title: A molecular characterization of the agonist binding site of a nematode cys-loop GABA receptor.
abstract: OBJECTIVE
  Cys-loop GABA receptors represent important targets for human chemotherapeutics and insecticides and are potential targets for novel anthelmintics (nematicides). However, compared with insect and mammalian receptors, little is known regarding the pharmacological characteristics of nematode Cys-loop GABA receptors. Here we have investigated the agonist binding site of the Cys-loop GABA receptor UNC-49 (Hco-UNC-49) from the parasitic nematode Haemonchus contortus.
  
  
  METHODS
  We used two-electrode voltage-clamp electrophysiology to measure channel activation by classical GABA receptor agonists on Hco-UNC-49 expressed in Xenopus laevis oocytes, along with site-directed mutagenesis and in silico homology modelling.
  
  
  RESULTS
  The sulphonated molecules P4S and taurine had no effect on Hco-UNC-49. Other classical Cys-loop GABAA receptor agonists tested on the Hco-UNC-49B/C heteromeric channel had a rank order efficacy of GABA > trans-4-aminocrotonic acid > isoguvacine > imidazole-4-acetic acid (IMA) > (R)-(-)-4-amino-3-hydroxybutyric acid [R(-)-GABOB] > (S)-(+)-4-amino-3-hydroxybutyric acid [S(+)-GABOB] > guanidinoacetic acid > isonipecotic acid > 5-aminovaleric acid (DAVA) (partial agonist) > β-alanine (partial agonist). In silico ligand docking revealed some variation in binding between agonists. Mutagenesis of a key serine residue in binding loop C to threonine had minimal effects on GABA and IMA but significantly increased the maximal response to DAVA and decreased twofold the EC50 for R(-)- and S(+)-GABOB.
  
  
  CONCLUSIONS
  The pharmacological profile of Hco-UNC-49 differed from that of vertebrate Cys-loop GABA receptors and insect resistance to dieldrin receptors, suggesting differences in the agonist binding pocket. These findings could be exploited to develop new drugs that specifically target GABA receptors of parasitic nematodes.
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Mark D
authors: MD
authors: Kaji
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Ariel
authors: A
authors: Kwaka
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Micah K
authors: MK
authors: Callanan
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Humza
authors: H
authors: Nusrat
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Jean-Paul
authors: JP
authors: Desaulniers
authors: Faculty of Science, University of Ontario Institute of Technology, Oshawa, ON, Canada.
authors: Sean G
authors: SG
authors: Forrester
mesh_terms: D000818:Animals; D001665:Binding Sites / Q000187:drug effects; D003198:Computer Simulation; D058785:GABA-A Receptor Agonists / Q000737:chemistry / Q000378:metabolism* / Q000494:pharmacology; D006189:Haemonchus / Q000737:chemistry*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008958:Models, Molecular; D009865:Oocytes / Q000187:drug effects / Q000502:physiology; D018079:Receptors, GABA / Q000737:chemistry* / Q000378:metabolism*; D014982:Xenopus laevis
chemical_list: D058785:GABA-A Receptor Agonists; D018079:Receptors, GABA