The Drosophila immune response against Gram-negative bacteria is mediated by a peptidoglycan recognition protein

Exploring the innate immune system of Urechis unicinctus: Insights from full-length transcriptome analysis

The Echiura worm Urechis unicinctus refers to a common benthic invertebrate found in the intertidal zone of Huanghai as well as Bohai Bay. U. unicinctus is known to contain various physiologically active substances, making it highly valuable in terms of its edibility, medicinal properties, and economic potential. Nonetheless, the limited study on the immune system of U. unicinctus poses difficulties for its aquaculture and artificial reproduction. Marine invertebrates, including shellfish and U. unicinctus, are thought to primarily depend on their innate immune system for disease protection, owing to the severalinnate immune molecules they possess. Herein, we employed PacBio single-molecule real-time (SMRT) sequencing technology to perform the full-length transcriptome analysis of U. unicinctus individuals under five different conditions (room temperature (RT), low temperature (LT), high temperature (HT), without water (DRY), ultraviolet irradiation (UV)). Concequently, we identified 59,371 unigenes that had a 2,779 bp average length, 2,613 long non-coding RNAs (lncRNAs), 59,190 coding sequences (CDSs), 35,166 simple sequence repeats (SSRs), and 1,733 transcription factors (TFs), successfully annotating 90.58 % (53,778) of the unigenes. Subsequently, key factors associated with immune-related processes, such as non-self-recognition, cellular immune defenses, and humoral immune defenses, were searched. Our study also identified pattern recognition receptors (PRRs) that included 17 peptidoglycan recognition proteins (PGRPs), 13 Gram-negative binding proteins (GNBPs), 18 scavenger receptors (SRs), 74 toll-like receptors (TLRs), and 89 C-type lectins (CLTs). Altogether, the high-quality transcriptome obtained data will offer valuable insights for further investigations into U. unicinctus innate immune response, laying the foundation for subsequent molecular biology studies and aquaculture.

A cryptic homotypic interaction motif of insect STING is required for its antiviral signaling

Stimulator of interferon genes (STING) mediates innate immune response upon binding to cyclic GMP-AMP (cGAMP). It recruits tank-binding kinase 1 (TBK1) and transcription factor interferon regulatory factor 3 (IRF3) through its C-terminal tail and facilitates TBK1-dependent phosphorylation of IRF3 via forming STING polymers in mammalian cells. However, the mechanism behind STING-mediated activation of NF-κB transcription factor, Relish, in insect cells is unknown. Our study revealed that insect STING formed oligomers and the cryptic RIP homotypic interaction motif (cRHIM) was required for its oligomerization and its anti-viral functions. Cells expressing cRHIM-deficient mutants exhibited lower levels of anti-viral molecules, higher viral load after viral infection and weak activation of Relish. Moreover, we observed that under cGAMP stimulation, insect STING interacted with IMD, and deletion of the cRHIM motif on either protein prevented this interaction. Finally, we demonstrated that cGAMP enhanced the amyloid-like property of insect STING aggregates by ThT staining. In summary, our research showed that insect STING employed a homotypic motif to form intermolecular interactions that are essential for its antiviral signaling.

Acute lipopolysaccharide (LPS)-induced cell membrane hyperpolarization is independent of voltage gated and calcium activated potassium channels

The gram-negative toxin lipopolysaccharides (LPS) are known to trigger inflammatory cytokines in mammals, which can result in pathological responses. Upon treatment of bacterial sepsis with antibiotics, the lysing bacteria can present a surge in LPS, inducing a cytokine storm. However, LPS can also have direct cellular effects, including transient rapid hyperpolarizing of the membrane potential, blocking glutamate receptors and even promoting release of glutamate. The detailed mechanism of action for these immediate responses is still unresolved. In addressing the membrane hyperpolarization, voltage gated K+ channel blockers 4-aminopyridine (4-AP, 3 mM), quinidine hydrochloride monohydrate (0.1 mM) and tetraethylammonium (TEA, 20 mM) were examined along with RNAi knockdowns of potential calcium activated K+ channels. The immediate responses of LPS were not blocked. Even in the presence of glutamate, the membrane still hyperpolarizes with LPS. When the driving gradient for the ionotropic glutamate receptors is enhanced during hyperpolarization, spontaneous quantal responses are dampened in amplitude. Thus, glutamate receptors are blocked, and the mechanism of hyperpolarization remains unresolved. The larval Drosophila glutamatergic neuromuscular junction is used as a model synaptic preparation to address the direct rapid actions by LPS.

Dietary lauric acid promoted antioxidant and immune capacity by improving intestinal structure and microbial population of swimming crab (Portunus trituberculatus)

Lauric acid (LA), a saturated fatty acid with 12 carbon atoms, is widely regarded as a healthy fatty acid that plays an important role in disease resistance and improving immune physiological function. The objective of this study was to determine the effects of dietary lauric acid on the growth performance, antioxidant capacity, non-specific immunity and intestinal microbiology, and evaluate the potential of lauric acids an environmentally friendly additive in swimming crab (Portunus trituberculatus) culture. A total of 192 swimming crabs with an initial body weight of 11.68 ± 0.02 g were fed six different dietary lauric acid levels, the analytical values of lauric acid were 0.09, 0.44, 0.80, 1.00, 1.53, 2.91 mg/g, respectively. There were four replicates per treatment and 8 juvenile swimming crabs per replicate. The results indicated that final weight, percent weight gain, specific growth rate, survival and feed intake were not significantly affected by dietary lauric acid levels; however, crabs fed diets with 0.80 and 1.00 mg/g lauric acid showed the lowest feed efficiency among all treatments. Proximate composition in hepatopancreas and muscle were not significantly affected by dietary lauric acid levels. The highest activities of amylase and lipase in hepatopancreas and intestine were found at crabs fed diet with 0.80 mg/g lauric acid (P < 0.05), the activity of carnitine palmityl transferase (CPT) in hepatopancreas and intestine significantly decreased with dietary lauric acid levels increasing from 0.09 to 2.91 mg/g (P < 0.05). The lowest concentration of glucose and total protein and the activity of alkaline phosphatase in hemolymph were observed at crabs fed diets with 0.80 and 1.00 mg/g lauric acid among all treatments. The activity of GSH-Px in hepatopancreas significantly increased with dietary lauric acid increasing from 0.09 to 1.53 mg/g, MDA in hepatopancreas and hemolymph was not significantly influenced by dietary lauric acid levels. The highest expression of cat and gpx in hepatopancreas were exhibited in crabs fed diet with 1.00 mg/g lauric acid, however, the expression of genes related to the inflammatory signaling pathway (relish, myd88, traf6, nf-κB) were up-regulated in the hepatopancreas with dietary lauric acid levels increasing from 0.09 to 1.00 mg/g, moreover, the expression of genes related to intestinal inflammatory, immune and antioxidant were significantly affected by dietary lauric acid levels (P < 0.05). Crabs fed diet without lauric acid supplementation exhibited higher lipid drop area in hepatopancreas than those fed the other diets (P < 0.05). The expression of genes related to lipid catabolism was up-regulated, however, and the expression of genes related to lipid synthesis was down-regulated in the hepatopancreas of crabs fed with 0.80 mg/g lauric acid. Lauric acid improved hepatic tubular integrity, and enhanced intestinal barrier function by increasing peritrophic membrane (PM) thickness and upregulating the expression of structural factors (per44, zo-1) and intestinal immunity-related genes. In addition, dietary 1.00 mg/g lauric acid significantly improved the microbiota composition of the intestinal, increased the abundance of Actinobacteria and Rhodobacteraceae, and decreased the abundance of Vibrio, thus maintaining the microbiota balance of the intestine. The correlation analysis showed that there was a relationship between intestinal microbiota and immune-antioxidant function. In conclusion, the dietary 1.00 mg/g lauric acid is beneficial to improve the antioxidant capacity and intestinal health of swimming crab.

