Lysinibacillus sphaericus binary toxin induces apoptosis in susceptible Culex quinquefasciatus larvae

Scavenger Receptor C1 Mediates Toxicity of Binary Toxin from Lysinibacillus sphaericus to Ag55 Cells

Lysinibacillus sphaericus harboring Binary (BinA and BinB) toxins is highly toxic against Anopheles and Culex mosquito larvae. The Anopheles Ag55 cell line is a suitable model for investigating the mode of Bin toxin action. Based on the low-levels of α-glycosidase Agm3 mRNA in Ag55 cells and the absence of detectable Agm3 proteins, we hypothesized that a scavenger receptor could be mediating Bin cytotoxicity. Preliminary RNA interference knockdown of the expressed scavenger receptors, combined with Bin cytotoxicity assays, was conducted. The scavenger Receptor C1 (SCRC1) became the focus of this study, as a putative receptor for Bin toxins in Ag55 cells, and SCRBQ2 was selected as a negative control. Open reading frames encoding SCRC1 and SCRBQ2 were cloned and expressed in vitro, and polyclonal antibodies were prepared for immunological analyses. The RNAi silencing of SCRC1 and SCRBQ2 resulted in the successful knockdown of both SCRC1 and SCRBQ2 transcripts and protein levels. The cytolytic toxicity of Bin against Ag55 cells was severely reduced after the SCRC1-RNAi treatment. The phagocytic receptor protein SCRC1 mediates endocytosis of the Bin toxin into Ag55 cells, thereby facilitating its internal cytological activity. The results support a mechanism of the Bin toxin entering Ag55 cells, possibly via SCRC1-mediated endocytosis, and encourage investigations into how Bin is transferred from its bound form on the midgut epithelial cells into the epithelial endocytic system.

[Deficiency in N-acetylglucosamine transport affects the sporulation process and increases the hemolytic activity of the S-layer protein in Lysinibacillus sphaericus ASB13052]

Lysinibacillus sphaericus is a bacterium that, along with Bacillus thuringiensis var. israelensis, is considered the best biological insecticide for controlling mosquito larvae and an eco-friendly alternative to chemical insecticides. It depends on peptidic molecules such as N-acetylglucosamine to obtain carbon sources and possesses a phosphotransferase system (PTS) for their incorporation. Some strains carry S-layer proteins, whose involvement in metal retention and larvicidal activity against disease-carrying mosquitoes has been demonstrated. Alterations in the amino sugar incorporation system could affect the protein profile and functionality. Strain ASB13052 and the isogenic mutant in the ptsH gene, which is predominant in the PTS signaling pathway, were used in this study. For the first time, the presence of N-glycosylated S-layer proteins was confirmed in both strains, with a variation in their molecular weight pattern depending on the growth phase. In the exponential phase, an S-layer protein greater than 130 kDa was found in the ptsH mutant, which was absent in the wild-type strain. The mutant strain exhibited altered and incomplete low quality sporulation processes. Hemolysis analysis, associated with larvicidal activity, showed that the ptsH mutant has higher lytic efficiency, correlating with the high molecular weight protein. The results allow us to propose the potential effects that arise as a result of the absence of amino sugar transport on hemolytic activity, S-layer isoforms, and the role of N-acetylglucosamine in larvicidal activity.

The Perpetual Vector Mosquito Threat and Its Eco-Friendly Nemeses

Mosquitoes are the most notorious arthropod vectors of viral and parasitic diseases for which approximately half the world's population, ~4,000,000,000, is at risk. Integrated pest management programs (IPMPs) have achieved some success in mitigating the regional transmission and persistence of these diseases. However, as many vector-borne diseases remain pervasive, it is obvious that IPMP successes have not been absolute in eradicating the threat imposed by mosquitoes. Moreover, the expanding mosquito geographic ranges caused by factors related to climate change and globalization (travel, trade, and migration), and the evolution of resistance to synthetic pesticides, present ongoing challenges to reducing or eliminating the local and global burden of these diseases, especially in economically and medically disadvantaged societies. Abatement strategies include the control of vector populations with synthetic pesticides and eco-friendly technologies. These "green" technologies include SIT, IIT, RIDL, CRISPR/Cas9 gene drive, and biological control that specifically targets the aquatic larval stages of mosquitoes. Regarding the latter, the most effective continues to be the widespread use of Lysinibacillus sphaericus (Ls) and Bacillus thuringiensis subsp. israelensis (Bti). Here, we present a review of the health issues elicited by vector mosquitoes, control strategies, and lastly, focus on the biology of Ls and Bti, with an emphasis on the latter, to which no resistance has been observed in the field.

