Differential hERG Subunit Transcription Drives Changes in IKr During Cardiac Maturation

Abstract

hERG conducts the cardiac repolarizing potassium current, IKr, which is one of the first currents to appear in the developing heart. Disrupting IKr is linked with sudden infant death syndrome and intrauterine fetal death. Despite this, the mechanisms regulating hERG during cardiac maturation and its impact on excitability in the immature heart are unknown. At least two subunits, hERG 1a and hERG 1b, comprise native hERG channels. Shifts in the relative abundance of hERG 1a and hERG 1b alter IKr gating kinetics and cardiac repolarization. Decreasing the relative abundance of hERG 1b slows IKr gating kinetics, reduces repolarizing IKr, and prolongs the action potential (AP) in cardiomyocytes derived from human induced pluripotent stem cells (hIPSC-CMs). A previous study demonstrated that hERG 1a mRNA was upregulated and hERG 1b mRNA downregulated in adult ventricular tissue compared to fetal ventricular tissue. We hypothesized that hERG subunits are selectively regulated to meet the physiological needs of the maturing myocardium. To define the mechanisms regulating hERG subunit abundance during cardiac maturation, we measured hERG 1a and hERG 1b mRNA and protein, as well as IKr and AP morphology in immature and matured hIPSC-CMs. We found that matured hIPSC-CMs had significantly slower IKr kinetics, increased peak tail IKr density, and reduced repolarizing charge conducted by IKr during a ventricular AP voltage command. In agreement with the observed shift in IKr kinetics, hERG 1a immunofluorescence significantly increased whereas hERG 1b immunofluorescence significantly decreased in matured hiPSC-CMs. Furthermore, qRT-PCR showed that upregulated Tbx20 levels selectively increased hERG 1a mRNA levels in matured hiPSC-CMs compared to immature hiPSC-CMs. These data demonstrate that cardiac maturation drives transcriptional subunit-specific regulation of hERG and modifies IKr. These data will help clinicians predict the pathogenicity of subunit-specific hERG variants during development.