Involvement of Manduca sexta peptidoglycan recognition protein-1 in the recognition of bacteria and activation of prophenoloxidase system

The role of a novel secretory peptidoglycan recognition protein with antibacterial ability from the Chinese Oak Silkworm Antheraea pernyi in humoral immunity

Peptidoglycan recognition proteins (PGRPs) are a family of pattern recognition receptors that play a critical role in the immune response of invertebrates and vertebrates. Herein, the short ApPGRP-D gene was cloned from the model lepidopteran Antheraea pernyi. Quantitative PCR (qPCR) confirmed that ApPGRP-D is an immune-related protein and that the expression of ApPGRP-D can be induced by microorganisms. ApPGRP-D is a broad-spectrum pattern recognition protein that activates the prophenoloxidase cascade activation system and promotes the agglutination of microbial cells. Likely due to its amidase activity, ApPGRP-D can inhibit the growth of E. coli and S. aureus. In addition, we demonstrated for the first time that zinc ions, as important metal coenzymes, could promote multiple functions of ApPGRP-D but not its amidase activity.

Identification of immunity-related genes distinctly regulated by Manduca sexta Spӓtzle-1/2 and Escherichia coli peptidoglycan

The immune system of Manduca sexta has been well studied to understand molecular mechanisms of insect antimicrobial responses. While evidence supports the existence of major immune signaling pathways in this species, it is unclear how induced production of defense proteins is specifically regulated by the Toll and Imd pathways. Our previous studies suggested that diaminopimelic acid-type peptidoglycans (DAP-PG) from Gram-negative and some Gram-positive bacteria, more than Lys-type peptidoglycans (Lys-PG) from other Gram-positive bacteria, triggers both pathways through membrane-bound receptors orthologous to Drosophila Toll and PGRP-LC. In this study, we produced M. sexta proSpätzle-1 and proSpätzle-2 in Sf9 cells, identified their processing enzymes, and used prophenoloxidase activating protease-3 to activate the cytokine precursors. After Spätzle-1 and -2 were isolated from the reaction mixtures, we separately injected the purified cytokines into larval hemocoel to induce gene transcription in fat body through the Toll pathway solely. On the other hand, we treated a M. sexta cell line with E. coli DAP-PG to only induce the Imd pathway and target gene expression. RNA-Seq analysis of the fat body and cultured cells collected at 0, 6, and 24 h after treatment indicated that expression of diapausin-4, -10, -12, -13, cecropin-2, -4, -5, attacin-5, -11, and lebocin D is up-regulated predominantly via Toll signaling, whereas transcription of cecropin-6, gloverin, lysozyme-1, and gallerimycin-2 is mostly induced by DAP-PG via Imd signaling. Other antimicrobial peptides are expressed in response to both pathways. Transcripts of most Toll-specific genes (e.g., lebocin D) peaked at 6 h, contrasting the gradual increase and plateauing of drosomycin mRNA level at 24-48 h in Drosophila. We also used T (oll)-I (md) ratios to estimate relative contributions of the two pathways to transcriptional regulation of other components of the immune system. The differences in pathway specificity and time course of transcriptional regulation call for further investigations in M. sexta and other insects.

Silencing gram-negative bacteria binding protein 1 decreases the immunity of Tribolium castaneum against bacteria

Gram-negative bacteria binding proteins (GNBPs) have the ability to recognize molecular patterns associated with microbial pathogens (PAMPs), leading to the activation of immune responses downstream. In the genome of Tribolium castaneum, three GNBP genes have been identified; however, their immunological roles remain unexplored. In our study, a GNBP1, designated as TcGNBP1, were identified from the cDNA library of T. castaneum. The coding sequence of TcGNBP1 consisted of 1137 bps and resulted in the synthesis of a protein comprising 378 amino acids. This protein encompasses a signal peptide, a low-complexity region, and a glycoside hydrolase 16 domain. TcGNBP1 was strongly expressed in early adult stages, and mainly distributed in hemolymph and gut. Upon being challenged with Escherichia coli or Staphylococcus aureus, the transcript levels of TcGNBP1 were significantly changed at different time points. Through molecular docking and ELISA analysis, it was observed that TcGNBP1 has the ability to interact with lipopolysaccharides, peptidoglycan, and β-1, 3-glucan. Based on these findings, it was further discovered that recombinant TcGNBP1 can directly bind to five different bacteria in a Ca2+-dependent manner. After knockdown of TcGNBP1 with RNA interference, expression of antimicrobial peptide genes and prophenoloxidase (proPO) activity were suppressed, the susceptibility of T. castaneum to E. coli or S. aureus infection was enhanced, leading to low survival rate. These results suggest a regulatory mechanism of TcGNBP1 in innate immunity of T. castaneum and provide a potential molecular target for dsRNA-based insect pest management.

Serine protease homolog pairs CLIPA4-A6, A4-A7Δ, and A4-A12 act as cofactors for proteolytic activation of prophenoloxidase-2 and -7 in Anopheles gambiae

Phenoloxidase (PO) catalyzed melanization and other insect immune responses are mediated by serine proteases (SPs) and their noncatalytic homologs (SPHs). Many of these SP-like proteins have a regulatory clip domain and are called CLIPs. In most insects studied so far, PO precursors are activated by a PAP (i.e., PPO activating protease) and its cofactor of clip-domain SPHs. Although melanotic encapsulation is a well-known refractory mechanism of mosquitoes against malaria parasites, it is unclear if a cofactor is required for PPO activation. In Anopheles gambiae, CLIPA4 is 1:1 orthologous to Manduca sexta SPH2; CLIPs A5-7, A12-14, A26, A31, A32, E6, and E7 are 11:4 orthologous to M. sexta SPH1a, 1b, 4, and 101, SPH2 partners in the cofactors. Here we produced proCLIPs A4, A6, A7Δ, A12, and activated them with CLIPB9 or M. sexta PAP3. A. gambiae PPO2 and PPO7 were expressed in Escherichia coli for use as PAP substrates. CLIPB9 was mutated to CLIPB9Xa by including a Factor Xa cleavage site. CLIPA7Δ was a deletion mutant with a low complexity region removed. After PAP3 or CLIPB9Xa processing, CLIPA4 formed a high Mr complex with CLIPA6, A7Δ or A12, which assisted PPO2 and PPO7 activation. High levels of specific PO activity (55-85 U/μg for PO2 and 1131-1630 U/μg for PO7) were detected in vitro, indicating that cofactor-assisted PPO activation also occurs in this species. The cleavage sites and mechanisms for complex formation and cofactor function are like those reported in M. sexta and Drosophila melanogaster. In conclusion, these data suggest that the three (and perhaps more) SPHI-II pairs may form cofactors for CLIPB9-mediated activation of PPOs for melanotic encapsulation in A. gambiae.

Differential expression of immunity-related genes in larval Manduca sexta tissues in response to gut and systemic infection

Introduction

The midgut epithelium functions as tissue for nutrient uptake as well as physical barrier against pathogens. Additionally, it responds to pathogen contact by production and release of various factors including antimicrobial peptides, similar to the systemic innate immune response. However, if such a response is restricted to a local stimulus or if it appears in response to a systemic infection, too is a rather underexplored topic in insect immunity. We addressed the role of the midgut and the role of systemic immune tissues in the defense against gut-borne and systemic infections, respectively.

Methods

Manduca sexta larvae were challenged with DAP-type peptidoglycan bacteria - Bacillus thuringiensis for local gut infection and Escherichia coli for systemic stimulation. We compared the immune response to both infection models by measuring mRNA levels of four selected immunity-related genes in midgut, fat body, hematopoietic organs (HOs), and hemocytes, and determined hemolymph antimicrobial activity. Hemocytes and HOs were tested for presence and distribution of lysozyme mRNA and protein.

Results

The midgut and circulating hemocytes exhibited a significantly increased level of lysozyme mRNA in response to gut infection but did not significantly alter expression in response to a systemic infection. Conversely, fat body and HOs responded to both infection models by altered mRNA levels of at least one gene monitored. Most, but not all hemocytes and HO cells contain lysozyme mRNA and protein.

Discussion

These data suggest that the gut recruits immune-related tissues in response to gut infection whereas systemic infections do not induce a response in the midgut. The experimental approach implies a skewed cross-talk: An intestinal infection triggers immune activity in systemic immune organs, while a systemic infection does not elicit any or only a restricted immune response in the midgut. The HOs, which form and release hemocytes in larval M. sexta, i) synthesize lysozyme, and ii) respond to immune challenges by increased immune gene expression. These findings strongly suggest that they not only provide phagocytes for the cellular immune response but also synthesize humoral immune components.

