Calcium Handling in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Abstract

Fibroblasts from human skin biopsies can be reprogrammed into pluripotent stem cells (iPSC), which can then be coaxed to differentiate into myocytes with cardiac-specific properties (iPSC-CMs). The field of iPSCs is still in its infancy, but it is increasingly clear that the excitation-contraction coupling (ECC) machinery of differentiating CMs undergoes proportionally incremental complexity and it remains to be seen whether it reaches complete maturity in cultured cells. We used patch-clamp and confocal Ca2+ imaging for a comparative assessment of ECC in human iPSC-CM and adult cardiomyocytes. In the latter, entry of Ca2+ through the L-type Ca2+ channel (ICa) triggers rapid, uniform release of Ca2+ from the sarcoplasmic reticulum (SR) via CICR. In iPSC-CMs at early stages of differentiation, the current-voltage relationship for ICa is remarkably similar to that of adult cardiomyocytes, indicating that the appearance of a “trigger” for contraction is an early event in the ontogenesis of ECC that doesn’t hinder efficient generation of Ca2+ signals. However, primitive iPSC-CMs commonly exhibit a poorly developed SR, as assessed by their variegated response to caffeine and their great dependence on extracellular Ca2+ for contraction. Cells are mostly rounded and t-tubules are absent. As a result, [Ca2+]i transient waveforms appear non-uniform and start at the periphery of the cell, as is expected of a Ca2+ front with focal initiation that propagates later to the interior of the cell. At more advanced stages of differentiation, iPSC-CMs display fairly uniform Ca2+ fronts, suggesting fast propagation of external Ca2+ signals to the interior of the cell. Thus, by this coarse functional estimate, it is expected that iPSC-CMs become accurate models of cardiomyopathies at late stages of differentiation, but the developmental characteristics of ECC is unclear and warrants a systematic approach, which we are currently performing.