Overexpression of Smad2 in Tgf-β3-null mutant mice rescues cleft palate

Transforming growth factor (TGF)-β3 is an important contributor to the regulation of medial edge epithelium (MEE) disappearance during palatal fusion. SMAD2 phosphorylation in the MEE has been shown to be directly regulated by TGF-β3. No phospho-SMAD2 was identified in the MEE in Tgf-β3-null mutant mice (Tgf-β3−/−), which was correlated with the persistence of the MEE and failure of palatal fusion. In the present study, the cleft palate phenotype in Tgf-β3−/− mice was rescued by overexpression of a Smad2 transgene in Keratin 14-synthesizing MEE cells following mating Tgf-β3 heterozygous mice with Keratin 14 promoter directed Smad2 transgenic mice (K14-Smad2). Success of the rescue could be attributed to the elevated phospho-SMAD2 level in the MEE, demonstrated by two indirect evidences. The rescued palatal fusion in Tgf-β3−/−/K14-Smad2 mice, however, never proceeded to the junction of primary and secondary palates and the most posterior border of the soft palate, despite phospho-SMAD2 expression in these regions at the same level as in the middle portion of the secondary palate. The K14-Smad2 transgene was unable to restore all the functional outcomes of TGF-β3. This may indicate an anterior-posterior patterning in the palatal shelves with respect to TGF-β3 signaling and the mechanism of secondary palatal fusion.