A novel Rel protein and shortened isoform that differentially regulate antibacterial peptide genes in the silkworm Bombyx mori

CTL10 has multiple functions in the innate immune responses of the silkworm, Bombyx mori

Insect C-type lectins (CTLs) play crucial roles in modulating the humoral and cellular immune responses. In the domesticated silkworm Bombyx mori L., BmCTL10 gene encodes an immulectin containing two carbohydrate recognition domains (CRDs). The phylogenetic analysis showed that BmCTL10 didn't cluster with other immulectin homologs in B. mori. BmCTL10 was mainly expressed in second to fifth instar larvae, wandering stage larvae, prepupa, and adults. In naïve fifth instar larvae, BmCTL10 was predominantly expressed in the fat body and epidermis. In second instar larvae, the topical application of Beauveria bassiana by immersion caused down-regulation of BmCTL10. The intra-hemocoel injection of E. coli, S. aureus, B. bassiana, and 20-hydroxyecdysone in fifth instar larvae caused tissue and time-specific inductions. The recombinant protein (rBmCTL10) can bind to larval hemocytes and various pathogen-associated molecular patterns to enhance hemocyte-mediated nodulation, phagocytosis, and encapsulation. rBmCTL10 caused significant upregulation of most antimicrobial peptides and nitric oxide synthase 1 in hemocytes in vivo. Yeast two-hybrid demonstrated that integrin β3 and β4 subunits can interact with BmCTL10. Furthermore, only CRD2 can interact with the β3, while both CRD1 and CRD2 can interact with the β4. Taken together, this study showed that BmCTL10 has multiple functions in the innate immune responses of B. mori and two integrin β subunits are their potential receptors.

Fungal immunity and pathogenesis in mammals versus the invertebrate model organism Galleria mellonella

In recent decades, Galleria mellonella (Lepidoptera: Pyralidae) have emerged as a model system to explore experimental aspects of fungal pathogenesis. The benefits of the G. mellonella model include being faster, cheaper, higher throughput and easier compared with vertebrate models. Additionally, as invertebrates, their use is subject to fewer ethical and regulatory issues. However, for G. mellonella models to provide meaningful insight into fungal pathogenesis, the G. mellonella-fungal interactions must be comparable to mammalian-fungal interactions. Indeed, as discussed in the review, studies suggest that G. mellonella and mammalian immune systems share many similarities, and fungal virulence factors show conserved functions in both hosts. While the moth model has opened novel research areas, many comparisons are superficial and leave large gaps of knowledge that need to be addressed concerning specific mechanisms underlying G. mellonella-fungal interactions. Closing these gaps in understanding will strengthen G. mellonella as a model for fungal virulence in the upcoming years. In this review, we provide comprehensive comparisons between fungal pathogenesis in mammals and G. mellonella from immunological and virulence perspectives. When information on an antifungal immune component is unknown in G. mellonella, we include findings from other well-studied Lepidoptera. We hope that by outlining this information available in related species, we highlight areas of needed research and provide a framework for understanding G. mellonella immunity and fungal interactions.

Influence of Hyperproteinemia on Insect Innate Immune Function of the Circulatory System in Bombyx mori

Simple Summary

Hyperproteinemia, a condition of elevated protein levels in the blood, is associated with a diverse range of human and animal diseases. However, there is no reliable hyperproteinemia disease models or modeling methods in mammal or other organisms, and the effect of hyperproteinemia on immunity is still unknown. Our work succeeded in constructing an animal model of hyperproteinemia with no primary disease effects and a controllable plasma protein concentration (PPC) in an invertebrate model organism, Bombyx mori. Our work confirmed that high PPC enhances hemolymph phagocytosis via a rapid increase in granulocytes and inhibited hemolymph melanization due to inhibition of the prophenoloxidase (PPO) signaling pathway, and also upregulated the gene expression of antimicrobial peptides via activating the Toll and Imd pathways in NF-κB signaling, and showed an inconsistent antibacterial activity for Gram-positive and Gram-negative bacteria. Our results show that high PPC had multiple significant effects on the innate immune function of the silkworm circulatory system and is expected to be improved by endocrine hormones. Our work explores the pathogenesis of hyperproteinemia in an invertebrate model, and expands the scope for silkworm biomedical applications, even use for a potential drug development platform.

Abstract

Metabolic disorders of the circulatory system of animals (e.g., hyperglycemia and hyperlipidemia) can significantly affect immune function; however, since there is currently no reliable animal model for hyperproteinemia, its effects on immunity remain unclear. In this study, we established an animal model for hyperproteinemia in an invertebrate silkworm model, with a controllable plasma protein concentration (PPC) and no primary disease effects. We evaluated the influence of hyperproteinemia on innate immunity. The results showed that high PPC enhanced hemolymph phagocytosis via inducing a rapid increase in granulocytes. Moreover, while oenocytoids increased, the plasmacytes quickly dwindled. High PPC inhibited hemolymph melanization due to decreased phenoloxidase (PO) activity in the hemolymph via inhibiting the expression of the prophenoloxidase-encoding genes, PPO1 and PPO2. High PPC upregulated the gene expression of antimicrobial peptides via differential activation of the Toll and Imd signaling pathways associated with NF-κB signaling, followed by an induction of inconsistent antibacterial activity towards Gram-positive and Gram-negative bacteria in an animal model of high PPC. Therefore, high PPC has multiple significant effects on the innate immune function of the silkworm circulatory system.

