Excessive Wnt/ß-catenin signaling disturbs tooth-root formation.

BACKGROUND AND OBJECTIVE: Wingless-type MMTV integration site family (Wnt)/ß-catenin signaling plays an essential role in cellular differentiation and matrix formation during skeletal development. However, little is known about its role in tooth-root formation. In a previous study, we found excessive formation of dentin and cementum in mice with constitutive ß-catenin stabilization in the dental mesenchyme. In the present study we analyzed the molar roots of these mice to investigate the role of Wnt/ß-catenin signaling in root formation in more detail. MATERIAL AND METHODS: We generated OC-Cre:Catnb(+/lox(ex3)) mice by intercrossing Catnb(+/lox(ex3)) and OC-Cre mice, and we analyzed their mandibular molars using radiography, histomorphometry and immunohistochemistry. RESULTS: OC-Cre:Catnb(+/lox(ex3)) mice showed impaired root formation. At the beginning of root formation in mutant molars, dental papilla cells did not show normal differentiation into odontoblasts; rather, they were prematurely differentiated and had a disorganized arrangement. Interestingly, SMAD family member 4 was upregulated in premature odontoblasts. In 4-wk-old mutant mice, molar roots were about half the length of those in their wild-type littermates. In contrast to excessively formed dentin in crown, root dentin was thin and hypomineralized in mutant mice. Biglycan and dentin sialophosphoprotein were downregulated in root dentin of mutant mice, whereas dentin matrix protein 1 and Dickkopf-related protein 1 were upregulated. Additionally, ectonucleotide pyrophosphatase/phosphodiesterase 1 was significantly downregulated in the cementoblasts of mutant molars. Finally, in the cementum of mutant mice, bone sialoprotein was downregulated but Dickkopf-related protein 2 was upregulated. CONCLUSION: These results suggest that temporospatial regulation of Wnt/ß-catenin signaling plays an important role in cell differentiation and matrix formation during root and cementum formation.

Disruption of Tgfbr2 in odontoblasts leads to aberrant pulp calcification.

Transforming growth factor ß (TGF-ß) signaling has been implicated in dentin formation and repair; however, the molecular mechanisms underlying dentin formation remain unclear. To address the role of TGF-ß signaling in dentin formation, we analyzed odontoblast-specific Tgfbr2 conditional knockout mice. The mutant mice had aberrant teeth with thin dysplastic dentin and pulpal obliteration, similar to teeth from human patients with dentinogenesis imperfecta type II and dentin dysplasia. In mutant, the odontoblasts lost their cellular polarity, and matrix secretion was disrupted after mantle dentin formation. As a consequence, the amount of predentin decreased significantly, and an ectopic fibrous matrix was formed below the odontoblast layer. This matrix gradually calcified and obliterated the pulp chamber with increasing age. Immunohistochemistry revealed decreased expression of alkaline phosphatase in mutant odontoblasts. In mutant dentin, Dsp expression was reduced, but Dmp1 expression increased significantly. Collagen type I, biglycan, and Dsp were expressed in the ectopic matrix. These results suggest that loss of responsiveness to TGF-ß in odontoblasts results in impaired matrix formation and pulpal obliteration. Our study indicates that TGF-ß signaling plays an important role in dentin formation and pulp protection. Furthermore, our findings may provide new insight into possible mechanisms underlying human hereditary dentin disorders and reparative dentin formation.