Regulation of apoptosis in Drosophila

Functional genomics of the horn fly, Haematobia irritans (Linnaeus, 1758)

Background

The horn fly, Haematobia irritans (Linnaeus, 1758) (Diptera: Muscidae) is one of the most important ectoparasites of pastured cattle. Horn flies infestations reduce cattle weight gain and milk production. Additionally, horn flies are mechanical vectors of different pathogens that cause disease in cattle. The aim of this study was to conduct a functional genomics study in female horn flies using Expressed Sequence Tags (EST) analysis and RNA interference (RNAi).

Results

A cDNA library was made from whole abdominal tissues collected from partially fed adult female horn flies. High quality horn fly ESTs (2,160) were sequenced and assembled into 992 unigenes (178 contigs and 814 singlets) representing molecular functions such as serine proteases, cell metabolism, mitochondrial function, transcription and translation, transport, chromatin structure, vitellogenesis, cytoskeleton, DNA replication, cell response to stress and infection, cell proliferation and cell-cell interactions, intracellular trafficking and secretion, and development. Functional analyses were conducted using RNAi for the first time in horn flies. Gene knockdown by RNAi resulted in higher horn fly mortality (protease inhibitor functional group), reduced oviposition (vitellogenin, ferritin and vATPase groups) or both (immune response and 5'-NUC groups) when compared to controls. Silencing of ubiquitination ESTs did not affect horn fly mortality and ovisposition while gene knockdown in the ferritin and vATPse functional groups reduced mortality when compared to controls.

Conclusions

These results advanced the molecular characterization of this important ectoparasite and suggested candidate protective antigens for the development of vaccines for the control of horn fly infestations.

Another tier for caspase regulation: IAPs as NEDD8 E3 ligases

Many inhibitor of apoptosis proteins (IAPs) function as E3 ligases to ubiquitinate important cell death proteins, including caspases. Broemer et al. (2010) report recently in Molecular Cell that IAPs can also inhibit caspases by promoting conjugation of the UBL NEDD8.

Two-color in vivo imaging of photoreceptor apoptosis and development in Drosophila

We report a new two-color fluorescent imaging system to visualize the mosaic adult photoreceptor neurons (PRs) in real-time. Using this method, we examined a collection of 434 mutants and identified genes required for PR survival, planar cell polarity (PCP), patterning and differentiation. We could track the progression of PR degeneration in living flies. By introducing the expression of p35, a caspase inhibitor, we found mutations that specifically activate caspase-dependent death. Moreover, we showed that grh is required in R3 for correct PCP establishment. The "Tomato/GFP-FLP/FRT" method allows high-throughput, rapid and precise identification of survival and developmental pathways in living adult PRs at single-cell resolution.

Canonical wingless signaling regulates cone cell specification in the Drosophila retina

Correct tissue patterning during development involves multiple morphogenetic events that include specification of different cell fates, cell proliferation, cell death, and coordinated changes in cell shape, position, and adhesion. Here, we use the Drosophila retina to explore the molecular mechanisms that regulate and integrate these various events. In a previous report, we found that wingless (wg) was required to induce a previously unknown surge of cell death ("early death") in the pupal retina. Here, we show that wg is also required to induce the more widely studied mid-pupal cell death ("late death") in a process that involves regulation of DIAP1. Furthermore, our data suggest that wg has a previously unreported role in specifying the glial-like cone cells. This activity requires canonical Wg signaling and is linked with Notch pathway activity. Our work broadens the role of canonical Wg signaling to encompass multiple patterning steps in the emerging Drosophila retina.

Sept4/ARTS is required for stem cell apoptosis and tumor suppression

Inhibitor of Apoptosis Proteins (IAPs) are frequently overexpressed in tumors and have become promising targets for developing anti-cancer drugs. IAPs can be inhibited by natural antagonists, but a physiological requirement of mammalian IAP antagonists remains to be established. Here we show that deletion of the mouse Sept4 gene, which encodes the IAP antagonist ARTS, promotes tumor development. Sept4-null mice have increased numbers of hematopoietic stem and progenitor cells, elevated XIAP protein, increased resistance to cell death, and accelerated tumor development in an Eμ-Myc background. These phenotypes are partially suppressed by inactivation of XIAP. Our results suggest that apoptosis plays an important role as a frontline defense against cancer by restricting the number of normal stem cells.

