Cloning and characterization of two glutathione S-transferases from pyrethroid-resistant Culex pipiens

Characterization of three glutathione S-transferases potentially associated with adaptation of the wheat blossom midge Sitodiplosis mosellana to host plant defense

BACKGROUND

Insect glutathione S-transferases (GSTs), a multifunctional protein family, play a crucial role in detoxification of plant defensive compounds. However, they have been rarely investigated in Sitodiplosis mosellana, a destructive pest of wheat worldwide. In this study, we characterized for the first time a delta (SmGSTd1) and two epsilon GST genes (SmGSTe1 and SmGSTe2) and analyzed their expression patterns and functions associated with adaptation to host plant defense in this species.

RESULTS

Expression of these SmGST genes greatly increased in S. mosellana larvae feeding on resistant wheat varieties Kenong1006, Shanmai139 and Jinmai47 which contain higher tannin and ferulic acid, the major defensive compounds of wheat against this pest, compared with those feeding on susceptible varieties Xinong822, Xinong88 and Xiaoyan22. Their expression was also tissue-specific, most predominant in larval midgut. Recombinant SmGSTs expressed in Escherichia coli could catalyze the conjugation of 1-chloro-2,4-dinitrobenzene, with activity peak at pH around 7.0 and temperature between 30 and 40 °C. Notably, they could metabolize tannin and ferulic acid, with the strongest metabolic ability by SmGSTe2 against two compounds, followed by SmGSTd1 on tannin, and SmGSTe1 on ferulic acid.

CONCLUSION

The results suggest that these SmGSTs are important in metabolizing wheat defensive chemicals during feeding, which may be related to host plant adaptation of S. mosellana. Our study has provided information for future investigation and development of strategies such as host-induced gene silencing of insect-detoxifying genes for managing pest adaptation. © 2023 Society of Chemical Industry.

Susceptibility monitoring and comparative gene expression of susceptible and resistant strains of Spodoptera frugiperda to lambda-cyhalothrin and chlorpyrifos

BACKGROUND

Spodoptera frugiperda (J. E. Smith) is a widespread agricultural pest with several records of resistance to different insecticides and Bt proteins, including the neurotoxic insecticides chlorpyrifos (organophosphate) and lambda-cyhalothrin (pyrethroid). Here, we (i) characterized and monitored the susceptibility of field populations of S. frugiperda to chlorpyrifos (194 populations) and lambda-cyhalothrin (197 populations) collected from major maize-growing regions of Brazil from 2003 to 2016, and (ii) compared gene expression levels of laboratory-selected, chlorpyrifos- and lambda-cyhalothrin-resistant strains to a susceptible reference strain (Sf-ss) of S. frugiperda.

RESULTS

The susceptibility monitoring detected average survival ranging from 29.3% to 36.0% for chlorpyrifos, and 23.1% to 68.0% for lambda-cyhalothrin. The resistance ratio of the chlorpyrifos-resistant strain (Clo-rr) was 25.4-fold and of the lambda-cyhalothrin-resistant strain (Lam-rr) was 21.5-fold. We identified 1098 differentially expressed genes (DEGs) between Clo-rr and Sf-ss, and 303 DEGs between Lam-rr and Sf-ss. Functional analyses of the DEGs revealed the up-regulation of several detoxification enzymes, mainly cytochrome P450 belonging to CYP3 and CYP6 clans. Genes associated with regulatory processes, such as the forkhead box class O (FoxO) transcription factor were also up-regulated. Variant analysis of target-site mutations for both pesticides identified the A201S and F290V mutations in acetylcholinesterase-1, both occurring in heterozigosis in the Clo-rr S. frugiperda strain.

CONCLUSION

Our data show that the overexpression of the enzymatic detoxification machinery is the main difference to explain the resistance of Clo-rr and Lam-rr strains of S. frugiperda to chlorpyrifos and lambda-cyhalothrin, although a target-site mutation also contributes to the Clo-rr resistance to chlorpyrifos. © 2023 Society of Chemical Industry.

Identification of 14 glutathione S-transferase genes from Lasioderma serricorne and characterization of LsGSTe1 involved in lambda-cyhalothrin detoxification

In insects, glutathione S-transferases (GSTs) play a pivotal role in the detoxification of a wide range of pesticides. The cigarette beetle, Lasioderma serricorne, is an economically important pest insect of stored products. Recently, pyrethroid insecticides have been used to control this pest. However, little is known concerning the responses and functions of GSTs in L. serricorne under pyrethroid exposure. In this study, transcriptome sequencing was performed on L. serricorne, and a total of 14 GSTs were identified by retrieving the unigene dataset. Of these, 13 predicted GSTs fell into six cytosolic classes, namely, delta, epsilon, omega, sigma, theta, and zeta, and one was assigned to an "unclassified" group. The GST genes were differentially expressed in various larval tissues and at different developmental stages. Exposure to the pyrethroid insecticide lambda-cyhalothrin (LCT) caused oxidative stress in L. serricorne larvae and led to significantly elevated expression levels of six genes, among which LsGSTe1 was the most upregulated. Recombinant LsGSTE1 protein displayed LCT-metabolizing activity. Furthermore, LsGSTE1 protects cells against oxidative stress. Moreover, knockdown of LsGSTe1 by RNA interference dramatically increased the susceptibility of L. serricorne larvae to LCT treatment. The results from this study provide sequence resources and expression data for GST genes in L. serricorne. Our findings indicate that LsGSTE1 plays a dual role in LCT detoxification by metabolizing the pesticide and by preventing LCT-induced oxidative stress. Thus, the LsGSTe1 gene could be used as a potential target for sustainable management of the cigarette beetle.

Recombinant expression and characterization of GSTd3 from a resistant population of Anopheles arabiensis and comparison of DDTase activity with GSTe2

The development of insecticide resistance in malaria vectors is a challenge for the global effort to control and eradicate malaria. Glutathione S-transferases (GSTs) are multifunctional enzymes involved in the detoxification of many classes of insecticides. For mosquitoes, it is known that overexpression of an epsilon GST, GSTe2, confers resistance towards DDT and pyrethroids. In addition to GSTe2, consistent overexpression of a delta class GST, GSTd3, has been observed in insecticide resistant populations of different malaria vector species. However, the functional role of GSTd3 towards DDT resistance has not yet been investigated. Here, we recombinantly expressed both GSTe2 and GSTd3 from Anopheles arabiensis and compared their metabolic activities against DDT. Both AaGSTd3 and AaGSTe2 exhibited CDNB-conjugating and glutathione peroxidase activity and DDT metabolism was observed for both GSTs. However, the DDT dehydrochlorinase activity exhibited by AaGSTe2 was much higher than for AaGSTd3, and AaGSTe2 was also able to eliminate DDE although the metabolite could not be identified. Molecular modeling revealed subtle differences in the binding pocket of both enzymes and a better fit of DDT within the H-site of AaGSTe2. The overexpression but much lower DDT metabolic activity of AaGSTd3, might suggest that AaGSTd3 sequesters DDT. These findings highlight the complexity of insecticide resistance in the major malaria vectors and the difficulties associated with control of the vectors using DDT, which is still used for indoor residual spraying.

