Characterization of major midgut proteinase cDNAs from Helicoverpa armigera larvae and changes in gene expression in response to four proteinase inhibitors in the diet

Implications of long-term exposure of a Lymantria dispar L. population to pollution for the response of larval midgut proteases and acid phosphatases to chronic cadmium treatment

Cadmium (Cd) presence in terrestrial ecosystems is a serious threat that requires continuous development of biomonitoring tools. Ideally, a suitable biomarker of exposure should respond to the toxicant consistently in different populations regardless of previous exposure to pollution. Here we considered the activities and isoform patterns of certain proteases and acid phosphatases (ACP) in the midgut of Lymantria dispar larvae as well as the integrated biomarker response (IBR) for application in Cd biomonitoring. We compared the responses of caterpillars originating from unpolluted and polluted localities after they had been chronically subjected to dietary Cd (50 and 100 μg Cd/g dry food). The population inhabiting the unpolluted forest was far more sensitive to Cd exposure as the activities of total proteases, trypsin (TRY) and leucine aminopeptidase (LAP) were mostly reduced while the activities of total and non-lysosomal ACP were increased. Non-lysosomal ACP activity was elevated in larvae from the contaminated site in response to the higher Cd concentration. Exposure to the metal resulted in numerous alterations in the pattern of enzyme isoforms, but the responses of the two populations were similar except that larvae from the polluted locality were more tolerant to the lower Cd concentration. Non-lysosomal ACP activity and the appearance of ACP isoforms 4 and 5 together with the IBR index are the most promising indicators of Cd presence, potentially applicable even in populations with a history of exposure to pollution. TRY and total ACP activities could be used to monitor populations at uncontaminated localities.

Intestinal proteases profiling from Anticarsia gemmatalis and their binding to inhibitors

Although the importance of intestinal hydrolases is recognized, there is little information on the intestinal proteome of lepidopterans such as Anticarsia gemmatalis. Thus, we carried out the proteomic analysis of the A. gemmatalis intestine to characterize the proteases by LC/MS. We examined the interactions of proteins identified with protease inhibitors (PI) using molecular docking. We found 54 expressed antigens for intestinal protease, suggesting multiple important isoforms. The hydrolytic arsenal featured allows for a more comprehensive understanding of insect feeding. The docking analysis showed that the soybean PI (SKTI) could bind efficiently with the trypsin sequences and, therefore, insect resistance does not seem to involve changing the sequences of the PI binding site. In addition, a SERPIN was identified and the interaction analysis showed the inhibitor binding site is in contact with the catalytic site of trypsin, possibly acting as a regulator. In addition, this SERPIN and the identified PI sequences can be targets for the control of proteolytic activity in the caterpillar intestine and serve as a support for the rational design of a molecule with greater stability, less prone to cleavage by proteases and viable for the control of insect pests such as A. gemmatalis.

Role of digestive protease enzymes and related genes in host plant adaptation of a polyphagous pest, Spodoptera frugiperda

The evolutionary success of phytophagous insects depends on their ability to efficiently exploit plants as a source of energy for survival. Herbivorous insects largely depend on the efficiency, flexibility, and diversity of their digestive physiology and sophistication of their detoxification system to use chemically diverse host plants as food sources. The fall armyworm, Spodoptera frugiperda (J.E. Smith), is a polyphagous pest of many commercially important crops. To elucidate the ability of this insect pest to adapt to host plant mechanisms, we evaluated the impact of primary (corn) and alternate (rice) host plants after 11 generations on gut digestive enzymatic activity and expression profiles of related genes. Results indicated that the total protease and class-specific trypsin- and chymotrypsin-like protease activity of S. frugiperda significantly differed among host plant treatments. The class-specific protease profiles greatly differed in S. frugiperda midguts upon larval exposure to different treatments with inhibitors compared with treatments without inhibitors. Similarly, the single and cumulative effects of the enzyme-specific inhibitors TLCK, TPCK, and E-64 significantly increased larval mortality and reduced larval growth/mass across different plant treatments. Furthermore, the quantitative reverse transcription polymerase chain reaction results revealed increased transcription of two trypsin (SfTry-3, SfTry-7) and one chymotrypsin gene (Sfchym-9), which indicated that they have roles in host plant adaptation. Knockdown of these genes resulted in significantly reduced mRNA expression levels of the trypsin genes. This was related to the increased mortality observed in treatments compared with the dsRED control. This result indicates possible roles of S. frugiperda gut digestive enzymes and related genes in host plant adaptation.

Regulation of gene expression encoding the digestive α-amylase in the larvae of Colorado potato beetle, Leptinotarsa decemlineata (Say) in response to plant protein extracts

Considering the relevance of insect α-amylases and natural α-amylase inhibitors present in plants to protect against insect damage, we investigated the effect of white bean and rapeseed protein extracts on digestive α-amylase gene expression of the Colorado potato beetle, Leptinotarsa decemlineata (Say). For this purpose, in vitro and in vivo trials were performed to determine the inhibitory activity of seed proteins on the third and fourth instar larvae. In both trials, the significant inhibitory effect of each extracts on the third and fourth instar larval α-amylase activity and considerable mortality in treatments were observed compared to control trials. In the RT-qPCR, expression ratio demonstrated that the α-amylase gene of two different larval stages grown on both proteins treated leaves had significantly differentiated expression and was up-regulated in third instar larvae and down-regulated in fourth instar larvae compared to control. Results suggest that the hyper-production of α-amylase in third instar larvae is elicited to compensate for the enzyme activity inhibition at an earlier stage and also down-regulation suggests the existence of a negative feedback of plant proteins on the last instar larvae via impaired food intake and digestive α-amylase activity in Colorado potato beetle. Therefore, disruption of the insect's digestive physiology by plant defensive proteins can be considered in the development of innovative controlling methods of this crucial potato pest.

Global gene expression changes induced by knockout of a protease gene cluster in Helicoverpa armigera with CRISPR/Cas9

Helicoverpa armigera is one of the most serious agricultural insect pests of global importance. It is highly polyphagous and depends on digestive serine proteases to degrade proteins to peptides and to amino acids. H. armigera has evolved adaptive ability to compensate for the inhibition of plant defensive protease inhibitors (PIs) in its diet by overproduction of digestive enzymes. As far as we know, compensation for deletion of serine protease genes has not yet been studied in any herbivorous insect. In this study, we used CRISPR/Cas9 to knock out a cluster of 18 trypsin-like genes in H. armigera. Compared with the wild type SCD strain, activities of the total proteases, trypsins and chymotrypsins were not significantly changed in the gene cluster knockout strain (Tryp-KO). RNA-seq data showed 1492 upregulated and 461 downregulated DEGs in Try-KO. GO function classification and KEGG pathway analyses revealed these differentially expressed genes were enriched for terms related to binding, catalytic activity, metabolic process and signal transduction. In regard to serine protease genes, 35 were upregulated and 12 downregulated in Tryp-KO strain. Our study indicated that H. armigera can compensate for the deleted protease genes by overexpression of other trypsin and chymotrypsin genes in order to maintain its genetic and metabolic robustness. It also suggests that genetic perturbations created by genome editing tools can induce global gene expression changes.

