Defective Trafficking of Annexins to the Site of Injury in ANO5-Knockout Muscle Fibers

Abstract

Mutations in ANO5 (TMEM16E) cause limb-girdle muscular dystrophy R12 (limb-girdle muscular dystrophy type 2L). Recent evidence implicates defective plasma membrane repair as a likely mechanism for the disorder. Here, we probe the ANO5-dependency of the membrane repair pathway using a laser wounding assay in Ano5 knockout mouse muscle fibers. Wounded myofibers from Ano5 knockout mice exhibit delayed membrane resealing relative to wild type fibers as revealed by an increased uptake of the membrane-impermeant FM1-43 dye and a prolonged elevation of intracellular Ca2+. The trafficking of several annexin proteins, which together form a cap at the site of injury, is altered in Ano5 knockout fibers. Annexin A2 accumulates at the wound to nearly twice the level observed in WT fibers, while annexin A6 accumulation is substantially inhibited in the absence of ANO5. Furthermore, trafficking of annexins A1 and A5 to the cap is decreased in the Ano5 knockout. These changes are correlated with an alteration in the fine structure of the annexin repair cap and the shedding of annexin-positive extracellular vesicles. Our results suggest that the meticulous coordination of the annexin repair machinery required to effectively reseal wounded sarcolemma is disrupted in Ano5 knockout mice. ANO5 is a putative phospholipid scramblase, responsible for exposure of intracellular phospholipids to the extracellular leaflet of the plasma membrane. However, because the membrane repair defect is rescued by overexpression of wild type ANO5 or a scramblase-defective mutant, we suggest that ANO5-mediated phospholipid scrambling is not essential for membrane repair. Significance StatementMutations in ANO5/TMEM16E cause myopathies of variable severity, with some patients losing ambulation entirely. Unfortunately, relatively little is known about the function of ANO5 at the protein level, but it has been suggested that ANO5 plays a role in the repair of injured muscle plasma membranes. Here, we investigate the mechanism of ANO5-mediated repair and find that annexin proteins, which in normal muscle form a cap to seal wounds, traffic abnormally to the cap when ANO5 is not expressed. Muscle fibers lacking ANO5 reseal more slowly and thus are exposed to prolonged intracellular calcium elevation that can damage the fibers. Our findings contribute to the growing literature implicating failed repair as a probable pathogenic mechanism in patients with ANO5 mutations.