The IRAK homolog Pelle is the functional counterpart of IκB kinase in the Drosophila Toll pathway

Capsid protein mediated evasion of IRAK1-dependent signalling is essential to Sindbis virus neuroinvasion and virulence in mice

ABSTRACTAlphaviruses are arthropod-borne, single-stranded positive-sense RNA viruses that are recognized as rapidly emerging pathogens. Despite being exquisitely sensitive to the effects of the innate immune response alphaviruses can readily replicate, disseminate, and induce pathogenesis in immunologically competent hosts. Nonetheless, how alphaviruses evade the induction of an innate immune response prior to viral gene expression, or in non-permissive infections, is unknown. Previously we reported the identification of a novel host/pathogen interaction between the viral Capsid (CP) protein and the host IRAK1 protein. The CP/IRAK1 interaction was determined to negatively impact IRAK1-dependent PAMP detection in vitro, however, the precise importance of the CP/IRAK1 interaction to alphaviral infection remained unknown. Here we detail the identification of the CP/IRAK1 interaction determinants of the Sindbis virus (SINV) CP protein and examine the importance of the interaction to alphaviral infection and pathogenesis in vivo using an interaction deficient mutant of the model neurotropic strain of SINV. Importantly, these interaction determinants are highly conserved across multiple Old-World alphaviruses, including Ross River virus (RRV), Mayaro virus (MAYV), Chikungunya virus (CHIKV), and Semliki Forest virus (SFV). In the absence of a functional CP/IRAK1 interaction, SINV replication is significantly restricted and fails to disseminate from the primary site of inoculation due to the induction of a robust type-I Interferon response. Altogether these data indicate that the evasion of IRAK1-dependent signalling is critical to overcoming the host innate immune response and the in vivo data presented here demonstrate the importance of the CP/IRAK1 interaction to neurovirulence and pathogenesis.

Mechanisms for controlling Dorsal nuclear levels

Formation of the Dorsal nuclear-cytoplasmic gradient is important for the proper establishment of gene expression patterns along the dorsal-ventral (DV) axis during embryogenesis in Drosophila melanogaster. Correct patterning of the DV axis leads to formation of the presumptive mesoderm, neurogenic ectoderm, dorsal ectoderm, and amnioserosa, which are tissues necessary for embryo viability. While Toll signaling is necessary for Dorsal gradient formation, a gradient still forms in the absence of Toll, suggesting there are additional mechanisms required to achieve correct nuclear Dorsal levels. Potential mechanisms include post-translational modification, shuttling, and nuclear spacing. Post-translational modification could affect import and export rates either directly through modification of a nuclear localization sequence or nuclear export sequence, or indirectly by affecting interactions with binding partners that alter import and export rates. Shuttling, which refers to the facilitated diffusion of Dorsal through its interaction with its cytoplasmic inhibitor Cactus, could regulate nuclear levels by delivering more Dorsal ventrally. Finally, nuclear spacing could result in higher nuclear levels by leaving fewer nuclei in the ventral domain to uptake Dorsal. This review details how each of these mechanisms may help establish Dorsal nuclear levels in the early fly embryo, which serves as a paradigm for understanding how the dynamics of graded inputs can influence patterning and target gene expression. Furthermore, careful analysis of nuclear Dorsal levels is likely to provide general insights as recent studies have suggested that the regulation of nuclear import affects the timing of gene expression at the maternal-to-zygotic transition.

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.

Neofunctionalization of Toll Signaling in Insects: From Immunity to Dorsoventral Patterning

Toll signaling plays a crucial role in pathogen defense throughout the animal kingdom. It was discovered, however, for its function in dorsoventral (DV) axis formation in Drosophila. In all other insects studied so far, but not outside the insects, Toll is also required for DV patterning. However, in insects more distantly related to Drosophila, Toll's patterning role is frequently reduced and substituted by an expanded influence of BMP signaling, the pathway implicated in DV axis formation in all major metazoan lineages. This suggests that Toll was integrated into an ancestral BMP-based patterning system at the base of the insects or during insect evolution. The observation that Toll signaling has an immune function in the extraembryonic serosa, an early differentiating tissue of most insect embryos, suggests a scenario of how Toll was co-opted from an ancestral immune function for its new role in axis formation.

