GRIP1 in GABAergic synapses

WWC2 modulates GABAA-receptor-mediated synaptic transmission, revealing class-specific mechanisms of synapse regulation by WWC family proteins

The WW and C2 domain-containing protein (WWC2) is implicated in several neurological disorders. Here, we demonstrate that WWC2 interacts with inhibitory, but not excitatory, postsynaptic scaffolds, consistent with prior proteomic identification of WWC2 as a putative component of the inhibitory postsynaptic density. Using mice lacking WWC2 expression in excitatory forebrain neurons, we show that WWC2 suppresses γ-aminobutyric acid type-A receptor (GABAAR) incorporation into the plasma membrane and regulates HAP1 and GRIP1, which form a complex promoting GABAAR recycling to the membrane. Inhibitory synaptic transmission is increased in CA1 pyramidal cells lacking WWC2. Furthermore, unlike the WWC2 homolog KIBRA (kidney/brain protein; WWC1), a key regulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) trafficking at excitatory synapses, the deletion of WWC2 does not affect synaptic AMPAR expression. In contrast, loss of KIBRA does not affect GABAAR membrane expression. These data reveal synapse class-selective functions for WWC proteins as regulators of ionotropic neurotransmitter receptors and provide insight into mechanisms regulating GABAAR membrane expression.

Modulation of GABA A receptor trafficking by WWC2 reveals class-specific mechanisms of synapse regulation by WWC family proteins

WWC2 (WW and C2 domain-containing protein) is implicated in several neurological disorders, however its function in the brain has yet to be determined. Here, we demonstrate that WWC2 interacts with inhibitory but not excitatory postsynaptic scaffolds, consistent with prior proteomic identification of WWC2 as a putative component of the inhibitory postsynaptic density. Using mice lacking WWC2 expression in excitatory forebrain neurons, we show that WWC2 suppresses GABA A R incorporation into the plasma membrane and regulates HAP1 and GRIP1, which form a complex promoting GABA A R recycling to the membrane. Inhibitory synaptic transmission is dysregulated in CA1 pyramidal cells lacking WWC2. Furthermore, unlike the WWC2 homolog KIBRA (WWC1), a key regulator of AMPA receptor trafficking at excitatory synapses, deletion of WWC2 does not affect synaptic AMPAR expression. In contrast, loss of KIBRA does not affect GABA A R membrane expression. These data reveal unique, synapse class-selective functions for WWC proteins as regulators of ionotropic neurotransmitter receptors and provide insight into mechanisms regulating GABA A R membrane expression.

Glutamate receptor-interacting protein 1 in D1- and D2-dopamine receptor-expressing medium spiny neurons differentially regulates cocaine acquisition, reinstatement, and associated spine plasticity

BACKGROUND

The nucleus accumbens (NAc) is involved in the expression of cocaine addictive phenotypes, including acquisition, extinction, and reinstatement. In the NAc, D1-medium spiny neurons (MSNs) encode cocaine reward, whereas D2-MSNs encode aversive responses in drug addiction. Glutamate receptor-interacting protein 1 (GRIP1) is known to be associated with cocaine addiction, but the role of GRIP1 in D1-MSNs and D2-MSNs of the NAc in cocaine acquisition and reinstatement remains unknown.

METHODS

A conditioned place preference apparatus was used to establish cocaine acquisition, extinction, and reinstatement in mouse models. GRIP1 expression was evaluated using Western blotting. Furthermore, GRIP1-siRNA and GRIP1 overexpression lentivirus were used to interfere with GRIP1 in the NAc. After the behavioral test, green fluorescent protein immunostaining of brain slices was used to detect spine density.

RESULTS

GRIP1 expression decreased during cocaine acquisition and reinstatement. GRIP1-siRNA enhanced cocaine-induced CPP behavior in acquisition and reinstatement and regulated associated spine plasticity. Importantly, the decreased GRIP1 expression that mediated cocaine acquisition and reinstatement was mainly driven by the interference of the GRIP1-GluA2 interaction in D1-MSNs and could be blocked by the interference of the GRIP1-GluA2 interaction in D2-MSNs. Interference with the GRIP1-GluA2 interaction in D1- and D2-MSNs decreased spine density in D1- and D2-MSNs, respectively.

CONCLUSION

GRIP1 in D1- and D2-MSNs of the NAc differentially modulates cocaine acquisition and reinstatement. GRIP1 downregulation in D1-MSNs has a positive effect on cocaine acquisition and reinstatement, while GRIP1 downregulation in D2-MSNs has a negative effect. Additionally, GRIP1 downregulation in D1-MSNs plays a leading role in cocaine acquisition and reinstatement.

The Yin and Yang of GABAergic and Glutamatergic Synaptic Plasticity: Opposites in Balance by Crosstalking Mechanisms

Synaptic plasticity is a critical process that regulates neuronal activity by allowing neurons to adjust their synaptic strength in response to changes in activity. Despite the high proximity of excitatory glutamatergic and inhibitory GABAergic postsynaptic zones and their functional integration within dendritic regions, concurrent plasticity has historically been underassessed. Growing evidence for pathological disruptions in the excitation and inhibition (E/I) balance in neurological and neurodevelopmental disorders indicates the need for an improved, more "holistic" understanding of synaptic interplay. There continues to be a long-standing focus on the persistent strengthening of excitation (excitatory long-term potentiation; eLTP) and its role in learning and memory, although the importance of inhibitory long-term potentiation (iLTP) and depression (iLTD) has become increasingly apparent. Emerging evidence further points to a dynamic dialogue between excitatory and inhibitory synapses, but much remains to be understood regarding the mechanisms and extent of this exchange. In this mini-review, we explore the role calcium signaling and synaptic crosstalk play in regulating postsynaptic plasticity and neuronal excitability. We examine current knowledge on GABAergic and glutamatergic synapse responses to perturbances in activity, with a focus on postsynaptic plasticity induced by short-term pharmacological treatments which act to either enhance or reduce neuronal excitability via ionotropic receptor regulation in neuronal culture. To delve deeper into potential mechanisms of synaptic crosstalk, we discuss the influence of synaptic activity on key regulatory proteins, including kinases, phosphatases, and synaptic structural/scaffolding proteins. Finally, we briefly suggest avenues for future research to better understand the crosstalk between glutamatergic and GABAergic synapses.

