Genetic control of programmed cell death (apoptosis) in Drosophila

Next Generation Sequencing Identifies Five Major Classes of Potentially Therapeutic Enzymes Secreted by Lucilia sericata Medical Maggots

Lucilia sericata larvae are used as an alternative treatment for recalcitrant and chronic wounds. Their excretions/secretions contain molecules that facilitate tissue debridement, disinfect, or accelerate wound healing and have therefore been recognized as a potential source of novel therapeutic compounds. Among the substances present in excretions/secretions various peptidase activities promoting the wound healing processes have been detected but the peptidases responsible for these activities remain mostly unidentified. To explore these enzymes we applied next generation sequencing to analyze the transcriptomes of different maggot tissues (salivary glands, gut, and crop) associated with the production of excretions/secretions and/or with digestion as well as the rest of the larval body. As a result we obtained more than 123.8 million paired-end reads, which were assembled de novo using Trinity and Oases assemblers, yielding 41,421 contigs with an N50 contig length of 2.22 kb and a total length of 67.79 Mb. BLASTp analysis against the MEROPS database identified 1729 contigs in 577 clusters encoding five peptidase classes (serine, cysteine, aspartic, threonine, and metallopeptidases), which were assigned to 26 clans, 48 families, and 185 peptidase species. The individual enzymes were differentially expressed among maggot tissues and included peptidase activities related to the therapeutic effects of maggot excretions/secretions.

Control of apoptosis by Drosophila DCAF12

Regulated Apoptosis (Programmed Cell Death, PCD) maintains tissue homeostasis in adults, and ensures proper growth and morphogenesis of tissues during development of metazoans. Accordingly, defects in cellular processes triggering or executing apoptotic programs have been implicated in a variety of degenerative and neoplastic diseases. Here, we report the identification of DCAF12, an evolutionary conserved member of the WD40-motif repeat family of proteins, as a new regulator of apoptosis in Drosophila. We find that DCAF12 is required for Diap1 cleavage in response to pro-apoptotic signals, and is thus necessary and sufficient for RHG (Reaper, Hid, and Grim)-mediated apoptosis. Loss of DCAF12 perturbs the elimination of supernumerary or proliferation-impaired cells during development, and enhances tumor growth induced by loss of neoplastic tumor suppressors, highlighting the wide requirement for DCAF12 in PCD.

Transgenic Drosophila for Investigating DUX4 and FRG1, Two Genes Associated with Facioscapulohumeral Muscular Dystrophy (FSHD)

Facioscapulohumeral muscular dystrophy (FSHD) is typically an adult onset dominant myopathy. Epigenetic changes in the chromosome 4q35 region linked to both forms of FSHD lead to a relaxation of repression and increased somatic expression of DUX4-fl (DUX4-full length), the pathogenic alternative splicing isoform of the DUX4 gene. DUX4-fl encodes a transcription factor expressed in healthy testis and pluripotent stem cells; however, in FSHD, increased levels of DUX4-fl in myogenic cells lead to aberrant regulation of target genes. DUX4-fl has proven difficult to study in vivo; thus, little is known about its normal and pathogenic roles. The endogenous expression of DUX4-fl in FSHD-derived human muscle and myogenic cells is extremely low, exogenous expression of DUX4-fl in somatic cells rapidly induces cytotoxicity, and, due in part to the lack of conservation beyond primate lineages, viable animal models based on DUX4-fl have been difficult to generate. By contrast, the FRG1 (FSHD region gene 1), which is linked to FSHD, is evolutionarily conserved from invertebrates to humans, and has been studied in several model organisms. FRG1 expression is critical for the development of musculature and vasculature, and overexpression of FRG1 produces a myopathic phenotype, yet the normal and pathological functions of FRG1 are not well understood. Interestingly, DUX4 and FRG1 were recently linked when the latter was identified as a direct transcriptional target of DUX4-FL. To better understand the pathways affected in FSHD by DUX4-fl and FRG1, we generated transgenic lines of Drosophila expressing either gene under control of the UAS/GAL4 binary system. Utilizing these lines, we generated screenable phenotypes recapitulating certain known consequences of DUX4-fl or FRG1 overexpression. These transgenic Drosophila lines provide resources to dissect the pathways affected by DUX4-fl or FRG1 in a genetically tractable organism and may provide insight into both muscle development and pathogenic mechanisms in FSHD.

The Cell-Cycle Arrest and Apoptotic Functions of p53 in Tumor Initiation and Progression

P53 is a transcription factor highly inducible by many stress signals such as DNA damage, oncogene activation, and nutrient deprivation. Cell-cycle arrest and apoptosis are the most prominent outcomes of p53 activation. Many studies showed that p53 cell-cycle and apoptosis functions are important for preventing tumor development. p53 also regulates many cellular processes including metabolism, antioxidant response, and DNA repair. Emerging evidence suggests that these noncanonical p53 activities may also have potent antitumor effects within certain context. This review focuses on the cell-cycle arrest and apoptosis functions of p53, their roles in tumor suppression, and the regulation of cell fate decision after p53 activation.

A new genetic model for calcium induced autophagy and ER-stress in Drosophila photoreceptor cells

Cytoplasmic Ca2+ overload is known to trigger autophagy and ER-stress. Furthermore, ER-stress and autophagy are commonly associated with degenerative pathologies, but their role in disease progression is still a matter of debate, in part, owing to limitations of existing animal model systems. The Drosophila eye is a widely used model system for studying neurodegenerative pathologies. Recently, we characterized the Drosophila protein, Calphotin, as a cytosolic immobile Ca2+ buffer, which participates in Ca2+ homeostasis in Drosophila photoreceptor cells. Exposure of calphotin hypomorph flies to continuous illumination, which induces Ca2+ influx into photoreceptor cells, resulted in severe Ca2+-dependent degeneration. Here we show that this degeneration is autophagy and ER-stress related. Our studies thus provide a new model in which genetic manipulations trigger changes in cellular Ca2+ distribution. This model constitutes a framework for further investigations into the link between cytosolic Ca2+, ER-stress and autophagy in human disorders and diseases.

