Dominant negative phenotype of Bacillus thuringiensis Cry1Ab, Cry11Aa and Cry4Ba mutants suggest hetero-oligomer formation among different Cry toxins

Characterization of the mode of action of eCry1Gb.1Ig, a fall armyworm (Spodoptera frugiperda) active protein, with a novel site of action

Insect pests cause immense agronomic losses worldwide. One of the most destructive of major crops is the Fall Armyworm (Spodoptera frugiperda, FAW). The ability to migrate long distances, a prodigious appetite, and a demonstrated ability to develop resistance to insecticides, make it a difficult target to control. Insecticidal proteins, for example those produced by the bacterium Bacillus thuringiensis, are among the safest and most effective insect control agents. Genetically modified (GM) crops expressing such proteins are a key part of a successful integrated pest management (IPM) program for FAW. However, due to the development of populations resistant to commercialized GM products, new GM traits are desperately needed. Herein, we describe a further characterization of the newly engineered trait protein eCry1Gb.1Ig. Similar to other well characterized Cry proteins, eCry1Gb.1Ig is shown to bind FAW midgut cells and induce cell-death. Binding competition assays using trait proteins from other FAW-active events show a lack of competition when binding FAW brush border membrane vesicles (BBMVs) and when utilizing non-pore-forming versions as competitors in in vivo bioassays. Similarly, insect cell lines expressing SfABCC2 and SfABCC3 (well characterized receptors of existing commercial Cry proteins) are insensitive to eCry1Gb.1Ig. These findings are consistent with results from our previous work showing that eCry1Gb.1Ig is effective in controlling insects with resistance to existing traits. This underscores the value of eCry1Gb.1Ig as a new GM trait protein with a unique site-of-action and its potential positive impact to global food production.

Structure of the Lysinibacillus sphaericus Tpp49Aa1 pesticidal protein elucidated from natural crystals using MHz-SFX

The Lysinibacillus sphaericus proteins Tpp49Aa1 and Cry48Aa1 can together act as a toxin toward the mosquito Culex quinquefasciatus and have potential use in biocontrol. Given that proteins with sequence homology to the individual proteins can have activity alone against other insect species, the structure of Tpp49Aa1 was solved in order to understand this protein more fully and inform the design of improved biopesticides. Tpp49Aa1 is naturally expressed as a crystalline inclusion within the host bacterium, and MHz serial femtosecond crystallography using the novel nanofocus option at an X-ray free electron laser allowed rapid and high-quality data collection to determine the structure of Tpp49Aa1 at 1.62 Å resolution. This revealed the packing of Tpp49Aa1 within these natural nanocrystals as a homodimer with a large intermolecular interface. Complementary experiments conducted at varied pH also enabled investigation of the early structural events leading up to the dissolution of natural Tpp49Aa1 crystals-a crucial step in its mechanism of action. To better understand the cooperation between the two proteins, assays were performed on a range of different mosquito cell lines using both individual proteins and mixtures of the two. Finally, bioassays demonstrated Tpp49Aa1/Cry48Aa1 susceptibility of Anopheles stephensi, Aedes albopictus, and Culex tarsalis larvae-substantially increasing the potential use of this binary toxin in mosquito control.

In vivo and in silico comparison analyses of Cry toxin activities toward the sugarcane giant borer

