MMP20-generated amelogenin cleavage products prevent formation of fan-shaped enamel malformations

Abstract

Dental enamel forms extracellularly as thin ribbons of amorphous calcium phosphate (ACP) that initiate on dentin mineral in close proximity to the ameloblast distal membrane. Secreted proteins are critical for this process. Enam−/− and Ambn−/− mice fail to form enamel. We characterize enamel ribbon formation in wild-type (WT), Amelx−/− and Mmp20−/− mouse mandibular incisors using focused ion beam scanning electron microscopy (FIB-SEM) in inverted backscatter mode. In Amelx−/− mice, initial enamel mineral ribbons extending from dentin are similar in form to those of WT mice. As early enamel development progresses, the Amelx−/− mineral ribbons develop multiple branches, resembling the staves of a Japanese fan. These striking fan-shaped structures cease growing after attaining ~ 20 µm of enamel thickness (WT is ~ 120 µm). The initial enamel mineral ribbons in Mmp20−/− mice, like those of the Amelx−/− and WT, extend from the dentin surface to the ameloblast membrane, but appear to be fewer in number and coated on their sides with organic material. Remarkably, Mmp20−/− mineral ribbons also form fan-like structures that extend to ~ 20 µm from the dentin surface. However, these fans are subsequently capped with a hard, disorganized outer mineral layer. Amelogenin cleavage products are the only matrix components absent in both Amelx−/− and Mmp20−/− mice. We conclude that MMP20 and amelogenin are not critical for enamel mineral ribbon initiation, orientation, or initial shape. The pathological fan-like plates in these mice may form from the lack of amelogenin cleavage products, which appear necessary to form ordered hydroxyapatite.

Structure and Chemical Composition of ca. 10-Million-Year-Old (Late Miocene of Western Amazon) and Present-Day Teeth of Related Species

“…Bartlett et al [ 21 ] showed that the knockout of Amelx and Mmp20 in mice resulted in the presence of “fan-like” crystal arrangements in the deeper enamel (20 m of enamel, close to the DEJ), while the middle and outer enamel of such animals show a dysplastic and more mineralized layer [ 21 ]. Interestingly, the enamel of the extant alligator exhibited mineral structures that resemble fans in form ( Figure 5 F–G), with the same orientation (the larger border of the fan towards the outside of the enamel).…”

“…To date, information on the association of morphological features of dental enamel with the composition (quantity and diversity) of proteins in forming enamel is still poorly understood. In recent years, this information has been obtained primarily from mouse models [ 18 , 19 , 20 , 21 ]. Little is known about other species, since the protein information is gained while enamel is forming, and this usually happens in early life.…”

Ganoin and acrodin formation on scales and teeth in spotted gar: A vital role of enamelin in the unique process of enamel mineralization

“…These findings agree that ENAM plays vital roles in mineral ribbon formation in both sarcopterygians and actinopterygians but do not necessarily support such roles for AMBN. Furthermore, SCPP5 resembles AMEL (Kawasaki et al, 2021) that is not important for the initial formation of mineral ribbons (Bartlett et al, 2021; Hu et al, 2016; Smith et al, 2016). It is thus likely that the evolution of ENAM was the crucial event for the origin of the unique mineralization process of the enamel matrix.…”

Enamel defects in Acp4R110C/R110C mice and human ACP4 mutations

“…Charging on the surfaces of block faces was reduced by coating them with a thin layer of platinum (3 nm) where required. Curtaining defects in TLD images were reduced using python language software described previously 85 . In an attempt to better define the DEJ area in Acp4 R110C/R110C mutant mice, images from Acp4 +/+ and Acp4 R110C/R110C mice at sequential levels of section across the first half of the secretory stage were normalized in ImageJ ( https://imagej.nih.gov/ij/ ) based on mean dentin intensity and then pseudocolored using the built in 16 color lookup table (LUT) (Fig.…”