Cloning and localization of a glutathione S-transferase class I gene from Anopheles gambiae

Molecular Cloning and Characterization of Five Glutathione S-Transferase Genes and Promoters from Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae) and Their Response to Tannic Acid Stress

Plants accumulate phenolic compounds such as tannic acid to resist insect herbivores. The survival of insects exposed to toxic secondary metabolites depends on the detoxification metabolism mediated by limited groups of glutathione S-transferases (GSTs). Micromelalopha troglodyta (Graeser) (Lepidoptera: Notodontidae) is an important foliar pest of poplar trees. GSTs play an important role in xenobiotic detoxification in M. troglodyta. Five GST genes were identified in M. troglodyta and were classified into five different cytosolic GST classes, delta, omega, sigma, theta, and zeta. Real-time fluorescent quantitative polymerase chain reaction (qPCR) was used to determine the mRNA expression of the five cloned GSTs in the midguts and fat bodies of M. troglodyta. The mRNA expression of the five GSTs was significantly induced when M. troglodyta was exposed to tannic acid. To further understand the tannic acid regulatory cascade, the 5′-flanking promoter sequences of the five MtGSTs were isolated by genome walking methods, and the promoters were very active and induced by tannic acid. In summary, the induction of GST mRNA expression was due to the response of five MtGST promoters to tannic acid. Therefore, MtGST promoters play an important role in the regulation of GST transcription.

Combining genetic crosses and pool targeted DNA-seq for untangling genomic variations associated with resistance to multiple insecticides in the mosquito Aedes aegypti

In addition to combating vector-borne diseases, studying the adaptation of mosquitoes to insecticides provides a remarkable example of evolution-in-action driving the selection of complex phenotypes. Actually, most resistant mosquito populations show multi-resistance phenotypes as a consequence of the variety of insecticides employed and of the complexity of selected resistance mechanisms. Such complexity makes the identification of alleles conferring resistance to specific insecticides challenging and prevents the development of molecular assays to track them in the field. Here we showed that combining simple genetic crosses with pool targeted DNA-seq can enhance the specificity of resistance allele's detection while maintaining experimental work and sequencing effort at reasonable levels. A multi-resistant population of the mosquito Aedes aegypti was exposed to three distinct insecticides (deltamethrin, bendiocarb and fenitrothion), and survivors to each insecticide were crossed with a susceptible strain to generate three distinct lines. F2 individuals from each line were then segregated based on their survival to two insecticide doses. Hundreds of genes covering all detoxifying enzymes and insecticide targets together with more than 7,000 intergenic regions equally spread over mosquito genome were sequenced from pools of F0 and F2 individuals unexposed or surviving insecticide. Differential coverage analysis identified 39 detoxification enzymes showing an increased gene copy number in association with resistance. Combining an allele frequency filtering approach with a Bayesian F ST-based genome scan identified multiple genomic regions showing strong selection signatures together with 50 nonsynonymous variations associated with resistance. This study provides a simple and cost-effective approach to improve the specificity of resistance allele's detection in multi-resistant populations while reducing false positives frequently arising when comparing populations showing divergent genetic backgrounds. The identification of novel DNA resistance markers opens new opportunities for improving the tracking of insecticide resistance in the field.

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

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

The transcription factor Maf-S regulates metabolic resistance to insecticides in the malaria vector Anopheles gambiae

Background

Malaria control in Africa is dependent upon the use insecticides but intensive use of a limited number of chemicals has led to resistance in mosquito populations. Increased production of enzymes that detoxify insecticides is one of the most potent resistance mechanisms. Several metabolic enzymes have been implicated in insecticide resistance but the processes controlling their expression have remained largely elusive.

Results

Here, we show that the transcription factor Maf-S regulates expression of multiple detoxification genes, including the key insecticide metabolisers CYP6M2 and GSTD1 in the African malaria vector Anopheles gambiae. Attenuation of this transcription factor through RNAi induced knockdown reduced transcript levels of these effectors and significantly increased mortality after exposure to the pyrethroid insecticides and DDT (permethrin: 9.2% to 19.2% (p = 0.015), deltamethrin: 3.9% to 21.6% (p = 0.036) and DDT: 1% to 11.7% (p = <0.01), whilst dramatically decreasing mortality induced by the organophosphate malathion (79.6% to 8.0% (p = <0.01)). Additional genes regulated by Maf-S were also identified providing new insight into the role of this transcription factor in insects.

