The homeobox gene Caudal regulates constitutive local expression of antimicrobial peptide genes in Drosophila epithelia

Functions and regulations of insect gut bacteria

The insect gut is a complicated ecosystem that inhabits a large number of symbiotic bacteria. As an important organ of the host insect, the symbiotic bacteria of the insect gut play very important roles in regulating physiological and metabolic processes. Recently, much progress has been made in the study of symbiotic bacteria in insect guts with the development of high-throughput sequencing technology and molecular biology. This review summarizes the primary functions of symbiotic bacteria in insect guts, such as enhancing insecticide resistance, facilitating food digestion, promoting detoxification, and regulating mating behavior and egg hatching. It also addresses some possible pathways of gut bacteria symbiont regulation governed by external habitats, physiological conditions and immunity of the host insect. This review provides solid foundations for further studies on novel theories, new technologies and practical applications of symbiotic bacteria in insect guts. © 2024 Society of Chemical Industry.

Lysine-36 of Drosophila histone H3.3 supports adult longevity

Aging is a multifactorial process that disturbs homeostasis, increases disease susceptibility, and ultimately results in death. Although the definitive set of molecular mechanisms responsible for aging remain to be discovered, epigenetic change over time is proving to be a promising piece of the puzzle. Several post-translational histone modifications have been linked to the maintenance of longevity. Here, we focus on lysine-36 of the replication-independent histone protein, H3.3 (H3.3K36). To interrogate the role of this residue in Drosophila developmental gene regulation, we generated a lysine-to-arginine mutant that blocks the activity of its cognate-modifying enzymes. We found that an H3.3BK36R mutation causes a significant reduction in adult lifespan, accompanied by dysregulation of the genomic and transcriptomic architecture. Transgenic co-expression of wild-type H3.3B completely rescues the longevity defect. Because H3.3 is known to accumulate in nondividing tissues, we carried out transcriptome profiling of young vs aged adult fly heads. The data show that loss of H3.3K36 results in age-dependent misexpression of NF-κB and other innate immune target genes, as well as defects in silencing of heterochromatin. We propose H3.3K36 maintains the postmitotic epigenomic landscape, supporting longevity by regulating both pericentric and telomeric retrotransposons and by suppressing aberrant immune signaling.

Lysine-36 of Drosophila histone H3.3 supports adult longevity

Aging is a multifactorial process that disturbs homeostasis, increases disease susceptibility, and ultimately results in death. Although the definitive set of molecular mechanisms responsible for aging remain to be discovered, epigenetic change over time is proving to be a promising piece of the puzzle. Several posttranslational histone modifications (PTMs) have been linked to the maintenance of longevity. Here, we focus on lysine-36 of the replication-independent histone protein, H3.3 (H3.3K36). To interrogate the role of this residue in Drosophila developmental gene regulation, we generated a lysine to arginine mutant that blocks the activity of its cognate modifying enzymes. We found that an H3.3BK36R mutation causes a significant reduction in adult lifespan, accompanied by dysregulation of the genomic and transcriptomic architecture. Transgenic co-expression of wild-type H3.3B completely rescues the longevity defect. Because H3.3 is known to accumulate in non-dividing tissues, we carried out transcriptome profiling of young vs aged adult fly heads. The data show that loss of H3.3K36 results in age-dependent misexpression of NF-κB and other innate immune target genes, as well as defects in silencing of heterochromatin. We propose H3.3K36 maintains the postmitotic epigenomic landscape, supporting longevity by regulating both pericentric and telomeric retrotransposons and by suppressing aberrant immune signaling.

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.

Recognition of commensal bacterial peptidoglycans defines Drosophila gut homeostasis and lifespan

Commensal microbes in animals have a profound impact on tissue homeostasis, stress resistance, and ageing. We previously showed in Drosophila melanogaster that Acetobacter persici is a member of the gut microbiota that promotes ageing and shortens fly lifespan. However, the molecular mechanism by which this specific bacterial species changes lifespan and physiology remains unclear. The difficulty in studying longevity using gnotobiotic flies is the high risk of contamination during ageing. To overcome this technical challenge, we used a bacteria-conditioned diet enriched with bacterial products and cell wall components. Here, we demonstrate that an A. persici-conditioned diet shortens lifespan and increases intestinal stem cell (ISC) proliferation. Feeding adult flies a diet conditioned with A. persici, but not with Lactiplantibacillus plantarum, can decrease lifespan but increase resistance to paraquat or oral infection of Pseudomonas entomophila, indicating that the bacterium alters the trade-off between lifespan and host defence. A transcriptomic analysis using fly intestine revealed that A. persici preferably induces antimicrobial peptides (AMPs), while L. plantarum upregulates amidase peptidoglycan recognition proteins (PGRPs). The specific induction of these Imd target genes by peptidoglycans from two bacterial species is due to the stimulation of the receptor PGRP-LC in the anterior midgut for AMPs or PGRP-LE from the posterior midgut for amidase PGRPs. Heat-killed A. persici also shortens lifespan and increases ISC proliferation via PGRP-LC, but it is not sufficient to alter the stress resistance. Our study emphasizes the significance of peptidoglycan specificity in determining the gut bacterial impact on healthspan. It also unveils the postbiotic effect of specific gut bacterial species, which turns flies into a "live fast, die young" lifestyle.

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.

Constitutively-expressed and induced immune effectors in the house fly (Musca domestica) and the transcription factors that may regulate them

Insects possess both infection-induced and constitutively expressed innate immune defences. Some effectors, such as lysozymes and antimicrobial peptides (AMPs), are constitutively expressed in flies, but expression patterns vary across tissues and species. The house fly (Musca domestica L.) has an impressive immune repertoire, with more effector genes than any other flies. We used RNA-seq to explore both constitutive and induced expression of immune effectors in flies. House flies were fed either Pseudomonas aeruginosa or Escherichia coli, or sterile control broth, and gene expression in the gut and carcass was analysed 4 h post-feeding. Flies fed either bacterium did not induce AMP expression, but some lysozyme and AMP genes were constitutively expressed. Prior transcriptome data from flies injected with bacteria also were analysed, and these constitutively expressed genes differed from those induced by bacterial injection. Binding sites for the transcription factor Myc were enriched upstream of constitutively expressed AMP genes, while upstream regions of induced AMPs were enriched for NF-κB binding sites resembling those of the Imd-responsive transcription factor Relish. Therefore, we identified at least two expression repertoires for AMPs in the house fly: constitutively expressed genes that may be regulated by Myc, and induced AMPs likely regulated by Relish.

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.

