| The Drosophila follicle cells form an epithelial tissue that surrounds the germline cells in the ovary. During oogenesis multiple signaling events between follicle cells and germline cells are critical for egg chamber patterning. Meanwhile, follicle cells undergo dynamic morphogenetic changes including proliferation, shape change and directional migration. To identify genes that regulate follicular epithelium morphogenesis, I collaborated in an EMS mutagenesis screen of the X chromosome. From the screen we isolated 252 mutant lines that exhibited defects in various aspects of follicle cell morphogenesis. To date we have characterized 20 mutant groups of 116 mutant lines through recombination mapping and complementation tests. In this dissertation four mutant groups are discussed in detail.We identified two novel genes, Rabconnectin-3 alpha and beta (Rbcn-3A and B), as important regulators of Notch signaling in Drosophila. Mutations in Rbcn-3 disrupt Notch signaling in follicle cells and imaginal disc cells. In addition to disrupting Notch signaling, mutations in Rbcn-3A and B cause defects in endocytic trafficking, where Notch and other membrane proteins accumulate in late endosomal compartments. Interestingly, the yeast homolog of Rbcn-3A, Rav1, regulates the V-ATPase proton pump responsible for acidifying intracellular organelles. We found that, similarly, Rbcn-3A and B appear to regulate V-ATPase function. Moreover, we identified mutants in VhaAC39, a V-ATPase subunit, and showed that they phenocopy Rbcn-3A and Rbcn-3B mutants. Our results demonstrate that Rbcn-3 affects Notch signaling and trafficking through regulating V-ATPase function, which implies that the acidification of an intracellular compartment in the receiving cells is crucial for signaling.We also analyzed mutations in CG10260, a phophatidylinositol 4-kinase (PI4KIIIalpha), and found that this gene plays an important role in the posterior follicle cells for oocyte polarization. PI4KIIIalpha mutant follicle cells exhibited phenotypes similar to those caused by mutations in the Hippo signaling pathway. Moreover, PI4KIIIalpha mutations lead to abnormal expression of Hippo signaling targets in the follicle cells. PI4KIIIalpha mutations affected the integrity of the apical domain structure in the follicle cells. Our analysis of PI4KIIIalpha mutants potentially provides a link to understanding Hippo signal transduction from the cell membrane to its core kinase cascade. |
