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

The Drosophila IMD pathway in the activation of the humoral immune response

The IMD pathway signaling plays a pivotal role in the Drosophila defense against bacteria. During the last two decades, significant progress has been made in identifying the components and deciphering the molecular mechanisms underlying this pathway, including the means of bacterial sensing and signal transduction. While these findings have contributed to the understanding of the immune signaling in insects, they have also provided new insights in studying the mammalian NF-κB signaling pathways. Here, we summarize the current view of the IMD pathway focusing on how it regulates the humoral immune response of Drosophila.

Conventional and non-conventional Drosophila Toll signaling

The discovery of Toll in Drosophila and of the remarkable conservation in pathway composition and organization catalyzed a transformation in our understanding of innate immune recognition and response. At the center of that picture is a cascade of interactions in which specific microbial cues activate Toll receptors, which then transmit signals driving transcription factor nuclear localization and activity. Experiments gave substance to the vision of pattern recognition receptors, linked phenomena in development, gene regulation, and immunity into a coherent whole, and revealed a rich set of variations for identifying non-self and responding effectively. More recently, research in Drosophila has illuminated the positive and negative regulation of Toll activation, the organization of signaling events at and beneath membranes, the sorting of information flow, and the existence of non-conventional signaling via Toll-related receptors. Here, we provide an overview of the Toll pathway of flies and highlight these ongoing realms of research.

Antiviral autophagy restrictsRift Valley fever virus infection and is conserved from flies to mammals

Autophagy has been implicated as a component of host defense, but the significance of antimicrobial autophagy in vivo and the mechanism by which it is regulated during infection are poorly defined. Here we found that antiviral autophagy was conserved in flies and mammals during infection with Rift Valley fever virus (RVFV), a mosquito-borne virus that causes disease in humans and livestock. In Drosophila, Toll-7 limited RVFV replication and mortality through activation of autophagy. RVFV infection also elicited autophagy in mouse and human cells, and viral replication was increased in the absence of autophagy genes. The mammalian Toll-like receptor adaptor, MyD88, was required for anti-RVFV autophagy, revealing an evolutionarily conserved requirement for pattern-recognition receptors in antiviral autophagy. Pharmacologic activation of autophagy inhibited RVFV infection in mammalian cells, including primary hepatocytes and neurons. Thus, autophagy modulation might be an effective strategy for treating RVFV infection, which lacks approved vaccines and therapeutics.

Advances in peripheral nerve regeneration

Rodent models of nerve injury have increased our understanding of peripheral nerve regeneration, but clinical applications have been scarce, partly because such models do not adequately recapitulate the situation in humans. In human injuries, axons are often required to extend over much longer distances than in mice, and injury leaves distal nerve fibres and target tissues without axonal contact for extended amounts of time. Distal Schwann cells undergo atrophy owing to the lack of contact with proximal neurons, which results in reduced expression of neurotrophic growth factors, changes in the extracellular matrix and loss of Schwann cell basal lamina, all of which hamper axonal extension. Furthermore, atrophy and denervation-related changes in target tissues make good functional recovery difficult to achieve even when axons regenerate all the way to the target tissue. To improve functional outcomes in humans, strategies to increase the speed of axonal growth, maintain Schwann cells in a healthy, repair-capable state and keep target tissues receptive to reinnervation are needed. Use of rodent models of chronic denervation will facilitate our understanding of the molecular mechanisms of peripheral nerve regeneration and create the potential to test therapeutic advances.

Sarm1-mediated axon degeneration requires both SAM and TIR interactions

Axon degeneration is an evolutionarily conserved pathway that eliminates damaged or unneeded axons. Manipulation of this poorly understood pathway may allow treatment of a wide range of neurological disorders. In an RNAi-based screen performed in cultured mouse DRG neurons, we observed strong suppression of injury-induced axon degeneration upon knockdown of Sarm1 [SARM (sterile α-motif-containing and armadillo-motif containing protein)]. We find that a SARM-dependent degeneration program is engaged by disparate neuronal insults: SARM ablation blocks axon degeneration induced by axotomy or vincristine treatment, while SARM acts in parallel with a soma-derived caspase-dependent pathway following trophic withdrawal. SARM is a multidomain protein that associates with neuronal mitochondria. Deletion of the N-terminal mitochondrial localization sequence disrupts SARM mitochondrial localization in neurons but does not alter its ability to promote axon degeneration. In contrast, mutation of either the SAM (sterile α motif) or TIR (Toll-interleukin-1 receptor) domains abolishes the ability of SARM to promote axonal degeneration, while a SARM mutant containing only these domains elicits axon degeneration and nonapoptotic neuronal death even in the absence of injury. Protein-protein interaction studies demonstrate that the SAM domains are necessary and sufficient to mediate SARM-SARM binding. SARM mutants lacking a TIR domain bind full-length SARM and exhibit strong dominant-negative activity. These results indicate that SARM plays an integral role in the dismantling of injured axons and support a model in which SAM-mediated multimerization is necessary for TIR-dependent engagement of a downstream destruction pathway. These findings suggest that inhibitors of SAM and TIR interactions represent therapeutic candidates for blocking pathological axon loss and neuronal cell death.

