Drosophila immunity. A sequence homologous to mammalian interferon consensus response element enhances the activity of the diptericin promoter

The effect of infection with the entomopathogenic fungus Conidiobolus coronatus (Entomopthorales) on eighteen cytokine-like proteins in Galleria mellonella (Lepidoptera) larvae

Background

In response to the replace mammal research models with insects in preliminary immunological studies, interest has grown in invertebrate defense systems. The immunological response is regulated by cytokines; however, while their role in mammals is well understood, little is known of their function in insects. A suitable target for studies into insect immunology is Galleria mellonella (Lepidoptera), the wax moth: a common host for human fungal and bacterial pathogens. G. mellonella is also a perfect subject for studies into the presence of cytokine-like proteins.

Specific objectives

The main goal of present research was detection in insect immunocompetent cells the 18 mammalian cytokines (IL-1α, IL-1β, IL-2, IL-3, IL-6, IL-7, IL-8, IL-12, IL-13, IL-15, IL-17, IL-19, IFN-γ, TNF-α, TNF-β, GM-CSF, M-CSF, G-CSF), which play important role in immunological response and indication how their level change after fungal infection.

Methodology

The changes of cytokine-like proteins level were detected in hemocytes taken from G. mellonella larvae infected with entomopathogenic fungus, C. coronatus. The presence of cytokine-proteins was confirmed with using fluorescence microscopy (in cultured hemocytes) and flow cytometry (in freshly collected hemolymph). The ELISA test was used to detect changes in concentration of examined cytokine-like proteins.

Results

Our findings indicated the presence of eighteen cytokine-like molecules in G. mellonella hemocytes during infection with C. coronatus. The hemocytes taken from infected larvae demonstrated higher fluorescence intensity for six cytokine-like proteins (GM-CSF, M-CSF, IL-3, IL-15, IL-1β and IL-19) compared to untreated controls. ELISA test indicated significantly higher IL-3 and IL-15. M-CSF, IL-1α and IL-19 concentration in the hemolymph after fungal infection, and significantly lower TNF-β and G-CSF.

Conclusions

Our findings confirm that the selected cytokine-like molecules are present in insect hemocytes and that their concentrations change after fungal infection, which might suggest that they play a role in the anti-fungal immunological response.

Intracellular cytokine detection based on flow cytometry in hemocytes from Galleria mellonella larvae: A new protocol

Invertebrates are becoming increasingly popular models for research on the immune system. The innate immunity possessed by insects shows both structural and functional similarity to the resistance displayed by mammals, and many processes occurring in insect hemocytes are similar to those that occur in mammals. However, the use of insects as research models requires the development of methods for working with hemocytes. The aim of this study was to develop a protocol for intracellular cytokine detection in Galleria mellonella larvae hemocytes based on flow cytometry. It describes the anticoagulant composition of the buffer, the optimal conditions for hemocyte permeabilization and fixation, as well as the conditions of cell centrifugation to prevent cell disintegration. A key element is the selection of staining conditions, especially the length of the incubation time with the primary antibody, which turned out to be much longer than recommended for mammalian cells. The development of these individual steps allowed for the creation of a reproducible protocol for cytokine detection using flow cytometry in wax moth hemocytes. This will certainly facilitate the development of further protocols allowing for wider use of insect cells in immunological research.

Cloning and Characterization of Drosophila melanogaster Juvenile Hormone Epoxide Hydrolases (JHEH) and Their Promoters

