An emerging blueprint for apoptosis in Drosophila

Distinct pathways mediate UV-induced apoptosis in Drosophila embryos

Cell death in Drosophila is regulated by many of the same signals that control apoptosis in mammalian systems. For all the three major cell death pathways that have been described in humans, homologous components have been identified in Drosophila. Here we report that distinct pathways mediate UV-induced apoptosis at different developmental stages in the Drosophila embryo. In midstage embryos, UVC irradiation induces reaper expression and cell death through a mei-41(dATM)-dependent pathway; UVB does not have the same effect. In contrast, in pregastrulation embryos, both UVB and UVC promote apoptosis via transcriptional induction of the Drosophila Apaf-1/ced-4 homolog. This early UV response requires E2F but not mei-41 function and appears to be independent of DNA damage.

Mutations in lozenge and D-Pax2 invoke ectopic patterned cell death in the developing Drosophila eye using distinct mechanisms

Mutations in the lozenge gene of Drosophila melanogaster elicit a pleiotropic set of adult phenotypes, including severe compound eye perturbations resulting from the defective recruitment of photoreceptors R1/6 and R7, cone and pigment cells. In this study, we show that excessive patterned apoptosis is evident at the same developmental stage in these lozenge mutants. In lozenge null mutants, apoptosis occurs prior to lozenge-dependent cell fate specification. A second gene, D-Pax2, genetically interacts with lozenge. Interestingly, D-Pax2 mutants also exhibit increased cell death, but slightly later in development than that in lozenge mutants. Although expression of the caspase inhibitor p35 eliminates death in both lozenge and D-Pax2 mutants, the lozenge mutant eye phenotypes persist because other normal Lozenge functions are still lacking. D-Pax2 eye phenotypes, in contrast, are dramatically altered in a p35 background, because cells that normally differentiate as cone and primary pigment cells are subsequently transformed into secondary pigment cells. This study leads us to propose that Lozenge, aside from its known role in gene regulation of cell-specific transcription factors, is required to contribute to the repression of cell death mechanisms, creating a permissive environment for the survival of undifferentiated cells in early eye development. Lack of lozenge expression increases the likelihood that an undifferentiated cell will initiate its default death program and die prematurely. The ectopic cell death evident in D-Pax2 mutants appears to arise from the cell fate transformation of cone cells into secondary pigment cells, either autonomously or as a result of defective signalling.

blue cheese mutations define a novel, conserved gene involved in progressive neural degeneration

A common feature of many human neurodegenerative diseases is the accumulation of insoluble ubiquitin-containing protein aggregates in the CNS. Although Drosophila has been helpful in understanding several human neurodegenerative disorders, a loss-of-function mutation has not been identified that leads to insoluble CNS protein aggregates. The study of Drosophila mutations may identify unique components that are associated with human degenerative diseases. The Drosophila blue cheese (bchs) gene defines such a novel degenerative pathway. bchs mutants have a reduced adult life span with the age-dependent formation of protein aggregates throughout the neuropil of the CNS. These inclusions contain insoluble ubiquitinated proteins and amyloid precursor-like protein. Progressive loss of CNS size and morphology along with extensive neuronal apoptosis occurs in aged bchs mutants. BCHS protein is widely expressed in the cytoplasm of CNS neurons and is present over the entire length of axonal projections. BCHS is nearly 3500 amino acids in size, with the last 1000 amino acids consisting of three functional protein motifs implicated in vesicle transport and protein processing. This region along with previously unidentified proteins encoded in the human, mouse, and nematode genomes shows striking homology along the full length of the BCHS protein. The high degree of conservation between Drosophila and human bchs suggests that study of the functional pathway of BCHS and associated mutant phenotype may provide useful insights into human neurodegenerative disorders.

Steroid-triggered programmed cell death of a motoneuron is autophagic and involves structural changes in mitochondria

Neuronal death occurs during normal development and disease and can be regulated by steroid hormones. In the hawkmoth, Manduca sexta, individual accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD) at pupation that is triggered directly and cell autonomously by the steroid hormone 20-hydroxyecdysone (20E). APRs from abdominal segment six [APR(6)s] die by 48 hours after pupal ecdysis (PE; entry into the pupal stage), whereas APR(4)s survive until adulthood. Cell culture experiments showed previously that 20E acts directly on APRs to trigger PCD, with intrinsic segmental identity determining which APRs die. The APR(6) death pathway includes caspase activation and loss of mitochondrial function. We used transmission electron microscopy to investigate the ultrastructure of APR somata before and during PCD. APR(4)s showed normal ultrastructure at all stages examined, as did APR(6)s until approximately stage PE. During APR(6) death, there was massive accumulation of autophagic bodies and vacuoles, mitochondria became ultracondensed and aggregated into compact clusters, and ribosomes aggregated in large blocks. Nuclear ultrastructure remained normal, without chromatin condensation, until the nuclear envelope fragmented late in the death process. Light microscopic immunocytochemistry showed that dying APR(6)s were TUNEL-positive, which is diagnostic of fragmented DNA. These observations indicate that the steroid-induced, caspase-dependent, cell-autonomous PCD of APR(6)s is autophagic, not apoptotic, and support an early role for mitochondrial alterations during PCD. This system permits the study of neuronal death in response to its bona fide developmental signal, the rise in a steroid hormone.

Activation of the innate immunity in Drosophila by endogenous chromosomal DNA that escaped apoptotic degradation

Apoptotic cell death is accompanied by degradation of chromosomal DNA. Here, we established in Drosophila a null mutation in the gene for inhibitor of caspase-activated DNase (ICAD) by P-element insertion. We also identified a loss-of-function mutant in Drosophila for DNase II-like acid DNase. The flies deficient in the ICAD gene did not express CAD, and did not undergo apoptotic DNA fragmentation during embryogenesis and oogenesis. In contrast, the deficiency of DNase II enhanced the apoptotic DNA fragmentation in the embryos and ovary, but paradoxically, the mutant flies accumulated a large amount of DNA, particularly in the ovary. This accumulation of DNA in the DNase II mutants caused the constitutive expression of the antibacterial genes for diptericin and attacin, which are usually activated during bacterial infection. The expression of these genes was further enhanced in flies lacking both dICAD and DNase II. These results indicated that CAD and DNase II work independently to degrade chromosomal DNA during apoptosis, and if the DNA is left undigested, it can activate the innate immunity in Drosophila.

