Colocalization of gephyrin and GABAA-receptor subunits in the rat retina

Innervation of orexin/hypocretin neurons by GABAergic, glutamatergic or cholinergic basal forebrain terminals evidenced by immunostaining for presynaptic vesicular transporter and postsynaptic scaffolding proteins

Orexin/hypocretin (Orx) neurons are critical for the maintenance of waking in association with behavioral arousal and postural muscle tone, since with their loss narcolepsy with cataplexy occurs. Given that basal forebrain (BF) neurons project to the hypothalamus and play important diverse roles in sleep/wake states, we sought to determine whether acetylcholine (ACh), glutamate (Glu), and/or GABA-releasing BF neurons innervate and could thereby differentially regulate the Orx neurons. From discrete injections of biotinylated dextran amine (BDA, 10,000 MW) into the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) in the rat, BDA-labeled fibers projected to the lateral hypothalamus (LH), perifornical area (PF), and dorsomedial hypothalamus (DMH), where approximately 41%, approximately 11%, and 9% of Orx-positive (+) neurons were respectively contacted in each region. Employing triple fluorescent staining for Orx, BDA, and presynaptic vesicular (V) transporters (T), we found that only 4% of the innervated Orx+ neurons in the LH were contacted by BDA+[VAChT+] terminals, whereas approximately 31% and approximately 67% were respectively contacted by BDA+[VGluT2+] and BDA+[VGAT+] terminals. In 3D-rendered and rotated confocal images, we confirmed the latter contacts and examined staining for postsynaptic proteins PSD-95, a marker for glutamatergic synapses, and gephyrin, a marker for GABAergic synapses, that were located on Orx+ neurons facing BDA-labeled terminals in approximately 20% and approximately 50% of contacts, respectively. With such synaptic input, BF glutamatergic neurons can excite Orx neurons and thus act to maintain behavioral arousal with muscle tone, whereas GABAergic neurons can inhibit Orx neurons and thus promote behavioral quiescence and sleep along with muscle atonia.

Vesicular glutamate (VGlut), GABA (VGAT), and acetylcholine (VACht) transporters in basal forebrain axon terminals innervating the lateral hypothalamus

The basal forebrain (BF) is known to play important roles in cortical activation and sleep, which are likely mediated by chemically differentiated cell groups including cholinergic, gamma-aminobutyric acid (GABA)ergic and other unidentified neurons. One important target of these cells is the lateral hypothalamus (LH), which is critical for arousal and the maintenance of wakefulness. To determine whether chemically specific BF neurons provide an innervation to the LH, we employed anterograde transport of 10,000 MW biotinylated dextran amine (BDA) together with immunohistochemical staining of the vesicular transporter proteins (VTPs) for glutamate (VGluT1, -2, and -3), GABA (VGAT), or acetylcholine (ACh, VAChT). In addition, we applied triple staining for the postsynaptic proteins (PSPs), PSD-95 with VGluT or Gephyrin (Geph) with VGAT, to examine whether the BDA-labeled varicosities may form excitatory or inhibitory synapses in the LH. Axons originating from BDA-labeled neurons in the magnocellular preoptic nucleus (MCPO) and substantia innominata (SI) descended within the medial forebrain bundle and extended collateral varicose fibers to contact LH neurons. In the LH, the BDA-labeled varicosities were immunopositive (+) for VAChT ( approximately 10%), VGluT2 ( approximately 25%), or VGAT ( approximately 50%), revealing an important influence of newly identified glutamatergic together with GABAergic BF inputs. Moreover, in confocal microscopy, VGluT2+ and VGAT+ terminals were apposed to PSD-95+ and Geph+ profiles respectively, indicating that they formed synaptic contacts with LH neurons. The important inputs from glutamatergic and GABAergic BF cells could thus regulate LH neurons in an opposing manner to stimulate vs. suppress cortical activation and behavioral arousal reciprocally.

Immunocytochemical study of glycine receptors in the retina of the frog Xenopus laevis

The expression of glycine receptors in the retina of clawed frog, Xenopus laevis was studied immunocytochemically. Glycine receptors (GlyRs), as revealed by means of several different antibodies, were mainly distributed in the inner (IPL) and the outer plexiform layers. Their composition was determined to include alpha2 and alpha3 subunits. Typical punctate appearance and specific lamination in the IPL were seen with each of the antibodies directed against the different GlyRs' subunits. A notion for diversity of the glycine receptors was put forward, according to which the alpha2 and alpha3 subunits are located in different subtypes of glycine synapses.

Differential regulation of GABA(A) receptor and gephyrin postsynaptic clustering in immature hippocampal neuronal cultures

Gephyrin is a postsynaptic scaffolding protein involved in clustering of glycine- and GABA(A) receptors at inhibitory synapses. The role of gephyrin in GABAergic synapses, the nature of its interactions with GABA(A) receptors, and the mechanisms of targeting to GABAergic synapses are largely unknown. To gain further insights into these questions, the formation of GABA(A) receptor and gephyrin clusters and their distribution relative to presynaptic terminals were investigated in immature cultures of embryonic hippocampal neurons using triple immunofluorescence staining. GABA(A) receptor clusters, labeled for the alpha2 subunit, formed independently of gephyrin clusters, and were distributed on neurites at constant densities, either extrasynaptically or, to a lesser extent, postsynaptically, apposed to synapsin-I-positive axon terminals. In contrast, gephyrin clusters were always associated with GABA(A) receptors and were preferentially localized postsynaptically. Their density increased linearly with the extent of innervation, which developed rapidly during the first week in vitro. These results suggested that GABA(A) receptor clustering is mediated by cell-autonomous mechanisms independent of synapse formation. Their association with gephyrin is dynamically regulated and may contribute to stabilization at postsynaptic sites. Labeling for vesicular glutamate transporters revealed that most synapses in these immature cultures were presumably glutamatergic, implying that postsynaptic GABA(A) receptor and gephyrin clusters initially were located in "mismatched" synapses. However, clusters appropriately localized in GABAergic synapses were distinctly larger and more intensely stained. Altogether, these results demonstrate that the targeting of GABA(A) receptor and gephyrin clusters to GABAergic synapses occurs secondarily and is regulated by presynaptic factors that are not essential for clustering.

Cellular localization and subcellular distribution of Unc-33-like protein 6, a brain-specific protein of the collapsin response mediator protein family that interacts with the neuronal glycine transporter 2

Unc-33-like protein (Ulip)6, a brain-specific phosphoprotein of the Ulip/collapsin response mediator protein family, was originally identified in our laboratory by yeast two-hybrid screening using the cytoplasmic N-terminal domain of the neuronal glycine transporter, glycine transporter (GlyT) 2, as a bait. Here, the interaction of Ulip6 with the N-terminal domain of GlyT2 was found to be specific for this member of the Ulip/collapsin response mediator protein family and to involve amino acids 135-184 of GlyT2. In pull-down assays and coimmunoprecipitation experiments with rat spinal cord extract, the presence of phosphatase inhibitors significantly enhanced binding of Ulip6 to GlyT2. Subcellular fractionation of spinal cord and retina homogenates at different developmental stages showed Ulip6 immunoreactivity to be associated with light vesicles that were distinct from GlyT2-containing and synaptic vesicles. Immunocytochemistry revealed punctate Ulip6 immunoreactivity in both somatic regions and processes of cultured spinal neurones; no colocalization with GlyT2 or other synaptic marker proteins was found. In retina, which expresses only GlyT1 but not GlyT2, Ulip6 was detected in the inner plexiform layer and along the somata and processes of selected bipolar, amacrine and ganglion cells. Our data support a model in which Ulip6 transiently interacts with GlyT2 in a phosphorylation-dependent manner.

