Transcriptome profiling of human papillary and reticular fibroblasts from adult interfollicular dermis pinpoints the ‘tissue skeleton’ gene network as a component of skin chrono-ageing

Proteomic and secretomic comparison of young and aged dermal fibroblasts highlights cytoskeleton as a key component during aging

Crucial for skin homeostasis, synthesis and degradation of extracellular matrix components are orchestrated by dermal fibroblasts. During aging, alterations of component expression, such as collagens and enzymes, lead to reduction of the mechanical cutaneous tension and defects of skin wound healing. The aim of this study was to better understand the molecular alterations underwent by fibroblasts during aging by comparing secretomic and proteomic signatures of fibroblasts from young (<35years) and aged (>55years) skin donors, in quiescence or TGF-stimulated conditions, using HLPC/MS. The comparison of the secretome from young and aged fibroblasts revealed that 16 proteins in resting condition, and 11 proteins after a 24h-lasting TGF-β1-treatment, were expressed in significant different ways between the two cell groups (fold change>2, p-value <0.05), with a 77% decrease in the number of secreted proteins in aged cells. Proteome comparison between young and aged fibroblasts identified a significant change of 63 proteins in resting condition, and 73 proteins in TGF-β1-stimulated condition, with a 67% increase in the number of proteins in aged fibroblasts. The majority of the differentially-expressed molecules belongs to the cytoskeleton-associated proteins and aging was characterized by an increase in Coronin 1C (CORO1C), and Filamin B (FLNB) expression in fibroblasts together with a decrease in Cofilin (CFL1), and Actin alpha cardiac muscle 1 (ACTC1) detection in aged cells, these proteins being involved in actin-filament polymerization and sharing co-activity in cell motility. Our present data reinforce knowledge about an age-related alteration in the synthesis of major proteins linked to the migratory and contractile functions of dermal human fibroblasts.

Assessment of Extracellular Matrix Fibrous Elements in Male Dermal Aging: A Ten-Year Follow-Up Preliminary Case Study

Skin aging is a complex phenomenon influenced by multiple internal and external factors that can lead to significant changes in skin structure, particularly the degradation of key extracellular matrix (ECM) components such as collagen and elastic fibers in the dermis. In this study, we aimed to meticulously assess the morphological changes within these critical fibrous ECM elements in the dermis of the same volunteer at age 47 and 10 years later (2012 to 2022). Using advanced histological staining techniques, we examined the distribution and characteristics of ECM components, including type I collagen, type III collagen, and elastic fibers. Morphological analysis, facilitated by hematoxylin and eosin staining, allowed for an accurate assessment of fiber bundle thickness and a quantification of collagen and elastic fiber areas. In addition, we used the generalized Pareto distribution for histogram modeling to refine our statistical analyses. This research represents a pioneering effort to examine changes in ECM fiber material, specifically within the male dermis over a decade-long period. Our findings reveal substantial changes in the organization of type I collagen within the ECM, providing insight into the dynamic processes underlying skin aging.

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.

Single-cell transcriptional profiling reveals immunomodulatory properties of stromal and epithelial cells in periodontal immune milieu with diabetes in rats

Periodontitis is the sixth major complication of diabetes. Gingiva, as an important component of periodontal tissues, serves as the first defense barrier against infectious stimuli. However, relatively little is known about cellular heterogeneity and cell-specific changes in gingiva in response to diabetes-associated periodontitis. To characterize molecular changes linking diabetes with periodontitis, we profiled single-cell transcriptome analyses of a total of 45,259 cells from rat gingiva with periodontitis under normoglycemic and diabetic condition. The single-cell profiling revealed that stromal and epithelial cells of gingiva contained inflammation-related subclusters enriched in functions of immune cell recruitment. Compared to normoglycemic condition, diabetes led to a reduction in epithelial basal cells, fibroblasts and smooth muscle cells in gingiva with periodontitis. Analysis of differentially expressed genes indicated that stromal and epithelial populations were reprogrammed towards pro-inflammatory phenotypes promoting immune cell recruitment in diabetes-related periodontitis. In aspect of immune cells, diabetes prominently enhanced neutrophil and M1 macrophage infiltration in periodontitis lesions. Cell-cell communications revealed enhanced crosstalk between stromal/epithelial cells and immune cells mediating by chemokine/chemokine receptor interplay in diabetes-associated periodontitis. Our findings deconvolved cellular heterogeneity of rat gingiva associated with periodontitis and diabetes, uncovered altered immune milieu caused by the disease, and revealed immunomodulatory functions of stromal and epithelial cells in gingival immune niche. The present study improves the understanding of the link between the diabetes and periodontitis and helps in formulating precise therapeutic strategies for diabetes-enhanced periodontitis.