Two clip-domain serine protease homologs, cSPH35 and cSPH242, act as a cofactor for prophenoloxidase-1 activation in Drosophila melanogaster

Insect phenoloxidases (POs) catalyze phenol oxygenation and o-diphenol oxidation to form reactive intermediates that kill invading pathogens and form melanin polymers. To reduce their toxicity to host cells, POs are produced as prophenoloxidases (PPOs) and activated by a serine protease cascade as required. In most insects studied so far, PPO activating proteases (PAPs) generate active POs in the presence of a high Mr cofactor, comprising two serine protease homologs (SPHs) each with a Gly residue replacing the catalytic Ser of an S1A serine protease (SP). These SPHs have a regulatory clip domain at the N-terminus, like most of the SP cascade members including PAPs. In Drosophila, PPO activation and PO-catalyzed melanization have been examined in genetic analyses but it is unclear if a cofactor is required for PPO activation. In this study, we produced the recombinant cSPH35 and cSPH242 precursors, activated them with Manduca sexta PAP3, and confirmed their predicted role as a cofactor for Drosophila PPO1 activation by MP2 (i.e., Sp7). The cleavage sites and mechanisms for complex formation and cofactor function are highly similar to those reported in M. sexta. In the presence of high Mr complexes of the cSPHs, PO at a high specific activity of 260 U/μg was generated in vitro. To complement the in vitro analysis, we measured hemolymph PO activity levels in wild-type flies, cSPH35, and cSPH242 RNAi lines. Compared with the wild-type flies, only 4.4% and 18% of the control PO level (26 U/μl) was detected in the cSPH35 and cSPH242 knockdowns, respectively. Consistently, percentages of adults with a melanin spot at the site of septic pricking were 82% in wild-type, 30% in cSPH35 RNAi, and 53% in cSPH242 RNAi lines; the survival rate of the control (45%) was significantly higher than those (30% and 15%) of the two RNAi lines. These data suggest that Drosophila cSPH35 and cSPH242 are components of a cofactor for MP2-mediated PPO1 activation, which are indispensable for early melanization in adults.

Mechanisms and roles of the first stage of nodule formation in lepidopteran insects

Nodule formation is a process of cellular immunity in insects and other arthropods with open circulatory systems. Based on histological observations, nodule formation occurs in 2 stages. The first stage occurs immediately after microbial inoculation and includes aggregate formation by granulocytes. The second stage occurs approximately 2-6 h later and involves the attachment of plasmatocytes to melanized aggregates produced during the first stage. The first stage response is thought to play a major role in the rapid capture of invading microorganisms. However, little is known regarding how granulocytes in the hemolymph form aggregates, or how the first stage of the immunological response protects against invading microorganisms. Since the late 1990s, our understanding of the molecules and immune pathways that contribute to nodule formation has improved. The first stage of nodule formation involves a hemocyte-induced response that is triggered by pathogen-associated molecular pattern (PAMP) recognition proteins in the hemolymph regulated by a serine proteinase cascade and cytokine (Spätzle) and Toll signaling pathways. Hemocyte agglutination proceeds through stepwise release of biogenic amine, 5-HT, and eicosanoids that act downstream of the Toll pathway. The first stage of nodule formation is closely linked to melanization and antimicrobial peptide (AMP) production, which is critical for insect humoral immunity. Nodule formation in response to artificial inoculation with millions of microorganisms has long been studied. It has recently been suggested that this system is the original natural immune system, and enables insects to respond to a single invading microorganism in the hemocoel.

Silencing of the immune gene BmPGRP-L4 in the midgut affects the growth of silkworm (Bombyx mori) larvae

Peptidoglycan recognition proteins (PGRPs) are one of the receptors in insects' immune pathways, essential for insects to recognize the exogenous pathogens in order to activate the Toll and immune deficiency (IMD) pathway. In the silkworm Bombyx mori, previous studies focused on the short PGRPs and less is known about the long PGRPs. In this study, a long PGRP in silkworm BmPGRP-L4 was cloned and its expression and function were analysed. The results showed that BmPGRP-L4 contains a transmembrane region, a conserved PGRP domain, and an amidase-2 domain. The expression profile demonstrated that BmPGRP-L4 existed in diverse tissues including epidermis, fat body, midgut, and silk glands, with remarkably high expression in the midgut in the 5th instar. Oral infection with Escherichia coli and Staphylococcus aureus significantly induced BmPGRP-L4 in the midgut and epidermis, as well as in the fat body and silk glands. Peptidoglycan also induced the expression of BmPGRP-L4 in midgut tissue ex vivo and BmN4 cells in vitro. RNAi of BmPGRP-L4 was effective in the midgut and epidermis, while the efficiency in the fat body was transient. RNAi-mediated knock-down of BmPGRP-L4 reduced the weight and growth of the silkworm, possibly due to its participation in the immune response and the regulation of the microbiota in the midgut lumen of the silkworm larvae.

Serine Protease Networks Mediate Immune Responses in Extra-Embryonic Tissues of Eggs in the Tobacco Hornworm, Manduca sexta

The melanization and Toll pathways, regulated by a network of serine proteases and noncatalytic serine protease homologs (SPHs), have been investigated mostly in adult and larval insects. However, how these innate immune reactions are regulated in insect eggs remains unclear. Here we present evidence from transcriptome and proteome analyses that extra-embryonic tissues (yolk and serosa) of early-stage Manduca sexta eggs are immune competent, with expression of immune effector genes including prophenoloxidase and antimicrobial peptides. We identified gene products of the melanization and Toll pathways in M. sexta eggs. Through in vitro reconstitution experiments, we demonstrated that constitutive and infection-induced serine protease cascade modules that stimulate immune responses exist in the extra-embryonic tissues of M. sexta eggs. The constitutive module (HP14b-SP144-GP6) may promote rapid early immune signaling by forming a cascade activating the cytokine Spätzle and regulating melanization by activating prophenoloxidase (proPO). The inducible module (HP14a-HP21-HP5) may trigger enhanced activation of Spätzle and proPO at a later phase of infection. Crosstalk between the two modules may occur in transition from the constitutive to the induced response in eggs inoculated with bacteria. Examination of data from two other well-studied insect species, Tribolium castaneum and Drosophila melanogaster, supports a role for a serosa-dependent constitutive protease cascade in protecting early embryos against invading pathogens.

Peptidoglycan recognition protein 6 (PGRP6) from Asian corn borer, Ostrinia furnacalis (Guenée) serve as a pattern recognition receptor in innate immune response

Peptidoglycan recognition proteins (PGRPs) recognize invading microbes via detecting peptidoglycans from microbial cell walls. PGRPs are highly conserved from insects to vertebrates and all play roles during the immune defensive response. Ten putative PGRPs have been identified through transcriptome analysis in the Asian corn borer, Ostrinia furnacalis (Guenée). Whereas, the biochemical functions of most of them have not yet been elucidated. In this study, we found PGRP6 messenger RNA exhibited extremely high expression levels in the midgut, and its transcript level increased dramatically upon bacterial infection. Moreover, the enzyme-linked immunosorbent assay indicated recombinant PGRP6 exhibited a strong binding affinity to peptidoglycans from Micrococcus luteus and Bacillus subtilis, which could agglutinate M. luteus and yeast Pichia pastoris. Additionally, we demonstrated that PGRP6 was involved in the pathway of antimicrobial peptides synthesis, but could not enhance encapsulation and melanization of hemocytes. Overall, our results indicated that O. furnacalis PGRP6 serves as a pattern recognition receptor and detects peptidoglycans from microbes to initiate the immune response.

Proteomic analysis of pharate pupal molting fluid from the tobacco hornworm, Manduca sexta

The insect cuticle is a key component of their success, being important for protection, communication, locomotion, and support. Conversely, as an exoskeleton, it also limits the size of the insect and must be periodically molted and a new one synthesized, to permit growth. To achieve this, the insect secretes a solution of chitinases, proteases and other proteins, known collectively as molting fluid, during each molting process to break down and recycle components of the old cuticle. Previous research has focused on the degradative enzymes in molting fluid and offered some characterization of their biochemical properties. However, identification of the specific proteins involved remained to be determined. We have used 2D SDS-PAGE and LC/MS-based proteomic analysis to identify proteins in the molting fluid of the tobacco hornworm, Manduca sexta, undergoing the larval to pupal molt. We categorized these proteins based on their proposed functions including chitin metabolism, proteases, peptidases, and immunity. This analysis complements previous reported work on M. sexta molting fluid and identifies candidate genes for enzymes involved in cuticle remodeling. Proteins classified as having an immune function highlight potential for molting fluid to act as an immune barrier to prevent infections during the cuticle degradation and ecdysis processes. Several proteins known to function in melanin synthesis as an immune response in hemolymph were present in molting fluid. We demonstrated that the bacterium Micrococcus luteus and the entomopathogenic fungus Beauveria bassiana can stimulate activation of phenoloxidase in molting fluid, indicating that the recognition proteins, protease cascade, and prophenoloxidase needed for melanin synthesis are present as a defense against infection during cuticle degradation. This analysis offers insights for proteins that may be important not only for molting in M. sexta but for insects in general.

