The Cell Biology of Synaptic Plasticity

DOI: 10.1126/science.1209236, 623 (2011);334 Science

, et al.Victoria M. HoThe Cell Biology of Synaptic Plasticity

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50. D. D. Bock et al., Nature 471, 177 (2011).51. V. Jain, H. S. Seung, S. C. Turaga, Curr. Opin. Neurobiol.

20, 653 (2010).Acknowledgments: We thank K. Briggman, H. Hess, J. Livet,

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10.1126/science.1209168

The Cell Biology of Synaptic PlasticityVictoria M. Ho,1 Ji-Ann Lee,2 Kelsey C. Martin2,3,4*

Synaptic plasticity is the experience-dependent change in connectivity between neurons thatis believed to underlie learning and memory. Here, we discuss the cellular and molecularprocesses that are altered when a neuron responds to external stimuli, and how these alterationslead to an increase or decrease in synaptic connectivity. Modification of synaptic componentsand changes in gene expression are necessary for many forms of plasticity. We focus onexcitatory neurons in the mammalian hippocampus, one of the best-studied model systemsof learning-related plasticity.

The circuitry of the human brain is com-posed of a trillion (1012) neurons and aquadrillion (1015) synapses, whose con-nectivity underlies all human perception, emo-tion, thought, and behavior. Studies in a rangeof species have revealed that the overall struc-ture of the nervous system is genetically hard-wired but that neural circuits undergo extensivesculpting and rewiring in response to a varietyof stimuli. This process of experience-dependentchanges in synaptic connectivity is called synap-tic plasticity.

Studies of synaptic plasticity have begun todetail the molecular mechanisms that underliethese synaptic changes. This research has exam-ined a variety of cell biological processes, in-cluding synaptic vesicle release and recycling,neurotransmitter receptor trafficking, cell adhe-sion, and stimulus-induced changes in gene ex-pression within neurons. Taken together, thesestudies have provided an initial molecular bio-logical understanding of how nature and nurturecombine to determine our identities. As a result,research on synaptic plasticity promises to pro-vide insight into the biological basis of manyneuropsychiatric disorders in which experience-dependent brain rewiring goes awry.

Here we focus on long-lasting forms of plas-ticity that underlie learning and memory. Weconsider, in turn, each component of the synapse:the presynaptic compartment, the postsynapticcompartment, and the synaptic cleft, and discuss

processes that undergo activity-dependent mod-ifications to alter synaptic efficacy. Long-lastingchanges in synaptic connectivity require newRNA and/or protein synthesis, and we discusshow gene expression is regulated within neu-rons.We concentrate on studies of learning-relatedplasticity at excitatory chemical synapses in therodent hippocampus because these provide ex-tensive evidence for the cell biological mecha-nisms of plasticity in the vertebrate brain. Space

constraints prevent us from addressing any sin-gle mechanism in depth; instead, our aim is toprovide a framework for understanding the cellbiology of synaptic plasticity.

Hippocampal Synaptic PlasticityThe successful study of the cell biology of syn-aptic plasticity requires a tractable experimentalmodel system. Ideally, such a model should con-sist of a defined population of identifiable neu-rons and be amenable to electrophysiological,genetic, and molecular cell biological manipu-lations. Awell-studiedmodel system for studyingplasticity in the adult vertebrate nervous systemis the rodent hippocampus (Fig. 1). Critical formemory formation, the anatomy of the hippo-campus renders it particularly suitable for electro-physiological investigation. It consists of threesequential synaptic pathways (perforant, mossyfiber, and Schaffer collateral pathways), eachwith discrete cell body layers and axonal anddendritic projections (Fig. 1). Synaptic plasticityhas been studied in all three hippocampal path-ways. Distinct stimuli elicit changes in synaptic

1Interdepartmental Program in Neurosciences, University ofCaliforniaLos Angeles (UCLA), BSRB 390B, 615 Charles E.Young Drive South, Los Angeles, CA 900951737, USA. 2De-partment of Biological Chemistry, UCLA, BSRB 310, 615Charles E. Young Drive South, Los Angeles, CA 900951737,USA. 3Department of Psychiatry and Biobehavioral Sciences,UCLA, BSRB 390B, 615 Charles E. Young Drive South, LosAngeles, CA 900951737, USA. 4Brain Research Institute,UCLA, BSRB 390B, 615 Charles E. Young Drive South, LosAngeles, CA 900951737, USA.

