THE ACTIVIN-BINDING PROTEIN FOLLISTATIN IS EXPRESSED IN DEVELOPING MURINE MOLAR AND INDUCES ODONTOBLAST-LIKE CELL-DIFFERENTIATION IN-VITRO

It has recently been shown that mice deficient in activin-beta A subun its and follistatin exhibit major defects in dentition. To increase un derstanding of the roles played by these molecules during tooth develo pment, we determined the temporospatial expression of activin-beta A s ubunit and follistatin messenger RNA and their corresponding proteins in developing murine molars (between day E 14 and 2 days after birth). The effects of recombinant human activin A and its binding protein fo llistatin on odontoblast differentiation were also studied in cultures of dental papillae (DP) isolated from the mandibular first molars of E-17-day mice. In situ hybridization indicated that transcripts for ac tivin-beta A subunit were abundant in pre-odontoblasts at the tips of forming cusps prior to odontoblast terminal differentiation, and trans cripts for follistatin in overlying inner enamel epithelial cells (pre -ameloblasts). Pre-odontoblasts were also weakly immunoreactive in rel ation to activin-beta A subunit, pre-ameloblasts in relation to follis tatin. When follistatin was added at different concentrations to a DP culture model (2-14 nmol/DP) together with heparin at constant concent ration, differentiation of odontoblast-like cells was induced, as evid enced by polarization and deposition of extracellular matrix in vitro, to extents depending on the follistatin concentration. In contrast, t he addition of activin A (2 nmol/DP) had no effect on the differentiat ion parameters studied. These findings suggest that the activin-follis tatin system regulates odontoblast differentiation during tooth develo pment. In particular, we suggest that binding of endogenous activin A by follistatin may allow odontoblast terminal differentiation to occur .