Role of Dynamin-2 in Neuromuscular Development and Disease

Abstract

Centronuclear myopathies (CNMs) are a group of inherited muscle disorders characterized by muscle weakness and centralized nuclei on biopsy. Autosomal dominant centronuclear myopathy is caused by mutations in the gene encoding dynamin 2 (DNM2), a protein involved in endocytosis and membrane trafficking pathways. There is little known about the muscle dysfunction underlying this disorder, and there are currently no treatments. The goal of this work is to gain greater insight into the role of DNM2 in normal muscle function and disease-related pathology. Since zebrafish muscle shares many features with higher vertebrates, we utilize zebrafish as a model system to characterize DNM2 function in muscle. We identify two zebrafish orthologs to human DNM2, and show that they are required for normal development. We further demonstrate that the knockdown of zebrafish dnm2 results in a severe motor phenotype and that organization of t-tubules and sarcoplasmic reticulum (SR) is disrupted in these animals. To determine if these changes are relevant in the pathogenesis of DNM2-CNM, we establish a novel zebrafish model of DNM2-CNM by transiently overexpressing a mutant version of DNM2 (DNM2-S619L) during development. We show that overexpression of DNM2-S619L leads to histopathologic changes in muscle and a severe motor phenotype. We further demonstrate that the muscle weakness seen in these animals can be significantly alleviated by treatment with an acetylcholinesterase inhibitor, suggesting that abnormal neuromuscular transmission is involved in the pathology of DNM2-CNM. This finding is supported by case histories of two patients with mutations in DNM2. In the muscle of DNM2-S619L fish, we also find substantial disorganization of the t-tubule and SR on muscle ultrastructure. DNM2-S619L overexpression in an ex vivo model of t-tubule formation provides additional evidence that CNM-associated mutations in DNM2 can disrupt t-tubule structure. Together, our results suggest that deficits at the both the neuromuscular junction and sarcotubular network may play an important role in the pathogenesis of DNM2-CNM and that treatments targeting this dysfunction may provide disease-modifying therapies for patients with this disorder.