Epidermal keratinocytes as the forefront of the sensory system

Cutaneous targets for topical pain medications in patients with neuropathic pain: individual differential expression of biomarkers supports the need for personalized medicine

Introduction

Numerous potential cutaneous targets exist for treating chronic pain with topically applied active pharmaceutical ingredients. This preliminary human skin tissue investigation was undertaken to characterize several key biomarkers in keratinocytes and provide proof-of-principle data to support clinical development of topical compounded formulations for peripheral neuropathic pain syndromes, such as postherpetic neuralgia (PHN).

Objectives

The study intended to identify objective biomarkers in PHN skin on a patient-by-patient personalized medicine platform. The totality of biopsy biomarker data can provide a tissue basis for directing individualized compounded topical preparations to optimize treatment efficacy.

Methods

Referencing 5 of the most common actives used in topical pain relief formulations (ketamine, gabapentin, clonidine, baclofen, and lidocaine), and 3 well-established cutaneous mediators (ie, neuropeptides, cannabinoids, and vanilloids), comprehensive immunolabeling was used to quantify receptor biomarkers in skin biopsy samples taken from ipsilateral (pain) and contralateral (nonpain) dermatomes of patients with PHN.

Results

Epidermal keratinocyte labeling patterns were significantly different among the cohort for each biomarker, consistent with potential mechanisms of action among keratinocytes. Importantly, the total biomarker panel indicates that the enriched PHN cohort contains distinct subgroups.

Conclusion

The heterogeneity of the cohort differences may explain studies that have not shown statistical group benefit from topically administered compounded therapies. Rather, the essential need for individual tissue biomarker evaluations is evident, particularly as a means to direct a more accurately targeted topical personalized medicine approach and generate positive clinical results.

Modeling of three-dimensional innervated epidermal like-layer in a microfluidic chip-based coculture system

Reconstruction of skin equivalents with physiologically relevant cellular and matrix architecture is indispensable for basic research and industrial applications. As skin-nerve crosstalk is increasingly recognized as a major element of skin physiological pathology, the development of reliable in vitro models to evaluate the selective communication between epidermal keratinocytes and sensory neurons is being demanded. In this study, we present a three-dimensional innervated epidermal keratinocyte layer as a sensory neuron-epidermal keratinocyte co-culture model on a microfluidic chip using the slope-based air-liquid interfacing culture and spatial compartmentalization. Our co-culture model recapitulates a more organized basal-suprabasal stratification, enhanced barrier function, and physiologically relevant anatomical innervation and demonstrated the feasibility of in situ imaging and functional analysis in a cell-type-specific manner, thereby improving the structural and functional limitations of previous coculture models. This system has the potential as an improved surrogate model and platform for biomedical and pharmaceutical research.

Ectodermal origins of the skin-brain axis: a novel model for the developing brain, inflammation, and neurodevelopmental conditions

Early life development and its divergence is influenced by multiple genetic, neurological, and environmental factors. Atypical neurodevelopment, such as that observed in autism spectrum disorder, likely begins in early gestation during a period of entwined growth between the brain and epithelial barriers of the skin, gastrointestinal tract, and airway. This review coalesces epidemiological and neuroinflammatory evidence linking cutaneous atopic disease with both reduced skin barrier integrity and determinants of neurodivergence. We consider the shared developmental origin of epidermal and neural tissue with related genetic and environmental risk factors to evaluate potential pre- and postnatal modifiers of the skin-brain connection. Initial postnatal skin barrier integrity may provide a useful marker for both cortical integrity and meaningful subgroups of children showing early neurodevelopmental delays. It may also modify known risk factors to neurodevelopment, such as pathogen caused immune system activation. These novel insights of a skin-brain-neurodevelopment connection may advance detection and intervention opportunities.

OR2AT4, an Ectopic Olfactory Receptor, Suppresses Oxidative Stress-Induced Senescence in Human Keratinocytes

Olfactory receptors (ORs) are the largest protein superfamily in mammals. Certain ORs are ectopically expressed in extranasal tissues and regulate cell type-specific signal transduction pathways. OR2AT4 is ectopically expressed in skin cells and promotes wound healing and hair growth. As the capacities of wound healing and hair growth decline with aging, we investigated the role of OR2AT4 in the aging and senescence of human keratinocytes. OR2AT4 was functionally expressed in human keratinocytes (HaCaT) and exhibited co-expression with G-protein-coupled receptor signaling components, Golfα and adenylate cyclase 3. The OR2AT4 ligand sandalore modulates the intracellular calcium, inositol phosphate, and cyclic adenosine monophosphate (cAMP) levels. The increased calcium level induced by sandalore was attenuated in cells with OR2AT4 knockdown. OR2AT4 activation by sandalore inhibited the senescent cell phenotypes and restored cell proliferation and Ki-67 expression. Sandalore also inhibited the expression of senescence-associated β-galactosidase and increased p21 expression in senescent HaCaT cells in response to hydrogen peroxide. Additionally, sandalore activated the CaMKKβ/AMPK/mTORC1/autophagy signaling axis and promoted autophagy. OR2AT4 knockdown attenuated the increased in the intracellular calcium level, cell proliferation, and AMPK phosphorylation induced by sandalore. These findings demonstrate that the effects of sandalore are mediated by OR2AT4 activation. Our findings suggest that OR2AT4 may be a novel therapeutic target for anti-aging and anti-senescence in human keratinocytes.

