Development of recombinant viral insecticides by expression of an insect-specific toxin and insect-specific enzyme in nuclear polyhedrosis viruses

Gut Transcription in Helicoverpa zea is Dynamically Altered in Response to Baculovirus Infection

The Helicoverpa zea transcriptome was analyzed 24 h after H. zea larvae fed on artificial diet laced with Helicoverpa zea single nucleopolyhedrovirus (HzSNPV). Significant differential regulation of 1,139 putative genes (p < 0.05 T-test with Benjamini and Hochberg False Discovery Rate) was detected in the gut epithelial tissue; where 63% of these genes were down-regulated and 37% of genes were up-regulated compared to the mock-infected control. Genes that play important roles in digestive physiology were noted as being generally down-regulated. Among these were aminopeptidases, trypsin-like serine proteases, lipases, esterases and serine proteases. Genes related to the immune response reacted in a complex nature having peptidoglycan binding and viral antigen recognition proteins and antiviral pathway systems down-regulated, whereas antimicrobial peptides and prophenoloxidase were up-regulated. In general, detoxification genes, specifically cytochrome P450 and glutathione S-transferase were down-regulated as a result of infection. This report offers the first comparative transcriptomic study of H. zea compared to HzSNPV infected H. zea and provides further groundwork that will lead to a larger understanding of transcriptional perturbations associated with viral infection and the host response to the viral insult in what is likely the most heavily infected tissue in the insect.

Zootoxic effects of reduviid Rhynocoris marginatus (Fab.) (Hemiptera: Reduviidae) venomous saliva on Spodoptera litura (Fab.)

Rhynocoris marginatus is a predominant and potential reduviid predator of many economically important pests in India. The venomous saliva (VS) was collected by milking method and diluted with HPLC grade water to prepare different concentrations (200, 400, 600, 800 and 1000ppm). The VS from R. marginatus was found to be toxic and the LD(50) of the VS in Spodoptera litura third instar were 768 and 929ppm at 48 and 96h for microinjection and oral toxicity studies, respectively. Level of hydrolase and detoxification enzymes significantly decreased in a dose-dependent manner after treating the host with VS for 96h. A decrease in carbohydrate (21%) and lipid (46%) contents and an increase in the protein content (50%) were prominent in the experimental category. The VS reduced the relative growth rate, approximate digestibility, efficiency of conversion of ingested and digested food of S. litura in the oral toxicity study. Salivary venom inhibits the haemocytes from aggregation and affects spreading behavior of haemocytes separated from the fifth stadium larvae of S. litura. The result showed that VS toxins caused mortality, changed the nutritional indices, and altered the levels of macromolecule quantity and digestive enzymes of S. litura. We concluded that the VS of R. marginatus is venomous to a prey species, S. litura.

Sea anemone venom as a source of insecticidal peptides acting on voltage-gated Na+ channels

Sea anemones produce a myriad of toxic peptides and proteins of which a large group acts on voltage-gated Na+ channels. However, in comparison to other organisms, their venoms and toxins are poorly studied. Most of the known voltage-gated Na+ channel toxins isolated from sea anemone venoms act on neurotoxin receptor site 3 and inhibit the inactivation of these channels. Furthermore, it seems that most of these toxins have a distinct preference for crustaceans. Given the close evolutionary relationship between crustaceans and insects, it is not surprising that sea anemone toxins also profoundly affect insect voltage-gated Na+ channels, which constitutes the scope of this review. For this reason, these peptides can be considered as insecticidal lead compounds in the development of insecticides.

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.

Persistence of extracellular baculoviral DNA in aquatic microcosms: extraction, purification, and amplification by the polymerase chain reaction (PCR)

Genetically-modified baculoviruses have potential uses as bio-pesticides in forestry. However, the baculoviral occlusion bodies (OBs) may release genetically-modified DNA into the forest environment. In this research, outdoor aquatic microcosms, spiked with 673 microg of genomic DNA (4.4 x 10(12) target copies) from the genetically modified baculovirus Choristoneura fumiferana MNPVegt-/lacZ+, were exposed to natural summer conditions. A 530 bp DNA fragment from the genome of CfMNPVegt-/lacZ+ was detected in field microcosm water samples for about 24 h. The introduced DNA may have persisted for a longer time, but was below the detection limit of the PCR analysis (13.5 pg DNA or 8.9 x 10(4) target copies ml(-1) water). The detection limit of PCR was determined by spiking water samples with a dilution series of CfMNPVegt-/lacZ+ genomic DNA, extracting and purifying the DNA, and then PCR analysis. This study provides some of the first information on the persistence and detection limits of this viral DNA under aquatic ecological conditions, and the methods that can be used to conduct such a molecular-based field study.

