DRONC, an ecdysone-inducible Drosophila caspase

Loss of non-muscle myosin II Zipper leads to apoptosis-induced compensatory proliferation in Drosophila

Drosophila Non-muscle myosin II Zipper (Zip) belongs to a functionally divergent class of molecular motors that play a vital role in various cellular processes including cell adhesion, cell migration, cell protrusion, and maintenance of polarity via its cross-linking property with actin. To further determine its role in cell proliferation and apoptosis, we carried out Zip loss of function studies that led to compromised epithelial integrity in Drosophila wing imaginal discs as evident from the perturbed expression pattern of cell-cell junction proteins Cadherin, Actin, and Armadillo. Disruption of these adhesion proteins resulted in the cells undergoing apoptosis as evident from the increased level of effector caspase, cDcp-1. The induction of cell death due to the loss of function of Zip was accompanied by proliferation as apparent from increased PH3 staining. The control of apoptosis-induced compensatory proliferation lies under the caspase cascade. We carried out experiments that suggested that the apical caspase Dronc is responsible for the apoptosis-induced compensatory proliferation due to the loss of Zip function and not the effector caspase Drice/Dcp-1. Further, it was observed that Dronc leads to the subsequent activation of Jun N-terminal kinase pathway (JNK) pathway and Wingless (Wg) mitogen that diffuse to the neighboring cells and prompt them to undergo cell division. Taken together, our results suggest that loss of function of Zip leads to apoptosis-induced compensatory proliferation.

The activation of caspases in immunocompetent cells is an important infection factor of the pathogenic fungus Conidiobolus coronatus (Entomophthorales: Ancylistaceae)

Apoptosis is a mechanism commonly used by pathogenic fungi to inhibit the host's immune response. One opportunistic pathogen is Conidiobolus coronatus, which causes fungal infection in mammals and insects. In a study, larvae of Galleria mellonella were exposed to the pathogen for 24 h. After this exposure, some larvae were used for hemolymph collection (F24), while others were incubated for an additional 24 h (F48). The activity of caspase-9 and caspase-3-like proteins in hemocytes was measured using a colorimetric method. The changes in caspase concentration were calculated using ELISA tests. Immunocytochemical analyses were employed to show changes in the levels of the examined proteins in both their pro- and active forms. Fluorescence microscopy was used to detect changes in cultured hemocytes and flow cytometry analysis was conducted to detect both forms of caspases in freshly collected hemocytes. To evaluate the effect of fungal infection, caspase inhibitors (Z-DEVD-FMK and Z-LEHD-FMK) were injected into the larvae, and their impact on insect development and resistance to fungal infection was determined. The exposure of larvae to the entomopathogen increased the detection levels, concentrations, and activity of both caspase-like proteins in hemocytes during fungal infection. The research has indicated that inhibition of these proteins disrupts larval development and increases resistance to infection. These results suggest that apoptosis might be an important mechanism for a pathogen to inhibit the insect immune response. Given the similarities between insects' and mammals' innate immune responses, the presented results may indicate a potential mechanism of fungal pathogenicity in both groups.

Ras promotes germline stem cell division in Drosophila ovaries

The Ras family genes are proto-oncogenes that are highly conserved from Drosophila to humans. In Drosophila, RasV12 is a constitutively activated form of the Ras oncoprotein, and its function in cell-cycle progression is context dependent. However, how it influences the cell cycle of female germline stem cells (GSCs) still remains unknown. Using both wild-type GSCs and bam mutant GSC-like cells as model systems, here we determined that RasV12 overexpression promotes GSC division, not growth, opposite to that in somatic wing disc cells. Ras performs this function through activating the mitogen-activated protein kinase (MAPK) signaling. This signaling is activated specifically in the M phase of mitotic germ cells, including both wild-type GSCs and bam mutant GSC-like cells. Furthermore, RasV12 overexpression triggers polyploid nurse cells to die through inducing mitotic stress. Given the similarities between Drosophila and mammalian GSCs, we propose that the Ras/MAPK signaling also promotes mammalian GSC division.

Spatial patterning controls neuron numbers in the Drosophila visual system

Neurons must be made in the correct proportions to communicate with the appropriate synaptic partners and form functional circuits. In the Drosophila visual system, multiple subtypes of distal medulla (Dm) inhibitory interneurons are made in distinct, reproducible numbers-from 5 to 800 per optic lobe. These neurons are born from a crescent-shaped neuroepithelium called the outer proliferation center (OPC), which can be subdivided into specific domains based on transcription factor and growth factor expression. We fate mapped Dm neurons and found that more abundant neural types are born from larger neuroepithelial subdomains, while less abundant subtypes are born from smaller ones. Additionally, morphogenetic Dpp/BMP signaling provides a second layer of patterning that subdivides the neuroepithelium into smaller domains to provide more granular control of cell proportions. Apoptosis appears to play a minor role in regulating Dm neuron abundance. This work describes an underappreciated mechanism for the regulation of neuronal stoichiometry.

Understanding Developmental Cell Death Using Drosophila as a Model System

Cell death plays an essential function in organismal development, wellbeing, and ageing. Many types of cell deaths have been described in the past 30 years. Among these, apoptosis remains the most conserved type of cell death in metazoans and the most common mechanism for deleting unwanted cells. Other types of cell deaths that often play roles in specific contexts or upon pathological insults can be classed under variant forms of cell death and programmed necrosis. Studies in Drosophila have contributed significantly to the understanding and regulation of apoptosis pathways. In addition to this, Drosophila has also served as an essential model to study the genetic basis of autophagy-dependent cell death (ADCD) and other relatively rare types of context-dependent cell deaths. Here, we summarise what is known about apoptosis, ADCD, and other context-specific variant cell death pathways in Drosophila, with a focus on developmental cell death.

Activation of caspase-9 on the apoptosome as studied by methyl-TROSY NMR

Mitochondrial apoptotic signaling cascades lead to the formation of the apoptosome, a 1.1-MDa heptameric protein scaffold that recruits and activates the caspase-9 protease. Once activated, caspase-9 cleaves and activates downstream effector caspases, triggering the onset of cell death through caspase-mediated proteolysis of cellular proteins. Failure to activate caspase-9 enables the evasion of programmed cell death, which occurs in various forms of cancer. Despite the critical apoptotic function of caspase-9, the structural mechanism by which it is activated on the apoptosome has remained elusive. Here, we used a combination of methyl-transverse relaxation-optimized NMR spectroscopy, protein engineering, and biochemical assays to study the activation of caspase-9 bound to the apoptosome. In the absence of peptide substrate, we observed that both caspase-9 and its isolated protease domain (PD) only very weakly dimerize with dissociation constants in the millimolar range. Methyl-NMR spectra of isotope-labeled caspase-9, within the 1.3-MDa native apoptosome complex or an engineered 480-kDa apoptosome mimic, reveal that the caspase-9 PD remains monomeric after recruitment to the scaffold. Binding to the apoptosome, therefore, organizes caspase-9 PDs so that they can rapidly and extensively dimerize only when substrate is present, providing an important layer in the regulation of caspase-9 activation. Our work highlights the unique role of NMR spectroscopy to structurally characterize protein domains that are flexibly tethered to large scaffolds, even in cases where the molecular targets are in excess of 1 MDa, as in the present example.

