The humoral antibacterial response of Drosophila

The Development of New Methods to Stimulate the Production of Antimicrobial Peptides in the Larvae of the Black Soldier Fly Hermetia illucens

(1) The global population is projected to reach a staggering 9.8 billion people by the year 2050, leading to major concerns about food security. The necessity to increase livestock production is inevitable. The black soldier fly (BSF) is known for its ability to consume a wide range of organic waste, and BSF larvae have already been used as a partial substitute for fishmeal. In contrast, the use of antibiotics in livestock feed for growth promotion and prophylaxis poses a severe threat to global health owing to antimicrobial resistance. Insect antimicrobial peptides (AMPs) have shown the potential to rapidly disrupt target bacterial membranes, making bacterial resistance to AMPs a less likely concern. (2) In this study, we explored various methods for stimulating AMP synthesis in BSF larvae and found that thermal injury effectively induced the production of various AMP types. Additionally, we investigated the activation of innate immune response pathways that lead to AMP production following thermal injury. (3) Interestingly, thermal injury treatment, although not involving bacteria, exhibited a similar response to that observed following Gram-positive bacterial infection in eliciting the expression of AMP genes. (4) Our findings offer support for the industrial use of BSF to enhance livestock production and promote environmental health.

Ecdysone signaling mediates the trade-off between immunity and reproduction via suppression of amyloids in the mosquito Aedes aegypti

The balance between immunity and reproduction is essential for many key physiological functions. We report that to maintain an optimal fertility, 20-hydroxyecdysone (20E) and the ecdysone receptor (EcR) downregulate the immune deficiency (IMD) pathway during the post blood meal phase (PBM) of the Aedes aegypti reproductive cycle. RNA interference-mediated depletion of EcR elicited an increased expression of the IMD pathway components, and these mosquitoes were more resistant to infection by Gram-negative bacteria. Moreover, 20E and EcR recruit Pirk-like, the mosquito ortholog of Drosophila melanogaster Pirk. CRISPR-Cas9 knockout of Pirk-like has shown that it represses the IMD pathway by interfering with IMD-mediated formation of amyloid aggregates. 20E and EcR disruption of the amyloid formation is pivotal for maintaining normal yolk protein production and fertility. Additionally, 20E and its receptor EcR directly induce Pirk-like to interfere with cRHIM-mediated formation of amyloid. Our study highlights the vital role of 20E in governing the trade-off between immunity and reproduction. Pirk-like might be a potential target for new methods to control mosquito reproduction and pathogen transmission.

OMN6 a novel bioengineered peptide for the treatment of multidrug resistant Gram negative bacteria

New antimicrobial agents are urgently needed, especially to eliminate multidrug resistant Gram-negative bacteria that stand for most antibiotic-resistant threats. In the following study, we present superior properties of an engineered antimicrobial peptide, OMN6, a 40-amino acid cyclic peptide based on Cecropin A, that presents high efficacy against Gram-negative bacteria with a bactericidal mechanism of action. The target of OMN6 is assumed to be the bacterial membrane in contrast to small molecule-based agents which bind to a specific enzyme or bacterial site. Moreover, OMN6 mechanism of action is effective on Acinetobacter baumannii laboratory strains and clinical isolates, regardless of the bacteria genotype or resistance-phenotype, thus, is by orders-of-magnitude, less likely for mutation-driven development of resistance, recrudescence, or tolerance. OMN6 displays an increase in stability and a significant decrease in proteolytic degradation with full safety margin on erythrocytes and HEK293T cells. Taken together, these results strongly suggest that OMN6 is an efficient, stable, and non-toxic novel antimicrobial agent with the potential to become a therapy for humans.

Septate junction components control Drosophila hematopoiesis through the Hippo pathway

Hematopoiesis requires coordinated cell signals to control the proliferation and differentiation of progenitor cells. In Drosophila, blood progenitors, called prohemocytes, which are located in a hematopoietic organ called the lymph gland, are regulated by the Salvador-Warts-Hippo pathway. In epithelial cells, the Hippo pathway integrates diverse biological inputs, such as cell polarity and cell-cell contacts, but Drosophila blood cells lack the conspicuous polarity of epithelial cells. Here, we show that the septate-junction components Cora and NrxIV promote Hippo signaling in the lymph gland. Depletion of septate-junction components in hemocytes produces similar phenotypes to those observed in Hippo pathway mutants, including increased differentiation of immune cells. Our analysis places septate-junction components as upstream regulators of the Hippo pathway where they recruit Merlin to the membrane. Finally, we show that interactions of septate-junction components with the Hippo pathway are a key functional component of the cellular immune response following infection.

