Myosin in Autoinhibited State(s), Stabilized by Mavacamten, Can Be Recruited Via Inotropic Effectors

Mavacamten is a novel, FDA-approved, small molecule therapeutic designed to regulate cardiac function by selectively but reversibly inhibiting the enzymatic activity of myosin. It shifts myosin towards ordered states close to the thick filament backbone. It remains unresolved whether mavacamten permanently sequesters these myosin heads in the state(s) or whether these heads can be recruited in response to physiological stimuli when required to boost cardiac output. We show that cardiac myosins stabilized in these state(s) by mavacamten are recruitable by Ca, increased heart rate, stretch, and β-adrenergic (β-AR) stimulation, all known physiological inotropic effectors. At the molecular level, we show that, in presence of mavacamten, Ca increases myosin ATPase activity by shifting myosin heads from the reserve super-relaxed (SRX) state to the active disordered relaxed (DRX) state. At the myofilament level, both Ca and passive lengthening can shift ordered myosin heads from positions close to the thick filament backbone to disordered states closer to the thin filaments in the presence of mavacamten. In isolated rat cardiomyocytes, increased stimulation rates enhanced shortening fraction in mavacamten-treated cells. This observation was confirmed in telemetered rats, where left-ventricular dP/dt an index of inotropy, increased with heart rate in mavacamten treated animals. Finally, we show that β-AR stimulation increases left-ventricular function and stroke volume in the setting of mavacamten. Our data demonstrate that the mavacamten-promoted states of myosin in the thick filament are activable, at least partially, thus leading to preservation of cardiac reserve mechanisms.