Pruritus: A Sensory Symptom Generated in Cutaneous Immuno-Neuronal Crosstalk

Pruritus or itch generated in the skin is one of the most widespread symptoms associated with various dermatological and systemic (immunological) conditions. Although many details about the molecular mechanisms of the development of both acute and chronic itch were uncovered in the last 2 decades, our understanding is still incomplete and the clinical management of pruritic conditions is one of the biggest challenges in daily dermatological practice. Recent research revealed molecular interactions between pruriceptive sensory neurons and surrounding cutaneous cell types including keratinocytes, as well as resident and transient cells of innate and adaptive immunity. Especially in inflammatory conditions, these cutaneous cells can produce various mediators, which can contribute to the excitation of pruriceptive sensory fibers resulting in itch sensation. There also exists significant communication in the opposite direction: sensory neurons can release mediators that maintain an inflamed, pruritic tissue-environment. In this review, we summarize the current knowledge about the sensory transduction of pruritus detailing the local intercellular interactions that generate itch. We especially emphasize the role of various pruritic mediators in the bidirectional crosstalk between cutaneous non-neuronal cells and sensory fibers. We also list various dermatoses and immunological conditions associated with itch, and discuss the potential immune-neuronal interactions promoting the development of pruritus in the particular diseases. These data may unveil putative new targets for antipruritic pharmacological interventions.

Do epidermal keratinocytes have sensory and information processing systems?

It was long considered that the role of epidermal keratinocytes is solely to construct a water-impermeable protective membrane, the stratum corneum, at the uppermost layer of the skin. However, in the last two decades, it has been found that keratinocytes contain multiple sensory systems that detect environmental changes, including mechanical stimuli, sound, visible radiation, electric fields, magnetic fields, temperature and chemical stimuli, and also a variety of receptor molecules associated with olfactory or taste sensation. Moreover, neurotransmitters and their receptors that play crucial roles in the brain are functionally expressed in keratinocytes. Recent studies have demonstrated that excitation of keratinocytes can induce sensory perception in the brain. Here, we review the sensory and information processing capabilities of keratinocytes. We discuss the possibility that epidermal keratinocytes might represent the earliest stage in the development of the brain during the evolution of vertebrates.

Ectopic Odorant Receptor Responding to Flavor Compounds: Versatile Roles in Health and Disease

Prompted by the ground-breaking discovery of the rodent odorant receptor (OR) gene family within the olfactory epithelium nearly 30 years ago, followed by that of OR genes in cells of the mammalian germ line, and potentiated by the identification of ORs throughout the body, our appreciation for ORs as general chemoreceptors responding to odorant compounds in the regulation of physiological or pathophysiological processes continues to expand. Ectopic ORs are now activated by a diversity of flavor compounds and are involved in diverse physiological phenomena varying from adipogenesis to myogenesis to hepatic lipid accumulation to serotonin secretion. In this review, we outline the key biological functions of the ectopic ORs responding to flavor compounds and the underlying molecular mechanisms. We also discuss research opportunities for utilizing ectopic ORs as therapeutic strategies in the treatment of human disease as well as challenges to be overcome in the future. The recognition of the potent function, signaling pathway, and pharmacology of ectopic ORs in diverse tissues and cell types, coupled with the fact that they belong to G protein-coupled receptors, a highly druggable protein family, unequivocally highlight the potential of ectopic ORs responding to flavor compounds, especially food-derived odorant compounds, as a promising therapeutic strategy for various diseases.

Phenotypic overlap between atopic dermatitis and autism

BACKGROUND

Autism, a childhood behavioral disorder, belongs to a large suite of diseases, collectively referred to as autism spectrum disorders (ASD). Though multifactorial in etiology, approximately 10% of ASD are associated with atopic dermatitis (AD). Moreover, ASD prevalence increases further as AD severity worsens, though these disorders share no common causative mutations. We assessed here the link between these two disorders in the standard, valproic acid mouse model of ASD. In prior studies, there was no evidence of skin involvement, but we hypothesized that cutaneous involvement could be detected in experiments conducted in BALB/c mice. BALB/c is an albino, laboratory-bred strain of the house mouse and is among the most widely used inbred strains used in animal experimentation.

