Gene name - breathless
Synonyms - DFGFR-1; Dtk2, Fgf-r Fibroblast-growth-factor-receptor, DFR2
Cytological map position - 70C6--70D6
Function - receptor kinase
Symbol - btl
Genetic map position - 3-
Classification - FGF receptor homolog - Ig superfamily
Cellular location - surface
breathless is involved in two distinctly different processes during the development of the fly. First it is used in the development of the trachea, and later it plays a role in migration of specific midline glia and the resultant effect on axonogenesis. These are similar developmental processes to those affected by Drifter, a POU homeodomain transcription factor which could be downstream of breathless.
The ligand is Branchless, and its developmentally regulated distribution is responsible for the spatially restricted activation of Breathless. Function of the extracellular domain of Breathless can be assessed by substituting a Torso sequence for that of the FGF-receptor sequence. Such a hybrid molecule corrects tracheal defects in breathless mutants, suggesting that the Breathless receptor does not receive spatial clues from a spatially restricted FGF ligand (Reichman-Fried, 1994).
Breathless shares a downstream effector pathway with Torso, Sevenless and the EGF-R/Torpedo receptor. Thus the all purpose RAS-RAF pathway is used for each of these receptors. How are specific downstream targets regulated when the signaling pathway is redundant? Tracheal cells specifically express many markers independently of breathless (Krüppel, Caudal and cut for example). These cells form a completely different chemical milieu when compared with retinal cells, follicle cells and terminal cells, all of which have receptors that use the RAS-RAF pathway.
Some cell functions are common to both tracheal cells and midline glia. Cell migration is regulated by Breathless in both types of cells. Use of a heat inducible dominant negative Breathless protein, lacking a functional tyrosine kinase, reveals that breathless is required at the initiation of tracheal cell migration. Distinct subsets of tracheal cells designated as terminal cells extend long processes toward target tissues. Blind-ended tubes called tracheoles are formed connecting to the main tracheal branches. Tracheoles insure the proper supply of air to growing larval tissues. Late induction of the dominant negative form of Breathless blocks tracheole formation. Thus breathless is implicated in a third developmental process, the formation of tracheoles (Reichman-Fried, 1995).
The patterned branching in the Drosophila tracheal system is triggered by Branchless (Bnl), which activates Breathless and the MAP kinase pathway. A single fusion cell at the tip of each fusion branch expresses the zinc-finger gene escargot, leads branch migration in a stereotypical pattern and contacts with another fusion cell to mediate fusion of the branches. A high level of MAP kinase activation is also limited to the tips of the branches. Restriction of such cell specialization events to the tip is essential for tracheal tubulogenesis. Notch signaling plays crucial roles in the singling out process of the fusion cell. Notch is activated in tracheal cells by Branchless signaling through stimulation of Delta (Dl) expression at the tips of tracheal branches and activated Notch represses the fate of the fusion cell. In addition, Notch is required to restrict activation of MAP kinase to the tips of the branches, in part through the negative regulation of Branchless expression. Notch-mediated lateral inhibition in sending and receiving cells is thus essential to restrict the inductive influence of Branchless on the tracheal tubulogenesis (Ikeya, 1999).
