TRPA1-dependent pruritus in IL-13-induced chronic atopic dermatitis

Interleukin-31: The "itchy" cytokine in inflammation and therapy

The cytokine interleukin-31 has been implicated in the pathophysiology of multiple atopic disorders such as atopic dermatitis (AD), allergic rhinitis, and airway hyper-reactivity. In AD, IL-31 has been identified as one of the main "drivers" of its cardinal symptom, pruritus. Here, we summarize the mechanisms by which IL-31 modulates inflammatory and allergic diseases. T_H 2 cells play a central role in AD and release high levels of T_H 2-associated cytokines including IL-31, thereby mediating inflammatory responses, initiating immunoregulatory circuits, stimulating itch, and neuronal outgrowth through activation of the heterodimeric receptor IL-31 receptor A (IL31RA)/Oncostatin M receptor (OSMRβ). IL31RA expression is found on human and murine dorsal root ganglia neurons, epithelial cells including keratinocytes and various innate immune cells. IL-31 is a critical cytokine involved in neuroimmune communication, which opens new avenues for cytokine modulation in neuroinflammatory diseases including AD/pruritus, as validated by recent clinical trials using an anti-IL-31 antibody. Accordingly, inhibition of IL-31-downstream signaling may be a beneficial approach for various inflammatory diseases including prurigo. However, as to whether downstream JAK inhibitors directly block IL-31-mediated-signaling needs to be clarified. Targeting the IL-31/IL31RA/OSMRβ axis appears to be a promising approach for inflammatory, neuroinflammatory, and pruritic disorders in the future.

Calcium Channels: Noteworthy Regulators and Therapeutic Targets in Dermatological Diseases

Dysfunctional skin barrier and impaired skin homeostasis may lead to or aggravate a series of dermatologic diseases. A large variety of biological events and bioactive molecules are involved in the process of skin wound healing and functional recovery. Calcium ions (Ca²⁺) released from intracellular stores as well as influx through plasma membrane are essential to skin function. Growing evidence suggests that calcium influx is mainly regulated by calcium-sensing receptors and channels, including voltage-gated, transient potential receptor, store-operated, and receptor-operated calcium channels, which not only maintain cellular Ca²⁺ homeostasis, but also participate in cell proliferation and skin cell homeostasis through Ca²⁺-sensitive proteins such as calmodulin (CaM). Furthermore, distinct types of Ca²⁺ channels not merely work separately, they may work concertedly to regulate cell function. In this review, we discussed different calcium-sensing receptors and channels, including voltage-gated, transient receptor potential, store-operated, and receptor-operated calcium channels, particularly focusing on their regulatory functions and inherent interactions as well as calcium channels-related reagents and drugs, which is expected to bridge basic research and clinical applications in dermatologic diseases.

Tralokinumab for the Treatment of Atopic Dermatitis

Atopic dermatitis (AD) is a relapsing or chronic heterogeneous inflammatory skin disorder with a substantial economic and social impact. AD is a multifactorial disease regulated by a diverse set of environmental and genetic determinants. The main factors involved in the pathogenesis of AD are epidermal barrier dysfunction, immune dysregulation, and dysbiosis. Current data have valued interleukin (IL)-13 as conceivably the crucial cytokine in the underlying inflammation of AD. Advances in understanding AD pathophysiology have driven the progress of targeted immunomodulatory treatments for the treatment of AD, including tralokinumab, a selective IL-13 inhibitor. A phase IIb clinical trial showed that a dosing regimen of 150 or 300 mg every 2 weeks effectively treated moderate-to-severe AD in adults with an acceptable tolerability profile. Phase III clinical trials demonstrated that results with tralokinumab in monotherapy were superior to those with placebo at 16 weeks of treatment. It was also well tolerated up to 52 weeks in the vast majority of patients. In addition, in association with topical corticosteroids, tralokinumab was well tolerated and effective and had a favorable risk-benefit profile. These data provide additional evidence that IL-13 is central to AD pathogenesis, suggesting that tralokinumab may be seen as an innovative option for the treatment of moderate-to-severe AD.

TRPV1 and TRPA1 Channels Are Both Involved Downstream of Histamine-Induced Itch

Two histamine receptor subtypes (HR), namely H1R and H4R, are involved in the transmission of histamine-induced itch as key components. Although exact downstream signaling mechanisms are still elusive, transient receptor potential (TRP) ion channels play important roles in the sensation of histaminergic and non-histaminergic itch. The aim of this study was to investigate the involvement of TRPV1 and TRPA1 channels in the transmission of histaminergic itch. The potential of TRPV1 and TRPA1 inhibitors to modulate H1R- and H4R-induced signal transmission was tested in a scratching assay in mice in vivo as well as via Ca²⁺ imaging of murine sensory dorsal root ganglia (DRG) neurons in vitro. TRPV1 inhibition led to a reduction of H1R- and H4R- induced itch, whereas TRPA1 inhibition reduced H4R- but not H1R-induced itch. TRPV1 and TRPA1 inhibition resulted in a reduced Ca²⁺ influx into sensory neurons in vitro. In conclusion, these results indicate that both channels, TRPV1 and TRPA1, are involved in the transmission of histamine-induced pruritus.

