An emerging blueprint for apoptosis in Drosophila

The conserved PI3'K/PTEN/Akt signaling pathway regulates both cell size and survival in Drosophila

Akt (or PKB) is an oncogene involved in the regulation of cell survival. Akt is regulated by phosphatidylinositol 3-OH kinase (PI3'K) signaling and has shown to be hyperactivated through the loss of the PTEN tumor suppressor. In Drosophila, insulin signaling as studied using the Drosophila IRS-4 homolog (Chico) has been shown to be a crucial regulator of cell size. We have studied Drosophila Akt (Dakt1) and have shown that it is also involved in the regulation of cell size. Furthermore we have performed genetic epistasis tests to demonstrate that in Drosophila, PI3'K, PTEN and Akt comprise a signaling cassette that is utilized during multiple stages of development. In addition, we show that this signaling cassette is also involved in the regulation of cell survival during embryogenesis. This study therefore establishes the evolutionary conservation of this signaling pathway in Drosophila. Oncogene (2000) 19, 3971 - 3977.

A screen for mutations that suppress the phenotype of Drosophila armadillo, the beta-catenin homolog

During development signaling pathways coordinate cell fates and regulate the choice between cell survival or programmed cell death. The well-conserved Wingless/Wnt pathway is required for many developmental decisions in all animals. One transducer of the Wingless/Wnt signal is Armadillo/beta-catenin. Drosophila Armadillo not only transduces Wingless signal, but also acts in cell-cell adhesion via its role in the epithelial adherens junction. While many components of both the Wingless/Wnt signaling pathway and adherens junctions are known, both processes are complex, suggesting that unknown components influence signaling and junctions. We carried out a genetic modifier screen to identify some of these components by screening for mutations that can suppress the armadillo mutant phenotype. We identified 12 regions of the genome that have this property. From these regions and from additional candidate genes tested we identified four genes that suppress arm: dTCF, puckered, head involution defective (hid), and Dpresenilin. We further investigated the interaction with hid, a known regulator of programmed cell death. Our data suggest that Wg signaling modulates Hid activity and that Hid regulates programmed cell death in a dose-sensitive fashion.

E93 directs steroid-triggered programmed cell death in Drosophila

Steroid hormones coordinate multiple cellular changes, yet the mechanisms by which these systemic signals are refined into stage- and tissue-specific responses remain poorly understood. Here we show that the Drosophila E93 gene determines the nature of a steroid-induced biological response. E93 mutants possess larval salivary glands that fail to undergo steroid-triggered programmed cell death, and E93 is expressed in cells immediately before the onset of death. E93 protein is bound to the sites of steroid-regulated and cell death genes on polytene chromosomes, and the expression of these genes is defective in E93 mutants. Furthermore, expression of E93 is sufficient to induce programmed cell death. We propose that the steroid induction of E93 determines a programmed cell death response during development.

Identification and developmental expression of inhibitor of caspase-activated DNase (ICAD) in Drosophila melanogaster

DNA fragmentation, a hallmark of apoptosis, is regulated by a specific nuclease called caspase-activated DNase (CAD) and its inhibitor (ICAD). When cell lysates from Drosophila S2 cells were chemically denatured and the denatured proteins were removed after dialysis, the supernatant inhibited Drosophila CAD (dCAD). To identify the inhibitor, we tested recombinant DREP-1, which was previously identified using the Drosophila EST data base and found it also inhibited dCAD DNase. An antibody against DREP-1 inhibited the ICAD activity in the S2 cell extracts, confirming the identification of DREP-1 as a Drosophila homolog of ICAD (dICAD). The recombinant DREP-1/dICAD was cleaved at a specific site by human caspase 3 as well as by extracts prepared from S2 cells undergoing apoptosis. Biochemical fractionation and immunoprecipitation of dICAD from S2 cell extracts indicated that dICAD is complexed with dCAD in proliferating cells. The expression of the caspase-resistant form of dICAD/DREP-1 in a Drosophila neuronal cell line prevented the apoptotic DNA fragmentation. Northern hybridization and the immunohistochemical analyses revealed that the expression of the dICAD gene is developmentally regulated.

