Impaired excitability of fast-spiking neurons in a novel mouse model of <i>KCNC1</i> epileptic encephalopathy.

Abstract

The recurrent pathogenic variant <i>KCNC1-</i>p.Ala421Val (A421V) is a cause of developmental and epileptic encephalopathy characterized by moderate-to-severe developmental delay/intellectual disability, and infantile-onset treatment-resistant epilepsy with multiple seizure types, including myoclonic seizures. Yet, the mechanistic basis of this disease, and of the <i>KCNC1</i> disease spectrum, remains unclear. <i>KCNC1</i> encodes Kv3.1, a voltage-gated potassium channel subunit that is strongly and selectively expressed in neurons capable of generating action potentials at high frequency, including parvalbumin-positive fast-spiking GABAergic inhibitory interneurons in cerebral cortex (PV-INs) that are known to be important for cognitive function and plasticity as well as control of network excitation to prevent seizures. In this study, we generate a novel transgenic mouse model with conditional expression of the A421V pathogenic missense variant (<i>Kcnc1</i>-A421V/+ mice) to explore the specific physiological mechanisms of <i>KCNC1</i> developmental and epileptic encephalopathy. Our results indicate that global heterozygous expression of the A421V variant leads to cognitive impairment, epilepsy, and premature lethality. We observe decreased PV-IN cell surface expression of Kv3.1 via immunohistochemistry, decreased voltage-gated potassium current density in PV-INs using outside-out nucleated macropatch recordings in brain slice, and profound impairments in the intrinsic excitability of cerebral cortex PV-INs (but not excitatory neurons) via current-clamp electrophysiology. <i>In vivo</i> two-photon calcium imaging revealed altered activity in <i>Kcnc1</i>-A421V/+ PV-INs and excitatory cells, as well as hypersynchronous discharges correlated with brief paroxysmal movements that were subsequently shown to be myoclonic seizures on electroencephalography. We found alterations in PV-IN-mediated inhibitory neurotransmission in young adult but not juvenile <i>Kcnc1</i>-A421V/+ mice relative to wild-type controls. Together, these results establish the specific impact of the recurrent Kv3.1-A421V variant on neuronal excitability and synaptic physiology across development to drive network dysfunction underlying <i>KCNC1</i> epileptic encephalopathy.