Compounds Inhibiting Noppera-bo, a Glutathione S-transferase Involved in Insect Ecdysteroid Biosynthesis: Novel Insect Growth Regulators

Glutathione S-transferases (GSTs) are conserved in a wide range of organisms, including insects. In 2014, an epsilon GST, known as Noppera-bo (Nobo), was shown to regulate the biosynthesis of ecdysteroid, the principal steroid hormone in insects. Studies on fruit flies, Drosophila melanogaster, and silkworms, Bombyx mori, demonstrated that loss-of-function mutants of nobo fail to synthesize ecdysteroid and die during development, consistent with the essential function of ecdysteroids in insect molting and metamorphosis. This genetic evidence suggests that chemical compounds that inhibit activity of Nobo could be insect growth regulators (IGRs) that kill insects by disrupting their molting and metamorphosis. In addition, because nobo is conserved only in Diptera and Lepidoptera, a Nobo inhibitor could be used to target IGRs in a narrow spectrum of insect taxa. Dipterans include mosquitoes, some of which are vectors of diseases such as malaria and dengue fever. Given that mosquito control is essential to reduce mosquito-borne diseases, new IGRs that specifically kill mosquito vectors are always in demand. We have addressed this issue by identifying and characterizing several chemical compounds that inhibit Nobo protein in both D. melanogaster and the yellow fever mosquito, Aedes aegypti. In this review, we summarize our findings from the search for Nobo inhibitors.

Cap 'n' Collar C and Aryl Hydrocarbon Receptor Nuclear Translocator Facilitate the Expression of Glutathione S-Transferases Conferring Adaptation to Tannic Acid and Quercetin in Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae)

Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae) is a notorious pest of poplar. Coevolution with poplars rich in plant secondary metabolites prompts M. troglodyta to expand effective detoxification mechanisms against toxic plant secondary metabolites. Although glutathione S-transferases (GSTs) play an important role in xenobiotic detoxification in M. troglodyta, it is unclear how GSTs act in response to toxic secondary metabolites in poplar. In this study, five GST gene core promoters were accurately identified by a 5' loss luciferase reporter assay, and the core promoters were significantly induced by two plant secondary metabolites in vitro. Two transcription factors, cap 'n' collar C (CncC) and aryl hydrocarbon receptor nuclear translocator (ARNT), were cloned in M. troglodyta. MtCncC and MtARNT clustered well with other insect CncCs and ARNTs, respectively. In addition, MtCncC and MtARNT could bind the MtGSTt1 promoter and strongly improve transcriptional activity, respectively. However, MtCncC and MtARNT had no regulatory function on the MtGSTz1 promoter. Our findings revealed the molecular mechanisms of the transcription factors MtCncC and MtARNT in regulating the GST genes of M. troglodyta. These results provide useful information for the control of M. troglodyta.

Molecular Mechanisms Underlying Metabolic Resistance to Cyflumetofen and Bifenthrin in Tetranychus urticae Koch on Cowpea

Tetranychus urticae Koch (T. urticae) is one of the most tremendous herbivores due to its polyphagous characteristics, and is resistant to most acaricides. In this study, enzyme-linked immunosorbent assay (ELISA), transcriptome sequencing (RNA-seq) and quantitative real-time PCR (qRT-PCR) were carried out to analyze the mechanisms of T. urticae metabolic resistance to cyflumetofen and bifenthrin on cowpea. The enzyme activity of UDP-glucuronosyltransferases (UGTs) and carboxylesterases (CarEs) in the cyflumetofen-resistant (R_cfm) strain significantly decreased, while that of cytochrome P450 monooxygenases (P450s) significantly increased. Meanwhile, the activities of glutathione-S-transferases (GSTs), CarEs and P450s in the bifenthrin-resistant (R_bft) strain were significantly higher than those in the susceptible strain (Lab_SS). According to the Kyoto Encyclopedia of Genes and Genomes (KEGG) and Gene Ontology (GO) analyses, in the R_cfm mite strain, two carboxyl/cholinesterase (CCE) genes and two P450 genes were upregulated and one gene was downregulated, namely CYP392E7; in the R_bft mite strain, eleven CCE, nine UGT, two P450, four GST and three ABC genes were upregulated, while four CCE and three P450 genes were downregulated. Additionally, 94 differentially expressed genes (DEGs) were common to the two resistant groups. Specifically, TuCCE46 and TuCCE70 were upregulated in both resistant groups. Furthermore, the qRT-PCR validation data were consistent with those from the transcriptome sequencing analysis. Specifically, TuCCE46 (3.37-fold) was significantly upregulated in the R_cfm strain, while in the R_bft strain, TeturUGT22 (5.29-fold), teturUGT58p (1.74-fold), CYP392A11 (2.89-fold) and TuGSTd15 (5.12-fold) were significantly upregulated and TuCCE01 (0.13-fold) and CYP392A2p (0.07-fold) were significantly downregulated. Our study indicates that TuCCE46 might play the most important role in resistance to cyflumetofen, and TuCCE01, teturUGT58p, teturUGT22, CYP392A11, TuGSTd15, TuGSTm09 and TuABCG-13 were prominent in the resistance to bifenthrin. These findings provide further insight into the critical genes involved in the metabolic resistance of T. urticae to cyflumetofen and bifenthrin.

New insights into cypermethrin insecticide resistance mechanisms of Culex pipiens pallens by proteome analysis

BACKGROUND

Due to the development of insecticide resistance in mosquitoes, with worldwide mosquito-borne diseases resurgence in recent years, recent advances in proteome technology have facilitated a proteome-wide analysis of insecticide resistance-associated proteins in mosquitoes. Understanding the complexity of the molecular basis of insecticide resistance mechanisms employed by mosquitoes will help in designing the most effective and sustainable mosquito control methods.

RESULTS

After 30 generations, insecticide-selected strains showed elevated resistance levels to the cypermethrin used for selection. Proteome data allowed the detection of 2892 proteins, of which 2885 differentially expressed proteins (DEPs) achieved quantitative significances in four stages (egg, larvae, pupae, adult) of Culex pipiens pallens cypermethrin-resistant strain as compared to the susceptible strain. Among them, a significant enrichment of proteins, including cuticular proteins, enzymes involved in the detoxification (cytochrome P450, glutathione S-transferases, esterase, ATP-binding cassette) and some biological pathways (oxidative phosphorylation, hippo signalling) that are potentially involved in cypermethrin resistance, was observed. Thirty-one representative DEPs (cytochrome P450, glutathione S-transferase, cuticle protein) during Cx. pipiens pallens developmental stages were confirmed by a parallel reaction monitoring strategy.

CONCLUSIONS

The present study confirmed the power of isobaric tags for relative and absolute quantification for identifying concomitantly quantitative proteome changes associated with cypermethrin in Cx. pipiens pallens. Proteome analysis suggests that proteome modifications can be selected rapidly by cypermethrin, and multiple resistance mechanisms operate simultaneously in cypermethrin-resistance of Cx. pipiens pallens, Our results interpret that an up-regulated expression of proteins and enzymes like cytochrome P450, glutathione S-transferases, esterase etc. has an impact in insecticide resistance. Previously neglected penetration resistance (cuticular proteins) may play an important role in the adaptive response of Cx. pipiens pallens to insecticides. This information may serve as a basis for future work concerning the possible role of these proteins in cypermethrin resistance in mosquito Cx. pipiens pallens. © 2022 Society of Chemical Industry.

