An industrial collaboration to develop new microscopy tools to image ATP usage in muscle

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Cardiac muscle contraction occurs when myosin II in the thick filament uses ATP to power the relative movement of the actincontaining thin filament. Given the vast numbers of myosin motors, if all were active, the heart would be rapidly drained of ATP. Therefore, to save energy, myosin can enter the 'super relaxed (SRX)' state when detached from actin, which has an inhibited ATPase. When needed, e.g., during exercise, these SRX myosins can be reactivated. Hypertrophic cardiomyopathy (HCM), the most common genetic heart disease, is thought to be due to hypercontractility, which may be caused by underpopulation of SRX. We will investigate how SRX is regulated in both relaxed and active conditions. Our approach is to study myosin and actin in their native myofibril environment. This highly ordered structure is capable of contracting in the presence of calcium and ATP. In this project, we will construct a device that uses glass microneedles to grab either end of a myofibril, this will enable the contractile force to be measured. Uniquely, we will combine this with single molecule imaging of ATP being turned over in the myofibrils ofporcine/human cardiac muscle, and stem-cell derived cardiomyocytes. This has not been achieved before and opens up a huge possibility for understanding how muscle is affected by calcium, phosphorylation, force, pharmacological interventions, and disease-associated mutations. By the end of this project, we will provide a comprehensive understanding of the role and regulation of myosin's SRX states in normal and diseased tissue.