A hallmark of muscle development is that myoblasts fuse to form myofibers. However, smooth muscles and cardiomyocytes do not generally fuse. In C. elegans, the body wall muscles (BWMs), the physiological equivalents of skeletal muscles, are mononuclear. Here, to determine what would be the consequences of fusing BWMs, we express the cell-cell fusogen EFF-1 in these cells. We find that EFF-1 induces paralysis and dumpy phenotypes. To determine whether EFF-1-induced muscle fusion results in these pathologies we injected viruses pseudotyped with AFF-1, a paralog of EFF-1, into the pseudocoelom of C. elegans. When these engineered viruses encounter cells expressing EFF-1 or AFF-1 they are able to infect them as revealed by GFP expression from the viral genome. We find that AFF-1 viruses can fuse to EFF-1-expressing muscles revealing multinucleated fibers that cause paralysis and abnormal muscle morphogenesis. Thus, aberrant fusion of otherwise non-syncytial muscle cells may lead to pathological conditions.Graphical abstractSignificance statementMost cells are individual units that do not mix their cytoplasms. However, some cells fuse to become multinucleated in placenta, bones and muscles. In most animals, muscles are formed by myofibers that originate by cell-cell fusion. In contrast, in C. elegans the body wall muscles are mononucleated cells that mediate worm-like movement. EFF-1 and AFF-1 fusogens mediate physiological cell fusion in C. elegans. By ectopically expressing EFF-1 in body wall muscles we induce their fusion resulting in behavioral and morphological deleterious effects, revealing possible causes of congenital myopathies in humans. Using AFF-1-coated pseudoviruses we infect EFF-1-expressing muscle cells retargeting viral infection into these cells. We suggest that virus retargeting can be utilized to study myogenesis, neuronal regeneration, gamete fusion and screens for new fusogens in different organisms. In addition, our virus retargeting system can be used in gene-therapy, viral-based oncolysis and to study viral-host interactions.
A screen for genetic loci required for body-wall muscle development during embryogenesis in Caenorhabditis elegans.