Bruce A. Hay

haybruce@caltech.edu
Ph.D., 1989, University of California - San Francisco

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Molecular Genetics of Cell Death

We are interested in several areas of biology. See our lab web site for more detailed information on each of these areas (http://www.its.caltech.edu/~haylab/). A primary focus within the lab is the study of programmed, or apoptotic, cell death. Apoptotic cell death 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, de-regulation of this pathway 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 immunity, 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. 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. Important cellular regulatory pathways are evolutionarily conserved; thus, molecules identified as important regulators of cell death in Drosophila are likely to have counterparts in vertebrates and the pathways that link these molecules are likely to be regulated similarly.

Our work on cell death is leading us in several new directions. One of these is the study of small, noncoding RNAs known as microRNAs. Almost all cell death regulatory genes identified to date encode proteins. However, within the last several years it has become clear that animal genomes encode on the order of hundreds (2), or perhaps thousands, of small noncoding RNAs known as microRNAs (miRNAs). It is generally thought that miRNAs downregulate gene expression by binding to 3' UTR regions of target transcripts. miRNA binding to target transcripts, as a part of a proteinaceous complex known as the RISC, targets these transcripts for translational inhibition if the miRNA and mRNA target show limited complementarity, or transcript cleavage and degradation if the miRNA and mRNA target show perfect or near perfect complementarity. The functions of most miRNAs have not been explored. We identified several miRNAs that function as potent cell death inhibitors. One of these miRNAs, mir-14, also functions to negatively regulate fat storage. We are interested in understanding the contexts in which these miRNAs function to regulate death and the nature of their targets.

Our work on cell death has also led us into the study of several nonapoptotic processes, spermatogenesis and compensatory proliferation. Spermatozoa throughout the animal kingdom are generated and mature within a germline syncytium. Differentiation of haploid syncytial spermatids into single motile sperm requires the encapsulation of each spermatid within an independent plasma membrane and the elimination of most sperm cytoplasm, a process known as individualization. Little is known about how individualization is carried out. However, the importance of one aspect of this process for human fertility, the elimination of excess cytoplasm, is suggested by the fact that many conditions or treatments resulting in infertility disrupt this process. We recently reported that multiple caspase family proteases and their activators were required for spermatid individualization in Drosophila. These observations were striking because caspases are the core of the evolutionarily conserved, apoptotic cell death machine. Once activated they typically cleave a number of cellular substrates that ultimately lead to cell death and corpse phagocytosis. Our observations raise a number of questions as to how caspases promote cell differentiation without inducing cell death.

Finally, we have also found that activation of death signaling promotes - indirectly - cell proliferation. Achieving proper organ size requires a balance between proliferation and cell death. For example, at least 40-60 percent of cells in the Drosophila wing disc can be lost, yet these discs go on to give rise to normal-looking adult wings as a result of compensatory proliferation. The signals that drive this proliferation are unknown. One intriguing possibility is that they derive, at least in part, from the dying cells. To explore this hypothesis we activated cell death signaling in specific populations of cells in the developing wing but prevented these cells from dying. We found that the presence of these "undead" cells resulted in proliferation of neighboring cells. Thus, dying cells send a signal that stimulates compensatory proliferation, thereby contributing to tissue homeostasis. This pathway may also contribute to deregulation of tissue growth in cancer. For example, cells expressing high levels of dMyc, the Drosophila homolog of the myc oncogene, are super-competitors. They survive and proliferate at the expense of neighboring cells, which are eliminated by apoptosis. As noted above, cell death leads to the generation of signals that drive the proliferation of neighbors. Therefore, upregulation of dMyc may induce a positive feedback cycle in which cells with increased dMyc levels promote the death of neighbors, which then send a signal back to the dMyc-overexpressing cells that further stimulates their proliferation, thereby driving Myc-dependent oncogenesis. The identity of the signal produced by dying cells that drive proliferation of neighbors is not known, nor is the mechanism of its activation. These are two topics of current interest.