Proteomic analysis of novel Cry1Ac binding proteins in Helicoverpa armigera (Hübner)

V-ATPase E mediates Cry2Ab binding and toxicity in Helicoverpa armigera

Cry2Ab is one of the important alternative Bt proteins that can be used to manage insect pests resistant to Cry1A toxins and to expand the insecticidal spectrum of pyramided Bt crops. Previous studies have showed that vacuolar H+-ATPase subunits A and B (V-ATPase A and B) may be involved in Bt insecticidal activities. The present study investigated the role of V-ATPases subunit E in the toxicity of Cry2Ab in Helicoverpa amigera. RT-PCR analysis revealed that oral exposure of H. amigera larvae to Cry2Ab led to a significant reduction in the expression of H. armigera V-ATPase E (HaV-ATPase E). Ligand blot, homologous and heterologous competition experiments confirmed that HaV-ATPases E physically and specifically bound to activated Cry2Ab toxin. Heterologous expressing of HaV-ATPase E in Sf9 cells made the cell line more susceptible to Cry2Ab, whereas knockdown of the endogenous V-ATPase E in H. zea midgut cells decreased Cry2Ab's cytotoxicity against this cell line. Further in vivo bioassay showed that H. armigera larvae fed a diet overlaid with both Cry2Ab and E. coli-expressed HaV-ATPase E protein suffered significantly higher mortality than those fed Cry2Ab alone. These results support that V-ATPases E is a putative receptor of Cry2Ab and can be used to improve Cry2Ab toxicity and manage Cry2Ab resistance at least in H. armigera.

ATP Synthase Subunit α from Helicoverpa armigera Acts as a Receptor of Bacillus thuringiensis Cry1Ac and Synergizes Cry1Ac Toxicity

Insect resistance to Bacillus thuringiensis (Bt) toxins has led to an urgent need to explore the insecticidal mechanisms of Bt. Previous studies indicated that Helicoverpa armigera ATP synthase subunit α (HaATPs-α) is involved in Cry1Ac resistance. In this study, a real-time quantitative polymerase chain reaction (RT-PCR) confirmed that HaATPs-α expression was significantly reduced in the Cry1Ac-resistant strain (BtR). Cry1Ac feeding induced the downregulated expression of HaATPs-α in the susceptible strain, but not in the BtR strain. Furthermore, the interaction between HaATPs-α and Cry1Ac was verified by ligand blotting and homologous competition experiments. The in vitro gain and loss of function analyses showed HaATPs-α involved in Cry1Ac toxicity by expressing endogenous HaATPs-α and HaATPs-α double-stranded RNAs in Sf9 and midgut cells, respectively. Importantly, purified HaATPs-α synergized Cry1Ac toxicity to H. armigera larvae. These findings provide the first evidence that HaATPs-α is a potential receptor of Cry1Ac, it shows downregulated participation in Cry1Ac resistance, and it exhibits higher enhancement of Cry1Ac toxicity to H. armigera larvae.

Insect chaperones Hsp70 and Hsp90 cooperatively enhance toxicity of Bacillus thuringiensis Cry1A toxins and counteract insect resistance

Bacillus thuringiensis (Bt) produces different insecticidal proteins effective for pest control. Among them, Cry insecticidal proteins have been used in transgenic plants for the control of insect pests. However, evolution of resistance by insects endangers this technology. Previous work showed that the lepidopteran insect Plutella xylostella PxHsp90 chaperone enhanced the toxicity of Bt Cry1A protoxins by protecting them from degradation by the larval gut proteases and by enhancing binding of the protoxin to its receptors present in larval midgut cells. In this work, we show that PxHsp70 chaperone also protects Cry1Ab protoxin from gut proteases degradation, enhancing Cry1Ab toxicity. We also show that both PxHsp70 and PxHsp90 chaperones act cooperatively, increasing toxicity and the binding of Cry1Ab439D mutant, affected in binding to midgut receptors, to cadherin receptor. Also, insect chaperones recovered toxicity of Cry1Ac protein to a Cry1Ac-highly resistant P. xylostella population, NO-QAGE, that has a disruptive mutation in an ABCC2 transporter linked to Cry1Ac resistance. These data show that Bt hijacked an important cellular function for enhancing its infection capability, making use of insect cellular chaperones for enhancing Cry toxicity and for lowering the evolution of insect resistance to these toxins.

The Plant V-ATPase

V-ATPase is the dominant proton pump in plant cells. It contributes to cytosolic pH homeostasis and energizes transport processes across endomembranes of the secretory pathway. Its localization in the trans Golgi network/early endosomes is essential for vesicle transport, for instance for the delivery of cell wall components. Furthermore, it is crucial for response to abiotic and biotic stresses. The V-ATPase's rather complex structure and multiple subunit isoforms enable high structural flexibility with respect to requirements for different organs, developmental stages, and organelles. This complexity further demands a sophisticated assembly machinery and transport routes in cells, a process that is still not fully understood. Regulation of V-ATPase is a target of phosphorylation and redox-modifications but also involves interactions with regulatory proteins like 14-3-3 proteins and the lipid environment. Regulation by reversible assembly, as reported for yeast and the mammalian enzyme, has not be proven in plants but seems to be absent in autotrophic cells. Addressing the regulation of V-ATPase is a promising approach to adjust its activity for improved stress resistance or higher crop yield.

Does Bacillus thuringiensis Affect the Stress and Immune Responses of Rhynchophorus ferrugineus Larvae, Females, and Males in the Same Way?

Simple Summary

Rhynchophorus ferrugineus is a destructive quarantine pest of palm trees, now widely distributed. Although broad-spectrum insecticides are often used to protect palm against R. ferrugineus, there is increasing concern about their effects on the environment and human health, especially where palm trees are located in urban areas. As an environmentally friendly entomopathogen, Bacillus thuringiensis (Bt) has been widely used to prevent other pest infestations. Although Bt products are the most sold bio-insecticides, there are still many interesting features to be investigated in the relationship of Bt and its hosts. We investigated the effect of Bt on larvae, females, and males. This research yielded experimental evidence of significant mortality and significant effects on immune system and stress answer. Within a few hours, stress due to Bt infection was detected in the hemocytes and in the brain providing better insights into the insect-pathogen interaction and highlighting the potential use of Bt in R. ferrugineus management.

