Small changes in bone structure of female a7 nicotinic acetylcholine receptor knockout mice

Abstract

BACKGROUND: Recently, analysis of bone from knockout mice identified muscarinic acetylcholine receptor subtype M3 (mAChR M3) and nicotinic acetylcholine receptor (nAChR) subunit a2 as positive regulator of bone mass accrual whereas of male mice deficient for a7-nAChR (a7KO) did not reveal impact in regulation of bone remodeling. Since female sex hormones are involved in fair coordination of osteoblast bone formation and osteoclast bone degradation we assigned the current study to analyze bone strength, composition and microarchitecture of female a7KO compared to their corresponding wild-type mice (a7WT). METHODS: Vertebrae and long bones of female 16-week-old a7KO (n = 10) and a7WT (n = 8) were extracted and analyzed by means of histological, radiological, biomechanical, cell- and molecular methods as well as time of flight secondary ion mass spectrometry (ToF-SIMS) and transmission electron microscopy (TEM). RESULTS: Bone of female a7KO revealed a significant increase in bending stiffness (p<0.05) and cortical thickness (p<0.05) compared to a7WT, whereas gene expression of osteoclast marker cathepsin K was declined. ToF-SIMS analysis detected a decrease in trabecular calcium content and an increase in C4H6N+ (p<0.05) and C4H8N+ (p<0.001) collagen fragments whereas a loss of osteoid was found by means of TEM. CONCLUSIONS: Our results on female a7KO bone identified differences in bone strength and composition. In addition, we could demonstrate that a7-nAChRs are involved in regulation of bone remodelling. In contrast to mAChR M3 and nAChR subunit a2 the a7-nAChR favours reduction of bone strength thereby showing similar effects as a7ß2-nAChR in male mice. nAChR are able to form heteropentameric receptors containing a- and ß-subunits as well as the subunits a7 can be arranged as homopentameric cation channel. The different effects of homopentameric and heteropentameric a7-nAChR on bone need to be analysed in future studies as well as gender effects of cholinergic receptors on bone homeostasis.