Identification and Characterization of Aminopeptidase-N as a Binding Protein for Cry3Aa in the Midgut of Monochamus alternatus (Coleoptera: Cerambycidae)

Comparative gut proteomics study revealing adaptive physiology of Eurasian spruce bark beetle, Ips typographus (Coleoptera: Scolytinae)

The bark beetle, Ips typographus (L.), is a major pest of Norway spruce, Picea abies (L.), causing enormous economic losses globally. The adult stage of the I. typographus has a complex life cycle (callow and sclerotized); the callow beetles feed ferociously, whereas sclerotized male beetles are more aggressive and pioneers in establishing new colonies. We conducted a comparative proteomics study to understand male and female digestion and detoxification processes in callow and sclerotized beetles. Proteome profiling was performed using high-throughput liquid chromatography-mass spectrometry. A total of >3000 proteins were identified from the bark beetle gut, and among them, 539 were differentially abundant (fold change ±2, FDR <0.05) between callow and sclerotized beetles. The differentially abundant proteins (DAPs) mainly engage with binding, catalytic activity, anatomical activity, hydrolase activity, metabolic process, and carbohydrate metabolism, and hence may be crucial for growth, digestion, detoxification, and signalling. We validated selected DAPs with RT-qPCR. Gut enzymes such as NADPH-cytochrome P450 reductase (CYC), glutathione S-transferase (GST), and esterase (EST) play a crucial role in the I. typographus for detoxification and digesting of host allelochemicals. We conducted enzyme activity assays with them and observed a positive correlation of CYC and GST activities with the proteomic results, whereas EST activity was not fully correlated. Furthermore, our investigation revealed that callow beetles had an upregulation of proteins associated with juvenile hormone (JH) biosynthesis and chitin metabolism, whereas sclerotized beetles exhibited an upregulation of proteins linked to fatty acid metabolism and the TCA cycle. These distinctive patterns of protein regulation in metabolic and functional processes are specific to each developmental stage, underscoring the adaptive responses of I. typographicus in overcoming conifer defences and facilitating their survival. Taken together, it is the first gut proteomic study comparing males and females of callow and sclerotized I. typographus, shedding light on the adaptive ecology at the molecular level. Furthermore, the information about bark beetle handling of nutritionally limiting and defence-rich spruce phloem diet can be utilized to formulate RNAi-mediated beetle management.

Identification of two Bacillus thuringiensis Cry3Aa toxin-binding aminopeptidase N from Rhynchophorus ferrugineus (Coleoptera: Curculionidae)

Rhynchophorus ferrugineus is a quarantine pest that mainly damages plants in tropical regions, which are essential economic resources. Cry3Aa has been used to control coleopteran pests and is known to be toxic to R. ferrugineus. The binding of the Cry toxin to specific receptors on the target insect plays a crucial role in the toxicological mechanism of Cry toxins. However, in the case of R. ferrugineus, the nature and identity of the receptor proteins involved remain unknown. In the present study, pull-down assays and mass spectrometry were used to identify two proteins of aminopeptidase N proteins (RfAPN2a and RfAPN2b) in the larval midguts of R. ferrugineus. Cry3Aa was able to bind to RfAPN2a (Kd = 108.5 nM) and RfAPN2b (Kd = 68.2 nM), as well as midgut brush border membrane vesicles (Kd = 482.5 nM). In silico analysis of both RfAPN proteins included the signal peptide and anchored sites for glycosyl phosphatidyl inositol. In addition, RfAPN2a and RfAPN2b were expressed in the human embryonic kidney 293T cell line, and cytotoxicity assays showed that the transgenic cells were not susceptible to activated Cry3Aa. Our results show that RfAPN2a and RfAPN2b are Cry3Aa-binding proteins involved in the Cry3Aa toxicity of R. ferrugineus. This study deepens our understanding of the action mechanism of Cry3Aa in R. ferrugineus larvae.

Bacillus thuringiensis toxins with nematocidal activity against the pinewood nematode Bursaphelenchus xylophilus

The pine wilt disease is caused by the pinewood nematode Bursaphelenchus xylophilus and it results in serious ecological and economic losses. Therefore, effective prevention and control methods for the pinewood nematode are urgently required. Bacillus thuringiensis (Bt), a widely used microbial insecticide, produces toxins that are toxic to several species of parasitic nematodes, however, its effects on B. xylophilus have not been determined. In this study, Cry5Ba3, App6Aa2, Cry12Aa1, Cry13Aa1, Cry14Aa1, Cry21Aa3, Cry21Fa1, Xpp55Aa1, and Cyt8Aa1 toxins' nematocidal activity against B. xylophilus was evaluated, six toxins with high toxicity were identified: App6Aa2 (LC₅₀ = 49.71 μg/mL), Cry13Aa1 (LC₅₀ = 53.17 μg/mL), Cry12Aa1 (LC₅₀ = 58.88 μg/mL), Cry5Ba3 (LC₅₀ = 63.99 μg/mL), Xpp55Aa1 (LC₅₀ = 65.14 μg/mL), and Cyt8Aa1 (LC₅₀ = 96.50 μg/mL). The six toxins caused shrinkage and thinning of the intestinal cells, contraction of the intestine from the body wall, vacuolization, and degenerated appearance of the pinewood nematodes. The results of this study provide basic information to study the action mechanism of nematocidal toxins on the pinewood nematode and direction for the use of nematocidal toxins in the biological control of B. xylophilus.

Engineering of Cry3Bb1 provides mechanistic insights toward countering western corn rootworm resistance

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Cry3Bb1 engineering for receptor retargeting to counter WCR resistance described

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Phage display screens against BBMV or recombinant insecticidal-protein receptors

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77 gut binding peptides selected and engineered into 284 new variants of Cry3Bb1

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112 variants were active against susceptible but not resistant WCR

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Engineering of Cry3Bb1 Domain II loops 1 and 2 disrupted insecticidal activity

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Sites for Cry3Bb1 engineering and implications for Cry3Bb1 resistance discussed

The western corn rootworm (WCR), Diabrotica virgifera virgifera LeConte (Coleoptera: Chrysomelidae), is an economically important pest of corn (maize) in North America and Europe. Current management practices for WCR involve transgenic expression of insecticidal proteins to minimize larval feeding damage to corn roots. The evolution of resistant WCR populations to transgenic corn expressing insecticidal proteins (e.g. Cry3Bb1, Gpp34Ab1/Tpp35Ab1) necessitates efforts to discover and deploy new modes of action for WCR control. Here, we tested the hypothesis that the addition of short peptides selected for binding to the WCR gut would restore insecticidal activity of Cry3Bb1 to resistant insects. Phage display technology coupled with deep sequencing was used to identify peptides selected for binding to WCR brush border membrane vesicles and to recombinant putative receptors aminopeptidase and cadherin. The binding and specificity of selected peptides was confirmed by ELISA and pull-down assays, and candidate gut surface binding partners were identified. Although production of 284 novel Cry3Bb1 variants with these peptides did not restore activity against resistant WCR in artificial diet bioassays, 112 variants were active against susceptible insects. These results provided insights for the mechanism of Cry3Bb1 activity and toward engineering a new mode-of-action via receptor re-targeting in the context of protein structure and function.