promote satellite cell quiescence (4, 100). Inaddition to endothelial cells, satellite cells and myoblasts,differentiating myofibers also generate VEGF within skele-



tal muscle (70, 92, 189). VEGF can function in an autocrinefashion through VEGF receptors present on various myo-genic cells to stimulate migration, prevent apoptosis, andpromote differentiation (92, 189). In addition, VEGF stim-ulation can reportedly activate the Akt pathway and pro-mote myofiber hypertrophy (70, 514).

In aged muscle, it was observed that while myofibers losetheir close contact with the microvascular network, there isa corresponding VEGF level decrease (451). In addition,endothelial nitric oxide synthase (eNOS) secreted by endo-thelial cells is also decreased in aged muscle (580; reviewedin Ref. 67).

Taken together, recent studies have begun to unveil inter-actions between satellite cells and cells within the microvas-culature, experiments that imply a potentially profoundfunctional role necessitating the close proximity of satellitecells to the microvasculature. Future studies are needed tofurther elucidate these interactions, particularly those ofperiendothelial cells, during muscle regeneration.

2. Systemic milieu

A) IMMUNE CELLS. Only a small number of immune cells residewithin intact skeletal muscle. During the early stages ofmuscle injury, immune cells are recruited and play impor-tant roles in the regeneration process (reviewed in Ref. 532).Upon injury, immune cells rapidly infiltrate the muscle toremove necrotic tissue and secrete soluble factors that serveto activate satellite cells (reviewed in Ref. 530). As such,immune cells constitute a transient local environment forsatellite cells. Depletion of macrophages and monocytesimpairs subsequent muscle regeneration (206). Satellitecells and immune cells attract one another through chemo-kines (chemoattraction) (440). Satellite cells have beendemonstrated to secrete a panel of pro-inflammatory cyto-kines, such as IL-1, IL-6, and TNF-�, to facilitate immunecell infiltration and function (reviewed in Ref. 92). In turn,immune cells secrete a wealth of diffusible factors, such asgrowth factors, IL-6, globular adiponectin, ECM compo-nents, and ECM remodeling MMPs. These diffusible fac-tors generate ECM chemoattractive fragments, help satel-lite cells escape from the basal lamina of myofibers, andpromote satellite cell proliferation. In addition, cell-to-cellcontact between immune cells and satellite cells protectssatellite cells from apoptosis (503).

Aged muscle displays a functional deficit in the immunecells surrounding the satellite cell niche. In vitro studiesindicate that the capabilities of free radical generation,phagocytosis, and chemotaxis are decreased in aged im-mune cells (25). The perturbation between satellite cells andimmune cells impedes satellite cell activation and migra-tion.

B) IL-6. IL-6 is a ubiquitous pleiotropic cytokine, which canbe produced by many cell types, such as hematopoietic cells,fibroblasts, and vascular endothelial cells. Although IL-6 isprimarily produced following an immune response, con-vincing evidence demonstrates that IL-6 is also involved insatellite cell-mediated muscle hypertrophy and regenera-tion. Chronic systematic elevation of IL-6 leads to muscleatrophy in aging and disease states (29, 212; reviewed inRef. 154). Likewise, transgenic mice overexpressing IL-6display a muscle wasting (cachexia) phenotype (538). It isalso well documented that exercise causes an increase inIL-6 levels within muscle (533; reviewed in Ref. 403). Dur-ing regeneration, satellite cells, myofibers, and neutrophilsare the principal sources of IL-6, the expression of which isregulated by calcineurin-NFAT, NF-�B, AP-1, IL-1, andNO signaling pathways (74, 333, 583; reviewed in Ref.403).

The IL-6 receptor (IL-6R�) is expressed on the myofibersarcolemma and is responsive to exercise (268). Muscle-derived IL-6 can act in an autocrine fashion by binding tothe IL-6R� on the sarcolemma and activate the JAK/STAT3signaling pathway (480; reviewed in Refs. 403, 430). Bind-ing of IL-6 to the receptor IL-6R� leads to JAK2 phosphor-ylation and subsequent STAT3 phosphorylation. Phos-phorylated STAT3 dimerizes and translocates to the nu-cleus where it acts as a transcription factor for target geneactivation.

It has been shown that IL-6 can also induce satellite cellproliferation (80). Recently, it was demonstrated thatIL-6 is involved in satellite cell-mediated muscle hyper-trophy (480). Although early studies demonstrated thatmuscle regeneration in IL-6�/� null mice was compara-ble to that of wild type (556), a recent study reported ablunted hypertrophic response and less contribution ofsatellite cell myonuclei to regenerating myofibers (480).Furthermore, it was shown that the proliferation of sat-ellite cells from IL-6�/� mice was impaired both in vivoand in vitro, which is due to the attenuation of STAT3signaling (480).

C) ANDROGENS. The musculature of men and woman differsin mass, fiber size, and various other physiological prop-erties. These differences are in part due to the anabolicactions of circulating androgens, e.g., testosterone, onmuscle size and muscle strength (reviewed in Refs. 97,228). Androgens bind to androgen receptors (AR) andlead to the dimerization and nuclear translocation of AR.In the nucleus, AR homodimers bind to the DNA motifsor androgen response elements (AREs) and activate tran-scriptional target genes.

Satellite cells express androgen receptors and are recep-tive to androgen signaling (143); however, the endoge-nous androgens acting on satellite cells have yet to be



determined. Exogenous testosterone increases satellitecell numbers by promoting satellite cell activation andproliferation (255, 494). This function is predicted toinvolve the induction of Notch signaling in satellite cells(494). Furthermore, androgen administration causes in-creased myonuclei and muscle hypertrophy in a dose-dependent manner (254, 493, 495). Multiple mecha-nisms have been implicated in androgen-induced musclehypertrophy. First, androgen administration has beenshown to elevate local IGF-I levels (165, 307) through apostulated induction of IGF1R expression on satellitecells (529). Androgen administration can also elevate in-tracellular calcium and inositol 1,4,5-trisphosphate (IP3)levels, which induces phosphorylation of ERK1/2 andcontributes to affect the IGF pathway (157). Addition-ally, androgen may promote muscle hypertrophy bycounteracting the catabolic effect of glucocorticoid hor-mones and promote the myogenic differentiation (48,496).

During aging, systemic levels of androgens decline, which isconcomitant with a loss in muscle mass and a gain in fatmass (228, 544).

D) NITRIC OXIDE. NO is a diffusible small molecule, which canfreely pass through cell membranes. It can be produced bynitric oxide synthase (NOS) in diverse cell types, includingepithelial cells, endothelial cells, fibroblasts, hepatocytes,macrophages, and skeletal muscle cells (60). Currentlythree NOS isoforms exist: endothelial NOS (eNOS), neuro-nal NOS (nNOS), and inducible NOS (iNOS) (394).

NO has profound functions in intact muscle as well as dur-ing muscle regeneration (reviewed in Ref. 505). In skeletalmuscle, nNOS is constitutively expressed within the sarco-lemma of myofibers (275). Loss of nNOS from the sarco-lemma (e.g., as a result of a dystrophin deficiency) leads toinflammation and myofiber lysis, implicating a protectivefunction for NO in skeletal muscle (89). iNOS levels areextremely low in intact muscle, yet drastically induced bymacrophages and myofibers in response to muscle injury(449; reviewed in Ref. 262). In the degenerating phase, NO,generated by macrophages and myofibers, promotes thelysis of necrotic myofibers by macrophages while reducingother inflammation-induced damage (375; reviewed in Ref.505). During regeneration, NO induces satellite cell activa-tion, as evidenced by the inhibitory effect of L-argininemethyl ester (L-NAME), a NOS inhibitor, on satellite cellactivation (16). NOS inhibition prevents the binding ofHGF to the c-Met receptor on satellite cells, whereas treat-ment with L-arginine, the NOS substrate, resulted in anincrease in satellite cell activation. Accordingly, iNOS�/�

mice display delayed satellite cell activation following mus-cle injury, although injured muscles were still able to regen-erate (16). The function of NO-dependent satellite cell ac-tivation stems from the ability of NO to promote the release

of HGF from the ECM (575, 576). MMP-2 was demon-strated to be involved in this process (575). In addition, itwas also suggested that NO may regulate satellite cell mi-gration (104) and prevent fibrosis by inhibiting transform-ing growth factor-� signaling during muscle regeneration(126).

IV. CONCLUDING REMARKSAND PERSPECTIVES

Compelling evidence indicates that adult satellite cells rep-resent a heterogeneous population of stem cells and com-mitted myogenic progenitors in skeletal muscle. At the pop-ulation level, the stem cell characteristics of satellite cells arewell represented by their remarkable and sustained abilityto support skeletal muscle regeneration. At the individuallevel, satellite cells vary in their self-renewal, proliferation,and myogenic differentiation potential. These intrinsic dif-ferences of satellite cells likely reflect the distinct needs of1) maintaining a sustainable reservoir of stem cells, 2) pro-ducing sufficient numbers of myogenic cells, and 3) gener-ating, during the course of muscle regeneration, functionalcontractile muscular structures. Throughout regeneration,the hierarchical and collaborative behaviors of satellite cellsare influenced by changes in their niche. These changesinclude diverse molecules, cells, and structures that consti-tute a dynamic niche continually influencing the multipletasks set forth for satellite cells.

Satellite cells are an ideal cellular model system to studyadult stem cells and tissue regeneration. Extensive studiesover the last three decades have accumulated much knowl-edge and insights. Satellite cells are currently not applicableto regenerative medicine due to difficulties in their isolationand loss of stemness ex vivo. However, satellite cell-basedcell therapy would greatly benefit from future studies inlineage progression to better delineate satellite cell hierar-chy along with physiologically relevant interactions be-tween satellite cells and other myogenic cells. Moreover,such issues may be resolved following the maturation oftechniques involving the generation of iPS or other types ofES-like multipotent cells. If so, the broad interest of futurestudies should focus on identification of the intrinsic andextrinsic regulatory mechanisms that govern satellite cellcommitment and differentiation throughout the muscle re-generation. Future advances in system biology and bioengi-neering may hopefully harness these previous achievementstowards developing therapeutic approaches for sarcopeniaand muscle dystrophic diseases.

ACKNOWLEDGMENTS

We credit L. Bucklin for the human leg anatomy image usedin FIGURE 3 and Alexander Grayston for critical review ofthe manuscript.



Address for reprint requests and other correspondence:M. A. Rudnicki, Ottawa Hospital Research Institute, 501Smyth Rd., Ottawa, Ontario, Canada K1H 8L6 (e-mail:[email protected]).

DISCLOSURES

No conflicts of interest, financial or otherwise, are declaredby the authors.

REFERENCES

1. Abedi M, Greer DA, Colvin GA, Demers DA, Dooner MS, Harpel JA, Weier HU,Lambert JF, Quesenberry PJ. Robust conversion of marrow cells to skeletal musclewith formation of marrow-derived muscle cell colonies: a multifactorial process. ExpHematol 32: 426–434, 2004.

2. Abiola M, Favier M, Christodoulou-Vafeiadou E, Pichard AL, Martelly I, Guillet-DeniauI. Activation of Wnt/beta-catenin signaling increases insulin sensitivity through a recip-rocal regulation of Wnt10b and SREBP-1c in skeletal muscle cells. PLoS One 4: e8509,2009.

3. Abou-Khalil R, Le Grand F, Pallafacchina G, Valable S, Authier FJ, Rudnicki MA, Gh-erardi RK, Germain S, Chretien F, Sotiropoulos A, Lafuste P, Montarras D, Chazaud B.Autocrine and paracrine angiopoietin 1/Tie-2 signaling promotes muscle satellite cellself-renewal. Cell Stem Cell 5: 298–309, 2009.

4. Abou-Khalil R, Mounier R, Chazaud B. Regulation of myogenic stem cell behavior byvessel cells: the “menage a trois” of satellite cells, periendothelial cells and endothelialcells. Cell Cycle 9: 892–896, 2010.

5. Adams GR. Autocrine and/or paracrine insulin-like growth factor-I activity in skeletalmuscle. Clin Orthop Relat Res S188–196, 2002.

6. Adams GR, McCue SA. Localized infusion of IGF-I results in skeletal muscle hypertro-phy in rats. J Appl Physiol 84: 1716–1722, 1998.

7. Alderton JM, Steinhardt RA. How calcium influx through calcium leak channels isresponsible for the elevated levels of calcium-dependent proteolysis in dystrophicmyotubes. Trends Cardiovasc Med 10: 268–272, 2000.

8. Alfaro LA, Dick SA, Siegel AL, Anonuevo AS, McNagny KM, Megeney LA, CornelisonDD, Rossi FM. CD34 promotes satellite cell motility and entry into proliferation tofacilitate efficient skeletal muscle regeneration. Stem Cells 29: 2030–2041, 2011.

9. Allbrook DB, Han MF, Hellmuth AE. Population of muscle satellite cells in relation toage and mitotic activity. Pathology 3: 223–243, 1971.

10. Allen RE, Boxhorn LK. Inhibition of skeletal muscle satellite cell differentiation bytransforming growth factor-beta. J Cell Physiol 133: 567–572, 1987.

11. Allen RE, Boxhorn LK. Regulation of skeletal muscle satellite cell proliferation anddifferentiation by transforming growth factor-beta, insulin-like growth factor I, andfibroblast growth factor. J Cell Physiol 138: 311–315, 1989.

12. Allen RE, Dodson MV, Luiten LS. Regulation of skeletal muscle satellite cell prolifer-ation by bovine pituitary fibroblast growth factor. Exp Cell Res 152: 154–160, 1984.

13. Allen RE, Sheehan SM, Taylor RG, Kendall TL, Rice GM. Hepatocyte growth factoractivates quiescent skeletal muscle satellite cells in vitro. J Cell Physiol 165: 307–312,1995.

14. Allouh MZ, Yablonka-Reuveni Z, Rosser BW. Pax7 reveals a greater frequency andconcentration of satellite cells at the ends of growing skeletal muscle fibers. J His-tochem Cytochem 56: 77–87, 2008.

15. Anastasi S, Giordano S, Sthandier O, Gambarotta G, Maione R, Comoglio P, Amati P.A natural hepatocyte growth factor/scatter factor autocrine loop in myoblast cells andthe effect of the constitutive Met kinase activation on myogenic differentiation. J CellBiol 137: 1057–1068, 1997.

16. Anderson JE. A role for nitric oxide in muscle repair: nitric oxide-mediated activationof muscle satellite cells. Mol Biol Cell 11: 1859–1874, 2000.

17. Angelini C, Di Mauro S, Margreth A. Relationship of serum enzyme changes to muscledamage in vitamin E deficiency of the rabbit. Sperimentale 118: 349–369, 1968.

18. Armand O, Boutineau AM, Mauger A, Pautou MP, Kieny M. Origin of satellite cells inavian skeletal muscles. Arch Anat Microsc Morphol Exp 72: 163–181, 1983.

19. Armstrong RB. Initial events in exercise-induced muscular injury. Med Sci Sports Ex-ercise 22: 429–435, 1990.

20. Arsic N, Zacchigna S, Zentilin L, Ramirez-Correa G, Pattarini L, Salvi A, Sinagra G,Giacca M. Vascular endothelial growth factor stimulates skeletal muscle regenerationin vivo. Mol Ther 10: 844–854, 2004.

21. Artavanis-Tsakonas S, Rand MD, Lake RJ. Notch signaling: cell fate control and signalintegration in development. Science 284: 770–776, 1999.

22. Asakura A, Hirai H, Kablar B, Morita S, Ishibashi J, Piras BA, Christ AJ, Verma M,Vineretsky KA, Rudnicki MA. Increased survival of muscle stem cells lacking the MyoDgene after transplantation into regenerating skeletal muscle. Proc Natl Acad Sci USA104: 16552–16557, 2007.

23. Asakura A, Komaki M, Rudnicki M. Muscle satellite cells are multipotential stem cellsthat exhibit myogenic, osteogenic, adipogenic differentiation. Differentiation 68: 245–253, 2001.

24. Asakura A, Seale P, Girgis-Gabardo A, Rudnicki MA. Myogenic specification of sidepopulation cells in skeletal muscle. J Cell Biol 159: 123–134, 2002.

25. Ashcroft GS, Mills SJ, Ashworth JJ. Ageing and wound healing. Biogerontology 3: 337–345, 2002.

26. Ates K, Yang SY, Orrell RW, Sinanan AC, Simons P, Solomon A, Beech S, Goldspink G,Lewis MP. The IGF-I splice variant MGF increases progenitor cells in ALS, dystrophic,and normal muscle. FEBS Lett 581: 2727–2732, 2007.

27. Bachrach E, Li S, Perez AL, Schienda J, Liadaki K, Volinski J, Flint A, Chamberlain J,Kunkel LM. Systemic delivery of human microdystrophin to regenerating mouse dys-trophic muscle by muscle progenitor cells. Proc Natl Acad Sci USA 101: 3581–3586,2004.

28. Baldwin KM, Haddad F. Effects of different activity and inactivity paradigms on myosinheavy chain gene expression in striated muscle. J Appl Physiol 90: 345–357, 2001.

29. Baltgalvis KA, Berger FG, Pena MM, Davis JM, Muga SJ, Carson JA. Interleukin-6 andcachexia in ApcMin/� mice. Am J Physiol Regul Integr Comp Physiol 294: R393–R401,2008.

30. Bansal D, Miyake K, Vogel SS, Groh S, Chen CC, Williamson R, McNeil PL, CampbellKP. Defective membrane repair in dysferlin-deficient muscular dystrophy. Nature423: 168–172, 2003.

31. Barberi T, Bradbury M, Dincer Z, Panagiotakos G, Socci ND, Studer L. Derivation ofengraftable skeletal myoblasts from human embryonic stem cells. Nat Med 13: 642–648, 2007.

32. Bark TH, McNurlan MA, Lang CH, Garlick PJ. Increased protein synthesis after acuteIGF-I or insulin infusion is localized to muscle in mice. Am J Physiol Endocrinol Metab275: E118–E123, 1998.

33. Baron P, Scarpini E, Meola G, Santilli I, Conti G, Pleasure D, Scarlato G. Expression ofthe low-affinity NGF receptor during human muscle development, regeneration, andin tissue culture. Muscle Nerve 17: 276–284, 1994.

