Biomechanical and Biochemical Changes in Murine Skin During Development and Aging.

Aging leads to biochemical and biomechanical changes in skin, with biological and functional consequences. Despite extensive literature on skin aging, there is a lack of studies which investigate the maturation of the tissue and connect the microscopic changes in the skin to its macroscopic biomechanical behavior as it evolves over time. The present work addresses this knowledge gap using multiscale characterization of skin in a murine model considering newborn, adult and aged mice. Monotonic uniaxial loading, tension relaxation with change of bath, and loading to failure tests were performed on murine skin samples from different age groups, complemented by inflation experiments and atomic force microscopy indentation measurements. In parallel, skin samples were characterized using histological and biochemical techniques to assess tissue morphology, collagen organization, as well as collagen content and cross-linking. We show that 1-week-old skin differs across nearly all measured parameters from adult skin, showing reduced strain stiffening and tensile strength, a thinner dermis, lower collagen content and altered crosslinking patterns. Surprisingly, adult and aged skin were similar across most biomechanical parameters in the physiologic loading range, while aged skin had lower stiffening behavior at large force values and lower tensile strength. This correlates with altered collagen content and cross-links. Based on a computational model, differences in mechanocoupled stimuli in the skin of the different age groups were calculated, pointing to a potential biological significance of the age-induced biomechanical changes in regulating the local biophysical environment of dermal cells. STATEMENT OF SIGNIFICANCE: Skin microstructure and the emerging mechanical properties change with age, leading to biological, functional and health-related consequences. Despite extensive literature on skin aging, only very limited quantitative data are available on microstructural changes and the corresponding macroscopic biomechanical behavior as they evolve over time. This work provides a wide-range multiscale mechanical characterization of skin of newborn, adult and aged mice, and quantifies microstructural correlations in tissue morphology, collagen content, organization and cross-linking. Remarkably, aged skin retained normal hydration and biomechanical function in the physiological loading range but showed significantly reduced properties at super-physiological loading. Our data show that age-related microstructural differences have a profound effect not only on tissue-level properties but also on the cell-level biophysical environment.

A lysyl oxidase-responsive collagen peptide illuminates collagen remodeling in wound healing.

Tissue repair and fibrosis involve the dynamic remodeling of collagen, and accurate detection of these sites is of utmost importance. Here, we use a collagen peptide sensor (1) to visualize collagen formation and remodeling during wound healing in mice and humans. We show that the probe binds selectively to sites of collagen formation and remodeling at different stages of healing. Compared to conventional methods, the peptide sensor localizes preferentially to areas of collagen synthesis and remodeling at the wound edge and not in matured fibrillar collagen. We also demonstrate its applicability for in vivo wound imaging and for discerning differential remodeling in wounds of transgenic mice with altered collagen dynamics. Our findings show the value of 1 as a diagnostic tool to rapidly identify the sites of matrix remodeling in tissue sections, which will aid in the conception of new therapeutic strategies for fibrotic disorders and defective tissue repair.

Release of miR-29 Target Laminin C2 Improves Skin Repair.

miRNAs are small noncoding RNAs that regulate mRNA targets in a cell-specific manner. miR-29 is expressed in murine and human skin, where it may regulate functions in skin repair. Cutaneous wound healing model in miR-29a/b1 gene knockout mice was used to identify miR-29 targets in the wound matrix, where angiogenesis and maturation of provisional granulation tissue was enhanced in response to genetic deletion of miR-29. Consistently, antisense-mediated inhibition of miR-29 promoted angiogenesis in vitro by autocrine and paracrine mechanisms. These processes are likely mediated by miR-29 target mRNAs released upon removal of miR-29 to improve cell-matrix adhesion. One of these, laminin (Lam)-c2 (also known as laminin γ2), was strongly up-regulated during skin repair in the wound matrix of knockout mice. Unexpectedly, Lamc2 was deposited in the basal membrane of endothelial cells in blood vessels forming in the granulation tissue of knockout mice. New blood vessels showed punctate interactions between Lamc2 and integrin α6 (Itga6) along the length of the proto-vessels, suggesting that greater levels of Lamc2 may contribute to the adhesion of endothelial cells, thus assisting angiogenesis within the wound. These findings may be of translational relevance, as LAMC2 was deposited at the leading edge in human wounds, where it formed a basal membrane for endothelial cells and assisted neovascularization. These results suggest a link between LAMC2, improved angiogenesis, and re-epithelialization.

