Activation of primary sensory neurons by the topical application of capsaicin on the epidermis of a re-innervated organotypic human skin model

Emergence of New Concepts in Skin Physiopathology through the Use of in vitro Human Skin Explants Models

BACKGROUND

This review summarizes uses and new applications for dermatological research of in vitro culture models of human skin explants (HSEs). In the last decade, many innovations have appeared in the literature and an exponential number of studies have been recorded in various fields of application such as process culture engineering, stem cell extractions methodology, or cell-to-cell interaction studies under physiological and pathological conditions, wound-healing, and inflammation. Most studies also concerned pharmacology, cosmetology, and photobiology. However, these topics will not be considered in our review.

SUMMARY

A better understanding of the mechanisms driving intercellular relationships, at work in the maintenance of 3D tissue architectures has led to the improvement of cell culture techniques. Many papers have focused on the physiological ways that govern in vitro tissue maintenance of HSEs. The analysis of the necessary mechanical stress, intercellular and cell-matrix interactions, allows the maintenance and prolonged use of HSEs in culture for up to 15 days, regardless of the great variability of study protocols from one laboratory to another and in accordance with the objectives set. Because of their close similarities to fresh skin, HSEs are increasingly used to study skin barrier repair and wound healing physiology. Easy to use in co-culture, this model allows a better understanding of the connections and interactions between the peripheral nervous system, the skin and the immune system. The development of the concept of an integrated neuro-immuno-cutaneous system at work in skin physiology and pathology highlighted by this article represents one of the new technical challenges in the field of in vitro culture of HSE. This review of the literature also reveals the importance of using such models in pathology. As sources of stem cells, HSEs are the basis for the development of new tissue engineering models such as organoids or optical clearing tissues technology. This study identifies the main advances and cross-cutting issues in the use of HSE.

In vitro models for investigating itch

Itch (pruritus) is a sensation that drives a desire to scratch, a behavior observed in many animals. Although generally short-lasting and not causing harm, there are several pathological conditions where chronic itch is a hallmark symptom and in which prolonged scratching can induce damage. Finding medications to counteract the sensation of chronic itch has proven difficult due to the molecular complexity that involves a multitude of triggers, receptors and signaling pathways between skin, immune and nerve cells. While much has been learned about pruritus from in vivo animal models, they have limitations that corroborate the necessity for a transition to more human disease-like models. Also, reducing animal use should be encouraged in research. However, conducting human in vivo experiments can also be ethically challenging. Thus, there is a clear need for surrogate models to be used in pre-clinical investigation of the mechanisms of itch. Most in vitro models used for itch research focus on the use of known pruritogens. For this, sensory neurons and different types of skin and/or immune cells are stimulated in 2D or 3D co-culture, and factors such as neurotransmitter or cytokine release can be measured. There are however limitations of such simplistic in vitro models. For example, not all naturally occurring cell types are present and there is also no connection to the itch-sensing organ, the central nervous system (CNS). Nevertheless, in vitro models offer a chance to investigate otherwise inaccessible specific cell–cell interactions and molecular pathways. In recent years, stem cell-based approaches and human primary cells have emerged as viable alternatives to standard cell lines or animal tissue. As in vitro models have increased in their complexity, further opportunities for more elaborated means of investigating itch have been developed. In this review, we introduce the latest concepts of itch and discuss the advantages and limitations of current in vitro models, which provide valuable contributions to pruritus research and might help to meet the unmet clinical need for more refined anti-pruritic substances.

A re-innervated in vitro skin model of non-histaminergic itch and skin neurogenic inflammation: PAR2-, TRPV1- and TRPA1-agonist induced functionality

Background

Skin, and epidermis, is innervated by sensory nerve fibres. Interactions between them and signal transduction are only partially elucidated in physiological/pathological conditions, especially in pruritus.

Objectives

To study the mechanisms involved in pruritus in vitro, we developed a skin explant model re‐innervated by sensory neurons.

Methods

This model is based on the co‐culture of human skin explants and sensory neurons from dorsal root ganglia of rats. Innervation and the expression of protease activated receptor 2 (PAR2), transient receptor potential vanilloid 1 (TRPV1) and transient receptor potential ankyrin one (TRPA1) was analysed by immunostaining. The response of the model to TRPV1, PAR2 and TRPA1 agonists was analysed by patch‐clamp, qPCR and enzyme‐linked immunosorbent assay.