Antimicrobial Peptides and Their Therapeutic Potential for Bacterial Skin Infections and Wounds

Alarming data about increasing resistance to conventional antibiotics are reported, while at the same time the development of new antibiotics is stagnating. Skin and soft tissue infections (SSTIs) are mainly caused by the so called ESKAPE pathogens (Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter species) which belong to the most recalcitrant bacteria and are resistant to almost all common antibiotics. S. aureus and P. aeruginosa are the most frequent pathogens isolated from chronic wounds and increasing resistance to topical antibiotics has become a major issue. Therefore, new treatment options are urgently needed. In recent years, research focused on the development of synthetic antimicrobial peptides (AMPs) with lower toxicity and improved activity compared to their endogenous counterparts. AMPs appear to be promising therapeutic options for the treatment of SSTIs and wounds as they show a broad spectrum of antimicrobial activity, low resistance rates and display pivotal immunomodulatory as well as wound healing promoting activities such as induction of cell migration and proliferation and angiogenesis. In this review, we evaluate the potential of AMPs for the treatment of bacterial SSTIs and wounds and provide an overview of the mechanisms of actions of AMPs that contribute to combat skin infections and to improve wound healing. Bacteria growing in biofilms are more resistant to conventional antibiotics than their planktonic counterparts due to limited biofilm penetration and distinct metabolic and physiological functions, and often result in chronification of infections and wounds. Thus, we further discuss the feasibility of AMPs as anti-biofilm agents. Finally, we highlight perspectives for future therapies and which issues remain to bring AMPs successfully to the market.

Host and Bacterial Factors Control Susceptibility of Drosophila melanogaster to Coxiella burnetii Infection

Coxiella burnetii is the causative agent of Q fever, a zoonotic disease that threatens both human and animal health. Due to the paucity of experimental animal models, little is known about how host factors interface with bacterial components and affect pathogenesis. Here, we used Drosophila melanogaster, in conjunction with the biosafety level 2 (BSL2) Nine Mile phase II (NMII) clone 4 strain of C. burnetii, as a model to investigate host and bacterial components implicated in infection. We demonstrate that adult Drosophila flies are susceptible to C. burnetii NMII infection and that this bacterial strain, which activates the immune deficiency (IMD) pathway, is able to replicate and cause mortality in the animals. We show that in the absence of Eiger, the only known tumor necrosis factor (TNF) superfamily homolog in Drosophila, Coxiella-infected flies exhibit reduced mortality from infection. We also demonstrate that the Coxiella type 4 secretion system (T4SS) is critical for the formation of the Coxiella-containing vacuole and establishment of infection in Drosophila Altogether, our data reveal that the Drosophila TNF homolog Eiger and the Coxiella T4SS are implicated in the pathogenesis of C. burnetii in flies. The Drosophila/NMII model mimics relevant aspects of the infection in mammals, such as a critical role of host TNF and the bacterial T4SS in pathogenesis. Our work also demonstrates the usefulness of this BSL2 model to investigate both host and Coxiella components implicated in infection.

The PLA2 gene mediates the humoral immune responses in Bactrocera dorsalis (Hendel)

The phospholipase A2 (PLA2) gene encodes the enzyme that catalyzes the hydrolysis of phospholipids (PLs) from the sn-2 position. However, little is known about its role in humoral immune responses. In this study, we investigated the expression profile of PLA2 in different tissues and developmental stages in Bactrocera dorsalis (Hendel), and the results showed that the transcriptional level of PLA2 was high in the egg and mature stage and in the testis tissue. Bacterial infection increased the expression of PLA2, and the highest degree of up-regulation appeared in the fat body. Silencing PLA2 influenced the expression of immune-related genes, including MyD88 and defensin in the Toll pathway and relish and diptericin in the Imd pathway. Moreover, the expression of MyD88 and defensin was down-regulated significantly in the ds-PLA2 group compared with those in the ds-egfp group when B. dorsalis was infected with L. monocytogenes and S. aureus, indicating that PLA2 was involved in the activation of the Toll pathway. Meanwhile, infection with L. monocytogenes and E. coli, which activate the Imd pathway, does not increase the mRNA levels of relish and diptericin in the ds-PLA2 group as severely as it increases those in the ds-egfp group, indicating that the Imd pathway was also repressed after silencing PLA2. Notably, the development of lipid droplets in fat body cells was influenced by silencing PLA2, implying that PLA2 affects the function of fat body tissue. These results suggest that the PLA2 gene may mediate humoral immune responses by reducing lipid storage in fat body cells in B. dorsalis.

Canalization of gene expression is a major signature of regulatory cold adaptation in temperate Drosophila melanogaster

BACKGROUND

Transcriptome analysis may provide means to investigate the underlying genetic causes of shared and divergent phenotypes in different populations and help to identify potential targets of adaptive evolution. Applying RNA sequencing to whole male Drosophila melanogaster from the ancestral tropical African environment and a very recently colonized cold-temperate European environment at both standard laboratory conditions and following a cold shock, we seek to uncover the transcriptional basis of cold adaptation.

