Extracellular epimorphin modulates epidermal differentiation signals mediated by epidermal growth factor receptor

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.

Role of syntaxin3 an apical polarity protein in poorly polarized keratinocytes: regulation of asymmetric barrier formations in the skin epidermis

The skin epidermis exhibits an asymmetric structure composed of multilayered keratinocytes and those in the outer layers form two-way physical barriers, cornified cell envelope (CCE), and tight junctions (TJs). While undifferentiated keratinocytes in the basal layer continuously deliver daughter cells outward, which undergo successive differentiation with losing their polarized characteristics, they retain the expression of several polarity proteins. In the present study, we revealed that the t-SNARE protein syntaxin3, a critical element for the formation of the apical compartment in simple epithelial cells, is required to confer the ability to organize the physical barriers on "poorly polarized" keratinocytes in epidermal outer layers. HaCaT keratinocytes with genetic ablation of syntaxin3 readily succumbed to hydrogen peroxide-induced cell death. Additionally, they lost the ability to organize TJ and CCE structures, accompanied by notable downregulation of transglutaminase1 and caspase14 (a cornification regulator) expression. These syntaxin3-knockout cells appeared to restore oxidative stress tolerance and functional TJ formation ability, in response to the inducible re-expression of exogenous syntaxin3. While plausible mechanisms underlying these phenomena remain unclear, syntaxin3, an apical polarity protein in the simple epithelia, has emerged as a potentially crucial element for barrier formation in poorly polarized keratinocytes in polarized epidermal tissue.

Current Strategies and Future Perspectives of Skin-on-a-Chip Platforms: Innovations, Technical Challenges and Commercial Outlook

The skin is the largest and most exposed organ in the human body. Not only it is involved in numerous biological processes essential for life but also it represents a significant endpoint for the application of pharmaceuticals. The area of in vitro skin tissue engineering has been progressing extensively in recent years. Advanced in vitro human skin models strongly impact the discovery of new drugs thanks to the enhanced screening efficiency and reliability. Nowadays, animal models are largely employed at the preclinical stage of new pharmaceutical compounds development for both risk assessment evaluation and pharmacokinetic studies. On the other hand, animal models often insufficiently foresee the human reaction due to the variations in skin immunity and physiology. Skin-on-chips devices offer innovative and state-of-the-art platforms essential to overcome these limitations. In the present review, we focus on the contribution of skin-on-chip platforms in fundamental research and applied medical research. In addition, we also highlighted the technical and practical difficulties that must be overcome to enhance skin-on-chip platforms, e.g. embedding electrical measurements, for improved modeling of human diseases as well as of new drug discovery and development.