Acute Activation of Oxidative Pentose Phosphate Pathway as First-Line Response to Oxidative Stress in Human Skin Cells.

Integrity of human skin is endangered by exposure to UV irradiation and chemical stressors, which can provoke a toxic production of reactive oxygen species (ROS) and oxidative damage. Since oxidation of proteins and metabolites occurs virtually instantaneously, immediate cellular countermeasures are pivotal to mitigate the negative implications of acute oxidative stress. We investigated the short-term metabolic response in human skin fibroblasts and keratinocytes to H2O2 and UV exposure. In time-resolved metabolomics experiments, we observed that within seconds after stress induction, glucose catabolism is routed to the oxidative pentose phosphate pathway (PPP) and nucleotide synthesis independent of previously postulated blocks in glycolysis (i.e., of GAPDH or PKM2). Through ultra-short (13)C labeling experiments, we provide evidence for multiple cycling of carbon backbones in the oxidative PPP, potentially maximizing NADPH reduction. The identified metabolic rerouting in oxidative and non-oxidative PPP has important physiological roles in stabilization of the redox balance and ROS clearance.

Integrated analysis of motif activity and gene expression changes of transcription factors.

The ability to predict transcription factors based on sequence information in regulatory elements is a key step in systems-level investigation of transcriptional regulation. Here, we have developed a novel tool, IMAGE, for precise prediction of causal transcription factors based on transcriptome profiling and genome-wide maps of enhancer activity. High precision is obtained by combining a near-complete database of position weight matrices (PWMs), generated by compiling public databases and systematic prediction of PWMs for uncharacterized transcription factors, with a state-of-the-art method for PWM scoring and a novel machine learning strategy, based on both enhancers and promoters, to predict the contribution of motifs to transcriptional activity. We applied IMAGE to published data obtained during 3T3-L1 adipocyte differentiation and showed that IMAGE predicts causal transcriptional regulators of this process with higher confidence than existing methods. Furthermore, we generated genome-wide maps of enhancer activity and transcripts during human mesenchymal stem cell commitment and adipocyte differentiation and used IMAGE to identify positive and negative transcriptional regulators of this process. Collectively, our results demonstrate that IMAGE is a powerful and precise method for prediction of regulators of gene expression.

An integrative metabolomics and transcriptomics study to identify metabolic alterations in aged skin of humans in vivo.

BACKGROUND: Aging human skin undergoes significant morphological and functional changes such as wrinkle formation, reduced wound healing capacity, and altered epidermal barrier function. Besides known age-related alterations like DNA-methylation changes, metabolic adaptations have been recently linked to impaired skin function in elder humans. Understanding of these metabolic adaptations in aged skin is of special interest to devise topical treatments that potentially reverse or alleviate age-dependent skin deterioration and the occurrence of skin disorders. RESULTS: We investigated the global metabolic adaptions in human skin during aging with a combined transcriptomic and metabolomic approach applied to epidermal tissue samples of young and old human volunteers. Our analysis confirmed known age-dependent metabolic alterations, e.g. reduction of coenzyme Q10 levels, and also revealed novel age effects that are seemingly important for skin maintenance. Integration of donor-matched transcriptome and metabolome data highlighted transcriptionally-driven alterations of metabolism during aging such as altered activity in upper glycolysis and glycerolipid biosynthesis or decreased protein and polyamine biosynthesis. Together, we identified several age-dependent metabolic alterations that might affect cellular signaling, epidermal barrier function, and skin structure and morphology. CONCLUSIONS: Our study provides a global resource on the metabolic adaptations and its transcriptional regulation during aging of human skin. Thus, it represents a first step towards an understanding of the impact of metabolism on impaired skin function in aged humans and therefore will potentially lead to improved treatments of age related skin disorders.

Genome-wide expression analysis of wounded skin reveals novel genes involved in angiogenesis.