RESULTS

In both the ancestral and the derived populations, the predominant characteristic of the cold shock response is the swift and massive upregulation of heat shock proteins and other chaperones. Although we find ~25 % of the genome to be differentially expressed following a cold shock, only relatively few genes (n = 16) are up- or down-regulated in a population-specific way. Intriguingly, 14 of these 16 genes show a greater degree of differential expression in the African population. Likewise, there is an excess of genes with particularly strong cold-induced changes in expression in Africa on a genome-wide scale.

CONCLUSIONS

The analysis of the transcriptional cold shock response most prominently reveals an upregulation of components of a general stress response, which is conserved over many taxa and triggered by a plethora of stressors. Despite the overall response being fairly similar in both populations, there is a definite excess of genes with a strong cold-induced fold-change in Africa. This is consistent with a detrimental deregulation or an overshooting stress response. Thus, the canalization of European gene expression might be responsible for the increased cold tolerance of European flies.

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

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

Analysis of genes expression of Spodoptera exigua larvae upon AcMNPV infection

Background

The impact of Autographa californica multicapsid nucleopolyhedrovirus (AcMNPV) infection on host gene expression in Spodoptera exigua 4th instar larvae was investigated through the use of 454 sequencing-based RNA-seq of cDNA libraries developed from insects challenged with active AcMNPV or heat-inactivated AcMNPV.

Methodology/Principal Findings

By comparing the two cDNA libraries, we show that 201 host genes are significantly up-regulated and 234 genes are significantly down-regulated by active AcMNPV infection. Down-regulated host genes included genes encoding antimicrobial peptides, namely three gloverin isoforms and an attacin, indicating that the viral infection actively repressed the expression of a portion of the host immune gene repertoire. Another interesting group of down-regulated host genes included genes encoding two juvenile hormone binding proteins and a hexamerin, all of which are involved in juvenile hormone regulation. The expression of these genes was enhanced by the topical application of Juvenile Hormone III (JHIII) in the insects challenged with heat-inactivated AcMNPV. However, infection with the active virus strongly suppresses the expression of these three genes, regardless of the absence or presence of JHIII.

Conclusions/Significance

Using RNA-seq, we have identified groups of immune-regulated and juvenile hormone-regulated genes that are suppressed by infection with active AcMNPV. This information and further studies on the regulation of host gene expression by AcMNPV will provide the tools needed to enhance the utility of the virus as an effective protein expression system and as an insecticide.

Substitution of the GalNAc-α-O-Thr¹¹ residue in drosocin with O-linked glyco-peptoid residue: effect on antibacterial activity and conformational change

One of the obvious disadvantages of natural peptides is their liability to proteases. Among the several solutions for this issue, peptoids or oligomers of N-substituted glycine have emerged as a promising tool that may enhance the stability of proteolysis-susceptible natural peptides. We have synthesized the drosocin and its glyco-peptoid analogues linked O-GalNAc at the Thr(11) residue. One of our glyco-peptoid analogues showed an increased antibacterial activity by the modification of the Thr(11) residue with glyco-peptoid. Structure-activity relationship studies revealed that the antibacterial activity by glyco-peptoid drosocin requires three key elements: free hydroxyl group on the carbohydrate moiety, γ-methyl group of the Thr(11) residue derivative and (S)-configuration over (R)-configuration.

Drosomycin, an innate immunity peptide of Drosophila melanogaster, interacts with the fly voltage-gated sodium channel

Several peptide families, including insect antimicrobial peptides, plant protease inhibitors, and ion channel gating modifiers, as well as blockers from scorpions, bear a common CSalphabeta scaffold. The high structural similarity between two peptides containing this scaffold, drosomycin and a truncated scorpion beta-toxin, has prompted us to examine and compare their biological effects. Drosomycin is the most expressed antimicrobial peptide in Drosophila melanogaster immune response. A truncated scorpion beta-toxin is capable of binding and inducing conformational alteration of voltage-gated sodium channels. Here, we show that both peptides (i) exhibit anti-fungal activity at micromolar concentrations; (ii) enhance allosterically at nanomolar concentration the activity of LqhalphaIT, a scorpion alpha toxin that modulates the inactivation of the D. melanogaster voltage-gated sodium channel (DmNa(v)1); and (iii) inhibit the facilitating effect of the polyether brevetoxin-2 on DmNa(v)1 activation. Thus, the short CSalphabeta scaffold of drosomycin and the truncated scorpion toxin can maintain more than one bioactivity, and, in light of this new observation, we suggest that the biological role of peptides bearing this scaffold should be carefully examined. As for drosomycin, we discuss the intriguing possibility that it has additional functions in the fly, as implied by its tight interaction with DmNa(v)1.

Cloning and expression profiling of four antibacterial peptide genes from the bumblebee Bombus ignitus

Four antibacterial peptide genes (apidaecin, hymenoptaecin, abaecin, and defensin) were cloned from the bumblebee Bombus ignitus, and cDNAs and their genomic structures were sequenced and characterized. Comparative analysis revealed that the four antibacterial peptides of B. ignitus had similar characteristics to other bee antibacterial peptides identified to date. The transcriptional expression profiles of the four antibacterial peptide genes in the fat body of B. ignitus workers revealed that all four antibacterial peptide genes were acutely induced in a similar manner by PBS injection or LPS stimulation, indicating that antibacterial peptides from various classes are simultaneously expressed in a single insect upon infection or injury.

