Ameloblastin Amphipathic Helix Motif mediates Ameloblast Polarization and Prismatic Enamel Formation via a RhoA Signaling Pathway.

Ameloblastin (Ambn) is a tooth-specific multifunctional protein crucial for enamel biomineralization and its prismatic structure. To examine the function of the evolutionarily conserved cell-binding Ambn amphipathic helix (AH) motif, we deleted the hydrophobic residues within Ambn AH motif in genetically engineered mice. Enamel in the homozygous (Ambn (ΔL76-P86)) mutants had normal thickness but was hypo-mineralized and lacked prismatic structure. Micro-CT analysis using conventional neural network revealed loss in mineral density and a delay in the initiation of secretory stage of amelogenesis. Ameloblasts in the mutants were stunted and exhibited loss of cell polarity, as demonstrated by the mis-localization of Pard3, Claudin-1 and GM130 immunosignals. In the Ambn (ΔL76-P86) mutants, a loss of Ambn-ameloblast distal membrane interaction was observed, with nuclear localization of β-catenin and p-Smad2/3, and a decrease in RhoA immunolabeling intensity. Our study demonstrates a novel signaling role of Ambn AH motif in mediating cell polarization and forming enamel prismatic structure.