Isolation and structure determination of a paralytic peptide from the hemolymph of the silkworm, Bombyx mori

Human gut associated Bacteroides and Akkermansia bacteria exhibit immunostimulatory activity in the silkworm muscle contraction assay

The immunoregulatory activity of human gut bacteria has attracted attention in recent years. To assess the innate immune-stimulatory activity of various samples in vivo efficiently, we previously introduced a silkwormbased assay as a novel alternative method. The method has been used for over a decade to screen for substances with potential physiological activity. In this study, we prepared heat-killed cells of four strains of human gut bacteria (Bacteroides ovatus, B. thetaiotaomicron, B. uniformis, and Akkermansia muciniphila) and assessed their innate immune-stimulatory activity within the silkworm model. Our findings indicate that the sample from either B. ovatus or B. thetaiotaomicron has immunostimulatory activity in the silkworm, in contrast to B. uniformis and A. muciniphila. Moreover, a pathogenicity assessment using the silkworm infection model was conducted to determine the safety of these bacterial strains for human consumption when considered as food ingredients. None of the four gut bacterial strains exhibited pathogenic effects in silkworms, with Pseudomonas aeruginosa serving as a positive control of the pathogenicity test. These results suggest that the silkworm-based assay can distinguish between the immunostimulatory effects of different human gut microbes and may enhance the safety evaluation of microbial ingredients.

The growth-blocking peptide is a dual regulator of development and immunity with biocontrol potential in Spodoptera frugiperda (Lepidoptera: Noctuidae)

Insect growth-blocking peptides (GBPs) are a family of cytokines found in several insect orders and are known for their roles in regulating development, paralysis, cell proliferation, and immune responses. Despite their diverse functions, the potential of GBPs as biocontrol targets against the pest Spodoptera frugiperda (Lepidoptera: Noctuidae) has not been fully explored. In this study, S. frugiperda GBP (SfGBP) was identified and functionally characterized. SfGBP is synthesized as a 146 amino acid proprotein with a 24 amino acid C-terminal active peptide (Glu123-Gly146). Predominant expression of SfGBP occurs in fourth to sixth instar larvae and in the larval fat body, with significant upregulation in response to pathogens and pathogen-associated molecular patterns. Injection of the synthetic active peptide into larvae induced growth retardation, delayed pupation, and increased survival against Beauveria bassiana infection. Conversely, RNA interference-mediated knockdown of SfGBP resulted in accelerated growth, earlier pupation, and decreased survival against B. bassiana infection. Further analysis revealed that SfGBP promoted SF9 cell proliferation and spreading, enhanced bacteriostatic activity of larval hemolymph, and directly inhibited germination of B. bassiana conidia. In addition, SfGBP enhanced humoral responses, such as upregulation of immunity-related genes and generation of reactive oxygen species, and cellular responses, such as nodulation, phagocytosis, and encapsulation. These results highlight the dual regulatory role of SfGBP in development and immune responses and establish it as a promising biocontrol target for the management of S. frugiperda.

Systematic Analysis of the Relationship among 3D Structure, Bioactivity, and Membrane Permeability of PF1171F, a Cyclic Hexapeptide with Paralyzing Effects on Silkworms

PF1171 hexapeptides, a family of cyclic hexapeptides produced by fungi, exhibit paralyzing effects on the larvae of silkworms via oral administration. To elucidate the structural features of PF1171 hexapeptides that are crucial for bioactivity, the relationship among 3D structure, bioactivity, and membrane permeability of PF1171F (the peptide with the highest bioavailability) was systematically analyzed through the synthesis of 22 analogues. The PF1171F analogues were prepared by the solid-phase synthesis of a linear precursor and subsequent solution-phase macrolactamization. Analysis by NMR spectroscopy and molecular modeling indicated that the major 3D conformations of PF1171F in various solvents resemble its X-ray crystal structure. The analogues with this conformation tend to exhibit potent paralysis against silkworms, indicating the significance of the conformation in the paralysis. The biological activity was dependent on the mode of administration, varying between hemolymph injection and oral administration. Parallel artificial membrane permeability assay (PAMPA) of the analogues revealed a correlation between membrane permeabilities and paralytic activity by hemolymph injection, indicating that the target molecule of PF1171F is present inside the cell membrane.