Bacterial peptidoglycan serves as a critical modulator of the gut-immune-brain axis in Drosophila

Metabolites and compounds derived from gut-associated bacteria can modulate numerous physiological processes in the host, including immunity and behavior. Using a model of oral bacterial infection, we previously demonstrated that gut-derived peptidoglycan (PGN), an essential constituent of the bacterial cell envelope, influences female fruit fly egg-laying behavior by activating the NF-κB cascade in a subset of brain neurons. These findings underscore PGN as a potential mediator of communication between gut bacteria and the brain in Drosophila, prompting further investigation into its impact on all brain cells. Through high-resolution mass spectrometry, we now show that PGN fragments produced by gut bacteria can rapidly reach the central nervous system. In Addition, by employing a combination of whole-genome transcriptome analyses, comprehensive genetic assays, and reporter gene systems, we reveal that gut bacterial infection triggers a PGN dose-dependent NF-κB immune response in perineurial glia, forming the continuous outer cell layer of the blood-brain barrier. Furthermore, we demonstrate that persistent PGN-dependent NF-κB activation in perineurial glial cells correlates with a reduction in lifespan and early neurological decline. Overall, our findings establish gut-derived PGN as a critical mediator of the gut-immune-brain axis in Drosophila.

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

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

Identification of immunity-related genes distinctly regulated by Manduca sexta Spӓtzle-1/2 and Escherichia coli peptidoglycan

The immune system of Manduca sexta has been well studied to understand molecular mechanisms of insect antimicrobial responses. While evidence supports the existence of major immune signaling pathways in this species, it is unclear how induced production of defense proteins is specifically regulated by the Toll and Imd pathways. Our previous studies suggested that diaminopimelic acid-type peptidoglycans (DAP-PG) from Gram-negative and some Gram-positive bacteria, more than Lys-type peptidoglycans (Lys-PG) from other Gram-positive bacteria, triggers both pathways through membrane-bound receptors orthologous to Drosophila Toll and PGRP-LC. In this study, we produced M. sexta proSpätzle-1 and proSpätzle-2 in Sf9 cells, identified their processing enzymes, and used prophenoloxidase activating protease-3 to activate the cytokine precursors. After Spätzle-1 and -2 were isolated from the reaction mixtures, we separately injected the purified cytokines into larval hemocoel to induce gene transcription in fat body through the Toll pathway solely. On the other hand, we treated a M. sexta cell line with E. coli DAP-PG to only induce the Imd pathway and target gene expression. RNA-Seq analysis of the fat body and cultured cells collected at 0, 6, and 24 h after treatment indicated that expression of diapausin-4, -10, -12, -13, cecropin-2, -4, -5, attacin-5, -11, and lebocin D is up-regulated predominantly via Toll signaling, whereas transcription of cecropin-6, gloverin, lysozyme-1, and gallerimycin-2 is mostly induced by DAP-PG via Imd signaling. Other antimicrobial peptides are expressed in response to both pathways. Transcripts of most Toll-specific genes (e.g., lebocin D) peaked at 6 h, contrasting the gradual increase and plateauing of drosomycin mRNA level at 24-48 h in Drosophila. We also used T (oll)-I (md) ratios to estimate relative contributions of the two pathways to transcriptional regulation of other components of the immune system. The differences in pathway specificity and time course of transcriptional regulation call for further investigations in M. sexta and other insects.

Larval microbiota primes the Drosophila adult gustatory response

The survival of animals depends, among other things, on their ability to identify threats in their surrounding environment. Senses such as olfaction, vision and taste play an essential role in sampling their living environment, including microorganisms, some of which are potentially pathogenic. This study focuses on the mechanisms of detection of bacteria by the Drosophila gustatory system. We demonstrate that the peptidoglycan (PGN) that forms the cell wall of bacteria triggers an immediate feeding aversive response when detected by the gustatory system of adult flies. Although we identify ppk23+ and Gr66a+ gustatory neurons as necessary to transduce fly response to PGN, we demonstrate that they play very different roles in the process. Time-controlled functional inactivation and in vivo calcium imaging demonstrate that while ppk23+ neurons are required in the adult flies to directly transduce PGN signal, Gr66a+ neurons must be functional in larvae to allow future adults to become PGN sensitive. Furthermore, the ability of adult flies to respond to bacterial PGN is lost when they hatch from larvae reared under axenic conditions. Recolonization of germ-free larvae, but not adults, with a single bacterial species, Lactobacillus brevis, is sufficient to restore the ability of adults to respond to PGN. Our data demonstrate that the genetic and environmental characteristics of the larvae are essential to make the future adults competent to respond to certain sensory stimuli such as PGN.

A novel short-type peptidoglycan recognition protein with unique polysaccharide recognition specificity in sea cucumber, Apostichopus japonicus

Pattern recognition receptors (PRRs) are the first line of immune defense in invertebrates against pathogen infection; they recognize pathogens and transmit signals to downstream immune pathways. Among these, peptidoglycan recognition proteins (PGRPs) are an important family in invertebrates that generally comprise of complicated isoforms. A comprehensive understanding of PGRPs in evolutionarily and economically important marine invertebrates, such as the sea cucumber, Apostichopus japonicus, is crucial. Previous studies have identified two PGRPs in sea cucumber, AjPGRP-S and AjPGRP-S1, and another novel short-type PGRP, AjPGRP-S3, was additionally identified here. The full-length cDNA sequence of AjPGRP-S3 was obtained here by PCR-RACE, followed by which showed its gene expression analyses by in situ hybridization that showed it to be relatively highly expressed in coelomocytes and tube feet. Based on an analysis of the recombinant protein, rAjPGRP-S3, a board-spectrum pathogen recognition ability was noted that covered diverse Gram-negative and -positive bacteria, and fungi. Moreover, according to the results of yeast two-hybridization, it was suggested that rAJPGRP-S3 interacted with multiple immune-related factors, including proteins involved in the complement system, extracellular matrix, vesicle trafficking, and antioxidant system. These findings prove the important functions of AjPGRP-S3 in the transduction of pathogen signals to downstream immune effectors and help explore the functional differences in the AjPGRP isoforms.

Glucocerebrosidase deficiency leads to neuropathology via cellular immune activation

Mutations in GBA (glucosylceramidase beta), which encodes the lysosomal enzyme glucocerebrosidase (GCase), are the strongest genetic risk factor for the neurodegenerative disorders Parkinson's disease (PD) and Lewy body dementia. Recent work has suggested that neuroinflammation may be an important factor in the risk conferred by GBA mutations. We therefore systematically tested the contributions of immune-related genes to neuropathology in a Drosophila model of GCase deficiency. We identified target immune factors via RNA-Seq and proteomics on heads from GCase-deficient flies, which revealed both increased abundance of humoral factors and increased macrophage activation. We then manipulated the identified immune factors and measured their effect on head protein aggregates, a hallmark of neurodegenerative disease. Genetic ablation of humoral (secreted) immune factors did not suppress the development of protein aggregation. By contrast, re-expressing Gba1b in activated macrophages suppressed head protein aggregation in Gba1b mutants and rescued their lifespan and behavioral deficits. Moreover, reducing the GCase substrate glucosylceramide in activated macrophages also ameliorated Gba1b mutant phenotypes. Taken together, our findings show that glucosylceramide accumulation due to GCase deficiency leads to macrophage activation, which in turn promotes the development of neuropathology.