TmDorX2 positively regulates antimicrobial peptides in Tenebrio molitor gut, fat body, and hemocytes in response to bacterial and fungal infection

Dorsal, a member of the nuclear factor-kappa B (NF-κB) family of transcription factors, is a critical downstream component of the Toll pathway that regulates the expression of antimicrobial peptides (AMPs) against pathogen invasion. In this study, the full-length ORF of Dorsal was identified from the RNA-seq database of the mealworm beetle Tenebrio molitor (TmDorX2). The ORF of TmDorX2 was 1,482 bp in length, encoding a polypeptide of 493 amino acid residues. TmDorX2 contains a conserved Rel homology domain (RHD) and an immunoglobulin-like, plexins, and transcription factors (IPT) domain. TmDorX2 mRNA was detected in all developmental stages, with the highest levels observed in 3-day-old adults. TmDorX2 transcripts were highly expressed in the adult Malpighian tubules (MT) and the larval fat body and MT tissues. After challenging the larvae with Staphylococcus aureus and Escherichia coli, the TmDorX2 mRNA levels were upregulated 6 and 9 h post infection in the whole body, fat body, and hemocytes. Upon Candida albicans challenge, the TmDorX2 mRNA expression were found highest at 9 h post-infection in the fat body. In addition, TmDorX2-knockdown larvae exposed to E. coli, S. aureus, or C. albicans challenge showed a significantly increased mortality rate. Furthermore, the expression of 11 AMP genes was downregulated in the gut and fat body of dsTmDorX2-injected larvae upon E. coli challenge. After C. albicans and S. aureus challenge of dsTmDorX2-injected larvae, the expression of 11 and 10 AMPs was downregulated in the gut and fat body, respectively. Intriguingly, the expression of antifungal transcripts TmTenecin-3 and TmThaumatin-like protein-1 and -2 was greatly decreased in TmDorX2-silenced larvae in response to C. albicans challenge, suggesting that TmDorX2 regulates antifungal AMPs in the gut in response to C. albicans infection. The AMP expression profiles in the fat body, hemocytes, gut, and MTs suggest that TmDorX2 might have an important role in promoting the survival of T. molitor larvae against all mentioned pathogens.

Immune gene regulation in the gut during metamorphosis in a holo- versus a hemimetabolous insect

During metamorphosis, holometabolous insects completely replace the larval gut and must control the microbiota to avoid septicaemia. Rapid induction of bactericidal activity in the insect gut at the onset of pupation has been described in numerous orders of the Holometabola and is best-studied in the Lepidoptera where it is under control of the 20-hydroxyecdysone (20E) moulting pathway. Here, using RNAseq, we compare the expression of immune effector genes in the gut during metamorphosis in a holometabolous (Galleria mellonella) and a hemimetabolous insect (Gryllus bimaculatus). We find that in G. mellonella, the expression of numerous immune effectors and the transcription factor GmEts are upregulated, with peak expression of three antimicrobial peptides (AMPs) and a lysozyme coinciding with delamination of the larval gut. By contrast, no such upregulation was detectable in the hemimetabolous Gr. bimaculatus. These findings support the idea that the upregulation of immune effectors at the onset of complete metamorphosis is an adaptive response, which controls the microbiota during gut replacement. This article is part of the theme issue 'The evolution of complete metamorphosis'.

Insect Transcription Factors: A Landscape of Their Structures and Biological Functions in Drosophila and beyond

Transcription factors (TFs) play essential roles in the transcriptional regulation of functional genes, and are involved in diverse physiological processes in living organisms. The fruit fly Drosophila melanogaster, a simple and easily manipulated organismal model, has been extensively applied to study the biological functions of TFs and their related transcriptional regulation mechanisms. It is noteworthy that with the development of genetic tools such as CRISPR/Cas9 and the next-generation genome sequencing techniques in recent years, identification and dissection the complex genetic regulatory networks of TFs have also made great progress in other insects beyond Drosophila. However, unfortunately, there is no comprehensive review that systematically summarizes the structures and biological functions of TFs in both model and non-model insects. Here, we spend extensive effort in collecting vast related studies, and attempt to provide an impartial overview of the progress of the structure and biological functions of current documented TFs in insects, as well as the classical and emerging research methods for studying their regulatory functions. Consequently, considering the importance of versatile TFs in orchestrating diverse insect physiological processes, this review will assist a growing number of entomologists to interrogate this understudied field, and to propel the progress of their contributions to pest control and even human health.

