Drosophila immunity: genes on the third chromosome required for the response to bacterial infection

From Drosophila Blood Cells to Human Leukemia

The hematopoietic system plays a critical role in establishing the proper response against invading pathogens or in removing cancerous cells. Furthermore, deregulations of the hematopoietic differentiation program are at the origin of numerous diseases including leukemia. Importantly, many aspects of blood cell development have been conserved from human to Drosophila. Hence, Drosophila has emerged as a potent genetic model to study blood cell development and leukemia in vivo. In this chapter, we give a brief overview of the Drosophila hematopoietic system, and we provide a protocol for the dissection and the immunostaining of the larval lymph gland, the most studied hematopoietic organ in Drosophila. We then focus on the various paradigms that have been used in fly to investigate how conserved genes implicated in leukemogenesis control blood cell development. Specific examples of Drosophila models for leukemia are presented, with particular attention to the most translational ones. Finally, we discuss some limitations and potential improvements of Drosophila models for studying blood cell cancer.

Peptidoglycan recognition by the Drosophila Imd pathway

The structural requirements for recognition of peptidoglycan (PGN) by PGRP-LC and activation of the Drosophila IMD pathway are not yet clear. In order to examine this question more carefully, the activity of peptidoglycan from different types of bacteria was compared in cell-based and whole animal assays. Drosophila S2* cells, but not adult flies, responded to Lys-type Micrococcus luteus PGN, but with significantly less potency compared to Dap-type Escherichia coli PGN, while intact Lys-type PGN from Staphylococcus aureus was inactive. After treatment with lysostaphin, which digests the cross-bridging peptides, S. aureus PGN weakly stimulated the IMD pathway, similar to M. luteus PGN. Further digestion with mutanolysin, which creates monomeric PGN fragments, abolished the activity of S. aureus PGN. On the other hand, monomeric E. coli PGN, generated by mutanolysin digestion, was still active but required different isoforms of PGRP-LC for recognition. Polymeric PGN required only PGRP-LCx, while monomeric E. coli PGN required both the PGRP-LCa and PGRP-LCx isoforms. These results suggest that the recognition by PGRP-LCx alone requires polymeric PGN, and that polymeric Dap-type PGN is a more potent PGRP-LCx agonist, compared to Lys-type PGN. These results also suggest that the heteromeric PGRP-LCa/LCx receptor complex recognizes monomeric Dap-type, but not Lys-type, PGN.

Genetic Screen in Drosophila Larvae Links ird1 Function to Toll Signaling in the Fat Body and Hemocyte Motility

To understand how Toll signaling controls the activation of a cellular immune response in Drosophila blood cells (hemocytes), we carried out a genetic modifier screen, looking for deletions that suppress or enhance the mobilization of sessile hemocytes by the gain-of-function mutation Toll10b (Tl10b). Here we describe the results from chromosome arm 3R, where five regions strongly suppressed this phenotype. We identified the specific genes immune response deficient 1 (ird1), headcase (hdc) and possibly Rab23 as suppressors, and we studied the role of ird1 in more detail. An ird1 null mutant and a mutant that truncates the N-terminal kinase domain of the encoded Ird1 protein affected the Tl10b phenotype, unlike mutations that affect the C-terminal part of the protein. The ird1 null mutant suppressed mobilization of sessile hemocytes, but enhanced other Tl10b hemocyte phenotypes, like the formation of melanotic nodules and the increased number of circulating hemocytes. ird1 mutants also had blood cell phenotypes on their own. They lacked crystal cells and showed aberrant formation of lamellocytes. ird1 mutant plasmatocytes had a reduced ability to spread on an artificial substrate by forming protrusions, which may explain why they did not go into circulation in response to Toll signaling. The effect of the ird1 mutation depended mainly on ird1 expression in hemocytes, but ird1-dependent effects in other tissues may contribute. Specifically, the Toll receptor was translocated from the cell membrane to intracellular vesicles in the fat body of the ird1 mutant, and Toll signaling was activated in that tissue, partially explaining the Tl10b-like phenotype. As ird1 is otherwise known to control vesicular transport, we conclude that the vesicular transport system may be of particular importance during an immune response.

