Depolarization block of inhibitory neurons impacts neuronal function in epileptic encephalopathy

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Contact PD/PI: Wengert, Eric Ryan. PROJECT SUMMARY SCN8A epileptic encephalopathy is a severe infantile epilepsy syndrome caused by mutations in the SCN8A gene encoding voltage-gated sodium channel isoform NaV1.6. NaV1.6 is not only expressed in excitatory neurons, where it is critically involved in action potential (AP) generation and propagation, but it is also expressed in inhibitory interneurons. Although they make up the minority (~15%) of cortical neurons, inhibitory interneurons in the neocortex powerfully sculpt network dynamics through various feed-forward, feed-back, and lateral inhibition circuit motifs. Previous work has implicated NaV1.6 dysfunction with abnormal excitability in excitatory neurons driven primarily by persistent and resurgent sodium currents. However, there have been no studies examining the effect of mutant NaV1.6 expression on inhibitory interneuron physiology and the subsequent contribution of these interneurons to behavioral seizures in SCN8A encephalopathy. This proposal seeks to test the hypothesis that inhibitory interneuron excitability is reduced in the Scn8aD/+ mouse model of SCN8A encephalopathy, leading to an increase in overall network excitability. My preliminary data suggest that somatostatin-positive inhibitory interneurons (SST) have reduced intrinsic excitability at high-firing frequencies due to entry into depolarization block, and have aberrantly large persistent sodium currents. In aim 1, I will record WT and Scn8aD/+ and fully characterize the voltage-gated sodium currents, intrinsic excitability, and alterations in synaptic physiology of the two major interneuron subpopulations: parvalbumin (PV) - and somatostatin (SST) -positive inhibitory interneurons. This aim will clarify the impact of a gain-of-function SCN8A mutation on interneuron function and network excitability. In aim 2, I will test the hypothesis that genetic knock-down of SCN8A specifically in inhibitory interneuron populations using a Cre-dependent shRNA will rescue the reduction in interneuron excitability, normalize the aberrant sodium channel physiology and have an impact on seizure frequency and severity in Scn8aD/+ mice. Overall, completion of these aims will resolve an important question in the field regarding how interneurons contribute to SCN8A encephalopathy and hopefully generate novel mechanistically-informed approaches to better treat SCN8A encephalopathy.