New insights on the role of gephyrin in regulating both phasic and tonic GABAergic inhibition in rat hippocampal neurons in culture

Gephyrin is a tubulin-binding protein that acts as a scaffold for clustering glycine and GABAA receptors at postsynaptic sites. In this study, the role of gephyrin on GABAA receptor function was assessed at the post-translational level, using gephyrin-specific single chain antibody fragments (scFv-gephyrin). When expressed in cultured rat hippocampal neurons as a fusion protein containing a nuclear localization signal, scFv-gephyrin were able to remove endogenous gephyrin from GABAA receptor clusters. Immunocytochemical experiments revealed a significant reduction in the number of synaptic γ2-subunit containing GABAA receptors and a significant decrease in the density of the GABAergic presynaptic marker vesicular GABA transporter (VGAT). These effects were associated with a slow down of the onset kinetics, a reduction in the amplitude and in the frequency of miniature inhibitory postsynaptic currents (mIPSCs). The quantitative analysis of current responses to ultrafast application of GABA suggested that changes in onset kinetics resulted from modifications in the microscopic gating of GABAA receptors and in particular from a reduced entry into the desensitized state. In addition, hampering gephyrin function with scFv-gephyrin induced a significant reduction in GABAA receptor-mediated tonic conductance. This effect was probably dependent on the decrease in GABAergic innervation and in GABA release from presynaptic nerve terminals. These results indicate that gephyrin is essential not only for maintaining synaptic GABAA receptor clusters in the right position but also for regulating both phasic and tonic inhibition.