GABAA receptor-associated protein regulates GABAA receptor cell-surface number in Xenopus laevis oocytes

GABA(A) receptor-associated protein (GABARAP) was isolated previously in a yeast two-hybrid screen using the intracellular loop of the gamma2 subunit of the GABA(A) receptor as bait. GABARAP has been shown to participate in the membrane-clustering and intracellular-trafficking of GABA(A) receptors, including a stimulation of the surface expression of GABA(A) receptors. To assess this quantitatively, we used Xenopus laevis oocytes expressing alpha1beta2gamma2S-containing GABA(A) receptors to demonstrate that coexpression of GABARAP increased net surface levels of GABA(A) receptors as shown by both increased GABA currents and surface-expressed protein. This GABARAP stimulation of GABA currents required the receptor gamma2 subunit and full-length GABARAP: deletion of the microtubule-binding domain (amino acids 1-22) or disrupting the polymerization of microtubules abolished the enhancement, indicating that the effect of GABARAP was derived from the interaction with microtubules. GABARAP coexpression did not alter the general properties of GABA(A) receptors such as sensitivity to GABA or benzodiazepines, but it increased surface levels of receptor protein in oocytes. Rather, it seems to supplement inadequate amounts of endogenous GABARAP to support optimum trafficking and/or stabilization of surface GABA(A) receptors.

Epilepsy and Medication Effects on the Pattern Visual Evoked Potential*

Visual disruption in patients diagnosed with epilepsy may be attributable to either the disease itself or to the anti-epileptic drugs prescribed to control the seizures. Effects on visual function may be due to perturbations of the GABAergic neurotransmitter system, since deficits in GABAergic cortical interneurons have been hypothesized to underlie some forms of epilepsy, some anti-epileptic medications increase cortical GABA levels, and GABAergic neural circuitry plays an important role in mediating the responses of cells in the visual cortex and retina. This paper characterizes the effects of epilepsy and epilepsy medications on the visual evoked response to patterned stimuli. Steady-state visual evoked potentials (VEP) evoked by onset-offset modulation of high-contrast sine-wave stimuli were measured in 24 control and 54 epileptic patients. Comparisons of VEP spectral amplitude as a function of spatial frequency were made between controls, complex partial, and generalized epilepsy groups. The effects of the GABA-active medication valproate were compared to those of carbamezepine. The amplitude of the fundamental (F1) component of the VEP was found to be sensitive to epilepsy type. Test subjects with generalized epilepsy had F1 spatial frequency-amplitude functions with peaks shifted to lower spatial frequencies relative to controls and test subjects with complex partial epilepsy. This shift may be due to reduced intracortical inhibition in the subjects with generalized epilepsy. The second harmonic component (F2) response was sensitive to medication effects. Complex partial epilepsy patients on VPA therapies showed reduced F2 response amplitude across spatial frequencies, consistent with previous findings that showed the F2 response is sensitive to GABA-ergic effects on transient components of the VEP.

Molecular structure and function of the glycine receptor chloride channel

The glycine receptor chloride channel (GlyR) is a member of the nicotinic acetylcholine receptor family of ligand-gated ion channels. Functional receptors of this family comprise five subunits and are important targets for neuroactive drugs. The GlyR is best known for mediating inhibitory neurotransmission in the spinal cord and brain stem, although recent evidence suggests it may also have other physiological roles, including excitatory neurotransmission in embryonic neurons. To date, four alpha-subunits (alpha1 to alpha4) and one beta-subunit have been identified. The differential expression of subunits underlies a diversity in GlyR pharmacology. A developmental switch from alpha2 to alpha1beta is completed by around postnatal day 20 in the rat. The beta-subunit is responsible for anchoring GlyRs to the subsynaptic cytoskeleton via the cytoplasmic protein gephyrin. The last few years have seen a surge in interest in these receptors. Consequently, a wealth of information has recently emerged concerning GlyR molecular structure and function. Most of the information has been obtained from homomeric alpha1 GlyRs, with the roles of the other subunits receiving relatively little attention. Heritable mutations to human GlyR genes give rise to a rare neurological disorder, hyperekplexia (or startle disease). Similar syndromes also occur in other species. A rapidly growing list of compounds has been shown to exert potent modulatory effects on this receptor. Since GlyRs are involved in motor reflex circuits of the spinal cord and provide inhibitory synapses onto pain sensory neurons, these agents may provide lead compounds for the development of muscle relaxant and peripheral analgesic drugs.

Biochemical Characterization of the High Affinity Binding between the Glycine Receptor and Gephyrin

Gephyrin is an essential and instructive molecule for the formation of inhibitory synapses. Gephyrin binds directly to the large cytoplasmic loop located between transmembrane helices three and four of the beta-subunit of the glycine receptor and to microtubules, thus promoting glycine receptor (GlyR) anchoring to the cytoskeleton and clustering in the postsynaptic membrane. Besides its structural role, gephyrin is involved in the biosynthesis of the molybdenum cofactor that is essential for all molybdenum-dependent enzymes in mammals. Gephyrin can be divided into an N-terminal trimeric G domain and a C-terminal E domain, which are connected by a central linker region. Here we have studied the in vitro interaction of gephyrin and its domains with the large cytoplasmic loop of the GlyR beta-sub-unit (GlyRbeta-loop). Binding of gephyrin to the GlyR is exclusively mediated by the E domain, and the binding site was mapped to one of its sub-domains (residues 496-654). By using isothermal titration calorimetry, a high affinity (K(d) = 0.2-0.4 microm) and low affinity (K(d) = 11-30 microm) binding site for the GlyRbeta-loop was found on holo-gephyrin and the E domain, respectively, with a binding stoichiometry of two GlyRbeta-loops per E domain in both cases. Binding of the GlyRbeta-loop does not change the oligomeric state of either full-length gephyrin or the isolated E domain.

GABAA receptor subunit and gephyrin protein changes differ in the globus pallidus in Huntington's diseased brain

Immunoreactivity of GABA(A) receptor subunits and the receptor anchoring protein gephyrin was investigated in the human globus pallidus using antibodies raised against the alpha(1) and gamma(2) subunits of the GABA(A) receptor complex and gephyrin. The results revealed increased GABA(A) receptor subunit immunoreactivity and unchanged levels of gephyrin immunoreactivity in Huntington's diseased (HD) globus pallidus (GP). The results demonstrate that gephyrin immunoreactivity did not change in unison with GABA(A) receptor changes in HD, suggesting that the receptor anchoring protein gephyrin is unaltered and maintains a stable lattice structure in the face of GABA(A) receptor changes in HD.