Aging-related upregulation of the homeobox gene caudal represses intestinal stem cell differentiation in Drosophila

The differentiation efficiency of adult stem cells undergoes a significant decline in aged animals, which is closely related to the decline in organ function and age-associated diseases. However, the underlying mechanisms that ultimately lead to this observed decline of the differentiation efficiency of stem cells remain largely unclear. This study investigated Drosophila midguts and identified an obvious upregulation of caudal (cad), which encodes a homeobox transcription factor. This factor is traditionally known as a central regulator of embryonic anterior-posterior body axis patterning. This study reports that depletion of cad in intestinal stem/progenitor cells promotes quiescent intestinal stem cells (ISCs) to become activate and produce enterocytes in the midgut under normal gut homeostasis conditions. However, overexpression of cad results in the failure of ISC differentiation and intestinal epithelial regeneration after injury. Moreover, this study suggests that cad prevents intestinal stem/progenitor cell differentiation by modulating the Janus kinase/signal transducers and activators of the transcription pathway and Sox21a-GATAe signaling cascade. Importantly, the reduction of cad expression in intestinal stem/progenitor cells restrained age-associated gut hyperplasia in Drosophila. This study identified a function of the homeobox gene cad in the modulation of adult stem cell differentiation and suggested a potential gene target for the treatment of age-related diseases induced by age-related stem cell dysfunction.

Adult stem cells undergo an aging-related decline of differentiation efficiency in aged animals. However, the underlying mechanisms that ultimately lead to this observed decline of differentiation efficiency in stem cells still remain largely unclear. By using the Drosophila midgut as a model system, this study identified the homeobox family transcription factor gene caudal (cad), the expression of which is significantly upregulated in intestinal stem cells (ISCs) and progenitor cells of aged Drosophila. Depletion of cad promoted quiescent ISCs to become activate and produce enterocytes (ECs) in midguts under normal gut homeostasis conditions; However, overexpression of cad resulted in the failure of ISC differentiation and intestinal epithelial regeneration after injury. Moreover, cad prevents ISC-to-EC differentiation by inhibiting JAK/STAT signaling, and the expressions of Sox21a and GATAe. Reduction of cad expression in intestinal stem/progenitor cells restrained age-associated gut hyperplasia in Drosophila. These findings enable a detailed understanding of the roles of homeobox genes in the modulation of adult stem cell aging in humans. This will be beneficial for the treatment of age-associated diseases that are caused by a functional decline of stem cells.

Regulatory mechanisms of microbial homeostasis in insect gut

Insects live in incredibly complex environments. The intestinal epithelium of insects is in constant contact with microorganisms, some of which are beneficial and some harmful to the host. Insect gut health and function are maintained through multidimensional mechanisms that can proficiently remove foreign pathogenic microorganisms while effectively maintaining local symbiotic microbial homeostasis. The basic immune mechanisms of the insect gut, such as the dual oxidase-reactive oxygen species (Duox-ROS) system and the immune deficiency (Imd)-signaling pathway, are involved in the maintenance of microbial homeostasis. This paper reviews the role of physical defenses, the Duox-ROS and Imd signaling pathways, the Janus kinase/signal transducers and activators of transcription signaling pathway, and intestinal symbiotic flora in the homeostatic maintenance of the insect gut microbiome.

Tissue-autonomous immune response regulates stress signaling during hypertrophy

Postmitotic tissues are incapable of replacing damaged cells through proliferation, but need to rely on buffering mechanisms to prevent tissue disintegration. By constitutively activating the Ras/MAPK-pathway via Ras^V12-overexpression in the postmitotic salivary glands (SGs) of Drosophila larvae, we overrode the glands adaptability to growth signals and induced hypertrophy. The accompanied loss of tissue integrity, recognition by cellular immunity, and cell death are all buffered by blocking stress signaling through a genuine tissue-autonomous immune response. This novel, spatio-temporally tightly regulated mechanism relies on the inhibition of a feedback-loop in the JNK-pathway by the immune effector and antimicrobial peptide Drosomycin. While this interaction might allow growing SGs to cope with temporary stress, continuous Drosomycin expression in Ras^V12-glands favors unrestricted hypertrophy. These findings indicate the necessity to refine therapeutic approaches that stimulate immune responses by acknowledging their possible, detrimental effects in damaged or stressed tissues.

Tissues and organs work hard to maintain balance in everything from taking up nutrients to controlling their growth. Ageing, wounding, sickness, and changes in the genetic code can all alter this balance, and cause the tissue or organ to lose some of its cells. Many tissues restore this loss by dividing their remaining cells to fill in the gaps. But some – like the salivary glands of fruit fly larvae – have lost this ability.

Tissues like these rely on being able to sense and counteract problems as they arise so as to not lose their balance in the first place. The immune system and stress responses are crucial for this process. They trigger steps to correct the problem and interact with each other to find a common decision about the fate of the affected tissue. To better understand how the immune system and stress response work together, Krautz, Khalili and Theopold genetically manipulated cells in the salivary gland of fruit fly larvae. These modifications switched on signals that stimulate cells to keep growing, causing the salivary gland’s tissue to slowly lose its balance and trigger the stress and immune response.

The experiments showed that while the stress response instructed the cells in the gland to die, a peptide released by the immune system called Drosomycin blocked this response and prevented the tissue from collapsing. The cells in the part of the gland not producing this immune peptide were consequently killed by the stress response. When all the cells in the salivary gland were forced to produce Drosomycin, none of the cells died and the whole tissue survived. But it also allowed the cells in the gland to grow uncontrollably, like a tumor, threatening the health of the entire organism.

Mapping the interactions between immune and stress pathways could help to fine-tune treatments that can prevent tissue damage. Fruit flies share many genetic features and molecular pathways with humans. So, the next step towards these kinds of treatments would be to screen for similar mechanisms that block stress activation in damaged human tissues. But this research carries a warning: careless activation of the immune system to protect stressed tissues could lead to uncontrolled tissue growth, and might cause more harm than good.

Leucine-Rich Immune Factor APL1 Is Associated With Specific Modulation of Enteric Microbiome Taxa in the Asian Malaria Mosquito Anopheles stephensi

The commensal gut microbiome is contained by the enteric epithelial barrier, but little is known about the degree of specificity of host immune barrier interactions for particular bacterial taxa. Here, we show that depletion of leucine-rich repeat immune factor APL1 in the Asian malaria mosquito Anopheles stephensi is associated with higher midgut abundance of just the family Enterobacteraceae, and not generalized dysbiosis of the microbiome. The effect is explained by the response of a narrow clade containing two main taxa related to Klebsiella and Cedecea. Analysis of field samples indicate that these two taxa are recurrent members of the wild Anopheles microbiome. Triangulation using sequence and functional data incriminated relatives of C. neteri and Cedecea NFIX57 as candidates for the Cedecea component, and K. michiganensis, K. oxytoca, and K.sp. LTGPAF-6F as candidates for the Klebsiella component. APL1 presence is associated with host ability to specifically constrain the abundance of a narrow microbiome clade of the Enterobacteraceae, and the immune factor may promote homeostasis of this clade in the enteric microbiome for host benefit.