Evidence of inter- and intra-keloid heterogeneity through analysis of dermal fibroblasts: A new insight in deciphering keloid physiopathology

Keloid scars are hypertrophic and proliferating pathological scars extending beyond the initial lesion and without tendency to regression. Usually, keloids are considered and treated as a single entity but clinical observations suggest heterogeneity in keloid morphologies with distinction of superficial/extensive and nodular entities. Within a keloid, heterogeneity could also be detected between superficial and deep dermis or centre and periphery. Focusing on fibroblasts as main actors of keloid formation, we aimed at evaluating intra- and inter-keloid fibroblast heterogeneity by analysing their gene expression and functional capacities (proliferation, migration, traction forces), in order to improve our understanding of keloid pathogenesis. Fibroblasts were obtained from centre, periphery, papillary and reticular dermis from extensive or nodular keloids and were compared to control fibroblasts from healthy skin. Transcriptional profiling of fibroblasts identified a total of 834 differentially expressed genes between nodular and extensive keloids. Quantification of ECM-associated gene expression by RT-qPCR brought evidence that central reticular fibroblasts of nodular keloids are the population which synthesize higher levels of mature collagens, TGFβ, HIF1α and αSMA as compared to control skin, suggesting that this central deep region is the nucleus of ECM production with a centrifuge extension in keloids. Although no significant variations were found for basal proliferation, migration of peripheral fibroblasts from extensive keloids was higher than that of central ones and from nodular cells. Moreover, these peripheral fibroblasts from extensive keloids exhibited higher traction forces than central cells, control fibroblasts and nodular ones. Altogether, studying fibroblast features demonstrate keloid heterogeneity, leading to a better understanding of keloid pathophysiology and treatment adaptation.

NRF2 Shortage in Human Skin Fibroblasts Dysregulates Matrisome Gene Expression and Affects Collagen Fibrillogenesis

NRF2 is a master regulator of the antioxidative response that was recently proposed as a potential regulator of extracellular matrix (ECM) gene expression. Fibroblasts are major ECM producers in all connective tissues, including the dermis. A better understanding of NRF2-mediated ECM regulation in skin fibroblasts is thus of great interest for skin homeostasis maintenance and aging protection. In this study, we investigate the impact of NRF2 downregulation on matrisome gene expression and ECM deposits in human primary dermal fibroblasts. RNA-sequencing‒based transcriptome analysis of NRF2 silenced dermal fibroblasts shows that ECM genes are the most regulated gene sets, highlighting the relevance of the NRF2-mediated matrisome program in these cells. Using complementary light and electron microscopy methods, we show that NRF2 deprivation in dermal fibroblasts results in reduced collagen I biosynthesis and impacts collagen fibril deposition. Moreover, we identify ZNF469, a putative transcriptional regulator of collagen biosynthesis, as a target of NRF2. Both ZNF469 silenced fibroblasts and fibroblasts derived from Brittle Corneal Syndrome patients carrying variants in ZNF469 gene show reduced collagen I gene expression. Our study shows that NRF2 orchestrates matrisome expression in human skin fibroblasts through direct or indirect transcriptional mechanisms that could be prioritized to target dermal ECM homeostasis in health and disease.

