In recent years, several novel aspects of GPCR pharmacology have been described, which are thought to play a role in determining the in vivo efficacy of a compound. Fluorescent ligands have been used to study many of these, which have also required the development of new experimental approaches. Fluorescent ligands offer the potential to use the same fluorescent probe to perform a broad range of experiments, from single‐molecule microscopy to in vivo BRET. This review provides an overview of the in vitro use of fluorescent ligands in further understanding emerging pharmacological paradigms within the GPCR field, including ligand‐binding kinetics, allosterism and intracellular signalling, along with the use of fluorescent ligands to study physiologically relevant therapeutic agents.
Previous research has indicated that allosteric interactions across the dimer interface of β 1‐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 pKi values at the human β 1‐adenoceptor, which may result from such allosterism interactions. Three fluorescent β 1‐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 human β 1‐adrenoceptor expressed in HEK 293 cells (NanoBRET). This proximity assay showed high‐affinity‐specific binding to the NanoLuc‐ β 1‐adrenoceptor with each of the three fluorescent ligands yielding K D values 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 with 3H‐CGP12177 and TIRF microscopy, to monitor NanoLuc bioluminescence, confirmed a high cell surface expression of the NanoLuc‐ β 1‐adrenoceptor in HEK 293 cells (circa 1500 fmol.mg protein−1). Following a 1 h incubation with fluorescent ligands and β 1‐adrenoceptor competing antagonists, there were significant differences (P < 0.001) in the pKi values obtained for CGP20712a and CGP 12177 with the different fluorescent ligands and 3H‐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 human β 1‐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.
Receptor internalization in response to prolonged agonist treatment is an important regulator of G protein–coupled receptor (GPCR) function. The adenosine A1 receptor (A1AR) is one of the adenosine receptor family of GPCRs, and evidence for its agonist-induced internalization is equivocal. The recently developed NanoBiT technology uses split NanoLuc Luciferase to monitor changes in protein interactions. We have modified the human A1AR on the N-terminus with the small high-affinity HiBiT tag. In the presence of the large NanoLuc subunit (LgBiT), complementation occurs, reconstituting a full-length functional NanoLuc Luciferase. Here, we have used complemented luminescence to monitor the internalization of the A1AR in living HEK293 cells. Agonist treatment resulted in a robust decrease in cell-surface luminescence, indicating an increase in A1AR internalization. These responses were inhibited by the A1AR-selective antagonist 1,3-dipropyl-8-cyclopentylxanthine (DPCPX), with an antagonist affinity that closely matched that measured using ligand binding with a fluorescent A1 receptor antagonist (CA200645). The agonist potencies for inducing A1AR internalization were very similar to the affinities previously determined by ligand binding, suggesting little or no amplification of the internalization response. By complementing the HiBiT tag to exogenous purified LgBiT, it was also possible to perform NanoBRET ligand-binding experiments using HiBiT–A1AR. This study demonstrates the use of NanoBiT technology to monitor internalization of the A1AR and offers the potential to combine these experiments with NanoBRET ligand-binding assays.
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