Sexual Dimorphisms in Innate Immunity and Responses to Infection in Drosophila melanogaster

The sexes show profound differences in responses to infection and the development of autoimmunity. Dimorphisms in immune responses are ubiquitous across taxa, from arthropods to vertebrates. Drosophila melanogaster shows strong sex dimorphisms in immune system responses at baseline, upon pathogenic challenge, and over aging. We have performed an exhaustive survey of peer-reviewed literature on Drosophila immunity, and present a database of publications indicating the sex(es) analyzed in each study. While we found a growing interest in the community in adult immunity and in reporting both sexes, the main body of work in this field uses only one sex, or does not stratify by sex. We synthesize evidence for sexually dimorphic responses to bacterial, viral, and fungal infections. Dimorphisms may be mediated by distinct immune compartments, and we review work on sex differences in behavioral, epithelial, cellular, and systemic (fat body-mediated) immunity. Emerging work on sexually dimorphic aging of immune tissues, immune senescence, and inflammation are examined. We consider evolutionary drivers for sex differences in immune investment, highlight the features of Drosophila biology that make it particularly amenable to studies of immune dimorphisms, and discuss areas for future exploration.

Ventx1.1 competes with a transcriptional activator Xcad2 to regulate negatively its own expression

Dorsoventral patterning of body axis in vertebrate embryo is tightly controlled by a complex regulatory network of transcription factors. Ventx1.1 is known as a transcriptional repressor to inhibit dorsal mesoderm formation and neural differentiation in Xenopus. In an attempt to identify, using chromatin immunoprecipitation (ChIP)-Seq, genome-wide binding pattern of Ventx1.1 in Xenopus gastrulae, we observed that Ventx1.1 associates with its own 5′-flanking sequence. In this study, we present evidence that Ventx1.1 binds a cis-acting Ventx1.1 response element (VRE) in its own promoter, leading to repression of its own transcription. Site-directed mutagenesis of the VRE in the Ventx1.1 promoter significantly abrogated this inhibitory autoregulation of Ventx1.1 transcription. Notably, Ventx1.1 and Xcad2, an activator of Ventx1.1 transcription, competitively co-occupied the VRE in the Ventx1.1 promoter. In support of this, mutation of the VRE down-regulated basal and Xcad2-induced levels of Ventx1.1 promoter activity. In addition, overexpression of Ventx1.1 prevented Xcad2 from binding to the Ventx1.1 promoter, and vice versa. Taken together, these results suggest that Ventx1.1 negatively regulates its own transcription in competition with Xcad2, thereby fine-tuning its own expression levels during dorsoventral patterning of Xenopus early embryo.

Microbial Control of Intestinal Homeostasis via Enteroendocrine Cell Innate Immune Signaling

A community of commensal microbes, known as the intestinal microbiota, resides within the gastrointestinal tract of animals and plays a role in maintenance of host metabolic homeostasis and resistance to pathogen invasion. Enteroendocrine cells, which are relatively rare in the intestinal epithelium, have evolved to sense and respond to these commensal microbes. Specifically, they express G-protein-coupled receptors and functional innate immune signaling pathways that recognize products of microbial metabolism and microbe-associated molecular patterns, respectively. Here we review recent evidence from Drosophila melanogaster that microbial cues recruit antimicrobial, mechanical, and metabolic branches of the enteroendocrine innate immune system and argue that this response may play a role not only in maintaining host metabolic homeostasis but also in intestinal resistance to invasion by bacterial, viral, and parasitic pathogens.

Regulation of immune and tissue homeostasis by Drosophila POU factors

The innate immune system of insects deploys both cellular and humoral reactions in immunocompetent tissues for protection of insects against a variety of infections, including bacteria, fungi, and viruses. Transcriptional regulation of genes encoding antimicrobial peptides (AMPs), cytokines, and other immune effectors plays a pivotal role in maintenance of immune homeostasis both prior to and after infections. The POU/Oct transcription factor family is a subclass of the homeodomain proteins present in all metazoans. POU factors are involved in regulation of development, metabolism and immunity. Their role in regulation of immune functions has recently become evident, and involves control of tissue-specific, constitutive expression of immune effectors in barrier epithelia as well as positive and negative control of immune responses in gut and fat body. In addition, they have been shown to affect the composition of gut microbiota and play a role in regulation of intestinal stem cell activities. In this review, we summarize the current knowledge of how POU transcription factors control Drosophila immune homeostasis in healthy and infected insects. The role of POU factor isoform specific regulation of stem cell activities in Drosophila and mammals is also discussed.

Insect Transcription Factors: A Landscape of Their Structures and Biological Functions in Drosophila and beyond

Transcription factors (TFs) play essential roles in the transcriptional regulation of functional genes, and are involved in diverse physiological processes in living organisms. The fruit fly Drosophila melanogaster, a simple and easily manipulated organismal model, has been extensively applied to study the biological functions of TFs and their related transcriptional regulation mechanisms. It is noteworthy that with the development of genetic tools such as CRISPR/Cas9 and the next-generation genome sequencing techniques in recent years, identification and dissection the complex genetic regulatory networks of TFs have also made great progress in other insects beyond Drosophila. However, unfortunately, there is no comprehensive review that systematically summarizes the structures and biological functions of TFs in both model and non-model insects. Here, we spend extensive effort in collecting vast related studies, and attempt to provide an impartial overview of the progress of the structure and biological functions of current documented TFs in insects, as well as the classical and emerging research methods for studying their regulatory functions. Consequently, considering the importance of versatile TFs in orchestrating diverse insect physiological processes, this review will assist a growing number of entomologists to interrogate this understudied field, and to propel the progress of their contributions to pest control and even human health.