Genetic architecture of ovary size and asymmetry in European honeybee workers

The molecular basis of complex traits is increasingly understood but a remaining challenge is to identify their co-regulation and inter-dependence. Pollen hoarding (pln) in honeybees is a complex trait associated with a well-characterized suite of linked behavioral and physiological traits. In European honeybee stocks bidirectionally selected for pln, worker (sterile helper) ovary size is pleiotropically affected by quantitative trait loci that were initially identified for their effect on foraging behavior. To gain a better understanding of the genetic architecture of worker ovary size in this model system, we analyzed a series of crosses between the selected strains. The crossing results were heterogeneous and suggested non-additive effects. Three significant and three suggestive quantitative trait loci of relatively large effect sizes were found in two reciprocal backcrosses. These loci are not located in genome regions of known effects on foraging behavior but contain several interesting candidate genes that may specifically affect worker-ovary size. Thus, the genetic architecture of this life history syndrome may be comprised of pleiotropic, central regulators that influence several linked traits and other genetic factors that may be downstream and trait specific.

The genomic underpinnings of apoptosis in the silkworm, Bombyx mori

Background

Apoptosis is regulated in an orderly fashion by a series of genes, and has a crucial role in important physiological processes such as growth development, immunological response and so on. Recently, substantial studies have been undertaken on apoptosis in model animals including humans, fruit flies, and the nematode. However, the lack of genomic data for silkworms limits their usefulness in apoptosis studies, despite the advantages of silkworm as a representative of Lepidoptera and an effective model system. Herein we have identified apoptosis-related genes in the silkworm Bombyx mori and compared them to those from insects, mammals, and nematodes.

Results

From the newly assembled genome databases, a genome-wide analysis of apoptosis-related genes in Bombyx mori was performed using both nucleotide and protein Blast searches. Fifty-two apoptosis-related candidate genes were identified, including five caspase family members, two tumor necrosis factor (TNF) superfamily members, one Bcl-2 family member, four baculovirus IAP (inhibitor of apoptosis) repeat (BIR) domain family members and 1 RHG (Reaper, Hid, Grim, and Sickle; Drosophila cell death activators) family member. Moreover, we identified a new caspase family member, BmCaspase-New, two splice variants of BmDronc, and Bm3585, a mammalian TNF superfamily member homolog. Twenty-three of these apoptosis-related genes were cloned and sequenced using cDNA templates isolated from BmE-SWU1 cells. Sequence analyses revealed that these genes could have key roles in apoptosis.

Conclusions

Bombyx mori possesses potential apoptosis-related genes. We hypothesized that the classic intrinsic and extrinsic apoptotic pathways potentially are active in Bombyx mori. These results lay the foundation for further apoptosis-related study in Bombyx mori.

Apoptosis, stem cells, and tissue regeneration

Most metazoans have at least some ability to regenerate damaged cells and tissues, although the regenerative capacity varies depending on the species, organ, or developmental stage. Cell replacement and regeneration occur in two contexts: renewal of spent cells during tissue homeostasis (homeostatic growth), and in response to external injury, wounding, or amputation (epimorphic regeneration). Model organisms that display remarkable regenerative capacity include amphibians, planarians, Hydra, and the vertebrate liver. In addition, several mammalian organs--including the skin, gut, kidney, muscle, and even the human nervous system--have some ability to replace spent or damaged cells. Although the regenerative response is complex, it typically involves the induction of new cell proliferation through formation of a blastema, followed by cell specification, differentiation, and patterning. Stem cells and undifferentiated progenitor cells play an important role in both tissue homeostasis and tissue regeneration. Stem cells are typically quiescent or passing slowly through the cell cycle in adult tissues, but they can be activated in response to cell loss and wounding. A series of studies, mostly performed in Drosophila as well as in Hydra, Xenopus, and mouse, has revealed an unexpected role of apoptotic caspases in the production of mitogenic signals that stimulate the proliferation of stem and progenitor cells to aid in tissue regeneration. This Review summarizes some of the key findings and discusses links to stem cell biology and cancer.

