A Single-Cell Transcriptomic Atlas of Human Skin Aging

New opportunities and challenges of natural products research: When target identification meets single-cell multiomics

Natural products, and especially the active ingredients found in traditional Chinese medicine (TCM), have a thousand-year-long history of clinical use and a strong theoretical basis in TCM. As such, traditional remedies provide shortcuts for the development of original new drugs in China, and increasing numbers of natural products are showing great therapeutic potential in various diseases. This paper reviews the molecular mechanisms of action of natural products from different sources used in the treatment of inflammatory diseases and cancer, introduces the methods and newly emerging technologies used to identify and validate the targets of natural active ingredients, enumerates the expansive list of TCM used to treat inflammatory diseases and cancer, and summarizes the patterns of action of emerging technologies such as single-cell multiomics, network pharmacology, and artificial intelligence in the pharmacological studies of natural products to provide insights for the development of innovative natural product-based drugs. Our hope is that we can make use of advances in target identification and single-cell multiomics to obtain a deeper understanding of actions of mechanisms of natural products that will allow innovation and revitalization of TCM and its swift industrialization and internationalization.

Apoptotic bodies for advanced drug delivery and therapy

Extracellular vesicles (EVs) have emerged as promising candidates for multiple biomedical applications. Major types of EVs include exosomes, microvesicles, and apoptotic bodies (ABs). ABs are conferred most properties from parent cells in the final stages of apoptosis. A wide variety of sources and stable morphological features are endowed to ABs by the rigorous apoptotic program. ABs accommodate more functional biomolecules by relying on the larger volume and maintaining their naturalness in circulation. The predominant body surface ratio of ABs facilitates their recognition by recipient cells and is advantageous for interactions with microenvironments. ABs can modulate and alleviate symptoms of numerous diseases for their origins, circulation, and high biocompatibility. In addition, ABs have been emerging in disease diagnosis, immunotherapy, regenerative therapy, and drug delivery. Here, we aim to present a thorough discussion on current knowledge about ABs. Of particular interest, we will summarize the application of AB-based strategies for diagnosis and disease therapy. Perspectives for the development of ABs in biomedical applications are highlighted.

The epiphany derived from T-cell–inflamed profiles: Pan-cancer characterization of CD8A as a biomarker spanning clinical relevance, cancer prognosis, immunosuppressive environment, and treatment responses

CD8A encodes the CD8 alpha chain of αβT cells, which has been proposed as a quantifiable indicator for the assessment of CD8⁺ cytotoxic T lymphocytes (CTLs) recruitment or activity and a robust biomarker for anti-PD-1/PD-L1 therapy responses. Nonetheless, the lack of research into the role of CD8A in tumor microenvironment predisposes to limitations in its clinical utilization. In the presented study, multiple computational tools were used to investigate the roles of CD8A in the pan-cancer study, revealing its essential associations with tumor immune infiltration, immunosuppressive environment formation, cancer progression, and therapy responses. Based on the pan-cancer cohorts of the Cancer Genome Atlas (TCGA) database, our results demonstrated the distinctive CD8A expression patterns in cancer tissues and its close associations with the prognosis and disease stage of cancer. We then found that CD8A was correlated with six major immune cell types, and immunosuppressive cells in multiple cancer types. Besides, epigenetic modifications of CD8A were related to CTL levels and T cell dysfunctional states, thereby affecting survival outcomes of specific cancer types. After that, we explored the co-occurrence patterns of CD8A mutation, thus identifying RMND5A, RNF103-CHMP3, CHMP3, CD8B, MRPL35, MAT2A, RGPD1, RGPD2, REEP1, and ANAPC1P1 genes, which co-occurred mutations with CD8A, and are concomitantly implicated in the regulation of cancer-related pathways. Finally, we tested CD8A as a therapeutic biomarker for multiple antitumor agents’ or compounds’ responsiveness on various cancer cell lines and cancer cohorts. Our findings denoted the underlying mechanics of CD8A in reflecting the T-cell-inflamed profiles, which has potential as a biomarker in cancer diagnosis, prognosis, and therapeutic responses.

Understanding Human Epidermal Stem Cells at Single-Cell Resolution

The human epidermis is one of the first tissues in which the existence of stem cells was recognized and is one of the few in which ex vivo expansion for tissue repair is established clinically. Nevertheless, the nature of stem cells has been elusive. Using clonal growth assays of cultured keratinocytes as a quantitative measure of their abundance, several candidate stem cell markers have been described. Recently, the volume and quality of single-cell RNA-sequencing datasets have increased exponentially, providing new opportunities to explore the nature of epidermal stem cells and test the validity of in vitro experimental models.

Differential gene screening and bioinformatics analysis of epidermal stem cells and dermal fibroblasts during skin aging

To explore the differentially expressed genes (DEGs) and potential therapeutic targets of skin aging in GEO database by bioinformatics methods. Dermal fibroblasts and skin aging related data sets GSE110978 and GSE117763 were downloaded from GEO database, and epidermal stem cells and skin aging related data sets GSE137176 were downloaded. GEO2R was used to screen DEGs of candidate samples from the three microarrays, GO function analysis and KEGG pathway analysis were performed. Protein interaction network was constructed using String database, and hub gene was obtained by Cytoscape. NetworkAnalys was used to analyze the coregulatory network of DEGs and MicroRNA (miRNA), interaction with TF, and protein-chemical interactions of DEGs. Finally, DSigDB was used to determine candidate drugs for DEGs. Six DEGs were obtained. It mainly involves the cytological processes such as response to metal ion, and is enriched in mineral absorption and other signal pathways. Ten genes were screened by PPI analysis. Gene-miRNA coregulatory network found that Peg3 and mmu-miR-1931 in DEGs were related to each other, and Cybrd1 was related to mmu-miR-290a-5p and mmu-miR-3082-5p. TF-gene interactions found that the transcription factor UBTF co-regulated two genes, Arhgap24 and Mpzl1. Protein-chemical Interactions analysis and identification of candidate drugs show results for candidate drugs. Try to explore the mechanism of hub gene action in skin aging progression, and to discover the key signaling pathways leading to skin aging, which may be a high risk of skin aging.

