Biochemical and immunocytochemical characterization of GRIP, a putative AMPA receptor anchoring protein, in rat brain

Splice-dependent trans-synaptic PTPδ-IL1RAPL1 interaction regulates synapse formation and non-REM sleep

Alternative splicing regulates trans‐synaptic adhesions and synapse development, but supporting in vivo evidence is limited. PTPδ, a receptor tyrosine phosphatase adhering to multiple synaptic adhesion molecules, is associated with various neuropsychiatric disorders; however, its in vivo functions remain unclear. Here, we show that PTPδ is mainly present at excitatory presynaptic sites by endogenous PTPδ tagging. Global PTPδ deletion in mice leads to input‐specific decreases in excitatory synapse development and strength. This involves tyrosine dephosphorylation and synaptic loss of IL1RAPL1, a postsynaptic partner of PTPδ requiring the PTPδ‐meA splice insert for binding. Importantly, PTPδ‐mutant mice lacking the PTPδ‐meA insert, and thus lacking the PTPδ interaction with IL1RAPL1 but not other postsynaptic partners, recapitulate biochemical and synaptic phenotypes of global PTPδ‐mutant mice. Behaviorally, both global and meA‐specific PTPδ‐mutant mice display abnormal sleep behavior and non‐REM rhythms. Therefore, alternative splicing in PTPδ regulates excitatory synapse development and sleep by modulating a specific trans‐synaptic adhesion.

Myosin IXa Binds AMPAR and Regulates Synaptic Structure, LTP, and Cognitive Function

Myosin IXa (Myo9a) is a motor protein that is highly expressed in the brain. However, the role of Myo9a in neurons remains unknown. Here, we investigated Myo9a function in hippocampal synapses. In rat hippocampal neurons, Myo9a localizes to the postsynaptic density (PSD) and binds the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR) GluA2 subunit. Myo9a(+/-) mice displayed a thicker PSD and increased levels of PSD95 and surface AMPAR expression. Furthermore, synaptic transmission, long-term potentiation (LTP) and cognitive functions were impaired in Myo9a(+/-) mice. Together, these results support a key role for Myo9a in controlling the molecular structure and function of hippocampal synapses.

Synaptic adhesion molecule IgSF11 regulates synaptic transmission and plasticity

Synaptic adhesion molecules regulate synapse development and plasticity through mechanisms that include trans-synaptic adhesion and recruitment of diverse synaptic proteins. We found that the immunoglobulin superfamily member 11 (IgSF11), a homophilic adhesion molecule that preferentially expressed in the brain, is a dual-binding partner of the postsynaptic scaffolding protein PSD-95 and AMPA glutamate receptors (AMPARs). IgSF11 required PSD-95 binding for its excitatory synaptic localization. In addition, IgSF11 stabilized synaptic AMPARs, as determined by IgSF11 knockdown-induced suppression of AMPAR-mediated synaptic transmission and increased surface mobility of AMPARs, measured by high-throughput, single-molecule tracking. IgSF11 deletion in mice led to the suppression of AMPAR-mediated synaptic transmission in the dentate gyrus and long-term potentiation in the CA1 region of the hippocampus. IgSF11 did not regulate the functional characteristics of AMPARs, including desensitization, deactivation or recovery. These results suggest that IgSF11 regulates excitatory synaptic transmission and plasticity through its tripartite interactions with PSD-95 and AMPARs.

Neto2 interacts with the scaffolding protein GRIP and regulates synaptic abundance of kainate receptors

Kainate receptors (KARs) are a class of ionotropic glutamate receptors that are expressed throughout the central nervous system. The function and subcellular localization of KARs are tightly regulated by accessory proteins. We have previously identified the single-pass transmembrane proteins, Neto1 and Neto2, to be associated with native KARs. In the hippocampus, Neto1, but not Neto2, controls the abundance and modulates the kinetics of postsynaptic KARs. Here we evaluated whether Neto2 regulates synaptic KAR levels in the cerebellum where Neto1 expression is limited to the deep cerebellar nuclei. In the cerebellum, where Neto2 is present abundantly, we found a ∼40% decrease in GluK2-KARs at the postsynaptic density (PSD) of Neto2-null mice. No change, however, was observed in total level of GluK2-KARs, thereby suggesting a critical role of Neto2 on the synaptic localization of cerebellar KARs. The presence of a putative class II PDZ binding motif on Neto2 led us to also investigate whether it interacts with PDZ domain-containing proteins previously implicated in regulating synaptic abundance of KARs. We identified a PDZ-dependent interaction between Neto2 and the scaffolding protein GRIP. Furthermore, coexpression of Neto2 significantly increased the amount of GRIP associated with GluK2, suggesting that Neto2 may promote and/or stabilize GluK2:GRIP interactions. Our results demonstrate that Neto2, like Neto1, is an important auxiliary protein for modulating the synaptic levels of KARs. Moreover, we propose that the interactions of Neto1/2 with various scaffolding proteins is a critical mechanism by which KARs are stabilized at diverse synapses.

Interacting partners of AMPA-type glutamate receptors

Glutamate is the principal excitatory neurotransmitter in the brain. The alpha-amino-3-hydroxyl-5-methyl-4-isoxazolepropionic (AMPA) receptors, as one of several types of endogenous ionotropic glutamate receptors, mediate the fast excitatory synaptic transmission that is essential for information processing and integration in the mammalian brain. Modifications of AMPA receptors are assumed to be the molecular basis underlying learning and memory, and impairments of AMPA receptors cause certain neurological diseases, including epilepsy, autism spectrum disorders, and Alzheimer's disease. Thus, extensive studies have been conducted, and these have revealed a complex protein-protein network controlling the expression, trafficking, and function of AMPA receptors in neurons. Here, we summarize the interacting partners of AMPA-type glutamate receptors and the functional implications of these interactions.

mGluR and NMDAR activation internalize distinct populations of AMPARs

Activation of metabotropic- (mGluRs) or NMDA-type glutamate receptors (NMDARs) each can induce long-term depression (LTD) of synaptic transmission in CA1 hippocampal neurons. These two forms of LTD are triggered by diverse signaling pathways yet both are expressed by the internalization of AMPA-type glutamate receptors (AMPARs). An unanswered question remains as to whether the convergence of the mGluR and NMDAR signaling pathways on AMPAR endocytosis renders these two forms of plasticity functionally equivalent, with both pathways inducing endocytosis of the same population of synaptic AMPARs. We now report evidence that these pathways couple to the endocytosis of distinct populations of AMPARs defined by their mobility in the membrane surface. NMDAR activation enhances removal of surface AMPARs that rapidly cycle into and out of the membrane surface, while activation of mGluRs with DHPG results in the internalization of a non-mobile population of AMPARs. Glutamate Receptor Interacting Proteins 1 and 2 (GRIP1/2) play a key role in defining the non-cycling receptor population. GRIP1/2 knockdown with siRNA increases the proportion of rapidly cycling surface AMPARs and inhibits mGluR- but not NMDAR-mediated AMPAR internalization. Additionally, we find that mGluR activation dissociates surface AMPARs from GRIP1/2 while stimulation of NMDARs elicits the loss of membrane receptors not bound to GRIP1/2. We propose that these two receptor pathways can drive the endocytosis of distinct populations of AMPARs: NMDARs activation induces the endocytosis of rapidly cycling surface AMPARs not directly associated with GRIP1/2 while mGluR activation induces the endocytosis of non-cycling GRIP-bound surface AMPARs.

