Molecular characterization of an insecticide-induced novel glutathione transferase in silkworm

Structure-activity analysis suggests an olfactory function for the unique antennal delta glutathione transferase of Apis mellifera

Glutathione transferases (GST) are detoxification enzymes that conjugate glutathione to a wide array of molecules. In the honey bee Apis mellifera, AmGSTD1 is the sole member of the delta class of GSTs, with expression in antennae. Here, we structurally and biochemically characterized AmGSTD1 to elucidate its function. We showed that AmGSTD1 can efficiently catalyse the glutathione conjugation of classical GST substrates. Additionally, AmGSTD1 exhibits binding properties with a range of odorant compounds. AmGSTD1 has a peculiar interface with a structural motif we propose to call 'sulfur sandwich'. This motif consists of a cysteine disulfide bridge sandwiched between the sulfur atoms of two methionine residues and is stabilized by CH…S hydrogen bonds and S…S sigma-hole interactions. Thermal stability studies confirmed that this motif is important for AmGSTD1 stability and, thus, could facilitate its functions in olfaction.

Characterization of glutathione-specific gamma glutamyl cyclotransferase (ChaC) in Bombyx mori

Glutathione (GSH) contributes to redox maintenance and detoxification of various xenobiotic and endogenous substances. γ-glutamyl cyclotransferase (ChaC) is involved in GSH degradation. However, the molecular mechanism underlying GSH degradation in silkworms (Bombyx mori) remains unknown. Silkworms are lepidopteran insects that are considered to be an agricultural pest model. We aimed to examine the metabolic mechanism underlying GSH degradation mediated by B. mori ChaC and successfully identified a novel ChaC gene in silkworms (herein, bmChaC). The amino acid sequence and phylogenetic tree revealed that bmChaC was closely related to mammalian ChaC2. We overexpressed recombinant bmChaC in Escherichia coli, and the purified bmChaC showed specific activity toward GSH. Additionally, we examined the degradation of GSH to 5-oxoproline and cysteinyl glycine via liquid chromatography-tandem mass spectrometry. Quantitative real-time polymerase chain reaction revealed that bmChaC mRNA expression was observed in various tissues. Our results suggest that bmChaC participates in tissue protection via GSH homeostasis. This study provides new insights into the activities of ChaC and the underlying molecular mechanisms that can aid the development of insecticides to control agricultural pests.

Role of Insect and Mammal Glutathione Transferases in Chemoperception

Glutathione transferases (GSTs) are ubiquitous key enzymes with different activities as transferases or isomerases. As key detoxifying enzymes, GSTs are expressed in the chemosensory organs. They fulfill an essential protective role because the chemosensory organs are located in the main entry paths of exogenous compounds within the body. In addition to this protective function, they modulate the perception process by metabolizing exogenous molecules, including tastants and odorants. Chemosensory detection involves the interaction of chemosensory molecules with receptors. GST contributes to signal termination by metabolizing these molecules. By reducing the concentration of chemosensory molecules before receptor binding, GST modulates receptor activation and, therefore, the perception of these molecules. The balance of chemoperception by GSTs has been shown in insects as well as in mammals, although their chemosensory systems are not evolutionarily connected. This review will provide knowledge supporting the involvement of GSTs in chemoperception, describing their localization in these systems as well as their enzymatic capacity toward odorants, sapid molecules, and pheromones in insects and mammals. Their different roles in chemosensory organs will be discussed in light of the evolutionary advantage of the coupling of the detoxification system and chemosensory system through GSTs.

Identification of genes and gene expression associated with dispersal capacity in the mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae)

Dispersal flights by the mountain pine beetle have allowed range expansion and major damage to pine stands in western Canada. We asked what the genetic and transcriptional basis of mountain pine beetle dispersal capacity is. Using flight mills, RNA-seq and a targeted association study, we compared strong-flying, weak-flying, and non-flying female beetles from the recently colonized northern end of their range. Nearly 3,000 genes were differentially expressed between strong and weak flying beetles, while weak fliers and nonfliers did not significantly differ. The differentially expressed genes were mainly associated with lipid metabolism, muscle maintenance, oxidative stress response, detoxification, endocrine function, and flight behavior. Three variant loci, two in the coding region of genes, were significantly associated with flight capacity but these genes had no known functional link to flight. Several differentially expressed gene systems may be important for sustained flight, while other systems are downregulated during dispersal and likely to conserve energy before host colonization. The candidate genes and SNPs identified here will inform further studies and management of mountain pine beetle, as well as contribute to understanding the mechanisms of insect dispersal flights.

