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

Abstract

Abstract PCDH19 encodes for protocadherin-19 (PCDH19), a cell-adhesion molecule of the cadherin superfamily preferentially expressed in the brain. PCDH19 mutations 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.

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