A specific requirement of Arc/Arg3.1 for visual experience-induced homeostatic synaptic plasticity in mouse primary visual cortex

Visual experience scales down excitatory synapses in the superficial layers of visual cortex in a process that provides an in vivo paradigm of homeostatic synaptic scaling. Experience-induced increases in neural activity rapidly upregulates mRNAs of immediate early genes involved in synaptic plasticity, one of which is Arc (activity-regulated cytoskeleton protein or Arg3.1). Cell biological studies indicate that Arc/Arg3.1 protein functions to recruit endocytic machinery for AMPA receptor internalization, and this action, together with its activity-dependent expression, rationalizes a role for Arc/Arg3.1 in homeostatic synaptic scaling. Here, we investigated the role of Arc/Arg3.1 in homeostatic scaling in vivo by examining experience-dependent development of layer 2/3 neurons in the visual cortex of Arc/Arg3.1 knock-out (KO) mice. Arc/Arg3.1 KOs show minimal changes in basal and developmental regulation of excitatory synaptic strengths but display a profound deficit in homeostatic regulation of excitatory synapses by visual experience. As additional evidence of specificity, we found that the visual experience-induced regulation of inhibitory synapses is normal, although the basal inhibitory synaptic strength is increased in the Arc/Arg3.1 KOs. Our results demonstrate that Arc/Arg3.1 plays a selective role in regulating visual experience-dependent homeostatic plasticity of excitatory synaptic transmission in vivo.

Regulation of AMPA receptor trafficking by delta-catenin

delta-catenin is a protein that binds to the classical cadherins and to synaptic scaffolding proteins in a manner which allows the protein to serve as a link between the adherens junction and the postsynaptic complex. Here we show the regulatory role of delta-catenin on synaptic transmission. delta-catenin increased the AMPA receptor-mediated EPSC, but had no significant effect on the NMDA receptor-mediated EPSC. The effect of delta-catenin on the AMPAR EPSC was mediated by its PDZ ligand. delta-catenin regulates the surface expression of GluR2 in the dendritic spines of neurons. Immunoprecipitation revealed that delta-catenin bound to GRIP-1. In COS7 cells, co-transfection of delta-catenin, GRIP and GluR2 showed that delta-catenin regulates the membrane localization of GRIP through its PDZ ligand and increased the surface expression of GluR2. This study directly shows that delta-catenin is essential for the trafficking and positioning GluR2 in the spine and thus further links delta-catenin to neuronal plasticity.

The efficiency of Xenopus primordial germ cell migration depends on the germplasm mRNA encoding the PDZ domain protein Grip2

A microarray analysis of vegetal pole sequences in the egg and early Xenopus laevis embryo identified Unigene Xl.14891 as a vegetally localized RNA. Analysis of the Xenopus tropicalis genome showed this Unigene to be localized near the 3' end of the Grip2 (glutamate receptor interacting protein 2) transcription unit. RACE showed that the Unigene represented the 3' UTR of Grip2 mRNA. Grip2 mRNA is present in the mitochondrial cloud of late pre-vitellogenic oocytes and then in the germplasm through oogenesis and early development until tailbud tadpole stages. Interference with Grip2 mRNA translation using two antisense morpholino oligos (MOs) impairs primordial germ cell (PGC) migration to the germinal ridges. Both MOs also inhibit swimming movements of the tailbud tadpole, known to involve glutamate receptors. We conclude that Grip2 has several functions in the embryo, including enabling efficient PGC migration.

Neuronal cotransport of glycine receptor and the scaffold protein gephyrin

The dynamics of postsynaptic receptor scaffold formation and remodeling at inhibitory synapses remain largely unknown. Gephyrin, which is a multimeric scaffold protein, interacts with cytoskeletal elements and stabilizes glycine receptors (GlyRs) and individual subtypes of γ-aminobutyric acid A receptors at inhibitory postsynaptic sites. We report intracellular mobility of gephyrin transports packets over time. Gephyrin units enter and exit active synapses within several minutes. In addition to previous reports of GlyR–gephyrin interactions at plasma membranes, we show cosedimentation and coimmunoprecipitation of both proteins from vesicular fractions. Moreover, GlyR and gephyrin are cotransported within neuronal dendrites and further coimmunoprecipitate and colocalize with the dynein motor complex. As a result, the blockade of dynein function or dynein–gephyrin interaction, as well as the depolymerization of microtubules, interferes with retrograde gephyrin recruitment. Our data suggest a GlyR–gephyrin–dynein transport complex and support the concept that gephyrin–motor interactions contribute to the dynamic and activity-dependent rearrangement of postsynaptic GlyRs, a process thought to underlie the regulation of synaptic strength.

Identification and characterization of two novel splice forms of GRIP1 in the rat brain

We cloned two novel alternatively-spliced mRNA isoforms of glutamate receptor interacting protein 1 (GRIP1) which we named GRIP1d and GRIP1e 4-7. GRIP1d is a 135 kDa, 7-PDZ-domain variant of GRIP1, containing the 12 amino acid C-terminus originally described for the 4-PDZ-domain GRIP1c 4-7. GRIP1e 4-7 is a 75 kDa 4-PDZ-domain variant of GRIP1, containing the 12 amino acid C-terminus originally described for the 7-PDZ-domain GRIP1a/b. Northern blots indicated that GRIP1d mRNA is 5.1 kb long and abundant in brain. An antibody to the C-terminus of the 75 kDa GRIP1c 4-7 also recognized an abundant 135 kDa protein, consistent with the predicted size of GRIP1d. Similarly, an antibody to the C-terminus of the 135 kDa GRIP1a/b also recognized a low abundance 75 kDa protein, consistent with the predicted size of GRIP1e 4-7. Immunocytochemistry of hippocampal cultures and intact brain using these antibodies showed that (i) these isoforms are present in both GABAergic and glutamatergic synapses, and (ii) the isoforms co-localize in individual synapses. While GRIP1a/b isoforms are abundant in interneurons and highly concentrated in GABAergic presynaptic terminals, the isoforms recognized by the antibody to the C-terminus common to GRIP1c 4-7 and GRIP1d are much less abundant in interneurons and preferentially concentrate at the postsynaptic complex.

GRIP1 in GABAergic synapses

The glutamate receptor-interacting protein GRIP1 is present in glutamatergic synapses and interacts with the GluR2/3/4c subunits of the AMPA receptors. This interaction plays important roles in trafficking, synaptic targeting, and recycling of AMPA receptors as well as in the plasticity of glutamatergic synapses. Although GRIP1 has been shown to be present at GABAergic synapses in cultured neurons, the use of EM (electron microscopy) immunocytochemistry in the intact brain has failed to convincingly reveal the presence of GRIP1 in GABAergic synapses. Therefore, most studies on GRIP1 have focused on glutamatergic synapses. By using mild tissue fixation and embedding in EM, we show that in the intact brain the 7-PDZ domain GRIP1a/b is present not only in glutamatergic synapses but also in GABAergic synapses. In GABAergic synapses GRIP1a/b localizes both at the presynaptic terminals and postsynaptically, being frequently localized on the synaptic membranes or the synaptic junctional complex. Considerably higher density of GRIP1a/b is found in the presynaptic GABAergic terminals than in the glutamatergic terminals, while the density of GRIP1a/b in the postsynaptic complex is similar in both types of synapses. The results also show that the 7-PDZ and the shorter 4-PDZ domain splice forms of GRIP1 (GRIP1c 4-7) frequently colocalize with each other in individual GABAergic and glutamatergic synapses. The results suggest that GRIP1 splice forms might play important roles in brain GABAergic synapses.

