Genetically modified baculoviruses: a historical overview and future outlook

Hemolin increases the immune response of a caterpillar to NPV infection

Hemolin, a member of the immunoglobulin superfamily, plays a crucial role in the immune responses of insects against pathogens. However, the innate immune response of Hemolin to baculovirus infection varies among different insects, and the antiviral effects of Hemolin in Hyphantria cunea (HcHemolin) remain poorly understood. Our results showed that HcHemolin was expressed throughout all developmental stages, with higher expressions observed during pupal and adult stages of H. cunea. Additionally, HcHemolin was expressed in reproductive and digestive organs. The expression levels of the HcHemolin were induced significantly following H. cunea nucleopolyhedrovirus (HcNPV) infection. The susceptibility of H. cunea larvae to HcNPV decreased upon silencing of HcHemolin, resulting in a 40% reduction in median lifespan compared to the control group. The relative growth rate (RGR), the relative efficiency of consumption rate (RCR), the efficiency of the conversion of ingested food (ECI), and efficiency of the conversion of digested food (ECD) of silenced H. cunea larvae were significantly lower than those of the control group. Immune challenge assays showed that the median lifespan of treated H. cunea larvae was two-fold longer than the control group after HcNPV and HcHemolin protein co-injection. Therefore, we propose that HcHemolin plays a crucial role in regulating the growth, development, and food utilization of H. cunea, as well as in the antiviral immune response against HcNPV. These findings provide implications for the development of targeted nucleic acid pesticides and novel strategies for pollution-free biological control synergists for HcNPV.

Enhanced virulence of genetically engineered Autographa californica nucleopolyhedrovirus owing to accelerated viral DNA replication aided by inserted ascovirus genes

Genetic engineering technology is an ideal method to improve insecticidal efficiency by combining the advantages of different pathogenic microorganisms. Thus, six ascovirus genes were introduced into the genomic DNA of Autographa californica nucleopolyhedrovirus (AcMNPV) to possibly transfer the intrinsically valuable insecticidal properties from ascovirus to baculovirus. The viral budded virus (BV) production and viral DNA replication ability of AcMNPV-111 and AcMNPV-165 were significantly stronger than that of AcMNPV-Egfp (used as the wild-type virus in this study), whereas AcMNPV-33 had reduced ones. AcMNPV-111 and AcMNPV-165 also exhibited excellent insecticidal efficiency in the in vivo bioassays: AcMNPV-111 showed a 24.1% decrease in the LT₅₀ value and AcMNPV-165 exhibited a 56.3% decrease in the LD₅₀ value compared with AcMNPV-Egfp against the 3rd instar of Spodoptera exigua larvae, respectively. Furthermore, the size of the occlusion bodies (OBs) of AcMNPV-33, AcMNPV-111, and AcMNPV-165 were significantly increased compared to that of AcMNPV-Egfp. AcMNPV-111 and AcMNPV-165 had stable virulence against the 2nd to 4th instars tested larvae and higher OB yield than AcMNPV-Egfp in the 3rd and 4th instar larvae. Correlation and regression analyses indicated that it is better to use 5 OBs/larva virus to infect the 2nd instar larvae to produce AcMNPV-111 and 50 OBs/larva virus to infect the 3rd instar larvae to produce AcMNPV-165. The results of this study obtained recombinant viruses with enhanced virulence and exhibited a diversity of ascovirus gene function based on the baculovirus platform, which provided a novel strategy for the improvement of baculovirus as a biological insecticide.

Improvement of Akanthomyces lecanii resistance to tebuconazole through UV-C radiation and selective pressure on microbial evolution and growth arenas

Tebuconazole (TEB) is a fungicide widely used in agriculture; however, its constant application has increased the emergence of resistant plant pathogenic fungal strains and reduced the effectiveness of fungi as biological control agents; for instance, the entomopathogenic and hyperparasitic fungus Akanthomyces lecanii, suitable for simultaneous biological control of insect pest and plant pathogenic fungi, is highly sensitive to fungicides. We carried out the induction of resistance to TEB in two wild type strains of A. lecanii by UV radiation and selective pressure in increasing fungicide gradients using a modified Microbial Evolution and Growth Arena (MEGA), to produce A. lecanii strains that can be used as biological control agent in the presence of tebuconazole. Nine UV-induced and three naturally adapted A. lecanii strains were resistant to TEB at the agriculturally recommended dose, and three irradiated strains were resistant to TEB concentration ten times higher; moreover, growth, sporulation rates, production of hydrolytic enzymes, and virulence against the hemipteran Coccus viridis, a major pest of coffee crops, were not affected in the TEB-resistant strains. These A. lecanii TEB-resistant strains would have a greater opportunity to develop and to establish themselves in fields where the fungicide is present and can be used in a combined biological-chemical strategy to improve insect and plant pathogenic fungal control in agriculture. Also, the selective pressure through modified MEGA plate methodology can be used for the adaptation of entomopathogenic filamentous fungi to withstand other chemical or abiotic stresses that limits its effectiveness for pest control.

