Illuminating Trafficking of KCNQ1/KCNE1 Channels in Heart

Abstract

In human heart, pore-forming KCNQ1 subunits associate with auxiliary KCNE1 subunits to generate the slowly activating, delayed rectifier potassium current, IKs. Decreases in IKs, due to either congenital mutations in KCNQ1/KCNE1 subunits or the adverse neuro-hormonal milieu accompanying heart failure, delay cardiac repolarization, leading to long QT syndrome (LQTS). LQTS predisposes to lethal ventricular arrhythmias and sudden cardiac death. How KCNQ1/KCNE1 channel trafficking in the heart proceeds and is regulated is unknown. However, understanding this process is critical both for fundamental mechanistic insights into LQTS, and for rational development of potential new therapies for this condition. We sought to develop optical tools that permit direct visualization of KCNQ1/KCNE1 subunit assembly and trafficking in heart cells. KCNQ1 and KCNE1 were tagged intra-cellularly with YFP and CFP, respectively, and extra-cellularly with a 13-residue high affinity α-bungarotoxin (BTX) binding site (BBS). The tagged proteins were viable as determined by functional electrophysiological assays in Chinese hamster ovary (CHO) cells– BBS-KCNQ1-YFP expressed alone gave rise to rapidly-activating outward currents, and co-expression with KCNE1-CFP resulted in the slowly activating kinetic signature of IKs. Cell surface BBS-tagged channel subunits were selectively detected with quantum dot. Optical pulse chase assays in human embryonic kidney (HEK 293) cells revealed that surface KCNQ1 channels undergo rapid endocytosis and recycling, and this process is largely unaffected by KCNE1. Tagged KCNQ1/KCNE1 subunits expressed in guinea pig heart cells using adenovirus were targeted to the surface sarcolemma, t-tubules and intercalated disks, similar to endogenous channels. Surprisingly, IKs amplitude remained unchanged with over-expression of either KCNQ1 or KCNE1, suggesting a tightly regulated system for maintaining KCNQ1/KCNE1 surface density.