Neuronal cotransport of glycine receptor and the scaffold protein gephyrin

KIF5B plays important roles in dendritic spine plasticity and dendritic localization of PSD95 and FMRP in the mouse cortex in vivo

Kinesin 1 (KIF5) is one major type of motor protein in neurons, but its members' function in the intact brain remains less studied. Using in vivo two-photon imaging, we find that conditional knockout of Kif5b (KIF5B cKO) in CaMKIIα-Cre-expressing neurons shows heightened turnover and lower stability of dendritic spines in layer 2/3 pyramidal neurons with reduced spine postsynaptic density protein 95 acquisition in the mouse cortex. Furthermore, the RNA-binding protein fragile X mental retardation protein (FMRP) is translocated to the proximity of newly formed spines several hours before the spine formation events in vivo in control mice, but this preceding transport of FMRP is abolished in KIF5B cKO mice. We further find that FMRP is localized closer to newly formed spines after fear extinction, but this learning-dependent localization is disrupted in KIF5B cKO mice. Our findings provide the crucial in vivo evidence that KIF5B is involved in the dendritic targeting of synaptic proteins that underlies dendritic spine plasticity.

Role of the Glycine Receptor β Subunit in Synaptic Localization and Pathogenicity in Severe Startle Disease

Startle disease is due to the disruption of recurrent inhibition in the spinal cord. Most common causes are genetic variants in genes (GLRA1, GLRB) encoding inhibitory glycine receptor (GlyR) subunits. The adult GlyR is a heteropentameric complex composed of α1 and β subunits that localizes at postsynaptic sites and replaces embryonically expressed GlyRα2 homomers. The human GlyR variants of GLRA1 and GLRB, dominant and recessive, have been intensively studied in vitro. However, the role of unaffected GlyRβ, essential for synaptic GlyR localization, in the presence of mutated GlyRα1 in vivo is not fully understood. Here, we used knock-in mice expressing endogenous mEos4b-tagged GlyRβ that were crossed with mouse Glra1 startle disease mutants. We explored the role of GlyRβ under disease conditions in mice carrying a missense mutation (shaky) or resulting from the loss of GlyRα1 (oscillator). Interestingly, synaptic targeting of GlyRβ was largely unaffected in both mouse mutants. While synaptic morphology appears unaltered in shaky animals, synapses were notably smaller in homozygous oscillator animals. Hence, GlyRβ enables transport of functionally impaired GlyRα1 missense variants to synaptic sites in shaky animals, which has an impact on the efficacy of possible compensatory mechanisms. The observed enhanced GlyRα2 expression in oscillator animals points to a compensation by other GlyRα subunits. However, trafficking of GlyRα2β complexes to synaptic sites remains functionally insufficient, and homozygous oscillator mice still die at 3 weeks after birth. Thus, both functional and structural deficits can affect glycinergic neurotransmission in severe startle disease, eliciting different compensatory mechanisms in vivo.

Loss of CDKL5 Causes Synaptic GABAergic Defects That Can Be Restored with the Neuroactive Steroid Pregnenolone-Methyl-Ether

CDKL5 deficiency disorder (CDD) is an X-linked neurodevelopmental disorder characterised by early-onset drug-resistant epilepsy and impaired cognitive and motor skills. CDD is caused by mutations in cyclin-dependent kinase-like 5 (CDKL5), which plays a well-known role in regulating excitatory neurotransmission, while its effect on neuronal inhibition has been poorly investigated. We explored the potential role of CDKL5 in the inhibitory compartment in Cdkl5-KO male mice and primary hippocampal neurons and found that CDKL5 interacts with gephyrin and collybistin, two crucial organisers of the inhibitory postsynaptic sites. Through molecular and electrophysiological approaches, we demonstrated that CDKL5 loss causes a reduced number of gephyrin puncta and surface exposed γ₂ subunit-containing GABA_A receptors, impacting the frequency of miniature inhibitory postsynaptic currents, which we ascribe to a postsynaptic function of CDKL5. In line with previous data showing that CDKL5 loss impacts microtubule (MT) dynamics, we showed that treatment with pregnenolone-methyl-ether (PME), which promotes MT dynamics, rescues the above defects. The impact of CDKL5 deficiency on inhibitory neurotransmission might explain the presence of drug-resistant epilepsy and cognitive defects in CDD patients. Moreover, our results may pave the way for drug-based therapies that could bypass the need for CDKL5 and provide effective therapeutic strategies for CDD patients.

Inhibitory postsynaptic density from the lens of phase separation

To faithfully transmit and decode signals released from presynaptic termini, postsynaptic compartments of neuronal synapses deploy hundreds of various proteins. In addition to distinct sets of proteins, excitatory and inhibitory postsynaptic apparatuses display very different organization features and regulatory properties. Decades of extensive studies have generated a wealth of knowledge on the molecular composition, assembly architecture and activity-dependent regulatory mechanisms of excitatory postsynaptic compartments. In comparison, our understanding of the inhibitory postsynaptic apparatus trails behind. Recent studies have demonstrated that phase separation is a new paradigm underlying the formation and plasticity of both excitatory and inhibitory postsynaptic molecular assemblies. In this review, we discuss molecular composition, organizational and regulatory features of inhibitory postsynaptic densities through the lens of the phase separation concept and in comparison with the excitatory postsynaptic densities.

