Stimulating Calcium Handling in hiPSC-Derived Engineered Cardiac Tissues Enhances Force Production

Human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) have profound utility in generating functional human engineered cardiac tissues (ECT) for heart repair. However, the field at large is concerned about the relative immaturity of these hiPSC-CMs as we aim to develop clinically relevant models for regenerative therapy and drug testing. Herein, we develop a novel calcium (Ca(2+)) conditioning protocol that maintains ECTs in a physiological range of Ca(2+) and assesses contractility in increasing calcium environments. Lactate-based selection served as a method to purify and shift the metabolic profile of hiPSC-CMs to evaluate the role of metabolism on Ca(2+) sensitivity. After 2 weeks, we observe 2-fold greater peak twitch stress in high-Ca(2+) conditioned ECTs, despite having lower stiffness and no change in Ca(2+) sensitivity of twitch force. Interestingly, the force-calcium relationship reveals higher Ca(2+) sensitivity in lactate conditioned tissues, suggesting that metabolic maturation alters mitochondrial Ca(2+) buffering and regulation. Ca(2+) sensitivity and force amplitude are not coupled, as lactate conditioned tissues produce force comparable to that of controls in high calcium environments. An upregulation of calcium handling protein gene expression likely contributes to the greater Ca(2+) sensitivity in lactate conditioned hiPSC-CMs. Our findings support the use of physiological Ca(2+) to enhance the functional maturation of excitation-contraction coupling in hiPSC-CMs and demonstrate that metabolic changes induced by lactate conditioning alter cardiomyocyte sensitivity to external Ca(2+). These conditioning methods may be used to advance the development of engineered human cardiac tissue for translational applications in vitro and in vivo as a regenerative therapy.