Establishment of a Novel Baculovirus–Silkworm Expression System

The baculovirus vector expression system is a well-established tool for foreign protein production and gene delivery. In this study, we constructed a recombinant baculovirus vector system. The UAS promotor region and Bombyx mori nucleopolyhedrovirus (BmNPV) polyhedrin coding region were ligated into a pFastBac Dual vector to obtain a BmBac-UPS recombinant bacmid. The recombinant bacmid BmBac-Gal4 was generated by the same strategy which has a Gal4 coding region controlled by the IE2 promoter. BmBac-UPS and BmBac-IGal4 were co-infected into silkworm BmN cells to confirm the ability of the UAS/Gal4 system to form polyhedrons in B. mori cells. Furthermore, the recombinant viruses were tested for infection efficiency and the ability to generate polyhedra in transgenic B. mori cell line BmE. The results showed that recombinant viruses have the ability to form polyhedrons and gain raised pathogenicity when orally infected B. mori larvae and are applied as the preferred tool for foreign gene delivery and expression

Recombinant Baculovirus: A Flexible Drug Screening Platform for Chikungunya Virus

Chikungunya virus (CHIKV) is a mosquito-transmitted infectious agent that causes an endemic or epidemic outbreak(s) of Chikungunya fever that is reported in almost all countries. This virus is an intense global threat, due to its high rate of contagion and the lack of effective remedies. In this study, we developed two baculovirus expression vector system (BEVS)-based approaches for the screening of anti-CHIKV drugs in Spodoptera frugiperda insect (Sf21) cells and U-2OS cells. First, structural protein of CHIKV was co-expressed through BEVS and thereby induced cell fusion in Sf21 cells. We used an internal ribosome entry site (IRES) to co-express the green fluorescent protein (EGFP) for identifying these fusion events. The EGFP-positive Sf21 cells fused with each other and with uninfected cells to form syncytia. We identified that ursolic acid has potential anti-CHIKV activity in vitro, by using this approach. Second, BacMam virus-based gene delivery has been successfully applied for the transient expression of non-structural proteins with a subgenomic promoter-EGFP (SP-EGFP) cassette in U-2OS cells to act as an in vitro CHIKV replicon system. Our BacMam-based screening system has identified that the potential effects of baicalin and baicalein phytocompounds can inhibit the replicon activity of CHIKV in U-2OS cells. In conclusion, our results suggested that BEVS can be a potential tool for screening drugs against CHIKV.

Identification of Multiple Replication Stages and Origins in the Nucleopolyhedrovirus of Anticarsia gemmatalis

To understand the mechanism of replication used by baculoviruses, it is essential to describe all the factors involved, including virus and host proteins and the sequences where DNA synthesis starts. A lot of work on this topic has been done, but there is still confusion in defining what sequence/s act in such functions, and the mechanism of replication is not very well understood. In this work, we performed an AgMNPV replication kinetics into the susceptible UFL-Ag-286 cells to estimate viral genome synthesis rates. We found that the viral DNA exponentially increases in two different phases that are temporally separated by an interval of 5 h, probably suggesting the occurrence of two different mechanisms of replication. Then, we prepared a plasmid library containing virus fragments (0.5-2 kbp), which were transfected and infected with AgMNPV in UFL-Ag-286 cells. We identified 12 virus fragments which acted as origins of replication (ORI). Those fragments are in close proximity to core genes. This association to the core genome would ensure vertical transmission of ORIs. We also predict the presence of common structures on those fragments that probably recruit the replication machinery, a structure also present in previously reported ORIs in baculoviruses.

Field Studies on the Horizontal Transmission Potential by Voluntary and Involuntary Carriers of Helicoverpa armigera nucleopolyhedrovirus (Baculoviridae)

Horizontal transmission of Helicoverpa armigera nucleopolyhedrovirus (HearNPV) has been found to occur through several pathways involving abiotic factors such as soil, wind, and rain, and biotic factors such as predators, parasitoids, and infected hosts. Previous studies examining horizontal transmission through certain biological carriers speculated they were likely not significant in increasing infection rates, however; these studies only focused on a relatively small number of arthropods present within a field setting. This study was conducted to evaluate the horizontal transmission potential of HearNPV by all potential biological carriers when applied as a foliar bioinsecticide or as virus-infected, nonmotile Helicoverpa zea (Boddie) (Lepidoptera: Noctuidae) larvae in a soybean field. Soybean plots were either sprayed with HearNPV or infested with late-stage HearNPV-infected larvae, and sample zones were sampled 3, 7, 10, 14, 17, and 21 days after the infestation, and analyzed for viral presence using PCR. We then identified HearNPV carriers through contamination from the application (involuntary) or through contact with a HearNPV-infected larva (voluntary). Both were confirmed through PCR analysis. Regardless of application technique, on average, HearNPV was capable of disseminating up to 61.0 m in 3 d after inoculation and was found within the sampled canopy 13-21 d after inoculation. Several arthropods were identified as novel carriers of HearNPV. Results from this study indicate that many novel HearNPV carriers are likely important in disseminating HearNPV.

Expressing Double-Stranded RNAs of Insect Hormone-Related Genes Enhances Baculovirus Insecticidal Activity

Baculoviruses have already been used for insect pest control, but the slow killing speed limits their further promotion and application. Here we provide a strategy for improving baculovirus insecticidal activity using Helicoverpa armigera nucleopolyhedrovirus (HearNPV) to express double-stranded RNAs (dsRNAs) targeting cotton bollworm (Helicoverpa armigera) juvenile hormone (JH)-related genes. Droplet-feeding bioassays show that the 50% lethal concentration (LC50) values of recombinant baculoviruses expressing the dsRNA of JH acid methyl transferase gene (HaJHAMT) and the JH acid binding protein gene (HaJHBP) were 1.24 × 10⁴ polyhedral inclusion bodies (PIB)/mL and 2.26 × 10⁴ PIB/mL, respectively. Both were much lower than the control value (8.12 × 10⁴ PIB/mL). Meanwhile, the LT50 of recombinant baculovirus expressing dsRNA of HaJHBP was only 54.2% of the control value, which means that larval death was accelerated. Furthermore, the mRNA level of target genes was reduced in recombinant baculovirus-treated cotton bollworm larvae. Transcription of several key genes involved in hormone signaling pathways-for example, ecdysone receptor gene (HaEcR)-was also altered. This study establishes a new strategy for pest management by interfering with insect hormone-related gene expression via baculoviruses, and the engineered baculoviruses have great potential application in cotton production.

