Molecular cloning of a glutathione S-transferase overproduced in an insecticide-resistant strain of the housefly (Musca domestica)

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.

The genome of the Java medaka (Oryzias javanicus): Potential for its use in marine molecular ecotoxicology

The Java medaka (Oryzias javanicus) is distributed in tropical brackish water and is considered as an ecotoxicological experimental organism for assessing diverse pollutions and global climate change effects in the ocean. In this study, we sequenced and assembled the genome of O. javanicus using the Oxford Nanopore technique and anchored the scaffolds to the 24 genetic linkage map of a sister species Oryzias melastigma. The assembled genome consisted of 773 scaffolds including 24 LG-based scaffolds, and the estimated genome length was 846.3 Mb (N50 = 19.3 Mb), containing 24,498 genes. As detoxification processes are crucial in aquatic organisms, antioxidant-related genes including glutathione S-transferases, superoxide dismutase, catalase, and glutathione peroxidase were identified in this study. In the genome of O. javanicus, a total of 21 GSTs, 4 SODs, 1 CAT, and 7 GPxs were identified and showed high similarities between sister species O. melastigma and Oryzias latipes. In addition, despite having 8 classes of cytosolic GSTs family, medaka showed no presence of GST pi and sigma classes, which are predominantly found in carp and salmon, but not in neoteleostei. This study adds another set to genome-library of Oryzias spp. and is a useful resource for better understanding of the molecular ecotoxicology.

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.

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.

Role of induced glutathione-S-transferase from Helicoverpa armigera (Lepidoptera: Noctuidae) HaGST-8 in detoxification of pesticides

The present study deals with glutathione-S-transferase (GST) based detoxification of pesticides in Helicoverpa armigera and its potential application in eliminating pesticides from the environment. Dietary exposure of a pesticide mixture (organophosphates - chlorpyrifos and dichlorvos, pyrethroid - cypermethrin; 2-15ppm each) to H. armigera larvae resulted in a dose dependant up-regulation of GST activity and gene expression. A variant GST from H. armigera (HaGST-8) was isolated from larvae fed with 10ppm pesticide mixture and it was recombinantly expressed in yeast (Pichia pastoris HaGST-8). HaGST-8 had a molecular mass of 29kDa and was most active at pH 9 at 30°C. GC-MS and LC-HRMS analysis validated that HaGST-8 was effective in eliminating organophosphate type of pesticides and partially reduced the cypermethrin content (53%) from aqueous solutions. Unlike the untransformed yeast, P. pastoris HaGST-8 grew efficiently in media supplemented with pesticide mixtures (200 and 400ppm each pesticide) signifying the detoxification ability of HaGST-8. The amino acid sequence of HaGST-8 and the already reported sequence of HaGST-7 had just 2 mismatches. The studies on molecular interaction strengths revealed that HaGST-8 had stronger binding affinities with organophosphate, pyrethroid, organochloride, carbamate and neonicotinoid type of pesticides. The abilities of recombinant HaGST-8 to eliminate pesticides and P. pastoris HaGST-8 to grow profusely in the presence of high level of pesticide content can be applied for removal of such residues from food, water resources and bioremediation.

APPEARANCE AND SWEEP OF A GENE DUPLICATION: ADAPTIVE RESPONSE AND POTENTIAL FOR NEW FUNCTIONS IN THE MOSQUITO CULEX PIPIENS

Evolution of a new gene function is a fundamental process of adaptation. Gene duplication followed by divergence due to relaxed selection on redundant copies has been viewed as the predominant mechanism involved in this process. At a macroevolutionary scale, evidence for this scenario came from the analysis of sequences of genes families. However, even if several genetic models have described the different potential microevolutionary scenario for a new function to evolve, little is really known about the initial evolutionary dynamics of such processes. We analyze such early dynamics in natural populations of the mosquito Culex pipiens polymorphic for a duplication at Ace.1, a locus involved in insecticide resistance. The date of occurrence and the selective advantages of the duplication were estimated using frequency data. We propose a scenario where the spread of a duplication is driven, from the very beginning, by selection due to insecticide treatment.

Age and prior blood feeding of Anopheles gambiae influences their susceptibility and gene expression patterns to ivermectin-containing blood meals

Background

Ivermectin has been proposed as a novel malaria transmission control tool based on its insecticidal properties and unique route of acquisition through human blood. To maximize ivermectin’s effect and identify potential resistance/tolerance mechanisms, it is important to understand its effect on mosquito physiology and potential to shift mosquito population age-structure. We therefore investigated ivermectin susceptibility and gene expression changes in several age groups of female Anopheles gambiae mosquitoes.

Methods

The effect of aging on ivermectin susceptibility was analyzed in three age groups (2, 6, and 14-days) of colonized female Anopheles gambiae mosquitoes using standard survivorship assays. Gene expression patterns were then analyzed by transcriptome sequencing on an Illumina HiSeq 2500 platform. RT-qPCR was used to validate transcriptional changes and also to examine expression in a different, colonized strain and in wild mosquitoes, both of which blood fed naturally on an ivermectin-treated person.

Results

Mosquitoes of different ages and blood meal history died at different frequencies after ingesting ivermectin. Mortality was lowest in 2-day old mosquitoes exposed on their first blood meal and highest in 6-day old mosquitoes exposed on their second blood meal. Twenty-four hours following ivermectin ingestion, 101 and 187 genes were differentially-expressed relative to control blood-fed, in 2 and 6-day groups, respectively. Transcription patterns of select genes were similar in membrane-fed, colonized, and naturally-fed wild vectors. Transcripts from several unexpected functional classes were highly up-regulated, including Niemann-Pick Type C (NPC) genes, peritrophic matrix-associated genes, and immune-response genes, and these exhibited different transcription patterns between age groups, which may explain the observed susceptibility differences. Niemann-Pick Type 2 genes were the most highly up-regulated transcripts after ivermectin ingestion (up to 160 fold) and comparing phylogeny to transcriptional patterns revealed that NPCs have rapidly evolved and separate members respond to either blood meals or to ivermectin.

Conclusion

We present evidence of increased ivermectin susceptibility in older An. gambiae mosquitoes that had previously bloodfed. Differential expression analysis suggests complex midgut interactions resulting from ivermectin ingestion that likely involve blood meal digestion physiological responses, midgut microflora, and innate immune responses. Thus, the transcription of certain gene families is consistently affected by ivermectin ingestion, and may provide important clues to ivermectin’s broad effects on malaria vectors. These findings contribute to the growing understanding of ivermectin’s potential as a transmission control tool.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2029-8) contains supplementary material, which is available to authorized users.

Genotype to phenotype, the molecular and physiological dimensions of resistance in arthropods

The recent accumulation of molecular studies on mutations in insects, ticks and mites conferring resistance to insecticides, acaricides and biopesticides is reviewed. Resistance is traditionally classified by physiological and biochemical criteria, such as target-site insensitivity and metabolic resistance. However, mutations are discrete molecular changes that differ in their intrinsic frequency, effects on gene dosage and fitness consequences. These attributes in turn impact the population genetics of resistance and resistance management strategies, thus calling for a molecular genetic classification. Mutations in structural genes remain the most abundantly described, mostly in genes coding for target proteins. These provide the most compelling examples of parallel mutations in response to selection. Mutations causing upregulation and downregulation of genes, both in cis (in the gene itself) and in trans (in regulatory processes) remain difficult to characterize precisely. Gene duplications and gene disruption are increasingly reported. Gene disruption appears prevalent in the case of multiple, hetero-oligomeric or redundant targets.

