Bidirectional synaptic plasticity regulated by phosphorylation of stargazin-like TARPs

The GLUR2 subunit of AMPA receptors: synaptic role

Ampa receptors mediate the majority of excitatory synaptic transmission in the brain. Thus, the mechanisms that control the developmental and activity-dependent changes in the functional synaptic expression of AMPA receptors are of fundamental importance. Here we focus on the role of GluR2 subunit in synaptic function and plasticity.

Coupled phosphatase and kinase switches produce the tristability required for long-term potentiation and long-term depression

Studies of long-term potentiation (LTP) and long-term depression (LTD) strongly suggest that individual synapses can be bidirectionally modified. A central question is the biochemical mechanisms that make LTP and LTD persistent. Previous theoretical models have proposed that the autophosphorylation properties of CaMKII could underlie a bistable molecular switch that maintains LTP, and there is experimental support for this mechanism. In contrast, there has been comparatively little theoretical or experimental work regarding the mechanisms that maintain LTD. Several lines of evidence indicate that LTD is not simply a reversal of previous LTP but rather involves separate biochemical reactions. These findings indicate that a minimal model of the synapse must involve a tristable system. Here, we describe a phosphatase (PP2A) switch, which together with a kinase switch form a tristable system. PP2A can be activated by a Ca(2+)-dependent process but can also be phosphorylated and inactivated by CaMKII. When dephosphorylated, PP2A can dephosphorylate itself. We show that these properties can lead to a persistent increase in PP2A during LTD (as reported experimentally), thus forming a phosphatase switch. We show that the coupled PP2A and CaMKII switches lead to a tristable system in which the kinase activity is high in the LTP state; the PP2A activity is high in the LTD state, and neither activity is high in the basal state. Our results provide an explanation for the recent finding that inhibition of PP2A prevents LTD induction.

Identification and investigation of Drosophila postsynaptic density homologs

AMPA receptors are responsible for fast excitatory transmission in the CNS and the trafficking of these receptors has been implicated in LTP and learning and memory. These receptors reside in the postsynaptic density, a network of proteins that links the receptors to downstream signaling components and to the neuronal cytoskeleton. To determine whether the fruit fly, Drosophila melanogaster, possesses a similar array of proteins as are found at the mammalian PSD, we identified Drosophila homologs of 95.8% of mammalian PSD proteins. We investigated, for the first time, the role of one of these PSD proteins, Pod1 in GluR cluster formation at the Drosophila neuromuscular junction and found that mutations in pod1 resulted in a specific loss of A-type receptors at the synapse.

Mechanisms of potentiation of mossy fiber EPSCs in the cerebellar nuclei by coincident synaptic excitation and inhibition

Neurons of the cerebellar nuclei receive synaptic excitation from cerebellar mossy fibers. Unlike in many principal neurons, coincident presynaptic activity and postsynaptic depolarization do not generate long-term potentiation at these synapses. Instead, EPSCs are potentiated by high-frequency trains of presynaptic activity applied with postsynaptic hyperpolarization, in patterns resembling mossy-fiber-mediated excitation and Purkinje-cell-mediated inhibition that are predicted to occur during delay eyelid conditioning. Here, we have used electrophysiology and Ca imaging to test how synaptic excitation and inhibition interact to generate long-lasting synaptic plasticity in nuclear cells in cerebellar slices. We find that the extent of plasticity varies with the relative timing of synaptic excitation and hyperpolarization. Potentiation is most effective when synaptic stimuli precede the postinhibitory rebound by approximately 400 ms, whereas with longer intervals, or with a reverse sequence, EPSCs tend to depress. When basal intracellular Ca is raised by spontaneous firing or reduced by voltage clamping at subthreshold potentials, potentiation is induced as long as the synaptic-rebound temporal sequence is maintained, suggesting that plasticity does not require Ca levels to exceed a threshold or attain a specific concentration. Although rebound and spike-dependent Ca influx are global, potentiation is synapse specific, and is disrupted by inhibitors of calcineurin or Ca-calmodulin-dependent protein kinase II, but not PKC. When IPSPs replace the hyperpolarizing step in the induction protocol, potentiation proceeds normally. These results lead us to propose that synaptic and inhibitory/rebound stimuli initiate separate processes, with local NMDA receptor-mediated Ca influx "priming" synapses, and Ca changes from the inhibition and rebound "triggering" potentiation at recently activated synapses.

Stable synaptic retention of serine-880-phosphorylated GluR2 in hippocampal neurons

Phosphorylation of S880 within the GluR2 C-terminus has been reported to promote endocytosis of AMPA receptors (AMPARs) by preventing GluR2 interaction with the putative synaptic anchoring proteins GRIP and ABP. It is not yet established however, whether S880 phosphorylation induces removal of AMPARs from synaptic sites, and the trafficking of phosphorylated GluR2 subunits with surface and endocytosed GluR2 has not been directly compared within the same intact neurons. Here we show that phosphorylation of GluR2 subunits by PKC activated with phorbol esters is compartmentally restricted to receptors located at the cell surface. Endogenous AMPARs containing S880-phosphorylated GluR2 remained highly synaptic and colocalized with postsynaptic markers to the same extent as AMPARs which did not contain S880-phosphorylated GluR2. Moreover, following S880 phosphorylation, exogenous GluR2 homomers were found specifically at the cell surface and did not co-traffic with the internalized endosomal GluR2 population. We also show that GluR2 is endogenously phosphorylated by a constitutively active kinase pharmacologically related to PKC, and this phosphorylation is opposed by the protein phosphatase PP1. Our results demonstrate a population of hippocampal AMPARs which do not require interaction with GRIP/ABP for synaptic anchorage.

Glutamate receptor dynamics in dendritic microdomains

Among diverse factors regulating excitatory synaptic transmission, the abundance of postsynaptic glutamate receptors figures prominently in molecular memory and learning-related synaptic plasticity. To allow for both long-term maintenance of synaptic transmission and acute changes in synaptic strength, the relative rates of glutamate receptor insertion and removal must be tightly regulated. Interactions with scaffolding proteins control the targeting and signaling properties of glutamate receptors within the postsynaptic membrane. In addition, extrasynaptic receptor populations control the equilibrium of receptor exchange at synapses and activate distinct signaling pathways involved in plasticity. Here, we review recent findings that have shaped our current understanding of receptor mobility between synaptic and extrasynaptic compartments at glutamatergic synapses, focusing on AMPA and NMDA receptors. We also examine the cooperative relationship between intracellular trafficking and surface diffusion of glutamate receptors that underlies the expression of learning-related synaptic plasticity.

