Amino acids outside of the loops that define the agonist binding site are important for ligand binding to insect nicotinic acetylcholine receptors

Disruption of nicotinic acetylcholine receptor α6 mediated by CRISPR/Cas9 confers resistance to spinosyns in Plutella xylostella

BACKGROUND

The clustered regularly interspaced short palindromic repeats (CRISPR)/Cas9 system provides some advantages over other reverse genetic techniques to investigate the causal relationship between insecticide resistance phenotype and candidate gene. Several studies published to date point to the nicotinic acetylcholine receptor (nAChR) α6 subunit strongly associated with spinosyns resistance in insects, including Plutella xylostella. However, reverse genetic verification of the P. xylostella nAChRα6 has not yet been achieved via an in vivo approach.

RESULTS

Here, we successfully constructed a homozygous strain (Pxα6-KO) with a 2-nt deletion mutation of nAChRα6 by CRISPR/Cas9 coupled with non-homologous end joining approach in P. xylostella. The manipulated mutation results in a frame shift in the open reading frame of transcripts, which produces a predicted protein truncated in the TM3-TM4 loop region. When compared to the background strain IPP-S, the knockout strain Pxα6-KO exhibited 229- and 1462-fold resistance to spinosad and spinetoram, respectively, but no or limited (resistance ratios <3-fold) effects on the toxicities of imidacloprid, abamectin, β-cypermethrin, indoxacarb, metaflumizone and chlorantraniliprole. Furthermore, the mode of inheritance of the acquired spinetoram resistance was autosomal recessive and significantly linked with the 2-nt deletion mutation of nAChRα6 in the Pxα6-KO strain.

CONCLUSION

In vivo functional investigation demonstrates the causality of the Pxα6 truncating mutation with high levels of resistance to spinosyns in P. xylostella. Our results suggest the Pxα6-KO strain underlies an autosomal, recessive mode of inheritance for spinetoram resistance, and reinforces the association of this gene to the mode of action of spinosyns. © 2019 Society of Chemical Industry.

Molecular characterization and expression profiles of nicotinic acetylcholine receptors in the rice striped stem borer, Chilo suppressalis (Lepidoptera: Crambidae)

Nicotinic acetylcholine receptors (nAChRs) are members of the cys-loop ligand-gated ion channel (cysLGIC) superfamily, mediating fast synaptic cholinergic transmission in the central nervous system in insects. Insect nAChRs are the molecular targets of economically important insecticides, such as neonicotinoids and spinosad. Identification and characterization of the nAChR gene family in the rice striped stem borer, Chilo suppressalis, could provide beneficial information about this important receptor gene family and contribute to the investigation of the molecular modes of insecticide action and resistance for current and future chemical control strategies. We searched our C. suppressalis transcriptome database using Bombyx mori nAChR sequences in local BLAST searches and obtained the putative nAChR subunit complementary DNAs (cDNAs) via reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends methods. Similar to B. mori, C. suppressalis possesses 12 nAChR subunits, including nine α-type and three β-type subunits. Quantitative RT-PCR analysis revealed the expression profiles of the nAChR subunits in various tissues, including the brain, subesophageal ganglion, thoracic ganglion, abdominal ganglion, hemocytes, fat body, foregut, midgut, hindgut and Malpighian tubules. Developmental expression analyses showed clear differential expression of nAChR subunits throughout the C. suppressalis life cycle. The identification of nAChR subunits in this study will provide a foundation for investigating the diverse roles played by nAChRs in C. suppressalis and for exploring specific target sites for chemicals that control agricultural pests while sparing beneficial species.

Pleiotropic Effects of Loss of the Dα1 Subunit in Drosophila melanogaster: Implications for Insecticide Resistance

Nicotinic acetylcholine receptors (nAChRs) are a highly conserved gene family that form pentameric receptors involved in fast excitatory synaptic neurotransmission. The specific roles individual nAChR subunits perform in Drosophila melanogaster and other insects are relatively uncharacterized. Of the 10 D. melanogaster nAChR subunits, only three have described roles in behavioral pathways; Dα3 and Dα4 in sleep, and Dα7 in the escape response. Other subunits have been associated with resistance to several classes of insecticides. In particular, our previous work has demonstrated that an allele of the Dα1 subunit is associated with resistance to neonicotinoid insecticides. We used ends-out gene targeting to create a knockout of the Dα1 gene to facilitate phenotypic analysis in a controlled genetic background. To our knowledge, this is the first report of a native function for any nAChR subunits known to be targeted by insecticides. Loss of Dα1 function was associated with changes in courtship, sleep, longevity, and insecticide resistance. While acetylcholine signaling had previously been linked with mating behavior and reproduction in D. melanogaster, no specific nAChR subunit had been directly implicated. The role of Dα1 in a number of behavioral phenotypes highlights the importance of understanding the biological roles of nAChRs and points to the fitness cost that may be associated with neonicotinoid resistance.

