Epithelial tissues cover surfaces and subdivide developing embryos into discrete compartments that give rise to the function units of tissues and organs. Epithelial cells are connected to one another and the underlying extracellular matrix via specialized junctions that are formed by cell adhesion proteins and cytoplasmic adapter proteins that dynamically interact with the cytoskeleton. These components enable epithelial cells to sense their mechanical environment and dynamically modify their interactions with neighboring cells to maintain epithelial integrity. Disruption of these junctions leads to a range of pathologies from congenital defects such as spina bifida to pathologies including blistering disease and cancer. During development, the shape and proportions of epithelial tissues are remodeled in a process known as morphogenesis. Contractile actomyosin networks that generate tensile forces power a range of morphogenetic processes. However, the mechanisms that activate and modulate these networks in space and time remain poorly understood. The objective of our current research is to elucidate the mechanisms that enable epithelial cell networks to remodel in precise spatial and temporal domains yet maintain their stability. To accomplish this goal, we focus on identifying and characterizing gene networks that control epithelial organization and remodeling. The approaches we use leverage the strength of genetic loss- and gain-of-function analyses, high-resolution time lapse imaging and quantitative image analysis to uncover dynamic molecular interactions and their effects on cellular behavior and tissue remodeling.
In previous work we screened for genes affecting epithelial elongation. We identified several regulators of Rho GTPase signaling whose depletion by RNAi impaired epithelial remodeling. One of the genes identified in the screen was RhoGAP68F, the fly homolog of vertebrate RhoGAP50C. We discovered that RhoGAP68F inhibits endocytic recycling of cell adhesion molecules back to the cell surface in Rab4 recycling endosomes to decrease the stability and increase the fluidity of epithelial cell-cell contacts during remodeling. We are investigating the involvement of other RhoGEFs and RhoGAPs in epithelial remodeling. More recently, we discovered that the WAVE regulatory complex (WRC), the Arp2/3 complex and F-actin branching dynamically counterbalance contractile actomyosin networks during epithelial remodeling. We found that these networks generate cyclical pushing and pulling forces at the level of adherens junctions that enable epithelial cells to search for and establish optimal cell shapes and packing geometry. As the role of the WRC and F-actin branching in remodeling epithelial cell-cell junctions has been elusive, our current goal is to determine the mechanisms that target the WRC to cell-cell junctions and the biomechanical signaling pathways that regulate the activity of the WRC at these junctions. We are investigating the involvement of Rho GTPase and phosphoinositide signaling in regulating the cyclical activation of the WRC and F-actin branching, and the role of the immunoglobulin superfamily adhesion protein Sidekick in this process. Understanding the role and regulation of the WRC and F-actin branching in epithelial junctional remodeling will provide fundamental insights to the understanding of epithelial morphogenesis in other tissues and model systems and a range of pathologies that disrupt the stability and plasticity of epithelial cell networks.