Nubbin isoform antagonism governs Drosophila intestinal immune homeostasis

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

A TALE of shrimps: Genome-wide survey of homeobox genes in 120 species from diverse crustacean taxa

The homeodomain-containing proteins are an important group of transcription factors found in most eukaryotes including animals, plants and fungi. Homeobox genes are responsible for a wide range of critical developmental and physiological processes, ranging from embryonic development, innate immune homeostasis to whole-body regeneration. With continued fascination on this key class of proteins by developmental and evolutionary biologists, multiple efforts have thus far focused on the identification and characterization of homeobox orthologs from key model organisms in attempts to infer their evolutionary origin and how this underpins the evolution of complex body plans. Despite their importance, the genetic complement of homeobox genes has yet been described in one of the most valuable groups of animals representing economically important food crops. With crustacean aquaculture being a growing industry worldwide, it is clear that systematic and cross-species identification of crustacean homeobox orthologs is necessary in order to harness this genetic circuitry for the improvement of aquaculture sustainability. Using publicly available transcriptome data sets, we identified a total of 4183 putative homeobox genes from 120 crustacean species that include food crop species, such as lobsters, shrimps, crayfish and crabs. Additionally, we identified 717 homeobox orthologs from 6 other non-crustacean arthropods, which include the scorpion, deer tick, mosquitoes and centipede. This high confidence set of homeobox genes will now serve as a key resource to the broader community for future functional and comparative genomics studies.

An IMD-like pathway mediates both endosymbiont control and host immunity in the cereal weevil Sitophilus spp

Many insects developing on nutritionally unbalanced diets have evolved symbiotic associations with vertically transmitted intracellular bacteria (endosymbionts) that provide them with metabolic components, thereby improving the host's abilities to thrive on such poor ecological niches. While host-endosymbiont coevolutionary constraints are known to entail massive genomic changes in the microbial partner, host's genomic evolution remains elusive, particularly with regard to the immune system. In the cereal weevil Sitophilus spp., which houses Sodalis pierantonius, endosymbionts are secluded in specialized host cells, the bacteriocytes that group together as an organ, the bacteriome. We previously reported that at standard conditions, the bacteriome highly expresses the coleoptericin A (colA) antimicrobial peptide (AMP), which was shown to prevent endosymbiont escape from the bacteriocytes. However, following the insect systemic infection by pathogens, the bacteriome upregulates a cocktail of AMP encoding genes, including colA. The regulations that allow these contrasted immune responses remain unknown. In this short report, we provide evidence that an IMD-like pathway is conserved in two sibling species of cereal weevils, Sitophilus oryzae and Sitophilus zeamais. RNA interference (RNAi) experiments showed that imd and relish genes are essential for (i) colA expression in the bacteriome under standard conditions, (ii) AMP up-regulation in the bacteriome following a systemic immune challenge, and (iii) AMP systemic induction following an immune challenge. Histological analyses also showed that relish inhibition by RNAi resulted in endosymbiont escape from the bacteriome, strengthening the involvement of an IMD-like pathway in endosymbiont control. We conclude that Sitophilus' IMD-like pathway mediates both the bacteriome immune program involved in endosymbiont seclusion within the bacteriocytes and the systemic and local immune responses to exogenous challenges. This work provides a striking example of how a conserved immune pathway, initially described as essential in pathogen clearance, also functions in the control of mutualistic associations.

The role of a single gene encoding the Single von Willebrand factor C-domain protein (SVC) in bumblebee immunity extends beyond antiviral defense

The Single von Willebrand factor C-domain proteins (SVCs) are a group of short proteins mainly found in arthropods. They are proposed to be responsive in relation to environmental challenges including the nutritional status, bacterial and viral infections. The SVC protein Vago acts as a cross-talk molecule between the small interfering RNA (siRNA) pathway and the Jak/STAT pathway upon viral infection in Drosophila melanogaster and Culex mosquito cells. Unlike flies and mosquitoes that possess diverse SVCs, most bee species only have one of which the function remains unclear. Here we investigated whether this single SVC within the genome of the bumblebee Bombus terrestris is also involved in the host antiviral immunity and whether links with other immune pathways can be found. We can show the presence of two key characteristics of Vago linked with the single SVC in B. terrestris (BtSVC). The antiviral character is proven by silencing BtSVC, which lead to increased Israeli acute paralysis virus (IAPV) levels in the fat body. Second, the silencing of BtDicer-2 resulted in a lower expression of BtSVC and increased IAPV levels, confirming the link between Dicer-2 and BtSVC. We were, however, unable to demonstrate a third known role of Vago in the activation of the Jak/STAT pathway. This is probably because we lack good markers for this pathway in bumblebees. Interestingly, we found that BtSVC contributes to the basal expression levels of four antimicrobial peptide (AMP)-coding genes in the fat body of the bumblebees. Therefore, the single SVC gene in bumblebees may be involved in both host antiviral immunity and basal AMPs expression.

Bacteria sensing mechanisms in Drosophila gut: Local and systemic consequences

All insects are colonized by microorganisms on their exoskeleton, their gut and even in some cases within their own somatic and germ line cells. This microbiota that can represent up to a few percent of the insect biomass may have a pervasive impact on many aspects of insect biology including physiology, nutrient acquisition, ageing, behaviour and resistance to infection. Mainly through ingestion of contaminated food, the mouth-gut axis represents the first and principal access of external bacteria to the host. Soon after ingestion, the feeding insect needs to rapidly and accurately identify the ingested microbes and decide whether to preserve them if beneficial or neutral, or to eliminate them if potentially harmful. We will review here the recent data acquired in Drosophila on the mechanisms that invertebrate enterocytes rely on to detect the presence of bacteria in the gut. We will compare these modes of bacteria sensing to those in other immune competent tissues and try to rationalize differences that may exist. We will also analyse the physiological consequences of bacteria detection not only locally for the gut itself but also for remote tissues. Finally, we will describe the physiological disorders that can occur due to inaccurate bacteria identification by the gut epithelium.

A critical role for the Drosophila dopamine D1-like receptor Dop1R2 at the onset of metamorphosis

BACKGROUND

Insect metamorphosis relies on temporal and spatial cues that are precisely controlled. Previous studies in Drosophila have shown that untimely activation of genes that are essential to metamorphosis results in growth defects, developmental delay and death. Multiple factors exist that safeguard these genes against dysregulated expression. The list of identified negative regulators that play such a role in Drosophila development continues to expand.

RESULTS

By using RNAi transgene-induced gene silencing coupled to spatio/temporal assessment, we have unraveled an important role for the Drosophila dopamine 1-like receptor, Dop1R2, in development. We show that Dop1R2 knockdown leads to pre-adult lethality. In adults that escape death, abnormal wing expansion and/or melanization defects occur. Furthermore we show that salivary gland expression of this GPCR during the late larval/prepupal stage is essential for the flies to survive through adulthood. In addition to RNAi-induced effects, treatment of larvae with the high affinity D1-like receptor antagonist flupenthixol, also results in developmental arrest, and in morphological defects comparable to those seen in Dop1R2 RNAi flies. To examine the basis for pupal lethality in Dop1R2 RNAi flies, we carried out transcriptome analysis. These studies revealed up-regulation of genes that respond to ecdysone, regulate morphogenesis and/or modulate defense/immunity.

