Drosophila immunity: a comparative analysis of the Rel proteins dorsal and Dif in the induction of the genes encoding diptericin and cecropin

The identification of regulatory ceRNA network involved in Drosophila Toll immune responses

Non-coding RNAs play important roles in the innate immunity of Drosophila, with various lncRNAs and miRNAs identified to maintain Drosophila innate immune homeostasis by regulating protein functions. However, it remains unclear whether interactions between lncRNAs and miRNAs give rise to a ceRNA network. In our previous study, we observed the highest differential expression levels of lncRNA-CR11538, lncRNA-CR33942, and lncRNA-CR46018 in wild-type flies after Gram-positive bacterial infection, prompting us to investigate their role in the regulation of Drosophila Toll immune response through RNA-seq analysis. Herein, our comprehensive bioinformatics analysis revealed that lncRNA-CR11538, lncRNA-CR33942, and lncRNA-CR46018 are involved in defense mechanisms and stimulus response. Moreover, lncRNA-CR11538 and lncRNA-CR46018 can also participate in the metabolic recovery processes following Gram-positive bacterial infection. Subsequently, we employed GSEA screening and RT-qPCR to identify seven miRNAs (miR-957, miR-1015, miR-982, miR-993, miR-1007, miR-193, and miR-978) that may be regulated by these three lncRNAs. Furthermore, we predicted the potential target genes in the Toll signaling pathway for these miRNAs and their interaction with the three lncRNAs using TargetScan and miRanda software and preliminary verification. As a result, we established a potential ceRNA regulatory network for Toll immune responses in Drosophila, comprising three lncRNAs and seven miRNAs. This study provides evidence of a ceRNA regulatory network in Drosophila Toll immune responses and offers novel insights into understanding the regulatory networks involved in the innate immunity of other animals.

Interaction of lncRNA-CR33942 with Dif/Dorsal Facilitates Antimicrobial Peptide Transcriptions and Enhances Drosophila Toll Immune Responses

The Drosophila Toll signaling pathway mainly responds to Gram-positive (G⁺) bacteria or fungal infection, which is highly conserved with mammalian TLR signaling pathway. Although many positive and negative regulators involved in the immune response of the Toll pathway have been identified in Drosophila, the roles of long noncoding RNAs (lncRNAs) in Drosophila Toll immune responses are poorly understood to date. In this study, our results demonstrate that lncRNA-CR33942 is mainly expressed in the nucleus and upregulated after Micrococcus luteus infection. Especially, lncRNA-CR33942 not only modulates differential expressions of multiple antimicrobial peptide genes but also affects the Drosophila survival rate during response to G⁺ bacterial infection based on the transiently overexpressing and the knockdown lncRNA-CR33942 assays in vivo. Mechanically, lncRNA-CR33942 interacts with the NF-κB transcription factors Dorsal-related immunity factor/Dorsal to promote the transcriptions of antimicrobial peptides drosomycin and metchnikowin, thus enhancing Drosophila Toll immune responses. Taken together, this study identifies lncRNA-CR33942 as a positive regulator of Drosophila innate immune response to G⁺ bacterial infection to facilitate Toll signaling via interacting with Dorsal-related immunity factor/Dorsal. It would be helpful to reveal the roles of lncRNAs in Toll immune response in Drosophila and provide insights into animal innate immunity.

Molecular characterization and functional analysis of a Rel gene in the Pacific oyster

The nuclear factor-κB (NF-κB) signaling pathway plays a crucial role in regulating many physiological processes such as development, inflammation, apoptosis, cell proliferation, differentiation and immune responses. And the NF-κB/Rel family members were considered as the most important transcription factors in the NF-κB signaling pathway. In this study, we cloned a Rel homolog gene (named as CgRel2) from the Pacific oyster, Crassostrea gigas. The 2115-bp open reading frame (ORF) encodes 704 amino acids and CgRel2 possesses a conserved Rel Homology Domain (RHD) at the N-terminus. Phylogenetic analysis revealed that CgRel2 is most closely related to Pinctada fucata dorsal protein. CgRel2 transcripts are widely expressed in all tested tissues, with the highest expression observed in the labial palp and the gill. Moreover, the expression of CgRel2 is significantly upregulated after lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid [poly(I:C)] challenge. CgRel2 transfection into human cell lines activated NF-κB, TNFα and oyster IL-17 (CgIL-17) reporter genes in a dose-dependent manner, while CgRel2 overexpression cannot induce ISRE (Interferon stimulation response element) reporter gene's transcriptional activity. Additionally, the results of co-immunoprecipitation showed that CgRel2 or CgRel1 could interact with oyster IκB1, IκB2 and IκB3 proteins strongly, which may be critical for the immune signaling transduction and the regulation of its immune functions. Together, these results suggest that CgRel2 could respond to pathogenic infection, participate in the immune signal transduction and activate NF-κB, TNFα and CgIL-17 reporter genes. Thus, CgRel2 could play an important role in the oyster immune system.

Cloning and analysis of peptidoglycan recognition protein-LC and immune deficiency from the diamondback moth, Plutella xylostella

Peptidoglycan (PGN) exists in both Gram-negative and Gram-positive bacteria as a component of the cell wall. PGN is an important target to be recognized by the innate immune system of animals. PGN recognition proteins (PGRP) are responsible for recognizing PGNs. In Drosophila melanogaster, PGRP-LC and IMD (immune deficiency) are critical for activating the Imd pathway. Here, we report the cloning and analysis of PGRP-LC and IMD (PxPGRP-LC and PxIMD) from diamondback moth, Plutella xylostella (L.), the insect pest of cruciferous vegetables. PxPGRP-LC gene consists of six exons encoding a polypeptide of 308 amino acid residues with a transmembrane region and a PGRP domain. PxIMD cDNA encodes a polypeptide of 251 amino acid residues with a death domain. Sequence comparisons indicate that they are characteristic of Drosophila PGRP-LC and IMD homologs. PxPGRP-LC and PxIMD were expressed in various tissues and developmental stages. Their mRNA levels were affected by bacterial challenges. The PGRP domain of PxPGRP-LC lacks key residues for the amidase activity, but it can recognize two types of PGNs. Overexpression of full-length and deletion mutants in Drosophila S2 cells induced expression of some antimicrobial peptide genes. These results indicate that PxPGRP-LC and PxIMD may be involved in the immune signaling of P. xylostella. This study provides a foundation for further studies of the immune system of P. xylostella.

