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

Introduction: Osteoarthritis (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 α7 nicotinic Ach receptor (α7nAchR), 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 α7nAchR in osteoblastic response to inflammation, and (iii) to study the role of α7nAchR in a spontaneous aging OA model. Methods: Primary cultures of WT and α7nAchR 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, IL1ß-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. Results: Murine WT osteoblasts express the main components of the cholinergic system and α7 subunit composing α7nAchR. Nicotine partially prevented the IL1ß-induced expression and production of IL6, MMP3, and RANKL in WT osteoblasts. The effect for IL6 and MMP was mediated by α7nAchR 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 α7 subunit as well as CHRFAM7A, the dominant negative duplicate of Chrna7. Nicotine pretreatment did not significantly reduce IL6 and MMP3 production in IL-1ß-stimulated human osteoarthritic osteoblasts (n = 4), possibly due to CHRFAM7A. Conclusion: Cholinergic system counteracts murine osteoblastic response to IL-1ß through α7nAchR. Since α7nAchR 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.

The Role of the Non-neuronal Cholinergic System in Inflammation and Degradation Processes in Osteoarthritis.

OBJECTIVE: The non-neuronal cholinergic system represents non-neuronal cells that have the biochemical machinery to synthetize de novo and/or respond to acetylcholine (ACh). We undertook this study to investigate this biochemical machinery in chondrocytes and its involvement in osteoarthritis (OA). METHODS: Expression of the biochemical machinery for ACh metabolism and nicotinic ACh receptors (nAChR), particularly α7-nAChR, in human OA and murine chondrocytes was determined by polymerase chain reaction and ligand-binding. We investigated the messenger RNA expression of the human duplicate α7-nACh subunit, called CHRFAM7A, which is responsible for truncated α7-nAChR. We assessed the effect of nAChR on chondrocytes activated by interleukin-1ß (IL-1ß) and the involvement of α7-nAChR using chondrocytes from wild-type (WT) and α7-deficient Chrna7-/- mice. The role of α7-nAChR in OA was explored after medial meniscectomy in WT and Chrna7-/- mice. RESULTS: Human and murine chondrocytes express the biochemical partners of the non-neuronal cholinergic system and a functional α7-nAChR at their cell surface (n = 5 experiments with 5 samples each). The expression of CHRFAM7A in human OA chondrocytes (n = 23 samples) correlated positively with matrix metalloproteinase 3 (MMP-3) (r = 0.38, P < 0.05) and MMP-13 (r = 0.48, P < 0.05) expression. Nicotine decreased the IL-1ß-induced IL-6 and MMP expression, in a dose-dependent manner, in WT chondrocytes but not in Chrna7-/- chondrocytes. Chrna7-/- mice that underwent meniscectomy (n = 7) displayed more severe OA cartilage damage (mean ± SD Osteoarthritis Research Society International [OARSI] score 4.46 ± 1.09) compared to WT mice that underwent meniscectomy (n = 9) (mean ± SD OARSI score 3.05 ± 0.9; P < 0.05). CONCLUSION: The non-neuronal cholinergic system is functionally expressed in chondrocytes. Stimulation of nAChR induces antiinflammatory and anticatabolic activity through α7-nAChR, but the anticatabolic activity may be mitigated by truncated α7-nAChR in human chondrocytes. In vivo experiments strongly suggest that α7-nAChR has a protective role in OA.