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. <i>Yap1</i> is a transcriptional coactivator that has been shown to be involved in regulating cell proliferation and differentiation. Although <i>Yap1</i> has been reported to play an important role in tooth development, the mechanism by which <i>Yap1</i> regulates enamel formation and ameloblast differentiation remains unclear. In this study, we report that ablation of <i>Yap1</i> in the dental epithelium using <i>Pitx2^Cre</i> leads to the defect of enamel formation. In the <i>Yap1^Pitx2Cre</i> mutant incisors, the expression of <i>Amelx</i>, <i>Ambn</i>, and <i>Mmp20</i> was greatly reduced in ameloblasts, indicating a defect in ameloblast differentiation. The proliferation of epithelial and mesenchymal cells was significantly reduced in <i>Yap1^Pitx2Cre</i> mutant incisors; however, there was no significant difference in apoptosis between wild type and mutant. Transcriptome analysis and in situ hybridization identified that <i>Shh</i>, <i>Ptch1</i>, <i>Fgf3</i>, <i>Fgf10</i>, <i>Etv4</i>, and <i>Etv5</i> were significantly downregulated after <i>Yap1</i> deletion. In the labial cervical loop of <i>Yap1^Pitx2Cre</i> 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 <i>Fgf3</i> expression in vitro, and activation of the hedgehog pathway upregulates FGF signaling in vivo. Overexpression of <i>Ihh</i> attenuates enamel formation and cell proliferation defects caused by <i>Yap1</i> deletion, confirming the genetic integration of Hedgehog signaling. In summary, our study shows that epithelial <i>Yap1</i> may regulate ameloblast differentiation by modulating SHH/FGF signaling.