The Drosophila inhibitor of apoptosis protein DIAP2 functions in innate immunity and is essential to resist gram-negative bacterial infection

Genetic Screening Revealed the Negative Regulation of miR-310~313 Cluster Members on Imd Pathway during Gram-Negative Bacterial Infection in Drosophila

Innate immune response is the first line of host defense against pathogenic microorganisms, and its excessive or insufficient activation is detrimental to the organism. Many individual microRNAs (miRNAs) have emerged as crucial post-transcriptional regulators of immune homeostasis in Drosophila melanogaster. However, the synergistical regulation of miRNAs located within a cluster on the Imd-immune pathway remains obscured. In our study, a genetic screening with 52 transgenic UAS-miRNAs was performed to identify ten miRNAs or miRNA clusters, including the miR310~313 cluster, which may function on Imd-dependent immune responses. The miRNA RT-qPCR analysis showed that the expression of miR-310~313 cluster members exhibited an increase at 6-12 h post E. coli infection. Furthermore, the overexpression of the miR-310~313 cluster impaired the Drosophila survival. And the overexpression of miR-310/311/312 reduced Dpt expression, an indication of Imd pathway induced by Gram-negative bacteria. Conversely, the knockdown of miR-310/311/312 led to increases in Dpt expression. The Luciferase reporter expression assays and RT-qPCR analysis confirmed that miR-310~313 cluster members directly co-targeted and inhibited Imd transcription. These findings reveal that the members of the miR-310~313 cluster synergistically inhibit Imd-dependent immune responses by co-targeting the Imd gene in Drosophila.

Dysregulation of innate immune signaling in animal models of spinal muscular atrophy

BACKGROUND

Spinal muscular atrophy (SMA) is a devastating neuromuscular disease caused by hypomorphic loss of function in the survival motor neuron (SMN) protein. SMA presents across a broad spectrum of disease severity. Unfortunately, genetic models of intermediate SMA have been difficult to generate in vertebrates and are thus unable to address key aspects of disease etiology. To address these issues, we developed a Drosophila model system that recapitulates the full range of SMA severity, allowing studies of pre-onset biology as well as late-stage disease processes.

RESULTS

Here, we carried out transcriptomic and proteomic profiling of mild and intermediate Drosophila models of SMA to elucidate molecules and pathways that contribute to the disease. Using this approach, we elaborated a role for the SMN complex in the regulation of innate immune signaling. We find that mutation or tissue-specific depletion of SMN induces hyperactivation of the immune deficiency (IMD) and Toll pathways, leading to overexpression of antimicrobial peptides (AMPs) and ectopic formation of melanotic masses in the absence of an external challenge. Furthermore, the knockdown of downstream targets of these signaling pathways reduced melanotic mass formation caused by SMN loss. Importantly, we identify SMN as a negative regulator of a ubiquitylation complex that includes Traf6, Bendless, and Diap2 and plays a pivotal role in several signaling networks.

CONCLUSIONS

In alignment with recent research on other neurodegenerative diseases, these findings suggest that hyperactivation of innate immunity contributes to SMA pathology. This work not only provides compelling evidence that hyperactive innate immune signaling is a primary effect of SMN depletion, but it also suggests that the SMN complex plays a regulatory role in this process in vivo. In summary, immune dysfunction in SMA is a consequence of reduced SMN levels and is driven by cellular and molecular mechanisms that are conserved between insects and mammals.

Ubiquitin signalling in Drosophila innate immune responses

Cells respond to invading pathogens and danger signals from the environment by adapting gene expression to meet the need for protective effector molecules. While this innate immune response is required for the cell and the organism to recover, excess immune activation may lead to loss of homeostasis, thereby promoting chronic inflammation and cancer progression. The molecular basis of innate immune defence is comprised of factors promoting survival and proliferation, such as cytokines, antimicrobial peptides and anti-apoptotic proteins. As the molecular mechanisms regulating innate immune responses are conserved through evolution, the fruit fly Drosophila melanogaster serves as a convenient, affordable and ethical model organism to enhance understanding of immune signalling. Fly immunity against bacterial infection is built up by both cellular and humoral responses, where the latter is regulated by the Imd and Toll pathways activating NF-κB transcription factors Relish, Dorsal and Dif, as well as JNK activation and JAK/STAT signalling. As in mammals, the Drosophila innate immune signalling pathways are characterised by ubiquitination of signalling molecules followed by ubiquitin receptors binding to the ubiquitin chains, as well as by rapid changes in protein levels by ubiquitin-mediated targeted proteasomal and lysosomal degradation. In this review, we summarise the molecular signalling pathways regulating immune responses to pathogen infection in Drosophila, with a focus on ubiquitin-dependent control of innate immunity and inflammatory signalling.

The Relish/miR-275/Dredd mediated negative feedback loop is crucial to restoring immune homeostasis of Drosophila Imd pathway

The NF-κB/Relish, as a core transcription factor of Drosophila immune deficiency (Imd) pathway, activates the transcriptions of antimicrobial peptides (AMPs) to combat gram-negative bacterial infections, but its role in regulating miRNA expression during immune response has less been reported. We here describe a negative feedback loop of Imd signaling mediated by Relish/miR-275/Dredd that controls Drosophila immune homeostasis after Escherichia coli (E. coli) infection. Our results demonstrate that Relish may directly activate the transcription of miR-275 via binding to its promoter in vitro and vivo, particularly miR-275 further inhibits the expression of Dredd through binding to its 3'UTR to negatively control Drosophila Imd immune response. Remarkably, the ectopic expression of miR-275 significantly reduces Drosophila lifespan. More importantly, our work uncovers a new mechanism by which Relish can flexibly switch its role to maintain Drosophila immune response and homeostasis during infection. Collectively, our study not only reveals the functional duality of Relish in regulating immune response of Drosophila Imd pathway, but also provides a new insight into the maintenance of animal innate immune homeostasis.

Signaling events induced by lipopolysaccharide-activated Toll in response to bacterial infection in shrimp

Toll-like receptors (TLR) play a crucial role in the detection of microbial infections in vertebrates and invertebrates. Mammalian TLRs directly recognize a variety of structurally conserved microbial components. However, invertebrates such as Drosophila indirectly recognize microbial products by binding to the cytokine-like ligand Spätzle, which activates signaling cascades that are not completely understood. In this study, we investigated the signaling events triggered by Toll in response to lipopolysaccharide (LPS), a cell wall component of gram-negative bacteria, and Vibrio parahaemolyticus infection in the arthropod shrimp Litopenaeus vannamei. We found that five of the nine Tolls from L. vannamei bound to LPS and the RNAi of LvToll1, LvToll2, LvToll3, LvToll5, and LvToll9 weakened LvDorsal-L phosphorylation induced by V. parahaemolyticus. All nine Tolls combined with MyD88 via the TIR domain, thereby conferring signals to the tumor necrosis factor receptor-associated factor 6 (TRAF6)-transforming growth factor-β activated kinase 1 binding protein 2 (TAB2)-transforming growth factor-β activated kinase 1 (TAK1) complex. Further examination revealed that the LvTRAF6-LvTAB2-LvTAK1 complex contributes to Dorsal-L phosphorylation and nuclear translocation during V. parahaemolyticus infection. Overall, shrimp Toll1/2/3/5/9-TRAF6/TAB2/TAK1-Dorsal cascades protect the host from V. parahaemolyticus infection, which provides a better understanding of how the innate immune system recognizes and responds to bacterial infections in invertebrates.

