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

Starvation-Induced Changes to the Midgut Proteome and Neuropeptides in Manduca sexta

Starvation is a complex physiological state that induces changes in protein expression to ensure survival. The insect midgut is sensitive to changes in dietary content as it is at the forefront of communicating information about incoming nutrients to the body via hormones. Therefore, a DIA proteomics approach was used to examine starvation physiology and, specifically, the role of midgut neuropeptide hormones in a representative lepidopteran, Manduca sexta. Proteomes were generated from midguts of M. sexta fourth-instar caterpillars, starved for 24 h and 48 h, and compared to fed controls. A total of 3047 proteins were identified, and 854 of these were significantly different in abundance. KEGG analysis revealed that metabolism pathways were less abundant in starved caterpillars, but oxidative phosphorylation proteins were more abundant. In addition, six neuropeptides or related signaling cascade proteins were detected. Particularly, neuropeptide F1 (NPF1) was significantly higher in abundance in starved larvae. A change in juvenile hormone-degrading enzymes was also detected during starvation. Overall, our results provide an exploration of the midgut response to starvation in M. sexta and validate DIA proteomics as a useful tool for quantifying insect midgut neuropeptide hormones.

Proteomic Analysis of the Midgut Contents of Silkworm in the Pupal Stage

The silkworm Bombyx mori, a lepidopteran insect, possesses an 8-10-day pupal stage, during which significant changes occur in the midgut, where it first condenses into the yellow body, and then undergoes decomposition. To gain insights into this transformation process, proteomics was performed on Bombyx mori midgut contents on day 2 and day 7 after pupation. The results revealed the identification of 771 proteins with more than one unique peptide. An analysis using AgriGO demonstrated that these proteins were predominantly associated with catalytic activity. Among the identified proteins, a considerable number were found to be involved in carbohydrate metabolism, amino acid metabolism, lipid metabolism, nucleic acid degradation, and energy support. Additionally, variations in the levels of certain proteases were observed between the midgut contents on day 2 and day 7 after pupation. An in-depth analysis of the two-dimensional electrophoresis of the midgut contents on day 7 after pupation led to the identification of twelve protein spots with potential gelatinolytic activity. Among these, six proteases were identified through mass spectrometry, including the p37k protease, vitellin-degrading protease, chymotrypsin-2, etc. These proteases may be responsible for the digestion of the yellow body during the later stages of pupal development.

Parasitoid Serpins Evolve Novel Functions to Manipulate Host Homeostasis

Parasitoids introduce various virulence factors when parasitism occurs, and some taxa generate teratocytes to manipulate the host immune system and metabolic homeostasis for the survival and development of their progeny. Host-parasitoid interactions are extremely diverse and complex, yet the evolutionary dynamics are still poorly understood. A category of serpin genes, named CvT-serpins, was discovered to be specifically expressed and secreted by the teratocytes of Cotesia vestalis, an endoparasitoid of the diamondback moth Plutella xylostella. Genomic and phylogenetic analysis indicated that the C. vestalis serpin genes are duplicated and most of them are clustered into 1 monophyletic clade. Intense positive selection was detected at the residues around the P1-P1' cleavage sites of the Cv-serpin reactive center loop domain. Functional analyses revealed that, in addition to the conserved function of melanization inhibition (CvT-serpins 1, 16, 18, and 21), CvT-serpins exhibited novel functions, i.e. bacteriostasis (CvT-serpins 3 and 5) and nutrient metabolism regulation (CvT-serpins 8 and 10). When the host-parasitoid system is challenged with foreign bacteria, CvT-serpins act as an immune regulator to reprogram the host immune system through sustained inhibition of host melanization while simultaneously functioning as immune effectors to compensate for this suppression. In addition, we provided evidence that CvT-serpin8 and 10 participate in the regulation of host trehalose and lipid levels by affecting genes involved in these metabolic pathways. These findings illustrate an exquisite tactic by which parasitoids win out in the parasite-host evolutionary arms race by manipulating host immune and nutrition homeostasis via adaptive gene evolution and neofunctionalization.

