Molecular cloning and transformation of cyclodiene resistance in Drosophila: an invertebrate gamma-aminobutyric acid subtype A receptor locus

Evaluation of fipronil efficacy and first molecular report of gamma-aminobutyric acid (GABA) gated chloride channel gene of Rhipicephalus microplus ticks in China and Pakistan

Southern cattle tick, Rhipicephalus (R) microplus, is a major challenge in transmitting Babesia spp., Theileria spp., and Anaplasma spp., worldwide. Mitigation of this tick and associated pathogens is crucial for animal and human-health. In present study, laboratory-based larval packet tests (LPTs) were performed to assess fipronil efficacy against R. microplus ticks collected from four Chinese localities, Nanning, Guangshui, Macheng, and Tengchong. Additionally, we reported first molecular characterization of the full-length gamma-aminobutyric acid (GABA)-gated chloride channel gene of R. microplus ticks from China and Pakistan (Khyber Pakhtunkhwa and Punjab). For LPTs bioassay, fipronil's concentrations (5-ppm, 25-ppm, 50-ppm, 75-ppm, 100-ppm) in acetone and DNase-free water were prepared. The highest average mortality rate of larvae was observed in Nanning (4.34-98.13 %) while lowest in Tengchong (5.70-82.29 %) via triplicate LPTs. The recorded LC50 values were 33.2 ppm, 35.7 ppm, 49.7 ppm, and 55.4 ppm, respectively, for the four localities (Guangshui, Macheng, Nanning, and Tengchong) while the LC99 values were 722 ppm, 827 ppm, 949 ppm, and 1342.3 ppm, respectively. The resistance factors (RR50) were 13.83, 14.88, 20.71, and 23.08, indicating the development of level-II resistance in the tick populations from these regions respectively. Molecularly, consensus sequences of R. microplus GABA-gene from China and Pakistan resulted 99.24-99.52 % and 99.10-99.39 % identities with dieldrin-susceptible NRFS (GQ398111.1) and dieldrin-resistant (GQ398112.1) R. microplus ticks, respectively. Findings revealed 21 single nucleotide polymorphisms (SNPs) [8-non-synonymous (NS) and 13-synonymous (S)], including specifically 10-SNPs from Nanning, 9-SNPs from Macheng, 7-SNPs from Guangshui, 10-SNPs from Tengchong, and 9-SNPs from Pakistani R. microplus full-length GABA gene. Explicitly, we obtained one NS-SNP in TM3 (T-871-G; F-291-V) and two NS-SNPs in TM4 (A-1438-G; N-480-D and A-1439-G; N-480-G) regions of Chinese and Pakistani, while one NS-SNP in TM2 (A-763-G; T-255-A) region of Pakistani R. microplus ticks was documented. Coexistence among SNPs in TM3 (T-871-G; F-291-V), TM4 (A-1439-G; N-480-G) and one linker SNP (A-1378-G; S-460-G) were also recorded. Presence of SNPs and their coexistence suggest the possible tri-dimensional structural modification in GABA-Cl channels that might interfere with binding of fipronil, resulting in development of resistance. This comprehensive research will provide a basis for governmental and pharmaceutical industries for development of effective tick control-strategies as well as to manage the effectiveness and functionality of the available acaricides.

Grapefruit-derived nootkatone potentiates GABAergic signaling and acts as a dual-action mosquito repellent and insecticide

Humanity has long battled mosquitoes and the diseases they transmit-a struggle intensified by climate change and globalization, which have expanded mosquito ranges and the spread of associated diseases.1 Additionally, widespread insecticide resistance has reduced the efficacy of current control methods, necessitating new solutions.2,3 Nootkatone, a natural compound found in grapefruit, shows promise as both a mosquito repellent and an insecticide.4,5 However, its mechanism of action remains unclear. Our study demonstrates that nootkatone acts as a potent spatial and contact repellent against multiple mosquito species. Nootkatone-induced spatial aversion, which is influenced by human odor, is in Aedes aegypti partially mediated by Orco- and ionotropic receptor (IR)-positive neurons, while contact aversion is robust and likely mediated via the proboscis and independent of TRPA1 and IRs. We further find that nootkatone potentiates γ-aminobutyric acid (GABA)-mediated signaling by modulating the broadly expressed major insect GABA-gated chloride channel resistant to dieldrin (Rdl). At low doses, the chemosensory-mediated spatial and contact repellency is likely strengthened by nootkatone's disruption of synaptic transmission in select mosquito sensory neurons. At higher doses, nootkatone induces paralysis and death, presumably through broad-range synaptic transmission disruption. These findings reveal nootkatone's unique mode of action and highlight its potential as an effective mosquito control agent. Its dual role as a repellent and an insecticide, combined with low-to-no toxicity to humans and a pleasant smell, underscores nootkatone's promise as a future tool in mosquito control efforts.

N318L Blocks the Interaction of Fluralaner but Not Broflanilide or Fipronil with the Insect GABA Receptor In Vivo

Fluralaner is a novel insecticide targeting the ionotropic GABA receptor (GABAR) subunit, RDL. A recent study revealed that N316L, a substitution of asparagine (N) with leucine (L), in the second transmembrane (M2)-spanning region reduced the antagonist action of fluralaner on the housefly Musca domestica RDL (MdRDL) in vitro. To verify the impact of N316L in vivo, the corresponding mutation (N318L) in the fruitfly Drosophila melanogaster RDL (DmRDL) was constructed using CRISPR/Cas9 genome editing. The homozygous DmRDLN318L mutant showed a 9.87-fold resistance to fluralaner compared with w1118 while still being highly sensitive to broflanilide and fipronil, which is consistent with those findings observed in the electrophysiology assays of the homomeric DmRDLWT or DmRDLN318L channel. Moreover, DmRDLN318L led to malformed ovaries, stunted eggs, and sterility in homozygous females. These results highlighted N318 as a molecular site for fluralaner in vivo and in vitro and might elucidate the resistance mechanisms of insects against fluralaner.

Heterogeneity in the projections and excitability of tyraminergic/octopaminergic neurons that innervate the Drosophila reproductive tract

Aminergic nuclei in mammals are generally composed of relatively small numbers of cells with broad projection patterns. Despite the gross similarity of many individual neurons, recent transcriptomic, anatomic and behavioral studies suggest previously unsuspected diversity. Smaller clusters of aminergic neurons in the model organism Drosophila melanogaster provide an opportunity to explore the ramifications of neuronal diversity at the level of individual cells. A group of approximately 10 tyraminergic/octopaminergic neurons innervates the female reproductive tract in flies and has been proposed to regulate multiple activities required for fertility. The projection patterns of individual neurons within the cluster are not known and it remains unclear whether they are functionally heterogenous. Using a single cell labeling technique, we show that each region of the reproductive tract is innervated by a distinct subset of tyraminergic/octopaminergic cells. Optogenetic activation of one subset stimulates oviduct contractions, indicating that the cluster as a whole is not required for this activity, and underscoring the potential for functional diversity across individual cells. Using whole cell patch clamp, we show that two adjacent and morphologically similar cells are tonically inhibited, but each responds differently to injection of current or activation of the inhibitory GluCl receptor. GluCl appears to be expressed at relatively low levels in tyraminergic/octopaminergic neurons within the cluster, suggesting that it may regulate their excitability via indirect pathways. Together, our data indicate that specific tyraminergic/octopaminergic cells within a relatively homogenous cluster have heterogenous properties and provide a platform for further studies to determine the function of each cell.

GABA-gated chloride channel mutation (Rdl) induces cholinergic physiological compensation resulting in cross resistance in Drosophila melanogaster

The Drosophila melanogaster MD-RR strain contains an Rdl mutation (A301S) resulting in resistance to several insecticide classes viz. phenyl pyrazoles (e.g., fipronil), cyclodienes (e.g., dieldrin), and chlorinated aliphatic hydrocarbons (e.g., lindane). Fitness costs are commonly observed with resistant insect populations as side effects of the genetic change conferring the resistant phenotype. Because of fitness costs, reversion from the resistant to susceptible genotype and phenotype is common. However, the Rdl genotype in D. melanogaster appears to allow the flies to maintain the resistant genotype/phenotype without selective pressure and with minimal fitness costs. We provide evidence that compensation for the Rdl mutation influences the cholinergic system, where an increase in acetylcholinesterase gene expression and enzyme activity results in neurophysiological changes and cross resistance to a carbamate insecticide (propoxur oral resistance ratio (RR) of 63) and an organophosphate insecticide (dichlorvos oral RR of 7). Such cross resistance was not previously reported with the initial collection and testing of this strain. In addition to acetylcholinesterase, the Rdl mutation influences the expression of the muscarinic acetylcholine receptor subtype-B, resulting in resistance to non-selective muscarinic compounds (pilocarpine and atropine). Collectively, these results indicate that the Rdl mutation (A301S) at GABA-gated ionophore complex influences the physiology of the cholinergic system, leading to resistance to established insecticide classes. Additionally, this mutation may impact the effectiveness of insecticides targeting novel sites, like muscarinic receptors.

