Ultrarapid Delayed Rectifier K(+) Channelopathies in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Atrial fibrillation (AF) is the most common cardiac arrhythmia. About 5–15% of AF patients have a mutation in a cardiac gene, including mutations in KCNA5, encoding the K(v)1.5 α-subunit of the ion channel carrying the atrial-specific ultrarapid delayed rectifier K(+) current (I(Kur)). Both loss-of-function and gain-of-function AF-related mutations in KCNA5 are known, but their effects on action potentials (APs) of human cardiomyocytes have been poorly studied. Here, we assessed the effects of wild-type and mutant I(Kur) on APs of human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). We found that atrial-like hiPSC-CMs, generated by a retinoic acid-based differentiation protocol, have APs with faster repolarization compared to ventricular-like hiPSC-CMs, resulting in shorter APs with a lower AP plateau. Native I(Kur), measured as current sensitive to 50 μM 4-aminopyridine, was 1.88 ± 0.49 (mean ± SEM, n = 17) and 0.26 ± 0.26 pA/pF (n = 17) in atrial- and ventricular-like hiPSC-CMs, respectively. In both atrial- and ventricular-like hiPSC-CMs, I(Kur) blockade had minimal effects on AP parameters. Next, we used dynamic clamp to inject various amounts of a virtual I(Kur), with characteristics as in freshly isolated human atrial myocytes, into 11 atrial-like and 10 ventricular-like hiPSC-CMs, in which native I(Kur) was blocked. Injection of I(Kur) with 100% density shortened the APs, with its effect being strongest on the AP duration at 20% repolarization (APD(20)) of atrial-like hiPSC-CMs. At I(Kur) densities < 100% (compared to 100%), simulating loss-of-function mutations, significant AP prolongation and raise of plateau were observed. At I(Kur) densities > 100%, simulating gain-of-function mutations, APD(20) was decreased in both atrial- and ventricular-like hiPSC-CMs, but only upon a strong increase in I(Kur). In ventricular-like hiPSC-CMs, lowering of the plateau resulted in AP shortening. We conclude that a decrease in I(Kur), mimicking loss-of-function mutations, has a stronger effect on the AP of hiPSC-CMs than an increase, mimicking gain-of-function mutations, whereas in ventricular-like hiPSC-CMs such increase results in AP shortening, causing their AP morphology to become more atrial-like. Effects of native I(Kur) modulation on atrial-like hiPSC-CMs are less pronounced than effects of virtual I(Kur) injection because I(Kur) density of atrial-like hiPSC-CMs is substantially smaller than that of freshly isolated human atrial myocytes.