Fig. 8

WNT signaling functions as a relay cue facilitating supernumerary tooth formation at late cap stage. A–F IHC staining of LEF-1, a putative downstream marker of canonical WNT signaling, on the coronal sections of lower incisors at E14.5-E16.5 showed an ectopic overexpression in both the dental epithelium and dental mesenchyme (arrows in D, E and F) at the presumptive location for supernumerary tooth formation (mainly at the mesial-lingual side of native enamel organ) in the K14^Cre/+;Fam20B^fl/fl mice compared to the control mice. Note that the LEF1 staining of enamel knots in (B) and (E) can be used as internal references for comparing the expression levels between control and knockout. d↔m indicates the orientation of distal and mesial sides. G, H To confirm the upregulation of WNT signaling, we introduced a WNT indicator allele, BAT-GAL, into the conditional knockout and control mice. LacZ staining on the coronal sections of E15.5 lower incisors demonstrated more and ectopic WNT activity in the enamel organ and adjacent dental mesenchyme (arrows in H) of K14^Cre/+;Fam20B^fl/fl mice compared to the control mice. Note that the LacZ staining of enamel knots can be used as internal references for comparing the WNT activity between control and knockout. I, J Accordingly, the transcription of a WNT inhibitor, Sostdc1, was downregulated in the dental epithelium and adjacent dental follicle at the presumptive location of replacement tooth formation in K14^Cre/+;Fam20B^fl/fl mice (arrow in J). Scale bars: 200 μm. K–M To clarify the significance of the overactivated WNT signaling in the supernumerary tooth formation, we overexpressed a WNT inhibitor, DKK1, in the GAG-deficient dental epithelium by introducing tetO^DKK1 and K14^rtTA alleles to the K14^Cre/+;Fam20B^fl/fl mice. Doxycycline was administrated to the resultant mice at 2 mg/kg at E13.5 to induce DKK1 expression. Overexpression of DKK1 fully rescued the supernumerary tooth phenotype in K14^rtTA;tetO^DKK1;K14^Cre/+;Fam20B^fl/fl mice compared with the normal control (Fam20B^fl/fl) and conditional knockout (K14^Cre/+;Fam20B^fl/fl) mice