Dynein and muskelin control myosin VI delivery towards the neuronal nucleus

Protein transport toward the nucleus is important for translating molecular signals into gene expression changes. Interestingly, the unconventional motor protein myosin VI regulates RNA polymerase II-dependent gene transcription. Whether actin-filament-dependent myosins are actively transported to nuclear compartments remains unknown. Here, we report that neurons also contain myosin VI inside their nucleus. Notably, nuclear appearance of this actin-dependent motor depends on functional cytoplasmic dynein, a minus end-directed microtubule motor. We find that the trafficking factor muskelin assists in the formation of dynein-myosin VI interactions and further localizes to nuclear foci, enriched in the myosin. Impairment of dynein, but not myosin VI function, reduces nuclear muskelin levels. In turn, muskelin represents a critical determinant in regulating myosin VI nuclear targeting. Our data reveal that minus end-directed microtubule transport determines myosin VI subcellular localization. They suggest a pathway of cytoplasm-to-nucleus trafficking that requires muskelin and is based on dynein-myosin cross talk.

The α3 subunit of GABAA receptors promotes formation of inhibitory synapses in the absence of collybistin

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. Collybistin (CB), a brain-specific guanine nucleotide exchange factor, is essential for the formation of γ-aminobutyric acidergic (GABAergic) postsynapses in defined regions of the mammalian forebrain, including the hippocampus and basolateral amygdala. This process depends on a direct interaction of CB with the scaffolding protein gephyrin, which leads to the redistribution of gephyrin into submembranous clusters at nascent inhibitory synapses. Strikingly, synaptic clustering of gephyrin and GABA_A type A receptors (GABA_ARs) in several brain regions, including the cerebral cortex and certain thalamic areas, is unperturbed in CB-deficient mice, indicating that the formation of a substantial subset of inhibitory postsynapses must be controlled by gephyrin-interacting proteins other than CB. Previous studies indicated that the α3 subunit of GABA_ARs (GABA_AR-α3) binds directly and with high affinity to gephyrin. Here, we provide evidence (i) that a homooligomeric GABA_AR-α3^A343W mutant induces the formation of submembranous gephyrin clusters independently of CB in COS-7 cells, (ii) that gephyrin clustering is unaltered in the neuronal subpopulations endogenously expressing the GABA_AR-α3 in CB-deficient brains, and (iii) that exogenous expression of GABA_AR-α3 partially rescues impaired gephyrin clustering in CB-deficient hippocampal neurons. Our results identify an important role of GABA_AR-α3 in promoting gephyrin-mediated and CB-independent formation of inhibitory postsynapses.

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.

Reciprocal stabilization of glycine receptors and gephyrin scaffold proteins at inhibitory synapses

Postsynaptic scaffold proteins immobilize neurotransmitter receptors in the synaptic membrane opposite to presynaptic vesicle release sites, thus ensuring efficient synaptic transmission. At inhibitory synapses in the spinal cord, the main scaffold protein gephyrin assembles in dense molecule clusters that provide binding sites for glycine receptors (GlyRs). Gephyrin and GlyRs can also interact outside of synapses, where they form receptor-scaffold complexes. Although several models for the formation of postsynaptic scaffold domains in the presence of receptor-scaffold interactions have been advanced, a clear picture of the coupled dynamics of receptors and scaffold proteins at synapses is lacking. To characterize the GlyR and gephyrin dynamics at inhibitory synapses, we performed fluorescence time-lapse imaging after photoconversion to directly visualize the exchange kinetics of recombinant Dendra2-gephyrin in cultured spinal cord neurons. Immuno-immobilization of endogenous GlyRs with specific antibodies abolished their lateral diffusion in the plasma membrane, as judged by the lack of fluorescence recovery after photobleaching. Moreover, the cross-linking of GlyRs significantly reduced the exchange of Dendra2-gephyrin compared with control conditions, suggesting that the kinetics of the synaptic gephyrin pool is strongly dependent on GlyR-gephyrin interactions. We did not observe any change in the total synaptic gephyrin levels after GlyR cross-linking, however, indicating that the number of gephyrin molecules at synapses is not primarily dependent on the exchange of GlyR-gephyrin complexes. We further show that our experimental data can be quantitatively accounted for by a model of receptor-scaffold dynamics that includes a tightly interacting receptor-scaffold domain, as well as more loosely bound receptor and scaffold populations that exchange with extrasynaptic pools. The model can make predictions for single-molecule data such as typical dwell times of synaptic proteins. Taken together, our data demonstrate the reciprocal stabilization of GlyRs and gephyrin at inhibitory synapses and provide a quantitative understanding of their dynamic organization.

Spatial control of membrane traffic in neuronal dendrites

Neuronal dendrites are highly branched and specialized compartments with distinct structures and secretory organelles (e.g., spines, Golgi outposts), and a unique cytoskeletal organization that includes microtubules of mixed polarity. Dendritic membranes are enriched with proteins, which specialize in the formation and function of the post-synaptic membrane of the neuronal synapse. How these proteins partition preferentially in dendrites, and how they traffic in a manner that is spatiotemporally accurate and regulated by synaptic activity are long-standing questions of neuronal cell biology. Recent studies have shed new insights into the spatial control of dendritic membrane traffic, revealing new classes of proteins (e.g., septins) and cytoskeleton-based mechanisms with dendrite-specific functions. Here, we review these advances by revisiting the fundamental mechanisms that control membrane traffic at the levels of protein sorting and motor-driven transport on microtubules and actin filaments. Overall, dendrites possess unique mechanisms for the spatial control of membrane traffic, which might have specialized and co-evolved with their highly arborized morphology.