Juvenile hormone epoxide hydrolase (JHEH) plays an important role in the metabolism of JH III in insects. To study the control of JHEH in female Drosophila melanogaster, JHEH 1, 2 and 3 cDNAs were cloned and sequenced. Northern blot analyses showed that the three transcripts are expressed in the head thorax, the gut, the ovaries and the fat body of females. Molecular modeling shows that the enzyme is a homodimer that binds juvenile hormone III acid (JH IIIA) at the catalytic groove better than JH III. Analyses of the three JHEH promoters and expressing short promoter sequences behind a reporter gene (lacZ) in D. melanogaster cell culture identified a JHEH 3 promoter sequence (626 bp) that is 10- and 25-fold more active than the most active promoter sequences of JHEH 2 and JHEH 1, respectively. A transcription factor (TF) Sp1 that is involved in the activation of JHEH 3 promoter sequence was identified. Knocking down Sp1 using dsRNA inhibited the transcriptional activity of this promoter in transfected D. melanogaster cells and JH III and 20HE downregulated the JHEH 3 promoter. On the other hand, JH IIIA and farnesoic acid did not affect the promoter, indicating that JH IIIA is JHEH's preferred substrate. A transgenic D. melanogaster expressing a highly activated JHEH 3 promoter behind a lacZ reporter gene showed promoter transcriptional activity in many D. melanogaster tissues.

Beyond Host Defense: Deregulation of Drosophila Immunity and Age-Dependent Neurodegeneration

Age-dependent neurodegenerative disorders are a set of diseases that affect millions of individuals worldwide. Apart from a small subset that are the result of well-defined inherited autosomal dominant gene mutations (e.g., those encoding the β-amyloid precursor protein and presenilins), our understanding of the genetic network that underscores their pathology, remains scarce. Genome-wide association studies (GWAS) especially in Alzheimer's disease patients and research in Parkinson's disease have implicated inflammation and the innate immune response as risk factors. However, even if GWAS etiology points toward innate immunity, untangling cause, and consequence is a challenging task. Specifically, it is not clear whether predisposition to de-regulated immunity causes an inadequate response to protein aggregation (such as amyloid or α-synuclein) or is the direct cause of this aggregation. Given the evolutionary conservation of the innate immune response in Drosophila and humans, unraveling whether hyperactive immune response in glia have a protective or pathological role in the brain could be a potential strategy in combating age-related neurological diseases.

Functional analysis of Fenneropenaeus chinensis anti-lipopolysaccharide factor promoter regulated by lipopolysaccharide and (1,3)-β-D-glucan

Current knowledge on cis-regulatory elements of immune genes of shrimp is poor. In this study, the genomic sequence of the Fenneropenaeus chinensis anti-lipopolysaccharide factor (ALFFc) gene was obtained by using PCR and genome walking techniques, and the promoter was identified. The ALFFc gene contained three exons interrupted by two introns. Immune-related transcription factor binding sites recognized by nuclear factor-kappa B, octamer binding protein 1, GATA binding factor 1 and specificity protein 1 were identified in the regin from +1 to -702. The activity of ALFFc promoter was analyzed in insect sf9 cell lines. The putative promoter sequence of pALF-702 drive the expression of reporter EGFP gene successfully by adding lipopolysaccharide or (1,3)-β-D-glucan, but the shorter promoter sequence pALF-318 is only by (1,3)-β-D-glucan. The results pointed out that these transcription elements might contribute to the differences in promoter of ALFFc. Our results would provide supports for future studies to identify the functional transcription elements in the ALF promoter and to expand our knowledge on regulation of innate immune genes in Chinese shrimp.

Identification of a cis-regulatory element required for 20-hydroxyecdysone enhancement of antimicrobial peptide gene expression in Drosophila melanogaster

The antimicrobial peptide diptericin plays an important role in defence against microorganisms. Drosophila melanogaster diptericin mRNA levels showed an increase during the late final instar larval stage when the ecdysteroid titres increase to initiate metamorphosis. Deletion analysis in Drosophila melanogaster mbn2 (mbn2) cells identified a cis-regulatory element (AAGAAAGATCCCCTG) necessary for 20-hydroxyecdysone enhancement of peptidoglycan-induced expression of diptericin in the 3 kb diptericin promoter. Proteins extracted from mbn2 cells treated with peptidoglycan plus 20-hydroxyecdysone specifically bound to this element. 20-hydroxyecdysone also enhanced peptidoglycan-induced expression of four other antimicrobial peptide (AMP) genes--drosomycin, attacin-A, metchnikowin and cecropin A1. Moreover, in silico promoter analysis using the meme program showed that an eight-nucleotide region of the identified cis-regulatory element is present in the promoters of these four AMP genes. These studies suggest that 20-hydroxyecdysone regulates the expression of AMP genes through a conserved cis-regulatory element.

