Temporal Layering of Signaling Effectors Drives Chromatin Remodeling during Hair Follicle Stem Cell Lineage Progression

Abstract

Tissue regeneration relies on resident stem cells (SCs), whose activity and lineage choices are influenced by the microenvironment. Exploiting the synchronized, cyclical bouts of tissue regeneration in hair follicles (HFs), we investigate how microenvironment dynamics shape the emergence of stem cell lineages. Employing epigenetic and ChIP-seq profiling, we uncover how signal-dependent transcription factors couple spatiotemporal cues to chromatin dynamics, thereby choreographing stem cell lineages. Using enhancer-driven reporters, mutagenesis, and genetics, we show that simultaneous BMP-inhibitory and WNT signals set the stage for lineage choices by establishing chromatin platforms permissive for diversification. Mechanistically, when binding of BMP effector pSMAD1 is relieved, enhancers driving HF-stem cell master regulators are silenced. Concomitantly, multipotent, lineage-fated enhancers silent in HF-stem cells become activated by exchanging WNT effectors TCF3/4 for LEF1. Throughout regeneration, lineage enhancers continue reliance upon LEF1 but then achieve specificity by accommodating additional incoming signaling effectors. Barriers to progenitor plasticity increase when diverse, signal-sensitive transcription factors shape LEF1-regulated enhancer dynamics.

miR-29a/b1 Inhibits Hair Follicle Stem Cell Lineage Progression by Spatiotemporally Suppressing WNT and BMP Signaling

“…Among the enriched genes, WNT and BMP signaling-related genes were notable because of the requirement of these signaling cascades in HFSC-lineage progression (Adam et al, 2018;Andl et al, 2002;Choi et al, 2013;Genander et al, 2014;Huelsken et al, 2001;Xu et al, 2015). To assess whether WNT signaling may be affected by miR-29a/b1 induction, we examined the expression levels of LEF1 and Axin2, which correlate with WNT pathway activity in the HF (Adam et al, 2018;Choi et al, 2013). Interestingly, the levels of LEF1 and Axin2 were dramatically downregulated in DTG mice induced during active hair growth (P3) or in telogen (P21) (Figures 3G, 5B, and 5C).…”

“…Hair follicle stem cells (HFSCs) can self-renew or differentiate to promote hair growth. To fuel hair-follicle (HF) regeneration, HFSCs undergo multiple steps toward lineage determination, a process tightly correlated with dynamic changes in the signaling effectors (Adam et al, 2018). At the onset of HF growth, signals from the dermal papilla (DP) first stimulate the activation of the hair germ (HG) and then bulge stem cells (SCs) (Chueh et al, 2013;Greco et al, 2009).…”

“…In mature HFs, the ORS extends from the bulge to the transit-amplifying (TA) matrix. Matrix cells envelop the DP at the HF base and then rapidly proliferate and differentiate upward into one of the seven distinct layers of the hair shaft (HS), the inner root sheath (IRS), or the companion layer between the IRS and the ORS (Adam et al, 2018;Hsu et al, 2011;Yang et al, 2017). HFSC self-renewal and differentiation are finely balanced to maintain HF homeostasis or promote regeneration.…”

“…Because the differentiation programs of the HF are spatially and temporally well-defined, the HF is an ideal model for studying the mechanisms that underlie HFSC lineage determination. WNT and BMP signaling are central to the signaling pathways that regulate HFSC behavior and hair differentiation (Adam et al, 2018;Andl et al, 2002;Choi et al, 2013;Genander et al, 2014;Huelsken et al, 2001;Xu et al, 2015). In the HFSC lineage, quiescent HFSCs are activated by WNT-activating cues and BMP inhibitory factors.…”

“…Furthermore, loss of Bmpr1a in mice blocks differentiation of the IRS and HS and leads to aberrant activation of quiescent HFSCs (Andl et al, 2004;Genander et al, 2014;Kandyba et al, 2013;Kobielak et al, 2003). Overall, HFSC lineage determination is mediated by the dynamic interaction of WNT and BMP signaling at distinct stages (Adam et al, 2018;Kandyba et al, 2013;Kobielak et al, 2003). Although the involvement of WNT and BMP signaling in HFSC lineage diversification has been extensively studied, the function of other factors and inputs underlying HF lineage choices has yet to be fully explored.…”