Maximizing Force Production from hiPSC-Derived Engineered Cardiac Tissues By Stimulating Calcium Handling

Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) have profound utility in generating functional human engineered cardiac tissue (ECTs) 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. Addressing the calcium handling properties and metabolic maturity will improve excitation-contraction (E-C) coupling and E-C metabolism coupling in hiPSC-CMs to facilitate functional integration in vivo. Herein, we develop a novel calcium (Ca²⁺) conditioning protocol that maintains ECTs in physiological levels of Ca²⁺ and assess contractility in increasing calcium environments. Lactate selection served as a method to shift the metabolic profile of hiPSC-CMs to evaluate the role of metabolism on Ca²⁺ sensitivity. We observe two-fold greater contractility in the calcium conditioning group with lower stiffness. Interestingly, the force-calcium relationship reveals higher calcium sensitivity in lactate conditioned tissues suggesting that metabolic maturation alters mitochondrial Ca²⁺ buffering capacity. We find that calcium sensitivity and amplitude of contraction are not coupled, as lactate conditioned tissues produce force comparable to that of controls in high calcium environments. An upregulation of calcium handling proteins further contributes to the greater Ca²⁺ sensitivity in lactate conditioned hiPSC-CMs. Our findings support the use of physiological Ca²⁺to enhance the functional maturation of E-C coupling in hiPSC-CMs and demonstrate that metabolic changes induced by lactate conditioning significantly alters cell sensitivity to external calcium. These conditioning methods may further the development of 3D human engineered cardiac tissue as a potential regenerative therapy.