Ubiquitin signalling in Drosophila innate immune responses

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

The virus-induced cyclic dinucleotide 2'3'-c-di-GMP mediates STING-dependent antiviral immunity in Drosophila

In mammals, the enzyme cGAS senses the presence of cytosolic DNA and synthesizes the cyclic dinucleotide (CDN) 2'3'-cGAMP, which triggers STING-dependent immunity. In Drosophila melanogaster, two cGAS-like receptors (cGLRs) produce 3'2'-cGAMP and 2'3'-cGAMP to activate STING. We explored CDN-mediated immunity in 14 Drosophila species covering 50 million years of evolution and found that 2'3'-cGAMP and 3'2'-cGAMP failed to control infection by Drosophila C virus in D. serrata and two other species. We discovered diverse CDNs produced in a cGLR-dependent manner in response to viral infection in D. melanogaster, including 2'3'-c-di-GMP. This CDN was a more potent STING agonist than cGAMP in D. melanogaster and it also activated a strong antiviral transcriptional response in D. serrata. Our results shed light on the evolution of cGLRs in flies and provide a basis for understanding the function and regulation of this emerging family of pattern recognition receptors in animal innate immunity.

Full-length transcriptome analysis provides new insights into the diversity of immune-related genes in the threatened freshwater shellfish Solenaia oleivora

Solenaia oleivora, a valuable and rare bivalve endemic to China, is becoming a threatened freshwater sepcies. However, the lack of research on its genome and immune system will hinder advances in its conservation and artificial breeding. In this study, we obtained the full-length transcriptome of S. oleivora using PacBio sequencing. A total of 21,415 transcripts with an average length of 1,726 bp were generated. Among these transcripts, 12,084 had coding sequences (CDS), of which 8,639 were annotated in 6 databases. The structure analysis identified 625 transcript factors (TFs), 8,005 long non-coding RNAs (lncRNAs), and 5,288 simple sequences repeat (SSRs). Meanwhile, massive immune genes were identified from the transcriptome of S. oleivora. In terms of non-self-identification, 97 transcripts of pattern recognition receptors (PRRs) were discovered, including peptidoglycan recognition proteins (PGRPs), gram-negative bacteria binding proteins (GNBPs), toll-like receptors (TLRs), scavenger receptors (SRs), galectins (GALs), C-type lectins (CLTs), and fibrinogen-related protein (FREPs). For pathogen elimination, 7 transcripts related to antimicrobial peptides, lysozymes, and lysosomal enzymes were identified. Moreover, 33 complement-associated transcripts were found. This study enriched the genome resources of S. oleivora and provided new insights for the study of the immune system of S. oleivora.

Induced expression modes of genes related to Toll, Imd, and JAK/STAT signaling pathway-mediated immune response in Spodoptera frugiperda infected with Beauveria bassiana

Spodoptera frugiperda is one of the most harmful pests that attack maize and other major food crops and causes huge economic loss every year in China and other countries and regions. Beauveria bassiana, a kind of entomological fungus that is highly pathogenic to pests, is harmless to the environment and human beings. However, at present, S. frugiperda has gradually developed resistance to many pesticides and microbial insecticides. In this study, transcriptome sequencing was conducted to analyze the differences in gene expression between B. bassiana-infected and -uninfected S. frugiperda. More than 160 Gb of clean data were obtained as 150-bp paired-end reads using the Illumina HiSeq™ 4000 platform, and 2,767 and 2,892 DEGs were identified in LH36vsCK36 and LH144vsCK144, respectively. In order to explore the roles of JAK/STAT, Toll, and Imd signaling pathways in antifungal immune response in S. frugiperda against B. bassiana infection, the expression patterns of those signaling pathway-related genes in B. bassiana-infected S. frugiperda were analyzed by quantitative real-time PCR. In addition, antifungal activity experiments revealed that the suppression of JAK/STAT, Toll, and Imd signaling pathways by inhibitors could inhibit the antifungal activity to a large extent and lead to increased sensitivity of S. frugiperda to B. bassiana infection, indicating that JAK/STAT, Toll, and Imd signaling pathways and their associated genes might be involved in the synthesis and secretion of antifungal substances. This study implied that JAK/STAT, Toll, and Imd signaling pathways played crucial roles in the antifungal immune response of the S. frugiperda larvae, in which the related genes of these signaling pathways could play special regulatory roles in signal transduction. This study would improve our understanding of the molecular mechanisms underlying innate immunity and provide the basis for a wide spectrum of strategies against antifungal resistance of S. frugiperda.

Role of Rab5 early endosomes in regulating Drosophila gut antibacterial response

Interactions between prokaryotes and eukaryotes require a dialogue between MAMPs and PRRs. In Drosophila, bacterial peptidoglycan is detected by PGRP receptors. While the components of the signaling cascades activated upon PGN/PGRP interactions are well characterized, little is known about the subcellular events that translate these early signaling steps into target gene transcription. Using a Drosophila enteric infection model, we show that gut-associated bacteria can induce the formation of intracellular PGRP-LE aggregates which colocalized with the early endosome marker Rab5. Combining microscopic and RNA-seq analysis, we demonstrate that RNAi inactivation of the endocytosis pathway in the Drosophila gut affects the expression of essential regulators of the NF-κB response leading not only to a disruption of the immune response locally in the gut but also at the systemic level. This work sheds new light on the involvement of the endocytosis pathway in the control of the gut response to intestinal bacterial infection.

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.

Recognition of Arboviruses by the Mosquito Immune System

Arthropod-borne viruses (arboviruses) pose a significant threat to both human and animal health worldwide. These viruses are transmitted through the bites of mosquitoes, ticks, sandflies, or biting midges to humans or animals. In humans, arbovirus infection often results in mild flu-like symptoms, but severe disease and death also occur. There are few vaccines available, so control efforts focus on the mosquito population and virus transmission control. One area of research that may enable the development of new strategies to control arbovirus transmission is the field of vector immunology. Arthropod vectors, such as mosquitoes, have coevolved with arboviruses, resulting in a balance of virus replication and vector immune responses. If this balance were disrupted, virus transmission would likely be reduced, either through reduced replication, or even through enhanced replication, resulting in mosquito mortality. The first step in mounting any immune response is to recognize the presence of an invading pathogen. Recent research advances have been made to tease apart the mechanisms of arbovirus detection by mosquitoes. Here, we summarize what is known about arbovirus recognition by the mosquito immune system, try to generate a comprehensive picture, and highlight where there are still gaps in our current understanding.

Immune functions of pattern recognition receptors in Lepidoptera

Pattern recognition receptors (PRRs), as the "sensors" in the immune response, play a prominent role in recognizing pathogen-associated molecular patterns (PAMPs) and initiating an effective defense response to pathogens in Lepidoptera. It is becoming increasingly clear that damage-associated molecular patterns (DAMPs) normally play a physiological role within cells; however, when exposed to extracellular, they may become "part-time" critical signals of the immune response. Based on research in recent years, we review herein typical PRRs of Lepidoptera, including peptidoglycan recognition protein (PGRP), gram-negative binding protein (GNBP), β-1,3-glucan recognition protein (βGRP), C-type lectin (CTL), and scavenger receptor (SR). We also outline the ways in which DAMPs participate in the immune response and the correlation between PRRs and immune escape. Taken together, these findings suggest that the role of PRRs in insect innate immunity may be much greater than expected and that it is possible to recognize a broader range of signaling molecules.