Paralytic peptide activates insect humoral immune response via epidermal growth factor receptor

Paralytic peptide (PP) activates innate immunity of silkworm Bombyx mori, inducing production of anti-microbial peptides (AMPs) and phagocytosis-related proteins; however the signal pathways of PP-dependent immune responses are not clear. In present study, we characterized BmE cells as a PP-responsive cell line by examining the expression of AMP genes and activation of p38 mitogen-activated protein kinase (p38 MAPK) under PP stimulation, and we also found PP directly binds to BmE cell membrane. Then we found that PP-dependent expression of AMP genes is suppressed by tyrosine kinase inhibitor (genistein) both in BmE cells and in fat body of silkworm larvae. Moreover, the specific tyrosine kinase epidermal growth factor receptor (EGFR) inhibitor (AG1478) attenuates PP-induced expression of AMP genes in BmE cells and fat body of silkworm and RNA interference (RNAi) to BmEGFR also suppresses PP-induced expression of AMP genes. Furthermore, the PP-induced p38 MAPK phosphorylation is inhibited by AG1478. Our results suggest that BmE cells can be used as a cell model to investigate the signal pathway of PP-dependent humoral immune response and receptor tyrosine kinase EGFR/p38 MAPK pathway is involved in the production of AMPs induced by PP.

Studies of host-pathogen interactions and immune-related drug development using the silkworm: interdisciplinary immunology, microbiology, and pharmacology studies

Innate immunity acts as a front-line barrier against invading pathogens, and the majority of the components are widely conserved among species. Regulation of innate immunity is important for overcoming infections and preventing self-damaging sepsis. Using the silkworm (Bombyx mori) as an animal model, we elucidated the activation processes of innate immunity with emphasis on a multifunctional insect cytokine called paralytic peptide. Moreover, we established an ex vivo system using silkworm larval specimens to quantitatively evaluate the immunostimulatory activity of natural compounds. We observed that overactivation of innate immunity in silkworms induces tissue damage followed by host death, resembling sepsis-induced multi-organ failure in humans. Here, we summarize our recent findings and propose the usefulness of the silkworm as an animal model for studying immune regulation and for evaluating compounds with the potential to regulate innate immunity.

Paralytic peptide: an insect cytokine that mediates innate immunity

Host animals combat invading pathogens by activating various immune responses. Modulation of the immune pathways by cytokines is critical for efficient pathogen elimination. Insects and mammals possess common innate immune systems, and individual immune pathways have been intensively studied over the last two decades. Relatively less attention, however, has been focused on the functions of cytokines in insect innate immunity. Here, we summarize our recent findings from studies of the insect cytokine, paralytic peptide, in the silkworm Bombyx mori. The content of this report was presented at the First Asian Invertebrate Immunity Symposium. Acute activation of paralytic peptide occurs via proteolysis after stimulation with the cell wall components of pathogens, leading to the induction of a wide range of cellular and humoral immune responses. The pathogenic bacterium Serratia marcescens suppresses paralytic peptide-dependent immune activation, which impairs host resistance. Studies of insect cytokines will broaden our understanding of the basic mechanisms underlying the interaction between host innate immunity and pathogenic agents.