Conclusion

Maf-S is a key regulator of detoxification genes in Anopheles mosquitoes. Disrupting this transcription factor has opposing effects on the mosquito’s response to different insecticide classes providing a mechanistic explanation to the negative cross resistance that has been reported between pyrethroids and organophosphates.

Electronic supplementary material

The online version of this article (10.1186/s12864-017-4086-7) contains supplementary material, which is available to authorized users.

Resistance to DDT in an urban setting: common mechanisms implicated in both M and S forms of Anopheles gambiae in the city of Yaoundé Cameroon

BACKGROUND

In the city of Yaoundé in Cameroon malaria is predominately transmitted by the M and S molecular forms of Anopheles gambiae and both are resistant to the pyrethroid insecticides and DDT. Mutations in the target site of these insecticides, present at a high frequency in malaria vectors in this city, contribute to this resistance profile. To identify additional resistance mechanisms, the expression profile of multiple DDT-resistant field populations of M and S molecular forms was compared to laboratory-susceptible populations.

METHODOLOGY/PRINCIPAL FINDINGS

The prevalence of DDT resistance was highest in the S form population originating from the cultivated site of Nkolondom (mortality after WHO bioassay = 4%). A high prevalence of DDT resistance was also found in two urban M form populations, Messa from a pristine unpolluted environment (DDT mortality = 54%), and Gare, where the breeding sites are heavily polluted with organic matter (DDT mortality = 38%). Microarray analysis showed that several transcripts coding for detoxification enzymes (P450s, GSTs and UDPGTs) and ABC transporters were upregulated in the three populations. Despite the presence of multiple detoxification genes over expressed in the DDT-resistant subset of these field populations, only three were commonly over expressed in resistant populations from all three environments. Two of these genes, CYP6M2 and GSTD1-6, encode enzymes that have been previously shown to metabolize DDT.

CONCLUSION/SIGNIFICANCE

Analogous to target site resistance, genes involved in metabolic resistance to DDT are also shared between the M and S forms of An gambiae. Alternative explanations for this occurrence are explored.

Purification and biochemical characterization of glutathione S-transferases from four field populations of Bactrocera dorsalis (Hendel) (Diptera: Tephritidae)

Glutathione S-transferases (GSTs) are a group of detoxification enzymes that catalyze the nucleophilic addition of glutathione to a wide variety of endogenous and xenobiotic compounds. In this study, GSTs were purified from four field populations of Bactrocera dorsalis with different insecticide susceptibilities by glutathione-agarose affinity chromatography. The populations were collected from Dongguan (DG) and Guangzhou (GZ) of the Guangdong Province, Haikou of the Hainan province (HN), and Kunming of the Yunnan province (YN), China. Differences in GST characteristics among the four populations were studied using purified enzyme samples through comparative SDS-PAGE, kinetic, and inhibition experiments. The specific activities of the purified enzymes were similar, but the purification yield of the GZ population (31.54%) was the lowest. SDS-PAGE analysis showed only one band at approximately 23 kDa for these four populations. Kinetic analyses showed that the affinities of the purified GSTs from the GZ and YN populations for 1-chloro-2.4-dinitrobenzene (CDNB) were much higher than those of GSTs from the other two populations, whereas the HN population had the highest catalytic capability in terms of V(max) value. The optimum temperature for CDNB conjugation was 37 °C and the optimum pH was 7.5 in all four populations. Inhibition kinetics showed that ethacrynic acid, diethyl maleate, tetraethylthiuram disulfide, curcumin, bromosulfalein, and β-cypermethrin had excellent inhibitory effects on GSTs in the four populations of B. dorsalis, but the low inhibitory effects of malathion and avermectin did not differ between populations. These results suggest that GSTs may have a role in detoxification of β-cypermethrin in B. dorsalis.

The role of the Aedes aegypti Epsilon glutathione transferases in conferring resistance to DDT and pyrethroid insecticides

The Epsilon glutathione transferase (GST) class in the dengue vector Aedes aegypti consists of eight sequentially arranged genes spanning 53,645 bp on super contig 1.291, which maps to chromosome 2. One Epsilon GST, GSTE2, has previously been implicated in conferring resistance to DDT. The amino acid sequence of GSTE2 in an insecticide susceptible and a DDT resistant strain differs at five residues two of which occur in the putative DDT binding site. Characterization of the respective recombinant enzymes revealed that both variants have comparable DDT dehydrochlorinase activity although the isoform from the resistant strain has higher affinity for the insecticide. GSTe2 and two additional Epsilon GST genes, GSTe5 and GSTe7, are expressed at elevated levels in the resistant population and the recombinant homodimer GSTE5-5 also exhibits low levels of DDT dehydrochlorinase activity. Partial silencing of either GSTe7 or GSTe2 by RNA interference resulted in an increased susceptibility to the pyrethroid, deltamethrin suggesting that these GST enzymes may also play a role in resistance to pyrethroid insecticides.