Genetic characterization of two gain-of-function alleles of the effector caspase DrICE in Drosophila

Caspases are the executioners of apoptosis. Although much is known about their physiological roles and structures, detailed analyses of missense mutations of caspases are lacking. As mutations within caspases are identified in various human diseases, the study of caspase mutants will help to elucidate how caspases interact with other components of the apoptosis pathway and how they may contribute to disease. DrICE is the major effector caspase in Drosophila required for developmental and stress-induced cell death. Here, we report the isolation and characterization of six de novo drICE mutants, all of which carry point mutations affecting amino acids conserved among caspases in various species. These six mutants behave as recessive loss-of-function mutants in a homozygous condition. Surprisingly, however, two of the newly isolated drICE alleles are gain-of-function mutants in a heterozygous condition, although they are loss-of-function mutants homozygously. Interestingly, they only behave as gain-of-function mutants in the presence of an apoptotic signal. These two alleles carry missense mutations affecting conserved amino acids in close proximity to the catalytic cysteine residue. This is the first time that viable gain-of-function alleles of caspases are described in any intact organism and provides a significant exception to the expectation that mutations of conserved amino acids always abolish the pro-apoptotic activity of caspases. We discuss models about how these mutations cause the gain-of-function character of these alleles.

The Mitochondrial Permeability Transition Pore: Channel Formation by F-ATP Synthase, Integration in Signal Transduction, and Role in Pathophysiology

The mitochondrial permeability transition (PT) is a permeability increase of the inner mitochondrial membrane mediated by a channel, the permeability transition pore (PTP). After a brief historical introduction, we cover the key regulatory features of the PTP and provide a critical assessment of putative protein components that have been tested by genetic analysis. The discovery that under conditions of oxidative stress the F-ATP synthases of mammals, yeast, and Drosophila can be turned into Ca(2+)-dependent channels, whose electrophysiological properties match those of the corresponding PTPs, opens new perspectives to the field. We discuss structural and functional features of F-ATP synthases that may provide clues to its transition from an energy-conserving into an energy-dissipating device as well as recent advances on signal transduction to the PTP and on its role in cellular pathophysiology.

The Sound of Silence: Signaling by Apoptotic Cells

Apoptosis is a carefully choreographed process of cellular self-destruction in the absence of inflammation. During the death process, apoptotic cells actively communicate with their environment, signaling to both their immediate neighbors as well as distant sentinels. Some of these signals direct the anti-inflammatory immune response, instructing specific subsets of phagocytes to participate in the limited and careful clearance of dying cellular debris. These immunomodulatory signals can also regulate the activation state of the engulfing phagocytes. Other signals derived from apoptotic cells contribute to tissue growth control with the common goal of maintaining tissue integrity. Derangements in these growth control signals during prolonged apoptosis can lead to excessive cell loss or proliferation. Here, we highlight some of the most intriguing signals produced by apoptotic cells during the course of normal development as well as during physiological disturbances such as atherosclerosis and cancer.

Rice ragged stunt virus-induced apoptosis affects virus transmission from its insect vector, the brown planthopper to the rice plant

Most plant viruses that seriously damage agricultural crops are transmitted by insects. However, the mechanisms enabling virus transmission by insect vectors are poorly understood. The brown planthopper (Nilaparvata lugens) is one of the most serious rice pests, causing extensive damage to rice plants by sucking the phloem sap and transmitting viruses, including Rice ragged stunt virus (RRSV). In this study, we investigated the mechanisms of RRSV transmission from its insect vector to the rice plant in vivo using the terminal deoxynucleotidyl transferase dUTP nick-end labeling assay and RNA interference technology. RRSV induced apoptosis in the salivary gland cells of its insect vector, N. lugens. The RRSV-induced apoptosis was regulated through a caspase-dependent manner, and inhibition of the expression of N. lugens caspase-1 genes significantly interfered with virus transmission. Our findings establish a link between virus-associated apoptosis and virus transmission from the insect vector to the host plant.

The role of the effector caspases drICE and dcp-1 for cell death and corpse clearance in the developing optic lobe in Drosophila

In the developing Drosophila optic lobe, cell death occurs via apoptosis and in a distinctive spatio-temporal pattern of dying cell clusters. We analyzed the role of effector caspases drICE and dcp-1 in optic lobe cell death and subsequent corpse clearance using mutants. Neurons in many clusters required either drICE or dcp-1 and each one is sufficient. This suggests that drICE and dcp-1 function in cell death redundantly. However, dying neurons in a few clusters strictly required drICE but not dcp-1, but required drICE and dcp-1 when drICE activity was reduced via hypomorphic mutation. In addition, analysis of the mutants suggests an important role of effecter caspases in corpse clearance. In both null and hypomorphic drICE mutants, greater number of TUNEL-positive cells were observed than in wild type, and many TUNEL-positive cells remained until later stages. Lysotracker staining showed that there was a defect in corpse clearance in these mutants. All the results suggested that drICE plays an important role in activating corpse clearance in dying cells, and that an additional function of effector caspases is required for the activation of corpse clearance as well as that for carrying out cell death.

Towards automatic image analysis and assessment of the multicellular apoptosis process

Apoptotic programmed cell death (PCD) is a fundamental aspect of developmental maturation. However, the authors' understanding of apoptosis, especially in the multi-cell regime, is incomplete because of the difficulty of identifying dying cells by conventional strategies. Real-time in vivo microscopy of Drosophila, an excellent model system for studying the PCD during development, has been used to uncover plausible collective apoptosis at the tissue level, although the dynamic regulation of the process remains to be deciphered. In this work, the authors have developed an image-analysis program that can quantitatively analyse time-lapse microscopy of live tissues undergoing apoptosis with a fluorescent nuclear marker, and subsequently extract the spatiotemporal patterns of multicellular response. The program can process a large number of cells (>10³) automatically tracked across sets of image frames. It is applied to characterise the apoptosis of Drosophila wing epithelium at eclosion. Using the natural anatomic structures as reference, the authors identify dynamic patterns in the progression of PCD within the Drosophila tissues. The results not only confirm the previously observed collective multi-cell behaviour from a quantitative perspective, but also reveal a plausible role played by the anatomic structures, such as the wing veins, in the PCD propagation across the Drosophila wing.

Xrn1/Pacman affects apoptosis and regulates expression of hid and reaper

Programmed cell death, or apoptosis, is a highly conserved cellular process that is crucial for tissue homeostasis under normal development as well as environmental stress. Misregulation of apoptosis is linked to many developmental defects and diseases such as tumour formation, autoimmune diseases and neurological disorders. In this paper, we show a novel role for the exoribonuclease Pacman/Xrn1 in regulating apoptosis. Using Drosophila wing imaginal discs as a model system, we demonstrate that a null mutation in pacman results in small imaginal discs as well as lethality during pupation. Mutant wing discs show an increase in the number of cells undergoing apoptosis, especially in the wing pouch area. Compensatory proliferation also occurs in these mutant discs, but this is insufficient to compensate for the concurrent increase in apoptosis. The phenotypic effects of the pacman null mutation are rescued by a deletion that removes one copy of each of the pro-apoptotic genes reaper, hid and grim, demonstrating that pacman acts through this pathway. The null pacman mutation also results in a significant increase in the expression of the pro-apoptotic mRNAs, hid and reaper, with this increase mostly occurring at the post-transcriptional level, suggesting that Pacman normally targets these mRNAs for degradation. Our results uncover a novel function for the conserved exoribonuclease Pacman and suggest that this exoribonuclease is important in the regulation of apoptosis in other organisms.