CONCLUSION

Taken together our findings suggest a role for Dop1R2 in the repression of genes that coordinate metamorphosis. Premature release of this inhibition is not tolerated by the developing fly.

Expression pattern of arenicins-the antimicrobial peptides of polychaete Arenicola marina

Immune responses of invertebrate animals are mediated through innate mechanisms, among which production of antimicrobial peptides play an important role. Although evolutionary Polychaetes represent an interesting group closely related to a putative common ancestor of other coelomates, their immune mechanisms still remain scarcely investigated. Previously our group has identified arenicins-new antimicrobial peptides of the lugworm Arenicola marina, since then these peptides were thoroughly characterized in terms of their structure and inhibitory potential. In the present study we addressed the question of the physiological functions of arenicins in the lugworm body. Using molecular and immunocytochemical methods we demonstrated that arencins are expressed in the wide range of the lugworm tissues-coelomocytes, body wall, extravasal tissue and the gut. The expression of arenicins is constitutive and does not depend on stimulation of various infectious stimuli. Most intensively arenicins are produced by mature coelomocytes where they function as killing agents inside the phagolysosome. In the gut and the body wall epithelia arenicins are released from producing cells via secretion as they are found both inside the epithelial cells and in the contents of the cuticle. Collectively our study showed that arenicins are found in different body compartments responsible for providing a first line of defense against infections, which implies their important role as key components of both epithelial and systemic branches of host defense.

The digestive tract of Drosophila melanogaster

The digestive tract plays a central role in the digestion and absorption of nutrients. Far from being a passive tube, it provides the first line of defense against pathogens and maintains energy homeostasis by exchanging neuronal and endocrine signals with other organs. Historically neglected, the gut of the fruit fly Drosophila melanogaster has recently come to the forefront of Drosophila research. Areas as diverse as stem cell biology, neurobiology, metabolism, and immunity are benefitting from the ability to study the genetics of development, growth regulation, and physiology in the same organ. In this review, we summarize our knowledge of the Drosophila digestive tract, with an emphasis on the adult midgut and its functional underpinnings.

Innate immune recognition of the microbiota promotes host-microbial symbiosis

Pattern-recognition receptors (PRRs) are traditionally known to sense microbial molecules during infection to initiate inflammatory responses. However, ligands for PRRs are not exclusive to pathogens and are abundantly produced by the resident microbiota during normal colonization. Mechanism(s) that underlie this paradox have remained unclear. Recent studies reveal that gut bacterial ligands from the microbiota signal through PRRs to promote development of host tissue and the immune system, and protection from disease. Evidence from both invertebrate and vertebrate models reveals that innate immune receptors are required to promote long-term colonization by the microbiota. This emerging perspective challenges current models in immunology and suggests that PRRs may have evolved, in part, to mediate the bidirectional cross-talk between microbial symbionts and their hosts.

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

Background

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

Results

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

Conclusions

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

Distinct antimicrobial peptide expression determines host species-specific bacterial associations

Animals are colonized by coevolved bacterial communities, which contribute to the host's health. This commensal microbiota is often highly specific to its host-species, inferring strong selective pressures on the associated microbes. Several factors, including diet, mucus composition, and the immune system have been proposed as putative determinants of host-associated bacterial communities. Here we report that species-specific antimicrobial peptides account for different bacterial communities associated with closely related species of the cnidarian Hydra. Gene family extensions for potent antimicrobial peptides, the arminins, were detected in four Hydra species, with each species possessing a unique composition and expression profile of arminins. For functional analysis, we inoculated arminin-deficient and control polyps with bacterial consortia characteristic for different Hydra species and compared their selective preferences by 454 pyrosequencing of the bacterial microbiota. In contrast to control polyps, arminin-deficient polyps displayed decreased potential to select for bacterial communities resembling their native microbiota. This finding indicates that species-specific antimicrobial peptides shape species-specific bacterial associations.

Mutations in the IMD pathway and mustard counter Vibrio cholerae suppression of intestinal stem cell division in Drosophila

Vibrio cholerae is an estuarine bacterium and an intestinal pathogen of humans that causes severe epidemic diarrhea. In the absence of adequate mammalian models in which to study the interaction of V. cholerae with the host intestinal innate immune system, we have implemented Drosophila melanogaster as a surrogate host. We previously showed that immune deficiency pathway loss-of-function and mustard gain-of-function mutants are less susceptible to V. cholerae infection. We find that although the overall burden of intestinal bacteria is not significantly different from that of control flies, intestinal stem cell (ISC) division is increased in these mutants. This led us to examine the effect of V. cholerae on ISC division. We report that V. cholerae infection and cholera toxin decrease ISC division. Because IMD pathway and Mustard mutants, which are resistant to V. cholerae, maintain higher levels of ISC division during V. cholerae infection, we hypothesize that suppression of ISC division is a virulence strategy of V. cholerae and that accelerated epithelial regeneration protects the host against V. cholerae. Extension of these findings to mammals awaits the development of an adequate experimental model.

Caudal is a negative regulator of the Anopheles IMD pathway that controls resistance to Plasmodium falciparum infection

Malaria parasite transmission depends upon the successful development of Plasmodium in its Anopheles mosquito vector. The mosquito's innate immune system constitutes a major bottleneck for parasite population growth. We show here that in Anopheles gambiae, the midgut-specific transcription factor Caudal acts as a negative regulator in the Imd pathway-mediated immune response against the human malaria parasite Plasmodium falciparum. Caudal also modulates the mosquito midgut bacterial flora. RNAi-mediated silencing of Caudal enhanced the mosquito's resistance to bacterial infections and increased the transcriptional abundance of key immune effector genes. Interestingly, Caudal's silencing resulted in an increased lifespan of the mosquito, while it impaired reproductive fitness with respect to egg laying and hatching.