Culex quinquefasciatus Resistant to the Binary Toxin from Lysinibacillus sphaericus Displays a Consistent Downregulation of Pantetheinase Transcripts

Culex quinquefasciatus resistance to the binary (Bin) toxin, the major larvicidal component from Lysinibacillus sphaericus, is associated with mutations in the cqm1 gene, encoding the Bin-toxin receptor. Downregulation of the cqm1 transcript was found in the transcriptome of larvae resistant to the L. sphaericus IAB59 strain, which produces both the Bin toxin and a second binary toxin, Cry48Aa/Cry49Aa. Here, we investigated the transcription profiles of two other mosquito colonies having Bin resistance only. These confirmed the cqm1 downregulation and identified transcripts encoding the enzyme pantetheinase as the most downregulated mRNAs in both resistant colonies. Further quantification of these transcripts reinforced their strong downregulation in Bin-resistant larvae. Multiple genes were found encoding this enzyme in Cx. quinquefasciatus and a recombinant pantetheinase was then expressed in Escherichia coli and Sf9 cells, with its presence assessed in the midgut brush border membrane of susceptible larvae. The pantetheinase was expressed as a ~70 kDa protein, potentially membrane-bound, which does not seem to be significantly targeted by glycosylation. This is the first pantetheinase characterization in mosquitoes, and its remarkable downregulation might reflect features impacted by co-selection with the Bin-resistant phenotype or potential roles in the Bin-toxin mode of action that deserve to be investigated.

Biotechnological Potential of Microorganisms for Mosquito Population Control and Reduction in Vector Competence

Mosquitoes transmit pathogens that cause human diseases such as malaria, dengue fever, chikungunya, yellow fever, Zika fever, and filariasis. Biotechnological approaches using microorganisms have a significant potential to control mosquito populations and reduce their vector competence, making them alternatives to synthetic insecticides. Ongoing research has identified many microorganisms that can be used effectively to control mosquito populations and disease transmission. However, the successful implementation of these newly proposed approaches requires a thorough understanding of the multipronged microorganism-mosquito-pathogen-environment interactions. Although much has been achieved in discovering new entomopathogenic microorganisms, antipathogen compounds, and their mechanisms of action, only a few have been turned into viable products for mosquito control. There is a discrepancy between the number of microorganisms with the potential for the development of new insecticides and/or antipathogen products and the actual available products, highlighting the need for investments in the intersection of basic research and biotechnology.

Culex quinquefasciatus alpha-glucosidase serves as a putative receptor of the Cry48Aa toxin from Lysinibacillus sphaericus

The Cry48Aa/Cry49Aa toxin of Lysinibacillus sphaericus shows specific toxicity towards larvae of Culex spp. Individual Cry48Aa and Cry49Aa subunits interact with distinct target sites in the larval midgut and overcome the resistance of Culex to the Bin toxin. However, the toxin-binding proteins have not yet been identified. The present study aimed to identify Cry48Aa-binding proteins in Culex quinquefasciatus. Pulldown assays using C. quinquefasciatus midgut brush-border membrane fractions (BBMFs) identified a class of proteins, including aminopeptidases (APNs), protease m1 zinc metalloproteases, alkaline phosphatases (ALPs), and maltases, that could be potentially involved in the mode of action of this toxin. RNA interference analysis showed that silenced larvae treated with dsRNA of the alpha-glucosidase (named Glu71) gene were more tolerant of the Cry48Aa/Cry49Aa toxin, which induced less than 20% mortality. The amino acid sequence of Glu71 exhibited 42% identity with Cqm1/Cpm1, which acted as a Bin toxin receptor. Toxin binding assays showed that Cry48Aa had a high specific binding capacity for the Glu71 protein, whereas Cry49Aa exhibited no specific binding. Overall, our results showed that Glu71 is a Cry48-binding protein involved in Cry48Aa/Cry49Aa toxicity.