The molecular pharmacology and cell biology of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate receptors (AMPARs) are of fundamental importance in the brain. They are responsible for the majority of fast excitatory synaptic transmission, and their overactivation is potently excitotoxic. Recent findings have implicated AMPARs in synapse formation and stabilization, and regulation of functional AMPARs is the principal mechanism underlying synaptic plasticity. Changes in AMPAR activity have been described in the pathology of numerous diseases, such as Alzheimer's disease, stroke, and epilepsy. Unsurprisingly, the developmental and activity-dependent changes in the functional synaptic expression of these receptors are under tight cellular regulation. The molecular and cellular mechanisms that control the postsynaptic insertion, arrangement, and lifetime of surface-expressed AMPARs are the subject of intense and widespread investigation. For example, there has been an explosion of information about proteins that interact with AMPAR subunits, and these interactors are beginning to provide real insight into the molecular and cellular mechanisms underlying the cell biology of AMPARs. As a result, there has been considerable progress in this field, and the aim of this review is to provide an account of the current state of knowledge.

LAR receptor protein tyrosine phosphatases in the development and maintenance of excitatory synapses

Leukocyte common antigen-related (LAR) family receptor protein tyrosine phosphatases (LAR-RPTP) bind to liprin-alpha (SYD2) and are implicated in axon guidance. We report that LAR-RPTP is concentrated in mature synapses in cultured rat hippocampal neurons, and is important for the development and maintenance of excitatory synapses in hippocampal neurons. RNA interference (RNAi) knockdown of LAR or dominant-negative disruption of LAR function results in loss of excitatory synapses and dendritic spines, reduction of surface AMPA receptors, impairment of dendritic targeting of the cadherin-beta-catenin complex, and reduction in the amplitude and frequency of miniature excitatory postsynaptic currents (mEPSCs). Cadherin, beta-catenin and GluR2/3 are tyrosine phosphoproteins that coimmunoprecipitate with liprin-alpha and GRIP from rat brain extracts. We propose that the cadherin-beta-catenin complex is cotransported with AMPA receptors to synapses and dendritic spines by a mechanism that involves binding of liprin-alpha to LAR-RPTP and tyrosine dephosphorylation by LAR-RPTP.

Rationale for and use of NMDA receptor antagonists in Parkinson's disease

N-Methyl-d-aspartate (NMDA) glutamate receptors are a class of excitatory amino acid receptors, which have several important functions in the motor circuits of the basal ganglia, and are viewed as important targets for the development of new drugs to prevent or treat Parkinson's disease (PD). NMDA receptors are ligand-gated ion channels composed of multiple subunits, each of which has distinct cellular and regional patterns of expression. They have complex regulatory properties, with both agonist and co-agonist binding sites and regulation by phosphorylation and protein-protein interactions. They are found in all of the structures of the basal ganglia, although the subunit composition in the various structures is different. NMDA receptors present in the striatum are crucial for dopamine-glutamate interactions. The abundance, structure, and function of striatal receptors are altered by the dopamine depletion and further modified by the pharmacological treatments used in PD. In animal models, NMDA receptor antagonists are effective antiparkinsonian agents and can reduce the complications of chronic dopaminergic therapy (wearing off and dyskinesias). Use of these agents in humans has been limited because of the adverse effects associated with nonselective blockade of NMDA receptor function, but the development of more potent and selective pharmaceuticals holds the promise of an important new therapeutic approach for PD.

A four PDZ domain-containing splice variant form of GRIP1 is localized in GABAergic and glutamatergic synapses in the brain

We have isolated, from a rat brain cDNA library, a clone corresponding to a 2779-bp cDNA encoding a novel splice form of the glutamate receptor interacting protein-1 (GRIP1). We call this 696-amino acid splice form GRIP1c 4-7 to differentiate it from longer splice forms of GRIP1a/b containing seven PDZ domains. The four PDZ domains of GRIP1c 4-7 are identical to PDZ domains 4-7 of GRIP1a/b. GRIP1c 4-7 also contains 35 amino acids at the N terminus and 12 amino acids at the C terminus that are different from GRIP1a/b. In transfected HEK293 cells, a majority of GRIP1c 4-7 was associated with the plasma membrane. GRIP1c 4-7 interacted with GluR2/3 subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor. In low density hippocampal cultures, GRIP1c 4-7 clusters colocalized with GABAergic (where GABA is gamma-aminobutyric acid) and glutamatergic synapses, although a higher percentage of GRIP1c 4-7 clusters colocalized with gamma-aminobutyric acid, type A, receptor (GABA(A)R) clusters than with alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptor clusters. Transfection of hippocampal neurons with hemagglutinin-tagged GRIP1c 4-7 showed that it could target to the postsynaptic complex of GABAergic synapses colocalizing with GABA(A)R clusters. GRIP1c 4-7-specific antibodies, which did not recognize previously described splice forms of GRIP1, recognized a 75-kDa protein that is enriched in a postsynaptic density fraction isolated from rat brain. EM immunocytochemistry experiments showed that in intact brain GRIP1c 4-7 concentrates at postsynaptic complexes of both type I glutamatergic and type II GABAergic synapses although it is also presynaptically localized. These results indicate that GRIP1c 4-7 plays a role not only in glutamatergic synapses but also in GABAergic synapses.

Semiquantitative proteomic analysis of rat forebrain postsynaptic density fractions by mass spectrometry

The postsynaptic density (PSD) of central excitatory synapses plays a key role in postsynaptic signal transduction and contains a high concentration of glutamate receptors and associated scaffold and signaling proteins. We report here a comprehensive analysis of purified PSD fractions by liquid chromatography coupled with tandem mass spectrometry (LC-MS/MS). We identified 374 different proteins that copurified with the PSD structure and discovered thirteen phosphorylated sites from eight proteins. These proteins were classified into numerous functional groups, implying that the signaling pathways in the PSD are complex and diverse. Furthermore, using quantitative mass spectrometry, we measured the molar concentration and relative stoichiometries of a number of glutamate receptor subunits and scaffold proteins in the postsynaptic density. Thus this proteomic study reveals crucial information about molecular abundance as well as molecular diversity in the PSD, and provides a basis for further studies on the molecular mechanisms of synaptic function and plasticity.

SHN-1, a Shank homologue inC. elegans, affects defecation rhythm via the inositol-1,4,5-trisphosphate receptor

Protein localization in the postsynaptic density (PSD) of neurons is mediated by scaffolding proteins such as PSD-95 and Shank, which ensure proper function of receptors at the membrane. The Shank family of scaffolding proteins contain PDZ (PSD-95, Dlg, and ZO-1) domains and have been implicated in the localizations of many receptor proteins including glutamate receptors in mammals. We have identified and characterized shn-1, the only homologue of Shank in Caenorhabditis elegans. The shn-1 gene shows approximately 40% identity over 1000 amino acids to rat Shanks. SHN-1 protein is localized in various tissues including neurons, pharynx and intestine. RNAi suppression of SHN-1 did not cause lethality or developmental abnormality. However, suppression of SHN-1 in the itr-1 (sa73) mutant, which has a defective inositol-1,4,5-trisphosphate (IP(3)) receptor, resulted in animals with altered defecation rhythm. Our data suggest a possible role of SHN-1 in affecting function of IP(3) receptors in C. elegans.