Role of the deubiquitylating enzyme DmUsp5 in coupling ubiquitin equilibrium to development and apoptosis in Drosophila melanogaster

Protein ubiquitylation is a dynamic process that affects the function and stability of proteins and controls essential cellular processes ranging from cell proliferation to cell death. This process is regulated through the balanced action of E3 ubiquitin ligases and deubiquitylating enzymes (DUB) which conjugate ubiquitins to, and remove them from target proteins, respectively. Our genetic analysis has revealed that the deubiquitylating enzyme DmUsp5 is required for maintenance of the ubiquitin equilibrium, cell survival and normal development in Drosophila. Loss of the DmUsp5 function leads to late larval lethality accompanied by the induction of apoptosis. Detailed analyses at a cellular level demonstrated that DmUsp5 mutants carry multiple abnormalities, including a drop in the free monoubiquitin level, the excessive accumulation of free polyubiquitins, polyubiquitylated proteins and subunits of the 26S proteasome. A shortage in free ubiquitins results in the induction of a ubiquitin stress response previously described only in the unicellular budding yeast. It is characterized by the induction of the proteasome-associated deubiquitylase DmUsp14 and sensitivity to cycloheximide. Removal of DmUsp5 also activates the pro-apoptotic machinery thereby resulting in widespread apoptosis, indicative of an anti-apoptotic role of DmUsp5. Collectively, the pleiotropic effects of a loss of DmUsp5 function can be explained in terms of the existence of a limited pool of free monoubiquitins which makes the ubiquitin-dependent processes mutually interdependent.

In vivo CaspaseTracker biosensor system for detecting anastasis and non-apoptotic caspase activity

The discovery that mammalian cells can survive late-stage apoptosis challenges the general assumption that active caspases are markers of impending death. However, tools have not been available to track healthy cells that have experienced caspase activity at any time in the past. Therefore, to determine if cells in whole animals can undergo reversal of apoptosis, known as anastasis, we developed a dual color CaspaseTracker system for Drosophila to identify cells with ongoing or past caspase activity. Transient exposure of healthy females to environmental stresses such as cold shock or starvation activated the CaspaseTracker coincident with caspase activity and apoptotic morphologies in multiple cell types of developing egg chambers. Importantly, when stressed flies were returned to normal conditions, morphologically healthy egg chambers and new progeny flies were labeled by the biosensor, suggesting functional recovery from apoptotic caspase activation. In striking contrast to developing egg chambers, which lack basal caspase biosensor activation under normal conditions, many adult tissues of normal healthy flies exhibit robust caspase biosensor activity in a portion of cells, including neurons. The widespread persistence of CaspaseTracker-positivity implies that healthy cells utilize active caspases for non-apoptotic physiological functions during and after normal development.

Temporal patterning of neuroblasts controls Notch-mediated cell survival through regulation of Hid or Reaper

Temporal patterning of neural progenitors is one of the core mechanisms generating neuronal diversity in the central nervous system. Here, we show that, in the tips of the outer proliferation center (tOPC) of the developing Drosophila optic lobes, a unique temporal series of transcription factors not only governs the sequential production of distinct neuronal subtypes but also controls the mode of progenitor division, as well as the selective apoptosis of Notch(OFF) or Notch(ON) neurons during binary cell fate decisions. Within a single lineage, intermediate precursors initially do not divide and generate only one neuron; subsequently, precursors divide, but their Notch(ON) progeny systematically die through Reaper activity, whereas later, their Notch(OFF) progeny die through Hid activity. These mechanisms dictate how the tOPC produces neurons for three different optic ganglia. We conclude that temporal patterning generates neuronal diversity by specifying both the identity and survival/death of each unique neuronal subtype.

F-ATPase of Drosophila melanogaster forms 53-picosiemen (53-pS) channels responsible for mitochondrial Ca2+-induced Ca2+ release

Mitochondria of Drosophila melanogaster undergo Ca(2+)-induced Ca(2+) release through a putative channel (mCrC) that has several regulatory features of the permeability transition pore (PTP). The PTP is an inner membrane channel that forms from F-ATPase, possessing a conductance of 500 picosiemens (pS) in mammals and of 300 pS in yeast. In contrast to the PTP, the mCrC of Drosophila is not permeable to sucrose and appears to be selective for Ca(2+) and H(+). We show (i) that like the PTP, the mCrC is affected by the sense of rotation of F-ATPase, by Bz-423, and by Mg(2+)/ADP; (ii) that expression of human cyclophilin D in mitochondria of Drosophila S2R(+) cells sensitizes the mCrC to Ca(2+) but does not increase its apparent size; and (iii) that purified dimers of D. melanogaster F-ATPase reconstituted into lipid bilayers form 53-pS channels activated by Ca(2+) and thiol oxidants and inhibited by Mg(2+)/γ-imino ATP. These findings indicate that the mCrC is the PTP of D. melanogaster and that the signature conductance of F-ATPase channels depends on unique structural features that may underscore specific roles in different species.

Non-canonical roles for Yorkie and Drosophila Inhibitor of Apoptosis 1 in epithelial tube size control

Precise control of epithelial tube size is critical for organ function, yet the molecular mechanisms remain poorly understood. Here, we examine the roles of cell growth and a highly conserved organ growth regulatory pathway in controlling the dimensions of the Drosophila tracheal (airway) system, a well-characterized system for investigating epithelial tube morphogenesis. We find that tracheal tube-size is regulated in unexpected ways by the transcription factor Yorkie (Yki, homolog of mammalian YAP and TAZ) and the Salvador/Warts/Hippo (SWH) kinase pathway. Yki activity typically promotes cell division, inhibits apoptosis, and can promote cell growth. However, reducing Yki activity in developing embryos increases rather than decreases the length of the major tracheal tubes, the dorsal trunks (DTs). Similarly, reduction of Hippo pathway activity, which antagonizes Yki, shortens tracheal DTs. yki mutations do not alter DT cell volume or cell number, indicating that Yki and the Hippo pathway regulate cell shape and apical surface area, but not volume. Yki does not appear to act through known tracheal pathways of apical extracellular matrix, septate junctions (SJs), basolateral or tubular polarity. Instead, the Hippo pathway and Yki appear to act downstream or in parallel to SJs because a double mutant combination of an upstream Hippo pathway activator, kibra, and the SJ component sinu have the short tracheal phenotype of a kibra mutant. We demonstrate that the critical target of Yki in tube size control is Drosophila Inhibitor of Apoptosis 1 (DIAP1), which in turn antagonizes the Drosophila effector caspase, Ice. Strikingly, there is no change in tracheal cell number in DIAP1 or Ice mutants, thus epithelial tube size regulation defines new non-apoptotic roles for Yki, DIAP1 and Ice.

