A Taste of Stem Cells

Continuous supply of mature differentiated cells by adult stem cells is required in most of adult tissues especially those with rapid turnover rates. In recent years, using advanced cell biological methods, many studies have uncovered homeostatic mechanisms that are driven by specific tissue resident stem cells. Mammalian lingual epithelium (tongue) always had been a focus for identifying diverse taste receptors, cells and their mechanism of action. However, stem cells for this high turnover tissue remained largely uncharacterized. In a recent study published in Nature Cell Biology, Tanaka et al., show how homeostasis and regeneration of lingual epithelium are maintained by distinct stem cell population.

Mammalian tongue is composed of taste buds and keratinized epithelial cells, the later providing rigidity for the organ. The authors of this study attempted to identify stem cells that generate these differentiated epithelial cells. They combine the knowledge of classical thymidine labeling studies and state-of-the-art multicolour cell lineage techniques. It has been proposed previously that the keratin 5/14 positive lingual epithelial cell population possesses characteristics of stem cells. Authors of this work proved that these cells were not a distinct population of cells that gives rise to differentiated cells, however the stem cells they have identified do express these markers. To identify the individual population origin of differentiated cells, they labeled cells with CreERT2-inducible multicolour fluorescent reporters (green, blue, orange, red). They identified few label retaining cells after 28 days at the interpapillary pit (IPP) of the lingual epithelium. To identify a specific marker within this small population of cells, they have crossed various stem cell marker-CreERT2 knock-in mice with the multicolour expressing rainbow mice and confirmed Bmi1 (Bmi1 polycomb ring finger oncogene) as a lingual stem cell marker! Previously, Bmi1 has been reported as intestinal stem cell marker. There is no obvious explanation on how they chose Bmi1 as a likely candidate (cherry pick?!). However, further experiments with RNA in-situ hybridization proved the highest expression of this gene in lingual epithelial stem cells. Taste bud cells were not labeled long-term in Bmi1^CreER/+/Rosa26^rbw/+ mice confirming that the Bmi1-positive cells identified are unipotent stem cells giving rise to keratinized epithelial cells.

Furthermore, to examine the regenerative capacity of Bmi1-positive stem cells, the authors injured the lingual epithelial cells with different doses of irradiation. Progeny of Bmi1-positive cells were detected on the surface of keratinized epithelial cells at day 7 after irradiation and regenerated the injured tissue. To analyze the regenerative ability in the absence of Bmi1-positive cells, they have effectively deleted Bmi1-positive cells by crossing Bmi1^CreER/+ mice with Rosa26^{loxp–stop–loxp–dta/+} mice. In this system, Tamoxifen induction of CreERT2 trigger the expression of Diphtheria toxin (DTA) which kills the cells. Removal of Bmi1-positive cells leads to decreased proliferation of basal cells, proving the hypothesis that the Bmi1-positive cells play a role in tissue regeneration.

The authors conclude that the Bmi1-positive cells are slow cycling long-term stem cells, giving a hint to hunt for rapid proliferating stem cell population in lingual epithelium, which is not identified yet. Also, further investigations on the role of Bmi1-positive cells in lingual origin of squamous cell carcinoma would allow targeting this gene for therapeutic interventions.

 
Tanaka T, et al., Identification of stem cells that maintain and regenerate lingual keratinized epithelial cells. Nature Cell Biology. 2013 May;15(5):511-8.

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