Molecular cloning and characterization of a β-1,3-glucan recognition protein from Plutella xylostella (L.)

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.

Molecular characterization and functional analysis of a beta-1,3-glucan recognition protein from oriental fruit moth Grapholita molesta (Lepidoptera: Tortricidae)

The beta-1,3-glucan recognition protein (BGRP) is an important pattern recognition protein (PRP), which plays an important role in immune recognition and signaling pathway of insect innate immunity. Herein, a BGRP gene was obtained from the transcriptome of Grapholita molesta and its expression was verified by PCR. The full cDNA of the GmBGRP gene was 1691 bp encoding 486 amino acid residues. The calculated molecular mass of the mature protein was 54.83 kDa with an estimated pI of 6.14. The amino acid sequence of GmBGRP was highly homologous to BGRPs of other lepidopterans including Leguminivora glycinivorella BGRP-3. Expression profile of GmBGRP at different developmental stages and different tissues of 5th instar larvae showed that the expression level of this gene tends to slightly increase and then decrease at the adult stage, with the highest at the pupa stage; and mainly expressed in the epidermis, fat body and hemocytes compared with other tissues. In addition, we investigated the transcription levels of other immune-related genes, such as Serine-1, Serine-2, Serine-3, Serpin, SRCB (scavenger receptor gene), Toll, PPO (prophenoloxidase) upon GmBGRP gene silencing, indicating that GmBGRP expression is associated with immune responses of G. molesta. This was further supported by the upregulation of the mRNA level of GmBGRP following fungal infection. Taken together, these results provide experimental evidence for the role of GmBGRP gene in immune defense in G. molesta larvae.

OfGNBP silencing enhances the toxicity of Serratia marcescens Bizio (SM1) to Odontotermes formosanus (Shiraki)

The immunity of insects plays a vital role in their survival. Our experiments found that lipopolysaccharide (LPS) and glucono-δ-lactone (GDL) could influence the virulence of Serratia marcescens Bizio (SM1) to Odontotermes formosanus (Shiraki) by affecting the immunity. Gram-negative binding proteins (GNBPs) are an important pattern recognition proteins that play a crucial role in the innate immune system. Therefore, two OfGNBPs were cloned in O. formosanus. The expression of OfGNBPs was significantly changed by LPS,SM1 and GDL, not prick. In addition, the immune-related gene expression, the phenoloxidase activity and antibacterial activity of donor termites and recipient termites were significantly induced by SM1. Furthermore, the knockdown of OfGNBP by RNA interference reduced not only individual immunity but also social immunity in O. formosanus, which increased the virulence of SM1 to O. formosanus. Importantly, dsOfGNBP alone also had good control effect on O. formosanus. In summary, we concluded that dsOfGNBPs are important termite immunosuppressants.

Silencing β-1,3-glucan binding protein enhances the susceptibility of Plutella xylostella to entomopathogenic fungus Isaria cicadae

BACKGROUND

The diamondback moth, Plutella xylostella is a notorious pest of brassicaceae crops globally and has developed serious resistance to insecticide. Insects primarily rely on their innate immunity to defense against various pathogens. In this study, we investigated the immunological functions of a β-1,3-glucan binding protein from P. xylostella (PxβGBP) and evaluated its potential for biocontrolling P. xylostella.

RESULTS

The open reading frame of PxβGBP is 1422 bp encoding 473 amino acids residues. PxβGBP contained a CBM39 domain, a PAC domain and a GH16 domain and shared evolutionary conservation with other lepidoptera βGRPs. PxβGBP was strongly expressed in the third instar larvae and fat body. PxβGBP transcript levels increased significantly after the challenge with microbes, including Isaria cicadae, Escherichia coli and Staphylococcus aureus. PxβGBP was identified in P. xylostella larvae challenged by I cicadae, but not in the naïve insects. Recombinant PxβGBP can directly bind fungal and bacterial cells, and also agglutinate the cells of I cicadae, S. aureus and E coli in a zinc-dependent manner. Knockdown of PxβGBP via RNA interference significantly down-regulated the expression of antimicrobial peptide gene gloverin, and enhanced the susceptibility of P. xylostella to I. cicadae infection, leading to high mortality.

CONCLUSION

These results indicated that PxβGBP plays an important role in the immune response of P. xylostella against I. cicadae infection, and could serve as a potential novel target for pest control. © 2022 Society of Chemical Industry.

