Establishment and characterization of a reconstructed Chinese human epidermis model

Construction of a pumpless gravity-driven vascularized Skin-on-a-Chip for the study of hepatocytotoxicity in percutaneous exposure to exogenous chemicals

The utilization of existing Skin-on-a-Chip (SoC) is constrained by the complex structures, the multiplicity of auxiliary devices, and the inability to evaluate exogenous chemicals that are hepatotoxic after percutaneous metabolism. In this study, a gravity-driven SoC without any auxiliary devices was constructed for the hepatocytotoxicity study of exogenous chemicals. The SoC possesses 3 layers of culture chambers, from top to bottom, for human skin equivalent (HSE), Human Umbilical Vein Endothelial Cells (HUVEC) and hepatocytes (HepG2), and the maintenance and expression capacity of the corresponding cells on the SoC were verified by specificity parameters. The reactivity of the SoC to exogenous chemicals was verified by 2-aminofluorene (2-AF). The SoC can realistically simulate the in vivo exposure process of exogenous chemicals that are percutaneously exposed and metabolized into the bloodstream and then to the liver to produce toxicity, and it can achieve the same effects on transcriptome as those of animal tests at lower exposure levels while examining multiple toxicological targets of the skin, vascular endothelial cells, and hepatocytes. Both in terms of species similarity, the principles of reduction, replacement and refinement (3R), or the level of exposure suggest that the present SoC has a degree of replacement for animal models in assessing exogenous chemicals, especially those that are hepatotoxic after percutaneous metabolism.

A modified protocol for studying filaggrin degradation using a reconstructed human epidermis model under low and high humidity

BACKGROUND

Filaggrin (FLG) is an essential protein that plays a vital role in maintaining skin barrier function and moisture levels, allowing the skin to adapt to dry environments. However, the precise temporal dynamics of FLG metabolism in the human epidermis remain poorly understood, and suitable tools to study these time-dependent effects are currently lacking.

OBJECTIVE

To investigate the molecular mechanisms and time course of FLG metabolism and skin barrier function under high- and low-humidity conditions, utilizing a reconstructed epidermis model.

METHODS

EpiSkin specimens cultured under humid or dry conditions for varying durations (2-48 h) were compared by assessing FLG degradation and skin barrier formation using immunofluorescence staining and western blotting.

RESULTS

Under conditions of low humidity, the proteolysis of FLG in EpiSkin increased between 4 and 12 h and was accompanied by elevated levels of cysteine-aspartic protease (caspase)-14. The expression of peptidyl arginine deiminase 1 and calpain 1 also increased at 4 h. However, after 24 h, the expression of these three FLG-degrading proteins significantly decreased. Conversely, the levels of pyrrolidone-5-carboxylic acid and urocanic acid initially decreased at 2 h and then increased between 12 and 24 h. Additionally, the expression of skin barrier proteins, such as FLG, transglutaminase 5, loricrin and zonula occludens-1, decreased starting from 12 h. Notably, epidermal cell viability and activity were also inhibited.

CONCLUSION

We propose a reliable and ethical model to study the temporal dynamics of FLG metabolism and its role in skin barrier function. Using a commercially reconstructed epidermis to mimic dry skin formation obviates the need for animal and human testing.

Glycolic acid-induced disruption of epidermal homeostasis in a skin equivalent model: Insights into temporal dynamics and mechanisms

Glycolic acid (GA) is extensively used in cosmetic formulations and skin peeling treatments but its adverse effects, notably severe disruption of epidermal structure, limit its clinical utility. However, the detailed impact of GA on epidermal homeostasis, including changes in structure and protein expression over time, is not fully understood. This study employed a reconstructed human epidermis (RHE) model to assess the effects of varying GA concentrations on epidermal proliferation, differentiation, and desquamation at different time points. Through histology, immunofluorescence, and immunohistochemistry, we observed that 35% GA concentration adversely caused abnormal epidermal homeostasis by affecting epidermal proliferation, differentiation and desquamation. Our findings reveal time-specific responses of key proteins to GA: Filaggrin, Involucrin, Loricrin, and Ki67 showed very early responses; KLK10 an early response; and AQP3 and K10 late responses. This research provides a detailed characterization of GA's effects in an RHE model, mimicking clinical superficial peeling and identifying optimal times for detecting GA-induced changes. Our results offer insights for designing interventions to mitigate GA's adverse effects on skin, enhancing the safety and efficacy of GA peeling treatments.

