Programmed cell death (PCD) is essential for normal metazoan development. Apoptosis is a caspase dependent form of PCD in which the dying cell undergoes morphologically conserved changes such as chromatin compaction, membrane blebbing, and cell shrinkage. While apoptosis has been the focus of intense research, as have been autophagic and necrotic cell deaths, other forms of cell death have been described. However, evidence for in vivo relevance of such cell death programs has not been obtained, nor have proteins participating in these non-apoptotic processes isolated. We are investigating the mechanism of linker cell death in C. elegans to understand the molecular basis of one such alternative cell death pathway. The linker cell is the migratory leading cell of the developing male gonad, and undergoes PCD at the L4/adult transition. Analysis of linker cell death has shown that its death is independent of
ced-3 caspase and all known cell death genes (1). The linker cell employs a cell-autonomous death program, and utilizes an unknown set of engulfment genes for its removal. Electron microscopy of the dying linker cell revealed non-apoptotic features, including nuclear crenellation, absence of chromatin condensation, organelle swelling, and accumulation of cytoplasmic membrane-bound structures. Remarkably, similar features are seen during developmental death of neurons in the vertebrate spinal cord and ciliary ganglia, suggesting that linker cell death is morphologically conserved (2,3). Since the linker cell dies at the L4/adult transition we speculated that developmental timing genes could regulate its death. Indeed, we found that two heterochronic genes,
let-7 and
lin-29, control linker cell death. The linker cell inappropriately survived in 53% of
lin-29(
n333) males examined, and in 20% of
let-7(
n2853ts) males examined. To identify additional genes involved in linker cell death we have performed a genome-wide RNAi screen covering ~94% of the C. elegans genome. Using the available RNAi feeding libraries, synchronized L1 animals were fed dsRNA expressing bacteria and screened for inappropriate linker cell survival in adult males. Analysis of potential positive clones is currently underway. (1) Abraham, M.C. et al. (2007) Dev. Cell 12, 73-86. (2) Oppenheim, R.W. et al. (2001) J. Neurosci. 21, 4752-4760. (3) Pilar, G., and Landmesser, L. (1976) J. Cell Biol. 68, 339-356.