Down-regulation of NF-kappaB target genes by the AP-1 and STAT complex during the innate immune response in Drosophila

How the Structure of Signaling Regulation Evolves: Insights From an Evolutionary Model

To monitor environmental changes, signaling pathways attenuate their activity with negative feedback loops (NFLs), where proteins produced upon stimulation downregulate the response. NFLs function both upstream of signaling to reduce input and downstream to reduce output. Unlike upstream NFLs, downstream NFLs regulate gene expression without the involvement of intermediate proteins. Thus, we hypothesized that downstream NFLs evolve under more stringent selection than upstream NFLs. Indeed, genes encoding downstream NFLs evolve at a slower and more consistent rate than upstream genes, suggesting that the latter may be under weaker or more context-specific selection. This suggests that downstream NFLs evolve more robustly, whereas upstream NFLs are more susceptible to changes in signaling proteins and stimuli. We tested these assumptions using a minimal model of immune signaling, which predicts robust evolution of downstream NFLs to changes in model parameters. This is consistent with their critical role in regulating signaling and the conservative rate of evolution. Furthermore, we show that the number of signaling steps needed to activate a downstream NFL is influenced by the cost of signaling. Our model predicts that upstream NFLs are more likely to evolve under a shorter half-life of signaling proteins, absence of host-pathogen co-evolution, and a high infection rate. Although it has been proposed that NFLs evolve to reduce the cost of signaling, we show that a high cost does not necessarily predict the evolution of upstream NFLs. The insights from our model have broad implications for understanding the evolution of regulatory mechanisms across signaling pathways.

How the Structure of Signaling Regulation Evolves: Insights from an Evolutionary Model

To remain responsive to environmental changes, signaling pathways attenuate their activity with negative feedback loops (NFLs), where proteins produced upon stimulation downregulate the response. NFLs function both upstream of signaling to reduce input and downstream to reduce output. Unlike upstream NFLs, downstream NFLs directly regulate gene expression without the involvement of intermediate proteins. Thus, we hypothesized that downstream NFLs evolve under more stringent selection than upstream NFLs. Indeed, genes encoding downstream NFLs exhibit a slower evolutionary rate than upstream genes. Such differences in selective pressures could result in the robust evolution of downstream NFLs while making the evolution of upstream NFLs more sensitive to changes in signaling proteins and stimuli. Here, we test these assumptions within the context of immune signaling. Our minimal model of immune signaling predicts robust evolution of downstream NFLs to changes in model parameters. This is consistent with their critical role in regulating signaling and the conservative rate of evolution. Furthermore, we show that the number of signaling steps needed to activate a downstream NFL is influenced by the cost of signaling. Our model predicts that upstream NFLs are more likely to evolve under a shorter half-life of signaling proteins, absence of host-pathogen co-evolution, and a high infection rate. Although it has been proposed that NFLs evolve to reduce the cost of signaling, we show that a high cost does not necessarily predict the evolution of upstream NFLs. The insights from our model have broad implications for understanding the evolution of regulatory mechanisms across signaling pathways.

The role of uncertainty and negative feedback loops in the evolution of induced immune defenses

Organisms use constitutive or induced defenses against pathogens and other external threats. Constitutive defenses are constantly on, whereas induced defenses are activated when needed. Each of these strategies has costs and benefits, which can affect the type of defense that evolves in response to pathogens. In addition, induced defenses are usually regulated by multiple negative feedback mechanisms that prevent overactivation of the immune response. However, it is unclear how negative feedback affects the costs, benefits, and evolution of induced responses. To address this gap, we developed a mechanistic model of the well-characterized Drosophila melanogaster immune signaling network that includes 3 separate mechanisms of negative feedback as a representative of the widespread phenomenon of multilevel regulation of induced responses. We show that, under stochastic fly-bacteria encounters, an induced defense is favored when bacterial encounters are rare or uncertain, but in ways that depend on the bacterial proliferation rate. Our model also predicts that the specific negative regulators that optimize the induced response depend on the bacterial proliferation rate, linking negative feedback mechanisms to the factors that favor induction.

Relish-mediated C2H2 zinc finger protein IMZF restores Drosophila immune homeostasis via inhibiting the transcription of Imd/Tak1

The dysregulation of intensity and duration in innate immunity can result in detrimental effects on the body, emphasizing the crucial need for precise regulation. However, the intricate and accurate nature of innate immunity implies the existence of numerous undiscovered innate immunomodulators, particularly transcription factors. In this study, we have identified a Drosophila C2H2 zinc finger protein CG18262, named Immune-mediated Zinc Finger protein (IMZF), capable of suppressing immune responses of Imd pathway. Mechanistically, IMZF serves as a transcription factor that represses the expression of Imd and Tak1. Intriguingly, our findings also reveal that Relish, an NF-κB transcription factor, positively regulates the expression of IMZF, consequently inhibiting the activation of Imd and Tak1 to prevent an exaggerated immune response. Additionally, we have elucidated the pivotal role played by the Relish-IMZF-Imd/Tak1 axis in restoring immune homeostasis of Drosophila Imd pathway. In summary, our findings not only unveil a novel C2H2 zinc finger immunoregulatory transcription factor, IMZF, along with its specific mechanism of immune regulation, but also shed light on the dual functionality of Relish in different stages of the immune response by modulating distinct effectors. This discovery provides new insights and enlightenment into the complex regulation of Drosophila innate immunity.

Evaluating the Impact of Vitamin D3 on NF-κB and JAK/STAT Signaling Pathways in Drosophila melanogaster

This study delved into the consequences of prolonged administration of vitamin D3 on innate immune systems, particularly NF-κB and JAK/STAT, in Drosophila melanogaster. The outcomes indicated that vitamin D3 treatment exhibited a notable capacity to improve the survival of adult flies with compromised immune functions, a condition induced by the loss of PGRP-LB, particularly when the flies were exposed to heat-killed Escherichia coli. The PGRP-LBΔ mutant line that was treated with heat-killed E. coli experienced reduced survival. Treatment of heat-killed E. coli-treated PGRP-LBΔ with vitamin D3 resulted in improved survival, and this phenotypic feature might be due to the downregulation of gene expression in the NF-κB and JAK/STAT pathways. However, a higher concentration of vitamin D3 was associated with decreased survival, potentially linked to intricate immunological responses. The research also underscored the influence of vitamin D3 on the expression of antioxidant genes, sod1 and sod2, indicating an augmented resistance to oxidative stress. Further, this study revealed the effect of vitamin D3 on the reproductive status of the autoinflammatory model, showing an increase in pupae and adult flies with a treatment of 10 mM vitamin D3, suggesting the potential benefits of vitamin D3 on the reproductive profile. Overall, this study provides preliminary insights into the complex interactions between vitamin D3, immune pathways, oxidative responses in the cell, and reproduction in Drosophila.

