Enhanced Muscular Dystrophy from Loss of Dysferlin Is Accompanied by Impaired Annexin A6 Translocation after Sarcolemmal Disruption.

Dysferlin is a membrane-associated protein implicated in membrane resealing; loss of dysferlin leads to muscular dystrophy. We examined the same loss-of-function Dysf mutation in two different mouse strains, 129T2/SvEmsJ (Dysf(129)) and C57BL/6J (Dysf(B6)). Although there are many genetic differences between these two strains, we focused on polymorphisms in Anxa6 because these variants were previously associated with modifying a pathologically distinct form of muscular dystrophy and increased the production of a truncated annexin A6 protein. Dysferlin deficiency in the C57BL/6J background was associated with increased Evan's Blue dye uptake into muscle and increased serum creatine kinase compared to the 129T2/SvEmsJ background. In the C57BL/6J background, dysferlin loss was associated with enhanced pathologic severity, characterized by decreased mean fiber cross-sectional area, increased internalized nuclei, and increased fibrosis, compared to that in Dysf(129) mice. Macrophage infiltrate was also increased in Dysf(B6) muscle. High-resolution imaging of live myofibers demonstrated that fibers from Dysf(B6) mice displayed reduced translocation of full-length annexin A6 to the site of laser-induced sarcolemmal disruption compared to Dysf(129) myofibers, and impaired translocation of annexin A6 associated with impaired resealing of the sarcolemma. These results provide one mechanism by which the C57BL/6J background intensifies dysferlinopathy, giving rise to a more severe form of muscular dystrophy in the Dysf(B6) mouse model through increased membrane leak and inflammation.

An actin-dependent annexin complex mediates plasma membrane repair in muscle.

Disruption of the plasma membrane often accompanies cellular injury, and in muscle, plasma membrane resealing is essential for efficient recovery from injury. Muscle contraction, especially of lengthened muscle, disrupts the sarcolemma. To define the molecular machinery that directs repair, we applied laser wounding to live mammalian myofibers and assessed translocation of fluorescently tagged proteins using high-resolution microscopy. Within seconds of membrane disruption, annexins A1, A2, A5, and A6 formed a tight repair "cap." Actin was recruited to the site of damage, and annexin A6 cap formation was both actin dependent and Ca(2+) regulated. Repair proteins, including dysferlin, EHD1, EHD2, MG53, and BIN1, localized adjacent to the repair cap in a "shoulder" region enriched with phosphatidlyserine. Dye influx into muscle fibers lacking both dysferlin and the related protein myoferlin was substantially greater than control or individual null muscle fibers, underscoring the importance of shoulder-localized proteins. These data define the cap and shoulder as subdomains within the repair complex accumulating distinct and nonoverlapping components.

Eps 15 Homology Domain (EHD)-1 Remodels Transverse Tubules in Skeletal Muscle.

We previously showed that Eps15 homology domain-containing 1 (EHD1) interacts with ferlin proteins to regulate endocytic recycling. Myoblasts from Ehd1-null mice were found to have defective recycling, myoblast fusion, and consequently smaller muscles. When expressed in C2C12 cells, an ATPase dead-EHD1 was found to interfere with BIN1/amphiphysin 2. We now extended those findings by examining Ehd1-heterozygous mice since these mice survive to maturity in normal Mendelian numbers and provide a ready source of mature muscle. We found that heterozygosity of EHD1 was sufficient to produce ectopic and excessive T-tubules, including large intracellular aggregates that contained BIN1. The disorganized T-tubule structures in Ehd1-heterozygous muscle were accompanied by marked elevation of the T-tubule-associated protein DHPR and reduction of the triad linker protein junctophilin 2, reflecting defective triads. Consistent with this, Ehd1-heterozygous muscle had reduced force production. Introduction of ATPase dead-EHD1 into mature muscle fibers was sufficient to induce ectopic T-tubule formation, seen as large BIN1 positive structures throughout the muscle. Ehd1-heterozygous mice were found to have strikingly elevated serum creatine kinase and smaller myofibers, but did not display findings of muscular dystrophy. These data indicate that EHD1 regulates the maintenance of T-tubules through its interaction with BIN1 and links T-tubules defects with elevated creatine kinase and myopathy.