The amyloid protein APin is highly expressed during enamel mineralization and maturation in rat incisors.

Abstract

This study investigated the expression and localization of APin (which was previouslyidentified and cloned from a rat odontoblast cDNA library), during ameloblast differentiationin rat incisors, by using in situ hybridization and immunohistochemistry.The subcellular localization of APin varied during ameloblast differentiation, but wasstage-specific. APin mRNA was not expressed in pre-ameloblasts, was weaklyexpressed in secretory ameloblasts, and was strongly expressed in maturation-stageameloblasts as well as in the junctional epithelium attached to the enamel of eruptedmolars. In the maturation-stage ameloblasts, APin protein was conspicuous in thesupranuclear area (Golgi complex) of smooth-ended ameloblasts as well as in both thesupranuclear area and the ruffle end of ruffle-ended ameloblasts. During ameloblastlineagecell culture, APin was expressed at a low level in the early stages of culture, butat a high level in the late stage of culture, which was equivalent to the maturationstage. APin protein was efficiently secreted from transfected cells in culture. Furthermore,its overexpression and inactivation caused an increase and decrease in matrix metalloproteinase-20 (MMP-20) and tuftelin expression, respectively. These findings indicate a functional role for APin in the mineralization and maturation of enamel that is mediated by the expression of MMP-20 and tuftelin.