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.

A Toll-dependent Bre1/Rad6-cact feedback loop in controlling host innate immune response

The Toll signaling pathway was initially identified for its involvement in the control of early embryogenesis. It was later shown to be also part of a major innate immune pathway controlling the expression of anti-microbial peptides in many eukaryotes including humans; cactus, the essential negative regulator of this pathway in flies, was found to be induced in parallel to the Toll-dependent activation process during immune defenses. We were interested in the mechanisms of this dual effect and provide here evidence that upon pathogenic stimuli, dorsal, one of the transcription factors of the fly Toll pathway, can induce the expression of the E3 ligase Bre1. We further show that Bre1 complexes with the E2 Rad6 to mono-ubiquitinate histone H2B and to promote the transcription of cactus to achieve homeostasis of the Toll immune response. Our studies characterize a Toll signal-dependent regulatory machinery in governing the Toll pathway in Drosophila.

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.

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.

A non-canonical Raf function is required for dorsal-ventral patterning during Drosophila embryogenesis

Proper embryonic development requires directional axes to pattern cells into embryonic structures. In Drosophila, spatially discrete expression of transcription factors determines the anterior to posterior organization of the early embryo, while the Toll and TGFβ signalling pathways determine the early dorsal to ventral pattern. Embryonic MAPK/ERK signaling contributes to both anterior to posterior patterning in the terminal regions and to dorsal to ventral patterning during oogenesis and embryonic stages. Here we describe a novel loss of function mutation in the Raf kinase gene, which leads to loss of ventral cell fates as seen through the loss of the ventral furrow, the absence of Dorsal/NFκB nuclear localization, the absence of mesoderm determinants Twist and Snail, and the expansion of TGFβ. Gene expression analysis showed cells adopting ectodermal fates much like loss of Toll signaling. Our results combine novel mutants, live imaging, optogenetics and transcriptomics to establish a novel role for Raf, that appears to be independent of the MAPK cascade, in embryonic patterning.

A genetic screen in Drosophila reveals the role of fucosylation in host susceptibility to Candida infection

Candida infections constitute a blind spot in global public health as very few new anti-fungal drugs are being developed. Genetic surveys of host susceptibilities to such infections using mammalian models have certain disadvantages in that obtaining results is time-consuming, owing to relatively long lifespans, and these results have low statistical resolution because sample sizes are usually small. Here, we report a targeted genetic screening of 5698 RNAi lines encompassing 4135 Drosophila genes with human homologues, several of which we identify as important for host survival after Candida albicans infection. These include genes in a variety of functional classes encompassing gene expression, intracellular signalling, metabolism and enzymatic regulation. Analysis of one of the screen hits, the infection-induced α-(1,3)-fucosylase FucTA, showed that N-glycan fucosylation has several targets among proteins involved in host defence, which provides multiple avenues of investigation for the mechanistic analysis of host survival to systemic C. albicans infection.

Expression and Function of Toll Pathway Components in the Early Development of the Wasp Nasonia vitripennis

The Toll signaling pathway is the main source of embryonic DV polarity in the fly Drosophila melanogaster. This pathway appears to have been co-opted from an ancestral innate immunity system within the insects and has been deployed in different ways among insect taxa. Here we report the expression and function of homologs of the important components of the D. melanogaster Toll pathway in the wasp Nasonia vitripennis. We found homologs for all the components; many components had one or more additional paralogs in the wasp relative the fly. We also found significant deviations in expression patterns of N. vitripennis homologs. Finally, we provide some preliminary functional analyses of the N. vitripennis homologs, where we find a mixture of conservation and divergence of function.