Integrated Transcriptomic and Proteomic Analyses Suggest the Participation of Endogenous Protease Inhibitors in the Regulation of Protease Gene Expression in Helicoverpa armigera

Insects adapt to plant protease inhibitors (PIs) present in their diet by differentially regulating multiple digestive proteases. However, mechanisms regulating protease gene expression in insects are largely enigmatic. Ingestion of multi-domain recombinant Capsicum annuum protease inhibitor-7 (CanPI-7) arrests growth and development of Helicoverpa armigera (Lepidoptera: Noctuidae). Using de novo RNA sequencing and proteomic analysis, we examined the response of H. armigera larvae fed on recombinant CanPI-7 at different time intervals. Here, we present evidence supporting a dynamic transition in H. armigera protease expression on CanPI-7 feeding with general down-regulation of protease genes at early time points (0.5 to 6 h) and significant up-regulation of specific trypsin, chymotrypsin and aminopeptidase genes at later time points (12 to 48 h). Further, coexpression of H. armigera endogenous PIs with several digestive protease genes were apparent. In addition to the differential expression of endogenous H. armigera PIs, we also observed a distinct novel isoform of endogenous PI in CanPI-7 fed H. armigera larvae. Based on present and earlier studies, we propose potential mechanism of protease regulation in H. armigera and subsequent adaptation strategy to cope with anti-nutritional components of plants.

Functional analysis of five trypsin-like protease genes in the oriental fruit fly, Bactrocera dorsalis (Diptera: Tephritidae)

Insect midgut proteases catalyze the release of free amino acids from dietary proteins and are essential for insect normal development. To date, digestive proteases as potential candidates have made great progress in pest control. To clarify the function of trypsin-like protease genes in the digestive system of Bactrocera dorsalis, a serious pest of a wide range of tropical and subtropical fruit and vegetable crops, five trypsin genes (BdTry1, BdTry2, BdTry3, BdTry4 and BdTry5) were identified from transcriptome dataset, and the effects of feeding condition on their expression levels were examined subsequently. RNA interference (RNAi) was applied to further explore their function on the growth of B. dorsalis. The results showed that all the BdTrys in starving midgut expressed at a minimal level but up-regulated upon feeding (except BdTry3). Besides, RNAi by feeding dsRNAs to larvae proved to be an effective method to cause gene silencing and the mixed dsRNAs of the five BdTrys slowed larvae growth of B. dorsalis. The current data suggest that trypsin genes are actively involved in digestion process of B. dorsalis larvae and thereafter play crucial roles in their development.

Trypsin inhibitor from Leucaena leucocephala seeds delays and disrupts the development of Aedes aegypti, a multiple-disease vector

BACKGROUND

Nowadays, the Aedes aegypti mosquito represents a serious public health issue in view of the large outbreaks of the arboviral diseases zika, dengue, chikungunya and yellow fever. This holometabolous insect has midgut digestive enzymes that are trypsin- and chymotrypsin-like proteins. Protease inhibitors are able to bind to proteolytic enzymes and promote a blockage in digestion and nutrition, leading to death. Thus, we investigated the effect of trypsin inhibitor of Leucaena leucocephala (LTI) seeds on egg hatching, larval development and digestive midgut proteases.

RESULTS

LTI was obtained by trichloroacetic acid precipitation followed by a single chromatography step on anhydrous trypsin sepharose. Polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate showed a single protein band with a molecular mass close to 20 kDa. After exposure of Ae. aegypti eggs to LTI (0.3 mg mL^-1 ), egg hatching was reduced (50%). LTI did not show acute toxicity on newly hatched larvae incubated under the same conditions, but after 10 days of exposure a high mortality rate (86%) was observed and the surviving larvae had a 25% delay in development. LTI was able to inhibit in vitro the midgut enzymatic activity (70%), and when larvae were incubated with LTI solution we observed an inhibition of 56%.

CONCLUSIONS

LTI is a promising new tool to control critical points of Ae. aegypti development. © 2016 Society of Chemical Industry.

Alkaline serine proteases from Helicoverpa armigera: potential candidates for industrial applications

We characterized trypsin- and chymotrypsin-like serine alkaline proteases from cotton bollworm, Helicoverpa armigera, for their probable potential application as additives in various bio-formulations. Purification was achieved by using hydroxylapatite, DEAE sephadex and CM sephadex columns, which resulted in increased enzyme activity by 13.76- and 14.05-fold for trypsin and chymotrypsin, respectively. Michaelis-Menten constants (K_m ) for substrates of trypsin and chymotrypsin, BApNA and SAAPFpNA, were found to be 1.25 and 0.085 mM, correspondingly. Fluorescent zymogram analysis indicated the presence of five trypsin bands with molecular masses of ∼21, 25, 38, 40, and 66 kDa and two chymotrypsin bands with molecular masses of ∼29 and 34 kDa in SDS-PAGE. The optimum pH was 10.0 and optimum temperature was 50°C for proteolytic activity for the purified proteases. The proteases were inhibited by synthetic inhibitors such as PMSF, aprotonin, leupeptin, pefabloc, and antipain. TLCK and TPCK inhibited about 94 and 90% of trypsin and chymotrypsin activity, respectively, while EDTA, EGTA, E64, pepstatin, idoacetamide, and bestatin did not affect the enzymes. The purified enzymes exhibited high stability and compatibility with metal ions; oxidizing, reducing, and bleaching agents; organic solvents; and commercial detergents. Short life cycles, voracious feeding behavior, and production of multiple forms of proteases in the midgut with rapid catalytic activity and chemostability can serve H. armigera as an excellent alternative source of industrially important proteases for use as additives in stain removers, detergents, and other bio-formulations. Identification of enzymes with essential industrial properties from insect species could be a bioresource.

Comparative analysis of expression profiling of the trypsin and chymotrypsin genes from Lepidoptera species with different levels of sensitivity to soybean peptidase inhibitors

Peptidase inhibitors (PIs) are essential proteins involved in plant resistance to herbivorous insects, yet many insect species are able to escape the negative effects of these molecules. We compared the effects of acute and chronic ingestion of soybean peptidase inhibitors (SPIs) on Spodoptera frugiperda and Diatraea saccharalis, two Lepidoptera species with different sensitivities to SPI ingestion. We analyzed the trypsin and chymotrypsin gene expression profiles in both species. Acute exposure of S. frugiperda to the inhibitors activated seven genes (SfChy5, SfChy9, SfChy19, SfChy22, SfTry6, SfTry8, and SfTry10), whereas chronic exposure activated 16 genes (SfChy2, SfChy4, SfChy5, SfChy8, SfChy9, SfChy11, SfChy12, SfChy15, SfChy17, SfChy21, SfChy22, SfTry6, SfTry8, SfTry9, SfTry10, and SfTry12). By contrast, the challenge of D. saccharalis with SPIs did not differentially induce the expression of trypsin- or chymotrypsin-encoding genes, with the exception of DsChy7. Bayesian phylogenetic analysis of S. frugiperda trypsin protein sequences revealed two gene clades: one composed of genes responsive to the SPIs and a second composed of the unresponsive genes. D. saccharalis trypsin proteins were clustered nearest to the S. frugiperda unresponsive genes. Overall, our findings support a hypothesized mechanism of resistance of Noctuidae moths to SPIs, involving gene number expansion of trypsin and chymotrypsin families and regulation of gene expression, which could also explain the variable susceptibility between S. frugiperda and D. saccharalis to these plant inhibitors.

