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

A zebrafish gephyrinb mutant distinguishes synaptic and enzymatic functions of Gephyrin

Gephyrin is thought to play a critical role in clustering glycine receptors at synapses within the central nervous system (CNS). The main in vivo evidence for this comes from Gephyrin (Gphn)-null mice, where glycine receptors are depleted from synaptic regions. However, these mice die at birth, possibly due to impaired molybdenum cofactor (MoCo) synthesis, an essential role Gephyrin assumes throughout an animal. This complicates the interpretation of synaptic phenotypes in Gphn-null mice and raises the question whether the synaptic and enzymatic functions of Gephyrin can be investigated separately. Here, we generated a gephyrinb zebrafish mutant, vo84, that almost entirely lacks Gephyrin staining in the spinal cord. gephyrinbvo84 mutants exhibit normal gross morphology at both larval and adult stages. In contrast to Gphn-null mice, gephyrinbvo84 mutants exhibit normal motor activity and MoCo-dependent enzyme activity. Instead, gephyrinbvo84 mutants display impaired rheotaxis and increased mortality in late development. To investigate what may mediate these defects in gephyrinbvo84 mutants, we examined the cell density of neurons and myelin in the spinal cord and found no obvious changes. Surprisingly, in gephyrinbvo84 mutants, glycine receptors are still present in the synaptic regions. However, their abundance is reduced, potentially contributing to the observed defects. These findings challenge the notion that Gephyrin is absolutely required to cluster glycine receptors at synapses and reveals a new role of Gephyrin in regulating glycine receptor abundance and rheotaxis. They also establish a powerful new model for studying the mechanisms underlying synaptic, rather than enzymatic, functions of Gephyrin.

Modulation of sleep/wake patterns by gephyrin phosphorylation status

Sleep/wake cycles intricately shape physiological activities including cognitive brain functions, yet the precise molecular orchestrators of sleep remain elusive. Notably, the clinical impact of benzodiazepine drugs underscores the pivotal role of GABAergic neurotransmission in sleep regulation. However, the specific contributions of distinct GABAA receptor subtypes and their principal scaffolding protein, gephyrin, in sleep dynamics remain unclear. The evolving role of synaptic phospho-proteome alterations at excitatory and inhibitory synapses suggests a potential avenue for modulating gephyrin and, consequently, GABAARs for sleep through on-demand kinase recruitment. Our study unveils the distinctive roles of two prevalent GABAA receptor subtypes, α1- and α2-GABAARs, in influencing sleep duration and electrical sleep activity. Notably, the absence of α1-GABAARs emerges as central in sleep regulation, manifesting significant alterations in both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep during dark or active phases, accompanied by altered electroencephalogram (EEG) patterns across various frequencies. Gephyrin proteomics analysis reveals sleep/wake-dependent interactions with a repertoire of known and novel kinases. Crucially, we identify the regulation of gephyrin interaction with ERK1/2, and phosphorylations at serines 268 and 270 are dictated by sleep/wake cycles. Employing AAV-eGFP-gephyrin or its phospho-null variant (S268A/S270A), we disrupt sleep either globally or locally to demonstrate gephyrin phosphorylation as a sleep regulator. In summary, our findings support the local cortical sleep hypothesis and we unveil a molecular mechanism operating at GABAergic synapses, providing critical insights into the intricate regulation of sleep.

Wheel-Running Exercise Alleviates Anxiety-Like Behavior via Down-Regulating S-Nitrosylation of Gephyrin in the Basolateral Amygdala of Male Rats

Physical exercise has beneficial effect on anxiety disorders, but the underlying molecular mechanism remains largely unknown. Here, it is demonstrated that physical exercise can downregulate the S-nitrosylation of gephyrin (SNO-gephyrin) in the basolateral amygdala (BLA) to exert anxiolytic effects. It is found that the level of SNO-gephyrin is significantly increased in the BLA of high-anxiety rats and a downregulation of SNO-gephyrin at cysteines 212 and 284 produced anxiolytic effect. Mechanistically, inhibition of SNO-gephyrin by either Cys212 or Cys284 mutations increased the surface expression of GABAAR γ2 and the subsequent GABAergic neurotransmission, exerting anxiolytic effect in male rats. On the other side, overexpression of neuronal nitric oxide synthase in the BLA abolished the anxiolytic-like effects of physical exercise. This study reveals a key role of downregulating SNO-gephyrin in the anxiolytic effects of physical exercise, providing a new explanation for protein post-translational modifications in the brain after exercise.

Thermodynamic modulation of gephyrin condensation by inhibitory synapse components

Phase separation drives compartmentalization of intracellular contents into various biomolecular condensates. Individual condensate components are thought to differentially contribute to the organization and function of condensates. However, how intermolecular interactions among constituent biomolecules modulate the phase behaviors of multicomponent condensates remains unclear. Here, we used core components of the inhibitory postsynaptic density (iPSD) as a model system to quantitatively probe how the network of intra- and intermolecular interactions defines the composition and cellular distribution of biomolecular condensates. We found that oligomerization-driven phase separation of gephyrin, an iPSD-specific scaffold, is critically modulated by an intrinsically disordered linker region exhibiting minimal homotypic attractions. Other iPSD components, such as neurotransmitter receptors, differentially promote gephyrin condensation through distinct binding modes and affinities. We further demonstrated that the local accumulation of scaffold-binding proteins at the cell membrane promotes the nucleation of gephyrin condensates in neurons. These results suggest that in multicomponent systems, the extent of scaffold condensation can be fine-tuned by scaffold-binding factors, a potential regulatory mechanism for self-organized compartmentalization in cells.

The History of Animal and Plant Sulfite Oxidase-A Personal View

Sulfite oxidase is one of five molybdenum-containing enzymes known in eukaryotes where it catalyzes the oxidation of sulfite to sulfate. This review covers the history of sulfite oxidase research starting out with the early years of its discovery as a hepatic mitochondrial enzyme in vertebrates, leading to basic biochemical and structural properties that have inspired research for decades. A personal view on sulfite oxidase in plants, that sulfates are assimilated for their de novo synthesis of cysteine, is presented by Ralf Mendel with numerous unexpected findings and unique properties of this single-cofactor sulfite oxidase localized to peroxisomes. Guenter Schwarz connects his research to sulfite oxidase via its deficiency in humans, demonstrating its unique role amongst all molybdenum enzymes in humans. In essence, in both the plant and animal kingdoms, sulfite oxidase represents an important player in redox regulation, signaling and metabolism, thereby connecting sulfur and nitrogen metabolism in multiple ways.