Investigation of the Substrate-Binding Site of a Prostaglandin E Synthase in Bombyx mori

Prostaglandin E synthase (PGES) catalyzes the conversion of prostaglandin H₂ to prostaglandin E₂ in the presence of glutathione (GSH) in mammals. Amid the limited knowledge on prostaglandin and its related enzymes in insects, we recently identified PGES from the silkworm Bombyx mori (bmPGES) and determined its crystal structure complexed with GSH. In the current study, we investigated the substrate-binding site of bmPGES by site-directed mutagenesis and X-ray crystallography. We found that the residues Tyr107, Val155, Met159, and Glu203 are located in the catalytic pockets of bmPGES, and mutagenesis of each residue reduced the bmPGES activity. Our results suggest that these four residues contribute to the catalytic activity of bmPGES. Overall, this structure-function study holds implications in controlling pests by designing rational and efficient pesticides.

Investigation of the active site of an unclassified glutathione transferase in Bombyx mori by alanine scanning

Glutathione transferase (GST) is an important class of detoxification enzymes that are vital for defense against various xenobiotics and cellular oxidative stress. Previously, we had reported an unclassified glutathione transferase 2 in Bombyx mori (bmGSTu2) to be responsible for detoxifying diazinon. In this study, we aimed to identify the amino acid residues that constitute a hydrogen-bonding network important for GST activity. Site-directed mutagenesis of bmGSTu2 suggested that residues Asn102, Pro162, and Ser166 contribute to its catalytic activity.

An omega-class glutathione S-transferase in the brown planthopper Nilaparvata lugens exhibits glutathione transferase and dehydroascorbate reductase activities

A complementary DNA that encodes an omega-class glutathione S-transferase (GST) of the brown planthopper, Nilaparvata lugens (nlGSTO), was isolated by reverse transcriptase polymerase chain reaction. A recombinant protein (nlGSTO) was obtained via overexpression in the Escherichia coli cells and purified. nlGSTO catalyzes the biotransformation of glutathione with 1-chloro-2,4-dinitrobenzene, a general substrate for GST, as well as with dehydroascorbate to synthesize ascorbate. Mutation experiments revealed that putative substrate-binding sites, including Phe28, Cys29, Phe30, Arg176, and Lue225, were important for glutathione transferase and dehydroascorbate reductase activities. As ascorbate is a reducing agent, nlGSTO may participate in antioxidant resistance.

Functional Analysis of an Epsilon-Class Glutathione S-Transferase From Nilaparvata lugens (Hemiptera: Delphacidae)

Glutathione conjugation is a crucial step in xenobiotic detoxification. In the current study, we have functionally characterized an epsilon-class glutathione S-transferase (GST) from a brown planthopper Nilaparvata lugens (nlGSTE). The amino acid sequence of nlGSTE revealed approximately 36-44% identity with epsilon-class GSTs of other species. The recombinant nlGSTE was prepared in soluble form by bacterial expression and was purified to homogeneity. Mutation experiments revealed that the putative substrate-binding sites, including Phe107, Arg112, Phe118, and Phe119, were important for glutathione transferase activity. Furthermore, inhibition study displayed that nlGSTE activity was affected by insecticides, proposing that, in brown planthopper, nlGSTE could recognize insecticides as substrates.

