Resistance to fipronil in Drosophila simulans: influence of two point mutations in the RDL GABA receptor subunit

Contributions of γ-Aminobutyric Acid (GABA) Receptors for the Activities of Pectis brevipedunculata Essential Oil against Drosophila suzukii and Pollinator Bees

The γ-aminobutyric acid (GABA) receptors play pivotal roles in the transmission of neuronal information in the nervous system of insects, which has led these proteins to be targeted by synthetic and natural products. Here, we assessed the insecticidal potential of the essential oil of Pectis brevipedunculata (Gardner) Sch. Bip., a neotropical Asteraceae plant used in traditional medicine, for controlling Drosophila suzukii (Matsumura) adults by feeding exposure. By using in silico approaches, we disentangle the contribution of GABA receptors and other potential neuronal targets (e.g., acetylcholinesterase, glutathione-S-transferases) in insects that may explain the essential oil differential activities against D. suzukii and two essential pollinator bees (Apis mellifera Linnaeus and Partamona helleri Friese). Neral (26.7%) and geranial (33.9%) were the main essential oil components which killed D. suzukii with an estimated median lethal concentration (LC50) of 2.25 µL/mL. Both pollinator forager bee species, which would likely contact this compound in the field, were more tolerant to the essential oil and did not have their diet consumptions affected by the essential oil. Based on the molecular predictions for the three potential targets and the essential oil main components, a higher affinity of interaction with the GABA receptors of D. suzukii (geranial -6.2 kcal/mol; neral -5.8 kcal/mol) in relation to A. mellifera (geranial -5.2 kcal/mol; neral -4.9 kcal/mol) would contribute to explaining the difference in toxicities observed in the bioassays. Collectively, our findings indicated the involvement of GABA receptors in the potential of P. brevipedunculata essential oil as an alternative tool for controlling D. suzukii.

Cys-loop ligand-gated ion channel superfamily of Pardosa pseudoannulata: Implication for natural enemy safety

Cys-loop ligand-gated channels mediate neurotransmission in insects and are receptors for many insecticides. Some insecticides acting on cysLGIC also have lethal effects on non-targeting organisms, but the mechanism of this negative effect is unclear due to information absence. The identification and analysis of cysLGIC family in Pardosa pseudoannulata, a pond wolf spider, can deepen the understanding of insecticides for natural enemy safety. Thirty-four cysLGIC genes were identified in P. pseudoannulata genome, including nicotinic acetylcholine receptors, γ-aminobutyric acid gated chloride channels, glutamate-gated chloride channels, histamine-gated chloride channels, and pH-sensitive chloride channels. The expansion of GABACls and HisCls accounts for the large number of cysLGICs in P. pseudoannulata, and the alternative splicing events in nAChR and RDL subunits enriched the diversity of the superfamily. Most cysLGIC genes show the highest expression in brain and lowest expression in the early-egg sac stage. Variable residues (R81, V83, R135, N137, F190, and W197) in P. pseudoannulata nAChR β subunits and critical differences in α6 subunit TM4 region compared with insects would apply for the insensitivity to neonicotinoids and spinosyn. In contrast, avermectin and dieldrin may be lethal to P. pseudoannulata due to the similar drugs binding sites in GluCls compared with insects. These findings will provide a valuable clue for natural enemy protection and environmentally friendly insecticide development.

Activity, selection response and molecular mode of action of the isoxazoline afoxolaner in Tetranychus urticae

BACKGROUND

Afoxolaner is a novel representative of the isoxazolines, a class of ectoparasiticides which has been commercialized for the control of tick and flea infestations in dogs. In this study, the biological efficacy of afoxolaner against the two-spotted spider mite Tetranychus urticae was evaluated. Furthermore, as isoxazolines are known inhibitors of γ-aminobutyric acid-gated chloride channels (GABACls), the molecular mode of action of afoxolaner on T. urticae GABACls (TuRdls) was studied using functional expression in Xenopus oocytes followed by two-electrode voltage-clamp (TEVC) electrophysiology, and results were compared with inhibition by fluralaner, fipronil and endosulfan. To examine the influence of known GABACl resistance mutations, H301A, I305T and A350T substitutions in TuRdl1 and a S301A substitution in TuRdl2 were introduced.

RESULTS

Bioasassays revealed excellent efficacy of afoxolaner against all developmental stages and no cross-resistance was found in a panel of strains resistant to most currently used acaricides. Laboratory selection over a period of 3 years did not result in resistance. TEVC revealed clear antagonistic activity of afoxolaner and fluralaner for all homomeric TuRdl1/2/3 channels. The introduction of single, double or triple mutations to TuRdl1 and TuRdl2 did not lower channel sensitivity. By contrast, both endosulfan and fipronil had minimal antagonistic activities against TuRdl1/2/3, and channels carrying single mutations, whereas the sensitivity of double and triple TuRdl1 mutants was significantly increased.

CONCLUSIONS

Our results demonstrate that afoxolaner is a potent antagonist of GABACls of T. urticae and has a powerful mode of action to control spider mites. © 2022 Society of Chemical Industry.

Recent progress in the structural study of ion channels as insecticide targets

Ion channels, many expressed in insect neural and muscular systems, have drawn huge attention as primary targets of insecticides. With the recent technical breakthroughs in structural biology, especially in cryo-electron microscopy (cryo-EM), many new high-resolution structures of ion channel targets, apo or in complex with insecticides, have been solved, shedding light on the molecular mechanism of action of the insecticides and resistance mutations. These structures also provide accurate templates for structure-based insecticide screening and rational design. This review summarizes the recent progress in the structural studies of 5 ion channel families: the ryanodine receptor (RyR), the nicotinic acetylcholine receptor (nAChR), the voltage-gated sodium channel (VGSC), the transient receptor potential (TRP) channel, and the ligand-gated chloride channel (LGCC). We address the selectivity of the channel-targeting insecticides by examining the conservation of key coordinating residues revealed by the structures. The possible resistance mechanisms are proposed based on the locations of the identified resistance mutations on the 3D structures of the target channels and their impacts on the binding of insecticides. Finally, we discuss how to develop "green" insecticides with a novel mode of action based on these high-resolution structures to overcome the resistance.

Combined metabolic and target-site resistance mechanisms confer fipronil and deltamethrin resistance in field-collected German cockroaches (Blattodea: Ectobiidae)

Despite insecticide resistance issues, pyrethroids and fipronil have continued to be used extensively to control the German cockroach, Blattella germanica (L.) (Blattodea: Ectobiidae) for more than two decades. We evaluated the physiological insecticide resistance in five German cockroach populations collected from 2018 to 2020 and measured the extent of metabolic detoxification and target-site insensitivity resistance mechanisms. Topically applied doses of the 3 x LD₉₅ of deltamethrin, fipronil, DDT, or dieldrin of a susceptible strain (UCR, Diagnostic Dose) failed to cause >23% mortality, and the 10 x LD₉₅ of deltamethrin or fipronil failed to cause >53% mortality. All field-collected strains possessed a combination of metabolic and target-site insensitivity mechanisms that cause reduced susceptibility. Elevated activities of esterase and glutathione S-transferase were measured, and the synergists piperonyl butoxide or S,S,S-tributyl phosphorotrithioate increased topical mortality up to 100% for deltamethrin and 93% for fipronil 10 x LD₉₅. The target-site mutations L993F of the para-homologous sodium channel and A302S of the GABA-gated chloride channel associated with pyrethroid and fipronil resistance, respectively, were found at ~80-100% frequency in field populations. Pyrethroid and fipronil spray formulations also were ineffective in a choice box assay against field-collected strains suggesting that these treatments would fail to control cockroaches under field conditions.