Lethal and Sublethal Effects of Three Microbial Biocontrol Agents on Spodoptera litura and Its Natural Predator Rhynocoris kumarii

Entomopathogenic microbes such as Spodoptera litura nucleopolyhedrovirus (SpltNPV), Metarhizium anisopliae, and Pseudomonas fluorescens are biological agents used for the control of multiple arthropod pests. The objective of this study was to assess their effects on the biological parameters of Spodoptera litura (Lepidoptera: Noctuidae) larvae, and its natural reduviid predator Rhynocoris kumarii (Hemiptera: Reduviidae) under laboratory conditions. Results suggested that P. fluorescens reduced the food consumption index, relative growth rate, approximate digestibility, the efficiency of conversion of ingested food, and the efficiency of conversion of digested food of S. litura third instar larvae compared to prey infected with M. anisopliae and SpltNPV. Both SpltNPV and M. anisopliae caused similar mortality of S. litura life stages after 96 h of observation. To observe the effect of an infected prey diet on predator behavior, infected S. litura larvae were offered to the third, fourth, and fifth instar nymphs of R. kumarii, and their prey handling time, predation rate (number/day/predator), developmental period, and the survival rate was recorded. When the life stages of R. kumarii were offered entomopathogen-infected S. litura larvae, their predation rate was comparable to or higher than the untreated control. The juvenile predator, after feeding on P. fluorescens-infected S. litura larvae, had a significantly longer developmental period (2–4 days) compared to those fed on larvae infected with other microbial control agents. However, feeding on P. fluorescens alone did not affect the predator nymphal survival rate or the adult sex ratio. Although three entomopathogens had some degree of effect on the biological parameters of R. kumarii, the outcome of this study suggests that integration of reduviids with the tested entomopathogens are a compatible and potentially effective strategy for the management of S. litura populations. However promising, this combined strategy needs to be tested under field conditions to confirm the laboratory findings.

Recombinant Helicoverpa armigera nucleopolyhedrovirus with arthropod-specific neurotoxin gene RjAa17f from Rhopalurus junceus enhances the virulence against the host larvae

A recombinant Helicoverpa armigera nucleopolyhedrovirus (HearNPV) expressing the insect-selective neurotoxin (RjAa17f) from Cuban scorpion Rhopalurus junceus was constructed by replacing the UDP-glucosyltransferase gene (egt) using λ-red homologous recombination system. Another egt deleted control HearNPV was constructed in a similar way by inserting egfp gene into the egt locus. One-step viral growth curve and viral DNA replication curve analysis confirmed that the recombination did not affect the viral growth and DNA replication in host cells. There is no discernable difference in occlusion-body morphogenesis between RjAa17f-HearNPV, Egfp-HearNPV and HZ8-HearNPV, which was confirmed by transmission electron microscopy analysis. However, the insecticidal activity of RjAa17f-HearNPV is enhanced against the third instar H. armigera larvae according to the bioassay on virulence comparison. There is a dramatic reduction (56.9%) in median lethal dose (LD₅₀ ) and also a reduction (13.4%) in median survival time (ST₅₀ ) for the recombinant RjAa17f-HearNPV compared to the HZ8-HearNPV, but only a 27.5% reduction in LD₅₀ and 10.1% reduction in ST₅₀ value when Egfp-HearNPV is compared with HZ8-HearNPV. The daily diet consumption analysis showed that the RjAa17f-HearNPV was able to inhibit the infected larvae feeding compared with the egt minus HearNPV. These results demonstrated that this novel recombinant RjAa17f-HearNPV could improve the insecticidal effect against its host insects and RjAa17f could be a considerable candidate for other recombinant baculovirus constructions.

Introduction to the Use of Baculoviruses as Biological Insecticides

Baculoviruses are widely used both as protein expression vectors and as insect pest control agents. This section provides an overview of the baculovirus life cycle and use of baculoviruses as insecticidal agents. This chapter includes discussion of the pros and cons for use of baculoviruses as insecticides, and progress made in genetic enhancement of baculoviruses for improved insecticidal efficacy. These viruses are used extensively for control of insect pests in a diverse range of agricultural and forest habitats.