Abstract

Bacillus thuringiensis (Bt) is considered a potentially useful entomopathogen against red palm weevil (RPW) Rhynchophorus ferrugineus. We compared the effects of Bt on mature larvae, females, and males. The pathogenicity of Bt was evaluated, estimating: Median Lethal Dose (LD₅₀), Median Lethal Time (LT₅₀), Total Hemocyte Count (THC), and Differential Hemocyte Counts (DHC), and the expression of the stress protein Heat Shock Protein 70 (Hsp 70) in hemocytes and the brain. Mortality exhibited a positive trend with the dosage and duration of exposure to Bt. Larvae were more susceptible than adults, and the LD₅₀ of females was almost double the value of that of the larvae. LT₅₀ value was higher for females than for males and larvae. Treatment with sub-lethal doses of Bt induced a decrease in THC in larvae, females, and males. In treated larvae, plasmatocytes decreased, while oenocytes and spherulocytes increased. In treated females, all types of hemocytes decreased, while in males the number of plasmatocytes decreased and granulocytes increased. We also registered the stress response directly on hemocytes showing that, already at 3 h after eating Bt, the expression of the stress protein Hsp 70 was modulated. This effect was also observed in brain tissue at 6 h after treatment. The results confirm that Bt treatment induces a pathogenic state in larvae and adults of both sexes, with effects after only a few hours from ingestion; however, the effects are different in magnitude and in type of target.

Genome-wide analysis of V-ATPase genes in Plutella xylostella (L.) and the potential role of PxVHA-G1 in resistance to Bacillus thuringiensis Cry1Ac toxin

The rapid development of insecticide resistance has hampered the use of Bacillus thuringiensis (Bt), a widely used bio-pesticide. Plutella xylostella (L.) is a globally distributed lepidopteran pest of cruciferous vegetables and has developed severe field resistance to the Bt toxin. Vacuolar H⁺-ATPases (VHA) are multi-subunit complexes and participate in multiple physiological processes. However, the characterization and functional studies of VHA genes are lacking in insects. This study performed a genome-wide analysis and identified 35 VHA gene family members divided into 15 subfamilies in P. xylostella. We cloned a V-ATPase subunit G gene, PxVHA-G1, in our previous midgut transcriptome profiles. Quantitative reverse transcriptase-polymerase chain reaction results showed that PxVHA-G1 was upregulated in the Cry1S1000-resistant strain than in the G88-susceptible strain, and its expression profile revealed that the midgut, Malpighian tubules, and larva stages generally showed high expression levels. RNAi-mediated knockdown of the PxVHA-G1 gene increased the susceptibility of P. xylostella (G88 and Cry1S1000) to Cry1Ac toxin. Our study is the first to explore the role of PxVHA-G1 on regulating Cry1Ac toxicity in P. xylostella, thus, providing new insights into the role of VHAs in the development of Cry1Ac resistance and sustainable development of pest management.

ABCC2 is a functional receptor of Bacillus thuringiensis Cry1Ca in Spodoptera litura

Spodoptera litura is a serious polyphagous pest in the whole world, which has developed resistance to most conventional insecticides and even some Bacillus thuringiensis (Bt) toxins. Cry1Ca has excellent insecticide activity against S. litura with potential application to control S. litura and delay the development of insect resistance. However, the mode of action of Cry1Ca in S. litura is poorly understood. Here, Cry1Ca-binding proteins were identified from S. litura by using pull down assays and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The results indicated that aminopeptidase-N (APN), ATP binding cassette subfamily C member 2 (ABCC2), polycalin, actin and V-type proton ATPase subunit A may bind with Cry1Ca. Further study confirmed that ABCC2 fragment expressed in vitro can bind to Cry1Ca as demonstrated by Ligand blot and homologous competition experiments. The over-expression of endogenous SlABCC2 in Sf9 cells increased Cry1Ca cytotoxicity. Correspondingly, the vivo loss of function analyses by SlABCC2 small interfering RNAs (siRNAs) in S. litura larvae decreased the toxicity of Cry1Ca to larvae. Altogether, these results show that ABCC2 of S. litura is a functional receptor that is involved in the action mode of Cry1Ca.

Docking-based generation of antibodies mimicking Cry1A/1B protein binding sites as potential insecticidal agents against diamondback moth (Plutella xylostella)

BACKGROUND

Broad use of insecticidal Cry proteins from Bacillus thuringiensis in biopesticides and transgenic crops has resulted in cases of practical field resistance, highlighting the need for novel approaches to insect control. Previously we described an anti-Cry1Ab idiotypic-antibody (B12-scFv) displaying toxicity against rice leafroller (Cnaphalocrocis medinalis) larvae, supporting the potential of antibodies for pest control. The goal of the present study was to generate insecticidal antibodies against diamondback moth (Plutella xylostella) larvae.

RESULTS

Four genetically engineered antibodies (GEAbs) were designed in silico from B12-scFv using three-dimensional (3D) structure and docking predictions to alkaline phosphatase (ALP) as a Cry1Ac receptor in P. xylostella. Among these GEAbs, the GEAb-dV_L antibody consisting of two light chains had overlapping binding sites with Cry1A and Cry1B proteins and displayed high binding affinity to P. xylostella midgut brush border membrane (BBM) proteins. Proteins in BBM identified by pull-down assays as binding to GEAb-dV_L included an ABC transporter and V-ATPase subunit A protein. Despite lacking the α-helical structures in Cry1A that are responsible for pore formation, ingestion of GEAb-dV_L disrupted the P. xylostella larval midgut epithelium and resulted in toxicity. Apoptotic genes were activated in gut cells upon treatment with GEAb-dV_L .

CONCLUSION

This study describes the first insecticidal GEAb targeting P. xylostella by mimicking Cry proteins. Data support that GEAb-dV_L toxicity is associated to activation of intracellular cell death pathways, in contrast to pore-formation associated toxicity of Cry proteins. This work provides a foundation for the design of novel insecticidal antibodies for insect control. © 2021 Society of Chemical Industry.

