Apoptosis in Drosophila: neither fish nor fowl (nor man, nor worm)

The interaction of DIAP1 with dOmi/HtrA2 regulates cell death in Drosophila

Mitochondrial proteins such as cytochrome c, Smac/DIABLO and Omi/HtrA2 play important roles in the cell death pathways of mammalian cells. In Drosophila, the role of mitochondria in cell death is less clear. Here, we report the identification and characterization of the Drosophila ortholog of human Omi/HtrA2. We show that Drosophila Omi/HtrA2 is imported into the mitochondria where it undergoes proteolytic maturation to yield two isoforms, dOmi-L and dOmi-S. dOmi-L contains a canonical N-terminal IAP-binding motif (AVVS), whereas dOmi-S contains a distinct N-terminal motif (SKMT). DIAP1 was able to bind to both isoforms via its BIR1 and BIR2 domains. This resulted in cleavage of the linker region of DIAP1 between the BIR1 and BIR2 domains and further degradation of the BIR1 domain by the proteolytic activity of dOmi. The binding of DIAP1 to dOmi also resulted in DIAP1-mediated polyubiquitination of dOmi, suggesting that DIAP1 could target dOmi for proteasomal degradation. Consistent with this, expression of DIAP1 in Drosophila eye discs protected them from dOmi-induced eye ablation, indicating that DIAP1 plays an important role in protecting cells from the potentially lethal effects of dOmi. The ability of IAPs to bind to and ubiquitinate mitochondrial proteins such as dOmi may be a key conserved function to counterbalance the lethal effects of these proteins if accidentally released into the cytosol.

The molecular archaeology of a mitochondrial death effector: AIF in Drosophila

Apoptosis-inducing factor (AIF) is a phylogenetically conserved redox-active flavoprotein that contributes to cell death and oxidative phosphorylation in Saccharomyces cerevisiae, Caenorhabditis elegans, mouse and humans. AIF has been characterized as a caspase-independent death effector that is activated by its translocation from mitochondria to the cytosol and nucleus. Here, we report the molecular characterization of AIF in Drosophila melanogaster, a species in which most cell deaths occur in a caspase-dependent manner. Interestingly, knockout of zygotic D. melanogaster AIF (DmAIF) expression using gene targeting resulted in decreased embryonic cell death and the persistence of differentiated neuronal cells at late embryonic stages. Although knockout embryos hatch, they undergo growth arrest at early larval stages, accompanied by mitochondrial respiratory dysfunction. Transgenic expression of DmAIF misdirected to the extramitochondrial compartment (DeltaN-DmAIF), but not wild-type DmAIF, triggered ectopic caspase activation and cell death. DeltaN-DmAIF-induced death was not blocked by removal of caspase activator Dark or transgenic expression of baculoviral caspase inhibitor p35, but was partially inhibited by Diap1 overexpression. Knockdown studies revealed that DeltaN-DmAIF interacts genetically with the redox protein thioredoxin-2. In conclusion, we show that Drosophila AIF is a mitochondrial effector of cell death that plays roles in developmentally regulated cell death and normal mitochondrial function.

Surprising complexity of the ancestral apoptosis network

A comparative genomics approach revealed that the genes for several components of the apoptosis network with single copies in vertebrates have multiple paralogs in cnidarian-bilaterian ancestors, suggesting a complex evolutionary history for this network.

Background

Apoptosis, one of the main types of programmed cell death, is regulated and performed by a complex protein network. Studies in model organisms, mostly in the nematode Caenorhabditis elegans, identified a relatively simple apoptotic network consisting of only a few proteins. However, analysis of several recently sequenced invertebrate genomes, ranging from the cnidarian sea anemone Nematostella vectensis, representing one of the morphologically simplest metazoans, to the deuterostomes sea urchin and amphioxus, contradicts the current paradigm of a simple ancestral network that expanded in vertebrates.

Results

Here we show that the apoptosome-forming CED-4/Apaf-1 protein, present in single copy in vertebrate, nematode, and insect genomes, had multiple paralogs in the cnidarian-bilaterian ancestor. Different members of this ancestral Apaf-1 family led to the extant proteins in nematodes/insects and in deuterostomes, explaining significant functional differences between proteins that until now were believed to be orthologous. Similarly, the evolution of the Bcl-2 and caspase protein families appears surprisingly complex and apparently included significant gene loss in nematodes and insects and expansions in deuterostomes.

Conclusion

The emerging picture of the evolution of the apoptosis network is one of a succession of lineage-specific expansions and losses, which combined with the limited number of 'apoptotic' protein families, resulted in apparent similarities between networks in different organisms that mask an underlying complex evolutionary history. Similar results are beginning to surface for other regulatory networks, contradicting the intuitive notion that regulatory networks evolved in a linear way, from simple to complex.