Structure, function and evolution of insect flight muscle

Insects, the largest group of animals on the earth, owe their prosperity to their ability of flight and small body sizes. The ability of flight provided means for rapid translocation. The small body size allowed access to unutilized niches. By acquiring both features, however, insects faced a new problem: They were forced to beat their wings at enormous frequencies. Insects have overcome this problem by inventing asynchronous flight muscle, a highly specialized form of striated muscle capable of oscillating at >1,000 Hz. This article reviews the structure, mechanism, and molecular evolution of this unique invention of nature.

Major effect genes or loose confederations? The development of insecticide resistance in the malaria vector Anopheles gambiae

Insecticide use in public health and agriculture presents a dramatic adaptive challenge to target and non-target insect populations. The rapid development of genetically modulated resistance to insecticides is postulated to develop in two distinct ways: By selection for single major effect genes or by selection for loose confederations in which several factors, not normally associated with each other, inadvertently combine their effects to produce resistance phenotypes. Insecticide resistance is a common occurrence and has been intensively studied in the major malaria vector Anopheles gambiae, providing a useful model for examining how insecticide resistance develops and what pleiotropic effects are likely to emerge as a consequence of resistance. As malaria vector control becomes increasingly reliant on successfully managing insecticide resistance, the characterisation of resistance mechanisms and their pleiotropic effects becomes increasingly important.

Malaria management: past, present, and future

The prospect of malaria eradication has been raised recently by the Bill and Melinda Gates Foundation with support from the international community. There are significant lessons to be learned from the major successes and failures of the eradication campaign of the 1960s, but cessation of transmission in the malaria heartlands of Africa will depend on a vaccine and better drugs and insecticides. Insect control is an essential part of reducing transmission. To date, two operational scale interventions, indoor residual spraying and deployment of long-lasting insecticide-treated nets (LLINs), are effective at reducing transmission. Our ability to monitor and evaluate these interventions needs to be improved so that scarce resources can be sensibly deployed, and new interventions that reduce transmission in a cost-effective and efficient manner need to be developed. New interventions could include using transgenic mosquitoes, larviciding in urban areas, or utilizing cost-effective consumer products. Alongside this innovative development agenda, the potential negative impact of insecticide resistance, particularly on LLINs, for which only pyrethroids are available, needs to be monitored.

The Aedes aegypti glutathione transferase family

In this report, we describe the glutathione transferase (GST) gene family in the dengue vector Aedes aegypti and suggest a novel role for a new class of mosquito GSTs. Twenty-six GST genes are present in Ae. aegypti, two of which are alternatively spliced to give a total of 29 transcripts for cytosolic GSTs. The six classes identified in other insect species are all represented and, as in Anopheles gambiae, the majority of the mosquito GSTs belong to the insect-specific Delta and Epsilon classes with eight members each. Sixteen secure 1:1 orthologs were identified between GSTs in Ae. aegypti and An. gambiae, but only four of these have recognisable orthologs in Drosophila melanogaster. Three mosquito-specific GSTs were identified which did not belong to any previously recognised GST classes. One of these, GSTx2, has been previously implicated in conferring 1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane (DDT) resistance in Ae. aegypti from South America. However, we found no evidence for increased levels of this GST protein in DDT/pyrethroid-resistant populations from Thailand. Furthermore, we show that the recombinant GSTX2-2 protein is unable to metabolise DDT. Interestingly, GSTX2-2 showed an affinity for hematin, and this, together with the restricted distribution of this class to haematophagous insects, may indicate a role for these enzymes in protecting mosquitoes against heme toxicity during blood feeding.