The DNA chaperone HMGB1 potentiates the transcriptional activity of Rel1A in the mosquito Aedes aegypti

High Mobility Group protein 1 (HMGB1) is a non-histone, chromatin-associated nuclear protein that functions in regulating eukaryotic gene expression. We investigated the influence and mechanism of action of Aedes aegypti HMGB1 (AaHMGB1) on mosquito Rel1A-mediated transcription from target gene promoters. The DNA-binding domain (RHD) of AaRel1A was bacterially expressed and purified, and AaHMGB1 dramatically enhanced RHD binding to consensus NF-kB/Rel DNA response elements. Luciferase reporter analyses using a cecropin gene promoter showed that AaHMGB1 potentiates the transcriptional activity of AaRel1A in Aag-2 cells. Moreover, overexpression of AaHMGB1 in Aag-2 cells led to an increase in mRNA levels of antimicrobial peptide genes. In vitro GST pull-down assays revealed that the presence of DNA is a pre-requisite for assembly of a possible ternary complex containing DNA, AaHMGB1 and AaRel1A. Notably, DNA bending by AaHMGB1 enhanced the binding of AaRel1A to a DNA fragment containing a putative NF-kB/Rel response element. Importantly, AaHMGB1 was identified as a potential immune modulator in A. aegypti through AaHMGB1 overexpression or RNAi silencing in Aag-2 cells followed by bacterial challenge or through AaHMGB1 RNAi knockdown in mosquitoes followed by Dengue virus (DENV) infection. We propose a model in which AaHMGB1 bends NF-kB/Rel target DNA to recruit and allow more efficient AaRel1A binding to activate transcription of effector genes, culminating in a stronger Toll pathway-mediated response against DENV infection.

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.

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.

Characterization of the Bombyx mori Cecropin A1 promoter regulated by IMD pathway

Cecropin A1 (CecA1) promoter from Bombyx mori was cloned and characterized to provide insight into the transcriptional control of this antimicrobial peptide gene upon immune challenges. Reporter gene assays demonstrated that both Escherichia coli and lipopolysaccharide could induce expression in BmE cells but B. bombyseptieus or peptidoglycan failed, and the induction pattern of the reporter gene was coincident with the endogenous CecA1. Analysis of deletion and mutation constructs revealed that the regulatory region was the κB motif located between -176 and -166, and no other predicted elements on CecA1 promoter affected its inducibility. Insertion of additional κB motifs increased the activity of CecA1 promoter. Furthermore, binding of Relish to κB motif was confirmed by electrophoretic mobility shift assay. These findings indicate the regulatory mechanism of CecA1 expression in IMD pathway and suggest an approach of engineering antimicrobial peptide promoter with enhanced activities that may lead to broad applications.

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.

A regulatory pathway, ecdysone-transcription factor relish-cathepsin L, is involved in insect fat body dissociation

Insect fat body is the organ for intermediary metabolism, comparable to vertebrate liver and adipose tissue. Larval fat body is disintegrated to individual fat body cells and then adult fat body is remodeled at the pupal stage. However, little is known about the dissociation mechanism. We find that the moth Helicoverpa armigera cathepsin L (Har-CL) is expressed heavily in the fat body and is released from fat body cells into the extracellular matrix. The inhibitor and RNAi experiments demonstrate that Har-CL functions in the fat body dissociation in H. armigera. Further, a nuclear protein is identified to be transcription factor Har-Relish, which was found in insect immune response and specifically binds to the promoter of Har-CL gene to regulate its activity. Har-Relish also responds to the steroid hormone ecdysone. Thus, the dissociation of the larval fat body is involved in the hormone (ecdysone)-transcription factor (Relish)-target gene (cathepsin L) regulatory pathway.

Insect fat body is the intermediary metabolism organ and the main source of hemolymph components, and it is crucial for insect development and metamorphosis. However, molecular mechanism for the fat body remodeling is almost unknown other than in Drosophila melanogaster. A pupal diapause species the cotton bollworm, Helicoverpa armigera (Har), is a useful model to study individual or tissue remodeling, because larval fat body will remain integral in diapause-type pupae for months, whereas the dissociation of larval fat body will start on day 0 after pupation in nondiapause-type ones. Here, we find that H. armigera cathepsin L (Har-CL) is released from fat body cells into the extracellular matrix for tissue dissociation. A nuclear protein is identified to be transcription factor Har-Relish, which regulates the promoter activity of Har-CL gene. Har-Relish also responds to the steroid hormone ecdysone. Thus, a new regulatory mechanism, ecdysone-Relish-cathepsin L signaling pathway, is involved in the larval fat body dissociation.

Activation of autophagic programmed cell death and innate immune gene expression reveals immuno-competence of integumental epithelium in Bombyx mori infected by a dipteran parasitoid

In insects, the integument forms the primary barrier between the environment and internal milieu, but cellular and immune responses of the integumental epithelium to infection by micro- and macro-parasites are mostly unknown. We elucidated cellular and immune responses of the epithelium induced through infection by a dipteran endoparasitoid, Exorista bombycis in the economically important silkworm Bombyx mori. Degradative autophagic vacuoles, lamella-like bodies, a network of cytoplasmic channels with cellular cargo, and an RER network that opened to vacuoles were observed sequentially with increase in age after infection. This temporal sequence culminated in apoptosis, accompanied by the upregulation of the caspase gene and fragmentation of DNA. The infection significantly enhanced the tyrosine level and phenol oxidase activity in the integument. Proteomic analysis revealed enhanced expression of innate immunity components of toll and melanization pathways, cytokines, signaling molecules, chaperones, and proteolytic enzymes demonstrating diverse host responses. qPCR analysis revealed the upregulation of spatzle, BmToll, and NF kappa B transcription factors Dorsal and BmRel. NF kappa B inhibitor cactus showed diminished expression when Dorsal and BmRel were upregulated, revealing a negative correlation (R = (-)0.612). During melanization, prophenol oxidase 2 was expressed, a novel finding in integumental epithelium. The integument showed a low level of melanin metabolism and localized melanism in order to prevent the spreading of cytotoxic quinones. The gene-encoding proteolytic enzyme, beta-N-acetylglucosaminidase, was activated at 24 h post-infection, whereas chitinase, was activated at 96 h post-infection; however, most of the immune genes enhanced their expression in the early stages of infection. Thus the integument contributes to humoral immune responses that enhance resistance against macroparasite invasion.