Effects of the isoflavone prunetin on gut health and stress response in male Drosophila melanogaster

The traditional Asian diet is rich in fruits, vegetables and soy, the latter representing a significant source of dietary isoflavones. The isoflavone prunetin was recently identified to improve intestinal epithelial barrier function in vitro and to ameliorate general survival and overall health state in vivo in male Drosophila melanogaster. However, the prunetin-mediated health benefits in the fruit fly were ascertained under standard living conditions. As the loss of intestinal integrity is closely related to a reduction in Drosophila lifespan and barrier dysfunction increases with age, effects on prunetin-modulated gut health under oxidative or pathogenic stress provocation remain to be elucidated. In this study, male adult D. melanogaster were administered either a prunetin or a control diet. Gut-derived junction protein expression and pathogen-induced antimicrobial peptide expressions as well as the stem cell proliferation in the gut were evaluated. Furthermore, survival following exposure to hydrogen peroxide was assessed. Prunetin ingestion did not attenuate bacterial infection and did not protect flies from oxidative stress. Intestinal mRNA expression levels of adherence and septate junction proteins as well as the stem cell proliferation were not altered by prunetin intake. Prunetin does not improve the resistance of flies against severe injuring, exogenous stress and therefore seems to function in a preventive rather than a therapeutic approach since the health-promoting benefits appear to be exclusively restricted to normal living circumstances.

•

Gram-negative bacterial strains induce AMP-mediated defense in the fruit fly.

•

Prunetin improves life and health span in male fruit flies independent of gut health.

•

Prunetin fails to ameliorate resistance of the flies towards severe injury.

•

AMP expression, stem cell proliferation & oxidative stress resistance are unaffected.

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.

Quantitative measurement of the immune response and sleep in Drosophila

A complex interaction between the immune response and host behavior has been described in a wide range of species. Excess sleep, in particular, is known to occur as a response to infection in mammals (1) and has also recently been described in Drosophila melanogaster(2). It is generally accepted that sleep is beneficial to the host during an infection and that it is important for the maintenance of a robust immune system(3,4). However, experimental evidence that supports this hypothesis is limited(4), and the function of excess sleep during an immune response remains unclear. We have used a multidisciplinary approach to address this complex problem, and have conducted studies in the simple genetic model system, the fruitfly Drosophila melanogaster. We use a standard assay for measuring locomotor behavior and sleep in flies, and demonstrate how this assay is used to measure behavior in flies infected with a pathogenic strain of bacteria. This assay is also useful for monitoring the duration of survival in individual flies during an infection. Additional measures of immune function include the ability of flies to clear an infection and the activation of NFκB, a key transcription factor that is central to the innate immune response in Drosophila. Both survival outcome and bacterial clearance during infection together are indicators of resistance and tolerance to infection. Resistance refers to the ability of flies to clear an infection, while tolerance is defined as the ability of the host to limit damage from an infection and thereby survive despite high levels of pathogen within the system(5). Real-time monitoring of NFκB activity during infection provides insight into a molecular mechanism of survival during infection. The use of Drosophila in these straightforward assays facilitates the genetic and molecular analyses of sleep and the immune response and how these two complex systems are reciprocally influenced.