Identification, sequencing and expression of selenium-dependent glutathione peroxidase transcript in the freshwater bivalve Unio tumidus exposed to Aroclor 1254

Glutathione peroxidases (GPx) and glutathione S-transferases (GST) are essential enzymes of the cellular defense system. The aim of this work was the identification of GPx transcript in a freshwater bivalve, Unio tumidus, and the effects of Aroclor 1254 on GPx and pi-class GST (pi-GST) expression pattern. The GPx full-length coding sequence was obtained by reverse transcription PCR using degenerated primers followed by 5' and 3' rapid amplification of cDNA ends. The GPx cDNA encodes a protein of 232 amino acids. The 72nd amino acid corresponds to a selenocysteine encoded by a TGA codon. Residues essential to the enzymatic function are conserved in GPx of U. tumidus. Specific amplifications of the Se-GPx mRNA from U. tumidus were performed on the digestive gland, the excretory system and the gills. Se-GPx expression level is highest in the digestive gland. No induction of the Se-GPx was observed at the transcriptional level in the digestive gland and the excretory system of Aroclor-treated mussels, while an increase of the pi-GST mRNA level was observed in the excretory system.

cDNA cloning and expression pattern of pi-class glutathione S-transferase in the freshwater bivalves Unio tumidus and Corbicula fluminea

Glutathione S-transferases (GSTs) are enzymes involved in major detoxification reactions of xenobiotics in many organisms. The aim of this work was the identification of GST transcripts in the freshwater bivalves Unio tumidus and Corbicula fluminea. We used degenerated primers designed in the highly conserved regions of GST to amplify the corresponding mRNA. Full-length coding sequences were obtained by 5' and 3' rapid amplification of cDNA ends. In the two species, the GST cDNAs identified encoded a protein of 205 amino acids. The comparison of the deduced amino acid sequences with GSTs from other species showed that the enzymes belong to the pi-class and the amino acids defining the binding sites of glutathione (G-site) and for xenobiotic substrates (H-site) are highly conserved. Specific amplifications of the GST mRNA from U. tumidus and C. fluminea were performed on the digestive gland, the excretory system and the gills. For each mussel, the results revealed that the pi-class GSTs are expressed at the same level in the three tissues.

Glutathione S‐transferase from Malarial Parasites: Structural and Functional Aspects

Malaria represents an emerging disease because of increasing parasite resistance against available drugs and because of increasing geographical distribution of the causative agent, Plasmodium falciparum. The complete genome of Plasmodium was sequenced recently, revealing that the parasite harbors only one glutathione S-transferase (PfGST). This observation was of particular interest: First, certain antimalarial drugs such as chloroquine and methylene blue presumably influence the glutathione metabolism in which PfGST is involved. Second, PfGST might play a significant role in drug resistance. PfGST was studied in parasite extracts and as recombinant protein, and its x-ray structure has been solved. The available data indicate that the homodimeric PfGST cannot be assigned to any of the previously known GST classes. PfGST exhibits significant structural differences to human GSTs, particularly at the so-called hydrophobic binding pocket (H-site) where the second substrate binds. Inhibition of PfGST is expected to act at different vulnerable metabolic sites of the parasite in parallel; it is likely to disturb GSH-dependent detoxification processes, to increase the levels of cytotoxic peroxides, and possibly to increase the concentration of toxic hemin. In this chapter, we summarize the current knowledge on PfGST, including aspects of structure, function, and future drug development.

Molecular cloning, expression and characterization of a novel class glutathione S-transferase from the fungus Cunninghamella elegans

The structural gene for glutathione S-transferase (CeGST1-1) in the fungus Cunninghamella elegans was cloned by screening a cDNA library using a degenerate oligonucleotide probe based on the N-terminal sequence of the purified protein. Open reading frame analysis indicated that the cegst1 gene encodes a protein of 210 amino acid residues. The deduced amino acid sequence showed 25% sequence identity with the sequence of the Pi-class GST from Danio rerio (zebrafish). Similarity was also shown with the Alpha-class GST from Fasciola hepatica (liver fluke; 23% identity), the Mu class from Mus musculus (22%) and the Sigma class from Ommastrephes sloani (squid; 21%). Further screening of a cDNA library with the cegst1 gene probe revealed the presence of another GST isoenzyme (CeGST2-2) in this fungus, which shows 84% sequence identity with CeGST1-1 at the amino acid level. Reverse transcription PCR revealed that cegst2 was also expressed at the mRNA level in the fungus C. elegans. Both cegst genes were overexpressed in Escherichia coli using the expression vector pQE51, displaying specific activities with 1-chloro-2,4-dinitrobenzene of 2.04 and 0.75 micromol/min per mg of protein respectively. Both enzymes exhibited a similar substrate specificity and inhibition profile, indicating that CeGST1-1 and CeGST2-2 belong to the same GST class. Mutagenesis analysis revealed that Tyr(10) in the N-terminal region is essential for catalysis of CeGST1-1. We propose from these results that the CeGSTs are novel Gamma-class GSTs and designated as GSTG1-1 and GSTG2-2 respectively.