Bacterial recognition by PGRP-SA and downstream signalling by Toll/DIF sustain commensal gut bacteria in Drosophila

The gut sets the immune and metabolic parameters for the survival of commensal bacteria. We report that in Drosophila, deficiency in bacterial recognition upstream of Toll/NF-κB signalling resulted in reduced density and diversity of gut bacteria. Translational regulation factor 4E-BP, a transcriptional target of Toll/NF-κB, mediated this host-bacteriome interaction. In healthy flies, Toll activated 4E-BP, which enabled fat catabolism, which resulted in sustaining of the bacteriome. The presence of gut bacteria kept Toll signalling activity thus ensuring the feedback loop of their own preservation. When Toll activity was absent, TOR-mediated suppression of 4E-BP made fat resources inaccessible and this correlated with loss of intestinal bacterial density. This could be overcome by genetic or pharmacological inhibition of TOR, which restored bacterial density. Our results give insights into how an animal integrates immune sensing and metabolism to maintain indigenous bacteria in a healthy gut.

Proteasome α6 Subunit Negatively Regulates the JAK/STAT Pathway and Blood Cell Activation in Drosophila melanogaster

JAK/STAT signaling regulates central biological functions such as development, cell differentiation and immune responses. In Drosophila, misregulated JAK/STAT signaling in blood cells (hemocytes) induces their aberrant activation. Using mass spectrometry to analyze proteins associated with a negative regulator of the JAK/STAT pathway, and by performing a genome-wide RNAi screen, we identified several components of the proteasome complex as negative regulators of JAK/STAT signaling in Drosophila. A selected proteasome component, Prosα6, was studied further. In S2 cells, Prosα6 silencing decreased the amount of the known negative regulator of the pathway, ET, leading to enhanced expression of a JAK/STAT pathway reporter gene. Silencing of Prosα6 in vivo resulted in activation of the JAK/STAT pathway, leading to the formation of lamellocytes, a specific hemocyte type indicative of hemocyte activation. This hemocyte phenotype could be partially rescued by simultaneous knockdown of either the Drosophila STAT transcription factor, or MAPKK in the JNK-pathway. Our results suggest a role for the proteasome complex components in the JAK/STAT pathway in Drosophila blood cells both in vitro and in vivo.

cGMP signaling pathway that modulates NF-κB activation in innate immune responses

The nuclear factor-kappa B (NF-κB) pathway is an evolutionarily conserved signaling pathway that plays a central role in immune responses and inflammation. Here, we show that Drosophila NF-κB signaling is activated via a pathway in parallel with the Toll receptor by receptor-type guanylate cyclase, Gyc76C. Gyc76C produces cyclic guanosine monophosphate (cGMP) and modulates NF-κB signaling through the downstream Tollreceptor components dMyd88, Pelle, Tube, and Dif/Dorsal (NF-κB). The cGMP signaling pathway comprises a membrane-localized cGMP-dependent protein kinase (cGK) called DG2 and protein phosphatase 2A (PP2A) and is crucial for host survival against Gram-positive bacterial infections in Drosophila. A membrane-bound cGK, PRKG2, also modulates NF-κB activation via PP2A in human cells, indicating that modulation of NF-κB activation in innate immunity by the cGMP signaling pathway is evolutionarily conserved.