*To whom correspondence should be addressed. E-mail:[email protected]

Dentate gyrus

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Perforant (axons from entorhinal cortex)Mossy fiber (axons from dentate granule cells)Schaffer collateral (axons from CA3 pyramidal cells)

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Fig. 1. Hippocampal synaptic plasticity. The rodent hippocampus can be dissected and cut into transverseslices that preserve all three synaptic pathways. In the perforant pathway (purple), axons from theentorhinal cortex project to form synapses (yellow circles) on dendrites of dentate granule cells; in themossy fiber pathway (green), dentate granule axons synapse on CA3 pyramidal neuron dendrites; and inthe Schaffer collateral pathway (brown), CA3 axons synapse on CA1 dendrites. The dentate, CA3, and CA1cell bodies form discrete somatic layers (dark blue lines), projecting axons and dendrites into definedregions. Electrodes can be used to stimulate axonal afferents and record from postsynaptic follower cells,as illustrated for the Schaffer collateral (CA3-CA1) pathway.

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efficacy; high-frequency stimuli pro-duce synaptic strengthening calledlong-term potentiation (LTP), andlow-frequency stimulation producessynaptic weakening, called long-term depression (LTD). LTP andLTD can also be produced by spiketimingdependent plasticity, inwhichthe relative timing of pre- and post-synaptic spikes leads to changes insynaptic strength (1). Different pat-terns of stimulation elicit changes insynaptic strength that persist over var-ious time domains, with long-lastingforms, but not short-term forms, re-quiring new RNA and protein syn-thesis (2).

Hippocampal plasticity is studiedin in vivo and in vitro preparations.Implanted electrodes can be used tostimulate and record from hippocam-pal pathways in living animals. Thehippocampus can be dissected outof the brain and cut into 300- to 500-mm-thick transverse slices that canbe maintained and recorded fromfor hours (Fig. 1). Slices can alsobe kept as organotypic slice culturesfor weeks, preserving many aspectsof their architecture. Finally, hippo-campal neurons can be studied indissociated cultures, which are par-ticularly amenable to manipulationand dynamic imaging of individ-ual neurons and synapses. The de-velopment of genetically modifiedmice and vectors for acute manipu-lation of gene expression completea rich tool-kit for studies of the celland molecular biology of hippocampal synapticplasticity.

Presynaptic Mechanisms of PlasticityCommunication at chemical synapses involvesthe release of neurotransmitter from the presyn-aptic terminal, diffusion across the cleft, andbinding to postsynaptic receptors (Figs. 2 and3). Chemical neurotransmission is rapid (occur-ring in milliseconds) and highly regulated. Thepresynaptic terminal contains synaptic vesiclesfilled with neurotransmitter and a dense matrixof cytoskeleton and scaffolding proteins at thesite of release, the active zone. Varying the prob-ability of neurotransmitter release provides onemechanism for altering synaptic strength duringneuronal plasticity.

Synaptic vesicle release can be subdividedinto distinct steps, including vesicle mobilization,docking, priming, fusion, and recycling. Althougheach of these steps may be regulated by activity,we will highlight three: vesicle mobilization,docking, and priming.

Synapsins and synaptic vesicle mobilization.The population of synaptic vesicles within a pre-synaptic terminal exist in three states: the readily

releasable pool docked at the active zone; therecycling pool, which can be released with mod-erate stimulation; and the reserve pool, which isonly released in response to strong stimuli. A fam-ily of phosphoproteins called synapsins tethersynaptic vesicles to the actin cytoskeleton andto one another. Neuronal stimulation activateskinases that phosphorylate synapsins to modulatesynaptic vesicle tethering and thereby alter thenumber of synaptic vesicles available for release(3). Synapsin knockout mice have reduced re-serve pools of synaptic vesicles and demon-strate deficits in learning and memory as wellas various forms of plasticity (4), indicating thatactivity-dependent modulation of synaptic ves-icle mobilization is critical to neuronal and be-havioral plasticity.