Adaptive regulation of digestive serine proteases in the larval midgut of Helicoverpa armigera in response to a plant protease inhibitor

Protease inhibitors (PIs) are direct defenses induced by plants in response to herbivory. PIs reduce herbivore digestive efficiency by inhibiting insects' digestive proteases; in turn insects can adapt to PIs by generally increasing protease levels and/or by inducing the expression of PI-insensitive proteases. Helicoverpa armigera, a highly polyphagous lepidopteran insect pest, is known for its ability to adapt to PIs. To advance our molecular and functional understanding of the regulation of digestive proteases, we performed a comprehensive gene expression experiment of H. armigera exposed to soybean Kunitz trypsin inhibitor (SKTI) using a custom-designed microarray. We observed poor larval growth on the SKTI diet until 24 h, however after 48 h larvae attained comparable weight to that of control diet. Although initially the expression of several trypsins and chymotrypsins increased, eventually the expression of some trypsins decreased, while the number of chymotrypsins and their expression increased in response to SKTI. Some of the diverged serine proteases were also differentially expressed. The expression of serine proteases observed using microarrays were further validated by qRT-PCR at different time points (12, 24, 48, 72 and 96 h) after the start of SKTI ingestion. There were also large changes in transcriptional patterns over time in the control diet. Carbohydrate metabolism and immune defense genes were affected in response to SKTI ingestion. Enzyme assays revealed reduced trypsin-specific activity and increased chymotrypsin-specific activity in response to SKTI. The differential regulation of trypsins and chymotrypsins at the transcript and protein levels accompanying a rebound in growth rate indicates that induction of SKTI-insensitive proteases is an effective strategy of H. armigera in coping with this protease inhibitor in its diet.

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.

Effects of different dietary conditions on the expression of trypsin- and chymotrypsin-like protease genes in the digestive system of the migratory locust, Locusta migratoria

While technological advancements have recently led to a steep increase in genomic and transcriptomic data, and large numbers of protease sequences are being discovered in diverse insect species, little information is available about the expression of digestive enzymes in Orthoptera. Here we describe the identification of Locusta migratoria serine protease transcripts (cDNAs) involved in digestion, which might serve as possible targets for pest control management. A total of 5 putative trypsin and 15 putative chymotrypsin gene sequences were characterized. Phylogenetic analysis revealed that these are distributed among 3 evolutionary conserved clusters. In addition, we have determined the relative gene expression levels of representative members in the gut under different feeding conditions. This study demonstrated that the transcript levels for all measured serine proteases were strongly reduced after starvation. On the other hand, larvae of L. migratoria displayed compensatory effects to the presence of Soybean Bowman Birk (SBBI) and Soybean Trypsin (SBTI) inhibitors in their diet by differential upregulation of multiple proteases. A rapid initial upregulation was observed for all tested serine protease transcripts, while only for members belonging to class I, the transcript levels remained elevated after prolonged exposure. In full agreement with these results, we also observed an increase in proteolytic activity in midgut secretions of locusts that were accustomed to the presence of protease inhibitors in their diet, while no change in sensitivity to these inhibitors was observed. Taken together, this paper is the first comprehensive study on dietary dependent transcript levels of proteolytic enzymes in Orthoptera. Our data suggest that compensatory response mechanisms to protease inhibitor ingestion may have appeared early in insect evolution.

The trypsin inhibitor from Entada acaciifolia seeds affects negatively the development of Mediterranean flour moth, Anagasta kuehniella

The Mediterranean flour moth (Anagasta kuehniella) is a pest insect that attacks stored foods. The difficulty in controlling this kind of pest promotes the development of alternatives for pest control, among them the use of proteins with insecticide effect. In this work, we evaluated the role of a trypsin inhibitor purified from Entada acaciifolia seeds (EATI) on the A. kuehniella development. Different concentrations of inhibitor were added to a diet to determine its effects on insect performance. At 0.4%, the EATI decreases the larval weight and survival rates by 54.6% and 15%, respectively; in addition to the extension of the life cycle of insect. The biochemical analysis showed that the inhibitor is refractory to the digestion by midgut proteases, and led to a reduction of 32% in general proteolytic activity. A detailed analysis of the enzymatic activity revealed a decrease of 50% in trypsin activity as the chymotrypsin activity increased by 12%; possibly to compensate the commitment of the digestive process. The trypsins from the EATI-fed group stayed sensitive to the inhibition by EATI, and based on kinetic assays no new trypsin enzymes were produced as adaptation attempt. The insecticides effects observed for the EATI against this pest encourage a more in depth study of its possible long-term use as a biotechnological tool.

Revisiting rubisco as a protein substrate for insect midgut proteases

Gene fragments encoding the large subunit (LS) of Rubisco (RBCL) were cloned from various species of host plants of phytophagous Lepidoptera and expressed as recombinant proteins in Escherichia coli. Recombinant RBCLs were compared among each other along with casein and native Rubisco as proteinaceous substrates for measuring total midgut protease activities of fourth instar larvae of Helicoverpa armigera feeding on casein, Pieris brassicae feeding on cauliflower, and Antheraea assamensis feeding on Litsea monopetala and Persea bombycina. Cognate rRBCL (from the pertinent host plant species) substrates performed similar to noncognate rRBCL reflecting the conserved nature of encoding genes and the versatile use of these recombinant proteins. Casein and recombinant RBCL generally outperformed native Rubisco as substrates, except where inclusion of a reducing agent in the enzyme assay likely unfolded the plant proteins. Levels of total midgut protease activities detected in A. assamensis larvae feeding on two primary host species were similar, suggesting that the suite(s) of digestive enzymes in these insects could hydrolyze a plant protein efficiently. Protease activities detected in the presence of protease inhibitors and the reducing agent dithiothreitol (DTT) suggested that recombinant RBCL was a suitable protein substrate for studying insect proteases using in vitro enzyme assays and substrate zymography.

Knocking-down Meloidogyne incognita proteases by plant-delivered dsRNA has negative pleiotropic effect on nematode vigor

The root-knot nematode Meloidogyne incognita causes serious damage and yield losses in numerous important crops worldwide. Analysis of the M. incognita genome revealed a vast number of proteases belonging to five different catalytic classes. Several reports indicate that M. incognita proteases could play important roles in nematode parasitism, besides their function in ordinary digestion of giant cell contents for feeding. The precise roles of these proteins during parasitism however are still unknown, making them interesting targets for gene silencing to address protein function. In this study we have knocked-down an aspartic (Mi-asp-1), a serine (Mi-ser-1) and a cysteine protease (Mi-cpl-1) by RNAi interference to get an insight into the function of these enzymes during a host/nematode interaction. Tobacco lines expressing dsRNA for Mi-ser-1 (dsSER), Mi-cpl-1 (dsCPL) and for the three genes together (dsFusion) were generated. Histological analysis of galls did not show clear differences in giant cell morphology. Interestingly, nematodes that infected plants expressing dsRNA for proteases produced a reduced number of eggs. In addition, nematode progeny matured in dsSER plants had reduced success in egg hatching, while progeny resulting from dsCPL and dsFusion plants were less successful to infect wild-type host plants. Quantitative PCR analysis confirmed a reduction in transcripts for Mi-cpl-1 and Mi-ser-1 proteases. Our results indicate that these proteases are possibly involved in different processes throughout nematode development, like nutrition, reproduction and embryogenesis. A better understanding of nematode proteases and their possible role during a plant-nematode interaction might help to develop new tools for phytonematode control.