Transcriptome analyses of Bactericera cockerelli adults in response to "Candidatus Liberibacter solanacearum" infection

The potato/tomato psyllid, Bactericera cockerelli (Šulc) is an economically important crop pest that not only causes damage through its feeding but also transmits the bacterium, "Candidatus Liberibacter solanacearum" (CLs), which causes zebra chip disease in potato. There is some information about the phenotypic effects of phytopathogenic bacteria on their insect vectors; however, there are no published reports of the molecular mechanisms underlying phytopathogenic bacteria-insect vector interaction. In order to investigate the effects of CLs infection on B. cockerelli, transcriptomic analyses of CLs-infected and uninfected adult psyllids that were reared on potato were performed. De novo assembly of cDNA sequences generated 136,518 and 109,983 contigs for infected and uninfected insect libraries with an average contig length of 514 bp. BlastX analysis against the NCBI-nr database revealed that 33.33 % had significant matches. Gene ontology data illustrated that the majority of the expressed psyllid genes are involved in metabolic process, biological regulation, binding and catalytic activity. The psyllid transcriptome had an abundance of genes such as vitellogenin, heat shock protein, ejaculatory bulb-specific protein, ferritin, and cytochrome oxidase. Notably absent in the psyllid transcriptome were innate immunity genes induced in response to Gram-negative bacteria (IMD pathway). Several functionally diverse contigs related to symbiotic bacteria including the primary endosymbiont Carsonella ruddii, Wolbachia, and CLs in the psyllid transcriptome were identified. A total of 247 contigs showed differential expression in response to CLs infection including immune and stress-related genes and vitellogenins. Expression analyses of selected psyllid genes were performed on psyllids that were exclusively reared on potato (host of the insects used for RNAseq) and psyllids exclusively reared on tomato (alternative host of psyllids). These genes showed similar expression patterns irrespective of the host plant on which the psyllids were reared, which suggests that host-plant association may not modulate expression of these genes. Our findings suggest that the impact of CLs on psyllid transcriptome was to a large extent on genes involved in metabolic processes and to a small extent on immune and stress response genes. This study is the first description of transcriptomic changes in an insect vector in response to infection with a naturally occurring bacterial plant pathogen. Data from this study provide new sequence and gene expression resources for functional genomics of potato psyllids.

The localisation of PtdIns3P in Drosophila fat responds to nutrients but not insulin: a role for Class III but not Class II phosphoinositide 3-kinases

PtdIns3P and PtdIns(3,4,5)P(3) are regulated differently in fat body in response to nutritional status and insulin signalling. In feeding larvae PtdIns(3,4,5)P(3) is upregulated at the cell membrane where it is generated in response to insulin signalling. In contrast PtdIns3P is down regulated in the fat body of well-fed larvae but on starvation it accumulates in punctate vesicles throughout the cytoplasm and on refeeding it relocalises to vesicles at the periphery of the cell. Both responses are independent of insulin signalling and on the presence of glutamine which has previously been linked to nutritional sensing. We find that both Class II and Class III PI3Ks are capable of generating PtdIns3P in vivo but the source of PtdIns3P in the fat body and the response to nutritional status can be exclusively accounted for by Class III PI3K activity.

Use of a Drosophila model to identify genes regulating Plasmodium growth in the mosquito

We performed a forward genetic screen, using Drosophila as a surrogate mosquito, to identify host factors required for the growth of the avian malaria parasite, Plasmodium gallinaceum. We identified 18 presumed loss-of-function mutants that reduced the growth of the parasite in flies. Presumptive mutation sites were identified in 14 of the mutants on the basis of the insertion site of a transposable element. None of the identified genes have been previously implicated in innate immune responses or interactions with Plasmodium. The functions of five Anopheles gambiae homologs were tested by using RNAi to knock down gene function followed by measuring the growth of the rodent parasite, Plasmodium berghei. Loss of function of four of these genes in the mosquito affected Plasmodium growth, suggesting that Drosophila can be used effectively as a surrogate mosquito to identify relevant host factors in the mosquito.

