The Crystal Structure of Cdc42 in Complex with Collybistin II, a Gephyrin-interacting Guanine Nucleotide Exchange Factor

Demixing is a default process for biological condensates formed via phase separation

Excitatory and inhibitory synapses do not overlap even when formed on one submicron-sized dendritic protrusion. How excitatory and inhibitory postsynaptic cytomatrices or densities (e/iPSDs) are segregated is not understood. Broadly, why membraneless organelles are naturally segregated in cellular subcompartments is unclear. Using biochemical reconstitutions in vitro and in cells, we demonstrate that ePSDs and iPSDs spontaneously segregate into distinct condensed molecular assemblies through phase separation. Tagging iPSD scaffold gephyrin with a PSD-95 intrabody (dissociation constant ~4 nM) leads to mistargeting of gephyrin to ePSD condensates. Unexpectedly, formation of iPSD condensates forces the intrabody-tagged gephyrin out of ePSD condensates. Thus, instead of diffusion-governed spontaneous mixing, demixing is a default process for biomolecules in condensates. Phase separation can generate biomolecular compartmentalization specificities that cannot occur in dilute solutions.

Intracellular signaling mechanisms that shape postsynaptic GABAergic synapses

Postsynaptic GABAergic receptors interact with various membrane and intracellular proteins to mediate inhibitory synaptic transmission. They form structural and/or signaling synaptic protein complexes that perform a variety of postsynaptic functions. In particular, the key GABAergic synaptic scaffold, gephyrin, and its interacting partners govern downstream signaling pathways that are essential for GABAergic synapse development, transmission, and plasticity. In this review, we discuss recent researches on GABAergic synaptic signaling pathways. We also outline the main outstanding issues that need to be addressed in this field and highlight the association of dysregulated GABAergic synaptic signaling with the onset of various brain disorders.

ARHGEF9 regulates melanoma morphogenesis in environments with diverse geometry and elasticity by promoting filopodial-driven adhesion

Rho GTP Exchange Factors (RhoGEFs) and Rho GTPase Activating Proteins (RhoGAPs) are large families of molecules that regulate shape determination in all eukaryotes. In pathologies such as melanoma, RhoGEF and RhoGAP activity underpins the ability of cells to invade tissues of varying elasticity. To identify RhoGEFs and RhoGAPs that regulate melanoma cell shape on soft and/or stiff materials, we performed genetic screens, in tandem with single-cell quantitative morphological analysis. We show that ARHGEF9/Collybistin (Cb) is essential for cell shape determination on both soft and stiff materials, and in cells embedded in 3D soft hydrogel. ARHGEF9 is required for melanoma cells to invade 3D matrices. Depletion of ARHGEF9 results in loss of tension at focal adhesions decreased cell-wide contractility, and the inability to stabilize protrusions. Taken together we show that ARHGEF9 promotes the formation of actin-rich filopodia, which serves to establish and stabilize adhesions and determine melanoma cell shape.

Differential modulation of collybistin conformational dynamics by the closely related GTPases Cdc42 and TC10

Interneuronal synaptic transmission relies on the proper spatial organization of presynaptic neurotransmitter release and its reception on the postsynaptic side by cognate neurotransmitter receptors. Neurotransmitter receptors are incorporated into and arranged within the plasma membrane with the assistance of scaffolding and adaptor proteins. At inhibitory GABAergic postsynapses, collybistin, a neuronal adaptor protein, recruits the scaffolding protein gephyrin and interacts with various neuronal factors including cell adhesion proteins of the neuroligin family, the GABA A receptor α2-subunit and the closely related small GTPases Cdc42 and TC10 (RhoQ). Most collybistin splice variants harbor an N-terminal SH3 domain and exist in an autoinhibited/closed state. Cdc42 and TC10, despite sharing 67.4% amino acid sequence identity, interact differently with collybistin. Here, we delineate the molecular basis of the collybistin conformational activation induced by TC10 with the aid of recently developed collybistin FRET sensors. Time-resolved fluorescence-based FRET measurements reveal that TC10 binds to closed/inactive collybistin leading to relief of its autoinhibition, contrary to Cdc42, which only interacts with collybistin when forced into an open state by the introduction of mutations destabilizing the closed state of collybistin. Taken together, our data describe a TC10-driven signaling mechanism in which collybistin switches from its autoinhibited closed state to an open/active state.

Neuronal Cytoskeleton in Intellectual Disability: From Systems Biology and Modeling to Therapeutic Opportunities

Intellectual disability (ID) is a pathological condition characterized by limited intellectual functioning and adaptive behaviors. It affects 1–3% of the worldwide population, and no pharmacological therapies are currently available. More than 1000 genes have been found mutated in ID patients pointing out that, despite the common phenotype, the genetic bases are highly heterogeneous and apparently unrelated. Bibliomic analysis reveals that ID genes converge onto a few biological modules, including cytoskeleton dynamics, whose regulation depends on Rho GTPases transduction. Genetic variants exert their effects at different levels in a hierarchical arrangement, starting from the molecular level and moving toward higher levels of organization, i.e., cell compartment and functions, circuits, cognition, and behavior. Thus, cytoskeleton alterations that have an impact on cell processes such as neuronal migration, neuritogenesis, and synaptic plasticity rebound on the overall establishment of an effective network and consequently on the cognitive phenotype. Systems biology (SB) approaches are more focused on the overall interconnected network rather than on individual genes, thus encouraging the design of therapies that aim to correct common dysregulated biological processes. This review summarizes current knowledge about cytoskeleton control in neurons and its relevance for the ID pathogenesis, exploiting in silico modeling and translating the implications of those findings into biomedical research.

Recruitment of Plasma Membrane GABA-A Receptors by Submembranous Gephyrin/Collybistin Clusters

It has been shown that subunit composition is the main determinant of the synaptic or extrasynaptic localization of GABA_A receptors (GABA_ARs). Synaptic and extrasynaptic GABA_ARs are involved in phasic and tonic inhibition, respectively. It has been proposed that synaptic GABA_ARs bind to the postsynaptic gephyrin/collybistin (Geph/CB) lattice, but not the typically extrasynaptic GABA_ARs. Nevertheless, there are no studies of the direct binding of various types of GABA_ARs with the submembranous Geph/CB lattice in the absence of other synaptic proteins, some of which are known to interact with GABA_ARs. We have reconstituted GABA_ARs of various subunit compositions, together with the Geph/CB scaffold, in HEK293 cells, and have investigated the recruitment of surface GABA_ARs by submembranous Geph/CB clusters. Results show that the typically synaptic α1β3γ2 GABA_ARs were trapped by submembranous Geph/CB clusters. The α5β3γ2 GABA_ARs, which are both synaptic and extrasynaptic, were also trapped by Geph/CB clusters. Extrasynaptic α4β3δ GABA_ARs consistently showed little or no trapping by the Geph/CB clusters. However, the extrasynaptic α6β3δ, α1β3, α6β3 (and less α4β3) GABA_ARs were highly trapped by the Geph/CB clusters. AMPA and NMDA glutamate receptors were not trapped. The results suggest: (I) in the absence of other synaptic molecules, the Geph/CB lattice has the capacity to trap not only synaptic but also several typically extrasynaptic GABA_ARs; (II) the Geph/CB lattice is important but does not play a decisive role in the synaptic localization of GABA_ARs; and (III) in neurons there must be mechanisms preventing the trapping of several typically extrasynaptic GABA_ARs by the postsynaptic Geph/CB lattice.

