Molecular regulation of epithelial tube size

Abstract

In nature, epithelial tubes are vital structures in organ design and are required for transport of gases and liquids in organs, such as the vascular system, the vertebrate lung and the kidneys. The tubular epithelium is single layered, but is often reinforced by layers of muscular support. It constitutes an apical side facing the lumen and a basal side that contacts surrounding tissues. To ensure optimal flow, it is critical that the tubes are correctly sized and shaped. Epithelial tube growth depends on apical membrane enlargements, as well as sub-apical rearrangements, but the mechanisms involved in the regulation of size and shape of epithelial tubes are yet to be revealed. In this thesis the Drosophila respiratory (tracheal) system has been used as a model organ to identify essential genes and clarify the mechanisms involved in the making and shaping of tubes. Through genetic and molecular analyses, new biological concepts have been uncovered. The main tracheal tube, the dorsal trunk (DT), expands three-fold in diameter during a short interval followed by tube elongation. In this thesis we have dissected the roles of five genes in tube regulation, called kkv, knk rtv, dBest2 and DAAM. Analysis of kkv, knk and rtv led us to identify an unprecedented need for luminal matrix components in modeling tube shape. A chitinous luminal matrix is deposited in newly formed tubes and constitutes an expanding cord inside the tube that is required for uniform tube diameter growth. kkv is required for chitin synthesis while knk and rtv are needed for chitin filament assembly. If chitin is missing or fail to form an organized matrix, the expanding tubes develop severe local dilations and constrictions. The subsequent tube elongation requires dBest2 and DAAM. dBest2 encodes an apical chloride channel and is essential for lumen growth during elongation, suggesting that elongation is driven by an increased luminal osmotic pressure. DAAM has a function in actin organization. In the wild type trachea, actin filaments arrange as sub-apical rings perpendicular to tube length, thus allowing for lumen elongation, but not diametrical expansion, upon the increase in lumen pressure. In DAAM mutants, the actin rings are disorganized, thus lumen elongation is inhibited. The luminal chitin matrix has a second role at this stage by preventing excess tube elongation. A balance between combinatorial physical forces exerted by the lumen and sub-apical actin cytoskeleton determines final tube size.