Changes in skeletal collagen cross-links and matrix hydration in high- and low-turnover chronic kidney disease

Abstract

Purpose/Introduction Clinical data have documented a clear increase in fracture risk associated with chronic kidney disease (CKD). Preclinical studies have shown reductions in bone mechanical properties although the tissue-level mechanisms for these differences remain unclear. The goal of this study was to assess collagen cross-links and matrix hydration, two variables known to affect mechanical properties, in animals with either high or low turnover CKD. Methods At 35 weeks of age (>75% reduction in kidney function), the femoral diaphysis of male Cy/+ rats with high or low bone turnover rates, along with normal littermate (NL) controls, were assessed for collagen cross-links (pyridinoline (PYD), deoxypyridinoline (DPD), and pentosidine (PE)) using a high performance liquid chromatography (HPLC) assay as well as pore and bound water per volume (pw and bw) using a 1H nuclear magnetic resonance (NMR) technique. Material-level biomechanical properties were calculated based on previously published whole bone mechanical tests. Results Cortical bone from animals with high turnover disease had lower Pyd and Dpd crosslink levels (−21% each), lower bw (−10%), higher PE (+71%), and higher pw (+46%), compared to NL. Animals with low turnover had higher Dpd, PE (+71%), and bw (+7%) along with lower pw (−60%) compared to NL. Both high and low turnover animals had reduced material-level bone toughness compared to NL animals as determined by three-point bending. Conclusions These data document an increase in skeletal PE with advanced CKD that is independent of bone turnover rate and inversely related to decline in kidney function. Although hydration changes occur in both high and low turnover disease, the data suggest that non-enzymatic collagen crosslinks may be a key factor in compromised mechanical properties of CKD.

Moderate chronic kidney disease impairs bone quality in C57Bl/6J mice

“…Iwasaki et al observed changes in whole bone storage modulus and tan delta in rats after 16 weeks with 5/6 Nx and a high-phosphate diet [Iwasaki et al, 2015]. Meanwhile, in a Cy/+ genetic CKD model in rats, diseased bones had significantly lower ultimate stress, a trend towards lowered toughness, and no change in modulus as measured from three-point bending [Allen et al, 2014]. In another study with Cy /+ rats, CKD significantly reduced ultimate stress and stiffness, while energy to failure was not changed [Moe et al, 2014].…”

“…The current work addressed an important inconsistency in whether microscale material properties change with CKD; namely, through placing arrays in a location of actively forming bone, lower bone material quality was readily observed. A preponderance of evidence collected from this study and others now suggests that CKD does affect bone material properties, and that these properties may involve the quantity of mineral or its maturation ( e.g., current work, Iwasaki et al, 2011, Iwasaki et al, 2015), or the collagen network supporting mineralization [Allen et al, 2014]. Looking ahead, a site-matched approach employing nanoindentation alongside Raman Spectroscopy may reveal greater detail about how formation and maturation of bone mineral and matrix are specifically altered with CKD, and how these changes contribute to diminished microscale mechanical properties.…”

What Animal Models Have Taught Us About the Safety and Efficacy of Bisphosphonates in Chronic Kidney Disease

“…This is quite surprising given the known preferential effect of CKD on cortical bone [37]. The effects of BP on tissue mineralization, or other aspects of the matrix for that matter (mineral crystallinity, heterogeneity, collagen structure/ cross-linking, hydration, microdamage), are nonexistent in part because the effects of CKD alone have only recently been explored [38,39]. Given that BPs are known to affect tissue properties in ways that have both positive and negative influence on bone integrity [40], these ultra structural assessments are desperately needed to understand the full picture of BP effects.…”

A materials science vision of extracellular matrix mineralization

“…The hydration shell of collagen is provided by small proteoglycans SLRPs (Small Leucine-Rich Proteoglycans) that decorate the surface of collagen fibrils 61 . The precisely located, enzymatic covalent crosslinks in the collagenous scaffold confine bound water to the interfibrillar space 62 . As a result of the crosslinking, the collagenous matrix is effectively a continuous framework with limited extensibility that cannot swell to accommodate bound water, and the osmotic pressure provided by proteoglycans builds up to the kPa range [63][64][65] .…”