Bacteria- and IMD pathway-independent immune defenses against Plasmodium falciparum in Anopheles gambiae

The mosquito Anopheles gambiae uses its innate immune system to control bacterial and Plasmodium infection of its midgut tissue. The activation of potent IMD pathway-mediated anti-Plasmodium falciparum defenses is dependent on the presence of the midgut microbiota, which activate this defense system upon parasite infection through a peptidoglycan recognition protein, PGRPLC. We employed transcriptomic and reverse genetic analyses to compare the P. falciparum infection-responsive transcriptomes of septic and aseptic mosquitoes and to determine whether bacteria-independent anti-Plasmodium defenses exist. Antibiotic treated aseptic mosquitoes mounted molecular immune responses representing a variety of immune functions upon P. falciparum infection. Among other immune factors, our analysis uncovered a serine protease inhibitor (SRPN7) and Clip-domain serine protease (CLIPC2) that were transcriptionally induced in the midgut upon P. falciparum infection, independent of bacteria. We also showed that SRPN7 negatively and CLIPC2 positively regulate the anti-Plasmodium defense, independently of the midgut-associated bacteria. Co-silencing assays suggested that these two genes may function together in a signaling cascade. Neither gene was regulated, nor modulated, by infection with the rodent malaria parasite Plasmodium berghei, suggesting that SRPN7 and CLIPC2 are components of a defense system with preferential activity towards P. falciparum. Further analysis using RNA interference determined that these genes do not regulate the anti-Plasmodium defense mediated by the IMD pathway, and both factors act as agonists of the endogenous midgut microbiota, further demonstrating the lack of functional relatedness between these genes and the bacteria-dependent activation of the IMD pathway. This is the first study confirming the existence of a bacteria-independent, anti-P. falciparum defense. Further exploration of this anti-Plasmodium defense will help clarify determinants of immune specificity in the mosquito, and expose potential gene and/or protein targets for malaria intervention strategies based on targeting the parasite in the mosquito vector.

Functional analysis of PGRP-LA in Drosophila immunity

PeptidoGlycan Recognition Proteins (PGRPs) are key regulators of the insect innate antibacterial response. Even if they have been intensively studied, some of them have yet unknown functions. Here, we present a functional analysis of PGRP-LA, an as yet uncharacterized Drosophila PGRP. The PGRP-LA gene is located in cluster with PGRP-LC and PGRP-LF, which encode a receptor and a negative regulator of the Imd pathway, respectively. Structure predictions indicate that PGRP-LA would not bind to peptidoglycan, pointing to a regulatory role of this PGRP. PGRP-LA expression was enriched in barrier epithelia, but low in the fat body. Use of a newly generated PGRP-LA deficient mutant indicates that PGRP-LA is not required for the production of antimicrobial peptides by the fat body in response to a systemic infection. Focusing on the respiratory tract, where PGRP-LA is strongly expressed, we conducted a genome-wide microarray analysis of the tracheal immune response of wild-type, Relish, and PGRP-LA mutant larvae. Comparing our data to previous microarray studies, we report that a majority of genes regulated in the trachea upon infection differ from those induced in the gut or the fat body. Importantly, antimicrobial peptide gene expression was reduced in the tracheae of larvae and in the adult gut of PGRP-LA-deficient Drosophila upon oral bacterial infection. Together, our results suggest that PGRP-LA positively regulates the Imd pathway in barrier epithelia.

Antimicrobial autophagy: a conserved innate immune response in Drosophila

Autophagy is a highly conserved degradative pathway that has rapidly emerged as a critical component of immunity and host defense. Studies have implicated autophagy genes in restricting the replication of a diverse array of pathogens, including bacteria, viruses and protozoans. However, in most cases, the in vivo role of antimicrobial autophagy against pathogens has been undefined. Drosophila provides a genetically tractable model system that can be easily adapted to study autophagy in innate immunity, and recent studies in flies have demonstrated that autophagy is an essential antimicrobial response against bacteria and viruses in vivo. These findings reveal striking conservation of antimicrobial autophagy between flies and mammals, and in particular, the role of pathogen-associated pattern recognition in triggering this response. This review discusses our current understanding of antimicrobial autophagy in Drosophila and its potential relevance to human immunity.