Targeting NRF2 to promote epithelial repair.

The transcription factor NRF2 is well known as a master regulator of the cellular stress response. As such, activation of NRF2 has gained widespread attention for its potential to prevent tissue injury, but also as a possible therapeutic approach to promote repair processes. While NRF2 activation affects most or even all cell types, its effect on epithelial cells during repair processes has been particularly well studied. In response to tissue injury, these cells proliferate, migrate and/or spread to effectively repair the damage. In this review, we discuss how NRF2 governs repair of epithelial tissues, and we highlight the increasing number of NRF2 targets with diverse roles in regulating epithelial repair.

Quantitative Proteomics Identifies Reduced NRF2 Activity and Mitochondrial Dysfunction in Atopic Dermatitis.

Atopic dermatitis is the most common inflammatory skin disease and is characterized by a deficient epidermal barrier and cutaneous inflammation. Genetic studies suggest a key role of keratinocytes in atopic dermatitis pathogenesis, but the alterations in the proteome that occur in the full epidermis have not been defined. Using a pressure-cycling technology and data-independent acquisition approach, we performed quantitative proteomics of epidermis from healthy volunteers and lesional and nonlesional patient skin. Results were validated by targeted proteomics using parallel reaction monitoring mass spectrometry and immunofluorescence staining. Proteins that were differentially abundant in the epidermis of patients with atopic dermatitis versus in healthy control reflect the strong inflammation in lesional skin and the defect in keratinocyte differentiation and epidermal stratification that already characterizes nonlesional skin. Most importantly, they reveal impaired activation of the NRF2-antioxidant pathway and reduced abundance of mitochondrial proteins involved in key metabolic pathways in the affected epidermis. Analysis of primary human keratinocytes with small interfering RNA‒mediated NRF2 knockdown revealed that the impaired NRF2 activation and mitochondrial abnormalities are partially interlinked. These results provide insight into the molecular alterations in the epidermis of patients with atopic dermatitis and identify potential targets for pharmaceutical intervention.

Activation of Nrf2 in fibroblasts promotes a skin aging phenotype via an Nrf2-miRNA-collagen axis.

Aging is associated with progressive skin fragility and a tendency to tear, which can lead to severe clinical complications. The transcription factor NRF2 is a key regulator of the cellular antioxidant response, and pharmacological NRF2 activation is a promising strategy for the prevention of age-related diseases. Using a combination of molecular and cellular biology, histology, imaging and biomechanical studies we show, however, that constitutive genetic activation of Nrf2 in fibroblasts of mice suppresses collagen and elastin expression, resulting in reduced skin strength as seen in aged mice. Mechanistically, the "aging matrisome" results in part from direct Nrf2-mediated overexpression of a network of microRNAs that target mRNAs of major skin collagens and other matrix components. Bioinformatics and functional studies revealed high NRF2 activity in aged human fibroblasts in 3D skin equivalents and human skin biopsies, highlighting the translational relevance of the functional mouse data. Together, these results identify activated NRF2 as a promoter of age-related molecular and biomechanical skin features.

Topical Wound Treatment with a Nitric Oxide-Releasing PDE5 Inhibitor Formulation Enhances Blood Perfusion and Promotes Healing in Mice.

Chronic, non-healing wounds constitute a major health problem, and the current therapeutic options are limited. Therefore, pharmaceuticals that can be locally applied to complicated wounds are urgently needed. Such treatments should directly target the underlying causes, which include diabetes mellitus, chronic local pressure and/or vascular insufficiency. A common consequence of these disorders is impaired wound angiogenesis. Here, we investigated the effect of topical application of a nitric oxide-releasing phosphodiesterase 5 inhibitor (TOP-N53)-containing liquid hydrogel on wound repair in mice. The drug-loaded hydrogel promoted re-epithelialization and angiogenesis in wounds of healthy and healing-impaired diabetic mice. Using a non-invasive label-free optoacoustic microscopy approach combined with automated vessel analysis, we show that the topical application of TOP-N53 formulation increases the microvascular network density and promotes the functionality of the newly formed blood vessels, resulting in enhanced blood perfusion of the wounds. These results demonstrate a remarkable healing-stimulating activity of topically applied TOP-N53 formulation, supporting its further development as a wound therapeutic.

