Molecular cloning and characterization of cDNA for insect biogenic peptide, growth-blocking peptide

Identification of a cytokine combination that protects insects from stress

Growth-blocking peptide (GBP) and stress-responsive peptide (SRP) are insect cytokines whose expression levels are elevated by various stressful conditions such as parasitization and high or low temperatures. Both GBP and SRP are synthesized as precursors and released into the hemolymph, where they are enzymatically processed to active peptides. Injection of active GBP or SRP into early last instar larvae elicits a reduction in feeding and consequent growth retardation in the armyworm Mythimna separata. Although such functions are thought to benefit insects under stressful conditions by affecting their physiologies and behaviors, the relationship between GBP and SRP remains elusive. Here we show that heat stress-induced reactive oxygen species (ROS) elevated hemolymph GBP, which activated SRP transcription and increased the SRP concentration in the hemolymph. Injection of both GBP and SRP elevated hemolymph antioxidant levels. We found that simultaneous increases in both active cytokines occurred in the larval hemolymph from 2 to 3 h after heat stress or H₂O₂ injection, suggesting a synergic action of the two factors. This speculation was confirmed by demonstrating that co-injection of GBP and SRP caused a more severe reduction in appetite and growth retardation than injection of an individual peptide alone. However, injection of GBP together with SRP did not elevate SRP expression at all, indicating the effect of negative feedback regulation. Furthermore, SRP RNAi larvae showed higher body weights compared to controls, and GBP-induced growth retardation was partially abrogated in SRP RNAi larvae. These results led us to conclude that GBP is an upstream cytokine in the regulation of SRP expression and that these cytokines synergistically retard larval growth by repressing feeding activities when insects are exposed to stress conditions.

Enhanced expression of stress-responsive cytokine-like gene retards insect larval growth

Growth rates of immature animals are governed by their feeding activities. A reduction in feeding sometimes causes serious growth retardation in insects; a typical case is often seen in host insects parasitized by a solitary endoparasitoid wasp. However, understanding of the mechanisms underlying the physiological repression of parasitized insects is fragmentary. Here we analyzed brain gene expression of the host common cutworm, Spodoptera litura, parasitized by a solitary endoparasitoid, Microplitis manilae, and identified a novel gene whose expression was significantly enhanced by parasitization. The gene encoded a pre-pro-peptide of a cytokine-like molecule and its expression was observed mainly in nervous tissues, hemocytes, and integuments. The 25 amino acid cytokine-like peptide encoded by the C-terminus of this gene was demonstrated to exist in the hemolymph of S. litura larvae and to change hemocytes from non-adhesive to adhesive in vitro. Further, injection of the active peptide reduced feeding activities of test larvae and consequently delayed their growth. The enhanced gene expression was also observed in larvae under severe stress conditions: abdominal ligature, proleg cutting, mechanical vibration, low temperature, and heat shock at 45°C. Elevated gene expression was maintained only in seriously growth-retarded larvae but not in recovered larvae at 24h or 48h after heat treatment. Thus, it is reasonable to conclude that stress-induced elevation of the peptide gene expression highly correlates with reduced feeding activities and growth retardation of the host larvae parasitized by M. manilae. Based on the conclusion, we named this peptide stress-responsive peptide (SRP).

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.

Adaptor protein is essential for insect cytokine signaling in hemocytes

Growth-blocking peptide (GBP) is an insect cytokine that stimulates a class of immune cells called plasmatocytes to adhere to one another and to foreign surfaces. Although extensive structure-activity studies have been performed on the GBP and its mutants in Lepidoptera Pseudaletia separata, the signaling pathway of GBP-dependent activation of plasmatocytes remains unknown. We identified an adaptor protein (P77) with a molecular mass of 77 kDa containing SH2/SH3 domain binding motifs and an immunoreceptor tyrosine-based activation motif (ITAM)-like domain in the cytoplasmic region of the C terminus. Although P77 showed no capacity for direct binding with GBP, its cytoplasmic tyrosine residues were specifically phosphorylated within seconds after GBP was added to a plasmatocyte suspension. Tyrosine phosphorylation of P77 also was observed when hemocytes were incubated with Enterobactor cloacae or Micrococcus luteus, but this phosphorylation was found to be induced by GBP released from hemocytes stimulated by the pathogens. Tyrosine phosphorylation of the integrin beta subunit also was detected in plasmatocytes stimulated by GBP. Double-stranded RNAs targeting P77 not only decreased GBP-dependent tyrosine phosphorylation of the integrin beta subunit, but also abolished GBP-induced spreading of plasmatocytes on foreign surfaces. P77 RNAi larvae also showed significantly higher mortality than control larvae after infection with Serratia marcescens, indicating that P77 is essential for GBP to mediate a normal innate cellular immunity in insects. These results demonstrate that GBP signaling in plasmatocytes requires the adaptor protein P77, and that active P77-assisted tyrosine phosphorylation of integrins is critical for the activation of plasmatocytes.

