MIDKINE (MK), A HEPARIN-BINDING GROWTH-DIFFERENTIATION FACTOR, IS REGULATED BY RETINOIC ACID AND EPITHELIAL-MESENCHYMAL INTERACTIONS IN THEDEVELOPING MOUSE TOOTH, AND AFFECTS CELL-PROLIFERATION AND MORPHOGENESIS

Midkine (MK) is the first cloned gene in a new family of heparin-bindi ng growth/differentiation factors involved in the regulation of growth and differentiation. We have analyzed the expression of MK mRNA and p rotein during tooth development in mouse embryos and studied the regul ation of MK expression and the biological effects of MK protein in org an cultures. MK expression was restricted and preferential in the toot h area as compared to the rest of the developing maxillary and mandibu lar processes suggesting specific functions for MK during tooth morpho genesis. MK mRNA and protein were expressed during all stages of tooth formation (initiation, morphogenesis, and cell differentiation), and shifts of expression were observed between the epithelial and mesenchy mal tissue components. However, the expression of mRNA and protein sho wed marked differences at some stages suggesting paracrine functions f or MK. Tissue recombination experiments showed that MK gene and protei n expression are regulated by epithelial-mesenchymal interactions, and , moreover, that dental tissue induces the ectopic expression of MK pr otein in non-dental tissue. The expression of MK gene and protein in t he mandibular arch mesenchyme from the tooth region were stimulated by local application of retinoic acid in beads. Cell proliferation was i nhibited in dental mesenchyme around the beads releasing MK, but this effect was modulated by simultaneous application of FGF-2. Morphogenes is and cell differentiation were inhibited in tooth germs cultured in the presence of neutralizing antibodies for MK, whereas the developmen t of other organs (e.g., salivary gland, kidney) was unaffected. These results suggest important roles for MK in the molecular cascade that regulates tooth development.