Stem Cell-Derived Exosomes: A New Method for Reversing Skin Aging

Senescence is an inevitable natural life process that involves structural and functional degeneration of tissues and organs. Recently, the process of skin aging has attracted much attention. Determining a means to delay or even reverse skin aging has become a research hotspot in medical cosmetology and anti-aging. Dysfunction in the epidermis and fibroblasts and changes in the composition and content of the extracellular matrix are common pathophysiological manifestations of skin aging. Reactive oxygen species and matrix metalloproteinases play essential roles in this process. Stem cells are pluripotent cells that possess self-replication abilities and can differentiate into multiple functional cells under certain conditions. These cells also possess a strong ability to facilitate tissue repair and regeneration. Stem cell transplantation has the potential for application in anti-aging therapy. Increasing studies have demonstrated that stem cells perform functions through paracrine processes, particularly those involving exosomes. Exosomes are nano-vesicular substances secreted by stem cells that participate in cell-to-cell communication by transporting their contents into target cells. In this chapter, the biological characteristics of exosomes were reviewed, including their effects on extracellular matrix formation, epidermal cell function, fibroblast function and antioxidation. Exosomes derived from stem cells may provide a new means to reverse skin aging.

Age-Related Alterations in Macrophage Distribution and Function Are Associated With Delayed Cutaneous Wound Healing

Ageing-related delays and dysregulated inflammation in wound healing are well-documented in both human and animal models. However, cellular and molecular changes underlying this impairment in healing progression are not fully understood. In this study, we characterised ageing-associated changes to macrophages in wounds of young and aged mice and investigated transcriptomic differences that may impact the progression of wound healing. Full-thickness wounds created on the dorsum of C57BL/6J young and aged mice were excised on Days 3 and 7 post-wounding for analysis by immunohistochemistry, flow cytometry, and RNA sequencing. Our data revealed that macrophages were significantly reduced in aged wounds in comparison to young. Functional transcriptomic analyses showed that macrophages from aged wounds exhibited significantly reduced expression of cell cycle, DNA replication, and repair pathway genes. Furthermore, we uncovered an elevated pro-inflammatory gene expression program in the aged macrophages correlated with poor inflammation resolution and excessive tissue damage observed in aged wounds. Altogether, our work provides insights into how poorly healing aged wounds are phenotypically defined by the presence of macrophages with reduced proliferative capacity and an exacerbated inflammatory response, both of which are pathways that can be targeted to improve healing in the elderly.

Urolithin A protects human dermal fibroblasts from UVA-induced photoaging through NRF2 activation and mitophagy

Photoaging, caused by exposure to sunlight and especially UVA, has been identified as one of the culprits for age-related skin deterioration. Here, we initially demonstrated that urolithin A (UroA), a metabolite derived from intestine microflora, possessed sufficient photoprotective capacity and attenuated UVA-induced senescent phenotypes in human fibroblasts, such as growth inhibition, senescence-associated β-galactosidase activity, breakdown of extracellular matrix, synthesis of senescence-associated secretory phenotypes and cell cycle arrest. Furthermore, UroA lessened the accumulation of intracellular reactive oxygen species, which promoted the phosphorylation and afterwards nuclear translocation of NRF2, subsequently driving the activation of downstream antioxidative enzymes. In parallel, we proved that UroA restored mitochondrial function by induction of mitophagy, which was regulated by the SIRT3-FOXO3-PINK1-PARKIN network. Taken together, our results showed that UroA protected dermal fibroblast from UVA damage through NRF2/ARE activation and mitophagy process, thus supporting UroA as a potential therapeutic agent for photoaging.

Circular RNA circ-ABCB10 Promotes Proliferation and Inhibits Apoptosis of Laryngeal Carcinoma by Inhibiting KLF6

Objective

To explore the effect of circular RNA circ-ABCB10 on the proliferation and apoptosis of laryngeal carcinoma via inhibiting KLF6.

Methods

RT-qPCR assay was adopted to detect the expression of circ-ABCB10 and KFL6 in laryngeal carcinoma tissues and cell lines. Cell counting kit-8 (CCK-8) and clone formation assay were employed to detect laryngeal cancer cell viability and proliferation when circ-ABCB10 was silenced or upregulated. In this study, the apoptosis rate was detected by flow cytometry and the protein expression was detected by Western blotting. Wound healing and cross-hole invasion were used to study the migration and invasion of laryngeal cancer cells when circ-ABCB10 was silenced or upregulated.

Results

The results of RT-qPCR detection indicated that the expression of circ-ABCB10 in all three laryngeal carcinoma cells was downregulated by 3.2 times compared with that of HaCat cells. There is low expression of circ-ABCB10 in most laryngeal carcinoma tissues, the diagnostic cutoff value of circ-ABCB10 is 0.0008, the area under the curve is 0.718, the sensitivity is 0.981, and the specificity is 0.556. The expression level of KLF6 in laryngeal carcinoma is on the rise, which is significantly higher compared to healthy tissues (P < 0.05); 48 hours after transfection, RT-qPCR analysis confirmed the transfection efficiency, and upregulation of circ-ABCB10 could significantly promote cell proliferation. Compared with the control group, silencing circ-MTCL1 could inhibit cell proliferation, overexpression of circ-ABCB10 promoted cell migration, and downregulation of circ-ABCB10 significantly inhibited cell movement (P < 0.001). Upregulation of circ-ABCB10 significantly enhanced the invasiveness and motility of laryngeal cancer cells, while downregulation of circ-ABCB10 was the opposite. Compared with the KLF6 NC group, KLF6 level increased significantly in the KLF6 group, while cell viability, colony formation, scratch healing rate, invasive cell number, and Bcl-2 expression level decreased significantly in the KLF6 group, while apoptosis rate and Bax expression level increased significantly (P < 0.05). KLF6 level in the si-circ-ABCB10+anti-KLF6 group was significantly lower than that in the si-circ-ABCB10+anti-KLF6-NC group (P < 0.05). Meanwhile, the cell activity, colony formation number, cell scratch healing rate, number of invaded cells, and Bcl-2 all indicated an upward trend, while the cell apoptosis rate and Bax expression indicated a downward trend (P < 0.05).

Conclusion

The expression of circ-ABCB10 in laryngeal carcinoma was significantly higher compared to that in paracancerous tissues. Silencing circ-ABCB10 could significantly inhibit the growth and proliferation of laryngeal adenocarcinoma cells, while overexpression of circ-ABCB10 could significantly promote the growth of laryngeal adenocarcinoma cells, probably by inhibiting KLF6 to enhance the proliferation of laryngeal carcinoma and inhibit apoptosis.