Transglutaminase-catalyzed incorporation of polyamines masks the DNA-binding region of the transcription factor Relish

In Drosophila, the final immune deficiency (IMD) pathway-dependent signal is transmitted through proteolytic conversion of the nuclear factor-κB (NF-κB)-like transcription factor Relish to the active N-terminal fragment Relish-N. Relish-N is then translocated from the cytosol into the nucleus for the expression of IMD-controlled genes. We previously demonstrated that transglutaminase (TG) suppresses the IMD pathway by polymerizing Relish-N to inhibit its nuclear translocation. Conversely, we also demonstrated that orally ingested synthetic amines, such as monodansylcadaverine (DCA) and biotin-labeled pentylamine, are TG-dependently incorporated into Relish-N, causing the nuclear translocation of modified Relish-N in gut epithelial cells. It remains unclear, however, whether polyamine-containing Relish-N retains transcriptional activity. Here, we used mass spectrometry analysis of a recombinant Relish-N modified with DCA by TG activity after proteolytic digestion and show that the DCA-modified Gln residues are located in the DNA-binding region of Relish-N. TG-catalyzed DCA incorporation inhibited binding of Relish-N to the Rel-responsive element in the NF-κB-binding DNA sequence. Subcellular fractionation of TG-expressing Drosophila S2 cells indicated that TG was localized in both the cytosol and nucleus. Of note, natural polyamines, including spermidine and spermine, competitively inhibited TG-dependent DCA incorporation into Relish-N. Moreover, in vivo experiments demonstrated that Relish-N was modified by spermine and that this modification reduced transcription of IMD pathway-controlled cecropin A1 and diptericin genes. These findings suggest that intracellular TG regulates Relish-N-mediated transcriptional activity by incorporating polyamines into Relish-N and via protein-protein cross-linking.

Identification and functional analysis of pointed homologs in Bombyx mori

Using gene-knockdown techniques, we searched for endogenous Ets family proteins involved in the regulation of Escherichia coli-dependent lebocin promoter activation in the E. coli-responsive silkworm cell line NIAS-Bm-aff3. Results showed that the gene knockdown of BmPointeds (BmPNTs), Drosophila Pointed orthologs, enhanced E. coli-dependent lebocin promoter activation, suggesting that endogenous BmPNTs repress the activation of this promoter. Furthermore, we found that i) the BmPNT gene produced at least two alternative splicing isoforms, BmPNT1 and BmPNT2, both of which function as repressors; ii) BmPNTs were not associated with an already-reported repressor element, most proximal GGAA/T motif (EtsRE3), in lebocin promoter, which plays a role in the repression of E. coli- and BmRelish1-dependent lebocin promoter activation; iii) although BmPNTs did not directly affect BmRelish1-dependent lebocin promoter activation, they were able to directly repress its activation on the promoter lacking EtsRE3, probably because of competitive inhibition of binding of BmRelish1 to κB sites by BmPNTs; and iv) BmPNTs were mainly expressed in larval hemocytes, and the gene expression levels of BmPNT2, but not of BmPNT1, were decreased in response to E. coli and Bacillus subtilis. These findings suggest that endogenous BmPNTs are directly and indirectly involved in the repression of E. coli-mediated lebocin promoter activation in NIAS-Bm-aff3 cells.

Annotation of the Asian Citrus Psyllid Genome Reveals a Reduced Innate Immune System

Citrus production worldwide is currently facing significant losses due to citrus greening disease, also known as Huanglongbing. The citrus greening bacteria, Candidatus Liberibacter asiaticus (CLas), is a persistent propagative pathogen transmitted by the Asian citrus psyllid, Diaphorina citri Kuwayama (Hemiptera: Liviidae). Hemipterans characterized to date lack a number of insect immune genes, including those associated with the Imd pathway targeting Gram-negative bacteria. The D. citri draft genome was used to characterize the immune defense genes present in D. citri. Predicted mRNAs identified by screening the published D. citri annotated draft genome were manually searched using a custom database of immune genes from previously annotated insect genomes. Toll and JAK/STAT pathways, general defense genes Dual oxidase, Nitric oxide synthase, prophenoloxidase, and cellular immune defense genes were present in D. citri. In contrast, D. citri lacked genes for the Imd pathway, most antimicrobial peptides, 1,3-β-glucan recognition proteins (GNBPs), and complete peptidoglycan recognition proteins. These data suggest that D. citri has a reduced immune capability similar to that observed in A. pisum, P. humanus, and R. prolixus. The absence of immune system genes from the D. citri genome may facilitate CLas infections, and is possibly compensated for by their relationship with their microbial endosymbionts.

Bombyx mori E26 transformation-specific 2 (BmEts2), an Ets family protein, represses Bombyx mori Rels (BmRels)-mediated promoter activation of antimicrobial peptide genes in the silkworm Bombyx mori

E26 transformation-specific (Ets) family transcription factors are known to play roles in various biological phenomena, including immunity, in vertebrates. However, the mechanisms by which Ets proteins contribute to immunity in invertebrates remain poorly understood. In this study, we identified a cDNA encoding BmEts2, which is a putative orthologue of Drosophila Yan and human translocation-ets-leukemia/Ets-variant gene 6, from the silkworm Bombyx mori. Expression of the BmEts2 gene was significantly increased in the fat bodies of silkworm larvae in response to injection with Escherichia coli and Staphylococcus aureus. BmEts2 overexpression dramatically repressed B. mori Rels (BmRels)-mediated promoter activation of antimicrobial peptide genes in silkworm cells. Conversely, gene knockdown of BmEts2 significantly enhanced BmRels activity. In addition, two κB sites located on the 5' upstream region of cecropin B1 were found to be involved in the repression of BmRels-mediated promoter activation. Protein-competition analysis further demonstrated that BmEts2 competitively inhibited binding of BmRels to κB sites. Overall, BmEts2 acts as a repressor of BmRels-mediated transactivation of antimicrobial protein genes by inhibiting the binding of BmRels to κB sites.

Alcohol resistance in Drosophila is modulated by the Toll innate immune pathway

A growing body of evidence has shown that alcohol alters the activity of the innate immune system and that changes in innate immune system activity can influence alcohol-related behaviors. Here, we show that the Toll innate immune signaling pathway modulates the level of alcohol resistance in Drosophila. In humans, a low level of response to alcohol is correlated with increased risk of developing an alcohol use disorder. The Toll signaling pathway was originally discovered in, and has been extensively studied in Drosophila. The Toll pathway is a major regulator of innate immunity in Drosophila, and mammalian Toll-like receptor signaling has been implicated in alcohol responses. Here, we use Drosophila-specific genetic tools to test eight genes in the Toll signaling pathway for effects on the level of response to ethanol. We show that increasing the activity of the pathway increases ethanol resistance whereas decreasing the pathway activity reduces ethanol resistance. Furthermore, we show that gene products known to be outputs of innate immune signaling are rapidly induced following ethanol exposure. The interaction between the Toll signaling pathway and ethanol is rooted in the natural history of Drosophila melanogaster.

