Studies of programmed cell death in the nematode caenorhabditis elegans

Abstract

Programmed cell death is an evolutionarily conserved process that plays critical roles in normal animal development and has been extensively studied in C. elegans. During programmed cell death, caspases are activated in the dying cell. The cell corpse is engulfed by a neighboring cell and degraded. Almost all cell deaths in C. elegans are "suicides"-they are caspase-dependent and apparently cell-autonomous, and do not require engulfment. During development of the C. elegans male, the cells B.alapaav and B.arapaav are generated during the late third-larval stage. During the early fourth-larval stage one of these cells undergoes programmed cell death, and the other survives. These two cells form an equivalence group; the decision of which cell dies and which survives is stochastic. The cell that dies is engulfed by the neighboring cell P12.pa and was speculated to be an engulfment-dependent cell "murder" or an "induced suicide." I have discovered that B.al/rapaav instead represents an "assisted suicide" that requires both the core apoptosis pathway and the engulfment pathway. egl-1 and ced-3 are expressed in the dying or undead cell in wild-type and engulfment-defective animals, and these genes are required for the B.al/rapaav cell death. In engulfment mutants the B.al/rapaav death process fails at a point after caspase activation, suggesting that the core cell-death pathway is necessary but not sufficient for this cell death. Previous genetic screens have not been designed to systematically identify essential genes with a role in cell death. Most somatic cell deaths in C. elegans occur during early development, but several male-specific cell deaths occur during the fourth larval stage. These late cell deaths provide an opportunity to examine essential genes for a role in programmed cell death, as RNAi treatment after hatching can eliminate gene function before these deaths occur but after embryogenesis. I performed an RNAi screen for 1,132 essential genes and assayed the effect on Rn.aap cell survival. I analyzed candidate genes for non-specific effects, such as affecting the Rn cell lineage rather than cell death processes, to find twenty-five essential genes that might have a role in the Rn.aap cell death.