The role of transmembrane AMPA receptor regulatory proteins (TARPs) in neurotransmission and receptor trafficking (Review)

AMPA receptors (AMPAR) mediate the majority of fast excitatory neurotransmission in the central nervous system (CNS). Transmembrane AMPAR regulatory proteins (TARPs) have been identified as a novel family of proteins which act as auxiliary subunits of AMPARs to modulate AMPAR trafficking and function. The trafficking of AMPARs to regulate the number of receptors at the synapse plays a key role in various forms of synaptic plasticity, including long-term potentiation (LTP) and long-term depression (LTD). Expression of the prototypical TARP, stargazin/TARPgamma2, is ablated in the stargazer mutant mouse, an animal model of absence epilepsy and cerebellar ataxia. Studies on the stargazer mutant mouse have revealed that failure to express TARPgamma2 has widespread effects on the balance of expression of both excitatory (AMPAR) and inhibitory receptors (GABA(A) receptors, GABAR). The understanding of TARP function has implications for the future development of AMPAR potentiators, which have been shown to have therapeutic potential in both psychological and neurological disorders such as schizophrenia, depression and Parkinson's disease.

The role of RIM1alpha in BDNF-enhanced glutamate release

Brain-derived neurotrophic factor (BDNF) is known to activate proline-directed Ser/Thr protein kinases and to enhance glutamatergic transmission via a Rab3a-dependent molecular pathway. The identity of molecular targets in BDNF's action on Rab3a pathway, a synaptic vesicle protein involved in vesicle trafficking and synaptic plasticity, is not fully known. Here we demonstrate that BDNF enhances depolarization-evoked efflux of [(3)H]-glutamate from nerve terminals isolated from the CA1 region of the hippocampus. BDNF also potentiated hyperosmotic shock-evoked [(3)H]-glutamate efflux, indicating an effect on the size of the readily releasable pool. This effect of BDNF was completely abolished in nerve terminals derived from Rim1alphaKO (Rab3 interacting molecule 1alpha null mutant) mice. Using in vitro phosphorylation assays we identified two novel phosphorylation sites, Ser447 and Ser745 that were substrates for ERK2, a proline-directed kinase known to be activated by BDNF. The pSer447 site was phosphorylated under resting conditions in hippocampal CA1 nerve terminals and its phosphorylation was enhanced by BDNF treatment, as indicated by the use of a pSer447-RIM1alpha antibody we have developed. Together these findings identify RIM1alpha, a component of the Rab3a molecular pathway in mediating presynaptic plasticity, as a necessary factor in BDNF's enhancement of [(3)H]-glutamate efflux from hippocampal CA1 nerve terminals and indicate a possible role for RIM1alpha phosphorylation in BDNF-dependent presynaptic plasticity.

Acute and chronic dopamine receptor stimulation modulates AMPA receptor trafficking in nucleus accumbens neurons cocultured with prefrontal cortex neurons

Postsynaptic interactions between dopamine (DA) and glutamate receptors in the nucleus accumbens (NAc) are critical for addiction. To determine the effect of acute and repeated DA receptor stimulation on AMPA receptor (AMPAR) synaptic targeting in medium spiny NAc neurons, we developed a model system consisting of rat NAc neurons cocultured with prefrontal cortex neurons from enhanced green fluorescent protein-expressing mice. Cortical neurons restore excitatory input onto NAc neurons but are distinguishable based on fluorescence. First, we showed that brief D1-like agonist exposure increased AMPAR insertion onto extrasynaptic regions of NAc neuronal processes through a mechanism requiring protein kinase A. This facilitated the Ca2+/calmodulin dependent protein kinase II (CaMKII)-dependent synaptic incorporation of AMPARs in response to subsequent NMDA receptor (NMDAR) stimulation. Through this mechanism, DA may promote reward- and drug-related plasticity in the NAc. Then, to model effects of repeated in vivo cocaine exposure, we treated cocultures with DA (1 microm, 30 min) on days 7, 9, and 11 in culture. On day 15, NAc neurons exhibited increased synaptic AMPAR levels. This was associated with CaMKII activation and was blocked by the CaMKII inhibitor KN-93 (N-[2-[N-(4-chlorocinnamyl)-N-methylaminomethyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulfonamide phosphate salt). Furthermore, D1-like agonist exposure on day 15 no longer increased AMPAR surface expression. This refractoriness was associated with decreased D1 receptor surface expression. NMDAR surface expression was not altered by acute or repeated DA receptor stimulation. These results suggest that (1) after repeated DA treatment, NAc neurons are more responsive to glutamate inputs but D(1)-like receptor regulation of plasticity is impaired, and (2) NAc/prefrontal cortex cocultures are useful for studying dopamine-induced neuroadaptations.

Okadaic acid-sensitive protein phosphatases constrain phrenic long-term facilitation after sustained hypoxia

Phrenic long-term facilitation (pLTF) is a serotonin-dependent form of pattern-sensitive respiratory plasticity induced by intermittent hypoxia (IH), but not sustained hypoxia (SH). The mechanism(s) underlying pLTF pattern sensitivity are unknown. SH and IH may differentially regulate serine/threonine protein phosphatase activity, thereby inhibiting relevant protein phosphatases uniquely during IH and conferring pattern sensitivity to pLTF. We hypothesized that spinal protein phosphatase inhibition would relieve this braking action of protein phosphatases, thereby revealing pLTF after SH. Anesthetized rats received intrathecal (C4) okadaic acid (25 nm) before SH (25 min, 11% O(2)). Unlike (vehicle) control rats, SH induced a significant pLTF in okadaic acid-treated rats that was indistinguishable from rats exposed to IH (three 5 min episodes, 11% O(2)). IH and SH with okadaic acid may elicit pLTF by similar, serotonin-dependent mechanisms, because intravenous methysergide blocks pLTF in rats receiving IH or okadaic acid plus SH. Okadaic acid did not alter IH-induced pLTF. In summary, pattern sensitivity in pLTF may reflect differential regulation of okadaic acid-sensitive serine/threonine phosphatases; presumably, these phosphatases are less active during/after IH versus SH. The specific okadaic acid-sensitive phosphatase(s) constraining pLTF and their spatiotemporal dynamics during and/or after IH and SH remain to be determined.

Accumulation of AMPA receptors in autophagosomes in neuronal axons lacking adaptor protein AP-4

AP-4 is a member of the adaptor protein complexes, which control vesicular trafficking of membrane proteins. Although AP-4 has been suggested to contribute to basolateral sorting in epithelial cells, its function in neurons is unknown. Here, we show that disruption of the gene encoding the beta subunit of AP-4 resulted in increased accumulation of axonal autophagosomes, which contained AMPA receptors and transmembrane AMPA receptor regulatory proteins (TARPs), in axons of hippocampal neurons and cerebellar Purkinje cells both in vitro and in vivo. AP-4 indirectly associated with the AMPA receptor via TARPs, and the specific disruption of the interaction between AP-4 and TARPs caused the mislocalization of endogenous AMPA receptors in axons of wild-type neurons. These results indicate that AP-4 may regulate proper somatodendritic-specific distribution of its cargo proteins, including AMPA receptor-TARP complexes and the autophagic pathway in neurons.

