Development expression of bone sialoprotein mRNA in rat mineralized connective tissues

Bone sialoprotein (BSP) is a phosphorylated and sulfated glycoprotein that is a major noncollagenous protein of bone and other mineralizing connective tissues. BSP is characterized by the presence of several polyglutamic acid segments and an RGD motif that mediates cell attachment through a vitronectin-like receptor. Although the precise function of BSP is unknown, the expression of BSP in conjunction with bone formation in vitro indicates a role for this protein in the biomineralization of connective tissues. In this study we used Northern hybridization and in situ hybridization to determine the tissue-specific and developmental expression of BSP during embryogenesis and growth of rat tissues. Analysis of tissues obtained from 13, 17, and 21 day fetuses, and from 4-, 14-, and 100-day-old animals indicates that BSP mRNA expression is restricted to cells actively forming the mineralizing tissues of bone, dentin and cementum. BSP mRNA transcripts were first evident in fully differentiated osteoblasts of 17 day fetal tissues at sites of de novo intramembranous and endochondral bone formation, with maximal expression observed at 21 days of gestation. Thereafter, BSP mRNA levels decreased markedly, and in adult bone hybridization was detected only in the primary spongiosa of long bones. In comparison, mRNAs for osteopontin (OPN), alkaline phosphatase (ALP), and osteocalcin (OC) peaked at 4-14 days postpartum before declining. In the tibiae, Northern hybridization revealed a second peak of mRNA for BSP, ALP, and OPN at 14 days, reflecting an increased osteogenic activity due to the formation of the secondary centers of ossification in the epiphyseal cartilage. In situ hybridization also revealed BSP mRNA in hypertrophic chondrocytes at sites of bone formation, in odontoblasts of the incisor during dentinogenesis, and in cementoblasts during cementogenesis. In view of the restricted distribution and temporal changes in the expression of BSP mRNA that we observed together with the chemical properties of BSP, we believe that this protein has a specific role in mediating the initial stages of connective tissue mineralization.