Incorporation of hair follicles in 3D bioprinted models of human skin

Current approaches fail to adequately introduce complex adnexal structures such as hair follicles within tissue engineered models of skin. Here, we report on the use of 3D bioprinting to incorporate these structures in engineered skin tissues. Spheroids, induced by printing dermal papilla cells (DPCs) and human umbilical vein cells (HUVECs), were precisely printed within a pregelled dermal layer containing fibroblasts. The resulting tissue developed hair follicle-like structures upon maturation, supported by migration of keratinocytes and melanocytes, and their morphology and composition grossly mimicked that of the native skin tissue. Reconstructed skin models with increased complexity that better mimic native adnexal structures can have a substantial impact on regenerative medicine as grafts and efficacy models to test the safety of chemical compounds.

How to facilitate the implementation of 3D models in China by applying good in vitro method practice for regulatory use

The Organization for Economic Co-operation and Development (OECD) Guidance Document No. 34 and No. 286 on Good In Vitro Method Practices (GIVIMPs) for the development and implementation of in vitro methods for regulatory use in human safety assessment have been endorsed. Considering that China is accelerating the development of alternative approaches in both research and acceptance, early application of these principles is beneficial to the implementation and acceptance of in vitro alternative methods in China. To promote the replacement of animal testing for regulatory use, L’Oréal initiated the EpiSkin™ skin irritation test (SIT) implementation program in China. More than 50 external scientists participated, and the method has been established in 34 organizations including authorities, industries, and testing service laboratories. Taking two collaborations with Guangdong CDC and Shanghai SGS for in vitro SIT as examples, we demonstrated a method implementation process in good alignment with the OECD principles. The current study illustrated the practical way in which both OECD Guidance documents assisted in the transfer and establishment of in vitro approaches and further promoted the future scientific recognition and acceptance of new OECD-accepted alternative testing methodologies in China.

Transferability and reproducibility of the EpiSkin™ Micronucleus Assay

A novel in vitro 3D micronucleus assay was developed in China using the EpiSkin™ 3D human skin model. This EpiSkin™ Micronucleus Assay showed good predictivity and reproducibility during internal validation and is expected to contribute to in vitro genotoxicity testing as a follow-up for positive results from 2D micronucleus assay. Having developed the assay in one laboratory, further work focused on the transferability and inter-laboratory reproducibility in two additional Chinese authority laboratories (Guangdong Provincial Center for Disease Control and Prevention and Zhejiang Institute for Food and Drug Control). Formal training was provided for both laboratories, which resulted in good transferability based on the results of two positive compounds, such as mitomycin C and vinblastine. Independent experiments were then performed, and inter-laboratory reproducibility was checked using 2-acetylaminofluorene, 5-fluorouracil, 2,4-dichlorophenol, and d-limonene. The dose-responses of the positive control chemical, mitomycin C, were similar to those of the developing laboratory, and all test chemicals were correctly classified by all laboratories. Overall, there was a good transferability as well as intra- and inter-laboratory reproducibility of the EpiSkin™ Micronucleus Assay. This study further confirmed the assay's robustness and provided confidence to enter following validation stages for scientific acceptance.

Evaluation of native and non-native biomaterials for engineering human skin tissue

A variety of human skin models have been developed for applications in regenerative medicine and efficacy studies. Typically, these employ matrix molecules that are derived from non-human sources along with human cells. Key limitations of such models include a lack of cellular and tissue microenvironment that is representative of human physiology for efficacy studies, as well as the potential for adverse immune responses to animal products for regenerative medicine applications. The use of recombinant extracellular matrix proteins to fabricate tissues can overcome these limitations. We evaluated animal- and non-animal-derived scaffold proteins and glycosaminoglycans for the design of biomaterials for skin reconstruction in vitro. Screening of proteins from the dermal-epidermal junction (collagen IV, laminin 5, and fibronectin) demonstrated that certain protein combinations when used as substrates increase the proliferation and migration of keratinocytes compared to the control (no protein). In the investigation of the effect of components from the dermal layer (collagen types I and III, elastin, hyaluronic acid, and dermatan sulfate), the primary influence on the viability of fibroblasts was attributed to the source of type I collagen (rat tail, human, or bovine) used as scaffold. Furthermore, incorporation of dermatan sulfate in the dermal layer led to a reduction in the contraction of tissues compared to the control where the dermal scaffold was composed primarily of collagen type I. This work highlights the influence of the composition of biomaterials on the development of complex reconstructed skin models that are suitable for clinical translation and in vitro safety assessment.