YAP/TAZ activity in stromal cells prevents ageing by controlling cGAS-STING

Ageing is intimately connected to the induction of cell senescence1,2, but why this is so remains poorly understood. A key challenge is the identification of pathways that normally suppress senescence, are lost during ageing and are functionally relevant to oppose ageing3. Here we connected the structural and functional decline of ageing tissues to attenuated function of the master effectors of cellular mechanosignalling YAP and TAZ. YAP/TAZ activity declines during physiological ageing in stromal cells, and mimicking such decline through genetic inactivation of YAP/TAZ in these cells leads to accelerated ageing. Conversely, sustaining YAP function rejuvenates old cells and opposes the emergence of ageing-related traits associated with either physiological ageing or accelerated ageing triggered by a mechano-defective extracellular matrix. Ageing traits induced by inactivation of YAP/TAZ are preceded by induction of tissue senescence. This occurs because YAP/TAZ mechanotransduction suppresses cGAS-STING signalling, to the extent that inhibition of STING prevents tissue senescence and premature ageing-related tissue degeneration after YAP/TAZ inactivation. Mechanistically, YAP/TAZ-mediated control of cGAS-STING signalling relies on the unexpected role of YAP/TAZ in preserving nuclear envelope integrity, at least in part through direct transcriptional regulation of lamin B1 and ACTR2, the latter of which is involved in building the peri-nuclear actin cap. The findings demonstrate that declining YAP/TAZ mechanotransduction drives ageing by unleashing cGAS-STING signalling, a pillar of innate immunity. Thus, sustaining YAP/TAZ mechanosignalling or inhibiting STING may represent promising approaches for limiting senescence-associated inflammation and improving healthy ageing.

Benefit of coupling heparin to crosslinked collagen I/III scaffolds for human dermal fibroblast subpopulations' tissue growth

Currently, there is a lack of models representing the skin dermal heterogeneity for relevant research and skin engineering applications. This is the first study reporting production of dermal equivalents reproducing features of papillary and reticular dermal compartments. Inspired from our current knowledge on the architecture and composition differences between the papillary and reticular dermis, we evaluated different collagen-based porous materials to serve as scaffolds for the three-dimensional expansion of freshly isolated papillary and/or reticular fibroblasts. The scaffolds, composed of either collagen I or collagen I and III mixtures, were prepared by lyophilization. Pore size and hydrolytic stability were controlled by crosslinking with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide (EDC) and N-hydroxysuccinimide (NHS) or EDC/NHS with covalently bound heparin. The evaluation of the resultant "papillary" and "reticular" dermal equivalents was based on the analysis of characteristic features of each dermal compartment, such as cell density and deposition of newly synthetized extracellular matrix components in histological sections. Crosslinking supported cell growth during dermal tissue formation independent on the fibroblast subpopulation. The presence of collagen III seemed to have some positive but non-specific effect only on the maintenance of the mechanical strength of the scaffolds during dermal formation. Histological analyses demonstrated a significant and specific effect of heparin on generating dermal equivalents reproducing the respective higher papillary than reticular cell densities and supporting distinct extracellular matrix components deposition (three to five times more carbohydrate material deposited by papillary fibroblasts in all scaffolds containing heparin, while higher collagen production was observed only in the presence of heparin).

Assessment of Mechanical Stress Induced by Radiofrequency Currents on Skin Interfaces

The epidermal-dermal (ED) and dermal-subcutaneous (DS) junctions are the most prominent skin interfaces, which are known to be of primary importance in different dermatological and aesthetic conditions. These interfaces are strongly modified in aging skin, and their effective targeting can lead to improvement of skin appearance in aging and by cellulite. Application of radiofrequency (RF) currents to the skin can selectively produce mechanical stress on these interfaces. Here, we assess the stresses induced by RF currents of different frequencies on EDJ and DSJ and discuss possible applications of the interfacial therapy in aesthetic medicine.