Eukaryotic-like gephyrin and cognate membrane receptor coordinate corynebacterial cell division and polar elongation

The order Corynebacteriales includes major industrial and pathogenic Actinobacteria such as Corynebacterium glutamicum or Mycobacterium tuberculosis. These bacteria have multi-layered cell walls composed of the mycolyl-arabinogalactan-peptidoglycan complex and a polar growth mode, thus requiring tight coordination between the septal divisome, organized around the tubulin-like protein FtsZ, and the polar elongasome, assembled around the coiled-coil protein Wag31. Here, using C. glutamicum, we report the discovery of two divisome members: a gephyrin-like repurposed molybdotransferase (Glp) and its membrane receptor (GlpR). Our results show how cell cycle progression requires interplay between Glp/GlpR, FtsZ and Wag31, showcasing a crucial crosstalk between the divisome and elongasome machineries that might be targeted for anti-mycobacterial drug discovery. Further, our work reveals that Corynebacteriales have evolved a protein scaffold to control cell division and morphogenesis, similar to the gephyrin/GlyR system that mediates synaptic signalling in higher eukaryotes through network organization of membrane receptors and the microtubule cytoskeleton.

Eukaryotic-like gephyrin and cognate membrane receptor coordinate corynebacterial cell division and polar elongation

The order Corynebacteriales includes major industrial and pathogenic actinobacteria such as Corynebacterium glutamicum or Mycobacterium tuberculosis . Their elaborate multi-layered cell wall, composed primarily of the mycolyl-arabinogalactan-peptidoglycan complex, and their polar growth mode impose a stringent coordination between the septal divisome, organized around the tubulin-like protein FtsZ, and the polar elongasome, assembled around the tropomyosin-like protein Wag31. Here, we report the identification of two new divisome members, a gephyrin-like repurposed molybdotransferase (GLP) and its membrane receptor (GLPR). We show that the interplay between the GLPR/GLP module, FtsZ and Wag31 is crucial for orchestrating cell cycle progression. Our results provide a detailed molecular understanding of the crosstalk between two essential machineries, the divisome and elongasome, and reveal that Corynebacteriales have evolved a protein scaffold to control cell division and morphogenesis similar to the gephyrin/GlyR system that in higher eukaryotes mediates synaptic signaling through network organization of membrane receptors and the microtubule cytoskeleton.

Automated Image Analysis Reveals Different Localization of Synaptic Gephyrin C4 Splice Variants

Postsynaptic scaffolding proteins function as central organization hubs, ensuring the synaptic localization of neurotransmitter receptors, trans-synaptic adhesion proteins, and signaling molecules. Gephyrin is the major postsynaptic scaffolding protein at glycinergic and a subset of GABAergic inhibitory synapses. In contrast to cells outside the CNS, where one gephyrin isoform is predominantly expressed, neurons express different splice variants. In this study, we characterized the expression and scaffolding of neuronal gephyrin isoforms differing in the inclusion of the C4 cassettes located in the central C-domain. In hippocampal and cortical neuronal populations, gephyrin P1, lacking additional cassettes, is the most abundantly expressed isoform. In addition, alternative splicing generated isoforms carrying predominantly C4a, and minor amounts of C4c or C4d cassettes. We detected no striking difference in C4 isoform expression between different neuron types and a single neuron can likely express all C4 isoforms. To avoid the cytosolic aggregates that are commonly observed upon exogenous gephyrin expression, we used adeno-associated virus (AAV)-mediated expression to analyze the scaffolding behavior of individual C4 isoforms in murine dissociated hippocampal glutamatergic neurons. While all isoforms showed similar clustering at GABAergic synapses, a thorough quantitative analysis revealed localization differences for the C4c isoform (also known as P2). Specifically, synaptic C4c isoform clusters showed a more distal dendritic localization and reduced occurrence at P1-predominating synapses. Additionally, inhibitory currents displayed faster decay kinetics in the presence of gephyrin C4c compared with P1. Therefore, inhibitory synapse heterogeneity may be influenced, at least in part, by mechanisms relating to C4 cassette splicing.

Bi-allelic gephyrin variants impair GABAergic inhibition in a patient with epileptic encephalopathy

Synaptic inhibition is essential for shaping the dynamics of neuronal networks, and aberrant inhibition is linked to epilepsy. Gephyrin (Geph) is the principal scaffolding protein at inhibitory synapses and is essential for postsynaptic clustering of glycine (GlyRs) and GABA type A receptors (GABAARs). Consequently, gephyrin is crucial for maintaining the relationship between excitation and inhibition in normal brain function and mutations in the gephyrin gene (GPHN) are associated with neurodevelopmental disorders and epilepsy. We identified bi-allelic variants in the GPHN gene, namely the missense mutation c.1264G > A and splice acceptor variant c.1315-2A > G, in a patient with developmental and epileptic encephalopathy (DEE). We demonstrate that the splice acceptor variant leads to nonsense-mediated mRNA decay (NMD). Furthermore, the missense variant (D422N) alters gephyrin structure, as examined by analytical size exclusion chromatography and CD-spectroscopy, thus leading to reduced receptor clustering and sensitivity towards calpain-mediated cleavage. Additionally, both alterations contribute to an observed reduction of inhibitory signal transmission in neurons, which likely contributes to the pathological encephalopathy.

Artemisinin-treatment in pre-symptomatic APP-PS1 mice increases gephyrin phosphorylation at Ser270: a modification regulating postsynaptic GABAAR density

Artemisinins, a group of plant-derived sesquiterpene lactones, are efficient antimalarial agents. They also share anti-inflammatory and anti-viral activities and were considered for treatment of neurodegenerative disorders like Alzheimer's disease (AD). Additionally, artemisinins bind to gephyrin, the multifunctional scaffold of GABAergic synapses, and modulate inhibitory neurotransmission in vitro. We previously reported an increased expression of gephyrin and GABA_A receptors in early pre-symptomatic stages of an AD mouse model (APP-PS1) and in parallel enhanced CDK5-dependent phosphorylation of gephyrin at S270. Here, we studied the effects of artemisinin on gephyrin in the brain of young APP-PS1 mice. We detected an additional increase of gephyrin protein level, elevated gephyrin phosphorylation at Ser270, and an increased amount of GABA_AR-γ2 subunits after artemisinin-treatment. Interestingly, the CDK5 activator p35 was also upregulated. Moreover, we demonstrate decreased density of postsynaptic gephyrin and GABA_AR-γ2 immunoreactivities in cultured hippocampal neurons expressing gephyrin with alanine mutations at two CDK5 phosphorylation sites. In addition, the activity-dependent modulation of synaptic protein density was abolished in neurons expressing gephyrin lacking one or both of these phosphorylation sites. Thus, our results reveal that artemisinin modulates expression as well as phosphorylation of gephyrin at sites that might have important impact on GABAergic synapses in AD.