Regulation of apoptosis in Drosophila

Insects have made major contributions to understanding the regulation of cell death, dating back to the pioneering work of Lockshin and Williams on death of muscle cells during postembryonic development of Manduca. A physically smaller cousin of moths, the fruit fly Drosophila melanogaster, offers unique advantages for studying the regulation of cell death in response to different apoptotic stimuli in situ. Different signaling pathways converge in Drosophila to activate a common death program through transcriptional activation of reaper, hid and grim. Reaper-family proteins induce apoptosis by binding to and antagonizing inhibitor of apoptosis proteins (IAPs), which in turn inhibit caspases. This switch from life to death relies extensively on targeted degradation of cell death proteins by the ubiquitin-proteasome pathway. Drosophila IAP-1 (Diap1) functions as an E3-ubiquitin ligase to protect cells from unwanted death by promoting the degradation of the initiator caspase Dronc. However, in response to apoptotic signals, Reaper-family proteins are produced, which promote the auto-ubiquitination and degradation of Diap1, thereby removing the 'brakes on death' in cells that are doomed to die. More recently, several other ubiquitin pathway proteins were found to play important roles for caspase regulation, indicating that the control of cell survival and death relies extensively on targeted degradation by the ubiquitin-proteasome pathway.

A quorum on bacterial programmed cell death

A recent article in Science (Kolodkin-Gal et al., 2007) reported a novel programmed cell death mechanism for Escherichia coli that occurs during cellular overcrowding via the release of a fratricidal pentapeptide derived from the metabolic enzyme glucose-6-phosphate dehydrogenase.

Flock house virus induces apoptosis by depletion of Drosophila inhibitor-of-apoptosis protein DIAP1

The molecular mechanisms by which RNA viruses induce apoptosis and apoptosis-associated pathology are not fully understood. Here we show that flock house virus (FHV), one of the simplest RNA viruses (family, Nodaviridae), induces robust apoptosis of permissive Drosophila Line-1 (DL-1) cells. To define the pathway by which FHV triggers apoptosis in this model invertebrate system, we investigated the potential role of Drosophila apoptotic effectors during infection. Suggesting the involvement of host caspases, the pancaspase inhibitor benzyloxycarbonyl-Val-Ala-Asp-fluromethylketone (z-VAD-fmk) prevented FHV-induced cytopathology and prolonged cell survival. RNA interference-mediated ablation of the principal Drosophila effector caspase DrICE or its upstream initiator caspase DRONC prevented FHV-induced apoptosis and demonstrated direct participation of this intrinsic caspase pathway. Prior to the FHV-induced activation of DrICE, the intracellular level of inhibitor-of-apoptosis (IAP) protein DIAP1, the principal caspase regulator in Drosophila melanogaster, was dramatically reduced. DIAP1 was depleted despite z-VAD-fmk-mediated caspase inhibition during infection, suggesting that the loss of DIAP1 was caused by an upstream FHV-induced signal. The RNA interference-mediated knockdown of DIAP1 caused rapid and uniform apoptosis of DL-1 cells and thus indicated that DIAP1 depletion is sufficient to trigger apoptosis. Confirming this conclusion, the elevation of intracellular DIAP1 levels in stable diap1-transfected cells blocked caspase activation and prevented FHV-induced apoptosis. Collectively, our findings suggest that DIAP1 is a critical sensor of virus infection, which upon virus-signaled depletion relieves caspase inhibition, which subsequently executes apoptotic death. Thus, our study supports the hypothesis that altering the level or the activity of cellular IAP proteins is a general mechanism by which RNA viruses trigger apoptosis.

Heat shock genes - integrating cell survival and death

Heat shock induced gene expression and other cellular responses help limit the damage caused by stress and thus facilitate cellular recovery. Cellular damage also triggers apoptotic cell death through several pathways. This paper briefly reviews interactions of the major heat shock proteins with components of the apoptotic pathways. Hsp90, which acts as a chaperone for unstable signal transducers to keep them poised for activation, interacts with RIP and Akt and promotes NF-kappa B mediated inhibition of apoptosis; in addition it also blocks some steps in the apoptotic pathways. Hsp70 is mostly anti-apoptotic and acts at several levels like inhibition of translocation of Bax into mitochondria, release of cytochrome c from mitochondria,formation of apoptosome and inhibition of activation of initiator caspases. Hsp70 also modulates JNK,NF-kappa B and Akt signaling pathways in the apoptotic cascade. In contrast, Hsp60 has both anti-and pro-apoptotic roles. Cytosolic Hsp60 prevents translocation of the pro-apoptotic protein Bax into mitochondria and thus promotes cell survival but it also promotes maturation of procaspase-3,essential for caspase mediated cell death. Our recent in vivo studies show that RNAi for the Hsp60D in Drosophila melanogaster prevents induced apoptosis. Hsp27 exerts its anti-apoptotic influence by inhibiting cytochrome c and TNF-mediated cell death. alpha beta crystallin suppresses caspase-8 and cytochrome c mediated activation of caspase-3. Studies in our laboratory also reveal that absence or reduced levels of the developmentally active as well as stress induced non-coding hsr omega transcripts, which are known to sequester diverse hnRNPs and related nuclear RNA-binding proteins,block induced apoptosis in Drosophila. Modulation of the apoptotic pathways by Hsps reflects their roles as "weak links" between various "hubs" in cellular networks. On the other hand, non-coding RNAs, by virtue of their potential to bind with multiple proteins,can act as "hubs" in these networks. In view of the integrative nature of living systems, it is not surprising that stress-induced genes,generally believed to primarily function in cell survival pathways, inhibit or even promote cell death pathways at multiple levels to ensure homeostasis at cell and/or organism level. The heat shock genes obviously do much more than merely help cells survive stress.

