Expression of full-length and truncated forms of crystal protein genes from Bacillus thuringiensis subsp. kurstaki in a baculovirus and pathogenicity of the recombinant viruses

Enhancing the insecticidal potential of a baculovirus by overexpressing the mammalian β-galactosyl binding protein galectin-1

BACKGROUND

Bio-pesticide development is an important area of research in agriculture, in which viruses are an essential tool. Infection by entomological pathogenic viruses kills agricultural pests, and viral progenies are disseminated to infect more pests, eventually achieving long-term pest control in the field. Of the current virus-based pest control models, Autographa californica multiple nucleopolyhedrovirus (AcMNPV) is the most studied. AcMNPV belongs to the Baculoviridae family and can infect many lepidopterans. Although AcMNPV has been previously demonstrated to be a potential pest-control tool, its long virus infection cycle has made field applications challenging. To overcome this, we generated a recombinant baculovirus that can express mammalian galectin-1, which is a galactoside-binding protein that binds to the peritrophic matrix in the midgut of lepidopteran pests and induces perforation of the membrane.

RESULTS

Hosts infected with a recombinant virus that expressed mammalian galectin-1 exhibited reduced appetite and died sooner in both laboratory and small-scale field studies, suggesting that the overexpression of galectin-1 can more efficiently eliminate pest hosts. In addition to disrupting the integrity of the peritrophic matrix, the immune system of hosts infected with recombinant baculovirus carrying the galectin-1 gene was suppressed, making hosts more vulnerable to secondary infection.

CONCLUSION

Galectin-1 has been shown to affect immune responses in mammals, including humans, but to our knowledge, the effect of galectin-1 on insect immune systems had not been previously reported. Our results demonstrated that the pest-control potential of baculoviruses can be improved by using a recombinant baculovirus that overexpresses mammalian galectin-1 in hosts. © 2022 Society of Chemical Industry.

Cytosolic delivery of CDK4/6 inhibitor p16 protein using engineered protein crystals for cancer therapy

The tumor suppressor p16 protein is an endogenous CDK4/6 inhibitor. Inactivation of its encoding gene is found in nearly half of human cancers. Restoration of p16 function via adenovirus-based gene delivery has been shown to be effective in suppressing aberrant cell growth in many types of cancer, however, the potential risk of insertional mutagenesis in genomic DNA remains a major concern. Thus, there has been great interest in developing efficient strategies to directly deliver proteins into cells as an alternative that can avoid such safety concerns while achieving a comparable therapeutic effect. Nevertheless, intracellular delivery of protein therapeutics remains a challenge. Our group has recently developed a protein delivery platform based on an engineered Pos3Aa protein that forms sub-micrometer-sized crystals in Bacillus thuringiensis cells. In this report, we describe the further development of this platform (Pos3AaTM) via rationally designed site-directed mutagenesis, and its resultant potency for the delivery of cargo proteins into cells. Pos3AaTM-based fusion protein crystals are shown to exhibit improved release of their cargo proteins as demonstrated using a model mCherry protein. Importantly, this Pos3AaTM platform is able to mediate the efficient intracellular delivery of p16 protein with significant endosomal escape, resulting in p16-mediated inhibition of CDK4/6 kinase activity and Rb phosphorylation, and as a consequence, significant cell cycle arrest and cell growth inhibition. These results validate the ability of these improved Pos3AaTM crystals to mediate enhanced cytosolic protein delivery and highlight the potential of using protein therapeutics as selective CDK4/6 inhibitors for cancer therapy. STATEMENT OF SIGNIFICANCE: Cytosolic delivery of bioactive therapeutic proteins capable of eliciting therapeutic benefit remains a significant challenge. We have previously developed a protein delivery platform based on engineered Pos3Aa protein crystals with excellent cell-permeability and endosomal escape properties. In this report, we describe the rational design of an improved Pos3Aa triple mutant (Pos3AaTM) with enhanced cargo release. We demonstrate that Pos3AaTM-mCherry-p16 fusion crystals can efficiently deliver p16 protein, a CDK4/6 inhibitor frequently inactivated in human cancers, into p16-deficient UM-SCC-22A cells, where it promotes significant G1 cell cycle arrest and cell growth inhibition. These results highlight the ability of the Pos3AaTM platform to promote potent cytosolic delivery of protein therapeutics, and the efficacy of p16 protein delivery as an effective strategy for treating cancer.

