Programmed Cell Death (Apoptosis)

Naturally occurring or “programmed” cell death is widespread during C. elegans development. We have characterized many genes that act in programmed cell death, including three that cause cells to die, six that function in the engulfment of dying cells by their neighbors, and one that is involved in destroying the debris generated by cell corpses. Most of these genes have human counterparts. For example, the killer gene ced-3 encodes a caspase (cysteine aspartate protease); mammalian caspases similarly cause programmed cell death. ced-3 action is facilitated by ced-4, which is similar to human Apaf-1, identified because it promotes caspase activation in vitro. ced-4 function is blocked by ced-9, which protects cells against programmed cell death and is similar to the human proto-oncogene bcl-2, which also protects against cell death. ced-9 activity is inhibited by the worm killer gene egl-1, which is similar to a number of mammalian killer genes. The activity of egl-1 is controlled in a cell-specific fashion by other genes that specify which cells are to live and which are to die. One such gene, tra-1, encodes a zinc-finger transcription factor that regulates the sexually dimorphic fates of neurons that die in males and control egg-laying in hermaphrodites. Programmed cell death appears to be initiated by the transcriptional activation of egl-1, the product of which binds the mitochondrial protein CED-9 and causes the release of CED-4 from CED-9 and the translocation of CED-4 from the mitochondrial to the nuclear membrane.

The engulfment process removes dying cells. Interestingly, the engulfment process also actively promotes the deaths of engulfed cells. The engulfment gene ced-1 encodes a receptor that allows an engulfing cell to recognize a neighboring cell corpse. ced-7 encodes an ABC transporter that we postulate to act by transporting a small molecule that marks a dying cell so that it can be recognized by the CED-1 protein, which in turn may initiate the death process by directly binding to and signaling via an adaptor protein that is the product of the ced-6 gene. Four other cell-death engulfment genes – ced-2, ced-5, ced-10 and ced-12 – encode components of an intracellular signal transduction system. Mediated by the CED-10 protein, a Rac/Rho family GTPase, this system probably effects the cell-shape changes of the engulfing cell by altering its cytoskeleton. The ced-11 gene affects some of the morphological changes that occur as cells undergo programmed cell death. The nuc-1 gene encodes a DNAse II-like nuclease that helps to degrade the DNA in dying cells.

Many aspects of programmed cell death remain to be elucidated. For example, how do 131 cells decide to die while another 816 decide to live? What actually causes cells to die, e.g., what are the functional targets of the CED-3 caspase? How does the engulfment process facilitate cell killing? What signal from dying cells allows them to be recognized by engulfing cells? What other genes act in the process of programmed cell death? We are currently continuing to elucidate the genetic, molecular and biochemical mechanisms responsible for these and other aspects of programmed cell death. One focus is on sexually dimorphic programmed cell deaths. We have found that the sexual fate of the HSN neurons, which survive in hermaphrodites but die in males, is specified in part by the direct action of the zinc-finger transcription factor TRA-1 (which determines overall organismic sexual identity) as a repressor of the killer gene egl-1: when TRA-1 is active (in hermaphrodites), egl-1 is repressed in the HSNs and these neurons survive, whereas when TRA-1 is inactive (in males), egl-1 is expressed and the HSNs die. By contrast, the male-specific CEM neurons survive in males but die in hermaphrodites. Using a green fluorescent protein (GFP) marker specific for the CEMs, we can very efficiently screen for mutants in which this marker is expressed in hermaphrodites (e.g., 60,000 genomes were screened in three days, yielding 192 independent mutants). This screen has identified a variety of interesting genes, including one that seems to act specifically to control the life vs. death decision of the CEMs. A second current focus involves the identification of new genes that act in the killing and engulfment steps of programmed cell death. We are also identifying suppressors and enhancers of known genes as well as systematically seeking genes that act not only in programmed cell death but also in other, essential processes. Interestingly, among our enhancers of a partial loss-of-function allele of the ced-3 caspase gene are mutations in the gene dpl-1, which encodes a protein similar to human DP. DP proteins act in transcriptional repression in both C. elegans and mammals, and we have characterized dpl-1 in some detail in our studies of C. elegans vulval development. These and other findings suggest that dpl-1 interacts with at least one newly discovered gene to regulate gene expression and promote cell killing after the process of programmed cell death has been initiated. We plan to identify those dpl-1 transcriptional targets that act in programmed cell death and to determine how those targets interact with the core ced-3 caspase pathway to cause cells to die by programmed cell death.