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.

Detection and geographical distribution of the organophosphate resistance-associated Δ3Q ace mutation in the olive fruit fly, Bactrocera oleae (Rossi)

BACKGROUND

The olive fruit fly, Bactrocera oleae (Rossi) (Diptera: Tephritidae), is the most important pest of olives. Its control is based mostly on organophosphate (OP) insecticides, a practice that has led to resistance development. OP resistance in B. oleae has been associated with three mutations in the acetylcholinesterase (AChE), the product of ace gene. The current study presents new diagnostic tests for the detection of the ace mutations and aims at monitoring the frequency of the Δ3Q mutation, which appears associated with resistance at higher OP doses in natural olive fly populations.

RESULTS

An allele-specific polymerase chain reaction (PCR), a PCR-RFLP (restriction fragment length polymorphism) and a Taq-Man test were developed for the Δ3Q mutation detection and a new duplex quantitative PCR assay was designed for the G488S and I214V mutations. Moreover, the frequency of Δ3Q mutation was examined in ten populations of eight countries around the Mediterranean basin. The highest frequencies (10%) were found in Greece and Italy, whereas a gradual decrease of Δ3Q frequency towards the western Mediterranean was noted.

CONCLUSION

Robust tests for insecticide resistance mutations at their incipient levels are essential tools to monitor the increase and geographical spread of such mutations. Three different tests were developed for AChE-Δ3Q that indicated its association with OP applications across the Mediterranean.

The mechanisms underlying α-amanitin resistance in Drosophila melanogaster: a microarray analysis

The rapid evolution of toxin resistance in animals has important consequences for the ecology of species and our economy. Pesticide resistance in insects has been a subject of intensive study; however, very little is known about how Drosophila species became resistant to natural toxins with ecological relevance, such as α-amanitin that is produced in deadly poisonous mushrooms. Here we performed a microarray study to elucidate the genes, chromosomal loci, molecular functions, biological processes, and cellular components that contribute to the α-amanitin resistance phenotype in Drosophila melanogaster. We suggest that toxin entry blockage through the cuticle, phase I and II detoxification, sequestration in lipid particles, and proteolytic cleavage of α-amanitin contribute in concert to this quantitative trait. We speculate that the resistance to mushroom toxins in D. melanogaster and perhaps in mycophagous Drosophila species has evolved as cross-resistance to pesticides, other xenobiotic substances, or environmental stress factors.

Structural analysis of an epsilon-class glutathione transferase from housefly, Musca domestica

Glutathione transferases (GSTs) play an important role in the detoxification of insecticides, and as such, they are a key contributor to enhanced resistance to insecticides. In the housefly (Musca domestica), two epsilon-class GSTs (MdGST6A and MdGST6B) that share high sequence homology have been identified, which are believed to be involved in resistance against insecticides. The structural determinants controlling the substrate specificity and enzyme activity of MdGST6s are unknown. The aim of this study was to crystallize and perform structural analysis of the GST isozyme, MdGST6B. The crystal structure of MdGST6B complexed with reduced glutathione (GSH) was determined at a resolution of 1.8 Å. MdGST6B was found to have a typical GST folding comprised of N-terminal and C-terminal domains. Arg113 and Phe121 on helix 4 were shown to protrude into the substrate binding pocket, and as a result, the entrance of the substrate binding pocket was narrower compared to delta- and epsilon-class GSTs from Africa malaria vector Anopheles gambiae, agGSTd1-6 and agGSTe2, respectively. This substrate pocket narrowing is partly due to the presence of a π-helix in the middle of helix 4. Among the six residues that donate hydrogen bonds to GSH, only Arg113 was located in the C-terminal domain. Ala substitution of Arg113 did not have a significant effect on enzyme activity, suggesting that the Arg113 hydrogen bond does not play a crucial role in catalysis. On the other hand, mutation at Phe108, located just below Arg113 in the binding pocket, reduced the affinity and catalytic activity to both GSH and the electrophilic co-substrate, 1-chloro-2,4-dinitrobenzene.

Cloning, expression and analysis of the olfactory glutathione S-transferases in coho salmon

The glutathione S-transferases (GSTs) provide cellular protection by detoxifying xenobiotics, maintaining redox status, and modulating secondary messengers, all of which are critical to maintaining olfaction in salmonids. Here, we characterized the major coho salmon olfactory GSTs (OlfGSTs), namely omega, pi, and rho subclasses. OlfGST omega contained an open reading frame of 720bp and encoded a protein of 239 amino acids. OlfGST pi and OlfGST rho contained open reading frames of 627 and 681nt, respectively, and encoded proteins of 208 and 226 amino acids. Whole-protein mass spectrometry yielded molecular weights of 29,950, 23,354, and 26,655Da, respectively, for the GST omega, pi, and rho subunits. Homology modeling using four protein-structure prediction algorithms suggest that the active sites in all three OlfGST isoforms resembled counterparts in other species. The olfactory GSTs conjugated prototypical GST substrates, but only OlfGST rho catalyzed the demethylation of the pesticide methyl parathion. OlfGST pi and rho exhibited thiol oxidoreductase activity toward 2-hydroxyethyl disulfide (2-HEDS) and conjugated 4-hydroxynonenal (HNE), a toxic aldehyde with neurodegenerative properties. The kinetic parameters for OlfGST pi conjugation of HNE were K(M)=0.16 ± 0.06mM and V(max)=0.5 ± 0.1μmolmin⁻¹mg⁻¹, whereas OlfGST rho was more efficient at catalyzing HNE conjugation (K(M)=0.022 ± 0.008 mM and V(max)=0.47 ± 0.05μmolmin⁻¹mg⁻¹). Our findings indicate that the peripheral olfactory system of coho expresses GST isoforms that detoxify certain electrophiles and pesticides and that help maintain redox status and signal transduction.

Identification and characterisation of ten glutathione S-transferase genes from oriental migratory locust, Locusta migratoria manilensis (Meyen)

BACKGROUND

Synthetic pyrethroids are the primary insecticides that are widely used for controlling Locusta migratoria manilensis (Meyen), a major pest in eastern and southern Asia and the Pacific region. In this paper, ten cDNAs encoding glutathione S-transferases (GSTs) were sequenced and characterised in L. migratoria manilensis. The effects of deltamethrin on the ten GST gene expressions were studied.

