Identification and characterization of Aedes aegypti aminopeptidase N as a putative receptor of Bacillus thuringiensis Cry11A toxin

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.

The larval midgut of Anopheles, Aedes, and Toxorhynchites mosquitoes (Diptera, Culicidae): a comparative approach in morphophysiology and evolution

The mosquito larval midgut is responsible for acquiring and storing most of the nutrients that will sustain the events of metamorphosis and the insect's adult life. Despite its importance, the basic biology of this larval organ is poorly understood. To help fill this gap, we carried out a comparative morphophysiological investigation of three larval midgut regions (gastric caeca, anterior midgut, and posterior midgut) of phylogenetically distant mosquitoes: Anopheles gambiae (Anopheles albimanus was occasionally used as an alternate), Aedes aegypti, and Toxorhynchites theobaldi. Larvae of Toxorhynchites mosquitoes are predacious, in contrast to the other two species, that are detritivorous. In this work, we show that the larval gut of the three species shares basic histological characteristics, but differ in other aspects. The lipid and carbohydrate metabolism of the An. gambiae larval midgut is different compared with that of Ae. aegypti and Tx. theobaldi. The gastric caecum is the most variable region, with differences probably related to the chemical composition of the diet. The peritrophic matrix is morphologically similar in the three species, and processes involved in the post-embryonic development of the organ, such as cell differentiation and proliferation, were also similar. FMRF-positive enteroendocrine cells are grouped in the posterior midgut of Tx. theobaldi, but individualized in An. gambiae and Ae. aegypti. We hypothesize that Tx. theobaldi larval predation is an ancestral condition in mosquito evolution.

Purification, characterization and proteolytic processing of mosquito larvicidal protein Cry11Aa from Bacillus thuringensis subsp. isralensis ISPC-12

Cry11Aa is the most potent mosquito larvicidal protein of Bacillus thuringiensis subsp. israelensis (Bti). Development of resistance against insecticidal proteins including Cry11Aa is known but no field resistance was observed with Bti. The phenomenon of increasing resistance in insect pests necessitates the development of new strategies and techniques to enhance efficacy of insecticidal proteins. Recombinant technology offers better control over the molecule and allows modification of protein to achieve maximal effect against target pests. In this study, we standardised protocol for recombinant purification of Cry11Aa. Recombinant Cry11Aa found active against larvae of Aedes and Culex mosquito species and LC₅₀ were estimated. Detailed biophysical characterization provides crucial insights into stability and in-vitro behaviour of the recombinant Cry11Aa. Moreover, trypsin hydrolysis doesn't improve overall toxicity of recombinant Cry11Aa. Proteolytic processing suggests domain I and II are more prone to proteolysis in comparison to domain III. Significance of structural features for proteolysis of Cry11Aa was observed after performing molecular dynamics simulations. Findings reported here are contributing significantly in method for purification, understanding in-vitro behaviour and proteolytic processing of Cry11Aa which could facilitate in efficient utilisation of Bti for insect pests and vectors control.

Function of CTLGA9 Amino Acid Residue Leucine-6 in Modulating Cry Toxicity

Aedes aegypti is a crucial vector for many arboviral diseases that cause millions of deaths worldwide and thus is of major public health concern. Crystal (Cry) proteins, which are toxins produced by Bacillus thuringiensis, are structurally organized into three-domains, of which domain II is the most variable in terms of binding towards various toxin receptors. The binding of Cry11Aa to putative receptor such as aminopeptidase-N (APN) is explicitly inhibited by midgut C-type lectins (CTLs). The similarity between the domain II fold of Cry11Aa toxin and the carbohydrate recognition domain in the CTLs is a possible structural basis for the involvement of Cry domain II in the recognition of carbohydrates on toxin receptors. In this study, a site-directed point mutation was introduced into the A. aegypti CTLGA9 gene on the basis of molecular docking findings, leading to substitution of the Leucine-6 (Leu-6) residue in the protein with alanine. Subsequently, functional monitoring of the mutated protein was carried out. Unlike the amino acid residues of wild-type CTLGA9, none of the residues of mutant (m) CTLGA9 were competed with Cry11Aa for binding to the APN receptor interface. Additionally, ligand blot analysis showed that both wild-type and mutant CTLGA9 had similar abilities to bind to APN and Cry11Aa. Furthermore, in the competitive ELISA in which labeled mutant CTLGA9 (10 nM) was mixed with increasing concentrations of unlabeled Cry11Aa (0–500 nM), the mutant showed no competition with Cry11Aa for binding to APN., By contrast, in the positive control sample of labeled wild type CTLGA9 mixed with same concentrations of Cry11Aa competition between the two ligands for binding to the APN was evident. These results suggest that Leucine-6 may be the key site involved in the competitive receptor binding between CTLGA9 and Cry11Aa. Moreover, according to the bioassay results, mutant CTLGA9 could in fact enhance the toxicity of Cry11Aa. Our novel findings provide further insights into the mechanism of Cry toxicity as well as a theoretical basis for enhancing the mosquitocidal activity of these toxin through molecular modification strategies.

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.

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.

Bacterial Toxins Active against Mosquitoes: Mode of Action and Resistance

Larvicides based on the bacteria Bacillus thuringiensis svar. israelensis (Bti) and Lysinibacillus sphaericus are effective and environmentally safe compounds for the control of dipteran insects of medical importance. They produce crystals that display specific and potent insecticidal activity against larvae. Bti crystals are composed of multiple protoxins: three from the three-domain Cry type family, which bind to different cell receptors in the midgut, and one cytolytic (Cyt1Aa) protoxin that can insert itself into the cell membrane and act as surrogate receptor of the Cry toxins. Together, those toxins display a complex mode of action that shows a low risk of resistance selection. L. sphaericus crystals contain one major binary toxin that display an outstanding persistence in field conditions, which is superior to Bti. However, the action of the Bin toxin based on its interaction with a single receptor is vulnerable for resistance selection in insects. In this review we present the most recent data on the mode of action and synergism of these toxins, resistance issues, and examples of their use worldwide. Data reported in recent years improved our understanding of the mechanism of action of these toxins, showed that their combined use can enhance their activity and counteract resistance, and reinforced their relevance for mosquito control programs in the future years.

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.