Transcriptome reveal the response to Cry1Ac toxin in susceptible Bombyx mori

Bombyx mori as a representative in Lepidoptera is an important economic insect in agriculture production. Bacillus thuringiensis (Bt) is a bacterial pathogen in silkworm production. Understanding how silkworm respond to Bt-toxin can provide guidance to cultivate resistant silkworm strains. Cry1Ac is one type of Bt-toxin. In current research, Dazao, a susceptible B. mori strain to Bt-toxin, was treated by Cry1Ac toxin and compared its transcriptome with untreated samples. This analysis detected 1234 differentially expressed genes (DEGs). Gene Ontology, KEGG, and UniProt keyword enrichment analysis showed that DEGs include ATP-binding cassette (ABC) transporter, stress response, cuticle, and protein synthesis, and folding process. Five ABC genes were upregulated after Cry1Ac treatment including ABCA2, ABCA3, and ABCC4. They are also known as the transporters of Bt-toxin in lepidopteran insect. Expression of cuticle proteins was significantly increased at 6 h after Cry1Ac treatment. Sex-specific storage-proteins and heat shock protein were also upregulated in Cry1Ac treated samples. Our data provide an expression profile about the response of Cry1Ac toxin in susceptible B. mori strain.

Suppression of Calcineurin Enhances the Toxicity of Cry1Ac to Helicoverpa armigera

Insect resistance to Bacillus thuringiensis (Bt) insecticidal proteins has rapidly evolved with the expansion of the planting area of transgenic Bt crops. Pyramiding RNA interference (RNAi) and Bt in crops is urgently needed to counter the rapid increase in pest resistance. The ideal “pyramid” strategy simultaneously targets different action pathways that exert synergetic effects on each other. Here, we identified a dephosphatase, namely, Helicoverpa armigera calcineurin (HaCAN), which might enhance the insecticidal activity of Cry1Ac against Helicoverpa armigera by regulating immune gene expression via dephosphatase activity, but not by acting as a receptor. Notably, blocking enzyme activity or knocking down endogenous HaCAN significantly promoted the enhancement in Cry1Ac toxicity to insect larvae and cells. Correspondingly, the increase in HaCAN activity reduced the cytotoxicity of Cry1Ac as shown by the heterologous expression of HaCAN. Our results provide a probable that HaCAN is an important candidate gene for pyramiding RNAi and Cry1Ac crops to control cotton bollworm.

In vivo nanoscale analysis of the dynamic synergistic interaction of Bacillus thuringiensis Cry11Aa and Cyt1Aa toxins in Aedes aegypti

The insecticidal Cry11Aa and Cyt1Aa proteins are produced by Bacillus thuringiensis as crystal inclusions. They work synergistically inducing high toxicity against mosquito larvae. It was proposed that these crystal inclusions are rapidly solubilized and activated in the gut lumen, followed by pore formation in midgut cells killing the larvae. In addition, Cyt1Aa functions as a Cry11Aa binding receptor, inducing Cry11Aa oligomerization and membrane insertion. Here, we used fluorescent labeled crystals, protoxins or activated toxins for in vivo localization at nano-scale resolution. We show that after larvae were fed solubilized proteins, these proteins were not accumulated inside the gut and larvae were not killed. In contrast, if larvae were fed soluble non-toxic mutant proteins, these proteins were found inside the gut bound to gut-microvilli. Only feeding with crystal inclusions resulted in high larval mortality, suggesting that they have a role for an optimal intoxication process. At the macroscopic level, Cry11Aa completely degraded the gastric caeca structure and, in the presence of Cyt1Aa, this effect was observed at lower toxin-concentrations and at shorter periods. The labeled Cry11Aa crystal protein, after midgut processing, binds to the gastric caeca and posterior midgut regions, and also to anterior and medium regions where it is internalized in ordered "net like" structures, leading finally to cell break down. During synergism both Cry11Aa and Cyt1Aa toxins showed a dynamic layered array at the surface of apical microvilli, where Cry11Aa is localized in the lower layer closer to the cell cytoplasm, and Cyt1Aa is layered over Cry11Aa. This array depends on the pore formation activity of Cry11Aa, since the non-toxic mutant Cry11Aa-E97A, which is unable to oligomerize, inverted this array. Internalization of Cry11Aa was also observed during synergism. These data indicate that the mechanism of action of Cry11Aa is more complex than previously anticipated, and may involve additional steps besides pore-formation activity.

Functional validation of DvABCB1 as a receptor of Cry3 toxins in western corn rootworm, Diabrotica virgifera virgifera

Western corn rootworm (WCR), Diabrotica virgifera virgifera (Coleoptera: Chrysomelidae), is a serious insect pest in the major corn growing areas of North America and in parts of Europe. WCR populations with resistance to Bacillus thuringiensis (Bt) toxins utilized in commercial transgenic traits have been reported, raising concerns over their continued efficacy in WCR management. Understanding the modes of action of Bt toxins is important for WCR control and resistance management. Although different classes of proteins have been identified as Bt receptors for lepidopteran insects, identification of receptors in WCR has been limited with no reports of functional validation. Our results demonstrate that heterologous expression of DvABCB1 in Sf9 and HEK293 cells conferred sensitivity to the cytotoxic effects of Cry3A toxins. The result was further validated using knockdown of DvABCB1 by RNAi which rendered WCR larvae insensitive to a Cry3A toxin. However, silencing of DvABCB2 which is highly homologous to DvABCB1 at the amino acid level, did not reduce the sensitivity of WCR larvae to a Cry3A toxin. Furthermore, our functional studies corroborate different mode-of-actions for other insecticidal proteins including Cry34Ab1/35Ab1, Cry6Aa1, and IPD072Aa against WCR. Finally, reduced expression and alternatively spliced transcripts of DvABCB1 were identified in a mCry3A-resistant strain of WCR. Our results provide the first clear demonstration of a functional receptor in the molecular mechanism of Cry3A toxicity in WCR and confirmed its role in the mechanism of resistance in a mCry3A resistant strain of WCR.