Evolution of the Insecticide Target Rdl in African Anopheles Is Driven by Interspecific and Interkaryotypic Introgression

The evolution of insecticide resistance mechanisms in natural populations of Anopheles malaria vectors is a major public health concern across Africa. Using genome sequence data, we study the evolution of resistance mutations in the resistance to dieldrin locus (Rdl), a GABA receptor targeted by several insecticides, but most notably by the long-discontinued cyclodiene, dieldrin. The two Rdl resistance mutations (296G and 296S) spread across West and Central African Anopheles via two independent hard selective sweeps that included likely compensatory nearby mutations, and were followed by a rare combination of introgression across species (from A. gambiae and A. arabiensis to A. coluzzii) and across nonconcordant karyotypes of the 2La chromosomal inversion. Rdl resistance evolved in the 1950s as the first known adaptation to a large-scale insecticide-based intervention, but the evolutionary lessons from this system highlight contemporary and future dangers for management strategies designed to combat development of resistance in malaria vectors.

A unique primary structure of RDL (resistant to dieldrin) confers resistance to GABA-gated chloride channel blockers in the two-spotted spider mite Tetranychus urticae Koch

The primary structure of the second transmembrane (M2) segment of resistant to dieldrin (RDL), an ionotropic γ-aminobutyric acid receptor (GABAR) subunit, and the structure-function relationships in RDL are well conserved among insect species. An amino acid substitution at the 2' position in the M2 segment (Ala to Ser or Gly) confers resistance to non-competitive antagonists (NCAs) of GABARs. Here, a cDNA encoding RDL was cloned from the two-spotted spider mite Tetranychus urticae Koch. Unlike insect homologs, native TuRDL has His at the 2' position (H305) and Ile at 6' (I309) in the M2 segment and is insensitive to NCAs. Single and multiple mutations were introduced in the M2 segment of TuRDL, and the mutant proteins were expressed in Xenopus oocytes and examined for the restoration of sensitivity to NCAs. The sensitivity of a double mutant (H305A and I309T in the M2 segment) was greatly increased but was still considerably lower than that of insect RDLs. We therefore constructed chimeric RDLs consisting of TuRDL and Drosophila melanogaster RDL and examined their sensitivities to NCAs. The results show that the N-terminal region containing the Cys-loop as well as the M2 segment confers functional specificity; thus, our current understanding of the mechanism underlying NCA binding to GABARs requires reappraisal.

Insights into the synergistic mechanism of target resistance: A case study of N. lugens RDL-GABA receptors and fipronil

It is known that a single mutation exerts moderate resistance to pesticide, while double mutations (DM) cause severe resistance problem through synergistic effect, and even result in failure application of pesticides. However, little is known about how double mutations would synergistically cause much high resistance level. In this work, computational studies were performed on the interaction of fipronil with N. lugens RDL-GABA receptors, to see how single and double mutations changed receptor structure properties and then conferred distinct resistance levels. The A2'S mutation displayed relative weak influence on receptor structure properties. The R0'Q mutation, which has not been detected in the absence of A2'S, however could deeply alter the electrostatic potential around the inner pore region and significantly narrow the bottom region around -2'Pro. For the DM system, the synergistic effect of two mutations lead to similar pore diameters to the WT system, except for the slightly reduced middle part. Docking study and binding free energy calculation revealed that fipronil displayed binding potencies in the order of WT > A2'S > R0'Q > DM systems, coinciding well with the reported fipronil sensitivity trends and resistance levels.

RDL mutations in Guangxi Anopheles sinensis populations along the China-Vietnam border: distribution frequency and evolutionary origin of A296S resistance allele

Background

Malaria is a deadly vector-borne disease in tropical and subtropical regions. Although indigenous malaria has been eliminated in Guangxi of China, 473 confirmed cases were reported in the Northern region of neighbouring Vietnam in 2014. Considering that frequent population movement occurs across the China–Vietnam border and insecticide resistance is a major obstacle in disease vector control, there is a need to know the genotype and frequency of insecticide resistance alleles in Anopheles sinensis populations along the China–Vietnam border and to take action to prevent the possible migration of insecticide resistance alleles across the border.

Methods

Two hundred and eight adults of An. sinensis collected from seven locations in Guangxi along the China–Vietnam border were used in the investigation of individual genotypes of the AsRDL gene, which encodes the RDL gamma-aminobutyric acid (GABA) receptor subunit in An. sinensis. PCR-RFLP (polymerase chain reaction-restriction fragment length polymorphism) analysis was deployed to genotype codon 345, while direct sequencing of PCR products was conducted to clarify the genotypes for codons 296 and 327 of the AsRDL gene. The genealogical relation of AsRDL haplotypes was analyzed using Network 5.0.

Results

Three putative insecticide resistance related mutations (A296S, V327I and T345S) were detected in all the seven populations of An. sinensis in Guangxi along the China–Vietnam border. The resistance-conferring A296S mutation was found to be widely distributed and present at notably high frequencies (78.8% to 100%). Relatively lower frequencies of mutations V327I (26.9% to 53.2%) and T345S (0% to 28.8%) were observed. The V327I or T345S always occurred in the presence of A296S. Evolutionary analysis of 21 AsRDL haplotypes indicated multiple origins of the A296S and V327I mutations.

Conclusion

The resistance A296S allele was present at high frequencies in the An. sinensis populations along the China–Vietnam border, indicating a risk of resistance to insecticides targeting RDL. The double mutations (A296S + V327I) may have evolved from alleles carrying the A296S mutation by scaffolding the additional mutation V327I, and A296S allele may have multiple evolutionary origins. These findings will help inform strategies for vector control and malaria prevention.

Molecular markers and their application in the monitoring of acaricide resistance in Rhipicephalus microplus

Monitoring acaricide resistance and understanding the underlying mechanisms are critically important in developing strategies for resistance management and tick control. Identification of single nucleotide polymorphisms in the acaricide-resistant associated gene of Rhipicephalus microplus has enabled the development of molecular markers for detection and monitoring of resistance against different types of acaricide. There are many molecular markers developed for resistance monitoring, including mutations on target genes such as sodium channel, acetylcholinesterase, carboxylesterase, β-adrenergic octopamine receptor, octopamine-tyramine etc. Molecular genotyping through molecular markers can detect the presence of resistance-associated genes in a tick population before it reaches high frequency. This review aims to provide an update on the various molecular markers discovered to date from different regions of the world.

Mutations in Rhipicephalus microplus GABA gated chloride channel gene associated with fipronil resistance

The tropical cattle tick, Rhipicephalus microplus, is one of the most damaging parasites that affects cattle in tropical and subtropical regions in the world. Tick resistance to acaricides is dispersed worldwide and a number of associated mutations in target site genes have been described. Phenylpyrazole (e.g. fipronil) and cyclodiene (e.g. lindane, dieldrin) insecticides both have the same mode of action blocking the GABA-gated chloride channel encoded by the GABA-Cl gene. A conserved mutation, rdl (resistance to dieldrin) is found across a number of arthropods resistant to cyclodienes and phenylpyrazoles. In ticks, the mutation T290 L, was identified in the second transmembrane (TM2) domain of the GABA-gated chloride channel of Australian cattle tick populations that are resistant to dieldrin. Recently, cross-resistance between fipronil and lindane was reported in R. microplus populations obtained from Uruguay and Brazil. The objective of the present study was to identify mutations in the GABA-Cl gene associated with fipronil resistance. Genomic DNA was obtained from engorged females from fipronil-susceptible and resistant populations sampled from Uruguay and Brazil (n = 166). Initially, it was searched the T290 L mutation described in Australia; however, this mutation was not detected in individuals from resistant populations from either country. The sequencing of a fragment of the GABA-Cl gene revealed nucleotide polymorphisms in fipronil- and lindane-resistant ticks in two populations from Uruguay and five from Brazil. Five amino acid substitutions were present in the resistant strains. Two different substitutions were found in an alanine residue (A286S and A286 L) that is homolog to rdl mutations in fipronil-resistant individuals of other arthropod species. Four other amino acid substitutions (S281 T, V317I, T328 A and A329S) were present in some resistant strains, always with the mutation A286S. This is the first documentation of mutations in the GABA-Cl gene associated with fipronil-resistant in R. microplus.