RIM proteins and synaptic vesicle dockingand priming. For synaptic vesicles to becomefusion competent, they must undergo dockingand priming, in which vesicle and plasma mem-brane soluble NSF-attachment protein receptor(SNARE) proteins are brought into close contactto allow rapid fusion following calcium influx.The Rab3-interacting molecule (RIM) familyof proteins is critical for this process (5). As

large, multidomain proteins, RIMs act as scaf-folding proteins to cluster calcium channels in theactive zone (6) and interact with Munc-13 (7),a priming factor required for efficient SNAREcomplex formation and membrane fusion. RIMis a substrate for phosphorylation by protein ki-nase A (PKA) and is required for mossy fiberLTP (8).

Postsynaptic Mechanisms of PlasticityMost principle neurons in the brain are studdedwith membrane protuberances called dendrit-ic spines, which are the postsynaptic compart-ments. Spines are heterogeneous in shape (Fig.2), but consist of a bulbous head and a thinnerneck that connects the spine to the dendriticshaft; the size of the spine head and the vol-ume of the spine correlate with synaptic strength(9, 10), with large spine heads containing moreneurotransmitter receptors, reflecting greater syn-aptic strength. Spines serve as compartmental-ized signaling units, and the number and shapeof spines change during synaptic plasticity (11).At the ultrastructural level, the postsynaptic com-partment is characterized by an electron-densepostsynaptic density (PSD), which consists ofneurotransmitter receptors and an extensive net-work of scaffolding proteins.

Activation of postsynaptic kinases in thespine: CaMKII and PKMz. LTP and LTD induc-tion are both dependent on postsynaptic increasesin intracellular calcium, with LTP requiring largeincreases in calcium concentrations and LTDbeing dependent on smaller calcium increases.The increase in calcium activates multiple down-stream signaling enzymes, including the kinasescalcium/calmodulin-dependent protein kinase II(CaMKII) and protein kinase C (PKC).

LTP induction in the CA1 region of the hip-pocampus requires CaMKII activity (12, 13),and transgenic mice lacking the a isoform havedefective LTP and spatial learning (14, 15).CaMKII undergoes autophosphorylation in re-sponse to increases in Ca2+-bound calmodulin,which renders the kinase autonomously active.Neuronal activity also translocates CaMKII tothe PSD, where it can phosphorylate many PSDproteins, including glutamate receptors. The auto-phosphorylation of CaMKII is essential for LTPinduction and, perhaps, its maintenance (16) [butsee (17)].

The brain-restricted atypical PKC isoform,protein kinase M zeta (PKMz), is constitutivelyactive and thus phosphorylates targets in the ab-sence of extracellular stimulation. PKMzmRNAis targeted to dendrites where activity-dependentsignaling cascades regulate its local translationduring LTP and LTD (18). PKMz is sufficientand necessary for LTP maintenance and for themaintenance of long-term memories, and PKMzactivation may perpetuate synaptic plasticity andmemory (18, 19).

Activity-dependent modulation of post-synaptic glutamate receptors. The main excitatoryneurotransmitter in the brain is glutamate, which

Fig. 2. The ultrastructure of the synapse. Neurons communicatewith one another at chemical synapses. (A) Electron micrographfrom area CA1 in adult rat hippocampus. The CA1 dendritic shaftis colorized in yellow, the spine neck and head in green, thepresynaptic terminal in orange, and astroglial processes in blue.Scale bar, 0.5 mm. (B) Three-dimensional reconstruction of an8.5-mm-long dendrite (yellow) with the PSDs labeled in red. Notethe variation in spine and PSD size and shape. Scale cube, 0.5 mm3.Reproduced with permission from Elsevier (63).

