Non-neuronal cholinergic stimulation favors bone mass accrual

Abstract

Introduction: Non-neuronal cholinergic receptors are expressed in immune cells and their stimulation has been shown to regulate the secretion of several cytokines. Some of these cytokines, such as interleukin-17 (IL-17), IL-23, interferon-γ (IFN-γ) and tumor necrosis factor-α (TNF-α), are known to regulate bone mass. Accordingly, we hypothesize that stimulating cholinergic receptors in non-neuronal cells, such as immune cells, promotes bone mass accrual.

Methods: To test this hypothesis, we used neostigmine, a drug that increases acetylcholine levels by inhibiting acetylcholinesterase activity in peripheral tissues. Female C57BL/6 mice were treated with neostigmine for six weeks, and μCT, histomorphometry, Raman spectroscopy, X-ray diffraction, and mechanical testing were used to analyze bone parameters. A rat model was used to assess bone defect healing and implant osseointegration. Serum cytokines were measured by ELISA, and IL-17 effects on osteoblast proliferation were evaluated in vitro.

Results: Here, we show that 6 weeks of neostigmine treatment promotes bone mass accrual in endochondral bones of both the axial and appendicular skeleton in mice. Moreover, the administration of neostigmine for 2 weeks accelerated the healing process of the surgically induced bone defects in rats. The body mass index, body weight, visceral fat pad weight and epinephrine levels in the neostigmine-treated mice were similar to those of saline-treated mice, indicating that neostigmine favored bone mass accrual by acting peripherally rather than centrally. The increased bone mass in the neostigmine-treated mice was caused by an increase in osteoblast proliferation and bone formation rate. We also observed an increase in circulating immunocytokine IL-17 levels in the neostigmine-treated mice. Statistical analysis showed that the increase in serum IL-17 level was associated with the increase in osteoblast number. In agreement with our findings from the in vivo experiments, IL-17 treatment increased the proliferation of MC3T3.E1 preosteoblasts in vitro, while acetylcholine or neostigmine did not have any significant effect.

Conclusion: Taken together, our findings indicate that peripheral cholinergic stimulation promotes bone mass accrual, in part through IL-17-mediated osteoblast activity. Although the evidence is correlative, these results highlight a potential neuro-immune pathway and suggest new therapeutic directions for enhancing bone formation and regeneration.

Keywords: IL-17; bone; bone remodeling; cholinergic neurons; immune system; muscarinic receptors.