Calcium Handling in Human Induced Pluripotent Stem Cell Derived Cardiomyocytes

Background: The ability to establish human induced pluripotent stem cells (hiPSCs) by reprogramming of adult fibroblasts and to coax their differentiation into cardiomyocytes opens unique opportunities for cardiovascular regenerative and personalized medicine. In the current study, we investigated the Ca2+-handling properties of hiPSCs derived-cardiomyocytes (hiPSC-CMs). Methodology/Principal Findings: RT-PCR and immunocytochemistry experiments identified the expression of key Ca2+-handling proteins. Detailed laser confocal Ca2+ imaging demonstrated spontaneous whole-cell [Ca2+](i) transients. These transients required Ca2+ influx via L-type Ca2+ channels, as demonstrated by their elimination in the absence of extracellular Ca2+ or by administration of the L-type Ca2+ channel blocker nifedipine. The presence of a functional ryanodine receptor (RyR)-mediated sarcoplasmic reticulum (SR) Ca2+ store, contributing to [Ca2+](i) transients, was established by application of caffeine (triggering a rapid increase in cytosolic Ca2+) and ryanodine (decreasing [Ca2+](i)). Similarly, the importance of Ca2+ reuptake into the SR via the SR Ca2+ ATPase (SERCA) pump was demonstrated by the inhibiting effect of its blocker (thapsigargin), which led to [Ca2+](i) transients elimination. Finally, the presence of an IP3-releasable Ca2+ pool in hiPSC-CMs and its contribution to whole-cell [Ca2+](i) transients was demonstrated by the inhibitory effects induced by the IP3-receptor blocker 2-Aminoethoxydiphenyl borate (2-APB) and the phosopholipase C inhibitor U73122. Conclusions/Significance: Our study establishes the presence of a functional, SERCA-sequestering, RyR-mediated SR Ca2+ store in hiPSC-CMs. Furthermore, it demonstrates the dependency of whole-cell [Ca2+](i) transients in hiPSC-CMs on both sarcolemmal Ca2+ entry via L-type Ca2+ channels and intracellular store Ca2+ release.