Ameloblastin and Ameloblast Differentiation in Enamel Formation

Ameloblastin (Ambn), one of the non-amelogenin enamel matrix proteins, is expressed and secreted by ameloblasts during both secretory and maturation stages. In Ambn mutant mice, lacking exons 5 and 6 (Ambn^mutant), ameloblasts are dysplastic and fail to form enamel. Objectives: The goal was to define the role of Ambn for ameloblasts and enamel formation by expressing Ambn from a transgene in Ambn^mutant mice. Methods: By reconstituting the enamel layer with Ambn from a transgene, enamel formation was analyzed in mice using mandibular incisors with all stages of enamel formation. Ambn^mutant mice were mated with mice from two different mouse lines expressing Ambn in a dose lower or similar to normal from a transgene driven by the amelogenin promoter. The transgenic Ambn level was determined by Western Blot analysis of molars at 5 days age. Ameloblasts and enamel mineralization were analyzed by histology and micro-computed tomography (microCT). Both analyses were performed for each stage of enamel formation (secretory, early, mid and late maturation) on cross-sections of mandibular incisors at 7 weeks age. Results: In Ambn^mutant animals, microCT and histological analyses displayed that an enamel layer failed to be deposited and that the ameloblasts were dysplastic. In Ambn^{mutant/Tg, low} animals, the deposition and mineralization of enamel were delayed compared to wildtype controls. In Ambn^{mutant/Tg, high} animals, enamel deposition and mineralization started earlier than wildtype controls. The histological analysis of ameloblasts revealed that at low dose of transgenic Ambn, a few ameloblasts displayed polarization. The enamel organ was lacking stage-specific layers, instead extensive cyst formation was observed. Ambn^{mutant/Tg, high} animals displayed ameloblasts with polarization and the enamel organ consisted of layers similar to wildtype controls. Conclusions: Ambn is required for enamel mineralization, for ameloblasts differentiation, and for stage-specific morphology in the enamel organ. This study was funded by the University Research Council, UTHSCSA.