Novel Insights into CB1 Receptor Signaling and the Anabolic Role of Cannabinoid Receptors in Bone

Abstract

Activation of the CB1 receptor is modulated by aspartate residue D2.63176 in transmembrane helix (TMH) II. Interestingly, D2.63 does not affect the affinity for ligand binding at the CB1 receptor. Studies in class A GPCRs have suggested an ionic interaction between residues of TMHII and VII. In this report, modeling studies identified residue K373, in the extracellular (EC)-3 loop, in charged interactions with D2.63. We investigated this possibility by performing reciprocal mutations and biochemical studies. D2.63176A, K373A, D2.63176A-K373A, and the reciprocal mutant with the interacting residues juxtaposed, D2.63176K-K373D were characterized using radioligand binding and guanosine 5'-3-O-(thio)triphosphate functional assays. None of the mutations resulted in a significant change in the binding affinity of CP55,940 or SR141716A. Computational results indicate that the D2.63176-K373 ionic interaction strongly influences the conformation(s) of the EC-3 loop, providing a structure-based rationale for the importance of the EC-3 loop to signal transduction in CB1. Specifically, the putative ionic interaction results in the EC-3 loop pulling over the top (extracellular side) of the receptor; this EC-3 loop conformation may serve protective and mechanistic roles. These results suggest that the ionic interaction between D2.63176 and K373 is crucial for CB1 signal transduction. This work may help to aide drug design efforts for the effective treatment of different diseases. The cannabinoid receptors of osteoblasts may represent a target for the treatment of bone disorders such as osteoporosis. Our research demonstrates that cannabinoids can affect important signaling molecules in osteoblasts. In MC3T3-E1 osteoblastic cells, the CB1 antagonist, AM251, has been reported to induce increases in Runx2 mRNA, mineralized bone nodule formation, and activation of signaling molecules such as ERK and AKT (Wu et al., 2011). Studies from our lab characterizing mice in which both CB1 and CB2 receptors were inactivated by homologous recombination have demonstrated increased bone mass coupled with enhanced osteoblast differentiation of bone marrow stromal cells in culture (manuscript in preparation). We explored the effect of antagonizing CB1 and CB2 cannabinoid receptors in osteoblastic cells to gain insights into molecular pathways that may help to explain the effects of the endocannabinoid system (ECS) in bone development. Our data was generated by running time course experiments with MC3T3-E1 cells under the influence of SR141716A, SR144528 or both in combination. The cells were harvested with a lysis buffer at specific time points and analyzed by western blot analysis. Quantification of protein activation was calculated using LiCor imaging equipment and software. Within 15 minutes, treatment with the CB1 receptor antagonist SR141716A resulted in several fold increases in pERK, pSMAD158, and pAKT. SR144528, a CB2 receptor antagonist, caused increases in pERK and pSMAD158, but not pAKT. When both antagonists were applied together, pERK and pSMAD158 levels increased, while pAKT signaling was diminished compared to SR141716A alone. The finding that cannabinoid receptor antagonists alter the activity of the SMAD158 complex is a novel finding, which suggests that cannabinoids can influence bone morphogenic signaling pathways, and therefore play a significant role in osteoblast differentiation and function.