Mosquito-larvicidal Binary (BinA/B) proteins for mosquito control programs —advancements, challenges, and possibilities

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Binary (BinAB) toxin is primarily responsible for the larvicidal action of the WHO recognized mosquito-larvicidal bacterium Lysinibacillus sphaericus.

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BinAB is a single receptor-specific toxin, active against larvae of Culex and Anopheles, but not Aedes aegypti.

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The target receptor in Culex is Cqm1 protein, a GPI-anchored amylomaltase located apically in the lipid-rafts of the larval-midgut epithelium.

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Interaction of the toxin components with the receptor is critical for the larvicidal activity of the toxin.

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Evidences support the pore formation model for BinAB toxin internalization and the role of toxin-glycan interactions in the endoplasmic reticulum in mediating larval death.

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Targeted R&D efforts are required to maintain the sustainability and improve efficacy of the eco-friendly BinAB proteins for efficient mosquito control interventions.

The increasing global burden of mosquito-borne diseases require targeted, environmentally friendly, and sustainable approaches for effective vector control without endangering the non-target beneficial insect population. Biological interventions such as biopesticides, Wolbachia-mediated biological controls, or sterile insect techniques are used worldwide. Here we review Binary or BinAB toxin—the mosquito-larvicidal component of WHO-recognized Lysinibacillus sphaericus bacterium employed in mosquito control programs. Binary (BinAB) toxin is primarily responsible for the larvicidal effect of the bacterium. BinAB is a single-receptor-specific toxin and is effective against larvae of Culex and Anopheles, but not against Aedes aegypti. The receptor in Culex, the Cqm1 protein, has been extensively studied. It is a GPI-anchored amylomaltase and is located apically in the lipid rafts of the larval-midgut epithelium. The interaction of the toxin components with the receptor is crucial for the mosquito larvicidal activity of the BinAB toxin. Here we extend support for the pore formation model of BinAB toxin internalization and the role of toxin-glycan interactions in the endoplasmic reticulum in mediating larval death. BinAB is phylogenetically safe for humans, as Cqm1-like protein is not expected in the human proteome. This review aims to initiate targeted R&D efforts, such as applying fusion technologies (chimera of BinA, chemical modification of BinA), for efficient mosquito control interventions. In addition, the review also examines other areas such as bioremediation and cancer therapeutics, in which L. sphaericus is proving useful and showing potential for further development.

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.

Cytotoxic Effects and Intracellular Localization of Bin Toxin from Lysinibacillus sphaericus in Human Liver Cancer Cell Line

Binary toxin (Bin toxin), BinA and BinB, produced by Lysinibacillus sphaericus has been used as a mosquito-control agent due to its high toxicity against the mosquito larvae. The crystal structures of Bin toxin and non-insecticidal but cytotoxic parasporin-2 toxin share some common structural features with those of the aerolysin-like toxin family, thus suggesting a common mechanism of pore formation of these toxins. Here we explored the possible cytotoxicity of Bin proteins (BinA, BinB and BinA + BinB) against Hs68 and HepG2 cell lines. The cytotoxicity of Bin proteins was evaluated using the trypan blue exclusion assay, MTT assay, morphological analysis and LDH efflux assay. The intracellular localization of Bin toxin in HepG2 cells was assessed by confocal laser scanning microscope. HepG2 cells treated with BinA and BinB (50 µg/mL) showed modified cell morphological features and reduced cell viability. Bin toxin showed no toxicity against Hs68 cells. The EC₅₀ values against HepG2 at 24 h were 24 ng/mL for PS2 and 46.56 and 39.72 µg/mL for BinA and BinB, respectively. The induction of apoptosis in treated HepG2 cells was confirmed by upregulation of caspase levels. The results indicated that BinB mediates the translocation of BinA in HepG2 cells and subsequently associates with mitochondria. The study supports the possible development of Bin toxin as either an anticancer agent or a selective delivery vehicle of anticancer agents to target mitochondria of human cancer cells in the future.