Drosophila as a model system for antibacterial peptides

As a defence against bacteria, infected insects synthesize cecropins and a large number of other bactericidal proteins and peptides. To understand this response and its possible relationship with similar systems in mammals, we need to characterize the induced components and how they act, as well as how this antibacterial response is initiated. To study the molecular basis for this response we cloned the genes for cecropins and other bactericidal peptides from Drosophila, 14 genes in total. The cecropin genes were selected as convenient markers for the immune response because they are strongly induced by different microbial substances. In contrast the lysozyme gene family is constitutively expressed in the digestive tract. We have developed an inducible blood cell line from Drosophila for studying the immune response in vitro. Using this system we are now investigating the function of membrane proteins and signal pathways in the transcriptional activation of immune genes in Drosophila.

Design of a functionally equivalent nonglycosylated analog of the glycopeptide antibiotic formaecin I

Various nonglycosylated analogs were designed in order to explore the role of glycosylation in formaecin I, an antibacterial glycopeptide of insect origin. The functional behavior of a designed nonglycosylated analog (P(7),endo P(8a),DeltaT(11))formaecin I was found to be similar to that of native glycosylated peptide. Both the peptides showed similar antibacterial activities against Escherichia coli and Salmonella strains. The designed nonglycosylated analog (P(7),endo P(8a),DeltaT(11))formaecin I has low binding affinity to LPS identical to that of native glycopeptide, formaecin I. Both the peptides have similar killing kinetics and are nontoxic to erythrocytes. Formaecin I and designed nonglycosylated (P(7),endo P(8a),DeltaT(11))formaecin I have no definite conformational features associated with them. The glycosylated residue of threonine in formaecin I and proline residues in designed peptide [(P(7),endo P(8a),DeltaT(11))formaecin I], possibly help in stabilizing the correct conformation that facilitates presentation of the peptide to its receptor. It is evident that a functionally equivalent nonglycosylated analog of native glycosylated antibacterial peptide can be designed by strategically modifying the sequence.

Cyclization of pyrrhocoricin retains structural elements crucial for the antimicrobial activity of the native peptide

Pyrrhocoricin is a naturally occurring antimicrobial peptide from the European fire bug Pyrrhocoris apterus. It has submicromolar activity against a range of Gram-negative bacterial strains and has created recent interest as a lead for the development of novel antibiotic compounds. In this study, we have used NMR spectroscopy to determine the solution structures of pyrrhocoricin and a synthetic macrocyclic derivative that has improved in vivo pharmaceutical properties. Native pyrrhocoricin is largely disordered in solution, but there is evidence of a subpopulation with ordered turn regions over residues 2-5, 4-7, and 16-19. The macrocyclic derivative incorporates a nine amino acid linker joining the N- and C-termini, which does not adversely affect the antimicrobial potency but leads to a broader spectrum of activity. The NMR data suggest that the turn conformations in the cyclic derivative are similar to those in the native form, thus implicating them in the biological function.

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.

Diptericin-like protein: an immune response gene regulated by the anti-bacterial gene induction pathway in Drosophila

Insects produce various anti-microbial peptides in response to injury and infection. In Drosophila, diptericin has previously been studied as an anti-bacterial immune response gene. Here, we report the cloning of the diptericin-like protein (dptlp) gene as a paralog of Drosophila diptericin. By comparison of their sequences, we found that the dptlp gene has all of the functional domains conserved in the diptericin gene and other anti-bacterial proteins. The dptlp gene was rapidly induced by bacterial infections and showed different time-dependent gene expression patterns from those of diptericin. Like diptericin, dptlp was specifically produced from the fat body, and its expression was strictly dependent on bacterial infections. In addition, the dptlp gene expression was almost completely abolished in the imd mutant, which implicates that its expression is regulated by the anti-bacterial arm of the Drosophila innate immune regulatory pathways. In support of this, we found GATA, interferon consensus responding element, and kappa B binding sites, which is known to be important for the proper expression of anti-bacterial genes, in the proximal promoter region of the dptlp gene. Taken together, our findings support that dptlp is a novel anti-bacterial peptide whose expression is regulated by the anti-bacterial immune response mechanism.