The chitinase C gene PsChiC from Pseudomonas sp. and its synergistic effects on larvicidal activity

Pseudomonas sp. strain TXG6-1, a chitinolytic gram-negative bacterium, was isolated from a vegetable field in Taixing city, Jiangsu Province, China. In this study, a Pseudomonas chitinase C gene (PsChiC) was isolated from the chromosomal DNA of this bacterium using a pair of specific primers. The PsChiC gene consisted of an open reading frame of 1443 nucleotides and encoded 480 amino acid residues with a calculated molecular mass of 51.66 kDa. The deduced PsChiC amino acid sequence lacked a signal sequence and consisted of a glycoside hydrolase family 18 catalytic domain responsible for chitinase activity, a fibronectin type III-like domain (FLD) and a C-terminal chitin-binding domain (ChBD). The amino acid sequence of PsChiCshowed high sequence homology (> 95%) with chitinase C from Serratia marcescens. SDS-PAGE showed that the molecular mass of chitinase PsChiC was 52 kDa. Chitinase assays revealed that the chitobiosidase and endochitinase activities of PsChiCwere 51.6- and 84.1-fold higher than those of pET30a, respectively. Although PsChiC showed little insecticidal activity towards Spodoptera litura larvae, an insecticidal assay indicated that PsChiC increased the insecticidal toxicity of SpltNPV by 1.78-fold at 192 h and hastened death. These results suggest that PsChiC from Pseudomonas sp. could be useful in improving the pathogenicity of baculoviruses.

The unfulfilled promises of scorpion insectotoxins

Since the description and biochemical characterization of the first insect-specific neurotoxins from scorpion venoms, almost all contributions have highlighted their potential application as leads for the development of potent bioinsecticides. Their practical use, however, has been hindered by different factors, some of which are intrinsically related to the toxins and other external determinants. Recent developments in the understanding of the action mechanisms of the scorpion insectotoxins and their bioactive surfaces, coupled with the exploration of novel bioinsecticide delivery systems have renewed the expectations that the scorpion insectotoxins could find their way into commercial applications in agriculture, as part of integrated pest control strategies. Herein, we review the current arsenal of available scorpion neurotoxins with a degree of specificity for insects, the progress made with alternative delivery methods, and the drawbacks that still preclude their practical use.

Regulation analysis of AcMNPV-mediated expression of a Chinese scorpion neurotoxin under the IE1, P10 and PH promoter in vivo and its use as a potential bio-insecticide

OBJECTIVE

To analyze the regulation mechanism of AcMNPV (Autographa californica multicapsid nucleopolyhedrovirus)-mediated expression of BmK IT under IE1, P10 and PH promoters in the larva of Heliothis armigera..

RESULTS

The transcription level of BmK IT gene in midgut and epidermal tissue was analyzed by quantitative PCR. The start time of transcription of recombinant BmK IT gene was early under the regulation of IE promoter, whereas transcription of BmK IT was high under the regulation of P10 promoter in the midgut tissue of infected larvae. TdT-UTP nick-end labeling (TUNEL) assay showed the degree of apoptotic cell death in the midgut tissue of AcMNPV-BmK IT-transfected insect larvae was higher than that in the AcMNPV treatment group at 8 h post-infection. The time-effect relationship between the insect's humoral immunity and regulation of promoters was confirmed in the phenoloxidase activity assay.

CONCLUSION

The anti-insect mechanism and regulation of different promoters in AcMNPV-BmK IT at molecular and cellular levels provide an experimental basis for the development of recombinant baculovirus biopesticides.

Baculovirus insecticides in Latin America: historical overview, current status and future perspectives

Baculoviruses are known to regulate many insect populations in nature. Their host-specificity is very high, usually restricted to a single or a few closely related insect species. They are amongst the safest pesticides, with no or negligible effects on non-target organisms, including beneficial insects, vertebrates and plants. Baculovirus-based pesticides are compatible with integrated pest management strategies and the expansion of their application will significantly reduce the risks associated with the use of synthetic chemical insecticides. Several successful baculovirus-based pest control programs have taken place in Latin American countries. Sustainable agriculture (a trend promoted by state authorities in most Latin American countries) will benefit from the wider use of registered viral pesticides and new viral products that are in the process of registration and others in the applied research pipeline. The success of baculovirus-based control programs depends upon collaborative efforts among government and research institutions, growers associations, and private companies, which realize the importance of using strategies that protect human health and the environment at large. Initiatives to develop new regulations that promote the use of this type of ecological alternatives tailored to different local conditions and farming systems are underway.

Improvement in the UV resistance of baculoviruses by displaying nano-zinc oxide-binding peptides on the surfaces of their occlusion bodies

The sensitivity of baculoviruses to UV radiation severely limits their large-scale application as biological insecticides. The polyhedron envelope of a baculovirus, which is composed of carbohydrate and polyhedron envelope protein (PEP), is a significant structure for the stability and persistence of occlusion bodies (OBs) under environmental conditions. The results of this study revealed that the rough pitted surface phenotype of a pep-null Autographa californica multiple nucleopolyhedrovirus (AcMNPV) could not be rescued by any of its homologues, such as Helicoverpa armigera nucleopolyhedrovirus pep or Cydia pomonella granulovirus putative peps. In contrast, the N-terminal and middle flexible region (NM region, 1-167 aa) of AcMNPV PEP were able to form an intact OB envelope. Furthermore, this region was capable of carrying eGFP to the surfaces of the OBs. To improve the UV resistance of AcMNPV OBs, two peptides capable of specifically binding to nano-ZnO were separately fused to the NM region of PEP. Under laboratory conditions, infectivity of the recombinant viruses binding to nano-ZnO particles was about ninefold higher than that without the nano-ZnO particles after UV-B irradiation. Pot experiments revealed that the half-life of the recombinant baculovirus binding nano-ZnO particles was 3.3 ± 0.15 days, which was significantly longer than that of the control virus (0.49 ± 0.06 days). These results therefore represent a new approach for the protection the baculoviral insecticides against UV irradiation in the field.

