β-Catenin Stabilization in Skin Fibroblasts Causes Fibrotic Lesions by Preventing Adipocyte Differentiation of the Reticular Dermis

Dermal white adipose tissue: Development and impact on hair follicles, skin defense, and fibrosis

Dermal white adipose tissue (DWAT) is a distinctive adipose depot located within the lower dermis of the skin. Its significance as an ancillary fat in skin homoeostasis has recently received increased attention. New research has revealed that DWAT responses to skin pathology and physiology changes, impacting skin development, hair cycling, defense mechanisms, and fibrotic conditions. In this review, we explore the developmental process of DWAT and the adipose commitment timing of hypodermal. We explore the development process of DWAT and its pivotal role in regulating the hair cycle. We conclude the antibacterial activity and reversible dedifferentiation of dermal adipocytes in response to skin defense. Furthermore, we underscore the potentially crucial yet underestimated anti-fibrotic functions of DWAT-derived adipokines and adipocyte-myofibroblast transition.

Clinical, mechanistic, and therapeutic landscape of cutaneous fibrosis

When dysregulated, skin fibrosis can lead to a multitude of pathologies. We provide a framework for understanding the wide clinical spectrum, mechanisms, and management of cutaneous fibrosis encompassing a variety of matrix disorders, fibrohistiocytic neoplasms, injury-induced scarring, and autoimmune scleroses. Underlying such entities are common mechanistic pathways that leverage morphogenic signaling, immune activation, and mechanotransduction to modulate fibroblast function. In light of the limited array of available treatments for cutaneous fibrosis, scientific insights have opened new therapeutic and investigative avenues for conditions that still lack effective interventions.

Developmental regulation of dermal adipose tissue by BCL11b

The distinct anatomic environment in which adipose tissues arise during organogenesis is a principle determinant of their adult expansion capacity. Metabolic disease results from a deficiency in hyperplastic adipose expansion within the dermal/subcutaneous depot; thus, understanding the embryonic origins of dermal adipose is imperative. Using single-cell transcriptomics throughout murine embryogenesis, we characterized cell populations, including Bcl11b + cells, that regulate the development of dermal white adipose tissue (dWAT). We discovered that BCL11b expression modulates the Wnt signaling microenvironment to enable adipogenic differentiation in the dermal compartment. Subcutaneous and visceral adipose arises from a distinct population of Nefl + cells during embryonic organogenesis, whereas Pi16 + /Dpp4 + fibroadipogenic progenitors support obesity-stimulated hypertrophic expansion in the adult. Together, these results highlight the unique regulatory pathways used by anatomically distinct adipose depots, with important implications for human metabolic disease.

Topographical changes in extracellular matrix during skin fibrosis and recovery can be evaluated using automated image analysis algorithms

Skin fibrosis is characterized by fibroblast activation and intradermal fat loss, resulting in excess deposition and remodeling of dermal extracellular matrix (ECM). The topography of the dominant ECM proteins, such as collagens, can indicate skin stiffness and remains understudied in evaluating fibrotic skin. Here, we adapted two different unbiased image analysis algorithms to define collagen topography and alignment in a genetically inducible and reversible Wnt activation fibrosis model. We demonstrated that Wnt-activated fibrotic skin has altered collagen fiber characteristics and a loss of collagen alignment, which were restored in the reversible model. This study highlights how unbiased algorithms can be used to analyze ECM topography, providing novel avenues to evaluate fibrotic skin onset, recovery, and treatment.

Profibrotic Subsets of SPP1+ Macrophages and POSTN+ Fibroblasts Contribute to Fibrotic Scarring in Acne Keloidalis

Acne keloidalis is a primary scarring alopecia characterized by longstanding inflammation in the scalp causing keloid-like scar formation and hair loss. Histologically, acne keloidalis is characterized by mixed leukocytic infiltrates in the acute stage followed by a granulomatous reaction and extensive fibrosis in the later stages. To further explore its pathogenesis, bulk RNA sequencing, single-cell RNA sequencing, and spatial transcriptomics were applied to occipital scalp biopsy specimens of lesional and adjacent no-lesional skin in patients with clinically active disease. Unbiased clustering revealed 19 distinct cell populations, including 2 notable populations: POSTN+ fibroblasts with enriched extracellular matrix signatures and SPP1+ myeloid cells with an M2 macrophage phenotype. Cell communication analyses indicated that fibroblasts and myeloid cells communicated by SPP1 signaling networks in lesional skin. A reverse transcriptomics in silico approach identified corticosteroids as possessing the capability to reverse the gene expression signatures of SPP1+ myeloid cells and POSTN+ fibroblasts. Intralesional corticosteroid injection greatly reduced SPP1 and POSTN gene expression as well as acne keloidalis disease activity. Spatial transcriptomics and immunofluorescence staining verified microanatomic specificity of SPP1+ myeloid cells and POSTN+ fibroblasts with disease activity. In summary, the communication between POSTN+ fibroblasts and SPP1+ myeloid cells by SPP1 axis may contribute to the pathogenesis of acne keloidalis.

Tissue fibrosis associated depletion of lipid-filled cells

Fibrosis is primarily described as the deposition of excessive extracellular matrix, but in many tissues it also involves a loss of lipid or lipid-filled cells. Lipid-filled cells are critical to tissue function and integrity in many tissues including the skin and lungs. Thus, loss or depletion of lipid-filled cells during fibrogenesis, has implications for tissue function. In some contexts, lipid-filled cells can impact ECM composition and stability, highlighting their importance in fibrotic transformation. Recent papers in fibrosis address this newly recognized fibrotic lipodystrophy phenomenon. Even in disparate tissues, common mechanisms are emerging to explain fibrotic lipodystrophy. These findings have implications for fibrosis in tissues composed of fibroblast and lipid-filled cell populations such as skin, lung, and liver. In this review, we will discuss the roles of lipid-containing cells, their reduction/loss during fibrotic transformation, and the mechanisms of that loss in the skin and lungs.

Modulating embryonic signaling pathways paves the way for regeneration in wound healing

Epithelial tissues, including the skin, are highly proliferative tissues with the capability to constant renewal and regeneration, a feature that is essential for survival as the skin forms a protective barrier against external insults and water loss. In adult mammalian skin, every injury will lead to a scar. The scar tissue that is produced to seal the wound efficiently is usually rigid and lacks elasticity and the skin's original resilience to external impacts, but also secondary appendages such as hair follicles and sebaceous glands. While it was long thought that hair follicles develop solely during embryogenesis, it is becoming increasingly clear that hair follicles can also regenerate within a wound. The ability of the skin to induce hair neogenesis following injury however declines with age. As fetal and neonatal skin have the remarkable capacity to heal without scarring, the recapitulation of a neonatal state has been a primary target of recent regenerative research. In this review we highlight how modulating dermal signaling or the abundance of specific fibroblast subsets could be utilized to induce de novo hair follicles within the wound bed, and thus to shift wound repair with a scar to scarless regeneration.

