Neonicotinoids, such as imidacloprid, are nicotinic acetylcholine receptor (nAChR) agonists with potent insecticidal activity. Since its introduction in the early 1990s, imidacloprid has become one of the most extensively used insecticides for both crop protection and animal health applications. As with other classes of insecticides, resistance to neonicotinoids is a significant threat and has been identified in several pest species, including the brown planthopper, Nilaparvata lugens, a major rice pest in many parts of Asia. In this study,

Recent studies have shown that RIC-3, originally identified inCaenorhabditis elegans as the protein encoded by the gene resistance to inhibitors of cholinesterase (ric-3), can enhance functional expression of ␣7 nicotinic acetylcholine receptors (nAChRs). In the present study, the influence of C. elegans and human RIC-3 upon multiple homomeric (␣7, ␣8, and ␣9) and heteromeric (␣3␤2, ␣3␤4, ␣4␤2, ␣4␤4, and ␣9␣10) nAChR subtypes has been examined in transfected mammalian cells by radioligand binding and functional characterization. Coexpression of RIC-3 facilitates a dramatic enhancement of the ability of ␣7 (and the closely related ␣8 subunit) to generate functional nAChRs in otherwise nonpermissive mammalian cells. In contrast, coexpression of RIC-3 did not facilitate functional expression of either homomeric ␣9 or heteromeric ␣9␣10 nAChRs in mammalian cell lines. It is noteworthy that whereas RIC-3 has been reported to cause a marked functional inhibition of heteromeric nAChRs such as ␣3␤4 and ␣4␤2 expressed in Xenopus laevis oocytes, RIC-3 significantly enhances levels of functional expression of these and other (␣3␤2 and ␣4␤4) heteromeric nAChRs when expressed in mammalian cell lines. In addition, the interaction of multiple nAChR subunits (␣3, ␣4, ␣7, ␤2, and ␤4) with RIC-3 has been demonstrated by coimmunoprecipitation from metabolically labeled transfected cells. It is significant that coimmunoprecipitation experiments have provided evidence that RIC-3 associates with unassembled nAChR subunits, a finding that is consistent with previous suggestions that RIC-3 may act by enhancing the maturation (subunit folding and assembly) of nAChRs. We conclude that RIC-3 is an nAChR-associated protein that can enhance functional expression of multiple nAChR subtypes in transfected mammalian cells.

Neonicotinoid insecticides are potent selective agonists of insect nicotinic acetylcholine receptors (nAChRs). Since their introduction in 1991, resistance to neonicotinoids has been slow to develop, but it is now established in some insect field populations such as the planthopper, Nilaparvata lugens, a major rice pest in many parts of Asia. We have reported recently the identification of a target-site mutation (Y151S) within two nAChR subunits (Nla1 and Nla3) from a laboratoryselected field population of N. lugens. In the present study, we have examined the influence of this mutation upon the functional properties of recombinant nAChRs expressed in Xenopus oocytes (as hybrid nAChRs, co-expressed with a rat b2 subunit). The agonist potency of several nicotinic agonists has been examined, including all of the neonicotinoid insecticides that are currently licensed for either crop protection or animal health applications (acetamiprid, clothianidin, dinotefuran, imidacloprid, nitenpyram, thiacloprid and thiamethoxam). The Y151S mutation was found to have no significant effect on the maximal current (I max ) observed with the endogenous agonist, acetylcholine. In contrast, a significant reduction in I max was observed for all neonicotinoids (the I max for mutant nAChRs ranged from 13 to 81% of that observed on wild-type receptors). In addition, nAChRs containing the Y151S mutation caused a significant rightward shift in agonist dose-response curves for all neonicotinoids, but of varying magnitude (shifts in EC 50 values ranged from 1.3 to 3.6-fold). The relationship between neonicotinoid structure and their potency on nAChRs containing the Y151S target-site mutation is discussed.

Heterologous expression of cloned Drosophila nicotinic acetylcholine receptor (nAChR) subunits indicates that these proteins misfold when expressed in mammalian cell lines at 37°C.This misfolding can, however, be overcome either by growing transfected mammalian cells at lower temperatures or by the expression of Drosophila nAChR subunits in a Drosophila cell line. Whereas the Drosophila nAChR /3 subunit (SBD) cDNA, reported previously, lacked part of the SBD coding sequence, here we report the construction and expression of a full-length SBD cDNA. We have examined whether problems in expressing functional Drosophila nAChRs in either Xenopus oocytes or mammalian cell lines can be attributed to an inability of these expression systems to assemble correctly Drosophila nAChRs. Despite expression in what might be considered a more native cellular environment, we have been unable to detect functional nAChRs in a Drosophila cell line unless Drosophila nAChR subunit cDNA5 are coexpressed with vertebrate nAChR subunits. Our results indicate that the folding of Drosophila nAChR subunits is temperature-sensitive and strongly suggest that the inability of these Drosophila nAChR subunits to generate functional channels in the absence of vertebrate subunits is due to a requirement for coassembly with as yet unidentified Drosophila nAChR subunits. Key Words: Protein folding -Assembly-Nicotinic acetylcholine receptor.

