Ectodermal organ development is initiated by inductive tissue interactions, and developing teeth, epidermis, hair, and limbs are classic examples of such inductive processes. Tooth development can be divided into the initiation, bud, cap, and bell stages. In mice, tooth development begins with the thickening of the dental epithelium. The dental lamina undergoes further proliferation and subsequently develops into the tooth bud and germ. The tooth bud is formed by the invagination of the placode and the condensation of mesenchyme cells adjacent to the bud. At the cap stage, dental epithelial cells differentiate into several cell types, such as the inner dental epithelium and the enamel knot cells. Cell death by apoptosis within the enamel knot is critical for cusp formation in molars. At the bell stage, the dental mesenchyme differentiates into dentin matrix-secreting odontoblasts, and the inner dental epithelial cells differentiate into enamel matrix-secreting ameloblasts. The goal of this project is to discover novel and previously uncharacterized genes in order to understand how tooth and craniofacial tissues develop, and to define molecular defects underlying the anomalies of these tissues. Ameloblastin (Ambn) is one of the enamel matrix proteins that is secreted by ameloblasts. Ambn localizes at the apical region of the cells, and serves as the adhesion molecule for ameloblasts. We previously created a gene-targeted mouse for Ambn, in which exons 4 and 5 of the Ambn gene were deleted. In the mutant mice, ameloblasts were detached from the enamel matrix, continued to proliferate and form multiple cell layers, and odontogenic tumors often developed in the maxilla with age. We also reported that the Ambn protein has heparin-binding domains, and that these domains are critical for binding to dental epithelial cells. In collaboration with Dr. Satoshi Fukumoto, we found that overexpression of full-length Ambn in human ameloblastoma AM-1 cells inhibited cell proliferation and reduced the expression of Msx2, which regulates the dental epithelial progenitor phenotype. Thus, Ambn promotes cell attachment through its heparin-binding sites, and may play a role in preventing odontogenic tumor development by suppressing cell proliferation and maintaining differentiation phenotypes. In our collaboration with Dr. Nanci, we noticed that the Ambn mutant mice secreted a truncated Ambn protein in their teeth, due to the skipping of exons 4 and 5. Therefore, the enamel deficiency could be due to an absence of functional ameloblastin, or to a potential dominant negative effect of the truncated ameloblastin protein. In collaboration with Dr. Simmer, transgenic mouse lines were generated that expressed full-length of Ambn under the control of the ameloblast-specific amelogenin promoter. All lines expressing detectable levels of ameloblastin recovered a more normal enamel phenotype. Thus, ameloblastin is essential for dental enamel formation.
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