Six primary branches form in the tracheal primordia, among which the dorsal branch (DB), anterior and posterior dorsal trunk (DTa, DTp), and anterior and posterior lateral trunk (LTa, LTp) migrate along a stereotyped path to be connected with other branches from adjacent primordia. These fusion branches are capped with fusion cells that express Esg. The remaining visceral branch (VB) migrates to reach the internal organs. Terminal cells expressing Drosophila serum response factor (DSRF) are formed in each primary branch except in DTs and later differentiate multiple tracheoles. High expression of DL mRNA and protein is expressed in the DT of stage-15 embryos. Cells in the tracheal primordium just after invagination expressed Dl uniformly. At early stage 11, Dl expression started to be elevated in 2-3 cells at the tip of the branches in which outgrowth had begun and the number of the Dl-expressing cells was reduced to one at late stage 11. At stage 14, high Dl expression remains only in the DT, which has completed fusion. Ser protein also accumulates at the apical side of the DT cells at the same stage. In the case of the trachea, the level of N protein expression remains uniform, suggesting that the expression level of Dl, or its potentiation, must be crucial for N activation. An esg-lacZ reporter is initially expressed in 2-3 cells at the tip of the fusion branches in mid stage 11 embryos, and is downregulated to be maintained in only a single cell at the tip of each fusion branch at late stage 11. These cells also express a high level of Dl. Therefore, localized elevation of Dl expression in stage 11 correlates well with the selection process of a single fusion competent cell (Ikeya, 1999).
When tracheal cells became unresponsive to Bnl due to btl mutation, no sign of primary branching and Dl upregulation is observed. On the contrary, when Btl is hyperactivated by overexpression of Bnl in all the tracheal cells, primary branching is severely inhibited and Dl expression is elevated. These results suggest that elevation of Dl expression is triggered by the external signal Bnl. The results also suggest that the N/Dl pathway may mediate the Bnl signal to control cell migration and cell fate decisions (Ikeya, 1999).
In Nts1 embryos grown under non-permissive condiditons a misrouting defect was observed in DB, which normally elongates to the dorsal midline where it meets its counterpart from the other side of the metamere. DBs are often curved in the anteroposterior direction and make contact with the tip of DB from the same side. The misrouted DBs accumulate a luminal component detectable by 2A12 antibody at the ectopic contact sites, but do not appear to fuse properly. Cell migration defect is also observed in DB. DB consists of a total of 5-7 cells in the case of Tr5, of which two specialized cells are located at the tip. One is the terminal cell, from which a thin terminal branch sprouts: the other is the fusion cell. The remaining stalk cells are located between the tip and DT at regular intervals. In Nts1 mutants, the number of cells at the tip is increased with a corresponding decrease in the number of stalk cells, the latter having become unusually elongated. The total number of cell nuclei do not change compared to the controls, so no additional mitosis occurs. This cell migration phenotype suggests that stalk cells and terminal cells acquire a property of fusion cells to become localized at the tip of DB. Since Esg represses DSRF expression and terminal branching, the loss of terminal branching in Nts1 embryos may be the consequence of ectopic Esg expression. Similar defects in Esg and DSRF expression are also observed in null N mutants. These results suggest that in N minus embryos, several terminal and stalk cells are recruited to the fate of fusion cells (Ikeya, 1999).
A three-step model is presented for tubule formation in fusion branches. During induction, exogenously supplied Bnl activates its receptor Btl in equivalent tracheal cells where N is inactive. The signal is transduced by activation of MAPK to stimulate Dl expression. The expression of Bnl is also regulated negatively by N signaling. During lateral inhibition, induced Dl activates N in neighboring cells, which in turn, represses esg transcription. N may also repress Dl expression. However, a high level of Dl inhibits N signaling in a cell autonomous manner, allowing activation of esg and MAPK. A small difference in the response to Bnl within equivalent tracheal cells is amplified to select out a single fusion cell with a high level of Dl and esg expression. During tubule formation, the fusion cell becomes the only cell that responds to Bnl and becomes motile. Maintenance of Btl activity by Bnl would limit the migration toward the source of Bnl. Other tracheal cells follow fusion cells to become stalk cells (Ikeya, 1999).
Bases in 5' UTR - 60
Exons - two
The extracellular region encodes five immunoglobulin-like domains, and an N-terminal signal sequence. The cytoplasmic kinase domain exhibits a high degree of similarity to the vertebrate FGF-Rs with the typical split kinase and comparably sized juxtamembrane and carboxy-terminal regions (Glazer, 1991 and Klambt, 1992).
date revised: 2 Jan 97
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