Alpha-7 Nicotinic Receptor Dampens Murine Osteoblastic Response to Inflammation and Age-Related Osteoarthritis

IntroductionOsteoarthritis (OA) is a whole-joint disease characterized by a low-grade inflammation that is involved in both cartilage degradation and subchondral bone remodeling. Since subchondral bone has a cholinergic innervation and that acetylcholine (Ach) might have an anti-inflammatory effect through the alpha 7 nicotinic Ach receptor (alpha 7nAchR), we aimed (i) to determine the expression of non-neuronal cholinergic system and nicotinic receptor subunits by murine and human osteoblasts, (ii) to address the role of alpha 7nAchR in osteoblastic response to inflammation, and (iii) to study the role of alpha 7nAchR in a spontaneous aging OA model. MethodsPrimary cultures of WT and alpha 7nAchR knock-out mice (Chrna7(-/-)) murine osteoblasts and of subchondral bone human OA osteoblasts were performed. The expressions of the non-neuronal cholinergic system and of the nAchR subunits were assessed by PCR. In vitro, IL1 beta-stimulated WT, Chrna7(-/-), and human osteoblasts were pretreated with nicotine. At 24 h, expressions of interleukin-6 (IL6) and metalloproteinase-3 and -13 (MMP), RANK-ligand (RANKL), and osteoprotegerin (OPG) were quantified by qPCR and ELISA. Spontaneous aging OA was evaluated and compared between male WT and Chrna7(-/-) mice of 9 and 12 months. ResultsMurine WT osteoblasts express the main components of the cholinergic system and alpha 7 subunit composing alpha 7nAchR. Nicotine partially prevented the IL1 beta-induced expression and production of IL6, MMP3, and RANKL in WT osteoblasts. The effect for IL6 and MMP was mediated by alpha 7nAchR since nicotine had no effect on Chrna7(-/-) osteoblasts while the RANKL decrease persisted. Chrna7(-/-) mice displayed significantly higher cartilage lesions than their WT counterparts at 9 and 12 months, without difference in subchondral bone remodeling. Human OA osteoblasts also expressed the non-neuronal cholinergic system and alpha 7 subunit as well as CHRFAM7A, the dominant negative duplicate of Chrna7. Nicotine pretreatment did not significantly reduce IL6 and MMP3 production in IL-1 beta-stimulated human osteoarthritic osteoblasts (n = 4), possibly due to CHRFAM7A. ConclusionCholinergic system counteracts murine osteoblastic response to IL-1 beta through alpha 7nAchR. Since alpha 7nAchR deletion may limit cartilage degradation during murine age-related OA, enhancing cholinergic system could be a new therapeutic target in OA but may depend on CHRFAM7A expression.