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PLoS One
2014 Jan 01;95:e97468. doi: 10.1371/journal.pone.0097468.
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Actions of agonists, fipronil and ivermectin on the predominant in vivo splice and edit variant (RDLbd, I/V) of the Drosophila GABA receptor expressed in Xenopus laevis oocytes.
Lees K
,
Musgaard M
,
Suwanmanee S
,
Buckingham SD
,
Biggin P
,
Sattelle D
.
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Ionotropic GABA receptors are the targets for several classes of insecticides. One of the most widely-studied insect GABA receptors is RDL (resistance to dieldrin), originally isolated from Drosophila melanogaster. RDL undergoes alternative splicing and RNA editing, which influence the potency of GABA. Most work has focussed on minority isoforms. Here, we report the first characterisation of the predominant native splice variant and RNA edit, combining functional characterisation with molecular modelling of the agonist-binding region. The relative order of agonist potency is GABA> muscimol> TACA> β-alanine. The I/V edit does not alter the potency of GABA compared to RDLbd. Docking calculations suggest that these agonists bind and activate RDLbdI/V through a similar binding mode. TACA and β-alanine are predicted to bind with lower affinity than GABA, potentially explaining their lower potency, whereas the lower potency of muscimol and isoguvacine cannot be explained structurally from the docking calculations. The A301S (resistance to dieldrin) mutation reduced the potency of antagonists picrotoxin, fipronil and pyrafluprole but the I/V edit had no measurable effect. Ivermectin suppressed responses to GABA of RDLbdI/V, RDLbd and RDLbdI/VA301S. The dieldrin resistant variant also showed reduced sensitivity to Ivermectin. This study of a highly abundant insect GABA receptor isoform will help the design of new insecticides.
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Figure 2. Binding models for the different agonists.(A) Overall structure of the homology model of RDLbd. Chains A and E are shown in cartoon representation, A in orange and E in cyan. Chains B, C and D are shown in spacefill representation in three different shades of grey. The agonist-binding site is indicated by the sidechains of R111 in chain A (light orange spacefill) and D204 in chain E (light cyan spacefill). (B–G): binding models for the docked agonists. The agonists are shown with grey carbon atoms, and relevant protein sidechains from chain A and chain E are shown with light orange and light blue carbon atoms, respectively. Only polar hydrogen atoms on agonists and sidechains are included. Potential hydrogen bonds (<3 Å) between ligand and protein are marked with black lines. (H) Superposition of the binding models in (B–F). The conformations of R111 and E204 shown are from the GABA binding model.
Figure 3. Concentration-inhibition curves showing responses elicited by PTX to 1 mM GABA in RDL, RDLbdIV and RDLbdIV/A301S.Each data point was normalised to the amplitude of the response to 1 mM GABA and represents the mean amplitude of the responses recorded in 7–13 oocytes from at least 3 separate batches. Structures of the two compounds found in picrotoxin (PTX), picrotoxinin and picrotin, are shown.
Figure 4. Concentration-inhibition curves showing responses elicited by the phenylpyrazoles (A) fipronil and (B) pyrafluprole to 1 mM GABA in wild-type, RDLbdIV and RDLbdIV/A301S.Each point represents the mean amplitude of the responses recorded in 2–13 oocytes from at least 3 separate batches. Structures of the two phenylpyrazoles are inserted.
Figure 5. Effect of ivermectin on RDLbd.Concentration inhibition curves showing responses elicited by IVM to 100 µM GABA in wild-type, RDLbdIV and RDLbdIV/A301S. Each point represents the mean amplitude of the responses recorded in 2–6 oocytes from at least 3 separate batches. The structure of ivermectin is inserted.
Figure 1. Concentration response curves for GABA and analogues mediated by variants of the RDLbd splice variant.(A) Concentration response curves for the three RDL subunits: RDLbd, RDLbdIV and RDLbdIVA301S to GABA (structure inserted). Data were normalised to 1 mM GABA. Each data point is the mean ± S.E.M from four to six oocytes from at least three separate frogs. Representative responses to 100 µM GABA are shown for each variant. Scale bars indicate 0.5 µA, 1 s. (B) Concentration response curves showing responses elicited by GABA and analogues at RDLbdIV receptors in Xenopus oocytes. GABA is the most potent agonist, and the rank order is GABA> muscimol> TACA>>β-alanine. Isoguvacine did not reach saturation. Data were normalised to the response to 1 mM GABA. Each data point represents mean±S.E.M from five to eight oocytes. (C) The chemical structures of the agonists tested in the present study: GABA (γ-amino butyric acid), TACA (trans-4-aminocrotonic acid), β-alanine, muscimol and isoguvacine.
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