Tramtrack69 regulates epithelial tube expansion in the Drosophila ovary through Paxillin, Dynamin, and the homeobox protein Mirror. Nathaniel Peters, Celeste Berg. Dept of Genome Sciences/MCB Program, University of Washington, Seattle, WA.

   Epithelial tubes serve as the infrastructure for organs and tissues and are essential for most multicellular life; faithful tube morphogenesis requires precise orchestration of cell signaling, shape, polarity, migration, and adhesion. In the Drosophila ovary, the follicular epithelium that encases each developing egg chamber forms a pair of epithelial tubes, the lumens of which act as molds for the eggshell respiratory filaments, or dorsal appendages (DAs). This system is a robust and accessible model for epithelial tube patterning, formation, and expansion. The Tramtrack69 (TTK69) transcription factor controls DA lumen volume by regulating tube expansion; the twin peaks (twk) mutation reduces TTK69 levels specifically during late oogenesis, inhibiting tube expansion and producing stunted eggshell DAs. Microarray analysis of wild type and twk ovaries, followed by in situ hybridization and RNAi of candidate genes, identified the focal adhesion scaffold Paxillin, the endocytotic regulator Shibire (Dynamin), and the homeodomain transcription factor Mirror as TTK69-regulated effectors of DA-tube expansion. These genes display enriched expression in DA-tube cells, reduced expression in twk, and RNAi phenotypes that are enhanced in a twk heterozygous background, indicating genetic interactions. Although Mirror is known to pattern the follicular epithelium prior to DA tube morphogenesis, we demonstrate that Mirror regulates DA-tube expansion independently of patterning, revealing a novel tube morphogenic role for this transcription factor. We show that Mirror, as well as TTK69, positively influences the expression of Paxillin, suggesting that these TTK69 effectors are in the same pathway. Finally, our results implicate several other genes, including shibire, as tube expansion effectors downstream of TTK69. Thus, our characterization of twk-differentially expressed genes identifies novel tube morphogenesis regulators, begins to elucidate the network of TTK69 effectors required for epithelial tube expansion, and significantly advances our understanding of this vital developmental process.