Baculovirus interactions in vitro and in vivo

Baculoviruses are promising viral insecticides and are safe for the environment. Interaction of baculoviruses in vitro and in vivo is a basic molecular and ecological question that has practical applications in agriculture. Cellular secretion is also a fundamental property in cell-cell communication. Here, we review recent investigations on how baculoviruses interact with insect cells and insect hosts. We focus particularly on a new interaction mechanism in which a secretion from cells infected with one virus enhances infection by a second virus. We also discuss a hypothesis that the secreted signals may serve as ligands that bind to the receptors on the surface of the cells that harbor the suppressed genomes of Thysanoplusia orichalcea MNPV (ThorMNPV) in Sf21 and Spodoptera exigua MNPV (SeMNPV) in High 5 to initiate signal transduction leading to the activation of genome replication of ThorMNPV in Sf21 and SeMNPV in High 5. We also discuss how the enhanced replication of SeMNPV replication by Autographa californica MNPV (AcMNPV) in nonpermissive insect cells depends on the types of cells. Interaction of baculoviruses in insects focused on mutualism and antagonism, even though the mechanism is not clear on mutualism. The antagonism of a Nucleopolyhedrovirus (NPV) with a Granulovirus (GV) has been extensively studied by a metalloprotein in the capsule of GV that disrupts the peritrophic membrane, a physical barrier to NPV entry to the midgut of larvae, to facilitate NPV infection.

Anti-lipopolysaccharide factors from the black tiger shrimp, Penaeus monodon, are encoded by two genomic loci

Different isoforms of the ALF homologues (ALFPm1-5) have been previously identified from Penaeus monodon expressed sequence tag (EST) database (http://pmonodon.biotec.or.th). The nucleotide and amino acid sequences of the P. monodon ALF homologues were analyzed and categorized into two groups, ALFPm1 and 2 in group A and ALFPm3-5 in group B. The genomic sequences of the two ALF gene groups were obtained by using the PCR and genome walking techniques. The ALF group A gene consisted of three exons interrupted by two introns whereas the ALF group B gene contained four exons interrupted by three introns. The alignment of genomic sequences with the ALF cDNA sequences revealed that different transcripts in both groups were generated by alternative RNA splicing of the pre-mRNA transcripts. The 5' upstream sequences of the two ALF groups contained the putative cis-regulatory elements, including the activator protein 1, the Octamer, the GATA, the nuclear factor-kappaB, and the GAAA motifs, which possibly promoted transcription in response to infection as in other antimicrobial peptide genes. The RT-PCR analysis revealed that although all ALF isoforms were expressed in individual shrimp, the ALFPm2 and 3 were the major or authentic ALFs in the hemocytes. The expression of both ALFPm2 and 3 were increased in response to Vibrio harveyi infection indicating the important function of the ALFs against bacterial invasion.

A novel association between clustered NF-kappaB and C/EBP binding sites is required for immune regulation of mosquito Defensin genes

A comparative analysis identified key cis-acting regulatory elements responsible for the temporal control of mosquito Defensin gene expression. The promoters of Anopheles gambiae Defensin 1 and two isoforms of Aedes aegypti Defensin A are up-regulated by immune challenge. This stimulated activity depends upon a cluster of three NF-kappaB binding sites and closely associated C/EBP-like motifs, which function as a unit for optimal promoter activity. Binding of NF-kappaB and C/EBP like transcription factors is confirmed by electrophoretic mobility shift assay, including supershifts with antibodies to C/EBP. KappaB-like motifs are abundant within antimicrobial peptide gene promoters and most are very closely associated with putative C/EBP binding sites. This novel association between NF-kappaB and C/EBP binding sites may, therefore, be of widespread significance.