34. Barton-Davis ER, Shoturma DI, Musaro A, Rosenthal N, Sweeney HL. Viral mediatedexpression of insulin-like growth factor I blocks the aging-related loss of skeletalmuscle function. Proc Natl Acad Sci USA 95: 15603–15607, 1998.

35. Barton-Davis ER, Shoturma DI, Sweeney HL. Contribution of satellite cells to IGF-Iinduced hypertrophy of skeletal muscle. Acta Physiol Scand 167: 301–305, 1999.

36. Bashir R, Britton S, Strachan T, Keers S, Vafiadaki E, Lako M, Richard I, Marchand S,Bourg N, Argov Z, Sadeh M, Mahjneh I, Marconi G, Passos-Bueno MR, Moreira Ede S,Zatz M, Beckmann JS, Bushby K. A gene related to Caenorhabditis elegans spermato-genesis factor fer-1 is mutated in limb-girdle muscular dystrophy type 2B. Nat Genet20: 37–42, 1998.

37. Beauchamp JR, Heslop L, Yu DS, Tajbakhsh S, Kelly RG, Wernig A, Buckingham ME,Partridge TA, Zammit PS. Expression of CD34 and Myf5 defines the majority ofquiescent adult skeletal muscle satellite cells. J Cell Biol 151: 1221–1234, 2000.



38. Beggs ML, Nagarajan R, Taylor-Jones JM, Nolen G, Macnicol M, Peterson CA. Alter-ations in the TGFbeta signaling pathway in myogenic progenitors with age. Aging Cell3: 353–361, 2004.

39. Belcastro AN, Shewchuk LD, Raj DA. Exercise-induced muscle injury: a calpain hy-pothesis. Mol Cell Biochem 179: 135–145, 1998.

40. Ben-Yair R, Kalcheim C. Lineage analysis of the avian dermomyotome sheet revealsthe existence of single cells with both dermal and muscle progenitor fates. Develop-ment 132: 689–701, 2005.

41. Benchaouir R, Meregalli M, Farini A, D’Antona G, Belicchi M, Goyenvalle A, BattistelliM, Bresolin N, Bottinelli R, Garcia L, Torrente Y. Restoration of human dystrophinfollowing transplantation of exon-skipping-engineered DMD patient stem cells intodystrophic mice. Cell Stem Cell 1: 646–657, 2007.

42. Benezra R, Davis RL, Lassar A, Tapscott S, Thayer M, Lockshon D, Weintraub H. Id:a negative regulator of helix-loop-helix DNA binding proteins. Control of terminalmyogenic differentiation. Ann NY Acad Sci 599: 1–11, 1990.

43. Benezra R, Davis RL, Lockshon D, Turner DL, Weintraub H. The protein Id: a nega-tive regulator of helix-loop-helix DNA binding proteins. Cell 61: 49–59, 1990.

44. Bergstrom DA, Penn BH, Strand A, Perry RL, Rudnicki MA, Tapscott SJ. Promoter-specific regulation of MyoD binding and signal transduction cooperate to pattern geneexpression. Mol Cell 9: 587–600, 2002.

45. Berkes CA, Tapscott SJ. MyoD and the transcriptional control of myogenesis. SeminCell Dev Biol 16: 585–595, 2005.

46. Besson V, Smeriglio P, Wegener A, Relaix F, Nait Oumesmar B, Sassoon DA, MarazziG. PW1 gene/paternally expressed gene 3 (PW1/Peg3) identifies multiple adult stemand progenitor cell populations. Proc Natl Acad Sci USA 108: 11470–11475, 2011.

47. Bhagavati S, Xu W. Generation of skeletal muscle from transplanted embryonic stemcells in dystrophic mice. Biochem Biophys Res Commun 333: 644–649, 2005.

48. Bhasin S, Taylor WE, Singh R, Artaza J, Sinha-Hikim I, Jasuja R, Choi H, Gonzalez-Cadavid NF. The mechanisms of androgen effects on body composition: mesenchy-mal pluripotent cell as the target of androgen action. J Gerontol A Biol Sci Med Sci 58:M1103–M1110, 2003.

49. Bischoff R. Chemotaxis of skeletal muscle satellite cells. Dev Dyn 208: 505–515, 1997.

50. Bischoff R. Interaction between satellite cells and skeletal muscle fibers. Development109: 943–952, 1990.

51. Bischoff R. Regeneration of single skeletal muscle fibers in vitro. Anat Rec 182: 215–235, 1975.

52. Bischoff R. The satellite cell and muscle regeneration. Myology 97–118, 1994.

53. Bischoff R. A satellite cell mitogen from crushed adult muscle. Dev Biol 115: 140–147,1986.

54. Bittner RE, Schofer C, Weipoltshammer K, Ivanova S, Streubel B, Hauser E, FreilingerM, Hoger H, Elbe-Burger A, Wachtler F. Recruitment of bone-marrow-derived cellsby skeletal and cardiac muscle in adult dystrophic mdx mice. Anat Embryol 199:391–396, 1999.

55. Bjerknes M, Cheng H. Gastrointestinal stem cells. II. Intestinal stem cells. Am J PhysiolGastrointest Liver Physiol 289: G381–G387, 2005.

56. Blais A, Tsikitis M, Acosta-Alvear D, Sharan R, Kluger Y, Dynlacht BD. An initialblueprint for myogenic differentiation. Genes Dev 19: 553–569, 2005.

57. Blaivas M, Carlson BM. Muscle fiber branching–difference between grafts in old andyoung rats. Mech Ageing Dev 60: 43–53, 1991.

58. Blaveri K, Heslop L, Yu DS, Rosenblatt JD, Gross JG, Partridge TA, Morgan JE.Patterns of repair of dystrophic mouse muscle: studies on isolated fibers. Dev Dyn 216:244–256, 1999.

59. Bockhold KJ, Rosenblatt JD, Partridge TA. Aging normal and dystrophic mouse mus-cle: analysis of myogenicity in cultures of living single fibers. Muscle Nerve 21: 173–183,1998.

60. Bogdan C. Nitric oxide and the regulation of gene expression. Trends Cell Biol 11:66–75, 2001.

61. Boonen KJ, Rosaria-Chak KY, Baaijens FP, van der Schaft DW, Post MJ. Essentialenvironmental cues from the satellite cell niche: optimizing proliferation and differ-entiation. Am J Physiol Cell Physiol 296: C1338–C1345, 2009.

62. Boppart MD, Burkin DJ, Kaufman SJ. Alpha7beta1-integrin regulates mechanotrans-duction and prevents skeletal muscle injury. Am J Physiol Cell Physiol 290: C1660–C1665, 2006.

63. Bosnakovski D, Xu Z, Li W, Thet S, Cleaver O, Perlingeiro RC, Kyba M. Prospectiveisolation of skeletal muscle stem cells with a Pax7 reporter. Stem Cells 26: 3194–3204,2008.

64. Bourke DL, Ontell M. Branched myofibers in long-term whole muscle transplants: aquantitative study. Anat Rec 209: 281–288, 1984.

65. Brack AS, Conboy IM, Conboy MJ, Shen J, Rando TA. A temporal switch from notchto Wnt signaling in muscle stem cells is necessary for normal adult myogenesis. CellStem Cell 2: 50–59, 2008.

66. Brack AS, Conboy MJ, Roy S, Lee M, Kuo CJ, Keller C, Rando TA. Increased Wntsignaling during aging alters muscle stem cell fate and increases fibrosis. Science 317:807–810, 2007.

67. Brandes RP, Fleming I, Busse R. Endothelial aging. Cardiovasc Res 66: 286–294, 2005.

68. Brazelton TR, Nystrom M, Blau HM. Significant differences among skeletal muscles inthe incorporation of bone marrow-derived cells. Dev Biol 262: 64–74, 2003.

69. Brockes JP, Kumar A. Plasticity and reprogramming of differentiated cells in amphibianregeneration. Nat Rev Mol Cell Biol 3: 566–574, 2002.

70. Bryan BA, Walshe TE, Mitchell DC, Havumaki JS, Saint-Geniez M, Maharaj AS, Mal-donado AE, D’Amore PA. Coordinated vascular endothelial growth factor expressionand signaling during skeletal myogenic differentiation. Mol Biol Cell 19: 994–1006,2008.

71. Buckingham M. Myogenic progenitor cells and skeletal myogenesis in vertebrates.Curr Opin Genet Dev 16: 525–532, 2006.

72. Buckingham M, Bajard L, Chang T, Daubas P, Hadchouel J, Meilhac S, Montarras D,Rocancourt D, Relaix F. The formation of skeletal muscle: from somite to limb. J Anat202: 59–68, 2003.

73. Burkin DJ, Kaufman SJ. The alpha7beta1 integrin in muscle development and disease.Cell Tissue Res 296: 183–190, 1999.

74. Cahill CM, Rogers JT. Interleukin (IL) 1beta induction of IL-6 is mediated by a novelphosphatidylinositol 3-kinase-dependent AKT/IkappaB kinase alpha pathway target-ing activator protein-1. J Biol Chem 283: 25900–25912, 2008.

75. Cairns J. Mutation selection and the natural history of cancer. Nature 255: 197–200,1975.

76. Calise S, Blescia S, Cencetti F, Bernacchioni C, Donati C, Bruni P. Sphingosine 1-phos-phate stimulates proliferation and migration of satellite cells: role of S1P receptors.Biochim Biophys Acta 1823: 439–450, 2012.

77. Camargo FD, Gokhale S, Johnnidis JB, Fu D, Bell GW, Jaenisch R, Brummelkamp TR.YAP1 increases organ size and expands undifferentiated progenitor cells. Curr Biol 17:2054–2060, 2007.

78. Camargo FD, Green R, Capetanaki Y, Jackson KA, Goodell MA. Single hematopoieticstem cells generate skeletal muscle through myeloid intermediates. Nat Med 9: 1520–1527, 2003.

79. Campion DR. The muscle satellite cell: a review. Int Rev Cytol 87: 225–251, 1984.

80. Cantini M, Carraro U. Macrophage-released factor stimulates selectively myogeniccells in primary muscle culture. J Neuropathol Exp Neurol 54: 121–128, 1995.

81. Cantini M, Giurisato E, Radu C, Tiozzo S, Pampinella F, Senigaglia D, Zaniolo G,Mazzoleni F, Vitiello L. Macrophage-secreted myogenic factors: a promising tool forgreatly enhancing the proliferative capacity of myoblasts in vitro and in vivo. Neurol Sci23: 189–194, 2002.

82. Cao B, Zheng B, Jankowski RJ, Kimura S, Ikezawa M, Deasy B, Cummins J, Epperly M,Qu-Petersen Z, Huard J. Muscle stem cells differentiate into haematopoietic lineagesbut retain myogenic potential. Nat Cell Biol 5: 640–646, 2003.



83. Cao Y, Kumar RM, Penn BH, Berkes CA, Kooperberg C, Boyer LA, Young RA,Tapscott SJ. Global and gene-specific analyses show distinct roles for Myod and Myogat a common set of promoters. EMBO J 25: 502–511, 2006.

84. Cao Y, Yao Z, Sarkar D, Lawrence M, Sanchez GJ, Parker MH, MacQuarrie KL,Davison J, Morgan MT, Ruzzo WL, Gentleman RC, Tapscott SJ. Genome-wide MyoDbinding in skeletal muscle cells: a potential for broad cellular reprogramming. Dev Cell18: 662–674, 2010.

85. Caplan AI. All MSCs are pericytes? Cell Stem Cell 3: 229–230, 2008.

86. Carmeli E, Moas M, Reznick AZ, Coleman R. Matrix metalloproteinases and skeletalmuscle: a brief review. Muscle Nerve 29: 191–197, 2004.

87. Chakravarthy MV, Davis BS, Booth FW. IGF-I restores satellite cell proliferative po-tential in immobilized old skeletal muscle. J Appl Physiol 89: 1365–1379, 2000.

88. Chang H, Yoshimoto M, Umeda K, Iwasa T, Mizuno Y, Fukada S, Yamamoto H,Motohashi N, Miyagoe-Suzuki Y, Takeda S, Heike T, Nakahata T. Generation oftransplantable, functional satellite-like cells from mouse embryonic stem cells. FASEBJ 23: 1907–1919, 2009.

89. Chao DS, Gorospe JR, Brenman JE, Rafael JA, Peters MF, Froehner SC, Hoffman EP,Chamberlain JS, Bredt DS. Selective loss of sarcolemmal nitric oxide synthase inBecker muscular dystrophy. J Exp Med 184: 609–618, 1996.

90. Charge SB, Brack AS, Hughes SM. Aging-related satellite cell differentiation defectoccurs prematurely after Ski-induced muscle hypertrophy. Am J Physiol Cell Physiol283: C1228–C1241, 2002.

91. Chazaud B, Brigitte M, Yacoub-Youssef H, Arnold L, Gherardi R, Sonnet C, Lafuste P,Chretien F. Dual and beneficial roles of macrophages during skeletal muscle regen-eration. Exerc Sport Sci Rev 37: 18–22, 2009.

92. Chazaud B, Sonnet C, Lafuste P, Bassez G, Rimaniol AC, Poron F, Authier FJ, DreyfusPA, Gherardi RK. Satellite cells attract monocytes and use macrophages as a supportto escape apoptosis and enhance muscle growth. J Cell Biol 163: 1133–1143, 2003.

93. Chen JF, Mandel EM, Thomson JM, Wu Q, Callis TE, Hammond SM, Conlon FL, WangDZ. The role of microRNA-1 and microRNA-133 in skeletal muscle proliferation anddifferentiation. Nature Genet 38: 228–233, 2006.

94. Chen JF, Tao Y, Li J, Deng Z, Yan Z, Xiao X, Wang DZ. microRNA-1 and microRNA-206 regulate skeletal muscle satellite cell proliferation and differentiation by repress-ing Pax7. J Cell Biol 190: 867–879, 2010.

95. Chen X, Mao Z, Liu S, Liu H, Wang X, Wu H, Wu Y, Zhao T, Fan W, Li Y, Yew DT,Kindler PM, Li L, He Q, Qian L, Fan M. Dedifferentiation of adult human myoblastsinduced by ciliary neurotrophic factor in vitro. Mol Biol Cell 16: 3140–3151, 2005.

96. Chen Y, Lin G, Slack JM. Control of muscle regeneration in the Xenopus tadpole tail byPax7. Development 133: 2303–2313, 2006.

97. Chen Y, Zajac JD, MacLean HE. Androgen regulation of satellite cell function. J Endo-crinol 186: 21–31, 2005.

98. Cheung TH, Quach NL, Charville GW, Liu L, Park L, Edalati A, Yoo B, Hoang P, RandoTA. Maintenance of muscle stem-cell quiescence by microRNA-489. Nature 482:524–528, 2012.

99. Chevrel G, Hohlfeld R, Sendtner M. The role of neurotrophins in muscle underphysiological and pathological conditions. Muscle Nerve 33: 462–476, 2006.

100. Christov C, Chretien F, Abou-Khalil R, Bassez G, Vallet G, Authier FJ, Bassaglia Y,Shinin V, Tajbakhsh S, Chazaud B, Gherardi RK. Muscle satellite cells and endothelialcells: close neighbors and privileged partners. Mol Biol Cell 18: 1397–1409, 2007.

101. Cinnamon Y, Ben-Yair R, Kalcheim C. Differential effects of N-cadherin-mediatedadhesion on the development of myotomal waves. Development 133: 1101–1112,2006.

102. Clow C, Jasmin BJ. Skeletal muscle-derived BDNF regulates satellite cell differentia-tion and muscle regeneration. Mol Biol Cell 2010.

103. Coleman ME, DeMayo F, Yin KC, Lee HM, Geske R, Montgomery C, Schwartz RJ.Myogenic vector expression of insulin-like growth factor I stimulates muscle celldifferentiation and myofiber hypertrophy in transgenic mice. J Biol Chem 270: 12109–12116, 1995.

104. Collins-Hooper H, Woolley TE, Dyson L, Patel A, Potter P, Baker RE, Gaffney EA,Maini PK, Dash PR, Patel K. Age-related changes in speed and mechanism of adultskeletal muscle stem cell migration. Stem Cells 30: 1182–1195, 2012.

105. Collins CA, Olsen I, Zammit PS, Heslop L, Petrie A, Partridge TA, Morgan JE. Stemcell function, self-renewal, and behavioral heterogeneity of cells from the adult musclesatellite cell niche. Cell 122: 289–301, 2005.

106. Collinsworth AM, Zhang S, Kraus WE, Truskey GA. Apparent elastic modulus andhysteresis of skeletal muscle cells throughout differentiation. Am J Physiol Cell Physiol283: C1219–C1227, 2002.

107. Conboy IM, Conboy MJ, Smythe GM, Rando TA. Notch-mediated restoration ofregenerative potential to aged muscle. Science 302: 1575–1577, 2003.

108. Conboy IM, Rando TA. The regulation of Notch signaling controls satellite cell acti-vation and cell fate determination in postnatal myogenesis. Dev Cell 3: 397–409, 2002.

109. Conboy MJ, Karasov AO, Rando TA. High incidence of non-random template strandsegregation and asymmetric fate determination in dividing stem cells and their prog-eny. PLoS Biol 5: e102, 2007.

110. Coolican SA, Samuel DS, Ewton DZ, McWade FJ, Florini JR. The mitogenic andmyogenic actions of insulin-like growth factors utilize distinct signaling pathways. J BiolChem 272: 6653–6662, 1997.

111. Cooper RN, Tajbakhsh S, Mouly V, Cossu G, Buckingham M, Butler-Browne GS. Invivo satellite cell activation via Myf5 and MyoD in regenerating mouse skeletal muscle.J Cell Sci 112: 2895–2901, 1999.

112. Corbel SY, Lee A, Yi L, Duenas J, Brazelton TR, Blau HM, Rossi FM. Contribution ofhematopoietic stem cells to skeletal muscle. Nat Med 9: 1528–1532, 2003.

113. Cornelison DD, Filla MS, Stanley HM, Rapraeger AC, Olwin BB. Syndecan-3 andsyndecan-4 specifically mark skeletal muscle satellite cells and are implicated in satel-lite cell maintenance and muscle regeneration. Dev Biol 239: 79–94, 2001.

114. Cornelison DD, Olwin BB, Rudnicki MA, Wold BJ. MyoD(�/�) satellite cells in single-fiber culture are differentiation defective and MRF4 deficient. Dev Biol 224: 122–137,2000.