Staying alive: dalmation mediated blocking of apoptosis is essential for tissue maintenance

In an EMS screen for mutations disrupting tracheal development, we identified new alleles of the dalmation (dmt) gene, which had previously been shown to affect peripheral nervous system (PNS) development. Here, we demonstrate that dmt loss results in programmed cell death, disrupting PNS patterning and leading to large gaps in the salivary duct and trachea. Dmt loss results in increased expression of the proapoptotic regulator genes head involution defective (hid) and reaper (rpr), and deletion of these genes or tissue-specific expression of the baculoviral apoptotic inhibitor P35 rescues the dmt defects. dmt is also required to protect cells from irradiation induced expression of hid and rpr during the irradiation resistant stage, which begins as cells become irreversibly committed to their final fates. Thus, we propose that Dmt keeps cells alive by blocking activation of hid and rpr as cells become irreversibly committed.

Drosophila Rel proteins are central regulators of a robust, multi-organ immune network

Survival of all animals depends on effective protection against infection. In Drosophila, opportunistic infection kills larvae if they lack the Rel/NF-kappaB proteins Dorsal and Dif. We have used tissue-specific expression of Dif and Dorsal to reveal that these Rel proteins act in three different tissues to defend larvae from infection. Dif and Dorsal act in circulating blood cells, where they are required autonomously to promote blood-cell survival and phagocytosis of microorganisms. We show that a major transcriptional target of Dorsal and Dif in blood cells is Drosophila IAP1, a gene protecting these cells from death. We find that in addition to their autonomous role in blood-cell survival, Dif and Dorsal also act in the fat body to produce factors that promote blood-cell viability. These Rel proteins act in the epidermis to prevent infection by maintaining a barrier to microbial entry. Dorsal or Dif in any one of the three tissues is sufficient to defend the animal from opportunistic infection. Thus Drosophila has a multi-pronged system of defense and each branch of this network requires Rel proteins. Based on similarities between Drosophila and mammals, we propose that a Rel-dependent network is an ancient and robust framework of animal immune systems.

A novel F-box protein is required for caspase activation during cellular remodeling in Drosophila

Terminal differentiation of male germ cells in Drosophila and mammals requires extensive cytoarchitectural remodeling, the elimination of many organelles, and a large reduction in cell volume. The associated process, termed spermatid individualization, is facilitated by the apoptotic machinery, including caspases, but does not result in cell death. From a screen for genes defective in caspase activation in this system, we isolated a novel F-box protein, which we termed Nutcracker, that is strictly required for caspase activation and sperm differentiation. Nutcracker interacts through its F-box domain with members of a Cullin-1-based ubiquitin ligase complex (SCF): Cullin-1 and SkpA. This ubiquitin ligase does not regulate the stability of the caspase inhibitors DIAP1 and DIAP2, but physically binds Bruce, a BIR-containing giant protein involved in apoptosis regulation. Furthermore, nutcracker mutants disrupt proteasome activity without affecting their distribution. These findings define a new SCF complex required for caspase activation during sperm differentiation and highlight the role of regulated proteolysis during this process.

Apoptosis: a way to maintain healthy individuals

Apoptosis, the best known form of programmed cell death, is tightly regulated by a number of sensors, signal transducers and effectors. Apoptosis is mainly active during embryonic development, when deletion of redundant cellular material is required for the correct morphogenesis of tissues and organs; moreover, it is essential for the maintenance of tissue homeostasis during cell life. Cells also activate apoptosis when they suffer from various insults, such as damage to DNA or to other cellular components, or impairment of basic processes, such as DNA replication and DNA repair. Removal of damaged cells is fundamental in maintaining the health of organisms. In addition, apoptosis induction following DNA damage is exploited to kill cancer cells. In this chapter we will review the main features of developmental and induced apoptosis.

Drosophila Myc

Myc genes play a major role in human cancer, and they are important regulators of growth and proliferation during normal development. Despite intense study over the last three decades, many aspects of Myc function remain poorly understood. The identification of a single Myc homolog in the model organism Drosophila melanogaster more than 10 years ago has opened new possibilities for addressing these issues. This review summarizes what the last decade has taught us about Myc biology in the fruit fly.

Regulation of cell death by the ubiquitin-proteasome system

The regulation of apoptosis (programmed cell death) has been the subject of a vast body of research because of its implications in normal development, tissue homeostasis and a wide range of diseases. The ubiquitin-proteasome system (UPS) plays a prominent role in the control of apoptosis by targeting key cell death proteins, including caspases, the central executioners of apoptosis. Here we summarize the major findings on the function of the UPS in both pro- and anti-apoptotic regulation.