C-terminal elongation of growth-blocking peptide enhances its biological activity and micelle binding affinity

Growth-blocking peptide (GBP) is a hormone-like peptide that suppresses the growth of the host armyworm. Although the 23-amino acid GBP (1-23 GBP) is expressed in nonparasitized armyworm plasma, the parasitization by wasp produces the 28-amino acid GBP (1-28 GBP) through an elongation of the C-terminal amino acid sequence. In this study, we characterized the GBP variants, which consist of various lengths of the C-terminal region, by comparing their biological activities and three-dimensional structures. The results of an injection study indicate that 1-28 GBP most strongly suppresses larval growth. NMR analysis shows that these peptides have basically the same tertiary structures and that the extension of the C-terminal region is disordered. However, the C-terminal region of 1-28 GBP undergoes a conformational transition from a random coiled state to an alpha-helical state in the presence of dodecylphosphocholine micelles. This suggests that binding of the C-terminal region would affect larval growth activity.

Plasmatocyte-spreading peptide (PSP) plays a central role in insect cellular immune defenses against bacterial infection

Insect hemocytes (blood cells) are a central part of the insect's cellular response to bacterial pathogens, and these specialist cells can both recognize and engulf bacteria. During this process, hemocytes undergo poorly characterized changes in adhesiveness. Previously, a peptide termed plasmatocyte-spreading peptide (PSP), which induces the adhesion and spreading of plasmatocytes on foreign surfaces, has been identified in lepidopteran insects. Here, we investigate the function of this peptide in the moth Manduca sexta using RNA interference (RNAi) to prevent expression of the precursor protein proPSP. We show that infection with the insect-specific bacterial pathogen Photorhabdus luminescens and non-pathogenic Escherichia coli induces proPSP mRNA transcription in the insect fat body but not in hemocytes; subsequently, proPSP protein can be detected in cell-free hemolymph. We used RNAi to silence this upregulation of proPSP and found that the knock-down insects succumbed faster to infection with P. luminescens, but not E. coli. RNAi-treated insects infected with E. coli showed a reduction in the number of circulating hemocytes and higher bacterial growth in hemolymph as well as a reduction in overall cellular immune function compared with infected controls. Interestingly,RNAi-mediated depletion of proPSP adversely affected the formation of melanotic nodules but had no additional effect on other cellular responses when insects were infected with P. luminescens, indicating that this pathogen employs mechanisms that suppress key cellular immune functions in M. sexta. Our results provide evidence for the central role of PSP in M. sexta cellular defenses against bacterial infections.

Regulation of growth-blocking peptide expression during embryogenesis of the cabbage armyworm

Growth-blocking peptide (GBP) is an insect cytokine with diverse biological functions. Northern blot analysis revealed high heterogeneity in the size distribution of GBP mRNAs as well as in the tissues where they are detected. The spatio-temporal transcription pattern is dynamic, especially during embryogenesis. Gel shift assays demonstrated that the cabbage armyworm embryo nuclear extract specifically binds to a 178-bp element, at position +234 to +411 from the transcription start site of the 1.3 kb GBP transcript, in which two Drosophila Deformed (Dfd) binding sites are repeated in tandem. The specific binding between this element and Dfd was demonstrated using recombinant cabbage armyworm Dfd protein. Silencing the Dfd expression in embryos by treating with Dfd double-stranded RNA did not reduce the expression level of GBP, but ectopic GBP expression was observed in the lateral region of the embryo, suggesting that Dfd could serve as a transcriptional repressor for the GBP gene.

