Glycine-receptor activation is required for receptor clustering in spinal neurons

The ability of nerve cells to receive up to several thousands of synaptic inputs from other neurons provides the anatomical basis for information processing in the vertebrate brain. The formation of functional synapses involves selective clustering of neurotransmitter receptors at presumptive postsynaptic regions of the neuronal plasma membrane. Receptor-associated proteins are believed to be crucial for this process. In spinal neurons, synaptic targeting of the inhibitory glycine receptor (GlyR) depends on the expression of the anchoring protein gephyrin. Here we show that the competitive GlyR antagonist strychnine and L-type Ca2+-channel blockers inhibit the accumulation of GlyR and gephyrin at postsynaptic membrane areas in cultured rat spinal neurons. Our data are consistent with a model in which GlyR activation that results in Ca2+ influx is required for the clustering of gephyrin and GlyR at developing postsynaptic sites. Similar activity-driven mechanisms may be of general importance in synaptogenesis.

Development of adult-type inhibitory glycine receptors in the central auditory system of rats

Inhibitory synaptic activity is crucial for many aspects of acoustic information processing and mainly mediated by glycine and gamma-aminobutyric acid, the two principal inhibitory neurotransmitters in the auditory system. Glycine exerts its inhibitory action via binding to postsynaptic receptors existing in various isoforms. Here we have investigated the spatiotemporal distribution of adult-type, strychnine-sensitive glycine receptors (GlyRs) in the rat auditory system by using a specific antibody against the ligand-binding alpha1 GlyR subunit. In adults, alpha1 GlyRs were found at all relay stations of the auditory pathway except for the medial geniculate body and the auditory cortex. In most brainstem nuclei, labeling was characterized by dense clusters of heavily immunoreactive puncta outlining the somata and proximal dendrites, indicative of a powerful glycinergic inhibition. No alpha1 immunoreactivity was seen in the auditory system of fetal rats, consistent with results obtained by others in the spinal cord. At birth, labeling was weak and restricted to defined nuclei of the cochlear nuclear complex and the superior olivary complex. By postnatal day 8, labeling was seen in all brainstem nuclei. At the first appearance of immunoreactivity, alpha1 GlyRs were diffusely distributed on the neuronal surface, yet they became clustered with age, finally densely incrusting the somata and proximal dendrites between the 3rd and 4th postnatal week, when the mature pattern of immunoreactivity was established. We never observed an overexpression of alpha1 GlyRs or a transient appearance in areas that are devoid of the receptor in adults. The late formation of glycinergic synapses harboring the adult-type GlyRs in the auditory system, at a time when internuclear connections have already formed, indicates that alpha1 GlyRs do not participate in early synaptogenesis.

[Acute delusional psychosis and neuroleptic malignant syndrome after emigration of a Russian German patient]

Emigration is often followed by psychic disorders. The special issue of Germans from the GUS-States immigrating to Germany is presented. The modus of paranoid reaction is discussed along the biography and the criteria of ICD 10. The acute paranoid psychosis was complicated by a neuroleptic malignant syndrome.

Analysis of the promoter region of the murine gephyrin gene

The peripheral membrane protein gephyrin is essential for the postsynaptic localization of the inhibitory glycine receptor and is thought to anchor the receptor complex to the subsynaptic cytoskeleton. Since gephyrin transcripts are also found in non-neuronal tissues, we analyzed the 5'-flanking region of the gephyrin gene to identify its promoter region. This region contains several SP1 consensus sites, which may account for the widespread expression of gephyrin mRNA. Indeed, reporter gene constructs encompassing at least the two SP1 consensus sites upstream from the transcription start site were efficiently transcribed in both a fibroblast and a neuronal cell line. Our data are consistent with the widespread expression of the gephyrin gene in most mammalian tissues.

[Noise field screening. Results of a television field study for detection of visual field defects]

White noise field campimetry is able to transform usually negative scotomas immediately into perceivable visual field defects. As this method needs a monitor, the obvious solution was to evaluate the suitability of the noise field stimulus broadcasted to home TV sets. Therefore, in cooperation with the "Süddeutscher Rundfunk" (SDR) as well as several health insurance companies ("AOK Baden-Württemberg" and other "RVO-Kassen"), approximately 300,000 viewers were invited to take this test after receiving brief information and instruction. There were 531 calls for questionnaires to document the subjects' findings and the results of a subsequent ophthalmological examination. In most cases this clarified the perceived noise field defect. Out of 127 evaluable questionnaires, 78 cases did not show any relevant ophthalmological pathology; this held true especially for lesions of the visual pathway. However, in 49 persons the ophthalmologists detected pathological findings; in 20 of these, the scotomas were previously unknown to the physicians. Glaucomatous optic neuropathy and macular degeneration were most frequently diagnosed as causing the white noise field defects. Finally, some preliminary estimates of the costs and benefit of this study are presented.

