Amelogenin Supra-Molecular Assembly in vitro Compared with the Architecture of the Forming Enamel Matrix

Abstract

Tooth enamel is formed in the extracellular space within an organic matrix enriched in amelogenin proteins. Amelogenin nanosphere assembly is a key factor in controlling the oriented and organized growth of enamel apatite crystals. Recently, we have reported the formation of higher ordered structures resulting from organized association and self-orientation of amelogenin nanospheres in vitro. This remarkable hierarchical organization includes self-assembly of amelogenin molecules into subunits of 4–6 nm in diameter followed by their assembly to form nanospheres of 15–25 nm in radii. Chains of >100 nm length are then formed as the result of nanosphere association. These linear arrays of nanospheres assemble to form the microribbons that are hundreds of microns in length, tens of microns in width, and a few microns in thickness. Here, we review the step by step process of amelogenin self-assembly during the formation of microribbon structures in vitro. Assembly properties of selected amelogenins lacking the hydrophilic C terminus will then be reviewed. We will consider amelogenin as a template for the organized growth of crystals in vitro. Finally, we will compare the structures formed in vitro with globular and periodic structures observed earlier, in vivo, by different sample preparation conditions. We propose that the alignment of amelogenin nanospheres into long chains is evident in vivo, and is an important indication for the function of this protein in controlling the oriented and elongated growth of apatite crystals during enamel biomineralization.

Partial Rescue of the Amelogenin Null Dental Enamel Phenotype

“…Although similar micro-ribbon-like structures are observed by transmission electron microscopy in vivo as can be reproduced in vitro with Escherichia coli-expressed 179-amino-acid amelogenins (see review by Ref. 42), the actual composition in vivo may include other enamel proteins, or other amelogenins.…”

Gene Duplication and the Evolution of Vertebrate Skeletal Mineralization

“…These facts show the essential role of these proteins in enamel mineralization. AMEL constitutes more than 90% of enamel ECM protein and serves as the transient scaffold for mineralizing enamel [Moradian-Oldak and Goldberg, 2005;Margolis et al, 2006], whereas the functions of ENAM and AMBN are less clear [Bouropoulos and Moradian-Oldak, 2004;Fukumoto et al, 2004]. During maturation, these enamel proteins are actively degraded by proteinases secreted by ameloblasts and subsequently removed from the mineralizing enamel [Zeichner-David et al, 1995;Simmer and Hu, 2002;Hu et al, 2005].…”

Mammalian Enamel Formation

“…The in vivo periodic structures reported earlier may well be related to the amelogenin protein nano-chain assembly observed in vitro [143][144][145][146]. For a detailed analysis of such comparison and an insight into the emergence of nanospheres as basic structural units of developing enamel see Moradian-Oldak and Goldberg [147] and Moradian-Oldak [148], respectively.…”