Proteolytic processing of native Cry1Ab toxin by midgut extracts and purified trypsins from the Mediterranean corn borer Sesamia nonagrioides

Enzyme-linked immunosorbent assay detection and bioactivity of Cry1Ab protein fragments

The continuing use of transgenic crops has led to an increased interest in the fate of insecticidal crystalline (Cry) proteins in the environment. Enzyme-linked immunosorbent assays (ELISAs) have emerged as the preferred detection method for Cry proteins in environmental matrices. Concerns exist that ELISAs are capable of detecting fragments of Cry proteins, which may lead to an overestimation of the concentration of these proteins in the environment. Five model systems were used to generate fragments of the Cry1Ab protein, which were then analyzed by ELISAs and bioassays. Fragments from 4 of the model systems were not detectable by ELISA and did not retain bioactivity. Fragments from the proteinase K model system were detectable by ELISA and retained bioactivity. In most cases, ELISAs appear to provide an accurate estimation of the amount of Cry proteins in the environment, as detectable fragments retained bioactivity and nondetectable fragments did not retain bioactivity. Environ Toxicol Chem 2016;35:3101-3112. © 2016 SETAC.

Investigation of larvae digestive β-glucosidase and proteases of the tomato pest Tuta absoluta for inhibiting the insect development

The tomato leaf miner Tuta absoluta is one of the most devastating pests for tomato crops. Digestive proteases and β-glucosidase enzymes were investigated using general and specific substrates and inhibitors. Maximal β-glucosidase and proteolytic activities occurred at temperature and pH optima of 30 and 40°C, 5 and 10-11 unit of pH, respectively. Zymogram analysis showed the presence of distinguished β-glucosidase exhibiting a specific activity of about 183 ± 15 µmol min-1 mg-1. In vitro inhibition experiments suggested that serine proteases were the primary gut proteases. Gel based protease inhibition assays demonstrated that the 28 and 73 kDa proteases might be trypsin-like and chymotrypsin-like enzymes, respectively. Overall gut trypsin-like and chymotrypsin-like activities were evaluated to be about 27.2 ± 0.84 and 1.68 ± 0.03 µmol min-1 mg-1, respectively. Sodium dodecyl sulfate polyacrylamide gel electrophoresis analysis showed that T. absoluta gut serine proteases are responsible for Bacillus thuringiensis Cry insecticidal proteins proteolysis. Additionally, bioassays showed that T. absoluta larvae development was more affected by the β-glucosidases inhibitor (D-glucono-δ-lactone) than the serine proteases inhibitor (soybean trypsin inhibitor). These results are of basic interest since they present interesting data of β-glucosidases and gut serine proteases of T. absoluta larvae.

Sugarcane giant borer transcriptome analysis and identification of genes related to digestion

Sugarcane is a widely cultivated plant that serves primarily as a source of sugar and ethanol. Its annual yield can be significantly reduced by the action of several insect pests including the sugarcane giant borer (Telchin licus licus), a lepidopteran that presents a long life cycle and which efforts to control it using pesticides have been inefficient. Although its economical relevance, only a few DNA sequences are available for this species in the GenBank. Pyrosequencing technology was used to investigate the transcriptome of several developmental stages of the insect. To maximize transcript diversity, a pool of total RNA was extracted from whole body insects and used to construct a normalized cDNA database. Sequencing produced over 650,000 reads, which were de novo assembled to generate a reference library of 23,824 contigs. After quality score and annotation, 43% of the contigs had at least one BLAST hit against the NCBI non-redundant database, and 40% showed similarities with the lepidopteran Bombyx mori. In a further analysis, we conducted a comparison with Manduca sexta midgut sequences to identify transcripts of genes involved in digestion. Of these transcripts, many presented an expansion or depletion in gene number, compared to B. mori genome. From the sugarcane giant borer (SGB) transcriptome, a number of aminopeptidase N (APN) cDNAs were characterized based on homology to those reported as Cry toxin receptors. This is the first report that provides a large-scale EST database for the species. Transcriptome analysis will certainly be useful to identify novel developmental genes, to better understand the insect's biology and to guide the development of new strategies for insect-pest control.

