Molecular and Functional Interaction between Protocadherin-γC5 and GABAA Receptors

We have found that the gamma 2 subunit of the GABA(A) receptor (gamma 2-GABA(A)R) specifically interacts with protocadherin-gamma C5 (Pcdh-gamma C5) in the rat brain. The interaction occurs between the large intracellular loop of the gamma 2-GABA(A)R and the cytoplasmic domain of Pcdh-gamma C5. In brain extracts, Pcdh-gamma C5 coimmunoprecipitates with GABA(A)Rs. In cotransfected HEK293 cells, Pcdh-gamma C5 promotes the transfer of gamma 2-GABA(A)R to the cell surface. We have previously shown that, in cultured hippocampal neurons, endogenous Pcdh-gamma C5 forms clusters, some of which associate with GABAergic synapses. Overexpression of Pcdh-gamma C5 in hippocampal neurons increases the density of gamma 2-GABA(A)R clusters but has no significant effect on the number of GABAergic contacts that these neurons receive, indicating that Pcdh-gamma C5 is not synaptogenic. Deletion of the cytoplasmic domain of Pcdh-gamma C5 enhanced its surface expression but decreased the association with both gamma 2-GABA(A)R clusters and presynaptic GABAergic contacts. Cultured hippocampal neurons from the Pcdh-gamma 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-gamma C5 expression with shRNA decreased gamma 2-GABA(A)R cluster density and GABAergic innervation. The results indicate that, although Pcdh-gamma C5 is not essential for GABAergic synapse formation or GABA(A)R clustering, (1) Pcdh-gamma C5 regulates the surface expression of GABAARs via cis-cytoplasmic interaction with gamma 2-GABA(A)R, and (2) Pcdh-gamma C5 plays a role in the stabilization and maintenance of some GABAergic synapses.