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Precise control of cell death in the nervous system is essential for development. Spatial and temporal factors activate the death of Drosophila neural stem cells (neuroblasts) by controlling the transcription of multiple cell death genes through a shared enhancer, enh1. The activity of enh1 is controlled byabdominalAandNotch, but additional inputs are needed for proper specificity. Here we show that the Cut DNA binding protein is required for neuroblast death, acting downstream of enh1. In the nervous system, Cut promotes an open chromatin conformation in the cell death gene locus, allowing cell death gene expression in response toabdominalA. We demonstrate a temporal increase in global H3K27me3 levels in neuroblasts, which is enhanced bycutknockdown. Furthermore,cutregulates the expression of the cohesin subunit Stromalin in the nervous system. The cohesin components Stromalin and NippedB are required for neuroblast death, and knockdown of Stromalin increases repressive histone modifications in neuroblasts. Thus Cut and cohesin regulate apoptosis in the developing nervous system by altering the chromatin landscape.Summary statementCut regulates the programmed death of neural stem cells by altering cohesin levels and promoting a more open chromatin conformation to allow cell death gene expression.

Cell death is a prevalent, well-controlled and fundamental aspect of development, particularly in the nervous system. In Drosophila, specific neural stem cells are eliminated by apoptosis during embryogenesis. In the absence of apoptosis, these stem cells continue to divide, resulting in a dramatically hyperplastic central nervous system and adult lethality. Although core cell death pathways have been well described, the spatial, temporal and cell identity cues that activate the cell death machinery in specific cells are largely unknown. We identified a cis-regulatory region that controls the transcription of the cell death activators reaper, grim and sickle exclusively in neural stem cells. Using a reporter generated from this regulatory region, we found that Notch activity is required for neural stem cell death. Notch regulates the expression of the abdominalA homeobox protein, which provides important spatial cues for death. Importantly, we show that pro-apoptotic Notch signaling is activated by the Delta ligand expressed on the neighboring progeny of the stem cell. Thus we identify a previously undescribed role for progeny in regulating the proper developmental death of their parental stem cells.

In this chapter we discuss methods that can be used to study apoptotic cell death in the Drosophila embryo, ovary, as well as in cultured cell lines. These methods assay various aspects of the cell death process, from mitochondrial changes to caspase activation and DNA cleavage. The assays are useful for examining apoptosis in normal development and in response to developmental perturbations and external stresses. These techniques include Acridine Orange staining, TUNEL, cleaved caspase staining, caspase activity assays and assays for mitochondrial fission and permeabilization.