115. Cornelison DD, Wilcox-Adelman SA, Goetinck PF, Rauvala H, Rapraeger AC, OlwinBB. Essential and separable roles for Syndecan-3 and Syndecan-4 in skeletal muscledevelopment and regeneration. Genes Dev 18: 2231–2236, 2004.

116. Cornelison DD, Wold BJ. Single-cell analysis of regulatory gene expression in quies-cent and activated mouse skeletal muscle satellite cells. Dev Biol 191: 270–283, 1997.

117. Cox DM, Du M, Marback M, Yang EC, Chan J, Siu KW, McDermott JC. Phosphory-lation motifs regulating the stability and function of myocyte enhancer factor 2A. J BiolChem 278: 15297–15303, 2003.

118. Crisan M, Yap S, Casteilla L, Chen CW, Corselli M, Park TS, Andriolo G, Sun B, ZhengB, Zhang L, Norotte C, Teng PN, Traas J, Schugar R, Deasy BM, Badylak S, Buhring HJ,Giacobino JP, Lazzari L, Huard J, Peault B. A perivascular origin for mesenchymal stemcells in multiple human organs. Cell Stem Cell 3: 301–313, 2008.

119. Crist CG, Montarras D, Pallafacchina G, Rocancourt D, Cumano A, Conway SJ,Buckingham M. Muscle stem cell behavior is modified by microRNA-27 regulation ofPax3 expression. Proc Natl Acad Sci USA 106: 13383–13387, 2009.

120. Csete M, Walikonis J, Slawny N, Wei Y, Korsnes S, Doyle JC, Wold B. Oxygen-mediated regulation of skeletal muscle satellite cell proliferation and adipogenesis inculture. J Cell Physiol 189: 189–196, 2001.

121. d’Albis A, Lenfant-Guyot M, Janmot C, Chanoine C, Weinman J, Gallien CL. Regula-tion by thyroid hormones of terminal differentiation in the skeletal dorsal muscle. I.Neonate mouse. Dev Biol 123: 25–32, 1987.

122. Danieli-Betto D, Peron S, Germinario E, Zanin M, Sorci G, Franzoso S, Sandona D,Betto R. Sphingosine 1-phosphate signaling is involved in skeletal muscle regeneration.Am J Physiol Cell Physiol 298: C550–C558, 2010.

123. Darabi R, Arpke RW, Irion S, Dimos JT, Grskovic M, Kyba M, Perlingeiro RC. HumanES- and iPS-derived myogenic progenitors restore dystrophin and improve contrac-tility upon transplantation in dystrophic mice. Cell Stem Cell 10: 610–619, 2012.

124. Darabi R, Gehlbach K, Bachoo RM, Kamath S, Osawa M, Kamm KE, Kyba M, Per-lingeiro RC. Functional skeletal muscle regeneration from differentiating embryonicstem cells. Nat Med 14: 134–143, 2008.



125. Darabi R, Santos FN, Filareto A, Pan W, Koene R, Rudnicki MA, Kyba M, PerlingeiroRC. Assessment of the myogenic stem cell compartment following transplantation ofPax3/Pax7-induced embryonic stem cell-derived progenitors. Stem Cells 29: 777–790, 2011.

126. Darmani H, Crossan J, McLellan SD, Meek D, Adam C. Expression of nitric oxidesynthase and transforming growth factor-beta in crush-injured tendon and synovium.Mediators Inflamm 13: 299–305, 2004.

127. Day K, Shefer G, Richardson JB, Enikolopov G, Yablonka-Reuveni Z. Nestin-GFPreporter expression defines the quiescent state of skeletal muscle satellite cells. DevBiol 304: 246–259, 2007.

128. Day K, Shefer G, Shearer A, Yablonka-Reuveni Z. The depletion of skeletal musclesatellite cells with age is concomitant with reduced capacity of single progenitors toproduce reserve progeny. Dev Biol 340: 330–343, 2010.

129. De Angelis L, Berghella L, Coletta M, Lattanzi L, Zanchi M, Cusella-De Angelis MG,Ponzetto C, Cossu G. Skeletal myogenic progenitors originating from embryonicdorsal aorta coexpress endothelial and myogenic markers and contribute to postnatalmuscle growth and regeneration. J Cell Biol 147: 869–878, 1999.

130. De Castro Rodrigues A, Andreo JC, de Mattos Rodrigues SP. Myonuclei and satellitecells in denervated rat muscles: an electron microscopy study. Microsurgery 26: 396–398, 2006.

131. De Strooper B, Annaert W. Where Notch and Wnt signaling meet. The presenilin hub.J Cell Biol 152: F17–20, 2001.

132. Deasy BM, Feduska JM, Payne TR, Li Y, Ambrosio F, Huard J. Effect of VEGF on theregenerative capacity of muscle stem cells in dystrophic skeletal muscle. Mol Ther J AmSoc Gene Ther 17: 1788–1798, 2009.

133. DeChiara TM, Efstratiadis A, Robertson EJ. A growth-deficiency phenotype inheterozygous mice carrying an insulin-like growth factor II gene disrupted by target-ing. Nature 345: 78–80, 1990.

134. deLapeyriere O, Ollendorff V, Planche J, Ott MO, Pizette S, Coulier F, Birnbaum D.Expression of the Fgf6 gene is restricted to developing skeletal muscle in the mouseembryo. Development 118: 601–611, 1993.

135. Dellavalle A, Maroli G, Covarello D, Azzoni E, Innocenzi A, Perani L, Antonini S,Sambasivan R, Brunelli S, Tajbakhsh S, Cossu G. Pericytes resident in postnatal skel-etal muscle differentiate into muscle fibres and generate satellite cells. Nature Com-mun 2: 499, 2011.

136. Dellavalle A, Sampaolesi M, Tonlorenzi R, Tagliafico E, Sacchetti B, Perani L, InnocenziA, Galvez BG, Messina G, Morosetti R, Li S, Belicchi M, Peretti G, Chamberlain JS,Wright WE, Torrente Y, Ferrari S, Bianco P, Cossu G. Pericytes of human skeletalmuscle are myogenic precursors distinct from satellite cells. Nat Cell Biol 9: 255–267,2007.

137. Delling U, Tureckova J, Lim HW, De Windt LJ, Rotwein P, Molkentin JD. A calcineurin-NFATc3-dependent pathway regulates skeletal muscle differentiation and slow my-osin heavy-chain expression. Mol Cell Biol 20: 6600–6611, 2000.

138. Deponti D, Buono R, Catanzaro G, De Palma C, Longhi R, Meneveri R, Bresolin N,Bassi MT, Cossu G, Clementi E, Brunelli S. The low-affinity receptor for neurotro-phins p75NTR plays a key role for satellite cell function in muscle repair acting viaRhoA. Mol Biol Cell 20: 3620–3627, 2009.

139. Dey BK, Gagan J, Dutta A. miR-206 and -486 induce myoblast differentiation bydownregulating Pax7. Mol Cell Biol 31: 203–214, 2011.

140. Dezawa M, Ishikawa H, Itokazu Y, Yoshihara T, Hoshino M, Takeda S, Ide C,Nabeshima Y. Bone marrow stromal cells generate muscle cells and repair muscledegeneration. Science 309: 314–317, 2005.

141. DiMario J, Buffinger N, Yamada S, Strohman RC. Fibroblast growth factor in theextracellular matrix of dystrophic (mdx) mouse muscle. Science 244: 688–690, 1989.

142. Doherty MJ, Ashton BA, Walsh S, Beresford JN, Grant ME, Canfield AE. Vascularpericytes express osteogenic potential in vitro and in vivo. J Bone Miner Res 13:828–838, 1998.

143. Doumit ME, Cook DR, Merkel RA. Testosterone up-regulates androgen receptorsand decreases differentiation of porcine myogenic satellite cells in vitro. Endocrinology137: 1385–1394, 1996.

144. Doumit ME, Merkel RA. Conditions for isolation and culture of porcine myogenicsatellite cells. Tissue Cell 24: 253–262, 1992.

145. Dourdin N, Balcerzak D, Brustis JJ, Poussard S, Cottin P, Ducastaing A. Potentialm-calpain substrates during myoblast fusion. Exp Cell Res 246: 433–442, 1999.

146. Doyle MJ, Zhou S, Tanaka KK, Pisconti A, Farina NH, Sorrentino BP, Olwin BB. Abcg2labels multiple cell types in skeletal muscle and participates in muscle regeneration. JCell Biol 195: 147–163, 2011.

147. Doyonnas R, LaBarge MA, Sacco A, Charlton C, Blau HM. Hematopoietic contribu-tion to skeletal muscle regeneration by myelomonocytic precursors. Proc Natl AcadSci USA 101: 13507–13512, 2004.

148. Dreyfus PA, Chretien F, Chazaud B, Kirova Y, Caramelle P, Garcia L, Butler-BrowneG, Gherardi RK. Adult bone marrow-derived stem cells in muscle connective tissueand satellite cell niches. Am J Pathol 164: 773–779, 2004.

149. Duckmanton A, Kumar A, Chang YT, Brockes JP. A single-cell analysis of myogenicdedifferentiation induced by small molecules. Chem Biol 12: 1117–1126, 2005.

150. Enesco M, Puddy D. Increase in the number of nuclei and weight in skeletal muscle ofrats of various ages. Am J Anat 114: 235–244, 1964.

151. Engert JC, Berglund EB, Rosenthal N. Proliferation precedes differentiation in IGF-I-stimulated myogenesis. J Cell Biol 135: 431–440, 1996.

152. Engler AJ, Griffin MA, Sen S, Bonnemann CG, Sweeney HL, Discher DE. Myotubesdifferentiate optimally on substrates with tissue-like stiffness: pathological implica-tions for soft or stiff microenvironments. J Cell Biol 166: 877–887, 2004.

153. Ernfors P, Wetmore C, Eriksdotter-Nilsson M, Bygdeman M, Stromberg I, Olson L,Persson H. The nerve growth factor receptor gene is expressed in both neuronal andnon-neuronal tissues in the human fetus. Int J Dev Neurosci 9: 57–66, 1991.

154. Ershler WB, Keller ET. Age-associated increased interleukin-6 gene expression, late-life diseases, and frailty. Annu Rev Med 51: 245–270, 2000.

155. Esner M, Meilhac SM, Relaix F, Nicolas JF, Cossu G, Buckingham ME. Smooth muscleof the dorsal aorta shares a common clonal origin with skeletal muscle of the myo-tome. Development 133: 737–749, 2006.

156. Espinosa A, Estrada M, Jaimovich E. IGF-I and insulin induce different intracellularcalcium signals in skeletal muscle cells. J Endocrinol 182: 339–352, 2004.

157. Estrada M, Espinosa A, Muller M, Jaimovich E. Testosterone stimulates intracellularcalcium release and mitogen-activated protein kinases via a G protein-coupled recep-tor in skeletal muscle cells. Endocrinology 144: 3586–3597, 2003.

158. Ewton DZ, Falen SL, Florini JR. The type II insulin-like growth factor (IGF) receptorhas low affinity for IGF-I analogs: pleiotypic actions of IGFs on myoblasts are appar-ently mediated by the type I receptor. Endocrinology 120: 115–123, 1987.

159. Fahim MA, Robbins N. Ultrastructural studies of young and old mouse neuromuscularjunctions. J Neurocytol 11: 641–656, 1982.

160. Fan Y, Maley M, Beilharz M, Grounds M. Rapid death of injected myoblasts in myoblasttransfer therapy. Muscle Nerve 19: 853–860, 1996.

161. Farrington-Rock C, Crofts NJ, Doherty MJ, Ashton BA, Griffin-Jones C, Canfield AE.Chondrogenic and adipogenic potential of microvascular pericytes. Circulation 110:2226–2232, 2004.

162. Fedorov YV, Rosenthal RS, Olwin BB. Oncogenic Ras-induced proliferation requiresautocrine fibroblast growth factor 2 signaling in skeletal muscle cells. J Cell Biol 152:1301–1305, 2001.

163. Feng J, Mantesso A, De Bari C, Nishiyama A, Sharpe PT. Dual origin of mesenchymalstem cells contributing to organ growth and repair. Proc Natl Acad Sci USA 108:6503–6508, 2011.

164. Fernando P, Kelly JF, Balazsi K, Slack RS, Megeney LA. Caspase 3 activity is requiredfor skeletal muscle differentiation. Proc Natl Acad Sci USA 99: 11025–11030, 2002.

165. Ferrando AA, Sheffield-Moore M, Yeckel CW, Gilkison C, Jiang J, Achacosa A, Lieber-man SA, Tipton K, Wolfe RR, Urban RJ. Testosterone administration to older menimproves muscle function: molecular and physiological mechanisms. Am J Physiol En-docrinol Metab 282: E601–E607, 2002.



166. Ferrari G, Cusella-De Angelis G, Coletta M, Paolucci E, Stornaiuolo A, Cossu G,Mavilio F. Muscle regeneration by bone marrow-derived myogenic progenitors. Sci-ence 279: 1528–1530, 1998.

167. Ferre PJ, Liaubet L, Concordet D, SanCristobal M, Uro-Coste E, Tosser-Klopp G,Bonnet A, Toutain PL, Hatey F, Lefebvre HP. Longitudinal analysis of gene expressionin porcine skeletal muscle after post-injection local injury. Pharm Res 24: 1480–1489,2007.

168. Fielding RA, Manfredi TJ, Ding W, Fiatarone MA, Evans WJ, Cannon JG. Acute phaseresponse in exercise. III. Neutrophil and IL-1 beta accumulation in skeletal muscle. AmJ Physiol Regul Integr Comp Physiol 265: R166–R172, 1993.

169. Florini JR, Ewton DZ, Coolican SA. Growth hormone and the insulin-like growthfactor system in myogenesis. Endocr Rev 17: 481–517, 1996.

170. Florini JR, Ewton DZ, Roof SL. Insulin-like growth factor-I stimulates terminal myo-genic differentiation by induction of myogenin gene expression. Mol Endocrinol 5:718–724, 1991.

171. Florini JR, Magri KA, Ewton DZ, James PL, Grindstaff K, Rotwein PS. “Spontaneous”differentiation of skeletal myoblasts is dependent upon autocrine secretion of insulin-like growth factor-II. J Biol Chem 266: 15917–15923, 1991.

172. Floss T, Arnold HH, Braun T. A role for FGF-6 in skeletal muscle regeneration. GenesDev 11: 2040–2051, 1997.

173. Friday BB, Horsley V, Pavlath GK. Calcineurin activity is required for the initiation ofskeletal muscle differentiation. J Cell Biol 149: 657–666, 2000.

174. Fuchs E. Finding one’s niche in the skin. Cell Stem Cell 4: 499–502, 2009.

175. Fuchtbauer EM, Westphal H. MyoD and myogenin are coexpressed in regeneratingskeletal muscle of the mouse. Dev Dyn 193: 34–39, 1992.

176. Fukada S. Molecular regulation of muscle stem cells by “quiescence genes.” Yakugakuzasshi J Pharmaceutical Soc Japan 131: 1329–1332, 2011.

177. Fukada S, Uezumi A, Ikemoto M, Masuda S, Segawa M, Tanimura N, Yamamoto H,Miyagoe-Suzuki Y, Takeda S. Molecular signature of quiescent satellite cells in adultskeletal muscle. Stem Cells 25: 2448–2459, 2007.

178. Fukushima K, Nakamura A, Ueda H, Yuasa K, Yoshida K, Takeda S, Ikeda S. Activationand localization of matrix metalloproteinase-2 and -9 in the skeletal muscle of themuscular dystrophy dog (CXMDJ). BMC Musculoskelet Disord 8: 54, 2007.

179. Gal-Levi R, Leshem Y, Aoki S, Nakamura T, Halevy O. Hepatocyte growth factorplays a dual role in regulating skeletal muscle satellite cell proliferation and differenti-ation. Biochim Biophys Acta 1402: 39–51, 1998.

180. Galbiati F, Volonte D, Engelman JA, Scherer PE, Lisanti MP. Targeted down-regula-tion of caveolin-3 is sufficient to inhibit myotube formation in differentiating C2C12myoblasts. Transient activation of p38 mitogen-activated protein kinase is requiredfor induction of caveolin-3 expression and subsequent myotube formation. J Biol Chem274: 30315–30321, 1999.

181. Galbiati F, Volonte D, Minetti C, Chu JB, Lisanti MP. Phenotypic behavior of caveo-lin-3 mutations that cause autosomal dominant limb girdle muscular dystrophy(LGMD-1C). Retention of LGMD-1C caveolin-3 mutants within the Golgi complex. JBiol Chem 274: 25632–25641, 1999.

182. Galvez BG, Sampaolesi M, Brunelli S, Covarello D, Gavina M, Rossi B, Constantin G,Torrente Y, Cossu G. Complete repair of dystrophic skeletal muscle by mesoangio-blasts with enhanced migration ability. J Cell Biol 174: 231–243, 2006.

183. Gao C, Chen YG. Dishevelled: the hub of Wnt signaling. Cell Signal 22: 717–727, 2010.

184. Gao Y, Kostrominova TY, Faulkner JA, Wineman AS. Age-related changes in themechanical properties of the epimysium in skeletal muscles of rats. J Biomech 41:465–469, 2008.

185. Gatchalian CL, Schachner M, Sanes JR. Fibroblasts that proliferate near denervatedsynaptic sites in skeletal muscle synthesize the adhesive molecules tenascin(J1), N-CAM, fibronectin, and a heparan sulfate proteoglycan. J Cell Biol 108: 1873–1890,1989.

186. Gauthier-Rouviere C, Vandromme M, Tuil D, Lautredou N, Morris M, Soulez M, KahnA, Fernandez A, Lamb N. Expression and activity of serum response factor is requiredfor expression of the muscle-determining factor MyoD in both dividing and differen-tiating mouse C2C12 myoblasts. Mol Biol Cell 7: 719–729, 1996.

187. Gayraud-Morel B, Chretien F, Flamant P, Gomes D, Zammit PS, Tajbakhsh S. A rolefor the myogenic determination gene Myf5 in adult regenerative myogenesis. Dev Biol312: 13–28, 2007.

188. Gensch N, Borchardt T, Schneider A, Riethmacher D, Braun T. Different autonomousmyogenic cell populations revealed by ablation of Myf5-expressing cells during mouseembryogenesis. Development 135: 1597–1604, 2008.

