Expression of NeuroD in the mouse taste buds.

NeuroD, a basic helix-loop-helix transcription factor, has been shown to play a role in the differentiation of neurons, olfactory cells, and neuroendocrine tissues. Since the taste buds have characteristics of typical paraneurons, we examined the expression of NeuroD in the taste buds of mice. By RT-PCR analysis, NeuroD mRNA was found to be expressed in the epithelium of circumvallate papillae-containing taste buds, but not in the lingual epithelium lacking them. NeuroD immunoreactivity was detected in a subset of taste bud cells in the circumvallate, foliate, and fungiform papillae and in the soft palate. NeuroD-expressing cells had a spindle-like shape, first appeared at postnatal day 3, and increased in number during postnatal development. After bisection of the glossopharyngeal nerves, NeuroD-expressing cells decreased in number at day 4 and disappeared from the trench wall of the circumvallate papillae by day 14. A few NeuroD-expressing taste buds reappeared at postoperative day 28. Denervation and regeneration experiments showed that expression of NeuroD in the taste buds was dependent upon gustatory innervation. Double immunolabeling with gustducin or with neural cell adhesion molecule (NCAM) showed that NeuroD-expressing cells did not express NCAM, but did express gustducin. These results suggest that NeuroD is expressed in a mature cell type, type-II cells, but not in type-III cells.