Characterisation of a diazinon-metabolising glutathione S-transferase in the silkworm Bombyx mori by X-ray crystallography and genome editing analysis

Previously, we found an unclassified glutathione S-transferase 2 (bmGSTu2) in the silkworm Bombyx mori that conjugates glutathione to 1-chloro-2,4-dinitrobenzene and also metabolises diazinon, an organophosphate insecticide. Here, we provide a structural and genome-editing characterisation of the diazinon-metabolising glutathione S-transferase in B. mori. The structure of bmGSTu2 was determined at 1.68 Å by X-ray crystallography. Mutation of putative amino acid residues in the substrate-binding site showed that Pro13, Tyr107, Ile118, Phe119, and Phe211 are crucial for enzymatic function. bmGSTu2 gene disruption resulted in a decrease in median lethal dose values to an organophosphate insecticide and a decrease in acetylcholine levels in silkworms. Taken together, these results indicate that bmGSTu2 could metabolise an organophosphate insecticide. Thus, this study provides insights into the physiological role of bmGSTu2 in silkworms, detoxification of organophosphate insecticides, and drug targets for the development of a novel insecticide.

Enzymatic characterization of two epsilon-class glutathione S-transferases of Spodoptera litura

Two cDNAs encoding glutathione S-transferase (GST) of the tobacco cutworm, Spodoptera litura, were cloned by reverse transcriptase-polymerase chain reaction. The deduced amino acid sequences of the resulting clones revealed 32-51% identities to the epsilon-class GSTs from other organisms. The recombinant proteins were functionally overexpressed in Escherichia coli cells in soluble form and were purified to homogeneity. The enzymes were capable of catalyzing the bioconjugation of glutathione with 1-chloro-2,4-dinitrobenzene, 1,2-epoxy-3-(4-nitrophenoxy)-propane, and ethacrynic acid. A competition assay revealed that the GST activity was inhibited by insecticides, suggesting that it could be conducive to insecticide tolerance in the tobacco cutworm.

Expression profiles of different glutathione S-transferase isoforms in scallop Chlamys farreri exposed to benzo[a]pyrene and chrysene in combination and alone

Aquatic organisms are increasingly exposed to polycyclic aromatic hydrocarbons (PAHs) due to anthropogenic pressure. This study aimed at evaluating the response of Glutathione S-transferases (GSTs) in scallop Chlamys farreri against benzo[a]pyrene (BaP) and chrysene (CHR) exposure under laboratory conditions. Nine published GST genes were classified into six subfamilies and a new member of rho family was identified for the first time. Twelve GSTs (including nine published GST genes and three in transcriptome established by our laboratory) mRNA transcript levels in the gills, digestive glands, adductor muscle, mantle, testis, ovaries, blood cells of scallops were measured by real-time PCR. The results showed that the mRNA transcript levels of twelve GSTs, except GST-zeta, GST-mu and GST-microsomal, were highest in digestive gland. Accordingly, the mRNA expression levels of GSTs were measured in digestive glands of scallops exposed to BaP (0.1μg/L and 1μg/L), CHR (0.1μg/L and 1μg/L) and their mixtures (0.1μg/L BaP +0.1μg/L CHR and 1μg/L BaP +1μg/L CHR). The results indicated that different GST had specific response to different pollution exposure. In BaP exposure experiment, the mRNA expression level of GST-theta was a potential suitable biomarker. GST-sigma-2 and GST-3, which belonged to sigma class, were sensitive to CHR exposure while GST-microsomal was considered a potential ideal bioindicator to joint exposure of BaP and CHR. In summary, this study investigated the classification of GSTs and provided information about the expression profiles of different class GSTs after PAHs exposure.

Biochemical characterization of an unclassified glutathione S-transferase of Plutella xylostella

cDNA encoding an unclassified glutathione S-transferase (GST) of the diamondback moth, Plutella xylostella, was cloned by reverse transcriptase-polymerase chain reaction. The resulting clone was sequenced and the amino acid sequence deduced, revealing 67%-73% identities with unclassified GSTs from other organisms. A recombinant protein was functionally overexpressed in Escherichia coli cells in a soluble form and purified to homogeneity. The enzyme was capable to catalyze the transformation of 1-chloro-2,4-dinitrobenzene and ethacrynic acid with glutathione. A competition assay revealed that GST activity was inhibited by insecticides, suggesting that the enzyme could contribute to insecticide metabolism in the diamondback moth.