Integrin regulation of cytoplasmic calcium in excitatory neurons depends upon glutamate receptors and release from intracellular stores

Integrins regulate cytoplasmic calcium levels ([Ca(2+)]i) in various cell types but information on activities in neurons is limited. The issue is of current interest because of the evidence that both integrins and changes in [Ca(2+)]i are required for Long-Term Potentiation. Accordingly, the present studies evaluated integrin ligand effects in cortical neurons. Integrin ligands or alpha5beta1 integrin activating antisera rapidly increased [Ca(2+)]i with effects greater in glutamatergic than GABAergic neurons, absent in astroglia, and blocked by beta1 integrin neutralizing antisera and the tyrosine kinase antagonist genistein. Increases depended upon extracellular calcium and intracellular store release. Ligand-induced effects were reduced by voltage-sensitive calcium channel and NMDA receptor antagonists, but blocked by tetrodotoxin or AMPA receptor antagonists. These results indicate that integrin ligation triggers AMPA receptor/depolarization-dependent calcium influx followed by intracellular store release and suggest the possibility that integrin modulation of activity-induced changes in [Ca(2+)]i contributes importantly to lasting synaptic plasticity in forebrain neurons.

The regulation of dendritic arbor development and plasticity by glutamatergic synaptic input: a review of the synaptotrophic hypothesis

The synaptotropic hypothesis, which states that synaptic inputs control the elaboration of dendritic (and axonal) arbors was articulated by Vaughn in 1989. Today the role of synaptic inputs in controlling neuronal structural development remains an area of intense research activity. Several recent studies have applied modern molecular genetic, imaging and electrophysiological methods to this question and now provide strong evidence that maturation of excitatory synaptic inputs is required for the development of neuronal structure in the intact brain. Here we critically review data concerning the hypothesis with the expectation that understanding the circumstances when the data do and do not support the hypothesis will be most valuable. The synaptotrophic hypothesis contributes at both conceptual and mechanistic levels to our understanding of how relatively minor changes in levels or function of synaptic proteins may have profound effects on circuit development and plasticity.

Functional modulation of AMPA receptors by transmembrane AMPA receptor regulatory proteins

The AMPA receptors are ligand-gated ion channels belonging to the family of ionotropic glutamate receptors. They play an essential role in fast excitatory synaptic transmission in the CNS of vertebrates. Their activity-dependent directed transport and fast turnover at the plasma membrane contribute to synaptic plasticity and require numerous trafficking and scaffolding proteins. Participating in the delivery and synaptic localization of AMPA receptors is a recently discovered protein family named transmembrane AMPA receptor regulatory proteins (TARPs). In addition to their function in trafficking, TARPs alter the biophysical properties of AMPA receptors in remarkable ways and thus contribute significantly to the functional plasticity of the synapse. The study of TARP-mediated functional plasticity of AMPA receptors, which has emerged only recently as a hot new field, promises to yield valuable insight into the regulation of neuronal communication.

Regulation of AMPA receptor recruitment at developing synapses

Fast synaptic current at most excitatory synapses in the brain is carried by AMPA and NMDA subtypes of ionotropic glutamate receptors (AMPARs and NMDARs). During development there is an increase in the ratio of AMPAR- to NMDAR-mediated current at these synapses. Recent studies indicate that NMDAR signaling early in development negatively regulates AMPAR expression and function at multiple levels, which likely accounts for the small AMPAR current at developing synapses. This contrasts with the positive role of NMDAR signaling in recruiting AMPARs to synapses during long-term potentiation in the adult brain. Thus, NMDARs exert differential effects on the recruitment of AMPA receptors to synapses depending on the developmental state of the neural circuit.

Synaptic plasticity: multiple forms, functions, and mechanisms

Experiences, whether they be learning in a classroom, a stressful event, or ingestion of a psychoactive substance, impact the brain by modifying the activity and organization of specific neural circuitry. A major mechanism by which the neural activity generated by an experience modifies brain function is via modifications of synaptic transmission; that is, synaptic plasticity. Here, we review current understanding of the mechanisms of the major forms of synaptic plasticity at excitatory synapses in the mammalian brain. We also provide examples of the possible developmental and behavioral functions of synaptic plasticity and how maladaptive synaptic plasticity may contribute to neuropsychiatric disorders.

Synaptic adhesion molecules and PSD-95

Synaptic adhesion molecules are known to participate in various steps of synapse development including initial contacts between dendrites and axons, formation of early synapses, and their maturation and plastic changes. Notably, a significant subset of synaptic adhesion molecules associates with synaptic scaffolding proteins, suggesting that they may act in concert to couple trans-synaptic adhesion to molecular organization of synaptic proteins. Here, we describe an emerging group of synaptic adhesion molecules that directly interact with the abundant postsynaptic scaffold PSD-95, which include neuroligins, NGLs, SALMs, and ADAM22, and discuss how these proteins and PSD-95 act together to regulate synaptic development. PSD-95 may be one of the central organizers of synaptic adhesion that recruits diverse proteins to sites of synaptic adhesion, promotes trans-synaptic signaling, and couples neuronal activity with changes in synaptic adhesion.

AMPA receptors and stargazin-like transmembrane AMPA receptor-regulatory proteins mediate hippocampal kainate neurotoxicity

Naturally occurring glutamate analogs, such as kainate and domoate, which cause excitotoxic shellfish poisoning, induce nondesensitizing responses at neuronal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors. In addition to acting on AMPA receptors, kainate and domoate also activate high-affinity kainate-type glutamate receptors. The receptor type that mediates their neurotoxicity remains uncertain. Here, we show that the transmembrane AMPA receptor-associated protein (TARP) gamma-2 (or stargazin) and the related TARP gamma-8 augment responses to kainate and domoate by making these neurotoxins more potent and more efficacious AMPA receptor agonists. Genetic deletion of hippocampal enriched gamma-8 selectively abolishes sustained depolarizations in hippocampus mediated by kainate activation of AMPA receptors. gamma-8 knockout mice display typical kainate-induced seizures; however, the associated neuronal cell death in the hippocampus is attenuated in mice lacking gamma-8. This work decisively demonstrates that TARP-associated AMPA receptors mediate kainate neurotoxicity and identifies TARPs as targets for modulating neurotoxic properties of AMPA receptors.