Co-expression of Dorsal and Rel2 Negatively Regulates Antimicrobial Peptide Expression in the Tobacco Hornworm Manduca sexta

Nuclear factor κB (NF-κB) plays an essential role in regulation of innate immunity. In mammals, NF-κB factors can form homodimers and heterodimers to activate gene expression. In insects, three NF-κB factors, Dorsal, Dif and Relish, have been identified to activate antimicrobial peptide (AMP) gene expression. However, it is not clear whether Dorsal (or Dif) and Relish can form heterodimers. Here we report the identification and functional analysis of a Dorsal homologue (MsDorsal) and two Relish short isoforms (MsRel2A and MsRel2B) from the tobacco hornworm, Manduca sexta. Both MsRel2A and MsRel2B contain only a Rel homology domain (RHD) and lack the ankyrin-repeat inhibitory domain. Overexpression of the RHD domains of MsDorsal and MsRel2 in Drosophila melanogaster S2 and Spodoptera frugiperda Sf9 cells can activate AMP gene promoters from M. sexta and D. melanogaster. We for the first time confirmed the interaction between MsDorsal-RHD and MsRel2-RHD, and suggesting that Dorsal and Rel2 may form heterodimers. More importantly, co-expression of MsDorsal-RHD with MsRel2-RHD suppressed activation of several M. sexta AMP gene promoters. Our results suggest that the short MsRel2 isoforms may form heterodimers with MsDorsal as a novel mechanism to prevent over-activation of antimicrobial peptides.

Epithelial immune response in Drosophila malpighian tubules: interplay between Diap2 and ion channels

Systemic immune response via the Immune deficiency pathway requires Drosophila inhibitor of apoptosis protein 2 to activate the NF-κB transcription factor Relish. Malpighian tubules (MTs), simple epithelial tissue, are the primary excretory organs, performing additional role in providing protection to Drosophila against pathogenic infections. MTs hold a strategic position in Drosophila as one of the larval tissues that are carried over to adults, unlike other larval tissues that are histolysed during pupation. In this paper we show that Diap2 is an important regulator of local epithelial immune response in MTs and depletion of Diap2 from MTs, increases susceptibility of flies to infection. In the absence of Diap2, activation and translocation of Relish to the nucleus is abolished and as a consequence the production of IMD pathway dependent AMPs are reduced. Ion channels, (Na(+)/K(+))-ATPase and V-ATPase, are important for the immune response of MTs and expression of AMPs and the IMD pathway genes are impaired on inhibition of transporters, and they restrict the translocation of Relish into the nucleus. We show that Diap2 could be regulating ion channels, as loss of Diap2 consequently reduces the expression of ion channels and affects the balance of ion concentrations which results in reduced uric acid deposition. Thus Diap2 seems to be a key regulator of epithelial immune response in MTs, perhaps by modulating ion channels.

Differential gene expression between summer and winter diapause pupae of the onion maggot Delia antiqua, detected by suppressive subtractive hybridization

To gain a better understanding of the molecular mechanisms regulating pupal diapause of the onion maggot Delia antiqua, PCR-based suppressive subtractive hybridization was performed to identify genes involved in summer and/or winter diapause. A total of 209 unique sequences were obtained including 89 in forward library for winter diapausing pupae and 120 in the reverse library for summer diapausing pupae. 76.4% (68/89) and 68.3% (82/120) unique sequences had significant hits to non-redundant proteins database. Gene functional annotation showed these non-redundant sequences are involved in stress response and innate immunity, metabolism and energy, information processing and regulation, binding, food storage, morphogenesis and development, cell skeleton and cycle, protein synthesis and folding. Approximately 28.2% (59/209) transcripts showed no significant similarity to any other sequence in the public databases, probably representing unique genes of the onion maggot. Semi-quantitative RT-PCR revealed that the relative expression levels of 18 genes were comparable between summer and winter diapause. This study elucidates the temporal expression of diapause-related genes in onion maggot, also provides new insights into the differences in the physiological changes in summer and winter pupae. Functional characterization of some candidate genes will further enhance the understanding of the generating, maintaining, and breaking mechanism of diapause.

BmEts upregulates promoter activity of lebocin in Bombyx mori

The Ets family protein BmEts is assumed to be implicated in determination of diapause in the embryogenesis of Bombyx mori. In this study, we found that expression of BmEts was increased in the fat body and other tissues of the 5th instar larvae in response to Escherichia coli injection. Cotransfection experiments using a silkworm cell line revealed that overexpression of BmEts significantly elevated the activity of lebocin promoter but not of cecropin B1, cecropin D, attacin, and moricin promoters. Activation of the lebocin promoter by BmEts was dependent on at least two κB elements and the most proximal GGAA/T motif located on the 5'-upstream region. BmEts further synergistically enhanced E. coli or BmRelish1-d2 (active form)-stimulated lebocin promoter activation. Two κB elements were also found to be involved in promoter activation by BmRelish1-d2 and in synergistic promoter activation by BmEts and BmRelish1-d2 in the silkworm cells. Specific binding of recombinant BmEts to the proximal κB element and the most proximal GGAA/T motif and interaction between BmEts and BmRelish1 were also observed. To our knowledge, this is the first report of an Ets family protein directly regulating immune-related genes in invertebrates.

Manduca sexta moricin promoter elements can increase promoter activities of Drosophila melanogaster antimicrobial peptide genes

Insects produce a variety of antimicrobial peptides (AMPs). Induction of insect AMP genes is regulated by the Toll and IMD (immune deficiency) pathways via NF-κB and GATA factors. Little is known about species-specific regulation of AMP genes. In this report, we showed that activities of most Manduca sexta and Drosophila melanogaster AMP gene promoters were regulated in a species-specific manner in Drosophila (Dipteran) S2 cells and Spodoptera frugiperda (Lepidopteran) Sf9 cells. A κB-GATA element (22 bp) from M. sexta moricin (MsMoricin) promoter could significantly increase activities of Drosophila AMP gene promoters in S2 cells, and an MsMoricin promoter activating element (MPAE) (140 bp) could increase activity of drosomycin promoter specifically in Sf9 cells. However, κB and GATA factors alone were not sufficient for MsMoricin gene activation, suggesting that other co-regulators may be required to fully activate AMP genes. Our results suggest that induction of insect AMP genes may require a transcription complex composed of common nuclear factors (such as NF-κB and GATA factors) and species-related co-regulators, and it is the co-regulators that may confer species-specific regulation of AMP genes. In addition, we showed that activity of Drosophila drosomycin promoter could be activated cooperatively by the inserted exogenous κB-GATA element and the endogenous κB element. These findings revealed an approach of engineering AMP genes with enhanced activities, which may lead to broad applications.

The Drosophila Toll signaling pathway

The identification of the Drosophila melanogaster Toll pathway cascade and the subsequent characterization of TLRs have reshaped our understanding of the immune system. Ever since, Drosophila NF-κB signaling has been actively studied. In flies, the Toll receptors are essential for embryonic development and immunity. In total, nine Toll receptors are encoded in the Drosophila genome, including the Toll pathway receptor Toll. The induction of the Toll pathway by gram-positive bacteria or fungi leads to the activation of cellular immunity as well as the systemic production of certain antimicrobial peptides. The Toll receptor is activated when the proteolytically cleaved ligand Spatzle binds to the receptor, eventually leading to the activation of the NF-κB factors Dorsal-related immunity factor or Dorsal. In this study, we review the current literature on the Toll pathway and compare the Drosophila and mammalian NF-κB pathways.

