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

Abstract

Neuronal KCNQ channels mediate the 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 different mutations have on channel function remains poorly defined, largely because of our limited understanding of the voltage sensing mechanisms that trigger channel gating. Here, we define parameters of voltage sensor movements in wt-KCNQ2 and channels bearing epilepsy-associated mutations using cysteine accessibility and voltage clamp fluorometry (VCF). Cysteine modification reveals that a stretch of 8-9 amino acids in the S4 become exposed upon VSD activation of KCNQ2 channels. VCF shows that the voltage dependence and the time course of S4 movement and channel opening/closing closely correlate. VCF reveals different mechanisms by which different epilepsy-associated mutations affect KCNQ2 channel voltage-dependent gating. This study provides insight into KCNQ2 channel function, which will aid in uncovering the mechanisms underlying channelopathies.

Electro-mechanical coupling of KCNQ channels is a target of epilepsy-associated mutations and retigabine

“…1A ) ( 7 , 8 , 37 , 41 ). Consistent with other studies ( 45 , 47 , 48 ), our VCF results showed that KCNQ3, unlike KCNQ1, exhibits a one-step VSD transition based on steady-state voltage-dependent activation ( Fig. 1, D and E ).…”

“…1D ). A recent VCF study of KCNQ2 showed a similar result ( 48 ). These results differ from the VCF recordings of KCNQ1, which exhibit two well-separated components in the F - V relation (F1 and F2) as the result of the two-step VSD activation ( Fig.…”

“…While the steady-state F - V relation only indicated one-step VSD movement for KCNQ2 and KCNQ3 ( Fig. 1E ) ( 45 , 47 , 48 ), it should be noted that the possibility of a transient “intermediate state” existing during VSD activation cannot be totally ruled out. It is possible that such a VSD state may be kinetically observable based on biexponential kinetics of VSD activation ( Fig.…”

“…In contrast, the KCNQ1 AO state exhibits a right-shifted G - V relation ( V 1/2 ~ +40 mV) and slower activation kinetics ( 12 – 18 , 64 ) compared to KCNQ2 and KCNQ3 ( Fig. 1, D and E ) ( 45 , 47 , 48 ). While these differences in the AO state activation properties may have important implications for the primary physiological roles of KCNQ2/3 (regulation of neuronal excitability) ( 25 , 26 ) versus KCNQ1 (cardiac action potential repolarization) ( 1 , 2 ), it remains unclear why the gating features of KCNQ2 and KCNQ3 resemble the KCNQ1 IO state, instead of the AO state.…”

The Voltage-Sensor S4 Rises to the Occasion in KCNQ2 Channel Activation

“…In the current article, Edmond et al investigate biophysical properties of KCNQ2 to elucidate how different mutations alter channel function. 7 They used 2 methods to investigate the components that are involved in voltage sensing. In each case, mutated KCNQ2 channel proteins are expressed in oocytes for electrophysiological analysis.…”