Baculovirus apoptotic suppressor P49 is a substrate inhibitor of initiator caspases resistant to P35 in vivo

Caspases play a critical role in the execution of metazoan apoptosis and are thus attractive therapeutic targets for apoptosis-associated diseases. Here we report that baculovirus P49, a homolog of pancaspase inhibitor P35, prevents apoptosis in invertebrates by inhibiting an initiator caspase that is P35 insensitive. Consequently P49 blocked proteolytic activation of effector caspases at a unique step upstream from that affected by P35 but downstream from inhibitor of apoptosis Op-IAP. Like P35, P49 was cleaved by and stably associated with its caspase target. Ectopically expressed P49 blocked apoptosis in cultured cells from a phylogenetically distinct organism, Drosophila melanogaster. Furthermore, P49 inhibited human caspase-9, demonstrating its capacity to affect a vertebrate initiator caspase. Thus, P49 is a substrate inhibitor with a novel in vivo specificity for a P35-insensitive initiator caspase that functions at an evolutionarily conserved step in the caspase cascade. These data indicate that activated initiator caspases provide another effective target for apoptotic intervention by substrate inhibitors.

Identification of X-Linked Genes Required for Migration and Programmed Cell Death of Drosophila melanogaster Germ Cells

Drosophila germ cells form at the posterior pole of the embryo and migrate to the somatic gonad. Approximately 50% of the germ cells that form reach their target. The errant cells within the embryo undergo developmentally regulated cell death. Prior studies have identified some autosomal genes that regulate germ cell migration, but the genes that control germ cell death are not known. To identify X-linked genes required for germ cell migration and/or death, we performed a screen for mutations that disrupt these processes. Here we report the identification of scattershot and outsiders, two genes that regulate the programmed death of germ cells. The scattershot gene is defined by a mutation that disrupts both germ cell migration and the death of germ cells ectopic to the gonad. Maternal and zygotic expression of scattershot is required, but the migration and cell death functions can be genetically uncoupled. Zygotic expression of wild-type scattershot rescues germ cell pathfinding, but does not restore the programmed death of errant cells. The outsiders gene is required zygotically. In outsiders mutant embryos, the appropriate number of germ cells is incorporated into the gonad, but germ cells ectopic to the gonad persist.

Competition and compensation: coupled to death in development and cancer

Developing systems exhibit robust and tolerant properties in the form of compensation and competition within cell groups. Fundamentally, these behaviors are governed by mechanisms that elicit proliferation and apoptosis. Recent insights from the Drosophila system implicate new determinants that specify "winners" and "losers" in this process.

Mitochondria, the killer organelles and their weapons

Apoptosis is a cell-autonomous mode of death that is activated to eradicate superfluous, damaged, mutated, or aged cells. In addition to their role as the cell's powerhouse, mitochondria play a central role in the control of apoptosis. Thus, numerous pro-apoptotic molecules act on mitochondria and provoke the permeabilization of mitochondrial membranes. Soluble proteins contained in the mitochondrial intermembrane space are released through the outer membrane and participate in the organized destruction of the cell. Several among these lethal proteins can activate caspases, a class of cysteine proteases specifically activated in apoptosis, whereas others act in a caspase-independent fashion, by acting as nucleases (e.g., endonuclease G), nuclease activators (e.g., apoptosis-inducing factor), or serine proteases (e.g., Omi/HtrA2). In addition, mitochondria can generate reactive oxygen species, following uncoupling and/or inhibition of the respiratory chain. The diversity of mitochondrial factors participating in apoptosis emphasizes the central role of these organelles in apoptosis control and unravels novel mechanisms of cell death execution.

GH3, a novel proapoptotic domain in Drosophila Grim, promotes a mitochondrial death pathway

Grim encodes a protein required for programmed cell death in Drosophila. The Grim N-terminus induces apoptosis by disrupting IAP blockage of caspases; however, N-terminally-deleted Grim retains pro apoptotic activity. We describe GH3, a 15 amino acid internal Grim domain absolutely required for its proapoptotic activity and sufficient to induce cell death when fused to heterologous carrier proteins. A GH3 homology region is present in the Drosophila proapoptotic proteins Reaper and Sickle. The GH3 domain and the homologous regions in Reaper and Sickle are predicted to be structured as amphipathic alpha-helixes. During apoptosis induction, Grim colocalizes with mitochondria and cytochrome c in a GH3-dependent but N-terminal- and caspase activity-independent manner. When Grim is overexpressed in vivo, both the N-terminal and the GH3 domains are equally necessary, and cooperate for apoptosis induction. The N-terminal and GH3 Grim domains thus activate independent apoptotic pathways that synergize to induce programmed cell death efficiently.

Death to flies: Drosophila as a model system to study programmed cell death

Programmed cell death (PCD) is essential for the removal of unwanted cells and is critical for both restricting cell numbers and for tissue patterning during development. Components of the cell death machinery are remarkably conserved through evolution, from worms to mammals. Central to the PCD process is the family of cysteine proteases, known as caspases, which are activated by death-inducing signals. Comparisons between C. elegans and mammalian PCD have shown that there is additional complexity in the regulation of PCD in mammals. The fruitfly, Drosophila melanogaster, is proving an ideal genetically tractable model organism, of intermediary complexity between C. elegans and mammals, in which to study the intricacies of PCD. Here, we review the literature on PCD during Drosophila development, highlighting the methods used in these studies.