Identification of a Novel Brevibacillus laterosporus Strain With Insecticidal Activity Against Aedes albopictus Larvae

Bacterial species able to produce proteins that are toxic against insects have been discovered at the beginning of the last century. However, up to date only two of them have been used as pesticides in mosquito control strategies targeting larval breeding sites: Bacillus thuringensis var. israelensis and Lysinibacillus sphaericus. Aiming to expand the arsenal of biopesticides, bacterial cultures from 44 soil samples were assayed for their ability to kill larvae of Aedes albopictus. A method to select, grow and test the larvicidal capability of spore-forming bacteria from each soil sample was developed. This allowed identifying 13 soil samples containing strains capable of killing Ae. albopictus larvae. Among the active isolates, one strain with high toxicity was identified as Brevibacillus laterosporus by 16S rRNA gene sequencing and by morphological characterization using transmission electron microscopy. The new isolate showed a larvicidal activity significantly higher than the B. laterosporus LMG 15441 reference strain. Its genome was phylogenomically characterized and compared to the available Brevibacillus genomes. Thus, the new isolate can be considered as a candidate adjuvant to biopesticides formulations that would help preventing the insurgence of resistance.

Identification of Lysinibacillus sphaericus Binary toxin binding proteins in a malarial mosquito cell line by proteomics: A novel approach towards improving mosquito control

Bacterial insecticidal proteins, such as the Bin toxin from Lysinibacillus sphaericus, could be used more extensively to control insecticide resistant mosquitoes. This study was aimed at identification of mosquito cell proteins binding Bin toxin. Results showed that purified toxin was toxic to Anopheles gambiae larvae and Ag55 cultured cells. Clathrin heavy chain (an endocytosis protein) and glycolytic enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase were identified as binders of Bin toxin. The viability of Ag55 cells in the presence of endocytosis inhibitor, pitstop2, was significantly decreased upon Bin treatment, while the inhibitor chlorpromazine did not affect Bin toxicity. Bin toxin treatment decreased ATP production and mitochondrial respiration in Ag55 cells, whereas non-mitochondrial oxygen consumption significantly increased after Bin toxin treatment. These findings are steps towards understanding how Bin toxin kills mosquitoes. SIGNIFICANCE: Mosquitoes are vectors of pathogens causing human diseases such as dengue fever, yellow fever, zika virus and malaria. An insecticidal toxin from Lysinibacillus sphaericus called Binary, or Bin, toxin could be used more extensively to control insecticide resistant mosquitoes. Bin toxin enter cells in susceptible mosquitoes and induces apoptosis or autophagy. In the current research, we used the malaria mosquito Anopheles gambiae Ag55 cell line as a model. A proteomic-based approach identified proteins that interact with Bin toxin. Interacting proteins include clathrin heavy chain (endocytosis protein) and glycolysis enzymes such as pyruvate kinase, enolase and dihydrolipoamide dehydrogenase. In Ag55 cell toxicity assays, an endocytosis inhibitor, pitstop2, increased Bin toxicity. Real time assays with a Seahorse™ flux analyzer showed that Bin significantly affects mitochondrial respiration, a result consistent with cell death via apoptosis or autophagy. These research findings add insights into how an unusual binary protein exploits cellular machinery to kill mosquitoes.

In vitro analysis of the anticancer activity of Lysinibacillus sphaericus binary toxin in human cancer cell lines

Binary or Bin toxin produced by Lysinibacillus sphaericus is composed of BinA (42 kDa) and BinB (51 kDa) subunits. These work together to exert maximal toxicity against mosquito larvae via pore formation and induction of apoptosis. The C-terminal domains in both subunits are homologous to those of aerolysin-type β pore-forming toxins, including parasporin-2 (PS2). The latter is one of the Bacillus thuringiensis toxins that exhibits specific cytotoxicity against human cancer cells. The present study investigates the possible anticancer activity of Bin toxin using PS2 as a control. We demonstrate that treatment with a high concentration of trypsin-activated Bin inhibits cell proliferation in human cancer cells A549, Caco-2, HepG2, HK-1 and KKU-M055. In the most susceptible cells, HK-1, Bin toxin exposure led to morphological alterations, decreased migration, decreased adhesion activity and apoptosis induction. Although these effects necessitated high concentrations, they suggest that Bin toxin may be optimized as a novel potential cancer-therapeutic agent.

