Enamel evolution: Back in time by a molecular manipulation

Biomineralization is one of the key processes during vertebrate evolution that incorporates calcium and phosphate ions into soft matrices in the form of hydroxyapatite. Occurrence of mineralized tissues have offered the basis for various adaptive phenotypes such as endoskeleton for locomotion (bone), body armor for protection and teeth (enamel, dentin) for predation. Enamel is unique among the mineralized tissues, as it is formed on specific network of enamel matrix proteins (EMPs) secreted by epithelial ameloblasts. The two key structural proteins of enamel, amelogenin (AMEL) and ameloblastin (AMBN), self‐assemble into higher‐ordered structures from monomeric intrinsically disordered subunits as does the type I collagen (COL1), the predominant matrix protein in bone and dentine. While COL1 undergoes self‐assembly via consecutive Gly‐X‐Y motif, the mechanism of self‐assembly of EMPs and their subsequent role in formation of organized layer of hydroxyapatite crystals remains poorly understood. We report here a novel evolutionary conserved self‐assembly motif common to the key structural enamel matrix proteins AMBN and AMEL. The presence of this motif is essential for self‐assembly of AMBN and AMEL into higher‐ordered structures. These structures are essential for proper enamel formation. Transgenic mice that were unable to produce supramolecular structures of AMBN due to point substitutions within the identified self‐assembly motif then produced severely affected enamel with simplified organization. Despite the normal cellular organization, EMPs secretion and Ca and P levels within the growing enamel of transgenic mouse, the affected enamel lacked organized prismatic structures and showed only radial organization without visible Hunter‐Schreger bands. Moreover, enamel of mutant mice contained an enormous portion of interprismatic matrix with hypomineralized, yet well recognizable, crystallites, while no formation of oriented crystallites was observed within the compromised prisms. This is the first in vivo evidence that the formation of supramolecular structures of enamel matrix proteins was essential for evolution of highly structured enamel in mammals.