Acting target of toosendanin locates in the midgut epithelium cells of Mythimna separate Walker larvae (lepidoptera: Noctuidae)

Toosendanin (TSN), which is extracted from the root bark of Melia toosendan Siebold and Zuccarini, has multiple modes of action against insects. Especially, this compound has a potent stomach poisoning activity against several lepidoptera pests. In this paper, the signs of toxicity, digestive enzymes activity, the histopathological changes and immuno-electron microscopic localization of TSN in the midgut epithelium of Mythimna separate Walker larvae were investigated for better understanding its action mechanism against insects. The bioassay results indicated that TSN has strong stomach poisoning against the fifth-instar larvae of M. separata (LC₅₀ = 252.23 μg/mL). The typical poisoned symptom were regurgitation and paralysis. Activities of digestive enzymes had no obvious changes after treatment with LC₈₀ dose of TSN. The midgut epithelial cells of insect were damaged by TSN, showing the degeneration of microvilli, hyperplasia of smooth endoplasmic reticulum and condensation of chromatin. Immunohistochemical analysis revealed that the gold particles existed on the microvilli of columnar cells and goblet cells, and gradually accumulated with the exacerbation of poisoning symptoms, showing that TSN targets on the microvilli of the midgutcells. Therefore, TSN acts on digestive system and locates in the microvilli of midgutcells of M. separata.

Insect Hsp90 Chaperone Assists Bacillus thuringiensis Cry Toxicity by Enhancing Protoxin Binding to the Receptor and by Protecting Protoxin from Gut Protease Degradation

Bacillus thuringiensis took advantage of important insect cellular proteins, such as chaperones, involved in maintaining protein homeostasis, to enhance its insecticidal activity. This constitutes a positive loop where the concentrations of Hsp90 and Hsp70 in the gut lumen are likely to increase as midgut cells burst due to Cry1A pore formation action. Hsp90 protects Cry1A protoxin from degradation and enhances receptor binding, resulting in increased toxicity. The effect of insect chaperones on Cry toxicity could have important biotechnological applications to enhance the toxicity of Cry proteins to insect pests, especially those that show low susceptibility to these toxins.

Bacillus thuringiensis Cry proteins are pore-forming insecticidal toxins with specificity against different crop pests and insect vectors of human diseases. Previous work suggested that the insect host Hsp90 chaperone could be involved in Cry toxin action. Here, we show that the interaction of Cry toxins with insect Hsp90 constitutes a positive loop to enhance the performance of these toxins. Plutella xylostella Hsp90 (PxHsp90) greatly enhanced Cry1Ab or Cry1Ac toxicity when fed together to P. xylostella larvae and also in the less susceptible Spodoptera frugiperda larvae. PxHsp90 bound Cry1Ab and Cry1Ac protoxins in an ATP- and chaperone activity-dependent interaction. The chaperone Hsp90 participates in the correct folding of proteins and may suppress mutations of some client proteins, and we show here that PxHsp90 recovered the toxicity of the Cry1AbG439D protoxin affected in receptor binding, in contrast to the Cry1AbR99E or Cry1AbE129K mutant, affected in oligomerization or membrane insertion, respectively, which showed a slight toxicity improvement. Specifically, PxHsp90 enhanced the binding of Cry1AbG439D protoxin to the cadherin receptor. Furthermore, PxHsp90 protected Cry1A protoxins from degradation by insect midgut proteases. Our data show that PxHsp90 assists Cry1A proteins by enhancing their binding to the receptor and by protecting Cry protoxin from gut protease degradation. Finally, we show that the insect cochaperone protein PxHsp70 also increases the toxicity of Cry1Ac in P. xylostella larvae, in contrast to a bacterial GroEL chaperone, which had a marginal effect, indicating that the use of insect chaperones along with Cry toxins could have important biotechnological applications for the improvement of Cry insecticidal activity, resulting in effective control of insect pests.

The midgut V-ATPase subunit A gene is associated with toxicity to crystal 2Aa and crystal 1Ca-expressing transgenic rice in Chilo suppressalis

Insecticidal crystal (Cry) proteins produced by the bacterium Bacillus thuringiensis (Bt) are toxic to a diverse range of insects. Transgenic rice expressing Cry1A, Cry2A and Cry1C toxins have been developed that are lethal to Chilo suppressalis, a devastating insect pest of rice in China. Identifying the mechanisms underlying the interactions of Cry toxins with susceptible hosts will improve both our understanding of Cry protein toxicology and long-term efficacy of Bt crops. In this study, we tested the hypothesis that V-ATPase subunit A contributes to the action of Cry1Ab/1Ac, Cry2Aa and Cry1Ca toxins in C. suppressalis. The full-length V-ATPase subunit A transcript was initially cloned from the C. suppressalis larval midgut and then used to generate double-stranded RNA (dsRNA)-producing bacteria. Toxicity assays using transgenic rice lines TT51 (Cry1Ab and Cry1Ac fusion genes), T2A-1 (Cry2Aa), and T1C-19 (Cry1Ca) in conjunction with V-ATPase subunit A dsRNA-treated C. suppressalis larvae revealed significantly reduced larval susceptibility to T2A-1 and T1C-19 transgenic rice, but not to TT51 rice. These results suggest that the V-ATPase subunit A plays a crucial role in mediating Cry2Aa and Cry1Ca toxicity in C. suppressalis. These findings will have significant implications on the development of future resistance management tools.

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.