Characterization of a homologue of the Rel/NF-kappaB transcription factor from a beetle, Allomyrina dichotoma

A cDNA encoding a Rel/NF-kappaB homologue was cloned from a beetle, Allomyrina dichotoma, by reverse transcriptase-polymerase chain reactions (RT-PCR) taking advantage of the conserved Rel homology domain (RHD) to synthesize primers. The Rel/NF-kappaB homologue was designated A. dichotoma (A.d.) Rel A. The amino acid sequence of the A.d. Rel A RHD was compared with those of insect RHDs. The result showed that it has 70% identity with Tribolium castaneum Dorsal, 66% with Drosophila melanogaster Dorsal, 61% with Anopheles gambiae Gambif1, and 55% with D. melanogaster Dif. A putative phosphorylation site in the RHD, RRPS, and two putative nuclear localization signals were conserved in A.d. Rel A. A recombinant fusion protein containing the A.d. Rel A RHD was confirmed to bind specifically to the NF-kappaB site of a gene encoding A.d. coleoptericin A, an antibacterial peptide from A. dichotoma. The activity of A.d. Rel A in modulating a gene construct of the A.d. coleoptericin A promoter-luciferase reporter by expressing the A.d. coleoptericin A cDNA in a Bombyx mori cell line was analyzed. The result showed that A.d. Rel A strongly activates the A.d. coleoptericin A gene construct, whereas A.d. Rel A failed to activate the gene construct containing the mutated NF-kappaB site, suggesting the importance of the interaction between the NF-kappaB site and A.d. Rel A in the signal transduction for gene expression of antibacterial peptides in A. dichotoma.

Cg-Rel, the first Rel/NF-kappaB homolog characterized in a mollusk, the Pacific oyster Crassostrea gigas

We report here the identification and functional characterization of Cg-Rel, a gene encoding the Crassostrea gigas homolog of Rel/NF-kappaB transcription factors found in insects and mammals. Sequence and phylogenetic analysis showed that Cg-Rel shares the structural organization of Rel/NF-kappaB transcription factors of class II. It includes a Rel homology domain as well as a C-terminal transactivation domain (TD). Overexpression of Cg-Rel in the Drosophila S2 cell line activated the expression of a NF-kappaB-dependent reporter gene, whereas transfection with a Cg-Rel construct containing a C-terminal deletion of the TD or using a reporter gene with mutated kappaB binding sites failed to activate expression. These results suggest that Cg-Rel is a functional member of the Rel family of transcription factors, making this the sixth structurally homologous component of the Rel/NF-kappaB pathway characterized in C. gigas. Based on homology to other invertebrates' Rel/NF-kappaB cascade, the function of the oyster pathway may serve to regulate genes involved in innate defense and/or development. These findings serve to highlight a potentially important regulatory pathway to the study of oyster immunology, hence allowing comparison of the immune system in vertebrates and invertebrates, an important key issue to understand its evolution.

The existence of a putative post-transcriptional regulatory element in 3'-UTR of Drosophila antibacterial peptide diptericin's mRNA

Antibacterial peptides' genes are rapidly and transiently expressed on immune stimulation, which is the characteristic of immediate early genes. It implies post-transcriptional regulation is an important pathway in antibacterial peptides' gene expression. In a search of putative post-transcriptional regulatory elements, we found a segment of an AU-rich sequence in 3'-untranslated region (UTR) of drosophila diptericin mRNA. 3'-UTR of diptericin mRNA can be specifically bound with Elav and this binding can be competed with the typical AU-rich element (ARE) of c-fos mRNA. These results suggest that the AU-rich sequence in the 3'-UTR of diptericin mRNA may be a cis-acting element and involved in post-transcriptional regulation.

