Positive and Negative Allosteric Modulators of the Ca2+-sensing Receptor Interact within Overlapping but Not Identical Binding Sites in the Transmembrane Domain

A three-dimensional model of the human extracellular Ca 2+ -sensing receptor (CaSR) has been used to identify specific residues implicated in the recognition of two negative allosteric CaSR modulators of different chemical structure, NPS 2143 and Calhex 231. To demonstrate the involvement of these residues, we have analyzed dose-inhibition response curves for the effect of these calcilytics on Ca 2+ -induced [ 3 H]inositol phosphate accumulation for the selected CaSR mutants transiently expressed in HEK293 cells. These mutants were further used for investigating the binding pocket of two chemically unrelated positive allosteric CaSR modulators, NPS R-568 and ( R )-2-[1-(1-naphthyl)ethylaminomethyl]-1 H -indole (Calindol), a novel potent calcimimetic that stimulates (EC 50 = 0.31 μ m ) increases in [ 3 H]inositol phosphate levels elicited by activating the wild-type CaSR by 2 m m Ca 2+ . Our data validate the involvement of Trp-818 6.48 , Phe-821 6.51 , Glu-837 7.39 , and Ile-841 7.43 located in transmembranes (TM) 6 and TM7, in the binding pocket for both calcimimetics and calcilytics, despite important differences observed between each family of compounds. The TMs involved in the recognition of both calcilytics include residues located in TM3 (Arg-680 3.28 , Phe-684 3.32 , and Phe-688 3.36 ). However, our study indicates subtle differences between the binding of these two compounds. Importantly, the observation that some mutations that have no effect on calcimimetics recognition but which affect the binding of calcilytics in TM3 and TM5, suggests that the binding pocket of positive and negative allosteric modulators is partially overlapping but not identical. Our CaSR model should facilitate the development of novel drugs of this important therapeutic target and the identification of the molecular determinants involved in the binding of allosteric modulators of class 3 G-protein-coupled receptors.