Programmed cell death, also referred to as apoptosis, is critical for the removal of 'unwanted' cells and is conserved from nematodes to humans. How programmed cell death is regulated is still not fully understood. The core programmed cell death pathway in C. elegans consists of four genes
egl-1 BH3-only,
ced-9 Bcl-2,
ced-4 Apaf-1 and
ced-3 caspase, which act in a simple linear pathway. The most downstream gene,
ced-3, encodes a caspase, which triggers cell death execution once its activity has reached a certain 'lethal' threshold. We present evidence that the C. elegans homolog of the mammalian RNA-binding proteins PUM1 and PUM2, PUF-8, plays a dual role in the control of programmed cell death during embryonic development. First, we found that the
puf-8 gene has anti-apoptotic activity and prevents the
ced-3caspase-dependent precocious death of mothers of cells programmed to die. Using single-molecule RNA fluorescent in situ hybridization (smRNA FISH), we show that the loss of
puf-8 causes a significant increase in the copy number of
ced-3 mRNA in the RID neuroblast, a 'mother' of a cell programmed to die, which suggests that
puf-8 prevents the precocious death of mothers by promoting
ced-3 mRNA turnover. Second, we found that
puf-8 has pro-apoptotic activity and promotes the death of cells programmed to die. However, the loss of
puf-8 does not affect the copy number of
ced-3 mRNA in cells programmed to die, which indicates that in this context, PUF-8 has targets other than
ced-3caspase and/or acts through mechanisms other than mRNA turnover. Interestingly, we found that the loss of
puf-8 increases the size of the RID sister cell, which is the smaller daughter of the RID neuroblast and which is programmed to die. An increase in the size of cells programmed to die has previously been shown to compromise the ability of these cells to undergo programmed cell death. Therefore,
puf-8 may promote programmed cell death by ensuring that the sizes of cells programmed to die are below a certain 'lethal' size. Finally, based on these findings, we propose that the dual role of PUF-8 -PUM1,2 in the control of programmed cell death is critical for the robustness with which the same 131 cells reproducibly die during C. elegans development.