Key result
MYH7 E525K mutation reduces single-cell contraction ~65% despite increasing maximal myofibril force.
Why the study?
The cardiac beta-myosin E525K mutation was linked to DCM, but its effects on contractile function had not been measured in myofilaments or cardiomyocytes.
The E525K mutation causes hypocontractility in cardiomyocytes through reduced sarcomere organization and increased IHM stability, despite increased maximal isometric force in isolated myofibrils.
E525K iPSC-cardiomyocytes show hypocontractility without kinetic changes; leaves open structural mechanisms in DCM pathogenesis for validation.
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 Ca 2+ activated isometric force in isolated myofibrils increased by 45% and sub-maximal Ca 2+ 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. Graphical Abstract A model for how the E525K mutation impacts contracting myofibrils Here, we show that the E525K mutation impacts contraction in multiple ways: (1) Decreased sarcomere organization in cells and tissues drives decreased force generation. (2) Increased binding affinity of E525K myosin for actin and faster Pi release contributes to increased force generation in isolated myofibrils. (3) Increased IHM stability reduces twitch force. (4) The rate-limiting step of loaded crossbridge cycling, ADP release, is unchanged by the E525K mutation, and the rates of loaded contraction and relaxation are unchanged at all scales of contraction.
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Robeson et al. (2026) studied Dilated cardiomyopathy. E525K β-myosin mutation vs. Wild type (WT) was evaluated on Contractile function (single-cell contraction and isometric twitch force). The E525K mutation decreased single-cell contraction by 65% and isometric twitch force by 39%, but increased maximal Ca2+ activated isometric force in isolated myofibrils by 45%.
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