Imaging and targeting LOX-mediated tissue remodeling with a reactive collagen peptide.

Collagens are fibrous proteins that are integral to the strength and stability of connective tissues. During collagen maturation, lysyl oxidases (LOX) initiate the cross-linking of fibers, but abnormal LOX activity is associated with impaired tissue function as seen in fibrotic and malignant diseases. Visualizing and targeting this dynamic process in healthy and diseased tissue is important, but so far not feasible. Here we present a probe for the simultaneous monitoring and targeting of LOX-mediated collagen cross-linking that combines a LOX-activity sensor with a collagen peptide to chemoselectively target endogenous aldehydes generated by LOX. This synergistic probe becomes covalently anchored and lights up in vivo and in situ in response to LOX at the sites where cross-linking occurs, as demonstrated by staining of normal skin and cancer sections. We anticipate that our reactive collagen-based sensor will improve understanding of collagen remodeling and provide opportunities for the diagnosis of fibrotic and malignant diseases.

The Nrf2 transcription factor: A multifaceted regulator of the extracellular matrix.

The transcription factor nuclear factor-erythroid 2-related factor 2 (Nrf2) is widely recognized as a master regulator of the cellular stress response by facilitating the transcription of cytoprotective genes. As such, the Nrf2 pathway is critical in guarding the cell from the harmful effects of excessive reactive oxygen species/reactive nitrogen species (ROS/RNS) and in maintaining cellular redox balance. While excessive ROS/RNS are harmful to the cell, physiological levels of ROS/RNS play important roles in regulating numerous signaling pathways important for normal cellular function, including the synthesis of extracellular matrix (ECM). Recent advances have underscored the importance of ROS/RNS, and by extension, factors that influence redox-balance such as Nrf2, in regulating ECM production and deposition. In addition to reducing the oxidative burden in the cell, the discovery that Nrf2 can also directly target genes that regulate and form the ECM has cemented it as a multifaceted player in the regulation of ECM proteins, and provides new insight into its potential usefulness as a target for treating ECM-related pathologies.

A Dual-Acting Nitric Oxide Donor and Phosphodiesterase 5 Inhibitor Promotes Wound Healing in Normal Mice and Mice with Diabetes.

Chronic wounds affect a large percentage of the population worldwide and cause significant morbidity. Unfortunately, efficient compounds for the treatment of chronic wounds are yet not available. Endothelial dysfunction, which is at least in part a result of compromised nitric oxide production and concomitant reduction in cGMP levels, is a major pathologic feature of chronic wounds. Therefore, we designed and synthesized a compound with a unique dual-acting activity (TOP-N53), acting as a nitric oxide donor and phosphodiesterase 5 inhibitor, and applied it locally to full-thickness skin wounds in healthy and healing-impaired mice with diabetes. TOP-N53 promoted keratinocyte proliferation, angiogenesis, and collagen maturation in healthy mice without accelerating the wound inflammatory response or scar formation. Most importantly, it partially rescued the healing impairment of mice with genetically determined type II diabetes (db/db) by stimulating re-epithelialization and granulation tissue formation, including angiogenesis. In vitro studies with human and murine primary cells showed a positive effect of TOP-N53 on keratinocyte and fibroblast migration, keratinocyte proliferation, and endothelial cell migration and tube formation. These results demonstrate a remarkable healing-promoting activity of TOP-N53 by targeting the major resident cells in the wound tissue.

Comprehensive characterization of myeloid cells during wound healing in healthy and healing-impaired diabetic mice.