Transcriptional regulation of limulus factor C: repression of an NFkappaB motif modulates its responsiveness to bacterial lipopolysaccharide

Serine proteases play fundamental roles in invertebrate development, hemostasis, and innate immunity. This is exemplified by the limulus Factor C, which is a serine protease that binds a pathogen-associated molecule, lipopolysaccharide (LPS) to trigger a blood coagulation cascade. As a central molecule in the limulus innate immunity and hemostasis, Factor C gene expression has been detected in two major immune defense tissues, the amebocytes and hepatopancreas. Infection of the limulus with live Gram-negative bacteria induces a 2-3-fold increase in mRNA transcripts in both tissues. However, in vitro studies in Drosophila cell lines using Factor C promoter-reporter chimera DNA constructs, and site-directed mutagenesis of the promoter demonstrated that a proximal kappaB binding site, aided by an adjacent dorsal-like binding motif responds dramatically to LPS and dorsal transcription factor overexpression. Electrophoretic mobility shift assay further confirmed a strong interaction of the limulus kappaB motif with Rel proteins. However, deletion constructs of the Factor C promoter harboring different numbers of dorsal-like binding sites upstream of the kappaB motif as well as the electrophoretic mobility shift assay of these motifs with Rel proteins strongly suggest that the up-regulation of Factor C gene expression is attenuated during microbial challenge. The repression of the dramatic activation of this pathogen-responsive gene by LPS is probably effected via competition between the dorsal-like motifs over the proximal LPS-responsive kappaB unit, or through inhibition from the upstream repressive element(s), which accounts for the gene expression pattern observed in vivo. Our findings demonstrate that blood coagulation and innate immune response are integrated at the transcriptional level in this ancient organism, and that this LPS-responsive serine protease is controlled by an evolutionarily conserved NFkappaB pathway.

The caudal homeodomain protein activates Drosophila E2F gene expression

The Drosophila caudal homeobox gene is required for definition of the anteroposterior axis and for gut development, and CDX1 and CDX2, human homologs, play crucial roles in the regulation of cell proliferation and differentiation in the intestine. Most studies have indicated tumor suppressor functions of Cdx2, with inhibition of proliferation, while the effects of Cdx1 are more controversial. The influence of Drosophila Caudal on cell proliferation is unknown. In this study, we found three potential Caudal binding sequences in the 5'-flanking region of the Drosophila E2F (DE2F) gene and showed by transient transfection assays that they are involved in Caudal transactivation of the dE2F gene promoter. Analyses with transgenic flies carrying an E2F-lacZ fusion gene, with and without mutation in the Caudal binding site, indicated that the Caudal binding sites are required for expression of dE2F in living flies. Caudal-induced E2F expression was also confirmed with a GAL4-UAS system in living flies. In addition, ectopic expression of Caudal with heat-shock promotion induced melanotic tumors in larvae. These results suggest that Caudal is involved in regulation of proliferation through transactivation of the E2F gene in Drosophila.

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.

Genomic organization and immune regulation of the defensin gene from the mosquito, Anopheles gambiae

The defensin gene from the mosquito, Anopheles gambiae, is present as a single copy per haploid genome. Two exons, encoding a 102 residue preprodefensin, are separated by a 105 bp intron bounded by consensus splice sites. The upstream regulatory sequence includes a TATA box, arthropod initiator and numerous motifs homologous to insect and mammalian immune response elements. This promoter is capable of upregulation by immune challenge in cultured cells and activity is further stimulated by Gambif1, a mosquito Rel protein known to translocate to the nucleus and bind NF-kappa B sites in target promoters. Activity is inhibited by p50, a mammalian Rel protein that competitively binds NF-kappa B sites, and virtually abolished by p40, an avian I kappa B protein that inhibits nuclear translocation.

