Programmed cell death is an essential process during metazoan development. During wildtype C. elegans development, nearly all cells slated to die activate caspases and undergo stereotypical morphological changes including chromatin compaction and cell shrinkage. The male-specific linker cell (LC), however, is an exception. The LC leads the migration of the developing gonad to the cloaca, and once migration is complete at the L4-to-adult transition, the LC dies using a cell-autonomous death program. Strong loss-of-function alleles of canonical cell-death pathway genes, including
ced-3,
ced-4, and
egl-1, as well as a
ced-9(gf) mutation, do not block LC death, indicating that LC death must be controlled by a novel program (1). Indeed, electron micrographs of dying LCs reveal 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 deaths of neurons in the vertebrate spinal cord and ciliary ganglia, suggesting that LC death is morphologically conserved. Previous studies revealed that LC death requires the developmental timing genes
let-7 and
lin-29, and that these genes act within the LC to promote cell death. To identify additional components required for LC death, we performed a genome-wide RNAi screen aimed at identifying genes whose loss prevents LC death. We identified several candidate genes, and have begun to verify these. One of the genes we identified is
tir-1: 30% of
tir-1(RNAi) mutants exhibit LC survival.
tir-1 encodes a Toll-like/interleukin-1 receptor adaptor protein that functions as a signaling protein in innate immunity and during neuronal cell-fate establishment. In these processes,
tir-1 functions upstream of
nsy-1/MAPKKK and
sek-1/MAPKK. We reasoned that
tir-1 could be activating a similar MAPK cascade in the LC and tested whether strong loss-of-function alleles of
nsy-1 and
sek-1 also block LC death. While the
nsy-1(
ky397) and
nsy-1(
ky542) alleles do not affect LC death, the
sek-1(
km4) allele prevents LC death in 20% of males. These results suggest that a MAPK cascade may control genes essential for LC death. Downstream targets of this cascade may emerge from studies of the other RNAi candidates we identified and/or from a genetic screen currently in progress (E.B. and R. Razi, unpublished). 1) Abraham et al., Dev. Cell 12:73-86, 2007.