Injection of insect membrane in Xenopus oocyte: An original method for the pharmacological characterization of neonicotinoid insecticides

The Xenopus Oocyte: A Tool for Membrane Biology

The Xenopus is a special study model in experimental research [...].

GABAA Receptor Subunit Composition Drives Its Sensitivity to the Insecticide Fipronil

Fipronil (FPN) is a worldwide-used neurotoxic insecticide, targeting, and blocking GABA_A receptors (GABA_ARs). Beyond its efficiency on insect GABA_ARs, FPN causes neurotoxic effects in humans and mammals. Here, we investigated the mode of action of FPN on mammalian α6-containing GABA_ARs to understand its inhibitory effects on GABA-induced currents, as a function of the synaptic or extrasynaptic localization of GABA_ARs. We characterized the effects of FPN by electrophysiology using Xenopus oocytes which were microtransplanted with cerebellum membranes or injected with α6β3, α6β3γ2S (synaptic), and α6β3δ (extrasynaptic) cDNAs. At micromolar concentrations, FPN dose-dependently inhibited cerebellar GABA currents. FPN acts as a non-competitive antagonist on ternary receptors. Surprisingly, the inhibition of GABA-induced currents was partial for extra-synaptic (α6β3δ) and binary (α6β3) receptors, while synaptic α6β3γ2S receptors were fully blocked, indicating that the complementary γ or δ subunit participates in FPN-GABA_AR interaction. FPN unexpectedly behaved as a positive modulator on β3 homopentamers. These data show that FPN action is driven by the subunit composition of GABA_ARs-highlighting the role of the complementary subunit-and thus their localization within a physiological synapse. We built a docking model of FPN on GABA_ARs, which reveals two putative binding sites. This is consistent with a double binding mode of FPN on GABA_ARs, possibly one being of high affinity and the other of low affinity. Physiologically, the γ/δ subunit incorporation drives its inhibitory level and has important significance for its toxicity on the mammalian nervous system, especially in acute exposure.

Partial Agonist Activity of Neonicotinoids on Rat Nicotinic Receptors: Consequences over Epinephrine Secretion and In Vivo Blood Pressure

Neonicotinoid insecticides are nicotine-derived molecules which exert acute neurotoxic effects over the insect central nervous system by activating nicotinic acetylcholine receptors (nAChRs). However, these receptors are also present in the mammalian central and peripheral nervous system, where the effects of neonicotinoids are faintly known. In mammals, cholinergic synapses are crucial for the control of vascular tone, blood pressure and skeletal muscle contraction. We therefore hypothesized that neonicotinoids could affect cholinergic networks in mammals and sought to highlight functional consequences of acute intoxication in rats with sub-lethal concentrations of the highly used acetamiprid (ACE) and clothianidin (CLO). In this view, we characterized their electrophysiological effects on rat α3β4 nAChRs, knowing that it is predominantly expressed in ganglia of the vegetative nervous system and the adrenal medulla, which initiates catecholamine secretion. Both molecules exhibited a weak agonist effect on α3β4 receptors. Accordingly, their influence on epinephrine secretion from rat adrenal glands was also weak at 100 μM, but it was stronger at 500 μM. Challenging ACE or CLO together with nicotine (NIC) ended up with paradoxical effects on secretion. In addition, we measured the rat arterial blood pressure (ABP) in vivo by arterial catheterization. As expected, NIC induced a significant increase in ABP. ACE and CLO did not affect the ABP in the same conditions. However, simultaneous exposure of rats to both NIC and ACE/CLO promoted an increase of ABP and induced a biphasic response. Modeling the interaction of ACE or CLO on α3β4 nAChR is consistent with a binding site located in the agonist pocket of the receptor. We present a transversal experimental approach of mammal intoxication with neonicotinoids at different scales, including in vitro, ex vivo, in vivo and in silico. It paves the way of the acute and chronic toxicity for this class of insecticides on mammalian organisms.

Nicotinic acetylcholine receptors in the synganglion of the tick Ixodes ricinus: Functional characterization using membrane microtransplantation

Nicotinic acetylcholine receptors are an important class of excitatory receptors in the central nervous system of arthropods. In the ticks Ixodes ricinus, the functional and pharmacological properties of nicotinic receptors located in their neurons are still unknown. The objective of this study was to characterize the pharmacological properties of tick nicotinic receptors using membrane microtransplantation in Xenopus laevis oocytes and two-electrodes voltage clamp method. The membranes microtransplanted were extracted from the tick synganglion. We found that oocytes microtransplanted with tick synganglion membranes expressed nicotinic acetylcholine receptor subtypes which were activated by acetylcholine (1 mM) and nicotine (1 mM). Currents induced by pressure application of acetylcholine and nicotine were diminished by 10 nM α-bungarotoxin and methyllycaconitine, suggesting that they expressed two subtypes of nicotinic receptors, α-bungarotoxin-sensitive and -insensitive, respectively. In addition, we found that nicotine receptors expressed in the synganglion membranes were poorly sensitive to the neonicotinoid insecticides clothianidin (CLT), imidacloprid (IMI), acetamiprid (ACE) and thiamethoxam (TMX), in agreement with their lack of activity as acaricides. Interestingly, current amplitudes were strongly potentialized in the presence of 1 μM PNU-120596. CLT was more active as an agonist than IMI, TMX and ACE. Finally, we demonstrated that microtransplantation of purified membrane from the tick synganglion can be a valuable tool for the development and screening of compounds targeting tick nicotinic acetylcholine receptor subtypes.