Wound healing is a multistage process involving collaborative efforts of different cell types and distinct cellular functions. Among others, the high metabolic activity at the wound site requires the formation and sprouting of new blood vessels (angiogenesis) to ensure an adequate supply of oxygen and nutrients for a successful healing process. Thus, a cutaneous wound healing model was established to identify new factors that are involved in vascular formation and remodeling in human skin after embryonic development. By analyzing global gene expression of skin biopsies obtained from wounded and unwounded skin, we identified a small set of genes that were highly significant differentially regulated in the course of wound healing. To initially investigate whether these genes might be involved in angiogenesis, we performed siRNA experiments and analyzed the knockdown phenotypes using a scratch wound assay which mimics cell migration and proliferation in vitro. The results revealed that a subset of these genes influence cell migration and proliferation in primary human endothelial cells (EC). Furthermore, histological analyses of skin biopsies showed that two of these genes, ALBIM2 and TMEM121, are colocalized with CD31, a well known EC marker. Taken together, we identified new genes involved in endothelial cell biology, which might be relevant to develop therapeutics not only for impaired wound healing but also for chronic inflammatory disorders and/or cardiovascular diseases.

Gluteal and abdominal subcutaneous adipose tissue depots as stroma cell source: gluteal cells display increased adipogenic and osteogenic differentiation potentials.

Human adipose-derived stroma cells (ADSCs) have successfully been employed in explorative therapeutic studies. Current evidence suggests that ADSCs are unevenly distributed in subcutaneous adipose tissue; therefore, the anatomical origin of ADSCs may influence clinical outcomes. This study was designed to investigate proliferation and differentiation capacities of ADSCs from the gluteal and abdominal depot of 8 females. All had normal BMI (22.01 ± 0.39 kg/m(2) ) and waist circumference (81.13 ± 2.33 cm). Examination by physicians and analysis of 31 laboratory parameters did not reveal possibly confounding medical disorders. Gluteal and abdominal adipose tissue was sampled by en bloc resection on day 7 (±1) after the last menses. Histological examination did not reveal significant depot-specific differences. As assessed by BrdU assay, proliferation of cells from both depots was similar after 24 h and analysis of 15 cell surface markers by flow cytometry identified the isolated cells as ADSCs, again without depot-specific differences. ADSCs from both depots differentiated poorly to chondroblasts. Gluteal ADSCs displayed significantly higher adipogenic differentiation potential than abdominal cells. Osteogenic differentiation was most pronounced in gluteal cells, whereas differentiation of abdominal ADSCs was severely impaired. Our data demonstrate a depot-specific difference in ADSC differentiation potential with abdominal cells failing to meet the criteria of multipotent ADSCs. This finding should be taken into account in future explorations of ADSC-derived therapeutic strategies.

Aging and chronic sun exposure cause distinct epigenetic changes in human skin.

Epigenetic changes are widely considered to play an important role in aging, but experimental evidence to support this hypothesis has been scarce. We have used array-based analysis to determine genome-scale DNA methylation patterns from human skin samples and to investigate the effects of aging, chronic sun exposure, and tissue variation. Our results reveal a high degree of tissue specificity in the methylation patterns and also showed very little interindividual variation within tissues. Data stratification by age revealed that DNA from older individuals was characterized by a specific hypermethylation pattern affecting less than 1% of the markers analyzed. Interestingly, stratification by sun exposure produced a fundamentally different pattern with a significant trend towards hypomethylation. Our results thus identify defined age-related DNA methylation changes and suggest that these alterations might contribute to the phenotypic changes associated with skin aging.

Development of an epigenetic clock to predict visual age progression of human skin.

Aging is a complex process characterized by the gradual decline of physiological functions, leading to increased vulnerability to age-related diseases and reduced quality of life. Alterations in DNA methylation (DNAm) patterns have emerged as a fundamental characteristic of aged human skin, closely linked to the development of the well-known skin aging phenotype. These changes have been correlated with dysregulated gene expression and impaired tissue functionality. In particular, the skin, with its visible manifestations of aging, provides a unique model to study the aging process. Despite the importance of epigenetic age clocks in estimating biological age based on the correlation between methylation patterns and chronological age, a second-generation epigenetic age clock, which correlates DNAm patterns with a particular phenotype, specifically tailored to skin tissue is still lacking. In light of this gap, we aimed to develop a novel second-generation epigenetic age clock explicitly designed for skin tissue to facilitate a deeper understanding of the factors contributing to individual variations in age progression. To achieve this, we used methylation patterns from more than 370 female volunteers and developed the first skin-specific second-generation epigenetic age clock that accurately predicts the skin aging phenotype represented by wrinkle grade, visual facial age, and visual age progression, respectively. We then validated the performance of our clocks on independent datasets and demonstrated their broad applicability. In addition, we integrated gene expression and methylation data from independent studies to identify potential pathways contributing to skin age progression. Our results demonstrate that our epigenetic age clock, VisAgeX, specifically predicting visual age progression, not only captures known biological pathways associated with skin aging, but also adds novel pathways associated with skin aging.

