Impaired muscle growth and response to insulin-like growth factor 1 in dysferlin-mediated muscular dystrophy.

Loss-of-function mutations in dysferlin cause muscular dystrophy, and dysferlin has been implicated in resealing membrane disruption in myofibers. Given the importance of membrane fusion in many aspects of muscle function, we studied the role of dysferlin in muscle growth. We found that dysferlin null myoblasts have a defect in myoblast-myotube fusion, resulting in smaller myotubes in culture. In vivo, dysferlin null muscle was found to have mislocalized nuclei and vacuolation. We found that myoblasts isolated from dysferlin null mice accumulate enlarged, lysosomal-associated membrane protein 2 (LAMP2)-positive lysosomes. Dysferlin null myoblasts accumulate transferrin-488, reflecting abnormal vesicular trafficking. Additionally, dysferlin null myoblasts display abnormal trafficking of the insulin-like growth factor (IGF) receptor, where the receptor is shuttled to LAMP2-positive lysosomes. We studied growth, in vivo, by infusing mice with the growth stimulant IGF1. Control IGF1-treated mice increased myofiber diameter by 30% as expected, whereas dysferlin null muscles had no response to IGF1, indicating a defect in myofiber growth. We also noted that dysferlin null fibroblasts also accumulate acidic vesicles, IGF receptor and transferrin, indicating that dysferlin is important for nonmuscle vesicular trafficking. These data implicate dysferlin in multiple membrane fusion events within the cell and suggest multiple pathways by which loss of dysferlin contributes to muscle disease.