Evaluation of Cry1Ac and Cry2Aa Toxin Binding to Two Important Beneficial Cotton Field Insects, Harmonia axyridis and Orius similis

Transgenic crops expressing Cry toxins are effective and considered environmentally friendly alternatives to synthetic pesticides, but assessment of environmental risks of their application on nontarget organisms is ongoing. The main risk is the transfer of Cry toxins to natural enemies through the food chain. There is reported evidence supporting that Cry toxins can be detected in the body and gut of some natural enemy insects. Considering that binding of Cry toxins to insect proteins is an essential step in the intoxication process, this work was conducted to evaluate interactions between Cry1Ac and Cry2Aa toxins with proteins from larvae/nymphs and adults of two important predatory natural enemies in cotton fields, Harmonia axyridis and Orius similis. Results support the absence of Cry1Ac or Cry2Aa binding proteins in immature stages of H. axyridis and O. similis, as well as in imaginal stage of H. axyridis. One same binding band about 70 kDa was found in imaginal total protein of O. similis when probed with the two Cry proteins, with the best match to Hsc70 of O. sauteri in the Uniprot database. However, nonspecific binding was verified by following competitive binding assays between the two Cry proteins and imaginal total protein of O. similis. From these results, we may infer that Cry1Ac and Cry2Aa have no likely detrimental effects on H. axyridis and O. similis.

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.

Identification of Bacillus thuringiensis Cry1AbMod binding-proteins from Spodoptera frugiperda

Bacillus thuringiensis Cry toxins are currently used for pest control in transgenic crops but evolution of resistance by the insect pests threatens the use of this technology. The Cry1AbMod toxin was engineered to lack the alpha helix-1 of the parental Cry1Ab toxin and was shown to counter resistance to Cry1Ab and Cry1Ac toxins in different insect species including the fall armyworm Spodoptera frugiperda. In addition, Cry1AbMod showed enhanced toxicity to Cry1Ab-susceptible S. frugiperda populations. To gain insights into the mechanisms of this Cry1AbMod-enhanced toxicity, we isolated the Cry1AbMod toxin binding proteins from S. frugiperda brush border membrane vesicles (BBMV), which were identified by pull-down assay and liquid chromatography-tandem mass spectrometry (LC-MS/MS). The LC-MS/MS results indicated that Cry1AbMod toxin could bind to four classes of aminopeptidase (N1, N3, N4 y N5) and actin, with the highest amino acid sequence coverage acquired for APN 1 and APN4. In addition to these proteins, we found other proteins not previously described as Cry toxin binding proteins. This is the first report that suggests the interaction between Cry1AbMod and APN in S. frugiperda.

Identification of Cry48Aa/Cry49Aa toxin ligands in the midgut of Culex quinquefasciatus larvae

A binary mosquitocidal toxin composed of a three-domain Cry-like toxin (Cry48Aa) and a binary-like toxin (Cry49Aa) was identified in Lysinibacillus sphaericus. Cry48Aa/Cry49Aa has action on Culex quinquefasciatus larvae, in particular, to those that are resistant to the Bin Binary toxin, which is the major insecticidal factor from L. sphaericus-based biolarvicides, indicating that Cry48Aa/Cry49Aa interacts with distinct target sites in the midgut and can overcome Bin toxin resistance. This study aimed to identify Cry48Aa/Cry49Aa ligands in C. quinquefasciatus midgut through binding assays and mass spectrometry. Several proteins, mostly from 50 to 120 kDa, bound to the Cry48Aa/Cry49Aa toxin were revealed by toxin overlay and pull-down assays. These proteins were identified against the C. quinquefasciatus genome and after analysis a set of 49 proteins were selected which includes midgut bound proteins such as aminopeptidases, amylases, alkaline phosphatases in addition to molecules from other classes that can be potentially involved in this toxin's mode of action. Among these, some proteins are orthologs of Cry receptors previously identified in mosquito larvae, as candidate receptors for Cry48Aa/Cry49Aa toxin. Further investigation is needed to evaluate the specificity of their interactions and their possible role as receptors.

Proteomic analysis of Cry2Aa-binding proteins and their receptor function in Spodoptera exigua

The bacterium Bacillus thuringiensis produces Crystal (Cry) proteins that are toxic to a diverse range of insects. Transgenic crops that produce Bt Cry proteins are grown worldwide because of their improved resistance to insect pests. Although Bt "pyramid" cotton that produces both Cry1A and Cry2A is predicted to be more resistant to several lepidopteran pests, including Spodoptera exigua, than plants that produce Cry1Ac alone, the mechanisms responsible for the toxicity of Cry2Aa in S. exigua are not well understood. We identified several proteins that bind Cry2Aa (polycalin, V-ATPase subunits A and B, actin, 4-hydroxybutyrate CoA-transferase [4-HB-CoAT]), and a receptor for activated protein kinase C (Rack), in S. exigua. Recombinant, expressed versions of these proteins were able to bind the Cry2Aa toxin in vitro assays. RNA interference gene knockdown of the Se-V-ATPase subunit B significantly decreased the susceptibility of S. exigua larvae to Cry2Aa, whereas knockdown of the other putative binding proteins did not. Moreover, an in vitro homologous competition assay demonstrated that the Se-V-ATPase subunit B binds specifically to the Cry2Aa toxin, suggesting that this protein acts as a functional receptor of Cry2Aa in S. exigua. This the first Cry2Aa toxin receptor identified in S. exigua brush-border membrane vesicles.

Proteomics-based identification of midgut proteins correlated with Cry1Ac resistance in Plutella xylostella (L.)

The diamondback moth, Plutella xylostella (L.), is a worldwide pest of cruciferous crops and can rapidly develop resistance to many chemical insecticides. Although insecticidal crystal proteins (i.e., Cry and Cyt toxins) derived from Bacillus thuringiensis (Bt) have been useful alternatives to chemical insecticides for the control of P. xylostella, resistance to Bt in field populations of P. xylostella has already been reported. A better understanding of the resistance mechanisms to Bt should be valuable in delaying resistance development. In this study, the mechanisms underlying P. xylostella resistance to Bt Cry1Ac toxin were investigated using two-dimensional differential in-gel electrophoresis (2D-DIGE) and ligand blotting for the first time. Comparative analyses of the constitutive expression of midgut proteins in Cry1Ac-susceptible and -resistant P. xylostella larvae revealed 31 differentially expressed proteins, 21 of which were identified by mass spectrometry. Of these identified proteins, the following fell into diverse eukaryotic orthologous group (KOG) subcategories may be involved in Cry1Ac resistance in P. xylostella: ATP-binding cassette (ABC) transporter subfamily G member 4 (ABCG4), trypsin, heat shock protein 70 (HSP70), vacuolar H(+)-ATPase, actin, glycosylphosphatidylinositol anchor attachment 1 protein (GAA1) and solute carrier family 30 member 1 (SLC30A1). Additionally, ligand blotting identified the following midgut proteins as Cry1Ac-binding proteins in Cry1Ac-susceptible P. xylostella larvae: ABC transporter subfamily C member 1 (ABCC1), solute carrier family 36 member 1 (SLC36A1), NADH dehydrogenase iron-sulfur protein 3 (NDUFS3), prohibitin and Rap1 GTPase-activating protein 1. Collectively, these proteomic results increase our understanding of the molecular resistance mechanisms to Bt Cry1Ac toxin in P. xylostella and also demonstrate that resistance to Bt Cry1Ac toxin is complex and multifaceted.

