Tracheal cells invaginate from the epidermis and subsequently migrate in a stereotyped pattern guided by the local expression of the Fibroblast growth factor (FGF) homolog Branchless (Bnl)

INTRODUCTION Tip cells are specialized endothelial cells that lead the migration of sprouting vessels and mediate anastomosis formation during vascular development in vertebrates (Geudens and Gerhardt, 2011; Herwig et al., 2011). Signaling through Vascular endothelial growth factor receptor (VEGFR), a receptor tyrosine kinase (RTK), is involved in selecting tip cells and in guiding their directed migration. However, the mechanisms that select between different tip cell behaviors (migration versus anastomosis formation) are not clear. In the Drosophila tracheal system, 20 groups of epidermal cells generate a tubular network through a sequence of branching and tube fusion events (Ghabrial et al., 2003; Uv et al., 2003; Affolter and Caussinus, 2008; Maruyama and Andrew, 2012). Tracheal cells invaginate from the epidermis and subsequently migrate in a stereotyped pattern guided by the local expression of the Fibroblast growth factor (FGF) homolog Branchless (Bnl) (Sutherland et al., 1996). Bnl activates the FGF receptor (FGFR) Breathless (Btl) on tracheal tip cells, which lead the concerted migration towards the Bnl source. Reminiscent of the role of VEGFR signaling in angiogenesis, Bnl promotes and guides cell motility, but also stimulates differentiation of tracheal tip cells through activating Ras-MAPK signaling (Samakovlis et al., 1996a; Samakovlis et al., 1996b). Whereas the branch stalk cells form tubes with extracellular lumina sealed by cell-cell junctions, tip cells generate two different types of seamless tubes with intracellular lumina. Terminal cells (TCs), which express the Drosophila Serum response factor homolog (DSRF; Bs – FlyBase) (Guillemin et al., 1996; Montagne et al., 1996) under the control of Ras-MAPK signaling, develop branched and lumenized cytoplasmic extensions that mediate gas exchange at the target tissues. The second type of seamless tubes, generated by fusion cells (FCs), mediates the connection of adjacent tracheal metameres. A single FC expressing the Zn-finger protein Escargot (Esg) (Samakovlis et al., 1996b; Tanaka-Matakatsu et al., 1996) and the bHLH protein Dysfusion (Dys) (Jiang and Crews, 2003) is specified at the tip of each branch that will connect with a cognate branch from a neighboring or contralateral tracheal metamere. FC specification involves FGF signaling and branch-specific Wingless (Wg) and TGFβ signals, which promote the FC fate (Steneberg et al., 1999; Chihara and Hayashi, 2000; Llimargas, 2000). Conversely, Delta/Notchdependent lateral inhibition prevents neighboring cells from assuming FC fate, thus ensuring that a single FC is specified at each branch tip (Ikeya and Hayashi, 1999; Llimargas, 1999). Although high-level MAPK activation in tip cells promotes both FC and TC specification, the choice between the two tip cell fates is regulated in a branch-specific fashion. The tips of most tracheal branches contain a single FC and at least one TC. However, the main longitudinal dorsal trunk (DT) tube contains only FCs, whereas the visceral branches (VB), which originate from the same region as the DT, contain only TCs. Although the correct choice between the two different seamless tube types is essential for the formation of a functional respiratory network, how tip cell fate choice is coordinated with branch identity remains unclear. It has been shown that DT identity and FC specification in the DT depend on Wg signaling (Chihara and Hayashi, 2000; Llimargas, 2000), but how Institute of Molecular Life Sciences and PhD Program in Molecular Life Sciences, University of Zurich, Winterthurer Str. 190, CH-8057 Zurich, Switzerland.