ced-2/crkII and ced-10/rac have previously unassigned negative roles in engulfment of apoptotic cells in C. elegans

In Caenorhabditis elegans, two partially redundant pathways containing at least eight genes (ced-1, ced-6, ced-7, dyn-1 and ced-2, ced-5, ced-10, ced-12) function in the engulfment of apoptotic cells. Mutations in any of these genes cause engulfment defects and hence result in the persistence of cell corpses. CED-2, a SH2 and SH3 containing adapter protein, may bind to the CED-5/CED-12 heterodimeric guanine nucleotide exchange factor complex to activate CED-10, a small Rac GTPase, leading to cytoskeletal reorganization during engulfment of apoptotic cells. Surprisingly, we found ced-10(n3246) significantly reduced cell corpse number in ced-5 mutants. Using four-dimensional microscopic recording, we analyzed the first 14 cell deaths in the AB and MS cell lineages and found that ced-10(n3246) partially suppressed the engulfment defect of ced-5 mutants. These results reveal a possible negative role of ced-10 in cell-corpse engulfment. Interestingly, ced-10(n3246) did not reduce cell corpse number of ced-12 single or ced-12; ced-5 double mutants, indicating that the negative role of ced-10 may require ced-12. We also found that ced-2(n1994), but not ced-2(e1752), mutation reduced cell corpse number in the ced-10(n3246) background; the reduction was caused by a partial suppression of the Ced-10 engulfment defect. These findings suggest that CED-2 may exert an inhibitory function through its N-terminal SH3 domain. Further genetic analyses showed that the negative role of ced-2 required ced-12 but not ced-5. In summary, our genetic and phenotypic analyses reveal novel roles of ced-2 and ced-10 in negative regulation of cell-corpse engulfment during programmed cell death.