Alternative splicing generates multiple transcripts of the inhibitor of apoptosis protein 1 in Aedes and Culex spp. mosquitoes

We determined the sequences of cDNA encoding Inhibitor of Apoptosis Protein 1 (IAP1) homologues from Aedes triseriatus, Aedes albopictus, Aedes aegypti, Culex pipiens and Culex tarsalis. The cDNAs encode translation products that share > or = 84% sequence similarity. The IAP1 mRNA of each mosquito species exists as 3-5 distinct variants due to the presence of heterogeneous sequences at the distal end of their 5'UTRs. Partial genomic sequencing upstream of the 5' end of the Ae. triseriatus IAP1 gene, and analysis of the Ae. aegypti genomic sequence, suggest that these mRNA variants are generated by alternative splicing. Each IAP1 mRNA variant from Ae. triseriatus and Cx. pipiens was detected by RT-PCR in all mosquito life-stages and adult tissues examined, and the relative concentration of each Ae. triseriatus IAP mRNA variant in various tissues was determined.

Detection of apoptosis by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling and acridine orange in Drosophila embryos and adult male gonads

In Drosophila, vast numbers of cells undergo apoptosis during normal development. In addition, excessive apoptosis can be induced in response to a variety of stress or injury paradigms, including DNA damage, oxidative stress, nutrient deprivation, unfolded proteins and mechanical tissue damage. Two of the most commonly used methods to label apoptotic cells in Drosophila are terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) for fixed tissues and acridine orange (AO) staining for live embryos or tissues. Here, we describe protocols for labeling apoptotic cells in Drosophila embryos and adult male gonads. Slightly modified protocols can also be applied for other Drosophila tissues. The AO protocol is quick, simple and allows real-time imaging of doomed cells in live tissues. However, it is difficult to combine with conventional counterstains or Ab labeling. On the other hand, this functionality is readily afforded by the TUNEL protocol, which permits the detection of apoptotic cells in fixed tissues. These staining procedures can be completed in 1-2 d.

A collective form of cell death requires homeodomain interacting protein kinase

We examined post-eclosion elimination of the Drosophila wing epithelium in vivo where collective "suicide waves" promote sudden, coordinated death of epithelial sheets without a final engulfment step. Like apoptosis in earlier developmental stages, this unique communal form of cell death is controlled through the apoptosome proteins, Dronc and Dark, together with the IAP antagonists, Reaper, Grim, and Hid. Genetic lesions in these pathways caused intervein epithelial cells to persist, prompting a characteristic late-onset blemishing phenotype throughout the wing blade. We leveraged this phenotype in mosaic animals to discover relevant genes and establish here that homeodomain interacting protein kinase (HIPK) is required for collective death of the wing epithelium. Extra cells also persisted in other tissues, establishing a more generalized requirement for HIPK in the regulation of cell death and cell numbers.

XIAP activity dictates Apaf-1 dependency for caspase 9 activation

The current model for the intrinsic apoptotic pathway holds that mitochondrial activation of caspases in response to cytotoxic drugs requires both Apaf-1-induced dimerization of procaspase 9 and Smac/Diablo-mediated sequestration of inhibitors of apoptosis proteins (IAPs). Here, we showed that either pathway can independently promote caspase 9 activation in response to apoptotic stimuli. In drug-treated Apaf-1(-/-) primary myoblasts, but not fibroblasts, Smac/Diablo accumulates in the cytosol and sequesters X-linked IAP (XIAP), which is expressed at lower levels in myoblasts than in fibroblasts. Consequently, caspase 9 activation proceeds in Apaf-1(-/-) myoblasts; concomitant ablation of Apaf-1 and Smac is required to prevent caspase 9 activation and the onset of apoptosis. Conversely, in stimulated Apaf-1(-/-) fibroblasts, the ratio of XIAP to Smac/Diablo is high compared to that for myoblasts and procaspase 9 is not activated. Suppressing XIAP with exogenous Smac/Diablo or a pharmacological inhibitor can still induce caspase 9 in drug-treated Apaf-1-null fibroblasts. Thus, caspase 9 activation in response to intrinsic apoptotic stimuli can be uncoupled from Apaf-1 in vivo by XIAP antagonists.

Autophagy induction rescues toxicity mediated by proteasome inhibition

The ubiquitin-proteasome and macroautophagy-lysosome pathways are major routes for intracytosolic protein degradation. In many systems, proteasome inhibition is toxic. A Nature article by Pandey et al. shows that this toxicity can be modulated by altering autophagic activity. Their tantalizing results suggest that overexpression of HDAC6 may increase flux through the autophagy pathway, thereby attenuating the toxicity resulting from proteasome inhibition.