Single-Cell Transcriptomics Uncover Key Regulators of Skin Regeneration in Human Long-Term Mechanical Stretch-Mediated Expansion Therapy

Tissue expansion is a commonly performed therapy to grow extra skin invivo for reconstruction. While mechanical stretch-induced epidermal changes have been extensively studied in rodents and cell culture, little is known about the mechanobiology of the human epidermis in vivo. Here, we employed single-cell RNA sequencing to interrogate the changes in the human epidermis during long-term tissue expansion therapy in clinical settings. We also verified the main findings at the protein level by immunofluorescence analysis of independent clinical samples. Our data show that the expanding human skin epidermis maintained a cellular composition and lineage trajectory that are similar to its non-expanding neighbor, suggesting the cellular heterogeneity of long-term expanded samples differs from the early response to the expansion. Also, a decrease in proliferative cells due to the decayed regenerative competency was detected. On the other hand, profound transcriptional changes are detected for epidermal stem cells in the expanding skin versus their non-expanding peers. These include significantly enriched signatures of C-FOS, EMT, and mTOR pathways and upregulation of AREG and SERPINB2 genes. CellChat associated ligand-receptor pairs and signaling pathways were revealed. Together, our data present a single-cell atlas of human epidermal changes in long-term tissue expansion therapy, suggesting that transcriptional change in epidermal stem cells is the major mechanism underlying long-term human skin expansion therapy. We also identified novel therapeutic targets to promote human skin expansion efficiency in the future.

Identification, discrimination and heterogeneity of fibroblasts

Fibroblasts, the principal cell type of connective tissue, secrete extracellular matrix components during tissue development, homeostasis, repair and disease. Despite this crucial role, the identification and distinction of fibroblasts from other cell types are challenging and laden with caveats. Rapid progress in single-cell transcriptomics now yields detailed molecular portraits of fibroblasts and other cell types in our bodies, which complement and enrich classical histological and immunological descriptions, improve cell class definitions and guide further studies on the functional heterogeneity of cell subtypes and states, origins and fates in physiological and pathological processes. In this review, we summarize and discuss recent advances in the understanding of fibroblast identification and heterogeneity and how they discriminate from other cell types.

In this review, the authors look at how recent progress in single-cell transcriptomics complement and enrich the classical, largely morphological, portraits of fibroblasts. The detailed molecular information now available provides new insights into fibroblast identity, heterogeneity and function.

Single-cell transcriptomics reveals lineage trajectory of human scalp hair follicle and informs mechanisms of hair graying

Hair conditions, such as hair loss and graying, are prevalent human conditions. But they are often poorly controlled due to our insufficient understanding of human scalp hair follicle (hsHF) in health and disease. Here we describe a comprehensive single-cell RNA-seq (scRNA-seq) analysis on highly purified black and early-stage graying hsHFs. Based on these, a concise single-cell atlas for hsHF and its early graying changes is generated and verified using samples from multiple independent individuals. These data reveal the lineage trajectory of hsHF in unprecedented detail and uncover its multiple unexpected features not found in mouse HFs, including the presence of an innerbulge like compartment in the growing phase, lack of a discrete companion layer, and enrichment of EMT features in HF stem cells (HFSCs). Moreover, we demonstrate that besides melanocyte depletion, early-stage human hair graying is also associated with specific depletion of matrix hair progenitors but not HFSCs. The hair progenitors' depletion is accompanied by their P53 pathway activation whose pharmaceutical blockade can ameliorate hair graying in mice, enlightening a promising therapeutic avenue for this prevalent hair condition.

Heterochronic parabiosis induces stem cell revitalization and systemic rejuvenation across aged tissues

The young circulatory milieu capable of delaying aging in individual tissues is of interest as rejuvenation strategies, but how it achieves cellular- and systemic-level effects has remained unclear. Here, we constructed a single-cell transcriptomic atlas across aged tissues/organs and their rejuvenation in heterochronic parabiosis (HP), a classical model to study systemic aging. In general, HP rejuvenated adult stem cells and their niches across tissues. In particular, we identified hematopoietic stem and progenitor cells (HSPCs) as one of the most responsive cell types to young blood exposure, from which a continuum of cell state changes across the hematopoietic and immune system emanated, through the restoration of a youthful transcriptional regulatory program and cytokine-mediated cell-cell communications in HSPCs. Moreover, the reintroduction of the identified rejuvenating factors alleviated age-associated lymphopoiesis decline. Overall, we provide comprehensive frameworks to explore aging and rejuvenating trajectories at single-cell resolution and revealed cellular and molecular programs that instruct systemic revitalization by blood-borne factors.

Single-cell transcriptomics reveals age-resistant maintenance of cell identities, stem cell compartments and differentiation trajectories in long-lived naked mole-rats skin

Naked mole-rats (NMR) are subterranean rodents characterized by an unusual longevity coupled with an unexplained resistance to aging. In the present study, we performed extensive in situ analysis and single-cell RNA-sequencing comparing young and older animals. At variance with other species, NMR exhibited a striking stability of skin compartments and cell types, which remained stable over time without aging-associated changes. Remarkably, the number of stem cells was constant throughout aging. We found three classical cellular states defining a unique keratinocyte differentiation trajectory that were not altered after pseudo-temporal reconstruction. Epidermal gene expression did not change with aging either. Langerhans cell clusters were conserved, and only a higher basal stem cell expression of Igfbp3 was found in aged animals. In accordance, NMR skin healing closure was similar in young and older animals. Altogether, these results indicate that NMR skin is characterized by peculiar genetic and cellular features, different from those previously demonstrated for mice and humans. The remarkable stability of the aging NMR skin transcriptome likely reflects unaltered homeostasis and resilience.

Nutrition Interventions of Herbal Compounds on Cellular Senescence

When cells undergo large-scale senescence, organ aging ensues, resulting in irreversible organ pathology and organismal aging. The study of senescence in cells provides an important avenue to understand the factors that influence aging and can be used as one of the useful tools for examining age-related human diseases. At present, many herbal compounds have shown effects on delaying cell senescence. This review summarizes the main characteristics and mechanisms of cell senescence, age-related diseases, and the recent progress on the natural products targeting cellular senescence, with the aim of providing insights to aid the clinical management of age-related diseases.

Growth factors-based platelet lysate rejuvenates skin against ageing through NF-κB signalling pathway: In vitro and in vivo mechanistic and clinical studies

Introduction

Platelets benefit tissue regeneration by secreting growth factors, and platelet products, for example, platelet lysate (PL), have been clinically applied for tissue rejuvenation. To determine the anti‐ageing efficacy and mechanism of human PL (hPL) on skin, this study conducted clinical retrospective analysis, nude mice‐based in vivo study and human dermal fibroblasts (HDFs)‐based in vitro study.

Methods

Flow cytometry was employed for quality control of hPL, and ELISA was used for quantification of growth factors (EGF, IGF‐1, PDGF and TGF‐β) in hPL. After d‐galactose modelling, skin texture grading, histopathological observation, immunofluorescence analysis and oxidative stress assays were conducted on nude mice, while SA‐β‐gal staining, CCK‐8 and wound healing assays were conducted on HDFs. qPCR and western blot were conducted to clarify hPL's mechanism.

