Identification and Characterization of a New ECM Protein, TM14 (Fibulin-7)

Objective: Identify and characterize novel genes important for tooth and cartilage development and define the molecular basis of anomalies.

Methods: cDNA microarrays, in situ hybridization, immunohistochemistry, protein expression, cell adhesion.

Results: We previously identified a gene encoding a novel ECM protein, TM14, which is preferentially expressed in teeth. TM14 is a new member of the fibulin family, fibulin-7. In situ hybridization revealed that TM14 mRNA was expressed by preodontoblasts and odontoblasts in developing teeth, hair follicles, and placenta. Immunostaining in teeth revealed that TM14 is localized at the apical pericellular regions of preodontoblasts, in the predentin matrix, and along the dentinal tubules. We showed that TM14 interacted with heparin, fibronectin, fibulin-1, and Dspp and preferentially bound dental mesenchyme cells and odontoblasts but not dental epithelial cells or nondental cells. Inhibition assays suggested that both a heparan sulfate-containing cell surface receptor and an integrin are involved in TM14 cell binding. In situ hybridization and immunostaining analyses in the growth plate showed that TM14 was also expressed in articular and proliferative chondrocytes. TM14 protein was expressed from mouse embryonic day (E) 13 to the adult stage in limbs and vertebrae. In E13, TM14 was expressed in mesenchyme condensation, and in later stages it was expressed around proliferative chondrocytes of the growth plate and vertebrae. TM14 was also expressed in the articular surface matrix. The expression pattern showed coexpression of TM14 and collagen II during cartilage development. TM14 was strongly expressed in RCS cells and induced during differentiation of chondrogenic ATDC5 cells.

Conclusion: TM14 is a cell adhesion molecule that interacts with ECM molecules in teeth and plays important roles in both the differentiation and maintenance of odontoblasts as well as in dentin formation. TM14 is also expressed in the proliferative and articular chondrocytes where it may be involved in chondrocyte differentiation.