Collybistin SH3-protein isoforms are expressed in the rat brain promoting gephyrin and GABA-A receptor clustering at GABAergic synapses

Collybistin (CB) is a guanine nucleotide exchange factor (GEF) selectively localized at GABAergic and glycinergic postsynapses. Analysis of mRNA shows that several isoforms of collybistin are expressed in the brain. Some of the isoforms have a SH3 domain (CBSH3+) and some have no SH3 domain (CBSH3-). The CBSH3+ mRNAs are predominantly expressed over CBSH3-. However, in an immunoblot study of mouse brain homogenates, only CBSH3+ protein isoforms were detected, proposing that CBSH3- protein might not be expressed in the brain. The expression or lack of expression of CBSH3- protein is an important issue because CBSH3- has a strong effect in promoting the postsynaptic clustering of gephyrin and GABA-A receptors (GABA_A Rs). Moreover CBSH3- is constitutively active; therefore lower expression of CBSH3- protein might play a relatively stronger functional role than the more abundant but self-inhibited CBSH3+ isoforms, which need to be activated. We are now showing that: (a) CBSH3- protein is expressed in the brain; (b) parvalbumin positive (PV+) interneurons show higher expression of CBSH3- protein than other neurons; (c) CBSH3- is associated with GABAergic synapses in various regions of the brain and (d) knocking down CBSH3- in hippocampal neurons decreases the synaptic clustering of gephyrin and GABA_A Rs. The results show that CBSH3- protein is expressed in the brain and that it plays a significant role in the size regulation of the GABAergic postsynapse.

A proline-rich motif in the large intracellular loop of the glycine receptor α1 subunit interacts with the Pleckstrin homology domain of collybistin

Introduction

The inhibitory glycine receptor (GlyR), a mediator of fast synaptic inhibition, is located and held at neuronal synapses through the anchoring proteins gephyrin and collybistin. Stable localization of neurotransmitter receptors is essential for synaptic function. In case of GlyRs, only beta subunits were known until now to mediate synaptic anchoring.

Objectives

We identified a poly-proline II helix (PPII) in position 365–373 of the intra-cellular TM3-4 loop of the human GlyRα1 subunit as a novel potential synaptic anchoring site. The potential role of the PPII helix as synaptic anchoring site was tested.

Methods

Glycine receptors and collybistin variants were generated and recombinantly expressed in HEK293 cells and cultured neurons. Receptor function was assessed using patch-clamp electrophysiology, protein-protein interaction was studied using co-immuno-precipitation and pulldown experiments.

Results

Recombinantly expressed collybistin bound to isolated GlyRα1 TM3-4 loops in GST-pulldown assays. When the five proline residues P365A, P366A, P367A, P369A, P373A (GlyRα1^P1-5A) located in the GlyRα1-PPII helix were replaced by alanines, the PPII secondary structure was disrupted. Recombinant GlyRα1^P1-5A mutant subunits displayed normal cell surface expression and wildtype-like ion channel function, but binding to collybistin was abolished. The GlyRα1-collybistin interaction was independently confirmed by o-immunoprecipitation assays using full-length GlyRα1 subunits. Surprisingly, the interaction was not mediated by the SH3 domain of collybistin, but by its Pleckstrin homology (PH) domain. The mutation GlyRα1^P366L, identified in a hyperekplexia patient, is also disrupting the PPII helix, and caused reduced collybistin binding.

Conclusion

Our data suggest a novel interaction between α1 GlyR subunits and collybistin, which is physiologically relevant in vitro and in vivo and may contribute to postsynaptic anchoring of glycine receptors.

Identification of a Core Amino Acid Motif within the α Subunit of GABAARs that Promotes Inhibitory Synaptogenesis and Resilience to Seizures

SUMMARY

The fidelity of inhibitory neurotransmission is dependent on the accumulation of γ-aminobutyric acid type A receptors (GABA_ARs) at the appropriate synaptic sites. Synaptic GABA_ARs are constructed from α(1–3), β(1–3), and γ2 subunits, and neurons can target these subtypes to specific synapses. Here, we identify a 15-amino acid inhibitory synapse targeting motif (ISTM) within the α2 subunit that promotes the association between GABA_ARs and the inhibitory scaffold proteins collybistin and gephyrin. Using mice in which the ISTM has been introduced into the α1 subunit (Gabra1–2 mice), we show that the ISTM is critical for axo-axonic synapse formation, the efficacy of GABAergic neurotransmission, and seizure sensitivity. The Gabra1–2 mutation rescues seizure-induced lethality in Gabra2–1 mice, which lack axo-axonic synapses due to the deletion of the ISTM from the α2 subunit. Taken together, our data demonstrate that the ISTM plays a critical role in promoting inhibitory synapse formation, both in the axonic and somatodendritic compartments.

In Brief

Molecular mechanisms regulating specific synaptic GABA_AR accumulation are critical for the fidelity of inhibitory neurotransmission. Nathanson et al. show that strengthening the interaction between α1-GABA_ARs and collybistin via genetic manipulation results in augmented synaptic targeting of these receptors, enhanced inhibitory neurotransmission, and seizure resilience.

A modified method for preparation of fluorescent MantGDP bound CDC42

Small GTPase cycled between the GDP-bound inactive state and GTP-bound active state, catalyzed by guanine nucleotide exchange factors (GEFs). Guanine nucleotide exchange assay was a direct way to investigate the specificity, activity, and kinetics of GEFs. The N-methylanthraniloyl derivative of GDP (mantGDP), which was bound to small GTPase, served as a substitution for labeled small GTPase involved in bioluminescent, colorimetric, or radioactive methods due to its safety and sensitivity. In this study, we present an economical and efficient approach to prepare qualified mantGDP-bound CDC42, a member of the Rho GTPase family. In our protocol, with a K_d value of 0.048 μM, alkaline phosphatase hydrolysis of CDC42 increased mantGDP binding affinity to CDC42, allowing mant-nucleotide associating onto CDC42 more easily. Only 1.5-fold molar excess of mantGDP was required to prepare mantGDP-bound CDC42 without nonhydrolyzable GTP analog and high performance liquid chromatography. The mantGDP-bound CDC42 was verified to be efficient for measuring the guanine nucleotide exchange activity of VAV2.

