Human reconstructed skin xenografts on mice to model skin physiology

Dose-dependent effects of Nrf2 on the epidermis in chronic skin inflammation

Atopic dermatitis (AD) is a chronic inflammatory skin disease, characterized by an impaired epidermal barrier and immunological alterations. The activity of the cytoprotective NRF2 transcription factor is reduced in the epidermis of AD patients. To determine the functional relevance of this deficiency, we used mice lacking fibroblast growth factor receptors 1 and 2 in keratinocytes (K5-R1/R2 mice), which exhibit several AD-like symptoms. Proteomics analysis of their epidermis revealed reduced Nrf2 activity. This was accompanied by an increase in DNA damage and in the number of senescent cells. Genetic deletion of Nrf2 in keratinocytes of these mice further promoted DNA damage and senescence, but time-limited pharmacological activation of Nrf2 in the skin had a mild protective effect. Surprisingly, long-term genetic activation of Nrf2 in keratinocytes of K5-R1/R2 mice caused strong hyperkeratosis, keratinocyte hyperproliferation, epidermal thickening, increased keratinocyte apoptosis and DNA damage, and altered immune cell composition. These results reveal a complex role of Nrf2 in the epidermis and show the necessity to optimize the duration and intensity of NRF2 activation for the treatment of epidermal alterations in patients with AD.

Oyster hydrolysate ameliorates UVB-induced skin dehydration and barrier dysfunction

Ultraviolet (UV) exposure triggers skin aging primarily by disrupting skin barrier function, resulting in dry skin and wrinkle formation. Oyster hydrolysate (OH), as a functional food, has been reported for anti-cancer, anti-oxidant and anti-apoptotic effects. This study investigated the underlying mechanism of OH effect on UVB-induced skin aging in SKH1 hairless mice. Mice were exposed to UVB three times per week while they were fed with a normal diet or diet containing OH for 10 weeks. Additionally, a randomized, double-blind, and placebo-controlled clinical trial was performed to investigate the OH effect on human skin moisturizing to evaluate its efficacy and safety. UVB exposure increased parameters of skin aging; dehydration, transepidermal water loss, and macroscopic dorsal skin lesions. OH significantly reduced these features of skin aging. Histological analysis demonstrated that OH decreased skin epidermal and dermal thickness and collagen degradation induced by UVB. OH significantly reduced ROS production, suppressed macrophage activation and neutrophil infiltration, and downregulated pro-inflammatory cytokine production. OH improved skin barrier function by increasing the expression of filaggrin, aquaporin-3, and hyaluronic acid synthesis enzymes and promoting recovery from skin damage. Importantly, the results from a human clinical trial demonstrated that OH improved skin moisturization and integrity with no side effects. Taken together, OH supplementation ameliorates skin damage via anti-oxidant and anti-inflammatory properties and enhances skin hydration and barrier function. OH has a therapeutic potential for skin photoaging.

Investigating Snake-Venom-Induced Dermonecrosis and Inflammation Using an Ex Vivo Human Skin Model

Snakebite envenoming is a neglected tropical disease that causes >100,000 deaths and >400,000 cases of morbidity annually. Despite the use of mouse models, severe local envenoming, defined by morbidity-causing local tissue necrosis, remains poorly understood, and human-tissue responses are ill-defined. Here, for the first time, an ex vivo, non-perfused human skin model was used to investigate temporal histopathological and immunological changes following subcutaneous injections of venoms from medically important African vipers (Echis ocellatus and Bitis arietans) and cobras (Naja nigricollis and N. haje). Histological analysis of venom-injected ex vivo human skin biopsies revealed morphological changes in the epidermis (ballooning degeneration, erosion, and ulceration) comparable to clinical signs of local envenoming. Immunostaining of these biopsies confirmed cell apoptosis consistent with the onset of necrosis. RNA sequencing, multiplex bead arrays, and ELISAs demonstrated that venom-injected human skin biopsies exhibited higher rates of transcription and expression of chemokines (CXCL5, MIP1-ALPHA, RANTES, MCP-1, and MIG), cytokines (IL-1β, IL-1RA, G-CSF/CSF-3, and GM-CSF), and growth factors (VEGF-A, FGF, and HGF) in comparison to non-injected biopsies. To investigate the efficacy of antivenom, SAIMR Echis monovalent or SAIMR polyvalent antivenom was injected one hour following E. ocellatus or N. nigricollis venom treatment, respectively, and although antivenom did not prevent venom-induced dermal tissue damage, it did reduce all pro-inflammatory chemokines, cytokines, and growth factors to normal levels after 48 h. This ex vivo skin model could be useful for studies evaluating the progression of local envenoming and the efficacy of snakebite treatments.

