Minimal expression of dysferlin prevents development of dysferlinopathy in dysferlin exon 40a knockout mice

Syntaxin 4-enhanced plasma membrane repair is independent of dysferlin in skeletal muscle

Plasma membrane repair (PMR) restores membrane integrity of cells, preventing cell death in vital organs, and has been studied extensively in skeletal muscle. Dysferlin, a sarcolemmal Ca2+-binding protein, plays a crucial role in PMR in skeletal muscle. Previous studies have suggested that PMR uses membrane trafficking and membrane fusion, similar to neurotransmission. Soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) mediate membrane fusion in neurotransmission with the help of synaptotagmin, a crucial Ca2+-binding protein. Interestingly, dysferlin shares structural similarity with synaptotagmin and was shown to promote SNARE-mediated membrane fusion in a liposome-based assay. However, whether dysferlin facilitates SNARE-mediated membrane fusion in PMR in muscle cells remains unclear. In this study, we aimed to test if SNARE-mediated PMR requires dysferlin in muscle cells with pharmacological and genetic approaches. TAT-NSF700, which disrupts the disassembly of SNARE complexes, was used to disrupt functions of SNAREs in muscle cells. We found that human-induced pluripotent stem cells-derived cardiomyocytes (hiPS-CMs) treated with TAT-NSF700 showed a higher loss of membrane integrity after repetitive mechanical strains. Moreover, laser-wounded mouse flexor digitorum brevis (FDB) fibers treated with TAT-NSF700 showed an increased Ca2+ influx, but a decreased FM1-43 uptake, which depends on dynamin-regulated endocytosis as we previously showed in FDB fibers. Importantly, overexpression of STX4-mCitrine or eGFP-SNAP23 decreased Ca2+ influx in laser-wounded FDB fibers. Furthermore, overexpression of STX4-mCitrine also decreased Ca2+ influx in laser-wounded dysferlin-deficient FDB fibers. Overall, these results suggest that disassembly of SNARE complexes is required for efficient PMR and STX4-enhanced PMR does not require dysferlin in skeletal muscle.NEW & NOTEWORTHY Dysferlin, a crucial Ca2+-binding protein in plasma membrane repair (PMR), shares homology with synaptotagmin, which binds Ca2+ and regulates SNARE-mediated vesicle fusion in neurons. Dysferlin was thus hypothesized to function as synaptotagmin in PMR. We demonstrate here that the activity of SNAREs is important for PMR, and overexpression of STX4 enhances PMR in both intact and dysferlin-deficient skeletal muscle. These data suggest that SNARE-mediated PMR may be independent of dysferlin in skeletal muscle.

Gene-editing in patient and humanized-mice primary muscle stem cells rescues dysferlin expression in dysferlin-deficient muscular dystrophy

Dystrophy-associated fer-1-like protein (dysferlin) conducts plasma membrane repair. Mutations in the DYSF gene cause a panoply of genetic muscular dystrophies. We targeted a frequent loss-of-function, DYSF exon 44, founder frameshift mutation with mRNA-mediated delivery of SpCas9 in combination with a mutation-specific sgRNA to primary muscle stem cells from two homozygous patients. We observed a consistent >60% exon 44 re-framing, rescuing a full-length and functional dysferlin protein. A new mouse model harboring a humanized Dysf exon 44 with the founder mutation, hEx44mut, recapitulates the patients' phenotype and an identical re-framing outcome in primary muscle stem cells. Finally, gene-edited murine primary muscle stem-cells are able to regenerate muscle and rescue dysferlin when transplanted back into hEx44mut hosts. These findings are the first to show that a CRISPR-mediated therapy can ameliorate dysferlin deficiency. We suggest that gene-edited primary muscle stem cells could exhibit utility, not only in treating dysferlin deficiency syndromes, but also perhaps other forms of muscular dystrophy.

Tetraspanin CD82 Associates with Trafficking Vesicle in Muscle Cells and Binds to Dysferlin and Myoferlin

Tetraspanins organize protein complexes at the cell membrane and are responsible for assembling diverse binding partners in changing cellular states. Tetraspanin CD82 is a useful cell surface marker for prospective isolation of human myogenic progenitors and its expression is decreased in Duchenne muscular dystrophy (DMD) cell lines. The function of CD82 in skeletal muscle remains elusive, partly because the binding partners of this tetraspanin in muscle cells have not been identified. CD82-associated proteins are sought to be identified in human myotubes via mass spectrometry proteomics, which identifies dysferlin and myoferlin as CD82-binding partners. In human dysferlinopathy (Limb girdle muscular dystrophy R2, LGMDR2) myogenic cell lines, expression of CD82 protein is near absent in two of four patient samples. In the cell lines where CD82 protein levels are unaffected, increased expression of the ≈72 kDa mini-dysferlin product is identified using an antibody recognizing the dysferlin C-terminus. These data demonstrate that CD82 binds dysferlin/myoferlin in differentiating muscle cells and its expression can be affected by loss of dysferlin in human myogenic cells.