Ligand Binding at the 4-4 Agonist-Binding Site of the 42 nAChR Triggers Receptor Activation through a Pre-Activated Conformational State

PLoS One. 2016 Aug 23;11(8):e0161154. doi: 10.1371/journal.pone.0161154. eCollection 2016.

Abstract

The α4β2 nicotinic acetylcholine receptor (nAChR) is the most abundant subtype in the brain and exists in two functional stoichiometries: (α4)3(β2)2 and (α4)2(β2)3. A distinct feature of the (α4)3(β2)2 receptor is the biphasic activation response to the endogenous agonist acetylcholine, where it is activated with high potency and low efficacy when two α4-β2 binding sites are occupied and with low potency/high efficacy when a third α4-α4 binding site is occupied. Further, exogenous ligands can bind to the third α4-α4 binding site and potentiate the activation of the receptor by ACh that is bound at the two α4-β2 sites. We propose that perturbations of the recently described pre-activation step when a third binding site is occupied are a key driver of these distinct activation properties. To investigate this, we used a combination of simple linear kinetic models and voltage clamp electrophysiology to determine whether transitions into the pre-activated state were increased when three binding sites were occupied. We separated the binding at the two different sites with ligands selective for the α4-β2 site (Sazetidine-A and TC-2559) and the α4-α4 site (NS9283) and identified that when a third binding site was occupied, changes in the concentration-response curves were best explained by an increase in transitions into a pre-activated state. We propose that perturbations of transitions into a pre-activated state are essential to explain the activation properties of the (α4)3(β2)2 receptor by acetylcholine and other ligands. Considering the widespread clinical use of benzodiazepines, this discovery of a conserved mechanism that benzodiazepines and ACh potentiate receptor activation via a third binding site can be exploited to develop therapeutics with similar properties at other cys-loop receptors.

MeSH terms

  • Acetylcholine / metabolism*
  • Acetylcholine / pharmacology
  • Animals
  • Azetidines / pharmacology
  • Binding Sites
  • Cell Membrane / drug effects
  • Cell Membrane / genetics
  • Humans
  • Ion Channel Gating / genetics*
  • Ligands
  • Molecular Conformation / drug effects
  • Oocytes / drug effects*
  • Oocytes / growth & development
  • Oocytes / metabolism
  • Oxadiazoles / pharmacology
  • Protein Binding
  • Protein Isoforms / drug effects
  • Pyridines / pharmacology
  • Receptors, Nicotinic / genetics*
  • Receptors, Nicotinic / metabolism
  • Xenopus laevis / genetics
  • Xenopus laevis / growth & development

Substances

  • 3-(3-(pyridine-3-yl)-1,2,4-oxadiazol-5-yl)benzonitrile
  • Azetidines
  • Ligands
  • Oxadiazoles
  • Protein Isoforms
  • Pyridines
  • Receptors, Nicotinic
  • TC 2559
  • sazetidine-A
  • Acetylcholine

Grants and funding

This work was funded by the National Health and Medical Research Council of Australia (NHMRC) Project Grant 1069417 (https://www.nhmrc.gov.au/grants-funding/apply-funding/project-grants). DCI is funded by an International Postgraduate Research Scholarship by the University of Sydney (http://sydney.edu.au/scholarships/prospective/international_postgraduate_scholarships.shtml#iprs) and received support of a John Lamberton Scholarship (http://agile2.ucc.usyd.edu.au/ro/opportunities/scholarships/1260). This work was co-funded by Imk Almene Fond, Denmark and Saniona A/S (http://saniona.com/EN/).