Fluxametamide: A novel isoxazoline insecticide that acts via distinctive antagonism of insect ligand-gated chloride channels

Fluxametamide is a novel wide-spectrum insecticide that was discovered and synthesized by Nissan Chemical Industries, Ltd. To identify the mode of action of fluxametamide, we first performed [³H]4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) binding assays. Fluxametamide potently inhibited the specific binding of [³H]EBOB to housefly-head membranes, suggesting that fluxametamide affects insect γ-aminobutyric acid (GABA)-gated chloride channels (GABACls). Next, the antagonism of housefly GABACls and glutamate-gated chloride channels (GluCls) was examined using the two-electrode voltage clamp (TEVC) method. Fluxametamide inhibited agonist responses in both ion channels expressed in Xenopus oocytes in the nanomolar range, indicating that this insecticide is a ligand-gated chloride channel (LGCC) antagonist. The insecticidal and LGCC antagonist potencies of fluxametamide against fipronil-susceptible and fipronil-resistant strains of small brown planthoppers and two-spotted spider mites, which are insensitive to fipronil, were evaluated. Fluxametamide exhibited similar levels of both activities in these fipronil-susceptible and fipronil-resistant arthropod pests. These data indicate that fluxametamide exerts distinctive antagonism of arthropod GABACls by binding to a site different from those for existing antagonists. In contrast to its profound actions on the arthropod LGCCs, the antagonistic activity of fluxametamide against rat GABACls and human glycine-gated chloride channels was nearly insignificant, suggesting that fluxametamide has high target-site selectivity for arthropods over mammals. Overall, fluxametamide is a new type of LGCC antagonist insecticide with excellent safety for mammals at the target-site level.

A model species for agricultural pest genomics: the genome of the Colorado potato beetle, Leptinotarsa decemlineata (Coleoptera: Chrysomelidae)

The Colorado potato beetle is one of the most challenging agricultural pests to manage. It has shown a spectacular ability to adapt to a variety of solanaceaeous plants and variable climates during its global invasion, and, notably, to rapidly evolve insecticide resistance. To examine evidence of rapid evolutionary change, and to understand the genetic basis of herbivory and insecticide resistance, we tested for structural and functional genomic changes relative to other arthropod species using genome sequencing, transcriptomics, and community annotation. Two factors that might facilitate rapid evolutionary change include transposable elements, which comprise at least 17% of the genome and are rapidly evolving compared to other Coleoptera, and high levels of nucleotide diversity in rapidly growing pest populations. Adaptations to plant feeding are evident in gene expansions and differential expression of digestive enzymes in gut tissues, as well as expansions of gustatory receptors for bitter tasting. Surprisingly, the suite of genes involved in insecticide resistance is similar to other beetles. Finally, duplications in the RNAi pathway might explain why Leptinotarsa decemlineata has high sensitivity to dsRNA. The L. decemlineata genome provides opportunities to investigate a broad range of phenotypes and to develop sustainable methods to control this widely successful pest.

Assessment of biochemical mechanisms of tolerance to chlorpyrifos in ancient and contemporary Daphnia pulicaria genotypes

The evolution of tolerance to environmental contaminants in non-target taxa has been largely studied by comparing extant populations experiencing contrasting exposure. Previous research has demonstrated that "resurrected" genotypes from a population of Daphnia pulicaria express temporal variation in sensitivity to the insecticide chlorpyrifos. Ancient genotypes (1301-1646AD.) were on average more sensitive to this chemical compared to the contemporary genotypes (1967-1977AD.). To determine the physiological mechanisms of tolerance, a series of biochemical assays was performed on three ancient and three contemporary genotypes; these six genotypes exhibited the most sensitive and most tolerant phenotypes within the population, respectively. Metabolic tolerance mechanisms were evaluated using acute toxicity testing, while target-site tolerance was assessed via in vitro acetylcholinesterase (AChE) assays. Acute toxicity tests were conducted using i) the toxic metabolite chlorpyrifos-oxon (CPF-oxon) and ii) CPF-oxon co-applied with piperonyl butoxide (PBO), a known Phase-I metabolic inhibitor. Both series of toxicity tests reduced the mean variation in sensitivity between tolerant and sensitive genotypes. Exposure to CPF-O reduced the disparity from a 4.7-fold to 1.6-fold difference in sensitivity. The addition of PBO further reduced the variation to a 1.2-fold difference in sensitivity. In vitro acetylcholinesterase assays yielded no significant differences in constitutive activity or target-site sensitivity. These findings suggest that pathways involving Phase-I detoxification and/or bioactivation of chlorpyrifos play a significant role in dictating the microevolutionary trajectories of tolerance in this population.

RDL mutations predict multiple insecticide resistance in Anopheles sinensis in Guangxi, China

Background

Anopheles sinensis is a major vector of malaria in China. The gamma-aminobutyric acid (GABA)-gated chloride channel, encoded by the RDL (Resistant to dieldrin) gene, is the important target for insecticides of widely varied structures. The use of various insecticides in agriculture and vector control has inevitably led to the development of insecticide resistance, which may reduce the control effectiveness. Therefore, it is important to investigate the presence and distribution frequency of the resistance related mutation(s) in An. sinensis RDL to predict resistance to both the withdrawn cyclodienes (e.g. dieldrin) and currently used insecticides, such as fipronil.

Methods

Two hundred and forty adults of An. sinensis collected from nine locations across Guangxi Zhuang Autonomous Region were used. Two fragments of An. sinensis RDL (AsRDL) gene, covering the putative insecticide resistance related sites, were sequenced respectively. The haplotypes of each individual were reconstructed by the PHASE2.1 software, and confirmed by clone sequencing. The phylogenetic tree was built using maximum-likelihood and Bayesian inference methods. Genealogical relations among different haplotypes were also analysed using Network 5.0.

Results

The coding region of AsRDL gene was 1674 bp long, encoding a protein of 557 amino acids. AsRDL had 98.0% amino acid identity to that from Anopheles funestus, and shared common structural features of Cys-loop ligand-gated ion channels. Three resistance-related amino acid substitutions (A296S, V327I and T345S) were detected in all the nine populations of An. sinensis in Guangxi, with the 296S mutation being the most abundant (77–100%), followed by 345S (22–47%) and 327I (8–60%). 38 AsRDL haplotypes were identified from 240 individuals at frequencies ranging from 0.2 to 34.8%. Genealogical analysis suggested multiple origins of the 345S mutation in AsRDL.

Conclusions

The near fixation of the 296S mutation and the occurrence of the 327I and 345S mutations in addition to 296S, in all the nine tested An. sinensis populations in Guangxi, strongly indicate a risk of multiple insecticide resistance. The haplotype diversity plus genetic heterogeneities in the geographical distribution, and multiple origins of AsRDL alleles call for a location-customized strategy for monitoring and management of insecticide resistance.

The novel isoxazoline ectoparasiticide lotilaner (Credelio™): a non-competitive antagonist specific to invertebrates γ-aminobutyric acid-gated chloride channels (GABACls)

BACKGROUND

The isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and, to a lesser extent, of inhibitory glutamate-gated chloride channels (GluCls). Lotilaner (Credelio™), a novel representative of this chemical class, is currently evaluated for its excellent ectoparasiticide properties.

