Weibin Zhanga, Sudipto Roya,b,c,*
a Institute of Molecular and Cell Biology, Agency for Science, Technology and Research (A*STAR), 61 Biopolis Drive, Proteos, Singapore 138673.
b Department of Pediatrics, Yong Loo Lin School of Medicine, National University of Singapore, 1E Kent Ridge Road, Singapore 119288, Singapore
c Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Singapore 117543, Singapore
Published online in Developmental Biology on 1 February 2017.
During skeletal muscle development, myocytes aggregate and fuse to form multinucleated muscle fibers.
Inhibition of myocyte fusion is thought to significantly derail the differentiation of functional muscle fibers.
Despite the purported importance of fusion in myogenesis, in vivo studies of this process in vertebrates are
rather limited. Myomaker, a multipass transmembrane protein, has been shown to be the first muscle-specific fusion protein essential for myocyte fusion in the mouse. We have generated loss-of-function alleles in zebrafish myomaker, and found that fusion of myocytes into syncytial fast-twitch muscles was significantly compromised.
However, mutant myocytes could be recruited to fuse with wild-type myocytes in chimeric embryos, albeit rather inefficiently. Conversely, overexpression of Myomaker was sufficient to induce hyperfusion among fasttwitch myocytes, and it also induced fusion among slow-twitch myocytes that are normally fusion-incompetent. In line with this, Myomaker overexpression also triggered fusion in another myocyte fusion mutant compromised in the function of the junctional cell adhesion molecule, Jam2a. We also provide evidence that Rac, a regulator of actin cytoskeleton, requires Myomaker activity to induce fusion, and that an approximately 3 kb of myomaker promoter sequence, with multiple E-box motifs, is sufficient to direct expression within the fast-twitch muscle lineage. Taken together, our findings underscore a conserved role for Myomaker in vertebrate myocyte fusion. Strikingly, and in contrast to the mouse, homozygous myomaker mutants are viable and do not exhibit discernible locomotory defects. Thus, in the zebrafish, myocyte fusion is not an absolute requirement for
skeletal muscle morphogenesis and function.
Muscle fibres were labelled with antibodies to fast-twitch myosin (red), the cell membranes were labelled with anti-β-catenin antibodies (green) and nuclei were highlighted with DAPI.
Legend for cover image
Confocal image of the myotome of zebrafish embryos showing multinucleated fast-twitch muscles of the wild-type (left), and unfused single-celled fast-twitch muscles of the tmem8c mutant.
For more information on Sudipto ROY's lab, please click here.