Tooth morphogenesis results from reciprocal interactions between oral epithelium and ectomesenchyme, culminating in the formation of mineralized tissues, enamel, and dentin. The earliest morphogenetic event of mouse tooth development occurs when the oral ectoderm invaginates into the underlying neural crest-derived mesenchyme. Continuation of this invagination results in the formation of epithelial tooth buds. Mesenchymal cells surrounding the bud form the dental papillae, which later develop into dentin-secreting odontoblasts and the tooth pulp. After the bud stage, the tooth germ progresses to the cap and bell stages, and the epithelium differentiates into enamel-secreting ameloblasts. In this oral and craniofacial project, our goal is to discover and characterize previously unknown genes to help understand how tooth and craniofacial tissues develop and to define the molecular defects underlying anomalies of these tissues or oral cancer. ? ? Using differential hybridization to tooth germ cDNA microchips, we have previously identified epiprofin, which is highly expressed in teeth. Epiprofin, a homologue of Sp6, a member of the Sp and Kruppel-Like-Factor (KLF) family containing three characteristic C2H2-type zinc-finger motifs. Epiprofin mRNA is expressed in certain ectodermal organs such as developing tooth, hair follicle and limb. In cell culture, transfection of the epiprofin expression vector into dental epithelium promotes differentiation into ameloblast phenotypes. To determine the role of epiprofin in ectodermal organ development, we created gene knockout mice for epiprofin and found that mutant mice develop an excess number of teeth (hyperdontia), enamel hypoplasia, and severe defects in hair formation. Our results suggest that epiprofin is an essential factor for the formation of teeth at the proper number and for dental epithelial cell differentiation. ? ? We have also identified a new extracellular protein, TM14, from a mouse tooth germ cDNA library. TM14 contains 3 EGF modules at the center, a C-terminal domain homologous to the fibulin module, and a unique Sushi domain at the N-terminus. TM14 mRNA was expressed by preodontoblasts and odontoblasts in developing teeth. Immunostaining revealed that TM14 was localized at the apical pericellular regions of preodontoblasts. When the dentin matrix was fully formed and dentin mineralization occurred, TM14 was present in the predentin matrix and along the dentinal tubules. We found that the recombinant TM14 protein interacted with heparin, fibronectin, fibulin-1, and dentin sialophosphoprotein. We also found that TM14 preferentially bound dental mesenchyme cells and odontoblasts but not dental epithelial cells or nondental cells such as HeLa, Cos7, or NIH3T3 cells. Heparin, EDTA, and anti-integrin beta1 antibody inhibited TM14 binding to dental mesenchyme cells, suggesting that both a heparan sulfate-containing cell surface receptor and an integrin are involved in TM14 cell binding. Our findings suggest that TM14 plays important roles in both the differentiation and maintenance of odontoblasts as well as in dentin formation. Because of its protein characteristics, TM14 can be classified as fibulin-7, a new member of the fibulin family.
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