Ecdysone triggered PGRP-LC expression controls Drosophila innate immunity

The involvement of tumor necrosis factor receptor-associated factor 6 in regulating immune response by NF-κB at pre-molt stage of Chinese mitten crab (Eriocheir sinensis)

Molting is a crucial biological process of crustaceans. Crustaceans go through three separate stages throughout their molting process, including pre-molt, post-molt and inter-molt. However, the exact mechanism of immunological modulation during molting remains unclear. Tumor necrosis factor receptor-associated factor 6 (TRAF6) has been extensively documented to participate in immune defense. In the present study, a TRAF6 gene with two TRAF-type zinc finger domains was identified from Eriocheir sinensis (designed as EsTRAF6), and its role in regulating immune response during molting process was explored. The mRNA expression level of EsTRAF6 at pre-molt stage was higher than that at post-molt stage and inter-molt stage. After Aeromonas hydrophila stimulation, the expression levels of EsTRAF6, EsRelish and anti-lipopolysaccharide factors (ALFs) genes exhibited a considerable increase at three molting stages. Subsequently, the expression patterns of EsTRAF6 and EsRelish in response to the treatment with 20-hydroxyecdysone (20E) were examined. The mRNA expression of EsTRAF6 and EsRelish were significantly increased at 12 h after 20E injection. Additionally, the protein expression level of TRAF6 was also up-regulated in 20E group compared to control group. Furthermore, the role of EsTRAF6 in regulating the anti- ALFs expression at pre-molt stage post A. hydrophila stimulation was investigated. Following the inhibition of the EsTRAF6 transcript using RNAi or the injection of inhibitor (TMBPS), there was a notable decrease of the EsALF1, EsALF2 and EsALF3 transcripts. Moreover, a significant reduction in the phosphorylation level of NF-κB at pre-molt stage was observed after A. hydrophila stimulation in TRAF6-inhibited crabs. Collectively, our results suggest that EsTRAF6 could be induced by 20E and promoted the EsALFs expression by activating NF-κB at pre-molt stage, which provides a novel insight into the research of immune regulatory mechanism during the process of molting of crustaceans.

Ecdysone promotes gene- and pathogen-specific immune responses to Micrococcus luteus and Bacillus subtilis in Drosophila S2 cells

In Drosophila, the 20-hydroxyecdysone (20E) hormone regulates numerous essential biological processes. Here, we studied the contribution of 20E to the activity of immune signaling pathways and antimicrobial activity using the model Drosophila S2 cells. We found that while 20E alone has no essential effect on this system, pretreating S2 cells with 20E followed by incubation with Escherichia coli or Micrococcus luteus stimulates the induction of a limited number of antimicrobial peptide (AMP) genes, such as Diptericin (Dpt) and Drosomycin (Drs). Contrary to this, cells pretreatment with 20E simulates the activity of numerous Bacillus subtilis-induced AMP genes. Interestingly, it also significantly promotes the expression of components of both the Toll (Dif, Dorsal, etc.) and the IMD pathways (Relish, IMD, etc.) in the presence of Bacillus subtilis. Unexpectedly, simultaneous treatment of S2 cells by 20E and all three bacteria shows another pattern of activity and leads to a suppression of Drosocin (Dro) induction, in particular. Our study reveals that the contribution of 20E to immune genes activity varies for different genes and depends on the mode of 20E interplay with the pathogen and the nature of the pathogen itself.

Transcriptome and proteome analyses reveal genes and signaling pathways involved in the response to two insect hormones in the insect-fungal pathogen Hirsutella satumaensis

The insect hormones ecdysone (20E) and juvenile hormone III (JH) have been demonstrated to stimulate the secretion of conidia mucilage and pigments in Hirsutella satumaensis. However, the underlying mechanisms remain elusive. Here, comparative transcriptome and proteome analyses were performed to identify the fungal genes and proteins of H. satumaensis that are up- or downregulated in response to insect hormones. A total of 17,407 unigenes and 1,016 proteins in conidia mucilage were identified. The genes involved in response to the hormones were classified into four functional groups: (1) stress response-related genes that are required for the removal of reactive oxygen species (glutathione synthetase, c7144) and genes involved in the response to osmotic stress in the hemocoel, such as those encoding proteins involved in the G, mTOR, and MAPK signaling pathways (2); insect hormone metabolic genes, including genes encoding ecdysteroid UDP-glucosyltransferase, ecdysteroid-22-kinase, and a key aldehyde dehydrogenase in a juvenile hormone synthesis pathway (3); secretory proteins that share homology with those of the host Bombyx mori, including fibrohexamerin, sericin 1, metalloprotease 1 protein, and silk gum protein, which were revealed by the omics data; and (4) proteins related to amino sugar metabolism and oxidative phosphorylation that were specifically expressed in mucilage in response to 20E and JH, respectively. These findings revealed that H. satumaensis can mount effective responses by modulating the expression of genes involved in the detoxification, adaptation, and evasion of insect hormone-mediated immune responses, providing fresh insights into fungal pathogen-host insect interactions.IMPORTANCEInsect hormones are highly important for the regulation of insect growth, development, and immune system function. Thus, the expansion of entomopathogenic fungi (EPF) could be affected by these hormones when they inhabit the host hemocoel. However, the molecular basis of EPF in response to insect hormones has yet to be determined. Our results revealed that EPF are impacted by 20E and JH, both of which act as signals, as these hormones lead to changes in metabolic pathways of the fungus, thus demonstrating a direct relationship between the fungus and the hormones. Furthermore, adaptive strategies, such as the use of ecdysone-inactivating enzymes and secreted filamentous proteins in H. satumaensis, which strongly resemble those of the host insect, have been discovered, thus illustrating the importance of adaptation to insect hormones for a better understanding of the interaction between insects and EPF.

Cereal weevils' antimicrobial peptides: at the crosstalk between development, endosymbiosis and immune response

Interactions between animals and microbes are ubiquitous in nature and strongly impact animal physiology. These interactions are shaped by the host immune system, which responds to infections and contributes to tailor the associations with beneficial microorganisms. In many insects, beneficial symbiotic associations not only include gut commensals, but also intracellular bacteria, or endosymbionts. Endosymbionts are housed within specialized host cells, the bacteriocytes, and are transmitted vertically across host generations. Host-endosymbiont co-evolution shapes the endosymbiont genome and host immune system, which not only fights against microbial intruders, but also ensures the preservation of endosymbionts and the control of their load and location. The cereal weevil Sitophilus spp. is a remarkable model in which to study the evolutionary adaptation of the immune system to endosymbiosis owing to its binary association with a unique, relatively recently acquired nutritional endosymbiont, Sodalis pierantonius. This Gram-negative bacterium has not experienced the genome size shrinkage observed in long-term endosymbioses and has retained immunogenicity. We focus here on the sixteen antimicrobial peptides (AMPs) identified in the Sitophilus oryzae genome and their expression patterns in different tissues, along host development or upon immune challenges, to address their potential functions in the defensive response and endosymbiosis homeostasis along the insect life cycle. This article is part of the theme issue 'Sculpting the microbiome: how host factors determine and respond to microbial colonization'.

Insect Antimicrobial Peptides as Guardians of Immunity and Beyond: A Review

Antimicrobial peptides (AMPs), as immune effectors synthesized by a variety of organisms, not only constitute a robust defense mechanism against a broad spectrum of pathogens in the host but also show promising applications as effective antimicrobial agents. Notably, insects are significant reservoirs of natural AMPs. However, the complex array of variations in types, quantities, antimicrobial activities, and production pathways of AMPs, as well as evolution of AMPs across insect species, presents a significant challenge for immunity system understanding and AMP applications. This review covers insect AMP discoveries, classification, common properties, and mechanisms of action. Additionally, the types, quantities, and activities of immune-related AMPs in each model insect are also summarized. We conducted the first comprehensive investigation into the diversity, distribution, and evolution of 20 types of AMPs in model insects, employing phylogenetic analysis to describe their evolutionary relationships and shed light on conserved and distinctive AMP families. Furthermore, we summarize the regulatory pathways of AMP production through classical signaling pathways and additional pathways associated with Nitric Oxide, insulin-like signaling, and hormones. This review advances our understanding of AMPs as guardians in insect immunity systems and unlocks a gateway to insect AMP resources, facilitating the use of AMPs to address food safety concerns.

