Induction of collagen mineralization by a bone sialoprotein--decorin chimeric protein

The observation that hydroxyapatite (HA) formation from metastable solutions can be induced by nucleating proteins such as bone sialoprotein (BSP) suggests a possible treatment for bone defects. The introduction of a mixture of nucleating protein and type I collagen should result in a defect becoming filled with a mineralized collagenous matrix that is biologically and mechanically compatible and capable of being remodeled. To create a nucleating protein that would interact with collagen fibrils, we combined the putative collagen-binding site of mouse decorin with one of two putative HA-nucleating sites of pig BSP. The resulting chimeric protein induced the formation of HA crystals in a steady-state agarose gel system and bound with high affinity to fibrillar type I collagen. The addition of chimeric protein to collagen gels perfused with low concentrations of calcium and phosphate resulted in the deposition of large, apparently needle-shaped HA crystals on the surface of collagen fibrils. These findings suggest that the BSP-decorin chimeric protein could be capable of inducing the mineralization of collagen in vivo.