Results

The clinical retrospective data showed that hPL obviously rejuvenated human skin appearances without adverse events. The animal data showed that hPL exerted rejuvenative effects on skin, and the cellular data showed that hPL significantly promoted the proliferation and migration of HDFs and suppressed senescence‐associated secretory protein secretion and senescence state of senescent HDFs by suppressing NF‐κB pathway. The NF‐κB‐dependent mechanism was verified positively by using P65 siRNA and negatively by using prostratin. Furthermore, EGF, IGF‐1, PDGF and TGF‐β were found as the main ingredients in hPL, which contributed to the efficacy and mechanism of hPL.

Conclusion

This study provided novel knowledge of hPL, making it ideal for skin rejuvenation.

Single-cell transcriptomics in human skin research: available technologies, technical considerations, and disease applications

Single‐cell technologies have revolutionized research in the last decade, including for skin biology. Single‐cell RNA sequencing has emerged as a powerful tool allowing the dissection of human disease pathophysiology at unprecedented resolution by assessing cell‐to‐cell variation, facilitating identification of rare cell populations and elucidating cellular heterogeneity. In dermatology, this technology has been widely applied to inflammatory skin disorders, fibrotic skin diseases, wound healing complications and cutaneous neoplasms. Here, we discuss the available technologies and technical considerations of single‐cell RNA sequencing and describe its applications to a broad spectrum of dermatological diseases.

Neutrophils and their extracellular traps impair ablative fractional carbon dioxide laser-induced dermal remolding in mice

OBJECTIVES

Ablative fractional CO₂ laser (AFL) therapy is an effective intervention to induce dermal remodeling. AFL treatment of the skin triggers the recruitment of immune cells, with neutrophils dominating the early phase. However, the role of recruited neutrophils in AFL-induced microinjuries and their subsequent dermal remodeling capacity remains elusive.

MATERIALS AND METHODS

A mouse model of AFL-induced dermal remodeling was established. RNA sequencing was used to identify the prominent features of AFL-treated tissues. Histological analysis, including H&E and Masson staining, ultrastructure observation by transmission microscopy, immunofluorescence, and quantitative real-time polymerase chain reaction were used for dermal remodeling analysis. Moreover, AFL-treated mice were intraperitoneally injected with anti-mouse Ly6G antibodies to deplete neutrophils. Neutrophil extracellular traps (NETs) were explored using immunofluorescence, transmission microscopy, and in vitro coculture experiments.

RESULTS

Dermal remodeling, characterized by an increased number of CD31-positve vessels and elevated messenger RNA (mRNA) expression of genes encoding transforming growth factor-β (TGF-β), collagen I, and collagen III, was observed at 15 days after AFL treatment. In the AFL-induced inflammation phase, RNA sequencing identified neutrophil chemotaxis, and degranulation genes were significantly enriched. Histology and immunofluorescence staining of human and mouse tissues harvested at Day 1 after AFL treatment revealed significant neutrophil infiltration surrounding thermal-induced microinjuries. Neutrophil depletion decreased the expression of stress-related genes such as S100A8 and S100A9 in the early phase following AFL treatment. Importantly, neutrophil depletion enhanced dermal remodeling at Day 15, as reflected by enrichment of the extracellular matrix and collagen biosynthesis genes based on RNA sequencing. Moreover, increased collagen I, collagen III, and TGF-β mRNA expression, increased cell proliferation, and vascularity were observed. Interestingly, NETs, which could be induced by AFL-treated fibroblasts in vitro, were identified in both human and mouse tissues on Day 1 after AFL treatment.

CONCLUSIONS

AFL-treated human and mouse skin recruited a large number of neutrophils. The neutrophil surge impaired dermal remodeling in mice. The microenvironment and fibroblast functional modulation mediated by neutrophil degranulation and NET formation were determined to be the underlying mechanisms. Our results indicate that modification of infiltrated neutrophil activity might be a potential therapeutic target for AFL-induced dermal remodeling.

Isolating and cryopreserving pig skin cells for single-cell RNA sequencing study

The pig skin architecture and physiology are similar to those of humans. Thus, the pig model is very valuable for studying skin biology and testing therapeutics. The single-cell RNA sequencing (scRNA-seq) technology allows quantitatively analyzing cell types, compositions, states, signaling, and receptor-ligand interactome at single-cell resolution and at high throughput. scRNA-seq has been used to study mouse and human skins. However, studying pig skin with scRNA-seq is still rare. A critical step for successful scRNA-seq is to obtain high-quality single cells from the pig skin tissue. Here we report a robust method for isolating and cryopreserving pig skin single cells for scRNA-seq. We showed that pig skin could be efficiently dissociated into single cells with high cell viability using the Miltenyi Human Whole Skin Dissociation kit and the Miltenyi gentleMACS Dissociator. Furthermore, the obtained single cells could be cryopreserved using 90% FBS + 10% DMSO without causing additional cell death, cell aggregation, or changes in gene expression profiles. Using the developed protocol, we were able to identify all the major skin cell types. The protocol and results from this study are valuable for the skin research scientific community.

Temporal modelling using single-cell transcriptomics

Methods for profiling genes at the single-cell level have revolutionized our ability to study several biological processes and systems including development, differentiation, response programmes and disease progression. In many of these studies, cells are profiled over time in order to infer dynamic changes in cell states and types, sets of expressed genes, active pathways and key regulators. However, time-series single-cell RNA sequencing (scRNA-seq) also raises several new analysis and modelling issues. These issues range from determining when and how deep to profile cells, linking cells within and between time points, learning continuous trajectories, and integrating bulk and single-cell data for reconstructing models of dynamic networks. In this Review, we discuss several approaches for the analysis and modelling of time-series scRNA-seq, highlighting their steps, key assumptions, and the types of data and biological questions they are most appropriate for.

Systems approaches to investigate the role of NF-κB signaling in aging

The nuclear factor-κB (NF-κB) signaling pathway is one of the most well-studied pathways related to inflammation, and its involvement in aging has attracted considerable attention. As aging is a complex phenomenon and is the result of a multi-step process, the involvement of the NF-κB pathway in aging remains unclear. To elucidate the role of NF-κB in the regulation of aging, different systems biology approaches have been employed. A multi-omics data-driven approach can be used to interpret and clarify unknown mechanisms but cannot generate mechanistic regulatory structures alone. In contrast, combining this approach with a mathematical modeling approach can identify the mechanistics of the phenomena of interest. The development of single-cell technologies has also helped clarify the heterogeneity of the NF-κB response and underlying mechanisms. Here, we review advances in the understanding of the regulation of aging by NF-κB by focusing on omics approaches, single-cell analysis, and mathematical modeling of the NF-κB network.

