Allosterism in the adenosine A_2A and cannabinoid CB₂ heteromer

Allosterism is a regulatory mechanism for GPCRs that can be attained by ligand-binding or protein-protein interactions with another GPCR. We have studied the influence of the dimer interface on the allosteric properties of the A_2A receptor and CB₂ receptor heteromer.

We have evaluated cAMP production, phosphorylation of signal-regulated kinases (pERK1/2), label-free dynamic mass redistribution, β-arrestin 2 recruitment and bimolecular fluorescence complementation assays in the absence and presence of synthetic peptides that disrupt the formation of the heteromer. Molecular dynamic simulations provided converging evidence that the heteromeric interface influences the allosteric properties of the A_2AR-CB₂R heteromer.

Apo A_2AR blocks agonist-induced signalling of CB₂R. The disruptive peptides, with the amino acid sequence of transmembrane (TM) 6 of A_2AR or CB₂R, facilitate CB₂R activation, suggesting that A_2AR allosterically prevents the outward movement of TM 6 of CB₂R for G protein binding. Significantly, binding of the selective antagonist SCH 58261 to A_2AR also facilitated agonist-induced activation of CB₂R.

It is proposed that the A_2AR-CB₂R heteromer contains distinct dimerization interfaces that govern its functional properties. The molecular interface between protomers of the A_2AR-CB₂R heteromer interconverted from TM 6 for apo or agonist-bound A_2AR, blocking CB₂R activation, to mainly the TM 1/7 interface for antagonist-bound A_2AR, facilitating the independent opening of intracellular cavities for G protein binding. These novel results shed light on a different type of allosteric mechanism and extend the repertoire of GPCR heteromer signalling.

This article is part of a themed issue Complexity of GPCR Modulation and Signaling (ERNST). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v182.14/issuetoc.