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.

The mushroom body development and learning ability of adult honeybees are influenced by cold exposure during their early pupal stage

The honeybees are the most important pollinator in the production of crops and fresh produce. Temperature affects the survival of honeybees, and determines the quality of their development, which is of great significance for beekeeping production. Yet, little was known about how does low temperature stress during development stage cause bee death and any sub-lethal effect on subsequent. Early pupal stage is the most sensitive stage to low temperature in pupal stage. In this study, early pupal broods were exposed to 20°C for 12, 16, 24, and 48 h, followed by incubation at 35°C until emergence. We found that 48 h of low temperature duration cause 70% of individual bees to die. Although the mortality at 12 and 16 h seems not very high, the association learning ability of the surviving individuals was greatly affected. The brain slices of honeybees showed that low temperature treatment could cause the brain development of honeybees to almost stop. Gene expression profiles between low temperature treatment groups (T24, T48) and the control revealed that 1,267 and 1,174 genes were differentially expressed respectively. Functional enrichment analysis of differentially expressed genes showed that the differential expression of Map3k9, Dhrs4, and Sod-2 genes on MAPK and peroxisome signaling pathway caused oxidative damage to the honeybee head. On the FoxO signal pathway, InsR and FoxO were upregulated, and JNK, Akt, and Bsk were downregulated; and on the insect hormone synthesis signal pathway, Phm and Spo genes were downregulated. Therefore, we speculate that low temperature stress affects hormone regulation. It was detected that the pathways related to the nervous system were Cholinergic synapse, Dopaminergic synapse, GABAergic synapse, Glutamatergic synapse, Serotonergic synapse, Neurotrophin signaling pathway, and Synaptic vesicle cycle. This implies that the synaptic development of honeybees is quite possibly greatly affected by low temperature stress. Understanding how low temperature stress affects the physiology of bee brain development and how it affects bee behavior provide a theoretical foundation for a deeper comprehension of the temperature adaptation mechanism that underlies the "stenothermic" development of social insects, and help to improve honeybee management strategies to ensure the healthy of colony.

Acclimatory gene expression of primed clams enhances robustness to elevated pCO2

Sub-lethal exposure to environmental challenges may enhance ability to cope with chronic or repeated change, a process known as priming. In a previous study, pre-exposure to seawater enriched with pCO₂ improved growth and reduced antioxidant capacity of juvenile Pacific geoduck Panopea generosa, suggesting that transcriptional shifts may drive phenotypic modifications post-priming. To this end, juvenile clams were sampled and TagSeq gene expression data analyzed after 1) a 110-day acclimation under ambient (921 μatm, naïve) and moderately-elevated pCO₂ (2870 μatm, pre-exposed); then following 2) a second 7-day exposure to three pCO₂ treatments (ambient: 754 μatm; moderately-elevated: 2750 μatm; severely-elevated: 4940 μatm), a 7-day return to ambient pCO₂ , and a third 7-day exposure to two pCO₂ treatments (ambient: 967 μatm; moderately-elevated: 3030 μatm). Pre-exposed geoducks frontloaded genes for stress and apoptosis/innate immune response, homeostatic processes, protein degradation, and transcriptional modifiers. Pre-exposed geoducks were also responsive to subsequent encounters, with gene sets enriched for mitochondrial recycling and immune defense under elevated pCO₂ and energy metabolism and biosynthesis under ambient recovery. In contrast, gene sets with higher expression in naïve clams were enriched for fatty-acid degradation and glutathione components, suggesting naïve clams could be depleting endogenous fuels, with unsustainable energetic requirements if changes in carbonate chemistry persist. Collectively, our transcriptomic data indicates pCO₂ priming during post-larval periods could, via gene expression regulation, enhance robustness in bivalves to environmental change. Such priming approaches may be beneficial for aquaculture, as seafood demand intensifies concurrent with increasing climate change in marine systems.

Hemocyte Clusters Defined by scRNA-Seq in Bombyx mori: In Silico Analysis of Predicted Marker Genes and Implications for Potential Functional Roles

Within the hemolymph, insect hemocytes constitute a heterogeneous population of macrophage-like cells that play important roles in innate immunity, homeostasis and development. Classification of hemocytes in different subtypes by size, morphology and biochemical or immunological markers has been difficult and only in Drosophila extensive genetic analysis allowed the construction of a coherent picture of hemocyte differentiation from pro-hemocytes to granulocytes, crystal cells and plasmatocytes. However, the advent of high-throughput single cell technologies, such as single cell RNA sequencing (scRNA-seq), is bound to have a high impact on the study of hemocytes subtypes and their phenotypes in other insects for which a sophisticated genetic toolbox is not available. Instead of averaging gene expression across all cells as occurs in bulk-RNA-seq, scRNA-seq allows high-throughput and specific visualization of the differentiation status of individual cells. With scRNA-seq, interesting cell types can be identified in heterogeneous populations and direct analysis of rare cell types is possible. Next to its ability to profile the transcriptomes of individual cells in tissue samples, scRNA-seq can be used to propose marker genes that are characteristic of different hemocyte subtypes and predict their functions. In this perspective, the identities of the different marker genes that were identified by scRNA-seq analysis to define 13 distinct cell clusters of hemocytes in larvae of the silkworm, Bombyx mori, are discussed in detail. The analysis confirms the broad division of hemocytes in granulocytes, plasmatocytes, oenocytoids and perhaps spherulocytes but also reveals considerable complexity at the molecular level and highly specialized functions. In addition, predicted hemocyte marker genes in Bombyx generally show only limited convergence with the genes that are considered characteristic for hemocyte subtypes in Drosophila.

TAK1 confers antibacterial protection through mediating the activation of MAPK and NF-κB pathways in shrimp

MAPK and NF-κB pathways are important components of innate immune system in multicellular animals. In some model organisms, the MAP3-kinase TGF-beta-activated kinase 1 (TAK1) have been shown to regulate both MAPK and NF-κB pathways activation to tailor immune responses to pathogens or infections. However, this process is not fully understood in shrimp. In this study, we investigated the effect of TAK1 on MAPK and NF-κB activation in shrimp Litopenaeus vannamei following Vibrio parahaemolyticus infection. We found that shrimp TAK1 could activate c-Jun and Relish, the transcription factors of MAPK pathway and NF-κB pathway, respectively. Specifically, over-expression of shrimp TAK1 was able to strongly induce the activities of both AP-1 and NF-κB reporters. TAK1 was shown to bind several MAP2-kinases, including MKK4, MKK6 and MKK7, and induced their phosphorylations, the hallmarks for MAPK pathways activation. TAK1 knockdown in vivo also inhibited the nuclear translocation of c-Jun and Relish during V. parahaemolyticus infection. Accordingly, ectopic expression of shrimp TAK1 in Drosophila S2 cells increased the cleavage of co-expressed shrimp Relish, and induced the promoter activity of Relish targeted gene Diptericin (Dpt). Furthermore, knockdown of c-Jun and Relish enhanced the sensitivity of shrimp to V. parahaemolyticus infection. These findings indicated that shrimp TAK1 conferred antibacterial protection through regulating the activation of both MAPK pathway and NF-κB pathway, and suggested that the TAK1-MAPK/NF-κB axis could be a potential therapeutic target for enhancing antibacterial responses in crustaceans.