Characterization and transcriptional analyses of cDNAs encoding three trypsin- and chymotrypsin-like proteinases in Cry1Ab-susceptible and Cry1Ab-resistant strains of sugarcane borer, Diatraea saccharalis

Diatraea saccharalis is a major corn borer pest. Midgut serine proteinases are essential for insect growth and development. Alteration of midgut proteinases is responsible for Bt resistance development in some species. To clone midgut trypsin and chymotrypsin cDNAs and to test if the Cry1Ab resistance in D. saccharalis is associated with changes in midgut proteinases, total midgut tryptic and chymotryptic activities, cDNA sequences, and gene expressions of three trypsin and three chymotrypsin genes were comparatively examined between Cry1Ab-susceptible (Cry1Ab-SS) and Cry1Ab-resistant (Cry1Ab-RR) strains. Full-length cDNAs encoding three trypsin- and three chymotrypsin-like proteinases were sequenced from Cry1Ab-SS and Cry1Ab-RR larvae. These cDNAs code for active forms of midgut serine proteinases with all functional motifs, including signal peptide, conserved His-Asp-Ser for the catalytic triad, three pairs of cysteines for disulfide bridge configurations, and conserved substrate specificity determination residues. In general, cDNA and putative protein sequences are highly similar between Cry1Ab-SS and Cry1Ab-RR strains, except for a few nucleotide and predicted amino acid substitutions, whose function need to be further clarified. Total trypsin and chymotrypsin activities were also similar between Cry1Ab-SS and Cry1Ab-RR strains. Transcriptional levels of the trypsin and chymotrypsin genes had numerical difference between Cry1Ab-SS and Cry1Ab-RR strains, but the difference was not statistically significant. Data suggest that the development of Cry1Ab resistance in D. saccharalis was not significantly associated with these trypsins and chymotrypsins. Results clarified the role of six midgut proteinases and provided a foundation for continuing examination of potential involvement of other midgut proteinases in Bt resistance development and other important biochemical processes.

Differential protease activity augments polyphagy in Helicoverpa armigera

Helicoverpa armigera (Lepidoptera: Noctuidae) and other polyphagous agricultural pests are extending their plant host range and emerging as serious agents in restraining crop productivity. Dynamic regulation, coupled with a diversity of digestive and detoxifying enzymes, play a crucial role in the adaptation of polyphagous insects. To investigate the functional intricacy of serine proteases in the development and polyphagy of H. armigera, we profiled the expression of eight trypsin-like and four chymotrypsin-like phylogenetically diverse mRNAs from different life stages of H. armigera reared on nutritionally distinct host plants. These analyses revealed diet- and stage-specific protease expression patterns. The trypsins expressed showed structural variations, which might result in differential substrate specificity and interaction with inhibitors. Protease profiles in the presence of inhibitors and their mass spectrometric analyses revealed insight into their differential activity. These findings emphasize the differential expression of serine proteases and their consequences for digestive physiology in promoting polyphagy in H. armigera.

Purification of a Kunitz-type inhibitor from Acacia polyphyllaDC seeds: characterization and insecticidal properties against Anagasta kuehniella Zeller (Lepidoptera: Pyralidae)

Anagasta kuehniella is a polyphagous pest that causes economic losses worldwide. This species produces serine proteases as its major enzymes for protein digestion. In this study, a new serine-protease inhibitor was isolated from Acacia polyphylla seeds (AcKI).Further analysis revealed that AcKI is formed by two polypeptide chains with a relative molecular mass of ∼20 kDa. The effects of AcKI on the development, survival, and enzymatic activity of Anagasta kuehniella larvae were evaluated, by incorporating AcKI in an artificial diet. Bioassays revealed a reduction in larval weight of ∼50% with the lower concentration of AcKI used in the study (0.5%). Although additionalassays showed an increase in endogenous trypsin and chymotrypsin activities, with a degree of AcKI-insensivity, AcKI produces an anti nutritional effect on A. kuehniella, indicating AcKI as a promising bioinsecticide protein for engineering plants that are resistant to insect pests.

The impact of ingested potato type II inhibitors on the production of the major serine proteases in the gut of Helicoverpa armigera

The flowers of the ornamental tobacco produce high levels of a series of 6 kDa serine protease inhibitors (NaPIs) that are effective inhibitors of trypsins and chymotrypsins from lepidopteran species. These inhibitors have a negative impact on the growth and development of lepidopteran larvae and have a potential role in plant protection. Here we investigate the effect of NaPIs on the activity and levels of serine proteases in the gut of Helicoverpa armigera larvae and explore the adaptive mechanisms larvae employ to overcome the negative effects of NaPIs in the diet. Polyclonal antibodies were raised against a Helicoverpa punctigera trypsin that is a target for NaPIs and two H. punctigera chymotrypsins; one that is resistant and one that is susceptible to inhibition by NaPIs. The antibodies were used to optimize procedures for extraction of proteases for immunoblot analysis and to assess the effect of NaPIs on the relative levels of the proteases in the gut and frass. We discovered that consumption of NaPIs did not lead to over-production of trypsins or chymotrypsins but did result in excessive loss of proteases to the frass.

Bacillus subtillis RTSBA6 6.00, a new strain isolated from gut of Helicoverpa armigera (Lepidoptera: Noctuidae) produces chymotrypsin-like proteases

Exploring bacterial communities with proteolytic activity from the gut of the Helicoverpa armigera (Hubner) (Lepidoptera: Noctuidae) insect pests was the purpose of this study. As initial efforts to achieve this goal here we report the isolation of new Bacillus subtillis RTSBA6 6.00 strain from the gut of H. armigera and demonstrated as proteases producer. Zymographic analysis revealed 12 proteolytic bands with apparent molecular weights ranging from 20 to 185 kDa. Although some activity was detected at acidic pH, the major activity was observed at slight alkaline pH (7.8). The optimum temperature was found to be 35 °C with complete loss of activity at 70 °C. All proteases were completely inactivated by PMSF (phenylmethylsulfonyl fluoride) and TPCK (N-tosyl-l-phenylalanine chloromethyl ketone), suggesting that proteases secreted by B. subtillis RTSBA6 6.00 belong to serine proteases class with chymotrypsin-like activity. The occurrence of protease producing bacterial community in the gut of the H. armigera advocates its probable assistance to insect in proteinaceous food digestion and adaptation to protease inhibitors of host plants.

Bioinsecticidal activity of Murraya koenigii miraculin-like protein against Helicoverpa armigera and Spodoptera litura

Miraculin-like proteins, belonging to the Kunitz superfamily, are natural plant defense agents against pests and predators, and therefore are potential biopesticides for incorporation into pest-resistant crops. Here, a miraculin-like protein from Murraya koenigii was assessed for its in vitro and in vivo effects against two polyphagous lepidopteran insect pests, Helicoverpa armigera and Spodoptera litura. M. koenigii miraculin-like protein (MKMLP) inhibited the trypsin-like activity and total protease activity of H. armigera gut proteinases (HGP) by 78.5 and 40%, respectively, and S.litura gut proteinases (SGP) by 81 and 48%, respectively. The inhibitor was stable and actively inhibited the proteolysis of both HGP and SGP enzymes for up to 72 h. Incorporation of MKMLP into artificial diet adversely affected the growth and development of pests in a dose-dependent manner. After 10 days of feeding on diets containing 200 µM MKMLP, larval weight was reduced to 69 and 44.8% and larval mortality was increased to 40 and 43.3% for H. armigera and S litura, respectively. The LC(50) of MKMLP was 0.34 and 0.22% of the diet for H.armigera and S. litura, respectively. These results demonstrate the efficacy of MKMLP as a potential plant defense agent against H. armigera and S. litura.

A midgut-specific chymotrypsin cDNA (Slctlp1) from Spodoptera litura: cloning, characterization, localization and expression analysis

Serine proteases play important roles in food digestion and immune response during insect development. A full-length cDNA (Slctlp1) encoding a chymotrypsin-like serine protease was cloned from Spodoptera litura and characterized for its cDNA structure, developmental and induced expression and localization. The deduced protein of the Slctlp1 cDNA contains a catalytic triad and a substrate specificity pocket found in most of the serine proteases. Both the transcripts and protein of Slctlp1 were predominately expressed in the midgut at the feeding stages during the larval development. Immunohistochemistry analysis indicated that the SLCTLP1 protein was predominately present in the midgut of the 6th instar feeding larvae. Starvation suppressed the expression of Slctlp1 gene and protein in 6th instar larvae and the protein expression was increased again by re-feeding the insect. The results suggest that the cloned Slctlp1 cDNA may be involved in food protein digestion at the feeding stages during larval development.