Activity-dependent regulation of h channel distribution in hippocampal CA1 pyramidal neurons

The hyperpolarization-activated cation current, I(h), plays an important role in regulating intrinsic neuronal excitability in the brain. In hippocampal pyramidal neurons, I(h) is mediated by h channels comprised primarily of the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel subunits, HCN1 and HCN2. Pyramidal neuron h channels within hippocampal area CA1 are remarkably enriched in distal apical dendrites, and this unique distribution pattern is critical for regulating dendritic excitability. We utilized biochemical and immunohistochemical approaches in organotypic slice cultures to explore factors that control h channel localization in dendrites. We found that distal dendritic enrichment of HCN1 is first detectable at postnatal day 13, reaching maximal enrichment by the 3rd postnatal week. Interestingly we found that an intact entorhinal cortex, which projects to distal dendrites of CA1 but not area CA3, is critical for the establishment and maintenance of distal dendritic enrichment of HCN1. Moreover blockade of excitatory neurotransmission using tetrodotoxin, 6-cyano-7-nitroquinoxaline-2,3-dione, or 2-aminophosphonovalerate redistributed HCN1 evenly throughout the dendrite without significant changes in protein expression levels. Inhibition of calcium/calmodulin-dependent protein kinase II activity, but not p38 MAPK, also redistributed HCN1 in CA1 pyramidal neurons. We conclude that activation of ionotropic glutamate receptors by excitatory temporoammonic pathway projections from the entorhinal cortex establishes and maintains the distribution pattern of HCN1 in CA1 pyramidal neuron dendrites by activating calcium/calmodulin-dependent protein kinase II-mediated downstream signals.

Sustained structural change of GABA(A) receptor-associated protein underlies long-term potentiation at inhibitory synapses on a cerebellar Purkinje neuron

Fast inhibitory synaptic transmission is predominantly mediated by GABA(A) receptor (GABA(A)R) in the CNS. Although several types of neuronal activity-dependent plasticity at GABAergic synapses have been reported, the detailed mechanism is elusive. Here we show that binding of structurally altered GABA(A)R-associated protein (GABARAP) to GABA(A)R gamma2 subunit and to tubulin is critical for long-term potentiation [called rebound potentiation (RP)] at inhibitory synapses on a cerebellar Purkinje neuron (PN). Either inhibition of GABARAP association with GABA(A)Rgamma2 or deletion of tubulin binding region of GABARAP impaired RP. Inhibition of tubulin polymerization also suppressed RP. Thus, precise regulation of GABA(A)Rgamma2-GABARAP-microtubule interaction is critical for RP. Furthermore, competitive inhibition of GABARAP binding to GABA(A)Rgamma2 after the RP establishment attenuated the potentiated response, suggesting that GABARAP is critical not only for the induction but also for the maintenance of RP. Fluorescence resonance energy transfer analysis revealed that GABARAP underwent sustained structural alteration after brief depolarization of a PN depending on the activity of Ca2+/calmodulin-dependent protein kinase II (CaMKII), which is required for the RP induction. The susceptibility of GABARAP to undergo structural alteration was abolished by an amino acid replacement in GABARAP. Furthermore, RP was impaired by expression of the mutant GABARAP with the replacement. Together, we conclude that GABA(A)R association with structurally altered GABARAP downstream of CaMKII activation is essential for RP.

Posttranslational modifications and receptor-associated proteins in AMPA receptor trafficking and synaptic plasticity

AMPA-type glutamate receptors (AMPARs) mediate most fast excitatory synaptic transmission in the mammalian brain. It is widely believed that the long-lasting, activity-dependent changes in synaptic strength, including long-term potentiation and long-term depression, could be the molecular and cellular basis of experience-dependent plasticities, such as learning and memory. Those changes of synaptic strength are directly related to AMPAR trafficking to and away from the synapse. There are many forms of synaptic plasticity in the mammalian brain, while the prototypic form, hippocampal CA1 long-term potentiation, has received the most intense investigation. After synthesis, AMPAR subunits undergo posttranslational modifications such as glycosylation, palmitoylation, phosphorylation and potential ubiquitination. In addition, AMPAR subunits spatiotemporally associate with specific neuronal proteins in the cell. Those posttranslational modifications and receptor-associated proteins play critical roles in AMPAR trafficking and regulation of AMPAR-dependent synaptic plasticity. Here, we summarize recent studies on posttranslational modifications and associated proteins of AMPAR subunits, and their roles in receptor trafficking and synaptic plasticity.

New transmembrane AMPA receptor regulatory protein isoform, gamma-7, differentially regulates AMPA receptors

AMPA-type glutamate receptors (GluRs) mediate most excitatory signaling in the brain and are composed of GluR principal subunits and transmembrane AMPA receptor regulatory protein (TARP) auxiliary subunits. Previous studies identified four mammalian TARPs, gamma-2 (or stargazin), gamma-3, gamma-4, and gamma-8, that control AMPA receptor trafficking, gating, and pharmacology. Here, we explore roles for the homologous gamma-5 and gamma-7 proteins, which were previously suggested not to serve as TARPs. Western blotting reveals high levels of gamma-5 and gamma-7 in the cerebellum, where gamma-7 is enriched in Purkinje neurons in the molecular layer and glomerular synapses in the granule cell layer. Immunoprecipitation proteomics shows that cerebellar gamma-7 avidly and selectively binds to AMPA receptor GluR subunits and also binds to the AMPA receptor clustering protein, postsynaptic density-95 (PSD-95). Furthermore, gamma-7 occurs together with PSD-95 and AMPA receptor subunits in purified postsynaptic densities. In heterologous cells, gamma-7 but not gamma-5 greatly enhances AMPA receptor glutamate-evoked currents and modulates channel gating. In granule cells from stargazer mice, transfection of gamma-7 but not gamma-5 increases AMPA receptor-mediated currents. Compared with stargazin, gamma-7 differentially modulates AMPA receptor glutamate affinity and kainate efficacy. These studies define gamma-7 as a new member of the TARP family that can differentially influence AMPA receptors in cerebellar neurons.

A new principle for information storage in an enzymatic pathway model

Strong experimental evidence indicates that protein kinase and phosphatase (KP) cycles are critical to both the induction and maintenance of activity-dependent modifications in neurons. However, their contribution to information storage remains controversial, despite impressive modeling efforts. For instance, plasticity models based on KP cycles do not account for the maintenance of plastic modifications. Moreover, bistable KP cycle models that display memory fail to capture essential features of information storage: rapid onset, bidirectional control, graded amplitude, and finite lifetimes. Here, we show in a biophysical model that upstream activation of KP cycles, a ubiquitous mechanism, is sufficient to provide information storage with realistic induction and maintenance properties: plastic modifications are rapid, bidirectional, and graded, with finite lifetimes that are compatible with animal and human memory. The maintenance of plastic modifications relies on negligible reaction rates in basal conditions and thus depends on enzyme nonlinearity and activation properties of the activity-dependent KP cycle. Moreover, we show that information coding and memory maintenance are robust to stochastic fluctuations inherent to the molecular nature of activity-dependent KP cycle operation. This model provides a new principle for information storage where plasticity and memory emerge from a single dynamic process whose rate is controlled by neuronal activity. This principle strongly departs from the long-standing view that memory reflects stable steady states in biological systems, and offers a new perspective on memory in animals and humans.