Sleep triggered by an immune response in Drosophila is regulated by the circadian clock and requires the NFkappaB Relish

Background

Immune challenge impacts behavior in many species. In mammals, this adaptive behavior is often manifested as an increase in sleep. Sleep has therefore been proposed to benefit the host by enhancing immune function and thereby overcome the challenge. To facilitate genetic studies on the relationship between sleep and immune function, we characterized the effect of the immune response on sleep in Drosophila melanogaster. Behavioral features of sleep as well as the innate immune response signaling pathways are well characterized in flies and are highly conserved in mammals.

Results

An immune response induced by infection with Gram-negative bacteria or by aseptic injury increased sleep in flies. The increase in sleep occurred during the morning hours after treatment and the magnitude of the effect was dependent on the time-of-day of inoculation or injury such that night-time treatment had a stronger effect than that during the daytime. This pattern persisted in constant darkness, indicating a role of the circadian clock. Mutants of the circadian clock gene, period, eliminated the increase in sleep observed in the morning, but instead showed enhanced sleep immediately after injury or infection.

Null mutants of the Nuclear Factor κB (NFκB) Relish, which is central to the innate immune response, do not increase sleep in response to injury or infection at any time of day. Instead, they maintain a normal sleep pattern until they die. Expression of a full-length Relish transgene in the fat bodies of Relish mutants restored the morning increase in sleep during an immune response. Fat bodies are a major site of immune signalling in flies and have a key role in host defense.

Conclusions

These data demonstrate that an immune response increases sleep in flies in a manner that is gated by the circadian clock and that requires the NFκB Relish. These findings support a role of sleep in a recovery process and demonstrate a conserved feature of the Drosophila model of sleep.

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

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

Differential regulation of mRNA stability controls the transient expression of genes encoding Drosophila antimicrobial peptide with distinct immune response characteristics

The tight regulation of transiently expressed antimicrobial peptides (AMPs) with a distinct antimicrobial spectrum and different expression kinetics contributes greatly to the properly regulated immune response for resistance to pathogens and for the maintenance of mutualistic microbiota in Drosophila. The important role of differential regulation of AMP expression at the posttranscriptional level needs to be elucidated. It was observed that the highly expressed Cecropin A1 (CecA1) mRNA encoding a broad antimicrobial spectrum AMP against both bacteria and fungi decayed more quickly than did the moderately expressed Diptericin mRNA encoding AMP against Gram negative bacteria. The mRNA stability of AMPs is differentially regulated and is attributed to the specific interaction between cis-acting ARE in 3'-UTR of AMP mRNA and the RNA destabilizing protein transactor Tis11 as shown in co-immunoprecipitation of the Tis11 RNP complex with CecA1 mRNA but not other AMP mRNA. The p38MAPK was further demonstrated to play a crucial role in stabilizing ARE-bearing mRNAs by inhibiting Tis11-mediated degradation in LPS induced AMP expression. This evidence suggests an evolutionarily conserved and functionally important molecular basis for and effective approach to exact control of AMP gene expression. These mechanisms thereby orchestrate a well balanced and dynamic antimicrobial spectrum of innate immunity to resist infection and maintain resident microbiota properly.

Akirins are highly conserved nuclear proteins required for NF-kappaB-dependent gene expression in drosophila and mice

During a genome-wide RNAi screen, we isolated CG8580 as a gene involved in the innate immune response of Drosophila. CG8580, which we named Akirin, acts in parallel with the NF-κB transcription factor downstream of the Imd pathway and was required for defense against Gram-negative bacteria. Akirin is highly conserved and the human genome contains two homologues, one of which was able to rescue the loss of function phenotype in Drosophila cells. Akirins had a strict nuclear localization. Knockout of both Akirin homologues in mice revealed that one had an essential function downstream of Toll-like receptor, tumor necrosis factor and interleukin 1-β (IL-1β) signaling pathways leading to the production of IL-6. Thus, Akirin is a conserved nuclear factor required for innate immune responses.

FOXO and insulin signaling regulate sensitivity of the circadian clock to oxidative stress

Circadian rhythms can be regulated by many environmental and endogenous factors. We show here a sensitivity of circadian clock function to oxidative stress that is revealed in flies lacking the foxo gene product. When exposed to oxidative stress, wild-type flies showed attenuated clock gene cycling in peripheral tissues, whereas foxo mutants also lost behavioral rhythms driven by the central clock. FOXO is expressed predominantly in the fat body, and transgenic expression in this tissue rescued the mutant behavioral phenotype, suggesting that foxo has non-cell-autonomous effects on central circadian clock function. Overexpression of signaling molecules that affect FOXO activity, such as the insulin receptor or Akt, in the fat body also increased susceptibility of the central clock to oxidative stress. Finally, foxo mutants showed a rapid decline in rest:activity rhythms with age, supporting the idea that the increase of oxidative stress contributes to age-associated degeneration of behavioral rhythms and indicating the importance of FOXO in mitigating this deterioration. Together these data demonstrate that metabolism affects central clock function and provide a link among insulin signaling, oxidative stress, aging, and circadian rhythms.

A kappaB sequence code for pathway-specific innate immune responses

The Toll and Imd pathways induce humoral innate immune responses in Drosophila by activating NF-kappaB proteins that bind kappaB target sites. Here, we delineate a kappaB site sequence code that directs pathway-specific expression of innate immune loci. Using bioinformatic analysis of expression and sequence data, we identify shared properties of Imd- and Toll-specific response elements. Employing synthetic kappaB sites in luciferase reporter and in vitro binding assays, we demonstrate that the length of the (G)(n) element in the 5' half-site and of the central (A,T)-rich region combine to specify responsiveness to one or both pathways. We also show that multiple sites function to enhance the response to either or both pathways. Together, these studies elucidate the mechanism by which kappaB motifs direct binding by particular Drosophila NF-kappaB family members and thereby induce specialized innate immune repertoires.

The host defense of Drosophila melanogaster

To combat infection, the fruit fly Drosophila melanogaster relies on multiple innate defense reactions, many of which are shared with higher organisms. These reactions include the use of physical barriers together with local and systemic immune responses. First, epithelia, such as those beneath the cuticle, in the alimentary tract, and in tracheae, act both as a physical barrier and local defense against pathogens by producing antimicrobial peptides and reactive oxygen species. Second, specialized hemocytes participate in phagocytosis and encapsulation of foreign intruders in the hemolymph. Finally, the fat body, a functional equivalent of the mammalian liver, produces humoral response molecules including antimicrobial peptides. Here we review our current knowledge of the molecular mechanisms underlying Drosophila defense reactions together with strategies evolved by pathogens to evade them.