Characterization of guanine nucleotide exchange activity of DH domain of human FGD2

FGD2, a member of FGD family, contains a Dbl homology domain (DH) and two pleckstrin homology domains segregated by a FYVE domain. The DH domain has been deduced to be responsible for guanine nucleotide exchange of CDC42 to activate downstream factors. Our aim was to build a prokaryotic expression system for the DH domain and to examine its guanine nucleotide exchange activity toward CDC42 in vitro. A recombinant vector, which was successfully constructed based on pGEX-6P-1, was employed to express the DH domain of human FGD2 (FGD2-DH) in E. coli BL21 (DE3). Purified FGD2-DH behaved as a homogeneous monomer with an estimated molecular weight that corresponded to the theoretical molecular weight and was predicted to be an α-helix protein by circular dichroism spectroscopy. FGD2-DH displayed weak guanine nucleotide exchange activity in vitro and very weak interactions with CDC42 following glutaraldehyde cross-linking.

Clinical and Molecular Characterization of Three Novel ARHGEF9 Mutations in Patients with Developmental Delay and Epilepsy

Mutations in the rho guanine nucleotide exchange factor 9 gene (ARHGEF9) are present in patients with heterogeneous phenotypes including psychomotor developmental delay and variable degrees of epilepsy. Malfunction of collybistin (CB) encoded by ARHGEF9 leading to impaired clustering of gephyrin-dependent glycine receptors and γ-aminobutyric acid type A (GABAα) receptors is a crucial pathogenic mechanism. Here, we report on three patients with epilepsy and mental retardation. We studied three male patients with epilepsy and mild to moderate mental retardation. We conducted targeted panel sequencing of genes known to cause inherited disorders. In vitro studies and transcriptional experiments were performed to evaluate the functional and splicing effects of these variants on CB. Two novel missense variants (p.I294T and p.R357I) and one novel splicing variant (c.381+3A>G) in ARHGEF9 were identified in the three patients, respectively. In vitro studies confirmed that the two missense variants disrupted CB-mediated accumulation of gephyrin in submembrane microclusters. Transcriptional experiments of the splicing variant revealed the presence of aberrant transcripts leading to truncated protein product. Significance: Our cases and functional studies enrich our understanding of the phenotypic and genotypic spectrum of ARHGEF9.

Preserving the balance: diverse forms of long-term GABAergic synaptic plasticity

Cellular mechanisms that regulate the interplay of synaptic excitation and inhibition are thought to be central to the functional stability of healthy neuronal circuits. A growing body of literature demonstrates the capacity for inhibitory GABAergic synapses to exhibit long-term plasticity in response to changes in neuronal activity. Here, we review this expanding field of research, focusing on the diversity of mechanisms that link glutamatergic signalling, postsynaptic action potentials and inhibitory synaptic strength. Several lines of evidence indicate that multiple, parallel forms of plasticity serve to regulate activity at both the input and output domains of individual neurons. Overall, these varied phenomena serve to promote both stability and flexibility over the life of the organism.

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.

Dendritic spine actin cytoskeleton in autism spectrum disorder

Dendritic spines are small actin-rich protrusions from neuronal dendrites that form the postsynaptic part of most excitatory synapses. Changes in the shape and size of dendritic spines correlate with the functional changes in excitatory synapses and are heavily dependent on the remodeling of the underlying actin cytoskeleton. Recent evidence implicates synapses at dendritic spines as important substrates of pathogenesis in neuropsychiatric disorders, including autism spectrum disorder (ASD). Although synaptic perturbations are not the only alterations relevant for these diseases, understanding the molecular underpinnings of the spine and synapse pathology may provide insight into their etiologies and could reveal new drug targets. In this review, we will discuss recent findings of defective actin regulation in dendritic spines associated with ASD.

Modulation of Angiopoietin 2 release from endothelial cells and angiogenesis by the synaptic protein Neuroligin 2

The synaptic protein Neuroligin 2, similarly to its isoform Neuroligin 1, is produced by endothelial cells, but its activity in the vascular context remains unknown. This study aimed at verifying the hypothesis that Neuroligin 2, in parallel with its extraneuronal involvement in pancreatic beta cells exocytosis, modulated cytokine release from endothelial cells and consequently angiogenesis. We used in vitro approaches to modulate Neuroligin 2 expression and Neuroligin 2 null mice to test our hypotheses. In vitro, upon VEGF stimulation, Neuroligin 2 silencing strongly reduces Angiopoietin 2 release in the medium and increases the endothelial cell retention of Weibel Palade Bodies, the specialized organelles that store Angiopoietin 2 and various other cytokines. On the contrary, Neuroligin 2 overexpression almost depletes cells of Weibel Palade Bodies, independent of VEGF. In vivo, both the retina and tumor xenografts grown in NLGN2- null mice display an immature vasculature, with lower pericyte coverage and lower Tie2 phosphorylation. At the molecular level NLGN2 colocalizes with its neuronal partner collibystin, a CDC42 guanine nucleotide exchange factor, which is also expressed by endothelial cells and in turn modulates Angiopoietin 2 release. Neuroligin 2, an inhibitory synaptic protein, modulates a peculiar aspect of vascular function and could represent a novel target of therapy in various fields, from tumor angiogenesis to vascular diseases.

Structural and functional dissection of the DH and PH domains of oncogenic Bcr-Abl tyrosine kinase

The two isoforms of the Bcr-Abl tyrosine kinase, p210 and p190, are associated with different leukemias and have a dramatically different signaling network, despite similar kinase activity. To provide a molecular rationale for these observations, we study the Dbl-homology (DH) and Pleckstrin-homology (PH) domains of Bcr-Abl p210, which constitute the only structural differences to p190. Here we report high-resolution structures of the DH and PH domains and characterize conformations of the DH-PH unit in solution. Our structural and functional analyses show no evidence that the DH domain acts as a guanine nucleotide exchange factor, whereas the PH domain binds to various phosphatidylinositol-phosphates. PH-domain mutants alter subcellular localization and result in decreased interactions with p210-selective interaction partners. Hence, the PH domain, but not the DH domain, plays an important role in the formation of the differential p210 and p190 Bcr-Abl signaling networks.

Receptor tyrosine kinase EphA7 is required for interneuron connectivity at specific subcellular compartments of granule cells

Neuronal transmission is regulated by the local circuitry which is composed of principal neurons targeted at different subcellular compartments by a variety of interneurons. However, mechanisms that contribute to the subcellular localisation and maintenance of GABAergic interneuron terminals are poorly understood. Stabilization of GABAergic synapses depends on clustering of the postsynaptic scaffolding protein gephyrin and its interaction with the guanine nucleotide exchange factor collybistin. Lentiviral knockdown experiments in adult rats indicated that the receptor tyrosine kinase EphA7 is required for the stabilisation of basket cell terminals on proximal dendritic and somatic compartments of granular cells of the dentate gyrus. EphA7 deficiency and concomitant destabilisation of GABAergic synapses correlated with impaired long-term potentiation and reduced hippocampal learning. Reduced GABAergic innervation may be explained by an impact of EphA7 on gephyrin clustering. Overexpression or ephrin stimulation of EphA7 induced gephyrin clustering dependent on the mechanistic target of rapamycin (mTOR) which is an interaction partner of gephyrin. Gephyrin interactions with mTOR become released after mTOR activation while enhanced interaction with the guanine nucleotide exchange factor collybistin was observed in parallel. In conclusion, EphA7 regulates gephyrin clustering and the maintenance of inhibitory synaptic connectivity via mTOR signalling.