Diversity of glycine receptors in the mouse retina: localization of the alpha3 subunit

Glycine receptors (GlyRs) and their role in retinal circuitry were analyzed immunocytochemically in wild-type and GlyR alpha3 subunit-deficient (Glra3(-/-)) mouse retinae. GlyRs are localized in the inner plexiform layer in brightly fluorescent puncta, which are likely to represent postsynaptically clustered GlyRs. Approximately one third of the clusters were found to contain the alpha1 subunit, and half possessed the alpha3 subunit. However, these two GlyR isoforms were localized at different glycinergic synapses. In the Glra3(-/-) mouse, alpha3 subunit clusters were completely eliminated, although the total number of GlyR clusters was only slightly reduced. This finding indicates that other GlyR subunits (such as alpha2 or alpha4) may have compensated for the loss of the alpha3 subunit. Characteristic expression patterns of the alpha1 and alpha3 subunits within the synaptic circuits of the retina were revealed by double labeling sections for GlyRs and markers that define specific retinal neurons. The alpha1 subunit mediates signal transfer in the rod pathway between AII amacrine cells and OFF-cone bipolar cells. In contrast, the alpha3 subunit appears to be predominantly involved with the cone pathways. Thus, expression of different GlyR alpha subunit genes correlates with anatomically defined connectivities.

Isoform heterogeneity of the human gephyrin gene (GPHN), binding domains to the glycine receptor, and mutation analysis in hyperekplexia

Gephyrin (GPHN) is an organizational protein that clusters and localizes the inhibitory glycine (GlyR) and GABAA receptors to the microtubular matrix of the neuronal postsynaptic membrane. Mice deficient in gephyrin develop a hereditary molybdenum cofactor deficiency and a neurological phenotype that mimics startle disease (hyperekplexia). This neuromotor disorder is associated with mutations in the GlyR alpha1 and beta subunit genes (GLRA1 and GLRB). Further genetic heterogeneity is suspected, and we hypothesized that patients lacking mutations in GLRA1 and GLRB might have mutations in the gephyrin gene (GPHN). In addition, we adopted a yeast two-hybrid screen, using the GlyR beta subunit intracellular loop as bait, in an attempt to identify further GlyR-interacting proteins implicated in hyperekplexia. Gephyrin cDNAs were isolated, and subsequent RT-PCR analysis from human tissues demonstrated the presence of five alternatively spliced GPHN exons concentrated in the central linker region of the gene. This region generated 11 distinct GPHN transcript isoforms, with 10 being specific to neuronal tissue. Mutation analysis of GPHN exons in hyperekplexia patients revealed a missense mutation (A28T) in one patient causing an amino acid substitution (N10Y). Functional testing demonstrated that GPHNN10Y does not disrupt GlyR-gephyrin interactions or collybistininduced cell-surface clustering. We provide evidence that GlyR-gephyrin binding is dependent on the presence of an intact C-terminal MoeA homology domain. Therefore, the N10Y mutation and alternative splicing of GPHN transcripts do not affect interactions with GlyRs but may affect other interactions with the cytoskeleton or gephyrin accessory proteins.

Distribution of gephyrin in the human brain: an immunohistochemical analysis

Gephyrin is an ubiquitously expressed protein that, in the central nervous system, generates a protein scaffold to anchor inhibitory neurotransmitter receptors in the postsynaptic membrane. It was first identified as a protein component of the glycine receptor complex. Recent studies have demonstrated that gephyrin is colocalized with several subtypes of GABA(A) receptors and is part of postsynaptic GABA(A) receptor clusters. Here, we describe a study of the regional and cellular distribution of gephyrin in the human brain, determined by immunohistochemical localisation at the light and confocal laser scanning microscopic levels. At the regional level, gephyrin immunoreactivity was observed in most of the major brain regions examined. The most intense staining was in the cerebral cortex, hippocampus and caudate-putamen, in various brainstem nuclei with more moderate levels in the thalamus and cerebellum. At the cellular level gephyrin immunoreactivity was present on the plasma membranes of the soma and dendrites of pyramidal neurons throughout the various cortical regions examined. In the hippocampus, intense staining was observed on the granule cells of the dentate gyrus, and neurons of the CA1 and CA3 regions showed intense punctate gephyrin staining on their apical dendrites and cell bodies. Gephyrin immunoreactivity was also observed on neurons in the thalamus, globus pallidus and substantia nigra. In the putamen intense labelling of the striosomes was observed; most of the medium-sized neurons in the caudate-putamen were weakly labelled and many large neurons of the striatum were conspicuously stained. Many of the brainstem nuclei, notably the dorsal motor nucleus of the vagus, hypoglossal nucleus, trigeminal nucleus and inferior olive were all labelled with gephyrin. The spinal cord also showed high levels of gephyrin immunoreactivity. Our results demonstrate that the anchoring protein gephyrin is ubiquitously present in the human brain. We therefore suggest that gephyrin may have a central organizer role in assembling and stabilizing inhibitory postsynaptic membranes in human brain and is similar in function to those observed in the rodent brain. These findings contribute towards elucidating the role of gephyrin in the human brain.

Formation and plasticity of GABAergic synapses: physiological mechanisms and pathophysiological implications

gamma-Aminobutyric acid(A) (GABA(A)) receptors mediate most of the fast inhibitory neurotransmission in the CNS. They represent a major site of action for clinically relevant drugs, such as benzodiazepines and ethanol, and endogenous modulators, including neuroactive steroids. Alterations in GABA(A) receptor expression and function are thought to contribute to prevalent neurological and psychiatric diseases. Molecular cloning and immunochemical characterization of GABA(A) receptor subunits revealed a multiplicity of receptor subtypes with specific functional and pharmacological properties. A major tenet of these studies is that GABA(A) receptor heterogeneity represents a key factor for fine-tuning of inhibitory transmission under physiological and pathophysiological conditions. The aim of this review is to highlight recent findings on the regulation of GABA(A) receptor expression and function, focusing on the mechanisms of sorting, targeting, and synaptic clustering of GABA(A) receptor subtypes and their associated proteins, on trafficking of cell-surface receptors as a means of regulating synaptic (and extrasynaptic) transmission on a short-time basis, on the role of endogenous neurosteroids for GABA(A) receptor plasticity, and on alterations of GABA(A) receptor expression and localization in major neurological disorders. Altogether, the findings presented in this review underscore the necessity of considering GABA(A) receptor-mediated neurotransmission as a dynamic and highly flexible process controlled by multiple mechanisms operating at the molecular, cellular, and systemic level. Furthermore, the selected topics highlight the relevance of concepts derived from experimental studies for understanding GABA(A) receptor alterations in disease states and for designing improved therapeutic strategies based on subtype-selective drugs.

Association of gephyrin with synaptic and extrasynaptic GABAA receptors varies during development in cultured hippocampal neurons

Several studies have reported extrasynaptic clusters of GABAA receptors in hippocampal neurons. Yet their functional relevance as well as their evolution in relation with gephyrin during synaptogenesis remain unknown. We have analyzed the expression pattern of the main proteins of the GABAergic synapses during synaptogenesis in cultured hippocampal neurons. We found that GABAergic terminals, characterized by VIAAT and GAD-65 expression, differentiated 3 to 7 days after the glutamatergic endings. At the postsynaptic side, the GABAAR- beta3 subunit was first diffuse and then clustered when GABAergic terminals differentiated and gephyrin formed large clusters. Colocalization of these proteins was high and increased with development. At later stages, GABAAR beta3 clusters colocalized with gephyrin at synaptic but also at extrasynaptic sites. GABAAR gamma2 subunits were directly expressed as clusters which were first extrasynaptic and not associated with gephyrin. Subsequently, the GABAAR gamma2 subunits associated with gephyrin at synaptic and/or extrasynaptic sites. Our data indicate that formation of GABAAR gamma2 subunit clusters is gephyrin independent.