Histological and functional characterization of 3D human skin models mimicking the inflammatory skin diseases psoriasis and atopic dermatitis

Three-dimensional (3D) human skin equivalents have emerged as valuable tools in skin research, replacing animal experimentation and precluding the need for patient biopsies. In this study, we advanced 3D skin equivalents to model the inflammatory skin diseases atopic dermatitis and psoriasis by cytokine stimulation, and were successful in integrating TH1 T cells into skin models to develop an immunocompetent 3D psoriasis model. We performed in-depth histological and functional characterization of 3D skin equivalents and validated them in terms of tissue architecture, pathological changes, expression of antimicrobial peptides and Staphylococcus aureus colonization using 3D reconstruction by multiphoton microscopy and phenotyping by highly multiplexed 'co-detection by indexing' (CODEX) microscopy. We show that our skin equivalents have a structural architecture with a well-developed dermis and epidermis, thus resembling human skin. In addition, the skin models of atopic dermatitis and psoriasis show several phenotypic features of inflammatory skin disease, including disturbed epidermal differentiation and alterations in the expression of epidermal barrier genes and antimicrobial peptides, and can be reliably used to test novel treatment strategies. Therefore, these 3D equivalents will be a valuable tool in experimental dermatological research.

NEK2 overexpression aggravates IL-22-induced keratinocyte proliferation and cytokine level increases and IMQ-induced psoriasis-like dermatitis

BACKGROUND

Psoriasis is a common inflammatory skin disease characterized by the excessive proliferation and abnormal differentiation of keratinocytes. Protein kinases could act on intracellular signaling pathways associated with cell proliferation.

OBJECTIVE

Identifying more hub protein kinases affecting cellular and molecular processes in psoriasis, and exploring the dynamic effects of baicalin and NEK2 on the IL-22-induced cellular inflammation and IMQ-induced psoriasis-like mice.

METHODS AND RESULTS

In this study, differentially expressed protein kinases playing a hub role in psoriasis initiation and development were identified using integrative bioinformatics analyses, and NEK2 has been chosen. NEK2 was significantly up-regulated in psoriatic samples according to online datasets and experimental analyses. In IL-22-induced cellular inflammation model in HaCaT cells, NEK2 overexpression promoted, whereas NEK2 knockdown partially abolished IL-22-induced alterations in cell viability, DNA synthesis, cytokine levels, as well as STAT3 phosphorylation and p-RB, cyclin D1, CDK4, and CDK6 protein contents. Baicalin treatment partially suppressed IL-22-induced HaCaT cell viability, DNA synthesis, and increases in cytokine levels, whereas NEK2 overexpression significantly abolished Baicalin-induced protection against cellular inflammation. In IMQ-induced psoriasis-like skin inflammation model in mice, baicalin markedly ameliorated IMQ-induced psoriasis-like symptoms and local skin inflammation, whereas NEK2 overexpression partially eliminated the therapeutic effects of baicalin.

CONCLUSION

NEK2, up-regulated in psoriatic lesion skin, could aggravate IMQ-induced psoriasis-like dermatitis and attenuate the therapeutic efficiency of baicalin through promoting keratinocyte proliferation and cytokine levels. The STAT3 signaling might be involved.

