Tcf3 and Tcf4 are essential for long-term homeostasis of skin epithelia

Abstract

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...

Wnt some lose some: transcriptional governance of stem cells by Wnt/β-catenin signaling

“…3A; Pereira et al 2006;Tam et al 2008), and when Wnt signaling is activated or TCF3 is absent, core pluripotency factors are up-regulated (Cole et al 2008;Yi et al 2011). When combined with the recent findings of Chodaparambil et al (2014) and the findings delineated for the hair follicle above (Merrill et al 2001;Nguyen et al 2006Nguyen et al , 2009Greco et al 2009;Hsu et al 2014;Lien et al 2014), these findings suggest that binding of b-catenin to TCF3 alters the TCF3-bound repressive chromatin state, in turn activating target genes (Fig. 3B).…”

“…Another possibility is that growth inhibitory signals from neighboring wild-type basal epidermal progenitors provide negative cues to adjacent Ctnnb1-null progenitors, which might restrict their otherwise hyperproliferative growth. Alternatively, there could be fundamental differences between nonhairy and hairy skin (Huelsken et al 2001;Nguyen et al 2009;Choi et al 2013;Lim et al 2013;Lien et al 2014). When taken together with additional ambiguities surrounding the extent to which Axin2 and b-catenin levels are pure readouts for Wnt signaling as well as the caveats discussed above regarding adult mouse epidermis as a model for stem cell biology, it will not be a simple task to sift through the plethora of possible mechanisms underlying Wnt/b-catenin signaling in the epidermis.…”

“…The preferential binding of these corepressors, TLE and CtBP, to TCF3 and TCF4 is interesting in light of the long-standing observation that in the hair follicle, TCF3 and TCF4 are present in quiescent stem cells, where Wnt reporter activity is silent. In contrast, LEF1 is present under conditions where Wnt reporter activity is active-initially in hair germ (HG) progenitors upon their activation and then at greatly amplified levels in the transit-amplifying matrix cells as they become fated to differentiate and make the hair shaft (DasGupta and Fuchs 1999;Merrill et al 2001;Nguyen et al 2006Nguyen et al , 2009Lien et al 2014). Indeed, TCF3 and TCF4 often appear to function to lower transcriptional levels of their targets, while TCF1 and LEF1 are more typically viewed as transcriptional enhancers of Wnt signaling (Merrill et al 2004;Yi et al 2011;Wu et al 2012).…”

Wnt signaling through T-cell factor phosphorylation

“…LEF1−/− mouse embryos lack teeth, mammary glands, and hair and are deficient in neural crest development [63], whereas double knockouts of TCF genes display more severe phenotypes [64][65][66]. Similarly, in Xenopus and zebrafish embryos, TCF proteins play diverse roles in dorsoventral patterning, CNS, neural crest and muscle development [67][68][69][70][71][72].…”

Development and Homeostasis of the Skin Epidermis

“…Tcf3 and Tcf4 are also expressed in early undifferentiated embryonic epidermal progenitors and in adult HF SCs and their progeny, and seem to act at least partially as transcriptional repressors in a Wnt-dependent and -independent manner. Their simultaneous deletion leads to rapid HF disappearance (Nguyen et al 2009). b-Catenin signaling acts upstream of the Eda-A1/Edar/NF-kB signaling pathway, which controls HF morphogenesis during the early stage of HF specification, but EDA/EDAR/NF-kB activity helps to sustain Wnt activity at latter stages (Zhang et al 2009a).…”