introduction
Stem cells are present in the body's tissues for tissue homeostasis and damage repair. In adults, the 500-70 billion cells lost daily to wear and tear in tissues are replenished by lineage-restricted resident stem cells [1]. Normally, resident stem cells settle in the unique niche of tissues, maintaining self-renewal and directed differentiation into specific cells through a complex microenvironment. Studies have found that tissue structure damage can disrupt the local microenvironment of such stem cells, leading to homeostatic imbalance. During this process, stem cells undergo a change called lineage plasticity and migrate to the site of tissue injury, proliferating and differentiating into other lineage cells [2]. Lineage plasticity is a typical injury-induced stress response that confers flexibility in the fate selection of stem cells and is characterized by the simultaneous expression of classical transcription factors of both new and old lineage cells[2]. After the damaged tissue is repaired, the stem cells exit the plastic state and resettle in the niche to regain homeostasis. This withdrawal process, once disrupted, can lead to a state of chronic injury or carcinogenesis (Science | Prostate cancer spectrum plasticity is dependent on the JAK/STAT inflammatory signaling pathway). However, it remains unclear how the pedigree plasticity state exits.
Stem cell lineage plasticity is mostly studied in the fields of skin injury, transplantation, and tumors. As an ideal model, there are two types of adult stem cells in the skin, hair follicle stem cells (HFSCs) located in the hair follicle protrusion and epidermal stem cells (EpSCs) in the basal layer of the epidermis [3, 4]. Under different damage depths, HFSCs and EpSCs of different degrees will migrate out of the ecological niche and enter a state of lineage plasticity to repair damaged tissues. Skin stem cell lineage plasticity is typically characterized by the simultaneous expression of the transcription factor Sox9 of HFSCs and the transcription factor Klf5 of EpSCs [3]. Interestingly, isolated HFSCs mimic a similar state of lineage plasticity when cultured with rich serum and trophic factors, with co-expression of Sox9 and Klf5 and extensive lineage plasticity-related chromatin remodeling [5]. The influencing factors and mechanisms behind this phenomenon remain unclear.
Recently, Elaine Fuchs' research team at the Howard Hughes Medical Research Institute at Rockefeller University United States published a research article titled Vitamin A resolves lineage plasticity to orchestrate stem cell lineage choices online in the journal Science. In this study, we used mouse skin lesions as a model to find that skin stem cells that did not withdraw from lineage plasticity could not effectively promote hair regrowth and skin repair, and for the first time, vitamin A was identified as a key active substance in the niche of hair follicle stem cells to regulate the lineage selection of hair follicle stem cells.
The team first constructed Klf5-EGFP-NLS HFSCs cells, which expressed Klf5 under serum-rich conditions, and Klf5 expression decreased after serum withdrawal. Using this in vitro model, the authors screened three small molecules that significantly reduced Klf5 levels, including ERK inhibitor (ERKi), proteinase C inhibitor (PKCi), and all-trans retinoic acid (atRA). Given the role of ERKi in the normal hair cycle, the authors focused on PKCi and atRA. Although atRA is inferior to PKCi in reducing Klf5 levels, it can significantly promote Sox9 expression, and the simultaneous treatment of both can promote the exit of phyloplastic states and support HFSCs identity. scRNA-seq further supports these findings on the single-cell transcriptome. Taken together, these data suggest a role for atRA and PKCi in maintaining HFSCs identity out of the state of lineage plasticity.
Subsequently, the authors transfected retinoic acid response element (RARE)-RFP adenovirus in HFSCs, which can interact with retinoic acid receptor (RAR)-retinoid X receptor (retinoid X receptor) in the presence of retinoic acid (RA) RXR) to monitor retinoic acid metabolic activity in combination with promoting RFP expression. atRA, retinol, or vitamin A can all activate RFP expression in these cells, suggesting that HFSCs can metabolize retinol for atRA and have atRA receptors. In vivo, RARE-RFP adenovirus transfection was also detected, and RFP activity decreased and lineage plasticity was induced during skin injury. RFP activity was restored and the number of SOX9+KLF5+ cells decreased after 2 weeks of injury, indicating that atRA was negatively correlated with lineage plasticity in injury repair. Using Sox9CreER; When RXRa was knocked out in HFSCs in Rxra-fl/fl mice, the HFSCs in the skin of the mice also showed a lineage plasticity phenotype in the absence of injury, which fully demonstrated the lineage plasticity of atRA signal inhibition and the maintenance of HFSCs identity. Subsequently, the team isolated HFSCs from WT and RXR knockout mice and performed ATAC-seq analysis after lineage plasticity induction (FBS) and lineage plasticity elimination (atRA+PKCi), which did show that atRA eliminated lineage plasticity and promoted HFSCs identity at the chromatin level.