Molecular identification of a bevy of serine proteinases in Manduca sexta hemolymph

Extracellular serine proteinase pathways control immune and homeostatic processes in insects. Our current knowledge of their components is limited-prophenoloxidase-activating proteinases (PAPs) are among the few hemolymph proteinases (HPs) with known functions. To identify components of proteinase systems in the hemolymph of Manduca sexta, we amplified cDNAs from larval fat body or hemocytes using degenerate primers coding for two conserved regions in S1 family serine proteinases. PCR yielded fragments encoding seven known (HP1-HP4, PAP-1, PAP-2 and PAP-3) and 18 unknown (HP5-HP22) serine proteinases. We screened cDNA libraries and isolated clones for 17 of the newly discovered HPs (HP5-HP22 except for HP11) and prepared antibodies to 14 recombinant proteins (HP6, HP8-HP10, HP12, HP14-HP19, HP21 and HP22). Fourteen of the HPs contain regulatory clip domain(s) at their amino-terminus--HP1, HP2, HP6, HP8, HP13, HP17, HP18, HP21, HP22 and PAP-1 have one, whereas HP12, HP15, PAP-2 and PAP-3 have two clip domains. Multiple sequence alignment of catalytic domains in these and other arthropod serine proteinases provided useful clues for future functional analysis. Northern blot and reverse transcription PCR (RT-PCR) analyses showed increases in HP2, HP7, HP9, HP10, HP12-HP22 mRNA levels at 24h after a bacterial challenge, and immunoblot analysis confirmed elevated concentrations of HP12, HP14-HP19, HP21 and HP22 proteins in plasma in response to injected bacteria. Hemocytes express HP13 and HP18; fat body produces HP12, HP20-HP22; both tissues synthesize the other HPs. These results collectively indicate the existence of a complex serine proteinase network in M. sexta hemolymph, predicted to mediate rapid defense responses upon wounding and/or microbial infection.

A cytokine secreted from the suboesophageal body is essential for morphogenesis of the insect head

The suboesophageal body of insects was identified over a century ago in the silkworm embryo, but its biological function is still unknown. We discovered that this tissue is differentiated in the earliest embryonic stages of the cabbage armyworm and secretes the insect cytokine, growth-blocking peptide (GBP), transiently from 24 to 60 h after oviposition when gastrulation is in progress. Over-expression of GBP, achieved by microinjection of the GBP gene driven by a cytomegalovirus (CMV) constitutive promoter, resulted in complex deformities of the procephalon (embryonic head). Severe abnormal phenotypes of the head structure were produced by silencing the GBP expression in the embryo by treating with GBP double-stranded RNA: the procephalon-containing optic lobes diminished and completely separated into bilateral halves. This indicates that GBP secreted from the suboesophageal body plays an essential role in the formation of the procephalic domain during early embryogenesis. The cytokine-induced fusion of bilateral procephalic lobes is thought to be evolutionarily conserved at least in insects, because of the widespread occurrence of the suboesophageal body in insect embryos.

Insect cytokine growth-blocking peptide triggers a termination system of cellular immunity by inducing its binding protein

Growth-blocking peptide (GBP) is a 25-amino acid cytokine found in lepidopteran insects that possesses diverse biological activities such as stimulation of immune cells (plasmatocytes), cell proliferation, and larval growth regulation. We found another novel function of GBP that induces a hemolysis of another class of blood cells (oenocytoids). In the lysate of oenocytoids we identified a GBP-binding protein that shows a specific affinity for GBP. The characterization of purified GBP-binding protein and its cDNA demonstrated it as a 49.5-kDa novel protein with a C-terminal region displaying limited homology to several insect lipoproteins. Results of Northern and Western blotting indicated that the GBP-binding protein should be synthesized only in blood cells. Immunoelectron microscopic analyses confirmed that indirect immunoreactive signals were mostly localized in oenocytoids. Kinetic and biological analyses of interaction between GBP and the binding protein showed their strong binding was followed by clearance of GBP from hemolymph, thus indicating that this protein might function as an inhibitory factor against GBP. Based on these results, we propose that insect cytokine GBP shows multifunctions even in cellular immunity: it serves to stimulate immune cells and afterward silences its own action by inducing the binding protein through specific hemolysis.

Expression and purification of a small cytokine growth-blocking peptide from armyworm Pseudaletia separata by an optimized fermentation method using the methylotrophic yeast Pichia pastoris

A small multifunctional cytokine, growth-blocking peptide (GBP), from the armyworm Pseudaletia separata larvae was expressed as a soluble and active recombinant peptide in the methylotrophic yeast Pichia pastoris. An expression vector for GBP secretion was constructed using vector pPIC9, and GBP was expressed under the control of the alcohol oxidase (AOX1) promoter. Although we first tried to cultivate GBP in shake flask cultures, the yield was low, probably due to proteolysis of the recombinant protein. To overcome this problem, we utilized a high-density fermentation method. The pH of the medium in the fermenter was kept at 3.0, and the medium was collected within 48h post methanol shift to minimize exposure of the target peptide to proteases. Recombinant GBP was purified through three reverse-phase HPLC columns. We characterized the 25 amino acid GBP by molecular mass spectrometry and amino acid sequencing. Plasmatocyte spreading, one of the activities of GBP, was similar between chemically synthesized GBP and purified recombinant GBP. Up to 50mg GBP was recovered per 1L of yeast culture supernatant.