A new 3D model for genotoxicity assessment: EpiSkin™ Micronucleus Assay

The European Regulation on Cosmetics (no. 1223/2009) has prohibited the use of animals in safety testing since March 2009 for ingredients used in cosmetics. Irreversible events at the chromosome level (clastogenesis and aneugenesis) are commonly evaluated by scoring either micronuclei or chromosome aberrations using cell-based genotoxicity assays. Like most in vitro genotoxicity assays, the 2D in vitro micronucleus assay exhibits a poor specificity and does not mimic the dermal route. To address these limitations, the current project aims to develop and validate a 3D micronucleus assay using the EpiSkin™ model. This project is scientifically supported by the Cosmetics Europe Genotoxicity Task Force. In a first step, two key criteria for the development of micronucleus assay, namely, the sufficient yield of cells from the EpiSkin™ model and an acceptable proliferation rate of the basal layer, were assessed and demonstrated. Subsequently, six chemicals (vinblastine, n-ethylnitrosourea, β-butyrolactone, 2-acetylaminofluorene, 2,4-dichlorophenoland d-limonene) were evaluated in the EpiSkin™ Micronucleus Assay. At least two independent experiments using 48- and 72-h incubations were performed for each chemical. Results showed good inter-experimental reproducibility, as well as the correct identification of all six tested chemicals. The metabolism of 2-acetylaminofluorene on the EpiSkin™ model was also investigated and confirmed by the formation of an intermediate metabolite (2-aminofluorene). These preliminary results from the EpiSkin™ Micronucleus Assay indicate that it is a promising in vitro assay for assessing genotoxicity. The availability and suitability of this test method contribute significantly to the development of non-animal testing methods in China and its impact on the worldwide field.

FoxO1 enhances differentiation and apoptosis in human primary keratinocytes

Forkhead box-O1 (FoxO1) is a key nutrient- and growth factor-dependent regulator of metabolism, but its functional role in human primary keratinocytes (HPKs) is less known. To investigate the role of FoxO1 in HPKs and effect of insulin-like growth factor 1 (IGF-1) and isotretinoin on FoxO1 expression, HPKs were treated with 1.2 mmol/L calcium chloride, 1-20 ng/mL IGF-1 and 0.1-10 μmol/L isotretinoin. Recombinant adenovirus expressing FoxO1 or FKHR shRNA lentivirus transfection was introduced to upregulate or silence FoxO1 expression. Epidermal FoxO1 immunostaining was lower in acne lesion than in normal skin. FoxO1 overexpression induced involucrin expression, G2/M arrest and apoptosis but suppressed proliferation, while FoxO1 silencing decreased involucrin expression but increased proliferation, S phase and viable cells in HPKs. IGF-1 downregulated FoxO1 and involucrin but upregulated p-Akt expression in HPKs, which was blocked by pretreatment with LY294002. Isotretinoin enhanced FoxO1, p53 and p21 but inhibited p-FoxO1 and involucrin expression in HPKs. These results demonstrate that FoxO1 promotes differentiation and apoptosis in HPKs. IGF-1 may reduce keratinocyte differentiation through PI3K/Akt/FoxO1 pathway, while isotretinoin can reinforce FoxO1 expression. FoxO1 may be involved in acne pathogenesis and could serve as a potential therapeutic target.

[Quality control study of engineered skin tissue]

OMERO service was used to annotate the cell line HaCaT microscope images by two independent expert groups. The images were obtained in the course of developing tissue-engineered epithelium which consisted of several layers of the keratinocytes. Evaluation of expert opinions was performed by calculation of specificity, sensitivity and accuracy. The best convergence of opinions (91%) was achieved for the confluence of the cell monolayers. Accuracy 70% was observed in determining the extent of cell differentiation after 10 days of incubation. The paper illustrates the usefulness of OMERO service for dynamic cross-validation of quality in the development and standardization of cell preparations.

A Trichophyton Rubrum Infection Model Based on the Reconstructed Human Epidermis - Episkin®

BACKGROUND

Trichophyton rubrum represents the most common infectious fungus responsible for dermatophytosis in human, but the mechanism involved is still not completely understood. An appropriate model constructed to simulate host infection is the prerequisite to study the pathogenesis of dermatophytosis caused by T. rubrum. In this study, we intended to develop a new T. rubrum infection model in vitro, using the three-dimensional reconstructed epidermis - EpiSkin ®, and to pave the way for further investigation of the mechanisms involved in T. rubrum infection.

METHODS

The reconstructed human epidermis (RHE) was infected by inoculating low-dose (400 conidia) and high-dose (4000 conidia) T. rubrum conidia to optimize the infection dose. During the various periods after infection, the samples were processed for pathological examination and scanning electron microscopy (SEM) observation.

RESULTS

The histological analysis of RHE revealed a fully differentiated epidermis with a functional stratum corneum, which was analogous to the normal human epidermis. The results of hematoxylin and eosin staining and the periodic acid-Schiff staining showed that the infection dose of 400 conidia was in accord with the pathological characteristics of host dermatophytosis caused by T. rubrum. SEM observations further exhibited the process of T. rubrum infection in an intuitionistic way.

CONCLUSIONS

We established the T. rubrum infection model on RHE in vitro successfully. It is a promising model for further investigation of the mechanisms involved in T. rubrum infection.