Structural and Biochemical Characterization of the Catalytic Core of the Metastatic Factor P-Rex1 and Its Regulation by PtdIns(3,4,5)P3

Phosphatidylinositol 3,4,5-trisphosphate (PIP3)-dependent Rac exchanger 1 (P-Rex1) is a Rho guanine nucleotide exchange factor synergistically activated by PIP3 and Gβγ that plays an important role in the metastasis of breast, prostate, and skin cancer, making it an attractive therapeutic target. However, the molecular mechanisms behind P-Rex1 regulation are poorly understood. We determined structures of the P-Rex1 pleckstrin homology (PH) domain bound to the headgroup of PIP3 and resolved that PIP3 binding to the PH domain is required for P-Rex1 activity in cells but not for membrane localization, which points to an allosteric activation mechanism by PIP3. We also determined structures of the P-Rex1 tandem Dbl homology/PH domains in complexes with two of its substrate GTPases, Rac1 and Cdc42. Collectively, this study provides important molecular insights into P-Rex1 regulation and tools for targeting the PIP3-binding pocket of P-Rex1 with a new generation of cancer chemotherapeutic agents.

Missense Mutation R338W in ARHGEF9 in a Family with X-linked Intellectual Disability with Variable Macrocephaly and Macro-Orchidism

Non-syndromal X-linked intellectual disability (NS-XLID) represents a broad group of clinical disorders in which ID is the only clinically consistent manifestation. Although in many cases either chromosomal linkage data or knowledge of the >100 existing XLID genes has assisted mutation discovery, the underlying cause of disease remains unresolved in many families. We report the resolution of a large family (K8010) with NS-XLID, with variable macrocephaly and macro-orchidism. Although a previous linkage study had mapped the locus to Xq12-q21, this region contained too many candidate genes to be analyzed using conventional approaches. However, X-chromosome exome sequencing, bioinformatics analysis and segregation analysis revealed a novel missense mutation (c.1012C>T; p.R338W) in ARHGEF9. This gene encodes collybistin (CB), a neuronal GDP-GTP exchange factor previously implicated in several cases of XLID, as well as clustering of gephyrin and GABAA receptors at inhibitory synapses. Molecular modeling of the CB R338W substitution revealed that this change results in the substitution of a long electropositive side-chain with a large non-charged hydrophobic side-chain. The R338W change is predicted to result in clashes with adjacent amino acids (K363 and N335) and disruption of electrostatic potential and local folding of the PH domain, which is known to bind phosphatidylinositol-3-phosphate (PI3P/PtdIns-3-P). Consistent with this finding, functional assays revealed that recombinant CB CB2SH3- (R338W) was deficient in PI3P binding and was not able to translocate EGFP-gephyrin to submembrane microaggregates in an in vitro clustering assay. Taken together, these results suggest that the R338W mutation in ARHGEF9 is the underlying cause of NS-XLID in this family.

Specificity of Collybistin-Phosphoinositide Interactions: IMPACT OF THE INDIVIDUAL PROTEIN DOMAINS

The regulatory protein collybistin (CB) recruits the receptor-scaffolding protein gephyrin to mammalian inhibitory glycinergic and GABAergic postsynaptic membranes in nerve cells. CB is tethered to the membrane via phosphoinositides. We developed an in vitro assay based on solid-supported 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine membranes doped with different phosphoinositides on silicon/silicon dioxide substrates to quantify the binding of various CB2 constructs using reflectometric interference spectroscopy. Based on adsorption isotherms, we obtained dissociation constants and binding capacities of the membranes. Our results show that full-length CB2 harboring the N-terminal Src homology 3 (SH3) domain (CB2SH3+) adopts a closed and autoinhibited conformation that largely prevents membrane binding. This autoinhibition is relieved upon introduction of the W24A/E262A mutation, which conformationally "opens" CB2SH3+ and allows the pleckstrin homology domain to properly bind lipids depending on the phosphoinositide species with a preference for phosphatidylinositol 3-monophosphate and phosphatidylinositol 4-monophosphate. This type of membrane tethering under the control of the release of the SH3 domain of CB is essential for regulating gephyrin clustering.

Catching Functional Modes and Structural Communication in Dbl Family Rho Guanine Nucleotide Exchange Factors

Computational approaches such as Principal Component Analysis (PCA) and Elastic Network Model-Normal Mode Analysis (ENM-NMA) are proving to be of great value in investigating relevant biological problems linked to slow motions with no demand in computer power. In this study, these approaches have been coupled to the graph theory-based Protein Structure Network (PSN) analysis to dissect functional dynamics and structural communication in the Dbl family of Rho Guanine Nucleotide Exchange Factors (RhoGEFs). They are multidomain proteins whose common structural feature is a DH-PH tandem domain deputed to the GEF activity that makes them play a central role in cell and cancer biology. While their common GEF action is accomplished by the DH domain, their regulatory mechanisms are highly variegate and depend on the PH and the additional domains as well as on interacting proteins. Major evolutionary-driven deformations as inferred from PCA concern the α6 helix of DH that dictates the orientation of the PH domain. Such deformations seem to depend on the mechanisms adopted by the GEF to prevent Rho binding, i.e. functional specialization linked to autoinhibition. In line with PCA, ENM-NMA indicates α6 and the linked PH domain as the portions of the tandem domain holding almost the totality of intrinsic and functional dynamics, with the α6/β1 junction acting as a hinge point for the collective motions of PH. In contrast, the DH domain holds a static scaffolding and hub behavior, with structural communication playing a central role in the regulatory actions by other domains/proteins. Possible allosteric communication pathways involving essentially DH were indeed found in those RhoGEFs acting as effectors of small or heterotrimeric RasGTPases. The employed methodology is suitable for deciphering structure/dynamics relationships in large sets of homologous or analogous proteins.