A multilayered defense against infection: combinatorial control of insect immune genes

The innate immune defense system involves the activity of endogenous antimicrobial peptides (AMPs), which inhibit the growth of most microbes. In insects, genes encoding AMPs are expressed at basal levels in barrier epithelia and are upregulated systemically in response to infection. To achieve this differentiated immune defense, Drosophila immune gene promoters combine tissue-specific enhancers and signal-dependent response elements. Transcription factors of the Hox, POU and GATA families control tissue-specific expression of AMP genes, either constitutively or in combination with NF-kappaB/Rel family factors that function as 'on-off switches' during infection. Here, we review these different modes of AMP expression and provide a model for transcriptional regulation of AMP genes.

Innate immune responses regulate trypanosome parasite infection of the tsetse fly Glossina morsitans morsitans

Tsetse flies transmit the protozoan parasite African trypanosomes, the agents of human sleeping sickness in sub-Saharan Africa. Parasite transmission in the insect is restricted by a natural resistance phenomenon (refractoriness). Understanding the mechanism of parasite resistance is important as strengthening fly's response(s) via transgenic approaches can prevent parasite transmission and lead to the development of novel vector control strategies. Here, we investigated the role of one of the two major pathways regulating innate immunity in invertebrates, the immunodeficiency (Imd) pathway, for Glossina morsitans morsitans's natural defence against Trypanosoma brucei spp. infections. We determined the molecular structure of the Imd pathway transcriptional activator Relish (GmmRel), which shows high amino acid identity and structural similarity to its Drosophila homologue. Through a double-stranded RNA-based interference approach, we showed that the pathogen-induced expression profile of the antimicrobial peptides (AMPs) attacin and cecropin is under the regulation of GmmRel. Unexpectedly, the AMP diptericin appears to be constitutively expressed in tsetse independent of the presence of the Rel factor. Through GmmRel knock-down, we could successfully block the induction of attacin and cecropin expression in the immune responsive tissues fat body and proventriculus (cardia) following microbial challenge. The midgut and salivary gland trypanosome infection prevalence, as well as the intensity of midgut parasite infections were found to be significantly higher in flies when attacin and relish expression were knocked down. Our results provide the first direct evidence for the involvement of antimicrobial peptides in trypanosome transmission in tsetse.

Weckle is a zinc finger adaptor of the toll pathway in dorsoventral patterning of the Drosophila embryo

BACKGROUND

The Drosophila Toll pathway takes part in both establishment of the embryonic dorsoventral axis and induction of the innate immune response in adults. Upon activation by the cytokine Spätzle, Toll interacts with the adaptor proteins DmMyD88 and Tube and the kinase Pelle and triggers degradation of the inhibitor Cactus, thus allowing the nuclear translocation of the transcription factor Dorsal/Dif. weckle (wek) was previously identified as a new dorsal group gene that encodes a putative zinc finger transcription factor. However, its role in the Toll pathway was unknown.

RESULTS

Here, we isolated new wek alleles and demonstrated that cactus is epistatic to wek, which in turn is epistatic to Toll. Consistent with this, Wek localizes to the plasma membrane of embryos, independently of Toll signaling. Wek homodimerizes and associates with Toll. Moreover, Wek binds to and localizes DmMyD88 to the plasma membrane. Thus, Wek acts as an adaptor to assemble/stabilize a Toll/Wek/DmMyD88/Tube complex. Remarkably, unlike the DmMyD88/tube/pelle/cactus gene cassette of the Toll pathway, wek plays a minimal role, if any, in the immune defense against Gram-positive bacteria and fungi.

CONCLUSIONS

We conclude that Wek is an adaptor to link Toll and DmMyD88 and is required for efficient recruitment of DmMyD88 to Toll. Unexpectedly, wek is dispensable for innate immune response, thus revealing differences in the Toll-mediated activation of Dorsal in the embryo and Dif in the fat body of adult flies.

WntD is a feedback inhibitor of Dorsal/NF-kappaB in Drosophila development and immunity

Regulating the nuclear factor-kappaB (NF-kappaB) family of transcription factors is of critical importance to animals, with consequences of misregulation that include cancer, chronic inflammatory diseases and developmental defects. Studies in Drosophila melanogaster have proved fruitful in determining the signals used to control NF-kappaB proteins, beginning with the discovery that the Toll/NF-kappaB pathway, in addition to patterning the dorsal-ventral axis of the fly embryo, defines a major component of the innate immune response in both Drosophila and mammals. Here, we characterize the Drosophila wntD (Wnt inhibitor of Dorsal) gene. We show that WntD acts as a feedback inhibitor of the NF-kappaB homologue Dorsal during both embryonic patterning and the innate immune response to infection. wntD expression is under the control of Toll/Dorsal signalling, and increased levels of WntD block Dorsal nuclear accumulation, even in the absence of the IkappaB homologue Cactus. The WntD signal is independent of the common Wnt signalling component Armadillo (beta-catenin). By engineering a gene knockout, we show that wntD loss-of-function mutants have immune defects and exhibit increased levels of Toll/Dorsal signalling. Furthermore, the wntD mutant phenotype is suppressed by loss of zygotic dorsal. These results describe the first secreted feedback antagonist of Toll signalling, and demonstrate a novel Wnt activity in the fly.

The lesswright mutation activates Rel-related proteins, leading to overproduction of larval hemocytes in Drosophila melanogaster

The lesswright (lwr) gene encodes an enzyme that conjugates a small ubiquitin-related modifier (SUMO). Since the conjugation of SUMO occurs in many different proteins, a variety of cellular processes probably require lwr function. Here, we demonstrate that lwr function regulates the production of blood cells (hemocytes) in Drosophila larvae. lwr mutant larvae develop many melanotic tumors in the hemolymph at the third instar stage. The formation of melanotic tumors is due to a large number of circulating hemocytes, which is approximately 10 times higher than those of wild type. This overproduction of hemocytes is attributed to the loss of lwr function primarily in hemocytes and the lymph glands, a hematopoietic organ in Drosophila larvae. High incidences of Dorsal (Dl) protein in the nucleus were observed in lwr mutant hemocytes, and the dl and Dorsal-related immunity factor (Dif) mutations were found to be suppressors of the lwr mutation. Therefore, the lwr mutation leads to the activation of these Rel-related proteins, key transcription factors in hematopoiesis. We also demonstrate that dl and Dif play different roles in hematopoiesis. dl primarily stimulates plasmatocyte production, but Dif controls both plasmatocyte and lamellocyte production.