Binding of gephyrin to microtubules is regulated by its phosphorylation at Ser270

Gephyrin is a multifunctional scaffolding protein anchoring glycine- and subtypes of GABA type A- receptors at inhibitory postsynaptic membrane specializations by binding to the microtubule (MT) and/or the actin cytoskeleton. However, the conditions under which gephyrin can bind to MTs and its regulation are currently unknown. Here, we demonstrate that during the purification of MTs from rat brain by sedimentation of polymerized tubulin using high-speed centrifugation a fraction of gephyrin was bound to MTs, whereas gephyrin phosphorylated at the CDK5-dependent site Ser270 was detached from MTs and remained in the soluble protein fraction. Moreover, after collybistin fostered phosphorylation at Ser270 the binding of a recombinant gephyrin to MTs was strongly reduced in co-sedimentation assays. Correspondingly, upon substitution of wild-type gephyrin with recombinant gephyrin carrying alanine mutations at putative CDK5 phosphorylation sites the binding of gephyrin to MTs was increased. Furthermore, the analysis of cultured HEK293T and U2OS cells by immunofluorescence-microscopy disclosed a dispersed and punctuated endogenous gephyrin immunoreactivity co-localizing with MTs which was evidently not phosphorylated at Ser270. Thus, our study provides additional evidence for the binding of gephyrin to MTs in brain tissue and in in vitro cell systems. More importantly, our findings indicate that gephyrin-MT binding is restricted to a specific gephyrin fraction and depicts phosphorylation of gephyrin as a regulatory mechanism of this process by showing that soluble gephyrin detached from MTs can be detected specifically with the mAb7a antibody, which recognizes the Ser270 phosphorylated- version of gephyrin.

Gephyrin-mediated formation of inhibitory postsynaptic density sheet via phase separation

Inhibitory synapses are also known as symmetric synapses due to their lack of prominent postsynaptic densities (PSDs) under a conventional electron microscope (EM). Recent cryo-EM tomography studies indicated that inhibitory synapses also contain PSDs, albeit with a rather thin sheet-like structure. It is not known how such inhibitory PSD (iPSD) sheet might form. Here, we demonstrate that the key inhibitory synapse scaffold protein gephyrin, when in complex with either glycine or GABA_A receptors, spontaneously forms highly condensed molecular assemblies via phase separation both in solution and on supported membrane bilayers. Multivalent and specific interactions between the dimeric E-domain of gephyrin and the glycine/GABA_A receptor multimer are essential for the iPSD condensate formation. Gephyrin alone does not form condensates. The linker between the G- and E-domains of gephyrin inhibits the iPSD condensate formation via autoinhibition. Phosphorylation of specific residues in the linker or binding of target proteins such as dynein light chain to the linker domain regulates gephyrin-mediated glycine/GABA_A receptor clustering. Thus, analogous to excitatory PSDs, iPSDs are also formed by phase separation-mediated condensation of scaffold protein/neurotransmitter receptor complexes.

High-throughput determination of protein affinities using unmodified peptide libraries in nanomolar scale

Protein-protein interactions (PPIs) are of fundamental importance for our understanding of physiology and pathology. PPIs involving short, linear motifs play a major role in immunological recognition, signaling, and regulation and provide attractive starting points for pharmaceutical intervention. Yet, state-of-the-art protein-peptide affinity determination approaches exhibit limited throughput and sensitivity, often resulting from ligand immobilization, labeling, or synthesis. Here, we introduce a high-throughput method for in-solution analysis of protein-peptide interactions using a phenomenon called temperature related intensity change (TRIC). We use TRIC for the identification and fine-mapping of low- and high-affinity protein interaction sites and the definition of sequence binding requirements. Validation is achieved by microarray-based studies using wild-type and mutated recombinant protein and the native protein within tissue lysates. On-chip neutralization and strong correlation with structural data establish TRIC as a quasi-label-free method to determine binding affinities of unmodified peptide libraries with large dynamic range.

Tuning GABAergic Inhibition: Gephyrin Molecular Organization and Functions

To be highly reliable, synaptic transmission needs postsynaptic receptors (Rs) in precise apposition to the presynaptic release sites. At inhibitory synapses, the postsynaptic protein gephyrin self-assembles to form a scaffold that anchors glycine and GABAARs to the cytoskeleton, thus ensuring the accurate accumulation of postsynaptic receptors at the right place. This protein undergoes several post-translational modifications which control protein-protein interaction and downstream signaling pathways. In addition, through the constant exchange of scaffolding elements and receptors in and out of synapses, gephyrin dynamically regulates synaptic strength and plasticity. The aim of the present review is to highlight recent findings on the functional role of gephyrin at GABAergic inhibitory synapses. We will discuss different approaches used to interfere with gephyrin in order to unveil its function. In addition, we will focus on the impact of gephyrin structure and distribution at the nanoscale level on the functional properties of inhibitory synapses as well as the implications of this scaffold protein in synaptic plasticity processes. Finally, we will emphasize how gephyrin genetic mutations or alterations in protein expression levels are implicated in several neuropathological disorders, including autism spectrum disorders, schizophrenia, temporal lobe epilepsy and Alzheimer's disease, all associated with severe deficits of GABAergic signaling. This article is part of a Special Issue entitled: Honoring Ricardo Miledi - outstanding neuroscientist of XX-XXI centuries.

Nanoscale Subsynaptic Domains Underlie the Organization of the Inhibitory Synapse

Inhibitory synapses mediate the majority of synaptic inhibition in the brain, thereby controlling neuronal excitability, firing, and plasticity. Although essential for neuronal function, the central question of how these synapses are organized at the subsynaptic level remains unanswered. Here, we use three-dimensional (3D) super-resolution microscopy to image key components of the inhibitory postsynaptic domain and presynaptic terminal, revealing that inhibitory synapses are organized into nanoscale subsynaptic domains (SSDs) of the gephyrin scaffold, GABA_ARs and the active-zone protein Rab3-interacting molecule (RIM). Gephyrin SSDs cluster GABA_AR SSDs, demonstrating nanoscale architectural interdependence between scaffold and receptor. GABA_AR SSDs strongly associate with active-zone RIM SSDs, indicating an important role for GABA_AR nanoscale organization near sites of GABA release. Finally, we find that in response to elevated activity, synapse growth is mediated by an increase in the number of postsynaptic SSDs, suggesting a modular mechanism for increasing inhibitory synaptic strength.

