Cutting-Edge Skin Ageing Research on tissue stem cell.

In developed economies, the growing number of older individuals is a pressing issue. As a result, research progress into ageing has emphasised the significance of staying healthy in one's later years. Stem cells have a fundamental role to play in fostering diverse cell types and necessary processes for tissue repair and regeneration. Stem cells experience the effects of ageing over time, which is caused by their functional deterioration. Changes to stem cells, their niches, and signals from other tissues they interact with are crucial factors in the ageing of stem cells. Progress in single-cell RNA sequencing (scRNA-seq) technology has greatly advanced stem cell research. This review examines the mechanisms of stem cell ageing, its impact on health, and investigates the potential of stem cell therapy, with a special emphasis on the skin.

CD201+ fascia progenitors choreograph injury repair.

Optimal tissue recovery and organismal survival are achieved by spatiotemporal tuning of tissue inflammation, contraction and scar formation1. Here we identify a multipotent fibroblast progenitor marked by CD201 expression in the fascia, the deepest connective tissue layer of the skin. Using skin injury models in mice, single-cell transcriptomics and genetic lineage tracing, ablation and gene deletion models, we demonstrate that CD201+ progenitors control the pace of wound healing by generating multiple specialized cell types, from proinflammatory fibroblasts to myofibroblasts, in a spatiotemporally tuned sequence. We identified retinoic acid and hypoxia signalling as the entry checkpoints into proinflammatory and myofibroblast states. Modulating CD201+ progenitor differentiation impaired the spatiotemporal appearances of fibroblasts and chronically delayed wound healing. The discovery of proinflammatory and myofibroblast progenitors and their differentiation pathways provide a new roadmap to understand and clinically treat impaired wound healing.

Vasculature atrophy causes a stiffened microenvironment that augments epidermal stem cell differentiation in aged skin.

Stem cell loss causes tissue deterioration associated with aging. The accumulation of genomic and oxidative stress-induced DNA damage is an intrinsic cue for stem cell loss1,2; however, whether there is an external microenvironmental cue that triggers stem cell loss remains unclear. Here we report that the involution of skin vasculature causes dermal stiffening that augments the differentiation and hemidesmosome fragility of interfollicular epidermal stem cells (IFESCs) in aged mouse skin. Aging-related IFESC dysregulation occurs in plantar and tail skin, and is correlated with prolonged calcium influx, which is contributed by the mechanoresponsive ion channel Piezo1 (ref. 3). Epidermal deletion of Piezo1 ameliorated IFESC dysregulation in aged skin, whereas Piezo1 activation augmented IFESC differentiation and hemidesmosome fragility in young mice. The dermis stiffened with age, which was accompanied by dermal vasculature atrophy. Conversely, induction of the dermal vasculature softened the dermis and ameliorated IFESC dysregulation in aged skin. Single-cell RNA sequencing of dermal fibroblasts identified an aging-associated anti-angiogenetic secretory molecule, pentraxin 3 (ref. 4), which caused dermal sclerotization and IFESC dysregulation in aged skin. Our findings show that the vasculature softens the microenvironment for stem cell maintenance and provide a potential mechanobiology-based therapeutic strategy against skin disorders in aging.

Vasculature-driven stem cell population coordinates tissue scaling in dynamic organs.

Stem cell (SC) proliferation and differentiation organize tissue homeostasis. However, how SCs regulate coordinate tissue scaling in dynamic organs remain unknown. Here, we delineate SC regulations in dynamic skin. We found that interfollicular epidermal SCs (IFESCs) shape basal epidermal proliferating clusters (EPCs) in expanding abdominal epidermis of pregnant mice and proliferating plantar epidermis. EPCs consist of IFESC-derived Tbx3+-basal cells (Tbx3+-BCs) and their neighboring cells where Adam8-extracellular signal-regulated kinase signaling is activated. Clonal lineage tracing revealed that Tbx3+-BC clones emerge in the abdominal epidermis during pregnancy, followed by differentiation after parturition. In the plantar epidermis, Tbx3+-BCs are sustained as long-lived SCs to maintain EPCs invariably. We showed that Tbx3+-BCs are vasculature-dependent IFESCs and identified mechanical stretch as an external cue for the vasculature-driven EPC formation. Our results uncover vasculature-mediated IFESC regulations, which explain how the epidermis adjusts its size in orchestration with dermal constituents in dynamic skin.