Overview on the recent study of antimicrobial peptides: origins, functions, relative mechanisms and application

Antimicrobial peptides (AMPs), which are produced by several species including insects, other animals, micro-organisms and synthesis, are a critical component of the natural defense system. With the growing problem of pathogenic organisms resistant to conventional antibiotics, especially with the emergence of NDM-1, there is increased interest in the pharmacological application of AMPs. They can protect against a broad array of infectious agents, such as bacteria, fungi, parasite, virus and cancer cells. AMPs have a very good future in the application in pharmaceuticals industry and food additive. This review focuses on the AMPs from different origins in these recent years, and discusses their various functions and relative mechanisms of action. It will provide some detailed files for clinical research of pharmaceuticals industry and food additive in application.

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.

Bombyx mori transcription factors: genome-wide identification, expression profiles and response to pathogens by microarray analysis

Transcription factors are present in all living organisms, and play vital roles in a wide range of biological processes. Studies of transcription factors will help reveal the complex regulation mechanism of organisms. So far, hundreds of domains have been identified that show transcription factor activity. Here, 281 reported transcription factor domains were used as seeds to search the transcription factors in genomes of Bombyx mori L. (Lepidoptera: Bombycidae) and four other model insects. Overall, 666 transcription factors including 36 basal factors and 630 other factors were identified in B. mori genome, which accounted for 4.56% of its genome. The silkworm transcription factors' expression profiles were investigated in relation to multiple tissues, developmental stages, sexual dimorphism, and responses to oral infection by pathogens and direct bacterial injection. These all provided rich clues for revealing the transcriptional regulation mechanism of silkworm organ differentiation, growth and development, sexual dimorphism, and response to pathogen infection.

Differential inhibition of BmRelish1-dependent transcription in lepidopteran cells by bracovirus ankyrin-repeat proteins

In the tripartite parasitization system of the lepidopteran host Manduca sexta, the endoparasitoid wasp Cotesia congregata and its endosymbiotic virus, C. congregata Bracovirus (CcBV), the expression of viral proteins is necessary for successful parasitization. Here we have examined the in vitro effects of six members of the ankyrin-repeat protein family (Ank) of CcBV, which are thought to interfere with the host's induced innate immune responses, on the transcriptional activity of a heterologous lepidopteran Rel/NFκB transcription factor, Relish1 of Bombyx mori. Using as transcriptional activator BmRelish1-d2 (R1d2), a constitutively active mutant of the major regulator of the Imd pathway, BmRelish1, in conjunction with a reporter gene controlled by a B. mori antimicrobial peptide gene promoter, we have found that 5 of the 6 examined Anks suppress R1d2-dependent transcriptional activity to various degrees. Immunofluorescence studies have also revealed that while some of the Ank proteins have a rather strict cytoplasmic localization, others are detected both in the cytoplasm and the nucleus of the expressing cells and that colocalization with R1d2 occurs exclusively in the nucleus. Thus, our results suggest that functional and spatial differences among the various CcBV Ank family members may be responsible for the observed differential inhibition of R1d2 activity.

Immunity in lepidopteran insects

Lepidopteran insects provide important model systems for innate immunity of insects, particularly for cell biology of hemocytes and biochemical analyses of plasma proteins. Caterpillars are also among the most serious agricultural pests, and understanding of their immune systems has potential practical significance. An early response to infection in lepidopteran larvae is the activation of hemocyte adhesion, leading to phagocytosis, nodule formation, or encapsulation. Plasmatocytes and granular cells are the hemocyte types involved in these responses. Infectious microorganisms are recognized by binding of hemolymph plasma proteins to microbial surface components. This "pattern recognition" triggers phagocytosis and nodule formation, activation of prophenoloxidase and melanization and the synthesis of antimicrobial proteins that are secreted into the hemolymph. Many hemolymph proteins that function in such innate immune responses of insects were first discovered in lepidopterans. Microbial proteinases and nucleic acids released from lysed host cells may also activate lepidopteran immune responses. Hemolymph antimicrobial peptides and proteins can reach high concentrations and may have activity against a broad spectrum of microorganisms, contributing significantly to clearing of infections. Serine proteinase cascade pathways triggered by microbial components interacting with pattern recognition proteins stimulate activation of the cytokine Spätzle, which initiates the Toll pathway for expression of antimicrobial peptides. A proteinase cascade also results inproteolytic activation of phenoloxidase and production of melanin coatings that trap and kill parasites and pathogens. The proteinases in hemolymph are regulated by specific inhibitors, including members of the serpin superfamily. New developments in lepidopteran functional genomics should lead to much more complete understanding of the immune systems of this insect group.

