Fgf9 promotes incisor dental epithelial stem cell survival and enamel formation

Background: Understanding the role of cytokines in tooth development is critical for advancing dental tissue engineering. Fibroblast growth factor 9 (FGF9) is the only FGF consistently expressed throughout dental epithelial tissue, from the initiation of tooth bud formation to tooth maturation. However, mice lacking Fgf9 (Fgf9) surprisingly show no obvious abnormalities in tooth development, suggesting potential compensation by other FGFs. Here we report findings from an Fgf9 mutation mouse model, a loss-of-function mutation with a dominant negative effect. Our study reveals that Fgf9 is crucial for dental epithelial stem cell (DESC) survival and enamel formation. Methods: To dissect the role of Fgf9 in tooth development, we performed the micro-CT, histomorphological analysis and gene expression assay in mice and embryos with S99N mutation. In addition, we assessed the effect of FGF9 on the DESC survival and dental epithelial differentiation by DESC sphere formation assay and tooth explant culture. Cell/tissue culture methods, gene expression analysis, specific inhibitors, and antibody blockage analysis were employed to explore how Fgf9 regulates enamel differentiation and DESC survival through both direct and indirect mechanisms. Results: The Fgf9 mutation in mice led to reduced ameloblasts, impaired enamel formation, and increased apoptosis in the cervical loop (CL). DESC sphere culture experiments revealed that FGF9 facilitated DESC survival via activating ERK/CREB signaling, without affecting cell proliferation. Furthermore, in vitro tissue culture experiments demonstrated that FGF9 promoted enamel formation in a manner dependent on the presence of mesenchyme. Interestingly, FGF9 stimulation inhibited enamel formation in isolated enamel epithelia and DESC spheres. Further investigation revealed that FGF9 supports DESC survival and promotes amelogenesis by stimulating the secretion of FGF3 and FGF10 in dental mesenchymal cells via the MAPK/ERK signaling pathway. Conclusions: Our study demonstrates that Fgf9 is essential for DESC survival and enamel formation. Fgf9 performs as a dual-directional regulator of the dental enamel epithelium, not only inhibiting DESC differentiation into ameloblasts to preserve the stemness of DESC, but also promoting ameloblast differentiation through epithelial-mesenchymal interactions.