Structural insights into the high basal activity and inverse agonism of the orphan receptor GPR6 implicated in Parkinson's disease.

GPR6 is an orphan G protein-coupled receptor with high constitutive activity found in D2-type dopamine receptor-expressing medium spiny neurons of the striatopallidal pathway, which is aberrantly hyperactivated in Parkinson's disease. Here, we solved crystal structures of GPR6 without the addition of a ligand (a pseudo-apo state) and in complex with two inverse agonists, including CVN424, which improved motor symptoms in patients with Parkinson's disease in clinical trials. In addition, we obtained a cryo-electron microscopy structure of the signaling complex between GPR6 and its cognate G(s) heterotrimer. The pseudo-apo structure revealed a strong density in the orthosteric pocket of GPR6 corresponding to a lipid-like endogenous ligand. A combination of site-directed mutagenesis, native mass spectrometry, and computer modeling suggested potential mechanisms for high constitutive activity and inverse agonism in GPR6 and identified a series of lipids and ions bound to the receptor. The structures and results obtained in this study could guide the rational design of drugs that modulate GPR6 signaling.