Mosquito E-20-Monooxygenase Gene Knockout Increases Dengue Virus Replication in Aedes aegypti Cells

E-20-monooxygenase (E20MO) is an enzymatic product of the shade (shd) locus (cytochrome p450, E20MO). Initially discovered in Drosophila, E20MO facilitates the conversion of ecdysone (E) into 20-hydroxyecdysone (20E) and is crucial for oogenesis. Prior research has implicated 20E in growth, development, and insecticide resistance. However, little attention has been given to the association between the E20MO gene and DENV2 infection. The transcriptome of Ae. aegypti cells (Aag2 cells) infected with DENV2 revealed the presence of the E20MO gene. The subsequent quantification of E20MO gene expression levels in Aag2 cells post-DENV infection was carried out. A CRISPR/Cas9 system was utilized to create an E20MO gene knockout cell line (KO), which was then subjected to DENV infection. Analyses of DENV2 copies in KO and wild-type (WT) cells were conducted at different days post-infection (dpi). Plasmids containing E20MO were constructed and transfected into KO cells, with pre- and post-transfection viral copy comparisons. Gene expression levels of E20MO increased after DENV infection. Subsequently, a successful generation of an E20MO gene knockout cell line and the verification of code-shifting mutations at both DNA and RNA levels were achieved. Furthermore, significantly elevated DENV2 RNA copies were observed in the mid-infection phase for the KO cell line. Viral RNA copies were lower in cells transfected with plasmids containing E20MO, compared to KO cells. Through knockout and plasmid complementation experiments in Aag2 cells, the role of E20MO in controlling DENV2 replication was demonstrated. These findings contribute to our understanding of the intricate biological interactions between mosquitoes and arboviruses.

A CTL - Lys immune function maintains insect metamorphosis by preventing gut bacterial dysbiosis and limiting opportunistic infections

BACKGROUND

Gut bacteria are beneficial to the host, many of which must be passed on to host offspring. During metamorphosis, the midgut of holometabolous insects undergoes histolysis and remodeling, and thus risks losing gut bacteria. Strategies employed by holometabolous insects to minimize this risk are obscure. How gut bacteria affect host insects after entering the hemocoel and causing opportunistic infections remains largely elusive.

RESULTS

We used holometabolous Helicoverpa armigera as a model and found low Lactobacillus load, high level of a C-type lectin (CTL) gene CD209 antigen-like protein 2 (CD209) and its downstream lysozyme 1 (Lys1) in the midgut of the wandering stage. CD209 or Lys1 depletion increased the load of midgut Lactobacillus, which further translocate to the hemocoel. In particular, CD209 or Lys1 depletion, injection of Lactobacillus plantarum, or translocation of midgut L. plantarum into the hemocoel suppressed 20-hydroxyecdysone (20E) signaling and delayed pupariation. Injection of L. plantarum decreased triacylglycerol and cholesterol storage, which may result in insufficient energy and 20E available for pupariation. Further, Lysine-type peptidoglycan, the major component of gram-positive bacterial cell wall, contributed to delayed pupariation and decreased levels of triacylglycerols, cholesterols, and 20E, in both H. armigera and Drosophila melanogaster.

CONCLUSIONS

A mechanism by which (Lactobacillus-induced) opportunistic infections delay insect metamorphosis was found, namely by disturbing the homeostasis of lipid metabolism and reducing 20E production. Moreover, the immune function of CTL - Lys was characterized for insect metamorphosis by maintaining gut homeostasis and limiting the opportunistic infections.

The genome of the ant Tetramorium bicarinatum reveals a tandem organization of venom peptides genes allowing the prediction of their regulatory and evolutionary profiles

BACKGROUND

Venoms have evolved independently over a hundred times in the animal kingdom to deter predators and/or subdue prey. Venoms are cocktails of various secreted toxins, whose origin and diversification provide an appealing system for evolutionary researchers. Previous studies of the ant venom of Tetramorium bicarinatum revealed several Myrmicitoxin (MYRTX) peptides that gathered into seven precursor families suggesting different evolutionary origins. Analysis of the T. bicarinatum genome enabling further genomic approaches was necessary to understand the processes underlying the evolution of these myrmicitoxins.

RESULTS

Here, we sequenced the genome of Tetramorium bicarinatum and reported the organisation of 44 venom peptide genes (vpg). Of the eleven chromosomes that make up the genome of T. bicarinatum, four carry the vpg which are organized in tandem repeats. This organisation together with the ML evolutionary analysis of vpg sequences, is consistent with evolution by local duplication of ancestral genes for each precursor family. The structure of the vpg into two or three exons is conserved after duplication events while the promoter regions are the least conserved parts of the vpg even for genes with highly identical sequences. This suggests that enhancer sequences were not involved in duplication events, but were recruited from surrounding regions. Expression level analysis revealed that most vpg are highly expressed in venom glands, although one gene or group of genes is much more highly expressed in each family. Finally, the examination of the genomic data revealed that several genes encoding transcription factors (TFs) are highly expressed in the venom glands. The search for binding sites (BS) of these TFs in the vpg promoters revealed hot spots of GATA sites in several vpg families.

CONCLUSION

In this pioneering investigation on ant venom genes, we provide a high-quality assembly genome and the annotation of venom peptide genes that we think can fosters further genomic research to understand the evolutionary history of ant venom biochemistry.

mosGILT controls innate immunity and germ cell development in Anopheles gambiae

Gene-edited mosquitoes lacking a gamma-interferon-inducible lysosomal thiol reductase-like protein, namely (mosGILTnull) have lower Plasmodium infection, which is linked to impaired ovarian development and immune activation. The transcriptome of mosGILTnull Anopheles gambiae was therefore compared to wild type (WT) mosquitoes by RNA-sequencing to delineate mosGILT-dependent pathways. Compared to WT mosquitoes, mosGILTnull A. gambiae demonstrated altered expression of genes related to oogenesis, 20-hydroxyecdysone synthesis, as well as immune-related genes. Serendipitously, the zero population growth gene, zpg, an essential regulator of germ cell development was found to be one of the most downregulated genes in mosGILTnull mosquitoes. These results provide a crucial missing link between two previous studies on the role of zpg and mosGILT in ovarian development. This study further demonstrates that mosGILT has the potential to serve as a target for the biological control of mosquito vectors and to influence the Plasmodium life cycle within the vector.

SetDB1 and Su(var)3-9 are essential for late stages of larval development of Drosophila melanogaster

Methylation of H3K9 histone residue is a marker of gene silencing in eukaryotes. Three enzymes responsible for adding this modification - G9a, SetDB1/Egg, and Su(var)3-9 - are known in Drosophila. To understand how simultaneous mutations of SetDB1 and Su(var)3-9 may affect the fly development, appropriate combinations were obtained. Double mutants egg; Su(var)3-9 displayed pronounced embryonic lethality, slower larval growth and died before or during metamorphosis. Analysis of transcription in larval salivary glands and wing imaginal disks indicated that the effect of double mutation is tissue-specific. In salivary gland chromosomes, affected genes display low H3K9me2 enrichment and are rarely bound by SetDB1 or Su(var)3-9. We suppose that each of these enzymes directly or indirectly controls its own set of gene targets in different organs, and double mutation results in an imbalanced developmental program. This also indicates that SetDB1 and Su(var)3-9 may affect transcription via H3K9-independent mechanisms. Unexpectedly, in double and triple mutants, amount of di- and tri-methylated H3K9 is drastically reduced, but not completely absent. We hypothesize that this residual methylation implies the existence of additional H3K9-specific methyltransferase in Drosophila.

Expression patterns and antifungal function study of KaSPI in Mythimna separata

Kazal-type serine protease inhibitors (KaSPI) play important roles in insect growth, development, digestion, metabolism and immune defence. In this study, based on the transcriptome of Mythimna separata, the cDNA sequence of MsKaSPI with Kazal domain was uploaded to GenBank (MN931651). Spatial and temporal expression analysis showed that MsKaSPI was expressed at different developmental stages and different tissues, and it was induced by 20-hydroxyecdysone in third-instar larvae of M. separata. After 24 h infection by Beauveria bassiana, the expression level of MsKaSPI and the corresponding MsKaSPI content were significantly up-regulated, being 6.42-fold and 1.91-fold to the control group, respectively, while the activities of serine protease, trypsin and chymotrypsin were inhibited. After RNA interference interfered with MsKaSPI for 6 h, the expression decreased by 73.44%, the corresponding content of MsKaSPI protein decreased by 55.66% after 12 h, and the activities of serine protease and trypsin were significantly enhanced. Meanwhile, both the larval and pupal stages of M. separata were prolonged, the weights were reduced and the number of eggs per female decreased by 181. Beauveria bassiana infection also increased the mortality of MsKaSPI-silenced M. separata by 18.96%. These prove MsKaSPI can not only result in slow growth and low fecundity of M. separata by regulating the activity of related protease, but also participate in the resistance to pathogenic fungi by regulating the serine protease inhibitor content and the activities of related serine protease.