The adaptor proteins HAP1a and GRIP1 collaborate to activate the kinesin-1 isoform KIF5C

Binding of motor proteins to cellular cargoes is regulated by adaptor proteins. HAP1 and GRIP1 are kinesin-1 adaptors that have been implicated individually in the transport of vesicular cargoes in the dendrites of neurons. We find that HAP1a and GRIP1 form a protein complex in the brain, and co-operate to activate the kinesin-1 subunit KIF5C in vitro. Based upon this co-operative activation of kinesin-1, we propose a modification to the kinesin activation model that incorporates stabilisation of the central hinge region known to be critical to autoinhibition of kinesin-1.

Summary: The adaptor proteins HAP1a and GRIP1 form a protein complex in the brain, and co-operate to activate the kinesin-1 subunit KIF5C in vitro.

Mutation p.R356Q in the Collybistin Phosphoinositide Binding Site Is Associated With Mild Intellectual Disability

The recruitment of inhibitory GABA_A receptors to neuronal synapses requires a complex interplay between receptors, neuroligins, the scaffolding protein gephyrin and the GDP-GTP exchange factor collybistin (CB). Collybistin is regulated by protein-protein interactions at the N-terminal SH3 domain, which can bind neuroligins 2/4 and the GABA_AR α2 subunit. Collybistin also harbors a RhoGEF domain which mediates interactions with gephyrin and catalyzes GDP-GTP exchange on Cdc42. Lastly, collybistin has a pleckstrin homology (PH) domain, which binds phosphoinositides, such as phosphatidylinositol 3-phosphate (PI3P/PtdIns3P) and phosphatidylinositol 4-monophosphate (PI4P/PtdIns4P). PI3P located in early/sorting endosomes has recently been shown to regulate the postsynaptic clustering of gephyrin and GABA_A receptors and consequently the strength of inhibitory synapses in cultured hippocampal neurons. This process is disrupted by mutations in the collybistin gene (ARHGEF9), which cause X-linked intellectual disability (XLID) by a variety of mechanisms converging on disrupted gephyrin and GABA_A receptor clustering at central synapses. Here we report a novel missense mutation (chrX:62875607C>T, p.R356Q) in ARHGEF9 that affects one of the two paired arginine residues in the PH domain that were predicted to be vital for binding phosphoinositides. Functional assays revealed that recombinant collybistin CB3_SH3- ^R356Q was deficient in PI3P binding and was not able to translocate EGFP-gephyrin to submembrane microaggregates in an in vitro clustering assay. Expression of the PI3P-binding mutants CB3_SH3- ^R356Q and CB3_SH3- ^R356N/R357N in cultured hippocampal neurones revealed that the mutant proteins did not accumulate at inhibitory synapses, but instead resulted in a clear decrease in the overall number of synaptic gephyrin clusters compared to controls. Molecular dynamics simulations suggest that the p.R356Q substitution influences PI3P binding by altering the range of structural conformations adopted by collybistin. Taken together, these results suggest that the p.R356Q mutation in ARHGEF9 is the underlying cause of XLID in the probands, disrupting gephyrin clustering at inhibitory GABAergic synapses via loss of collybistin PH domain phosphoinositide binding.

Alterations in GABAA-Receptor Trafficking and Synaptic Dysfunction in Brain Disorders

GABA_A receptors (GABA_AR) are the major players in fast inhibitory neurotransmission in the central nervous system (CNS). Regulation of GABA_AR trafficking and the control of their surface expression play important roles in the modulation of the strength of synaptic inhibition. Different pieces of evidence show that alterations in the surface distribution of GABA_AR and dysregulation of their turnover impair the activity of inhibitory synapses. A diminished efficacy of inhibitory neurotransmission affects the excitatory/inhibitory balance and is a common feature of various disorders of the CNS characterized by an increased excitability of neuronal networks. The synaptic pool of GABA_AR is mainly controlled through regulation of internalization, recycling and lateral diffusion of the receptors. Under physiological condition these mechanisms are finely coordinated to define the strength of GABAergic synapses. In this review article, we focus on the alteration in GABA_AR trafficking with an impact on the function of inhibitory synapses in various disorders of the CNS. In particular we discuss how similar molecular mechanisms affecting the synaptic distribution of GABA_AR and consequently the excitatory/inhibitory balance may be associated with a wide diversity of pathologies of the CNS, from psychiatric disorders to acute alterations leading to neuronal death. A better understanding of the cellular and molecular mechanisms that contribute to the impairment of GABAergic neurotransmission in these disorders, in particular the alterations in GABA_AR trafficking and surface distribution, may lead to the identification of new pharmacological targets and to the development of novel therapeutic strategies.

Calpain-GRIP Signaling in Nucleus Accumbens Core Mediates the Reconsolidation of Drug Reward Memory

