Induction of apoptosis by Drosophila reaper, hid and grim through inhibition of IAP function.

Induction of apoptosis in Drosophila requires the activity of three closely linked genes, reaper, hid and grim. Here we show that the proteins encoded by reaper, hid and grim activate cell death by inhibiting the anti-apoptotic activity of the Drosophila IAP1 (diap1) protein. In a genetic modifier screen, both loss-of-function and gain-of-function alleles in the endogenous diap1 gene were obtained, and the mutant proteins were functionally and biochemically characterized. Gain-of-function mutations in diap1 strongly suppressed reaper-, hid- and grim-induced apoptosis. Sequence analysis of these alleles revealed that they were caused by single amino acid changes in the baculovirus IAP repeat domains of diap1, a domain implicated in binding REAPER, HID and GRIM. Significantly, the corresponding mutant DIAP1 proteins displayed greatly reduced binding of REAPER, HID and GRIM, indicating that REAPER, HID and GRIM kill by forming a complex with DIAP1. These data provide strong in vivo evidence for a previously published model of cell death regulation in Drosophila.

The Drosophila gene hid is a direct molecular target of Ras-dependent survival signaling.

Extracellular growth factors are required for the survival of most animal cells. They often signal through the activation of the Ras pathway. However, the molecular mechanisms by which Ras signaling inhibits the intrinsic cell death machinery are not well understood. Here, we present evidence that in Drosophila, activation of the Ras pathway specifically inhibits the proapoptotic activity of the gene head involution defective (hid). By using transgenic animals and cultured cells, we show that MAPK phosphorylation sites in Hid are critical for this response. These findings define a novel mechanism by which growth factor signaling directly inactivates a critical component of the intrinsic cell death machinery. These studies provide further insights into the function of ras as an oncogene.

Mechanisms and control of programmed cell death in invertebrates.

Apoptosis is a morphologically distinct form of programmed cell death that plays important roles in development, tissue homeostasis and a wide variety of diseases, including cancer, AIDS, stroke, myopathies and various neurodegenerative disorders (see Thompson (1995) for review). It is now clear that apoptosis occurs by activating an intrinsic cell suicide program which is constitutively expressed in most animal cells, and that key components of this program have been conserved in evolution from worms to insects to man. Genetic studies of programmed cell death in experimentally highly accessible invertebrate model systems have provided important clues about the molecular nature of the death program, and the intracellular mechanisms that control its activation. This review summarizes some of the key findings in this area, but also touches on some of the many unresolved questions and challenges that remain.

Disruption of a behavioral sequence by targeted death of peptidergic neurons in Drosophila.

The neuropeptide eclosion hormone (EH) is a key regulator of insect ecdysis. We tested the role of the two EH-producing neurons in Drosophila by using an EH cell-specific enhancer to activate cell death genes reaper and head involution defective to ablate the EH cells. In the EH cell knockout flies, larval and adult ecdyses were disrupted, yet a third of the knockouts emerged as adults, demonstrating that EH has a significant but nonessential role in ecdysis. The EH cell knockouts had discrete behavioral deficits, including slow, uncoordinated eclosion and an insensitivity to ecdysis-triggering hormone. The knockouts lacked the lights-on eclosion response despite having a normal circadian eclosion rhythm. This study represents a novel approach to the dissection of neuropeptide regulation of a complex behavioral program.

Cooperative functions of the reaper and head involution defective genes in the programmed cell death of Drosophila central nervous system midline cells.

In Drosophila, the chromosomal region 75C1-2 contains at least three genes, reaper (rpr), head involution defective (hid), and grim, that have important functions in the activation of programmed cell death. To better understand how cells are killed by these genes, we have utilized a well defined set of embryonic central nervous system midline cells that normally exhibit a specific pattern of glial cell death. In this study we show that both rpr and hid are expressed in dying midline cells and that the normal pattern of midline cell death requires the function of multiple genes in the 75C1-2 interval. We also utilized the P[UAS]/P[Gal4] system to target expression of rpr and hid to midline cells. Targeted expression of rpr or hid alone was not sufficient to induce ectopic midline cell death. However, expression of both rpr and hid together rapidly induced ectopic midline cell death that resulted in axon scaffold defects characteristic of mutants with abnormal midline cell development. Midline-targeted expression of the baculovirus p35 protein, a caspase inhibitor, blocked both normal and ectopic rpr- and hid-induced cell death. Taken together, our results suggest that rpr and hid are expressed together and cooperate to induce programmed cell death during development of the central nervous system midline.