Identification of CYCLE targets that contribute diverse features of circadian rhythms in the mosquito Culex pipiens

Culex pipiens demonstrates robust circadian rhythms in adult eclosion, flight activity, mating, and development. These rhythmic patterns are believed to be controlled by the endogenous light-entrainable circadian clock that consists of positive and negative regulators working in a transcription-translation feedback loop. Moreover, these mosquitoes undergo seasonal diapause in exposure to the short photoperiod of late summer or early fall. However, the exact genetic and cellular mechanism behind the clock gene-mediated activity pattern, seasonal time measurement, and subsequent diapause initiation still need to be unraveled. To determine the possible linkage between clock genes and downstream processes, here we employed ChIP-sequencing to identify the direct targets of one of the core clock proteins, Cycle (CYC). The nearest genes with peaks mapping to their 1Kb upstream region of the transcription start site were extracted and scanned for consensus E box sequences, resulting in a dataset comprising the target genes possibly regulated by CYC. Based on the highest fold enrichment and functional relevance, we identified genes relating to five gene categories of potential interest, including peptide/receptors, neurotransmission, olfaction, immunity, and reproductive growth. Of these, we validated fourteen genes with ChIP-qPCR and qRT-PCR. These genes showed a significantly high expression in dusk compared to dawn in concert with the activity level of the CYC transcription factor and are thus strong candidates for mediating circadian rhythmicity and possibly regulating seasonal shifts in mosquito reproductive activity.

Effects of TmTak1 silencing on AMP production as an Imd pathway component in Tenebrio molitor

Mealworms beetles, Tenebrio molitor, are the limelight next-generation food for humans due to their high nutrient contents. Since Tenebrio molitor is used as feed for pets and livestock in addition to their ability to decompose polystyrene and plastic waste, it is recognized as an insect with an industrial core value. Therefore, it is important to study the immune mechanism related to the development and infection of mealworms for mass breeding purposes. The immune deficiency (Imd) signaling is one of the main pathways with pivotal roles in the production of antimicrobial peptides (AMPs). Transforming growth factor-β activated kinase (TAK1) is one of the Imd pathway components, forms a complex with TAK1 binding protein 2 (TAB2) to ultimately help activate the transcription factor Relish and eventually induce host to produce AMPs. Relatively, little has been revealed about TAK1 in insect models, especially in the T. molitor. Therefore, this study was conducted to elucidate the function of TmTak1 in T. molitor. Our results showed that the highest and lowest mRNA expression of TmTak1 were found in egg and young larvae respectively. The tissue-specific expression patterns were reported in the gut of T. molitor larvae and the fat bodies of adults. Systemic microbial challenge illustrated TmTak1 high expression following the fungal infection in all dissected tissues except for the whole body. However, silencing TmTak1 experiments showed that the survivability of T. molitor larvae affected significantly following Escherichia coli infection. Accordingly, AMP induction after TmTak1 knock down was mainly reported in the integument and the fat bodies.

Feeding Spodoptera exigua larvae with gut-derived Escherichia sp. increases larval juvenile hormone levels inhibiting cannibalism

Feed quality influences insect cannibalistic behavior and gut microbial communities. In the present study, Spodoptera exigua larvae were fed six different artificial diets, and one of these diets (Diet 3) delayed larval cannibalistic behavior and reduced the cannibalism ratio after ingestion. Diet 3-fed larvae had the highest gut bacterial load (1.396 ± 0.556 × 1014 bacteria/mg gut), whereas Diet 2-fed larvae had the lowest gut bacterial load (3.076 ± 1.368 × 1012 bacteria/mg gut). The gut bacterial composition and diversity of different diet-fed S. exigua larvae varied according to the 16S rRNA gene sequence analysis. Enterobacteriaceae was specific to the Diet 3-fed larval gut. Fifteen culturable bacterial isolates were obtained from the midgut of Diet 3-fed larvae. Of these, ten belonged to Escherichia sp. After administration with Diet 1- or 2-fed S. exigua larvae, two bacterial isolates (SePC-12 and -37) delayed cannibalistic behavior in both tested larval groups. Diet 2-fed larvae had the lowest Juvenile hormone (JH) concentration and were more aggressive against intraspecific predation. However, SePC-12 loading increased the JH hormone levels in Diet 2-fed larvae and inhibited their cannibalism. Bacteria in the larval midgut are involved in the stabilization of JH levels, thereby regulating host larval cannibalistic behavior.

The reproductive status determines tolerance and resistance to Mycobacterium marinum in Drosophila melanogaster

Sex and reproductive status of the host have a major impact on the immune response against infection. Our aim was to understand their impact on host tolerance or resistance in the systemic Mycobacterium marinum infection of Drosophila melanogaster. We measured host survival and bacillary load at time of death, as well as expression by quantitative real-time polymerase chain reaction of immune genes (diptericin and drosomycin). We also assessed the impact of metabolic and hormonal regulation in the protection against infection by measuring expression of upd3, impl2 and ecR. Our data showed increased resistance in actively mating flies and in mated females, while reducing their tolerance to infection. Data suggests that Toll and immune deficiency (Imd) pathways determine tolerance and resistance, respectively, while higher basal levels of ecR favours the stimulation of the Imd pathway. A dual role has been found for upd3 expression, linked to increased/decreased mycobacterial load at the beginning and later in infection, respectively. Finally, impl2 expression has been related to increased resistance in non-actively mating males. These results allow further assessment on the differences between sexes and highlights the role of the reproductive status in D. melanogaster to face infections, demonstrating their importance to determine resistance and tolerance against M. marinum infection.

Kruppel homolog 1 modulates ROS production and antimicrobial peptides expression in shrimp hemocytes during infection by the Vibrio parahaemolyticus strain that causes AHPND

Shrimp aquaculture has been seriously affected by acute hepatopancreatic necrosis disease (AHPND), caused by a strain of Vibrio parahaemolyticus that carries the Pir toxin plasmids (V. parahaemolyticus (AHPND)). In this study, the transcription factor, Kruppel homolog 1-like of Peneaus vannamei (PvKr-h1), was significantly induced in shrimp hemocytes after V. parahaemolyticus (AHPND) challenge, suggesting that PvKr-h1 is involved in shrimp immune response. Knockdown of PvKr-h1 followed by V. parahaemolyticus (AHPND) challenge increased bacterial abundance in shrimp hemolymph coupled with high shrimp mortality. Moreover, transcriptome and immunofluorescence analyses revealed that PvKr-h1 silencing followed by V. parahaemolyticus (AHPND) challenge dysregulated the expression of several antioxidant-related enzyme genes, such as Cu-Zu SOD, GPX, and GST, and antimicrobial peptide genes, i.e., CRUs and PENs, and reduced ROS activity and nuclear translocation of Relish. These data reveal that PvKr-h1 regulates shrimps' immune response to V. parahaemolyticus (AHPND) infection by suppressing antioxidant-related enzymes, enhancing ROS production and promoting nuclei import of PvRelish to stimulate antimicrobial peptide genes expression.

Long non-coding RNAs regulate Aedes aegypti vector competence for Zika virus and reproduction

Long non-coding RNAs (lncRNAs) play critical regulatory roles in various cellular and metabolic processes in mosquitoes and all other organisms studied thus far. In particular, their involvement in essential processes such as reproduction makes them potential targets for the development of novel pest control approaches. However, their function in mosquito biology remains largely unexplored. To elucidate the role of lncRNAs in mosquitoes' reproduction and vector competence for arboviruses, we have implemented a computational and experimental pipeline to mine, screen, and characterize lncRNAs related to these two biological processes. Through analysis of publicly available Zika virus (ZIKV) infection-regulated Aedes aegypti transcriptomes, at least six lncRNAs were identified as being significantly upregulated in response to infection in various mosquito tissues. The roles of these ZIKV-regulated lncRNAs (designated Zinc1, Zinc2, Zinc3, Zinc9, Zinc10 and Zinc22), were further investigated by dsRNA-mediated silencing studies. Our results show that silencing of Zinc1, Zinc2, and Zinc22 renders mosquitoes significantly less permissive to ZIKV infection, while silencing of Zinc22 also reduces fecundity, indicating a potential role for Zinc22 in trade-offs between vector competence and reproduction. We also found that silencing of Zinc9 significantly increases fecundity but has no effect on ZIKV infection, suggesting that Zinc9 may be a negative regulator of oviposition. Our work demonstrates that some lncRNAs play host factor roles by facilitating viral infection in mosquitoes. We also show that lncRNAs can influence both mosquito reproduction and permissiveness to virus infection, two biological systems with important roles in mosquito vectorial capacity.

