Examining the Causes and Consequences of Calcium Overload in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Abstract

Intracellular calcium overload has been linked to arrhythmias in conditions such as ischemia reperfusion, heart failure, catecholaminergic polymorphic ventricular tachycardia and digitalis intoxication. While this link has been extensively studied at the cellular level using animal models, there is a paucity of information on the causes and consequences of Ca2+ overload in healthy human myocytes. With the advent of induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), human myocytes are becoming more readily available. However, it remains unclear if these myocytes faithfully recapitulate all aspects of adult cardiomyocyte physiology. In order to determine if iPSC-CM will be a useful platform to study the causes and consequences of Ca2+ overload, we developed methodology to examine Ca2+ overload in beating clusters of iPSC-CM and in isolated myocytes. Spontaneous and field stimulated action potentials were measured with high resistance microelectrodes in spontaneously beating clusters of iPSC-CM while measuring cluster contraction with simultaneous video edge detection. Free intracellular Ca2+ was measured with fluo-4 and confocal microscopy in beating clusters and in isolated iPSC-CM. Ca2+ overload was induced in both preparations by treatment with ouabain (2.5-5 uM or with isoproterenol (1 uM) plus 5.4 mM extracellular Ca2+. Both maneuvers produced a rise in diastolic Ca2+ as well as the appearance of oscillatory action potentials, putative delayed afterdepolarizations and triggered activityies in beating clusters. These results indicate that iPS-derived cardiomyocytes provide a useful platform to study the mechanisms of Ca2+ overload-induced arrhythmias as well as possible treatments. However, given the spontaneous nature of iPS-derived cardiomyocytes, these cells may more readily recapitulate the effects of Ca2+ overload on sinoatrial or latent pacemaker cells.