The absence of muscle segment homeobox 2 leads to the pyroptosis of ameloblasts by inducing squamous epithelial hyperplasia in the enamel organ.

Muscle segment homeobox 2 (MSX2) has been confirmed to be involved in the regulation of early tooth development. However, the role of MSX2 has not been fully elucidated in enamel development. To research the functions of MSX2 in enamel formation, we used a Msx2-/- (KO) mouse model with no full Msx2 gene. In the present study, the dental appearance and enamel microstructure were detected by scanning electron microscopy and micro-computed tomography. The results showed that the absence of Msx2 resulted in enamel defects, leading to severe tooth wear in KO mice. To further investigate the mechanism behind the phenotype, we performed detailed histological analyses of the enamel organ in KO mice. We discovered that ameloblasts without Msx2 could secrete a small amount of enamel matrix protein in the early stage. However, the enamel epithelium occurred squamous epithelial hyperplasia and partial keratinization in the enamel organ during subsequent developmental stages. Ameloblasts depolarized and underwent pyroptosis. Overall, during the development of enamel, MSX2 affects the formation of enamel by regulating the function of epithelial cells in the enamel organ.

RUNX2 contributes to TGF-ß1-induced expression of Wdr72 in ameloblasts during enamel mineralization.

The elaborate modulation of the transforming growth factor ß (TGF-ß) superfamily signaling network plays an essential role in tooth morphogenesis and differentiation. In our previous studies, we have demonstrated that TGF-ß1 promotes enamel mineralization and maturation using TGF-ß1 gene conditional knockout (TGF-ß1-cKO) mice. However, the specific regulatory mechanisms of TGF-ß1 during enamel development remain unclear. Furthermore, we have previously found that the expression of WD repeat-containing protein 72(WDR72)in mouse enamel epithelium is decreased significantly in the absence of TGF-ß1. Therefore, the aim of the present study was to investigate how TGF-ß1 affects amelogenesis by regulating the expression of Wdr72. Histological examination showed that the absence of TGF-ß1 in ameloblastic epithelial cells resulted in a reduction in enamel mineralization and a delay in enamel matrix protein absorption. TGF-ß1, Runt-related transcription factor 2(RUNX2) and WDR72 were revealed to be colocalized in ameloblasts by immunohistochemistry, and it was also found that the expression of Runx2 and Wdr72 was markedly different between TGF-ß1-cKO mice and wild type(TGF-ß1-WT)mice. In addition, the effect of exogenous TGF-ß1 on Wdr72 was more significant when RUNX2 was present than when RUNX2 was absent. Furthermore, we found that there were binding sites for RUNX2 on the promoter of Wdr72 and that Wdr72 expression was regulated by RUNX2. Collectively, our results suggest that TGF-ß1 affects enamel mineralization by modulating RUNX2 and thus affecting the expression of Wdr72.