A Sam68-dependent alternative splicing program shapes postsynaptic protein complexes

Alternative splicing is one of the key mechanisms to increase the diversity of cellular transcriptomes, thereby expanding the coding capacity of the genome. This diversity is of particular importance in the nervous system with its elaborated cellular networks. Sam68, a member of the Signal Transduction Associated RNA-binding (STAR) family of RNA-binding proteins, is expressed in the developing and mature nervous system but its neuronal functions are poorly understood. Here, we perform genome-wide mapping of the Sam68-dependent alternative splicing program in mice. We find that Sam68 is required for the regulation of a set of alternative splicing events in pre-mRNAs encoding several postsynaptic scaffolding molecules that are central to the function of GABAergic and glutamatergic synapses. These components include Collybistin (Arhgef9), Gephyrin (Gphn), and Densin-180 (Lrrc7). Sam68-regulated Lrrc7 variants engage in differential protein interactions with signalling proteins, thus, highlighting a contribution of the Sam68 splicing program to shaping synaptic complexes. These findings suggest an important role for Sam68-dependent alternative splicing in the regulation of synapses in the central nervous system.

Synaptic structural protein dysfunction leads to altered excitation inhibition ratios in models of autism spectrum disorder

Genetics is believed to play a key role in the development of Autism Spectrum Disorder (ASD) and a plethora of potential candidate genes have been identified by genetic characterization of patients, their family members and controls. To make sense of this information investigators have searched for common pathways and downstream properties of neural networks that are regulated by these genes. For instance, several candidate genes encode synaptic proteins, and one hypothesis that has emerged is that disruption of the synaptic excitation and inhibition (E/I) balance would destabilize neural processing and lead to ASD phenotypes. Some compelling evidence for this has come from the analyses of mouse and culture models with defects in synaptic structural proteins, which influence several aspects of synapse biology and is the subject of this review. Remaining challenges include identifying the specifics that distinguish ASD from other psychiatric diseases and designing more direct tests of the E/I balance hypothesis.

Structure-Function Relationships of Glycine and GABAA Receptors and Their Interplay With the Scaffolding Protein Gephyrin

Glycine and γ-aminobutyric acid (GABA) are the major determinants of inhibition in the central nervous system (CNS). These neurotransmitters target glycine and GABA_A receptors, respectively, which both belong to the Cys-loop superfamily of pentameric ligand-gated ion channels (pLGICs). Interactions of the neurotransmitters with the cognate receptors result in receptor opening and a subsequent influx of chloride ions, which, in turn, leads to hyperpolarization of the membrane potential, thus counteracting excitatory stimuli. The majority of glycine receptors and a significant fraction of GABA_A receptors (GABA_ARs) are recruited and anchored to the post-synaptic membrane by the central scaffolding protein gephyrin. This ∼93 kDa moonlighting protein is structurally organized into an N-terminal G-domain (GephG) connected to a C-terminal E-domain (GephE) via a long unstructured linker. Both inhibitory neurotransmitter receptors interact via a short peptide motif located in the large cytoplasmic loop located in between transmembrane helices 3 and 4 (TM3-TM4) of the receptors with a universal receptor-binding epitope residing in GephE. Gephyrin engages in nearly identical interactions with the receptors at the N-terminal end of the peptide motif, and receptor-specific interaction toward the C-terminal region of the peptide. In addition to its receptor-anchoring function, gephyrin also interacts with a rather large collection of macromolecules including different cytoskeletal elements, thus acting as central scaffold at inhibitory post-synaptic specializations. Dysfunctions in receptor-mediated or gephyrin-mediated neurotransmission have been identified in various severe neurodevelopmental disorders. Although biochemical, cellular and electrophysiological studies have helped to understand the physiological and pharmacological roles of the receptors, recent high resolution structures of the receptors have strengthened our understanding of the receptors and their gating mechanisms. Besides that, multiple crystal structures of GephE in complex with receptor-derived peptides have shed light into receptor clustering by gephyrin at inhibitory post-synapses. This review will highlight recent biochemical and structural insights into gephyrin and the GlyRs as well as GABA_A receptors, which provide a deeper understanding of the molecular machinery mediating inhibitory neurotransmission.

GABA type a receptor trafficking and the architecture of synaptic inhibition

Ubiquitous expression of GABA type A receptors (GABAA R) in the central nervous system establishes their central role in coordinating most aspects of neural function and development. Dysregulation of GABAergic neurotransmission manifests in a number of human health disorders and conditions that in certain cases can be alleviated by drugs targeting these receptors. Precise changes in the quantity or activity of GABAA Rs localized at the cell surface and at GABAergic postsynaptic sites directly impact the strength of inhibition. The molecular mechanisms constituting receptor trafficking to and from these compartments therefore dictate the efficacy of GABAA R function. Here we review the current understanding of how GABAA Rs traffic through biogenesis, plasma membrane transport, and degradation. Emphasis is placed on discussing novel GABAergic synaptic proteins, receptor and scaffolding post-translational modifications, activity-dependent changes in GABAA R confinement, and neuropeptide and neurosteroid mediated changes. We further highlight modern techniques currently advancing the knowledge of GABAA R trafficking and clinically relevant neurodevelopmental diseases connected to GABAergic dysfunction. © 2017 Wiley Periodicals, Inc. Develop Neurobiol 78: 238-270, 2018.

Sequences Flanking the Gephyrin-Binding Site of GlyRβ Tune Receptor Stabilization at Synapses

The efficacy of synaptic transmission is determined by the number of neurotransmitter receptors at synapses. Their recruitment depends upon the availability of postsynaptic scaffolding molecules that interact with specific binding sequences of the receptor. At inhibitory synapses, gephyrin is the major scaffold protein that mediates the accumulation of heteromeric glycine receptors (GlyRs) via the cytoplasmic loop in the β-subunit (β-loop). This binding involves high- and low-affinity interactions, but the molecular mechanism of this bimodal binding and its implication in GlyR stabilization at synapses remain unknown. We have approached this question using a combination of quantitative biochemical tools and high-density single molecule tracking in cultured rat spinal cord neurons. The high-affinity binding site could be identified and was shown to rely on the formation of a 3₁₀-helix C-terminal to the β-loop core gephyrin-binding motif. This site plays a structural role in shaping the core motif and represents the major contributor to the synaptic confinement of GlyRs by gephyrin. The N-terminal flanking sequence promotes lower affinity interactions by occupying newly identified binding sites on gephyrin. Despite its low affinity, this binding site plays a modulatory role in tuning the mobility of the receptor. Together, the GlyR β-loop sequences flanking the core-binding site differentially regulate the affinity of the receptor for gephyrin and its trapping at synapses. Our experimental approach thus bridges the gap between thermodynamic aspects of receptor-scaffold interactions and functional receptor stabilization at synapses in living cells.