Bioactive materials for in vivo sweat gland regeneration

Loss of sweat glands (SwGs) commonly associated with extensive skin defects is a leading cause of hyperthermia and heat stroke. In vivo tissue engineering possesses the potential to take use of the body natural ability to regenerate SwGs, making it more conducive to clinical translation. Despite recent advances in regenerative medicine, reconstructing SwG tissue with the same structure and function as native tissue remains challenging. Elucidating the SwG generation mechanism and developing biomaterials for in vivo tissue engineering is essential for understanding and developing in vivo SwG regenerative strategies. Here, we outline the cell biology associated with functional wound healing and the characteristics of bioactive materials. We critically summarize the recent progress in bioactive material-based cell modulation approaches for in vivo SwG regeneration, including the recruitment of endogenous cells to the skin lesion for SwG regeneration and in vivo cellular reprogramming for SwG regeneration. We discussed the re-establishment of microenvironment via bioactive material-mediated regulators. Besides, we offer promising perspectives for directing in situ SwG regeneration via bioactive material-based cell-free strategy, which is a simple and effective approach to regenerate SwG tissue with both fidelity of structure and function. Finally, we discuss the opportunities and challenges of in vivo SwG regeneration in detail. The molecular mechanisms and cell fate modulation of in vivo SwG regeneration will provide further insights into the regeneration of patient-specific SwGs and the development of potential intervention strategies for gland-derived diseases.

Identifying characteristics of dermal fibroblasts in skin homeostasis and disease

Heterogeneous dermal fibroblasts are the main components that constitute the dermis. Distinct fibroblast subgroups show specific characteristics and functional plasticity that determine dermal structure during skin development and wound healing. Although researchers have described the roles of fibroblast subsets, this is not completely understood. We review recent evidence supporting understanding about the heterogeneity of fibroblasts. We summarize the origins and the identified profiles of fibroblast subpopulations. The characteristics of fibroblast subpopulations in both healthy and diseased states are highlighted, and the potential of subpopulations to be involved in wound healing in different ways was discussed. Additionally, we review the plasticity of subpopulations and the underlying signalling mechanisms. This review may provide greater insights into potential novel therapeutic targets and tissue regeneration strategies for the future.

CD201+ fascia progenitors choreograph injury repair

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

Macrophages and fibrosis: how resident and infiltrating mononuclear phagocytes account for organ injury, regeneration or atrophy

Mononuclear phagocytes (MP), i.e., monocytes, macrophages, and dendritic cells (DCs), are essential for immune homeostasis via their capacities to clear pathogens, pathogen components, and non-infectious particles. However, tissue injury-related changes in local microenvironments activate resident and infiltrating MP towards pro-inflammatory phenotypes that contribute to inflammation by secreting additional inflammatory mediators. Efficient control of injurious factors leads to a switch of MP phenotype, which changes the microenvironment towards the resolution of inflammation. In the same way, MP endorses adaptive structural responses leading to either compensatory hypertrophy of surviving cells, tissue regeneration from local tissue progenitor cells, or tissue fibrosis and atrophy. Under certain circumstances, MP contribute to the reversal of tissue fibrosis by clearance of the extracellular matrix. Here we give an update on the tissue microenvironment-related factors that, upon tissue injury, instruct resident and infiltrating MP how to support host defense and recover tissue function and integrity. We propose that MP are not intrinsically active drivers of organ injury and dysfunction but dynamic amplifiers (and biomarkers) of specific tissue microenvironments that vary across spatial and temporal contexts. Therefore, MP receptors are frequently redundant and suboptimal targets for specific therapeutic interventions compared to molecular targets upstream in adaptive humoral or cellular stress response pathways that influence tissue milieus at a contextual level.

Cutaneous changes in diabetic patients: Primed for aberrant healing?

Cutaneous manifestations affect most patients with diabetes mellitus, clinically presenting with numerous dermatologic diseases from xerosis to diabetic foot ulcers (DFUs). Skin conditions not only impose a significantly impaired quality of life on individuals with diabetes but also predispose patients to further complications. Knowledge of cutaneous biology and the wound healing process under diabetic conditions is largely limited to animal models, and studies focusing on biology of the human condition of DFUs remain limited. In this review, we discuss the critical molecular, cellular, and structural changes to the skin in the hyperglycaemic and insulin-resistant environment of diabetes with a focus specifically on human-derived data. Elucidating the breadth of the cutaneous manifestations coupled with effective diabetes management is important for improving patient quality of life and averting future complications including wound healing disorders.

Rapid mechanosensitive migration and dispersal of newly divided mesenchymal cells aid their recruitment into dermal condensates

Embryonic mesenchymal cells are dispersed within an extracellular matrix but can coalesce to form condensates with key developmental roles. Cells within condensates undergo fate and morphological changes and induce cell fate changes in nearby epithelia to produce structures including hair follicles, feathers, or intestinal villi. Here, by imaging mouse and chicken embryonic skin, we find that mesenchymal cells undergo much of their dispersal in early interphase, in a stereotyped process of displacement driven by 3 hours of rapid and persistent migration followed by a long period of low motility. The cell division plane and the elevated migration speed and persistence of newly born mesenchymal cells are mechanosensitive, aligning with tissue tension, and are reliant on active WNT secretion. This behaviour disperses mesenchymal cells and allows daughters of recent divisions to travel long distances to enter dermal condensates, demonstrating an unanticipated effect of cell cycle subphase on core mesenchymal behaviour.

Skin fibroblast functional heterogeneity in health and disease

Fibroblasts are the major cell population of connective tissue, including the skin dermis, and are best known for their function in depositing and remodelling the extracellular matrix. Besides their role in extracellular matrix homeostasis, fibroblasts have emerged as key players in many biological processes ranging from tissue immunity and wound healing to hair follicle development. Recent advances in single-cell RNA-sequencing technologies have revealed an astonishing transcriptional fibroblast heterogeneity in the skin and other organs. A key challenge in the field is to understand the functional relevance and significance of the identified new cell clusters in health and disease. Here, we discuss the functionally distinct fibroblast subtypes identified in skin homeostasis and repair and how they evolve in fibrotic disease conditions, in particular keloid scars and cancer. © 2023 The Authors. The Journal of Pathology published by John Wiley & Sons Ltd on behalf of The Pathological Society of Great Britain and Ireland.

