Genetics of Apoptosis

damage checkpoint pathway that already exists in yeasts and the metazoan specific
apoptotic machinery. It will be of great interest to determine the molecular link
between cep-1 and the core apoptotic machinery, which is likely to involve
transcriptional activation of cep-1 target genes. Furthermore, additional genetic
screens are likely to uncover novel potentially metazoan-specific DNA damage
checkpoint genes.

12.

Genes required for the engulfment of dying cells

The final executing steps in apoptosis are the engulfment by other cells and the
degradation of dying cells (cell corpses). The engulfment process, which removes cell
corpses before they can lyse and release harmful cytoplasmic contents, is highly
efficient and lasts less than 1 h. Cell corpse engulfment is a multistep process that
involves the recognition of the dying cell followed by the subsequent enclosure by
expanding pseudopodia of the engulfing cell (Ellis et al., 1991). The engulfment,
observed by ultrastructural studies, can already occur even before any morphologic
changes in the dying cell appear (Ellis et al., 1991). In C. elegans, no professional
phagocytes exist. Therefore, neighboring cells usually take up this function. Many
cells have the potential to recognize and engulf apoptotic cells. During embryogenesis,
dying cells are often engulfed by their sister cells, whereas postembryonic cell corpses
are removed, usually by epithelial cells (Sulston and Horvitz, 1977; Hedgecock et al.,
1983). In contrast to the fixed fate of dying cells, engulfment involves stochastic
events, as various cells can engulf a specific dying cell (Hoeppner et al., 2001). In the
C. elegans germ line, engulfment by gonadal sheath cells (these somatic cell surround
the germ line) starts to occur immediately after the syncytial germ-line nuclei become
cellularized during programmed cell death (Liu and Hengartner, 1998; Wu and
Horvitz, 1998; Gumienny et al., 1999).
So far, seven genes, ced-1, ced-2, ced-5, ced-6, ced-7, ced-10, and ced-12, have been
found to be required for efficient engulfment of dying cells (Hedgecock et al., 1983;
Ellis et al., 1991) (Figure 3). Mutations in any of these genes result in a persistent
corpse phenotype. Dying cells in engulfment-defective worms display all morphologic
changes indicative of apoptotic corpses, including the appearance of the highly
refractive disks under Normarski optics. Corpses in engulfment-defective animals can
stay for an extended period of time, disappear by secondary necrosis, or be
phagocytized with delay (Hedgecock et al., 1983; Ellis et al., 1991; Wu and Horvitz,
1998).
Interestingly, two recent papers suggest that there is an unexpected link between
the cell-death machinery and the corpse-engulfment machinery that can be best
explained by a positive feedback regulation between cell-death and cell-engulfment
signals (Hoeppner et al., 2001; Reddien et al., 2001). These studies took advantage
of properties conferred by weak loss-of-function alleles of ced-3 and ced-4 that
presumably result in threshold levels of caspase activity (Hoeppner et al., 2001;
Reddien et al., 2001). These alleles result in only a weak cell-death defect, and

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