Field efficacy and transmission of fast- and slow-killing nucleopolyhedroviruses that are infectious to Adoxophyes honmai (Lepidoptera: Tortricidae)

The smaller tea tortrix, Adoxophyes honmai (Lepidoptera: Tortricidae), is an economically important pest of tea in Japan. Previous work showed that a fast-killing nucleopolyhedrovirus (NPV) isolated from A. orana (AdorNPV) and a slow-killing NPV isolated from A. honmai (AdhoNPV) are both infectious to A. honmai larvae. Field application of these different NPVs was conducted against an A. honmai larval population in tea plants, and the control efficacy and transmission rate of the two NPVs were compared. The slow-killing AdhoNPV showed lower field efficacy, in terms of preventing damage caused by A. honmai larvae against the tea plants, than the fast-killing AdorNPV. However, AdhoNPV had a significantly higher horizontal transmission rate than AdorNPV. These results show that AdorNPV is suitable as an inundative agent, while AdhoNPV is an appropriate inoculative agent.

Single-stranded DNA fragments of insect-specific nuclear polyhedrosis virus act as selective DNA insecticides for gypsy moth control

This paper focuses on the DNA insecticides as a novel preparation against gypsy moth (Lymantria dispar) based on DNA fragments of the anti-apoptotic gene of its nuclear polyhedrosis virus. It was found that the external application of a solution with two single-stranded DNA fragments from BIR and RING domains of LdMNPV (L.dispar multicapsid nuclear polyhedrosis virus) IAP-3 (inhibitor of apoptosis) gene induces a significantly higher mortality of gypsy moth caterpillars in comparison with the application of the control solutions. This effect does not depend on the infection of caterpillars with LdMNPV. The results also show that DNA insecticides based on LdMNPV IAP-3 gene fragments can be selective in action, and at least are not harmful to tobacco hornworm (Manduca sexta) and black cutworm (Agrotis ipsilon). Part of the gypsy moth genome cloned with the fragments of BIR and RING domains of LdMNPV IAP-3 gene as primers, has an overlap with the corresponding part of the LdMNPV IAP-3 gene and L.dispar IAP-1 mRNA for an inhibitor of apoptosis protein with the high cover by query, allows assuming that we cloned a part of gypsy moth anti-apoptosis gene. This finding gives the grounding that proposed here DNA insecticides might act through the blocking of the mechanisms involved in post transcriptional expression of insect anti-apoptosis genes. The results show the insecticidal potential of the viral genome fragments that can be used to create safe and relatively fast-acting DNA insecticides to control the quantity of gypsy moth populations, important task for forestry and agriculture.

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.

Baculovirus replication induces the expression of heat shock proteins in vivo and in vitro

A recent handful of studies have linked baculovirus infection with the induction of heat shock proteins, a highly conserved family of cytoprotective proteins. Here, we demonstrate baculovirus-stimulated upregulation of hsp70 transcription in the natural host, Helicoverpa zea. Larvae lethally infected with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV) accumulated hsp70 transcripts throughout the 72-hour course of infection in the midgut, hemocytes, and fat body. While a maximal 17- or 15-fold induction of hsp70 was noted in the midgut and hemocytes, respectively, by 72 hours postinfection, the level of hsp70 transcription in the fat body of larvae was greater than two orders of magnitude higher than in mock-infected larvae. These results were largely mirrored in cultures of infected cells, and a potentiation effect was observed in cells that were both heat shocked and infected. In contrast, Spodoptera frugiperda multiple nucleopolyhedrovirus and ultraviolet-inactivated HzSNPV did not stimulate hsp70 transcription in these non-permissive larvae and in cell culture, respectively. Taken together, this report documents baculovirus-mediated upregulation of hsp70 in the host and demonstrates the requirement for productive infection for hsp70 induction in vitro and in vivo.

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.

The ac53, ac78, ac101, and ac103 genes are newly discovered core genes in the family Baculoviridae

The family Baculoviridae is a large group of insect viruses containing circular double-stranded DNA genomes of 80 to 180 kbp, which have broad biotechnological applications. A key feature to understand and manipulate them is the recognition of orthology. However, the differences in gene contents and evolutionary distances among the known members of this family make it difficult to assign sequence orthology. In this study, the genome sequences of 58 baculoviruses were analyzed, with the aim to detect previously undescribed core genes because of their remote homology. A routine based on Multi PSI-Blast/tBlastN and Multi HaMStR allowed us to detect 31 of 33 accepted core genes and 4 orthologous sequences in the Baculoviridae which were not described previously. Our results show that the ac53, ac78, ac101 (p40), and ac103 (p48) genes have orthologs in all genomes and should be considered core genes. Accordingly, there are 37 orthologous genes in the family Baculoviridae.