The sugarcane giant borer, Telchin licus licus, is an insect pest that causes significant losses in sugarcane crops and in the sugar-alcohol sector. Chemical and manual control methods are not effective. As an alternative, in the current study, we have screened Bacillus thuringiensis (Bt) Cry toxins with high toxicity against this insect. Bioassays were conducted to determine the activity of four Cry toxins (Cry1A (a, b, and c) and Cry2Aa) against neonate T. licus licus larvae. Notably, the Cry1A family toxins had the lowest LC₅₀ values, in which Cry1Ac presented 2.1-fold higher activity than Cry1Aa, 1.7-fold larger than Cry1Ab, and 9.7-fold larger than Cry2Aa toxins. In silico analyses were performed as a perspective to understand putative interactions between T. licus licus receptors and Cry1A toxins. The molecular dynamics and docking analyses for three putative aminopeptidase N (APN) receptors (TlAPN1, TlAPN3, and TlAPN4) revealed evidence for the amino acids that may be involved in the toxin-receptor interactions. Notably, the properties of Cry1Ac point to an interaction site that increases the toxin's affinity for the receptor and likely potentiate toxicity. The interacting amino acid residues predicted for Cry1Ac in this work are probably those shared by the other Cry1A toxins for the same region of APNs. Thus, the presented data extend the existing knowledge of the effects of Cry toxins on T. licus licus and should be considered in further development of transgenic sugarcane plants resistant to this major occurring insect pest in sugarcane fields.

De novo determination of mosquitocidal Cry11Aa and Cry11Ba structures from naturally-occurring nanocrystals

Cry11Aa and Cry11Ba are the two most potent toxins produced by mosquitocidal Bacillus thuringiensis subsp. israelensis and jegathesan, respectively. The toxins naturally crystallize within the host; however, the crystals are too small for structure determination at synchrotron sources. Therefore, we applied serial femtosecond crystallography at X-ray free electron lasers to in vivo-grown nanocrystals of these toxins. The structure of Cry11Aa was determined de novo using the single-wavelength anomalous dispersion method, which in turn enabled the determination of the Cry11Ba structure by molecular replacement. The two structures reveal a new pattern for in vivo crystallization of Cry toxins, whereby each of their three domains packs with a symmetrically identical domain, and a cleavable crystal packing motif is located within the protoxin rather than at the termini. The diversity of in vivo crystallization patterns suggests explanations for their varied levels of toxicity and rational approaches to improve these toxins for mosquito control.

Comparison of in vitro and in vivo binding site competition of Bacillus thuringiensis Cry1 proteins in two important maize pests

BACKGROUND

Binding site models, derived from in vitro competition binding studies, have been widely used for predicting potential cross-resistance among insecticidal proteins from Bacillus thuringiensis. However, because discrepancies have been found between binding data and observed cross-resistance patterns in some insect species, new tools are required to study the functional relevance of the shared binding sites.

RESULTS

Here, an in vivo approach has been applied to the competition studies to establish the functional relevance of shared binding sites as determined by in vitro competition assays. Using Cry disabled proteins as competitors in mixed protein overlay assays, we assessed the preference of Cry1Ab, Cry1Fa, and Cry1A.105 proteins for shared binding sites in vivo in two important corn pests, Ostrinia nubilalis and Spodoptera frugiperda.

CONCLUSION

This study shows that in vivo and in vitro binding site competition assays can provide useful information to better ascertain whether different Cry proteins share binding sites and, consequently, whether cross-resistance due to binding site alteration can occur. © 2021 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

In vivo competition assays between Vip3 proteins confirm the occurrence of shared binding sites in Spodoptera littoralis

Due to their different specificity, the use of Vip3 proteins from Bacillus thuringiensis in combination with the conventionally used Cry proteins in crop protection is being essential to counteract the appearance of insect resistance. Therefore, understanding the mode of action of Vip3 proteins is crucial for their better application, with special interest on the binding to membrane receptors as the main step for specificity. Derived from in vitro heterologous competition binding assays using ¹²⁵I-Vip3A and other Vip3 proteins as competitors, it has been shown that Vip3 proteins share receptors in Spodoptera frugiperda and Spodoptera exigua brush border membrane vesicles (BBMV). In this study, using ¹²⁵I-Vip3Aa, we have first extended the in vitro competition binding site model of Vip3 proteins to Spodoptera littoralis. With the aim to understand the relevance (in terms of toxicity) of the binding to the midgut sites observed in vitro on the insecticidal activity of these proteins, we have performed in vivo competition assays with S. littoralis larvae, using disabled mutant (non-toxic) Vip3 proteins as competitors for blocking the toxicity of Vip3Aa and Vip3Af. The results of the in vivo competition assays confirm the occurrence of shared binding sites among Vip3 proteins and help understand the functional role of the shared binding sites as revealed in vitro.