METHODS

In this study, we investigated the molecular mode of action and pharmacology of lotilaner. We report the successful gene identification, cDNA cloning and functional expression in Xenopus oocytes of Drosohpila melanogaster (wild type and dieldrin/fipronil-resistant forms), Lepeophtheirus salmonis (an ectoparasite copepod crustacean of salmon), Rhipicephalus microplus and Canis lupus familiaris GABACls. Automated Xenopus oocyte two-electrode voltage clamp electrophysiology was used to assess GABACls functionality and to compare ion channel inhibition by lotilaner with that of established insecticides addressing GABACls as targets.

RESULTS

In these assays, we demonstrated that lotilaner is a potent non-competitive antagonist of insects (fly) GABACls. No cross-resistance with dieldrin or fipronil resistance mutations was detected, suggesting that lotilaner might bind to a site at least partly different from the one bound by known GABACl blockers. Using co-application experiments, we observed that lotilaner antagonism differs significantly from the classical open channel blocker fipronil. We finally confirmed for the first time that isoxazoline compounds are not only powerful antagonists of GABACls of acari (ticks) but also of crustaceans (sea lice), while no activity on a dog GABA_A receptor was observed up to a concentration of 10 μM.

CONCLUSIONS

Together, these results demonstrate that lotilaner is a non-competitive antagonist specific to invertebrate's γ-aminobutyric acid-gated chloride channels (GABACls). They contribute to our understanding of the mode of action of this new ectoparasiticide compound.

Influence of the RDL A301S mutation in the brown planthopper Nilaparvata lugens on the activity of phenylpyrazole insecticides

We discovered the A301S mutation in the RDL GABA-gated chloride channel of fiprole resistant rice brown planthopper, Nilaparvata lugens populations by DNA sequencing and SNP calling via RNASeq. Ethiprole selection of two field N. lugens populations resulted in strong resistance to both ethiprole and fipronil and resulted in fixation of the A301S mutation, as well as the emergence of another mutation, Q359E in one of the selected strains. To analyse the roles of these mutations in resistance to phenylpyrazoles, three Rdl constructs: wild type, A301S and A301S+Q359E were expressed in Xenopus laevis oocytes and assessed for their sensitivity to ethiprole and fipronil using two-electrode voltage-clamp electrophysiology. Neither of the mutant Rdl subtypes significantly reduced the antagonistic action of fipronil, however there was a significant reduction in response to ethiprole in the two mutated subtypes compared with the wild type. Bioassays with a Drosophila melanogaster strain carrying the A301S mutation showed strong resistance to ethiprole but not fipronil compared to a strain without this mutation, thus further supporting a causal role for the A301S mutation in resistance to ethiprole. Homology modelling of the N. lugens RDL channel did not suggest implications of Q359E for fiprole binding in contrast to A301S located in transmembrane domain M2 forming the channel pore. Synergist bioassays provided no evidence of a role for cytochrome P450s in N. lugens resistance to fipronil and the molecular basis of resistance to this compound remains unknown. In summary this study provides strong evidence that target-site resistance underlies widespread ethiprole resistance in N. lugens populations.

Molecular characterization and expression pattern of three GABA receptor-like subunits in the small brown planthopper Laodelphax striatellus (Hemiptera: Delphacidae)

Ionotropic γ-aminobutyric acid (GABA)-gated chloride channel receptors mediate rapid inhibitory neurotransmission in vertebrates and invertebrates. GABA receptors are well known to be the molecular targets of synthetic insecticides or parasiticides. Three GABA receptor-like subunits, LsLCCH3, LsGRD and LS8916, of the small brown planthopper, Laodelphax striatellus (Fallén), a major insect pest of crop systems in East Asia, had been identified and characterized in this study. All three genes were cloned using the reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). They shared common structural features with known Cys-loop ligand-gated ion channels (LGICs): the well-conserved dicysteine-loop structures, an extracellular N-terminal domain containing six distinct regions (loops A-F) that form the ligand binding sites and four transmembrane regions (TM1-4). Additionally, temporal and spatial transcriptional profiling analysis indicated that Lslcch3 was significantly higher than the other two genes. All of them were expressed at higher levels in fifth-instar nymph and adults than in eggs and from first- to fourth-instar nymph. They were predominantly expressed in the heads of 2-d old female adults. These findings enhanced our understanding of cys-loop LGIC functional characterization in Hemiptera and provided a useful basis for the development of improved insecticides that targeting this important agricultural pest.

Ion channels as insecticide targets

Ion channels remain the primary target of most of the small molecule insecticides. This review examines how the subunit composition of heterologously expressed receptors determines their insecticide-specific pharmacology and how the pharmacology of expressed receptors differs from those found in the insect nervous system. We find that the insecticide-specific pharmacology of some receptors, like that containing subunits of the Rdl encoded GABA receptor, can be reconstituted with very few of the naturally occurring subunits expressed. In contrast, workers have struggled even to express functional insect nicotinic acetylcholine receptors (nAChRs), and work has therefore often relied upon the expression of vertebrate receptor subunits in their place. We also examine the extent to which insecticide-resistance-associated mutations, such as those in the para encoded voltage-gated sodium channel, can reveal details of insecticide-binding sites and mode of action. In particular, we examine whether mutations are present in the insecticide-binding site and/or at sites that allosterically affect the drug preferred conformation of the receptor. We also discuss the ryanodine receptor as a target for the recently developed diamides. Finally, we examine the lethality of the genes encoding these receptor subunits and discuss how this might determine the degree of conservation of the resistance-associated mutations found.

Synergistic and compensatory effects of two point mutations conferring target-site resistance to fipronil in the insect GABA receptor RDL

Insecticide resistance can arise from a variety of mechanisms, including changes to the target site, but is often associated with substantial fitness costs to insects. Here we describe two resistance-associated target-site mutations that have synergistic and compensatory effects that combine to produce high and persistent levels of resistance to fipronil, an insecticide targeting on γ-aminobytyric acid (GABA) receptors. In Nilaparvata lugens, a major pest of rice crops in many parts of Asia, we have identified a single point mutation (A302S) in the GABA receptor RDL that has been identified previously in other species and which confers low levels of resistance to fipronil (23-fold) in N. lugans. In addition, we have identified a second resistance-associated RDL mutation (R300Q) that, in combination with A302S, is associated with much higher levels of resistance (237-fold). The R300Q mutation has not been detected in the absence of A302S in either laboratory-selected or field populations, presumably due to the high fitness cost associated with this mutation. Significantly, it appears that the A302S mutation is able to compensate for deleterious effects of R300Q mutation on fitness cost. These findings identify a novel resistance mechanism and may have important implications for the spread of insecticide resistance.

Susceptibility of Adult Cat Fleas (Siphonaptera: Pulicidae) to Insecticides and Status of Insecticide Resistance Mutations at the Rdl and Knockdown Resistance Loci

The susceptibility of 12 field-collected isolates and 4 laboratory strains of cat fleas, Ctenocephalides felis was determined by topical application of some of the insecticides used as on-animal therapies to control them. In the tested field-collected flea isolates the LD50 values for fipronil and imidacloprid ranged from 0.09 to 0.35 ng/flea and 0.02 to 0.19 ng/flea, respectively, and were consistent with baseline figures published previously. The extent of variation in response to four pyrethroid insecticides differed between compounds with the LD50 values for deltamethrin ranging from 2.3 to 28.2 ng/flea, etofenprox ranging from 26.7 to 86.7 ng/flea, permethrin ranging from 17.5 to 85.6 ng/flea, and d-phenothrin ranging from 14.5 to 130 ng/flea. A comparison with earlier data for permethrin and deltamethrin implied a level of pyrethroid resistance in all isolates and strains. LD50 values for tetrachlorvinphos ranged from 20.0 to 420.0 ng/flea. The rdl mutation (conferring target-site resistance to cyclodiene insecticides) was present in most field-collected and laboratory strains, but had no discernible effect on responses to fipronil, which acts on the same receptor protein as cyclodienes. The kdr and skdr mutations conferring target-site resistance to pyrethroids but segregated in opposition to one another, precluding the formation of genotypes homozygous for both mutations.

