Bruce A. Hay
Division of Biology 156-29
California Institute of Technology
Pasadena, CA 91125
1997 Searle Scholar
Mechanisms Controlling Programmed Cell Death
We are interested in how cell fate choice is regulated and carried out. A large focus of our work is directed towards understanding the genetic and molecular mechanisms that regulate and bring about apoptotic cell death. Apoptosis is a form of regulated cell death in which superfluous or harmful cells are removed from an organism. Apoptosis is required for many aspects of normal development, tissue size homeostasis, and as a defense against potentially harmful cells, such as self reactive cells in the immune system, virally infected cells, cells that have damaged DNA, or cells that are being induced to proliferate inappropriately. Because cell death is widespread during the development and normal function of organisms, deregulation of this process has dire consequences. Inappropriate cell death is associated with degenerative neurological diseases such as Alzheimer's disease and Parkinson's disease; inhibition of normally occurring cell death can contribute to the development of auto immune disorders, persistent viral infections, and can set the stage for cancer by preventing the death of cells that would normally die, allowing them to undergo mutations that could lead to transformation.
The identification in worms, flies and mammals of homologous proteins that function similarly to regulate cell death indicates that, as with other important signal transduction pathways, components and modes of death regulation are likely to be conserved throughout evolution. We use Drosophila melanogaster as a model system to identify genes that function to regulate cell death, and to identify important roles that cell death plays in normal development. A long term goal is to take the molecules and pathways uncovered in Drosophila, and to apply this information to the study of cell death in vertebrates, with the ultimate goal of determining the role that aberrations in this process play in human pathologies. In this context we see Drosophila as a powerful tool for uncovering it conserved components and modes of death regulation. We use Drosophila as a system in which to study cell death because cell death in flies is regulated in response to many of the same kinds of stimuli that are used in mammalian cells, because of the powerful genetics available in flies, and because large amounts of material can easily be obtained for biochemical studies
One tool we use to identify cell death regulators are genetic screens. Our screens take advantage of the power of Drosophila genetics and the biology of the developing eye. Our general strategy is to alter cell death signaling in the fly eye by driving the eye-specific expression of cell death regulators. We can measure the strength of cell death signaling in the eye in response to this expression by scoring the size and morphology of the adult eye (large, rough appearing eye = decreased normally occurring cell death; small eye = increased cell death).We can use flies with altered patterns of eye cell death as sensitive genetic backgrounds in which to screen for new cell death regulators as modifiers of the initial overexpression-dependent phenotype. During the course of one such screen, in which we searched for modifiers of an induced increase in cell death-dependent small eye phenotype, we identified a family of cell death inhibitors, the Drosophila IAPs (DIAPs), homologous to baculovirus inhibitors of apoptosis (IAPs). Death preventing IAPs are also found in mammals. We are using genetics and biochemical approaches to identify the mechanisms by which these proteins regulate cell death, and by which their activity is regulated.
We have also developed a general screening approach to identify genes that are important regulators of cell fate. This approach takes advantage of the fact that an eye-specific promoter placed in a transposable element (a P element) can function to drive the expression of genes near the genomic site of insertion. By mobilizing such a P element throughout the genome we can rapidly search through a large fraction of the genome for genes that function to regulate cell death and other developmental processes in the eye based on overexpression-dependent phenotypes. Drosophila it is uniquely suited for carrying out gene overexpression screens because P elements can be mobilized throughout the genome at a high frequency, and because they have a preference for insertion near the 5' end of a gene. The engineered P element we use has been designed with features that make it easy for us to isolate the genes being overexpressed, and to knock them out. Thus we are able to move quickly from an overexpression-dependent phenotype to the sequence of the overexpressed gene and a characterization of its loss-of-function phenotype.
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