Single-Cell RNA Profiling of Human Skin Reveals Age-Related Loss of Dermal Sheath Cells and Their Contribution to a Juvenile Phenotype

The dermal sheath (DS) is a population of mesenchyme-derived skin cells with emerging importance for skin homeostasis. The DS includes hair follicle dermal stem cells, which exhibit self-renewal and serve as bipotent progenitors of dermal papilla (DP) cells and DS cells. Upon aging, stem cells exhibit deficiencies in self-renewal and their number is reduced. While the DS of mice has been examined in considerable detail, our knowledge of the human DS, the pathways contributing to its self-renewal and differentiation capacity and potential paracrine effects important for tissue regeneration and aging is very limited. Using single-cell RNA sequencing of human skin biopsies from donors of different ages we have now analyzed the transcriptome of 72,048 cells, including 50,149 fibroblasts. Our results show that DS cells that exhibit stem cell characteristics were lost upon aging. We further show that HES1, COL11A1, MYL4 and CTNNB1 regulate DS stem cell characteristics. Finally, the DS secreted protein Activin A showed paracrine effects on keratinocytes and dermal fibroblasts, promoting proliferation, epidermal thickness and pro-collagen production. Our work provides a detailed description of human DS identity on the single-cell level, its loss upon aging, its stem cell characteristics and its contribution to a juvenile skin phenotype.

Single-Cell Transcriptomics Reveals the Molecular Anatomy of Sheep Hair Follicle Heterogeneity and Wool Curvature

Wool is the critical textile raw material which is produced by the hair follicle of sheep. Therefore, it has important implications to investigate the molecular mechanism governing hair follicle development. Due to high cellular heterogeneity as well as the insufficient cellular, molecular, and spatial characterization of hair follicles on sheep, the molecular mechanisms involved in hair follicle development and wool curvature of sheep remains largely unknown. Single-cell RNA sequencing (scRNA-seq) technologies have made it possible to comprehensively dissect the cellular composition of complex skin tissues and unveil the differentiation and spatial signatures of epidermal and hair follicle development. However, such studies are lacking so far in sheep. Here, single-cell suspensions from the curly wool and straight wool lambskins were prepared for unbiased scRNA-seq. Based on UAMP dimension reduction analysis, we identified 19 distinct cell populations from 15,830 single-cell transcriptomes and characterized their cellular identity according to specific gene expression profiles. Furthermore, novel marker gene was applied in identifying dermal papilla cells isolated in vitro. By using pseudotime ordering analysis, we constructed the matrix cell lineage differentiation trajectory and revealed the dynamic gene expression profiles of matrix progenitors' commitment to the hair shaft and inner root sheath (IRS) cells. Meanwhile, intercellular communication between mesenchymal and epithelial cells was inferred based on CellChat and the prior knowledge of ligand–receptor pairs. As a result, strong intercellular communication and associated signaling pathways were revealed. Besides, to clarify the molecular mechanism of wool curvature, differentially expressed genes in specific cells between straight wool and curly wool were identified and analyzed. Our findings here provided an unbiased and systematic view of the molecular anatomy of sheep hair follicle comprising 19 clusters; revealed the differentiation, spatial signatures, and intercellular communication underlying sheep hair follicle development; and at the same time revealed the potential molecular mechanism of wool curvature, which will give important new insights into the biology of the sheep hair follicle and has implications for sheep breeding.

Single cell analysis reveals inhibition of angiogenesis attenuates the progression of heterotopic ossification in Mkx-/- mice

Tendon heterotopic ossification (HO) is characterized by bone formation inside tendon tissue, which severely debilitates people in their daily life. Current therapies fail to promote functional tissue repair largely due to our limited understanding of HO pathogenesis. Here, we investigate the pathological mechanism and propose a potential treatment method for HO. Immunofluorescence assays showed that the Mohawk (MKX) expression level was decreased in human tendon HO tissue, coinciding with spontaneous HO and the upregulated expression of osteochondrogenic and angiogenic genes in the tendons of Mkx^-/- mice. Single-cell RNA sequencing analyses of wild-type and Mkx^-/- tendons identified three cell types and revealed the excessive activation of osteochondrogenic genes during the tenogenesis of Mkx^-/- tendon cells. Single-cell analysis revealed that the gene expression program of angiogenesis, which is strongly associated with bone formation, was activated in all cell types during HO. Moreover, inhibition of angiogenesis by the small-molecule inhibitor BIBF1120 attenuated bone formation and angiogenesis in the Achilles tendons of both Mkx mutant mice and a rat traumatic model of HO. These findings provide new insights into the cellular mechanisms of tendon HO and highlight the inhibition of angiogenesis with BIBF1120 as a potential treatment strategy for HO.

Challenges and Opportunities for the Translation of Single-Cell RNA Sequencing Technologies to Dermatology

Skin is a complex and heterogeneous organ at the cellular level. This complexity is beginning to be understood through the application of single-cell genomics and computational tools. A large number of datasets that shed light on how the different human skin cell types interact in homeostasis-and what ceases to work in diverse dermatological diseases-have been generated and are publicly available. However, translation of these novel aspects to the clinic is lacking. This review aims to summarize the state-of-the-art of skin biology using single-cell technologies, with a special focus on skin pathologies and the translation of mechanistic findings to the clinic. The main implications of this review are to summarize the benefits and limitations of single-cell analysis and thus help translate the emerging insights from these novel techniques to the bedside.

Role of distinct fibroblast lineages and immune cells in dermal repair following UV radiation-induced tissue damage

Solar ultraviolet radiation (UVR) is a major source of skin damage, resulting in inflammation, premature ageing, and cancer. While several UVR-induced changes, including extracellular matrix reorganisation and epidermal DNA damage, have been documented, the role of different fibroblast lineages and their communication with immune cells has not been explored. We show that acute and chronic UVR exposure led to selective loss of fibroblasts from the upper dermis in human and mouse skin. Lineage tracing and in vivo live imaging revealed that repair following acute UVR is predominantly mediated by papillary fibroblast proliferation and fibroblast reorganisation occurs with minimal migration. In contrast, chronic UVR exposure led to a permanent loss of papillary fibroblasts, with expansion of fibroblast membrane protrusions partially compensating for the reduction in cell number. Although UVR strongly activated Wnt signalling in skin, stimulation of fibroblast proliferation by epidermal β-catenin stabilisation did not enhance papillary dermis repair. Acute UVR triggered an infiltrate of neutrophils and T cell subpopulations and increased pro-inflammatory prostaglandin signalling in skin. Depletion of CD4- and CD8-positive cells resulted in increased papillary fibroblast depletion, which correlated with an increase in DNA damage, pro-inflammatory prostaglandins, and reduction in fibroblast proliferation. Conversely, topical COX-2 inhibition prevented fibroblast depletion and neutrophil infiltration after UVR. We conclude that loss of papillary fibroblasts is primarily induced by a deregulated inflammatory response, with infiltrating T cells supporting fibroblast survival upon UVR-induced environmental stress.