Functional Bacillus thuringiensis Cyt1Aa Is Necessary To Synergize Lysinibacillus sphaericus Binary Toxin (Bin) against Bin-Resistant and -Refractory Mosquito Species

The binary (Bin) toxin from Lysinibacillus sphaericus is effective to mosquito larvae, but its utilization is threatened by the development of insect resistance. Bin toxin is composed of the BinB subunit required for binding to midgut receptors and the BinA subunit that causes toxicity after cell internalization, mediated by BinB. Culex quinquefasciatus resistance to this toxin is caused by mutations that prevent expression of Bin toxin receptors in the midgut. Previously, it was shown that the Cyt1Aa toxin from Bacillus thuringiensis subsp. israelensis restores Bin toxicity to Bin-resistant C. quinquefasciatus and to Aedes aegypti larvae, which are naturally devoid of functional Bin receptors. Our goal was to elucidate the mechanism involved in Cyt1Aa synergism with Bin in such larvae. In vivo assays showed that the mixture of Bin toxin, or its BinA subunit, with Cyt1Aa was effective to kill resistant larvae. However, no specific binding interaction between Cyt1Aa and the Bin toxin, or its subunits, was observed. The synergy between Cyt1Aa and Bin toxins is dependent on functional Cyt1Aa, as demonstrated by using the nontoxic Cyt1AaV122E mutant toxin affected in oligomerization and membrane insertion, which was unable to synergize Bin toxicity in resistant larvae. The synergism correlated with the internalization of Bin or BinA into anterior and medium midgut epithelial cells, which occurred only in larvae treated with wild-type Cyt1Aa toxin. This toxin is able to overcome failures in the binding step involving BinB receptor by allowing the internalization of Bin toxin, or its BinA subunit, into the midgut cells.IMPORTANCE One promising management strategy for mosquito control is the utilization of a mixture of L. sphaericus and B. thuringiensis subsp. israelensis insecticidal toxins. From this set, Bin and Cyt1Aa toxins synergize and display toxicity to resistant C. quinquefasciatus and to A. aegypti larvae, whose midgut cells lack Bin toxin receptors. Our data set provides evidence that functional Cyt1Aa is essential for internalization of Bin or its BinA subunit into such cells, but binding interaction between Bin and Cyt1Aa is not observed. Thus, this mechanism contrasts with that for the synergy between Cyt1Aa and the B. thuringiensis subsp. israelensis Cry toxins, where active Cyt1Aa is not necessary but a specific binding between Cry and Cyt1Aa is required. Our study established the initial molecular basis of the synergy between Bin and Cyt1Aa, and these findings enlarge our knowledge of their mode of action, which could help to develop improved strategies to cope with insect resistance.

A differential transcriptional profile by Culex quinquefasciatus larvae resistant to Lysinibacillus sphaericus IAB59 highlights genes and pathways associated with the resistance phenotype

Background

The study of the mechanisms by which larvae of the Culex quinquefasciatus mosquito survive exposure to the entomopathogen Lysinibacillus sphaericus has benefited substantially from the generation of laboratory-selected colonies resistant to this bacterium. One such colony, RIAB59, was selected after regular long-term exposure of larvae to the L. sphaericus IAB59 strain. This strain is characterized by its ability to produce the well known Binary (Bin) toxin, and the recently characterized Cry48Aa/Cry49Aa toxin, able to kill Bin-resistant larvae. Resistance to Bin is associated with the depletion of its receptor, Cqm1 α-glucosidase, from the larvae midgut. This study aimed to identify novel molecules and pathways associated with survival of the RIAB59 larvae and the resistance phenotype.