Identification and Spatiotemporal Expression of a Putative New GABA Receptor Subunit in the Human Body Louse Pediculus humanus humanus

The human louse (Pediculus humanus) is an obligatory blood feeding ectoparasite with two ecotypes: the human body louse (Pediculus humanus humanus), a competent vector of several bacterial pathogens, and the human head louse (Pediculus humanus capitis), responsible for pediculosis and affecting millions of people around the globe. GABA (γ-aminobutyric acid) receptors, members of the cys-loop ligand gated ion channel superfamily, are among the main pharmacological targets for insecticides. In insects, there are four subunits of GABA receptors: resistant-to-dieldrin (RDL), glycin-like receptor of drosophila (GRD), ligand-gated chloride channel homologue3 (LCCH3), and 8916 are well described and form distinct phylogenetic clades revealing orthologous relationships. Our previous studies in the human body louse confirmed that subunits Phh-RDL, Phh-GRD, and Phh-LCCH3 are well clustered in their corresponding clades. In the present work, we cloned and characterized a putative new GABA receptor subunit in the human body louse that we named HoCas, for Homologous to Cys-loop α like subunit. Extending our analysis to arthropods, HoCas was found to be conserved and clustered in a new (fifth) phylogenetic clade. Interestingly, the gene encoding this subunit is ancestral and has been lost in some insect orders. Compared to the other studied GABA receptor subunits, HoCas exhibited a relatively higher expression level in all development stages and in different tissues of human body louse. These findings improved our understanding of the complex nature of GABA receptors in Pediculus humanus and more generally in arthropods.

Functional validation of A2'N mutation of the RDL GABA receptor against fipronil via molecular modeling and genome engineering in drosophila

BACKGROUND

Insect RDL (resistant to dieldrin) receptors are essential pentameric ligand-gated chloride channels that mediate the neuroinhibitory effect of GABA, the chief inhibitory neurotransmitter in the central nervous system. These receptors serve as primary targets for various insecticides, including noncompetitive antagonists (NCAs) such as cyclodiene organochlorines and phenylpyrazoles, as well as allosteric modulators like meta-diamides and isoxazolines. This study focuses on a newly discovered A2'N mutation within the RDL receptors, identified in fipronil-resistant planthoppers. Despite in vitro electrophysiological studies have proposed its role in conferring target-site resistance, in vivo genetic functional validation of this mutation remains unexplored.

RESULTS

Our research employed toxicity bioassays, assessing various Rdl genotypes against a spectrum of insecticides, including fipronil, α-endosulfan, broflanilide, and isocycloseram. Results revealed distinct resistance profiles for A2'N and A2'S mutants, indicating different binding interactions of RDL receptors with NCAs. Significantly, the A2'N heterozygote showed substantial resistance to fipronil, despite its homozygous lethality. Molecular modeling and docking simulations further supported these findings, highlighting unique binding poses for fipronil and α-endosulfan.

CONCLUSION

This study confirmed that A2'N mutation of the RDL GABA receptor confer high resistance to fipronil in vivo. The observed resistance in A2'N mutants is likely attributable to a steric hindrance mechanism, wherein the introduction of larger side chains hampers fipronil binding, even in a heterozygous state. © 2023 Society of Chemical Industry.

Transgenic line for characterizing GABA-receptor expression to study the neural basis of olfaction in the yellow-fever mosquito

The mosquito Aedes aegypti is an important vector of diseases including dengue, Zika, chikungunya, and yellow fever. Olfaction is a critical modality for mosquitoes enabling them to locate hosts, sources of nectar, and sites for oviposition. GABA is an essential neurotransmitter in olfactory processing in the insect brain, including the primary olfactory center, the antennal lobe. Previous work with Ae. aegypti has suggested that antennal lobe inhibition via GABA may be involved in the processing of odors. However, little is known about GABA receptor expression in the mosquito brain, or how they may be involved in odor attraction. In this context, generating mutants that target the mosquito's olfactory responses, and particularly the GABAergic system, is essential to achieve a better understanding of these diverse processes and olfactory coding in these disease vectors. Here we demonstrate the potential of a transgenic line using the QF2 transcription factor, GABA-B1QF2-ECFP, as a new neurogenetic tool to investigate the neural basis of olfaction in Ae. aegypti. Our results show that the gene insertion has a moderate impact on mosquito fitness. Moreover, the line presented here was crossed with a QUAS reporter line expressing the green fluorescent protein and used to determine the location of the metabotropic GABA-B1 receptor expression. We find high receptor expression in the antennal lobes, especially the cell bodies surrounding the antennal lobes. In the mushroom bodies, receptor expression was high in the Kenyon cells, but had low expression in the mushroom body lobes. Behavioral experiments testing the fruit odor attractants showed that the mutants lost their behavioral attraction. Together, these results show that the GABA-B1QF2-ECFP line provides a new tool to characterize GABAergic systems in the mosquito nervous system.

Mechanistic insights into the selectivity of bicyclophosphorothionate antagonists for housefly versus rat GABA receptors

BACKGROUND

γ-Aminobutyric acid (GABA) receptors (GABARs) are validated targets of insecticides. Bicyclophosphorothionates are a group of insecticidal compounds that act as noncompetitive antagonists of GABARs. We previously reported that the analogs exhibit various degrees of selectivity for housefly versus rat GABARs, depending on substitutions at the 3- and 4-positions. We here sought to elucidate the unsolved mechanisms of the receptor selectivity using quantitative structure-activity relationship (QSAR), molecular docking, and molecular dynamics approaches.

RESULTS

Three-dimensional (3D)-QSAR studies using Topomer comparative molecular field analysis quantitatively demonstrated how the introduction of a small alkyl group at the 3-position of bicyclophosphorothionates contributes to the housefly versus rat GABAR selectivity. To investigate the molecular mechanisms of the selective action, bicyclophosphorothionates were docked into housefly Resistance to dieldrin (RDL) GABAR and rat α1β2γ2 GABAR homology models built using the published 3D-structures of human GABARs as templates. The results of molecular docking and molecular dynamics simulations revealed that the 2'Ala and 6'Thr residues of the RDL subunit within the channel are the key amino acids for binding to the housefly GABARs, whereas the 2'Ser residue of γ2 subunit plays a crucial role in binding to rat GABARs.

CONCLUSION

We revealed the molecular mechanisms underlying the selective antagonistic action of bicyclophosphorothionates on housefly versus rat GABARs. The information presented should help design and develop novel, safe GABAR-targeting insecticides. © 2023 Society of Chemical Industry.

Investigating open access new approach methods (NAM) to assess biological points of departure: A case study with 4 neurotoxic pesticides

Open access new approach methods (NAM) in the US EPA ToxCast program and NTP Integrated Chemical Environment (ICE) were used to investigate activities of four neurotoxic pesticides: endosulfan, fipronil, propyzamide and carbaryl. Concordance of in vivo regulatory points of departure (POD) adjusted for interspecies extrapolation (AdjPOD) to modelled human Administered Equivalent Dose (AEDHuman) was assessed using 3-compartment or Adult/Fetal PBTK in vitro to in vivo extrapolation. Model inputs were from Tier 1 (High throughput transcriptomics: HTTr, high throughput phenotypic profiling: HTPP) and Tier 2 (single target: ToxCast) assays. HTTr identified gene expression signatures associated with potential neurotoxicity for endosulfan, propyzamide and carbaryl in non-neuronal MCF-7 and HepaRG cells. The HTPP assay in U-2 OS cells detected potent effects on DNA endpoints for endosulfan and carbaryl, and mitochondria with fipronil (propyzamide was inactive). The most potent ToxCast assays were concordant with specific components of each chemical mode of action (MOA). Predictive adult IVIVE models produced fold differences (FD) < 10 between the AEDHuman and the measured in vivo AdjPOD. The 3-compartment model was concordant (i.e., smallest FD) for endosulfan, fipronil and carbaryl, and PBTK was concordant for propyzamide. The most potent AEDHuman predictions for each chemical showed HTTr, HTPP and ToxCast were mainly concordant with in vivo AdjPODs but assays were less concordant with MOAs. This was likely due to the cell types used for testing and/or lack of metabolic capabilities and pathways available in vivo. The Fetal PBTK model had larger FDs than adult models and was less predictive overall.

