Stable expression and functional characterization of a human nicotinic acetylcholine receptor with α6ß2 properties: discovery of selective antagonists.

BACKGROUND AND PURPOSE: Despite growing evidence that inhibition of α6ß2-containing (α6ß2*) nicotinic acetylcholine receptors (nAChRs) may be beneficial for the therapy of tobacco addiction, the lack of good sources of α6ß2*-nAChRs has delayed the discovery of α6ß2-selective antagonists. Our aim was to generate a cell line stably expressing functional nAChRs with α6ß2 properties, to enable pharmacological characterization and the identification of novel α6ß2-selective antagonists. EXPERIMENTAL APPROACH: Different combinations of the α6, ß2, ß3, chimeric α6/3 and mutant ß3(V273S) subunits were transfected in human embryonic kidney cells and tested for activity in a fluorescent imaging plate reader assay. The pharmacology of rat immune-immobilized α6ß2*-nAChRs was determined with ¹²5I-epibatidine binding. KEY RESULTS: Functional channels were detected after co-transfection of α6/3, ß2 and ß3(V273S) subunits, while all other subunit combinations failed to produce agonist-induced responses. Stably expressed α6/3ß2ß3(V273S)-nAChR pharmacology was unique, and clearly distinct from α4ß2-, α3ß4-, α7- and α1ß1δε-nAChRs. Antagonist potencies in inhibiting α6/3ß2ß3(V273S) -nAChRs was similar to their binding affinity for rat native α6ß2*-nAChRs. Agonist affinities for α6ß2*-nAChRs was higher than their potency in activating α6/3ß2ß3(V273S)-nAChRs, but their relative activities were equivalent. Focussed set screening at α6/3ß2ß3(V273S)-nAChRs, followed by cross-screening with the other nAChRs, led to the identification of novel α6ß2-selective antagonists. CONCLUSIONS AND IMPLICATIONS: We generated a mammalian cell line stably expressing nAChRs, with pharmacological properties similar to native α6ß2*-nAChRs, and used it to identify novel non-peptide, low molecular weight, α6ß2-selective antagonists. We also propose a pharmacophore model of α6ß2 antagonists, which offers a starting point for the development of new smoking cessation agents.

Cloning and pharmacological characterization of the cat urotensin-II receptor (UT).

Urotensin-II (U-II), acting through its G-protein-coupled receptor, UT, is a possible contributor to hypertension. Variable functional responses to U-II, both within and between species studied to date, complicate the characterization of UT antagonists. In the cat, however, U-II causes systemic hypertension and constricts arterial segments isolated from several vascular beds. The purpose of this study was to clone and pharmacologically characterize cat recombinant UT to determine whether this system represents a model for characterizing UT antagonists. Cloned cat UT displayed 74% identity to primate UT, and 77% identity to rodent UT. [(125)I] hU-II bound in a saturable manner to a single site on recombinant cat UT with high affinity (K(D) 288+/-13pM) and high density (B(max) 747+/-66fmol/mg protein). U-II isopeptides displayed equipotent, high affinity binding to cat UT (K(i) 1.8-5.3nM). Cat UT was coupled to intracellular [Ca(2+)] release (EC(50) 0.6+/-0.2nM) and total inositol phosphate (IP) formation (EC(50) 0.4+/-0.1nM). Protein kinase C activation desensitized cat, but not human, UT-mediated IP formation. UT mRNA expression was detected in cat blood vessels, trachea, lung, and kidney, where the medulla (K(D) 815+/-34) and cortex and (K(D) 316+/-39pM) displayed high affinity binding for human U-II (hU-II). The cat urotensin-II receptor represents a suitable in vitro model to examine the role of the U-II/UT system in the etiology of hypertension, assisting in the evaluation of the UT antagonists to help treat cardiovascular disease.

Heterologous expression of G protein-coupled receptors in U-2 OS osteosarcoma cells.

Recombinant baculoviruses, in which the insect cell-specific polyhedrin promoter has been replaced with a mammalian cell-active expression cassette (BacMam viruses), are efficient gene delivery vehicles for many mammalian cell types. BacMam viruses have been generated for expression of G protein-coupled receptors (GPCRs) and used to establish Ca2+mobilization assays in HEK-293 human embryonic kidney cells and U-2 OS human osteosarcoma cells. U-2 OS cells are highly susceptible to BacMam-based gene delivery and lack many of the endogenous receptors present on HEK-293 and other mammalian cell lines typically used for heterologous expression of GPCRs. U-2 OS cells were found to have a null background for muscarine, ADP, ATP, UTP, UDP, and lysophosphatidic acid (LPA). Consequently, U-2 OS cells transduced with BacMam constructs encoding the muscarinic acetylcholine receptors (M1, M2, M3, M4, and M5subtypes), the P2Y receptors (P2Y1, P2Y2), or the LPA receptors (EDG-2, EDG-7) were used for the establishment of whole-cell Ca2+mobilization assays, assays that cannot readily be established in HEK-293 cells. U-2 OS cells were susceptible to simultaneous expression of multiple genes delivered by BacMam vectors. In U-2 OS cells the functional expression of the Gi-coupled M2and M4receptors was dependent on co-expression of the receptor and a G protein chimera, both of which were delivered to the cells via BacMam viruses. The use of U-2 OS cells and BacMam-based gene delivery has facilitated development of whole-cell-based GPCR functional assays, especially for P2Y, muscarininc acetylcholine, and LPA receptors.