Baculovirus caspase inhibitors P49 and P35 block virus-induced apoptosis downstream of effector caspase DrICE activation in Drosophila melanogaster cells

Baculoviruses induce widespread apoptosis in invertebrates. To better understand the pathways by which these DNA viruses trigger apoptosis, we have used a combination of RNA silencing and overexpression of viral and host apoptotic regulators to identify cell death components in the model system of Drosophila melanogaster. Here we report that the principal effector caspase DrICE is required for baculovirus-induced apoptosis of Drosophila DL-1 cells as demonstrated by RNA silencing. proDrICE was proteolytically cleaved and activated during infection. Activation was blocked by overexpression of the cellular inhibitor-of-apoptosis proteins DIAP1 and SfIAP but not by the baculovirus caspase inhibitor P49 or P35. Rather, the substrate inhibitors P49 and P35 prevented virus-induced apoptosis by arresting active DrICE through formation of stable inhibitory complexes. Consistent with a two-step activation mechanism, proDrICE was cleaved at the large/small subunit junction TETD(230)-G by a DIAP1-inhibitable, P49/P35-resistant protease and then at the prodomain junction DHTD(28)-A by a P49/P35-sensitive protease. Confirming that P49 targeted DrICE and not the initiator caspase DRONC, depletion of DrICE by RNA silencing suppressed virus-induced cleavage of P49. Collectively, our findings indicate that whereas DIAP1 functions upstream to block DrICE activation, P49 and P35 act downstream by inhibiting active DrICE. Given that P49 has the potential to inhibit both upstream initiator caspases and downstream effector caspases, we conclude that P49 is a broad-spectrum caspase inhibitor that likely provides a selective advantage to baculoviruses in different cellular backgrounds.

Mitochondrial disruption in Drosophila apoptosis

Mitochondrial disruption is a conserved aspect of apoptosis, seen in many species from mammals to nematodes. Despite significant conservation of other elements of the apoptotic pathway in Drosophila, a broad role for mitochondrial changes in apoptosis in flies remains unconfirmed. Here, we show that Drosophila mitochondria become permeable in response to the expression of Reaper and Hid, endogenous regulators of developmental apoptosis. Caspase activation in the absence of Reaper and Hid is not sufficient to permeabilize mitochondria, but caspases play a role in Reaper- and Hid-induced mitochondrial changes. Reaper and Hid rapidly localize to mitochondria, resulting in changes in mitochondrial ultrastructure. The dynamin-related protein, Drp1, is important for Reaper- and DNA-damage-induced mitochondrial disruption. Significantly, we show that inhibition of Reaper or Hid mitochondrial localization or inhibition of Drp1 significantly inhibits apoptosis, indicating a role for mitochondrial disruption in fly apoptosis.

Drosophila Omi, a mitochondrial-localized IAP antagonist and proapoptotic serine protease

Although essential in mammals, in flies the importance of mitochondrial outer membrane permeabilization for apoptosis remains highly controversial. Herein, we demonstrate that Drosophila Omi (dOmi), a fly homologue of the serine protease Omi/HtrA2, is a developmentally regulated mitochondrial intermembrane space protein that undergoes processive cleavage, in situ, to generate two distinct inhibitor of apoptosis (IAP) binding motifs. Depending upon the proapoptotic stimulus, mature dOmi is then differentially released into the cytosol, where it binds selectively to the baculovirus IAP repeat 2 (BIR2) domain in Drosophila IAP1 (DIAP1) and displaces the initiator caspase DRONC. This interaction alone, however, is insufficient to promote apoptosis, as dOmi fails to displace the effector caspase DrICE from the BIR1 domain in DIAP1. Rather, dOmi alleviates DIAP1 inhibition of all caspases by proteolytically degrading DIAP1 and induces apoptosis both in cultured cells and in the developing fly eye. In summary, we demonstrate for the first time in flies that mitochondrial permeabilization not only occurs during apoptosis but also results in the release of a bona fide proapoptotic protein.

Drosophila Bcl-2 proteins participate in stress-induced apoptosis, but are not required for normal development

Many developing tissues require programmed cell death (PCD) for proper formation. In mice and C. elegans, developmental PCD is regulated by the Bcl-2 family of proteins. Two bcl-2 genes are encoded in the Drosophila genome (debcl/dBorg1/Drob-1/dBok and buffy/dBorg2) and previous RNAi-based studies suggested a requirement for these in embryonic development. However, we report here that, despite the fact that many tissues in fruit flies are shaped by PCD, deletion of the bcl-2 genes does not perturb normal development. We investigated whether the fly bcl-2 genes regulate non-apoptotic processes that require caspases, but found these to be bcl-2 gene-independent. However, irradiation of the mutants demonstrates that DNA damage-induced apoptosis, mediated by Reaper, is blocked by buffy and that debcl is required to inhibit buffy. Our results demonstrate that developmental PCD regulation in the fly does not rely upon the Bcl-2 proteins, but that they provide an added layer of protection in the apoptotic response to stress.

