Lgr5+ ductal cells of von Ebner's glands: Candidate stem cells for turnover of posterior tongue taste buds.

Taste bud cells have a limited lifespan and must be continuously replaced along with the papilla epithelium in which they reside. Previous work has shown that expression of leucine-rich G protein-coupled receptor 5 (Lgr5), a Wnt pathway agonist, serves as a marker of adult stem/progenitor cells for taste buds located in posterior tongue (circumvallate and foliate), but not anterior tongue (fungiform), taste papillae. However, the specific location/niche of the Lgr5-expressing cells supporting renewal and their phenotypic properties have not been fully explored. To address this, the genesis and fate of Lgr5+ cells were examined in developing and adult mice using genetic reporter strains. Evidence from Lgr5-lacZ and Lgr5-GFP mice shows that, while Lgr5 is broadly expressed in the epithelium of nascent circumvallate papillae and their trenches during embryonic development, it becomes concentrated within the ducts of adjacent von Ebner's salivary glands during the first postnatal week, co-incident with the appearance of differentiated taste buds. In posterior tongue taste papillae of adult animals, sites of highest Lgr5-lacZ and Lgr5-GFP expression are found in excretory ducts, restricted to the outer (basal) layer of the bi-layered excretory zone. These Lgr5+ cells are immunoreactive for keratin 14, like cells in the basal layer of extragemmal taste epithelium, and are often seen to express Sox9, a marker of exocrine gland duct cells. Lineage tracing experiments with an Lgr5-EGFP-IRES-CreERT2; mTmG reporter show that Lgr5+ ductal cells become labeled one day following Cre induction, prior to the appearance of descendent cells in taste buds. Overall, the data support a role for Lgr5+ ductal cells as stem cells and suggest that a cooperative interaction exists between posterior taste epithelium and its associated salivary glands in taste cell turnover.