A mechanistic analysis of bacterial recognition and serine protease cascade initiation in larval hemolymph of Manduca sexta

Serine protease cascades have evolved in vertebrates and invertebrates to mediate rapid defense responses. Previous biochemical studies showed that in hemolymph of a caterpillar, Manduca sexta, recognition of fungi by β-1,3-glucan recognition proteins (βGRP1 and βGRP2) or recognition of bacteria by peptidoglycan recognition protein-1 (PGRP1) and microbe binding protein (MBP) results in autoactivation of hemolymph protease-14 precursor (proHP14). HP14 then activates downstream members of a protease cascade leading to the melanization immune response. ProHP14 has a complex domain architecture, with five low-density lipoprotein receptor class A repeats at its amino terminus, followed by a Sushi domain, a Sushi domain variant called Wonton, and a carboxyl-terminal serine protease catalytic domain. Its zymogen form is activated by specific proteolytic cleavage at the amino-terminal end of the protease domain. While a molecular mechanism for recognition and triggering the response to β-1,3-glucan has been delineated, it is unclear how bacterial recognition stimulates proHP14 activation. To fill this knowledge gap, we expressed the two domains of M. sexta MBP and found that the amino-terminal domain binds to diaminopimelic acid-peptidoglycan (DAP-PG). ProHP14 bound to both the carboxyl-terminal domain (MBP-C) and amino-terminal domain (MBP-N) of MBP. In the mixture of DAP-PG, MBP, and larval plasma, inclusion of an HP14 fragment composed of LDLa repeats 2-5 (LDLa2-5) or MBP-C significantly reduced prophenoloxidase activation, likely by competing with the interactions of the full-length proteins, and suggesting that molecular interactions involving these regions of proHP14 and MBP take part in proHP14 activation in response to peptidoglycan. Using a series of N-terminally truncated versions of proHP14, we found that autoactivation required LDLa2-5. The optimal ratio of PGRP1, MBP, and proHP14 is close to 3:2:1. In summary, proHP14 autoactivation by DAP-type peptidoglycan requires binding of DAP-PG by PGRP1 and the MBP N-terminal domain and association of the LDLa2-5 region of proHP14 with the MBP C-terminal domain. These interactions may concentrate the proHP14 zymogen at the bacterial cell wall surface and promote autoactivation.

Preferential binding of DAP-PGs by major peptidoglycan recognition proteins found in cell-free hemolymph of Manduca sexta

Peptidoglycan recognition proteins (PGRPs) detect invading bacteria to trigger or modulate immune responses in insects. While these roles are established in Drosophila, functional studies are not yet achieved at the PGRP family level in other insects. To attain this goal, we selected Manduca sexta PGRP12 and five of the nine secreted PGRPs for recombinant expression and biochemical characterization. We cloned PGRP2-5, 12 and 13 cDNAs, produced the proteins in full (PGRP2-5, 13) or in part (PGRP3s, 12e, 13N, 13C) in Sf9 cells, and tested their bindings of two muramyl pentapeptides by surface plasmon resonance, two soluble peptidoglycans by competitive ELISA, and four insoluble peptidoglycans and eight whole bacteria by a pull-down assay. Preferential binding of meso-diaminopimelic acid-peptidoglycans (DAP-PGs) was observed in all the proteins containing a peptidoglycan binding domain and, since PGRP6, 7 and 9 proteins were hardly detected in cell-free hemolymph, the reportoire of PGRPs (including PGRP1 published previously) in M. sexta hemolymph is likely adapted to mainly detect Gram-negative bacteria and certain Gram-positive bacteria with DAP-PGs located on their surface. After incubation with plasma from naïve larvae, PGRP2, 3f, 4, 5, 13f and 13N considerably stimulated prophenoloxidase activation in the absence of a bacterial elicitor. PGRP3s and 12e had much smaller effects. Inclusion of the full-length PGRPs and their regions in the plasma also led to proHP8 activation, supporting their connections to the Toll pathway, since HP8 is a Spӓtzle-1 processing enzyme in M. sexta. Together, these findings raised concerns on the common belief that the Toll-pathway is specific for Gram-positive bacteria in insects.

Cleavage activation and functional comparison of Manduca sexta serine protease homologs SPH1a, SPH1b, SPH4, and SPH101 in conjunction with SPH2

Phenoloxidase (PO) is a crucial component of the insect immune response against microbial infection. In the tobacco hornworm Manduca sexta, PO is generated from its precursor proPO by prophenoloxidase activating proteases (PAPs) in the presence of two noncatalytic serine protease homologs (SPHs). cDNA cloning and genome analysis indicate that SPH1a (formerly known as SPH1), SPH1b, SPH4, SPH101, and SPH2 contain a clip domain, a linker, and a protease-like domain (PLD). The first 22 residues of the SPH1b, SPH4, and SPH101 PLDs are identical, and differ from SPH1a only at position 4, Thr¹⁵⁴ substituted with Asn¹⁵⁴ in SPH1a. While the sequence from Edman degradation was used to establish PAP cofactor as a high M_r complex of SPH1a and SPH2, this assignment needed further validation, especially because SPH1b mRNA levels are much higher than SPH1a's and better correlate with SPH2 transcription. Thus, here we determined expression profiles of these SPH genes in different tissues from various developmental stages using highly specific primers. High levels of SPH1b and SPH2 proteins, low SPH4, and no SPH1a or SPH101 were detected in hemolymph from larvae in the feeding, wandering and bar stages, pupae, and adults by targeted LC-MS/MS analysis, based on unique peptides from the trypsin-treated SPHs. We expressed the five proSPHs in baculovirus-infected Sf9 cells for use as standards to identify and quantify their counterparts in plasma samples. Moreover, we tested their cleavage by PAP3 and efficacy of the SPH1a, 1b, 4, and 101 as SPH2 partners in PAP3-mediated proPO activation. PAP3 processed proSPH1b and 101 more readily than proSPH1a and 4; PAP3 activated proPO more efficiently in the presence of SPH2 with SPH101 or SPH1b than with SPH1a or SPH4. These results generally agree with their order of appearance or sequence similarity: SPH101 > SPH1b (98%) > SPH1a (90%) > SPH4 (83%). In other words, likely due to positive selection, products of the newly duplicated genes (SPH1b and SPH101) are more favorable substrates of PAP3 and better SPH2 partners in forming a high M_r cofactor than SPH1a or SPH4 is. Electrophoresis on native gel and immunoblot analysis further indicated that SPH101 or 1b form high M_r complexes more readily than SPH1a or 4 does. In comparison, SPH2 showed a small mobility decrease and then increase on native gel after PAP3 cleavage at the first site. Since the natural cofactor in bar-stage hemolymph is complexes of SPH1 and 2 with an average M_r of 790 kDa, PAP3-activated SPH2 may associate with the higher M_r SPH1b scaffolds to form super-complexes. Their structures and formation in relation to cleavage of SPH1b at different sites await further exploration.

Peptidoglycan recognition protein-S1 (PGRP-S1) from Diaphania pyloalis (Walker) is involved in the agglutination and prophenoloxidase activation pathway

Recognition of invading foreign exogenous pathogen is the first step to initiate the innate immune response of insects, which accomplished by the pattern recognition receptors (PRRs). Peptidoglycan recognition proteins (PGRPs) serve as an important type of PRRs, which activate immune response by detecting peptidoglycan of microbial cell wall. In this study, we have cloned the full-length cDNA of PGRP gene called PGRP-S1 from the Diaphania pyloalis (Walker). The open reading frame (ORF) of D. pyloalis PGRP-S1 encodes 211 amino acids which containing a secretion signal peptide and a canonical PGRP domain. Multisequence alignment revealed that PGRP-S1 possess the amino acid residues responsible for zinc binding and amidase activity. D. pyloalis PGRP-S1 exhibited the highest transcript level in fat body and followed in head. The mRNA concentration dramatically increased after an injection of Escherichia coli or Micrococcus luteus. Purified recombinant PGRP-S1 exhibit binding ability to peptidoglycans from Staphylococcus aureus or Bacillus subtilis and cause intensive agglutination of E. coli, M. luteus or S. aureus in the presence of zinc ions. Furthermore, phenoloxidase activity significantly increased when the plasma from larvae was incubated with recombinant PGPR-S1 and peptidoglycans from B. subtilis or M. luteus simultaneously. These results implied that PGRP-S1 was a member involving the prophenoloxidase activation pathway. Overall, our results indicated that D. pyloalis PGRP-S1 serve as a PRR to participate in the recognition of foreign pathogen and prophenoloxidase pathway stimulation.

A Short-Type Peptidoglycan Recognition Protein 1 (PGRP1) Is Involved in the Immune Response in Asian Corn Borer, Ostrinia furnacalis (Guenée)

The insect immune response is initiated by the recognition of invading microorganisms. Peptidoglycan recognition proteins (PGRPs) function primarily as pattern recognition receptors by specifically binding to peptidoglycans expressed on microbial surfaces. We cloned a full-length cDNA for a PGRP from the Asian corn borer Ostrinia furnacalis (Guenée) and designated it as PGRP1. PGRP1 mRNA was mainly detected in the fat bodies and hemocytes. Its transcript levels increased significantly upon bacterial and fungal challenges. Purified recombinant PGRP1 exhibited binding activity to the gram-positive Micrococcus luteus, gram-negative Escherichia coli, entomopathogenic fungi Beauveria bassiana, and yeast Pichia pastoris. The binding further induced their agglutination. Additionally, PGRP1 preferred to bind to Lys-type peptidoglycans rather than DAP-type peptidoglycans. The addition of recombinant PGRP1 to O. furnacalis plasma resulted in a significant increase in phenoloxidase activity. The injection of recombinant PGRP1 into larvae led to a significantly increased expression of several antimicrobial peptide genes. Taken together, our results suggest that O. furnacalis PGRP1 potentially recognizes the invading microbes and is involved in the immune response in O. furnacalis.