189. Germani A, Di Carlo A, Mangoni A, Straino S, Giacinti C, Turrini P, Biglioli P, Capo-grossi MC. Vascular endothelial growth factor modulates skeletal myoblast function.Am J Pathol 163: 1417–1428, 2003.

190. Gibson MC, Schultz E. The distribution of satellite cells and their relationship tospecific fiber types in soleus and extensor digitorum longus muscles. Anat Rec 202:329–337, 1982.

191. Gillespie MA, Le Grand F, Scime A, Kuang S, von Maltzahn J, Seale V, Cuenda A, RanishJA, Rudnicki MA. p38-�-dependent gene silencing restricts entry into the myogenicdifferentiation program. J Cell Biol 187: 991–1005, 2009.

192. Gnocchi VF, White RB, Ono Y, Ellis JA, Zammit PS. Further characterisation of themolecular signature of quiescent and activated mouse muscle satellite cells. PLoS One4: e5205, 2009.

193. Golding JP, Calderbank E, Partridge TA, Beauchamp JR. Skeletal muscle stem cellsexpress anti-apoptotic ErbB receptors during activation from quiescence. Exp Cell Res313: 341–356, 2007.

194. Goldspink G. Research on mechano growth factor: its potential for optimising physicaltraining as well as misuse in doping. Br J Sports Med 39: 787–788, 2005.

195. Goldspink G, Fernandes K, Williams PE, Wells DJ. Age-related changes in collagengene expression in the muscles of mdx dystrophic and normal mice. NeuromusculDisord 4: 183–191, 1994.

196. Goljanek-Whysall K, Sweetman D, Abu-Elmagd M, Chapnik E, Dalmay T, HornsteinE, Munsterberg A. MicroRNA regulation of the paired-box transcription factor Pax3confers robustness to developmental timing of myogenesis. Proc Natl Acad Sci USA108: 11936–11941, 2011.

197. Gonzalez I, Tripathi G, Carter EJ, Cobb LJ, Salih DA, Lovett FA, Holding C, Pell JM.Akt2, a novel functional link between p38 mitogen-activated protein kinase and phos-phatidylinositol 3-kinase pathways in myogenesis. Mol Cell Biol 24: 3607–3622, 2004.

198. Goodpaster BH, Wolf D. Skeletal muscle lipid accumulation in obesity, insulin resis-tance, and type 2 diabetes. Pediatr Diabetes 5: 219–226, 2004.

199. Grass S, Arnold HH, Braun T. Alterations in somite patterning of Myf-5-deficientmice: a possible role for FGF-4 and FGF-6. Development 122: 141–150, 1996.

200. Greco AV, Mingrone G, Giancaterini A, Manco M, Morroni M, Cinti S, Granzotto M,Vettor R, Camastra S, Ferrannini E. Insulin resistance in morbid obesity: reversal withintramyocellular fat depletion. Diabetes 51: 144–151, 2002.

201. Griesbeck O, Parsadanian AS, Sendtner M, Thoenen H. Expression of neurotrophinsin skeletal muscle: quantitative comparison and significance for motoneuron survivaland maintenance of function. J Neurosci Res 42: 21–33, 1995.

202. Grimby G, Saltin B. The ageing muscle. Clin Physiol 3: 209–218, 1983.

203. Gros J, Manceau M, Thome V, Marcelle C. A common somitic origin for embryonicmuscle progenitors and satellite cells. Nature 435: 954–958, 2005.

204. Gros J, Scaal M, Marcelle C. A two-step mechanism for myotome formation in chick.Dev Cell 6: 875–882, 2004.

205. Gros J, Serralbo O, Marcelle C. WNT11 acts as a directional cue to organize theelongation of early muscle fibres. Nature 457: 589–593, 2009.

206. Grounds MD. Phagocytosis of necrotic muscle in muscle isografts is influenced by thestrain, age, and sex of host mice. J Pathol 153: 71–82, 1987.

207. Grounds MD, Garrett KL, Lai MC, Wright WE, Beilharz MW. Identification of skeletalmuscle precursor cells in vivo by use of MyoD1 and myogenin probes. Cell Tissue Res267: 99–104, 1992.

208. Gullberg D, Tiger CF, Velling T. Laminins during muscle development and in musculardystrophies. Cell Mol Life Sci 56: 442–460, 1999.



209. Guo K, Wang J, Andres V, Smith RC, Walsh K. MyoD-induced expression of p21inhibits cyclin-dependent kinase activity upon myocyte terminal differentiation. MolCell Biol 15: 3823–3829, 1995.

210. Gussoni E, Soneoka Y, Strickland CD, Buzney EA, Khan MK, Flint AF, Kunkel LM,Mulligan RC. Dystrophin expression in the mdx mouse restored by stem cell trans-plantation. Nature 401: 390–394, 1999.

211. Guttinger M, Tafi E, Battaglia M, Coletta M, Cossu G. Allogeneic mesoangioblasts giverise to alpha-sarcoglycan expressing fibers when transplanted into dystrophic mice.Exp Cell Res 312: 3872–3879, 2006.

212. Haddad F, Zaldivar F, Cooper DM, Adams GR. IL-6-induced skeletal muscle atrophy.J Appl Physiol 98: 911–917, 2005.

213. Haldar M, Karan G, Tvrdik P, Capecchi MR. Two cell lineages, myf5 and myf5-independent, participate in mouse skeletal myogenesis. Dev Cell 14: 437–445, 2008.

214. Halevy O, Cantley LC. Differential regulation of the phosphoinositide 3-kinase andMAP kinase pathways by hepatocyte growth factor vs. insulin-like growth factor-I inmyogenic cells. Exp Cell Res 297: 224–234, 2004.

215. Halevy O, Novitch BG, Spicer DB, Skapek SX, Rhee J, Hannon GJ, Beach D, LassarAB. Correlation of terminal cell cycle arrest of skeletal muscle with induction of p21by MyoD. Science 267: 1018–1021, 1995.

216. Halevy O, Piestun Y, Allouh MZ, Rosser BW, Rinkevich Y, Reshef R, Rozenboim I,Wleklinski-Lee M, Yablonka-Reuveni Z. Pattern of Pax7 expression during myogen-esis in the posthatch chicken establishes a model for satellite cell differentiation andrenewal. Dev Dyn 231: 489–502, 2004.

217. Hall-Craggs EC, Seyan HS. Histochemical changes in innervated and denervated skel-etal muscle fibers following treatment with bupivacaine (marcain). Exp Neurol 46:345–354, 1975.

218. Hamer PW, McGeachie JM, Davies MJ, Grounds MD. Evans Blue Dye as an in vivomarker of myofibre damage: optimising parameters for detecting initial myofibremembrane permeability. J Anat 200: 69–79, 2002.

219. Hammond CL, Hinits Y, Osborn DP, Minchin JE, Tettamanti G, Hughes SM. Signalsand myogenic regulatory factors restrict pax3 and pax7 expression to dermomyo-tome-like tissue in zebrafish. Dev Biol 302: 504–521, 2007.

220. Han B, Tong J, Zhu MJ, Ma C, Du M. Insulin-like growth factor-1 (IGF-1) and leucineactivate pig myogenic satellite cells through mammalian target of rapamycin (mTOR)pathway. Mol Reprod Dev 75: 810–817, 2008.

221. Han J, Jiang Y, Li Z, Kravchenko VV, Ulevitch RJ. Activation of the transcription factorMEF2C by the MAP kinase p38 in inflammation. Nature 386: 296–299, 1997.

222. Hannon K, Kudla AJ, McAvoy MJ, Clase KL, Olwin BB. Differentially expressed fibro-blast growth factors regulate skeletal muscle development through autocrine andparacrine mechanisms. J Cell Biol 132: 1151–1159, 1996.

223. Hannun YA, Obeid LM. Principles of bioactive lipid signalling: lessons from sphingo-lipids. Nat Rev Mol Cell Biol 9: 139–150, 2008.

224. Hansen-Smith FM, Picou D, Golden MH. Muscle satellite cells in malnourished andnutritionally rehabilitated children. J Neurol Sci 41: 207–221, 1979.

225. Harel I, Nathan E, Tirosh-Finkel L, Zigdon H, Guimaraes-Camboa N, Evans SM,Tzahor E. Distinct origins and genetic programs of head muscle satellite cells. Dev Cell16: 822–832, 2009.

226. He L, Papoutsi M, Huang R, Tomarev SI, Christ B, Kurz H, Wilting J. Three differentfates of cells migrating from somites into the limb bud. Anat Embryol 207: 29–34,2003.

227. Hellmuth AE, Allbrook DB. Muscle satellite cell numbers during the postnatal period.J Anat 110: 503, 1971.

228. Herbst KL, Bhasin S. Testosterone action on skeletal muscle. Curr Opin Clin NutrMetab Care 7: 271–277, 2004.

229. Hermanson JW, Moschella MC, Ontell M. Effect of neonatal denervation-reinnerva-tion on the functional capacity of a 129ReJ dy/dy murine dystrophic muscle. Exp Neurol102: 210–216, 1988.

230. Hinescu ME, Gherghiceanu M, Suciu L, Popescu LM. Telocytes in pleura: two- andthree-dimensional imaging by transmission electron microscopy. Cell Tissue Res 343:389–397, 2011.

231. Hirai H, Verma M, Watanabe S, Tastad C, Asakura Y, Asakura A. MyoD regulatesapoptosis of myoblasts through microRNA-mediated down-regulation of Pax3. J CellBiol 191: 347–365, 2010.

232. Hjiantoniou E, Anayasa M, Nicolaou P, Bantounas I, Saito M, Iseki S, Uney JB, Phylac-tou LA. Twist induces reversal of myotube formation. Differ Res Biol Diversity 76:182–192, 2008.

233. Hollemann D, Budka H, Loscher WN, Yanagida G, Fischer MB, Wanschitz JV. Endo-thelial and myogenic differentiation of hematopoietic progenitor cells in inflammatorymyopathies. J Neuropathol Exp Neurol 67: 711–719, 2008.

234. Hollenberg SM, Cheng PF, Weintraub H. Use of a conditional MyoD transcriptionfactor in studies of MyoD trans-activation and muscle determination. Proc Natl AcadSci USA 90: 8028–8032, 1993.

235. Holterman CE, Le Grand F, Kuang S, Seale P, Rudnicki MA. Megf10 regulates theprogression of the satellite cell myogenic program. J Cell Biol 179: 911–922, 2007.

236. Horowitz A, Tkachenko E, Simons M. Fibroblast growth factor-specific modulation ofcellular response by syndecan-4. J Cell Biol 157: 715–725, 2002.

237. Horsley V, Friday BB, Matteson S, Kegley KM, Gephart J, Pavlath GK. Regulation of thegrowth of multinucleated muscle cells by an NFATC2-dependent pathway. J Cell Biol153: 329–338, 2001.

238. Horsley V, Pavlath GK. Forming a multinucleated cell: molecules that regulate myo-blast fusion. Cells Tissues Organs 176: 67–78, 2004.

239. Hu E, Tontonoz P, Spiegelman BM. Transdifferentiation of myoblasts by the adipo-genic transcription factors PPAR gamma and C/EBP alpha. Proc Natl Acad Sci USA 92:9856–9860, 1995.

240. Huckins C. The spermatogonial stem cell population in adult rats. I. Their morphol-ogy, proliferation and maturation. Anat Rec 169: 533–557, 1971.

241. Hughes SM, Blau HM. Migration of myoblasts across basal lamina during skeletalmuscle development. Nature 345: 350–353, 1990.

242. Hutcheson DA, Zhao J, Merrell A, Haldar M, Kardon G. Embryonic and fetal limbmyogenic cells are derived from developmentally distinct progenitors and have dif-ferent requirements for beta-catenin. Genes Dev 23: 997–1013, 2009.

243. Ikezawa M, Cao B, Qu Z, Peng H, Xiao X, Pruchnic R, Kimura S, Miike T, Huard J.Dystrophin delivery in dystrophin-deficient DMDmdx skeletal muscle by isogenicmuscle-derived stem cell transplantation. Hum Gene Ther 14: 1535–1546, 2003.

244. Irintchev A, Zeschnigk M, Starzinski-Powitz A, Wernig A. Expression pattern of M-cadherin in normal, denervated, and regenerating mouse muscles. Dev Dyn 199:326–337, 1994.

245. Jackson KA, Mi T, Goodell MA. Hematopoietic potential of stem cells isolated frommurine skeletal muscle. Proc Natl Acad Sci USA 96: 14482–14486, 1999.

246. James PL, Jones SB, Busby WH Jr, Clemmons DR, Rotwein P. A highly conservedinsulin-like growth factor-binding protein (IGFBP-5) is expressed during myoblastdifferentiation. J Biol Chem 268: 22305–22312, 1993.

247. Jankowski RJ, Huard J. Establishing reliable criteria for isolating myogenic cell fractionswith stem cell properties and enhanced regenerative capacity. Blood Cells Mol Dis 32:24–33, 2004.

248. Jejurikar SS, Marcelo CL, Kuzon WM Jr. Skeletal muscle denervation increases satellitecell susceptibility to apoptosis. Plast Reconstr Surg 110: 160–168, 2002.

249. Jenniskens GJ, Veerkamp JH, van Kuppevelt TH. Heparan sulfates in skeletal muscledevelopment and physiology. J Cell Physiol 206: 283–294, 2006.

250. Jirmanova I, Thesleff S. Ultrastructural study of experimental muscle degenerationand regeneration in the adult rat. Z Zellforsch Mikrosk Anat 131: 77–97, 1972.

251. Jockusch H, Voigt S. Migration of adult myogenic precursor cells as revealed byGFP/nLacZ labelling of mouse transplantation chimeras. J Cell Sci 116: 1611–1616,2003.



252. Joe AW, Yi L, Natarajan A, Le Grand F, So L, Wang J, Rudnicki MA, Rossi FM. Muscleinjury activates resident fibro/adipogenic progenitors that facilitate myogenesis. NatCell Biol 12: 153–163, 2010.

253. Jones JI, Clemmons DR. Insulin-like growth factors and their binding proteins: biolog-ical actions. Endocr Rev 16: 3–34, 1995.

254. Joubert Y, Tobin C. Satellite cell proliferation and increase in the number of myonucleiinduced by testosterone in the levator ani muscle of the adult female rat. Dev Biol 131:550–557, 1989.

255. Joubert Y, Tobin C. Testosterone treatment results in quiescent satellite cells beingactivated and recruited into cell cycle in rat levator ani muscle. Dev Biol 169: 286–294,1995.

256. Kablar B, Asakura A, Krastel K, Ying C, May LL, Goldhamer DJ, Rudnicki MA. MyoDand Myf-5 define the specification of musculature of distinct embryonic origin.Biochem Cell Biol 76: 1079–1091, 1998.

257. Kablar B, Krastel K, Tajbakhsh S, Rudnicki MA. Myf5 and MyoD activation defineindependent myogenic compartments during embryonic development. Dev Biol 258:307–318, 2003.

258. Kablar B, Krastel K, Ying C, Asakura A, Tapscott SJ, Rudnicki MA. MyoD and Myf-5differentially regulate the development of limb versus trunk skeletal muscle. Develop-ment 124: 4729–4738, 1997.

259. Kablar B, Krastel K, Ying C, Tapscott SJ, Goldhamer DJ, Rudnicki MA. Myogenicdetermination occurs independently in somites and limb buds. Dev Biol 206: 219–231,1999.

260. Kablar B, Rudnicki MA. Skeletal muscle development in the mouse embryo. HistolHistopathol 15: 649–656, 2000.

261. Kalcheim C, Ben-Yair R. Cell rearrangements during development of the somite andits derivatives. Curr Opin Genet Dev 15: 371–380, 2005.

262. Kaminski HJ, Andrade FH. Nitric oxide: biologic effects on muscle and role in musclediseases. Neuromuscul Disord 11: 517–524, 2001.

263. Kanisicak O, Mendez JJ, Yamamoto S, Yamamoto M, Goldhamer DJ. Progenitors ofskeletal muscle satellite cells express the muscle determination gene, MyoD. Dev Biol332: 131–141, 2009.

264. Kardon G, Campbell JK, Tabin CJ. Local extrinsic signals determine muscle andendothelial cell fate and patterning in the vertebrate limb. Dev Cell 3: 533–545,2002.

265. Kassar-Duchossoy L, Giacone E, Gayraud-Morel B, Jory A, Gomes D, Tajbakhsh S.Pax3/Pax7 mark a novel population of primitive myogenic cells during development.Genes Dev 19: 1426–1431, 2005.

266. Kastner S, Elias MC, Rivera AJ, Yablonka-Reuveni Z. Gene expression patterns of thefibroblast growth factors and their receptors during myogenesis of rat satellite cells. JHistochem Cytochem 48: 1079–1096, 2000.

267. Kazuki Y, Hiratsuka M, Takiguchi M, Osaki M, Kajitani N, Hoshiya H, Hiramatsu K,Yoshino T, Kazuki K, Ishihara C, Takehara S, Higaki K, Nakagawa M, Takahashi K,Yamanaka S, Oshimura M. Complete genetic correction of ips cells from Duchennemuscular dystrophy. Mol Ther 18: 386–393, 2010.

268. Keller P, Penkowa M, Keller C, Steensberg A, Fischer CP, Giralt M, Hidalgo J, Ped-ersen BK. Interleukin-6 receptor expression in contracting human skeletal muscle:regulating role of IL-6. FASEB J 19: 1181–1183, 2005.

269. Kelly AM. Perisynaptic satellite cells in the developing and mature rat soleus muscle.Anat Rec 190: 891–903, 1978.

270. Kherif S, Lafuma C, Dehaupas M, Lachkar S, Fournier JG, Verdiere-Sahuque M,Fardeau M, Alameddine HS. Expression of matrix metalloproteinases 2 and 9 inregenerating skeletal muscle: a study in experimentally injured and mdx muscles. DevBiol 205: 158–170, 1999.

271. Kim CH, Neiswender H, Baik EJ, Xiong WC, Mei L. Beta-catenin interacts with MyoDand regulates its transcription activity. Mol Cell Biol 28: 2941–2951, 2008.

272. Kitamoto T, Hanaoka K. Notch3 null mutation in mice causes muscle hyperplasia byrepetitive muscle regeneration. Stem Cells 28: 2205–2216, 2010.