The frontline of alternatives to animal testing: novel in vitro skin model application in drug development and evaluation

The FDA Modernization Act 2.0 has brought nonclinical drug evaluation into a new era. In vitro models are widely used and play an important role in modern drug development and evaluation, including early candidate drug screening and preclinical drug efficacy and toxicity assessment. Driven by regulatory steering and facilitated by well-defined physiology, novel in vitro skin models are emerging rapidly, becoming the most advanced area in alternative testing research. The revolutionary technologies bring us many in vitro skin models, either laboratory-developed or commercially available, which were all built to emulate the structure of the natural skin to recapitulate the skin's physiological function and particular skin pathology. During the model development, how to achieve balance among complexity, accessibility, capability, and cost-effectiveness remains the core challenge for researchers. This review attempts to introduce the existing in vitro skin models, align them on different dimensions, such as structural complexity, functional maturity, and screening throughput, and provide an update on their current application in various scenarios within the scope of chemical testing and drug development, including testing in genotoxicity, phototoxicity, skin sensitization, corrosion/irritation. Overall, the review will summarize a general strategy for in vitro skin model to enhance future model invention, application, and translation in drug development and evaluation.

Single-cell analysis of human dermal fibroblasts isolated from a single male donor over 35 years

The aim of this study is to examine the effects of ageing on dermal fibroblast heterogeneity based on samples obtained from the same donor. We used a dermal fibroblast lineage (named ASF-4 cell lines) isolated from the inner side of the upper arm of a healthy male donor over a 35-year period, beginning at 36 years of age. Because clonal analysis of ASF-4 cell lines demonstrated a donor age-dependent loss of proliferative capacity and acquisition of senescent traits at the single-cell level, cultured cells frozen at passage 10 at ages 36 and 72 years were subjected to single-cell RNA sequencing. Transcriptome analysis revealed an increase in senescent fibroblasts and downregulation of genes associated with extracellular matrix remodelling with ageing. In addition, two putative differentiation pathways, with one endpoint consisting of senescent fibroblasts and the other without, were speculated using a pseudo-time analysis. Knockdown of the characteristic gene of the non-senescent fibroblast cluster endpoint, EFEMP2, accelerated cellular senescence. This was also confirmed in two other normal human dermal fibroblast cell lines. The detection of a common cellular senescence-related gene from single-donor analysis is notable. This study provides new insights into the behaviour of dermal fibroblasts during skin ageing.

Comparing the application of various engineered xenografts for skin defects: A systematic review

INTRODUCTION

Xenografts are a now a cornerstone in the management of wound dressings. Promising results were achieved since 1960 in the application of skin substitute for skin defects.

OBJECTIVE

The objective of this study was to evaluate the efficacy of various xenografts.

METHODS

A literature research was conducted using the following query: 'Porcine skin dermatology substitute', 'bovine skin dermatology substitute', 'xenograft skin substitute dermatology', 'xenografts skin defect', 'porcine skin defect', 'bovine skin defect'.

RESULTS

The review yielded 35 articles pertaining to the topic. Main indications for porcine and bovine xenograft application were burn wounds and post-traumatic wounds, respectively. Mean discharge date or length of stay was at the 6th day after porcine application, and the time of graft healing was reported for 33.7% (n = 510) of patients. Promising results were seen with Matriderm and split-thickness skin graft. Most wounds achieved an excellent cosmetic result with full range of motion and a smooth contour appearance. A great variety of tissue substitutes exist, and the choice of graft application should depend on a patient's factors, product availability, wound type, size, and physician's factors.

CONCLUSION

In summary, xenografts are more economic and affordable but have higher risk of infections compared to allografts.

Novel strategies for designing regenerative skin products for accelerated wound healing

Healthy skin protects from pathogens, water loss, ultraviolet rays, and also maintains homeostasis conditions along with sensory perceptions in normal circumstances. Skin wound healing mechanism is a multi-phased biodynamic process that ultimately triggers intercellular and intracellular mechanisms. Failure to implement the normal and effective healing process may result in chronic injuries and aberrant scarring. Chronic wounds lead to substantial rising healthcare expenditure, and innovative methods to diagnose and control severe consequences are urgently needed. Skin tissue engineering (STE) has achieved several therapeutic accomplishments during the last few decades, demonstrating tremendous development. The engineered skin substitutes provide instant coverage for extensive wounds and facilitate the prevention of microbial infections and fluid loss; furthermore, they help in fighting inflammation and allow rapid neo-tissue formation. The current review primarily focused on the wound recovery and restoration process and the current conditions of STE with various advancements and complexities associated with different strategies such as cell sources, biopolymers, innovative fabrication techniques, and growth factors delivery systems.

