Rho/Rok-dependent regulation of actomyosin contractility at tricellular junctions controls epithelial permeability in Drosophila

Cell contacts in epithelia are remodeled to regulate paracellular permeability and to control passage of migrating cells, but how barrier function is modulated while preserving epithelial integrity is not clear. In the follicular epithelium of Drosophila ovaries, tricellular junctions (TCJs) open transiently in a process termed patency to allow passage of externally produced yolk proteins for uptake by the oocyte. Here we show that modulation of actomyosin contractility at cell vertices controls TCJ permeability. Before patency, circumferential actomyosin bundles are anchored at apical follicle cell vertices, where tension-sensing junctional proteins, Rho-associated kinase (Rok), and active Myosin II accumulate and maintain vertices closed. TCJ opening is initiated by redistribution of Myosin II from circumferential bundles to a medial pool, accompanied by decreasing tension on vertices. This transition requires activation of Cofilin-dependent F-actin disassembly by the phosphatase Slingshot and Myosin II inactivation by Myosin light chain phosphatase, and is counteracted by Rok. Accordingly, constitutive activation of Myosin or of Rho signaling prevent vertex opening, whereas reduced Myosin II or Rok activity cause excessive vertex opening. Thus, the opening of intercellular gaps in the follicular epithelium relies on relaxation of actomyosin contractility rather than active actomyosin-based pulling forces. Conversely, F-actin assembly is required for closing intercellular gaps after patency. Our findings are consistent with a force transduction model in which TCJ integrity is maintained by vertex-anchored contractile actomyosin. We propose that the cell-type-specific organization of actomyosin at cell vertices determines the mode of contractility-dependent regulation of epithelial permeability.