Nicotinic acetylcholine receptors: Ex-vivo expression of functional, non-hybrid, heteropentameric receptors from a marine arthropod, Lepeophtheirus salmonis

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels mostly located in the post-synaptic membrane of cholinergic synapses. The natural neurotransmitter is acetylcholine, but they are also the direct targets for neonicotinoids, chemicals widely used against ectoparasites, arthropod vectors and agricultural pests. There are significant concerns regarding adverse effects of neonicotinoids on beneficial insects. In arthropods, functional nAChRs made of α subunits have been expressed from Drosophila genes, and hybrid receptors (sometimes also referred to as chimeric receptors) using species-specific α subunits and vertebrate β subunits have been expressed ex-vivo. Arthropod-specific nAChRs made of both α and β subunits from the target species have not been expressed ex-vivo. The aim of the current study was to express such receptors in Xenopus oocytes using only genes from Lepeophtheirus salmonis, to characterize them and study their modulation. Genes encoding α and β subunits of the nAChRs and three ancillary proteins, RIC-3, UNC-50 and UNC-74 were identified in the L. salmonis genome, subjected to RACE-PCR, cloned into an expression vector and the cRNA produced was then injected into Xenopus laevis oocytes. Co-expression of the ancillary proteins was essential for the successful expression of the L. salmonis nAChRs with both α and β subunits. Two functional nAChRs were identified: Lsa-nAChR1 consisting of α1, α2, β1 and β2 subunits, reconstituted to one distinct receptor, while Lsa-nAChR2, consisting of α3, β1 and β2 subunits reconstitutes receptors with two distinct characteristics. Out of seven neonicotinoids tested, six worked as partial agonist of Lsa-nAChR1 while only three did so for Lsa-nAChR2. Four non-neonicotinoid compounds tested had no effect on either of the nAChRs. The study demonstrated that fully functional, non-hybrid nAChRs containing both α and β subunits from an arthropod can be reconstituted ex-vivo by co-expression of essential ancillary proteins. Such models would be valuable for in-depth studies of effects by neonicotinoids and other compounds on target pests, as well as for studies of adverse effects on non-target arthropods.

Nicotinic acetylcholine receptors, nAChRs, respond to the neurotransmitter acetylcholine or drugs like nicotine. These receptors are targets for neonicotinoids, the most commonly used compounds against ectoparasites and agricultural pests. In-depth studies of the function of these channels in arthropods are sparse, as no groups managed to reconstitute functional nAChRs made of both α and β subunits using genes only from the target arthropod in an ex-vivo system. We report the successful assembly of non-hybrid, fully functional nAChRs containing both α and β subunits from a marine arthropod, assembled and expressed in Xenopus laevis oocytes. We identified two possible combinations of α and β subunits producing functional receptors. We found ancillary proteins to be essential for successful expression and assembly of both α and β subunits into a functional receptor. The findings of the present study provide a basis for studying native nAChRs from arthropods, with a switch from hybrid nAChRs to species-specific native nAChRs.

Microinjection of Xenopus Oocytes

Microinjection of Xenopus oocytes has proven to be a valuable tool in a broad array of studies that require expression of DNA or RNA into functional protein. These studies are diverse and range from expression cloning to receptor-ligand interaction to nuclear programming. Oocytes offer a number of advantages for such studies, including their large size (∼1.2 mm in diameter), capacity for translation, and enormous nucleus (0.3-0.4 mm). They are cost effective, easily manipulated, and can be injected in large numbers in a short time period. Oocytes have a large maternal stockpile of all the essential components for transcription and translation. Consequently, the investigator needs only to introduce by microinjection the specific DNA or RNA of interest for synthesis. Oocytes translate virtually any exogenous RNA regardless of source, and the translated proteins are folded, modified, and transported to the correct cellular locations. Here we present procedures for the efficient microinjection of oocytes and their subsequent care.

Concentration-dependent effects of acute and chronic neonicotinoid exposure on the behaviour and development of the nematode Caenorhabditis elegans

BACKGROUND

Neonicotinoid insecticides are under review owing to emerging toxicity to non-target species. Interest has focused on biological pollinators while their effects on other organisms that are key contributors to the ecosystem remain largely unknown. To advance this, we have tested the effects of representatives of three major classes of neonicotinoids, thiacloprid, clothianidin and nitenpyram, on the free-living nematode Caenorhabditis elegans (C. elegans), as a representative of the Nematoda, an ecologically important phylum contributing to biomass.

RESULTS

Concentrations that are several-fold higher than those with effects against target species had limited impact on locomotor function. However, increased potency was observed in a mutant with a hyperpermeable cuticle, which shows that drug access limits the effects of the neonicotinoids in C. elegans. Thiacloprid was most potent (EC₅₀ 714 μm). In addition, it selectively delayed larval development in wild-type worms at 1 mm.

CONCLUSION

C. elegans is less susceptible to neonicotinoids than target species of pest insect. We discuss an approach in which this defined low sensitivity may be exploited by heterologous expression of insect nicotinic acetylcholine receptors from both pest and beneficial insects in transgenic C. elegans with increased cuticle permeability to provide a whole organism assay for species-dependent neonicotinoid effects. © 2017 Society of Chemical Industry.