Stable chromosomal inversion polymorphisms and insecticide resistance in the malaria vector mosquito Anopheles gambiae (Diptera: Culicidae)

Anopheles gambiae Giles has been implicated as a major vector of malaria in Africa. A number of paracentric chromosomal inversions have been observed as polymorphisms in wild and laboratory populations of this species. These polymorphisms have been used to demonstrate the existence of five reproductive units in West African populations that are currently described as incipient species. They have also been correlated with various behavioral characteristics such as adaptation to aridity and feeding preference and have been associated with insecticide resistance. Two paracentric inversions namely 2La and 2Rb are highly ubiquitous in the wild and laboratory populations sampled. Both inversions are easily conserved during laboratory colonization of wild material and one shows significant positive heterosis with respect to Hardy-Weinberg proportions. Inversion 2La has previously been associated with dieldrin resistance and inversion 2Rb shows an association with DDT resistance based on this study. The stability and maintenance of these inversions as polymorphisms provides an explanation for the transmission and continued presence of DDT and dieldrin resistance in a laboratory strain of An. gambiae in the absence of insecticide selection pressure. This effect may also be operational in wild populations. Stable inversion polymorphism also provides a possible mechanism for the continual inheritance of suitable genetic factors that otherwise compromise the fitness of genetically modified malaria vector mosquitoes.

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.

Overproduction of a P450 that metabolizes diazinon is linked to a loss-of-function in the chromosome 2 ali-esterase (MdalphaE7) gene in resistant house flies

Up-regulation of detoxifying enzymes in insecticide-resistant strains of the house fly is a common mechanism for metabolic resistance. However, the molecular basis of this increased insecticide metabolism is not well understood. In the multiresistant Rutgers strain, several cytochromes P450 and glutathione S-transferases are constitutively overexpressed at the transcriptional level. Overexpression is the result of trans-regulation, and a regulatory gene has been located on chromosome 2. A Gly137 to Asp point mutation in alphaE7 esterase gene, leading to the loss of carboxylesterase activity, has been associated with organophosphate resistance in the house fly and the sheep blowfly. We show here that purified recombinant CYP6A1 is able to detoxify diazinon with a high efficiency. We also show that either the Gly137 to Asp point mutation in alphaE7 esterase gene or a deletion at this locus confer resistance and overproduction of the CYP6A1 protein. Based on these findings, we propose it is the absence of the wild-type Gly137 allele of the alphaE7 gene that releases the transcriptional repression of genes coding for detoxification enzymes such as CYP6A1, thereby leading to metabolic resistance to diazinon.

Identification of a novel class of insect glutathione S-transferases involved in resistance to DDT in the malaria vector Anopheles gambiae

The sequence and cytological location of five Anopheles gambiae glutathione S-transferase (GST) genes are described. Three of these genes, aggst1-8, aggst1-9 and aggst1-10, belong to the insect class I family and are located on chromosome 2R, in close proximity to previously described members of this gene family. The remaining two genes, aggst3-1 and aggst3-2, have a low sequence similarity to either of the two previously recognized classes of insect GSTs and this prompted a re-evaluation of the classification of insect GST enzymes. We provide evidence for seven possible classes of insect protein with GST-like subunits. Four of these contain sequences with significant similarities to mammalian GSTs. The largest novel insect GST class, class III, contains functional GST enzymes including two of the A. gambiae GSTs described in this report and GSTs from Drosophila melanogaster, Musca domestica, Manduca sexta and Plutella xylostella. The genes encoding the class III GST of A. gambiae map to a region of the genome on chromosome 3R that contains a major DDT [1,1,1-trichloro-2,2-bis-(p-chlorophenyl)ethane] resistance gene, suggesting that this gene family is involved in GST-based resistance in this important malaria vector. In further support of their role in resistance, we show that the mRNA levels of aggst3-2 are approx. 5-fold higher in a DDT resistant strain than in the susceptible strain and demonstrate that recombinant AgGST3-2 has very high DDT dehydrochlorinase activity.

