Essentially all animal cells have the ability to kill themselves when they are no longer needed by activating a gene-encoded cell suicide program. This self-destruction program leads to apoptosis, a morphologically distinct form of cell death. Apoptosis is of central importance for development and tissue homeostasis. On the other hand, aberrant regulation of apoptosis is intimately associated with a variety of human diseases, including cancer, autoimmunity, AIDS, neurodegenerative disorders, liver diseases, and ischemic stroke. Since the basic cell death program is constitutively expressed in virtually all animal cells, precise regulatory mechanisms must exist that restrict the activation of apoptosis to unwanted cells.
More than a decade ago, we discovered and characterized a family of novel proteins (Reaper proteins) that act as central integrators of many different signaling pathways to ensure that the death program is specifically activated in cells that are doomed to die. Reaper, Hid, and Grim (now termed RHG proteins) activate apoptosis by binding to and inactivating inhibitor of apoptosis proteins (IAPs), which in turn directly inhibit caspases, the key executioners of apoptosis. In this way, Reaper and related proteins remove powerful "brakes on death." We have also shown that, in addition to liberating caspases from their inhibitors by disrupting IAP-caspase protein complexes, Reaper stimulates the autoubiquitination and degradation of IAPs and in this way irreversibly destroys key protection against cell death. Significantly, reaper, hid, and grim are transcriptionally regulated by a variety of death-inducing stimuli, including steroid hormones, segmentation and patterning genes, and DNA-damaging agents. Therefore, these genes act as integrators for relaying different apoptotic signals to the core death program.
Our finding that transcription of reaper, hid, and grim is regulated by many different signaling pathways provided the first molecular explanation for the role of de novo gene expression in initiating apoptosis. Furthermore, it explains how integration of many different signaling pathways can be achieved to regulate cell death. RHG proteins are also extensively regulated at the post-transcriptional level, and in this way they connect a large number of signaling pathways with the core cell death program.
More recently, we have shown that caspase activity and an apoptosis-like phenomenon (apoptosis without death) is necessary to generate mature sperm, and that caspase activation in this system is strictly regulated by mitochondria and cytochrome c. A similar mechanism contributes to the removal of bulk cytoplasm during mammalian spermiogenesis, and we have found that mice mutant for a caspase-activating protein are defective in removing the residual sperm cytoplasm and are male sterile. Drosophila spermatogenesis is a unique and powerful system to identify new cell death genes because it permits bringing the full power of Drososphila genetics to study the role of mitochondria for caspase regulation. We have conducted systematic screens for mutants that affect caspase activation in this system, and we are characterizing many of the corresponding genes. In this way, we expect to gain insights into how mitochondria contribute to the regulation of cell death.
Developing tissues can often compensate for the massive loss of cells in response to injury and/or stress (such as radiation). We discovered that Drosophila cells that undergo apoptosis can stimulate their own replacement by secreting mitogens to induce proliferation of adjacent progenitor cells. These secreted mitogens include Wingless (Wnt) and BMP/TGFβ family proteins. Since these pathways have been highly conserved in evolution, similar phenomena may occur in mammals as well, and we are investigating this possibility. These findings have profound implications for cancer therapy, stem cell biology, and regenerative medicine.
While we use Drosophila as our primary model to discover cell death genes and order them into pathways, we are also testing whether concepts originally developed in Drosophila can be applied to mammalian systems. For this purpose, we have generated mouse strains with mutations in select cell death genes. For example, we showed that inactivation of ARTS (apoptosis-related protein in the TGFβ signaling pathway), a mammalian IAP antagonist, causes male sterility and spontaneous tumor formation in mice. This is the first direct evidence for a role of mammalian IAP antagonists in caspase regulation and tumor suppression in vivo. More generally, our current work is aimed at understanding the coordinate regulation of apoptosis and cell proliferation in normal development and several disease models. We are particularly interested in the regulation of stem cell survival, apoptosis and compensatory proliferation in cancer, and wound repair/tissue regeneration. We expect that knowledge gained from this work can ultimately be exploited to manipulate apoptosis for therapeutic benefits.