Voltage Activation Mechanisms of Epilepsy- and ASD-Associated Mutations in Neuronal KCNQ Chanels

Abstract

The voltage-gated potassium KCNQ2/3 channel mediates the M-current (IKM) which contributes to set the resting membrane potential, shapes action potentials, and hinders repetitive neuronal firing. Inherited mutations in the KCNQ2/3 channel has been linked to the onset of epileptic encephalopathy, which is often co-morbid in patients with autism spectrum disorder (ASD). However, the molecular mechanisms by which these mutations disrupt KCNQ2/3 channel function remain elusive. Moreover, because one in three patients fails to respond to available medications, there is a need to develop alternative therapies. Small lipophilic molecules have been shown to alleviate the symptoms of some intractable epileptic seizures, although the mechanisms are unknown. Here, we use voltage-clamp fluorometry (VCF) to simultaneously measure voltage sensor movement and gate opening in a set of epilepsy- and ASD-associated mutations in the voltage sensor (S4) of KCNQ2/3 channels expressed in Xenopus oocytes. We also investigate the effects of small lipophilic molecules on KCNQ2/3 channel activity and study how these molecules improve mutated channel function. We find that, compared to the wild type channel, the ASD-linked mutations in the S4 of KCNQ3 (R227Q and R236C) shift the voltage dependence of current G(V) and fluorescence F(V) activation towards negative and positive voltages, respectively. VCF also shows that, compared to wild type, the time courses of the fluorescence and current in R227Q and R236C are faster and slower, respectively. This suggests that whereas the mutation R227Q destabilizes the resting conformation of the S4, R236C mainly act by destabilizing the S4 activated state. Finally, we find a set of small molecules that partially reverse the defects caused by the mutations, thereby laying the groundwork for the development of mutation-specific therapies.