The Role of FAM20B-Catalyzed Proteoglycans in Tooth Development
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Project summary/Supernumerary teeth can cause a broad range of dental complications. As extra teeth are formed onexisting dentition, unraveling the molecular mechanism of supernumerary tooth formation will not only helpdevelop the therapeutic strategy for this disease but also provide insights into tooth regeneration. Despitethe significant progress in understanding the regulatory role of morphogens, growth factors, andtranscriptional factors in supernumerary tooth formation, little is known about the role of extracellularcomponents such as proteoglycans in this pathological process. Our recent studies show that inactivationof dental epithelial Fam20B, a newly discovered xylose kinase essential for glycosaminoglycan (GAG)assembly, leads to supernumerary incisors in mice. Our pilot study reveals that GAG deficiency in thedental epithelium leads to ectopic activation of WNT signaling, and that an ectopic Sox2 expression islocated in the same area, which normally should disappear from this site after E14.5. Our in vitro studyshows that GAGs on certain FAM20B-catalyzed proteoglycans suppress WNT signaling but facilitate Wise-mediated inhibition on WNT. Conversely, administering WNT inhibitor to the mutant embryos rescued thetooth phenotype in some cases. These data led us to form our central hypothesis that certain FAM20B-catalyzed proteoglycans regulate tooth renewal by mediating the stem cell renewal via negative regulationon WNT signaling in the dental epithelium. To test this hypothesis, we propose the following three specificaims: (1) To determine if FAM20B-catalyzed proteoglycans mediate tooth renewal via negative regulationon WNT signaling, and if GAG-mediated Wise inhibition on WNT underlies the supernumerary toothformation. We will perform ''rescue'' experiments by overexpressing DKK1 or Wise in the dental epitheliumto inhibit the overactivated WNT signaling in K14Cre/+;Fam20Bfl/fl mice. (2) To determine whether FAM20B-catalyzed proteoglycans regulate tooth renewal by mediating the stem cell renewal in the dental epithelium.We will perform lineage tracing and inducible knockout experiments to determine the role of stem cellrenewal in GAG deficiency-caused supernumerary teeth, and if the FAM20B-catalyzed PGs mediate stemcell renewal in the dental epithelium in a cell-autonomous manner. (3) To identify the FAM20B-catalyzedproteoglycans responsible for the supernumerary tooth formation. We will determine the expression patternof FAM20B-catalyzed proteoglycans in tooth and identify those negatively regulating WNT signaling. Thebiological function of candidate proteoglycans will be determined by gene knockdown and organ culturemethods. The completion of this study will advance our understanding about the molecular mechanismunderlying supernumerary tooth formation and help in laying the groundwork for tooth regeneration. AsFAM20B and proteoglycans are extensively expressed in many tissues, the knowledge gained from thisstudy may also provide valuable insights into the proteoglycan-mediated signaling in other tissues.