Localization of Phosphorylated Serine, Osteopontin, and Bone Sialoprotein on Bone Fracture Surfaces

Bone mineral density or bone mass alone cannot reliably predict fracture risk in patients. It is generally accepted that bone quality, including the properties of the organic matrix of bone, should also be considered. Collagen type I accounts for about 90% of this organic matrix. The other 10% are accounted for by various proteins and proteoglycans usually summarized by the term noncollagenous proteins (NCPs). These NCPs have a large influence on the nanoscale organization of bone. In addition, some NCPs have intriguing properties that could strongly influence bone matrix material properties; they can form self-healing networks based on ion-mediated bonds. Such behavior was also reported for trabecular bone fracture surfaces, rejoined after cleavage. To obtain proof that this behavior of bone is due to NCPs, an immunohistochemical approach was chosen for the work presented in this communication. Antibodies for phosphoserine, which is abundant in many NCPs but not in collagen type I, as well as antibodies for osteopontin and bone sialoprotein, were used on human trabecular bone fracture surfaces and microfractured trabeculae. Signals were detected using secondary gold-labeled antibodies and backscattered scanning electron microscopy. We found homogenous NCP coverage of fracture surfaces and elevated signals on bridging ligaments. Osteopontin and bone sialoprotein were detected in localized patches. Overall, this work suggests that the self-healing effect of trabecluar bone fracture surfaces, rejoined after cleavage, can be explained by the presence of NCPs. In addition, we conclude that NCPs also constitute the interface that is disrupted when bone fails, attributing them high importance for bone matrix material properties and fracture risk.