RESULTS

Phylogenetic analysis revealed nine GSTs in three different classes, including seven in sigma, one in delta and one in theta. The remaining GST (LmGSTu1) was unclassified. RT-PCR analysis showed that most GST genes were expressed in all tissues examined, including the foregut, midgut, gastric caecum, hindgut, Malpighian tubules, fat bodies, muscles, spermaries and ovaries, except that LmGSTs2, LmGSTs4, LmGSTs7 and LmGSTu1 were expressed in several tissues. LmGSTu1 appeared to be the only gene whose expressions could not be detected in eggs. Real-time quantitative PCR showed that deltamethrin at 0.08 and/or 0.12 µg mL⁻¹ increased almost all ten GST gene expressions in third-instar nymph locusts. However, deltamethrin at 0.16 and/or 0.2 µg mL⁻¹ decreased the expressions of LmGSTd1, LmGSTs1, LmGSTs5 and LmGSTs6.

CONCLUSION

The increases in GST gene expressions after deltamethrin exposure in L. migratoria manilensis might result in its elevating tolerance to other insecticides and xenobiotics.

Resistance to the organophosphate temephos: mechanisms, evolution and reversion in an Aedes aegypti laboratory strain from Brazil

Insecticide resistance is one of the main problems in vector control programs. Because insects have developed resistance to all classes of available chemical insecticides, a proper surveillance and management of resistance in areas where these compounds are being utilized is crucial for the success of control programs. Since the mechanisms and molecular bases of resistance are various, they must be characterized to allow efficient monitoring strategies. Here we report the establishment of an Aedes aegypti strain resistant to temephos, named RecR, selected under laboratory conditions. The parental A. aegypti population was obtained from eggs collected in an area where temephos had been used for 8 years, and presented a baseline resistance ratio (RR) of 7. After 17 generations under selective pressure, the RR has increased to 180. Biochemical assays indicate that metabolic mechanisms are involved on temephos resistance in the selected strain. These experiments showed that, compared to the susceptible colony Rockefeller, RecR present higher activity of glutathione S-transferases (GSTs), alpha- and beta-esterases, and, to a lesser degree, mixed function oxidases (MFO). At the 14th or 17th generations, there was no cross resistance of these insects to deltamethrin, cypermethrin and malathion, while a low resistance level (RR=3) was observed for pyriproxyfen, a juvenile hormone analogue. Experiments on resistance reversal, performed through three different field simulated schemes using the resistant strain, showed that temephos susceptibility can be recovered. The establishment of an A. aegypti colony resistant to temephos is extremely valuable for a deeper understanding of resistance mechanisms and thus for further improvements in control strategies against this vector. With the urgent need on improving methodologies to monitor resistance, molecular studies such as microarrays, and resistant colonies such as RecR will certainly hasten such studies.

Expression of the cytochrome P450s, CYP6P3 and CYP6M2 are significantly elevated in multiple pyrethroid resistant populations of Anopheles gambiae s.s. from Southern Benin and Nigeria

Background

Insecticide resistance in Anopheles mosquitoes is threatening the success of malaria control programmes. This is particularly true in Benin where pyrethroid resistance has been linked to the failure of insecticide treated bed nets. The role of mutations in the insecticide target sites in conferring resistance has been clearly established. In this study, the contribution of other potential resistance mechanisms was investigated in Anopheles gambiae s.s. from a number of localities in Southern Benin and Nigeria. The mosquitoes were sampled from a variety of breeding sites in a preliminary attempt to investigate the role of contamination of mosquito breeding sites in selecting for resistance in adult mosquitoes.

Results

All mosquitoes sampled belonged to the M form of An. gambiae s.s. There were high levels of permethrin resistance in an agricultural area (Akron) and an urban area (Gbedjromede), low levels of resistance in mosquito samples from an oil contaminated site (Ojoo) and complete susceptibility in the rural Orogun location. The target site mutation kdrW was detected at high levels in two of the populations (Akron f = 0.86 and Gbedjromede f = 0.84) but was not detected in Ojoo or Orogun. Microarray analysis using the Anopheles gambiae detox chip identified two P450s, CYP6P3 and CYP6M2 up regulated in all three populations, the former was expressed at particularly high levels in the Akron (12.4-fold) and Ojoo (7.4-fold) populations compared to the susceptible population. Additional detoxification and redox genes were also over expressed in one or more populations including two cuticular pre-cursor genes which were elevated in two of the three resistant populations.

Conclusion

Multiple resistance mechanisms incurred in the different breeding sites contribute to resistance to permethrin in Benin. The cytochrome P450 genes, CYP6P3 and CYP6M2 are upregulated in all three resistant populations analysed. Several additional potential resistance mechanisms were also identified that warrant further investigation. Metabolic genes were over expressed irrespective of the presence of kdr, the latter resistance mechanism being absent in one resistant population. The discovery that mosquitoes collected from different types of breeding sites display differing profiles of metabolic genes at the adult stage may reflect the influence of a range of xenobiotics on selecting for resistance in mosquitoes.

Ohno's dilemma: evolution of new genes under continuous selection

New genes with novel functions arise by duplication and divergence, but the process poses a problem. After duplication, an extra gene copy must rise to sufficiently high frequency in the population and remain free of common inactivating lesions long enough to acquire the rare mutations that provide a new selectable function. Maintaining a duplicated gene by selection for the original function would restrict the freedom to diverge. (We refer to this problem as Ohno's dilemma). A model is described by which selection continuously favors both maintenance of the duplicate copy and divergence of that copy from the parent gene. Before duplication, the original gene has a trace side activity (the innovation) in addition to its original function. When an altered ecological niche makes the minor innovation valuable, selection favors increases in its level (the amplification), which is most frequently conferred by increased dosage of the parent gene. Selection for the amplified minor function maintains the extra copies and raises the frequency of the amplification in the population. The same selection favors mutational improvement of any of the extra copies, which are not constrained to maintain their original function (the divergence). The rate of mutations (per genome) that improve the new function is increased by the multiplicity of target copies within a genome. Improvement of some copies relaxes selection on others and allows their loss by mutation (becoming pseudogenes). Ultimately one of the extra copies is able to provide all of the new activity.

Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotics

Xenobiotic resistance in insects has evolved predominantly by increasing the metabolic capability of detoxificative systems and/or reducing xenobiotic target site sensitivity. In contrast to the limited range of nucleotide changes that lead to target site insensitivity, many molecular mechanisms lead to enhancements in xenobiotic metabolism. The genomic changes that lead to amplification, overexpression, and coding sequence variation in the three major groups of genes encoding metabolic enzymes, i.e., cytochrome P450 monooxygenases (P450s), esterases, and glutathione-S-transferases (GSTs), are the focus of this review. A substantial number of the adaptive genomic changes associated with insecticide resistance that have been characterized to date are transposon mediated. Several lines of evidence suggest that P450 genes involved in insecticide resistance, and perhaps insecticide detoxification genes in general, may share an evolutionary association with genes involved in allelochemical metabolism. Differences in the selective regime imposed by allelochemicals and insecticides may account for the relative importance of regulatory or structural mutations in conferring resistance.

