The E525K β-Myosin Mutation Causes Hypocontractility in Cardiomyocytes Without Altering Loaded Crossbridge Cycling

The cardiac β-myosin (MYH7) mutation E525K was identified in 2012 in a patient with dilated cardiomyopathy (DCM). Work using engineered constructs has shown that this mutation stabilizes the interacting heads motif (IHM) of myosin and increases the ATPase activity of mutant motor S1 heads. However, no measurements have been made in myofilaments or cardiomyocytes to determine its effect on contractile function. Here, we present force and contractile kinetics measurements from induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) engineered for heterozygous expression of E525K. Single-cell contraction for E525K hiPSC-CMs decreased by 65%, and isometric twitch force in engineered heart tissues (EHTs) decreased by 39%. In contrast, maximal Ca²⁺ activated isometric force in isolated myofibrils increased by 45% and sub-maximal Ca²⁺ activated force was similar to WT myofibrils. Structural analysis revealed reduced myofibril content (13.7% decrease) and decreased organization (increased z-disk dispersion angle) in E525K cells. We confirmed that E525K S1 myosin has higher actin affinity than WT S1 and elevated ATPase activity. However, there was no change in S1 ADP release rate. There was also no change in either the rate of force development or relaxation in myofibrils, cells, or EHTs. These findings suggest that E525K myosin crossbridge cycling is not altered during loaded contractions. To understand how twitch force of myocytes was reduced but maximal isometric force was increased, we used a spatially explicit sarcomere model. The results were explained by changing three rates: reduced recruitment from the OFF/IHM state, and increased rates of actin binding and Pi release. Additional force deficits in cells and EHTs likely result from the myofibrillar disorganization. This study demonstrates the value of multi-scale analysis and coupled, computational modeling to understand the molecular mechanisms of sarcomere mutations in cardiomyocytes.