In vivo transcriptional governance of hair follicle stem cells by canonical Wnt regulators

Abstract

Hair follicle stem cells (HFSCs) regenerate hair in response to Wnt signalling. Here, we unfold genome-wide transcriptional and chromatin landscapes of β-catenin–TCF3/4–TLE circuitry, and genetically dissect their biological roles within the native HFSC niche. We show that during HFSC quiescence, TCF3, TCF4 and TLE (Groucho) bind coordinately and transcriptionally repress Wnt target genes. We also show that β-catenin is dispensable for HFSC viability, and that if TCF3/4 levels are sufficiently reduced, it is dispensable for proliferation. However, β-catenin is essential to activate genes that launch hair follicle fate and suppress sebocyte fate determination. TCF3/4 deficiency mimics Wnt–β-catenin-dependent activation of these hair follicle fate targets; TCF3 overexpression parallels their TLE4-dependent suppression. Our studies unveil TCF3/4–TLE histone deacetylases as a repressive rheostat, whose action can be relieved by Wnt–β-catenin signalling. When TCF3/4 and TLE levels are high, HFSCs can maintain stemness, but remain quiescent. When these levels drop or when Wnt–β-catenin levels rise, this balance is shifted and hair regeneration initiates.

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

“…These findings agree well with recent in vivo ChIP and Illumina deep sequencing (ChIP-seq) and RNA sequencing (RNA-seq) on purified quiescent hair follicle stem cells (HFSCs), which show that TCF3, TCF4, and TLEs bind to common chromatin sites in the absence of Wnt signaling (Lien et al 2014). These TCF3/TCF4/TLE-bound genes include chromatin-repressed genes that must be derepressed by canonical Wnt signaling in order to activate hair follicle fate specification (Lien et al 2014).…”

“…These findings agree well with recent in vivo ChIP and Illumina deep sequencing (ChIP-seq) and RNA sequencing (RNA-seq) on purified quiescent hair follicle stem cells (HFSCs), which show that TCF3, TCF4, and TLEs bind to common chromatin sites in the absence of Wnt signaling (Lien et al 2014). These TCF3/TCF4/TLE-bound genes include chromatin-repressed genes that must be derepressed by canonical Wnt signaling in order to activate hair follicle fate specification (Lien et al 2014). Although it was initially surmised that nuclear b-catenin directly binds LEF/TCF and displaces Groucho/TLE repressors (Daniels and Weis 2005), derepression may not necessarily involve a competitive mechanism (Chodaparambil et al 2014).…”

“…Similar findings have been reported for adult HFSCs, which can self-renew and be passaged long-term in the absence of b-catenin when they are maintained in culture conditions that favor TCF3/4 repression (Lien et al 2014). In contrast, in vivo, the TCF3/4-high stem cells can persist for months in the absence of b-catenin, but the cells are unable to induce hair lineage differentiation (Choi et al 2013;Lien et al 2014).…”

“…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).…”

“…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).…”