Changes in gene expression in the permissive larval host lightbrown apple moth (Epiphyas postvittana, Tortricidae) in response to EppoNPV (Baculoviridae) infection

Host cell and virus gene expression were measured five days after per os inoculation of 3rd instar lightbrown apple moth (LBAM) larvae with the Epiphyas postvittana nucleopolyhedrovirus (EppoNPV). Microarray analysis identified 84 insect genes that were up-regulated and 18 genes that were down-regulated in virus-infected larvae compared with uninfected larvae. From the 134 viral open reading frames represented on the microarray, 81 genes showed strong expression. Of the 38 functionally identifiable regulated insect genes, 23 coded for proteins that have roles in one of five processes; regulation of transcription and translation, induction of apoptosis, and maintenance of both juvenility and actin cytoskeletal integrity. Of the 34 functionally identifiable viral genes that were most strongly expressed, 12 had functions associated with these five processes, as did a further seven viral genes which were expressed at slightly lower levels. A survey of the LBAM-expressed sequence tag library identified further genes involved in these processes. In total, 135 insect genes and 38 viral genes were analysed by quantitative polymerase chain reaction. Twenty-one insect genes were strongly up-regulated and 31 genes strongly down-regulated. All 38 viral genes examined were highly expressed. These data suggest that induction of apoptosis and regulation of juvenility are the major 'battlegrounds' between virus and insect, with the majority of changes observed representing viral control of insect gene expression. Transcription and translational effects seem to be exerted largely through modulation of mRNA and protein degradation. Examples of attempts by the insect to repel the infection via changes in gene expression within these same processes were, however, also noted. The data also showed the extent to which viral transcription dominated in the infected insects at five days post inoculation.

Post-translational modification of CASK leads to its proteasome-dependent degradation

CASK is a member of the membrane-associated guanylate kinase family. In mammals it is an essential protein, as CASK knockout mice die after birth and its deletion in humans has developmental consequences. CASK plays a role in the transcription of genes required for forebrain development, and in the nervous systems of Drosophila and C. elegans, it participates in receptor localization at the plasma membrane. This role in organizing supramolecular protein complexes to appropriate subcellular regions is shared in mammals and is regulated by phosphorylation. CASK is a kinase and regulator of cell proliferation and adhesion, which adds to an expanding list of roles. In this study we report for the first time that CASK is degraded in a characteristic fashion in mammalian cells. We found that CASK is a long-lived protein despite the fact that it contains three putative PEST sequences. Finally, we provide detailed evidence that CASK degradation is mediated through a ubiquitin-proteasome pathway and this is phosphorylation-dependent. Together, these results provide evidence that post-translational modifications to CASK are major regulatory steps leading to its proteasomal degradation. This regulation not only has important implications on how CASK participates in its many disparate roles, but highlights how altering this regulation may contribute to the pathogenesis of human disease.

Can't live without them, can live with them: roles of caspases during vital cellular processes

Since the pioneering discovery that the genetic cell death program in C. elegans is executed by the cysteine-aspartate protease (caspase) CED3, caspase activation has become nearly synonymous with apoptosis. A critical mass of data accumulated in the past few years, have clearly established that apoptotic caspases can also participate in a variety of non-apoptotic processes. The roles of caspases during these processes and the regulatory mechanisms that prevent unrestrained caspase activity remain to be fully investigated, and may vary in different cellular contexts. Significantly, some of these processes, such as terminal differentiation of vertebrate lens fiber cells and red blood cells, as well as spermatid terminal differentiation and dendritic pruning of sensory neurons in Drosophila, all involve proteolytic degradation of major cellular compartments, and are conceptually, molecularly, biochemically, and morphologically reminiscent of apoptosis. Moreover, some of these model systems bear added values for the study of caspase activation/apoptosis. For example, the Drosophila sperm differentiation is the only system known in invertebrate which absolutely requires the mitochondrial pathway (i.e. Cyt c). The existence of testis-specific genes for many of the components in the electron transport chain, including Cyt c, facilitates the use of the Drosophila sperm system to investigate possible roles of these otherwise essential proteins in caspase activation. Caspases are also involved in a wide range of other vital processes of non-degenerative nature, indicating that these proteases play much more diverse roles than previously assumed. In this essay, we review genetic, cytological, and molecular studies conducted in Drosophila, vertebrate, and cultured cells, which underlie the foundations of this newly emerging field.