Caspase inhibitor P35 and inhibitor of apoptosis Op-IAP block in vivo proteolytic activation of an effector caspase at different steps

Signal-induced activation of caspases, the critical protease effectors of apoptosis, requires proteolytic processing of their inactive proenzymes. Consequently, regulation of procaspase processing is critical to apoptotic execution. We report here that baculovirus pancaspase inhibitor P35 and inhibitor of apoptosis Op-IAP prevent caspase activation in vivo, but at different steps. By monitoring proteolytic processing of endogenous Sf-caspase-1, an insect group II effector caspase, we show that Op-IAP blocked the first activation cleavage at TETD downward arrowG between the large and small caspase subunits. In contrast, P35 failed to affect this cleavage, but functioned downstream to block maturation cleavages (DXXD downward arrow(G/A)) of the large subunit. Substitution of P35's reactive site residues with TETDG failed to increase its effectiveness for blocking TETD downward arrowG processing of pro-Sf-caspase-1, despite wild-type function for suppressing apoptosis. These data are consistent with the involvement of a novel initiator caspase that is resistant to P35, but directly or indirectly inhibitable by Op-IAP. The conservation of TETD downward arrowG processing sites among insect effector caspases, including Drosophila drICE and DCP-1, suggests that in vivo activation of these group II caspases involves a P35-insensitive caspase and supports a model wherein apical and effector caspases function through a proteolytic cascade to execute apoptosis in insects.

The Drosophila bcl-2 family member dBorg-1 functions in the apoptotic response to UV-irradiation

As with all metazoans, the fly makes extensive use of selective programmed cell death (PCD) to remove excess cells and properly sculpt developing tissues. Several core components of the cell death machinery have been identified in flies, including caspases and an Apaf-1 ortholog [1] [2] [3] [4]. One missing component has been a member of the Bcl-2 family of proteins, which act either pro- or anti-apoptotically as upstream regulatory proteins. Here, we report the identification of Bcl-2 family members in Drosophila - dBorg-1 (Drosophila Bcl-2 ortholog), also identified by Igaki et al. [5], and dBorg-2. Removal of dBorg-1 function during Drosophila embryonic development resulted in excess glial cells, demonstrating its pro-apoptotic function. In cell culture assays, dBorg-1 efficiently induced apoptosis but, remarkably, also demonstrated protective activity when death stimuli were introduced. Finally, ectopic expression of dBorg-1 in the eye led to subtle defects that were strongly potentiated by ultra violet (UV) irradiation, resulting in a dramatic loss of retinal cells.

Regulation and execution of apoptosis during Drosophila development

The development of the Drosophila embryo into an adult fly is a process that integrates cell proliferation and differentiation with programmed cell death, or apoptosis. Apoptosis is an evolutionarily conserved process that is controlled in the developing fly by the products of the genes reaper, grim, and hid. We discuss the role of programmed cell death in the establishment and maintenance of correct patterning in the embryo, and examine the coordination of apoptosis with the hormonally controlled degeneration of larval tissues during metamorphosis. Finally, we address the architecture of the adult eye as an example of how programmed cell death plays a key role in the development of many adult structures.

Drosophila p53 is a structural and functional homolog of the tumor suppressor p53

The importance of p53 in carcinogenesis stems from its central role in inducing cell cycle arrest or apoptosis in response to cellular stresses. We have identified a Drosophila homolog of p53 ("Dmp53"). Like mammalian p53, Dmp53 binds specifically to human p53 binding sites, and overexpression of Dmp53 induces apoptosis. Importantly, inhibition of Dmp53 function renders cells resistant to X ray-induced apoptosis, suggesting that Dmp53 is required for the apoptotic response to DNA damage. Unlike mammalian p53, Dmp53 appears unable to induce a G1 cell cycle block when overexpressed, and inhibition of Dmp53 activity does not affect X ray-induced cell cycle arrest. These data reveal an ancestral proapoptotic function for p53 and identify Drosophila as an ideal model system for elucidating the p53 apoptotic pathway(s) induced by DNA damage.