Relish-facilitated lncRNA-CR11538 suppresses Drosophila Imd immune response and maintains immune homeostasis via decoying Relish away from antimicrobial peptide promoters

Innate immunity plays a crucial role in host defense against pathogen invasion and its strength and duration requires precise control. Long non-coding RNAs (lncRNAs) have become important regulators of innate immunity, yet their roles in Drosophila immune responses remain largely unknown. In this study, we identified that the overexpression of lncRNA-CR11538 inhibits the expression of antimicrobial peptides (AMPs) Dpt and AttA in Drosophila upon Escherichia coli (E. coli) infection, and influences the survival rate of flies after E. cloacae infection. Mechanically, lncRNA-CR11538 decoys Relish away from AMPs promoter region. We further revealed that Relish can promote the transcription of lncRNA-CR11538. After analyzing the dynamic expression profile of lncRNA-CR11538 during Imd immune response, we put forward a hypothesis that in the late stage of Imd immune response, lncRNA-CR11538 can be activated by Relish and further decoy Relish away from the AMPs promoter to suppress excessive immune signal and maintain immune homeostasis. This mechanism we proposed provides insights into the complex regulatory networks controlling immune responses in Drosophila and suggests potential targets for therapeutic intervention in diseases involving dysregulated immune responses.

Atg2 Regulates Cellular and Humoral Immunity in Drosophila

Autophagy is a process that promotes the lysosomal degradation of cytoplasmic proteins and is highly conserved in eukaryotic organisms. Autophagy maintains homeostasis in organisms and regulates multiple developmental processes, and autophagy disruption is related to human diseases. However, the functional roles of autophagy in mediating innate immune responses are largely unknown. In this study, we sought to understand how Atg2, an autophagy-related gene, functions in the innate immunity of Drosophila melanogaster. The results showed that a large number of melanotic nodules were produced upon inhibition of Atg2. In addition, inhibiting Atg2 suppressed the phagocytosis of latex beads, Staphylococcus aureus and Escherichia coli; the proportion of Nimrod C1 (one of the phagocytosis receptors)-positive hemocytes also decreased. Moreover, inhibiting Atg2 altered actin cytoskeleton patterns, showing longer filopodia but with decreased numbers of filopodia. The expression of AMP-encoding genes was altered by inhibiting Atg2. Drosomycin was upregulated, and the transcript levels of Attacin-A, Diptericin and Metchnikowin were decreased. Finally, the above alterations caused by the inhibition of Atg2 prevented flies from resisting invading pathogens, showing that flies with low expression of Atg2 were highly susceptible to Staphylococcus aureus and Erwinia carotovora carotovora 15 infections. In conclusion, Atg2 regulated both cellular and humoral innate immunity in Drosophila. We have identified Atg2 as a crucial regulator in mediating the homeostasis of immunity, which further established the interactions between autophagy and innate immunity.

Heuristic-enabled active machine learning: A case study of predicting essential developmental stage and immune response genes in Drosophila melanogaster

Computational prediction of absolute essential genes using machine learning has gained wide attention in recent years. However, essential genes are mostly conditional and not absolute. Experimental techniques provide a reliable approach of identifying conditionally essential genes; however, experimental methods are laborious, time and resource consuming, hence computational techniques have been used to complement the experimental methods. Computational techniques such as supervised machine learning, or flux balance analysis are grossly limited due to the unavailability of required data for training the model or simulating the conditions for gene essentiality. This study developed a heuristic-enabled active machine learning method based on a light gradient boosting model to predict essential immune response and embryonic developmental genes in Drosophila melanogaster. We proposed a new sampling selection technique and introduced a heuristic function which replaces the human component in traditional active learning models. The heuristic function dynamically selects the unlabelled samples to improve the performance of the classifier in the next iteration. Testing the proposed model with four benchmark datasets, the proposed model showed superior performance when compared to traditional active learning models (random sampling and uncertainty sampling). Applying the model to identify conditionally essential genes, four novel essential immune response genes and a list of 48 novel genes that are essential in embryonic developmental condition were identified. We performed functional enrichment analysis of the predicted genes to elucidate their biological processes and the result evidence our predictions. Immune response and embryonic development related processes were significantly enriched in the essential immune response and embryonic developmental genes, respectively. Finally, we propose the predicted essential genes for future experimental studies and use of the developed tool accessible at http://heal.covenantuniversity.edu.ng for conditional essentiality predictions.

Drosophila as a Model Organism to Study Basic Mechanisms of Longevity

The spatio-temporal regulation of gene expression determines the fate and function of various cells and tissues and, as a consequence, the correct development and functioning of complex organisms. Certain mechanisms of gene activity regulation provide adequate cell responses to changes in environmental factors. Aside from gene expression disorders that lead to various pathologies, alterations of expression of particular genes were shown to significantly decrease or increase the lifespan in a wide range of organisms from yeast to human. Drosophila fruit fly is an ideal model system to explore mechanisms of longevity and aging due to low cost, easy handling and maintenance, large number of progeny per adult, short life cycle and lifespan, relatively low number of paralogous genes, high evolutionary conservation of epigenetic mechanisms and signalling pathways, and availability of a wide range of tools to modulate gene expression in vivo. Here, we focus on the organization of the evolutionarily conserved signaling pathways whose components significantly influence the aging process and on the interconnections of these pathways with gene expression regulation.

Dynamic Regulation of NF-κB Response in Innate Immunity: The Case of the IMD Pathway in Drosophila

Metazoans have developed strategies to protect themselves from pathogenic attack. These preserved mechanisms constitute the immune system, composed of innate and adaptive responses. Among the two kinds, the innate immune system involves the activation of a fast response. NF-κB signaling pathways are activated during infections and lead to the expression of timely-controlled immune response genes. However, activation of NF-κB pathways can be deleterious when uncontrolled. Their regulation is necessary to prevent the development of inflammatory diseases or cancers. The similarity of the NF-κB pathways mediating immune mechanisms in insects and mammals makes Drosophila melanogaster a suitable model for studying the innate immune response and learning general mechanisms that are also relevant for humans. In this review, we summarize what is known about the dynamic regulation of the central NF-κB-pathways and go into detail on the molecular level of the IMD pathway. We report on the role of the nuclear protein Akirin in the regulation of the NF-κB Relish immune response. The use of the Drosophila model allows the understanding of the fine-tuned regulation of this central NF-κB pathway.

Mutations of γCOP Gene Disturb Drosophila melanogaster Innate Immune Response to Pseudomonas aeruginosa

Drosophila melanogaster (the fruit fly) is a valuable experimental platform for modeling host–pathogen interactions. It is also commonly used to define innate immunity pathways and to understand the mechanisms of both host tolerance to commensal microbiota and response to pathogenic agents. Herein, we investigate how the host response to bacterial infection is mirrored in the expression of genes of Imd and Toll pathways when D. melanogaster strains with different γCOP genetic backgrounds are infected with Pseudomonas aeruginosa ATCC 27853. Using microarray technology, we have interrogated the whole-body transcriptome of infected versus uninfected fruit fly males with three specific genotypes, namely wild-type Oregon, γCOP^S057302/TM6B and γCOP^14a/γCOP^14a. While the expression of genes pertaining to Imd and Toll is not significantly modulated by P. aeruginosa infection in Oregon males, many of the components of these cascades are up- or downregulated in both infected and uninfected γCOP^S057302/TM6B and γCOP^14a/γCOP^14a males. Thus, our results suggest that a γCOP genetic background modulates the gene expression profiles of Imd and Toll cascades involved in the innate immune response of D. melanogaster, inducing the occurrence of immunological dysfunctions in γCOP mutants.