Sodium molybdate does not inhibit sulfate-reducing bacteria but increases shell growth in the Pacific oyster Magallana gigas

Recent work on microbe-host interactions has revealed an important nexus between the environment, microbiome, and host fitness. Marine invertebrates that build carbonate skeletons are of particular interest in this regard because of predicted effects of ocean acidification on calcified organisms, and the potential of microbes to buffer these impacts. Here we investigate the role of sulfate-reducing bacteria, a group well known to affect carbonate chemistry, in Pacific oyster (Magallana gigas) shell formation. We reared oyster larvae to 51 days post fertilization and exposed organisms to control and sodium molybdate conditions, the latter of which is thought to inhibit bacterial sulfate reduction. Contrary to expectations, we found that sodium molybdate did not uniformly inhibit sulfate-reducing bacteria in oysters, and oysters exposed to molybdate grew larger shells over the experimental period. Additionally, we show that microbiome composition, host gene expression, and shell size were distinct between treatments earlier in ontogeny, but became more similar by the end of the experiment. Although additional testing is required to fully elucidate the mechanisms, our work provides preliminary evidence that M. gigas is capable of regulating microbiome dysbiosis caused by environmental perturbations, which is reflected in shell development.

Drice restrains Diap2-mediated inflammatory signalling and intestinal inflammation

The Drosophila IAP protein, Diap2, is a key mediator of NF-κB signalling and innate immune responses. Diap2 is required for both local immune activation, taking place in the epithelial cells of the gut and trachea, and for mounting systemic immune responses in the cells of the fat body. We have found that transgenic expression of Diap2 leads to a spontaneous induction of NF-κB target genes, inducing chronic inflammation in the Drosophila midgut, but not in the fat body. Drice is a Drosophila effector caspase known to interact and form a stable complex with Diap2. We have found that this complex formation induces its subsequent degradation, thereby regulating the amount of Diap2 driving NF-κB signalling in the intestine. Concordantly, loss of Drice activity leads to accumulation of Diap2 and to chronic intestinal inflammation. Interestingly, Drice does not interfere with pathogen-induced signalling, suggesting that it protects from immune responses induced by resident microbes. Accordingly, no inflammation was detected in transgenic Diap2 flies and Drice-mutant flies reared in axenic conditions. Hence, we show that Drice, by restraining Diap2, halts unwanted inflammatory signalling in the intestine.

Protein Phosphatase 4 Negatively Regulates the Immune Deficiency-NF-κB Pathway during the Drosophila Immune Response

The evolutionarily conserved immune deficiency (IMD) signaling pathway shields Drosophila against bacterial infections. It regulates the expression of antimicrobial peptides encoding genes through the activation of the NF-κB transcription factor Relish. Tight regulation of the signaling cascade ensures a balanced immune response, which is otherwise highly harmful. Several phosphorylation events mediate intracellular progression of the IMD pathway. However, signal termination by dephosphorylation remains largely elusive. Here, we identify the highly conserved protein phosphatase 4 (PP4) complex as a bona fide negative regulator of the IMD pathway. RNA interference-mediated gene silencing of PP4-19c, PP4R2, and Falafel, which encode the catalytic and regulatory subunits of the phosphatase complex, respectively, caused a marked upregulation of bacterial-induced antimicrobial peptide gene expression in both Drosophila melanogaster S2 cells and adult flies. Deregulated IMD signaling is associated with reduced lifespan of PP4-deficient flies in the absence of any infection. In contrast, flies overexpressing this phosphatase are highly sensitive to bacterial infections. Altogether, our results highlight an evolutionarily conserved function of PP4c in the regulation of NF-κB signaling from Drosophila to mammals.

Verloren negatively regulates the expression of IMD pathway dependent antimicrobial peptides in Drosophila

Drosophila immune deficiency (IMD) pathway is similar to the human tumor necrosis factor receptor (TNFR) signaling pathway and is preferentially activated by Gram-negative bacterial infection. Recent studies highlighted the importance of IMD pathway regulation as it is tightly controlled by numbers of negative regulators at multiple levels. Here, we report a new negative regulator of the IMD pathway, Verloren (Velo). Silencing of Velo led to constitutive expression of the IMD pathway dependent antimicrobial peptides (AMPs), and Escherichia coli stimulation further enhanced the AMP expression. Epistatic analysis indicated that Velo knock-down mediated AMP upregulation is dependent on the canonical members of the IMD pathway. The immune fluorescent study using overexpression constructs revealed that Velo resides both in the nucleus and cytoplasm, but the majority (~ 75%) is localized in the nucleus. We also observed from in vivo analysis that Velo knock-down flies exhibit significant upregulation of the AMP expression and reduced bacterial load. Survival experiments showed that Velo knock-down flies have a short lifespan and are susceptible to the infection of pathogenic Gram-negative bacteria, P. aeruginosa. Taken together, these data suggest that Velo is an additional new negative regulator of the IMD pathway, possibly acting in both the nucleus and cytoplasm.

Tick Immune System: What Is Known, the Interconnections, the Gaps, and the Challenges

Ticks are ectoparasitic arthropods that necessarily feed on the blood of their vertebrate hosts. The success of blood acquisition depends on the pharmacological properties of tick saliva, which is injected into the host during tick feeding. Saliva is also used as a vehicle by several types of pathogens to be transmitted to the host, making ticks versatile vectors of several diseases for humans and other animals. When a tick feeds on an infected host, the pathogen reaches the gut of the tick and must migrate to its salivary glands via hemolymph to be successfully transmitted to a subsequent host during the next stage of feeding. In addition, some pathogens can colonize the ovaries of the tick and be transovarially transmitted to progeny. The tick immune system, as well as the immune system of other invertebrates, is more rudimentary than the immune system of vertebrates, presenting only innate immune responses. Although simpler, the large number of tick species evidences the efficiency of their immune system. The factors of their immune system act in each tick organ that interacts with pathogens; therefore, these factors are potential targets for the development of new strategies for the control of ticks and tick-borne diseases. The objective of this review is to present the prevailing knowledge on the tick immune system and to discuss the challenges of studying tick immunity, especially regarding the gaps and interconnections. To this end, we use a comparative approach of the tick immune system with the immune system of other invertebrates, focusing on various components of humoral and cellular immunity, such as signaling pathways, antimicrobial peptides, redox metabolism, complement-like molecules and regulated cell death. In addition, the role of tick microbiota in vector competence is also discussed.

IAPs Gene Expansion in the Scallop Patinopecten yessoensis and Their Expression Profiles After Exposure to the Toxic Dinoflagellate

Inhibitors of apoptosis proteins (IAPs) are conserved regulators involved in cell cycle, cell migration, cell death, immunity and inflammation, should be due to the fact that they can assist with the ability to cope with different kinds of extrinsic or intrinsic stresses. Bivalve molluscs are well adapted to highly complex marine environments. As free-living filter feeders that may take toxic dinoflagellates as food, bivalves can accumulate and put up with significant levels of paralytic shellfish toxins (PSTs). PSTs absorption and accumulation could have a deleterious effect on bivalves, causing negative impact on their feeding and digestion capabilities. In the present study, we analyzed IAP genes (PyIAPs) in Yesso scallop (Patinopecten yessoensis), a major fishery and aquaculture species in China. Forty-seven PyIAPs from five sub-families were identified, and almost half of the PyIAP genes were localized in clusters on two chromosomes. Several sites under positive selection was revealed in the significantly expanded sub-families BIRC4 and BIRC5. After exposure to PST-producing dinoflagellates, Alexandrium catenella, fourteen PyIAPs showed significant responses in hepatopancreas and kidney, and more than eighty-five percent of them were from the expanded sub-families BIRC4 and BIRC5. The regulation pattern of PyIAPs was similar between the two tissues, with more than half exhibited expression suppression within three days after exposure. In contrast to hepatopancreas, more acute changes of PyIAPs expression could be detected in kidney, suggesting the possible involvement of these PyIAPs in tissue-specific PST tolerance. These findings also imply the adaptive expansion of bivalve IAP genes in response to algae derived biotoxins.

