Post on 20-Apr-2017
MOLECULAR BASIS OF GROWTH
INDIAN DENTAL ACADEMY
Leader in continuing dental education www.indiandentalacademy.com
www.indiandentalacademy.com
INDEX INTRODUCTION DEFINITION OF GROWTH GROWTH – EXTERNAL & MOLECULAR GFOWTH FACTORS MATRIXMETALLOPROTEINASES CYTOKINES BONE & TGF B(RECEPTORS & BIOLOGICAL
ACTIVITY) BMP – CLASSIFICATION PDGF,FGF, IGF CONCLUSION REFERENCES www.indiandentalacademy.com
www.indiandentalacademy.com
Craniofacial development
Facial development in the embryo involves the origin of the facial mesenchyme which arises from neural crest cells.
Unusually, they disrupt the ectodermal-mesodermal junction and migrate into the underlying tissue as ectomesenchymal cells.
www.indiandentalacademy.com
Among the derivatives of the cephalic neural crest cells are the maxilla, mandible, zygomatic, nasal bones, and bones of the cranial vault.
This process is presumed to be under the control of genes known as homeobox genes (1984), which endow neural crest cells (NCC) with a positional identity, which mediates aspects of craniofacial morphogenesis and patterning.
www.indiandentalacademy.com
Role of Homeobox genes
genes which are highly conserved throughout evolution of diverse organisms
fundamental in evolution of the specialised body parts of many animal species
www.indiandentalacademy.com
master genes of the head and face controlling patterning, induction, programmed cell death, and epithelial mesenchymal interaction during development of the craniofacial complex.
Those of particular interest in craniofacial development include the Hox group, Msx1 and Msx2 (muscle segment), Dlx (distalless), Otx (orthodontical), Gsc (goosecoid), and Shh (sonic hedgehog).
www.indiandentalacademy.com
Proteins encoded by these homeobox genes are transcription factors, which can switch genes on and off by activating or repressing gene expression, and therefore control other genes producing a co-ordinated cascade of molecular events which, in turn, control patterning and morphogenesis .
(Thesleff, 1995).
www.indiandentalacademy.com
At a cellular level this control is expressed through two main groups of regulatory proteins, the growth factor family and the steroid/ thyroid/retinoic acid super family (Evans, 1988), that regulate growth (Johnston and Bronsky, 1995).
These regulatory molecules in the mesenchyme are fibroblast growth factor (FGF), epidermal growth factor (EGF), transforming growth factor alpha (TGF ), transforming growth factor beta (TGF ), and bone morphogenetic proteins (BMPs)www.indiandentalacademy.com
MSX genes
Homologous to drosophila msh (muscle segment homeobox)
Murine msx family – msx1, msx2, msx3
Msx1 & 2 – transcriptina; repressors in cellular differentiation.
Along with dlx genes (activator)- mutually modulate their own transcriptional activities www.indiandentalacademy.com
Mice studies
Msx1- targeted disruption – loss of palatal shelves & maxillary bones , slight shortening of maxilla &/or mandible, no tooth development beyond bud stage
Msx2-(less well defined role in lip & palate development) deficiency-skull ossification defects & persistent calvarial foramen; mutation- craniofacial malformation- mandibular hypoplasia, cleft palate medial facial palate.
www.indiandentalacademy.com
DLX genes
Homologous to drosophila distal-less (Dll gene)
At least 6 members in mice- dlx1,2, 3, 5,6 &7 –encode transcription factors involved in orofacial patterning from neural crest cells
www.indiandentalacademy.com
Dlx 1 & 2-development of maxi arch (development of palatine & pterygoid bones of palate)
Dlx 1, 2, 3, 5, 6 –development of mandible
www.indiandentalacademy.com
LHX genes
(lim homeobox genes)
Tissue patterning/specification & differentiation of different cell types
Lhx 6 & 7- expressed prior to initiation of tooth formation in oral & odontogenic mesenchyme of maxi & mandi processes (mice)
www.indiandentalacademy.com
Lhx 8(L3)-differentially expressed in maxilla, mandible & ventral forebrain; continuous expression in mesenchyme during diff stages of palatogenesis- candidate gene for isolated nonsyndromic form of cleft palate
www.indiandentalacademy.com
PRRX gene
Paired related homeobox
Homologous to drosophila paired & gooseberry genes & to mouse pax3, 6 & 7 genes.
2 best studied prrx genes-prrx1(mhox/phox1/prx1/K2) & 2(S8/prx2)-similar expressions in cranium, branchial arches, body wall & limbs
www.indiandentalacademy.com
Prrx 1- detected in most of the ectoderm including brain precursors- inactivation (mice)- microcephaly, low set ears, pointed snout, cleft palate & mild mandibular hypoplasia.
Prrx 2 – no expression in developing brain – inactivation is compensated by prrx 1 functionality
www.indiandentalacademy.com
GOOSECOID genes
Involved in the final stages of formation of craniofacial structures like the ear, nose & mouth.
Disruption – lower mandible & associated musculature including tongue, nasal cavity, nasal pits, malleus & external auditory meatus, malformation of various bones of base of skull (e.g;palatine)
www.indiandentalacademy.com
RYK gene
Related to tyrosine kinases (receptor proteins).
Rugulate diverse cellular functions-mitogenesis, differentiation & morphogenesis
www.indiandentalacademy.com
Disruption (mice study– Halford 2000) – some limb abnormalities with slightly smaller & more rounded cranial vault, shorter snout premaxillary/maxillary hypoplasia(flattened midface) & reduced mandible with clefting of palate.
www.indiandentalacademy.com
GROWTH FACTORS - FGF
FUNCTIONS – angiogenesis, wound healing, embryonic development & malignant transformation, regulate cell proliferation, differentiation & migration in different tissues
At least 7 members are expressed in the developing face – Fgf 1,2,4,5,8,9 & 12
www.indiandentalacademy.com
Receptors – FGFRs – fgfrs 1,2 & 3- expressed in development of face & later associated with some regions of chondrogenesis
FGFR activating mutations – embryonic lethality & limb & craniofacial abnormalities such as CP & reduced maxillary bone (Apert, Crouzon syndromes)
www.indiandentalacademy.com
PDGF & PDGFR 2 RECEPTORS – pdgfr-alpha & pdgfr-beta
Pdgfs & Pdgfrs –regulatory roles in development of CNS, vascular system, in maintenance of tissue homeostasis & in wound healing along with imp role in palate development
Homozygous knockout of Pdgfra – midline defects & underdevelopment of face with absence of some facial bones (Soriano 1997)
www.indiandentalacademy.com
TGF TGF ALPHA & BETA-contribute to facial
development, especially palate formation.
Tgf A – some human studies indicate a positive association between tgfa & CL with/without CP(Ardinger 1989)
Rather than a necessary & sufficient determinant , it has been postulated that tgfa acts as a modifier gene.
www.indiandentalacademy.com
TGF Beta superfamily
Tgf beta 1,2,3,4 & 5 and more distantly related bone morphogenetic proteins
Tgh beta 1,2, 3-expressed in early embryogenesis & are associated later with some regions of skeletal development
www.indiandentalacademy.com
Depletion of tgf beta 3(Brunet 1995)-prevents in vitro fusion of palatal shelves.