METHODS

We performed our studies in valproic acid (VPA)-treated BALB/c hairless mice, a standard mouse model of ASD. Mid-trimester pregnant mice received a single intraperitoneal injection of either valproic acid sodium salt dissolved in saline or saline alone on embryonic day 12.5 and were housed individually until postnatal day 21. Only the brain and epidermis appeared to be affected, while other tissues remain unchanged. At various postnatal time points, brain, skin and blood samples were obtained for histology and for quantitation of tissue sphingolipid content and cytokine levels.

RESULTS

AD-like changes in ceramide content occurred by day one postpartum in both VPA-treated mouse skin and brain. The temporal co-emergence of AD and ASD, and the AD phenotype-dependent increase in ASD prevalence correlated with early appearance of cytokine markers (i.e., interleukin [IL]-4, 5, and 13), as well as mast cells in skin and brain. The high levels of interferon (IFN)γ not only in skin, but also in brain likely account for a significant decline in esterified very-long-chain N-acyl fatty acids in brain ceramides, again mimicking known IFNγ-induced changes in AD.

CONCLUSION

Baseline involvement of both AD and ASD could reflect concurrent neuro- and epidermal toxicity, possibly because both epidermis and neural tissues originate from the embryonic neuroectoderm. These studies illuminate the shared susceptibility of the brain and epidermis to a known neurotoxin, suggesting that the atopic diathesis could be extended to include ASD.

Microarray patches enable the development of skin-targeted vaccines against COVID-19

The COVID-19 pandemic is a serious threat to global health and the global economy. The ongoing race to develop a safe and efficacious vaccine to prevent infection by SARS-CoV-2, the causative agent for COVID-19, highlights the importance of vaccination to combat infectious pathogens. The highly accessible cutaneous microenvironment is an ideal target for vaccination since the skin harbors a high density of antigen-presenting cells and immune accessory cells with broad innate immune functions. Microarray patches (MAPs) are an attractive intracutaneous biocargo delivery system that enables safe, reproducible, and controlled administration of vaccine components (antigens, with or without adjuvants) to defined skin microenvironments. This review describes the structure of the SARS-CoV-2 virus and relevant antigenic targets for vaccination, summarizes key concepts of skin immunobiology in the context of prophylactic immunization, and presents an overview of MAP-mediated cutaneous vaccine delivery. Concluding remarks on MAP-based skin immunization are provided to contribute to the rational development of safe and effective MAP-delivered vaccines against emerging infectious diseases, including COVID-19.

Modified tamarind kernel polysaccharide-based matrix alters neuro-keratinocyte cross-talk and serves as a suitable scaffold for skin tissue engineering

Advanced technologies like skin tissue engineering are requisite of various disorders where artificially synthesized materials need to be used as a scaffold in vivo, which in turn can allow the formation of functional skin and epidermal layer with all biological sensory functions. In this work, we present a set of hydrogels which have been synthesized by the method utilizing radical polymerization of a natural polymer extracted from kernel of Tamarindus indica, commonly known as Tamarind Kernel Powder (TKP) modified by utilizing the monomer acrylic acid (AA) in different mole ratios. These materials are termed as TKP: AA hydrogels and characterized by Atomic Force Microscopy (AFM), surface charge, and particle size distribution using Dynamic Light Scattering measurements. These materials are biocompatible with mouse dermal fibroblasts (NIH- 3T3) and human skin keratinocytes (HaCaT), as confirmed by MTT and biocompatibility assays. These TKP: AA hydrogels do not induce unwanted ROS signaling as confirmed by mitochondrial functionality determined by DCFDA staining, Mitosox imaging, and measuring the ATP levels. We demonstrate that in the co-culture system, TKP: AA allows the establishment of proper neuro-keratinocyte contact formation, suggesting that this hydrogel can be suitable for developing skin with sensory functions. Skin corrosion analysis on SD rats confirms that TKP: AA is appropriate for in vivo applications as well. This is further confirmed by in vivo compatibility and toxicity studies, including hemocompatibility and histopathology of liver and kidney upon direct introduction of hydrogel into the body. We propose that TKP: AA (1: 5) offers a suitable surface for skin tissue engineering with sensory functions applicable in vitro, in vivo, and ex vivo. These findings may have broad biomedical and clinical importance.

Functionally expressed bitter taste receptor TAS2R14 in human epidermal keratinocytes serves as a chemosensory receptor

Traditionally, it is theorized that skin sensation is initiated when cutaneous sensory afferents and Merkel cells receive sensory stimuli, while epidermal keratinocytes were deemed to have no role. However, mounting evidence has shown that keratinocytes can initiate skin sensation by receiving sensory stimuli and transmitting sensory information to sensory afferents. Knowledge regarding the mechanisms by which keratinocytes receive exogenous stimuli is limited, with TRP channels and olfactory receptors having been proposed to serve as receptors for exogenous stimuli in keratinocytes. Recently, expression analyses have demonstrated the expression of multiple TAS2R genes in human skin. TAS2Rs are chemosensory GPCRs employed by taste cells to detect bitter-tasting substances. However, only subtypes TAS2R1 and TAS2R38 have been characterized in epidermal keratinocytes. We present evidence suggesting that subtype TAS2R14 is functionally expressed in epidermal keratinocytes. TAS2R14 transcripts and protein were detected in primary and N/TERT-1 keratinocytes. Additionally, keratinocytes responded to α-thujone, a TAS2R14 ligand, with an increase in intracellular free Ca²⁺ concentration. The tastant-evoked Ca²⁺ signals were found to be mediated by wild-type TAS2R14 and heterotrimeric G proteins. We conclude that TAS2R14 serves as a chemosensory receptor in epidermal keratinocytes and hypothesize that it enables the cells to recognize potentially harmful chemical substances.

