Control of Myoblast Fusion by a Guanine Nucleotide Exchange Factor, Loner, and Its Effector ARF6

Abstract

Myoblast fusion is essential for the formation and regeneration of skeletal muscle. In a genetic screen for regulators of muscle development in Drosophila, we discovered a gene encoding a guanine nucleotide exchange factor, called loner, which is required for myoblast fusion. Loner localizes to subcellular sites of fusion and acts downstream of cell surface fusion receptors by recruiting the small GTPase ARF6 and stimulating guanine nucleotide exchange. Accordingly, a dominant-negative ARF6 disrupts myoblast fusion in Drosophila embryos and in mammalian myoblasts in culture, mimicking the fusion defects caused by loss of Loner. Loner and ARF6, which also control the proper membrane localization of another small GTPase, Rac, are key components of a cellular apparatus required for myoblast fusion and muscle development. In muscle cells, this fusigenic mechanism is coupled to fusion receptors; in other fusion-competent cell types it may be triggered by different upstream signals.

FuRMAS: triggering myoblast fusion in Drosophila

“…In myogenesis, Arf6 probably has an overlapping and, therefore, redundant function with another Arf GTPase (see Ultrastructural Features of Myoblast Fusion section). A dominant-negative version of Arf6 was reported to reduce fusion efficiency in transfected C2C12 murine myoblast cell culture (Chen et al, 2003), which implies that Arf6 might also play a role during vertebrate myogenesis. Indeed, also using C2C12 murine myoblast cell culture in an RNAi approach, Pajcini et al, (2008) analysed the role of GEF Brag2 (Schizo/Loner in Drosophila) and GEF Dock180 (Mbc in Drosophila) and observed a lowered fusion index.…”

“…Schizo/Loner, a GEF for the Adenosin ribosylation factor (Arf) family of GTPases, is suggested to be another signalling molecule within fcs/growing myotubes and was shown to bind Arf6 in in vitro assays (Chen et al, 2003). arf6 loss-of-function mutants exhibit a wild-type muscle pattern and mutant flies are vital but male sterile (Dyer et al, 2007).…”

“…Moreover, the Arf6 guanine nucleotide exchange factor Schizo/Loner and the Arf6-GTPase are evolutionarily conserved in both vertebrates and Drosophila (Chen et al, 2003;Ö nel et al, 2004). In myogenesis, Arf6 probably has an overlapping and, therefore, redundant function with another Arf GTPase (see Ultrastructural Features of Myoblast Fusion section).…”

Regulation of actin cytoskeleton dynamics by Arf-family GTPases

“…Interestingly, the Drosophila Arf6 GEF, Loner, promotes Rac activation and localization during myoblast fusion [41], a process that also requires the Drosophila ortholog of Dock180, myoblast city [42].…”

Visualizing new dimensions in Drosophila myoblast fusion

“…For example, mbc, Rac, kette and loner all have RNA expression in the early embryo that could be maternally loaded and could therefore affect the final phenotype. (41,47,51,57) Inefficient fusion could thus explain the differences in final nuclei number observed between kette, mbc, loner and Rac mutants and the intermediate fusion blocks often observed with rescue experiments. (21,49) rols, in contrast, is clearly not maternally loaded.…”