Expression of diverse midgut serine proteinases in the sericigenous Lepidoptera Antheraea assamensis (Helfer) is influenced by choice of host plant species

Antheraea assamensis is reared on various species of the Lauraceae family from north-east India for its distinctive cocoon silk. We demonstrate differential expression of digestive trypsin and chymotrypsins in larvae feeding on a primary host, Persea bombycina Kosterm., in comparison to larvae feeding on Litsea monopetala Roxb. using in vitro proteolytic assays, zymogram analyses with proteinase inhibitors, restriction digestion of RNA-PCR amplicons and quantitative real-time PCR (RT-PCR). Eight novel members of the serine proteinase gene family were identified, including an intron-spliced trypsin (AaPb4) and seven putative chymotrypsins (AaPb2, AaPb4, AaPb12, AaLm4, AaLm6, AaLm19 and AaLm29). Midgut transcript levels of the putative trypsin were higher in larvae fed P. bombycina whereas levels of transcripts encoding putative chymotrypsins were higher in larvae reared on L. monopetala. Complex, differential expression of sequence divergent midgut serine proteinases may reflect the ability of lepidopteran larvae to feed on different species of host plants. Possible implications of host plant choice on the digestive physiology of A. assamensis are discussed.

Coexpression of potato type I and II proteinase inhibitors gives cotton plants protection against insect damage in the field

Potato type I and II serine protease inhibitors are produced by solanaceous plants as a defense mechanism against insects and microbes. Nicotiana alata proteinase inhibitor (NaPI) is a multidomain potato type II inhibitor (pin II) that is produced at high levels in the female reproductive tissues of the ornamental tobacco, Nicotiana alata. The individual inhibitory domains of NaPI target the major classes of digestive enzymes, trypsin and chymotrypsin, in the gut of lepidopteran larval pests. Although consumption of NaPI dramatically reduced the growth and development of a major insect pest, Helicoverpa punctigera, we discovered that surviving larvae had high levels of chymotrypsin activity resistant to inhibition by NaPI. We found a potato type I inhibitor, Solanum tuberosum potato type I inhibitor (StPin1A), was a strong inhibitor of the NaPI-resistant chymotrypsin activity. The combined inhibitory effect of NaPI and StPin1A on H. armigera larval growth in the laboratory was reflected in the increased yield of cotton bolls in field trials of transgenic plants expressing both inhibitors. Better crop protection thus is achieved using combinations of inhibitors in which one class of proteinase inhibitor is used to match the genetic capacity of an insect to adapt to a second class of proteinase inhibitor.

A chymotrypsin-like serine protease cDNA involved in food protein digestion in the common cutworm, Spodoptera litura: Cloning, characterization, developmental and induced expression patterns, and localization

A full-length cDNA (Slctlp2) encoding a chymotrypsin-like serine protease was cloned from Spodoptera litura. This cDNA encoded a putative serine protease with a predicted molecular mass of 30.6kDa, which contained a serine protease catalytic motif GDSGGPL. Temporal and spatial expression of Slctlp2 mRNA and protein detected by Northern blotting, RT-PCR, qPCR and Western blotting analyses revealed that both Slctlp2 mRNA and protein were mainly present in the foregut and midgut of the 5th and 6th instar larvae during the feeding stages. In situ hybridization and immunohistochemistry confirmed that both Slctlp2 mRNA and protein were predominately present in the midgut. Expression of the gene was not induced by bacterial infection. Juvenile hormone III induced the gene expression, while 20-hydroxyecdysone had no impact on the expression. The expression of Slctlp2 mRNA and protein was down-regulated by starvation but up-regulated by re-feeding. The SlCTLP2 protein was detected in the lumen residues of the anterior, middle and posterior midgut and feces of the feeding 6th instar larvae, suggesting that it was secreted from the epithelium into the lumen of the gut. The results suggest that this Slctlp2 gene may be involved in digestive process of food proteins during the feeding stages of the larval development.

Exogenous and endogenous protease inhibitors in the gut of the fall armyworm larvae, Spodoptera frugiperda

A dose-dependent inhibition of endogenous trypsin and aminopeptidase occurs in the lumen of Spodoptera frugiperda after feeding L6 larvae exogenous inhibitors soybean trypsin inhibitor (SBTI), tosyl-L-lysine chloromethyl ketone-HCl (TLCK), or bestatin, respectively, for 3 days. TLCK inhibits trypsin in tissue extracts and in secretions more strongly than SBTI. The aminopeptidase released into the lumen (containing the peritrophic membrane) is strongly inhibited by bestatin, but the membrane-bound enzyme is not. A bound enzyme may be more resistant to an inhibitor than unbound. A cross-class elevation of aminopeptidase activity occurs in response to ingested trypsin inhibitor, but there was no cross-class effect of aminopeptidase inhibitor (bestatin) on trypsin activity. An endogenous trypsin and aminopeptidase inhibitor is present in the lumen and ventricular cells. The strength of the endogenous trypsin inhibition seems to be in the same range as that resulting from ingestion of the exogenous inhibitor SBTI. In some insect species, considerable trypsin secretion occurs in unfed as well as in fed animals, and endogenous protease inhibitors might function to protect the ventricular epithelium by inactivation of trypsin when less food is available.

A salivary serine protease of the haematophagous reduviid Panstrongylus megistus: sequence characterization, expression pattern and characterization of proteolytic activity

A cDNA encoding a trypsin-like protease from the salivary glands of the haematophagous reduviid Panstrongylus megistus was cloned and sequenced. The deduced protein sequence showed similarities to serine proteases of other hemipterans but with substitutions in the catalytic triad and the substrate binding site. The expression of the gene increased more than sixfold after feeding. Saliva showed the highest proteolytic activity at neutral to slightly basic pH. Substrate and inhibitor profiles and zymography indicated the presence of a trypsin-like protease with preference for Arg and Lys at P1. Using chromatography, a fibrinolytic enzyme was purified whose sequence was identified by tandem mass spectrometry as that encoded by the cDNA.

The effect of diet on the expression of lipase genes in the midgut of the lightbrown apple moth (Epiphyas postvittana Walker; Tortricidae)

We have identified lipase-like genes from an Epiphyas postvittana larval midgut EST library. Of the 10 pancreatic lipase family genes, six appear to encode active lipases and four encode inactive lipases, based on the presence/absence of essential catalytic residues. The four gastric lipase family genes appear to encode active proteins. Phylogenetic analysis of 54 lepidopteran pancreatic lipase proteins resolved the clade into five groups of midgut origin and a sixth of non-midgut lipases. The inactive proteins formed two separate groups with highly conserved mutations. The lepidopteran midgut lipases formed a ninth subfamily of pancreatic lipases. Eighteen insect and human gastric lipases were analysed phylogenetically with only very weak support for any groupings. Gene expression was measured in the larval midgut following feeding on five artificial diets and on apple leaves. The artificial diets contained different levels of triacylglycerol, linoleic acid and cholesterol. Significant changes in gene expression (more than 100-fold for active pancreatic lipases) were observed. All the inactive lipases were also highly expressed. The gastric lipase genes were expressed at lower levels and suppressed in larvae feeding on leaves. Together, protein motif analysis and the gene expression data suggest that, in phytophagous lepidopteran larvae, the pancreatic lipases may function in vivo as galactolipases and phospholipases whereas the gastric lipases may function as triacylglycerol hydrolases.