Transferrin-mediated iron sequestration suggests a novel therapeutic strategy for controlling Nosema disease in the honey bee, Apis mellifera

Nosemosis C, a Nosema disease caused by microsporidia parasite Nosema ceranae, is a significant disease burden of the European honey bee Apis mellifera which is one of the most economically important insect pollinators. Nevertheless, there is no effective treatment currently available for Nosema disease and the disease mechanisms underlying the pathological effects of N. ceranae infection in honey bees are poorly understood. Iron is an essential nutrient for growth and survival of hosts and pathogens alike. The iron tug-of-war between host and pathogen is a central battlefield at the host-pathogen interface which determines the outcome of an infection, however, has not been explored in honey bees. To fill the gap, we conducted a study to investigate the impact of N. ceranae infection on iron homeostasis in honey bees. The expression of transferrin, an iron binding and transporting protein that is one of the key players of iron homeostasis, in response to N. ceranae infection was analysed. Furthermore, the functional roles of transferrin in iron homeostasis and honey bee host immunity were characterized using an RNA interference (RNAi)-based method. The results showed that N. ceranae infection causes iron deficiency and upregulation of the A. mellifera transferrin (AmTsf) mRNA in honey bees, implying that higher expression of AmTsf allows N. ceranae to scavenge more iron from the host for its proliferation and survival. The suppressed expression levels of AmTsf via RNAi could lead to reduced N. ceranae transcription activity, alleviated iron loss, enhanced immunity, and improved survival of the infected bees. The intriguing multifunctionality of transferrin illustrated in this study is a significant contribution to the existing body of literature concerning iron homeostasis in insects. The uncovered functional role of transferrin on iron homeostasis, pathogen growth and honey bee's ability to mount immune responses may hold the key for the development of novel strategies to treat or prevent diseases in honey bees.

Toxicity of Dithiothreitol (DTT) to Drosophila melanogaster

The thiol-containing compound Dithiothreitol (DTT) has been shown to be toxic to cultured cells by inducing the generation of reactive oxygen species that ultimately cause cell death. However, its effects on multicellular organisms and the environment have not been investigated yet in detail. In this work, we tested the toxicity of DTT to the model insect Drosophila melanogaster. We show that DTT is lethal to D. melanogaster by topical application but not through feeding. DTT treatment triggers the transcription of the canonical apoptosis regulators grim, hid and rpr at low amounts. The amplitude of this induction declines with elevating DTT amounts. By live microscopy, we observe apoptotic cells especially in the gut of DTT treated flies. In parallel, low DTT amounts also activate the expression of the cuticle barrier component gene snsl. This indicates that a physical defence response is launched upon DTT contact. This combined measure is seemingly successful in preventing fly death. The expression of a number of known detoxification genes including cyp6a2, cyp6a8, cyp12d1 and GstD2 is also enhanced through DTT contact. The degree of upregulation of these genes is proportional to the applied DTT amounts. Despite this effort, flies exposed to high amounts of DTT eventually die. Together, D. melanogaster is able to sense DTT toxicity and adjust the defence response successfully at least at low concentrations. This is the first time to analyse the molecular consequences of DTT exposure in a multicellular organism. Our work provides a new model to discuss the physiological response of animals against thiol toxins and to resurvey the effect of redox agents on the environment.

Non-apoptotic caspase activation preserves Drosophila intestinal progenitor cells in quiescence

Caspase malfunction in stem cells often precedes the appearance and progression of multiple types of cancer, including human colorectal cancer. However, the caspase-dependent regulation of intestinal stem cell properties remains poorly understood. Here, we demonstrate that Dronc, the Drosophila ortholog of caspase-9/2 in mammals, limits the number of intestinal progenitor cells and their entry into the enterocyte differentiation programme. Strikingly, these unexpected roles for Dronc are non-apoptotic and have been uncovered under experimental conditions without epithelial replenishment. Supporting the non-apoptotic nature of these functions, we show that they require the enzymatic activity of Dronc, but are largely independent of the apoptotic pathway. Alternatively, our genetic and functional data suggest that they are linked to the caspase-mediated regulation of Notch signalling. Our findings provide novel insights into the non-apoptotic, caspase-dependent modulation of stem cell properties that could improve our understanding of the origin of intestinal malignancies.

Interplay between Fungal Infection and Bacterial Associates in the Wax Moth Galleria mellonella under Different Temperature Conditions

Various insect bacterial associates are involved in pathogeneses caused by entomopathogenic fungi. The outcome of infection (fungal growth or decomposition) may depend on environmental factors such as temperature. The aim of this study was to analyze the bacterial communities and immune response of Galleria mellonella larvae injected with Cordyceps militaris and incubated at 15 °C and 25 °C. We examined changes in the bacterial CFUs, bacterial communities (Illumina MiSeq 16S rRNA gene sequencing) and expression of immune, apoptosis, ROS and stress-related genes (qPCR) in larval tissues in response to fungal infection at the mentioned temperatures. Increased survival of larvae after C. militaris injection was observed at 25 °C, although more frequent episodes of spontaneous bacteriosis were observed at this temperature compared to 15 °C. We revealed an increase in the abundance of enterococci and enterobacteria in the midgut and hemolymph in response to infection at 25 °C, which was not observed at 15 °C. Antifungal peptide genes showed the highest expression at 25 °C, while antibacterial peptides and inhibitor of apoptosis genes were strongly expressed at 15 °C. Cultivable bacteria significantly suppressed the growth of C. militaris. We suggest that fungi such as C. militaris may need low temperatures to avoid competition with host bacterial associates.

Drosophila Myc restores immune homeostasis of Imd pathway via activating miR-277 to inhibit imd/Tab2

Drosophila Myc (dMyc), as a broad-spectrum transcription factor, can regulate the expression of a large number of genes to control diverse cellular processes, such as cell cycle progression, cell growth, proliferation and apoptosis. However, it remains largely unknown about whether dMyc can be involved in Drosophila innate immune response. Here, we have identified dMyc to be a negative regulator of Drosophila Imd pathway via the loss- and gain-of-function screening. We demonstrate that dMyc inhibits Drosophila Imd immune response via directly activating miR-277 transcription, which further inhibit the expression of imd and Tab2-Ra/b. Importantly, dMyc can improve the survival of flies upon infection, suggesting inhibiting Drosophila Imd pathway by dMyc is vital to restore immune homeostasis that is essential for survival. Taken together, our study not only reports a new dMyc-miR-277-imd/Tab2 axis involved in the negative regulation of Drosophila Imd pathway, and provides a new insight into the complex regulatory mechanism of Drosophila innate immune homeostasis maintenance.

