Characterization of Enamelysin (MMP-20) Expressed in the Porcine Odontoblast

Enamelysin (MMP-20) is a matrix metalloproteinase isolated from the cDNA library of the enamel organ. Although enamelysin is also expressed in the tooth pulp, especially in the odontoblast, the functional significance of enamelysin protein in dentinogenesis has not been elucidated. Thus, the present study was designed to clarify the characteristics of enamelysin in the pulp from the porcine deciduous molar tooth germ and developing teeth. Western blot analysis of proteins extracted from the pulp of the tooth germ revealed that immunoreactive enamelysin occurred as 54 kDa and 51 kDa proteins. None of the protein bands lower than 51 kDa was stained. Light and electron microscopic immunohistochemistry of tooth germs and erupting teeth demonstrated positive reactions over the cytoplasmic matrix of odontoblast cell bodies and processes (dentinal fibers). The secretoty machinery of the odontoblast was shown to be devoid of immunoreactivity. Intensity of the immunoreaction in the odontoblast was variable but essentially dependent upon the position and differentiation stage of the odontoblast. The highest immunoreactivity was found in some odontoblasts located at the cuspal region of the tooth germ. During tooth eruption, cell bodies of odontoblasts in the crown showed faint or no immunoreactivity. No immunoreaction was detected in the odontoblast facing the root dentin, although dentinal fibers of the root dentin showed faint immunoreactivity. RT-PCR and cDNA sequence analysis failed to find any differences between enamelysin mRNA extracted from the enamel organ and that from the pulp. These results suggest that odontoblasts transiently express enamelysin during tooth formation and that the enamelysin in the odontoblast is not secreted from the cell.