Poplar defense against insects: genome analysis, full-length cDNA cloning, and transcriptome and protein analysis of the poplar Kunitz-type protease inhibitor family

*Kunitz protease inhibitors (KPIs) feature prominently in poplar defense responses against insects. The increasing availability of genomics resources enabled a comprehensive analysis of the poplar (p)KPI family. *Using genome analysis, expressed sequence tag (EST) mining and full-length (FL)cDNA cloning we established an inventory and phylogeny of pKPIs. Microarray and real-time PCR analyses were used to profile pKPI gene expression following real or simulated insect attack. Proteomics of insect midgut content was used to monitor stability of pKPI protein. *We identified 31 pKPIs in the genome and validated gene models by EST mining and cloning of 41 unique FLcDNAs. Genome organization of the pKPI family, with six poplar-specific subfamilies, suggests that tandem duplications have played a major role in its expansion. pKPIs are expressed throughout the plant and many are strongly induced by insect attack, although insect-specific signals seem initially to suppress the tree pKPI response. We found substantial peptide coverage for a potentially intact pKPI protein in insect midgut after eating poplar leaves. *These results highlight the complexity of an important defense gene family in poplar with regard to gene family size, differential constitutive and insect-induced gene expression, and resilience of at least one pKPI protein to digestion by herbivores.

Structural and functional diversities in lepidopteran serine proteases

Primary protein-digestion in Lepidopteran larvae relies on serine proteases like trypsin and chymotrypsin. Efforts toward the classification and characterization of digestive proteases have unraveled a considerable diversity in the specificity and mechanistic classes of gut proteases. Though the evolutionary significance of mutations that lead to structural diversity in serine proteases has been well characterized, detailing the resultant functional diversity has continually posed a challenge to researchers. Functional diversity can be correlated to the adaptation of insects to various host-plants as well as to exposure of insects to naturally occurring antagonistic biomolecules such as plant-derived protease inhibitors (PIs) and lectins. Current research is focused on deciphering the changes in protease specificities and activities arising from altered amino acids at the active site, specificity-determining pockets and other regions, which influence activity. Some insight has been gained through in silico modeling and simulation experiments, aided by the limited availability of characterized proteases. We examine the structurally and functionally diverse Lepidopteran serine proteases, and assess their influence on larval digestive processes and on overall insect physiology.

Digestive alpha-amylases of the flour moth Ephestia kuehniella--adaptation to alkaline environment and plant inhibitors

The digestive tract of lepidopteran insects is extremely alkaline. In the present work, molecular adaptation of amylolytic enzymes to this environment was investigated in the flour moth Ephestia kuehniella, an important stored-product pest. Three digestive alpha-amylases [Ephestia kuehniella alpha-amylase isoenzymes 1-3 (EkAmy1-3)] with an alkaline pH optimum were purified from larvae and biochemically characterized. These isoenzymes differ significantly in their sensitivity to alpha-amylase inhibitors of plant origin that are directed against herbivores as antifeedants. Such functional variability renders the amylolytic system less vulnerable to suppression by plant defensive molecules. Moreover, we found that expression of alpha-amylases is upregulated in larvae feeding on a diet enriched with an alpha-amylase inhibitor. The alpha-amylases are secreted into the larval midgut by an exocytotic mechanism, as revealed by immunogold microscopy. The cDNA sequence of EkAmy3 was determined, and a homology model of EkAmy3 was built in order to analyze the structural features responsible for adaptation to alkaline pH. First, the overall fold was found to be stabilized by remodeling of ion pairs. Second, molecular simulations supported by activity measurements showed that EkAmy3 does not bind a Cl(-), owing to an Arg-to-Gln mutation in a conserved binding site. The Cl(-)-binding residues are in contact with the catalytic residues, and this change might help to fine-tune the catalytic pK(a) values to an alkaline pH optimum. We conclude that lepidopteran alpha-amylases are evolutionarily adapted in terms of structure and expression dynamics for effective functioning in the digestive system.

Serine proteases identified from a Costelytra zealandica (White) (Coleoptera: Scarabaeidae) midgut EST library and their expression through insect development

Costelytra zealandica larvae are pests of New Zealand pastures causing damage by feeding on the roots of grasses and clovers. The major larval protein digestive enzymes are serine proteases (SPs), which are targets for disruption in pest control. An expressed sequence tag (EST) library from healthy, third instar larval midgut tissue was constructed and analysed to determine the composition and regulation of proteases in the C. zealandica larval midgut. Gene mining identified three trypsin-like and 11 chymotrypsin-like SPs spread among four major subgroups. Representative SPs were examined by quantitative PCR and enzyme activity assayed across developmental stages. The serine protease genes examined were expressed throughout feeding stages and downregulated in nonfeeding stages. The study will improve targeting of protease inhibitors and bacterial disruptors of SP synthesis.

Digestive duet: midgut digestive proteinases of Manduca sexta ingesting Nicotiana attenuata with manipulated trypsin proteinase inhibitor expression

BACKGROUND

The defensive effect of endogenous trypsin proteinase inhibitors (NaTPIs) on the herbivore Manduca sexta was demonstrated by genetically altering NaTPI production in M. sexta's host plant, Nicotiana attenuata. To understand how this defense works, we studied the effects of NaTPI on M. sexta gut proteinase activity levels in different larval instars of caterpillars feeding freely on untransformed and transformed plants.

METHODOLOGY/ PRINCIPAL FINDINGS

Second and third instars larvae that fed on NaTPI-producing (WT) genotypes were lighter and had less gut proteinase activity compared to those that fed on genotypes with either little or no NaTPI activity. Unexpectedly, NaTPI activity in vitro assays not only inhibited the trypsin sensitive fraction of gut proteinase activity but also halved the NaTPI-insensitive fraction in third-instar larvae. Unable to degrade NaTPI, larvae apparently lacked the means to adapt to NaTPI in their diet. However, caterpillars recovered at least part of their gut proteinase activity when they were transferred from NaTPI-producing host plants to NaTPI-free host plants. In addition extracts of basal leaves inhibited more gut proteinase activity than did extracts of middle stem leaves with the same protein content.

CONCLUSIONS/ SIGNIFICANCE

Although larvae can minimize the effects of high NaTPI levels by feeding on leaves with high protein and low NaTPI activity, the host plant's endogenous NaTPIs remain an effective defense against M. sexta, inhibiting gut proteinase and affecting larval performance.

Biochemical characterization of midgut digestive proteases from Mamestra brassicae (cabbage moth; Lepidoptera: Noctuidae) and effect of soybean Kunitz inhibitor (SKTI) in feeding assays

Proteolytic activities in soluble protein extracts from Mamestra brassicae (cabbage moth) larval midgut were analysed using specific peptide substrates and proteinase inhibitors. Serine proteinases were the major activities detected, with chymotrypsin-like and trypsin-like activities being responsible for approximately 62% and 19% of the total proteolytic activity towards a non-specific protein substrate. Only small amounts of elastase-like activities could be detected. The serine proteinases were active across the pH range 7-12.5, with both trypsin-like and chymotrypsin-like activities maximal at pH 11.5. The digestive proteinases were stable to the alkaline environment of the lepidopteran gut over the timescale of passage of food through the gut, with 50% of trypsin and 40% of chymotrypsin activity remaining after 6h at pH 12, 37 degrees C. Soybean Kunitz trypsin inhibitor (SKTI) ingestion by the larvae had a growth-inhibitory effect, and induced inhibitor-insensitive trypsin-like activity. Qualitative and quantitative changes in proteinase activity bands after gel electrophoresis of gut extracts were evident in SKTI-fed larvae when compared with controls, with increases in levels of most bands, appearance of new bands, and a decrease in the major proteinase band present in extracts from control insects.

