Wnt extracellular signaling molecules have essential roles as regulators of cell proliferation, migration, differentiation, and in epithelial-mesenchymal interactions involved in tissue morphogenesis. Frizzled integral membrane proteins have been shown to function as receptors for Wnt signaling molecules. Vertebrates also produce secreted proteins related to Frizzled receptors, Frizzled-related proteins (FRPs), which contain the cysteine-rich domain of Frizzleds and appear to function as Wnt antagonists. Tooth development is regulated by a reciprocal series of epithelial-mesenchymal interactions, and many Wnt signaling pathway genes are expressed in the developing tooth at these sites. Here we report the expression of one FRP gene, Mfrzbl, in the rostral mesenchyme of the mandibular primordium. Using explant cultures, we show that expression of Mfrzbl in the mandibular mesenchyme is under the control of signals derived from the overlying epithelium. Bead implantation experiments in vitro show that FGF8 induces Mfrzbl expression, whereas BMP4 and SHH proteins have no effect. We studied the effect of ectopic MFrzbl protein on the developing tooth germs by transplanting explants treated with Mfrzbl protein into renal capsules, and found it to retard tooth development. This suggests that Wnt signaling is required early in tooth germ formation and that interference with signaling via addition of an antagonist results in retarded development and formation of smaller teeth.
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