A new function of BMP4: dual role for BMP4 in regulation of Sonic hedgehogexpression in the mouse tooth germ

The murine tooth development is governed by sequential and reciprocal epith elial-mesenchymal interactions. Multiple signaling molecules are expressed in the developing tooth germ and interact each other to mediate the inducti ve tissue interactions. Among them are Sonic hedgehog (SHH), Bone Morphogen etic Protein-2 (BMP2) and Bone Morphogenetic Protein-4 (BMP4). We have inve stigated the interactions between these signaling molecules during early to oth development. We found that the expression of Shh and Bmp2 is downregula ted at E12.5 and E13.5 in the dental epithelium of the Msx1 mutant tooth ge rm where Bmp4 expression is significantly reduced in the dental mesenchyme, Inhibition of BMP4 activity by noggin resulted in repression of Shh and Bm p2 in wild-type dental epithelium, When implanted into the dental mesenchym e of Msx1 mutants, beads soaked with BMP4 protein were able to restore the expression of both Shh and Bmp2 in the Msx1 mutant epithelium. These result s demonstrated that mesenchymal BMP4 represents one component of the signal acting on the epithelium to maintain Shh and Bmp2 expression, In contrast, BMP4-soaked beads repressed Shh and Bmp2 expression in the wild-type denta l epithelium. TUNEL assay indicated that this suppression of gene expressio n by exogenous BMP4 was not the result of an increase in programmed cell de ath in the tooth germ. Ectopic expression of human Bmp4 to the dental mesen chyme driven by the mouse Msx1 promoter restored Shh expression in the Msx1 mutant dental epithelium but repressed Shh in the wild-type tooth germ in vivo, We further demonstrated that this regulation of Shh expression by BMP 4 is conserved in the mouse developing limb bud, In addition, Shh expressio n was unaffected in the developing limb buds of the transgenic mice in whic h a constitutively active Bmpr-IB is ectopically expressed in the forelimb posterior mesenchyme and throughout the hindlimb mesenchyme, suggesting tha t the repression of Shh expression by BMP4 may not be mediated by BMP recep tor-IB, These results provide evidence for a new function of BMP4 BMP4 can act upstream to Shh by regulating Shh expression in mouse developing tooth germ and limb bud, Taken together, our data provide insight into a new regu latory mechanism for Shh expression, and suggest that this BMP4-mediated pa thway in Shh regulation may have a general implication in vertebrate organo genesis.