Growing up and growing pores in myoblast fusion

Dhanyasi et al. describe how myoblasts form muscle syncytia in Drosophila flight muscles.

Development of mammalian skeletal muscle involves fusion of myoblasts with myotubes to form multinucleate muscle fibers. Myoblast fusion also takes place during Drosophila embryogenesis and during development of the adult flight muscles. Although flight muscle development generally shares more features with mammalian skeletal muscle development than does embryonic myogenesis, it remains less well studied.

Dhanyasi et al. used electron microscopy to get a good view of developing flight muscles, preparing their samples with a technique that maximizes cell membrane preservation. Knockdown of adhesion molecules or mediators of actin polymerization known to be important for myoblast fusion revealed that cell adhesion molecules facilitated an initial association between the myoblast and myotube. Then, polymerization of branched actin filaments in the myoblast flattened it against the myotube, bringing their membranes into close apposition. Finally, the membranes of the two cells directly contacted each other at several spots, culminating in the formation and expansion of multiple fusion pores. Senior author Eyal Schejter says that future studies will explore the role of actin in pore formation in greater detail.

Interestingly, fusion pores have also been implicated in embryonic myoblast fusion, although recent studies have suggested that the process instead involves the formation of fingerlike projections from the myoblast surface into the myotube. Dhanyasi et al. found such projections were rare in fusing flight muscles. This may reflect stage-specific differences, says Schejter, or may also be relevant to the embryonic fusion mechanism.