Activation of the unfolded protein response downregulates cardiac ion channels in human induced pluripotent stem cell-derived cardiomyocytes

Rationale: Heart failure is characterized by electrical remodeling that contributes to arrhythmic risk. The unfolded protein response (UPR) is active in heart failure and can decrease protein levels by increasing mRNA decay, accelerating protein degradation, and inhibiting protein translation. Objective: Therefore, we investigated whether the UPR downregulated cardiac ion channels that may contribute to arrhythmogenic electrical remodeling. Methods: Human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) were used to study cardiac ion channels. Action potentials (APs) and ion channel currents were measured by patch clamp recording. The mRNA and protein levels of channels and the UPR effectors were determined by quantitative RT-PCR and Western blotting. Tunicamycin (TM, 50 ng/mL and 5 μg/mL), GSK2606414 (GSK, 300 nmol/L), and 4μ8C (5 μmol/L) were utilized to activate the UPR, inhibit protein kinase-like ER kinase (PERK) and inositol-requiring protein-1 (IRE1), respectively. Results: TM-induced activation of the UPR caused significant prolongation of the AP duration (APD) and a reduction of the maximum upstroke velocity (dV/dt _max ) of the AP phase 0 in both acute (20–24 h) and chronic treatment (6 days). These changes were explained by reductions in the sodium, L-type calcium, the transient outward and rapidly/slowly activating delayed rectifier potassium currents. Na _v 1.5, Ca _v 1.2, K _v 4.3, and K _v LQT1 channels showed concomitant reductions in mRNA and protein levels under activated UPR. Inhibition of PERK or IRE1 shortened the APD and reinstated dV/dt _max . The PERK branch regulated Na _v 1.5, K _v 4.3, hERG, and K _v LQT1. The IRE1 branch regulated Na _v 1.5, hERG, K _v LQT1, and Ca _v 1.2. Conclusions: Activated UPR downregulates all major cardiac ion currents and results in electrical remodeling in hiPSC-CMs. Both PERK and IRE1 branches downregulate Na _v 1.5, hERG, and K _v LQT1. The PERK branch specifically downregulates K _v 4.3, while the IRE1 branch downregulates Ca _v 1.2. Therefore, the UPR contributed to electrical remodeling, and targeting the UPR might be anti-arrhythmic.