Molecular and Functional Characterization of Peptidoglycan Recognition Proteins OfPGRP-A and OfPGRP-B in Ostrinia furnacalis (Lepidoptera: Crambidae)

Simple Summary

The Asian corn borer, Ostrinia furnacalis (Guenée), is the most destructive lepidopteran insect pest of corn (Zea mays L.) in China. Pathogenic microorganisms play an important role in the population control of the Asian corn borer. Although microorganisms can cause the death of O. furnacalis, an immune response also occurs as an attempt to fight off and eliminate invading pathogens. If the molecular mechanism of interaction between O. furnacalis and pathogenic bacteria is clarified, the lethal effect of pathogenic microorganisms can be better exerted by inhibiting the natural immune response of O. furnacalis. As an important member of the pattern-recognition receptor family, peptidoglycan recognition protein (PGRP) plays a key role in the insect innate immune response. In this study, we cloned two PGRP genes from O. furnacalis and analyzed their spatiotemporal expression. In combination with bacterial induction experiments, we revealed the immune signal recognition pathway involved in the two proteins. The results of this study deepen the understanding of the natural immune response of O. furnacalis and provide new ideas for better utilization of pathogenic microorganisms in biological control of the Asian corn borer.

Abstract

Peptidoglycan recognition proteins (PGRPs) are important components of insect immune systems, in which they play key roles. We cloned and sequenced two full-length PGRP, named OfPGRP-A and OfPGRP-B, from the Asian corn borer, Ostrinia furnacalis. These two genes comprise open reading frames of 658 and 759 bp, encoding proteins of 192 and 218 amino acids, respectively. qPCR showed that OfPGRP-A and OfPGRP-B are prominently expressed in the midgut of O. furnacalis fourth instar larvae. After inoculation with Staphylococcus aureus and Bacillus thuringiensis, the expression of OfPGRP-A was significantly upregulated, whereas the expression of OfPGRP-B was enhanced after inoculation with Escherichia coli. This suggests that OfPGRP-A mainly recognizes Gram-positive bacteria and may participate in the Toll signaling pathways, while OfPGRP-B identifies Gram-negative bacteria and may participate in Imd signaling pathways. Our results provide insights into the roles of PGRPs in O. furnacalis immune function and a foundation for using pathogens for the biological control of O. furnacalis.

Bombyx mori β-1,3-Glucan Recognition Protein 4 (BmβGRP4) Could Inhibit the Proliferation of B. mori Nucleopolyhedrovirus through Promoting Apoptosis

β-1,3-glucan recognition proteins (βGRPs) as pattern recognition receptors (PRRs) play an important role in recognizing various pathogens and trigger complicated signaling pathways in insects. In this study, we identified a Bombyx mori β-1,3-glucan recognition protein gene named BmβGRP4, which showed differential expression, from a previous transcriptome database. The full-length cDNA sequence was 1244 bp, containing an open reading frame (ORF) of 1128 bp encoding 375 amino acids. BmβGRP4 was strongly expressed in the larval stages and highly expressed in the midgut of B. mori larvae in particular. After BmNPV infection, the expression of BmβGRP4 was reduced significantly in the midgut. Furthermore, a significant increase in the copy number of BmNPV was observed after the knockdown of BmβGRP4 in 5th instar larvae, while the overexpression of BmβGRP4 suppressed the proliferation of BmNPV in BmN cells. Subsequently, the expression analysis of several apoptosis-related genes and observation of the apoptosis morphology demonstrated that overexpression of BmβGRP4 facilitated apoptosis induced by BmNPV in BmN cells. Moreover, BmβGRP4 positively regulated the phosphatase and tensin homolog gene (BmPTEN), while expression of the inhibitor of apoptosis gene (BmIAP) was negatively regulated by BmβGRP4. Hence, we hypothesize that BmNPV infection might suppress BmPTEN and facilitate BmIAP to inhibit cell apoptosis by downregulating the expression of BmβGRP4 to escape host antiviral defense. Taken together, these results show that BmβGRP4 may play a role in B. mori response to BmNPV infection and lay a foundation for studying its functions.

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.