Antibacterial peptides isolated from insects

Insects are amazingly resistant to bacterial infections. To combat pathogens, insects rely on cellular and humoral mechanisms, innate immunity being dominant in the latter category. Upon detection of bacteria, a complex genetic cascade is activated, which ultimately results in the synthesis of a battery of antibacterial peptides and their release into the haemolymph. The peptides are usually basic in character and are composed of 20-40 amino acid residues, although some smaller proteins are also included in the antimicrobial repertoire. While the proline-rich peptides and the glycine-rich peptides are predominantly active against Gram-negative strains, the defensins selectively kill Gram-positive bacteria and the cecropins are active against both types. The insect antibacterial peptides are very potent: their IC50 (50% of the bacterial growth inhibition) hovers in the submicromolar or low micromolar range. The majority of the peptides act through disintegrating the bacterial membrane or interfering with membrane assembly, with the exception of drosocin, apidaecin and pyrrhocoricin which appear to deactivate a bacterial protein in a stereospecific manner. In accordance with their biological function, the membrane-active peptides form ordered structures, e.g. alpha-helices or beta-pleated sheets and often cast permeable ion-pores. Their cytotoxic properties were exploited in in vivo studies targeting tumour progression. Although the native peptides degrade quickly in biological fluids other than insect haemolymph, structural modifications render the peptides resistant against proteases without sacrificing biological activity. Indeed, a pyrrhocoricin analogue shows lack of toxicity in vitro and in vivo and protects mice against experimental Escherichia coli infection. Careful selection of lead molecules based on the insect antibacterial peptides may extend their utility and produce viable alternatives to the conventional antimicrobial compounds for mammalian therapy.

Horseshoe crab hemocyte-derived antimicrobial polypeptides, tachystatins, with sequence similarity to spider neurotoxins

Antimicrobial peptides, named tachystatins A, B, and C, were identified from hemocytes of the horseshoe crab Tachypleus tridentatus. Tachystatins exhibited a broad spectrum of antimicrobial activity against Gram-negative and Gram-positive bacteria and fungi. Of these tachystatins, tachystatin C was most effective. Tachystatin A is homologous to tachystatin B, but tachystatin C has no significant sequence similarity to tachystatins A and B. Tachystatins A and B showed sequence similarity to omega-agatoxin-IVA of funnel web spider venom, a potent blocker of voltage-dependent calcium channels. However, they exhibited no blocking activity of the P-type calcium channel in rat Purkinje cells. Tachystatin C also showed sequence similarity to several insecticidal neurotoxins of spider venoms. Tachystatins A, B, and C bound significantly to chitin. A causal relationship was observed between chitin binding activity and antifungal activity. Tachystatins caused morphological changes against a budding yeast, and tachystatin C had a strong cell lysis activity. The septum between mother cell and bud, a chitin-rich region, was stained by fluorescence-labeled tachystatin C, suggesting that the primary recognizing substance on the cell wall is chitin. As horseshoe crab is a close relative of the spider, tachystatins and spider neurotoxins may have evolved from a common ancestral peptide, with adaptive functions.

Range of activity and metabolic stability of synthetic antibacterial glycopeptides from insects

Antibacterial glycopeptides isolated from insects are exciting bio-oligomers because they represent a family of compounds in which the structural and functional effects of incorporating short O-linked sugars to protein fragments can be studied. Additionally, their high activity in vitro warrants detailed further drug development efforts. Due to the limited availability of the isolated material, we used synthetic glycopeptides and some analogs to investigate the range of activity of drosocin and pyrrhocoricin. While addition of the Gal-GalNAc disaccharide to the natural mid-chain position generally increased the antibacterial activity of drosocin, pyrrhocoricin lacking sugar appeared to be more potent, with an IC50 against Escherichia coli D22 of 150 nM. Although glycosylated drosocin was active against E. coli in the low microM range in vitro, this peptide was completely inactive when injected into mice. The lack of in vivo activity of drosocin could be explained by the unusually high degradation rate of the peptides in mammalian sera. The early degradation products were inactive in vitro. In contrast, the peptides were considerably more stable in insect hemolymph, where their natural activity is manifested.

A novel lectin from Sarcophaga. Its purification, characterization, and cDNA cloning

A novel C-type lectin that agglutinates rabbit red cells was purified from NIH-Sape-4 cells derived from the flesh fly (Sarcophaga peregrina), and its cDNA was isolated. This lectin, named granulocytin, appeared to be a trimer of a 20-kDa subunit consisting of 151 amino acid residues. The gene for granulocytin was activated in third instar larvae, and its expression was enhanced when the larval body wall was injured. In third instar larvae, granulocytin was found to be synthesized by hemocytes and secreted into the hemolymph. The molecular mass and gene expression patterns of granulocytin were very similar to those of Drosophila lectin that we reported previously (Haq, S., Kubo, T., Kurata, S., Kobayashi, A., and Natori, S. (1996) J. Biol. Chem. 271, 20213-20218). However, these two lectins showed amino acid identities of 20% at most, and no significant hapten sugar for granulocytin was identified.

