Pharmacological Inhibition of β Myosin II Disrupts Sarcomere Assembly in Human iPSC-Derived Cardiac Myocytes

Sarcomeres are the fundamental contractile units of striated muscle. The functional roles of the cardiac-specific myosin heavy chains, MYH6 (α myosin II) and MYH7 (β myosin II) during sarcomere assembly remain controversial. To address this, we utilized a selective MYH7 inhibitor, mavacamten, in combination with siRNA-mediated knockdown of MYH6 or MYH7 in human induced pluripotent stem cell-derived cardiomyocytes (hiCMs). Our results demonstrate that sarcomere assembly proceeds when either MYH6 or MYH7 is independently depleted, suggesting functional redundancy. However, pharmacological inhibition of MYH7 contractility by mavacamten disrupts sarcomere assembly in a concentration-dependent manner. Sensitivity to mavacamten correlated with the relative abundance of MYH6 and MYH7: sarcomere assembly by MYH7-enriched (i.e., MYH6-depleted) hiCMs was more sensitive to mavacamten (IC₅₀ = 0.1 μM), while assembly by MYH6-enriched (i.e., MYH7-depleted) hiCMs was less sensitive (IC₅₀ = 0.5 μM). These findings suggest that MYH7-mediated contractility is required for sarcomere assembly, but only when MYH7 is present within a cardiac myocyte. We conclude that the MYH7/MYH6 ratio impacts the susceptibility of sarcomere assembly to pharmacological inhibition.