Reversed binding of a small molecule ligand in homologous chemokine receptors - differential role of extracellular loop 2

BACKGROUND AND PURPOSE: The majority of small molecule compounds targeting chemokine receptors share a similar pharmacophore with a centrally located aliphatic positive charge and flanking aromatic moieties. Here we describe a novel piperidine-based compound with structural similarity to previously described CCR8-specific agonists, but containing a unique phenyl-tetrazol moiety which, in addition to activity at CCR8 was also active at CCR1. EXPERIMENTAL APPROACH: Single point mutations were introduced in CCR1 and CCR8, and their effect on small molecule ligand-induced receptor activation was examined through inositol trisphosphate (IP(3)) accumulation. The molecular interaction profile of the agonist was verified by molecular modeling. KEY RESULTS: The chemokine receptor conserved glutamic acid in TM-VII served as a common anchor for the positively charged amine in the piperidine ring. However, whereas the phenyl-tetrazol group interacted with TyrIV:24 (Tyr(172)) and TyrIII:09 (Tyr(114)) in the major binding pocket (delimited by TM-III to VII) of CCR8, it also interacted with TrpII:20 (Trp(90)) and LysII:24 (Lys(94)) in the minor counterpart (delimited TM-I to III, plus TM-VII) in CCR1. A straightening of TM-II by Ala-substitution of ProII:18 confirmed its unique role in CCR1. The extracellular loop 2 (ECL-2) contributed directly to the small molecule binding site in CCR1, whereas it contributed to efficacy, but not potency in CCR8. CONCLUSION AND IMPLICATIONS: Despite high ligand potency and efficacy and receptor similarity, this dual-active and bitopic compound binds oppositely in CCR1 and CCR8 with different roles of ECL-2, thereby expanding and diversifying the influence of extracellular receptor regions in drug action