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

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.

The cytoskeleton and neurotransmitter receptors

The neuronal cytoskeleton consists of microtubules and microfilaments that can interact with membrane proteins including neurotransmitter receptors and ion channels. Ligand-gated ion channels, such as nicotinic acetylcholine receptors, glycine receptors, glutamate receptors and gamma-aminobutryic acidA (GABAA) receptors, are known to cluster in plasma membranes. Studies suggest that postsynaptic ligand-gated channels form clusters that are anchored in the plasma membrane by interacting with cytoskeletal components and these clusters may serve to optimize delivery of neurotransmitters to the channels. Other findings indicate that the interaction of clustered ligand-gated ion channels with cytoskeletal components may also play a role in channel function. For example, studies suggest that the interaction of microtubules with GABAA receptors regualtes GABA binding affinity. Regulation of neurotransmitter function may be significant in the study of neuropathological processes, such as Alzheimer's disease, neurotrauma, and experimental epilepsy, in which the cytoskeleton is vulnerable to disruption.

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.

Cerebellar granule cells in vitro recapitulate the in vivo pattern of GABAA-receptor subunit expression

GABAA-receptor heterogeneity is based on the combinatorial assembly of a family of 15 subunits (alpha 1-6, beta 1-3, gamma 1-3, delta, rho 1-2) into multiple receptor subtypes. The factors regulating the differential expression of GABAA-receptor subtypes in the CNS are largely unknown. In the present study, we have used primary cultures of rat cerebellar granule cells as model system to analyze to which extent the expression, subunit composition, and subcellular localization of GABAA-receptors depend on signals available in the cerebellum in vivo, in particular GABAergic input which is lost in vitro. GABAA-receptor subunits were visualized immunohistochemically with subunit-specific antibodies and their subcellular localization was investigated by confocal laser microscopy with selective markers for dendritic proteins (microtubule-associated protein 2, non-phosphorylated neurofilaments) and synaptic vesicles (synaptophysin). Triple immunofluorescence staining experiments revealed that dissociated granule cells taken from 7-day-old rats and maintained in culture for 9 days co-expressed the subunits alpha 1 alpha 6 beta 2,3 gamma 2 delta, a subunit repertoire which is identical to that seen in vivo in the adult cerebellum. GABAA-receptor subunits were localized selectively in granule cell somata and in proximal neurites immunopositive for MAP-2. No staining was detected in distal neurites immunopositive for neurofilaments. GABAA-receptor subunits frequently were aggregated in clusters closely apposed to synaptophysin-immunoreactive varicosities, suggesting a post-synaptic localization. Thus, major functional determinants of GABAA-receptors in granule cells (subunit repertoire, subcellular segregation and clustering in post-synaptic sites) develop in vitro, indicating that they are regulated to a large extent by intrinsic factors, independently of GABAergic input.

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.

Co-stratification of GABAA receptors with the directionally selective circuitry of the rat retina

Direction-selective (DS) ganglion cells of the mammalian retina have their dendrites in the inner plexiform layer (IPL) confined to two narrow strata. The same strata are also occupied by the dendrites of cholinergic amacrine cells which are probably presynaptic to the DS ganglion cells. GABA is known to play a crucial role in creating DS responses. We examined the types of GABAA receptors expressed by the cholinergic amacrine cells and also those expressed by their presynaptic and postsynaptic neurons, by applying immunocytochemical markers to vertical sections of rat retinas. Double-labelling experiments with antibodies against choline acetyltransferase (ChAT) and specific antibodies against different GABAA receptor subunits were performed. Cholinergic amacrine cells seem to express an unusual combination of GABAA receptor subunits consisting of alpha 2-, beta 1-, beta 2/3-, gamma 2-, and delta-subunits. Bipolar cells, which could provide synaptic input to the DS circuitry, were stained with antibodies against the glutamate transporter GLT-1. The axon terminals of these bipolar cells are narrowly stratified in close proximity to the dendritic plexus of displaced cholinergic amacrine cells. The retinal distribution of synaptoporin, a synaptic vesicle associated protein, was studied. Strong reduction of immunolabelling was observed in the two cholinergic strata. The anatomical findings are discussed in the context of models of the DS circuitry of the mammalian retina.