B56-associated protein phosphatase 2A is required for survival and protects from apoptosis in Drosophila melanogaster

Protein phosphorylation and specific protein kinases can initiate signal transduction pathways leading to programmed cell death. The specific protein phosphatases regulating apoptosis have been more elusive. Using double-stranded RNA-mediated interference (RNAi), the role of protein phosphatase 2A (PP2A) in cellular signaling was investigated. Knockdown of A or C subunits individually or of combined B subunits led to concurrent loss of nontargeted PP2A subunits, suggesting that PP2A is an obligate heterotrimer in vivo. Global knockdown of PP2A activity or specific loss of redundant B56 regulatory subunits caused cell death with the morphological and biochemical changes characteristic of apoptosis in cultured S2 cells. B56:PP2A-regulated apoptosis required caspases and the upstream regulators dark, reaper, head involution defective, and dp53. In Drosophila embryos, knockdown of B56-regulated PP2A activity resulted in apoptosis and failure of gastrulation, an effect that was blocked by concurrent RNAi of the caspase DRICE: B56-regulated PP2A activity appears to be required upstream of dp53 to maintain a critical proapoptotic substrate in a dephosphorylated, inactive state, thereby preventing apoptosis in Drosophila S2 cells.

Drosophila Morgue is an F box/ubiquitin conjugase domain protein important for grim-reaper mediated apoptosis

In Drosophila melanogaster, apoptosis is controlled by the integrated actions of the Grim-Reaper (Grim-Rpr) and Drosophila Inhibitor of Apoptosis (DIAP) proteins (reviewed in refs 1 4). The anti-apoptotic DIAPs bind to caspases and inhibit their proteolytic activities. DIAPs also bind to Grim-Rpr proteins, an interaction that promotes caspase activity and the initiation of apoptosis. Using a genetic modifier screen, we identified four enhancers of grim-reaper-induced apoptosis that all regulate ubiquitination processes: uba-1, skpA, fat facets (faf), and morgue. Strikingly, morgue encodes a unique protein that contains both an F box and a ubiquitin E2 conjugase domain that lacks the active site Cys required for ubiquitin linkage. A reduction of morgue activity suppressed grim-reaper-induced cell death in Drosophila. In cultured cells, Morgue induced apoptosis that was suppressed by DIAP1. Targeted morgue expression downregulated DIAP1 levels in Drosophila tissue, and Morgue and Rpr together downregulated DIAP1 levels in cultured cells. Consistent with potential substrate binding functions in an SCF ubiquitin E3 ligase complex, Morgue exhibited F box-dependent association with SkpA and F box-independent association with DIAP1. Morgue may thus have a key function in apoptosis by targeting DIAP1 for ubiquitination and turnover.

AP-1, but not NF-kappa B, is required for efficient steroid-triggered cell death in Drosophila

Extensive studies in vertebrate cells have assigned a central role to Rel/NF-kappa B and AP-1 family members in the control of apoptosis. We ask here whether parallel pathways might function in Drosophila by determining if Rel/NF-kappa B or AP-1 family members contribute to the steroid-triggered death of larval salivary glands during Drosophila metamorphosis. We show that two of the three Drosophila Rel/NF-kappa B genes are expressed in doomed salivary glands and that one family member, Dif, is induced in a stage-specific manner immediately before the onset of programmed cell death. Similarly, Djun is expressed for many hours before salivary gland cell death while Dfos is induced in a stage-specific manner, immediately before this tissue is destroyed. We show that null mutations in the three Drosophila Rel/NF-kappa B family members, either alone or in combination, have no apparent effect on this death response. In contrast, Dfos is required for the proper timing of larval salivary gland cell death as well as the proper induction of key death genes. This study demonstrates a role for AP-1 in the stage-specific steroid-triggered programmed cell death of larval tissues during Drosophila metamorphosis.

Unrestrained caspase-dependent cell death caused by loss of Diap1 function requires the Drosophila Apaf-1 homolog, Dark

In mammals and Drosophila, apoptotic caspases are under positive control via the CED-4/Apaf-1/Dark adaptors and negative control via IAPs (inhibitor of apoptosis proteins). However, the in vivo genetic relationship between these opposing regulators is not known. In this study, we demonstrate that a dark mutation reverses catastrophic defects seen in Diap1 mutants and rescues cells specified for Diap1- regulated cell death in development and in response to genotoxic stress. We also find that dark function is required for hyperactivation of caspases which occurs in the absence of Diap1. Since the action of dark is epistatic to that of Diap1, these findings demonstrate that caspase-dependent cell death requires concurrent positive input through Apaf-1-like proteins together with disruption of IAP-caspase complexes.

The role of ARK in stress-induced apoptosis in Drosophila cells

The molecular mechanisms of apoptosis are highly conserved throughout evolution. The homologs of genes essential for apoptosis in Caenorhabditis elegans and Drosophila melanogaster have been shown to be important for apoptosis in mammalian systems. Although a homologue for CED-4/apoptotic protease-activating factor (Apaf)-1 has been described in Drosophila, its exact function and the role of the mitochondrial pathway in its activation remain unclear. Here, we used the technique of RNA interference to dissect apoptotic signaling pathways in Drosophila cells. Inhibition of the Drosophila CED-4/Apaf-1-related killer (ARK) homologue resulted in pronounced inhibition of stress-induced apoptosis, whereas loss of ARK did not protect the cells from Reaper- or Grim-induced cell death. Reduction of DIAP1 induced rapid apoptosis in these cells, whereas the inhibition of DIAP2 expression did not but resulted in increased sensitivity to stress-induced apoptosis; apoptosis in both cases was prevented by inhibition of ARK expression. Cells in which cytochrome c expression was decreased underwent apoptosis induced by stress stimuli, Reaper or Grim. These results demonstrate the central role of ARK in stress-induced apoptosis, which appears to act independently of cytochrome c. Apoptosis induced by Reaper or Grim can proceed via a distinct pathway, independent of ARK.

Apoptosis: letting slip the dogs of war

Recent studies have shown that, during cell death, the protein Omi is released from the mitochondrial intermembrane space into the cytosol, where it augments caspase-dependent apoptosis by blocking inhibitors and may induce caspase-independent cell death via its serine protease activity.