Identification of Drosophila mutants altering defense of and endurance to Listeria monocytogenes infection

We extended the use of Drosophila beyond being a model for signaling pathways required for pattern recognition immune signaling and show that the fly can be used to identify genes required for pathogenesis and host-pathogen interactions. We performed a forward genetic screen to identify Drosophila mutations altering sensitivity to the intracellular pathogen Listeria monocytogenes. We recovered 18 mutants with increased susceptibility to infection, none of which were previously shown to function in a Drosophila immune response. Using secondary screens, we divided these mutants into two groups: In the first group, mutants have reduced endurance to infections but show no change in bacterial growth. This is a new fly immunity phenotype that is not commonly studied. In the second group, mutants have a typical defense defect in which bacterial growth is increased and survival is decreased. By further challenging mutant flies with L. monocytogenes mutants, we identified subgroups of fly mutants that affect specific stages of the L. monocytogenes life cycle, exit from the vacuole, or actin-based movement. There is no overlap between our genes and the hundreds of genes identified in Drosophila S2 cells fighting L. monocytogenes infection, using genomewide RNAi screens in vitro. By using a whole-animal model and screening for host survival, we revealed genes involved in physiologies different from those that were found in previous screens, which all had defects in defensive immune signaling.

Dissecting innate immunity by germline mutagenesis

The innate arm of our immune system is the first line of defence against infections. In addition, it is believed to drive adaptive immune responses, which help fight pathogens and provide long-term memory. As such, the innate immune system is instrumental for protection against pathogens that would otherwise destroy their host. Although our understanding of the innate immune components involved in pathogen sensing and fighting is improving, it is still limited. This is particularly exemplified by increased documentation of innate immune deficiencies in humans that often result in high and recurrent susceptibility to infections or even death, without the genetic cause being evident. To provide further insight into the mechanisms by which pathogen sensing and eradication occur, several strategies can be used. The current review focuses on the forward genetic approaches that have been used to dissect innate immunity in the fruit fly and the mouse. For both animal models, forward genetics has been instrumental in the deciphering of innate immunity and has greatly improved our understanding of how we respond to invading pathogens.

Microarray analyses reveal distinct roles for Rel proteins in the Drosophila immune response

The NF-kappaB group of transcription factors play an important role in mediating immune responses in organisms as diverse as insects and mammals. The fruit fly Drosophila melanogaster express three closely related NF-kappaB-like transcription factors: Dorsal, Dif, and Relish. To study their roles in vivo, we used microarrays to determine the effect of null mutations in individual Rel transcription factors on larval immune gene expression. Of the 188 genes that were significantly up-regulated in wild-type larvae upon bacterial challenge, overlapping but distinct groups of genes were affected in the Rel mutants. We also ectopically expressed Dorsal or Dif and used cDNA microarrays to determine the genes that were up-regulated in the presence of these transcription factors. This expression was sufficient to drive expression of some immune genes, suggesting redundancy in the regulation of these genes. Combining this data, we also identified novel genes that may be specific targets of Dif.

Drosophila eiger mutants are sensitive to extracellular pathogens

We showed previously that eiger, the Drosophila tumor necrosis factor homolog, contributes to the pathology induced by infection with Salmonella typhimurium. We were curious whether eiger is always detrimental in the context of infection or if it plays a role in fighting some types of microbes. We challenged wild-type and eiger mutant flies with a collection of facultative intracellular and extracellular pathogens, including a fungus and Gram-positive and Gram-negative bacteria. The response of eiger mutants divided these microbes into two groups: eiger mutants are immunocompromised with respect to extracellular pathogens but show no change or reduced sensitivity to facultative intracellular pathogens. Hence, eiger helps fight infections but also can cause pathology. We propose that eiger activates the cellular immune response of the fly to aid clearance of extracellular pathogens. Intracellular pathogens, which can already defeat professional phagocytes, are unaffected by eiger.

We show that the gene eiger, which is the sole tumor necrosis factor homolog in the fruit fly, can play opposing roles in the fly's response to infections. Sometimes eiger contributes to the disease induced by an infection, while at other times it is required to fight an infection. Commonly, the fly's immune response is described as dividing microbes into two groups with Gram-positive bacteria and fungi lying in one group and Gram-negative bacteria lying in the other. Pathogenic bacteria can also be divided into two groups based on their behavior in eiger mutant flies, but these two groups differ from past descriptions. eiger tends to be required for the innate immune response against extracellular pathogens but tends to cause pathology during an infection with an intracellular pathogen. We suggest that eiger is required for innate immune responses that are effective at fighting extracellular pathogens but are wasteful or simply ineffective when fighting intracellular pathogens. We show here that the fly immune response is more complex than previously recognized and suggest new directions for studying pathogenesis in addition to innate immunity in the fly.

