title: Different binding modes of tropeines mediating inhibition and potentiation of alpha1 glycine receptors.
abstract: Tropeines are bidirectional modulators of native and recombinant glycine receptors (GlyRs) and promising leads for the development of novel modulatory agents. Tropisetron potentiates and inhibits agonist-triggered GlyR currents at femto- to nanomolar and micromolar concentrations respectively. Here, the potentiating and inhibitory effects of another tropeine, 3alpha-(3'-methoxy-benzoyloxy)nortropane (MBN) were examined by voltage-clamp electrophysiology at wild type and mutant alpha1 GlyRs expressed in Xenopus laevis oocytes. Several substitutions around the agonist-binding cavity of the alpha1 subunit interface (N46C, F63A, N102A, R119K, R131A, E157C, K200A, Y202L and F207A) were found to reduce or eliminate MBN inhibition of glycine activation. In contrast, the binding site mutations Q67A, R119A and S129A which did not affect MBN inhibition abolished the potentiation of chloride currents elicited by low concentrations of the partial agonist taurine following pre-incubation with MBN. Thus, potentiation and inhibition involve distinct binding modes of MBN in the inter-subunit agonist-binding pocket of alpha1 GlyRs. Homology modelling and molecular dynamics simulations disclosed two distinct docking modes for MBN, which are consistent with the differential effects of individual binding site substitutions on MBN inhibition and potentiation respectively. Together these results suggest that distinct binding modes at adjacent binding sites located within the agonist-binding pocket of the GlyR mediate the bidirectional modulatory effects of tropeines.
authors.affiliation: Department of Molecular Pharmacology, Institute of Biomolecular Chemistry, Chemical Research Center, Hungarian Academy of Sciences, Budapest, Hungary. maksay@chemres.hu
authors.forename: Gábor
authors.initials: G
authors.lastname: Maksay
authors.forename: Bodo
authors.initials: B
authors.lastname: Laube
authors.forename: Rudolf
authors.initials: R
authors.lastname: Schemm
authors.forename: Joanna
authors.initials: J
authors.lastname: Grudzinska
authors.forename: Malgorzata
authors.initials: M
authors.lastname: Drwal
authors.forename: Heinrich
authors.initials: H
authors.lastname: Betz
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000187:drug effects / Q000235:genetics; D001703:Biophysics; D004305:Dose-Response Relationship, Drug; D004558:Electric Stimulation; D006801:Humans; D007211:Indoles / Q000737:chemistry / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008845:Microinjections / Q000379:methods; D008958:Models, Molecular; D016297:Mutagenesis, Site-Directed / Q000379:methods; D009154:Mutation; D009662:Nortropanes / Q000494:pharmacology; D009865:Oocytes; D018408:Patch-Clamp Techniques / Q000379:methods; D011485:Protein Binding / Q000187:drug effects / Q000235:genetics; D017434:Protein Structure, Tertiary; D018009:Receptors, Glycine / Q000037:antagonists & inhibitors* / Q000235:genetics / Q000502:physiology*; D000077526:Tropisetron; D014982:Xenopus laevis
chemical_list: C542092:3-(3'-methoxybenzoyloxy)nortropane; D007211:Indoles; D009662:Nortropanes; D018009:Receptors, Glycine; D000077526:Tropisetron