Drosophila Thor participates in host immune defense and connects a translational regulator with innate immunity

Thor has been identified as a new type of gene involved in Drosophila host immune defense. Thor is a member of the 4E-binding protein (4E-BP) family, which in mammals has been defined as critical regulators in a pathway that controls initiation of translation through binding eukaryotic initiation factor 4E (eIF4E). Without an infection, Thor is expressed during all developmental stages and transcripts localize to a wide variety of tissues, including the reproductive system. In response to bacterial infection and, to a lesser extent, by wounding, Thor is up-regulated. The Thor promoter has the canonical NFkappaB and associated GATA recognition sequences that have been shown to be essential for immune induction, as well as other sequences commonly found for Drosophila immune response genes, including interferon-related regulatory sequences. In survival tests, Thor mutants show symptoms of being immune compromised, indicating that Thor may be critical in host defense. In contrast to Thor, Drosophila eIF4E is not induced by bacterial infection. These findings for Thor provide the first evidence that a 4E-BP family member has a role in immune induction in any organism. Further, no gene in the translation initiation pathway that includes 4E-BP has been previously found to be immune induced. Our results suggest either a role for translational regulation in humoral immunity or a new, nontranslational function for 4E-BP type genes.

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.

Gene structure and polymorphism of an invertebrate nitric oxide synthase gene

Nitric oxide synthases (NOSs) are ubiquitous in living organisms. However, little is known about the evolution of this large gene family. The first inducible NOS to be described from an invertebrate regulates malaria parasite (Plasmodium spp.) development in the mosquito Anopheles stephensi. This single copy gene shows the highest homology to the vertebrate neuronal isoforms, followed by decreasing homology to endothelial and inducible isoforms. The open reading frame of 1247 amino acids is encoded by 19 exons, which span approximately 33 kilobases. More than 50% of the mosquito exons, distributed around the putative heme, calmodulin, and FAD/NADPH cofactor-binding domains, are conserved with those of the three human genes. Repetitive elements identified within the larger introns include a polymorphic dinucleotide repeat, two tandem repeats, and a putative miniature inverted repeat transposable element. Sequence analysis and primer extension indicate that the upstream promoter is 'TATA-less' with multiple transcription start sites within approximately 250 base pairs of the initiation methionine. Transcription factor binding sites in the 5'-flanking sequence demonstrate a bipartite distribution of lipopolysaccharide- and inflammatory cytokine-responsive elements that is strikingly similar to that described for vertebrate inducible NOS gene promoters.

WRS-85D: A tryptophanyl-tRNA synthetase expressed to high levels in the developing Drosophila salivary gland

In a screen for genes expressed in the Drosophila embryonic salivary gland, we identified a tryptophanyl-tRNA synthetase gene that maps to cytological position 85D (WRS-85D). WRS-85D expression is dependent on the homeotic gene Sex combs reduced (Scr). In the absence of Scr function, WRS-85D expression is lost in the salivary gland primordia; conversely, ectopic expression of Scr results in expression of WRS-85D in new locations. Despite the fact that WRS-85D is a housekeeping gene essential for protein synthesis, we detected both WRS-85D mRNA and protein at elevated levels in the developing salivary gland. WRS-85D is required for embryonic survival; embryos lacking the maternal contribution were unrecoverable, whereas larvae lacking the zygotic component died during the third instar larval stage. We showed that recombinant WRS-85D protein specifically charges tRNATrp, and WRS-85D is likely to be the only tryptophanyl-tRNA synthetase gene in Drosophila. We characterized the expression patterns of all 20 aminoacyl-tRNA synthetases and found that of the four aminoacyl-tRNA synthetase genes expressed at elevated levels in the salivary gland primordia, WRS-85D is expressed at the highest level throughout embryogenesis. We also discuss the potential noncanonical activities of tryptophanyl-tRNA synthetase in immune response and regulation of cell growth.

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.

Two distinct pathways can control expression of the gene encoding the Drosophila antimicrobial peptide metchnikowin

Metchnikowin is a recently discovered proline-rich peptide from Drosophila with antibacterial and antifungal properties. Like most other antimicrobial peptides from insects, its expression is immune-inducible. Here we present evidence that induction of metchnikowin gene expression can be mediated either by the TOLL pathway or by the imd gene product. We show that the gene remains inducible in Toll-deficient mutants, in which the antifungal response is blocked, as well as in imd mutants, which fail to mount an antibacterial response. However, in Toll-deficient;imd double mutants, metchnikowin gene expression can no longer be detected after immune challenge. Our results suggest that expression of this peptide with dual activity can be triggered by signals generated by either bacterial or fungal infection. Cloning of the metchnikowin gene revealed the presence in the 5' flanking region of several putative cis-regulatory motifs characterized in the promoters of insect immune genes: namely, Rel sites, GATA motifs, interferon consensus response elements and NF-IL6 response elements. Establishment of transgenic fly lines in which the GFP reporter gene was placed under the control of 1.5 kb of metchnikowin gene upstream sequences indicates that this fragment is able to confer full immune inducibility and tissue specificity of expression on the transgene.