Characteristics common to a cytokine family spanning five orders of insects

Growth-blocking peptide (GBP) is a member of an insect cytokine family with diverse functions including growth and immunity controls. Members of this cytokine family have been reported in 15 species of Lepidoptera, and we have recently identified GBP-like peptides in Diptera such as Lucilia cuprina and Drosophila melanogaster, indicating that this peptide family is not specific to Lepidoptera. In order to extend our knowledge of this peptide family, we purified the same family peptide from one of the tenebrionids, Zophobas atratus,(1) isolated its cDNA, and sequenced it. The Z. atratus GBP sequence together with reported sequence data of peptides from the same family enabled us to perform BLAST searches against EST and genome databases of several insect species including Coleoptera, Diptera, Hymenoptera, and Hemiptera and identify homologous peptide genes. Here we report conserved structural features in these sequence data. They consist of 19-30 amino acid residues encoded at the C terminus of a 73-152 amino acid precursor and contain the motif C-x(2)-G-x(4,6)-G-x(1,2)-C-[KR], which shares a certain similarity with the motif in the mammalian EGF peptide family. These data indicate that these small cytokines belonging to one family are present in at least five insect orders.

Drosophila growth-blocking peptide-like factor mediates acute immune reactions during infectious and non-infectious stress

Antimicrobial peptides (AMPs), major innate immune effectors, are induced to protect hosts against invading microorganisms. AMPs are also induced under non-infectious stress; however, the signaling pathways of non-infectious stress-induced AMP expression are yet unclear. We demonstrated that growth-blocking peptide (GBP) is a potent cytokine that regulates stressor-induced AMP expression in insects. GBP overexpression in Drosophila elevated expression of AMPs. GBP-induced AMP expression did not require Toll and immune deficiency (Imd) pathway-related genes, but imd and basket were essential, indicating that GBP signaling in Drosophila did not use the orthodox Toll or Imd pathway but used the JNK pathway after association with the adaptor protein Imd. The enhancement of AMP expression by non-infectious physical or environmental stressors was apparent in controls but not in GBP-knockdown larvae. These results indicate that the Drosophila GBP signaling pathway mediates acute innate immune reactions under various stresses, regardless of whether they are infectious or non-infectious.

Insect cytokine paralytic peptide (PP) induces cellular and humoral immune responses in the silkworm Bombyx mori

In the blood (hemolymph) of the silkworm Bombyx mori, the insect cytokine paralytic peptide (PP) is converted from an inactive precursor to an active form in response to the cell wall components of microorganisms and contributes to silkworm resistance to infection. To investigate the molecular mechanism underlying the up-regulation of host resistance induced by PP, we performed an oligonucleotide microarray analysis on RNA of blood cells (hemocytes) and fat body tissues of silkworm larvae injected with active PP. Expression levels of a large number of immune-related genes increased rapidly within 3 h after injecting active PP, including phagocytosis-related genes such as tetraspanin E, actin A1, and ced-6 in hemocytes, and antimicrobial peptide genes cecropin A and moricin in the fat body. Active PP promoted in vitro and in vivo phagocytosis of Staphyloccocus aureus by the hemocytes. Moreover, active PP induced in vivo phosphorylation of p38 mitogen-activated protein kinase (p38 MAPK) in the fat body. Pretreatment of silkworm larvae with ML3403, a pharmacologic p38 MAPK inhibitor, suppressed the PP-dependent induction of cecropin A and moricin genes in the fat body. Injection of active PP delayed the killing of silkworm larvae by S. aureus, whereas its effect was abolished by preinjection of the p38 MAPK inhibitor, suggesting that p38 MAPK activation is required for PP-dependent defensive responses. These findings suggest that PP acts on multiple tissues in silkworm larvae and acutely activates cellular and humoral immune responses, leading to host protection against infection.

Characterization of feeding-delaying factors from the silkworm Bombyx mori

In several phytophagous insects, feeding behavior occurs regularly. Recently, we demonstrated that feeding behavior in larvae of the silkworm Bombyx mori had a regular frequency. To address the control of the feeding cycle in B. mori, we aimed to characterize factors influencing feeding initiation and termination. Injection of extracts of the midgut, foregut, and fat body into starved Bombyx larvae delayed the initiation of feeding. This result indicates the presence in these tissues of factors capable of decreasing the likelihood of feeding initiation.

