Small changes in bone structure of female [alpha]7 nicotinic acetylcholine receptor knockout mice

Background Recently, analysis of bone from knockout mice identified muscarinic acetylcholine receptor subtype M3 (mAChR M3) and nicotinic acetylcholine receptor (nAChR) subunit [alpha]2 as positive regulator of bone mass accrual whereas of male mice deficient for [alpha]7-nAChR ([alpha]7KO) 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 [alpha]7KO compared to their corresponding wild-type mice ([alpha]7WT). Methods Vertebrae and long bones of female 16-week-old [alpha]7KO (n = 10) and [alpha]7WT (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 [alpha]7KO revealed a significant increase in bending stiffness (p < 0.05) and cortical thickness (p < 0.05) compared to [alpha]7WT, whereas gene expression of osteoclast marker cathepsin K was declined. ToF-SIMS analysis detected a decrease in trabecular calcium content and an increase in C.sub.4H.sub.6N.sup.+ (p < 0.05) and C.sub.4H.sub.8N.sup.+ (p < 0.001) collagen fragments whereas a loss of osteoid was found by means of TEM. Conclusions Our results on female [alpha]7KO bone identified differences in bone strength and composition. In addition, we could demonstrate that [alpha]7-nAChRs are involved in regulation of bone remodelling. In contrast to mAChR M3 and nAChR subunit [alpha]2 the [alpha]7-nAChR favours reduction of bone strength thereby showing similar effects as [alpha]7[beta]2-nAChR in male mice. nAChR are able to form heteropentameric receptors containing [alpha]- and [beta]-subunits as well as the subunits [alpha]7 can be arranged as homopentameric cation channel. The different effects of homopentameric and heteropentameric [alpha]7-nAChR on bone need to be analysed in future studies as well as gender effects of cholinergic receptors on bone homeostasis. Keywords: Nicotinic receptor, Bone strength, Bending stiffness, Cathepsin K, ToF-SIMS, Osteoid, Knockout mice, Micro-CT