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Drosophila NF-κB signaling is activated by Gyc76C in parallel with the Toll receptor

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Gyc76C modulates NF-κB signaling through downstream Toll receptor components

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In Drosophila, the pathway comprises a cGMP-dependent protein kinase (cGK) and PP2A

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In human cells, a membrane-bound cGK, PRKG2, also modulates NF-κB signaling via PP2A

Genetic determinants of antiviral immunity in dipteran insects - Compiling the experimental evidence

The genetic basis of antiviral immunity in dipteran insects is extensively studied in Drosophila melanogaster and advanced technologies for genetic manipulation allow a better characterization of immune responses also in non-model insect species. Especially, immunity in vector mosquitoes is recently in the spotlight, due to the medical impact that these insects have by transmitting viruses and other pathogens. Here, we review the current state of experimental evidence that supports antiviral functions for immune genes acting in different cellular pathways. We discuss the well-characterized RNA interference mechanism along with the less well-defined JAK-STAT, Toll, and IMD signaling pathways. Furthermore, we highlight the initial evidence for antiviral activity observed for the autophagy pathway, transcriptional pausing, as well as piRNA production from endogenous viral elements. We focus our review on studies from Drosophila and mosquito species from the lineages Aedes, Culex, and Anopheles, which contain major vector species responsible for virus transmission.

A reaction-diffusion network model predicts a dual role of Cactus/IκB to regulate Dorsal/NFκB nuclear translocation in Drosophila

Dorsal-ventral patterning of the Drosophila embryo depends on the NFκB superfamily transcription factor Dorsal (Dl). Toll receptor activation signals for degradation of the IκB inhibitor Cactus (Cact), leading to a ventral-to-dorsal nuclear Dl gradient. Cact is critical for Dl nuclear import, as it binds to and prevents Dl from entering the nuclei. Quantitative analysis of cact mutants revealed an additional Cact function to promote Dl nuclear translocation in ventral regions of the embryo. To investigate this dual Cact role, we developed a predictive model based on a reaction-diffusion regulatory network. This network distinguishes non-uniform Toll-dependent Dl nuclear import and Cact degradation, from the Toll-independent processes of Cact degradation and reversible nuclear-cytoplasmic Dl flow. In addition, it incorporates translational control of Cact levels by Dl. Our model successfully reproduces wild-type data and emulates the Dl nuclear gradient in mutant dl and cact allelic combinations. Our results indicate that the dual role of Cact depends on the dynamics of Dl-Cact trimers along the dorsal-ventral axis: In the absence of Toll activation, free Dl-Cact trimers retain Dl in the cytoplasm, limiting the flow of Dl into the nucleus; in ventral-lateral regions, Dl-Cact trimers are recruited by Toll activation into predominant signaling complexes and promote Dl nuclear translocation. Simulations suggest that the balance between Toll-dependent and Toll-independent processes are key to this dynamics and reproduce the full assortment of Cact effects. Considering the high evolutionary conservation of these pathways, our analysis should contribute to understanding NFκB/c-Rel activation in other contexts such as in the vertebrate immune system and disease.

In Drosophila, Toll pathway establishes spatially distinct gene expression territories that define the embryonic dorsal-ventral axis. Toll activation leads to degradation of the IκB inhibitor Cactus, releasing the NFκB superfamily transcription factor Dorsal for nuclear entry. Recently, quantitative analysis of cact mutants revealed that Cact displays an additional function to promote Dl nuclear translocation in ventral regions of the embryo. To understand this novel activity, we developed a predictive theoretical model that shows that the kinetics of Dorsal-Cactus complex formation prior to their recruitment to Toll-signaling complexes is an essential regulatory hub. Cactus controls the balance between the recruitment of these complexes by active Toll receptor and association-dissociation events that generate free Dorsal for direct nuclear import.

Regulators and signalling in insect antimicrobial innate immunity: Functional molecules and cellular pathways

Insects possess an immune system that protects them from attacks by various pathogenic microorganisms that would otherwise threaten their survival. Immune mechanisms may deal directly with the pathogens by eliminating them from the host organism or disarm them by suppressing the synthesis of toxins and virulence factors that promote the invasion and destructive action of the intruder within the host. Insects have been established as outstanding models for studying immune system regulation because innate immunity can be explored as an integrated system at the level of the whole organism. Innate immunity in insects consists of basal immunity that controls the constitutive synthesis of effector molecules such as antimicrobial peptides, and inducible immunity that is activated after detection of a microbe or its product(s). Activation and coordination of innate immune defenses in insects involve evolutionary conserved immune factors. Previous research in insects has led to the identification and characterization of distinct immune signalling pathways that modulate the response to microbial infections. This work has not only advanced the field of insect immunology, but it has also rekindled interest in the innate immune system of mammals. Here we review the current knowledge on key molecular components of insect immunity and discuss the opportunities they present for confronting infectious diseases in humans.