Proteolytic processing of Bacillus thuringiensis Vip3A proteins by two Spodoptera species

Vip3 proteins have been described to be secreted by Bacillus thuringiensis during the vegetative growth phase and to display a broad insecticidal spectrum against lepidopteran larvae. Vip3Aa protoxin has been reported to be significantly more toxic to Spodoptera frugiperda than to Spodoptera exigua and differences in the midgut processing have been proposed to be responsible. In contrast, we have found that Vip3Ae is essentially equally toxic against these two species. Proteolysis experiments were performed to study the stability of Vip3A proteins to peptidase digestion and to see whether the differences found could explain differences in toxicity against these two Spodoptera species. It was found that activation of the protoxin form and degradation of the 62kDa band took place at lower concentrations of trypsin when using Vip3Aa than when using Vip3Ae. The opposite effect was observed for chymotrypsin. Vip3Aa and Vip3Ae protoxins were effectively processed by midgut content extracts from the two Spodoptera species and the proteolytic activation did not produce a peptidase resistant core under these in vitro conditions. Digestion experiments performed with S. frugiperda chromatography-purified digestive serine peptidases showed that the degradation of the Vip3A toxins active core is mainly due to the action of cationic chymotrypsin-like peptidase. Although the digestion patterns of Vip3A proteins do not always correlate with toxicity, the peptidase stability of the 62kDa core is in agreement with intraspecific differences of toxicity of the Vip3Aa protein.

Resistance to Bt maize in Mythimna unipuncta (Lepidoptera: Noctuidae) is mediated by alteration in Cry1Ab protein activation

Bt maize cultivars based on the event MON810 (expressing Cry1Ab) have shown high efficacy for controlling corn borers. However, their efficiency for controlling some secondary lepidopteran pests such as Mythimna unipuncta has been questioned, raising concerns about potential outbreaks and its economic consequences. We have selected a resistant strain (MR) of M. unipuncta, which is capable of completing its life cycle on Bt maize and displays a similar performance when feeding on both Bt and non-Bt maize. The proteolytic activation of the protoxin and the binding of active toxin to brush border membrane vesicles were investigated in the resistant and a control strain. A reduction in the activity of proteolytic enzymes, which correlates with impaired capacity of midgut extracts to activate the Cry1Ab protoxin has been observed in the resistant strain. Moreover, resistance in larvae of the MR strain was reverted when treated with Cry1Ab toxin activated with midgut juice from the control strain. All these data indicate that resistance in the MR strain is mediated by alteration of toxin activation rather than to an increase in the proteolytic degradation of the protein. By contrast, binding assays performed with biotin labelled Cry1Ab suggest that binding to midgut receptors does not play a major role in the resistance to Bt maize. Our results emphasize the risk of development of resistance in field populations of M. unipuncta and the need to consider this secondary pest in ongoing resistance management programs to avoid the likely negative agronomic and environmental consequences.

An insect trypsin-like serine protease as a target of microRNA: utilization of microRNA mimics and inhibitors by oral feeding

MicroRNAs (miRNAs) are non-coding small RNAs (18-25 nt) that play crucial roles in various biological processes, including development, as gene regulators. In this study, we identified a miRNA, har-miR-2002b, that is specifically expressed during larval stages of Helicoverpa armigera (cotton bollworm). For the first time, we explored the possibility of utilizing synthetic inhibitor and mimic of a miRNA by oral feeding and confirmed that the inhibitor and mimic can successfully inhibit/oversupply har-miR-2002b in H. armigera larvae. Bioassays by oral feeding indicated 70% reduction in fecundity and 40% larval mortality in the presence of har-miR-2002b mimic. In addition, a large proportion of those larvae that made it to the pupal stage were deformed and only few moths emerged from them. This implied an important role of har-miR-2002b in larval development and adult fecundity. A trypsin-like serine protease (Ha-TLP) was identified as a target of har-miR-2002b. In vivo and in vitro studies revealed that in the presence of the miRNA mimic Ha-TLP transcript/protein levels and enzyme activity were significantly reduced. In contrast, inhibition of har-miR-2002b led to significant up-regulation of Ha-TLP transcript levels. This is the first study which shows the potential of utilization of miRNA mimics and inhibitors in pest control by targeting specific insect genes.