Tri-trophic effects of transgenic insect-resistant tobacco expressing a protease inhibitor or a biotin-binding protein on adults of the predatory carabid beetle Ctenognathus novaezelandiae

Tri-trophic impacts on adult predatory carabid beetles, Ctenognathus novaezelandiae, of insect-resistant transgenic tobacco plants expressing a serine protease inhibitor, bovine spleen trypsin inhibitor (BSTI), or a biotin-binding protein, avidin, were investigated. Both proteins could potentially affect this beetle, since avidin is known to be insecticidal to many beetle species and C. novaezelandiae midguts were shown to contain high levels of trypsin, a protease powerfully inhibited by bovine pancreatic trypsin inhibitor (a BSTI homologue) in vitro. Newly emerged field-collected adult C. novaezelandiae were fed exclusively for 280 days on Spodoptera litura larvae raised either on non-transgenic control, transgenic avidin (55 ppm) or transgenic BSTI (68 ppm) tobacco. Despite this long-term exclusive diet, there was no treatment effect on survival or fecundity and only minor and transient effects on beetles were observed. Data pooled across time and genders showed control-prey-fed beetles weighed 3% more than BSTI-prey-fed beetles and avidin-prey-fed beetles consumed 3-4% fewer prey than control- or BSTI-prey-fed individuals. Females in all treatments gained more mass and survived longer than males. Low exposure to the proteins because of dilution and deactivation within the prey is the most likely explanation for the lack of tri-trophic effects observed. Aditionally, the presence of a digestive chymotrypsin only partially inhibited by BSTI may provide an alternative path for proteolysis.

Expressed sequence tags from the midgut of Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae)

The midgut is a key tissue in insect science. Physiological roles include digestion and peritrophic membrane function, as well as being an important target for insecticides. We used an expressed sequence tag (EST) approach to identify candidate genes and gene families involved in these processes in the light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae). Two cDNA libraries were constructed from dissected midgut of third to fifth instar larvae. Clustering analysis of 6416 expressed sequence tags produced 1178 tentative unique genes comprising 725 tentative contigs and 453 singletons. The sequences show similar codon usage to sequences from other lepidopterans, a Kozak consensus sequence similar to Drosophila and single nucleotide polymorphisms (SNPs) were detected at a frequency of 1.35/kb. The identity of the most common Interpro families correlates well with major known functions of the midgut. Phylogenetic analysis was conducted on representative sequences from selected multigene families. Gene families include a broad range of digestive proteases, lipases and carbohydrases that appear to have degradative capacity against the major food components found in leaves, the diet of these larvae; and carboxylesterases, glutathione-S-transferases and cytochrome P450 monooxygenases, potentially involved in xenobiotic degradation. Two of the larger multigene families, serine proteases and lipases, expressed a high proportion of genes that are likely to be catalytically inactive.

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.

Bioinsecticidal activity of Archidendron ellipticum trypsin inhibitor on growth and serine digestive enzymes during larval development of Spodoptera litura

The roles of serine proteases involved in the digestion mechanism of the cutworm Spodoptera litura (Lepidoptera: Noctuidae) were examined (in vitro and in vivo) following feeding of plant protease inhibitors. A trypsin inhibitor from Archidendron ellipticum (AeTI) was purified by ammonium sulfate fractionation, ion-exchange chromatography and size-exclusion chromatography (HPLC) and its bioinsecticidal properties against S. litura were compared with Soybean Kunitz trypsin inhibitor (SBTI). AeTI inhibited the trypsin-like activities of the midgut proteases of fifth instar larvae of S. litura by over 70%. Dixon plot analysis revealed competitive inhibition of larval midgut trypsin and chymotrypsin by AeTI, with an inhibition constant (K(i)) of 3.5x10(-9) M and 1.5x10(-9) M, respectively. However, inhibitor kinetics using double reciprocal plots for both trypsin and chymotrypsin inhibitions demonstrated a mixed inhibition pattern. Feeding experiments conducted on different (neonate to ultimate) instars suggested a dose-dependent decrease for both the larval body weight as well as % survival of larva fed on diet containing 50, 100 and 150 microM AeTI. Influence of AeTI on the larval gut physiology indicated a 7-fold decrease of trypsin-like protease activity and a 5-fold increase of chymotrypsin-like protease activity, after being fed with a diet supplemented with 150 microM AeTI. This study suggests that although the early (1st to 3rd) larval instars of S. litura are susceptible to the trypsin inhibitory action of AeTI, the later instars may facilitate the development of new serine proteases, insensitive to the inhibitor.

Assessment of novel inhibitors of Helicoverpa aminopeptidases as anti-insect agents

Helicoverpa species present problems worldwide as pests on a variety of agricultural crops. In Australia, the costs of controlling H. armigera (Hübn.) and H. punctigera (Wall.) are a major burden on the cotton industry, and novel mechanisms are continually sought to combat these pests. Potential new targets for insecticides are the digestive proteases of the insect, including the aminopeptidases (APs). A variety of compounds, designed to be similar in structure to known AP inhibitors, were synthesized and screened for activity in inhibiting H. armigera larval growth and AP activity. The most effective compounds in both assays proved to be hydroxamic acids and methylphosphonic acids. Compounds that incorporated both of these groups were also found to have significant potential as control agents. The most inhibitory compounds included valine methylphosphonic acid and a leucine methylphosphonic acid/hydroxamic acid derivative. The valine methylphosphonic acid compound was tested further in vitro, with the aim of producing a new active capable of restricting the viability of Helicoverpa populations on commercial crops.

Podborer (Helicoverpa armigera Hübn.) does not show specific adaptations in gut proteinases to dietary Cicer arietinum Kunitz proteinase inhibitor

We investigated the response of Helicoverpa armigera larvae towards ingestion of Cicer arietinum Kunitz proteinase inhibitor (CaKPI), which caused antagonistic effects on developing H. armigera larvae. CaKPI-degrading proteinases were not detectable in either control or sensitized larvae. There were negligible increases in total proteinase activity, as well as in trypsin-like and chymotrypsin-like activities of H. armigera gut proteinases (HGPs). Decrease in sensitivity of HGPs to inhibition by CaKPI was not observed when the inhibitor was fed suggesting that the insect had not shown a specific adaptive response to dietary CaKPI. Semi-quantitative reverse transcriptase polymerase chain reaction (Q RT-PCR) analysis showed a general up-regulation of proteases in larvae that ingested CaKPI and a specific regulation of individual transcripts was not observed. CaKPI had maximum inhibitory activity against HGP derived from fourth instar larvae. CaKPI was equally potent in inhibition of HGPs derived from larvae fed on different host plants, as well as various proteinase inhibitors (PIs) to which larval adaptation was previously reported. The lack of larval response to CaKPI was attributable to the atypical active site sequence and inhibitory activity of CaKPI and/or to the pre-adaptation of H. armigera larvae due to the constant exposure to basal levels of CaKPI in chickpea seeds or a chickpea seed-based diet.

Evolutionary mechanisms acting on proteinase inhibitor variability

The interaction of proteinase inhibitors produced, in most cases, by host organisms and the invasive proteinases of pathogens or parasites or the dietary proteinases of predators, results in an evolutionary 'arms race' of rapid and ongoing change in both interacting proteins. The importance of these interactions in pathogenicity and predation is indicated by the high level and diversity of observable evolutionary activity that has been found. At the initial level of evolutionary change, recruitment of other functional protein-folding families has occurred, with the more recent evolution of one class of proteinase inhibitor from another, using the same mechanism and proteinase contact residues. The combination of different inhibitor domains into a single molecule is also observed. The basis from which variation is possible is shown by the high rate of retention of gene duplication events and by the associated process of inhibitory domain multiplication. At this level of reorganization, mutually exclusive splicing is also observed. Finally, the major mechanism by which variation is achieved rapidly is hypervariation of contact residues, an almost ubiquitous feature of proteinase inhibitors. The diversity of evolutionary mechanisms in a single class of proteins is unlikely to be common, because few systems are under similar pressure to create variation. Proteinase inhibitors are therefore a potential model system in which to study basic evolutionary process such as functional diversification.