Exposure to drug-paired cues causes drug memories to be in a destabilized state and interfering with memory reconsolidation can inhibit relapse. Calpain, a calcium-dependent neutral cysteine protease, is involved in synaptic plasticity and the formation of long-term fear memory. However, the role of calpain in the reconsolidation of drug reward memory is still unknown. In the present study, using a conditioned place preference (CPP) model, we found that exposure to drug-paired contextual stimuli induced the activation of calpain and decreased the expression of glutamate receptor interacting protein 1 (GRIP1) in the nucleus accumbens (NAc) core, but not shell, of male rats. Infusions of calpain inhibitors in the NAc core immediately after retrieval disrupted the reconsolidation of cocaine/morphine cue memory and blocked retrieval-induced calpain activation and GRIP1 degradation. The suppressive effect of calpain inhibitors on the expression of drug-induced CPP lasted for at least 14 d. The inhibition of calpain without retrieval 6 h after retrieval or after exposure to an unpaired context had no effects on the expression of reward memory. Calpain inhibition after retrieval also decreased cocaine seeking in a self-administration model and this effect did not recover spontaneously after 28 d. Moreover, the knock-down of GRIP1 expression in the NAc core by lentivirus-mediated short-hairpin RNA blocked disruption of the reconsolidation of drug cue memories that was induced by calpain inhibitor treatment. These results suggest that calpain activity in the NAc core is crucial for the reconsolidation of drug reward memory via the regulation of GRIP1 expression.SIGNIFICANCE STATEMENT Calpain plays an important role in synaptic plasticity and long-term memory consolidation, however, its role in the reconsolidation of drug cue memory remains unknown. Using conditioned place preference and self-administration procedures, we found that exposure to drug-paired cues induced the activation of calpain and decreased glutamate receptor interacting protein 1 (GRIP1) expression in the nucleus accumbens (NAc) core. The inhibition of calpain activity in the NAc core immediately after retrieval disrupted the reconsolidation of cocaine/morphine cue memory that was blocked by prior GRIP1 knock-down. Our findings indicate that calpain-GRIP signaling is essential for the restabilization process that is associated with drug cue memory and the inhibition of calpain activity may be a novel strategy for the prevention of drug relapse.

Mice lacking GRIP1/2 show increased social interactions and enhanced phosphorylation at GluA2-S880

Glutamate receptor interacting proteins 1 and 2 (GRIP1/2) play an important role in regulating synaptic trafficking of AMPA receptor 2/3 (GluA2/3) and synaptic strength. Gain-of-function GRIP1 mutations are implicated in social behavioral deficits in autism. To study mechanisms of Grip1/2-mediated AMPA signaling in the regulation of social behaviors, we performed social behavioral testing on neuron-specific Grip1/2-double knockout (DKO) and wild type (WT) mice that are matched for age, sex, and strain background. We determined the expression profile of key signaling proteins in AMPAR, mGluR, mTOR, and GABA pathways in frontal cortex, striatum, and cerebellum of DKO mice. Compared to WT mice, DKO mice show increased sociability in a modified three-chamber social behavioral test [mean±sem for interaction time in seconds; WT: 44.0±5.0; n=10; DKO: 81.0±9.0; n=9; two factor repeated measures ANOVA: F(1,37)=14.45; p<0.01 and planned t-test; p<0.01] and in a dyadic male-male social interaction test (mean±sem for total time in seconds: sniffing, WT-WT, 18.9±1.1; WT-DKO, 42.5±2.1; t-test: p<0.001; following, WT-WT, 7.7±0.72; WT-DKO,14.4±1.8; t-test: p<0.001). Immunoblot studies identified an increase in phosphorylation at GluA2-Serine 880 (GluA2-pS880) in frontal cortex (mean±sem; WT: 0.69±0.06, n=5; DKO: 0.96±0.06, n=6; t-test; p<0.05) and reduced GABAβ3 expression in striatum (mean±sem; WT: 1.16±0.04, n=4; DKO: 0.95±0.06, n=4; t-test; p<0.05) in DKO mice. GluA2-S880 phosphorylation is known to regulate GluA2synaptic recycling, AMPA signaling strength and plasticity. GABAβ3 has been implicated in the etiology and pathogenesis in autism. These data support an important role of Grip1/2-mediated AMPA signaling in regulating social behaviors and disturbance of glutamate- and GABA-signaling in specialized brain regions in autism-related social behavioral deficits.

Postsynaptic Neurotransmitter Receptor Reserve Pools for Synaptic Potentiation

At excitatory and inhibitory synapses, an immediate transfer of additional neurotransmitter receptors from non-synaptic positions to the synapse mediates synaptic long-term potentiation (LTP). Different types of non-synaptic reserve pools permit the rapid supply of transmembrane neurotransmitter receptors. Recycling endosomes (REs) serve as an intracellular reservoir of receptors that is delivered to the plasma membrane on LTP induction. Furthermore, AMPA receptors at the non-synaptic plasma membrane provide an extrasynaptic reserve pool that is also important to potentiate synapse function. Finally, bidirectional synaptic versus extrasynaptic trapping of freely diffusing plasma membrane GABAA receptors (GABAARs) by scaffolding proteins modulates synaptic transmission. Here we discuss novel findings regarding neurotransmitter receptor reservoirs and potential reserve pool mechanisms for synaptic potentiation.

Ring finger protein 34 (RNF34) interacts with and promotes γ-aminobutyric acid type-A receptor degradation via ubiquitination of the γ2 subunit

We have found that the large intracellular loop of the γ2 GABAA receptor (R) subunit (γ2IL) interacts with RNF34 (an E3 ubiquitin ligase), as shown by yeast two-hybrid and in vitro pulldown assays. In brain extracts, RNF34 co-immunoprecipitates with assembled GABAARs. In co-transfected HEK293 cells, RNF34 reduces the expression of the γ2 GABAAR subunit by increasing the ratio of ubiquitinated/nonubiquitinated γ2. Mutating several lysines of the γ2IL into arginines makes the γ2 subunit resistant to RNF34-induced degradation. RNF34 also reduces the expression of the γ2 subunit when α1 and β3 subunits are co-assembled with γ2. This effect is partially reversed by leupeptin or MG132, indicating that both the lysosomal and proteasomal degradation pathways are involved. Immunofluorescence of cultured hippocampal neurons shows that RNF34 forms clusters and that a subset of these clusters is associated with GABAergic synapses. This association is also observed in the intact rat brain by electron microscopy immunocytochemistry. RNF34 is not expressed until the 2nd postnatal week of rat brain development, being highly expressed in some interneurons. Overexpression of RNF34 in hippocampal neurons decreases the density of γ2 GABAAR clusters and the number of GABAergic contacts that these neurons receive. Knocking down endogenous RNF34 with shRNA leads to increased γ2 GABAAR cluster density and GABAergic innervation. The results indicate that RNF34 regulates postsynaptic γ2-GABAAR clustering and GABAergic synaptic innervation by interacting with and ubiquitinating the γ2-GABAAR subunit promoting GABAAR degradation.