Developing velocity sensitivity in a model neuron by local synaptic plasticity

Sensor neurons, like those in the visual cortex, display specific functional properties, e.g., tuning for the orientation, direction and velocity of a moving stimulus. It is still unclear how these properties arise from the processing of the inputs which converge at a given cell. Specifically, little is known how such properties can develop by ways of synaptic plasticity. In this study we investigate the hypothesis that velocity sensitivity can develop at a neuron from different types of synaptic plasticity at different dendritic sub-structures. Specifically we are implementing spike-timing dependent plasticity at one dendritic branch and conventional long-term potentiation at another branch, both driven by dendritic spikes triggered by moving inputs. In the first part of the study, we show how velocity sensitivity can arise from such a spatially localized difference in the plasticity. In the second part we show how this scenario is augmented by the interaction between dendritic spikes and back-propagating spikes also at different dendritic branches. Recent theoretical (Saudargiene et al. in Neural Comput 16:595-626, 2004) and experimental (Froemke et al. in Nature 434:221-225, 2005) results on spatially localized plasticity suggest that such processes may play a major role in determining how synapses will change depending on their site. The current study suggests that such mechanisms could be used to develop the functional specificities of a neuron.

Phospholipase C is required for changes in postsynaptic structure and function associated with NMDA receptor-dependent long-term depression

NMDA receptor (NMDAR)-dependent hippocampal synaptic plasticity underlying learning and memory coordinately regulates dendritic spine structure and AMPA receptor (AMPAR) postsynaptic strength through poorly understood mechanisms. Induction of long-term depression (LTD) activates protein phosphatase 2B/calcineurin (CaN), leading to dendritic spine shrinkage through actin depolymerization and AMPAR depression through receptor dephosphorylation and internalization. The scaffold proteins A-kinase-anchoring protein 79/150 (AKAP79/150) and postsynaptic density 95 (PSD95) form a complex that controls the opposing actions of the cAMP-dependent protein kinase (PKA) and CaN in regulation of AMPAR phosphorylation. The AKAP79/150-PSD95 complex is disrupted in hippocampal neurons during LTD coincident with internalization of AMPARs, decreases in PSD95 levels, and loss of AKAP79/150 and PKA from spines. AKAP79/150 is targeted to spines through binding F-actin and the phospholipid phosphatidylinositol-(4,5)-bisphosphate (PIP2). Previous electrophysiological studies have demonstrated that inhibition of phospholipase C (PLC)-catalyzed hydrolysis of PIP2 inhibits NMDAR-dependent LTD; however, the signaling mechanisms that link PLC activation to alterations in dendritic spine structure and AMPAR function in LTD are unknown. We show here that NMDAR stimulation of PLC in cultured hippocampal neurons is necessary for AKAP79/150 loss from spines and depolymerization of spine actin. Importantly, we demonstrate that NMDAR activation of PLC is also necessary for decreases in spine PSD95 levels and AMPAR internalization. Thus, PLC signaling is required for structural and functional changes in spine actin, PSD scaffolding, and AMPAR trafficking underlying postsynaptic expression of LTD.

Calcium channel γ subunits: a functionally diverse protein family

The calcium channel gamma subunits comprise an eight-member protein family that share a common topology consisting of four transmembrane domains and intracellular N- and C-termini. Although the first gamma subunit was identified as an auxiliary subunit of a voltage-dependent calcium channel, a review of phylogenetic, bioinformatic, and functional studies indicates that they are a functionally diverse protein family. A cluster containing gamma1 and gamma6 conforms to the original description of the protein family as they seem to act primarily as subunits of calcium channels expressed in muscle. Members of a second cluster (gamma2, gamma3, gamma4, gamma8) function as regulators of AMPA receptor localization and function in the brain and are collectively known as TARPs. The function of members of the third cluster (gamma5, gamma7) remains unclear. Our analysis shows that the members of each cluster contain conserved regulatory motifs that help to differentiate the groups. However, the physiological significance of these motifs in many cases remains to be demonstrated.

The interaction between Stargazin and PSD-95 regulates AMPA receptor surface trafficking

Accumulation of AMPA receptors at synapses is a fundamental feature of glutamatergic synaptic transmission. Stargazin, a member of the TARP family, is an AMPAR auxiliary subunit allowing interaction of the receptor with scaffold proteins of the postsynaptic density, such as PSD-95. How PSD-95 and Stargazin regulate AMPAR number in synaptic membranes remains elusive. We show, using single quantum dot and FRAP imaging in live hippocampal neurons, that exchange of AMPAR by lateral diffusion between extrasynaptic and synaptic sites mostly depends on the interaction of Stargazin with PSD-95 and not upon the GluR2 AMPAR subunit C terminus. Disruption of interactions between Stargazin and PSD-95 strongly increases AMPAR surface diffusion, preventing AMPAR accumulation at postsynaptic sites. Furthermore, AMPARs and Stargazin diffuse as complexes in and out synapses. These results propose a model in which the Stargazin-PSD-95 interaction plays a key role to trap and transiently stabilize diffusing AMPARs in the postsynaptic density.

Regulatory mechanisms of AMPA receptors in synaptic plasticity

Activity-dependent changes in the strength of excitatory synapses are a cellular mechanism for the plasticity of neuronal networks that is widely recognized to underlie cognitive functions such as learning and memory. AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid)-type glutamate receptors (AMPARs) are the main transducers of rapid excitatory transmission in the mammalian CNS, and recent discoveries indicate that the mechanisms which regulate AMPARs are more complex than previously thought. This review focuses on recent evidence that alterations to AMPAR functional properties are coupled to their trafficking, cytoskeletal dynamics and local protein synthesis. These relationships offer new insights into the regulation of AMPARs and synaptic strength by cellular signalling.

Running to stand still: ionotropic receptor dynamics at central and peripheral synapses

For synapses to form and function, neurotransmitter receptors must be recruited to a location on the postsynaptic cell in direct apposition to presynaptic neurotransmitter release. However, once receptors are inserted into the postsynaptic membrane, they are not fixed in place but are continually exchanged between synaptic and extrasynaptic regions, and they cycle between the surface and intracellular compartments. This article highlights and compares the current knowledge about the dynamics of acetylcholine receptors at the vertebrate peripheral neuromuscular junction and AMPA, N-methyl-D-aspartate, and gamma-aminobutyric acid receptors in central synapses.

TARPs and the AMPA Receptor Trafficking Paradox

AMPA receptors (AMPARs) conduct fast, excitatory currents that depolarize neurons and trigger action potentials. AMPARs took on new importance when it was shown that AMPAR transport can increase or decrease the number of AMPARs at synapses and give rise to synapse plasticity, including long-term potentiation (LTP) and long-term depression (LTD). This review considers how transmembrane AMPAR regulatory proteins (TARPs), a novel family of AMPAR auxiliary subunits, have changed our view of AMPAR transport and raised some perplexing questions.

