Mutations in the dysferlin gene cause limb-girdle muscular dystrophy type 2B, Miyoshi myopathy, and distal anterior compartment myopathy. Dysferlin mainly localizes to the sarcolemma in mature skeletal muscle where it is implicated in membrane fusion and repair. In different forms of muscular dystrophy, a predominantly cytoplasmic localization of dysferlin can be observed in regenerating myofibers, but the subcellular compartment responsible for this labeling pattern is not yet known. We have previously demonstrated an association of dysferlin with the developing T-tubule system in vitro. To investigate the role of dysferlin in adult skeletal muscle regeneration, we studied dysferlin localization at high resolution in a rat model of regeneration and found that the subcellular labeling of dysferlin colocalizes with the developing T-tubule system. Furthermore, ultrastructural analysis of dysferlin-deficient muscle revealed primary T-tubule anomalies similar to those seen in caveolin-3-deficient muscle. These findings indicate that dysferlin is necessary for correct T-tubule formation, and dysferlin-deficient skeletal muscle is characterized by abnormally configured T-tubules.

Skeletal muscle requires an efficient and active membrane repair system to overcome the rigours of frequent contraction. Dysferlin is a component of that system and absence of dysferlin causes muscular dystrophy (dysferlinopathy) characterized by adult onset muscle weakness, high serum creatine kinase levels and a prominent inflammatory infiltrate. We have observed that dysferlinopathy patient biopsies show an excess of immature fibres and therefore investigated the role of dysferlin in muscle regeneration. Using notexin-induced muscle damage, we have shown that regeneration is attenuated in a mouse model of dysferlinopathy, with delayed removal of necrotic fibres, an extended inflammatory phase and delayed functional recovery. Satellite cell activation and myoblast fusion appear normal, but there is a reduction in early neutrophil recruitment in regenerating and also needle wounded muscle in dysferlin-deficient mice. Primary mouse dysferlinopathy myoblast cultures show reduced cytokine release upon stimulation, indicating that the secretion of chemotactic molecules is impaired. We suggest an extension to the muscle membrane repair model, where in addition to fusing patch repair vesicles with the sarcolemma dysferlin is also involved in the release of chemotactic agents. Reduced neutrophil recruitment results in incomplete cycles of regeneration in dysferlinopathy which combines with the membrane repair deficit to ultimately trigger dystrophic pathology. This study reveals a novel pathomechanism affecting muscle regeneration and maintenance in dysferlinopathy and highlights enhancement of the neutrophil response as a potential therapeutic avenue in these disorders.

Insulin-secreting β cells were thought to reside only in the pancreas. Here, we show that β cells are also present in the extra-hepatic bile ducts of mice. They are characterised by insulin and C-peptide content, the presence of secretory granules that are immunoreactive for insulin, and the ducts exhibit glucose-stimulated insulin secretion. Genetic lineage labelling shows that these β cells arise from the liver domain rather than the pancreas and, by histological study, they appear to be formed directly from the bile duct epithelium in late embryogenesis. Other endocrine cell types (producing somatostatin and pancreatic polypeptide) are also found in close association with the bile-duct-derived β cells, but exocrine pancreatic tissue is not present. This discovery of β cells outside the mammalian pancreas has implications for regenerative medicine, indicating that biliary epithelium might offer a new source of β cells for the treatment of diabetes. The finding also has evolutionary significance, because it is known that certain basal vertebrates usually form all of their β cells from the bile ducts. The mammalian bile-duct-derived β cells might therefore represent an extant trace of the evolutionary origin of the vertebrate β cell.

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