Hemolymph-dependent and -independent responses inDrosophila immune tissue

Insects possess an antimicrobial defense response that is similar to the mammalian innate immune response. The innate immune system is designed to recognize conserved components of microorganisms called pathogen-associated molecular patterns (PAMPs). How host receptors detect PAMPs and transmit the signals to mount the immune response is being elucidated. Using GFP-Dorsal, -Dif, and -Relish reporter proteins in ex vivo assays, we demonstrate that Drosophila fat bodies, a major immune tissue, have both hemolymph-dependent and -independent responses. Microbial preparations such as lipoteichoic acid (LTA) and peptidoglycan (PGN) can stimulate some responses from dissected and rinsed larval fat bodies. Therefore, at least some aspects of recognition can occur on fat body cell surfaces, bypassing the requirement of hemolymph. Our results also show that supernatants from bacterial cultures can stimulate the nuclear translocation of Dorsal in dissected fat bodies, but this stimulation is strictly hemolymph-dependent. Various biochemical assays suggest that the factors from bacterial supernatants that stimulate the hemolymph-dependent nuclear translocation are likely made up of proteins. We further show that Dorsal mutant larvae have much lower phenoloxidase activity, consistent with a more important role of Dorsal in innate immunity than previously shown.

An intron enhancer activates the immunoglobulin-related Hemolin gene in Hyalophora cecropia

Hemolin is the only insect member of the immunoglobulin (Ig) superfamily reported to be up-regulated during an immune response. In diapausing pupae of Hyalophora cecropia the gene is expressed in fat body cells and in haemocytes. Like the mammalian Ig kappa light chain gene, the Hemolin gene harbours an enhancer including a kappaB motif in one of its introns. This motif binds the H. cecropia Rel factor Cif (Cecropia immunoresponsive factor). The Hemolin third intron also mediates transient reporter gene expression in immunoresponsive Drosophila mbn-2 cells. Co-transfections of Drosophila SL2 cells showed that the Drosophila Rel factor Dif (Dorsal-related immunity factor), transactivates reporter gene constructs through the intron. Moreover, a 4.8-fold synergistic activation was obtained when Dif is combined with the rat C/EBP (CCAAT/enhancer element-binding protein) and human HMGI (high mobility group protein I). This is the first report of an insect immune-related gene that is up-regulated by an enhancer activity conferred through an intron.

Regulation of antibacterial and antifungal innate immunity in fruitflies and humans

Insects have been very successful in adapting to their environment, and the ability of the insect immune system to detect and elicit the appropriate response against various invading pathogens has helped in this success. Unlike the vertebrate immune system, which consists of both innate and adaptive components, insect immunity probably consists entirely of an innate immune response, as no evidence of an adaptive response has been found. The innate immune response is described as either a reaction against "lack of self," or the interaction between host germline-encoded receptors and molecules unique to a particular class of invading organisms. Once the invading organism is recognized, the host immune response can be activated via signaling pathways that lead to the appropriate reaction. This review endeavors to put forth how through genetic, molecular, and biochemical studies of the fruit fly Drosophila melanogaster, as well as other insects, it is now understood that aspects of the insect and vertebrate innate immune system are very similar.

A genome-wide analysis of immune responses in Drosophila

Oligonucleotide DNA microarrays were used for a genome-wide analysis of immune-challenged Drosophila infected with Gram-positive or Gram-negative bacteria, or with fungi. Aside from the expression of an established set of immune defense genes, a significant number of previously unseen immune-induced genes were found. Genes of particular interest include corin- and Stubble-like genes, both of which have a type II transmembrane domain; easter- and snake-like genes, which may fulfil the roles of easter and snake in the Toll pathway; and a masquerade-like gene, potentially involved in enzyme regulation. The microarray data has also helped to greatly reduce the number of target genes in large gene groups, such as the proteases, helping to direct the choices for future mutant studies. Many of the up-regulated genes fit into the current conceptual framework of host defense, whereas others, including the substantial number of genes with unknown functions, offer new avenues for research.

Patterns of gene expression during Drosophila mesoderm development

The transcription factor Twist initiates Drosophila mesoderm development, resulting in the formation of heart, somatic muscle, and other cell types. Using a Drosophila embryo sorter, we isolated enough homozygous twist mutant embryos to perform DNA microarray experiments. Transcription profiles of twist loss-of-function embryos, embryos with ubiquitous twist expression, and wild-type embryos were compared at different developmental stages. The results implicate hundreds of genes, many with vertebrate homologs, in stage-specific processes in mesoderm development. One such gene, gleeful, related to the vertebrate Gli genes, is essential for somatic muscle development and sufficient to cause neural cells to express a muscle marker.

Cactus-independent nuclear translocation of Drosophila RELISH

Insects can effectively and rapidly clear microbial infections by a variety of innate immune responses including the production of antimicrobial peptides. Induction of these antimicrobial peptides in Drosophila has been well established to involve NF-kappaB elements. We present evidence here for a molecular mechanism of Lipopolysaccharide (LPS)-induced signaling involving Drosophila NF-kappaB, RELISH, in Drosophila S2 cells. We demonstrate that LPS induces a rapid processing event within the RELISH protein releasing the C-terminal ankyrin-repeats from the N-terminal Rel homology domain (RHD). Examination of the cellular localization of RELISH reveals that the timing of this processing coincides with the nuclear translocation of the RHD and the retention of the ankyrin-repeats within the cytoplasm. Both the processing and the nuclear translocation immediately precede the expression of antibacterial peptide genes cecropin A1, attacin, and diptericin. Over-expression of the RHD but not full-length RELISH results in an increase in the promoter activity of the cecropin A1 gene in the absence of LPS. Furthermore, the LPS-induced expression of these antibacterial peptides is greatly reduced when RELISH expression is depleted via RNA-mediated interference. In addition, loss of cactus expression via RNAi revealed that RELISH activation and nuclear translocation is not dependent on the presence of cactus. Taken together, these results suggest that this signaling mechanism involving the processing of RELISH followed by nuclear translocation of the RHD is central to the induction of at least part of the antimicrobial response in Drosophila, and is largely independent of cactus regulation.

The biology of Toll-like receptors

In 1997, a human homologue of the Drosophila Toll protein was described, a protein later to be designated Toll-like receptor 4 (TLR4). Since that time, additional human and murine TLR proteins have been identified. Mammalian TLR proteins appear to represent a conserved family of innate immune recognition receptors. These receptors are coupled to a signaling pathway that is conserved in mammals, insects, and plants, resulting in the activation of genes that mediate innate immune defenses. Numerous studies have now identified a wide variety of chemically-diverse bacterial products that serve as putative ligands for TLR proteins. More recent studies have identified the first endogenous protein ligands for TLR proteins. TLR signaling represents a key feature of innate immune response to pathogen invasion.

A novel lipopolysaccharide response element in the Bombyx mori cecropin B promoter

Cecropin B is one of the major antibacterial peptides in the silkworm, Bombyx mori. Transcription of the cecropin B gene (CecB) occurs rapidly after bacterial invasion. Using 235 base pairs (bp) of the CecB promoter region, a kappaB-related protein and two additional DNA-binding complexes (designated F2BPI and F4BP) were identified in nuclear extracts from immunized larval fat body by the electrophoretic mobility shift assay (EMSA) (1). Further EMSA analyses indicated that the F2BPI-binding site was CATTA, and that F2BPI translocated from the cytoplasm to the nucleus after infection. In a recently established B. mori cell line, NISES-BoMo-DZ, 235 bp of CecB promoter linked to a reporter luciferase was activated 6-fold by stimulation with lipopolysaccharide (LPS), which is a major trigger of CecB expression in larvae. Truncation of the F2BPI-binding site from the promoter reduced the activation 2-fold. Deletion of either of two kappaB motifs also reduced promoter activation 2-fold. Elimination of both the F2BPI-binding site and the kappaB motifs resulted in the complete loss of LPS inducibility. These results indicate that the F2BPI-binding site is an LPS-responsive cis-element that is necessary for full activation of CecB.