Sas-Ptp10D shapes germ-line stem cell niche by facilitating JNK-mediated apoptosis

The function of the stem cell system is supported by a stereotypical shape of the niche structure. In Drosophila ovarian germarium, somatic cap cells form a dish-like niche structure that allows only two or three germ-line stem cells (GSCs) reside in the niche. Despite extensive studies on the mechanism of stem cell maintenance, the mechanisms of how the dish-like niche structure is shaped and how this structure contributes to the stem cell system have been elusive. Here, we show that a transmembrane protein Stranded at second (Sas) and its receptor Protein tyrosine phosphatase 10D (Ptp10D), effectors of axon guidance and cell competition via epidermal growth factor receptor (Egfr) inhibition, shape the dish-like niche structure by facilitating c-Jun N-terminal kinase (JNK)-mediated apoptosis. Loss of Sas or Ptp10D in gonadal apical cells, but not in GSCs or cap cells, during the pre-pupal stage results in abnormal shaping of the niche structure in the adult, which allows excessive, four to six GSCs reside in the niche. Mechanistically, loss of Sas-Ptp10D elevates Egfr signaling in the gonadal apical cells, thereby suppressing their naturally-occurring JNK-mediated apoptosis that is essential for the shaping of the dish-like niche structure by neighboring cap cells. Notably, the abnormal niche shape and resulting excessive GSCs lead to diminished egg production. Our data propose a concept that the stereotypical shaping of the niche structure optimizes the stem cell system, thereby maximizing the reproductive capacity.

Drosophila caspases as guardians of host-microbe interactions

An intact cell death machinery is not only crucial for successful embryonic development and tissue homeostasis, but participates also in the defence against pathogens and contributes to a balanced immune response. Centrally involved in the regulation of both cell death and inflammatory immune responses is the evolutionarily conserved family of cysteine proteases named caspases. The Drosophila melanogaster genome encodes for seven caspases, several of which display dual functions, participating in apoptotic signalling and beyond. Among the Drosophila caspases, the caspase-8 homologue Dredd has a well-characterised role in inflammatory signalling activated by bacterial infections, and functions as a driver of NF-κB-mediated immune responses. Regarding the other Drosophila caspases, studies focusing on tissue-specific immune signalling and host-microbe interactions have recently revealed immunoregulatory functions of the initiator caspase Dronc and the effector caspase Drice. The aim of this review is to give an overview of the signalling cascades involved in the Drosophila humoral innate immune response against pathogens and of their caspase-mediated regulation. Furthermore, the apoptotic role of caspases during antibacterial and antiviral immune activation will be discussed.

Glial TGFβ activity promotes neuron survival in peripheral nerves

Maintaining long, energetically demanding axons throughout the life of an animal is a major challenge for the nervous system. Specialized glia ensheathe axons and support their function and integrity throughout life, but glial support mechanisms remain poorly defined. Here, we identified a collection of secreted and transmembrane molecules required in glia for long-term axon survival in vivo. We showed that the majority of components of the TGFβ superfamily are required in glia for sensory neuron maintenance but not glial ensheathment of axons. In the absence of glial TGFβ signaling, neurons undergo age-dependent degeneration that can be rescued either by genetic blockade of Wallerian degeneration or caspase-dependent death. Blockade of glial TGFβ signaling results in increased ATP in glia that can be mimicked by enhancing glial mitochondrial biogenesis or suppressing glial monocarboxylate transporter function. We propose that glial TGFβ signaling supports axon survival and suppresses neurodegeneration through promoting glial metabolic support of neurons.

Retromer deficiency in Tauopathy models enhances the truncation and toxicity of Tau

Alteration of the levels, localization or post-translational processing of the microtubule associated protein Tau is associated with many neurodegenerative disorders. Here we develop adult-onset models for human Tau (hTau) toxicity in Drosophila that enable age-dependent quantitative measurement of central nervous system synapse loss and axonal degeneration, in addition to effects upon lifespan, to facilitate evaluation of factors that may contribute to Tau-dependent neurodegeneration. Using these models, we interrogate the interaction of hTau with the retromer complex, an evolutionarily conserved cargo-sorting protein assembly, whose reduced activity has been associated with both Parkinson’s and late onset Alzheimer’s disease. We reveal that reduction of retromer activity induces a potent enhancement of hTau toxicity upon synapse loss, axon retraction and lifespan through a specific increase in the production of a C-terminal truncated isoform of hTau. Our data establish a molecular and subcellular mechanism necessary and sufficient for the depletion of retromer activity to exacerbate Tau-dependent neurodegeneration.

Tau and the Retromer complex are both linked to Parkinson’s and Alzheimer’s disease. Using Drosophila neurodegeneration models, this study finds that low retromer activity induces a specific increase of a highly toxic truncated form of human Tau.

Integrin-ECM interactions and membrane-associated Catalase cooperate to promote resilience of the Drosophila intestinal epithelium

Balancing cellular demise and survival constitutes a key feature of resilience mechanisms that underlie the control of epithelial tissue damage. These resilience mechanisms often limit the burden of adaptive cellular stress responses to internal or external threats. We recently identified Diedel, a secreted protein/cytokine, as a potent antagonist of apoptosis-induced regulated cell death in the Drosophila intestinal midgut epithelium during aging. Here, we show that Diedel is a ligand for RGD-binding Integrins and is thus required for maintaining midgut epithelial cell attachment to the extracellular matrix (ECM)-derived basement membrane. Exploiting this function of Diedel, we uncovered a resilience mechanism of epithelial tissues, mediated by Integrin-ECM interactions, which shapes cell death spreading through the regulation of cell detachment and thus cell survival. Moreover, we found that resilient epithelial cells, enriched for Diedel-Integrin-ECM interactions, are characterized by membrane association of Catalase, thus preserving extracellular reactive oxygen species (ROS) balance to maintain epithelial integrity. Intracellular Catalase can relocalize to the extracellular membrane to limit cell death spreading and repair Integrin-ECM interactions induced by the amplification of extracellular ROS, which is a critical adaptive stress response. Membrane-associated Catalase, synergized with Integrin-ECM interactions, likely constitutes a resilience mechanism that helps balance cellular demise and survival within epithelial tissues.

Impact of Group II Baculovirus IAPs on Virus-Induced Apoptosis in Insect Cells

Apoptosis plays an important role in virus-host interactions and is a major element of the insect immune response. Exploring the regulatory mechanisms of virus-induced apoptosis through the expression of apoptotic genes holds important research and application value. Functional research on the reported inhibitor of apoptosis proteins (IAPs) mainly focuses on the group I baculovirus, while the functions of the group II baculovirus IAPs remains unclear. To explore its role in the regulation of the apoptosis of insect cells, we constructed the transient expression vector (pIE1 vectors) and the recombinant baculovirus expressing Bsiap genes (from the Buzura suppressaria nucleopolyhedrovirus) of the group II baculovirus. Apoptosis gene expression results and the virus-induced apoptosis rate show that the overexpression of BsIAP1 could promote apoptosis in insect cells. However, the overexpression of BsIAP2 and BsIAP3 decreases the expression of apoptotic genes, revealing an inhibitory effect. Results on the impact of baculovirus-induced apoptosis also confirm that BsIAP1 reduces viral nucleocapsid expression and the baculovirus titer, while BsIAP2 and BsIAP3 increase them significantly. Furthermore, compared with single expression, the co-expression of BsIAP2 and BsIAP3 significantly reduces the rate of virus-induced apoptosis and improves the expression of nucleocapsids and the titer of offspring virus, indicating the synergistic effect on BsIAP2 and BsIAP3. In addition, combined expression of all three BsIAPs significantly reduced levels of intracellular apoptosis-related genes (including apoptosis and anti-apoptosis genes), as well as apoptosis rate and progeny virus titer, indicating that life activities in insect cells are also inhibited. These findings reveal the relationship between apoptosis and group II baculovirus IAP, which provide an experimental and theoretical basis for further exploration of the molecular mechanism between group II baculoviruses and insect cells.

