Project Summary Led by a multi-PI team of a cell biologist and a biophysicist, this project is a renewal of our investigation into the cellular detection of and responses to wounds. For our model system, we use the Drosophila pupal notum, a diploid epithelial monolayer, and we wound it by laser ablation. Although the tissue is diploid, the region at and near the wound margin is dominated by giant syncytial cells. The origin, function, and fate of these syncytial cells are all unknown. Using live imaging, we have found that the giant syncytia are formed via cell-cell fusion of multiple diploid cells. These fusions occur within ~20 minutes of wounding and the resulting syncytial cells migrate more quickly and close wounds faster than diploid cells. By the end of tissue repair, most of these giant syncytia are eliminated from the epithelium. Interestingly, we have found that the amount of cell fusion and syncytia formation depends on the mode of wounding. We will compare wound healing behaviors in wounds that lack syncytia and those that have syncytia to investigate how these giant cells increase the rate of wound closure (Aim 1). In Aim 2, we will investigate how wounds induce mononuclear diploid cells to fuse into syncytia. In Aim 3, we will analyze the long-term fate of these syncytia, which appear to die by apoptosis and extrusion as wound closure is ending. Syncytial and polyploid cells have been observed in other organisms and tissues in response to wounds, but our system is the first to make a detailed analysis of their formation, contribution, and elimination possible using live imaging. Cells involved in wound-healing generally share behaviors with tumor cells, and the wound- induced giant syncytial cells may represent the wound equivalent of Giant Polyploid Cancer Cells, a syncytial cell type found in many cancers. Giant Polyploid Cancer Cells are malignant, resistant to all therapies, and appear to be a major source of tumor cells fueling metastasis and relapse. We expect that our studies into the adaptive functions of wound-induced syncytia will be important for understanding the biology, origin, and potential therapies for maladaptive Giant Polyploid Cancer Cells.