Regulated nuclear import of Rel proteins in the Drosophila immune response

The Drosophila immune response uses many of the same components as the mammalian innate immune response, including signalling pathways that activate transcription factors of the Rel/NK-kappaB family. In response to infection, two Rel proteins, Dif and Dorsal, translocate from the cytoplasm to the nuclei of larval fat-body cells. The Toll signalling pathway, which controls dorsal-ventral patterning during Drosophila embryogenesis, regulates the nuclear import of Dorsal in the immune response, but here we show that the Toll pathway is not required for nuclear import of Dif. Cytoplasmic retention of both Dorsal and Dif depends on Cactus protein; nuclear import of Dorsal and Dif is accompanied by degradation of Cactus. Therefore the two signalling pathways that target Cactus for degradation must discriminate between Cactus-Dorsal and Cactus-Dif complexes. We identified new genes that are required for normal induction of transcription of an antibacterial peptide during the immune response. Mutations in three of these genes prevent nuclear import of Dif in response to infection, and define new components of signalling pathways involving Rel. Mutations in three other genes cause constitutive nuclear localization of Dif; these mutations may block Rel protein activity by a novel mechanism.

The genes encoding the antibacterial sex-specific peptides ceratotoxins are clustered in the genome of the medfly Ceratitis capitata

Ceratotoxins are antibacterial peptides produced in the female reproductive accessory glands of the medfly Ceratitis capitata. Their expression is not affected by bacterial infection, but is enhanced after mating and is modulated by juvenile hormone. Three different peptides, named ceratotoxins A, B and C, have been previously purified from the female accessory gland secretion and their amino acid and cDNA sequences have been determined. We report here the complete nucleotide sequences of four genes encoding closely related ceratotoxin peptides. One of them encodes a novel peptide, which we named ceratotoxin D. Restriction and nucleotide sequence analysis indicate that these ceratotoxin genes are organized in a large cluster spanning more than 26 kilobases of DNA. All ceratotoxin genes are coordinately expressed. Ceratotoxin transcripts appear in 2-3 day old adult females, and they reach a maximum in 6-7 day old females. The presence of highly conserved motifs in the upstream regions of all the sequenced ceratotoxin genes suggests the presence of common regulatory elements for all ceratotoxins.

A novel member of lebocin gene family from the silkworm, Bombyx mori

We screened genomic clones encoding lebocin, an antibacterial peptide from the silkworm, Bombyx mori. Two positive clones were obtained and their nucleotide sequences indicated that they contain no introns. The deduced amino acid sequences revealed that one clone (Leb 3) encoded lebocin 3 and another (Leb 4) is a new member of the lebocin gene family. Gene expression of both Leb 3 and Leb 4 was shown to be induced by lipopolysaccharide and to occur tissue-specifically in the fat body and hemocytes. Our results suggest that lebocin as well as cecropin forms a multiple gene family in B. mori.

A JNK signal transduction pathway that mediates morphogenesis and an immune response in Drosophila

The Drosophila MAP kinase DJNK is a homolog of the mammalian c-Jun amino-terminal kinase (JNK). Mutations in the DJNK gene correspond to the complementation group basket. DJNK is phosphorylated and activated by the Drosophila MAP kinase kinase HEP. Substrates of DJNK include the transcription factor DJun. DJNK participates in multiple physiological processes. Exposure to endotoxic lipopolysaccharide initiates an insect immune response and leads to DJNK activation. In addition, embryos lacking DJNK are defective in dorsal closure, a process in which the lateral epithelial cells migrate over the embryo and join at the dorsal midline. These data demonstrate that the DJNK signal transduction pathway mediates an immune response and morphogenesis in vivo.

Enlarged scale chemical synthesis and range of activity of drosocin, an O-glycosylated antibacterial peptide of Drosophila

Insects respond to a bacterial challenge by rapidly synthesizing a diverse range of antibacterial and antifungal peptides. One of them, drosocin, a 19-residue proline-rich antibacterial peptide, was isolated from Drosophila. This peptide carries a disaccharide moiety attached to a threonine residue in mid-chain position. The present report describes the enlarged-scale chemical synthesis of drosocin, glycosylated with Gal (beta 1 --> 3)GalNAc(alpha 1 --> O). We have studied the range of activity of the synthetic glycopeptide, of two truncated glycosylated isoforms, and of the unglycosylated L and D enantiomers. Both isolated and chemically synthesized drosocins carrying the disaccharide display the same antibacterial activity. Using circular dichroic spectroscopy we demonstrated that the O-linked disaccharidic motif did not affect the backbone conformation of drosocin. The antibacterial activity of the synthetic glycopeptide was directed against gram-negative strains with the exception of the gram-positive bacteria Micrococcus luteus. Deletion of the first five N-terminal residues completely abolished the activity of drosocin. As a first approach to the study of the mode of action of drosocin, we have synthesized a non-glycosylated D enantiomer and, using this molecule, we have shown that drosocin may act on the gram-negative bacteria through a stereospecific target.