His180 in the pore-lining α4 of the Bacillus thuringiensis Cry4Aa δ-endotoxin is crucial for structural arrangements of the α4-α5 transmembrane hairpin and hence biotoxicity

One proposed toxic mechanism of Bacillus thuringiensis Cry δ-endotoxins involves pore formation in target membranes by the α4-α5 transmembrane hairpin constituting their pore-forming domain. Here, nine selected charged and uncharged polar residues in the pore-lining α4 of the Cry4Aa mosquito-active toxin were substituted with Ala. All mutant toxins, i.e., D169A, R171A, Q173A, H178A, Y179A, H180A, Q182A, N183A and E187A, were over-expressed in Escherichia coli as 130-kDa protoxin inclusions at levels comparable to the wild-type toxin. Bioassays against Aedes aegypti larvae revealed that only H178A and H180A mutants displayed a drastic reduction in biotoxicity, albeit almost complete insolubility observed for H178A, but not for H180A inclusions. Further mutagenic analysis showed that replacements of His¹⁸⁰ with charged (Arg, Lys, Asp, Glu), small uncharged polar (Ser, Cys) or small non-polar (Gly, Val) residues severely impaired the biotoxicity, unlike substitutions with relatively large uncharged (Asn, Gln, Leu) or aromatic (Phe, Tyr, Trp) residues. Similar to the trypsin-activated wild-type toxin, both bio-active and -inactive H180 mutants were still capable of releasing entrapped calcein from lipid vesicles and producing cation-selective channels with ~130-pS maximum conductance. Analysis of the Cry4Aa structure revealed the existence of a hydrophobic cavity near the critical His¹⁸⁰ side-chain. Analysis of simulated structures revealed that His¹⁸⁰-to-smaller residue conversions create a gap disrupting such cavity's hydrophobicity and hence structural arrangements of the α4-α5 hairpin. Altogether, our data disclose a critical involvement in Cry4Aa-biotoxicity of His¹⁸⁰ exclusively present in the lumen-facing α4 for providing proper environment for the α4-α5 hairpin prior to membrane-inserted pore formation.

Potential for Bacillus thuringiensis and Other Bacterial Toxins as Biological Control Agents to Combat Dipteran Pests of Medical and Agronomic Importance

The control of dipteran pests is highly relevant to humans due to their involvement in the transmission of serious diseases including malaria, dengue fever, Chikungunya, yellow fever, zika, and filariasis; as well as their agronomic impact on numerous crops. Many bacteria are able to produce proteins that are active against insect species. These bacteria include Bacillus thuringiensis, the most widely-studied pesticidal bacterium, which synthesizes proteins that accumulate in crystals with insecticidal properties and which has been widely used in the biological control of insects from different orders, including Lepidoptera, Coleoptera, and Diptera. In this review, we summarize all the bacterial proteins, from B. thuringiensis and other entomopathogenic bacteria, which have described insecticidal activity against dipteran pests, including species of medical and agronomic importance.

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

Bacillus thuringiensis Cry proteins have been widely used over the past decades for many different insect pests, which are safe for users and the environment. The coleopteran-specific Cry3Aa toxin from B. thuringiensis exhibits toxicity to the larvae of Monochamus alternatus. Receptors play a key role in the mechanisms underlying the toxic action of Cry. However, the binding receptor for Cry3Aa has yet to be identified in the midgut of M. alternatus larvae. Therefore, the aim of this study was to identify the receptor for Cry3Aa toxin in the brush border membrane vesicles (BBMVs) of M. alternatus larvae. Our results indicate that the Cry3Aa toxin binds to the BBMVs (Kd = 247 nM) of M. alternatus via a 107 kDa aminopeptidase N (APN) (Kd = 57 nM). In silico analysis of the APN protein predicted that an 18 amino acid sequence in the N-terminal acted as a signal peptide, and that the Asn residue, located at position 918 in the C-terminus is an anchored site for glycosyl phosphatidyl inositol. Further analysis showed that M. alternatus APN exhibits 75% homology to the APN from Anoplophora glabripenis. Our work, therefore, confirmed that APN, which is localized in the BBMVs in the midgut of M. alternatus larvae, acts as a binding protein for Cry3Aa toxins.

The great potential of entomopathogenic bacteria Xenorhabdus and Photorhabdus for mosquito control: a review

The control of insects of medical importance, such as Aedes aegypti and Aedes albopictus are still the only effective way to prevent the transmission of diseases, such as dengue, chikungunya and Zika. Their control is performed mainly using chemical products; however, they often have low specificity to non-target organisms, including humans. Also, studies have reported resistance to the most commonly used insecticides, such as the organophosphate and pyrethroids. Biological control is an ecological and sustainable method since it has a slow rate of insect resistance development. Bacterial species of the genera Xenorhabdus and Photorhabdus have been the target of several research groups worldwide, aiming at their use in agricultural, pharmaceutical and industrial products. This review highlights articles referring to the use of Xenorhabdus and Photorhabdus for insects and especially for mosquito control proposing future ways for their biotechnological applicability. Approximately 24 species of Xenorhabdus and five species of Photorhabdus have been described to have insecticidal properties. These studies have shown genes that are capable of encoding low molecular weight proteins, secondary toxin complexes and metabolites with insecticide activities, as well as antibiotic, fungicidal and antiparasitic molecules. In addition, several species of Xenorhabdus and Photorhabdus showed insecticidal properties against mosquitoes. Therefore, these biological agents can be used in new control methods, and must be, urgently considered in short term, in studies and applications, especially in mosquito control.

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.

Function of Aedes aegypti galectin-6 in modulation of Cry11Aa toxicity

Galectins are a family of β-galactoside binding proteins, and insect galectins play a role in immune responses and may also affect Cry toxin activity. In this study, we aimed to further understand the function and molecular mechanism of Aedes aegypti galectin-6 in modulation of Cry11Aa toxicity. A. aegypti galectin-6 was cloned, and the recombinant galectin-6 was expressed and purified. Bioassays indicated that galectin-6 could reduce the toxicity of Cry11Aa, protecting A. aegypti larvae. To determine interactions among galectin-6, Cry11Aa and putative toxin receptors, Octet Red System, western blotting, far-western blotting and ELISA assays were performed. Octet Red System showed that galectin-6 bound to BBMVs of A. aegypti larvae with lower affinity than that of Cry11Aa. Western blotting and far-western blotting analyses demonstrated that galectin-6 bound to A. aegypti ALP1 and APN2 as well as to BBMVs, consistent with the results of ELISA and protein docking simulations. However, galectin-6 did not bind to Cadherin in far-western blotting or ELISA assay, though the protein docking simulations suggested their binding potential. These findings support the conclusion that galectin-6 may block Cry11Aa from binding to ALP1 and APN2 due to structural similarity, which might decrease the mosquitocidal toxicity of Cry11Aa.

