Transgenic Rescue of Enamel Phenotype in Ambn Null Mice

Abstract

Ambn−/− mice fail to make an enamel layer, but the defects could be due to an absence of functional Ambn or to the secretion of a potential toxic mutant Ambn. We hypothesized that the enamel phenotype could be rescued by the transgenic expression of normal Ambn in Ambn−/− mice. We established and analyzed five transgenic lines that expressed Ambn from the amelogenin (AmelX) promoter. We identified transgenic lines that express virtually no transgene, slightly less than normal (Tg+), somewhat higher than normal (Tg++), and much higher than normal (Tg+++) levels of Ambn. All lines expressing detectable levels of Ambn protein at least partially recovered the enamel phenotype. When Ambn expression was only somewhat higher than normal, the enamel covering the molars and incisors was of normal thickness, looked normal, had clearly defined rod and interrod enamel, and held up well in function. We conclude that Ambn is essential for dental enamel formation.

The Importance of Serine Phosphorylation of Ameloblastin on Enamel Formation

“…A previous study (Chun et al 2010) showed that overexpression of WT Ambn led to minor or mild enamel defects when the transgene dosage was slightly higher than normal (~3 folds based on the Western blot) or much higher (~6 to 9 folds). As the transgene is randomly inserted into the genomic DNA, no 2 transgenic lines are genetically identical, whereas their phenotypes could be very similar.…”

“…As the transgene is randomly inserted into the genomic DNA, no 2 transgenic lines are genetically identical, whereas their phenotypes could be very similar. In this regard, the mice overexpressing WT Ambn (Chun et al 2010) can serve as controls in comparison with those overexpressing the mutant versions. In our study, D-Tg + and D-Tg ++ mice showed minor and mild enamel defects when the transgene dosage was slightly higher than normal (~3 folds) and much higher (~7 folds).…”

“…The Ambn-and Enam-mutant subjects develop autosomal-dominant amelogenesis imperfecta in a dosedependent manner, while those bearing Amel mutations show X-linked traits. Studies of Ambn knockout (Fukumoto et al 2004;Wazen et al 2009) and rescue (Chun et al 2010) suggest that the amelogenesis imperfecta caused by AMBN truncation was most likely due to haploinsufficiency. However, it remains unclear whether point mutations of AMBN can interfere with WT protein in a dominant negative manner, as point mutation of Ambn has not been reported in humans and animals.…”

Protein Interaction between Ameloblastin and Proteasome Subunit α Type 3 Can Facilitate Redistribution of Ameloblastin Domains within Forming Enamel

“…Moreover, in mouse models that express a truncated ameloblastin (46 -48) or overexpress ameloblastin (49), the resulting enamel shows structural imperfections, with disturbances to the canonical pattern of rod-interrod boundaries. In the truncated ameloblastin animal, rescue of the enamel rod microstructure abnormalities has been achieved with expression of a full-length ameloblastin transgene (10). These observations suggest that the distributions of ameloblastin domains within the forming enamel matrix play important roles in establishing the enamel microstructure comprising the rod-interrod pattern of organization and hence in producing the favorable material properties found in mature enamel.…”

“…As a consequence of cellular fabrication, the boundaries between cells give rise to a hierarchically integrated microstructure of crystallite bundles, which, in rodents, forms a decussating pattern of woven bioceramic structure (1,2). This highly organized hierarchical structure provides enamel with its unique material properties of wear resistance, fracture toughness, and in rodents, self-sharpening edges (3)(4)(5)(6)(7)(8)(9)(10)(11).…”