Dysferlin regulates cell membrane repair by facilitating injury-triggered acid sphingomyelinase secretion

International audience; Dysferlin deficiency compromises the repair of injured muscle, but the underlying cellular mechanism remains elusive. To study this phenomenon, we have developed mouse and human myoblast models for dysferlinopathy. These dysferlinopathic myoblasts undergo normal differentiation but have a deficit in their ability to repair focal injury to their cell membrane. Imaging cells undergoing repair showed that dysferlin-deficit decreased the number of lysosomes present at the cell membrane, resulting in a delay and reduction in injury-triggered lysosomal exocytosis. We find repair of injured cells does not involve formation of intracellular membrane patch through lysosome–lysosome fusion; instead, individual lysosomes fuse with the injured cell membrane, releasing acid sphingomyelinase (ASM). ASM secretion was reduced in injured dysferlinopathic cells, and acute treatment with sphingomyelinase restored the repair ability of dysferlinopathic myoblasts and myofibers. Our results provide the mechanism for dysferlin-mediated repair of skeletal muscle sarcolemma and identify ASM as a potential therapy for dysferlinopathy. Dysferlinopathy is a progressive muscle wasting disease, which is classified as limb-girdle muscular dystrophy type 2B (LGMD2B) or Miyoshi muscular dystrophy 1, based on its muscle involvement. 1,2 Dysferlin deficit leads to altered vesicle formation and trafficking, 3,4 poor repair of injured cell membranes, 5,6 and increased muscle inflammation. 7,8 Dys-ferlin contains C2 domains that are found in Ca 2 þ-dependent membrane fusion proteins such as synaptotagmins. 9 Thus, dysferlin is thought to regulate muscle function by regulating vesicle trafficking and fusion. 10–13 Dysferlin deficiency has also been implicated in conflicting reports regarding the fusion ability of dysferlinopathic myoblasts. 4,14–16 With such diverse roles for dysferlin, the mechanism through which dysferlin deficiency results in muscle pathology is unresolved. As skeletal muscle-specific re-expression of dysferlin rescues all dysferlinopathic pathologies, 17,18 myofiber repair has been suggested to be the unifying deficit underlying muscle pathology in dysferlinopathy.