Alk5 mediated TGF-beta signaling acts upstream of FGF10 to regulate the proliferation and maintenance of dental epithelial stem cells

Mouse incisors grow continuously throughout life. This growth is supported by the division of dental epithelial stem cells that reside in the cervical loop region. Little is known about the maintenance and regulatory mechanisms of dental epithelial stem cells. In the present study, we investigated how TGF-beta signal mediated mesenchymal-epithelial interactions control dental epithelium stem cells. We designed two approaches using incisor organ culture and BrdU pulse-chase experiments to identify and evaluate stem cell functions. We showed that loss of TGF-beta type I receptor (Alk5) in the cranial neural crest derived dental mesenchyme severely affects the proliferation of TA (transit amplifying) cells and the maintenance of dental epithelium stem cells. Incisors of Wnt1--Alk5 fl/fl mice lose their ability to continue to grow in vitro. The number of BrdU label retaining cells (LRC) is dramatically reduced in Alk5 mutant mice. Fgf10, Fgf3 and Fgf9 signaling in the dental mesenchyme are downregulated in Wnt1--Alk5fl/fl incisors. Strikingly, the addition of exogenous FGF10 into incisor cultures rescues dental epithelial stem cells in Wnt1--Alk5 fl/fl mice. Therefore we propose that Alk5 functions upstream of Fgf10 to regulate TA cell proliferation and stem cell maintenance, and this signaling mechanism is crucial for stem cell mediated tooth regeneration.