Immune-related genes of the larval Holotrichia parallela in response to entomopathogenic nematodes Heterorhabditis beicherriana LF

Background

Entomopathogenic nematodes (EPNs) emerge as compatible alternatives to conventional insecticides in controlling Holotrichia parallela larvae (Coleoptera: Scarabaeidae). However, the immune responses of H. parallela against EPNs infection remain unclear.

Results

In present research, RNA-Seq was firstly performed. A total of 89,427 and 85,741 unigenes were achieved from the midgut of H. parallela larvae treated with Heterorhabditis beicherriana LF for 24 and 72 h, respectively; 2545 and 3156 unigenes were differentially regulated, respectively. Among those differentially expressed genes (DEGs), 74 were identified potentially related to the immune response. Notably, some immune-related genes, such as peptidoglycan recognition protein SC1 (PGRP-SC1), pro-phenoloxidase activating enzyme-I (PPAE-I) and glutathione s-transferase (GST), were induced at both treatment points. Bioinformatics analysis showed that PGRP-SC1, PPAE-I and GST were all involved in anti-parasitic immune process. Quantitative real-time PCR (qRT-PCR) results showed that the three immune-related genes were expressed in all developmental stages; PGRP-SC1 and PPAE-I had higher expressions in midgut and fat body, respectively, while GST exhibited high expression in both of them. Moreover, in vivo silencing of them resulted in increased susceptibility of H. parallela larvae to H. beicherriana LF.

Conclusion

These results suggest that H. parallela PGRP-SC1, PPAE-I and GST are involved in the immune responses to resist H. beicherriana LF infection. This study provides the first comprehensive transcriptome resource of H. parallela exposure to nematode challenge that will help to support further comparative studies on host-EPN interactions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07506-4.

Structure-function analysis of PGRP-S1 from the oriental armyworm, Mythimna separata

Peptidoglycan recognition proteins (PGRPs) are well known for their abilities to recognize or hydrolyze peptidoglycan (PGN), one of the major bacterial cell wall components. However, much less is known about their antifungal activities. PGRP-S1 was previously identified from a crop pest, Mythimna separata (Walker) (Lepidoptera: Noctuidae). PGRP-S1 showed bacteriolytic activities against Gram-positive and Gram-negative bacteria. In this study, tissue expression analysis showed that PGRP-S1 was mainly expressed in the midgut of naïve larvae. The induction analysis showed that it was significantly induced in the larval midgut 12 h post the injection of Beauveria bassiana conidia. To identify the key residues that are related to its microbicidal activities, the structure of PGPR-S1 was predicted for structural comparison and molecular docking analysis. Six residues (H61, H62, Y97, H171, T175, and C179) were mutated to Ala individually by site-directed mutagenesis. The recombinant wild-type (WT) and mutant proteins were expressed and purified. The recombinant proteins bound to different polysaccharides, PGNs, and bacteria. H61A, Y97A, H171A, and C179A lost amidase activity. Accordingly, antibacterial assay and scanning electron microscopy confirmed that only H62A and T175A retained bacteriolytic activities. The germination of B. bassiana conidia was significantly inhibited by WT, H61A, Y97A, T175A, and C179A mutants. Electron microscopy showed that some conidia became ruptured after treatment. The growth of hyphae was inhibited by the WT, H61A, H62A, and T175A. In summary, our data showed that different residues of PGRP-S1 are involved in the antibacterial and antifungal activities.

Hemolymph protease-5 links the melanization and Toll immune pathways in the tobacco hornworm, Manduca sexta

Proteolytic activation of phenoloxidase (PO) and the cytokine Spätzle during immune responses of insects is mediated by a network of hemolymph serine proteases (HPs) and noncatalytic serine protease homologs (SPHs) and inhibited by serpins. However, integration and conservation of the system and its control mechanisms are not fully understood. Here we present biochemical evidence that PO-catalyzed melanin formation, Spätzle-triggered Toll activation, and induced synthesis of antimicrobial peptides are stimulated via hemolymph (serine) protease 5 (HP5) in Manduca sexta Previous studies have demonstrated a protease cascade pathway in which HP14 activates proHP21; HP21 activates proPAP2 and proPAP3, which then activate proPO in the presence of a complex of SPH1 and SPH2. We found that both HP21 and PAP3 activate proHP5 by cleavage at ESDR176*IIGG. HP5 then cleaves proHP6 at a unique site of LDLH112*ILGG. HP6, an ortholog of Drosophila Persephone, activates both proHP8 and proPAP1. HP8 activates proSpätzle-1, whereas PAP1 cleaves and activates proPO. HP5 is inhibited by Manduca sexta serpin-4, serpin-1A, and serpin-1J to regulate its activity. In summary, we have elucidated the physiological roles of HP5, a CLIPB with unique cleavage specificity (cutting after His) that coordinates immune responses in the caterpillar.

Pattern recognition receptors from lepidopteran insects and their biological functions

Lepidopteran insects have potent innate immunity to fight against the invading pathogens. As the initiation step, pattern recognition receptors (PRRs) recognize and bind microbial surface configurations known as pathogen-associated molecular patterns (PAMPs). Aftermath, they initiate both cellular and humoral immune responses, including phagocytosis, agglutination, nodulation, encapsulation, prophenoloxidase activation, and synthesis of antimicrobial peptides. In this review, we summarize the recent findings concerning PRRs in lepidoptaeran insects, mostly agriculture pests including Helicoverpa armigera, Plutella xylostella, and Spodoptera exigua. We mainly focus on the function and phylogeny of C-type lectins (CTLs), peptidoglycan recognition proteins (PGRPs), β-1,3-glucan recognition proteins (βGRPs), and galectins (GALEs). It enriches our understanding of the immune system of lepidopteran insects and provides directions in the future research.

Immune responses to Bacillus thuringiensis in the midgut of the diamondback moth, Plutella xylostella

The diamondback moth, Plutella xylostella, is the first insect to develop resistance to Bacillus thuringiensis (Bt) in the field. To date, little is known about the molecular mechanism of the interaction between Bt and midgut immunity in P. xylostella. Here, we report immune responses in the P. xylostella midgut to Bt strain Bt8010 using a combined approach of transcriptomics and quantitative proteomics. Many genes in the Toll, IMD, JNK and JAK-STAT pathways and antimicrobial peptide genes were activated at 18 h post-infection. In the prophenoloxidase (PPO) cascade, four serpin genes were activated, and the PPO1 gene was suppressed by Bt8010. Inhibition of the two PPO proteins was observed at 18 h post-infection. Feeding Bt8010-infected larvae recombinant PPOs enhanced their survival. These results revealed that the Toll, IMD, JNK and JAK-STAT pathways were triggered and participated in the immune defence of the midgut against Bt8010, while the PPO cascade was inhibited and played an important role in this process.

Inhibition of immune pathway-initiating hemolymph protease-14 by Manduca sexta serpin-12, a conserved mechanism for the regulation of melanization and Toll activation in insects

A network of serine proteases (SPs) and their non-catalytic homologs (SPHs) activates prophenoloxidase (proPO), Toll pathway, and other insect immune responses. However, integration and conservation of the network and its control mechanisms have not yet been fully understood. Here we present evidence that these responses are initiated through a conserved serine protease and negatively regulated by serpins in two species, Manduca sexta and Anopheles gambiae. We have shown that M. sexta serpin-12 reduces the proteolytic activation of HP6, HP8, proPO activating proteases (PAPs), SPHs, and POs in larval hemolymph, and we hypothesized that these effects are due to the inhibition of the immune pathway-initiating protease HP14. To test whether these changes are due to HP14 inhibition, we isolated a covalent complex of HP14 with serpin-12 from plasma using polyclonal antibodies against the HP14 protease domain or against serpin-12, and confirmed formation of the complex by 2D-electrophoresis, immunoblotting, and mass spectrometry. Upon recognition of bacterial peptidoglycans or fungal β-1,3-glucan, the zymogen proHP14 became active HP14, which formed an SDS-stable complex with serpin-12 in vitro. Activation of proHP21 by HP14 was suppressed by serpin-12, consistent with the decrease in steps downstream of HP21, proteolytic activation of proPAP3, proSPH1/2 and proPO in hemolymph. Guided by the results of phylogenetic analysis, we cloned and expressed A. gambiae proSP217 (an ortholog of HP14) and core domains of A. gambiae serpin-11 and -17. The recombinant SP217 zymogen became active during expression, with cleavage between Tyr394 and Ile395. Both MsHP14 and AgSP217 cleaved MsSerpin-12 and AgSRPN11 at Leu*Ser (P1*P1') and formed complexes in vitro. ProPO activation in M. sexta plasma increased after recombinant AgSP217 had been added, indicating that it may function in a similar manner as the endogenous initiating protease HP14. Based on these data, we propose that inhibition of an initiating modular protease by a serpin may be a common mechanism in holometabolous insects to regulate proPO activation and other protease-induced immune responses.

