Biglycan and chondroitin sulfate play pivotal roles in bone toughness via retaining bound water in bone mineral matrix.

Recent in vitro evidence shows that glycosaminoglycans (GAGs) and proteoglycans (PGs) in bone matrix may functionally be involved in the tissue-level toughness of bone. In this study, we showed the effect of biglycan (Bgn), a small leucine-rich proteoglycan enriched in extracellular matrix of bone and the associated GAG subtype, chondroitin sulfate (CS), on the toughness of bone in vivo, using wild-type (WT) and Bgn deficient mice. The amount of total GAGs and CS in the mineralized compartment of Bgn KO mouse bone matrix decreased significantly, associated with the reduction of the toughness of bone, in comparison with those of WT mice. However, such differences between WT and Bgn KO mice diminished once the bound water was removed from bone matrix. In addition, CS was identified as the major subtype in bone matrix. We then supplemented CS to both WT and Bgn KO mice to test whether supplemental GAGs could improve the tissue-level toughness of bone. After intradermal administration of CS, the toughness of WT bone was greatly improved, with the GAGs and bound water amount in the bone matrix increased, while such improvement was not observed in Bgn KO mice or with supplementation of dermatan sulfate (DS). Moreover, CS supplemented WT mice exhibited higher bone mineral density and reduced osteoclastogenesis. Interestingly, Bgn KO bone did not show such differences irrespective of the intradermal administration of CS. In summary, the results of this study suggest that Bgn and CS in bone matrix play a pivotal role in imparting the toughness to bone most likely via retaining bound water in bone matrix. Moreover, supplementation of CS improves the toughness of bone in mouse models.

Measurements of mobile and bound water by nuclear magnetic resonance correlate with mechanical properties of bone.

Since clinical measures of bone mineral density do not necessarily predict whether a person will fracture a bone without an intervention, there is a need to find supplementary tools for assessing bone quality. Presently, we hypothesized that measures of mobile and bound water by a Nuclear Magnetic Resonance (NMR) technique are correlated with bone strength and toughness, respectively. To test this, bending specimens from the mid-diaphysis of 18 human femurs were collected from 18 male donors and divided into middle aged and elderly groups. After NMR measurements of each hydrated specimen, an inversion technique was used to convert the free induction decay data into a distribution of spin-spin (T2) relaxation rates. Then, the distribution resolved into three distinct components that likely represent solid hydrogen, water bound to bone tissue, and mobile water that occupy microscopic pores within the bone specimen. The integrated signal intensities of the bound and mobile components were normalized by the wet mass of the specimen. Following NMR measurements, three point bending tests were conducted to determine the modulus of elasticity, flexure strength, and work to fracture of each specimen. Next, the porosity, mineral-to-collagen ratio, and pentosidine concentration were measured. In this sample of human cortical bone, there was no age-related difference in the amount of mobile water, but the decrease in the amount of bound water with increasing age was statistically significant. Moreover, bound water was associated with both strength and work to fracture of bone, while mobile water was correlated with modulus of elasticity and appeared to quantify the level of microscopic pores within bone. On the other hand, bound water was correlated with the concentration of non-enzymatic collagen cross-links. The results of this study indicate that quantifying mobile and bound water with magnetic resonance techniques could potentially serve as indicators of bone quality.