CTLGA9 Interacts with ALP1 and APN Receptors To Modulate Cry11Aa Toxicity in Aedes aegypti

The mosquito Aedes aegypti is associated with the spread of many viral diseases in humans, including Dengue virus (DENVs), Yellow fever virus (YFV), Zika virus (ZIKV), and Chikungunya virus (CHIKV). Bacillus thuringiensis (Bt) is widely used as a biopesticide, which produces Cry toxins for mosquito control. The Cry toxins bind mainly to important receptors, including alkaline phosphatase (ALP) and aminopeptidase-N (APN). This work investigated the function of a C-type lectin, CTLGA9, in A. aegypti in response to Cry toxins. Our results showed by far-western blot and ELISA methods that the CTLTGA9 protein interacted with brush border membrane vesicles (BBMVs) of A. aegypti larvae and with ALP1, APN, and Cry11Aa proteins. Furthermore, molecular docking showed overlapping binding sites in ALP1 and APN for binding to Cry11Aa and CTLGA9. The toxicity assays further demonstrated that CTLGA9 inhibited the larvicidal activity of Cry toxins. According to the results of molecular docking, CTLGA9 may compete with Cry11Aa for binding to ALP1 and APN receptors and thus decreases the mosquitocidal toxicity of Cry11Aa. Our results provide further insights into better understanding the mechanism of Cry toxins and help improve the Cry toxicity for mosquito control.

Cloning, expression and activity of ATP-binding protein in Bacillus thuringiensis toxicity modulation against Aedes aegypti

Background

Bacillus thuringiensis israelensis (Bti) is a widely used mosquitocidal microbial pesticide due to its high toxicity. ATP-binding proteins (ABP) are prevalently detected in insects and are related to reaction against Bti toxins. However, the function of ABP in mosquito biocontrol is little known, especially in Aedes aegypti. Therefore, this study aimed to clarify the function of ABP in Ae. aegypti against Bti toxin.

Results

Aedes aegypti ABP (GenBank: XM_001661856.2) was cloned, expressed and purified in this study. Far-western blotting and ELISA were also carried out to confirm the interaction between ABP and Cry11Aa. A bioassay of Cry11Aa was performed both in the presence and absence of ABP, which showed that the mortality of Ae. aegypti is increased with an increase in ABP.

Conclusions

Our results suggest that ABP in Ae. aegypti can modulate the toxicity of Cry11Aa toxin to mosquitoes by binding to Bti toxin. This could not only enrich the mechanism of Bt toxin, but also provide more data for the biocontrol of this transmission vector.

Function and Role of ATP-Binding Cassette Transporters as Receptors for 3D-Cry Toxins

When ABC transporter family C2 (ABCC2) and ABC transporter family B1 (ABCB1) were heterologously expressed in non-susceptible cultured cells, the cells swelled in response to Cry1A and Cry3 toxins, respectively. Consistent with the notion that 3D-Cry toxins form cation-permeable pores, Bombyx mori ABCC2 (BmABCC2) facilitated cation-permeable pore formation by Cry1A when expressed in Xenopus oocytes. Furthermore, BmABCC2 had a high binding affinity (KD) to Cry1Aa of 3.1 × 10-10 M. These findings suggest that ABC transporters, including ABCC2 and ABCB1, are functional receptors for 3D-Cry toxins. In addition, the Cry2 toxins most distant from Cry1A toxins on the phylogenetic tree used ABC transporter A2 as a receptor. These data suggest that 3D-Cry toxins use ABC transporters as receptors. In terms of inducing cell swelling, ABCC2 has greater activity than cadherin-like receptor. The pore opening of ABC transporters was hypothesized to be linked to their receptor function, but this was repudiated by experiments using mutants deficient in export activity. The synergistic relationship between ABCC2 and cadherin-like receptor explains their ability to cause resistance in one species of insect.

Aedes aegypti Galectin Competes with Cry11Aa for Binding to ALP1 To Modulate Cry Toxicity

The key step for the toxicity of Bacillus thuringiensis subsp. israelensis (Bti) is the interaction between toxins and putative receptors; thus, many studies focus on identification of new toxin receptors and engineering of toxins with higher affinity/specificity for receptors. In the larvae of Aedes aegypti, galectin-14 was one of the genes upregulated by Bti treatment. RNAi knockdown expression of galectin-14 and feeding recombinant galectin-14-thioredoxin fusion protein significantly affected survival of Ae. aegypti larvae treated with Bti toxins. Recombinant galectin-14 protein bound to brush border membrane vesicles (BBMVs) of Ae. aegypti larvae, ALP1 and APN2, and galectin-14 and Cry11Aa bound to BBMVs with a similarly high affinity. Competitive binding results showed that galectin-14 competed with Cry11Aa for binding to BBMVs and ALP1 to prevent effective binding of toxin to receptors. These novel findings demonstrated that midgut proteins other than receptors play an important role in modulating the toxicity of Cry toxins.

Inhibition of Sodium-Hydrogen Antiport by Antibodies to NHA1 in Brush Border Membrane Vesicles from Whole Aedes aegypti Larvae

The present research report describes Na⁺/H⁺ antiport by brush border membrane vesicles isolated from whole larvae of Aedes aegypti (AeBBMVw). Our hypothesis is that acid quenching of acridine orange by AeBBMVw is predominantly mediated by Na⁺/H⁺ antiport via the NHA1 component of the AeBBMVw in the absence of amino acids and ATP. AeNHA1 is a Na⁺/H⁺ antiporter that has been postulated to exchange Na⁺ and H⁺ across the apical plasma membrane in posterior midgut of A. aegypti larvae. Its principal function is to recycle the H⁺ and Na⁺ that are transported during amino acid uptake, e.g., phenylalanine. This uptake is mediated, in part, by a voltage-driven, Na⁺-coupled, nutrient amino acid transporter (AeNAT8). The voltage is generated by an H⁺ V-ATPase. All three components, V-ATPase, antiporter, and nutrient amino acid transporter (VAN), are present in brush border membrane vesicles isolated from whole larvae of A. aegypti. By omitting ATP and amino acids, Na⁺/H⁺ antiport was measured by fluorescence quenching of acridine orange (AO) caused by acidification of either the internal vesicle medium (Na⁺_in > Na⁺_out) or the external fluid-membrane interface (Na⁺_in < Na⁺_out). Vesicles with 100 micromolar Na⁺ inside and 10 micromolar Na⁺ outside or with 0.01 micromolar Na⁺ inside and 100 micromolar Na⁺ outside quenched fluorescence of AO by as much as 30%. Acidification did not occur in the absence of AeBBMVw. Preincubation of AeBBMVw with antibodies to NHA1 inhibit Na⁺/H⁺ antiport dependent fluorescence quenching, indicating that AeNHA1 has a significant role in Na⁺/H⁺ exchange.