Hemocyte Changes During Immune Melanization in Bombyx Mori Infected with Escherichia coli

Hemolymph melanization is a conserved immune response in insects and other arthropods. However, the physiological process of the hemolymph system in the melanization response is hardly studied. Here, alterations of hemocytes in immune melanization were observed by Escherichia coli infection in Bombyx mori. Results first showed that there were cells aggregating into clusters. However, it vanished, and only part of clustered hemocytes were melanized during the period of intense immunity. The hemocyte numbers immediately decreased following an immune challenge, slowly increased to a peak, then reduced and finally returned to normalization. Granulocytes participated in cells aggregation at the early and later immune stage, while plasmatocytes were responsible for hemocytes agglomerate and melanization for the longest time, and more oenocytoids appeared at the peak stage of melanization. Moreover, hemocytes played a crucial role in resisting invasion of pathogens by agglomerate and melanization, and the circulatory system maintained higher hemocyte numbers and stronger antibacterial activity in fifth than fourth instar larvae after infection. In vitro immune melanization was most likely preferentially implemented in an independent process. These were the main characteristics reflecting the physiological process of hemolymph immune melanization, which provided an important foundation for further study of the complete mechanisms in the immunity of silkworm.

The three-dimensional structure and recognition mechanism of Manduca sexta peptidoglycan recognition protein-1

Peptidoglycan recognition proteins (PGRPs) recognize bacteria through their unique cell wall constituent, peptidoglycans (PGs). PGRPs are conserved from insects to mammals and all function in antibacterial defense. In the tobacco hornworm Manduca sexta, PGRP1 and microbe binding protein (MBP) interact with PGs and hemolymph protease-14 precursor (proHP14) to yield active HP14. HP14 triggers a serine protease network that produces active phenoloxidase (PO), Spätzle, and other cytokines to stimulate immune responses. PGRP1 binds preferentially to diaminopimelic acid (DAP)-PGs of Gram-negative bacteria and Gram-positive Bacillus and Clostridium species than Lys-PGs of other Gram-positive bacteria. In this study, we synthesized DAP- and Lys-muramyl pentapeptide (MPP) and monitored their associations with M. sexta PGRP1 by surface plasmon resonance. The K_d values (0.57 μM for DAP-MPP and 45.6 μM for Lys-MPP) agree with the differential recognition of DAP- and Lys-PGs. To reveal its structural basis, we produced the PGRP1 in insect cells and determined its structure at a resolution of 2.1 Å. The protein adopts a fold similar to those from other PGRPs with a classical L-shaped PG-binding groove. A unique loop lining the shallow groove suggests a different ligand-binding mechanism. In summary, this study provided new insights into the PG recognition by PGRPs, a critical first step that initiates the serine protease cascade.

Characterization of PGRP-S1 from the oriental armyworm, Mythimna separata

Peptidoglycan is the key component forming the backbone of bacterial cell wall. It can be recognized by a group of pattern recognition receptors, known as peptidoglycan recognition proteins (PGRPs) in insects and higher animals. PGRPs may serve as immune receptors or N-acetylmuramoyl-L-alanine amidases (EC 3.5.1.28). Here, we report the characterization of a short PGRP, PGRP-S1, from the oriental armyworm, Mythimna separata. MsePGRP-S1 cDNA encodes a protein of 197 amino acids (aa) with a PGRP domain of about 150 aa. MsePGRP-S1 was expressed in several tissues of naïve larvae, including hemocytes, midgut, fat body and epidermis. Bacterial challenges caused variable changes in different tissues at the mRNA level. The recombinant protein bound strongly to Staphylococcus aureus and purified peptidoglycans from Staphylococcus aureus and Bacillus subtilis. It can inhibit the growth of gram-negative and gram-positive bacteria by disrupting bacterial surface. It can degrade peptidoglycans from Escherichia coli and Staphylococcus aureus. Taken together, these data demonstrate that M. separata PGRP-S1 is involved in defending against bacteria.

Peptidoglycan recognition proteins in insect immunity

Insects lack an acquired immune system and rely solely on the innate immune system to combat microbial infection. The innate immunity of insects mainly depends on the interaction between the host's pattern recognition receptor (PRR) and pathogen-associated molecular pattern (PAMP). The peptidoglycan recognition proteins (PGRPs) family is the most important pattern recognition receptor (PRR) for insects. It can recognize the main component of the cell wall of the pathogenic microorganism, peptidoglycan (PGN), and plays an important role in the innate immunity of insects. In this paper, the structure, classification, and function of PGRPs is summarized, and the role of PGRPs in the innate immunity of insects is also discussed.

Involvement of a short-type peptidoglycan recognition protein (PGRP) from Chinese giant salamanders Andrias davidianus in the immune response against bacterial infection

PGRPs (Peptidoglycan recognition proteins) could recognize peptidoglycan and play vital roles in innate immunity among different animals. Till present, the functions of PGRP have been studied in various animals, but few reports have studied the amphibian PGRPs. In the current research, a short type PGRP was identified from Chinese giant salamander and its involvement in the innate immunity was studied. The ORF of AdPGRP-SC2 cDNA was 573 bp, which encoded 190 amino acids, and contained a PGRP and an amidase_2 domain. The qPCR analysis revealed that AdPGRP-SC2 mRNA transcripts expressed in different tissues, with the highest expression level in muscle, intestine and spleen. Results of immune challenges with peptidoglycan (PGN) demonstrated that expression patterns of AdPGRP-SC2 were significantly up-regulated in erythrocyte and spleen at the early injection stage. The recombinant AdPGRP-SC2 protein was successfully produced and purified, and it could show binding affinity to different bacteria. In the presence of Zn²⁺, the rAdPGRP-SC2 could exhibit a broad PAMPs binding activities, strongly agglutinate bacteria and exhibit amidase enzyme activity. Collectively, these data indicate AdPGRP-SC2 could act as PRR to recognize the invading microorganisms and as the antimicrobial effectors during the innate immune response of A. davidianus.

Manduca sexta hemolymph protease-2 (HP2) activated by HP14 generates prophenoloxidase-activating protease-2 (PAP2) in wandering larvae and pupae

Melanization is a universal defense mechanism of insects against microbial infection. During this response, phenoloxidase (PO) is activated from its precursor by prophenoloxidase activating protease (PAP), the terminal enzyme of a serine protease (SP) cascade. In the tobacco hornworm Manduca sexta, hemolymph protease-14 (HP14) is autoactivated from proHP14 to initiate the protease cascade after host proteins recognize invading pathogens. HP14, HP21, proHP1*, HP6, HP8, PAP1-3, and non-catalytic serine protease homologs (SPH1 and SPH2) constitute a portion of the extracellular SP-SPH system to mediate melanization and other immune responses. Here we report the expression, purification, and functional characterization of M. sexta HP2. The HP2 precursor is synthesized in hemocytes, fat body, integument, nerve and trachea. Its mRNA level is low in fat body of 5th instar larvae before wandering stage; abundance of the protein in hemolymph displays a similar pattern. HP2 exists as an active enzyme in plasma of the wandering larvae and pupae in the absence of an infection. HP14 cleaves proHP2 to yield active HP2. After incubating active HP2 with larval hemolymph, we detected higher levels of PO activity, i.e. an enhancement of proPO activation. HP2 cleaved proPAP2 (but not proPAP3 or proPAP1) to yield active PAP2, responsible for a major increase in IEARpNA hydrolysis. PAP2 activates proPOs in the presence of a cofactor of SPH1 and SPH2. In summary, we have identified a new member of the proPO activation system and reconstituted a pathway of HP14-HP2-PAP2-PO. Since high levels of HP2 mRNA were present in integument and active HP2 in plasma of wandering larvae, HP2 likely plays a role in cuticle melanization during pupation and protects host from microbial infection in a soil environment.

Comparative analysis of the Monochamus alternatus immune system

The pine sawyer beetle, Monochamus alternatus, is regarded as a notorious forest pest in Asia, vectoring an invasive pathogenic nematode, Bursaphelenchus xylophilus, which is known to cause pine wilt disease. However, little sequence information is available for this vector beetle. This hampered the research on its immune system. Based on the transcriptome of M. alternatus, we have identified and characterized 194 immunity-related genes in M. alternatus, and compared them with homologues molecules from other species known to exhibit immune responses against invading microbes. The lower number of putative immunity-related genes in M. alternatus were attributed to fewer C-type lectin, serine protease (SP) and anti-microbial peptide (AMP) genes. Phylogenetic analysis revealed that M. alternatus had a unique recognition gene, galectin3, orthologues of which were not identified in Tribolium castaneum, Drosophila melanogastor, Anopheles gambiae and Apis mellifera. This suggested a lineage-specific gene evolution for coleopteran insects. Our study provides the comprehensive sequence resources of the immunity-related genes of M. alternatus, presenting valuable information for better understanding of the molecular mechanism of innate immunity processes in M. alternatus against B. xylophilus.