A glutamate receptor-interacting protein homolog organizes muscle guidance in Drosophila

During Drosophila embryogenesis, developing muscles extend growth-cone-like structures to navigate toward specific epidermal attachment sites. Here, we show that the homolog of Glutamate Receptor-Interacting Proteins (DGrip) acts as a key component of proper muscle guidance. Mutations in dgrip impair patterning of ventral longitudinal muscles (VLMs), whereas lateral transverse muscles (LTMs) that attach to intrasegmental attachment sites develop normally. Myoblast fusion, stabilization of muscle contacts, and general muscle function are not impaired in the absence of DGrip. Instead, the proper formation of cellular extensions during guidance fails in dgrip mutant VLMs. DGrip protein concentrates at the ends of VLMs while these muscles guide toward segment border attachment sites. Conversely, LTMs overexpressing DGrip form ectopic cellular extensions that can cause attachment of these muscles to other muscles at segment borders. Our data suggest that DGrip participates in the reception of an attractive signal that emanates from the epidermal attachment sites to direct the motility of developing muscles. This dgrip phenotype should be valuable to study mechanistic principles of Grip function.

CASK and Dlg form a PDZ protein complex at the mammalian neuromuscular junction

Membrane-associated guanylate kinases (MAGUKs) are modular adapter proteins that serve as scaffolding molecules and anchor channels and receptors via their PDZ (PSD-95, Dlg, Zo-1) domains. Calcium, calmodulin-associated serine/threonine kinase (CASK) is a MAGUK that is critical at synapses in the central nervous system and at cell-cell junctions because of its interactions with channels, receptors, and structural proteins. We show via confocal microscopy that CASK and another MAGUK, Discs Large (Dlg), are present at the mammalian neuromuscular junction in skeletal muscle. Immunoprecipitation data from mouse muscle show that CASK associates with Dlg, providing evidence of a MAGUK protein complex at this synapse. These data indicate that CASK and Dlg may act as a scaffold for organizing receptors and channels at the postsynaptic membrane of the neuromuscular junction.

Interaction of the ERC family of RIM-binding proteins with the liprin-alpha family of multidomain proteins

Liprin-alpha/SYD-2 is a family of multidomain proteins with four known isoforms. One of the reported functions of liprin-alpha is to regulate the development of presynaptic active zones, but the underlying mechanism is poorly understood. Here we report that liprin-alpha directly interacts with the ERC (ELKS-Rab6-interacting protein-CAST) family of proteins, members of which are known to bind RIMs, the active zone proteins that regulate neurotransmitter release. In vitro results indicate that ERC2/CAST, an active zone-specific isoform, interacts with all of the known isoforms of liprin-alpha and that liprin-alpha1 associates with both ERC2 and ERC1b, a splice variant of ERC1 that distributes to both cytosolic and active zone regions. ERC2 colocalizes with liprin-alpha1 in cultured neurons and forms a complex with liprin-alpha1 in brain. Liprin-alpha1, when expressed alone in cultured neurons, shows a partial synaptic localization. When coexpressed with ERC2, however, liprin-alpha1 is redistributed to synaptic sites. Moreover, roughly the first half of ERC2, which contains the liprin-alpha-binding region, is sufficient for the synaptic localization of liprin-alpha1 while the second half is not. These results suggest that the interaction between ERC2 and liprin-alpha may be involved in the presynaptic localization of liprin-alpha and the molecular organization of presynaptic active zones.

Specific involvement in neuropathic pain of AMPA receptors and adapter proteins for the GluR2 subunit

Chronic pain states arise from peripheral nerve injury and are inadequately treated with current analgesics. Using intrathecal drug administration in a rat model of neuropathic pain, we demonstrate that AMPA receptors play a role in the central sensitisation that is thought to underpin chronic pain. The GluR2 subunit of the AMPA receptor binds to a number of intracellular adapter proteins including GRIP, PICK1 and NSF, which may link the receptor to proteins with signalling, scaffolding and other roles. We implicate for the first time a possible role for GRIP, PICK1 and NSF in neuropathic sensitisation from experiments with cell-permeable blocking peptides mimicking their GluR2 interaction motifs and also demonstrate differential changes in expression of these proteins following peripheral nerve injury. These studies suggest a critical involvement of protein:protein complexes associated with the AMPA receptor in neuropathic pain, and the possibility that they may have potential as novel therapeutic targets.

Lipid rafts in the maintenance of synapses, dendritic spines, and surface AMPA receptor stability

Cholesterol/sphingolipid microdomains (lipid rafts) in the membrane are involved in protein trafficking, formation of signaling complexes, and regulation of actin cytoskeleton. Here, we show that lipid rafts exist abundantly in dendrites of cultured hippocampal neurons, in which they are associated with several postsynaptic proteins including surface AMPA receptors. Depletion of cholesterol/sphingolipid leads to instability of surface AMPA receptors and gradual loss of synapses (both inhibitory and excitatory) and dendritic spines. The remaining synapses and spines in raft-depleted neurons become greatly enlarged. The importance of lipid rafts for normal synapse density and morphology could explain why cholesterol promotes synapse maturation in retinal ganglion cells (Mauch et al., 2001) and offers a potential link between disordered cholesterol metabolism and the synapse loss seen in neurodegenerative disease.

Association of the kinesin motor KIF1A with the multimodular protein liprin-alpha

Liprin-alpha/SYD-2 is a multimodular scaffolding protein important for presynaptic differentiation and postsynaptic targeting of alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid glutamate receptors. However, the molecular mechanisms underlying these functions remain largely unknown. Here we report that liprin-alpha interacts with the neuron-specific kinesin motor KIF1A. KIF1A colocalizes with liprin-alpha in various subcellular regions of neurons. KIF1A coaccumulates with liprin-alpha in ligated sciatic nerves. KIF1A cofractionates and coimmunopreciptates with liprin-alpha and various liprin-alpha-associated membrane, signaling, and scaffolding proteins including alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid receptors, GRIP/ABP, RIM, GIT1, and beta PIX. These results suggest that liprin-alpha functions as a KIF1A receptor, linking KIF1A to various liprin-alpha-associated proteins for their transport in neurons.

Interaction between liprin-alpha and GIT1 is required for AMPA receptor targeting

Liprin-alpha is a multidomain protein that interacts with the LAR family of receptor protein tyrosine phosphatases and the GRIP/ABP family of AMPA receptor-interacting proteins. Previous studies have indicated that liprin-alpha regulates the development of presynaptic active zones and that the association of liprin-alpha with GRIP is required for postsynaptic targeting of AMPA receptors. However, the underlying molecular mechanisms are not well understood. Here we report that liprin-alpha directly interacts with GIT1, a multidomain protein with GTPase-activating protein activity for the ADP-ribosylation factor family of small GTPases known to regulate protein trafficking and the actin cytoskeleton. Electron microscopic analysis indicates that GIT1 distributes to the region of postsynaptic density (PSD) as well as presynaptic active zones. GIT1 is enriched in PSD fractions and forms a complex with liprin-alpha, GRIP, and AMPA receptors in brain. Expression of dominant-negative constructs interfering with the GIT1-liprin-alpha interaction leads to a selective and marked reduction in the dendritic and surface clustering of AMPA receptors in cultured neurons. These results suggest that the GIT1-liprin-alpha interaction is required for AMPA receptor targeting and that GIT1 may play an important role in the organization of presynaptic and postsynaptic multiprotein complexes.