Noncanonical cell death pathways act during Drosophila oogenesis

Programmed cell death (PCD) is a highly conserved process that occurs during development and in response to adverse conditions. In Drosophila, most PCDs require the genes within the H99 deficiency, the adaptor molecule Ark, and caspases. Here we investigate 10 cell death genes for their potential roles in two distinct types of PCD that occur in oogenesis: developmental nurse cell PCD and starvation-induced PCD. Most of the genes investigated were found to have little effect on late stage developmental PCD in oogenesis, although ark mutants showed a partial inhibition. Mid-stage starvation-induced germline PCD was found to be independent of the upstream activators and ark although it requires caspases, suggesting an apoptosome-independent mechanism of caspase activation in mid-oogenesis. These results indicate that novel pathways must control PCD in the ovary.

Characterization of Aedes Dredd: a novel initiator caspase from the yellow fever mosquito, Aedes aegypti

Caspases play an essential role during programmed cell death in all metazoans. These enzymes are cysteine proteases and comprise a multi-gene family with more than a dozen mammalian family members. Although caspases have been characterized in many animals, including Drosophila melanogaster, little is known about the caspases that exist in mosquitoes. Here we describe the identification and characterization of Aedes Dredd (AeDredd), a novel caspase in the yellow fever mosquito, Aedes aegypti. AeDredd contains two N-terminal death effector domains and the well conserved caspase catalytic domain. Multiple sequence alignments and functional substrate assays of recombinant protein suggest that AeDredd is an orthologue of Drosophila Dredd and human caspase-8, both central effectors of the death receptor-mediated apoptotic pathway. AeDredd exhibits substrate specificity most similar to human caspase-8. AeDredd transcripts were found in all developmental stages with highest expression in early pupae. Within adults, AeDredd was found in all the tissues examined, with the highest transcript levels detected in fat body tissues. This is the first functional characterization of a death domain-containing caspase in an insect vector of human disease, and will initiate studies on the role of apoptosis in the innate immune response of vectors towards intracellular parasites such as viruses.

The antiapoptotic activity of insect IAPs requires activation by an evolutionarily conserved mechanism

Apoptosis represents a fundamental biological process that relies on the activation of caspases. Inhibitor of apoptosis (IAP) proteins represent a group of negative regulators of both caspases and cell death. The current model dictates that IAPs suppress apoptosis by blocking the catalytic pocket of effector caspases thereby preventing substrate entry. Here, we provide evolutionary evidence for the functional interplay between insect IAPs and the N-end rule-associated ubiquitylation machinery in neutralising effector caspases and cell death. We find that IAPs require 'priming' in order to function as antiapoptotic molecules. Consistently, we demonstrate that the antiapoptotic activity of diverse insect IAPs is activated by effector caspases, providing the cell with a sensitive strategy to monitor and neutralise active caspases. Almost 300 million years of evolutionary selection pressure has preserved a caspase cleavage site in insect IAPs that, following processing by a caspase, exposes a binding motif for the N-end-rule-associated degradation machinery. Recruitment of this ubiquitylation machinery into the 'cleaved-IAP:caspase' complex provides a mechanism to negatively regulate effector caspases and block apoptosis. Furthermore, comparisons between cellular and several viral IAPs suggest differences in their modes of action, as OpIAP3, CpGV-IAP3 and HcNPV-IAP3 fail to associate with several effector caspases. Evolutionary conservation of the N-end-rule degradation pathway in IAP-mediated regulation of apoptosis further corroborates the physiological relevance of this ubiquitylation-associated process.

The N-terminus and alpha-5, alpha-6 helices of the pro-apoptotic protein Bax, modulate functional interactions with the anti-apoptotic protein Bcl-xL

Background

Bcl-2 family proteins are key regulators of mitochondrial integrity and comprise both pro- and anti-apoptotic proteins. Bax a pro-apoptotic member localizes as monomers in the cytosol of healthy cells and accumulates as oligomers in mitochondria of apoptotic cells. The Bcl-2 homology-3 (BH3) domain regulates interactions within the family, but regions other than BH3 are also critical for Bax function. Thus, the N-terminus has been variously implicated in targeting to mitochondria, interactions with BH3-only proteins as well as conformational changes linked to Bax activation. The transmembrane (TM) domains (α5-α6 helices in the core and α9 helix in the C-terminus) in Bax are implicated in localization to mitochondria and triggering cytotoxicity. Here we have investigated N-terminus modulation of TM function in the context of regulation by the anti-apoptotic protein Bcl-x_L.

Results

Deletion of 29 amino acids in the Bax N-terminus (Bax 30–192) caused constitutive accumulation at mitochondria and triggered high levels of cytotoxicity, not inhibited by Bcl-x_L. Removal of the TM domains (Bax 30–105) abrogated mitochondrial localization but resulted in Bcl-x_L regulated activation of endogenous Bax and Bax-Bak dependent apoptosis. Inclusion of the α5-α6 helices/TMI domain (Bax 30–146) phenocopied Bax 30–192 as it restored mitochondrial localization, Bcl-x_L independent cytotoxicity and was not dependent on endogenous Bax-Bak. Inhibition of function and localization by Bcl-x_L was restored in Bax 1–146, which included the TM1 domain. Regardless of regulation by Bcl-x_L, all N-terminal deleted constructs immunoprecipitated Bcl-x_Land converged on caspase-9 dependent apoptosis consistent with mitochondrial involvement in the apoptotic cascade. Sub-optimal sequence alignments of Bax and Bcl-x_L indicated a sequence similarity between the α5–α6 helices of Bax and Bcl-x_L. Alanine substitutions of three residues (T14A-S15A-S16A) in the N-terminus (Bax-Ala3) attenuated regulation by the serine-threonine kinase Akt/PKB but not by Bcl-x_L indicative of distinct regulatory mechanisms.