Artesunate treatment ameliorates ultraviolet irradiation-driven skin photoaging via increasing β-catenin expression

OBJECTIVE

Artesunate, a semi-synthetic derivative of artemisinin, exerts various pharmacological activities. Nevertheless, the effects of Art on skin photoaging remain unclear. Herein, we investigated whether Art ameliorated ultraviolet-irradiated skin photoaging in HaCaT cells and mice.

METHODS

To construct skin photoaging cellular models, HaCaT cells were irradiated by UV (UVB, 20mJ/cm²) for 5 days. HaCaT cells were pretreated with three concentrations of Art (1, 5 and 20 μg/ml) for 2 h each day. After 5 days, cell senescence, ROS production, SOD levels, p16^INK4a and β-catenin expression, proliferation and apoptosis were detected in HaCaT cells. Effects of Art on normal cells were investigated. After sh-β-catenin transfection or XAV-939 treatment, HaCaT cells were pretreated with 20 μg/ml Art and irradiated by UVB. After 5 days, skin photoaging was then observed. Furthermore, skin photoaging mouse models were established and the effects of Art and β-catenin silencing on skin photoaging were investigated.

RESULTS

Art treatment suppressed cell senescence, intracellular ROS production, p16^INK4a expression and apoptosis and promoted proliferation and SOD and β-catenin expression in UVB irradiated HaCaT cells. But Art had no toxic effects on normal cells. Silencing β-catenin by sh-β-catenin or XAV-939 exacerbated UVB irradiation-mediated cell senescence, apoptosis, and ROS production in HaCaT cells, which was ameliorated by Art treatment. The therapeutic effects of Art on skin photoaging were also confirmed in mouse models.

CONCLUSIONS

These findings suggested that Art treatment alleviated UVB irradiation-driven skin photoaging through enhancing β-catenin expression, which offered novel clues for pharmacological activity of Art.

Single-Cell RNA Sequencing and Assay for Transposase-Accessible Chromatin Using Sequencing Reveals Cellular and Molecular Dynamics of Aortic Aging in Mice

OBJECTIVE

The impact of vascular aging on cardiovascular diseases has been extensively studied; however, little is known regarding the cellular and molecular mechanisms underlying age-related vascular aging in aortic cellular subpopulations. Approach and Results: Transcriptomes and transposase-accessible chromatin profiles from the aortas of 4-, 26-, and 86-week-old C57/BL6J mice were analyzed using single-cell RNA sequencing and assay for transposase-accessible chromatin sequencing. By integrating the heterogeneous transcriptome and chromatin accessibility data, we identified cell-specific TF (transcription factor) regulatory networks and open chromatin states. We also determined that aortic aging affects cell interactions, inflammation, cell type composition, dysregulation of transcriptional control, and chromatin accessibility. Endothelial cells 1 have higher gene set activity related to cellular senescence and aging than do endothelial cells 2. Moreover, construction of senescence trajectories shows that endothelial cell 1 and fibroblast senescence is associated with distinct TF open chromatin states and an mRNA expression model.

CONCLUSIONS

Our data provide a system-wide model for transcriptional and epigenetic regulation during aortic aging at single-cell resolution.

A single-cell transcriptomic landscape of the lungs of patients with COVID-19

The lung is the primary organ targeted by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), making respiratory failure a leading coronavirus disease 2019 (COVID-19)-related mortality. However, our cellular and molecular understanding of how SARS-CoV-2 infection drives lung pathology is limited. Here we constructed multi-omics and single-nucleus transcriptomic atlases of the lungs of patients with COVID-19, which integrate histological, transcriptomic and proteomic analyses. Our work reveals the molecular basis of pathological hallmarks associated with SARS-CoV-2 infection in different lung and infiltrating immune cell populations. We report molecular fingerprints of hyperinflammation, alveolar epithelial cell exhaustion, vascular changes and fibrosis, and identify parenchymal lung senescence as a molecular state of COVID-19 pathology. Moreover, our data suggest that FOXO3A suppression is a potential mechanism underlying the fibroblast-to-myofibroblast transition associated with COVID-19 pulmonary fibrosis. Our work depicts a comprehensive cellular and molecular atlas of the lungs of patients with COVID-19 and provides insights into SARS-CoV-2-related pulmonary injury, facilitating the identification of biomarkers and development of symptomatic treatments.

Transcriptomic analysis of human skin wound healing and rejuvenation following ablative fractional laser treatment

Ablative fractional laser treatment is considered the gold standard for skin rejuvenation. In order to understand how fractional laser works to rejuvenate skin, we performed microarray profiling on skin biopsies to identify temporal and dose-response changes in gene expression following fractional laser treatment. The backs of 14 women were treated with ablative fractional laser (Fraxel®) and 4 mm punch biopsies were collected from an untreated site and at the treated sites 1, 3, 7, 14, 21 and 28 days after the single treatment. In addition, in order to understand the effect that multiple fractional laser treatments have on skin rejuvenation, several sites were treated sequentially with either 1, 2, 3, or 4 treatments (with 28 days between treatments) followed by the collection of 4 mm punch biopsies. RNA was extracted from the biopsies, analyzed using Affymetrix U219 chips and gene expression was compared between untreated and treated sites. We observed dramatic changes in gene expression as early as 1 day after fractional laser treatment with changes remaining elevated even after 1 month. Analysis of individual genes demonstrated significant and time related changes in inflammatory, epidermal, and dermal genes, with dermal genes linked to extracellular matrix formation changing at later time points following fractional laser treatment. When comparing the age-related changes in skin gene expression to those induced by fractional laser, it was observed that fractional laser treatment reverses many of the changes in the aging gene expression. Finally, multiple fractional laser treatments, which cover different regions of a treatment area, resulted in a sustained or increased dermal remodeling response, with many genes either differentially regulated or continuously upregulated, supporting previous observations that maximal skin rejuvenation requires multiple fractional laser treatments. In conclusion, fractional laser treatment of human skin activates a number of biological processes involved in wound healing and tissue regeneration.

Cross-tissue single-cell transcriptomic landscape reveals the key cell subtypes and their potential roles in the nutrient absorption and metabolism in dairy cattle

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Discover 55 cell types and their specific markers in the first single-cell atlas of cattle;

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Identify and verify 3 epithelial progenitor-like cell subtypes in the forestomach

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Reveal vital but nonimmune functions of neutrophils in the mammary gland;

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Uncover key cell subtypes with preferential nutrient uptake;

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Find Th17 cells regulate epithelial cells responding to nutrient transport in the forestomach.

