Intracellular Association of Glycine Receptor with Gephyrin Increases Its Plasma Membrane Accumulation Rate

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Abstract
Gephyrin, a tubulin-binding protein, is the core of inhibitory postsynaptic scaffolds stabilizing glycine receptors (GlyRs) and/or GABAAreceptors. Previous ultrastructural studiesin vivoandin vitrohave reported a localization of gephyrin to intracellular cisternas during development or after glycinergic denervation (Seitanidou et al., 1992; Colin et al., 1996, 1998). These data were compatible with a traffic of this cytoplasmic, but membrane-associated, protein together with membrane proteins such as GlyR after exocytosis and/or endocytosis pathways. We have now investigated the consequences of a GlyR–gephyrin interaction on the localization and the dynamics of these two molecules in African green monkey kidney cells (COS-7) cells and in neurons transfected with green fluorescent protein-tagged-gephyrin and myc-tagged GlyR α1subunits. In these experiments, myc-tagged GlyR α1contained, or did not contain, the gephyrin-binding sequence (βgb) of the GlyR β subunit. We report here that GlyR–gephyrin interaction localizes gephyrin to GlyR-containing organelles. Videomicroscopy and nocodazole treatment indicate that the movements of these vesicles are microtubule dependent. Expressing GlyR α1with a thrombin cleavage site between the myc-tag and the N terminal of the GlyR α1subunit (Rosenberg et al., 2001) allowed monitoring of newly inserted receptors in the cell surface. Using temperature changes to block GlyR in, and then release it from, the trans-Golgi network, we show that gephyrin accelerates the accumulation of GlyR at the cell surface. Therefore, our data strongly suggest that some GlyR clusters are associated with gephyrin on their way to the cell surface and that this association increases the accumulation of GlyR at the plasma membrane.