Sarco/Endoplasmic Reticulum Ca2+-ATPase is a more effective calcium remover than Sodium-Calcium Exchanger in human embryonic stem cell-derived cardiomyocytes

Abstract

Human pluripotent stem cell (hPSCs)-derived ventricular (V) cardiomyocytes (CMs) display immature Ca2+-handing properties with smaller transient amplitudes and slower kinetics due to such differences in crucial Ca2+-handling proteins as the poor sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) pumpbut robust Na-Ca exchanger (NCX) activities in human embryonic stem cell (ESC)-derived VCMs compared to adult. Despite their fundamental importance in excitation-contraction coupling, the relative contribution of SERCA and NCX to Ca2+-handling of hPSC-VCMs remains unexplored.We systematically altered the activities of SERCA and NCX in hESC-VCMs and their engineered micro-tissues, followed by examining the resultant phenotypic consequences. SERCA overexpression in hESC-VCMs shortened the decay of Ca2+transient at low frequencies (0.5Hz) without affecting the amplitude, SR Ca2+content and Ca2+ baseline. Interestingly, shRNA-based NCX suppression did not prolong the transient decay, indicating a compensatory response for Ca2+removal. Although hESC-VCMs and their derived microtissues exhibited negative frequency-transient/force responses, SERCA overexpression rendered them less negative at high frequencies (>2 Hz) by accelerating Ca2+sequestration. We conclude that forhESC-VCMs and their microtissues, SERCA, rather than NCX, is the main Ca2+remover during diastole; poor SERCA expression is the leading cause for immature negative-frequency/force responses, which can be partially reverted by forced expression. Combinatorial approach to mature calcium handling in hESC-VCMs may help shed further mechanistic insights.