THE ENGULFMENT PROCESS OF PROGRAMMED CELL DEATH IN CAENORHABDITIS ELEGANS

Programmed cell death involves the removal of cell corpses by other cells in a process termed engulfment. Genetic studies of the nematode Caenorhabditis elegans have led to a framework not only for the killing step of programmed cell death but also for the process of cell-corpse engulfment. This work has defined two signal transduction pathways that act redundantly to control engulfment. Signals expressed by dying cells probably regulate these C. elegans pathways. Components of the cell-corpse recognition system of one of the C. elegans pathways include the CED-7 ABC transporter, which likely presents a death ligand on the surface of the dying cell; the CED-1 transmembrane receptor, which recognizes this signal; and the CED-6 adaptor protein, which may transduce a signal from CED-1. The second C. elegans pathway acts in parallel and involves a novel Rac GTPase signaling pathway, with the components CED-2 CrkII, CED-5 DOCK180, CED-12 ELMO, and CED-10 Rac. The cell-corpse recognition system that activates this pathway remains to be characterized. In C. elegans, and possibly in mammals, the process of cell-corpse engulfment promotes the death process itself. The known mechanisms for cell-corpse engulfment leave much to be discovered concerning this fundamental aspect of metazoan biology.