Multiple mechanisms modulate distinct cellular susceptibilities toward apoptosis in the developing Drosophila eye

Although apoptosis is mechanistically well understood, a comprehensive understanding of how cells modulate their susceptibility toward apoptosis in a developing tissue is lacking. Here, we reveal striking dynamics in the apoptotic susceptibilities of different cell types in the Drosophila retina over a period of only 24 hr. Mitotic cells are extremely susceptible to apoptotic signals, while postmitotic cells have developed several strategies to promote survival. For example, photoreceptor neurons accumulate the inhibitor of apoptosis, Diap1. In unspecified cells, Cullin-3-mediated degradation keeps Diap1 levels low. These cells depend on EGFR signaling for survival. As development proceeds, developmentally older photoreceptors degrade Diap1, resulting in increased apoptosis susceptibility. Finally, R8 photoreceptors have very efficient survival mechanisms independent of EGFR or Diap1. These examples illustrate how complex cellular susceptibility toward apoptosis is regulated in a developing organ. Similar complexities may regulate apoptosis susceptibilities in mammalian development, and tumor cells may take advantage of it.

Male-killing Spiroplasma induces sex-specific cell death via host apoptotic pathway

Some symbiotic bacteria cause remarkable reproductive phenotypes like cytoplasmic incompatibility and male-killing in their host insects. Molecular and cellular mechanisms underlying these symbiont-induced reproductive pathologies are of great interest but poorly understood. In this study, Drosophila melanogaster and its native Spiroplasma symbiont strain MSRO were investigated as to how the host's molecular, cellular and morphogenetic pathways are involved in the symbiont-induced male-killing during embryogenesis. TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) staining, anti-cleaved-Caspase-3 antibody staining, and apoptosis-deficient mutant analysis unequivocally demonstrated that the host's apoptotic pathway is involved in Spiroplasma-induced male-specific embryonic cell death. Double-staining with TUNEL and an antibody recognizing epidermal marker showed that embryonic epithelium is the main target of Spiroplasma-induced male-specific apoptosis. Immunostaining with antibodies against markers of differentiated and precursor neural cells visualized severe neural defects specifically in Spiroplasma-infected male embryos as reported in previous studies. However, few TUNEL signals were detected in the degenerate nervous tissues of male embryos, and the Spiroplasma-induced neural defects in male embryos were not suppressed in an apoptosis-deficient host mutant. These results suggest the possibility that the apoptosis-dependent epidermal cell death and the apoptosis-independent neural malformation may represent different mechanisms underlying the Spiroplasma-induced male-killing. Despite the male-specific progressive embryonic abnormality, Spiroplasma titers remained almost constant throughout the observed stages of embryonic development and across male and female embryos. Strikingly, a few Spiroplasma-infected embryos exhibited gynandromorphism, wherein apoptotic cell death was restricted to male cells. These observations suggest that neither quantity nor proliferation of Spiroplasma cells but some Spiroplasma-derived factor(s) may be responsible for the expression of the male-killing phenotype.

Symbiotic bacteria are ubiquitously associated with diverse insects, and affect their host biology in a variety of ways. In Drosophila fruit flies, infection with Spiroplasma symbionts often causes male-specific embryonic mortality, resulting in the production of all-female offspring. This striking phenotype is called “male-killing”, whose underlying mechanisms are of great interest. Here we investigated Drosophila melanogaster and its native Spiroplasma symbiont strain to understand how the host's molecular, cellular and morphogenetic pathways are involved in the symbiont-induced male-killing. Specifically in Spiroplasma-infected male embryos, pathogenic phenotypes including massive cell death throughout the body and neural malformation were observed. We unequivocally identified that the male-specific cell death preferentially occurs in the embryonic epithelium via the host's apoptotic pathway. Meanwhile, we found that, unexpectedly, the male-specific neural defects occur independently of host's apoptosis, suggesting that at least two different mechanisms may be involved in the Spiroplasma-induced male-killing. Also unexpected was the finding that Spiroplasma titers are almost constant throughout embryogenesis irrespective of sex despite the male-specific severe apoptosis. We serendipitously found Spiroplasma-infected sexual mosaic embryos, wherein apoptosis was associated with male cells, which suggests that some Spiroplasma-derived factor(s) may selectively act on male cells and cause male-killing.

Detecting caspase activity in Drosophila larval imaginal discs

Caspases are a highly specialized class of cell death proteases. Since they are synthesized as inactive full-length zymogens, activation--at least of effector caspases and to some extent also of initiator caspases-requires a proteolytic cleavage event, generating a large and a small subunit, two of each forming the active caspase. The proteolytic cleavage event generates neo-epitopes at both the C-terminus of the large subunit and the N-terminus of the small subunit. The cleaved Caspase-3 (CC3) antibody was raised against the neo-epitope of the large subunit and thus detects only cleaved, but not full-length, Caspase-3. Although raised against human cleaved Caspase-3, the CC3 antibody cross-reacts in other species and detects cleaved caspases, most notably DrICE and Dcp-1, in Drosophila. This protocol describes the procedure for use of the CC3 antibody to detect caspase activity in larval imaginal discs in Drosophila.

Diversity of cell death pathways: insight from the fly ovary

Multiple types of cell death exist including necrosis, apoptosis, and autophagic cell death. The Drosophila ovary provides a valuable model to study the diversity of cell death modalities, and we review recent progress to elucidate these pathways. At least five distinct types of cell death occur in the ovary, and we focus on two that have been studied extensively. Cell death of mid-stage egg chambers occurs through a novel caspase-dependent pathway that involves autophagy and triggers phagocytosis by surrounding somatic epithelial cells. For every egg, 15 germline nurse cells undergo developmental programmed cell death, which occurs independently of most known cell death genes. These forms of cell death are strikingly similar to cell death observed in the germlines of other organisms.

