Mechanisms and Consequences of Apoptosis and Apoptosis-induced Proliferation in Drosophila Principal Investigator: Andreas Bergmann, Ph.D. University of Massachusetts Medical School, Worcester, MA Apoptosis is the major form of cell death that is critical for normal development and tissue homeostasis of multi-cellular organisms. Defects in the regulation of apoptosis contribute to the pathogenesis of multiple diseases including those associated with reduced rates of cell death (cancer, autoimmunity) or with excessive cell death (neurodegeneration, stroke, myocardial infarction). Apoptotic cells interact with and influence the behavior of their cellular environment by releasing anti-inflammatory, pro- and anti-apoptotic as well as mitogenic signals. The release of the latter triggers Apoptosis-induced Proliferation (AiP) which describes the ability of apoptotic cells to induce regenerative proliferation of neighboring surviving cells, thus compensating for their loss. Unexpectedly, evidence obtained in several organisms including Drosophila, Xenopus, Hydra, Mouse and human cancer suggests that regenerative AiP of amputated or otherwise damaged tissues including tumors depends on apoptotic caspases (highly specific cell death proteases) in addition to, but independently of, their apoptotic function. The overall objective of this scientific program is to gain a comprehensive understanding of the biological principles that underlie the regulation of apoptosis and AiP in a multi-cellular organism, to identify and characterize the genes involved in these processes, and to develop methods to manipulate them. We are using the powerful genetic model organism Drosophila melanogaster for these studies. We have developed genetic models of apoptosis and AiP, and initiated forward genetic screens that directly assessed the genetic basis of these fundamental processes. This application focuses on four key questions. 1. How is the fine-tuning of caspase activity achieved? 2. What are the proteolytic targets of caspases for non-apoptotic functions? 3. How do caspases control the generation of reactive oxygen species (ROS) for AiP? 4. How do macrophages (hemocytes) adopt an activated phenotype for growth control? This program is very relevant for understanding of human cancer. Our studies elucidate mechanisms by which potential tumor cells increase their resistance to apoptosis, a hallmark of cancer, which may generate immortalized (undead) cells. Moreover, apoptotic tumor cells promote caspase-dependent AiP. For example, although radio- and chemotherapy attempt to cure cancer by killing tumor cells, relapse of treated tumors is frequently observed which may be due to an AiP-promoting activity of dying tumor cells. Therefore, the results of this research program will significantly improve our understanding of apoptosis and regenerative proliferation under normal conditions, and tumor phenotypes under pathological conditions.