Distinct Compartmentalization of Dentin Matrix Protein 1 Fragments in Mineralized Tissues and Cells

Abstract

Dentin matrix protein 1 (DMP1) has been shown to be critical for the formation of dentin and bone. However, the precise pathway by which DMP1 participates in dentinogenesis and osteogenesis remains to be clarified. DMP1 is present in the extracellular matrix of dentin and bone as processed NH₂- and COOH-terminal fragments. The NH₂-terminal fragment occurs as a proteoglycan, whereas the COOH-terminal fragment is highly phosphorylated. The differences in biochemical properties suggest that these fragments may have different tissue and cell distribution in association with distinct functions. In this study, we analyzed the distribution of the NH₂- and COOH-terminal fragments of DMP1 in tooth, bone, osteocytes as well as MC3T3-E1 and HEK-293 cells. Immunohistochemical analyses were performed using antibodies specific to the NH₂- or COOH-terminal region of DMP1. Clear differences in the distribution of these fragments were observed. In the teeth and bone, the NH₂-terminal fragment was primarily located in the nonmineralized predentin and cartilage of the growth plate, while the COOH-terminal fragment accumulated in the mineralized zones. In osteocytes, the NH₂-terminal fragment appeared more abundant along cell membrane and processes of osteocytes, while the COOH-terminal fragment was often found in the nuclei. This pattern of distribution in cellular compartments was further confirmed by analyses on MC3T3-E1 and HEK-293 cells transfected with a construct containing DMP1 cDNA. In these cell lines, the COOH-terminal fragment accumulated in cell nuclei, while the NH₂-terminal fragment was in the cytosol. The different distribution of DMP1 fragments indicates that these DMP1 variants must perform distinct functions.