Programmed Cell Death in C. elegans
Programmed cell death is a naturally-occurring cellular process in which cells self-destruct by activation of an intrinsic suicide program. Like cell proliferation, cell death is an essential aspect of animal development and homeostasis. Both processes are tightly controlled so that cell numbers in tissues and organs are maintained at appropriate levels. Misregulation of programmed cell death may underlie many human diseases including cancers, autoimmune disorders, neurodegenerative disorders and immunodeficiency diseases.
We use the nematode Caenorhabditis elegans as a model system to study how programmed cell death is regulated and executed, because C. elegans is uniquely amenable to both molecular genetic and biochemical analyses and because the cell death pathway is conserved between nematodes and mammals. The study of cell death in nematodes can provide crucial information towards understanding cell death mechanisms in humans, and ultimately, may identify means to combat human diseases caused by misregulation of cell death.
Genetic studies in C. elegans have identified many genes that are important for four sequential steps of programmed cell death (see Figure): the decision-making step of which cells should die, the execution of cell death, the engulfment of cell corpses, and the degradation of cellular debris. Several genes have been cloned and found to encode proteins homologous to proteins involved in mammalian cell death, including a transcription factor (ces-2), a cysteine protease that executes the suicide program in all the cells that are doomed to die (ced-3), and a death inhibitor similar to the mammalian cell survival factor Bcl-2 (ced-9). The CED-3 death protease is particularly interesting. CED-3 is first synthesized as an inactive proenzyme and later is activated through proteolysis, which then triggers the activation of the death program and a series of cellular and morphological changes in the dying cells. Thus the study of how CED-3 acts to cause cell death offers a great paradigm for studying the regulation and the execution of programmed cell death.
We will focus on addressing three key issues about the CED-3 death protease: 1) how is CED-3 expressed and activated in the right cell and at the right time? 2) what are the substrates of the CED-3 death protease? and 3) how do proteolytic cleavages of these substrates by CED-3 elicit the cellular and morphological changes that lead to the demise of a cell?
Using a combination of genetic, molecular biological and biochemical approaches, we will examine the expression of the ced-3 gene in C. elegans and the interaction of the CED-3 protease with proteins in the cell death pathway. We will perform both genetic and biochemical screens to identify proteins that regulate the activity of the CED-3 protease and proteins that are substrates of the CED-3 protease. Finally, we will express protease substrates in mammalian cells and in nematodes to examine their roles in the execution of programmed cell death.