Cloning and Characterization of Bombyx mori PP-BP a Gene Induced by Viral Infection

The ENF peptide family, so termed after the consensus sequence in their amino termini (Glu-Asn-Phe-), is assumed to play multiple important roles in defense reactions, growth regulation, and homeostasis of Lepidopteran insects. The paralytic peptide of Bombyx mori (BmPP) is one such peptide that is involved in the paralytic and plasmatocyte-spreading activities in the hemocyte immune reaction. The growth-blocking peptide of Pseudaletia separata (PsGBP), which is also a member of the ENF peptide family, has similar functions that can reportedly be attenuated by the growth-blocking peptide-binding protein (GBP-BP). Using the fluorescent differential display (FDD) technique, the differential expression pattern of genes in highly susceptible silkworm strain 306 were analyzed, following infection with B. mori nuclear polyhedrosis virus (BmNPV), and a differential band (G12(782)) was obtained from the hemolymph RNA pools. Using 5'-RACE with a specially designed primer based on the FDD study, a 1,401 bp cDNA clone was obtained containing a 1,311 bp open reading frame (ORF, GenBank accession number DQ306881). The deduced protein was highly homologous in primary structure to GBP-BP and was termed B. mori paralytic peptide-binding protein (PP-BP). The B. mori PP-BP gene is organized into two exons and only one intron, using bioinformatics searches.Using RT-PCR analysis, it was found that the B. mori PP-BP gene was expressed almost exclusively in the hemolymph. Real-time quantitative PCR analysis indicated that the B. mori PP-BP mRNA level in B. mori strain 306 exposed to BmNPV was much higher than that in B. mori strain without the virus infection. This result implies that the B. mori PP-BP is related to the cellular immune response after BmNPV invades the hemolymph.

Insulin signaling is involved in hematopoietic regulation in an insect hematopoietic organ

Only a few extracellular hematopoietic factors have been identified in insects. We previously developed an in vitro culture system for the larval hematopoietic organ (HPO) of the silkworm Bombyx mori, and found that cell proliferation is linked to hemocyte discharge from the HPO. In this study, we tested hematopoietic activity of bombyxin, a peptide in the insulin family. When silkworm HPO was cultured with synthetic bombyxin-II, the number of discharged hemocytes increased in a dose-dependent manner, indicating that bombyxin promoted cell proliferation in the HPO. However, a neutralization experiment using anti-bombyxin-II antibody revealed that bombyxin is not the primary effector in larval plasma. Similarly, bovine insulin showed hematopoietic activity. Addition of molting hormone, 20-hydroxyecdysone, circumstantially enhanced the hematopoietic activity of bombyxin and insulin. Bombyxin and insulin induced phosphorylation of different sets of proteins in the HPO, suggesting that their signaling pathways are different.

The Gly-Gly Linker Region of the Insect Cytokine Growth-blocking Peptide Is Essential for Activity

Growth-blocking peptide (GBP) is a 25-amino acid cytokine isolated from the lepidopteran insect Pseudaletia separata. GBP exhibits various biological activities such as regulation of larval growth of insects, proliferation of a few kinds of cultured cells, and stimulation of a class of insect immune cells called plasmatocytes. The tertiary structure of GBP consists of a well structured core domain and disordered N and C termini. Our previous studies revealed that, in addition to the structured core, specific residues in the unstructured N-terminal region (Glu1 and Phe3) are also essential for the plasmatocyte-stimulating activity. In this study, a number of deletion, insertion, and site-directed mutants targeting the unstructured N-terminal residues of GBP were constructed to gain more detailed insight into the mode of interaction between the N-terminal region and GBP receptor. Alteration of the backbone length of the linker region between the core structure and N-terminal domain reduced plasmatocyte-stimulating activity. The substitutions of Gly5 or Gly6 in this linker region with more bulky residues, such as Phe and Pro, also remarkably reduced this activity. We conclude that the interaction of GBP with its receptor depends on the relative position of the N-terminal domain to the core structure, and therefore the backbone flexibility of Gly residues in the linker region is necessary for adoption of a proper conformation suited to receptor binding. Additionally, antagonistic experiments using deletion mutants confirmed that not only the core domain but also the N-terminal region of GBP are required for "receptor-binding," and furthermore Phe3 is a binding determinant of the N-terminal domain.

