Molecular and Functional Interaction between Protocadherin-γC5 and GABA_AReceptors

We have found that the γ2 subunit of the GABA A receptor (γ2-GABA A R) specifically interacts with protocadherin-γC5 (Pcdh-γC5) in the rat brain. The interaction occurs between the large intracellular loop of the γ2-GABA A R and the cytoplasmic domain of Pcdh-γC5. In brain extracts, Pcdh-γC5 coimmunoprecipitates with GABA A Rs. In cotransfected HEK293 cells, Pcdh-γC5 promotes the transfer of γ2-GABA A R to the cell surface. We have previously shown that, in cultured hippocampal neurons, endogenous Pcdh-γC5 forms clusters, some of which associate with GABAergic synapses. Overexpression of Pcdh-γC5 in hippocampal neurons increases the density of γ2-GABA A R clusters but has no significant effect on the number of GABAergic contacts that these neurons receive, indicating that Pcdh-γC5 is not synaptogenic. Deletion of the cytoplasmic domain of Pcdh-γC5 enhanced its surface expression but decreased the association with both γ2-GABA A R clusters and presynaptic GABAergic contacts. Cultured hippocampal neurons from the Pcdh-γ triple C-type isoform knock-out (TCKO) mouse ( Pcdhg tcko/tcko ) showed plenty of GABAergic synaptic contacts, although their density was reduced compared with sister cultures from wild-type and heterozygous mice. Knocking down Pcdh-γC5 expression with shRNA decreased γ2-GABA A R cluster density and GABAergic innervation. The results indicate that, although Pcdh-γC5 is not essential for GABAergic synapse formation or GABA A R clustering, (1) Pcdh-γC5 regulates the surface expression of GABA A Rs via cis -cytoplasmic interaction with γ2-GABA A R, and (2) Pcdh-γC5 plays a role in the stabilization and maintenance of some GABAergic synapses.