Two attacin antibacterial genes of Drosophila melanogaster

Insects express a battery of potent antimicrobial proteins in response to injury and infection. Recent work from several laboratories has demonstrated that this response is neither stereotypic nor completely nonspecific, and that different pathways are responsible for inducing the expression of antifungal and antibacterial peptides. Here we report the cloning of two closely linked attacin genes from Drosophila melanogaster. We compare their protein coding sequences and find the amino acid sequences to be more highly conserved than the nucleotide sequences, suggesting that both genes are expressed. Like other antimicrobial peptides, attacin expression is strongly induced in infected and injured flies. Unlike others, attacin transcription is uniquely sensitive to mutations in the 18-Wheeler receptor protein, and thus may be regulated by a distinct signaling pathway. The number and organization of binding sites for kappaB and other transcription factors in the promoter regions of both attacin genes are consistent with strong and rapid immune induction. We demonstrate that these promoter regions are sufficient to direct beta-galactosidase expression in transformed Drosophila third-instar larval fat body in a bacterially inducible manner. We present a comparison of the promoter regions of the two attacin genes to those cloned from other antimicrobial peptide genes to assist a better understanding of how antimicrobial genes are differentially regulated.

Lipopolysaccharide-activated kinase, an essential component for the induction of the antimicrobial peptide genes in Drosophila melanogaster cells

Eukaryotic organisms use a similar Rel/NF-kappaB signaling cascade for the induction of innate immune genes. In Drosophila, lipopolysaccharide (LPS) signal-induced activation of the Rel/NF-kappaB family transcription factors is an essential step in the transcriptional activation of inducible antimicrobial peptide genes. However, the mechanism by which the LPS-induced signaling pathway proceeds remains largely unknown. Here we have cloned a novel Drosophila LPS-activated kinase (DLAK) that is structurally related to mammalian IkappaB kinases. DLAK is expressed and transiently activated in LPS-responsive Drosophila cells following LPS stimulation. Furthermore, DLAK can interact with Cactus, a Drosophila IkappaB and phosphorylate recombinant Cactus, in vitro. Overexpression of dominant-negative mutant DLAK (DLAK(K50A)) blocks LPS-induced Cactus degradation. DLAK-bound Cactus can be degraded in a LPS signal-dependent fashion, whereas the DLAK(K50A) mutant-bound Cactus is completely resistant to degradation in the presence of LPS. The DLAK(K50A) mutant also inhibits nuclear kappaB binding activity and kappaB-dependent diptericin reporter gene activity in a dose-dependent manner, but the kappaB-dependent diptericin reporter gene activity can be rescued by overexpression of wild type DLAK. Moreover, mRNA analysis of various kappaB-dependent antimicrobial peptide genes shows that LPS inducibility of these genes is greatly impaired in cells overexpressing DLAK(K50A). These results establish that DLAK is a novel LPS-activated kinase, which is an essential signaling component for the induction of antimicrobial peptide genes following LPS treatment in Drosophila cells.

Interaction and specificity of Rel-related proteins in regulating Drosophila immunity gene expression

NF-kappaB/Rel family proteins regulate genes that are critical for many cellular processes including apoptosis, inflammation, immune response, and development. NF-kappaB/Rel proteins function as homodimers or heterodimers, which recognize specific DNA sequences within target promoters. We examined the activity of different Drosophila Rel-related proteins in modulating Drosophila immunity genes by expressing the Rel proteins in stably transfected cell lines. We also compared how different combinations of these transcriptional regulators control the activity of various immunity genes. The results show that Rel proteins are directly involved in regulating the Drosophila antimicrobial response. Furthermore, the drosomycin and defensin expression is best induced by the Relish/Dif and the Relish/Dorsal heterodimers, respectively, whereas the attacin activity can be efficiently up-regulated by the Relish homodimer and heterodimers. These results illustrate how the formation of Rel protein dimers differentially regulate target gene expression.

A mosaic analysis in Drosophila fat body cells of the control of antimicrobial peptide genes by the Rel proteins Dorsal and DIF

Expression of the gene encoding the antifungal peptide Drosomycin in Drosophila adults is controlled by the Toll signaling pathway. The Rel proteins Dorsal and DIF (Dorsal-related immunity factor) are possible candidates for the transactivating protein in the Toll pathway that directly regulates the drosomycin gene. We have examined the requirement of Dorsal and DIF for drosomycin expression in larval fat body cells, the predominant immune-responsive tissue, using the yeast site-specific flp/FRT recombination system to generate cell clones homozygous for a deficiency uncovering both the dorsal and the dif genes. Here we show that in the absence of both genes, the immune-inducibility of drosomycin is lost but can be rescued by overexpression of either dorsal or dif under the control of a heat-shock promoter. This result suggests a functional redundancy between both Rel proteins in the control of drosomycin gene expression in the larvae of Drosophila. Interestingly, the gene encoding the antibacterial peptide Diptericin remains fully inducible in the absence of the dorsal and dif genes. Finally, we have used fat body cell clones homozygous for various mutations to show that a linear activation cascade Spaetzle--> Toll-->Cactus-->Dorsal/DIF leads to the induction of the drosomycin gene in larval fat body cells.

Induction and regulation of antimicrobial peptides in Drosophila

Activation of the innate immune response involves recognition of the infectious agent and the subsequent activation of cellular and humoral reactions. In insects, a number of immunity genes are activated at the level of transcription leading to the synthesis of antimicrobial peptides. Genetic analyses in Drosophila have identified several signal transduction pathways that promote activation of these immunity genes. Recent data suggest that the insect immune system is able to discriminate between a bacterial and a fungal infection, and responds by higher levels of activation of the appropriate peptides to repel the infection. These and other recent data on transcription factors and regulation of antimicrobial genes are integrated into a model to suggest how differential activation of antifungal and antibacterial peptides can occur in response to fungal and bacterial infection.

Toll receptor-mediated Drosophila immune response requires Dif, an NF-kappaB factor

The induction of immunity genes in Drosophila has been proposed to be dependent on Dorsal, Dif, and Relish, the NF-kappaB-related factors. Here we provide genetic evidence that Dif is required for the induction of only a subset of antimicrobial peptide genes. The results show that the presence of Dif without Dorsal is sufficient to mediate the induction of drosomycin and defensin. We also demonstrate that Dif is a downstream component of the Toll signaling pathway in activating the drosomycin expression. These results reveal that individual members of the NF-kappaB family in Drosophila have distinct roles in immunity and development.