Alpha subunit-dependent glycine receptor clustering and regulation of synaptic receptor numbers

Accumulation of glycine receptors at synapses requires the interaction between the beta subunit of the receptor and the scaffold protein gephyrin. Here, we questioned whether different alpha subunits could modulate the receptors' diffusion and propensity to cluster at spinal cord synapses. Using quantitative photoactivated localisation microscopy we found that alpha-1 and alpha-3 containing glycine receptors display the same α₃:β₂ stoichiometry and gephyrin binding. Despite these similarities, alpha-3 containing receptors are less mobile and cluster at higher density compared to alpha-1, with 1500 versus 1100 complexes µm^-2, respectively. Furthermore, we identified a subunit-specific regulation of glycine receptor copy numbers at synapses: when challenged with interleukin 1β, the synaptic occupancy of alpha-1 but not alpha-3 receptors was reduced. This mechanism may play a role in the cell-type dependent regulation of glycinergic currents in response to interleukin 1β and highlights the capacity of the alpha subunits to affect receptor-gephyrin binding at synapses.

Shared function and moonlighting proteins in molybdenum cofactor biosynthesis

The biosynthesis of the molybdenum cofactor (Moco) is a highly conserved pathway in bacteria, archaea and eukaryotes. The molybdenum atom in Moco-containing enzymes is coordinated to the dithiolene group of a tricyclic pyranopterin monophosphate cofactor. The biosynthesis of Moco can be divided into three conserved steps, with a fourth present only in bacteria and archaea: (1) formation of cyclic pyranopterin monophosphate, (2) formation of molybdopterin (MPT), (3) insertion of molybdenum into MPT to form Mo-MPT, and (4) additional modification of Mo-MPT in bacteria with the attachment of a GMP or CMP nucleotide, forming the dinucleotide variants of Moco. While the proteins involved in the catalytic reaction of each step of Moco biosynthesis are highly conserved among the Phyla, a surprising link to other cellular pathways has been identified by recent discoveries. In particular, the pathways for FeS cluster assembly and thio-modifications of tRNA are connected to Moco biosynthesis by sharing the same protein components. Further, proteins involved in Moco biosynthesis are not only shared with other pathways, but additionally have moonlighting roles. This review gives an overview of Moco biosynthesis in bacteria and humans and highlights the shared function and moonlighting roles of the participating proteins.

An aggregation-removal model for the formation and size determination of post-synaptic scaffold domains

The formation and stability of synapses are key questions in neuroscience. Post-synaptic domains have been classically conceived as resulting from local insertion and turnover of proteins at the synapse. However, insertion is likely to occur outside the post-synaptic domains and advances in single-molecule imaging have shown that proteins diffuse in the plane of the membrane prior to their accumulation at synapses. We quantitatively investigated this scenario using computer simulations and mathematical analysis, taking for definiteness the specific case of inhibitory synapse components, i.e., the glycine receptor (GlyR) and the associated gephyrin scaffolding protein. The observed domain sizes of scaffold clusters can be explained by a dynamic balance between the aggregation of gephyrin proteins diffusing while bound to GlyR and their turnover at the neuron membrane. We also predict the existence of extrasynaptic clusters with a characteristic size distribution that significantly contribute to the size fluctuations of synaptic domains. New super-resolution data for gephyrin proteins established the existence of extrasynaptic clusters the sizes of which are consistent with the model predictions in a range of model parameters. At a general level, our results highlight aggregation with removal as a non-equilibrium phase separation which produces structures of tunable size.

Synapses mediate information transmission between neurons and are the physical support of memory. It has been realized that synapses are dynamic biological structures. Neurotransmitter receptors diffuse in the neuron membrane and synaptic scaffolding proteins are constantly renewed. We propose a biophysical model that links these different measured quantities for inhibitory synapse components and show how they determine the size of postsynaptic domains. The model predicts that synaptic scaffolds also exist extrasynaptically and that they contribute to fluctuations of synaptic sizes. We confirm by super-resolution microscopy the existence of extrasynaptic scaffolds. Their measured size distribution agrees with the model predictions for specific parameters. The model should be helpful to better understand the dynamics of synapses and their possible levels of regulation.

Gephyrin and the regulation of synaptic strength and dynamics at glycinergic inhibitory synapses

Glycinergic synapses predominate in brainstem and spinal cord where they modulate motor and sensory processing. Their postsynaptic mechanisms have been considered rather simple because they lack a large variety of glycine receptor isoforms and have relatively simple postsynaptic densities at the ultrastructural level. However, this simplicity is misleading being their postsynaptic regions regulated by a variety of complex mechanisms controlling the efficacy of synaptic inhibition. Early studies suggested that glycinergic inhibitory strength and dynamics depend largely on structural features rather than on molecular complexity. These include regulation of the number of postsynaptic glycine receptors, their localization and the amount of co-localized GABA_A receptors and GABA-glycine co-transmission. These properties we now know are under the control of gephyrin. Gephyrin is the first postsynaptic scaffolding protein ever discovered and it was recently found to display a large degree of variation and regulation by splice variants, posttranslational modifications, intracellular trafficking and interactions with the underlying cytoskeleton. Many of these mechanisms are governed by converging excitatory activity and regulate gephyrin oligomerization and receptor binding, the architecture of the postsynaptic density (and by extension the whole synaptic complex), receptor retention and stability. These newly uncovered molecular mechanisms define the size and number of gephyrin postsynaptic regions and the numbers and proportions of glycine and GABA_A receptors contained within. All together, they control the emergence of glycinergic synapses of different strength and temporal properties to best match the excitatory drive received by each individual neuron or local dendritic compartment.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Simultaneous impairment of neuronal and metabolic function of mutated gephyrin in a patient with epileptic encephalopathy

