Epithelial cell fusion is required for tissue repair following UV-A irradiation

Cell cycle-dependent and independent mechanisms lead to the generation of mononucleated and multinucleated polyploid cells. The more than doubling of a cell's nuclear genome by endoreplication has been found to be an adaptation to genotoxic stress, enabling cell survival despite DNA damage. However, it remains unknown whether cells that increase ploidy via multinucleation also arise in response to genotoxic stress. Here, we use ultraviolet light A (UV-A) to induce permanent DNA damage in cells within the adult fruit fly epithelium. UV-A irradiation causes an injury-like response where giant multinucleated, polyploid cells arise following cell death. The epithelial cells undergo endoreplication, which is required to compensate for cell loss, but is surprisingly dispensable for tissue repair. UV-A irradiation also induces cell fusion, which generates multinucleated cells that encompass almost the entire epithelial area post injury. Cell fusion can be inhibited by expression of a dominant negative Rac or Cdc42 GTPase, which then blocks epithelial tissue repair postirradiation. Apoptotic nuclei were detected at the site of cell junction breakdown, suggesting that apoptosis itself or an apoptotic signal is required for polyploidization in this model. Expression of the effector caspase inhibitor, p35, led to inhibition of apoptosis, the endocycle, and cell fusion post UV-A. Therefore, we have found that caspase activation is necessary for polyploidization post injury and enhancing cell ploidy via multinucleation is another strategy to enable cell survival and tissue repair following genotoxic stress.