Introduction

Dairy cattle are a vitally important ruminant in meeting the demands for high-quality animal protein production worldwide. The complicated biological process of converting human indigestible biomass into highly digestible and nutritious milk is orchestrated by various tissues. However, poorly understanding of the cellular composition and function of the key metabolic tissues hinders the improvement of health and performance of domestic ruminants.

Objectives

The cellular heterogeneity, metabolic features, interactions across ten tissue types of lactating dairy cattle were studied at single-cell resolution in the current study.

Methods

Unbiased single-cell RNA-sequencing and analysis were performed on the rumen, reticulum, omasum, abomasum, ileum, rectum, liver, salivary gland, mammary gland, and peripheral blood of lactating dairy cattle. Immunofluorescences and fluorescence in situ hybridization were performed to verify cell identity.

Results

In this study, we constructed a single-cell landscape covering 88,013 high-quality (500 < genes < 4,000, UMI < 50, 000, and mitochondrial gene ratio < 40% or 15%) single cells and identified 55 major cell types in lactating dairy cattle. Our systematic survey of the gene expression profiles and metabolic features of epithelial cells related to nutrient transport revealed cell subtypes that have preferential absorption of different nutrients. Importantly, we found that T helper type 17 (Th17) cells (highly expressing CD4 and IL17A) were specifically enriched in the forestomach tissues and predominantly interacted with the epithelial cell subtypes with high potential uptake capacities of short-chain fatty acids through IL-17 signaling. Furthermore, the comparison between IL17RA^highIL17RC^high cells (epithelial cells with IL17RA and IL17RC expression levels both greater than 0.25) and other cells explained the importance of Th17 cells in regulating the epithelial cellular transcriptional response to nutrient transport in the forestomach.

Conclusion

The findings enhance our understanding of the cellular biology of ruminants and open new avenues for improved animal production of dairy cattle.

Promises and challenges of senolytics in skin regeneration, pathology and ageing

The research of the last two decades has defined a crucial role of cellular senescence in both the physiology and pathology of skin, and senescent cells have been detected in conditions including development, regeneration, aging, and disease. The pathophysiology of cellular senescence in skin is complex as the phenotype of senescence pertains to several different cell types including fibroblasts, keratinocytes and melanocytes, among others. Paradoxically, the transient presence of senescent cells is believed to be beneficial in the context of development and wound healing, while the chronic presence of senescent cells is detrimental in the context of aging, diseases, and chronic wounds, which afflict predominantly the elderly. Identifying strategies to prevent senescence induction or reduce senescent burden in the skin could broadly benefit the aging population. Senolytics, drugs known to specifically eliminate senescent cells while preserving non-senescent cells, are being intensively studied for use in the clinical setting. Here, we review recent research on skin senescence, on the methods for the detection of senescent cells and describe promises and challenges related to the application of senolytic drugs. This article is part of the Special Issue - Senolytics - Edited by Joao Passos and Diana Jurk.

A stress-induced miR-31-CLOCK-ERK pathway is a key driver and therapeutic target for skin aging

Regressive changes in epithelial stem cells underlie mammalian skin aging, but the driving mechanisms are not well understood. Here, we report that mouse skin hair follicle stem cell (HFSC) aging is initiated by their intrinsic upregulation of miR-31, a microRNA that can be induced by physical injury or genotoxic stress and is also strongly upregulated in aged human skin epithelium. Using transgenic and conditional knockout mouse models plus a lineage-tracing technique, we show that miR-31 acts as a key driver of HFSC aging by directly targeting Clock, a core circadian clock gene whose deregulation activates a MAPK/ERK cascade to induce HFSC depletion via transepidermal elimination. Notably, blocking this pathway by either conditional miR-31 ablation or clinically approved MAPK/ERK inhibitors provides safe and effective protection against skin aging, enlightening a promising therapeutic avenue for treating skin aging and other genotoxic stress-induced skin conditions such as radiodermatitis.

Exploring Human Skin Aging at the Single-Cell Level

In this issue of Developmental Cell, Zou et al. utilize eyelid samples to examine human skin aging at the single-cell level. They discover photo- and inflammation-related changes already in middle age and find that restoring youthful expression of KLF6 and HES1 may dial back some age-associated changes.

The need to reassess single-cell RNA sequencing datasets: the importance of biological sample processing

Background: The advent of single-cell RNA sequencing (scRNAseq) and additional single-cell omics technologies have provided scientists with unprecedented tools to explore biology at cellular resolution. However, reaching an appropriate number of good quality reads per cell and reasonable numbers of cells within each of the populations of interest are key to infer relevant conclusions about the underlying biology of the dataset. For these reasons, scRNAseq studies are constantly increasing the number of cells analysed and the granularity of the resultant transcriptomics analyses. Methods: We aimed to identify previously described fibroblast subpopulations in healthy adult human skin by using the largest dataset published to date (528,253 sequenced cells) and an unsupervised population-matching algorithm. Results: Our reanalysis of this landmark resource demonstrates that a substantial proportion of cell transcriptomic signatures may be biased by cellular stress and response to hypoxic conditions. Conclusions: We postulate that careful design of experimental conditions is needed to avoid long processing times of biological samples. Additionally, computation of large datasets might undermine the extent of the analysis, possibly due to long processing times.

The need to reassess single-cell RNA sequencing datasets: more is not always better

Background: The advent of single-cell RNA sequencing (scRNAseq) and additional single-cell omics technologies have provided scientists with unprecedented tools to explore biology at cellular resolution. However, reaching an appropriate number of good quality reads per cell and reasonable numbers of cells within each of the populations of interest are key to infer conclusions from otherwise limited analyses. For these reasons, scRNAseq studies are constantly increasing the number of cells analysed and the granularity of the resultant transcriptomics analyses. Methods: We aimed to identify previously described fibroblast subpopulations in healthy adult human skin by using the largest dataset published to date (528,253 sequenced cells) and an unsupervised population-matching algorithm. Results: Our reanalysis of this landmark resource demonstrates that a substantial proportion of cell transcriptomic signatures may be biased by cellular stress and response to hypoxic conditions. Conclusions: We postulate that the "more is better" approach, currently prevalent in the scientific community, might undermine the extent of the analysis, possibly due to long computational processing times inherent to large datasets.