A novel virus-induced cyclic dinucleotide, 2'3'-c-di-GMP, mediates STING-dependent antiviral immunity in Drosophila

In mammals, the enzyme cGAS senses the presence of cytosolic DNA and synthesizes the cyclic dinucleotide (CDN) 2'3'-cGAMP. This CDN binds to and activates the protein STING to trigger immunity. We recently discovered in the model organism Drosophila melanogaster two cGAS-like receptors (cGLRs) that activate STING-dependent antiviral immunity and can produce 3'2'-cGAMP, in addition to 2'3'-cGAMP. Here we explore CDN-mediated immunity in 14 different Drosophila species covering 50 million years of evolution and report that 2'3'-cGAMP and 3'2'-cGAMP fail to control infection by Drosophila C virus in D. serrata, D. sechellia and D. mojavensis . Using an accurate and sensitive mass spectrometry method, we discover an unexpected diversity of CDNs produced in a cGLR-dependent manner in response to viral infection in D. melanogaster , including a novel CDN, 2'3'-c-di-GMP. We show that 2'3'-c-di-GMP is the most potent STING agonist identified so far in D. melanogaster and that this molecule also activates a strong antiviral transcriptional response in D. serrata . Our results shed light on the evolution of cGLRs in flies and provide a basis for the understanding of the function and regulation of this emerging family of PRRs in animal innate immunity.

Very-long-chain fatty acids induce glial-derived sphingosine-1-phosphate synthesis, secretion, and neuroinflammation

VLCFAs (very-long-chain fatty acids) are the most abundant fatty acids in myelin. Hence, during demyelination or aging, glia are exposed to higher levels of VLCFA than normal. We report that glia convert these VLCFA into sphingosine-1-phosphate (S1P) via a glial-specific S1P pathway. Excess S1P causes neuroinflammation, NF-κB activation, and macrophage infiltration into the CNS. Suppressing the function of S1P in fly glia or neurons, or administration of Fingolimod, an S1P receptor antagonist, strongly attenuates the phenotypes caused by excess VLCFAs. In contrast, elevating the VLCFA levels in glia and immune cells exacerbates these phenotypes. Elevated VLCFA and S1P are also toxic in vertebrates based on a mouse model of multiple sclerosis (MS), experimental autoimmune encephalomyelitis (EAE). Indeed, reducing VLCFA with bezafibrate ameliorates the phenotypes. Moreover, simultaneous use of bezafibrate and fingolimod synergizes to improve EAE, suggesting that lowering VLCFA and S1P is a treatment avenue for MS.

Rift Valley Fever Virus Primes Immune Responses in Aedes aegypti Cells

The ongoing global emergence of arthropod-borne (arbo) viruses has accelerated research into the interactions of these viruses with the immune systems of their vectors. Only limited information exists on how bunyaviruses, such as Rift Valley fever virus (RVFV), are sensed by mosquito immunity or escape detection. RVFV is a zoonotic phlebovirus (Bunyavirales; Phenuiviridae) of veterinary and human public health and economic importance. We have shown that the infection of mosquitoes with RVFV triggers the activation of RNA interference pathways, which moderately restrict viral replication. Here, we aimed to better understand the interactions between RVFV and other vector immune signaling pathways that might influence RVFV replication and transmission. For this, we used the immunocompetent Aedes aegypti Aag2 cell line as a model. We found that bacteria-induced immune responses restricted RVFV replication. However, virus infection alone did not alter the gene expression levels of immune effectors. Instead, it resulted in the marked enhancement of immune responses to subsequent bacterial stimulation. The gene expression levels of several mosquito immune pattern recognition receptors were altered by RVFV infection, which may contribute to this immune priming. Our findings imply that there is a complex interplay between RVFV and mosquito immunity that could be targeted in disease prevention strategies.

Regulating metabolism to shape immune function: Lessons from Drosophila

Infection with pathogenic microbes is a severe threat that hosts manage by activating the innate immune response. In Drosophila melanogaster, the Toll and Imd signaling pathways are activated by pathogen-associated molecular patterns to initiate cellular and humoral immune processes that neutralize and kill invaders. The Toll and Imd signaling pathways operate in organs such as fat body and gut that control host nutrient metabolism, and infections or genetic activation of Toll and Imd signaling also induce wide-ranging changes in host lipid, carbohydrate and protein metabolism. Metabolic regulation by immune signaling can confer resistance to or tolerance of infection, but it can also lead to pathology and susceptibility to infection. These immunometabolic phenotypes are described in this review, as are changes in endocrine signaling and gene regulation that mediate survival during infection. Future work in the field is anticipated to determine key variables such as sex, dietary nutrients, life stage, and pathogen characteristics that modify immunometabolic phenotypes and, importantly, to uncover the mechanisms used by the immune system to regulate metabolism.

BdNub Is Essential for Maintaining gut Immunity and Microbiome Homeostasis in Bactrocera dorsalis

Insects face immune challenges posed by invading and indigenous bacteria. They rely on the immune system to clear these microorganisms. However, the immune response can be harmful to the host. Therefore, fine-tuning the immune response to maintain tissue homeostasis is of great importance to the survival of insects. The Nub gene of the OCT/POU family regulates the intestinal IMD pathway. However, the role of the Nub gene in regulating host microbiota remains unstudied. Here, a combination of bioinformatic tools, RNA interference, and qPCR methods were adopted to study BdNub gene function in Bactrocera dorsalis gut immune system. It's found that BdNubX1, BdNubX2, and antimicrobial peptides (AMPs), including Diptcin (Dpt), Cecropin (Cec), AttcinA (Att A), AttcinB (Att B) and AttcinC (Att C) are significantly up-regulated in Tephritidae fruit fly Bactrocera dorsalis after gut infection. Silencing BdNubX1 leads to down-regulated AMPs expression, while BdNubX2 RNAi leads to increased expression of AMPs. These results indicate that BdNubX1 is a positive regulatory gene of the IMD pathway, while BdNubX2 negatively regulates IMD pathway activity. Further studies also revealed that BdNubX1 and BdNubX2 are associated with gut microbiota composition, possibly through regulation of IMD pathway activity. Our results prove that the Nub gene is evolutionarily conserved and participates in maintaining gut microbiota homeostasis.

Drosophila caspases as guardians of host-microbe interactions

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

The endocrine disruptor, fenoxycarb modulates gut immunity and gut bacteria titer in the cotton bollworm, Helicoverpa armigera

The endocrine system modulates several physiological functions such as development and metamorphosis in insects. The normal growth and development of insects is interfered with insect growth regulators (IGRs), which act as mimics of insect hormones like juvenile hormone (JH) and ecdysone hormone. The effects of JH and its analogs on systemic immunity have been identified. However the effect of these compounds on local gut immunity is largely unknown. In this study, the effects of JH analog fenoxycarb on the local gut immunity of Helicoverpa armigera, gut bacteria population, and their role in the pathogenicity of Bacillus thuringiensis (Bt) were analyzed. The results showed that feeding fenoxycarb causes a decrease in the transcription level of IMD (Relish and PGPR-LC), ROS (DUOX and SOD) related genes and antimicrobial peptides (AMPs), followed by an overpopulation of gut bacteria. The fenoxycarb-treated larvae showed higher susceptibility to Bt compared to the control larvae. Overall, these findings collectively suggest that JH analog affects local gut immunity and gut bacteria titer.

Differential Gene Expression of Innate Immune Response Genes Consequent to Solenopsis invicta Virus-3 Infection

The red imported fire ant Solenopsis invicta Buren (fire ant hereafter) is a global pest that inflicts billions of dollars in damages to the United States economy and poses a major threat on a global scale. Concerns with the broad-spectrum application of insecticides have facilitated the hunt for natural enemy-mediated controls. One of these, the virus Solenopsis invicta virus-3 (SINV-3 hereafter) is exceptionally virulent in laboratory settings. However, despite high mortality rates in the laboratory and documented widespread SINV-3 prevalence in the southern United States, the fire ant remains a major pest. To explore this paradox, we document the immune response elicited by the fire ant when infected with SINV-3. We sequence the fire ant transcriptome prior to and following infection with SINV-3, and identify and discuss in detail genes in immune response pathways differentially expressed following infection with SINV-3. This information provides insights into genes and pathways involved in the SINV-3 infection response in the fire ant and offers avenues to pursue, to suppress key immune response genes and force the fire ant to succumb to SINV-3 infection in the field.