Cloning, expression and characterization of theta-class glutathione S-transferase from the silkworm, Bombyx mori

This study focused on glutathione S-transferase (GST), one of the detoxification enzymes, from the silkworm, Bombyx mori (GSTT1). A cDNA encoding a putative GST was amplified by reverse transcriptase-polymerase chain reaction and sequenced. The deduced amino acid sequence revealed 59%, 57% and 56% identities to theta-class GSTs of Musca domestica, Anopheles gambiae and Drosophila melanogaster, respectively. GSTT1 was also estimated to be close to those GSTs in a phylogenetic tree. Recombinant GST (rGSTT1) was functionally overexpressed in Escherichia coli in a soluble form, purified to homogeneity, and characterized. The pH-optimum of rGSTT1 was broad from pH 4 to 9 and rGSTT1 retained more than 75% of its original activity after incubation at pH 5-11. Incubation for 30 min at temperatures below 50 degrees C also affected the activity insignificantly. The Michaelis constant for 1-chloro-2,4-dinitrobenzene was 0.48 mM.

Insect glutathione transferases and insecticide resistance

Glutathione transferases (GSTs) are a diverse family of enzymes found ubiquitously in aerobic organisms. They play a central role in the detoxification of both endogenous and xenobiotic compounds and are also involved in intracellular transport, biosynthesis of hormones and protection against oxidative stress. Interest in insect GSTs has primarily focused on their role in insecticide resistance. GSTs can metabolize insecticides by facilitating their reductive dehydrochlorination or by conjugation reactions with reduced glutathione, to produce water-soluble metabolites that are more readily excreted. In addition, they contribute to the removal of toxic oxygen free radical species produced through the action of pesticides. Annotation of the Anopheles gambiae and Drosophila melanogaster genomes has revealed the full extent of this enzyme family in insects. This mini review describes the insect GST enzyme family, focusing specifically on their role in conferring insecticide resistance.

Mosquito Glutathione Transferases

The glutathione transferases (glutathione S-transferases, GSTs) are a diverse family of enzymes involved in a wide range of biological processes, many of which involve the conjugation of the tripeptide glutathione to an electrophilic substrate. Relatively little is known about the endogenous substrates of mosquito GSTs, and most studies have focused on their role in insecticide metabolism, because elevated levels of GST activity have been associated with resistance to all the major classes of insecticides. In addition, there is growing interest in the role of this enzyme family in maintaining the redox status of the mosquito cell, particularly in relation to vectorial capacity. Most GSTs are cytosolic dimeric proteins, although a smaller class of microsomal GSTs exists in insects, mammals, and plants. Each GST subunit has a G site that binds glutathione and a substrate-binding site or H site. There are more than 30 GST genes in mosquitoes. Additional diversity is contributed by alternative splicing to produce GSTs with differing substrate specificities. In this review, we first discuss the diversity of insect GST enzymes and their mode of action before focusing on the various functions that have been attributed to specific mosquito GSTs.

Drosophila glutathione S-transferases

The Drosophila glutathione S-transferases (GSTs; EC2.5.1.18) comprise a host of cytosolic proteins that are encoded by a gene superfamily and a homolog of the human microsomal GST. Biochemical studies of certain recombinant GSTs have linked their enzymatic functions to important substrates such as the pesticide DDT and 4-hydroxynonenal, a reactive lipid metabolite. Moreover, a correspondence has been observed between resistance to insecticide substrates-such as DDT-and elevated enzyme levels in resistant strains. Such significant, recurring connections suggest that these gst genes may feature in a model for the development of insecticide resistance. We have amassed substantial biochemical support for relating the overexpression of a particular gst gene to insecticide resistance but are still short of solid genetic evidence to affirm a causal relationship. With the Drosophila system, we have at our disposal genetic and molecular techniques such as p-element mutagenesis and excision, siRNA technology, and versatile transgenic techniques. We can use these methods to effect loss-of-function and gain-of-function conditions and, in these rendered contexts, study other potentially important functions of the gst gene superfamily. An immediate problem that comes to mind is the possible causal relationship between GST substrate specificity and chemical resistance phenotype(s). In this chapter, we present an analysis of selected strategies and laboratory methods that may be useful in pursuing a variety of interesting problems. We will cover three kinds of approaches-biochemistry, genetics, and genomics-as important instruments in a toolkit for studies of the Drosophila gst superfamily. We make the case that these approaches (biochemistry, genetics, and genomics) have helped us gain important insights and can continue to help the community gain a more complete understanding of the biological functions of GSTs. Such knowledge may be key in addressing questions about the detoxification of pesticides and how oxidative stresses affect life span. We hope that these techniques will prove fruitful in studying a host of other physiologic functions as well.

The molecular basis of insecticide resistance in mosquitoes

Insecticide resistance is an inherited characteristic involving changes in one or more insect gene. The molecular basis of these changes are only now being fully determined, aided by the availability of the Drosophila melanogaster and Anopheles gambiae genome sequences. This paper reviews what is currently known about insecticide resistance conferred by metabolic or target site changes in mosquitoes.

Genome-wide transcription profile of field- and laboratory-selected dichlorodiphenyltrichloroethane (DDT)-resistant Drosophila

Genome-wide microarray analysis (Affymetrix array) was used (i) to determine whether only one gene, the cytochrome P450 enzyme Cyp6g1, is differentially transcribed in dichlorodiphenyltrichloroethane (DDT)-resistant vs. -susceptible Drosophila; and (ii) to profile common genes differentially transcribed across a DDT-resistant field isolate [Rst(2)DDT(Wisconsin)] and a laboratory DDT-selected population [Rst(2)DDT(91-R)]. Statistical analysis (ANOVA model) identified 158 probe sets that were differentially transcribed among Rst(2)DDT(91-R), Rst(2)DDT(Wisconsin), and the DDT-susceptible genotype Canton-S (P < 0.01). The cytochrome P450 Cyp6a2 and the diazepam-binding inhibitor gene (Dbi) were over transcribed in the two DDT-resistant genotypes when compared to the wild-type Drosophila, and this difference was significant at the most stringent statistical level, a Bonferroni correction. The list of potential candidates differentially transcribed also includes 63 probe sets for which molecular function ontology annotation of the probe sets did not exist. A total of four genes (Cyp6a2, Dbi, Uhg1, and CG11176) were significantly different (P < 5.6 e(-06)) between Rst(2)DDT(91-R) and Canton-S. Additionally, two probe sets encoding Cyp12d1 and Dbi were significantly different between Rst(2)DDT(Wisconsin) and Canton-S after a Bonferroni correction. Fifty-two probe sets, including those associated with pesticide detoxification, ion transport, signal transduction, RNA transcription, and lipid metabolism, were commonly expressed in both resistant lines but were differentially transcribed in Canton-S. Our results suggest that more than Cyp6g1 is overtranscribed in field and laboratory DDT-resistant genotypes, and the number of commonalities suggests that similar resistance mechanisms may exist between laboratory- and field-selected DDT-resistant fly lines.