Regulation of Drosophila melanogaster pro-apoptotic gene hid

Key decisions one makes in a lifetime include whether and how often to reproduce, what role to play in the community and, under certain conditions, whether to live or die. Similar decisions are also made at the level of cells: whether to divide, what fate to assume in the multicellular context of metazoan development and, under certain conditions, whether to live or to die. The pro-apoptotic gene hid plays an important role in the execution of cell death in Drosophila. Here, we review the various levels of control that exist to regulate Hid according to the life-or-death choice of a cell.

Cracking open cell death in the Drosophila ovary

The Drosophila melanogaster ovary is a powerful yet simple system with only a few cell types. Cell death in the ovary can be induced in response to multiple developmental and environmental signals. These cell deaths occur at distinct stages of oogenesis and involve unique mechanisms utilizing apoptotic, autophagic and perhaps necrotic processes. In this review, we summarize recent progress characterizing cell death mechanisms in the fly ovary.

The unique Morgue ubiquitination protein is conserved in a diverse but restricted set of invertebrates

Drosophila Morgue is a unique ubiquitination protein that facilitates programmed cell death and associates with DIAP1, a critical cell death inhibitor with E3 ubiquitin ligase activity. Morgue possesses a unique combination of functional domains typically associated with distinct types of ubiquitination enzymes. This includes an F box characteristic of the substrate-binding subunit in Skp, Cullin, and F box (SCF)-type ubiquitin E3 ligase complexes and a variant ubiquitin E2 conjugase domain where the active site cysteine is replaced by a glycine. Morgue also contains a single C4-type zinc finger motif. This architecture suggests potentially novel ubiquitination activities for Morgue. In this study, we address the evolutionary origins of this distinctive protein utilizing a combination of bioinformatics and molecular biology approaches. We find that Morgue exhibits widespread but restricted phylogenetic distribution among metazoans. Morgue proteins were identified in a wide range of Protostome phyla, including Arthropoda, Annelida, Mollusca, Nematoda, and Platyhelminthes. However, with one potential exception, Morgue was not detected in Deuterostomes, including Chordates, Hemichordates, or Echinoderms. Morgue was also not found in Ctenophora, Cnidaria, Placozoa, or Porifera. Characterization of Morgue sequences within specific animal lineages suggests that gene deletion or acquisition has occurred during divergence of nematodes and that at least one arachnid expresses an atypical form of Morgue consisting only of the variant E2 conjugase domain. Analysis of the organization of several morgue genes suggests that exon-shuffling events have contributed to the evolution of the Morgue protein. These results suggest that Morgue mediates conserved and distinctive ubiquitination functions in specific cell death pathways.

Inhibitor of apoptosis proteins in Drosophila: gatekeepers of death

Regulation of apoptosis is crucial to ensure cellular viability, and failure to do so is linked to several human pathologies. The apoptotic cell death programme culminates in the activation of caspases, a family of highly specific cysteine proteases essential for the destruction of the cell. Although best known for their role in executing apoptosis, caspases also play important signalling roles in non-apoptotic processes, such as regulation of actin dynamics, innate immunity, cell proliferation, differentiation and survival. Under such conditions, caspases are activated without killing the cell. Caspase activation and activity is subject to complex regulation, and various cellular and viral inhibitors have been identified that control the activity of caspases in their apoptotic and non-apoptotic roles. Members of the Inhibitor of APoptosis (IAP) protein family ensure cell viability in Drosophila by directly binding to caspases and regulating their activities in a ubiquitin-dependent manner. The observation that IAPs are essential for cell survival in Drosophila, and are frequently deregulated in human cancer, contributing to tumourigenesis, chemoresistance, disease progression and poor patient survival, highlights the importance of this family of caspase regulators in health and disease. Here we summarise recent advances from Drosophila that start to elucidate how the cellular response to caspase activation is modulated by IAPs and their regulators.

Letting go: modification of cell adhesion during apoptosis

Apoptosis appears to be a carefully orchestrated process for the ordered dismantling of cells. A recent paper in BMC Developmental Biology shows that the disassembly of adherens junctions during apoptosis in Drosophila is progressive and requires the amino-terminal cleavage of the β-catenin Armadillo by the apoptotic effector caspase DrICE.

Programmed cell death in the nervous system--a programmed cell fate?

