Apoptotic priming is regulated during Drosophila development. Yunsik Kang, Arash Bashirullah. Sch Pharmacy, Univ Wisconsin, Madison, Madison, WI.

   Resistance to apoptosis is a hallmark of cancer cells meditated in part by an increased threshold for initiating caspase activation. Despite its importance in disease, however, the role of apoptotic thresholds under normal physiological conditions remains poorly understood. Here we demonstrate that apoptotic thresholds vary dramatically during development and, as a result, not all developing cells are primed to trigger apoptosis. Primed cells initiate caspase activation and apoptosis in response to expression of death activator proteins like the IAP-antagonist reaper (rpr) or to loss of the IAP diap1. In contrast, we identified unprimed cells that are resistant to ectopic expression of IAP-antagonists. Surprisingly, these unprimed cells are also resistant to diap1 knockdown, challenging the notion that IAPs are the final barrier to initiation of apoptosis. We show that unprimed cells are 50-fold more resistant than primed cells and that these unprimed resistant cells are characterized by reduced levels of core death genes like Ark and Nc (the Apaf1 and caspase-9 homologs, respectively). Importantly, increasing expression of Ark and Nc is sufficient to prime previously resistant cells to respond to death activators. Conversely, reducing levels of Ark and Nc is sufficient to confer apoptotic resistance to primed cells. We show that apoptotic priming precedes, and is essential for, programmed cell death. Finally, our data suggests that apoptotic priming is regulated by ecdysone in a tissue- and stage-specific manner during major developmental transitions. Thus, regulation of apoptotic priming provides a novel and critical cellular protection mechanism during development.