Expression of Six1 and Six4 in mouse taste buds.

Members of the Six gene family are expressed in various tissues including sensory organs, such as the inner ear and olfactory epithelium. We examined the expression of Six1 and Six4 mRNAs in mouse taste buds by using in situ hybridization. Six1 was detected immunohistochemically in the nuclei of taste bud cells, in a subset of type-II cells, as shown by double-immunolabeling with anti-Six1 together with anti-PLCβ2 or anti-IP(3)R3 antibodies. Six1-immunoreactive (IR) nuclei appeared at embryonic day 17.5 in the dorsal epithelium, and in the trench wall epithelium of circumvallate papillae at postnatal day 5. At this stage, Six1-IR nuclei were observed in all newly-formed type-II cells. During postnatal development, type-II cells increased in number, but those with Six1-IR nuclei showed no apparent increase. After transection of the bilateral glossopharyngeal nerve, type-II cells gradually disappeared; but some of them remained in the epithelium even at 11-17 days post-transection. The remaining type-II cells showed Six1-immunoreactivity. At 24 days after nerve transection, regenerating type-II cells appeared; and strong Six1-immunoreactivity was observed in them. Also, enhanced green fluorescent protein-immunoreactivity and β-galactosidase-immunoreactivity, which were indicators for Six1 transcripts and Six4 transcripts, respectively, overlapped. These results suggest that Six1 and Six4 genes are expressed in the taste bud cells, in newly formed or surviving type-II cells.