Msx2 function in tooth crown morphogenesis

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DESCRIPTION (provided by applicant): Teeth are epithelial appendages whose morphogenesis is regulated by conserved signaling cascades. The patterning and shape of tooth crowns is determined by the location, orientation, shape and height of tooth cusps, and by the proliferation and folding of the adjacent epithelium. At the molecular level, the patterning and shape of tooth crowns is controlled by signals and their transcriptional mediators derived from the enamel knot, an epithelial structure considered the organizing center for tooth morphogenesis. More than 10 signaling proteins have restricted expression in the enamel knots, including fibroblast growth factors (FGFs), hedgehog (HH), WNTs and transforming growth factor b (TGFb) superfamilies as well as mediators of signaling such as p21, Edar, Lef1 and Msx2. Mice lacking the homeobox gene Msx2 exhibit defects in crown morphogenesis. Our studies show that in the Msx2 knock out mice (i) the cusps are formed but they are misshapen, (ii) the folding of the dental epithelium is aberrant in the molars exhibiting localized, increased cell proliferation and that (iii) Bmp4 signaling in the enamel knot is Msx2-dependent suggesting that Bmp4 and Msx2 genes function within the same genetic pathway. These observations serve as the basis for our proposed hypothesis that an Msx2-dependent signal BMP4 is a potential major effector of the Msx2 function in the enamel knot by controlling the shape of cusps and crown normal morphogenesis. In this context and to further investigate the role of Msx2 during tooth development, we will (i) identify genes regulated by Msx2 in the enamel knot, by investigating the expression patterns of genes that, like Bmp4, are co-expressed with Msx2 in the enamel knot and (ii) determine whether the Msx2 downstream gene, BMP4, is a major effector of the Msx2 function in the enamel knot, by testing the sufficiency of BMP4 to rescue the cusp shape and crown morphogenesis defect in vitro and in vivo. Collectively these experiments should define a regulatory hierarchy between Msx2 and its downstream gene Bmp4 in both genetic and molecular terms during late tooth development and provide a molecular framework for understanding cusp shape and crown morphogenesis. Narrative: Teeth are epithelial appendages whose morphogenesis is regulated by complex genetic pathways. Several craniofacial and ectodermal dysplasia disorders affect tooth development by affecting their early development (tooth agenesis) or late development (dysplastic crown, enamel hypoplasia). Msx2 mutant teeth are dysplastic, exhibiting crown shape abnormalities. Although, Msx2 is critical for tooth crown morphogenesis when mutated, its function remains unknown. This project aims at understanding the role of Msx2 in the pathogenesis of tooth dysmorphias. To accomplish this aim, a search for genes regulated by Msx2 will be conducted, using powerful molecular biology, transgenic and organ culture approaches. It is anticipated that these results will provide valuable insight on the function of Msx2 gene, on the pathogenetic mechanism of tooth development and may also provide valuable information towards treatment of tooth dysmorphias. 4