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.

Gimmicks of gamma-aminobutyric acid (GABA) in pancreatic β-cell regeneration through transdifferentiation of pancreatic α- to β-cells

In vivo regeneration of lost or dysfunctional islet β cells can fulfill the promise of improved therapy for diabetic patients. To achieve this, many mitogenic factors have been attempted, including gamma-aminobutyric acid (GABA). GABA remarkably affects pancreatic islet cells' (α cells and β cells) function through paracrine and/or autocrine binding to its membrane receptors on these cells. GABA has also been studied for promoting the transformation of α cells to β cells. Nonetheless, the gimmickry of GABA-induced α-cell transformation to β cells has two different perspectives. On the one hand, GABA was found to induce α-cell transformation to β cells in vivo and insulin-secreting β-like cells in vitro. On the other hand, GABA treatment showed that it has no α- to β-cell transformation response. Here, we will summarize the physiological effects of GABA on pancreatic islet β cells with an emphasis on its regenerative effects for transdifferentiation of islet α cells to β cells. We will also critically discuss the controversial results about GABA-mediated transdifferentiation of α cells to β cells.

Cytoskeletal makeup of the synapse: Shaft versus spine

The ability of neurons to communicate and store information depends on the activity of synapses which can be located on small protrusions (dendritic spines) or directly on the dendritic shaft. The formation, plasticity, and stability of synapses are regulated by the neuronal cytoskeleton. Actin filaments together with microtubules, neurofilaments, septins, and scaffolding proteins orchestrate the structural organization of both shaft and spine synapses, enabling their efficacy in response to synaptic activation. Synapses critically depend on several factors, which are also mediated by the cytoskeleton, including transport and delivery of proteins from the soma, protein synthesis, as well as surface diffusion of membrane proteins. In this minireview, we focus on recent progress made in the field of cytoskeletal elements of the postsynapse and discuss the differences and similarities between synapses located in the spines versus dendritic shaft.

Gephyrin Palmitoylation in Basolateral Amygdala Mediates the Anxiolytic Action of Benzodiazepine

BACKGROUND

Benzodiazepines (BZDs) have been used to treat anxiety disorders for more than five decades as the allosteric modulator of the gamma-aminobutyric acid A receptor (GABA_AR). Little is known about other mechanisms of BZDs. Here, we describe how the rapid stabilization of postsynaptic GABA_AR is essential and sufficient for the anxiolytic effect of BZDs via a palmitoylation-dependent mechanism.

METHODS

Palmitoylated proteins in the basolateral amygdala (BLA) of rats with different anxious states were assessed by a biotin exchange protocol. Both pharmacological and genetic approaches were used to investigate the role of palmitoylation in anxiety behavior. Electrophysiological recording, reverse transcription polymerase chain reaction, Western blotting, and coimmunoprecipitation were used to investigate the mechanisms.

RESULTS

Highly anxious rats were accompanied by the deficiency of gephyrin palmitoylation and decreased the synaptic function of GABA_AR in the BLA. We then identified that the dysfunction of DHHC12, a palmitoyl acyltransferase that specifically palmitoylates gephyrin, contributed to the high-anxious state. Furthermore, diazepam, as an anxiolytic drug targeting GABA_ARs, was found to increase gephyrin palmitoylation in the BLA via a GABA_AR-dependent manner to activate DHHC12. The anxiolytic effect of diazepam was nearly abolished by the DHHC12 knockdown. Specifically, similar to the effect of BZD, the overexpression of DHHC12 in the BLA exerted a significant anxiolytic action, which was prevented by flumazenil.

CONCLUSIONS

Our results support the view that the strength of inhibitory synapse was controlled by gephyrin palmitoylation in vivo and proposes a previously unknown palmitoylation-centered mode of BZD's action.

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.

Impaired Glycine Receptor Trafficking in Neurological Diseases

Ionotropic glycine receptors (GlyRs) enable fast synaptic neurotransmission in the adult spinal cord and brainstem. The inhibitory GlyR is a transmembrane glycine-gated chloride channel. The immature GlyR protein undergoes various processing steps, e.g., folding, assembly, and maturation while traveling from the endoplasmic reticulum to and through the Golgi apparatus, where post-translational modifications, e.g., glycosylation occur. The mature receptors are forward transported via microtubules to the cellular surface and inserted into neuronal membranes followed by synaptic clustering. The normal life cycle of a receptor protein includes further processes like internalization, recycling, and degradation. Defects in GlyR life cycle, e.g., impaired protein maturation and degradation have been demonstrated to underlie pathological mechanisms of various neurological diseases. The neurological disorder startle disease is caused by glycinergic dysfunction mainly due to missense mutations in genes encoding GlyR subunits (GLRA1 and GLRB). In vitro studies have shown that most recessive forms of startle disease are associated with impaired receptor biogenesis. Another neurological disease with a phenotype similar to startle disease is a special form of stiff-person syndrome (SPS), which is most probably due to the development of GlyR autoantibodies. Binding of GlyR autoantibodies leads to enhanced receptor internalization. Here we focus on the normal life cycle of GlyRs concentrating on assembly and maturation, receptor trafficking, post-synaptic integration and clustering, and GlyR internalization/recycling/degradation. Furthermore, this review highlights findings on impairment of these processes under disease conditions such as disturbed neuronal ER-Golgi trafficking as the major pathomechanism for recessive forms of human startle disease. In SPS, enhanced receptor internalization upon autoantibody binding to the GlyR has been shown to underlie the human pathology. In addition, we discuss how the existing mouse models of startle disease increased our current knowledge of GlyR trafficking routes and function. This review further illuminates receptor trafficking of GlyR variants originally identified in startle disease patients and explains changes in the life cycle of GlyRs in patients with SPS with respect to structural and functional consequences at the receptor level.