The Phosphatidylinositol (3,4,5)-Trisphosphate-dependent Rac Exchanger 1·Ras-related C3 Botulinum Toxin Substrate 1 (P-Rex1·Rac1) Complex Reveals the Basis of Rac1 Activation in Breast Cancer Cells

The P-Rex (phosphatidylinositol (3,4,5)-trisphosphate (PIP3)-dependent Rac exchanger) family (P-Rex1 and P-Rex2) of the Rho guanine nucleotide exchange factors (Rho GEFs) activate Rac GTPases to regulate cell migration, invasion, and metastasis in several human cancers. The family is unique among Rho GEFs, as their activity is regulated by the synergistic binding of PIP3 and Gβγ at the plasma membrane. However, the molecular mechanism of this family of multi-domain proteins remains unclear. We report the 1.95 Å crystal structure of the catalytic P-Rex1 DH-PH tandem domain in complex with its cognate GTPase, Rac1 (Ras-related C3 botulinum toxin substrate-1). Mutations in the P-Rex1·Rac1 interface revealed a critical role for this complex in signaling downstream of receptor tyrosine kinases and G protein-coupled receptors. The structural data indicated that the PIP3/Gβγ binding sites are on the opposite surface and markedly removed from the Rac1 interface, supporting a model whereby P-Rex1 binding to PIP3 and/or Gβγ releases inhibitory C-terminal domains to expose the Rac1 binding site.

Lipid binding defects and perturbed synaptogenic activity of a Collybistin R290H mutant that causes epilepsy and intellectual disability

Signaling at nerve cell synapses is a key determinant of proper brain function, and synaptic defects--or synaptopathies--are at the basis of many neurological and psychiatric disorders. In key areas of the mammalian brain, such as the hippocampus or the basolateral amygdala, the clustering of the scaffolding protein Gephyrin and of γ-aminobutyric acid type A receptors at inhibitory neuronal synapses is critically dependent upon the brain-specific guanine nucleotide exchange factor Collybistin (Cb). Accordingly, it was discovered recently that an R290H missense mutation in the diffuse B-cell lymphoma homology domain of Cb, which carries the guanine nucleotide exchange factor activity, leads to epilepsy and intellectual disability in human patients. In the present study, we determined the mechanism by which the Cb(R290H) mutation perturbs inhibitory synapse formation and causes brain dysfunction. Based on a combination of biochemical, cell biological, and molecular dynamics simulation approaches, we demonstrate that the R290H mutation alters the strength of intramolecular interactions between the diffuse B-cell lymphoma homology domain and the pleckstrin homology domain of Cb. This defect reduces the phosphatidylinositol 3-phosphate binding affinity of Cb, which limits its normal synaptogenic activity. Our data indicate that impairment of the membrane lipid binding activity of Cb and a consequent defect in inhibitory synapse maturation represent a likely molecular pathomechanism of epilepsy and mental retardation in humans.

Shaping inhibition: activity dependent structural plasticity of GABAergic synapses

Inhibitory transmission through the neurotransmitter γ-aminobutyric acid (GABA) shapes network activity in the mammalian cerebral cortex by filtering synaptic incoming information and dictating the activity of principal cells. The incredibly diverse population of cortical neurons that use GABA as neurotransmitter shows an equally diverse range of mechanisms that regulate changes in the strength of GABAergic synaptic transmission and allow them to dynamically follow and command the activity of neuronal ensembles. Similarly to glutamatergic synaptic transmission, activity-dependent functional changes in inhibitory neurotransmission are accompanied by alterations in GABAergic synapse structure that range from morphological reorganization of postsynaptic density to de novo formation and elimination of inhibitory contacts. Here we review several aspects of structural plasticity of inhibitory synapses, including its induction by different forms of neuronal activity, behavioral and sensory experience and the molecular mechanisms and signaling pathways involved. We discuss the functional consequences of GABAergic synapse structural plasticity for information processing and memory formation in view of the heterogenous nature of the structural plasticity phenomena affecting inhibitory synapses impinging on somatic and dendritic compartments of cortical and hippocampal neurons.

A conformational switch in collybistin determines the differentiation of inhibitory postsynapses

The formation of neuronal synapses and the dynamic regulation of their efficacy depend on the assembly of the postsynaptic neurotransmitter receptor apparatus. Receptor recruitment to inhibitory GABAergic and glycinergic synapses is controlled by the scaffold protein gephyrin and the adaptor protein collybistin. We derived new insights into the structure of collybistin and used these to design biochemical, cell biological, and genetic analyses of collybistin function. Our data define a collybistin-based protein interaction network that controls the gephyrin content of inhibitory postsynapses. Within this network, collybistin can adopt open/active and closed/inactive conformations to act as a switchable adaptor that links gephyrin to plasma membrane phosphoinositides. This function of collybistin is regulated by binding of the adhesion protein neuroligin-2, which stabilizes the open/active conformation of collybistin at the postsynaptic plasma membrane by competing with an intramolecular interaction in collybistin that favors the closed/inactive conformation. By linking trans-synaptic neuroligin-dependent adhesion and phosphoinositide signaling with gephyrin recruitment, the collybistin-based regulatory switch mechanism represents an integrating regulatory node in the formation and function of inhibitory postsynapses.

Gephyrin: a master regulator of neuronal function?

The neurotransmitters GABA and glycine mediate fast synaptic inhibition by activating ligand-gated chloride channels--namely, type A GABA (GABA(A)) and glycine receptors. Both types of receptors are anchored postsynaptically by gephyrin, which self-assembles into a scaffold and interacts with the cytoskeleton. Current research indicates that postsynaptic gephyrin clusters are dynamic assemblies that are held together and regulated by multiple protein-protein interactions. Moreover, post-translational modifications of gephyrin regulate the formation and plasticity of GABAergic synapses by altering the clustering properties of postsynaptic scaffolds and thereby the availability and function of receptors and other signalling molecules. Here, we discuss the formation and regulation of the gephyrin scaffold, its role in GABAergic and glycinergic synaptic function and the implications for the pathophysiology of brain disorders caused by abnormal inhibitory neurotransmission.

Collybistin activation by GTP-TC10 enhances postsynaptic gephyrin clustering and hippocampal GABAergic neurotransmission

In many brain regions, gephyrin and GABAA receptor clustering at developing inhibitory synapses depends on the guanine nucleotide exchange factor collybistin (Cb). The vast majority of Cb splice variants contain an autoinhibitory src homology 3 domain, and several synaptic proteins are known to bind to this SH3 domain and to thereby activate gephyrin clustering. However, many functional GABAergic synapses form independently of the known Cb-activating proteins, indicating that additional Cb activators must exist. Here we show that the small Rho-like GTPase TC10 stimulates Cb-dependent gephyrin clustering by binding in its active, GTP-bound state to the pleckstrin homology domain of Cb. Overexpression of a constitutively active TC10 variant in neurons causes an increase in the density of synaptic gephyrin clusters and mean miniature inhibitory postsynaptic current amplitudes, whereas a dominant negative TC10 variant has opposite effects. The enhancement of Cb-induced gephyrin clustering by GTP-TC10 does not depend on the guanine nucleotide exchange activity of Cb but involves an interaction that resembles reported interactions of other small GTPases with their effectors. Our data indicate that GTP-TC10 activates the major src homology 3 domain-containing Cb variants by relieving autoinhibition and thus define an alternative GTPase-driven signaling pathway in the genesis of inhibitory synapses.

