Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and Delta-Notch signaling to the air sac primordium

RESEARCH ARTICLE elifesciences.org Myoblast cytonemes mediate Wg signaling from the wing imaginal disc and Delta-Notch signaling to the air sac primordium Hai Huang, Thomas B Kornberg* Cardiovascular Research Institute, University of California, San Francisco, San Francisco, United States Abstract The flight muscles, dorsal air sacs, wing blades, and thoracic cuticle of the Drosophila adult function in concert, and their progenitor cells develop together in the wing imaginal disc. The wing disc orchestrates dorsal air sac development by producing decapentaplegic and fibroblast growth factor that travel via specific cytonemes in order to signal to the air sac primordium (ASP). Here, we report that cytonemes also link flight muscle progenitors (myoblasts) to disc cells and to the ASP, enabling myoblasts to relay signaling between the disc and the ASP. Frizzled (Fz)-containing myoblast cytonemes take up Wingless (Wg) from the disc, and Delta (Dl)-containing myoblast cytonemes contribute to Notch activation in the ASP. Wg signaling negatively regulates Dl expression in the myoblasts. These results reveal an essential role for cytonemes in Wg and Notch signaling and for a signal relay system in the myoblasts. DOI: 10.7554/eLife.06114.001 *For correspondence: Competing interests: The authors declare that no competing interests exist. Funding: See page 20 Received: 17 December 2014 Accepted: 16 April 2015 Published: 07 May 2015 Reviewing editor: K VijayRaghavan, National Centre for Biological Sciences, Tata Institute for Fundamental Research, India Copyright Huang and Kornberg. This article is distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use and redistribution provided that the original author and source are credited. Introduction Flight muscles of the Drosophila adult drive the coordinated movements of the wings and thoracic cuticle to power flight, and many thin tubes (tracheoles) that emanate from the thoracic dorsal air sacs penetrate the muscles to oxygenate them. Thus, the functions of the muscles, wings, thoracic cuticle, and trachea are linked, and the physical associations are intimate. The progenitor cells that produce these tissues develop together in the wing imaginal disc. Previous studies from this lab showed that the air sac primordium (ASP), which is the progenitor of the dorsal air sacs, depends on Branchless/ FGF (FGF) and Dpp signaling proteins that the wing disc produces (Sato and Kornberg, 2002; Roy et al., 2014). Here, we describe two other signaling systems that coordinate the progenitors of the flight muscles with the wing disc and trachea. The wing disc can be described as a flattened sac that juxtaposes the apical surfaces of two connected epithelial sheets across a narrow lumen. One of the sheets, called the columnar epithelium because its cells are highly elongated along their apical/basal axis, generates the wing blade and most of the notum, the dorsal cuticle of the thorax. The wing disc is encapsulated by a basement membrane, but a branch of the tracheal system (the transverse connective) penetrates the basement membrane at several sites in the dorsal region of the disc (Guha et al., 2009). Transverse connective that is within the basement membrane lies adjacent to the basal surface of the columnar epithelium, and during the third instar (L3), this segment of the transverse connective sprouts a tubular outgrowth—the ASP—in response to FGF expressed by a group of nearby columnar epithelial cells (Sato and Kornberg, 2002). Myoblasts that are the progenitors of the flight muscles are also at the basal surface of the columnar epithelium, underneath the basement membrane, and in the vicinity of the tracheal branches. They proliferate during L3 to extend over most of the dorsal part of the disc where the cells that will produce the notum cuticle grow (Sudarsan et al., 2001; Gunage et al., 2014). Huang and Kornberg. eLife 2015;4:e06114. DOI: 10.7554/eLife.06114 1 of 22 Research article Cell biology | Developmental biology and stem cells eLife digest Fruit fly larvae undergo a remarkable physical transformation to become an adult fly. During this transformation, the tissues in the larvae change into the structures found in the adult. For example, the adult wings, flight muscles, and other structures needed for coordinated flight form from a pair of disc-like tissues called the wing imaginal discs. For these structures to develop correctly, the cells in the wing imaginal discs need to receive coordinated instructions about what types of cells they need to become. Within the wing discs, finger-like projections called cytonemes link specific cells together to allow signal molecules to move between the cells; this controls the development of the wing disc itself as well as structures called dorsal air sacs, which supply oxygen to the flight muscles in the adult fly. However, it is not known if cytonemes allow the exchange of signal molecules between cells involved in the formation of other structures needed for flight. Here, Huang and Kornberg investigated the role of cytonemes in the development of the flight muscles in fruit flies. The experiments reveal that cells called myoblasts—which will later become the flight muscle cells—form two sets of cytonemes with other cells. One set connects the myoblasts to cells in the developing air sac, which allows a signal protein called Delta to signal from the myoblasts into the air sac cells. The other set of cytonemes connects the myoblasts to wing disc cells. This enables another signal molecule called Wingless, which is produced in wing disc cells, to move into the myoblasts and block the production of Delta. Huang and Kornberg’s findings reveal a new role for cytonemes in coordinating the development of the flight muscles and the dorsal air sacs. A future challenge will be to understand how individual cytonemes are able to connect to specific cells. DOI: 10.7554/eLife.06114.002 Signaling proteins that contribute to the growth and diversification of the cells of the wing disc have been extensively characterized. Three that are relevant to the ASP and myoblasts are Notch, Dpp, and Wg (Couso et al., 1995; Ng et al., 1996; Brennan et al., 1999; Steneberg et al., 1999; Sudarsan et al., 2001; Baena-Lopez et al., 2003; Giraldez and Cohen, 2003; Marois et al., 2006; Herranz et al., 2008; Gunage et al., 2014). Notch signaling has essential roles at both the dorsal/ ventral and anterior/posterior compartment borders of the disc, and although it has been shown to specify fusion cell fate and branch identity during formation of tracheal system in the embryo, a role in larval trachea has not been reported. Studies in several other contexts indicate that Notch signaling may be mediated by cytonemes that make direct contacts between signaling cells (Renaud and Simpson, 2001; Cohen et al., 2010). Dpp-expressing cells line the anterior side of the anterior/pos (...truncated)