The Epilepsy-Related Protein PCDH19 Regulates Tonic Inhibition, GABA_AR Kinetics, and the Intrinsic Excitability of Hippocampal Neurons

PCDH19encodes for protocadherin-19 (PCDH19), a cell-adhesion molecule of the cadherin superfamily preferentially expressed in the brain.PCDH19mutations cause a neurodevelopmental syndrome named epileptic encephalopathy, early infantile, 9 (EIEE9) characterized by seizures associated with cognitive and behavioral deficits. We recently reported that PCDH19 binds the alpha subunits of GABA(A)receptors (GABA(A)Rs), modulating their surface availability and miniature inhibitory postsynaptic currents (mIPSCs). Here, we investigated whether PCDH19 regulatory function on GABA(A)Rs extends to the extrasynaptic receptor pool that mediates tonic current. In fact, the latter shapes neuronal excitability and network properties at the base of information processing. By combining patch-clamp recordings in whole-cell and cell-attached configurations, we provided a functional characterization of primary hippocampal neurons from embryonic rats of either sex expressing a specific PCDH19 short hairpin (sh)RNA. We first demonstrated that PCDH19 downregulation reduces GABA(A)R-mediated tonic current, evaluated by current shift and baseline noise analysis. Next, by single-channel recordings, we showed that PCDH19 regulates GABA(A)Rs kinetics without altering their conductance. In particular, GABA(A)Rs of shRNA-expressing neurons preferentially exhibit brief openings at the expense of long ones, thus displaying a flickering behavior. Finally, we showed that PCDH19 downregulation reduces the rheobase and increases the frequency of action potential firing, thus indicating neuronal hyperexcitability. These findings establish PCDH19 as a critical determinant of GABA(A)R-mediated tonic transmission and GABA(A)Rs gating, and provide the first mechanistic insights into PCDH19-related hyperexcitability and comorbidities.