Critical role of tgf beta2& 3 in molecular control of orofacial clefting (Lidral, 1998, Sanford 1997)
www.indiandentalacademy.com
EGF & EGFR
Necessary for normal craniofacial development.
Increases matrix metalloproteinases(MMPs) secretion (downstream signal transduction effector)
www.indiandentalacademy.com
Mice studies –(Miettinen 1999) egfr knockout-facial mediolateral defects like narrow & elongated snouts, underdeveloped lower jaws, CP & diminished secretion of MMPs in palatal shelf tissues
www.indiandentalacademy.com
GABA
Major inhibitory neurotransmitter with many critical functions as an intercellular signaling molecule in the CNS
Receptors – gaba A,B & C. gaba A can be modulated by steroids, benzodiazepines.
www.indiandentalacademy.com
Gaba- capable of promoting survival, differentiation & migration of embryonic neurons
Mice studies(Miller 1975, Homanics 1997) Abberations in gaba/gaba A – induces clefting of palate
www.indiandentalacademy.com
Growth factors
This collective term originally referred to substances that promote cell growth.
www.indiandentalacademy.com
The genes and mechanisms of our body are tirelessly operating day in, day out, and are part of a long interconnected chain of reactions that make our body work.
There are various factors involved that affect growth. It is the genetic coding of our bodies that determine the way we are and how we work, with the external environment either emphasizing or inhibiting the effectiveness of some of these genes.
www.indiandentalacademy.com
Growth factors
Introduction Growth factors are proteins that bind to
receptors on the cell surface, with the primary result of activating cellular proliferation and/or differentiation. Many growth factors are quite versatile, stimulating cellular division in numerous different cell types; while others are specific to a particular cell-type.
www.indiandentalacademy.com
Growth factors comprises of molecules that function as growth stimulators but also as growth inhibitors (sometimes referred to as negative growth factors ), factors that stimulate cell migration or as chemotactic agents or inhibit cell migration or invasion of tumor cells, factors that modulate differentiated functions of cells, factors involved in apoptosis , factors involved in angiogenesis , or factors that promote survival of cells without influencing growth and differentiation.
www.indiandentalacademy.com
Growth factors are polypeptides that belongs to a number of families.
Cell surface receptors capture them. Upon capturing receptor interacts with
membrane and cytoplasmic bound components to bring about alteration in gene expression of a cell.
Thus a growth factor is an inductive agent.
www.indiandentalacademy.com
A growth factor produced by one cell and acting on another is described as paracrine regulation.
Whereas the process of a cell that recaptures its own product is known as autocrine regulation.
Few growth factors act during embryogenesis.
www.indiandentalacademy.com
By contrast, the retinoic acid family freely enters a cell to complex with intracellular receptors, which eventually affect gene expression.
Both growth factors and the retinoids regulate the expression of the homeobox genes, which, in turn, regulate the expression of growth factors(role of regulatory loops in development).
www.indiandentalacademy.com
Epidermal Growth Factor (EGF) Platelet-Derived Growth Factor (PDGF) Fibroblast Growth Factors (FGFs) Transforming Growth Factors-b (TGFs-b) Transforming Growth Factor-a (TGF-a) Erythropoietin (Epo) Insulin-Like Growth Factor-I (IGF-I) Insulin-Like Growth Factor-II (IGF-II) Interleukin-1 (IL-1) Interleukin-2 (IL-2) Interleukin-6 (IL-6) Interleukin-8 (IL-8) Tumor Necrosis Factor-a (TNF-a) Tumor Necrosis Factor-b (TNF-b) Interferon-g (INF-g) Colony Stimulating Factors (CSFs) www.indiandentalacademy.com
Osteoblasts synthesize and regulate the deposition and mineralization of the extracellular matrix of bone. Systemic and locally active hormones, growth factors, ions, lipid metabolites and steroids are regulators of osteoblastic activity and/or differentiation.
www.indiandentalacademy.com
Members of the transforming growth factor beta (TGF-b) family, particularly TGF-b and the bone morphogenetic proteins (BMPs) are important to bone homeostasis. These factors modulate osteoblast proliferation and differentiation .
www.indiandentalacademy.com
Matrixmetalloproteinases
Extracellular matrix degrading metallo enzymes are known collectively as Matrixmetalloproteinases(MMPs).
Tissue inhibitors of metalloproteinases(TIMPs).
Depend on Zn²+ and Ca²+ for activity.
www.indiandentalacademy.com
Matrixmetalloproteinases
Rather than being primarily involved in matrix degradation MMPs have equally or more important roles as efficient processing enzymes of many bioactive mediators such as cytokines, chemokines, growth factors, their receptors and specific matrix protein anchors for these molecules.
www.indiandentalacademy.com
Cytokines
Includes a family of molecules which are small proteins with either paracrine or endocrine functions which are involved in local inflammation or immunoregulation.
Within this definition growth factors could be included.
www.indiandentalacademy.com
Cytokines.
Cytokines are a unique family of growth factors.
Secreted primarily from leukocytes, cytokines stimulate both the humoral and cellular immune responses, as well as the activation of phagocytic cells.
Lymphokines Monokines.
www.indiandentalacademy.com
Cytokines
A large family of cytokines are produced by various cells of the body.
Many of the lymphokines are also known as interleukins (ILs), since they are not only secreted by leukocytes but also able to affect the cellular responses of leukocytes.
www.indiandentalacademy.com
Cytokines
Specifically, interleukins are growth factors targeted to cells of hematopoietic origin.
The list of identified interleukins grows continuously with the total number of individual activities now at 22.
