Abstract
Tough natural nanocomposites like bone, nacre and sea sponges contain within their hierarchy, a mineral (phosphate, silicate or carbonate) phase that interacts with an organic phase. In bone, the role of mineral ultrastructure (organization, morphology, composition) is crucial to the mechanical and biological properties of the tissue. Better understanding of mineral interaction with the organic matrix, in particular non-collagenous proteins, osteocalcin (OC) and osteopontin (OPN), can lead to better design of biomimetic materials. Using small angle x-ray scattering (SAXS) and wavelength dispersive spectroscopy (WDS) on single (OC−/− and OPN−/−) and double (OC-OPN−/−;−/−) genetic knockout mice bones, we demonstrate that both osteocalcin and osteopontin have specific roles in the biomolecular regulation of mineral in bone and together they are major determinants of the quality of bone mineral. Specifically, for the first time, we show that proteins osteocalcin and osteopontin regulate bone mineral crystal size and organization in a codependent manner, while they independently determine crystal shape. We found that OC is more dominant in the regulation of the physical properties of bone mineral, while OPN is more dominant in the regulation of the mineral composition.
Highlights
Tough natural nanocomposites like bone, dentin, nacre[1] and sea sponges[2] contain within their hierarchy, a mineral phase[3]
Quantitative analysis of 2D small angle x-ray scattering (SAXS) spectra revealed that the absence of either OC and/or OPN reduced crystal thickness (Fig. 1a) and reduced crystal orientation (Fig. 1b) in the OC−/−, OPN−/− and OC-OPN−/−;−/− mice bones
NCPs are secreted during osteoid mineralization and their physico-chemical interactions with bone mineral have been well documented in vitro[17]
Summary
Tough natural nanocomposites like bone, dentin, nacre[1] and sea sponges[2] contain within their hierarchy, a mineral (phosphate, silicate or carbonate) phase[3]. Mineralization in various natural nanocomposites is affected by the coordinated action of various extracellular organic molecules, including proteins, on the inorganic phase. Bone crystals exhibit a plate or needle-like morphology and are aligned such that their c-axis lies along the long axis of collagen fibrils[8]. They vary in crystallinity (crystal size and perfection) due to trace impurities including trace element ions like carbonate (CO32−), fluoride (F−), sodium (Na+) and magnesium (Mg2+) ions as well as vacancies. Non-collagenous proteins (NCPs) including osteocalcin (OC) and osteopontin (OPN) have long been associated with regulating bone mineral. OPN−/− mice bones do not show distinct morphological differences as compared to WT bones[19], OPN−/− bones show greater mineral to matrix ratio and increased crystal maturity in comparison with WT21 suggesting that OPN may influence bone’s nanostructure via physicochemical pathways
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