Steroid hormone regulation of innate immunity in Drosophila melanogaster

Endocrine signaling networks control diverse biological processes and life history traits across metazoans. In both invertebrate and vertebrate taxa, steroid hormones regulate immune system function in response to intrinsic and environmental stimuli, such as microbial infection. The mechanisms of this endocrine-immune regulation are complex and constitute an ongoing research endeavor facilitated by genetically tractable animal models. The 20-hydroxyecdysone (20E) is the major steroid hormone in arthropods, primarily studied for its essential role in mediating developmental transitions and metamorphosis; 20E also modulates innate immunity in a variety of insect taxa. This review provides an overview of our current understanding of 20E-mediated innate immune responses. The prevalence of correlations between 20E-driven developmental transitions and innate immune activation are summarized across a range of holometabolous insects. Subsequent discussion focuses on studies conducted using the extensive genetic resources available in Drosophila that have begun to reveal the mechanisms underlying 20E regulation of immunity in the contexts of both development and bacterial infection. Lastly, I propose directions for future research into 20E regulation of immunity that will advance our knowledge of how interactive endocrine networks coordinate animals' physiological responses to environmental microbes.

Croquemort elicits activation of the immune deficiency pathway in ticks

The immune deficiency (IMD) pathway directs host defense in arthropods upon bacterial infection. In Pancrustacea, peptidoglycan recognition proteins sense microbial moieties and initiate nuclear factor-κB-driven immune responses. Proteins that elicit the IMD pathway in non-insect arthropods remain elusive. Here, we show that an Ixodes scapularis homolog of croquemort (Crq), a CD36-like protein, promotes activation of the tick IMD pathway. Crq exhibits plasma membrane localization and binds the lipid agonist 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol. Crq regulates the IMD and jun N-terminal kinase signaling cascades and limits the acquisition of the Lyme disease spirochete B. burgdorferi. Additionally, nymphs silenced for crq display impaired feeding and delayed molting to adulthood due to a deficiency in ecdysteroid synthesis. Collectively, we establish a distinct mechanism for arthropod immunity outside of insects and crustaceans.

Drosophila melanogaster as a model to study age and sex differences in brain injury and neurodegeneration after mild head trauma

Repetitive physical insults to the head, including those that elicit mild traumatic brain injury (mTBI), are a known risk factor for a variety of neurodegenerative conditions including Alzheimer's disease (AD), Parkinson's disease (PD), and chronic traumatic encephalopathy (CTE). Although most individuals who sustain mTBI typically achieve a seemingly full recovery within a few weeks, a subset experience delayed-onset symptoms later in life. As most mTBI research has focused on the acute phase of injury, there is an incomplete understanding of mechanisms related to the late-life emergence of neurodegeneration after early exposure to mild head trauma. The recent adoption of Drosophila-based brain injury models provides several unique advantages over existing preclinical animal models, including a tractable framework amenable to high-throughput assays and short relative lifespan conducive to lifelong mechanistic investigation. The use of flies also provides an opportunity to investigate important risk factors associated with neurodegenerative conditions, specifically age and sex. In this review, we survey current literature that examines age and sex as contributing factors to head trauma-mediated neurodegeneration in humans and preclinical models, including mammalian and Drosophila models. We discuss similarities and disparities between human and fly in aging, sex differences, and pathophysiology. Finally, we highlight Drosophila as an effective tool for investigating mechanisms underlying head trauma-induced neurodegeneration and for identifying therapeutic targets for treatment and recovery.

BmMD-2A responds to 20-hydroxyecdysone and regulates Bombyx mori silkworm innate immunity in larva-to-pupa metamorphosis

20E-hydroxyecdysone (20E) plays important roles in larval molting and metamorphosis in insects and is also involved in the insect innate immune response. Insect metamorphosis is a highly successful strategy for environmental adaptation and is the most vulnerable stage during which the insect is susceptible to various pathogens. 20E regulates a series of antimicrobial peptides (AMPs) through the immunodeficiency (IMD) pathway activation in Drosophila; nevertheless, whether other immune pathways are involved in 20E-regulated insect immunity is unknown. Our previous studies showed that BmMD-2A is a member of the MD-2-related lipid recognition (ML) family of proteins that are involved in the Bombyx mori innate immunity Toll signaling pathway. In this study, we further demonstrate that BmMD-2A is also positively regulated by 20E, and the BmMD-2A neutralization experiment suggested that 20E activates some downstream immune effect factors, the AMP genes against Escherichia coli and Staphylococcus aureus, through the regulation of BmMD-2A in larval metamorphosis, implying that B. mori may use the Toll-ML signaling pathway to maintain innate immune balance in the larval-pupal metamorphosis stage, which is a different innate immunity pathway regulated by 20E compared to the IMD pathway in Drosophila.

Aedes aegypti Malpighian tubules are immunologically activated following systemic Toll activation

BACKGROUND

Canine heartworm is a widespread and potentially fatal mosquito-borne disease caused by infections with the parasitic nematode, Dirofilaria immitis. We have previously shown that systemic activation of the Toll immune pathway via silencing of the negative regulator Cactus in Aedes aegypti blocks parasite development in the Malpighian tubules (MT), the mosquito renal organ. However, it was not established whether the MT were directly responding to Toll activation or were alternatively responding to upregulated proteins or other changes to the hemolymph driven by other tissues. Distinguishing these possibilities is crucial for developing more precise strategies to block D. immitis while potentially avoiding the fitness cost to the mosquito associated with Cactus silencing.

METHODS

This study defines the transcriptional response of the MT and changes to the hemolymph proteome of Ae. aegypti after systemic Toll activation via intra-thoracic injection of double-stranded Cactus (dsCactus) RNA.

RESULTS

Malpighian tubules significantly increased expression of the Toll pathway target genes that significantly overlapped expression changes occurring in whole mosquitoes. A significant overlap between the transcriptional response of the MT and proteins upregulated in the hemolymph was also observed.

CONCLUSIONS

Our data show that MT are capable of RNA interference-mediated gene silencing and directly respond to dsCactus treatment by upregulating targets of the canonical Toll pathway. Although not definitive, the strong correspondence between the MT transcriptional response and the hemolymph proteomic responses provides evidence that the MT may contribute to mosquito humoral immunity.

The role of micro RNAs (miRNAs) in the regulation of Drosophila melanogaster's innate immunity

MicroRNAs (miRNAs) are a class of small non-coding RNAs ~19-22 nt long which post-transcriptionally regulate gene expression. Their ability to exhibit dynamic expression patterns coupled with their wide variety of targets allows miRNAs to regulate many processes, including the innate immune response of Drosophila melanogaster. Recent studies have identified miRNAs in Drosophila which are differentially expressed during infection with different pathogens as well as miRNAs that may affect immune signalling when differentially expressed. This review provides an overview of miRNAswhich have been identified to play a role in the immune response of Drosophila through targeting of the Toll and IMD signalling pathways and other immune processes. It will also explore the role of miRNAs in fine-tuning the immune response in Drosophila and highlight current gaps in knowledge regarding the role of miRNAs in immunity and areas for further research.

Vibrio cholerae high cell density quorum sensing activates the host intestinal innate immune response

Quorum sensing fundamentally alters the interaction of Vibrio cholerae with aquatic environments, environmental hosts, and the human intestine. At high cell density, the quorum-sensing regulator HapR represses not only expression of cholera toxin and the toxin co-regulated pilus, virulence factors essential in human infection, but also synthesis of the Vibrio polysaccharide (VPS) exopolysaccharide-based matrix required for abiotic and biotic surface attachment. Here, we describe a feature of V. cholerae quorum sensing that shifts the host-pathogen interaction toward commensalism. By repressing pathogen consumptive anabolic metabolism and, in particular, tryptophan uptake, V. cholerae HapR stimulates host intestinal serotonin production. This, in turn, activates host intestinal innate immune signaling to promote host survival.

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.

