Function Of Dental Epithelium Wnt Production In Tooth Differentiation

Wnt ligands display distinct spatiotemporal expression in the epithelium and mesenchyme during developing teeth.However,it is not clear about the actions of differently produced Wnt ligands.Shh expression specifies the odontogenic epithelium during tooth initiation and is consistently restricted to the dental epithelium.Wntless is a kind of specific membrane proteins,and Wnt ligands cannot transported out of cell without function of Wntless(Wls).Therefore.,Wls conditional knockout mice can be used as a tool to study function of Wnt ligand production in different tissues.In order to investigate the function of Wnt in dental epithelium,we use Shh-Cre to knockout the Wls of the dental epithelium.We want to know the effect of Wls deletion on teeth development.We find that conditional knockout of Wls by Shh-Cre(WlsShh-Cre)leads to defections of ameloblast and odontoblast differentiation.Moreover,the key markers for odontoblast differentiation(Dspp,Osterxi,Decorin,and Colla1)are down-regulated in WlsShh-Cre teeth.We use Wnt canonical signaling reporter mouse line BAT-GAL to display the influence of Wls knockout to Wnt signaling activity.We find that WlsShh-Cre tooth displays reduced canonical Wnt signaling activity in the inner enamel epithelium and underlying mesenchyme in E 16.5,as exhibited also by decreased Axin2 and Dkkl expression.In addition,Shh signaling pathway molecules including Shh,Ptcl and Gli1 were also significantly down-regulated in tooth at E16.5.We also find that epithelial Wls loss-of-function in Shh lineage lead to aberrant cell proliferation in both dental epithelium and mesenchyme at E16.5;however,the cell apoptosis is unaffected.Moreover,we find that Decorin and Colla1,the key markers for odontoblast differentiation are down-regulated in WlsShh-Cre teeth.We show these two genes are direct downstream targets of canonical Wnt signaling pathway,as supported by chromatin immunoprecipitation(ChIP)analysis.Additionally,both Decorin and Colla1 expression can be increased by LiCl treatment in the in vitro tooth explants.Taken together,our results suggest that the spatial expression of Wnt ligands within the dental epithelial lineage specifies the differentiation of tooth structures in later stages.