The head involution defective gene of Drosophila melanogaster functions in programmed cell death.

Deletions of chromosomal region, 75C1,2 block virtually all programmed cell death (PCD) in the Drosophila embryo. We have identified a gene previously in this interval, reaper (rpr), which encodes an important regulator of PCD. Here we report the isolation of a second gene in this region, head involution defective (hid), which plays a similar role in PCD. hid mutant embryos have decreased levels of cell death and contain extra cells in the head. We have cloned the hid gene and find that its expression is sufficient to induce PCD in cell death defective mutants. The hid gene appears to encode a novel 410-amino-acid protein, and its mRNA is expressed in regions of the embryo where cell death occurs. Ectopic expression of hid in the Drosophila retina results in eye ablation. This phenotype can be suppressed completely by expression of the anti-apoptotic p35 protein from baculovirus, indicating that p35 may act genetically downstream from hid.

Yeast ADA2 protein binds to the VP16 protein activation domain and activates transcription.

Previously it was shown that yeast ADA2 protein is necessary for the full activity of some activation domains, such as VP16 and GCN4, in vivo and in vitro. These results suggest that ADA2 protein functions as a transcriptional coactivator or adaptor that bridges the interaction between certain acidic activation domains and the basal transcription machinery. Here we present two findings consistent with this model. (i) ADA2 protein interacts with a region of the VP16 acidic activation domain that requires ADA2 for activity in vivo. (ii) ADA2 protein, when fused to a heterologous DNA-binding domain, can stimulate the activity of the basal transcription factors in vivo. This ability of ADA2 to activate transcription is mediated by ADA3, a gene with properties similar to ADA2. These findings suggest that ADA2 protein has at least some of the properties expected of a transcriptional adaptor.

ADA3: a gene, identified by resistance to GAL4-VP16, with properties similar to and different from those of ADA2.

We describe the isolation of a yeast gene, ADA3, mutations in which prevent the toxicity of GAL4-VP16 in vivo. Toxicity was previously proposed to be due to the trapping of general transcription factors required at RNA polymerase II promoters (S. L. Berger, B. Piña, N. Silverman, G. A. Marcus, J. Agapite, J. L. Regier, S. J. Triezenberg, and L. Guarente, Cell 70:251-265, 1992). trans activation by VP16 as well as the acidic activation domain of GCN4 is reduced in the mutant. Other activation domains, such as those of GAL4 and HAP4, are only slightly affected in the mutant. This spectrum is similar to that observed for mutants with lesions in ADA2, a gene proposed to encode a transcriptional adaptor. The ADA3 gene is not absolutely essential for cell growth, but gene disruption mutants grow slowly and are temperature sensitive. Strains doubly disrupted for ada2 and ada3 grow no more slowly than single mutants, providing further evidence that these genes function in the same pathway. Selection of initiation sites by the general transcriptional machinery in vitro is altered in the ada3 mutant, providing a clue that ADA3 could be a novel general transcription factor involved in the response to acidic activators.

Genetic isolation of ADA2: a potential transcriptional adaptor required for function of certain acidic activation domains.

We have devised a genetic strategy to isolate the target of acidic activation domains of transcriptional activators based on toxicity in yeast cells of the chimeric activator, GAL4-VP16. Toxicity required the integrity of both the VP16 acidic activation domain and the GAL4 DNA-binding domain, suggesting that inhibition resulted from trapping of general transcription factors at genomic sites. Mutations that break the interaction between GAL4-VP16 and general factors would alleviate toxicity and identify transcriptional adaptors, if adaptors bridged the interaction between activators and general factors. We thus identified ADA1, ADA2, and ADA3. Mutations in ADA2 reduced the activity of GAL4-VP16 and GCN4 in vivo. ada2 mutant extracts exhibited normal basal transcription, but were defective in responding to GAL4-VP16, GCN4, or the dA:dT activator. Strikingly, the mutant extract responded like wild type to GAL4-HAP4. We conclude that ADA2 potentiates the activity of one class of acidic activation domain but not a second class.