Safety and efficacy of a novel home-use device for light-potentiated (LED) skin treatment

Skin structure and function results from a dynamic interplay between dermal and epidermal cell types. Optimizing skin health through an effective and long-lasting skin care regime therefore requires a global approach, encompassing various mechanisms to stimulate this interplay beyond the action scope of a classical topical solution. This study evaluates the impact of a novel home-use device combining a topical serum, light-emitting diodes and massage on the clinical signs of extrinsic skin aging. The innovative principle relies on potentiating the effect of active ingredients contained in the topical serum with visible and near infra-red photons to prevent extracellular matrix degradation and promote its reconstruction. After in vitro and ex vivo investigations, a clinical study assessed the safety and efficacy of a daily treatment with the home-use device for 28 days. A significant increases in skin density and radiance while reducing the wrinkles was obtained with no side effects.

Pain-associated Mediators and Axon Pathfinders in Fibromyalgia Skin Cells

Objective.

To investigate whether the expression of cytokine, nociception-associated ion channel, and axon guidance genes in patients with skin cell fibromyalgia syndrome (FMS) differs from healthy controls, potentially contributing to pain and small-fiber degeneration in FMS.

Methods.

We prospectively recruited 128 patients and 26 healthy controls. All study participants underwent neurological examination, and a skin punch biopsy was obtained from the lateral calf and thigh. Skin samples were processed to histologically determine intraepidermal nerve fiber density (IENFD) and for primary fibroblast and keratinocyte cell cultures. Gene expression of selected pro- and antiinflammatory cytokines, nociception-associated ion channels, and axon guidance cues was assessed with quantitative real-time PCR.

Results.

In fibroblasts, transforming growth factor–ß1 (TGF-ß1) gene expression was higher in patients with FMS compared to controls (calf and thigh: p < 0.001). Also, expression was higher in patients than in controls for these variables: hyperpolarization-activated cyclic nucleotide-gated ion channel 2 (calf: p < 0.01), ephrin-A4 (EFNA4; calf: p < 0.05, thigh: p < 0.001), and ephrin receptor-A4 (EPHA4; thigh: p < 0.05). In keratinocytes, interleukin 10 gene expression was higher in patients with FMS than in controls (thigh: p < 0.05). While no intergroup difference was found for nociception-associated ion channels, EFNA4 and EPHA4 (calf: p < 0.01 each) expression was higher in patients with FMS than in controls. Axon guide expression did not correlate with IENFD.

Conclusion.

In FMS, skin cells may contribute to cutaneous nociception by differentially expressing membrane-bound and soluble pain mediators and axon pathfinders.

Cannabinoid Signaling in the Skin: Therapeutic Potential of the "C(ut)annabinoid" System

The endocannabinoid system (ECS) has lately been proven to be an important, multifaceted homeostatic regulator, which influences a wide-variety of physiological processes all over the body. Its members, the endocannabinoids (eCBs; e.g., anandamide), the eCB-responsive receptors (e.g., CB₁, CB₂), as well as the complex enzyme and transporter apparatus involved in the metabolism of the ligands were shown to be expressed in several tissues, including the skin. Although the best studied functions over the ECS are related to the central nervous system and to immune processes, experimental efforts over the last two decades have unambiguously confirmed that cutaneous cannabinoid ("c[ut]annabinoid") signaling is deeply involved in the maintenance of skin homeostasis, barrier formation and regeneration, and its dysregulation was implicated to contribute to several highly prevalent diseases and disorders, e.g., atopic dermatitis, psoriasis, scleroderma, acne, hair growth and pigmentation disorders, keratin diseases, various tumors, and itch. The current review aims to give an overview of the available skin-relevant endo- and phytocannabinoid literature with a special emphasis on the putative translational potential, and to highlight promising future research directions as well as existing challenges.

