Abstract
Mutations of Ca(2+)-activated proteases (calpains) cause muscular dystrophies. Nevertheless, the specific role of calpains in Ca(2+) signalling during the onset of dystrophies remains unclear. We investigated Ca(2+) handling in skeletal cells from calpain 3-deficient mice. [Ca(2+)](i) responses to caffeine, a ryanodine receptor (RyR) agonist, were decreased in -/- myotubes and absent in -/- myoblasts. The -/- myotubes displayed smaller amplitudes of the Ca(2+) transients induced by cyclopiazonic acid in comparison to wild type cells. Inhibition of L-type Ca(2+) channels (LCC) suppressed the caffeine-induced [Ca(2+)](i) responses in -/- myotubes. Hence, the absence of calpain 3 modifies the sarcoplasmic reticulum (SR) Ca(2+) release, by a decrease of the SR content, an impairment of RyR signalling, and an increase of LCC activity. We propose that calpain 3-dependent proteolysis plays a role in activating support proteins of intracellular Ca(2+) signalling at a stage of cellular differentiation which is crucial for skeletal muscle regeneration.