Drosophila p53 binds a damage response element at the reaper locus

The tumor suppressor gene p53 regulates multiple cellular responses to DNA damage, but the transcriptional targets that specify these responses are incompletely understood. We describe a Drosophila p53 homolog and demonstrate that it can activate transcription from a promoter containing binding sites for human p53. Dominant-negative forms of Drosophila p53 inhibit both transactivation in cultured cells and radiation-induced apoptosis in developing tissues. The cis-regulatory region of the proapoptotic gene reaper contains a radiation-inducible enhancer that includes a consensus p53 binding site. Drosophila p53 can activate transcription from this site in yeast and a multimer of this site is sufficient for radiation induction in vivo. These results indicate that reaper is a direct transcriptional target of Drosophila p53 following DNA damage.

Comparative genomics of the eukaryotes

A comparative analysis of the genomes of Drosophila melanogaster, Caenorhabditis elegans, and Saccharomyces cerevisiae-and the proteins they are predicted to encode-was undertaken in the context of cellular, developmental, and evolutionary processes. The nonredundant protein sets of flies and worms are similar in size and are only twice that of yeast, but different gene families are expanded in each genome, and the multidomain proteins and signaling pathways of the fly and worm are far more complex than those of yeast. The fly has orthologs to 177 of the 289 human disease genes examined and provides the foundation for rapid analysis of some of the basic processes involved in human disease.

A steroid-triggered transcriptional hierarchy controls salivary gland cell death during Drosophila metamorphosis

The steroid hormone ecdysone signals the stage-specific programmed cell death of the larval salivary glands during Drosophila metamorphosis. This response is preceded by an ecdysone-triggered switch in gene expression in which the diap2 death inhibitor is repressed and the reaper (rpr) and head involution defective (hid) death activators are induced. Here we show that rpr is induced directly by the ecdysone-receptor complex through an essential response element in the rpr promoter. The Broad-Complex (BR-C) is required for both rpr and hid transcription, while E74A is required for maximal levels of hid induction. diap2 induction is dependent on betaFTZ-F1, while E75A and E75B are each sufficient to repress diap2. This study identifies transcriptional regulators of programmed cell death in Drosophila and provides a direct link between a steroid signal and a programmed cell death response.

Cell death: drosophila Apaf-1 - no longer in the (d)Ark

The recent discovery and characterization of Ark, the Drosophila homolog of the mammalian cell-death adaptor protein Apaf-1, have revealed that, like Apaf-1, this protein is important in multiple apoptosis pathways. The new findings also suggest that cell death in flies is very similar to that in mammals after all.

Debcl, a proapoptotic Bcl-2 homologue, is a component of the Drosophila melanogaster cell death machinery

Bcl-2 family of proteins are key regulators of apoptosis. Both proapoptotic and antiapoptotic members of this family are found in mammalian cells, but no such proteins have been described in insects. Here, we report the identification and characterization of Debcl, the first Bcl-2 homologue in Drosophila melanogaster. Structurally, Debcl is similar to Bax-like proapoptotic Bcl-2 family members. Ectopic expression of Debcl in cultured cells and in transgenic flies causes apoptosis, which is inhibited by coexpression of the baculovirus caspase inhibitor P35, indicating that Debcl is a proapoptotic protein that functions in a caspase-dependent manner. debcl expression correlates with developmental cell death in specific Drosophila tissues. We also show that debcl genetically interacts with diap1 and dark, and that debcl-mediated apoptosis is not affected by gene dosage of rpr, hid, and grim. Biochemically, Debcl can interact with several mammalian and viral prosurvival Bcl-2 family members, but not with the proapoptotic members, suggesting that it may regulate apoptosis by antagonizing prosurvival Bcl-2 proteins. RNA interference studies indicate that Debcl is required for developmental apoptosis in Drosophila embryos. These results suggest that the main components of the mammalian apoptosis machinery are conserved in insects.