Nociceptin/orphanin FQ opioid peptide-receptor expression in pachyonychia congenita

Nociceptin/orphanin FQ opioid peptide (NOP)-receptor (NOP-R) is a member of the opioid receptor family. NOP-R activation has demonstrated analgesic effects in preclinical pain models without the addiction risks associated with other opiate targets. Pachyonychia congenita (PC) is a palmoplantar keratoderma characterized by neuropathic pain in affected skin. A cohort of KRT6A gene mutation PC patients with no other explanation for their neuropathic pain offered a unique opportunity to assess potential of NOP-R as a therapeutic target. Plantar biopsies from 10 PC patients and 10 age/gender matched controls were performed at the ball (PC-affected) and the arch (PC-unaffected) of the foot. NOP-R expression was assessed by immunohistochemistry. Localization of NOP-R in subsets of epidermal nerve fibers was investigated using the pan-neuronal marker PGP9.5, markers for unmyelinated peptidergic fibers (calcitonin gene-related peptide [CGRP] and substance P [SP]), as well as for myelinated Aδ and Aβ fibers (neurofilament H [NFH]). Robust NOP-R expression was detected in epidermal keratinocytes and in a subset of PGP9.5+ fibers in both epidermis and dermis, confirmed by western blot and absorption experiments with NOP-R peptide. NOP-R expression in keratinocytes was significantly reduced in PC-affected plantar skin compared with PC-unaffected skin. In addition, NOP-R expression occurred in dermal NFH+ myelinated fibers in all groups, although few CGRP+ fibers co-expressed NOP-R. Furthermore, most SP+ fibers also co-expressed NOP-R. These findings indicate that NOP-R is expressed on epidermal keratinocytes, as well as on epidermal and dermal nerve fibers and has potential as a promising target to treat neuropathic pain in PC.

Physico-chemical characterization of Antheraea mylitta silk mats for wound healing applications

In the field of plastic reconstructive surgery, development of new innovative matrices for skin repair is in demand. The ideal biomaterial should promote attachment, proliferation and growth of cells. Additionally, it should degrade in an appropriate time period without releasing harmful substances, not exerting a pathological immune response. The materials used should display optimized mechanical properties to sustain cell growth and limit scaffold contraction. Wound healing is a biological process directed towards restoration of tissue that has suffered an injury. An important phase of wound healing is the generation of a basal epithelium wholly replacing the epidermis of the wound. Wild silk from Antheraea mylitta meets these demands to a large extent. To evaluate the effects of the treatment, Antheraea mylitta and Bombyx mori samples were characterized by SEM-EDX, FT-IR, XRD and TGA-DSC techniques. Preliminary cell growth behavior was carried out by culturing epidermal cells and proliferation was quantified via viability assay. Moreover, Antheraea mylitta possesses excellent cell-adhesive capability, effectively promoting cell attachment and proliferation. Antheraea mylitta serves as a delivery vehicle for cells. With all these unique features, it is expected that Antheraea mylitta mat will have wide utility in the areas of tissue engineering and regenerative medicine.

Sensory Neuron-Specific Deletion of TRPA1 Results in Mechanical Cutaneous Sensory Deficits

The nonselective cation channel transient receptor potential ankyrin 1 (TRPA1) is known to be a key contributor to both somatosensation and pain. Recent studies have implicated TRPA1 in additional physiologic functions and have also suggested that TRPA1 is expressed in nonneuronal tissues. Thus, it has become necessary to resolve the importance of TRPA1 expressed in primary sensory neurons, particularly since previous research has largely used global knock-out animals and chemical TRPA1 antagonists. We therefore sought to isolate the physiological relevance of TRPA1 specifically within sensory neurons. To accomplish this, we used Advillin-Cre mice, in which the promoter for Advillin is used to drive expression of Cre recombinase specifically within sensory neurons. These Advillin-Cre mice were crossed with Trpa1^fl/fl mice to generate sensory neuron-specific Trpa1 knock-out mice. Here, we show that tissue-specific deletion of TRPA1 from sensory neurons produced strong deficits in behavioral sensitivity to mechanical stimulation, while sensitivity to cold and heat stimuli remained intact. The mechanical sensory deficit was incomplete compared to the mechanosensory impairment of TRPA1 global knock-out mice, in line with the incomplete (∼80%) elimination of TRPA1 from sensory neurons in the tissue-specific Advillin-Cre knock-out mice. Equivalent findings were observed in tissue-specific knock-out animals originating from two independently-generated Advillin-Cre lines. As such, our results show that sensory neuron TRPA1 is required for mechanical, but not cold, responsiveness in noninjured skin.

Real-time imaging of human epidermal calcium dynamics in response to point laser stimulation

BACKGROUND

Changes of epidermal calcium ion concentration are involved in regulation of barrier homeostasis and keratinocyte differentiation. Moreover, intracellular calcium dynamics might play a role in skin sensation. But, although calcium dynamics of cultured keratinocytes in response to mechanical stresses has been well studied, calcium propagation in stimulated human epidermis is still poorly understood.

OBJECTIVE

The aim of this study was to demonstrate a novel method for real-time measurement of calcium dynamics in response to point stimulation of human epidermis at the single-cell level.

METHODS

We examined calcium propagation in cross-sectional samples of living human epidermis ex vivo, as well as in cultured human keratinocytes, by means of two-photon microscopy after stimulating cells in stratum granulosum with the emission laser of a two-photon microscope.