Hyperpolarization Induced by Lipopolysaccharides but Not by Chloroform Is Inhibited by Doxapram, an Inhibitor of Two-P-Domain K+ Channel (K2P)

Bacterial septicemia is commonly induced by Gram-negative bacteria. The immune response is triggered in part by the secretion of bacterial endotoxin lipopolysaccharide (LPS). LPS induces the subsequent release of inflammatory cytokines which can result in pathological conditions. There is no known blocker to the receptors of LPS. The Drosophila larval muscle is an amendable model to rapidly screen various compounds that affect membrane potential and synaptic transmission such as LPS. LPS induces a rapid hyperpolarization in the body wall muscles and depolarization of motor neurons. These actions are blocked by the compound doxapram (10 mM), which is known to inhibit a subtype of the two-P-domain K+ channel (K2P channels). However, the K2P channel blocker PK-THPP had no effect on the Drosophila larval muscle at 1 and 10 mM. These channels are activated by chloroform, which also induces a rapid hyperpolarization of these muscles, but the channels are not blocked by doxapram. Likewise, chloroform does not block the depolarization induced by doxapram. LPS blocks the postsynaptic glutamate receptors on Drosophila muscle. Pre-exposure to doxapram reduces the LPS block of these ionotropic glutamate receptors. Given that the larval Drosophila body wall muscles are depolarized by doxapram and hyperpolarized by chloroform, they offer a model to begin pharmacological profiling of the K2P subtype channels with the potential of identifying blockers for the receptors to mitigate the actions of the Gram-negative endotoxin LPS.

Induction of Multiple Immune Signaling Pathways in Gryllodes sigillatus Crickets during Overt Viral Infections

Despite decades of focus on crickets (family: Gryllidae) as a popular commodity and model organism, we still know very little about their immune responses to microbial pathogens. Previous studies have measured downstream immune effects (e.g., encapsulation response, circulating hemocytes) following an immune challenge in crickets, but almost none have identified and quantified the expression of immune genes during an active pathogenic infection. Furthermore, the prevalence of covert (i.e., asymptomatic) infections within insect populations is becoming increasingly apparent, yet we do not fully understand the mechanisms that maintain low viral loads. In the present study, we measured the expression of several genes across multiple immune pathways in Gryllodes sigillatus crickets with an overt or covert infection of cricket iridovirus (CrIV). Crickets with overt infections had higher relative expression of key pathway component genes across the Toll, Imd, Jak/STAT, and RNAi pathways. These results suggests that crickets can tolerate low viral infections but can mount a robust immune response during an overt CrIV infection. Moreover, this study provides insight into the immune strategy of crickets following viral infection and will aid future studies looking to quantify immune investment and improve resistance to pathogens.

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.

Drosophila hemocytes recognize lymph gland tumors of mxc mutants and activate the innate immune pathway in a reactive oxygen species-dependent manner

Mechanisms of cancer cell recognition and elimination by the innate immune system remains unclear. The immune signaling pathways are activated in the fat body to suppress the tumor growth in mxcmbn1 hematopoietic tumor mutants in Drosophila by inducing antimicrobial peptides (AMP). Here, we investigated the regulatory mechanism underlying the activation in the mutant. Firstly, we found that reactive oxygen species (ROS) accumulated in the hemocytes due to induction of dual oxidase and one of its activators. This was required for the AMP induction and the tumor growth suppression. Next, more hemocytes transplanted from normal larvae were associated with the mutant tumor than normal lymph glands (LGs). Matrix metalloproteinase 1 and 2 (MMP2) were highly expressed in the tumors. The basement membrane components in the tumors were reduced and ultimately lost inside. Depletion of the MMP2 rather than MMP1 resulted in a significantly reduced AMP expression in the mutant larvae. The hemocytes may recognize the disassembly of basement membrane in the tumors and activate the ROS production. Our findings highlight the mechanism via which macrophage-like hemocytes recognize tumor cells and subsequently convey the information to induce AMPs in the fat body. They contribute to uncover the role of innate immune system against cancer.

Four peptidoglycan recognition proteins are indispensable for antibacterial immunity in the cigarette beetle Lasioderma serricorne (Fabricius)

The peptidoglycan recognition protein (PGRP), an important pattern recognition receptor of insects, is significant for reducing innate immunity and effective pest control. We cloned four PGRP genes (LsPGRP-LB, LsPGRP-LB1, LsPGRP-LE, and LsPGRP-SC2) from the cigarette beetle, Lasioderma serricorne (Fabricius), which encoded proteins of 216, 197, 317, and 190 amino acids, respectively. Three LsPGRPs were predominantly expressed in the larval and pupal stages, whereas LsPGRP-LE displayed high expression in adults. All the four LsPGRPs genes were highly expressed in the midgut and integument. Pathogen inoculation revealed that the four LsPGRPs actively responded to Escherichia coli and its peptidoglycan. The transcription levels of LsPGRP-LE and LsPGRP-SC2 increased significantly after Staphylococcus aureus stimulation. RNA interference-mediated knockdown of the four LsPGRPs led to increased larval mortality when challenged by E. coli, and the expression of four antimicrobial peptide genes (LsCole, LsAtt2, LsDef1 and LsDef2) had a significant decrease. Higher mortality and lower AMP expression were also observed in L. serricorne under S. aureus infection after silencing LsPGRP-LE and LsPGRP-SC2. Our results suggest that the four LsPGRP genes play important and distinct regulatory roles in the antibacterial defense response of L. serricorne.

The circular RNA Edis regulates neurodevelopment and innate immunity

Circular RNAs (circRNAs) are widely expressed in eukaryotes. However, only a subset has been functionally characterized. We identify and validate a collection of circRNAs in Drosophila, and show that depletion of the brain-enriched circRNA Edis (circ_Ect4) causes hyperactivation of antibacterial innate immunity both in cultured cells and in vivo. Notably, Edis depleted flies display heightened resistance to bacterial infection and enhanced pathogen clearance. Conversely, ectopic Edis expression blocks innate immunity signaling. In addition, inactivation of Edis in vivo leads to impaired locomotor activity and shortened lifespan. Remarkably, these phenotypes can be recapitulated with neuron-specific depletion of Edis, accompanied by defective neurodevelopment. Furthermore, inactivation of Relish suppresses the innate immunity hyperactivation phenotype in the fly brain. Moreover, we provide evidence that Edis encodes a functional protein that associates with and compromises the processing and activation of the immune transcription factor Relish. Importantly, restoring Edis expression or ectopic expression of Edis-encoded protein suppresses both innate immunity and neurodevelopment phenotypes elicited by Edis depletion. Thus, our study establishes Edis as a key regulator of neurodevelopment and innate immunity.

Lipopolysaccharide detection by the innate immune system may be an uncommon defence strategy used in nature

Since the publication of the Janeway's Pattern Recognition hypothesis in 1989, study of pathogen-associated molecular patterns (PAMPs) and their immuno-stimulatory activities has accelerated. Most studies in this area have been conducted in model organisms, which leaves many open questions about the universality of PAMP biology across living systems. Mammals have evolved multiple proteins that operate as receptors for the PAMP lipopolysaccharide (LPS) from Gram-negative bacteria, but LPS is not immuno-stimulatory in all eukaryotes. In this review, we examine the history of LPS as a PAMP in mammals, recent data on LPS structure and its ability to activate mammalian innate immune receptors, and how these activities compare across commonly studied eukaryotes. We discuss why LPS may have evolved to be immuno-stimulatory in some eukaryotes but not others and propose two hypotheses about the evolution of PAMP structure based on the ecology and environmental context of the organism in question. Understanding PAMP structures and stimulatory mechanisms across multi-cellular life will provide insights into the evolutionary origins of innate immunity and may lead to the discovery of new PAMP variations of scientific and therapeutic interest.