Hsa-miR-132 regulates apoptosis in non-small cell lung cancer independent of acetylcholinesterase

MiR-132 is enriched in the central nerve system and is thought to be involved in neuronal development, maturation and function, and to be associated with several neurological disorders including Alzheimer's disease. In addition to its documented neuronal functions, an emerging role for miR-132 in tumorigenesis has been suggested. Recently, hsa-miR-132 was shown to be modulated in different tumor types. However, its role in non-small cell lung cancer (NSCLC) remains unclear. Here, we show that hsa-miR-132 can initiate apoptosis in NSCLC cells to dramatically attenuate tumor formation in nude mice independent of its effect on the proliferation/apoptosis-associated gene, acetylcholinesterase (AChE). Interestingly, hsa-miR-132 has no pro-apoptotic effect in normal pulmonary trachea epithelium. Taken together, these results suggest that hsa-miR-132 represses NSCLC growth by inducing apoptosis independent of AChE.

Caspase activity is required for engulfment of apoptotic cells

Clearance of apoptotic cells by phagocytic neighbors is crucial for normal development of multicellular organisms. However, how phagocytes discriminate between healthy and dying cells remains poorly understood. We focus on glial phagocytosis of apoptotic neurons during development of the Drosophila central nervous system. We identified phosphatidylserine (PS) as a ligand on apoptotic cells for the phagocytic receptor Six Microns Under (SIMU) and report that PS alone is not sufficient for engulfment. Our data reveal that, additionally to PS exposure, caspase activity is required for clearance of apoptotic cells by phagocytes. Here we demonstrate that SIMU recognizes and binds PS on apoptotic cells through its N-terminal EMILIN (EMI), Nimrod 1 (NIM1), and NIM2 repeats, whereas the C-terminal NIM3 and NIM4 repeats control SIMU affinity to PS. Based on the structure-function analysis of SIMU, we discovered a novel mechanism of internal inhibition responsible for differential affinities of SIMU to its ligand which might prevent elimination of living cells exposing PS on their surfaces.

SfDronc, an initiator caspase involved in apoptosis in the fall armyworm Spodoptera frugiperda

Initiator caspases are the first caspases that are activated following an apoptotic stimulus, and are responsible for cleaving and activating downstream effector caspases, which directly cause apoptosis. We have cloned a cDNA encoding an ortholog of the initiator caspase Dronc in the lepidopteran insect Spodoptera frugiperda. The SfDronc cDNA encodes a predicted protein of 447 amino acids with a molecular weight of 51 kDa. Overexpression of SfDronc induced apoptosis in Sf9 cells, while partial silencing of SfDronc expression in Sf9 cells reduced apoptosis induced by baculovirus infection or by treatment with UV or actinomycin D. Recombinant SfDronc exhibited several expected biochemical characteristics of an apoptotic initiator caspase: 1) SfDronc efficiently cleaved synthetic initiator caspase substrates, but had very little activity against effector caspase substrates; 2) mutation of a predicted cleavage site at position D340 blocked autoprocessing of recombinant SfDronc and reduced enzyme activity by approximately 10-fold; 3) SfDronc cleaved the effector caspase Sf-caspase-1 at the expected cleavage site, resulting in Sf-caspase-1 activation; and 4) SfDronc was strongly inhibited by the baculovirus caspase inhibitor SpliP49, but not by the related protein AcP35. These results indicate that SfDronc is an initiator caspase involved in caspase-dependent apoptosis in S. frugiperda, and as such is likely to be responsible for the initiator caspase activity in S. frugiperda cells known as Sf-caspase-X.

Chromatin occupancy patterns of the ETS repressor Yan: a mechanism for buffering gene expression against noise?

Developmental programs are driven by transcription factors that coordinate precise patterns of gene expression. While recent publications have described the importance of coordinated action of transcriptional activators at multiple cis-regulatory modules or enhancers, the contribution of sequence-specific repressors to overall regulation and robustness of gene expression has been difficult to ascertain. The Ets transcriptional repressor Yan functions as part of a conserved network downstream of receptor tyrosine kinase (RTK) signaling in Drosophila. This network displays switch-like responsiveness to RTK signaling, with the transition from a high-Yan to a low-Yan state induced by mitogen-activated protein kinase (MAPK)-mediated phosphorylation and inactivation of Yan. The ability of Yan to self-associate through a conserved sterile α motif (SAM) is essential for Yan's repressive ability, and has been suggested to allow spreading of Yan repressive complexes along chromatin. Such a mechanism has the potential to confer both signal responsiveness and robustness to the Yan network. To explore this spreading model, we compared the genome-wide chromatin binding profiles of wild-type vs. monomeric Yan. Consistent with the starting prediction, we found that wild type chromatin occupancy at genes encoding crucial developmental regulators and core signaling pathway components occurs as clusters of peaks that "spread" over multiple kilobases. However monomeric Yan, which fails to rescue a yan null mutation and displays significantly impaired repressive ability, exhibits a broadly similar occupancy profile to that of wild-type Yan, with multi-kilobase binding at developmentally important genes. This unexpected result suggests that SAM-mediated self-association does not mediate Yan recruitment to DNA or chromatin spreading, and raises the questions of why developmentally important genes require extensive Yan chromatin occupancy and how SAM-mediated polymerization might contribute to active repressive mechanisms in this context. In this Extra View article we discuss potential mechanisms by which Yan self-association and extended chromatin occupancy may contribute to robust regulation of gene expression.

Caspase signalling in the absence of apoptosis drives Jnk-dependent invasion

Tumours evolve several mechanisms to evade apoptosis, yet many resected carcinomas show significantly elevated caspase activity. Moreover, caspase activity is positively correlated with tumour aggression and adverse patient outcome. These observations indicate that caspases might have a functional role in promoting tumour invasion and metastasis. Using a Drosophila model of invasion, we show that precise effector caspase activity drives cell invasion without initiating apoptosis. Affected cells express the matrix metalloprotinase Mmp1 and invade by activating Jnk. Our results link Jnk and effector caspase signalling during the invasive process and suggest that tumours under apoptotic stresses from treatment, immune surveillance or intrinsic signals might be induced further along the metastatic cascade.

Apoptotic and nonapoptotic caspase functions in animal development

A developing animal is exposed to both intrinsic and extrinsic stresses. One stress response is caspase activation. Caspase activation not only controls apoptosis but also proliferation, differentiation, cell shape, and cell migration. Caspase activation drives development by executing cell death or nonapoptotic functions in a cell-autonomous manner, and by secreting signaling molecules or generating mechanical forces, in a noncell autonomous manner.