Declusterization of GABAA receptors affects the kinetic properties of GABAergic currents in cultured hippocampal neurons

Speed and reliability of synaptic transmission are essential for information coding in neuronal networks and require the presence of clustered neurotransmitter receptors at the plasma membrane in precise apposition to presynaptic terminals. Receptor clusterization is the result of highly regulated processes involving functional and structural proteins. Among the structural elements, microtubules are known to play a crucial role in anchoring of gamma-aminobutyric acid, type A (GABA(A)) receptors. Here we show that microtubule depolymerization with nocodazole induces the declusterization of GABA(A) receptors and modifies the kinetic properties of GABAergic currents in cultured hippocampal neurons. In particular, this drug, applied either in the bath or via the patch pipette, induced the acceleration of the onset kinetics of miniature inhibitory postsynaptic currents (mIPSCs) without significantly affecting their frequency, thus suggesting a main postsynaptic site of action. After nocodazole treatment, current responses to ultrafast applications of GABA exhibited a faster rise time and an accelerated onset of desensitization. A quantitative analysis of GABA-evoked currents and model simulations suggest that declusterization affects the gating properties of GABA(A) receptors. In particular, a faster entry into the desensitized state of declustered GABA(A) receptors may account for the changes in the kinetic properties of mIPSCs after nocodazole treatment. Hence it appears that the clustered condition of GABA(A) receptors contributes in shaping GABAergic currents.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Dynamic regulation of GABA(A) receptors at synaptic sites

gamma-Aminobutyric acid type A receptors (GABA(A)Rs) mediate fast synaptic inhibition in brain and spinal cord. They are ligand-gated ion channels composed of numerous distinct subunit combinations. For efficient synaptic transmission, GABA(A)Rs need to be localized to and anchored at postsynaptic sites in precise apposition to presynaptic nerve terminals that release the neurotransmitter GABA. Neurons therefore require distinct mechanisms to regulate intracellular vesicular protein traffic, plasma membrane insertion, synaptic clustering and turnover of GABA(A)Rs. The GABA(A) receptor-associated protein GABARAP interacts with the gamma2 subunit of GABA(A)Rs and displays high homology to proteins involved in membrane fusion underlying Golgi transport and autophagic processes. The binding of GABARAP with NSF, microtubules and gephyrin together with its localization at intracellular membranes suggests a role in GABA(A)R targeting and/or degradation. Growth factor tyrosine kinase receptor activation is involved in the control of GABA(A)R levels at the plasma membrane. In particular insulin recruits GABA(A)Rs to the cell surface. Furthermore, the regulation of GABA(A)R surface half-life can also be the consequence of negative modulation at the proteasome level. Plic-1, a ubiquitin-like protein binds to both the proteasome and GABA(A)Rs and the Plic1-GABA(A)R interaction is important for the maintenance of GABA-activated current amplitudes. At synaptic sites, GABA(A)Rs are clustered via gephyrin-dependent and gephyrin-independent mechanisms and may subsequently become internalized via clathrin-mediated endocytosis underlying receptor recycling or degradation processes. This article discusses these recent data in the field of GABA(A)R dynamics.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

The development of inhibitory synaptic specializations in the mouse deep cerebellar nuclei

Using confocal laser scanning microscopy and immunohistochemistry, this study shows the complete morphological development of GABAergic synaptic contacts between Purkinje cells and neurons of the deep cerebellar nuclei of the mouse. Firstly, presynaptic varicosities visualized with antibodies against synaptophysin, synapsin or glutamic acid decarboxylase, were detected when the postsynaptic GABA(A) receptors were not yet aggregated in the membrane but had a diffuse cytoplasmic distribution, which indicated a lead in maturation of presynaptic terminals over target cells. Secondly, receptor aggregates developed suddenly after an initial week of diffuse expression and these clusters matured into more numerous and larger synaptic aggregates. During this postsynaptic maturation, the presynaptic varicosities develop into numerous and well-defined spots. As soon as both pre- and postsynaptic clusters were detectable, these sites are always colocalized. We therefore consider the aggregation of postsynaptic receptor during development as a landmark of synapse formation. Our observations are consistent with a developmental model in which the presynaptic neuron differentiates its axon before the target neuron expresses the mature form of its receptors on the membrane. The presynaptic neuron can therefore instruct the target neuron about the distribution and aggregation of the postsynaptic receptors at the synapse.

A new benzodiazepine pharmacology

Classical benzodiazepine drugs are in wide clinical use as anxiolytics, hypnotics, anticonvulsants, and muscle relaxants. They act by enhancing the gamma-aminobutyric acid(A) (GABA(A)) receptor function in the central nervous system. The pharmacological relevance of the multitude of structurally diverse GABA(A) receptor subtypes has only recently been identified. Based on an in vivo point mutation strategy, alpha(1)-GABA(A) receptors were found to mediate sedation, anterograde amnesia, and part of the seizure protection, whereas alpha(2)-GABA(A) receptors, but not alpha(3)-receptors, mediate anxiolysis. Rational drug targeting to specific receptor subtypes has now become possible. Only restricted neuronal networks will be modulated by the new subtype-selective drugs. Promising new anxiolytics have already been developed. A new pharmacology of the benzodiazepine site is on the horizon.

GPHN, a novel partner gene fused to MLL in a leukemia with t(11;14)(q23;q24)

We report a novel MLL-associated chromosome translocation t(11;14)(q23;q24) in a child who showed signs of acute undifferentiated leukemia 3 years after intensive chemotherapy that included the topoisomerase-II inhibitor VP 16. Screening of a cDNA library of the patient's leukemic cells showed a novel fusion transcript between MLL and the Gephyrin (GPHN) gene on 14q24. The resulting MLL-GPHN fusion gene encodes MLL AT hook motifs and a DNA methyltransferase homology domain fused to the C-terminal half of Gephyrin, including a presumed tubulin binding site and a domain homologous to the Escherichia coli molybdenum cofactor biosynthesis protein MoeA. Genomic breakpoint analysis showed potential in vitro topoisomerase-II DNA-binding sites spanning the breakpoints in both MLL and GPHN but no flanking sequences that might mediate homologous recombination. This suggests that MLL-GPHN may have been generated by VP 16/topoisomerase-II-induced DNA double-strand breaks, followed by error-prone DNA repair via non-homologous end joining. Gephyrin was originally identified as a submembraneous scaffold protein that anchors and immobilizes postsynaptic membrane neurotransmitter receptors to underlying cytoskeletal elements. It also is reported to bind to phosphatidylinositol 3,4,5-triphosphate binding proteins involved in actin dynamics and downstream signaling and interacts with ATM-related family member RAFT1. Gephyrin domains in the chimeric protein therefore could contribute novel signal sequences or might modify MLL activity by oligomerization or intracellular redistribution.