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activates several postsynaptic receptors. Two typesof ionotropic glutamate receptorsa-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid(AMPA) and N-methyl D-aspartate (NMDA)have central roles in hippocampal synaptic plas-ticity. Both are ligand-gated ion channels andhave unique properties that subserve differentphases of synaptic plasticity. NMDA-type glu-tamate receptors (NMDARs) are calcium per-meable and, when activated, allow an influx ofcalcium needed for the induction of LTP. How-ever, NMDARs do not conduct current at restingpotentials because their channel pores are blockedby magnesium cations. Consequently, NMDARshave been called coincidence detectors because,to conduct current, they require both presynaptictransmitter release as well as postsynaptic de-polarization to relieve the magnesium block.AMPA-type glutamate receptors (AMPARs) areimportant for the expression and maintenanceof LTP. Unlike NMDARs, AMPARs can be ac-

tivated by ligand binding at resting potentialsto allow current flow. Increased conductancethrough AMPARs is responsible for the increasein synaptic strength during NMDAR-dependentLTP at CA1 synapses.

Given the importance of AMPARs in de-termining synaptic strength, much effort hasfocused on delineating the mechanisms thatregulate their function. Regulated phosphoryl-ation can change AMPAR function by changingthe open probabilities and conductances of thereceptors. However, changes in channel proper-ties are unlikely to account for the drastic changesin AMPAR function seen with LTP (20). Instead,changes inAMPAR function during synaptic plas-ticity are mostly due to phosphorylation-inducedchanges in its abundance at the synapse.

AMPARs traffic constitutively to and fromthe plasma membrane via recycling endosomes(21) (Fig. 3). Delivery of AMPARs to synapses isbelieved to occur first by exocytosis at extra-

synaptic sites followed by lateral diffusion withinthe plasma membrane to PSDs, where the mo-bility of the receptors is greatly reduced. Duringremoval of synaptic AMPARs, receptors diffuseaway from the PSD and then undergo clathrin-mediated, dynamin-dependent endocytosis. Afterendocytosis, small GTP-binding proteins of theRab family and effector proteins direct AMPARseither to early (sorting) endosomes or back tothe plasma membrane (22).

AMPAR trafficking occurs constitutively un-der basal conditions and is modulated by activitythrough changes in actin and myosin dynamics(23), as well as AMPAR interactions with scaf-folding proteins and accessory subunits. One ofthese accessory subunits, Stargazin, mediates theinteraction between AMPARs and the PSD pro-tein PSD-95, and this interaction is importantfor synaptic localization of AMPARs (24). Ac-tivity alters the phosphorylation of Stargazin,with phosphorylated Stargazin decreasing the

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Fig. 3. Activity-dependent modulation of pre-, post-, and trans-synapticcomponents. Presynaptic: Neurotransmitter vesicle cycling. Neurotransmitterrelease starts with the filling of synaptic vesicles, which then dock andundergo priming at the active zone. Arrival of an action potential inducescalcium influx through voltage-sensitive calcium channels (VSCCs), whichtriggers membrane fusion and exocytosis. The synaptic vesicles are thenrecycled via local reuse (a; kiss and stay), fast recycling (b; kiss andrun), or clathrin-mediated endocytosis (c). Neurotransmitter release can beregulated during plasticity as exemplified by the regulation of synapsinphosphorylation (1) and the regulation of RIM protein phosphorylation(2). Postsynaptic: AMPA receptor trafficking. Locally and somatically syn-thesized AMPARs enter a pool of endosomes that undergo constitutive

and regulated membrane trafficking. During potentiation, greater receptorinsertion (3) increases the concentration of AMPARs at the synapse, wherethey are anchored by interactions at the PSD. During synaptic depression,AMPARs are endocytosed (3). The preferential location of endocytosis andexocytosis is probably extrasynaptic. Within the plasma membrane, traf-ficking of AMPARs between the synapse and the point of insertion or removaloccurs by lateral diffusion. Extrasynaptic movement of AMPARs increaseswith neuronal activity (4). Receptor trafficking is modulated by phosphoryl-ation of AMPAR subunits (5), which influences interactions with scaffoldingproteins. Trans-synaptic: Synaptic cell adhesion molecules. PSA-NCAM isincreased following neuronal activity (6). Lightning bolts indicate activity-dependent processes.


