Naturally occurring or "programmed"
cell death (often referred to as "apoptosis") appears to
be a universal feature of animal development, and abnormalities in
programmed cell death have been associated with a broad variety of
human diseases, including certain cancers and neurodegenerative disorders.
Of the 947 non-gonadal cells generated by the C. elegans cell
lineage, 131 undergo programmed cell death. Our laboratory is defining
a molecular genetic pathway for programmed cell death. We have characterized
genes that cause cells to die, that protect cells from dying, that
specify which cells are to live and which are to die, that function
in the engulfment of dying cells by their neighbors, and that are
involved in degrading the debris from the 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 was the first
caspase gene identified to function in programmed cell death, and
its discovery led to the identification of numerous apoptotic caspases
that act in mammals and other animals. ced-3 action is facilitated
by ced-4, which is similar to human Apaf-1, identified because
it promotes caspase activation in a biochemical system. 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 "BH-3 only" 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. Three such genes –
ces-1, ces-2 and tra-1 – encode transcription factors.
The ces-2 gene is similar to human E2A-HLF, the product of
an oncogene implicated in leukemia and thought to act by perturbing
apoptosis. The ced-8 gene contributes to the process of cell
killing.
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.
The current molecular genetic pathway
for programmed cell death.
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 (see SIGNAL
TRANSDUCTION, CHROMATIN REMODELING AND TRANSCRIPTIONAL REGULATION).
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.
Publications:
Programmed Cell Death
Abstracts:
Programmed Cell Death