GABA(C) rho(1) subunits form functional receptors but not functional synapses in hippocampal neurons

The ability to control the physiological and pharmacological properties of synaptic receptors is a powerful tool for studying neuronal function and may be of therapeutic utility. We designed a recombinant adenovirus to deliver either a GABA(C) receptor rho(1) subunit or a mutant GABA(A) receptor beta(2) subunit lacking picrotoxin sensitivity [beta2(mut)] to hippocampal neurons. A green fluorescent protein (GFP) reporter molecule was simultaneously expressed. Whole cell patch-clamp recordings demonstrated somatic expression of both bicuculline-resistant GABA(C) receptor-mediated and picrotoxin-resistant GABA(A) receptor-mediated GABA-evoked currents in rho(1)- and beta(2)(mut)-transduced hippocampal neurons, respectively. GABAergic miniature inhibitory postsynaptic currents (mIPSCs) recorded in the presence of 6-cyano-7-nitroquinoxalene-2,3-dione, Mg(2+), and TTX revealed synaptic events with monoexponential activation and biexponential decay phases. Despite the robust expression of somatic GABA(C) receptors in rho(1)-neurons, no bicuculline-resistant mIPSCs were observed. This suggested either a kinetic mismatch between the relatively brief presynaptic GABA release and slow-activating rho(1) receptors or failure of the rho(1) subunit to target properly to the subsynaptic membrane. Addition of ruthenium red, a presynaptic release enhancer, failed to unmask GABA(C) receptor-mediated mIPSCs. Short pulse (2 ms) application of 1 mM GABA to excised outside-out patches from rho(1) neurons proved that a brief GABA transient is sufficient to activate rho(1) receptors. The simulated-IPSC experiment strongly suggests that if postsynaptic GABA(C) receptors were present, bicuculline-resistant mIPSCs would have been observed. In contrast, in beta(2)(mut)-transduced neurons, picrotoxin-resistant mIPSCs were observed; they exhibited a smaller peak amplitude and faster decay compared with control. Confocal imaging of transduced neurons revealed rho(1) immunofluorescence restricted to the soma, whereas punctate beta(2)(mut) immunofluorescence was seen throughout the neuron, including the dendrites. Together, the electrophysiological and imaging data show that despite robust somatic expression of the rho(1) subunit, the GABA(C) receptor fails to be delivered to the subsynaptic target. On the other hand, the successful incorporation of beta(2)(mut) subunits into subsynaptic GABA(A) receptors demonstrates that viral transduction is a powerful method for altering the physiological properties of synapses.

Identification of a gephyrin-binding motif in the GDP/GTP exchange factor collybistin

The brain-specific GDP/GTP exchange factor collybistin interacts with the receptor-anchoring protein gephyrin and activates the Rho-like GTPase Cdc42, which is known to regulate actin cytoskeleton dynamics. Alternative splicing creates two collybistin variants, I and II. In coexpression experiments, collybistin II has been shown to induce the formation of submembraneous gephyrin aggregates which cluster with hetero-oligomeric glycine receptors (GlyRs). Here we identified residues critical for interaction with gephyrin in the linker region between the SH3 and the DH domains of collybistin. Respective collybistin deletion mutants failed to bind gephyrin upon coexpression in heterologous cells, in GST pull-down assays and in the yeast two-hybrid system. Site-directed mutagenesis revealed polar amino acid residues as essential determinants of gephyrin binding. Furthermore, in vitro gephyrin bound simultaneously to both collybistin and the GlyR beta-subunit binding motif. Our data are consistent with collybistin-gephyrin interactions occuring during inhibitory postsynaptic membrane formation.

IPSC kinetics at identified GABAergic and mixed GABAergic and glycinergic synapses onto cerebellar Golgi cells

In the rat cerebellum, Golgi cells receive serotonin-evoked inputs from Lugaro cells (L-IPSCs), in addition to spontaneous inhibitory inputs (S-IPSCs). In the present study, we analyze the pharmacology of these IPSCs and show that S-IPSCs are purely GABAergic events occurring at basket and stellate cell synapses, whereas L-IPSCs are mediated by GABA and glycine. Corelease of the two transmitters at Lugaro cell synapses is suggested by the fact that both GABA(A) and glycine receptors open during individual L-IPSCs. Double immunocytochemical stainings demonstrate that GABAergic and glycinergic markers are coexpressed in Lugaro cell axonal varicosities, together with the mixed vesicular inhibitory amino acid transporter. Lugaro cell varicosities are found apposed to glycine receptor (GlyR) clusters that are localized on Golgi cell dendrites and participate in postsynaptic complexes containing GABA(A) receptors (GABA(A)Rs) and the anchoring protein gephyrin. GABA(A)R and GlyR/gephyrin appear to form segregated clusters within individual postsynaptic loci. Basket and stellate cell varicosities do not face GlyR clusters. For the first time the characteristics of GABA and glycine cotransmission are compared with those of GABAergic transmission at identified inhibitory synapses converging onto the same postsynaptic neuron. The ratio of the decay times of L-IPSCs and of S-IPSCs is a constant value among Golgi cells. This indicates that, despite a high cell-to-cell variability of the overall IPSC decay kinetics, postsynaptic Golgi cells coregulate the kinetics of their two main inhibitory inputs. The glycinergic component of L-IPSCs is responsible for their slower decay, suggesting that glycinergic transmission plays a role in tuning the IPSC kinetics in neuronal networks.

Structure of ligand-gated ion channels: critical assessment of biochemical data supports novel topology

Rapid signaling across the synaptic junction is partially mediated by the ligand-gated ion channel superfamily (LGICS), which includes inhibitory glycine and GABA receptors and excitatory acetylcholine and serotonin receptors. The glycine receptor (GlyR) can assemble as homopentamers of alpha subunits, and baculovirus expression systems are capable of overexpressing large quantities of active receptors. Limited proteolysis coupled to mass spectrometry on reconstituted alpha1 GlyR homopentamers identified proteolytic cleavages within proposed transmembrane domains postulated to fold as bilayer-spanning alpha helices in the "classical" model and identified unexpected membrane-associated regions in the N-terminal domain (J. F. Leite et al., 2000, J. Biol. Chem. 275, 13683-13689). In this review, optimized sequence alignments were used to integrate these proteolysis data with biochemical information determined in studies of all the LGICS members in order to construct a novel topological model.

Regulation of the subcellular distribution and gene expression of GABA(A) receptor by microtubules and microfilaments in cultured brain neurons

Mechanisms underlying the intracellular transport of gamma-aminobutyric acid(A) receptor (GABA(A)R) were examined in the cultured neurons derived from chicken embryo brains. In situ trypsinization of the cultures and (3)H-flunitrazepam (FNZ) binding assay were employed to determine the cell surface and intracellular distribution of the receptor. A 3-h treatment of the cells with 1 microM of colchicine, a microtubule depolymerizer, reversibly raised the proportion of intracellular GABA(A)R density by about 36% and decreased that of the cell surface receptors by 18% from respective control values, whereas the 3-h incubation with 2 microM of cytochalasin D, a microfilament disrupter, did not cause significant changes. These treatments failed to alter the total number of the (3)H-FNZ binding sites of the neurons and the affinity of the ligand. Moreover, the exposure to colchicine seemed to produce a stronger cytoplasmic immunostaining of the GABA(A)R alpha subunits in many neurons without affecting the total cellular level of the proteins, in accordance with the increased fraction of intracellular (3)H-FNZ binding. However, in the neurons exposed to cytochalasin D, there was an increase of around 28% in the total content of alpha(1)+51kDa proteins. In addition, the colchicine or cytochalasin D treatment inhibited approximately 21 or 18% of the rate of general protein synthesis in the culture. Notably, in situ hybridization assay showed that the GABA(A)R alpha(1) or alpha(2) mRNA was present in 92 +/- 2% or 94 +/- 2% of the cytochalasin D-treated neurons, both of which were higher than 71 +/- 2-74 +/- 3% of the control and colchicine-treated cells. The data suggest that by regulating the intracellular transport, the microtubular system participates in the maintenance of normal subcellular distribution of GABA(A)R in the neurons. By contrast, the organization of microfilaments may play a role in modulating the gene expression of GABA(A)R subunits.

