Characterization of a hypercontraction-induced myopathy in Drosophila caused by mutations in Mhc

The Myosin heavy chain (Mhc) locus encodes the muscle-specific motor mediating contraction in Drosophila. In a screen for temperature-sensitive behavioral mutants, we have identified two dominant Mhc alleles that lead to a hypercontraction-induced myopathy. These mutants are caused by single point mutations in the ATP binding/hydrolysis domain of Mhc and lead to degeneration of the flight muscles. Electrophysiological analysis in the adult giant fiber flight circuit demonstrates temperature-dependent seizure activity that requires neuronal input, as genetic blockage of neuronal activity suppresses the electrophysiological seizure defects. Intracellular recordings at the third instar neuromuscular junction show spontaneous muscle movements in the absence of neuronal stimulation and extracellular Ca²⁺, suggesting a dysregulation of intracellular calcium homeostasis within the muscle or an alteration of the Ca²⁺ dependence of contraction. Characterization of these new Mhc alleles suggests that hypercontraction occurs via a mechanism, which is molecularly distinct from mutants identified previously in troponin I and troponin T.