Oral and Craniofacial Development and Disease

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Ectodermal organ development is initiated by inductive tissue interactions. Developing teeth, epidermis, hair, and limbs are classic examples of these types of inductive processes. Tooth development can be divided into the initiation, bud, cap, and bell stages. In mice, tooth development begins at embryonic day (E) 11.5 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 (E14.5), 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 (E17.5), 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 anomalies of these tissues. Tooth enamel is the hardest mineralized tissue in the body, and is located on the surface of the crown that protects the tooth from damage when chewing food and insulates it from temperature and chemicals. Unlike regenerative bones and dentin, self-repair of the damaged enamel is impossible once enamel is broken. This is because the live ameloblast, a cell type responsible for forming enamel by secreting enamel matrix, is lost forever when teeth erupt. In collaboration with Dr. Satoshi Fukumoto, we succeeded for the first time in differentiating iPS cells to enamel matrix-secreting ameloblasts by co-culturing them with ameloblasts as feeder cells. These findings suggest that the local environment is essential for iPS differentiation to ameloblasts and may provide a novel approach to tooth bioengineering. We previously identified epiprofin (Epfn) as a member of the Sp zinc-finger transcription factor family that is expressed in certain developing ectodermal tissues such as teeth, hair follicles, skin, and limbs. In Epfn knockout mice (Epfn-/-), development of teeth is delayed. However, at later stages, mutant incisors and molars erupt in excess (hyperdontia) and show enamel deficiency and abnormal dentin structure. However, the mechanism of the defects remains unknown. Tight and adherens junctions are indispensable for ameloblast and odontoblast cell differentiation, since establishment of a firm junctional complex is fundamental for these cells to polarize, become functional, and ultimately secrete enamel and dentin tissue, respectively. We studied the role of Epiprofin / Sp6 in the formation of tight and adherens junction complexes during molar and incisor development. In bell-stage developing incisor and molar teeth of Epiprofin-null mice, we discovered a clear decrease in tight junction and adherens junction proteins, with loss of these junctions correlating with failure of ameloblast differentiation. Conversely, overexpression of Epiprofin in dental pulp MDPC-23 cells results in activation of Wnt-β-catenin signalling, resulting in an increased cellular accumulation and nuclear redistribution of β-catenin protein but not cadherin protein. These results suggest that Epiprofin increases Wnt-β-catenin signaling in mesenchymal cells, which subsequently affects cell adhesion and ameloblast and odontoblast differentiation.