Epithelial cell rearrangements during tubular organ formation

The tracheal system of Drosophila is a simple branched tubular organ that consists of different branches with distinct cellular architectures. We have analyzed how these branches are formed during embryonic development and how this process is controlled genetically. In particular we focused our analysis on the intercalation process that underlies the transition from thick, multicellular branches to finer branches, in which single cells reach around the lumen and seal it with autocellular AJs. To study tracheal development at the cellular level, I developed a technique that allows for the visualization and manipulation of single tracheal cells in vivo. Using this technique we characterized the intercalation process during dorsal branch formation. Based on these findings we proposed a model of intercalation that involves four steps. We also found that intercalation is specifically blocked in the dorsal trunk by the expression of the transcription factor spalt (sal) and that sal is sufficient to block intercalation also in other branches. To mediate this function, sal expression is required in a group of cells. In the following I tried to identify genes that are involved in this process using different approaches. In a candidate approach I tested a variety of genes that have been implicated in cell rearrangement and intercalation in other systems. Furthermore I tested several candidates with potentially interesting domains that show a trachea-specific expression pattern. I also searched for phenotypes that resemble the gain or loss of function phenotype of sal in a genetic screen using molecularly mapped deficiencies. Finally I initiated a screen using gene chips to identify sal target genes. Using these different approaches I characterized the molecular basis of intercalation. On the one hand I ruled out that some obvious candidates contribute to the process. On the other hand I identified candidates that affect intercalation. In the deficiency screen I identified a deletion that shows ectopic intercalation. This deficiency also exhibits cargospecific secretion defects. I am currently mapping the gene responsible for these defects by generating new deficiencies. Finally I attempted to characterize the forces that drive intercalation. Investigating the role of the FGF pathway I identified a putative positive input of the FGF pathway on intercalation via the transcription factor pointed.