Expression profiles of glutathione S-transferase superfamily in Spodoptera litura tolerated to sublethal doses of chlorpyrifos

Chlorpyrifos (CPF) is a broad-spectrum organophosphate insecticide. Glutathione S-transferases (GSTs) in insects are a family of detoxification enzymes and they play critical roles in CPF detoxification. Spodoptera litura is one of the most destructive agricultural pests in tropical and subtropical areas in the world. In this study, 37 Slgsts from 46 unique transcripts of gsts in S. litura transcriptome data, including eight previously reported GSTs, were identified and their expression patterns in susceptible and 12-generation-CPF-treated strains were analyzed to understand the roles of these Slgsts in sublethal doses of CPF tolerance. The results indicate that the members of the S. litura GST superfamily could be distinguished into three major groups: one group, including six cytosolic Slgsts (SlGSTe1, SlGSTe3, SlGSTe10, SlGSTe15, SlGSTo2 and SlGSTs5) and two microsomal Slgsts (SlMGST1-2 and SlMGST1-3), was directly responsible for CPF induction in both 12-generation-treated and susceptible strains; the second group, including three cytosolic Slgsts (SlGSTe13, SlGSTt1 and SlGSTz1) and one microsomal Slgst (SlMGST1-1), was induced only in the 12-generation-treated strain; the third group, including eight cytosolic Slgsts (two epsilon, three delta, one omega, one zeta and one unclassified Slgst), was expressed 1.52-5.15-fold higher in the 12-generation-treated strain than in the susceptible strain.

Identification of glutathione S-transferase genes in Leptinotarsa decemlineata and their expression patterns under stress of three insecticides

Glutathione S-transferases (GSTs) is a family of multifunctional enzymes that are involved in detoxification of poisonous compounds. In the present paper, the Leptinotarsa decemlineata genome and transcriptome dataset were mined and 30 GST genes were identified. These GSTs belonged to cytosolic (29 genes) and microsomal (1 gene) classes. Among them 3 GSTs (LdGSTe2, LdGSTs4, and LdGSTo3) possessed splice variants. Of the 29 cytosolic LdGSTs, 3, 10, 5, 4, 4, and 1 members were classified as delta, epsilon, omega, sigma, theta, and zeta subclasses respectively, along with 2 unclassified genes. Phylogenetic analysis suggest that epsilon, omega and sigma subclasses appear to undergo species-specific bloom. Moreover, most epsilon, omega and sigma GSTs are tandemly arranged in three chromosome scaffolds. To find GST candidates involving in insecticide detoxification, we tested the mRNA levels of 20 GST transcripts under stress of cyhalothrin, fipronil or endosulfan. Out of them, LdGSTe2a, LdGSTe2b, LdGSTo5 and LdGSTt1 were significantly overexpressed after exposure to each of the three insecticides. Two other genes were respectively upregulated after cyhalothrin (LdGSTe10 and LdGSTu2) or endosulfan (LdGSTd1 and LdGSTu2) treatment. The diversified expression responses to insecticide exposure suggest that the LdGSTs may depend on a functionally complex system to detoxify different classes of insecticides. In addition, our findings provide a base for a better understanding of the evolution of insecticide resistance, and functional research on specific GST genes.

Identification of a diazinon-metabolizing glutathione S-transferase in the silkworm, Bombyx mori

The glutathione S-transferase superfamily play key roles in the metabolism of numerous xenobiotics. We report herein the identification and characterization of a novel glutathione S-transferase in the silkworm, Bombyx mori. The enzyme (bmGSTu2) conjugates glutathione to 1-chloro-2,4-dinitrobenzene, as well as metabolizing diazinon, one of the organophosphate insecticides. Quantitative reverse transcription–polymerase chain reaction analysis of transcripts demonstrated that bmGSTu2 expression was induced 1.7-fold in a resistant strain of B. mori. Mutagenesis of putative amino acid residues in the glutathione-binding site revealed that Ile54, Glu66, Ser67, and Asn68 are crucial for enzymatic function. These results provide insights into the catalysis of glutathione conjugation in silkworm by bmGSTu2 and into the detoxification of organophosphate insecticides.