Proteins interactions implicated in AMPA receptor trafficking: a clear destination and an improving route map

The mechanisms that regulate alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor (AMPAR), synthesis, transport, targeting and surface expression are of fundamental importance to understand the molecular basis of fast excitatory neurotransmission and synaptic plasticity in the mammalian CNS. An area of intense current interest is how AMPARs are directed to the correct locations in the neuron as and when required. This is a multi-layered problem, which involves complex spatio-temporal coordination of multiple protein interactions. Considerable progress has been achieved in identifying a number of proteins that bind directly to AMPAR subunits and the functional consequences of blocking some of these interactions have been determined. This review highlights recent developments in the field.

The proteoglycan NG2 is complexed with alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors by the PDZ glutamate receptor interaction protein (GRIP) in glial progenitor cells. Implications for glial-neuronal signaling

The proteoglycan NG2 is expressed by immature glial cells in the developing and adult central nervous system. Using the COOH-terminal region of NG2 as bait in a yeast two-hybrid screen, we identified the glutamate receptor interaction protein GRIP1, a multi-PDZ domain protein, as an interacting partner. NG2 exhibits a PDZ binding motif at the extreme COOH terminus which binds to the seventh PDZ domain of GRIP1. In addition to the published expression in neurons, GRIP1 is expressed by immature glial cells. GRIP1 is known to bind to the GluRB subunit of the AMPA glutamate receptor expressed by subpopulations of neurons and immature glial cells. In cultures of primary oligodendrocytes, cells coexpress GluRB and NG2. A complex of NG2, GRIP1, and GluRB can be precipitated from transfected mammalian cells and from cultures of primary oligodendrocytes. Furthermore, NG2 and GRIP can be coprecipitated from developing brain tissue. These data suggest that GRIP1 acts as a scaffolding molecule clustering NG2 and AMPA receptors in immature glia. In view of the presence of synaptic contacts between neurons and NG2-positive glial cells in the hippocampus and the close association of NG2-expressing glial cells with axons, we suggest a role for the NG2.AMPA receptor complex in glial-neuronal recognition and signaling.

Immunolocalization of PICK1 in the ascending auditory pathways of the adult rat

Protein that interacts with C-kinase alpha (PICK1) is a PDZ domain protein that interacts with many binding partners in the central nervous system (CNS), including activated protein kinase Calpha and subunits of the AMPA subtype of glutamate receptor. Almost nothing is known about the anatomic distribution of PICK1 in the intact adult CNS. By using PICK1 antisera and peroxidase immunocytochemistry, we report on the distribution of PICK1 in the ascending pathways of the central auditory system of the adult rat. PICK1-immunoreactivity (ir) was observed in many component nuclei of the central auditory system, including the dorsal cochlear nucleus, anteroventral cochlear nucleus, posteroventral cochlear nucleus, some divisions of the superior olivary complex, inferior colliculus, medial geniculate body, and primary auditory cortex. The general staining pattern for PICK1-immunoreactivity was somatodendritic with scattered puncta in neuropil and somatodendritic regions. The distribution of PICK1 partially overlaps with PKCalpha and glutamate receptor subunits such as GluR2. These data suggest that PICK1 may function in the regulation of PKCalpha and GluR2 localization in components of the rat auditory system, which may be a fundamental mechanism of synaptic transmission and/or plasticity. J. Comp. Neurol.

Lipid rafts at postsynaptic sites: distribution, function and linkage to postsynaptic density

Accumulating evidence suggests that special lipid microdomains in the lipid membrane play various roles in cellular functions. Neurons also have such microdomains, non-caveolar lipid rafts. However, the rafts at the synaptic sites had not been reported until 2001, when a raft-like fraction was purified from synaptic plasma membrane of the rat forebrain (Mol. Brain Res. 89 (2001) 20). This article reviews recent findings on lipid rafts, especially those in the brain, and discusses the possible interaction between the postsynaptic raft and the postsynaptic density, both of which are essential for the structure and function of the postsynaptic side of the synapse.

Association of the kinesin superfamily motor protein KIF1Balpha with postsynaptic density-95 (PSD-95), synapse-associated protein-97, and synaptic scaffolding molecule PSD-95/discs large/zona occludens-1 proteins

Mutation in KIF1B, a kinesin superfamily motor protein, causes a peripheral neuropathy known as Charcot-Marie-Tooth disease type 2A (CMT2A). Little is known, however, about how a defective KIF1B gene leads to CMT2A. Here we report that KIF1Balpha, one of the two splice variants of KIF1B, directly interacts through its C-terminal postsynaptic density-95 (PSD-95)/discs large/zona occludens (PDZ) domain-binding motif with PDZ proteins including PSD-95/synapse-associated protein-90 (SAP90), SAP97, and synaptic scaffolding molecule (S-SCAM)-90 (SAP90). KIF1Balpha selectively interacts with PSD-95, SAP97, and S-SCAM in yeast two-hybrid, pull-down, and in vivo coimmunoprecipitation experiments. KIF1Balpha, SAP97, and S-SCAM are widely distributed to both dendrites and axons of cultured neurons and are enriched in the small membrane fraction of the brain. In the flotation assay, KIF1Balpha cofractionates and coimmunoprecipitates with PSD-95, SAP97, and S-SCAM. These results suggest that the PSD-95 family proteins and S-SCAM have a novel function as KIF1Balpha receptors, linking KIF1Balpha to its specific cargos, and are involved in peripheral neuropathies.

Association of the AMPA receptor-related postsynaptic density proteins GRIP and ABP with subsets of glutamate-sensitive neurons in the rat retina

We used specific antibodies against two postsynaptic density proteins, GRIP (glutamate receptor interacting protein) and ABP (AMPA receptor-binding protein), to study their distribution in the rat retina. In the central nervous system, it has been shown that both proteins bind strongly to the AMPA glutamate receptor (GluR) 2/3 subunits, but not other GluRs, through a set of three PDZ domains. Western blots detected a single GRIP protein that was virtually identical in retina and brain, whereas retinal ABP corresponded to only one of three ABP peptides found in brain. The retinal distributions of GluR2/3, GRIP, and ABP immunoreactivity (IR) were similar but not identical. GluR2/3 immunoreactivity (IR) was abundant in both plexiform layers and in large perikarya. ABP IR was concentrated in large perikarya but was sparse in the plexiform layers, whereas GRIP IR was relatively more abundant in the plexiform layers than in perikarya. Immunolabel for these three antibodies consisted of puncta < or = 0.2 microm in diameter. The cellular localization of GRIP and ABP IR was examined by double labeling subclasses of retinal neuron with characteristic marker proteins, e.g., calbindin. GRIP, ABP, and GluR2/3 IR were detected in horizontal cells, dopaminergic and glycinergic AII amacrine cells and large ganglion cells. Immunolabel was absent in rod bipolar and weak or absent in cholinergic amacrine cells. By using the tyramide method of signal amplification, a colocalization of GluR2/3 was found with either GRIP or ABP in horizontal cell terminals, and perikarya of amacrine and ganglion cells. Our results show that ABP and GRIP colocalize with GluR2/3 in particular subsets of retinal neuron, as was previously established for certain neurons in the brain.

