The Role of FAM20B-catalyzed Proteoglycans in Tooth Development

Objectives: Proteoglycans(PGs) on the cell surface and in the extracellular space are important signaling orchestrators. Although the expression of PGs has been documented in dental tissues for decades, their exact roles in tooth development remain poorly understood. In this study, we inactivated the Fam20B in both dental epithelium and mesenchyme to identify the role of FAM20B-catalyzed PGs in tooth development.
Methods: We generated a mouse line with floxed allele of Fam20B, a newly identified xylose kinase essential for glycosaminoglycan (GAG) assembly, and removed this kinase from the dental epithelium and dental mesenchyme by crossing the Fam20B-floxed mice with K14-Cre and Wnt1-Cre mice. To determine molecular mechanism underlying the supernumerary tooth formation, we examined the transcriptome changes in E12.5 lower incisors of K14-Cre;FamB-cKO mice compared with those of the WT. To confirm the signaling changes, we crossbred tetO-Dkk1 mice and K14-Cre;FamB-cKO mice mice to overexpress Dkk1 in the dental epithelium at E13.5.
Results: The K14-Cre;FamB-cKO mice mice showed duplicate incisors and enamel defects, while the Wnt1-Cre;FamB-cKO mice had striking phenotypes in crown morphology and root length, suggesting that GAGs are essential to enamel formation and the signaling balance controlling tooth renewal, root development and crown morphogenesis. We identified an overactivity of WNT signaling in the K14-Cre;FamB-cKO mice, and confirmed this change by qPCR and WNT indicator mice (BAT-GAL). In vitro analyses showed that the GAGs on certain PGs inhibit WNT signaling but facilitate Wise-mediated inhibition on WNT. Moreover, among 12 Dkk1-overexpressed K14-Cre;FamB-cKO mice, 9 showed complete rescue of the supernumerary incisors. More interestingly, we found ectopic SOX2 expression in the dental epithelium of mutant mice, suggesting that the GAG-mediated signaling may govern stem cell renewal in the dental epithelium.
Conclusions: Our results suggest that FAM20B-catalyzed PGs are pivotal orchestrators for tooth development. The GAGs on certain PGs may regulate stem cell renewal in the dental epithelium by negatively mediating the WNT signaling. As FAM20B and PGs are extensively expressed in many tissues, the knowledge gained from this study may have broader implication in the GAG-mediated signaling in other tissues.