Qualitative and Quantitative Analyses of Protein Phosphorylation in Naive and Stimulated Mouse Synaptosomal Preparations

Activity-dependent protein phosphorylation is a highly dynamic yet tightly regulated process essential for cellular signaling. Although recognized as critical for neuronal functions, the extent and stoichiometry of phosphorylation in brain cells remain undetermined. In this study, we resolved activity-dependent changes in phosphorylation stoichiometry at specific sites in distinct subcellular compartments of brain cells. Following highly sensitive phosphopeptide enrichment using immobilized metal affinity chromatography and mass spectrometry, we isolated and identified 974 unique phosphorylation sites on 499 proteins, many of which are novel. To further explore the significance of specific phosphorylation sites, we used isobaric peptide labels and determined the absolute quantity of both phosphorylated and non-phosphorylated peptides of candidate phosphoproteins and estimated phosphorylation stoichiometry. The analyses of phosphorylation dynamics using differentially stimulated synaptic terminal preparations revealed activity-dependent changes in phosphorylation stoichiometry of target proteins. Using this method, we were able to differentiate between distinct isoforms of Ca2+/calmodulin-dependent protein kinase (CaMKII) and identify a novel activity-regulated phosphorylation site on the glutamate receptor subunit GluR1. Together these data illustrate that mass spectrometry-based methods can be used to determine activity-dependent changes in phosphorylation stoichiometry on candidate phosphopeptides following large scale phosphoproteome analysis of brain tissue.

Lamina-specific abnormalities of AMPA receptor trafficking and signaling molecule transcripts in the prefrontal cortex in schizophrenia

Ampakines, positive AMPA receptor modulators, can improve cognitive function in schizophrenia, and enhancement of AMPA receptor-mediated currents by them potentiates the activity of antipsychotics. In vitro studies have revealed that trafficking of AMPA receptors is mediated by specific interactions of a complex network of proteins that also target and anchor them at the postsynaptic density (PSD). The aim of this study was to determine whether there are abnormalities of the molecules associated with trafficking and localization of AMPA receptors at the PSD in the dorsolateral prefrontal cortex (DLPFC) in schizophrenia. We analyzed AMPA receptor expression in DLPFC in schizophrenia, major depression, bipolar disorder, and a control group, by examining transcript levels of all four AMPA receptor subunits by in situ hybridization. We found decreased GluR2 subunit expression in all three illnesses, decreased GluR3 in major depression, and decreased GluR4 in schizophrenia. However, autoradiography experiments showed no changes in AMPA receptor binding; thus, we hypothesized that these changes in receptor subunit stoichiometry do not alter binding to the assembled receptor, but rather intracellular processing. In situ hybridization for AMPA-trafficking molecules showed decreased expression of PICK1 and increased expression of stargazin in DLPFC in schizophrenia, both restricted to large cells of cortical layer III. These data suggest that AMPA-mediated glutamatergic neurotransmission is compromised in schizophrenia, particularly at the level of AMPA-related PSD proteins that mediate AMPA receptor trafficking, synaptic surface expression, and intracellular signaling.

Synapse-specific and developmentally regulated targeting of AMPA receptors by a family of MAGUK scaffolding proteins

Trafficking of AMPA receptors (AMPA-Rs) to and from synapses controls the strength of excitatory synaptic transmission. However, proteins that cluster AMPA-Rs at synapses remain poorly understood. Here we show that PSD-95-like membrane-associated guanylate kinases (PSD-MAGUKs) mediate this synaptic targeting, and we uncover a remarkable functional redundancy within this protein family. By manipulating endogenous neuronal PSD-MAGUK levels, we find that both PSD-95 and PSD-93 independently mediate AMPA-R targeting at mature synapses. We also reveal unanticipated synapse heterogeneity as loss of either PSD-95 or PSD-93 silences largely nonoverlapping populations of excitatory synapses. In adult PSD-95 and PSD-93 double knockout animals, SAP-102 is upregulated and compensates for the loss of synaptic AMPA-Rs. At immature synapses, PSD-95 and PSD-93 play little role in synaptic AMPA-R clustering; instead, SAP-102 dominates. These studies establish a PSD-MAGUK-specific regulation of AMPA-R synaptic expression that establishes and maintains glutamatergic synaptic transmission in the mammalian central nervous system.

Stargazin interacts functionally with the AMPA receptor glutamate-binding module

Neuronal AMPA receptors comprise pore forming glutamate receptor (GluR) proteins and auxiliary transmembrane AMPA receptor regulatory (TARP) subunits. TARPs traffic AMPA receptors to synapses and regulate channel gating. Both intracellular and extracellular regions in TARPs regulate AMPA receptors; however, the details for these interactions remain unknown. Here, we employ site-directed mutagenesis to determine functional interactions between GluR1 and the prototypical TARP, stargazin. We find that a point mutation in the glutamate-binding region of GluR1 corresponding to the Lurcher allele of GluRdelta2, abolishes stargazin's effects on receptor trafficking and channel gating. A point mutation that prevents receptor desensitization modulates the effects of stargazin on channel gating but preserves receptor trafficking. These studies identify a functional interaction of stargazin with the extracellular glutamate-binding domain of AMPA receptors.

Factors critical for the plasticity of dendritic spines and memory storage

The structure of dendritic spines is highly plastic and responds to synaptic activity, including activity patterns that induce long-term potentiation (LTP) and depression (LTD). Induction of LTP causes enlargement of spine heads, while LTD causes spine head shrinkage. In addition, spine structure is well associated with synaptic weight and the extent of synaptic plasticity, such that structural changes of the spine may represent forms of memory storage. While the correlation between structural and functional plasticity appears to be simple, the underlying mechanisms of spine plasticity are intricate. Spine plasticity requires multiple molecular interactions, and is affected by the surrounding environment and by cellular metabolic state. Here, I synthesize the latest progress in this field by defining six determinants of spine plasticity, and discuss the role of each factor in memory storage.

Abundant distribution of TARP gamma-8 in synaptic and extrasynaptic surface of hippocampal neurons and its major role in AMPA receptor expression on spines and dendrites

Transmembrane alpha-amino-3-hydroxyl-5-isoxazolepropionate (AMPA) receptor regulatory proteins (TARPs) play pivotal roles in AMPA receptor trafficking and gating. Here we examined cellular and subcellular distribution of TARP gamma-8 in the mouse brain. Immunoblot and immunofluorescence revealed the highest concentration of gamma-8 in the hippocampus. Immunogold electron microscopy demonstrated dense distribution of gamma-8 on the synaptic and extrasynaptic surface of hippocampal neurons with very low intracellular labeling. Of the neuronal surface, gamma-8 was distributed at the highest level on asymmetrical synapses of pyramidal cells and interneurons, whereas their symmetrical synapses selectively lacked immunogold labeling. Then, the role of gamma-8 in AMPA receptor expression was pursued in the hippocampus using mutant mice defective in the gamma-8 gene. In the mutant cornu ammonis (CA)1 region, synaptic and extrasynaptic AMPA receptors on dendrites and spines were severely reduced to 35-37% of control levels, whereas reduction was mild for extrasynaptic receptors on somata (74%) and no significant decrease was seen for intracellular receptors within spines. In the mutant CA3 region, synaptic AMPA receptors were reduced mildly at asymmetrical synapses in the stratum radiatum (67% of control level), and showed no significant decrease at mossy fiber-CA3 synapses. Therefore, gamma-8 is abundantly distributed on hippocampal excitatory synapses and extrasynaptic membranes, and plays an important role in increasing the number of synaptic and extrasynaptic AMPA receptors on dendrites and spines, particularly, in the CA1 region. Variable degrees of reduction further suggest that other TARPs may also mediate this function at different potencies depending on hippocampal subregions, input sources and neuronal compartments.