Knockdown of Helicoverpa armigera protease genes affects its growth and mortality via RNA interference

Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), the cotton bollworm, is a destructive pest which is famous for its resistance to a variety of insecticides. RNA interference is a posttranscriptional gene silencing mechanism that has become a popular tool to control insect pests, triggered by double-stranded RNAs (dsRNAs). The effect of ingestion and injection delivery methods of dsRNA related to some protease genes including Trypsin (Ha-TRY39 and Ha-TRY96), Chymotrypsin (Ha-CHY), and Cathepsin L (Ha-CAT) on growth and development of H. armigera was investigated in this study. All protease genes encoded full ORFs and were expressed in all H. armigera larvae stages and tissues. In both injection and feeding bioassays, Ha-RNAi CHY's performance outperformed that of other protease genes. CHY enzyme activity in the midgut of larvae was significantly reduced after treatment with ds-HaCHY. Oral administration of ds-CHY also resulted in significant mortality of H. armigera larvae. However, because of the high RNase activity in the midgut lumen of lepidoptera, a large amount of dsRNA was needed to effectively kill instars of H. armigera. To reduce dsRNA degradation, bacterial expression and dsRNA formulation were used. After oral administration, it was toxic to H. armigera larvae. Before oral administration, bacterial cells were sonicated to increase dsRNA release. The RNA interference efficiency of sonicated bacteria was significantly increased, resulting in higher larval mortality when administered orally. All of these findings point to Ha-CHY as a new candidate for developing an effective dsRNA-based pesticide for H. armigera control.

Black Soldier Fly Larvae Adapt to Different Food Substrates through Morphological and Functional Responses of the Midgut

Modulation of nutrient digestion and absorption is one of the post-ingestion mechanisms that guarantees the best exploitation of food resources, even when they are nutritionally poor or unbalanced, and plays a pivotal role in generalist feeders, which experience an extreme variability in diet composition. Among insects, the larvae of black soldier fly (BSF), Hermetia illucens, can grow on a wide range of feeding substrates with different nutrient content, suggesting that they can set in motion post-ingestion processes to match their nutritional requirements. In the present study we address this issue by investigating how the BSF larval midgut adapts to diets with different nutrient content. Two rearing substrates were compared: a nutritionally balanced diet for dipteran larvae and a nutritionally poor diet that mimics fruit and vegetable waste. Our data show that larval growth performance is only moderately affected by the nutritionally poor diet, while differences in the activity of digestive enzymes, midgut cell morphology, and accumulation of long-term storage molecules can be observed, indicating that diet-dependent adaptation processes in the midgut ensure the exploitation of poor substrates. Midgut transcriptome analysis of larvae reared on the two substrates showed that genes with important functions in digestion and absorption are differentially expressed, confirming the adaptability of this organ.

Silk properties and overwinter survival in gregarious butterfly larvae

All organisms are challenged by encounters with parasites, which strongly select for efficient escape strategies in the host. The threat is especially high for gregarious species entering immobile periods, such as diapause. Larvae of the Glanville fritillary butterfly, Melitaea cinxia, spend the winter in diapause in groups of conspecifics each sheltered in a silk nest. Despite intensive monitoring of the population, we have little understanding of the ecological factors influencing larval survival over the winter in the field. We tested whether qualitative and quantitative properties of the silk nest contribute to larval survival over diapause. We used comparative proteomics, metabarcoding analyses, microscopic imaging, and in vitro experiments to compare protein composition of the silk, community composition of the silk-associated microbiota, and silk density from both wild-collected and laboratory-reared families, which survived or died in the field. Although most traits assessed varied across families, only silk density was correlated with overwinter survival in the field. The silk nest spun by gregarious larvae before the winter acts as an efficient breathable physical shield that positively affects larval survival during diapause. Such benefit may explain how this costly trait is conserved across populations of this butterfly species and potentially across other silk-spinning insect species.

A midgut-specific serine protease, BmSP36, is involved in dietary protein digestion in the silkworm, Bombyx mori

Serine proteases play important roles in digestion and immune responses during insect development. In the present study, the serine protease gene BmSP36, which encodes a 292-residue protein, was cloned from the midgut cells of Bombyx mori. BmSP36 contains an intact catalytic triad (H57, D102 and S195) and a conserved substrate-binding site (G189, H216 and G226), suggesting that it is a serine protease with chymotrypsin-like specificity. The temporal and spatial expression patterns of BmSP36 indicated that its messenger RNA and protein expression mainly occurred in the midgut at the feeding stages. Western blotting, immunofluorescence and liquid chromatography-tandem mass spectrometry analyses revealed secretion of BmSP36 protein from epithelial cells into the midgut lumen. The transcriptional and translational expression of BmSP36 was down-regulated after starvation but up-regulated after refeeding. Moreover, expression of the BmSP36 gene could be up-regulated by a juvenile hormone analogue. These results enable us to better define the potential role of BmSP36 in dietary protein digestion at the feeding stages during larval development.

