UC San Diego

Dilated cardiomyopathy (DCM) leads to cardiac contractile deficits and pathological dilation of the left ventricle, which may result in heart failure. Inherited DCM can be caused by mutations in contractile proteins (e.g. myosin). Though myosin DCM mutations are generally thought to reduce myosin function, the structural and molecular origins leading to cardiac dilation are not well understood. Here, we took advantage of the powerful genetic tools available in Drosophila to generate the first fly models of myosin-induced DCM and determine the mechanistic basis of disease. The S532P and R369H mutations, located within the actin-binding region at residues mutated in DCM patients, were introduced into Drosophila myosin heavy chain. We implemented an integrative approach to determine how these mutations disrupt intramolecular interactions and cause biochemical, structural and physiological defects in muscles. Given the location of these mutations within known actin binding sites, we hypothesized that they cause DCM by disrupting actomyosin interactions, leading to reduced myosin motor function and hypo-contractility of muscles.

To define the exact intramolecular interactions disrupted by the S532P DCM mutation, we are using cryo-electron microscopy to solve the structure of the mutant motor domain bound to F-actin. Preliminary analyses of negatively stained samples suggest that the S532P mutation disrupts the transition from weak to strong actin binding. Biochemical and muscle mechanics experiments revealed that the S532P mutation leads to depressed power output by increasing the rate of actin detachment and reducing and the rates of actin binding and actin-dependent enzymatic activity. Additionally, the R369H mutation reduces maximal actin binding in vitro. Both mutations reduce skeletal muscle function, suggesting that these myosin forms are underfunctional. S532P mutant hearts exhibit a dilated phenotype, which may compensate for reduced myosin function by preserving cardiac output. In support of our hypothesis, the S532P and R369H mutations impair actomyosin interactions and thereby decrease muscle function.

These studies provide insight into the molecular basis by which contractile deficits induced by mutations of these residues in humans lead to compensatory enlargement of the left ventricle. Future work will exploit our fly models for drug screening to provide insight into potential therapeutic treatments for human patients.