Rpr- and hid-driven cell death in Drosophila photoreceptors

The reaper (rpr) and head involution defective (hid) genes mediate programmed cell death (PCD) during Drosophila development. We show that expression of either rpr or hid under control of a rhodopsin promoter induces rapid cell death of adult photoreceptor cells. Ultrastructural analysis revealed that the dying photoreceptor cells share morphological features with other cells undergoing PCD. The anti-apoptotic baculoviral P35 protein acts downstream of hid activity to suppress the photoreceptor cell death driven by rpr and hid. These results establish that the Drosophila photoreceptors are sensitive to the rpr- and hid-driven cell death pathways.

The damage-responsive Drosophila gene sickle encodes a novel IAP binding protein similar to but distinct from reaper, grim, and hid

In flies and mammals, critical regulators of cell death function by antagonizing Inhibitor of Apoptosis Proteins (IAPs), which themselves directly block caspase action. The three currently known IAP antagonists in Drosophila map to the H99 genomic interval required for all programmed cell death. Here we describe a fourth member of this genetic group, sickle (skl), which maps just outside of the H99 deletion. At its N terminus, Skl shares residues in common with other IAP antagonists in flies (Rpr, Grim, and Hid) and in mammals (Smac/DIABLO and Omi/Htra2). Like other activators of apoptosis mapping in the Reaper region, full-length skl induced apoptosis when overexpressed, and the N terminus of this protein specifically bound to the BIR2 domain of DIAP1. However, unlike the N termini of Grim, Hid, and Rpr, the N terminus of Skl did not induce apoptosis. skl transcripts accumulate in cells that are fated to die in some but not all regions of the embryo. Genotoxic stimuli induced skl expression, but skl was not responsive to all signals that trigger premature apoptosis. skl is potentially a fourth IAP antagonist in the "Reaper region" and a new candidate transducer of apoptotic damage signaling in Drosophila.

sickle, a novel Drosophila death gene in the reaper/hid/grim region, encodes an IAP-inhibitory protein

Inhibitors of apoptosis proteins (IAPs) interact with caspases and inhibit their protease activity, whereas the IAP-inhibitory proteins Smac/DIABLO in mammals and Reaper, Hid, and Grim in flies relieve IAP-mediated inhibition to induce cell death. Here we describe the functional characterization of the novel Drosophila cell death protein Sickle (Skl), which binds to IAPs and neutralizes their apoptotic inhibitory activity. Skl exhibits no sequence homology to Reaper, Hid, Grim, or Smac/DIABLO, except within the 4 residue N-terminal IAP binding motif. Skl interacts with Drosophila and mammalian IAPs and can promote caspase activation in the presence of IAPs. Consistent with these findings, expression of Skl in Drosophila and mammalian cell lines or in Drosophila embryos induces apoptosis. Skl can also synergize with Grim to induce cell death in the Drosophila eye imaginal disc. Based on biochemical and structural data, the N terminus of Skl, like that of the mammalian Smac/DIABLO, is absolutely required for its apoptotic and caspase-promoting activities and its ability to interact with IAPs. These findings point to conservation in the structure and function of the IAP-inhibitory proteins across species and suggest the existence of other family members.

Drosophila sickle is a novel grim-reaper cell death activator

The Drosophila genes reaper, head involution defective (hid), and grim all reside at 75C on chromosome three and encode related proteins that have crucial functions in programmed cell death (reviewed in ). In this report, we describe a novel grim-reaper gene, termed sickle, that resides adjacent to reaper. The sickle gene, like reaper and grim, encodes a small protein which contains an RHG motif and a Trp-block. In wild-type embryos, sickle expression was detected in cells of the developing central nervous system. Unlike reaper, hid, and grim, the sickle gene is not removed by Df(3L)H99, and strong ectopic sickle expression was detected in the nervous system of this cell death mutant. sickle very effectively induced cell death in cultured Spodoptera Sf-9 cells, and this death was antagonized by the caspase inhibitors p35 or DIAP1. Strikingly, unlike the other grim-reaper genes, targeted sickle expression did not induce cell death in the Drosophila eye. However, sickle strongly enhanced the eye cell death induced by expression of either an r/grim chimera or reaper.

Identification of Omi/HtrA2 as a mitochondrial apoptotic serine protease that disrupts inhibitor of apoptosis protein-caspase interaction

To identify human proteins that bind to the Smac and caspase-9 binding pocket on the baculoviral inhibitor of apoptosis protein (IAP) repeat 3 (BIR3) domain of human XIAP, we used BIR3 as an affinity reagent, followed by elution with the BIR3 binding peptide AVPIA, microsequencing, and mass spectrometry. The mature serine protease Omi (also known as HtrA2) was identified as a mitochondrial direct BIR3-binding protein and a caspase activator. Like mature Smac (also known as Diablo), mature Omi contains a conserved IAP-binding motif (AVPS) at its N terminus, which is exposed after processing of its N-terminal mitochondrial targeting sequence upon import into the mitochondria. Mature Omi is released together with mature Smac from the mitochondria into the cytosol upon disruption of the outer mitochondrial membrane during apoptosis. Finally, mature Omi can induce apoptosis in human cells in a caspase-independent manner through its protease activity and in a caspase-dependent manner via its ability to disrupt caspase-IAP interaction. Our results provide clear evidence for the involvement of a mitochondrial serine protease in the apoptotic pathway, emphasizing the critical role of the mitochondria in cell death.