Expression, delivery and function of insecticidal proteins expressed by recombinant baculoviruses

Since the development of methods for inserting and expressing genes in baculoviruses, a line of research has focused on developing recombinant baculoviruses that express insecticidal peptides and proteins. These recombinant viruses have been engineered with the goal of improving their pesticidal potential by shortening the time required for infection to kill or incapacitate insect pests and reducing the quantity of crop damage as a consequence. A wide variety of neurotoxic peptides, proteins that regulate insect physiology, degradative enzymes, and other potentially insecticidal proteins have been evaluated for their capacity to reduce the survival time of baculovirus-infected lepidopteran host larvae. Researchers have investigated the factors involved in the efficient expression and delivery of baculovirus-encoded insecticidal peptides and proteins, with much effort dedicated to identifying ideal promoters for driving transcription and signal peptides that mediate secretion of the expressed target protein. Other factors, particularly translational efficiency of transcripts derived from recombinant insecticidal genes and post-translational folding and processing of insecticidal proteins, remain relatively unexplored. The discovery of RNA interference as a gene-specific regulation mechanism offers a new approach for improvement of baculovirus biopesticidal efficacy through genetic modification.

A novel recombinant baculovirus overexpressing a Bacillus thuringiensis Cry1Ab toxin enhances insecticidal activity

Baculoviruses have been genetically modified to express foreign genes under powerful promoters in order to accelerate their speed of killing. In this study a truncated form of cry1Ab gene derived from Bacillus thuringinsis (Bt) subsp. aegypti isolate Bt7 was engineered into the genome of the baculovirus Autographa californica multiple nuclearpolyhedrosis wild type virus, in place of the polyhedrin gene by using homologous recombination in Spodoptera frugiperda (Sf) cells between a transfer vector carrying the Bt gene and the wild type virus linearized DNA. Recombinant wild type virus containing the cry1Ab gene was detected as blue occlusion-negative plaques in monolayers of Sf cells grown in the presence of X-Gal. In Sf cells infected with plaque-purified recombinant virus, the cry1Ab gene was expressed to yield a protein of approximately 82-kDa, as determined by immunoblot analysis. The toxicity of the recombinant virus expressing the insecticidal crystal protein (ICP) was compared to that of the wild-type virus. Infected-cell extract was toxic to cotton leaf worm Spodoptera littoralis second instar larvae and the estimated LC50 was 1.7 μg/ml for the recombinant virus compared with that of wild-type virus which was 10 μg/ml.

A recombinant Anticarsia gemmatalis MNPV harboring chiA and v-cath genes from Choristoneura fumiferana defective NPV induce host liquefaction and increased insecticidal activity

One of the interesting features of Anticarsia gemmatalis multiple nucleopolyhedrovirus isolate 2D (AgMNPV-2D) genome is the absence of chitinase (chiA) and cathepsin (v-cath) genes. This characteristic may be responsible for the lack of liquefaction and melanization in A. gemmatalis larvae killed by AgMNPV-2D infection. This study aimed to test the hypothesis that CHIA and V-CATH proteins from Choristonera fumiferana DEF multiple nucleopolyhedrovirus (CfDEFNPV) are able to liquefy and melanize the cuticle of A. gemmatalis larvae infected by a recombinant AgMNPV containing chiA and v-cath genes inserted in its genome. A fragment from the CfDefNPV genome containing chiA and v-cath genes was inserted into the genome of AgMNPV-2D. The recombinant virus (vAgp2100Cf.chiA/v-cath) was purified and used to infect insect cells and larvae. Transcripts of v-cath and chiA genes were detected along the infection of insect cells by qRT-PCR, from early to late phases of infection. The analysis of A. gemmatalis larvae killed by vAgp2100Cf.chiA/v-cath infection confirmed the hypothesis proposed. The vAgp2100Cf.chiA/v-cath showed higher insecticidal activity against third instar A. gemmatalis larvae when compared to AgMNPV-2D. The mean time to death was also lower for the vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D at 10 days post infection. Occlusion body production was higher in A. gemmatalis larvae infected with vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D. Enzyme assays showed higher chitinase and cysteine protease activities in insect cells and insects infected with vAgp2100Cf.chiA/v-cath when compared to AgMNPV-2D. The introduction of chiA and v-cath genes into the genome of AgMNPV improves its insecticidal activity against A. gemmatalis larvae and this recombinant virus could be used as an alternative to the wild type virus to control this important insect pest.