New approaches for solving old problems in neuronal protein trafficking

Fundamental cellular properties are determined by the repertoire and abundance of proteins displayed on the cell surface. As such, the trafficking mechanisms for establishing and maintaining the surface proteome must be tightly regulated for cells to respond appropriately to extracellular cues, yet plastic enough to adapt to ever-changing environments. Not only are the identity and abundance of surface proteins critical, but in many cases, their regulated spatial positioning within surface nanodomains can greatly impact their function. In the context of neuronal cell biology, surface levels and positioning of ion channels and neurotransmitter receptors play essential roles in establishing important properties, including cellular excitability and synaptic strength. Here we review our current understanding of the trafficking pathways that control the abundance and localization of proteins important for synaptic function and plasticity, as well as recent technological advances that are allowing the field to investigate protein trafficking with increasing spatiotemporal precision.

Development of inhibitory synaptic inputs on layer 2/3 pyramidal neurons in the rat medial prefrontal cortex

Inhibitory control of pyramidal neurons plays a major role in governing the excitability in the brain. While spatial mapping of inhibitory inputs onto pyramidal neurons would provide important structural data on neuronal signaling, studying their distribution at the single cell level is difficult due to the lack of easily identifiable anatomical proxies. Here, we describe an approach where in utero electroporation of a plasmid encoding for fluorescently tagged gephyrin into the precursors of pyramidal cells along with ionotophoretic injection of Lucifer Yellow can reliably and specifically detect GABAergic synapses on the dendritic arbour of single pyramidal neurons. Using this technique and focusing on the basal dendritic arbour of layer 2/3 pyramidal cells of the medial prefrontal cortex, we demonstrate an intense development of GABAergic inputs onto these cells between postnatal days 10 and 20. While the spatial distribution of gephyrin clusters was not affected by the distance from the cell body at postnatal day 10, we found that distal dendritic segments appeared to have a higher gephyrin density at later developmental stages. We also show a transient increase around postnatal day 20 in the percentage of spines that are carrying a gephyrin cluster, indicative of innervation by a GABAergic terminal. Since the precise spatial arrangement of synaptic inputs is an important determinant of neuronal responses, we believe that the method described in this work may allow a better understanding of how inhibition settles together with excitation, and serve as basics for further modelling studies focusing on the geometry of dendritic inhibition during development.

Mechanisms regulating dendritic arbor patterning

The nervous system is populated by diverse types of neurons, each of which has dendritic trees with strikingly different morphologies. These neuron-specific morphologies determine how dendritic trees integrate thousands of synaptic inputs to generate different firing properties. To ensure proper neuronal function and connectivity, it is necessary that dendrite patterns are precisely controlled and coordinated with synaptic activity. Here, we summarize the molecular and cellular mechanisms that regulate the formation of cell type-specific dendrite patterns during development. We focus on different aspects of vertebrate dendrite patterning that are particularly important in determining the neuronal function; such as the shape, branching, orientation and size of the arbors as well as the development of dendritic spine protrusions that receive excitatory inputs and compartmentalize postsynaptic responses. Additionally, we briefly comment on the implications of aberrant dendritic morphology for nervous system disease.

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.

Regulation of dynein-dynactin-driven vesicular transport

Most of the long-range intracellular movements of vesicles, organelles and other cargoes are driven by microtubule (MT)-based molecular motors. Cytoplasmic dynein, a multisubunit protein complex, with the aid of dynactin, drives transport of a wide variety of cargoes towards the minus end of MTs. In this article, I review our current understanding of the mechanisms underlying spatiotemporal regulation of dynein-dynactin-driven vesicular transport with a special emphasis on the many steps of directional movement along MT tracks. These include the recruitment of dynein to MT plus ends, the activation and processivity of dynein, and cargo recognition and release by the motor complex at the target membrane. Furthermore, I summarize the most recent findings about the fine control mechanisms for intracellular transport via the interaction between the dynein-dynactin motor complex and its vesicular cargoes.

Endosomal Phosphatidylinositol 3-Phosphate Promotes Gephyrin Clustering and GABAergic Neurotransmission at Inhibitory Postsynapses

The formation of neuronal synapses and the dynamic regulation of their efficacy depend on the proper assembly of the postsynaptic neurotransmitter receptor apparatus. Receptor recruitment to inhibitory GABAergic postsynapses requires the scaffold protein gephyrin and the guanine nucleotide exchange factor collybistin (Cb). In vitro, the pleckstrin homology domain of Cb binds phosphoinositides, specifically phosphatidylinositol 3-phosphate (PI3P). However, whether PI3P is required for inhibitory postsynapse formation is currently unknown. Here, we investigated the role of PI3P at developing GABAergic postsynapses by using a membrane-permeant PI3P derivative, time-lapse confocal imaging, electrophysiology, as well as knockdown and overexpression of PI3P-metabolizing enzymes. Our results provide the first in cellula evidence that PI3P located at early/sorting endosomes regulates the postsynaptic clustering of gephyrin and GABA_A receptors and the strength of inhibitory, but not excitatory, postsynapses in cultured hippocampal neurons. In human embryonic kidney 293 cells, stimulation of gephyrin cluster formation by PI3P depends on Cb. We therefore conclude that the endosomal pool of PI3P, generated by the class III phosphatidylinositol 3-kinase, is important for the Cb-mediated recruitment of gephyrin and GABA_A receptors to developing inhibitory postsynapses and thus the formation of postsynaptic membrane specializations.