A serine protease, HtrA2, is released from the mitochondria and interacts with XIAP, inducing cell death

X chromosome-linked inhibitor of apoptosis (XIAP) is an endogenous inhibitor of caspase-3, -7, and -9. Smac/DIABLO, an inhibitor of XIAP, is released from mitochondria upon receiving apoptotic stimuli and binds to the BIR2 and BIR3 domains of XIAP, thereby inhibiting its caspase-inhibitory activity. Here we report that a serine protease called HtrA2/Omi is released from mitochondria and inhibits the function of XIAP by direct binding in a similar way to Smac. Moreover, when overexpressed extramitochondrially, HtrA2 induces atypical cell death, which is neither accompanied by a significant increase in caspase activity nor inhibited by caspase inhibitors, including XIAP. A catalytically inactive mutant of HtrA2, however, does not induce cell death. In short, HtrA2 is a Smac-like inhibitor of IAP activity with a serine protease-dependent cell death-inducing activity.

Steroid-triggered death by autophagy

Programmed cell death is a critical part of normal development, removing obsolete tissues or cells and sculpting body parts to assume their appropriate form and function. Most programmed cell death occurs by apoptosis of individual cells or autophagy of groups of cells. Although these pathways have distinct morphological characteristics, they also have a number of features in common, suggesting some overlap in their regulation. A recent paper by Lee and Baehrecke provides further support for this proposal.(1) These authors present, for the first time, a genetic analysis of autophagy, using the steroid-triggered metamorphosis of Drosophila as a model system. They demonstrate a remarkable degree of overlap between the control of apoptosis and autophagy as well as a key role for the steroid-inducible gene E93 in directing the autophagic death response. This paper also shows that E93 can direct cell death independently from the known death-inducer genes, defining a novel death pathway in Drosophila.

Structural analysis of a functional DIAP1 fragment bound to grim and hid peptides

The inhibitor of apoptosis protein DIAP1 suppresses apoptosis in Drosophila, with the second BIR domain (BIR2) playing an important role. Three proteins, Hid, Grim, and Reaper, promote apoptosis, in part by binding to DIAP1 through their conserved N-terminal sequences. The crystal structures of DIAP1-BIR2 by itself and in complex with the N-terminal peptides from Hid and Grim reveal that these peptides bind a surface groove on DIAP1, with the first four amino acids mimicking the binding of the Smac tetrapeptide to XIAP. The next 3 residues also contribute to binding through hydrophobic interactions. Interestingly, peptide binding induces the formation of an additional alpha helix in DIAP1. Our study reveals the structural conservation and diversity necessary for the binding of IAPs by the Drosophila Hid/Grim/Reaper and the mammalian Smac proteins.

Apoptosis-regulating proteins as targets for drug discovery

Defects in the regulation of apoptosis (programmed cell death) contribute to many diseases, including pathologies associated with cell loss (e.g. stroke, heart failure, neurodegeneration and AIDS), and disorders characterized by a failure to eliminate harmful cells (e.g. cancer, autoimmunity). Apoptosis is caused by activation of intracellular proteases, known as caspases, which are responsible directly or indirectly for the morphological and biochemical events that characterize the apoptotic cell. Numerous caspase regulators have been discovered, which respond to environmental stimuli and influence the decision of cell death and survival. Knowledge of the molecular details of apoptosis regulation, and the three-dimensional structures of proteins constituting the apoptosis core machinery has revealed new strategies for identifying small-molecule drugs that could one day yield more effective treatments for a wide variety of illnesses.

Developmental change in the steroid hormone signal for cell-autonomous, segment-specific programmed cell death of a motoneuron

During metamorphosis of the hawkmoth, Manduca sexta, accessory planta retractor (APR) motoneurons undergo a segment-specific pattern of programmed cell death (PCD): e.g., APRs from abdominal segment six [APR(6)s] die at pupation in direct response to the prepupal rise in 20-hydroxyecdysone (20E), whereas APR(4)s survive through the pupal stage and die at eclosion (adult emergence). The hypothesis that the death of APR(4)s is triggered by the decline in 20E at eclosion was supported by findings that injection of 20E into developing pupae to delay the fall in 20E delayed APR(4) death. Furthermore, abdomen isolation to advance the fall in 20E caused precocious APR(4) death. In other experiments, APR(4)s were placed in primary cell culture 4 days before eclosion in medium containing 1 microg/ml 20E. A switch to hormone-free medium induced PCD in a significant proportion of APR(4)s, compared to APR(4)s that remained in 20E. Process fragmentation was a reliable early indicator of PCD. These results show that a decline in 20E triggers cell-autonomous PCD of APR(4)s, in contrast to the rise in 20E that triggers cell-autonomous PCD of APR(6)s. Thus, the PCD of homologous motoneurons in different body segments at different developmental times is triggered by different steroid hormone signals.

Apoptotic death sensor: an organelle's alter ego?

Caspases are intracellular cysteine proteases that are primarily responsible for the stereotypic morphological and biochemical changes that are associated with apoptosis. Caspases are often activated by the apoptotic protease-activating factor 1 (APAF-1) apoptosome, a complex that is formed following mitochondrial release of cytochrome c in response to many death-inducing stimuli. Both pro- and anti-apoptotic BCL-2 family members regulate apoptosis, primarily by their effects on mitochondria, whereas many inhibitor of apoptosis proteins (IAPs) regulate apoptosis by directly inhibiting distinct caspases. Exposure of cells to chemicals and radiation, as well as loss of trophic stimuli, perturb cellular homeostasis and, depending on the type of cellular stress, particular or multiple organelles appear to 'sense' the damage and signal the cell to undergo apoptosis by stimulating the formation of unique and/or common caspase-activating complexes.

Mouse models of cell death

Cell death is critical for the development and orderly maintenance of cellular homeostasis in metazoans. Developmental genetics in model systems, including Caenorhabditis elegans and Drosophila melanogaster, have helped to identify and order the components of cell-death pathways. An even more complex network of apoptotic pathways has evolved in higher organisms that possess homologs within each set of cell-death regulators. Whereas biochemical studies provide details of molecular mechanisms, genetic models reveal the essential physiologic roles. Transgenic and gene-ablated mice have helped to elucidate mammalian apoptotic pathways and identify the principal effect of each cell death regulator. Here, we review the details of the apoptotic machinery as revealed by mice deficient in critical components of cell-death pathways; we concentrate on cell-death regulators classified as members of the caspase and Bcl2 families or, broadly, as adaptors and mitochondrial released factors.

