Cell fusion after wounding requires plasma membrane damage and endocytosis

Tissue wounds comprise both dead and damaged cells. In epithelial wounds, repair is accomplished by cells at the wound edges, which are themselves often damaged. In the Drosophila pupal notum, wound-adjacent epithelial cells with plasma membrane damage often fuse to form syncytia; when plasma membrane damage is prevented, syncytia do not form. Damaged cells share cytoplasm as soon as milliseconds after wounding, and fusion pores connecting cell membranes form minutes later. A genetic screen reveals that wound-induced fusion requires endocytosis machinery, and dynamin localization indicates that endocytosis preferentially targets plasma membrane removed during fusion. Endocytosis promotes cell fusion by specifically promoting fusion pore expansion, indicated by quantitative analysis of cytoplasmic sharing between cells over time. Without endocytosis-mediated cell fusion, wound healing is slowed. Together, our results support a model of damage-induced cell fusion in which plasma membrane damage initiates fusion pores and endocytosis expands fusion pores, resulting in cellular fusion as an integration of single cell damage with tissue repair.