CB1 and CB2 cannabinoid receptors are G protein-coupled receptors which have been described to couple mainly to the Gi/o subfamily of G proteins. However, in some cell types and upon stimulation with certain cannabinoid agonists, activation of other G protein subtypes has also been observed. GPR55 is an orphan G protein-coupled receptor which has been suggested to be a novel member of the cannabinoid receptor family.
In this study we wanted to characterize the G protein activation properties of the two known cannabinoid receptors and GPR55 following stimulation with different cannabinoid ligands, using bioluminescence resonance energy transfer (BRET). We monitored the activation of different G protein subtypes (Go, Gq, Gs or G12) using Renilla luciferase-tagged wild type or chimeric Gαo subunits (i.e. Gαo with the C-terminal 5 amino acids replaced with those of Gαq, Gαs or Gα12, respectively) co-expressed with EYFP-tagged α1α11 subunit and the receptor in CHO cells.
We found that CB1 was able to activate all four subtypes of G proteins, with different pharmacokinetic properties, following stimulation by non-selective (WIN55 and 2-AG) or CB1-selective (ACEA) cannabinoid agonists. Basal activity of CB1 could also be detected with Go and G12 subtypes, as the CB1 inverse agonist AM251 caused significant BRET increase (i.e. G protein subunit association) when tested with these G proteins. In contrast, CB2 showed no G protein activation other than Go, upon either WIN55 or 2-AG stimuli. Stimulation of GPR55 with WIN55, 2-AG or AM251 did not alter the activity of the tested G proteins even at considerably high ligand concentrations.