C-Type Lectin-20 Interacts with ALP1 Receptor to Reduce Cry Toxicity in Aedes aegypti

Aedes aegypti is a crucial vector for human diseases, such as yellow fever, dengue, chikungunya, and Zika viruses. Today, a major challenge throughout the globe is the insufficient availability of antiviral drugs and vaccines against arboviruses, and toxins produced by Bacillus thuringiensis (Bt) are still used as biological agents for mosquito control. The use of Cry toxins to kill insects mainly depends on the interaction between Cry toxins and important toxin receptors, such as alkaline phosphatase (ALP). In this study, we investigated the function of A. aegypti C-type lectin-20 (CTL-20) in the tolerance of Cry toxins. We showed that recombinant CTL-20 protein interacted with both Cry11Aa and ALP1 by the Far-Western blot and ELISA methods, and CTL-20 bound to A. aegypti larval brush border membrane vesicles (BBMVs). Binding affinity of CTL-20 to ALP1 was higher than that of Cry11Aa to ALP1. Furthermore, the survival rate of A. aegypti larvae fed with Cry11Aa toxin mixed with recombinant CTL-20 fusion protein was significantly increased compared with that of the control larvae fed with Cry11Aa mixed with thioredoxin. Our novel results suggest that midgut proteins like CTLs may interfere with interactions between Cry toxins and toxin receptors by binding to both Cry toxins and receptors to alter Cry toxicity.

Cell lines as models for the study of Cry toxins from Bacillus thuringiensis

Cell lines have been use extensively for the study of the mode of action of different pore forming toxins produced by different bacterial species. Bacillus thuringiensis Cry toxins are not the exception and their mechanism of action has been analyzed in different cell lines. Here we review the data obtained with different cell lines, including those that are naturally susceptible to the three domain Cry toxins (3d-Cry) and other non-susceptible cell lines that have been transformed with 3d-Cry toxin binding molecules cloned from the susceptible insects. The effects on Cry toxin action after expressing different insect gut proteins, such as glycosyl-phosphatidyl-inositol (GPI) anchored proteins (like alkaline phosphatase (ALP) aminopeptidase (APN)), or trans-membrane proteins (like cadherin (CAD) or ATP-binding cassette subfamily C member 2 (ABCC2) transporter) in cell lines showed that, with few exceptions, expression of GPI-anchored proteins do not correlated with increased susceptibility to the toxin, while the expression of CAD or ABCC2 proteins correlated with induced susceptibility to Cry toxins in the transformed cells lines. Also, that the co-expression of CAD and ABCC2 transporter induced a synergistic effect in the toxicity of 3d-Cry toxins. Overall the data show that in susceptible cell lines, the 3d-Cry toxins induce pore formation that correlates with toxicity. However, the intracellular responses remain controversial since it was shown that the same 3d-Cry toxin in different cell lines activated different responses such as adenylate cyclase-PKA death response or apoptosis. Parasporins are Cry toxins that are toxic to cancer cell lines that have structural similarities with the insecticidal Cry toxins. They belong to the 3d-Cry toxin or to MTX-like Cry toxin families but also show important differences with the insecticidal Cry proteins. Some parasporins are pore-forming toxins, and some activate apoptosis. In this review we summarized the results of the different studies about the Cry toxins mode of action using cultured cell lines and discuss their relation with the studies performed in insect larvae.

Cry11Aa Interacts with the ATP-Binding Protein from Culex quinquefasciatus To Improve the Toxicity

Cry11Aa displays high toxicity to the larvae of several mosquito species, including Aedes, Culex, and Anopheles. To study its binding characterization against Culex quinquefasciatus, Cry11Aa was purified and western blot results showed that Cry11Aa could bind successfully to the brush border membrane vesicles. To identify Cry11Aa-binding proteins in C. quinquefasciatus, a biotin-based protein pull-down experiment was performed and seven Cry11Aa-binding proteins were isolated from the midgut of C. quinquefasciatus larvae. Analysis of liquid chromatography-tandem mass spectrometry showed that one of the Cry11Aa-binding proteins is the ATP-binding domain 1 family member B. To investigate its binding property and effect on the toxicity of Cry11Aa, western blot, far-western blot, enzyme-linked immunosorbent assay, and bioassays of Cry11Aa in the presence and absence of the recombinant ATP-binding protein were performed. Our results showed that the ATP-binding protein interacted with Cry11Aa and increased the toxicity of Cry11Aa against C. quinquefasciatus. Our study suggests that midgut proteins other than the toxin receptors may modulate the toxicity of Cry toxins against mosquitoes.

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.

Adsorption of Insecticidal Crystal Protein Cry11Aa onto Nano-Mg(OH)2: Effects on Bioactivity and Anti-Ultraviolet Ability

The traditional Bacillus thuringiensis (Bt) formulations for field applications are not resistant to harsh environmental conditions. Hence, the active ingredients of the Bt bioinsecticides could degrade quickly and has low anti-ultraviolet ability in the field, which significantly limits its practical application. In the present study, we developed an efficient and stable delivery system for Bt Cry11Aa toxins. We coated Cry11Aa proteins with Mg(OH)₂ nanoparticles (MHNPs), and then assessed the effects of MHNPs on bioactivity and anti-ultraviolet ability of the Cry11Aa proteins. Our results indicated that MHNPs, like "coating clothes", could effectively protect the Cry protein and enhance the insecticidal bioactivity after UV radiation (the degradation rate was decreased from 64.29% to 16.67%). In addtion, MHNPs could improve the proteolysis of Cry11Aa in the midgut and aggravate the damage of the Cry protein to the gut epithelial cells, leading to increased insecticidal activity against Culex quinquefasciatus. Our results revealed that MHNPs, as an excellent nanocarrier, could substantially improve the insecticidal bioactivity and anti-ultraviolet ability of Cry11Aa.