Expression of antimicrobial peptides associated with different susceptibilities to environmental chemicals in Drosophila suzukii and Drosophila melanogaster

Drosophila suzukii is a serious agricultural pest. The evolved morphology of the female D. suzukii assists in penetrating the surface of fresh fruit and spawns eggs with its unique ovipositor. Conversely, Drosophila melanogaster, a taxonomically close species with D. suzukii, largely inhabits decaying and fermenting fruits and is consistently exposed to extensive environmental chemicals, such as 2-phenylethanol, ethanol, and acetic acid, produced by microorganisms. Considering the distinct habitats of the two flies, D. suzukii is thought to be more susceptible to environmental chemicals than D. melanogaster. We investigated the significantly higher survival rate of D. melanogaster following exposure to 2-phenylethanol, ethanol, and acetic acid. A comparison of the expression of antimicrobial peptides (AMPs) between the two flies treated with chemicals established that AMPs were generally more abundantly induced in D. melanogaster than in D. suzukii, particularly in the gut and fat body. Among the AMPs, the induction of genes (Diptericin A, Diptericin B, and Metchnikowin), which are regulated by the immune deficiency (IMD) pathway, was significantly higher than that of Drosomycin, which belongs to the Toll pathway in chemical-treated D. melanogaster. A transgenic RNAi fly (D. melanogaster) with silenced expression of AMPs and Relish, a transcription factor of the IMD pathway, exhibited significantly reduced survival rates than the control fly. Our results suggest that AMPs regulated by the IMD pathway play an important role in the chemical tolerance of D. melanogaster, and these flies are adapted to their habitats by physiological response.

M1-linked ubiquitination facilitates NF-κB activation and survival during sterile inflammation

Methionine 1 (M1)-linked ubiquitination plays a key role in the regulation of inflammatory nuclear factor-κB (NF-κB) signalling and is important for clearance of pathogen infection in Drosophila melanogaster. M1-linked ubiquitin (M1-Ub) chains are assembled by the linear ubiquitin E3 ligase (LUBEL) in flies. Here, we have studied the role of LUBEL in sterile inflammation induced by different types of cellular stresses. We have found that the LUBEL catalyses formation of M1-Ub chains in response to hypoxic, oxidative and mechanical stress conditions. LUBEL is shown to be important for flies to survive low oxygen conditions and paraquat-induced oxidative stress. This protective action seems to be driven by stress-induced activation of the NF-κB transcription factor Relish via the immune deficiency (Imd) pathway. In addition to LUBEL, the intracellular mediators of Relish activation, including the transforming growth factor activating kinase 1 (Tak1), Drosophila inhibitor of apoptosis (IAP) Diap2, the IκB kinase γ (IKKγ) Kenny and the initiator caspase Death-related ced-3/Nedd2-like protein (Dredd), but not the membrane receptor peptidoglycan recognition protein (PGRP)-LC, are shown to be required for sterile inflammatory response and survival. Finally, we showed that the stress-induced upregulation of M1-Ub chains in response to hypoxia, oxidative and mechanical stress is also induced in mammalian cells and protects from stress-induced cell death. Taken together, our results suggest that M1-Ub chains are important for NF-κB signalling in inflammation induced by stress conditions often observed in chronic inflammatory diseases and cancer.

In Silico Characterization and Gene Expression Analysis of Toll Signaling Pathway-Related Genes in Diaphorina citri

The Asian citrus psyllid, Diaphorina citri is the main vector of citrus greening disease, also known as Huanglongbing (HLB). Currently, mitigating HLB depends on the control of D. citri using insecticides. To design innovative control strategies, we should investigate various biological aspects of D. citri at the molecular level. Herein we explored the Toll signaling system-related proteins in D. citri using in silico analyzes. Additionally, the transcripts of the identified genes were determined in all life stages from eggs to adults. Our findings reveal that D. citri genome possesses Toll signaling pathway-related genes similar to the insect model, Drosophila melanogaster, with slight differences. These genes include cact, TI, Myd88, Dif/DI, pll, tub, and spz encoding Cactus, Toll, Myeloid differentiation factor 88, Dorsal related immunity factor/Dorsal, Pelle, Tube, and Spaetzle, respectively. Unlike D. melanogaster, in D. citri Dorsal, immunity factor and Dorsal are the same protein. In addition, in D. citri, Pelle protein possesses a kinase domain, which is absent in Pelle of D. melanogaster. Gene expression analysis showed the transcript for cact, TI, Myd88, pll, tub, and spz are maximum in adults, suggesting the immunity increases with maturity. Instead, Dif/DI transcripts were maximal in eggs and adults and minimal in nymphal stages, indicating its role in embryonic development. The overall findings will help in designing pioneering control strategies of D. citri based on repressing its immunity by RNAi or CRISPR and combining that with biological control.

Renal NF-κB activation impairs uric acid homeostasis to promote tumor-associated mortality independent of wasting

Tumor-induced host wasting and mortality are general phenomena across species. Many groups have previously demonstrated endocrinal impacts of malignant tumors on host wasting in rodents and Drosophila. Whether and how environmental factors and host immune response contribute to tumor-associated host wasting and survival, however, are largely unknown. Here, we report that flies bearing malignant yki^3SA-gut tumors exhibited the exponential increase of commensal bacteria, which were mostly acquired from the environment, and systemic IMD-NF-κB activation due to suppression of a gut antibacterial amidase PGRP-SC2. Either gut microbial elimination or specific IMD-NF-κB blockade in the renal-like Malpighian tubules potently improved mortality of yki^3SA-tumor-bearing flies in a manner independent of host wasting. We further indicate that renal IMD-NF-κB activation caused uric acid (UA) overload to reduce survival of tumor-bearing flies. Therefore, our results uncover a fundamental mechanism whereby gut commensal dysbiosis, renal immune activation, and UA imbalance potentiate tumor-associated host death.

Redclaw crayfish (Cherax quadricarinatus) responds to Vibrio parahaemolyticus infection by activating toll and immune deficiency signaling pathways and transcription of associated immune response genes

In invertebrates, several genes controlled by the Toll and immunodeficiency (IMD) signaling cascade are altered during microbial infection. However, little is known about the expression patterns of innate immune response genes in red-clawed crayfish (Cherax quadricarinatus). In the present study, the transcription of five genes was assessed in C. quadricarinatus challenged with Vibrio parahaemolyticus (V. parahaemolyticus). The expression of Relish, Toll-like receptor (TLR), tumor necrosis factor receptor-related factor 6 (TRAF6), Akirin, and IMD in different tissues and at different time points after infection were assessed. In addition, the Relish gene was amplified, the protein conformation of the Relish gene was predicted, and gene expression changes associated with antimicrobial peptide production in C. quadricarinatus were analyzed using RNA interference (RNAi). During V. parahaemolyticus infection, the transcripts of the above five genes were significantly increased in the hepatopancreas of C. quadricarinatus (P < 0.05). In contrast, TLR was significantly downregulated in muscle tissue at the initial stage of infection (P < 0.05); TRAF6 and IMD were significantly down-regulated throughout infection (P < 0.05); Akirin transcripts had the lowest abundance at 24 h post-infection; Relish, IMD and Akirin genes were significantly up-regulated in gill tissue at the early stage of infection (P < 0.05); only TRAF6 was significantly up-regulated at 6, 24 and 48 h after infection. The Relish gene of C. quadricarinatus is closely related to the Exopalaemon carinicauda. When the Relish gene was knocked down by RNAi, the V. parahaemolyticus challenge showed that the mortality rate of the RANi group was significantly higher than that of the NC group; pathological sections showed that the hepatopancreatic tissue damage was the most severe 12 h after the interference; and the interference significantly inhibited IRF4, NF-κB, ALF, laccase, SOD1, and lectin genes. Therefore, it can be hypothesized that the Toll and IMD pathways are activated in C. quadricarinatus in response to bacterial infection and that genes associated with these pathways are differentially transcribed in different tissues. This study provides insights into the Toll and IMD signaling pathways and the spatiotemporal expression of key genes regulating bacterial infection resistance in C. quadricarinatus.