Regulation of small GTPases by GEFs, GAPs, and GDIs

Small GTPases use GDP/GTP alternation to actuate a variety of functional switches that are pivotal for cell dynamics. The GTPase switch is turned on by GEFs, which stimulate dissociation of the tightly bound GDP, and turned off by GAPs, which accelerate the intrinsically sluggish hydrolysis of GTP. For Ras, Rho, and Rab GTPases, this switch incorporates a membrane/cytosol alternation regulated by GDIs and GDI-like proteins. The structures and core mechanisms of representative members of small GTPase regulators from most families have now been elucidated, illuminating their general traits combined with scores of unique features. Recent studies reveal that small GTPase regulators have themselves unexpectedly sophisticated regulatory mechanisms, by which they process cellular signals and build up specific cell responses. These mechanisms include multilayered autoinhibition with stepwise release, feedback loops mediated by the activated GTPase, feed-forward signaling flow between regulators and effectors, and a phosphorylation code for RhoGDIs. The flipside of these highly integrated functions is that they make small GTPase regulators susceptible to biochemical abnormalities that are directly correlated with diseases, notably a striking number of missense mutations in congenital diseases, and susceptible to bacterial mimics of GEFs, GAPs, and GDIs that take command of small GTPases in infections. This review presents an overview of the current knowledge of these many facets of small GTPase regulation.

A comprehensive small interfering RNA screen identifies signaling pathways required for gephyrin clustering

The postsynaptic scaffold protein gephyrin is clustered at inhibitory synapses and serves for the stabilization of GABA(A) receptors. Here, a comprehensive kinome-wide siRNA screen in a human HeLa cell-based model for gephyrin clustering was used to identify candidate protein kinases implicated in the stabilization of gephyrin clusters. As a result, 12 hits were identified including FGFR1 (FGF receptor 1), TrkB, and TrkC as well as components of the MAPK and mammalian target of rapamycin (mTOR) pathways. For confirmation, the impact of these hits on gephyrin clustering was analyzed in rat primary hippocampal neurons. We found that brain-derived neurotrophic factor (BDNF) acts on gephyrin clustering through MAPK signaling, and this process may be controlled by the MAPK signaling antagonist sprouty2. BDNF signaling through phosphatidylinositol 3-kinase (PI3K)-Akt also activates mTOR and represses GSK3β, which was previously shown to reduce gephyrin clustering. Gephyrin is associated with inactive mTOR and becomes released upon BDNF-dependent mTOR activation. In primary neurons, a reduction in the number of gephyrin clusters due to manipulation of the BDNF-mTOR signaling is associated with reduced GABA(A) receptor clustering, suggesting functional impairment of GABA signaling. Accordingly, application of the mTOR antagonist rapamycin leads to disinhibition of neuronal networks as measured on microelectrode arrays. In conclusion, we provide evidence that BDNF regulates gephyrin clustering via MAPK as well as PI3K-Akt-mTOR signaling.

Structurally unique interaction of RBD-like and PH domains is crucial for yeast pheromone signaling

The Ras-binding domain is conserved among fungal Ste11 MAPKKKs and is critical for mating in fungi. Its interaction with Ras1 is critical for Schizosaccharomyces pombe mating, whereas in Saccharomyces cerevisiae its interaction with the Ste5 PH domain plays the crucial role. The binding partner of RBD for fungal mating is shifted from Ras to a PH domain in fungi in which Ste5 exists.

The Ste5 protein forms a scaffold that associates and regulates the components of the mitogen-activated protein (MAP) kinase cascade that controls mating-pheromone-mediated signaling in the yeast Saccharomyces cerevisiae. Although it is known that the MEK kinase of the pathway, Ste11, associates with Ste5, details of this interaction have not been established. We identified a Ras-binding-domain-like (RBL) region in the Ste11 protein that is required specifically for the kinase to function in the mating pathway. This module is structurally related to domains in other proteins that mediate Ras-MAP kinase kinase kinase associations; however, this RBL module does not interact with Ras, but instead binds the PH domain of the Ste5 scaffold. Structural and functional studies suggest that the key role of this PH domain is to mediate the Ste5–Ste11 interaction. Overall these two evolutionarily conserved modules interact with each other through a unique interface, and thus in the pheromone pathway the structural context of the RBL domain contribution to kinase activation has been shifted through a change of its interaction partner from Ras to a PH domain.

Molecular and functional heterogeneity of GABAergic synapses

Knowledge of the functional organization of the GABAergic system, the main inhibitory neurotransmitter system, in the CNS has increased remarkably in recent years. In particular, substantial progress has been made in elucidating the molecular mechanisms underlying the formation and plasticity of GABAergic synapses. Evidence available ascribes a key role to the cytoplasmic protein gephyrin to form a postsynaptic scaffold anchoring GABA(A) receptors along with other transmembrane proteins and signaling molecules in the postsynaptic density. However, the mechanisms of gephyrin scaffolding remain elusive, notably because gephyrin can auto-aggregate spontaneously and lacks PDZ protein interaction domains found in a majority of scaffolding proteins. In addition, the structural diversity of GABA(A) receptors, which are pentameric channels encoded by a large family of subunits, has been largely overlooked in these studies. Finally, the role of the dystrophin-glycoprotein complex, present in a subset of GABAergic synapses in cortical structures, remains ill-defined. In this review, we discuss recent results derived mainly from the analysis of mutant mice lacking a specific GABA(A) receptor subtype or a core protein of the GABAergic postsynaptic density (neuroligin-2, collybistin), highlighting the molecular diversity of GABAergic synapses and its relevance for brain plasticity and function. In addition, we discuss the contribution of the dystrophin-glycoprotein complex to the molecular and functional heterogeneity of GABAergic synapses.

Splice-specific glycine receptor binding, folding, and phosphorylation of the scaffolding protein gephyrin

The multimeric scaffolding protein gephyrin forms post-synaptic clusters at inhibitory sites, thereby anchoring inhibitory glycine (GlyR) and subsets of γ-aminobutyric acid type A (GABAA) receptors. Gephyrin is composed of three domains, the conserved N-terminal G- and C-terminal E-domain, connected by the central (C-) domain. In this study we investigated the oligomerization, folding and stability, GlyR β-loop binding, and phosphorylation of three gephyrin splice variants (Geph, Geph-C3, Geph-C4) after expression and purification from insect cells (Sf9). In contrast to Escherichia coli-derived trimeric gephyrin, we found that Sf9 gephyrins form hexamers as basic oligomeric form. In the case of Geph and Geph-C4, also high-oligomeric forms (∼900 kDa) were isolated. Partial proteolysis revealed a compact folding of the Gephyrin G and C domain in one complex, whereas a much lower stability for the E domain was found. After GlyR β-loop binding, the stability of the E domain increased in Geph and Geph-C4 significantly. In contrast, the E domain in Geph-C3 is less stable and binds the GlyR β-loop with one order of magnitude lower affinity. Finally, we identified 18 novel phosphorylation sites in gephyrin, of which all except one are located within the C domain. We propose two models for the domain arrangement in hexameric gephyrin based on the oligomerization of either the E or C domains, with the latter being crucial for the regulation of gephyrin clustering.

