In vitro effects of dentin matrix protein-1 on hydroxyapatite formation provide insights into in vivo functions

Dentin matrix protein-1 (DMP1) is a mineralized tissue matrix protein synthesized by osteoblasts, hypertrophic chondrocytes, and ameloblasts as well as odontoblasts. DMP1 is believed to have multiple in vivo functions, acting both as a signaling molecule and a regulator of biomineralization. Using a cell-free system in vitro, we evaluated the action of DMP1 in the regulation of hydroxylapatite (HA) formation and crystal growth. The non-phosphorylated recombinant protein acted as an HA nucleator, increasing the amount of mineral formed in a gelatin gel HA growth system relative to protein-free controls. The recombinant protein phosphorylated in vitro had no detectable effect on HA formation and growth. In contrast, phosphorylated bovine DMP1 expressed in marrow stromal cells with an adenovirus vector containing 29.7 phosphates/mol was an effective inhibitor of HA formation and growth. The native full-length protein appeared to be absent or present in only small amounts in the extracellular matrix of bones and teeth. However, two highly phosphorylated fragments representing the N- and C-terminal portions of DMP1 have been identified, apparently arising from proteolytic cleavage of four X-Asp bonds. The highly phosphorylated C-terminal 57-kDa fragment (containing 42 phosphates/mol), like the non-phosphorylated DMP1, was an HA nucleator. These data suggest that, in its native form, DMP1 inhibits mineralization, but when cleaved or dephosphorylated, it initiates mineralization. These in vitro data are consistent with the findings in the DMP1 knockout mouse.