The effect of TGFβRI inhibition on fibroblast heterogeneity in hypertrophic scar 2D in vitro models

In burn patients, wound healing is often accompanied by hypertrophic scarring (HTS), resulting in both functional and aesthetic problems. HTSs are characterized by abundant presence of myofibroblasts (MFs) residing in the dermis. HTS development and MF persistence is primarily regulated by TGF-β signalling. A promising method to target the transforming growth factor receptor I (TGFβRI; also known as activin-like kinase 5 (ALK5)) is by making use of exon skipping through antisense oligonucleotides. In HTS the distinguishing border between the papillary dermis and the reticular dermis is completely abrogated, thus exhibiting a one layered dermis containing a heterogenous fibroblast population, consisting of papillary fibroblasts (PFs), reticular fibroblasts (RFs) and MFs. It has been proposed that PFs, as opposed to RFs, exhibit anti-fibrotic properties. Currently, it is still unclear which fibroblast subtype is most affected by exon skipping treatment. Therefore, the aim of this study was to investigate the effect of TGFβRI inhibition by exon skipping in PF, RF and HTS fibroblast monocultures. Morphological analyses revealed the presence of a PF-like population after exon skipping in the different fibroblast cultures. This observation was further confirmed by the expression of genes specific for PFs, demonstrated by qPCR analyses. Further investigations on mRNA and protein level revealed that indeed MFs and to a lesser extent RFs are targeted by exon skipping. Furthermore, collagen gel contraction analysis showed that ALK5 exon skipping reduced TGF-β- induced contraction together with decreased alpha-smooth muscle actin expression levels. In conclusion, we show for the first time that exon skipping primarily targets pro-fibrotic fibroblasts. This could be a promising step towards reduced HTS development of burn tissue.

Global Gene Expression of Cultured Human Dermal Fibroblasts: Focus on Cell Cycle and Proliferation Status in Improving the Condition of Face Skin

Chronological skin ageing is an inevitable physiological process that results in thin and sagging skin, fine wrinkles, and gradual dermal atrophy. The main therapeutic approaches to soft tissue augmentation involve using dermal fillers, where natural fillers, such as autologous fibroblasts, are involved in generating dermal matrix proteins. The aim of this study was to determine the global transcriptome profile of three passages of dermal autologous fibroblasts from a male volunteer, focusing on the processes of the cell cycle and cell proliferation status to estimate the optimal passage of the tested cells with respect to their reimplantation. We performed K-means clustering and validation of the expression of the selected mRNA by qRT-PCR. Ten genes were selected (ANLN, BUB1, CDC20, CCNA2, DLGAP5, MKI67, PLK1, PRC1, SPAG5, and TPX2) from the top five processes annotated to cluster 5. Detailed microarray analysis of the fibroblast genes indicated that the cell population of the third passage exhibited the highest number of upregulated genes involved in the cell cycle and cell proliferation. In all cases, the results of qRT-PCR confirmed the differences in expression of the selected mRNAs between fibroblasts from the primary culture (C0) and from the first (C1), second (C2), and third (C3) cell passage. Our results thus suggest that these cells might be useful for increasing fibroblast numbers after reimplantation into a recipient's skin, and the method used in this study seems to be an excellent tool for autologous transplantation allowing the rejuvenation of aging skin.

[Skin aging: Pathophysiology and innovative therapies]

One of the major challenges of the 21^st century is the fight against aging, defined as a set of physiological mechanisms altering the physical and intellectual capacities of human beings. Aging of the skin is only one visible part of this process. It is associated with major healing defects linked in part to the alteration of the biomechanical properties of skin cells, mainly dermal fibroblasts. The immune system, another key component in maintaining skin homeostasis and the efficient healing of wounds, also suffers the effects of time: the consequent skin immunosenescence would limit the anti-infectious and vaccine response, while promoting a pro-tumor environment. The main skin damages due to aging, whether intrinsic or extrinsic, will be detailed before listing the effective anti-aging strategies to combat age-related dermal and epidermal stigmas.