Hetero-oligomerization of Bacillus thuringiensis Cry1A proteins enhance binding to the ABCC2 transporter of Spodoptera exigua

The ATP binding cassette (ABC) transporters are membrane proteins that can act as putative receptors for Cry proteins from Bacillus thuringiensis (Bt) in the midgut of different insects. For the beet armyworm, Spodoptera exigua, ABCC2 and ABCC3 have been found to interact with Cry1A proteins, the main insecticidal proteins used in Bt crops, as well as Bt-based pesticides. The ABCC2 has shown to have specific binding towards Cry1Ac and is involved in the toxic process of Cry1A proteins, but the role of this transporter and how it relates with the Cry1A proteins is still unknown. Here, we have characterized the interactions between the SeABCC2 and the main proteins that bind to the receptor. By labeling the Cry1Aa protein, we have found that virtually all of the binding is in an oligomeric state, a conformation that allowed higher levels of specific binding that could not be achieved by the monomeric protein on its own. Furthermore, we have observed that Cry1A proteins can hetero-oligomerize in the presence of the transporter, which is reflected in an increase in binding and toxicity to SeABCC2-expressing cells. This synergism can be one of the reasons why B. thuringiensis co-expresses different Cry1 proteins that can apparently have similar binding preferences. The results from in vitro competition and ex vivo competition showed that Cry1Aa, Cry1Ab and Cry1Ac share functional binding sites. By using Cry1Ab-Cry1Ac chimeras, the presence of domain I from Cry1A proteins was revealed to be critical for oligomer formation.

Study of the Bacillus thuringiensis Cry1Ia Protein Oligomerization Promoted by Midgut Brush Border Membrane Vesicles of Lepidopteran and Coleopteran Insects, or Cultured Insect Cells

Bacillus thuringiensis (Bt) produces insecticidal proteins that are either secreted during the vegetative growth phase or accumulated in the crystal inclusions (Cry proteins) in the stationary phase. Cry1I proteins share the three domain (3D) structure typical of crystal proteins but are secreted to the media early in the stationary growth phase. In the generally accepted mode of action of 3D Cry proteins (sequential binding model), the formation of an oligomer (tetramer) has been described as a major step, necessary for pore formation and subsequent toxicity. To know if this could be extended to Cry1I proteins, the formation of Cry1Ia oligomers was studied by Western blot, after the incubation of trypsin activated Cry1Ia with insect brush border membrane vesicles (BBMV) or insect cultured cells, using Cry1Ab as control. Our results showed that Cry1Ia oligomers were observed only after incubation with susceptible coleopteran BBMV, but not following incubation with susceptible lepidopteran BBMV or non-susceptible Sf21 insect cells, while Cry1Ab oligomers were persistently detected after incubation with all insect tissues tested, regardless of its host susceptibility. The data suggested oligomerization may not necessarily be a requirement for the toxicity of Cry1I proteins.

Disabled insecticidal proteins: A novel tool to understand differences in insect receptor utilization

The development of insect resistance to pesticides via natural selection is an acknowledged agricultural issue. Likewise, resistance development in target insect populations is a significant challenge to the durability of crop traits conferring insect protection and has driven the need for novel insecticidal proteins (IPs) with alternative mechanism of action (MOA) mediated by different insect receptors. The combination or "stacking" of transgenes encoding different insecticidal proteins in a single crop plant can greatly delay the development of insect resistance, but requires sufficient knowledge of MOA to identify proteins with different receptor preferences. Accordingly, a rapid technique for differentiating the receptor binding preferences of insecticidal proteins is a critical need. This article introduces the Disabled Insecticidal Protein (DIP) method as applied to the well-known family of three-domain insecticidal proteins from Bacillus thuringiensis and related bacteria. These DIP's contain amino acid substitutions in domain 1 that render the proteins non-toxic but still capable of competing with active proteins in insect feeding assays, resulting in a suppression of the expected insecticidal activity. A set of insecticidal proteins with known differences in receptor binding (Cry1Ab3, Cry1Ac.107, Cry2Ab2, Cry1Ca, Cry1A.105, and Cry1A.1088) has been studied using the DIP method, yielding results that are consistent with previous MOA studies. When a native IP and an excess of DIP are co-administered to insects in a feeding assay, the outcome depends on the overlap between their MOAs: if receptors are shared, then the DIP saturates the receptors to which the native protein would ordinarily bind, and acts as an antidote whereas, if there is no shared receptor, the toxicity of the native insecticidal protein is not inhibited. These results suggest that the DIP methodology, employing standard insect feeding assays, is a robust and effective method for rapid MOA differentiation among insecticidal proteins.

