Ameloblastin amphipathic helix motif mediates ameloblast polarization and prismatic enamel formation.

Ameloblastin (Ambn) is a tooth-specific multifunctional protein essential for enamel biomineralization and the formation of its prismatic microstructure. To examine the function of the evolutionarily conserved cell-binding Ambn amphipathic helix (AH) motif, we deleted the hydrophobic residues within the Ambn AH motif in genetically engineered mice. Enamel in the homozygous (AmbnΔL76-P86) mutants had normal thickness but was hypo-mineralized and lacked prismatic structure. Micro-CT analysis further revealed that both the secretory and maturation stages of amelogenesis were delayed and proceeded slower than in the controls. Ameloblasts in the mutants were stunted and exhibited loss of cell polarity, as demonstrated by the mislocalization of Pard3, Claudin-1 and GM130 immunosignals. In the AmbnΔL76-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, suggesting that changes in known signaling pathways may connect Ambn-cell interactions to the establishment of cell polarity. Together, these findings support a model in which AH-dependent Ambn engagement at the distal ameloblast membrane contributes to secretory-stage polarity and prism patterning, with downstream consequences for enamel organization and maturation-stage mineral density.