In vitro studies of hematopoiesis in the silkworm: cell proliferation in and hemocyte discharge from the hematopoietic organ

The lepidopteran hematopoietic process is poorly understood. We therefore examined the fundamental properties of hematopoiesis in the silkworm Bombyx mori using hematopoietic organ culture. In a medium containing larval plasma taken from the fourth day of the final larval stadium, over 50,000 hemocytes per hematopoietic organ were discharged within 48 h, with the number of cells comprising the hematopoietic organ simultaneously increasing from approximately 20,000 to 40,000. However, in the absence of plasma, cell numbers comprising the hematopoietic organ were unchanged and the number of discharged cells was much less. Hematopoietic organs cultured with plasma showed strong mitotic indices in a BrdU incorporation assay, but did not when cultured without plasma, indicating that plasma contains hematopoietic factor(s). The hematopoietic stimulation ability of larval plasma was observed from the last day of the penultimate larval stadium to the prepupal stage. The response of the hematopoietic organs to larval plasma was highest at the beginning of the final larval stadium and decreased with aging. Most cells discharged from the hematopoietic organ were plasmatocytes and prohemocytes, irrespective of location and developmental stage. Using this in vitro culture method, we tested the effects of 20-hydroxyecdysone (20E) and juvenile hormone-I (JH-I) on B. mori hematopoiesis. 20E showed a weak, but significant, hematopoietic activity, whereas JH-I did not, suggesting that a part of larval hematopoiesis is endocrinally regulated.

Effects of silkworm paralytic peptide on in vitro hematopoiesis and plasmatocyte spreading

Bombyx mori paralytic peptide (BmPP), a multifunctional cytokine-like molecule, is expressed in the hematopoietic organ-wing imaginal disc complex, suggesting that BmPP is involved in both immune response and the hematopoietic process. We studied the effects of BmPP on plasmatocytes and hematopoietic organs of the silkworm. BmPP (1 microM) stimulated spreading of circulating plasmatocytes, but the percentage of spread plasmatocytes was only 20%. Over 10 nM of BmPP, however, elicited prominent spreading in 70% of young plasmatocytes discharged from cultured hematopoietic organs. Cells in hematopoietic organs that were enzymatically dispersed did not spread even after adding 100 nM of BmPP, indicating that plasmatocytes acquired BmPP-sensitivity immediately after discharge. When cultured in a medium containing larval plasma, hematopoietic organs grew markedly and discharged a large number of hemocytes, over 95% of which were morphologically plasmatocytes. The hemocyte discharge was blocked in the medium containing BmPP dose-dependently, although hematopoietic organ growth was not suppressed. These results suggest that BmPP plays important roles both in hematopoietic regulation and in the hemocyte immune reaction of the silkworm.

Roles of aromatic residues in the structure and biological activity of the small cytokine, growth-blocking peptide (GBP)

Growth-blocking peptide (GBP) is a small (25 amino acids) insect cytokine with a variety of functions: controlling the larval development of lepidopteran insects, acting as a mitogen for various types of cultured cells, and stimulating insect blood cells. The aromatic residues of GBP (Phe-3, Tyr-11, and Phe-23) are highly conserved in the ENF peptide family found in lepidopteran insects. We investigated the relationship between the biological activities and structural properties of a series of GBP mutants, in which each of the three aromatic residues is replaced by a different residue. The results of the hemocytes-stimulating assays of GBP mutants indicated that Phe-3 is the key residue in this activity: Ala or Tyr replacement resulted in significant loss of the activity, but Leu replacement did not. The replacements of other aromatic residues hardly affected the activity. On the other hand, NMR analysis of the mutants suggested that Tyr-11 is a key residue for maintaining the core structure of GBP. Surprisingly, the Y11A mutant maintained its biological activity, although its native-like secondary structure was disordered. Detailed analyses of the (15)N-labeled Y11A mutant by heteronuclear NMR spectroscopy showed that the native-like beta-sheet structure of Y11A was induced by the addition of 2,2,2-trifluoroethanol. The results suggest that Y11A has a tendency to form a native-like structure, and this property may give the Y11A mutant native-like activity.