Molecular Characterization of Two Mitogen-Activated Protein Kinases: p38 MAP Kinase and Ribosomal S6 Kinase From Bombyx mori (Lepidoptera: Bombycidae), and Insight Into Their Roles in Response to BmNPV Infection

Proteins p38 map kinase and ribosomal S6 kinase (S6K) as members of mitogen-activated protein kinases (MAPKs) play important roles against pathogens. In this study, Bmp38 and BmS6K were identified as differentially expressed proteins from iTRAQ database. Bmp38 and BmS6K were expressed, and recombinant proteins were purified. The bioinformatics analysis showed that both proteins have serine/threonine-protein kinases, catalytic domain (S_TKc) with 360 and 753 amino acids, respectively. The real-time quantitative polymerase chain reaction (RT-qPCR) results suggest that Bmp38 and BmS6K had high expression in the midgut and hemolymph. The comparative expression level of Bmp38 and BmS6K in BC9 was upregulated than in P50 in the midgut after Bombyx mori nucleopolyhedrovirus (BmNPV) infection. Western bolt results showed a positive correlation between RT-qPCR and iTRAQ data for Bmp38, but BmS6K data showed partial correlation with iTRAQ. Injection of anti-Bmp38 and anti-BmS6K serum suggested that Bmp38 may be involved against BmNPV infection, whereas BmS6K may require phosphorylation modification to inhibit BmNPV infection. Taken together, our results suggest that Bmp38 and BmS6k might play an important role in innate immunity of silkworm against BmNPV.

β-1,3-Glucan recognition protein 3 activates the prophenoloxidase system in response to bacterial infection in Ostrinia furnacalis Guenée

Pattern recognition receptors (PRRs) are biosensor proteins that bind to non-self pathogen associated molecular patterns (PAMPs). β-1,3-glucan recognition proteins (βGRPs) play an essential role in immune recognition and signaling pathway of insect innate immunity. Here, we report the cloning and characterization of cDNA of OfβGRP3 from Ostrinia furnacalis larvae. The OfβGRP3 contains 1455 bp open reading frame, encoding a predicted 484 amino acid residue protein. In hemocytes, the expression levels of OfβGRP3 in Escherichia coli-challenged group were higher than those of Bacillus subtilis-challenged group at 2, 4, 8, 10 and 12 h post injection (HPI). In fat body, OfβGRP3 expression in both B. subtilis and E. coli-challenged group was significantly higher than that in untreated group from 4 to 10 HPI, and then the expression continuously dropped from 12 to 36 HPI. The OfβGRP3 expression in laminarin-injected group was higher than that in lipopolysaccharides (LPS)-injected group in various test tissues from 4 to 24 HPI. The LT₅₀ of E. coli-infected OfβGRP3-RNAi larvae (1.0 days) was significantly lower compared with that of E. coli infected wild-type larvae (3.0 days) (p < 0.01). Only 10.2% Sephadex G50 beads (degree 3) were completely melanized in the larvae inoculated with OfβGRP3 dsRNA, as compared to 48.8% in control larvae (p < 0.01). A notable reduction in the PO activity and IEARase activity in hemolymph was also detected in the OfβGRP3 knockdown larvae. Our study demonstrates that OfβGRP3 is one of PRR members involved the PPO-activating system in O. furnacalis larvae.

An anionic defensin from Plutella xylostella with potential activity against Bacillus thuringiensis

Insect defensins, are cationic peptides that play an important role in immunity against microbial infection. In the present study, an anionic defensin from Plutella xylostella, (designated as PxDef) was first cloned and characterized. Amino acid sequence analysis showed that the mature peptide owned characteristic six-cysteine motifs with predicted isoelectric point of 5.57, indicating an anionic defensin. Quantitative real-time polymerase chain reaction analysis showed that PxDef was significantly induced in epidermis, fat body, midgut and hemocytes after injection of heat-inactivated Bacillus thuringiensis, while such an induction was delayed by the injection of live B. thuringiensis in the 4th instar larvae of P. xylostella. Knocking down the expression of nuclear transcription factor Dorsal in P. xylostella by RNA interference significantly decreased the mRNA level of PxDef, and increased the sensitivity of P. xylostella larvae to the infection by live B. thuringiensis. The purified recombinant mature peptide (PxDef) showed higher activity against Gram-positive bacteria, with the minimum inhibition concentrations of 1.6 and 2.6 µM against B. thuringiensis and Bacillus subtilis, respectively. To our knowledge, this is the first report about an anionic PxDef, which may play an important role in the immune system of P. xylostella against B. thuringiensis.