Inducible expression of an antibiotic peptide gene in lipopolysaccharide-challenged tracheal epithelial cells

Mammals continually confront microbes at mucosal surfaces. A current model suggests that epithelial cells contribute to defense at these sites, in part through the production of broad-spectrum antibiotic peptides. Previous studies have shown that invertebrates can mount a host defense response characterized by the induction in epithelia] cells of a variety of antibiotic proteins and peptides when they are challenged with microorganisms, bacterial cell wall/membrane components, or traumatic injury [Boman, H.G. & Hultmark, D. (1987) Annu. Rev. Microbiol. 41, 103-126J. However, factors that govern the expression of similar defense molecules in mammalian epithelial cells are poorly understood. Here, a 13-fold induction of the endogenous gene encoding tracheal antimicrobial peptide was found to characterize a host response of tracheal epithelia] cells (TECs) exposed to bacterial lipopolysaccharide (LPS). Northern blot data indicated that TECs express CD14, a well-characterized LPS-binding protein known to mediate many LPS responses. A monoclonal antibody to CD14 blocked the observed tracheal antimicrobial peptide induction by LPS under serum-free conditions. Together the data support that CD14 of epithelial cell origin mediates the LPS induction of an antibiotic peptide gene in TECs, providing evidence for the active participation of epithelial cells in the host's local defense response to bacteria. Furthermore, the data allude to a conservation of this host response in evolution and suggest that a similar inducible pathway of host defense is prevalent at mucosal surfaces of mammals.

Characterization of an immunodeficiency mutant in Drosophila

Drosophila immunity and embryogenesis appear to be linked by an evolutionarily ancient signalling pathway, which includes the Rel-domain transcription factors Dif and dorsal, respectively, as well as a common inhibitor, cactus. Previous genetic screens have centered on maternal mutants that disrupt the dorsal pathway. In an effort to identify additional components that influence Rel-domain gene function we have conducted a search for immunodeficiency mutants in Drosophila. One such mutant, which maps near the Black cells (Bc) gene, causes a severe impairment of the normal immune response, including attenuated induction of several immunity genes. Survival assays indicate a positive correlation between the induction of these genes, particularly diptericin, and resistance to bacterial infection. These studies are consistent with the notion that insect anti-microbial peptides work synergistically by binding distinct targets within infecting pathogens. Evidence is also presented that non-specific acquired immunity results from the persistence of bacterial metabolites long after primary infection. We discuss the potential usefulness of this study with regard to the identification of conserved components of Rel signalling pathways.

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

Bacterial challenge of larvae or adults of Drosophila induces the rapid transcription of several genes encoding antibacterial peptides with a large spectrum of activity. One of these peptides, the 82-residue anti-gram negative diptericin, is encoded by a single intronless gene and we are investigating the control of expression of this gene. Previous studies using both transgenic experiments and footprint analysis have highlighted the role in the induction of this gene of a 30 nucleotide region which contains three partially overlapping motifs with sequence homology to mammalian NF-kappa B and NF-IL6 response elements and to the GAAANN sequence present in the interferon consensus response elements of some mammalian interferon-induced genes. We now show that the latter sequence binds in immune responsive tissues (fat body, blood cells) of Drosophila a approximately 45 kDa polypeptide which cross-reacts with a polyserum directed against mammalian interferon Regulatory Factor-I. Using a transfection assay of Drosophila tumorous blood cells, we show that the GAAANN sequence positively regulates the activity of the diptericin promoter. We propose that this motif cooperatively interacts with the other response elements in the regulation of the diptericin gene expression.

Identification of early genes in the Drosophila immune response by PCR-based differential display: the Attacin A gene and the evolution of attacin-like proteins

We are using the PCR-based differential display technique to isolate genes which are induced during the immune response in Drosophila. In this way, a cDNA clone for a member of the attacin family of antibacterial proteins was isolated. The corresponding Attacin A (Att A) gene is localized at 51A-B on the second chromosome, and it is closely linked to at least one more cross-hybridizing gene. Injection of bacteria induces a 0.8 kb transcript, with expression kinetics similar to that of cecropin. Drosophila attacin is most closely related to sarcotoxin II of Sarcophaga peregrina, but it lacks the extra domains that are unique to this protein, and the overall domain structure of the Att A gene product is identical to that of the attacins from Hyalophora cecropia.

cDNA sequence and expression of the ceratotoxin gene encoding an antibacterial sex-specific peptide from the medfly Ceratitis capitata (diptera)

Ceratotoxins are antibacterial 3-kDa molecular mass amphiphilic peptides isolated from the female reproductive accessory glands of the medfly Ceratitis capitata. They are physiologically related to bee melittin and show amino acid sequence homology with magainin peptides. In this paper, we report the complete sequence of cDNA coding for ceratotoxin A and the expression of the gene during the life cycle of the insect. Experimental data show that the ceratotoxin is a gene expressed exclusively in the imaginal stages and that it is female-specific, related to sexual maturity, and stimulated by mating. Differently from most antibacterial insect hemolymph peptides, it is not induced by microbial infection. Western blot analysis using an anti-ceratotoxin antibody indicates the female accessory glands as the only site where the production of the ceratotoxin peptide occurs.