Innate immune responses in the Chinese oak silkworm, Antheraea pernyi

Innate immunity, the evolutionarily conserved defense system, has been extensively analyzed in insect models over recent decades. The significant progress in this area has formed our dominant conceptual framework of the innate immune system, but critical advances in other insects have had a profound impact on our insights into the mystery of innate immunity. In recent years, we focused on the immune responses in Antheraea pernyi, an important commercial silkworm species reared in China. Here, we review the immune responses of A. pernyi based on immune-related gene-encoded proteins that are divided into five categories, namely pattern recognition receptors, hemolymph proteinases and their inhibitors, prophenoloxidase, Toll pathway factors and antimicrobial peptides, and others. Although the summarized information is limited since the research on A. pernyi immunity is in its infancy, we hope to provide evidence for further exploration of innate immune mechanisms.

Integrin β3 plays a novel role in innate immunity in silkworm, Bombyx mori

Integrins are transmembrane receptors that play essential roles in many physiological and pathological processes through cell-to-cell and cell-to-extracellular matrix (ECM) interactions. In the current study, a 2653-bp full-length cDNA of a novel integrin β subunit (designated Bmintegrin β3) was obtained from silkworm hemocytes. Bmintegrin β3 has the typical conserved structure of the integrin β family. The qRT-PCR results showed that Bmintegrin β3 was specifically expressed in the hematological system and that its expression was significantly increased after challenge with different types of PAMPs and bacteria. The recombinant Bmintegrin β3 protein displayed increased aggregation with S. aureus, suggesting that Bmintegrin β3 might directly bind to PAMPs. Interestingly, Bmintegrin β3 knockdown promoted PPO1, PPO2, BAEE, SPH78, SPH125, and SPH127 expression and accelerated the melanization process. Unexpectedly, the expression of genes related to phagocytosis, the Toll pathway, and the IMD pathway was also up-regulated after Bmintegrin β3 knockdown. Thus, Bmintegrin β3 might be a pattern recognition protein (PRP) for PAMPs and might directly bind to bacteria and enhance the phagocytosis activity of hemocytes. Moreover, Bmintegrin β3 and its ligand might negatively regulate the expression of immune-related genes through an unknown mechanism. In summary, our studies provide new insights into the immune functions of Bmintegrin β3 from the silkworm, Bombyx mori.

20-Hydroxyecdysone activates PGRP-SA mediated immune response in Locusta migratoria

20-hydroxyecdysone (20E) has been implicated in regulating the immune response in insects. Conflicting conclusions on 20E regulating immunity have been reported in model holometabolous species. However, in hemimetabolous insects, the role of 20E as an immune-suppressor or activator and the mechanism remains unclear. The migratory locust Locusta migratoria is a representative member of hemimetabolous insects. Here, digital gene expression (DGE) profiles of Locusta migratoria treated with 20E were analyzed. Pattern recognition receptors [peptidoglycan recognition protein (PGRP-SA), PGRP-LE, and gram-negative binding protein (GNBP3)] and antimicrobial peptides (defensin, diptericin, and i-type lysozyme) were significantly induced by 20E in fat body. These immune-related genes significantly increased their mRNA levels during the high-20E stage. Antibacterial activities in plasma were enhanced after 20E injection and during the high-20E developmental stage. Conversely, when 20E signal was suppressed by RNAi of EcR (ecdysone receptor), the expression levels of these genes and antibacterial activities failed to be increased by 20E injection and during the high-20E developmental stage, and the mortality increased after being infected by entomogenous fungus. The knockdown of PGRP-SA inhibited the expression level of defensin, diptericin and i-type lysozyme in fat body and reduced antibacterial activities in plasma. 20E injection could not significantly induce the expression of antimicrobial peptides after RNAi of PGRP-SA. These results demonstrated that 20E enhanced the immune response by activating PGRP-SA in L. migratoria.

Prophenoloxidase activation and antimicrobial peptide expression induced by the recombinant microbe binding protein of Manduca sexta

Manduca sexta microbe binding protein (MBP) is a member of the β-1,3-glucanase-related protein superfamily that includes Gram-negative bacteria-binding proteins (GNBPs), β-1,3-glucan recognition proteins (βGRPs), and β-1,3-glucanases. Our previous and current studies showed that the purified MBP from baculovirus-infected insect cells had stimulated prophenoloxidase (proPO) activation in the hemolymph of naïve and immune challenged larvae and that supplementation of the exogenous MBP and peptidoglycans (PGs) had caused synergistic increases in PO activity. To explore the underlying mechanism, we separated by SDS-PAGE naïve and induced larval plasma treated with buffer or MBP and detected on immunoblots changes in intensity and/or mobility of hemolymph (serine) proteases [HP14, HP21, HP6, HP8, proPO-activating proteases (PAPs) 1-3] and their homologs (SPH1, SPH2). In a nickel pull-down assay, we observed association of MBP with proHP14 (slightly), βGRP2, PG recognition protein-1 (PGRP1, indirectly), SPH1, SPH2, and proPO2. Further experiments indicated that diaminopimelic acid (DAP) or Lys PG, MBP, PGRP1, and proHP14 together trigger the proPO activation system in a Ca²⁺-dependent manner. Injection of the recombinant MBP into the 5th instar naïve larvae significantly induced the expression of several antimicrobial peptide genes, revealing a possible link between HP14 and immune signal transduction. Together, these results suggest that the recognition of Gram-negative or -positive bacteria via their PGs induces the melanization and Toll pathways in M. sexta.

Identification of transcripts involved in digestion, detoxification and immune response from transcriptome of Empoasca vitis (Hemiptera: Cicadellidae) nymphs

Tea production has been significantly impacted by the false-eye leafhopper, Empoasca vitis (Göthe), around Asia. To identify the key genes which are responsible for nutrition absorption, xenobiotic metabolism and immune response, the transcriptome of either alimentary tracts or bodies minus alimentary tract of E. vitis was sequenced and analyzed. Over 31 million reads were obtained from Illumina sequencing. De novo sequence assembly resulted in 52,182 unigenes with a mean size of 848nt. The assembled unigenes were then annotated using various databases. Transcripts of at least 566 digestion-, 224 detoxification-, and 288 immune-related putative genes in E. vitis were identified. In addition, relative expression of highly abundant transcripts was verified through quantitative real-time PCR. Results from this investigation provide genomic information about E. vitis, which will be helpful in further study of E. vitis biology and in the development of novel strategies to control this devastating pest.

In search of a function of Manduca sexta hemolymph protease-1 in the innate immune system

Extracellular serine protease cascades mediate immune signaling and responses in insects. In the tobacco hornworm Manduca sexta, nearly 30 serine proteases (SPs) and their homologs (SPHs) are cloned from hemocytes and fat body. Some of them participate in prophenoloxidase (proPO) activation and proSpätzle processing. Here we report the cDNA cloning of hemolymph protease-1b (HP1b), which is 90% identical and 95% similar to HP1a (formerly HP1). The HP1a and HP1b mRNA levels in hemocytes was down- and up-regulated after an immune challenge, respectively. Quantitative real-time polymerase chain reactions revealed their tissue-specific and development-dependent expression, mostly in hemocytes of the feeding larvae. We isolated HP1 precursor (proHP1) from larval hemolymph and observed micro-heterogeneity caused by N-linked glycosylation. Supplementation of the purified proHP1 to plasma samples from naïve larvae or induced ones injected with bacteria caused a small PO activity increase, much lower than those elicited by recombinant proHP1a/b, but no proteolytic cleavage was detected in the zymogens. Incubation of proHP1a/b or their catalytic domains with a cationic detergent, cetylpyridinium chloride, induced an amidase activity that hydrolyzed LDLH-p-nitroanilide. Since LDLH corresponds to the P4-P1 region before the proteolytic activation site of proHP6, we propose that the active but uncleaved proHP1 may cut proHP6 to generate HP6 that in turn activates proPAP1 and proHP8. The catalytic domain of HP1a/b, which by itself does not activate purified proHP6 or hydrolyze LDLH-p-nitroanilide, somehow generated active HP6, HP8, PAP1 and PO in plasma. Together, these results indicate that proHP1 participates in the proPO activation system, although detailed mechanism needs further exploration.

Peptidoglycan recognition protein-S5 functions as a negative regulator of the antimicrobial peptide pathway in the silkworm, Bombyx mori

Prophenoloxidase (proPO), immune deficiency (IMD), and Toll are the major signaling pathways leading to melanization and antimicrobial peptide production in insect hemolymph. Peptidoglycan recognition proteins (PGRPs) act as receptors and negative regulators in these pathways, and some PGRPs exhibit antimicrobial activity. Previously, we demonstrated that silkworm PGRP-S5 recognizes peptidoglycans (PGs) and triggers activation of the proPO pathway. It also acts as a bactericide, via its amidase activity (Chen et al., 2014). Here, we generated a C177S site-mutated silkworm PGRP-S5 protein that lacked amidase activity but retained its PG-binding capacity. Functional studies showed that the mutation caused loss of its receptor function for activation of the proPO pathway, suggesting that processing of PG by PGRP-S5 is necessary for formation of the pathway initiation complex. Further, we found that PGRP-S5 negatively regulates antimicrobial peptides generation in an amidase-dependent manner, likely through the IMD pathway. Thus, silkworm PGRP-S5 acts as a sensor, a modulator, and an effector in the silkworm humoral immune system.

