Myosin in autoinhibited <i>off</i> state(s), stabilized by mavacamten, can be recruited in response to inotropic interventions.

Abstract

Mavacamten is a FDA-approved small-molecule therapeutic designed to regulate cardiac function at the sarcomere level by selectively but reversibly inhibiting the enzymatic activity of myosin. It shifts myosin toward ordered <i>off</i> states close to the thick filament backbone. It remains elusive whether these myosin heads in the <i>off</i> state(s) can be recruited in response to physiological stimuli when required to boost cardiac output. We show that cardiac myosins stabilized in these <i>off</i> state(s) by mavacamten are recruitable by 1) Ca²⁺, 2) increased chronotropy [heart rate (HR)], 3) stretch, and 4) β-adrenergic (β-AR) stimulation, all known physiological inotropic interventions. At the molecular level, we show that Ca²⁺ increases myosin ATPase activity by shifting mavacamten-stabilized myosin heads from the inactive super-relaxed state to the active disordered relaxed state. At the myofilament level, both Ca²⁺ and passive lengthening can shift mavacamten-ordered <i>off</i> myosin heads from positions close to the thick filament backbone to disordered <i>on</i> states closer to the thin filaments. In isolated rat cardiomyocytes, increased stimulation rates enhanced shortening fraction in mavacamten-treated cells. This observation was confirmed in vivo in telemetered rats, where left-ventricular dP/dt_max, an index of inotropy, increased with HR in mavacamten-treated animals. Finally, we show that β-AR stimulation in vivo increases left-ventricular function and stroke volume in the setting of mavacamten. Our data demonstrate that the mavacamten-promoted <i>off</i> states of myosin in the thick filament are at least partially activable, thus preserving cardiac reserve mechanisms.