Effects of NF-kB Signaling Inhibitors on Bed Bug Resistance to Orally Provisioned Entomopathogenic Bacteria

Bed bugs are globally important pests and there is an ongoing need for the development and improvement of bed bug control tools. Though promising against other insect pests, the exploration of biological methods for bed bug control is limited. Previously, we identified several species of bacteria that have entomopathogenic effects against bed bugs when ingested. We also described the conservation of several antibacterial responses in bed bugs, including the expression of immune effector genes regulated by NF-kB transcription factors through the Toll and immune deficiency (IMD) signaling pathways. Accordingly, we predicted that chemical inhibition of NF-kB signaling could reduce bed bug resistance to orally provisioned entomopathogenic bacteria, potentially improving their effectiveness as biological control agents. In the present study, we administered four small molecule inhibitors of NF-kB signaling (BMS345541, IKK16, IMD0354, Takinib) to bed bugs by feeding them in a blood meal. We then quantified basal mortality and mortality in response to oral infection with two different entomopathogenic bacteria (Pseudomonas entomophila and Bacillus thuringiensis israelensis). None of the NF-kB signaling inhibitors tested increased mortality above control levels when administered alone, suggesting a lack of direct toxicity. However, one inhibitor (IKK16) significantly enhanced the rate of mortality from oral infection with P. entomophila. Enhanced mortality was independent of direct effects of IKK16 on P. entomophila growth in vitro but was associated with higher bacterial loads in vivo (i.e., reduced resistance). Together, these results provide new insight into the regulation of the bed bug immune system and suggest that administration of entomopathogens in combination with inhibition of immune signaling pathways to reduce infection resistance may be effective for biological control of bed bugs.

Robustness of the Dorsal morphogen gradient with respect to morphogen dosage

In multicellular organisms, the timing and placement of gene expression in a developing tissue assigns the fate of each cell in the embryo in order for a uniform field of cells to differentiate into a reproducible pattern of organs and tissues. This positional information is often achieved through the action of spatial gradients of morphogens. Spatial patterns of gene expression are paradoxically robust to variations in morphogen dosage, given that, by definition, gene expression must be sensitive to morphogen concentration. In this work we investigate the robustness of the Dorsal/NF-κB signaling module with respect to perturbations to the dosage of maternally-expressed dorsal mRNA. The Dorsal morphogen gradient patterns the dorsal-ventral axis of the early Drosophila embryo, and we found that an empirical description of the Dorsal gradient is highly sensitive to maternal dorsal dosage. In contrast, we found experimentally that gene expression patterns are highly robust. Although the components of this signaling module have been characterized in detail, how their function is integrated to produce robust gene expression patterns to variations in the dorsal maternal dosage is still unclear. Therefore, we analyzed a mechanistic model of the Dorsal signaling module and found that Cactus, a cytoplasmic inhibitor for Dorsal, must be present in the nucleus for the system to be robust. Furthermore, active Toll, the receptor that dissociates Cactus from Dorsal, must be saturated. Finally, the vast majority of robust descriptions of the system require facilitated diffusion of Dorsal by Cactus. Each of these three recently-discovered mechanisms of the Dorsal module are critical for robustness. These mechanisms synergistically contribute to changing the amplitude and shape of the active Dorsal gradient, which is required for robust gene expression. Our work highlights the need for quantitative understanding of biophysical mechanisms of morphogen gradients in order to understand emergent phenotypes, such as robustness.