Changes in the GRIP 1&2 scaffolding proteins in the cerebellum of the ataxic stargazer mouse

Glutamate receptor-interacting proteins (GRIP1&2) and protein-interacting with C kinase-1 (PICK1) are synaptic scaffold proteins associated with the stabilization and recycling of synaptic GluA2-, 3- and 4c-containing α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs). PICK1-mediated phosphorylation of GluA serine880 uncouples GRIP1&2 leading to AMPAR endocytosis, important in mediating forms of synaptic plasticity underlying learning and memory. Ataxic and epileptic stargazer mice possess a mutation in the CACNG2 gene encoding the transmembrane AMPAR-regulatory protein (TARP)-γ2 (stargazin). TARPs are AMPAR-auxiliary subunits required for efficient AMPAR trafficking to synapses. Stargazin is abundantly expressed in the cerebellum and its loss results in severe deficits in AMPAR trafficking to cerebellar synapses, particularly at granule cell (GC) synapses, leading to the ataxic phenotype of stargazers. However, how the stargazin mutation impacts on the expression of other AMPAR-interacting scaffold proteins is unknown. This study shows a significant increase in GRIP1&2, but not PICK1, levels in whole tissue and synapse-enriched extracts from stargazer cerebella. Post-embedding immunogold-cytochemistry electron microscopy showed GRIP1&2 levels were unchanged at mossy fiber-GC synapses in stargazers, which are silent due to virtual total absence of synaptic and extrasynaptic GluA2/3-AMPARs. These results indicate that loss of synaptic AMPARs at this excitatory synapse does not affect GRIP1&2 expression within the postsynaptic region of mossy fiber-GC synapses. Interestingly, increased GRIP and reduced GluA2-AMPARexpression also occur in cerebella of autistic patients. Further research establishing the role of elevated cerebellar GRIP1&2 in stargazers may help identify common cellular mechanisms in the comorbid disorders ataxia, epilepsy and autism leading to more effective treatment strategies.

Genome-wide genetic diversity and differentially selected regions among Suffolk, Rambouillet, Columbia, Polypay, and Targhee sheep

Sheep are among the major economically important livestock species worldwide because the animals produce milk, wool, skin, and meat. In the present study, the Illumina OvineSNP50 BeadChip was used to investigate genetic diversity and genome selection among Suffolk, Rambouillet, Columbia, Polypay, and Targhee sheep breeds from the United States. After quality-control filtering of SNPs (single nucleotide polymorphisms), we used 48,026 SNPs, including 46,850 SNPs on autosomes that were in Hardy-Weinberg equilibrium and 1,176 SNPs on chromosome × for analysis. Phylogenetic analysis based on all 46,850 SNPs clearly separated Suffolk from Rambouillet, Columbia, Polypay, and Targhee, which was not surprising as Rambouillet contributed to the synthesis of the later three breeds. Based on pair-wise estimates of F_ST, significant genetic differentiation appeared between Suffolk and Rambouillet (F_ST = 0.1621), while Rambouillet and Targhee had the closest relationship (F_ST = 0.0681). A scan of the genome revealed 45 and 41 differentially selected regions (DSRs) between Suffolk and Rambouillet and among Rambouillet-related breed populations, respectively. Our data indicated that regions 13 and 24 between Suffolk and Rambouillet might be good candidates for evaluating breed differences. Furthermore, ovine genome v3.1 assembly was used as reference to link functionally known homologous genes to economically important traits covered by these differentially selected regions. In brief, our present study provides a comprehensive genome-wide view on within- and between-breed genetic differentiation, biodiversity, and evolution among Suffolk, Rambouillet, Columbia, Polypay, and Targhee sheep breeds. These results may provide new guidance for the synthesis of new breeds with different breeding objectives.

Molecular and functional interaction between protocadherin-γC5 and GABAA receptors

We have found that the γ2 subunit of the GABA(A) receptor (γ2-GABA(A)R) specifically interacts with protocadherin-γC5 (Pcdh-γC5) in the rat brain. The interaction occurs between the large intracellular loop of the γ2-GABA(A)R and the cytoplasmic domain of Pcdh-γC5. In brain extracts, Pcdh-γC5 coimmunoprecipitates with GABA(A)Rs. In cotransfected HEK293 cells, Pcdh-γC5 promotes the transfer of γ2-GABA(A)R to the cell surface. We have previously shown that, in cultured hippocampal neurons, endogenous Pcdh-γC5 forms clusters, some of which associate with GABAergic synapses. Overexpression of Pcdh-γC5 in hippocampal neurons increases the density of γ2-GABA(A)R clusters but has no significant effect on the number of GABAergic contacts that these neurons receive, indicating that Pcdh-γC5 is not synaptogenic. Deletion of the cytoplasmic domain of Pcdh-γC5 enhanced its surface expression but decreased the association with both γ2-GABA(A)R clusters and presynaptic GABAergic contacts. Cultured hippocampal neurons from the Pcdh-γ triple C-type isoform knock-out (TCKO) mouse (Pcdhg(tcko/tcko)) showed plenty of GABAergic synaptic contacts, although their density was reduced compared with sister cultures from wild-type and heterozygous mice. Knocking down Pcdh-γC5 expression with shRNA decreased γ2-GABA(A)R cluster density and GABAergic innervation. The results indicate that, although Pcdh-γC5 is not essential for GABAergic synapse formation or GABA(A)R clustering, (1) Pcdh-γC5 regulates the surface expression of GABA(A)Rs via cis-cytoplasmic interaction with γ2-GABA(A)R, and (2) Pcdh-γC5 plays a role in the stabilization and maintenance of some GABAergic synapses.