Apoptosis in motion. An apical, P35-insensitive caspase mediates programmed cell death in insect cells

Activation of caspases by proteolytic processing is a critical step during apoptosis in metazoans. Here we use high resolution time lapse microscopy to show a tight link between caspase activation and the morphological events delineating apoptosis in cultured SF21 cells from the moth Spodoptera frugiperda, a model insect system. The principal effector caspase, Sf-caspase-1, is proteolytically activated during SF21 apoptosis. To define the potential role of initiator caspases in vivo, we tested the effect of cell-permeable peptide inhibitors on pro-Sf-caspase-1 processing. Anti-caspase peptide analogues prevented apoptosis induced by diverse signals, including UV radiation and baculovirus infection. IETD-fmk potently inhibited the initial processing of pro-Sf-caspase-1 at the junction (TETD-G) of the large and small subunit, a cleavage that is blocked by inhibitor of apoptosis Op-IAP but not pancaspase inhibitor P35. Because Sf-caspase-1 was inhibited poorly by IETD-CHO, our data indicated that the protease responsible for the first step in pro-Sf-caspase-1 activation is a distinct apical caspase. Thus, Sf-caspase-1 activation is mediated by a novel, P35-resistant caspase. These findings support the hypothesis that apoptosis in insects, like that in mammals, involves a cascade of caspase activations.

The RHG motifs of Drosophila Reaper and Grim are important for their distinct cell death-inducing abilities

Reaper, Hid, and Grim are three Drosophila cell death activators that each contain a conserved NH(2)-terminal Reaper, Hid, Grim (RHG) motif. We have analyzed the importance of the RHG motifs in Reaper and Grim for their different abilities to activate cell death during development. Analysis of chimeric R/Grim and G/Reaper proteins indicated that the Reaper and Grim RHG motifs are functionally distinct and help to determine specific cell death activation properties. A truncated GrimC protein lacking the RHG motif retained an ability to induce cell death, and unlike Grim, R/Grim, or G/Reaper, its actions were not efficiently blocked by the cell death inhibitors, Diap1, Diap2, p35, or a dominant/negative Dronc caspase. Finally, we identified a second region of sequence similarity in Reaper, Hid, and Grim, that may be important for shared RHG motif-independent activities.

A conserved XIAP-interaction motif in caspase-9 and Smac/DIABLO regulates caspase activity and apoptosis

X-linked inhibitor-of-apoptosis protein (XIAP) interacts with caspase-9 and inhibits its activity, whereas Smac (also known as DIABLO) relieves this inhibition through interaction with XIAP. Here we show that XIAP associates with the active caspase-9-Apaf-1 holoenzyme complex through binding to the amino terminus of the linker peptide on the small subunit of caspase-9, which becomes exposed after proteolytic processing of procaspase-9 at Asp315. Supporting this observation, point mutations that abrogate the proteolytic processing but not the catalytic activity of caspase-9, or deletion of the linker peptide, prevented caspase-9 association with XIAP and its concomitant inhibition. We note that the N-terminal four residues of caspase-9 linker peptide share significant homology with the N-terminal tetra-peptide in mature Smac and in the Drosophila proteins Hid/Grim/Reaper, defining a conserved class of IAP-binding motifs. Consistent with this finding, binding of the caspase-9 linker peptide and Smac to the BIR3 domain of XIAP is mutually exclusive, suggesting that Smac potentiates caspase-9 activity by disrupting the interaction of the linker peptide of caspase-9 with BIR3. Our studies reveal a mechanism in which binding to the BIR3 domain by two conserved peptides, one from Smac and the other one from caspase-9, has opposing effects on caspase activity and apoptosis.

Role of caspases and mitochondria in the steroid-induced programmed cell death of a motoneuron during metamorphosis

Accessory planta retractor (APR) motoneurons of the hawk moth, Manduca sexta, undergo a segment-specific pattern of programmed cell death (PCD) 24 to 48 h after pupal ecdysis (PE). Cell culture experiments show that the PCD of APRs in abdominal segment 6 [APR(6)s] is a cell-autonomous response to the steroid hormone 20-hydroxyecdysone (20E) and involves mitochondrial demise and cell shrinkage. Twenty-four hours before PE, at stage W3-noon, APR(6)s require further 20E exposure and protein synthesis (as tested with cycloheximide) to undergo PCD, and death can be blocked by a broad-spectrum caspase inhibitor. By PE, death is 20E- and protein synthesis-independent and the caspase inhibitor blocks cell shrinkage but not loss of mitochondrial function. Thus, the commitment to mitochondrial demise precedes the commitment to execution events. The phenotype of necrotic cell death induced by a mitochondrial electron transfer inhibitor differs unambiguously from 20E-induced PCD. By inducing PCD pharmacologically, the readiness of APR(6)s to execute PCD was found to increase during the final larval instar. These data suggest that the 20E-induced PCD of APR(6)s includes a premitochondrial phase which includes 20E-induced synthetic events and apical caspase activity, a mitochondrial phase which culminates in loss of mitochondrial function, and a postmitochondrial phase during which effector caspases are activated and APR(6) is destroyed.

An essential role for the caspase dronc in developmentally programmed cell death in Drosophila

Dronc is a caspase recruitment domain-containing Drosophila caspase that is expressed in a temporally and spatially restricted fashion during development. Dronc is the only fly caspase known to be regulated by the hormone ecdysone. Here we show that ectopic expression of dronc in the developing fly eye leads to increased cell death and an ablated eye phenotype that can be suppressed by halving the dosage of the genes in the H99 complex (reaper, hid, and grim) and enhanced by mutations in diap1. In contrast to previous reports, we show that the dronc eye ablation phenotype can be suppressed by coexpression of the baculoviral caspase inhibitor p35. Dronc also interacts, both genetically and biochemically, with the CED-4/Apaf-1 fly homolog, Dark. Furthermore, extracts made from Dark homozygous mutant flies have reduced ability to process Dronc, showing that Dark is required for Dronc processing. Finally, using the RNA interference technique, we show that loss of Dronc function in early Drosophila embryos results in a dramatic decrease in cell death, indicating that Dronc is important for programmed cell death during embryogenesis. These results suggest that Dronc is a key caspase mediating programmed cell death in Drosophila.