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.

MmTX1 and MmTX2 from coral snake venom potently modulate GABAA receptor activity

GABAA receptors shape synaptic transmission by modulating Cl(-) conductance across the cell membrane. Remarkably, animal toxins that specifically target GABAA receptors have not been identified. Here, we report the discovery of micrurotoxin1 (MmTX1) and MmTX2, two toxins present in Costa Rican coral snake venom that tightly bind to GABAA receptors at subnanomolar concentrations. Studies with recombinant and synthetic toxin variants on hippocampal neurons and cells expressing common receptor compositions suggest that MmTX1 and MmTX2 allosterically increase GABAA receptor susceptibility to agonist, thereby potentiating receptor opening as well as desensitization, possibly by interacting with the α(+)/β(-) interface. Moreover, hippocampal neuron excitability measurements reveal toxin-induced transitory network inhibition, followed by an increase in spontaneous activity. In concert, toxin injections into mouse brain result in reduced basal activity between intense seizures. Altogether, we characterized two animal toxins that enhance GABAA receptor sensitivity to agonist, thereby establishing a previously unidentified class of tools to study this receptor family.

Selective synaptic targeting of the excitatory and inhibitory presynaptic organizers FGF22 and FGF7

Specific formation of excitatory and inhibitory synapses is crucial for proper functioning of the brain. Fibroblast growth factor 22 (FGF22) and FGF7 are postsynaptic-cell-derived presynaptic organizers necessary for excitatory and inhibitory presynaptic differentiation, respectively, in the hippocampus. For the establishment of specific synaptic networks, these FGFs must localize to appropriate synaptic locations - FGF22 to excitatory and FGF7 to inhibitory postsynaptic sites. Here, we show that distinct motor and adaptor proteins contribute to intracellular microtubule transport of FGF22 and FGF7. Excitatory synaptic targeting of FGF22 requires the motor proteins KIF3A and KIF17 and the adaptor protein SAP102 (also known as DLG3). By contrast, inhibitory synaptic targeting of FGF7 requires the motor KIF5 and the adaptor gephyrin. Time-lapse imaging shows that FGF22 moves with SAP102, whereas FGF7 moves with gephyrin. These results reveal the basis of selective targeting of the excitatory and inhibitory presynaptic organizers that supports their different synaptogenic functions. Finally, we found that knockdown of SAP102 or PSD95 (also known as DLG4), which impairs the differentiation of excitatory synapses, alters FGF7 localization, suggesting that signals from excitatory synapses might regulate inhibitory synapse formation by controlling the distribution of the inhibitory presynaptic organizer.

Design and synthesis of high-affinity dimeric inhibitors targeting the interactions between gephyrin and inhibitory neurotransmitter receptors

Gephyrin is the central scaffolding protein for inhibitory neurotransmitter receptors in the brain. Here we describe the development of dimeric peptides that inhibit the interaction between gephyrin and these receptors, a process which is fundamental to numerous synaptic functions and diseases of the brain. We first identified receptor-derived minimal gephyrin-binding peptides that displayed exclusive binding towards native gephyrin from brain lysates. We then designed and synthesized a series of dimeric ligands, which led to a remarkable 1220-fold enhancement of the gephyrin affinity (KD=6.8 nM). In X-ray crystal structures we visualized the simultaneous dimer-to-dimer binding in atomic detail, revealing compound-specific binding modes. Thus, we defined the molecular basis of the affinity-enhancing effect of multivalent gephyrin inhibitors and provide conceptually novel compounds with therapeutic potential, which will allow further elucidation of the gephyrin-receptor interplay.

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.

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.

Tubulin binds to the cytoplasmic loop of TRESK background K⁺ channel in vitro

The cytoplasmic loop between the second and third transmembrane segments is pivotal in the regulation of TRESK (TWIK-related spinal cord K+ channel, K2P18.1, KCNK18). Calcineurin binds to this region and activates the channel by dephosphorylation in response to the calcium signal. Phosphorylation-dependent anchorage of 14-3-3 adaptor protein also modulates TRESK at this location. In the present study, we identified molecular interacting partners of the intracellular loop. By an affinity chromatography approach using the cytoplasmic loop as bait, we have verified the specific association of calcineurin and 14-3-3 to the channel. In addition to these known interacting proteins, we observed substantial binding of tubulin to the intracellular loop. Successive truncation of the polypeptide and pull-down experiments from mouse brain cytosol narrowed down the region sufficient for the binding of tubulin to a 16 amino acid sequence: LVLGRLSYSIISNLDE. The first six residues of this sequence are similar to the previously reported tubulin-binding region of P2X2 purinergic receptor. The tubulin-binding site of TRESK is located close to the protein kinase A (PKA)-dependent 14-3-3-docking motif of the channel. We provide experimental evidence suggesting that 14-3-3 competes with tubulin for the binding to the cytoplasmic loop of TRESK. It is intriguing that the 16 amino acid tubulin-binding sequence includes the serines, which were previously shown to be phosphorylated by microtubule-affinity regulating kinases (MARK kinases) and contribute to channel inhibition. Although tubulin binds to TRESK in vitro, it remains to be established whether the two proteins also interact in the living cell.

