Tcf3 and Lef1 regulate lineage differentiation of multipotent stem cells in skin

Abstract

In skin, multipotent stem cells generate the keratinocytes of the epidermis, sebaceous gland, and hair follicles. In this paper, we show that Tcf3 and Lef1 control these differentiation lineages. In contrast to Lef1, which requires Wnt signaling and stabilized ␤-catenin to express the hair-specific keratin genes and control hair differentiation, Tcf3 can act independently of its ␤-catenin interacting domain to suppress features of epidermal terminal differentiation, in which Tcf3 is normally shut off, and promote features of the follicle outer root sheath (ORS) and multipotent stem cells (bulge), the compartments which naturally express Tcf3. These aspects of Tcf3's action are dependent on its DNA binding and Groucho repressor-binding domains. In the absence of its ␤-catenin interacting domain, Lef1's behavior (⌬NLef1) seems to be markedly distinct from that of ⌬NTcf3. ⌬NLef1 does not suppress epidermal differentiation and promote ORS/bulge differentiation, but rather suppresses hair differentiation and gives rise to sebocyte differentiation. Taken together, these findings provide powerful evidence that the status of Tcf3/Lef complexes has a key role in controlling cell fate lineages in multipotent skin stem cells. The canonical Wnt/Wingless signaling pathway directs cell fate in a variety of cell types and has an integral role in organogenesis. At the core of the pathway is the stability of ␤-catenin, a protein that serves a dual role in intercellular junction formation and in transcriptional regulation. Wnt signaling inhibits the amino-terminal phosphorylation and subsequent degradation of ␤-catenin, allowing cytoplasmic pools of ␤-catenin to enter the nucleus and bind to members of the Lef1/Tcf family of HMG domain-containing DNA-binding proteins. ␤-catenin's activation domain and Lef1/Tcf's DNA-binding domain combine to form a bipartite transcriptional activator of target genes (for review, see Willert and Nusse 1998;Korswagen and Clevers 1999;Waltzer and Bienz 1999).Although Lef1/Tcf proteins often function as transactivators, these proteins can also interact with, and behave as, transcriptional repressors. CtBP represses a Wnt/Wingless response and selectively interacts with the carboxy-terminal domain of Tcf proteins, missing in Lef1 (Brannon et al. 1999). In contrast, the Groucho family of repressor proteins associate with the amino-terminal domain of all known Tcf/Lef family members (Cavallo et al. 1998;Roose et al. 1998;Brantjes et al. 2001). Genetically, a Drosophila dTcf-Groucho interaction antagonizes Wnt/Wingless signaling (Cavallo et al. 1998), and expression of Groucho proteins in Xenopus embryos can block axis formation and activation of ␤-catenin target genes, such as siamois and xnr3 ). In zebrafish, embryonic Tcf3 appears to function as a transcriptional repressor, as an amino-terminally truncated Tcf3 (missing its ␤-catenin binding domain) complements the headless phenotype caused by loss-of-function of wild-type Tcf3 (Kim et al. 2000). Therefore, in the absence of a Wnt/Wingless signal, the...

Identification and characterization of Grainyhead‐like epithelial transactivator (GET‐1), a novel mammalian Grainyhead‐like factor

“…In large part, this regulation is achieved at the level of gene transcription, and in recent years, several signaling pathways and transcription factors important for epithelial morphogenesis have been described. These include pathways involving Wnt ligands, ␤-catenin and Lef/Tcf transcription factors (Zhou et al, 1995;Kratochwil et al, 1996;Gat et al, 1998;DasGupta and Fuchs, 1999;Huelsken et al, 2001;Merrill et al, 2001), as well as Shh ligands, and patched and Gli transcription factors (Bitgood and McMahon, 1995;Oro et al, 1997;StJacques et al, 1998;Chiang et al, 1999;Wang et al, 2000). Of interest, the developmental roles of these pathways are conserved from Drosophila to mammals, suggesting the possibility that the function of other regulators involved in developmental control of epidermis in Drosophila may be conserved in mammals (Oro and Scott, 1998;Fuchs et al, 2001).…”

Transdetermination: Drosophila imaginal disc cells exhibit stem cell-like potency

“…Mouse lung progenitor cells, for example, TD to intestinal cell types by elevated and/or prolonged Wnt signaling (Okubo & Hogan, 2004). Lineage switches were also observed when Wnt signaling is altered in hair follicle and epidermal cells (Merrill, Gat, DasGupta, & Fuchs, 2001;Niemann, 2006) and in the mammary gland and prostate (Bierie et al, 2003;Miyoshi et al, 2002).…”

Transgenic Wnt/TCF pathway reporters: all you need is Lef?

“…This 'default repression' (Barolo and Posakony, 2002) has been demonstrated to be critical for the proper regulation of many Wnt target genes. In some cases, default repression by TCFs appears to be at least as important as Wnt/TCFmediated activation for proper target gene regulation (e.g., Brannon et al, 1997;Crawford et al, 1999;Yang et al, 2000;Knirr and Frasch, 2001;Merrill et al, 2001;Park et al, 2005). It is quite possible that Wnt signaling events that direct the de-repression of TCF target genes, but that do not trigger robust activation by TCFs, might not be detectable by multimerized TCF reporters, which have a baseline of zero and thus cannot register changes in repression (Merrill et al, 2004).…”