Characterization of receptors of insect cytokine, growth-blocking peptide, in human keratinocyte and insect Sf9 cells

Insect cytokine, growth-blocking peptide (GBP), enhances cell proliferation of human keratinocyte cells with a potency almost equivalent to that of human epidermal growth factor (EGF). GBP consists of 25 amino acid residues containing a core region that shows a striking similarity to the C-terminal beta-loop domain of EGF and disordered N and C termini. The present study demonstrates that, although GBP lacks the N-terminal half-portion of EGF molecule, at least five amino acids of the disordered N-terminal six-amino acid region are indispensable for affecting the cell growth activity of GBP. Upon stimulating mitogenesis in keratinocyte cells, GBP directly binds and activates their EGF receptors. GBP also effects proliferative activity on insect Sf9 cells through the binding and activation of the specific receptor, which consists of a heterodimeric complex: a binding subunit (60 kDa) and a tyrosine phosphorylation subunit (58 kDa). These results indicate that GBP enhances cell proliferation of human keratinocyte and insect Sf9 cells through the activation of EGF and GBP receptors, respectively.

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.

Neuropeptides and neuropeptide receptors in the Drosophila melanogaster genome

Recent genetic analyses in worms, flies, and mammals illustrate the importance of bioactive peptides in controlling numerous complex behaviors, such as feeding and circadian locomotion. To pursue a comprehensive genetic analysis of bioactive peptide signaling, we have scanned the recently completed Drosophila genome sequence for G protein-coupled receptors sensitive to bioactive peptides (peptide GPCRs). Here we describe 44 genes that represent the vast majority, and perhaps all, of the peptide GPCRs encoded in the fly genome. We also scanned for genes encoding potential ligands and describe 22 bioactive peptide precursors. At least 32 Drosophila peptide receptors appear to have evolved from common ancestors of 15 monophyletic vertebrate GPCR subgroups (e.g., the ancestral gastrin/cholecystokinin receptor). Six pairs of receptors are paralogs, representing recent gene duplications. Together, these findings shed light on the evolutionary history of peptide GPCRs, and they provide a template for physiological and genetic analyses of peptide signaling in Drosophila.

Plasmatocyte spreading peptide (PSP1) and growth blocking peptide (GBP) are multifunctional homologs

Recently, we identified Plasmatocyte spreading peptide (PSP1) from the moth Pseudoplusia includens and reported that it mediates adhesion of hemocytes to foreign surfaces. PSP1 is structurally very similar to three classes of peptides identified earlier from other species of Lepidoptera: growth blocking peptide (GBP) originally identified in Pseudaletia separata, and a series of related peptides from other species designated as paralytic (PP) or cardioactive (CAP) peptides. In this study, we conducted parallel experiments in P. includens and P. separata to determine whether PSP1 and GBP have distinct or multiple biological activities. Both peptides affected the adhesive state of hemocytes from each moth very similarly. PSP1 and GBP exhibited significant growth blocking and paralytic activity in P. separata. Both peptides also had growth blocking activity in P. includens although larvae had to be injected with higher doses of each peptide to reduce weight gain than was observed for P. separata. However, GBP and PSP1 had little paralytic activity in P. includens. Collectively, our results indicate that GBP and PSP1 are multifunctional, but that some interspecific variation also exists in their growth blocking and paralytic activities. We suggest that all PSP1, GBP, PP and CAP family members are homologs that likely have multiple biological activities. Based upon the unique consensus sequence of their N termini, we propose that these molecules be henceforth referred to as members of the "ENF" peptide family.

