Understanding how cardiac myosin modulators regulate function in health and disease

Cardiac myosin is the molecular motor that drives heart contraction, powered by the energy-source ATP and through its interaction with actin tracks. Direct modulators of cardiac myosin function are promising treatments for inherited heart disease and heart failure. The inherited heart disease hypertrophic cardiomyopathy affects 1 in 500 people and is the most common cause of sudden onset cardiac death in the young, causing approximately 12 deaths in the UK every week. Heart failure is an increasing worldwide public health issue with an estimated prevalence of 64 million people, a mortality rate of approximately 50 % within five years of diagnosis, and an estimated cost to the economy of $346 billion. Current treatments improve heart function but not patient survival. Direct myosin modulators have the potential to do both. A first-in-class cardiac myosin inhibitor, Mavacamten, was approved by the FDA for the treatment of symptomatic obstructive hypertrophic cardiomyopathy (oHCM) in 2022. Whilst the muscle activator Omecamtiv mecarbil (OM) has been shown to improve cardiac function in patients with systolic heart failure, modestly decreasing heart-failure events and deaths in a Phase III trial. Consequently, the potential use of direct myosin modulators for the long-term treatment of various muscle diseases has been more broadly recognised and many other myosin modulators have entered clinical trials. Importantly, despite a range of pre-clinical studies, we still have very little idea as to how these modulators work at the molecular level. This hinders their clinical usage and development. Mavacamten is only available under a restricted programme because it can reduce systolic function (ventricular contraction). OM has just been withdrawn from further clinical trials after failing to obtain FDA approval following a lack of evidence for long-term effectiveness. Thus, understanding the molecular modes of action of these modulators is crucial to understanding how they may work as therapeutics, to enable the identification of patient populations that can most benefit from them, to predict prospective adverse effects, and to inform on the design of improved myosin modulators with better long-term outcomes. Here, we will study the effects of Mavacamten and OM on diverse structural states of myosin, and the dynamics of those states, as myosin cycles through non-force producing off-actin states crucial for relaxation of muscle and on-actin force-producing states crucial for contraction. We will achieve this using our well-established cryo electron microscopy pipeline to study myosin structural states coupled with structural mass spectrometry approaches to capture myosin dynamics, complemented by kinetic assays to assess myosin function. With this structural, dynamic, and functional knowledge, we will predict the effects of these modulators on myosin function in the presence of HCM disease-causing mutations and test these predictions for specific mutations. This will provide unprecedented detail of the effects of the modulators Mavacamten and OM on the cardiac myosin functional cycle in health and disease, provide an experimental pipeline for the assessment of the molecular mode of action of other modulators, currently in clinical trials, and for the structure-guided design of improved myosin modulators with the potential for improved patient outcomes.