H2AK119ub dynamics controls hair follicle stem cell quiescence [RNA-Seq 1]

The transition of stem cells from a quiescent state to an active state is a finely tuned process that requires the dismantling of the quiescence program and the establishment of a cell cycle-promoting transcriptional landscape. Whether epigenetic processes control stem cell states to promote the regeneration of adult tissues remains elusive. In this study, we show that a repressive histone modification, H2AK119ub, is dynamic between quiescent and active hair follicle stem cells (HFSCs) in the adult murine skin. Ablation of H2AK119ub in HFSCs leads to failure in maintaining quiescence leading to premature activation and an eventual exhaustion of HFSC pool. Transcriptional and chromatin studies revealed that H2AK119ub directly represses a proliferation promoting transcriptional program in the HFSCs to preserve quiescence. Lastly, we identify that the inhibitory FGF signaling produced by the hair follicle niche keratinocytes maintains H2AK119ub in quiescent HFSCs. Together, these findings reveal that a repressive histone mark, H2AK119ub, is under the dynamic regulation of inhibitory niche signaling to prevent the untimely establishment of an activated state to preserve SC function and longevity. Overall design: Control and PRC1 i2KO mice were induced starting at P50. 10 days after completion of induction treatment, the back skin was collected and the adipose layer from the dorsal side was scraped off using a surgical scalpel. The scraped skin sample was washed with PBS prior to incubation with 0.25% Trypsin/EDTA at 37°C for 1 hour on a rotation plate set at 60 r.p.m. After incubation, the epidermal cells, including the HFSCs, was scraped off from the trypsinized skin into the plate. 25 mLs of E-media was added to the cell suspension and was then sequentially strained through 100µm and 40µm filters. The cell pellet was washed twice with DPBS before proceeding with staining with cell surface markers. The cells were stained with 1:200 PerCP-Cy5.5-Sca1 (Thermo Fisher Scientific, 45-5981-82), 1:200 FITC-a6-integrin (eBioscience, 11-0493-81), 1:100 APC-EpCAM (Biolegend, 324207) and 1:20 Alexa700-CD34 (eBioscience, 56-0341-82) in staining buffer (HBSS + 2% Fetal Bovine Serum) for 30 minutes on ice and then washed twice with DPBS before cell sorting. HFSCs were sorted by gating on EpCAM (+), Sca1(-), a6-integrin(high) and CD34(+).