Identification of ABCC2 as a binding protein of Cry1Ac on brush border membrane vesicles from Helicoverpa armigera by an improved pull-down assay

Cry1Ac toxin‐binding proteins from Helicoverpa armigera brush border membrane vesicles were identified by an improved pull‐down method that involves coupling Cry1Ac to CNBr agarose combined with liquid chromatography–tandem mass spectrometry (LC‐MS/MS). According to the LC‐MS/MS results, Cry1Ac toxin could bind to six classes of aminopeptidase‐N, alkaline phosphatase, cadherin‐like protein, ATP‐binding cassette transporter subfamily C protein (ABCC2), actin, ATPase, polycalin, and some other proteins not previously characterized as Cry toxin‐binding molecules such as dipeptidyl peptidase or carboxyl/choline esterase and some serine proteases. This is the first report that suggests the direct binding of Cry1Ac toxin to ABCC2 in H. armigera.

Validation of the Target Protein of Insecticidal Dihydroagarofuran Sesquiterpene Polyesters

A series of insecticidal dihydroagarofuran sesquiterpene polyesters were isolated from the root bark of Chinese bittersweet (Celastrus angulatus Max). A previous study indicated that these compounds affect the digestive system of insects, and aminopeptidase N3 and V-ATPase have been identified as the most putative target proteins by affinity chromatography. In this study, the correlation between the affinity of the compounds to subunit H and the insecticidal activity or inhibitory effect on the activity of V-ATPase was analyzed to validate the target protein. Results indicated that the subunit H of V-ATPase was the target protein of the insecticidal compounds. In addition, the possible mechanism of action of the compounds was discussed. The results provide new ideas for developing pesticides acting on V-ATPase of insects.

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.

Separation of Binding Protein of Celangulin V from the Midgut of Mythimna separata Walker by Affinity Chromatography

Celangulin V, an insecticidal compound isolated from the root bark of Chinese bittersweet, can affect the digestive system of insects. However, the mechanism of how Celangulin V induces a series of symptoms is still unknown. In this study, affinity chromatography was conducted through coupling of Celangulin V-6-aminoacetic acid ester to the CNBr-activated Sepharose 4B. SDS-PAGE was used to analyze the collected fraction eluted by Celangulin V. Eight binding proteins (Zinc finger protein, Thioredoxin peroxidase (TPx), Glyceraldehyde 3-phosphate dehydrogenase (GAPDH), SUMO E3 ligase RanBP2, Transmembrane protein 1, Actin, APN and V-ATPase) were obtained and identified by LC/Q-TOF-MS from the midgut of Mythimna separata larvae. The potential of these proteins to serve as target proteins involved in the insecticidal activity of Celangulin V is discussed.

Prohibitin, an essential protein for Colorado potato beetle larval viability, is relevant to Bacillus thuringiensis Cry3Aa toxicity

Bacillus thuringienesis (Bt) Cry toxins constitute the most extensively used environmentally safe biopesticide and their mode of action relies on the interaction of the toxins with membrane proteins in the midgut of susceptible insects that mediate toxicity and insect specificity. Therefore, identification of Bt Cry toxin interacting proteins in the midgut of target insects and understanding their role in toxicity is of great interest to exploit their insecticidal action. Using ligand blot, we demonstrated that Bt Cry3Aa toxin bound to a 30kDa protein in Colorado potato beetle (CPB) larval midgut membrane, identified by sequence homology as prohibitin-1 protein. Prohibitins comprise a highly conserved family of proteins implicated in important cellular processes. We obtained the complete CPB prohibitin-1 DNA coding sequence of 828pb, in silico translated into a 276-amino acid protein. The analysis at the amino acid level showed that the protein contains a prohibitin-homology domain (Band7_prohibitin, cd03401) conserved among prohibitin proteins. A striking feature of the CPB identified prohibitin-1 is the predicted presence of cadherin elements, potential binding sites for Cry toxins described in other Bt susceptible insects. We also showed that CPB prohibitin-1 protein partitioned into both, detergent soluble and insoluble membrane fractions, as well as a prohibitin-2 homologous protein, previously reported to form functional complexes with prohibitin-1 in other organisms. Prohibitin complexes act as membrane scaffolds ensuring the recruitment of membrane proteases to facilitate substrate processing. Accordingly, sequestration of prohibitin-1 by an anti-prohibitin-1 antibody impaired the Cry3Aa toxin inhibition of the proteolytic cleavage of a fluorogenic synthetic substrate of an ADAM-like metalloprotease previously reported to proteolize this toxin. In this work, we also demonstrated that prohibitin-1 RNAi silencing in CPB larvae produced deleterious effects and together with a LD50 Cry3Aa toxin treatment resulted in a highly efficient short term response since 100% larval mortality was achieved just 5days after toxin challenge. Therefore, the combination of prohibitin RNAi and Cry toxin reveals as an effective strategy to improve crop protection.

