Alteration in Calcium Handling at the Subcellular Level in mdx Myotubes

In this study, we have tested the hypothesis that augmented [Ca ] in subcellular regions or organelles, which are known to play a key role in cell survival, is the missing link between Ca homeostasis alterations and muscular degeneration associated with muscular dystrophy. To this end, different targeted chimeras of the Ca -sensitive photoprotein aequorin have been transiently expressed in subcellular compartments of skeletal myotubes of mdx mice, the animal model of Duchenne muscular dystrophy. Direct measurements of the [Ca] in the sarcoplasmic reticulum, [Ca], show a higher steady state level at test and a larger drop after KCl-induced depolarization in mdx compared with control myotubes. The peaks in [Ca] occurring in the mitochondrial matrix of mdx myotubes are significantly larger than in controls upon KCl-induced depolarization or caffeine application. The augmented response of mitochondria precedes the alterations in the Ca responses of the cytosol and of the cytoplasmic region beneath the membrane, which become significant only at a later stage of myotube differentiation. Taking into account the key role played by mitochondria Ca handling in the control of cell death, our data suggest that mitochondria are potential targets of impaired Ca homeostasis in muscular dystrophy.