Functional Variation of IL-1R-Associated Kinases in the Conserved MyD88-TRAF6 Pathway during Evolution

IL-1R-associated kinases (IRAK) are important regulators in the TLR/IL-1R pathways, but their function appears inconsistent between Drosophila, bony fishes, and vertebrates. This causes a difficulty to understand the IRAK functions. As a step to reveal the evolution of IRAKs, in this study, we performed comparative and functional analysis of IRAKs by exploiting the amphioxus, a pivotal taxon connecting invertebrates and vertebrates. Sequence and phylogenetic analysis indicated three major IRAK lineages: IRAK1/2/3 is a vertebrate-specific lineage, IRAK4 is an ancient lineage conserved between invertebrate and vertebrates, and Pelle is another ancient lineage that is preserved in protostomes and invertebrate deuterostomes but lost in vertebrate deuterostomes. Pelle is closer neither to IRAK4 nor to IRAK1/2/3, hence suggesting no clear functional analogs to IRAK1/2/3 in nonvertebrates. Functional analysis showed that both amphioxus IRAK4 and Pelle could suppress NF-κB activation induced by MyD88 and TRAF6, which are unlike mammalian and Drosophila IRAKs, but, surprisingly, similar to bony fish IRAK4. Also unlike Drosophila IRAKs, no interaction was detected between amphioxus IRAK4 and Pelle, although both of them were shown capable of binding MyD88. These findings, together with previous reports, show that unlike other signal transducers in the TLR/IL-1R pathways, such as MyD88 and TRAF6, the functions of IRAKs are highly variable during evolution and very specialized in different major animal taxa. Indeed, we suggest that the functional variability of IRAKs might confer plasticity to the signal transduction of the TLR/IL-1R pathways, which in return helps the species to evolve against the pathogens.

Formation, interpretation, and regulation of the Drosophila Dorsal/NF-κB gradient

The morphogen gradient of the transcription factor Dorsal in the early Drosophila embryo has become one of the most widely studied tissue patterning systems. Dorsal is a Drosophila homolog of mammalian NF-κB and patterns the dorsal-ventral axis of the blastoderm embryo into several tissue types by spatially regulating upwards of 100 zygotic genes. Recent studies using fluorescence microscopy and live imaging have quantified the Dorsal gradient and its target genes, which has paved the way for mechanistic modeling of the gradient. In this review, we describe the mechanisms behind the initiation of the Dorsal gradient and its regulation of target genes. The main focus of the review is a discussion of quantitative and computational studies of the Dl gradient system, including regulation of the Dl gradient. We conclude with a discussion of potential future directions.

Life history alterations upon oral and hemocoelic bacterial exposure in the butterfly Melitaea cinxia

Life history strategies often shape biological interactions by specifying the parameters for possible encounters, such as the timing, frequency, or way of exposure to parasites. Consequentially, alterations in life-history strategies are closely intertwined with such interaction processes. Understanding the connection between life-history alterations and host-parasite interactions can therefore be important to unveil potential links between adaptation to environmental change and changes in interaction processes. Here, we studied how two different host-parasite interaction processes, oral and hemocoelic exposure to bacteria, affect various life histories of the Glanville fritillary butterfly Melitaea cinxia. We either fed or injected adult butterflies with the bacterium Micrococcus luteus and observed for differences in immune defenses, reproductive life histories, and longevity, compared to control exposures. Our results indicate differences in how female butterflies adapt to the two exposure types. Orally infected females showed a reduction in clutch size and an earlier onset of reproduction, whereas a reduction in egg weight was observed for hemocoelically exposed females. Both exposure types also led to shorter intervals between clutches and a reduced life span. These results indicate a relationship between host-parasite interactions and changes in life-history strategies. This relationship could cast restrictions on the ability to adapt to new environments and consequentially influence the population dynamics of a species in changing environmental conditions.