Extraction, detection and persistence of extracellular DNA in forest litter microcosms

A DNA extraction method was developed that preferentially extracted extracellular DNA rather than intracellular DNA from forest litter. The method purposely avoided the use of harsh chemicals and physical disruption steps used in total DNA extraction to release DNA from cells. The detection limit of PCR, determined by spiking forest litter samples with a dilution series of Choristoneura fumiferana MNPVegt(-)/lacZ(+) genomic DNA, was about 1 ng DNA or 6.85 x 10(6) target copies 0.5 g(-1) moist forest litter or 0.14 g(-1) dry forest litter. In this study, outdoor terrestrial microcosms, each spiked with 49.2 microg of genomic DNA (from the baculovirus CfMNPVegt(-)/lacZ(+)), were exposed to summer conditions. A 530 bp DNA fragment from the genome of the baculovirus CfMNPVegt(-)/lacZ(+) was detected in these microcosms for about 3 months. The DNA may have persisted for a longer period but was below the detection limit of the PCR analysis.

Further enhancement of baculovirus insecticidal efficacy with scorpion toxins that interact cooperatively

We have studied whether the cooperative insecticidal effect of certain scorpion toxin pairs, namely either a combination of excitatory and depressant, or alpha and depressant scorpion toxins, would improve the efficacy of Autographa californica nucleopolyhedrovirus (AcMNPV) over a virus expressing only a single toxin, towards Heliothis virescens, Helicoverpa armigera, and Spodoptera littoralis larvae. The best result was achieved by combined expression of the excitatory toxin, LqhIT1, and the depressant toxin, LqhIT2, that provided an ET(50) value of 46.9 h on H. virescens neonates, an improvement of 40% over the efficacy of wild-type AcMNPV, and of 18% and 22% over baculoviruses that express each of the toxins independently. These results demonstrate that significant improvement in efficacy of recombinant baculoviruses is obtainable with toxins that exhibit a cooperative effect, and may contribute to employ baculoviruses to replace hazardous chemicals in insect control.

Purification and characterization of a short insect toxin from the venom of the scorpion Buthus tamulus

A short chain peptide has been isolated from the venom of a red scorpion of Indian origin, Buthus tamulus. This peptide was purified using ion exchange and reverse phase chromatography and was characterized by molecular weight determination and amino acid sequence. The primary structure analysis shows that BtITx3 is a short peptide of 35 amino acid residues having a molecular weight of 3796 Da. The toxin shows toxicity towards the Lepidopteran species of insect Helicoverpa armigera causing flaccid paralysis and even death within 24 h. It shows more than 50% homology with the short insectotoxins having four disulfide bridges, which suggests that the toxin belongs to the class of short chain toxins blocking the chloride ion channels. This sequence homology study has also helped to bring out the structure-function relationship between the various short toxins. Homology modeling done by using template structure of a known toxin indicated that this toxin consists of a similar alpha/beta scaffold, as present in other scorpion toxins.

Purification, amino-acid sequence and partial characterization of two toxins with anti-insect activity from the venom of the South American scorpion Tityus bahiensis (Buthidae)

We report here the isolation by a two-step chromatographic procedure of two new toxins from the South American scorpion Tityus bahiensis. Their amino-acid sequences and some of their biological features were established. The two toxins have different biological properties. Toxin TbIT-I had almost no activity or pharmacological effects in vertebrate tissues whereas it was lethal to house flies (LD50 80.0 ng/house fly). In contrast, Tb2-II was active against both mammals (intracerebroventricular injection of 100 ng/mouse was lethal) and insects (LD50 40.0 ng/house fly). The amino-acid sequences of these toxins were established and found to be similar (60-95%) to previously described beta-toxins from the Tityus genus. Based on the available comparative information, this study attempts identify possible structure-function relationships that may be responsible for the differences in bioactivity displayed by these toxins.

Extraction and detection of baculoviral DNA from lake water, detritus and forest litter

AIMS

This paper describes a quick, reproducible, sensitive method for baculoviral DNA extraction, purification and detection from freshwater and forest litter environments.

METHODS AND RESULTS

The extraction protocol utilizes enzymatic and chemical lysis and physical disruption. To assess the efficiency of the extraction and purification protocol, PCR was used to detect a 530 bp DNA fragment from the genome of a genetically-modified baculovirus, Choristoneura fumiferana NPVegt-/lacZ+. The detection limit of PCR amplification was routinely about 4.1 x 102 occlusion bodies (OBs) 450 microl-1 lake water. Template DNA from the detritus and forest litter samples required 100-fold dilutions before use in PCR reactions. The detection limits for detritus and forest litter samples were routinely about 7.41 x 103 and 2.08 x 104 OBs 0.5 g-1 dry weight, respectively.

CONCLUSION

The DNA extraction and purification methodology is reproducible, sensitive and can be used in lieu of, or in conjunction with, insect bioassays.

SIGNIFICANCE AND IMPACT OF THE STUDY

The DNA extraction and purification protocol described in this paper will facilitate risk assessment and ecological studies of both wild-type and genetically-modified baculoviruses.

