Dentin matrix protein 1 is predominantly expressed in chicken and rat osteocytes but not in osteoblasts

Although osteocytes are the most abundant cells in bone, little is known ab out their function, and no specific marker protein for osteocytes has been described. Dentin matrix protein 1 (DMP1) is an acidic phosphoprotein expre ssed in tooth organ and bone. Our previous work showed that in the chicken, which is not capable of forming tooth, DMP1 messenger RNA (mRNA) is highly expressed in bone by Northern blot analysis. To clarify the significance o f DMP1 expression in bone, the expression of DMP1 mRNA and its protein was examined in the chicken and rat. In the chicken, DMP1 mRNA was detected onl y in bone tissues and was localized in osteocytes and preosteocytes but not in osteoblasts. Similarly, in the rat, DMP1 mRNA was predominantly express ed in osteocytes and preosteocytes. in bone matrix but not in osteoblasts l ocated at the bone surface. Antiserum. was raised against the peptide from rat DMP1, and the localization of DMP1 was examined by immunohistochemistry . In the development of bone, DMP1 was first detected in newly formed bone matrix after osteoblastic cells had been embedded within it. After the appe arance of typical osteocytes, DMP1 was localized in the pericellular bone m atrix of osteocytes, including their processes. These data show that DMP1 i s a bone matrix protein specifically expressed in osteocytes. and preosteoc ytes and suggest that DMP1 plays a role in bone homeostasis because of its high calcium ion-binding capacity.