Conclusion

Collectively, the analysis of Bax deletion constructs indicates that the N-terminus drives conformational changes facilitating inhibition of cytotoxicity by Bcl-x_L. We speculate that the TM1 helices may serve as 'structural antagonists' for BH3-Bcl-x_L interactions, with this function being regulated by the N-terminus in the intact protein.

Fork head controls the timing and tissue selectivity of steroid-induced developmental cell death

Cell death during Drosophila melanogaster metamorphosis is controlled by the steroid hormone 20-hydroxyecdysone (20E). Elements of the signaling pathway that triggers death are known, but it is not known why some tissues, and not others, die in response to a particular hormone pulse. We found that loss of the tissue-specific transcription factor Fork head (Fkh) is both required and sufficient to specify a death response to 20E in the larval salivary glands. Loss of fkh itself is a steroid-controlled event that is mediated by the 20E-induced BR-C gene, and that renders the key death regulators hid and reaper hormone responsive. These results implicate the D. melanogaster FOXA orthologue Fkh with a novel function as a competence factor for steroid-controlled cell death. They explain how a specific tissue is singled out for death, and why this tissue survives earlier hormone pulses. More generally, they suggest that cell identity factors like Fkh play a pivotal role in the normal control of developmental cell death.

The Aedes albopictus inhibitor of apoptosis 1 gene protects vertebrate cells from bluetongue virus-induced apoptosis

We sequenced and characterized the inhibitor of apoptosis (iap) 1 gene from Aedes albopictus, designated as Aaiap1. The Aaiap1 gene rescued Spodoptera frugiperda (Sf9) cells from apoptosis when cotransfected with the Drosophila pro-apoptotic hid gene. The antiapoptotic function of the Aaiap1 gene was evaluated in the bluetongue virus (BTV)-induced apoptosis system. BTV infection induced apoptosis in vertebrate cells via the intrinsic apoptotic pathway. This was shown by the translocation of cytochrome C and the second mitochondria-derived activator of caspase (Smac, also known as DIABLO) from the mitochondria and the subsequent activation of caspase-9 and -3. Stable expression of the Aaiap1 gene in derivative baby hamster kidney cells delayed BTV-induced apoptosis by 24 h and reduced the BTV progeny yield by 10-fold. This study provides the first evidence that the mosquito AaIAP1 protein possesses antiapoptotic activity.

The genomic underpinnings of apoptosis in Strongylocentrotus purpuratus

Programmed cell death through apoptosis is a pan-metazoan character involving intermolecular signaling networks that have undergone substantial lineage-specific evolution. A survey of apoptosis-related proteins encoded in the sea urchin genome provides insight into this evolution while revealing some interesting novelties, which we highlight here. First, in addition to a typical CARD-carrying Apaf-1 homologue, sea urchins have at least two novel Apaf-1-like proteins that are each linked to a death domain, suggesting that echinoderms have evolved unique apoptotic signaling pathways. Second, sea urchins have an unusually large number of caspases. While the set of effector caspases (caspases-3/7 and caspase-6) in sea urchins is similar to that found in other basal deuterostomes, signal-responsive initiator caspase subfamilies (caspases-8/10 and 9, which are respectively linked to DED and CARD adaptor domains) have undergone echinoderm-specific expansions. In addition, there are two groups of divergent caspases, one distantly related to the vertebrate interleukin converting enzyme (ICE)-like subfamily, and a large clan that does not cluster with any of the vertebrate caspases. Third, the complexity of proteins containing an anti-apoptotic BIR domain and of Bcl-2 family members approaches that of vertebrates, and is greater than that found in protostome model systems such as Drosophila or Caenorhabditis elegans. Finally, the presence of Death receptor homologues, previously known only in vertebrates, in both Strongylocentrotus purpuratus and Nematostella vectensis suggests that this family of apoptotic signaling proteins evolved early in animals and was subsequently lost in the nematode and arthropod lineage(s). Our results suggest that cell survival is contingent upon a diverse array of signals in sea urchins, more comparable in complexity to vertebrates than to arthropods or nematodes, but also with unique features that may relate to specific requirements imposed by the biphasic life cycle and/or immunological idiosyncrasies of this organism.

miRNAs and apoptosis: RNAs to die for

MicroRNAs (miRNAs) are small non-coding RNAs of about 18-24 nucleotides in length that negatively regulate gene expression. Discovered only recently, it has become clear that they are involved in many biological processes such as developmental timing, differentiation and cell death. Data that connect miRNAs to various kinds of diseases, particularly cancer, are accumulating. miRNAs can influence cancer development in many ways, including the regulation of cell proliferation, cell transformation, and cell death. In this review, we focus on miRNAs that have been shown to play a role in the regulation of apoptosis. We first describe in detail how Drosophila has been utilized as a model organism to connect several miRNAs with the cell death machinery. We discuss the genetic approaches that led to the identification of those miRNAs and subsequent work that helped to establish their function. In the second part of the review article, we focus on the involvement of miRNAs in apoptosis regulation in mammals. Intriguingly, many of the miRNAs that regulate apoptosis have been shown to affect cancer development. In the end, we discuss a virally encoded miRNA that influences the cell death response in the mammalian host cell. In summary, the data gathered over the recent years clearly show the potential and important role of miRNAs to regulate apoptosis at various levels and in several organisms.

