The mechanism of action of external protons on hERG potassium channels

Myocardial ischemia occurs when there is a reduction of blood flow to the heart due to blockage of an artery, causing inefficient delivery of oxygen and nutrients to the heart muscle. Acidosis is one of the major consequences during myocardial ischemia and affects a wide array of ion channels in the heart that may predispose individuals to cardiac arrhythmia. The human ether-à-go-go (hERG) related gene encodes the potassium channel that conducts the IKr current, which is responsible for the repolarization phase of the cardiac action potential and is particularly sensitive to external acidosis. External acidosis had been well observed to reduce hERG channel overall conductance, right-shift the voltage-dependence of activation, and accelerate deactivation rate, all of which lead to a loss-of-function effect on the hERG channels. We hypothesized that this can prolong action potential duration, increase susceptibility of individuals to long QT syndrome and reduce the protective current against premature ectopic beat. My first study aimed to determine the site and mechanism for the right-shift in the voltage-dependence of activation. I found that external protons disrupt stable interactions formed by the cation binding pocket, compose of three acidic residues: D456, D460 in S2, and D509 in S3, with positive charges in S4 to destabilize the activated voltage sensor. My second study aims to determine the site and mechanism for the acceleration of deactivation induced by external protons. Using voltage-clamp fluorimetry and gating current measurement with long duration protocols to measure voltage sensor mode-shift, we determined that external protons reduced the voltage sensor mode-shift by right shifting the voltage-dependence of deactivation suggesting that the relaxed conformation of the voltage sensor is destabilized and that D509 is a critical protonation site. In my last study, I used zebrafish heart as a translational model, with the optical mapping technique to investigate the effect of external protons on the overall cardiac action potential. We discovered that at low pH, the cardiac action potential is significantly prolonged, triangulation is increased, and the electrical restitution curve is flattened, which are all predictors for arrhythmogenicity.