NeuroBactrus, a novel, highly effective, and environmentally friendly recombinant baculovirus insecticide

A novel recombinant baculovirus, NeuroBactrus, was constructed to develop an improved baculovirus insecticide with additional beneficial properties, such as a higher insecticidal activity and improved recovery, compared to wild-type baculovirus. For the construction of NeuroBactrus, the Bacillus thuringiensis crystal protein gene (here termed cry1-5) was introduced into the Autographa californica nucleopolyhedrovirus (AcMNPV) genome by fusion of the polyhedrin-cry1-5-polyhedrin genes under the control of the polyhedrin promoter. In the opposite direction, an insect-specific neurotoxin gene, AaIT, from Androctonus australis was introduced under the control of an early promoter from Cotesia plutellae bracovirus by fusion of a partial fragment of orf603. The polyhedrin-Cry1-5-polyhedrin fusion protein expressed by the NeuroBactrus was not only occluded into the polyhedra, but it was also activated by treatment with trypsin, resulting in an ∼65-kDa active toxin. In addition, quantitative PCR revealed that the neurotoxin was expressed from the early phase of infection. NeuroBactrus showed a high level of insecticidal activity against Plutella xylostella larvae and a significant reduction in the median lethal time against Spodoptera exigua larvae compared to those of wild-type AcMNPV. Rerecombinant mutants derived from NeuroBactrus in which AaIT and/or cry1-5 were deleted were generated by serial passages in vitro. Expression of the foreign proteins (B. thuringiensis toxin and AaIT) was continuously reduced during the serial passage of the NeuroBactrus. Moreover, polyhedra collected from S. exigua larvae infected with the serially passaged NeuroBactrus showed insecticidal activity similar to that of wild-type AcMNPV. These results suggested that NeuroBactrus could be recovered to wild-type AcMNPV through serial passaging.

Virulence and transmission of infectious diseases in humans and insects: evolutionary and demographic patterns

The rate and degree of proliferation of disease organisms determine their pathogenicity and the efficiency of their transmission. These traits dictate the impact of a disease on individuals as well as populations. Virulence and transmission of diseases are molded by evolutionary forces - pathogens and hosts are each selected to reproduce and persist. New ideas about the evolution of human diseases also apply to the relationships between insects and their diseases. Evidence for close associations between insects and pathogens include the viral suppression of insect molting hormones and the occurrence of latent virus that can be activated by foreign viruses.

Insecticidal activity of two proteases against Spodoptera frugiperda larvae infected with recombinant baculoviruses

Background

Baculovirus comprise the largest group of insect viruses most studied worldwide, mainly because they efficiently kill agricutural insect pests. In this study, two recombinant baculoviruses containing the ScathL gene from Sarcophaga peregrina (vSynScathL), and the Keratinase gene from the fungus Aspergillus fumigatus (vSynKerat), were constructed. and their insecticidal properties analysed against Spodoptera frugiperda larvae.

Results

Bioassays of third-instar and neonate S. frugiperda larvae with vSynScathL and vSynKerat showed a decrease in the time needed to kill the infected insects when compared to the wild type virus. We have also shown that both recombinants were able to increase phenoloxidase activity in the hemolymph of S. frugiperda larvae. The expression of proteases in infected larvae resulted in destruction of internal tissues late in infection, which could be the reason for the increased viral speed of kill.

Conclusions

Baculoviruses and their recombinant forms constitute viable alternatives to chemical insecticides. Recombinant baculoviruses containing protease genes can be added to the list of engineered baculoviruses with great potential to be used in integrated pest management programs.