Glutathione S-Transferase May Contribute to the Detoxification of (S)-(-)-Palasonin in Plutella xylostella (L.) via Direct Metabolism

The control of P. xylostella primarily involves chemical insecticides, but overuse has brought about many negative effects. Our previous study reported that (S)-(-)-palasonin (PLN) is a plant-derived active substance with significant insecticidal activity against P. xylostella. However, we noticed a possible cross-resistance between (S)-(-)-palasonin and other insecticides which may be related to metabolic detoxification. In order to further explore the detoxification effect of detoxification enzymes on (S)-(-)-palasonin in P. xylostella, the effects of (S)-(-)-palasonin on enzyme activity and transcription level were determined, and the detoxification and metabolism of GSTs on (S)-(-)-palasonin were studied by in vitro inhibition and metabolism experiments. During this study, GST enzyme activity was significantly increased in P. xylostella after (S)-(-)-palasonin treatment. The expression levels of 19 GSTs genes were significantly increased whereas the expression levels of 1 gene decreased. Furthermore, (S)-(-)-palasonin is shown to be stabilized with GSTs and metabolized GSTs (GSTd1, GSTd2, GSTs1 and GSTs2) in vitro, with the highest metabolic rate of 80.59% for GSTs1. This study advances the beneficial utilization of (S)-(-)-palasonin as a botanical pesticide to control P. xylostella in the field.

Metabolism and antioxidant activity of SlGSTD1 in Spodoptera litura as a detoxification enzyme to pyrethroids

Glutathione S-transferase (GSTs) are members of multifunction enzymes in organisms and mostly known for their roles in insecticide resistance by conjugation. Spodoptera litura (Fabricius) is a voracious agricultural pest widely distributed in the world with high resistance to various insecticides. The function of GSTs in the delta group of S. litura is still lacking. Significantly up-regulation of SlGSTd1 was reported in four pyrethroids-resistant populations and a chlorpyrifos-selected population. To further explore its role in pyrethroids and organophosphates resistance, the metabolism and peroxidase activity of SlGSTD1 were studied by heterologous expression, RNAi, and disk diffusion assay. The results showed that K_m and V_max for 1-chloro-2,4-dinitrobenzene (CDNB) conjugating activity of SlGSTD1were 1.68 ± 0.11 mmol L⁻¹ and 76.0 ± 2.7 nmol mg⁻¹ min⁻¹, respectively. Cyhalothrin, beta-cypermethrin, and chlorpyrifos had an obvious inhibitory effect on SlGSTD1 activity, especially for fenvalerate, when using CDNB as substrate. Fenvalerate and cyhalothrin can be metabolized by SlGSTD1 in E. coli and in vitro. Also, silencing of SlGSTd1 significantly increased the toxicity of fenvalerate and cyhalothrin, but had no significant effect on the mortality of larvae treated by beta-cypermethrin or chlorpyrifos. SlGSTD1 possesses peroxidase activity using cumene hydroperoxide as a stress inducer. The comprehensive results indicate that SlGSTD1 is involved in fenvalerate and cyhalothrin resistance of S. litura by detoxication and antioxidant capacity.

Persistent, bioaccumulative, and toxic chemicals in insects: Current state of research and where to from here?

The ongoing decline in the biomass, abundance, and species number of insects is an established fact. Persistent, bioaccumulative, and toxic chemicals (PBTs) - persistent organic pollutants (POPs) and, in the case of our study, mercury (Hg) - play an important role, but their effect on insect populations is insufficiently investigated. Here, the current state of research on PBTs related to insects is examined with a systematic literature study using Web of Science™. We investigate time trends of research intensity compared with other organisms, insect orders and chemicals analyzed, chemicals' effects on insects, and geographical aspects. We show that research intensity increased in the early 1990s, but studies on PBTs in insects are still underrepresented compared with other organisms. The taxonomic focus lies strongly on dipterans. The predominance of studies on DDT suggests its relevance in the context of disease-vector management. Phenotypic and acute effects on insects were more often investigated than genotypic and chronic effects. Laboratory-bred insects and wild-bred insects were examined equally often, pollutant exposure and analysis were conducted predominantly in the laboratory. Mostly habitats with a medium or high human impact were studied, and natural and near-natural habitats are understudied. The sources of the substances are often unknown. Most studies were carried out in economically rich continents, including North America, Europe, and Australia. The numbers of publications dealing with Asia, South America, and Africa are comparatively low, although the control of vector-borne diseases with POPs is still intensively practiced there. We identify gaps in the research - among others, refined analytical methods for biomarkers and for the examination of chronic effects, combinations of field and laboratory experiments to analyze the same problem, and a global approach for the monitoring of PBTs will be needed for accelerating the dearly needed progress in the research of PBTs in insects.

Amplification and Characterization of DDT Metabolizing Delta Class GST in Sand Fly, Phlebotomus argentipes (Diptera: Psychodidae) From Bihar, India

Phlebotomus argentipes is an established vector for Visceral leishmaniasis prevalent in the Indian subcontinent. Insect Glutathione S-transferases (GST) enzyme plays a pivotal role in the metabolism of xenobiotics and chemical insecticides. We report herein the identification and characterization of a delta class GST from the sandfly, P. argentipes. The resulting clone (rParg-GSTδ) is successfully sequenced, which revealed 76.43% and 66.32% gene identity with GST from Phlebotomus papatasi (Scopoli; Diptera: Psychodidae) and Lutzomiya longipalpis (Lutz and Neiva; Diptera: Psychodidae), respectively. The identified rParg-GST amino acid Blast results revealed 82.6% homology to delta class GST of Phlebotomus papatasi and more than 50% homology to Lepidoptera which comprises butterflies and moths. The Phylogenetic analysis of Parg-GST with different classes of Insect GSTs further supported its classification as delta class. A functional recombinant Parg-GSTδ protein (rParg-GSTδ) was expressed in Escherichia coli (Migula; Enterobacterales: Enterobacteriaceae) cells in a soluble form, purified to homogeneity and found to be active against a substrate 1-chloro-2,4-dintrobenzene (CDNB) and lipid peroxidation by-product 4-Hydrxynonenal (4-HNE). Interestingly, rParg-GSTδ demonstrates high dehydrochlorination activity against dichlorodiphenyltrichloroethane (DDT) i.e., 16.27 nM/µg in high performance liquid chromatography (HPLC) assay. These results provide evidence of direct DDT metabolism property exhibited by P. argentipes GST and set the foundation to decipher the metabolic resistance mechanism in P. argentipes against insecticides.

Susceptibility of Dermanyssus gallinae from China to acaricides and functional analysis of glutathione S-transferases associated with beta-cypermethrin resistance

Dermanyssus gallinae poses a significant threat to poultry production, and the resistance to pyrethroids has been identified worldwide. Periodic monitoring of acaricide resistance in D. gallinae is very important for its control, and molecular mechanism associated with beta-cypermethrin resistance in D. gallinae is not fully clear. Results showed, four field isolates of CBP-1, CBP-2, CBP-5 and CBY-1 from China remained either susceptible or with decreased susceptibility (resistance ratio < 5.0) to phoxim, amitraz, propoxur and carbaryl. Four field isolates of CBP-1, CBP-3, CBY-2 and CBH-1 had developed high or extremely high level of resistance (resistance ratio ≥ 40.0) to beta-cypermethrin or permethrin. Detoxification enzyme activity of GSTs was significantly higher in beta-cypermethrin resistant (RS) than susceptible strain (SS), indicating that GSTs are probably involved in beta-cypermethrin resistance in D. gallinae. The recombinant GSTs (rGST-1, 2, 3) showed a pronounced activity toward the conjugates of 1-chloro-2, 4 dinitrobenzene (CDNB) and glutathione (GSH), with rGST-1 presenting the highest enzymatic activity. Constitutive over-expression of Deg-GST-2 was detected in RS strain, and GSTs genes were all inducible with the treatment of beta-cypermethrin in SS and RS strains. More importantly, knocking down Deg-GST-2 gene expression by RNAi increased the susceptibility of RS strain to beta-cypermethrin. HPLC analysis indicated that rGST-1 protein could metabolize phoxim directly, but rGSTs could not directly metabolize beta-cypermethrin. Our results indicated that some field isolates of D. gallinae from China had developed high level of resistance to pyrethroids, and elevated GSTs activity as well as increased GSTs expression levels were involved in beta-cypermethrin resistance, but the three evaluated GSTs did not play a direct role in the metabolism of beta-cypermethrin.

