Mitochondrial Calcium Uptake Regulates Rapid Calcium Transients in Skeletal Muscle during Excitation-Contraction (E-C) Coupling

Defective coupling between sarcoplasmic reticulum and mitochondria during control of intracellular Ca2+ signaling has been implicated in the progression of neuromuscular diseases. Our previous study showed that skeletal muscles derived from an amyotrophic lateral sclerosis (ALS) mouse model displayed segmental loss of mitochondrial function that was coupled with elevated and uncontrolled sarcoplasmic reticulum Ca2+ release activity. The localized mitochondrial defect in the ALS muscle allows for examination of the mitochondrial contribution to Ca2+ removal during excitation-contraction coupling by comparing Ca2+ transients in regions with normal and defective mitochondria in the same muscle fiber. Here we show that Ca2+ transients elicited by membrane depolarization in fiber segments with defective mitochondria display an similar to 10% increased amplitude. These regional differences in Ca2+ transients were abolished by the application of 1,2-bis(O-aminophenoxy) ethane-N,N,N',N'-tetraacetic acid, a fast Ca2+ chelator that reduces mitochondrial Ca2+ uptake. Using a mitochondria-targeted Ca2+ biosensor (mt11-YC3.6) expressed in ALS muscle fibers, we monitored the dynamic change of mitochondrial Ca2+ levels during voltage-induced Ca2+ release and detected a reduced Ca2+ uptake by mitochondria in the fiber segment with defective mitochondria, which mirrored the elevated Ca2+ transients in the cytosol. Our study constitutes a direct demonstration of the importance of mitochondria in shaping the cytosolic Ca2+ signaling in skeletal muscle during excitation-contraction coupling and establishes that malfunction of this mechanism may contribute to neuromuscular degeneration in ALS.