Identification of dihydromyricetin as a natural DNA methylation inhibitor with rejuvenating activity in human skin.

Changes in DNA methylation patterning have been reported to be a key hallmark of aged human skin. The altered DNA methylation patterns are correlated with deregulated gene expression and impaired tissue functionality, leading to the well-known skin aging phenotype. Searching for small molecules, which correct the aged methylation pattern therefore represents a novel and attractive strategy for the identification of anti-aging compounds. DNMT1 maintains epigenetic information by copying methylation patterns from the parental (methylated) strand to the newly synthesized strand after DNA replication. We hypothesized that a modest inhibition of this process promotes the restoration of the ground-state epigenetic pattern, thereby inducing rejuvenating effects. In this study, we screened a library of 1800 natural substances and 640 FDA-approved drugs and identified the well-known antioxidant and anti-inflammatory molecule dihydromyricetin (DHM) as an inhibitor of the DNA methyltransferase DNMT1. DHM is the active ingredient of several plants with medicinal use and showed robust inhibition of DNMT1 in biochemical assays. We also analyzed the effect of DHM in cultivated keratinocytes by array-based methylation profiling and observed a moderate, but significant global hypomethylation effect upon treatment. To further characterize DHM-induced methylation changes, we used published DNA methylation clocks and newly established age predictors to demonstrate that the DHM-induced methylation change is associated with a reduction in the biological age of the cells. Further studies also revealed re-activation of age-dependently hypermethylated and silenced genes in vivo and a reduction in age-dependent epidermal thinning in a 3-dimensional skin model. Our findings thus establish DHM as an epigenetic inhibitor with rejuvenating effects for aged human skin.

DNA methylation regulates lineage-specifying genes in primary lymphatic and blood endothelial cells.

During embryonic development, the lymphatic system emerges by transdifferentiation from the cardinal vein. Although lymphatic and blood vasculature share a close molecular and developmental relationship, they display distinct features and functions. However, even after terminal differentiation, transitions between blood endothelial cells (BEC) and lymphatic endothelial cells (LEC) have been reported. Since phenotypic plasticity and cellular differentiation processes frequently involve epigenetic mechanisms, we hypothesized that DNA methylation might play a role in regulating cell type-specific expression in endothelial cells. By analyzing global gene expression and methylation patterns of primary human dermal LEC and BEC, we identified a highly significant set of genes, which were differentially methylated and expressed. Pathway analyses of the differentially methylated and upregulated genes in LEC revealed involvement in developmental and transdifferentiation processes. We further identified a set of novel genes, which might be implicated in regulating BEC-LEC plasticity and could serve as therapeutic targets and/or biomarkers in vascular diseases associated with alterations in the endothelial phenotype.

Macrophages Are Polarized toward an Inflammatory Phenotype by their Aged Microenvironment in the Human Skin.

Aging of the skin is accompanied by cellular as well as tissue environmental changes, ultimately reducing the ability of the tissue to regenerate and adequately respond to external stressors. Macrophages are important gatekeepers of tissue homeostasis, and it has been reported that their number and phenotype change during aging in a site-specific manner. How aging affects human skin macrophages and what implications this has for the aging process in the tissue are still not fully understood. Using single-cell RNA-sequencing analysis, we show that there is at least a 50% increase of macrophages in human aged skin, which appear to have developed from monocytes and exhibit more proinflammatory M1-like characteristics. In contrast, the cell-intrinsic ability of aged monocytes to differentiate into M1 macrophages was reduced. Using coculture experiments with aged dermal fibroblasts, we show that it is the aged microenvironment that drives a more proinflammatory phenotype of macrophages in the skin. This proinflammatory M1-like phenotype in turn negatively influenced the expression of extracellular matrix proteins by fibroblasts, emphasizing the impact of the aged macrophages on the skin phenotype.

Modeling transcriptomic age using knowledge-primed artificial neural networks.