Optimization of recombinant bacteria expressing dsRNA to enhance insecticidal activity against a lepidopteran insect, Spodoptera exigua

Double-stranded RNA (dsRNA) has been applied to control insect pests due to its induction of RNA interference (RNAi) of a specific target gene expression. However, developing dsRNA-based insecticidal agent has been a great challenge especially against lepidopteran insect pests due to variations in RNAi efficiency. The objective of this study was to screen genes of chymotrypsins (SeCHYs) essential for the survival of the beet armyworm, Spodoptera exigua, to construct insecticidal dsRNA. In addition, an optimal oral delivery method was developed using recombinant bacteria. At least 7 SeCHY genes were predicted from S. exigua transcriptomes. Subsequent analyses indicated that SeCHY2 was widely expressed in different developmental stages and larval tissues by RT-PCR and its expression knockdown by RNAi caused high mortality along with immunosuppression. However, a large amount of dsRNA was required to efficiently kill late instars of S. exigua because of high RNase activity in their midgut lumen. To minimize dsRNA degradation, bacterial expression and formulation of dsRNA were performed in HT115 Escherichia coli using L4440 expression vector. dsRNA (300 bp) specific to SeCHY2 overexpressed in E. coli was toxic to S. exigua larvae after oral administration. To enhance dsRNA release from E. coli, bacterial cells were sonicated before oral administration. RNAi efficiency of sonicated bacteria was significantly increased, causing higher larval mortality at oral administration. Moreover, targeting young larvae possessing weak RNase activity in the midgut lumen significantly enhanced RNAi efficiency and subsequent insecticidal activity against S. exigua.

CHARACTERIZATION OF TRYPSIN-LIKE AND CHYMOTRYPSIN-LIKE SERINE PROTEASES FROM MIDGUT OF Mythimna separata Walker

Two cDNA sequences encoding a trypsin-like and a chymotrypsin-like serine protease (MsT and MsCT, GenBank accession Nos. KP730443 and KP730444, respectively) were cloned from midgut of oriental armyworm, Mythimna separata Walker. Multiple alignments revealed that the deduced amino acid sequences of MsT and MsCT contained a serine protease catalytic motif GDSGGPL and catalytic triads (His, Asp, and Ser). Analyses of tissue and developmental expression of MsT and MsCT showed that they were mainly expressed in midguts and could be detected in first to sixth instar larvae, prepupal and pupal stages. Expressions of both MsT and MsCT were downregulated after 24 h of starvation and upregulated by subsequent insect refeeding. MsT expression in response to 20-hydroxyecdysone (20E) was dose dependent and upregulated after 24 h. However, MsCT expression in response to 20E was downregulated compared with controls. MsCT, but not MsT, transcripts were upregulated after 24 h of Cry1Ac protoxin exposure. These results suggested that MsT was most likely involved in food protein digestion and molting in M. separata whereas MsCT was most likely involved in food protein digestion and Bacillus thuringiensis (Bt) protoxin activation. RNA interference indicated that MsT and MsCT expression levels decreased 76.7 and 86.2% after treated with MsT and MsCT dsRNA, respectively. This study showed that M. separata expressed midgut proteases in line with known lepidopteran counterparts and contributed valuable sequence resource information regarding insect proteases.

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.

Characterization of Bactrocera dorsalis serine proteases and evidence for their indirect role in insecticide tolerance

The oriental fruit fly Bactrocera dorsalis (Hendel) causes devastating losses to agricultural crops world-wide and is considered to be an economically important pest. Little is known about the digestive enzymes such as serine proteases (SPs) in B. dorsalis, which are important both for energy supply and mitigation of fitness cost associated with insecticide tolerance. In this study, we identified five SP genes in the midgut of B. dorsalis, and the alignments of their deduced amino acid sequences revealed the presence of motifs conserved in the SP superfamily. Phylogenetic analyses with known SPs from other insect species suggested that three of them were trypsin-like proteases. Analyses of the expression profiles among the different developmental stages showed that all five genes were most abundant in larvae than in other stages. When larvae were continuously fed on diet containing 0.33 μg/g β-Cypermethrin, expression of all five genes were upregulated in the midgut but the larval development was delayed. Biochemical assays were consistent with the increased protease activity exhibited by SPs in the midgut after treatment with β-Cypermethrin. Taken together, these findings provide evidence for the hypothesis that enhanced SP activity may play an indirect role in relieving the toxicity stress of insecticide in B. dorsalis.