Patterns of colocalization of GABA, glutamate and glycine immunoreactivities in terminals that synapse on dendrites of noradrenergic neurons in rat locus coeruleus

Amino acid transmitters play a key role in regulating the activity of noradrenergic neurons in the locus coeruleus. We investigated the anatomical substrate for this regulation by quantifying immunoreactivity for GABA, glutamate and glycine in terminals that contacted the dendrites of tyrosine hydroxylase-immunoreactive principal neurons in rat locus coeruleus. Pre-embedding peroxidase immunocytochemistry was used to detect tyrosine hydroxylase-immunoreactivity in Vibratome sections of tissue perfused with 2.5% glutaraldehyde. GABA, glutamate and glycine were localized with postembedding immunogold labelling. Gold particle densities over terminals were measured in three semiserial ultrathin sections, each reacted for a different amino acid. More than 90% (range among rats, 89%-95%) of the terminals analyzed (n = 288) were immunoreactive for at least one amino acid. A high proportion (39%-49%) were positive for two or three amino acids. About two-thirds (60%-69%) of the boutons contained GABA, of which more than half (51%-55%) also contained glycine. More than one-third (36%-38%) of the terminals were positive for glycine. Terminals immunoreactive for glycine alone were rare (0%-2%). About one-third of the terminals showed glutamate-immunoreactivity (32%-37%). GABA and/or glycine occurred in one-fifth to one-third of these. These results show that amino acid-immunoreactivity is present in almost all of the terminals that synapse on tyrosine hydroxylase-positive dendrites in locus coeruleus. Glutamate provides a major excitatory input. The almost complete colocalization of glycine with GABA suggests that the inhibitory input to locus coeruleus is predominantly GABAergic with a contribution from glycine in about half of the GABAergic boutons.

Regional distribution of glycine receptor messenger RNA in the central nervous system of zebrafish

We report the cloning of the zebrafish beta subunit of the glycine receptor and compare the anatomical distribution of three glycine receptor subunit constituents in adult zebrafish brain (alphaZ1, alphaZ2 and betaZ) to the expression pattern of homologous receptor subunits (alpha1, alpha2 and beta) in the mammalian adult CNS. Non-radioactive hybridization was used to map the distribution of the alphaZ1, alphaZ2 and betaZ glycine receptor subunit messenger RNAs in the adult zebrafish brain. The anterior-posterior expression gradient found in adult zebrafish brain was similar to that reported in mammalian CNS. However, the glycine receptor transcripts, notably the alphaZ1 subunit, were more widely distributed in the anterior regions of the zebrafish than in the adult mammalian brain. The isoform-specific distribution pattern was less regionalized in zebrafish than in the rat mammalian CNS. Nevertheless, there was some regionalization of alphaZ1, alphaZ2 and betaZ transcripts in the diencephalic and mesencephalic nuclei where different sensory and motor centers express either alphaZ1/betaZ or alphaZ2 subunits. In contrast to the widespread distribution of the beta subunit in adult mammalian brain, alphaZ2 messenger RNA presented the widest expression territory of all three glycine receptor subunits tested. alphaZ2 messenger RNA was expressed in the absence of alphaZ1 and betaZ messenger RNA in the outer nuclear layer of the retina, the inferior olive and the raphe of the medulla oblongata, as well as in the nucleus of Cajal of the medulla spinalis. In contrast, an identified central neuron of the reticular formation, the Mauthner cell, expresses all three glycine receptor subunits (alphaZ1, alphaZ2 and betaZ). This report extends the already described glycine receptor expression in the vertebrate CNS and confirms the importance of glycine-mediated inhibition in spinal cord and brainstem.

Gephyrin-independent clustering of postsynaptic GABA(A) receptor subtypes

Gephyrin has been shown to be essential for the synaptic localization of the inhibitory glycine receptor and major GABA(A) receptor (GABA(A)R) subtypes. However, in retina certain GABA(A)R subunits are found at synaptic sites in the absence of gephyrin. Here, we quantitatively analyzed GABA(A)R alpha1, alpha2, alpha3, alpha5, beta2/3, and gamma2 subunit immunoreactivities in spinal cord sections derived from wild-type and gephyrin-deficient (geph -/-) mice. The punctate staining of GABA(A)R alpha1 and alpha5 subunits was unaltered in geph -/- mice, whereas the numbers of alpha2-, alpha3-, beta2/3-, and gamma2-subunit-immunoreactive synaptic sites were significantly or even strikingly reduced in the mutant animals. Immunostaining with an antibody specific for the vesicular inhibitory amino acid transporter revealed that the number of inhibitory presynaptic terminals is unaltered upon gephyrin deficiency. These data show that in addition to gephyrin other clustering proteins must exist that mediate the synaptic localization of selected GABA(A)R subtypes.

Localization of two major GABA(A) receptor subunits in the dentate gyrus of the rat and cell type-specific up-regulation following entorhinal cortex lesion

GABA(A) receptor subunits show a specific regional distribution in the CNS during development and in the adult animal. In the hippocampal formation, individual subsets of GABAergic interneurons are highly immunoreactive for the alpha1-subunit, whereas granule and pyramidal cells show a strong expression of the alpha2-subunit. Using confocal microscopy and digital image analysis, we demonstrate that in the dentate gyrus the alpha1-subunit immunolabeling appears in differently sized clusters. The large clusters, which are confined to dendrites of interneurons, show no alpha2 labeling, whereas the smaller ones coincide with alpha2-subunit-positive clusters. In the molecular layer, the clusters of both alpha-subunits co-localize with the anchoring protein gephyrin. In the granule cell layer and hilus, we found alpha1- and alpha2-subunit-positive clusters which were devoid of gephyrin labeling. Lesions of the medial entorhinal cortex led to the deafferentation of dendrites in the middle molecular layer of the dentate gyrus. This resulted in a significantly increased concentration of alpha2-subunit-positive clusters. We also observed an increase of alpha1-subunit immunolabeling in the deafferented area. We found no change in the co-localization between alpha1 and alpha2, and no significant change in the number of large alpha1-positive clusters along individual dendritic segments of interneurons. In a previous study, we demonstrated that calbindin-immunoreactive dendrites of granule cells revealed a significant increase in gephyrin immunoreactivity following lesion, whereas parvalbumin-positive dendrites showed no such alterations. The predominant localization of small gephyrin clusters in dendrites of granule cells, which was also described in this study, leads to the conclusion that the increase of the alpha2-subunit-positive clusters, demonstrated in the present study, indicates that, following entorhinal cortex lesion, new GABAergic synapses may be formed and that they contact predominantly granule cell dendrites.

Constructing inhibitory synapses

Control of nerve-cell excitability is crucial for normal brain function. Two main groups of inhibitory neurotransmitter receptors--GABA(A) and glycine receptors--fulfil a significant part of this role. To mediate fast synaptic inhibition effectively, these receptors need to be localized and affixed opposite nerve terminals that release the appropriate neurotransmitter at multiple sites on postsynaptic neurons. But for this to occur, neurons require intracellular anchoring molecules, as well as mechanisms that ensure the efficient turnover and transport of mature, functional inhibitory synaptic receptor proteins. This review describes the dynamic regulation of synaptic GABA(A) and glycine receptors and discusses recent advances in this rapidly evolving field.

