GABAergic Terminals Are Required for Postsynaptic Clustering of Dystrophin but Not of GABA_A Receptors and Gephyrin

In rat hippocampal cultures, we show by multilabeling immunocytochemistry that pyramidal cells, which receive little or no GABAergic input, mistarget α₂-GABA_A receptors and gephyrin to glutamatergic terminals. This mismatch does not occur in neurons innervated by numerous GABAergic terminals. A similar phenomenon has been reported for isolated autaptic hippocampal neurons (Rao et al., 2000). GABAergic synapses typically form multiple release sites apposed to GABA_A receptor and gephyrin clusters. Remarkably, dystrophin, a protein highly abundant in skeletal muscle membranes, is extensively colocalized with α₂-GABA_A receptors exclusively opposite GABAergic terminals. In addition, selective apposition of syntrophin and β-dystroglycan to GABAergic presynaptic terminals suggests that the entire dystrophin-associated protein complex (DPC) clusters at GABAergic synapses. In contrast to gephyrin and GABA_A receptors, DPC proteins are not mistargeted to glutamatergic synapses, indicating independent clustering mechanisms. This was confirmed in hippocampal neurons cultured from GABA_A receptor γ₂ subunit-deficient mice. Clustering of GABA_A receptor and gephyrin in these neurons was strongly impaired, whereas clustering of dystrophin and associated proteins was unaffected by the absence of the γ₂ subunit. Our results indicate that accumulation of dystrophin and DPC proteins at GABAergic synapses occurs independently of postsynaptic GABA_A receptors and gephyrin. We suggest that selective signaling from GABAergic terminals contributes to postsynaptic clustering of dystrophin.