An efficient binary system for gene expression in the silkworm, Bombyx mori, using GAL4 variants

A binary gene expression system using the yeast GAL4 DNA-binding protein and the upstream activating sequence (UAS) of galactose-driven yeast genes is an established and powerful tool for the analysis of gene function. However, in the domesticated silkworm, Bombyx mori, this system has been limited in its utility by the relatively low transcriptional activation activity of GAL4 and by its toxicity. In this study, we investigated the potential of several established GAL4 variants (GAL4Δ, GAL4VP16, GAL4VPmad2, GAL4VPmad3, and GAL4NFκB) and of two new GAL4 variants, GAL4Rel and GAL4Relish, which contain the transcription-activating regions of the BmRel and BmRelish genes, respectively, to improve the utility of the GAL4/UAS system in B. mori. We generated constructs containing these GAL4 variants under the control of constitutive or inducible promoters and investigated their transcription-activating activity in cultured B. mori cells and embryos and in transgenic silkworms. GAL4VP16 and GAL4NFκB exhibited high transactivation activity but appeared to be toxic when used as transgenes under the control of a constitutive promoter. Similarly, GAL4VPmad2 and GAL4VPmad3 exhibited higher transactivation activity than GAL4, combined with strong toxicity. The transcription-activating activity of GAL4Δ was about twice that of GAL4. The two new GAL4 variants, GAL4Rel and GAL4Relish, were less active than GAL4. Using GAL4VP16 and GAL4NFκB constructs, we have developed a very efficient GAL4/UAS binary gene expression system for use in cultured B. mori cells and embryos and in transgenic silkworms.

Expression profiling of novel bacteria-induced genes from the silkworm, Bombyx mori

In this study, we have newly identified three bacteria-induced genes from the silkworm Bombyx mori by quantitative reverse transcriptase-polymerase chain reaction. One of these, eukaryotic initiation factor 4E-1 (eIF4E-1), is assumed to encode an eIF4E family, which plays a role in the initiation of translation as a mRNA cap-binding protein. The second gene is BmFOXG1, belonging to a family of forkhead transcription factors, FOXG1. The third gene is MBF2-related (MBF2-R) whose product has high homology to a co-activator protein MBF2 from B. mori. Although BmFOXG1 was up-regulated in the fat body in response to three kinds of bacteria, Escherichia coli, Staphylococcus aureus, and Bacillus subtilis, eIF4E-1 and MBF2-R were up-regulated by E. coli and B. subtilis, but not S. aureus, suggesting that bacteria possessing meso-diaminopimelic acid-containing peptidoglycan but not lysine-containing peptidoglycan activate eIF4E-1 and MBF2-R, probably through a conserved immune deficiency pathway. We further profiled the expression of three genes in different tissues and a silkworm cell line, NIAS-Bm-aff3, in response to bacteria, and at different times after bacterial challenge in the fat body.

A genome-wide survey for host response of silkworm, Bombyx mori during pathogen Bacillus bombyseptieus infection

Host-pathogen interactions are complex relationships, and a central challenge is to reveal the interactions between pathogens and their hosts. Bacillus bombysepticus (Bb) which can produces spores and parasporal crystals was firstly separated from the corpses of the infected silkworms (Bombyx mori). Bb naturally infects the silkworm can cause an acute fuliginosa septicaemia and kill the silkworm larvae generally within one day in the hot and humid season. Bb pathogen of the silkworm can be used for investigating the host responses after the infection. Gene expression profiling during four time-points of silkworm whole larvae after Bb infection was performed to gain insight into the mechanism of Bb-associated host whole body effect. Genome-wide survey of the host genes demonstrated many genes and pathways modulated after the infection. GO analysis of the induced genes indicated that their functions could be divided into 14 categories. KEGG pathway analysis identified that six types of basal metabolic pathway were regulated, including genetic information processing and transcription, carbohydrate metabolism, amino acid and nitrogen metabolism, nucleotide metabolism, metabolism of cofactors and vitamins, and xenobiotic biodegradation and metabolism. Similar to Bacillus thuringiensis (Bt), Bb can also induce a silkworm poisoning-related response. In this process, genes encoding midgut peritrophic membrane proteins, aminopeptidase N receptors and sodium/calcium exchange protein showed modulation. For the first time, we found that Bb induced a lot of genes involved in juvenile hormone synthesis and metabolism pathway upregulated. Bb also triggered the host immune responses, including cellular immune response and serine protease cascade melanization response. Real time PCR analysis showed that Bb can induce the silkworm systemic immune response, mainly by the Toll pathway. Anti-microorganism peptides (AMPs), including of Attacin, Lebocin, Enbocin, Gloverin and Moricin families, were upregulated at 24 hours post the infection.

