Elucidating the role of Polycomb Repressive Complexes in Lingual Papillae Development

Summary The tongue is a complex organ that is essential for mastication, taste sensation, and communication. Proper development of the tongue is dependent on complex interactions between different tissues, including muscle, connective tissue, and epithelium. Importantly, failure to establish these interactions leads to developmental defects, such as aglossia, microglossia, and ankyloglossia. The lingual epithelium is an array of epithelial-mesenchymal appendages termed papillae. Fungiform papillae harbor taste buds required for taste sensation. They are surrounded by keratinized, non-gustatory filiform papillae, which provide essential barrier functions. Wnt and Shh signaling pathways are central players in regulating the patterning and development of fungiform and filiform papillae; however, the role of cell-intrinsic mechanisms in the specification and maintenance of these structures remains unclear. Polycomb repressive complex (PRC) 1 is a major chromatin transcriptional regulator and has been shown to play a critical role in lineage identity and early embryonic development; however, PRC1-mediated regulation of cell lineages has yet to be explored in oral tissues. To investigate the role of PRC1 in the development of lingual papillae, we generated mice in which essential PRC1 subunits, Ring1a and Ring1b, were conditionally ablated in the basal lingual epithelial cells (Ring1a/b 2KO). Our results showed that in Ring1a/b 2KO mice, the fungiform papillae are not maintained and the filiform papillae are not formed. Further analysis of the Ring1a/b-null lingual epithelium revealed widespread expression of the Sonic Hedgehog (Shh) ligand and the transcription factor Sox2, whereas in control mice, the expression of these genes was confined to taste progenitors. The expression pattern of components of the Wnt signaling pathway, Wnt10b and Lef1, was also altered in Ring1a/b 2KO; however, this did not result in aberrant canonical Wnt signaling. We hypothesize that PRC1 restricts gene expression and thus promotes lingual papilla patterning and formation. To dissect the molecular mechanisms of PRC1's control of the lingual epithelium, we will carry out an RNA-seq analysis of FACS-purified control and Ring1a/b- null lingual epithelial cells. This analysis will be coupled with ChIP-seq with antibodies against PRC1 subunits to identify the direct targets of PRC1. To identify the specific molecular pathways disrupted in the Ring1a/b-null lingual epithelium, we will carry out Gene Ontology analysis and focus on candidate genes that regulate epithelial-mesenchymal interactions, transcriptional regulation, and lineage identity. We will analyze the functional significance of the candidate genes by performing genetic experiments in vivo. Together, our studies will allow us to elucidate PRC1's regulation of the lingual epithelium, including its interaction with developmental signaling pathways. Understanding the complex molecular interactions between lingual tissues may also help us to reveal the mechanisms that cause congenital tongue abnormalities to form.