Heterologous expression of four glutathione transferase genes genetically linked to a major insecticide-resistance locus from the malaria vector Anopheles gambiae

A cluster of eight genes encoding glutathione transferases (GSTs) are located on division 33B of polytene chromosome arm 3R of the African malaria mosquito, Anopheles gambiae. This region of the genome contains a major 1,1,1-trichloro-2,2-bis-( p -chlorophenyl)ethane (DDT)-resistance locus, rtd1. These GSTs belong to the insect-specific Epsilon class and share between 22.6 and 65.2% identity at the amino acid level. Two distinct allelic variants of the Epsilon GST, GSTe1, differing at 12 out of 224 amino acid residues, are present in laboratory and field populations of A. gambiae. To investigate the possible role of these GSTs in conferring resistance to the insecticide DDT, both GSTe1 alleles, plus three additional members of this gene cluster, were expressed in Escherichia coli and the recombinant proteins biochemically characterized. The five putative glutathione transferases encoded catalytically active subunits with variable biochemical properties. For example, the two allelic variants of GSTE1-1 encoded proteins with over 100-fold variation in peroxidase activity, while the three remaining GSTs had no detectable peroxidase activity. Only GSTE2-2 was able to metabolize DDT. Western blots using antibodies raised against these GSTs indicated that the expression of GSTE2-2 is elevated in a DDT-resistant strain of A. gambiae.

Resistance to coumaphos and diazinon in Boophilus microplus (Acari: Ixodidae) and evidence for the involvement of an oxidative detoxification mechanism

The levels of resistance to two organophosphate acaricides, coumaphos and diazinon, in several Mexican strains of Boophilus microplus (Canestrini) were evaluated using the FAO larval packet test. Regression analysis of LC50 data revealed a significant cross-resistance pattern between those two acaricides. Metabolic mechanisms of resistance were investigated with synergist bioassays. Piperonyl butoxide (PBO) reduced coumaphos toxicity in susceptible strains, but synergized coumaphos toxicity in resistant strains. There was a significant correlation between PBO synergism ratios and the coumaphos resistance ratios. The results suggest that an enhanced cytochrome P450 monooxygenase (cytP450)-mediated detoxification mechanism may exist in the resistant strains, in addition to the cytP450-mediated metabolic pathway that activates coumaphos. PBO failed to synergize diazinon toxicity in resistant strains, suggesting the cytP450 involved in detoxification were specific. Triphenylphosphate (TPP) synergized toxicity of both acaricides in both susceptible and resistant strains, and there was no correlation between TPP synergism ratios and the LC50 estimates for either acaricide. Esterases may not play a major role in resistance to coumaphos and diazinon in those strains. Bioassays with diethyl maleate (DEM) revealed a significant correlation between DEM synergism ratios and LC50 estimates for diazinon, suggesting a possible role for glutathione S-transferases in diazinon detoxification. Resistance to coumaphos in the Mexican strains of B. microplus was likely to be conferred by both a cytP450-mediated detoxification mechanism described here and the mechanism of insensitive acetylcholinesterases reported elsewhere. The results of this study also underscore the potential risk of coumaphos resistance in B. microplus from Mexico to the U.S. cattle fever tick eradication program.

Relationship between biomarker activity and developmental endpoints in Chironomus riparius Meigen exposed to an organophosphate insecticide

The biomarkers acetylcholinesterase (AChE) and glutathione S-transferase (GST) were measured in fourth-instar Chironomus riparius Meigen larvae exposed to the organophosphate insecticide pirimiphos methyl (0, 5, 10, and 50ng/g) for 48 or 96h, and at high or low food ration. Larvae exposed to 50ng/g pirimiphos methyl died within 48h. The weight of larvae exposed to 10ng/g pirimiphos methyl was significantly lower than those exposed to 0 and 5ng/g. AChE activity was significantly reduced in larvae exposed to 10ng/g, but GST activity remained unaffected. Lower food ration reduced larval weights across all treatments but did not affect biomarker measurements. Insecticide exposure was associated with a longer time to adult emergence and oviposition, fewer egg masses, a greater proportion of deformed egg masses, and fewer eggs.

Effect of temperature and pirimiphos methyl on biochemical biomarkers in Chironomus riparius Meigen

Fourth-instar Chironomus riparius Meigen larvae were exposed to the organophosphate (OP) insecticide pirimiphos methyl (0, 0.1, 1.0, and 10 microg/L) for 48, 72, or 96 h at three temperatures (3, 12, or 22 degrees C). Two biochemical biomarkers, acetylcholinesterase (AChE) and glutathione S-transferase (GST), were measured in individual larvae from each treatment. AChE activity was inhibited by the OP in a dose-responsive fashion. This response remained similar at all three temperatures, demonstrating that AChE is a robust and specific biomarker. Exposure duration had little effect on AChE activity. In contrast, GST activity was induced at the highest OP insecticide concentration, but induction was also evident at 3 degrees C. There was a significant effect of exposure duration, with an overall decline in GST activity over time. This result agrees with previous work suggesting that GSTs are not particularly suitable for use as a biomarker of pesticide exposure or effect in Chironomus.

In vitro generation of organophosphate resistant boophilus microplus (Acari: Ixodidae) cell lines

Three organophosphate resistant Boophilus microplus Canestrini cell lines were generated by exposing B. microplus VIII-SCC cell line to incrementally increased toxic concentrations of the acaricide coumaphos. The development of resistance was evidenced by LC50 values elevated over those of control cells. The resistant cell lines selected in higher concentrations of organophosphate, designated C44 and C54, also had significantly slower duplication rates than a resistant cell line selected in lower concentrations of coumaphos (C34) and the nonresistant control cells. Resistant cell lines C44 and C54 also had significantly higher levels of esterase after exposure to coumaphos than resistant cell line C34 and the nonresistant controls. These in vitro results agree with reports of increased esterase activity associated with organophosphate resistance in B. microplus ticks in vivo.

Purification, molecular cloning and heterologous expression of a glutathione S-transferase involved in insecticide resistance from the rice brown planthopper, Nilaparvata lugens

A novel glutathione S-transferase (GST)-based pyrethroid resistance mechanism was recently identified in Nilaparvata lugens [Vontas, Small and Hemingway (2001) Biochem. J. 357, 65-72]. To determine the nature of GSTs involved in conferring this resistance, the GSTs from resistant and susceptible strains of N. lugens were partially purified by anion exchange and affinity chromatography. The majority of peroxidase activity, previously correlated with resistance, was confined to the fraction that bound to the affinity column, which was considerably elevated in the resistant insects. A cDNA clone encoding a GST (nlgst1-1) - the first reported GST sequence from Hemiptera with up to 54% deduced amino-acid identity with other insect class I GSTs - was isolated from a pyrethroid-resistant strain. Northern analysis showed that nlgst1-1 was overexpressed in resistant insects. nlgst1-1 was expressed in Escherichia coli, purified and characterized. The ability of the recombinant protein to bind to the S-hexylglutathione affinity matrix, its substrate specificities and its immunological properties confirmed that this GST was one from the elevated subset of N. lugens GSTs. Peroxidase activity of the recombinant nlgst1-1 indicated that it had a role in resistance, through detoxification of lipid peroxidation products induced by pyrethroids. Southern analysis of genomic DNA from the resistant and susceptible strains indicated that GST-based insecticide resistance may be associated with gene amplification in N. lugens.