Cloning and characterization of Dorsal homologues in the hemipteran Rhodnius prolixus

Rhodnius prolixus is an ancient haematophagous hemipteran insect capable of mounting a powerful immune response. This response is transcriptionally regulated in part by transcription factors of the Rel/Nuclear Factor kappa B (Rel/NF-kappaB) family. We have cloned and characterized three members of this transcription factor family in this insect. Dorsal 1A is primarily expressed in early developmental stages. In contrast, dorsal 1B and 1C, both differentially spliced products of dorsal 1A, are expressed primarily in the adult fat body in response to septic injury, suggesting their exclusive role in immunity. Additionally, we identified putative kappaB binding sites in the 5' upstream regions of target genes known to be involved in the innate immune response of insects.

Differential activation of the lectin and antimicrobial peptide genes in Sarcophaga peregrina (the flesh fly)

Sarcophaga lectin is an immune defense protein which is transcriptionally induced upon immune challenge in the flesh fly, Sarcophaga peregrina. So far, we have revealed that the Sarcophaga lectin gene has multiple NF-kappaB -binding motifs in its promoter. Here we showed that the nuclear extracts from Sarcophaga-derived culture cells, NIH-Sape-4, and larval fat bodies have binding activity to the multiple kappaB motifs in the lectin gene promoter, some of which were responsive to immune stimuli. We also compared the expression profiles of the lectin gene with those of the antibacterial peptide genes from the point of view of inducers, expression tissues and local induction in digestive tracts. In each case, the lectin gene was activated in different manners from other inducible defense genes. These results indicate the complex regulation of the lectin gene, possibly by NF-kappaB -related transcription factors.

Functional analysis of four Gloverin-like genes in the silkworm, Bombyx mori

To identify genes involved in the innate immunity of the silkworm Bombyx mori, we constructed a cDNA library from the fat body of Escherichia coli-challenged B. mori larvae. Based on the expressed sequence tag (EST) data and whole genome shotgun sequence analysis, we found four Gloverin-like genes, BmGlov1-4, in the Bombyx genome. Northern blot and RT-PCR analysis showed that BmGlov1-4 were induced in the larval fat body after an immune challenge by the injection of E. coli; however, less induction was observed after the injection of a yeast Candida albicans. In silico sequence analysis revealed the presence of a motif homologous to NF-kappaB binding site in the upstream region of each BmGlov gene. Moreover, we expressed recombinant BmGlov1-4 proteins using the baculovirus expression system, and found that all the recombinant BmGlov1-4 significantly inhibited the growth of E. coli.

Identification and analysis of Toll-related genes in the domesticated silkworm, Bombyx mori

Silkworm (Bombyx mori), a model system for Lepidoptera, has contributed enormously to the study of insect immunology especially in humoral immunity. But little is known about the molecular mechanism of immune response in the silkworm. Toll receptors are a group of evolutionarily ancient proteins, which play a crucial role in the innate immunity of both insects and vertebrates. In human, Toll-like receptors (TLRs) are the typical pattern recognition receptors for different kinds of pathogen molecules. Toll-related receptors in Drosophila, however, were thought to function as cytokine receptors in immune response and embryogenesis. We have identified 11 putative Toll-related receptors and two Toll analogs in the silkworm genome. Phylogenetic analysis of insect Toll family and human TLRs showed that BmTolls is grouped with Drosophila Tolls and Anopheles Tolls. These putative proteins are typical transmembrane receptors flanked by the extracellular leucine-rich repeat (LRR) domain and the cytoplasmic TIR domain. Structural prediction of the TIR domain alignment found five stranded sheets and five helices, which are alternatingly joined. Microarray data indicated that BmToll and BmToll-2 were expressed with remarkable enrichment in the ovary, suggesting that they might play a role in the embryogenesis. However, the enriched expression of BmToll-2 and -4 in the midgut suggested that the proteins they encode may be involved in immune defense. Testis-specific expression of BmToll-10 and -11 and BmToLK-2 implies that these may be involved in sex-specific biological functions. The RT-PCR results indicated that 10 genes were induced or suppressed with different degrees after their immune system was challenged by different invaders. Expression profiles of BmTolls and BmToLKs reported here provide insight into their role in innate immunity and development.

Prominent down-regulation of storage protein genes after bacterial challenge in eri-silkworm, Samia cynthia ricini

We constructed two independent cDNA libraries from the fat body of Escherichia coli- or Candida albicans-challenged eri-silkworm Samia cynthia ricini larvae. We performed comparative expressed sequence tag (EST) analysis of the two cDNA libraries and found that two putative storage protein genes, ScSP1 and ScSP2, were markedly repressed by E. coli injection as compared with C. albicans injection. By quantitative real-time RT-PCR analysis, we showed that ScSP1 mRNA significantly reduced to 1/32-1/3 in the fat body of the female larvae, and ScSP2 mRNA reduced to 1/7-1/3 and 1/22-1/5 in the females and males, respectively, 12-36 h after E. coli injection as compared with PBS injection. In addition, SDS-PAGE analysis revealed that the accumulation of both the ScSP proteins in the larval hemolymph apparently decreased up to 36 h after E. coli injection. However, the amounts of the two ScSP proteins returned to the same level as those in the larvae injected with PBS by 48 h after injection, showing that the reduction in ScSPs caused by the bacterial challenge was transient. Moreover, potential binding sites for the Drosophila Rel/NF-kappaB protein Dorsal were found in the 5' upstream regulatory regions of ScSP1 and ScSP2, suggesting the participation of the Rel/NF-kappaB proteins in controlling the bacterial suppression of the ScSP genes. These results suggested the hypothesis that S. c. ricini has a genetic program to shut down temporarily dispensable gene expression in order to induce an acute and efficient expression of immune-related genes. These findings may provide new insight into the innate immune system in lepidopteran insects.