Expression of chronic myeloid leukemia oncogenes BCR-ABL P210 and BCR-ABL T315I affect cellular and humoral innate immunity in Drosophila melanogaster

Chronic myeloid leukemia (CML) is a myeloproliferative neoplasm that results from a chromosomal translocation between chromosome 9 and chromosome 22. The resulting fusion gene ( BCR-ABL ) encodes a constitutively active BCR-ABL tyrosine kinase. Some mutations of this oncogene, especially the Threonine 315 to Isoleucine substitution of the ABL kinase is resistant to first and second-generation tyrosine kinase inhibitors (TKIs) conventionally used in CML therapy. We have previously validated a CML fruit fly model for drug screening using the adult fly compound eye. Here we expressed wild-type BCR-ABL P210 and mutated BCR-ABL T315I in Drosophila melanogaster hematopoietic system to understand the phenotypic consequences of this expression and its impact on innate immune pathways. Flies expressing both wild-type BCR-ABL P210 and mutant BCR-ABL T315I showed increased number of circulating hemocytes, disruption in sessile patterning of resident hemocytes, dysregulation in the humoral Toll, ImD, and JAK/STAT pathways at the mRNA level in both the 3 rd instar larva and adult stages. Of note, BCR-ABL T315I flies presented more severe phenotypes and a higher deviation in humoral dysregulation than BCR -ABL P210 flies pointing towards more complex oncogenic effect of this mutant which requires further investigation.

SUMOylation of Jun fine-tunes the Drosophila gut immune response

Post-translational modification by the small ubiquitin-like modifier, SUMO can modulate the activity of its conjugated proteins in a plethora of cellular contexts. The effect of SUMO conjugation of proteins during an immune response is poorly understood in Drosophila. We have previously identified that the transcription factor Jra, the Drosophila Jun ortholog and a member of the AP-1 complex is one such SUMO target. Here, we find that Jra is a regulator of the Pseudomonas entomophila induced gut immune gene regulatory network, modulating the expression of a few thousand genes, as measured by quantitative RNA sequencing. Decrease in Jra in gut enterocytes is protective, suggesting that reduction of Jra signaling favors the host over the pathogen. In Jra, lysines 29 and 190 are SUMO conjugation targets, with the Jra^K29R+K190R double mutant being SUMO conjugation resistant (SCR). Interestingly, a Jra^SCR fly line, generated by CRISPR/Cas9 based genome editing, is more sensitive to infection, with adults showing a weakened host response and increased proliferation of Pseudomonas. Transcriptome analysis of the guts of Jra^SCR and Jra^WT flies suggests that lack of SUMOylation of Jra significantly changes core elements of the immune gene regulatory network, which include antimicrobial agents, secreted ligands, feedback regulators, and transcription factors. Mechanistically, SUMOylation attenuates Jra activity, with the TFs, forkhead, anterior open, activating transcription factor 3 and the master immune regulator Relish being important transcriptional targets. Our study implicates Jra as a major immune regulator, with dynamic SUMO conjugation/deconjugation of Jra modulating the kinetics of the gut immune response.

The intestine has a resident population of commensal microorganisms against which the immune machinery is tuned to show low or no reactivity. In contrast, when pathogenic microorganisms are ingested, the gut responds by activating signaling cascades that lead to the killing and clearance of the pathogen. In this study, we examine the role played by the well-known transcription factor Jun in regulating the immune response in the Drosophila gut. We find that loss of Jun leads to the change in intensity and kinetics of the gut immune transcriptome. The transcriptional profile indicates a stronger response when Jun activity is reduced. Also, animals infected with Pseudomonas entomophila live longer when Jun signaling is reduced. Further, we find that Jun is post-translationally modified on Lys29 and Lys190 by SUMO. To understand the effect of SUMO-conjugation of Jun, we create by state-of-the-art CRISPR/Cas9 genome editing a Drosophila line where Jun is resistant to SUMOylation. This line is more sensitive to infection, with a weaker host-defense response. Our data suggest that Jun Signaling favors the pathogen by dampening the immune response. SUMO conjugation of Jun reverses the dampening and strengthens the immune response in favor of the host. Dynamic SUMOylation of Jun thus fine-tunes the gut immune response to pathogens.

Chromosome-Level Genome Assembly Reveals Significant Gene Expansion in the Toll and IMD Signaling Pathways of Dendrolimus kikuchii

A high-quality genome is of significant value when seeking to control forest pests such as Dendrolimus kikuchii, a destructive member of the order Lepidoptera that is widespread in China. Herein, a high quality, chromosome-level reference genome for D. kikuchii based on Nanopore, Pacbio HiFi sequencing and the Hi-C capture system is presented. Overall, a final genome assembly of 705.51 Mb with contig and scaffold N50 values of 20.89 and 24.73 Mb, respectively, was obtained. Of these contigs, 95.89% had unique locations on 29 chromosomes. In silico analysis revealed that the genome contained 15,323 protein-coding genes and 63.44% repetitive sequences. Phylogenetic analyses indicated that D. kikuchii may diverged from the common ancestor of Thaumetopoea. Pityocampa, Thaumetopoea ni, Heliothis virescens, Hyphantria armigera, Spodoptera frugiperda, and Spodoptera litura approximately 122.05 million years ago. Many gene families were expanded in the D. kikuchii genome, particularly those of the Toll and IMD signaling pathway, which included 10 genes in peptidoglycan recognition protein, 19 genes in MODSP, and 11 genes in Toll. The findings from this study will help to elucidate the mechanisms involved in protection of D. kikuchii against foreign substances and pathogens, and may highlight a potential channel to control this pest.

Drosophila as a model to explore secondary injury cascades after traumatic brain injury

Drosophilae are emerging as a valuable model to study traumatic brain injury (TBI)-induced secondary injury cascades that drive persisting neuroinflammation and neurodegenerative pathology that imposes significant risk for long-term neurological deficits. As in mammals, TBI in Drosophila triggers axonal injury, metabolic crisis, oxidative stress, and a robust innate immune response. Subsequent neurodegeneration stresses quality control systems and perpetuates an environment for neuroprotection, regeneration, and delayed cell death via highly conserved cell signaling pathways. Fly injury models continue to be developed and validated for both whole-body and head-specific injury to isolate, evaluate, and modulate these parallel pathways. In conjunction with powerful genetic tools, the ability for longitudinal evaluation, and associated neurological deficits that can be tested with established behavioral tasks, Drosophilae are an attractive model to explore secondary injury cascades and therapeutic intervention after TBI. Here, we review similarities and differences between mammalian and fly pathophysiology and highlight strategies for their use in translational neurotrauma research.

JNK Signaling in Drosophila Aging and Longevity

The evolutionarily conserved c-Jun N-terminal kinase (JNK) signaling pathway is a critical genetic determinant in the control of longevity. In response to extrinsic and intrinsic stresses, JNK signaling is activated to protect cells from stress damage and promote survival. In Drosophila, global JNK upregulation can delay aging and extend lifespan, whereas tissue/organ-specific manipulation of JNK signaling impacts lifespan in a context-dependent manner. In this review, focusing on several tissues/organs that are highly associated with age-related diseases-including metabolic organs (intestine and fat body), neurons, and muscles-we summarize the distinct effects of tissue/organ-specific JNK signaling on aging and lifespan. We also highlight recent progress in elucidating the molecular mechanisms underlying the tissue-specific effects of JNK activity. Together, these studies highlight an important and comprehensive role for JNK signaling in the regulation of longevity in Drosophila.