The innate immune response transcription factor relish is necessary for neurodegeneration in a Drosophila model of ataxia-telangiectasia

Neurodegeneration is a hallmark of the human disease ataxia-telangiectasia (A-T) that is caused by mutation of the A-T mutated (ATM) gene. We have analyzed Drosophila melanogaster ATM mutants to determine the molecular mechanisms underlying neurodegeneration in A-T. Previously, we found that ATM mutants upregulate the expression of innate immune response (IIR) genes and undergo neurodegeneration in the central nervous system. Here, we present evidence that activation of the IIR is a cause of neurodegeneration in ATM mutants. Three lines of evidence indicate that ATM mutations cause neurodegeneration by activating the Nuclear Factor-κB (NF-κB) transcription factor Relish, a key regulator of the Immune deficiency (Imd) IIR signaling pathway. First, the level of upregulation of IIR genes, including Relish target genes, was directly correlated with the level of neurodegeneration in ATM mutants. Second, Relish mutations inhibited upregulation of IIR genes and neurodegeneration in ATM mutants. Third, overexpression of constitutively active Relish in glial cells activated the IIR and caused neurodegeneration. In contrast, we found that Imd and Dif mutations did not affect neurodegeneration in ATM mutants. Imd encodes an activator of Relish in the response to gram-negative bacteria, and Dif encodes an immune responsive NF-κB transcription factor in the Toll signaling pathway. These data indicate that the signal that causes neurodegeneration in ATM mutants activates a specific NF-κB protein and does so through an unknown activator. In summary, these findings suggest that neurodegeneration in human A-T is caused by activation of a specific NF-κB protein in glial cells.

Gut-associated microbes of Drosophila melanogaster

There is growing interest in using Drosophila melanogaster to elucidate mechanisms that underlie the complex relationships between a host and its microbiota. In addition to the many genetic resources and tools Drosophila provides, its associated microbiota is relatively simple (1-30 taxa), in contrast to the complex diversity associated with vertebrates (> 500 taxa). These attributes highlight the potential of this system to dissect the complex cellular and molecular interactions that occur between a host and its microbiota. In this review, we summarize what is known regarding the composition of gut-associated microbes of Drosophila and their impact on host physiology. We also discuss these interactions in the context of their natural history and ecology and describe some recent insights into mechanisms by which Drosophila and its gut microbiota interact.

Conserved microRNA miR-8 in fat body regulates innate immune homeostasis in Drosophila

Antimicrobial peptides (AMPs) constitute a major arm of the innate immune system across diverse organisms. In Drosophila, septic injury by microbial pathogens rapidly induces the production of the AMPs in fat body via well elucidated pathways such as Toll and IMD. However, several epithelial tissues were reported to locally express AMPs without septic injury via poorly characterized ways. Here, we report that microRNA miR-8 regulates the levels of AMPs basally expressed in Drosophila. The levels of AMPs such as Drosomycin and Diptericin are significantly increased in miR-8 null animals in non-pathogen stimulated conditions. Analysis of various larval tissues revealed that the increase of Drosomycin is fat body specific. Supporting this observation, re-introduction of miR-8 only in the fat body restored the altered AMP expression in miR-8 null flies. Although loss of miR-8 impedes PI3K in the fat body, inhibition of PI3K does not phenocopy the AMP expression of miR-8 null flies, indicating that miR-8 regulates AMP independently of PI3K. Together, our findings suggest a role of miR-8 in systemic immune homeostasis in generally non-pathogenic conditions in flies.

The Drosophila protein mustard tailors the innate immune response activated by the immune deficiency pathway

In this study, we describe a Drosophila melanogaster transposon insertion mutant with tolerance to Vibrio cholerae infection and markedly decreased transcription of diptericin as well as other genes regulated by the immune deficiency innate immunity signaling pathway. We present genetic evidence that this insertion affects a locus previously implicated in pupal eclosion. This genetic locus, which we have named mustard (mtd), contains a LysM domain, often involved in carbohydrate recognition, and a TLDc domain of unknown function. More than 20 Mtd isoforms containing one or both of these conserved domains are predicted. We establish that the mutant phenotype represents a gain of function and can be replicated by increased expression of a short, nuclearly localized Mtd isoform comprised almost entirely of the TLDc domain. We show that this Mtd isoform does not block Relish cleavage or translocation into the nucleus. Lastly, we present evidence suggesting that the eclosion defect previously attributed to the Mtd locus may be the result of the unopposed action of the NF-κB homolog, Relish. Mtd homologs have been implicated in resistance to oxidative stress. However, to our knowledge this is the first evidence that Mtd or its homologs alter the output of an innate immunity signaling cascade from within the nucleus.

A shared role for RBF1 and dCAP-D3 in the regulation of transcription with consequences for innate immunity

Previously, we discovered a conserved interaction between RB proteins and the Condensin II protein CAP-D3 that is important for ensuring uniform chromatin condensation during mitotic prophase. The Drosophila melanogaster homologs RBF1 and dCAP-D3 co-localize on non-dividing polytene chromatin, suggesting the existence of a shared, non-mitotic role for these two proteins. Here, we show that the absence of RBF1 and dCAP-D3 alters the expression of many of the same genes in larvae and adult flies. Strikingly, most of the genes affected by the loss of RBF1 and dCAP-D3 are not classic cell cycle genes but are developmentally regulated genes with tissue-specific functions and these genes tend to be located in gene clusters. Our data reveal that RBF1 and dCAP-D3 are needed in fat body cells to activate transcription of clusters of antimicrobial peptide (AMP) genes. AMPs are important for innate immunity, and loss of either dCAP-D3 or RBF1 regulation results in a decrease in the ability to clear bacteria. Interestingly, in the adult fat body, RBF1 and dCAP-D3 bind to regions flanking an AMP gene cluster both prior to and following bacterial infection. These results describe a novel, non-mitotic role for the RBF1 and dCAP-D3 proteins in activation of the Drosophila immune system and suggest dCAP-D3 has an important role at specific subsets of RBF1-dependent genes.

The retinoblastoma protein (pRB) is a tumor suppressor protein known for its ability to repress transcription of E2F-dependent genes and induce cell cycle arrest. We have previously shown that RB proteins in Drosophila and human cells interact with the Condensin II subunit, CAP-D3, in an E2F-independent manner. Condensins promote condensation of chomosomes in mitosis. Our previous studies suggested that the Drosophila pRB and CAP-D3 homologs, RBF1 and dCAP-D3, co-localize on DNA and may share a function in cells that never undergo mitosis. In this study, we show that one non-mitotic function shared between RBF1 and dCAP-D3 is the regulation of many non-cell-cycle-related, clustered, and cell-type-specific transcripts including a conserved family of genes that are important for the immune response in the fly. In fact, results show that normal levels of dCAP-D3 and RBF1 expression are necessary for the ability of the fly to clear infection with human bacterial pathogens. This work demonstrates that dCAP-D3 proteins can regulate a unique subset of RBF1-dependent transcripts in vivo and identifies a novel role for both RBF1 and dCAP-D3 protein in activation of innate immune genes, which may be conserved in human cells.