Proximate larval epidermal cell layer generates forces for Pupal thorax closure in Drosophila

During tissue closures, such as embryonic dorsal closure in Drosophila melanogaster, a proximate extra-embryonic layer, amnioserosa, generates forces that drive migration of the flanking lateral embryonic epidermis, thereby zip-shutting the embryo. Arguably, this paradigm of tissue closure is also recapitulated in mammalian wound healing wherein proximate fibroblasts transform into contractile myofibroblasts, develop cell junctions, and form a tissue layer de novo: contraction of the latter then aids in wound closure. Given this parallelism between disparate exemplars, we posit a general principle of tissue closure via proximate cell layer-generated forces. Here, we have tested this hypothesis in pupal thorax closure wherein 2 halves of the presumptive adult thorax of Drosophila, the contralateral heminotal epithelia, migrate over an underlying larval epidermal cell layer. We show that the proximate larval epidermal cell layer promotes thorax closure by its active contraction, orchestrated by its elaborate actomyosin network-driven epithelial cell dynamics, cell delamination, and death-the latter being prefigured by the activation of caspases. Larval epidermal cell dynamics generate contraction forces, which when relayed to the flanking heminota-via their mutual integrin-based adhesions-mediate thorax closure. Compromising any of these contraction force-generating mechanisms in the larval epidermal cell layer slows down heminotal migration, while loss of its relay to the flanking heminota abrogates the thorax closure altogether. Mathematical modeling further reconciles the biophysical underpinning of this emergent mechanism of thorax closure. Revealing mechanism of thorax closure apart, these findings show conservation of an essential principle of a proximate cell layer-driven tissue closure.

The p53-caspase-2 axis in the cell cycle and DNA damage response

Caspase-2 was discovered almost three decades ago. It was one of the first two mammalian homologs of CED-3, the other being interleukin 1β-converting enzyme (ICE/caspase-1). Despite high similarity with CED-3 and its fly and mammalian counterparts (DRONC and caspase-9, respectively), the function of caspase-2 in apoptosis has remained enigmatic. A number of recent studies suggest that caspase-2 plays an important role in the regulation of p53 in response to cellular stress and DNA damage to prevent the proliferation and accumulation of damaged or aberrant cells. Here, we review these recent observations and their implications in caspase-2-mediated cellular death, senescence, and tumor suppression.

Analysis of the lamprey genotype provides insights into caspase evolution and functional divergence

The cysteine-containing aspartate specific proteinase (caspase) family plays important roles in apoptosis and the maintenance of homeostasis in lampreys. We conducted genomic and functional comparisons of six distinct lamprey caspase groups with human counterparts to determine how these expanded molecules evolved to adapt to the changing caspase-mediated signaling pathways. Our results showed that lineage-specific duplication and rearrangement were responsible for expanding lamprey caspases 3 and 7, whereas caspases 1, 6, 8, and 9 maintained a relatively stable genome and protein structure. Lamprey caspase family molecules displayed various expression patterns and were involved in the innate immune response. Caspase 1 and 7 functioned as a pattern recognition receptor with a broad-spectrum of microbial recognition and bactericidal effect. Additionally, caspases 1 and 7 may induce cell apoptosis in a time- and dose-dependent manner; however, apoptosis was inhibited by caspase inhibitors. Thus, these molecules may reflect the original state of the vertebrates caspase family. Our phylogenetic and functional data provide insights into the evolutionary history of caspases and illustrate their functional characteristics in primitive vertebrates.

Comparative transcriptome analyses of the Drosophila pupal eye

Tissue function is dependent on correct cellular organization and behavior. As a result, the identification and study of genes that contribute to tissue morphogenesis is of paramount importance to the fields of cell and developmental biology. Many of the genes required for tissue patterning and organization are highly conserved between phyla. This has led to the emergence of several model organisms and developmental systems that are used to study tissue morphogenesis. One such model is the Drosophila melanogaster pupal eye that has a highly stereotyped arrangement of cells. In addition, the pupal eye is postmitotic that allows for the study of tissue morphogenesis independent from any effects of proliferation. While the changes in cell morphology and organization that occur throughout pupal eye development are well documented, less is known about the corresponding transcriptional changes that choreograph these processes. To identify these transcriptional changes, we dissected wild-type Canton S pupal eyes and performed RNA-sequencing. Our analyses identified differential expression of many loci that are documented regulators of pupal eye morphogenesis and contribute to multiple biological processes including signaling, axon projection, adhesion, and cell survival. We also identified differential expression of genes not previously implicated in pupal eye morphogenesis such as components of the Toll pathway, several non-classical cadherins, and components of the muscle sarcomere, which could suggest these loci function as novel patterning factors.

Ibuprofen as an emerging pollutant on non-target aquatic invertebrates: Effects on Chironomus riparius

The concern about pharmaceuticals has been increased over the last decade due to their burgeoning consumption. Ibuprofen has an extensive presence in surface water with risks for the aquatic biota. This study focuses on the effects of ibuprofen at environmental concentrations on the survival, transcriptional level, and enzymatic activity for 24, 96 h on Chironomus riparius. Ibuprofen developed a substantial effect on survival by all the conditions. mRNA levels of EcR, Dronc, and Met (endocrine system), hsp70, hsp24, and hsp27 (stress response), and Proph and Def (immune system) were modified, joined to increased GST and PO activity. The results confirmed alterations on the development of C. riparius, as well as two essential mechanisms, involved in protection against external toxicological challenge. Ibuprofen poses an incipient risk to C. riparius and could at an organismal level by compromising their survival, development, and ability to respond to adverse conditions on the future populations.

Identification and functional characterization of SlDronc in Spodoptera littoralis

Background

Apoptosis is responsible for eliminating damaged and virus-infected cells, regulating normal cell turnover, and maintaining the immune system’s development and function. Caspases play a vital role in both mammal and invertebrate apoptosis. Spodoptera littoralis is a generalist insect herbivore that is one of the most destructive pests in tropical and subtropical areas and attacks a wide range of commercially important crops. Although S. littoralis is a model organism in the study of baculovirus infection, its apoptotic pathway has not been explored.

Methods

We cloned a new caspase gene named sldronc in S. littoralis using Rapid Amplification of cDNA Ends (RACE). We then measured caspase activity on synthetic caspase substrates and S. littoralis’ effector caspase. SlDronc’s function in the apoptotic pathway and its interaction with caspase inhibitors were also tested in SL2 cells.

