Characterization of a β-1,3-glucan recognition protein from the beet armyworm, Spodoptera exigua (Insecta: Lepidoptera: Noctuidae)

RNAi-mediated silencing of NlGRP3 augments the insecticidal virulence of Metarhizium anisopliae to the brown planthopper Nilaparvata lugens

The rapid development of insecticide resistance reinforces the urgent need to develop eco-friendly strategies for controlling Nilaparvata lugens (brown planthopper, BPH), the most destructive insect pest of rice. Both entomopathogens and RNA interference (RNAi) provide attractive alternatives to chemical insecticides. In this study, we demonstrated the synergistic potential of the combination use of entomopathogen- and RNAi-mediated approaches to control BPH. The β-1, 3-glucan recognition protein (βGRP) encoding gene NlGRP3 was identified and its potential role in immune defense was characterized in BPH. The open reading frame (ORF) of NlGRP3 is 1740 bp in length, encoding a 65.8 kDa protein with conserved CBM39 and GH16 domains that typically existed in the βGRP family members. NlGRP3 was shown to be differentially expressed across developmental stages and highly transcribed in the immune responsive tissues haemolymph and fat body. Topical infection with a fungal entomopathogen Metarhizium anisopliae could significantly up-regulate its expression level. RNAi-mediated silencing of NlGRP3 resulted in significantly decreased survival rate and increased susceptibility to fungal challenge in the fifth-instar BPH nymphs. The greatly enhanced mortality of NlGRP3-silenced BPH following fungal infection might be in part directly due to the immune suppression by down-regulating expressions of antimicrobial peptide genes and the imbalance of the bacterial community harboring in BPH body. Our results highly demonstrated that suppressing the insect innate immune defense through RNAi targeting the immune-related genes could effectively strengthen the biocontrol efficacy of fungal entomopathogens, providing clues to the combination use of RNAi and entomopathogens as a promising approach for BPH control.

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.

Toll signal pathway activating eicosanoid biosynthesis shares its conserved upstream recognition components in a lepidopteran Spodoptera exigua upon infection by Metarhizium rileyi, an entomopathogenic fungus

Eicosanoids play crucial roles in mediating immune responses in insects. Upon a fungal infection, Toll signal pathway can mediate immune responses of Spodoptera exigua, a lepidopteran insect, by activating eicosanoid biosynthesis. However, upstream signal components of the Toll signal pathway activating eicosanoid biosynthesis remain unclear. This study predicted pattern recognition receptors (PRRs) and serine proteases (SPs) as upstream components of the Toll pathway with reference to known signal components of Manduca sexta, another lepidopteran insect. S. exigua infected with Metarhizium rileyi, an entomopathogenic fungus, activated phospholipase A₂ (PLA₂) and phenoloxidase (PO) enzymes along with marked increases of expression levels of genes encoding three specific antimicrobial peptides, cecropin, gallerimycin, and hemolin. Among ten Toll receptors encoded in the genome of S. exigua, seven Toll genes were associated with immune responses against fungal infection by M. rileyi through individual RNA interference (RNAi) screening. In addition, two Spätzles (ligands of Toll receptor) were required for Toll signaling against the fungal infection. All predicted upstream components of the Toll pathway were inducible by the fungal infection. Individual RNAi screening showed that three PRRs (βGRP-1, βGRP-2, and GNBP3) and five SPs (ModSP, HP21, HP5, HP6, and HP8) were required for immune responses of S. exigua mediated by Toll signal pathway against the fungal infection. However, two PO-activating proteases (PAP1 and PAP3) were not required for PLA₂ activation, although they were required for PO activation. These results suggest that PRRs and SPs conserved as upstream components in Toll signal pathway play crucial roles in triggering eicosanoid biosynthesis of S. exigua to mediate various immune responses against fungal infection.

Immune functions of insect βGRPs and their potential application

Insects rely completely on the innate immune system to sense the foreign bodies and to mount the immune responses. Germ-line encoded pattern recognition receptors play crucial roles in recognizing pathogen-associated molecular patterns. Among them, β-1,3-glucan recognition proteins (βGRPs) and gram-negative bacteria-binding proteins (GNBPs) belong to the same pattern recognition receptor family, which can recognize β-1,3-glucans. Typical insect βGRPs are comprised of a tandem carbohydrate-binding module in the N-terminal and a glucanase-like domain in the C-terminal. The former can recognize triple-helical β-1,3-glucans, whereas the latter, which normally lacks the enzymatic activity, can recruit adapter proteins to initiate the protease cascade. According to studies, insect βGRPs possess at least three types of functions. Firstly, some βGRPs cooperate with peptidoglycan recognition proteins to recognize the lysine-type peptidoglycans upstream of the Toll pathway. Secondly, some directly recognize fungal β-1,3-glucans to activate the Toll pathway and melanization. Thirdly, some form the 'attack complexes' with other immune effectors to promote the antifungal defenses. The current review will focus on the discovery of insect βGRPs, functions of some well-characterized members, structure-function studies and their potential application.

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.

Identification of immunity-related genes in the larvae of Protaetia brevitarsis seulensis (Coleoptera: Cetoniidae) by a next-generation sequencing-based transcriptome analysis

To identify immune-related genes in the larvae of white-spotted flower chafers, next-generation sequencing was conducted with an Illumina HiSeq2000, resulting in 100 million cDNA reads with sequence information from over 10 billion base pairs (bp) and >50× transcriptome coverage. A subset of 77,336 contigs was created, and ∼35,532 sequences matched entries against the NCBI nonredundant database (cutoff, e < 10(-5)). Statistical analysis was performed on the 35,532 contigs. For profiling of the immune response, samples were analyzed by aligning 42 base sequence tags to the de novo reference assembly, comparing levels in immunized larvae to control levels of expression. Of the differentially expressed genes, 3,440 transcripts were upregulated and 3,590 transcripts were downregulated. Many of these genes were confirmed as immune-related genes such as pattern recognition proteins, immune-related signal transduction proteins, antimicrobial peptides, and cellular response proteins, by comparison to published data.