Differential role of Manduca sexta aminopeptidase-N and alkaline phosphatase in the mode of action of Cry1Aa, Cry1Ab, and Cry1Ac toxins from Bacillus thuringiensis

CRISPR-Cas9 knockout of membrane-bound alkaline phosphatase or cadherin does not confer resistance to Cry toxins in Aedes aegypti

The Aedes aegypti cadherin-like protein (Aae-Cad) and the membrane-bound alkaline phosphatase (Aae-mALP) are membrane proteins identified as putative receptors for the larvicidal Cry toxins produced by Bacillus thuringiensis subsp. israelensis bacteria. Cry toxins are the most used toxins in the control of different agricultural pest and mosquitos. Despite the relevance of Aae-Cad and Aae-mALP as possible toxin-receptors in mosquitoes, previous efforts to establish a clear functional connection among them and Cry toxins activity have been relatively limited. In this study, we used CRISPR-Cas9 to generate knockout (KO) mutations of Aae-Cad and Aae-mALP. The Aae-mALP KO was successfully generated, in contrast to the Aae-Cad KO which was obtained only in females. The female-linked genotype was due to the proximity of aae-cad gene to the sex-determining loci (M:m). Both A. aegypti KO mutant populations were viable and their insect-development was not affected, although a tendency on lower egg hatching rate was observed. Bioassays were performed to assess the effects of these KO mutations on the susceptibility of A. aegypti to Cry toxins, showing that the Aae-Cad female KO or Aae-mALP KO mutations did not significantly alter the susceptibility of A. aegypti larvae to the mosquitocidal Cry toxins, including Cry11Aa, Cry11Ba, Cry4Ba, and Cry4Aa. These findings suggest that besides the potential participation of Aae-Cad and Aae-mALP as Cry toxin receptors in A. aegypti, additional midgut membrane proteins are involved in the mode of action of these insecticidal toxins.

A quantitative micro-tomographic gut atlas of the lepidopteran model insect Manduca sexta

The tobacco hornworm is used extensively as a model system for ecotoxicology, immunology and gut physiology. Here, we established a micro-computed tomography approach based on the oral application of the clinical contrast agent iodixanol, allowing for a high-resolution quantitative analysis of the Manduca sexta gut. This technique permitted the identification of previously unknown and understudied structures, such as the crop or gastric ceca, and revealed the underlying complexity of the hindgut folding pattern, which is involved in fecal pellet formation. The acquired data enabled the volume rendering of all gut parts, the reliable calculation of their volumes, and the virtual endoscopy of the entire alimentary tract. It can provide information for accurate orientation in histology uses, enable quantitative anatomical phenotyping in three dimensions, and allow the calculation of locally effective midgut concentrations of applied chemicals. This atlas will provide critical insights into the evolution of the alimentary tract in lepidopterans.

Molecular characterization and functional analysis of Cry toxin receptor-like genes from the model insect Galleria mellonella

Crystal (Cry) toxins produced from the soil bacterium, Bacillus thuringiensis (Bt), have gained worldwide attention for long due to their insecticidal potential. A number of receptor proteins located on the epithelial cells of the larval midgut were shown to be crucial for Cry intoxication in different insect pests belonging to order Lepidoptera, Diptera and Coleoptera. A beehive pest, Galleria mellonella, serves as an excellent insect model for biochemical research. However, information on the Cry receptor-like genes in G. mellonella is limited. In the present study, the full-length sequences of four putative Cry receptor genes (ABC transporter, alkaline phosphatase, aminopeptidase N and cadherin) were cloned from G. mellonella. All these receptor genes were substantially upregulated in the midgut tissue of fourth-instar G. mellonella larvae upon early exposure (6 h) to a sub-lethal dose of Cry1AcF toxin. Oral and independent delivery of bacterially-expressed dsRNAs corresponding to four receptor genes in G. mellonella suppressed the transcription of target receptors which in turn significantly reduced the larval sensitivity to Cry1AcF toxin. As the laboratory populations of G. mellonella develop Bt resistance in a relatively short time, molecular characterization of Cry receptor genes in G. mellonella performed in the present study may provide some useful information for future research related to the genetic basis of Bt resistance in the model insect.

Role of Lectin in the Response of Aedes aegypti Against Bt Toxin

Aedes aegypti is one of the world’s most dangerous mosquitoes, and a vector of diseases such as dengue fever, chikungunya virus, yellow fever, and Zika virus disease. Currently, a major global challenge is the scarcity of antiviral medicine and vaccine for arboviruses. Bacillus thuringiensis var israelensis (Bti) toxins are used as biological mosquito control agents. Endotoxins, including Cry4Aa, Cry4Ba, Cry10Aa, Cry11Aa, and Cyt1Aa, are toxic to mosquitoes. Insect eradication by Cry toxin relies primarily on the interaction of cry toxins with key toxin receptors, such as aminopeptidase (APN), alkaline phosphatase (ALP), cadherin (CAD), and ATP-binding cassette transporters. The carbohydrate recognition domains (CRDs) of lectins and domains II and III of Cry toxins share similar structural folds, suggesting that midgut proteins, such as C-type lectins (CTLs), may interfere with interactions among Cry toxins and receptors by binding to both and alter Cry toxicity. In the present review, we summarize the functional role of C-type lectins in Ae. aegypti mosquitoes and the mechanism underlying the alteration of Cry toxin activity by CTLs. Furthermore, we outline future research directions on elucidating the Bti resistance mechanism. This study provides a basis for understanding Bti resistance, which can be used to develop novel insecticides.

Tobacco Hornworm (Manduca sexta) caterpillars as a novel host model for the study of fungal virulence and drug efficacy

The two leading yeast pathogens of humans, Candida albicans and Cryptococcus neoformans, cause systemic infections in >1.4 million patients worldwide with mortality rates approaching 75%. It is thus imperative to study fungal virulence mechanisms, efficacy of antifungal drugs, and host response pathways. While this is commonly done in mammalian models, which are afflicted by ethical and practical concerns, invertebrate models, such as wax moth larvae and nematodes have been introduced over the last two decades. To complement existing invertebrate host models, we developed fifth instar caterpillars of the Tobacco Hornworm moth Manduca sexta as a novel host model. These caterpillars can be maintained at 37°C, are suitable for injections with defined amounts of yeast cells, and are susceptible to the most threatening yeast pathogens, including C. albicans, C. neoformans, C. auris, and C. glabrata. Importantly, fungal burden can be assessed daily throughout the course of infection in a single caterpillar’s feces and hemolymph. Infected caterpillars can be rescued by treatment with antifungal drugs. Notably, these animals are large enough for weight to provide a reliable and reproducible measure of fungal disease and to facilitate host tissue-specific expression analyses. M. sexta caterpillars combine a suite of parameters that make them suitable for the study of fungal virulence.

Knockout of Two Cry-Binding Aminopeptidase N Isoforms Does Not Change Susceptibility of Aedes aegypti Larvae to Bacillus thuringiensis subsp. israelensis Cry4Ba and Cry11Aa Toxins

Simple Summary

The midgut aminopeptidase N (APN) isoforms have been identified as the binding receptor of insecticidal Cry toxins in numerous insects, including the major arbovirus vector Aedes aegypti (Ae. aegypti). However, whether the Cry-binding APN acts as an essential functional receptor to mediate Bacillus thuringiensis subsp. israelensis (Bti) toxicity in Ae. aegypti larvae remains to be determined. In this study, our results provide the direct molecular evidence demonstrating that two Cry-binding APN isoforms (AeAPN1 and AeAPN2) did not play a key role in mediating Bti Cry4Ba and Cry11Aa toxicity in Ae. aegypti larvae.

Abstract

The insecticidal Cry4Ba and Cry11Aa crystal proteins from Bacillus thuringiensis subsp. israelensis (Bti) are highly toxic to Ae. aegypti larvae. The glycosylphosphatidylinositol (GPI)-anchored APN was identified as an important membrane-bound receptor for multiple Cry toxins in numerous Lepidoptera, Coleoptera, and Diptera insects. However, there is no direct molecular evidence to link APN of Ae. aegypti to Bti toxicity in vivo. In this study, two Cry4Ba/Cry11Aa-binding Ae. aegypti GPI-APN isoforms (AeAPN1 and AeAPN2) were individually knocked-out using CRISPR/Cas9 mutagenesis, and the AeAPN1/AeAPN2 double-mutant homozygous strain was generated using the reverse genetics approach. ELISA assays showed that the high binding affinity of Cry4Ba and Cry11Aa protoxins to the midgut brush border membrane vesicles (BBMVs) from these APN knockouts was similar to the background from the wild-type (WT) strain. Likewise, the bioassay results showed that neither the single knockout of AeAPN1 or AeAPN2, nor the simultaneous disruption of AeAPN1 and AeAPN2 resulted in significant changes in susceptibility of Ae. aegypti larvae to Cry4Ba and Cry11Aa toxins. Accordingly, our results suggest that AeAPN1 and AeAPN2 may not mediate Bti Cry4Ba and Cry11Aa toxicity in Ae. aegypti larvae as their binding proteins.

