Many stem cells (SCs) respond to Wnt signaling, but whether beta-catenin's DNA binding partners, the Tcfs, play a role in SCs in the absence of Wnts, is unknown. In adult skin, quiescent multipotent progenitors express Tcf3 and commit to a hair cell fate in response to Wnt signaling. We find that embryonic skin progenitors also express Tcf3. Using an inducible system in mice, we show that upon Tcf3 reactivation, committed epidermal cells induce genes associated with an undifferentiated, Wnt-inhibited state and Tcf3 promotes a transcriptional program shared by embryonic and postnatal SCs. Further, Tcf3-repressed genes include transcriptional regulators of the epidermal, sebaceous gland and hair follicle differentiation programs, and correspondingly, all three terminal differentiation pathways are suppressed when Tcf3 is induced postnatally. These data suggest that in the absence of Wnt signals, Tcf3 may function in skin SCs to maintain an undifferentiated state and, through Wnt signaling, directs these cells along the hair lineage.

Single-layered embryonic skin either stratifies to form epidermis or responds to Wnt signaling (stabilized β-catenin) to form hair follicles. Postnatally, stem cells continue to differentially use Wnt signaling in long-term tissue homeostasis. We have discovered that embryonic progenitor cells and postnatal hair follicle stem cells coexpress Tcf3 and Tcf4, which can act as transcriptional activators or repressors. Using loss-of-function studies and transcriptional analyses, we uncovered consequences to the absence of Tcf3 and Tcf4 in skin that only partially overlap with those caused by β-catenin deficiency. We established roles for Tcf3 and Tcf4 in long-term maintenance and wound repair of both epidermis and hair follicles, suggesting that Tcf proteins have both Wnt-dependent and Wnt-independent roles in lineage determination.Members of the Lef1 and Tcf family of DNA binding proteins bind β-catenin and transactivate Wnt target genes 1 . We previously discovered that Tcf3 is essential for gastrulation, where it differs from other Lef1 and Tcf binding proteins in acting not only in the presence but also in the apparent absence of Wnt signaling [2][3][4] . In embryonic skin, Tcf3 and Lef1 are expressed in basal progenitors 4,5 . As morphogenesis proceeds, Tcf3 becomes restricted to the early hair follicle 'bulge' region, a site where slow-cycling multipotent stem cells will reside, whereas Lef1 becomes confined to the base of the growing hair follicle, where committed, transitamplifying matrix cells and precortical cells differentiate to produce the hair 4,6 . In postnatal skin, Tcf3 remains a faithful marker of hair follicle stem cells: its expression is maintained both in the bulge and in the outer root sheath (ORS) cells that trail down from the bulge. The ORS cells are thought to represent activated stem cells in transit to the hair follicle bulb, where they fuel production of the matrix cells, hair shaft and surrounding channel [6][7][8] . We have shown that bulge stem cell activation is dependent on Wnt signaling and stabilized β-catenin, whereas Note: Supplementary information is available on the Nature Genetics website.Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions/. AUTHOR CONTRIBUTIONSH.N. designed and conducted experiments, analyzed data and wrote the paper. B.J.M. generated the Tcf3 cKO mice. M.R. conducted the microarray analysis. L.P. conducted the skin grafting. M.N. and T.M.S. provided technical assistance. H.A.P. conducted the histological analysis. E.F. designed experiments, analyzed data and wrote the paper. All authors read and contributed to the manuscript. during the resting stage of the hair cycle, Tcf3 persists in the apparent absence of nuclear β-catenin 2,5,9 . NIH Public AccessLef1 and Tcf DNA binding proteins have not been identified in resident stem cells of epidermis or sebaceous glands. However, expression of a dominant-negative Lef1 promotes sebaceous gland cell proliferation and differentiation 2,10 , and ectopic ind...