Pairing orphan G protein–coupled receptors (GPCRs) with their cognate endogenous ligands is expected to have a major impact on our understanding of GPCR biology. It follows that the reproducibility of orphan receptor ligand pairs should be of fundamental importance to guide meaningful investigations into the pharmacology and function of individual receptors. GPR17 is an orphan receptor characterized by some as a dualistic uracil nucleotide/cysteinyl leukotriene receptor and by others as inactive toward these stimuli altogether. Whereas regulation of central nervous system myelination by GPR17 is well established, verification of activity of its putative endogenous ligands has proven elusive so far. Herein we report that uracil nucleotides and cysteinyl leukotrienes do not activate human, mouse, or rat GPR17 in various cellular backgrounds, including primary cells, using eight distinct functional assay platforms based on label-free pathway-unbiased biosensor technologies, as well as canonical second-messenger or biochemical assays. Appraisal of GPR17 activity can be accomplished with neither the coapplication of both ligand classes nor the exogenous transfection of partner receptors nucleotide purinergic G protein–coupled receptor, cysteinyl leukotriene 1, to reconstitute the elusive pharmacology. Moreover, our study does not support the inhibition of GPR17 by the marketed antiplatelet drugs cangrelor and ticagrelor, previously suggested to antagonize GPR17. Whereas our data do not disagree with a role of GPR17 per se as an orchestrator of central nervous system functions, they challenge the utility of the proposed (ant)agonists as tools to imply direct contribution of GPR17 in complex biologic settings.
- Received December 16, 2016.
- Accepted March 1, 2017.
K.S., N.M., and Ral.S. contributed equally to this work.
↵1 Current affiliation: CNS Discovery Research, UCB Pharma, Chemin du Foriest R4, B-1420 Braine-l’Alleud, Belgium.
This work was funded by a grant from the German Federal Ministry for Education and Research within the BioPharma Initiative “Neuroallianz” [Grant 1615609B]. K.S. and E.K. are members of the German Research Foundation (DFG)-funded Research Training Group RTG1873.
Note in memoriam: We dedicate this manuscript to our coworker, colleague, and friend Lucas Peters, who deceased while this study was in progress.
- Copyright © 2017 by The American Society for Pharmacology and Experimental Therapeutics