Promising small molecule anti-fibrotic agents: Newly developed or repositioned drugs targeting myofibroblast transdifferentiation

Fibrosis occurs in all organs and tissues except the brain, and its progression leads to dysfunction of affected organs. Fibrosis-induced organ dysfunction results from the loss of elasticity, strength, and functionality of tissues due to the extracellular matrix secreted by myofibroblasts that express smooth muscle-type actin as a marker. Myofibroblasts, which play a major role in fibrosis, were once thought to originate exclusively from activated fibroblasts; however, it is now clear that myofibroblasts are diverse in origin, from epithelial cells, endothelial cells, adipocytes, macrophages, and other cells. Fibrosis of vital organs, such as the heart, lungs, kidneys, and liver, is a serious chronic disease that ultimately leads to death. Currently, anti-cancer drugs have made remarkable progress, as evidenced by the development of many molecular-targeted drugs, and are making a significant contribution to improving the prognosis of cancer treatment. However, the development of anti-fibrotic agents, which also play an important role in prognosis, has lagged. In this review, the current knowledge regarding myofibroblasts is summarized, with particular attention given to their origin and transdifferentiation signaling pathways (e.g., TGF-β, Wnt/β-catenin, YAP/TAZ and AMPK signaling pathways). The development of new small molecule anti-fibrotic agents and the repositioning of existing drugs targeting myofibroblast transdifferentiation are discussed.

Dynamic interplay between IL-1 and WNT pathways in regulating dermal adipocyte lineage cells during skin development and wound regeneration

Dermal adipocyte lineage cells are highly plastic and can undergo reversible differentiation and dedifferentiation in response to various stimuli. Using single-cell RNA sequencing of developing or wounded mouse skin, we classify dermal fibroblasts (dFBs) into distinct non-adipogenic and adipogenic cell states. Cell differentiation trajectory analyses identify IL-1-NF-κB and WNT-β-catenin as top signaling pathways that positively and negatively associate with adipogenesis, respectively. Upon wounding, activation of adipocyte progenitors and wound-induced adipogenesis are mediated in part by neutrophils through the IL-1R-NF-κB-CREB signaling axis. In contrast, WNT activation, by WNT ligand and/or ablation of Gsk3, inhibits the adipogenic potential of dFBs but promotes lipolysis and dedifferentiation of mature adipocytes, contributing to myofibroblast formation. Finally, sustained WNT activation and inhibition of adipogenesis is seen in human keloids. These data reveal molecular mechanisms underlying the plasticity of dermal adipocyte lineage cells, defining potential therapeutic targets for defective wound healing and scar formation.

Inflammation in Wound Healing and Pathological Scarring

Significance: The aberrant inflammation during wound healing results in pathological scarring, such as hypertrophic scars and keloids. This adversely affects the quality of life of patients due to the disfiguring appearance as well as the symptoms of itch and pain. This review summarizes the up-to-date knowledge of the immunopathogenesis and treatment options for pathological scars. Recent Advances: With the advent of new technologies, combined with in vitro and in vivo wound models, several inflammatory cells have been shown to have both direct and indirect effects on both wound healing and pathological scarring. Critical Issues: Expansion of pro-fibrotic immune cells such as M2 macrophages, dendritic cells, mast cells, and Th2 cells leads to fibroblast transition to myofibroblasts via transforming growth factor-β1 signaling pathway. Appropriate management of such inflammatory responses during wound healing remains a critical issue during clinical practice. Future Directions: Regulating inflammation response during wound healing may be a potential therapeutic option for avoiding or reducing pathological scars.

Wound-Induced Hair Neogenesis: A Portal to the Development of New Therapies for Hair Loss and Wound Regeneration

Adult mammals retain the remarkable ability to regenerate hair follicles after wounding. Wound-induced hair neogenesis (WIHN) in many ways recapitulates embryogenesis. The origin of the stem cells that give rise to a nascent hair follicle after wounding and the role of mesenchymal cells and signaling pathways responsible for this regenerative phenomenon are slowly being elucidated. WIHN provides a potential therapeutic window for manipulating cell fate by the introduction of factors during the wound healing process to enhance hair follicle formation.

The origins of skin diversity: lessons from dermal fibroblasts

Skin is largely composed of an epidermis that overlies a supporting dermis. Recent advancements in our understanding of how diverse groups of dermal fibroblasts regulate epidermal and hair follicle growth and differentiation have been fueled by tools capable of resolving molecular heterogeneity at a single-cell level. Fibroblast heterogeneity can be traced back to their developmental origin before their segregation into spatially distinct fibroblast subtypes. The mechanisms that drive this lineage diversification during development are being unraveled, with studies showing that both large- and small-scale positional signals play important roles during dermal development. Here, we first delineate what is known about the origins of the dermis and the central role of Wnt/β-catenin signaling in its specification across anatomical locations. We then discuss how one of the first morphologically recognizable fibroblast subtypes, the hair follicle dermal condensate lineage, emerges. Leveraging the natural variation of skin and its appendages between species and between different anatomical locations, these collective studies have identified shared and divergent factors that contribute to the extraordinary diversity of skin.

Fat transplantation induces dermal adipose regeneration and reverses skin fibrosis through dedifferentiation and redifferentiation of adipocytes

Background

Localized scleroderma causes cosmetic disfigurement, joint contractures, and other functional impairment, but no currently available medications can reverse the resulting skin lesions. Fat grafting is beneficial for reversing skin fibrosis; however, the mechanism by which adipose tissue transplantation contributes to lesion improvement has not been fully clarified. The purpose of our study was to verify the therapeutic effect of fat grafts in reversing skin fibrosis.

Methods

Inguinal fat pads from AdipoqCreER+;mT/mG mice, which were treated with tamoxifen, were transplanted to the skin lesion in bleomycin-treated wild-type C57 mice. Tdtomato transgenic mice-derived adipocytes, adipose-derived stem cells (ASCs), dedifferentiated adipocytes (DAs) were embedded in matrigel and transplanted beneath the skin lesion of bleomycin-treated wild-type C57 mice. A transwell co‐culture system was used to verify the effect of ASCs, adipocytes or DAs on scleroderma fibroblasts or monocytes.

Results

Adipocytes from the fat grafts could undergo dedifferentiation and redifferentiation for dermal adipose tissue re-accumulation within the skin lesion. Moreover, compared with ASCs and adipocytes, DAs show greater potency of inducing adipogenesis. ASCs and DAs showed comparable effect on inducing angiogenesis and suppressing macrophage infiltration in fibrotic skin. Co-culture assay showed that DAs and ASCs were able to reduce fibrosis-related genes in human scleroderma fibroblasts and drive M2 macrophage polarization.

Conclusion

Our results indicated that adipocytes would transform into a more functional and dedifferentiated state and reverse dermal fibrosis, by promoting dermal adipose tissue regeneration, improving angiogenesis, suppressing macrophage-mediated inflammation and myofibroblast accumulation.

