Reciprocal amplification of ROS and Ca2+ signals in stressed mdx dystrophic skeletal muscle fibers

Muscular dystrophies are among the most severe inherited muscle diseases. The genetic defect is a mutation in the gene for dystrophin, a cytoskeletal protein which protects muscle cells from mechanical damage. Mechanical stress, applied as osmotic shock, elicits an abnormal surge of Ca²⁺ spark-like events in skeletal muscle fibers from dystrophin deficient (mdx) mice. Previous studies suggested a link between changes in the intracellular redox environment and appearance of Ca²⁺ sparks in normal mammalian skeletal muscle. Here, we tested whether the exaggerated Ca²⁺ responses in mdx fibers are related to oxidative stress. Localized intracellular and mitochondrial Ca²⁺ transients, as well as ROS production, were assessed with confocal microscopy. The rate of basal cellular but not mitochondrial ROS generation was significantly higher in mdx cells. This difference was abolished by pre-incubation of mdx fibers with an inhibitor of NAD(P)H oxidase. In addition, immunoblotting showed a significantly stronger expression of NAD(P)H oxidase in mdx muscle, suggesting a major contribution of this enzyme to oxidative stress in mdx fibers. Osmotic shock produced an abnormal and persistent Ca²⁺ spark activity, which was suppressed by ROS-reducing agents and by inhibitors of NAD(P)H oxidase. These Ca²⁺ signals resulted in mitochondrial Ca²⁺ accumulation in mdx fibers and an additional boost in cellular and mitochondrial ROS production. Taken together, our results indicate that the excessive ROS production and the simultaneous activation of abnormal Ca²⁺ signals amplify each other, finally culminating in a vicious cycle of damaging events, which may contribute to the abnormal stress sensitivity in dystrophic skeletal muscle.