Differential palmitoylation directs the AMPA receptor-binding protein ABP to spines or to intracellular clusters

Long-term changes in excitatory synapse strength are thought to reflect changes in synaptic abundance of AMPA receptors mediated by receptor trafficking. AMPA receptor-binding protein (ABP) and glutamate receptor-interacting protein (GRIP) are two similar PDZ (postsynaptic density 95/Discs large/zona occludens 1) proteins that interact with glutamate receptors 2 and 3 (GluR2 and GluR3) subunits. Both proteins have proposed roles during long-term potentiation and long-term depression in the delivery and anchorage of AMPA receptors at synapses. Here we report a variant of ABP-L (seven PDZ form of ABP) called pABP-L that is palmitoylated at a cysteine residue at position 11 within a novel 18 amino acid N-terminal leader sequence encoded through differential splicing. In cultured hippocampal neurons, nonpalmitoylated ABP-L localizes with internal GluR2 pools expressed from a Sindbis virus vector, whereas pABP-L is membrane targeted and associates with surface-localized GluR2 receptors at the plasma membrane in spines. Mutation of Cys-11 to alanine blocks the palmitoylation of pABP-L and targets the protein to intracellular clusters, confirming that targeting the protein to spines is dependent on palmitoylation. Non-palmitoylated GRIP is primarily intracellular, but a chimera with the pABP-L N-terminal palmitoylation sequence linked to the body of the GRIP protein is targeted to spines. We suggest that pABP-L and ABP-L provide, respectively, synaptic and intracellular sites for the anchorage of AMPA receptors during receptor trafficking to and from the synapse.

Phosphorylation of stargazin by protein kinase A regulates its interaction with PSD-95

Stargazin is the first transmembrane protein known to associate with AMPA (alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate) glutamate receptors (AMPARs) and regulate their synaptic targeting by two distinct mechanisms, specifically via delivery of AMPARs to the surface membrane and synaptic targeting of these receptors by binding to PSD-95/SAP-90 and related PDZ proteins. However, it is not known whether and how this stargazin-mediated synaptic targeting of AMPARs is regulated. Stargazin interacts with the PDZ domains of PSD-95 through the C-terminal PDZ-binding motif. The stargazin C terminus contains a consensus sequence for phosphorylation by cAMP-dependent protein kinase A (PKA). Phosphorylation site-specific stargazin antibodies reveal that the stargazin C terminus is phosphorylated at the Thr-321 residue in heterologous cells and in vivo. Stargazin phosphorylation is enhanced by the catalytic subunit of PKA. Mutations mimicking stargazin phosphorylation (T321E and T321D) lead to elimination of yeast two-hybrid interactions, in vitro coimmunoprecipitation, and coclustering between stargazin and PSD-95. Phosphorylated stargazin shows a selective loss of coimmunoprecipitation with PSD-95 in heterologous cells and limited enrichment in postsynaptic density fractions of rat brain. These results suggest that phosphorylation of the stargazin C terminus by PKA regulates its interaction with PSD-95 and synaptic targeting of AMPARs.

Interaction between GRIP and liprin-alpha/SYD2 is required for AMPA receptor targeting

Interaction with the multi-PDZ protein GRIP is required for the synaptic targeting of AMPA receptors, but the underlying mechanism is unknown. We show that GRIP binds to the liprin-alpha/SYD2 family of proteins that interact with LAR receptor protein tyrosine phosphatases (LAR-RPTPs) and that are implicated in presynaptic development. In neurons, liprin-alpha and LAR-RPTP are enriched at synapses and coimmunoprecipitate with GRIP and AMPA receptors. Dominant-negative constructs that interfere with the GRIP-liprin interaction disrupt the surface expression and dendritic clustering of AMPA receptors in cultured neurons. Thus, by mediating the targeting of liprin/GRIP-associated proteins, liprin-alpha is important for postsynaptic as well as presynaptic maturation.

Glutamate receptors in the rod pathway of the mammalian retina

Rod bipolar (RB) cells of the mammalian retina release glutamate in a graded, light-dependent fashion from 20 to 40 ribbon synapses (dyads). At the dyads, two classes of amacrine cells, the AI and AII cells, are the postsynaptic partners. We examined the glutamate receptors (GluRs) that are expressed by AI and AII cells using immunocytochemistry with specific antibodies against GluR subunits. Sections of macaque monkey and rabbit retina were examined by confocal microscopy. AII amacrine cells were selectively labeled for calretinin, and AI cells in rabbits were labeled for 5-HT uptake. Thus, double- and triple-labeling for these markers and GluR subunits was possible. Electron microscopy using postembedding immunocytochemistry and double-labeling was applied to show the synaptic expression of GluRs. We also studied the synaptic localization of the two postsynaptic density proteins PSD-95 and glutamate receptor-interacting protein (GRIP). We found that AII amacrine cells express the AMPA receptor subunits GluR2/3 and GluR4 at the RB cell dyads, and they are clustered together with PSD-95. In contrast, AI amacrine cells express the delta1/2 subunits that appear to be associated with kainate receptor subunits and to be clustered together with GRIP. The RB cell dyad is therefore a synapse that initiates two functionally and molecularly distinct pathways: a "through conducting" pathway based on AMPA receptors and a modulatory pathway mediated by a combination of delta1/2 subunits and kainate receptors.

Biochemical and morphological characterization of an intracellular membrane compartment containing AMPA receptors

AMPA receptors cycle rapidly in and out of the postsynaptic membrane, while NMDA receptors are relatively immobile. Changing the distribution of AMPA receptors between intracellular and surface synaptic pools is an important means of controlling synaptic strength. However, little is known about the intracellular membrane compartments of neurons that contain AMPA receptors. Here we describe biochemical and morphological characteristics of an intracellular pool of AMPA receptors in rat brain. By velocity gradient centrifugation of microsomal light membranes from rat brain, we identified a membrane fraction enriched for AMPA receptor subunits GluR2/3 but lacking NMDA receptors. This membrane compartment sedimented more slowly than synaptosomes but faster than synaptic vesicles and cofractionated with GRIP, PICK-1 and syntaxin-13. Morphological examination of this fraction revealed round and tubular vesicles ranging from approximately 50 to 300 nm in diameter. Immunocytochemistry of cultured hippocampal neurons showed that a significant portion of AMPA receptors colocalized with syntaxin-13 (a SNARE protein associated with tubulovesicular recycling endosomes) and with transferrin receptors. Taken together, these results suggest that a pool of intracellular GluR2/3 resides in a syntaxin 13-positive tubulovesicular membrane compartment, which might serve as a reservoir for the dendritic recycling of AMPA receptors.