Effects and mechanisms of Bacillus thuringiensis crystal toxins for mosquito larvae

Bacillus thuringiensis is a Gram-positive aerobic bacterium that produces insecticidal crystalline inclusions during sporulation phases of the mother cell. The virulence factor, known as parasporal crystals, is composed of Cry and Cyt toxins. Most Cry toxins display a common 3-domain topology. Cry toxins exert intoxication through toxin activation, receptor binding and pore formation in a suitable larval gut environment. The mosquitocidal toxins of Bt subsp. israelensis (Bti) were found to be highly active against mosquito larvae and are widely used for vector control. Bt subsp. jegathesan is another strain which possesses high potency against broad range of mosquito larvae. The present review summarizes characterized receptors for Cry toxins in mosquito larvae, and will also discuss the diversity and effects of 3-D mosquitocidal Cry toxin and the ongoing research for Cry toxin mechanisms generated from investigations of lepidopteran and dipteran larvae.

Specificity and putative mode of action of a mosquito larvicidal toxin from the bacterium Xenorhabdus innexi

Reduction of mosquito-borne diseases relies, in part, on the use of synthetic pesticides to control pest mosquitoes. This reliance has led to genetic resistance, environmental contamination and the nondiscriminatory elimination of both pest and non-pest species. To expand our options for control, we screened entomopathogenic bacteria for potential larvicidal activity. A lipopeptide from the bacterium, Xenorhabdus innexi, was discovered that displayed potent larvicidal activity. The LC₅₀s of the lipopeptide towards Aedes aegypti, Culex pipiens and Anopheles gambiae larvae were 1.81, 1.25 and 1.86 parts-per-million, respectively. No mortality was observed in other insect species tested. The putative mode of action of the lipopeptide suggested that after orally ingestion, it bound to the apical membrane of anterior midgut cells and created pores in the cellular membranes. The rapid neutralization of midgut pH suggested the pores disabled the H⁺-V-ATPase on the basal membrane and led to epithelial cell death. Specificity and toxicity towards mosquito larvae and the unique mode of action makes this lipopeptide a potentially attractive bacterial insecticide for control of mosquitoes.

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 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.

Aedes cadherin mediates the in vivo toxicity of the Cry11Aa toxin to Aedes aegypti

Cadherin plays an important role in the toxicity of Bacillus thuringiensis Cry proteins. We previously cloned a full-length cadherin from Aedes aegypti larvae and reported this protein binds Cry11Aa toxin from B. thuringiensis subsp. israelensis with high affinity, ≈16.7nM. Based on these results, we investigated if Aedes cadherin is involved in the in vivo toxicity of Cry11Aa toxin to Ae. aegypti. We established a mosquito cell line stably expressing the full-length Aedes cadherin and transgenic mosquitoes with silenced Aedes cadherin expression. Cells expressing the Aedes cadherin showed increased sensitivity to Cry11Aa toxin. Cry11Aa toxin at 400nM killed approximately 37% of the cells in 3h. Otherwise, transgenic mosquitoes with silenced Aedes cadherin expression showed increased tolerance to Cry11Aa toxin. Furthermore, cells expressing Aedes cadherin triggered Cry11Aa oligomerization. These results show the Aedes cadherin plays a pivotal role in Cry11Aa toxicity to Ae. aegypti larvae by mediating Cry11Aa oligomerization. However, since high toxicity was not obtained in cadherin-expressing cells, an additional receptor may be needed for manifestation of full toxicity. Moreover, cells expressing Aedes cadherin were sensitive to Cry4Aa and Cry11Ba, but not Cry4Ba. However transgenic mosquitoes with silenced Aedes cadherin expression showed no tolerance to Cry4Aa, Cry4Ba, and Cry11Ba toxins. These results suggest that while Aedes cadherin may mediate Cry4Aa and Cry11Ba toxicity, this cadherin but is not the main receptor of Cry4Aa, Cry4Ba and Cry11Ba toxin in Ae. aegypti.

Chitosan/DsiRNA nanoparticle targeting identifies AgCad1 cadherin in Anopheles gambiae larvae as an in vivo receptor of Cry11Ba toxin of Bacillus thuringiensis subsp. jegathesan

The Cry11Ba protein of Bacillus thuringiensis subsp. jegathesan crystals has uniquely high toxicity against a spectrum of mosquito species. The high potency of Cry11Ba against Anopheles gambiae is caused by recognition of multiple midgut proteins including glycosyl phosphatidylinositol-anchored alkaline phosphatase AgALP1, aminopeptidase AgAPN2, α-amylase AgAmy1 and α-glucosidase Agm3 that bind Cry11Ba with high affinity and function as putative receptors. The cadherin AgCad2 in An. gambiae larvae also binds Cry11Ba with high affinity (Kd = 12 nM) and is considered a putative receptor, while cadherin AgCad1 bound Cry11Ba with low affinity (Kd = 766 nM), a property not supportive for a Cry11Ba receptor role. Here, we show the in vivo involvement of AgCad1 in Cry11Ba toxicity in An. gambiae larvae using chitosan/DsiRNA nanoparticles to inhibit AgCad expression in larvae. Cry11Ba was significantly less toxic to AgCad1-silenced larvae than to control larvae. Because AgCad1 was co-suppressed by AgCad2 DsRNAi, the involvement of AgCad2 in Cry11Ba toxicity could not be ascertained. The ratio of AgCad1:AgCad2 transcript level is 36:1 for gut tissue in 4th instar larvae. Silencing AgCad expression had no effect on transcript levels of other binding receptors of Cry11Ba. We conclude that AgCad1 and possibly AgCad2 in An. gambiae larvae are functional receptors of Cry11Ba toxin in vivo.