www.indiandentalacademy.com
www.indiandentalacademy.com
Factor Principal Source Primary Activity
Comments
PDGF platelets, endothelial cells, placenta
promotes proliferation of connective tissue, glial and smooth muscle cells
two different protein chains form 3 distinct dimer forms; AA, AB and BB
www.indiandentalacademy.com
Factor Principal Source Primary Activity
EGF submaxillary gland, Brunners gland
promotes proliferation of mesenchymal, glial and epithelial cells
www.indiandentalacademy.com
Factor Principal Source Primary Activity
Comments
TGF-a common in transformed cells
may be important for normal wound healing
related to EGF
www.indiandentalacademy.com
Factor Principal Source
Primary Activity Comments
FGF wide range of cells; protein is associated with the ECM
promotes proliferation of many cells; inhibits some stem cells; induces mesoderm to form in early embryos
at least 19 family members, 4 distinct receptors
www.indiandentalacademy.com
Factor Principal Source
Primary Activity Comments
NGF promotes neurite (axites & dendrites) outgrowth and neural cell survival
several related proteins first identified as proto-oncogenes; trkA (trackA), trkB, trkC
www.indiandentalacademy.com
Factor Principal Source
Primary Activity Comments
Erythropoietin kidney promotes proliferation and differentiation of erythrocytes
www.indiandentalacademy.com
Factor Principal Source
Primary Activity Comments
TGF-b activated TH1 cells (T-helper) and natural killer (NK) cells
anti-inflammatory (suppresses cytokine production and class II MHC expression), promotes wound healing, inhibits macrophage and lymphocyte proliferation
at least 100 different family members
www.indiandentalacademy.com
Factor Principal Source Primary Activity
Comments
IGF-I primarily liver promotes proliferation of many cell types
related to IGF-II and proinsulin, also called Somatomedin C
www.indiandentalacademy.com
Factor Principal Source Primary Activity
Comments
IGF-II variety of cells promotes proliferation of many cell types primarily of fetal origin
related to IGF-I and proinsulin
www.indiandentalacademy.com
Factors modulating growth, chemotactic behavior and/or functional activities of smooth muscle cells include Activin A , Adrenomedullin , aFGF, ANF , Angiogenin , Angiotensin-2 , Betacellulin , bFGF , CLAF , ECDGF (endothelial cell-derived growth factor ), ET (Endothelins ), Factor X , Factor Xa , HB-EGF , Heart derived inhibitor of vascular cell proliferation , IFN-gamma , IL1 , LDGF (Leiomyoma-derived growth factor ), MDGF (macrophage-derived growth factor , monocyte-derived growth factor ), Oncostatin M , PD-ECGF , PDGF , Prolactin , Protein S , SDGF (smooth muscle cell-derived growth factor ), SDMF (Smooth muscle cell-derived migration factor ), Tachykinins , TGF-beta , Thrombospondin .
www.indiandentalacademy.com
Factors modulating growth, chemotactic behavior and/or functional activities of vascular endothelial cells include AcSDKP , aFGF , ANF , Angiogenin , angiomodulin , Angiotropin , AtT20-ECGF , B61 , bFGF , bFGF inducing activity , CAM-RF , ChDI , CLAF , ECGF , ECI , EDMF , EGF , EMAP-2 , Neurothelin , Endostatin , Endothelial cell growth inhibitor , Endothelial cell-viability maintaining factor , Epo , FGF-5 , IGF-2 , HBNF , HGF , HUAF , IFN-gamma , IL1 , K-FGF , LIF , MD-ECI , MECIF , NPY , Oncostatin M , PD-ECGF , PDGF , PF4 , PlGF , Prolactin , TNF-alpha , TNF-beta , Transferrin , VEGF .
www.indiandentalacademy.com
A mind once stretched by a new idea, never regains its original dimensions.
www.indiandentalacademy.com
Bone – enormous reservoir
1.PDGF2.IGF – I IGF – II3.BMP (PART OF TGF-β FAMILY) Binding proteins to keep these factors within
bone itself.
www.indiandentalacademy.com
Transforming Growth Factors-β (TGFs- β)
ALTERNATIVE NAMES CIF-B ( cartilage inducing factor B =
TGF-beta-2 ) DIF ( differentiation-inhibiting factor ) DSF ( decidual suppressor factor =
TGF-beta-2 ) EIF ( Epstein-Barr virus inducing factor ) EGI (epithelial growth inhibitor )
www.indiandentalacademy.com
MDGF ( milk-derived growth factor ) MGF ( milk growth factor ) MGF-a ( milk growth factor =TGF-beta-2 ) MGF-b ( milk growth factor =TGF-beta-1 ) TCGF ( transformed cell growth factor ) TGI ( tissue-derived growth inhibitor ) TIF-1 ( tumor inducing factor-1 ).
www.indiandentalacademy.com
Sources of TGFs- β
Found predominantly in spleen and bone tissues. Platelets - milligrams of TGF-beta/ kg. other tissues - microgram TGF/kg. Human milk – MGF. Synthesized also by - macrophages(TGF-beta-1 ),
lymphocytes(TGF-beta-1 ), endothelial cells(TGF-beta-1 ), keratinocytes(TGF-beta-2 ), granulosa cells(TGF-beta-2 ), chondrocytes (TGF-beta-1 ), glioblastoma cells(TGF-beta-2 ), leukemia cells(TGF-beta-1 ).
www.indiandentalacademy.com
Inducers of TGFs- β
secretion can be induced by a no. of different stimuli including: steroids, retinoids, EGF , NGF , activators of lymphocytes, vitamin D3 , and IL1 .
www.indiandentalacademy.com
Inhibitors of TGFs- β
The synthesis of TGF-beta can be inhibited by: EGF , FGF , dexamethasone, calcium, retinoids and follicle stimulating hormone .
TGF-beta also influences the expression of its own gene and this may be important in Wound healing .
www.indiandentalacademy.com
TGFs- β
With the extracellular matrix as a complex with betaglycan and decorin.
Stored in a biologically inactive form. The exact molecular mechanisms underlying
its release from these reservoirs is unknown.
www.indiandentalacademy.com
PROTEIN CHARACTERISTICS Five isoforms TGF-beta-1 , TGF-beta-2 , TGF-beta-3 ,
TGF-beta-4 , TGF-beta-5 . They are not related to TGF-alpha . Their amino acid sequences display
homologies on the order of 70-80 percent. TGF-beta-1 - prevalent form.
www.indiandentalacademy.com
PROTEIN CHARACTERISTICS The biologically active forms of all isoforms
are disulfide-linked homodimers. Sometimes hetrodimers. The isoforms of TGF-beta arise by proteolytic
cleavage of longer precursors.
www.indiandentalacademy.com
PROTEIN CHARACTERISTICS Isoforms isolated from different species are
evolutionarily closely conserved and have sequence identities on the order of 98 percent.
Mature human, porcine, simian and bovine TGF-beta-1 are identical and differ from murine TGF-beta-1 in a single amino acid position.
Carboxy terminal end and Amino terminal end of precursor.
www.indiandentalacademy.com
Biosynthesis and processing of mature Biosynthesis and processing of mature TGF-betaTGF-beta (dark blue) (dark blue)
www.indiandentalacademy.com
L - TGF Almost all forms of TGF-β are released as
biologically inactive forms that are known also as L-TGF ( latent TGF ).
Latent forms are complexes of TGF-β, an aminoterminal portion of the TGF-beta precursor, designatedTGF-LAP ( TGF-latency associated peptide ), and a specific binding protein, known as LT-BP ( latent TGF binding protein).
www.indiandentalacademy.com
L-TGF
L-TGF - localized at the cell surface by binding to the mannose-6-phosphate/IGF-2 receptor.
Biologically active TGF-beta results after dissociation from the LAP complex.
The nature of the activation mechanism of L-TGF in vivo is unclear.
Direct cell-to-cell contacts, proteases, specifically plasmin, transglutaminases Thrombospondin .
www.indiandentalacademy.com
Alpha2M/TGF-beta complexes The main fraction of the factor in the serum is
covalently attached to one of the acute phase proteins , Alpha-2-Macroglobulin (Alpha2M).