Exogenous Gene Expression and Insect Resistance in Dual Bt Toxin Populus × euramericana 'Neva' Transgenic Plants

Bacillus thuringiensis (Bt) insecticidal protein genes are important tools in efforts to develop insect resistance in poplar. In this study, the Cry1Ac and Cry3A Bt toxin genes were simultaneously transformed into the poplar variety Populus × euramericana 'Neva' by Agrobacterium-mediated transformation to explore the exogenous gene expression and insect resistance, and to examine the effects of Bt toxin on the growth and development of Anoplophora glabripennis larvae after feeding on the transgenic plant. Integration and expression of the transgenes were determined by molecular analyses and the insect resistance of transgenic lines was evaluated in feeding experiments. Sixteen transgenic dual Bt toxin genes Populus × euramericana 'Neva' lines were obtained. The dual Bt toxin genes were expressed at both the transcriptional and translational levels; however, Cry3A protein levels were much higher than those of Cry1Ac. Some of the transgenic lines exhibited high resistance to the first instar larvae of Hyphantria cunea and Micromelalopha troglodyta, and the first and second instar larvae and adults of Plagiodera versicolora. Six transgenic lines inhibited the growth and development of A. glabripennis larvae. The differences in the transcriptomes of A. glabripennis larvae fed transgenic lines or non-transgenic control by RNA-seq analyses were determined to reveal the mechanism by which Bt toxin regulates the growth and development of longicorn beetle larvae. The expression of genes related to Bt prototoxin activation, digestive enzymes, binding receptors, and detoxification and protective enzymes showed significant changes in A. glabripennis larvae fed Bt toxin, indicating that the larvae responded by regulating the expression of genes related to their growth and development. This study lay a theoretical foundation for developing resistance to A. glabripennis in poplar, and provide a foundation for exploring the mechanism of Bt toxin action on Cerambycidae insects.

A toxin complex protein from Photorhabdus akhurstii conferred oral insecticidal activity against Galleria mellonella by targeting the midgut epithelium

The nematode-bacterium pair Heterorhabditis indica-Photorhabdus akhurstii is a malleable model system to investigate mutualistic relations. A number of toxins produced by P. akhurstii allow the bacterium to kill the insect host. However, a few of these heterologously expressed toxins are orally active against different insects which possibly caused neglected attention to Photorhabdus toxins compared to Bt (Bacillus thuringiensis). In the current study, a functional subunit of orally active toxin complex (Tc) protein, TcaB (63 kDa), isolated from two strains of P. akhurstii namely IARI-SGHR2 and IARI-SGMS1, was tested for biological activity against Galleria mellonella. A force feeding-based administration of the toxin translated into LD₅₀ values of 45.63-58.90 ng/g which was even lower compared to injection LD₅₀ values (51.48-64.30 ng/g) at 48 h after inoculation. An oral uptake of 500 ng toxin caused extensive gut damage in G. mellonella during 6-24 h incubation period coupled with a gradual disruption of gut integrity leading to escape of TcaB into the hemocoel. This finding was supported by the cytotoxic and immune-stimulatory effect of TcaB in the insect hemocoel at 6-24 h after force feeding. The circulatory hemocyte numbers and cell viability was markedly reduced to 0.66-0.68 × 10⁶ ml^-1 and 49-52 %, respectively, in TcaB force fed insect at 24 h, compared to control (2.55 × 10⁶ ml^-1; 100 %). The hemolymph phenoloxidase (PO) activity was elevated by 10.2-fold in force fed larvae than control at 24 h. An in silico docking study revealed that TcaB putatively interacts with a number of G. mellonella receptor proteins in order to become a gut-active toxin. Present research reinforces the potential of gut-active Photorhabdus toxins for their inclusion in sustainable insect management tactics and strengthens the existing Bt-dominated management repository.

Bacillus thuringiensis Cry1Ab Domain III β-22 Mutants with Enhanced Toxicity to Spodoptera frugiperda (J. E. Smith)

The fall armyworm, Spodoptera frugiperda, is an invasive maize pest that has spread from the Americas into Africa and Asia and causes severe crop damage worldwide. Most populations of S. frugiperda show low susceptibility to Bacillus thuringiensis (Bt) Cry1Ab or Cry1Ac toxins, which have been proved to be effective against several other lepidopteran pests. In addition, S. frugiperda has evolved resistance to transgenic maize expressing Cry1Fa toxin. The specificity and toxicity of Cry toxins are determined by their binding to different larval midgut proteins, such as aminopeptidase N (APN), alkaline phosphatase (ALP), and cadherin (CAD), among other proteins, by means of exposed domain II loop regions and also by the domain III β-sheets β-16 and β-22. Here, we analyzed different Cry1Ab mutants with mutations in the domain III β-22 region. Alanine-scanning mutagenesis of this region revealed that all mutants showed increased toxicity against a nonsusceptible Cry1Ab S. frugiperda population. Further analysis of the mutant toxin Cry1AbS587A (bearing a mutation of S to A at position 587) revealed that, compared to Cry1Ab, it showed significantly increased toxicity to three other S. frugiperda populations from Mexico but retained similar toxicity to Manduca sexta larvae. Cry1AbS587A bound to brush border membrane vesicles (BBMV), and its higher toxicity correlated with higher binding affinities to APN, ALP, and CAD recombinant proteins. Furthermore, silencing the expression of APN1 and CAD receptors in S. frugiperda larvae by RNA interference (RNAi) showed that Cry1AbS587A toxicity relied on CAD expression, in contrast to Cry1Ab. These data support the idea that the increased toxicity of Cry1AbS587A to S. frugiperda is in part due to an improved binding interaction with the CAD receptor.IMPORTANCE Spodoptera frugiperda is an important worldwide pest of maize and rice crops that has evolved resistance to Cry1Fa-expressing maize in different countries. Therefore, identification of additional toxins with different modes of action is needed to provide alternative tools to control this insect pest. Bacillus thuringiensis (Bt) Cry1Ab and Cry1Ac toxins are highly active against several important lepidopteran pests but show varying and low levels of toxicity against different S. frugiperda populations. Thus, the identification of Cry1A mutants that gain toxicity to S. frugiperda and retain toxicity to other pests could be of great value to produce transgenic crops that resist a broader spectrum of lepidopteran pests. Here, we characterized Cry1Ab domain III β-22 mutants, and we found that a Cry1AbS587A mutant displayed increased toxicity against different S. frugiperda populations. Thus, Cry1AbS587A could be a good toxin candidate to produce transgenic maize with broader efficacy against this important insect pest in the field.

Differences in midgut transcriptomes between resistant and susceptible strains of Chilo suppressalis to Cry1C toxin

Background

Chilo suppressalis is a widespread rice pest that poses a major threat to food security in China. This pest can develop resistance to Cry toxins from Bacillus thuringiensis (Bt), threatening the sustainable use of insect-resistant transgenic Bt rice. However, the molecular basis for the resistance mechanisms of C. suppressalis to Cry1C toxin remains unknown. This study aimed to identify genes associated with the mechanism of Cry1C resistance in C. suppressalis by comparing the midgut transcriptomic responses of resistant and susceptible C. suppressalis strains to Cry1C toxin and to provide information for insect resistance management.

Results

A C. suppressalis midgut transcriptome of 139,206 unigenes was de novo assembled from 373 million Illumina HiSeq and Roche 454 clean reads. Comparative analysis identified 5328 significantly differentially expressed unigenes (DEGs) between C. suppressalis Cry1C-resistant and -susceptible strains. DEGs encoding Bt Cry toxin receptors, aminopeptidase-P like protein, the ABC subfamily and alkaline phosphatase were downregulated, suggesting an association with C. suppressalis Cry1C resistance. Additionally, Cry1C resistance in C. suppressalis may be related to changes in the transcription levels of enzymes involved in hydrolysis, digestive, catalytic and detoxification processes.

Conclusion

Our study identified genes potentially involved in Cry1C resistance in C. suppressalis by comparative transcriptome analysis. The assembled and annotated transcriptome data provide valuable genomic resources for further study of the molecular mechanisms of C. suppressalis resistance to Cry toxins.

Txp40, a protein from Photorhabdus akhurstii, conferred potent insecticidal activity against the larvae of Helicoverpa armigera, Spodoptera litura and S. exigua

BACKGROUND

Txp40, a 37 kDa protein, previously characterized from the Gram-negative bacterium Photorhabdus akhurstii (symbiotically associates with insect-parasitic nematode, Heterorhabditis indica), conferred insecticidal activity against Galleria mellonella. Here, the biological activity of Txp40 was evaluated against economically important insects, including Helicoverpa armigera, Spodoptera litura and S. exigua.

