DISTRIBUTION AND SYNTHESIS OF BONE SIALOPROTEIN IN METAPHYSEAL BONE OF YOUNG-RATS SHOW A DISTINCTLY DIFFERENT PATTERN FROM THAT OF OSTEOPONTIN

Bone sialoprotein (BSP) and osteopontin (OPN) are two phosphorylated a nd highly glycosylated cell-binding proteins in bone. Both proteins bi nd to hydroxylapatite. The cell binding is mediated via an Arg-Gly-Asp (RGD) sequence and previous work indicates that both proteins can bin d to the vitronectin receptor (alpha(nu)beta(3)). The present work sho ws that a prevailing localization of BSP in metaphyseal bone of the yo ung rat is at the interface between calcified cartilage and bone. Thus BSP shows a conspicuous enrichment in the osteoid laid down by the in vading osteoblasts immediately next to the calcified cartilage. Furthe rmore, the most prominent amount of BSP mRNA was detected in cells at the epiphyseal/metaphyseal border. As opposed to OPN, no prominent acc umulation of BSP immunoreactivity was observed at bone surfaces that f ace cells. Also the synthesis OPN was most pronounced at sites very di fferent from those of BSP. Thus, the most prominent amount of OPN mRNA was observed in cells close to the metaphyseal/diaphyseal border, whe re osteoclastic bone resorption is particularly active. Indeed, messag e was often found in cells surrounding osteoclasts without any detecta ble message. The distintly different patterns of synthesis and express ion of the two proteins indicate different roles in bone turnover at t his stage of development. Thus, it appears that BSP has a specific rol e during the initial phases of bone formation at the cartilage/bone in terface. On the other hand, the pattern of OPN synthesis and expressio n support and extend our previous data showing OPN particularly enrich ed at attachment sites of osteoclasts resorbing bone.