RESULTS

Cells in different epidermal layers showed different responses, and those in stratum basale showed the greatest elevation of intracellular calcium. Calcium propagation in epidermis was inhibited in the presence of apyrase (which degrades adenosine triphosphate; ATP) or gap-junction blockers. In cultured keratinocytes, on the other hand, calcium propagated in a simple concentric wave-like manner from the stimulation site, and propagation was strongly suppressed by apyrase.

CONCLUSION

Our results suggested that ATP and gap junctions play important roles in calcium propagation induced by point laser stimulation of the uppermost layer of epidermis. Our method should be broadly useful to study calcium dynamics, epidermal physiological mechanisms, and mechanisms of skin sensation at the single-cell level.

Evidence for physiological and pathological roles for sensory nerves in the microvasculature and skin

This review highlights the progress from the initial finding of neurogenic inflammation up to the most recent development in the field of sensory nerves research, focusing on their roles in the microvasculature and the skin. Recent discovery of Transient Receptor Potential (TRP) channels highlight their important roles in detecting a range of environmental stimuli, including chemical and temperature. This provides us novel mechanisms for driving neurogenic inflammation upstream of neuropeptide release in addition to promising potential therapeutic targets in various diseases, including pain, itching and skin inflammation.

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.

Sensitive skin

Sensitive skin is a clinical condition defined by the self-reported facial presence of different sensory perceptions, including tightness, stinging, burning, tingling, pain and pruritus. Sensitive skin may occur in individuals with normal skin, with skin barrier disturbance, or as a part of the symptoms associated with facial dermatoses such as rosacea, atopic dermatitis and psoriasis. Although experimental studies are still pending, the symptoms of sensitive skin suggest the involvement of cutaneous nerve fibres and neuronal, as well as epidermal, thermochannels. Many individuals with sensitive skin report worsening symptoms due to environmental factors. It is thought that this might be attributed to the thermochannel TRPV1, as it typically responds to exogenous, endogenous, physical and chemical stimuli. Barrier disruptions and immune mechanisms may also be involved. This review summarizes current knowledge on the epidemiology, potential mechanisms, clinics and therapy of sensitive skin.

Keratinocytes at the uppermost layer of epidermis might act as sensors of atmospheric pressure change

It has long been suggested that climate, especially atmospheric pressure change, can cause health problems ranging from migraine to myocardial infarction. Here, I hypothesize that the sensory system of epidermal keratinocytes mediates the influence of atmospheric pressure change on the human physiological condition. We previously demonstrated that even subtle changes of atmospheric pressure (5–20 hPa) induce elevation of intracellular calcium level in cultured human keratinocytes (excitation of keratinocytes). It is also established that communication occurs between epidermal keratinocytes and peripheral nerve systems. Moreover, various neurotransmitters and hormones that influence multiple systems (nervous, cardiovascular, endocrine, and immune systems) are generated and released from epidermal keratinocytes in response to various external stimuli. Thus, I suggest that pathophysiological phenomena induced by atmospheric pressure changes might be triggered by epidermal keratinocytes.

Newly discovered olfactory receptors in epidermal keratinocytes are associated with proliferation, migration, and re-epithelialization of keratinocytes

Skin contains receptors for various environmental factors. In this issue of the Journal, Busse et al. cloned a new olfactory receptor, OR2AT4, in keratinocytes. They show that the activation of OR2AT4 induces phosphorylation of extracellular signal-regulated kinases and p38 mitogen-activated protein kinases, and that it accelerates wound healing. OR2AT4 may be a promising candidate as a target in clinical drug development.

Sensory Neurons, Ion Channels, Inflammation and the Onset of Neuropathic Pain

Neuropathic pain often fails to respond to conventional pain management procedures. here we review the aetiology of neuropathic pain as would result from peripheral neuropathy or injury. We show that inflammatory mediators released from damaged nerves and tissue are responsible for triggering ectopic activity in primary afferents and that this, in turn, provokes increased spinal cord activity and the development of ‘central sensitization’. Although evidence is mounting to support the role of interleukin-1β, prostaglandins and other cytokines in the onset of neuropathic pain, the clinical efficacy of drugs which antagonize or prevent the actions of these mediators is yet to be determined. basic science findings do, however, support the use of pre-emptive analgesia during procedures which involve nerve manipulation and the use of anti-inflammatory steroids as soon as possible following traumatic nerve injury.