Differential gene expression in insects: transcriptional control

Studies on transcriptional control of gene expression play a pivotal role in many areas of biology. In non-Drosophilid insects, the cuticle, chorion, immune response, silk gland, storage proteins, and vitellogenin are foci for advances in basic research on promoter elements and transcription factors. Insects offer other advantages for gene regulation studies, including the availability of applied problems. In non-Drosophilid insects, the most serious problem for transcriptional control studies is the lack of homologous in vivo expression systems. Once this deficiency is addressed, the full impact of research on transcription control will be realized throughout the field of entomology.

Antimicrobial peptide defense in Drosophila

Drosophila responds to a septic injury by the rapid synthesis of antimicrobial peptides. These molecules are predominantly produced by the fat body, a functional equivalent of mammalian liver, and are secreted into the hemolymph where their concentrations can reach up to 100 microM. Six distinct antibacterial peptides (plus isoforms) and one antifungal peptide have been characterized in Drosophila and their genes cloned. The induction of the gene encoding the antifungal peptide relies on the spätzle/Toll/cactus gene cassette, which is involved in the control of dorsoventral patterning in the embryo, and shows interesting structural and functional similarities with cytokine-induced activation of NF-kappa B in mammalian cells. An additional pathway, dependent on the as yet unidentified imd (for immune-deficiency) gene, is required for the full induction of the antibacterial peptide genes. Mutants deficient for the Toll and imd pathways exhibit a severely reduced survival to fungal and bacterial infections, respectively. Recent data on the molecular mechanisms underlying recognition of non-self are also discussed in this review.

Drosophila immunity

Septic injury induces in Drosophila the rapid and transient transcription of several genes encoding potent antimicrobial peptides. Significant structural and functional similarities exist between the injury-induced signalling cascades leading to antimicrobial peptide gene expression in Drosophila and cytokine-induced expression of mammalian acute-phase proteins. Here, the authors discuss their understanding of these pathways and their relationships to those found in mammalian cells. They also analyse non-self recognition and the role of blood cells in Drosophila host defence.

Biological mediators of insect immunity

Infection in insects stimulates a complex defensive response. Recognition of pathogens may be accomplished by plasma or hemocyte b1p4eins that bind specifically to bacterial or fungal polysaccharides. Several morphologically distinct hemocyte cell types cooperate in the immune response. Hemocytes attach to invading organisms and then isolate them by phagocytosis, by trapping them in hemocyte aggregates called nodules, or by forming an organized multicellular capsule around large parasites. These responses are often accompanied by proteolytic activation of the phenoloxidase zymogen that is present in the hemolymph. A component of insect immune responses to bacteria is the synthesis by fat body and hemocytes of a variety of antibacterial proteins and peptides, which are secreted into the hemolymph. These molecules attack bacteria by several mechanisms. Inducible antifungal proteins have also been recently discovered in insect hemolymph. The promoters for several antibacterial protein genes in insects are regulated by transcription factors similar to those involved in mammalian acute phase responses.

A tick homologue of the human Ki nuclear autoantigen

A clone isolated from a tick salivary gland cDNA library encodes a homologue of the human Ki lupus autoantigen, a protein of unknown function that is related to the subunits of the PA28 proteasome activator. The Ki sequences appear to be well conserved between mammals and invertebrates, with 55% identity between the tick and human primary structures. This is the first report of a Ki homologue in invertebrates.