Epithelial Cell Transformation and Senescence as Indicators of Genome Aging: Current Advances and Unanswered Questions

The recent advances in deciphering the human genome allow us to understand and evaluate the mechanisms of human genome age-associated transformations, which are largely unclear. Genome sequencing techniques assure comprehensive mapping of human genetics; however, understanding of gene functional interactions, specifically of time/age-dependent modifications, remain challenging. The age of the genome is defined by the sum of individual (inherited) and acquired genomic traits, based on internal and external factors that impact ontogenesis from the moment of egg fertilization and embryonic development. The biological part of genomic age opens a new perspective for intervention. The discovery of single cell-based mechanisms for genetic change indicates the possibility of influencing aging and associated disease burden, as well as metabolism. Cell populations with transformed genetic background were shown to serve as the origin of common diseases during extended life expectancy (superaging). Consequently, age-related cell transformation leads to cancer and cell degeneration (senescence). This article aims to describe current advances in the genomic mechanisms of senescence and its role in the spatiotemporal spread of epithelial clones and cell evolution.

A genome-wide CRISPR-based screen identifies KAT7 as a driver of cellular senescence

Understanding the genetic and epigenetic bases of cellular senescence is instrumental in developing interventions to slow aging. We performed genome-wide CRISPR-Cas9-based screens using two types of human mesenchymal precursor cells (hMPCs) exhibiting accelerated senescence. The hMPCs were derived from human embryonic stem cells carrying the pathogenic mutations that cause the accelerated aging diseases Werner syndrome and Hutchinson-Gilford progeria syndrome. Genes whose deficiency alleviated cellular senescence were identified, including KAT7, a histone acetyltransferase, which ranked as a top hit in both progeroid hMPC models. Inactivation of KAT7 decreased histone H3 lysine 14 acetylation, repressed p15^INK4b transcription, and alleviated hMPC senescence. Moreover, lentiviral vectors encoding Cas9/sg-Kat7, given intravenously, alleviated hepatocyte senescence and liver aging and extended life span in physiologically aged mice as well as progeroid Zmpste24^-/- mice that exhibit a premature aging phenotype. CRISPR-Cas9-based genetic screening is a robust method for systematically uncovering senescence genes such as KAT7, which may represent a therapeutic target for developing aging interventions.

Cutting into wound repair

The skin is home to an assortment of fibroblastic lineages that shape the wound repair response toward scars or regeneration. In this review, we discuss the distinct embryonic origins, anatomic locations, and functions of fibroblastic lineages, and how these distinct lineages of fibroblasts dictate the skin's wound response across injury depths, anatomic locations, and embryonic development to promote either scarring or regeneration. We highlight the supportive role of the fascia in dictating scarring outcomes; we then discuss recent findings that indicate fascia mobilization by its resident fibroblasts supersede the classical de novo deposition program of wound matrix formation. These recent findings reconfigure our traditional view of wound repair and present exciting new therapeutic avenues to treat scarring and fibrosis across a range of medical settings.

Transcriptomic Analysis of Cellular Senescence: One Step Closer to Senescence Atlas

Senescent cells that gradually accumulate during aging are one of the leading causes of aging. While senolytics can improve aging in humans as well as mice by specifically eliminating senescent cells, the effect of the senolytics varies in different cell types, suggesting variations in senescence. Various factors can induce cellular senescence, and the rate of accumulation of senescent cells differ depending on the organ. In addition, since the heterogeneity is due to the spatiotemporal context of senescent cells, in vivo studies are needed to increase the understanding of senescent cells. Since current methods are often unable to distinguish senescent cells from other cells, efforts are being made to find markers commonly expressed in senescent cells using bulk RNA-sequencing. Moreover, single-cell RNA (scRNA) sequencing, which analyzes the transcripts of each cell, has been utilized to understand the in vivo characteristics of the rare senescent cells. Recently, transcriptomic cell atlases for each organ using this technology have been published in various species. Novel senescent cells that do not express previously established marker genes have been discovered in some organs. However, there is still insufficient information on senescent cells due to the limited throughput of the scRNA sequencing technology. Therefore, it is necessary to improve the throughput of the scRNA sequencing technology or develop a way to enrich the rare senescent cells. The in vivo senescent cell atlas that is established using rapidly developing single-cell technologies will contribute to the precise rejuvenation by specifically removing senescent cells in each tissue and individual.

Tissue Mechanics in Haired Murine Skin: Potential Implications for Skin Aging

During aging, the skin undergoes changes in architecture and composition. Skin aging phenotypes occur due to accumulated changes in the genome/epigenome, cytokine/cell adhesion, cell distribution/extracellular matrix (ECM), etc. Here we review data suggesting that tissue mechanics also plays a role in skin aging. While mouse and human skin share some similarities, their skin architectures differ in some respects. However, we use recent research in haired murine skin because of the available experimental data. Skin suffers from changes in both its appendages and inter-appendage regions. The elderly exhibit wrinkles and loose dermis and are more likely to suffer from wounds and superficial abrasions with poor healing. They also have a reduction in the number of skin appendages. While telogen is prolonged in aging murine skin, hair follicle stem cells can be rejuvenated to enter anagen if transplanted to a young skin environment. We highlight recent single-cell analyses performed on epidermis and aging human skin which identified new basal cell subpopulations that shift in response to wounding. This may be due to alterations of basement membrane stiffness which would change tissue mechanics in aging skin, leading to altered homeostatic dynamics. We propose that the extracellular matrix (ECM) may play a key role as a chemo-mechanical integrator of the multi-layered senescence-associated signaling pathways, dictating the tissue mechanical landscape of niche microenvironments in aging phenotypes. We show examples where failed chemo-mechanical signaling leads to deteriorating homeostasis during skin aging and suggest potential therapeutic strategies to guide future research to delay the aging processes.

Mtor inhibition by INK128 extends functions of the ovary reconstituted from germline stem cells in aging and premature aging mice

Stem cell transplantation has been generally considered as promising therapeutics in preserving or recovering functions of lost, damaged, or aging tissues. Transplantation of primordial germ cells (PGCs) or oogonia stem cells (OSCs) can reconstitute ovarian functions that yet sustain for only short period of time, limiting potential application of stem cells in preservation of fertility and endocrine function. Here, we show that mTOR inhibition by INK128 extends the follicular and endocrine functions of the reconstituted ovaries in aging and premature aging mice following transplantation of PGCs/OSCs. Follicular development and endocrine functions of the reconstituted ovaries by transplanting PGCs into kidney capsule of the recipient mice were maintained by INK128 treatment for more than 12 weeks, in contrast to the controls for only about 4 weeks without receiving the mTOR inhibitors. Comparatively, rapamycin also can prolong the ovarian functions but for limited time. Furthermore, our data reveal that INK128 promotes mitochondrial function in addition to its known function in suppression of immune response and inflammation. Taken together, germline stem cell transplantation in combination with mTOR inhibition by INK128 improves and extends the reconstituted ovarian and endocrine functions in reproductive aging and premature aging mice.