Solution structure of paralytic peptide of silkworm, Bombyx mori

Paralytic peptide of Bombyx mori (BmPP) is one of the multifunctional ENF-peptides; the name of "ENF" is the consensus N-terminal amino acid sequence of the family peptides. We revealed that BmPP significantly possesses growth-blocking activity and plasmatocyte-spreading activity and that its activity profiles are different from those of another ENF-family peptide, namely, the growth-blocking peptide of Pseudaletia separata (PsGBP). We also determined the NMR structures of BmPP and PsGBP under the same conditions, which revealed the structural differences of the first and second beta-turn regions between the two peptides. On the basis of our results, it can be considered that the tertiary structural difference in these peptides may cause their different profiles of growth-blocking activity.

Structure and activity of the insect cytokine growth-blocking peptide. Essential regions for mitogenic and hemocyte-stimulating activities are separate

Growth-blocking peptide (GBP) is a 25-amino acid insect cytokine found in Lepidopteran insects that possesses diverse biological activities such as larval growth regulation, cell proliferation, and stimulation of immune cells (plasmatocytes). The tertiary structure of GBP consists of a structured core that contains a disulfide bridge and a short antiparallel beta-sheet (Tyr(11)-Arg(13) and Cys(19)-Pro(21)) and flexible N and C termini (Glu(1)-Gly(6) and Phe(23)-Gln(25)). In this study, deletion and point mutation analogs of GBP were synthesized to investigate the relationship between the structure of GBP and its mitogenic and plasmatocyte spreading activity. The results indicated that deletion of the N-terminal residue, Glu(1), eliminated all plasmatocyte spreading activity but did not reduce mitogenic activity. In contrast, deletion of Phe(23) along with the remainder of the C terminus destroyed all mitogenic activity but only slightly reduced plasmatocyte spreading activity. Therefore, the minimal structure of GBP containing mitogenic activity is 2-23 GBP, whereas that with plasmatocyte spreading activity is 1-22 GBP. NMR analysis indicated that these N- and C-terminal deletion mutants retained a similar core structure to wild-type GBP. Replacement of Asp(16) with either a Glu, Leu, or Asn residue similarly did not alter the core structure of GBP. However, these mutants had no mitogenic activity, although they retained about 50% of their plasmatocyte spreading activity. We conclude that specific residues in the unstructured and structured domains of GBP differentially affect the biological activities of GBP, which suggests the possibility that multifunctional properties of this peptide may be mediated by different forms of a GBP receptor.

Molecular cloning of silkworm paralytic peptide and its developmental regulation

The silkworm paralytic peptide (PP) is a member of the ENF peptide family that exerts multiple biological activities involved in defense reaction and growth regulation. We isolated its cDNA and examined mRNA expression profiles. cDNA encoded 131 amino acids from which the 23-residue PP sequence was found at the C-terminal portion. Immunoblot analysis and paralytic activity assay indicated that inactive pro-protein in larval hemolymph was processed into active peptide immediately after bleeding. In the last larval instar, 0.6-kb PP mRNA was expressed in various tissues, of which the fat body was predominant. Its expression in the fat body decreased during the feeding period and then increased during metamorphic process. Juvenile hormone and 20-hydroxyecdysone upregulated its expression. At the embryonic stage, 1.5-kb mRNA, in addition to 0.6-kb mRNA, was expressed from 1 day after oviposition to hatching. PP was thus expressed stage-specifically under hormonal control.