A Novel Xenograft Model Demonstrates Human Fibroblast Behavior During Skin Wound Repair and Fibrosis

Objective: Xenografts of human skin in immunodeficient mice provide a means of assessing human skin physiology and its response to wounding. Approach: We describe a novel xenograft model using full-thickness human neonatal foreskin to examine human skin wound repair. Full-thickness 8 mm human neonatal foreskin biopsies were sutured into the dorsum of NOD scid gamma (NSG; NOD.Cg-Prkdc scidIl2rgtm1Wjl/SzJ) pups as subcutaneous grafts. At postnatal day 21 the subcutaneous grafts were exposed to cutaneous grafts. Following maturation of 2 months, xenografts were then wounded with 5 mm linear incisions and monitored until postwound day (PWD) 14 to study skin repair and fibrosis. To explore whether our model can be used to test the efficacy of topical therapies, wounded xenografts were injected with antifibrotic fibroblast growth factor 2 (FGF2) for the first four consecutive PWDs. Xenografts were harvested for analysis by histology and fluorescence-activated cell sorting (FACS). Results: Xenografts were successfully engrafted with evidence of mouse-human anastomoses and resembled native neonatal foreskin at the gross and microscopic level. Wounded xenografted skin scarred with human collagen and an expansion of CD26-positive human fibroblasts. Collagen scar was quantitated by neural network analysis, which revealed distinct clustering of collagen fiber networks from unwounded skin and wounded skin at PWD7 and PWD14. Collagen fiber networks within FGF2-treated wounds at PWD14 resembled those in untreated wounded xenografts at PWD7, suggesting that FGF2 treatment at time of wounding can reduce fibrosis. Innovation and Conclusion: This novel xenograft model can be used to investigate acute fibrosis, fibroblast heterogeneity, and the efficacy of antifibrotic agents during wound repair in human skin.

The Rho guanosine nucleotide exchange factors Vav2 and Vav3 modulate epidermal stem cell function

It is known that Rho GTPases control different aspects of the biology of skin stem cells (SSCs). However, little information is available on the role of their upstream regulators under normal and tumorigenic conditions in this process. To address this issue, we have used here mouse models in which the activity of guanosine nucleotide exchange factors of the Vav subfamily has been manipulated using both gain- and loss-of-function strategies. These experiments indicate that Vav2 and Vav3 regulate the number, functional status, and responsiveness of hair follicle bulge stem cells. This is linked to gene expression programs related to the reinforcement of the identity and the quiescent state of normal SSCs. By contrast, in the case of cancer stem cells, they promote transcriptomal programs associated with the identity, activation state, and cytoskeletal remodeling. These results underscore the role of these Rho exchange factors in the regulation of normal and tumor epidermal stem cells.

Generation and characterization of hair-bearing skin organoids from human pluripotent stem cells

Human skin uses millions of hairs and glands distributed across the body surface to function as an external barrier, thermoregulator and stimuli sensor. The large-scale generation of human skin with these appendages would be beneficial, but is challenging. Here, we describe a detailed protocol for generating hair-bearing skin tissue entirely from a homogeneous population of human pluripotent stem cells in a three-dimensional in vitro culture system. Defined culture conditions are used over a 2-week period to induce differentiation of pluripotent stem cells to surface ectoderm and cranial neural crest cells, which give rise to the epidermis and dermis, respectively, in each organoid unit. After 60 d of incubation, the skin organoids produce hair follicles. By day ~130, the skin organoids reach full complexity and contain stratified skin layers, pigmented hair follicles, sebaceous glands, Merkel cells and sensory neurons, recapitulating the cell composition and architecture of fetal skin tissue at week 18 of gestation. Skin organoids can be maintained in culture using this protocol for up to 150 d, enabling the organoids to be used to investigate basic skin biology, model disease and, further, reconstruct or regenerate skin tissue.

