We observed similar stem cell plasticity when we purified and tested the myoepithelial stem cells from sweat glands (Lu et al., 2012; Blanpain and Fuchs, 2014). Similar to myoepithelial stem cells of mammary glands, these stem cells normally act unipotently and only replenish dying myoepithelial cells of the gland. However, when purified by fluorescence activated cell sorting (FACS) and transplanted directly into a mammary fat pad, the stem cells can regenerate the complete bi-layered gland, and the new luminal cells secrete sweat. Moreover, when engrafted to the skin, these stem cells can make epidermis. An area of interest in my lab is to understand the environmental cues that dictate the fascinating plasticity of epithelial stem cells, and to elucidate the chromatin remodeling that leads to the changes in gene expression necessary to generate different tissues from a common progenitor.

To understand how a stem cell chooses its differentiation pathway, we have taken several approaches. An ongoing approach of the lab is to express different fluorescent proteins under the control of various skin promoters, active at different stages in stem cells and their lineages. Through FACS, we've purified cells at different time points along the lineages and generated a battery of lineage-specific profiles, enabling us to define at an mRNA (RNA-seq) and chromatin (ChIP-seq) level how stem cells change as they transition from quiescence to activation to lineage determination. Our global objective is to exploit this information to understand how stem cells receive signals, change their program of gene expression and select a lineage. We also want to understand the functional significance of these changes. The beauty of the hair follicle as a model is that it is currently the only system where sufficient quantities of stem cells can be isolated directly from their native niche in order to carry out whole-genome wide analyses in vivo. This eliminates the caveats arising from culturing cells, namely induction of a stress response and large-scale epigenetic changes in gene expression.

For the hair follicle, >150 mRNAs are selectively upregulated in the bulge stem cells relative to their short-lived progeny (Tumbar et al., 2004; Blanpain et al., 2004; Keyes et al., 2013). A number of these changes are in transcription factors and epigenetic regulators. Weve conducted in vivo chromatin immunoprecipitation and high throughput sequencing (ChIP-seq) on chromatin from hair follicle stem cells (HFSCs) and their short-lived progeny. Bioinformatics reveals which genes bind these transcription factors, and how this changes as the stem cells progress to form transiently dividing cells that then terminally differentiate along one of the 7 distinct concentric cell layers that constitute the hair and its channel. By conducting high throughput RNA sequencing (RNA-seq) on HFSCs lacking each of these genes, weve learned which target genes depend upon binding these transcription factors. Finally, by engineering inducible-conditional knockouts to selectively remove these transcription factors in the stem cells, weve learned the physiological relevance of these factors.

Based upon these analyses, TCF3/TCF4, LHX2 and SOX9 are all essential for maintaining the hair follicle stem cells in their native niche (Nguyen et al., 2006; 2009; Rhee et al., 2006; Folgueras et al., 2013; Lien et al., 2011; 2014; Nowak et al., 2008; Kadaja et al., 2014). In addition, LHX2 represses sebaceous gland differentiation: following its loss, the stem cell niche soon becomes a sebaceous gland (Folgueras et al., 2013). SOX9 represses epidermal differentiation: following its loss, the niche becomes an epidermal cyst (Kadaja et al., 2014). TCF3 and TCF4 repress HF differentiation: following their loss, quiescent HFSCs precociously activate a new hair cycle (Lien et al., 2014). TCF3 and TCF4 can partner with -catenin, which is stabilized and becomes nuclear upon Wnt signaling: if -catenin is silenced in the quiescent HFSCs, they never reenter a new hair cycle. In their native niche, quiescent HFSCs express a transcriptional repressor TLE4 which binds to TCF3 and TCF4: our findings are consistent with the view that Wnt signaling functions by relieving TCF3/4/TLE4-mediated repression (Lien et al., 2014).

NFATc1 is required for maintaining HFSC quiescence, and in its absence, HFs cycle precociously (Horsley et al., 2008). Additionally, NFATc1 is downstream of BMP signaling, offering a potential explanation as to why BMP signaling must be lowered to activate hair cycling. A major feature of the aging HFSC signature is elevated NFATc1 target genes, and we can stimulate old follicles by inhibiting NFATc1 (Keyes et al., 2013). A major question still to be answered is whether HFSCs have an endless capacity for hair cycling and whether this same phenomenon operates in aging scalp hairs in humans. If so, these findings may open new doors for future therapeutics.

NFiB is a transcription factor which is specific to the HFSCs, but functions by repressing the expression of genes that are essential for the differentiation of the melanocyte stem cells, which reside within the same stem cell niche (Chang et al., 2013). These two stem cell populations must be activated at the same time so that differentiating melanocytes can transfer pigment to the differentiating hair cells to provide the natural coloring to our hair. Loss of NFiB uncouples this crosstalk and leads to the precocious activation of a key NFiB target gene that encodes a secreted melanocyte differentiation factor (Chang et al., 2013).

There are a number of additional transcription factors and epigenetic regulators which are enhanced in the complex milieu of HF stem cell chromatin, and there is still much to be learned. Of the epigenetic regulators, weve thus far examined only the role of polycomb chromatin repressor complexes, which weve shown function critically in controlling the fate switch from a stem cell to a committed, transit-amplifying state (Ezhkova et al., 2009; 2011; Lien et al., 2011). In coming years, we will continue to systematically work our way through the functional significance and mechanism of action of epigenetic and transcriptional controls on stem cells as they transit from a quiescent to activated to committed state. When coupled with our recent ability to efficiently knockdown genes in a few days using lentiviral-mediated shRNA delivery (Beronja et al., 2010), this now becomes a powerful tool for exploiting bioinformatics analyses to gain biological insights.

Our ultimate goal is to understand how external signals from the surrounding niche microenvironment impact chromatin dynamics to achieve tissue production. Equally important will be the expression of specific genes that enables them to remodel their cytoskeleton and adhesive contacts and either form a stratified epidermis or an epithelial bud that can then develop into a hair follicle (Perez-Moreno et al., 2003; Blanpain and Fuchs, 2009; Hsu et al., 2014). While our model is the skin, the problem is a general one of how a single epithelial stem cell gives rise to a spatially organized, functional tissue. It is also integrally linked to understanding the basis of cancer progression.

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The Rockefeller University Stem Cells of the Skin and ...