The glycine binding site of the N-methyl-D-aspartate receptor subunit NR1: identification of novel determinants of co-agonist potentiation in the extracellular M3-M4 loop region

The N-methyl-D-aspartate (NMDA) subtype of ionotropic glutamate receptors is a heterooligomeric membrane protein composed of homologous subunits. Here, the contribution of the M3-M4 loop of the NR1 subunit to the binding of glutamate and the co-agonist glycine was investigated by site-directed mutagenesis. Substitution of the phenylalanine residues at positions 735 or 736 of the M3-M4 loop produced a 15- to 30-fold reduction in apparent glycine affinity without affecting the binding of glutamate and the competitive glycine antagonist 7-chlorokynurenic acid; mutation of both residues caused a >100-fold decrease in glycine affinity. These residues are found in a C-terminal region of the M3-M4 loop that shows significant sequence similarity to bacterial amino acid-binding proteins. Epitope tagging revealed both the N-terminus and the M3-M4 loop to be exposed extracellularly, whereas a C-terminal epitope was localized intracellularly. These results indicate that the M3-M4 loop is part of the ligand-binding pocket of the NR1 subunit and provide the basis for a refined model of the glycine-binding site of the NMDA receptor.

Expression of glycine receptor alpha subunits and gephyrin in cultured spinal neurons

The inhibitory glycine receptor is a pentameric membrane protein composed of alpha and beta subunits. In the postsynaptic membrane, the glycine receptor and the copurifying peripheral membrane protein gephyrin are clustered underneath glycine-releasing nerve terminals. Here, we describe the expression of gephyrin and the neonatal and adult glycine receptor alpha subunit isoforms alpha1 and alpha2 during in vitro differentiation of rat spinal neurons. Analysis by immunoassays and the reverse transcriptase-polymerase chain reaction showed that gephyrin and alpha subunit mRNA and protein levels exhibited a marked increase from 1 to 5 days in vitro, i.e. prior to the formation of functional synaptic contacts. Using confocal and standard immunofluorescence, we determined the number of immunoreactive cells and the cellular localization of the alpha subunits and gephyrin. At 3 days in vitro, glycine receptor immunoreactivity revealed by the monoclonal antibody mAb4a was found in < 10% of cells and was mainly localized intracellularly; in contrast, gephyrin was detected in in vitro, gephyrin was essentially localized at the neuronal surface. At this stage, the number of glycine receptor-positive cells approached that of gephyrin-containing neurons (50%), and glycine receptor antigen was found both intracellularly and at the periphery of the cells. The antibody mAb2b, which binds exclusively to the alpha1 subunit, revealed aggregates at the surface of a few neurons. At 10 days in vitro, glycine receptor and gephyrin staining was localized in clusters at the periphery of the soma and the neurites. This quantitative analysis corroborates temporal differences in the cellular distribution of gephyrin and glycine receptor alpha subunits, the former being accumulated first at the neuronal surface.

Targeting of glycine receptor subunits to gephyrin-rich domains in transfected human embryonic kidney cells

In adult spinal neurons inhibitory glycine receptors (GlyR) are localized at postsynaptic membrane specializations underlying glycinergic nerve terminals. The peripheral membrane protein gephyrin has been shown to be essential for the formation of postsynaptic GlyR clusters. Here, we coexpressed GlyR polypeptides and gephyrin in 293 cells and observed rerouting of hetero-oligomeric GlyR and its beta, but not of alpha subunits to intracellular gephyrin aggregates. A GlyR chimeric alpha 1/beta protein was also accumulated at these gephyrin aggregates, indicating that colocalization with gephyrin depends on cytoplasmic domains of the beta subunit. gamma-Aminobutyric acid type-A receptor (GABAAR) subunits were not targeted to intracellular gephyrin aggregates with the exception of the GABAAR beta 3 subunit, which partially colocalized with gephyrin. These data show that gephyrin alters the subcellular localization of the GlyR beta and, to some extent, GABAAR beta 3 subunits. Thus, gephyrin-binding subunits might target hetero-oligomeric ion channels to a gephyrin matrix underlying the differentiating postsynaptic membrane.