Methods

A transcriptomic approach and bioinformatic tools were used to compare the profiles derived from the midguts of larvae resistant and susceptible to L. sphaericus IAB59.

Results

The RNA-seq profiles identified 1355 differentially expressed genes (DEGs), with 673 down- and 682 upregulated transcripts. One of the most downregulated DEGs was cqm1, which validates the approach. Other strongly downregulated mRNAs encode the enzyme pantetheinase, apolipoprotein D, lipases, heat-shock proteins and a number of lesser known and hypothetical polypeptides. Among the upregulated DEGs, the top most encodes a peroxisomal enzyme involved in lipid metabolism, while others encode enzymes associated with juvenile hormone synthesis, ion channels, DNA binding proteins and defense polypeptides. Further analyses confirmed a strong downregulation of several enzymes involved in lipid catabolism while the assignment of DEGs into metabolic pathways highlighted the upregulation of those related to DNA synthesis and maintenance, confirmed by their clustering into related protein networks. Several other pathways were also identified with mixed profiles of down- and upregulated transcripts. Quantitative RT-PCR confirmed the changes in levels seen for selected mRNAs.

Conclusions

Our transcriptome-wide dataset revealed that the RIAB59 colony, found to be substantially more resistant to Bin than to the Cry48Aa/Cry49Aa toxin, developed a differential expression profile as well as metabolic features co-selected during the long-term adaptation to IAB59 and that are most likely linked to Bin resistance.

Electronic supplementary material

The online version of this article (10.1186/s13071-019-3661-y) contains supplementary material, which is available to authorized users.

Mechanisms of resistance to commercially relevant entomopathogenic bacteria

Bacteria represent the most commercially successful entomopathogenic microbial group, with most commercialized insecticides containing gram-positive bacteria in the Bacillaceae family. Resistance to entomopathogenic bacteria threatens sustainable agriculture, and information on the mechanisms and genes involved is vital to develop management practices aimed at reducing this risk. We provide an integrative summary on mechanisms responsible for resistance to commercialized entomopathogenic bacteria, including information on resistance to transgenic crops producing insecticidal proteins from Bacillus thuringiensis (Bt crops). The available experimental evidence identifies alterations in binding of insecticidal proteins to receptors in the host as the main mechanism for high levels of resistance to entomopathogenic bacteria.

Mosquito-larvicidal BinA toxin displays affinity for glycoconjugates: Proposal for BinA mediated cytotoxicity

Lysinibacillus sphaericus parasporal BinAB toxin displays mosquito larvicidal activity against Culex and Anopheles, but several Aedes species are refractory. Recently reported crystal structure of BinAB revealed the presence of N-terminal lectin-like domain in BinA. Hemagglutination and hemolytic activities were not observed for BinA in the present studies. We attempted to characterize carbohydrate specificity of BinA by high-throughput approaches using extrinsic fluorescence and thermofluor shift assay. A total of 34 saccharides (mono-, di- and polysaccharides, and glycoproteins) were used for initial high-throughput screening. The promising glycans were identified based on significant change in the fluorescence intensity. Surface plasmon resonance revealed differential binding of BinA with glycoproteins (fetuin, asialofetuin and thyroglobulin) and affinity for simple sugars, l-fucose and l-arabinose. In the limited carbohydrate competition assay, arabinose, fucose and fetuin inhibited BinA toxicity towards Culex larvae. This study for the first time provides direct evidence that BinA is competent to bind diverse and structurally different glycosylated proteins. This activity may be linked to its intracellular cytotoxicity, as protein N-glycosylation is thought to be critical for development and survival of insect larvae. The glycoproteins do not form stable complexes with BinA, however, as observed in the pull-down assay using affinity immobilized BinA and in native-PAGE analysis. As BinA displays only mild affinity with receptor polypeptide, we hypothesize that toxin-receptor specificity of BinA in Culex may be mediated by dual interaction of BinA with glycan core of GPI anchor and receptor polypeptide. The study shall be useful for refining strategies for improving larvicidal activity and for broadening target specificity of BinAB toxin.