ER-localized Hrd1 ubiquitinates and inactivates Usp15 to promote TLR4-induced inflammation during bacterial infection

The special organelle-located MAVS, STING and TLR3 are important for clearing viral infections. Although TLR4 triggers NF-κB activation to produce pro-inflammatory cytokines for bacterial clearance, effectors with special organelle localization have not been identified. Here, we screened more than 280 E3 ubiquitin ligases and discovered that the endoplasmic reticulum-located Hrd1 regulates TLR4-induced inflammation during bacterial infection. Hrd1 interacts directly with the deubiquitinating enzyme Usp15. Unlike the classical function of Hrd1 in endoplasmic reticulum-associated degradation, Usp15 is not degraded but loses its deubiquitinating activity for IκBα deubiquitination, resulting in excessive NF-κB activation. Importantly, Hrd1 deficiency in macrophages protects mice against lipopolysaccharide-induced septic shock, and knockdown of Usp15 in Hrd1-knockout macrophages restores the reduced IL-6 production. This study proposes that there is crosstalk between Hrd1 and TLR4, thereby linking the endoplasmic reticulum-plasma membrane function during bacterial infection.

Inhibitor of apoptosis-1 gene as a potential target for pest control and its involvement in immune regulation during fungal infection

BACKGROUND

Environmentally friendly insect management technologies, including RNA interference (RNAi) and entomopathogenic fungi, have attracted increasing attention as options for pest control. Here, we sought to extend RNAi-directed targeting of the inhibitor of apoptosis protein 1 (IAP1) gene to the locust, and to examine its relationship to immune responses and susceptibility to Metarhizium acridum, a locust-specific fungal pathogen.

RESULTS

Expression of the locust LmIAP gene was induced in the hemolymph and fat body after M. acridum infection. RNAi-directed silencing of locust LmIAP1 resulted in increased caspase 3 activity, degeneration of the gut and dose-dependent mortality. Synergistic mortality was seen in RNAi-LmIAP/fungal co-infection experiments with median survival time (MST) values decreasing from ∼ 5 days for RNAi and M. acridum treatments alone, to 2.6 days for co-treatments. Reduced hemocyte numbers and antimicrobial peptide levels were seen in co-treated locusts, with changes in gut opportunistic pathogenic bacteria seen between treatments. Enhanced fungal sporulation on co-treated insect cadavers was also compared with fungal infection alone.

CONCLUSIONS

Silencing of the locust LmIAP1 gene results in direct mortality and increases insect susceptibility to insect fungal pathogens, in part by decreasing immunity and altering the gut microbiome. © 2019 Society of Chemical Industry.

No Evidence of Apoptotic Response of the Potato Psyllid Bactericera cockerelli to "Candidatus Liberibacter solanacearum" at the Gut Interface

"Candidatus Liberibacter solanacearum" is a pathogen transmitted by the potato psyllid Bactericera cockerelli (Šulc) (Hemiptera: Triozidae) in a persistent manner. In this study, we investigated the molecular interaction between "Ca. Liberibacter solanacearum" and the potato psyllid at the gut interface. Specifically, we focused on the apoptotic response of potato psyllids to the infection by two "Ca. Liberibacter solanacearum" haplotypes, LsoA and LsoB. To this end, we first quantified and localized "Ca. Liberibacter solanacearum" in the gut of adult psyllids. We then evaluated the existence of an apoptotic response in the insect gut using microscopy analyses to visualize the nuclei and the actin cytoskeleton of the gut cells and DNA fragmentation analyses by agarose gel electrophoresis. We also performed annexin V cell death assays to detect apoptosis. Finally, we annotated apoptosis-related genes from the potato psyllid transcriptome and evaluated their expression in response to "Ca. Liberibacter solanacearum" infection. The results showed no cellular markers of apoptosis despite the large amount of "Ca. Liberibacter solanacearum" present in the psyllid gut. In addition, only three genes potentially involved in apoptosis were regulated in the psyllid gut in response to "Ca. Liberibacter solanacearum": the apoptosis-inducing factor AIF3 was downregulated in LsoA-infected psyllids, while the inhibitor of apoptosis IAPP5 was downregulated and IAP6 was upregulated in LsoB-infected psyllids. Overall, no evidence of apoptosis was observed in the gut of potato psyllid adults in response to either "Ca. Liberibacter solanacearum" haplotype. This study represents a first step toward understanding the interactions between "Ca. Liberibacter solanacearum" and the potato psyllid, which is crucial to developing approaches to disrupt their transmission.

Protective effects of Phaseolus vulgaris lectin against viral infection in Drosophila

Phytohemagglutinin (PHA) isolated from the family of Phaseolus vulgaris beans is a promising agent against viral infection; however, it has not yet been demonstrated in vivo. We herein investigated this issue using Drosophila as a host. Adult flies were fed lectin approximately 12 h before they were subjected to a systemic viral infection. After a fatal infection with Drosophila C virus, death was delayed and survival was longer in flies fed PHA-P, a mixture of L4, L3E1, and L2E2, than in control unfed flies. We then examined PHA-L4, anticipating subunit L as the active form, and confirmed the protective effects of this lectin at markedly lower concentrations than PHA-P. In both experiments, lectin feeding reduced the viral load prior to the onset of fly death. Furthermore, we found a dramatic increase in the levels of the mRNAs of phagocytosis receptors in flies after feeding with PHA-L4 while a change in the levels of the mRNAs of antimicrobial peptides was marginal. We concluded that P. vulgaris PHA protects Drosophila against viral infection by augmenting the level of host immunity.

Recent advances in understanding inhibitor of apoptosis proteins

The inhibitor of apoptosis proteins (IAPs) are a family of proteins that were chiefly known for their ability to inhibit apoptosis by blocking caspase activation or activity. Recent research has shown that cellular IAP1 (cIAP1), cIAP2, and X-linked IAP (XIAP) also regulate signaling by receptors of the innate immune system by ubiquitylating their substrates. These IAPs thereby act at the intersection of pathways leading to cell death and inflammation. Mutation of IAP genes can impair tissue homeostasis and is linked to several human diseases. Small-molecule IAP antagonists have been developed to treat certain malignant, infectious, and inflammatory diseases. Here, we will discuss recent advances in our understanding of the functions of cIAP1, cIAP2, and XIAP; the consequences of their mutation or dysregulation; and the therapeutic potential of IAP antagonist drugs.

The NF-κB Factor Relish Regulates Atg1 Expression and Controls Autophagy

Macroautophagy and cell death both contribute to innate immunity, but little is known about how these processes integrate. Drosophila larval salivary glands require autophagy for developmentally programmed cell death, and innate immune signaling factors increase in these dying cells. Here, we show that the nuclear factor κB (NF-κB) factor Relish, a component of the immune deficiency (Imd) pathway, is required for salivary gland degradation. Surprisingly, of the classic Imd pathway components, only Relish and the PGRP receptors were involved in salivary gland degradation. Significantly, Relish controls salivary gland degradation by regulating autophagy but not caspases. In addition, expression of either Relish or PGRP-LC causes premature autophagy induction and subsequent gland degradation. Relish controls autophagy by regulating the expression of Atg1, a core component and activator of the autophagy pathway. Together these findings demonstrate that a NF-κB pathway regulates autophagy during developmentally programmed cell death.

