Human P2Y1 receptor: molecular modeling and site-directed mutagenesis as tools to identify agonist and antagonist recognition sites

The role of amino acids in extracellular loops of the human P2Y1 receptor in surface expression and activation processes

The P2Y1 receptor is a membrane-bound G protein-coupled receptor stimulated by adenine nucleotides. Using alanine scanning mutagenesis, the role in receptor activation of charged amino acids (Asp, Glu, Lys, and Arg) and cysteines in the extracellular loops (EL) of the human P2Y1 receptor has been investigated. The mutant receptors were expressed in COS-7 cells and measured for stimulation of phospholipase C induced by the potent agonist 2-methylthioadenosine-5'-diphosphate (2-MeSADP). In addition to single point mutations, all receptors carried the hemagglutinin epitope at the N- terminus for detection of cell-surface expression. The C124A and C202A mutations, located near the exofacial end of transmembrane helix 3 and in EL2, respectively, ablated phospholipase C stimulation by 1000-fold greater than for the wild-type receptor. The double mutant receptor C42A/C296A exhibited no additive shift in the concentration-response curve for 2-MeSADP. These data suggest that Cys42 and Cys296 form another disulfide bridge in the extracellular region, which is critical for activation. Replacement of charged amino acids produced only minor changes in receptor activation, with two remarkable exceptions. The E209A mutant receptor (EL2) exhibited a >1000-fold shift in EC50. However, if Glu209 were substituted with amino acids capable of hydrogen bonding (Asp, Gln, or Arg), the mutant receptors responded like the wild-type receptor. Arg287 in EL3 was impaired similarly to Glu209 when substituted by alanine. Substitution of Arg287 by lysine, another positively charged residue, failed to fully restore wild-type activity.

Role of the extracellular loops of G protein-coupled receptors in ligand recognition: a molecular modeling study of the human P2Y1 receptor

The P2Y1 receptor is a G protein-coupled receptor (GPCR) and is stimulated by extracellular ADP and ATP. Site-directed mutagenesis of the three extracellular loops (ELs) of the human P2Y1 receptor indicates the existence of two essential disulfide bridges (Cys124 in EL1 and Cys202 in EL2; Cys42 in the N-terminal segment and Cys296 in EL3) and several specific ionic and H-bonding interactions (involving Glu209 and Arg287). Through molecular modeling and molecular dynamics simulations, an energetically sound conformational hypothesis for the receptor has been calculated that includes transmembrane (TM) domains (using the electron density map of rhodopsin as a template), extracellular loops, and a truncated N-terminal region. ATP may be docked in the receptor, both within the previously defined TM cleft and within two other regions of the receptor, termed meta-binding sites, defined by the extracellular loops. The first meta-binding site is located outside of the TM bundle, between EL2 and EL3, and the second higher energy site is positioned immediately underneath EL2. Binding at both the principal TM binding site and the lower energy meta-binding sites potentially affects the observed ligand potency. In meta-binding site I, the side chain of Glu209 (EL2) is within hydrogen-bonding distance (2.8 A) of the ribose O3', and Arg287 (EL3) coordinates both alpha- and beta-phosphates of the triphosphate chain, consistent with the insensitivity in potency of the 5'-monophosphate agonist, HT-AMP, to mutation of Arg287 to Lys. Moreover, the selective reduction in potency of 3'NH2-ATP in activating the E209R mutant receptor is consistent with the hypothesis of direct contact between EL2 and nucleotide ligands. Our findings support ATP binding to at least two distinct domains of the P2Y1 receptor, both outside and within the TM core. The two disulfide bridges present in the human P2Y1 receptor play a major role in the structure and stability of the receptor, to constrain the loops within the receptor, specifically stretching the EL2 over the opening of the TM cleft and thus defining the path of access to the binding site.

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.

A3 Adenosine Receptors: Protective vs. Damaging Effects Identified Using Novel Agonists and Antagonists

Investigation of the physiologic role of the A3 adenosine receptor has been facilitated by the availability of selective agonists and antagonists. Selective agonists include IB-MECA and the 2-chloro derivative Cl-IB-MECA. Selective antagonists have been identified and designed with the aid of molecular modeling among various nonpurine classes of heterocycles: flavonoids, 1,4-dihydropyridine derivatives, triazoloquinazolines, isoquinolines, and a triazolonaphthyridine. The dihydropyridine 3-ethyl 5-benzyl 2-methyl-6-phenyl-4-phenylethynyl-1,4-(±)-dihydropyridine-3,5-dicarboxylate (MRS 1191) is 1,300-fold selective for human A3 (Ki of 31 nM) vs. A1/A2A adenosine receptors and also 28-fold A3 selective in rat tissue (Ki of 1.42 mM). 9-Chloro-2-(2-furyl)-5-phenylacetylamino[1,2,4]-triazolo[1,5-c]quinazoline (MRS 1220) is useful as an A3 selective antagonist only in human tissue, with a Ki value of 0.65 nM. The pyridine derivative 5-propyl 2-ethyl-4-propyl-3-(ethylsulfanylcarbonyl)-6-phenylpyridine-5-carboxylate (MRS 1523) is a selective antagonist of both rat and human A3 receptors, with Ki values of 113 and 19 nM, respectively. Paradoxical effects of A3 agonists in the brain, heart and other tissues indicate that acute activation of A3 receptors at greater than 10 mM concentrations acts as a lethal input to cells, whereas low, nanomolar concentrations of A3 receptor agonists protect against apoptosis or ischemic damage. Adenosine A3 receptor agonists, antagonists, or both, may be useful in treating inflammatory conditions.