Combinatory Actions of Co-transmitters in Dopaminergic Systems Modulate Drosophila Olfactory Memories

Dopamine neurons (DANs) are extensively studied in the context of associative learning, in both vertebrates and invertebrates. In the acquisition of male and female Drosophila olfactory memory, the PAM cluster of DANs provides the reward signal, and the PPL1 cluster of DANs sends the punishment signal to the Kenyon cells (KCs) of mushroom bodies, the center for memory formation. However, thermo-genetical activation of the PPL1 DANs after memory acquisition impaired aversive memory, and that of the PAM DANs impaired appetitive memory. We demonstrate that the knockdown of glutamate decarboxylase, which catalyzes glutamate conversion to GABA in PAM DANs, potentiated the appetitive memory. In addition, the knockdown of glutamate transporter in PPL1 DANs potentiated aversive memory, suggesting that GABA and glutamate co-transmitters act in an inhibitory manner in olfactory memory formation. We also found that, in γKCs, the Rdl receptor for GABA and the mGluR DmGluRA mediate the inhibition. Although multiple-spaced training is required to form long-term aversive memory, a single cycle of training was sufficient to develop long-term memory when the glutamate transporter was knocked down, in even a single subset of PPL1 DANs. Our results suggest that the mGluR signaling pathway may set a threshold for memory acquisition to allow the organisms' behaviors to adapt to changing physiological conditions and environments.SIGNIFICANCE STATEMENT In the acquisition of olfactory memory in Drosophila, the PAM cluster of dopamine neurons (DANs) mediates the reward signal, while the PPL1 cluster of DANs conveys the punishment signal to the Kenyon cells of the mushroom bodies, which serve as the center for memory formation. We found that GABA co-transmitters in the PAM DANs and glutamate co-transmitters in the PPL1 DANs inhibit olfactory memory formation. Our findings demonstrate that long-term memory acquisition, which typically necessitates multiple-spaced training sessions to establish aversive memory, can be triggered with a single training cycle in cases where the glutamate co-transmission is inhibited, even within a single subset of PPL1 DANs, suggesting that the glutamate co-transmission may modulate the threshold for memory acquisition.

GABAergic signaling shapes multiple aspects of Drosophila courtship motor behavior

Inhibitory neurons are essential for orchestrating and structuring behavior. We use one of the best studied behaviors in Drosophila, male courtship, to analyze how inhibitory, GABAergic neurons shape the different steps of this multifaceted motor sequence. RNAi-mediated knockdown of the GABA-producing enzyme GAD1 and the ionotropic receptor Rdl in sex specific, fruitless expressing neurons in the ventral nerve cord causes uncoordinated and futile copulation attempts, defects in wing extension choice and severe alterations of courtship song. Altered song of GABA depleted males fails to stimulate female receptivity, but rescue of song patterning alone is not sufficient to rescue male mating success. Knockdown of GAD1 and Rdl in male brain circuits abolishes courtship conditioning. We characterize the around 220 neurons coexpressing GAD1 and Fruitless in the Drosophila male nervous system and propose inhibitory circuit motifs underlying key features of courtship behavior based on the observed phenotypes.

Convergent resistance to GABA receptor neurotoxins through plant-insect coevolution

The molecular mechanisms of coevolution between plants and insects remain elusive. GABA receptors are targets of many neurotoxic terpenoids, which represent the most diverse array of natural products known. Over deep evolutionary time, as plant terpene synthases diversified in plants, so did plant terpenoid defence repertoires. Here we show that herbivorous insects and their predators evolved convergent amino acid changing substitutions in duplicated copies of the Resistance to dieldrin (Rdl) gene that encodes the GABA receptor, and that the evolution of duplicated Rdl and terpenoid-resistant GABA receptors is associated with the diversification of moths and butterflies. These same substitutions also evolved in pests exposed to synthetic insecticides that target the GABA receptor. We used in vivo genome editing in Drosophila melanogaster to evaluate the fitness effects of each putative resistance mutation and found that pleiotropy both facilitates and constrains the evolution of GABA receptor resistance. The same genetic changes that confer resistance to terpenoids across 300 Myr of insect evolution have re-evolved in response to synthetic analogues over one human lifespan.

Physiological Function of RDL1 and RDL2 Subunits of the Ionotropic GABA Receptor in the Spodoptera litura with the CRISPR/Cas9 System In Vivo

In insect ionotropic γ-aminobutyric acid receptor (iGABAR) subunits, only resistance to dieldrin (RDL) can be individually and functionally expressed in vitro. In lepidopteran, two to three RDL subtypes are identified; however, their physiological roles have not been distinguished in vivo. In this study, SlRdl1 and SlRdl2 of S. litura were individually knocked out using CRISPR/Cas9, respectively. The mortality and larval and pupal duration of KOSlRdl1 and KOSlRdl2 were increased. The flight time and distance were increased by 43.30%-80.66% and 58.96%-198.22%, respectively, in KOSlRdl1. The GABA-induced current was significantly decreased by 53.57%-74.28% and 46.91%-63.34% in the ventral nerve cord, and the GABA titer was significantly reduced by 17.65%-28.05% and 19.85%-42.46% in KOSlRdl1 and KOSlRdl2, respectively. In conclusion, SlRdl1 and SlRdl2 are necessary for the transmission of GABA-induced neural signals; however, only SlRdl1 could regulate the flight capability of S. litura. Our results provided a new avenue to study lepidopteran iGABARs.

Biochemical alterations in cotton leafworm, Spodoptera littoralis (Boisd.) related to emamectin benzoate and fipronil compared to their joint action

Cotton leafworm, Spodoptera littoralis (Boisduval), is one of the major destructive pests of ornamental, industrial, and vegetable crops. The efficacy of technical emamectin benzoate (EMB) and fipronil (FPR) was assessed against the 4th larval instar using leaf-dip bioassay method. EMB was more efficient than FPR based on 96 h LC50 values of 0.004 and 0.023 μg/ml, respectively. Joint toxic action of the dual exposure in sequence with time interval 24 h and in mix were evaluated at LC10:LC10, LC25:LC25 and LC50:LC50 after 96 h posttreatment, as well. Their impacts on detoxification enzymes, esterases (ESTs); alkaline phosphatase (ALP); and glutathione S-transferase (GST) as well as acetylcholine esterase (AChE) were also determined. The sequential exposure of EMB after FPR (S1) produced antagonism, potentiation, and potentiation effects, respectively while sequential exposure of FPR after EMB (S2) interacted as addition, potentiation, and potentiation respectively. The rest of binary mixtures (Mix) revealed antagonistic effect regardless of concentration. Orthogonal contrast analysis showed that the highest elevations of AChE, α-EST, β- EST and ALP enzymes were obtained from Mix at LC50:LC50 (181.6%, 288.4, 229.2 and 460.9%, respectively), LC25:LC25 (131.5%, 252.8, 205.60 and 252.0, respectively) and LC10:LC10 (106.6%, 215.6%, 201.8% and 170.0%, respectively). Differently, the greatest elevation of GST activity (157.7%) resulted from S1 at LC50:LC50, while it was significantly lower at LC25:LC25 and LC10:LC10 as well as Mix and S2 at all concentrations than corresponding concentrations of FPR. These findings shed some light on the role of GST in FPR toxicity and clarified the risk of these dual exposures in elevating detoxification enzymes dangerously compared to their individual insecticides. These dual exposures should be carefully handled. Although rotational exposure at low concentrations may enhance performance and mitigate resistance risk, rotational exposure at high concentrations and Mix may indirectly contribute to the evolution of cross-resistance to other insecticides.

Inference of selection pressures that drive insecticide resistance in Anopheles and Culex mosquitoes in Korea

Pyrethroids are primarily used for mosquito control in Korea. However, high frequencies of mutations conferring resistance to not only pyrethroids but also to other insecticides have been found in mosquito populations. This study aimed to examine the hypothesis that insecticides used outside of public health may play a role in selection. Briefly, the resistance mutation frequencies to three insecticide groups (pyrethroids, organophosphates, and cyclodienes) were estimated in two representative groups of mosquito species (Anopheles Hyrcanus Group and Culex pipiens complex). The relationship between these frequencies and the land-use status of the collection sites was investigated through multiple regression analysis. In the Anopheles Hyrcanus Group, the frequencies of both ace1 (organophosphate resistance) and rdl (cyclodiene resistance) mutations were positively correlated with 'proximity to golf course', possibly be due to the insecticides used for turf maintenance. They also showed positive correlations with field area and rice paddy area, respectively, suggesting the role of agricultural insecticides in the selection of these resistance traits. For the Cx. pipiens complex, the kdr (pyrethroid resistance), ace1, and rdl mutations were positively correlated with the residential area, field, and rice paddy, respectively. Therefore, pyrethroids used for public health could serve as a direct source of resistance selection pressure against kdr, whereas non-public health insecticides may pose primary selection pressure against the ace1 and rdl traits. The current findings suggest that the insecticides used in agriculture and the golf industry play a significant role in mosquito selection, despite variations in the extent of indirect selection pressure according to the mosquito groups and insecticide classes.

