The hindsight gene is required for epithelial maintenance and differentiation of the tracheal system in Drosophila

During animal development, morphogenesis of tissues and organs requires dynamic cell shape changes and movements that are accomplished without loss of epithelial integrity. Data from vertebrate and invertebrate systems have implicated several cell surface and cytoskeleton-associated molecules in the establishment and maintenance of epithelial architecture, but there has been little analysis of the genetic regulatory hierarchies that control epithelial morphogenesis in specific tissues. Here we show that the Drosophila Hindsight nuclear zinc-finger protein is required during tracheal morphogenesis for the maintenance of epithelial integrity and assembly of apical extracellular structures known as taenidia. In hindsight (hnt) mutants tracheal placodes form, invaginate, and undergo primary branching as well as early fusion events. Starting at midembryogenesis, however, the tracheal epithelium collapses or expands to give rise to sacs of tissue. While a subset of hnt mutant tracheal cells enters the apoptotic pathway, genetic suppression of apoptosis indicates that this is not the cause of the epithelial defects. Surviving hnt mutant tracheal cells retain cell-cell junctions and a normal subcellular distribution of apical markers such as Crumbs and DE-Cadherin. However, taenidia do not form on the lumenal surface of tracheal cells. While loss of epithelial integrity is a common feature of crumbs, stardust, and hnt mutants, defective assembly of taenidia is unique to hnt mutants. These data suggest that HNT is a tissue-specific factor that regulates maintenance of the tracheal epithelium as well as differentiation of taenidia.