Psidin is required in Drosophila blood cells for both phagocytic degradation and immune activation of the fat body

Phagocytic blood cells are critical to innate immune defense: They internalize and destroy microbial invaders and produce signals that trigger other immune responses. Despite this central role, the in vivo contributions of phagocytosis to systemic immune activation are not well understood. Drosophila has proven a fruitful model for the investigation of evolutionarily conserved innate immune mechanisms, including NF-kappaB-dependent transcriptional induction, RNAi in antiviral responses, and phagocytosis. The phagocytes of Drosophila encounter bacterial invaders early in infection and contribute to survival of infection. Phagocytosis in flies and mammals is highly homologous: Both rely on scavenger receptors, opsonins, and actin rearrangements for engulfment; have phagosomal cysteine proteases active at low pH; and can be subverted by similar intracellular pathogens. Although the role of Drosophila phagocytes in the activation of other immune tissues has not been clear, we show that induction of the antibacterial-peptide gene Defensin in the fat body during infection requires blood-cell contributions. We identify a gene, psidin, that encodes a lysosomal protein required in the blood cells for both degradation of engulfed bacteria and activation of fat-body Defensin. These data establish a role for the phagocytic blood cells of Drosophila in detection of infection and activation of the humoral immune response.

ird1 is a Vps15 homologue important for antibacterial immune responses in Drosophila

The immune response-deficient 1 (ird1) gene was identified in a forward genetic screen as a novel regulator for the activation of Imd NFkappaB immune signalling pathway in Drosophila. ird1 animals are also more susceptible to Escherichia coli and Micrococcus luteus bacterial infection. ird1 encodes the Drosophila homologue of the Vps15/p150 serine/threonine kinase that regulates a class III phosphoinositide 3-kinase and is necessary for phagosome maturation and starvation-induced autophagy in yeast and mammalian cells. To gain insight into the role of ird1 in the immune response, we examine how amino acid starvation affects the immune signalling pathways in Drosophila. Starvation, in the absence of infection, leads to expression of antimicrobial peptide (AMP) genes and this response is dependent on ird1 and the Imd immune signalling pathway. Starvation, in addition to bacterial infection, suppresses the AMP response in wild-type animals and reduces the ability to survive M. luteus infection. Our results suggest that starvation and innate immune signalling may be intimately linked processes.

Dorsoventral axis formation in the Drosophila embryo--shaping and transducing a morphogen gradient

The graded nuclear location of the transcription factor Dorsal along the dorsoventral axis of the early Drosophila embryo provides positional information for the determination of different cell fates. Nuclear uptake of Dorsal depends on a complex signalling pathway comprising two parts: an extracellular proteolytic cascade transmits the dorsoventral polarity of the egg chamber to the early embryo and generates a gradient of active Spätzle protein, the ligand of the receptor Toll; an intracellular cascade downstream of Toll relays this graded signal to embryonic nuclei. The slope of the Dorsal gradient is not determined by diffusion of extracellular or intracellular components from a local source, but results from self-organised patterning, in which positive and negative feedback is essential to create and maintain the ratio of key factors at different levels, thereby establishing and stabilising the graded spatial information for Dorsal nuclear uptake.