Distribution and residual activity of two insecticidal proteins, avidin and aprotinin, expressed in transgenic tobacco plants, in the bodies and frass of Spodoptera litura larvae following feeding

To understand how a major cosmopolitan pest responds to two very different insecticidal proteins and to determine whether herbivorous insects and their frass could be environmental sources of recombinant proteins from transgenic plants, Spodoptera litura (Fab.) (Lepidoptera, Noctuidae) larvae were fed on tobacco leaves expressing either the biotin-binding protein, avidin, or the protease inhibitor, aprotinin. Control larvae received non-transgenic tobacco. Samples of larvae were taken after 5, 6 or 7 days' feeding and frass was collected after two 24-h periods at 6 and 7 days. Insects in all treatments grew significantly during the experiment, but the avidin-fed larvae were significantly smaller than the others on Day 7. Avidin was found in all samples of avidin-fed larvae (7.0+/-0.86 ng mg(-1), n=45), at a lower level than in their frass (31.9+/-5.08 ng mg(-1), n=30), and these frass levels were lower than those of the the leaves fed to the larvae (69.0+/-6.71 ng mg(-1), n=45). All of the avidin detected in these samples was capable of binding biotin. On average, between 10 and 28% of avidin was recovered with the methods used, whereas almost full recovery of aprotinin was effected. Aprotinin levels in larvae (8.2+/-0.53 ng mg(-1), n=45) were also lower than aprotinin levels in frass (77.4+/-6.9 ng mg(-1), n=30), which were somewhat lower than those in the leaves fed to the larvae (88.6+/-2.51 ng mg(-1), n=45). Approximately half the trypsin-binding ability of aprotinin was lost in larvae, and in frass, aprotinin had lost about 90% of its ability to bind trypsin.

Diversity of trypsins in the Mediterranean corn borer Sesamia nonagrioides (Lepidoptera: Noctuidae), revealed by nucleic acid sequences and enzyme purification

The existence of a diverse trypsin gene family with a main role in the proteolytic digestion process has been proved in vertebrate and invertebrate organisms. In lepidopteran insects, a diversity of trypsin-like genes expressed in midgut has also been identified. Genomic DNA and cDNA trypsin-like sequences expressed in the Mediterranean corn Borer (MCB), Sesamia nonagrioides, midgut are reported in this paper. A phylogenetic analysis revealed that at least three types of trypsin-like enzymes putatively involved in digestion are conserved in MCB and other lepidopteran species. As expected, a diversity of sequences has been found, including four type-I (two subtypes), four type-II (two subtypes) and one type-III. In parallel, four different trypsins have been purified from midgut lumen of late instar MCB larvae. N-terminal sequencing and mass spectrometric analyses of purified trypsins have been performed in order to identify cDNAs coding for major trypsins among the diversity of trypsin-like sequences obtained. Thus, it is revealed that the four purified trypsins in MCB belong to the three well-defined phylogenetic groups of trypsin-like sequences detected in Lepidoptera. Major active trypsins present in late instar MCB lumen guts are trypsin-I (type-I), trypsin-IIA and trypsin-IIB (type-II), and trypsin-III (type-III). Trypsin-I, trypsin-IIA and trypsin-III showed preference for Arg over Lys, but responded differently to proteinaceous or synthetic inhibitors. As full-length cDNA clones coding for the purified trypsins were available, three-dimensional protein models were built in order to study the implication of specific residues on their response to inhibitors. Thus, it is predicted that Arg73, conserved in type-I lepidopteran trypsins, may favour reversible inhibition by the E-64. Indeed, the substitution of Val213Cys, unique for type-II lepidopteran trypsins, may be responsible for their specific inhibition by HgCl2. The implication of these results on the optimisation of the use of protease inhibitors for pest control, and on the identification of endoprotease-mediated resistance to Bacillus thuringiensis Cry-toxins is discussed.

Molecular dynamics of detoxification and toxin-tolerance genes in brown planthopper (Nilaparvata lugens Stål., Homoptera: Delphacidae) feeding on resistant rice plants

To investigate the molecular response of brown planthopper, Nilaparvata lugens (BPH) to BPH-resistant rice plants, we isolated cDNA fragments of the genes encoding for carboxylesterase (CAR), trypsin (TRY), cytochrome P450 monooxygenase (P450), NADH-quinone oxidoreductase (NQO), acetylcholinesterase (ACE), and Glutathione S-transferase (GST). Expression profiles of the genes were monitored on fourth instar nymphs feeding on rice varieties with different resistance levels. Northern blot hybridization showed that, compared with BPH reared on susceptible rice TN1, expression of the genes for P450 and CAR was apparently up-regulated and TRY mRNA decreased in BPH feeding on a highly resistant rice line B5 and a moderately resistant rice variety MH63, respectively. Two transcripts of GST increased in BPH feeding on B5; but in BPH feeding on MH63, this gene was inducible and its expression reached a maximum level at 24 h, and then decreased slightly. The expression of NQO gene was enhanced in BPH on B5 plants but showed a constant expression in BPH on MH63 plants. No difference in ACE gene expression among BPH on different rice plants was detected by the RT-PCR method. The results suggest these genes may play important roles in the defense response of BPH to resistant rice.

Characterization of two midgut proteinases of Helicoverpa armigera and their interaction with proteinase inhibitors

Two serine proteinases from the midgut of Helicoverpa armigera have been partially purified and characterized. One proteinase, HGP-1, was capable of hydrolyzing a synthetic substrate of elastase and was inhibited by elastatinal. The second proteinase, HGP-2, was inhibited by a trypsin inhibitor. Molecular weights of HGP-1 and HGP-2 were approximately 26.0 and 29.0kDa, respectively. Both the proteinases exhibited alkaline pH optima in the range of 10-11. Furthermore, interaction of HGP-1 and HGP-2 with proteinase inhibitors (PIs) from host and non-host plants was studied. HGP-1 was not only insensitive to a PI from chickpea (host) but was also able to degrade it. The same PI from chickpea was able to inhibit over 50% activity of HGP-2. On the contrary, PIs from potato (non-host) showed strong inhibition of both, HGP-1 and HGP-2 and also demonstrated protection of chickpea seed proteins from digestion by both the HGPs. These results could provide important clues in designing strategies for sustainable use of plant PIs in developing insect-tolerant transgenic plants.

Gene expression patterns of Helicoverpa armigera gut proteases

Relative quantification of reported gut proteinase cDNAs from Helicoverpa armigera larvae fed on various host plants (cotton, chickpea, pigeonpea, tomato and okra), non-host plant PIs (winged bean, bitter gourd, ground nut, and capsicum) and during larval development has been carried out using semi-quantitative RT-PCR. Five trypsin-like and three chymotrypsin-like proteinases were categorized as insensitive or sensitive to most of the proteinase inhibitors (PIs) and insensitive/sensitive to specific PIs based on their expression analysis. These results were supported by amino acid sequence analysis, where diverged amino acids were observed in the regions, which are reported to be involved in typical trypsin-trypsin inhibitor interactions and critical for proteinase inhibitor resistance. Among exopeptidases (five aminopeptidase and three carboxypeptidase), HaAmi4 and HaAmi5 of aminopeptidase and HaCar1 of carboxypeptidase exhibited considerable differential expression. Elastase and cathepsin B-like proteinases were expressed at negligible levels. The proteases identified in the study would be ideal candidates for further interactions studies with protease inhibitors to understand the structural reasons of protease inhibitor insensitivity.