Wound healing involves the concerted action of various lymphoid and in particular myeloid cell populations. To characterize and quantitate different types of myeloid cells and to obtain information on their kinetics during wound healing, we performed multiparametric flow cytometry analysis. In healthy mice, neutrophil numbers increased early after injury and returned to near basal levels after completion of healing. Macrophages, monocyte-derived dendritic cells (DCs), and eosinophils were abundant throughout the healing phase, in particular in early wounds, and Langerhans cells increased after wounding and remained elevated after epithelial closure. Major differences in healing-impaired diabetic mice were a much higher percentage of immune cells in late wounds, mainly as a result of neutrophil, macrophage, and monocyte persistence; reduced numbers and percentages of macrophages and monocyte-derived DCs in early wounds; and of Langerhans cells, conventional DCs, and eosinophils throughout the healing process. Finally, unbiased cluster analysis (PhenoGraph) identified a large number of different clusters of myeloid cells in skin wounds. These results provide insight into myeloid cell diversity and dynamics during wound repair and highlight the abnormal inflammatory response associated with impaired healing.

Tussilagonone Ameliorates Psoriatic Features in Keratinocytes and Imiquimod-Induced Psoriasis-Like Lesions in Mice via NRF2 Activation.

Psoriasis is a common inflammatory skin disorder that is characterized by keratinocyte hyperproliferation and abnormal differentiation, resulting in the thickening of the epidermis and stratum corneum. In this study, we investigated in vitro and in vivo pharmacological effects of tussilagonone (TGN), a sesquiterpenoid isolated from Tussilago farfara, on transcription factors relevant for the pathogenesis of psoriasis. TGN inhibited activation of NF-κB and STAT3, leading to the attenuated expression of psoriasis-related inflammatory genes and suppression of keratinocyte hyperproliferation. Mechanistically, we show that the inhibition of NF-κB and STAT3 by TGN is mediated through activation of the cytoprotective transcription factor NRF2. Evaluation of in vivo antipsoriatic effects of topical TGN in the imiquimod-induced psoriasis-like dermatitis mouse model demonstrated amelioration of imiquimod-induced phenotypical changes, lesion severity score, epidermal thickening, and reduction in dermal cellularity. The spleen index also diminished in TGN-treated mice, suggesting anti-inflammatory properties of TGN. Moreover, TGN significantly attenuated the imiquimod-induced mRNA levels of psoriasis-associated inflammatory cytokines and antimicrobial peptides and reduced epidermal hyperproliferation. Taken together, TGN, as a potent NRF2 activator, is a promising therapeutic candidate for the development of antipsoriatic agents derived from medicinal plants.

Tissue Repair: Guarding against Friendly Fire.

Following tissue injury, cells produce reactive molecules that fight off invading pathogens, but these factors might also damage the host tissue. A new study has characterized a network of defense pathways that synergize to protect cells from collateral damage and drive repair.

Regulation of Wound Healing by the NRF2 Transcription Factor-More Than Cytoprotection.

The nuclear factor-erythroid 2-related factor 2 (NRF2) transcription factor plays a central role in mediating the cellular stress response. Due to their antioxidant properties, compounds activating NRF2 have received much attention as potential medications for disease prevention, or even for therapy. Accumulating evidence suggests that activation of the NRF2 pathway also has a major impact on wound healing and may be beneficial in the treatment of chronic wounds, which remain a considerable health and economic burden. While NRF2 activation indeed shows promise, important considerations need to be made in light of corresponding evidence that also points towards pro-tumorigenic effects of NRF2. In this review, we discuss the evidence to date, highlighting recent advances using gain- and loss-of-function animal models and how these data fit with observations in humans.

Nrf2-Mediated Fibroblast Reprogramming Drives Cellular Senescence by Targeting the Matrisome.

Nrf2 is a key regulator of the antioxidant defense system, and pharmacological Nrf2 activation is a promising strategy for cancer prevention and promotion of tissue repair. Here we show, however, that activation of Nrf2 in fibroblasts induces cellular senescence. Using a combination of transcriptomics, matrix proteomics, chromatin immunoprecipitation and bioinformatics we demonstrate that fibroblasts with activated Nrf2 deposit a senescence-promoting matrix, with plasminogen activator inhibitor-1 being a key inducer of the senescence program. In vivo, activation of Nrf2 in fibroblasts promoted re-epithelialization of skin wounds, but also skin tumorigenesis. The pro-tumorigenic activity is of general relevance, since Nrf2 activation in skin fibroblasts induced the expression of genes characteristic for cancer-associated fibroblasts from different mouse and human tumors. Therefore, activated Nrf2 qualifies as a marker of the cancer-associated fibroblast phenotype. These data highlight the bright and the dark sides of Nrf2 and the need for time-controlled activation of this transcription factor.

