ML277 specifically enhances pore opening of KCNQ1 with VSD at the activated state by modulating VSD-pore coupling

In response to membrane depolarization, the KCNQ1 potassium channel opens at the intermediate (IO) and activated (AO) states that correspond to the stepwise activation of the voltage sensing domain (VSD) to the intermediate (I) and activated (A) states. In the heart, KCNQ1 associates with the auxiliary subunit KCNE1 to form the IKs channel that regulates heart rhythm. Hundreds of loss-of-function KCNQ1 mutations cause long QT syndrome (LQTS). KCNE1 suppresses the IO state so that the IKs channel opens only to the AO state. Thus, enhancing AO state presents a potential therapy for anti-LQTS. Here, we systematically tested modulations of KCNQ1 channels by a recently identified KCNQ1 activator, ML277. It enhances the current amplitude, slows down activation, deactivation and inactivation kinetics, shifts the voltage dependence of activation to more positive voltages, decreases the Rb+/K+ permeability ratio, and selectively increases currents of mutant KCNQ1 channels that open only to the AO state. All these observations are consistent with the mechanism that ML277 specifically enhances the AO state. On the other hand, ML277 does not affect the VSD activation, suggesting that it enhances the AO state by altering the electromechanical (E-M) coupling when the VSD moves to the activated state. Our results suggest that ML277 provides a unique tool to investigate the gating mechanism of KCNQ1 and IKs channels. The specificity of ML277 to enhance the AO state of native IKs currents also suggests a new strategy for anti-LQTS therapy.