miR-34 Modulates Innate Immunity and Ecdysone Signaling in Drosophila

microRNAs are endogenous small regulatory RNAs that modulate myriad biological processes by repressing target gene expression in a sequence-specific manner. Here we show that the conserved miRNA miR-34 regulates innate immunity and ecdysone signaling in Drosophila. miR-34 over-expression activates antibacterial innate immunity signaling both in cultured cells and in vivo, and flies over-expressing miR-34 display improved survival and pathogen clearance upon Gram-negative bacterial infection; whereas miR-34 knockout animals are defective in antibacterial defense. In particular, miR-34 achieves its immune-stimulatory function, at least in part, by repressing the two novel target genes Dlg1 and Eip75B. In addition, our study reveals a mutual repression between miR-34 expression and ecdysone signaling, and identifies miR-34 as a node in the intricate interplay between ecdysone signaling and innate immunity. Lastly, we identify cis-regulatory genomic elements and trans-acting transcription factors required for optimal ecdysone-mediated repression of miR-34. Taken together, our study enriches the repertoire of immune-modulating miRNAs in animals, and provides new insights into the interplay between steroid hormone signaling and innate immunity.

The N-terminal loop of IRAK-4 death domain regulates ordered assembly of the Myddosome signalling scaffold

Activation of Toll-like receptors induces dimerization and the recruitment of the death domain (DD) adaptor protein MyD88 into an oligomeric post receptor complex termed the Myddosome. The Myddosome is a hub for inflammatory and oncogenic signaling and has a hierarchical arrangement with 6-8 MyD88 molecules assembling with exactly 4 of IRAK-4 and 4 of IRAK-2. Here we show that a conserved motif in IRAK-4 (Ser8-X-X-X-Arg12) is autophosphorylated and that the phosphorylated DD is unable to form Myddosomes. Furthermore a mutant DD with the phospho-mimetic residue Asp at this position is impaired in both signalling and Myddosome assembly. IRAK-4 Arg12 is also essential for Myddosome assembly and signalling and we propose that phosphorylated Ser8 induces the N-terminal loop to fold into an α-helix. This conformer is stabilised by an electrostatic interaction between phospho-Ser8 and Arg12 and would destabilise a critical interface between IRAK-4 and MyD88. Interestingly IRAK-2 does not conserve this motif and has an alternative interface in the Myddosome that requires Arg67, a residue conserved in paralogues, IRAK-1 and 3(M).

Toll Receptor-Mediated Hippo Signaling Controls Innate Immunity in Drosophila

The Hippo signaling pathway functions through Yorkie to control tissue growth and homeostasis. How this pathway regulates non-developmental processes remains largely unexplored. Here, we report an essential role for Hippo signaling in innate immunity whereby Yorkie directly regulates the transcription of the Drosophila IκB homolog, Cactus, in Toll receptor-mediated antimicrobial response. Loss of Hippo pathway tumor suppressors or activation of Yorkie in fat bodies, the Drosophila immune organ, leads to elevated cactus mRNA levels, decreased expression of antimicrobial peptides, and vulnerability to infection by Gram-positive bacteria. Furthermore, Gram-positive bacteria acutely activate Hippo-Yorkie signaling in fat bodies via the Toll-Myd88-Pelle cascade through Pelle-mediated phosphorylation and degradation of the Cka subunit of the Hippo-inhibitory STRIPAK PP2A complex. Our studies elucidate a Toll-mediated Hippo signaling pathway in antimicrobial response, highlight the importance of regulating IκB/Cactus transcription in innate immunity, and identify Gram-positive bacteria as extracellular stimuli of Hippo signaling under physiological settings.

How Toll Met Hippo

Toll and Hippo are two seemingly disparate pathways that regulate innate immunity and tissue growth, respectively. Reporting recently in Cell, Liu et al. (2016) tie them together by demonstrating that microbial infection activates Toll, which then signals to both pathways to modulate the antimicrobial response.