The determination of Plutella xylostella (L.) GSTs (PxGSTs) involved in the detoxification metabolism of Tolfenpyrad

BACKGROUND

Insect glutathione S-transferases (GSTs) play a crucial role in insecticide detoxification. However, there remains a distinct lack of information regarding the role of GSTs in the detoxification of Tolfenpyrad (TFP) in insects.

RESULTS

Real-time quantitative PCR showed significant upregulation of PxGSTs after exposure to TFP for 6 h. An in vitro inhibition assay showed that TFP could inhibit PxGSTδ, PxGSTε and PxGSTσ, and the most pronounced inhibitory effect was on PxGSTσ. Metabolism assays displayed that PxGSTσ was superior to other test PxGSTs in metabolizing TFP. The molecular docking of TFP and PxGSTσ revealed that the H-bond provided by the sidechains of Tyr107 and Tyr162 were key to the detoxification of TFP by PxGSTσ. Further tests using mutant PxGSTσ proteins at the sites of Tyr107 (PxGSTσY107A) and Tyr162 (PxGSTσY162A) corroborated that the individual replacement of Tyr107 and Tyr162 could greatly weaken the binding and metabolic abilities to TFP.

CONCLUSION

Metabolic interactions between the Plutella xylostella (L.) GSTs (PxGSTs) and TFP were deciphered. This study illustrates the molecular metabolism mechanism of PxGSTσ towards TFP and provides theoretical underpinnings for the design and optimization of novel TFP-like insecticides. © 2020 Society of Chemical Industry.

Functional diversification of three delta-class glutathione S-transferases involved in development and detoxification in Tribolium castaneum

Glutathione S-transferases (GSTs) are members of a multifunctional enzyme superfamily. Forty-one GSTs have been identified in Tribolium castaneum; however, none of the 41 GSTs has been functionally characterized. Here, three delta-class GSTs, TcGSTd1, TcGSTd2 and TcGSTd3, of T. castaneum were successfully cloned and expressed in Escherichia coli. All of the studied GSTs catalysed the conjugation of reduced glutathione with 1-chloro-2,4-dinitrobenzene. Insecticide treatment showed that the expression levels of TcGSTd3 and TcGSTd2 were significantly increased after exposure to phoxim and lambda-cyhalothrin, whereas TcGSTd1 was slightly upregulated only in response to phoxim. A disc diffusion assay showed that overexpression of TcGSTD3, but not TcGSTD1 or TcGSTD2, in E. coli increased resistance to paraquat-induced oxidative stress. RNA interference knockdown of TcGSTd1 caused metamorphosis deficiencies and reduced fecundity by regulating insulin/target-of-rapamycin signalling pathway-mediated ecdysteroid biosynthesis, and knockdown of TcGSTd3 led to reduced fertility and a decreased hatch rate of the offspring, probably caused by the reduced antioxidative activity in the reproductive organs. These results indicate that TcGSTd3 and TcGSTd2 may play vital roles in cellular detoxification, whereas TcGSTd1 may play essential roles in normal development of T. castaneum. These delta-class GSTs in T. castaneum have obtained different functions during the evolution.

Characterisation of GST genes from the Hyphantria cunea and their response to the oxidative stress caused by the infection of Hyphantria cunea nucleopolyhedrovirus (HcNPV)

The fall webworm, Hyphantria cunea (Drury) (Lepidoptera: Noctuidae), is a major pest found in forests. In this study, the effects of Hyphantria cunea nucleopolyhedrovirus (HcNPV) infection on the transcription levels and activities of glutathione S-transferases (GSTs) in H. cunea were determined. In the present study, 18 GST family genes were identified from the H. cunea transcriptome dataset by using bioinformatic analyses. These GST genes were classified into cytosolic (15 genes) and microsomal (three genes) classes. The 15 cytosolic GST genes belonged to four different subclasses (epsilon, sigma and delta). The all GST genes, especially GSTe4, showed high expression levels in egg and 1st~4th instar larval stage while their low expression levels in 5th~7th instar larvae using real-time quantitative PCR analysis. However, the expression levels of the 18 GST genes were varied after exposure to sublethal doses of HcNPV. The expression levels of most GSTs were downregulated and upregulated at low and high concentrations of HcNPV, respectively. The corresponding total GST activities also showed similar patterns. In H. cunea, changes in the expression levels and enzymatic activities of GSTs after exposure to HcNPV indicated that they may have important functions in the defense against HcNPV, and the stress, which may be reflected by the high GST enzymatic activities.

Detection and functional characterization of sigma class GST in Phlebotomus argentipes and its role in stress tolerance and DDT resistance

Several Glutathione S-transferases (GSTs) enzymes, in insects, have previously been implicated in resistance developed against DDT and other insecticides. The GST enzyme particularly sigma class have important physiological role in detoxification of lipid peroxidation by-products in insects. Phlebotomus argentipes has been intensely exposed to DDT over years due to Indoor Residual Spray (IRS) programme for Kala-azar elimination in Bihar, India. However, in P. argentipes, role of GSTs in DDT resistance have not been elucidated. Here, sigma class GST of P. argentipes (Parg-GSTσ) was successfully cloned, expressed and purified by affinity chromatography. The recombinant Parg-GSTσ was found to be highly active towards cumene hydroperoxide and 4-HNE having specific activity 92.47 & 203.92 µM/min/mg of protein, respectively and exhibited low activity towards universal substrate CDNB i.e., 8.75 µM/min/mg of protein. RT-PCR and immunoblot analysis showed at least 2 and 1.8 fold overexpression of Parg-GSTσ in the single exposed and non exposed DDT resistant P. argentipes as compared to susceptible, implicating Parg-GSTσ also involved in DDT resistance probably by imparting enhanced stress tolerance. The DDT, H₂O₂ and temperature induction assays demonstrated stress-dependent induction of Parg-GSTσ expression indicating its important role in oxidative stress redressal.

Identification and Functional Analysis of a Delta Class Glutathione S-Transferase Gene Associated with Insecticide Detoxification in Bradysia odoriphaga

A delta class glutathione S-transferase gene (BoGSTd2) is identified from Bradysia odoriphaga for the first time. Developmental expression analysis showed that expression of BoGSTd2 is significantly higher in the fourth instar larval stage and the adult stage. Tissue-specific expression analysis found that BoGSTd2 was expressed predominantly in the midgut and Malpighian tubules in the fourth instar larvae and the abdomen of adults. Expression of BoGSTd2 was significantly upregulated following exposure to chlorpyrifos and clothianidin. In vitro inhibition and metabolic assays indicated that recombinant BoGSTd2 could not directly metabolize chlorpyrifos and clothianidin. Nevertheless, disk diffusion assays indicated that BoGSTd2 plays an important role in protection against oxidative stress. RNAi assays showed that BoGSTd2 participates in the elimination of reactive oxygen species induced by chlorpyrifos and clothianidin. These results strongly suggest that BoGSTd2 plays an important role in chlorpyrifos and clothianidin detoxification in B. odoriphaga by protecting tissues from oxidative stress induced by these insecticides.

