Distinctive mechanisms of epilepsy-causing mutants discovered by measuring S4 movement in KCNQ2 channels

Neuronal KCNQ channels mediate the muscarine-regulated M-current, a key regulator of membrane excitability in the central and peripheral nervous systems. Mutations in KCNQ2 channels cause severe neurodevelopmental disorders, including epileptic encephalopathies. However, the impact that mutations have on channel function remain poorly defined, largely because of our limited understanding of the voltage sensing mechanisms that trigger channel gating. Here, we present measurements of voltage sensor movements in wt-KCNQ2 and channels bearing epilepsy-causing mutations using mutagenesis, cysteine accessibility, and voltage clamp fluorometry (VCF). Cysteine modification reveals that a stretch of 8-9 amino acids in the S4 become exposed upon opening of KCNQ2 channels. VCF shows that the voltage dependence and kinetics of S4 movement and channel opening/closing closely correlate, suggesting an activation scheme in which channel opening does not require multiple voltage-sensor movements. VCF and kinetic modeling reveal different mechanisms by which epilepsy-causing mutations affect KCNQ2 channel voltage-dependent gating. This study provides insight into KCNQ2 channel function, which will aid in uncovering the mechanisms underlying channelopathies.