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

This study investigated the expression and localization of APin (which was previously identified and cloned from a rat odontoblast cDNA library), during ameloblast differentiation in rat incisors, by using in situ hybridization and immunohistochemistry. The subcellular localization of APin varied during ameloblast differentiation, but was stage-specific. APin mRNA was not expressed in pre-ameloblasts, was weakly expressed in secretory ameloblasts, and was strongly expressed in maturation-stage ameloblasts as well as in the junctional epithelium attached to the enamel of erupted molars. In the maturation-stage ameloblasts, APin protein was conspicuous in the supranuclear area (Golgi complex) of smooth-ended ameloblasts as well as in both the supranuclear area and the ruffle end of ruffle-ended ameloblasts. During ameloblast-lineage cell culture, APin was expressed at a low level in the early stages of culture, but at a high level in the late stage of culture, which was equivalent to the maturation stage. 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.