RESULTS

When both intra-hemocoel injected and orally fed to test insects, comparatively greater oral LD₅₀ (187.7-522 ng g^-1 ) than injection LD₅₀ (32.33-150.6 ng g^-1 ) was obtained with Txp40 derived from P. akhurstii strain IARI-SGMG3. Injection of purified Txp40 caused a dose-dependent reduction in the total circulatory hemocytes and hemocyte viability of fourth-instar larvae of the test insects at 12 h post incubation; unlike healthy cells toxin-treated ones displayed aggregated distribution. Injection of Txp40 significantly elevated the phenoloxidase activity of insect hemolymph, which potentially led to unrestrained melanization reaction and ultimately larval death. Histological analyses showed the primary site of action of Txp40 in the insect midgut. Extensive damage to midgut epithelium 24 h after injection of the Txp40 explains the access of the toxin from hemocoel to midgut via leaky septate junctions. In silico analyses suggested that Txp40 can potentially interact with H. armigera midgut receptor proteins cadherin, ATP-binding cassettes, aminopeptidase N1 and alkaline phosphatase to exert toxicity.

CONCLUSION

We propose Txp40 as an attractive alternative to Cry toxins of Bacillus thuringiensis, the transgenic expression of which is reported to cause resistance development in insects. © 2019 Society of Chemical Industry.

Knockout of three aminopeptidase N genes does not affect susceptibility of Helicoverpa armigera larvae to Bacillus thuringiensis Cry1A and Cry2A toxins

Bacillus thuringiensis (Bt) insecticidal toxins have been globally utilized for control of agricultural insects through spraying or transgenic crops. Binding of Bt toxins to special receptors on midgut epithelial cells of target insects is a key step in the mode of action. Previous studies suggested aminopeptidase N1 (APN1) as a receptor or putative receptor in several lepidopteran insects including Helicoverpa armigera through evidence from RNA interefence-based gene silencing approaches. In the current study we tested the role of APNs in the mode of action of Bt toxins using clustered regularly interspaced palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated gene knockout. Three APN genes (HaAPN1, HaAPN2 and HaAPN5) were individually knocked out in a susceptible strain (SCD) of H. armigera to establish three homozygous knockout strains. Qualitative in vitro binding studies indicated binding of Cry1Ac or Cry2Ab to midgut brush border membrane vesicles was not obviously affected by APN knockout. Bioassay results showed that none of the three knockouts had significant changes in susceptibility to Cry1A or Cry2A toxins when compared with the SCD strain. This suggests that the three HaAPN genes we tested may not be critical in the mode of action of Cry1A or Cry2A toxins in H. armigera.

Polycalin is involved in the toxicity and resistance to Cry1Ac toxin in Helicoverpa armigera (Hübner)

Polycalin has been confirmed as a binding protein of the Cry toxins in a few Lepidoptera insects, but its function in the action mechanism of Cry1Ac and whether it is involved in resistance evolution are still unclear. In this study, Ligand blot and enzyme-linked immunosorbent assays showed that Helicoverpa armigera polycalin could specifically interact with Cry1Ac with a high affinity (K_d  = 118.80 nM). Importantly, antisera blocking polycalin in H. armigera larvae decreased the toxicity of Cry1Ac by 31.84%. Furthermore, the relative gene and protein expressions were lower in Cry1Ac-resistant strain (LF60) than that in Cry1Ac-susceptible strain (LF). These findings indicated that H. armigera polycalin was a possible receptor of Cry1Ac and may be contributed to the resistance to Cry1Ac.

Identification of Cry1Ah-binding proteins through pull down and gene expression analysis in Cry1Ah-resistant and susceptible strains of Ostrinia furnacalis

Bacillus thuringiensis produces insecticidal Cry toxins used in the control of multiple insect pests. Evolution of insect resistance to Bt toxins endangers the use of Cry toxins for pest control. Analysis of the Cry1Ah-binding proteins from brush border membrane vesicles (BBMV) of Ostrinia furnacalis, Asian corn borer (ACB) from the Cry1Ah-resistant (ACB-AhR) and susceptible (ACB-BtS) strains was performed by an improved pull down assay that includes coupling Cry1Ah to NHS-activated Sepharose combined with liquid chromatography-tandem mass spectrometry (LC-MS/MS). Our data show that Cry1Ah bound to alkaline phosphatase (ALP), cadherin-like (CAD), actin, aminopeptidase-N (APN), prophenoloxidase (proPO), serine proteinase inhibitor (SPI), immulectin, and V-ATPase and to other proteins that were not previously characterized as Cry-binding proteins in ACB-BtS strain. Analysis of Cry1Ah-pulled down proteins of the BBMV from ACB-AhR revealed that Cry1Ah toxin did not bind to ALP in ACB-AhR strain, suggesting that this protein may correlate with the resistant phenotype of this strain. Additionally, we analyzed the expression of representative genes coding for Cry1Ah-binding proteins such as ALP, APN, CAD, proPO, SPI, and immulectin by qRT-PCR. ACB-AhR showed increased expression levels of proPO (7.5 fold), ALP (6.2 fold) and APN (1.4 fold) in comparison to ACB-BtS strain. In contrast, the cad gene showed slight decreased expression in ACB-AhR strain (0.7 fold) compared with ACB-BtS strain. Our data suggest that differences in the susceptibility to Cry1Ah toxin in the ACB-AhR strain may be associated with reduced ALP binding sites and with an increased immune response. This study also brings evidence of a possible binding interaction of Cry1Ah toxin to immune related proteins like proPO.

Cry1A Proteins are Cytotoxic to HeLa but not to SiHa Cervical Cancer Cells

BACKGROUND

Bacillus thuringiensis toxins are effective against multiple biological targets such as insects, nematodes, mites, protozoa, and importantly, human cancer cells. One of the main mechanisms by which Cry toxins to trigger cell death is the specific recognition of cadherin-like membrane cell receptors.

OBJECTIVE

This work aimed to assess the cytotoxicity of the Cry1Ab and Cry1Ac toxins from Bacillus thuringiensis in HeLa, cervical cancer cell line, as well as their antitumor activity in mouse models.

METHODS

We analyzed several biological targets of Cry1Ab and Cry1Ac including erythrocytes, insect larvae, as well as cancer and non-cancer cell lines. The viability of HeLa, SiHa, MCF7 and HaCat cells was assessed by MTT 24 h after the administration of Cry toxins. We also studied apoptosis as a possible cytotoxicity mechanism in HeLa. The capacity of Cry toxins to eliminate tumors in xenograft mouse models was also analyzed.

RESULTS

Both toxins, Cry1Ab and Cry1Ac, showed specific cytotoxic activity in HeLa (HPV18+) cervical cancer cell line, with a Cry1Ab LC50 of 2.5 µg/ml, and of 0.5 µg/ml for Cry1Ac. Apoptosis was differentially induced in HeLa cells using the same concentration of Cry1Ab and Cry1Ac toxins. Cry1Ac eliminated 50% of the tumors at 10 µg/ml, and eliminate 100% of the tumors at 30 and 50 µg/ml.

CONCLUSION

Bacillus thuringiensis Cry1A toxins show dual cytotoxic activity, in insects as well as in HeLa cancer cell line.

Knockdown of the aminopeptidase N genes decreases susceptibility of Chilo suppressalis larvae to Cry1Ab/Cry1Ac and Cry1Ca

Bacillus thuringiensis (Bt) insecticide is currently the most widely used bioinsecticide. Bt expressing cry genes are some of the most successful foreign-genome-inserting genes used in transgenic insect-resistant crop development. Cry toxins are resistant to lepidopteran pests, such as Chilo suppressalis, a major insect pest of rice worldwide. Since Cry toxins exert their activity by binding to specific receptors in the midgut of target insects, identification of functional Cry toxin receptors in the midgut of C. suppressalis larvae is crucial to evaluate potential resistance mechanisms and develop effective strategies for inhibiting insect resistance. In this study, we isolated two aminopeptidase N genes (APN6 and APN8) from C. suppressalis and determined that they were expressed in the foregut. APN6 was highly expressed at the fourth instar, and APN8 was highly expressed in adult and pupa. Knockdown of CsAPN6 and CsAPN8 by RNA interference resulted in significantly decreased susceptibility of larvae to Bt rice varieties TT51 (expressing cry1Ac/cry1Ab fusion genes) and T1C-19 (expressing cry1Ca), but not T2A-1 (expressing cry2Aa). These findings suggest that both APN6 and APN8 are involved in the toxicity of Cry1Ac/Cry1Ab and Cry1Ca toxins.