PDZ domains and the organization of supramolecular complexes

PDZ domains are modular protein interaction domains that bind in a sequence-specific fashion to short C-terminal peptides or internal peptides that fold in a beta-finger. The diversity of PDZ binding specificities can be explained by variable amino acids lining the peptide-binding groove of the PDZ domain. Abundantly represented in Caenorhabditis elegans, Drosophila melanogaster, and mammalian genomes, PDZ domains are frequently found in multiple copies or are associated with other protein-binding motifs in multidomain scaffold proteins. PDZ-containing proteins are typically involved in the assembly of supramolecular complexes that perform localized signaling functions at particular subcellular locations. Organization around a PDZ-based scaffold allows the stable localization of interacting proteins and enhances the rate and fidelity of signal transduction within the complex. Some PDZ-containing proteins are more dynamically regulated in distribution and may also be involved in the trafficking of interacting proteins within the cell.

Molecular organization of the postsynaptic specialization

A specific set of molecules including glutamate receptors is targeted to the postsynaptic specialization of excitatory synapses in the brain, gathering in a structure known as the postsynaptic density (PSD). Synaptic targeting of glutamate receptors depends on interactions between the C-terminal tails of receptor subunits and specific PDZ domain-containing scaffold proteins in the PSD. These scaffold proteins assemble a specialized protein complex around each class of glutamate receptor that functions in signal transduction, cytoskeletal anchoring, and trafficking of the receptors. Among the glutamate receptor subtypes, the N-methyl-d-aspartate receptor is relatively stably integrated in the PSD, whereas the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor moves in and out of the postsynaptic membrane in highly dynamic fashion. The distinctive cell biological behaviors of N-methyl-d-aspartate and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors can be explained by their differential interactions with cytoplasmic proteins.

Proteolysis of glutamate receptor-interacting protein by calpain in rat brain: implications for synaptic plasticity

Activation of the calcium-dependent protease calpain has been proposed to be a key step in synaptic plasticity in the hippocampus. However, the exact pathway through which calpain mediates or modulates changes in synaptic function remains to be clarified. Here we report that glutamate receptor-interacting protein (GRIP) is a substrate of calpain, as calpain-mediated GRIP degradation was demonstrated using three different approaches: (i) purified calpain I digestion of synaptic membranes, (ii) calcium treatment of frozen-thawed brain sections, and (iii) NMDA-stimulated organotypic hippocampal slice cultures. More importantly, calpain activation resulted in the disruption of GRIP binding to the GluR2 subunit of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors. Because GRIP has been proposed to function as an AMPA receptor-targeting and synaptic-stabilizing protein, as well as a synaptic-organizing molecule, calpain-mediated degradation of GRIP and disruption of AMPA receptor anchoring are likely to play important roles in the structural and functional reorganization accompanying synaptic modifications in long-term potentiation and long-term depression.

Biochemical evidence for localization of AMPA-type glutamate receptor subunits in the dendritic raft

A low density Triton-insoluble fraction with characteristic lipid composition was prepared from synaptic plasma membrane from the rat forebrain. The fraction was named dendritic raft based on its absence of the presynaptic marker synaptophysin, the presence of postsynaptic Glutamate receptor (GluR) subunits, and its resemblance to raft, caveolae-like structure. We found a differential distribution of NMDA-type and AMPA-type GluR subunits in the dendritic raft and postsynaptic density (PSD) fractions; the latter type GluR subunits were localized to the dendritic raft as well as PSD fraction, whereas the former type was mostly localized to the PSD fraction. We also found the differential distribution of the components of ras/mitogen-activated protein kinase (MAPK) pathway to the dendritic raft and PSD fractions. Dendritic raft and PSD may possibly interact at the postsynaptic sites for efficient signal processing that is required for expression of synaptic plasticity.

Differential cellular and subcellular localization of ampa receptor-binding protein and glutamate receptor-interacting protein

Excitatory synaptic currents in the mammalian brain are typically mediated by the neurotransmitter glutamate, acting at AMPA receptors. We used immunocytochemistry to investigate the distribution of AMPA receptor-binding protein (ABP) in the cerebral neocortex. ABP was most prominent in pyramidal neurons, although it was also present (at lower levels) in interneurons. ABP and its putative binding partners, the GluR2/3 subunits of the AMPA receptor, exhibited prominent cellular colocalization. Under appropriate processing conditions, colocalization could also be documented in puncta, many of which could be recognized as dendritic spines. However, a sizable minority of GluR2/3-positive puncta were immunonegative for ABP. Because glutamate receptor-interacting protein (GRIP) may also anchor GluR2, we studied the relative distribution of ABP and GRIP. There was extensive colocalization of these two antigens at the cellular level, although GRIP, unlike ABP, was strongest in nonpyramidal neurons. Different parts of a single dendrite could stain selectively for ABP or GRIP. To further characterize this heterogeneity, we investigated punctate staining of neuropil using synaptophysin and the membrane tracer DiA to identify probable synapses. Some puncta were comparably positive for both ABP and GRIP, but the majority were strongly positive for one antigen and only weakly positive or immunonegative for the other. This heterogeneity could be seen even within adjacent spines of a single dendrite. These data suggest that ABP may act as a scaffold for AMPA receptors either in concert with or independently from GRIP.

Modification of AMPA receptor clustering regulates cerebellar synaptic plasticity

Cerebellar long-term depression (LTD) induced at parallel fiber-Purkinje neuron synapses is proposed to underlie certain types of motor learning. alpha-Amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptors, which mediate chemical transmission in these synapses, are clustered on the postsynaptic membrane. By increasing local density of the receptors, clustering is believed to increase synaptic efficacy. This article focuses on molecular mechanisms regulating the synaptic AMPA receptor clustering in Purkinje cells, which could underlie the expression of cerebellar LTD. Synaptic AMPA receptor clusters in dendritic spines of Purkinje cells are disrupted upon protein kinase C (PKC)-mediated phosphorylation of serine 880 in the C-terminal domain of GluR2. Phosphorylation of this residue causes significant reduction in the affinity of GluR2 C-terminal tail for glutamate receptor interacting protein (GRIP), a molecule known to be crucial for AMPA receptor clustering. Consequently, AMPA receptors on the synaptic membrane are destabilized and internalized by endocytosis. Based on these findings, a model for the expression of cerebellar LTD is proposed, in which a decrease in the number of postsynaptic AMPA receptors, initiated by phosphorylation of GluR2 serine 880, is the major mechanism underlying cerebellar LTD.

Sharpin, a novel postsynaptic density protein that directly interacts with the shank family of proteins

The Shank family of proteins (also termed CortBP, ProSAP, or Synamon) is highly enriched in the postsynaptic density (PSD) of excitatory synapses in brain. Shank contains multiple domains for protein-protein interactions, including ankyrin repeats, SH3 domain, PDZ domain, SAM domain, and an extensive proline-rich region. We have identified a novel protein, termed Sharpin, that directly interacts with the ankyrin repeats of Shank. Sharpin is enriched in the PSD and forms a complex with Shank in heterologous cells and brain. Immunostaining reveals the presence of Sharpin at excitatory synapses and its colocalization with Shank. While the C-terminal half of Sharpin interacts with Shank, the N-terminal half of Sharpin mediates homomultimerization. Considering the fact that the ankyrin repeats and the SH3 domain of Shank can be truncated by alternative splicing, these results define Sharpin as a novel PSD protein that may regulate the complexity of the Shank-based protein network in an alternative splicing-dependent manner.

