The actin nucleator WASp is required for myoblast fusion during adultDrosophilamyogenesis

Abstract

SUMMARYMyoblast fusion provides a fundamental, conserved mechanism for muscle fiber growth. We demonstrate here that the functional contribution of Wsp, the Drosophila homolog of the conserved actin nucleation-promoting factor (NPF) WASp, is essential for myoblast fusion during the formation of muscles of the adult fly. Disruption of Wsp function results in complete arrest of myoblast fusion in all muscles examined. Wsp activity during adult Drosophila myogenesis is specifically required for muscle cell fusion and is crucial both for the formation of new muscle fibers and for the growth of muscles derived from persistent larval templates. Although Wsp is expressed both in fibers and individual myoblasts, its activity in either one of these cell types is sufficient. SCAR, a second major Arp2/3 NPF, is also required during adult myoblast fusion. Formation of fusionassociated actin 'foci' is dependent on Arp2/3 complex function, but appears to rely on a distinct, unknown nucleator. The comprehensive nature of these requirements identifies Arp2/3-based branched actin polymerization as a universal mechanism underlying myoblast fusion.

PI(4,5)P2 regulates myoblast fusion through Arp2/3 regulator localization at the fusion site

“…DWip (WASp) and kette (Scar), are defective in focus formation. This also agrees with data from adult Drosophila reporting that Arp2/3 knockdown results in an absence of focus formation (Mukherjee et al, 2011). Nevertheless, small foci are found in our PI(4,5)2 sequestration myoblasts.…”

“…This phenotype is reminiscent of published phenotypes in which either the Mbc-Rac-Kette-Scar or the Blow-DWip-WASp pathway is disrupted, resulting in characteristic teardrop-shaped FCMs (Abmayr and Pavlath, 2012). The FCM teardrop morphology (Chen and Olson, 2001;HakedaSuzuki et al, 2002;Haralalka et al, 2011;Mukherjee et al, 2011;Richardson et al, 2007) observed in these mutants via time-lapse analysis suggests an additional function of Arp2/3 in the expansion of the fusion interface between the fusion partners through changes in FCM morphology. We note that the use of live imaging is key in this regard, as the movement of the FCMs could be monitored, and the integrity of the filopodial processes, particularly in the sequestering background, could be maintained.…”

The actin regulator N-WASp is required for muscle-cell fusion in mice

“…Noteworthy in this context is the observed requirement for Nap1, a regulator of the WAVE NPF, during fusion of cultured murine myogenic cell lines (25). An essential involvement of WASp/WAVE-family elements in myoblast fusion has been previously established for the different phases of Drosophila myogenesis (21,(26)(27)(28)(29)(30)(31)(32).…”

“…Using cell-type-specific expression methods, we have shown that expression of Drosophila WASp and its associated element WIP in either partner of a fusing cell pair is sufficient for fusion to occur (21,28). This mechanistic distinction may be reflective of the inherent difference between the invertebrate and mammalian systems, in which the former is based on fusion A strong β-catenin signal is displayed along contact surfaces of 4.2 ± 1.6% of control cell pairs and 4.8 ± 1.5% of N-WASp-depleted cell pairs (n ∼ 900, P = 0.31).…”

Adult Muscle Formation Requires Drosophila Moleskin for Proliferation of Wing Disc-Associated Muscle Precursors

“…Fiber formation in the pupal stages was analyzed by comparing the total number of 22C10(+) developing fibers per hemi-segment between the control and experimental animals at specific time points. IFM myonuclei were monitored by counting the total number of Ewg(+) nuclei per fiber (Mukherjee et al 2011). N = 6-8 individuals for each genotype were quantified.…”