Distinctive spatiotemporal expression patterns for neurotrophins develop in gustatory papillae and lingual tissues in embryonic tongue organ cultures

Brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) mRNAs ar e expressed in the developing rat tongue and taste organs in specific spati otemporal patterns. BDNF mRNA is present in the early lingual gustatory pap illa epithelium, from which taste buds eventually arise, prior to the arriv al of gustatory nerve fibers at the epithelium, whereas NT-3 initially dist ributes in the mesenchyme. However, a direct test for neural dependence of neurotrophin expression on the presence of innervation in tongue has not be en made, nor is it known whether the patterns of neurotrophin expression ca n be replicated in an in vitro system. Therefore, we used a tongue organ cu lture model that supports taste papilla formation while eliminating the inf luence from sensory nerve fibers, to study neurotrophin mRNAs in lingual ti ssues. Rat tongue cultures were begun at embryonic day 13 or 14 (E13, E14), and BDNF NT-3, nerve growth factor (NGF) and neurotrophin-4 (NT-4) mRNAs w ere studied at 0, 2, 3 and 6 days in culture. BDNF transcripts were localiz ed in the gustatory epithelium of both developing fungiform and circumvalla te papillae after 2 or 3 days in culture, and NT-3 transcripts were in the subepithelial mesenchyme. The neurotrophin distributions were comparable to those in vivo at E13-E16. In 6-day tongue cultures, however, BDNF transcri pts in anterior tongue were not restricted to fungiform papillae but were m ore widespread in the lingual epithelium, while the circumvallate trench ep ithelium exhibited restricted BDNF labeling. The NT-3 expression pattern sh ifted in 6-day organ cultures in a manner comparable to that in the embryo in vivo, and was expressed in the lingual epithelium as well as mesenchyme. NGF mRNA expression was subepithelial throughout 6 days in cultures. NT-4 mRNA was not detected. The neurotrophin mRNA distributions demonstrate that temporospatial localization of neurotrophins observed during development i n vivo is retained in the embryonic tongue organ culture system. Furthermor e, initial neurotrophin expression in the developing lingual epithelium, me senchyme, and/or taste papillae is not dependent on intact sensory innervat ion. We suggest that patterns of lingual neurotrophin mRNA expression are c ontrolled by the influence of local tissue interactions within the tongue a t early developmental stages. However, the eventual loss of restricted BDNF mRNA localization from fungiform papillae in anterior tongue suggests that sensory innervation may be important for restricting the localized express ion of neurotrophins at later developmental stages, and for maintaining the unique phenotypes of gustatory papillae.