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

ABSTRACT

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.