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

Knockdown of a Nicotinic Acetylcholine Receptor Subunit Gene Bdorβ1 Decreases Susceptibility to Oxa-Bridged trans- instead of cis-Nitromethylene Neonicotinoid Insecticides in Bactrocera dorsalis

Nicotinic acetylcholine receptors (nAChRs) are ligand-gated ion channels that mediate the fast action of acetylcholine in synaptic cholinergic transmissions. Insect nAChRs are the target of several classes of insecticides. Here, the full-length cDNA encoding a nAChR beta1 subunit (Bdorβ1) was identified and characterized from a destructive pest, Bactrocera dorsalis. The amino acid sequence of Bdorβ1 shows high identities to other insect nAChRs β1 subunits. Double injection of dsBdorβ1 reduced the expression of Bdorβ1 and in turn significantly decreased susceptibility to oxa-bridged trans- instead of cis-nitromethylene neonicotinoids. Our results support the involvement of Bdorβ1 in the susceptibility of B. dorsalis to oxa-bridged trans- instead of cis-nitromethylene neonicotinoids and imply that these two classes of neonicotinoids might be acting at different nAChR subtypes.

The actions of neonicotinoid insecticides on nicotinic acetylcholine subunits Ldα1 and Ldα8 of Leptinotarsa decemlineata and assembled receptors

The insect nicotinic acetylcholine receptor (nAChR) is a pentameric channel protein and also a target for neonicotinoids. There are few reported studies on the molecular interactions of Leptinotarsa decemlineata nAChRs with neonicotinoids. In this study, we analyzed the response of acetylcholine and neonicotinoids (thiamethoxam [TMX], imidacloprid [IMI], and clothianidin [CLO]) on hybrid receptors constructed by nAChR α1 and α8 subunits of L. decemlineata (Ldα1 and Ldα8) co-expressed with rat β2 subunit (rβ2) at different capped RNA (cRNA) ratios in Xenopus oocytes. In addition, we evaluated the expression changes of Ldα1 and Ldα8 after median lethal dose of TMX treatment for 72 h by quantitative polymerase chain reaction (qPCR). The resulting functional nAChRs Ldα1/rβ2 and Ldα1/Ldα8/rβ2 showed different pharmacological characteristics. The neonicotinoids tested showed lower agonist affinity on Ldα1/Ldα8/rβ2 compared to Ldα1/rβ2 at same ratios of subunit cRNAs. The sensitivities of neonicotinoids tested for Ldα1/rβ2 and Ldα1/Ldα8/rβ2 at cRNA ratios of 5:1, 1:1 and 5:5:1, 1:1:1, respectively, were lower than those for nAChRs at ratios of 1:5 and 1:1:5, respectively, whereas the values of maximum response (I_max ) varied. For Ldα1/Ldα8/rβ2, a reduction of Lda8 cRNA resulted in increased sensitivity to IMI and decreased sensitivity to TMX. The expression of Ldα1 and Ldα8 significantly decreased in adults by 82.12% and 47.02%, respectively, while Ldα8 was significantly upregulated by 2.44 times in 4th instar larvae after exposure to TMX. We infer that Ldα1 and Ldα8 together play an important role in the sensitivity of L. decemlineata to neonicotinoids.

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.

The Apis mellifera alpha 5 nicotinic acetylcholine receptor subunit expresses as a homomeric receptor that is sensitive to serotonin

Insect nicotinic acetylcholine receptors (nAChRs) are molecular targets of highly effective insecticides such as neonicotinoids. Functional expression of these receptors provides useful insights into their functional and pharmacological properties. Here, we report that the α5 nAChR subunit of the honey bee, Apis mellifera, functionally expresses in Xenopus laevis oocytes, which is the first time a homomeric insect nAChR has been robustly expressed in a heterologous system without the need for chaperone proteins. Using two-electrode voltage-clamp electrophysiology we show that the α5 receptor has low sensitivity to acetylcholine with an EC₅₀ of 2.37 mM. However, serotonin acts as an agonist with a considerably lower EC₅₀ at 119 μM that is also more efficacious than acetylcholine in activating the receptor. Molecular modelling indicates that residues in the complementary binding site may be involved in the selectivity towards serotonin. This is the first report of a ligand-gated ion channel activated by serotonin from an insect and phylogenetic analysis shows that the α5 subunit of A. mellifera and other non-Dipteran insects, including pest species, belong to a distinct subgroup of subunits, which may represent targets for the development of novel classes of insecticides.

