Identification of two Lynx proteins inNilaparvata lugensand the modulation on insect nicotinic acetylcholine receptors

A consensus phosphoserine within the large cytoplasmic loop of insect nAChR α8 subunits modulated interaction between 14-3-3ε and nAChRs to regulate neonicotinoid efficacy

Neonicotinoids are insect-selective nicotinic acetylcholine receptors (nAChRs) agonists that are used extensively for plant protection and animal health care. Some chaperone proteins, such as 14-3-3 proteins, importantly modulate nAChRs to display the physiological and pharmacological properties. Here we found that there is a 14-3-3 binding motif RSPSTH within the cytoplasmic loop of most insect α8 subunits. In the motif, a potential phosphorylated serine residue, serine 337, was a putative protein kinase A (PKA) substrate. Using Locusta migratoria α8 subunit as a representative, here we demonstrated that Loc14-3-3ε interacted with the unique phosphoserine (α8^S337) of Locα8 subunit to regulate agonist efficacy on hybrid Locα8/β2 nAChRs in Xenopus oocytes. Co-expression of Loc14-3-3ε caused a dramatic rise of maximal inward currents (I_max) of Locα8/β2 for acetylcholine and imidacloprid to 2.9-fold and 3.1-fold of that of Locα8/β2 alone. The S337A substitution of Locα8 reduced the I_max rise when Locα8^S337A/β2 and Loc14-3-3ε were co-expressed. The increased agonist currents by exogenous Loc14-3-3ε on Locα8/β2 could be almost abolished by either PKA inhibitor KT5720 or 14-3-3 inhibitor difopein. The findings revealed that serine 337 within motif RSPSTH was important for the interaction between insect nAChRs and 14-3-3ε, and inhibiting the interaction would change the pharmacological property of insect nAChRs to agonist such as neonicotinoids which may provide insights to develop new targets for insecticide design.

Dual effects of insect nAChR chaperone RIC-3 on hybrid receptor: Promoting assembly on endoplasmic reticulum but suppressing transport to plasma membrane on Xenopus oocytes

Resistance to inhibitors of cholinesterase (RIC) -3 promotes the maturation (folding and assembly) of neuronal nicotinic acetylcholine receptors (nAChRs) as a molecular chaperone. The modulation effects of RIC-3 on homomeric α7 nAChRs are always positive, but its effects on heteromeric subtypes are inconsistent among reports. In this study, five RIC-3 isoforms were identified from Locusta migratoria. Four isoforms showed obvious effects on hybrid receptor Locα1/rβ2 expressed in Xenopus oocytes. As a representative, the co-expression of RIC-3v4 exhibited the decreased agonist responses (I_max) on oocytes, lower specific [³H]epibatidine binding (B_max) on plasma membrane protein (PMP), and reduced subunit levels in PMP, which showed that the mature Locα1/rβ2 on the plasma membrane was decreased by the co-expression of RIC-3. In contrast, the [³H]epibatidine binding and mature Locα1/rβ2 levels in the endoplasmic reticulum membrane protein (ERMP) were much increased when co-expressing with RIC-3v4. The [³H]epibatidine binding and mature Locα1/rβ2 levels in total membrane protein (TMP) gave the similar results as that in ERMP. Taking data together, the results showed that the co-expression of RIC-3 increased the mature Locα1/rβ2 receptor levels on ER of Xenopus oocytes, but these mature receptors were mostly kept on ER and suppressed to transport to plasma membrane.

The insecticidal activity and action mode of an imidacloprid analogue, 1-(3-pyridylmethyl)-2-nitroimino-imidazolidine

Neonicotinoids, such as imidacloprid, are key insecticides extensively used for control of Nilaparvata lugens. However, imidacloprid resistance has been reported in many Asian countries in recent years. To understand the roles of the chlorine atom of pyridyl group on insecticidal activity and resistance, the atom was removed to generate an imidacloprid analogue DC-Imi (DesChlorine Imidacloprid). DC-Imi showed significantly higher toxicity than imidacloprid in the susceptible strain of N. lugens, but had medium level cross-resistance in an imidacloprid-resistant strain. In Xenopus oocyte expressed nicotinic acetylcholine receptors (nAChRs) Nlα1/rβ2, the inward currents evoked by DC-Imi were detected and could be blocked by typical nAChRs antagonist dihydro-β-erythroidine (DHβE), which demonstrated that DC-Imi acted as an agonist on insect nAChRs. The efficacy of DC-Imi on Nlα1/rβ2 was 1.8-fold higher than that of imidacloprid. In addition, the influence of an imidacloprid resistance associated mutation (Y151S) on agonist potencies was evaluated. Compared with the wild-type receptor, the mutation reduced maximal inward current of DC-Imi to 55.6% and increased half maximal effective concentration (EC50 ) to 3.53-fold. Compared with imidacloprid (increasing EC50 to 2.38-fold of wild-type receptor), Y151S mutation decreased DC-Imi potency more significantly. The results indicated that the selective and possibly high toxicities could be achieved through the modification of 6-chloro-3-pyridyl group in imidacloprid and other neonicotinoids.