Laboratory and field evaluations of the efficacy of a fast-killing baculovirus isolate from Spodoptera frugiperda

Three biopesticide parameters were evaluated for a fast-killing isolate (3AP2) and a wild-type isolate (Sf3) of Spodoptera frugiperda multiple nucleopolyhedrovirus (SfMNPV). Both isolates were evaluated for virus production using in vivo methods, for speed of kill based on bioassay of applications to glasshouse-grown and field-grown plants, and for residual insecticidal activity of unformulated virus and an encapsulating formulation to provide UV protection. Two inoculation rates comparing relative in vivo production of the isolates demonstrated 3AP2 inoculated larvae were significantly smaller than Sf3 inoculated larvae at death. At the lower inoculation rate, Sf3 inoculated larvae produced approximately twofold more occlusion bodies as the 3AP2 inoculated larvae. A model system of applications to cabbage plants and a bioassay to observe mortality of neonate S. frugiperda (J.E. Smith) after feeding on samples of treated leaves was used to evaluate speed of kill and residual insecticidal activity. The LT(50) for the 3AP2 isolate was at least 30 h less than the LT(50) for the Sf3 isolate when applied to either glasshouse-grown or field-grown plants. The spray-dried lignin encapsulating formulation provided similar benefits to both virus isolates when exposed to simulated sunlight in the laboratory and to natural sunlight in the field. For treatment applications to field grown cabbage in June, the half-life for efficacy of unformulated virus was <7.5 h compared with a half-life of >26.7 h for encapsulated virus. These results demonstrate that improved technologies can be combined to address characteristics which otherwise can limit the commercial potential of microbial-based biological insecticides.

Baculovirus: molecular insights on their diversity and conservation

The Baculoviridae is a large group of insect viruses containing circular double-stranded DNA genomes of 80 to 180 kbp. In this study, genome sequences from 57 baculoviruses were analyzed to reevaluate the number and identity of core genes and to understand the distribution of the remaining coding sequences. Thirty one core genes with orthologs in all genomes were identified along with other 895 genes differing in their degrees of representation among reported genomes. Many of these latter genes are common to well-defined lineages, whereas others are unique to one or a few of the viruses. Phylogenetic analyses based on core gene sequences and the gene composition of the genomes supported the current division of the Baculoviridae into 4 genera: Alphabaculovirus, Betabaculovirus, Gammabaculovirus, and Deltabaculovirus.

Effect of starvation upon baculovirus replication in larval Bombyx mori and Heliothis virescens

The progression of baculovirus (BmNPV, BmCysPD, AcMNPV or AcAaIT) infection in larval Bombyx mori and Heliothis virescens (1st, 3rd or 5th instar) was investigated following various starvation regimes. When the larvae were starved for 12 or 24h immediately following inoculation, the median lethal time to death (LT(50)) was delayed by 9.5-19.2h in comparison to non-starved controls. This corresponded to a delay of 10-23% depending upon the larval stage and virus that was used for inoculation. When a 24h-long starvation period was initiated at 1 or 2 days post inoculation (p.i.), a statistically significant difference in LT(50) was not found indicating that the early stages of infection are more sensitive to the effects of starvation. Viral titers in the hemolymph of 5th instar B. mori that were starved for 24h immediately following inoculation were 10-fold lower (p<0.01) than that found in non-starved control larvae. Histochemical analyses indicated that virus transmission was reduced in 5th instar B. mori that were starved for 24h immediately following inoculation in comparison to non-starved control larvae. In general, the mass of larvae that were starved immediately after inoculation was 30% lower than that of non-starved control insects. Our findings indicate that starvation of the larval host at the time of baculovirus exposure has a negative effect on the rate baculovirus transmission and pathogenesis.

Effects of fetal bovine serum deprivation in cell cultures on the production of Anticarsia gemmatalis multinucleopolyhedrovirus

BACKGROUND

Anticarsia gemmatalis is a pest in South America's soybean crops, which could be controlled by the Multinucleopolyhedrovirus of A. gemmatalis (AgMNPV). Currently, its commercial production is based on infected larvae. However, the possibility of using modified baculoviruses in Integrated Pest Management programs has stimulated an interest to develop alternative multiplication processes. This study evaluated the AgMNPV production in UFL-Ag-286 cells previously deprived Fetal Bovine Serum.

RESULTS

Culture media containing 1% FBS during the previous 48 hours achieved a synchronized condition where 90% of cells were found in G0/G1 stage, showing the presence of non-filamentous actin. All characteristics were estimated from cellular viability tests, cell actin detection trials and flow cytometer cell cycle analysis. AgMNPV production was tested by transcript studies and budded viruses (BVs) and occlusion bodies (OBs) yield quantitation. Results showed that the productivity in FBS deprived cells was 9.8 times more in BVs and 3.8 times more in OBs with respect to non-treated cells.

CONCLUSIONS

UFL-Ag-286 cells previously deprived in FBS shown to be a better host for AgMNPV propagation, increasing the useful for both in vitro bioinsecticide production and applications such as recombinant protein expression or gene delivery.

Juvenile hormone esterase: biochemistry and structure

Normal insect development requires a precisely timed, precipitous drop in hemolymph juvenile hormone (JH) titer. This drop occurs through a coordinated halt in JH biosynthesis and increase in JH metabolism. In many species, JH esterase (JHE) is critical for metabolism of the resonance-stabilized methyl ester of JH. JHE metabolizes JH with a high kcat/KM ratio that results primarily from an exceptionally low KM. Here we review the biochemistry and structure of authentic and recombinant JHEs from six insect orders, and present updated diagnostic criteria that help to distinguish JHEs from other carboxylesterases. The use of a JHE-encoding gene to improve the insecticidal efficacy of biopesticides is also discussed.

Proteases as insecticidal agents

Proteases from a variety of sources (viruses, bacteria, fungi, plants, and insects) have toxicity towards insects. Some of these insecticidal proteases evolved as venom components, herbivore resistance factors, or microbial pathogenicity factors, while other proteases play roles in insect development or digestion, but exert an insecticidal effect when over-expressed from genetically engineered plants or microbial pathogens. Many of these proteases are cysteine proteases, although insect-toxic metalloproteases and serine proteases have also been examined. The sites of protease toxic activity range from the insect midgut to the hemocoel (body cavity) to the cuticle. This review discusses these insecticidal proteases along with their evaluation and use as potential pesticides.