M1-linked ubiquitination by LUBEL is required for inflammatory responses to oral infection in Drosophila

Post-translational modifications such as ubiquitination play a key role in regulation of inflammatory nuclear factor-κB (NF-κB) signalling. The Drosophila IκB kinase γ (IKKγ) Kenny is a central regulator of the Drosophila Imd pathway responsible for activation of the NF-κB Relish. We found the Drosophila E3 ligase and HOIL-1L interacting protein (HOIP) orthologue linear ubiquitin E3 ligase (LUBEL) to catalyse formation of M1-linked linear ubiquitin (M1-Ub) chains in flies in a signal-dependent manner upon bacterial infection. Upon activation of the Imd pathway, LUBEL modifies Kenny with M1-Ub chains. Interestingly, the LUBEL-mediated M1-Ub chains seem to be targeted both directly to Kenny and to K63-linked ubiquitin chains conjugated to Kenny by DIAP2. This suggests that DIAP2 and LUBEL work together to promote Kenny-mediated activation of Relish. We found LUBEL-mediated M1-Ub chain formation to be required for flies to survive oral infection with Gram-negative bacteria, for activation of Relish-mediated expression of antimicrobial peptide genes and for pathogen clearance during oral infection. Interestingly, LUBEL is not required for mounting an immune response against systemic infection, as Relish-mediated antimicrobial peptide genes can be expressed in the absence of LUBEL during septic injury. Finally, transgenic induction of LUBEL-mediated M1-Ub drives expression of antimicrobial peptide genes and hyperplasia in the midgut in the absence of infection. This suggests that M1-Ub chains are important for Imd signalling and immune responses in the intestinal epithelia, and that enhanced M1-Ub chain formation is able to drive chronic intestinal inflammation in flies.

Beyond immunity: The Imd pathway as a coordinator of host defense, organismal physiology and behavior

The humoral arm of host defense in Drosophila relies on two evolutionarily conserved NFκB signaling cascades, the Toll and the immune deficiency (Imd) pathways. The Imd signaling pathway senses and neutralizes Gram-negative bacteria. Its activity is tightly adjusted, allowing the host to simultaneously prevent infection by pathogenic bacteria and tolerate beneficial gut microbiota. Over-activation of Imd signaling is detrimental at least in part by causing gut dysbiosis that further exacerbates intestinal pathologies. Furthermore, it is increasingly recognized that the Imd pathway or its components also play non-immune roles. In this review, we summarize recent advances in Imd signal transduction, discuss the gut-microbiota interactions mediated by Imd signaling, and finally elaborate on its diverse physiological functions beyond immunity. Understanding the multifaceted physiological outputs of Imd activation will help integrate its immune role into the regulation of whole organismal physiology.

Transcriptomic responses of the endangered freshwater mussel Margaritifera margaritifera to trace metal contamination in the Dronne River, France

The freshwater pearl mussel Margaritifera margaritifera is one of the most threatened freshwater bivalves worldwide. In this study, we aimed (i) to study the processes by which water quality might affect freshwater mussels in situ and (ii) to provide insights into the ecotoxicological significance of water pollution to natural populations in order to provide necessary information to enhance conservation strategies. M. margaritifera specimens were sampled in two close sites located upstream or downstream from an illegal dumping site. The renal transcriptome of these animals was assembled and gene transcription determined by RNA-seq. Correlations between transcription levels of each single transcript and the bioaccumulation of nine trace metals, age (estimated by sclerochronology), and condition index were determined in order to identify genes likely to respond to a specific factor. Amongst the studied metals, Cr, Zn, Cd, and Ni were the main factors correlated with transcription levels, with effects on translation, apoptosis, immune response, response to stimulus, and transport pathways. However, the main factor explaining changes in gene transcription appeared to be the age of individuals with a negative correlation with the transcription of retrotransposon-related genes. To investigate this effect further, mussels were classified into three age classes. In young, middle-aged and old animals, transcription levels were mainly explained by Cu, Zn and age, respectively. This suggests differences in the molecular responses of this species to metals during its lifetime that must be better assessed in future ecotoxicology studies.

Signal Integration by the IκB Protein Pickle Shapes Drosophila Innate Host Defense

Pattern recognition receptors are activated following infection and trigger transcriptional programs important for host defense. Tight regulation of NF-κB activation is critical to avoid detrimental and misbalanced responses. We describe Pickle, a Drosophila nuclear IκB that integrates signaling inputs from both the Imd and Toll pathways by skewing the transcriptional output of the NF-κB dimer repertoire. Pickle interacts with the NF-κB protein Relish and the histone deacetylase dHDAC1, selectively repressing Relish homodimers while leaving other NF-κB dimer combinations unscathed. Pickle's ability to selectively inhibit Relish homodimer activity contributes to proper host immunity and organismal health. Although loss of pickle results in hyper-induction of Relish target genes and improved host resistance to pathogenic bacteria in the short term, chronic inactivation of pickle causes loss of immune tolerance and shortened lifespan. Pickle therefore allows balanced immune responses that protect from pathogenic microbes while permitting the establishment of beneficial commensal host-microbe relationships.

Innate immune signaling in Drosophila is regulated by transforming growth factor β (TGFβ)-activated kinase (Tak1)-triggered ubiquitin editing

Coordinated regulation of innate immune responses is necessary in all metazoans. In Drosophila the Imd pathway detects Gram-negative bacterial infections through recognition of diaminopimelic acid (DAP)-type peptidoglycan and activation of the NF-κB precursor Relish, which drives robust antimicrobial peptide gene expression. Imd is a receptor-proximal adaptor protein homologous to mammalian RIP1 that is regulated by proteolytic cleavage and Lys-63-polyubiquitination. However, the precise events and molecular mechanisms that control the post-translational modification of Imd remain unclear. Here, we demonstrate that Imd is rapidly Lys-63-polyubiquitinated at lysine residues 137 and 153 by the sequential action of two E2 enzymes, Ubc5 and Ubc13-Uev1a, in conjunction with the E3 ligase Diap2. Lys-63-ubiquitination activates the TGFβ-activated kinase (Tak1), which feeds back to phosphorylate Imd, triggering the removal of Lys-63 chains and the addition of Lys-48 polyubiquitin. This ubiquitin-editing process results in the proteasomal degradation of Imd, which we propose functions to restore homeostasis to the Drosophila immune response.

Inhibition of a NF-κB/Diap1 Pathway by PGRP-LF Is Required for Proper Apoptosis during Drosophila Development

NF-κB pathways are key signaling cascades of the Drosophila innate immune response. One of them, the Immune Deficiency (IMD) pathway, is under a very tight negative control. Although molecular brakes exist at each step of this signaling module from ligand availability to transcriptional regulation, it remains unknown whether repressors act in the same cells or tissues and if not, what is rationale behind this spatial specificity. We show here that the negative regulator of IMD pathway PGRP-LF is epressed in ectodermal derivatives. We provide evidence that, in the absence of any immune elicitor, PGRP-LF loss-of-function mutants, display a constitutive NF-κB/IMD activation specifically in ectodermal tissues leading to genitalia and tergite malformations. In agreement with previous data showing that proper development of these structures requires induction of apoptosis, we show that ectopic activation of NF-κB/IMD signaling leads to apoptosis inhibition in both genitalia and tergite primordia. We demonstrate that NF-κB/IMD signaling antagonizes apoptosis by up-regulating expression of the anti-apoptotic protein Diap1. Altogether these results show that, in the complete absence of infection, the negative regulation of NF-κB/IMD pathway by PGRP-LF is crucial to ensure proper induction of apoptosis and consequently normal fly development. These results highlight that IMD pathway regulation is controlled independently in different tissues, probably reflecting the different roles of this signaling cascade in both developmental and immune processes.