NMDA receptors in GABAergic synapses during postnatal development

GABA (gamma-aminobutyric-acid), the main inhibitory neurotransmitter in the adult brain, exerts depolarizing (excitatory) actions during development and this GABAergic depolarization cooperates with NMDARs (N-methyl-D-aspartate receptors) to drive spontaneous synchronous activity (SSA) that is fundamentally important for developing neuronal networks. Although GABAergic depolarization is known to assist in the activation of NMDARs during development, the subcellular localization of NMDARs relative to GABAergic synapses is still unknown. Here, we investigated the subcellular distribution of NMDARs in association with GABAergic synapses at the developmental stage when SSA is most prominent in mice. Using multiple immunofluorescent labeling and confocal laser-scanning microscopy in the developing mouse hippocampus, we found that NMDARs were associated with both glutamatergic and GABAergic synapses at postnatal day 6–7 and we observed a direct colocalization of GABA_A- and NMDA-receptor labeling in GABAergic synapses. Electron microscopy of pre-embedding immunogold-immunoperoxidase reactions confirmed that GluN1, GluN2A and GluN2B NMDAR subunits were all expressed in glutamatergic and GABAergic synapses postsynaptically. Finally, quantitative post-embedding immunogold labeling revealed that the density of NMDARs was 3 times higher in glutamatergic than in GABAergic synapses. Since GABAergic synapses were larger, there was little difference in the total number of NMDA receptors in the two types of synapses. In addition, receptor density in synapses was substantially higher than extrasynaptically. These data can provide the neuroanatomical basis of a new interpretation of previous physiological data regarding the GABA_AR-NMDAR cooperation during early development. We suggest that during SSA, synaptic GABA_AR-mediated depolarization assists NMDAR activation right inside GABAergic synapses and this effective spatial cooperation of receptors and local change of membrane potential will reach developing glutamatergic synapses with a higher probability and efficiency even further away on the dendrites. This additional level of cooperation that operates within the depolarizing GABAergic synapse, may also allow its own modification triggered by Ca²⁺-influx through the NMDA receptors.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

GABAA receptor trafficking-mediated plasticity of inhibitory synapses

Proper developmental, neural cell-type-specific, and activity-dependent regulation of GABAergic transmission is essential for virtually all aspects of CNS function. The number of GABA(A) receptors in the postsynaptic membrane directly controls the efficacy of GABAergic synaptic transmission. Thus, regulated trafficking of GABA(A) receptors is essential for understanding brain function in both health and disease. Here we summarize recent progress in the understanding of mechanisms that allow dynamic adaptation of cell surface expression and postsynaptic accumulation and function of GABA(A) receptors. This includes activity-dependent and cell-type-specific changes in subunit gene expression, assembly of subunits into receptors, as well as exocytosis, endocytic recycling, diffusion dynamics, and degradation of GABA(A) receptors. In particular, we focus on the roles of receptor-interacting proteins, scaffold proteins, synaptic adhesion proteins, and enzymes that regulate the trafficking and function of receptors and associated proteins. In addition, we review neuropeptide signaling pathways that affect neural excitability through changes in GABA(A)R trafficking.

Differential regulation of the postsynaptic clustering of γ-aminobutyric acid type A (GABAA) receptors by collybistin isoforms

Collybistin promotes submembrane clustering of gephyrin and is essential for the postsynaptic localization of gephyrin and γ-aminobutyric acid type A (GABA(A)) receptors at GABAergic synapses in hippocampus and amygdala. Four collybistin isoforms are expressed in brain neurons; CB2 and CB3 differ in the C terminus and occur with and without the Src homology 3 (SH3) domain. We have found that in transfected hippocampal neurons, all collybistin isoforms (CB2(SH3+), CB2(SH3-), CB3(SH3+), and CB3(SH3-)) target to and concentrate at GABAergic postsynapses. Moreover, in non-transfected neurons, collybistin concentrates at GABAergic synapses. Hippocampal neurons co-transfected with CB2(SH3-) and gephyrin developed very large postsynaptic gephyrin and GABA(A) receptor clusters (superclusters). This effect was accompanied by a significant increase in the amplitude of miniature inhibitory postsynaptic currents. Co-transfection with CB2(SH3+) and gephyrin induced the formation of many (supernumerary) non-synaptic clusters. Transfection with gephyrin alone did not affect cluster number or size, but gephyrin potentiated the clustering effect of CB2(SH3-) or CB2(SH3+). Co-transfection with CB2(SH3-) or CB2(SH3+) and gephyrin did not affect the density of presynaptic GABAergic terminals contacting the transfected cells, indicating that collybistin is not synaptogenic. Nevertheless, the synaptic superclusters induced by CB2(SH3-) and gephyrin were accompanied by enlarged presynaptic GABAergic terminals. The enhanced clustering of gephyrin and GABA(A) receptors induced by collybistin isoforms was not accompanied by enhanced clustering of neuroligin 2. Moreover, during the development of GABAergic synapses, the clustering of gephyrin and GABA(A) receptors preceded the clustering of neuroligin 2. We propose a model in which the SH3- isoforms play a major role in the postsynaptic accumulation of GABA(A) receptors and in GABAergic synaptic strength.

Gain-of-function glutamate receptor interacting protein 1 variants alter GluA2 recycling and surface distribution in patients with autism

Glutamate receptor interacting protein 1 (GRIP1) is a neuronal scaffolding protein that interacts directly with the C termini of glutamate receptors 2/3 (GluA2/3) via its PDZ domains 4 to 6 (PDZ4-6). We found an association (P<0.05) of a SNP within the PDZ4-6 genomic region with autism by genotyping autistic patients (n=480) and matched controls (n=480). Parallel sequencing identified five rare missense variants within or near PDZ4-6 only in the autism cohort, resulting in a higher cumulative mutation load (P=0.032). Two variants correlated with a more severe deficit in reciprocal social interaction in affected sibling pairs from proband families. These variants were associated with altered interactions with GluA2/3 and faster recycling and increased surface distribution of GluA2 in neurons, suggesting gain-of-function because GRIP1/2 deficiency showed opposite phenotypes. Grip1/2 knockout mice exhibited increased sociability and impaired prepulse inhibition. These results support a role for GRIP in social behavior and implicate GRIP1 variants in modulating autistic phenotype.

