The Enamel Matrix Protein Amelogenin is Essential for Enamel Mineral Transport and Deposition

Abstract

Amelogenin, the primary protein of the enamel matrix, has long been implicated in regulating crystal nucleation, growth, and spatial organization during tooth development. This study investigates how the absence of amelogenin affects enamel structure and mineralization. Using amelogenin knockout mice, we examine its role in maintaining enamel integrity, modulating ameloblast vesicle dynamics, and facilitating calcium ion transport through specific channels to the enamel surface. The goal is to uncover the mechanistic contributions of amelogenin to enamel biomineralization and its broader implications for dental tissue engineering and pathology. Our study demonstrates that the absence of amelogenin leads to profound disruptions in enamel formation and mineral transport. In amelogenin-null mice, the typical enamel layer was absent and replaced by peg-like, tapered mineral structures. These pegs stained positively for calcium (via alizarin red) and inorganic phosphate (via von Kossa’s method), indicating aberrant mineral deposition. Electron diffraction revealed that the pegs contained bundles of thin, parallel-aligned crystals with patterns consistent with calcium hydroxyapatite, confirming their mineralized nature. At the cellular level, ameloblasts in wild-type mice displayed large, bilayered vesicles (∼200 nm in diameter) at their apical poles, containing inorganic phosphate as detected by modified submicroscopic von Kossa staining. In contrast, amelogenin-deficient ameloblasts lacked both the bilayer membrane structure and phosphate labeling within these vesicles, suggesting disrupted vesicular transport and ion packaging. Further, in vivo calcium labeling with Fluo-4 showed successful apical transport of calcium to the enamel surface in wild-type mice. However, in the absence of amelogenin, calcium was aberrantly retained at the basal ameloblast pole and in the stratum intermedium. This mislocalization correlated with altered expression and distribution of intracellular calcium channel proteins, as shown by immunoreactivity. Together, these findings expand the functional role of amelogenin beyond structural organization during early enamel crystal formation. They reveal a previously underappreciated role in mediating vesicle architecture, phosphate loading, and directional calcium ion transport essential for proper enamel mineralization.