Assessment of deltamethrin toxicity in Macrobrachium nipponense based on histopathology, oxidative stress and immunity damage

Deltamethrin (Del), a commonly used broad-spectrum insecticide, has been reported to have a toxic effect on aquatic animals, but knowledge in freshwater prawns is limited. This study revealed that Del is highly toxic to Macrobrachium nipponens with the 24 h, 48 h, 72 h, and 96 h LC₅₀ values to be 0.268, 0.165, 0.104, and 0.066 μg/L, respectively. To further investigate the toxic effect of Del in M. nipponense and the reversibility of damage, prawns were exposed to 0.05 μg/L Del for four days and then transferred into fresh water for seven days. Histopathological examination, oxidative stress, hepatopancreas function, respiration system, and immune system were analyzed through multiple biomarkers. Results showed that Del exposure caused severe histopathological damage to hepatopancreas and gill in M. nipponense, and the prominent decrease of acid phosphatase (ACP) and alkaline phosphatase (AKP) activity further enhanced the hepatopancreas damage; the accumulation of malonaldehyde (MDA) and hydrogen peroxide (H₂O₂), and the decrease of superoxide dismutase (SOD) and glutathione peroxidase (GSH-Px) activity, indicated severe oxidative stress caused by Del. Besides, Del exposure also induced remarkably increased lactic acid (LD) level, decreased lactate dehydrogenase (LDH) activity, and decreased expression of immune-related genes, which demonstrated the respiration disruption and immunosuppression caused by Del. After 7-day decontamination in freshwater, the indicator of hepatopancreas function (ACP and AKP activity) and respiration (LD level and LDH activity) improved to the control group level. However, the histopathological damage and the biomarker in oxidative stress and immune system did not recover to the initial level.

Microenvironmental innate immune signaling and cell mechanical responses promote tumor growth

Tissue homeostasis is achieved by balancing stem cell maintenance, cell proliferation and differentiation, as well as the purging of damaged cells. Elimination of unfit cells maintains tissue health; however, the underlying mechanisms driving competitive growth when homeostasis fails, for example, during tumorigenesis, remain largely unresolved. Here, using a Drosophila intestinal model, we find that tumor cells outcompete nearby enterocytes (ECs) by influencing cell adhesion and contractility. This process relies on activating the immune-responsive Relish/NF-κB pathway to induce EC delamination and requires a JNK-dependent transcriptional upregulation of the peptidoglycan recognition protein PGRP-LA. Consequently, in organisms with impaired PGRP-LA function, tumor growth is delayed and lifespan extended. Our study identifies a non-cell-autonomous role for a JNK/PGRP-LA/Relish signaling axis in mediating death of neighboring normal cells to facilitate tumor growth. We propose that intestinal tumors "hijack" innate immune signaling to eliminate enterocytes in order to support their own growth.

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.

Immunopathology and immune homeostasis during viral infection in insects

Organisms clear infections by mounting an immune response that is normally turned off once the pathogens have been cleared. However, sometimes this immune response is not properly or timely arrested, resulting in the host damaging itself. This immune dysregulation may be referred to as immunopathology. While our knowledge of immune and metabolic pathways in insects, particularly in response to viral infections, is growing, little is known about the mechanisms that regulate this immune response and hence little is known about immunopathology in this important and diverse group of organisms. In this chapter we focus both on documenting the molecular mechanisms described involved in restoring immune homeostasis in insects after viral infections and on identifying potential mechanisms for future investigation. We argue that learning about the immunopathological consequences of an improperly regulated immune response in insects will benefit both insect and human health.

Positive and negative regulatory effects of transcription factor activator protein 1 (AP1) on the expression of antimicrobial peptides in Macrobrachium nipponense

Transcription factor activator protein 1 (AP1) plays an irreplaceable role in the response to a variety of external stimulants, such as cellar stress, bacterial and viral infections, and inflammatory cytokines. In this study, we identified a novel AP1 gene from Macrobrachium nipponense and named it MnAP1, which has a full length of 1747 bp contains an 882 bp open reading frame, and encodes a protein with 293 amino acids. The MnAP1 protein contains Pfam and bZIP domains. MnAP1 is widely distributed in hemocytes, heart, hepatopancreas, gill, stomach, and intestinal tissues. The expression levels of MnAP1 in the gills and stomach were significantly upregulated after Vibrio parahaemolyticus and Staphylococcus aureus attacks. We studied the relationship between MnAP1 and the transcripts of antimicrobial peptides (AMPs) in gills through RNA interference. Interestingly, the regulatory effects of MnAP1 on the expression of different AMPs were different. We found that the expression levels of crustins, including Cru1, Cru3, and Cru4 in the gills were evidently decreased, whereas the synthesis of Cru5 and anti-lipopolysaccharide factors (ALF3 and ALF4) were obviously increased. We further explored the effect of MnAP1 on the expression of transcription factor relish from M. nipponense. The result showed that the knockdown of MnAP1 can remarkably upregulate the expression of MnRelish. Relish as a member of the nuclear factor κB family that regulates the expression of AMPs in the innate immunity of crustacean. Hence, we also detected the expression levels of Cru5, ALF3, and ALF4 in the gills of MnRelish-silenced prawns. The Data showed that the expression levels of these three AMPs were evidently reduced after MnRelish silencing. Our results indicated that MnAP1 plays a positive role in regulating the expression of AMPs, promotes the JNK/AP1 signaling pathway, and exerts a negative regulatory effect on the synthesis of AMPs by inhibiting the transcription of NF-κB factor in the innate immunity of M. nipponense.

Back to homeostasis: Negative regulation of NF-κB immune signaling in insects

Maintenance of homeostasis requires prompt activation and down-regulation of immune signaling pathways. This review attempts to summarize our current knowledge regarding the negative regulation of two NF-κB signaling pathways in insects, Toll and IMD pathway, which are mostly essential for host defense against bacteria and fungus. Various types of negative regulators and their mechanisms are discussed here with the emphasis on the prominent roles of ubiquitination. The counterbalance between these two pathways, the crosstalk with other physiological pathways, and the difference in their repertoires of negative regulators are also discussed.

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.

Dynamic miRNA-mRNA regulations are essential for maintaining Drosophila immune homeostasis during Micrococcus luteus infection

Pathogen bacteria infections can lead to dynamic changes of microRNA (miRNA) and mRNA expression profiles, which may control synergistically the outcome of immune responses. To reveal the role of dynamic miRNA-mRNA regulation in Drosophila innate immune responses, we have detailedly analyzed the paired miRNA and mRNA expression profiles at three time points during Drosophila adult males with Micrococcus luteus (M. luteus) infection using RNA- and small RNA-seq data. Our results demonstrate that differentially expressed miRNAs and mRNAs represent extensively dynamic changes over three time points during Drosophila with M. luteus infection. The pathway enrichment analysis indicates that differentially expressed genes are involved in diverse signaling pathways, including Toll and Imd as well as orther signaling pathways at three time points during Drosophila with M. luteus infection. Remarkably, the dynamic change of miRNA expression is delayed by compared to mRNA expression change over three time points, implying that the "time" parameter should be considered when the function of miRNA/mRNA is further studied. In particular, the dynamic miRNA-mRNA regulatory networks have shown that miRNAs may synergistically regulate gene expressions of different signaling pathways to promote or inhibit innate immune responses and maintain homeostasis in Drosophila, and some new regulators involved in Drosophila innate immune response have been identified. Our findings strongly suggest that miRNA regulation is a key mechanism involved in fine-tuning cooperatively gene expressions of diverse signaling pathways to maintain innate immune response and homeostasis in Drosophila. Taken together, the present study reveals a novel role of dynamic miRNA-mRNA regulation in immune response to bacteria infection, and provides a new insight into the underlying molecular regulatory mechanism of Drosophila innate immune responses.