G3'MTMD3 in the insect GABA receptor subunit, RDL, confers resistance to broflanilide and fluralaner

Meta-diamides (e.g. broflanilide) and isoxazolines (e.g. fluralaner) are novel insecticides that target the resistant to dieldrin (RDL) subunit of insect γ-aminobutyric acid receptors (GABARs). In this study, we used in silico analysis to identify residues that are critical for the interaction between RDL and these insecticides. Substitution of glycine at the third position (G3') in the third transmembrane domain (TMD3) with methionine (G3'M TMD3), which is present in vertebrate GABARs, had the strongest effect on fluralaner binding. This was confirmed by expression of RDL from the rice stem borer, Chilo suppressalis (CsRDL) in oocytes of the African clawed frog, Xenopus laevis, where the G3'MTMD3 mutation almost abolished the antagonistic action of fluralaner. Subsequently, G3'MTMD3 was introduced into the Rdl gene of the fruit fly, Drosophila melanogaster, using the CRISPR/Cas9 system. Larvae of heterozygous lines bearing G3'MTMD3 did not show significant resistance to avermectin, fipronil, broflanilide, and fluralaner. However, larvae homozygous for G3'MTMD3 were highly resistant to broflanilide and fluralaner whilst still being sensitive to fipronil and avermectin. Also, homozygous lines showed severely impaired locomotivity and did not survive to the pupal stage, indicating a significant fitness cost associated with the G3'MTMD3. Moreover, the M3'GTMD3 in the mouse Mus musculus α1β2 GABAR increased sensitivity to fluralaner. Taken together, these results provide convincing in vitro and in vivo evidence for both broflanilide and fluralaner acting on the same amino acid site, as well as insights into potential mechanisms leading to target-site resistance to these insecticides. In addition, our findings could guide further modification of isoxazolines to achieve higher selectivity for the control of insect pests with minimal effects on mammals.

Disynaptic inhibition shapes tuning of OFF-motion detectors in Drosophila

The circuitry underlying the detection of visual motion in Drosophila melanogaster is one of the best studied networks in neuroscience. Lately, electron microscopy reconstructions, algorithmic models, and functional studies have proposed a common motif for the cellular circuitry of an elementary motion detector based on both supralinear enhancement for preferred direction and sublinear suppression for null-direction motion. In T5 cells, however, all columnar input neurons (Tm1, Tm2, Tm4, and Tm9) are excitatory. So, how is null-direction suppression realized there? Using two-photon calcium imaging in combination with thermogenetics, optogenetics, apoptotics, and pharmacology, we discovered that it is via CT1, the GABAergic large-field amacrine cell, where the different processes have previously been shown to act in an electrically isolated way. Within each column, CT1 receives excitatory input from Tm9 and Tm1 and provides the sign-inverted, now inhibitory input signal onto T5. Ablating CT1 or knocking down GABA-receptor subunit Rdl significantly broadened the directional tuning of T5 cells. It thus appears that the signal of Tm1 and Tm9 is used both as an excitatory input for preferred direction enhancement and, through a sign inversion within the Tm1/Tm9-CT1 microcircuit, as an inhibitory input for null-direction suppression.

Alkaline taste sensation through the alkaliphile chloride channel in Drosophila

The sense of taste is an important sentinel governing what should or should not be ingested by an animal, with high pH sensation playing a critical role in food selection. Here we explore the molecular identities of taste receptors detecting the basic pH of food using Drosophila melanogaster as a model. We identify a chloride channel named alkaliphile (Alka), which is both necessary and sufficient for aversive taste responses to basic food. Alka forms a high-pH-gated chloride channel and is specifically expressed in a subset of gustatory receptor neurons (GRNs). Optogenetic activation of alka-expressing GRNs is sufficient to suppress attractive feeding responses to sucrose. Conversely, inactivation of these GRNs causes severe impairments in the aversion to high pH. Altogether, our discovery of Alka as an alkaline taste receptor lays the groundwork for future research on alkaline taste sensation in other animals.

CRISPR/Cas9-mediated D472N substitution in the Rdl1 of Plutella xylostella confers low resistance to abamectin

BACKGROUND

Abamectin is one of the main insecticides used for the control of Plutella xylostella, a destructive pest of cruciferous crops. Target-site mutation plays an important role in insecticide resistance. A point mutation (D472N) has been reported in the Rdl1 γ-aminobutyric acid receptor (GABAR) in P. xylostella, but its roles in insecticide resistance remain unknown.

RESULTS

In this study, the D472N mutation of the Rdl1 GABAR was detected in several field populations of P. xylostella and showed a positive correlation with abamectin resistance. A knock-in homozygous mutation strain (D472N-KI) of P. xylostella was successfully constructed using CRISPR/Cas9 coupled with homology-directed repair, and the bioassay results demonstrated that compared with the susceptible strain, the D472N-KI strain had 11.1- and 3.7-fold increased resistance to abamectin and endosulfan, respectively. There was no difference in resistance to fipronil, broflanilide or isocycloseram, which also target the GABAR. In addition, the total fecundity of the D472N-KI strain was significantly reduced by 50.0%.

CONCLUSION

Our results suggest that the homozygous D472N mutation in Rdl1 confers a low level of resistance to abamectin in P. xylostella but causes significant fecundity disadvantages, which may delay the development of resistance to some extent. These results lay a foundation for further understanding the mechanisms of abamectin resistance in insect pests. © 2022 Society of Chemical Industry.

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.

Transcriptome of the synganglion in the tick Ixodes ricinus and evolution of the cys-loop ligand-gated ion channel family in ticks

BACKGROUND

Ticks represent a major health issue for humans and domesticated animals. Exploring the expression landscape of the tick's central nervous system (CNS), known as the synganglion, would be an important step in understanding tick physiology and in managing tick-borne diseases, but studies on that topic are still relatively scarce. Neuron-specific genes like the cys-loop ligand-gated ion channels (cys-loop LGICs, or cysLGICs) are important pharmacological targets of acaricides. To date their sequence have not been well catalogued for ticks, and their phylogeny has not been fully studied.

RESULTS

We carried out the sequencing of transcriptomes of the I. ricinus synganglion, for adult ticks in different conditions (unfed males, unfed females, and partially-fed females). The de novo assembly of these transcriptomes allowed us to obtain a large collection of cys-loop LGICs sequences. A reference meta-transcriptome based on synganglion and whole body transcriptomes was then produced, showing high completeness and allowing differential expression analyses between synganglion and whole body. Many of the genes upregulated in the synganglion were associated with neurotransmission and/or localized in neurons or the synaptic membrane. As the first step of a functional study of cysLGICs, we cloned the predicted sequence of the resistance to dieldrin (RDL) subunit homolog, and functionally reconstituted the first GABA-gated receptor of Ixodes ricinus. A phylogenetic study was performed for the nicotinic acetylcholine receptors (nAChRs) and other cys-loop LGICs respectively, revealing tick-specific expansions of some types of receptors (especially for Histamine-like subunits and GluCls).

CONCLUSIONS

We established a large catalogue of genes preferentially expressed in the tick CNS, including the cysLGICs. We discovered tick-specific gene family expansion of some types of cysLGIC receptors, and a case of intragenic duplication, suggesting a complex pattern of gene expression among different copies or different alternative transcripts of tick neuro-receptors.

5-(4-Pyridinyl)-3-isothiazolols as Competitive Antagonists of Insect GABA Receptors: Design, Synthesis, and a New Mechanism Leading to Insecticidal Effects

Ionotropic γ-aminobutyric acid (GABA) receptors (iGABARs) are validated targets of drugs and insecticides. Our previous studies showed that the competitive antagonists of insect iGABARs exhibit insecticidal activities and that the 3-isothiazolol scaffold is used as a lead for developing novel iGABAR antagonists. Here, we designed a novel series of 4-aryl-5-(4-pyridinyl)-3-isothiazolol (4-API) analogs that have various aromatic substituents at the 4-position. Two-electrode voltage clamp experiments showed that all synthesized 4-APIs exhibited antagonistic activity against Musca domestica and Spodoptera litura iGABARs (RDL) expressed in oocytes of Xenopus laevis at 100 μM. Of the 4-APIs, the 4-(1,1'-biphenylyl) analog was the most potent antagonist with IC₅₀s of 7.1 and 9.9 μM against M. domestica and S. litura RDL receptors, respectively. This analog also showed a certain insecticidal activity against S. litura larvae, with >75% mortality at 100 μg/g diet. Molecular docking studies with a M. domestica iGABAR model indicated that the π-π stacking interactions formed between the pyridinyl ring and Y252 and between the 4-substituted aromatic group and Y107 might be important for antagonism by the 4-(1,1'-biphenylyl) analog. Our studies provide important information for designing novel iGABAR antagonists and suggest that the 4-APIs acting on iGABARs are promising insecticide leads for further studies.