Dominant Inheritance of Field-Evolved Resistance to Fipronil in Plutella xylostella (Lepidoptera: Plutellidae)

A field-collected strain (HF) of Plutella xylostella (L.) showed 420-fold resistance to fipronil compared with a susceptible laboratory strain (Roth). The HF-R strain, derived from the HF strain by 25 generations of successive selection with fipronil in the laboratory, developed 2,200-fold resistance to fipronil relative to the Roth strain. The F(1) progeny of the reciprocal crosses between HF-R and Roth showed 640-fold (R♀ × S♂) and 1,380-fold (R♂ × S♀) resistance to fipronil, indicating resistance is inherited as an incompletely dominant trait. Analysis of progeny from a backcross (F1♂ × S♀) suggests that resistance is controlled by one major locus. The LC(50) of the R♂ × S♀ cross F(1) progeny is slightly but significantly higher than that of the R♀ × S♂ cross F(1) progeny, suggesting a minor resistance gene on the Z chromosome. Sequence analysis of PxGABARα1 (an Rdl-homologous GABA receptor gene of P. xylostella) from the HF-R strain identified two mutations A282S and A282G (corresponding to the A302S mutation of the Drosophila melanogaster Rdl gene), which have been previously implicated in fipronil resistance in several insect species including P. xylostella. PxGABARα1 was previously mapped to the Z chromosome of P. xylostella. In conclusion, fipronil resistance in the HF-R strain of P. xylostella was incompletely dominant, and controlled by a major autosomal locus and a sex-linked minor gene (PxGABARα1) on the Z chromosome.

The neonicotinoid imidacloprid, and the pyrethroid deltamethrin, are antagonists of the insect Rdl GABA receptor

A mutation in the second transmembrane domain of the GABA receptor subunit, Rdl, is associated with resistance to insecticides such as dieldrin and fipronil. Molecular cloning of Rdl cDNA from a strain of the malaria mosquito, Anopheles gambiae, which is highly resistant to dieldrin revealed this mutation (A296G) as well as another mutation in the third transmembrane domain (T345M). Wild-type, A296G, T345M and A296G + T345M homomultimeric Rdl were expressed in Xenopus laevis oocytes and their sensitivities to fipronil, deltamethrin, 1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane (DDT), imidacloprid and spinosad were measured using two-electrode voltage-clamp electrophysiology. Spinosad and DDT had no agonist or antagonist actions on Rdl. However, fipronil, deltamethrin and imidacloprid decreased GABA-evoked currents. These antagonistic actions were either reduced or abolished with the A296G and the A296G + T345M mutations while T345M alone appeared to have no significant effect. In conclusion, this study identifies another mutation in the mosquito Rdl that is associated with insecticide resistance. While T345M itself does not affect insecticide sensitivity, it may serve to offset the structural impact of A296G. The present study also highlights Rdl as a potential secondary target for neonicotinoids and pyrethroids. We show for the first time that deltamethrin (a pyrethroid insecticide) and imidacloprid (a neonicotinoid insecticide) act directly on the insect GABA receptor, Rdl. Our findings highlight Rdl as a potential secondary target of pyrethroids and neonicotinoids mutations in which may contribute to resistance to these widely used insecticides.

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.

Knockdown of the ionotropic γ-aminobutyric acid receptor (GABAR) RDL gene decreases fipronil susceptibility of the small brown planthopper, Laodelphax striatellus (Hemiptera: Delphacidae)

Insect γ-aminobutyric acid receptors (GABARs) are important molecular targets of cyclodiene and phenylpyrazole insecticides. Previously GABARs encoding rdl (resistant to dieldrin) genes responsible for dieldrin and fipronil resistance were identified in various economically important insect pests. In this study, we cloned the open reading frame cDNA sequence of rdl gene from fipronil-susceptible and fipronil-resistant strains of Laodelphax striatellus (Lsrdl). Sequence analysis confirmed the presence of a previously identified resistance-conferring mutation. Different alternative splicing variants of Lsrdl were noted. Injection of dsLsrdl reduced the mRNA abundance of Lsrdl by 27-82%, and greatly decreased fipronil-induced mortality of individuals from both susceptible and resistant strains. These data indicate that Lsrdl encodes a functional RDL subunit that mediates susceptibility to fipronil. Additionally, temporal and spatial expression analysis showed that Lsrdl was expressed at higher levels in eggs, fifth-instar nymphs, and female adults than in third-instar and fourth-instar nymphs. Lsrdl was predominantly expressed in the heads of 2-day-old female adults. All these results provide useful background knowledge for better understanding of fipronil resistance related ionotropic GABA receptor rdl gene expressed variants and potential functional differences in insects.

Differential mechanisms of action of the novel γ-aminobutyric acid receptor antagonist ectoparasiticides fluralaner (A1443) and fipronil

BACKGROUND

Fluralaner (A1443) is an isoxazoline ectoparasiticide that is a novel antagonist of γ-aminobutyric acid (GABA) receptors (GABARs), with a potency comparable to that of fipronil, a phenylpyrazole ectoparasiticide. To clarify the biological effectiveness of fluralaner against fipronil-resistant pests, differences in the actions of fluralaner and fipronil on GABARs that possess resistance to dieldrin (rdl)-type mutations were evaluated.

RESULTS

Fipronil had neither pest control nor GABAR-antagonistic activities against two-spotted spider mites (Tetranychus urticae) that had two different rdl-type amino acids (A(301) → H and T(350) → A: Drosophila melanogaster GABAR numbering) and against small brown planthoppers (Laodelphax striatellus) that had a novel rdl-type (A(283) → N) mutation in GABARs. In contrast, fluralaner showed not only high pest control activities against these pests, but also excellent antagonistic activities for these rdl-type GABARs.

CONCLUSION

The findings indicate that rdl-type fipronil-resistant pests do not show cross-resistance to fluralaner owing to the differential actions of fluralaner and fipronil on the GABAR.

Binding difference of fipronil with GABAARs in fruitfly and zebrafish: insights from homology modeling, docking, and molecular dynamics simulation studies

Fipronil, which targets GABAA receptors (GABAARs), is the first phenylpyrazole insecticide widely used in crop protection and public hygiene. However, its high toxicity on fishes greatly limited its applications. In the present study, a series of computational methods including homology modeling, docking, and molecular dynamics simulation studies were integrated to explore the binding difference of fipronil with GABAARs from fruitfly and zebrafish systems. It was found that, in the zebrafish system, the H-bond between 6'Thr and fipronil exerted key effects on the recognition of fipronil, which was absent in the fruitfly system. On the other hand, in the fruitfly system, strong electrostatic interaction between 2'Ala and fipronil was favorable to the binding of fipronil but detrimental to the binding in the zebrafish system. These findings marked the binding difference of fipronil with different GABAARs, which might be helpful in designing selective insecticides against pests instead of fishes.