Mechanisms of nodule-specific melanization in the hemocoel of the silkworm, Bombyx mori

In the insect immune system, nodules are known to be a product of the cellular response against microorganisms and may be a preferential target for melanization. However, the mechanism of nodule-preferential melanization remains to be explored. In this study, we identified several mechanisms of nodule-preferential melanization by analyzing congregation and the activation of several factors involved in the prophenoloxidase (proPO)-activating system in the silkworm, Bombyx mori. Microorganism-binding assays revealed that B. mori larval plasma have an effective invading microorganism-surveillance network consisting of at least six pattern-recognition receptors (PRRs). We also found that a hemolymph serine proteinase, BmHP14, can bind to Saccharomyces cerevisiae. Pull-down assays showed that PRR C-type lectins form protein complexes with serine proteinase homologs, BmSPH1 and BmSPH2, which leads to the activated forms of BmSPH1 and BmSPH2 being gathered on microorganisms and trapped in nodules. Immunostaining analysis revealed that most factors in the proPO-activating system and some factors in the triggering system for antimicrobial peptide production exist in the granules of hemocytes which can gather in nodules. Western blot analysis showed that factors in the proPO-activating system are congregated in formed nodules by their concentration in plasma and aggregating hemocytes.

Activity Augmentation of Amphioxus Peptidoglycan Recognition Protein BbtPGRP3 via Fusion with a Chitin Binding Domain

Peptidoglycan recognition proteins (PGRPs), which have been identified in most animals, are pattern recognition molecules that involve antimicrobial defense. Resulting from extraordinary expansion of innate immune genes, the amphioxus encodes many PGRPs of diverse functions. For instance, three isoforms of PGRP encoded by Branchiostoma belcheri tsingtauense, termed BbtPGRP1~3, are fused with a chitin binding domain (CBD) at the N-terminus. Here we report the 2.7 Å crystal structure of BbtPGRP3, revealing an overall structure of an N-terminal hevein-like CBD followed by a catalytic PGRP domain. Activity assays combined with site-directed mutagenesis indicated that the individual PGRP domain exhibits amidase activity towards both DAP-type and Lys-type peptidoglycans (PGNs), the former of which is favored. The N-terminal CBD not only has the chitin-binding activity, but also enables BbtPGRP3 to gain a five-fold increase of amidase activity towards the Lys-type PGNs, leading to a significantly broadened substrate spectrum. Together, we propose that modular evolution via domain shuffling combined with gene horizontal transfer makes BbtPGRP1~3 novel PGRPs of augmented catalytic activity and broad recognition spectrum.

Insect parents improve the anti-parasitic and anti-bacterial defence of their offspring by priming the expression of immune-relevant genes

Insect parents that experienced an immune challenge are known to prepare (prime) the immune activity of their offspring for improved defence. This phenomenon has intensively been studied by analysing especially immunity-related proteins. However, it is unknown how transgenerational immune priming affects transcript levels of immune-relevant genes of the offspring upon an actual threat. Here, we investigated how an immune challenge of Manduca sexta parents affects the expression of immune-related genes in their eggs that are attacked by parasitoids. Furthermore, we addressed the question whether the transgenerational immune priming of expression of genes in the eggs is still traceable in adult offspring. Our study revealed that a parental immune challenge did not affect the expression of immune-related genes in unparasitised eggs. However, immune-related genes in parasitised eggs of immune-challenged parents were upregulated to a higher level than those in parasitised eggs of unchallenged parents. Hence, this transgenerational immune priming of the eggs was detected only "on demand", i.e. upon parasitoid attack. The priming effects were also traceable in adult female progeny of immune-challenged parents which showed higher transcript levels of several immune-related genes in their ovaries than non-primed progeny. Some of the primed genes showed enhanced expression even when the progeny was left unchallenged, whereas other genes were upregulated to a greater extent in primed female progeny than non-primed ones only when the progeny itself was immune-challenged. Thus, the detection of transgenerational immune priming strongly depends on the analysed genes and the presence or absence of an actual threat for the offspring. We suggest that M. sexta eggs laid by immune-challenged parents "afford" to upregulate the transcription of immunity-related genes only upon attack, because they have the chance to be endowed by parentally directly transferred protective proteins.

Transcriptomic insight into the immune defenses in the ghost moth, Hepialus xiaojinensis, during an Ophiocordyceps sinensis fungal infection

Hepialus xiaojinensis is an economically important species of Lepidopteran insect. The fungus Ophiocordyceps sinensis can infect its larvae, which leads to mummification after 5-12 months, providing a valuable system with which to study interactions between the insect hosts and pathogenic fungi. However, little sequence information is available for this insect. A time-course analysis of the fat body transcriptome was performed to explore the host immune response to O. sinensis infection. In total, 50,164 unigenes were obtained by assembling the reads from two high-throughput approaches: 454 pyrosequencing and Illumina Hiseq2000. Hierarchical clustering and functional examination revealed four major gene clusters. Clusters 1-3 included transcripts markedly induced by the fungal infection within 72 h. Cluster 4, with a lower number of transcripts, was suppressed during the early phase of infection but returned to normal expression levels sometime before 1 year. Based on sequence similarity to orthologs known to participate in immune defenses, 258 candidate immunity-related transcripts were identified, and their functions were hypothesized. The genes were more primitive than those in other Lepidopteran insects. In addition, lineage-specific family expansion of the clip-domain serine proteases and C-type lectins were apparent and likely caused by selection pressures. Global expression profiles of immunity-related genes indicated that H. xiaojinensis was capable of a rapid response to an O. sinensis challenge; however, the larvae developed tolerance to the fungus after prolonged infection, probably due to immune suppression. Specifically, antimicrobial peptide mRNAs could not be detected after chronic infection, because key components of the Toll pathway (MyD88, Pelle and Cactus) were downregulated. Taken together, this study provides insights into the defense system of H. xiaojinensis, and a basis for understanding the molecular aspects of the interaction between the host and the entomopathogen.

Phylogenetic analysis and expression profiling of the pattern recognition receptors: Insights into molecular recognition of invading pathogens in Manduca sexta

Pattern recognition receptors (PRRs) detect microbial pathogens and trigger innate immune responses. Previous biochemical studies have elucidated the physiological functions of eleven PRRs in Manduca sexta but our understanding of the recognition process is still limited, lacking genomic perspectives. While 34 C-type lectin-domain proteins and 16 Toll-like receptors are reported in the companion papers, we present here 120 other putative PRRs identified through the genome annotation. These include 76 leucine-rich repeat (LRR) proteins, 14 peptidoglycan recognition proteins, 6 EGF/Nim-domain proteins, 5 β-1,3-glucanase-related proteins, 4 galectins, 4 fibrinogen-related proteins, 3 thioester proteins, 5 immunoglobulin-domain proteins, 2 hemocytins, and 1 Reeler. Sequence alignment and phylogenetic analysis reveal the evolution history of a diverse repertoire of proteins for pathogen recognition. While functions of insect LRR proteins are mostly unknown, their structure diversification is phenomenal: In addition to the Toll homologs, 22 LRR proteins with a signal peptide are expected to be secreted; 18 LRR proteins lacking signal peptides may be cytoplasmic; 36 LRRs with a signal peptide and a transmembrane segment may be non-Toll receptors on the surface of cells. Expression profiles of the 120 genes in 52 tissue samples reflect complex regulation in various developmental stages and physiological states, including some likely by Rel family transcription factors via κB motifs in the promoter regions. This collection of information is expected to facilitate future biochemical studies detailing their respective roles in this model insect.

Molecular characterization of a peptidoglycan recognition protein from the cotton bollworm, Helicoverpa armigera and its role in the prophenoloxidase activation pathway

Peptidoglycan recognition proteins (PGRPs), which are evolutionarily conserved from invertebrates to vertebrates, function as pattern-recognition and effector molecules in innate immunity. In this study, a PGRP (HaPGRP-A) from the cotton bollworm, Helicoverpa armigera was identified and characterized. Sequence analysis indicated that HaPGRP-A is not an amidase-type PGRP. Increased levels of HaPGRP-A mRNA were observed in the fat body and hemocytes of H. armigera larvae following the injection of microbes or Sephadex beads. Analysis using purified recombinant HaPGRP-A showed that it (i) could bind and agglutinate Gram-negative Escherichia coli and Gram-positive Staphylococcus aureus, (ii) enhanced prophenoloxidase activation in the presence of microbes, (iii) promoted the formation of melanotic nodules in vivo, and (iv) enhanced the melanization of Sephadex beads in vivo. RNA interference assays were performed to further confirm the function of HaPGRP-A. When the expression of HaPGRP-A in H. armigera larvae was inhibited by dsHaPGRP-A injection, the phenoloxidase activity in larval hemolymph was significantly decreased and RNAi-treated insects infected with bacteria showed higher bacterial growth in hemolymph compared with infected control larvae. These results indicated that HaPGRP-A acts as a pattern recognition receptor and binds to the invading organism to trigger the prophenoloxidase activation pathway of H. armigera, and the activated phenoloxidase may participate in the melanization process of nodulation and encapsulation responses.