Nubbin isoform antagonism governs Drosophila intestinal immune homeostasis

Gut immunity is regulated by intricate and dynamic mechanisms to ensure homeostasis despite a constantly changing microbial environment. Several regulatory factors have been described to participate in feedback responses to prevent aberrant immune activity. Little is, however, known about how transcriptional programs are directly tuned to efficiently adapt host gut tissues to the current microbiome. Here we show that the POU/Oct gene nubbin (nub) encodes two transcription factor isoforms, Nub-PB and Nub-PD, which antagonistically regulate immune gene expression in Drosophila. Global transcriptional profiling of adult flies overexpressing Nub-PB in immunocompetent tissues revealed that this form is a strong transcriptional activator of a large set of immune genes. Further genetic analyses showed that Nub-PB is sufficient to drive expression both independently and in conjunction with nuclear factor kappa B (NF-κB), JNK and JAK/STAT pathways. Similar overexpression of Nub-PD did, conversely, repress expression of the same targets. Strikingly, isoform co-overexpression normalized immune gene transcription, suggesting antagonistic activities. RNAi-mediated knockdown of individual nub transcripts in enterocytes confirmed antagonistic regulation by the two isoforms and that both are necessary for normal immune gene transcription in the midgut. Furthermore, enterocyte-specific Nub-PB expression levels had a strong impact on gut bacterial load as well as host lifespan. Overexpression of Nub-PB enhanced bacterial clearance of ingested Erwinia carotovora carotovora 15. Nevertheless, flies quickly succumbed to the infection, suggesting a deleterious immune response. In line with this, prolonged overexpression promoted a proinflammatory signature in the gut with induction of JNK and JAK/STAT pathways, increased apoptosis and stem cell proliferation. These findings highlight a novel regulatory mechanism of host-microbe interactions mediated by antagonistic transcription factor isoforms.

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.

Possible Insecticidal Mechanisms Mediated by Immune-Response-Related Cry-Binding Proteins in the Midgut Juice of Plutella xylostella and Spodoptera exigua

Cry toxins are insecticidal toxin proteins produced by a spore-forming Gram-positive bacterium Bacillus thuringiensis. Interactions between the Cry toxins and the receptors from midgut brush border membrane vesicles (BBMVs), such as cadherin, alkaline phosphatase, and aminopeptidase, are key steps for the specificity and insecticidal activity of Cry proteins. However, little is known about the midgut juice proteins that may interfere with Cry binding to the receptors. To validate the hypothesis that there exist Cry-binding proteins that can interfere with the insecticidal process of Cry toxins, we applied Cry1Ab1-coupled Sepharose beads to isolate Cry-binding proteins form midgut juice of Plutella xylostella and Spodoptera exigua. Trypsin-like serine proteases and Dorsal were found to be Cry1Ab1-binding proteins in the midgut juice of P. xylostella. Peroxidase-C (POX-C) was found to be the Cry1Ab1-binding protein in the midgut juice of S. exigua. We proposed possible insecticidal mechanisms of Cry1Ab1 mediated by the two immune-related proteins: Dorsal and POX-C. Our results suggested that there exist, in the midgut juice, Cry-binding proteins, which are different from BBMV-specific receptors.

Identification and functional analysis of pointed homologs in Bombyx mori

Using gene-knockdown techniques, we searched for endogenous Ets family proteins involved in the regulation of Escherichia coli-dependent lebocin promoter activation in the E. coli-responsive silkworm cell line NIAS-Bm-aff3. Results showed that the gene knockdown of BmPointeds (BmPNTs), Drosophila Pointed orthologs, enhanced E. coli-dependent lebocin promoter activation, suggesting that endogenous BmPNTs repress the activation of this promoter. Furthermore, we found that i) the BmPNT gene produced at least two alternative splicing isoforms, BmPNT1 and BmPNT2, both of which function as repressors; ii) BmPNTs were not associated with an already-reported repressor element, most proximal GGAA/T motif (EtsRE3), in lebocin promoter, which plays a role in the repression of E. coli- and BmRelish1-dependent lebocin promoter activation; iii) although BmPNTs did not directly affect BmRelish1-dependent lebocin promoter activation, they were able to directly repress its activation on the promoter lacking EtsRE3, probably because of competitive inhibition of binding of BmRelish1 to κB sites by BmPNTs; and iv) BmPNTs were mainly expressed in larval hemocytes, and the gene expression levels of BmPNT2, but not of BmPNT1, were decreased in response to E. coli and Bacillus subtilis. These findings suggest that endogenous BmPNTs are directly and indirectly involved in the repression of E. coli-mediated lebocin promoter activation in NIAS-Bm-aff3 cells.

Drosophila Sulf1 is required for the termination of intestinal stem cell division during regeneration

Stem cell division is activated to trigger regeneration in response to tissue damage. The molecular mechanisms by which this stem cell mitotic activity is properly repressed at the end of regeneration are poorly understood. Here, we show that a specific modification of heparan sulfate is crucial for regulating Drosophila intestinal stem cell (ISC) division during normal midgut homeostasis and regeneration. Loss of the extracellular heparan sulfate endosulfatase Sulf1 resulted in increased ISC division during normal homeostasis, which was caused by upregulation of mitogenic signaling including the JAK-STAT, EGFR and Hedgehog pathways. Using a regeneration model, we found that ISCs failed to properly halt division at the termination stage in Sulf1 mutants, showing that Sulf1 is required for terminating ISC division at the end of regeneration. We propose that post-transcriptional regulation of mitogen signaling by heparan sulfate structural modifications provides a new regulatory step for precise temporal control of stem cell activity during regeneration.

Canalization of gene expression is a major signature of regulatory cold adaptation in temperate Drosophila melanogaster

BACKGROUND

Transcriptome analysis may provide means to investigate the underlying genetic causes of shared and divergent phenotypes in different populations and help to identify potential targets of adaptive evolution. Applying RNA sequencing to whole male Drosophila melanogaster from the ancestral tropical African environment and a very recently colonized cold-temperate European environment at both standard laboratory conditions and following a cold shock, we seek to uncover the transcriptional basis of cold adaptation.

RESULTS

In both the ancestral and the derived populations, the predominant characteristic of the cold shock response is the swift and massive upregulation of heat shock proteins and other chaperones. Although we find ~25 % of the genome to be differentially expressed following a cold shock, only relatively few genes (n = 16) are up- or down-regulated in a population-specific way. Intriguingly, 14 of these 16 genes show a greater degree of differential expression in the African population. Likewise, there is an excess of genes with particularly strong cold-induced changes in expression in Africa on a genome-wide scale.

