Unique patterns of organization and migration of FGF-expressing cells during Drosophila morphogenesis

Abstract

Fibroblast growth factors (FGF) are essential signaling proteins that regulate diverse cellular functions in developmental and metabolic processes. In Drosophila, the FGF homolog, branchless (bnl) is expressed in a dynamic and spatiotemporally restricted pattern to induce branching morphogenesis of the trachea, which expresses the Bnl-receptor, breathless (btl). Here we have developed a new strategy to determine bnl-expressing cells and study their interactions with the btl-expressing cells in the range of tissue patterning during Drosophila development. To enable targeted gene expression specifically in the bnl expressing cells, a new LexA based bnl enhancer trap line was generated using CRISPR/Cas9 based genome editing. Analyses of the spatiotemporal expression of the reporter in various embryonic stages, larval or adult tissues and in metabolic hypoxia, confirmed its target specificity and versatility. With this tool, new bnl expressing cells, their unique organization and functional interactions with the btl-expressing cells were uncovered in a larval tracheoblast niche in the leg imaginal discs, in larval photoreceptors of the developing retina, and in the embryonic central nervous system. The targeted expression system also facilitated live imaging of simultaneously labeled Bnl sources and tracheal cells, which revealed a unique morphogenetic movement of the embryonic bnl- source. Migration of bnl- expressing cells may create a dynamic spatiotemporal pattern of the signal source necessary for the directional growth of the tracheal branch. The genetic tool and the comprehensive profile of expression, organization, and activity of various types of bnl-expressing cells described in this study provided us with an important foundation for future research investigating the mechanisms underlying Bnl signaling in tissue morphogenesis.

Development and Function of the Drosophila Tracheal System

“…Each branch migrates toward a specific target tissue; for example, the dorsal branch toward the dorsal epidermis, the visceral branch toward the intestine, and the ganglionic branch toward the ventral nerve cord. Branch migration is guided by Bnl, which is expressed dynamically in a number of nontracheal tissues (Sutherland et al 1996;Ohshiro et al 2002;Du et al 2017) (Figure 2A). At stage 15 and later, Bnl-Btl signaling stimulates the differentiation and outgrowth of a tracheal cell subtype called terminal cells, which send cytoplasmic branches out toward target tissues to support respiration ( Figure 2, B and C).…”

Imaging Cytonemes in Drosophila Embryos

“…Usually growing tips of the dorsal and ventral branches project many cytonemes toward the FGF (fibroblast growth factor) source situated ahead of them (Fig. 4) [38]. Cytonemes from cells in the en stripes extend toward the neighboring cells (Fig.…”

“…A higher number of parent flies provide better chances to get synchronized embryos within a certain time window. For a repeated collection of the embryos, establishing a genetically combined or recombined fly stock harboring the driver and the reporter genes is convenient and recommended (e.g., recombined btl-Gal4, UAS -CD8:GFP homozygous stock on second chromosome [38]).…”

Multiple Requirements for Rab GTPases in the Development ofDrosophilaTracheal Dorsal Branches and Terminal Cells

“…Selection of tip cells within the DB group, followed by migration of the tip cells and intercalation of the remaining cells to form the DB stalk. During this competitive process, cells with higher Btl activity migrate to the tip of the group (Kato et al, 2004;Le Droguen et al, 2015;Du et al, 2017), and lateral inhibition mediated by Delta and Notch restricts the number of tip cells to two (Ohshiro et al, 2002;Schottenfeld et al, 2010). 3.…”