In situ hybridization shows dmp1 (AG1) to be a developmentally regulated dentin-specific protein produced by mature odontoblasts

Acidic phosphorylated proteins are prominent constituents of the extracellular matrix of bone and dentin. It has been postulated that they may have important structural and regulatory roles in the process of tissue mineralization. Studies of a cDNA library, prepared from cells of the rat incisor odontoblast-pulp complex of 3 week old Sprague-Dawley rats, led to the identification of a serine-rich acidic protein, designated AG1, which appeared to be a dentin matrix component. In order to determine which cells of the odontoblast-pulp complex were responsible for the making of AG1, in situ hybridization was carried out using digoxigenin-labeled probes. The full length AG1 cDNA was subcloned into the pBluescript vector, which contains two strong promoters, T3 and T7. The sense and antisense complementary RNA (cRNA) hybridization probes were prepared by in vitro transcription using T3 and T7 polymerases in the presence of 11-dUTP. Incisor sections were obtained from rat embryos at days 16, and 20, and newborns at days 2 and 5. No AG1 mRNA was detected in the embryonic sections, but digoxigenin labeling was evident in odontoblasts secreting mineralizing dentin at postnatal days 2 and 5. Sense probes showed no hybridization. Pulp cells, Meckel's cartilage, and alveolar bone were free of hybridization with the antisense probe. Unexpectedly, a low level of digoxigenin staining was seen in the cytoplasm of secretory ameloblasts, but not in the preameloblasts, stratum intermedium or stellate reticulum of the enamel organ. These data show that AG1 expression is regulated developmentally and is restricted to secretory stage mature odontoblasts.