The development of 'age clocks', machine learning models predicting age from biological data, has been a major milestone in the search for reliable markers of biological age and has since become an invaluable tool in aging research. However, beyond their unquestionable utility, current clocks offer little insight into the molecular biological processes driving aging, and their inner workings often remain non-transparent. Here we propose a new type of age clock, one that couples predictivity with interpretability of the underlying biology, achieved through the incorporation of prior knowledge into the model design. The clock, an artificial neural network constructed according to well-described biological pathways, allows the prediction of age from gene expression data of skin tissue with high accuracy, while at the same time capturing and revealing aging states of the pathways driving the prediction. The model recapitulates known associations of aging gene knockdowns in simulation experiments and demonstrates its utility in deciphering the main pathways by which accelerated aging conditions such as Hutchinson-Gilford progeria syndrome, as well as pro-longevity interventions like caloric restriction, exert their effects.

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.

A new protocol for functional analysis of adipogenesis using reverse transfection technology and time-lapse video microscopy.

Since the worldwide increase in obesity represents a growing challenge for healthcare systems, research focusing on fat cell metabolism has become a focal point of interest. Here, we describe a small interfering RNA (siRNA)-technology-based screening method to study fat cell differentiation in human primary preadipocytes that could be further developed towards an automated middle-throughput screening procedure. First, we established optimal conditions for the reverse transfection of human primary preadipocytes demonstrating that an efficient reverse transfection of preadipocytes is technically feasible. Aligning the processes of reverse transfection and fat cell differentiation utilizing peroxisome proliferator-activated receptor gamma (PPAR gamma)-siRNA, we showed that preadipocyte differentiation was suppressed by knock-down of PPAR gamma, the key regulator of fat cell differentiation. The use of fluorescently labelled fatty acids in combination with fluorescence time-lapse microscopy over a longer period of time enabled us to quantify the PPAR gamma phenotype. Additionally, our data demonstrate that reverse transfection of human cultured preadipocytes with TIP60 (HIV-1 Tat-interacting protein 60)-siRNA lead to a TIP60 knock-down and subsequently inhibits fat cell differentiation, suggesting a role of this protein in human adipogenesis. In conclusion, we established a protocol that allows for an efficient functional and time-dependent analysis by quantitative time-lapse microscopy to identify novel adipogenesis-associated genes.

Multi-omics network analysis reveals distinct stages in the human aging progression in epidermal tissue.

In recent years, reports of non-linear regulations in age- and longevity-associated biological processes have been accumulating. Inspired by methodological advances in precision medicine involving the integrative analysis of multi-omics data, we sought to investigate the potential of multi-omics integration to identify distinct stages in the aging progression from ex vivo human skin tissue. For this we generated transcriptome and methylome profiling data from suction blister lesions of female subjects between 21 and 76 years, which were integrated using a network fusion approach. Unsupervised cluster analysis on the combined network identified four distinct subgroupings exhibiting a significant age-association. As indicated by DNAm age analysis and Hallmark of Aging enrichment signals, the stages captured the biological aging state more clearly than a mere grouping by chronological age and could further be recovered in a longitudinal validation cohort with high stability. Characterization of the biological processes driving the phases using machine learning enabled a data-driven reconstruction of the order of Hallmark of Aging manifestation. Finally, we investigated non-linearities in the mid-life aging progression captured by the aging phases and identified a far-reaching non-linear increase in transcriptional noise in the pathway landscape in the transition from mid- to late-life.

Concomitant DNA methylation and transcriptome signatures define epidermal responses to acute solar UV radiation.

The simultaneous analysis of different regulatory levels of biological phenomena by means of multi-omics data integration has proven an invaluable tool in modern precision medicine, yet many processes ultimately paving the way towards disease manifestation remain elusive and have not been studied in this regard. Here we investigated the early molecular events following repetitive UV irradiation of in vivo healthy human skin in depth on transcriptomic and epigenetic level. Our results provide first hints towards an immediate acquisition of epigenetic memories related to aging and cancer and demonstrate significantly correlated epigenetic and transcriptomic responses to irradiation stress. The data allowed the precise prediction of inter-individual UV sensitivity, and molecular subtyping on the integrated post-irradiation multi-omics data established the existence of three latent molecular phototypes. Importantly, further analysis suggested a form of melanin-independent DNA damage protection in subjects with higher innate UV resilience. This work establishes a high-resolution molecular landscape of the acute epidermal UV response and demonstrates the potential of integrative analyses to untangle complex and heterogeneous biological responses.

Human Adipose-Derived Mesenchymal Stromal/Stem Cell Spheroids Possess High Adipogenic Capacity and Acquire an Adipose Tissue-like Extracellular Matrix Pattern.

