Identification of differentially expressed cDNA transcripts from a rat odontoblast cell line

Odontoblasts and osteoblasts are two among the myriads of cell types present in the craniofacial complex. Both have a common ectomesenchymal origin and secrete macromolecules that are necessary for the formation of dentin and alveolar bone via matrix-mediated mechanisms. The mineralized matrices of bone and dentin differ in morphology and function but several mineral associated proteins, formerly thought to be tissue specific, have been found to be common in both tissues. To decipher the complex molecular mechanisms involved in mineralized dentin formation, the suppressive subtraction hybridization (SSH) approach has been used to identify the genes expressed by polarized odontoblasts. Employing SSH, 187 cDNA clones were identified from the subtracted cDNA library. Many of these genes have not been previously reported to be expressed by terminally differentiated odontoblasts. Genes were classified into seven groups based on the predicted function of the encoded proteins: extracellular matrix; cytoskeletal components, molecules involved in adhesion and cell-cell interaction; metabolic enzymes, transporters, ion channels; protein processing, protein transport and protein folding molecules; nuclear proteins (transcription factors, DNA processing enzymes); signaling molecules and genes of yet unknown function. Northern blot and in situ hybridization analysis performed for five putative novel genes and one new isoform of amelogenin revealed differential expression levels in the osteoblasts, ameloblasts and the odontoblasts of the developing rat molars. Some of the known genes isolated from this enriched pool were the cleavage products of dentin sialophosphoprotein (DSPP) namely, phosphophoryn (PP) and dentin sialoprotein (DSP). Interestingly amelogenin, ameloblastin and enamelin were also expressed in the odontoblasts during dentin formation.