A sustainable strategy for generating highly stable human skin equivalents based on fish collagen

Tissue engineered skin equivalents are increasingly recognized as potential alternatives to traditional skin models such as human ex vivo skin or animal skin models. However, most of the currently investigated human skin equivalents (HSEs) are constructed using mammalian collagen which can be expensive and difficult to extract. Fish skin is a waste product produced by fish processing industries and identified as a cost-efficient and sustainable source of type I collagen. In this work, we describe a method for generating highly stable HSEs based on fibrin fortified tilapia fish collagen. The fortified fish collagen (FFC) formulation is optimized to enable reproducible fabrication of full-thickness HSEs that undergo limited contraction, facilitating the incorporation of human donor-derived skin cells and formation of biomimetic dermal and epidermal layers. The morphology and barrier function of the FFC HSEs are compared with a commercial skin model and validated with immunohistochemical staining and transepithelial electrical resistance testing. Finally, the potential of a high throughput screening platform with FFC HSE is explored by scaling down its fabrication to 96-well format.

The Development of an Advanced Model for Multilayer Human Skin Reconstruction In Vivo

Human skin reconstruction on immune-deficient mice has become indispensable for in vivo studies performed in basic research and translational laboratories. Further advancements in making sustainable, prolonged skin equivalents to study new therapeutic interventions rely on reproducible models utilizing patient-derived cells and natural three-dimensional culture conditions mimicking the structure of living skin. Here, we present a novel step-by-step protocol for grafting human skin cells onto immunocompromised mice that requires low starting cell numbers, which is essential when primary patient cells are limited for modeling skin conditions. The core elements of our method are the sequential transplantation of fibroblasts followed by keratinocytes seeded into a fibrin-based hydrogel in a silicone chamber. We optimized the fibrin gel formulation, timing for gel polymerization in vivo, cell culture conditions, and seeding density to make a robust and efficient grafting protocol. Using this approach, we can successfully engraft as few as 1.0 × 106 fresh and 2.0 × 106 frozen-then-thawed keratinocytes per 1.4 cm2 of the wound area. Additionally, it was concluded that a successful layer-by-layer engraftment of skin cells in vivo could be obtained without labor-intensive and costly methodologies such as bioprinting or engineering complex skin equivalents. Key features • Expands upon the conventional skin chamber assay method (Wang et al., 2000) to generate high-quality skin grafts using a minimal number of cultured skin cells. • The proposed approach allows the use of frozen-then-thawed keratinocytes and fibroblasts in surgical procedures. • This system holds promise for evaluating the functionality of skin cells derived from induced pluripotent stem cells and replicating various skin phenotypes. • The entire process, from thawing skin cells to establishing the graft, requires 54 days. Graphical overview.

[Use of 2D and 3D cell cultures in dermatology]

The skin is a complex organ that performs a number of vital functions, including forming a physical barrier that protects our body from the penetration of pathogens and irritants and from excessive transepidermal water loss. In addition to its passive properties, the skin is also actively involved in the immune process. A complex structure of different cell types and structures allows the skin to fulfil these functions. In vitro research often faces the problem that simple 2D cell cultures are not able to adequately map these functions. Here 3D skin models offer a possible solution. In recent years, there has been significant development in this field; the reproducibility of the method as well as the physiological structure and tissue architecture of the 3D skin models have been improved. Depending on the research question, protocols for 3D skin models have been published, ranging from simple multilayer epidermis models to highly complex vascularized 3D full skin models.

IGF-1R deficiency in human keratinocytes disrupts epidermal homeostasis and stem cell maintenance

BACKGROUND

Epidermal stem cells (ESCs) are keratinocytes that reside in the basal layer of the epidermis and mediate epidermal homeostasis. Insulin-like growth factor 1 (IGF-1) signaling through its receptor (IGF-1R) has been identified as an important regulator in rodent skin development and differentiation. However, the role of IGF-1/IGF-1R signaling in human keratinocytes is not yet well understood.

OBJECTIVE

This study aimed to clarify the role of IGF-1/IGF-1R signaling in human epidermal homeostasis.

METHODS

IGF-1R specific knockout (KO) HaCaT keratinocytes were generated by CRISPR-Caspase-9-mediated non-homologous end joining frame-shift mutations. Further, the behavior of these keratinocytes in epidermal homeostasis was investigated using reconstructed epidermis and human skin equivalents.

RESULTS

IGF-1R KO HaCaT keratinocytes were successfully established and produced thin epidermis in three-dimensional culture models. Keratin10-positive cells were frequently found in the basal layer of the reconstructed epidermis.

CONCLUSIONS

IGF-1/IGF-1R signaling was demonstrated to play a key role in maintaining human epidermal homeostasis. This method provides a new framework to investigate gene function in human epidermal homeostasis.