Downregulation of a chitin deacetylase-like protein in response to baculovirus infection and its application for improving baculovirus infectivity

Several expressed sequence tags (ESTs) with homology to chitin deacetylase-like protein (CDA) were selected from a group of Helicoverpa armigera genes whose expression changed after infection with H. armigera single nucleopolyhedrovirus (HearNPV). Some of these ESTs coded for a midgut protein containing a chitin deacetylase domain (CDAD). The expressed protein, HaCDA5a, did not show chitin deacetylase activity, but it showed a strong affinity for binding to chitin. Sequence analysis showed the lack of any chitin binding domain, described for all currently known peritrophic membrane (PM) proteins. HaCDA5a has previously been detected in the H. armigera PM. Such localization, together with its downregulation after pathogen infection, led us to hypothesize that this protein might be responsible for the homeostasis of the PM structure and that, by reduction of its expression, the insect may reduce PM permeability, decreasing the entrance of baculovirus. To test this hypothesis, we constructed a recombinant nucleopolyhedrovirus to express HaCDA5a in insect cells and tested its influence on PM permeability as well as the influence of HaCDA5a expression on the performance of the baculovirus. The experiments showed that HaCDA5a increased PM permeability, in a concentration-dependent manner. Bioassays on Spodoptera frugiperda and Spodoptera exigua larvae revealed that NPV expressing HaCDA5a was more infective than its parental virus. However, no difference in virulence was observed when the viruses were injected intrahemocoelically. These findings support the downregulation of a midgut-specific CDA-like protein as a possible mechanism used by H. armigera to reduce susceptibility to baculovirus by decreasing PM permeability.

Development of a prokaryotic-like polycistronic baculovirus expression vector by the linkage of two internal ribosome entry sites

Recombinant baculoviruses are suitable for the high-level production of large multi-protein complexes. A tri-cistronic expression vector was constructed by the inclusion of two internal ribosome entry sites (IRESs). In this novel polycistronic vector, one single polyhedrin promoter controlled the transcription of a tri-cistronic transcript. Also, the first cistron was translated through a cap-dependent mechanism, while the second and third cistrons were translated by the IRESs derived from the 5' UTR of Rhopalosiphum padi virus (RhPV) and Perina nuda virus (PnV), respectively. The ratio of tri-cistronic gene expression levels produced by the three translational initiation modules is about 2:1:1 (cap:PnV IRES:RhPV IRES). This study indicates that polycistronic genes can be co-expressed at the translational level as in prokaryotic expression system by baculovirus biotechnology.

Innovative applications for insect viruses: towards insecticide sensitization

The effective management of emerging insect-borne disease is dependent on the use of safe and efficacious chemical insecticides. Given the inherent ability of insects to develop resistance, it is essential to propose innovative strategies because insecticides remain the most important element of integrated approaches to vector control. Recently, intracellular phosphorylation and dephosphorylation of membrane receptors and ion channels targeted by insecticides have been described as new processes for increasing the sensitivity of insecticides. An efficient method might be to infect host insects with recombinant viruses overexpressing specific protein phosphatases/kinases known to regulate specific insecticide-sensitive targets. This attractive strategy could lead to sensitization of the insects, thus reducing the doses of insecticides and increasing the efficacy of treatments.

Mixed infections and the competitive fitness of faster-acting genetically modified viruses

Faster-acting recombinant baculoviruses have shown potential for improved suppression of insect pests, but their ecological impact on target and nontarget hosts and naturally occurring pathogens needs to be assessed. Previous studies have focused on the fitness of recombinants at the between-hosts level. However, the population structure of the transmission stages will also be decided by within-host selection. Here we have experimentally quantified the within-host competitive fitness of a fast-acting recombinant Autographa californica multicapsid nucleopolyhedrovirus missing the endogenous egt gene (vEGTDEL), by means of direct competition in single- and serial-passage experiments with its parental virus. Quantitative real-time PCR was employed to determine the ratio of these two viruses in passaged mixtures. We found that vEGTDEL had reduced within-host fitness: per passage the ratio of wild type to vEGTDEL was on average enhanced by a factor of 1.53 (single passage) and 1.68 (serial passage). There is also frequency-dependence: the higher the frequency of vEGTDEL, the stronger the selection against it is. Additionally, the virus ratio is a predictor of time to host death and virus yield. Our results show that egt is important to within-host fitness and allow for a more complete assessment of the ecological impact of recombinant baculovirus release.

Active DNA photolyase encoded by a baculovirus from the insect Chrysodeixis chalcites

The genome of Chrysodeixis chalcites nucleopolyhedrovirus (ChchNPV) contains two open reading frames, Cc-phr1 and Cc-phr2, which encode putative class II CPD-DNA photolyases. CPD-photolyases repair UV-induced pyrimidine cyclobutane dimers using visible light as an energy source. Expression of Cc-phr2 provided photolyase deficient Escherichia coli cells with photoreactivating activity indicating that Cc-phr2 encodes an active photolyase. In contrast, Cc-phr1 did not rescue the photolyase deficiency. Cc-phr2 was overexpressed in E. coli and the resulting photolyase was purified till apparent homogeneity. Spectral measurements indicated the presence of FAD, but a second chromophore appeared to be absent. Recombinant Cc-phr2 photolyase was found to bind specifically F0 (8-hydroxy-7,8-didemethyl-5-deazariboflavin), which is an antenna chromophore present in various photolyases.. After reconstitution, FAD and F0 were present in approximately equimolar amounts. In reconstituted photolyase the F0 chromophore is functionally active as judged from the increase in the in vitro repair activity. This study demonstrates for the first time that a functional photolyase is encoded by an insect virus, which may have implications for the design of a new generation of baculoviruses with improved performance in insect pest control.