Identification of a gephyrin binding motif on the glycine receptor beta subunit

The tubulin-binding protein gephyrin copurifies with the inhibitory glycine receptor (GlyR) and is essential for its postsynaptic localization. Here we have analyzed the interaction between the GlyR and recombinant gephyrin and identified a gephyrin binding site in the cytoplasmic loop between the third and fourth transmembrane segments of the beta subunit. GlyR alpha subunits and GABAA receptor proteins failed to bind recombinant gephyrin. However, insertion of an 18 residue segment of the GlyR beta subunit into the GABAA receptor beta 1 subunit conferred gephyrin binding both in an overlay assay and in transfected mammalian cells. These results indicate that beta subunit expression is essential for the formation of a postsynaptic GlyR matrix.

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

Gephyrin is a protein that copurifies with the glycine receptor (GlyR) and is required for the clustering of GlyRs at postsynaptic sites. Previously, it was thought that antibody mAb 7a, directed against gephyrin, was a specific marker for GlyR. However, there is evidence that gephyrin can also be found at nonglycinergic synapses. Here, immunocytochemistry was applied to show this directly for the rat retina. Both gephyrin and different subunits of the gamma-aminobutyric acid (GABA)A receptor were localized to discrete puncta in the inner plexiform layer, and these puncta were shown by electron microscopy to represent synaptic sites. Double immunocytochemistry revealed that GABAA receptors and GlyRs are not colocalized. However, gephyrin and different subunits of GABAA receptors were found to occur at the same synapses. The amount of colocalization varied with the GABAA receptor subunit composition and was most extensive for the alpha 2 subunit, less for the alpha 3 subunit, and minimal for the alpha 1 subunit. The gephyrin present at GABAergic synapses of the retina might also be involved with clustering of receptors at the postsynaptic sites. Hence, localization of gephyrin can no longer be considered as a unique marker of glycinergic synapses.

The postsynaptic localization of the glycine receptor-associated protein gephyrin is regulated by the cytoskeleton

The mechanisms underlying the postsynaptic localization of neurotransmitter receptors are poorly understood. Recently, the peripheral membrane protein gephyrin has been shown to be essential for the formation of inhibitory glycine receptor clusters in cultured rat spinal cord neurons. In vitro gephyrin binds with high affinity to polymerized tubulin. Here, the interaction of gephyrin with different components of the cytoskeleton was investigated in primary cultures of rat spinal neurons. After treatment with alkaloids affecting the cytoskeleton, the morphology of post-synaptic gephyrin clusters was analyzed by confocal immunofluorescence microscopy. Depolymerization of microtubules by demecolcine reduced both the percentage of cells with postsynaptic gephyrin clusters and the number of clusters/cell. The size of the remaining gephyrin clusters was increased whereas their gephyrin density was significantly lower than under control conditions. Depolymerization of microfilaments by cytochalasin D in contrast generated smaller clusters of increased gephyrin density. Demecolcine also dispersed postsynaptic glycine receptor clusters as revealed by immunostaining with a specific monoclonal antibody. These findings support the view that in vivo gephyrin anchors receptor polypeptides to the cytoskeleton by a complex interaction with microtubules and microfilaments.

The inhibitory glycine receptor: architecture, synaptic localization and molecular pathology of a postsynaptic ion-channel complex

Significant progress has been made towards the identification of functional domains of the inhibitory glycine receptor. Several residues crucial for ligand binding, ion-channel properties and stoichiometric subunit assembly have been identified. A major recent advance has been the finding that the biogenesis of postsynaptic glycine receptor clusters requires the tubulin-binding protein, gephyrin. Another area of exciting research has focused on mutations of glycine receptor alpha and beta subunit genes, which have been found to be causal for different hereditary motor disorders.

Modulation by zinc ions of native rat and recombinant human inhibitory glycine receptors

1. The effect of the divalent cation Zn2+ on inhibitory glycine receptor (GlyR) currents was investigated in rat embryonic spinal cord neurons and Xenopus oocytes expressing recombinant GlyRs. 2. In cultured spinal neurons, Zn2+ potentiated glycine-induced whole-cell currents about 3-fold when applied extracellularly at concentrations of 0.5-10 microM. In contrast, higher concentrations (> 100 microM) of Zn2+ decreased the glycine response. 3. A similar biphasic modulation of glycine-induced currents by Zn2+ was also found with recombinant homo- and hetero-oligomeric GlyRs generated in Xenopus oocytes. Dose-response analysis showed that both the potentiating and inhibitory effects of Zn2+ result from changes in apparent agonist affinity. 4. Analysis of chimeric constructs of the GlyR alpha 1- and beta-subunits revealed that the positive and negative modulatory effects of Zn2+ are mediated by different regions of the alpha 1-subunit. 5. Our data indicate the existence of distinct high- and low-affinity Zn2+ binding sites on the ligand-binding alpha-subunits of the GlyR. These sites may be implicated in the regulation of synaptic efficacy within glycinergic pathways.