Mouse Models for Human Skin Transplantation: A Systematic Review

Immunodeficient mouse models with human skin xenografts have been developed in the past decades to study different conditions of the skin. Features such as follow-up period and size of the graft are of different relevance depending on the purpose of an investigation. The aim of this study is to analyze the different mouse models grafted with human skin. A systematic review of the literature was performed in line with the PRISMA statement using MEDLINE/PubMed databases from January 1970 to June 2020. Articles describing human skin grafted onto mice were included. Animal models other than mice, skin substitutes, bioengineered skin, postmortem or fetal skin, and duplicated studies were excluded. The mouse strain, origin of human skin, graft dimensions, follow-up of the skin graft, and goals of the study were analyzed. Ninety-one models were included in the final review. Five different applications were found: physiology of the skin (25 models, mean human skin graft size 1.43 cm2 and follow-up 72.92 days), immunology and graft rejection (17 models, mean human skin graft size 1.34 cm2 and follow-up 86 days), carcinogenesis (9 models, mean human skin graft size 1.98 cm2 and follow-up 253 days), skin diseases (25 models, mean human skin graft size 1.55 cm2 and follow-up 86.48 days), and would healing/scars (15 models, mean human skin graft size 2.54 cm2 and follow-up 129 days). The follow-up period was longer in carcinogenesis models (253 ± 233.73 days), and the skin graft size was bigger in wound healing applications (2.54 ± 3.08 cm2). Depending on the research application, different models are suggested. Careful consideration regarding graft size, follow-up, immunosuppression, and costs should be analyzed and compared before choosing any of these mouse models. To our knowledge, this is the first systematic review of mouse models with human skin transplantation.

Human Three-Dimensional Models for Studying Skin Pathogens

Skin is the most exposed surface of the human body, separating the microbe-rich external environment, from the sterile inner part. When skin is breached or its homeostasis is perturbed, bacterial, fungal and viral pathogens can cause local infections or use the skin as an entry site to spread to other organs. In the last decades, it has become clear that skin provides niches for permanent microbial colonization, and it actively interacts with microorganisms. This crosstalk promotes skin homeostasis and immune maturation, preventing expansion of harmful organisms. Skin commensals, however, are often found to be skin most prevalent and dangerous pathogens. Despite the medical interest, mechanisms of colonization and invasion for most skin pathogens are poorly understood. This limitation is due to the lack of reliable skin models. Indeed, animal models do not adequately mimic neither the anatomy nor the immune response of human skin. Human 3D skin models overcome these limitations and can provide new insights into the molecular mechanisms of microbial pathogenesis. Herein, we address the strengths and weaknesses of different types of human skin models and we review the main findings obtained using these models to study skin pathogens.

Comparative intravital imaging of human and rodent malaria sporozoites reveals the skin is not a species-specific barrier

Malaria infection starts with the injection of Plasmodium sporozoites into the host’s skin. Sporozoites are motile and move in the skin to find and enter blood vessels to be carried to the liver. Here, we present the first characterization of P. falciparum sporozoites in vivo, analyzing their motility in mouse skin and human skin xenografts and comparing their motility to two rodent malaria species. These data suggest that in contrast to the liver and blood stages, the skin is not a species‐specific barrier for Plasmodium. Indeed, P. falciparum sporozoites enter blood vessels in mouse skin at similar rates to the rodent malaria parasites. Furthermore, we demonstrate that antibodies targeting sporozoites significantly impact the motility of P. falciparum sporozoites in mouse skin. Though the sporozoite stage is a validated vaccine target, vaccine trials have been hampered by the lack of good animal models for human malaria parasites. Pre‐clinical screening of next‐generation vaccines would be significantly aided by the in vivo platform we describe here, expediting down‐selection of candidates prior to human vaccine trials.

Transcriptomic Analysis of a Diabetic Skin-Humanized Mouse Model Dissects Molecular Pathways Underlying the Delayed Wound Healing Response

Defective healing leading to cutaneous ulcer formation is one of the most feared complications of diabetes due to its consequences on patients' quality of life and on the healthcare system. A more in-depth analysis of the underlying molecular pathophysiology is required to develop effective healing-promoting therapies for those patients. Major architectural and functional differences with human epidermis limit extrapolation of results coming from rodents and other small mammal-healing models. Therefore, the search for reliable humanized models has become mandatory. Previously, we developed a diabetes-induced delayed humanized wound healing model that faithfully recapitulated the major histological features of such skin repair-deficient condition. Herein, we present the results of a transcriptomic and functional enrichment analysis followed by a mechanistic analysis performed in such humanized wound healing model. The deregulation of genes implicated in functions such as angiogenesis, apoptosis, and inflammatory signaling processes were evidenced, confirming published data in diabetic patients that in fact might also underlie some of the histological features previously reported in the delayed skin-humanized healing model. Altogether, these molecular findings support the utility of such preclinical model as a valuable tool to gain insight into the molecular basis of the delayed diabetic healing with potential impact in the translational medicine field.