Glutathione S-transferases as antioxidant defence agents confer pyrethroid resistance in Nilaparvata lugens

Selection of a laboratory colony of the brown planthopper Nilaparvata lugens with the pyrethroids permethrin and lambda-cyhalothrin increased its resistance to both insecticides. Biochemical analysis and synergistic studies with metabolic inhibitors indicated that elevated glutathione S-transferases (GSTs) with a predominant peroxidase activity conferred resistance to both pyrethroids, whereas esterases conferred part of the resistance to permethrin. Purified esterases hydrolysed permethrin at a slow rate, but incubation of either pyrethroid or their primary metabolites with partially purified GSTs had no effect on the metabolic profile. Although GSTs were sensitive to inhibition by both pyrethroids, they did not serve as binding proteins, as previously hypothesized [Grant and Matsumura (1988) Insect Biochem. 18, 615-622]. We demonstrate that pyrethroids, in addition to their neurotoxic effect, induce oxidative stress and lipid peroxidation in insects. Pyrethroid exposure induced lipid peroxides, protein oxidation and depleted reduced glutathione. Elevated GSTs in the resistant strains attenuated the pyrethroid-induced lipid peroxidation and reduced mortality, whereas their in vivo inhibition eliminated their protective role. We therefore hypothesize that the main role of elevated GSTs in conferring resistance in N. lugens is through protecting tissues from oxidative damage. Our study extends the GSTs' range of efficacy to pyrethroid insecticides and possibly explains the role of elevated GSTs in other pyrethroid-resistant insects.

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.

Genomic organization and putative promoters of highly conserved glutathione S-transferases originating by alternative splicing in Anopheles dirus

The genomic DNA of a GST class I alternative splicing gene has been characterized from Anopheles dirus, a Thai malaria vector. This gene organization is highly conserved in An. dirus and Anopheles gambiae (aggst1alpha), with >80% nucleotide identity in the coding region. Their gene organization contains six exons for four mature GST transcripts, which share exon 1 and exon 2 but vary between four different exon 3 sequences (exon 3A-3D). The deduced amino acid sequence of the GST transcripts from these two genes also shows very high conservation, with 85-93% identity for each orthologous gene. Two putative promoters and possible regulatory elements were predicted by a combination of the TSSW and MatInspector programs. The Ad214 promoter is proposed to be involved in developmental stage regulation. The Ad2112 promoter would appear to respond to intra- or extracellular stimuli. These two Anopheline species appear to have diverged in the distant past based on gene neighbors and phylogenetic data, yet these GST genes are still conserved. Therefore it is highly probable that this GST gene organization has one or more important roles.

Genetic and physical mapping in mosquitoes: molecular approaches

The genetic background of individual mosquito species and populations within those species influences the transmission of mosquito-borne pathogens to humans. Technical advances in contemporary genomics are contributing significantly to the detailed genetic analysis of this mosquito-pathogen interaction as well as all other aspects of mosquito biology, ecology, and evolution. A variety of DNA-based marker types are being used to develop genetic maps for a number of mosquito species. Complex phenotypic traits such as vector competence are being dissected into their discrete genetic components, with the intention of eventually using this information to develop new methods to prevent disease transmission. Both genetic- and physical-mapping techniques are being used to define and compare genome architecture among and within mosquito species. The integration of genetic- and physical-map information is providing a sound framework for map-based positional cloning of target genes of interest. This review focuses on advances in genome-based analysis and their specific applications to mosquitoes.

Genetic mapping of two loci affecting DDT resistance in the malaria vector Anopheles gambiae

Resistance to the insecticide DDT in the mosquito vectors of malaria has severely hampered efforts to control this disease and has contributed to the increase in prevalence of malaria cases seen in recent years. Over 90% of the 300-500 million annual cases of malaria occur in Africa, where the major vector is Anopheles gambiae. DDT resistance in the ZAN/U strain of An. gambiae is associated with an increased metabolism of the insecticide, catalysed by members of the glutathione S-transferase (GST) enzyme family, but the molecular mechanism underlying this metabolic resistance is not known. Genetic crosses show that resistance is autosomal and semidominant. We have used microsatellite markers to identify two quantitative trait loci (QTL), which together explain over 50% of the variance in susceptibility to DDT in the ZAN/U strain of An. gambiae. The first locus, rtd1, is on chromosome 3 between markers H341 and H88 and has a recessive effect with respect to susceptibility. The second locus, rtd2 is on chromosome 2L, close to marker H325 and has an additive genetic effect. The markers flanking these two QTL have been physically mapped to An. gambiae polytene chromosomes. They do not coincide with any of the GST genes that have been cloned and mapped in this species. Characterization of these QTL will lead to a clearer understanding of the mechanisms of metabolic resistance to DDT.