Ly6h regulates trafficking of alpha7 nicotinic acetylcholine receptors and nicotine-induced potentiation of glutamatergic signaling

α7 nAChRs are expressed widely throughout the brain, where they are important for synaptic signaling, gene transcription, and plastic changes that regulate sensory processing, cognition, and neural responses to chronic nicotine exposure. However, the mechanisms by which α7 nAChRs are regulated are poorly understood. Here we show that trafficking of α7-subunits is controlled by endogenous membrane-associated prototoxins in the Ly6 family. In particular, we find that Ly6h reduces cell-surface expression and calcium signaling by α7 nAChRs. We detect Ly6h in several rat brain regions, including the hippocampus, where we find it is both necessary and sufficient to limit the magnitude of α7-mediated currents. Consistent with such a regulatory function, knockdown of Ly6h in rat hippocampal pyramidal neurons enhances nicotine-induced potentiation of glutamatergic mEPSC amplitude, which is known to be mediated by α7 signaling. Collectively our data suggest a novel cellular role for Ly6 proteins in regulating nAChRs, which may be relevant to plastic changes in the nervous system including rewiring of glutamatergic circuitry during nicotine addiction.

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.

Functional interaction of nicotinic acetylcholine receptors and Na+/K+ ATPase from Locusta migratoria manilensis (Meyen)

Associated proteins are important for the correct functioning of nicotinic acetylcholine receptors (nAChRs). In the present study, a neonicotinoid-agarose affinity column was used to isolate related proteins from a solubilized membrane preparation from the nervous system of Locusta migratoria manilensis (Meyen). 1530 peptides were identified and most of them were involved in the membranous structure, molecular interaction and cellular communication. Among these peptides, Na⁺/K⁺ ATPase had the highest MASCOT score and were involved in the molecular interaction, which suggested that Na⁺/K⁺ ATPase and nAChRs might have strong and stable interactions in insect central nervous system. In the present study, functional interactions between nAChRs and Na⁺/K⁺ ATPase were examined by heterologous expression in Xenopus oocytes. The results showed that the activated nAChRs increased pump currents of Na⁺/K⁺ ATPase, which did not require current flow through open nAChRs. In turn, Na⁺/K⁺ ATPase significantly increased agonist sensitivities of nAChRs in a pump activity-independent manner and reduced the maximum current (I_max) of nAChRs. These findings provide novel insights concerning the functional interactions between insect nAChRs and Na⁺/K⁺ ATPase.

Transcriptional Changes in nAChRs, Interactive Proteins and P450s in Locusta migratoria manilensis (Orthoptera: Acrididae) CNS in Response to High and Low Oral Doses of Imidacloprid

The insect central nervous system (CNS) is the target for many insecticides, and changes in transcript levels could be expected after insecticide applications. In this study, differentially expressed genes in the locust (Locusta migratoria manilensis) CNS in response to imidacloprid treatments at low dose (LD, 10% mortality) and high dose (HD, 80% mortality) were identified. Two nicotine acetylcholine receptor (nAChR) subunits genes and 18 interacting protein genes were regulated at LD, and only one nAChR subunit gene and 11 interacting proteins were regulated at HD. Among the 110 annotated P450 unigenes, 43 unigenes were regulated at LD and 34 unigenes were regulated at HD. Most of the differentially expressed P450 unigenes were mapped to CYP4, in which most unigenes were upregulated at LD, but downregulated at HD. Totally, the numbers and regulation levels of the regulated genes were more at LD than that at HD. Seventeen unigenes were selected to test their expression changes following insecticide treatments by qRT-PCR, in which the changes in more than half of the selected genes were verified. The results revealed the variation in the response of locusts to different insecticide pressure, such as different doses.

