The paradoxical effects of K+ channel gain-of-function are mediated by GABAergic neuron hypoexcitability and hyperconnectivity.

Gain-of-function (GOF) variants in K+ channels cause severe childhood epilepsies, but there are no mechanisms to explain how increased K+ currents lead to network hyperexcitability. Here, we introduced a human Na+-activated K+ (KNa) channel variant (KCNT1-Y796H) into mice and, using a multiplatform approach, found motor cortex hyperexcitability and early-onset seizures, phenotypes strikingly similar to those of human patients. Although the variant increased KNa currents in cortical excitatory and inhibitory neurons, there was a selective increase in the KNa current across subthreshold voltages in inhibitory neurons, particularly in those with non-fast spiking properties, resulting in impaired excitability and AP generation. We further observed evidence of synaptic rewiring associated with hyperexcitable networks, including increases in homotypic synaptic connectivity and the ratio of excitatory-to-inhibitory synaptic input. These findings support inhibitory neuron-specific mechanisms in mediating the epileptogenic effects of K+ channel GOF, offering cell-type-specific currents and effects as promising targets for therapeutic intervention.