Age‐Related Deterioration of Bone Toughness Is Related to Diminishing Amount of Matrix Glycosaminoglycans (GAGs)

Abstract

Hydration status significantly affects the toughness of bone. In addition to the collagen phase, recent evidence shows that glycosaminoglycans (GAGs) of proteoglycans (PGs) in the extracellular matrix also play a pivotal role in regulating the tissue-level hydration status of bone, thereby affecting the tissue-level toughness of bone. In this study, we hypothesized that the amount of GAGs in bone matrix decreased with age and such changes would lead to reduction in bound water and subsequently result in a decrease in the tissue-level toughness of bone. To test the hypothesis, nanoscratch tests were conducted to measure the tissue-level toughness of human cadaveric bone specimens, which were procured only from male donors in three different age groups: young (26 ± 6 years old), mid-aged (52 ± 5 years old) and elderly (73 ± 5 years old), with six donors in each group. Biochemical and histochemical assays were performed to determine the amount and major subtypes of GAGs and proteoglycans in bone matrix. In addition, low-field NMR measurements were implemented to determine bound water content in bone matrix. The results demonstrated that aging resulted in a statistically significant reduction (17%) of GAGs in bone matrix. Concurrently, a significant deterioration (20%) of tissue-level toughness of bone with age was observed. Most importantly, the deteriorated tissue-level toughness of bone was associated significantly with the age-related reduction (40%) of bound water, which was partially induced by the decrease of GAGs in bone matrix. Furthermore, we identified that chondroitin sulfate (CS) was a major subtype of GAGs and the amount of CS decreased with aging in accompany with a decrease of biglycan that is a major subtype of PGs in bone. The findings of this study suggests that reduction of GAGs in bone matrix is likely one of the molecular origins for age-related deterioration of bone quality.

Biglycan in the Skeleton

“…With aging, changes in PG secretion and post-translational modifications influence their binding to collagen. In fact, the glycanation state of SLRPs can serve as a chronological age “footprint.” 58–61 In mineralized matrix, the synthesis of Biglycan is reduced with age, and though the abundance of the PG form of Biglycan in bones appears to be stable throughout life, in articular cartilage and the intervertebral disc, there are noticeable changes: the GAG-free form of Biglycan is low in juveniles, but increases in adults until it becomes the predominant form. 62,63 For Decorin, though the GAG-free form is always a minor component, the level of glycosylated Decorin decreases with age, thereby slightly increasing the proportion of the GAG-free form.…”

Syndecan-3 enhances anabolic bone formation through WNT signalling

“…The transmembrane domain allows for dimerization, whereas the cytoplasmic domain (composed of conserved regions C1 and C2 flanking a variable region [V]) interacts with kinases, the cytoskeleton and C1 mediates endocytosis 11,12 . GAGs, abundant in the extracellular matrix (ECM), are known to enhance bone regeneration and osteoblastogenesis by binding of the WNT inhibitor, sclerostin. However, with ageing the amount of GAGs, and in particular CS decreases in human cortical mineralised bone matrix, which has a detrimental effect on bone toughness and likely contributes to the agerelated deterioration of bone quality 13 . Previous research 14 has shown that HS GAGs are important in osteoblast differentiation and activity, and SDCs with attached GAGs, can modulate WNT signalling 15,16 , however SDC-WNT crosstalk in bone has not been clearly deciphered 4,17 and involvement of SDCs in bone physiology is poorly defined.…”

Material properties and osteoporosis

“…Bone water content has recently attracted renewed interest as it has been shown to contribute to the overall toughness of bone, acting like a plasticizer 62– 65 . This indirectly emphasizes the importance of proteoglycans in the determination of bone strength 66 . Proteoglycans are non-collagenous components of the bone extracellular matrix and are characterized by the presence of one or more glycosaminoglycan polymers attached to a protein core 67 .…”