Identification of multiple gephyrin variants in different organs of the adult rat

The neurotransmitter receptor anchoring protein gephyrin is encoded by a highly mosaic gene whose primary transcript is subject to extensive alternative splicing. Gephyrin mRNAs are widely expressed in various mammalian tissues, and gephyrin has been implicated in neuron-specific and general metabolic functions. Using a novel affinity isolation procedure, we report the identification of different gephyrin variants in various organs of the adult rat. In particular, polypeptides of 52, 56, 60, and 91 kDa were detected in addition to the previously characterized 93-kDa protein. Our results suggest tissue-specific functional differences between gephyrin variants.

Reduced synaptic clustering of GABA and glycine receptors in the retina of the gephyrin null mutant mouse

Clustering of neurotransmitter receptors in postsynaptic densities involves proteins that aggregate the receptors and link them to the cytoskeleton. In the case of glycine and GABA(A) receptors, gephyrin has been shown to serve this function. However, it is unknown whether gephyrin is involved in the clustering of all glycine and GABA(A) receptors or whether it interacts only with specific isoforms. This was studied in the retinae of mice, whose gephyrin gene was disrupted, with immunocytochemistry and antibodies that recognize specific subunits of glycine and GABA(A) receptors. Because homozygous (geph -/-) mutants die around birth, an organotypic culture system of the mouse retina was established to study the clustering of gephyrin and the receptors in vitro. We found that all gephyrin and all glycine receptor clusters (hot spots) were abolished in the geph (-/-) mouse retina. In the case of GABA(A) receptors, there was a significant reduction of clusters incorporating the gamma2, alpha2, and alpha3 subunits; however, a substantial number of hot spots was still present in geph (-/-) mutant retinae. This shows that gephyrin interacts with all glycine receptor isoforms but with only certain forms of GABA(A) receptors. In heterozygous geph (+/-) mutants, no reduction of hot spots was observed in the retina in vivo, but a significant reduction was found in the organotypic cultures. This suggests that mechanisms may exist in vivo that allow for the compensation of a partial gephyrin deficit.

Formation of mixed glycine and GABAergic synapses in cultured spinal cord neurons

In the spinal cord, GABA and glycine mediate inhibition at separate or mixed synapses containing glycine and/or GABA(A) receptors (GlyR and GABA(A)R, respectively). We have analysed here the sequence of events leading to inhibitory synapse formation during synaptogenesis of embryonic spinal cord neurons between 1 and 11 days in vitro (DIV). We used immunocytochemical methods to detect simultaneously an antigen specific to inhibitory terminals, the vesicular inhibitory amino acid transporter (VIAAT), and one of the following postsynaptic elements: GlyR, GABA(A)R or gephyrin, the anchoring protein of GlyR, which is also associated with GABA(A)R. Quantitative analysis revealed that until 5 DIV most gephyrin clusters were not adjacent to VIAAT-positive profiles, but became associated with them at later stages. In contrast, GlyR and GABAAR clustered predominantly in front of VIAAT-containing terminals at all stages. However, about 10% of receptor aggregates were detected at nonsynaptic loci. The two receptors colocalized in 66.2+/-2.5% of the inhibitory postsynaptic domains after 11 DIV, while 30.3+/-2.6% and 3.4+/-0.8% of them contained only GlyR and GABA(A)R, respectively. Interestingly, at 3 DIV GABA(A)R clustered at a postsynaptic location prior to gephyrin and GlyR; GABA(A)R could thus be the initiating element in the construction of mixed glycine and GABAergic synapses. The late colocalization of gephyrin with GABA(A)R, and the demonstration by other groups that, in the absence of gephyrin, postsynaptic GABA(A)R is not detected, suggest that gephyrin is involved in the stabilization of GABA(A)R rather than in its initial accumulation at synaptic sites.

Regulation of somatodendritic GABAA receptor channels in rat hippocampal neurons: evidence for a role of the small GTPase Rac1

The role of the cytoskeleton in the activity of GABA(A) receptors was investigated in cultured hippocampal neurons. Receptor currents were measured with the whole-cell patch-clamp technique during repetitive stimulation with 1 microm muscimol. After destruction of the microtubular system with nocodazol, muscimol-induced currents showed a rundown by 78%. A similar rundown was observed when actin fibers were destroyed with latrunculin B or C2 toxin of Clostridium botulinum. Because the small GTPases of the Rho family RhoA, Rac1, and Cdc42 are known to control the organization of actin fibers, we investigated their possible involvement. Inactivation of the GTPases with clostridial toxins, as well as intracellular application of recombinant Rho GTPases, indicated that active Rac1 was necessary for full GABA(A) receptor activity. Immunocytochemical labeling of the receptors showed that the disappearance of receptor clusters in the somatic membrane as induced by muscimol stimulation was enhanced by Rac1 inactivation. It is suggested that Rac1 participates in the regulation of GABA(A) receptor clustering and/or recycling.

The gamma-aminobutyric acid type A (GABAA) receptor-associated protein (GABARAP) promotes GABAA receptor clustering and modulates the channel kinetics

A microtubule-associated protein, gamma-aminobutyric acid type A (GABA(A)) receptor-associated protein (GABARAP), was previously identified as binding to the intracellular domain of GABA(A) receptors by using the yeast two-hybrid screen. In the present work, immunofluorescent staining and green fluorescent protein-tagged receptor subunits showed that GABARAP is associated with and promotes the clustering of GABA(A) receptors in QT-6 quail fibroblasts. The tubulin-binding motif of GABARAP and the gamma2 subunit of the receptor are required. Disruption of microtubules prevents the clustering in a time-dependent manner. When green fluorescent protein-tagged alpha1 or gamma2 subunit coexpressed with beta2, gamma2L, and GABARAP was used, recordings from visually identified cells revealed that clustered GABA(A) receptor had an EC(50) of about 20 microM, vs. 5.7 microM for the diffuse receptor. Clustered receptors deactivated faster and desensitized slower than the diffuse receptors, because of decrease in the apparent affinity of GABA binding. Different properties for clustered receptors relative to unclustered receptors in heterologous cells suggest that homologous differences between extrasynaptic and synaptic clustered receptors in neurons may be due to the organization of the postsynaptic machinery.

Postnatal maturation of gephyrin/glycine receptor clusters on developing Renshaw cells

Adult mammalian Renshaw cells express large and complex postsynaptic gephyrin/glycine receptor clusters on their surface. Larger gephyrin clusters correlate with more "efficacious" inhibitory synapses, in terms of larger postsynaptic quantal size amplitudes, in part because they likely contain more postsynaptic receptors (Lim et al. [1999] J. Physiol. (Lond.) 516:505-512; Oleskevich et al. [1999] J. Neurophysiology 82:312-319). Here, we studied the postnatal development of the gephyrin/glycine receptor cluster size on Renshaw cells. Renshaw cells were identified by their calbindin immunoreactivity, location and morphology, and presence of cholinergic input. The populations of clusters over developing Renshaw cells immunoreactive to gephyrin or glycine receptor alpha1 subunits were comparable in number, size, and complexity and displayed a high degree of colocalization (>90%) at all ages. Quantitative morphologic analysis was performed on gephyrin-immunoreactive clusters. In neonatal animals, Renshaw cells expressed small punctate gephyrin-immunoreactive clusters (mean cluster size +/- SD = 0.19 +/- 0.19 microm(2)at 2 days; 0.22 +/- 0. 19 microm(2)at 5 days). By 10 and 15 days of age, Renshaw cells exhibited gephyrin-immunoreactive clusters that were larger and more complex (0.32 +/- 0.19 microm(2) at 10 days; 0.41 +/- 0.32 microm(2) at 15 days). Cluster growth reached a plateau in 25- and 60-day-old Renshaw cells (0.45 +/- 0.43 microm(2); 0.56 +/- 0.55 microm(2), respectively). By using electron microscopy, we confirmed that gephyrin-immunoreactive clusters were located at postsynaptic sites at both early and late postnatal ages on Renshaw cells. The potential significance of this gephyrin/glycine receptor cluster size maturation that sets Renshaw cells apart from other interneurons is discussed.

