Enamelin Is Critical for Ameloblast Integrity and Enamel Ultrastructure Formation

Abstract

Mutations in the human enamelin gene cause autosomal dominant hypoplastic amelogenesis imperfecta in which the affected enamel is thin or absent. Study of enamelin knockout NLS-lacZ knockin mice revealed that mineralization along the distal membrane of ameloblast is deficient, resulting in no true enamel formation. To determine the function of enamelin during enamel formation, we characterized the developing teeth of the Enam−/− mice, generated amelogenin-driven enamelin transgenic mouse models, and then introduced enamelin transgenes into the Enam−/− mice to rescue enamel defects. Mice at specific stages of development were subjected to morphologic and structural analysis using β-galactosidase staining, immunohistochemistry, and transmission and scanning electron microscopy. Enamelin expression was ameloblast-specific. In the absence of enamelin, ameloblasts pathology became evident at the onset of the secretory stage. Although the aggregated ameloblasts generated matrix-containing amelogenin, they were not able to create a well-defined enamel space or produce normal enamel crystals. When enamelin is present at half of the normal quantity, enamel was thinner with enamel rods not as tightly arranged as in wild type suggesting that a specific quantity of enamelin is critical for normal enamel formation. Enamelin dosage effect was further demonstrated in transgenic mouse lines over expressing enamelin. Introducing enamelin transgene at various expression levels into the Enam−/− background did not fully recover enamel formation while a medium expresser in the Enam+/− background did. Too much or too little enamelin abolishes the production of enamel crystals and prism structure. Enamelin is essential for ameloblast integrity and enamel formation.

Retinoic Acid Excess Impairs Amelogenesis Inducing Enamel Defects

“…The most outer enamel was less mineralized when compared with the control tooth, as outer enamel appeared darker in RA-treated animals. Enamel rods of RA-supplemented animals were less densely packed, and as a consequence, areas normally filled with interprismatic enamel seemed empty and showed holes-like pattern (Figure 2H), similar to Enam haploinsufficent mice (Hu et al, 2014). …”

“…When RA treatments were performed from E12.5 to 16.5, they were found to produce a chalky lightening and length reduction of incisors, changes more pronounced in lower incisors (Figures 1B,F). The whiter color and less shiny surface may reflect reduced enamel thickness typical of mouse enamel hypoplasia models (Gibson et al, 2001; Masuya et al, 2005; Hu et al, 2014). Treatments performed at later stages (E13.5–16.5: Figures 1C,G, or E14.5–17.5: Figures 1D,H) had less severe effects on enamel, suggesting RA has early roles in the oral epithelium starting at the placode stage of tooth initiation.…”

“…These diseases can be recapitulated in several mouse models. For example, hypoplastic or aplastic enamel deficiencies are produced by amelogenin (Amel), ameloblastin ( Ambn ), and enamelin ( Enam ) mutations, recapitulating defects in patients (Gibson et al, 2001; Fukumoto et al, 2004; Masuya et al, 2005; Hu et al, 2008, 2014). Studies in mouse models have contributed to understanding how defective ameloblast protein secretion contributes to these diseases.…”

Enamel Formation Genes Associated with Dental Erosive Wear

“…Hu et al [2014] suggested that the enamelin protein is essential for the formation of crystals with prism structures and obtaining optimal thickness of the enamel. Defects in the ENAM gene during evolution are associated with defective enamel formation [Meredith et al, 2009].…”

Accelerated enamel mineralization in Dspp mutant mice

“…Porcine DGP possesses one glycosylated asparagine and 4 phosphorylated serine residues [23]. DPP specifically binds to collagen fibrils [33–35] and can act as a mineralization promoter or inhibitor at low and high concentrations, respectively [36].…”