Synaptic and nonsynaptic localization of protocadherin-gammaC5 in the rat brain

It has been proposed that gamma-protocadherins (Pcdh-gammas) are involved in the establishment of specific patterns of neuronal connectivity. Contrary to the other Pcdh-gammas, which are expressed in the embryo, Pcdh-gammaC5 is expressed postnatally in the brain, coinciding with the peak of synaptogenesis. We have developed an antibody specific for Pcdh-gammaC5 to study the expression and localization of Pcdh-gammaC5 in brain. Pcdh-gammaC5 is highly expressed in the olfactory bulb, corpus striatum, dentate gyrus, CA1 region of the hippocampus, layers I and II of the cerebral cortex, and molecular layer of the cerebellum. Pcdh-gammaC5 is expressed in both neurons and astrocytes. In hippocampal neuronal cultures, and in the absence of astrocytes, a significant percentage of synapses, more GABAergic than glutamatergic, have associated Pcdh-gammaC5 clusters. Some GABAergic axons show Pcdh-gammaC5 in the majority of their synapses. Nevertheless, many Pcdh-gammaC5 clusters are not associated with synapses. In the brain, significant numbers of Pcdh-gammaC5 clusters are located at contact points between neurons and astrocytes. Electron microscopic immunocytochemistry of the rat brain shows that 1) Pcdh-gammaC5 is present in some GABAergic and glutamatergic synapses both pre- and postsynaptically; 2) Pcdh-gammaC5 is also extrasynaptically localized in membranes and in cytoplasmic organelles of neurons and astrocytes; and 3) Pcdh-gammaC5 is also localized in perisynaptic astrocyte processes. The results support the notions that 1) Pcdh-gammaC5 plays a role in synaptic specificity and/or synaptic maturation and 2) Pcdh-gammaC5 is involved in neuron-neuron synaptic interactions and in neuron-astrocyte interactions, including perisynaptic neuron-astrocyte interactions.

Gephyrin interacts with the glutamate receptor interacting protein 1 isoforms at GABAergic synapses

We have previously shown that the glutamate receptor interacting protein 1 (GRIP1) splice forms GRIP1a/b and GRIP1c4-7 are present at the GABAergic post-synaptic complex. Nevertheless, the role that these GRIP1 protein isoforms play at the GABAergic post-synaptic complex is not known. We are now showing that GRIP1c4-7 and GRIP1a/b interact with gephyrin, the main post-synaptic scaffold protein of GABAergic and glycinergic synapses. Gephyrin coprecipitates with GRIP1c4-7 or GRIP1a/b from rat brain extracts and from extracts of human embryonic kidney 293 cells that have been cotransfected with gephyrin and one of the GRIP1 protein isoforms. Moreover, purified gephyrin binds to purified GRIP1c4-7 or GRIP1a/b, indicating that gephyrin directly interacts with the common region of these GRIP1 proteins, which includes PDZ domains 4-7. An engineered deletion construct of GRIP1a/b (GRIP1a4-7), which both contains the aforementioned common region and binds to gephyrin, targets to the post-synaptic GABAergic complex of transfected cultured hippocampal neurons. In these hippocampal cultures, endogenous gephyrin colocalizes with endogenous GRIP1c4-7 and GRIP1a/b in over 90% of the GABAergic synapses. Double-labeling electron microscopy immunogold reveals that in the rat brain GRIP1c4-7 and GRIP1a/b colocalize with gephyrin at the post-synaptic complex of individual synapses. These results indicate that GRIP1c4-7 and GRIP1a/b colocalize and interact with gephyrin at the GABAergic post-synaptic complex and suggest that this interaction plays a role in GABAergic synaptic function.

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.

Septin 11 is present in GABAergic synapses and plays a functional role in the cytoarchitecture of neurons and GABAergic synaptic connectivity

Mass spectrometry and immunoblot analysis of a rat brain fraction enriched in type-II postsynaptic densities and postsynaptic GABAergic markers showed enrichment in the protein septin 11. Septin 11 is expressed throughout the brain, being particularly high in the spiny branchlets of the Purkinje cells in the molecular layer of cerebellum and in the olfactory bulb. Immunofluorescence of cultured hippocampal neurons showed that 54 +/- 4% of the GABAergic synapses and 25 +/- 2% of the glutamatergic synapses had colocalizing septin 11 clusters. Similar colocalization numbers were found in the molecular layer of cerebellar sections. In cultured hippocampal neurons, septin 11 clusters were frequently present at the base of dendritic protrusions and at the bifurcation points of the dendritic branches. Electron microscopy immunocytochemistry of the rat brain cerebellum revealed the accumulation of septin 11 at the neck of dendritic spines, at the bifurcation of dendritic branches, and at some GABAergic synapses. Knocking down septin 11 in cultured hippocampal neurons with septin 11 small hairpin RNAs showed (i) reduced dendritic arborization; (ii) decreased density and increased length of dendritic protrusions; and (iii) decreased GABAergic synaptic contacts that these neurons receive. The results indicate that septin 11 plays important roles in the cytoarchitecture of neurons, including dendritic arborization and dendritic spines, and that septin 11 also plays a role in GABAergic synaptic connectivity.