Fibroblast Heterogeneity in Healthy and Wounded Skin

Fibroblasts are the main cell type in the dermis. They are responsible for the synthesis and deposition of structural proteins such as collagen and elastin, which are integrated into the extracellular matrix (ECM). Mouse and human studies using flow cytometry, cell culture, skin reconstitution, and lineage tracing experiments have shown the existence of different subpopulations of fibroblasts, including papillary fibroblasts, reticular fibroblasts, and fibroblasts comprising the dermal papilla at the base of the hair follicle. In recent years, the technological advances in single-cell sequencing have allowed researchers to study the repertoire of cells present in full-thickness skin including the dermis. Multiple groups have confirmed that distinct fibroblast populations can be identified in mouse and human dermis on the basis of differences in the transcriptional profile. Here, we discuss the current state of knowledge regarding dermal fibroblast heterogeneity in healthy mouse and human skin, highlighting the similarities and differences between mouse and human fibroblast subpopulations. We also discuss how fibroblast heterogeneity may provide insights into physiological wound healing and its dysfunction in pathological states such as hypertrophic and keloid scars.

Skin Fibrosis and Recovery Is Dependent on Wnt Activation via DPP4

Fibrosis is the life-threatening, excessive accumulation of the extracellular matrix and is sometimes associated with a loss of lipid-filled cells in the skin and other organs. Understanding the mechanisms of fibrosis and associated lipodystrophy and their reversal may reveal new targets for therapeutic intervention. In vivo genetic models are needed to identify key targets that induce recovery from established fibrosis. Wnt signaling is activated in animal and human fibrotic diseases across organs. Here, we developed a genetically inducible and reversible Wnt activation model and showed that it is sufficient to cause fibrotic dermal remodeling, including extracellular matrix expansion and shrinking of dermal adipocytes. Upon withdrawal from Wnt activation, Wnt-induced fibrotic remodeling was reversed in mouse skin-fully restoring skin architecture. Next, we demonstrated CD26/ DPP4 is a Wnt/β-catenin-responsive gene and a functional mediator of fibrotic transformation. We provide genetic evidence that the Wnt/DPP4 axis is required to drive fibrotic dermal remodeling and is associated with human skin fibrosis severity. Remarkably, DPP4 inhibitors can be repurposed to accelerate recovery from established Wnt-induced fibrosis. Collectively, this study identifies Wnt/DPP4 axis as a key driver of extracellular matrix homeostasis and dermal fat loss, providing therapeutic avenues to manipulate the onset and reversal of tissue fibrosis.

β-Catenin Signaling Evokes Hair Follicle Senescence by Accelerating the Differentiation of Hair Follicle Mesenchymal Progenitors

Rationale: β-catenin signaling controls multiple fibroblast subsets, with its overactivity promoting the differentiation of hair follicle dermal stem cells (hfDSCs) and the hyperactivation of interfollicular fibroblasts. Understanding the concept of hfDSC activation and modulation offers hope towards the therapeutic armamentarium in dermatology and related comorbidities, as well as their potential applications in gerontology (the study of physiological aging). Having a comprehensive understanding in this stochastic process could also further yield important, novel insights into the molecular basis of skin aging to improve lifespan and preventing aging-related diseases.

Methods: A new CD34CrePGR mouse line was generated. Through fate-tracing models and a series of β-catenin genetic experiments, our study depicts how the wound environment increases phosphorylated β-catenin in hfDSCs and facilitates their differentiation into dermal papilla (DP) and dermal sheath (DS). In mice carrying hfDSC-specific activated allele of β-catenin, hfDSCs accelerated their differentiation into DP cells.

Results: Notably, with β-catenin stabilization in CD34-expressing cells and potential activation of canonical Wnt signaling, the mutant mice showed a brief increase of hair density in the short term, but over time leads to a senescence phenotype developing premature canities and thinning [hair follicle (HF) miniaturization].

Conclusion: β-catenin signaling drove HF senescence by accelerating differentiation of CD34⁺ hfDSCs, resulting in phenotypes attributable to the differentiation of the hfDSCs into DP cells and the loss of their stem cell potential. Therefore, our study reveals that the regulation of β-catenin signaling in hfDSCs may potentially become an important subject for future exploration in development of clinically effective therapies for hair loss treatment and an excellent model for revealing new therapeutic approaches to reverse aging or retarding the development of alopecia.

Mechanical stretching can modify the papillary dermis pattern and papillary fibroblast characteristics during skin regeneration

Clinical application of mechanical stretching is a reconstructive method for skin repair. Although studies have reported dermal fibroblast heterogeneity, whether stretching affects individual fibroblast subpopulations equally remains unclear. Here, we show the changes in dermal structure and papillary fibroblast (Fp) in regenerated human skin. Exhausted skin regeneration caused dermal-epidermal junction (DEJ) flattening, papillary dermis thinning, and an increase in the type III collagen (COL3)/type I collagen (COL1) ratio with upregulated hallmarks of aging. Well-regenerated skin displayed a notable increase in the Fp population. Consistent changes were observed in the rat expansion model. Moreover, we found that TGFβ1 expression was especially increased in skin showing good regeneration. Activation of the TGFβ1/Smad2/3 pathway improved exhausted skin regeneration and resulted in increased collagen content and Fp proliferation, while pharmacological inhibition of TGFβ1 action impacted well-regenerated skin. Short-term mechanical stretching that promoted skin regeneration enhanced Fp proliferation, extracellular matrix (ECM) synthesis, and increased TGFβ1 expression, leading to good regeneration. Conversely, long-term stretching induced premature Fp senescence, leading to poor regeneration. This work shows the mechanism of mechanical stretching in well skin regeneration that enhances Fp proliferation and ECM synthesis via the TGFβ1/Smad2/3 pathway, and highlights a crucial role of Fps in stretching-induced skin regeneration.

Hedgehog-signalling in papillary fibroblasts is essential for hair follicle regeneration during wound healing

Patients suffering from large scars such as burn victims not only encounter aesthetical challenges but also ongoing itching or pain that substantially deteriorates their quality of life. Skin appendages such as hair follicles (HFs) rarely regenerate within the healing wound. As they are crucial for skin homeostasis and the lack thereof constitutes one of the main limitations to scarless wound healing, their regeneration represents a major objective for regenerative medicine. Fibroblasts, the main resident cell type of the skin dermis, mediate embryonic HF morphogenesis and are particularly involved in wound healing as they orchestrate extracellular matrix (ECM) remodeling and collagen deposition in the wound bed. Importantly, dermal fibroblasts originate from two distinct developmental lineages with unique functions that differently mediate the response to epidermal signals such as Hedgehog (Hh)-signalling. In the present study, we show that Hh-signalling in the reticular fibroblast lineage promotes the initial phase of wound repair, possibly by modulating angiogenesis and fibroblast proliferation, while Hh-signalling in papillary fibroblasts is essential to induce de novo HF formation within the healing wound.