Effects of phenol on glutathione S-transferase expression and enzyme activity in Chironomus kiiensis larvae

Detoxifying enzyme mRNAs are potentially useful stress biomarkers. Glutathione S-transferase (GST) metabolises lipophilic organic contaminants and mitigates oxidative damage caused by environmental pollutants. Herein, 12 Chironomus kiiensis GSTs (CkGSTs1-6, CkGSTt1-2, CkGSTd1-2, CkGSTm1-2) were cloned and grouped into sigma, theta, delta and microsomal subclasses. Open reading frames (450-699 bp) encode 170-232 amino acid proteins with predicted molecular masses of 17.31-26.84 kDa and isoelectric points from 4.94 to 9.58. All 12 GSTs were expressed during all tested developmental stages, and 11 displayed higher expression in fourth-instar larvae than eggs. GST activity after 24 h of phenol exposure was used to estimate environmental phenol contamination. After exposure to sublethal concentrations of phenol for 48 h, expression and activity of CkGSTs were inhibited in C. kiiensis larvae. Expression of CkGSTd1-2 and CkGSTs1-2 varied with phenol concentration, indicating potential use as biomarkers for monitoring environmental phenol contamination.

Transcriptomic analysis of insecticide resistance in the lymphatic filariasis vector Culex quinquefasciatus

Culex quinquefasciatus plays an important role in transmission of vector-borne diseases of public health importance, including lymphatic filariasis (LF), as well as many arboviral diseases. Currently, efforts to tackle C. quinquefasciatus vectored diseases are based on either mass drug administration (MDA) for LF, or insecticide-based interventions. Widespread and intensive insecticide usage has resulted in increased resistance in mosquito vectors, including C. quinquefasciatus. Herein, the transcriptome profile of Ugandan bendiocarb-resistant C. quinquefasciatus was explored to identify candidate genes associated with insecticide resistance. High levels of insecticide resistance were observed for five out of six insecticides tested, with the lowest mortality (0.97%) reported to permethrin, while for DDT, lambdacyhalothrin, bendiocarb and deltamethrin the mortality rate ranged from 1.63-3.29%. Resistance to bendiocarb in exposed mosquitoes was marked, with 2.04% mortality following 1 h exposure and 58.02% after 4 h. Genotyping of the G119S Ace-1 target site mutation detected a highly significant association (p < 0.0001; OR = 25) between resistance and Ace1-119S. However, synergist assays using the P450 inhibitor PBO, or the esterase inhibitor TPP resulted in markedly increased mortality (to ≈80%), suggesting a role of metabolic resistance in the resistance phenotype. Using a novel, custom 60 K whole-transcriptome microarray 16 genes significantly overexpressed in resistant mosquitoes were detected, with the P450 Cyp6z18 showing the highest differential gene expression (>8-fold increase vs unexposed controls). These results provide evidence that bendiocarb resistance in Ugandan C. quinquefasciatus is mediated by both target-site mechanisms and over-expression of detoxification enzymes.

Transcription factors CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GSTs conferring resistance to chlorpyrifos and cypermethrin in Spodoptera exigua

BACKGROUND

Glutathione S-transferases (GSTs) are a superfamily of multifunctional dimeric proteins existing in both prokaryotic and eukaryotic organisms. They are involved in the detoxification of both endogenous and exogenous electrophiles, including insecticides. However, the molecular mechanisms underlying the regulation of GST genes in insects are poorly understood.

RESULTS

We first identified at least three GST genes involved in resistance to the insecticides chlorpyrifos and cypermethrin. Analysis of upstream sequences revealed that three GSTs (SeGSTo2, SeGSTe6 and SeGSTd3) harbor the same cap 'n' collar C/muscle aponeurosis fibromatosis (CncC/Maf) binding site, and SeGSTo2 and SeGSTe6 contain the aryl hydrocarbon receptor/aryl hydrocarbon receptor nuclear translocator (AhR/ARNT) binding site. Luciferase reporter assay showed co-transfection of reporter plasmid containing the SeGSTe6 promoter with CncC and/or Maf expressing constructs significantly boosted transcription. Similarly, AhR and/or ARNT expressing constructs also significantly increased the promoter activities. The co-transfection of mutated reporter plasmid with CncC/Maf or AhR/ARNT did not increase transcription activity anymore. Constitutive over-expression of CncC, Maf and AhR was also found in the HZ16 strain, which might be the molecular mechanism for up-regulated expression of multiple detoxification genes conferring resistance to insecticides.

CONCLUSION

These results suggest that CncC/Maf and AhR/ARNT coordinately regulate the expression of multiple GST genes involved in insecticide resistance in Spodoptera exigua. © 2019 Society of Chemical Industry.

Detection of alleles associated with resistance to chemical insecticide in the malaria vector Anopheles arabiensis in Santiago, Cabo Verde

BACKGROUND

Mosquitoes of the Anopheles gambiae complex are the main malaria vectors worldwide. Due to the lack of a vaccine to prevent malaria, the principal way to reduce the impact of this disease relies on the use of chemical insecticides to control its vectors. However, the intensive use of such compounds has led to the emergence of insecticide resistance in several Anopheles populations in Africa. This study aimed to investigate the presence of resistance alleles in an Anopheles arabiensis population from the City of Praia, capital of the Archipelago Cabo Verde, one of the countries on the World Health Organization list of countries that are on a path to eliminate local transmission of malaria.

METHODS

Larvae from the Anopheles genus were collected using a one-pint dipper in three areas of City of Praia. Larvae were fed and maintained until the emergence of adult mosquitoes, and these were morphologically identified. In addition, molecular identification was performed using IGS markers and all An. arabiensis samples were subjected to PCR to screen for mutations associated to resistance in the Ace-1, Na_v and GSTE2 genes.

RESULTS

From a total of 440 mosquitoes collected, 52.3% were morphologically identified as An. gambiae sensu lato (s.l.) and 46.7% as Anopheles pretoriensis. The molecular identification showed that 100% of the An. gambiae s.l. were An. arabiensis. The mutations G119S in the Ace-1 gene and L119F in the GSTE2 gene were screened but not found in any sample. However, sequencing analysis for GSTE2 revealed the presence of 37 haplotypes, 16 polymorphic sites and a high genetic diversity (π = 2.67). The L1014S mutation in the Na_v (voltage-gated sodium channel gene) was detected at a frequency of 7.3%.

CONCLUSION

This is the first study to investigate the circulation of insecticide resistance alleles in An. arabiensis from Cabo Verde. The circulation of the L1014S allele in the population of An. arabiensis in the city of Praia suggests that pyrethroid resistance may arise, be quickly selected, and may affect the process of malaria elimination in Cabo Verde. Molecular monitoring of resistance should continue in order to guide the development of strategies to be used in vector control in the study region.

CpGSTd3 is a lambda-Cyhalothrin Metabolizing Glutathione S-Transferase from Cydia pomonella (L.)