Synaptic inhibition is essential for shaping the dynamics of neuronal networks, and aberrant inhibition plays an important role in neurological disorders. Gephyrin is a central player at inhibitory postsynapses, directly binds and organizes GABA_A and glycine receptors (GABA_ARs and GlyRs), and is thereby indispensable for normal inhibitory neurotransmission. Additionally, gephyrin catalyzes the synthesis of the molybdenum cofactor (MoCo) in peripheral tissue. We identified a de novo missense mutation (G375D) in the gephyrin gene (GPHN) in a patient with epileptic encephalopathy resembling Dravet syndrome. Although stably expressed and correctly folded, gephyrin‐G375D was non‐synaptically localized in neurons and acted dominant‐negatively on the clustering of wild‐type gephyrin leading to a marked decrease in GABA_AR surface expression and GABAergic signaling. We identified a decreased binding affinity between gephyrin‐G375D and the receptors, suggesting that Gly375 is essential for gephyrin–receptor complex formation. Surprisingly, gephyrin‐G375D was also unable to synthesize MoCo and activate MoCo‐dependent enzymes. Thus, we describe a missense mutation that affects both functions of gephyrin and suggest that the identified defect at GABAergic synapses is the mechanism underlying the patient's severe phenotype.

Repetitive magnetic stimulation induces plasticity of inhibitory synapses

Repetitive transcranial magnetic stimulation (rTMS) is used as a therapeutic tool in neurology and psychiatry. While repetitive magnetic stimulation (rMS) has been shown to induce plasticity of excitatory synapses, it is unclear whether rMS can also modify structural and functional properties of inhibitory inputs. Here we employed 10-Hz rMS of entorhinohippocampal slice cultures to study plasticity of inhibitory neurotransmission on CA1 pyramidal neurons. Our experiments reveal a rMS-induced reduction in GABAergic synaptic strength (2-4 h after stimulation), which is Ca(2+)-dependent and accompanied by the remodelling of postsynaptic gephyrin scaffolds. Furthermore, we present evidence that 10-Hz rMS predominantly acts on dendritic, but not somatic inhibition. Consistent with this finding, a reduction in clustered gephyrin is detected in CA1 stratum radiatum of rTMS-treated anaesthetized mice. These results disclose that rTMS induces coordinated Ca(2+)-dependent structural and functional changes of specific inhibitory postsynapses on principal neurons.

Gephyrin: a central GABAergic synapse organizer

Gephyrin is a central element that anchors, clusters and stabilizes glycine and γ-aminobutyric acid type A receptors at inhibitory synapses of the mammalian brain. It self-assembles into a hexagonal lattice and interacts with various inhibitory synaptic proteins. Intriguingly, the clustering of gephyrin, which is regulated by multiple posttranslational modifications, is critical for inhibitory synapse formation and function. In this review, we summarize the basic properties of gephyrin and describe recent findings regarding its roles in inhibitory synapse formation, function and plasticity. We will also discuss the implications for the pathophysiology of brain disorders and raise the remaining open questions in this field.

Human gephyrin is encompassed within giant functional noncoding yin-yang sequences

Gephyrin is a highly-conserved gene that is vital for the organization of proteins at inhibitory receptors, molybdenum cofactor biosynthesis, and other diverse functions. Its specific function is intricately regulated and its aberrant activities have been observed for a number of human diseases. Here we report a remarkable yin-yang haplotype pattern encompassing gephyrin. Yin-yang haplotypes arise when a stretch of DNA evolves to present two disparate forms that bear differing states for nucleotide variations along their lengths. The gephyrin yin-yang pair consists of 284 divergent nucleotide states and both variants vary drastically from their mutual ancestral haplotype, suggesting rapid evolution. Several independent lines of evidence indicate strong positive selection on the region and suggest these high-frequency haplotypes represent two distinct functional mechanisms. This discovery holds potential to deepen our understanding of variable human-specific regulation of gephyrin while providing clues for rapid evolutionary events and allelic migrations buried within human history.

Modulation of gephyrin-glycine receptor affinity by multivalency

Gephyrin is a major determinant for the accumulation and anchoring of glycine receptors (GlyRs) and the majority of γ-aminobutyric acid type A receptors (GABAARs) at postsynaptic sites. Here we explored the interaction of gephyrin with a dimeric form of a GlyR β-subunit receptor-derived peptide. A 2 Å crystal structure of the C-terminal domain of gephyrin (GephE) in complex with a 15-residue peptide derived from the GlyR β-subunit defined the core binding site, which we targeted with the dimeric peptide. Biophysical analyses via differential scanning calorimetry (DSC), thermofluor, and isothermal titration calorimetry (ITC) demonstrated that this dimeric ligand is capable of binding simultaneously to two receptor binding sites and that this multivalency results in a 25-fold enhanced affinity. Our study therefore suggests that the oligomeric state of gephyrin and the number of gephyrin-binding subunits in the pentameric GABAARs and GlyRs together control postsynaptic receptor clustering.

Pin1-dependent signalling negatively affects GABAergic transmission by modulating neuroligin2/gephyrin interaction

The cell adhesion molecule Neuroligin2 (NL2) is localized selectively at GABAergic synapses, where it interacts with the scaffolding protein gephyrin in the post-synaptic density. However, the role of this interaction for formation and plasticity of GABAergic synapses is unclear. Here, we demonstrate that endogenous NL2 undergoes proline-directed phosphorylation at its unique S714-P consensus site, leading to the recruitment of the peptidyl-prolyl cis-trans isomerase Pin1. This signalling cascade negatively regulates NL2's ability to interact with gephyrin at GABAergic post-synaptic sites. As a consequence, enhanced accumulation of NL2, gephyrin and GABAA receptors was detected at GABAergic synapses in the hippocampus of Pin1-knockout mice (Pin1-/-) associated with an increase in amplitude of spontaneous GABAA-mediated post-synaptic currents. Our results suggest that Pin1-dependent signalling represents a mechanism to modulate GABAergic transmission by regulating NL2/gephyrin interaction.

Diffusion dynamics of synaptic molecules during inhibitory postsynaptic plasticity

The plasticity of inhibitory transmission is expected to play a key role in the modulation of neuronal excitability and network function. Over the last two decades, the investigation of the determinants of inhibitory synaptic plasticity has allowed distinguishing presynaptic and postsynaptic mechanisms. While there has been a remarkable progress in the characterization of presynaptically-expressed plasticity of inhibition, the postsynaptic mechanisms of inhibitory long-term synaptic plasticity only begin to be unraveled. At postsynaptic level, the expression of inhibitory synaptic plasticity involves the rearrangement of the postsynaptic molecular components of the GABAergic synapse, including GABA_A receptors, scaffold proteins and structural molecules. This implies a dynamic modulation of receptor intracellular trafficking and receptor surface lateral diffusion, along with regulation of the availability and distribution of scaffold proteins. This Review will focus on the mechanisms of the multifaceted molecular reorganization of the inhibitory synapse during postsynaptic plasticity, with special emphasis on the key role of protein dynamics to ensure prompt and reliable activity-dependent adjustments of synaptic strength.