Expression of antimicrobial peptide genes encoding Enbocin and Gloverin isoforms in the silkworm, Bombyx mori

Antimicrobial peptides, Enbocin and Gloverin isoforms from the silkworm Bombyx mori, were analyzed for expression of these peptide genes. Tissue-specific expression of Enbocin and Bmgloverin isoform genes was observed mainly in the fat body upon injection of Escherichia coli. Peptidoglycan and lipopolysaccharide triggered expression of these genes in vivo. On the other hand, lipid A activated Bmgloverin isoform genes but not Enbocin isoform genes. These results illustrate the fact that expression of Enbocin and Bmgloverin isoform genes is inducible by bacteria and that the effects of bacterial cell wall components on the activation of these peptide genes are not necessarily the same. In addition, selective activation of the Enbocin2, Bmgloverin2, and Bmgloverin4 genes by BmRelB rather than BmRelA was observed, providing additional evidence for the occurrence of selective activation of antimicrobial peptide genes by a Rel protein. These results suggest complex regulatory mechanisms in insect antimicrobial peptide genes by bacterial cell wall components.

Identification and functional analysis of Relish homologs in the silkworm, Bombyx mori

Two cDNAs designated BmRelish1 and 2, that encode Relish homologs, were cloned from the silkworm, Bombyx mori. BmRelish1 had an IkappaB-like domain with 5 ankyrin repeats in addition to Rel homology domain (RHD), nuclear localization signal (NLS), and acidic and hydrophobic amino acids (AHAA) rich regions. On the other hand, BmRelish2 lacked the AHAA and ankyrin repeats (ANK). Knockdown of the BmRelish gene in transgenic silkworms resulted in failure of the activation of antimicrobial peptide genes by Escherichia coli, suggesting that BmRelish plays an important role in antimicrobial peptide gene expression. Functional analysis of BmRelish1 and 2 in mbn-2 cells showed that both Relish homologs do not activate promoters of B. mori antimicrobial peptide genes encoding cecropin B1, attacin, lebocin 3 and lebocin 4. However, a gene construct BmRelish1-d2 lacking the ANK strongly activated promoters of these genes. Another gene construct lacking AHAA and ANK failed to activate these genes, suggesting that BmRelish becomes active by removal of the ANK and that the AHAA-rich region is a transactivation domain. BmRelish2 was shown to repress activation of Cecropin B1 gene expression by BmRelish1-d2, suggesting that BmRelish2 plays a role as a dominant negative factor against the BmRelish1 active form. Necessity of kappaB sites of Cecropin B1, Attacin and Lebocin 4 genes for the full activation of these genes by BmRelish1-d2 was confirmed. The requirement of the mandatory kappaB sites for Lebocin 4 gene expression was different between BmRelish1 active form and BmRelA, suggesting differential roles for kappaB sites in antimicrobial peptide gene activation by different transcription factors. The binding of the RHD portion of BmRelish1 fusion protein to the kappaB sites of Cecropin B1 and Attacin genes was also confirmed.

Characterization and expression of a cecropin-like gene from Helicoverpa armigera

A cDNA encoding a cecropin-like antibacterial peptide was obtained by RT-PCR from cotton budworm (Helicoverpa armigera). The cloned cDNA consists of 773 nucleotides with a 192 bp open reading frame encoding a peptide of 63 aa, which is comprised of a 21 aa signal peptide and a 42 amino acids mature peptide. The amino acid sequence of the mature peptide is highly similar to those D-type cecropins. The peptide was named as HacD. RT-PCR revealed that the transcript of HacD gene was inducible and could be detected in fatbodies, midguts, hemocytes and Malpighian tubules. HacD was highly expressed in E. coli M15 by fusing with green fluorescent protein (GFP). After purification, desalting and cleavage with factor Xa, HacD was released and showed antibacterial activity to both Gram-positive and Gram-negative bacteria. The genomic DNA of HacD was amplified by TAIL-PCR. NF-kappaB and NF-IL6 binding sites were found in the 5'-upstream regulatory region of HacD gene. EMSA (electrophoretic mobility shift assay) revealed that nuclear proteins from the immunized larvae could bind to the NF-kappaB site, but no nuclear proteins were found to bind to the NF-IL6 site. It was proposed that the NF-kappaB site might contribute to the expression of HacD.

