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

摘要

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 ∼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.