A cypovirus encoded microRNA negatively regulates the NF-κB pathway to enhance viral multiplication in Silkworm, Bombyx mori

MicroRNAs (miRNAs) are small non-coding RNAs that function as novel gene expression regulators at the post-transcriptional level. Not with standing that the biogenesis and function of miRNAs are well-understood in eukaryotes, little is known about RNA virus-encoded miRNAs. Bombyx mori cypovirus (BmCPV) is a double-stranded RNA virus with a segmented genome that causes cytoplasmic polyhedrosis disease in silkworm larvae. To date, the interaction between BmCPV and silkworm remains largely unclear. 22 candidate BmCPV-encoded miRNAs were identified in this study through small RNA sequencing, stem-loop RT-PCR and qRT-PCR. Then, generation and function analyses were conducted on one of the candidate miRNAs, BmCPV-miR-1, in the BmN cells and the silkworm larvae by RNA interference, quantitative PCR, dual-luciferase assay. Our results revealed that BmCPV-miR-1 was encoded by BmCPV genome RNA rather than the degraded fragments of the viral genome. Its generation depended on Dicer-1 and might also be correlated with Dicer-2, Argonaute-1 and Argonaute-2. Moreover, BmCPV-miR-1 could suppress the expression of the target gene, B. mori inhibitor of nuclear factor kappa-B kinase subunit beta (BmIKKβ), via binding to the target mRNA 3'-untranslated region, which fine-tuned the host NF-κB signaling pathway and consequently enhanced viral replication. Our results provide new evidence supporting the hypothesis that RNA viruses could generate miRNAs to modulate antiviral host defense.

Meta-Analysis of Immune Induced Gene Expression Changes in Diverse Drosophila melanogaster Innate Immune Responses

Simple Summary

Organisms can be infected by a wide range of pathogens, including bacteria, viruses, and parasites. Following infection, the host mounts an immune response to attempt to eliminate the pathogen. These responses are often specific to the type of pathogen and mediated by the expression of specialized genes. We have characterized the expression changes induced in host Drosophila fruit flies following infection by multiple types of pathogens, and identified a small number of genes that show expression changes in each infection. This includes genes that are known to be involved in pathogen resistance, and others that have not been previously studied as immune response genes. These findings provide new insight into transcriptional changes that accompany Drosophila immunity. They may suggest possible roles for the differentially expressed genes in innate immune responses to diverse classes of pathogens, and serve to identify candidate genes for further empirical study of these processes.

Abstract

Organisms are commonly infected by a diverse array of pathogens and mount functionally distinct responses to each of these varied immune challenges. Host immune responses are characterized by the induction of gene expression, however, the extent to which expression changes are shared among responses to distinct pathogens is largely unknown. To examine this, we performed meta-analysis of gene expression data collected from Drosophila melanogaster following infection with a wide array of pathogens. We identified 62 genes that are significantly induced by infection. While many of these infection-induced genes encode known immune response factors, we also identified 21 genes that have not been previously associated with host immunity. Examination of the upstream flanking sequences of the infection-induced genes lead to the identification of two conserved enhancer sites. These sites correspond to conserved binding sites for GATA and nuclear factor κB (NFκB) family transcription factors and are associated with higher levels of transcript induction. We further identified 31 genes with predicted functions in metabolism and organismal development that are significantly downregulated following infection by diverse pathogens. Our study identifies conserved gene expression changes in Drosophila melanogaster following infection with varied pathogens, and transcription factor families that may regulate this immune induction.

Molecular and Functional Characterization of Peptidoglycan Recognition Proteins OfPGRP-A and OfPGRP-B in Ostrinia furnacalis (Lepidoptera: Crambidae)

Simple Summary

The Asian corn borer, Ostrinia furnacalis (Guenée), is the most destructive lepidopteran insect pest of corn (Zea mays L.) in China. Pathogenic microorganisms play an important role in the population control of the Asian corn borer. Although microorganisms can cause the death of O. furnacalis, an immune response also occurs as an attempt to fight off and eliminate invading pathogens. If the molecular mechanism of interaction between O. furnacalis and pathogenic bacteria is clarified, the lethal effect of pathogenic microorganisms can be better exerted by inhibiting the natural immune response of O. furnacalis. As an important member of the pattern-recognition receptor family, peptidoglycan recognition protein (PGRP) plays a key role in the insect innate immune response. In this study, we cloned two PGRP genes from O. furnacalis and analyzed their spatiotemporal expression. In combination with bacterial induction experiments, we revealed the immune signal recognition pathway involved in the two proteins. The results of this study deepen the understanding of the natural immune response of O. furnacalis and provide new ideas for better utilization of pathogenic microorganisms in biological control of the Asian corn borer.

Abstract

Peptidoglycan recognition proteins (PGRPs) are important components of insect immune systems, in which they play key roles. We cloned and sequenced two full-length PGRP, named OfPGRP-A and OfPGRP-B, from the Asian corn borer, Ostrinia furnacalis. These two genes comprise open reading frames of 658 and 759 bp, encoding proteins of 192 and 218 amino acids, respectively. qPCR showed that OfPGRP-A and OfPGRP-B are prominently expressed in the midgut of O. furnacalis fourth instar larvae. After inoculation with Staphylococcus aureus and Bacillus thuringiensis, the expression of OfPGRP-A was significantly upregulated, whereas the expression of OfPGRP-B was enhanced after inoculation with Escherichia coli. This suggests that OfPGRP-A mainly recognizes Gram-positive bacteria and may participate in the Toll signaling pathways, while OfPGRP-B identifies Gram-negative bacteria and may participate in Imd signaling pathways. Our results provide insights into the roles of PGRPs in O. furnacalis immune function and a foundation for using pathogens for the biological control of O. furnacalis.

Gut-derived peptidoglycan remotely inhibits bacteria dependent activation of SREBP by Drosophila adipocytes

Bacteria that colonize eukaryotic gut have profound influences on the physiology of their host. In Drosophila, many of these effects are mediated by adipocytes that combine immune and metabolic functions. We show here that enteric infection with some bacteria species triggers the activation of the SREBP lipogenic protein in surrounding enterocytes but also in remote fat body cells and in ovaries, an effect that requires insulin signaling. We demonstrate that by activating the NF-κB pathway, the cell wall peptidoglycan produced by the same gut bacteria remotely, and cell-autonomously, represses SREBP activation in adipocytes. We finally show that by reducing the level of peptidoglycan, the gut born PGRP-LB amidase balances host immune and metabolic responses of the fat body to gut-associated bacteria. In the absence of such modulation, uncontrolled immune pathway activation prevents SREBP activation and lipid production by the fat body.

An increasing body of evidence indicates that microbes, which live closely associated with animals, significantly influence their development, physiology and even their behavior. The mechanisms that underly these mutual interactions are not yet completely understood. Using Drosophila as a model system, we study the impact of gut bacteria on the host physiology. We present here data showing that some bacteria present in the fly gut can stimulate the production of lipids in the remote fat body tissue via gut autophagy and insulin signaling. However, these bacteria produce many compounds and metabolites such as the cell wall peptidoglycan. Our data show that by cell-autonomously activating the NF-κB signaling pathway in the remote fat body, cell wall peptidoglycan antagonizes bacteria-triggered lipogenesis. We finally show that to prevent this antagonistic effect, flies produce an enzyme, called PGRP-LB, that cleaves the peptidoglycan into its inactive form. Our data highlight the multiple layers of interactions that take place between gut-associated bacteria and a eukaryotic host.

