Detection of Novel Functional Selectivity at M-3 Muscarinic Acetylcholine Receptors Using a Saccharomyces cerevisiae Platform

Functional selectivity, although new to many chemists and biologists only a few years ago, has now become a dominant theme in drug discovery. This concept posits that different ligands engender unique receptor conformations such that only a subset of signaling pathways linked to a given receptor are recruited. However, successful exploitation of the phenomenon to achieve pathway-based selectivity requires the ability to routinely detect it when assessing ligand behavior. We have utilized different strains of the yeasts. cerevislae, each expressing a specific human G alpha/yeast Gpa1 protein chimera coupled to a MAP kinase-linked reporter gene readout, to investigate the signaling of the M-3 muscarinic receptor, a G protein-coupled receptor (GPCR) for which various antagonists are used clinically. Using this novel platform, we found that the antagonists, atropine, N-methylscopolamine, and pirenzepine, were inverse agonists for Gpa1/G alpha(q) but low efficacy agonists for Gpa1/G alpha(12) Subsequent studies with atropine performed in mammalian 3T3 cells validated these findings by demonstrating inverse agonism for G(q/11)-mediated calcium mobilization but positive agonism for G(12)-mediated membrane ruffling. This is the first study to utilize a yeast platform to discover pathway-biased functional selectivity in a GPCR. In addition to the likely applicability of this approach for identifying biased signaling by novel chemical entities, our findings also suggest that currently marketed medications may exhibit hitherto unappreciated functional selectivity.