Understanding The Molecular Mechanisms Of Hair Follicle Stem Cell Quiescence And Genome Plasticity

Adult stem cells (SCs) utilize their abilities of self-renewal and differentiation to maintain proper homeostasis of their residing tissue. Malfunctioning of tissue SCs can result in multiple developmental disorders and cancers. To maintain balance between proliferation and quiescence, and faithfully execute fate determination, tissue SCs must be tightly regulated. Thus, understanding the molecular mechanisms of tissue SCs proliferation/quiescence and how their fate is determined are of critical importance in stem cell biology and medicine. Hair follicle stem cells reside in the bulge and remain relatively quiescent throughout the cycling of the hair follicle. During quiescence, hair follicle stem cells choose between the two fates prior to activation - migrate out from their niche and become differentiated progenitors or remain in the niche and self-renew. To understand how 1) hair follicle stem cells quiescence is maintained and 2) their quiescence relates to fate decision, we explored different aspects of regulation. In our effort to address the first question, we revealed how a key transcription factor of hair follicle stem cell activation and proliferation, Runx1, plays a role in regulating multiple Cyclin-Dependent Kinase inhibitors (CDKis). We found that multiple CDKis including p15, p21, p27, and p57 are highly expressed during quiescence. In the absence of p21, hair follicle stem cells fail to enter quiescence in a timely manner. Moreover, we discovered that Runx1 and p21 interaction plays a context-dependent role under different proliferative environments. Finally, we discovered a novel function of CDKis in regulating transcription of other CDKis, independent of kinase-inhibitory function. Together, we unveil the robust mechanism of hair follicle stem cell quiescence by transcriptional regulation of cell cycle regulators. To address the second question, we took a very different approach. Given the importance of histone methylation in regulating embryonic SCs self-renewal and differentiation, we asked how such regulation might take part in maintaining hair follicle stem cells quiescence and fate determination. We show that a distinct low level of different histone methylation marks characterizes quiescent hair follicle stem cells, and this level seems to be regulated by growth factor signaling including Bmp. These marks are associated with both transcriptional activation (H3K4me3) and repression (H3K9me3 and H3K27me3) of chromatin at specific gene promoters. Strikingly, globally low status of H3K4me3, H3K9me3, and H3K27me3 during quiescence is associated with higher genome plasticity, which likely allows hair follicle stem cells to make a fate decision prior to activation. Collectively, our findings suggest that the mechanism of maintaining hair follicle stem cells quiescence is robust and adds more biological significance to quiescence than previously realized by associating this state with plasticity for fate determination