The Unfolded-Protein Response Triggers the Arthropod Immune Deficiency Pathway

The insect immune deficiency (IMD) pathway is a defense mechanism that senses and responds to Gram-negative bacteria. Ticks lack genes encoding upstream components that initiate the IMD pathway. Despite this deficiency, core signaling molecules are present and functionally restrict tick-borne pathogens. The molecular events preceding activation remain undefined. Here, we show that the unfolded-protein response (UPR) initiates the IMD network. The endoplasmic reticulum (ER) stress receptor IRE1α is phosphorylated in response to tick-borne bacteria but does not splice the mRNA encoding XBP1. Instead, through protein modeling and reciprocal pulldowns, we show that Ixodes IRE1α complexes with TRAF2. Disrupting IRE1α-TRAF2 signaling blocks IMD pathway activation and diminishes the production of reactive oxygen species. Through in vitro, in vivo, and ex vivo techniques, we demonstrate that the UPR-IMD pathway circuitry limits the Lyme disease-causing spirochete Borrelia burgdorferi and the rickettsial agents Anaplasma phagocytophilum and A. marginale (anaplasmosis). Altogether, our study uncovers a novel linkage between the UPR and the IMD pathway in arthropods. IMPORTANCE The ability of an arthropod to harbor and transmit pathogens is termed "vector competency." Many factors influence vector competency, including how arthropod immune processes respond to the microbe. Divergences in innate immunity between arthropods are increasingly being reported. For instance, although ticks lack genes encoding key upstream molecules of the immune deficiency (IMD) pathway, it is still functional and restricts causative agents of Lyme disease (Borrelia burgdorferi) and anaplasmosis (Anaplasma phagocytophilum). How the IMD pathway is activated in ticks without classically defined pathway initiators is not known. Here, we found that a cellular stress response network, the unfolded-protein response (UPR), functions upstream to induce the IMD pathway and restrict transmissible pathogens. Collectively, this explains how the IMD pathway can be activated in the absence of canonical pathway initiators. Given that the UPR is highly conserved, UPR-initiated immunity may be a fundamental principle impacting vector competency across arthropods.

SPS1 deficiency-triggered PGRP-LC and Toll expression controls innate immunity in Drosophila S2 cells

Selenophosphate synthetase 1 (SPS1) is an essential gene for the cell growth and embryogenesis in Drosophila melanogaster. We have previously reported that SPS1 deficiency stimulates the expression of genes responsible for the innate immune system, including antimicrobial peptides (AMPs), in Drosophila S2 cells. However, the underlying mechanism has not been elucidated. Here, we investigated the immune pathways that control the SPS1-deficiency-induced expression of AMPs in S2 cells. It was found that the activation of AMP expression is regulated by both immune deficiency (IMD) and the Toll pathway. Double knockdown of the upstream genes of each pathway with SPS1 showed that the peptidoglycan recognition protein-LC (PGRP-LC) and Toll genes are targeted by SPS1 for regulating these pathways. We also found that the IMD and Toll pathway regulate AMP expression by cross-talking. The levels of PGRP-LC and Toll mRNAs were upregulated upon Sps1 knockdown (6.4±0.36 and 3.2±0.45-fold, respectively, n=3). Overexpression of each protein also upregulated AMPs. Interestingly, PGRP-LC overexpression upregulated AMP more than Toll overexpression. These data strongly suggest that SPS1 controls the innate immune system of D. melanogaster through regulating PGRP-LC and Toll expression.

Drosophila Innate Immunity Involves Multiple Signaling Pathways and Coordinated Communication Between Different Tissues

The innate immune response provides the first line of defense against invading pathogens, and immune disorders cause a variety of diseases. The fruit fly Drosophila melanogaster employs multiple innate immune reactions to resist infection. First, epithelial tissues function as physical barriers to prevent pathogen invasion. In addition, macrophage-like plasmatocytes eliminate intruders through phagocytosis, and lamellocytes encapsulate large particles, such as wasp eggs, that cannot be phagocytosed. Regarding humoral immune responses, the fat body, equivalent to the mammalian liver, secretes antimicrobial peptides into hemolymph, killing bacteria and fungi. Drosophila has been shown to be a powerful in vivo model for studying the mechanism of innate immunity and host-pathogen interactions because Drosophila and higher organisms share conserved signaling pathways and factors. Moreover, the ease with which Drosophila genetic and physiological characteristics can be manipulated prevents interference by adaptive immunity. In this review, we discuss the signaling pathways activated in Drosophila innate immunity, namely, the Toll, Imd, JNK, JAK/STAT pathways, and other factors, as well as relevant regulatory networks. We also review the mechanisms by which different tissues, including hemocytes, the fat body, the lymph gland, muscles, the gut and the brain coordinate innate immune responses. Furthermore, the latest studies in this field are outlined in this review. In summary, understanding the mechanism underlying innate immunity orchestration in Drosophila will help us better study human innate immunity-related diseases.

An Integrated Transcriptomics and Lipidomics Analysis Reveals That Ergosterol Is Required for Host Defense Against Bacterial Infection in Drosophila

Animals adjust their lipid metabolism states in response to pathogens infection. However, the underlying molecular mechanisms for how lipid metabolism responds to infection remain to be elusive. In this study, we assessed the temporal changes of lipid metabolism profiles during infection by an integrated transcriptomics and lipidomics analysis. Ergosterol is identified to be required for proper host defense to pathogens. Notably, ergosterol level is increased in the hemolymph upon bacterial infection. We show that the increase of ergosterol level by food supplement or genetic depletion of Acsl, a long-chain fatty acid-CoA synthetase, promotes host survival against bacterial challenges. Together, our results suggest a critical role of lipid metabolism adaption in the process of host defense against invading pathogens.

Calcium and integrin-binding protein 1-like interacting with an integrin α-cytoplasmic domain facilitates cellular immunity in Helicoverpa armigera

Integrins are transmembrane receptor heterodimers composed of α and β subunits. They are known to mediate extracellular signals to promote cell adhesion and spreading, and are therefore essential for cellular immunity. However, proteins that bind to integrin cytoplasmic domains and mediate intracellular signaling to promote cell adhesion require identification. Calcium and integrin-binding protein 1 (CIB1) that binds to the integrin α-cytoplasmic domain has rarely been examined in insects. In this study, we found that 20-hydroxyecdysone promoted cell phagocytosis and spreading in Helicoverpa armigera. Transcriptomic analyses of hemocytes identified an integrin α gene (HaINTα-PS1) whose expression could be induced by either 20-hydroxyecdysone injection or bead challenge. Isothermal titration calorimetry assays showed that H. armigera CIB1-like (HaCIB1-like) weakly bound to the cytoplasmic domain of HaINTα-PS1 in the presence of calcium. HaINTα-PS1 or HaCIB1-like knockdown inhibited hemocytic encapsulation and phagocytosis, and plasmatocyte spreading. Moreover, HaCIB1-like overexpression in a H. armigera epidermal cell line overexpanded cells and impaired cell phagocytosis. Thus, insect CIB1-like potentially interacted with integrin α-cytoplasmic domain and facilitated cell adhesion. This study enriches our understanding of the molecular mechanism underlying integrin-mediated cellular immunity in insects.

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.

Endocrine Regulation of Lifespan in Insect Diapause

Diapause is a physiological adaptation to conditions that are unfavorable for growth or reproduction. During diapause, animals become long-lived, stress-resistant, developmentally static, and non-reproductive, in the case of diapausing adults. Diapause has been observed at all developmental stages in both vertebrates and invertebrates. In adults, diapause traits weaken into adaptations such as hibernation, estivation, dormancy, or torpor, which represent evolutionarily diverse versions of the traditional diapause traits. These traits are regulated through modifications of the endocrine program guiding development. In insects, this typically includes changes in molting hormones, as well as metabolic signals that limit growth while skewing the organism's energetic demands toward conservation. While much work has been done to characterize these modifications, the interactions between hormones and their downstream consequences are incompletely understood. The current state of diapause endocrinology is reviewed here to highlight the relevance of diapause beyond its use as a model to study seasonality and development. Specifically, insect diapause is an emerging model to study mechanisms that determine lifespan. The induction of diapause represents a dramatic change in the normal progression of age. Hormones such as juvenile hormone, 20-hydroxyecdysone, and prothoracicotropic hormone are well-known to modulate this plasticity. The induction of diapause-and by extension, the cessation of normal aging-is coordinated by interactions between these pathways. However, research directly connecting diapause endocrinology to the biology of aging is lacking. This review explores connections between diapause and aging through the perspective of endocrine signaling. The current state of research in both fields suggests appreciable overlap that will greatly contribute to our understanding of diapause and lifespan determination.