Newly identified APN splice isoforms suggest novel splicing mechanisms may underlie circRNA circularization in moth

Circular RNA (circRNA) have long been considered by-products of splicing errors, but the coordination of RNA transcription and exon circularization events remains poorly understood. Here, we investigated this question using genes encoding aminopeptidases N (APNs), which are receptors of Bacillus thuringiensis toxins, in the cotton bollworm, Helicoverpa armigera. We cloned and sequenced the cDNA of ten APN genes (HaAPN1-10) located in the same APN gene cluster, and detected 20 and 14 novel splicing isoforms with exon skipping in HaAPN1 and HaAPN3, respectively, whereas no or very few variants were found in the remaining genes. Further study identified 14 and 6 circular RNA (circRNA) in HaAPN1 and HaAPN3, respectively. Neither novel splicing isoforms nor circRNA were detected in HaAPN2 and HaAPN5. Distinct from the conventional GT/AG splicing signal, short co-directional repeats were involved in the splicing of the linear and circular isoforms of HaAPN1 and HaAPN3. Identification of the splice sites revealed that the linear isoforms may be related in some way to the circularization. Moreover, phylogenetic analysis and detection of circRNA of the APN gene of the diamondback moth, Plutella xylostella (PxAPN3), showed that circRNA formation is relatively conserved during the lepidopteran evolutionary process. These results contribute to an improved understanding of lepidopteran APNs and this novel class of insect circRNA.

Alkaline phosphatase 2 is a functional receptor of Cry1Ac but not Cry2Ab in Helicoverpa zea

Although membrane-bound alkaline phosphatases (ALPs) have been proposed as a receptor for Cry1Ac in a few lepidopteran species, their potential functions as a Cry2Ab receptor are yet to be verified. To determine if ALP2 also serves as a receptor for Cry1Ac and even for Cry2Ab in Helicoverpa zea, we measured the potency of activated Cry1Ac and Cry2Ab against midgut and fat body cell lines of H. zea and the ovarian cell line of Spodoptera frugiperda (Sf9) expressing H. zea ALP2 (HzALP2) or transfected with HzALP2 double-stranded RNA (dsRNA). Relative to the control cells, the three cell lines expressing HzALP2 were more susceptible to Cry1Ac but there was no difference for Cry2Ab. By contrast, the two H. zea cell lines transfected with HzALP2 dsRNA were resistant to Cry1Ac while kept susceptible to Cry2Ab. Furthermore, RNA interference knockdown of HzALP2 in H. zea larvae enhanced larval survival on Cry1Ac-containing diets. These findings indicate that HzALP2 functions as a receptor of Cry1Ac but not Cry2Ab.

RNA Interference in the Tobacco Hornworm, Manduca sexta, Using Plastid-Encoded Long Double-Stranded RNA

RNA interference (RNAi) is a promising method for controlling pest insects by silencing the expression of vital insect genes to interfere with development and physiology; however, certain insect Orders are resistant to this process. In this study, we set out to test the ability of in planta-expressed dsRNA synthesized within the plastids to silence gene expression in an insect recalcitrant to RNAi, the lepidopteran species, Manduca sexta (tobacco hornworm). Using the Manduca vacuolar-type H+ ATPase subunit A (v-ATPaseA) gene as the target, we first evaluated RNAi efficiency of two dsRNA products of different lengths by directly feeding the in vitro-synthesized dsRNAs to M. sexta larvae. We found that a long dsRNA of 2222 bp was the most effective in inducing lethality and silencing the v-ATPaseA gene, when delivered orally in a water droplet. We further transformed the plastid genome of the M. sexta host plant, Nicotiana tabacum, to produce this long dsRNA in its plastids and performed bioassays with M. sexta larvae on the transplastomic plants. In the tested insects, the plastid-derived dsRNA had no effect on larval survival and no statistically significant effect on expression of the v-ATPaseA gene was observed. Comparison of the absolute quantities of the dsRNA present in transplastomic leaf tissue for v-ATPaseA and a control gene, GFP, of a shorter size, revealed a lower concentration for the long dsRNA product compared to the short control product. We suggest that stability and length of the dsRNA may have influenced the quantities produced in the plastids, resulting in inefficient RNAi in the tested insects. Our results imply that many factors dictate the effectiveness of in planta RNAi, including a likely trade-off effect as increasing the dsRNA product length may be countered by a reduction in the amount of dsRNA produced and accumulated in the plastids.

The C-terminal protoxin region of Bacillus thuringiensis Cry1Ab toxin has a functional role in binding to GPI-anchored receptors in the insect midgut

Bacillus thuringiensis Cry toxins are used worldwide for controlling insects. Cry1Ab is produced as a 130-kDa protoxin that is activated by proteolytic removal of an inert 500 amino-acid-long C-terminal region, enabling the activated toxin to bind to insect midgut receptor proteins, leading to its membrane insertion and pore formation. It has been proposed that the C-terminal region is only involved in toxin crystallization, but its role in receptor binding is undefined. Here we show that the C-terminal region of Cry1Ab protoxin provides additional binding sites for alkaline phosphatase (ALP) and aminopeptidase N (APN) insect receptors. ELISA, ligand blot, surface plasmon resonance, and pulldown assays revealed that the Cry1Ab C-terminal region binds to both ALP and APN but not to cadherin. Thus, the C-terminal region provided a higher binding affinity of the protoxin to the gut membrane that correlated with higher toxicity of protoxin than activated toxin. Moreover, Cry1Ab domain II loop 2 or 3 mutations reduced binding of the protoxin to cadherin but not to ALP or APN, supporting the idea that protoxins have additional binding sites. These results imply that two different regions mediate the binding of Cry1Ab protoxin to membrane receptors, one located in domain II-III of the toxin and another in its C-terminal region, suggesting an active role of the C-terminal protoxin fragment in the mode of action of Cry toxins. These results suggest that future manipulations of the C-terminal protoxin region could alter the specificity and increase the toxicity of B. thuringiensis proteins.

Identification of midgut membrane proteins from different instars of Helicoverpa armigera (Lepidoptera: Noctuidae) that bind to Cry1Ac toxin

Helicoverpa armigera is a polyphagous pest sensitive to Cry1Ac protein from Bacillus thuringiensis (Bt). The susceptibility of the different larval instars of H. armigera to Cry1Ac protoxin showed a significant 45-fold reduction in late instars compared to early instars. A possible hypothesis is that gut surface proteins that bind to Cry1Ac differ in both instars, although higher Cry toxin degradation in late instars could also explain the observed differences in susceptibility. Here we compared the Cry1Ac-binding proteins from second and fifth instars by pull-down assays and liquid chromatography coupled to mass spectrometry analysis (LC-MS/MS). The data show differential protein interaction patterns of Cry1Ac in the two instars analyzed. Alkaline phosphatase, and other membrane proteins, such as prohibitin and an anion selective channel protein were identified only in the second instar, suggesting that these proteins may be involved in the higher toxicity of Cry1Ac in early instars of H. armigera. Eleven Cry1Ac binindg proteins were identified exclusively in late instar larvae, like different proteases such as trypsin-like protease, azurocidin-like proteinase, and carboxypeptidase. Different aminopeptidase N isofroms were identified in both instar larvae. We compared the Cry1Ac protoxin degradation using midgut juice from late and early instars, showing that the midgut juice from late instars is more efficient to degrade Cry1Ac protoxin than that of early instars, suggesting that increased proteolytic activity on the toxin could also explain the low Cry1Ac toxicity in late instars.

Transcriptomic Responses to Different Cry1Ac Selection Stresses in Helicoverpa armigera

Helicoverpa armigera can develop resistance to Bacillus thuringiensis (Bt), which threaten the long-term success of Bt crops. In the present study, RNAseq was employed to investigate the midgut genes response to strains with different levels of resistance (LF5, LF10, LF20, LF30, LF60, and LF120) in H. armigera. Results revealed that a series of differentially expressed unigenes (DEGs) were expressed significantly in resistant strains compared with the LF-susceptible strain. Nine trypsin genes, ALP2, were downregulated significantly in all the six resistant strains and further verified by qRT-PCR, indicating that these genes may be used as markers to monitor and manage pest resistance in transgenic crops. Most importantly, the differences in DEG functions in the different resistant strains revealed that different resistance mechanisms may develop during the evolution of resistance. The immune and detoxification processes appear to be associated with the low-level resistance (LF5 strain). Metabolic process-related macromolecules possibly lead to resistance to Cry1Ac in the LF10 and LF20 strains. The DEGs involved in the “proton-transporting V-type ATPase complex” and the “proton-transporting two-sector ATPase complex” were significantly expressed in the LF30 strain, probably causing resistance to Cry1Ac in the LF30 strain. The DEGs involved in binding and iron ion homeostasis appear to lead to high-level resistance in the LF60 and LF120 strains, respectively. The multiple genes and different pathways seem to be involved in Cry1Ac resistance depending on the levels of resistance. Although the mechanisms of resistance are very complex in H. armigera, a main pathway seemingly exists, which contributes to resistance in each level of resistant strain. Altogether, the findings in the current study provide a transcriptome-based foundation for identifying the functional genes involved in Cry1Ac resistance in H. armigera.