Bid-induced cytochrome c release is mediated by a pathway independent of mitochondrial permeability transition pore and Bax

Bid, a pro-apoptosis "BH3-only" member of the Bcl-2 family, can be cleaved by caspase-8 after Fas/TNF-R1 engagement. The p15 form of truncated Bid (tBid) translocates to mitochondria and induces cytochrome c release, leading to the activation of downstream caspases and apoptosis. In the current study, we investigated the mechanism by which tBid regulated cytochrome c release in terms of its relationship to mitochondrial permeability transition and Bax, another Bcl-2 family protein. We employed an in vitro reconstitution system as well as cell cultures and an animal model to reflect the physiological environment where Bid could be functional. We found that induction of cytochrome c release by tBid was not accompanied by a permeability transition even at high doses. Indeed, inhibition of permeability transition did not suppress the activity of tBid in vitro nor could they block Fas activation-induced, Bid-dependent hepatocyte apoptosis in cultures. Furthermore, Mg(2+), although inhibiting permeability transition, actually enhanced the ability of tBid to induce cytochrome c release. We also found that tBid did not require Bax to induce cytochrome c release in vitro. In addition, mice deficient in bax were still highly susceptible to anti-Fas-induced hepatocyte apoptosis, in which cytochrome c release was unaffected. Moreover, although Bax-induced cytochrome c release was not dependent on tBid, the two proteins could function synergistically. We conclude that Bid possesses the biochemical activity to induce cytochrome c release through a mechanism independent of mitochondrial permeability transition pore and Bax.

Early genes required for salivary gland fate determination and morphogenesis in Drosophila melanogaster

Studies of Drosophila salivary gland formation have elucidated the regulatory pathway by which the salivary gland fate is determined and the morphogenetic processes by which the primordial cells are internalized to form the tubular glands. Both the position of the salivary primordia and the number of cells recruited to a salivary gland fate are established through a combination of the localized expression of the transcription factors SEX COMBS REDUCED (SCR), TEASHIRT (TSH) and ABDOMINAL-B (ABD-B), and localized DPP-signaling. Similarly, the distinction between the two major cell types, duct and secretory, is determined by spatially limited EGF-signaling. Salivary gland formation also requires the function of two transcription factors expressed in nearly all cells of the developing embryo, EXTRADENTICLE (EXD) and HOMOTHORAX (HTH). Once the salivary gland fate is determined, cells of the secretory primordia are internalized by an apical constriction mode of invagination. We have characterized three genes encoding transcription factors, trachealess (trh), hückebein (hkb), and fork head (fkh), that are downstream targets of the salivary gland regulators. Mutations in these transcription factors profoundly affect salivary gland morphogenesis. trh is required for the formation of the salivary duct tubes. hkb determines the order of secretory cell invagination, a regulated process critical for determining the final shape of the salivary gland. fkh has two early roles in salivary gland formation. fkh both promotes secretory cell survival and facilitates secretory cell internalization. trh, hkb, and fkh are involved in the formation of not only the salivary duct and secretory tubes, but also of other tubular structures, such as the trachea and the gut endoderm. We propose that trh, hkb, and fkh may serve as "morphogenetic cassettes" responsible for forming tubular structures in a variety of tissues.

The midline glia of Drosophila: a molecular genetic model for the developmental functions of glia

The Midline Glia of Drosophila are required for nervous system morphogenesis and midline axon guidance during embryogenesis. In origin, gene expression and function, this lineage is analogous to the floorplate of the vertebrate neural tube. The expression or function of over 50 genes, summarised here, has been linked to the Midline Glia. Like the floorplate, the cells which generate the Midline Glia lineage, the mesectoderm, are determined by the interaction of ectoderm and mesoderm during gastrulation. Determination and differentiation of the Midline Glia involves the Drosophila EGF, Notch and segment polarity signaling pathways, as well as twelve identified transcription factors. The Midline Glia lineage has two phases of cell proliferation and of programmed cell death. During embryogenesis, the EGF receptor pathway signaling and Wrapper protein both function to suppress apoptosis only in those MG which are appropriately positioned to separate and ensheath midline axonal commissures. Apoptosis during metamorphosis is regulated by the insect steroid, Ecdysone. The Midline Glia participate in both the attraction of axonal growth cones towards the midline, as well as repulsion of growth cones from the midline. Midline axon guidance requires the Drosophila orthologs of vertebrate genes expressed in the floorplate, which perform the same function. Genetic and molecular evidence of the interaction of attractive (Netrin) and repellent (Slit) signaling is reviewed and summarised in a model. The Midline Glia participate also in the generation of extracellular matrix and in trophic interactions with axons. Genetic evidence for these functions is reviewed.

Drug discovery opportunities from apoptosis research

Cell suicide is a normal process that participates in a wide variety of physiological processes, including tissue homeostasis, immune regulation, and fertility. Physiological cell death typically occurs by apoptosis, as opposed to necrosis. Defects in apoptotic cell-death regulation contribute to many diseases, including disorders associated with cell accumulation (e.g. cancer, autoimmunity, inflammation and restenosis) or where cell loss occurs (e.g. stroke, heart failure, neurodegeneration, AIDS and osteoporosis). At the center of the apoptosis machinery is a family of intracellular proteases, known as 'caspases', that are responsible directly or indirectly for the morphological and biochemical events that characterize apoptosis. Multiple positive and negative regulators of these cell-death proteases have been discovered in the genomes of mammals, amphibians, insects, nematodes, and other animal species, as well as a variety of animal viruses. Inputs from signal-transduction pathways into the core of the cell-death machinery have also been identified, demonstrating ways of linking environmental stimuli to cell-death responses or cell-survival maintenance. Knowledge of the molecular mechanisms of apoptosis has provided important insights into the causes of multiple diseases where aberrant cell-death regulation occurs and has revealed new approaches for identifying small-molecule drugs for more effectively treating these illnesses.