Identification and cloning of a key insecticide-metabolizing glutathione S-transferase (MdGST-6A) from a hyper insecticide-resistant strain of the housefly Musca domestica

Strains of the housefly, Musca domestica, highly resistant to organophosphate (OP) and other insecticides are known because they overproduce glutathione S-transferases (GSTs). Previous work has shown that overproduction in these strains involved numerous isozymes with glutathione conjugating activities (Pesticide Biochem. Physiol., 25 (1986) 169; Mol. General Genetics, 227 (1991) 355; J. Biol. Chem., 267 (1992) 1840; Mol. General Genetics, 245 (1994) 236; J. Mol. Evol., 43 (1996) 236). The current work describes the purification and identification of a M. domestica GST isozyme (pI 7.1) broadly specific for substrates from a housefly strain, Cornell-HR, that is highly resistant against OP-insecticides, and the isolation of two new MdGST genes using the antibody made against it. This isozyme, which was identified from amongst more than 20 isoelectric forms of GSTs of the same subunit size, was highly active for conjugating GSH to the model substrate 3,4-dichloronitrobenzne (DCNB). When expressed in Escherichia coli, one of the cloned GSTs, MdGST-6A, produces an enzyme that conjugates glutathione to the insecticides methyl parathion and lindane. On indication that it was the most active isozyme toward several xenobiotics among several MdGSTs tested, we advance the notion that MdGST-6A probably plays an important role in M. domestica Cornell-HR's resistance towards OP-insecticides. MdGST-6A and a second closely related one found in this work, MdGST-6B, are members of the traditional insect class I family (theta-class) and share the greatest homologies with a cluster of Drosophila GSTs on locus 55. In addition to having the unusually broad substrate specificity, the sequence of the new group of enzymes reveals that it has a highly diverged hydrophobic motif in its active site as compared to other class I GSTs from insects.

Heterologous expression and characterization of alternatively spliced glutathione S-transferases from a single Anopheles gene

Three cDNA sequences of glutathione S-transferase (GST), adgst1-2, adgst1-3 and adgst1-4, which are alternatively spliced products of the adgst1AS1 gene, were obtained from fourth instar larvae of Anopheles dirus mosquito by reverse transcriptase PCR reactions. The nucleotide sequences of these three cDNAs share >67% identity and the translated amino acid sequences share 61-64% identity. A comparison of the An. dirus to the An. gambiae enzymes shows that adGST1-2 versus agGST1-4, adGST1-3 versus agGST1-5 and adGST1-4 versus agGST1-3 have 85, 92 and 85% amino acid sequence identity, respectively, which confirms that orthologous isoenzymes occur across anopheline species. These three proteins were expressed at high levels, approximately 15-20 mg from 200 ml of E. coli culture. The recombinant enzymes were purified by affinity chromatography on an S-hexylglutathione agarose column. The subunit sizes of adGST1-2, adGST1-3 and adGST1-4 are 24.3, 23.9 and 25.1 kDa. The recombinant enzymes have high activities with 1-chloro-2,4-dinitrobenzene (CDNB), detectable activity with 1,2-dichloro-4-nitrobenzene but markedly low activity with ethacrynic acid and p-nitrophenethyl bromide. adGST1-3 was shown to be the most active enzyme from the kinetic studies. Permethrin inhibition of CDNB activity, at varying concentrations of CDNB, was significantly different, being uncompetitive for adGST1-2, noncompetitive for adGST1-3 and competitive for adGST1-4. In contrast, permethrin inhibition with varying glutathione concentrations was noncompetitive for all three GSTs. Despite the enzymes being splicing products of the same gene and sharing identical sequence in the N-terminal 45 amino acids, these GSTs show distinct substrate specificities, kinetic properties and inhibition properties modulated by the differences in the C-terminus.

Quantification of pyrethroid insecticides from treated bednets using a mosquito recombinant glutathione S-transferase

Recombinant glutathione S-transferase (agGST1-6) from the malaria vector mosquito Anopheles gambiae Giles (Diptera: Culicidae) was expressed in Escherichia coli using a pET3a vector system. The expressed enzyme was biochemically active with reduced glutathione (GSH) and 1-chloro-2,4-dinitrobenzene (CDNB). Activity of agGST1-6 with GSH and CDNB was inhibited to different degrees by both alpha-cyano and non-alpha-cyano pyrethroid insecticides. This inhibition was used to develop an assay for quantification of pyrethroids. Standard curves of insecticide concentration against percentage of enzyme inhibition or volume of iodine solution were established by spectrophotometry and iodine volumetric titration, respectively, for permethrin and deltamethrin. These assays allowed estimation of pyrethroid concentrations both spectrophotometrically and visually. For the residue assay of each insecticide, a cut-off point of 50% of the initial pyrethroid impregnation concentration was used, which should differentiate between biologically active and inactive treated bednets. The cross-reactivity of the primary permethrin photodegradants (3-phenoxyalcohol and 3-phenoxybenzoic acid) with the recombinant agGST1-6 was assayed in the same system. No agGST1-6 inhibition by the insecticide metabolites was observed, suggesting that the system is unaffected by primary permethrin metabolites and will accurately measure insecticide parent compound concentrations. The estimated pyrethroid insecticide concentrations, given spectrophotometrically and by iodine titration assay, were comparable to those obtained by direct HPLC quantification of residues extracted from bednets. Hence, it should be relatively easy to adapt this method to produce a test kit for residue quantification in the field.

Developmental expression and stress induction of glutathione S-transferase in the spruce budworm, Choristoneura fumiferana

Developmental and stress-induced expression of Choristoneura fumiferana glutathione S-transferase (CfGST) mRNA and protein were examined using Northern blots and Western blots. High levels of CfGST mRNA and protein were detected in 1st instar larvae and diapausing 2nd instar larvae. Expression of CfGST gradually decreased during larval development from 3rd to 5th instar, after which the expression increased once again, reaching peak levels in 6th instar larvae. CfGST mRNA and protein were undetectable in the pupal stage. Exposure to low temperature did not induce an increase in CfGST expression. Feeding on balsam fir foliage resulted in an increase in the expression of CfGST as compared to larvae that fed on artificial diet. The bacterial insecticide, Bacillus thuringiensis delta-endotoxin (Bt), the non-steroidal ecdysone analog, tebufenozide, and the synthetic pyrethroid, permethrin, induced the expression of CfGST mRNA in 5th instar larvae, whereas the chitin synthesis inhibitor, diflubenzuron, did not have any such effect. These results suggest that CfGST plays an important role in detoxifying various allelochemicals and insecticides in the spruce budworm. The developmental expression pattern strongly suggests that in addition to detoxification, CfGST might be involved in other functions.

Disruption of the microsomal glutathione S-transferase-like gene reduces life span of Drosophila melanogaster

Microsomal glutathione S-transferase-I (MGST-I) has been thought to be important for protecting the cell from oxidative damages and/or xenobiotics. We have previously identified the Microsomal glutathione S-transferase-like (Mgstl) gene, a Drosophila homologue of human MGST-I. To investigate the function of the enzyme using Drosophila as a model system, we examined the expression pattern of Mgstl during development, and generated loss-of-function mutants to assess its in-vivo function. Mgstl was expressed in all developmental stages. It is expressed ubiquitously with the highest expression in the larval fat body, an insect organ thought to be functionally corresponding to mammalian liver, while relatively low in the central nervous system. This tissue distribution is consistent with that of MGST-I in humans or Rats. Mgstl null mutants generated from a P element insertion line showed no obvious defects in morphology, indicating that it is not essential for the development. However, their life span was significantly reduced compared to control flies, suggesting that the MGSTL protein is involved in processes somehow contributing to aging. We found an Mgstl pseudogene, which is apparently derived through the reverse transcription of Mgstl mRNA and subsequent integration into the genome.