Reprogramming the chiA expression profile of Autographa californica multiple nucleopolyhedrovirus

Expression of chiA and v-cath RNA and enzyme activity in wild-type Autographa californica multiple nucleopolyhedrovirus (AcMNPV) was compared with that of recombinant AcMNPV viruses reprogrammed for expression of the endogenous chiA. To establish a baseline for our recombinant AcMNPV studies, we compared, for the first time, the temporal expression profiles of both AcMNPV chiA transcription and translation simultaneously. The rate of intracellular chitinase accumulation during AcMNPV infection followed the same pattern observed for chiA transcription but was delayed by about 6 h. Replacement of 21 nucleotides containing the native late chiA and v-cath promoters with a selectable polh-EGFP cassette was sufficient to eliminate expression of both chiA and v-cath. Viruses were generated that express chiA from either the late p6.9 or very late polh promoters of AcMNPV, replacing the native chiA promoter. There was a marked difference in the temporal chiA transcription profiles from the native, p6.9 and polh promoters, resulting in respective specific activities of chitinase at 48 h p.i. of 62, 160 and 219 mU (mg lysate total protein)(-1). Based on temporal analysis of v-cath transcription by Northern blot, AcMNPV v-cath was transcribed from 9 h p.i. in Sf21 cells. However, expression of v-cath RNA or enzyme from a reconstructed v-cath promoter in the chiA-reprogrammed viruses was not detected at 48 h of virus replication. Reprogramming for increased chitinase (and putatively cathepsin) expression with native baculovirus promoters might provide a means for designing environmentally benign biological insecticides.

Biosafety of recombinant and wild type nucleopolyhedroviruses as bioinsecticides

The entomopathogenic Autographa californica (Speyer) nucleopolyhedrovirus (AcMNPV) has been genetically modified to increase its speed of kill. The potential adverse effects of a recombinant AcMNPV (AcAaIT) as well as wild type AcMNPV and wild type Spodoptera littoralis NPV (SlNPV) were studied. Cotton plants were treated with these viruses at concentrations that were adjusted to resemble the recommended field application rate (4 x 10(12) PIBs/feddan, feddan = 4,200 m2) and 3rd instar larvae of S. littoralis were allowed to feed on the contaminated plants. SDS-PAGE, ELISA, and DNA analyses were used to confirm that larvae that fed on these plants were virus-infected. Polyhedra that were purified from the infected larvae were subjected to structural protein analysis. A 32 KDa protein was found in polyhedra that were isolated from all of the viruses. Subtle differences were found in the size and abundance of ODV proteins. Antisera against polyhedral proteins isolated from AcAaIT polyhedra were raised in rabbits. The terminal bleeds from rabbits were screened against four coating antigens (i.e., polyhedral proteins from AcAaIT, AcAaIT from field-infected larvae (AcAaIT-field), AcMNPV, and SlNPV) using a two-dimensional titration method with the coated antigen format. Competitive inhibition experiments were conducted in parallel to optimize antibody and coating antigen concentrations for ELISA. The IC50 values for each combination ranged from 1.42 to 163 microg/ml. AcAaIT-derived polyhedrin gave the lowest IC50 value, followed by those of SlNPV, AcAaIT-field, and AcMNPV. The optimized ELISA system showed low cross reactivity for AcMNPV (0.87%), AcAaIT-field (1.2%), and SlNPV (4.0%). Genomic DNAs isolated from AcAaIT that were passaged in larvae of S. littoralis that were reared in the laboratory or field did not show any detectable differences. Albino rats (male and female) that were treated with AcAaIT, AcMNPV or SlNPV (either orally or by intraperitoneal injection at doses of 1 x 10(8) or 1 x 10(7) PIBs/rat, respectively) appeared to be healthy and showed increased body weight at 21 days posttreatment. The effect of virus administration on hematological, serum biochemical, and histopathological parameters were determined. Slight to moderate differences were observed in most of the hematological parameters. Specifically, serum proteins were decreased markedly in female rats treated orally with SlNPV, and in male rats injected with AcAaIT. SDS-PAGE analysis also showed some changes in serum protein profiles. No marked changes in acetylcholine esterase (AChE) activity were found. Changes in serum glucose, alanine aminotransferase (ALT), aspartate aminotransferase (AST), alkaline phosphatase (ALP), bilirubin, creatinin, and urea were also observed. Immunohistochemical observation of tissues from stomach, intestine, liver, kidney, brain, spleen, and lung also showed slight changes. Fish (Tilapia nilotica) were also exposed to AcAaIT, AcMNPV or SlNPV by incorporating each of the viruses into diet (1 x 10(9) PIBs/group). No mortality was found in treated or untreated fish during the experimental period (28 days). Macrophage phagocytic activity of fish head kidney cells increased with time, reaching maximum values at 180 min for both treated and control fish.