Selective actions of Lynx proteins on different nicotinic acetylcholine receptors in the locust, Locusta migratoria manilensis

Nicotinic acetylcholine receptors (nAChRs) are major neurotransmitter receptors and targets of neonicotinoid insecticides in the insect nervous system. The full function of nAChRs is often dependent on associated proteins, such as chaperones, regulators and modulators. Here, three Lynx (Ly-6/neurotoxin) proteins, Loc-lynx1, Loc-lynx2 and Loc-lynx3, were identified in the locust, Locusta migratoria manilensis. Co-expression with Lynx resulted in a dramatic increase in agonist-evoked macroscopic currents on nAChRs Locα1/β2 and Locα2/β2 in Xenopus oocytes, but no changes in agonist sensitivity. Loc-lynx1 and Loc-lynx3 only modulated nAChRs Locα1/β2 while Loc-lynx2 modulated Locα2/β2 specifically. Meanwhile, Loc-lynx1 induced a more significant increase in currents evoked by imidacloprid and epibatidine than Loc-lynx3, and the effects of Loc-lynx1 on imidacloprid and epibatidine were significantly higher than those on acetylcholine. Among three lynx proteins, only Loc-lynx1 significantly increased [(3) H]epibatidine binding on Locα1/β2. The results indicated that Loc-lynx1 had different modulation patterns in nAChRs compared to Loc-lynx2 and Loc-lynx3. Taken together, these findings indicated that three Lynx proteins were nAChR modulators and had selective activities in different nAChRs. Lynx proteins might display their selectivities from three aspects: nAChR subtypes, various agonists and different modulation patterns. Insect Lynx (Ly-6/neurotoxin) proteins act as the allosteric modulators on insect nicotinic acetylcholine receptors (nAChRs), the important targets of insecticides. We found that insect lynx proteins showed their selectivities from at least three aspects: nAChR subtypes, various agonists and different modulation patterns.

New insights on the molecular features and electrophysiological properties of dinotefuran, imidacloprid and acetamiprid neonicotinoid insecticides

Structural features and hydrogen-bond interactions of dinotefuran (DIN), imidacoloprid (IMI) and acetamiprid (ACE) have been investigated experimentally through analyses of new crystal structures and observations in structural databases, as well as by Density Functional Theory quantum chemical calculations. Several conformations are observed experimentally in the solid state, highlighting the large flexibility of these compounds. This feature is confirmed by the theoretical calculations in the gas phase, the numerous and different energetic minima of the three neonicotinoids being located within a 10kJ/mol range. Comparisons of the observed and simulated data sheds light on the hydrogen-bond (HB) strength of the functional group at the tip of the electronegative fragment of each pharmacophore (NO(2) for DIN and IMI and CN for ACE). This effect originates in the 'push-pull' nature of these fragments and the related extensive electron delocalization. Molecular electrostatic potential calculations provide a ranking of the two fragments of the three neonicotinoid in terms of HB strength. Thus, the NO(2) group of DIN is the strongest HB acceptor of the electronegative fragment, closely followed by the cyano group of ACE. These two groups are significantly more potent than the NO(2) group of IMI. With respect to the other fragments of the three neonicotinoids, the nitrogen atom of the pyridine of IMI and ACE are stronger HB acceptors than the oxygen atom of the furanyl moiety of DIN. Finally, compared to electrophysiological studies obtained from cockroach synaptic and extrasynaptic receptors, DIN appears more effective than IMI and ACE because it strongly increases dose-dependently the ganglionic depolarisation and the currents amplitudes. These data suggest that DIN, IMI and ACE belong to two subgroups which act differently as agonists of insect nicotinic receptors.

Thiamethoxam, a poor agonist of nicotinic acetylcholine receptors expressed on isolated cell bodies, acts as a full agonist at cockroach cercal afferent/giant interneuron synapses

Thiamethoxam (TMX) is a second-generation neonicotinoid which is known to induce toxic effects on insects and mammalians. Recently, it has been proposed that TMX is a poor agonist of insect nicotinic acetylcholine receptors (nAChRs) on isolated cell bodies. Here, we have studied its effect on synaptic transmission. Our results demonstrate that TMX acts as an agonist of nAChRs expressed on cockroach cercal afferent giant/interneuron synapses as bath applications of TMX induce a strong reversible depolarization of the sixth abdominal ganglion. This response was reduced by the nicotinic antagonists mecamylamine and methyllicaconitine, but was insensitive to d-tubocurarine. Interestingly, TMX-induced depolarization was partially reduced by the muscarinic antagonist atropine, suggesting that TMX could bind to a 'mixed nicotinic/muscarinic' receptor. Compared to previous studies, we proposed that TMX is able to act as agonist of insect nAChRs expressed at cercal afferent/giant interneuron synapses. Moreover, our results suggest that nAChRs expressed on synaptic ganglion are distinct to nAChRs expressed on isolated cell bodies and that synaptic receptors have higher affinity to TMX resulting to a depolarization of postsynaptic nicotinic receptors.