Transient elevation of cytoplasmic calcium ion concentration at a single cell level precedes morphological changes of epidermal keratinocytes during cornification.

Epidermal keratinocytes achieve sequential differentiation from basal to granular layers, and undergo a specific programmed cell death, cornification, to form an indispensable barrier of the body. Although elevation of the cytoplasmic calcium ion concentration ([Ca2+]i) is one of the factors predicted to regulate cornification, the dynamics of [Ca2+]i in epidermal keratinocytes is largely unknown. Here using intravital imaging, we captured the dynamics of [Ca2+]i in mouse skin. [Ca2+]i was elevated in basal cells on the second time scale in three spatiotemporally distinct patterns. The transient elevation of [Ca2+]i also occurred at the most apical granular layer at a single cell level, and lasted for approximately 40 min. The transient elevation of [Ca2+]i at the granular layer was followed by cornification, which was completed within 10 min. This study demonstrates the tightly regulated elevation of [Ca2+]i preceding the cornification of epidermal keratinocytes, providing possible clues to the mechanisms of cornification.

Tbx3-dependent amplifying stem cell progeny drives interfollicular epidermal expansion during pregnancy and regeneration.

The skin surface area varies flexibly in response to body shape changes. Skin homeostasis is maintained by stem cells residing in the basal layer of the interfollicular epidermis. However, how the interfollicular epidermal stem cells response to physiological body shape changes remains elusive. Here, we identify a highly proliferative interfollicular epidermal basal cell population in the rapidly expanding abdominal skin of pregnant mice. These cells express Tbx3 that is necessary for their propagation to drive skin expansion. The Tbx3+ basal cells are generated from Axin2+ interfollicular epidermal stem cells through planar-oriented asymmetric or symmetric cell divisions, and express transit-amplifying cell marker CD71. This biased division of Axin2+ interfollicular epidermal stem cells is induced by Sfrp1 and Igfbp2 proteins secreted from dermal cells. The Tbx3+ basal cells promote wound repair, which is enhanced by Sfrp1 and Igfbp2. This study elucidates the interfollicular epidermal stem cell/progeny organisation during pregnancy and suggests its application in regenerative medicine.The abdominal skin expands rapidly during pregnancy. Here the authors show that a population of highly proliferative stem cell progenies expressing the transcription factor Tbx3 is required for abdominal skin expansion in pregnant mice.

Essential roles of Tbx3 in embryonic skin development during epidermal stratification.

Stepwise differentiation of epidermal cells is essential for development of stratified epithelium, but the underlying mechanisms remain unclear. Here, we show that Tbx3, a member of the T-box family of transcription factors, plays a pivotal role in this mechanism. Tbx3 is expressed in both basal and suprabasal cells in the interfollicular epidermis of mouse embryos. Epidermis-specific Tbx3 conditional knockout (cKO) embryos are small in size and display a thinner epidermis with an impaired barrier function. In the Tbx3 cKO epidermis, keratin 5-positive undifferentiated cells, which reside in both basal and suprabasal layers of wild-type embryos, are localized exclusively in the basal layer. In addition, mRNA expression levels of granular cell markers are increased in the Tbx3 cKO epidermis, suggesting that Tbx3 prevents premature differentiation of spinous cells. We further show that Tbx3 maintains the proliferative potential of basal cells and ensures their planar-oriented cell division. Moreover, Tbx3 is shown to be required for the expression of Hes1, a well-known Notch signaling target protein that is essential for epidermal development. We therefore propose that Tbx3 functions upstream of Hes1 to regulate proliferation and differentiation of basal and suprabasal cells during epidermal development.