PGRP-LC and PGRP-LE have essential yet distinct functions in the drosophila immune response to monomeric DAP-type peptidoglycan

Drosophila rely entirely on an innate immune response to combat microbial infection. Diaminopimelic acid-containing peptidoglycan, produced by Gram-negative bacteria, is recognized by two receptors, PGRP-LC and PGRP-LE, and activates a homolog of transcription factor NF-kappaB through the Imd signaling pathway. Here we show that full-length PGRP-LE acted as an intracellular receptor for monomeric peptidoglycan, whereas a version of PGRP-LE containing only the PGRP domain functioned extracellularly, like the mammalian CD14 molecule, to enhance PGRP-LC-mediated peptidoglycan recognition on the cell surface. Interaction with the imd signaling protein was not required for PGRP-LC signaling. Instead, PGRP-LC and PGRP-LE signaled through a receptor-interacting protein homotypic interaction motif-like motif. These data demonstrate that like mammals, drosophila use both extracellular and intracellular receptors, which have conserved signaling mechanisms, for innate immune recognition.

Regulators of the Toll and Imd pathways in the Drosophila innate immune response

The innate immune response is the first line of defense against microbial infections in both insects and mammals. Systematic analysis of the innate immune response in the model organism Drosophila melanogaster has provided important insights into the mechanisms of pathogen recognition and host response. Recognition of pathogen-associated molecules, such as peptidoglycans, stimulates the Toll and immune deficiency (Imd) pathways to induce antimicrobial responses. The Toll and Imd pathways are homologous to the mammalian Toll-like receptor (TLR) and tumor necrosis factor receptor (TNFR) signaling pathways, respectively, and are essential for Drosophila to survive infection. In this Review, we will discuss the recent genetic, genomic and RNA interference analyses that have unveiled additional intricacy in the Toll and Imd pathways.

Domains controlling cell polarity and proliferation in the Drosophila tumor suppressor Scribble

Cell polarity and cell proliferation can be coupled in animal tissues, but how they are coupled is not understood. In Drosophila imaginal discs, loss of the neoplastic tumor suppressor gene scribble (scrib), which encodes a multidomain scaffolding protein, disrupts epithelial organization and also causes unchecked proliferation. Using an allelic series of mutations along with rescuing transgenes, we have identified domain requirements for polarity, proliferation control, and other Scrib functions. The leucine-rich repeats (LRR) tether Scrib to the plasma membrane, are both necessary and sufficient to organize a polarized epithelial monolayer, and are required for all proliferation control. The PDZ domains, which recruit the LRR to the junctional complex, are dispensable for overall epithelial organization. PDZ domain absence leads to mild polarity defects accompanied by moderate overproliferation, but the PDZ domains alone are insufficient to provide any Scrib function in mutant discs. We suggest a model in which Scrib, via the activity of the LRR, governs proliferation primarily by regulating apicobasal polarity.

Drosophila peptidoglycan recognition protein LC (PGRP-LC) acts as a signal-transducing innate immune receptor

Drosophila peptidoglycan recognition protein LC (PGRP-LC), a transmembrane protein required for the response to bacterial infection, acts at the top of a cytoplasmic signaling cascade that requires the death-domain protein Imd and an IkappaB kinase to activate Relish, an NF-kappaB family member. It is not clear how binding of peptidoglycan to the extracellular domain of PGRP-LC activates intracellular signaling because its cytoplasmic domain has no homology to characterized proteins. Here, we demonstrate that PGRP-LC binds Imd and that its cytoplasmic domain is critical for its activity, suggesting that PGRP-LC acts as a signal-transducing receptor. The PGRP-LC cytoplasmic domain is also essential for the formation of dimers, and results suggest that dimerization may be required for receptor activation. The PGRP-LC cytoplasmic domain can mediate formation of heterodimers between different PGRP-LC isoforms, thereby potentially expanding the diversity of ligands that can be recognized by the receptor.