273. Kitzmann M, Carnac G, Vandromme M, Primig M, Lamb NJ, Fernandez A. The muscleregulatory factors MyoD and myf-5 undergo distinct cell cycle-specific expression inmuscle cells. J Cell Biol 142: 1447–1459, 1998.

274. Knudsen KA, Horwitz AF. Tandem events in myoblast fusion. Dev Biol 58: 328–338,1977.

275. Kobzik L, Reid MB, Bredt DS, Stamler JS. Nitric oxide in skeletal muscle. Nature 372:546–548, 1994.

276. Konigsberg IR. Clonal analysis of myogenesis. Science 140: 1273–1284, 1963.

277. Konigsberg UR, Lipton BH, Konigsberg IR. The regenerative response of single ma-ture muscle fibers isolated in vitro. Dev Biol 45: 260–275, 1975.

278. Kovanen V. Intramuscular extracellular matrix: complex environment of muscle cells.Exerc Sport Sci Rev 30: 20–25, 2002.

279. Kuang S, Charge SB, Seale P, Huh M, Rudnicki MA. Distinct roles for Pax7 and Pax3 inadult regenerative myogenesis. J Cell Biol 172: 103–113, 2006.

280. Kuang S, Kuroda K, Le Grand F, Rudnicki MA. Asymmetric self-renewal and commit-ment of satellite stem cells in muscle. Cell 129: 999–1010, 2007.

281. Kurosu H, Ogawa Y, Miyoshi M, Yamamoto M, Nandi A, Rosenblatt KP, Baum MG,Schiavi S, Hu MC, Moe OW, Kuro-o M. Regulation of fibroblast growth factor-23signaling by klotho. J Biol Chem 281: 6120–6123, 2006.

282. Kuschel R, Yablonka-Reuveni Z, Bornemann A. Satellite cells on isolated myofibersfrom normal and denervated adult rat muscle. J Histochem Cytochem 47: 1375–1384,1999.

283. Kust BM, Copray JC, Brouwer N, Troost D, Boddeke HW. Elevated levels of neu-rotrophins in human biceps brachii tissue of amyotrophic lateral sclerosis. Exp Neurol177: 419–427, 2002.

284. Kutcher ME, Herman IM. The pericyte: cellular regulator of microvascular blood flow.Microvasc Res 77: 235–246, 2009.

285. L’Honore A, Lamb NJ, Vandromme M, Turowski P, Carnac G, Fernandez A. MyoDdistal regulatory region contains an SRF binding CArG element required for MyoDexpression in skeletal myoblasts and during muscle regeneration. Mol Biol Cell 14:2151–2162, 2003.

286. L’Honore A, Rana V, Arsic N, Franckhauser C, Lamb NJ, Fernandez A. Identificationof a new hybrid serum response factor and myocyte enhancer factor 2-binding ele-ment in MyoD enhancer required for MyoD expression during myogenesis. Mol BiolCell 18: 1992–2001, 2007.

287. LaBarge MA, Blau HM. Biological progression from adult bone marrow to mononu-cleate muscle stem cell to multinucleate muscle fiber in response to injury. Cell 111:589–601, 2002.

288. Langsdorf A, Do AT, Kusche-Gullberg M, Emerson CP Jr, Ai X. Sulfs are regulators ofgrowth factor signaling for satellite cell differentiation and muscle regeneration. DevBiol 311: 464–477, 2007.

289. Lansdorp PM. Immortal strands? Give me a break. Cell 129: 1244–1247, 2007.

290. Larsen BD, Rampalli S, Burns LE, Brunette S, Dilworth FJ, Megeney LA. Caspase3/caspase-activated DNase promote cell differentiation by inducing DNA strandbreaks. Proc Natl Acad Sci USA 107: 4230–4235, 2010.

291. Lassar AB, Davis RL, Wright WE, Kadesch T, Murre C, Voronova A, Baltimore D,Weintraub H. Functional activity of myogenic HLH proteins requires hetero-oli-gomerization with E12/E47-like proteins in vivo. Cell 66: 305–315, 1991.

292. Laterza OF, Lim L, Garrett-Engele PW, Vlasakova K, Muniappa N, Tanaka WK, John-son JM, Sina JF, Fare TL, Sistare FD, Glaab WE. Plasma MicroRNAs as sensitive andspecific biomarkers of tissue injury. Clin Chem 55: 1977–1983, 2009.

293. Le Grand F, Jones AE, Seale V, Scime A, Rudnicki MA. Wnt7a activates the planar cellpolarity pathway to drive the symmetric expansion of satellite stem cells. Cell StemCell 4: 535–547, 2009.

294. Lee JY, Qu-Petersen Z, Cao B, Kimura S, Jankowski R, Cummins J, Usas A, Gates C,Robbins P, Wernig A, Huard J. Clonal isolation of muscle-derived cells capable ofenhancing muscle regeneration and bone healing. J Cell Biol 150: 1085–1100, 2000.



295. Lefaucheur JP, Gjata B, Lafont H, Sebille A. Angiogenic and inflammatory responsesfollowing skeletal muscle injury are altered by immune neutralization of endogenousbasic fibroblast growth factor, insulin-like growth factor-1 and transforming growthfactor-beta 1. J Neuroimmunol 70: 37–44, 1996.

296. Lefaucheur JP, Sebille A. Basic fibroblast growth factor promotes in vivo muscleregeneration in murine muscular dystrophy. Neurosci Lett 202: 121–124, 1995.

297. Lefaucheur JP, Sebille A. Muscle regeneration following injury can be modified in vivoby immune neutralization of basic fibroblast growth factor, transforming growth fac-tor beta 1 or insulin-like growth factor I. J Neuroimmunol 57: 85–91, 1995.

298. Lemercier C, To RQ, Carrasco RA, Konieczny SF. The basic helix-loop-helix tran-scription factor Mist1 functions as a transcriptional repressor of myoD. EMBO J 17:1412–1422, 1998.

299. Lepper C, Conway SJ, Fan CM. Adult satellite cells and embryonic muscle progenitorshave distinct genetic requirements. Nature 460: 627–631, 2009.

300. Lepper C, Fan CM. Inducible lineage tracing of Pax7-descendant cells reveals embry-onic origin of adult satellite cells. Genesis 48: 424–436, 2010.

301. Lepper C, Partridge TA, Fan CM. An absolute requirement for Pax7-positive satellitecells in acute injury-induced skeletal muscle regeneration. Development 138: 3639–3646, 2011.

302. Leri A, Kajstura J, Anversa P, Frishman WH. Myocardial regeneration and stem cellrepair. Curr Probl Cardiol 33: 91–153, 2008.

303. Lescaudron L, Peltekian E, Fontaine-Perus J, Paulin D, Zampieri M, Garcia L, ParrishE. Blood borne macrophages are essential for the triggering of muscle regenerationfollowing muscle transplant. Neuromuscul Disord 9: 72–80, 1999.

304. Leshem Y, Gitelman I, Ponzetto C, Halevy O. Preferential binding of Grb2 or phos-phatidylinositol 3-kinase to the met receptor has opposite effects on HGF-inducedmyoblast proliferation. Exp Cell Res 274: 288–298, 2002.

305. Leshem Y, Halevy O. Phosphorylation of pRb is required for HGF-induced muscle cellproliferation and is p27kip1-dependent. J Cell Physiol 191: 173–182, 2002.

306. Leshem Y, Spicer DB, Gal-Levi R, Halevy O. Hepatocyte growth factor (HGF) inhibitsskeletal muscle cell differentiation: a role for the bHLH protein twist and the cdkinhibitor p27. J Cell Physiol 184: 101–109, 2000.

307. Lewis MI, Horvitz GD, Clemmons DR, Fournier M. Role of IGF-I and IGF-bindingproteins within diaphragm muscle in modulating the effects of nandrolone. Am J PhysiolEndocrinol Metab 282: E483–E490, 2002.

308. Li L, Heller-Harrison R, Czech M, Olson EN. Cyclic AMP-dependent protein kinaseinhibits the activity of myogenic helix-loop-helix proteins. Mol Cell Biol 12: 4478–4485, 1992.

309. Li L, Zhou J, James G, Heller-Harrison R, Czech MP, Olson EN. FGF inactivatesmyogenic helix-loop-helix proteins through phosphorylation of a conserved proteinkinase C site in their DNA-binding domains. Cell 71: 1181–1194, 1992.

310. Li Y, Foster W, Deasy BM, Chan Y, Prisk V, Tang Y, Cummins J, Huard J. Transforminggrowth factor-beta1 induces the differentiation of myogenic cells into fibrotic cells ininjured skeletal muscle: a key event in muscle fibrogenesis. Am J Pathol 164: 1007–1019, 2004.

311. Lindstrom M, Pedrosa-Domellof F, Thornell LE. Satellite cell heterogeneity withrespect to expression of MyoD, myogenin, Dlk1 and c-Met in human skeletal muscle:application to a cohort of power lifters and sedentary men. Histochem Cell Biol 134:371–385, 2010.

312. Lipton BH, Konigsberg IR. A fine-structural analysis of the fusion of myogenic cells. JCell Biol 53: 348–364, 1972.

313. Liu H, Fergusson MM, Castilho RM, Liu J, Cao L, Chen J, Malide D, Rovira II, SchimelD, Kuo CJ, Gutkind JS, Hwang PM, Finkel T. Augmented Wnt signaling in a mammalianmodel of accelerated aging. Science 317: 803–806, 2007.

314. Liu J, Aoki M, Illa I, Wu C, Fardeau M, Angelini C, Serrano C, Urtizberea JA, Hentati F,Hamida MB, Bohlega S, Culper EJ, Amato AA, Bossie K, Oeltjen J, Bejaoui K, McK-enna-Yasek D, Hosler BA, Schurr E, Arahata K, de Jong PJ, Brown RH Jr. Dysferlin, anovel skeletal muscle gene, is mutated in Miyoshi myopathy and limb girdle musculardystrophy. Nat Genet 20: 31–36, 1998.

315. Liu JP, Baker J, Perkins AS, Robertson EJ, Efstratiadis A. Mice carrying null mutationsof the genes encoding insulin-like growth factor I (Igf-1) and type 1 IGF receptor(Igf1r). Cell 75: 59–72, 1993.

316. Liu Y, Gao L, Zuba-Surma EK, Peng X, Kucia M, Ratajczak MZ, Wang W, Enzmann V,Kaplan HJ, Dean DC. Identification of small Sca-1(�), Lin(�), and CD45(�) multipo-tential cells in the neonatal murine retina. Exp Hematol 37: 1096–1107, 2009.

317. Loh KC, Leong WI, Carlson ME, Oskouian B, Kumar A, Fyrst H, Zhang M, Proia RL,Hoffman EP, Saba JD. Sphingosine-1-phosphate enhances satellite cell activation indystrophic muscles through a S1PR2/STAT3 signaling pathway. PLoS One 7: e37218,2012.

318. Lu A, Cummins JH, Pollett JB, Cao B, Sun B, Rudnicki MA, Huard J. Isolation ofmyogenic progenitor populations from Pax7-deficient skeletal muscle based on ad-hesion characteristics. Gene Ther 15: 1116–1125, 2008.

319. Lu J, McKinsey TA, Zhang CL, Olson EN. Regulation of skeletal myogenesis by asso-ciation of the MEF2 transcription factor with class II histone deacetylases. Mol Cell 6:233–244, 2000.

320. Luo D, Renault VM, Rando TA. The regulation of Notch signaling in muscle stem cellactivation and postnatal myogenesis. Semin Cell Dev Biol 16: 612–622, 2005.

321. Luque E, Pena J, Martin P, Jimena I, Vaamonde R. Capillary supply during developmentof individual regenerating muscle fibers. Anat Histol Embryol 24: 87–89, 1995.

322. Ma DK, Bonaguidi MA, Ming GL, Song H. Adult neural stem cells in the mammaliancentral nervous system. Cell Res 19: 672–682, 2009.

323. Machida S, Booth FW. Insulin-like growth factor 1 and muscle growth: implication forsatellite cell proliferation. Proc Nutr Soc 63: 337–340, 2004.

324. Mackey AL, Kjaer M, Charifi N, Henriksson J, Bojsen-Moller J, Holm L, Kadi F. As-sessment of satellite cell number and activity status in human skeletal muscle biopsies.Muscle Nerve 40: 455–465, 2009.

325. Mal A, Sturniolo M, Schiltz RL, Ghosh MK, Harter ML. A role for histone deacetylaseHDAC1 in modulating the transcriptional activity of MyoD: inhibition of the myogenicprogram. EMBO J 20: 1739–1753, 2001.

326. Mansouri A, Stoykova A, Torres M, Gruss P. Dysgenesis of cephalic neural crestderivatives in Pax7�/� mutant mice. Development 122: 831–838, 1996.

327. Matheny RW Jr, Nindl BC, Adamo ML. Minireview: Mechano-growth factor: a puta-tive product of IGF-I gene expression involved in tissue repair and regeneration.Endocrinology 151: 865–875, 2010.

328. Matsuda R, Nishikawa A, Tanaka H. Visualization of dystrophic muscle fibers in mdxmouse by vital staining with Evans blue: evidence of apoptosis in dystrophin-deficientmuscle. J Biochem 118: 959–964, 1995.

329. Mauro A. Satellite cell of skeletal muscle fibers. J Biophys Biochem Cytol 9: 493–495,1961.

330. Mayer U. Integrins: redundant or important players in skeletal muscle? J Biol Chem278: 14587–14590, 2003.

331. McGann CJ, Odelberg SJ, Keating MT. Mammalian myotube dedifferentiation inducedby newt regeneration extract. Proc Natl Acad Sci USA 98: 13699–13704, 2001.

332. McGeachie JK, Grounds MD. Initiation and duration of muscle precursor replicationafter mild and severe injury to skeletal muscle of mice. An autoradiographic study. CellTissue Res 248: 125–130, 1987.

333. McKay BR, De Lisio M, Johnston AP, O’Reilly CE, Phillips SM, Tarnopolsky MA, PariseG. Association of interleukin-6 signalling with the muscle stem cell response followingmuscle-lengthening contractions in humans. PLoS One 4: e6027, 2009.

334. McLoon LK, Wirtschafter J. Activated satellite cells in extraocular muscles of normaladult monkeys and humans. Invest Ophthalmol Vis Sci 44: 1927–1932, 2003.

335. Meech R, Gomez M, Woolley C, Barro M, Hulin JA, Walcott EC, Delgado J, Makar-enkova HP. The homeobox transcription factor Barx2 regulates plasticity of youngprimary myofibers. PLoS One 5: e11612, 2010.

336. Meech R, Gonzalez KN, Barro M, Gromova A, Zhuang L, Hulin JA, Makarenkova HP.Barx2 is expressed in satellite cells and is required for normal muscle growth andregeneration. Stem Cells 30: 253–265, 2012.



337. Meeson AP, Hawke TJ, Graham S, Jiang N, Elterman J, Hutcheson K, Dimaio JM,Gallardo TD, Garry DJ. Cellular and molecular regulation of skeletal muscle sidepopulation cells. Stem Cells 22: 1305–1320, 2004.

338. Megeney LA, Kablar B, Garrett K, Anderson JE, Rudnicki MA. MyoD is required formyogenic stem cell function in adult skeletal muscle. Genes Dev 10: 1173–1183, 1996.

339. Meignin C, Alvarez-Garcia I, Davis I, Palacios IM. The salvador-warts-hippo pathwayis required for epithelial proliferation and axis specification in Drosophila. Curr Biol 17:1871–1878, 2007.

340. Menetrey J, Kasemkijwattana C, Day CS, Bosch P, Vogt M, Fu FH, Moreland MS,Huard J. Growth factors improve muscle healing in vivo. J Bone Joint Surg Br 82:131–137, 2000.

341. Merly F, Lescaudron L, Rouaud T, Crossin F, Gardahaut MF. Macrophages enhancemuscle satellite cell proliferation and delay their differentiation. Muscle Nerve 22:724–732, 1999.

342. Miller KJ, Thaloor D, Matteson S, Pavlath GK. Hepatocyte growth factor affectssatellite cell activation and differentiation in regenerating skeletal muscle. Am J PhysiolCell Physiol 278: C174–C181, 2000.

343. Minasi MG, Riminucci M, De Angelis L, Borello U, Berarducci B, Innocenzi A, CaprioliA, Sirabella D, Baiocchi M, De Maria R, Boratto R, Jaffredo T, Broccoli V, Bianco P,Cossu G. The meso-angioblast: a multipotent, self-renewing cell that originates fromthe dorsal aorta and differentiates into most mesodermal tissues. Development 129:2773–2783, 2002.

344. Minetti C, Sotgia F, Bruno C, Scartezzini P, Broda P, Bado M, Masetti E, Mazzocco M,Egeo A, Donati MA, Volonte D, Galbiati F, Cordone G, Bricarelli FD, Lisanti MP, ZaraF. Mutations in the caveolin-3 gene cause autosomal dominant limb-girdle musculardystrophy. Nat Genet 18: 365–368, 1998.

345. Mitchell KJ, Pannerec A, Cadot B, Parlakian A, Besson V, Gomes ER, Marazzi G,Sassoon DA. Identification and characterization of a non-satellite cell muscle residentprogenitor during postnatal development. Nat Cell Biol 12: 257–266, 2010.

346. Mizuno Y, Chang H, Umeda K, Niwa A, Iwasa T, Awaya T, Fukada SI, Yamamoto H,Yamanaka S, Nakahata T, Heike T. Generation of skeletal muscle stem/progenitorcells from murine induced pluripotent stem cells. FASEB J 2010.

347. Moelling K, Schad K, Bosse M, Zimmermann S, Schweneker M. Regulation of Raf-AktCross-talk. J Biol Chem 277: 31099–31106, 2002.

348. Mokalled MH, Johnson AN, Creemers EE, Olson EN. MASTR directs MyoD-depen-dent satellite cell differentiation during skeletal muscle regeneration. Genes Dev 26:190–202, 2012.

349. Montarras D, Lindon C, Pinset C, Domeyne P. Cultured myf5 null and myoD nullmuscle precursor cells display distinct growth defects. Biol Cell 92: 565–572, 2000.

350. Montarras D, Morgan J, Collins C, Relaix F, Zaffran S, Cumano A, Partridge T, Buck-ingham M. Direct isolation of satellite cells for skeletal muscle regeneration. Science309: 2064–2067, 2005.