NACHO permits functional heterologous expression of an insect homomeric α6 nicotinic acetylcholine receptor

Insect nicotinic acetylcholine receptors (nAChR) are molecular targets of highly effective insecticides. The use of chaperone proteins has been key to successful functional expression of these receptors in heterologous systems, permitting functional and pharmacological studies of insect nAChRs with particular subunit composition. Here, we report the first use of the chaperone protein, NACHO, to enable functional expression of an insect nAChR, the α6 subunit from Apis mellifera, in Xenopus laevis oocytes. This is also the first report of functional expression of a homomeric insect α6 nAChR. Using two-electrode voltage-clamp electrophysiology we show that the acetylcholine EC₅₀ of the α6 receptor is 0.88 μM and that acetylcholine responses are antagonized by α-bungarotoxin. Spinosad showed agonist actions and kept the ion channel open when co-applied with acetylcholine, reinforcing the α6 nAChR subunit as a key molecular target for the spinosyn class of insecticide. The use of NACHO may provide a basis for future expression studies of insect α6 nAChRs, potentially providing a tool for the discovery of novel insecticides.

Ligand-gated ion channels as targets of neuroactive insecticides

The Cys-loop superfamily of ligand-gated ion channels (Cys-loop receptors) is one of the most ubiquitous ion channel families in vertebrates and invertebrates. Despite their ubiquity, they are targeted by several classes of pesticides, including neonicotinoids, phenylpyrazols, and macrolides such as ivermectins. The current commercialized compounds have high target site selectivity, which contributes to the safety of insecticide use. Structural analyses have accelerated progress in this field; notably, the X-ray crystal structures of acetylcholine binding protein and glutamate-gated Cl channels revealed the details of the molecular interactions between insecticides and their targets. Recently, the functional expression of the insect nicotinic acetylcholine receptor (nAChR) has been described, and detailed evaluations using the insect nAChR have emerged. This review discusses the basic concepts and the current insights into the molecular mechanisms of neuroactive insecticides targeting the ligand-gated ion channels, particularly Cys-loop receptors, and presents insights into target-based selectivity, resistance, and future drug design.

Nicotinic acetylcholine receptor modulator insecticides act on diverse receptor subtypes with distinct subunit compositions

Insect nicotinic acetylcholine receptors (nAChRs) are pentameric ligand-gated ion channels mainly expressed in the central nervous system of insects. They are the directed targets of many insecticides, including neonicotinoids, which are the most widely used insecticides in the world. However, the development of resistance in pests and the negative impacts on bee pollinators affect the application of insecticides and have created a demand for alternatives. Thus, it is very important to understand the mode of action of these insecticides, which is not fully understood at the molecular level. In this study, we systematically examined the susceptibility of ten Drosophila melanogaster nAChR subunit mutants to eleven insecticides acting on nAChRs. Our results showed that there are several subtypes of nAChRs with distinct subunit compositions that are responsible for the toxicity of different insecticides. At least three of them are the major molecular targets of seven structurally similar neonicotinoids in vivo. Moreover, spinosyns may act exclusively on the α6 homomeric pentamers but not any other nAChRs. Behavioral assays using thermogenetic tools further confirmed the bioassay results and supported the idea that receptor activation rather than inhibition leads to the insecticidal effects of neonicotinoids. The present findings reveal native nAChR subunit interactions with various insecticides and have important implications for the management of resistance and the development of novel insecticides targeting these important ion channels.

Neonicotinoids and spinosyns account for approximately 24% and 3% of the world market value of insecticides, respectively. However, the negative effects of neonicotinoids on pollinators have led to the development of novel insecticides, such as sulfoxaflor, flupyradifurone and triflumezopyrim. Although all act via insect nicotinic acetylcholine receptors, their modes of action are not fully understood. Our work shows that these insecticides act on diverse receptor subtypes with distinct subunit compositions. This finding could lead to the development of more selective insecticides to control pests with minimal effects on beneficial insects.