Persistent Wnt/β-catenin signaling disables soft palatogenesis and palatal osteogenesis by inducing mesenchymal condensation.

INTRODUCTION: Mammalian palates are composed of the anterior hard palate and the posterior soft palate. However, the correlation of the genesis, pattern formation, and morphogenesis between the hard and soft palates remains elusive. METHODS: In this study, we explicated the complicated palatal defects in Osr2-cre (KI) ;Ctnnb1 (ex3f) mice, in which canonical Wnt activity was persistent due to constitutively active β-catenin in the palatal mesenchyme. Osr2-cre (KI) ;Ctnnb1 (ex3f) palates displayed an ectopic mesenchymal condensation extending from the proximal-posterior area to the distal-anterior area, along with impaired osteogenesis and agenesis of soft palate. RESULTS: Immunohistochemistry showed the overlapping active canonical Wnt domain with the ectopic mesenchymal condensation, indicating that the condensation was induced by persistent canonical Wnt signaling. Wnt5a, a chemokine that induces posterior-anterior migration of palatal mesenchymal cells, was activated in the anterior and middle palatal mesenchyme of Osr2-cre (KI) ;Ctnnb1 (ex3f) mice. Exogenous supplementation of Wnt5a into wild-type (WT) palates recapitulated the mesenchymal condensation. These findings indicate that the persistent canonical Wnt signaling in the palatal mesenchyme extended Wnt5a expression, which enforced posterior mesenchymal migration toward the anterior to form the convoluted condensation, thereby impairing the genesis of the soft palate in Osr2-cre (KI) ;Ctnnb1 (ex3f) mice. Moreover, the medially osteogenic markers Sox9, Runx2, and Osx; the laterally Shh, Foxf1, and Fgf10; and another Wnt inhibitor, Sfrp2, were significantly reduced or even diminished in Osr2-cre (KI) ;Ctnnb1 (ex3f) palatal shelves. In contrast, the condensed Osr2-cre (KI) ;Ctnnb1 (ex3f) palatal mesenchyme displayed the medial markers Dlx5 and p-Smad1/5/8, along with the fibrosis/dermal markers ɑ-SMA and Tbx15. The Wnt and TGF-β/BMP inhibitors Ectodin and Noggin were also ectopically activated in the palatal epithelium overlying the condensed mesenchyme Osr2-cre (KI) ;Ctnnb1 (ex3f) mice. DISCUSSION: These findings indicate a transition of palatal mesenchymal cells from an osteogenic fate into fibrosis commitment, along with disrupted mediolateral patterning of the palatal shelves due to persistent canonical Wnt activity. Our study provides molecular clues that fine-tuning the mesenchymal canonical Wnt activity and Wnt5a-directed cell migration correlates with the morphogenesis of hard palates and the genesis of soft palates.