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mobility of AMPARs and enhancing AMPARfunction. Blocking Stargazin phosphorylationor dephosphorylation blocks LTP and LTD, re-spectively (25).

AMPARs exist as tetramers made up of dif-ferent combinations of the four subunits, GluA1through 4. The cytoplasmic tails of each subunitcontain multiple phosphorylation sites that reg-ulate the trafficking of AMPARs. For example,PKA phosphorylation of S845 in the long cyto-plasmic tail of GluA1 increases GluA1 surfaceexpression due to both enhanced insertion andattenuated internalization (26). Conversely, LTDof dissociated cultures and brain slices results indephosphorylation of S845 and is correlated withan increased rate of AMPAR endocytosis (27).Knock-in mice with phosphorylation-deficientmutations at both S831A and S845A display aloss of NMDA-induced AMPAR internalization,deficits in LTP and LTD, and have impaired spa-tial memory (28).

Although studies of posttranslational mod-ifications at individual sites have established arole for regulating GluA1 trafficking and chan-nel properties, they do not fully account for thechanges in GluA1 function observed with syn-aptic plasticity (29). Activity-modified residuescontinue to be discovered, including, for exam-ple, the highly conserved T840 phosphorylationsite, the phosphorylation of which correlates re-markably well with synaptic strength (30). It islikely that complex patterns of phosphorylationand of other post-translational modifications (e.g.,palmitoylation or ubiquitination) combine to reg-ulate AMPAR localization.

Trans-Synaptic Signaling; the Synaptic CleftThe synaptic cleft is a ~20-nm junction betweenthe pre- and postsynaptic compartments, consist-ing of a space through which neurotransmittersdiffuse to bind postsynaptic receptors, as well asa network of cell adhesion molecules (CAMs)

that keeps the synapse together. These adhesiveinteractions are so strong that it is impossible toseparate intact pre- from postsynaptic compart-ments biochemically.

Role of CAMs in synaptic plasticity.TheCAMsthat localize to the synaptic cleft include membersof the cadherin, integrin, and immunoglobulin-containing CAMs, as well as neurexins and neu-roligins. Much research has focused on trying tounderstand whether and how CAMs mediatesynapse specificity during neural circuit forma-tion. Here we focus on the regulation of synapticCAMs during experience-dependent synapticplasticity, limiting our discussion to just two ofmany examples.

One such example involves the addition oflarge sialic acid homopolymers to the neuralcell adhesion molecule (NCAM) to form poly-sialylated NCAM (PSA-NCAM), which decreaseshomophilic adhesion to allow new synaptic re-modeling and growth. The ratio of PSA-NCAM

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Fig. 4. Local regulation of the synaptic proteome. Synaptic plasticity modifiesgene expression at many levels. Strong stimulation of synapses triggers signalsthat are sent to the nucleus to modify RNA synthesis. Synaptic activity alsomodifies protein synthesis, and has been found to act at several key stepsduring translation: (1) Relief of repression, e.g., RISC-mediated repression; (2)modification of translational initiation to allow 4E-4G interaction and re-cruitment of 40S; (3) formation of the preinitiation complex; and (4) de-phosphorylation of eEF2 to allow for catalysis of ribosome translocation

during translational elongation. To counterbalance local protein synthesis,local protein degradation also occurs at synapses (5). Together, these regu-lated steps in protein addition and removal allow for rapid, spatially restrictedcontrol of the synaptic proteome. Lightning bolts indicate activity-dependentprocesses. RBP, RNA binding proteins such as exon junction complexes, RISCmachinery, Staufen, CPEB, etc. (Note: Although local translation in dendrites isa well-accepted phenomenon, it has not been demonstrated to occur inspines.)


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to NCAM increases following hippocampallearning tasks, and inactivation of the enzymethat adds the polysialic moieties blocks hippo-campal learning and plasticity (31). The in-crease in PSA-NCAM is thought to promotesynaptic remodeling during persistent forms ofplasticity.