Gephyrin expression and clustering affects the size of glutamatergic synaptic contacts

We have recently shown that disrupting the expression and post-synaptic clustering of gephyrin in cultured hippocampal pyramidal cells, by either gephyrin RNAi (RNA interference) or over-expression of a dominant negative gephyrin-enhanced green fluorescent protein (EGFP) fusion protein, leads to decreased number of post-synaptic gephyrin and GABA(A) receptor clusters and to reduced GABAergic innervation of these cells. On the other hand, increasing gephyrin expression led to a small increase in the number of gephyrin and GABA(A) receptor clusters and to little or no effect on GABAergic innervation. We are now reporting that altering gephyrin expression and clustering affects the size but not the density of glutamatergic synaptic contacts. Knocking down gephyrin with gephyrin RNAi, or preventing gephyrin clustering by over-expression of the dominant negative gephyrin-enhanced green fluorescent protein fusion protein, leads to larger post-synaptic PSD-95 clusters and larger pre-synaptic glutamatergic terminals. On the other hand, over-expression of gephyrin leads to slightly smaller PSD-95 clusters and pre-synaptic glutamatergic terminals. The change in size of PSD-95 clusters were accompanied by a parallel change in the size of NR2-NMDA receptor clusters. It is concluded that the levels of expression and clustering of gephyrin, a protein that concentrates at the post-synaptic complex of the inhibitory synapses, not only has homotypic effects on GABAergic synaptic contacts, but also has heterotypic effects on glutamatergic synaptic contacts. We are proposing that gephyrin is a counterpart of the post-synaptic glutamatergic scaffold protein PSD-95 in regulating the number and/or size of the excitatory and inhibitory synaptic contacts.

NMDA receptor activation potentiates inhibitory transmission through GABA receptor-associated protein-dependent exocytosis of GABA(A) receptors

The trafficking of postsynaptic AMPA receptors (AMPARs) is a powerful mechanism for regulating the strength of excitatory synapses. It has become clear that the surface levels of inhibitory GABA(A) receptors (GABA(A)Rs) are also subject to regulation and that GABA(A)R trafficking may contribute to inhibitory plasticity, although the underlying mechanisms are not fully understood. Here, we report that NMDA receptor activation, which has been shown to drive excitatory long-term depression through AMPAR endocytosis, simultaneously increases expression of GABA(A)Rs at the dendritic surface of hippocampal neurons. This NMDA stimulus increases miniature IPSC amplitudes and requires the activity of Ca2+ calmodulin-dependent kinase II and the trafficking proteins N-ethylmaleimide-sensitive factor, GABA receptor-associated protein (GABARAP), and glutamate receptor interacting protein (GRIP). These data demonstrate for the first time that endogenous GABARAP and GRIP contribute to the regulated trafficking of GABA(A)Rs. In addition, they reveal that the bidirectional trafficking of AMPA and GABA(A) receptors can be driven by a single glutamatergic stimulus, providing a potent postsynaptic mechanism for modulating neuronal excitability.

Two pools of Triton X-100-insoluble GABA(A) receptors are present in the brain, one associated to lipid rafts and another one to the post-synaptic GABAergic complex

Rat forebrain synaptosomes were extracted with Triton X-100 at 4 degrees C and the insoluble material, which is enriched in post-synaptic densities (PSDs), was subjected to sedimentation on a continuous sucrose gradient. Two pools of Triton X-100-insoluble gamma-aminobutyric acid type-A receptors (GABA(A)Rs) were identified: (i) a higher-density pool (rho = 1.10-1.15 mg/mL) of GABA(A)Rs that contains the gamma2 subunit (plus alpha and beta subunits) and that is associated to gephyrin and the GABAergic post-synaptic complex and (ii) a lower-density pool (rho = 1.06-1.09 mg/mL) of GABA(A)Rs associated to detergent-resistant membranes (DRMs) that contain alpha and beta subunits but not the gamma2 subunit. Some of these GABA(A)Rs contain the delta subunit. Two pools of GABA(A)Rs insoluble in Triton X-100 at 4 degrees C were also identified in cultured hippocampal neurons: (i) a GABA(A)R pool that forms clusters that co-localize with gephyrin and remains Triton X-100-insoluble after cholesterol depletion and (ii) a GABA(A)R pool that is diffusely distributed at the neuronal surface that can be induced to form GABA(A)R clusters by capping with an anti-alpha1 GABA(A)R subunit antibody and that becomes solubilized in Triton X-100 at 4 degrees C after cholesterol depletion. Thus, there is a pool of GABA(A)Rs associated to lipid rafts that is non-synaptic and that has a subunit composition different from that of the synaptic GABA(A)Rs. Some of the lipid raft-associated GABA(A)Rs might be involved in tonic inhibition.

More than just synaptic building blocks: scaffolding proteins of the post-synaptic density regulate dendritic patterning

The dendritic arbor is responsible for receiving and consolidating neuronal input. Outgrowth and morphogenesis of the arbor are complex stages of development that are poorly understood. However, recent findings have identified synaptic scaffolding proteins as novel regulators of these important events. Scaffolding proteins are enriched in the post-synaptic density where they bind and bring into close proximity neurotransmitter receptors, signaling molecules, and regulators of the actin cytoskeleton. This property is important for dendritic spine morphogenesis and maintenance in the mature neuron. Scaffolding proteins are now being described as key regulators of neurite outgrowth, dendritic development, and pattern formation in immature neurons. These proteins, which include post-synaptic-95, Shank and Densin-180, as well as many of their interacting partners, appear to regulate both the microtubule and actin cytoskeleton to influence dendrite morphology. Through a large array of protein-protein interaction domains, scaffolding proteins are able to form large macromolecular complexes that include cytoskeletal motor proteins as well as microtubule and actin regulatory molecules. Together, the new findings form a persuasive argument that scaffolding proteins deliver critical regulatory elements to sites of dendritic outgrowth and branching to modulate the formation and maintenance of the dendritic arbor.