Comparative digestive physiology

In vertebrates and invertebrates, morphological and functional features of gastrointestinal (GI) tracts generally reflect food chemistry, such as content of carbohydrates, proteins, fats, and material(s) refractory to rapid digestion (e.g., cellulose). The expression of digestive enzymes and nutrient transporters approximately matches the dietary load of their respective substrates, with relatively modest excess capacity. Mechanisms explaining differences in hydrolase activity between populations and species include gene copy number variations and single-nucleotide polymorphisms. Transcriptional and posttranscriptional adjustments mediate phenotypic changes in the expression of hydrolases and transporters in response to dietary signals. Many species respond to higher food intake by flexibly increasing digestive compartment size. Fermentative processes by symbiotic microorganisms are important for cellulose degradation but are relatively slow, so animals that rely on those processes typically possess special enlarged compartment(s) to maintain a microbiota and other GI structures that slow digesta flow. The taxon richness of the gut microbiota, usually identified by 16S rRNA gene sequencing, is typically an order of magnitude greater in vertebrates than invertebrates, and the interspecific variation in microbial composition is strongly influenced by diet. Many of the nutrient transporters are orthologous across different animal phyla, though functional details may vary (e.g., glucose and amino acid transport with K+ rather than Na+ as a counter ion). Paracellular absorption is important in many birds. Natural toxins are ubiquitous in foods and may influence key features such as digesta transit, enzymatic breakdown, microbial fermentation, and absorption.

Effect of diet delivered various concentrations of double-stranded RNA in silencing a midgut and a non-midgut gene of Helicoverpa armigera

Ribonucleic acid interference (RNAi) is a sequence-specific gene silencing mechanism induced by double-stranded RNA (dsRNA). Recently, RNAi has gained popularity as a reverse genetics tool owing to its tremendous potential in insect pest management, which includes Helicoverpa armigera. However, its efficiency is mainly governed by dsRNA concentration, frequency of application, target gene, etc. Therefore, to obtain a robust RNAi response in H. armigera, we evaluated various concentrations of dsRNA and its frequency of applications delivered through diet in silencing a midgut gene, chymotrypsin and a non-midgut gene, juvenile hormone acid methyl transferase (jhamt) of H. armigera. The extent of target gene silencing was determined by employing reverse transcription quantitative real-time polymerase chain reaction (RT-qPCR). Our study revealed four significant findings: (i) single application of dsRNA elicited a delayed and transient silencing, while multiple applications resulted in early and persistent silencing of the above genes; (ii) silencing of the non-midgut gene (jhamt) through diet delivered dsRNA revealed prevalence of systemic silencing probably due to communication of silencing signals in this pest; (iii) the extent of silencing of chymotrypsin was positively correlated with dsRNA concentration and was negatively correlated with jhamt; (iv) interestingly, over-expression (15–18 folds) of an upstream gene, farnesyl diphosphate synthase (fpps), in juvenile hormone (JH) biosynthetic pathway at higher concentrations of jhamt dsRNA was the plausible reason for lesser silencing of jhamt. This study provides an insight into RNAi response of target genes, which is essential for RNAi design and implementation as a pest management strategy.

Four serine protease cDNAs from the midgut of Plutella xylostella and their proteinase activity are influenced by the endoparasitoid, cotesia vestalis

Serine proteinases, which include trypsins and chymotrypsins, play numerous roles in lepidopteran larvae, such as digestion, zymogen activation, and immune defense. Studies of lepidopteran serine proteinases could increase understanding of their feeding preference (polyphagous and monophagous) and facilitate identification of protease inhibitors, which can be engineered for pest management. In this paper, four full-length cDNAs encoding one chymotrypsin and three trypsins were cloned from larval midguts of the diamondback moth, Plutella xylostella. Real-time quantitative polymerase chain reaction (PCR) analysis showed all four serine protease genes were downregulated after P. xylostella was parasitized by the parasitoid wasp Cotesia vestalis. Trypsin and chymotrypsin enzymatic activities within the midgut of nonparasitized and C. vestalis-parasitized P. xylostella larvae were examined using N-a-benzoyl-arg p-nitroanilide and N-succinyl-ala-ala-pro-phe p-nitroanilide as substrates. Trypsin and chymotrypsin activities were decreased in parasitized larvae as compared to untreated larvae, with the effect being more pronounced over time. Chymotrypsin activity in particular exhibited a significant decrease in activity. The correlation of decreased enzymatic activity and transcript abundance suggests parasitization induced downregulation of serine proteinase gene transcripts.