Delivery methods for peptide and protein toxins in insect control

Since the introduction of DDT in the 1940s, arthropod pest control has relied heavily upon chemical insecticides. However, the development of insect resistance, an increased awareness of the real and perceived environmental and health impacts of these chemicals, and the need for systems with a smaller environmental footprint has stimulated the search for new insecticidal compounds, novel molecular targets, and alternative control methods. In recent decades a variety of biocontrol methods employing peptidic or proteinaceous insect-specific toxins derived from microbes, plants and animals have been examined in the laboratory and field with varying results. Among the many interdependent factors involved with the production of a cost-effective pesticide--production expense, kill efficiency, environmental persistence, pest-specificity, pest resistance-development, public perception and ease of delivery--sprayable biopesticides have not yet found equal competitive footing with chemical counterparts. However, while protein/peptide-based biopesticides continue to have limitations, advances in the technology, particularly of genetically modified organisms as biopesticidal delivery systems, has continually progressed. This review highlights the varieties of delivery methods currently practiced, examining the strengths and weaknesses of each method.

Genetically modified baculoviruses: a historical overview and future outlook

The concept of using genetic engineering to improve the natural insecticidal activity of baculoviruses emerged during the 1980s. Both academic and industrial laboratories have since invested a great deal of effort to generate genetically modified (GM) or recombinant baculoviruses with dramatically improved speeds of kill. Optimal production methodologies and formulations have also been developed, and the safety and ecology of the recombinant baculoviruses have been thoroughly investigated. Unfortunately, the initial excitement that was generated by these technologies was tempered when industry made a critical decision to not complete the registration process of GM baculoviruses for pest insect control. In this chapter, we summarize the developments in the field from a historical perspective and provide our opinions as to the current status and future potential of the technology. We will argue that GM baculoviruses are valuable and viable tools for pest insect control both alone and in combination with wild-type viruses. We believe that these highly effective biopesticides still have a bright future in modern agriculture as public awareness and acceptance of GM organisms, including GM baculoviruses, increases.

A recombinant truncated Cry1Ca protein is toxic to lepidopteran insects and forms large cuboidal crystals in insect cells

A truncated version of the cry1Ca gene from Bacillus thuringiensis was introduced into the genome of Autographa californica multiple nucleopolyhedrovirus (AcMNPV) under the control of two promoters. A recombinant virus (vSyncry1c) was isolated and used to infect insect cells in culture and insect larvae. Structural and ultrastructural analysis of insects infected with vSyncry1C showed the formation of large cuboidal crystals inside the cytoplasm of insect cells in culture and in insect cadavers late in infection. Infected insect cell extracts were analyzed by SDS-PAGE and Western blot and showed the presence of a 65-kDa polypeptide probably corresponding to the protease processed form of the toxin. Bioassays using purified recombinant toxin crystals showed a CL(50) of 19.49 ng/ml for 2(nd) instar A. gemmatalis larvae and 114.1 ng/ml for S. frugiperda.

An improved baculovirus insecticide producing occlusion bodies that contain Bacillus thuringiensis insect toxin

Baculovirus occlusion bodies, large proteinaceous structures which contain virions, have recently been engineered to incorporate foreign proteins. The major constituent protein of occlusion bodies from the baculovirus Autographa californica nucleopolyhedrovirus is polyhedrin, and assembly of recombinant occlusion bodies which incorporate a foreign protein depends on an interaction between native polyhedrin and a polyhedrin-foreign protein fusion. This technology has now been applied to the generation of a recombinant baculovirus (ColorBtrus) that produces occlusion bodies incorporating the Bacillus thuringiensis (Bt) insecticidal Cry1Ac toxin protein. ColorBtrus coexpresses native polyhedrin and a fusion protein in which polyhedrin is fused to the Bt toxin, which is in turn fused to green fluorescent protein (GFP). Analysis of ColorBtrus occlusion bodies confirmed that they include both Bt toxin and GFP, yet still incorporate virions. Bioassay of ColorBtrus demonstrated that its speed of action and pathogenicity are strikingly enhanced compared to wild-type virus. ColorBtrus represents a novel, powerful biological insecticide that combines positive attributes of both Bt toxin and baculovirus based systems.