Mechanisms of programmed cell death during oogenesis in Drosophila virilis

We describe the features of programmed cell death occurring in the egg chambers of Drosophila virilis during mid-oogenesis and late oogenesis. During mid-oogenesis, the spontaneously degenerating egg chambers exhibit typical characteristics of apoptotic cell death. As revealed by propidium iodide, rhodamine-conjugated phalloidin staining, and the TUNEL assay, respectively, the nurse cells contain condensed chromatin, altered actin cytoskeleton, and fragmented DNA. In vitro caspase activity assays and immunostaining procedures demonstrate that the atretic egg chambers possess high levels of caspase activity. Features of autophagic cell death are also observed during D. virilis mid-oogenesis, as shown by monodansylcadaverine staining, together with an ultrastructural examination by transmission electron microscopy. During the late stages of oogenesis in D. virilis, once again, the two mechanisms, viz., nurse cell cluster apoptosis and autophagy, operate together, manifesting features of cell death similar to those detailed above. Moreover, an altered form of cytochrome c seems to be released from the mitochondria in the nurse cells proximal to the oocyte. We propose that apoptosis and autophagy function synergistically during oogenesis in D. virilis in order to achieve a more efficient elimination of the degenerated nurse cells and abnormal egg chambers.

A gradient of epidermal growth factor receptor signaling determines the sensitivity of rbf1 mutant cells to E2F-dependent apoptosis

The inactivation of retinoblastoma (Rb) family members sensitizes cells to apoptosis. This cell death affects the development of mutant animals and also provides a critical constraint to the malignant potential of Rb mutant tumor cells. The extent of apoptosis caused by the inactivation of Rb is highly cell type and tissue specific, but the underlying reasons for this variation are poorly understood. Here, we characterize a specific time and place during Drosophila melanogaster development where rbf1 mutant cells are exquisitely sensitive to apoptosis. During the third larval instar, many rbf1 mutant cells undergo E2F-dependent cell death in the morphogenetic furrow. Surprisingly, this pattern of apoptosis is not caused by inappropriate cell cycle progression but instead involves the action of Argos, a secreted protein that negatively regulates Drosophila epidermal growth factor receptor (EGFR [DER]) activity. Apoptosis of rbf1 mutant cells is suppressed by the activation of DER, ras, or raf or by the inactivation of argos, sprouty, or gap1, and inhibition of DER strongly enhances apoptosis in rbf1 mutant discs. We show that RBF1 and a DER/ras/raf signaling pathway cooperate in vivo to suppress E2F-dependent apoptosis and that the loss of RBF1 alters a normal program of cell death that is controlled by Argos and DER. These results demonstrate that a gradient of DER/ras/raf signaling that occurs naturally during development provides the contextual signals that determine when and where the inactivation of rbf1 results in dE2F1-dependent apoptosis.

Cytochrome c-d regulates developmental apoptosis in the Drosophila retina

The role of cytochrome c (Cyt c) in caspase activation has largely been established from mammalian cell-culture studies, but much remains to be learned about its physiological relevance in situ. The role of Cyt c in invertebrates has been subject to considerable controversy. The Drosophila genome contains distinct cyt c genes: cyt c-p and cyt c-d. Loss of cyt c-p function causes embryonic lethality owing to a requirement of the gene for mitochondrial respiration. By contrast, cyt c-d mutants are viable but male sterile. Here, we show that cyt c-d regulates developmental apoptosis in the pupal eye. cyt c-d mutant retinas show a profound delay in the apoptosis of superfluous interommatidial cells and perimeter ommatidial cells. Furthermore, there is no apoptosis in mutant retinal tissues for the Drosophila homologues of apoptotic protease-activating factor 1 (Ark) and caspase 9 (Dronc). In addition, we found that cyt c-d--as with ark and dronc-regulates scutellar bristle number, which is known to depend on caspase activity. Collectively, our results indicate a role of Cyt c in caspase regulation of Drosophila somatic cells.

Programmed cell death of follicular epithelium during the late developmental stages of oogenesis in the fruit flies Bactrocera oleae and Ceratitis capitata (Diptera, Tephritidae) is mediated by autophagy

In the present study, we describe the features of programmed cell death of ovarian follicle cells, occurring during the late developmental stages of oogenesis in the olive fruit fly, Bactrocera oleae and the medfly, Ceratitis capitata. During stage 14, the follicle cells contain autophagic vacuoles, and they do not exhibit caspase activity in all parts of the egg chamber. Their nuclei are characterized by condensed chromatin, accompanied with high- but not low-molecular weight DNA fragmentation events exclusively detected in distinct cells of the anterior pole. These data argue for the presence of an autophagy-mediated cell death program in the ovarian follicle cell layer in both species. The above results are likely associated with the abundant phagocytosis observed at the entry of the lateral oviducts, where numerous cell bodies are massively engulfed by epithelial cells. We strongly believe that during the termination of the above Dipteran oogenesis, an efficient mechanism of absorption of the degenerated follicle cells is selectively activated, in order to prevent the blockage of the ovarioles and thus robustly support the physiological completion of the ovulation process.