Studies of developmental cell death in the nervous system have revealed two different modes of programmed cell death (PCD). One results from competition for target-derived trophic factors and leads to the stochastic removal of neurons and/or glia. A second, hard-wired form of PCD involves the lineage-specific, stereotypical death of identifiable neurons, glia or undifferentiated cells. Although traditionally associated with invertebrates, this 'programmed PCD' can also occur in vertebrates. Recent studies have shed light on its genetic control and have revealed that activation of the apoptotic machinery can be under the same complex, combinatorial control as the expression of terminal differentiation genes. This review will highlight these findings and will suggest why such complex control evolved.

dXNP/DATRX increases apoptosis via the JNK and dFOXO pathway in Drosophila neurons

Mutation of the XNP/ATRX gene, which encodes an SNF2 family ATPase/helicase protein, leads to ATR-X syndrome and several other X-linked mental retardation syndromes. Although XNP/ATRX is a chromatin remodeler, the molecular mechanism by which mental retardation occurs in patients with ATR-X has yet to be determined. To better understand the role of XNP/ATRX in neuronal development, we expressed Drosophila XNP (dXNP/DATRX) ectopically in Drosophila neurons. Neuronal expression of dXNP/DATRX resulted in various developmental defects and induced strong apoptosis. These defects and apoptosis were suppressed by Drosophila inhibitor of apoptosis protein 1. Expression of dXNP/DATRX also increased JNK activity and the levels of reaper and hid transcripts, which are pro-apoptotic factors that activate caspase. Furthermore, dXNP/DATRX-induced rough eye phenotype and apoptosis were suppressed by dFOXO deficiency. These results suggest that dXNP/DATRX is involved in caspase-dependent apoptosis in Drosophila neurons via regulation of the JNK and dFOXO pathway.

p53-independent apoptosis limits DNA damage-induced aneuploidy

DNA damage or unprotected telomeres can trigger apoptosis via signaling pathways that directly sense abnormal DNA structures and activate the p53 transcription factor. We describe a p53-independent mechanism that acts in parallel to the canonical DNA damage response pathway in Drosophila to induce apoptosis after exposure to ionizing radiation. Following recovery from damage-induced cell cycle arrest, p53 mutant cells activate the JNK pathway and expression of the pro-apoptotic gene hid. Mutations in grp, a cell cycle checkpoint gene, and puc, a negative regulator of the JNK pathway, sensitize p53 mutant cells to ionizing radiation (IR)-induced apoptosis. Induction of chromosome aberrations by DNA damage generates cells with segmental aneuploidy and heterozygous for mutations in ribosomal protein genes. p53-independent apoptosis limits the formation of these aneuploid cells following DNA damage. We propose that reduced copy number of haploinsufficient genes following chromosome damage activates apoptosis and helps maintain genomic integrity.

RNAi-mediated knockdown showing impaired cell survival in Drosophila wing imaginal disc

The genetically amenable organism Drosophila melanogaster has been estimated to have 14,076 protein coding genes in the genome, according to the flybase release note R5.13 (http://flybase.bio.indiana.edu/static_pages/docs/release_notes.html). Recent application of RNA interference (RNAi) to the study of developmental biology in Drosophila has enabled us to carry out a systematic investigation of genes affecting various specific phenotypes. In order to search for genes supporting cell survival, we conducted an immunohistochemical examination in which the RNAi of 2,497 genes was independently induced within the dorsal compartment of the wing imaginal disc. Under these conditions, the activities of a stress-activated protein kinase JNK (c-Jun N-terminal kinase) and apoptosis-executing factor Caspase-3 were monitored. Approximately half of the genes displayed a strong JNK or Caspase-3 activation when their RNAi was induced. Most of the JNK activation accompanied Caspase-3 activation, while the opposite did not hold true. Interestingly, the area activating Caspase-3 was more broadly seen than that activating JNK, suggesting that JNK is crucial for induction of non-autonomous apoptosis in many cases. Furthermore, the RNAi of essential factors commonly regulating transcription and translation showed a severe and cell-autonomous apoptosis but also elicited another apoptosis at an adjacent area in a non-autonomous way. We also found that the frequency of apoptosis varies depending on the tissues.