Immune response in the barrier epithelia: lessons from the fruit fly Drosophila melanogaster

The barrier epithelia of multicellular organisms frequently come into direct contact with microorganisms and thus need to fulfill the important task of preventing the penetration of pathogens that could cause systemic infections. A functional immune defence in the epithelial linings of the digestive, respiratory and reproductive organs as well as the epidermis/skin of animals is therefore of crucial importance for survival. Epithelial defence reactions are likely to be evolutionarily ancient, and the use of invertebrate animal models, such as insects and nematodes, has been crucial in unravelling the mechanisms underlying epithelial immunity. This review addresses basic questions of epithelial immunity in animals and humans. It focuses on recent developments in the understanding of the immune responses in the fruit fly Drosophila melanogaster and how the innate immune system acts locally in the epidermis and cuticle, tracheae, gut and genital organs. Both basal immune activities in epithelia that are constantly exposed to microbes as well as positive and negative regulation in response to pathogenic organisms are covered. Important immuno-physiological aspects of epithelial defence mechanisms are also discussed, such as wound healing, re-epithelialization and intestinal homeostasis.

Toll-8/Tollo negatively regulates antimicrobial response in the Drosophila respiratory epithelium

Barrier epithelia that are persistently exposed to microbes have evolved potent immune tools to eliminate such pathogens. If mechanisms that control Drosophila systemic responses are well-characterized, the epithelial immune responses remain poorly understood. Here, we performed a genetic dissection of the cascades activated during the immune response of the Drosophila airway epithelium i.e. trachea. We present evidence that bacteria induced-antimicrobial peptide (AMP) production in the trachea is controlled by two signalling cascades. AMP gene transcription is activated by the inducible IMD pathway that acts non-cell autonomously in trachea. This IMD-dependent AMP activation is antagonized by a constitutively active signalling module involving the receptor Toll-8/Tollo, the ligand Spätzle2/DNT1 and Ect-4, the Drosophila ortholog of the human Sterile alpha and HEAT/ARMadillo motif (SARM). Our data show that, in addition to Toll-1 whose function is essential during the systemic immune response, Drosophila relies on another Toll family member to control the immune response in the respiratory epithelium.

Invertebrates solely rely on innate immune responses for defense against microbial infections. Taking advantage of its powerful genetics, the fly Drosophila melanogaster has been extensively used as a model system to dissect the molecular mechanisms that control innate immunity. This work led to the discovery of the essential role of the Toll-1 receptor in triggering the systemic immune response in flies, and paved the way for the discovery of the function of members of the Toll-like receptor (TLR) family in mammalian immunity. Whereas all TLRs are implicated in the mammalian immune response, Toll-1 was, so far, the only Drosophila Toll family member to be involved in the regulation of the immune response. In the present study, we show that another Toll family member, Toll-8 (Tollo), plays an important role in controlling the respiratory epithelium immune response. Our data indicate that, by antagonizing the IMD pathway, Tollo is preventing over-activation of the antibacterial response in the airway epithelium.

Drosophila phagocytosis - still many unknowns under the surface

In mammals, phagocytosis coordinates host defence on two levels: It acts both as an effector of the innate immunity, as well as an initiator of the adaptive immunity. The fruit fly Drosophila melanogaster (D. melanogaster) lacks the adaptive immune response, and the role of Drosophila plasmatocytes, cells that resemble phagocytosing mammalian macrophages, is limited to innate immune responses. During the past years, several studies have shed light on the role of phagocytosis in the Drosophila host defence. At least in some infection models, the systemic production of potent antimicrobial peptides (AMPs) does not completely compensate for the need for cellular immune responses. As a model, Drosophila offers powerful tools for studying phagocytosis including, large-scale RNA interference (RNAi) based in vitro screens that can be combined with classical Drosophila genetics. These kinds of approaches have led to important discoveries related especially to microbial recognition by Drosophila plasmatocytes. Events following initial recognition, however, have remained more elusive. This review summarizes the current knowledge on Drosophila phagocytosis focusing on the most recent advancements in the field, and highlighting the benefits the Drosophila system has to offer for research on phagocytosis.

The role of mucosal immunity and host genetics in defining intestinal commensal bacteria

PURPOSE OF REVIEW

Dramatic advances in molecular characterization of the largely noncultivable enteric microbiota have facilitated better understanding of the composition of this complex ecosystem at broad phylogenetic levels. This review outlines current understanding of mechanisms by which commensal bacteria are controlled and shaped into functional communities by innate and adaptive immune responses, antimicrobial peptides produced by epithelial cells and host genetic factors.

RECENT FINDINGS

Secretory IgA, which targets enteric bacteria, regulates the number, composition, and function of luminal bacteria. Likewise, epithelial production of antimicrobial peptides helps control enteric microbiota growth, translocation, and perhaps composition. The developing role of innate signaling pathways, such as Toll-like receptors and NOD2, is beginning to be studied, with dysbiosis following their genetic deletion. Inflammation and effector immune responses lead to decreased diversity and selective alterations of functionally active bacterial species such as Escherichia coli and Faecalibacterium prausnitzii that have proinflammatory and protective activities, respectively. Studies of humans, mice, and comparative species indicate that both genetic and early environmental factors influence the development of a stable intestinal microbiota.

SUMMARY

Genetic and mucosal immunity strongly influence the composition and function of enteric commensal bacteria. This understanding should help develop strategies to correct dysfunctional altered microbiota in genetically susceptible individuals, better diagnose and correct potential dysbiosis in high-risk individuals at a preclinical stage, and therapeutically target pathogenic bacterial species that help drive chronic inflammatory conditions.

The POU transcription factor Drifter/Ventral veinless regulates expression of Drosophila immune defense genes

Innate immunity operates as a first line of defense in multicellular organisms against infections caused by different classes of microorganisms. Antimicrobial peptides (AMPs) are synthesized constitutively in barrier epithelia to protect against microbial attack and are also upregulated in response to infection. Here, we implicate Drifter/Ventral veinless (Dfr/Vvl), a class III POU domain transcription factor, in tissue-specific regulation of the innate immune defense of Drosophila. We show that Dfr/Vvl is highly expressed in a range of immunocompetent tissues, including the male ejaculatory duct, where its presence overlaps with and drives the expression of cecropin, a potent broad-spectrum AMP. Dfr/Vvl overexpression activates transcription of several AMP genes in uninfected flies in a Toll pathway- and Imd pathway-independent manner. Dfr/Vvl activates a CecA1 reporter gene both in vitro and in vivo by binding to an upstream enhancer specific for the male ejaculatory duct. Further, Dfr/Vvl and the homeodomain protein Caudal (Cad) activate transcription synergistically via this enhancer. We propose that the POU protein Dfr/Vvl acts together with other regulators in a combinatorial manner to control constitutive AMP gene expression in a gene-, tissue-, and sex-specific manner, thus promoting a first-line defense against infection in tissues that are readily exposed to pathogens.