Pelle Modulates dFoxO-Mediated Cell Death in Drosophila

Interleukin-1 receptor-associated kinases (IRAKs) are crucial mediators of the IL-1R/TLR signaling pathways that regulate the immune and inflammation response in mammals. Recent studies also suggest a critical role of IRAKs in tumor development, though the underlying mechanism remains elusive. Pelle is the sole Drosophila IRAK homolog implicated in the conserved Toll pathway that regulates Dorsal/Ventral patterning, innate immune response, muscle development and axon guidance. Here we report a novel function of pll in modulating apoptotic cell death, which is independent of the Toll pathway. We found that loss of pll results in reduced size in wing tissue, which is caused by a reduction in cell number but not cell size. Depletion of pll up-regulates the transcription of pro-apoptotic genes, and triggers caspase activation and cell death. The transcription factor dFoxO is required for loss-of-pll induced cell death. Furthermore, loss of pll activates dFoxO, promotes its translocation from cytoplasm to nucleus, and up-regulates the transcription of its target gene Thor/4E-BP. Finally, Pll physically interacts with dFoxO and phosphorylates dFoxO directly. This study not only identifies a previously unknown physiological function of pll in cell death, but also shed light on the mechanism of IRAKs in cell survival/death during tumorigenesis.

Ex vivo genome-wide RNAi screening of the Drosophila Toll signaling pathway elicited by a larva-derived tissue extract

Damage-associated molecular patterns (DAMPs), so-called "danger signals," play important roles in host defense and pathophysiology in mammals and insects. In Drosophila, the Toll pathway confers damage responses during bacterial infection and improper cell-fate control. However, the intrinsic ligands and signaling mechanisms that potentiate innate immune responses remain unknown. Here, we demonstrate that a Drosophila larva-derived tissue extract strongly elicits Toll pathway activation via the Toll receptor. Using this extract, we performed ex vivo genome-wide RNAi screening in Drosophila cultured cells, and identified several signaling factors that are required for host defense and antimicrobial-peptide expression in Drosophila adults. These results suggest that our larva-derived tissue extract contains active ingredients that mediate Toll pathway activation, and the screening data will shed light on the mechanisms of damage-related Toll pathway signaling in Drosophila.

Maternal control of the Drosophila dorsal-ventral body axis

The pathway that generates the dorsal-ventral (DV) axis of the Drosophila embryo has been the subject of intense investigation over the previous three decades. The initial asymmetric signal originates during oogenesis by the movement of the oocyte nucleus to an anterior corner of the oocyte, which establishes DV polarity within the follicle through signaling between Gurken, the Drosophila Transforming Growth Factor (TGF)-α homologue secreted from the oocyte, and the Drosophila Epidermal Growth Factor Receptor (EGFR) that is expressed by the follicular epithelium cells that envelop the oocyte. Follicle cells that are not exposed to Gurken follow a ventral fate and express Pipe, a sulfotransferase that enzymatically modifies components of the inner vitelline membrane layer of the eggshell, thereby transferring DV spatial information from the follicle to the egg. These ventrally sulfated eggshell proteins comprise a localized cue that directs the ventrally restricted formation of the active Spätzle ligand within the perivitelline space between the eggshell and the embryonic membrane. Spätzle activates Toll, a transmembrane receptor in the embryonic membrane. Transmission of the Toll signal into the embryo leads to the formation of a ventral-to-dorsal gradient of the transcription factor Dorsal within the nuclei of the syncytial blastoderm stage embryo. Dorsal controls the spatially specific expression of a large constellation of zygotic target genes, the Dorsal gene regulatory network, along the embryonic DV circumference. This article reviews classic studies and integrates them with the details of more recent work that has advanced our understanding of the complex pathway that establishes Drosophila embryo DV polarity. For further resources related to this article, please visit the WIREs website.

CONFLICT OF INTEREST

The authors have declared no conflicts of interest for this article.