Collybistin splice variants differentially interact with gephyrin and Cdc42 to regulate gephyrin clustering at GABAergic synapses

Collybistin (CB) is a guanine-nucleotide-exchange factor (GEF) selectively activating Cdc42. CB mutations cause X-linked mental retardation due to defective clustering of gephyrin, a postsynaptic protein associated with both glycine and GABA(A) receptors. Using a combination of biochemistry and cell biology we provide novel insights into the roles of the CB2 splice variants, CB2(SH3+) and CB2(SH3-), and their substrate, Cdc42, in regulating gephyrin clustering at GABAergic synapses. Transfection of Myc-tagged CB2(SH3+) and CB2(SH3-) into cultured neurons revealed strong, but distinct, effects promoting postsynaptic gephyrin clustering, denoting mechanistic differences in their function. In addition, overexpression of constitutively active or dominant-negative Cdc42 mutants identified a new function of Cdc42 in regulating the shape and size of postsynaptic gephyrin clusters. Using biochemical assays and native brain tissue, we identify a direct interaction between gephyrin and Cdc42, independent of its activation state. Finally, our data show that CB2(SH3-), but not CB2(SH3+), can form a ternary complex with gephyrin and Cdc42, providing a biochemical substrate for the distinct contribution of these CB isoforms in gephyrin clustering at GABAergic postsynaptic sites. Taken together, our results identify CB and Cdc42 as major regulators of GABAergic postsynaptic densities.

The role of collybistin in gephyrin clustering at inhibitory synapses: facts and open questions

Collybistin (Cb) is a brain-specific GDP/GTP-exchange factor, which interacts with the inhibitory receptor anchoring protein gephyrin. Data from mice carrying an inactivated Cb gene indicate that Cb is required for the formation and maintenance of gephyrin and gephyrin-dependent GABA(A) receptor (GABA(A)R) clusters at inhibitory postsynapses in selected regions of the mammalian forebrain. However, important aspects of how Cb's GDP/GTP-exchange activity, structure, and regulation contribute to gephyrin and GABA(A)R clustering, as well as its role in synaptic plasticity, remain poorly understood. Here we review the current state of knowledge about Cb's function and address open questions concerning its contribution to synapse formation, maintenance, plasticity, and adaptive changes in response to altered network activity.

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.

Mechanistic insights into specificity, activity, and regulatory elements of the regulator of G-protein signaling (RGS)-containing Rho-specific guanine nucleotide exchange factors (GEFs) p115, PDZ-RhoGEF (PRG), and leukemia-associated RhoGEF (LARG)

The multimodular guanine nucleotide exchange factors (GEFs) of the Dbl family mostly share a tandem Dbl homology (DH) and pleckstrin homology (PH) domain organization. The function of these and other domains in the DH-mediated regulation of the GDP/GTP exchange reaction of the Rho proteins is the subject of intensive investigations. This comparative study presents detailed kinetic data on specificity, activity, and regulation of the catalytic DH domains of four GEFs, namely p115, p190, PDZ-RhoGEF (PRG), and leukemia-associated RhoGEF (LARG). We demonstrate that (i) these GEFs are specific guanine nucleotide exchange factors for the Rho isoforms (RhoA, RhoB, and RhoC) and inactive toward other members of the Rho family, including Rac1, Cdc42, and TC10. (ii) The DH domain of LARG exhibits the highest catalytic activity reported for a Dbl protein till now with a maximal acceleration of the nucleotide exchange by 10(7)-fold, which is at least as efficient as reported for GEFs specific for Ran or the bacterial toxin SopE. (iii) A novel regulatory region at the N terminus of the DH domain is involved in its association with GDP-bound RhoA monitored by a fluorescently labeled RhoA. (iv) The tandem PH domains of p115 and PRG efficiently contribute to the DH-mediated nucleotide exchange reaction. (v) In contrast to the isolated DH or DH-PH domains, a p115 fragment encompassing both the regulator of G-protein signaling and the DH domains revealed a significantly reduced GEF activity, supporting the proposed models of an intramolecular autoinhibitory mechanism for p115-like RhoGEFs.

Complex role of collybistin and gephyrin in GABAA receptor clustering

Gephyrin and collybistin are key components of GABA(A) receptor (GABA(A)R) clustering. Nonetheless, resolving the molecular interactions between the plethora of GABA(A)R subunits and these clustering proteins is a significant challenge. We report a direct interaction of GABA(A)R α2 and α3 subunit intracellular M3-M4 domain (but not α1, α4, α5, α6, β1-3, or γ1-3) with gephyrin. Curiously, GABA(A)R α2, but not α3, binds to both gephyrin and collybistin using overlapping sites. The reciprocal binding sites on gephyrin for collybistin and GABA(A)R α2 also overlap at the start of the gephyrin E domain. This suggests that although GABA(A)R α3 interacts with gephyrin, GABA(A)R α2, collybistin, and gephyrin form a trimeric complex. In support of this proposal, tri-hybrid interactions between GABA(A)R α2 and collybistin or GABA(A)R α2 and gephyrin are strengthened in the presence of gephyrin or collybistin, respectively. Collybistin and gephyrin also compete for binding to GABA(A)R α2 in co-immunoprecipitation experiments and co-localize in transfected cells in both intracellular and submembrane aggregates. Interestingly, GABA(A)R α2 is capable of "activating " collybistin isoforms harboring the regulatory SH3 domain, enabling targeting of gephyrin to the submembrane aggregates. The GABA(A)R α2-collybistin interaction was disrupted by a pathogenic mutation in the collybistin SH3 domain (p.G55A) that causes X-linked intellectual disability and seizures by disrupting GABA(A)R and gephyrin clustering. Because immunohistochemistry in retina revealed a preferential co-localization of collybistin with α2 subunit containing GABA(A)Rs, but not GlyRs or other GABA(A)R subtypes, we propose that the collybistin-gephyrin complex has an intimate role in the clustering of GABA(A)Rs containing the α2 subunit.