Competitive chrodrimanin B interactions with rat brain GABAA receptors revealed by radioligand binding assays

Meroterpenoid compounds chrodrimanins produced by Talaromyces sp. YO-2 have been shown to act as competitive antagonists of silkworm larval GABA_A receptors using electrophysiology, yet no further evidence has been provided to support such an action. We have investigated the actions of chrodrimanin B on rat brain GABA_A receptors by binding assays with non-competitive ligand of GABA_A receptors [³H]EBOB and competitive ligands [³H]gabazine and [³H]muscimol. Chrodrimanin B did not significantly affect the binding of [³H]EBOB while reducing the binding of [³H]gabazine and [³H]muscimol to the rat membrane preparations. Chrodrimanin B increased the dissociation constant K_d of [³H]gabazine and [³H]muscimol without significantly affecting the maximum binding, pointing to competitive interactions of chrodrimanin B with rat GABA_A receptors in support of our previous observation that the compound acts as a competitive antagonist on the silkworm larval GABA receptor.

Characterisation of thymol effects on RDL receptors from the bee parasite Varroa destructor

A major contributor to bee colony decline is infestation with its most devastating pest, the mite Varroa destructor. To control these mites, thymol is often used, although how it achieves this is not understood. One well-documented action of thymol is to modulate GABA-activated ion channels, which includes insect RDL receptors, a known insecticidal target. Here we have cloned two Varroa RDL subunits, one of which is similar to the canonical RDL subunit, while the other has some differences in M4, and, to a lesser extent, M2 and its binding site loops. Expression of this unusual RDL receptor in Xenopus oocytes reveals GABA-activated receptors, with an EC50 of 56 μM. In contrast to canonical RDL receptors, thymol does not enhance GABA-elicited responses in this receptor, and concentration response curves reveal a decrease in GABA Imax in its presence; this decrease is not seen when similar data are obtained from Apis RDL receptors. We conclude that an M2 T6'M substitution is primarily responsible for the different thymol effects, and suggest that understanding how and where thymol acts could assist in the design of novel bee-friendly miticides.

Distinct roles of two RDL GABA receptors in fipronil action in the diamondback moth (Plutella xylostella)

The phenylpyrazole insecticide fipronil blocks resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors in insects, thereby impairing inhibitory neurotransmission. Some insect species, such as the diamondback moth (Plutella xylostella), possess more than one Rdl gene. The involvement of multiple Rdls in fipronil toxicity and resistance remains largely unknown. In this study, we investigated the roles of two Rdl genes, PxRdl1 and PxRdl2, in P. xylostella fipronil action. In Xenopus oocytes, PxRDL2 receptors were 40 times less sensitive to fipronil than PxRDL1. PxRDL2 receptors were also less sensitive to GABA compared with PxRDL1. Knockout of the fipronil-sensitive PxRdl1 reduced the fipronil potency 10-fold, whereas knockout of the fipronil-resistant PxRdl2 enhanced the fipronil potency 4.4-fold. Furthermore, in two fipronil-resistant diamondback moth field populations, PxRdl2 expression was elevated 3.7- and 4.1-fold compared with a susceptible strain, whereas PxRdl1 expression was comparable among the resistant and susceptible strains. Collectively, our results indicate antagonistic effects of PxRDL1 and PxRDL2 on fipronil action in vivo and suggest that enhanced expression of fipronil-resistant PxRdl2 is potentially a new mechanism of fipronil resistance in insects.

Functional Characteristics of the Lepidopteran Ionotropic GABA Receptor 8916 Subunit Interacting with the LCCH3 or the RDL Subunit

The ionotropic γ-aminobutyric acid (iGABA) receptor is commonly considered as a fast inhibitory channel and is an important insecticide target. Since 1990, RDL, LCCH3, and GRD have been successively isolated and found to be potential subunits of the insect iGABA receptor. More recently, one orphan gene named 8916 was found and considered to be another potential iGABA receptor subunit according to its amino acid sequence. However, little information about 8916 has been reported. Here, the 8916 subunit from Chilo suppressalis was studied to determine whether it can form part of a functional iGABA receptor by co-expressing this subunit with CsRDL1 or CsLCCH3 in the Xenopus oocyte system. Cs8916 or CsLCCH3 did not form functional ion channels when expressed alone. However, Cs8916 was able to form heteromeric ion channels when expressed with either CsLCCH3 or CsRDL1. The recombinant heteromeric Cs8916/LCCH3 channel was a cation-selective channel, which was sensitive to GABA or β-alanine. The current of the Cs8916/LCCH3 channel was inhibited by dieldrin, endosulfan, fipronil, or ethiprole. In contrast, fluralaner, broflanilide, and avermectin showed little effect on the Cs8916/LCCH3 channel (IC₅₀s > 10 000 nM). The Cs8916/RDL1 channel was sensitive to GABA, but was significantly different in EC₅₀ and I_max for GABA to those of homomeric CsRDL1. Fluralaner, fipronil, or dieldrin showed antagonistic actions on Cs8916/RDL1. In conclusion, Cs8916 is a potential iGABA receptor subunit, which can interact with CsLCCH3 to generate a cation-selective channel that is sensitive to several insecticides. Also, as Cs8916/RDL1 has a higher EC₅₀ than homomeric CsRDL1, Cs8916 may affect the physiological functions of CsRDL1 and therefore play a role in fine-tuning GABAergic signaling.

TDP-43 regulates GAD1 mRNA splicing and GABA signaling in Drosophila CNS

Alterations in the function of the RNA-binding protein TDP-43 are largely associated with the pathogenesis of amyotrophic lateral sclerosis (ALS), a devastating disease of the human motor system that leads to motoneurons degeneration and reduced life expectancy by molecular mechanisms not well known. In our previous work, we found that the expression levels of the glutamic acid decarboxylase enzyme (GAD1), responsible for converting glutamate to γ-aminobutyric acid (GABA), were downregulated in TBPH-null flies and motoneurons derived from ALS patients carrying mutations in TDP-43, suggesting that defects in the regulation of GAD1 may lead to neurodegeneration by affecting neurotransmitter balance. In this study, we observed that TBPH was required for the regulation of GAD1 pre-mRNA splicing and the levels of GABA in the Drosophila central nervous system (CNS). Interestingly, we discovered that pharmacological treatments aimed to potentiate GABA neurotransmission were able to revert locomotion deficiencies in TBPH-minus flies, revealing novel mechanisms and therapeutic strategies in ALS.

Application of computational methods in the analysis of pesticide target-site and resistance mechanisms

Meta-diamide insecticides including broflanilide have a high insecticidal activity by acting on RDL GABA receptors. Both membrane potential assays and docking studies suggest that the target site of meta-diamides is different from that of conventional noncompetitive inhibitors, such as fipronil. In fact, meta-diamides are effective against cyclodiene- and fipronil-resistant pests that carry target-site mutations. Dinotefuran uniquely possesses a tetrahydrofuran ring, whereas other neonicotinoids possess aromatic rings. Moreover, dinotefuran has been reported to be effective against imidacloprid-resistant strains. A docking study predicted the weak binding of dinotefuran to cytochrome P450s which are associated with imidacloprid resistance. Metabolic assays revealed that dinotefuran was not metabolized by these cytochrome P450s. These findings suggest that the lack of metabolic activity of P450s against dinotefuran causes a low level of cross-resistance.

Mechanisms underlying the selectivity of meta-diamides between insect resistance to dieldrin (RDL) and human γ-aminobutyric acid (GABA) and glycine receptors

BACKGROUND

Meta-diamides [3-benzamido-N-(4-(perfluoropropan-2-yl)phenyl)benzamides] show high insecticide activity by acting as antagonists to the insect resistance to dieldrin (RDL) γ-aminobutyric acid (GABA) receptors. In contrast, low-level antagonist activities of meta-diamides have been demonstrated against the human GABA type A receptor (GABA_A R) α1β2γ2S, mammalian GABA_A R α1β3γ2S, and the human glycine receptor (GlyR) α1β. Glycine residue 336 in the membrane-spanning region M3 of the Drosophila RDL GABA receptor is essential for its high sensitivity to meta-diamide 7, [3-benzamido-N-(2-bromo-4-(perfluoropropan-2-yl)-6-(trifluoromethyl)phenyl)-2-fluorobenzamide].