Glycine receptors in the retinas of normal and spastic mutant mice

PURPOSE

Spastic mutant mice have abnormal gait and righting behavior, and the responses of their retinal ganglion cells have recently been shown to be abnormal. The former defects have been linked to a reduction of glycine-receptor density in the spinal cord of spastic mutants, but the cause of the retinal defects has not yet been determined. The authors thus tested for reduced glycine-receptor density in the mutant retina by comparing the levels of glycine receptors in the retinas of spastic mutant mice with those found in normal mice.

METHODS

Indirect immunofluorescence histochemistry was employed, using monoclonal antibodies directed against the alpha- and beta-subunits of the receptor and against the 93-kd cytoplasmic receptor-associated protein, gephyrin.

RESULTS

In normal mice, all glycine-receptor antibodies labeled two laminae of the inner plexiform layer (IPL): a broad band in the distal third of the IPL and a narrow band in the middle of the IPL. Lighter labeling was also seen in the outer plexiform layer with these antibodies. In spastic mutant mice, the glycine-receptor labeling of the IPL was reduced markedly. However, the overall structure of the spastic mutant retina was not disrupted because the distribution and intensity of both a presynaptic marker (synaptophysin) and a marker for the rod bipolar cell (protein kinase C) in the mutant retina were indistinguishable from those in normal retinas.

CONCLUSIONS

The glycine-receptor distribution in normal mice was consistent with that previously reported for the rat and with the distribution of glycine responsiveness of dissociated rodent bipolar cells. The reduced levels of glycine receptors in spastic mice help explain the abnormal ganglion cell responses in the spastic mutant.

Differential expression of glycine receptor subunits in the retina of the rat: a study using immunohistochemistry and in situ hybridization

Immunohistochemistry and in situ hybridization were used to study the distribution of glycine receptor (GlyR) subunits and the GlyR-associated protein gephyrin in the rat retina. Monoclonal antibodies against the alpha and beta subunits of the GlyR and gephyrin showed a strong punctate labeling pattern in the inner plexiform layer. Glycine receptor mRNAs were found in the inner nuclear layer and the ganglion cell layer. The alpha 1 subunit mRNA is predominantly present in the outer half of the INL and on some but not all ganglion cells. GlyR alpha 2 subunit mRNA is predominantly present in the inner half of the INL and on nearly all cells in the ganglion cell layer. GlyR alpha 3-, GlyR beta-, and gephyrin-mRNAs are present in the entire INL and in cells in the ganglion cell layer. The differential expression of glycine receptor subunits indicates a functional diversity of glycine receptors in the retina.

Heterogeneous length and in-series arrangement of orbicularis oculi muscle: individual myofibers do not extend the length of the eyelid

Images

Importance of Arg-219 for correct biogenesis of alpha 1 homooligomeric glycine receptors

The inhibitory glycine receptor is characterized by a pentameric arrangement of subunits with four predicted transmembrane segments (M1-M4) each. Here, we have mutagenized arginine residues located at both termini of the alpha 1 subunit segment, M2, which lines the receptor's anion channel. No glycine-gated channel formation could be detected in the plasma membrane of expressing cells for any of the mutants. In addition, mutating the arginine at the cytoplasmic terminus of M2 (R219) generated proteins which were only core-glycosylated, retained within intracellular compartments, and aggregated to high molecular weight complexes. Thus, residue R219, which corresponds to an arginine/lysine conserved in other ligand-gated ion channel polypeptides, is essential for correct biogenesis of the receptor.

Gephyrin antisense oligonucleotides prevent glycine receptor clustering in spinal neurons

Each neuron in the mammalian brain carries many postsynaptic membrane specializations containing high densities of receptors that mediate signal transduction upon neurotransmitter release from the apposed nerve terminal. Little is known about the mechanisms by which receptors are transported to and anchored at postsynaptic sites, but extracellular as well as intracellular components may be involved. Ultrastructural studies have shown that the peripheral membrane protein gephyrin, which co-purifies with the postsynaptic inhibitory glycine receptor (GlyR) upon affinity chromatography, is situated on the cytoplasmic face of glycinergic postsynaptic membranes. Moreover, gephyrin binds with high affinity to polymerized tubulin and has been postulated to link the GlyR to the subsynaptic cytoskeleton. Here we report that treatment of rat spinal neurons in culture with gephyrin antisense oligonucleotides prevents the formation of GlyR clusters in the dendritic plasma membrane. Thus, gephyrin is essential for localizing the GlyR to presumptive postsynaptic plasma membrane specializations.