In vitro digestion of Cry1Ab proteins and analysis of the impact on their immunoreactivity

A pepsin resistance test performed at pH 1.2 and with high pepsin to protein ratio is one of the steps of the weight-of-evidence approach used for assessment of allergenicity of new proteins. However, the use of other in vitro digestibility tests, performed in more physiologically relevant conditions and in combination with immunological assays so as to increase the value of the information gained from the studies of stability of a novel protein to digestion for the overall allergenicity assessment, has been proposed. This study then aimed to investigate the stability to digestion of Cry1Ab protoxin and toxin, insecticidal proteins expressed in genetically modified crops, using simulated gastric fluid (SGF) at different pH values and pepsin-to-substrate ratios, in the presence or absence of physiological surfactant phosphatidylcholine (PC). Electrophoresis and immunoblot patterns and residual immunoreactivity of digesta were analyzed. Although Cry1Ab protoxin is extensively degraded at pH 1.2 with high pepsin-to-protein ratio, it is only slightly degraded at pH 2.0 and conserved its immunoreactivity. Furthermore, Cry1Ab proteins were demonstrated to be stable in a more physiologically relevant in vitro digestibility test (pH 2.5, pepsin-to-substrate ratio 1:20 (w/w) with PC). Factors such as pH, SGF composition, and pepsin-to-substrate ratio then greatly influence the digestion of Cry1Ab proteins, confirming that new and more physiologically relevant in vitro digestibility tests should be also considered to study the relationship between the resistance of a protein to digestion and its allergenicity.

The impacts of classical insect hormones on the expression profiles of a new digestive trypsin-like protease (TLP) from the cotton bollworm, Helicoverpa armigera

Trypsin proteinases perform important roles in the protein digestion of an insect midgut. A 1042 bp full-length cDNA was cloned from Helicoverpa armigera. The gene encoded a 32 kDa protein, with a predicted isoelectric point of 5.7. The amino acid sequence of the protein had a trypsin-like serine protease domain, and the gene was named Ha-TLP. The expression of the gene was tissue-specific and the transcript of Ha-TLP existed only in the midgut and was not found in the head-thorax, integument, fat body and haemocytes from 5th instar larvae, with similar expression levels between those in feeding larvae and in molting larvae. In the midgut, the gene transcription level declined from 6th instar 72 h after the larvae entered the wandering stage, and disappeared from 6th instar at 96 h until the pupal stage. By immunohistochemistry, Ha-TLP was detected in the cytoplasm of the midgut epithelial cells of the 6th instar feeding stage worms. The expression of Ha-TLP could be up-regulated by a juvenile hormone (JH) analog methoprene and down-regulated by 20-hydroxyecdysone (20E). These facts indicate that Ha-TLP was involved in food digestion during larval growth and probably up-regulated by JH and suppressed by extra 20E in vivo.

Prey mediated effects of Bt maize on fitness and digestive physiology of the red spider mite predator Stethorus punctillum Weise (Coleoptera: Coccinellidae)

The present study investigated prey-mediated effects of two maize varieties expressing a truncated Cry1Ab, Compa CB (event Bt176) and DKC7565 (event MON810), on the biology of the ladybird Stethorus punctillum. Although immuno-assays demonstrated the presence of Cry1Ab in both prey and predator collected from commercial maize-growing fields, neither transgenic variety had any negative effects on survival of the predator, nor on the developmental time through to adulthood. Furthermore, no subsequent effects on ladybird fecundity were observed. As a prerequisite to studying the interaction of ladybird proteases with Cry1Ab, proteases were characterised using a range of natural and synthetic substrates with diagnostic inhibitors. These results demonstrated that this predator utilises both serine and cysteine proteases for digestion. In vitro studies demonstrated that T. urticae were not able to process or hydrolyze Cry1Ab, suggesting that the toxin passes through the prey to the third trophic level undegraded, thus presumably retaining its insecticidal properties. In contrast, S. punctillum was able to activate the 130 kDa protoxin into the 65 kDa fragment; a fragment of similar size was also obtained with bovine trypsin, which is known to cleave the protoxin to the active form. Thus, despite a potential hazard to the ladybird of Bt-expressing maize (since the predator was both exposed to, and able to proteolytically cleave the toxin, at least in vitro), no deleterious effects were observed.

Identification and characterization of midgut proteases in Achaea janata and their implications

Insect midgut proteases are excellent targets for insecticidal agents such as Bacillus thuringiensis Cry toxins and protease inhibitors. The midgut proteases of Achaea janata have been characterized and Casein zymograms indicated at least five distinct activities corresponding to approx 17, 20, 29 and 80, and 90 kDa. Using a combination of synthetic substrates and specific inhibitors in casein zymograms, photometric assays and activity blots, three trypsin-like and one elastase-like serine proteases were identified but no chymotrypsin-like activity. Various proteinase inhibitors displayed differential inhibitory effects towards the midgut proteases.