Another family of CAMs that play a role inhippocampal plasticity includes the synapticallylocalized receptor tyrosine kinase ephrins andephrin receptors (Eph receptors). Initially studiedin the context of neural development, ephrins andEph receptors have also been found to be essen-tial for hippocampal LTP and LTD in the adultbrain (32). Specific ephrins and Eph receptorsregulate the localization and function of NMDAreceptors, and can thereby modulate synapticstrength in response to activity. Experimentsusing inhibitory ephrin and Eph receptor peptideshave revealed that both molecules are required,in a kinase-independent manner, for mossy fiberhippocampal LTP (33).

Trans-synaptic signaling by retrogrademessen-gers. Another means of trans-synaptic signalinginvolves diffusible, membrane-soluble messen-gers. The CB1 and CB2 cannabinoid receptorswere initially identified as receptors for canna-binoid, the active ingredient of THC/marijuana.This led to the identification of endogenous CB1and CB2 ligands, called endocannabinoids. Endo-cannabinoids have emerged as important modu-lators of plasticity, initially at inhibitory synapses,and more recently at excitatory synapses (34). De-polarization and activation of a variety of receptorshave been shown to activate release of endo-cannabinoids from the postsynaptic compartmentand binding to presynaptic CB receptors, result-ing in a suppression of neurotransmitter release(and thus regulating presynaptic plasticity). Thisform of plasticity is called endocannabinoid-LTD,or eCB-LTD. Endocannabinoid signaling is re-quired for extinction but not acquisition of spatialmemories (35).

The Tripartite Synapse: Glia andSynaptic PlasticityOnce thought of as the support cells of thenervous systems, glial cells are now consideredessential partners in synapse formation, synaptictransmission, and plasticity (36). Astrocytes sur-round the synapse (Fig. 2), forming a tripartitesynapse, composed of neuronal pre- and post-synaptic compartments as well as surroundingastrocytes. Synaptically localized glia release neu-roactive molecules that influence neuronal com-munication. For example, release of D-serine (acoactivator of the NMDA receptor) from glia isrequired for LTP of hippocampal Schaffer col-lateral synapses (37) [although see also (38)].Ephrin and Eph receptor signaling between neu-rons and glia regulates the uptake of glutamatethrough glial glutamate transporters and there-by affects neurotransmission and synaptic plas-ticity (39). The release of lactate from astrocytesand uptake by neurons has also been reported

to be required for long-term hippocampal mem-ory and plasticity (40).

Regulating Gene Expression WithinNeurons During PlasticitySignaling from synapse to nucleus to regulatetranscription. Long-lasting forms of synaptic plas-ticity, such as those underlying long-term memo-ry, require new RNA synthesis (2). This indicatesthat synaptic signals must be relayed to the nu-cleus to regulate transcription. Synapse-to-nucleussignaling poses a unique set of challenges inneurons, where the distance between the syn-apse and nucleus can be appreciable. Neuronsare specialized for rapid communication betweencompartments via electrochemical signaling, withdepolarization at the synaptic terminal leadingto depolarization at the soma in less than amillisecond. Calcium influx can occur throughvoltage- and ligand-gated ion channels. Cytosol-ic calcium can also be released from intracellularpools following activation of Gq-coupled recep-tors such as metabotropic GluRs (mGluRs). Eachroute of calcium influx induces different programsof gene induction (41).

Soluble signals can also be transported fromthe synapse to the nucleus by slower, microtubule-andmotor proteindependent pathways (42). Thisclass of signals includes kinases and transcrip-tional regulators that function to alter transcrip-tion. These slower pathways of signaling to thenucleus may sustain changes in gene expressionfor time periods extending beyond the initialstimulus.