The dual delivery of growth factors and antimicrobial peptide by PLGA/GO composite biofilms to promote skin-wound healing

Biodegradable biomaterials coated with active factors are effective medical devices to promote wound healing.

Insights into atopic dermatitis gained from genetically defined mouse models

Atopic dermatitis (AD) is characterized by severe pruritus and recurrent eczema with a chronic disease course. Impaired skin barrier function, hyperactivated T_H2 cell-type inflammation, and pruritus-induced scratching contribute to the disease pathogenesis of AD. Skin microbial alterations complicate the pathogenesis of AD further. Mouse models are a powerful tool to analyze such intricate pathophysiology of AD, with a caution that anatomy and immunology of the skin differ between human subjects and mice. Here we review recent understanding of AD etiology obtained using mouse models, which address the epidermal barrier, skin microbiome, T_H2 immune response, and pruritus.

Lessons learned from the development of a hidradenitis suppurativa xenograft mouse model

Hidradenitis suppurativa (HS) is a chronic inflammatory skin disease originating from the pilosebaceous unit, in which patients develop painful abscesses, sinus tracts, nodules and scarring, typically in intertriginous areas. Major gaps in our understanding of HS exist, and these may be partially due to the lack of an animal model for experimental studies. We developed an HS xenograft mouse model using human HS lesions grafted onto immunocompromised mice. Although the model had its limitations, several informative lessons were learned, which may contribute to future attempts at an HS animal model.

Intravital microscopy: Imaging host-parasite interactions in the brain

Intravital fluorescence microscopy (IVM) is a powerful technique for imaging multiple organs, including the brain of living mice and rats. It enables the direct visualisation of cells in situ providing a real-life view of biological processes that in vitro systems cannot. In addition, to the technological advances in microscopy over the last decade, there have been supporting innovations in data storage and analytical packages that enable the visualisation and analysis of large data sets. Here, we review the advantages and limitations of techniques predominantly used for brain IVM, including thinned skull windows, open skull cortical windows, and a miniaturised optical system based on microendoscopic probes that can be inserted into deep tissues. Further, we explore the relevance of these techniques for the field of parasitology. Several protozoan infections are associated with neurological symptoms including Plasmodium spp., Toxoplasma spp., and Trypanosoma spp. IVM has led to crucial findings on these parasite species, which are discussed in detail in this review.

PEG-Plasma Hydrogels Increase Epithelialization Using a Human Ex Vivo Skin Model

In vitro cell culture methods are used extensively to study cellular migration, proliferation, and differentiation, which play major roles in wound healing but the results often do not translate to the in vivo environment. One alternative would be to establish an ex vivo model utilizing human discarded skin to evaluate therapies in a more natural setting. The purpose of this study was to institute such a model by creating ‘wounds’ in the center of a piece of discarded skin and treating them with three different biomaterials: collagen, polyethylene glycol (PEG)-fibrin, or PEG-platelet free plasma (PFP). Explants were cultured for 14 days with supernatant and microscopy images collected every 3 days to assess cytotoxicity and epithelialization. After 14 days, the explants were fixed, sectioned, and stained for cytokeratin-10 (CK-10), alpha-smooth muscle actin (α-SMA), and wheat germ (WG). Compared to controls, similar levels of cytotoxicity were detected for 12 days which decreased slightly at day 14. The PEG-PFP hydrogel-treated wounds epithelialized faster than other treatments at days 6 to 14. A 6-8 cell layer thick CK-10+ stratified epidermis had developed over the PEG-PFP hydrogel and cells co-stained by WG and α-SMA were observed within the hydrogel. An ex vivo model was established that can be used practically to screen different therapies exploring wound healing.

Oral implant osseointegration model in C57Bl/6 mice: microtomographic, histological, histomorphometric and molecular characterization

Despite the successful clinical application of titanium (Ti) as a biomaterial, the exact cellular and molecular mechanisms responsible for Ti osseointegration remains unclear, especially because of the limited methodological tools available in this field.