Monomeric and Polymeric Gram-Negative Peptidoglycan but Not Purified LPS Stimulate the Drosophila IMD Pathway

Insects depend solely upon innate immune responses to survive infection. These responses include the activation of extracellular protease cascades, leading to melanization and clotting, and intracellular signal transduction pathways inducing antimicrobial peptide gene expression. In Drosophila, the IMD pathway is required for antimicrobial gene expression in response to gram-negative bacteria. The exact molecular component(s) from these bacteria that activate the IMD pathway remain controversial. We found that highly purified LPS did not stimulate the IMD pathway. However, lipid A, the active portion of LPS in mammals, activated melanization in the silkworm Bombyx morii. On the other hand, the IMD pathway was remarkably sensitive to polymeric and monomeric gram-negative peptidoglycan. Recognition of peptidoglycan required the stem-peptide sequence specific to gram-negative peptidoglycan and the receptor PGRP-LC. Recognition of monomeric and polymeric peptidoglycan required different PGRP-LC splice isoforms, while lipid A recognition required an unidentified soluble factor in the hemolymph of Bombyx morii.

Drosophila: The Genetics of Innate Immune Recognition and Response

Because of the evolutionary conservation of innate mechanisms of host defense, Drosophila has emerged as an ideal animal in which to study the genetic control of immune recognition and responses. The discovery that the Toll pathway is required for defense against fungal infection in Drosophila was pivotal in studies of both mammalian and Drosophila immunity. Subsequent genetic screens in Drosophila to isolate additional mutants unable to induce humoral responses to infection have identified and ordered the function of components of two signaling cascades, the Toll and Imd pathways, that activate responses to infection. Drosophila blood cells also contribute to host defense through phagocytosis and signaling, and may carry out a form of self-nonself recognition that is independent of microbial pattern recognition. Recent work suggests that Drosophila will be a useful model for dissecting virulence mechanisms of several medically important pathogens.

The genetic architecture of odor-guided behavior in Drosophila: epistasis and the transcriptome

We combined transcriptional profiling and quantitative genetic analysis to elucidate the genetic architecture of olfactory behavior in Drosophila melanogaster. We applied whole-genome expression analysis to five coisogenic smell-impaired (smi) mutant lines and their control. We used analysis of variance to partition variation in transcript abundance between males and females and between smi genotypes and to determine the genotype-by-sex interaction. A total of 666 genes showed sexual dimorphism in transcript abundance, and 530 genes were coregulated in response to one or more smi mutations, showing considerable epistasis at the level of the transcriptome in response to single mutations. Quantitative complementation tests of mutations at these coregulated genes with the smi mutations showed that in most cases (67%) epistatic interactions for olfactory behavior mirrored epistasis at the level of transcription, thus identifying new candidate genes regulating olfactory behavior.

Model systems in drug discovery: chemical genetics meets genomics

Animal model systems are an intricate part of the discovery and development of new medicines. The sequencing of not only the human genome but also those of the various pathogenic bacteria, the nematode Caenorhabditis elegans, the fruitfly Drosophila, and the mouse has enabled the discovery of new drug targets to push forward at an unprecedented pace. The knowledge and tools in these "model" systems are allowing researchers to carry out experiments more efficiently and are uncovering previously hidden biological connections. While the history of bacteria, yeast, and mice in drug discovery are long, their roles are ever evolving. In contrast, the history of Drosophila and C. elegans at pharmaceutical companies is short. We will briefly review the historic role of each model organism in drug discovery and then update the readers as to the abilities and liabilities of each model within the context of drug development.

Scribble is essential for olfactory behavior in Drosophila melanogaster

The ability to discriminate and respond to chemical signals from the environment is an almost universal prerequisite for survival. Here, we report that the scaffold protein Scribble is essential for odor-guided behavior in Drosophila. Previously, we identified a P-element insert line with generalized sexually dimorphic smell impairment, smi97B. We found that the transposon in this line is located between the predicted promoter region and the transcription initiation site of scrib. A deficiency in this region, Df(3R)Tl-X, and two scrib null alleles fail to complement the smell-impaired phenotype of smi97B. Wild-type behavior is restored by precise excision of the P element, scrib mRNA levels correspond with mutant and wild-type phenotypes, and introduction of a full-length scrib transgene in the smi97B mutant rescues the olfactory deficit. Expression of Scrib is widespread in olfactory organs and the central nervous system. Finally, alternative splicing of scrib generates transcripts that differ in the number of leucine-rich repeats and PDZ domains.

