Role of expansion in Drosophila tracheal tube diameter regulation. Ekaterini Iordanou, Rachana R. Chandran, Mina Essak, Lan Jiang. Biological Sciences, Oakland University, Rochester, MI.

   The regulation of optimal tubular sizes is a fundamental process that is critical for the function of human lungs, kidneys, and blood vessels. Aberrant alterations in tube sizes during development lead to devastating diseases such as polycystic kidney disease. The Drosophila tracheal system is one of the most powerful model systems used to study tubular epithelial morphogenesis. Despite the recent advances in understanding tubular organ formation, the mechanisms by which cells assemble into tubes, with highly regulated lengths and diameters, are still not well understood. The apical luminal matrix has been shown to be important in the prevention of tube over-expansion; however, mechanisms that mediate apical secretion of specific luminal components are poorly understood. We identified a novel, evolutionarily conserved, Drosophila protein, Expansion (Exp), which is required for tracheal tube-size regulation. In expansion mutants, uni-cellular tracheal branches develop bubble-like cysts. In addition, the secretion of certain luminal proteins is defective. We further demonstrate that the apical localization of Rab11, a member of the family of small GTPases, and Rip11, a Rab11-interacting protein, is significantly reduced in expansion mutants. In addition, Rab11-mediated apical secretion is required for the secretion of certain luminal proteins. Therefore, expansion is required for tube-size regulation partially by controlling Rab11/Rip11-mediated apical section of the luminal matrix. The expansion phenotype exemplifies a role for this novel protein in epithelial lumen formation and tube-size control.