Direct response to Notch activation: signaling crosstalk and incoherent logic

Notch is the receptor in one of a small group of conserved signaling pathways that are essential at multiple stages in development. Although the mechanism of transduction impinges directly on the nucleus to regulate transcription through the CSL [CBF-1/Su(H)/LAG-1] [corrected] DNA binding protein, there are few known direct target genes. Thus, relatively little is known about the immediate cellular consequences of Notch activation. We therefore set out to determine the genome-wide response to Notch activation by analyzing the changes in messenger RNA (mRNA) expression and the sites of CSL occupancy within 30 minutes of activating Notch in Drosophila cells. Through combining these data, we identify high-confidence direct targets of Notch that are implicated in the maintenance of adult muscle progenitors in vivo. These targets are enriched in cell morphogenesis genes and in components of other cell signaling pathways, especially the epidermal growth factor receptor (EGFR) pathway. Also evident are examples of incoherent network logic, where Notch stimulates the expression of both a gene and the repressor of that gene, which may result in a transient window of competence after Notch activation. Furthermore, because targets comprise both positive and negative regulators, cells become poised for both outcomes, suggesting one mechanism through which Notch activation can lead to opposite effects in different contexts.

Genetic control of programmed cell death during animal development

The elimination of unwanted cells by programmed cell death is a common feature of animal development. Genetic studies in the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster, and the mouse have not only revealed the molecular machineries that cause the programmed demise of specific cells, but have also allowed us to get a glimpse of the types of pathways that regulate these machineries during development. Rather than serving as a broad overview of programmed cell death during development, this review focuses on recent advances in our understanding of the regulation of specific programmed cell death events during nematode, fly, and mouse development. Recent studies have revealed that many of the regulatory pathways involved play additional important roles in development, which confirms that the programmed cell death fate is an integral aspect of animal development.

Staying alive: apoptosome feedback inhibition

Studies in Drosophila melanogaster reveal a mechanism for regulating caspases, the key executioners of the apoptotic cell-death program. An initiator caspase and its activating partner promote degradation of each other, thereby limiting the levels of the active protease complex. This negative-feedback inhibition helps to explain how cells avoid unwanted caspase activation and apoptosis.

The Toll-->NFkappaB signaling pathway mediates the neuropathological effects of the human Alzheimer's Abeta42 polypeptide in Drosophila

Alzheimer's (AD) is a progressive neurodegenerative disease that afflicts a significant fraction of older individuals. Although a proteolytic product of the Amyloid precursor protein, the Alphabeta42 polypeptide, has been directly implicated in the disease, the genes and biological pathways that are deployed during the process of Alphabeta42 induced neurodegeneration are not well understood and remain controversial. To identify genes and pathways that mediated Alphabeta42 induced neurodegeneration we took advantage of a Drosophila model for AD disease in which ectopically expressed human Alphabeta42 polypeptide induces cell death and tissue degeneration in the compound eye. One of the genes identified in our genetic screen is Toll (Tl). It encodes the receptor for the highly conserved Tl-->NFkB innate immunity/inflammatory pathway and is a fly homolog of the mammalian Interleukin-1 (Ilk-1) receptor. We found that Tl loss-of-function mutations dominantly suppress the neuropathological effects of the Alphabeta42 polypeptide while gain-of-function mutations that increase receptor activity dominantly enhance them. Furthermore, we present evidence demonstrating that Tl and key downstream components of the innate immunity/inflammatory pathway play a central role in mediating the neuropathological activities of Alphabeta42. We show that the deleterious effects of Alphabeta42 can be suppressed by genetic manipulations of the Tl-->NFkB pathway that downregulate signal transduction. Conversely, manipulations that upregulate signal transduction exacerbate the deleterious effects of Abeta42. Since postmortem studies have shown that the Ilk-1-->NFkB innate immunity pathway is substantially upregulated in the brains of AD patients, the demonstration that the Tl-->NFkB signaling actively promotes the process of Alphabeta42 induced cell death and tissue degeneration in flies points to possible therapeutic targets and strategies.

Cell survival and polarity of Drosophila follicle cells require the activity of ecdysone receptor B1 isoform

Proper assembly and maintenance of epithelia are critical for normal development and homeostasis. Here, using the Drosophila ovary as a model, we identify a role for the B1 isoform of the ecdysone receptor (EcR-B1) in this process. We performed a reverse genetic analysis of EcR-B1 function during oogenesis and demonstrate that silencing of this receptor isoform causes loss of integrity and multilayering of the follicular epithelium. We show that multilayered follicle cells lack proper cell polarity with altered distribution of apical and basolateral cell polarity markers including atypical-protein kinase C (aPKC), Discs-large (Dlg), and Scribble (Scrib) and aberrant accumulation of adherens junctions and F-actin cytoskeleton. We find that the EcR-B1 isoform is required for proper follicle cell polarity both during early stages of oogenesis, when follicle cells undergo the mitotic cell cycle, and at midoogenesis when these cells stop dividing and undergo several endocycles. In addition, we show that the EcR-B1 isoform is required during early oogenesis for follicle cell survival and that disruption of its function causes apoptotic cell death induced by caspase.