Cytotoxicity and binding profiles of activated Cry1Ac and Cry2Ab to three insect cell lines

While Cry1Ac has been known to bind with larval midgut proteins cadherin, APN (amino peptidase N), ALP (alkaline phosphatase) and ABCC2 (adenosine triphosphate-binding cassette transporter subfamily C2), little is known about the receptors of Cry2Ab. To provide a clue to the receptors of Cry2Ab, we tested the baseline cytotoxicity of activated Cry1Ac and Cry2Ab against the midgut and fat body cell lines of Helicoverpa zea and the ovary cell line of Spodoptera frugiperda (SF9). As expected, the descending order of cytotoxicity of Cry1Ac against the three cell lines in terms of 50% lethal concetration (LC₅₀ ) was midgut (31.0 μg/mL) > fat body (59.0 μg/mL) and SF9 cell (99.6 μg/mL). By contrast, the fat body cell line (LC₅₀ = 7.55 μg/mL) was about twice more susceptible to Cry2Ab than the midgut cell line (16.0 μg/mL), the susceptibility of which was not significantly greater than that of SF9 cells (27.0 μg/mL). Further, ligand blot showed the binding differences between Cry1Ac and Cry2Ab in the three cell lines. These results indicated that the receptors of Cry2Ab were enriched in fat body cells and thus largely different from the receptors of Cry1Ac, which were enriched in midgut cells.

Alimentary Tract Transcriptome Analysis of the Tea Geometrid, Ectropis oblique (Lepidoptera: Geometridae)

Ectropis oblique Prout (Lepidoptera: Geometridae) is one of the main pests that damages the tea crop in Southeast Asia. To understand the molecular mechanisms of its feeding biology, transcriptomes of the alimentary tract (AT) and of the body minus the AT of E. oblique were successfully sequenced and analyzed in this study. A total of 36,950 unigenes from de novo sequences were assembled. After analysis using six annotation databases (e.g., Gene Ontology, Kyoto Encyclopedia of Genes and Genome, and NCBI nr), a series of putative genes were found for this insect species that were related to digestion, detoxification, the immune system, and Bacillus thuringiensis (Bt) receptors. From this series of genes, 21 were randomly selected to verify the relative expression levels of transcripts using quantitative real-time polymerase chain reaction. These results will provide an invaluable genomic resource for future studies on the molecular mechanisms of E. oblique, which will be useful in developing biological control strategies for this pest.

Engineering Bacillus thuringiensis Cyt1Aa toxin specificity from dipteran to lepidopteran toxicity

The Cyt and Cry toxins are different pore-forming proteins produced by Bacillus thuringiensis bacteria, and used in insect-pests control. Cry-toxins have a complex mechanism involving interaction with several proteins in the insect gut such as aminopeptidase N (APN), alkaline phosphatase (ALP) and cadherin (CAD). It was shown that the loop regions of domain II of Cry toxins participate in receptor binding. Cyt-toxins are dipteran specific and interact with membrane lipids. We show that Cry1Ab domain II loop3 is involved in binding to APN, ALP and CAD receptors since point mutation Cry1Ab-G439D affected binding to these proteins. We hypothesized that construction of Cyt1A-hybrid proteins providing a binding site that recognizes gut proteins in lepidopteran larvae could result in improved Cyt1Aa toxin toward lepidopteran larvae. We constructed hybrid Cyt1Aa-loop3 proteins with increased binding interaction to Manduca sexta receptors and increased toxicity against two Lepidopteran pests, M. sexta and Plutella xylostella. The hybrid Cyt1Aa-loop3 proteins were severely affected in mosquitocidal activity and showed partial hemolytic activity but retained their capacity to synergize Cry11Aa toxicity against mosquitos. Our data show that insect specificity of Cyt1Aa toxin can be modified by introduction of loop regions from another non-related toxin with different insect specificity.

Midgut transcriptomal response of the rice leaffolder, Cnaphalocrocis medinalis (Guenée) to Cry1C toxin

Cnaphalocrocis medinalis (Guenée) is one of the important insect pests in rice field. Bt agents were recommended in the C. medinalis control and Bt rice is bred as a tactic to control this insect. However, the tolerance or resistance of insect to Bt protein is a main threat to the application of Bt protein. In order to investigate the response of C. medinalis transcriptome in defending a Cry1C toxin, high-through RNA-sequencing was carried in the C. medinalis larvae treated with and without Cry1C toxin. A total of 35,586 high-quality unigenes was annotated in the transcriptome of C. medinalis midgut. The comparative analysis identified 6,966 differently expressed unigenes (DEGs) between the two treatments. GO analysis showed that these genes involved in proteolysis and extracellular region. Among these DEGs, carboxylesterase, glutathione S-transferase and P450 were differently expressed in the treated C. medinalis midgut. Furthermore, trypsin, chymotrypsin, and carboxypeptidase were identified in DEGs, and most of them up-regulated. In addition, thirteen ABC transporters were downregulated and three upregulated in Cry1C-treated C. medinalis midgut. Based on the pathway analysis, antigen processing and presentation pathway, and chronic myeloid leukemia pathway were significant in C. medinalis treated with Cry1C toxin. These results indicated that serine protease, detoxification enzymes and ABC transporter, antigen processing and presentation pathway, and chronic myeloid leukemia pathway may involved in the response of C. medinalis to Cry1C toxin. This study provides a transcriptomal foundation for the identification and functional characterization of genes involved in the toxicity of Bt Cry protein against C. medinalis, and provides potential clues to the studies on the tolerance or resistance of an agriculturally important insect pest C. medinalis to Cry1C toxin.

Identification of Aminopeptidase-N2 as a Cry2Ab binding protein in Manduca sexta

Bacillus thuringiensis Cry2Ab toxin has been used in combination with Cry1Ac for resistance management on the Bt-cotton that is widely planted worldwide. However, little is known regarding Cry2Ab mode of action. Particularly, there is a gap of knowledge on the identification of insect midgut proteins that bind Cry2Ab and mediate toxicity. In the case of Cry1Ab toxin, a transmembrane cadherin protein and glycosyl-phosphatidylinositol (GPI) anchored proteins like aminopeptidase-N1 (APN1) or alkaline-phosphatase (ALP) from Manduca sexta, have been shown to be important for oligomer formation and insertion into the membrane. Binding competition experiments showed that Cry2Ab toxin does not share binding sites with Cry1Ab toxin in M. sexta brush border membrane vesicles (BBMV). Also, that Cry2Ab shows reduced binding to the Cry1Ab binding molecules cadherin, APN1 or ALP. Finally, ligand blot experiments and protein sequence by LC-MS/MS identified APN2 isoform as a Cry2Ab binding protein. Cloning and expression of APN2 confirmed that APN2 is a Cry2Ab binding protein.

Identification and Characterization of Hyphantria cunea Aminopeptidase N as a Binding Protein of Bacillus thuringiensis Cry1Ab35 Toxin

The fall webworm, Hyphantria cunea (Drury) is a major invasive pest in China. Aminopeptidase N (APN) isoforms in lepidopteran larvae midguts are known for their involvement in the mode of action of insecticidal crystal (Cry) proteins from Bacillus thuringiensis. In the present work, we identified a putative Cry1Ab toxin-binding protein, an APN isoform designated HcAPN3, in the midgut of H. cunea by ligand blot and mass spectrometry. HcAPN3 was highly expressed throughout all larval developmental stages and was abundant in the midgut and hindgut tissues. HcAPN3 was down-regulated at 6 h, then was up-regulated significantly at 12 h and 24 h after Cry1Ab toxin treatment. We expressed HcAPN3 in insect cells and detected its interaction with Cry1Ab toxin by ligand blot assays. Furthermore, RNA interference (RNAi) against HcAPN3 using oral delivery and injection of double-stranded RNA (dsRNA) resulted in a 61–66% decrease in transcript level. Down-regulating of the expression of HcAPN3 was closely associated with reduced susceptibility of H. cunea to Cry1Ab. In addition, the HcAPN3E fragment peptide expressed in Escherichia coli enhanced Cry1Ab toxicity against H. cunea larvae. This work represents the first evidence to suggest that an APN in H. cunea is a putative binding protein involved in Cry1Ab susceptibility.