Results

After 5 days of re‐innervating nerve fibres was evidenced in the epidermis. Re‐innervation was correlated with decrease of epidermal thickness and the number of apoptotic cells in the tissue. The major actors of non‐histaminergic itch (PAR‐2, thymic stromal lymphopoietin [TSLP], TSLP‐R, TRPA1 and TRPV1) were expressed in neurons and/or epidermal cells of skin explants. After topical exposure of TRPV1‐(Capsaicin), TRPA1‐(Polygodial) and PAR2‐agonist (SLIGKV‐NH₂) activation of reinnervating neurons could be shown in patch‐clamp analysis. The release of TSLP was increased with capsaicin or SLIGKV but decreased with polygodial. Release of CGRP was increased by capsaicin and polygodial but decreased with SLIGKV. Activation by SLIGKV showed a decrease of VEGF; polygodial induced an increase of TSLP, Tumour necrosis factor (TNF) and nerve growth factor and capsaicin lead to a decrease of sema3 and TNF expression.

Conclusion

The present model is suitable for studying itch and neurogenic inflammation pathways in vitro. We observed that activation of TRPV1, TRPA1 and PAR‐2 leads to different response profiles in re‐innervated skin explants.

Cutaneous innervation in impaired diabetic wound healing

Type 2 diabetes is associated with several potential comorbidities, among them impaired wound healing, chronic ulcerations, and the requirement for lower extremity amputation. Disease-associated abnormal cellular responses, infection, immunological and microvascular dysfunction, and peripheral neuropathy are implicated in the pathogenesis of the wound healing impairment and the diabetic foot ulcer. The skin houses a dense network of sensory nerve afferents and nerve-derived modulators, which communicate with epidermal keratinocytes and dermal fibroblasts bidirectionally to effect normal wound healing after trauma. However, the mechanisms through which cutaneous innervation modulates wound healing are poorly understood, especially in humans. Better understanding of these mechanisms may provide the basis for targeted treatments for chronic diabetic wounds. This review provides an overview of wound healing pathophysiology with a focus on neural involvement in normal and diabetic wound healing, as well as future therapeutic perspectives to address the unmet needs of diabetic patients with chronic wounds.

Sensitive Skins May Be Neuropathic Disorders: Lessons from Studies on Skin and Other Organs

Sensitive skin can be considered a neuropathic disorder. Sensory disorders and the decrease in intra-epidermal nerve ending density are strong arguments for small-fiber neuropathies. Sensitive skin is frequently associated with irritable bowel syndrome or sensitive eyes, which are also considered neuropathic disorders. Consequently, in vitro co-cultures of skin and neurons are adequate models for sensitive skin.

[Tridimensional in vitro models of nervous and immune systems in the skin]

The immune system and the sensory nervous system are responsible for perceiving danger under distinct yet complementary forms. In the last few years, neuroimmune interactions have become an important topic of dermatological research for conditions including wound healing, atopic dermatitis and psoriasis. We present here a selection of tridimensional in vitro models that reproduce skin structure and integrate an immune or a sensory function. Future evolutions of such models are expected to greatly contribute in a better understanding of reciprocal influences between sensory nervous system and immune system.

Fabrication of a Co-Culture System with Human Sweat Gland-Derived Cells and Peripheral Nerve Cells

The interaction of peripheral nerves with different cells of the skin is a relevant aspect of many physiological processes including nociception, temperature control, and wound healing. Here we describe a protocol for the setup of an indirect co-culture system of peripheral nerve cells and sweat gland-derived stem cells, which can be used to quantify neurite outgrowth.

