approximately 80% of the programmed cell deaths that normally occur still happen.
When the sequence of various cell-death stages was precisely followed by lineage
analysis in these weak cell-death mutants, several unexpected observations were made
(Hoeppner et al., 2001; Reddien et al., 2001). In many cases, the onset of death was
later than normal, and the execution of cell death was slowed down as in the situation
in ced-8 mutants, as one would expect if there were only threshold levels of caspase
activity. Similarly, in some instances, early stages of cell death occurred, but the cells
eventually returned to normal nonapoptotic morphology and remained alive. In some
cases, however, potential interactions between the cell-death and the cell-engulfment
machinery were revealed. It was observed that corpses failed to be engulfed, indicating
a caspase defect that might block the engulfment process. In other cases, corpses were
engulfed at stages that occurred much earlier than in wild-type animals. In addition
to these morphologic observations, double mutant analysis between weak ced-3 and
engulfment mutations also suggests an intricate link between the regulation of
apoptosis and corpse engulfment, as engulfment mutations enhance the cell-death
defects conferred by weak cell-death mutations (Hoeppner et al., 2001; Reddien et
al., 2001). It will be interesting to clarify the pathways that communicate between
the apoptosis and the engulfment pathways.
On the basis of genetic studies on the generation of many double-mutant
combinations, engulfment genes can be placed into two distinct, partially redundant
pathways involving the ced-1, ced-6, and ced-7 and the ced-2, ced-5, ced-10, and ced-12
group of engulfment genes (Ellis et al., 1991). Various double-mutants between the
two groups of mutants, but not within a single group, show a significantly elevated
number of persistent cell corpses compared to single mutants. In addition to defects
in corpse engulfment, ced-2, ced-5, ced-10, and ced-12 also show a defect in the
migrations of the gonadal distal tip cells (DTCs), resulting in abnormally shaped
gonads that frequently include extragonadal bends or bends at inappropriate positions
(Ellis et al., 1991; Gumienny et al., 2001). The migration defect seems to be specific
to the DTC, as ced-5 mutants appear to be normal in the migration of other cells,
such as the HSN neurons.
All seven C.elegans engulfment genes have been cloned, and their homologs have
been identified in vertebrate species, including humans. These studies have allowed
us to begin to understand how engulfing cells recognize dying cells, and what the ‘eat
me’ signals sensed by the dying cells are (Figure 6). Studies in vertebrates have
identified a number of molecules that may participate in the recognition step of the
engulfment process (for review, see Fadok et al., 2001; Feller, 2001; Schlegel et al.,
2000; May and Machesky, 2001). At a certain stage, apoptotic cells induce the
exposure of phosphatidylserins on the surface, an effect that is associated with a general
loss of the phospholipid asymmetry. Further changes in surface carbohydrates and
surface charges might also be involved in the recognition by the phagocytes, and a
number of mammalian receptors have been thought to interact with ligands on the
apoptotic cell surface. The C.elegans CED-1 protein bears structural similarities to
many transmembrane receptors, the most similar of which is the human SREC
(scavenger receptor from endothelial cells) that contains 16 EGF-like motifs (Zhou
178 GENETICS OF APOPTOSIS