Transcriptomic analysis of the hepatopancreas induced by eyestalk ablation in shrimp, Litopenaeus vannamei

Although eyestalk ablation (ESA) is currently considered the most effective method to facilitate molting and maturation, its physiological responses are still not clearly explained in decapod crustaceans. In this study, we analyzed the hepatopancreatic transcriptomes of Litopenaeus vannamei after ESA using the Illumina Miseq platform. After screening 53,029 contigs with high cutoff values (fold change>|10|; P-value<0.05; RPKM>1), we were able to identify 105 differentially expressed genes (DEGs), of which 100 were up-regulated and five were down-regulated. Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis showed that many DEGs were involved in the synthetic pathways for glycerol and trehalose, which are known to function as the major protectants under conditions of low temperature and osmotic stress in arthropods. Additional analysis of the other DEGs enabled us to classify them in four categories: immunity; cellular trafficking; transcriptional regulation; molting and maturation. Many DEGs were involved in immunity and stress responses, in particular the proPO activation system, which is the major immune and wound-healing system in arthropods. In addition to immunity and stress responses, we were also able to identify DEGs involved in molting and maturation processes (e.g., group I chitinase), as well as those involved in hormone metabolism and trafficking. Collectively, based on the transcriptomic analysis, ESA causes not only stress and immune responses, but also molting and maturation in L. vannamei. The DEGs identified in this study could be useful markers to understand the physiological responses that ESA induces in shrimp, such as molting, maturation, and immunity.

Transgenic cotton coexpressing Vip3A and Cry1Ac has a broad insecticidal spectrum against lepidopteran pests

Although farmers in China have grown transgenic Bt-Cry1Ac cotton to resist the major pest Helicoverpa armigera since 1997 with great success, many secondary lepidopteran pests that are tolerant to Cry1Ac are now reported to cause considerable economic damage. Vip3AcAa, a chimeric protein with the N-terminal part of Vip3Ac and the C-terminal part of Vip3Aa, has a broad insecticidal spectrum against lepidopteran pests and has no cross resistance to Cry1Ac. In the present study, we tested insecticidal activities of Vip3AcAa against Spodoptera litura, Spodoptera exigua, and Agrotis ipsilon, which are relatively tolerant to Cry1Ac proteins. The bioassay results showed that insecticidal activities of Vip3AcAa against these three pests are superior to Cry1Ac, and after an activation pretreatment, Vip3AcAa retained insecticidal activity against S. litura, S. exigua and A. ipsilon that was similar to the unprocessed protein. The putative receptor for this chimeric protein in the brush border membrane vesicle (BBMV) in the three pests was also identified using biotinylated Vip3AcAa toxin. To broaden Bt cotton activity against a wider spectrum of pests, we introduced the vip3AcAa and cry1Ac genes into cotton. Larval mortality rates for S. litura, A. ipsilon and S. exigua that had fed on this new cotton increased significantly compared with larvae fed on non-Bt cotton and Bt-Cry1Ac cotton in a laboratory experiment. These results suggested that the Vip3AcAa protein is an excellent option for a "pyramid" strategy for integrated pest management in China.

Functional roles of cadherin, aminopeptidase-N and alkaline phosphatase from Helicoverpa armigera (Hübner) in the action mechanism of Bacillus thuringiensis Cry2Aa

A pyramid strategy combining the Cry1A and Cry2A toxins in Bt crops has been widely used throughout the world to delay pest adaption to transgenic crops and broaden the insecticidal spectrum. Midgut membrane-bound cadherin (CAD), aminopeptidase-N (APN) and alkaline phosphatase (ALP) are important for Cry1A toxicity in some lepidopteran larvae, but the proteins that bind Cry2A in the midgut of target insects and their role in the Cry2A mechanism of action are still unclear. In this study, we found that heterologously expressed CAD, APN4 and ALP2 peptides from the midgut of Helicoverpa armigera could bind to the Cry2Aa toxin with a high affinity. Additionally, the efficiency of Cry2Aa insecticidal activity against H. armigera larvae was obviously reduced after the genes encoding these proteins were silenced with specific siRNAs: CAD- and ALP2-silenced larvae showed significantly similar reductions in mortality due to the Cry2Aa toxin (41.67% and 43.06%, respectively), whereas a larger reduction in mortality was observed in APN4-silenced larvae (61.11%) than in controls. These results suggest that CAD, APN4 and ALP2 are involved in the mechanism of action of Cry2Aa in H. armigera and may play important functional roles in the toxicity of the Cry2Aa toxin.

New insights on the role of alkaline phosphatase 2 from Spodoptera exigua (Hübner) in the action mechanism of Bt toxin Cry2Aa

Cry1A and Cry2A toxins, which are widely used in Bt transgenic crops, can specifically bind to insect midguts and exert their insecticidal effects. There are interactions between insect midgut-binding proteins and Cry1A toxins; however, little is known about the insect protein that specifically binds to Cry2A. Midgut membrane-bound alkaline phosphatases (ALPs), which are important for the binding of proteins to Cry1A, play dominant roles in Cry1A-mediated toxicity in some lepidopteran larvae. In this study, we cloned and expressed one partial ALP2 peptide from susceptible Spodoptera exigua larvae and studied the binding characteristics of SeALP2 with Cry2Aa. The ALPs proteins was expressed at all larval stages and highly expressed in the first and second instar larvae. The heterologously expressed SeALP2 peptide bound specifically to Cry2Aa with a high affinity. Knocking down ALP2 in vivo revealed that it plays an important role in the susceptibility of S. exigua to Cry2Aa. Based on these findings, we propose that ALP2 in S. exigua serves as a functional receptor for Cry2Aa.

Aminopeptidase N1 is involved in Bacillus thuringiensis Cry1Ac toxicity in the beet armyworm, Spodoptera exigua

Understanding how insecticidal proteins from the bacterium Bacillus thuringiensis (Bt) interact with their hosts is crucial to fully explain the molecular bases of Bt specificity and insecticidal activity. Previous studies support ATP binding cassette transporters (ABCC2/3) and one cadherin-like protein are Cry1Ac functional receptors in the beet armyworm (Spodoptera exigua). In this study, a combined one-dimensional gel electrophoresis and immunoblotting approach identified aminopeptidase N (APNs) as putative Cry1Ac binding proteins in the midgut brush border membrane of S. exigua larvae. Functional analyses by gene silencing of six different S. exigua APN genes (SeAPN1, SeAPN2, SeAPN3, SeAPN4, SeAPN5 and SeAPN6) showed that only suppression of SeAPN1 resulted in decreased larval susceptibility to Cry1Ac toxin. These results support that SeAPN1 plays important functional role in Cry1Ac toxicity in S. exigua.

Aminopeptidase N5 (APN5) as a Putative Functional Receptor of Cry1Ac Toxin in the Larvae of Athetis lepigone

Athetis lepigone was a new lepidopteran pest and caused severe damage to maize crops in China. We have detected that Cry1Ac protoxin and toxin were highly active against the larvae of A. lepigone. However, there is no report about the mode of action of Bt Cry1Ac toxin against this pest until now. A 110 kDa APN5 protein from BBMV of A. lepigone was identified as the binding receptor of Cry1Ac toxin using Ligand blotting. The Cry1Ac receptor APN5 was cloned from A. lepigone larval midgut mRNA and named as AlAPN5 (GenBank accession no.: KU950745). AlAPN5 had a GATEN motif and been classified to Class 5 APNs. 79.2% reduction in mortality was observed when A. lepigone larvae were injected with siRNA of the AlAPN5 gene and treated with Cry1Ac toxin. These data demonstrate that AlAPN5 is a putative functional receptor and maybe the only receptor of Cry1Ac in A. lepigone.

Identification of an alkaline phosphatase as a putative Cry1Ac binding protein in Ostrinia furnacalis (Guenée)

Asian corn borer (ACB), Ostrinia furnacalis, is an important insect pest of maize susceptible to different Cry1A toxins. Based on amino acid sequence alignment of ALP sequences from lepidopteran larvae an alp gene was cloned from ACB, named ofalp. Pull dawn assays using biotinylated Cry1Ac and brush border membrane vesicles isolated from second instar ACB larvae showed that four proteins of 50, 65, 68 and 70kDa precipitated with the Cry1Ac. The 65kDa band cross-reacted with the anti-OfALP monoclonal antibody. GalNac was able to release the binding of Cry1Ac to the 65kDa OfALP in pull down assays. A 37kDa fragment from residues D173 to D473 of OfALP was cloned and expressed in Escherichia coli cells. We show that this ALP-fragment was able to bind Cry1Ac in ligand blot analysis. Our data also indicate that different ALP isoforms or variants may be also Cry1Ac binding proteins since more ALP enzymatic activity was pull down with Cry1Ac than with anti-OfALP antibody. We also analyzed the expression levels of ALP throughout the larval development by qPCR and ALP enzymatic activity. Our data indicated that ALP expression in ACB was observed preferentially in young instar larvae. Finally, we show that resistance in O. furnacalis ACB-AcR strain resistant to Cry1Ac did not correlate with changes in expression of this ALP protein since it shows similar gene expression of ofalp than the susceptible insect strain. Identification of Cry1Ac receptors will help to understand mechanism of action of Cry1Ac in O. furnacalis and to understand mechanism of Cry toxin resistance. Our data indicate that at least one ALP protein is involved in the binding interaction with Cry1Ac in O. furnacalis.