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.

Proteomic analysis of glycine receptor β subunit (GlyRβ)-interacting proteins: evidence for syndapin I regulating synaptic glycine receptors

Glycine receptors (GlyRs) mediate inhibitory neurotransmission in spinal cord and brainstem. They are clustered at inhibitory postsynapses via a tight interaction of their β subunits (GlyRβ) with the scaffolding protein gephyrin. In an attempt to isolate additional proteins interacting with GlyRβ, we performed pulldown experiments with rat brain extracts using a glutathione S-transferase fusion protein encompassing amino acids 378-455 of the large intracellular loop of GlyRβ as bait. This identified syndapin I (SdpI) as a novel interaction partner of GlyRβ that coimmunoprecipitates with native GlyRs from brainstem extracts. Both SdpI and SdpII bound efficiently to the intracellular loop of GlyRβ in vitro and colocalized with GlyRβ upon coexpression in COS-7 cells. The SdpI-binding site was mapped to a proline-rich sequence of 22 amino acids within the intracellular loop of GlyRβ. Deletion and point mutation analysis disclosed that SdpI binding to GlyRβ is Src homology 3 domain-dependent. In cultured rat spinal cord neurons, SdpI immunoreactivity was found to partially colocalize with marker proteins of inhibitory and excitatory synapses. When SdpI was acutely knocked down in cultured spinal cord neurons by viral miRNA expression, postsynaptic GlyR clusters were significantly reduced in both size and number. Similar changes in GlyR cluster properties were found in spinal cultures from SdpI-deficient mice. Our results are consistent with a role of SdpI in the trafficking and/or cytoskeletal anchoring of synaptic GlyRs.

Postnatal development of glycine receptor subunits α1, α2, α3, and β immunoreactivity in multiple brain stem respiratory-related nuclear groups of the rat

The respiratory system is immature at birth and significant development occurs postnatally. A critical period of respiratory development occurs in rats around postnatal days 12-13, when enhanced inhibition dominates over suppressed excitation. The mechanisms underlying the heightened inhibition are not fully understood. The present study tested our hypothesis that the inhibition is marked by a switch in glycine receptor subunits from neonatal to adult form around the critical period. An in-depth immunohistochemical and single neuron optical densitometric study was undertaken on four respiratory-related nuclear groups (the pre-Bötzinger complex, nucleus ambiguus, hypoglossal nucleus, and ventrolateral subnucleus of solitary tract nucleus), and a non-respiratory cuneate nucleus in P2-21 rats. Our data revealed that in the respiratory-related nuclear groups: (1) the expressions of GlyRα2 and GlyRα3 were relatively high at P2, but declined after 1-1½ weeks to their lowest levels at P21; (2) the expression of GlyRα1 increased with age and reached significance at P12; and (3) the expression of GlyRβ rose from P2 to P12 followed by a slight decline until P21. No distinct increase in GlyRα1 at P12 was noted in the cuneate nucleus. Thus, there is a switch in dominance of expression from neonatal GlyRα2/α3 to the adult GlyRα1 and a heightened expression of GlyRα1 around the critical period in all respiratory-related nuclear groups, thereby supporting enhanced inhibition at that time. The rise in the expression of GlyRβ around P12 indicates that it plays an important role in forming the mature heteropentameric glycine receptors in these brain stem nuclear groups.

Structural characterization of gephyrin by AFM and SAXS reveals a mixture of compact and extended states

Gephyrin is a trimeric protein involved in the final steps of molybdenum-cofactor (Moco) biosynthesis and in the clustering of inhibitory glycine and GABAAreceptors at postsynaptic specializations. Each protomer consists of stably folded domains (referred to as the G and E domains) located at either terminus and connected by a proteolytically sensitive linker of ∼150 residues. Both terminal domains can oligomerize in their isolated forms; however, in the context of the full-length protein only the G-domain trimer is permanently present, whereas E-domain dimerization is prevented. Atomic force microscopy (AFM) and small-angle X-ray scattering (SAXS) reveal a high degree of flexibility in the structure of gephyrin. The results imply an equilibrium between compact and extended conformational states in solution, with a preference for compact states. CD spectroscopy suggests that a partial compaction is achieved by interactions of the linker with the G and E domains. Taken together, the data provide a rationale for the role of the linker in the overall structure and the conformational dynamics of gephyrin.

Dynein interacts with the neural cell adhesion molecule (NCAM180) to tether dynamic microtubules and maintain synaptic density in cortical neurons

Cytoplasmic dynein is well characterized as an organelle motor, but dynein also acts to tether and stabilize dynamic microtubule plus-ends in vitro. Here we identify a novel and direct interaction between dynein and the 180-kDa isoform of the neural cell adhesion molecule (NCAM). Optical trapping experiments indicate that dynein bound to beads via the NCAM180 interaction domain can tether projecting microtubule plus-ends. Live cell assays indicate that the NCAM180-dependent recruitment of dynein to the cortex leads to the selective stabilization of microtubules projecting to NCAM180 patches at the cell periphery. The dynein-NCAM180 interaction also enhances cell-cell adhesion in heterologous cell assays. Dynein and NCAM180 co-precipitate from mouse brain extract and from synaptosomal fractions, consistent with an endogenous interaction in neurons. Thus, we examined microtubule dynamics and synaptic density in primary cortical neurons. We find that depletion of NCAM, inhibition of the dynein-NCAM180 interaction, or dampening of microtubule dynamics with low dose nocodazole all result in significantly decreased in synaptic density. Based on these observations, we propose a working model for the role of dynein at the synapse, in which the anchoring of the motor to the cortex via binding to an adhesion molecule mediates the tethering of dynamic microtubule plus-ends to potentiate synaptic stabilization.

