Use of a new proximity assay (NanoBRET) to investigate the ligand‐binding characteristics of three fluorescent ligands to the humanβ1‐adrenoceptor expressed in HEK‐293 cells

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Abstract
Previous research has indicated that allosteric interactions across the dimer interface ofβ1‐adrenoceptors may be responsible for a secondary low affinity binding conformation. Here we have investigated the potential for probe dependence, in the determination of antagonist pKivalues at the humanβ1‐adenoceptor, which may result from such allosterism interactions. Three fluorescentβ1‐adrenoceptor ligands were used to investigate this using bioluminescence energy transfer (BRET) between the receptor‐bound fluorescent ligand and the N‐terminal NanoLuc tag of a humanβ1‐adrenoceptor expressed in HEK 293 cells (NanoBRET). This proximity assay showed high‐affinity‐specific binding to the NanoLuc‐β1‐adrenoceptor with each of the three fluorescent ligands yieldingKDvalues of 87.1 ± 10 nmol/L (n = 8), 38.1 ± 12 nmol/L (n = 7), 13.4 ± 2 nmol/L (n = 14) for propranolol‐Peg8‐BY630, propranolol‐β(Ala‐Ala)‐BY630 and CGP‐12177‐TMR, respectively. Parallel radioligand‐binding studies with3H‐CGP12177 and TIRF microscopy, to monitor NanoLuc bioluminescence, confirmed a high cell surface expression of the NanoLuc‐β1‐adrenoceptor in HEK 293 cells (circa 1500 fmol.mg protein−1). Following a 1 h incubation with fluorescent ligands andβ1‐adrenoceptor competing antagonists, there were significant differences (P < 0.001) in the pKivalues obtained for CGP20712a and CGP 12177 with the different fluorescent ligands and3H‐CGP 12177. However, increasing the incubation time to 2 h removed these significant differences. The data obtained show that the NanoBRET assay can be applied successfully to study ligand‐receptor interactions at the humanβ1‐adrenoceptor. However, the study also emphasizes the importance of ensuring that both the fluorescent and competing ligands are in true equilibrium before interpretations regarding probe dependence can be made.
Funding Information
  • Medical Research Council (0800006)
  • University of Nottingham

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