CONCLUSIONS

The analysis of the transcriptional cold shock response most prominently reveals an upregulation of components of a general stress response, which is conserved over many taxa and triggered by a plethora of stressors. Despite the overall response being fairly similar in both populations, there is a definite excess of genes with a strong cold-induced fold-change in Africa. This is consistent with a detrimental deregulation or an overshooting stress response. Thus, the canalization of European gene expression might be responsible for the increased cold tolerance of European flies.

WntD and Diedel: Two immunomodulatory cytokines in Drosophila immunity

Remarkable progress has been made on the understanding of the basic mechanisms of innate immunity in flies, from sensing infection to production of effector molecules. However, how the immune response is orchestrated at the level of the organism remains poorly understood. While cytokines activating immune responses, such as Spaetzle or Unpaired-3, have been identified and characterized in Drosophila, much less is known regarding immunosuppressor cytokines. In a recent publication, we reported the identification of a novel cytokine, Diedel, which acts as systemic negative regulator of the IMD pathway. Here, we discuss the similarities between Diedel and WntD, another immunomodulatory cytokine and present evidence that the 2 molecules act independently from one another.

The POU/Oct Transcription Factor Pdm1/nub Is Necessary for a Beneficial Gut Microbiota and Normal Lifespan of Drosophila

Maintenance of a stable gut microbial community relies on a delicate balance between immune defense and immune tolerance. We have used Drosophila to study how the microbial gut flora is affected by changes in host genetic factors and immunity. Flies with a constitutively active gut immune system, due to a mutation in the POU transcriptional regulator Pdm1/nubbin (nub) gene, had higher loads of bacteria and a more diverse taxonomic composition than controls. In addition, the microbial composition shifted considerably during the short lifespan of the nub1 mutants. This shift was characterized by a loss of relatively few OTUs (operational taxonomic units) and a remarkable increase in a large number of Acetobacter spp. and Leuconostoc spp. Treating nub1 mutant flies with antibiotics prolonged their lifetime survival by more than 100%. Immune gene expression was also persistently high in the presence of antibiotics, indicating that the early death was not a direct consequence of an overactive immune defense but rather an indirect consequence of the microbial load and composition. Thus, changes in host genotype and an inability to regulate the normal growth and composition of the gut microbiota leads to a shift in the microbial community, dysbiosis and early death.

Ebi, a Drosophila homologue of TBL1, regulates the balance between cellular defense responses and neuronal survival

Transducin β-like 1 (TBL1), a transcriptional co-repressor complex, is a causative factor for late-onset hearing impairments. Transcriptional co-repressor complexes play pivotal roles in gene expression by making a complex with divergent transcription factors. However, it remained to be clarified how co-repressor complex regulates cellular survival. We herein demonstrated that ebi, a Drosophila homologue of TBL1, suppressed photoreceptor cell degeneration in the presence of excessive innate immune signaling. We also showed that the balance between NF-κB and AP-1 is a key component of cellular survival under stress conditions. Given that Ebi plays an important role in innate immune responses by regulating NF-κB activity and inhibition of apoptosis induced by associating with AP-1, it may be involved in the regulation of photoreceptor cell survival by modulating cross-talk between NF-κB and AP-1.

The role of peroxiredoxin 4 in inflammatory response and aging

In prior studies, we determined that the moderate overexpression of the Drosophila endoplasmic reticulum (ER)-localized peroxiredoxin (Prx), dPrx4, reduced oxidative damage and conferred beneficial effects on life span, while a high-level expression increased the incidence of tissue-specific apoptosis and dramatically shortened longevity. The detrimental pro-apoptotic and life-shortening effects were attributed to aberrant localization of dPrx4 and the apparent ER stress elicited by dPrx4 overexpression. In addition, the activation of both the NF-κB- and the JAK/STAT-mediated stress responses was detected, although it was not clear whether these served as functional alarm signals. Here we extend these findings to show that the activation of the NF-κB-dependent immunity-related/inflammatory genes, associated with life span shortening effects, is dependent on the activity of a Drosophila NF-κB ortholog, Relish. In the absence of Relish, the pro-inflammatory effects typically elicited by dPrx4 overexpression were not detected. The absence of Relish not only prevented the hyperactivation of the immunity-related genes but also significantly rescued the severe shortening of life span normally observed in dPrx4 overexpressors. The overactivation of the immune/inflammatory responses was also lessened by JAK/STAT signaling. In addition, we found that cellular immune/pro-inflammatory responses provoked by the oxidant paraquat but not bacteria are mediated via dPrx4 activity in the ER, as the upregulation of the immune-related genes was eliminated in flies underexpressing dPrx4, whereas immune responses triggered by bacteria were unaffected. Finally, efforts to reveal critical tissues where dPrx4 modulates longevity showed that broad targeting of dPrx4 to neuronal tissue had strong beneficial effects, while targeting expression to the fat body had deleterious effects.

Functional Conservation of the Glide/Gcm Regulatory Network Controlling Glia, Hemocyte, and Tendon Cell Differentiation in Drosophila

High-throughput screens allow us to understand how transcription factors trigger developmental processes, including cell specification. A major challenge is identification of their binding sites because feedback loops and homeostatic interactions may mask the direct impact of those factors in transcriptome analyses. Moreover, this approach dissects the downstream signaling cascades and facilitates identification of conserved transcriptional programs. Here we show the results and the validation of a DNA adenine methyltransferase identification (DamID) genome-wide screen that identifies the direct targets of Glide/Gcm, a potent transcription factor that controls glia, hemocyte, and tendon cell differentiation in Drosophila. The screen identifies many genes that had not been previously associated with Glide/Gcm and highlights three major signaling pathways interacting with Glide/Gcm: Notch, Hedgehog, and JAK/STAT, which all involve feedback loops. Furthermore, the screen identifies effector molecules that are necessary for cell-cell interactions during late developmental processes and/or in ontogeny. Typically, immunoglobulin (Ig) domain-containing proteins control cell adhesion and axonal navigation. This shows that early and transiently expressed fate determinants not only control other transcription factors that, in turn, implement a specific developmental program but also directly affect late developmental events and cell function. Finally, while the mammalian genome contains two orthologous Gcm genes, their function has been demonstrated in vertebrate-specific tissues, placenta, and parathyroid glands, begging questions on the evolutionary conservation of the Gcm cascade in higher organisms. Here we provide the first evidence for the conservation of Gcm direct targets in humans. In sum, this work uncovers novel aspects of cell specification and sets the basis for further understanding of the role of conserved Gcm gene regulatory cascades.

Cellular Defense and Sensory Cell Survival Require Distinct Functions of ebi in Drosophila

The innate immune response and stress-induced apoptosis are well-established signaling pathways related to cellular defense. NF-κB and AP-1 are redox-sensitive transcription factors that play important roles in those pathways. Here we show that Ebi, a Drosophila homolog of the mammalian co-repressor molecule transducin β-like 1 (TBL1), variously regulates the expression of specific genes that are targets of redox-sensitive transcription factors. In response to different stimuli, Ebi activated gene expression to support the acute immune response in fat bodies, whereas Ebi repressed genes that are involved in apoptosis in photoreceptor cells. Thus, Ebi seems to act as a regulatory switch for genes that are activated or repressed in response to different external stimuli. Our results offer clear in vivo evidence that the Ebi-containing co-repressor complex acts in a distinct manner to regulate transcription that is required for modulating the output of various processes during Drosophila development.