The inflammatory regulation of TRPA1 expression in human A549 lung epithelial cells

Transient receptor potential ankyrin-1 (TRPA1) is an ion channel mediating pain and cough signals in sensory neurons. We and others have shown that TRPA1 is also expressed in some non-neuronal cells and supports inflammatory responses. To address the pathogenesis and to uncover potential targets for pharmacotherapy in inflammatory lung diseases, we set out to study the expression of TRPA1 in human A549 lung epithelial cells under inflammatory conditions. TRPA1 expression was determined by RT-qPCR and Western blotting at a mRNA and protein level, respectively and its function was studied by Fluo 3-AM intracellular Ca²⁺ measurement in A549 lung epithelial cells. TRPA1 promoter activity was assessed by reporter gene assay. TRPA1 expression was very low in A549 cells in the absence of inflammatory stimuli. Tumor necrosis factor-α (TNF-α) significantly increased TRPA1 expression and a synergy was found between TNF-α, interleukin-1β (IL-1β) and interferon-γ (IFN-γ). Reporter gene experiments indicate that the combination of TNF-α and IL-1β increases TRPA1 promoter activity while the effect of IFN-γ seems to be non-transcriptional. Interestingly, the glucocorticoid dexamethasone downregulated TRPA1 expression in A549 cells by reducing TRPA1 mRNA stability in a transcription-dependent manner. Furthermore, pharmacological blockade of TRPA1 reduced the production of the pro-inflammatory cytokine IL-8. In conclusion, TRPA1 was found to be expressed and functional in human A549 lung epithelial cells under inflammatory conditions. The anti-inflammatory steroid dexamethasone reduced TRPA1 expression through post-transcriptional mechanisms. The results reveal TRPA1 as a potential mediator and drug target in inflammatory lung conditions.

Common and discrete mechanisms underlying chronic pain and itch: peripheral and central sensitization

Normally, an obvious antagonism exists between pain and itch. In normal conditions, painful stimuli suppress itch sensation, whereas pain killers often generate itch. Although pain and itch are mediated by separate pathways under normal conditions, most chemicals are not highly specific to one sensation in chronic pathologic conditions. Notably, in patients with neuropathic pain, histamine primarily induces pain rather than itch, while in patients with atopic dermatitis, bradykinin triggers itch rather than pain. Accordingly, repetitive scratching even enhances itch sensation in chronic itch conditions. Physicians often prescribe pain relievers to patients with chronic itch, suggesting common mechanisms underlying chronic pain and itch, especially peripheral and central sensitization. Rather than separating itch and pain, studies should investigate chronic itch and pain including neuropathic and inflammatory conditions. Here, we reviewed chronic sensitization leading to chronic pain and itch at both peripheral and central levels. Studies investigating the connection between pain and itch facilitate the development of new therapeutics against both chronic dysesthesias based on the underlying pathophysiology.

Th2 Modulation of Transient Receptor Potential Channels: An Unmet Therapeutic Intervention for Atopic Dermatitis

Atopic dermatitis (AD) is a multifaceted, chronic relapsing inflammatory skin disease that affects people of all ages. It is characterized by chronic eczema, constant pruritus, and severe discomfort. AD often progresses from mild annoyance to intractable pruritic inflammatory lesions associated with exacerbated skin sensitivity. The T helper-2 (Th2) response is mainly linked to the acute and subacute phase, whereas Th1 response has been associated in addition with the chronic phase. IL-17, IL-22, TSLP, and IL-31 also play a role in AD. Transient receptor potential (TRP) cation channels play a significant role in neuroinflammation, itch and pain, indicating neuroimmune circuits in AD. However, the Th2-driven cutaneous sensitization of TRP channels is underappreciated. Emerging findings suggest that critical Th2-related cytokines cause potentiation of TRP channels, thereby exaggerating inflammation and itch sensation. Evidence involves the following: (i) IL-13 enhances TRPV1 and TRPA1 transcription levels; (ii) IL-31 sensitizes TRPV1 via transcriptional and channel modulation, and indirectly modulates TRPV3 in keratinocytes; (iii) The Th2-cytokine TSLP increases TRPA1 synthesis in sensory neurons. These changes could be further enhanced by other Th2 cytokines, including IL-4, IL-25, and IL-33, which are inducers for IL-13, IL-31, or TSLP in skin. Taken together, this review highlights that Th2 cytokines potentiate TRP channels through diverse mechanisms under different inflammatory and pruritic conditions, and link this effect to distinct signaling cascades in AD. This review strengthens the notion that interrupting Th2-driven modulation of TRP channels will inhibit transition from acute to chronic AD, thereby aiding the development of effective therapeutics and treatment optimization.

