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

SfMBP: A novel microbial binding protein and pattern recognition receptor in the fall armyworm, Spodoptera frugiperda (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda, poses a significant threat as a highly destructive agricultural pest in many countries. Understanding the complex interplay between the insect immune system and entomopathogens is critical for optimizing biopesticide efficacy. In this study, we identified a novel microbial binding protein, SfMBP, in S. frugiperda. However, the specific role of SfMBP in the immune response of S. frugiperda remains elusive. Encoded by the LOC118269163 gene, SfMBP shows significant induction in S. frugiperda larvae infected with the entomopathogen Beauveria bassiana. Consisting of 115 amino acids with a signal peptide, an N-terminal flexible region and a C-terminal β-sheet, SfMBP lacks any known functional domains. It is expressed predominantly during early larval stages and in the larval epidermis. Notably, SfMBP is significantly induced in larvae infected with bacteria and fungi and in SF9 cells stimulated by peptidoglycan. While recombinant SfMBP (rSfMBP) does not inhibit bacterial growth, it demonstrates binding capabilities to bacteria, fungal spores, peptidoglycan, lipopolysaccharides, and polysaccharides. This binding is inhibited by monosaccharides and EDTA. Molecular docking reveals potential Zn2+-interacting residues and three cavities. Furthermore, rSfMBP induces bacterial agglutination in the presence of Zn2+. It also binds to insect hemocytes and SF9 cells, enhancing phagocytosis and agglutination responses. Injection of rSfMBP increased the survival of S. frugiperda larvae infected with B. bassiana, whereas blocking SfMBP with the antibody decreased survival. These results suggest that SfMBP acts as a pattern recognition receptor that enhances pathogen recognition and cellular immune responses. Consequently, this study provides valuable insights for the development of pest control measures.

Immune functions of pattern recognition receptors in Lepidoptera

Pattern recognition receptors (PRRs), as the "sensors" in the immune response, play a prominent role in recognizing pathogen-associated molecular patterns (PAMPs) and initiating an effective defense response to pathogens in Lepidoptera. It is becoming increasingly clear that damage-associated molecular patterns (DAMPs) normally play a physiological role within cells; however, when exposed to extracellular, they may become "part-time" critical signals of the immune response. Based on research in recent years, we review herein typical PRRs of Lepidoptera, including peptidoglycan recognition protein (PGRP), gram-negative binding protein (GNBP), β-1,3-glucan recognition protein (βGRP), C-type lectin (CTL), and scavenger receptor (SR). We also outline the ways in which DAMPs participate in the immune response and the correlation between PRRs and immune escape. Taken together, these findings suggest that the role of PRRs in insect innate immunity may be much greater than expected and that it is possible to recognize a broader range of signaling molecules.

Peptidoglycan recognition protein MsPGRP in largemouth bass (Micropterus salmoides) mediates immune functions with broad nonself recognition ability

Peptidoglycan (PGN) recognition proteins (PGRPs) are important immune factors in innate immunity that function in recognising pathogens and activating the immune system. These ubiquitous proteins are conserved in invertebrates and vertebrates. In this study, a PGRP gene (MsPGRP) from largemouth bass (Micropterus salmoides) was identified and characterised, and its transcription distribution was explored. Recombinant protein (rMsPGRP) exhibited dose-dependent binding to PGN and glucan (GLU), but weak binding to lipopolysaccharide (LPS). MsPGRP exhibited agglutinating activity against several Gram-negative bacteria, Gram-positive bacteria and fungi, and it promoted phagocytosis activity of leukocytes against Micrococcus luteus and Aeromonas hydrophila. The protein also possessed amidase activity in the presence of Zn²⁺, degraded PGN, and disrupted the M. luteus cell wall. The results suggest that MsPGRP plays an important role in pathogen recognition, and acts as a opsonin during immune system responses and elimination of 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.

Four peptidoglycan recognition proteins are indispensable for antibacterial immunity in the cigarette beetle Lasioderma serricorne (Fabricius)

The peptidoglycan recognition protein (PGRP), an important pattern recognition receptor of insects, is significant for reducing innate immunity and effective pest control. We cloned four PGRP genes (LsPGRP-LB, LsPGRP-LB1, LsPGRP-LE, and LsPGRP-SC2) from the cigarette beetle, Lasioderma serricorne (Fabricius), which encoded proteins of 216, 197, 317, and 190 amino acids, respectively. Three LsPGRPs were predominantly expressed in the larval and pupal stages, whereas LsPGRP-LE displayed high expression in adults. All the four LsPGRPs genes were highly expressed in the midgut and integument. Pathogen inoculation revealed that the four LsPGRPs actively responded to Escherichia coli and its peptidoglycan. The transcription levels of LsPGRP-LE and LsPGRP-SC2 increased significantly after Staphylococcus aureus stimulation. RNA interference-mediated knockdown of the four LsPGRPs led to increased larval mortality when challenged by E. coli, and the expression of four antimicrobial peptide genes (LsCole, LsAtt2, LsDef1 and LsDef2) had a significant decrease. Higher mortality and lower AMP expression were also observed in L. serricorne under S. aureus infection after silencing LsPGRP-LE and LsPGRP-SC2. Our results suggest that the four LsPGRP genes play important and distinct regulatory roles in the antibacterial defense response of L. serricorne.

