The Roles of WT1 and BASP1 in the Development and Maintenance of the Posterior Taste Fiel

Taste is one of the fundamental senses that organisms have evolved, which is critical for survival. Taste receptor cells have the function to detect chemicals in the oral cavity and transmit the chemical information to the brain. Those taste receptor cells are housed in taste buds. Within each taste bud, there are type I, type II, type III and basal cells, which have different morphological structures and different cellular functions. In mammals, taste buds are located in 3 distinct sets of specialized taste papillae within the oral cavity, including circumvallate papillae, foliate papillae and fungiform papillae. As those cells are in contact with the external environment and easy to damage, they keep turning over throughout an organism's life. However, either aging or disease can cause loss of taste. Despite the importance of taste to our life, currently very little is known about the development and maintenance of the taste system.;Some factors have been identified to be important in regulating taste development, including sonic hedgehog, bone morphogenetic protein 4 and multiple members of the Wnt/beta-catenin signaling pathway. However, the understanding about the regulation of these factors is lacking. Here I focused on studying the role for the Wilms' tumor 1 protein (WT1), which is important for the development of other sensory tissues, and in the development of circumvallate papillae. I found WT1 is expressed in developing circumvallate papillae (CV) since the placode formation, and its expression is gradually confined to the taste epithelium by birth. The CV of mice lacking WT1 fails to develop normally and early taste development markers are dysregulated. ChIP assay results show that WT1 directly binds to the promoter region of Lef1, Ptch1 and Bmp4. The expression levels of WT1 target genes are significantly reduced in WT1 KO tongue. WT1's transcription function on Lef1 and Ptch1 is confirmed by primary cultured taste cells. Our results demonstrate that WT1 is a critical transcription factor in the development of the CV by regulating multiple factors that have known roles in taste placode formation (Chapter 2).;Since multiple studies have shown that WT1's transcriptional function is regulated by its corepressor BASP1, I hypothesized that in the taste system, WT1 is also regulated by BASP1. I found BASP1 is exclusively expressed in the gustatory nerve during embryonic development. However, BASP1 is highly expressed in taste cells starting around birth and this expression pattern is maintained until adulthood. This suggests a potential role of BASP1 in the renewal/maintenance of taste buds. BASP1 is co-localized with WT1 in lots of taste cells and occupies the promoter of WT1 targets Lef1 and Ptch1. Conditional deletion of BASP1 in Krt8-positive taste cells causes elevated levels of Lef1 and Ptch . Immunohistochemistry experiments with different taste cell markers reveal that BASP1 KO taste buds gain more type I taste cell features and lose type II and III taste cell features. This is consistent with previous findings that different expression levels of Wnt signaling and Shh signaling bias cell fate. Physiological studies using calcium imaging show that significantly less taste cells from BASP1 KO mice show detectible responses to taste solutions compared to wild type. The amplitudes of the remaining responses of taste cells from BASP1 KO mice were significantly smaller than wild type. Behavior study show that BASP1 KO mice have less sensitivity to different taste solutions. These data demonstrate that BASP1 regulates WT1 targets in the adult taste system and plays an important role in the maintenance of the adult CV.;My data have identified a new role for WT1 and WT1-BASP1 complex in the development and maintenance of the taste system. These findings provide new insights into the current understanding of the molecular mechanisms of the taste development and maintenance.