Biological activity of Manduca sexta paralytic and plasmatocyte spreading peptide and primary structure of its hemolymph precursor

A family of hemolymph peptides was previously identified in several lepidopteran insects, which exhibited multiple biological activities including rapid paralysis, blockage of growth and development, or stimulation of plasmatocyte spreading and aggregation. We synthesized Manduca sexta paralytic peptide 1 (PP1) and found that after it was injected into larvae, bleeding from wounds was dramatically reduced. PP1 also stimulated spreading and aggregation behavior of M. sexta plasmatocytes in vitro. Stimulation of plasmatocyte aggregation and adherence to the body wall may explain a decrease observed in the number of circulating plasmatocytes after injection of PP1. Such aggregates might rapidly form plugs in wounds to prevent bleeding. We cloned a cDNA for a Manduca paralytic peptide precursor, using polymerase chain reactions and cDNA library screening. The active 23-residue PP2 peptide encoded by this clone is at the carboxyl-terminal end of a precursor protein predicted to be 107 amino acid residues long after cleavage of a secretion signal peptide. Active PP2 was produced by processing of recombinant proPP2 by bovine factor Xa. A single proPP2 mRNA was present in fat body but not in hemocytes. The level of this mRNA was not affected by injection of bacteria into larvae. We produced recombinant proPP2 in Escherichia coli and used this protein to produce an antiserum. The antiserum detected proPP2 in plasma and was used to observe rapid proteolytic processing of proPP2 after hemolymph collection.

Structure of a paralytic peptide from an insect, Manduca sexta

Paralytic peptide 1 (PP1) from a moth, Manduca sexta, is a 23-residue peptide (Glu-Asn-Phe-Ala-Gly-Gly-Cys-Ala-Thr-Gly-Tyr-Leu-Arg-Thr-Ala-Asp-Gly-Arg -Cys-Lys-Pro-Thr-Phe) that was first found to have paralytic activity when injected into M. sexta larvae. Recent studies demonstrated that PP1 also stimulated the spreading and aggregation of a blood cell type called plasmatocytes and inhibited bleeding from wounds. We determined the solution structure of PP1 by two-dimensional 1H NMR spectroscopy to begin to understand structural-functional relationships of this peptide. PP1 has an ordered structure, which is composed of a short antiparallel beta-sheet at residues Tyr11-Thr14 and Arg18-Pro21, three beta turns at residues Phe3-Gly6, Ala8-Tyr11 and Thr14-Gly17, and a half turn at the carboxyl-terminus (residues Lys20-Phe23). The well-defined secondary and tertiary structure was stabilized by hydrogen bonding and side-chain hydrophobic interactions. In comparison with two related insect peptides, whose structures have been solved recently, the amino-terminal region of PP1 is substantially more ordered. The short antiparallel beta-sheet of PP1 has a folding pattern similar to the carboxyl-terminal subdomain of epidermal growth factor (EGF). Therefore, PP1 may interact with EGF receptor-like molecules to trigger its different biological activities.

Serine proteinase inhibitors in arthropod immunity

Arthropod hemolymph contains proteins with serine proteinase inhibitory activity. These inhibitors may exist in plasma or in hemocyte granules. Serine proteinase inhibitors from the Kazal, Kunitz, alpha-macroglobulin, and serpin families have been identified in arthropod hemolymph and have been characterized biochemically. Two new families of low molecular weight serine proteinase inhibitors have recently been discovered: one in silkworms (the Bombyx family) and another in locusts and a crayfish. The serine proteinase inhibitors in arthropod hemolymph are likely to function in protecting their hosts from infection by pathogens or parasites. Some may inhibit fungal or bacterial proteinases. Others probably have roles in regulating endogenous proteinases involved in coagulation, prophenol oxidase activation, or cytokine activation.

Solution structure of an insect growth factor, growth-blocking peptide

Growth-blocking peptide (GBP) is an insect growth factor consisting of 25 amino acid residues that retards the development of lepidopteran larvae at high concentration while it stimulates larval growth at low concentration. In this study, we determined the solution structure of GBP by two-dimensional 1H NMR spectroscopy. The structure contains a short segment of double-stranded beta-sheet involving residues 11-13 and 19-21 and a type-II beta-turn in the loop region (residues 8-11), whereas the N and C termini are disordered. This is the first report of the three-dimensional structure of the peptiderigic insect growth factor, and the structure of the well defined region of GBP was found to share similarity with that of the C-terminal domain of the epidermal growth factor (EGF). Because GBP has been reported to stimulate DNA synthesis of not only insect cells but also human keratinocyte cells at the same level with EGF, the structural similarity between GBP and EGF may lead to the interaction of GBP to EGF receptor.