Invertebrate models of fungal infection

The morbidity, mortality and economic burden associated with fungal infections, together with the emergence of fungal strains resistant to current antimicrobial agents, necessitate broadening our understanding of fungal pathogenesis and discovering new agents to treat these infections. Using invertebrate hosts, especially the nematode Caenorhabditis elegans and the model insects Drosophila melanogaster and Galleria mellonella, could help achieve these goals. The evolutionary conservation of several aspects of the innate immune response between invertebrates and mammals makes the use of these simple hosts an effective and fast screening method for identifying fungal virulence factors and testing potential antifungal compounds. The purpose of this review is to compare several model hosts that have been used in experimental mycology to-date and to describe their different characteristics and contribution to the study of fungal virulence and the detection of compounds with antifungal properties. This article is part of a Special Issue entitled: Animal Models of Disease.

Transcriptional pausing controls a rapid antiviral innate immune response in Drosophila

Innate immune responses are characterized by precise gene expression whereby gene subsets are temporally induced to limit infection, although the mechanisms involved are incompletely understood. We show that antiviral immunity in Drosophila requires the transcriptional pausing pathway, including negative elongation factor (NELF) that pauses RNA polymerase II (Pol II) and positive elongation factor b (P-TEFb), which releases paused Pol II to produce full-length transcripts. We identify a set of genes that is rapidly transcribed upon arbovirus infection, including components of antiviral pathways (RNA silencing, autophagy, JAK/STAT, Toll, and Imd) and various Toll receptors. Many of these genes require P-TEFb for expression and exhibit pausing-associated chromatin features. Furthermore, transcriptional pausing is critical for antiviral immunity in insects because NELF and P-TEFb are required to restrict viral replication in adult flies and vector mosquito cells. Thus, transcriptional pausing primes virally induced genes to facilitate rapid gene induction and robust antiviral responses.

MyD88-deficient Hydra reveal an ancient function of TLR signaling in sensing bacterial colonizers

Toll-like receptor (TLR) signaling is one of the most important signaling cascades of the innate immune system of vertebrates. Studies in invertebrates have focused on the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans, and there is little information regarding the evolutionary origin and ancestral function of TLR signaling. In Drosophila, members of the Toll-like receptor family are involved in both embryonic development and innate immunity. In C. elegans, a clear immune function of the TLR homolog TOL-1 is controversial and central components of vertebrate TLR signaling including the key adapter protein myeloid differentiation primary response gene 88 (MyD88) and the transcription factor NF-κB are not present. In basal metazoans such as the cnidarians Hydra magnipapillata and Nematostella vectensis, all components of the vertebrate TLR signaling cascade are present, but their role in immunity is unknown. Here, we use a MyD88 loss-of-function approach in Hydra to demonstrate that recognition of bacteria is an ancestral function of TLR signaling and that this process contributes to both host-mediated recolonization by commensal bacteria as well as to defense against bacterial pathogens.

A negative role for MyD88 in the resistance to starvation as revealed in an intestinal infection of Drosophila melanogaster with the Gram-positive bacterium Staphylococcus xylosus

Drosophila melanogaster is a useful model to investigate mucosal immunity. The immune response to intestinal infections is mediated partly by the Immune deficiency (IMD) pathway, which only gets activated by a type of peptidoglycan lacking in several medically important Gram-positive bacterial species such as Staphylococcus. Thus, the intestinal host defense against such bacterial strains remains poorly known. Here, we have used Staphylococcus xylosus to develop a model of intestinal infections by Gram-positive bacteria. S. xylosus behaves as an opportunistic pathogen in a septic injury model, being able to kill only flies immunodeficient either for the Toll pathway or the cellular response. When ingested, it is controlled by IMD-independent host intestinal defenses, yet flies eventually die. Having excluded an overreaction of the immune response and the action of toxins, we find that flies actually succumb to starvation, likely as a result of a competition for sucrose between the bacteria and the flies. Fat stores of wild-type flies are severely reduced within a day, a period when sucrose is not yet exhausted in the feeding solution. Interestingly, the Toll pathway mutant MyD88 is more resistant to the ingestion of S. xylosus and to starvation than wild-type flies. MyD88 flies do not rapidly deplete their fat stores when starved, in contrast to wild-type flies. Thus, we have uncovered a novel function of MyD88 in the regulation of metabolism that appears to be independent of its known roles in immunity and development.