Scavenger receptor C regulates antimicrobial peptide expression by activating toll signaling in silkworm, Bombyx mori

Scavenger receptor is pattern-recognition receptor (PRR) that plays a crucial function in host defense against pathogens. Scavenger receptor C (SR-C) is present only in invertebrates and its function has not been studied in detail. In this study, an SR-C homologous gene from the silkworm, Bombyx mori, was identified and characterized. SR-C was largely expressed in hemocytes and Malpighian tubules, with continuous expression in hemocytes. The peak expression was observed in hemocytes during molting and wandering stages both at mRNA and protein levels. Furthermore, immunofluorescence demonstrated it to be mainly distributed in the cell membranes of hemocytes, including oenocytoids and granulocytes. The recombinant SR-C protein (rSR-C) could bind to different types of bacteria and pathogen-associated molecular patterns (PAMPs), with strong binding to gram-positive bacteria and Lys-type peptidoglycans. The overexpression of SR-C induced the expression of genes related to the Toll pathway and antibacterial peptides. While the knockdown of SR-C reduced the expression of AMPs and inhibited the Toll pathway, it impaired the bacterial clearance ability of silkworm larvae, thus decreasing silkworm larvae's survival rate. Altogether, SR-C is a PRR that protect silkworms against bacterial pathogens by enhancing the expression of AMPs expression via the Toll pathway in hemocytes.

Co-option of immune effectors by the hormonal signalling system triggering metamorphosis in Drosophila melanogaster

Insect metamorphosis is triggered by the production, secretion and degradation of 20-hydroxyecdysone (ecdysone). In addition to its role in developmental regulation, increasing evidence suggests that ecdysone is involved in innate immunity processes, such as phagocytosis and the induction of antimicrobial peptide (AMP) production. AMP regulation includes systemic responses as well as local responses at surface epithelia that contact with the external environment. At pupariation, Drosophila melanogaster increases dramatically the expression of three AMP genes, drosomycin (drs), drosomycin-like 2 (drsl2) and drosomycin-like 5 (drsl5). We show that the systemic action of drs at pupariation is dependent on ecdysone signalling in the fat body and operates via the ecdysone downstream target, Broad. In parallel, ecdysone also regulates local responses, specifically through the activation of drsl2 expression in the gut. Finally, we confirm the relevance of this ecdysone dependent AMP expression for the control of bacterial load by showing that flies lacking drs expression in the fat body have higher bacterial persistence over metamorphosis. In contrast, local responses may be redundant with the systemic effect of drs since reduction of ecdysone signalling or of drsl2 expression has no measurable negative effect on bacterial load control in the pupa. Together, our data emphasize the importance of the association between ecdysone signalling and immunity using in vivo studies and establish a new role for ecdysone at pupariation, which impacts developmental success by regulating the immune system in a stage-dependent manner. We speculate that this co-option of immune effectors by the hormonal system may constitute an anticipatory mechanism to control bacterial numbers in the pupa, at the core of metamorphosis evolution.

Guadipyr, a new insecticide, induces microbiota dysbiosis and immune disorders in the midgut of silkworms (Bombyx mori)

Guadipyr, which combines neonicotinoid and semicarbazone functional groups in one molecule, exhibits good activity on several pests and high acute and chronic toxicity to silkworms (Bombyx mori). In this report, the effects of low-dose guadipyr on the midgut microbiota and immune system of silkworms were studied. Results showed that the structure and richness of the midgut microbiota of silkworms were altered after being treated with 5.25 mg/L (1/10 of LC₅₀) of guadipyr. The abundance of Pseudomonas was evidently increased, whereas Curvibacter was substantially reduced, which might be related to the growth and immunity of silkworms. The expression of key genes in the Toll, IMD, and JAK/STAT pathways, which ultimately led to the downregulation of antimicrobial peptide genes (AMPs), such as CecA, Defensin1, Leb, and glv2, was reduced upon guadipyr exposure. Simultaneously, the suppression of steroid hormone 20-hydroxyecdysone receptor and response genes, such as BR-C Z4, was detected in the exposed groups. The decreased expression of these immune regulatory pathway-related and 20-hydroxyecdysone signal pathway-related genes indicated that the immune system of silkworms was affected by low-dose guadipyr. Our results revealed the negative effects of guadipyr on silkworms and highlighted the unneglectable toxicity of low-dose guadipyr to this economic insect. Given the risk, it is necessary to control the application of guadipyr in or around the mulberry fields.

Drosophila H2Av negatively regulates the activity of the IMD pathway via facilitating Relish SUMOylation

Insects depend on the innate immune response for defense against a wide array of pathogens. Central to Drosophila immunity are antimicrobial peptides (AMPs), released into circulation when pathogens trigger either of the two widely studied signal pathways, Toll or IMD. The Toll pathway responds to infection by Gram-positive bacteria and fungi while the IMD pathway is activated by Gram-negative bacteria. During activation of the IMD pathway, the NF-κB-like transcription factor Relish is phosphorylated and then cleaved, which is crucial for IMD-dependent AMP gene induction. Here we show that loss-of-function mutants of the unconventional histone variant H2Av upregulate IMD-dependent AMP gene induction in germ-free Drosophila larvae and adults. After careful dissection of the IMD pathway, we found that Relish has an epistatic relationship with H2Av. In the H2Av mutant larvae, SUMOylation is down-regulated, suggesting a possible role of SUMOylation in the immune phenotype. Eventually we demonstrated that Relish is mostly SUMOylated on amino acid K823. Loss of the potential SUMOylation site leads to significant auto-activation of Relish in vivo. Further work indicated that H2Av regulates Relish SUMOylation after physically interacting with Su(var)2-10, the E3 component of the SUMOylation pathway. Biochemical analysis suggested that SUMOylation of Relish prevents its cleavage and activation. Our findings suggest a new mechanism by which H2Av can negatively regulate, and thus prevent spontaneous activation of IMD-dependent AMP production, through facilitating SUMOylation of the NF-κB like transcription factor Relish.

Toll and IMD signaling pathways should be involved in the production of antimicrobial peptides in animals upon infection. Immunity responses are energy consuming. Thus, these two pathways are fine-tuned. Animal H2A variant histones are involved in many physiological functions. In Drosophila, the production of antibacterial peptides is out of control in the mutant of H2A variant (H2Av⁸¹⁰). After careful examination, we found that Relish, the transcription factor of the IMD pathway, was activated in this mutant. Eventually we demonstrate that Relish can be SUMOylated with the involvement of H2Av. Loss of the main SUMOylation site in Relish induces it to auto-activate following over-expression. Therefore, H2Av is a negative regulator of the IMD signaling pathway by maintaining the normal level of Relish SUMOylation in Drosophila.

Relish plays a dynamic role in the niche to modulate Drosophila blood progenitor homeostasis in development and infection

Immune challenges demand the gearing up of basal hematopoiesis to combat infection. Little is known about how during development, this switch is achieved to take care of the insult. Here, we show that the hematopoietic niche of the larval lymph gland of Drosophila senses immune challenge and reacts to it quickly through the nuclear factor-κB (NF-κB), Relish, a component of the immune deficiency (Imd) pathway. During development, Relish is triggered by ecdysone signaling in the hematopoietic niche to maintain the blood progenitors. Loss of Relish causes an alteration in the cytoskeletal architecture of the niche cells in a Jun Kinase-dependent manner, resulting in the trapping of Hh implicated in progenitor maintenance. Notably, during infection, downregulation of Relish in the niche tilts the maintenance program toward precocious differentiation, thereby bolstering the cellular arm of the immune response.

Microbiota-derived acetate activates intestinal innate immunity via the Tip60 histone acetyltransferase complex

Microbe-derived acetate activates the Drosophila immunodeficiency (IMD) pathway in a subset of enteroendocrine cells (EECs) of the anterior midgut. In these cells, the IMD pathway co-regulates expression of antimicrobial and enteroendocrine peptides including tachykinin, a repressor of intestinal lipid synthesis. To determine whether acetate acts on a cell surface pattern recognition receptor or an intracellular target, we asked whether acetate import was essential for IMD signaling. Mutagenesis and RNA interference revealed that the putative monocarboxylic acid transporter Tarag was essential for enhancement of IMD signaling by dietary acetate. Interference with histone deacetylation in EECs augmented transcription of genes regulated by the steroid hormone ecdysone including IMD targets. Reduced expression of the histone acetyltransferase Tip60 decreased IMD signaling and blocked rescue by dietary acetate and other sources of intracellular acetyl-CoA. Thus, microbe-derived acetate induces chromatin remodeling within enteroendocrine cells, co-regulating host metabolism and intestinal innate immunity via a Tip60-steroid hormone axis that is conserved in mammals.