Alpha2M/TGF-beta complexes are believed to represent TGF-beta molecules released by platelets after tissue injuries and destined to degradation.
www.indiandentalacademy.com
Mutant TGF- β
Mutant forms of TGF- β have been created. They form wild-type/mutant heterodimers
deficient in assembly or processing. Such mutants behave as dominant negative
mutants and are useful in investigation of the role of TGF- β in normal and pathological conditions.
www.indiandentalacademy.com
GENE STRUCTURE
The different isoforms of TGF-β are encoded by different genes.
All genes have a length of more than 100 kb and contain seven exons.
The genes map to different chromosomes.
www.indiandentalacademy.com
GENE STRUCTURE The TGF-beta-1 gene maps to human chromosome
19q13. The TGF-beta-2 gene maps to human chromosome 1q41. The TGF-beta-3 gene maps to human chromosome
14q24. These genes are expressed differentially. The TGF-beta-3 gene is expressed strongly in embryonic
heart and lung tissues but only marginally in liver, spleen, and kidney tissues. TGF-beta-1 is expressed strongly in spleen tissues.
www.indiandentalacademy.com
RELATED FACTORS
TGF-beta is the prototype of a protein family known as the TGF-beta superfamily.
This family includes Inhibins , Activin A , MIS (Müllerian inhibiting substance ), BMP (bone morphogenetic proteins ), dpp (decapentaplegic ) and Vg-1 .
www.indiandentalacademy.com
RECEPTORS
An entire family of glycoprotein receptors for TGF-beta has emerged.
Some of these proteins do not bind TGFbeta-related factors belonging to the TGF-beta family.
Type-1 receptors (hematopoietic progenitor cells) and type-2 receptors.
www.indiandentalacademy.com
RECEPTOR AFFINITY
Individual TGF-b isotypes - varying affinities. E.g., TGF-beta-1 binds approximately tenfold
better than TGF-beta-2. Expression of the TGF-beta receptors is
decreased by EGF (Receptor transmodulation).
In endothelial cells the expression of the TGF-beta receptor is decreased by bFGF .
www.indiandentalacademy.com
RECEPTORS
Almost all types of cells express, type-3 receptor.
This receptor type is not expressed in primary epithelial, endothelial, and lymphoid cells .
The type-3 receptor is a proteoglycan (Betaglycan), binds TGF-beta-1 and TGF-beta-2 equally well.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Not species-specific. TGF-beta-2 is the only variant that does not
inhibit the growth of endothelial cells. Most pronounced differences in the TGF-beta
isoforms is their spatially and temporally distinct expression of both the mRNAs and proteins in developing tissues, regenerating tissues, and in pathologic responses.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
TGF-beta is the most potent known growth inhibitor for normal and transformed epithelial cells, endothelial cells, fibroblasts, neuronal cells, lymphoid cells and other hematopoietic cell types (CFU-S ), hepatocytes, and keratinocytes.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Inhibits the proliferation of T-lymphocytes. Inhibits the growth of natural killer cells in
vivo. Deactivates macrophages. Blocks the antitumor activity mediated in vivo
by IL2 and transferred lymphokine-activated or tumor infiltrating lymphocytes .
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Inhibits the growth of immature hematopoietic progenitor cells .
In particular growth of CFU-GEMM . Inhibits megakaryocytopoiesis. Antagonizes the biological activities of EGF ,
PDGF , aFGF and bFGF . Latent form of TGF-beta is a strong inhibitor
of erythroleukemia cell lines.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
The extent of growth inhibition induced by TGF-beta depends on the cell type, on the concentration of TGF-beta, and on the presence of other factors.
The growth-inhibitory activities of TGF-beta can be abolished by HGF (hepatocyte growth factor ).
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES At concentrations of 1-2 fg/cell - growth inhibition for
smooth muscle cells, fibroblasts, and chondrocytes. At higher concentrations - stimulation. This bimodal activity is mediated in part by PDGF . Low concentrations of TGF-beta - synthesis and
secretion of PDGF. Higher concentrations – lower expression of the
PDGF receptors and hence diminish the biological effects of PDGF .
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Overproduction of TGF-beta-1 by tumor cells - neovascularization and may help promote tumor development in vivo.
TGF-beta is an autocrine growth modulator for malignant gliomas.
It stimulates the growth of fibroblasts and osteoblasts in vivo and in vitro.
TGF-beta induces the synthesis of bone matrix proteins in osteoblasts.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Factors that promote bone resorption (IL1 , vitamin D3 , parathormone) induce the synthesis of TGF-beta in bone cells.
While calcitonin, an inhibitor of bone resorption, reduces the synthesis of TGF-beta.
It suppresses the expression of class II MHC antigens .
Microglial cells.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
TGF-beta stimulates the synthesis of the major matrix proteins including collagen, proteoglycans , glycosaminoglycans, fibronectin , integrins, Thrombospondin , osteonectin, osteopontin .
It inhibits degradation mainly by inhibiting the synthesis of neutral metalloproteinases and by increasing the synthesis of proteinase inhibitors.
www.indiandentalacademy.com
BIOLOGICAL ACTIVITIES
Involved in metastatic processes. It is responsible for the transformation of
epithelial cells into mesenchymal cells. Suppressive effects on the immune system . TGF-beta-1 is the most potent known
chemoattractant for neutrophils .
www.indiandentalacademy.com
CLINICAL USE AND SIGNIFICANCE It may be a potent regulator of Wound
healing and of bone fracture healing. Local application of TGF-beta has been
shown to accelerate wound repair. In combination with bone morphogenetic
protein-2 it causes development of ossification of the posterior longitudinal ligament of the cervical spine.
www.indiandentalacademy.com
CLINICAL USE AND SIGNIFICANCE The factor may be helpful in the treatment of
traumatic tissue injuries. Treatment of osteoporosis. Reverses age- or glucocorticoid-impaired
Wound healing even if given 24 hours before wounding.
www.indiandentalacademy.com
Bone morphogenetic protein (BMP) Responsible for osteoinductive activity in
bone matrix. Non-collagenous. Water soluble.
www.indiandentalacademy.com
Bone morphogenetic protein (BMP) The cellular and molecular events governing
bone formation in the embryo, healing of a fractured bone, and induced bone fusion follow a similar pattern.
Bone is unique of all the tissues. When injured, it heals by formation of new
bone.
www.indiandentalacademy.com
BMP-Introduction
The molecular and cellular processes that lead to the development of the skeletal structures within the embryo are very similar to the cascades that occur in the healing process in an injured bone.
Thus, there is a common theme in the development of bone from primitive mesenchymal tissues to a well-structured, well-organized mature bone.
www.indiandentalacademy.com
BMP-Introduction
The ongoing remodeling process in an adult organism, which is exposed to external physical and hormonal influences, is also modulated through a similar molecular mechanism.
Intracartilaginous process. Intramembranous process.
www.indiandentalacademy.com
BMP-Introduction
Postfracture healing - intracartilaginous ossification process.
Very high concentration of BMP - intramembranous route may be taken.