Results

We found that the initiator caspase SlDronc cleaved and activated effector caspase in S. littoralis. SlDronc overexpression induced apoptosis in SL2 cells, and Sldronc knockdown decreased apoptosis induced by UV irradiation in SL2 cells. Our results indicate that SlDronc acts as an apoptotic initiator caspase in S. littoralis. Additionally, we found that processed forms of SlDronc increased in the presence of N-terminally truncated S. littoralis inhibitors of apoptosis (SlIAP) and that SlDronc was inhibited by P49. This study contributes to the further understanding of S. littoralis’ apoptotic pathway and may facilitate future studies on baculovirus infection-induced apoptosis.

Ecdysone controlled cell and tissue deletion

The removal of superfluous and unwanted cells is a critical part of animal development. In insects the steroid hormone ecdysone, the focus of this review, is an essential regulator of developmental transitions, including molting and metamorphosis. Like other steroid hormones, ecdysone works via nuclear hormone receptors to direct spatial and temporal regulation of gene transcription including genes required for cell death. During insect metamorphosis, pulses of ecdysone orchestrate the deletion of obsolete larval tissues, including the larval salivary glands and the midgut. In this review we discuss the molecular machinery and mechanisms of ecdysone-dependent cell and tissue removal, with a focus on studies in Drosophila and Lepidopteran insects.

Apoptosis-mediated vasa down-regulation controls developmental transformation in Japanese Copidosoma floridanum female soldiers

Copidosoma floridanum is a polyembryonic, caste-forming, wasp species. The ratio of investment in different castes changes with environmental stressors (e.g. multi-parasitism with competitors). The vasa gene was first identified in Drosophila melanogaster as a germ-cell-determining factor, and C. floridanum vasa (Cf-vas) gene positive cells have been known to develop into reproductive larvae. Cf-vas seems to control the ratio of investment in C. floridanum larval castes. In this study, we identified environmental factors that control Cf-vas mRNA expression in Japanese C. floridanum by examining Cf-vas mRNA expression under competitor (Meteorus pulchricornis) venom stress; we treated the male and female morulae with M. pulchricornis venom. We also assessed the effects of multi-parasitism of Japanese C. floridanum with M. pulchricornis and found an increasing number of female soldier larvae. The results showed that several amino acid sequences differ between the Japanese and US Cf-vas. Quantitative reverse transcription polymerase chain reaction (qRT-PCR) showed that Japanese Cf-vas mRNA is expressed in both male and female larvae and pupae, but mRNA expression decreases in adults. Cf-vas mRNA expression significantly decreased, while C. floridanum dronc (Cf-dronc) mRNA expression increased, in female morulae after M. pulchricornis venom treatment at 20 h and 0 h of the culture period, respectively. Females and males showed different Cf-vas or Cf-dronc mRNA expression after M. pulchricornis venom treatment. Therefore, M. pulchricornis venom could affect the ratio of investment in different female castes of Japanese C. floridanum by decreasing Cf-vas mRNA expression via apoptosis.

Disassembly of dying cells in diverse organisms

Programmed cell death (PCD) is a conserved phenomenon in multicellular organisms required to maintain homeostasis. Among the regulated cell death pathways, apoptosis is a well-described form of PCD in mammalian cells. One of the characteristic features of apoptosis is the change in cellular morphology, often leading to the fragmentation of the cell into smaller membrane-bound vesicles through a process called apoptotic cell disassembly. Interestingly, some of these morphological changes and cell disassembly are also noted in cells of other organisms including plants, fungi and protists while undergoing 'apoptosis-like PCD'. This review will describe morphologic features leading to apoptotic cell disassembly, as well as its regulation and function in mammalian cells. The occurrence of cell disassembly during cell death in other organisms namely zebrafish, fly and worm, as well as in other eukaryotic cells will also be discussed.

A virus-acquired host cytokine controls systemic aging by antagonizing apoptosis

Aging is characterized by degeneration of unique tissues. However, dissecting the interconnectedness of tissue aging remains a challenge. Here, we employ a muscle-specific DNA damage model in Drosophila to reveal secreted factors that influence systemic aging in distal tissues. Utilizing this model, we uncovered a cytokine—Diedel—that, when secreted from muscle or adipose, can attenuate age-related intestinal tissue degeneration by promoting proliferative homeostasis of stem cells. Diedel is both necessary and sufficient to limit tissue degeneration and regulate lifespan. Secreted homologs of Diedel are also found in viruses, having been acquired from host genomes. Focusing on potential mechanistic overlap between cellular aging and viral-host cell interactions, we found that Diedel is an inhibitor of apoptosis and can act as a systemic rheostat to modulate cell death during aging. These results highlight a key role for secreted antagonists of apoptosis in the systemic coordination of tissue aging.

Aging in multicellular organisms is characterized by a progressive decline in the proper function of organs. This deterioration of organ function is a risk factor for many diseases. However, it is unlikely that organs age in isolation, as damage in one organ can presumably impact aging of other organs through either beneficial or detrimental cross-talk. Our work attempts to explore this aspect of aging using fruit flies as a model system. We uncovered that damaged fly muscle can protect against aging in other organs, such as the intestine, through the secretion of a blood-borne factor named Diedel. This blood-borne factor presumably allows damaged organs to communicate with each other during aging. Related factors are also found in certain viruses, which have been hijacked from insect genomes to promote viral spreading during infection. Using this information, we found that viral Diedel inhibits death of infected cells, allowing viruses to spread. Similarly, host (insect) Diedel also blocks cell death in organs during aging, thus limiting deterioration of organ function and extending the organism’s lifespan.

Starfish Apaf-1 activates effector caspase-3/9 upon apoptosis of aged eggs

Caspase-3-related DEVDase activity is initiated upon apoptosis in unfertilized starfish eggs. In this study, we cloned a starfish procaspase-3 corresponding to mammalian effector caspase containing a CARD that is similar to the amino terminal CARD of mammalian capsase-9, and we named it procaspase-3/9. Recombinant procaspase-3/9 expressed at 15 °C was cleaved to form active caspase-3/9 which has DEVDase activity. Microinjection of the active caspase-3/9 into starfish oocytes/eggs induced apoptosis. An antibody against the recombinant protein recognized endogenous procaspase-3/9 in starfish oocytes, which was cleaved upon apoptosis in aged unfertilized eggs. These results indicate that caspase-3/9 is an effector caspase in starfish. To verify the mechanism of caspase-3/9 activation, we cloned starfish Apaf-1 containing a CARD, a NOD, and 11 WD40 repeat regions, and we named it sfApaf-1. Recombinant sfApaf-1 CARD interacts with recombinant caspase-3/9 CARD and with endogenous procaspase-3/9 in cell-free preparations made from starfish oocytes, causing the formation of active caspase-3/9. When the cell-free preparation without mitochondria was incubated with inactive recombinant procaspase-3/9 expressed at 37 °C, DEVDase activity increased and apoptosome-like complexes were formed in the high molecular weight fractions containing both sfApaf-1 and cleaved caspase-3/9. These results suggest that sfApaf-1 activation is not dependent on cytochrome c.