In multicellular organism such as mammals or insects, activation of innate immune responses occurs following detection of microbes by dedicated receptors called pattern recognition receptors. Such immune activation is taking place in immune competent tissue such as the skin, the digestive and respiratory epithelia and is under a tight negative control. Negative control is essential to finely adjust the duration and the intensity of the immune response to the level of infection. We found that the Drosophila innate immunity negative regulator PGRP-LF, is specifically expressed in non-immune tissues and plays an essential role during development, in absence of any infection. Lack of PGRP-LF function in these tissues inhibits apoptosis leading to incomplete genitalia rotation and tergite malformations. We show that such apoptosis inhibition results from the over expression of the negative regulator of apoptosis Diap1 specifically in PGRP-LF expressing cells. Our data highlight that proper negative regulation of immune signaling pathway in non-immune tissues is contributing to normal development and illustrate the growing evidence of the dual role of immune signaling pathway contribution to both immunity and in development processes.

Targeting Inhibitor of Apoptosis Proteins Protects from Bleomycin-Induced Lung Fibrosis

Accumulation of apoptosis-resistant fibroblasts is a hallmark of pulmonary fibrosis. We hypothesized that disruption of inhibitor of apoptosis protein (IAP) family proteins would limit lung fibrosis. We first show that transforming growth factor-β1 and bleomycin increase X-linked IAP (XIAP) and cellular IAP (cIAP)-1 and -2 in murine lungs and mesenchymal cells. Functional blockade of XIAP and the cIAPs with AT-406, an orally bioavailable second mitochondria-derived activator of caspases (Smac) mimetic, abrogated bleomycin-induced lung fibrosis when given both prophylactically and therapeutically. To determine whether the reduction in fibrosis was predominantly due to AT-406-mediated inhibition of XIAP, we compared the fibrotic response of XIAP-deficient mice (XIAP(-/y)) with littermate controls and found no difference. We found no alterations in total inflammatory cells of either wild-type mice treated with AT-406 or XIAP(-/y) mice. AT-406 treatment limited CCL12 and IFN-γ production, whereas XIAP(-/y) mice exhibited increased IL-1β expression. Surprisingly, XIAP(-/y) mesenchymal cells had increased resistance to Fas-mediated apoptosis. Functional blockade of cIAPs with AT-406 restored sensitivity to Fas-mediated apoptosis in XIAP(-/y) mesenchymal cells in vitro and increased apoptosis of mesenchymal cells in vivo, indicating that the increased apoptosis resistance in XIAP(-/y) mesenchymal cells was the result of increased cIAP expression. Collectively, these results indicate that: (1) IAPs have a role in the pathogenesis of lung fibrosis; (2) a congenital deficiency of XIAP may be overcome by compensatory mechanisms of other IAPs; and (3) broad functional inhibition of IAPs may be an effective strategy for the treatment of lung fibrosis by promoting mesenchymal cell apoptosis.

The regulatory isoform rPGRP-LC induces immune resolution via endosomal degradation of receptors

The innate immune system needs to distinguish between harmful and innocuous stimuli to adapt its activation to the level of threat. How Drosophila mounts differential immune responses to dead and live Gram-negative bacteria using the single peptidoglycan receptor PGRP-LC is unknown. Here we describe rPGRP-LC, an alternative splice variant of PGRP-LC that selectively dampens immune response activation in response to dead bacteria. rPGRP-LC-deficient flies cannot resolve immune activation after Gram-negative infection and die prematurely. The alternative exon in the encoding gene, here called rPGRP-LC, encodes an adaptor module that targets rPGRP-LC to membrane microdomains and interacts with the negative regulator Pirk and the ubiquitin ligase DIAP2. We find that rPGRP-LC-mediated resolution of an efficient immune response requires degradation of activating and regulatory receptors via endosomal ESCRT sorting. We propose that rPGRP-LC selectively responds to peptidoglycans from dead bacteria to tailor the immune response to the level of threat.

Role of Penaeus monodon hemocyte homeostasis associated protein (PmHHAP) in regulation of caspase-mediated apoptosis

The viral responsive protein, PmHHAP, plays an important role in the control of hemocyte homeostasis in shrimps during viral infection. In this study, we further investigate the role of PmHHAP in the regulation of hemocyte apoptosis. RNA interference (RNAi) mediated gene silencing was used to suppress the PmHHAP expression and the change in hemocyte apoptosis was determined in the knockdown shrimp. Within circulating hemocytes, PmHHAP knockdown increased the number of annexin V-positive apoptotic cells and the combined caspase-3/-7 activity and induced the characteristic apoptotic DNA ladder. Furthermore, PmHHAP down-regulation was accompanied by significantly altered expression of apoptosis-related proteins including the effector caspases, PmCaspase and PmCasp. Yeast two-hybrid and co-immunoprecipitation assays showed that PmHHAP binds to the p20 domain of PmCasp. Moreover, the recombinant PmHHAP protein was able to reduce the caspase activity in the actinomycin D-treated hemocyte cells and rPmCasp-treated hemocyte cells. Taken together, our data indicate that PmHHAP regulates hemocyte homeostasis by inhibits apoptotic cell death through caspase activation.

Characterization of the effect of Cr(VI) on humoral innate immunity using Drosophila melanogaster

With the advancement of human race, different anthropogenic activities have heaped the environment with chemicals that can cause alteration in the immune system of exposed organism. As a first line of barrier, the evolutionary conserved innate immunity is crucial for the health of an organism. However, there is paucity of information regarding in vivo assessment of the effect of environmental chemicals on innate immunity. Therefore, we examined the effect of a widely used environmental chemical, Cr(VI), on humoral innate immune response using Drosophila melanogaster. The adverse effect of Cr(VI) on host humoral response was characterized by decreased gene expression of antimicrobial peptides (AMPs) in the exposed organism. Concurrently, a significantly decreased transcription of humoral pathway receptors (Toll and PGRP) and triglyceride level along with inhibition of antioxidant enzyme activities were observed in exposed organism. This in turn weakened the immune response of exposed organism that was manifested by their reduced resistance against bacterial infection. In addition, overexpression of the components of humoral immunity particularly Diptericin benefits Drosophila from Cr(VI)-induced humoral immune-suppressive effect. To our knowledge, this is the first report regarding negative impact of an environmental chemical on humoral innate immune response of Drosophila along with subsequent protection by AMPs, which may provide novel insight into host-chemical interactions. Also, our data validate the utility and sensitivity of Drosophila as a model that could be used for screening the possible risk of environmental chemicals on innate immunity with minimum ethical concern that can be further extrapolated to higher organisms.

The Role of the Phylogenetically Conserved Cochaperone Protein Droj2/DNAJA3 in NF-κB Signaling

The NF-κB pathway is a phylogenetically conserved signaling pathway with a central role in inflammatory and immune responses. Here we demonstrate that a cochaperone protein, Droj2/DNAJA3, is involved in the activation of canonical NF-κB signaling in flies and in human cultured cells. Overexpression of Droj2 induced the expression of an antimicrobial peptide in Drosophila. Conversely, Droj2 knockdown resulted in reduced expression of antimicrobial peptides and higher susceptibility to Gram-negative bacterial infection in flies. Similarly, Toll-like receptor-stimulated IκB phosphorylation and NF-κB activation were suppressed by DNAJA3 knockdown in HEK293 cells. IκB kinase overexpression-induced NF-κB phosphorylation was also compromised in DNAJA3 knockdown cells. Our study reveals a novel conserved regulator of the NF-κB pathway acting at the level of IκB phosphorylation.

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.

