Ryanodine Receptor Ca2+ Leak-Induced Redistribution of Ca2+ in Dystrophic mdx Mouse Muscle.

AIM: The dystrophic mdx mouse is a widely used model of Duchenne muscular dystrophy. Altered Ca2+ handling is a key feature, including increased Ca2+ leak through the ryanodine receptor (RyR1's), the primary Ca2+ release channel in skeletal muscle. Such leak has important downstream consequences for intracellular Ca2+ homeostasis. Here, we quantified basal compartmentalized Ca2+ levels in mdx muscle compared with wild-type (WT).

METHODS

Single extensor digitorum longus muscle fibers from WT and mdx mice were mechanically skinned. Transverse tubule Ca2+ dynamics were assessed using confocal microscopy with fluorescent Ca2+ indicators during caffeine-induced RyR1-mediated Ca2+ release. Sarcoplasmic reticulum (SR) and mitochondrial Ca2+ contents were quantified using established depletion protocols combined with force measurements.

RESULTS

Consistent with previous reports, mdx fibers exhibited increased RyR1 Ca2+ leak. Absolute quantification revealed a reduction in SR Ca2+ content accompanied by a ~4-fold increase in mitochondrial Ca2+ content. These shifts indicate a redistribution of intracellular Ca2+, triggered by the RyR1 Ca2+ leak to lower SR Ca2+ content and increase the Ca2+ permeability of the t-system membrane, leading to an elevation in cytoplasmic and mitochondrial Ca2+ levels in mdx muscle.

CONCLUSION

Redistribution of Ca2+ is a regulated process, proportional to RyR1 Ca2+ leak. In mdx muscle fibers, there is reduced SR and elevated mitochondrial and cytoplasmic Ca2+ compared to WT fibers. These alterations contribute to the dystrophic muscle pathology, likely through promotion of oxidative stress through increased reactive oxygen species production.