To obtain a global view of how transcrip-tion is altered during activity-dependent plastic-ity, expression profiling has been used to identifychanges in transcription following depolariza-tion of cultured mouse neurons. Such studieshave identified several hundred activity-regulatedgenes (41). Genome-wide analyses of transcrip-tion factor binding sites of the activated geneshave revealed that the transcription factors CREB,MEF2, and Npas4 control the activity-dependenttranscription of a large number of downstreamactivity-regulated genes (41). These downstreamtranscription factors regulate the expression ofoverlapping but distinct subsets of activity-regulatedgenes, suggesting that the precise temporal, spa-tial, and stimulus-specific cellular response isachieved by the combinatorial control by differ-ent transcription factors.

Local protein synthesis. Despite requiringnew transcription, LTP and LTD can occur in aspatially restricted manner, raising the questionof how gene expression in neurons can be lim-ited to subsets of synapses and not generalizedto the entire cell. One way of locally changingthe proteome in neurons is through regulatedtranslation of localized mRNAs (Fig. 4).

The existence of local translation in dendritesof mature neurons was first suggested by elec-tron micrographic identification of polyribosomesin hippocampal dendrites (43). Studies in hip-pocampal slices in which dendrites had been se-

vered from cell bodies found that such dendritesretain the ability to express long-lasting LTP andLTD, indicating that local translation can medi-ate long-term modification of synaptic strength(44, 45).

Studies of mRNA localization have led tothe identification of cis-acting RNA elementsthat bind to RNA-binding proteins to undergoexport from the soma into the dendrite (46). Al-though several dendritic localization elementshave been identified, there is to date no consensuson their sequence or structure. Among the best-studied RNA binding proteins involved in den-dritic mRNA localization are Staufen, Zipcodebinding protein 1 (ZBP1), and hnRNPA2 (46).These proteins bind cis-acting elements and as-semble transcripts into larger RNA transportgranules, which travel in a kinesin-dependentmanner along microtubules to their final destina-tion. Whether localized RNAs undergo directedtargeting, anchoring, or stabilization at specificsites remains an open question.

In terms of translational regulation, studieshave revealed activity-dependent regulation oftranslation initiation and elongation. A mech-anism of translational regulation known to occurat synapses involves the cytoplasmic polyade-nylation element binding protein (CPEB). CPEBbinding to 3 untranslated regions (3UTRs) re-presses translation. However, CPEB undergoesphosphorylation in an activity-dependent man-ner to recruit other proteins that increase thepolyadenylate [poly(A)] tails of mRNAs. Sub-sequently, poly(A) binding protein (PABP) isrecruited to the elongated poly(A) tail, which inturn recruits eukaryotic translation initiation fac-tor 4g (eIF4G) to interact with eukaryotic trans-lation initiation factor 4E (eIF4E) to promotetranslation initiation (47). CPEB localizes tosynapses, where it regulates translation of den-dritically localized CamKIIa mRNA (48, 49).

Another activity-dependent means of regu-lating translation initiation involves phospho-rylation of eIF4E-binding proteins (4E-BPs).Hypophosphorylated 4E-BPs bind eIF4E andprevent translation initiation; phosphorylated4E-BP dissociates from eIF4E and relieves trans-lational inhibition. In neurons, activity increases4E-BP phosphorylation and stimulates translation(50). Studies in 4E-BP2 knockout mice foundthat E-LTP stimulation protocols could induceL-LTP in brain slices. Recently, two additional4E-BPs have been identified in neurons: neuro-guidin and the cytoplasmic FMRP interactingprotein (CYFIP). Whereas 4E-BP1 and 2 are be-lieved to affect general translation, these new4E-BPs may preferentially affect subgroups oftranscripts within dendrites (51, 52).

Activity can also regulate translational elon-gation during synaptic plasticity. For example, theelongation factor eukaryotic translation elonga-tion factor 2 (eEF2) undergoes activity-dependentchanges in phosphorylation. Phosphorylation ofeEF2 decreases the rate of translation. Whereasaction potentials decrease eEF2 phosphorylation


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(thereby increasing translation), spontaneous re-lease of neurotransmitter increases eEF2 phos-phorylation and decreases translation (53). Theseeffects occur locally at synapses, indicating thatone function of spontaneous release may be tosuppress local translation and thereby stabilizesynapses.