The role of Rdl in resistance to phenylpyrazoles in Drosophila melanogaster

Extensive use of older generation insecticides may result in pre-existing cross-resistance to new chemical classes acting at the same target site. Phenylpyrazole insecticides block inhibitory neurotransmission in insects via their action on ligand-gated chloride channels (LGCCs). Phenylpyrazoles are broad-spectrum insecticides widely used in agriculture and domestic pest control. So far, all identified cases of target site resistance to phenylpyrazoles are based on mutations in the Rdl (Resistance to dieldrin) LGCC subunit, the major target site for cyclodiene insecticides. We examined the role that mutations in Rdl have on phenylpyrazole resistance in Drosophila melanogaster, exploring naturally occurring variation, and generating predicted resistance mutations by mutagenesis. Natural variation at the Rdl locus in inbred strains of D. melanogaster included gene duplication, and a line containing two Rdl mutations found in a highly resistant line of Drosophila simulans. These mutations had a moderate impact on survival following exposure to two phenylpyrazoles, fipronil and pyriprole. Homology modelling suggested that the Rdl chloride channel pore contains key residues for binding fipronil and pyriprole. Mutagenesis of these sites and assessment of resistance in vivo in transgenic lines showed that amino acid identity at the Ala(301) site influenced resistance levels, with glycine showing greater survival than serine replacement. We confirm that point mutations at the Rdl 301 site provide moderate resistance to phenylpyrazoles in D. melanogaster. We also emphasize the beneficial aspects of testing predicted mutations in a whole organism to validate a candidate gene approach.

Actions of agonists, fipronil and ivermectin on the predominant in vivo splice and edit variant (RDLbd, I/V) of the Drosophila GABA receptor expressed in Xenopus laevis oocytes

Ionotropic GABA receptors are the targets for several classes of insecticides. One of the most widely-studied insect GABA receptors is RDL (resistance to dieldrin), originally isolated from Drosophila melanogaster. RDL undergoes alternative splicing and RNA editing, which influence the potency of GABA. Most work has focussed on minority isoforms. Here, we report the first characterisation of the predominant native splice variant and RNA edit, combining functional characterisation with molecular modelling of the agonist-binding region. The relative order of agonist potency is GABA> muscimol> TACA> β-alanine. The I/V edit does not alter the potency of GABA compared to RDLbd. Docking calculations suggest that these agonists bind and activate RDLbdI/V through a similar binding mode. TACA and β-alanine are predicted to bind with lower affinity than GABA, potentially explaining their lower potency, whereas the lower potency of muscimol and isoguvacine cannot be explained structurally from the docking calculations. The A301S (resistance to dieldrin) mutation reduced the potency of antagonists picrotoxin, fipronil and pyrafluprole but the I/V edit had no measurable effect. Ivermectin suppressed responses to GABA of RDLbdI/V, RDLbd and RDLbdI/VA301S. The dieldrin resistant variant also showed reduced sensitivity to Ivermectin. This study of a highly abundant insect GABA receptor isoform will help the design of new insecticides.

The novel isoxazoline ectoparasiticide fluralaner: selective inhibition of arthropod γ-aminobutyric acid- and L-glutamate-gated chloride channels and insecticidal/acaricidal activity

Isoxazolines are a novel class of parasiticides that are potent inhibitors of γ-aminobutyric acid (GABA)-gated chloride channels (GABACls) and L-glutamate-gated chloride channels (GluCls). In this study, the effects of the isoxazoline drug fluralaner on insect and acarid GABACl (RDL) and GluCl and its parasiticidal potency were investigated. We report the identification and cDNA cloning of Rhipicephalus (R.) microplus RDL and GluCl genes, and their functional expression in Xenopus laevis oocytes. The generation of six clonal HEK293 cell lines expressing Rhipicephalus microplus RDL and GluCl, Ctenocephalides felis RDL-A285 and RDL-S285, as well as Drosophila melanogaster RDLCl-A302 and RDL-S302, combined with the development of a membrane potential fluorescence dye assay allowed the comparison of ion channel inhibition by fluralaner with that of established insecticides addressing RDL and GluCl as targets. In these assays fluralaner was several orders of magnitude more potent than picrotoxinin and dieldrin, and performed 5-236 fold better than fipronil on the arthropod RDLs, while a rat GABACl remained unaffected. Comparative studies showed that R. microplus RDL is 52-fold more sensitive than R. microplus GluCl to fluralaner inhibition, confirming that the GABA-gated chloride channel is the primary target of this new parasiticide. In agreement with the superior RDL on-target activity, fluralaner outperformed dieldrin and fipronil in insecticidal screens on cat fleas (Ctenocephalides felis), yellow fever mosquito larvae (Aedes aegypti) and sheep blowfly larvae (Lucilia cuprina), as well as in acaricidal screens on cattle tick (R. microplus) adult females, brown dog tick (Rhipicephalus sanguineus) adult females and Ornithodoros moubata nymphs. These findings highlight the potential of fluralaner as a novel ectoparasiticide.

Cloning, expression analysis, and molecular modeling of the gamma-aminobutyric acid receptor alpha2 subunit gene from the common cutworm, Spodoptera litura

Intensive research on the molecule structures of the gamma-nminobutyric acid (GABA) receptor in agricultural pests has great significance to the mechanism investigation, resistance prevention, and molecular design of novel pesticides. The GABA receptor a2 (SlGABARα2) subunit gene in Spodoptera litura (Fabricius) (Lepidoptera: Noctuidae) was cloned using the technologies of reverse transcription PCR and rapid amplification of cDNA ends. The gemonic DNA sequence of SlGABARα2 has 5164 bp with 8 exons and 7 introns that were in accordance with the GT-AG splicing formula. The complete mRNA sequence of SlGABARα2 was 1965 bp, with an open reading frame of 1500 bp encoding a protein of 499 amino acids. The GABA receptor is highly conserved among insects. The conserved regions include several N-glycosylation, Oglycosylation, and phosphorylation sites, as well as 4 transmembrane domains. The identities that SlGABARα2 shared with the GABA receptor a2 subunit of Spodoptera exigua, Heliothis virescens, Chilo suppressalis, Plutella xylostella, Bombyx mori ranged from 99.2% to 87.2% at the amino acid level. The comparative 3-dimensional model of SlGABARα2 showed that its tertiary structure was composed of 4 major α-helixes located at the 4 putative transmembrane domains on one side, with some β-sheets and 1 small α-helix on the other side. SlGABARα2 may be attached to the membrane by 4 α-helixes that bind ions in other conserved domains to transport them through the membrane. The results of quantitative real time PCR demonstrated that SlGABARα2 was expressed in all developmental stages of S. litura. The relative expression level of SlGABARα2 was the lowest in eggs and increased with larval growth, while it declined slightly in pupae and reached the peak in adults. The expressions of SlGABARα2 in larvae varied among different tissues; it was extremely high in the brain but was low in the midgut, epicuticle, Malpighian tube, and fat body.

Concentration-dependent effects of GABA on insensitivity to fipronil in the A2'S mutant RDL GABA receptor from fipronil-resistant Oulema oryzae (Coleoptera: Chrysomelidae)

The beetle Oulema oryzae Kuwayama (Coleoptera: Chrysomelidae), an important pest of rice, has developed fipronil resistance in Japan. Molecular cloning and sequence analysis of O. oryzae RDL gamma-aminobutyric acid (GABA) receptor subunit (OO-RDL) genes from fipronil-susceptible and -resistant O. oryzae identified the A2'S mutation (index number for the M2 membrane-spanning region). To investigate the effect of the A2'S mutation on fipronil resistance, we stably expressed the wild-type and mutant OO-RDL homomers in Drosophila Mel-2 cells. A membrane potential assay exhibited that the IC50 values of fipronil for inhibition of the response to EC80 GABA of the wild-type and A2'S mutant OO-RDL homomers were 0.09 microM and 0.11 microM, respectively. However, the IC50 values of fipronil for inhibition of the response to EC95 GABA of the wild-type and A2'S mutant OO-RDL homomers were 0.11 microM and approximately equal to 5 microM, respectively. These results suggest that the GABA concentration is an important factor affecting fipronil resistance in O. oryzae carrying the A2'S mutation in OO-RDL.