Nematicidal Activity of Cry1Ea11 from Bacillus thuringiensis BRC-XQ12 Against the Pine Wood Nematode (Bursaphelenchus xylophilus)

The nematicidal activity of 92 Bacillus thuringiensis strains against the pine wood nematode Bursaphelenchus xylophilus, one of the world's top 10 plant-parasitic nematodes, was determined. The insecticidal crystal proteins (ICPs) from Bacillus thuringiensis BRC-XQ12 were the most toxic to Bursaphelenchus xylophilus, with a lethal concentration 50 (LC₅₀) of 32.13 μg/ml. Because the ICPs expressed by Bacillus thuringiensis BRC-XQ12 were closest to Cry1Ea6 and B. thuringiensis BRC-XQ12 contained four kinds of cry1 subgenes (cry1Aa, cry1Cb, cry1Ea, and cry1Ia), Cry1Ea was most likely to be the key active component against the nematode. The 3,516-bp cry1Ea11 gene from BRC-XQ12, as designated by the B. thuringiensis δ-endotoxin nomenclature committee, was expressed in Escherichia coli. Purified Cry1Ea11 showed an LC₅₀ of 32.53 and 23.23 μg/ml at 24 and 48 h, with corresponding virulence equations of Y = 32.15X + 1.38 (R² = 0.9951) and Y = 34.29X + 3.16 (R² = 0.9792), respectively. In order to detect the pathway of B. thuringiensis Cry1Ea11 into Bursaphelenchus xylophilus, the nematode was fed with NHS-rhodamine-labeled GST-Cry1Ea11. The results of confocal laser-scanning microscopy showed that the 159-kDa GST-Cry1Ea11 could be detected in the stylet and the esophageal lumen of the pine wood nematode, indicating that GST-Cry1Ea11 could enter into the nematode through the stylet. As far as we know, no Cry1 proteins have been shown to have activity against plant-parasitic nematodes before. These results demonstrate that Cry1Ea11 is a promising nematicidal protein for controlling pine wilt disease rendered by B. xylophilus, further dramatically broadening the spectrum of Bacillus thuringiensis ICPs.

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

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

Transcriptional cellular responses in midgut tissue of Aedes aegypti larvae following intoxication with Cry11Aa toxin from Bacillus thuringiensis

Background

Although much is known about the mechanism of action of Bacillus thuringiensis Cry toxins, the target tissue cellular responses to toxin activity is less understood. Previous transcriptomic studies indicated that significant changes in gene expression occurred during intoxication. However, most of these studies were done in organisms without a sequenced and annotated reference genome. A reference genome and transcriptome is available for the mosquito Aedes aegypti, and its importance as a disease vector has positioned its biological control as a primary health concern. Through RNA sequencing we sought to determine the transcriptional changes observed during intoxication by Cry11Aa in A. aegypti and to analyze possible defense and recovery mechanisms engaged after toxin ingestion.

