Matrix metalloproteinase-2 in dentin matrix mineralization

In the serum-free culture medium of bovine odontoblasts we detected active gelatinolytic metalloproteinases, matrix metalloproteinase (MMP)-2 and MMP- 9 (gelatinases A and B). The activity of MMP-2, in particular, appeared sud denly around day 21 in the culture, coinciding with the development of odon toblastic cell processes and the loss of alkaline phosphatase. Reverse tran scriptase-polymerase chain reaction analysis of these odontoblasts demonstr ated that messages of MMP-2 but not MMP-9 increased significantly between d ay 15 and day 21. The in vitro observation indicates that medium conditione d by these odontoblasts and containing significant amounts of MMP-2 degrade s not only the collagenous substrates but also purified dentin phosphophory n as well. We have also observed that dephosphorylated dentin phosphoprotei n becomes a better substrate for casein kinase II after limited proteolysis with MMP-2. These results support our working hypothesis that MMP-2-mediat ed proteolytic processing is an important step in accelerating the process of dentin matrix maturation, which includes phosphorylation and subsequent mineralization. As has been suggested previously, extracellular phosphoryla tion of matrix proteins is an important step in biomineralization both in b one and in dentin (Mikuni-Takagaki et al., J Bone Miner Res 1995;10:231-42; Zhu et al., Biochem J 1997; 323:637-43), Our present histochemical analysi s in MMP-2 knockout mice confirms the concept with the delayed formation of mineralized tissues, dentin, and bone.