We exploit a genetic model, Drosophila, to understand the physiology of cell death during normal development and after cell injury. In this animal, deletion of a complex genomic interval, designated the Reaper region, prevents all programmed cell death (PCD).
We discovered four genes in this region that function as pivotal activators of the apoptotic pathway: Reaper, Grim, Hid, and Sickle. In germline transformation experiments, these genes partially rescue cell death defects and, in development, these death activators are expressed in patterns that precede the onset of PCD. In cultured cells and in transgenic animals, each gene is sufficient to provoke apoptosis that can be suppressed by caspase inhibitors.
The Reaper genes function through parallel circuits to engage a common set of effectors including caspases. Proteins encoded by the Reaper region function, in part, to liberate active caspases from inhibition by inhibitor-of-apoptosis proteins (IAPs) through activities that are homologous to mammalian IAP antagonists (e.g. Smac/Diablo).
We also discovered another essential regulator of apical caspase activation in flies: Dark. This gene is homologous to human Apaf-1 and to nematode CED-4. Like its mammalian counterpart, the protein is essential for the apoptogenic action of apical caspases and mutations at this gene exhibit profound failures in apoptosis. In related studies, we also established that Dark is a central effector of histolytic cell death and pivotal in some models of pathologic injury, where cell death is provoked by genotoxic stress or polyglutamine toxicity.
Our lab conducted genetic analyses of the two apical caspases in flies: Dronc and Dredd. Dronc encodes the only caspase-recruitment-domain-containing caspase in the Drosophila genome and, as such, exhibits shared structural features with initiator caspases in mammals. Animals lacking Dronc present a range of defects, including extensive hyperplasia of hematopoietic tissues and persisting neuronal cells. Furthermore, in ex vivo preparations, Dronc- cells were completely insensitive to induction of cell killing by diverse stimuli.
Together, these studies place Dronc, and its activator Dark, at an obligate position for converging pathways that specify PCD and stress-induced apoptosis.