Virulence of the Bio-Control Fungus Purpureocillium lilacinum Against Myzus persicae (Hemiptera: Aphididae) and Spodoptera frugiperda (Lepidoptera: Noctuidae)

Eco-friendly entomopathogenic fungi are widely used to control agricultural insect pests. Purpureocillium lilacinum (Thom.) Luangsa-ard et al. (Hypocreales: Ophiocordycipitaceae) is a nematophagous fungus used for the bio-control of destructive root-knot nematodes. However, its insecticidal activities against agricultural insect pests haven't been widely studied. In this study, P. lilacinum PL-1 was isolated from soil (Hefei, China) and identified by molecular and morphological analyses. The growth rate, spore production, proteinase, and chitinase activities of the isolate were analyzed. Virulence tests against green peach aphid, Myzus persicae (Sulzer) (Hemiptera: Aphididae) and fall armyworm (FAW), Spodoptera frugiperda (Smith) (Lepidoptera: Noctuidae) were performed. The median lethal concentration (LC50) and median lethal time (LT50) against aphids (via immersion) and LT50 against FAW (via injection) were determined. FAW eggs immersed in aqueous conidia suspension were infected after 60 h. Differentially expressed genes (DEGs) in the infection of FAW larvae by P. lilacinum were analyzed by quantitative reverse transcription PCR. The significantly upregulated DEGs include FAW immune genes (antimicrobial peptides, C-type lectins, lysozymes, prophenoloxidase, and peptidoglycan recognition proteins) and fungal pathogenic genes (ligase, chitinase, and hydrophobin). Our data demonstrate that P. lilacinum can be used as an entomopathogenic fungus against agricultural insect pests.

Identification of 35 C-Type Lectins in the Oriental Armyworm, Mythimna separata (Walker)

Simple Summary

The oriental armyworm Mythimna separata is a lepidopteral agricultural pest that causes serious damage to many crops, such as maize, wheat, and sorghum. To control this pest, it is advisable to take comprehensive measures, including the use of chemical pesticides, microbial pesticides, and cultural practices. However, microbial pesticides (entomopathogens) can be eliminated by the insect immune system. C-type lectins (CTLs) are a family of pattern-recognition receptors that recognize carbohydrates and mediate immune responses. C-type lectins in the oriental armyworm have not yet been identified and characterized. In this study, a transcriptome of M. separata larvae was constructed and a total of 35 CTLs containing single or dual carbohydrate-recognition domains (CRDs) were identified from unigenes. Phylogenetic analyses, sequence alignments and structural predictions were performed. Gene expression profiles in different developmental stages, naïve larval tissues, and bacteria/fungi-challenged larvae were analyzed. Overall, our findings indicate that most dual-CRD CTLs are expressed in mid-late-stage larvae, pupae, and adults. Bacterial and fungal challenges can stimulate the expression of many CTLs in larval hemocytes, fat body, and midgut. Our data suggest the importance of CTLs in immune responses of M. separata.

Abstract

Insect C-type lectins (CTLs) play vital roles in modulating humoral and cellular immune responses. The oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae) is a migratory pest that causes significant economic loss in agriculture. CTLs have not yet been systematically identified in M. separata. In this study, we first constructed a transcriptome of M. separata larvae, generating a total of 45,888 unigenes with an average length of 910 bp. Unigenes were functionally annotated in six databases: NR, GO, KEGG, Pfam, eggNOG, and Swiss-Prot. Unigenes were enriched in functional pathways, such as those of signal transduction, endocrine system, cellular community, and immune system. Thirty-five unigenes encoding C-type lectins were identified, including CTL-S1~CTL-S6 (single CRD) and IML-1~IML-29 (dual CRD). Phylogenetic analyses showed dramatic lineage-specific expansions of IMLs. Sequence alignment and structural modeling identified potential ligand-interacting residues. Real-time qPCR revealed that CTL-Ss mainly express in eggs and early stage larvae, while IMLs mainly express in mid-late-stage larvae, pupae, and adults. In naïve larvae, hemocytes, fat body, and epidermis are the major tissues that express CTLs. In larvae challenged by Escherichia coli, Staphylococcus aureus, or Beauveria bassiana, the expression of different CTLs was stimulated in hemocytes, fat body and midgut. The present study will help further explore functions of M. separata CTLs.

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.

Peptidoglycan recognition protein-S1 acts as a receptor to activate AMP expression through the IMD pathway in the silkworm Bombyx mori

Peptidoglycan recognition proteins (PGRPs) are the most important pattern recognition receptors (PRRs) in insects. PGRPs can recognize pathogenic microorganism peptidoglycans (PGs) and play an important role in innate immunity. Twelve PGRPs have been identified in silkworms. However, the specific roles played by these PGPRs in the silkworm innate immune system have not been elucidated to date. In this study, we systematically investigated the biological functions of BmPGRP-S1 in silkworms. We observed that BmPGRP-S1 was highly expressed in silkworm immune-related organs and was upregulated in response to bacterial challenges. Furthermore, we determined that BmPGRP-S1 can bind to bacteria or PGs and activate antimicrobial peptide (AMP) expression. Inhibition of the expression of BmPGRP-S1 by siRNA reduced AMP gene expression in silkworms. Further experiments demonstrated that BmPGRP-S1 is involved in IMD pathway activation to induce AMP expression. Taken together, these results demonstrate that BmPGRP-S1 serves as a receptor to activate AMP gene expression through the IMD pathway to address bacterial challenges.