Innate immunity of insects

Insects are particularly resistant to microorganisms. Their host-defense system relies on several innate reactions: upon injury, the immediate onset of two proteolytic cascades leading to localized blood clotting and to melanization, the latter process involving production of cytotoxic molecules (namely reactive oxygen intermediates); the phagocytosis of bacteria and the encapsulation of larger parasites by blood cells; the induced synthesis by the fat body of a battery of potent antimicrobial peptides/polypeptides which are secreted into the hemolymph where they act synergistically to kill the invading microorganisms. The insect host defence system shares many of the basic characteristics of the mammalian acute phase response, especially at the level of the coordinate control of gene expression, where similar cis-regulatory and inducible transactivators appear to play key functions. The powerful techniques developed to study the genetics of Drosophila provide a unique opportunity to dissect the development and differentiation of this primordial immune system and may contribute to our understanding of the innate immune response in higher organisms.

Permeabilization of the mitochondrial inner membrane by short cecropin-A-melittin hybrid peptides

A number of cecropin-A-melittin hybrid peptides have previously been shown to be potent antibacterial agents [Andreu, D., Ubach, J., Boman, A., Wahlin, B., Wade, D., Merrifield, R. B. & Boman, H. G. (1992) FEBS Lett. 296, 190-194]. In the present report we analyze their action on biological systems using rat liver mitochondria as a test system. We demonstrate that the longest peptide, cecropin-A-(1-8)-melittin(1-18) permeabilizes the mitochondrial inner membrane allowing the movement of both charged and non-charged solutes. Concentrations used have already been shown to be bactericidal. This effect is also demonstrated under respiring conditions where succinate oxidation is uncoupled. Shorter analogs also permeabilize mitochondria although at ten-fold higher concentrations. Heparin potentiates the peptide effects at low concentrations, while at high concentration it becomes inhibitory. We propose that the cecropin-melittin analogs disrupt the mitochondrial membrane in a detergent-like mode rather than by creating selective channels as had been previously suggested.

Individual and synergistic effects of rabbit granulocyte proteins on Escherichia coli

Affinity purification of crude acid extracts of rabbit polymorphonuclear leukocytes using Escherichia coli (J5) as adsorbent yields the bactericidal/permeability-increasing protein (BPI), two 15-kD species (p15s), and the two most potent (cationic) defensin species (neutrophil peptides [NP] -1 and -2). Tested in buffered isotonic medium, the relative antibacterial potency of these proteins against E. coli J5 is BPI (IC50 0.2 nM) > p15A (10 nM) > NP -1 (400 nM). Sublethal doses of p15A or NP-1 can synergize with BPI to decrease the dose required to inhibit the growth of E. coli by up to 50-fold. BPI and p15A display similar features of antibacterial action distinct from defensin NP-1, but NP-1 acts synergistically only with BPI and not with p15A. All aspects of the combined action of BPI and NP-1 resemble those observed with higher concentrations of BPI alone, implying that NP-1 enhances BPI potency. Neither NP-1 nor p15A alter the amount of BPI binding to E. coli but BPI enhances binding of p15A to E. coli, raising the possibility that synergy between these two proteins may occur at least partially at the level of binding. The potent synergistic actions of these proteins can also be demonstrated against serum-resistant clinical isolates of encapsulated E. coli tested in whole blood and plasma ex vivo, suggesting that such combined action may contribute to host defense in vivo.

Characterization and transcriptional profiles of a Drosophila gene encoding an insect defensin. A study in insect immunity

Insect defensins are a family of 4-kDa, cationic, inducible antibacterial peptides which bear six cysteine residues engaged in three intramolecular disulfide bridges. They owe their name to certain sequence similarities with defensins from mammalian neutrophiles and macrophages. We report the characterization of a novel defensin isoform from Drosophila and the cloning of the gene encoding a preprodefensin. The gene, which is intronless and present in a single copy/haploid genome, maps at position 46CD on the right arm of the second chromosome. The analysis of the upstream region of the gene reveals the presence of multiple putative cis-regulatory sequences similar to mammalian regulatory motifs of acute-phase-response genes. Transcriptional profiles indicate that the Drosophila defensin gene is induced by bacterial challenge with acute-phase kinetics. It is also expressed in the absence of immune challenge during metamorphosis. These and other data on the Drosophila defensin gene lead us to suggest that insect and mammalian defensins have evolved independently.

The inducible antibacterial peptides of insects

Insects respond to bacterial challenge by the rapid and transient synthesis of a large number of potent antibacterial peptides that are active against many different bacteria. Two families of inducible antibacterial peptides are well characterized: the cecropins and the insect defensins. A rapidly increasing number of proline- and glycine-rich peptides are reported from various insect species together with cecropins and insect defensins. In this review, Stéphane Cociancich, Philippe Bulet, Charles Hetru and Jules A. Hoffmann give an update of our current information on the induced antibacterial peptides.