Opposing Crosstalk between Rho1 and Cdc42 in Epithelial Morphogenesis

Epithelial cells line all surfaces of the body exposed to external environments where they perform critical roles for maintaining homeostasis. In addition, epithelial cells are implicated in several disease processes and are the most common cell type implicated in cancer. Therefore, understanding the regulation of epithelial cell development and function has important implications for adult homeostasis and disease states. The Rho family of small GTPases functions in a wide array of cellular processes in epithelial cells. However, in mammals Rho subfamilies have multiple members, often with overlapping roles, complicating the precise determination of Rho protein function in epithelial cells, especially in vivo. In Drosophila, two Rho subfamilies have only one member, Rho1 and Cdc42, which allows for straightforward loss-of-function analysis, in vivo. To determine the role of Rho1 and Cdc42 in Drosophila epithelia, we used mosaic clonal analysis and targeted RNA interference expression to perform loss-of-function studies with Rho1 and Cdc42 during pupal eye and larval imaginal disc morphogenesis. First, clonal analysis in the post-mitotic Drosophila pupal eye epithelium demonstrated that Rho1 was required to maintain AJ integrity independent of its role in sustaining apical cell tension. Rho1 depletion disrupted adherens junctions only when depleted in adjacent cells. Rho1 maintained AJs by inhibiting DE-cadherin endocytosis in a Cdc42/Par6-dependent manner. In contrast, depletion of Rho1 in single cells decreased apical tension, and Rok and Myosin were necessary downstream of Rho1 to sustain apical cell tension. Second, clonal analysis in the pupal eye epithelium further demonstrated that Cdc42 was also critical in limiting apical cell tension. It did so by localizing Par6/aPKC to AJs, where this complex limited Rho1 activity, and thus, acto-myosin contractility. Lastly, studies in larval imaginal discs identified Cdc42 and the Par polarity complex as novel regulators of apoptosis-induced compensatory proliferation. Depletion or disruption of this complex from AJs induced JNK-dependent apoptosis and compensatory proliferation. This was mediated by increased Rho1-Rok activation downstream of Cdc42 depletion, and Rok\u27s regulation of Myosin activity but not F-actin activated JNK. Therefore, opposing crosstalk between Rho1 and Cdc42 dictates epithelial cell shape, junctions, and compensatory proliferation during morphogenesis