Translation may also be regulated throughthe microRNA (miRNA) pathway, where eachmiRNA can potentially regulate hundreds oftranscripts and hence coordinate the expressionof many genes. Many miRNAs are relativelymore abundant in, or restricted to, the brain.While miRNAs can regulate cell-wide levels oftranslation, their posttranscriptional mode ofaction makes them especially well suited to reg-ulating distally localized transcripts. SpecificmiRNAs have been found in dendrites and syn-apses, and components of the RNA-induced si-lencing complex (RISC) machinery itself havebeen found to be altered by activity (54).

Consistent with the importance of regulatingsynaptic AMPAR concentrations during plasticity,the mRNAs encoding GluA1 and GluA2 haveboth been detected in hippocampal dendritesand found to undergo activity-dependent changesin localization and translation (55, 56). Furtherlinking local translation with synaptic AMPARabundance, local eEF2-dependent translationof Arc mRNA has been shown to trigger endo-cytosis of AMPARs during mGluR-mediatedhippocampal LTD (57, 58).

Local protein degradation. The local pro-teome is regulated not only by local translationbut also by protein degradation through theubiquitin proteasome system (Fig. 4). Bothprotein synthesis and degradation are requiredfor the maintenance of late-phase LTP, suggestingthat protein degradation is needed to counter-balance protein synthesis during plasticity (59).Like local translation, protein degradation canbe regulated within dendrites. Ubiquitin and pro-teasomal subunits have been found in dendritesand at synapses, and stimulation of hippocampalneurons triggers proteasome-dependent changesin the composition of PSD proteins (60). Activity-dependent degradation involves redistributionof proteasomes from dendritic shafts to spines(61). Notably, the ubiquitin proteasome pathwayalters AMPAR trafficking and degradation atsynapses during plasticity (62).

PerspectivesAs the above examples illustrate, cell biologicalapproaches have provided a detailed under-standing of many aspects of activity-dependentplasticity. By focusing on molecular processesoccurring within individual neurons and sub-cellular compartments, we now understand spe-cific processes that are modulated by experienceto change synaptic strength. These involve alter-ations in neurotransmitter release, trans-synapticsignaling, postsynaptic receptor dynamics, andgene expression within neurons. Distinct plastic-ity mechanisms are used at different types of syn-

apses. For instance, LTP at mossy fiber synapsesoccurs primarily through presynaptic changes,whereas LTP at Schaffer collateral synapses oc-curs mostly through postsynaptic mechanisms.The end result of many of the processes we havedescribed is to regulate the concentration of glu-tamate receptors, indicating that this is a majorpostsynaptic determinant of synaptic strength dur-ing plasticity.

Together, each of these cell biological mech-anisms provides potential therapeutic targetsfor diseases in which brain plasticity is dysfunc-tional. However, they fall short of elucidatinghow complex circuits are altered by experienceto store information and alter behavior. This willrequire the development of tools for investigatingboth the dynamic nano-architecture of the syn-apse and the neural circuit as a whole. A par-ticular challenge is to study plasticity in neuralcircuits in living animals, and to developmethodsto examine, and computational frameworks tounderstand, how all components of a circuit areregulated to alter circuit function dynamically.The development of methodologies for super-resolution time-lapse imaging of synapses, neu-rons, and circuits in live animals promises tomove the field forward toward a more nuancedand complete understanding of the experience-dependent plastic changes in the brain that me-diate learning and memory.

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(2007).Acknowledgments: We apologize to those whose

primary work could not be cited because of spaceconstraints. We thank J.T. Braslow, T. J. ODell,and F. E. Schweizer for comments on the manuscript;J. Bourne and K. M. Harris for hippocampalelectron microscopy images; and all membersof the Martin lab for helpful discussions. Supportcomes from NIH grants R01 MH077022 and R01NS045324 (to K.C.M.), the Medical ScientistTraining Program (NIH T32 GM008042), and theNeurobehavioral Genetics Training Program (NIHgrant T32 MH073526) (to V.M.H.). A glossary of termsused is included in the online (HTML) version of thisarticle (Box 1).

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