Cloning and characterization of a new theta-class glutathione-S-transferase (GST) gene, gst-3, from Drosophila melanogaster

We report here on the cloning and characterization of a new theta-class glutathione-S-transferase (GST) gene, gst-3, from Drosophila melanogaster. Its sequence is distinct from previously characterized Drosophila GST genes, and Southern blotting shows no other closely related genes in the genome. In-situ hybridization localizes the gene to chromosome 2 (55D), near gst-2 (53F), and clearly separate from the gst-D cluster at 87B. The gene is intronless and appears to possess conventional 5' TATA, Cap and 3' polyadenylation signals. A single transcript, approximately 1kb in size, appears to be expressed at high levels in all developmental stages examined. When this gene is overexpressed using various upstream GAL4 driver systems, no striking phenotypes are observed; however, we detect bristle morphology defects in some progeny. The gst-3 gene does not appear to be essential, based upon our observation that mutant flies homozygous for an EP element insertion 5' to the TATA box produce little or no detectable gst-3 mRNA; these flies are viable and fertile at 25 and 29 degrees C. Nevertheless, the gst-3 gene appears to be evolutionarily conserved in other Drosophila species, suggesting that it may be functionally important.

Glutathione S-transferase from the spruce budworm, Choristoneura fumiferana: identification, characterization, localization, cDNA cloning, and expression

A 23-kDa protein that was present at higher levels in diapausing 2nd instar larvae than in feeding 2nd instar larvae of Choristoneura fumiferana was purified, and polyclonal antibodies were raised against this protein. The antibodies were subsequently used to screen a cDNA library that was constructed using RNA from 2nd instar larvae. Eight identical cDNA clones were isolated. The cDNA clone had a 665-bp insert and the longest open reading frame coded for a 203-amino acid protein with a predicted molecular mass of 23.37 kDa. The deduced amino acid sequence showed high similarity to glutathione S-transferases and therefore, the cDNA clone was named C. fumiferana glutathione S-transferase (CfGST). Identity of CfGST was confirmed by using affinity-purification as well as enzyme activity assay. CfGST was closer in similarity to insect GST2 members than GST1 members. The apparent Vmax of the purified CfGST towards the substrates glutathione and 1-chloro-2,4-dinitrobenezene (CDNB) were similar. However, the enzyme had a three-fold higher affinity towards CDNB than glutathione. Analyses using Northern blot, immunoblot and immunocytochemistry demonstrated that the fat body was the major tissue where the enzyme was synthesized and stored. Higher levels of CfGST protein were present in diapausing 2nd instar larvae compared to feeding 2nd and 6th instar larvae, suggesting that besides detoxification CfGST may have other roles during insect development that are not readily apparent at present. The CfGST cDNA was expressed in a recombinant baculovirus expression system and an active enzyme was produced.

Is the insect glutathione S-transferase I gene family intronless?

The genes coding for class I glutathione S-transferases in insects were believed to be intronless because the coding sequence was not interrupted by an intron. But sequences of the untranslated 5' end of transcripts revealed the presence of an intron in housefly and Drosophila genes suggesting that most insect GSTI genes are in fact interrupted.

Molecular cloning and heterologous expression of a glutathione S-transferase involved in insecticide resistance from the diamondback moth, Plutella xylostella

Four glutathione S-transferase (GST, EC 2.5.1.18) isozymes have been characterized in the larvae of the diamondback moth (DBM), Plutella xylostella L., a cosmopolitan insect pest of crucifiers. This work aimed at cloning and heterologously expressing the cDNA of DBM GST-3, an isozyme involved in this insect resistance to some organophosphorus insecticides, and studying the molecular basis for its increased expression in the resistant strains. Reverse-transcription polymerase chain reaction (RT-PCR) using midgut mRNA from a methyl parathion resistant MPA strain and degenerate primers complimentary to the N-terminal and internal amino acid sequences of GST-3 generated a 128 bp DNA product. A clone of 809 bp, obtained by screening a midgut cDNA library of MPA strain using this PCR product as probe, encoded a protein of 216 amino acids (calculated Mr 24,083 and pI 8.50). This GST of DBM, PxGST3, shared the highest (46.3%) amino acid sequence identity, among insects, to MsGST1 of Manduca sexta. PxGST3 mRNA level was considerably higher in MPA than in susceptible strains, and Southern blots suggested that gene amplification was probably not involved in the increased expression of this GST isozyme. Enzymatically active PxGST3 expressed heterologously in E. coli exhibited similar biochemical and toxicological properties as GST-3 purified from DBM larvae. It is the first cloned GST with a well-defined role in insecticide resistance.

Molecular phylogeny of glutathione-S-transferases

The glutathione-S-transferase (GST) protein superfamily is currently composed of nearly 100 sequences. This study documents a greater phylogenetic diversity of GSTs than previously realized. Parsimony and distance phylogenetic methods of GST amino acid sequences yielded virtually the same results. There appear to be at least 25 groups (families) of GST-like proteins, as different from one another as are the currently recognized classes. This diversity will require the design of a new nomenclature for this large protein superfamily. There is one well-supported large clade containing the mammalian mu, pi, and alpha classes as well as GSTs from molluscs, helminths, nematodes, and arthropods.

Cloning and characterization of two glutathione S-transferases from a DDT-resistant strain of Anopheles gambiae

Two cDNA species, aggst1-5 and aggst1-6, comprising the entire coding region of two distinct glutathione S-transferases (GSTs) have been isolated from a 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) resistant strain (ZANDS) of Anopheles gambiae. The nucleotide sequences of these cDNA species share 80.2% identity and their derived amino acid sequences are 82.3% similar. They have been classified as insect class I GSTs on the basis of their high sequence similarity to class I GSTs from Drosophila melanogaster and Musca domestica and they are localized to a region of an An. gambiae chromosome known to contain further class I GSTs. The genes aggst1-5 and aggst1-6 were expressed at high levels in Escherichia coli and the recombinant GSTs were purified by affinity chromatography and characterized. Both agGST1-5 and agGST1-6 showed high activity with the substrates 1-chloro-2,4-dinitrobenzene and 1, 2-dichloro-4-nitrobenzene but negligible activity with the mammalian theta class substrates, 1,2-epoxy-3-(4-nitrophenoxy)propane and p-nitrophenyl bromide. Despite their high level of sequence identity, agGST1-5 and agGST1-6 displayed different kinetic properties. Both enzymes were able to metabolize DDT and were localized to a subset of GSTs that, from earlier biochemical studies, are known to be involved in insecticide resistance in An. gambiae. This subset of enzymes is one of three in which the DDT metabolism levels are elevated in resistant insects.