It is unclear what factor(s) direct(s) one process as opposed to the other in the embryonic phase or during fracture healing.
www.indiandentalacademy.com
Stages of bone healing and remodelingStages of bone healing and remodeling
www.indiandentalacademy.com
GF and cytokines involved in generation of new bone
www.indiandentalacademy.com
History
Senn(1889)-Decalcified ox bone promotes healing of osteomyelitic defects.
Lexer(1908)-Necrotic bone tissue released stimulating factors that affect osteoblasts.
Polettini(1922)-Substance released from graft tissue resulted in differentiation of fibroblasts into bone and cartilage forming cells.
www.indiandentalacademy.com
History
Leriche(1928)-Ca materials contained in the graft tissue were the agents inducing new bone formation.
Levander(1934)-crude alcohol extracts of bone induce bone formation in muscle.
Sharrard and Collin(1961)-EDTA decalcified allograft induced spinal fusion in children.
www.indiandentalacademy.com
History
Urist(1965)-acid-decalcified bone induced ectopic bone in rat model. He coined the term "bone morphogenetic protein" or "osteogenic protein" .
Reddi and Sampath(1983)-crude but reproducible bioassay for BMP; bone matrix when dissociated from BMP ineffective in bone induction; reconstituted matrix effective.
www.indiandentalacademy.com
History
Johnson(1992)-first clinical study; purified human BMP successful clinically.
Creative biomolecules and genetic institute(1990s)-simultaneous gene sequencing for various BMP’s and related patent dispute.
Stryker Corp., Medtronic Sofamor Danek(2002)-FDA approval of OP-1 (BMP-7) for long bone defects and BMP-2 in a collagen carrier within a cage for anterior lumbar interbody fusions.
www.indiandentalacademy.com
Classification of BMP
Bone morphogenic proteins are members of TGF superfamily.
The BMP subfamily comprises more than 10 proteins, and newer ones are being discovered.
Several structural homologies between BMPs and TGF growth factors.
The amino acid sequence of BMPs is considered to be as old as 600 million years.
www.indiandentalacademy.com
Classification
Because of this conservation, human recombinant BMPs are highly effective in lower life forms, including fruit flies.
BMPs are synthesized as precursor proteins. The mature portion of the protein is located at
the carboxy terminal of the precursor molecule.
www.indiandentalacademy.com
www.indiandentalacademy.com
BMP
It is the only morphogen of all known growth factors that has the ability to transform connective tissue cell into osteoprogenitor cells.
Thus, it is not only a mitogen but can be a morphogen as well.
All other growth factors such as TGF, insulin-like growth factor, fibroblast growth factor, PDGF, and vascular endothelial growth factor all induce multiplication of cells but do not transform one cell type into the other.
www.indiandentalacademy.com
Signaling Mechanism of BMP
BMP receptors - Type I and Type II serine/threonine kinase proteins.
The binding of the ligand to the Types I and II serine/threonine kinase transmembrane receptors results in the activation of the signaling cascade.
Type II receptor kinase phosphorylates the Type I receptor.
www.indiandentalacademy.com
Signaling Mechanism of BMP
Type I receptor phosphorylates the intracytoplasmic signaling molecules Smads 1, 5, and 8.
Smads 1, 5, and 8 bind to Smad 4. Translocate into the cell nucleus. Activation of transcriptional factors for the
early BMP response genes.
www.indiandentalacademy.com
Dosage
Normal bone contains approximately 0.002 mg of BMP per kilogram of pulverized bone.
At a fracture site, presumably the BMP is released at a higher concentration.
www.indiandentalacademy.com
Dosage
The concentration required for ideal induced bone bridging in osseous defects depends on several factors.
- state of the organism in the evolutionary scale.
- type of defect. Bone induced under the influence of BMP
matures faster than natural healing of the bone.
www.indiandentalacademy.com
Other uses
Brain protective agent. kidneys are their primary source in the
human adult. In chronic renal disease levels of BMP are lower. systemic administration of BMP may restore some of the renal functions.
Local application for dialysis patient in osteodystrophy.
www.indiandentalacademy.com
BMP
Named because of their osteoinductive ability.
Role in embryonic and post embryonic development.
Signaling molecules in no. of tissues. Implicated in mesodermal patterning,
neurogenesis and organogenesis.
www.indiandentalacademy.com
BMP
BMP signaling pathways ↔ other growth factors ↔ hormonal signaling pathways.
Cross talk between them must be evaluated to avoid side effects of BMP based therapies.
www.indiandentalacademy.com
BMP
Mutations perturbing functions of BMP genes: BMP-5 gene mutation – short ear – abnormal
growth & patterning of skeletal structures and diminished repair of bone fracture.
www.indiandentalacademy.com
BMP
GDF-5 gene mutation – brachypod phenotype in mice & in autosomal recessive syndromes Hunter – Thompson chondrodysplasia in humans – shortening of appendicular skeleton and loss or abnormal development of some joints.
www.indiandentalacademy.com
BMP
BMP-2 & BMP-4 knock out mice die early in embryonic development, long before development of skeleton, because of defects in gastrulation.
BMP-7 knock out – eye and kidney defects, only mild skeletal defects.
www.indiandentalacademy.com
BMP
Several extracellular proteins regulate activities of BMP.
Noggin (BMP-4 & BMP-2) Follistatin (BMP-4 & BMP-7) Protein chordin
Astacin family of metalloproteases – cleaves chordin.
www.indiandentalacademy.com
Platelet-Derived Growth Factor (PDGF)ALTERNATIVE NAMES MDGF ( monocyte-derived growth factor ) ODGF ( osteosarcoma-derived growth factor ) SOURCES megakaryocytes stored in the alpha granules of
platelets(PDGF-BB/AB) released after cell activation of platelets for example
by thrombin .
www.indiandentalacademy.com
Sources
Unstimulated cells of osteoblastic lineage – PDGF-AA.
Other cells - macrophages, endothelial cells, fibroblasts, glial cells, astrocytes, myoblasts, smooth muscle cells, and a number of tumor cell lines.
Synthesis of PDGF can be induced by IL1 , IL6 , TNF-alpha , TGF-beta and EGF .
www.indiandentalacademy.com
Platelet-Derived Growth Factor (PDGF) PDGF is composed of two distinct
polypeptide chains, A and B, that form homodimers (AA or BB) or heterodimers (AB).
3 isoforms. PDGF receptors have intrinsic tyrosine
kinase activity.
www.indiandentalacademy.com
PDGF
PDGF-BB – binds to receptor – activates extracellular signal regulated kinase 1&2 – cellular proliferation by accelerating cell recycle & inducing quiescent cells into the proliferation portion of the cell cycle.
This effect is mediated by protein kinase B, a serine-threonine protein kinase.
TGFb1 – inhibits receptor autophosphorylation – neutralizes mitogenic effect of PDGF.
www.indiandentalacademy.com
PDGF
Proliferative responses to PDGF action are exerted on many mesenchymal cell types.
Two related receptors, called PDGFR alpha or PDGFR beta.
