Sfrp4 Is Required for Proper Mouse Incisor Growth

Objectives: Mouse incisors grow continuously thanks to the presence of adult stem cells. Despite recent progress, how dental mesenchymal stem cells (MSCs) are supported in their niche and give rise to odontoblasts remain elusive. Among the signaling pathways involved in tooth development the Wnt signaling is of primordial importance. Our studies have established that loss of function mutations in Secreted Frizzled Receptor Protein 4 (SFRP4), a Wnt antagonist, lead to Pyle disease, a rare skeletal disorder characterized by severe limb deformity and often accompanied by oral manifestations including carious and misplaced teeth, taurodontism, retained deciduous teeth, and delayed tooth eruption. These clinical manifestations support a potential involvement of Sfrp4 in bone and tooth development. We have shown that Sfrp4 deletion in mice causes skeletal deformities closely mimicking Pyle disease. We therefore sought to assess whether Sfrp4^-/- mice might be a model to explore the oral anomalies.
Methods: In vivo studies on 6 weeks old Sfrp4^-/- and wt mice as well as in vitro studies were performed and analyzed by immunohistochemistry, histomorphometry, and microCT.
Results: We found that while in Sfrp4^-/- mice, skull size and dentition appear to be normal, the incisors are markedly shorter. Confirming this phenotype, Sfrp4 deletion leads to decreased dental pulp MSC proliferation and their rate of migration in the incisor pulp. Sfrp4 is expressed in the epithelial cells, MSCs and mesenchymal Transient Amplifying Cells (mTACs). Sfrp4 deletion leads to a decrease in Gli1 expression in the epithelial and mesenchymal compartments and in Ki67 levels in mTACs. Clipping studies demonstrates a slower growth in Sfrp4^-/- incisors compared to wt (p<0.05). Supporting a defect in odontoblast differentiation/function, dentin mineral apposition rate, measured by dynamic histomorphometry, was markedly decreased in Sfrp4^-/- mice (p<0.05). In vitro studies show that Sfrp4 deleted dental pulp MSCs display impaired ability to differentiate and migrate.
Conclusions: These findings support a role for Sfrp4 in the incisor stem cells. Elucidating the molecular processes that drive dental tissue development might open novel regenerative approaches in humans.