A Single-Cell Transcriptomic Atlas of Human Skin Aging

Skin undergoes constant self-renewal, and its functional decline is a visible consequence of aging. Understanding human skin aging requires in-depth knowledge of the molecular and functional properties of various skin cell types. We performed single-cell RNA sequencing of human eyelid skin from healthy individuals across different ages and identified eleven canonical cell types, as well as six subpopulations of basal cells. Further analysis revealed progressive accumulation of photoaging-related changes and increased chronic inflammation with age. Transcriptional factors involved in the developmental process underwent early-onset decline during aging. Furthermore, inhibition of key transcription factors HES1 in fibroblasts and KLF6 in keratinocytes not only compromised cell proliferation, but also increased inflammation and cellular senescence during aging. Lastly, we found that genetic activation of HES1 or pharmacological treatment with quercetin alleviated cellular senescence of dermal fibroblasts. These findings provide a single-cell molecular framework of human skin aging, providing a rich resource for developing therapeutic strategies against aging-related skin disorders.

Integrated Skin Transcriptomics and Serum Multiplex Assays Reveal Novel Mechanisms of Wound Healing in Diabetic Foot Ulcers

Nonhealing diabetic foot ulcers (DFUs) are characterized by low-grade chronic inflammation, both locally and systemically. We prospectively followed a group of patients who either healed or developed nonhealing chronic DFUs. Serum and forearm skin analysis, both at the protein expression and the transcriptomic level, indicated that increased expression of factors such as interferon-γ (IFN-γ), vascular endothelial growth factor, and soluble vascular cell adhesion molecule-1 were associated with DFU healing. Furthermore, foot skin single-cell RNA sequencing analysis showed multiple fibroblast cell clusters and increased inflammation in the dorsal skin of patients with diabetes mellitus (DM) and DFU specimens compared with control subjects. In addition, in myeloid cell DM and DFU upstream regulator analysis, we observed inhibition of interleukin-13 and IFN-γ and dysregulation of biological processes that included cell movement of monocytes, migration of dendritic cells, and chemotaxis of antigen-presenting cells pointing to an impaired migratory profile of immune cells in DM skin. The SLCO2A1 and CYP1A1 genes, which were upregulated at the forearm of nonhealers, were mainly expressed by the vascular endothelial cell cluster almost exclusively in DFU, indicating a potential important role in wound healing. These results from integrated protein and transcriptome analyses identified individual genes and pathways that can potentially be targeted for enhancing DFU healing.

Transmembrane collagens-Unexplored mediators of epidermal-dermal communication and tissue homeostasis

Tissue homeostasis is maintained through constant, dynamic and heterogeneous communication between cells and their microenvironment. Proteins that are at the same time active at the intracellular, cell periphery and deeper extracellular levels possess the ability to, on the individual molecular level, influence the cells and their microenvironment in a bidirectional manner. The transmembrane collagens are a family of such proteins, which are of notable interest for tissue development and homeostasis. In skin, expression of all transmembrane collagens has been reported and deficiency of transmembrane collagen XVII manifests with distinct skin phenotypes. Nevertheless, transmembrane collagens in skin remain understudied despite the association of them with epidermal wound healing and dermal fibrotic processes. Here, we present an overview of transmembrane collagens and put a spotlight on them as regulators of epidermal-dermal communication and as potential players in fibrinogenesis.

Fibroblast Heterogeneity in and Its Implications for Plastic and Reconstructive Surgery: A Basic Science Review

Fibroblasts’ integral role in tissue development, maintenance, and disease represents a fast-growing field of basic science research. Although fibroblasts were long thought to be a homogeneous cell population, recent research has illuminated the unforeseen complexity of these cells, giving rise to the rapidly expanding research field of “fibroblast heterogeneity.” Fibroblasts play a critical role in states of tissue fibrosis such as skin scarring, which affects hundreds of millions of patients annually and causes severe aesthetic, developmental, and functional morbidity. Beyond scarring, major organ fibrosis is an enormous public health concern responsible for nearly half of all deaths in the United States. Because fibrosis is a conserved response to tissue damage in all organs, the study of fibroblasts throughout the body may help us to understand their role in the conditions most relevant to plastic and reconstructive surgery—for instance, skin scarring (eg, from burns, traumatic lacerations, or surgical incisions), “pathological” scarring (hypertrophic scars, keloids), and capsular contracture. Here, we present a basic science review of fibroblast heterogeneity in wound healing, cancer, organ fibrosis, and human dermal architecture. The field of fibroblast heterogeneity is young, and many of the insights discussed have yet to be translated clinically. However, plastic surgeons stand in a unique position to bridge these discoveries into clinical realities. We hope this information can spur readers to consider both what questions in plastic surgery can be studied from the lens of fibroblast heterogeneity, and how these preclinical insights can be translated to improving care of our patients.

