Mechanisms by which atrial fibrillation‐associated mutations in the S1 domain of KCNQ1 slow deactivation of I_Ks channels

Abstract

The slow delayed rectifier K + current (I Ks ) is a major determinant of action potential repolarization in the heart. I Ks channels are formed by coassembly of pore-forming KCNQ1 α-subunits and ancillary KCNE1 β-subunits. Two gain of function mutations in KCNQ1 subunits (S140G and V141M) have been associated with atrial fibrillation (AF). Previous heterologous expression studies found that both mutations caused I Ks to be instantaneously activated, presumably by preventing channel closure. The purpose of this study was to refine our understanding of the channel gating defects caused by these two mutations located in the S1 domain of KCNQ1. Site-directed mutagenesis was used to replace S140 or V141 with several other natural amino acids. Wild-type and mutant channels were heterologously expressed in Xenopus oocytes and channel function was assessed with the two-microelectrode voltage clamp technique. Long intervals between voltage clamp pulses revealed that S140G and V141M KCNQ1-KCNE1 channels are not constitutively active as previously reported, but instead exhibit extremely slow deactivation. The slow component of I Ks deactivation was decreased 62-fold by S140G and 140-fold by the V141M mutation. In addition, the half-point for activation of these mutant I Ks channels was ∼50 mV more negative than wild-type channels. Other substitutions of S140 or V141 in KCNQ1 caused variable shifts in the voltage dependence of activation, but slowed I Ks deactivation to a much lesser extent than the AF-associated mutations. Based on a published structural model of KCNQ1, S140 and V141 are located near E160 in S2 and R237 in S4, two charged residues that could form a salt bridge when the channel is in the open state. In support of this model, mutational exchange of E160 and R237 residues produced a constitutively open channel. Together our findings suggest that altered charge-pair interactions within the voltage sensor module of KCNQ1 subunits may account for slowed I Ks deactivation induced by S140 or V141.

Single-channel basis for the slow activation of the repolarizing cardiac potassium current, I Ks

“…The very slow deactivation of this channel at −40 mV suggests that the channel is stabilized in the open conformation at this potential, similar to other gain-of-function mutations described in KCNQ1 (31). The shorter component of open dwell times was twice as long as in WT (25.9 ± 5.0 ms vs. 14.3 ± 2.7 ms), and the longer component to opening was about 1.5 times as long (69.9 ± 4.8 vs. 47.0 ± 8.2 ms), confirming this hypothesis.…”

Pitx2 prevents susceptibility to atrial arrhythmias by inhibiting left-sided pacemaker specification

“…Moreover, separate mutations in Kcnq1-a serine-to-proline (S209P) substitution and a valine-to-methionine (V141M) substitution-also were reported in familial AF (37,38). In vitro experiments indicate that the gain-of-function mutations in Kcnq1 may be caused by defects in channel inactivation (39). Our data indicate that Kcnq1 is up-regulated in Pitx2c mutants.…”

Location of KCNE1 relative to KCNQ1 in the I KS potassium channel by disulfide cross-linking of substituted cysteines

“…Xu et al (23) proposed that ''wobble'' of E1 between S1 and S4 might contribute to the unique slow activation of I KS . Restier et al (40) proposed that E1 might slow activation by altering the making and breaking of salt bridges between S2 and S4, presumably through an indirect interaction with S1 within the same voltage-sensor bundle. Additionally, they investigated the effects of the gain-of-function atrial fibrillation associated mutations in S1, S140G and V141M, and proposed that these mutations might exert their effect through altered docking between S1 and E1, which, in turn, alters interactions between E1 and S5 and S6.…”