Confronting complexity: strategies for understanding the microcircuitry of the retina

The mammalian retina contains upward of 50 distinct functional elements, each carrying out a specific task. Such diversity is not rare in the central nervous system, but the retina is privileged because its physical location, the distinctive morphology of its neurons, the regularity of its architecture, and the accessibility of its inputs and outputs permit a unique variety of experiments. Recent strategies for confronting the retina's complexity attempt to marry genetic approaches to new kinds of anatomical and electrophysiological techniques.

Clustering of inhibitory neurotransmitter receptors at developing postsynaptic sites: the membrane activation model

Recent studies indicate an important role of cytoskeleton-associated and lipid-anchored proteins in the formation of inhibitory postsynaptic membrane specializations. Membrane apposition of the tubulin-binding protein gephyrin is essential for the recruitment of inhibitory glycine receptors and GABAA receptors to developing postsynaptic sites. Newly disclosed interactions between gephyrin, exchange factors for G proteins of the Rho and Rac families, the translational regulator RAFT1, and actin-binding proteins like profilin might integrate activity-dependent and trophic-factor-mediated signals at developing postsynaptic sites. A model of inhibitory neurotransmitter receptor clustering, is proposed, in which this process is initiated by receptor-driven activation of phosphatidylinositol 3-kinase.

Development and influence of inhibition in the lateral superior olivary nucleus

While studies of neuronal development and plasticity have focused on excitatory pathways, the inhibitory projection from the MNTB to the LSO provides a favorable model for studies of synaptic inhibition. This review covers recent studies from our laboratories indicating that inhibitory connections are quite dynamic during development. These findings suggest that there are two phases inhibitory transmission. During an initial depolarizing phase is growth and branching of pre- and postsynaptic elements in the LSO. During a second hyperpolarizing phase there is refinement of inhibitory afferent arborizations and the LSO dendrites that they innervate.

Diversity and phylogeny of gephyrin: tissue-specific splice variants, gene structure, and sequence similarities to molybdenum cofactor-synthesizing and cytoskeleton-associated proteins

Gephyrin is essential for both the postsynaptic localization of inhibitory neurotransmitter receptors in the central nervous system and the biosynthesis of the molybdenum cofactor (Moco) in different peripheral organs. Several alternatively spliced gephyrin transcripts have been identified in rat brain that differ in their 5' coding regions. Here, we describe gephyrin splice variants that are differentially expressed in non-neuronal tissues and different regions of the adult mouse brain. Analysis of the murine gephyrin gene indicates a highly mosaic organization, with eight of its 29 exons corresponding to the alternatively spliced regions identified by cDNA sequencing. The N- and C-terminal domains of gephyrin encoded by exons 3-7 and 16-29, respectively, display sequence similarities to bacterial, invertebrate, and plant proteins involved in Moco biosynthesis, whereas the central exons 8, 13, and 14 encode motifs that may mediate oligomerization and tubulin binding. Our data are consistent with gephyrin having evolved from a Moco biosynthetic protein by insertion of protein interaction sequences.

Distribution of the integrin beta 1 subunit on radial cells in the embryonic and adult avian retina

The distribution of the beta1 integrin subunit was investigated in the developing and adult chick retina at the light and electron microscopic levels, using two different monoclonal antibodies. Western blotting revealed a single band with a molecular weight of approximately 130 kDa in the retina and in a number of other tissues, indicating the specificity of the antibodies. In the retina, immunoreactivity was detected on radial cells spanning the entire width between the pigment epithelium and the vitreal border. These cells were undifferentiated neuroepithelial cells at early stages and radial Müller glial cells at later stages of development. At all stages, the beta1 subunit was concentrated at the vitreal border of the retina around the inner limiting membrane. Mechanical isolation of the inner limiting membrane, as well as immunoelectron microscopy, demonstrated that this immunoreactivity was due to a concentration of the beta1 subunit in the endfeet of neuroepithelial and Müller glial cells. Injection of collagenase into the vitreous of live embryos, a procedure that selectively removes the inner limiting membrane, but does not proteolytically degrade the integrin protein, resulted in a redistribution of the integrin immunoreactivity, demonstrating that the integrity of the basal lamina is required for the maintenance of the concentration of the beta1 subunit in the endfeet. These results suggest a role for the beta1 subunit-containing integrin heterodimers in the adhesion of neuroepithelial and Müller glial cells to extracellular matrix components of the inner limiting membrane, possibly stabilizing the radial morphology of these cells.

The gamma-aminobutyric acid type A receptor (GABAAR)-associated protein GABARAP interacts with gephyrin but is not involved in receptor anchoring at the synapse

gamma-Aminobutyric acid type A receptors (GABA(A)Rs) are ligand-gated chloride channels that exist in numerous distinct subunit combinations. At postsynaptic membrane specializations, different GABA(A)R isoforms colocalize with the tubulin-binding protein gephyrin. However, direct interactions of GABA(A)R subunits with gephyrin have not been reported. Recently, the GABA(A)R-associated protein GABARAP was found to bind to the gamma2 subunit of GABA(A)Rs. Here we show that GABARAP interacts with gephyrin in both biochemical assays and transfected cells. Confocal analysis of neurons derived from wild-type and gephyrin-knockout mice revealed that GABARAP is highly enriched in intracellular compartments, but not at gephyrin-positive postsynaptic membrane specializations. Our data indicate that GABARAP-gephyrin interactions are not important for postsynaptic GABA(A)R anchoring but may be implicated in receptor sorting and/or targeting mechanisms. Consistent with this idea, a close homolog of GABARAP, p16, has been found to function as a late-acting intra-Golgi transport factor.

Distribution of GABA and glycine receptors on bipolar and ganglion cells in the mammalian retina

The amino acids GABA and glycine mediate synaptic transmission via specific neurotransmitter receptors. Molecular cloning studies have shown that there is a great diversity of GABA and glycine receptors. In the present article, the distribution of GABA and glycine receptors on identified bipolar and ganglion cell types in the mammalian retina is reviewed. Immunofluorescence obtained with antibodies against GABA and glycine receptors is punctate. Electron microscopy shows that the puncta represent a cluster of receptors at synaptic sites. Bipolar cell types were identified with immunohistochemical markers. Double immunofluorescence with subunit-specific antibodies was used to analyze the distribution of receptor clusters on bipolar axon terminals. The OFF cone bipolar cells seem to be dominated by glycinergic input, whereas the ON cone bipolar and rod bipolar cells are dominated by GABAergic input. Ganglion cells were intracellularly injected with Neurobiotin, visualized with Streptavidin coupled to FITC, and subsequently stained with subunit specific antibodies. The distribution and density of receptor clusters containing the alpha1 subunit of the GABA(A) receptor and the alpha1 subunit of the glycine receptor, respectively, were analyzed on midget and parasol cells in the marmoset (a New World monkey). Both GABA(A) and glycine receptors are distributed uniformly along the dendrites of ON and OFF types of parasol and midget ganglion cells, indicating that functional differences between these subtypes of ganglion cells are not determined by GABA or glycinergic input.