Cell growth activity of growth-blocking peptide

Growth-blocking peptide (GBP) is an insect biogenic peptide that retards the development of lepidopteran larvae. cDNAs encoding GBP were isolated from three lepidopteran species: Pseudaletia separata, Mamestra brassicae and Spodoptera litura. Comparison of these molecules revealed that the GBP coding regions were 70% homologous. In contrast, the upstream regions of the deduced propeptides were only 33% identical. Sequence analysis further suggested that GBP shares some structural similarities with human epidermal growth factors. Bioassay data revealed that several pmol/ml of GBP stimulated DNA synthesis of a human keratinocyte cell line and of SF-9 insect cells. However, several nmol/ml of GBP did not stimulate cell proliferation at all. In vivo studies similarly indicated that low concentrations of GBP stimulated larval growth while high concentrations of GBP retarded growth. These data suggest that GBP acts as a growth factor that regulates insect larval development.

Plasmatocyte spreading peptide is encoded by an mRNA differentially expressed in tissues of the moth Pseudoplusia includens

It has long been known that blood cells (hemocytes) are an essential component of the invertebrate immune system, yet little is known about the molecules mediating their function. Recently, we identified plasmatocyte spreading peptide (PSP1) from the moth Pseudoplusia includens which regulates the trafficking and adhesion of a hemocyte subclass called plasmatocytes. Here, we report the cloning of a cDNA (p15) that encodes a PSP1 precursor protein. Northern blot analysis revealed that a homologous prepro-PSP1 mRNA is expressed in fat body, and that other PSP1-related transcripts are expressed in nervous tissue and hemocytes. Coupled in vitro transcription/translation reactions indicated that p15 produces a protein recognized by a PSP1 polyclonal antibody. Immunoblotting experiments further revealed that a putative pro-PSP1 protein is present in P. includens plasma and fat body.

Mechanism of parasitism-induced elevation of haemolymph growth-blocking peptide levels in host insect larvae (Pseudaletia separata)

Growth-blocking peptide (GBP) has been purified for the first time from the haemolymph of the host armyworm Pseudaletia separata whose growth is inhibited and shows developmental arrest in the last larval instar stage when parasitized by the parasitoid wasp Cotesia kariyai. GBP naturally occurs in the haemolymph of lepidopteran larvae but its concentration is very low during the last larval instar in comparison with that in the penultimate larval instar. However, by 24h after parasitization or polydnavirus (PdV)-infection on day 0 of the last larval instar, a four-fold increase in GBP level, compared with synchronous non-parasitized control larvae, is observed. Although Northern blot analysis indicates that GBP mRNA is transcribed in brain-nerve cord and fat body, plasma GBP is likely to be secreted mainly from fat body because the GBP mRNA level is approximately 100-fold higher in fat body than that in brain-nerve cord. RT-PCR analysis demonstrates the constant expression of GBP mRNA in both parasitized (or PdV-infected) and non-parasitized larval fat body, which suggests that parasitism does not influence transcriptional level, but might influence post-transcriptional level to elevate plasma GBP concentration. This interpretation was supported by estimating GBP precursor levels in fat body of PdV-infected and non-infected larvae. Virus infection appears to elevate the GBP precursor levels in fat body to about six times greater than that in non-infected last instar larvae by 6h after PdV-injection. The GBP processing enzyme activity that occurs in Golgi body-rich extract of the fat body is increased by about 90% after parasitization or PdV-injection.

Isolation and identification of a plasmatocyte-spreading peptide from the hemolymph of the lepidopteran insect Pseudoplusia includens

Insect blood cells (hemocytes) play an essential role in defense against parasites and other pathogenic organisms that infect insects. A key class of hemocytes involved in insect cellular immunity is plasmatocytes. Here we describe the isolation and identification of a peptide from the moth Pseudoplusia includens that mediates the spreading of plasmatocytes to foreign surfaces. This peptide, designated plasmatocyte-spreading peptide (PSP1), contains 23 amino acid residues in the following sequence: H-ENFNGGCLAGYMRTADGRCKPTF-OH. In vitro assays using the synthetic peptide at concentrations >/=2 nM induced plasmatocytes from P. includens to spread on the surface of culture dishes. Injection of this peptide into P. includens larvae caused a transient depletion of plasmatocytes from circulation. Labeling studies indicated that this peptide induced 75% of plasmatocytes that were double-labeled by the monoclonal antibodies 49G3A3 and 43E9A8 to spread, whereas plasma induced significantly more plasmatocytes to spread. This suggests that only a certain subpopulation of plasmatocytes responds to the peptide and that other peptidyl factors mediate plasmatocyte adhesion responses.