RESULTS

We examined the effects of an equivalent mutation (M288G) in spontaneously opened human GABA_A R β3 homomers using membrane potential assay. Picrotoxin and fipronil blocked spontaneously opened human GABA_A Rs β3 and β3-M286G in a concentration-dependent manner. In contrast, meta-diamide 7 did not block spontaneously opened GABA_A R β3 homomers, although meta-diamide 7 blocked spontaneously opened GABA_A R β3-M286G homomers. In addition, inhibitory potency of meta-diamide 7 for GABA-induced membrane potential change in cells expressing GABA_A R α1β3-M286G was much higher than that in cells expressing GABA_A R α1β3. In the same way, the equivalent mutation (A288G) in GlyR α1 increased the inhibitory potency of meta-diamide 7 for GlyRs α1 and α1β.

CONCLUSION

Studies substituting an equivalent mutation (M288G) in spontaneously opening human GABA_A R β3 homomers and human GABA_A Rs α1β3 heteromers suggest that M286 in human GABA_A R β3 is important for the low sensitivity to meta-diamide 7. In this study, we summarize the mechanisms underlying the selectivity of meta-diamides between insect RDL and human GABA and glycine receptors. © 2020 Society of Chemical Industry.

Change the channel: CysLoop receptor antagonists from nature

Vertebrate and invertebrate ligand-gated ion channels (LGICs) exhibit significant structural homology and often share ligands. As a result, ligands with activity against one class can be brought to bear against another, including for development as insecticides. Receptor selectivity, metabolism and distribution must then be optimized using chemical synthesis. Here we review natural products (NPs) that ligate and inhibit the Cys-loop family of LGICs, which benefit from the unique physicochemical properties of natural product space but often present a high synthetic burden. Recent advances in chemical synthesis, however, have opened practical entries into these complex structures, several of which are highlighted. © 2020 Society of Chemical Industry.

Important amino acids for function of the insect Rdl GABA receptor

BACKGROUND

Insect Rdl GABA receptor is an important insecticide target. To design a novel insecticide, studies on the structures of homologous pentameric ligand-gated ion channels provide information about important amino acids that are necessary for the function of insect Rdl GABA receptors.

RESULTS

L9'A, T12'A, T13'A, T13'S, M15'S, and M15'N mutations in the Drosophila Rdl GABA receptor subunit caused the protein to spontaneously adopt the open state conformation. In contrast, the S16'A, S16'T, S17'A, and S17'H mutant homomers showed the same levels of agonist and antagonist sensitivity as the wild-type receptor. The G336M mutation in the Drosophila Rdl GABA receptor abolished the agonist activities of ivermectin and milbemectin, but the F339M mutation did not. Additionally, the F339M mutation caused spontaneous opening of the receptor. In the Drosophila Rdl model, the hydrophobic girdle plays an important role in stabilization of the closed state. Mutations which decrease hydrophobic interactions resulted in spontaneous opening, supporting the importance of the hydrophobic girdle for keeping the channel closed. Through a mutational study of transmembrane 3 (TM3) cytoplasmic domain and Rdl GABA receptor modeling, hydrophobic interactions between TM3 and TM4 and intersubunit interaction were demonstrated to be important for channel gating. Alternatively, the intrasubunit interaction between TM2 and TM3 domains were less important for channel gating in case of Drosophila Rdl GABA receptor.

CONCLUSION

This study demonstrates important amino acids critical to the function of the Drosophila Rdl GABA receptor based on the mutational studies and Drosophila Rdl GABA receptor modeling approach. © 2020 Society of Chemical Industry.

Resistance to Fipronil in the Common Bed Bug (Hemiptera: Cimicidae)

Cimex lectularius L. populations have been documented worldwide to be resistant to pyrethroids and neonicotinoids, insecticides that have been widely used to control bed bugs. There is an urgent need to discover new active ingredients with different modes of action to control bed bug populations. Fipronil, a phenylpyrazole that targets the GABA receptor, has been shown to be highly effective on bed bugs. However, because fipronil shares the same target site with dieldrin, we investigated the potential of fipronil resistance in bed bugs. Resistance ratios in eight North American populations and one European population ranged from 1.4- to >985-fold, with highly resistant populations on both continents. We evaluated metabolic resistance mechanisms mediated by cytochrome P450s, esterases, carboxylesterases, and glutathione S-transferases using synergists and a combination of synergists. All four detoxification enzyme classes play significant but variable roles in bed bug resistance to fipronil. Suppression of P450s and esterases with synergists eliminated resistance to fipronil in highly resistant bed bugs. Target-site insensitivity was evaluated by sequencing a fragment of the Rdl gene to detect the A302S mutation, known to confer resistance to dieldrin and fipronil in other species. All nine populations were homozygous for the wild-type genotype (susceptible phenotype). Highly resistant populations were also highly resistant to deltamethrin, suggesting that metabolic enzymes that are responsible for pyrethroid detoxification might also metabolize fipronil. It is imperative to understand the origins of fipronil resistance in the development or adoption of new active ingredients and implementation of integrated pest management programs.

Agonist and antagonist properties of an insect GABA-gated chloride channel (RDL) are influenced by heterologous expression conditions

Pentameric ligand-gated ion channels (pLGICs) activated by the inhibitory neurotransmitter γ-aminobutyric acid (GABA) are expressed widely in both vertebrate and invertebrate species. One of the best characterised insect GABA-gated chloride channels is RDL, an abbreviation of ‘resistance to dieldrin’, that was originally identified by genetic screening in Drosophila melanogaster. Here we have cloned the analogous gene from the bumblebee Bombus terrestris audax (BtRDL) and examined its pharmacological properties by functional expression in Xenopus oocytes. Somewhat unexpectedly, the sensitivity of BtRDL to GABA, as measured by its apparent affinity (EC₅₀), was influenced by heterologous expression conditions. This phenomenon was observed in response to alterations in the amount of cRNA injected; the length of time that oocytes were incubated before functional analysis; and by the presence or absence of a 3’ untranslated region. In contrast, similar changes in expression conditions were not associated with changes in apparent affinity with RDL cloned from D. melanogaster (DmRDL). Changes in apparent affinity with BtRDL were also observed following co-expression of a chaperone protein (NACHO). Similar changes in apparent affinity were observed with an allosteric agonist (propofol) and a non-competitive antagonist (picrotoxinin), indicating that expression-depended changes are not restricted to the orthosteric agonist binding site. Interestingly, instances of expression-dependent changes in apparent affinity have been reported previously for vertebrate glycine receptors, which are also members of the pLGIC super-family. Our observations with BtRDL are consistent with previous data obtained with vertebrate glycine receptors and indicates that agonist and antagonist apparent affinity can be influenced by the level of functional expression in a variety of pLGICs.

Potential Roles for Gamma-Aminobutyric Acid Signaling in Bacterial Communities

It is now established that the gut microbiome influences human neurology and behavior, and vice versa. Distinct mechanisms underlying this bidirectional communication pathway, termed the gut-brain axis, are becoming increasingly uncovered. This review summarizes recent interkingdom signaling research focused on gamma-aminobutyric acid (GABA), a human neurotransmitter and ubiquitous signaling molecule found in bacteria, fungi, plants, invertebrates, and mammals. We detail how GABAergic signaling has been shown to be a crucial component of the gut-brain axis. We further describe how GABA is also being found to mediate interkingdom signaling between algae and invertebrates, plants and invertebrates, and plants and bacteria. Based on these emerging results, we argue that obtaining a complete understanding of GABA-mediated communication in the gut-brain axis will involve deciphering the role of GABA signaling and metabolism within bacterial communities themselves.

Ion channels and G protein-coupled receptors as targets for invertebrate pest control: from past challenges to practical insecticides

In the late 1970s, we discovered that toxic bicyclic phosphates inhibit the generation of miniature inhibitory junction potentials, implying their antagonism of γ-aminobutyric acid (GABA) receptors (GABARs; GABA-gated chloride channels). This unique mode of action provided a strong incentive for our research on GABARs in later years. Furthermore, minor structural changes conferred insect GABAR selectivity to this class of compounds, convincing us of the possibility of GABARs as targets for insecticides. Forty years later, third-generation insecticides acting as allosteric modulator antagonists at a distinctive site of action in insect GABARs were developed. G protein-coupled receptors (GPCRs) are also promising targets for pest control. We characterized phenolamine receptors functionally and pharmacologically. Of the tested receptors, β-adrenergic-like octopamine receptors were revealed to be the most sensitive to the acaricide/insecticide amitraz. Given the presence of multiple sites of action, ion channels and GPCRs remain potential targets for invertebrate pest control.

