Asporin and the mineralization process in fluoride-treated rats

Microarray analysis of odontoblastic cells treated with sodium fluoride has identified the asporin gene as a fluoride target. Asporin is a member of the small leucine-rich repeat proteoglycan/protein (SLRP) family that is believed to be important in the mineralization process. In this study, asporin expression and distribution were investigated by systematic analysis of dentin and enamel, with and without fluoride treatment. Specific attention was focused on a major difference between the two mineralized tissues: the presence of a collagenous scaffold in dentin, and its absence in enamel. Normal and fluorotic, continually growing incisors from Wistar rats treated with 2.5 to 7.5 mM sodium fluoride (NaF) were studied by immunochemistry, in situ hybridization, Western blotting, and RT-qPCR. Asporin was continuously expressed in odontoblasts throughout dentin formation as expected. Asporin was also found, for the first time, in dental epithelial cells, particularly in maturation-stage ameloblasts. NaF decreased asporin expression in odontoblasts and enhanced it in ameloblasts, both in vivo and in vitro. The inverse response in the two cell types suggests that the effector, fluoride, is a trigger that elicits a cell-type-specific reaction. Confocal and ultrastructural immunohistochemistry evidenced an association between asporin and type 1 collagen in the pericellular nonmineralized compartments of both bone and dentin. In addition, transmission electron microscopy revealed asporin in the microenvironment of all cells observed. Thus, asporin is produced by collagen-matrix-forming and non-collagen-matrix-forming cells but may have different effects on the mineralization process. A model is proposed that predicts impaired mineral formation associated with the deficiency and excess of asporin.