A Novel Role of TGF-β Signaling in Postnatal Tooth Root Formation

Objectives: Previous studies revealed a critical role of TGF-β signaling during early tooth development. However, roles of TGF-β signaling in postnatal tooth formation remain largely unclear. The purpose of this study was to uncover the role of TGF-β signaling during postnatal root formation
Methods: The TGF-βR2^flox/flox mice were crossed with Gli1-Cre^ERT2 mice in R26R^tdTomato background to study the role of TGF-β signaling in mesenchymal progenitors, which are responsible for tooth root formation. Tamoxifen was administrated at postnatal day 5 and mice were sacrificed at the age of 1 month. A combination of X-ray, μ-CT, SEM, histology, and immunostaining techniques were used to study the specific role of TGF-β signaling in tooth root formation
Results: Loss of TGF-βR2 in tooth Gli1⁺ cells resulted in severe defects of molar root formation characterized by short roots and thin root dentin with little change in HERS and tooth crown. Histologic analyses showed a significant decrease in dentin mineralization rates and great changes in odontoblast morphology, including disorganization of cell distributions, a lack of cellular polarity and a loss of dentinal tubules. The μ-CT quantitative analyses showed a significant reduction in root length, mineral density and root dentin volume in cKO mice compared to the age-matched controls (n=6, P＜0.001). The molecular mechanism studies demonstrated: 1) dental pulp progenitors failed to differentiate into mature odontoblasts with great reductions of expressions of DSP, nestin, and OSX; 2) a great increase in DMP1 and OCN (early markers for odontoblasts); and 3) the deposition and mineralization of dentin matrix were notably reduced; 4) no apparent changes in cell cycle markers
Conclusions: The above data demonstrated that TGF-β signaling specifically regulates tooth root but not crown formation by a direct regulation of cell differentiations with little impact on cell cycles