Frontiers in epidermal barrier homeostasis--an approach to mathematical modelling of epidermal calcium dynamics

Intact epidermal barrier function is crucial for survival and is associated with the presence of gradients of both calcium ion concentration and electric potential. Although many molecules, including ion channels and pumps, are known to contribute to maintenance of these gradients, the mechanisms involved in epidermal calcium ion dynamics have not been clarified. We have established that a variety of neurotransmitters and their receptors, originally found in the brain, are expressed in keratinocytes and are also associated with barrier homeostasis. Moreover, keratinocytes and neurons show some similarities of electrochemical behaviour. As mathematical modelling and computer simulation have been employed to understand electrochemical phenomena in brain science, we considered that a similar approach might be applicable to describe the dynamics of epidermal electrochemical phenomena associated with barrier homeostasis. Such methodology would also be potentially useful to address a number of difficult problems in clinical dermatology, such as ageing and itching. Although this work is at a very early stage, in this essay, we discuss the background to our approach and we present some preliminary results of simulation of barrier recovery.

A synthetic sandalwood odorant induces wound-healing processes in human keratinocytes via the olfactory receptor OR2AT4

As the outermost barrier of the body, the skin is exposed to multiple environmental factors, including temperature, humidity, mechanical stress, and chemical stimuli such as odorants that are often used in cosmetic articles. Keratinocytes, the major cell type of the epidermal layer, express a variety of different sensory receptors that enable them to react to various environmental stimuli and process information in the skin. Here we report the identification of a novel type of chemoreceptors in human keratinocytes, the olfactory receptors (ORs). We cloned and functionally expressed the cutaneous OR, OR2AT4, and identified Sandalore, a synthetic sandalwood odorant, as an agonist of this receptor. Sandalore induces strong Ca(2+) signals in cultured human keratinocytes, which are mediated by OR2AT4, as demonstrated by receptor knockdown experiments using RNA interference. The activation of OR2AT4 induces a cAMP-dependent pathway and phosphorylation of extracellular signal-regulated kinases (Erk1/2) and p38 mitogen-activated protein kinases (p38 MAPK). Moreover, the long-term stimulation of keratinocytes with Sandalore positively affected cell proliferation and migration, and regeneration of keratinocyte monolayers in an in vitro wound scratch assay. These findings combined with our studies on human skin organ cultures strongly indicate that the OR 2AT4 is involved in human keratinocyte re-epithelialization during wound-healing processes.

Cytokine-related and histological biomarkers for neuropathic pain assessment

SUMMARY Neuropathic pain (NP) is a disabling condition that may occur following a disease or a lesion of the somatosensory nervous system. With an estimated prevalence of up to 8.2% in the general population, NP is common, and robust and objective diagnostic tools are warranted for pain assessment and follow-up. In the last years research has focused on defining biochemical and histological markers for this purpose, and possible systemic (blood, cerebrospinal fluid) and local (skin and nerve) targets have been investigated in a number of different NP disorders. This article focuses on an update giving an overview over some potential biomarkers for the diagnosis of NP, and will discuss their clinical relevance.

External negative electric potential accelerates exocytosis of lamellar bodies in human skin ex vivo

Exocytosis of lamellar bodies at the uppermost nucleated layer of the epidermis is a crucial process for epidermal permeability barrier homoeostasis. We have previously suggested that skin surface electric potential might be associated with barrier homoeostasis. Thus, we hypothesized that the potential might drive exocytosis of lamellar bodies. In this study, we tested this idea by applying negative electric potential (-0.5 V) to human skin samples ex vivo for 2 h and observing the ultrastructure of the uppermost layer. The secretion of lamellar bodies was accelerated in the potential-applied skin, compared to that in untreated control skin. Multiphoton observation indicated that extracellular lipid domains were more extensive in treated skin than in control skin. Moreover, the calcium ion gradient was greater at the uppermost layer of the epidermis of treated skin, compared to that in control skin. These results indicate that electric potential may regulate lamellar body secretion in healthy human skin.

Kv7/M-type potassium channels in rat skin keratinocytes

Skin keratinocytes fulfil important signalling and protective functions. Immunocytochemical experiments revealed the unexpected presence of immunoreactivity for the M-type potassium channel subunit Kv7.2 in the keratinocyte layer of intact rat paw skin and in keratinocytes isolated from the skin of 1-day-old rats and cultured in vitro for 3–10 days. Application of the M-channel enhancer retigabine (3–10 μM) to isolated cultured rat keratinocytes: (a) increased outward membrane currents recorded under voltage clamp, (b) produced ~3 mV hyperpolarization at rest, (c) enhanced ~3-fold the release of ATP induced by the TRPV3 agonist carvacrol (1 mM) and (d) increased the amplitude of the carvacrol-induced intracellular Ca²⁺ transient measured with Fura-2. The effect of retigabine on ATP release was prevented by the M-channel blocking agent XE991. We conclude that rat skin keratinocytes possess M-channels that, when activated, can modify their physiological properties, with potential significance for their sensory and other biological functions.