Exploring the Interaction Mechanism of Desmethyl-broflanilide in Insect GABA Receptors and Screening Potential Antagonists by In Silico Simulations

Broflanilide, a novel insecticide, is classified as a negative allosteric modulator (NAM) of insect γ-aminobutyric acid (GABA) receptors (GABARs) as desmethyl-broflanilide (DMBF) allosterically inhibits the GABA-induced responses. The G277M mutation of the Drosophila melanogaster GABAR subunit has been reported to abolish the inhibitory activity of DMBF. The binding mode of DMBF in insect GABARs needs to be clarified to understand the underlying mechanism of this mutation and to develop novel, efficient NAMs of insect GABARs. Here, we found that a hydrogen bond formed between DMBF and G277 of the D. melanogaster GABAR model might be the key interaction for the antagonism of DMBF by in silico simulations. The volume increase induced by the G277M mutation blocks the entrance of the binding pocket, making it difficult for DMBF to enter the binding pocket and thereby decreasing its activity. The following virtual screening and bioassay results identified a novel NAM candidate of insect GABARs. Overall, we reported a possible binding mode of DMBF in insect GABARs and proposed the insensitivity mechanism of the G277M mutant GABAR to DMBF using molecular simulations. The identified NAM candidates might provide more alternatives or potentials for the design of GABAR-targeting insecticides.

Reverse genetics reveals contrary effects of two Rdl-homologous GABA receptors of Helicoverpa armigera on the toxicity of cyclodiene insecticides

The resistance to dieldrin gene (Rdl) encodes a subunit of the insect γ-amino butyric acid (GABA) receptor, and the encoded Rdl subunit is a major target site for cyclodiene and phenylpyrazole insecticides. Since the substitution of a single amino acid (Ala to Ser/Gly at position 302) of the Drosophila melanogaster Rdl gene was first identified to confer high level resistance to dieldrin, mutations at the equivalent positions have been reported to confer resistance to dieldrin and/or fipronil in a wide range of different insects. In the cotton bollworm Helicoverpa armigera, there are two Rdl homologs (HaRdl-1 and HaRdl-2) in close proximity on the Z chromosome, which as wild-type sequences, encode alanine and serine respectively at amino acid position 302. In the present study, we used the CRISPR/Cas9 gene editing approach to knock out HaRdl-1 and HaRdl-2 and establish two homozygous knockout strains (ΔRdl-1 and ΔRdl-2). The ΔRdl-1 strain showed low levels of resistance (8.0- to 9.3-fold) to three cyclodiene insecticides (endosulfan, aldrin and dieldrin) compared with the background SCD strain. In contrast, toxicity of the three cyclodiene insecticides to the ΔRdl-2 strain increased significantly (3.6- to 6.3-fold) when compared with the SCD strain. Genetic analysis indicated the obtained resistance to endosulfan and dieldrin in the ΔRdl-1 strain was sex-linked, which is consistent with the fact that HaRdl-1 locus is located on the Z chromosome. The above results demonstrate that both HaRdl-1 and HaRdl-2 are important determinants for the susceptibility of H. armigera SCD strain to the three cyclodiene insecticides, but have opposite effects. It was also found that HaRdl-1 and HaRdl-2 are involved, to some extent, in mediating sensitivity of H. armigera to avermectin and fipronil respectively. We speculate that the HaRdl-1 and HaRdl-2 subunits have different pharmacological properties, which contribute to the differential sensitivities of H. armigera to the tested cyclodienes and other insecticides.

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.

Heterogeneous expression of GABA receptor-like subunits LCCH3 and GRD reveals functional diversity of GABA receptors in the honeybee Apis mellifera

BACKGROUND AND PURPOSE

Despite a growing awareness, annual losses of honeybee colonies worldwide continue to reach threatening levels for food safety and global biodiversity. Among the biotic and abiotic stresses probably responsible for these losses, pesticides, including those targeting ionotropic GABA receptors, are one of the major drivers. Most insect genomes include the ionotropic GABA receptor subunit gene, Rdl, and two GABA-like receptor subunit genes, Lcch3 and Grd. Most studies have focused on Rdl which forms homomeric GABA-gated chloride channels, and a complete analysis of all possible molecular combinations of GABA receptors is still lacking.

EXPERIMENTAL APPROACH

We cloned the Rdl, Grd, and Lcch3 genes of Apis mellifera and systematically characterized the resulting GABA receptors expressed in Xenopus oocytes, using electrophysiological assays, fluorescence microscopy and co-immunoprecipitation techniques.

KEY RESULTS

The cloned subunits interacted with each other, forming GABA-gated heteromeric channels with particular properties. Strikingly, these heteromers were always more sensitive than AmRDL homomer to all the pharmacological agents tested. In particular, when expressed together, Grd and Lcch3 form a non-selective cationic channel that opens at low concentrations of GABA and with sensitivity to insecticides similar to that of homomeric Rdl channels.

CONCLUSION AND IMPLICATIONS

For off-target species like the honeybee, chronic sublethal exposure to insecticides constitutes a major threat. At these concentration ranges, homomeric RDL receptors may not be the most pertinent target to study and other ionotropic GABA receptor subtypes should be considered in order to understand more fully the molecular mechanisms of sublethal toxicity to insecticides.

Potential of Competitive Antagonists of Insect Ionotropic γ-Aminobutyric Acid Receptors as Insecticides

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) represent an important insecticide target. Currently used GABAR-targeting insecticides are non-competitive antagonists (NCAs) of these receptors. Recent studies have demonstrated that competitive antagonists (CAs) of GABARs have functions of inhibiting insect GABARs similar to NCAs and that they also exhibit insecticidal activity. CAs have different binding sites and different mechanisms of action compared to those of NCAs. Therefore, GABAR CAs should have the potential to be developed into novel insecticides, which could be used to overcome the developed resistance of insect pests to conventional NCA insecticides. Although research on insect GABAR CAs has lagged behind that on mammalian GABAR CAs, research on the CAs of insect ionotropic GABARs has made great progress in recent years, and several series of heterocyclic compounds, such as 3-isoxazolols and 6-iminopyridazines, have been identified as insect GABAR CAs. In this review, we briefly summarize the design strategies, structures, and biological activities of the novel GABAR CAs that have been found in the past decade. Updated information about GABAR CAs may benefit the design and development of novel GABAR-targeting insecticides.

Sigillins from Springtails Are Potent Natural Insecticides

Sigillins are highly chlorinated natural products from the springtail Ceratophysella sigillata (Collembola) that are used to deter arthropod predators. We report here the isolation and structure elucidation of sigillin F, a hydrogenated benzopyranone compound bearing two trichloromethyl groups, and the synthesis of trideoxysigillin (8), a non-natural compound representing the basic scaffold of the sigillins. Sigillins A and F showed insecticidal activity toward various insects, similar to the commercial insecticide imidacloprid, whereas 8 was inactive. The highest mortality was observed for the aphids Megoura viciae and Myzus persicae, but other insect species were also susceptible. Sigillins act as noncompetitive antagonists of the GABA receptor. This mode of action is identical to that of known insecticides with high chlorine content such as dieldrin or endosulfan. The high content of sigillins in C. sigillata, more than 4 mM in concentration, indicates self-resistance. Strikingly, the Collembola and humans have both arrived at the same target with related types of compounds to combat insects.

Toxicity of Fipronil and Emamectin Benzoate and Their Mixtures Against Cotton Leafworm, Spodoptera littoralis (Lepidoptera: Noctuidae) With Relation to GABA Content

Fipronil and emamectin benzoate (EMB) are effective insecticides for controlling cotton leafworm, Spodoptera littoralis. Fipronil works by blocking gamma-amino butyric acid (GABA) gated chloride. In contrast, EMB is activating GABA transporters. The objectives of our study were to assess relative toxicity of the technical fipronil and EMB alone and in mixture against S. littoralis. In addition, the GABA content was simultaneously determined using HPLC. Technical fipronil and EMB and their mixtures were applied topically to the fourth-instar larvae, and their LD50 values were estimated after 48 h. Results demonstrated that the LD50 for EMB applied alone was 0.751 ng/larva which was much less than for fipronil 7.271 ng/larva. Each of the two insecticides alone showed a significant decrease in GABA content at LD10, LD25, and LD50 doses, while their mixtures induced GABA levels. The highest potentiation was observed when both insecticides were in a mixture at the ratio of LD10:LD10 which was associated with higher increase in GABA levels. Moreover, the weight of the alive larvae was less than that was in the untreated control. However, all mixtures exhibited potentiation effect, except for the mixture of fipronil at LD50 with EMB at LD10, LD25, and LD50 that had antagonistic effect correlated with the lowest decrease in GABA level. Results suggested that lower doses of both insecticides in a binary mixture had potentiation effect against S. littoralis. This mixture could be used in combination as field application for successful and effective control of S. littoralis and would also help in managing insecticide resistance.

