ENAMEL EPITHELIUM EXPRESSES BONE SIALOPROTEIN (BSP)

Bone sialoprotein (BSP) is a major non-collagenous extracellular matri x protein in bone and other mineralized connective tissues. BSP is syn thesized and secreted by bone-, dentin- and cementum-forming cells. In this study we hypothesized that BSP may be also involved in enamel fo rmation through its postulated role in matrix mineralization. In situ hybridization with cRNA probes for rat and hamster BSP, respectively, showed strong mRNA signals in ameloblasts actively synthesizing enamel matrix in developing incisors. However, no hybridization signals were observed at an earlier developmental stage when bell-shaped molar too th germs were being formed. Immunohistochemical analysis of tooth tiss ues from transgenic mice harboring a 2.7 kb rat BSP promoter ligated t o a luciferase reporter gene revealed strong staining for luciferase i n the enamel epithelium of the developing tooth germ. Interestingly, B SP expression was also observed in epithelial cells of an ameloblastom a. The neoplastic epithelial nests and cords demonstrated strong mRNA signals to the human BSP probe while the connective tissue stroma show ed only a background level of silver grains. Immunostaining also showe d deposition of BSP by the odontogenic cells of the tumor. These resul ts demonstrate that BSP is expressed by the enamel-forming epithelium of developing teeth, suggesting a possible role for BSP in enamel form ation and its subsequent mineralization. Expression of the BSP gene in ameloblastomas is consistent with the expression of BSP by the enamel epithelium and also with the expression of BSP by neoplastic tissues, suggesting a possible role in tumorigenesis.