Bivalency of Shh Modulates Cell Fate Decision of Dental Epithelium Stem Cells

Abstract

Rodent incisor grows throughout whole life of the animal and owns every stage of dental epithelium development, thus it provides an ideal model to study the fate decision of dental epithelial stem cells (DESCs) as well as to understand the mechanisms of tooth regeneration. Bivalency, characterized as concurrence of H3K27me3 and H3K4me3, manages key developmental genes transcript in some organs, however no evidences answer whether it works on enamel organ development. Here using mouse incisor models, we first of time reveal the dynamic alterations of bivalency during DESCs differentiation, which is dominated via H3K27me3 rather than H3K4me3. Ezh2 is identified to catalyze this H3K27me3 change, then determining cell fate of DESCs. Ezh2 defecient DESCs show impaired stemness but activated amelogenic gene amelogenin (Amelx). Conversely, elevating Ezh2 maintains pluripotency and represses Amelx, which is rescued by deleting SET trimethylase activity domain in Ezh2. Mechanistically, Ezh2 is validated to catalyze H3K27me3 within promoter of Shh in a methytransferase activity dependent manner, thus determining Shh transcript. In vitro silencing Ezh2 results in activation of Shh expression, increasing the recruitment of GLI1 within Amelx promoter, however, Ezh2 overexpression represses such Shh signaling. In vivo bivalency related Shh activation occurs in pre-ameloblasts (PAB), which directly triggers Ptch1/Gli1 signaling to enhance the enrichment of Gli1 within promoter of Amelx, finally opens the door of differentiation. These results bring light to brand new insights on role of bivalency in enamel organ formation and cell fate choices of stem cell. Funding Sources: This work was supported by National Natural Science Foundation of China (NSFC grant 81470711&81771033 to LWZ), preeminent youth fund of Sichuan province (2016JQ0054 to LWZ), Sichuan Province Science and Technology Innovation Team Program (2015TD0011 to YL, 2017TD0016 to Y.Q). Conflicts of Interest: The authors declare no potential conflicts of interest with respect to the authorship and publication of this article. Ethics Approval Statement: All mouse experiments were conducted in accordance with protocols approved by ethical committees of the West China School of Stomatology, Sichuan University and State Key Laboratory of Oral Diseases.