Mechanism of action of the insecticides, lindane and fipronil, on glycine receptor chloride channels

BACKGROUND AND PURPOSE

Docking studies predict that the insecticides, lindane and fipronil, block GABA(A) receptors by binding to 6' pore-lining residues. However, this has never been tested at any Cys-loop receptor. The neurotoxic effects of these insecticides are also thought to be mediated by GABA(A) receptors, although a recent morphological study suggested glycine receptors mediated fipronil toxicity in zebrafish. Here we investigated whether human α1, α1β, α2 and α3 glycine receptors were sufficiently sensitive to block by either compound as to represent possible neurotoxicity targets. We also investigated the mechanisms by which lindane and fipronil inhibit α1 glycine receptors.

EXPERIMENTAL APPROACH

Glycine receptors were recombinantly expressed in HEK293 cells and insecticide effects were studied using patch-clamp electrophysiology.

KEY RESULTS

Both compounds completely inhibited all tested glycine receptor subtypes with IC(50) values ranging from 0.2-2 µM, similar to their potencies at vertebrate GABA(A) receptors. Consistent with molecular docking predictions, both lindane and fipronil interacted with 6' threonine residues via hydrophobic interactions and hydrogen bonds. In contrast with predictions, we found no evidence for lindane interacting at the 2' level. We present evidence for fipronil binding in a non-blocking mode in the anaesthetic binding pocket, and for lindane as an excellent pharmacological tool for identifying the presence of β subunits in αβ heteromeric glycine receptors.

CONCLUSIONS AND IMPLICATIONS

This study implicates glycine receptors as novel vertebrate toxicity targets for fipronil and lindane. Furthermore, lindane interacted with pore-lining 6' threonine residues, whereas fipronil may have both pore and non-pore binding sites.

The cys-loop ligand-gated ion channel gene family of Tetranychus urticae: implications for acaricide toxicology and a novel mutation associated with abamectin resistance

The cys-loop ligand-gated ion channel (cysLGIC) super family of Tetranychus urticae, the two-spotted spider mite, represents the largest arthropod cysLGIC super family described to date and the first characterised one within the group of chelicerates. Genome annotation, phylogenetic analysis and comparison of the cysLGIC subunits with their counterparts in insects reveals that the T. urticae genome encodes for a high number of glutamate- and histamine-gated chloride channel genes (GluCl and HisCl) compared to insects. Three orthologues of the insect γ-aminobutyric acid (GABA)-gated chloride channel gene Rdl were detected. Other cysLGIC groups, such as the nAChR subunits, are more conserved and have clear insect orthologues. Members of cysLGIC family mediate endogenous chemical neurotransmission and they are prime targets of insecticides. Implications for toxicology associated with the identity and specific features of T. urticae family members are discussed. We further reveal the accumulation of known and novel mutations in different GluCl channel subunits (Tu_GluCl1 and Tu_GluCl3) associated with abamectin resistance in T. urticae, and provide genetic evidence for their causality. Our study provides useful toxicological insights for the exploration of the T. urticae cysLGIC subunits as putative molecular targets for current and future chemical control strategies.

A single amino acid determines the toxicity of Ginkgo biloba extracts

Ginkgo biloba extracts are currently used for a wide range of health-related conditions. Some of the medical benefits of these extracts are controversial, but their lack of toxicity in humans is not in doubt. These extracts are, however, highly toxic to insects. Their active components (ginkgolides and bilobalide) have structures similar to the convulsant picrotoxin, a GABA(A) receptor antagonist, so their lack of toxicity in mammals is puzzling. Here, we show that the different compositions of insect and vertebrate GABA receptor pores are responsible for the differing toxicities. Insect GABA receptors contain Ala at their 2' position in the pore. Substitution with Val, which is the equivalent residue in vertebrate GABA(A) receptor α-subunits, decreases ginkgolide potency by up to 10,000-fold. The reverse mutation in vertebrate GABA(A) α1 subunits increased the sensitivity of α1β2 and α1β2γ2 receptors to ginkgolides. Mutant cycle analysis demonstrates a strong interaction between the ginkgolides and the 2' residue, a result supported by in silico docking of compounds into a model of the pore. We conclude that the insecticidal activity of G. biloba extracts can be attributed to their effects at insect GABA receptors, and the presence of a Val at the 2' position in vertebrate GABA(A) receptors explains why these compounds are not similarly toxic to humans.

Mechanisms underlying fipronil resistance in a multiresistant field strain of the German cockroach (Blattodea: Blattellidae)

German cockroaches (Blattella germanica L.) have significant impacts on human health, most notably they are implicated as causes of childhood asthma. Gel bait formulations of fipronil, a phenylpyrazole insecticide, have been in use for German cockroach control in the United States since 1998. Previously, dieldrin resistant German cockroach strains were shown to have 7- to 17-fold cross-resistance to fipronil. More recently, a field-collected strain (GNV-R) displayed approximately 36-fold resistance to topically applied fipronil at the LD50 level, which is the highest level of fipronil resistance reported to date in the German cockroach. The aim of the current research was to identify mechanism(s) responsible for high-level fipronil resistance in the GNV-R strain. Synergist bioassays conducted using topical and injection application methods implicated cytochrome P450-mediated detoxification in resistance. Electrophysiological recordings using the suction-electrode technique revealed the nervous system of the GNV-R strain is insensitive to fipronil. In agreement with electrophysiology results, the alanine to serine (A302S) mutation encoded by the gamma-amino butyric acid-gated chloride channel subunit gene resistance to dieldrin, which confers limited cross-resistance to fipronil, was detected in 95% of GNV-R strain individuals. Logistic regression analysis showed that A302S mutation frequency correlates with neurological insensitivity as shown by electrophysiology data. Overall, results of synergism bioassays, electrophysiological recordings, and A302S mutation frequency measurements suggest that fipronil resistance in the GNV-R strain is caused by the combined effects of enhanced metabolism by cytochrome P450s and target-site insensitivity caused by the A302S-encoding mutation in the resistance to dieldrin gene.

Probing structural features and binding mode of 3-arylpyrimidin-2,4-diones within housefly γ-aminobutyric acid (GABA) receptor

In order to obtain structural features of 3-arylpyrimidin-2,4-diones emerged as promising inhibitors of insect γ-aminobutyric acid (GABA) receptor, a set of ligand-/receptor-based 3D-QSAR models for 60 derivatives are generated using Comparative Molecular Field Analysis (CoMFA) and Comparative Molecular Similarity Index Analysis (CoMSIA). The statistically optimal CoMSIA model is produced with highest q² of 0.62, r²_ncv of 0.97, and r²_pred of 0.95. A minor/bulky electronegative hydrophilic polar substituent at the 1-/6-postion of the uracil ring, and bulky substituents at the 3′-, 4′- and 5′-positions of the benzene ring are beneficial for the enhanced potency of the inhibitors as revealed by the obtained 3D-contour maps. Furthermore, homology modeling, molecular dynamics (MD) simulation and molecular docking are also carried out to gain a better understanding of the probable binding modes of these inhibitors, and the results show that residues Ala-183(C), Thr-187(B), Thr-187(D) and Thr-187(E) in the second transmembrane domains of GABA receptor are responsible for the H-bonding interactions with the inhibitor. The good correlation between docking observations and 3D-QSAR analyses further proves the model reasonability in probing the structural features and the binding mode of 3-arylpyrimidin-2,4-dione derivatives within the housefly GABA receptor.

