Cell migration and extension during tracheal branching morphogenesis

 

 

Branching morphogenesis is a widely used strategy to increase the surface area of a given organ. A number of tissues undergo branching morphogenesis during vertebrate development, including the lung, the kidney, the mammary and numerous glands. Until recently, very little was known about the genetic principles underlying the branching process and about the molecules participating in organ specification and branch formation.

The tracheal system of insects represents one of the best-characterised branched organs. The tracheal network provides air to most tissues and we and others have intensively studied its development during embryogenesis at the morphological, cellular and genetic level. More than 30 genes have been identified and ordered into sequential steps controlling branching morphogenesis. These studies have revealed a number of important principles that might be conserved in other systems.

One of the most important factors for tracheal branching morphogenesis is Branchless (Bnl), a Drosophila FGF homologue. Bnl acts as a chemoattractant for tracheal cells and its developmentally regulated expression dictates the branching pattern. How this happens at the molecular level remains elusive, especially since each branch that forms requires, in addition to Bnl, different signalling input for proper development.

Using 4D confocal videomicroscopy in living embryos we have demonstrated that Bnl regulates filopodial activity in tracheal tip cells. While other signalling systems are also required for branch outgrowth, these other systems appear not to be required for motility per se, but for cells to be capable to undergo cell rearrangements allowing them to move towards Bnl sources. At present we try to decipher the molecular basis for Bnl-regulated chemotaxis and for signal-regulated branch formation and outgrowth.

Epithelial cells are fated to become tracheal cells during gastrulation upon the expression of the PAS-domain transcription factor Trachealess (Trh). It is the presence of this particular transcriptional regulator that instructs epithelial cells to undergo branching morphogenesis and to respond correctly to Bnl with directed motility. In order to understand this commitment at the molecular level, we are undertaking transcriptional profiling experiments using Affymetrix gene chips and different RNA populations. We hope to eventually isolate genes required for cells to undergo Trh-induced tubulogenesis and branching morphogenesis, with special emphasis on genes that allow the induction of filopodial activity in response to Bnl signalling. The identification of such genes would greatly contribute to understanding how branching processes can remodel pre-existing epithelial structures.