Pharmacological Implications of A2Ar-D2R Heteromerization and the Significance for Parkinson's Disease

Abstract

Background: Recently, heteromeric GPCR complexes have become attractive targets for drug development since they exhibit distinct signaling and cell-specific localization from their homomeric counterparts. Yet, the effect of heteromerization on the pharmacology of many GPCR homomers remains unknown. We have undertaken the task to examine the effect of heteromerization on Gs signaling through the adenosine 2A receptor (A2AR) and Gi signaling through the dopamine receptor type 2 (D2R) because the A2AR-D2R heteromer is an emerging therapeutic target for Parkinson's disease. Previous work suggests A2AR-D2R heteromerization induces reciprocal antagonism of homomeric signaling.Methods: We examined the effect of heteromerization on A2AR and D2R homomeric signaling using electrophysiology and the Xenopus oocyte heterologous expression system. GIRK channels were used as reporters for Gi signaling because activation leads to direct Gbeta-gamma-mediated stimulation of the GIRK current. We also coupled GIRK channels to Gs signaling by over expressing Gαs. Our electrophysiological assay is innovative because it allows us to optimize the conditions of heteromerization. We anticipate that specific ligand combinations targeting the A2AR-D2R heteromer will be more efficacious than individual drug administration targeting A2AR or D2R homomers.Results: Preliminary data have demonstrated that heteromer formation decreases dopamine-elicited Gi signaling through the D2R and CGS-21680-elicited Gs signaling through the A2AR. Furthermore, this reciprocal antagonism seemed to occur through a wide GPCR (cRNA) injection ratio. Currently, we are examining crosstalk by assessing whether A2AR agonists or inverse agonists will decrease or increase D2R-mediated Gi signaling through the A2AR-D2R heteromer. Modulation of Gs signaling through the A2AR by D2R ligands is also being examined.Conclusions: Our electrophysiological assay demonstrates A2AR-D2R heteromerization induces reciprocal antagonism reducing both A2AR and D2R signaling from that of homomeric levels.