Innate immunity and gut-microbe mutualism in Drosophila

Metazoan guts face a wide variety of microorganisms upon exposure to the environment, including beneficial symbionts, non-symbionts, food-borne microbes and life-threatening pathogens. Recent evidence has shown that the innate immunity of gut epithelia, such as anti-microbial peptide- and reactive oxygen species-based immune systems, actively participate in gut-microbe homeostasis by shaping the commensal community while efficiently eliminating unwanted bacteria. Therefore, elucidation of the regulatory mechanism by which gut innate immunity occurs at the molecular level will provide a novel perspective of gut-microbe mutualisms as well as of gut diseases caused by alterations in the innate immunity.

Evolution of host innate defence: insights from Caenorhabditis elegans and primitive invertebrates

The genetically tractable model organism Caenorhabditis elegans was first used to model bacterial virulence in vivo a decade ago. Since then, great strides have been made in identifying the host response pathways that are involved in its defence against infection. Strikingly, C. elegans seems to detect, and respond to, infection without the involvement of its homologue of Toll-like receptors, in contrast to the well-established role for these proteins in innate immunity in mammals. What, therefore, do we know about host defence mechanisms in C. elegans and what can they tell us about innate immunity in higher organisms?

NF-κB/Rel proteins and the humoral immune responses of Drosophila melanogaster

Nuclear Factor-κB (NF-κB)/Rel transcription factors form an integral part of innate immune defenses and are conserved throughout the animal kingdom. Studying the function, mechanism of activation and regulation of these factors is crucial for understanding host responses to microbial infections. The fruit fly Drosophila melanogaster has proved to be a valuable model system to study these evolutionarily conserved NF-κB mediated immune responses. Drosophila combats pathogens through humoral and cellular immune responses. These humoral responses are well characterized and are marked by the robust production of a battery of anti-microbial peptides. Two NF-κB signaling pathways, the Toll and the IMD pathways, are responsible for the induction of these antimicrobial peptides. Signal transduction in these pathways is strikingly similar to that in mammalian TLR pathways. In this chapter, we discuss in detail the molecular mechanisms of microbial recognition, signal transduction and NF-κB regulation, in both the Toll and the IMD pathways. Similarities and differences relative to their mammalian counterparts are discussed, and recent advances in our understanding of the intricate regulatory networks in these NF-κB signaling pathways are also highlighted.

Long-range activation of systemic immunity through peptidoglycan diffusion in Drosophila

The systemic immune response of Drosophila is known to be induced both by septic injury and by oral infection with certain bacteria, and is characterized by the secretion of antimicrobial peptides (AMPs) into the haemolymph. To investigate other possible routes of bacterial infection, we deposited Erwinia carotovora (Ecc15) on various sites of the cuticle and monitored the immune response via expression of the AMP gene Diptericin. A strong response was observed to deposition on the genital plate of males (up to 20% of a septic injury response), but not females. We show that the principal response to genital infection is systemic, but that some AMPs, particularly Defensin, are induced locally in the genital tract. At late time points we detected bacteria in the haemolymph of immune deficient Relish^E20 flies, indicating that the genital plate can be a route of entry for pathogens, and that the immune response protects flies against the progression of genital infection. The protective role of the immune response is further illustrated by our observation that Relish^E20 flies exhibit significant lethality in response to genital Ecc15 infections. We next show that a systemic immune response can be induced by deposition of the bacterial elicitor peptidoglycan (PGN), or its terminal monomer tracheal cytotoxin (TCT), on the genital plate. This immune response is downregulated by PGRP-LB and Pirk, known regulators of the Imd pathway, and can be suppressed by the overexpression of PGRP-LB in the haemolymph compartment. Finally, we provide strong evidence that TCT can activate a systemic response by crossing epithelia, by showing that radiolabelled TCT deposited on the genital plate can subsequently be detected in the haemolymph. Genital infection is thus an intriguing new model for studying the systemic immune response to local epithelial infections and a potential route of entry for naturally occurring pathogens of Drosophila.

Innate immunity is the first line of antimicrobial defence for vertebrates and the only immune response present in invertebrates such as the fruitfly Drosophila, which provides a powerful model system to study innate immunity. Interestingly, local infections of epithelia like the gut and, in our study, the genital tract, result not only in a local immune response, but in an immune response of the whole body. The latter seems to protect Drosophila against the potential spread of local infections. We have investigated the immune response to bacteria placed on the genitalia, at the entrance to both the genital tract and hindgut. This could be a natural entry route of pathogens, possibly linked to sexually transmitted infections. We observe a strong immune response to Gram-negative bacteria, mediated by the immune responsive Imd signalling pathway. This response depends on peptidoglycan, a crucial component of the bacterial cell wall, as pure peptidoglycan placed on the genitalia is sufficient to trigger a whole body immune response. Finally, we present strong evidence that peptidoglycan fragments within the genital tract or hindgut can cross these epithelia, enter the body cavity and thus induce a system wide immune response to a local infection.

Drosomycin, an essential component of antifungal defence in Drosophila

Drosomycin is an inducible antifungal peptide of 44 residues initially isolated from bacteria-challenged Drosophila melanogaster. The systemic expression of drosomycin is regulated by the Toll pathway present in fat body, whereas inducible local expression in the respiratory tract is controlled by the Immune Deficiency (IMD) pathway. Drosomycin belongs to the cysteine-stabilized alpha-helical and beta-sheet (CSalphabeta) superfamily and is composed of an alpha-helix and a three-stranded beta-sheet stabilized by four disulphide bridges. Drosomycin exhibits a narrow antimicrobial spectrum and is only active against some filamentous fungi. However, recent work using recombinant drosomycin expressed in Escherichia coli revealed its antiparasitic and anti-yeast activities. Two evolutionary epitopes (alpha- and gamma-patch) and the m-loop have been proposed as putative functional regions of drosomycin for interaction with fungi and parasites, respectively. Similarity in sequence, structure and biological activity suggests that drosomycin and some defensin molecules from plants and fungi could originate from a common ancestor.