Comparison of enamel and ameloblasts in Amelotin-overexpression and -deficient mice

Background: Amelotin (Amtn) is one of the secretory calcium-binding phosphoprotein gene cluster, expresses predominantly during the maturation stage in ameloblasts and in a basal lamina-like structure at the interface between ameloblasts and mineralized enamel. Thus, it has been speculated that AMTN may mediate, modulate mineralization and/or regulate ion transport. However, the biological role of AMTN remains to be determined. Objectives: To compare the phenotype of the mice that overexpressed (Tg) and lack (knockout: KO) of the amelotin gene, focused on enamel structure and ameloblasts. Methods: The resulting phenotypes of both mice were analyzed by immunohistochemistry (IHC), light microscopy, scanning (SEM and TEM). Results: Severe attrition of mandibular incisors and molars in Tg mice and of only incisors in KO mice were observed from 9 weeks of age. Tg mice showed thinner enamel, irregular interwoven structure in several direction and bent ameloblasts layer. While histological and TEM examination of mandibular incisors in transverse sections of KO mice revealed that enamel mineralization was significantly delayed, and the ultimate enamel reached normal thickness, but was hypomineralized. SEM analysis revealed structural defects in the outer, “final” enamel layer of KO mice. Conclusion: AMTN-overexpression mice showed a unique tooth phenotype due to AMTN expression at the secretory stage. Moreover AMTN-knockout mice also displayed a specific tooth phenotype that was consistent with a role for AMTN in establishing the specialized microstructure of the final enamel layer. These roles were further supported by the temporo-spatial distribution of AMTN in normal mice mainly during the later stages of amelogenesis, and its localization at the basal lamina. These alterations of the enamel surface were sufficient to cause severe attrition of lower incisors, might be suggested that AMTN plays a key role in enamel maturation.