Dorsal-B, a splice variant of the Drosophila factor Dorsal, is a novel Rel/NF-kappaB transcriptional activator

The Drosophila transcription factor Dorsal, a member of the Rel/NF-kappaB family of proteins, plays a key role in the establishment of dorsoventral polarity in the early embryo and is also involved in the immune response. Here, we present evidence that the primary transcript of dorsal can be alternatively spliced, generating Dorsal-B, a new Rel/NF-kappaB family member. Dorsal and Dorsal-B are identical in the N-terminal region, which comprises both a DNA-binding domain and a dimerization domain. However, Dorsal-B lacks the nuclear localization signal located at the end of the Rel domain of Dorsal and is totally divergent in the C-terminal portion. Although Dorsal-B by itself is not able to induce the expression of a kappaB-controlled Luciferase reporter gene, we demonstrate that its C-terminal portion has transactivating properties. Analysis of the dorsal-B expression pattern indicates that the splicing is tissue-specific and excludes a putative role in early embryogenesis. However, dorsal-B synthesis is enhanced upon septic injury, and this challenge induces a nuclear accumulation of the protein in fat body cells suggesting that it may be involved in the immune response.

The Dorsal-related immunity factor (Dif) can define the dorsal-ventral axis of polarity in the Drosophila embryo

In Drosophila embryos, dorsal-ventral polarity is defined by a signal transduction pathway that regulates nuclear import of the Dorsal protein. Dorsal protein's ability to act as a transcriptional activator of some zygotic genes and a repressor of others defines structure along the dorsal-ventral axis. Dorsal is a member of a group of proteins, the Rel-homologous proteins, whose activity is regulated at the level of nuclear localization. Dif, a more recently identified Drosophila Rel-homologue, has been proposed to act as a mediator of the immune response in Drosophila. In an effort to understand the function and regulation of Rel-homologous proteins in Drosophila, we have expressed Dif protein in Drosophila embryos derived from dorsal mutant mothers. We found that the Dif protein was capable of restoring embryonic dorsal-ventral pattern elements and was able to define polarity correctly with respect to the orientation of the egg shell. This, together with the observation that the ability of Dif to restore a dorsal-ventral axis depended on the signal transduction pathway that normally regulates Dorsal, suggests that Dif protein formed a nuclear concentration gradient similar to that seen for Dorsal. By studying the expression of Dorsal target genes we found that Dif could activate the zygotic genes that Dorsal activates and repress the genes repressed by Dorsal. Differences in the expression of these target genes, as well as the results from interaction studies carried out in yeast, suggest that Dif is not capable of synergizing with the basic helix-loop-helix transcription factors with which Dorsal normally interacts, and thereby lacks an important component of Dorsal-mediated pattern formation.

A conserved p38 mitogen-activated protein kinase pathway regulates Drosophila immunity gene expression

Accumulating evidence suggests that the insect and mammalian innate immune response is mediated by homologous regulatory components. Proinflammatory cytokines and bacterial lipopolysaccharide stimulate mammalian immunity by activating transcription factors such as NF-kappaB and AP-1. One of the responses evoked by these stimuli is the initiation of a kinase cascade that leads to the phosphorylation of p38 mitogen-activated protein (MAP) kinase on Thr and Tyr within the motif Thr-Gly-Tyr, which is located within subdomain VIII. We have investigated the possible involvement of the p38 MAP kinase pathway in the Drosophila immune response. Two genes that are highly homologous to the mammalian p38 MAP kinase were molecularly cloned and characterized. Furthermore, genes that encode two novel Drosophila MAP kinase kinases, D-MKK3 and D-MKK4, were identified. D-MKK3 is an efficient activator of both Drosophila p38 MAP kinases, while D-MKK4 is an activator of D-JNK but not D-p38. These data establish that Drosophila indeed possesses a conserved p38 MAP kinase signaling pathway. We have examined the role of the D-p38 MAP kinases in the regulation of insect immunity. The results revealed that one of the functions of D-p38 is to attenuate antimicrobial peptide gene expression following exposure to lipopolysaccharide.

Kappa-B like DNA-binding activity is enhanced after spaced training that induces long-term memory in the crab Chasmagnathus

Regulation of gene expression has been involved in long-term memory consolidation. Present results support the role of Rel/ NFkappa-B like activation in this process. In the crab Chasmagnathus, the spaced presentation of 15 or more danger stimuli induces long-term habituation (LTH), while no LTH is observed after a massed training of 600 trials. When a group trained with 30 spaced trials was compared with a passive control group and massed trained groups, a higher level of specific Rel/kappa-B like DNA-binding activity was found in brain nuclear extracts. These results strongly suggest that the enhancement of Rel/kappa-B like DNA-binding activity in the brain is specifically related to LTH formation.

The 18-wheeler mutation reveals complex antibacterial gene regulation in Drosophila host defense

Mammals and insects employ similar Rel/NF-kappaB signaling cascades in their humoral immune responses. The mammalian interleukin-1 type I receptor (IL-1R) is one way of activating this cascade. The Drosophila Toll protein, whose cytoplasmic domain shows striking similarity to that of the IL-1R, acts in the humoral antimicrobial response. Here we demonstrate that a second IL-1R-related Drosophila protein, 18-Wheeler (18W), is a critical component of the humoral immune response. 18-wheeler is expressed in the larval fat body, the primary organ of antimicrobial peptide synthesis. In the absence of the 18W receptor, larvae are more susceptible to bacterial infection. Nuclear translocation of the Rel protein Dorsal-like immunity factor (Dif) is inhibited, though nuclear translocation of another Rel protein, Dorsal, is unaffected. Induction of several antibacterial genes is reduced following infection, relative to wild-type: attacin is reduced by 95%, cecropin by 65% and diptericin by 12%. Finally, 18-wheeler (18w) expression is induced in response to infection and, in addition to the receptor form, four immune-specific transcripts and proteins are produced.

Treatment of l(2)mbn Drosophila tumorous blood cells with the steroid hormone ecdysone amplifies the inducibility of antimicrobial peptide gene expression

Insects rely on both humoral and cellular mechanisms to defend themselves against microbial infections. The humoral response involves synthesis of a battery of potent antimicrobial peptides by the fat body and, to a lesser extent, by blood cells. The cellular response on the other hand consists of phagocytosis of small microorganisms and melanization and encapsulation of larger parasites. The l(2)mbn cell line, established from tumorous larval hemocytes, represents a system of choice to dissect the molecular events controlling cellular immunity. We report here that l(2)mbn cells can be efficiently induced to differentiate in adherent, macrophage-like cells by treatment with 20-hydroxyecdysone. Ecdysone treatment increases both the phagocytic capacity of l(2)mbn cells and their competence to express antimicrobial genes in response to immune challenge. We also report that expression of several regulatory molecules thought to be involved in the immune response is up-regulated by ecdysone in l(2)mbn cells.