Bacillus thuringiensis toxin resistance mechanisms among Lepidoptera: progress on genomic approaches to uncover causal mutations in the European corn borer, Ostrinia nubilalis

Transgenic plants that express Bacillus thuringiensis (Bt) crystal (Cry) protein toxins (Bt crops) effectively control feeding by the European corn borer, Ostrinia nubilalis, although documented resistance evolution among a number of species in both the laboratory and field has heightened concerns about the durability of this technology. Research has provided major insights into the mutations that alter Bt toxin binding receptor structure and function within the midgut of Lepidoptera that directly impacts the efficacy of Bt toxins, and potentially leads to the evolution of resistance to Bt crops in the field. In this manuscript we provide an overview of available data on the identification of genes involved in high levels of resistance to Cry toxins, with emphasis on resistance described for O. nubilalis as the main target of Bt corn.

Anthelmintic Effect of Bacillus thuringiensis Strains against the Gill Fish Trematode Centrocestus formosanus

Parasitic agents, such as helminths, are the most important biotic factors affecting aquaculture, and the fluke Centrocestus formosanus is considered to be highly pathogenic in various fish species. There have been efforts to control this parasite with chemical helminthicides, but these efforts have had unsuccessful results. We evaluated the anthelmintic effect of 37 strains of Bacillus thuringiensis against C. formosanus metacercariae in vitro using two concentrations of total protein, and only six strains produced high mortality. The virulence (CL₅₀) on matacercariae of three strains was obtained: the GP308, GP526, and ME1 strains exhibited a LC₅₀ of 146.2 μg/mL, 289.2 μg/mL, and 1721.9 μg/mL, respectively. Additionally, these six B. thuringiensis strains were evaluated against the cercariae of C. formosanus; the LC₅₀ obtained from the GP526 strain with solubilized protein was 83.8 μg/mL, and it could be considered as an alternative control of the metacercariae and cercariae of this parasite in the productivity systems of ornamental fishes.

Identification of Genes Relevant to Pesticides and Biology from Global Transcriptome Data of Monochamus alternatus Hope (Coleoptera: Cerambycidae) Larvae

Monochamus alternatus Hope is the main vector in China of the Pine Wilt Disease caused by the pine wood nematode Bursaphelenchus xylophilus. Although chemical control is traditionally used to prevent pine wilt disease, new strategies based in biological control are promising ways for the management of the disease. However, there is no deep sequence analysis of Monochamus alternatus Hope that describes the transcriptome and no information is available about gene function of this insect vector. We used next generation sequencing technology to sequence the whole fourth instar larva transcriptome of Monochamus alternatus Hope and successfully built a Monochamus alternatus Hope transcriptome database. In total, 105,612 unigenes were assigned for Gene Ontology (GO) terms, information for 16,730 classified unigenes was obtained in the Clusters of Orthologous Groups (COGs) database, and 13,024 unigenes matched with 224 predicted pathways in the Kyoto Encyclopedia of Genes and Genome (KEGG). In addition, genes related to putative insecticide resistance-related genes, RNAi, the Bt receptor, intestinal digestive enzymes, possible future insect control targets and immune-related molecules are described. This study provides valuable basic information that can be used as a gateway to develop new molecular tools for Monochamus alternatus Hope control strategies.

APN1 is a functional receptor of Cry1Ac but not Cry2Ab in Helicoverpa zea

Lepidopteran midgut aminopeptidases N (APNs) are phylogenetically divided into eight clusters, designated as APN1-8. Although APN1 has been implicated as one of the receptors for Cry1Ac in several species, its potential role in the mode of action of Cry2Ab has not been functionally determined so far. To test whether APN1 also acts as one of the receptors for Cry1Ac in Helicoverpa zea and even for Cry2Ab in this species, we conducted a gain of function analysis by heterologously expressing H. zea APN1 (HzAPN1) in the midgut and fat body cell lines of H. zea and the ovarian cell line of Spodoptera frugiperda (Sf9) and a loss of function analysis by RNAi (RNA interference) silencing of the endogenous APN1 in the three cell lines using the HzAPN1 double strand RNA (dsRNA). Heterologous expression of HzAPN1 significantly increased the susceptibility of the three cell lines to Cry1Ac, but had no effects on their susceptibility to Cry2Ab. Knocking down of the endogenous APN1 made the three cell lines resistant to Cry1Ac, but didn't change cell lines susceptibility to Cry2Ab. The findings from this study demonstrate that HzAPN1 is a functional receptor of Cry1Ac, but not Cry2Ab.

Bacillus thuringiensis Cry1AbMod toxin counters tolerance associated with low cadherin expression but not that associated with low alkaline phosphatase expression in Manduca sexta

To exert their toxic effect, Bacillus thuringiensis Cry1Ab toxin undergoes a sequential binding mechanism with different larval gut proteins including glycosyl-phosphatidyl-inositol anchored proteins like aminopeptidase-N (APN) or alkaline-phosphatase (ALP) and a transmembrane cadherin to form pre-pore structures that insert into the membrane. Cadherin binding induces oligomerization of the toxin by facilitating removal of the N-terminal region, while APN/ALP binding helps in oligomer membrane insertion. Cry1AbMod toxin was engineered to lack N-terminal region of the toxin and shown to counter resistance linked to cadherin mutations. In this manuscript we determined the toxicity of Cry1AbMod to Manduca sexta larvae silenced in the expression of cadherin, ALP or APN receptors. As previously reported Cry1Ab toxicity relied principally in ALP and cadherin in comparison to APN. Our data shows that Cry1AbMod counters resistance associated with low cadherin expression but was not effective against ALP silenced larvae. These results show that Cry1AbMod could be effective against resistance insects linked to mutations on binding molecules involved in toxin oligomerization but not against resistant insects linked to mutations on binding molecules involved in oligomer membrane insertion.

A toxin-binding alkaline phosphatase fragment synergizes Bt toxin Cry1Ac against susceptible and resistant Helicoverpa armigera

Evolution of resistance by insects threatens the continued success of pest control using insecticidal crystal (Cry) proteins from the bacterium Bacillus thuringiensis (Bt) in sprays and transgenic plants. In this study, laboratory selection with Cry1Ac yielded five strains of cotton bollworm, Helicoverpa armigera, with resistance ratios at the median lethal concentration (LC50) of activated Cry1Ac ranging from 22 to 1700. Reduced activity and reduced transcription of an alkaline phosphatase protein that binds Cry1Ac was associated with resistance to Cry1Ac in the four most resistant strains. A Cry1Ac-binding fragment of alkaline phosphatase from H. armigera (HaALP1f) was not toxic by itself, but it increased mortality caused by Cry1Ac in a susceptible strain and in all five resistant strains. Although synergism of Bt toxins against susceptible insects by toxin-binding fragments of cadherin and aminopeptidase N has been reported previously, the results here provide the first evidence of synergism of a Bt toxin by a toxin-binding fragment of alkaline phosphatase. The results here also provide the first evidence of synergism of a Bt toxin by any toxin-binding peptide against resistant insects.

MAPK signaling pathway alters expression of midgut ALP and ABCC genes and causes resistance to Bacillus thuringiensis Cry1Ac toxin in diamondback moth

Insecticidal crystal toxins derived from the soil bacterium Bacillus thuringiensis (Bt) are widely used as biopesticide sprays or expressed in transgenic crops to control insect pests. However, large-scale use of Bt has led to field-evolved resistance in several lepidopteran pests. Resistance to Bt Cry1Ac toxin in the diamondback moth, Plutella xylostella (L.), was previously mapped to a multigenic resistance locus (BtR-1). Here, we assembled the 3.15 Mb BtR-1 locus and found high-level resistance to Cry1Ac and Bt biopesticide in four independent P. xylostella strains were all associated with differential expression of a midgut membrane-bound alkaline phosphatase (ALP) outside this locus and a suite of ATP-binding cassette transporter subfamily C (ABCC) genes inside this locus. The interplay between these resistance genes is controlled by a previously uncharacterized trans-regulatory mechanism via the mitogen-activated protein kinase (MAPK) signaling pathway. Molecular, biochemical, and functional analyses have established ALP as a functional Cry1Ac receptor. Phenotypic association experiments revealed that the recessive Cry1Ac resistance was tightly linked to down-regulation of ALP, ABCC2 and ABCC3, whereas it was not linked to up-regulation of ABCC1. Silencing of ABCC2 and ABCC3 in susceptible larvae reduced their susceptibility to Cry1Ac but did not affect the expression of ALP, whereas suppression of MAP4K4, a constitutively transcriptionally-activated MAPK upstream gene within the BtR-1 locus, led to a transient recovery of gene expression thereby restoring the susceptibility in resistant larvae. These results highlight a crucial role for ALP and ABCC genes in field-evolved resistance to Cry1Ac and reveal a novel trans-regulatory signaling mechanism responsible for modulating the expression of these pivotal genes in P. xylostella.