Biophysical model of AMPA receptor trafficking and its regulation during long-term potentiation/long-term depression

AMPA receptors mediate the majority of fast excitatory synaptic transmission in the CNS, and evidence suggests that AMPA receptor trafficking regulates synaptic strength, a phenomenon implicated in learning and memory. There are two major mechanisms of AMPA receptor trafficking: exocytic/endocytic exchange of surface receptors with intracellular receptor pools, and the lateral diffusion or hopping of surface receptors between the postsynaptic density and the surrounding extrasynaptic membrane. In this paper, we present a biophysical model of these trafficking mechanisms under basal conditions and during the expression of long-term potentiation (LTP) and depression (LTD). We show how our model reproduces a wide range of physiological data, and use this to make predictions regarding possible targets of second-messenger pathways activated during the induction phase of LTP/LTD.

PKC Anchoring to GluR4 AMPA Receptor Subunit Modulates PKC-Driven Receptor Phosphorylation and Surface Expression

Changes in the synaptic content of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)-type glutamate receptors lead to synaptic efficacy modifications, involved in synaptic plasticity mechanisms believed to underlie learning and memory formation. Early in development, GluR4 is highly expressed in the hippocampus, and GluR4-containing AMPA receptors are inserted into synapses. During synapse maturation, the number of AMPA receptors at the synapse is dynamically regulated, and both addition and removal of receptors from postsynaptic sites occur through regulated mechanisms. GluR4 delivery to synapses in rat hippocampal slices was shown to require protein kinase A (PKA)-mediated phosphorylation of GluR4 at serine 842 (Ser842). Protein kinase C (PKC) can also phosphorylate Ser842, and we have shown that PKCgamma can associate with GluR4. Here we show that activation of PKC in retina neurons, or in human embryonic kidney 293 cells cotransfected with GluR4 and PKCgamma, increases GluR4 surface expression and Ser842 phosphorylation. Moreover, mutation of amino acids R821A, K825A and R826A at the GluR4 C-terminal, within the interacting region of GluR4 with PKCgamma, abolishes the interaction between PKCgamma and GluR4 and prevents the stimulatory effect of PKCgamma on GluR4 Ser842 phosphorylation and surface expression. These data argue for a role of anchored PKCgamma in Ser842 phosphorylation and targeting to the plasma membrane. The triple GluR4 mutant is, however, phosphorylated by PKA, and it is targeted to the synapse in CA1 hippocampal neurons in organotypic rat hippocampal slices. The present findings show that the interaction between PKCgamma and GluR4 is specifically required to assure PKC-driven phosphorylation and surface membrane expression of GluR4.

Protein serine/threonine phosphatases in neuronal plasticity and disorders of learning and memory

Phosphorylation and dephosphorylation of cellular proteins by protein kinases and phosphatases represent important mechanisms for controlling major biological events. In the nervous system, protein phosphatases are contained in highly dynamic complexes localized within specialized subcellular compartments and they ensure timely dephosphorylation of multiple neuronal phosphoproteins. This modulates the responsiveness of individual synapses to neural activity and controls synaptic plasticity. These enzymes in turn play a key role in many forms of learning and memory, and their dysfunction contributes to cognitive deficits associated with aging and dementias or neurodegenerative diseases. Here, we review key modes of regulation of neuronal protein serine/threonine phosphatases and their contribution to disorders of learning and memory.

Synaptic plasticity and phosphorylation

A number of neuronal functions, including synaptic plasticity, depend on proper regulation of synaptic proteins, many of which can be rapidly regulated by phosphorylation. Neuronal activity controls the function of these synaptic proteins by exquisitely regulating the balance of various protein kinase and protein phosphatase activity. Recent understanding of synaptic plasticity mechanisms underscores important roles that these synaptic phosphoproteins play in regulating both pre- and post-synaptic functions. This review will focus on key postsynaptic phosphoproteins that have been implicated to play a role in synaptic plasticity.

Molecular organization and assembly of the postsynaptic density of excitatory brain synapses

The postsynaptic density (PSD) is a postsynaptic membrane specialization at excitatory synapses. The PSD is made of macromolecular multiprotein complexes, which contain a variety of synaptic proteins including membrane, scaffolding, and signaling proteins. By coaggregating with postsynaptic cell adhesion molecules, PSD proteins promote the formation and maturation of excitatory synapses. PSD proteins organize signaling pathways to coordinate structural and functional changes in synapses, and they regulate trafficking and recycling of glutamate receptors, which determines synaptic strength and plasticity. Synaptic activity dynamically regulates the assembly of the PSD through mechanisms including protein phosphorylation, palmitoylation, and protein degradation. PSD proteins associate with diverse motor proteins, suggesting that they function as adaptors linking motors to their specific cargoes.

Stargazin mutation impairs cerebellar synaptogenesis, synaptic maturation and synaptic protein distribution

Stargazin mutation results in absence epilepsy and cerebellar ataxia in stargazer (stg) mice. We have previously discovered defects of AMPA receptor function, failure of BDNF expression and immature morphology specifically in the cerebellar cortex of stg mice. To further characterize the nature of synaptic abnormalities, we examined the ultrastructure of cerebellar granule cell output synapses and measured the expression levels of several synaptic proteins in different brain regions of stg mutant. Electron microscopic examination revealed a number of immature features in the molecular layer of the mutant cerebellar cortex, including the presence of desmosoid plaques, concentric profiles of parallel fibers, smaller presynaptic terminal and fewer synaptic vesicles. Quantitative measurement showed a significantly lower number of synapses and smaller area of presynaptic terminals in adult stg cerebellum when compared with age-matched wildtype. Immunoblotting analysis of the SNARE proteins revealed selective reduction of the levels of synaptobrevin and synaptophysin in synaptosomes from stg cerebellum. The expression levels of synapsins were not altered in stg cerebellum, but showed a significant upregulation in stg cerebral cortex and hippocampus. Our results suggest that, despite the relatively normal gross morphology of cerebellum, stargazin mutation results in abnormal ultrastructure of cerebellar synapses, and stargazin-induced regional failure of BDNF expression may be responsible for abnormal SNARE protein distribution and partially attributes to the defects in the synaptic ultrastructure.