Two specific membrane-bound aminopeptidase N isoforms from Aedes aegypti larvae serve as functional receptors for the Bacillus thuringiensis Cry4Ba toxin implicating counterpart specificity

The interaction between Bacillus thuringiensis Cry toxins and their receptors on midgut cells of susceptible insect larvae is the critical determinant in toxin specificity. Besides GPI-linked alkaline phosphatase in Aedes aegypti mosquito-larval midguts, membrane-bound aminopeptidase N (AaeAPN) is widely thought to serve as a Cry4Ba receptor. Here, two full-length AaeAPN isoforms, AaeAPN2778 and AaeAPN2783, predicted to be GPI-linked were cloned and successfully expressed in Spodoptera frugiperda (Sf9) cells as 112- and 107-kDa membrane-bound proteins, respectively. In the cytotoxicity assay, Sf9 cells expressing each of the two AaeAPN isoforms showed increased sensitivity to the Cry4Ba mosquito-active toxin. Double immunolocalization revealed specific binding of Cry4Ba to each individual AaeAPN expressed on the cell membrane surface. Sequence analysis and homology-based modeling placed these two AaeAPNs to the M1 aminopeptidase family as they showed similar four-domain structures, with the most conserved domain II being the catalytic component. Additionally, the most variable domain IV containing negatively charged surface patches observed only in dipteran APNs could be involved in insect specificity. Overall results demonstrated that these two membrane-bound APN isoforms were responsible for mediating Cry4Ba toxicity against AaeAPN-expressed Sf9 cells, suggesting their important role as functional receptors for the toxin counterpart in A. aegypti mosquito larvae.

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Two novel Aedes GPI-APN isoforms were functionally expressed in Sf9 cells.

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Cells expressing each AaeAPN were more sensitive to Cry4Ba toxin cytolysis.

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Specific binding of Cry4Ba to individual AaeAPN-expressing Sf9 cells was demonstrated.

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These two AaeAPNs mediating Cry4Ba cytotoxicity serve as receptors in Aedes larvae.

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AaeAPN models reveal four-domain organization with implications for toxin-counterpart specificity.

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.

Gene expression patterns and sequence polymorphisms associated with mosquito resistance to Bacillus thuringiensis israelensis toxins

BACKGROUND

Despite the intensive use of Bacillus thuringiensis israelensis (Bti) toxins for mosquito control, little is known about the long term effect of exposure to this cocktail of toxins on target mosquito populations. In contrast to the many cases of resistance to Bacillus thuringiensis Cry toxins observed in other insects, there is no evidence so far for Bti resistance evolution in field mosquito populations. High fitness costs measured in a Bti selected mosquito laboratory strain suggest that evolving resistance to Bti is costly. The aim of the present study was to identify transcription level and polymorphism variations associated with resistance to Bti toxins in the dengue vector Aedes aegypti. We used RNA sequencing (RNA-seq) for comparing a laboratory-selected strain showing elevated resistance to Bti toxins and its parental non-selected susceptible strain. As the resistant strain displayed two marked larval development phenotypes (slow and normal), each phenotype was analyzed separately in order to evidence potential links between resistance mechanisms and mosquito life-history traits.

RESULTS

A total of 12,458 genes were detected of which 844 were differentially transcribed between the resistant and susceptible strains. Polymorphism analysis revealed a total of 68,541 SNPs of which 12,571 SNPs exhibited more than 40% frequency difference between the resistant and susceptible strains, affecting 2,953 genes. Bti resistance is associated with changes in the transcription level of enzymes involved in detoxification and chitin metabolism. Among previously described Bti-toxin receptors, four alkaline phosphatases (ALPs) were differentially transcribed between resistant and susceptible larvae, and non-synonymous changes affected the protein sequence of one cadherin, six aminopeptidases (APNs) and four α-amylases. Other putative Cry receptors located in lipid rafts, such as flotillin and glycoside hydrolases, were under-transcribed and/or contained non-synonymous substitutions. Finally, immunity-related genes showed contrasted transcription and polymorphisms patterns between the two developmental resistant phenotypes, suggesting the existence of trade-offs between Bti-resistance, life-history traits and immunity.

CONCLUSIONS

The present study is the first to analyze the whole transcriptome of Bti-resistant mosquitoes by RNA-seq, shedding light on the importance of studying both transcription levels and sequence polymorphism variations to get a comprehensive view of insecticide resistance.

Non conserved residues between Cqm1 and Aam1 mosquito α-glucosidases are critical for the capacity of Cqm1 to bind the Binary toxin from Lysinibacillus sphaericus

The Binary (Bin) toxin from the entomopathogenic bacterium Lysinibacillus sphaericus acts on larvae of the culicid Culex quinquefasciatus through its binding to Cqm1, a midgut-bound α-glucosidase. Specific binding by the BinB subunit to the Cqm1 receptor is essential for toxicity however the toxin is unable to bind to the Cqm1 ortholog from the refractory species Aedes aegypti (Aam1). Here, to investigate the molecular basis for the interaction between Cqm1 and BinB, recombinant Cqm1 and Aam1 were first expressed as soluble forms in Sf9 cells. The two proteins were found to display the same glycosilation patterns and BinB binding properties as the native α-glucosidases. Chimeric constructs were then generated through the exchange of reciprocal fragments between the corresponding cqm1 and aam1 cDNAs. Subsequent expression and binding experiments defined a Cqm1 segment encompassing residues S129 and A312 as critical for the interaction with BinB. Through site directed mutagenesis experiments, replacing specific sets of residues from Cqm1 with those of Aam1, the 159GG160 doublet was required for this interaction. Molecular modeling mapped these residues to an exposed loop within the Cqm1's structure, compatible with a target site for BinB and providing a possible explanation for its lack of binding to Aam1.

Pre-selecting resistance against individual Bti Cry toxins facilitates the development of resistance to the Bti toxins cocktail

The bioinsecticide Bacillus thuringiensis subsp. israelensis is a larvicide used worldwide for mosquito control, which contains three Cry toxins and one Cyt toxin. We investigated for the first time in Aedes aegypti (1) the evolution of resistance and cross-resistance of strains selected with each Cry toxin, and (2) the effect of pre-selection with Cry toxin on the evolution of resistance to a mix of Bti toxins. Cross resistance was higher between Cry4Ba and Cry11Aa than between Cry4Aa and either Cry4Ba or Cry11Aa, suggesting both common and specific mechanisms of resistance. Pre-selecting resistance to each Cry toxins facilitated the development of resistance to the full Bti toxins cocktail.