Lepidopteran ortholog of Drosophila breathless is a receptor for the baculovirus fibroblast growth factor

The Bombyx mori nucleopolyhedrovirus (BmNPV) encodes a gene homologous to the mammalian fibroblast growth factor (FGF) family. We report the cloning of B. mori and Spodoptera frugiperda orthologous genes (Bmbtl and Sfbtl, respectively) of Drosophila melanogaster breathless (btl) encoding a receptor for Branchless/FGF and show that these genes encode the receptor for a baculovirus-encoded FGF (vFGF). Sequence analysis showed that BmBtl is composed of 856 amino acid residues, which potentially encodes a 97.3-kDa polypeptide and shares structural features and sequence similarities with the FGF receptor family. Reverse transcription-PCR experiments showed that Bmbtl was abundantly expressed in the trachea and midgut in B. mori larvae, with moderate expression observed in the hemocytes and the B. mori cultured cell line BmN. We generated Sf-9 cells that stably expressed His-tagged BmBtl. Western blot analysis revealed that BmBtl was an approximately 110-kDa protein. Immunoprecipitation experiments showed that BmNPV vFGF markedly phosphorylated BmBtl in Sf-9 cells. In addition, we found that BmBtl overexpression enhanced the migration activity for BmNPV vFGF. Furthermore, we generated Sf-9 cells in which Sfbtl was knocked down by transfection with double-strand RNA-expressing plasmids. In these cells, cell motility triggered by vFGF was markedly reduced. These results strongly suggest that the Btl orthologs, BmBtl and SfBtl, are the receptors for vFGF, which mediate vFGF-induced host cell chemotaxis.

SAGE analysis of early oogenesis in the silkworm, Bombyx mori

To identify genes involved in the differentiation of Bombyx cystoblast, we constructed two 3' long serial analysis of gene expression (Long SAGE) libraries from stage 1-3 or stage 2-3 egg chambers and compared their gene expression profiles. In both libraries, the most frequent tags were derived from the same novel transcript. The transcript does not have any open reading frame capable of encoding a protein with over 100 amino acids in length. RNA blot analysis revealed that this transcript is specifically and abundantly expressed in the Bombyx ovary, mainly the germ line cells in the ovarioles. These results suggest that Bombyx oogenesis may be regulated by a previously unidentified non-coding RNA. Comparison of the gene expression profiles between the stage 1-3 and stage 2-3 egg chamber libraries revealed that 272 tags were significantly more abundant in stage 1-3 egg chambers (p<0.05 and at least two-fold change) than in library 2. Among the differentially expressed transcripts were the sequences that correspond to ATP synthase subunit d (3.1-fold enriched) and ATP synthase coupling factor 6 (9.1-fold enriched), suggesting that they are involved in regulation of cell cycle of cystocytes.

Magnetocapture of abalone transcription factor NF-κB: A new strategy for isolation and detection of NF-κB both in vitro and in vivo

Rel\NF-kappaB plays an important role in the immune response. Recently, a homologue of Rel\NF-kappaB transcription factor was identified from a gastropod abalone, Haliotis diversicolor supertexta, named Ab-Rel. We present here a new strategy, so called magnetocapture, based on magnetic separation that was developed to isolate and detect this abalone transcriptional factor. With this method, Ab-Rel was successfully isolated from abalone haemocytes and the recombinant Ab-Rel protein from an insect cell line. Western blot analysis was performed to verify the effectiveness of this magnetocapture assay. Since only the activated NF-kappaB can be captured and separated from the whole cell extracts, this method was simultaneously used to detect the NF-kappaB activity in vivo, as well as the biological activity of the recombinant protein in vitro. In addition, several proteins which might interact with the abalone NF-kappaB were together isolated, suggesting that the magnetocapture method would be useful for identifying interaction molecules. These results illustrated that the magnetocapture strategy is rapid, reliable and versatile, providing a promising tool for studying Rel\NF-kappaB protein.

Nuclear factor-kappa B pathways in Drosophila

The nuclear factor kappa B (NF-kappaB) pathways in Drosophila are multi-component pathways, as in vertebrates, that regulate the expression of many genes responsible for the formation of dorsal-ventral polarity in the early embryo, the innate immune response to infection with Gram- negative and positive bacteria and fungi, the cellular immune response and hematopoiesis. Overactivation of the fly pathway can result in developmental defects, overproliferation of hemocytes and the formation of melanotic tumors or nodules. The extracellular events leading to the maturation of the ligand for initiation of the Drosophila NF-kappaB pathway is not conserved between flies and vertebrates, but the Toll receptor and downstream events are remarkably similar. NF-kappaB proteins have been identified in mollusks, and arthropods such as horseshoe crabs and beetles, indicating that this pathway has been established more than 500 million years ago. The fly NF-kappaB pathways are less complex than those in vertebrates, with the involvement of fewer proteins, but they are, nonetheless, just as important as their vertebrate counterparts for the life of the fly.

A transgenic silkworm expressing the immune-inducible cecropin B-GFP reporter gene

To analyze cecropin B promoter (P-CecB) activity in vivo, we constructed transgenic silkworms that expressed EGFP under the control of P-CecB using the piggyBac transposable element. Genomic Southern blot analysis of the G1 and G2 generations indicated the stable insertion of EGFP in the genome. Injection of Escherichia coli cells into the larvae strongly induced EGFP expression in the fat bodies and all five hemocyte cell types. Northern blot analysis indicated that the expression kinetics of EGFP in the fat bodies following bacterial injection were correlated with that of endogenous CecB. Flow cytometric analysis of the hemocytes revealed that EGFP expression was increased by bacteria, but not by yeast. Our results indicate that the features of EGFP expression in the transgenic silkworm are equivalent to those of endogenous CecB and that P-CecB activation can be monitored by EGFP expression using transgenic silkworms.