Synaptic plasticity, a symphony in GEF

Dendritic spines are the postsynaptic sites for the majority of excitatory synapses in the mammalian forebrain. While many spines display great stability, others change shape in a matter of seconds to minutes. These rapid alterations in dendritic spine number and size require tight control of the actin cytoskeleton, the main structural component of dendritic spines. The ability of neurons to alter spine number and size is essential for the expression of neuronal plasticity. Within spines, guanine nucleotide exchange factors (GEFs) act as critical regulators of the actin cytoskeleton by controlling the activity of Rho-GTPases. In this review we focus on the Rho-GEFs expressed in the nucleus accumbens and localized to the postsynaptic density, and thus positioned to effect rapid alterations in the structure of dendritic spines. We review literature that ties these GEFs to different receptor systems and intracellular signaling cascades and discuss the effects these interactions are likely to have on synaptic plasticity.

PH-domain-driven targeting of collybistin but not Cdc42 activation is required for synaptic gephyrin clustering

Collybistin (Cb) is a brain-specific guanine nucleotide exchange factor (GEF) that is essential for the synaptic clustering of gephyrin and GABAA receptors in selected regions of the mammalian central nervous system. It has been previously proposed that Cb regulates gephyrin clustering by activating Cdc42, and thus acts as a signal transducer in a membrane activation process which labels postsynaptic membrane domains for inhibitory synapse formation. Here, we dissected the functional roles of the Dbl-homology (DH) and pleckstrin homology (PH) domains of the constitutively active splice variant Cb II by substituting conserved amino acid residues that are required for GEF activity towards Cdc42 and phosphoinositide binding, respectively. A Cb II mutant lacking any detectable GEF activity towards Cdc42 was still fully active in inducing gephyrin scaffold formation, both in transfected NIH-3T3 cells and in cultured hippocampal neurons. Furthermore, mice with a forebrain-specific inactivation of the Cdc42 gene displayed normal densities of gephyrin and GABA(A) receptor clusters in the hippocampus. In contrast, substitution of Cb II PH-domain residues essential for phosphoinositide binding abolished gephyrin recruitment to synaptic sites. Our results provide evidence that the formation of gephyrin scaffolds at inhibitory synapses requires an intact Cb II PH-domain but is Cdc42-independent.

Galpha q allosterically activates and relieves autoinhibition of p63RhoGEF

Galpha(q) directly activates p63RhoGEF and closely related catalytic domains found in Trio and Kalirin, thereby linking G(q)-coupled receptors to the activation of RhoA. Although the crystal structure of G alpha(q) in complex with the catalytic domains of p63RhoGEF is available, the molecular mechanism of activation has not yet been defined. In this study, we show that membrane translocation does not appear to play a role in G alpha(q)-mediated activation of p63RhoGEF, as it does in some other RhoGEFs. G alpha(q) instead must act allosterically. We next identify specific structural elements in the PH domain that inhibit basal nucleotide exchange activity, and provide evidence that G alpha(q) overcomes this inhibition by altering the conformation of the alpha 6-alpha N linker that joins the DH and PH domains, a region that forms direct contacts with RhoA. We also identify residues in G alpha(q) that are important for the activation of p63RhoGEF and that contribute to G alpha subfamily selectivity, including a critical residue in the G alpha(q) C-terminal helix, and demonstrate the importance of these residues for RhoA activation in living cells.

Cellular transport and membrane dynamics of the glycine receptor

Regulation of synaptic transmission is essential to tune individual-to-network neuronal activity. One way to modulate synaptic strength is to regulate neurotransmitter receptor numbers at postsynaptic sites. This can be achieved either through plasma membrane insertion of receptors derived from intracellular vesicle pools, a process depending on active cytoskeleton transport, or through surface membrane removal via endocytosis. In parallel, lateral diffusion events along the plasma membrane allow the exchange of receptor molecules between synaptic and extrasynaptic compartments, contributing to synaptic strength regulation. In recent years, results obtained from several groups studying glycine receptor (GlyR) trafficking and dynamics shed light on the regulation of synaptic GlyR density. Here, we review (i) proteins and mechanisms involved in GlyR cytoskeletal transport, (ii) the diffusion dynamics of GlyR and of its scaffolding protein gephyrin that control receptor numbers, and its relationship with synaptic plasticity, and (iii) adaptative changes in GlyR diffusion in response to global activity modifications, as a homeostatic mechanism.

A balanced chromosomal translocation disrupting ARHGEF9 is associated with epilepsy, anxiety, aggression, and mental retardation

Clustering of inhibitory gamma-aminobutyric acid(A) (GABA(A)) and glycine receptors at synapses is thought to involve key interactions between the receptors, a "scaffolding" protein known as gephyrin and the RhoGEF collybistin. We report the identification of a balanced chromosomal translocation in a female patient presenting with a disturbed sleep-wake cycle, late-onset epileptic seizures, increased anxiety, aggressive behavior, and mental retardation, but not hyperekplexia. Fine mapping of the breakpoint indicates disruption of the collybistin gene (ARHGEF9) on chromosome Xq11, while the other breakpoint lies in a region of 18q11 that lacks any known or predicted genes. We show that defective collybistin transcripts are synthesized and exons 7-10 are replaced by cryptic exons from chromosomes X and 18. These mRNAs no longer encode the pleckstrin homology (PH) domain of collybistin, which we now show binds phosphatidylinositol-3-phosphate (PI3P/PtdIns-3-P), a phosphoinositide with an emerging role in membrane trafficking and signal transduction, rather than phosphatidylinositol 3,4,5-trisphosphate (PIP3/PtdIns-3,4,5-P) as previously suggested in the "membrane activation model" of gephyrin clustering. Consistent with this finding, expression of truncated collybistin proteins in cultured neurons interferes with synaptic localization of endogenous gephyrin and GABA(A) receptors. These results suggest that collybistin has a key role in membrane trafficking of gephyrin and selected GABA(A) receptor subtypes involved in epilepsy, anxiety, aggression, insomnia, and learning and memory.

The role of the conserved switch II glutamate in guanine nucleotide exchange factor-mediated nucleotide exchange of GTP-binding proteins

Guanine nucleotide exchange factors (GEFs) regulate the activity of small G proteins by catalysing the intrinsically slow exchange of GDP for GTP. The mechanism involves the formation of trimeric G protein-nucleotide-GEF complexes, followed by the release of nucleotide to form stable binary G protein-GEF complexes. A number of structural studies of G protein-GEF complexes have shown large structural changes induced in the nucleotide binding site. Together with a recent structure of a trimeric complex, these studies have suggested not only some common principles but also large differences in detail in the GEF-mediated exchange reaction. Several structures suggested that a glutamic acid residue in switch II, which is part of the DxxGQE motif and highly conserved in Ras-like G proteins, might have a decisive mechanistic role in GEF-mediated nucleotide exchange reactions. Here we show that mutation of the switch II glutamate to Ala severely impairs GEF-catalysed nucleotide exchange in most, but not all, Ras family G proteins, explaining its high sequence conservation. The residue determines the initial approach of GEF to the nucleotide-loaded G protein and does not appreciably affect the formation of a binary nucleotide-free complex. Its major effect thus appears to be the removal of the P-loop lysine from its interaction with the nucleotide.