Differentiation of the tumor microenvironment: are CAFs the Organizer?

Cancers contain a suite of genetically stable cells within an extracellular matrix, collectively termed the tumor microenvironment (TME). The TME strongly influences disease outcome for patients. Gleaning clues from the literature, we propose that the TME should be viewed not as disparate populations of cells constituting a pathological lesion, but as a cohesive tissue constituting a novel pathological organ, arising from the coordinated differentiation of its constituent cell types - a process we have termed tumor-associated neodifferentiation (TAND). We also discuss why cancer-associated fibroblasts (CAFs) may assume the role of Organizer of this organ, directing the recruitment and differentiation of cells within the TME. Viewing the microenvironment in this way will reveal new cancer vulnerabilities that may be exploited for therapy.

Systemic mesalazine treatment prevents spontaneous skin fibrosis in PLK2-deficient mice

Skin fibrosis is a complex biological remodeling process occurring in disease like systemic sclerosis, morphea, or eosinophilic fasciitis. Since the knowledge about the underlying pathomechanisms is still incomplete, there is currently no therapy, which prevents or reverses skin fibrosis sufficiently. The present study investigates the role of polo-like kinase 2 (PLK2) and the pro-fibrotic cytokine osteopontin (OPN) in the pathogenesis of cutaneous fibrosis and demonstrates the antifibrotic effects of systemic mesalazine treatment in vivo. Isolated primary dermal fibroblasts of PLK2 wild-type (WT) and knockout (KO) mice were characterized in vitro. Skin thickness and histoarchitecture were studied in paraffin-embedded skin sections. The effects of mesalazine treatment were examined in isolated fibroblasts and PLK2 KO mice, which were fed 100 µg/g mesalazine for 6 months via the drinking water. Compared to WT, PLK2 KO fibroblasts displayed higher spontaneous myofibroblast differentiation, reduced proliferation rates, and overexpression of the fibrotic cytokine OPN. In vitro, 72 h of treatment with 10 mmol/L mesalazine induced phenotype conversion in PLK2 KO fibroblasts and attenuated OPN expression by inhibiting ERK1/2. In vivo, dermal myofibroblast differentiation, collagen accumulation, and skin thickening were prevented by mesalazine in PLK2 KO. Plasma creatinine levels indicated good tolerability of systemic long-term mesalazine treatment. The current study reveals a spontaneous fibrotic skin phenotype and ERK1/2-dependent OPN overexpression in PLK2 KO mice. We provide experimental evidence for the antifibrotic effectiveness of systemic mesalazine treatment to prevent fibrosis of the skin, suggesting further investigation in experimental and clinical settings.

Insight into the role of dermal white adipose tissue loss in dermal fibrosis

The loss of dermal white adipose tissue (dWAT) is vital to the formation of dermal fibrosis (DF), but the specific mechanism is not well understood. A few studies are reviewed to explore the role of dWAT in the formation of DF. Recent findings indicated that the adipocytes-to-myofibroblasts transition in dWAT reflects the direct contribution to the DF formation. While adipose-derived stem cells (ADSCs) contained in dWAT express antifibrotic cytokines, the loss of ADSCs leads to skin protection decreased, which indirectly exacerbates DF and tissue damage. Therefore, blocking or reversing the adipocytes-to-myofibroblasts transition or improving the survival of ADSCs in dWAT and the expression of antifibrotic cytokines may be an effective strategy for the treatment of DF.

The role of Wnt signaling in skin fibrosis

Skin fibrosis is the excessive deposition of extracellular matrix in the dermis. Cutaneous fibrosis can occur following tissue injury, including burns, trauma, and surgery, resulting in scars that are disfiguring, limit movement and cause significant psychological distress for patients. Many molecular pathways have been implicated in the development of skin fibrosis, yet effective treatments to prevent or reverse scarring are unknown. The Wnt signaling pathways are known to play an important role in skin homeostasis, skin injury, and in the development of fibrotic skin diseases. This review provides a detailed overview of the role of the canonical Wnt signaling pathways in regulating skin scarring. We also discuss how Wnt signaling interacts with other known fibrotic molecular pathways to cause skin fibrosis. We further provide a summary of the different Wnt inhibitor types available for treating skin scarring. Understanding the role of the Wnt pathway in cutaneous fibrosis will accelerate the development of effective Wnt modulators for the treatment of skin fibrosis.

A Scarless Healing Tale: Comparing Homeostasis and Wound Healing of Oral Mucosa With Skin and Oesophagus

Epithelial tissues are the most rapidly dividing tissues in the body, holding a natural ability for renewal and regeneration. This ability is crucial for survival as epithelia are essential to provide the ultimate barrier against the external environment, protecting the underlying tissues. Tissue stem and progenitor cells are responsible for self-renewal and repair during homeostasis and following injury. Upon wounding, epithelial tissues undergo different phases of haemostasis, inflammation, proliferation and remodelling, often resulting in fibrosis and scarring. In this review, we explore the phenotypic differences between the skin, the oesophagus and the oral mucosa. We discuss the plasticity of these epithelial stem cells and contribution of different fibroblast subpopulations for tissue regeneration and wound healing. While these epithelial tissues share global mechanisms of stem cell behaviour for tissue renewal and regeneration, the oral mucosa is known for its outstanding healing potential with minimal scarring. We aim to provide an updated review of recent studies that combined cell therapy with bioengineering exporting the unique scarless properties of the oral mucosa to improve skin and oesophageal wound healing and to reduce fibrotic tissue formation. These advances open new avenues toward the ultimate goal of achieving scarless wound healing.

Fibroblasts: Origins, definitions, and functions in health and disease

Fibroblasts are diverse mesenchymal cells that participate in tissue homeostasis and disease by producing complex extracellular matrix and creating signaling niches through biophysical and biochemical cues. Transcriptionally and functionally heterogeneous across and within organs, fibroblasts encode regional positional information and maintain distinct cellular progeny. We summarize their development, lineages, functions, and contributions to fibrosis in four fibroblast-rich organs: skin, lung, skeletal muscle, and heart. We propose that fibroblasts are uniquely poised for tissue repair by easily reentering the cell cycle and exhibiting a reversible plasticity in phenotype and cell fate. These properties, when activated aberrantly, drive fibrotic disorders in humans.