Regulation of the Drosophila apoptosome through feedback inhibition

Apoptosis is induced by caspases, which are members of the cysteine protease family 1. Caspases are synthesized as inactive zymogens and initiator caspases first gain activity by associating with an oligomeric complex of their adaptor proteins, such as the apoptosome 2,3. Activated initiator caspases subsequently cleave and activate effector caspases. While such a proteolytic cascade would predict that a small number of active caspases could irreversibly amplify caspase activity and trigger apoptosis, many cells can maintain moderate levels of caspase activity to perform non-apoptotic roles in cellular differentiation, shape change and migration 4. Here we show that the Drosophila apoptosome engages in a feedback inhibitory loop, thereby moderating its activation level in vivo. Specifically, the adaptor protein Apaf-1 lowers the level of its associated initiator caspase, Dronc, without triggering apoptosis. Conversely, Dronc lowers Apaf-1 protein levels. This mutual suppression depends upon Dronc’s catalytic site and a caspase cleavage site within Apaf-1. Moreover, the Drosophila Inhibitor of Apoptosis Protein 1 (Diap1) is required for this process. We speculate that this feedback inhibition allows cells to regulate the degree of caspase activation for apoptotic and non-apoptotic purposes.

The role of the RING-finger protein Elfless in Drosophila spermatogenesis and apoptosis

elfless (CG15150, FBgn0032660) maps to polytene region 36DE 5' (left) of reduced ocelli/Pray for Elves (PFE) on chromosome 2L and is predicted to encode a 187 amino acid RING finger E3 ubiquitin ligase that is putatively involved in programmed cell death (PCD, e.g., apoptosis). Several experimental approaches were used to characterize CG15150/elfless and test whether defects in this gene underlie the male sterile phenotype associated with overlapping chromosomal deficiencies of region 36DE. elfless expression is greatly enhanced in the testes and the expression pattern of UAS-elfless-EGFP driven by elfless-Gal4 is restricted to the tail cyst cell nuclei of the testes. Despite this, elfless transgenes failed to rescue the male sterile phenotype in Df/Df flies. Furthermore, null alleles of elfless, generated either by imprecise excision of an upstream P-element or by FLP-FRT deletion between two flanking piggyBac elements, are fertile. In a gain-of-function setting in the eye, we found that elfless genetically interacts with key members of the apoptotic pathway including the initiator caspase Dronc and the ubiquitin conjugating enzyme UbcD1. DIAP1, but not UbcD1, protein levels are increased in heads of flies expressing Elfless-EGFP in the eye, and in testes of flies expressing elfless-Gal4 driven Elfless-EGFP. Based on these findings, we speculate that Elfless may regulate tail cyst cell degradation to provide an advantageous, though not essential, function in the testis.

Regulation of apoptosis by XIAP ubiquitin-ligase activity

Inhibitor of Apoptosis Proteins (IAPs) can bind to and inhibit caspases, the key executioners of apoptosis. Because IAPs are frequently overexpressed in human tumors, they have become major pharmacological targets for developing new cancer therapeutics. However, the precise physiological function of individual mammalian IAPs and their role as E3 ubiquitin-ligases in situ remain largely obscure. Here, we investigated the function of XIAP ubiquitin-ligase activity by inactivating the RING motif via gene targeting in the mouse. Removing the RING stabilized XIAP in apoptotic thymocytes, demonstrating that XIAP ubiquitin-ligase activity is a major determinant of XIAP protein stability. Surprisingly, the increased amounts of "XIAP-BIR-only" protein did not lead to attenuated but rather increased caspase activity and apoptosis. DeltaRING embryonic stem cells and fibroblasts had elevated caspase-3 enzyme activity, and XIAP DeltaRING embryonic fibroblasts were strongly sensitized to TNF-alpha-induced apoptosis. Similar results were obtained with XIAP deficient mice. Furthermore, deletion of the RING also improved the survival of mice in the Emu-Myc lymphoma model. This demonstrates a physiological requirement of XIAP ubiquitin-ligase activity for the inhibition of caspases and for tumor suppression in vivo.