Dehydroabietic Acid Induces Regeneration of Collagen Fibers in Ultraviolet B-Irradiated Human Dermal Fibroblasts and Skin Equivalents

BACKGROUND/AIMS

Dehydroabietic acid (DAA) is a natural phytochemical found in red pine trees and herbal plants. While DAA and its derivatives are known for improving diabetes and hyperlipidemia, the antiaging effect and its underlying mechanisms of DAA on skin have not been fully examined. Here, we assessed the antiaging effects of DAA on human dermal fibroblasts and skin equivalents.

METHODS

We investigated the effect of DAA on the secretion of type I procollagen and matrix metalloproteinase-1 (MMP-1) in ultraviolet B (UVB)-irradiated neonatal normal human dermal fibroblasts (NHDFn). Using nonlinear optical imaging techniques, we visualized quantitative and qualitative changes of collagen fibers by DAA treatment in human skin equivalent models.

RESULTS

DAA induces increases in type I procollagen secretion when treated on UVB-irradiated NHDFn. DAA also downregulates secretion of MMP-1 through the inhibition of the JNK signaling pathway. In human skin equivalent models, we successfully visualized the spatial distribution of collagen fibers in the dermis and found that quantity, diameter, and arrangement of collagen fibers in the dermis were significantly improved by DAA treatment.

CONCLUSION

Our results suggest that DAA could be a useful agent for improving skin photoaging through the protection and regeneration of collagen fibers in skin.

Full-Thickness Human Skin Equivalent Models of Atopic Dermatitis

Atopic dermatitis is a chronic inflammatory skin disease caused by complex multifactorial etiology. In the recent years, there have been significant advances in tissue engineering and the generation of in vitro skin models representative of healthy and diseased states. This chapter describes the methodology for the fabrication of in vitro human skin equivalent (HSE) from human keratinocytes and fibroblasts using a fibrin-based dermal matrix and serum-free culture conditions. Modification of the culture conditions with the supplementation of Th2 cytokines such as interleukin-4 induces the development of atopic dermatitis-like skin model. The chapter also describes the histological and immunohistochemical tools for characterization of the HSE model. The reconstruction of tissue-engineered HSE models that recapitulate the essential features of atopic dermatitis provides powerful tools for deeper understanding of the underlying pathological mechanisms on epidermal level, identification and testing of novel treatment options, and safety and toxicological evaluation in a pathophysiologically relevant system.

3D biomaterial matrix to support long term, full thickness, immuno-competent human skin equivalents with nervous system components

Current commercially available human skin equivalents (HSEs) are used for relatively short term studies (∼1 week) due in part to the time-dependent contraction of the collagen gel-based matrix and the limited cell types and skin tissue components utilized. In contrast, here we describe a new matrix consisting of a silk-collagen composite system that provides long term, stable cultivation with reduced contraction and degradation over time. This matrix supports full thickness skin equivalents which include nerves. The unique silk-collagen composite system preserves cell-binding domains of collagen while maintaining the stability and mechanics of the skin system for long-term culture with silk. The utility of this new composite protein-based biomaterial was demonstrated by bioengineering full thickness human skin systems using primary cells, including nerves and immune cells to establish an HSE with a neuro-immuno-cutaneous system. The HSEs with neurons and hypodermis, compared to in vitro skin-only HSEs controls, demonstrated higher secretion of pro-inflammatory cytokines. Proteomics analysis confirmed the presence of several proteins associated with inflammation across all sample groups, but HSEs with neurons had the highest amount of detected protein due to the complexity of the model. This improved, in vitro full thickness HSE model system utilizes cross-linked silk-collagen as the biomaterial and allows reduced reliance on animal models and provides a new in vitro tissue system for the assessment of chronic responses related to skin diseases and drug discovery.

Combined LC/MS-platform for analysis of all major stratum corneum lipids, and the profiling of skin substitutes

Ceramides (CERs), cholesterol, and free fatty acids (FFAs) are the main lipid classes in human stratum corneum (SC, outermost skin layer), but no studies report on the detailed analysis of these classes in a single platform. The primary aims of this study were to 1) develop an LC/MS method for (semi-)quantitative analysis of all main lipid classes present in human SC; and 2) use this method to study in detail the lipid profiles of human skin substitutes and compare them to human SC lipids. By applying two injections of 10μl, the developed method detects all major SC lipids using RPLC and negative ion mode APCI-MS for detection of FFAs, and NPLC using positive ion mode APCI-MS to analyze CERs and cholesterol. Validation showed this lipid platform to be robust, reproducible, sensitive, and fast. The method was successfully applied on ex vivo human SC, human SC obtained from tape strips and human skin substitutes (porcine SC and human skin equivalents). In conjunction with FFA profiles, clear differences in CER profiles were observed between these different SC sources. Human skin equivalents more closely mimic the lipid composition of human stratum corneum than porcine skin does, although noticeable differences are still present. These differences gave biologically relevant information on some of the enzymes that are probably involved in SC lipid processing. For future research, this provides an excellent method for (semi-)quantitative, 'high-throughput' profiling of SC lipids and can be used to advance the understanding of skin lipids and the biological processes involved.