Cross-species infections and their analysis

The ability of certain pathogens to infect multiple hosts has led to the development of genetically tractable nonvertebrate hosts to elucidate the molecular mechanisms of interactions between these pathogens and their hosts. The use of plant, insect, nematode, and protozoan hosts to study human pathogens has facilitated the elucidation of molecular nature of pathogenesis and host responses. Analyses of virulence of multihost pathogens on their respective hosts revealed that pathogens utilize many universal offensive strategies to overcome host defenses, irrespective of the evolutionary lineage of the host. Likewise, genetic dissections of the defense response of the nonvertebrate hosts have also shown that key features underlying host defense responses are highly conserved. This review summarizes how the information gained from the analysis of cross-species infections contributes to our understanding of host-pathogen interactions.

Bacterial infection of a model insect: Photorhabdus luminescens and Manduca sexta

Invertebrates, including insects, are being developed as model systems for the study of bacterial virulence. However, we understand little of the interaction between bacteria and specific invertebrate tissues or the immune system. To establish an infection model for Photorhabdus, which is released directly into the insect blood system by its nematode symbiont, we document the number and location of recoverable bacteria found during infection of Manduca sexta. After injection into the insect larva, P. luminescens multiplies in both the midgut and haemolymph, only later colonizing the fat body and the remaining tissues of the cadaver. Bacteria persist by suppressing haemocyte-mediated phagocytosis and culture supernatants grown in vitro, as well as plasma from infected insects, suppress phagocytosis of P. luminescens. Using GFP-labelled bacteria, we show that colonization of the gut begins at the anterior of the midgut and proceeds posteriorly. Within the midgut, P. luminescens occupies a specific niche between the extracellular matrix and basal membrane (lamina) of the folded midgut epithelium. Here, the bacteria express the gut-active Toxin complex A (Tca) and an RTX-like metalloprotease PrtA. This close association of the bacteria with the gut, and the production of toxins and protease, triggers a massive programmed cell death of the midgut epithelium.

Requirement for a peptidoglycan recognition protein (PGRP) in Relish activation and antibacterial immune responses in Drosophila

Components of microbial cell walls are potent activators of innate immune responses in animals. For example, the mammalian TLR4 signaling pathway is activated by bacterial lipopolysaccharide and is required for resistance to infection by Gram-negative bacteria. Other components of microbial surfaces, such as peptidoglycan, are also potent activators of innate immune responses, but less is known about how those components activate host defense. Here we show that a peptidoglycan recognition protein, PGRP-LC, is absolutely required for the induction of antibacterial peptide genes in response to infection in Drosophila and acts by controlling activation of the NF-kappaB family transcription factor Relish.

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

The antimicrobial defence of Drosophila relies largely on the challenge-induced synthesis of an array of potent antimicrobial peptides by the fat body. The defence against Gram-positive bacteria and natural fungal infections is mediated by the Toll signalling pathway, whereas defence against Gram-negative bacteria is dependent on the Immune deficiency (IMD) pathway. Loss-of-function mutations in either pathway reduce the resistance to corresponding infections. The link between microbial infections and activation of these two pathways has remained elusive. The Toll pathway is activated by Gram-positive bacteria through a circulating Peptidoglycan recognition protein (PGRP-SA). PGRPs appear to be highly conserved from insects to mammals, and the Drosophila genome contains 13 members. Here we report a mutation in a gene coding for a putative transmembrane protein, PGRP-LC, which reduces survival to Gram-negative sepsis but has no effect on the response to Gram-positive bacteria or natural fungal infections. By genetic epistasis, we demonstrate that PGRP-LC acts upstream of the imd gene. The data on PGRP-SA with respect to the response to Gram-positive infections, together with the present report, indicate that the PGRP family has a principal role in sensing microbial infections in Drosophila.