Non-cell autonomous control of apoptosis by ligand-independent Hedgehog signaling in Drosophila

Hedgehog (Hh) signaling is important for development and homeostasis in vertebrates and invertebrates. Ligand-independent, deregulated Hh signaling caused by loss of negative regulators such as Patched causes excessive cell proliferation, leading to overgrowth in Drosophila and tumors in humans, including basal-cell carcinoma and medulloblastoma. We show that in Drosophila deregulated Hh signaling also promotes cell survival by increasing the resistance to apoptosis. Surprisingly, cells with deregulated Hh activity do not protect themselves from apoptosis; instead, they promote cell survival of neighboring wild-type cells. This non-cell autonomous effect is mediated by Hh-induced Notch signaling, which elevates the protein levels of Drosophila inhibitor of apoptosis protein-1 (Diap-1), conferring resistance to apoptosis. In summary, we demonstrate that deregulated Hh signaling not only promotes proliferation but also cell survival of neighboring cells. This non-cell autonomous control of apoptosis highlights an underappreciated function of deregulated Hh signaling, which may help to generate a supportive micro-environment for tumor development.

Caspase levels and execution efficiencies determine the apoptotic potential of the cell

Differences in expression level of the effector caspases Drice and Dcp-1 and in their intrinsic abilities to induce apoptosis and to control the rate of cell death underlie the differential sensitivities of cells to apoptosis.

Essentially, all metazoan cells can undergo apoptosis, but some cells are more sensitive than others to apoptotic stimuli. To date, it is unclear what determines the apoptotic potential of the cell. We set up an in vivo system for monitoring and comparing the activity levels of the two main effector caspases in Drosophila melanogaster, Drice and Dcp-1. Both caspases were activated by the apoptosome after irradiation. However, whereas each caspase alone could induce apoptosis, Drice was a more effective inducer of apoptosis than Dcp-1, which instead had a role in establishing the rate of cell death. These functional differences are attributed to their intrinsic properties rather than merely their tissue specificities. Significantly, the levels of the procaspases are directly proportional to their activity levels and play a key role in determining the cell’s sensitivity to apoptosis. Finally, we provide evidence for the existence of a cellular execution threshold of caspase activity, which must be reached to induce apoptosis.

The HIV-1 Vpu protein induces apoptosis in Drosophila via activation of JNK signaling

The genome of the human immunodeficiency virus type 1 (HIV-1) encodes the canonical retroviral proteins, as well as additional accessory proteins that enhance the expression of viral genes, the infectivity of the virus and the production of virions. The accessory Viral Protein U (Vpu), in particular, enhances viral particle production, while also promoting apoptosis of HIV-infected human T lymphocytes. Some Vpu effects rely on its interaction with the ubiquitin-proteasome protein degradation system, but the mechanisms responsible for its pro-apoptotic effects in vivo are complex and remain largely to be elucidated.We took advantage of the Drosophila model to study the effects of Vpu activity in vivo. Expression of Vpu in the developing Drosophila wing provoked tissue loss due to caspase-dependent apoptosis. Moreover, Vpu induced expression of the pro-apoptotic gene reaper, known to down-regulate Inhibitor of Apoptosis Proteins (IAPs) which are caspase-antagonizing E3 ubiquitin ligases. Indeed, Vpu also reduced accumulation of Drosophila IAP1 (DIAP1). Though our results demonstrate a physical interaction between Vpu and the proteasome-addressing SLIMB/β-TrCP protein, as in mammals, both SLIMB/βTrCP-dependent and -independent Vpu effects were observed in the Drosophila wing. Lastly, the pro-apoptotic effect of Vpu in this tissue was abrogated upon inactivation of the c-Jun N-terminal Kinase (JNK) pathway. Our results in the fly thus provide the first functional evidence linking Vpu pro-apoptotic effects to activation of the conserved JNK pathway.

Effects of manipulating apoptosis on Sindbis virus infection of Aedes aegypti mosquitoes

Improved control of vector-borne diseases requires an understanding of the molecular factors that determine vector competence. Apoptosis has been shown to play a role in defense against viruses in insects and mammals. Although some observations suggest a correlation between apoptosis and resistance to arboviruses in mosquitoes, there is no direct evidence tying apoptosis to arbovirus vector competence. To determine whether apoptosis can influence arbovirus replication in mosquitoes, we manipulated apoptosis in Aedes aegypti mosquitoes by silencing the expression of genes that either positively or negatively regulate apoptosis. Silencing of the A. aegypti anti-apoptotic gene iap1 (Aeiap1) caused apoptosis in midgut epithelium, alterations in midgut morphology, and 60 to 70% mosquito mortality. Mortality induced by Aeiap1 silencing was rescued by cosilencing the initiator caspase gene Aedronc, indicating that the mortality was due to apoptosis. When mosquitoes which had been injected with Aeiap1 double-stranded RNA (dsRNA) were orally infected with Sindbis virus (SINV), increased midgut infection and virus dissemination to other organs were observed. This increase in virus infection may have been due to the effects of widespread apoptosis on infection barriers or innate immunity. In contrast, silencing the expression of Aedronc, which would be expected to inhibit apoptosis, reduced SINV midgut infection and virus dissemination. Thus, our data suggest that some level of caspase activity and/or apoptosis may be necessary for efficient virus replication and dissemination in mosquitoes. This is the first study to directly test the roles of apoptosis and caspases in determining mosquito vector competence for arboviruses.

Autophagy can promote but is not required for epithelial cell extrusion in the amnioserosa of the Drosophila embryo

During Drosophila embryogenesis the majority of the extra-embryonic epithelium known as the amnioserosa (AS) undergoes programmed cell death (PCD) following the completion of the morphogenetic process of dorsal closure. Approximately ten percent of AS cells, however, are eliminated during dorsal closure by extrusion from the epithelium. Using biosensors that report autophagy and caspase activity in vivo, we demonstrate that AS cell extrusion occurs in the context of elevated autophagy and caspase activation. Furthermore, we evaluate AS extrusion rates, autophagy, and caspase activation in embryos in which caspase activity or autophagy are altered by genetic manipulation. This includes using the GAL4/UAS system to drive expression of p35, reaper, dINR (ACT) and Atg1 in the AS; we also analyze embryos lacking both maternal and zygotic expression of Atg1. Based on our results we suggest that autophagy can promote, but is not required for, epithelial extrusion and caspase activation in the amnioserosa.

