Rap1 promotes epithelial integrity and cell viability in a growing tissue

Having intact epithelial tissues is critical for embryonic development and adult homeostasis. How epithelia respond to damaging insults or tissue growth while still maintaining intercellular connections and barrier integrity during development is poorly understood. The conserved small GTPase Rap1 is critical for establishing cell polarity and regulating cadherin-catenin cell junctions. Here, we identified a new role for Rap1 in maintaining epithelial integrity and tissue shape during Drosophila oogenesis. Loss of Rap1 activity disrupted the follicle cell epithelium and the shape of egg chambers during a period of major growth. Rap1 was required for proper E-Cadherin localization in the anterior epithelium and for epithelial cell survival. Both Myo-II and the adherens junction-cytoskeletal linker protein α-Catenin were required for normal egg chamber shape but did not strongly affect cell viability. Blocking the apoptotic cascade failed to rescue the cell shape defects caused by Rap1 inhibition. One consequence of increased cell death caused by Rap1 inhibition was the loss of polar cells and other follicle cells, which later in development led to fewer cells forming a migrating border cell cluster. Our results thus indicate dual roles for Rap1 in maintaining epithelia and cell survival in a growing tissue during development.