Identification and functional characterization of a peptidoglycan recognition protein from the cotton bollworm, Helicoverpa armigera

Peptidoglycan recognition proteins (PGRPs) specifically bind to peptidoglycans, and play crucial roles as pattern recognition receptors (PRRs) in mediating innate immune responses. In this study, we identified and characterized a PGRP (HaPGRP-D) from the cotton bollworm, Helicoverpa armigera. Sequence analysis indicated that HaPGRP-D is an amidase-type PGRP. Expression of HaPGRP-D was upregulated in the hemocytes of H. armigera larvae after injecting Gram-negative Escherichia coli, Gram-positive Staphylococcus aureus, or chromatography beads. To test the biological activity of HaPGRP-D, purified recombinant protein was prepared. Subsequent analysis showed that rHaPGRP-D (i) could bind and agglutinate Gram-negative E. coli and Gram-positive S. aureus in a zinc-dependent manner, (ii) functioned as an amidase to degrade peptidoglycans in the presence of Zn(2+) , (iii) strongly inhibited the growth of E. coli and S. aureus in the presence of Zn(2+) , (iv) could bind to the surface of hemocytes, (v) increased the phagocytosis of E. coli cells by hemocytes in vitro, and (vi) promoted hemocyte encapsulation on chromatography beads in vitro. These results suggest that HaPGRP-D plays important roles as PRR, amidase, and opsonin in H. armigera humoral and cellular immune responses.

Distinct regulation patterns of the two prophenoloxidase activating enzymes corresponding to bacteria challenge and their compensatory over expression feature in white shrimp (Litopenaeus vannamei)

Prophenoloxidase activating enzyme 2 (PPAE2), which belongs to the second PPAE family of prawns, was isolated from white shrimp Litopenaeus vannamei. The currently identified lvPPAE2 and lvPPAE1 from our former report were taken as model candidates to analyze the relationship of the two shrimp PPAE families as well as the regulation mechanism of shrimp PPAEs. The tissue expression of lvPPAE2 was more ubiquitous than lvPPAE1. The mRNA abundance of lvPPAE2 was about 10 percent of lvPPAE1 in co-existed tissues. When challenged with Vibrio harveyi. LvPPAE2 showed a distinct transcriptional regulation pattern compared to lvPPAE1. Silence of lvPPAE2 significantly increased shrimp's susceptibility to V. harveyi, suggesting the lvPPAE2 plays essential role in shrimp host defense. A novel PPAE specific compensatory over expression feature was found in the two lvPPAEs. Single gene specific silence of lvPPAE1 and lvPPAE2 resulted in a significant difference in reduction of hemolymph PO activity. Double silence of the two lvPPAEs failed to cause a further reduction on PO activity or shrimp mortality to bacteria, despite that double silence sufficiently suppressed both of the two lvPPAEs. Our findings suggest both lvPPAEs contribute to shrimp melanization cascade and host defense against bacteria. Distinct regulation pattern corresponding to the same pathogen invasion suggests the two lvPPAEs are actually under different regulation ways. A novel PPAE specific compensatory over expression mechanism found in our study offered us a clue in understanding the robustness of shrimp innate immunity and network of crustacean proPO activating system.

A short-type peptidoglycan recognition protein from the silkworm: expression, characterization and involvement in the prophenoloxidase activation pathway

Recognition of invading microbes as non-self is the first step of immune responses. In insects, peptidoglycan recognition proteins (PGRPs) detect peptidoglycans (PGs) of bacterial cell wall, leading to the activation of defense responses. Twelve PGRPs have been identified in the silkworm, Bombyx mori, through bioinformatics analysis. However, their biochemical functions are mostly uncharacterized. In this study, we found PGRP-S5 transcript levels were up-regulated in fat body and midgut after bacterial infection. Using recombinant protein isolated from Escherichia coli, we showed that PGRP-S5 binds to PGs from certain bacterial strains and induces bacteria agglutination. Enzyme activity assay confirmed PGRP-S5 is an amidase; we also showed it is an antibacterial protein effective against both Gram-positive and -negative bacteria. Additionally, we demonstrated that specific recognition of PGs by PGRP-S5 is involved in the prophenoloxidase activation pathway. Together, these data suggest the silkworm PGRP-S5 functions as a pattern recognition receptor for the prophenoloxidase pathway initiation and as an effecter to inhibit bacterial growth as well. We finally discussed possible roles of PGRP-S5 as a receptor for antimicrobial peptide gene induction and as an immune modulator in the midgut.

Identification of conserved and novel microRNAs in Manduca sexta and their possible roles in the expression regulation of immunity-related genes

The tobacco hornworm Manduca sexta has served as a model for insect biochemical and physiological research for decades. However, knowledge of the posttranscriptional regulation of gene expression by microRNAs is still rudimentary in this species. Our previous study (Zhang et al., 2012) identified 163 conserved and 13 novel microRNAs in M. sexta, most of which were present at low levels in pupae. To identify additional M. sexta microRNAs and more importantly to examine their possible roles in the expression regulation of immunity-related genes, we constructed four small RNA libraries using fat body and hemocytes from naïve or bacteria-injected larvae and obtained 32.9 million reads of 18-31 nucleotides by Illumina sequencing. Mse-miR-929 and mse-miR-1b (antisense microRNA of mse-miR-1) were predicted in the previous study and now found to be conserved microRNAs in the tissue samples. We also found four novel microRNAs, two of which result from a gene cluster. Mse-miR-281-star, mse-miR-965-star, mse-miR-31-star, and mse-miR-9b-star were present at higher levels than their respective mature strands. Abundance changes of microRNAs were observed after the immune challenge. Based on the quantitative data of mRNA levels in control and induced fat body and hemocytes as well as the results of microRNA target site prediction, we suggest that certain microRNAs and microRNA*s regulate gene expression for pattern recognition, prophenoloxidase activation, cellular responses, antimicrobial peptide synthesis, and conserved intracellular signal transduction (Toll, IMD, JAK-STAT, MAPK-JNK-p38, and small interfering RNA pathways). In summary, this study has enriched our knowledge on M. sexta microRNAs and how some of them may participate in the expression regulation of immunity-related genes.

Identification of immunity-related genes in Ostrinia furnacalis against entomopathogenic fungi by RNA-seq analysis

BACKGROUND

The Asian corn borer (Ostrinia furnacalis (Guenée)) is one of the most serious corn pests in Asia. Control of this pest with entomopathogenic fungus Beauveria bassiana has been proposed. However, the molecular mechanisms involved in the interactions between O. furnacalis and B. bassiana are unclear, especially under the conditions that the genomic information of O. furnacalis is currently unavailable. So we sequenced and characterized the transcriptome of O. furnacalis larvae infected by B. bassiana with special emphasis on immunity-related genes.

METHODOLOGY/PRINCIPAL FINDINGS

Illumina Hiseq2000 was used to sequence 4.64 and 4.72 Gb of the transcriptome from water-injected and B. bassiana-injected O. furnacalis larvae, respectively. De novo assembly generated 62,382 unigenes with mean length of 729 nt. All unigenes were searched against Nt, Nr, Swiss-Prot, COG, and KEGG databases for annotations using BLASTN or BLASTX algorithm with an E-value cut-off of 10(-5). A total of 35,700 (57.2%) unigenes were annotated to at least one database. Pairwise comparisons resulted in 13,890 differentially expressed genes, with 5,843 up-regulated and 8,047 down-regulated. Based on sequence similarity to homologs known to participate in immune responses, we totally identified 190 potential immunity-related unigenes. They encode 45 pattern recognition proteins, 33 modulation proteins involved in the prophenoloxidase activation cascade, 46 signal transduction molecules, and 66 immune responsive effectors, respectively. The obtained transcriptome contains putative orthologs for nearly all components of the Toll, Imd, and JAK/STAT pathways. We randomly selected 24 immunity-related unigenes and investigated their expression profiles using quantitative RT-PCR assay. The results revealed variant expression patterns in response to the infection of B. bassiana.

CONCLUSIONS/SIGNIFICANCE

This study provides the comprehensive sequence resource and expression profiles of the immunity-related genes of O. furnacalis. The obtained data gives an insight into better understanding the molecular mechanisms of innate immune processes in O. furnacalis larvae against B. bassiana.

Isolation and characterization of peptidoglycan recognition protein 1 from antler base of sika deer (Cervus nippon)

Peptidoglycan recognition proteins (PGRPs) are secreted innate immunity pattern recognition molecules. In this study, a new peptidoglycan recognition protein 1 named cnPGRP1 was isolated from an antler base of sika deer Cervus nippon. The antler base antimicrobial proteins (AAP) were subjected to consecutive chromatographic methods connected to Sephadex G-25 gel filtration column (CM) anion-exchange column, and RP-HPLC. The molecular weight of cnPGRP1 was 17.2 kDa under SDS-PAGE, and peptide mass fingerprint analysis by MALDI-TOF-MS as peptidoglycan recognition protein 1 matched to Dasypus novemcinctus. The matched amino acids sequences were RLYEIIQKWPHYRA. Both Gram-positive and Gram-negative bacteria can be killed by cnPGRP1 in the 50-250 μg/mL range through in vitro. Furthermore, cnPGRP1 has been found to bind Gram-positive bacteria, Gram-negative bacteria, and even fungus.