Cloning of the gene encoding the antibacterial peptide drosocin involved in Drosophila immunity. Expression studies during the immune response

A potent inducible antibacterial peptide carrying an O-glycosylated substitution has recently been isolated from Drosophila [Bulet, P., Dimarcq, J. L., Hetru, C., Lagueux, M., Charlet, M., Hegy, G., Van Dorsselaer, A. and Hoffmann, J. A. (1993) J. Biol. Chem. 268, 14893-14897]. Here we report cloning studies that show that Drosophila contains a single, intronless gene, located at position 51C1-6, which encodes the precursor protein from which drosocin is processed. The upstream and the downstream sequences of the drosocin gene contain putative cis-regulatory elements similar to mammalian regulatory motifs, namely three kappa B-related decameric sequences. The drosocin gene is silent in naive animals, and is strongly induced with acute phase kinetics after immune challenge in larvae and in adults. We have established several transgenic fly lines in which reporter genes were placed under the control of various drosocin promoter sequences. Our results indicate that 2.5 kb of upstream sequences confer inducibility and tissue specificity to the transgene, but that the level of its expression in the fat body after immune challenge is low. Addition of genomic regions downstream of the drosocin transcribed sequences results in increased transcription levels, which are similar for the fusion and the resident drosocin genes upon infection. Analysis of transgenic fly lines showed that the drosocin reporter gene is constitutively expressed in the oviducts of egg-laying females.

Immune factor Gambif1, a new rel family member from the human malaria vector, Anopheles gambiae

A novel rel family member, Gambif1 (gambiae immune factor 1), has been cloned from the human malaria vector, Anopheles gambiae, and shown to be most similar to Drosophila Dorsal and Dif. Gambif1 protein is translocated to the nucleus in fat body cells in response to bacterial challenge, although the mRNA is present at low levels at all developmental stages and is not induced by infection. DNA binding activity to the kappaB-like sites in the A.gambiae Defensin and the Drosophila Diptericin and Cecropin promoters is also induced in larval nuclear extracts following infection. Gambif1 has the ability to bind to kappaB-like sites in vitro. Co-transfection assays in Drosophila mbn-2 cells show that Gambif1 can activate transcription by interacting with the Drosophila Diptericin regulatory elements, but is not functionally equivalent to Dorsal in this assay. Gambif1 protein translocation to the nucleus and the appearance of kappaB-like DNA binding activity can serve as molecular markers of activation of the immune system and open up the possibility of studying the role of defence reactions in determining mosquito susceptibility/refractoriness to malaria infection.

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

In Drosophila, bacterial challenge induces the rapid transcription of several genes encoding potent antibacterial peptides. The upstream sequences of the diptericin and cecropin Al genes, which have been investigated in detail, contain two, respectively one sequence element homologous to the binding site of the mammalian nuclear factor kappaB. These elements have been shown to be mandatory for immune-induced transcription of both genes. Functional studies have shown that these kappaB-related elements can be the target for the Drosophila Rel proteins dorsal and Dif. Here we present a comparative analysis of the transactivating capacities of these proteins on reporter genes fused to either the diptericin or the cecropin kappaB-related motifs. We conclude from our results: (i) the kappaB motifs of the diptericin and cecropin genes are not functionally equivalent; (ii) the dorsal and Dif proteins have distinct DNA-binding characteristics; (iii) dorsal and Dif can heterodimerize in vitro; (iv) mutants containing no copies of dorsal and a single copy of Dif retain their full capacity to express the diptericin and cecropin genes in response to challenge.

Innate immunity in higher insects

The hallmark of the innate immune response of higher insects is the rapid and transient synthesis of a battery of broad spectrum antimicrobial peptides by the fat body. The control of the genes encoding these peptides involves cis-regulatory promoter elements homologous to sequences functional in mammalian acute-phase genes. Study of immune-deficient mutants of Drosophila has indicated that distinct pathways control the antibacterial and antifungal responses in this species. Novel receptors potentially involved in the initiation of the immune response have been recently characterized.