JNK is antagonized to ensure the correct number of interommatidial cells pattern the Drosophila retina

Apoptosis is crucial during the morphogenesis of most organs and tissues, and is utilized for tissues to achieve their proper size, shape and patterning. Many signaling pathways contribute to the precise regulation of apoptosis. Here we show that Jun N-terminal Kinase (JNK) activity contributes to the coordinated removal of interommatidial cells via apoptosis in the Drosophila pupal retina. This is consistent with previous findings that JNK activity promotes apoptosis in other epithelia. However, we found that JNK activity is repressed by Cindr (the CIN85 and CD2AP ortholog) in order to promote cell survival. Reducing the amount of Cindr resulted in ectopic cell death. Increased expression of the Drosophila JNK basket in the setting of reduced cindr expression was found to result in even more severe apoptosis, whilst ectopic death was found to be reduced if retinas were heterozygous for basket. Hence Cindr is required to properly restrict JNK-mediated apoptosis in the pupal eye, resulting in the correct number of interommatidial cells. A lack of precise control over developmental apoptosis can lead to improper tissue morphogenesis.

Insecticidal effects of dsRNA targeting the Diap1 gene in dipteran pests

The Drosophila melanogaster (fruit fly) gene Diap1 encodes a protein referred to as DIAP1 (D rosophila Inhibitor of Apoptosis Protein 1) that acts to supress apoptosis in "normal" cells in the fly. In this study we investigate the use of RNA interference (RNAi) to control two dipteran pests, Musca domestica and Delia radicum, by disrupting the control of apoptosis. Larval injections of 125-500 ng of Diap1 dsRNA resulted in dose-dependent mortality which was shown to be attributable to down-regulation of target mRNA. Insects injected with Diap1 dsRNA have approx. 1.5-2-fold higher levels of caspase activity than controls 24 hours post injection, providing biochemical evidence that inhibition of apoptotic activity by the Diap1 gene product has been decreased. By contrast adults were insensitive to injected dsRNA. Oral delivery failed to induce RNAi effects and we suggest this is attributable to degradation of ingested dsRNA by intra and extracellular RNAses. Non-target effects were demonstrated via mortality and down-regulation of Diap1 mRNA levels in M. domestica larvae injected with D. radicum Diap1 dsRNA, despite the absence of 21 bp identical sequence regions in the dsRNA. Here we show that identical 15 bp regions in dsRNA are sufficient to trigger non-target RNAi effects.

HDAC Inhibitors Disrupt Programmed Resistance to Apoptosis During Drosophila Development

We have previously shown that the ability to respond to apoptotic triggers is regulated during Drosophila development, effectively dividing the fly life cycle into stages that are either sensitive or resistant to apoptosis. Here, we show that the developmentally programmed resistance to apoptosis involves transcriptional repression of critical proapoptotic genes by histone deacetylases (HDACs). Administration of HDAC inhibitors (HDACi), like trichostatin A or suberoylanilide hydroxamic acid, increases expression of proapoptotic genes and is sufficient to sensitize otherwise resistant stages. Conversely, reducing levels of proapoptotic genes confers resistance to otherwise sensitive stages. Given that resistance to apoptosis is a hallmark of cancer cells, and that HDACi have been recently added to the repertoire of FDA-approved agents for cancer therapy, our results provide new insights for how HDACi help kill malignant cells and also raise concerns for their potential unintended effects on healthy cells.

An inhibitory mono-ubiquitylation of the Drosophila initiator caspase Dronc functions in both apoptotic and non-apoptotic pathways

Apoptosis is an evolutionary conserved cell death mechanism, which requires activation of initiator and effector caspases. The Drosophila initiator caspase Dronc, the ortholog of mammalian Caspase-2 and Caspase-9, has an N-terminal CARD domain that recruits Dronc into the apoptosome for activation. In addition to its role in apoptosis, Dronc also has non-apoptotic functions such as compensatory proliferation. One mechanism to control the activation of Dronc is ubiquitylation. However, the mechanistic details of ubiquitylation of Dronc are less clear. For example, monomeric inactive Dronc is subject to non-degradative ubiquitylation in living cells, while ubiquitylation of active apoptosome-bound Dronc triggers its proteolytic degradation in apoptotic cells. Here, we examined the role of non-degradative ubiquitylation of Dronc in living cells in vivo, i.e. in the context of a multi-cellular organism. Our in vivo data suggest that in living cells Dronc is mono-ubiquitylated on Lys78 (K78) in its CARD domain. This ubiquitylation prevents activation of Dronc in the apoptosome and protects cells from apoptosis. Furthermore, K78 ubiquitylation plays an inhibitory role for non-apoptotic functions of Dronc. We provide evidence that not all of the non-apoptotic functions of Dronc require its catalytic activity. In conclusion, we demonstrate a mechanism whereby Dronc’s apoptotic and non-apoptotic activities can be kept silenced in a non-degradative manner through a single ubiquitylation event in living cells.

Apoptosis is a programmed cell death mechanism which is conserved from flies to humans. Apoptosis is mediated by proteases, termed caspases that cleave cellular proteins and trigger the death of the cell. Activation of caspases is regulated at various levels such as protein-protein interaction for initiator caspases and ubiquitylation. Caspase 9 in mammals and its Drosophila ortholog Dronc carry a protein-protein interaction domain (CARD) in their prodomain which interacts with scaffolding proteins to form the apoptosome, a cell-death platform. Here, we show that Dronc is mono-ubiquitylated at Lysine 78 in its CARD domain. This ubiquitylation interferes with the formation of the apoptosome, causing inhibition of apoptosis. In addition to its apoptotic function, Dronc also participates in events where caspase activity is not required for cell killing, but for regulating other functions, so-called non-apoptotic functions of caspases such as apoptosis-induced proliferation. We found that mono-ubiquitylation of Lysine 78 plays an inhibitory role for these non-apoptotic functions of Dronc. Interestingly, we demonstrate that the catalytic activity of Dronc is not strictly required in these processes. Our in vivo study sheds light on how a single mono-ubiquitylation event could inhibit both apoptotic and non-apoptotic functions of a caspase.

A Near-Atomic Structure of the Dark Apoptosome Provides Insight into Assembly and Activation

In Drosophila, the Apaf-1-related killer (Dark) forms an apoptosome that activates procaspases. To investigate function, we have determined a near-atomic structure of Dark double rings using cryo-electron microscopy. We then built a nearly complete model of the apoptosome that includes 7- and 8-blade β-propellers. We find that the preference for dATP during Dark assembly may be governed by Ser325, which is in close proximity to the 2' carbon of the deoxyribose ring. Interestingly, β-propellers in V-shaped domains of the Dark apoptosome are more widely separated, relative to these features in the Apaf-1 apoptosome. This wider spacing may be responsible for the lack of cytochrome c binding to β-propellers in the Dark apoptosome. Our structure also highlights the roles of two loss-of-function mutations that may block Dark assembly. Finally, the improved model provides a framework to understand apical procaspase activation in the intrinsic cell death pathway.