351. Moresi V, Pristera A, Scicchitano BM, Molinaro M, Teodori L, Sassoon D, Adamo S,Coletti D. Tumor necrosis factor-alpha inhibition of skeletal muscle regeneration ismediated by a caspase-dependent stem cell response. Stem Cells 26: 997–1008, 2008.

352. Morosetti R, Mirabella M, Gliubizzi C, Broccolini A, De Angelis L, Tagliafico E, Sam-paolesi M, Gidaro T, Papacci M, Roncaglia E, Rutella S, Ferrari S, Tonali PA, Ricci E,Cossu G. MyoD expression restores defective myogenic differentiation of humanmesoangioblasts from inclusion-body myositis muscle. Proc Natl Acad Sci USA 103:16995–17000, 2006.

353. Morrison JI, Loof S, He P, Simon A. Salamander limb regeneration involves the acti-vation of a multipotent skeletal muscle satellite cell population. J Cell Biol 172: 433–440, 2006.

354. Morrison SJ, Kimble J. Asymmetric and symmetric stem-cell divisions in developmentand cancer. Nature 441: 1068–1074, 2006.

355. Moss FP, Leblond CP. Nature of dividing nuclei in skeletal muscle of growing rats. JCell Biol 44: 459–462, 1970.

356. Mourikis P, Sambasivan R, Castel D, Rocheteau P, Bizzarro V, Tajbakhsh S. A criticalrequirement for notch signaling in maintenance of the quiescent skeletal muscle stemcell state. Stem Cells 30: 243–252, 2012.

357. Mourkioti F, Rosenthal N. IGF-1, inflammation and stem cells: interactions duringmuscle regeneration. Trends Immunol 26: 535–542, 2005.

358. Mousavi K, Jasmin BJ. BDNF is expressed in skeletal muscle satellite cells and inhibitsmyogenic differentiation. J Neurosci 26: 5739–5749, 2006.

359. Mu X, Peng H, Pan H, Huard J, Li Y. Study of muscle cell dedifferentiation after skeletalmuscle injury of mice with a Cre-Lox system. PLoS One 6: e16699, 2011.

360. Mulvaney DR, Marple DN, Merkel RA. Proliferation of skeletal muscle satellite cellsafter castration and administration of testosterone propionate. Proc Soc Exp Biol Med188: 40–45, 1988.

361. Murphy MM, Lawson JA, Mathew SJ, Hutcheson DA, Kardon G. Satellite cells, con-nective tissue fibroblasts and their interactions are crucial for muscle regeneration.Development 138: 3625–3637, 2011.

362. Murre C, McCaw PS, Baltimore D. A new DNA binding and dimerization motif inimmunoglobulin enhancer binding, daughterless, MyoD, and myc proteins. Cell 56:777–783, 1989.

363. Murre C, McCaw PS, Vaessin H, Caudy M, Jan LY, Jan YN, Cabrera CV, Buskin JN,Hauschka SD, Lassar AB. Interactions between heterologous helix-loop-helix pro-teins generate complexes that bind specifically to a common DNA sequence. Cell 58:537–544, 1989.

364. Musaro A, Barberi L. Isolation and culture of mouse satellite cells. Methods Mol Biol633: 101–111, 2010.

365. Musaro A, McCullagh K, Paul A, Houghton L, Dobrowolny G, Molinaro M, Barton ER,Sweeney HL, Rosenthal N. Localized Igf-1 transgene expression sustains hypertrophyand regeneration in senescent skeletal muscle. Nat Genet 27: 195–200, 2001.

366. Musaro A, McCullagh KJ, Naya FJ, Olson EN, Rosenthal N. IGF-1 induces skeletalmyocyte hypertrophy through calcineurin in association with GATA-2 and NF-ATc1.Nature 400: 581–585, 1999.

367. Musaro A, Rosenthal N. Maturation of the myogenic program is induced by postmi-totic expression of insulin-like growth factor I. Mol Cell Biol 19: 3115–3124, 1999.

368. Muskiewicz KR, Frank NY, Flint AF, Gussoni E. Myogenic potential of muscle side andmain population cells after intravenous injection into sub-lethally irradiated mdx mice.J Histochem Cytochem 53: 861–873, 2005.

369. Nagata Y, Kobayashi H, Umeda M, Ohta N, Kawashima S, Zammit PS, Matsuda R.Sphingomyelin levels in the plasma membrane correlate with the activation state ofmuscle satellite cells. J Histochem Cytochem 54: 375–384, 2006.

370. Nagata Y, Partridge TA, Matsuda R, Zammit PS. Entry of muscle satellite cells into thecell cycle requires sphingolipid signaling. J Cell Biol 174: 245–253, 2006.

371. Nakagawa M, Koyanagi M, Tanabe K, Takahashi K, Ichisaka T, Aoi T, Okita K,Mochiduki Y, Takizawa N, Yamanaka S. Generation of induced pluripotent stem cellswithout Myc from mouse and human fibroblasts. Nat Biotechnol 26: 101–106, 2008.

372. Negroni E, Riederer I, Chaouch S, Belicchi M, Razini P, Di Santo J, Torrente Y,Butler-Browne GS, Mouly V. In vivo myogenic potential of human CD133� muscle-derived stem cells: a quantitative study. Mol Ther 17: 1771–1778, 2009.

373. Nelson WJ, Nusse R. Convergence of Wnt, beta-catenin, and cadherin pathways.Science 303: 1483–1487, 2004.

374. Nguyen HT, Frasch M. MicroRNAs in muscle differentiation: lessons from Drosophilaand beyond. Curr Opin Genet Dev 16: 533–539, 2006.

375. Nguyen HX, Tidball JG. Interactions between neutrophils and macrophages promotemacrophage killing of rat muscle cells in vitro. J Physiol 547: 125–132, 2003.

376. Nicolas N, Marazzi G, Kelley K, Sassoon D. Embryonic deregulation of muscle stresssignaling pathways leads to altered postnatal stem cell behavior and a failure in post-natal muscle growth. Dev Biol 281: 171–183, 2005.

377. Nishimura T, Nakamura K, Kishioka Y, Kato-Mori Y, Wakamatsu J, Hattori A. Inhibi-tion of matrix metalloproteinases suppresses the migration of skeletal muscle cells. JMuscle Res Cell Motil 29: 37–44, 2008.

378. Novitch BG, Mulligan GJ, Jacks T, Lassar AB. Skeletal muscle cells lacking the retino-blastoma protein display defects in muscle gene expression and accumulate in S andG2 phases of the cell cycle. J Cell Biol 135: 441–456, 1996.



379. Odelberg SJ, Kollhoff A, Keating MT. Dedifferentiation of mammalian myotubes in-duced by msx1. Cell 103: 1099–1109, 2000.

380. Ohlstein B, Kai T, Decotto E, Spradling A. The stem cell niche: theme and variations.Curr Opin Cell Biol 16: 693–699, 2004.

381. Ohnishi T, Daikuhara Y. Hepatocyte growth factor/scatter factor in development,inflammation and carcinogenesis: its expression and role in oral tissues. Arch Oral Biol48: 797–804, 2003.

382. Ojima K, Uezumi A, Miyoshi H, Masuda S, Morita Y, Fukase A, Hattori A, Nakauchi H,Miyagoe-Suzuki Y, Takeda S. Mac-1(low) early myeloid cells in the bone marrow-derived SP fraction migrate into injured skeletal muscle and participate in muscleregeneration. Biochem Biophys Res Commun 321: 1050–1061, 2004.

383. Okada S, Nonaka I, Chou SM. Muscle fiber type differentiation and satellite cellpopulations in normally grown and neonatally denervated muscles in the rat. ActaNeuropathol 65: 90–98, 1984.

384. Okita K, Ichisaka T, Yamanaka S. Generation of germline-competent induced pluri-potent stem cells. Nature 448: 313–317, 2007.

385. Olguin HC, Yang Z, Tapscott SJ, Olwin BB. Reciprocal inhibition between Pax7 andmuscle regulatory factors modulates myogenic cell fate determination. J Cell Biol 177:769–779, 2007.

386. Olwin BB, Rapraeger A. Repression of myogenic differentiation by aFGF, bFGF, andK-FGF is dependent on cellular heparan sulfate. J Cell Biol 118: 631–639, 1992.

387. Ono Y, Boldrin L, Knopp P, Morgan JE, Zammit PS. Muscle satellite cells are afunctionally heterogeneous population in both somite-derived and branchiomericmuscles. Dev Biol 337: 29–41, 2010.

388. Ono Y, Masuda S, Nam HS, Benezra R, Miyagoe-Suzuki Y, Takeda S. Slow-dividingsatellite cells retain long-term self-renewal ability in adult muscle. J Cell Sci 125:1309–1317, 2012.

389. Orimo S, Hiyamuta E, Arahata K, Sugita H. Analysis of inflammatory cells and com-plement C3 in bupivacaine-induced myonecrosis. Muscle Nerve 14: 515–520, 1991.

390. Ornatsky OI, Cox DM, Tangirala P, Andreucci JJ, Quinn ZA, Wrana JL, Prywes R, YuYT, McDermott JC. Post-translational control of the MEF2A transcriptional regula-tory protein. Nucleic Acids Res 27: 2646–2654, 1999.

391. Otto A, Schmidt C, Luke G, Allen S, Valasek P, Muntoni F, Lawrence-Watt D, Patel K.Canonical Wnt signalling induces satellite-cell proliferation during adult skeletal mus-cle regeneration. J Cell Sci 121: 2939–2950, 2008.

392. Otto A, Schmidt C, Patel K. Pax3 and Pax7 expression and regulation in the avianembryo. Anat Embryol 211: 293–310, 2006.

393. Oustanina S, Hause G, Braun T. Pax7 directs postnatal renewal and propagation ofmyogenic satellite cells but not their specification. EMBO J 23: 3430–3439, 2004.

394. Pacher P, Beckman JS, Liaudet L. Nitric oxide and peroxynitrite in health and disease.Physiol Rev 87: 315–424, 2007.

395. Pajcini KV, Corbel SY, Sage J, Pomerantz JH, Blau HM. Transient inactivation of Rb andARF yields regenerative cells from postmitotic mammalian muscle. Cell Stem Cell 7:198–213, 2010.

396. Palacio J, Galdiz JB, Alvarez FJ, Orozco-Levi M, Lloreta J, Gea J. Procion orange tracerdye technique vs. identification of intrafibrillar fibronectin in the assessment of sar-colemmal damage. Eur J Clin Invest 32: 443–447, 2002.

397. Paliwal P, Conboy IM. Inhibitors of tyrosine phosphatases and apoptosis reprogramlineage-marked differentiated muscle to myogenic progenitor cells. Chem Biol 18:1153–1166, 2011.

398. Pallafacchina G, Francois S, Regnault B, Czarny B, Dive V, Cumano A, Montarras D,Buckingham M. An adult tissue-specific stem cell in its niche: a gene profiling analysisof in vivo quiescent and activated muscle satellite cells. Stem Cell Res 4: 77–91, 2010.

399. Palumbo R, Sampaolesi M, De Marchis F, Tonlorenzi R, Colombetti S, Mondino A,Cossu G, Bianchi ME. Extracellular HMGB1, a signal of tissue damage, induces me-soangioblast migration and proliferation. J Cell Biol 164: 441–449, 2004.

400. Parise G, McKinnell IW, Rudnicki MA. Muscle satellite cell and atypical myogenicprogenitor response following exercise. Muscle Nerve 37: 611–619, 2008.

401. Pasquinelli G, Pacilli A, Alviano F, Foroni L, Ricci F, Valente S, Orrico C, Lanzoni G,Buzzi M, Luigi Tazzari P, Pagliaro P, Stella A, Paolo Bagnara G. Multidistrict humanmesenchymal vascular cells: pluripotency and stemness characteristics. Cytotherapy12: 275–287, 2010.

402. Patruno M, Caliaro F, Martinello T, Mascarello F. Expression of the paired box domainPax7 protein in myogenic cells isolated from the porcine semitendinosus muscle afterbirth. Tissue Cell 40: 1–6, 2008.

403. Pedersen BK, Febbraio MA. Muscle as an endocrine organ: focus on muscle-derivedinterleukin-6. Physiol Rev 88: 1379–1406, 2008.

404. Peng H, Huard J. Muscle-derived stem cells for musculoskeletal tissue regenerationand repair. Transpl Immunol 12: 311–319, 2004.

405. Penn BH, Bergstrom DA, Dilworth FJ, Bengal E, Tapscott SJ. A MyoD-generatedfeed-forward circuit temporally patterns gene expression during skeletal muscle dif-ferentiation. Genes Dev 18: 2348–2353, 2004.

406. Peppiatt CM, Howarth C, Mobbs P, Attwell D. Bidirectional control of CNS capillarydiameter by pericytes. Nature 443: 700–704, 2006.

407. Perez-Ruiz A, Ono Y, Gnocchi VF, Zammit PS. beta-Catenin promotes self-renewalof skeletal-muscle satellite cells. J Cell Sci 121: 1373–1382, 2008.

408. Petropoulos H, Skerjanc IS. Beta-catenin is essential and sufficient for skeletal myo-genesis in P19 cells. J Biol Chem 277: 15393–15399, 2002.

409. Philippou A, Halapas A, Maridaki M, Koutsilieris M. Type I insulin-like growth factorreceptor signaling in skeletal muscle regeneration and hypertrophy. J MusculoskeletNeuronal Interact 7: 208–218, 2007.

410. Phillips WD, Bennett MR. Elimination of distributed synaptic acetylcholine receptorclusters on developing avian fast-twitch muscle fibres accompanies loss of polyneu-ronal innervation. J Neurocytol 16: 785–797, 1987.

411. Pietsch J. The effects of colchicine on regeneration of mouse skeletal muscle. Anat Rec167–172, 1961.

412. Polesello C, Tapon N. Salvador-warts-hippo signaling promotes Drosophila posteriorfollicle cell maturation downstream of notch. Curr Biol 17: 1864–1870, 2007.

413. Polesskaya A, Seale P, Rudnicki MA. Wnt signaling induces the myogenic specificationof resident CD45� adult stem cells during muscle regeneration. Cell 113: 841–852,2003.

414. Popescu LM, Manole E, Serboiu CS, Manole CG, Suciu LC, Gherghiceanu M, PopescuBO. Identification of telocytes in skeletal muscle interstitium: implication for muscleregeneration. J Cell Mol Med 15: 1379–1392, 2011.

415. Pouget C, Pottin K, Jaffredo T. Sclerotomal origin of vascular smooth muscle cells andpericytes in the embryo. Dev Biol 315: 437–447, 2008.

416. Prior BM, Lloyd PG, Yang HT, Terjung RL. Exercise-induced vascular remodeling.Exerc Sport Sci Rev 31: 26–33, 2003.

417. Prockop DJ. Marrow stromal cells as stem cells for nonhematopoietic tissues. Science276: 71–74, 1997.

418. Przybylski RJ, Blumberg JM. Ultrastructural aspects of myogenesis in the chick. LabInvest 15: 836–863, 1966.

419. Puri PL, Avantaggiati ML, Balsano C, Sang N, Graessmann A, Giordano A, Levrero M.p300 is required for MyoD-dependent cell cycle arrest and muscle-specific genetranscription. EMBO J 16: 369–383, 1997.

420. Puri PL, Sartorelli V. Regulation of muscle regulatory factors by DNA-binding, inter-acting proteins, and post-transcriptional modifications. J Cell Physiol 185: 155–173,2000.

421. Qu-Petersen Z, Deasy B, Jankowski R, Ikezawa M, Cummins J, Pruchnic R, Myt-inger J, Cao B, Gates C, Wernig A, Huard J. Identification of a novel population ofmuscle stem cells in mice: potential for muscle regeneration. J Cell Biol 157: 851–864,2002.

422. Qu Z, Balkir L, van Deutekom JC, Robbins PD, Pruchnic R, Huard J. Development ofapproaches to improve cell survival in myoblast transfer therapy. J Cell Biol 142:1257–1267, 1998.



423. Quinlan JG, Lyden SP, Cambier DM, Johnson SR, Michaels SE, Denman DL. Radiationinhibition of mdx mouse muscle regeneration: dose and age factors. Muscle Nerve 18:201–206, 1995.

424. Rampalli S, Li L, Mak E, Ge K, Brand M, Tapscott SJ, Dilworth FJ. p38 MAPK signalingregulates recruitment of Ash2L-containing methyltransferase complexes to specificgenes during differentiation. Nat Struct Mol Biol 14: 1150–1156, 2007.

425. Rando TA, Blau HM. Primary mouse myoblast purification, characterization, andtransplantation for cell-mediated gene therapy. J Cell Biol 125: 1275–1287, 1994.

426. Rantanen J, Hurme T, Lukka R, Heino J, Kalimo H. Satellite cell proliferation and theexpression of myogenin and desmin in regenerating skeletal muscle: evidence for twodifferent populations of satellite cells. Lab Invest 72: 341–347, 1995.

427. Rapraeger AC. Syndecan-regulated receptor signaling. J Cell Biol 149: 995–998, 2000.

428. Rash JE, Fambrough D. Ultrastructural and electrophysiological correlates of cellcoupling and cytoplasmic fusion during myogenesis in vitro. Dev Biol 30: 166–186,1973.

429. Ratajczak MZ, Majka M, Kucia M, Drukala J, Pietrzkowski Z, Peiper S, Janowska-Wieczorek A. Expression of functional CXCR4 by muscle satellite cells and secretionof SDF-1 by muscle-derived fibroblasts is associated with the presence of both muscleprogenitors in bone marrow and hematopoietic stem/progenitor cells in muscles.Stem Cells 21: 363–371, 2003.

430. Rawlings JS, Rosler KM, Harrison DA. The JAK/STAT signaling pathway. J Cell Sci 117:1281–1283, 2004.

431. Relaix F, Montarras D, Zaffran S, Gayraud-Morel B, Rocancourt D, Tajbakhsh S,Mansouri A, Cumano A, Buckingham M. Pax3 and Pax7 have distinct and overlappingfunctions in adult muscle progenitor cells. J Cell Biol 172: 91–102, 2006.

432. Relaix F, Polimeni M, Rocancourt D, Ponzetto C, Schafer BW, Buckingham M. Thetranscriptional activator PAX3-FKHR rescues the defects of Pax3 mutant mice butinduces a myogenic gain-of-function phenotype with ligand-independent activation ofMet signaling in vivo. Genes Dev 17: 2950–2965, 2003.