Three-finger snake neurotoxins and Ly6 proteins targeting nicotinic acetylcholine receptors: pharmacological tools and endogenous modulators

Snake venom neurotoxins and lymphocyte antigen 6 (Ly6) proteins, most of the latter being membrane tethered by a glycosylphosphatidylinositol (GPI) anchor, have a variety of biological activities, but their three-finger (3F) folding combines them in one Ly6/neurotoxin family. Subsets of two groups, represented by α-neurotoxins and Lynx1, respectively, interact with nicotinic acetylcholine receptors (nAChR) and, hence, are of therapeutic interest for the treatment of neurodegenerative diseases, pain, and cancer. Information on the mechanisms of action and 3D structure of the binding sites, which is required for drug design, is available from the 3D structure of α-neurotoxin complexes with nAChR models. Here, I compare the structural and functional features of α-neurotoxins versus Lynx1 and its homologs to get a clearer picture of Lynx1-nAChR interactions that is necessary for fundamental science and practical applications.

Membrane-tethered ligands: tools for cell-autonomous pharmacological manipulation of biological circuits

Detection of secreted signaling molecules by cognate cell surface receptors is a major intercellular communication pathway in cellular circuits that control biological processes. Understanding the biological significance of these connections would allow us to understand how cellular circuits operate as a whole. Membrane-tethered ligands are recombinant transgenes with structural modules that allow them to act on cell-surface receptors and ion channel subtypes with pharmacological specificity in a cell-autonomous manner. Membrane-tethered ligands have been successful in the specific manipulation of ion channels as well as G-protein-coupled receptors, and, in combination with cell-specific promoters, such manipulations have been restricted to genetically defined subpopulations within cellular circuits in vivo to induce specific phenotypes controlled by those circuits. These studies establish the membrane-tethering approach as a generally applicable method for dissecting neural and physiological circuits.

Tethering toxins and peptide ligands for modulation of neuronal function

Tethering genetically encoded peptide toxins or ligands close to their point of activity at the cell plasma membrane provides a new approach to the study of cell networks and neuronal circuits, as it allows selective targeting of specific cell populations, enhances the working concentration of the ligand or blocker peptide, and permits the engineering of a large variety of t-peptides (e.g., including use of fluorescent markers, viral vectors and point mutation variants). This review describes the development of tethered toxins (t-toxins) and peptides derived from the identification of the cell surface nicotinic acetylcholine receptor (nAChR) modulator lynx1, the existence of related endogenous cell surface modulators of nAChR and AMPA receptors, and the application of the t-toxin and t-neuropeptide technology to the dissection of neuronal circuits in metazoans.

NMR structure and action on nicotinic acetylcholine receptors of water-soluble domain of human LYNX1

Discovery of proteins expressed in the central nervous system sharing the three-finger structure with snake α-neurotoxins provoked much interest to their role in brain functions. Prototoxin LYNX1, having homology both to Ly6 proteins and three-finger neurotoxins, is the first identified member of this family membrane-tethered by a GPI anchor, which considerably complicates in vitro studies. We report for the first time the NMR spatial structure for the water-soluble domain of human LYNX1 lacking a GPI anchor (ws-LYNX1) and its concentration-dependent activity on nicotinic acetylcholine receptors (nAChRs). At 5-30 μM, ws-LYNX1 competed with (125)I-α-bungarotoxin for binding to the acetylcholine-binding proteins (AChBPs) and to Torpedo nAChR. Exposure of Xenopus oocytes expressing α7 nAChRs to 1 μM ws-LYNX1 enhanced the response to acetylcholine, but no effect was detected on α4β2 and α3β2 nAChRs. Increasing ws-LYNX1 concentration to 10 μM caused a modest inhibition of these three nAChR subtypes. A common feature for ws-LYNX1 and LYNX1 is a decrease of nAChR sensitivity to high concentrations of acetylcholine. NMR and functional analysis both demonstrate that ws-LYNX1 is an appropriate model to shed light on the mechanism of LYNX1 action. Computer modeling, based on ws-LYNX1 NMR structure and AChBP x-ray structure, revealed a possible mode of ws-LYNX1 binding.

Differential sensitivity of Ctenocephalides felis and Drosophila melanogaster nicotinic acetylcholine receptor α1 and α2 subunits in recombinant hybrid receptors to nicotinoids and neonicotinoid insecticides