The biology of insecticidal activity and resistance

Identifying insecticide resistance mechanisms is paramount for pest insect control, as the understandings that underpin insect control strategies must provide ways of detecting and managing resistance. Insecticide resistance studies rely heavily on detailed biochemical and genetic analyses. Although there have been many successes, there are also many examples of resistance that still challenge us. As a precursor to rational pest insect control, the biology of the insect, within the contexts of insecticide modes of action and insecticide metabolism, must be well understood. It makes sense to initiate this research in the best model insect system, Drosophila melanogaster, and translate these findings and methodologies to other insects. Here we explore the usefulness of the D. melanogaster model in studying metabolic-based insecticide resistances, target-site mediated resistances and identifying novel insecticide targets, whilst highlighting the importance of having a more complete understanding of insect biology for insecticide studies.

The A2'N mutation of the RDL gamma-aminobutyric acid receptor conferring fipronil resistance in Laodelphax striatellus (Hemiptera: Delphacidae)

The planthopper Laodelphax striatellus (Fallén) (Hemiptera: Delphacidae) is a serious insect pest of rice, Oryza sativa L., and has developed resistance to fipronil in Japan. Sequence analysis of L. striatellus RDL gamma-aminobutyric acid (GABA) receptor subunit (LS-RDL) genes from a fipronil-resistant population and a fipronil-susceptible strain identified the A2'N mutation (index number for M2 membrane-spanning region), that was previously implicated in fipronil resistance in the planthopper Sogatella furcifera (Horváth) (Hemiptera: Delphacidae). Nineteen of 21 fipronil-resistant L. striatellus individuals were genotyped as heterozygous for the A2'N mutation, suggesting that this mutation is associated with fipronil resistance and that most fipronil-resistant L. striatellus express wild-type and A2'N mutant LS-RDL simultaneously. To confirm the role of the A2'N mutation of LS-RDL, Drosophila Mel-2 cells were transfected with wild-type and A2'N mutant LS-RDL genes, either individually or together. A membrane potential assay showed that fipronil had no inhibitory effect at 10 microM on cells transfected with the A2'N mutant LS-RDL gene with or without the wild-type LS-RDL gene. By contrast, the IC50 value of fipronil for wild-type LS-RDL homomers was 14 nM. These results suggest that the A2'N mutation of the RDL GABA receptor subunit confers fipronil resistance in L. striatellus as well as S. furcifera.

The ionotropic γ-aminobutyric acid receptor gene family of the silkworm, Bombyx mori

γ-Aminobutyric acid (GABA) is a very important inhibitory neurotransmitter in both vertebrate and invertebrate nervous systems. GABA receptors (GABARs) are known to be the molecular targets of a class of insecticides. Members of the GABAR gene family of the silkworm, Bombyx mori, a model insect of Lepidoptera, have been identified and characterized in this study. All putative silkworm GABAR cDNAs were cloned using the reverse transcriptase polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Bombyx mori appears to have the largest insect GABAR gene family known to date, including three RDL, one LCCH3, and one GRD subunit. The silkworm RDL1 gene has RNA-editing sites, and the RDL1 and RDL3 genes possess alternative splicing. These mRNA modifications enhance the diversity of the silkworm's GABAR gene family. In addition, truncated transcripts were found for the RDL1 and LCCH3 genes. In particular, the three RDL subunits may have arisen from two duplication events.

Identification of a dieldrin resistance-associated mutation in Rhipicephalus (Boophilus) microplus (Acari: Ixodidae)

The southern cattle tick, Rhipicephalus (Boophilus) microplus (Canestrini) (Acari: Ixodidae), is a major vector of tick fever organisms affecting cattle in many parts of the world, including Australia, Africa, and South America. Control of the southern cattle tick through acaricide use is an important approach in disease management. Resistance has emerged to many of the acaricides currently and previously used, including the cyclodienes. Although cyclodiene resistance mechanisms have been characterized in many insect species, this report is the first to identify mutations associated with dieldrin resistance in the cattle tick. A novel two base pair mutation in the GABA-gated chloride channel gene has been identified at position 868-9 and causes a codon change from threonine to leucine. Analysis of a small number of field-collected samples resistant to dieldrin shows this mutation has been maintained without selection pressure since the withdrawal of dieldrin in Australia > 20 yr ago. The mutation is not found in other laboratory-maintained strains of R. microplus that were subject to selection pressure with various acaricides.

RNA editing regulates insect gamma-aminobutyric acid receptor function and insecticide sensitivity

A-to-I pre-mRNA editing by adenosine deaminase enzymes has been reported to enhance protein diversity in the nervous system. In Drosophila, the resistance to dieldrin (RDL) gamma-aminobutyric acid (GABA) receptor subunit displays an editing site (R122) that is close to the putative GABA-binding site. We assessed the functional effects of editing at this site by expressing homomeric RDL receptors in Xenopus oocytes. After replacement of arginine 122 with a glycine, both agonist and fipronil potencies were shifted to the right in either fipronil-sensitive receptors or mutated resistant receptors (A301G/T350M). These data provide the first insight on the influence of RNA editing on GABA receptor function.

Role of a serine residue (S278) in the pore-facing region of the housefly L-glutamate-gated chloride channel in determining sensitivity to noncompetitive antagonists

Gamma-hexachlorocyclohexane (gamma-HCH), fipronil and picrotoxinin are noncompetitive antagonists (NCAs) of L-glutamate-gated chloride channels (GluCls), yet their potencies are weaker than those on gamma-aminobutyric acid receptors (GABARs). The A302S mutation of Drosophila RDL (resistant to dieldrin) GABAR confers NCA resistance, and housefly GluCls (MdGluCls) possess S278 as the residue corresponding to the A302. Thus, the effects of S278A mutation on the NCA actions on MdGluCls were investigated. The S278A mutation resulted in enhanced blocking by NCAs of the MdGluCl response to 30 microM L-glutamate. However, such actions of gamma-HCH and picrotoxinin, but not of fipronil, on the S278A mutant were reduced with 200 microM L-glutamate. Further increases in the L-glutamate concentration led to potentiation by NCAs of the mutant response to L-glutamate.

Molecular cloning of a GABA receptor subunit from Laodelphax striatella (Fallén) and patch clamp analysis of the homo-oligomeric receptors expressed in a Drosophila cell line

A cDNA encoding a gamma-aminobutyric acid (GABA) receptor subunit was cloned from the small brown planthopper Laodelphax striatella. The L. striatella GABA receptor subunit was found to have high amino acid sequence similarity to the bd-type splice variant of the Drosophila GABA receptor Rdl subunit and several other GABA receptor subunits, with identities of over 70%. The cDNA was inserted into the expression vector pAc5.1-lac-Hygro. Clonal cell lines stably expressing homo-oligomeric L. striatella GABA receptors were generated by transfecting the vector into D.mel-2 cells. Expression of functional GABA receptors in the cell lines was demonstrated by whole-cell patch clamp recordings. GABA induced inward currents with an EC(50) value of 29 microM and a Hill coefficient of 1.7. The GABA-evoked responses reversed close to the Nernst equilibrium potential for chloride ions. The amplitudes of agonist-induced currents were found to be in the order muscimol (100 microM) >/= GABA (100 microM) > isoguvacine (100 microM) > cis-4-aminocrotonic acid (CACA) (100 microM) > 5-(4-piperidyl)-3-isoxazolol (4-PIOL) (1 mM). Antagonists such as fipronil (100 nM), 4'-ethynyl-4-n-propylbicycloorthobenzoate (EBOB) (100 nM), dieldrin (100 nM) and SR95531 (gabazine) (1 microM) suppressed GABA-induced currents. The functional expression of a GABA receptor from an agricultural pest presents a unique opportunity to discover new molecules active at this important target site.