In this study, we have identified expression of the stem cell factor Sox2 in several adult epithelial tissues where it has not previously been characterized, including the testes, fore- and glandular stomach, anus, cervix, esophagus, lens as well as glands associated with oral cavity, trachea and cervix. Importantly, we provide unequivocal fate mapping evidence that Sox2+ cells contain long-term stem cells in some of these tissues. We further assign a critical requirement for Sox2+ cells in tissue homeostasis by showing that their ablation causes a fatal disruption of epithelial integrity.
To our knowledge, Sox2 is the only transcriptional regulator that is commonly expressed between ESCs, fetal progenitors and a number of adult stem cells and may therefore point towards molecular similarities in the regulation of pluripotent and different adult stem cells. Our developmental lineage tracing data show that Sox2+ adult stem cells originate from early Sox2+ epithelial progenitors in fetal development. These results suggest that Sox2 expression plays key roles at multiple stages of pre- and postnatal development. While it is initially required for the establishment of pluripotent founder cells within the embryo, it subsequently controls the formation of fetal ectodermal and endodermal primordia as well as fetal germ cells and eventually becomes confined to some derivative adult tissues where it marks unipotent and multipotent stem cells.
Recent elegant fate mapping studies have found that Lgr5 expression marks stem cells in several epithelial tissues (Barker et al., 2007
; Jaks et al., 2008
) including the pyloric stomach (Barker et al.), raising the question of whether the same or different stem cells are labeled compared with Sox2+
cells. Based on our co-expression results, we conclude that Sox2 expression labels a different and more rare population of cells in the glandular stomach than Lgr5 expression. Another pronounced difference between Lgr5+
stem cells is the finding that Lgr5+
cells are seen in the intestine and pyloric stomach whereas Sox2+
cells are found in both the pylorus and corpus compartment of the adult glandular stomach but not in the intestine. A previous study has identified a rare population of Villin-expressing cells in antral glands that may coincide with the Sox2-expressing cells described here. In contrast to our study and the report by Barker et al., however, lineage tracing experiments showed that these Villin+
cells only give rise to entirely labeled gastric units in response to interferon treatment. Future studies will be needed to conclusively establish whether Villin+
cells are different types of stem cells that independently replenish the stomach or whether these cells are hierarchically related with each other. The notion of distinct stomach stem cells is reminiscent of recently identified stem cell markers in the small intestine that seem to label different cell populations (Barker et al., 2007
; Montgomery et al.; Sangiorgi and Capecchi, 2008
In the testis, we detected rare individual Sox2+
cells that may represent As
spermatogonia, which are thought to be the most primitive cells in testes (Yamada et al., 2006
). This interpretation is supported by our findings that (i) Sox2-GFP+
testis cells successfully restore spermatogenesis upon transplantation into infertile mice, (ii) lineage tracing analysis permanently labels clones containing immature spermatogonia and mature sperm, (iii) ablation of Sox2+
cells results in loss of spermatogenesis, and (iv) derivation of spermatogonial stem cell lines selects for Sox2-expressing cells. The observation that Sox2 is also expressed in c-kit+
testis cells that do not reconstitute spermatogenesis and in some transit-amplifying cells of squamous epithelia including forestomach, esophagus, tongue, anus and cervix suggests that in these tissues Sox2 is not exclusively expressed in stem cells. It is noteworthy that Sox2 expression was seen in basal cells of squamous epithelia lining inner organs (anus, cervix, esophagus, forestomach, tongue) but was absent from the interfollicular epidermis of the skin, which is organized in a similar fashion. This may either reflect functional differences of these epithelia or their distinct developmental origin despite a remarkably similar structure.
Lastly, we have identified Sox2+ stem cells in the lens epithelium that gives rise to secondary lens fibers throughout adult life, as well as in ganglia, salivary glands and glands associated with trachea and cervix. Additional experiments are required to unequivocally test whether Sox2+ cells in those tissues also harbor stem cell activity.
In summary, our results establish Sox2 as a widespread marker of pluripotent and many adult stem/progenitor cell types, and suggest that common target genes and pathways may be activated that are crucial for maintaining their self-renewal and differentiation potentials. Given the recent recognition that Sox2 can act as an oncogene in squamous cell carcinoma of the lung and esophagus (Bass et al., 2009
), it should further be interesting to investigate whether the Sox2-expressing cells, identified here, can act as tumor initiating cells and whether Sox2 expression itself may be oncogenic in these tissues.