Adipose-derived mesenchymal stromal/stem cells (ASCs) represent a commonly used cell source for adipose tissue engineering. In this context, ASCs have routinely been cultured in conventional 2D culture and applied as single cell suspension for seeding onto scaffold materials or direct injection. However, this approach is associated with the loss of their intrinsic 3D microenvironment and leads to impaired regenerative capacity of the cells. Thus, the application of ASCs as self-assembled 3D spheroids with cells residing in their own matrix is an attractive alternative. However, characterization of the structural features and differentiation capacity of the spheroids is necessary to effectively apply them as building blocks in adipose tissue engineering. In this study, we focus on extracellular matrix (ECM) development in ASC spheroids, as well as adipogenic differentiation in comparison to conventional 2D culture using different induction protocols. Reproducible assembly of ASCs into spheroids was achieved within 24 h using the liquid overlay technique. Undifferentiated spheroids displayed a stromal ECM pattern, with fibronectin, collagen V, and VI as the main components. In the course of adipogenesis, a dynamic shift in the ECM composition toward an adipogenic phenotype was observed, associated with enhanced expression of laminin, collagen I, IV, V, and VI, similar to native fat. Furthermore, adipogenic differentiation was enhanced in spheroids as compared with 2D cultured cells, with the spheroids needing a distinctly shorter adipogenic stimulus to sustain adipogenesis, which was demonstrated based on analysis of triglyceride content and adipogenic marker gene expression. In summary, culturing ASCs as spheroids can enhance their adipogenic capacity and generate adipose-like microtissues, which may be a promising cell delivery strategy for adipose tissue engineering approaches. Impact statement Adipose-derived mesenchymal stromal/stem cells (ASCs) as a widely used cell source for adipose tissue engineering have been shown to be limited in their regenerative capacity when applied as single cells. As an alternative approach, the delivery as spheroids, consisting of cells in a 3D context, may be favorable. However, insights into extracellular matrix (ECM) development and efficient adipogenic differentiation are required for their effective application. In this study, we show that differentiated ASC spheroids develop an ECM, resembling native adipose tissue. Furthermore, the ASC spheroids exhibited a superior differentiation capacity as compared with conventional 2D culture, and required only a short adipogenic induction stimulus. Our results identify ASC-derived spheroids as an attractive cell delivery method for adipose tissue engineering approaches.

Methylation profiling identifies two subclasses of squamous cell carcinoma related to distinct cells of origin.

Cutaneous squamous cell carcinoma (cSCC) is the second most common skin cancer and usually progresses from a UV-induced precancerous lesion termed actinic keratosis (AK). Despite various efforts to characterize these lesions molecularly, the etiology of AK and its progression to cSCC remain partially understood. Here, we use Infinium MethylationEPIC BeadChips to interrogate the DNA methylation status in healthy, AK and cSCC epidermis samples. Importantly, we show that AK methylation patterns already display classical features of cancer methylomes and are highly similar to cSCC profiles. Further analysis identifies typical features of stem cell methylomes, such as reduced DNA methylation age, non-CpG methylation, and stem cell-related keratin and enhancer methylation patterns. Interestingly, this signature is detected only in half of the samples, while the other half shows patterns more closely related to healthy epidermis. These findings suggest the existence of two subclasses of AK and cSCC emerging from distinct keratinocyte differentiation stages.

Methacrylated gelatin/hyaluronan-based hydrogels for soft tissue engineering.

In vitro-generated soft tissue could provide alternate therapies for soft tissue defects. The aim of this study was to evaluate methacrylated gelatin/hyaluronan as scaffolds for soft tissue engineering and their interaction with human adipose-derived stem cells (hASCs). ASCs were incorporated into methacrylated gelatin/hyaluronan hydrogels. The gels were photocrosslinked with a lithium phenyl-2,4,6-trimethylbenzoylphosphinate photoinitiator and analyzed for cell viability and adipogenic differentiation of ASCs over a period of 30 days. Additionally, an angiogenesis assay was performed to assess their angiogenic potential. After 24 h, ASCs showed increased viability on composite hydrogels. These results were consistent over 21 days of culture. By induction of adipogenic differentiation, the mature adipocytes were observed after 7 days of culture, their number significantly increased until day 28 as well as expression of fatty acid binding protein 4 and adiponectin. Our scaffolds are promising as building blocks for adipose tissue engineering and allowed long viability, proliferation, and differentiation of ASCs.