Structural Insights into CB1 Receptor Biased Signaling

Abstract

The endocannabinoid system has emerged as a promising target for the treatment of numerous diseases, including cancer, neurodegenerative disorders, and metabolic syndromes. Thus far, two cannabinoid receptors, CB1 and CB2, have been discovered, which are found predominantly in the central nervous system (CB1) or the immune system (CB2), among other organs and tissues. CB1 receptor ligands have been shown to induce a complex pattern of intracellular effects. The binding of a ligand induces distinct conformational changes in the receptor, which will eventually translate into distinct intracellular signaling pathways through coupling to specific intracellular effector proteins. These proteins can mediate receptor desensitization, trafficking, or signaling. Ligand specificity and selectivity, complex cellular components, and the concomitant expression of other proteins (which either regulate the CB1 receptor or are regulated by the CB1 receptor) will affect the therapeutic outcome of its targeting. With an increased interest in G protein-coupled receptors (GPCR) research, in-depth studies using mutations, biological assays, and spectroscopic techniques (such as NMR, EPR, MS, FRET, and X-ray crystallography), as well as computational modelling, have begun to reveal a set of concerted structural features in Class A GPCRs which relate to signaling pathways and the mechanisms of ligand-induced activation, deactivation, or activity modulation. This review will focus on the structural features of the CB1 receptor, mutations known to bias its signaling, and reported studies of CB1 receptor ligands to control its specific signaling.

Keywords: CB1 receptor; biased signaling; cannabinoids; functional selectivity.

Figure 1

Human CB1 Helix net. (I1–I3 for the intracellular (IC) loops, E1–E3 for the extracellular (EC) loops). The most conserved amino acid residue in each helix in red. Purple highlights for putative phosphorylation sites on the C-terminus of the receptor. Blue highlights for negatively-charged residues in the C-terminus. Other residues discussed are highlighted in orange; in order, they are: F3.36, L3.43, T3.46, Y3.49, D3.51, W6.48, L341(6.33), A342(6.34), D6.30, and Y7.53.

Figure 2

Sequence alignment (hCB1/mCB1) of the C-terminus of CB1. Negatively-charged residues are in red, possible phosphorylation sites are highlighted red.

Figure 3

Canonical CB1 receptor signaling and β-arrestin2 interaction with the receptor. (A) Inactive state of the receptor with closed IC domain. (B) Agonist binding activates the receptor with opening at IC domain, mainly due to the TMH6 conformational change followed by G-protein coupling. (C) Phosphorylation of proximal (S425, S429) and distal (possibly at T460–S468) phosphorylation sites, by GRK3 and GRKs5/6, respectively. (D) β-arrestin2 binding to the C-terminus starts with an interaction with the distal phosphorylated C-terminus, which can mediate receptor internalization; followed by (E) possible conformational change at the IC domain of the receptor and the arrestin interacts with the proximal phosphorylated C-terminus, resulting in receptor desensitization.

Figure 4

Structures of the CB1 functionally-selective endocannabinoids AEA, 2-AG, mAEA, and NADA.

Figure 5

Structure of the CB1 endogenous allosteric modulator pregnenolone.

Figure 6

Structures of the plant-derived ligands (−)Δ⁹-tetrahydrocannabinol (Δ⁹-THC) and cannabidiol (CBD).

Figure 7

Structures of the phytocannabinoid synthetic ligands HU210 and CP55,940; indole derivatives WIN55,212-2, ORG27569, and PNR-4-20; and biphenylureas PSNCBAM-1, LDJ1288, and LDK1285.