The regulation of programmed and pathological cell death in C. elegans

Abstract

Programmed cell death, or apoptosis, is important in the development and homeostasis of metazoans. In the nematode C. elegans, four genes, egl-1, ced-9, ced-4, and ced-3, constitute the core pathway acting in all somatic programmed cell deaths. This pathway is evolutionarily conserved in humans. The BH3-only protein EGL-1 is transcriptionally upregulated in cells fated to undergo programmed cell death, and EGL-1 blocks cell-death inhibition by the cell-death regulator CED-9, a Bcl-2 family member. The binding of EGL- 1 to CED-9 releases the Apaf- 1-like adaptor protein CED-4 from CED-9, so that CED-4 can activate the caspase CED-3, a protease that is the effector of programmed cell death. In this thesis, I describe three projects, each of which examines one aspect of C. elegans cell death. From. screens for mutations that increase cell death in a sensitized genetic background, I identified a gene that protects cells from programmed cell death.

(cont.) This gene, spk-1, encodes a homolog of SR protein kinases, which regulate alternative splicing. Previous work has shown that ced-4 pre-mRNA is alternatively spliced to generate two transcripts that function oppositely in cell death. I found that spk-1 regulates ced-4 transcript splicing, thereby influencing the amount of programmed cell death that occurs. From a screen for genes that promote programmed cell death, I isolated a mutation in a conserved non-coding element in the transcriptionally regulated cell-death activator gene egl-1. This element regulates the deaths of specific cells in the C. elegans ventral nervous system. I found a novel C. elegans transcription factor, Y38C9A. 1, that binds this element and might function to regulate egl-1 transcription and programmed cell death in the ventral nervous system. In addition to the programmed cell deaths that occur in C. elegans, pathological death of specific cells can be caused by mutations in some genes. I characterized two genes, lin-24 and lin-33, that can mutate to cause the inappropriate death of specific hypodermal blast cells. One of these genes, lin-24, contains a domain similar to that found in some bacterial toxins.

(cont.) By morphological and genetic criteria, I show that the lin-24- and lin-33-mediated deaths are unlike previously characterized necrotic and apoptotic cell deaths in C. elegans. These deaths require some of the genes responsible for engulfing the corpses generated by programmed cell death, even though the deaths do not require the core genes of the genetic pathway of programmed cell death.