Calpains in muscle: selective and protective?

Calpains, cleaved mini-dysferlinC72, and L-type channels underpin calcium-dependent muscle membrane repair

Dysferlin is proposed as a key mediator of calcium-dependent muscle membrane repair, although its precise role has remained elusive. Dysferlin interacts with a new membrane repair protein, mitsugumin 53 (MG53), an E3 ubiquitin ligase that shows rapid recruitment to injury sites. Using a novel ballistics assay in primary human myotubes, we show it is not full-length dysferlin recruited to sites of membrane injury but an injury-specific calpain-cleavage product, mini-dysferlinC72. Mini-dysferlinC72-rich vesicles are rapidly recruited to injury sites and fuse with plasma membrane compartments decorated by MG53 in a process coordinated by L-type calcium channels. Collective interplay between activated calpains, dysferlin, and L-type channels explains how muscle cells sense a membrane injury and mount a specialized response in the unique local environment of a membrane injury. Mini-dysferlinC72 and MG53 form an intricate lattice that intensely labels exposed phospholipids of injury sites, then infiltrates and stabilizes the membrane lesion during repair. Our results extend functional parallels between ferlins and synaptotagmins. Whereas otoferlin exists as long and short splice isoforms, dysferlin is subject to enzymatic cleavage releasing a synaptotagmin-like fragment with a specialized protein- or phospholipid-binding role for muscle membrane repair.

Calpains, skeletal muscle function and exercise

1. Skeletal muscle fibres contain ubiquitous (mu-calpain and m-calpain) and muscle-specific (calpain-3) Ca(2+)-dependent proteases. The physiological roles of the calpains are not well understood, although ubiquitous calpains have been associated with apoptosis and myogenesis and calpain-3 is likely involved in sarcomeric remodelling. A defect in the expression of calpain-3 results in limb-girdle muscular dystrophy Type 2A. 2. At resting [Ca(2+)](i), calpains are present predominantly in their full-length, unautolysed/unactivated forms. Once activated, mu-calpain and calpain-3 appear in their autolysed forms and this measurement can be used to determine when in vivo activation occurs. Endogenously expressed mu-calpain and calpain-3 are activated within a physiological [Ca(2+)] range in a Ca(2+)- and time-dependent manner. 3. In skeletal muscle, mu-calpain is a freely diffusible protein that binds rapidly when [Ca(2+)](i) is increased. Calpain-3 is tightly bound in skeletal muscle fibres at the N2A line of the large elastic protein titin. 4. Overall, neither mu-calpain nor calpain-3 are activated immediately following sprint, endurance or eccentric exercise, despite the frequent episodes of high cytoplasmic [Ca(2+)] that would occur during these types of muscle contractions. Importantly, however, a substantial proportion of calpain-3, but not mu-calpain, is activated 24 h after a single bout of eccentric exercise. 5. In vitro studies have shown that calpain-3 becomes activated if exposed for a prolonged period of time (> 1 h) to resting cytoplasmic [Ca(2+)] that are approximately two- to fourfold higher than normal. This suggests that the small but sustained increase in [Ca(2+)](i) that likely occurs after eccentric contractions is both high and long enough to result in calpain-3 activation and supports the role for calpain-3 in sarcomeric remodelling.