Distinct Fibroblast Lineages Give Rise to NG2+ Pericyte Populations in Mouse Skin Development and Repair

We have examined the developmental origins of Ng2+ perivascular cell populations that adhere to the basement membrane of blood vessels, and their contribution to wound healing. Neural/glial antigen 2 (Ng2) labeled most perivascular cells (70-80%) in developing and adult mouse back skin, a higher proportion than expressed by other pericyte markers Tbx18, Nestin and Pdgfrβ. In adult mouse back skin Ng2+ perivascular cells could be categorized into 4 populations based on whether they expressed Pdgfrα and Pdgfrβ individually or in combination or were Pdgfr-negative. Lineage tracing demonstrated that although Ng2+ cells in embryonic and neonatal back skin contributed to multiple cell types they did not give rise to interfollicular fibroblasts within the dermis. Lineage tracing of distinct fibroblast populations during skin development showed that papillary fibroblasts (Lrig1+) gave rise to Ng2+ perivascular cells in the upper dermis, whilst Ng2+ perivascular cells in the lower dermis were primarily derived from reticular Dlk1+ fibroblasts. Following wounding of adult skin, Ng2+ dermal cells only give rise to Ng2+ blood vessel associated cells and did not contribute to other fibroblast lineages. The relative abundance of Ng2+ Pdgfrβ+ perivascular populations was comparable in wounded and non-wounded skin, indicating that perivascular heterogeneity was maintained during full thickness skin repair. In the wound bed Ng2+ perivascular populations were primarily derived from Lrig1+ papillary or Dlk1+ reticular fibroblast lineages, according to the location of the regenerating blood vessels. We conclude that Ng2+ perivascular cells represent a heterogeneous lineage restricted population that is primarily recruited from the papillary or reticular fibroblast lineages during tissue regeneration.

Cellular heterogeneity and microenvironmental control of skin cancer

Healthy tissues harbour a surprisingly high number of cells that carry well-known cancer-causing mutations without impacting their physiological function. In recent years, strong evidence accumulated that the immediate environment of mutant cells profoundly impact their prospect of malignant progression. In this review, focusing on the skin, we investigate potential key mechanisms that ensure tissue homeostasis despite the presence of mutant cells, as well as critical factors that may nudge the balance from homeostasis to tumour formation. Functional in vivo studies and single-cell transcriptome analyses have revealed a tremendous cellular heterogeneity and plasticity within epidermal (stem) cells and their respective niches, revealing for example wild-type epithelial cells, fibroblasts or immune-cell subsets as critical in preventing cancer formation and malignant progression. It's the same cells, however, that can drive carcinogenesis. Therefore, understanding the abundance and molecular variation of cell types in health and disease, and how they interact and modulate the local signalling environment will thus be key for new therapeutic avenues in our battle against cancer.

Adipose-derived stem cells: Pathophysiologic implications vs therapeutic potential in systemic sclerosis

Adipose-derived stem cells (ADSCs) residing in the stromal vascular fraction (SVF) of white adipose tissue are recently emerging as an alternative tool for stem cell-based therapy in systemic sclerosis (SSc), a complex connective tissue disorder affecting the skin and internal organs with fibrotic and vascular lesions. Several preclinical and clinical studies have reported promising therapeutic effects of fat grafting and autologous SVF/ADSC-based local treatment for facial and hand cutaneous manifestations of SSc patients. However, currently available data indicate that ADSCs may represent a double-edged sword in SSc, as they may exhibit a pro-fibrotic and anti-adipogenic phenotype, possibly behaving as an additional pathogenic source of pro-fibrotic myofibroblasts through the adipocyte-to-myofibroblast transition process. Thus, in the perspective of a larger employ of SSc-ADSCs for further therapeutic applications, it is important to definitely unravel whether these cells present a comparable phenotype and similar immunosuppressive, anti-inflammatory, anti-fibrotic and pro-angiogenic properties in respect to healthy ADSCs. In light of the dual role that ADSCs seem to play in SSc, this review will provide a summary of the most recent insights into the preclinical and clinical studies employing SVF and ADSCs for the treatment of the disease and, at the same time, will focus on the main findings highlighting the possible involvement of these stem cells in SSc-related fibrosis pathogenesis.

A Framework of Major Tumor-Promoting Signal Transduction Pathways Implicated in Melanoma-Fibroblast Dialogue

Simple Summary

Melanoma cells reside in a complex stromal microenvironment, which is a critical component of disease onset and progression. Mesenchymal or fibroblastic cell type are the most abundant cellular element of tumor stroma. Factors secreted by melanoma cells can activate non-malignant associated fibroblasts to become melanoma associate fibroblasts (MAFs). MAFs promote tumorigenic features by remodeling the extracellular matrix, supporting tumor cells proliferation, neo-angiogenesis and drug resistance. Additionally, environmental factors may contribute to the acquisition of pro-tumorigenic phenotype of fibroblasts. Overall, in melanoma, perturbed tissue homeostasis contributes to modulation of major oncogenic intracellular signaling pathways not only in tumor cells but also in neighboring cells. Thus, targeted molecular therapies need to be considered from the reciprocal point of view of melanoma and stromal cells.

Abstract

The development of a modified stromal microenvironment in response to neoplastic onset is a common feature of many tumors including cutaneous melanoma. At all stages, melanoma cells are embedded in a complex tissue composed by extracellular matrix components and several different cell populations. Thus, melanomagenesis is not only driven by malignant melanocytes, but also by the altered communication between melanocytes and non-malignant cell populations, including fibroblasts, endothelial and immune cells. In particular, cancer-associated fibroblasts (CAFs), also referred as melanoma-associated fibroblasts (MAFs) in the case of melanoma, are the most abundant stromal cells and play a significant contextual role in melanoma initiation, progression and metastasis. As a result of dynamic intercellular molecular dialogue between tumor and the stroma, non-neoplastic cells gain specific phenotypes and functions that are pro-tumorigenic. Targeting MAFs is thus considered a promising avenue to improve melanoma therapy. Growing evidence demonstrates that aberrant regulation of oncogenic signaling is not restricted to transformed cells but also occurs in MAFs. However, in some cases, signaling pathways present opposite regulation in melanoma and surrounding area, suggesting that therapeutic strategies need to carefully consider the tumor–stroma equilibrium. In this novel review, we analyze four major signaling pathways implicated in melanomagenesis, TGF-β, MAPK, Wnt/β-catenin and Hyppo signaling, from the complementary point of view of tumor cells and the microenvironment.