Results

In this work the changes in the transcriptome of 4^th instar A. aegypti larvae exposed to Cry11Aa toxin for 0, 3, 6, 9, and 12 h were analyzed. A total of 1060 differentially expressed genes after toxin ingestion were identified with two bioconductoR packages: DESeq2 and EdgeR. The most important transcriptional changes were observed after 9 or 12 h of toxin exposure. GO enrichment analysis of molecular function and biological process were performed as well as Interpro protein functional domains and pBLAST analyses. Up regulated processes include vesicular trafficking, small GTPase signaling, MAPK pathways, and lipid metabolism. In contrast, down regulated functions are related to transmembrane transport, detoxification mechanisms, cell proliferation and metabolism enzymes. Validation with RT-qPCR showed large agreement with Cry11Aa intoxication since these changes were not observed with untreated larvae or larvae treated with non-toxic Cry11Aa mutants, indicating that a fully functional pore forming Cry toxin is required for the observed transcriptional responses.

Conclusions

This study presents the first transcriptome of Cry intoxication response in a fully sequenced insect, and reveals possible conserved cellular processes that enable larvae to contend with Cry intoxication in the disease vector A. aegypti. We found some similarities of the mosquito responses to Cry11Aa toxin with previously observed responses to other Cry toxins in different insect orders and in nematodes suggesting a conserved response to pore forming toxins. Surprisingly some of these responses also correlate with transcriptional changes observed in Bti-resistant and Cry11Aa-resistant mosquito larvae.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2240-7) contains supplementary material, which is available to authorized users.

Aedes aegypti Mos20 cells internalizes cry toxins by endocytosis, and actin has a role in the defense against Cry11Aa toxin

Bacillus thuringiensis (Bt) Cry toxins are used to control Aedes aegypti, an important vector of dengue fever and yellow fever. Bt Cry toxin forms pores in the gut cells, provoking larvae death by osmotic shock. Little is known, however, about the endocytic and/or degradative cell processes that may counteract the toxin action at low doses. The purpose of this work is to describe the mechanisms of internalization and detoxification of Cry toxins, at low doses, into Mos20 cells from A. aegypti, following endocytotic and cytoskeletal markers or specific chemical inhibitors. Here, we show that both clathrin-dependent and clathrin-independent endocytosis are involved in the internalization into Mos20 cells of Cry11Aa, a toxin specific for Dipteran, and Cry1Ab, a toxin specific for Lepidoptera. Cry11Aa and Cry1Ab are not directed to secretory lysosomes. Instead, Mos20 cells use the Rab5 and Rab11 pathways as a common mechanism, most probably for the expulsion of Cry11Aa and Cry1Ab toxins. In conclusion, we propose that endocytosis is a mechanism induced by Cry toxins independently of specificity, probably as part of a basal immune response. We found, however, that actin is necessary for defense-specific response to Cry11Aa, because actin-silenced Mos20 cells become more sensitive to the toxic action of Cry11A toxin. Cry toxin internalization analysis in insect cell lines may contribute to a better understanding to Cry resistance in mosquitoes.

Cytotoxicity analysis of three Bacillus thuringiensis subsp. israelensis δ-endotoxins towards insect and mammalian cells

Three members of the δ-endotoxin group of toxins expressed by Bacillus thuringiensis subsp. israelensis, Cyt2Ba, Cry4Aa and Cry11A, were individually expressed in recombinant acrystalliferous B. thuringiensis strains for in vitro evaluation of their toxic activities against insect and mammalian cell lines. Both Cry4Aa and Cry11A toxins, activated with either trypsin or Spodoptera frugiperda gastric juice (GJ), resulted in different cleavage patterns for the activated toxins as seen by SDS-PAGE. The GJ-processed proteins were not cytotoxic to insect cell cultures. On the other hand, the combination of the trypsin-activated Cry4Aa and Cry11A toxins yielded the highest levels of cytotoxicity to all insect cells tested. The combination of activated Cyt2Ba and Cry11A also showed higher toxic activity than that of toxins activated individually. When activated Cry4Aa, Cry11A and Cyt2Ba were used simultaneously in the same assay a decrease in toxic activity was observed in all insect cells tested. No toxic effect was observed for the trypsin-activated Cry toxins in mammalian cells, but activated Cyt2Ba was toxic to human breast cancer cells (MCF-7) when tested at 20 µg/mL.