Identification and expression of SRF targeted by miR-133a during early development of Paralichthys olivaceus

Serum response factor (SRF) is a MADS-box transcription factor that regulates the expression of genes involved in development, metabolism, cell proliferation, and differentiation. In the present study, we cloned the full-length SRF cDNA which includes the coding region of 1503 bp, a 573-bp 5'untranslated region (UTR) and a 400-bp 3'-UTR. The deduced 501 amino acid sequence of the SRF protein contained a MADS domain and NLS at the N terminus, similar to other organisms, and it also is highly phylogenetically conserved. SRF mRNA is ubiquitously expressed in various tissues, with the highest level in the kidneys, and it is also highly expressed during the embryonic and metamorphic stages. During metamorphosis, the SRF mRNA levels are down-regulated by exogenous thyroid hormone (TH) at 17 dph and by thiourea (TU) at 29, 36, and 41 dph, whereas SRF mRNA levels were significantly up-regulated by the added exogenous TH to the TU-treated larvae at 41 dph, which indicates that thyroid hormone is essential for expression of SRF mRNA, so, higher levels of TH did not result in changes of SRF mRNA levels, while TH deficiency or inhibited by the non-specific TU toxicity cause down-regulation of SRF mRNA, which indicated that TH can indirectly affect the SRF mRNA levels. Meanwhile, using a luciferase reporter assay, we verified that SRF is a common target gene of miR-133a which is a muscle-specific microRNA (miRNA), which indicated that SRF may be involved in the signaling pathway of miRNA that regulates muscle development.

The c-Fos and c-Jun from Litopenaeus vannamei play opposite roles in Vibrio parahaemolyticus and white spot syndrome virus infection

Growing evidence indicates that activator protein-1 (AP-1) plays a major role in stimulating the transcription of immune effector molecules in cellular response to an incredible array of stimuli, including growth factors, cytokines, cellular stresses and bacterial and viral infection. Here, we reported the isolation and characterization of a cDNA from Litopenaeus vannamei encoding the full-length c-Fos protein (named as Lvc-Fos). The predicted amino acid sequences of Lvc-Fos contained a basic-leucine zipper (bZIP) domain, which was characteristic of members of the AP-1 family. Immunoprecipitation and native-PAGE assays determined that Lvc-Fos could interact with the Lvc-Jun, a homolog of c-Jun family in L. vannamei, in a heterodimer manner. Further investigation demonstrated that Lvc-Fos and Lvc-Jun were expressed in all tested tissues and located in the nucleus. Real-time RT-PCR analysis showed both Lvc-Fos and Lvc-Jun in gills were up-regulated during Vibrio parahaemolyticus and white spot syndrome virus (WSSV) challenges. In addition, reporter gene assays indicated Lvc-Fos and Lvc-Jun could activate the expression of antimicrobial peptides (AMPs) of Drosophila and shrimp, as well as WSSV immediate early (IE) genes wsv069 and wsv249, in a different manner. Knockdown of Lvc-Fos or Lvc-Jun by RNA interference (RNAi) resulted in higher mortalities of L. vannamei after infection with V. parahaemolyticus, suggesting that Lvc-Fos and Lvc-Jun might play protective roles in bacterial infection. However, silencing of Lvc-Fos or Lvc-Jun in shrimp caused lower mortalities and virus loads under WSSV infection, suggesting that Lvc-Fos and Lvc-Jun could be engaged for WSSV replication and pathogenesis. In conclusion, our results provided experimental evidence and novel insight into the roles of L. vannamei AP-1 in bacterial and viral infection.

The epigenetic regulator G9a mediates tolerance to RNA virus infection in Drosophila

Little is known about the tolerance mechanisms that reduce the negative effects of microbial infection on host fitness. Here, we demonstrate that the histone H3 lysine 9 methyltransferase G9a regulates tolerance to virus infection by shaping the response of the evolutionary conserved Jak-Stat pathway in Drosophila. G9a-deficient mutants are more sensitive to RNA virus infection and succumb faster to infection than wild-type controls, which was associated with strongly increased Jak-Stat dependent responses, but not with major differences in viral load. Genetic experiments indicate that hyperactivated Jak-Stat responses are associated with early lethality in virus-infected flies. Our results identify an essential epigenetic mechanism underlying tolerance to virus infection.

Multicellular organisms deploy various strategies to fight microbial infections. Invading pathogens may be eradicated directly by antimicrobial effectors of the immune system. Another strategy consists of increasing the tolerance of the host to infection, for example, by limiting the adverse effects of the immune response. The molecular mechanisms underlying this novel concept remain largely uncharacterized. Here, we demonstrate that the epigenetic regulator G9a mediates tolerance to virus infection in Drosophila. We found that G9a-deficient flies succumb faster than control flies to infection with RNA viruses, but that the viral burden did not significantly differ. Unexpectedly, mutant flies express higher levels of genes that are regulated by the Jak-Stat signaling pathway, which in other studies was found to be important for antiviral defense. Exploiting the genetic toolbox in Drosophila, we demonstrate that Jak-Stat hyperactivation induces early mortality after virus infection. Precise control of immune pathways is essential to ensure efficient immunity, while preventing damage due to excessive immune responses. Our results indicate that G9a, an epigenetic modifier, dampens Jak-Stat responses to prevent immunopathology. Therefore, we propose epigenetic regulation of immunity as a new paradigm for disease tolerance.

Myosin VI regulates gene pairing and transcriptional pause release in T cells

Naive CD4 T cells differentiate into several effector lineages, which generate a stronger and more rapid response to previously encountered immunological challenges. Although effector function is a key feature of adaptive immunity, the molecular basis of this process is poorly understood. Here, we investigated the spatiotemporal regulation of cytokine gene expression in resting and restimulated effector T helper 1 (Th1) cells. We found that the Lymphotoxin (LT)/TNF alleles, which encode TNF-α, were closely juxtaposed shortly after T-cell receptor (TCR) engagement, when transcription factors are limiting. Allelic pairing required a nuclear myosin, myosin VI, which is rapidly recruited to the LT/TNF locus upon restimulation. Furthermore, transcription was paused at the TNF locus and other related genes in resting Th1 cells and released in a myosin VI-dependent manner following activation. We propose that homologous pairing and myosin VI-mediated transcriptional pause release account for the rapid and efficient expression of genes induced by an external stimulus.

PI3K signaling and Stat92E converge to modulate glial responsiveness to axonal injury

Activation of glial cells following axon injury is mediated by a positive feedback loop downstream of the glial phagocytic receptor Draper, allowing the strength of the response to match the severity of injury.