Association of dystrophin-related protein 2 (DRP2) with postsynaptic densities in rat brain

The fundamental function of the membrane-associated cytoskeletal proteins dystrophin and utrophin remains unclear. To gain further insights into the dystrophin family of proteins, we have studied dystrophin-related protein 2 (DRP2), whose expression is largely confined to the vertebrate central nervous system. Both human and rat DRP2 are expressed from two alternative but neighboring transcriptional start sites and have simple transcript structures. Antibodies raised against DRP2 detect a characteristic quartet of bands ( approximately 100-120 kDa) in Western blots of rat brain. The DRP2 protein is associated with brain membrane fractions and highly enriched in the postsynaptic density. Immunohistochemistry shows DRP2 to be widely distributed in a punctate pattern on neuronal dendrites and in neuropil, with particular concentration in regions of the brain involved in cholinergic synaptic transmission. Given the presence of utrophin in the cholinergic neuromuscular junction, and perturbations of cholinergic transmission in dystrophin-deficient nematodes, our findings may suggest a role for DRP2 in the organization of central cholinergic synapses.

Cerebellar long-term depression requires PKC-regulated interactions between GluR2/3 and PDZ domain-containing proteins

Cerebellar LTD requires activation of PKC and is expressed, at least in part, as postsynaptic AMPA receptor internalization. Recently, it was shown that AMPA receptor internalization requires clathrin-mediated endocytosis and depends upon the carboxy-terminal region of GluR2/3. Phosphorylation of Ser-880 in this region by PKC differentially regulates the binding of the PDZ domain-containing proteins GRIP/ABP and PICK1. Peptides, corresponding to the phosphorylated and dephosphorylated GluR2 carboxy-terminal PDZ binding motif, were perfused in cerebellar Purkinje cells grown in culture. Both the dephospho form (which blocks binding of GRIP/ABP and PICK1) and the phospho form (which selectively blocks PICK1) attenuated LTD induction by glutamate/depolarization pairing, as did antibodies directed against the PDZ domain of PICK1. These findings indicate that expression of cerebellar LTD requires PKC-regulated interactions between the carboxy-terminal of GluR2/3 and PDZ domain-containing proteins.

Ligand-gated ion channel interactions with cytoskeletal and signaling proteins

In recent years, it has become apparent that ligand-gated ion channels (ionotropic receptors) in the neuronal plasma membrane interact via their cytoplasmic domains with a multitude of intracellular proteins. Different classes of ligand-gated channels associate with distinct sets of intracellular proteins, often through specialized scaffold proteins containing PDZ domains. These specific interactions link the receptor channel to the cortical cytoskeleton and to appropriate signal transduction pathways in the cell. Thus ionotropic receptors are components of extensive protein complexes that are likely involved in the subcellular targeting, cytoskeletal anchoring, and localized clustering of the receptors at specific sites on the neuronal surface. In addition to structural functions, receptor-associated proteins can play important roles as activity modulators or downstream effectors of ligand-gated channels.

Interaction of the postsynaptic density-95/guanylate kinase domain-associated protein complex with a light chain of myosin-V and dynein

NMDA receptors interact directly with postsynaptic density-95 (PSD-95), a scaffold protein that organizes a cytoskeletal- signaling complex at the postsynaptic membrane. The molecular mechanism by which the PSD-95-based protein complex is trafficked to the postsynaptic site is unknown but presumably involves specific motor proteins. Here we demonstrate a direct interaction between the PSD-95-associated protein guanylate kinase domain-associated protein (GKAP) and dynein light chain (DLC), a light chain subunit shared by myosin-V (an actin-based motor) and cytoplasmic dynein (a microtubule-based motor). A yeast two-hybrid screen with GKAP isolated DLC2, a novel protein 93% identical to the previously cloned 8 kDa dynein light chain (DLC1). A complex containing PSD-95, GKAP, DLC, and myosin-V can be immunoprecipitated from rat brain extracts. DLC colocalizes with PSD-95 and F-actin in dendritic spines of cultured neurons and is enriched in biochemical purifications of PSD. Immunogold electron microscopy reveals a concentration of DLC in the postsynaptic compartment of asymmetric synapses of brain in which it is associated with the PSD and the spine apparatus. We discuss the possibility that the GKAP/DLC interaction may be involved in trafficking of the PSD-95 complex by motor proteins.

Disruption of AMPA receptor GluR2 clusters following long-term depression induction in cerebellar Purkinje neurons

Cerebellar long-term depression (LTD) is thought to play an important role in certain types of motor learning. However, the molecular mechanisms underlying this event have not been clarified. Here, using cultured Purkinje cells, we show that stimulations inducing cerebellar LTD cause phosphorylation of Ser880 in the intracellular C-terminal domain of the AMPA receptor subunit GluR2. This phosphorylation is accompanied by both a reduction in the affinity of GluR2 to glutamate receptor interacting protein (GRIP), a molecule known to be critical for AMPA receptor clustering, and a significant disruption of postsynaptic GluR2 clusters. Moreover, GluR2 protein released from GRIP is shown to be internalized. These results suggest that the dissociation of postsynaptic GluR2 clusters and subsequent internalization of the receptor protein, initiated by the phosphorylation of Ser880, are the mechanisms underlying the induction of cerebellar LTD.

GRASP-1: a neuronal RasGEF associated with the AMPA receptor/GRIP complex

The PDZ domain-containing proteins, such as PSD-95 and GRIP, have been suggested to be involved in the targeting of glutamate receptors, a process that plays a critical role in the efficiency of synaptic transmission and plasticity. To address the molecular mechanisms underlying AMPA receptor synaptic localization, we have identified several GRIP-associated proteins (GRASPs) that bind to distinct PDZ domains within GRIP. GRASP-1 is a neuronal rasGEF associated with GRIP and AMPA receptors in vivo. Overexpression of GRASP-1 in cultured neurons specifically reduced the synaptic targeting of AMPA receptors. In addition, the subcellular distribution of both AMPA receptors and GRASP-1 was rapidly regulated by the activation of NMDA receptors. These results suggest that GRASP-1 may regulate neuronal ras signaling and contribute to the regulation of AMPA receptor distribution by NMDA receptor activity.

Interactions between AMPA receptors and intracellular proteins

alpha-Amino-3-hydroxy-5-methylisoxazolepropionate (AMPA) receptors mediate most fast excitatory synaptic transmission in the mammalian CNS. They play a central role in synapse stabilisation and plasticity and their prolonged activation is potently neurotoxic. Developmental and activity-dependent changes in the functional synaptic expression of these receptors are subject to tight cellular regulation. The molecular and cellular mechanisms which control the postsynaptic insertion and arrangement of individual AMPA receptor variants are therefore the subject of intense investigation and in the last two years there has been significant progress towards elucidating some of the processes involved. Much of the new information has come from the application of the yeast two-hybrid assay, which has led to the discovery of a hitherto unexpected complexity of proteins which selectively interact with individual AMPA receptor subunits. These proteins have been implicated in the regulation of AMPA receptor post-translational modification, targeting and trafficking, surface expression and anchoring. The aim of this article is to present an overview of the major interacting proteins described so far and to place these in the context of how they may participate in the well ordered series of events controlling the cell biology of AMPA receptors.