Bacillus thuringiensis subsp. israelensis and its dipteran-specific toxins

Bacillus thuringiensis subsp. israelensis (Bti) is the first Bacillus thuringiensis to be found and used as an effective biological control agent against larvae of many mosquito and black fly species around the world. Its larvicidal activity resides in four major (of 134, 128, 72 and 27 kDa) and at least two minor (of 78 and 29 kDa) polypeptides encoded respectively by cry4Aa, cry4Ba, cry11Aa, cyt1Aa, cry10Aa and cyt2Ba, all mapped on the 128 kb plasmid known as pBtoxis. These six δ-endotoxins form a complex parasporal crystalline body with remarkably high, specific and different toxicities to Aedes, Culex and Anopheles larvae. Cry toxins are composed of three domains (perforating domain I and receptor binding II and III) and create cation-selective channels, whereas Cyts are composed of one domain that acts as well as a detergent-like membrane perforator. Despite the low toxicities of Cyt1Aa and Cyt2Ba alone against exposed larvae, they are highly synergistic with the Cry toxins and hence their combinations prevent emergence of resistance in the targets. The lack of significant levels of resistance in field mosquito populations treated for decades with Bti-bioinsecticide suggests that this bacterium will be an effective biocontrol agent for years to come.

A 104 kDa Aedes aegypti aminopeptidase N is a putative receptor for the Cry11Aa toxin from Bacillus thuringiensis subsp. israelensis

The Cry11Aa protein produced in Bacillus thuringiensis subsp. israelensis, a bacterial strain used worldwide for the control of Aedes aegypti larvae, binds midgut brush border membrane vesicles (BBMV) with an apparent K(d) of 29.8 nM. Previously an aminopeptidase N (APN), named AaeAPN2, was identified as a putative Cry11Aa toxin binding protein by pull-down assays using biotinylated Cry11Aa toxin (Chen et al., 2009. Insect Biochem. Mol. Biol. 39, 688-696). Here we show this protein localizes to the apical membrane of epithelial cells in proximal and distal regions of larval caeca. The AaeAPN2 protein binds Cry11Aa with high affinity, 8.6 nM. The full-length and fragments of AaeAPN2 were cloned and expressed in Escherichia coli. The toxin-binding region was identified and further competitive assays demonstrated that Cry11Aa binding to BBMV was efficiently competed by the full-length AaeAPN2 and the fragments of AaeAPN2b and AaeAPN2e. In bioassays against Ae. aegypti larvae, the presence of full-length and a partial fragment (AaeAPN2b) of AaeAPN2 enhanced Cry11Aa larval mortality. Taken together, we conclude that AaeAPN2 is a binding protein and plays a role in Cry11Aa toxicity.

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.

Investigating the genetics of Bti resistance using mRNA tag sequencing: application on laboratory strains and natural populations of the dengue vector Aedes aegypti

Mosquito control is often the main method used to reduce mosquito-transmitted diseases. In order to investigate the genetic basis of resistance to the bio-insecticide Bacillus thuringiensis subsp. israelensis (Bti), we used information on polymorphism obtained from cDNA tag sequences from pooled larvae of laboratory Bti-resistant and susceptible Aedes aegypti mosquito strains to identify and analyse 1520 single nucleotide polymorphisms (SNPs). Of the 372 SNPs tested, 99.2% were validated using DNA Illumina GoldenGate® array, with a strong correlation between the allelic frequencies inferred from the pooled and individual data (r = 0.85). A total of 11 genomic regions and five candidate genes were detected using a genome scan approach. One of these candidate genes showed significant departures from neutrality in the resistant strain at sequence level. Six natural populations from Martinique Island were sequenced for the 372 tested SNPs with a high transferability (87%), and association mapping analyses detected 14 loci associated with Bti resistance, including one located in a putative receptor for Cry11 toxins. Three of these loci were also significantly differentiated between the laboratory strains, suggesting that most of the genes associated with resistance might differ between the two environments. It also suggests that common selected regions might harbour key genes for Bti resistance.

A Tenebrio molitor GPI-anchored alkaline phosphatase is involved in binding of Bacillus thuringiensis Cry3Aa to brush border membrane vesicles

Bacillus thuringiensis Cry toxins recognizes their target cells in part by the binding to glycosyl-phosphatidyl-inositol (GPI) anchored proteins such as aminopeptidase-N (APN) or alkaline phosphatases (ALP). Treatment of Tenebrio molitor brush border membrane vesicles (BBMV) with phospholipase C that cleaves out GPI-anchored proteins from the membranes, showed that GPI-anchored proteins are involved in binding of Cry3Aa toxin to BBMV. A 68 kDa GPI-anchored ALP was shown to bind Cry3Aa by toxin overlay assays. The 68 kDa GPI-anchored ALP was preferentially expressed in early instar larvae in comparison to late instar larvae. Our work shows for the first time that GPI-anchored ALP is important for Cry3Aa binding to T. molitor BBMV suggesting that the mode of action of Cry toxins is conserved in different insect orders.

Proteome analysis of Cry4Ba toxin-interacting Aedes aegypti lipid rafts using geLC-MS/MS

Lipid rafts are microdomains in the plasma membrane of eukaryotic cells. Among their many functions, lipid rafts are involved in cell toxicity caused by pore forming bacterial toxins including Bacillus thuringiensis (Bt) Cry toxins. We isolated lipid rafts from brush border membrane vesicles (BBMV) of Aedes aegypti larvae as a detergent resistant membrane (DRM) fraction on density gradients. Cholesterol, aminopeptidase (APN), alkaline phosphatase (ALP) and the raft marker flotillin were preferentially partitioned into the lipid raft fraction. When mosquitocidal Cry4Ba toxin was preincubated with BBMV, Cry4Ba localized to lipid rafts. A proteomic approach based on one-dimensional gel electrophoresis, in-gel trypsin digestion, followed by liquid chromatography-mass spectrometry (geLC-MS/MS) identified a total of 386 proteins. Of which many are typical lipid raft marker proteins including flotillins and glycosylphosphatidylinositol (GPI)-anchored proteins. Identified raft proteins were annotated in silico for functional and physicochemical characteristics. Parameters such as distribution of isoelectric point, molecular mass, and predicted post-translational modifications relevant to lipid raft proteins (GPI anchorage and myristoylation or palmitoylation) were analyzed for identified proteins in the DRM fraction. From a functional point of view, this study identified proteins implicated in Cry toxin interactions as well as membrane-associated proteins expressed in the mosquito midgut that have potential relevance to mosquito biology and vector management.

