The E706K IBM3 Myosin Mutation Depresses the Chemomechanical Properties and Increases the Lability of the Molecular Motor

Abstract

Hereditary myosin myopathies are muscle diseases with variable clinical features and age of onset. Inclusion body myopathy 3 (IBM3) is an autosomal dominant myopathy associated with a missense mutation (E706K) in the myosin heavy chain IIa gene (MYH2). The disease is mild in childhood but appears progressive in adulthood, with proximal muscle weakness affecting movement. Biopsies from adult patients reveal dystrophic alterations and rimmed vacuoles consistent with an increased expression of the mutant motor with advanced age. We constructed a transgene encoding E706K myosin and expressed it in Drosophila (E699K) indirect flight and jump muscles. Flight and jump abilities were reduced in heterozygous flies and were nearly absent in homozygotes consistent with the possible age-related dose-dependent response observed in patients. The mutant myosin displayed 80% lower actin sliding velocity and 74 and 83% reductions in basal and actin stimulated ATPase activities compared to wild-type myosin. Electron microscopy revealed E706K (E699K) myosin heads bear a dramatic propensity to collapse and to aggregate relative to wildtype heads. At 23°C, 77.5% (n=1146) of control molecules exhibited two well-resolved independent heads compared to only 22.5% (n=1014) of mutant myosin molecules. A five minute, 37°C incubation induced 80.9% (n=1181) of the control molecules’ heads to form intra- or intermolecular aggregates versus 95.3% (n=1192) of the mutant myosin heads. This test directly assessed motor integrity and suggests E706K (E699K) myosin is far more labile than wildtype myosin. We are imaging mutant myocytes to determine if the ultrastructural hallmarks seen in adult patients also appear in our fly model. The depressed motor properties and the propensity for the mutant myosin to collapse and aggregate likely contribute to the muscle weakness observed in our fly model and possibly in senescent patients.