Electron tomography on γ-aminobutyric acid-ergic synapses reveals a discontinuous postsynaptic network of filaments

The regulation of synaptic strength at γ-aminobutyric acid (GABA)-ergic synapses is dependent on the dynamic capture, retention, and modulation of GABA A-type receptors by cytoplasmic proteins at GABAergic postsynaptic sites. How these proteins are oriented and organized in the postsynaptic cytoplasm is not yet established. To better understand these structures and gain further insight into the mechanisms by which they regulate receptor populations at postsynaptic sites, we utilized electron tomography to examine GABAergic synapses in dissociated rat hippocampal cultures. GABAergic synapses were identified and selected for tomography by using a set of criteria derived from the structure of immunogold-labeled GABAergic synapses. Tomography revealed a complex postsynaptic network composed of filaments that extend ∼ 100 nm into the cytoplasm from the postsynaptic membrane. The distribution of these postsynaptic filaments was strikingly similar to that of the immunogold label for gephyrin. Filaments were interconnected through uniform patterns of contact, forming complexes composed of 2-12 filaments each. Complexes did not link to form an integrated, continuous scaffold, suggesting that GABAergic postsynaptic specializations are less rigidly organized than glutamatergic postsynaptic densities.

Cyclin-dependent kinase 5 is involved in the phosphorylation of gephyrin and clustering of GABAA receptors at inhibitory synapses of hippocampal neurons

CDK5 has been implicated in neural functions including growth, neuronal migration, synaptic transmission and plasticity of excitatory chemical synapses. Here we report robust effects of CDK5 on phosphorylation of the postsynaptic scaffold protein gephyrin and clustering of inhibitory GABA_A receptors in hippocampal neurons. shRNA-mediated knockdown of CDK5 and pharmacological inhibition of cyclin-dependent kinases reduced phosphorylated gephyrin clusters and postsynaptic γ2-containing GABA_A receptors. Phosphorylation of S270 is antagonized by PP1/PP2a phosphatase and site-directed mutagenesis and in vitro phosphorylation experiments indicate that S270 is a putative CDK5 phosphorylation site of gephyrin. Our data suggest that CDK5 plays an essential role for the stability of gephyrin-dependent GABA_A receptor clusters in 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.

Neuronal nitric oxide synthase-dependent S-nitrosylation of gephyrin regulates gephyrin clustering at GABAergic synapses

Gephyrin, the principal scaffolding protein at inhibitory synapses, is essential for postsynaptic clustering of glycine and GABA type A receptors (GABA(A)Rs). Gephyrin cluster formation, which determines the strength of GABAergic transmission, is modulated by interaction with signaling proteins and post-translational modifications. Here, we show that gephyrin was found to be associated with neuronal nitric oxide synthase (nNOS), the major source of the ubiquitous and important signaling molecule NO in brain. Furthermore, we identified that gephyrin is S-nitrosylated in vivo. Overexpression of nNOS decreased the size of postsynaptic gephyrin clusters in primary hippocampal neurons. Conversely, inhibition of nNOS resulted in a loss of S-nitrosylation of gephyrin and the formation of larger gephyrin clusters at synaptic sites, ultimately increasing the number of cell surface expressed synaptic GABA(A)Rs. In conclusion, S-nitrosylation of gephyrin is important for homeostatic assembly and plasticity of GABAergic synapses.

Palmitoylation of gephyrin controls receptor clustering and plasticity of GABAergic synapses

Gephyrin, the principal scaffolding protein at inhibitory synapses, needs to be palmitoylated in order to cluster and to assemble functional synapses.

Postsynaptic scaffolding proteins regulate coordinated neurotransmission by anchoring and clustering receptors and adhesion molecules. Gephyrin is the major instructive molecule at inhibitory synapses, where it clusters glycine as well as major subsets of GABA type A receptors (GABA_ARs). Here, we identified palmitoylation of gephyrin as an important mechanism of strengthening GABAergic synaptic transmission, which is regulated by GABA_AR activity. We mapped palmitoylation to Cys212 and Cys284, which are critical for both association of gephyrin with the postsynaptic membrane and gephyrin clustering. We identified DHHC-12 as the principal palmitoyl acyltransferase that palmitoylates gephyrin. Furthermore, gephyrin pamitoylation potentiated GABAergic synaptic transmission, as evidenced by an increased amplitude of miniature inhibitory postsynaptic currents. Consistently, inhibiting gephyrin palmitoylation either pharmacologically or by expression of palmitoylation-deficient gephyrin reduced the gephyrin cluster size. In aggregate, our study reveals that palmitoylation of gephyrin by DHHC-12 contributes to dynamic and functional modulation of GABAergic synapses.

Efficient signal transmission at synapses is essential for higher brain functions. Inhibitory signaling in the brain takes place primarily at GABA (γ-aminobutyric acid)-ergic synapses. GABA type A receptors (GABA_ARs) are clustered at the postsynaptic side by a scaffold composed of the peripheral membrane protein gephyrin. We demonstrate that gephyrin is modulated by palmitoylation, a reversible posttranslational fatty acid modification. Palmitoylation facilitates the membrane association of gephyrin and is therefore essential for normal clustering of gephyrin at GABAergic synapses. Reciprocally, palmitoylation of gephyrin is regulated by GABA_AR activity. Of the 23 known palmitoyl transferases that catalyze the palmitoylation of proteins in human cells, we identified one enzyme, DHHC-12, to specifically modify gephyrin. Our results provide a new aspect to the posttranslational control of synaptic plasticity.

The biosynthesis of the molybdenum cofactors

The biosynthesis of the molybdenum cofactors (Moco) is an ancient, ubiquitous, and highly conserved pathway leading to the biochemical activation of molybdenum. Moco is the essential component of a group of redox enzymes, which are diverse in terms of their phylogenetic distribution and their architectures, both at the overall level and in their catalytic geometry. A wide variety of transformations are catalyzed by these enzymes at carbon, sulfur and nitrogen atoms, which include the transfer of an oxo group or two electrons to or from the substrate. More than 50 molybdoenzymes were identified to date. In all molybdoenzymes except nitrogenase, molybdenum is coordinated to a dithiolene group on the 6-alkyl side chain of a pterin called molybdopterin (MPT). The biosynthesis of Moco can be divided into three general steps, with a fourth one present only in bacteria and archaea: (1) formation of the cyclic pyranopterin monophosphate, (2) formation of MPT, (3) insertion of molybdenum into molybdopterin to form Moco, and (4) additional modification of Moco in bacteria with the attachment of a nucleotide to the phosphate group of MPT, forming the dinucleotide variant of Moco. This review will focus on the biosynthesis of Moco in bacteria, humans and plants.

