In this study, we have tested the hypothesis that augmented [Ca 2+] in subcellular regions or organelles, which are known to play a key role in cell survival, is the missing link between Ca2+ homeostasis alterations and muscular degeneration associated with muscular dystrophy. To this end, different targeted chimeras of the Ca 2+-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 [Ca2+] in the sarcoplasmic reticulum, [Ca2+]sr, 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 [Ca2+] 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 Ca2+ 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 Ca2+ handling in the control of cell death, our data suggest that mitochondria are potential targets of impaired Ca2+ homeostasis in muscular dystrophy.
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