Actin-associated neurabin-protein phosphatase-1 complex regulates hippocampal plasticity

Protein phosphatase-1 (PP1) has been implicated in the control of long-term potentiation (LTP) and depression (LTD) in rat hippocampal CA1 neurons. PP1 catalytic subunits associate with multiple postsynaptic regulatory subunits, but the PP1 complexes that control hippocampal LTP and LTD in the rat hippocampus remain unidentified. The neuron-specific actin-binding protein, neurabin-I, is enriched in dendritic spines, and tethers PP1 to actin-rich postsynaptic density to regulate morphology and maturation of spines. The present studies utilized Sindbis virus-mediated expression of wild-type and mutant neurabin-I polypeptides in organotypic cultures of rat hippocampal slices to investigate their role in synaptic plasticity. While wild-type neurabin-I elicited no change in basal synaptic transmission, it enhanced LTD and inhibited LTP in CA1 pyramidal neurons. By comparison, mutant neurabins, specifically those unable to bind PP1 or F-actin, decreased basal synaptic transmission, attenuated LTD and increased LTP in slice cultures. Biochemical and cell biological analyses suggested that, by mislocalizing synaptic PP1, the mutant neurabins impaired the functions of endogenous neurabin-PP1 complexes and modulated LTP and LTD. Together, these studies provided the first biochemical and physiological evidence that a postsynaptic actin-bound neurabin-I-PP1 complex regulates synaptic transmission and bidirectional changes in hippocampal plasticity.

Role of the neurogranin concentrated in spines in the induction of long-term potentiation

Synaptic plasticity in CA1 hippocampal neurons depends on Ca2+ elevation and the resulting activation of calmodulin-dependent enzymes. Induction of long-term depression (LTD) depends on calcineurin, whereas long-term potentiation (LTP) depends on Ca2+/calmodulin-dependent protein kinase II (CaMKII). The concentration of calmodulin in neurons is considerably less than the total concentration of the apocalmodulin-binding proteins neurogranin and GAP-43, resulting in a low level of free calmodulin in the resting state. Neurogranin is highly concentrated in dendritic spines. To elucidate the role of neurogranin in synaptic plasticity, we constructed a computational model with emphasis on the interaction of calmodulin with neurogranin, calcineurin, and CaMKII. The model shows how the Ca2+ transients that occur during LTD or LTP induction affect calmodulin and how the resulting activation of calcineurin and CaMKII affects AMPA receptor-mediated transmission. In the model, knockout of neurogranin strongly diminishes the LTP induced by a single 100 Hz, 1 s tetanus and slightly enhances LTD, in accord with experimental data. Our simulations show that exchange of calmodulin between a spine and its parent dendrite is limited. Therefore, inducing LTP with a short tetanus requires calmodulin stored in spines in the form of rapidly dissociating calmodulin-neurogranin complexes.

AMPA and NMDA glutamate receptor trafficking: multiple roads for reaching and leaving the synapse

Glutamate receptor trafficking in and out of synapses is one of the core mechanisms for rapid changes in the number of functional receptors during synaptic plasticity. Recent data have shown that the fast gain and loss of receptors from synaptic sites are accounted for by endocytic/exocytic processes and by their lateral diffusion in the plane of the membrane. These events are interdependent and regulated by neuronal activity and interactions with scaffolding proteins. We review here the main cellular steps for AMPA and NMDA receptor synthesis, traffic within intracellular organelles, membrane exocytosis/endocytosis and surface trafficking. We focus on new findings that shed light on the regulation of receptor cycling events and surface trafficking and the way that this might reshape our thinking about the specific regulation of receptor accumulation at synapses.

Synaptic Incorporation of AMPA Receptors during LTP Is Controlled by a PKC Phosphorylation Site on GluR1

Incorporation of GluR1-containing AMPA receptors into synapses is essential to several forms of neural plasticity, including long-term potentiation (LTP). Numerous signaling pathways that trigger this process have been identified, but the direct modifications of GluR1 that control its incorporation into synapses are unclear. Here, we show that phosphorylation of GluR1 by PKC at a highly conserved serine 818 residue is increased during LTP and critical for LTP expression. GluR1 is phosphorylated by PKC at this site in vitro and in vivo. In addition, acute phosphorylation at GluR1 S818 by PKC, as well as a phosphomimetic mutation, promotes GluR1 synaptic incorporation. Conversely, preventing GluR1 S818 phosphorylation reduces LTP and blocks PKC-driven synaptic incorporation of GluR1. We conclude that the phosphorylation of GluR1 S818 by PKC is a critical event in the plasticity-driven synaptic incorporation of AMPA receptors.

Learning from stargazin: the mouse, the phenotype and the unexpected

The stargazin gene (also referred to as Cacng2) has been identified by forward genetics in a spontaneous mouse mutant with ataxic gait, upward head-elevating movements (hence the name stargazer for the mouse) and episodes of spike-wave discharges. Stargazin is related to the gamma-1 subunit of skeletal muscle voltage-dependent calcium channel (VDCC), and a deficit in its role as auxiliary VDCC subunit was proposed to underlie the epileptic phenotype of the mouse; yet, a conclusive demonstration of stargazin function in VDCC regulation is still lacking. In contrast, stargazin and its three closely related isoforms gamma-3, gamma-4 and gamma-8 were shown to function as auxiliary subunits for a very different ion channel - the AMPA-type glutamate receptor - prominently regulating early intracellular transport, synaptic targeting and anchoring, and ion channel functions of this major excitatory receptor in the brain.

Substrate Localization Creates Specificity in Calcium/Calmodulin-dependent Protein Kinase II Signaling at Synapses

Calcium/calmodulin-dependent protein kinase II (CaMKII), a major component of the postsynaptic density (PSD) of excitatory synapses, plays a key role in the regulation of synaptic function in the mammalian brain. Although many postsynaptic substrates for CaMKII have been characterized in vitro, relatively little is known about their phosphorylation in vivo. By tagging synaptic proteins with a peptide substrate specific for CaMKII and expressing them in cultured neurons, we have visualized substrate phosphorylation by CaMKII at intact synapses. All substrates tested were strongly phosphorylated by CaMKII in HEK293 cells. However, activity-dependent phosphorylation of substrates at synapses was highly selective in that the glutamate receptor subunits NR2B and GluR1 were poorly phosphorylated whereas PSD-95 and Stargazin, proteins implicated in the scaffolding and trafficking of AMPA receptors, were robustly phosphorylated. Phosphatase activity limited phosphorylation of Stargazin but not NR2B and GluR1. These results suggest that the unique molecular architecture of the PSD results in highly selective substrate discrimination by CaMKII.