Expression of GDNF and its receptors in developing tooth is developmentally regulated and suggests multiple roles in innervation and organogenesis.

Glial cell line-derived neurotrophic factor (GDNF) is a recently identified survival factor for several populations of neurons in the central and peripheral nervous system that also regulates kidney development. To study the roles of GDNF in the regulation of tooth innervation and formation, we analyzed by in situ hybridization the expression patterns of GDNF and its receptors Ret, GDNF family receptor alpha-1 (GFRalpha-1), and GFRalpha-2 from the initiation of first molar formation to the completion of crown morphogenesis. At the time of trigeminal axon ingrowth, GDNF mRNAs were expressed in the mesenchyme around the tooth germ (i.e., target field of the dental innervation), suggesting that it is involved in the regulation of the embryonic tooth innervation. This hypothesis was supported by the ability of GDNF to induce neurite outgrowth from embryonic day 12 (E12) to E15 trigeminal ganglia. This timing correlated with the appearance of Ret in the subset of cells in the trigeminal ganglion at E12, whereas GFRalpha-1 and GFRalpha-2 receptors were constantly expressed in trigeminal ganglion during E11-E15. After birth, GDNF expression showed apparent correlation with the ingrowth and presence of trigeminal nerve fibers in the tooth, suggesting that GDNF is involved in the regulation of innervation of the dental papilla and dentin postnatally. Ret, GFRalpha-1, and GFRalpha-2 mRNAs were expressed in the dental epithelial and mesenchymal cells at stages when epithelial-mesenchymal signalling regulates critical steps of tooth morphogenesis. Ret and GFRalpha-2 were colocalized in the dental mesenchyme during bud and cap stages. Expression of GFRalpha-1 associated with the formation of the epithelial enamel knot, which is a putative embryonic signalling center regulating tooth shape. During postnatal development, GDNF and its receptors were expressed in dental papilla mesenchyme. In addition, GDNF and GFRalpha-1 transcripts were seen in the preodontoblasts and odontoblasts, suggesting that they may be involved in differentiation and maintenance of functional properties of the odontoblasts. Taken together, these results suggest that GDNF acts as a target-derived neurotrophic factor during tooth innervation. In addition, GDNF and its receptors may have nonneuronal organogenetic functions during tooth morphogenesis.