Characterization of cDNAs encoding serine proteases and their transcriptional responses to Cry1Ab protoxin in the gut of Ostrinia nubilalis larvae

Serine proteases, such as trypsin and chymotrypsin, are the primary digestive enzymes in lepidopteran larvae, and are also involved in Bacillus thuringiensis (Bt) protoxin activation and protoxin/toxin degradation. We isolated and sequenced 34 cDNAs putatively encoding trypsins, chymotrypsins and their homologs from the European corn borer (Ostrinia nubilalis) larval gut. Our analyses of the cDNA-deduced amino acid sequences indicated that 12 were putative trypsins, 12 were putative chymotrypsins, and the remaining 10 were trypsin and chymotrypsin homologs that lack one or more conserved residues of typical trypsins and chymotrypsins. Reverse transcription PCR analysis indicated that all genes were highly expressed in gut tissues, but one group of phylogenetically-related trypsin genes, OnTry-G2, was highly expressed in larval foregut and midgut, whereas another group, OnTry-G3, was highly expressed in the midgut and hindgut. Real-time quantitative PCR analysis indicated that several trypsin genes (OnTry5 and OnTry6) were significantly up-regulated in the gut of third-instar larvae after feeding on Cry1Ab protoxin from 2 to 24 h, whereas one trypsin (OnTry2) was down-regulated at all time points. Four chymotrypsin and chymotrypsin homolog genes (OnCTP2, OnCTP5, OnCTP12 and OnCTP13) were up-regulated at least 2-fold in the gut of the larvae after feeding on Cry1Ab protoxin for 24 h. Our data represent the first in-depth study of gut transcripts encoding expanded families of protease genes in O. nubilalis larvae and demonstrate differential expression of protease genes that may be related to Cry1Ab intoxication and/or resistance.

Cloning and characterization of trypsin- and chymotrypsin-like genes in the striped rice stem borer, Chilo suppressalis

Serine proteinases including trypsins and chymotrypsins play various important roles in insects, including food digestion, immune defense, and zymogen activation. Studies on insect serine proteinases could reveal their feeding preference (polyphagous and monophagous) and facilitate identification of protease inhibitors, which can be engineered for pest management. In this paper, 11 transcripts encoding trypsin- and chymotrypsin-like proteins were cloned from the striped rice stem borer, Chilo suppressalis (Walker). All the predicted proteins share high sequence similarity with known trypsin- and chymotrypsin-like proteins from either lepidopterans or dipterans, and most of the proteins have conserved motifs that are characteristics of serine proteinases. Among the 11 cloned genes, six were expressed predominantly and one exclusively in the midgut of the insect, three were expressed relatively evenly in examined tissues, and one was not expressed in either the gut or hemolymph based on RT–PCR results. The seven genes that were predominantly or exclusively expressed in the gut were also affected by feeding on different host plants. The genes that were expressed in the gut and were affected by host plants are likely to encode digestive proteinases. The identification of trypsin- and chymotrypsin-like genes in this insect species is the first step towards further comparative studies and for identification of insect-specific proteinase inhibitors, which might be engineered to protect rice plants against the striped rice stem borer, which is one of the destructive pests of rice.

Cloning and characterization of a midgut-specific fatty acid binding protein in Spodoptera litura

A fatty acid binding protein (FABP) gene (Slfabp1) was cloned from the midgut of Spodoptera litura larvae. The gene consists of four exons and three introns and encodes a peptide of 134 amino acid residues with a predicted molecular mass of 14.7 kDa, which was confirmed by in vitro protein expression. Northern blot and Western blot analyses indicated that both of Slfabp1 mRNA and protein were highly and specifically expressed in the midgut during the fifth and sixth instar feeding larval stages. In situ hybridization and immunohistochemistry analyses confirmed the midgut-specific localization of Slfabp1 mRNA and protein. The result of Western blot showed that expression of the protein was downregulated by starvation and upregulated by refeeding in sixth instar larvae. All of the results taken together suggest that the SlFABP1 plays important role(s) in FA uptake and transport in the midgut during the larval feeding stages of the insect.

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.