PDGF is not released into the circulation. The biological half-life is less than two
minutes after intravenous administration.
www.indiandentalacademy.com
PDGF
In the adult organism PDGF is involved in Wound healing processes.
The dimeric form of PDGF is mainly mitogenic for cells of mesenchymal origin while monomeric forms of PDGF are mainly chemotactic.
Disruption of PDGF signaling – perinatal lethality > 50%.
www.indiandentalacademy.com
PDGF
At low concentrations PDGF is a chemoattractant for fibroblasts.
PDGF is also chemotactic and activating for monocytes and neutrophils.
PDGF (alone and in combination) may be useful in promoting bone formation.
Promotes fracture healing. Doesn’t provides entire osteogenic properties
itself.
www.indiandentalacademy.com
PDGF
Osteoblasts can specifically bind and proliferate in response to PDGF.
Enhanced proliferation of both osteoblasts and osteoclasts.
In tissue culture, PDGF alone has not yet been proved to be osteoinductive in vivo.
In osteosarcoma – positive feedback loop. Platelet gel.
www.indiandentalacademy.com
PDGF
PDGF is chemotactic for both alkaline phosphatase positive and negative cells.
It may so contribute to recruitment of bone cells during remodeling and repair.
Used in implants and periodontal therapies. With or without IGF-I.
www.indiandentalacademy.com
Fibroblast Growth Factors (FGFs) 19 distinct members FGF1 (acidic-FGF, aFGF) and FGF2 (basic-
FGF, bFGF). Studies of human disorders & gene knock-out
studies in mice show the prominent role for FGFs is in the development of the skeletal system in mammals.
www.indiandentalacademy.com
aFGF
SOURCE Best sources of aFGF is brain tissue.
The mechanism underlying the release of aFGF is unknown.
www.indiandentalacademy.com
bFGF
SOURCES Almost all tissues of mesodermal and
neuroectodermal origin. Also in tumors derived from these tissues. Endothelial cells.
www.indiandentalacademy.com
FGF
potent inducers of mesodermal differentiation in early embryos.
Specific cell-surface receptors. 4 distinct receptor types identified as FGFR1
- FGFR4. Receptors has intrinsic tyrosine kinase
activity . autophosphorylation of the receptor is the
immediate response to FGF binding.
www.indiandentalacademy.com
FGF
FGFs also bind to cell-surface heparan-sulfated proteoglycans with low affinity
The FGF receptors are widely expressed in developing bone.
Mutations in the FGFR genes-autosomal dominant disorders of bone growth e.g. achondroplasia(FGFR3).
FGFR3 is predominantly expressed in quiescent chondrocytes & it restricts chondrocyte proliferation and differentiation.
www.indiandentalacademy.com
FGF
bFGF stimulates the growth of fibroblasts, myoblasts, osteoblasts, endothelial cells, chondrocytes, and many other cell types.
bFGF is not only a mitogen for chondrocytes but also inhibits their terminal differentiation.
www.indiandentalacademy.com
FGF
Animals experiments with bFGF - promotes endosteal, but not periosteal, bone formation.
bFGF may thus be a potential agent for treatment of osteoporosis which may increase bone mass without causing outward deformation of the skeletal bones.
www.indiandentalacademy.com
FGF
Craniosynostosis syndromes have been shown to result from mutations in FGFR1, FGFR2 and FGFR3.
Sometimes the same mutation can cause two or more different craniosynostosis syndromes.
www.indiandentalacademy.com
Affected Receptor
Syndrome Phenotypes
FGFR1 Pfeiffer broad first digits, hypertelorism
FGFR2 Apert mid-face hypoplasia, fusion of digits
FGFR2 Beare-Stevenson
mid-face hypoplasia, corrugated skin
www.indiandentalacademy.com
Affected Receptor
Syndrome Phenotypes
FGFR2 Crouzon mid-face hypoplasia
FGFR2 Jackson-Weiss
mid-face hypoplasia, foot anamolies
FGFR2 Pfeiffer same as for FGFR1 mutations
www.indiandentalacademy.com
Affected Receptor
Syndrome Phenotypes
FGFR3 Crouzon mid-face hypoplasia, acanthosis nigricans
FGFR3 Non-syndromatic craniosynostosis
digit defects, hearing loss.
www.indiandentalacademy.com
Insulin-Like Growth Factor IGFs are single chain peptides. 2 isoforms (IGF-I and IGF-II). 40-50% homology with insulin. Still all 3 have
unique binding site to their receptors. IGF also has general activity (metabolic &
growth promoting) in many tissue types.
www.indiandentalacademy.com
IGF-I
SOURCE Mainly liver.
IGF – responsible for fetal and postnatal growth and development in general.
www.indiandentalacademy.com
IGF-I
IGF-I receptor gene – deleted mice died at birth putatively due to poor muscular development.
IGF-I: pre + postnatal development. IGF-II: prenatal stages only. IGF-I/ IGF-II ratio increases with age in many
tissues. Role in skeletal development and skeletal mass
maintenance and development of teeth.
www.indiandentalacademy.com
Insulin-Like Growth Factor-I (IGF-I) Called somatomedin C(considered as
circulating mediator of growth hormone). Primary protein involved in responses of cells
to growth hormone (GH) IGF-I is produced in response to GH and then
induces subsequent cellular activities, particularly on bone growth.
www.indiandentalacademy.com
IGF-I
IGF-I has autocrine and paracrine activities in addition to endocrine activities on bone.
Family of transmembrane IGF-I(tyrosine kinase), IGF-II(mannose-6-phosphate receptor) & insulin receptor.
Receptor has intrinsic tyrosine kinase activity. Plays role in general growth and maintenance of
body skeleton.
www.indiandentalacademy.com
Insulin-Like Growth Factor-II (IGF-II) Exclusively expressed in embryonic and
neonatal tissues. Following birth, the level of detectable IGF-II
protein falls significantly. The IGF-II receptor is identical to the
mannose-6-phosphate receptor.
www.indiandentalacademy.com
IGF
Osteoblast aging is associated with impaired production of the stimulatory components of the IGF-system, that may contribute to age-related decline in osteoblast functions.
Of all IFG binding proteins, IGFBP-5 is abundant in bone matrix.
www.indiandentalacademy.com
IGF
IGF-I & II are potent survival factors for fibroblasts, hematopoetic cells, cardiac muscle cells & pancreatic beta cell.
IGF-I has anti apoptotic activity in these cell types and in certain tumors.
Autocrine loop – tumor promoting effect. IGF-I has chemotactic effect on osteoblasts
in a dose dependent manner. IGF-II effects only at lowest conc.
www.indiandentalacademy.com
IGF
It promotes expression of bone specific protein e.g. bone sialoprotein, and osteopontin.
In vivo, systemic application of IGF-I – rapidly activated bone turnover – increase in serum osteocalcin, increased collagen marker of bone formation, and an increased urinary ratio of Ca/creatinine.
www.indiandentalacademy.com
IGF
In inflammatory tissue (e.g. fracture repair) – IL1 increases IGF-I production.
