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

Abstract

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.

Expression Profiling of a Hypercontraction-induced Myopathy in Drosophila Suggests a Compensatory Cytoskeletal Remodeling Response

“…In addition to muscle hypercontraction and degeneration, Mhc mutants display abnormal TS behavior at restrictive temperatures, reflected in an acute loss of motor coordination and walking ability at 38°C. Most surprisingly, we found that TS-induced behavioral dysfunction was a common feature of all Drosophila hypercontraction muscle mutants assayed, in contrast to muscle mutants that lead to hypocontraction (5). These shared behavioral defects suggest that hypercontracted muscles have fundamentally different electrical properties than hypocontracted and normal muscles, resulting in TS dysfunction at elevated temperatures.…”

“…This downregulation may be responsible for potential Ca 2ϩ homeostasis defects in Mhc flight muscles. Indeed, our previous analysis suggested that intracellular Ca 2ϩ is dysregulated in Mhc mutant fibers, as larval body wall muscles exhibit spontaneous contraction cycles even in the absence of external Ca 2ϩ (5). A subset of the genes that were identified as differentially regulated in our hypercontraction-induced myopathy model have been implicated previously in activity-dependent synaptic plasticity in mammals.…”

“…Immunostaining-Immunostaining of third instar larval fillets was done as described previously (5). For detecting MSP-300, we utilized ␣-MSP-300 antisera at 1:500 (17).…”

“…In a large scale screen for TS 2 behavioral mutants in Drosophila, we identified dominant mutations in Mhc that lead to hypercontraction and degeneration of the adult indirect flight muscles (5). The Mhc locus encodes the muscle-specific motor that mediates contraction of sarcomeres in Drosophila muscles.…”

Myosin Transducer Mutations Differentially Affect Motor Function, Myofibril Structure, and the Performance of Skeletal and Cardiac Muscles

“…Drosophila age in weeks, and they share common mechanisms that determine aging rates and longevity with higher organisms (Parkes et al, 1999;Finch and Ruvkun, 2001;Tatar et al, 2003;Wessells and Bodmer, 2007). Thus, the fly is an extremely powerful model for studying the progression of myosin-related skeletal and cardiac muscle dysfunction.Two myosin point mutations in the Drosophila Mhc gene, D45 and Mhc 5 , were localized in proximity to coding regions for loops and linkers of the transducer (Kronert et al, 1999;Montana and Littleton, 2004). These mutations are in Mhc constitutive exons 5 and 4, respectively.…”