433. Relaix F, Rocancourt D, Mansouri A, Buckingham M. Divergent functions of murinePax3 and Pax7 in limb muscle development. Genes Dev 18: 1088–1105, 2004.

434. Relaix F, Rocancourt D, Mansouri A, Buckingham M. A Pax3/Pax7-dependent popu-lation of skeletal muscle progenitor cells. Nature 435: 948–953, 2005.

435. Relaix F, Weng X, Marazzi G, Yang E, Copeland N, Jenkins N, Spence SE, Sassoon D.Pw1, a novel zinc finger gene implicated in the myogenic and neuronal lineages. DevBiol 177: 383–396, 1996.

436. Ren H, Yin P, Duan C. IGFBP-5 regulates muscle cell differentiation by binding toIGF-II and switching on the IGF-II auto-regulation loop. J Cell Biol 182: 979–991, 2008.

437. Reznik M. Thymidine-3H uptake by satellite cells of regenerating skeletal muscle. J CellBiol 40: 568–571, 1969.

438. Riuzzi F, Sorci G, Sagheddu R, Donato R. HMGB1-RAGE regulates muscle satellite cellhomeostasis through p38-MAPK- and myogenin-dependent repression of Pax7 tran-scription. J Cell Sci 125: 1440–1454, 2012.

439. Rivier F, Alkan O, Flint AF, Muskiewicz K, Allen PD, Leboulch P, Gussoni E. Role ofbone marrow cell trafficking in replenishing skeletal muscle SP and MP cell popula-tions. J Cell Sci 117: 1979–1988, 2004.

440. Robertson TA, Maley MA, Grounds MD, Papadimitriou JM. The role of macrophagesin skeletal muscle regeneration with particular reference to chemotaxis. Exp Cell Res207: 321–331, 1993.

441. Rodeheffer MS, Birsoy K, Friedman JM. Identification of white adipocyte progenitorcells in vivo. Cell 135: 240–249, 2008.

442. Rodrigues Ade C, Schmalbruch H. Satellite cells and myonuclei in long-term dener-vated rat muscles. Anat Rec 243: 430–437, 1995.

443. Rosania GR, Chang YT, Perez O, Sutherlin D, Dong H, Lockhart DJ, Schultz PG.Myoseverin, a microtubule-binding molecule with novel cellular effects. Nature Bio-technol 18: 304–308, 2000.

444. Rosant C, Nagel MD, Perot C. Aging affects passive stiffness and spindle function ofthe rat soleus muscle. Exp Gerontol 42: 301–308, 2007.

445. Rosen GD, Sanes JR, LaChance R, Cunningham JM, Roman J, Dean DC. Roles for theintegrin VLA-4 and its counter receptor VCAM-1 in myogenesis. Cell 69: 1107–1119,1992.

446. Rosenblatt JD, Lunt AI, Parry DJ, Partridge TA. Culturing satellite cells from livingsingle muscle fiber explants. In Vitro Cell Dev Biol Anim 31: 773–779, 1995.

447. Rosu-Myles M, Stewart E, Trowbridge J, Ito CY, Zandstra P, Bhatia M. A uniquepopulation of bone marrow cells migrates to skeletal muscle via hepatocyte growthfactor/c-met axis. J Cell Sci 118: 4343–4352, 2005.

448. Ruberti F, Capsoni S, Comparini A, Di Daniel E, Franzot J, Gonfloni S, Rossi G, BerardiN, Cattaneo A. Phenotypic knockout of nerve growth factor in adult transgenic micereveals severe deficits in basal forebrain cholinergic neurons, cell death in the spleen,and skeletal muscle dystrophy. J Neurosci 20: 2589–2601, 2000.

449. Rubinstein I, Abassi Z, Coleman R, Milman F, Winaver J, Better OS. Involvement ofnitric oxide system in experimental muscle crush injury. J Clin Invest 101: 1325–1333,1998.

450. Rudnicki MA, Le Grand F, McKinnell I, Kuang S. The molecular regulation of musclestem cell function. Cold Spring Harb Symp Quant Biol 73: 323–331, 2008.

451. Ryan NA, Zwetsloot KA, Westerkamp LM, Hickner RC, Pofahl WE, Gavin TP. Lowerskeletal muscle capillarization and VEGF expression in aged vs. young men. J ApplPhysiol 100: 178–185, 2006.

452. Sabourin LA, Girgis-Gabardo A, Seale P, Asakura A, Rudnicki MA. Reduced differen-tiation potential of primary MyoD�/� myogenic cells derived from adult skeletalmuscle. J Cell Biol 144: 631–643, 1999.

453. Sabourin LA, Rudnicki MA. The molecular regulation of myogenesis. Clin Genet 57:16–25, 2000.

454. Sacco A, Doyonnas R, Kraft P, Vitorovic S, Blau HM. Self-renewal and expansion ofsingle transplanted muscle stem cells. Nature 456: 502–506, 2008.

455. Sakurai H, Okawa Y, Inami Y, Nishio N, Isobe K. Paraxial mesodermal progenitorsderived from mouse embryonic stem cells contribute to muscle regeneration viadifferentiation into muscle satellite cells. Stem Cells 26: 1865–1873, 2008.

456. Sambasivan R, Gayraud-Morel B, Dumas G, Cimper C, Paisant S, Kelly RG, TajbakhshS. Distinct regulatory cascades govern extraocular and pharyngeal arch muscle pro-genitor cell fates. Dev Cell 16: 810–821, 2009.

457. Sambasivan R, Yao R, Kissenpfennig A, Van Wittenberghe L, Paldi A, Gayraud-MorelB, Guenou H, Malissen B, Tajbakhsh S, Galy A. Pax7-expressing satellite cells areindispensable for adult skeletal muscle regeneration. Development 138: 3647–3656,2011.

458. Sampaolesi M, Torrente Y, Innocenzi A, Tonlorenzi R, D’Antona G, Pellegrino MA,Barresi R, Bresolin N, De Angelis MG, Campbell KP, Bottinelli R, Cossu G. Celltherapy of alpha-sarcoglycan null dystrophic mice through intra-arterial delivery ofmesoangioblasts. Science 301: 487–492, 2003.

459. Sanes JR. The basement membrane/basal lamina of skeletal muscle. J Biol Chem 278:12601–12604, 2003.

460. Santa Maria L, Rojas CV, Minguell JJ. Signals from damaged but not undamaged skeletalmuscle induce myogenic differentiation of rat bone-marrow-derived mesenchymalstem cells. Exp Cell Res 300: 418–426, 2004.

461. Sassoli C, Formigli L, Bini F, Tani A, Squecco R, Battistini C, Zecchi-Orlandini S,Francini F, Meacci E. Effects of S1P on skeletal muscle repair/regeneration duringeccentric contraction. J Cell Mol Med 15: 2498–2511, 2011.

462. Scaal M, Christ B. Formation and differentiation of the avian dermomyotome. AnatEmbryol 208: 411–424, 2004.

463. Scata KA, Bernard DW, Fox J, Swain JL. FGF receptor availability regulates skeletalmyogenesis. Exp Cell Res 250: 10–21, 1999.

464. Schienda J, Engleka KA, Jun S, Hansen MS, Epstein JA, Tabin CJ, Kunkel LM, Kardon G.Somitic origin of limb muscle satellite and side population cells. Proc Natl Acad Sci USA103: 945–950, 2006.

465. Schmalbruch H. The morphology of regeneration of skeletal muscles in the rat. TissueCell 8: 673–692, 1976.



466. Schmalbruch H, Hellhammer U. The number of nuclei in adult rat muscles with specialreference to satellite cells. Anat Rec 189: 169–175, 1977.

467. Schmalbruch H, Lewis DM. Dynamics of nuclei of muscle fibers and connective tissuecells in normal and denervated rat muscles. Muscle Nerve 23: 617–626, 2000.

468. Schultz E. Changes in the satellite cells of growing muscle following denervation. AnatRec 190: 299–311, 1978.

469. Schultz E. A quantitative study of the satellite cell population in postnatal mouselumbrical muscle. Anat Rec 180: 589–595, 1974.

470. Schultz E. Satellite cell behavior during skeletal muscle growth and regeneration. MedSci Sports Exerc 21: S181–186, 1989.

471. Schultz E. Satellite cell proliferative compartments in growing skeletal muscles. DevBiol 175: 84–94, 1996.

472. Schultz E, Gibson MC, Champion T. Satellite cells are mitotically quiescent in maturemouse muscle: an EM and radioautographic study. J Exp Zool 206: 451–456, 1978.

473. Schultz E, Jaryszak DL, Valliere CR. Response of satellite cells to focal skeletal muscleinjury. Muscle Nerve 8: 217–222, 1985.

474. Schwander M, Leu M, Stumm M, Dorchies OM, Ruegg UT, Schittny J, Muller U. Beta1integrins regulate myoblast fusion and sarcomere assembly. Dev Cell 4: 673–685,2003.

475. Sciorati C, Galvez BG, Brunelli S, Tagliafico E, Ferrari S, Cossu G, Clementi E. Ex vivotreatment with nitric oxide increases mesoangioblast therapeutic efficacy in musculardystrophy. J Cell Sci 119: 5114–5123, 2006.

476. Seale P, Ishibashi J, Holterman C, Rudnicki MA. Muscle satellite cell-specific genesidentified by genetic profiling of MyoD-deficient myogenic cell. Dev Biol 275: 287–300,2004.

477. Seale P, Ishibashi J, Scime A, Rudnicki MA. Pax7 is necessary and sufficient for themyogenic specification of CD45�:Sca1� stem cells from injured muscle. PLoS Biol 2:E130, 2004.

478. Seale P, Sabourin LA, Girgis-Gabardo A, Mansouri A, Gruss P, Rudnicki MA. Pax7is required for the specification of myogenic satellite cells. Cell 102: 777–786,2000.

479. Semsarian C, Wu MJ, Ju YK, Marciniec T, Yeoh T, Allen DG, Harvey RP, Graham RM.Skeletal muscle hypertrophy is mediated by a Ca2�-dependent calcineurin signallingpathway. Nature 400: 576–581, 1999.

480. Serrano AL, Baeza-Raja B, Perdiguero E, Jardi M, Munoz-Canoves P. Interleukin-6 isan essential regulator of satellite cell-mediated skeletal muscle hypertrophy. CellMetab 7: 33–44, 2008.

481. Shafiq SA, Gorycki MA, Mauro A. Mitosis during postnatal growth in skeletal andcardiac muscle of the rat. J Anat 103: 135–141, 1968.

482. Shea KL, Xiang W, LaPorta VS, Licht JD, Keller C, Basson MA, Brack AS. Sprouty1regulates reversible quiescence of a self-renewing adult muscle stem cell pool duringregeneration. Cell Stem Cell 6: 117–129, 2010.

483. Sheehan SM, Allen RE. Skeletal muscle satellite cell proliferation in response to mem-bers of the fibroblast growth factor family and hepatocyte growth factor. J Cell Physiol181: 499–506, 1999.

484. Sheehan SM, Tatsumi R, Temm-Grove CJ, Allen RE. HGF is an autocrine growthfactor for skeletal muscle satellite cells in vitro. Muscle Nerve 23: 239–245, 2000.

485. Shefer G, Wleklinski-Lee M, Yablonka-Reuveni Z. Skeletal muscle satellite cells canspontaneously enter an alternative mesenchymal pathway. J Cell Sci 117: 5393–5404,2004.

486. Sherwood RI, Christensen JL, Conboy IM, Conboy MJ, Rando TA, Weissman IL,Wagers AJ. Isolation of adult mouse myogenic progenitors: functional heterogeneityof cells within and engrafting skeletal muscle. Cell 119: 543–554, 2004.

487. Sherwood RI, Christensen JL, Weissman IL, Wagers AJ. Determinants of skeletalmuscle contributions from circulating cells, bone marrow cells, and hematopoieticstem cells. Stem Cells 22: 1292–1304, 2004.

488. Shi D, Reinecke H, Murry CE, Torok-Storb B. Myogenic fusion of human bone mar-row stromal cells, but not hematopoietic cells. Blood 104: 290–294, 2004.

489. Shinin V, Gayraud-Morel B, Gomes D, Tajbakhsh S. Asymmetric division and coseg-regation of template DNA strands in adult muscle satellite cells. Nat Cell Biol 8:677–687, 2006.

490. Siegel AL, Atchison K, Fisher KE, Davis GE, Cornelison DD. 3D timelapse analysis ofmuscle satellite cell motility. Stem Cells 27: 2527–2538, 2009.

491. Simone C, Forcales SV, Hill DA, Imbalzano AN, Latella L, Puri PL. p38 pathway targetsSWI-SNF chromatin-remodeling complex to muscle-specific loci. Nat Genet 36: 738–743, 2004.

492. Simons M, Mlodzik M. Planar cell polarity signaling: from fly development to humandisease. Annu Rev Genet 42: 517–540, 2008.

493. Sinha-Hikim I, Artaza J, Woodhouse L, Gonzalez-Cadavid N, Singh AB, Lee MI, StorerTW, Casaburi R, Shen R, Bhasin S. Testosterone-induced increase in muscle size inhealthy young men is associated with muscle fiber hypertrophy. Am J Physiol EndocrinolMetab 283: E154–E164, 2002.

494. Sinha-Hikim I, Cornford M, Gaytan H, Lee ML, Bhasin S. Effects of testosteronesupplementation on skeletal muscle fiber hypertrophy and satellite cells in communi-ty-dwelling older men. J Clin Endocrinol Metab 91: 3024–3033, 2006.

495. Sinha-Hikim I, Roth SM, Lee MI, Bhasin S. Testosterone-induced muscle hypertrophyis associated with an increase in satellite cell number in healthy, young men. Am JPhysiol Endocrinol Metab 285: E197–E205, 2003.

496. Sinha-Hikim I, Taylor WE, Gonzalez-Cadavid NF, Zheng W, Bhasin S. Androgenreceptor in human skeletal muscle and cultured muscle satellite cells: up-regulation byandrogen treatment. J Clin Endocrinol Metab 89: 5245–5255, 2004.

497. Smith CK 2nd, Janney MJ, Allen RE. Temporal expression of myogenic regulatorygenes during activation, proliferation, differentiation of rat skeletal muscle satellitecells. J Cell Physiol 159: 379–385, 1994.

498. Snow MH. An autoradiographic study of satellite cell differentiation into regeneratingmyotubes following transplantation of muscles in young rats. Cell Tissue Res 186:535–540, 1978.

499. Snow MH. Myogenic cell formation in regenerating rat skeletal muscle injured bymincing. II. An autoradiographic study. Anat Rec 188: 201–217, 1977.

500. Snow MH. A quantitative ultrastructural analysis of satellite cells in denervated fast andslow muscles of the mouse. Anat Rec 207: 593–604, 1983.

501. Song WK, Wang W, Foster RF, Bielser DA, Kaufman SJ. H36-alpha 7 is a novel integrinalpha chain that is developmentally regulated during skeletal myogenesis. J Cell Biol117: 643–657, 1992.

502. Song YH, Li Y, Du J, Mitch WE, Rosenthal N, Delafontaine P. Muscle-specific expres-sion of IGF-1 blocks angiotensin II-induced skeletal muscle wasting. J Clin Invest 115:451–458, 2005.

503. Sonnet C, Lafuste P, Arnold L, Brigitte M, Poron F, Authier FJ, Chretien F, GherardiRK, Chazaud B. Human macrophages rescue myoblasts and myotubes from apoptosisthrough a set of adhesion molecular systems. J Cell Sci 119: 2497–2507, 2006.

504. Spicer DB, Rhee J, Cheung WL, Lassar AB. Inhibition of myogenic bHLH and MEF2transcription factors by the bHLH protein Twist. Science 272: 1476–1480, 1996.

505. Stamler JS, Meissner G. Physiology of nitric oxide in skeletal muscle. Physiol Rev 81:209–237, 2001.

506. Stark DA, Karvas RM, Siegel AL, Cornelison DD. Eph/ephrin interactions modulatemuscle satellite cell motility and patterning. Development 138: 5279–5289, 2011.

507. Starkey JD, Yamamoto M, Yamamoto S, Goldhamer DJ. Skeletal muscle satellite cellsare committed to myogenesis and do not spontaneously adopt nonmyogenic fates. JHistochem Cytochem 59: 33–46, 2011.

508. Steinhardt RA, Bi G, Alderton JM. Cell membrane resealing by a vesicular mechanismsimilar to neurotransmitter release. Science 263: 390–393, 1994.

509. Summer R, Fitzsimmons K, Dwyer D, Murphy J, Fine A. Isolation of an adult mouselung mesenchymal progenitor cell population. Am J Respir Cell Mol Biol 37: 152–159,2007.

510. Sun D, Martinez CO, Ochoa O, Ruiz-Willhite L, Bonilla JR, Centonze VE, Waite LL,Michalek JE, McManus LM, Shireman PK. Bone marrow-derived cell regulation ofskeletal muscle regeneration. FASEB J 23: 382–395, 2009.



511. Sun H, Li L, Vercherat C, Gulbagci NT, Acharjee S, Li J, Chung TK, Thin TH, Taneja R.Stra13 regulates satellite cell activation by antagonizing Notch signaling. J Cell Biol 177:647–657, 2007.

512. Suzuki J, Yamazaki Y, Li G, Kaziro Y, Koide H. Involvement of Ras and Ral in chemot-actic migration of skeletal myoblasts. Mol Cell Biol 20: 4658–4665, 2000.

513. Suzuki S, Yamanouchi K, Soeta C, Katakai Y, Harada R, Naito K, Tojo H. Skeletalmuscle injury induces hepatocyte growth factor expression in spleen. Biochem BiophysRes Commun 292: 709–714, 2002.

514. Takahashi A, Kureishi Y, Yang J, Luo Z, Guo K, Mukhopadhyay D, Ivashchenko Y,Branellec D, Walsh K. Myogenic Akt signaling regulates blood vessel recruitmentduring myofiber growth. Mol Cell Biol 22: 4803–4814, 2002.

515. Takahashi K, Tanabe K, Ohnuki M, Narita M, Ichisaka T, Tomoda K, Yamanaka S.Induction of pluripotent stem cells from adult human fibroblasts by defined factors.Cell 131: 861–872, 2007.

516. Takahashi K, Yamanaka S. Induction of pluripotent stem cells from mouse embryonicand adult fibroblast cultures by defined factors. Cell 126: 663–676, 2006.