The class 4 semaphorin Sema4D promotes the rapid assembly of GABAergic synapses in rodent hippocampus

Proper circuit function in the mammalian nervous system depends on the precise assembly and development of excitatory and inhibitory synaptic connections between neurons. Through a loss-of-function genetic screen in cultured hippocampal neurons, we previously identified the class 4 Semaphorin Sema4D as being required for proper GABAergic synapse development. Here we demonstrate that Sema4D is sufficient to promote GABAergic synapse formation in rodent hippocampus and investigate the kinetics of this activity. We find that Sema4D treatment of rat hippocampal neurons increases the density of GABAergic synapses as detected by immunocytochemistry within 30 min, much more rapidly than has been previously described for a prosynaptogenic molecule, and show that this effect is dependent on the Sema4D receptor PlexinB1 using PlxnB1(-/-) mice. Live imaging studies reveal that Sema4D elicits a rapid enhancement (within 10 min) in the rate of addition of synaptic proteins. Therefore, we demonstrate that Sema4D, via PlexinB1, acts to initiate synapse formation by recruiting molecules to both the presynaptic and the postsynaptic terminals; these nascent synapses subsequently become fully functional by 2 h after Sema4D treatment. In addition, acute treatment of an organotypic hippocampal slice epilepsy model with Sema4D reveals that Sema4D rapidly and dramatically alters epileptiform activity, which is consistent with a Sema4D-mediated shift in the balance of excitation and inhibition within the circuit. These data demonstrate an ability to quickly assemble GABAergic synapses in response to an appropriate signal and suggest a potential area of exploration for the development of novel antiepileptic drugs.

The possible role of GABAA receptors and gephyrin in epileptogenesis

The term epileptogenesis refers to a dynamic alteration in neuronal excitability that promotes the appearance of spontaneous seizures. Temporal lobe epilepsy, the most common type of acquired epilepsy, often develops after an insult to the brain such as trauma, febrile seizures, encephalitis, or status epilepticus. During the pre-epileptic state (also referred as latent or silent period) there is a plethora of molecular, biochemical, and structural changes that lead to the generation of recurrent spontaneous seizures (or epilepsy). The specific contribution of these alterations to epilepsy development is unclear, but a loss of inhibition has been associated with the increased excitability detected in the latent period. A rapid increase in neuronal hyperexcitability could be due, at least in part, to a decline in the number of physiologically active GABAA receptors (GABAAR). Altered expression of scaffolding proteins involved in the trafficking and anchoring of GABAAR could directly impact the stability of GABAergic synapses and promote a deficiency in inhibitory neurotransmission. Uncovering the molecular mechanisms operating during epileptogenesis and its possible impact on the regulation of GABAAR and scaffolding proteins may offer new targets to prevent the development of epilepsy.

Distribution of the glycine receptor β-subunit in the mouse CNS as revealed by a novel monoclonal antibody

Inhibitory glycine receptors (GlyRs) are composed of homologous α- (α1-4) and β-subunits. The β-subunits (GlyRβ) interact via their large cytosolic loops with the postsynaptic scaffolding protein gephyrin and are therefore considered essential for synaptic localization. In situ hybridization studies indicate a widespread distribution of GlyRβ transcripts throughout the mammalian central nervous system (CNS), whereas GlyRα mRNAs and proteins display more restricted expression patterns. Here we report the generation of a monoclonal antibody that specifically recognizes rodent GlyRβ (mAb-GlyRβ) and does not exhibit crossreactivity with any of the GlyRα1-4 subunits. Immunostaining with this antibody revealed high densities of punctate GlyRβ immunoreactivity at inhibitory synapses in mouse spinal cord, brainstem, midbrain, and olfactory bulb but not in the neocortex, cerebellum, or hippocampus. This contrasts the abundance of GlyRβ transcripts in all major regions of the rodent brain and suggests that GlyRβ protein levels are regulated posttranscriptionally. When mAb-GlyRβ was used in double-labeling experiments with GlyRα1-, α2-, α3-, or α4-specific antibodies to examine the colocalization of GlyRβ with these GlyR subunits in the mouse retina, >90% of the GlyRα1-3 clusters detected were found to be GlyRβ-immunoreactive. A subset (about 50%) of the GlyRα4 puncta in the inner plexiform layer, however, was found to lack GlyRβ and gephyrin immunostaining. These GlyRα4-only clusters were apposed to bassoon immunoreactivity and hence synaptically localized. Their existence points to a gephyrin-independent synaptic localization mechanism for a minor subset of GlyRs.

Inhibitory glycine receptors: an update

Strychnine-sensitive glycine receptors (GlyRs) mediate synaptic inhibition in the spinal cord, brainstem, and other regions of the mammalian central nervous system. In this minireview, we summarize our current view of the structure, ligand-binding sites, and chloride channel of these receptors and discuss recently emerging functions of distinct GlyR isoforms. GlyRs not only regulate the excitability of motor and afferent sensory neurons, including pain fibers, but also are involved in the processing of visual and auditory signals. Hence, GlyRs constitute promising targets for the development of therapeutically useful compounds.