IGF-I activity can be suppressed by NSAIDs e.g. indomethacin.
www.indiandentalacademy.com
www.indiandentalacademy.com
Taken from the AJO-DO on CD-ROM (Copyright © 1997 AJO-DO), Volume 1993 Aug (121 - 131): Heritability of skeletodental relationships - King, Harris, and Tolley, Fig. 6.
-------------------------------- Fig. 6. The epigenetic
landscape (redrawn from Waddington82).
www.indiandentalacademy.com
Taken from the AJO-DO on CD-ROM (Copyright © 1997 AJO-DO), Volume 1981 Oct (366 - 375): Genetics, epigenetics, and causation - Moss, Fig. 5.
-------------------------------- Fig. 5. This figure shows how boundaries
might be analogously demonstrated to operate in the epigenetic regulation. If the ball at the top of the figure is taken to represent a cell, or a tissue, capable of rolling down the convoluted surface shown, then the developmental pathway will be regulated by the convex ridges of that inclined surface. These epigenetic boundaries will effectively prevent the ball from passing into adjacent grooves, and thus determine the developmental pathway. (Adapted from Waddington: New Patterns in Genetics and Development, New York, 1962, Columbia University Press.)
www.indiandentalacademy.com
The ball may be considered, for example, as a simple, totipotential cell with its complete, genomically encoded information regulatory of the full range of species-specific polypeptide synthesis. The subsequent history of this cell and its descendants is a function of which developmental pathway (or "Chreod") it moves along. Some initial epigenetic factor or process determines the initial path selected, at which time portions of the genome become, respectively, repressed and derepressed, so that initial cytodifferentiation occurs. This new state, or epigenetic environment, keeps the vital material moving along this particular pathway until another bifurcation point occurs (Fig. 5). Once again, the instantaneous epigenetic state regulates this decision, and a "catastrophic" event occurs; that is, a new structually higher-order state or path is evolved. These pathways become "deeper" (have higher "walls") as they progressively become hierarchically more complex. This represents, in such a model, the fact that there is an increasing ability to withstand lateral, homeostatic perturbations during "movement'' along the landscape. This movement is termed homeorrhesis. In this model, the selection of pathways is not genomically but epigenetically regulated. Yet the genomic information must be present to permit synthetic activity by the cell and is one type of intrinsic, necessary information needed for ontogenesis to occur. The constantly added epigenetic information is the other type of necessary causation required. There is no reason for conflict between the genomic and epigenetic hypotheses of ontogenetic regulation when it is perceived that they are interdependent, yet different, categories of necessary causes and that only their unity provides the sufficient condition for growth and development to occur.74
www.indiandentalacademy.com
(1) all of the extrinsic (extraorganismal) factors impinging on vital structures, including importantly mechanical loadings and electroelectric states and (2) all of the intrinsic (intraorganismal) biophysical, biomechanical, biochemical, and bioelectric microenvironmental events occurring on, in, and between individual cells, extracellular materials, and cells and extracellular substances.
As previously noted,99 epigenetic factors include (1) all of the extrinsic, extraorganismal, macroenvironmental factors impinging on vital structures (for example, food, light, temperature), including mechanical loadings and electromagnetic fields, and (2) all of the intrinsic, intraorganismal, biophysical, biomechanical, biochemical, and bioelectric microenvironmental events occuring on, in, and between individual cells, extracellular materials, and cells and extracellular substances.
www.indiandentalacademy.com
www.indiandentalacademy.com
www.indiandentalacademy.com
www.indiandentalacademy.com
AJO-DO:1993 Mar - Sandy, Farndale, and Meikle Simplified diagram depicting
interactions of molecules at focal contacts. The model depicts how the cytoskeleton is linked through the membrane glycoprotein integrin to the extracellular matrix. Many of the extracellular matrix proteins which are responsible for cell adhesion contain common peptide sequences as cell recognition sites. These sites are recognized by integrins that are a family of glycoproteins, which span the cell membrane from the cytoplasm to the extracellular matrix. Integrin does not bind directly to microfilament structures, such as actin, but is dependent on associated proteins for this function. Integrin binds to fibronectin in the extracellular matrix and to talin on the cytoplasmic surface. Actin and vinculin then bind to this talin-integrin complex.
www.indiandentalacademy.com
www.indiandentalacademy.com
From macro to - micro
www.indiandentalacademy.com
From macro to - micro
www.indiandentalacademy.com
www.indiandentalacademy.com
Terminology Cell – basic living, structural & functional unit of
body Chromosome – highly coiled & folded DNA molecule
combined with protein molecules, present in the nucleus of cell
Gene-The unit of heredity: one or more nucleic acid sequences incorporating information necessary for the generation of a particular peptide or RNA product
(AJODO-1997 : MOSS - Meier AE, editor. A is for . . . gene. Sci Med 1996;3:72.)
These are located on the chromosomeswww.indiandentalacademy.com
Terminology Apoptosis – physiological cell death occuring
in normal tissues/in diseased organs, not associated with inflammatory reactions
Chemotaxis – process of migration of cells towards an attractant
Ontogenesis - growth and development of the cell
www.indiandentalacademy.com
Growth 3 possibilities : Hypertrophy – increase in the size of cells Hyperplasia – increase in the number of cells Secretion of extra cellular material MAINLY SEEN IN HARD TISSUES – BONE,
CARTILAGE, TEETH Atrophy – diminished size and number of cells due
to extreme failure of development Interstitial growth – occurring at all points within the
tissue; hyperplasia primarily & hypertrophy secondarily, with/without secretion of ECM
IN SOFT TISSUES & UNCALCIFIED CARTILAGE www.indiandentalacademy.com
Resolving synthesis (AJODO-1997 : MOSS) Morphogenesis is regulated (controlled, caused) by
the activity of both genomic and epigenetic processes and mechanisms. Both are necessary causes; neither alone are sufficient causes; and only their integrated activities provides the necessary and sufficient causes of growth and development. Genomic factors are considered as intrinsic and prior causes; epigenetic factors are considered as extrinsic and proximate causes.
It is probable that ontogeny involves nonlinear processes and is not fully predictable; that is, growth and development, to a significant extent, exhibit both random behaviors and frequent perturbations. www.indiandentalacademy.com
Resolving synthesis - Complexity and self-organization The highly ordered morphological properties of adult
complex biological systems (for example, functional matrices and skeletal units) result from the operation of a series of spontaneous and self-organized ontogenetic processes and mechanisms.
Environmental factors play a decisive role in all ontogenetic processes. But it is the organism itself that, as an integrated system, dictates the nature of each and every developmental response . . . the living organism self-organizes on the basis of its own internal structuring, in continuous interaction with the environment in which it finds itself.
www.indiandentalacademy.com
Mechanotransduction (AJODO-1997 : MOSS) Occurs in single bone cells
All vital cells are "irritable" or perturbed by and respond to alterations in their external environment. Mechanosensing processes enable a cell to sense and to respond to extrinsic loadings by using the processes of mechanoreception and of mechanotransduction.