Insect metamorphosis: out with the old, in with the new

During insect metamorphosis, the steroid hormone ecdysone activates programmed cell death of larval tissues and the further development of adult tissues. Recent studies suggest that the E93 gene is both necessary and sufficient to target tissues for ecdysone-induced apoptosis.

Guardian ancestry: fly p53 and damage-inducible apoptosis

The tumor suppressor, p53, is among the most commonly mutated genes in human cancers. Recent reports describe shared and divergent properties of a Drosophila p53 homolog Dmp53. Like its mammalian counterpart, Dmp53 also functions in damage-induced cell death. In this model system, the apoptosis activator reaper has emerged as an important target gene. Together with the wealth of genomic data available in Drosophila, continued studies on Dmp53 promise new insights into the regulation and function of this important gene family.

Steroid regulation of programmed cell death during Drosophila development

Steroid hormones play an important role in the regulation of numerous physiological responses, but the mechanisms that enable these systemic signals to trigger specific cell changes remain poorly characterized. Recent studies of Drosophila illustrate several important features of steroid-regulated programmed cell death. A single steroid hormone activates both cell differentiation and cell death in different tissues and at multiple stages during development. While several steroid-regulated genes are required for cell execution, most of these genes function in both cell differentiation and cell death, and require more specific factors to kill cells. Genes that regulate apoptosis during Drosophila embryogenesis are induced by steroids in dying cells later in development. These apoptosis genes likely function downstream of hormone-induced factors to serve a more direct role in the death response. This article reviews the current knowledge of steroid signaling and the regulation of programmed cell death during development of Drosophila.

Understanding IAP function and regulation: a view from Drosophila

Apoptosis is an active form of cell suicide that results in the orderly death and phagocytosis of cells during normal development and in the adult. Many death signals lead to the activation of members of a family of cysteine proteases known as caspases. These proteins act to transduce death signals from different cellular compartments and they cleave a number of cellular proteins, leading ultimately to many of the biochemical and morphological events associated with death. Many mechanisms act to inhibit cell death upstream of caspase activation. However, only one family of cellular proteins, the inhibitors of apoptosis (IAPs), has been identified that inhibit caspase activation and/or activity. The observations that IAP function is essential for cell survival in Drosophila, and that IAP expression is deregulated in many forms of cancer in humans, argue that IAPs are important cell death inhibitors and that deregulation of their function is likely to be important in human disease. Here we review IAP function, with particular reference to insights that study of the Drosophila IAPs has provided. We also discuss some directions for future study.

Mechanisms of apoptosis

Programmed cell death plays critical roles in a wide variety of physiological processes during fetal development and in adult tissues. In most cases, physiological cell death occurs by apoptosis as opposed to necrosis. Defects in apoptotic cell death regulation contribute to many diseases, including disorders where cell accumulation occurs (cancer, restenosis) or where cell loss ensues (stroke, heart failure, neurodegeneration, AIDS). In recent years, the molecular machinery responsible for apoptosis has been elucidated, revealing a family of intracellular proteases, the caspases, which are responsible directly or indirectly for the morphological and biochemical changes that characterize the phenomenon of apoptosis. Diverse regulators of the caspases have also been discovered, including activators and inhibitors of these cell death proteases. Inputs from signal transduction pathways into the core of the cell death machinery have also been identified, demonstrating ways of linking environmental stimuli to cell death responses or cell survival maintenance. Knowledge of the molecular mechanisms of apoptosis is providing insights into the causes of multiple pathologies where aberrant cell death regulation occurs and is beginning to provide new approaches to the treatment of human diseases.

The biochemistry of apoptosis

Apoptosis--the regulated destruction of a cell--is a complicated process. The decision to die cannot be taken lightly, and the activity of many genes influence a cell's likelihood of activating its self-destruction programme. Once the decision is taken, proper execution of the apoptotic programme requires the coordinated activation and execution of multiple subprogrammes. Here I review the basic components of the death machinery, describe how they interact to regulate apoptosis in a coordinated manner, and discuss the main pathways that are used to activate cell death.

Mitochondria as the central control point of apoptosis

Mitochondria play a major role in apoptosis triggered by many stimuli. They integrate death signals through Bcl-2 family members and coordinate caspase activation through the release of cytochrome c as a result of the outer mitochondrial membrane becoming permeable. The mechanisms that lead to this permeability are not yet completely understood. Here, we attempt to summarize our current view of the mechanisms that lead to the efflux of many proteins from mitochondria during apoptosis and the role played by Bcl-2 family proteins in the control of this event.

Bcl-2 and Bax mammalian regulators of apoptosis are functional in Drosophila

Studies of apoptosis in C. elegans have allowed the identification of three genes, ced-3, ced-4 and ced-9. Their products constitute the components of an induction pathway of apoptosis conserved in the nematode and mammals. In Drosophila, homologues have been found for CED-3, CED-4 and CED-9. CED-9 belongs to the Bcl-2 family which includes negative (Bcl-2) and positive (Bax) regulators of apoptosis. The recently discovered Bcl-2 family member named Drob-1 acts as a positive regulator of cell death. To address whether a Bcl-2 anti-apoptotic pathway exists in the fly, we studied the effects of expressing the mammalian genes bcl-2 in Drosophila. In embryos, expression of bcl-2 inhibits developmental and X-ray-induced apoptosis. Expressing bcl-2 or the pro-apoptotic mammalian bax in the developing eye and wing alters these structures, bcl-2 increasing the number of cells, while bax reduces the number of cells. In addition, the functional interaction between Bcl-2 and Bax is conserved. These results indicate that factors necessary for the activity of bcl-2 and bax are present in Drosophila. Therefore, a Bcl-2 pathway for inhibition of cell death may exist in the fly.