Heterogeneity of the glutathione transferase genes encoding enzymes responsible for insecticide degradation in the housefly

One of the four glutathione-S-transferases (GST) that is overproduced in the insecticide-resistant Cornell-R strain of the housefly (Musca domestica) produces an activity that degrades the insecticide dimethyl parathion and conjugates glutathione to lindane. In earlier work, it was shown that the resistant Cornell-R carries an amplification, probably a duplication, of one or more of its GST loci and that this amplification is directly related to resistance. Using polymerase chain reaction (PCR) amplification with genomic DNA, multiple copies of the gene encoding the parathion-degrading activity (called MdGst-3) were subcloned from both the ancestral, insecticide-susceptible strain BPM and from the insecticide-resistant Cornell-R. In BPM, three different MdGst-3 genes were identified while in Cornell-R, 12 different MdGst-3 sequences were found that, though closely related to ancestral genes, had diverged by a few nucleotides. This diversity in MdGst-3 genomic sequences in Cornell-R is reflected in the expressed sequences, as sampled through a cDNA bank. Population heterozygosity cannot account for these multiple GST genes. We suggest that selection for resistance to insecticides has resulted in not only amplification of the MdGst-3 genes but also in the divergence of sequence between the amplified copies.

Purification and characterization of a major glutathione S-transferase from the mosquito Anopheles dirus (species B)

The major form of glutathione S-transferase (GST) activity from the mosquito Anopheles dirus (species B), a vector of malaria in Thailand has been purified 421-fold. It constituted approx. 20% of the total measured CDNB conjugating activity in the homogenate. This enzyme appeared as a single band of 25.0 +/- 0.26 kDa on SDS-PAGE and was kinetically characterized with 10 substrates and 4 inhibitors. The enzyme is capable of catalysing dehydrochlorination of 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) in vitro at a rate of 4.4 nmol of 1,1-dichloro-2,2-bis-(p-chlorophenyl)ethane (DDE) formation per mg protein. This is comparable to the rate of catalysis of the orthologous isoenzyme from An. gambiae reported previously. The IC50 plots of the inhibitor data (fractional velocity vs log [I]) for three of the inhibitors indicate the homogenous nature of this enzyme. However, inhibition by ethacrynic acid demonstrates more than a single affinity site for interaction. The six N-terminal amino acids of the purified enzyme are identical to a GST reported from Aedes aegypti, which was indicated to play a role in DDT-resistance in this species. The results suggest that the two enzymes may belong to the same class, however each possesses a different specificity.

The hermit transposable element of the Australian sheep blowfly, Lucilia cuprina, belongs to the hAT family of transposable elements

We report the cloning of hermit, a member of the hAT family of transposable elements from the genome of the Australian sheep blowfly, Lucilia cuprina. Hermit is 2716 bp long and is 49% homologous to the autonomous hobo element, HFL1, at the nucleic acid level. Hermit has 15 bp terminal inverted repeats that share 10 bp with the terminal inverted repeats of HFL1. Conceptual translation reveals a 583 residue open reading frame (ORF) that is 64% similar and 42% identical to the HFL1 ORF. However, the sequence of the hermit element contains two frameshifts within the putative ORF, indication that hermit is an inactive element. Analysis of L. cuprina strains from within and outside Australia suggested that hermit is present as a single copy in all the genomes analysed.

Glutathione S-transferases from larval Manduca sexta midgut: sequence of two cDNAs and enzyme induction

Two glutathione S-transferase (GST) clones from a larval midgut cDNA library of the tobacco hornworm, Manduca sexta were sequenced. The nucleotide sequence of the first clone, M. sexta GST1, encoded a protein of 217 amino acids with a predicted molecular weight of 24,644 and isoelectric point of 4.8. The M. sexta GST1 was 45.9-48.6% identical to GSTs from Musca domestica and several Drosophila species. The M. sexta GST2 cDNA encoded a protein of 203 amino acids with a predicted molecular weight of 23,596 and isoelectric point of 5.5. The M. sexta GST2 shared 44.8-50.0% sequence identity to a second cluster of insect GSTs from M. domestica, D. melanogaster and Anopheles gambiae. GST1 and GST2 were only 24.1% identical in amino acid sequence. The divergence of these two classes of insect GSTs occurred before the radiation of Diptera and Lepidoptera. Northern analysis of the expression of these GSTs showed increased GST1 mRNA levels in midguts of larvae fed diets containing 2-undecanone, or phenobarbital. Midgut and fat body cytosolic GST activities were induced when larvae were fed diets containing 2-tridecanone, 2-undecanone, or phenobarbital. Partial purification of midgut GSTs by size-exclusion and glutathione affinity chromatography resulted in a series of isoelectric focusing bands, with the major one corresponding to the predicted isoelectric point of the M. sexta GST1. In summary, two midgut GSTs have been identified on the basis of cDNA sequence and one of these, GST1, was inducible by dietary chemicals.

Glutathione S-transferases in housefly (Musca domestica): location of GST-1 and GST-2 families

The two families of dipteran glutathione S-transferases (GST-1 and GST-2) were located separately by immunohistology on sections of adult houseflies. GST-1 was distributed in haemolymph cells, whereas GST-2 was found in the indirect flight muscles of the thorax and in the central nervous system. In the muscles, the distribution of GST-2 seemed to be uniform in cells, whereas in the brain and the thoracic ganglia GST-2 was found mainly in the cortical areas which are made up by cell bodies. Comparison of the GSTs' location between an insecticide susceptible strain of housefly and resistant ones indicated no variation due to resistance. Enzyme-linked immunosorbent assay tests were used to dose GST-2. In houseflies, there were 60 pmol of GST-2/fly, 80-90% being found in the thorax, about 10% in the head and the remainder in the abdomen. Furthermore, the roles of these GSTs are discussed in relation to their location and our knowledge on their catalytic activities or their transport ability in invertebrates and mammals.

Separation of multiple forms of acidic glutathione S-transferase isozymes in a susceptible and a resistant strain of house fly, Musca domestica (L.)

The acidic glutathione S-transferases from a CSMA (susceptible) strain and a Cornell-R (resistant) strain of houseflies were purified and separated utilizing affinity chromatography followed by chromatofocusing. Nine fractions were isolated from each house fly strain. Fraction 1 had the highest 1-chloro-2,4-dinitrobenzene vs. 1,2-dichloro-4-nitrobenzene ratio (CDNB/DCNB ratio) in both strains and the ratio of all the other fractions tended to decrease as the isoelectrical points decreased except for fractions 4 and 9. Most fractions from the CSMA strain had higher CDNB conjugation activities than the fractions from the Cornell-R strain, but all the fractions from the CSMA strain had lower DCNB conjugation activities than fractions from the Cornell-R strain. Steady-state kinetics of all the fractions were examined. The Km values obtained from both strains ranged from 0.36 to 1.12 mM, while the Vmax value ranged from 3.0 to 32.6 mumol/min/mg. In the 100,000 g supernatant, the CDNB specific activities in the CSMA strain was about 1/3 of the activity in the Cornell-R strain but it was about 1.5-fold following affinity chromatography. The specific activity for DCNB measured in the CSMA strain was only 1/5 of the activities of the Cornell-R strain in the 100,000 g supernatant, but was about the same after affinity chromatography. The difference was due to the selectivity of the affinity column used in the current study.