Dias JMC, Ribeiro BM. Characterization ofBacillus thuringiensissubsp.kurstakistrain S93 effective against the fall armywormSpodoptera frugiperda)

A Brazilian strain of Bacillus thuringiensis subsp. kurstaki, designated S93, was analyzed regarding its cry gene and protein contents and activity against the fall armyworm (Spodoptera frugiperda, Smith 1797). Bioassays using lyophilized powders of S93 or HD-1 and third instar larvae of S. frugiperda showed a 12.3-fold lower LC50for the S93 strain when compared with the standard HD-1 strain. The spore-crystal mixture, analyzed by SDS-PAGE, showed two major polypeptides of 130 and 65 kDa, corresponding to Cry1 and Cry2 toxins, respectively. Western blot analysis showed that these proteins were immunologically related to the Cry1A protein from B. thuringiensis subsp. kurstaki HD-73. The polymerase chain reaction technique (PCR) using total DNA from the S93 strain and specific primers showed the presence of cry1Aa, cry1Ab, and cry1Ac genes, and a cry1A-type gene was localized in a plasmid of about 44 MDa. A cry1Ab gene was isolated from a S93 plasmid DNA library and completely sequenced. Computer analysis showed that the gene sequence (GenBank acession number AF059670) is identical to cry1Ab1 and has 91.6 and 85.9% identity with cry1Aa1 and cry1Ac1 genes, respectively. The deduced amino-acid sequence showed a high degree of similarity with the amino-acid sequences of the Cry1Ab1 (100%), Cry1Aa1 (93.8%), and Cry1Ac1 (90.6%) proteins.Key words: Bacillus thuringiensis, Spodoptera frugiperda, biological control, crystal protein, cry genes.

Assessment of the application of baculoviruses for control of Lepidoptera

Baculoviruses, among other insect viruses, are regarded as safe and selective bioinsecticides, restricted to invertebrates. They have been used worldwide against many insect pests, mainly Lepidoptera. Their application as microbial pesticides, however, has not met their potential to control pests in crops, forests, and pastures, with the exception of the nuclear polyhedrosis virus of the soybean caterpillar (Anticarsia gemmatalis), which is used on approximately 1 million ha annually in Brazil. Problems that have limited expansion of baculovirus use include narrow host range, slow killing speed, technical and economical difficulties for in vitro commercial production, timing of application based on frequent host population monitoring, variability of field efficacy due to climatic conditions, and farmers' attitudes toward pest control, which have been based on application of fast-killing chemical insecticides. Farmer education regarding use of biological insecticides and their characteristics is considered one of the major actions necessary for increased use of baculoviruses. Strategies to counteract some of the limitations of baculoviruses, especially their slow killing activity, have been investigated and are promising. These include the use of chemical or biological substances added to virus formulations and genetic engineering of the viruses themselves to express insect toxins or hormones. Such strategies can enhance viral activity and increase speed of kill as well as reduce larval feeding activity. The use of baculoviruses against Lepidoptera is reviewed, with the utilization of the nuclear polyhedrosis virus of A. gemmatalis in Brazil serving as a case-study.

Construction of occluded recombinant baculoviruses containing the full-length cry1Ab and cry1Ac genes from Bacillus thuringiensis

The administration of baculoviruses to insects for bioassay purposes is carried out, in most cases, by contamination of food surfaces with a known amount of occlusion bodies (OBs). Sinceper os infection is the natural route of infection, occluded recombinant viruses containing crystal protein genes (cry1Ab and cry1Ac) from Bacillus thuringiensis were constructed for comparison with the baculovirus prototype Autographa californica nucleopolyhedrovirus (AcNPV). The transfer vector pAcUW2B was used for construction of occluded recombinant viruses. The transfer vector containing the crystal protein genes was cotransfected with linearized DNA from a non-occluded recombinant virus. The isolation of recombinant viruses was greatly facilitated by the reduction of background "wild type" virus and the increased proportion of recombinant viruses. Since the recombinant viruses containing full-length and truncated forms of the crystal protein genes did not seem to improve the pathogenicity of the recombinant viruses when compared with the wild type AcNPV, and in order to compare expression levels of the full-length crystal proteins produced by non-occluded and occluded recombinant viruses the full-length cry1Ab and cry1Ac genes were chosen for construction of occluded recombinant viruses. The recombinant viruses containing full-length and truncated forms of the crystal protein genes did not seem to improve its pathogenicity but the size of the larvae infected with the recombinant viruses was significantly smaller than that of larvae infected with the wild type virus.