The former transmits an extracellular physical stimulus into a receptor cell; the latter transduces or transforms the stimulus's energetic and/or informational content into an intracellular signalwww.indiandentalacademy.com
There are several mechanotransductive processes, for example, mechanoelectrical and mechanochemical. Whichever are used, bone adaptation requires the subsequent intercellular transmission of the transduced signals.
When an appropriate stimulus parameter exceeds threshold values, the loaded tissue responds by the triad of bone cell adaptation processes…..trio of possible osteoblastic responses to loading (deposition, resorption, or maintenance of bone tissue)
www.indiandentalacademy.com
Osseous mechanotransduction is unique in four ways:
(1) Most other mechanosensory cells are cytologically specialized, but bone cells are not;
(2) one bone-loading stimulus can evoke three adaptational responses, whereas nonosseous processes generally evoke one;
(3) osseous signal transmission is aneural, whereas all other mechanosensational signals use some afferent neural pathways and,
(4) the evoked bone adaptational responses are confined within each "bone organ" independently, e.g., within a femur, so there is no necessary "interbone" or organismal involvement.
www.indiandentalacademy.com
1. Electromechanical As in most cells, the osteocytic plasma membrane
contains voltage-activated ion channels, and transmembrane ion flow may be a significant osseous mechanotransductive process.
Stretch-activated channels - Several types of deformation may occur in strained bone tissue. One of these involves the plasma membrane stretch-activated (S-A) ion channels, a structure found in bone cells, in many other cell types and significantly in fibroblasts. When activated in strained osteocytes, they permit passage of a certain sized ion or set of ions, including K+, Ca2+, Na+. Such ionic flow may, in turn, initiate intracellular electrical events.
www.indiandentalacademy.com
2. Electrokinetic Bound and unbound electric charges exist in
bone tissue, many associated with the bone fluid(s) in the several osseous spaces or compartments. Electrical effects in fluid-filled bone are not piezoelectric, but rather of electrokinetic, that is, streaming potential (SP) origin. The SP is a measure of the strain-generated potential (SGP) of convected electric charges in the fluid flow of deformed bone. The usually observed SPG of ±2 mV can initiate both osteogenesis and osteocytic action potentials.
www.indiandentalacademy.com
3. Electric field strength
Bone responds to exogenous electrical fields. Although the extrinsic electrical parameter is unclear, field strength may play an important role. A significant parallel exists between the parameters of these exogenous electrical fields and the endogenous fields produced by muscle activity.
www.indiandentalacademy.com
Gap junctions & Connected cellular network (CCN) Gap junctions are regions on the lateral surfaces of
cells where the gap between the adjoining plasma membranes is reduced from 20 nm to 2nm in width. Pits/holes may be present in these regions, permeable to small tracer particles
All bone cells, except osteoclasts, are extensively interconnected by gap junctions that form an osseous CCN. In these junctions, connexin is the major protein. Each osteocyte, enclosed within its mineralized lacuna, has many (n = ± 80) cytoplasmic (canalicular) processes, 15 mm long and arrayed three-dimensionally, that interconnect with similar processes of up to 12 neighboring cells. These processes lie within mineralized bone matrix channels (canaliculi). www.indiandentalacademy.com
Gap junctions are found where the plasma membranes of a pair of markedly overlapping canalicular processes meet. Gap junctions also connect superficial osteocytes to periosteal and endosteal osteoblasts. All osteoblasts are similarly interconnected laterally. Vertically, gap junctions connect periosteal osteoblasts with preosteoblastic cells, and these, in turn, are similarly interconnected.
In addition to permitting the intercellular transmission of ions and small molecules, gap junctions exhibit both electrical and fluorescent dye transmission. Gap junctions are electrical synapses, in contradistinction to interneuronal, chemical synapses, and, significantly, they permit bidirectional signal traffic, e.g., biochemical, ionic. www.indiandentalacademy.com
Mechanotransductively activated bone cells, e.g., osteocytes, can initiate membrane action potentials capable of transmission through interconnecting gap junctions.
The CCNs show oscillation, i.e.,reciprocal signaling (feedback) between layers. This attribute enables them to adjustively self-organize.
Gap junctions, permitting bidirectional flow of information, are the cytological basis for the oscillatory behavior of a CCN.
www.indiandentalacademy.com
Integrins A family of membrane integral proteins that
span the cell membrane from the cytoplasm to the extracellular matrix.
Many of the extracellular matrix proteins which are responsible for cell adhesion contain common peptide sequences as cell recognition sites, these sites are recognized by integrins.
Changes in cell shape produce a range of effects mediated by integrins and the cytoskeleton, which may be important in transducing mechanical deformation into a meaningful biologic response.
www.indiandentalacademy.com
Integrins A family of glycoproteins. These are
connected extracellularly with the macromolecular collagen of the organic matrix and intracellularly with the cytoskekeletal actin. The molecules of the latter, in turn, are connected to the nuclear membrane,
These aid in transmitting signals from the ECM directly to the intranuclear genome. This informational transfer between cells and ECM is dynamic, reciprocal, and continuous.
www.indiandentalacademy.com
Osteoblasts (AJO-DO:1993 Mar - Sandy, Farndale, and Meikle) They are now recognized as the cells that
control both the resorptive and the formative phases of the remodeling cycle, and receptor studies have shown them to be the target cells for resorptive agents in bone.
The osteoblast is perceived as a pivotal cell, controlling many of the responses of bone to stimulation with hormones and mechanical forces.
www.indiandentalacademy.com
Genes All somatic cells commonly share approximately
5000 different polypeptide chains, each specific cell type is characterized only by approximately 100 specific proteins. And it is claimed that "these quantitative (protein) differences are related to differences in cell size, shape and internal architecture”.
The encoding of the DNA exists in two families; the vastly preponderant "housekeeping" genes and the nonabundant "structural" genes. The former regulate the normal molecular synthesis of agents involved in (1) the common energetic (metabolic, respiratory) activities of all cells and, (2) the specific activities of special cell types (e.g., neurons, osteoblasts, ameloblasts etc.)www.indiandentalacademy.com
Prenatal craniofacial development is controlled by two interrelated, temporally sequential, processes: (1) initial regulatory (homeobox) gene activity and (2) subsequent activity of two regulatory molecular groups: growth factor families and steroid/thyroid/retinoic acid super-family. For example, "homeobox genes coordinate the development of complex craniofacial structures" and in "both normal and abnormal development, much of the regulation of the development of virtually all of the skeletal and connective tissue of the face is dependent on a cascade of overlapping activity of homeobox genes.“
www.indiandentalacademy.com
It is claimed that regulatory molecules can (1) "alter the manner in which homeobox genes coordinate cell migration and subsequent cell interactions that regulate growth" and (2) be involved in the "genetic variations causing, or contributing to, the abnormal development of relatively common craniofacial malformations . . . perhaps modifying Hox gene activity."
www.indiandentalacademy.com
www.indiandentalacademy.com
Thank you
For more details please visit www.indiandentalacademy.com