Recent Advances in Understandings Towards Pathogenesis and Treatment for Intrauterine Adhesion and Disruptive Insights from Single-Cell Analysis

Intrauterine adhesion is a major cause of menstrual irregularities, infertility, and recurrent pregnancy losses and the progress towards its amelioration and therapy is slow and unsatisfactory. We aim to summarize and evaluate the current treatment progress and research methods for intrauterine adhesion. We conducted literature review in January 2020 by searching articles at PubMed on prevention and treatment, pathogenesis, the repair of other tissues/organs, cell plasticity, and the stem cell–related therapies for intrauterine adhesion. A total of 110 articles were selected for review. Uterine cell heterogeneity, expression profile, and cell-cell interaction were investigated based on scRNA-seq of uterus provided by Human Cell Landscape (HCL) project. Previous knowledge on intrauterine adhesion (IUA) pathogenesis was mostly derived from correlation studies by differentially expressed genes between endometrial tissue of intrauterine adhesion patients/animal models and normal endometrial tissue. Although the TGF-β1/SMAD pathway was suggested as the key driver for IUA pathogenesis, uterine cell heterogeneity and distinct expression profile among different cell types highlighted the importance of single-cell investigations. Cell-cell interaction in the uterus revealed the central hub of endothelial cells interacting with other cells, with endothelial cells in endothelial to mesenchymal transition and fibroblasts as the strongest interaction partners. The potential of stem cell–related therapies appeared promising, yet suffers from largely animal studies and nonstandard study design. The need to dissect the roles of endometrial cells, endothelial cells, and fibroblasts and their interaction is evident in order to elucidate the molecular and cellular mechanisms in both intrauterine adhesion pathogenesis and treatment.

Lef1 expression in fibroblasts maintains developmental potential in adult skin to regenerate wounds

Scars are a serious health concern for burn victims and individuals with skin conditions associated with wound healing. Here, we identify regenerative factors in neonatal murine skin that transforms adult skin to regenerate instead of only repairing wounds with a scar, without perturbing development and homeostasis. Using scRNA-seq to probe unsorted cells from regenerating, scarring, homeostatic, and developing skin, we identified neonatal papillary fibroblasts that form a transient regenerative cell type that promotes healthy skin regeneration in young skin. These fibroblasts are defined by the expression of a canonical Wnt transcription factor Lef1 and using gain- and loss of function genetic mouse models, we demonstrate that Lef1 expression in fibroblasts primes the adult skin macroenvironment to enhance skin repair, including regeneration of hair follicles with arrector pili muscles in healed wounds. Finally, we share our genomic data in an interactive, searchable companion website (https://skinregeneration.org/). Together, these data and resources provide a platform to leverage the regenerative abilities of neonatal skin to develop clinically tractable solutions that promote the regeneration of adult tissue.

A framework for advancing our understanding of cancer-associated fibroblasts

Cancer-associated fibroblasts (CAFs) are a key component of the tumour microenvironment with diverse functions, including matrix deposition and remodelling, extensive reciprocal signalling interactions with cancer cells and crosstalk with infiltrating leukocytes. As such, they are a potential target for optimizing therapeutic strategies against cancer. However, many challenges are present in ongoing attempts to modulate CAFs for therapeutic benefit. These include limitations in our understanding of the origin of CAFs and heterogeneity in CAF function, with it being desirable to retain some antitumorigenic functions. On the basis of a meeting of experts in the field of CAF biology, we summarize in this Consensus Statement our current knowledge and present a framework for advancing our understanding of this critical cell type within the tumour microenvironment.

Phagocytosis of Wnt inhibitor SFRP4 by late wound macrophages drives chronic Wnt activity for fibrotic skin healing

Human and murine skin wounding commonly results in fibrotic scarring, but the murine wounding model wound-induced hair neogenesis (WIHN) can frequently result in a regenerative repair response. Here, we show in single-cell RNA sequencing comparisons of semi-regenerative and fibrotic WIHN wounds, increased expression of phagocytic/lysosomal genes in macrophages associated with predominance of fibrotic myofibroblasts in fibrotic wounds. Investigation revealed that macrophages in the late wound drive fibrosis by phagocytizing dermal Wnt inhibitor SFRP4 to establish persistent Wnt activity. In accordance, phagocytosis abrogation resulted in transient Wnt activity and a more regenerative healing. Phagocytosis of SFRP4 was integrin-mediated and dependent on the interaction of SFRP4 with the EDA splice variant of fibronectin. In the human skin condition hidradenitis suppurativa, phagocytosis of SFRP4 by macrophages correlated with fibrotic wound repair. These results reveal that macrophages can modulate a key signaling pathway via phagocytosis to alter the skin wound healing fate.

Premature cell senescence in human skin: Dual face in chronic acquired pigmentary disorders

Although senescence was originally described as an in vitro acquired cellular characteristic, it was recently recognized that senescence is physiologically and pathologically involved in aging and age-related diseases in vivo. The definition of cellular senescence has expanded to include the growth arrest caused by various cellular stresses, including DNA damage, inadequate mitochondria function, activated oncogene or tumor suppressor genes and oxidative stress. While senescence in normal aging involves various tissues over time and contributes to a decline in tissue function even with healthy aging, disease-induced premature senescence may be restricted to one or a few organs triggering a prolonged and more intense rate of accumulation of senescent cells than in normal aging. Organ-specific high senescence rate could lead to chronic diseases, especially in post-mitotic rich tissue. Recently, two opposite acquired pathological conditions related to skin pigmentation were described to be associated with premature senescence: vitiligo and melasma. In both cases, it was demonstrated that pathological dysfunctions are not restricted to melanocytes, the cell type responsible for melanin production and transport to surrounding keratinocytes. Similar to physiological melanogenesis, dermal and epidermal cells contribute directly and indirectly to deregulate skin pigmentation as a result of complex intercellular communication. Thus, despite senescence usually being reported as a uniform phenotype sharing the expression of characteristic markers, skin senescence involving mainly the dermal compartment and its paracrine function could be associated with the disappearance of melanocytes in vitiligo lesions and with the exacerbated activity of melanocytes in the hyperpigmentation spots of melasma. This suggests that the difference may arise in melanocyte intrinsic differences and/or in highly defined microenvironment peculiarities poorly explored at the current state of the art. A similar dualistic phenotype has been attributed to intratumoral stromal cells as cancer-associated fibroblasts presenting a senescent-like phenotype which influence the behavior of neoplastic cells in either a tumor-promoting or tumor-inhibiting manner. Here, we present a framework dissecting senescent-related molecular alterations shared by vitiligo and melasma patients and we also discuss disease-specific differences representing new challenges for treatment.

