Genetics and Molecular Biology of Striated Muscle Myosin

We are probing the role of the myosin motor in aging and in progressive, genetically based diseases of skeletal and cardiac muscles using transgenic models of Drosophila melanogaster. We take advantage of the powerful genetic tools available for Drosophila to study myosin in an integrative manner, from atomic structural details and biochemical function through muscle ultrastructure, fiber mechanics, cardiac physiology and locomotion. We are examining the functional significance of specific residues within the skeletal muscle myosin motor and rod domains that are post-translationally modified during human aging. These modifications cause indirect flight muscle atrophy and dysfunction in our transgenic lines. To understand the basis of these defects, we are assessing myosin ATPase activity, in vitro motility, thick filament formation and fiber mechanics. We will determine if aging-related accumulation of ubiquinated protein aggregates and inclusion bodies is augmented in these lines, and will examine whether expression of the autophagy- enhancer FOXO ameliorates such disrupted proteostasis. Secondly, we will define the roles of proteins that anomalously accumulate in aggregates in our Drosophila models of inclusion body myopathy type 3 and myosin storage myopathy. Both are progressive skeletal muscle disorders that we found to exhibit aberrant proteostasis, yet they display phenotypically different protein aggregates. We will examine the structural and functional defects in skeletal muscles that arise from up- or down-regulating expression of the aberrant aggregate proteins as well as the effects of over-expressing FOXO. This will yield insights into roles of these proteins in aging and in degenerative muscle diseases. Finally, we are examining how mutations associated with human age-exacerbated dilated cardiomyopathy affect the structure and function of the Drosophila heart. We will determine mutation effects on cardiac proteostasis and examine if FOXO ameliorates mutation effects. Using a unique in vivo method to produce mutant myosins, we will gain mechanistic insights into biochemical and structural defects through ATPase and in vitro motility assays and by solving cardiac protein structures via crystallography. Further, we will express these mutant cardiac myosins in skeletal muscles to understand functional effects on muscle contractile properties. Overall, our project will test significant questions regarding basic myosin function during aging of healthy and diseased skeletal and cardiac muscles and assess whether improving proteostasis aids in rectifying muscle degeneration.