Formation of Golgi-derived active zone precursor vesicles

Vesicular trafficking of presynaptic and postsynaptic components is emerging as a general cellular mechanism for the delivery of scaffold proteins, ion channels, and receptors to nascent and mature synapses. However, the molecular mechanisms leading to the selection of cargos and their differential transport to subneuronal compartments are not well understood, in part because of the mixing of cargos at the plasma membrane and/or within endosomal compartments. In the present study, we have explored the cellular mechanisms of active zone precursor vesicle assembly at the Golgi in dissociated hippocampal neurons of Rattus norvegicus. Our studies show that Piccolo, Bassoon, and ELKS2/CAST exit the trans-Golgi network on a common vesicle that requires Piccolo and Bassoon for its proper assembly. In contrast, Munc13 and synaptic vesicle proteins use distinct sets of Golgi-derived transport vesicles, while RIM1α associates with vesicular membranes in a post-Golgi compartment. Furthermore, Piccolo and Bassoon are necessary for ELKS2/CAST to leave the Golgi in association with vesicles, and a core domain of Bassoon is sufficient to facilitate formation of these vesicles. While these findings support emerging principles regarding active zone differentiation, the cellular and molecular analyses reported here also indicate that the Piccolo-Bassoon transport vesicles leaving the Golgi may undergo further changes in protein composition before arriving at synaptic sites.

Homeostatic regulation of gephyrin scaffolds and synaptic strength at mature hippocampal GABAergic postsynapses

Gephyrin is a scaffolding protein important for the postsynaptic clustering of inhibitory neurotransmitter receptors. Here, we investigated the properties of gephyrin scaffolds at γ-aminobutyric acid- (GABA-)ergic synapses in organotypic entorhino-hippocampal cultures prepared from a transgenic mouse line, which expresses green fluorescent protein-tagged gephyrin under the control of the Thy1.2 promoter. Fluorescence recovery after photobleaching revealed a developmental stabilization of postsynaptic gephyrin clusters concomitant with an increase in cluster size and synaptic strength between 1 and 4 weeks in vitro. Prolonged treatment of the slice cultures with diazepam or a GABAA receptor antagonist disclosed a homeostatic regulation of both inhibitory synaptic strength and gephyrin cluster size and stability in 4-weeks-old cultures, whereas at 1 week in vitro, the same drug treatments modulated GABAergic postsynapse and gephyrin cluster properties following a Hebbian mode of synaptic plasticity. Our data are consistent with a model in which the postnatal maturation of the hippocampal network endows CA1 pyramidal neurons with the ability to homeostatically adjust the strength of their inhibitory postsynapses to afferent GABAergic drive by regulating gephyrin scaffold properties.

Gephyrin, the enigmatic organizer at GABAergic synapses

GABA_A receptors are clustered at synaptic sites to achieve a high density of postsynaptic receptors opposite the input axonal terminals. This allows for an efficient propagation of GABA mediated signals, which mostly result in neuronal inhibition. A key organizer for inhibitory synaptic receptors is the 93 kDa protein gephyrin that forms oligomeric superstructures beneath the synaptic area. Gephyrin has long been known to be directly associated with glycine receptor β subunits that mediate synaptic inhibition in the spinal cord. Recently, synaptic GABA_A receptors have also been shown to directly interact with gephyrin and interaction sites have been identified and mapped within the intracellular loops of the GABA_A receptor α1, α2, and α3 subunits. Gephyrin-binding to GABA_A receptors seems to be at least one order of magnitude weaker than to glycine receptors (GlyRs) and most probably is regulated by phosphorylation. Gephyrin not only has a structural function at synaptic sites, but also plays a crucial role in synaptic dynamics and is a platform for multiple protein-protein interactions, bringing receptors, cytoskeletal proteins and downstream signaling proteins into close spatial proximity.

Expression and subcellular distribution of gephyrin in non-neuronal tissues and cells

Gephyrin is a scaffolding protein required for the accumulation of inhibitory neurotransmitter receptors at neuronal postsynaptic membranes. In non-neuronal tissues, gephyrin is indispensible for the biosynthesis of molybdenum cofactor, the prosthetic group of oxidoreductases including sulfite oxidase and xanthine oxidase. However, the molecular and cellular basis of gephyrin's non-neuronal function is poorly understood; in particular, the roles of its splice variants remain enigmatic. Here, we used cDNA screening as well as Northern and immunoblot analyses to show that mammalian liver contains only a limited number of gephyrin splice variants, with the C3-containing variant being the predominant isoform. Using new and established anti-gephyrin antibodies in immunofluorescence and subcellular fractionation studies, we report that gephyrin localizes to the cytoplasm of both tissue hepatocytes and cultured immortalized cells. These findings were corroborated by RNA interference studies in which the cytosolic distribution was found to be abolished. Finally, by blue-native PAGE we show that cytoplasmic gephyrin is part of a ~600 kDa protein complex of yet unknown composition. Our data suggest that the expression pattern of non-neuronal gephyrin is simpler than indicated by previous evidence. In addition, gephyrin's presence in a cytosolic 600 kDa protein complex suggests that its metabolic and/or other non-neuronal functions are exerted in the cytoplasm and are not confined to a particular subcellular compartment.