Critical Roles of Spätzle5 in Antimicrobial Peptide Production Against Escherichia coli in Tenebrio molitor Malpighian Tubules

The dimeric cytokine ligand Spätzle (Spz) is responsible for Toll pathway activation and antimicrobial peptide (AMP) production upon pathogen challenge in Tenebrio molitor. Here, we indicated that TmSpz5 has a functional role in response to bacterial infections. We showed that the highest expression of TmSpz5 is induced by Candida albicans. However, TmSpz5 knockdown reduced larval survival against Escherichia coli and Staphylococcus aureus. To evaluate the molecular mechanism underlying the observed survival differences, the role of TmSpz5 in AMP production was examined by RNA interference and microbial injection. T. molitor AMPs that are active against Gram-negative and -positive bacteria, including Tmtenecins, Tmattacins, Tmcoleoptericins, Tmtaumatin-like-proteins, and Tmcecropin-2, were significantly downregulated by TmSpz-5 RNAi in the Malpighian tubules (MTs) following a challenge with E. coli and S. aureus. However, upon infection with C. albicans the mRNA levels of most AMPs in the dsTmSpz5-injected group were similar to those in the control groups. Likewise, the expression of the transcription factors NF-κB, TmDorX2, and TmRelish were noticeably suppressed in the MTs of TmSpz5-silenced larvae. Moreover, E. coli-infected TmSpz5 knockdown larvae showed decreased antimicrobial activity in the MTs and hindgut compared with the control group. These results demonstrate that TmSpz5 has a defined role in T. molitor innate immunity by regulating AMP expression in MTs in response to E. coli.

Bacterial peptidoglycan muropeptides benefit mitochondrial homeostasis and animal physiology by acting as ATP synthase agonists

The symbiotic relationship between commensal microbes and host animals predicts unidentified beneficial impacts of individual bacterial metabolites on animal physiology. Peptidoglycan fragments (muropeptides) from the bacterial cell wall are known for their roles in pathogenicity and for inducing host immune responses. However, the potential beneficial usage of muropeptides from commensal bacteria by the host needs exploration. We identified a striking role for muropeptides in supporting mitochondrial homeostasis, development, and behaviors in Caenorhabditis elegans. We determined that the beneficial molecules are disaccharide muropeptides containing a short AA chain, and they enter intestinal-cell mitochondria to repress oxidative stress. Further analyses indicate that muropeptides execute this role by binding to and promoting the activity of ATP synthase. Therefore, given the exceptional structural conservation of ATP synthase, the role of muropeptides as a rare agonist of the ATP synthase presents a major conceptual modification regarding the impact of bacterial cell metabolites on animal physiology.

TmIKKε Is Required to Confer Protection Against Gram-Negative Bacteria, E. coli by the Regulation of Antimicrobial Peptide Production in the Tenebrio molitor Fat Body

The inhibitor of nuclear factor-kappa B (NF-κB) kinase (IKK) is the core regulator of the NF-κB pathway against pathogenic invasion in vertebrates or invertebrates. IKKβ, -ε and -γ have pivotal roles in the Toll and immune deficiency (IMD) pathways. In this study, a homolog of IKKε (TmIKKε) was identified from Tenebrio molitor RNA sequence database and functionally characterized for its role in regulating immune signaling pathways in insects. The TmIKKε gene is characterized by two exons and one intron comprising an open reading frame (ORF) of 2,196 bp that putatively encodes a polypeptide of 731 amino acid residues. TmIKKε contains a serine/threonine protein kinases catalytic domain. Phylogenetic analysis established the close homology of TmIKKε to Tribolium castaneum IKKε (TcIKKε) and its proximity with other IKK-related kinases. The expression of TmIKKε mRNA was elevated in the gut, integument, and hemocytes of the last-instar larva and the fat body, Malpighian tubules, and testis of 5-day-old adults. TmIKKε expression was significantly induced by Escherichia coli, Staphylococcus aureus, and Candida albicans challenge in whole larvae and tissues, such as hemocytes, gut, and fat body. The knockdown of the TmIKKε messenger RNA (mRNA) expression significantly reduced the survival of the larvae against microbial challenges. Further, we investigated the induction patterns of 14 T. molitor antimicrobial peptides (AMPs) genes in TmIKKε gene-silencing model after microbial challenges. While in hemocytes, the transcriptional regulation of most AMPs was negatively regulated in the gut and fat body tissue of T. molitor, AMPs, such as TmTenecin 1, TmTenecin 4, TmDefensin, TmColeoptericin A, TmColeoptericin B, TmAttacin 1a, and TmAttacin 2, were positively regulated in TmIKKε-silenced individuals after microbial challenge. Collectively, the results implicate TmIKKε as an important factor in antimicrobial innate immune responses in T. molitor.

The Negative Regulative Roles of BdPGRPs in the Imd Signaling Pathway of Bactrocera dorsalis

Peptidoglycan recognition proteins (PGRPs) are key regulators in insects' immune response, functioning as sensors to detect invading pathogens and as scavengers of peptidoglycan (PGN) to reduce immune overreaction. However, the exact function of PGRPs in Bactrocera dorsalis is still unclear. In this study, we identified and functionally characterized the genes BdPGRP-LB, BdPGRP-SB₁ and BdPGRP-SC₂ in B. dorsalis. The results showed that BdPGRP-LB, BdPGRP-SB₁ and BdPGRP-SC₂ all have an amidase-2 domain, which has been shown to have N-Acetylmuramoyl-l-Alanine amidase activity. The transcriptional levels of BdPGRP-LB and BdPGRP-SC₂ were both high in adult stages and midgut tissues; BdPGRP-SB₁ was found most abundantly expressed in the 2nd instar larvae stage and adult fat body. The expression of BdPGRP-LB and BdPGRP-SB₁ and AMPs were significantly up-regulated after injury infected with Escherichia coli at different time points; however, the expression of BdPGRP-SC₂ was reduced at 9 h, 24 h and 48 h following inoculation with E. coli. By injection of dsRNA, BdPGRP-LB, BdPGRP-SB₁ and BdPGRP-SC₂ were knocked down by RNA-interference. Silencing of BdPGRP-LB, BdPGRP-SB₁ and BdPGRP-SC₂ separately in flies resulted in over-activation of the Imd signaling pathway after bacterial challenge. The survival rate of the ds-PGRPs group was significantly reduced compared with the ds-egfp group after bacterial infection. Taken together, our results demonstrated that three catalytic PGRPs family genes, BdPGRP-LB, BdPGRP-SB₁ and BdPGRP-SC₂, are important negative regulators of the Imd pathway in B. dorsalis.

Drice restrains Diap2-mediated inflammatory signalling and intestinal inflammation

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

The potential of antimicrobial peptides isolated from freshwater crayfish species in new drug development: A review

The much-publicised increased resistance of pathogenic bacteria to conventional antibiotics has focused research effort on the characterization of new antimicrobial drugs. In this context, antimicrobial peptides (AMPs) extracted from animals are considered a promising alternative to conventional antibiotics. In recent years, freshwater crayfish species have emerged as an important source of bioactive compounds. In fact, these invertebrates rely on an innate immune system based on cellular responses and on the production of important effectors in the haemolymph, such as AMPs, which are produced and stored in granules in haemocytes and released after stimulation. These effectors are active against both Gram-positive and Gram-negative bacteria. In this review, we summarise the recent progress on AMPs isolated from the several species of freshwater crayfish and their prospects for future pharmaceutical applications to combat infectious agents.