Detoxification of insecticides, allechemicals and heavy metals by glutathione S-transferase SlGSTE1 in the gut of Spodoptera litura

Insect glutathione S-transferases (GSTs) play important roles in detoxifying toxic compounds and eliminating oxidative stress caused by these compounds. In this study, detoxification activity of the epsilon GST SlGSTE1 in Spodoptera litura was analyzed for several insecticides and heavy metals. SlGSTE1 was significantly up-regulated by chlorpyrifos and xanthotoxin in the midgut of S. litura. The recombinant SlGSTE1 had Vmax (reaction rate of the enzyme saturated with the substrate) and Km (michaelis constant and equals to the substrate concentration at half of the maximum reaction rate of the enzyme) values of 27.95 ± 0.88 μmol/min/mg and 0.87 ± 0.028 mmol/L for glutathione, respectively, and Vmax and Km values of 22.96 ± 0.78 μmol/min/mg and 0.83 ± 0.106 mmol/L for 1-chloro-2,4-dinitrobenzene, respectively. In vitro enzyme indirect activity assay showed that the recombinant SlGSTE1 possessed high binding activities to the insecticides chlorpyrifos, deltamethrin, malathion, phoxim and dichloro-diphenyl-trichloroethane (DDT). SlGSTE1 showed higher binding activity to toxic heavy metals cadmium, chromium and lead than copper and zinc that are required for insect normal growth. Western blot analysis showed that SlGSTE1 was induced in the gut of larvae fed with chlorpyrifos or cadmium. SlGSTE1 also showed high peroxidase activity. All the results together indicate that SlGSTE1 may play an important role in the gut of S. litura to protect the insect from the toxic effects of these compounds and heavy metals.

Characterization and expression profiling of glutathione S-transferases in the diamondback moth, Plutella xylostella (L.)

Background

Glutathione S-transferases (GSTs) are multifunctional detoxification enzymes that play important roles in insects. The completion of several insect genome projects has enabled the identification and characterization of GST genes over recent years. This study presents a genome-wide investigation of the diamondback moth (DBM), Plutella xylostella, a species in which the GSTs are of special importance because this pest is highly resistant to many insecticides.

Results

A total of 22 putative cytosolic GSTs were identified from a published P. xylostella genome and grouped into 6 subclasses (with two unclassified). Delta, Epsilon and Omega GSTs were numerically superior with 5 genes for each of the subclasses. The resulting phylogenetic tree showed that the P. xylostella GSTs were all clustered into Lepidoptera-specific branches. Intron sites and phases as well as GSH binding sites were strongly conserved within each of the subclasses in the GSTs of P. xylostella. Transcriptome-, RNA-seq- and qRT-PCR-based analyses showed that the GST genes were developmental stage- and strain-specifically expressed. Most of the highly expressed genes in insecticide resistant strains were also predominantly expressed in the Malpighian tubules, midgut or epidermis.

Conclusions

To date, this is the most comprehensive study on genome-wide identification, characterization and expression profiling of the GST family in P. xylostella. The diversified features and expression patterns of the GSTs are inferred to be associated with the capacity of this species to develop resistance to a wide range of pesticides and biological toxins. Our findings provide a base for functional research on specific GST genes, a better understanding of the evolution of insecticide resistance, and strategies for more sustainable management of the pest.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1343-5) contains supplementary material, which is available to authorized users.

Structural insight into the active site of a Bombyx mori unclassified glutathione transferase

Glutathione transferases (GSTs) are major detoxification enzymes that play central roles in the defense against various environmental toxicants as well as oxidative stress. Here, we identify amino acid residues of an unclassified GST from Bombyx mori, bmGSTu-interacting glutathione (GSH). Site-directed mutagenesis of bmGSTu mutants indicated that amino acid residues Asp103, Ser162, and Ser166 contribute to catalytic activity.