Gephyrin clustering is required for the stability of GABAergic synapses

Although gephyrin is an important postsynaptic scaffolding protein at GABAergic synapses, the role of gephyrin for GABAergic synapse formation and/or maintenance is still under debate. We report here that knocking down gephyrin expression with small hairpin RNAs (shRNAs) in cultured hippocampal pyramidal cells decreased both the number of gephyrin and GABA(A) receptor clusters. Similar results were obtained by disrupting the clustering of endogenous gephyrin by overexpressing a gephyrin-EGFP fusion protein that formed aggregates with the endogenous gephyrin. Disrupting postsynaptic gephyrin clusters also had transsynaptic effects leading to a significant reduction of GABAergic presynaptic boutons contacting the transfected pyramidal cells. Consistent with the morphological decrease of GABAergic synapses, electrophysiological analysis revealed a significant reduction in both the amplitude and frequency of the spontaneous inhibitory postsynaptic currents (sIPSCs). However, no change in the whole-cell GABA currents was detected, suggesting a selective effect of gephyrin on GABA(A) receptor clustering at postsynaptic sites. It is concluded that gephyrin plays a critical role for the stability of GABAergic synapses.

GABAA receptors: properties and trafficking

Fast synaptic inhibition in the brain and spinal cord is mediated largely by ionotropic gamma-aminobutyric acid (GABA) receptors. GABAA receptors play a key role in controlling neuronal activity; thus modulating their function will have important consequences for neuronal excitation. GABAA receptors are important therapeutic targets for a range of sedative, anxiolytic, and hypnotic agents and are involved in a number of CNS diseases, including sleep disturbances, anxiety, premenstrual syndrome, alcoholism, muscle spasms, Alzheimer's disease, chronic pain, schizophrenia, bipolar affective disorders, and epilepsy. This review focuses on the functional and pharmacological properties of GABAA receptors and trafficking as an essential mechanism underlying the dynamic regulation of synaptic strength.

Osteopoikilosis, short stature and mental retardation as key features of a new microdeletion syndrome on 12q14

This report presents the detection of a heterozygous deletion at chromosome 12q14 in three unrelated patients with a similar phenotype consisting of mild mental retardation, failure to thrive in infancy, proportionate short stature and osteopoikilosis as the most characteristic features. In each case, this interstitial deletion was found using molecular karyotyping. The deletion occurred as a de novo event and varied between 3.44 and 6 megabases (Mb) in size with a 3.44 Mb common deleted region. The deleted interval was not flanked by low-copy repeats or segmental duplications. It contains 13 RefSeq genes, including LEMD3, which was previously shown to be the causal gene for osteopoikilosis. The observation of osteopoikilosis lesions should facilitate recognition of this new microdeletion syndrome among children with failure to thrive, short stature and learning disabilities.

Molecular organization and assembly of the central inhibitory postsynapse

gamma-Amino butyric acid type A (GABAA) receptors are the major sites of fast synaptic inhibition in the brain. GABAA receptors play an important role in regulating neuronal excitability and in addition have been implicated in numerous neurological disorders. In order to understand synaptic inhibition it is important to comprehend the cellular mechanisms, that neurons utilize to regulate the accumulation and regulation of GABAA receptors at postsynaptic inhibitory specializations. Over the past decade a number of GABAA receptor interacting proteins have been identified allowing us to further understand the trafficking, targeting and clustering of these receptors as well as the regulation of receptor stability. In the following review we examine the proteins identified as GABAA receptor binding partners and other components of the inhibitory postsynaptic scaffold, and how they contribute to the construction of inhibitory synapses and the dynamic modulation of synaptic inhibition.

Synaptic and nonsynaptic localization of GABAA receptors containing the alpha5 subunit in the rat brain

The alpha5 subunit of the GABA(A) receptors (GABA(A)Rs) has a restricted expression in the brain. Maximum expression of this subunit occurs in the hippocampus, cerebral cortex, and olfactory bulb. Hippocampal pyramidal cells show high expression of alpha5 subunit-containing GABA(A)Rs (alpha5-GABA(A)Rs) both in culture and in the intact brain. A large pool of alpha5-GABA(A)Rs is extrasynaptic and it has been proposed to be involved in the tonic GABAergic inhibition of the hippocampus. Nevertheless, there are no studies on the localization of the alpha5-GABA(A)Rs at the electron microscope (EM) level. By using both immunofluorescence of cultured hippocampal pyramidal cells and EM postembedding immunogold of the intact hippocampus we show that, in addition to the extrasynaptic pool, there is a pool of alpha5-GABA(A)Rs that concentrates at the GABAergic synapses in dendrites of hippocampal pyramidal cells. The results suggest that the synaptic alpha5-GABA(A)Rs might play a role in the phasic GABAergic inhibition of pyramidal neurons in hippocampus and cerebral cortex.

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.

Disruption of postsynaptic GABA receptor clusters leads to decreased GABAergic innervation of pyramidal neurons

We have used RNA interference (RNAi) to knock down the expression of the gamma2 subunit of the GABA(A) receptors (GABA(A)Rs) in pyramidal neurons in culture and in the intact brain. Two hairpin small interference RNAs (shRNAs) for the gamma2 subunit, one targeting the coding region and the other one the 3'-untranslated region (UTR) of the gamma2 mRNA, when introduced into cultured rat hippocampal pyramidal neurons, efficiently inhibited the synthesis of the GABA(A) receptor gamma2 subunit and the clustering of other GABA(A)R subunits and gephyrin in these cells. More significantly, this effect was accompanied by a reduction of the GABAergic innervation that these neurons received. In contrast, the gamma2 shRNAs had no effect on the clustering of postsynaptic alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, postsynaptic density protein 95 (PSD-95) or presynaptic glutamatergic innervation. A gamma2-enhanced green fluorescent protein (EGFP) subunit construct, whose mRNA did not contain the 3'-UTR targeted by gamma2 RNAi, rescued both the postsynaptic clustering of GABA(A)Rs and the GABAergic innervation. Decreased GABA(A)R clustering and GABAergic innervation of pyramidal neurons in the post-natal rat cerebral cortex was also observed after in utero transfection of these neurons with the gamma2 shRNAs. The results indicate that the postsynaptic clustering of GABA(A)Rs in pyramidal neurons is involved in the stabilization of the presynaptic GABAergic contacts.