A new tool to test active ingredient using lactic acid in vitro, a help to understand cellular mechanism involved in stinging test: An example using a bacterial polysaccharide (Fucogel® )

The stinging test is an in vivo protocol that evaluates sensitive skin using lactic acid (LA). A soothing sensation of cosmetics or ingredients can be also appreciated through a decrease in stinging score. To predict the soothing sensation of a product before in vivo testing, we developed a model based on an LA test and substance P (SP) release using a co-culture of human keratinocytes and NGF-differentiated PC12 cells. A bacterial fucose-rich polysaccharide present in Fucogel^® was evaluated as the soothing molecule in the in vivo stinging test and our in vitro model. Excluding toxic concentrations, the release of SP was significant from 0.2% of lactic acid for the PC12 cells and from 0.1% of lactic acid for the keratinocytes. When the pH was adjusted to approximately 7.4, LA did not provoke SP release. At these concentrations of LA, 0.1% of polysaccharide showed a significant decrease in SP release from the two cellular types and in co-cultures without modifying the pH of the medium. In vivo, a stinging test using the polysaccharide showed a 30% decrease in prickling intensity vs the placebo in 19 women between the ages of 21 and 69. Our in vitro model is ethically interesting and is adapted for cosmetic ingredients screening because it does not use animal experimentation and limits human volunteers. Moreover, Fucogel^® reduced prickling sensation as revealed by the in vivo stinging test and inhibits the neurogenic inflammation as showed by our new in vitro stinging test based on SP release.

Exploring pain pathophysiology in patients

Although animal models of pain have brought invaluable information on basic processes underlying pain pathophysiology, translation to humans is a problem. This Review will summarize what information has been gained by the direct study of patients with chronic pain. The techniques discussed range from patient phenotyping using quantitative sensory testing to specialized nociceptor neurophysiology, imaging methods of peripheral nociceptors, analyses of body fluids, genetics and epigenetics, and the generation of sensory neurons from patients via inducible pluripotent stem cells.

In vitro activation of the neuro-transduction mechanism in sensitive organotypic human skin model

Recent advances in tissue engineering have encouraged researchers to endeavor the production of fully functional three-dimensional (3D) thick human tissues in vitro. Here, we report the fabrication of a fully innervated human skin tissue in vitro that recapitulates and replicates skin sensory function. Previous attempts to innervate in vitro 3D skin models did not demonstrate an effective functionality of the nerve network. In our approach, we initially engineer functional human skin tissue based on fibroblast-generated dermis and differentiated epidermis; then, we promote rat dorsal root ganglion (DRG) neurons axon ingrowth in the de-novo developed tissue. Neurofilaments network infiltrates the entire native dermis extracellular matrix (ECM), as demonstrated by immunofluorescence and second harmonic generation (SHG) imaging. To prove sensing functionality of the tissue, we use topical applications of capsaicin, an agonist of transient receptor protein-vanilloid 1 (TRPV1) channel, and quantify calcium currents resulting from variations of Ca⁺⁺ concentration in DRG neurons innervating our model. Calcium currents generation demonstrates functional cross-talking between dermis and epidermis compartments. Moreover, through a computational fluid dynamic (CFD) analysis, we set fluid dynamic conditions for a non-planar skin equivalent growth, as proof of potential application in creating skin grafts tailored on-demand for in vivo wound shape.

Usefulness of Skin Explants for Histologic Analysis after Fractional Photothermolysis

Background

Fractional laser resurfacing treatment has been extensively investigated and is widely used. However, the mechanism underlying its effects is poorly understood because of the ethical and cosmetic problems of obtaining skin biopsies required to study the changes after laser treatment.

Objective

To evaluate the usefulness of human skin explants for the investigation of fractional photothermolysis.

Methods

Full-thickness discarded skin was treated in 4 ways: no treatment (control), fractional carbon dioxide laser, fractional Er:YAG laser, and fractional 1,550-nm erbium-doped fiber laser. Both treated and non-treated skin samples were cultured ex vivo at the air-medium interface for 7 days. Frozen tissue was sectioned and stained with hematoxylin & eosin for histologic examination and nitro blue tetrazolium chloride for viability testing.

Results

Skin explants cultured for up to 3 days exhibited histologic changes similar to those observed in in vivo studies, including microscopic treatment zones surrounded by a thermal coagulation zone, re-epithelialization, and formation of microscopic epidermal necrotic debris. However, the explant structure lost its original form within 7 days of culture. The viability of skin explants was maintained for 3 days of culture but was also lost within 7 days.

Conclusion

The skin explant model may be a useful tool for investigating the immediate or early changes following fractional photothermolysis, but further improvements are required to evaluate the long-term and dermal changes.