Glutathione-binding site of a bombyx mori theta-class glutathione transferase

The glutathione transferase (GST) superfamily plays key roles in the detoxification of various xenobiotics. Here, we report the isolation and characterization of a silkworm protein belonging to a previously reported theta-class GST family. The enzyme (bmGSTT) catalyzes the reaction of glutathione with 1-chloro-2,4-dinitrobenzene, 1,2-epoxy-3-(4-nitrophenoxy)-propane, and 4-nitrophenethyl bromide. Mutagenesis of highly conserved residues in the catalytic site revealed that Glu66 and Ser67 are important for enzymatic function. These results provide insights into the catalysis of glutathione conjugation in silkworm by bmGSTT and into the metabolism of exogenous chemical agents.

Antenna-specific glutathione S-transferase in male silkmoth Bombyx mori

Glutathione S-transferases (GSTs) are multifunctional enzymes that are widely distributed in different species. GSTs detoxify exogenous and endogenous substances by conjugation to reduced glutathione. We characterized BmGSTD4, an antenna-specific GST, in male silkmoths. The full-length mRNA of Bmgstd4 was cloned by RACE-PCR and contained an open reading frame of 738 bp encoding a 245 amino acid protein. The antenna specificity of BmGSTD4 was validated at the mRNA and protein levels and BmGSTD4 was shown to localize in the sensillum of male silkmoth antennae. Homology modeling and multi-sequence alignment suggested that BmGSTD4 was a typical GST belonging to the δ class and had a canonical GST fold with a conserved N-terminus, including a glutathione-binding site and a C-terminal domain harboring a hydrophobic substrate-binding site. Restricted expression of BmGSTD4 in silkmoth antennae combined with GST activity suggested that BmGSTD4 was involved in the detoxification of harmful chemicals.

Catalytic function of an ε-class glutathione S-transferase of the silkworm

The glutathione S-transferase (GST) superfamily is involved in the detoxification of various xenobiotics. A silkworm GST, belonging to a previously reported Epsilon-class GST family, was identified, named bmGSTE, cloned, and produced in Escherichia coli. Investigation of this enzyme's properties showed that it was able to catalyse glutathione (GSH) with 1-chloro-2,4-dinitrobenzene and ethacrynic acid, and also that it possessed GSH-dependent peroxidase activity. The enzyme's highly conserved amino acid residues, including Ser11, His53, Val55, Ser68 and Arg112, were of interest regarding their possible involvement in its catalytic activity. These residues were replaced with alanine by site-directed mutagenesis and subsequent kinetic analysis of bmGSTE mutants indicated that His53, Val55, and Ser68 were important for enzyme function.

Induction of detoxification enzymes by quercetin in the silkworm

Quercetin is one of the most abundant flavonoids and the defense secondary metabolites in plants. In this study, the effect of quercetin on the growth of the silkworm larvae was investigated. Cytochrome P450 monooxygenases (P450s), glutathione S-transferases (GSTs), and carboxylesterases (COE) were assayed after exposure to different concentrations of quercetin for 3 d (short-term) and 7 d (long-term), respectively. The results showed that the weight gain of the silkworm larvae significantly decreased after the larvae were treated by different concentrations of quercetin except for the treatment with 0.5% quercetin. Activities of P450, GST, and COE were induced by 0.5 or 1% concentration of quercetin. In the midgut, the induction activity of P450s was reached to the highest level (2.3-fold) by 1% quercetin for 7 d, the highest induction activities of GSTs toward CHP and CDNB were 4.1-fold and 2.6-fold of controls by 1% quercetin after 7 d exposure, respectively. For COEs, the highest activity (2.3-fold) was induced by 0.5% quercetin for 7 d. However, P450s in whole body were higher inducible activities in short-term treatment than those in long-term treatment. The responses of eight cytochrome P450 (CYP) genes belonged to CYP6 and CYP9 families and seven GST genes were detected with real-time polymerase chain reaction. In addition, the genes induced by quercetin significantly were confirmed by qRT-PCR. CYP6AB5, CYP6B29, and GSTe8 were identified as inducible genes, of which the highest induction levels were 10.9-fold (0.5% quercetin for 7 d), 6.2-fold (1% quercetin for 7 d), and 7.1-fold (1% quercetin for 7 d), respectively.