Midgut aminopeptidase N isoforms from Ostrinia nubilalis: activity characterization and differential binding to Cry1Ab and Cry1Fa proteins from Bacillus thuringiensis

Aminopeptidase N (APN) isoforms from Lepidoptera are known for their involvement in the mode of action of insecticidal Cry proteins from Bacillus thuringiensis. These enzymes belong to a protein family with at least eight different members that are expressed simultaneously in the midgut of lepidopteran larvae. Here, we focus on the characterization of the APNs from Ostrinia nubilalis (OnAPNs) to identify potential Cry receptors. We expressed OnAPNs in insect cells using a baculovirus system and analyzed their enzymatic activity by probing substrate specificity and inhibitor susceptibility. The interaction with Cry1Ab and Cry1Fa proteins (both found in transgenic insect-resistant maize) was evaluated by ligand blot assays and immunocytochemistry. Ligand blots of brush border membrane proteins showed that both Cry proteins bound mainly to a 150 kDa-band, in which OnAPNs were greatly represented. Binding analysis of Cry proteins to the cell-expressed OnAPNs showed that OnAPN1 interacted with both Cry1Ab and Cry1Fa, whereas OnAPN3a and OnAPN8 only bound to Cry1Fa. Two isoforms, OnAPN2 and OnAPN3b, did not interact with any of these two proteins. This work provides the first evidence of a differential role of OnAPN isoforms in the mode of action of Cry proteins in O. nubilalis.

Sodium solute symporter and cadherin proteins act as Bacillus thuringiensis Cry3Ba toxin functional receptors in Tribolium castaneum

Understanding how Bacillus thuringiensis (Bt) toxins interact with proteins in the midgut of susceptible coleopteran insects is crucial to fully explain the molecular bases of Bt specificity and insecticidal action. In this work, aminopeptidase N (TcAPN-I), E-cadherin (TcCad1), and sodium solute symporter (TcSSS) have been identified by ligand blot as putative Cry3Ba toxin-binding proteins in Tribolium castaneum (Tc) larvae. RNA interference knockdown of TcCad1 or TcSSS proteins resulted in decreased susceptibility to Cry3Ba toxin, demonstrating the Cry toxin receptor functionality for these proteins. In contrast, TcAPN-I silencing had no effect on Cry3Ba larval toxicity, suggesting that this protein is not relevant in the Cry3Ba toxin mode of action in Tc. Remarkable features of TcSSS protein were the presence of cadherin repeats in its amino acid sequence and that a TcSSS peptide fragment containing a sequence homologous to a binding epitope found in Manduca sexta and Tenebrio molitor Bt cadherin functional receptors enhanced Cry3Ba toxicity. This is the first time that the involvement of a sodium solute symporter protein as a Bt functional receptor has been demonstrated. The role of this novel receptor in Bt toxicity against coleopteran insects together with the lack of receptor functionality of aminopeptidase N proteins might account for some of the differences in toxin specificity between Lepidoptera and Coleoptera insect orders.

Differential proteomic analysis of Trichoplusia ni cells after continuous selection with activated Cry1Ac toxin

Development of insect resistance to Bacillus thuringiensis (Bt) toxins threatens the sustained successful application of Bt-based biological control tactics. Multi-mechanisms of resistance have been proposed, such as alteration of toxin-binding proteins, changes of proteases in midgut and so on. The other responses of the Cry1Ac-selected insects might also contribute to the evolution of resistance. Here, the Cry1Ac-selected Trichoplusia ni TnH5 cells with high resistance were subjected to analysis of proteome and the differentially expressed proteins were identified using mass spectrometry. The differential proteins included transporter, molecular chaperon, structural molecules and many other molecules involved in protein metabolism, signal transduction, nucleotide binding, lipid biosynthesis, carbohydrates metabolism and energy production, suggesting that a complex mechanisms involved in the development of insect resistance to Bt Cry1Ac toxins at cellular levels. The decrease of protein synthesis, changes of signal transduction, more rapid energy production, the enhanced lipid synthesis and the decline of possible Cry1Ac-binding proteins in cytoplasm and other events might contribute to the development of resistance in the selected cells. Our results provide some new cues for understanding the mechanism of Bt resistance.

The molecular characterization of antioxidant enzyme genes in Helicoverpa armigera adults and their involvement in response to ultraviolet-A stress

Ultraviolet (UV) radiation can cause oxidative stress in insects. To gain insight into the roles of different antioxidant enzymes of Helicoverpa armigera adults in response to oxidative stress caused by UV-A at a molecular level, three antioxidant enzyme genes [encoding copper zinc superoxide dismutase (Cu/ZnSOD), catalase (CAT) and glutathione peroxidase (GPX)] were cloned. The deduced amino acid sequences of these genes are similar to that of other insects. In addition, the expression profiles of genes from a classic antioxidant defense system, comprising Cu/ZnSOD, manganese superoxide dismutase (MnSOD), CAT, thioredoxin peroxidase, GPX and glutathione-S-transferase, were also determined. The results indicated that all antioxidant enzyme genes (except MnSOD in females) were significantly upregulated at certain time points (from 30 to 90min) in both male and female adults of H. armigera following UV-A radiation. We also found that longer periods of radiation exposure did not yield higher levels of mRNA expression. Furthermore, changes of determined physiological parameters (glutathione, Cu/ZnSOD and MnSOD activity) were basically consistent with the gene expression profiles. We therefore infer that the different antioxidant enzymes of H. armigera act in a coordinated manner at the transcriptional level against oxidative stress caused by UV-A radiation.

Identification of a calmodulin-binding site within the domain I of Bacillus thuringiensis Cry3Aa toxin

Bacillus thuringiensis Cry3Aa toxin is a coleopteran specific toxin highly active against Colorado Potato Beetle (CPB).We have recently shown that Cry3Aa toxin is proteolytically cleaved by CPB midgut membrane associated metalloproteases and that this cleavage is inhibited by ADAM metalloprotease inhibitors. In the present study, we investigated whether the Cry3Aa toxin is a calmodulin (CaM) binding protein, as it is the case of several different ADAM shedding substrates. In pull-down assays using agarose beads conjugated with CaM, we demonstrated that Cry3Aa toxin specifically binds to CaM in a calcium-independent manner. Furthermore, we used gel shift assays and (1)H NMR spectra to demonstrate that CaM binds to a 16-amino acid synthetic peptide corresponding to residues N256-V271 within the domain I of Cry3Aa toxin. Finally, to investigate whether CaM has any effect on Cry3Aa toxin CPB midgut membrane associated proteolysis, cleavage assays were performed in the presence of the CaM-specific inhibitor trifluoperazine. We showed that trifluoperazine significantly increased Cry3Aa toxin proteolysis and also decreased Cry3Aa larval toxicity.