Autophagy and the invisible line between life and death

For a considerable time cell death has been considered to represent mutually exclusive states with cell death modalities that are governed by their inherent and unique mode of action involving specific molecular entities and have therefore been studied primarily in isolation. It is now, however, becoming increasingly clear that these modalities are regulated by similar pathways and share a number of initiator and effector molecules that control both cell death as well as cell survival mechanisms, demanding a newly aligned and integrative approach of cell death assessment. Frequently cell death is triggered through a dual action that incorporates signaling events associated with more than one death modality. Apoptosis and necrosis regularly co-operate in a tightly balanced interplay that involves autophagy to serve context dependently either as a pro-survival or a pro-death mechanism. In this review we will assess current cell death modalities and their molecular overlap with the goal of clarifying the controversial role of autophagy in the cell death response. By dissecting the key molecular pathways and their positioning within a network of regulatory signalling hubs and checkpoints we discuss a distinct approach that integrates autophagy with a resultant cell death manifestation. In doing so, former classifications of cell death modalities fade and reveal the intricate molecular proportions and complexities of the cell death response that may contribute towards an enhanced means of cell death control.

Queen pheromone regulates programmed cell death in the honey bee worker ovary

In social insect colonies the presence of a queen, secreting her pheromones, is a key environmental cue for regulating the reproductive state of workers. However, until recently the proximate molecular mechanisms underlying facultative worker sterility were unidentified. Studies into worker oogenesis in the honey bee (Apis mellifera) have indicated that programmed cell death is central to the regulation of oogenesis. Here we investigate how queen pheromone, age of the worker and ovary state affect both programmed cell death and cell number in worker ovaries. We describe a novel method to simultaneously measure programmed cell death (caspase activity) and live cell number (estimated from the amount of adenosine triphosphate) in an insect tissue. Workers exposed to queen pheromone have higher levels of caspase activity in the ovary than those not exposed. Our results suggest that queen pheromone triggers programmed cell death at the mid-oogenesis checkpoint causing the abortion of worker oocytes and reproductive inhibition of the worker caste. Nonetheless, high caspase activity is present in activated ovaries from workers not exposed to queen pheromone. This caspase activity is most likely to be from the nurse cells undergoing programmed cell death, in late oogenesis, for normal oocyte development. Our study shows that the social environment of an organism can influence programmed cell death within a tissue.

Identification of AaCASPS7, an effector caspase in Aedes albopictus

Aedes albopictus mosquito is a vector of various arboviruses and is becoming a significant threat to public health due to its rapid global expansion. Several reports suggest that apoptosis could be a factor limiting arbovirus infection in mosquitoes. Thus, it is significant to identify apoptosis pathway and study the correlation between apoptosis and virus infection in mosquitoes. Apoptosis is a type of programmed cell death that plays a vital role in immunity, development, and tissue homeostasis. Caspases are a family of conserved proteases playing important roles in apoptosis. In this study, we identified Aedes albopictus AaCASPS7, a caspase shared high identity with dipteran insect drICE orthologs. Phylogenetic analysis showed the closest relative of AaCASPS7 was Aedes aegypti AeCASPS7. AaCASPS7 displayed several features that were typical of an effector caspase and showed significant activity to effector caspase substrates. Aacasps7 transcripts were expressed ubiquitously in developmental and adult stages in Aedes albopictus mosquitoes. Transient expression of AaCASPS7 induced caspase-dependent apoptosis in C6/36 cells. Taken together the above data, this study identified a novel caspase, AaCASPS7, which might function as an apoptotic caspase. Further study the function of AaCASPS7 would facilitate better understanding the apoptotic mechanism in Aedes albopictus mosquito.

Programmed cell death acts at different stages of Drosophila neurodevelopment to shape the central nervous system

Nervous system development is a process that integrates cell proliferation, differentiation, and programmed cell death (PCD). PCD is an evolutionary conserved mechanism and a fundamental developmental process by which the final cell number in a nervous system is established. In vertebrates and invertebrates, PCD can be determined intrinsically by cell lineage and age, as well as extrinsically by nutritional, metabolic, and hormonal states. Drosophila has been an instrumental model for understanding how this mechanism is regulated. We review the role of PCD in Drosophila central nervous system development from neural progenitors to neurons, its molecular mechanism and function, how it is regulated and implemented, and how it ultimately shapes the fly central nervous system from the embryo to the adult. Finally, we discuss ideas that emerged while integrating this information.

The Strica Homolog AaCASPS16 Is Involved in Apoptosis in the Yellow Fever Vector, Aedes albopictus

Caspases are a family of cysteine proteases playing essential roles during apoptosis. Seven caspases identified in Drosophila were Dronc, Dredd, Strica, Dcp-1, Decay, Drice and Damm. Among them, Strica is an insect-specific caspase containing a long serine- and threonine- rich prodomain, of which function is not yet well studied. Here we identified a homolog of strica from Aedes albopictus, named as Aacasps16. Aacasps16 encoded a protein containing a putative serine- and threonine-rich prodomain and a well conserved caspase catalytic domain. AaCASPS16 shared high identity with dipteran insects Strica homologs. Alignment showed that the closest relative of AaCASPS16 was Aedes aegypti AeCASPS16. The expression profiles of Aacasps16 during developmental and adult stages were analyzed. Purified recombinant AaCASPS16 exhibited the highest caspase activity to WEHD, which is the substrate preferred by human caspase-9. AaCASPS16 induced apoptosis when over-expressed in C6/36 cells. AaCASPS16 was processed during apoptosis induced by actinomycin D and ultraviolet irradiation treatment, whereas partial silencing of Aacasps16 reduced actinomycin D- and ultraviolet irradiation-triggered apoptosis in C6/36 cells. Taken together, our study identified AaCASPS16 as a novel apoptotic caspase in Aedes albopictus.

Suppression of induced but not developmental apoptosis in Drosophila by Ayurvedic Amalaki Rasayana and Rasa-Sindoor

Earlier we showed formulation-specific beneficial effects of dietary supplement of Ayurvedic Amalaki Rasayana (AR, a herbal formulation) and Rasa-Sindoor (RS, a mercury-based organo-metallic formulation) on various biological parameters in Drosophila, parallel to traditional Ayurvedic literature. These formulations also suppressed cell death and pathology in fly models of neurodegeneration. To understand basis of inhibition of apoptosis, we examined effects of AR and RS on induced and developmental apoptosis in Drosophila. Dietary AR or RS significantly reduced apoptosis induced by GMR-GAL4-, sev-GAL4- or hs-GAL4-directed expression of Rpr, Hid or Grim (RHG) proapoptotic proteins or by GMR-GAL4-directed DIAP1-RNAi, resulting in significant restoration of organism's viability and eye morphology. AR or RS supplement enhanced levels of inhibitor of apoptosis proteins, DIAP1 and DIAP2, and of Bancal/Hrb57A, while the levels of RHG proteins and of initiator Dronc and effecter Drice caspases were reduced in non-apoptotic wild type as well as in RHG over-expressing tissues. Levels of Dronc or Drice remained unaffected in cells developmentally destined to die so that developmental apoptosis occurred normally. Elevated levels of DIAPs and reduced levels of RHG proteins and caspases reflect a more robust physiological state of AR or RS fed organisms allowing them to tolerate greater insults without triggering the cell-death response. Such homeostatic effects of these Rasayanas seem to contribute to 'healthy ageing', one of their effects suggested in traditional Ayurvedic practices.