Midgut-based resistance of Heliothis virescens to baculovirus infection mediated by phytochemicals in cotton

The decrease in susceptibility to polyhedrosis disease when Heliothis virescens larvae feed on cotton is profound, limiting the utility of baculoviruses for controlling noctuids on this important crop. We observed that the mortalities of H. virescens larvae challenged with a reporter-gene construct of Autographa californica M nucleopolyhedrovirus (AcMNPV-hsp70/lacZ) and fed either lettuce or artificial diet were approximately 2.5-fold higher than that of cotton-fed insects. This decrease in susceptibility on cotton was observed following oral but not intrahemocoelic inoculation of virus, and it was negatively correlated with levels of foliar peroxidase. The rates of development of both infected and uninfected larvae also were correlated negatively with levels of foliar peroxidase, and hence, were significantly lower for insects fed cotton. When Calcofluor White M2R, an optical brightener reported to enhance the retention of AcMNPV-infected midgut cells, was included in inoculum administered orally to larvae, mortality levels were equivalent regardless of diet. These results suggest that sloughing of infected midgut cells occurred at a higher rate in insects that fed on cotton compared to the other two diets, and that midgut cell sloughing is the mechanism whereby susceptibility to mortal infection by AcMNPV-hsp70/lacZ is decreased on cotton. This conclusion is consistent with previous reports that ingestion of cotton can generate reactive oxygen species within the midgut lumen that may damage midgut epithelial cells. As far as we know, this is the first study to link resistance intrinsic to the physiology of the insect (e.g., developmental resistance) and resistance conferred by host plant chemistry to a single mechanism, i.e., midgut cell sloughing.

Enhancement of oocyte growth in the cockroach Blaberus craniifer by a scorpion toxin, charybdotoxin

We investigated the ovarian response of Blaberus craniifer to charybdotoxin in both imaginal molt headless females and isolated fed females, at three criteria. 1--Vitellogenesis onset, detected by immunocytochemical localisation of sites binding anti-eggs antibodies in the basal oocyte in headless females: 60% of treated females present a positive response. 2--Oocyte length at D4 during vitellogenesis of isolated fed females which was enhanced by 0.1 - 0.2 microg toxin; doses higher than 0.5 microg/female decrease ovarian growth. 3--Time of oviposition of paired females which was shortened by 24 hrs by the toxin. These results suggest that low doses charybdotoxin enhance vitellogenesis, possibly via the nervous system by means of a substance conditioning both protein synthesis by the fat body and ovarian uptake.

Species-specific effects of the hcf-1 gene on baculovirus virulence

The host cell-specific factor 1 gene (hcf-1) of the baculovirus Autographa californica nuclear polyhedrosis virus (AcMNPV) is required for the efficient replication and/or stability of reporter plasmids carrying an AcMNPV-derived origin of DNA replication in a cell-specific manner; hcf-1 is required for reporter plasmid replication or stability in TN-368 cells, a cell line derived from the cabbage looper Trichoplusia ni, but not in IPLB-SF-21 (SF-21) cells, a cell line derived from the fall armyworm Spodoptera frugiperda (A. Lu and L. K. Miller, J. Virol. 69:6265-6272, 1995). To further define the function of hcf-1, recombinant viruses with null mutations in hcf-1 were constructed in SF-21 cells and the phenotype of the mutants was determined in selected cell lines as well as in insect larvae. In S.frugiperda larvae and SF-21 cells, the phenotype of hcf-1 mutants was indistinguishable from that of wild-type AcMNPV. In T. ni larvae as well as T. ni-derived cell lines, hcf-1 mutants exhibited a mutant phenotype. In TN-368 cells, the replication of hcf-1 mutants was extremely impaired; the phenotype included a defect in viral DNA replication, late gene transcription, and virus production as well as a complete cessation of host and viral protein synthesis. In another cell line derived from T. ni, the BTI-TN5B1-4 cell line, the hcf-1 mutants exhibited a less severe phenotype. In T. ni larvae, the infectivity of the budded form of hcf-1 mutants was decreased significantly (50-fold), although no difference in the oral infectivity of the occluded form was observed. T. ni larvae infected with hcf-1 mutants by either oral or hemocoelic routes, however, died 20 to 30% more slowly than those infected with wild-type AcMNPV. These data indicate that there is a host-specific requirement for hcf-1 and that it exerts cell line-specific effects and possibly tissue-specific effects on the rate at which the virus replicates, thereby affecting the virulence of the virus in a species-specific manner.

Larva lights: a decade of photoaffinity labeling with juvenile hormone analogues

The introduction of photoaffinity labeling into the mode of action of insect hormones and pheromones started 12 yr ago with the photoaffinity labeling of juvenile hormone binding proteins (JHBPs) from cockroaches in the laboratory of the late John K. Koeppe. Applying this technique to Manduca sexta led ultimately to a three-laboratory collaborative project that has begun to dissect the molecular basis for JH transport, metabolism, and nuclear binding and gene activation in Lepidoptera. This review provides (1) a history of the first experiments; (2) an idea of the breadth of the technique in the arthropod classes Insecta, Crustacea, and Arachnida; and (3) evidence for the depth of the technique in unearthing key details about three different types of the molecular action of JH in M. sexta.