Myosin Sequestration Regulates Sarcomere Function, Cardiomyocyte Energetics, and Metabolism, Informing the Pathogenesis of Hypertrophic Cardiomyopathy.
Toepfer CN., Garfinkel AC., Venturini G., Repetti G., Wakimoto H., Alamo L., Ewoldt JF., Agarwal R., Letendre J., Cloonan P., Sharma A., Olivotto I., Lun M., Colan S., Ashley E., Watkins HC., Redwood CS., Michels M., Jacoby D., Staples JF., Day SM., Padrón R., Chopra A., Pereira AC., Seidman JG.
Background: Hypertrophic cardiomyopathy (HCM) is caused by pathogenic variants in sarcomere protein genes that evoke hypercontractility, poor relaxation, and increased energy consumption by the heart and increased patient risks for arrhythmias and heart failure. Recent studies show that pathogenic missense variants in myosin, the molecular motor of the sarcomere, are clustered in residues that participate in dynamic conformational states of sarcomere proteins. We hypothesized that these conformations are essential to adapt contractile output for energy conservation and that pathophysiology of HCM results from destabilization of these conformations. Methods: We assayed myosin ATP binding to define the proportions of myosin in SRX or DRX conformations in healthy rodent and human hearts, at baseline and in response to reduced hemodynamic demands of hibernation or pathogenic HCM variants. To determine the relationships between myosin conformations, sarcomere function, and cell biology we assessed contractility, relaxation, and cardiomyocyte morphology and metabolism, with and without an allosteric modulator of myosin ATPase activity. We then tested whether the positions of myosin variants with unknown clinical significance (VUS) that were identified in HCM patients, predicted functional consequences and associations with heart failure and arrhythmias. Results: Myosins undergo physiologic shifts between SRX conformations that maximized energy-conservation and active states (DRX) that enable cross-bridge formation with greater ATP consumption. Systemic hemodynamic requirements, pharmacologic modulators of myosin, and pathogenic myosin missense mutations influenced the proportions of these conformations. Hibernation increased SRX conformations while pathogenic variants destabilized these and increased the proportion of DRX myosins, which enhanced cardiomyocyte contractility but impaired relaxation, and evoked hypertrophic remodeling with increased energetic stress. Using structural locations to stratify VUS, we showed that variants that unbalanced myosin conformations were associated with higher rates of heart failure and arrhythmias in HCM patients. Conclusions: Myosin conformations establish work-energy equipoise that is essential for life-long cellular homeostasis and heart function. Destabilization of myosin energy conserving states promotes contractile abnormalities, morphological and metabolic remodeling and adverse clinical outcomes in HCM patients. Therapeutic restabilization corrects cellular contractile and metabolic phenotypes and may limit these adverse clinical outcomes in HCM patients.