Larval midgut modifications associated with Bti resistance in the yellow fever mosquito using proteomic and transcriptomic approaches

BACKGROUND

Bacillus thuringiensis var. israelensis (Bti) is a natural larval mosquito pathogen producing pore-forming toxins targeting the midgut of Diptera larvae. It is used worldwide for mosquito control. Resistance mechanisms of an Aedes aegypti laboratory strain selected for 30 generations with field-collected leaf litter containing Bti toxins were investigated in larval midguts at two levels: 1. gene transcription using DNA microarray and RT-qPCR and 2. differential expression of brush border membrane proteins using DIGE (Differential In Gel Electrophoresis).

RESULTS

Several Bti Cry toxin receptors including alkaline phosphatases and N-aminopeptidases and toxin-binding V-ATPases exhibited altered expression levels in the resistant strain. The under-expression of putative Bti-receptors is consistent with Bt-resistance mechanisms previously described in Lepidoptera. Four soluble metalloproteinases were found under-transcribed together with a drastic decrease of metalloproteinases activity in the resistant strain, suggesting a role in resistance by decreasing the amount of activated Cry toxins in the larval midgut.

CONCLUSIONS

By combining transcriptomic and proteomic approaches, we detected expression changes at nearly each step of the ingestion-to-infection process, providing a short list of genes and proteins potentially involved in Bti-resistance whose implication needs to be validated. Collectively, these results open the way to further functional analyses to better characterize Bti-resistance mechanisms in mosquitoes.

Comparative proteomic analysis of Aedes aegypti larval midgut after intoxication with Cry11Aa toxin from Bacillus thuringiensis

Cry toxins produced by Bacillus thuringiensis bacteria are environmentally safe alternatives to control insect pests. They are pore-forming toxins that specifically affect cell permeability and cellular integrity of insect-midgut cells. In this work we analyzed the defensive response of Aedes aegypti larva to Cry11Aa toxin intoxication by proteomic and functional genomic analyses. Two dimensional differential in-gel electrophoresis (2D-DIGE) was utilized to analyze proteomic differences among A. aegypti larvae intoxicated with different doses of Cry11Aa toxin compared to a buffer treatment. Spots with significant differential expression (p<0.05) were then identified by liquid chromatography-tandem mass spectrometry (LC-MS/MS), revealing 18 up-regulated and seven down-regulated proteins. The most abundant subcategories of differentially expressed proteins were proteins involved in protein turnover and folding, energy production, and cytoskeleton maintenance. We selected three candidate proteins based on their differential expression as representatives of the different functional categories to perform gene silencing by RNA interference and analyze their functional role. The heat shock protein HSP90 was selected from the proteins involved in protein turnover and chaperones; actin, was selected as representative of the cytoskeleton protein group, and ATP synthase subunit beta was selected from the group of proteins involved in energy production. When we affected the expression of ATP synthase subunit beta and actin by silencing with RNAi the larvae became hypersensitive to toxin action. In addition, we found that mosquito larvae displayed a resistant phenotype when the heat shock protein was silenced. These results provide insight into the molecular components influencing the defense to Cry toxin intoxication and facilitate further studies on the roles of identified genes.

Transcription profiling of resistance to Bti toxins in the mosquito Aedes aegypti using next-generation sequencing

The control of mosquitoes transmitting infectious diseases relies mainly on the use of chemical insecticides. However, resistance to most chemical insecticides threatens mosquito control programs. In this context, the spraying of toxins produced by the bacteria Bacillus thuringiensis subsp. israelensis (Bti) in larval habitats represents an alternative to chemical insecticides and is now widely used for mosquito control. Recent studies suggest that resistance of mosquitoes to Bti toxin may occur locally but mechanisms have not been characterized so far. In the present study, we investigated gene transcription level variations associated with Bti toxin resistance in the mosquito Aedes aegypti using a next-generation sequencing approach. More than 6 million short cDNA tags were sequenced from larvae of two strains sharing the same genetic background: a Bti toxins-resistant strain and a susceptible strain. These cDNA tags were mapped with a high coverage (308 reads per position in average) to more than 6000 genes of Ae. aegypti genome and used to quantify and compare the transcription level of these genes between the two mosquito strains. Among them, 86 genes were significantly differentially transcribed more than 4-fold in the Bti toxins resistant strain comparatively to the susceptible strain. These included gene families previously associated with Bti toxins resistance such as serine proteases, alkaline phosphatase and alpha-amylase. These results are discussed in regards of potential Bti toxins resistance mechanisms in mosquitoes.

Histopathology and ultrastructure of midgut of Alabama argillacea (Hübner) (Lepidoptera: Noctuidae) fed Bt-cotton

The interaction of Cry toxins from Bacillus thuringiensis in the midgut of some insect larvae determines their efficacies as insecticides, due to the expression and availability of sites of action of the toxin in the midgut. Researches point out cases of resistance to Cry toxin due to alterations in the binding sites in columnar cell membrane. We analyzed the effects of Cry1Ac toxin expressed by Bt-cotton plants on Alabama argillacea midgut morphophysiology clarifying in levels of morphological and ultrastructural. Larvae in the 4th instar of A. argillacea after 20 min from ingesting Bt-cotton leaves expressing 0.183 ng of Cry1Ac exhibited ultrastructural and morphological modifications in the columnar cells with significant changes in the mitochondrial polymorphism, cytoplasmic vacuolization, microvillus and basal labyrinth. Expressive morphological alterations were also observed in the goblet cells indicating that the columnar cells are not the only target of the Cry1Ac toxin. The regenerative cells did not modify their structures and exhibited decrease in regeneration capacity. In conclusion, the ingestion of 0.183 ± 0.077 ng of Cry1Ac was enough to promote alterations in the columnar and goblet cells, besides reducing significantly the number of regenerative cells, which may have contributed to larval death. Nevertheless, further studies are necessary to determine the true cause of death.