Manipulation of host ecdysteroid hormone levels facilitates infection by the fungal insect pathogen, Metarhizium rileyi

Entomopathogenic fungi such as Metarhizium rileyi and Beauveria bassiana are widely used insect biological control agents. Little, however, is known concerning genetic or enzymatic factors that differentiate the mechanisms employed by these two fungal pathogens to infect target hosts. Infection by either of these organisms is known to increase levels of the growth and molting hormone, ecdysone, which also regulates the expression of a number of innate immune pathways. M. rileyi, but not B. bassiana, has apparently evolved an ecdysteroid-22-oxidase (MrE22O) that inactivate ecdysone. We show that deletion of MrE22O impaired virulence compared with the wild-type strain, with an increase in ecdysone titer seen in hosts that was coupled to an increase in the expression of antimicrobial genes. An M. rileyi strain engineered to overexpress MrE22O (MrE22O^OE ), as well as trans-expression in B. bassiana (Bb::MrE220^OE ) resulted, in strains displaying enhanced virulence and dampening of host immune responses compared with their respective wild-type parental strains. These results indicate that ecdysone plays an important role in mediating responses to fungal infection and that some insect pathogenic fungi have evolved mechanisms for targeting this hormone as a means for facilitating infection.

Non-immune Traits Triggered by Blood Intake Impact Vectorial Competence

Blood-feeding arthropods are considered an enormous public health threat. They are vectors of a plethora of infectious agents that cause potentially fatal diseases like Malaria, Dengue fever, Leishmaniasis, and Lyme disease. These vectors shine due to their own physiological idiosyncrasies, but one biological aspect brings them all together: the requirement of blood intake for development and reproduction. It is through blood-feeding that they acquire pathogens and during blood digestion that they summon a collection of multisystemic events critical for vector competence. The literature is focused on how classical immune pathways (Toll, IMD, and JAK/Stat) are elicited throughout the course of vector infection. Still, they are not the sole determinants of host permissiveness. The dramatic changes that are the hallmark of the insect physiology after a blood meal intake are the landscape where a successful infection takes place. Dominant processes that occur in response to a blood meal are not canonical immunological traits yet are critical in establishing vector competence. These include hormonal circuitries and reproductive physiology, midgut permeability barriers, midgut homeostasis, energy metabolism, and proteolytic activity. On the other hand, the parasites themselves have a role in the outcome of these blood triggered physiological events, consistently using them in their favor. Here, to enlighten the knowledge on vector-pathogen interaction beyond the immune pathways, we will explore different aspects of the vector physiology, discussing how they give support to these long-dated host-parasite relationships.

Secreted virulence factors from Heterorhabditis bacteriophora highlight its utility as a model parasite among Clade V nematodes

Much of the available knowledge of entomopathogenic virulence factors has been gleaned from studies in the nematode parasite Steinernema carpocapsae, but there is good reason to complement this knowledge with similar studies in Heterorhabditis bacteriophora. Three candidate virulence factors from H. bacteriophora have recently been characterised, and each was demonstrated to contribute to infection. This information can be used not only to advance efforts in the biocontrol of insect pests, but also to make inferences about the emergence of parasitism among Clade V nematodes.

Regulation of antimicrobial peptides by juvenile hormone and its receptor, Methoprene-tolerant, in the mosquito Aedes aegypti

The trade-off between reproduction and immunity has been established for a number of insect species. However, the regulatory mechanisms governing this event is not well understood. In the mosquito Aedes aegypti, the vector of dangerous human arboviral diseases, juvenile hormone (JH) is required for the female post-eclosion development and reproductive maturation. In this study, we have revealed the JH negative effect on the expression of immunity-related genes, such as antimicrobial peptides (AMPs), during the post-eclosion phase of the female mosquito gonadotrophic reproductive cycle. Mosquitoes treated with JH became more sensitive to microbial infection. Mosquitoes subjected to the RNA interference knockdown (RNAi) of the JH receptor, Methoprene-tolerant (Met), showed increased expression of several AMP genes. Met binds to the E-box-like recognition motifs in the regulatory region of the diptericin (Dpt) gene, indicating that JH can suppress the Dpt gene expression through its receptor Met. Hence, JH is involved in the modulation of immune responses during the post-eclosion phase of reproduction. The RNAi knockdown of the peptidoglycan recognition protein (PGRP-LC) led to a significant reduction of the Dpt transcript level, indicating the PGRP-LC activating role on this AMP gene. Thus, Dpt appeared to be under the dual regulation of both the JH and the immune deficiency (IMD) signaling pathways. Our study provides a better understanding of how JH regulates insect immunity in adult mosquitoes.

Azadirachtin interferes with basal immunity and microbial homeostasis in the Rhodnius prolixus midgut

Rhodnius prolixus is an insect vector of two flagellate parasites, Trypanosoma rangeli and Trypanosoma cruzi, the latter being the causative agent of Chagas disease in Latin America. The R. prolixus neuroendocrine system regulates the synthesis of the steroid hormone ecdysone, which is essential for not only development and molting but also insect immunity. Knowledge for how this modulates R. prolixus midgut immune responses is essential for understanding interactions between the vector, its parasites and symbiotic microbes. In the present work, we evaluated the effects of ecdysone inhibition on R. prolixus humoral immunity and homeostasis with its microbiota, using the triterpenoid natural product, azadirachtin. Our results demonstrated that azadirachtin promoted a fast and lasting inhibitory effect on expression of both RpRelish, a nuclear factor kappa B transcription factor (NF-kB) component of the IMD pathway, and several antimicrobial peptide (AMP) genes. On the other hand, RpDorsal, encoding the equivalent NF-kB transcription factor in the Toll pathway, and the defC AMP gene were upregulated later in azadirachtin treated insects. The treatment also impacted on proliferation of Serratia marcescens, an abundant commensal bacterium. The simultaneous administration of ecdysone and azadirachtin in R. prolixus blood meals counteracted the azadirachtin effects on insect molting and also on expression of RpRelish and AMPs genes. These results support the direct involvement of ecdysone in regulation of the IMD pathway in the Rhodnius prolixus gut.

Endocrine regulation of immunity in insects

Organisms have constant contact with potentially harmful agents that can compromise their fitness. However, most of the times these agents fail to cause serious disease by virtue of the rapid and efficient immune responses elicited in the host that can range from behavioural adaptations to immune system triggering. The immune system of insects does not comprise the adaptive arm, making it less complex than that of vertebrates, but key aspects of the activation and regulation of innate immunity are conserved across different phyla. This is the case for the hormonal regulation of immunity as a part of the broad organismal responses to external conditions under different internal states. In insects, depending on the physiological circumstances, distinct hormones either enhance or suppress the immune response integrating individual (and often collective) responses physiologically and behaviourally. In this review, we provide an overview of our current knowledge on the endocrine regulation of immunity in insects, its mechanisms and implications on metabolic adaptation and behaviour. We highlight the importance of this multilayered regulation of immunity in survival and reproduction (fitness) and its dependence on the hormonal integration with other mechanisms and life-history traits.

The involvement of ecdysone and ecdysone receptor in regulating the expression of antimicrobial peptides in Chinese mitten crab, Eriocheir sinensis

The ecdysone, 20-hydroxyecdysone (20E) and ecdysone receptor (EcR), are regarded as the key regulators of development, metamorphosis, and growth in arthropods. In the present study, the role of 20E and EsEcR in regulating the expression of antimicrobial peptides (AMPs) was investigated in Chinese mitten crab, Eriocheir sinensis. The concentration of 20E in plasma was significantly (p < 0.05) up-regulated from 3 h to 12 h after lipopolysaccharide (LPS) stimulation. The mRNA expression level of EsEcR-4 in hemocytes was significantly (p < 0.01) up-regulated from 6 h to 24 h after LPS stimulation, while no significant changes of EsEcR-2 and EsEcR-3 transcripts were observed. After 20E injection, EsEcR-4 expression level was significantly increased from 12 h to 48 h with the highest level at 24 h (4.34-fold compared to the control group, p < 0.01), and the mRNA expression levels of AMPs (EsALF-2, EsLYZ and EsCrus) in hemocytes were significantly increased from 6 h to 24 h with the peak level of 2.93-fold (p < 0.01), 2.33-fold (p < 0.01) and 2.75-fold (p < 0.01) at 12 h, respectively. After EsEcR-4 expression was interfered with specific dsRNA, a significant reduction of EsALF-2 (0.56-fold compared to the control group, p < 0.01), EsLYZ (0.27-fold, p < 0.01) and EsCrus (0.41-fold, p < 0.01) mRNA expression level was observed in dsEsEcR-4+LPS group at 12 h post LPS stimulation. Moreover, the mRNA expression levels of EsDorsal and EsJNK in hemocytes were significantly (p < 0.05) increased from 6 h to 24 h post 20E injection, and the phosphorylation of Dorsal and JNK in the hemocytes were significantly (p < 0.01) up-regulated at 3 h post 20E injection, while that in dsEsEcR-4+LPS group were significantly decreased after LPS stimulation compared to dsEsEGFP+LPS group. Taken together, these results suggested that 20E and EsEcR-4 play important roles in regulating the expression level of AMPs in the immune responses of E. sinensis by regulating the mRNA expression level and phosphorylation of Dorsal and JNK.