Little is known about the role of specific delta GST genes in the detoxification of lambda-cyhalothrin in the global quarantine fruit pest codling moth, Cydia pomonella (L.). Real-time quantitative PCR shows that CpGSTd3 was ubiquitously expressed at all developmental stages and is most abundant in the larval stage and lowest in the egg stage; the mRNA level of CpGSTd3 is higher in the midgut and Malpighian tubules of fourth-instar larvae and abdomens of adults than in other tissues. Exposure of fourth-instar larvae to an LD₁₀ dosage of lambda-cyhalothrin significantly induced the transcript of CpGSTd3 at 3 h, but the mRNA level was down-regulated after 12 h of treatment. Recombinant CpGSTd3 expressed in Escherichia coli was able to catalyze the conjugation of 1-chloro-2,4-dinitrobenzene (CDNB) and with an IC₅₀ value of 0.65 mM for lambda-cyhalothrin. Metabolism assays indicate that recombinant CpGSTd3 could metabolize lambda-cyhalothrin. These results suggest that CpGSTd3 is probably a lambda-cyhalothrin metabolizing GST in C. pomonella.

Molecular characterization of kappa class glutathione S-transferase from the disk abalone (Haliotis discus discus) and changes in expression following immune and stress challenges

Glutathione S-transferase (GST; EC 2.5.1.18) isoenzymes represent a complex group of proteins that are involved in phase II detoxification in several organisms. In this study, GST kappa (GSTκ) from the disk abalone (Haliotis discus discus; AbGSTκ) was characterized at both the transcriptional and functional levels to determine its potential capacity to perform as a detoxification agent under conditions of different stress. The predicted AbGSTκ protein consists of 227 amino acids, with a predicted molecular weight of 25.6 kDa and a theoretical isoelectric point (pI) of 7.78. In silico analysis reveals that AbGSTκ is a disulfide bond formation protein A (DsbA), consisting of a thioredoxin domain, GSH binding sites (G-sites), and a catalytic residue. In contrast, no hydrophobic ligand binding site (H-site), or signal peptides, were detected. AbGSTκ showed the highest sequence identity with the orthologue from pufferfish (Takifugu obscurus) (60.0%). In a phylogenetic tree, AbGSTκ clustered closely together with other fish GSTκs, and was evolutionarily distanced from other cytosolic GSTs. The predicted three-dimensional structure clearly demonstrates that the dimer adopts a butterfly-like shape. A tissue distribution analysis revealed that GSTκ was highly expressed in the digestive tract, suggesting it has detoxification ability. Depending on the tissue and time, AbGSTκ showed different expression patterns, and levels of expression, following challenge of the abalone with immune stimulants. Enzyme kinetics of the purified recombinant proteins demonstrated its conjugating ability using 1-Chloro-2,4-dinitrobenzene (CDNB) and glutathione (GSH) as substrates, and suggested it has a low affinity for both substrates. The optimum temperature and pH for the rAbGSTκ GSH: CDNB conjugating activity were found to be 35 °C and pH 8, respectively indicating that the abalone is well adapted to a wide range of environmental conditions. Cibacron blue (100 μM) was capable of completely inhibiting rAbGSTκ (100%) with an IC₅₀ (half maximal inhibitory concentration) of 0.05 μM. A disk diffusion assay revealed that rAbGSTκ could significantly protect cells from H₂O₂, CdCl_2, and ZnCl₂. Altogether, this current study suggests that AbGSTκ is involved in detoxification and immunological host defense mechanisms and allows abalones to overcome stresses in order for them to have an increased chance of survival.

Identification, Expression, and Regulation of an Omega Class Glutathione S-transferase in Rhopalosiphum padi (L.) (Hemiptera: Aphididae) Under Insecticide Stress

Glutathione S-transferases (GSTs) play an essential role in the detoxification of xenobiotic toxins in insects, including insecticides. However, few data are available for the bird cherry-oat aphid, Rhopalosiphum padi (L.). In this study, we cloned and sequenced the full-length cDNA of an omega GST gene (RpGSTO1) from R. padi, which contains 720 bp in length and encodes 239 amino acids. A phylogenetic analysis revealed that RpGSTO1 belongs to the omega class of insect GSTs. RpGSTO1 gene was highly expressed in transformed Escherichia coli and the protein was purified by affinity chromatography. The recombinant RpGSTO1 displayed reduced glutathione (GSH)-dependent conjugating activity toward the substrate 1-chloro-2, 4-dinitrobenzene (CDNB) substrate. The recombinant RpGSTO1 protein exhibited optimal activity at pH 7.0 and 30°C. In addition, a disk diffusion assay showed that E. coli overexpressing RpGSTO1 increased resistance to cumene hydroperoxide-induced oxidative stress. Real-time quantitative PCR analysis showed that the relative expression level of RpGSTO1 was different in response to different insecticides, suggesting that the enzyme could contribute to insecticide metabolism in R. padi. These findings indicate that RpGSTO1 may play a crucial role in counteracting oxidative stress and detoxifying the insecticides. The results of our study contribute to a better understanding the mechanisms of insecticide detoxification and resistance in R. padi.

Identification and characterisation of seventeen glutathione S-transferase genes from the cabbage white butterfly Pieris rapae

Insect glutathione S-transferases (GSTs) play essential roles in the detoxification of insecticides and other xenobiotic compounds. The cabbage white butterfly, Pieris rapae, is an economically important agricultural pest. In this study, 17 cDNA sequences encoding putative GSTs were identified in P. rapae. All cDNAs include a complete open reading frame and were designated PrGSTd1-PrGSTz2. Based on phylogenetic analysis, PrGSTs were divided into six classes (delta, epsilon, omega, sigma, theta and zeta). The exon-intron organizations of these PrGSTs were also analysed. Recombinant proteins of eight PrGSTs (PrGSTD1, PrGSTD2, PrGSTE1, PrGSTE2, PrGSTO1, PrGSTS1, PrGSTT1 and PrGSTZ1) were heterologously expressed in Escherichia coli, and all of these proteins displayed glutathione-conjugating activity towards 1-chloro-2,4-dinitrobenzene (CDNB). Expression patterns in various larval tissues, at different life stages, and following exposure to sublethal doses of abamectin, chlorantraniliprole or lambda-cyhalothrin were determined by reverse transcription-quantitative PCR. The results showed that PrGSTe3, PrGSTs1, PrGSTs2, and PrGSTs4 were mainly transcribed in the fat body, while PrGSTe2 was expressed predominantly in the Malpighian tubules. Four genes (PrGSTe2, PrGSTo4, PrGSTs4 and PrGSTt1) were mainly expressed in fourth-instar larvae, while others were ubiquitously expressed in egg, larval, pupa and/or adult stages. Abamectin treatment significantly upregulated ten genes (PrGSTd1, PrGSTd3, PrGSTe1, PrGSTe2, PrGSTo1, PrGSTo3, PrGSTs1, PrGSTs3, PrGSTs4 and PrGSTt1). Chlorantraniliprole and lambda-cyhalothrin treatment significantly upregulated nine genes (PrGSTd1, PrGSTd2, PrGSTe1, PrGSTe2, PrGSTe3, PrGSTs1, PrGSTs3, PrGSTs4 and PrGSTz1) and ten genes (PrGSTd1, PrGSTd3, PrGSTe1, PrGSTe2, PrGSTo1, PrGSTo2, PrGSTs1, PrGSTs2, PrGSTs3 and PrGSTz2), respectively. These GSTs are potentially involved in the detoxification of insecticides.