Biopesticide and transgenic crops based on Bacillus thuringiensis (Bt) Cry toxins are widely used worldwide, yet the development of field resistance seriously threatens their sustainability. Unraveling these resistance mechanisms are of great importance for delaying insect field resistance evolution. The diamondback moth was the first insect to evolve field resistance to Bt biopesticides and it is an excellent model for the study of Bt resistance mechanisms. In this work, we present strong empirical evidence supporting that (1) field-evolved resistance to Bt in P. xylostella is tightly associated with differential expression of a membrane-bound alkaline phosphatase (ALP) and a suite of ATP-binding cassette transporter subfamily C (ABCC) genes, and (2) a constitutively transcriptionally-activated upstream gene (MAP4K4) in the MAPK signaling pathway is responsible for this trans-regulatory signaling mechanism. These findings identify key resistance genes and provide the first comprehensive mechanistic description responsible for the field-evolved Bt resistance in P. xylostella. Given that expression alterations of multiple receptor genes result in Bt resistance in many other insects, it can now be tested to determine whether the previously unidentified trans-regulatory mechanism characterized in this study is also involved in these cases.

Expression of recombinant and mosaic Cry1Ac receptors from Helicoverpa armigera and their influences on the cytotoxicity of activated Cry1Ac to Spodoptera litura Sl-HP cells

Bacillus thuringiensis (Bt) toxin receptors play important roles in the killing of pests, and investigation on characterization of the receptors is essential for utilization of Bt and management of insect resistance. Here, recombinant and mosaic receptors of Bt Cry1Ac toxin from Helicoverpa armigera were expressed in Spodoptera litura Sl-HP cells and their influences on cytotoxicity of activated Cry1Ac toxin were investigated. When H. armigera aminopeptidase N1 (APN1), alkaline phosphatase 2 (ALP2) and cadherin fused with or without GFP tag were, respectively, expressed in Sl-HP cells, live cell-immunofluorescence staining detection revealed that the quantity of the toxin binding to cadherin or cadherin-GFP was much more than that binding to ALP2 and APN1 or their fusion proteins with GFP, and only the cadherin- or cadherin-GFP-expressing cells showed aberrant cell morphology after the treatment of the toxin at low concentrations. ALP2 and APN1 fused with or without GFP tag did not significantly enhance the cadherin-mediated cytotoxicity of the toxin. The mosaic ALP-TBR-GFP-GPI was located on cell membrane, but did not bind to the toxin. The mosaic truncated cadherin-GFP-GPI was not located on cell membrane even if the signal peptide was sustained. The concentrations of the toxin resulting in swelling of 50 % cells for noncadherin-expressing Sl-HP cells and cadherin-expressing Hi5 cells were 5.08 and 9.50 µg/ml within 1 h, respectively. Taken together, our data have indicated that the binding affinity of ALP2 and APN1 to activated Cry1Ac toxin is much weaker than that of cadherin and both ALP2 and APN1 do not enhance the cytotoxicity of the toxin even though cadherin is co-expressed, and the mosaic receptor of ALP2 inserted with cadherin toxin binding domain does not mediate cytotoxicity of the toxin. In addition, the noncadherin-expressing Sl-HP cells are more susceptible to activated Cry1Ac than the cadherin-expressing Hi5 cells.

Alkaline phosphatases and aminopeptidases are altered in a Cry11Aa resistant strain of Aedes aegypti

Bacillus thuringiensis subsp. israelensis (Bti) is widely used for the biological control of mosquito populations. However, the mechanism of Bti toxins is still not fully understood. To further elucidate the mechanism of Bti toxins, we developed an Aedes aegypti resistant strain that shows high-level resistance to Cry11Aa toxin. After 27 selections with Cry11Aa toxin, the larvae showed a 124-fold resistance ratio for Cry11Aa (strain G30). G30 larvae showed cross-resistance to Cry4Aa (66-fold resistance), less to Cry4Ba (13-fold), but not to Cry11Ba (2-fold). Midguts from these resistant larvae did not show detectable difference in the processing of the Cry11Aa toxin compared to that in susceptible larvae (WT). Brush border membrane vesicles (BBMV) from resistant larvae bound slightly less Cry11Aa compared to WT BBMV. To identify potential proteins associated with Cry11A resistance, not only transcript changes in the larval midgut were analyzed using Illumina sequencing and qPCR, but alterations of previously identified receptor proteins were investigated using immunoblots. The transcripts of 375 genes were significantly increased and those of 208 genes were down regulated in the resistant larvae midgut compared to the WT. None of the transcripts for previously identified receptors of Cry11Aa (Aedes cadherin, ALP1, APN1, and APN2) were altered in these analyses. The genes for the identified functional receptors in resistant larvae midgut did not contain any mutation in their sequences nor was there any change in their transcript expression levels compared to WT. However, ALP proteins were expressed at reduced levels (∼ 40%) in the resistant strain BBMV. APN proteins and their activity were also slightly reduced in resistance strain. The transcript levels of ALPs (AAEL013330 and AAEL015070) and APNs (AAEL008158, AAEL008162) were significantly reduced. These results strongly suggest that ALPs and APNs could be associated with Cry11Aa resistance in Ae. aegypti.

Bacillus thuringiensis Cry1A toxins are versatile proteins with multiple modes of action: two distinct pre-pores are involved in toxicity

Cry proteins from Bacillus thuringiensis are insecticidal PFTs (pore-forming toxins). In the present study, we show that two distinct functional pre-pores of Cry1Ab are formed after binding of the protoxin or the protease-activated toxin to the cadherin receptor, but before membrane insertion. Both pre-pores actively induce pore formation, although with different characteristics, and contribute to the insecticidal activity. We also analysed the oligomerization of the mutant Cry1AbMod protein. This mutant kills different insect populations that are resistant to Cry toxins, but lost potency against susceptible insects. We found that the Cry1AbMod-protoxin efficiently induces oligomerization, but not the activated Cry1AbMod-toxin, explaining the loss of potency of Cry1AbMod against susceptible insects. These data are relevant for the future control of insects resistant to Cry proteins. Our data support the pore-formation model involving sequential interaction with different midgut proteins, leading to pore formation in the target membrane. We propose that not only different insect targets could have different receptors, but also different midgut proteases that would influence the rate of protoxin/toxin activation. It is possible that the two pre-pore structures could have been selected for in evolution, since they have differential roles in toxicity against selected targets, increasing their range of action. These data assign a functional role for the protoxin fragment of Cry PFTs that was not understood previously. Most PFTs produced by other bacteria are secreted as protoxins that require activation before oligomerization, to finally form a pore. Thus different pre-pores could be also part of the general mechanism of action of other PFTs.

Two distinct functional pre-pore oligomers of the Cry1Ab insecticidal toxin are formed before membrane insertion. These oligomers are formed after binding of either the protoxin or the protease-activated toxin to the cadherin receptor. Both pre-pores have different characteristics and contribute to insecticidal activity.

New insight to structure-function relationship of GalNAc mediated primary interaction between insecticidal Cry1Ac toxin and HaALP receptor of Helicoverpa armigera

Over the last few decades Cry1Ac toxin has been widely used in controlling the insect attack due to its high specificity towards target insects. The pore-forming toxin undergoes a complex mechanism in the insect midgut involving sequential interaction with specific glycosylated receptors in which terminal GalNAc molecule plays a vital role. Recent studies on Cry toxins interactions with specific receptors revealed the importance of several amino acid residues in domain III of Cry1Ac, namely Q509, N510, R511, Y513 and W545, serve as potential binding sites that surround the putative GalNAc binding pocket and mediate the toxin-receptor interaction. In the present study, alanine substitution mutations were generated in the Cry1Ac domain III region and functional significance of those key residues was monitored by insect bioassay on Helicoverpa armigera larvae. In addition, ligand blot analysis and SPR binding assay was performed to monitor the binding characteristics of Cry1Ac wild type and mutant toxins towards HaALP receptor isolated from Helicoverpa armigera. Mutagenesis data revealed that, alanine substitutions in R511, Y513 and W545 substantially impacted the relative affinity towards HaALP receptor and toxicity toward target insect. Furthermore, in silico study of GalNAc-mediated interaction also confirmed the important roles of these residues. This structural analysis will provide a detail insight for evaluating and engineering new generation Cry toxins to address the problem of change in insect behavioral patterns.