Enamel Defects and Ameloblast-specific Expression in Enam Knock-out/lacZ Knock-in Mice

Abstract

Enamelin is critical for proper dental enamel formation, and defects in the human enamelin gene cause autosomal dominant amelogenesis imperfecta. We used gene targeting to generate a knock-in mouse carrying a null allele of enamelin (Enam) that has a lacZ reporter gene replacing the Enam translation initiation site and gene sequences through exon 7. Correct targeting of the transgene was confirmed by Southern blotting and PCR analyses. No enamelin protein could be detected by Western blotting in the Enam-null mice. Histochemical 5-bromo-4-chloro-3-indolyl-␤-D-galactopyranoside (X-gal) staining demonstrated ameloblast-specific expression of enamelin. The enamel of the Enam ؉/؊ mice was nearly normal in the maxillary incisors, but the mandibular incisors were discolored and tended to wear rapidly where they contacted the maxillary incisors. The Enam ؊/؊ mice showed no true enamel. Radiography, microcomputed tomography, and light and scanning electron microscopy were used to document changes in the enamel of Enam ؊/؊ mice but did not discern any perturbations of bone, dentin, or any other tissue besides the enamel layer. Although a thick layer of enamel proteins covered normal-appearing dentin of unerupted teeth, von Kossa staining revealed almost a complete absence of mineral formation in this protein layer. However, a thin, highly irregular, mineralized crust covered the dentin on erupted teeth, apparently arising from the formation and fusion of small mineralization foci (calcospherites) in the deeper part of the accumulated enamel protein layer. These results demonstrate ameloblast-specific expression of enamelin and reveal that enamelin is essential for proper enamel matrix organization and mineralization.

Ctip2/Bcl11b controls ameloblast formation during mammalian odontogenesis

“…2 and 3), which was not correlated with cranial dysmorphogenesis. Ctip2 Ϫ/Ϫ ameloblasts failed to synthesize appreciable amounts of ameloblast-specific proteins, which are required for enamel formation, and Ctip2 appeared to regulate the expression of genes encoding some of these proteins, as well as transcription factors that are implicated in the regulation of ameloblast differentiation and enamel formation (16,28).…”

“…Enamelin comprises only 1 to 5% of enamel, yet plays an essential role in enamel formation by promoting and catalyzing growth of enamel crystals at the mineralization front of the ameloblast surface (28). Enamel crystals are organized into rods, and each rod is the product of a single ameloblast.…”

Amelogenesis Imperfecta and Other Biomineralization Defects in Fam20a and Fam20c Null Mice

“…7,88 Deficiencies in any of the secreted ameloblast proteins result in the production of defective enamel. In recent years, mice deficient for amelogenin, 34 kallikrein 4 (KLK4), 84 matrix metalloproteinase 20 (MMP-20), 12 ameloblastin, 111 and enamelin 43 have been shown to develop AI phenotypes. The dental lesions displayed by each of these models have helped elucidate the functions of these matrix components in enamel formation.…”

“…84 MMP-20 deficiency produces thin enamel with disrupted rod patterns, 12 while deficiencies in either ameloblastin or enamelin result in formation of an abnormally thin, disorganized, and poorly mineralized enamel layer. 43,111 The disorganized thin layer of calcified material covering the incisor and molar teeth of both ameloblastin and enamelin deficient mice only contains 68% mineral by weight (similar to bone). 90 The precise mechanisms are not yet understood, but it appears that ameloblastin and enamelin must be present within the enamel matrix for the mineral-induction event to occur normally.…”

Enamel Tissue Engineering

“…The enamelin gene has also been implicated in human amelogenesis imperfecta. Enamelin knockout mice do not form normal enamel, because of the lack of mineralization at the secretory surface of the ameloblasts (16), which suggests that enamelin is also necessary for enamel formation.…”