Epithelial YAP1 Regulates Ameloblast Differentiation through SHH/FGF Signaling

Enamel produced by ameloblasts derived from the oral epithelium is the hardest and most mineralized tissue. Developmental enamel defects have significant clinical implications, such as enamel hypoplasia or opacities, increased tooth sensitivity, and impaired mastication. Yap1 is a transcriptional coactivator that has been shown to be involved in regulating cell proliferation and differentiation. Although Yap1 has been reported to play an important role in tooth development, the mechanism by which Yap1 regulates enamel formation and ameloblast differentiation remains unclear. In this study, we report that ablation of Yap1 in the dental epithelium using Pitx2^Cre leads to the defect of enamel formation. In the Yap1^Pitx2Cre mutant incisors, the expression of Amelx, Ambn, and Mmp20 was greatly reduced in ameloblasts, indicating a defect in ameloblast differentiation. The proliferation of epithelial and mesenchymal cells was significantly reduced in Yap1^Pitx2Cre mutant incisors; however, there was no significant difference in apoptosis between wild type and mutant. Transcriptome analysis and in situ hybridization identified that Shh, Ptch1, Fgf3, Fgf10, Etv4, and Etv5 were significantly downregulated after Yap1 deletion. In the labial cervical loop of Yap1^Pitx2Cre mutant incisors, both SHH signaling and FGF signaling were significantly decreased. Furthermore, our results suggest that FGF signaling is regulated by SHH. SHH treatment induces Fgf3 expression in vitro, and activation of the hedgehog pathway upregulates FGF signaling in vivo. Overexpression of Ihh attenuates enamel formation and cell proliferation defects caused by Yap1 deletion, confirming the genetic integration of Hedgehog signaling. In summary, our study shows that epithelial Yap1 may regulate ameloblast differentiation by modulating SHH/FGF signaling.