Nicotinic acetylcholine receptors (nAChRs) are the binding sites for nicotinoid drugs, such as nicotine and epibatidine, and are the molecular targets of the selectively insecticidal neonicotinoids. In this study we report the full length cDNA cloning of the three Ctenocephalides (C.) felis (cat flea) nAChR α subunits Cfα1, Cfα2, and Cfα3. When expressed in Xenopus oocytes as hybrid receptors with the Gallus gallus (chicken) β2 (Ggβ2) subunit, these cat flea α subunits formed acetylcholine-responsive ion channels. Acetylcholine-evoked currents of Cfα2/Ggβ2 were resistant to α-bungarotoxin, while those of Cfα1/Ggβ2 were sensitive to this snake toxin. The pharmacological profiles of Cfα1/Ggβ2, Cfα2/Ggβ2 and the chicken neuronal receptor Ggα4/Ggβ2 for acetylcholine, two nicotinoids and 6 insecticidal neonicotinoids were determined and compared. Particularly remarkable was the finding that Cfα1/Ggβ2 was far more sensitive to acetylcholine, nicotine and neonicotinoid agonists than either Cfα2/Ggβ2 or Ggα4/Ggβ2: for the anti flea neonicotinoid market compound imidacloprid the respective EC₅₀s were 0.02 μM, 1.31 μM and 10 μM. These results were confirmed for another insect species, Drosophila melanogaster, where the pharmacological profile of the Dmα1 and Dmα2 subunits as hybrid receptors with Ggβ2 in Xenopus oocyte expressions resulted in a similar sensitivity pattern as those identified for the C. felis orthologs. Our results show that at least in a Ggβ2 hybrid receptor setting, insect α1 subunits confer higher sensitivity to neonicotinoids than α2 subunits, which may contribute in vivo to the insect-selective action of this pesticide class.

Crooked, coiled and crimpled are three Ly6-like proteins required for proper localization of septate junction components

Cellular junction formation is an elaborate process that is dependent on the regulated synthesis, assembly and membrane targeting of constituting components. Here, we report on three Drosophila Ly6-like proteins essential for septate junction (SJ) formation. SJs provide a paracellular diffusion barrier and appear molecularly and structurally similar to vertebrate paranodal septate junctions. We show that Crooked (Crok), a small GPI-anchored Ly6-like protein, is required for septa formation and barrier functions. In embryos that lack Crok, SJ components are produced but fail to accumulate at the plasma membrane. Crok is detected in intracellular puncta and acts tissue-autonomously, which suggests that it resides in intracellular vesicles to assist the cell surface localization of SJ components. In addition, we demonstrate that two related Ly6 proteins, Coiled (Cold) and Crimpled (Crim), are required for SJ formation and function in a tissue-autonomous manner, and that Cold also localizes to intracellular vesicles. Specifically, Crok and Cold are required for correct membrane trafficking of Neurexin IV, a central SJ component. The non-redundant requirement for Crok, Cold, Crim and Boudin (Bou; another Ly6 protein that was recently shown to be involved in SJ formation) suggests that members of this conserved family of proteins cooperate in the assembly of SJ components, possibly by promoting core SJ complex formation in intracellular compartments associated with membrane trafficking.

Selectivity of lynx proteins on insect nicotinic acetylcholine receptors in the brown planthopper, Nilaparvata lugens

Neuronal nicotinic acetylcholine receptors (nAChRs) are major excitatory neurotransmitter receptors in both vertebrates and invertebrates. Two lynx proteins (Nl-lynx1 and Nl-lynx2) have been identified in the brown planthopper, Nilaparvata lugens, which act as modulators on insect nAChRs. In the present study, two lynx proteins were found to act on the triplet receptor Nlalpha1/Nlalpha2/beta2 expressed in Xenopus oocytes, increasing agonist-evoked macroscopic currents, but not changing agonist sensitivity and desensitization properties. Nl-lynx1 and Nl-lynx2 increased I(max) (maximum responses) of acetylcholine to 4.85-fold and 2.40-fold of that of Nlalpha1/Nlalpha2/beta2 alone, and they also increased I(max) of imidacloprid to 2.57-fold and 1.25-fold. Although, on another triplet nAChRs Nlalpha3/Nlalpha8/beta2, Nl-lynx2 increased I(max) of acetylcholine and imidacloprid to 3.63-fold and 2.16-fold, Nl-lynx1 had no effects on I(max) of either acetylcholine or imidacloprid. The results demonstrate the selectivity of lynx proteins for different insect nAChR subtypes. This selectivity was also identified in native N. Lugens. Co-immunoprecipitation was found between Nlalpha1/Nlalpha2-containing receptors and both Nl-lynx1 and Nl-lynx2, but was only found between Nlalpha3/Nlalpha8-containing receptors and Nl-lynx2. When the previously identified Nlalpha1(Y151S) and Nlalpha3(Y151S) mutations were included (Nlalpha1(Y151S)/Nlalpha2/beta2 and Nlalpha3(Y151S)/Nlalpha8/beta2), the increase in I(max) of imidacloprid, but not acetylcholine, caused by co-expression of Nl-lynx1 and Nl-lynx2 was more noticeable than that of their wildtype counterparts. Taken together, these data suggest that two modulators, Nl-lynx1 and Nl-lynx2, might serve as an influencing factor in target site insensitivity in N. lugens, such as Y151S mutation.