517. Tamaki T, Akatsuka A, Yoshimura S, Roy RR, Edgerton VR. New fiber formation in theinterstitial spaces of rat skeletal muscle during postnatal growth. J Histochem Cy-tochem 50: 1097–1111, 2002.

518. Tanaka KK, Hall JK, Troy AA, Cornelison DD, Majka SM, Olwin BB. Syndecan-4-expressing muscle progenitor cells in the SP engraft as satellite cells during muscleregeneration. Cell Stem Cell 4: 217–225, 2009.

519. Tang W, Zeve D, Suh JM, Bosnakovski D, Kyba M, Hammer RE, Tallquist MD, GraffJM. White fat progenitor cells reside in the adipose vasculature. Science 322: 583–586,2008.

520. Tatsumi R, Allen RE. Active hepatocyte growth factor is present in skeletal muscleextracellular matrix. Muscle Nerve 30: 654–658, 2004.

521. Tatsumi R, Anderson JE, Nevoret CJ, Halevy O, Allen RE. HGF/SF is present in normaladult skeletal muscle and is capable of activating satellite cells. Dev Biol 194: 114–128,1998.

522. Tatsumi R, Hattori A, Ikeuchi Y, Anderson JE, Allen RE. Release of hepatocyte growthfactor from mechanically stretched skeletal muscle satellite cells and role of pH andnitric oxide. Mol Biol Cell 13: 2909–2918, 2002.

523. Tatsumi R, Liu X, Pulido A, Morales M, Sakata T, Dial S, Hattori A, Ikeuchi Y, Allen RE.Satellite cell activation in stretched skeletal muscle and the role of nitric oxide andhepatocyte growth factor. Am J Physiol Cell Physiol 290: C1487–C1494, 2006.

524. Tatsumi R, Sankoda Y, Anderson JE, Sato Y, Mizunoya W, Shimizu N, Suzuki T,Yamada M, Rhoads RP Jr, Ikeuchi Y, Allen RE. Possible implication of satellite cells inregenerative motoneuritogenesis: HGF upregulates neural chemorepellent Sema3Aduring myogenic differentiation. Am J Physiol Cell Physiol 297: C238–C252, 2009.

525. Tatsumi R, Sheehan SM, Iwasaki H, Hattori A, Allen RE. Mechanical stretch inducesactivation of skeletal muscle satellite cells in vitro. Exp Cell Res 267: 107–114, 2001.

526. Taulli R, Bersani F, Foglizzo V, Linari A, Vigna E, Ladanyi M, Tuschl T, Ponzetto C. Themuscle-specific microRNA miR-206 blocks human rhabdomyosarcoma growth inxenotransplanted mice by promoting myogenic differentiation. J Clin Invest 119:2366–2378, 2009.

527. Taylor-Jones JM, McGehee RE, Rando TA, Lecka-Czernik B, Lipschitz DA, PetersonCA. Activation of an adipogenic program in adult myoblasts with age. Mech Ageing Dev123: 649–661, 2002.

528. Theise ND. Gastrointestinal stem cells. III. Emergent themes of liver stem cell biology:niche, quiescence, self-renewal, and plasticity. Am J Physiol Gastrointest Liver Physiol290: G189–G193, 2006.

529. Thompson SH, Boxhorn LK, Kong WY, Allen RE. Trenbolone alters the responsive-ness of skeletal muscle satellite cells to fibroblast growth factor and insulin-like growthfactor I. Endocrinology 124: 2110–2117, 1989.

530. Tidball JG. Inflammatory cell response to acute muscle injury. Med Sci Sports Exerc 27:1022–1032, 1995.

531. Tidball JG, Daniel TL. Myotendinous junctions of tonic muscle cells: structure andloading. Cell Tissue Res 245: 315–322, 1986.

532. Tidball JG, Villalta SA. Regulatory interactions between muscle and the immune sys-tem during muscle regeneration. Am J Physiol Regul Integr Comp Physiol 2010.

533. Tomiya A, Aizawa T, Nagatomi R, Sensui H, Kokubun S. Myofibers express IL-6 aftereccentric exercise. Am J Sports Med 32: 503–508, 2004.

534. Torrente Y, Belicchi M, Marchesi C, Dantona G, Cogiamanian F, Pisati F, Gavina M,Giordano R, Tonlorenzi R, Fagiolari G, Lamperti C, Porretti L, Lopa R, Sampaolesi M,Vicentini L, Grimoldi N, Tiberio F, Songa V, Baratta P, Prelle A, Forzenigo L, GuglieriM, Pansarasa O, Rinaldi C, Mouly V, Butler-Browne GS, Comi GP, Biondetti P,Moggio M, Gaini SM, Stocchetti N, Priori A, D’Angelo MG, Turconi A, Bottinelli R,Cossu G, Rebulla P, Bresolin N. Autologous transplantation of muscle-derivedCD133� stem cells in Duchenne muscle patients. Cell Transplant 16: 563–577, 2007.

535. Torrente Y, Belicchi M, Sampaolesi M, Pisati F, Meregalli M, D’Antona G, TonlorenziR, Porretti L, Gavina M, Mamchaoui K, Pellegrino MA, Furling D, Mouly V, Butler-Browne GS, Bottinelli R, Cossu G, Bresolin N. Human circulating AC133(�) stemcells restore dystrophin expression and ameliorate function in dystrophic skeletalmuscle. J Clin Invest 114: 182–195, 2004.

536. Torrente Y, Tremblay JP, Pisati F, Belicchi M, Rossi B, Sironi M, Fortunato F, El FahimeM, D’Angelo MG, Caron NJ, Constantin G, Paulin D, Scarlato G, Bresolin N. Intra-arterial injection of muscle-derived CD34(�)Sca-1(�) stem cells restores dystrophinin mdx mice. J Cell Biol 152: 335–348, 2001.

537. Toti P, Villanova M, Vatti R, Schuerfeld K, Stumpo M, Barbagli L, Malandrini A, Costan-tini M. Nerve growth factor expression in human dystrophic muscles. Muscle Nerve27: 370–373, 2003.

538. Tsujinaka T, Ebisui C, Fujita J, Kishibuchi M, Morimoto T, Ogawa A, Katsume A,Ohsugi Y, Kominami E, Monden M. Muscle undergoes atrophy in association withincrease of lysosomal cathepsin activity in interleukin-6 transgenic mouse. BiochemBiophys Res Commun 207: 168–174, 1995.

539. Tureckova J, Wilson EM, Cappalonga JL, Rotwein P. Insulin-like growth factor-medi-ated muscle differentiation: collaboration between phosphatidylinositol 3-kinase-Akt-signaling pathways and myogenin. J Biol Chem 276: 39264–39270, 2001.

540. Tzahor E. Heart and craniofacial muscle development: a new developmental theme ofdistinct myogenic fields. Dev Biol 327: 273–279, 2009.

541. Uezumi A, Fukada S, Yamamoto N, Takeda S, Tsuchida K. Mesenchymal progenitorsdistinct from satellite cells contribute to ectopic fat cell formation in skeletal muscle.Nat Cell Biol 12: 143–152, 2010.

542. Ustanina S, Carvajal J, Rigby P, Braun T. The myogenic factor Myf5 supports efficientskeletal muscle regeneration by enabling transient myoblast amplification. Stem Cells25: 2006–2016, 2007.

543. Valadi H, Ekstrom K, Bossios A, Sjostrand M, Lee JJ, Lotvall JO. Exosome-mediatedtransfer of mRNAs and microRNAs is a novel mechanism of genetic exchange be-tween cells. Nature Cell Biol 9: 654–659, 2007.

544. Van den Beld AW, de Jong FH, Grobbee DE, Pols HA, Lamberts SW. Measures ofbioavailable serum testosterone and estradiol and their relationships with musclestrength, bone density, and body composition in elderly men. J Clin Endocrinol Metab85: 3276–3282, 2000.

545. Van Rooij E, Quiat D, Johnson BA, Sutherland LB, Qi X, Richardson JA, Kelm RJ Jr,Olson EN. A family of microRNAs encoded by myosin genes governs myosin expres-sion and muscle performance. Dev Cell 17: 662–673, 2009.

546. Van Rooij E, Sutherland LB, Qi X, Richardson JA, Hill J, Olson EN. Control of stress-dependent cardiac growth and gene expression by a microRNA. Science 316: 575–579, 2007.

547. Vandenburgh HH, Karlisch P, Shansky J, Feldstein R. Insulin and IGF-I induce pro-nounced hypertrophy of skeletal myofibers in tissue culture. Am J Physiol Cell Physiol260: C475–C484, 1991.

548. Velleman SG, Li X, Coy CS, McFarland DC. The effect of fibroblast growth factor 2 onthe in vitro expression of syndecan-4 and glypican-1 in turkey satellite cells. Poult Sci87: 1834–1840, 2008.

549. Vertino AM, Taylor-Jones JM, Longo KA, Bearden ED, Lane TF, McGehee RE Jr,MacDougald OA, Peterson CA. Wnt10b deficiency promotes coexpression of myo-genic and adipogenic programs in myoblasts. Mol Biol Cell 16: 2039–2048, 2005.



550. Viguie CA, Lu DX, Huang SK, Rengen H, Carlson BM. Quantitative study of the effectsof long-term denervation on the extensor digitorum longus muscle of the rat. Anat Rec248: 346–354, 1997.

551. Villena J, Brandan E. Dermatan sulfate exerts an enhanced growth factor response onskeletal muscle satellite cell proliferation and migration. J Cell Physiol 198: 169–178,2004.

552. Volonte D, Liu Y, Galbiati F. The modulation of caveolin-1 expression controls satel-lite cell activation during muscle repair. FASEB J 19: 237–239, 2005.

553. Wakelam MJ. The fusion of myoblasts. Biochem J 228: 1–12, 1985.

554. Walker BE. An investigation of skeletal muscle regeneration with radioautography.Anat Rec 350, 1960.

555. Walsh K. Coordinate regulation of cell cycle and apoptosis during myogenesis. ProgCell Cycle Res 3: 53–58, 1997.

556. Warren GL, Hulderman T, Jensen N, McKinstry M, Mishra M, Luster MI, SimeonovaPP. Physiological role of tumor necrosis factor alpha in traumatic muscle injury. FASEBJ 16: 1630–1632, 2002.

557. Watt DJ, Morgan JE, Clifford MA, Partridge TA. The movement of muscle precursorcells between adjacent regenerating muscles in the mouse. Anat Embryol 175: 527–536, 1987.

558. Wehrman TS, von Degenfeld G, Krutzik PO, Nolan GP, Blau HM. Luminescentimaging of beta-galactosidase activity in living subjects using sequential reporter-en-zyme luminescence. Nat Methods 3: 295–301, 2006.

559. Weinberg RA. The retinoblastoma protein and cell cycle control. Cell 81: 323–330,1995.

560. Wen Y, Bi P, Liu W, Asakura A, Keller C, Kuang S. Constitutive notch activationupregulates pax7 and promotes the self-renewal of skeletal muscle satellite cells. MolCell Biol 32: 2300–2311, 2012.

561. Weyman CM, Wolfman A. Mitogen-activated protein kinase kinase (MEK) activity isrequired for inhibition of skeletal muscle differentiation by insulin-like growth factor 1or fibroblast growth factor 2. Endocrinology 139: 1794–1800, 1998.

562. Whalen RG, Harris JB, Butler-Browne GS, Sesodia S. Expression of myosin isoformsduring notexin-induced regeneration of rat soleus muscles. Dev Biol 141: 24–40, 1990.

563. Wheeler EF, Bothwell M. Spatiotemporal patterns of expression of NGF and thelow-affinity NGF receptor in rat embryos suggest functional roles in tissue morpho-genesis and myogenesis. J Neurosci 12: 930–945, 1992.

564. White JD, Scaffidi A, Davies M, McGeachie J, Rudnicki MA, Grounds MD. Myotubeformation is delayed but not prevented in MyoD-deficient skeletal muscle: studies inregenerating whole muscle grafts of adult mice. J Histochem Cytochem 48: 1531–1544, 2000.

565. Williams AH, Liu N, van Rooij E, Olson EN. MicroRNA control of muscle develop-ment and disease. Curr Opin Cell Biol 21: 461–469, 2009.

566. Wilson A, Trumpp A. Bone-marrow haematopoietic-stem-cell niches. Nat Rev Immu-nol 6: 93–106, 2006.

567. Wokke JH, Van den Oord CJ, Leppink GJ, Jennekens FG. Perisynaptic satellite cells inhuman external intercostal muscle: a quantitative and qualitative study. Anat Rec 223:174–180, 1989.

568. Woodley DT, Rao CN, Hassell JR, Liotta LA, Martin GR, Kleinman HK. Interactions ofbasement membrane components. Biochim Biophys Acta 761: 278–283, 1983.

569. Wozniak AC, Anderson JE. Nitric oxide-dependence of satellite stem cell activationand quiescence on normal skeletal muscle fibers. Dev Dyn 236: 240–250, 2007.

570. Wu Z, Luby-Phelps K, Bugde A, Molyneux LA, Denard B, Li WH, Suel GM, GarbersDL. Capacity for stochastic self-renewal and differentiation in mammalian spermato-gonial stem cells. J Cell Biol 187: 513–524, 2009.

571. Wu Z, Woodring PJ, Bhakta KS, Tamura K, Wen F, Feramisco JR, Karin M, Wang JY,Puri PL. p38 and extracellular signal-regulated kinases regulate the myogenic programat multiple steps. Mol Cell Biol 20: 3951–3964, 2000.

572. Yablonka-Reuveni Z, Rivera AJ. Temporal expression of regulatory and structuralmuscle proteins during myogenesis of satellite cells on isolated adult rat fibers. Dev Biol164: 588–603, 1994.

573. Yablonka-Reuveni Z, Rudnicki MA, Rivera AJ, Primig M, Anderson JE, Natanson P. Thetransition from proliferation to differentiation is delayed in satellite cells from micelacking MyoD. Dev Biol 210: 440–455, 1999.

574. Yaffe D. Cellular aspects of muscle differentiation in vitro. Curr Top Dev Biol 4: 37–77,1969.

575. Yamada M, Sankoda Y, Tatsumi R, Mizunoya W, Ikeuchi Y, Sunagawa K, Allen RE.Matrix metalloproteinase-2 mediates stretch-induced activation of skeletal musclesatellite cells in a nitric oxide-dependent manner. Int J Biochem Cell Biol 40: 2183–2191, 2008.

576. Yamada M, Tatsumi R, Kikuiri T, Okamoto S, Nonoshita S, Mizunoya W, Ikeuchi Y,Shimokawa H, Sunagawa K, Allen RE. Matrix metalloproteinases are involved in me-chanical stretch-induced activation of skeletal muscle satellite cells. Muscle Nerve 34:313–319, 2006.

577. Yan D, Dong Xda E, Chen X, Wang L, Lu C, Wang J, Qu J, Tu L. MicroRNA-1/206targets c-Met and inhibits rhabdomyosarcoma development. J Biol Chem 284: 29596–29604, 2009.

578. Yang SY, Goldspink G. Different roles of the IGF-I Ec peptide (MGF) and mature IGF-Iin myoblast proliferation and differentiation. FEBS Lett 522: 156–160, 2002.

579. Yeow K, Phillips B, Dani C, Cabane C, Amri EZ, Derijard B. Inhibition of myogenesisenables adipogenic trans-differentiation in the C2C12 myogenic cell line. FEBS Lett506: 157–162, 2001.

580. Yildiz O. Vascular smooth muscle and endothelial functions in aging. Ann NY Acad Sci1100: 353–360, 2007.

581. Yoshimoto M, Chang H, Shiota M, Kobayashi H, Umeda K, Kawakami A, Heike T,Nakahata T. Two different roles of purified CD45�c-Kit�Sca-1�Lin� cells aftertransplantation in muscles. Stem Cells 23: 610–618, 2005.

582. Yu J, Vodyanik MA, Smuga-Otto K, Antosiewicz-Bourget J, Frane JL, Tian S, Nie J,Jonsdottir GA, Ruotti V, Stewart R, Slukvin II, Thomson JA. Induced pluripotentstem cell lines derived from human somatic cells. Science 318: 1917–1920, 2007.

583. Yu Y, Ge N, Xie M, Sun W, Burlingame S, Pass AK, Nuchtern JG, Zhang D, Fu S,Schneider MD, Fan J, Yang J. Phosphorylation of Thr-178 and Thr-184 in the TAK1T-loop is required for interleukin (IL)-1-mediated optimal NFkappaB and AP-1activation as well as IL-6 gene expression. J Biol Chem 283: 24497–24505, 2008.

584. Zammit PS, Golding JP, Nagata Y, Hudon V, Partridge TA, Beauchamp JR. Musclesatellite cells adopt divergent fates: a mechanism for self-renewal? J Cell Biol 166:347–357, 2004.

585. Zammit PS, Heslop L, Hudon V, Rosenblatt JD, Tajbakhsh S, Buckingham ME, Beau-champ JR, Partridge TA. Kinetics of myoblast proliferation show that resident satellitecells are competent to fully regenerate skeletal muscle fibers. Exp Cell Res 281: 39–49, 2002.

586. Zarnegar R, Michalopoulos GK. The many faces of hepatocyte growth factor: fromhepatopoiesis to hematopoiesis. J Cell Biol 129: 1177–1180, 1995.

587. Zhang JM, Wei Q, Zhao X, Paterson BM. Coupling of the cell cycle and myogenesisthrough the cyclin D1-dependent interaction of MyoD with cdk4. EMBO J 18: 926–933, 1999.

588. Zhang P, Wong C, Liu D, Finegold M, Harper JW, Elledge SJ. p21(CIP1) and p57(KIP2)control muscle differentiation at the myogenin step. Genes Dev 13: 213–224, 1999.

589. Zhao M, New L, Kravchenko VV, Kato Y, Gram H, di Padova F, Olson EN, Ulevitch RJ,Han J. Regulation of the MEF2 family of transcription factors by p38. Mol Cell Biol 19:21–30, 1999.

590. Zhong W, Chia W. Neurogenesis and asymmetric cell division. Curr Opin Neurobiol 18:4–11, 2008.



SATELLITE CELLS AND THE MUSCLE STEM CELL.pdf

Documents

Transcript of SATELLITE CELLS AND THE MUSCLE STEM CELL.pdf