Air-stimulated ATP release from keratinocytes occurs through connexin hemichannels

Cutaneous ATP release plays an important role in both epidermal stratification and chronic pain, but little is known about ATP release mechanisms in keratinocytes that comprise the epidermis. In this study, we analyzed ATP release from cultured human neonatal keratinocytes briefly exposed to air, a process previously demonstrated to trigger ATP release from these cells. We show that exposing keratinocytes to air by removing media for 15 seconds causes a robust, long-lasting ATP release. This air-stimulated ATP release was increased in calcium differentiated cultures which showed a corresponding increase in connexin 43 mRNA, a major component of keratinocyte hemichannels. The known connexin hemichannel inhibitors 1-octanol and carbenoxolone both significantly reduced air-stimulated ATP release, as did two drugs traditionally used as ABC transporter inhibitors (glibenclamide and verapamil). These same 4 inhibitors also prevented an increase in the uptake of a connexin permeable dye induced by air exposure, confirming that connexin hemichannels are open during air-stimulated ATP release. In contrast, activity of the MDR1 ABC transporter was reduced by air exposure and the drugs that inhibited air-stimulated ATP release had differential effects on this transporter. These results indicate that air exposure elicits non-vesicular release of ATP from keratinocytes through connexin hemichannels and that drugs used to target connexin hemichannels and ABC transporters may cross-inhibit. Connexins represent a novel, peripheral target for the treatment of chronic pain and dermatological disease.

UV light phototransduction activates transient receptor potential A1 ion channels in human melanocytes

Human skin is constantly exposed to solar ultraviolet radiation (UVR), the most prevalent environmental carcinogen. Humans have the unique ability among mammals to respond to UVR by increasing their skin pigmentation, a protective process driven by melanin synthesis in epidermal melanocytes. The molecular mechanisms used by melanocytes to detect and respond to long-wavelength UVR (UVA) are not well understood. We recently identified a UVA phototransduction pathway in melanocytes that is mediated by G protein-coupled receptors and leads to rapid calcium mobilization. Here we report that in human epidermal melanocytes physiological doses of UVR activate a retinal-dependent current mediated by transient receptor potential A1 (TRPA1) ion channels. The TRPA1 photocurrent is UVA-specific and requires G protein and phospholipase C signaling, thus contributing to UVA-induced calcium responses to mediate downstream cellular effects and providing evidence for TRPA1 function in mammalian phototransduction. Remarkably, TRPA1 activation is required for the UVR-induced and retinal-dependent early increase in cellular melanin. Our results show that TRPA1 is essential for a unique extraocular phototransduction pathway in human melanocytes that is activated by physiological doses of UVR and results in early melanin synthesis.

Topical antihistamines display potent anti-inflammatory activity linked in part to enhanced permeability barrier function

Systemic antagonists of the histamine type 1 and 2 receptors (H1/2r) are widely used as anti-pruritics and central sedatives, but demonstrate only modest anti-inflammatory activity. Because many inflammatory dermatoses result from defects in cutaneous barrier function, and because keratinocytes express both Hr1 and Hr2, we hypothesized that H1/2r antagonists might be more effective, if they were used topically to treat inflammatory dermatoses. Topical H1/2r antagonists additively enhanced permeability barrier homeostasis in normal mouse skin by: i) stimulation of epidermal differentiation, leading to thickened cornified envelopes; and ii) enhanced epidermal lipid synthesis and secretion. Since barrier homeostasis was enhanced to a comparable extent in mast cell-deficient mice, with no further improvement following application of topical H1/2r antagonists, H1/2r antagonists likely oppose mast cell-derived histamine. In four immunologically-diverse, murine disease models, characterized by either inflammation alone (acute irritant contact dermatitis, acute allergic contact dermatitis), or by prominent barrier abnormalities (subacute allergic contact dermatitis, atopic dermatitis), topical H1/2r agonists aggravated, while H1/2r antagonists improved inflammation and/or barrier function. The apparent ability of topical H1r/2r antagonists to target epidermal H1/2r could translate into increased efficacy in the treatment of inflammatory dermatoses, likely due to decreased inflammation and enhanced barrier function. These results could shift current paradigms of antihistamine utilization from a predominantly-systemic to a topical approach.

Regulation of permeability barrier homeostasis

A major function of the skin is to provide a barrier to the movement of water and electrolytes, which is required for life in a terrestrial environment. This permeability barrier is localized to the stratum corneum and is mediated by extracellular lipid-enriched lamellar membranes, which are delivered to the extracellular spaces by the secretion of lamellar bodies by stratum granulosum cells. A large number of factors have been shown to regulate the formation of this permeability barrier. Specifically, lamellar body secretion and permeability barrier formation are accelerated by decreases in the calcium content in the stratum granulosum layer of the epidermis. In addition, increased expression of cytokines and growth factors and the activation of nuclear hormone receptors (peroxisome proliferator-activated receptors, liver X receptors, vitamin D receptor) accelerate permeability barrier formation. In contrast, nitric oxide, protease-activated receptor 2 activation, glucocorticoids, and testosterone inhibit permeability barrier formation. The ability of a variety of factors to regulate permeability barrier formation allows for a more precise and nuanced regulation.