Niche signaling promotes stem cell survival in the Drosophila testis via the JAK-STAT target DIAP1

Tissue-specific stem cells are thought to resist environmental insults better than their differentiating progeny, but this resistance varies from one tissue to another, and the underlying mechanisms are not well-understood. Here, we use the Drosophila testis as a model system to study the regulation of cell death within an intact niche. This niche contains sperm-producing germline stem cells (GSCs) and accompanying somatic cyst stem cells (or CySCs). Although many signals are known to promote stem cell self-renewal in this tissue, including the highly conserved JAK-STAT pathway, the response of these stem cells to potential death-inducing signals, and factors promoting stem cell survival, have not been characterized. Here we find that both GSCs and CySCs resist cell death better than their differentiating progeny, under normal laboratory conditions and in response to potential death-inducing stimuli such as irradiation or starvation. To ask what might be promoting stem cell survival, we characterized the role of the anti-apoptotic gene Drosophila inhibitor of apoptosis 1 (diap1) in testis stem cells. DIAP1 protein is enriched in the GSCs and CySCs and is a JAK-STAT target. diap1 is necessary for survival of both GSCs and CySCs, and ectopic up-regulation of DIAP1 in somatic cyst cells is sufficient to non-autonomously rescue stress-induced cell death in adjacent differentiating germ cells (spermatogonia). Altogether, our results show that niche signals can promote stem cell survival by up-regulation of highly conserved anti-apoptotic proteins, and suggest that this strategy may underlie the ability of stem cells to resist death more generally.

Plasmodium falciparum evades mosquito immunity by disrupting JNK-mediated apoptosis of invaded midgut cells

The malaria parasite, Plasmodium, must survive and develop in the mosquito vector to be successfully transmitted to a new host. The Plasmodium falciparum Pfs47 gene is critical for malaria transmission. Parasites that express Pfs47 (NF54 WT) evade mosquito immunity and survive, whereas Pfs47 knockouts (KO) are efficiently eliminated by the complement-like system. Two alternative approaches were used to investigate the mechanism of action of Pfs47 on immune evasion. First, we examined whether Pfs47 affected signal transduction pathways mediating mosquito immune responses, and show that the Jun-N-terminal kinase (JNK) pathway is a key mediator of Anopheles gambiae antiplasmodial responses to P. falciparum infection and that Pfs47 disrupts JNK signaling. Second, we used microarrays to compare the global transcriptional responses of A. gambiae midguts to infection with WT and KO parasites. The presence of Pfs47 results in broad and profound changes in gene expression in response to infection that are already evident 12 h postfeeding, but become most prominent at 26 h postfeeding, the time when ookinetes invade the mosquito midgut. Silencing of 15 differentially expressed candidate genes identified caspase-S2 as a key effector of Plasmodium elimination in parasites lacking Pfs47. We provide experimental evidence that JNK pathway regulates activation of caspases in Plasmodium-invaded midgut cells, and that caspase activation is required to trigger midgut epithelial nitration. Pfs47 alters the cell death pathway of invaded midgut cells by disrupting JNK signaling and prevents the activation of several caspases, resulting in an ineffective nitration response that makes the parasite undetectable by the mosquito complement-like system.

Epithelial immune response in Drosophila malpighian tubules: interplay between Diap2 and ion channels

Systemic immune response via the Immune deficiency pathway requires Drosophila inhibitor of apoptosis protein 2 to activate the NF-κB transcription factor Relish. Malpighian tubules (MTs), simple epithelial tissue, are the primary excretory organs, performing additional role in providing protection to Drosophila against pathogenic infections. MTs hold a strategic position in Drosophila as one of the larval tissues that are carried over to adults, unlike other larval tissues that are histolysed during pupation. In this paper we show that Diap2 is an important regulator of local epithelial immune response in MTs and depletion of Diap2 from MTs, increases susceptibility of flies to infection. In the absence of Diap2, activation and translocation of Relish to the nucleus is abolished and as a consequence the production of IMD pathway dependent AMPs are reduced. Ion channels, (Na(+)/K(+))-ATPase and V-ATPase, are important for the immune response of MTs and expression of AMPs and the IMD pathway genes are impaired on inhibition of transporters, and they restrict the translocation of Relish into the nucleus. We show that Diap2 could be regulating ion channels, as loss of Diap2 consequently reduces the expression of ion channels and affects the balance of ion concentrations which results in reduced uric acid deposition. Thus Diap2 seems to be a key regulator of epithelial immune response in MTs, perhaps by modulating ion channels.

The Drosophila IMD pathway in the activation of the humoral immune response

The IMD pathway signaling plays a pivotal role in the Drosophila defense against bacteria. During the last two decades, significant progress has been made in identifying the components and deciphering the molecular mechanisms underlying this pathway, including the means of bacterial sensing and signal transduction. While these findings have contributed to the understanding of the immune signaling in insects, they have also provided new insights in studying the mammalian NF-κB signaling pathways. Here, we summarize the current view of the IMD pathway focusing on how it regulates the humoral immune response of Drosophila.

Analysis of expression, cellular localization, and function of three inhibitors of apoptosis (IAPs) from Litopenaeus vannamei during WSSV infection and in regulation of antimicrobial peptide genes (AMPs)

Inhibitors of apoptosis (IAPs) play important roles in apoptosis and NF-κB activation. In this study, we cloned and characterized three IAPs (LvIAP1-3) from the Pacific white shrimp, Litopenaeusvannamei. LvIAP1-3 proteins shared signature domains and exhibited significant similarities with other IAP family proteins. The tissue distributions of LvIAP1-3 were studied. The expression of LvIAP1-3 was induced in the muscle after white spot syndrome virus (WSSV) infection. LvIAP1 expression in the gill, hemocytes, hepatopancreas, and intestine was responsive to WSSV and Vibrioalginolyticus infections. LvIAP2 expression in the gill, hemocytes, and hepatopancreas was also responsive to WSSV infection. The expression of LvIAP3 in the gill, hemocytes, and intestine was reduced after V. alginolyticus infection. When overexpressed in Drosophila S2 cells, GFP labeled-LvIAP2 was distributed in the cytoplasm and appeared as speck-like aggregates in the nucleus. Both LvIAP1 and LvIAP3 were widely distributed throughout the cytoplasm and nucleus. The expression of LvIAP1, LvIAP2, and LvIAP3 was significantly knocked down by dsRNA-mediated gene silencing. In the gill of LvIAP1- or LvIAP3-silenced shrimp, the expression of WSSV VP28 was significantly higher than that of the dsGFP control group, suggesting that LvIAP1 and LvIAP3 may play protective roles in host defense against WSSV infection. Intriguingly, the LvIAP2-silenced shrimp all died within 48 hours after dsLvIAP2 injection. In the hemocytes of LvIAP2-silenced shrimps, the expression of antimicrobial peptide genes (AMPs), including Penaeidins, lysozyme, crustins, Vibriopenaeicidae-induced cysteine and proline-rich peptides (VICPs), was significantly downregulated, while the expression of anti-lipopolysaccharide factors (ALFs) was upregulated. Moreover, LvIAP2 activated the promoters of the NF-κB pathway-controlled AMPs, such as shrimp Penaeidins and Drosophila drosomycin and attacin A, in Drosophila S2 cells. Taken together, these results reveal that LvIAP1 and LvIAP3 might participate in the host defense against WSSV infection, and LvIAP2 might be involved in the regulation of shrimp AMPs.