Lgr5-EGFP Marks Taste Bud Stem/Progenitor Cells in Posterior Tongue

Abstract

Until recently, reliable markers for adult stem cells have been lacking for many regenerative mammalian tissues. Lgr5 (leucine-rich repeat-containing G-protein coupled receptor 5) has been identified as a marker for adult stem cells in intestine, stomach, and hair follicle; Lgr5-expressing cells give rise to all types of cells in these tissues. Taste epithelium also regenerates constantly, yet the identity of adult taste stem cells remains elusive. In this study, we found that Lgr5 is strongly expressed in cells at the bottom of trench areas at the base of circumvallate and foliate taste papillae and weakly expressed in the basal area of taste buds and that Lgr5-expressing cells in posterior tongue are a subset of K14-positive epithelial cells. Lineage-tracing experiments using an inducible Cre knock-in allele in combination with Rosa26-LacZ and Rosa26-tdTomato reporter strains showed that Lgr5-expressing cells gave rise to taste cells, perigemmal cells, along with self-renewing cells at the bottom of trench areas at the base of circumvallate and foliate papillae. Moreover, using subtype-specific taste markers, we found that Lgr5-expressing cell progeny include all three major types of adult taste cells. Our results indicate that Lgr5 may mark adult taste stem or progenitor cells in the posterior portion of the tongue.

Single Lgr5- or Lgr6-expressing taste stem/progenitor cells generate taste bud cells ex vivo

“…Another possibility is that at least two types of organoids originated from Lgr5 + or Lgr6 + cells, with distinct regeneration potentials. As reported previously, there are strong Lgr5 + cells under the trench areas of circumvallate and weak Lgr5 + cells in the basal areas of taste buds (6). Therefore, Lgr5 + cells may be heterogeneous, and we propose that the strong Lgr5 + cells are stem cells and the weak Lgr5 + cells are more committed progenitor cells.…”

“…Although Lgr5 is present in fungiform papillae in anterior tongue during embryonic stages and early life, based on the intrinsic GFP signal from the Lgr5-EGFP transgene, Lgr5-EGFP signal could not be detected in fungiform papillae cells in adult mice (6,7). Therefore, taste stem/progenitor cells remain to be identified in fungiform papillae in anterior tongue.…”

“…Lgr5 (leucine-rich repeat-containing G protein-coupled receptor 5), encoded by a Wnt (wingless-type MMTV integration site family) target gene, marks adult stem/progenitor cells in taste tissue in posterior tongue that in vivo give rise to all major types of taste bud cells, as well as perigemmal cells (6,7). Lgr5 is also known to mark actively cycling stem cells in small intestine, colon, stomach, and hair follicle, as well as quiescent stem cells in liver, pancreas, and cochlea (8).…”

“…Lgr5 marks adult taste stem/progenitor cells in posterior tongue, which was shown using an engineered mouse model in which enhanced green fluorescent protein (EGFP) and tamoxifen-inducible Cre recombinase (CreERT2) are knocked-in to replace the coding sequence of Lgr5 and act as surrogate markers for Lgr5 (6,7). Although Lgr5 is present in fungiform papillae in anterior tongue during embryonic stages and early life, based on the intrinsic GFP signal from the Lgr5-EGFP transgene, Lgr5-EGFP signal could not be detected in fungiform papillae cells in adult mice (6,7).…”

“…Our RT-PCR results show that in addition to Lgr5, Lgr6 can be amplified from our cultured organoids. Lgr5 marks adult taste stem/progenitor cells only in posterior tongue (6,7). Prompted by this finding, as well as the result that Lgr6 is preferentially expressed in fungiform taste tissue but not in the surrounding epithelium devoid of taste tissue (12), we set out to determine whether Lgr6 marks adult taste stem/progenitor cells in anterior taste fields (fungiform papillae).…”