MicroRNAs (miRNAs) are small regulatory RNAs that are between 21 and 25 nucleotides in length and repress gene function through interactions with target mRNAs. The genomes of metazoans encode on the order of several hundred miRNAs, but the processes they regulate have been defined for only two in C. elegans. We searched for new inhibitors of apoptotic cell death by testing existing collections of P element insertion lines for their ability to enhance a small-eye phenotype associated with eye-specific expression of the Drosophila cell death activator Reaper. Here we report the identification of the Drosophila miRNA mir-14 as a cell death suppressor. Loss of mir-14 enhances Reaper-dependent cell death, whereas ectopic expression suppresses cell death induced by multiple stimuli. Animals lacking mir-14 are viable. However, they are stress sensitive and have a reduced lifespan. Mir-14 mutants have elevated levels of the apoptotic effector caspase Drice, suggesting one potential site of action. Mir-14 also regulates fat metabolism. Deletion of mir-14 results in animals with increased levels of triacylglycerol and diacylglycerol, whereas increases in mir-14 copy number have the converse effect. We discuss possible relationships between these phenotypes.

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Spermatozoa are generated and mature within a germline syncytium. Differentiation of haploid syncytial spermatids into single motile sperm requires the encapsulation of each spermatid by an independent plasma membrane and the elimination of most sperm cytoplasm, a process known as individualization. Apoptosis is mediated by caspase family proteases. Many apoptotic cell deaths in Drosophila utilize the REAPER/HID/GRIM family proapoptotic proteins. These proteins promote cell death, at least in part, by disrupting interactions between the caspase inhibitor DIAP1 and the apical caspase DRONC, which is continually activated in many viable cells through interactions with ARK, the Drosophila homolog of the mammalian death-activating adaptor APAF-1. This leads to unrestrained activity of DRONC and other DIAP1-inhibitable caspases activated by DRONC. Here we demonstrate that ARK- and HID-dependent activation of DRONC occurs at sites of spermatid individualization and that all three proteins are required for this process. dFADD, the Drosophila homolog of mammalian FADD, an adaptor that mediates recruitment of apical caspases to ligand-bound death receptors, and its target caspase DREDD are also required. A third apoptotic caspase, DRICE, is activated throughout the length of individualizing spermatids in a process that requires the product of the driceless locus, which also participates in individualization. Our results demonstrate that multiple caspases and caspase regulators, likely acting at distinct points in time and space, are required for spermatid individualization, a nonapoptotic process.

Bruce is a large protein (530 kDa) that contains an N-terminal baculovirus IAP repeat (BIR) and a C-terminal ubiquitin conjugation domain (E2). BRUCE upregulation occurs in some cancers and contributes to the resistance of these cells to DNA-damaging chemotherapeutic drugs. However, it is still unknown whether Bruce inhibits apoptosis directly or instead plays some other more indirect role in mediating chemoresistance, perhaps by promoting drug export, decreasing the efficacy of DNA damage-dependent cell death signaling, or by promoting DNA repair. Here, we demonstrate, using gain-of-function and deletion alleles, that Drosophila Bruce (dBruce) can potently inhibit cell death induced by the essential Drosophila cell death activators Reaper (Rpr) and Grim but not Head involution defective (Hid). The dBruce BIR domain is not sufficient for this activity, and the E2 domain is likely required. dBruce does not promote Rpr or Grim degradation directly, but its antiapoptotic actions do require that their N termini, required for interaction with DIAP1 BIR2, be intact. dBruce does not block the activity of the apical cell death caspase Dronc or the proapoptotic Bcl-2 family member Debcl/Drob-1/dBorg-1/Dbok. Together, these results argue that dBruce can regulate cell death at a novel point.

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