Amelogenin Nanoribbons In Enamel Development And Engineering

Enamel is the only epithelial derived tissue that mineralizes. It develops in an extracellular matrix secreted by highly specialized epithelial cells, the ameloblasts, which synthesizes a host of specific matrix proteins with little homology to any other known proteins. Amelogenin is by far the largest constituent of the developing enamel matrix (DEM) comprising ~90% of all secreted protein. Amelogenin’s primary structure encodes a series of critical functions of the DEM that ultimately allow for control over the uniaxial growth of apatite nanofibers and their three-dimensional organization into a stiff, hard and fracture-resistant tissue optimized for mastication and integration with the underlying dentin. Previously, we have demonstrated the biological significance of the amyloid-like character of amelogenin which allows the protein to adopt ß-sheet structure and to self-assemble into nanoribbons (Amel-NR) that guide the growth of mineral ribbons. Based on this data, a new model of enamel biomineralization founded on the activation of the mineralization process by the enzymatic cleavage of amelogenin by matrix metalloprotease-20 (MMP20) has been proposed and will be further evaluated in this application. We have hitherto shown, that removal of the hydrophilic C-terminal domain allowed an acidic polymer to interact with Amel-NRs thereby initiating the deposition of an amorphous calcium phosphate (ACP) layer. Subsequently, ACP transformed into crystalline apatite in the form of ribbon-like mineral about 15-20 nm wide that followed the morphology of the Amel-NRs. The sequential process of biomineralization observed in our in vitro model correlates with biological events of tissue mineralization and reinforces emerging concepts of biomineralization not only valid to enamel but other hard tissues. Such concepts include: a) biomineralization is regulated by proteolysis; b) presence of a carrier-protein that acts as a process-directing agent and delivers mineral ions to a self-assembled protein framework; c) carrier-protein interactions produce amorphous mineral deposits onto the protein framework; d) the supramolecular structure of the organic phase directs the phase transformation into defined crystal habits by oriented crystallization. In continuation of previous studies, we propose to identify constituents of the DEM that are critical for the templated growth of crystalline apatite nanofibers in association with the protein nanoribbons. By determining the three-dimensional structure of Amel-NRs at near-atomic resolution, further functional domains will be identified and associated with critical molecular and structural mechanisms in enamel formation. Ultimately, we plan to use the unique ability of Amel-NRs to direct the fibrous growth of apatite and synthesize nanomaterials through hierarchical design at the micro- and nanometer length scale.