Drosophila melanogaster: a new model to study cisplatin-induced neurotoxicity

Platinum-based compounds are widely used and effective chemotherapeutic agents; however, sensory peripheral neuropathy is a dose-limiting and long term side effect for 20-30% of patients. A critical question is whether the mechanisms of cell death underlying clinical efficacy can be separated from the effects on neurons in order to develop strategies that prevent platinum-induced neuropathy. In rodent dorsal root ganglion neurons (DRG), cisplatin has been shown to bind and damage neuronal DNA, inducing apoptosis; however genetic manipulation in order to study mechanisms of this phenomenon in the rodent model system is costly and time-consuming. Drosophila melanogaster are commonly used to study neurological disorders, have DNA damage-apoptosis mechanisms homologous to mammalian systems, and have readily-available, inexpensive tools for rapid genetic manipulation. We therefore sought to develop adult Drosophila as a new model to study cisplatin-induced neurotoxicity. Adult Drosophila were exposed to 10, 25, 50, 100, 200 and 400 μg/ml cisplatin for 3 days and observed for fly survival and geotactic climbing behavior, cisplatin-DNA binding and cellular apoptosis. On day 3, 50 μg/ml cisplatin reduced the number of flies able to climb above 2 cm to 43% while fly survival was maintained at 92%. 100% lethality was observed at 400 μg/ml cisplatin. Whole fly platinum-genomic DNA adducts were measured and found to be comparable to adduct levels previously measured in rat DRG neurons. Brain, ovaries, kidney and heart harvested from cisplatin treated flies were stained for active caspase 3. Apoptosis was found in ovaries and brain but not in heart and kidney. Brain apoptosis was confirmed by transmission electron microscopy. Expression of the anti-apoptotic baculoviral protein, p35, in neurons using the GAL4-UAS system prevented cisplatin-induced apoptosis in the brain and restored climbing behavior. In conclusion, cisplatin-induced behavioral and apoptotic changes in Drosophila resemble those seen in mammals. Furthermore, the use of lethality and climbing assays combined with powerful gene manipulation, make Drosophila a suitable model to study mechanisms of cisplatin neurotoxicity.

A link between impaired purine nucleotide synthesis and apoptosis in Drosophila melanogaster

The biosynthetic pathways and multiple functions of purine nucleotides are well known. However, the pathways that respond to alterations in purine nucleotide synthesis in vivo in an animal model organism have not been identified. We examined the effects of inhibiting purine de novo synthesis in vivo and in cultured cells of Drosophila melanogaster. The purine de novo synthesis gene ade2 encodes phosphoribosylformylglycinamidine synthase (EC 6.3.5.3). An ade2 deletion, generated by P-element transposon excision, causes lethality in early pupal development, with darkening, or necrosis, of leg and wing imaginal disc tissue upon disc eversion. Together with analysis of a previously isolated weaker allele, ade2(4), and an allele of the Prat gene, which encodes an enzyme for the first step in the pathway, we determined that the lethal arrest and imaginal disc phenotypes involve apoptosis. A transgene expressing the baculovirus caspase inhibitor p35, which suppresses apoptosis caused by other stresses such as DNA damage, suppresses both the imaginal disc tissue darkening and the pupal lethality of all three purine de novo synthesis mutants. Furthermore, we showed the presence of apoptosis at the cellular level in both ade2 and Prat mutants by detecting TUNEL-positive nuclei in wing imaginal discs. Purine de novo synthesis inhibition was also examined in tissue culture by ade2 RNA interference followed by analysis of genome-wide changes in transcript levels. Among the upregulated genes was HtrA2, which encodes an apoptosis effector and is thus a candidate for initiating apoptosis in response to purine depletion.

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.

Bcl-2 proteins and autophagy regulate mitochondrial dynamics during programmed cell death in the Drosophila ovary

The Bcl-2 family has been shown to regulate mitochondrial dynamics during cell death in mammals and C. elegans, but evidence for this in Drosophila has been elusive. Here, we investigate the regulation of mitochondrial dynamics during germline cell death in the Drosophila melanogaster ovary. We find that mitochondria undergo a series of events during the progression of cell death, with remodeling, cluster formation and uptake of clusters by somatic follicle cells. These mitochondrial dynamics are dependent on caspases, the Bcl-2 family, the mitochondrial fission and fusion machinery, and the autophagy machinery. Furthermore, Bcl-2 family mutants show a striking defect in cell death in the ovary. These data indicate that a mitochondrial pathway is a major mechanism for activation of cell death in Drosophila oogenesis.

A dp53-dependent mechanism involved in coordinating tissue growth in Drosophila

A study in the Drosophila wing suggests a crucial role of p53 in the coordination of growth between adjacent cell populations to maintain organ proportions and shape.

Coordination of growth between and within organs contributes to the generation of well-proportioned organs and functionally integrated adults. The mechanisms that help to coordinate the growth between different organs start to be unravelled. However, whether an organ is able to respond in a coordinated manner to local variations in growth caused by developmental or environmental stress and the nature of the underlying molecular mechanisms that contribute to generating well-proportioned adult organs under these circumstances remain largely unknown. By reducing the growth rates of defined territories in the developing wing primordium of Drosophila, we present evidence that the tissue responds as a whole and the adjacent cell populations decrease their growth and proliferation rates. This non-autonomous response occurs independently of where growth is affected, and it is functional all throughout development and contributes to generate well-proportioned adult structures. Strikingly, we underscore a central role of Drosophila p53 (dp53) and the apoptotic machinery in these processes. While activation of dp53 in the growth-depleted territory mediates the non-autonomous regulation of growth and proliferation rates, effector caspases have a unique role, downstream of dp53, in reducing proliferation rates in adjacent cell populations. These new findings indicate the existence of a stress response mechanism involved in the coordination of tissue growth between adjacent cell populations and that tissue size and cell cycle proliferation can be uncoupled and are independently and non-autonomously regulated by dp53.

The coordination of growth within and between organs contributes to the generation of functionally integrated structures and well-proportioned animals and plants. Though these issues have fascinated biologists for centuries, the responsible molecular mechanisms remain largely uncharacterized. In this work, we have used the Drosophila wing primordium to show that adjacent cell populations grow and proliferate in a coordinated manner. By reducing growth rates in specific territories within the developing wing, we showed that the tissue responds as a whole and that in adjacent cell populations the growth and cell cycle rates are concomitantly reduced, thus maintaining tissue proportions and normal wing shape. Interestingly, we show that the Drosophila tumour suppressor protein dp53 and apoptotic machinery play a key role in coordinating this tissue-wide response. Both growth and proliferation rates are regulated in a coordinated and non-autonomous manner by the activity of dp53, whilst the apoptotic pathway has a specific and non-autonomous role in regulating cell proliferation rates. Our studies describe a novel mechanism for regulating tissue growth in developing organs that may ultimately be relevant for other processes involving coordination of growth, such as tissue renewal, regeneration, and cancer.