Single-cell transcriptomes of the human skin reveal age-related loss of fibroblast priming

Fibroblasts are an essential cell population for human skin architecture and function. While fibroblast heterogeneity is well established, this phenomenon has not been analyzed systematically yet. We have used single-cell RNA sequencing to analyze the transcriptomes of more than 5,000 fibroblasts from a sun-protected area in healthy human donors. Our results define four main subpopulations that can be spatially localized and show differential secretory, mesenchymal and pro-inflammatory functional annotations. Importantly, we found that this fibroblast 'priming' becomes reduced with age. We also show that aging causes a substantial reduction in the predicted interactions between dermal fibroblasts and other skin cells, including undifferentiated keratinocytes at the dermal-epidermal junction. Our work thus provides evidence for a functional specialization of human dermal fibroblasts and identifies the partial loss of cellular identity as an important age-related change in the human dermis. These findings have important implications for understanding human skin aging and its associated phenotypes.

Fibroblasts from the Human Skin Dermo-Hypodermal Junction are Distinct from Dermal Papillary and Reticular Fibroblasts and from Mesenchymal Stem Cells and Exhibit a Specific Molecular Profile Related to Extracellular Matrix Organization and Modeling

Human skin dermis contains fibroblast subpopulations in which characterization is crucial due to their roles in extracellular matrix (ECM) biology. This study investigates the properties of fibroblasts localized at the frontier of deep dermis and hypodermis, i.e., dermo-hypodermal junction fibroblasts (F-DHJ), which were compared to intermediate reticular dermis (Fr) and superficial papillary dermis (Fp) fibroblasts. F-DHJ differed from Fr and Fp cells in their wider potential for differentiation into mesodermal lineages and in their absence of contractility when integrated in a three-dimensional dermal equivalent. The transcriptomic profile of F-DHJ exhibited specificities in the expression of genes involved in ECM synthesis-processing and "tissue skeleton" organization. In accordance with transcriptome data, ECM proteins, notably Tenascin C, distributions differed between the reticular dermis and the dermo-hypodermal junction areas, which was documented in normal adult skin. Finally, genome-wide transcriptome profiling was used to evaluate the molecular proximity of F-DHJ with the two dermal fibroblast populations (Fp and Fr) and with the mesenchymal stem cells (MSCs) corresponding to five tissue origins (bone marrow, fat, amnion, chorion, and cord). This comparative analysis classified the three skin fibroblast types, including F-DHJ, as a clearly distinct group from the five MSC sample origins.

Deciphering the functional heterogeneity of skin fibroblasts using single-cell RNA sequencing

Though skin fibroblasts (FB) are the main cell population within the dermis, the different skin FB subsets are not well characterized and the traditional classification into reticular and papillary FBs has little functional relevance. To fill the gap of knowledge on FB diversity in human skin, we performed single-cell RNA sequencing. Investigation of marker genes for the different skin cell subtypes revealed a heterogeneous picture of FBs. When mapping reticular and papillary FB markers, we could not detect cluster specificity, suggesting that these two populations show a higher transcriptional heterogeneity than expected. This finding was further confirmed by in situ hybridization, showing that DPP4 was expressed in both dermal layers. Our analysis identified six FB clusters with distinct transcriptional signatures. Importantly, we could demonstrate that in human skin DPP4⁺ FBs are the main producers of factors involved in extracellular matrix (ECM) assembly. In conclusion, we provide evidence that hitherto considered FB markers are not ideal to characterize skin FB subpopulations in single-cell sequencing analyses. The identification of DPP4⁺ FBs as the main ECM-producing cells in human skin will foster the development of anti-fibrotic treatments for the skin and other organs.