Identification of Aadnr1, a novel gene related to innate immunity and apoptosis in Aedes albopictus

Innate immunity and apoptosis play critical roles in defending pathogens in insects. In Drosophila, Dnr1 was reported as a negative regulator of apoptosis and immune deficiency (Imd) pathway which belongs to innate immunity. Aedes albopictus is an important kind of arbovirus vector and becoming a significant threat to public health due to its rapid global expansion. Here we identified an ortholog of dnr1 from A. albopictus, named as Aadnr1. Aadnr1 encoded a putative protein containing an N-terminal FERM domain and a C-terminal RING domain. AaDnr1 shared high identity with dipteran insects Dnr1 orthologs. Phylogenetic analyses showed that the closest relative of AaDnr1 was Aedes aegypti Dnr1. Real-time PCR proved that Aadnr1 mRNA was expressed ubiquitously during developmental and adult stages. Transcriptional levels of Aadnr1 were decreased drastically in C6/36 cells underwent apoptosis induced by Actinomycin D (Act D) treatment. Partial silence of Aadnr1 enhanced Act D-induced caspase activity. When challenged by heat-inactivated E. coli, transcriptional level of Aadnr1 was also decreased dramatically in C6/36 cells. While when C6/36 cells were infected with Sindbis virus TE/GFP, transcriptional level of Aadnr1 was reduced and recovered repeatedly, with an overall decreasing trend. It was also shown in this study that similar to Drosophila Dnr1, RING domain destabilized AaDnr1 protein. Taken together, the study identified an innate immunity and apoptosis related gene Aadnr1 in A. albopictus.

Nerve-racking - apoptotic and non-apoptotic roles of caspases in the nervous system of Drosophila

Studies using Drosophila as a model system have contributed enormously to our knowledge of caspase function and regulation. Caspases are best known as central executioners of apoptosis but also control essential physiological processes in a non-apoptotic manner. The Drosophila genome codes for seven caspases and in this review we provide an overview of current knowledge about caspase function in the nervous system. Caspases regulate neuronal death at all developmental stages and in various neuronal populations. In contrast, non-apoptotic roles are less well understood. The development of new genetically encoded sensors for caspase activity provides unprecedented opportunities to study caspase function in the nervous system in more detail. In light of these new tools we discuss the potential of Drosophila as a model to discover new apoptotic and non-apoptotic neuronal roles of caspases.

SfDredd, a Novel Initiator Caspase Possessing Activity on Effector Caspase Substrates in Spodoptera frugiperda

Sf9, a cell line derived from Spodoptera frugiperda, is an ideal model organism for studying insect apoptosis. The first notable study that attempted to identify the apoptotic pathway in Sf9 was performed in 1997 and included the discovery of Sf-caspase-1, an effector caspase of Sf9. However, it was not until 2013 that the first initiator caspase in Sf9, SfDronc, was discovered, and the apoptotic pathway in Sf9 became clearer. In this study, we report another caspase of Sf9, SfDredd. SfDredd is highly similar to insect initiator caspase Dredd homologs. Experimentally, recombinant SfDredd underwent autocleavage and exhibited different efficiencies in cleavage of synthetic caspase substrates. This was attributed to its caspase activity for the predicted active site mutation blocked the above autocleavage and synthetic caspase substrates cleavage activity. SfDredd was capable of not only cleaving Sf-caspase-1 in vitro but also cleaving Sf-caspase-1 and inducing apoptosis when it was co-expressed with Sf-caspase-1 in Sf9 cells. The protein level of SfDredd was increased when Sf9 cells were treated by Actinomycin D, whereas silencing of SfDredd reduced apoptosis and Sf-caspase-1 cleavage induced by Actinomycin D treatment. These results clearly indicate that SfDredd functioned as an apoptotic initiator caspase. Apoptosis induced in Sf9 cells by overexpression of SfDredd alone was not as obvious as that induced by SfDronc alone, and the cleavage sites of Sf-caspase-1 for SfDredd and SfDronc are different. In addition, despite sharing a sequence homology with initiator caspases and possessing weak activity on initiator caspase substrates, SfDredd showed strong activity on effector caspase substrates, making it the only insect caspase reported so far functioning similar to human caspase-2 in this aspect. We believe that the discovery of SfDredd, and its different properties from SfDronc, will improve the understanding of apoptosis pathway in Sf9 cells.

Detection of Cell Death in Drosophila Tissues

Drosophila has served as a particularly attractive model to study cell death due to the vast array of tools for genetic manipulation under defined spatial and temporal conditions in vivo as well as in cultured cells. These genetic methods have been well supplemented by enzymatic assays and a panel of antibodies recognizing cell death markers. This chapter discusses reporters, mutants, and assays used by various laboratories to study cell death in the context of development and in response to external insults.

Studying Apoptosis in Drosophila

The apoptotic machinery is highly conserved throughout evolution, and central to the regulation of apoptosis is the caspase family of cysteine proteases. Insights into the regulation and function of apoptosis in mammals have come from studies using model organisms. Drosophila provides an exceptional model system for identifying the function of conserved mechanisms regulating apoptosis, especially during development. The characteristic patterns of apoptosis during Drosophila development have been well described, as has the apoptotic response following DNA damage. The focus of this discussion is to introduce methodologies for monitoring apoptosis during Drosophila development and also in Drosophila cell lines.

Independent Proteolytic Activities Control the Stability and Size of Drosophila Inhibitor of Apoptosis 2 Protein

The Drosophila immune deficiency pathway defends many bacterial pathogens and bears striking molecular similarities to the mammalian tumor necrosis factor signal transduction pathway. Orthologous inhibitors of apoptosis ubiquitin ligases act at a proximal stage of both responses to coordinate the assembly of signal transduction platforms that shape host immune responses. Despite the importance of inhibitor of apoptosis proteins within evolutionarily conserved innate immune responses, we know relatively little about the cellular machinery that controls inhibitor of apoptosis activity. In this study, we examined the molecular basis for inhibitor of apoptosis 2 protein regulation in the immune deficiency pathway. Our studies identified two distinct proteolytic events that determine the stability and composition of cellular inhibitor of apoptosis 2 protein pools. We found that apoptotic caspase activity cleaves inhibitor of apoptosis 2 at an N-terminal aspartate to generate a truncated protein that retains the ability to interact with immune deficiency pathway members. We also showed that a C-terminal ubiquitin ligase activity within inhibitor of apoptosis 2 directs the proteasomal destruction of full-length and truncated inhibitor of apoptosis 2 isoforms. These studies add to our appreciation of the regulation of innate immunity and suggest potential links between apoptotic caspases and innate defenses.

The multifaceted activity of insect caspases

Caspases are frequently considered synonymous with apoptotic cell death. Increasing evidence demonstrates that these proteases may exert their activities in non-apoptotic functions. The non-apoptotic roles of caspases may include developmentally regulated autophagy during insect metamorphosis, as well as neuroblast self-renewal and the immune response. Here, we summarize the established knowledge and the recent advances in the multiple roles of insect caspases to highlight their relevance for physiological processes and survival.

Cell death in development: Signaling pathways and core mechanisms

Programmed cell death eliminates unneeded and dangerous cells in a timely and effective manner during development. In this review, we examine the role cell death plays during development in worms, flies and mammals. We discuss signaling pathways that regulate developmental cell death, and describe how they communicate with the core cell death pathways. In most organisms, the majority of developmental cell death is seen in the nervous system. Therefore we focus on what is known about the regulation of developmental cell death in this tissue. Understanding how the cell death is regulated during development may provide insight into how this process can be manipulated in the treatment of disease.