Although atRA is essential for the exit of the phyloplastic state, it is not sufficient to fully replicate the in vivo features of HFSCs. The team further investigated how atRA functions with signaling pathways known to regulate HFSCs. BMPs are generated from the inner overhang of the HFSCs niche and maintain the resting state of HFSCs. While atRA allows HFSCs to proliferate, low-dose atRA (10 nM)+BMPs elicit cell cycle withdrawal and exhibit resting characteristics, suggesting that atRA establishes the necessary chromatin base and synergistically promotes resting and maintains stemness with BMPs. In contrast, HFSCs changed from resting to active states under the regulation of BMP inhibitors and WNT activation during the new germinal cycle. Activated HFSCs give rise to two stem cell populations, SOX9+, TCF3/4+, and LEF1+, TCF1+. Interestingly, the combination of R-spondins (WNT agonists) and atRA can also promote the transition of HFSCs from resting to activation, but the optimal concentration of atRA required is 10 times higher than that of atRA at rest, suggesting that the relative concentration of atRA determines whether HFSCs remain at rest (low level) or activated (high level). Subsequently, the authors used atRA-mediated epidermal inhibition and lineage plasticity exit characteristics to establish a culture platform to study the effects of different signals on the differentiation of HFSCs. The results showed that the phyloplasticity withdrawal state of HFSCs was a prerequisite for the coordinated development and differentiation of multiple signals in the hair follicle microenvironment.
Finally, the authors used Sox9CreER; Rxra-fl/fl mice and skin Cyp26b1 (atRA degrading enzyme) transfection experiments confirmed that HFSCs with defective atRA metabolism were more likely to enter the damaged skin epithelium when skin was damaged. However, hair growth is limited at this time, suggesting that restoration of atRA activity after skin injury is necessary for lineage plasticity exit as well as hair growth. At the same time, in order to further understand the clinical situation, the authors also found that more HFSCs enter the spectrum plasticity when skin lesions occur in mice with vitamin A diet deficiency, but this process can be inhibited by skin atRA application supplementation. Although atRA metabolic defects promote the recruitment of HFSCs to the epidermis at the time of skin injury, the recovery of the actual skin barrier is still delayed, suggesting that skin plasticity withdrawal is also the basis of skin repair.
In summary, this study not only pointed out that the withdrawal of stem cell lineage plasticity is an important factor in the maintenance of tissue homeostasis, but also confirmed that vitamin A metabolism plays a key role in stem cell lineage selection, which has therapeutic significance for hair growth, damage repair and tumors.
模式图（Credit: Science）
bibliography
1. Yejing, Ge., Elaine, Fuchs. (2018). Stretching the limits: from homeostasis to stem cell plasticity in wound healing and cancer. Nat Rev Genet, 19(5), 0. doi:10.1038/nrg.2018.92. Yejing, Ge., Nicholas C, Gomez., Rene C, Adam., Maria, Nikolova., Hanseul, Yang., Akanksha, Verma., Catherine Pei-Ju, Lu., Lisa, Polak., Shaopeng, Yuan., Olivier, Elemento., Elaine, Fuchs. (2017). Stem Cell Lineage Infidelity Drives Wound Repair and Cancer. Cell, 169(4), 0. doi:10.1016/j.cell.2017.03.0423. Cedric, Blanpain., William E, Lowry., Andrea, Geoghegan., Lisa, Polak., Elaine, Fuchs. (2004). Self-renewal, multipotency, and the existence of two cell populations within an epithelial stem cell niche. Cell, 118(5), 0. doi:10.1016/j.cell.2004.08.0124. Kevin Andrew Uy, Gonzales., Lisa, Polak., Irina, Matos., Matthew T, Tierney., Anita, Gola., Ellen, Wong., Nicole R, Infarinato., Maria, Nikolova., Shijing, Luo., Siqi, Liu., Jesse S S, Novak., Kenneth, Lay., Hilda Amalia, Pasolli., Elaine, Fuchs. (2021). Stem cells expand potency and alter tissue fitness by accumulating diverse epigenetic memories. Science, 374(6571), 0. doi:10.1126/science.abh24445. V R, Iyer., M B, Eisen., D T, Ross., G, Schuler., T, Moore., J C, Lee., J M, Trent., L M, Staudt., J Jr, Hudson., M S, Boguski., D, Lashkari., D, Shalon., D, Botstein., P O, Brown. (1999). The transcriptional program in the response of human fibroblasts to serum. Science, 283(5398), 0. doi:10.1126/science.283.5398.83https://doi.org/10.1126/science.adi7342
Editor-in-charge|Explore Jun
Typography |
Article Source | BioArt”
End