Site-Specific In Vivo Calcification and Osteogenesis Stimulated by Bone Sialoprotein

Abstract

Bone sialoprotein (BSP) is one of the major non-collagenous glycosylated phosphoproteins of the extracellular matrix in bone. In vitro studies suggest that BSP may play important roles in the initiation and/or growth of calcium-phosphate crystals. To investigate the potential role of BSP in more complex in vivo environments, we implanted purified bovine BSP with type-I collagen as a carrier into surgically created rat calvarial defects and thoracic subcutaneous pouches. The responses to the implants were assessed by histochemistry, immunohistochemistry, in situ hybridization, quantitative real-time PCR, and biochemical analyses. BSP-collagen, but not collagen alone, elicited mineral deposition in the matrix of proliferating cells near the dura at days 4-5 followed by osteoblast differentiation and synthesis of new bone in the mid-portion of the calvarial defects. In contrast, implantation of BSP-collagen into subcutaneous pouches did not induce calcification or osteogenesis over the same experimental period. We explored the underlying mechanisms for the site-specific responses to BSP-collagen implants and found that higher levels of calcium content and alkaline phosphatase activity at the cranial site at days 2-5 were associated with the BSP-mediated calcification. We also found that BSP stimulated osteoblast differentiation through up-regulation of cbfa1 and osterix, key transcription factors of osteoblast differentiation, which occurred in the calvarial defects but not in the subcutaneous tissue. These results demonstrate that BSP stimulates calcification and osteogenesis in a site-specific manner, and that local environment and the specificities of responding cells may play critical roles in the function of BSP in vivo.

The importance of the SIBLING family of proteins on skeletal mineralisation and bone remodelling

“…Indeed, in the presence of BSP, high levels of ALP activity can promote the initiation of mineral deposition (Wang et al 2006). This is further confirmed in BSP-overexpressing cell cultures that have a higher ALP activity (Valverde et al 2008).…”

“…This certainly seems convincing as numerous studies have documented BSP, which is localised to MVs, to be involved in the initial formation of HA (Harris et al 2000, Fisher et al 2001, Tye et al 2003, Wang et al 2006, Nahar et al 2008. Indeed, the Bsp null mouse displays shorter, hypomineralised bones with associated higher trabecular bone mass with low bone turnover (Malaval et al 2008).…”

Bone Material Properties in Osteogenesis Imperfecta

“…In remodeling sites in the rest of the skeleton, a variety of interlinked mechanisms have been identified that regulate the site and degree of mineralization activity, including the presence of mineralization inhibitors such as pyrophosphate (32) and osteopontin, (33) and the potential mineral nucleation initiator bone sialoprotein, which may act in concert with alkaline phosphatase. (34) Multiple other factors may also have a role, recently reviewed in Staines and colleagues. (35) In both endochondral ossification and matrix mineralization, however, it seems likely that an initial phase of amorphous mineral deposition precedes the formation of more highly organized mineral.…”

The Isolation and Characterization of Glycosylated Phosphoproteins from Herring Fish Bones

“…The posttranslational modifications of bone and dentin ECM proteins have been found to impact their biological functions. These range from effects on hydroxyapatite nucleation and accumulation of calcium phosphate crystals (1, 7, 16 -19, 60 -62) to cell attachment and behavior (36,42,63). Other studies have indicated that post-translational modifications influence in vitro osteoclast activity and bone resorption (40,42,64) and osteoblast differentiation (37).…”