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One nicotinic acetylcholine receptor non-alpha subunit was cloned from the pond wolf spider, Pardosa pseudoannulata, an important predatory enemy of some insect pests with agricultural importance, such as the green peach aphid Myzus persicae. The subunit shows high amino acid identities to insect beta1 subunits (74-78%), and was denoted as Ppbeta1. Although high identities are found between Ppbeta1 and insect beta1 subunits, amino acid differences are found within loops D, E and F, important segments contributing to ligand binding. The effects of amino acid differences within these loops were evaluated by introducing loops of insect or spider beta1 subunits into rat beta2 subunit and co-expressing with insect alpha subunit. The corresponding regions of rat beta2 chimera beta2(Mpbeta1) (beta2 with loops D, E and F from M. persicae beta1 subunit Mpbeta1) were replaced by loops D, E and F of Ppbeta1 singly or together to construct different chimeras. When these chimeras were co-expressed with insect Nlalpha1, it was found that the replacement of loops D, E and F of beta2(Mpbeta1) by that of Ppbeta1 resulted in a right-ward shift of the imidacloprid dose-response curves, reflecting increases in EC(50), compared to Nlalpha1/beta2(Mpbeta1). By contrast, the influences on ACh potency were minimal. The further study showed that R81Q, N137G and F190W differences, within loops D, E and F respectively, contributed mainly to these sensitivity changes. This study contributes to our understanding of the molecular mechanism underlying selectivity of neonicotinoids against insects over spiders.

Nicotinic acetylcholine (ACh) receptors (nAChRs) are ligand-gated ion channels which mediate fast cholinergic synaptic transmission in insect and vertebrate nervous systems. The nAChR agonist-binding site is formed by loops A-C present in alpha subunits together with loops D-F present in either non-alpha subunits or homomer-forming alpha subunits. A new non-alpha subunit was cloned from Nilaparvata lugens, a major rice pest in many parts of Asia, showing very high amino acid identity to other insect beta1 subunits, and was denoted as N. lugens beta1 (Nlbeta1). Six A-to-I RNA editing sites were found in Nlbeta1 N-terminal domain, in which only one site was previously reported in Drosophila melanogaster Dbeta1 and the other five were newly identified. Among the six editing sites, four caused amino acid changes, in which the site 2 (E2) and site 5 (E5) caused an N to D change in loop D (N73D) and loop E (N133D) respectively. E2 frequency was high in Sus (susceptible) strain and E5 frequency was high in Res (resistant) strain. By expressing in Xenopus oocytes, N73D editing was found to reduce the agonist potency of both ACh and imidacloprid, and the influence on ACh was more significant than on imidacloprid. By contrast, N133D editing only affected imidacloprid potency. These results indicated, although E2 and E5 editings both caused an N to D change in important loops, their roles in neonicotinoid insensitivity might be different.

Nicotinic acetylcholine (ACh) receptors (nAChRs) are ligand-gated ion channels which mediate fast cholinergic synaptic transmission in insect and vertebrate nervous systems. The nAChR agonist-binding site is present at the interface of adjacent subunits and is formed by loops A-C present in alpha subunits together with loops D-F present in either non-alpha subunits or homomer-forming alpha subunits. To investigate the mechanism of neonicotinoid selectivity, we have examined the effects of altering insect-specific loops D, E and F in hybrid nAChRs containing insect and mammalian subunits (Nlalpha1 from the brown planthopper Nilaparvata lugens and beta2 from rat). Introduction of the insect-specific loops D, E and F, singly or together, into rat beta2 subunit resulted in a leftward shift of the imidacloprid dose-response curves for nAChRs Nlalpha1-beta2 chimeras, reflecting decreases in EC(50), compared to wildtype nAChRs Nlalpha1-beta2. By contrast, the influences on ACh potency were minimal or negligible. The effects of loop D could be interpreted by the earlier findings of Shimomura et al. [2006. Role in the selectivity of neonicotinoids of insect-specific basic residues in loop D of the nicotinic acetylcholine receptor agonist-binding site. Mol. Pharmacol. 70, 1255-1263.], in which T77R and E79V were shown to be responsible for neonicotinoid selectivity. In the present study, S131Y(R) and D133N in loop E and T191W and P192K in loop F were found to contribute to the neonicotinoid selectivity of insect-specific loops E and F. These results indicated the insect-specific loops D, E and F each play important roles in neonicotinoids selectivity. This study contributes to our understanding of the molecular mechanism underlying selectivity of neonicotinoids against insects over vertebrates.