Cypermethrin resistance conferred by increased target insensitivity and metabolic detoxification in Culex pipiens pallens Coq

In order to elucidate the molecular mechanisms of cypermethrin resistance in Culex pipiens pallens Coq, the susceptible strain (SS strain) and cypermethrin resistant strain (CR strain) of Cx. p. pallens were investigated in this paper. The cypermethrin resistance ratio of CR strain to SS strain was measured by biological assays method, the cDNA sequence of sodium channel was cloned and analyzed. Real-time quantitative RT-PCR was used to detect the expression levels of the detoxification-related genes across between CR strain and SS strain of Cx. p. pallens. Bioassays indicated that CR strain was 283.06 and 80.68-fold resistance to cypermethrin and permethrin as compared to the susceptible strain, respectively. The sequence variability analysis of sodium channel gene between SS strain and CR strain shows that 4 point mutations (R954Q, L1023F, S1775G and A1989E) appear on the amino acid sequence of sodium channel of CR strain. The transcriptional levels of CYP6Z10, CYP9M10, CPGSTd1 and CPGSTd2 in the resistant strain are significantly higher than it is in the susceptible. The transcripts of CYP4H34 and E4 esterase have no significant difference between the CR strain and SS strain. The results indicated that sodium channel mutations, combined with elevated levels of P450s and GSTs, are associated with cypermethrin resistance in CR strain.

Identification of Four ATP-Binding Cassette Transporter Genes in Cnaphalocrocis medinalis and Their Expression in Response to Insecticide Treatment

The ATP-binding cassette (ABC) transporters belong to a superfamily of genes involved in the transport of specific molecules across lipid membranes, as well as insecticide resistance, present in all living organisms. In this study, we combined the Cnaphalocrocis medinals transcriptome database with a bioinformatics approach to identify four C. medinals ABCs (CmABCs), including CmABCG1, CmABCG4, CmABCC2 and CmABCC3. Tissue expression analysis showed that these genes had a tissue-specific expression pattern. CmABCG1 had significantly higher expression in the haemolymph and head compared to the other tissues. The expression of CmABCG4, CmABCC2 and CmABCC3 was highest in the midgut, followed by expression in the fat body. The developmental stage expression analysis showed that CmABCG1, CmABCG4, CmABCC2 and CmABCC3 were mainly expressed in adults. The transcription of CmABCG1, CmABCG4 and CmABCC2 was significantly induced by chlorpyrifos. Taken together, the results of our study provided useful information for understanding of the detoxification system of C. medinalis.

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.

Transcriptome and Difference Analysis of Fenpropathrin Resistant Predatory Mite, Neoseiulus barkeri (Hughes)

Several fenpropathrin-resistant predatory mites have been reported. However, the molecular mechanism of the resistance remains unknown. In the present study, the Neoseiulus barkeri (N. barkeri) transcriptome was generated using the Illumina sequencing platform, 34,211 unigenes were obtained, and 15,987 were manually annotated. After manual annotation, attentions were attracted to resistance-related genes, such as voltage-gated sodium channel (VGSC), cytochrome P450s (P450s), and glutathione S-transferases (GSTs). A polymorphism analysis detected two point mutations (E1233G and S1282G) in the linker region between VGSC domain II and III. In addition, 43 putative P450 genes and 10 putative GST genes were identified from the transcriptome. Among them, two P450 genes, NbCYP4EV2 and NbCYP4EZ1, and four GST genes, NbGSTd01, NbGSTd02, NbGSTd03 and NbGSTm03, were remarkably overexpressed 3.64-46.69-fold in the fenpropathrin resistant strain compared to that in the susceptible strain. These results suggest that fenpropathrin resistance in N. barkeri is a complex biological process involving many genetic changes and provide new insight into the N. barkeri resistance mechanism.

Quantification of GST and esterase activities in pyrethrin-resistant mosquitoes using pyrethroid-like fluorescent substrates

Botanical pyrethrins and synthetic pyrethroids are highly potent and environmentally safe insecticides that are used to control a wide range of disease vector and pest arthropods. Unfortunately, resistance to these insecticides has been demonstrated in numerous medically important mosquito species. In this study adult Culex pipiens sensu lato were captured in agricultural and urban locations in Fresno County, California, and subsequently exposed to a commercial formulation of pyrethrin insecticide by ultra low volume spraying. Following insecticide exposure, two pyrethroid-like, fluorescent substrates (cis-DCVC and cis-TFMCVC) and CDNB were used to measure esterase and glutathione S-transferase (GST) activities in surviving mosquitoes. Elevated esterase activity (2.5-fold) was found in surviving urban mosquitoes at 12 h post pyrethrin exposure (in comparison to non-insecticide exposed control mosquitoes) when cis-TFMCVC was used as a substrate. Additionally, when CDNB was used as a substrate, 2.8-fold higher GST activity was found. A simple assay was established using our pyrethroid-like, fluorescent substrates that was able to detect low-level esterase activities in homogenates made from individual mosquitoes. The cis-TFMCVC-based assay suggested that esterase activity plays a role in pyrethrin resistance in urban mosquitoes in California.

GLUTATHIONE S-TRANSFERASE Genes IN THE RICE LEAFFOLDER, Cnaphalocrocis medinalis (LEPIDOPTERA: PYRALIDAE): IDENTIFICATION AND EXPRESSION PROFILES

In insects, glutathione S-transferases (GSTs) play critical roles in the detoxification of various insecticides, resulting in insecticide resistance. The rice leaffolder, Cnaphalocrocis medinalis, is an economically important pest of rice in Asia. GST genes have not been largely identified in this insect species. In the present study, by searching the transcriptome dataset, 25 candidate GST genes were identified in C. medinalis for the first time. Of these, 23 predicted GST proteins fell into five cytosolic classes (delta, epsilon, omega, sigma, and zeta), and two were assigned to the "unclassified" subgroup. Real-time quantitative PCR analysis showed that these GST genes were differentially expressed in various tissues, including the midgut, Malpighian tubules, and fat body of larvae, and the antenna, abdomen, and leg of adults, indicating diversified functions for these genes. Transcription levels of CmGSTd2, CmGSTe6, and CmGSTe7 increased significantly in larvae following exposure to chlorpyrifos, suggesting that these GST genes could be involved in the detoxification of this insecticide. The results of our study pave the way to a better understanding of the detoxification system of C. medinalis.

Identification and characterisation of multiple glutathione S-transferase genes from the diamondback moth, Plutella xylostella

BACKGROUND

The diamondback moth (DBM), Plutella xylostella, is one of the most harmful insect pests on crucifer crops worldwide. In this study, 19 cDNAs encoding glutathione S-transferases (GSTs) were identified from the genomic and transcriptomic database for DBM (KONAGAbase) and further characterized.

RESULTS

Phylogenetic analysis showed that the 19 GSTs were classified into six different cytosolic classes, including four in delta, six in epsilon, three in omega, two in sigma, one in theta and one in zeta. Two GSTs were unclassified. RT-PCR analysis revealed that most GST genes were expressed in all developmental stages, with higher expression in the larval stages. Six DBM GSTs were expressed at the highest levels in the midgut tissue. Twelve purified recombinant GSTs showed varied enzymatic properties towards 1-chloro-2,4-dinitrobenzene and glutathione, whereas rPxGSTo2, rPxGSTz1 and rPxGSTu2 had no activity. Real-time quantitative PCR revealed that expression levels of the 19 DBM GST genes were varied and changed after exposure to acephate, indoxacarb, beta-cypermethrin and spinosad. PxGSTd3 was significantly overexpressed, while PxGSTe3 and PxGSTs2 were significantly downregulated by all four insecticide exposures.

CONCLUSION

The changes in DBM GST gene expression levels exposed to different insecticides indicate that they may play individual roles in tolerance to insecticides and xenobiotics.

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.