C-type lectin-mediated microbial homeostasis is critical for Helicoverpa armigera larval growth and development

The immune system of a host functions critically in shaping the composition of the microbiota, and some microbes are involved in regulating host endocrine system and development. However, whether the immune system acts on endocrine and development by shaping the composition of the microbiota remains unclear, and few molecular players or microbes involved in this process have been identified. In the current study, we found that RNA interference of a C-type lectin (HaCTL3) in the cotton bollworm Helicoverpa armigera suppresses ecdysone and juvenile hormone signaling, thus reducing larval body size and delaying pupation. Depletion of HaCTL3 also results in an increased abundance of Enterocuccus mundtii in the hemolymph, which may escape from the gut. Furthermore, HaCTL3 and its controlled antimicrobial peptides (attacin, lebocin, and gloverin) are involved in the clearance of E. mundtii from the hemolymph via phagocytosis or direct bactericidal activity. Injection of E. mundtii into larval hemocoel mimics HaCTL3-depleted phenotypes and suppresses ecdysone and juvenile hormone signaling. Taken together, we conclude that HaCTL3 maintains normal larval growth and development of H. armigera via suppressing the abundance of E. mundtii in the hemolymph. Our results provide the first evidence of an immune system acting on an endocrine system to modulate development via shaping the composition of microbiota in insect hemolymph. Thus, this study will deepen our understanding of the interaction between immunity and development.

Considering that a large number of hemocytes and multiple soluble effectors are present in insect hemolymph, it is conventionally believed that healthy insect hemolymph is a hostile environment for bacteria and is, therefore, sterile. However, increasing evidences disprove this opinion, although the interactive mechanism between hemolymph microbiota and insect host, as well as the function of hemolymph microbiota, remain unclear. C-type lectin (CTL), as pattern recognition receptor (PRR), plays important roles in defending against various pathogens. Here we found that various bacteria colonized the hemolymph of the cotton bollworm Helicoverpa armigera. We first reported that an H. armigera CTL (HaCTL3) was involved in modulating larval growth and development. Further study indicated that HaCTL3-mediated homeostasis of Enterocuccus mundtii in the hemolymph is critical for normal larval growth and development. Our study demonstrated that this PRR modulated insect development through shaping hemolymph microbiota, which may represent a novel mechanism of immune system regulation during insect development.

20-Hydroxyecdysone regulates the prophenoloxidase cascade to immunize Metarhizium anisopliae in Locusta migratoria

BACKGROUND

PPO (prophenoloxidase) cascade plays an important role in resisting invasion of entomogenous fungus. The 20-hydroxyecdysone (20E) exerts potent effect on the innate immunity in many insects. However, whether 20E controls the PPO cascade system against fungi and the regulatory mechanism in insects remains unclear.

RESULTS

In this study, both the proteome and transcriptome of Locusta migratoria were determined followed by the induction of 20E. Pattern recognition receptor GNBP-2 (Gram-negative binding proteins) has been identified that responded to 20E at both messenger RNA (mRNA) and protein levels. The PPO gene expression in fat body and PO (phenoloxidase) activity in plasma was found significantly induced after 20E injection and during the high-20E developmental stage. However, when 20E signal was blocked by RNA interference (RNAi) of ecdysone receptor, the expression level of PPO and PO activity failed to be increased by 20E. Thus, 20E could not significantly induce the expression of PPO gene and PO activity after RNAi of GNBP-2. Furthermore, 20E treatment notably enhanced the resistance of L. migratoria against Metarhizium anisopliae. Followed by of GNBP-2 silencing, the mortality of nymphs was significantly increased under the stress of Metarhizium anisopliae, and 20E injection could not increase the resistance.

CONCLUSION

The 20E regulates the PPO system to resist fungal invasion via regulating GNBP-2 in worldwide pest L. migratoria. Our results provide insight into the mechanism of how 20E enhances the antimicrobial immunity, and will be beneficial for modification of entomogenous fungi targeting on hormones and the immune system. © 2020 Society of Chemical Industry.

A putative UDP-glycosyltransferase from Heterorhabditis bacteriophora suppresses antimicrobial peptide gene expression and factors related to ecdysone signaling

Insect pathogens have adopted an array of mechanisms to subvert the immune pathways of their respective hosts. Suppression may occur directly at the level of host-pathogen interactions, for instance phagocytic capacity or phenoloxidase activation, or at the upstream signaling pathways that regulate these immune effectors. Insect pathogens of the family Baculoviridae, for example, are known to produce a UDP-glycosyltransferase (UGT) that negatively regulates ecdysone signaling. Normally, ecdysone positively regulates both molting and antimicrobial peptide production, so the inactivation of ecdysone by glycosylation results in a failure of host larvae to molt, and probably a reduced antimicrobial response. Here, we examine a putative ecdysteroid glycosyltransferase, Hba_07292 (Hb-ugt-1), which was previously identified in the hemolymph-activated transcriptome of the entomopathogenic nematode Heterorhabditis bacteriophora. Injection of recombinant Hb-ugt-1 (rHb-ugt-1) into Drosophila melanogaster flies resulted in diminished upregulation of antimicrobial peptides associated with both the Toll and Immune deficiency pathways. Ecdysone was implicated in this suppression by a reduction in Broad Complex expression and reduced pupation rates in r Hb-ugt-1-injected larvae. In addition to the finding that H. bacteriophora excreted-secreted products contain glycosyltransferase activity, these results demonstrate that Hb-ugt-1 is an immunosuppressive factor and that its activity likely involves the inactivation of ecdysone.

Pattern recognition receptors from lepidopteran insects and their biological functions

Lepidopteran insects have potent innate immunity to fight against the invading pathogens. As the initiation step, pattern recognition receptors (PRRs) recognize and bind microbial surface configurations known as pathogen-associated molecular patterns (PAMPs). Aftermath, they initiate both cellular and humoral immune responses, including phagocytosis, agglutination, nodulation, encapsulation, prophenoloxidase activation, and synthesis of antimicrobial peptides. In this review, we summarize the recent findings concerning PRRs in lepidoptaeran insects, mostly agriculture pests including Helicoverpa armigera, Plutella xylostella, and Spodoptera exigua. We mainly focus on the function and phylogeny of C-type lectins (CTLs), peptidoglycan recognition proteins (PGRPs), β-1,3-glucan recognition proteins (βGRPs), and galectins (GALEs). It enriches our understanding of the immune system of lepidopteran insects and provides directions in the future research.

Nuclear receptor E75 is a transcription suppressor of the Litopenaeus vannamei small subunit hemocyanin gene

Hemocyanin is a respiratory protein that possesses multiple physiological and immunological functions in shrimp. However, the transcriptional regulation of the hemocyanin gene is still poorly understood. Here, the nuclear receptor E75 of Litopenaeus vannamei (LvE75) was identified as one of the transcriptional regulators that modulates the transcription of the small molecular weight hemocyanin gene of L. vannamei (LvHMCs) by inhibiting its core promoter activity in a Dual-luciferase assay. In silico analysis revealed that the core promoter (designated HsP3), which is located at +1517/+1849 bp of LvHMCs contained a putative E75 binding motif ("ACGGAAT", spanning +1812/+1818 bp). Further, LvE75 was shown to inhibit the core promoter activity by direct binding. Importantly, in vivo silencing of LvE75 resulted in a significant upregulation in the mRNA and protein expression of LvHMCs gene. Taken together, our present results provide direct evidence that LvE75 is a transcriptional suppressor of the LvHMCs gene expression.