Glial cells are exquisitely sensitive to neuronal injury but mechanisms by which glia establish competence to respond to injury, continuously gauge neuronal health, and rapidly activate reactive responses remain poorly defined. Here, we show glial PI3K signaling in the uninjured brain regulates baseline levels of Draper, a receptor essential for Drosophila glia to sense and respond to axonal injury. After injury, Draper levels are up-regulated through a Stat92E-modulated, injury-responsive enhancer element within the draper gene. Surprisingly, canonical JAK/STAT signaling does not regulate draper expression. Rather, we find injury-induced draper activation is downstream of the Draper/Src42a/Shark/Rac1 engulfment signaling pathway. Thus, PI3K signaling and Stat92E are critical in vivo regulators of glial responsiveness to axonal injury. We provide evidence for a positive auto-regulatory mechanism whereby signaling through the injury-responsive Draper receptor leads to Stat92E-dependent, transcriptional activation of the draper gene. We propose that Drosophila glia use this auto-regulatory loop as a mechanism to adjust their reactive state following injury.

Acute injuries of the central nervous system (CNS) trigger a robust reaction from glial cells—a non-neuronal population of cells that regulate and support neural development and physiology. Although this process occurs after all types of CNS trauma in mammals, how it is activated and its precise role in recovery remain poorly understood. Using the fruit fly Drosophila melanogaster as a model, we previously identified a cell surface receptor called Draper, which is required for the activation of glia after local axon injury (“axotomy”) and for the removal of degenerating axonal debris by phagocytosis. Here, we show that regulation of Draper protein levels and glial activation through the Draper signaling pathway are mediated by the well-conserved PI3K and signal transducer and activator of transcription (STAT) signaling cascades. We find that STAT transcriptional activity is activated in glia in response to axotomy, and identify an injury-responsive regulatory element within the draper gene that appears to be directly modulated by STAT. Interestingly, the intensity of STAT activity in glial cells after axotomy correlates tightly with the number of local severed axons, indicating that Drosophila glia are able to fine-tune their response to neuronal injury according to its severity. In summary, we propose that the initial phagocytic competence of glia is regulated by setting Draper baseline levels (via PI3K), whereas injury-activated glial phagocytic activity is modulated through a positive feedback loop that requires STAT-dependent activation of draper. We speculate that the level of activation of this cascade is determined by glial cell recognition of Draper ligands present on degenerating axon material, thereby matching the levels of glial reactivity to the amount of injured axonal material.

Oct-1 regulates IL-17 expression by directing interchromosomal associations in conjunction with CTCF in T cells

Interchromosomal associations can regulate gene expression, but little is known about the molecular basis of such associations. In response to antigen stimulation, naive T cells can differentiate into Th1, Th2, and Th17 cells expressing IFN-γ, IL-4, and IL-17, respectively. We previously reported that in naive T cells, the IFN-γ locus is associated with the Th2 cytokine locus. Here we show that the Th2 locus additionally associates with the IL-17 locus. This association requires a DNase I hypersensitive region (RHS6) at the Th2 locus. RHS6 and the IL-17 promoter both bear Oct-1 binding sites. Deletion of either of these sites or Oct-1 gene impairs the association. Oct-1 and CTCF bind their cognate sites cooperatively, and CTCF deficiency similarly impairs the association. Finally, defects in the association lead to enhanced IL-17 induction. Collectively, our data indicate Th17 lineage differentiation is restrained by the Th2 locus via interchromosomal associations organized by Oct-1 and CTCF.

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.

A novel ortholog of serum response factor (SRF) with immune defense function identified in Crassostrea hongkongensis

Serum response factor (SRF) function is essential for transcriptional regulation of numerous growth-factor-inducible genes and triggers proliferation, differentiation and apoptosis of the cells. In this report, the first mollusk serum response factor like homolog gene (designated ChSRF) was identified and characterized from the Hong Kong oyster, Crassostrea hongkongensis. The full-length cDNA of ChSRF was 1716 bp in length and encodes a putative protein of 434 amino acids respectively, and shares the MADS domain at the N-terminal. ChSRF is ubiquitously expressed in various tissues, with the highest expression level observed in muscle. Temporal expression of ChSRF following microbe infection shows that the expression of ChSRF in hemocytes increases from 3 to 24 h post-challenge. As a target gene of SRF, β-actin demonstrates a similar gene expression mode in constitutive tissue and pathogen infection. Furthermore, some protein profiles of ChSRF was revealed, fluorescence microscopy results show that ChSRF located in the nuclei of HeLa cells and over-expression of ChSRF activated the transcriptional activities of MAPK signal pathway in HEK293T cells. These results indicate that ChSRF maybe play an important role in signal transduction in the immunity and development response of oysters. Furthermore, we found that ChSRF could regulate the expression of β-actin gene, which indicate that ChSRF is a muscle differentiation regulator in the oyster and it will help us to improve aquaculture production.

Insect antiviral innate immunity: pathways, effectors, and connections

Insects are infected by a wide array of viruses some of which are insect restricted and pathogenic, and some of which are transmitted by biting insects to vertebrates. The medical and economic importance of these viruses heightens the need to understand the interaction between the infecting pathogen and the insect immune system in order to develop transmission interventions. The interaction of the virus with the insect host innate immune system plays a critical role in the outcome of infection. The major mechanism of antiviral defense is the small, interfering RNA pathway that responds through the detection of virus-derived double-stranded RNA to suppress virus replication. However, other innate antimicrobial pathways such as Imd, Toll, and Jak-STAT and the autophagy pathway have also been shown to play important roles in antiviral immunity. In this review, we provide an overview of the current understanding of the main insect antiviral pathways and examine recent findings that further our understanding of the roles of these pathways in facilitating a systemic and specific response to infecting viruses.

The Oct1 homolog Nubbin is a repressor of NF-κB-dependent immune gene expression that increases the tolerance to gut microbiota

Background

Innate immune responses are evolutionarily conserved processes that provide crucial protection against invading organisms. Gene activation by potent NF-κB transcription factors is essential both in mammals and Drosophila during infection and stress challenges. If not strictly controlled, this potent defense system can activate autoimmune and inflammatory stress reactions, with deleterious consequences for the organism. Negative regulation to prevent gene activation in healthy organisms, in the presence of the commensal gut flora, is however not well understood.

Results

We show that the Drosophila homolog of mammalian Oct1/POU2F1 transcription factor, called Nubbin (Nub), is a repressor of NF-κB/Relish-driven antimicrobial peptide gene expression in flies. In nub¹ mutants, which lack Nub-PD protein, excessive expression of antimicrobial peptide genes occurs in the absence of infection, leading to a significant reduction of the numbers of cultivatable gut commensal bacteria. This aberrant immune gene expression was effectively blocked by expression of Nub from a transgene. We have identified an upstream regulatory region, containing a cluster of octamer sites, which is required for repression of antimicrobial peptide gene expression in healthy flies. Chromatin immunoprecipitation experiments demonstrated that Nub binds to octamer-containing promoter fragments of several immune genes. Gene expression profiling revealed that Drosophila Nub negatively regulates many genes that are involved in immune and stress responses, while it is a positive regulator of genes involved in differentiation and metabolism.

Conclusions

This study demonstrates that a large number of genes that are activated by NF-κB/Relish in response to infection are normally repressed by the evolutionarily conserved Oct/POU transcription factor Nub. This prevents uncontrolled gene activation and supports the existence of a normal gut flora. We suggest that Nub protein plays an ancient role, shared with mammalian Oct/POU transcription factors, to moderate responses to immune challenge, thereby increasing the tolerance to biotic stress.