A Tuned Tension Regulates the Contractility of Cardiomyocytes Differentiated from Induced Pluripotent Stem Cells

The ability to differentiate human cardiomyocytes (heart muscle cells) from induced pluripotent stem cells (iPSCs) presents high potential to model heart contractility. Shortening of sarcomeres in series along intracellular myofibrils enables the beating of cardiomyocytes. We developed a platform with patterned single iPSC-cardiomyocytes in arrays and tested the contractility of these cells as a function of their shape and of substrate stiffness. We fluorescently labelled sarcomeres with LifeAct to analyze their shortening and organization. We measured the mechanical output of contractile cycles with traction force microscopy and adapted cross-correlation algorithms to characterize movement of sarcomeres. We assayed single cells on patterns of 2000 μm2 with aspect ratios (length:width) ranging from 1:1 to 7:1 and on substrates with varied stiffnesses: 6 kPa, 10 kPa and 35 kPa. Preliminary studies suggested that cell morphology and substrate stiffness affect the organization of sarcomeres with a potential impact in cardiomyocyte contractility. We aimed to understand how the contractility of iPSC-cardiomyocytes is affected by the improved organization of sarcomeres induced by these cues. For a substrate stiffness of 10 kPa, we observed that cells with a 7:1 aspect ratio produced higher contractile forces per μm of sarcomere shortening. Improved sarcomere alignment seemed to drive increased contractility of cells with this shape. Substrate stiffness also affected the contractility of 7:1 iPSC-cardiomyocytes. 35 kPa substrates induced sarcomere ruptures and lower contractile forces. Cells on 6 kPa substrates developed sarcomeres that buckled. These results suggested a role of intracellular tension in contractility. We further tested how tension affects contractility and sarcomere organization in 7:1 cells through cell stretching, culture in high calcium and by inhibiting non-muscle myosin. Results showed evidence that a tuned intracellular tension mechanism drives myofibril alignment and improved contractility in iPSC-cardiomyocytes.