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High levels of resistance to spinosad, a macrocyclic lactone insecticide, have been reported previously in western flower thrips, Frankliniella occidentalis, an economically important insect pest of vegetables, fruit and ornamental crops. We have cloned the nicotinic acetylcholine receptor (nAChR) α6 subunit from F. occidentalis (Foα6) and compared the nucleotide sequence of Foα6 from susceptible and spinosad-resistant insect populations (MLFOM and R1S respectively). A single nucleotide change has been identified in Foα6, resulting in the replacement of a glycine (G) residue in susceptible insects with a glutamic acid (E) in resistant insects. The resistance-associated mutation (G275E) is predicted to lie at the top of the third α-helical transmembrane domain of Foα6. Although there is no direct evidence identifying the location of the spinosad binding site, the analogous amino acid in the C. elegans glutamate-gated chloride channel lies in close proximity (4.4 Å) to the known binding site of ivermectin, another macrocyclic lactone pesticide. The functional consequences of the resistance-associated mutation have been examined in the human nAChR α7 subunit. Introduction of an analogous (A272E) mutation in α7 abolishes the modulatory effects of spinosad whilst having no significant effect upon activation by acetylcholine, consistent with spinosad having an allosteric mechanism of action.

RIC‐3 is a transmembrane protein which enhances maturation (folding and assembly) of neuronal nicotinic acetylcholine receptors (nAChRs). In this study, we report the cloning and characterisation of 11 alternatively spliced isoforms of Drosophila melanogaster RIC‐3 (DmRIC‐3). Heterologous expression studies of alternatively spliced DmRIC‐3 isoforms demonstrate that nAChR chaperone activity does not require a predicted coiled‐coil domain which is located entirely within exon 7. In contrast, isoforms containing an additional exon (exon 2), which is located within a proline‐rich N‐terminal region, have a greatly reduced ability to enhance nAChR maturation. The ability of DmRIC‐3 to influence nAChR maturation was examined in co‐expression studies with human α7 nAChRs and with hybrid nAChRs containing both Drosophila and rat nAChR subunits. When expressed in a Drosophila cell line, several of the DmRIC‐3 splice variants enhanced nAChR maturation to a significantly greater extent than observed with human RIC‐3. In contrast, when expressed in a human cell line, human RIC‐3 enhanced nAChR maturation more efficiently than DmRIC‐3. The cloning and characterisation of 11 alternatively spliced DmRIC‐3 isoforms has helped to identify domains influencing RIC‐3 chaperone activity. In addition, studies conducted in different expression systems suggest that additional host cell factors may modulate the chaperone activity of RIC‐3.

Abstract:The recent introduction of the chloronicotinyl insecticide imidacloprid, targeting insect nicotinic acetylcholine receptors (nAChRs), emphasises the importance of a detailed molecular characterisation of these receptors. We are investigating the molecular diversity of insect nAChR subunit genes in an important agricultural pest, the peach-potato aphid Myzus persicae. Two M. persicae ␣-subunit cDNAs, Mp␣1 and Mp␣2, have been cloned previously. Here we report the isolation of three novel ␣-subunit genes (Mp␣3-5) with overall amino acid sequence identities between 43 and 76% to characterised insect nAChR subunits. Alignment of their amino acid sequences with other invertebrate and vertebrate nAChR subunits suggests that the insect ␣ subunits evolved in parallel to the vertebrate neuronal nAChRs and that the insect non-␣ subunits are clearly different from vertebrate neuronal ␤ and muscle non-␣ subunits. The discovery of novel subtypes in M. persicae is a further indicator of the complexity of the insect nAChR gene family. Heterologous co-expression of M. persicae nAChR ␣-subunit cDNAs with the rat ␤2 in Drosophila S2 cells resulted in high-affinity binding of nicotinic radioligands. The affinity of recombinant nAChRs for [ 3 H]imidacloprid was influenced strongly by the ␣ subtype. This is the first demonstration that imidacloprid selectively acts on Mp␣2 and Mp␣3 subunits, but not Mp␣1, in M. persicae.