The C. elegans endonuclease NUC-1 acts in engulfing cells to degrade the apoptotic cell DNA

During C. elegans embryonic development, cells undergoing programmed cell death are engulfed by neighboring cells and degraded inside phagosomes. Here we characterize a DNase responsible for the degradation of the chromatin DNA of apoptotic cells. In the past, NUC-1, a homolog of mammalian DNase II, which is only active at acidic pH, was claimed to act in apoptotic cell nuclei for chromatin DNA degradation by some researchers, yet proposed to act in engulfing cells by others. We found that NUC-1 acts exclusively in engulfing cells to degrade apoptotic cell DNA. In nuc-1 mutant embryos, apoptotic cell chromatin DNA remains undegraded. We observed that being engulfed is necessary for the apoptotic chromatin DNA to be degraded. In addition, specific expression of nuc-1 in the engulfing but not dying cells rescues the nuc-1 mutant phenotype. Furthermore, blocking the fusion between lysosomes and a phagosome in engulfing cells blocks apoptotic chromatin DNA degradation. NUC-1 was reported to be a lysosome-located enzyme. We not only confirmed this localization pattern, but also further determined that NUC-1 does not reside in the nuclei of either apoptotic or live cells. This, together with our finding that the nucleus of an apoptotic cell is not acidic, indicates that NUC-1 does not act in the apoptotic cell nucleus; rather, it acts in the engulfing cell phagosomal lumen to degrade apoptotic chromatin DNA. Our work clarified a long-standing controversy regarding the action of NUC-1 and advanced our knowledge of the mechanisms that drive the degradation of specific components of dying cells.