Aedes aegypti alkaline phosphatase ALP1 is a functional receptor of Bacillus thuringiensis Cry4Ba and Cry11Aa toxins

Bacillus thuringiensis subs. israelensis produces at least three Cry toxins (Cry4Aa, Cry4Ba, and Cry11Aa) that are active against Aedes aegypti larvae. Previous work characterized a GPI-anchored alkaline phosphatase (ALP1) as a Cry11Aa binding molecule from the gut of A. aegypti larvae. We show here that Cry4Ba binds ALP1, and that the binding and toxicity of Cry4Ba mutants located in loop 2 of domain II is correlated. Also, we analyzed the contribution of ALP1 toward the toxicity of Cry4Ba and Cry11Aa toxins by silencing the expression of this protein though RNAi. Efficient silencing of ALP1 was demonstrated by real-time quantitative PCR (qPCR) and Western blot. ALP1 silenced larvae showed tolerance to both Cry4Ba and Cry11Aa although the silenced larvae were more tolerant to Cry11Aa in comparison to Cry4Ba. Our results demonstrate that ALP1 is a functional receptor that plays an important role in the toxicity of the Cry4Ba and Cry11Aa proteins.

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.

Cadherin binding is not a limiting step for Bacillus thuringiensis subsp. israelensis Cry4Ba toxicity to Aedes aegypti larvae

Bacillus thuringiensis subsp. israelensis produces three Cry toxins (Cry4Aa, Cry4Ba and Cry11Aa) that are active against Aedes aegypti larvae. The identification of the rate-limiting binding steps of Cry toxins that are used for insect control in the field, such as those of B. thuringiensis subsp. israelensis, should provide targets for improving insecticides against important insect pests. Previous studies showed that Cry11Aa binds to cadherin receptor fragment CR7-11 (cadherin repeats 7-11) with high affinity. Binding to cadherin has been proposed to facilitate Cry toxin oligomer formation. In the present study, we show that Cry4Ba binds to CR7-11 with 9-fold lower binding affinity compared with Cry11Aa. Oligomerization assays showed that Cry4Ba is capable of forming oligomers when proteolytically activated in vitro in the absence of the CR7-11 fragment in contrast with Cry11Aa that formed oligomers only in the presence of CR7-11. Pore-formation assays in planar lipid bilayers showed that Cry4Ba oligomers were proficient in opening ion channels. Finally, silencing the cadherin gene by dsRNA (double-stranded RNA) showed that silenced larvae were more tolerant to Cry11Aa in contrast with Cry4Ba, which showed similar toxic levels to those of control larvae. These findings show that cadherin binding is not a limiting step for Cry4Ba toxicity to A. aegypti larvae.

Identifying insecticide resistance genes in mosquito by combining AFLP genome scans and 454 pyrosequencing

AFLP-based genome scans are widely used to study the genetics of adaptation and to identify genomic regions potentially under selection. However, this approach usually fails to detect the actual genes or mutations targeted by selection owing to the difficulties of obtaining DNA sequences from AFLP fragments. Here, we combine classical AFLP outlier detection with 454 sequencing of AFLP fragments to obtain sequences from outlier loci. We applied this approach to the study of resistance to Bacillus thuringiensis israelensis (Bti) toxins in the dengue vector Aedes aegypti. A genome scan of Bti-resistant and Bti-susceptible A. aegypti laboratory strains was performed based on 432 AFLP markers. Fourteen outliers were detected using two different population genetic algorithms. Out of these, 11 were successfully sequenced. Three contained transposable elements (TEs) sequences, and the 10 outliers that could be mapped at a unique location in the reference genome were located on different supercontigs. One outlier was in the vicinity of a gene coding for an aminopeptidase potentially involved in Bti toxin-binding. Patterns of sequence variability of this gene showed significant deviation from neutrality in the resistant strain but not in the susceptible strain, even after taking into account the known demographic history of the selected strain. This gene is a promising candidate for future functional analysis.

Bacillus thuringiensis: A story of a successful bioinsecticide

Bacillus thuringiensis (Bt) bacteria are insect pathogens that rely on insecticidal pore forming proteins known as Cry and Cyt toxins to kill their insect larval hosts. At least four different non-structurally related families of proteins form the Cry toxin group of toxins. The expression of certain Cry toxins in transgenic crops has contributed to an efficient control of insect pests resulting in a significant reduction in chemical insecticide use. The mode of action of the three domain Cry toxin family involves sequential interaction of these toxins with several insect midgut proteins facilitating the formation of a pre-pore oligomer structure and subsequent membrane insertion that leads to the killing of midgut insect cells by osmotic shock. In this manuscript we review recent progress in understanding the mode of action of this family of proteins in lepidopteran, dipteran and coleopteran insects. Interestingly, similar Cry-binding proteins have been identified in the three insect orders, as cadherin, aminopeptidase-N and alkaline phosphatase suggesting a conserved mode of action. Also, recent data on insect responses to Cry toxin attack is discussed. Finally, we review the different Bt based products, including transgenic crops, that are currently used in agriculture.

Multiple receptors as targets of Cry toxins in mosquitoes

Bacillus thuringiensis (Bt) produces inclusions that are composed of proteins known as crystal proteins or Cry toxins. Due to their high specificity and their safety to humans and the environment, these Cry toxins are considered to be valuable alternatives to chemical pesticides in insect control programs. It is believed that Cry toxin-induced membrane pore formation is responsible for insect toxicity. The molecular mechanism of pore formation involves recognition and subsequent binding of the toxin to membrane receptors. This binding is accompanied by toxin oligomerization and transfer of domain I helices of the toxin to the lipid-water interface. This toxin insertion creates pores that lyse the cells. Several receptors from lepidopteran, coleopteran, and dipteran insects have been well characterized. This paper provides an overview of the understanding of the interactions between Cry toxin and multiple receptors in mosquitoes, in particular Aedes aegypti and reviews the manner by which the receptors were identified and characterized, with a focus on three proteins, cadherin, alkaline phosphatase, and aminopeptidase-N.

Molecular characterization of three genes encoding aminopeptidases N in the poplar leaf beetle Chrysomela tremulae

Three genes encoding proteins showing sequence similarity and features typical of insect APNs were characterized in C. tremulae and designed as CtAPN1, CtAPN2 and CtAPN3. Expression analysis of the three C. tremulae APN genes showed that CtAPN2 transcript is more abundant in the fat body, whereas both CtAPN1 and CtAPN3 are specifically expressed in the midgut. Despite a similar genomic organization, lepidopteran and coleopteran APNs are phylogenetically distant, suggesting that APN gene duplication events occurred after these two insect orders split. Sequence and expression comparisons of CtAPN1, CtAPN2 and CtAPN3 cDNAs in a C. tremulae Bacillus thuringiensis (Bt)-susceptible and in a Bt-resistant strain did not show any polymorphism at the amino acid level or difference at the transcription level.