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.

Biochemical characterization of insecticide resistance in the German cockroach, Blattella germanica, from Malaysia

The possible insecticide resistance mechanisms of four Malaysian field-collected strains of the German cockroach, Blattella germanica (Linnaeus) (Dictyoptera: Blattellidae), were characterized with biochemical assays and native polyacrylamide gel electrophoresis (PAGE). Elevated esterase activity (at low to moderate frequency) and altered acetylcholinesterase (low frequency) were detected in all field strains, while elevated glutathione S-transferase levels were present in only two strains. Seven esterase bands were separated by native PAGE; a greater intensity occurred in three bands in the resistant strains compared to the susceptible strain. Inhibition studies using specific inhibitors on polyacrylamide gels suggested that the slowest of these three esterases is a cholinesterase, while the other two are carboxylesterases with a preference for beta- over alpha-naphthyl acetate.

Insecticide resistance in insect vectors of human disease

Insecticide resistance is an increasing problem in many insect vectors of disease. Our knowledge of the basic mechanisms underlying resistance to commonly used insecticides is well established. Molecular techniques have recently allowed us to start and dissect most of these mechanisms at the DNA level. The next major challenge will be to use this molecular understanding of resistance to develop novel strategies with which we can truly manage resistance. State-of-the-art information on resistance in insect vectors of disease is reviewed in this context.

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.

The role of alternative mRNA splicing in generating heterogeneity within the Anopheles gambiae class I glutathione S-transferase family

The class I glutathione S-transferases (GSTs) of Anopheles gambiae are encoded by a complex gene family. We describe the genomic organization of three members of this family, which are sequentially arranged on the chromosome in divergent orientations. One of these genes, aggst1-2, is intronless and has been described. In contrast, the two A. gambiae GST genes (aggst1alpha and aggst1beta) reported within are interrupted by introns. The gene aggst1alpha contains five coding exons that are alternatively spliced to produce four mature GST transcripts, each of which contains a common 5' exon encoding the N termini of the GST protein spliced to one of four distinct 3' exons encoding the carboxyl termini. All four of the alternative transcripts of aggst1alpha are expressed in A. gambiae larvae, pupae, and adults. We report on the involvement of alternative RNA splicing in generating multiple functional GST transcripts. A cDNA from the aggst1beta gene was detected in adult mosquitoes, demonstrating that this GST gene is actively transcribed. The percentage similarity of the six cDNAs transcribed from the three GST genes range from 49.5% to 83.1% at the nucleotide level.

The role of gene splicing, gene amplification and regulation in mosquito insecticide resistance

The primary routes of insecticide resistance in all insects are alterations in the insecticide target sites or changes in the rate at which the insecticide is detoxified. Three enzyme systems, glutathione S-transferases, esterases and monooxygenases, are involved in the detoxification of the four major insecticide classes. These enzymes act by rapidly metabolizing the insecticide to non-toxic products, or by rapidly binding and very slowly turning over the insecticide (sequestration). In Culex mosquitoes, the most common organophosphate insecticide resistance mechanism is caused by co-amplification of two esterases. The amplified esterases are differentially regulated, with three times more Est beta 2(1) being produced than Est alpha 2(1). Cis-acting regulatory sequences associated with these esterases are under investigation. All the amplified esterases in different Culex species act through sequestration. The rates at which they bind with insecticides are more rapid than those for their non-amplified counterparts in the insecticide-susceptible insects. In contrast, esterase-based organophosphate resistance in Anopheles is invariably based on changes in substrate specificities and increased turnover rates of a small subset of insecticides. The up-regulation of both glutathione S-transferases and monooxygenases in resistant mosquitoes is due to the effects of a single major gene in each case. The products of these major genes up-regulate a broad range of enzymes. The diversity of glutathione S-transferases produced by Anopheles mosquitoes is increased by the splicing of different 5' ends of genes, with a single 3' end, within one class of this enzyme family. The trans-acting regulatory factors responsible for the up-regulation of both the monooxygenase and glutathione S-transferases still need to be identified, but the recent development of molecular tools for positional cloning in Anopheles gambiae now makes this possible.

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.