Widespread expression of gephyrin, a putative glycine receptor-tubulin linker protein, in rat brain

The peripheral membrane protein gephyrin co-purifies with the inhibitory postsynaptic glycine receptor (GlyR) of mammalian spinal cord. By immunoelectron microscopy, gephyrin has been localized at the cytoplasmic face of glycinergic postsynaptic membrane specializations. Here, we used specific monoclonal antibodies to demonstrate the presence of gephyrin in all regions of rat brain known to contain synapses and compared its histochemical distribution with that of GlyR antigens. In most brain structures, gephyrin is expressed independently of the GlyR alpha 1 subunit, but its distribution is very similar to the pattern obtained with mAb 4a, a monoclonal antibody recognizing the known GlyR alpha and beta subunits. Our data suggest a much wider distribution of gephyrin and GlyR proteins in the mammalian CNS than anticipated previously.

Distribution of gephyrin transcripts in the adult and developing rat brain

The peripheral membrane protein gephyrin copurifies with the inhibitory glycine receptor of mammalian spinal cord. It binds with high affinity to polymerized tubulin and has been implicated in the anchoring of the glycine receptor to cytoskeletal elements. Recently, cDNA cloning has identified variants of the gephyrin mRNA, which originate from alternative splicing of four exonic regions (cassettes 1-4). In this study, the expression patterns of gephyrin splice variants were determined in the adult and developing rat brain by in situ hybridization with synthetic oligonucleotide probes. Gephyrin transcripts were detected throughout the brain and spinal cord, with mRNAs containing cassette 2 (C2 transcripts) being predominant in adult animals. C3 and C4 transcripts were seen in cerebellar granule cells and in the dentate gyrus, whereas a C1 probe did not produce detectable hybridization signals. During development, C2 and C3 mRNAs were found in most brain regions. Generally, the spatial and temporal distribution of gephyrin transcripts is similar to that of the glycine receptor beta subunit mRNA reported previously.

Coexpression of the receptor-associated protein gephyrin changes the ligand binding affinities of alpha 2 glycine receptors

The inhibitory glycine receptor (GlyR) is a ligand-gated chloride channel protein, whose ligand binding alpha subunit occurs in several isoforms in the mammalian central nervous system. Here we show that coexpression of the GlyR-associated protein gephyrin changes the agonist and antagonist binding affinities of GlyRs generated by alpha 2 subunit expression in 293 kidney cells. Thus, a receptor-associated protein modifies the functional properties of a neurotransmitter receptor. This may contribute to an optimization of the postsynaptic neurotransmitter response.

Primary structure and alternative splice variants of gephyrin, a putative glycine receptor-tubulin linker protein

A 93 kd polypeptide associated with the mammalian inhibitory glycine receptor (GlyR) is localized at central synapses and binds with high affinity to polymerized tubulin. This protein, named gephyrin (from the Greek gamma epsilon phi upsilon rho alpha, bridge), is thought to anchor the GlyR to subsynaptic microtubules. Here we report its primary structure deduced from cDNA and show that corresponding transcripts are found in all rat tissues examined. In brain, at least five different gephyrin mRNAs are generated by alternative splicing. Expression of gephyrin cDNAs in 293 kidney cells yields polypeptides reactive with a gephyrin-specific antibody, which coprecipitate with polymerized tubulin. Thus, gephyrin may define a novel type of microtubule-associated protein involved in membrane protein-cytoskeleton interactions.

The 93-kDa glycine receptor-associated protein binds to tubulin

A peripheral membrane protein with a relative molecular mass of 93,000 Da is associated with cytoplasmic domains of the inhibitory glycine receptor of mammalian spinal cord. Here, evidence is given that this 93-kDa protein binds to polymerized tubulin. First, tubulin cofractionated with the 93-kDa protein upon affinity purification of the glycine receptor. Second, tubulin bound to the isolated 93-kDa protein in an overlay procedure. Third, in assays containing the purified glycine receptor, the 93-kDa protein as well as the glycine receptor alpha and beta subunits coassembled with tubulin and microtubules. The interaction of the 93-kDa protein with tubulin displayed high affinity (KD approximately 2.5 nM) and significant cooperativity (Hill coefficient approximately 2.1) and approached a stoichiometry of approximately 1:4 under saturating conditions. These data suggest that the 93-kDa protein anchors the glycine receptor at postsynaptic sites via binding to subsynaptic tubulin.