Chromatin condensation of ovarian nurse and follicle cells is regulated independently from DNA fragmentation during Drosophila late oogenesis

Programmed cell death constitutes a common fundamental incident occurring during oogenesis in a variety of different organisms. In Drosophila melanogaster, it plays a significant role in the maturation process of the egg chamber. In the present study, we have used an in vitro development system for studying the effects of inducers and inhibitors of programmed cell death during the late stages of oogenesis. Treatment of the developing egg chambers with two widely used inducers of cell death, etoposide and staurosporine, blocks further development and induces chromatin condensation but not DNA fragmentation in nurse and follicle cells, as revealed by propidium iodide staining and terminal transferase-mediated dUTP nick-end labeling assay. Moreover, incubation of the developing egg chambers with the caspase-3 inhibitor Z-DEVD-FMK significantly delays development, prevents DNA fragmentation, but does not affect chromatin condensation. The above results demonstrate, for the first time, that chromatin condensation in Drosophila ovarian nurse and follicle cells is a caspase-3-like independent process and is regulated independently from DNA fragmentation.

Requirement of cytochrome c for apoptosis in human cells

During apoptosis, cytochrome c released from mitochondria activates Apaf-1, a cofactor of caspase-9. The evidence that cytochrome c can activate Apaf-1 is abundant, but the proof that cytochrome c is required for apoptosis is limited to two studies that used genetically modified mice. One of these studies concluded that in some tissues apoptosis may require Apaf-1 but not cytochrome c, which indicated the need to analyze the requirement of cytochrome c beyond the mouse models, and in human tumor cells in particular. In this study, we designed tools to silence cytochrome c expression in human cells and tested these tools in an experimental system of oncogenic transformation. We found that cytochrome c was required for apoptosis induced by both DNA damage and, unexpectedly, TNFalpha. Overall, this study established that cytochrome c is required for apoptosis in human cells and provided tools to dissect mechanisms of apoptosis in various experimental models.

Programmed cell death mechanisms of identifiable peptidergic neurons in Drosophila melanogaster

The molecular basis of programmed cell death (PCD) of neurons during early metamorphic development of the central nervous system (CNS) in Drosophila melanogaster are largely unknown, in part owing to the lack of appropriate model systems. Here, we provide evidence showing that a group of neurons (vCrz) that express neuropeptide Corazonin (Crz) gene in the ventral nerve cord of the larval CNS undergo programmed death within 6 hours of the onset of metamorphosis. The death was prevented by targeted expression of caspase inhibitor p35, suggesting that these larval neurons are eliminated via a caspase-dependent pathway. Genetic and transgenic disruptions of ecdysone signal transduction involving ecdysone receptor-B (EcR-B) isoforms suppressed vCrz death, whereas transgenic re-introduction of either EcR-B1 or EcR-B2 isoform into the EcR-B-null mutant resumed normal death. Expression of reaper in vCrz neurons and suppression of vCrz-cell death in a reaper-null mutant suggest that reaper functions are required for the death, while no apparent role was found for hid or grim as a death promoter. Our data further suggest that diap1 does not play a role as a central regulator of the PCD of vCrz neurons. Significant delay of vCrz-cell death was observed in mutants that lack dronc or dark functions, indicating that formation of an apoptosome is necessary, but not sufficient, for timely execution of the death. These results suggest that activated ecdysone signaling determines precise developmental timing of the neuronal degeneration during early metamorphosis, and that subsequent reaper-mediated caspase activation occurs through a novel DIAP1-independent pathway.

The two Drosophila cytochrome C proteins can function in both respiration and caspase activation

Cytochrome C has two apparently separable cellular functions: respiration and caspase activation during apoptosis. While a role of the mitochondria and cytochrome C in the assembly of the apoptosome and caspase activation has been established for mammalian cells, the existence of a comparable function for cytochrome C in invertebrates remains controversial. Drosophila possesses two cytochrome c genes, cyt-c-d and cyt-c-p. We show that only cyt-c-d is required for caspase activation in an apoptosis-like process during spermatid differentiation, whereas cyt-c-p is required for respiration in the soma. However, both cytochrome C proteins can function interchangeably in respiration and caspase activation, and the difference in their genetic requirements can be attributed to differential expression in the soma and testes. Furthermore, orthologues of the apoptosome components, Ark (Apaf-1) and Dronc (caspase-9), are also required for the proper removal of bulk cytoplasm during spermatogenesis. Finally, several mutants that block caspase activation during spermatogenesis were isolated in a genetic screen, including mutants with defects in spermatid mitochondrial organization. These observations establish a role for the mitochondria in caspase activation during spermatogenesis.