Skin Equivalent Models: Protocols for In Vitro Reconstruction for Dermal Toxicity Evaluation

Development of pluripotent stem cell-derived epidermal organoids that generate effective extracellular vesicles in skin regeneration

Cellular skin substitutes such as epidermal constructs have been developed for various applications, including wound healing and skin regeneration. These cellular models are mostly derived from primary cells such as keratinocytes and fibroblasts in a two-dimensional (2D) state, and further development of three-dimensional (3D) cultured organoids is needed to provide insight into the in vivo epidermal phenotype and physiology. Here, we report the development of epidermal organoids (EpiOs) generated from induced pluripotent stem cells (iPSCs) as a novel epidermal construct and its application as a source of secreted biomolecules recovered by extracellular vesicles (EVs) that can be utilized for cell-free therapy of regenerative medicine. Differentiated iPSC-derived epidermal organoids (iEpiOs) are easily cultured and expanded through multiple organoid passages, while retaining molecular and functional features similar to in vivo epidermis. These mature iEpiOs contain epidermal stem cell populations and retain the ability to further differentiate into other skin compartment lineages, such as hair follicle stem cells. By closely recapitulating the epidermal structure, iEpiOs are expected to provide a more relevant microenvironment to influence cellular processes and therapeutic response. Indeed, iEpiOs can generate high-performance EVs containing high levels of the angiogenic growth factor VEGF and miRNAs predicted to regulate cellular processes such as proliferation, migration, differentiation, and angiogenesis. These EVs contribute to target cell proliferation, migration, and angiogenesis, providing a promising therapeutic tool for in vivo wound healing. Overall, the newly developed iEpiOs strategy as an organoid-based approach provides a powerful model for studying basic and translational skin research and may also lead to future therapeutic applications using iEpiOs-secreted EVs.

Combining a microphysiological system of three organ equivalents and transcriptomics to assess toxicological endpoints for cosmetic ingredients

Animal testing for cosmetic ingredients and final products has been banned in Europe and is gaining legal force worldwide. However, the need for reliable testing methodologies remains for safety assessment of cosmetic ingredients. While new approach methodologies exist for many toxicological endpoints, some complex ones lack appropriate testing methods. Microphysiological systems (MPSs) have emerged as a promising tool to address this gap in pre-clinical testing, offering higher predictivity compared to animal models due to the phylogenetic distance between humans and animals. Moreover, they provide a more physiological approach than traditional in vitro testing by mimicking interconnections between different culture compartments as seen in complex organisms. This study presents a three-organ microfluidic MPS comprising skin, liver, and intestine equivalents. Combining this model with gene expression analysis, we evaluated toxicological endpoints of chemicals, demonstrating its potential for diverse applications. Our findings highlight the MPS model as a reliable and ethical method to be applied in an integrated approach for safety assessment in the cosmetic industry. It offers a promising strategy to evaluate toxicological endpoints for cosmetic ingredients and other chemicals, supporting the elimination of animal testing while ensuring consumer safety.

Glutamic acid promotes hair growth in mice

Glutamic acid is the main excitatory neurotransmitter acting both in the brain and in peripheral tissues. Abnormal distribution of glutamic acid receptors occurs in skin hyperproliferative conditions such as psoriasis and skin regeneration; however, the biological function of glutamic acid in the skin remains unclear. Using ex vivo, in vivo and in silico approaches, we showed that exogenous glutamic acid promotes hair growth and keratinocyte proliferation. Topical application of glutamic acid decreased the expression of genes related to apoptosis in the skin, whereas glutamic acid increased cell viability and proliferation in human keratinocyte cultures. In addition, we identified the keratinocyte glutamic acid excitotoxic concentration, providing evidence for the existence of a novel skin signalling pathway mediated by a neurotransmitter that controls keratinocyte and hair follicle proliferation. Thus, glutamic acid emerges as a component of the peripheral nervous system that acts to control cell growth in the skin. These results raise the perspective of the pharmacological and nutritional use of glutamic acid to treat skin diseases.