The CB1 cannabinoid receptor (CB1R) is the major cannabinoid receptor in neuronal cells and the brain, but it also occurs in endocrine cells and other peripheral tissues. CB1R is a member of the superfamily of G-protein-coupled receptors (GPCRs), which are characterized by seven transmembrane helices. The major mediators of CB1R are the G proteins of the Gi/o family, which inhibit adenylyl cyclases in most tissues and cells, and regulate ion channels, including calcium and potassium ion channels. Regulation of ion channels is an important component of neurotransmission modulation by endogenous cannabinoid compounds released in response to depolarization and Ca2+-mobilizing hormones. However, evidence exists that CB1Rs can also stimulate adenylyl cyclase via Gs, induce receptor-mediated Ca2+ fluxes and stimulate phospholipases in some experimental models. Stimulation of CB1R also leads to phosphorylation and activation of mitogen-activated protein kinases (MAPK), such as p42/p44 MAPK, p38 MAPK and c-Jun N-terminal kinase, which can regulate nuclear transcription factors. Activated and phosphorylated CB1Rs also associate with β-arrestin molecules, which can induce the formation of signalling complexes and participate in the regulation of GPCR signalling. Recent data also suggest that CB1Rs can form homo- and heterodimers/oligomers, and the altered pharmacological properties of these receptor complexes may explain the pharmacological differences observed in various tissues.
Gábor Turu and László Hunyady
Pál Gyombolai, András D Tóth, Dániel Tímár, Gábor Turu and László Hunyady
The role of the highly conserved ‘DRY’ motif in the signaling of the CB1 cannabinoid receptor (CB1R) was investigated by inducing single-, double-, and triple-alanine mutations into this site of the receptor. We found that the CB1R-R3.50A mutant displays a partial decrease in its ability to activate heterotrimeric Go proteins (∼80% of WT CB1R (CB1R-WT)). Moreover, this mutant showed an enhanced basal β-arrestin2 (β-arr2) recruitment. More strikingly, the double-mutant CB1R-D3.49A/R3.50A was biased toward β-arrs, as it gained a robustly increased β-arr1 and β-arr2 recruitment ability compared with the WT receptor, while its G-protein activation was decreased. In contrast, the double-mutant CB1R-R3.50A/Y3.51A proved to be G-protein-biased, as it was practically unable to recruit β-arrs in response to agonist stimulus, while still activating G-proteins, although at a reduced level (∼70% of CB1R-WT). Agonist-induced ERK1/2 activation of the CB1R mutants showed a good correlation with their β-arr recruitment ability but not with their G-protein activation or inhibition of cAMP accumulation. Our results suggest that G-protein activation and β-arr binding of the CB1R are mediated by distinct receptor conformations, and the conserved ‘DRY’ motif plays different roles in the stabilization of these conformations, thus mediating both G-protein- and β-arr-mediated functions of CB1R.