Autophagic degradation of dBruce controls DNA fragmentation in nurse cells during late Drosophila melanogaster oogenesis

Blocking autophagy protects the apoptosis inhibitor dBruce from destruction and promotes nurse cell survival in developing egg chambers.

Autophagy is an evolutionarily conserved pathway responsible for degradation of cytoplasmic material via the lysosome. Although autophagy has been reported to contribute to cell death, the underlying mechanisms remain largely unknown. In this study, we show that autophagy controls DNA fragmentation during late oogenesis in Drosophila melanogaster. Inhibition of autophagy by genetically removing the function of the autophagy genes atg1, atg13, and vps34 resulted in late stage egg chambers that contained persisting nurse cell nuclei without fragmented DNA and attenuation of caspase-3 cleavage. The Drosophila inhibitor of apoptosis (IAP) dBruce was found to colocalize with the autophagic marker GFP-Atg8a and accumulated in autophagy mutants. Nurse cells lacking Atg1 or Vps34 in addition to dBruce contained persisting nurse cell nuclei with fragmented DNA. This indicates that autophagic degradation of dBruce controls DNA fragmentation in nurse cells. Our results reveal autophagic degradation of an IAP as a novel mechanism of triggering cell death and thereby provide a mechanistic link between autophagy and cell death.

Evaluation of potential reference genes for real time RT-PCR studies in Atlantic halibut (Hippoglossus Hippoglossus L.); during development, in tissues of healthy and NNV-injected fish, and in anterior kidney leucocytes

Background

Real time RT-PCR has become an important tool for analyzing gene expression in fish. Although several housekeeping genes have been evaluated in Atlantic halibut (Hippoglossus Hippoglossus L.), appropriate reference genes for low copy mRNA transcripts at the earliest developmental stages have not been identified. No attempts have been reported to identify suitable reference genes in halibut infected with NNV or in stimulated halibut leucocytes. In this study, β-actin1 (ACTB1), elongation factor 1 alpha (EF1A1), hypoxanthine-guanine phosphoribosyltransferase 1 (HPRT1), ribosomal protein L7 (RPL7), tubulin beta 2C (Tubb2C), and ubiquitin-conjugating enzyme (UbcE) were evaluated as reference genes for normalization of real time RT-PCR data during Atlantic halibut development, in tissue of healthy and NNV-infected fish, and in in vivo and in vitro stimulated anterior kidney leucocytes.

Results

The expression of all six genes was relatively stable from the unfertilized egg until 12 day degrees post fertilization (ddpf). However, none of the selected genes were found to be stably expressed throughout halibut development. The mRNA levels of the six genes increased from 18 ddpf, when zygotic transcription is likely to be activated, and stabilized at different time points. The Excel-based software programs BestKeeper, geNorm, and NormFinder ranked EF1A1 and UbcE as the best candidate reference genes before activation of zygotic transcription, and RPL7 and EF1A1 as the best candidates after hatching. EF1A1 and RPL7 were also listed as the best reference genes when exploring the expression levels of the six genes in various halibut organs, both in non-injected fish and in mock- and NNV-injected fish. None of the reference genes were found optimal for normalization of real time RT-PCR data from in vitro stimulated anterior kidney leucocytes.

Conclusion

Generally, it was found that EF1A1 and RPL7 were the genes that showed least variation, with HPRT1 and UbcE as intermediate genes, and ACTB1 and Tubb2C as the least stable ones. None of the six reference genes can be recommended as reference gene candidates in ConA-PMA stimulated leucocytes. However, UbcE can be a good candidate in other experimental setups. This study emphasizes the need for reference gene evaluation, as universal reference genes have not been identified.

The role of ubiquitylation for the control of cell death in Drosophila

Ubiquitylation describes a process in which ubiquitin, a 76-amino-acid polypeptide, is covalently attached to target proteins. Traditionally, ubiquitin-conjugated proteins are targeted for degradation by the 26S proteasome. However, non-proteolytic roles in histone regulation, DNA repair and signal transduction have been reported. Here, the role of ubiquitylation in the cell death pathway in Drosophila is reviewed. Interestingly, ubiquitylation serves both pro- and anti-apoptotic functions. Although pro-apoptotic ubiquitylation leads to proteolytic degradation, recent evidence suggests that anti-apoptotic ubiquitylation may involve, at least in part, non-proteolytic functions.

The cleaved-Caspase-3 antibody is a marker of Caspase-9-like DRONC activity in Drosophila

The cleaved-Caspase-3 antibody is a popular tool in apoptosis research in Drosophila. As the antibody was raised against cleaved human Caspase-3, it was assumed that it detects cleaved DRICE and DCP-1, Caspase-3-like effector caspases in Drosophila. However, as shown here, strong immunoreactivity persists in apoptotic models doubly mutant for drICE and dcp-1. In contrast, mutants of the apoptosome components DRONC (Caspase-9-like) and ARK (Apaf-1 related) do not label with the cleaved-Caspase-3 antibody. By peptide blocking experiments and further genetic studies, we provide evidence that the cleaved-Caspase-3 antibody recognizes multiple proteins including DCP-1 and likely DRICE, but also at least one additional unknown protein, all of which require DRONC for epitope exposure. The unknown substrate may be involved in non-apoptotic functions of DRONC. Because the cleaved-Caspase-3 antibody not only detects cleaved Caspase-3-like proteins in Drosophila, but also other proteins in a DRONC-dependent manner, it is more accurate to consider the cleaved-Caspase-3 antibody as a marker for DRONC activity, rather than effector caspase activity.

Dual roles of Drosophila p53 in cell death and cell differentiation

The mammalian p53-family consists of p53, p63 and p73. While p53 accounts for tumor suppression through cell cycle arrest and apoptosis, the functions of p63 and p73 are more diverse and also include control of cell differentiation. The Drosophila genome contains only one p53 homolog, Dp53. Previous work has established that Dp53 induces apoptosis, but not cell cycle arrest. Here, by using the developing eye as a model, we show that Dp53-induced apoptosis is primarily dependent on the pro-apoptotic gene hid, but not reaper, and occurs through the canonical apoptosis pathway. Importantly, similar to p63 and p73, expression of Dp53 also inhibits cellular differentiation of photoreceptor neurons and cone cells in the eye independently of its apoptotic function. Intriguingly, expression of the human cell cycle inhibitor p21 or its Drosophila homolog dacapo can suppress both Dp53-induced cell death and differentiation defects in Drosophila eyes. These findings provide new insights into the pathways activated by Dp53 and reveal that Dp53 incorporates functions of multiple p53-family members.

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.