Mechanisms of myoblast fusion during muscle development
Introduction
Skeletal muscle is a unique tissue composed of bundles of multinucleated muscle fibers. Each myofiber is the product of fusion of hundreds or thousands of mononucleated muscle cells known as myoblasts. Myoblast fusion is critical not only for skeletal muscle development during embryogenesis, but also for satellite cell-mediated muscle regeneration in adults [1, 2]. For myoblast fusion to occur, two fusion partners must recognize each other, adhere their plasma membranes, open up fusion pores to allow cytoplasmic material exchange, and, ultimately, merge into one cell. As with any membrane fusion event, the rate-limiting step for successful myoblast fusion is bringing two cell membranes into close proximity to facilitate fusion pore formation. Recent studies in multiple model organisms, including Drosophila, zebrafish, and mouse, have uncovered many molecular components required for myoblast fusion in vivo [3•]. Mechanistic studies of these components suggest that muscle cells take at least three consecutive steps toward fusion pore formation (Figure 1) — first, muscle cell adhesion mediated by cell adhesion molecules (CAMs); second, closer cell membrane apposition mediated by a pair of pushing and resisting forces from the two fusion partners; and third, destabilization of the lipid bilayers, which makes them prone to fusion. Here, we review the in vivo evidence from Drosophila, zebrafish, and mouse that supports the three-step model of myoblast fusion. Insights from myoblast fusion are likely to apply to other cell–cell fusion events, such as fusion between macrophages, osteoclasts, and sperm and egg.
Section snippets
Drosophila
In Drosophila embryos, myoblast fusion occurs between two types of muscle cells, muscle founder cells and fusion competent myoblasts (FCMs), the fates of which are specified by the action of transcription factors [4, 5]. Muscle founder cells act as ‘seeds’ that attract the FCMs and ultimately determine the position, orientation, size, epidermal attachment, and nerve innervation patterns of the future multinucleated muscle fibers. Recognition and adhesion between founder cells and FCMs are
The second step toward myoblast fusion — enhancing cell membrane proximity
It is apparent that cell adhesion molecules are not sufficient to bring the membranes of fusion partners into close enough proximity required for fusion. Recent studies have revealed that the two fusion partners rearrange their actin cytoskeleton and the actomyosin network to achieve greater membrane proximity.
The third step toward myoblast fusion — destabilizing the lipid bilayer
Once the two muscle cell membranes are brought into close proximity by the interplay between the protrusive and resisting forces from the two fusion partners, the lipid bilayers need to be destabilized to facilitate fusion pore formation. Studies in Drosophila have yet to reveal any molecular components that directly facilitate this step of myoblast fusion. However, studies in cultured mouse C2C12 myoblasts in vitro suggested that transient exposure of phosphatidylserine (PS) on the cell
Exocytosis
Electron microscopy (EM) and genetic analyses in Drosophila have indicated a role for exocytosis in myoblast fusion. EM analyses of Drosophila embryos revealed vesicles with electron-dense rims at muscle cell contact sites [20, 22, 24, 25, 26, 54, 55, 56]. These vesicles appear to bud from the Golgi and being transported on microtubules [24]. Although similar vesicles have not been observed in the vicinity of actin-propelled membrane protrusions at the asymmetric fusogenic synapse, they may be
Concluding remarks
The past decade has witnessed unprecedented progress in our understanding of myoblast fusion, owing to the application of multifaceted experimental approaches and studies in multiple genetically amenable model systems. The discovery of the asymmetric fusogenic synapse has overturned the conventional view that myoblast fusion is a symmetrical process with equal contributions from both fusion partners. A biophysical framework has emerged in that protrusive and resisting forces from the two fusion
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgements
We thank the members of the Chen lab for discussions and Khurts Shilagardi for comments on the manuscript. Supported by NIH/NIAMS R01AR053173, NIH/NIGMS R01GM098816, the Muscular Dystrophy Association, and a National Established Investigator Award from the American Heart Association.
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