The I Ion Channel Activator Mefenamic Acid Requires KCNE1 and Modulates Channel Gating in a Subunit-Dependent Manner

The pairing of KCNQ1 and KCNE1 subunits together mediates the cardiac slow delayed rectifier current (I ), which is partly responsible for cardiomyocyte repolarization and physiologic shortening of the cardiac action potential. Mefenamic acid, a nonsteroidal anti-inflammatory drug, has been identified as an I activator. Here, we provide a biophysical and pharmacological characterization of mefenamic acid's effect on I Using whole-cell patch clamp, we show that mefenamic acid enhances I activity in both a dose- and stoichiometry-dependent fashion by changing the slowly activating and deactivating I current into an almost linear current with instantaneous onset and slowed tail current decay, sensitive to the I blocker (3R,4S)-(+)-N-[3-hydroxy-2,2-dimethyl-6-(4,4,4-trifluorobutoxy) chroman-4-yl]-N-methylmethanesulfonamide (HMR1556). Both single channels, which reveal no change in the maximum conductance, and whole-cell studies, which reveal a dramatically altered conductance-voltage relationship despite increasingly longer interpulse intervals, suggest mefenamic acid decreases the voltage sensitivity of the I channel and shifts channel gating kinetics toward more negative potentials. Modeling studies revealed that changes in voltage sensor activation kinetics are sufficient to reproduce the dose and frequency dependence of mefenamic acid action on I channels. Mutational analysis showed that mefenamic acid's effect on I required residue K41 and potentially other surrounding residues on the extracellular surface of KCNE1, and explains why the KCNQ1 channel alone is insensitive to up to 1 mM mefenamic acid. Given that mefenamic acid can enhance all I channel complexes containing different ratios of KCNQ1 to KCNE1, it may provide a promising therapeutic approach to treating life-threatening cardiac arrhythmia syndromes. SIGNIFICANCE STATEMENT: The channels which generate the cardiac slow delayed rectifier K⁺ current (I ) are composed of KCNQ1 and KCNE1 subunits. Due to the critical role played by I in heartbeat regulation, enhancing I current has been identified as a promising therapeutic strategy to treat various heart rhythm diseases. Most I activators, unfortunately, only work on KCNQ1 alone and not the physiologically relevant I channel. We have demonstrated that mefenamic acid can enhance I in a dose- and stoichiometry-dependent fashion, regulated by its interactions with KCNE1.