UTX coordinates steroid hormone-mediated autophagy and cell death

Correct spatial and temporal induction of numerous cell type-specific genes during development requires regulated removal of the repressive histone H3 lysine 27 trimethylation (H3K27me3) modification. Here we show that the H3K27me3 demethylase dUTX is required for hormone-mediated transcriptional regulation of apoptosis and autophagy genes during ecdysone-regulated programmed cell death of Drosophila salivary glands. We demonstrate that dUTX binds to the nuclear hormone receptor complex Ecdysone Receptor/Ultraspiracle, and is recruited to the promoters of key apoptosis and autophagy genes. Salivary gland cell death is delayed in dUTX mutants, with reduced caspase activity and autophagy that coincides with decreased apoptosis and autophagy gene transcripts. We further show that salivary gland degradation requires dUTX catalytic activity. Our findings provide evidence for an unanticipated role for UTX demethylase activity in regulating hormone-dependent cell death and demonstrate how a single transcriptional regulator can modulate a specific complex functional outcome during animal development.

Persephone/Spätzle pathogen sensors mediate the activation of Toll receptor signaling in response to endogenous danger signals in apoptosis-deficient Drosophila

Apoptosis is an evolutionarily conserved mechanism that removes damaged or unwanted cells, effectively maintaining cellular homeostasis. It has long been suggested that a deficiency in this type of naturally occurring cell death could potentially lead to necrosis, resulting in the release of endogenous immunogenic molecules such as damage-associated molecular patterns (DAMPs) and a noninfectious inflammatory response. However, the details about how danger signals from apoptosis-deficient cells are detected and translated to an immune response are largely unknown. In this study, we found that Drosophila mutants deficient for Dronc, the key initiator caspase required for apoptosis, produced the active form of the endogenous Toll ligand Spätzle (Spz). We speculated that, as a system for sensing potential DAMPs in the hemolymph, the dronc mutants constitutively activate a proteolytic cascade that leads to Spz proteolytic processing. We demonstrated that Toll signaling activation required the action of Persephone, a CLIP domain serine protease that usually reacts to microbial proteolytic activities. Our findings show that the Persephone proteolytic cascade plays a crucial role in mediating DAMP-induced systemic responses in apoptosis-deficient Drosophila mutants.

Genetic models of apoptosis-induced proliferation decipher activation of JNK and identify a requirement of EGFR signaling for tissue regenerative responses in Drosophila

Recent work in several model organisms has revealed that apoptotic cells are able to stimulate neighboring surviving cells to undergo additional proliferation, a phenomenon termed apoptosis-induced proliferation. This process depends critically on apoptotic caspases such as Dronc, the Caspase-9 ortholog in Drosophila, and may have important implications for tumorigenesis. While it is known that Dronc can induce the activity of Jun N-terminal kinase (JNK) for apoptosis-induced proliferation, the mechanistic details of this activation are largely unknown. It is also controversial if JNK activity occurs in dying or in surviving cells. Signaling molecules of the Wnt and BMP families have been implicated in apoptosis-induced proliferation, but it is unclear if they are the only ones. To address these questions, we have developed an efficient assay for screening and identification of genes that regulate or mediate apoptosis-induced proliferation. We have identified a subset of genes acting upstream of JNK activity including Rho1. We also demonstrate that JNK activation occurs both in apoptotic cells as well as in neighboring surviving cells. In a genetic screen, we identified signaling by the EGFR pathway as important for apoptosis-induced proliferation acting downstream of JNK signaling. These data underscore the importance of genetic screening and promise an improved understanding of the mechanisms of apoptosis-induced proliferation.

Work in recent years has revealed that apoptotic caspases not only induce apoptosis, but also have non-apoptotic functions. One of these functions is apoptosis-induced proliferation, a relatively recently discovered phenomenon by which apoptotic cells induce proliferation of surviving neighboring cells. This phenomenon may have important implications for stem cell activity, tissue regeneration and tumorigenesis. Here, we describe the development of a genetic model of apoptosis-induced proliferation and the use of this model for convenient and unbiased genetic screening to identify genes involved in the process. We tested mutants of our RNAi transgenic lines targeting the core components of the apoptotic pathway and of JNK signaling, a known mediator of apoptosis-induced proliferation. These assays demonstrate the feasibility of the system for systematic genetic screening and identified several new genes upstream of JNK that are involved in apoptosis-induced proliferation. Finally, we tested the model in a pilot screen for chromosome arm 2L and identified spi, the EGF ligand in flies, as important for apoptosis-induced proliferation. We confirmed the involvement of EGF in a genuine apoptosis-induced regeneration system. These data underscore the importance of genetic screening and promise an improved understanding of the mechanisms of apoptosis-induced proliferation and regeneration.

Apoptosome and inflammasome: conserved machineries for caspase activation

Apoptosome and inflammasome are multimeric protein complexes that mediate the activation of specific caspases at the onset of apoptosis and inflammation. The central component of apoptosome or inflammasome is a tripartite scaffold protein, exemplified by Apaf-1 and NLRC4, which contains an amino-terminal homotypic interaction motif, a central nucleotide-binding oligomerization domain and a carboxyl-terminal ligand-sensing domain. In the absence of death cue or an inflammatory signal, Apaf-1 or NLRC4 exists in an auto-inhibited, monomeric state, which is stabilized by adenosine diphosphate (ADP). Binding to an apoptosis- or inflammation-inducing ligand, together with replacement of ADP by adenosine triphosphate (ATP), results in the formation of a multimeric apoptosome or inflammasome. The assembled apoptosome and inflammasome serve as dedicated machineries to facilitate the activation of specific caspases. In this review, we describe the structure and functional mechanisms of mammalian inflammasome and apoptosomes from three representative organisms. Emphasis is placed on the molecular mechanism of caspase activation and the shared features of apoptosomes and inflammasomes.

Genetic control of specificity to steroid-triggered responses in Drosophila

Steroid hormones trigger a wide variety of biological responses through stage- and tissue-specific activation of target gene expression. The mechanisms that provide specificity to systemically released pulses of steroids, however, remain poorly understood. We previously completed a forward genetic screen for mutations that disrupt the destruction of larval salivary glands during metamorphosis in Drosophila melanogaster, a process triggered by the steroid hormone 20-hydroxyecdysone (ecdysone). Here, we characterize 10 complementation groups mapped to genes from this screen. Most of these mutations disrupt the ecdysone-induced expression of death activators, thereby failing to initiate tissue destruction. However, other responses to ecdysone, even within salivary glands, occur normally in mutant animals. Many of these newly identified regulators of ecdysone signaling, including brwd3, med12, med24, pak, and psg2, represent novel components of the ecdysone-triggered transcriptional hierarchy. These genes function combinatorially to provide specificity to ecdysone pulses, amplifying the hormonal cue in a stage-, tissue-, and target gene-specific manner. Most of the ecdysone response genes identified in this screen encode homologs of mammalian nuclear receptor coregulators, demonstrating an unexpected degree of functional conservation in the mechanisms that regulate steroid signaling between insects and mammals.