Bmp-Wnt Interaction Determines Dental Epithelia Cytodifferentiation through Regulation of EMT

Objectives: Cytodifferentiation of dental epithelia to enamel-producing ameloblasts or root epithelium cell lineage compartmentalizes teeth into crowns and roots. Bmp and Wnt signaling pathways are essential for early tooth morphogenesis, and activated in dental epithelia at cytodifferentiation stage. This study aims to clarify the direct roles of Bmp and Wnt signaling in cytodifferentiationd of dental epithelia.

Methods: Epithelial Bmp and Wnt signaling activities during cytodifferentiation of dental epithelia were modulated with inducible genetic mouse models based on the tet-on system, including Keratin5-rtTA/tetO-Dkk1, Keratin5-rtTA/tetO-Wnt1, and   Keratin5-rtTA/tetO-Cre mediated knockout of Bmpr1a or/and b-catenin. Micro-CT scaning, histolgoy, immunohistochemistry, in situ hybridization, quantitative RT-PCR were used to analyse tooth phenotypes and underlying mechanisms.

Results: Bmpr1a knockout during cytodifferentiation stage switched the direction of dental epithelial differentiation from ameloblasts to cementoblasts at both molar crowns and labial side of incisors, which was mediated by promotion of epithelial-mesenchymal transition (EMT) via upregulation of EMT associated transcription factors and downregulation of miR-200 family. Either activation or inhibition of Wnt/β-catenin pathway in dental epithelia inhibited amelogensis in incisors, but not in molars; and Wnt activation also promoted EMT and formation of root analogue instead of crown analogue at labial side of incisor, which was similarly mediated by upregulation of EMT factors and downregulation of miR-200. Notably, simultaneous β-catenin knockout rescued ectopic cementogenesis caused by Bmpr1a knockout by inhibiting EMT; while simultaneous Wnt1 over-expression advanced Bmpr1a knockout-induced ectopic cementogenesis by further promoting EMT.

Conclusions: These data indicated that BMP and Wnt/β-catenin signaling interact to determine cytodifferentiation of dental epithelia via regulation of EMT, and suggestd the potential of manipulating these two pathways to promote tooth root regeneration in future. In addition, for the first time as far as we know, this study provided the direct in vivo evidences for epithelia-originated cementogenesis through EMT.