A cardiomyocyte show of force: A fluorescent alpha-actinin reporter line sheds light on human cardiomyocyte contractility versus substrate stiffness

Cardiovascular disease is often associated with cardiac remodeling, including cardiac fibrosis, which may lead to increased stiffness of the heart wall. This stiffness in turn may cause subsequent failure of cardiac myocytes, however the response of these cells to increased substrate stiffness is largely unknown. To investigate the contractile response of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) to increased substrate stiffness, we generated a stable transgenic human pluripotent stem cell line expressing a fusion protein of α-Actinin and fluorescent mRubyII in a previously characterized NKX2.5-GFP reporter line. Cardiomyocytes differentiated from this line were subjected to a substrate with stiffness ranging from 4 kPa to 101 kPa, while contraction of sarcomeres and bead displacement in the substrate were measured for each single cardiomyocyte. We found that sarcomere dynamics in hPSC-CMs on polyacrylamide gels of increasing stiffness are not affected above physiological levels (21 kPa), but that contractile force increases up to a stiffness of 90 kPa, at which cell shortening, deducted from bead displacement, is significantly reduced compared to physiological stiffness. We therefore hypothesize that this discrepancy may be the cause of intracellular stress that leads to hypertrophy and consequent heart failure in vivo.