The Transcription Factor FOXN1 Regulates Skin Adipogenesis and Affects Susceptibility to Diet-Induced Obesity

FOXN1, a transcription factor expressed in the epidermis, regulates keratinocyte differentiation and participates in skin wound healing. In this study, we explored the impact of FOXN1 insufficiency on diet-stimulated weight gain and dermal white adipose tissue regulation in the intact and wounded skin of FOXN1^eGFP/+ (heterozygotes, FOXN1-insufficient) mice in the context of age and diet. The results showed that on a high-fat diet, FOXN1^eGFP/+ mice gained significantly less body weight than their FOXN1^+/+ counterparts (FOXN1-sufficient mice). The intact and wounded skin of FOXN1^eGFP/+ mice displayed abrogated expression of the master regulators of adipogenesis, PPARγ, FABP4, and leptin, which decreased with age in FOXN1^+/+ mice. FOXN1 insufficiency also resulted in a decreased percentage of adipocyte-committed precursor cells (CD24⁺) in the skin. The proadipogenic pathway genes Bmp2, Igf2, and Mest showed a gradual decrease in expression that accompanied the gradual inactivation of FOXN1 in the skin of FOXN1^+/+, FOXN1^eGFP/+, and FOXN1^eGFP/eGFP (lack of FOXN1) mice. Bone morphogenetic protein 2 and insulin-like growth factor 2 signals colocalized with FOXN1-eGFP in the epidermis and in hair follicles. These data demonstrated that FOXN1 initiates the cascade of adipogenic signaling that regulates skin homeostasis and wound healing and affects susceptibility to diet-induced obesity.

Regeneration of Dermis: Scarring and Cells Involved

There are many studies on certain skin cell specifications and their contribution to wound healing. In this review, we provide an overview of dermal cell heterogeneity and their participation in skin repair, scar formation, and in the composition of skin substitutes. The papillary, reticular, and hair follicle associated fibroblasts differ not only topographically, but also functionally. Human skin has a number of particular characteristics that are different from murine skin. This should be taken into account in experimental procedures. Dermal cells react differently to skin wounding, remodel the extracellular matrix in their own manner, and convert to myofibroblasts to different extents. Recent studies indicate a special role of papillary fibroblasts in the favorable outcome of wound healing and epithelial-mesenchyme interactions. Neofolliculogenesis can substantially reduce scarring. The role of hair follicle mesenchyme cells in skin repair and possible therapeutic applications is discussed. Participation of dermal cell types in wound healing is described, with the addition of possible mechanisms underlying different outcomes in embryonic and adult tissues in the context of cell population characteristics and extracellular matrix composition and properties. Dermal white adipose tissue involvement in wound healing is also overviewed. Characteristics of myofibroblasts and their activity in scar formation is extensively discussed. Cellular mechanisms of scarring and possible ways for its prevention are highlighted. Data on keloid cells are provided with emphasis on their specific characteristics. We also discuss the contribution of tissue tension to the scar formation as well as the criteria and effectiveness of skin substitutes in skin reconstruction. Special attention is given to the properties of skin substitutes in terms of cell composition and the ability to prevent scarring.

The role of metabolism in the pathogenesis of systemic sclerosis

Systemic sclerosis (SSc) is an immune-mediated autoimmune disease characterized by fibrosis and vascular abnormalities. The cellular and molecular mechanisms remain unclear, and current therapies are limited. Cell metabolism has been shown to play an essential role in cancer survival and tumour invasion as well as in rheumatic diseases such as systemic lupus erythematosus, rheumatoid arthritis and osteoarthritis. Although little is known about SSc, cell metabolism may provide new clues for understanding its pathogenesis. In this review, we summarize recent studies of metabolism in SSc and fibrotic disease, specifically focusing on glycolysis, fatty acid metabolism and oxidative stress. We highlight the role of metabolism in fibroblast differentiation and emphasize its potential therapeutic prospects in SSc.

Skin Cell Heterogeneity in Development, Wound Healing, and Cancer

Skin architecture and function depend on diverse populations of epidermal cells and dermal fibroblasts. Reciprocal communication between the epidermis and dermis plays a key role in skin development, homeostasis and repair. While several stem cell populations have been identified in the epidermis with distinct locations and functions, it is now recognised that there is additional heterogeneity within the mesenchymal cells of the dermis. Here, we discuss recent insights into how these distinct cell populations are maintained and coordinated during development, homeostasis, and wound healing. We highlight the importance of the local environment, or niche, in cellular plasticity. We also discuss new mechanisms that have been identified as influencing wound repair and cancer progression.

Lineage Identity and Location within the Dermis Determine the Function of Papillary and Reticular Fibroblasts in Human Skin

Human skin dermis is composed of the superficial papillary dermis and the reticular dermis in the lower layers, which can easily be distinguished histologically. In vitro analyses of fibroblasts from explant cultures from superficial and lower dermal layers suggest that human skin comprises at least two fibroblast lineages with distinct morphology, expression profiles, and functions. However, while for mouse skin cell surface markers have been identified, allowing the isolation of pure populations of one lineage or the other via FACS, this has not been achieved for human skin fibroblasts. We have now discovered two cell surface markers that discriminate between papillary and reticular fibroblasts. While FAP⁺CD90^- cells display increased proliferative potential, express PDPN and NTN1, and cannot be differentiated into adipocytes, FAP^-CD90⁺ fibroblasts express high levels of ACTA2, MGP, PPARγ, and CD36 and readily undergo adipogenic differentiation, a hallmark of reticular fibroblasts. Flow cytometric analysis of fibroblasts isolated from superficial and lower layers of human dermis showed that FAP⁺CD90^- cells are enriched in the papillary dermis. Altogether, functional analysis and expression profiling confirms that FAP⁺CD90^- cells represent papillary fibroblasts, whereas FAP^-CD90⁺ fibroblasts derive from the reticular lineage. Although papillary and reticular fibroblasts are enriched in the upper or lower dermis, respectively, they are not spatially restricted, and the microenvironment seems to affect their function.

Foxn1 in Skin Development, Homeostasis and Wound Healing

Intensive research effort has focused on cellular and molecular mechanisms that regulate skin biology, including the phenomenon of scar-free skin healing during foetal life. Transcription factors are the key molecules that tune gene expression and either promote or suppress gene transcription. The epidermis is the source of transcription factors that regulate many functions of epidermal cells such as proliferation, differentiation, apoptosis, and migration. Furthermore, the activation of epidermal transcription factors also causes changes in the dermal compartment of the skin. This review focuses on the transcription factor Foxn1 and its role in skin biology. The regulatory function of Foxn1 in the skin relates to physiological (development and homeostasis) and pathological (skin wound healing) conditions. In particular, the pivotal role of Foxn1 in skin development and the acquisition of the adult skin phenotype, which coincides with losing the ability of scar-free healing, is discussed. Thus, genetic manipulations with Foxn1 expression, specifically those introducing conditional Foxn1 silencing in a Foxn1+/+ organism or its knock-in in a Foxn1−/− model, may provide future perspectives for regenerative medicine.