Role of osteopontin and osteocalcin in the organic-inorganic interface of bone tissue

Abstract

The decrease in bone mechanical properties occurs with age. The associated fragility fractures present a global public health concern. The use of bone mineral density as a predictor of risk of fracture is, however, limited. A more comprehensive understanding of bone quality and its link to bone fragility is thus desirable.

Based on the above results, it is concluded that OC and OPN are present in bone as structural elements, and that they contribute to tissue mechanical properties via ionic interactions at the interfaces between mineralized fibrils.

The link between the organic-mineral interface and the tissue-level properties was also studied using a synthetic model. A materials chemistry approach allowed evaluation of binding of anionic groups found in the organic matrix, including the OC and OPN, to HA in various surrounding media. Ethanol made the anionic interactions harder to dissociate. Ethanol soaking of whole bone led to delayed crack initiation when OC and OPN were present at the interface. OC and OPN thus contribute to bone mechanical properties via strong ionic interactions with HA surfaces.

Via tissue-level mechanical testing on a genetically modified animal model lacking OC and/or OPN, it was shown that OC and OPN contribute, as individual proteins and as a complex, to energy dissipation during static and cyclic loading. The OC and OPN were also found to be involved in tissue recovery when loading is removed. To verify that changes in recovery and energy dissipation in bone are linked to presence or absence of OC and OPN, and not due to collateral changes in collagen and mineral structure, a robust NMR method was developed to study bone structure at the atomic length scale. A comprehensive set of NMR experiments confirmed that the absence of OC and OPN does not appear to alter the order of collagen or the mineral. Instead, removal of OC and OPN leads to small changes in the organic-mineral interface that imply an increased exposure to hydroxyapatite (HA) mineral for some charged amino acids and for noncollagenous components including glucosaminoglycans and citrate. The results support a model where OC and OPN are present in the extrafibrillar interfaces.

Besides the brittleness caused by nonenzymatic glycation of collagen, bone fracture resistance is also influenced by noncollagenous components such as osteocalcin (OC) and osteopontin (OPN). The structural role of OC and OPN in bone and how they contribute to mechanical properties remains unclear. This project aims to elucidate these two aspects.