Molybdenum in human health and disease

Molybdenum is an essential trace element and crucial for the survival of animals. Four mammalian Mo-dependent enzymes are known, all of them harboring a pterin-based molybdenum cofactor (Moco) in their active site. In these enzymes, molybdenum catalyzes oxygen transfer reactions from or to substrates using water as oxygen donor or acceptor. Molybdenum shuttles between two oxidation states, Mo(IV) and Mo(VI). Following substrate reduction or oxidation, electrons are subsequently shuttled by either inter- or intra-molecular electron transfer chains involving prosthetic groups such as heme or iron-sulfur clusters. In all organisms studied so far, Moco is synthesized by a highly conserved multi-step biosynthetic pathway. A deficiency in the biosynthesis of Moco results in a pleitropic loss of all four human Mo-enzyme activities and in most cases in early childhood death. In this review we first introduce general aspects of molybdenum biochemistry before we focus on the functions and deficiencies of two Mo-enzymes, xanthine dehydrogenase and sulfite oxidase, caused either by deficiency of the apo-protein or a pleiotropic loss of Moco due to a genetic defect in its biosynthesis. The underlying molecular basis of Moco deficiency, possible treatment options and links to other diseases, such as neuropsychiatric disorders, will be discussed.

Synaptic recruitment of gephyrin regulates surface GABAA receptor dynamics for the expression of inhibitory LTP

Postsynaptic long-term potentiation of inhibition (iLTP) can rely on increased GABAA receptors (GABA(A)Rs) at synapses by promoted exocytosis. However, the molecular mechanisms that enhance the clustering of postsynaptic GABA(A)Rs during iLTP remain obscure. Here we demonstrate that during chemically induced iLTP (chem-iLTP), GABA(A)Rs are immobilized and confined at synapses, as revealed by single-particle tracking of individual GABA(A)Rs in cultured hippocampal neurons. Chem-iLTP expression requires synaptic recruitment of the scaffold protein gephyrin from extrasynaptic areas, which in turn is promoted by CaMKII-dependent phosphorylation of GABA(A)R-β3-Ser(383). Impairment of gephyrin assembly prevents chem-iLTP and, in parallel, blocks the accumulation and immobilization of GABA(A)Rs at synapses. Importantly, an increase of gephyrin and GABA(A)R similar to those observed during chem-iLTP in cultures were found in the rat visual cortex following an experience-dependent plasticity protocol that potentiates inhibitory transmission in vivo. Thus, phospho-GABA(A)R-β3-dependent accumulation of gephyrin at synapses and receptor immobilization are crucial for iLTP expression and are likely to modulate network excitability.

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.

Brain-derived neurotrophic factor promotes gephyrin protein expression and GABAA receptor clustering in immature cultured hippocampal cells

Fast synaptic inhibition in the adult brain is largely mediated by GABAA receptors (GABAAR). GABAAR are anchored to synaptic sites by gephyrin, a scaffolding protein that appears to be assembled as a hexagonal lattice beneath the plasma membrane. Brain derived neurotrophic factor (BDNF) alters the clustering and synaptic distribution of GABAAR but mechanisms behind this regulation are just starting to emerge. The current study was aimed to examine if BDNF alters the protein levels and/or clustering of gephyrin and to investigate whether the modulation of gephyrin is accompanied by changes in the distribution and/or clustering of GABAAR. Exogenous application of BDNF to immature neuronal cultures from rat hippocampus increased the protein levels and clustering of gephyrin. BDNF also augmented the association of gephyrin with GABAAR and promoted the formation of GABAAR clusters. Together, these observations indicate that BDNF might regulate the assembly of GABAergic synapses by promoting the association of GABAAR with gephyrin.

GABAA receptor subunit composition and competition at synapses are tuned by GABAB receptor activity

GABABRs have a well-established role in controlling neuronal excitability and presynaptic neurotransmitter release. We examined the role of GABABR activity in modulating the number and lateral diffusion of GABAARs at inhibitory synapses. Changes in diffusion of GABAARs at synapses were observed when subunit heterogeneity was taken into account. While α1-GABAARs were unaffected, α2- and α5-GABAARs showed inverse changes in enrichment and diffusion. The intracellular TM3-4 loop of α2 was sufficient to observe the changes in diffusion by GABABR activity, whereas the loop of α5 was not. The opposing effect on α2- and α5-GABAARs was caused by a competition between GABAARs for binding slots at synapses. Receptor immobilization by cross-linking revealed that α5-GABAAR trapping at synapses is regulated by modulation of α2-GABAAR mobility. Finally, PKC activity was determined to be part of the signaling pathway through which GABABR activity modulates α2-GABAAR diffusion at synapses. These results outline a novel mechanism for tuning inhibitory transmission in a subunit-specific manner, and for the first time describe competition between GABAARs with different subunit compositions for binding slots at synapses.

Gephyrin phosphorylation in the functional organization and plasticity of GABAergic synapses

Gephyrin is a multifunctional scaffold protein essential for accumulation of inhibitory glycine and GABA_A receptors at post-synaptic sites. The molecular events involved in gephyrin-dependent GABA_A receptor clustering are still unclear. Evidence has been recently provided that gephyrin phosphorylation plays a key role in these processes. Gephyrin post-translational modifications have been shown to influence the structural remodeling of GABAergic synapses and synaptic plasticity by acting on post-synaptic scaffolding properties as well as stability. In addition, gephyrin phosphorylation and the subsequent phosphorylation-dependent recruitment of the chaperone molecule Pin1 provide a mechanism for the regulation of GABAergic signaling. Extensively characterized as pivotal enzyme controlling cell proliferation and differentiation, the prolyl-isomerase activity of Pin1 has been shown to regulate protein synthesis necessary to sustain the late phase of long-term potentiation at excitatory synapses, which suggests its involvement at synaptic sites. In this review we summarize the current state of knowledge of the signaling pathways responsible for gephyrin post-translational modifications. We will also outline future lines of research that might contribute to a better understanding of molecular mechanisms by which gephyrin regulates synaptic plasticity at GABAergic synapses.