Regulatory T cells are required for normal and activin-promoted wound repair in mice.

Healing of skin wounds is orchestrated by various types of immune cells, but little is known about the role of FoxP3+ regulatory T cells (Tregs) in this process. Here, we determined if Tregs are important for wound healing in normal mice and if they contribute to the accelerated healing of mice overexpressing the growth and differentiation factor activin. Diphtheria toxin induced Treg depletion prior to injury caused impaired healing characterized by delayed reepithelialization, reduced wound contraction, and impaired vessel maturation. The accelerated wound repair of activin-transgenic mice was also abrogated. Mechanistically, we found a strong increase in IL-4 levels combined with overrepresentation of T-bet+ and GATA-3+ αß T cells in Treg-depleted 7-day wounds. In addition, numbers of IFN-γ- or IL-17A-producing CD4+ and CD4- T cells were elevated. These results demonstrate that Treg depletion in wounds facilitates the expansion of an αß T-cell population with features of Th1 and Th2 cells, and suggest that concomitant changes in the cytokine milieu disturb the healing process.

The nucleotide-binding domain, leucine-rich repeat protein 3 inflammasome/IL-1 receptor I axis mediates innate, but not adaptive, immune responses after exposure to particulate matter under 10 µm.

Exposure to particulate matter (PM), a major component of air pollution, contributes to increased morbidity and mortality worldwide. Inhaled PM induces innate immune responses by airway epithelial cells that may lead to the exacerbation or de novo development of airway disease. We have previously shown that 10-µm PM (PM10) activates the nucleotide-binding domain, leucine-rich repeat protein (NLRP) 3 inflammasome in human airway epithelial cells. Our objective was to determine the innate and adaptive immune responses mediated by the airway epithelium NLRP3 inflammasome in response to PM10 exposure. Using in vitro cultures of human airway epithelial cells and in vivo studies with wild-type and Nlrp3(-/-) mice, we investigated the downstream consequences of PM10-induced NLPR3 inflammasome activation on cytokine production, cellular inflammation, dendritic cell activation, and PM10-facilitated allergic sensitization. PM10 activates an NLRP3 inflammasome/IL-1 receptor I (IL-1RI) axis in airway epithelial cells, resulting in IL-1ß, CC chemokine ligand-20, and granulocyte/macrophage colony-stimulating factor production, which is associated with dendritic cell activation and lung neutrophilia. Despite these profound innate immune responses in the airway epithelium, the NLRP3 inflammasome/IL-1RI axis is dispensable for PM10-facilitated allergic sensitization. We demonstrate the importance of the lung NLRP3 inflammasome in mediating PM10 exposure-associated innate, but not adaptive, immune responses. Our study highlights a mechanism by which PM10 exposure can contribute to the exacerbation of airway disease, but not PM10-facilitated allergic sensitization.

Perforin and granzyme B have separate and distinct roles during atherosclerotic plaque development in apolipoprotein E knockout mice.

The granzyme B/perforincytotoxic pathway is a well established mechanism of initiating target cell apoptosis. Previous studies have suggested a role for the granzyme B/perforin cytotoxic pathway in vulnerable atherosclerotic plaque formation. In the present study, granzyme B deficiency resulted in reduced atherosclerotic plaque development in the descending aortas of apolipoprotein E knockout mice fed a high fat diet for 30 weeks while perforindeficiency resulted in greater reduction in plaque development with significantly less plaque area than granzyme Bdeficient mice. In contrast to the descending aorta, no significant change in plaque size was observed in aortic roots from either granzyme Bdeficient or perforindeficient apolipoprotein E knockout mice. However, atherosclerotic plaques in the aortic roots did exhibit significantly more collagen in granzyme B, but not perforin deficient mice. Together these results suggest significant, yet separate roles for granzyme B and perforin in the pathogenesis of atherosclerosis that go beyond the traditional apoptotic pathway with additional implications in plaque development, stability and remodelling of extracellular matrix.