NaV1.4 mutations cause hypokalaemic periodic paralysis by disrupting IIIS4 movement during recovery

Abstract

Cations leaking through the voltage sensor of mutant sodium or calcium channels underlie hypokalaemic periodic paralysis. Groome et al. use muscle fibre recordings, voltage clamp, and molecular dynamics, to investigate recently discovered Nav1.4 channel mutations. They identify a novel voltage sensor movement that may explain the muscle pathology.

Domain III S4 in closed-state fast inactivation: Insights from a periodic paralysis mutation

“…Consistent with this hypothesis are findings from molecular dynamics simulations for rNa V 1.4 and rR1128C channels in silico, predicting that a limited translocation of the voltage sensor brings the third arginine in DIIIS4 through the gating pore constriction. 11 While our results show that increased charge movement for R3C at sub-threshold voltages is correlated with enhanced closed-state inactivation, it remains to be discerned what mechanisms couple that movement to the inactivation process. During recovery of skeletal muscle sodium channels, some portion of the activated gating charge is slow to return to a hyperpolarized favored position, a phenomenon termed charge immobilization.…”

“…11 In that study, we found that action potentials in R1135H patient muscle fibers were slow to develop and reduced in amplitude. Expression of that mutation and a novel R1135C mutation in mammalian cells and in oocytes revealed gating defects that include enhancement of fast and slow inactivation, prolonged recovery, depolarization-induced outward omega current and impaired deactivation.…”

“…11 We compared the effect of the rR1128C mutation (R3C; rat Na V 1.4 homolog to human R1135C) on fast inactivation and charge movement to explore the biophysical mechanisms underlying effects of that mutation on excitability. First, we confirmed that R3C, constructed in the rat Na V 1.4 isoform and expressed in oocytes, produces similar effects on fast inactivation as reported for R1135C in mammalian cells.…”

“…13 We previously showed that defective deactivation in rR1128H/C inhibits the initial phase of recovery from fast inactivation. 11 We wished to determine if the hypokalemic periodic paralysis R3C mutation also limits remobilization of the gating charge to prolong recovery from channels inactivated from closed, or from open states. To do this we compared recovery of channels to the kinetics of charge remobilization.…”

“…10 We characterized this (R1135H) and a novel cysteine mutation (R1135C) using heterologous expression in mammalian cells to study gating properties, and in Xenopus oocytes to study omega currents. 11 Here, we extend our analysis of the DIIIS4 R3C mutation to include investigation of charge movement and immobilization of the gating charge. We compared the effects of the R3C mutation on gating properties and gating currents with a focus on closed-state fast inactivation.…”