NMR Investigation of the Role of Osteocalcin and Osteopontin at the Organic-Inorganic Interface in Bone

Mechanical resilience of bone tissue decreases with age. The ability to comprehensively probe and understand bone properties could help alleviate this problem. One important aspect of bone quality that has recently been made evident is the presence of dilatational bands formed by osteocalcin (OC) and osteopontin (OPN), which contribute to fracture toughness. However, experimental evidence of the structural role of these two proteins at the organic-mineral interface in bone is still needed. Solid state nuclear magnetic resonance (SSNMR) is emerging as a useful technique in probing molecular level aspects of bone. Here, we present the first SSNMR study of bone tissue from genetically modified mice lacking OC and/or OPN. Probing the mineral phase, the organic matrix and their interface revealed that, despite the absence of OC and OPN, the organic matrix and mineral were well preserved, and the overall exposure of collagen to hydroxyapatite (HA) nanoparticles was hardly affected. However, the proximity to the HA surface was slightly increased for a number of bone components including less abundant amino acids like lysine, suggesting that this is how the tissue compensates for the lack of OC and OPN. Taken together, the NMR data supports the recently proposed model, in which the contribution of OC-OPN to fracture toughness is related to their presence at the extrafibrillar organic-mineral interfaces, where they reinforce the network of mineralized fibrils and form dilatational bands. In an effort toward further understanding the structural role of individual amino acids of low abundance in bone, we then explored the possibility of specific C-13 enrichment of mouse bone, and report the first SSNMR spectra of 97% C-13 lysine-enriched tissue. Results show that such isotopic enrichment allows valuable molecular-level structural information to be extracted, and sheds light on post-translational modifications undergone by specific amino acids in vivo.