Highly Effective Broad Spectrum Chimeric Larvicide That Targets Vector Mosquitoes Using a Lipophilic Protein

Two mosquitocidal bacteria, Bacillus thuringiensis subsp. israelensis (Bti) and Lysinibacillus sphaericus (Ls) are the active ingredients of commercial larvicides used widely to control vector mosquitoes. Bti’s efficacy is due to synergistic interactions among four proteins, Cry4Aa, Cry4Ba, Cry11Aa, and Cyt1Aa, whereas Ls’s activity is caused by Bin, a heterodimer consisting of BinA, the toxin, and BinB, a midgut-binding protein. Cyt1Aa is lipophilic and synergizes Bti Cry proteins by increasing midgut binding. We fused Bti’s Cyt1Aa to Ls’s BinA yielding a broad-spectrum chimeric protein highly mosquitocidal to important vector species including Anopheles gambiae, Culex quinquefasciatus, and Aedes aegypti, the latter an important Zika and Dengue virus vector insensitive to Ls Bin. Aside from its vector control potential, our bioassay data, in contrast to numerous other reports, provide strong evidence that BinA does not require conformational interactions with BinB or microvillar membrane lipids to bind to its intracellular target and kill mosquitoes.

Molecular analysis of Culex quinquefasciatus larvae responses to Lysinibacillus sphaericus Bin toxin

Lysinibacillus sphaericus produces the mosquito larvicidal binary toxin consisting of BinA and BinB, which are both required for toxicity against Culex and Anopheles larvae. The molecular mechanisms behind Bin toxin-induced damage remain unexplored. We used whole-genome microarray-based transcriptome analysis to better understand how Culex larvae respond to Bin toxin treatment at the molecular level. Our analyses of Culex quinquefasciatus larvae transcriptome changes at 6, 12, and 18 h after Bin toxin treatment revealed a wide range of transcript signatures, including genes linked to the cytoskeleton, metabolism, immunity, and cellular stress, with a greater number of down-regulated genes than up-regulated genes. Bin toxin appears to mainly repress the expression of genes involved in metabolism, the mitochondrial electron transport chain, and the protein transporter of the outer/inner mitochondrial membrane. The induced genes encode proteins linked to mitochondrial-mediated apoptosis and cellular detoxification including autophagic processes and lysosomal compartments. This study is, to our knowledge, the first microarray analysis of Bin toxin-induced transcriptional responses in Culex larvae, providing a basis for an in-depth understanding of the molecular nature of Bin toxin-induced damage.

The pesticidal Cry6Aa toxin from Bacillus thuringiensis is structurally similar to HlyE-family alpha pore-forming toxins

Background

The Cry6 family of proteins from Bacillus thuringiensis represents a group of powerful toxins with great potential for use in the control of coleopteran insects and of nematode parasites of importance to agriculture. These proteins are unrelated to other insecticidal toxins at the level of their primary sequences and the structure and function of these proteins has been poorly studied to date. This has inhibited our understanding of these toxins and their mode of action, along with our ability to manipulate the proteins to alter their activity to our advantage. To increase our understanding of their mode of action and to facilitate further development of these proteins we have determined the structure of Cry6Aa in protoxin and trypsin-activated forms and demonstrated a pore-forming mechanism of action.

Results

The two forms of the toxin were resolved to 2.7 Å and 2.0 Å respectively and showed very similar structures. Cry6Aa shows structural homology to a known class of pore-forming toxins including hemolysin E from Escherichia coli and two Bacillus cereus proteins: the hemolytic toxin HblB and the NheA component of the non-hemolytic toxin (pfam05791). Cry6Aa also shows atypical features compared to other members of this family, including internal repeat sequences and small loop regions within major alpha helices. Trypsin processing was found to result in the loss of some internal sequences while the C-terminal region remains disulfide-linked to the main core of the toxin. Based on the structural similarity of Cry6Aa to other toxins, the mechanism of action of the toxin was probed and its ability to form pores in vivo in Caenorhabditis elegans was demonstrated. A non-toxic mutant was also produced, consistent with the proposed pore-forming mode of action.

