Development of pharmacological modulators for the orphan G protein-coupled receptor GPR17 and their application for the investigation of molecular signal transduction pathways

Abstract

In order to discover a set of tools suitable to be employed in its in vitro characterization, the recently deorphanized G protein coupled receptor GPR17 was investigated by using different classical second messenger assays and the new dynamic mass redistribution technology. <br /> Contrary to reports from the literature which claim a function of GPR17 as a nucleotide receptor or as ligand and signalling independent, the receptor was found to be activated by a new class of GPCR ligands sharing an indole scaffold. Besides agonists, different classes of natural and synthetic compounds have been identified and characterised to display antagonistic activity at the receptor. <br /> Detailed compound analysis shows first evidence of functional selectivity. Both, the identified agonists and antagonists are potent lead structures for the development of novel therapies for ischemia and inflammation, two diseases which are promoted or modulated by GPR17. <br /> Furthermore, it could be shown that the receptor, upon activation, dependent on the cellular background is linked to the Gq, Gi and Gs signal transduction pathways. <br /> A detailed analysis of the signal transduction network reveals a novel calcium release mechanism in 1321N1 astrocytoma cells recombinatly expressing GPR17. This mechanism is characterized by a Gi dependent activation of a phosphatidylinositol (PI) specific phospholipase C. According to a hypothesis this in turn leads to a stimulation of intracellular calcium stores by IP3 receptors which is independent from inositol phosphate formation. Our model proposes the activation of IP3 receptors by the formation of a Gi-PI-PLC-IP3 receptor complex.