Identification of The Fipronil Resistance Associated Mutations in Nilaparvata lugens GABA Receptors by Molecular Modeling

Fipronil, as the first commercialized member of phenylpyrazole insecticides, has been widely used to control planthoppers in China due to its high insecticidal activity and low toxicity to mammals. However, insects have developed resistance to phenylpyrazoles after their long-term use. The resistance mechanism of insects to fipronil has not been well identified, which limited the development of phenylpyrazole insecticides. In the present study, we aimed to elucidate the related fipronil-resistance mechanism in N. lugens GABA receptors by homology modeling, molecular docking, and molecular dynamics. The results indicated that fipronil showed the weakest interaction with the mutant (R0'Q + A2'S) GABA receptors, which is consistent with the experimental study. The binding poses of fipronil were found to be changed when mutations were conducted. These findings verified the novel fipronil-resistance mechanism in silico and provide important information for the design of novel GABAR-targeting insecticides.

RDL A301S alone does not confer high levels of resistance to cyclodiene organochlorine or phenyl pyrazole insecticides in Plutella xylostella

Mutations in the GABA-gated chloride channel are associated with resistance to cyclodiene organochlorine and phenyl pyrazole insecticides. The best characterised of these is A301S, which was initially identified in a Dieldrin resistant strain of Drosophila melanogaster. The orthologous mutation has been found in a variety of different crop pests including the diamond back moth Plutella xylostella. However, the contribution of this mutation to resistance in this species remains unclear. We have used the CRISPR/Cas9 system in order to edit Plutella xylostella PxGABARalpha1 to Serine at the 301 orthologous position (282 in PxGABARalpha1) in an insecticide sensitive strain isolated from Vero Beach (VB) USA. In this edited line, no high level of resistance is conferred to Dieldrin, Endosulfan or Fipronil, rather only a subtle shift in sensitivity which could not confer commercially important resistance. We conclude that the high level of commercial resistance to cyclodiene organochlorine and phenyl pyrazole insecticides observed in some field isolates of Plutella xylostella cannot arise from A282S in PxGABARalpha1 alone.

Synthesis of the Tricyclic Picrotoxane Motif by an Oxidative Cascade Cyclization

An oxidative cascade cyclization of β-keto esters has been developed for the construction of the tricyclic picrotoxane motif in a single step, and DFT calculations suggested a possible cationic cyclization mechanism. This cascade cyclization can be operated on a 20 g scale to obtain a 77% total yield of the tricyclic products, which in turn can be converted to versatile intermediates for further elaboration to picrotoxanes and their structurally related compounds.

Functional integrity of honeybee (Apis mellifera L.) resistant to dieldrin γ-aminobutyric acid receptor channels conjugated with three fluorescent proteins

To generate an efficient tool used in Xenopus oocyte expression for in situ investigation of channel receptor expression, distribution and function, the C-terminus of the honeybee (Apis mellifera L.) resistant to dieldrin (RDL) subunit was fused with *FP, including monomeric red, enhanced yellow or enhanced green fluorescent protein (referred to as mRFP, EYFP and EGFP, respectively). In the present study, all fused *FP-AmRDLs could be visualized using fluorescence and laser confocal microscopy in cRNA-injected oocytes. Fluorescence was distributed isotropically in the cellular membrane. The potencies of the agonist γ-aminobutyric acid (GABA), but not β-alanine, and the test antagonists (fipronil, flufiprole, dieldrin, α-endosulfan, bifenazate and avermectin B1a) in the *FP-AmRDL receptor did not significantly differ from that of the untagged receptor with two-electrode voltage clamp detection. The half maximal effective concentrations (EC50 s) of GABA in AmRDL, EGFP-AmRDL, EYFP-AmRDL and mRFP-AmRDL receptors were 11.98, 12.61, 18.92 and 22.11 μM, respectively, and those of β-alanine were 651.6, 629.6, 1643.0 and 2146.0 μM, respectively. Inhibition percentages of test antagonists against *FP-AmRDL and AmRDL were not significantly different from each other. Overall, the consistency in functional properties between *FP-AmRDL and AmRDL receptors makes pGH19-*FP a promising tool for further in situ investigation of GABA receptors.

Spatiotemporally different expression of alternatively spliced GABA receptor subunit transcripts in the housefly Musca domestica

Insect γ-aminobutyric acid (GABA) receptors are important as major inhibitory neurotransmitter receptors and targets for insecticides. The housefly GABA receptor subunit gene MdRdl is alternatively spliced at exons 3 (a or b) and 6 (c or d) to yield the variants of ac, ad, bc, and bd combinations. In the present study, the expression of the MdRdl transcript in the body parts and in the developmental stages of the housefly Musca domestica was examined by quantitative polymerase chain reaction using specific primers that amplify the combinations of alternative exons. The results indicated that the transcripts of MdRdl, including four combinations, were highly expressed in the adult stage. MdRdl_bd was the most abundant in the adult head. The expression pattern did not change in the adult stage over 7 days after eclosion. The expression level of the MdRdl _bd transcript in the female head was similar to that of the male head. In contrast, MdRdl _bc was the predominant transcript in the pupal head and the adult leg. Because the homomeric Rdl _bc GABA receptor has a high affinity for GABA, our results provide grounds for designing agonist or competitive-antagonist insecticides that target the orthosteric site of the GABA receptor containing this Rdl variant.

Distinctions among electroconvulsion- and proconvulsant-induced seizure discharges and native motor patterns during flight and grooming: quantitative spike pattern analysis in Drosophila flight muscles

In Drosophila, high-frequency electrical stimulation across the brain triggers a highly stereotypic repertoire of spasms. These electroconvulsive seizures (ECS) manifest as distinctive spiking discharges across the nervous system and can be stably assessed throughout the seizure repertoire in the large indirect flight muscles dorsal longitudinal muscles (DLMs) to characterize modifications in seizure-prone mutants. However, the relationships between ECS-spike patterns and native motor programs, including flight and grooming, are not known and their similarities and distinctions remain to be characterized. We employed quantitative spike pattern analyses for the three motor patterns including: (1) overall firing frequency, (2) spike timing between contralateral fibers, and (3) short-term variability in spike interval regularity (CV₂) and instantaneous firing frequency (ISI^-1). This base-line information from wild-type (WT) flies facilitated quantitative characterization of mutational effects of major neurotransmitter systems: excitatory cholinergic (Cha), inhibitory GABAergic (Rdl) and electrical (ShakB) synaptic transmission. The results provide an initial glimpse on the vulnerability of individual motor patterns to different perturbations. We found marked alterations of ECS discharge spike patterns in terms of either seizure threshold, spike frequency or spiking regularity. In contrast, no gross alterations during grooming and a small but noticeable reduction of firing frequency during Rdl mutant flight were found, suggesting a role for GABAergic modulation of flight motor programs. Picrotoxin (PTX), a known pro-convulsant that inhibits GABA_A receptors, induced DLM spike patterns that displayed some features, e.g. left-right coordination and ISI^-1 range, that could be found in flight or grooming, but distinct from ECS discharges. These quantitative techniques may be employed to reveal overlooked relationships among aberrant motor patterns as well as their links to native motor programs.

Identification of the ionotropic GABA receptor-like subunits from the striped stem borer, Chilo suppressalis Walker (Lepidoptera: Pyralidae)

Ionotropic γ-aminobutyric acid (GABA) receptors (GABARs) mediate rapid inhibitory neurotransmission in both vertebrates and invertebrates, and are important molecular targets of insecticides. However, components of insect GABARs remain elusive. In addition to CsRDL1 and CsRDL2, the complementary DNAs (cDNAs) of another two GABA receptor-like subunits, CsLCCH3 and Cs8916, were identified from the rice striped stem borer, Chilo suppressalis Walker in the present study. Both CsLCCH3 and Cs8916 subunits shared common structural features, such as a highly-conserved Cys-loop structure, six distinct regions involved in ligand binding (loops A-F), and four transmembrane domains (TM 1-4). Transcript analysis demonstrated that the relative mRNA expression levels of both CsLCCH3 and Cs8916 subunits were the highest in the ventral nerve cord. Regarding developmental stage, transcript levels of both subunits were highest in eggs. Injections of double-stranded RNAs (dsRNAs), including dsRDL1, dsRDL2, dsLCCH3, or ds8916, significantly reduced mRNA abundance after 24 and 48 h. However, no observable effects on the development of C. suppressalis were observed. Injection of dsRDL1 or dsRDL2 did significantly reduce the mortality of C. suppressalis treated with fluralaner. Our results indicated that CsRDLs mediated the susceptibility of C. suppressalis to fluralaner, whereas CsLCCH3 and CsL8916 did not. The current investigation enhances our knowledge of Lepidopteran GABARs and offers a molecular basis for the development of novel insecticides to control C. suppressalis.