Conclusions

Cry6 proteins are members of the alpha helical pore-forming toxins – a structural class not previously recognized among the Cry toxins of B. thuringiensis and representing a new paradigm for nematocidal and insecticidal proteins. Elucidation of both the structure and the pore-forming mechanism of action of Cry6Aa now opens the way to more detailed analysis of toxin specificity and the development of new toxin variants with novel activities.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0295-9) contains supplementary material, which is available to authorized users.

Safety considerations derived from Cry34Ab1/Cry35Ab1 structure and function

Insecticidal proteins developed for in-plant protection against crop pests undergo extensive safety testing during the product development process. Safety considerations for insecticidal proteins expressed in crops follow recommended, science-based guidelines and specific studies are conducted on a case by case basis. Corn events expressing Bacillus thuringiensis (Bt) Cry34Ab1 and Cry35Ab1 were developed to protect maize from Diabrotica virgifera virgifera (western corn rootworm) feeding damage. The protein crystal structures of Cry34Ab1 and Cry35Ab1 are different from the more common three-domain Cry or Vip3 proteins expressed in insect resistant maize varieties. Cry34Ab1 is a single domain protein that folds into a beta sandwich structure that resembles membrane-active proteins, including several cytolysins, from a variety of natural sources. Cry35Ab1 has two domains, one domain with structural relatedness to sugar binding motifs and a second domain with an extended beta sheet structure that is clearly related to beta pore forming proteins, some of which are insecticidal, e.g. B. sphaericus BinA/BinB. In this review we discuss Cry34Ab1/Cry35Ab1 structure and function in the context of protein safety studies for insect resistant crops.

Anopheles gambiae Ag55 cell line as a model for Lysinibacillus sphaericus Bin toxin action

Binary toxin (Bin) produced by Lysinibacillus sphaericus is toxic to Culex and Anopheles mosquito larvae. It has been used world-wide for control of mosquitoes that vector disease. The Bin toxin interacts with the glucosidase receptor, Cpm1, in Culex and its orthologue, Agm3, in Anopheles mosquitoes. However, the exact mechanism of its mode of action is not clearly understood. It is essential to understand mode of action of Bin toxin to circumvent the resistance that develops over generations of exposure. A suitable model cell line will facilitate investigations of the molecular action of Bin toxin. Here we report Bin toxin activity on Ag55 cell line that has been derived from an actual target, Anopheles gambiae larvae. The Bin toxin, both in pro and active forms, kills the Ag55 cells within 24h. Bin toxin internalizes in Ag55 cells and also induces vacuolation as tracked by Lysotracker dye. The dose response studies showed that 1.5nM of Bin toxin is sufficient to induce vacuolation and Ag55 cell death. Presence of α-glucosidase gene (Agm3) expression in the Ag55 cells was also confirmed. Thus, Ag55 cells constitute an appropriate model system to decipher the mode of Bin action in mosquito larvae.

Interaction of Lysinibacillus sphaericus binary toxin with mosquito larval gut cells: Binding and internalization

The binary toxin produced by Lysinibacillus sphaericus is composed of BinA and BinB subunits. Together, but not separately, the two subunits are highly toxic to Culex quinquefasciatus larvae, but show no toxicity to Aedes aegypti. The molecular mechanism underlying intoxication has not been clearly elucidated. The present study compares the binding and the internalization of binary toxin into the midgut epithelial cells of susceptible C. quinquefasciatus mosquito larvae with those of Bin-refractory A. aegypti. The guts from larvae fed with fluorescently labeled toxin were dissected and analyzed using a confocal laser scanning microscope. When fed with a mixture of both components, co-localization of BinA and BinB was detected both on the cell surface and in the cytoplasm of Culex larval gut cells. However, administration of BinA alone resulted in localization only on the cell membrane, whereas BinB alone was detected both on the cell membrane and inside the cytoplasm. In contrast, when a mixture of both components, or each individual component, was fed to Aedes larvae, BinA and BinB were unable to reach the cytoplasm and were localized only on the cell membrane. These results are consistent with the suggestion that the internalization of BinA is essential for toxicity, and that BinB is required for this internalization into susceptible larval gut cells.