Characterization of glutathione S-transferases from Sus scrofa, Cydia pomonella and Triticum aestivum: their responses to cantharidin

Glutathione S-transferases (GSTs) play a key role in detoxification of xenobiotics in organisms. However, their other functions, especially response to the natural toxin cantharidin produced by beetles in the Meloidae and Oedemeridae families, are less known. We obtained GST cDNAs from three sources: Cydia pomonella (CpGSTd1), Sus scrofa (SsGSTα1), and Triticum aestivum (TaGSTf3). The predicted molecular mass is 24.19, 25.28 and 24.49 kDa, respectively. These proteins contain typical N-terminal and C-terminal domains. Recombinant GSTs were heterologously expressed in Escherichia coli as soluble fusion proteins. Their optimal activities are exhibited at pH 7.0-7.5 at 30 °C. Activity of CpGSTd1 is strongly inhibited by cantharidin and cantharidic acid, but is only slightly suppressed by the demethylated analog of cantharidin and cantharidic acid. Enzymatic assays revealed that cantharidin has no effect on SsGSTα1 activity, while it significantly stimulates TaGSTf3 activity, with an EC50 value of 0.3852 mM. Activities of these proteins are potently inhibited by the known GST competitive inhibitor: S-hexylglutathione (GTX). Our results suggest that these GSTs from different sources share similar structural and biochemical characteristics. Our results also suggest that CpGSTd1 might act as a binding protein with cantharidin and its analogs.

Molecular cloning, expression and enzymatic characterization of glutathione S-transferase from Antarctic sea-ice bacteria Pseudoalteromonas sp. ANT506

A glutathione S-transferase (GST) gene from Antarctic sea-ice bacteria Pseudoalteromonas sp. ANT506 (namely PsGST), was cloned and expressed in Escherichia coli. The open reading frame of PsGST comprised 654 bp encoding a protein of 217 amino acids with a calculated molecular size of 24.3 kDa. The rPsGST possesses the conserved amino acid defining the binding sites of glutathione (G-site) and substrate binding pocket (H-site) in GST N_3 family. PsGST was expressed in E. coli and the recombinant PsGST (rPsGST) was purified by Ni-affinity chromatography with a high specific activity of 74.21 U/mg. The purified rPsGST showed maximum activity at 40 °C and exhibited 14.2% activity at 0 °C. It was completely inactivated at 50 °C for 40 min. These results indicated that rPsGST was a typical cold active GST with low thermostability. The enzyme was little affected by H2O2 and Triton X-100, and 50.2% of the remaining activity was detected in the presence of high salt concentrations (2M NaCl). The enzymatic Km values for CDNB and GSH was 0.22 mM and 1.01 mM, respectively. These specific enzyme properties may be related to the survival environment of Antarctic sea ice bacteria.

Characterization and functional analysis of four glutathione S-transferases from the migratory locust, Locusta migratoria

Glutathione S-transferases (GSTs) play an important role in detoxification of xenobiotics in both prokaryotic and eukaryotic cells. In this study, four GSTs (LmGSTd1, LmGSTs5, LmGSTt1, and LmGSTu1) representing different classes were identified from the migratory locust, Locusta migratoria. These four proteins were heterologously expressed in Escherichia coli as soluble fusion proteins, purified by Ni(2+)-nitrilotriacetic acid agarose column and biochemically characterized. LmGSTd1, LmGSTs5, and LmGSTu1 showed high activities with 1-chloro-2, 4-dinitrobenzene (CDNB), detectable activity with p-nitro-benzyl chloride (p-NBC) and 1, 2-dichloro-4-nitrobenzene (DCNB), whereas LmGSTt1 showed high activity with p-NBC and detectable activity with CDNB. The optimal pH of the locust GSTs ranged between 7.0 to 9.0. Ethacrynic acid and reactive blue effectively inhibited all four GSTs. LmGSTs5 was most sensitive to heavy metals (Cu(2+) and Cd(2+)). The maximum expression of the four GSTs was observed in Malpighian tubules and fat bodies as evaluated by western blot. The nymph mortalities after carbaryl treatment increased by 28 and 12% after LmGSTs5 and LmGSTu1 were silenced, respectively. The nymph mortalities after malathion and chlorpyrifos treatments increased by 26 and 18% after LmGSTs5 and LmGSTu1 were silenced, respectively. These results suggest that sigma GSTs in L. migratoria play a significant role in carbaryl detoxification, whereas some of other GSTs may also involve in the detoxification of carbaryl and chlorpyrifos.

Cloning and functional characterization of inward-rectifying potassium (Kir) channels from Malpighian tubules of the mosquito Aedes aegypti

Inward-rectifying K(+) (Kir) channels play critical physiological roles in a variety of vertebrate cells/tissues, including the regulation of membrane potential in nerve and muscle, and the transepithelial transport of ions in osmoregulatory epithelia, such as kidneys and gills. It remains to be determined whether Kir channels play similar physiological roles in insects. In the present study, we sought to 1) clone the cDNAs of Kir channel subunits expressed in the renal (Malpighian) tubules of the mosquito Aedes aegypti, and 2) characterize the electrophysiological properties of the cloned Kir subunits when expressed heterologously in oocytes of Xenopus laevis. Here, we reveal that three Kir subunits are expressed abundantly in Aedes Malpighian tubules (AeKir1, AeKir2B, and AeKir3); each of their full-length cDNAs was cloned. Heterologous expression of the AeKir1 or the AeKir2B subunits in Xenopus oocytes elicits inward-rectifying K(+) currents that are blocked by barium. Relative to the AeKir2B-expressing oocytes, the AeKir1-expressing oocytes 1) produce larger macroscopic currents, and 2) exhibit a modulation of their conductive properties by extracellular Na(+). Attempts to functionally characterize the AeKir3 subunit in Xenopus oocytes were unsuccessful. Lastly, we show that in isolated Aedes Malpighian tubules, the cation permeability sequence of the basolateral membrane of principal cells (Tl(+) > K(+) > Rb(+) > NH(4)(+)) is consistent with the presence of functional Kir channels. We conclude that in Aedes Malpighian tubules, Kir channels contribute to the majority of the barium-sensitive transepithelial transport of K(+).

Development of pyrethroid-like fluorescent substrates for glutathione S-transferase

The availability of highly sensitive substrates is critical for the development of precise and rapid assays for detecting changes in glutathione S-transferase (GST) activity that are associated with GST-mediated metabolism of insecticides. In this study, six pyrethroid-like compounds were synthesized and characterized as substrates for insect and mammalian GSTs. All of the substrates were esters composed of the same alcohol moiety, 7-hydroxy-4-methylcoumarin, and acid moieties that structurally mimic some commonly used pyrethroid insecticides, including cypermethrin and cyhalothrin. CpGSTD1, a recombinant Delta class GST from the mosquito Culex pipiens pipiens, metabolized our pyrethroid-like substrates with both chemical and geometric preference (i.e., the cis-isomers were metabolized at 2- to 5-fold higher rates than the corresponding trans-isomers). A GST preparation from mouse liver also metabolized most of our pyrethroid-like substrates with both chemical and geometric preference but at 10- to 170-fold lower rates. CpGSTD1 and mouse GSTs metabolized 1-chloro-2,4-dinitrobenezene (CDNB), a general GST substrate, at more than 200-fold higher rates than our novel pyrethroid-like substrates. There was a 10-fold difference in the specificity constant (k(cat)/K(M) ratio) of CpGSTD1 for CDNB and those of CpGSTD1 for cis-DCVC and cis-TFMCVC, suggesting that cis-DCVC and cis-TFMCVC may be useful for the detection of GST-based metabolism of pyrethroids in mosquitoes.