Current Insights into Immunology and Novel Therapeutics of Atopic Dermatitis

Atopic dermatitis (AD) is one of the most prevalent inflammatory disease among non-fatal skin diseases, affecting up to one fifth of the population in developed countries. AD is characterized by recurrent pruritic and localized eczema with seasonal fluctuations. AD initializes the phenomenon of atopic march, during which infant AD patients are predisposed to progressive secondary allergies such as allergic rhinitis, asthma, and food allergies. The pathophysiology of AD is complex; onset of the disease is caused by several factors, including strong genetic predisposition, disrupted epidermal barrier, and immune dysregulation. AD was initially characterized by defects in the innate immune system and a vigorous skewed adaptive Th2 response to environmental agents; there are compelling evidences that the disorder involves multiple immune pathways. Symptomatic palliative treatment is the only strategy to manage the disease and restore skin integrity. Researchers are trying to more precisely define the contribution of different AD genotypes and elucidate the role of various immune axes. In this review, we have summarized the current knowledge about the roles of innate and adaptive immune responsive cells in AD. In addition, current and novel treatment strategies for the management of AD are comprehensively described, including some ongoing clinical trials and promising therapeutic agents. This information will provide an asset towards identifying personalized targets for better therapeutic outcomes.

Transient Receptor Potential Ankyrin 1 (TRPA1)-An Inflammation-Induced Factor in Human HaCaT Keratinocytes

Transient receptor potential ankyrin 1 (TRPA1) is an ion channel mainly studied in sensory neurons where it mediates itch, pain and neurogenic inflammation. Recently, some nonneuronal cells have also been shown to express TRPA1 to support inflammatory responses. To address the role of TRPA1 in skin inflammation, we aimed to investigate TRPA1 expression in keratinocytes. HaCaT cells (a model of human keratinocytes) and skin biopses from wild-type and TRPA1 deficient mice were used in the studies. TRPA1 expression in nonstimulated keratinocytes was very low but significantly inducible by the proinflammatory cytokine tumor necrosis factor (TNF) in an nuclear factor kappa B (NF-κB), and mitogen-activated protein (MAP) kinase (p38 and c-Jun N-terminal kinase, JNK)-dependent manner. Interestingly, drugs widely used to treat skin inflammation, the calcineurin inhibitors tacrolimus and cyclosporine and the glucocorticoid dexamethasone, significantly decreased TRPA1 expression. Furthermore, pharmacological inhibition and genetic deletion of TRPA1 reduced the synthesis of TNF-induced monocyte chemoattractant protein 1 (MCP-1) in keratinocytes and mouse skin biopsies. In conclusion, these findings point to an inflammatory role for TRPA1 in keratinocytes and present TRPA1 as a potential drug target in inflammatory skin diseases.

Pruritus as a Distinctive Feature of Type 2 Inflammation

Pruritus is a common symptom of several skin diseases, both inflammatory and neoplastic. Pruritus might have a tremendous impact on patients’ quality of life and strongly interfere with sleep, social, and work activities. We review the role of type-2 inflammation and immunity in the pathogenesis of chronic pruritic conditions of the skin. Type 2 cytokines, including IL-4, IL-13, thymic stromal lymphopoietin, periostin, IL-31, IL-25, and IL-33 are released by mast cells, innate lymphoid cells 2, keratinocytes, and type 2 T lymphocytes, and are master regulators of chronic itch. These cytokines might act as direct pruritogen on primary sensory neurons (pruriceptors) or alter the sensitivity to other itch mediators Type 2 inflammation- and immunity-dominated skin diseases, including atopic dermatitis, prurigo nodularis, bullous pemphigoid, scabies, parasitic diseases, urticaria, and Sézary syndrome are indeed conditions associated with most severe pruritus. In contrast, in other skin diseases, such as scleroderma, lupus erythematosus, hidradenitis suppurativa, and acne, type 2 inflammation is less represented, and pruritus is milder or variable. Th2 inflammation and immunity evolved to protect against parasites, and thus, the scratching response evoked by pruritus might have developed to alert about the presence and to remove parasites from the skin surface.

The Role of TRPA1 in Skin Physiology and Pathology

The transient receptor potential ankyrin 1 (TRPA1), a member of the TRP superfamily of channels, acts as 'polymodal cellular sensor' on primary sensory neurons where it mediates the peripheral and central processing of pain, itch, and thermal sensation. However, the TRPA1 expression extends far beyond the sensory nerves. In recent years, much attention has been paid to its expression and function in non-neuronal cell types including skin cells, such as keratinocytes, melanocytes, mast cells, dendritic cells, and endothelial cells. TRPA1 seems critically involved in a series of physiological skin functions, including formation and maintenance of physico-chemical skin barriers, skin cells, and tissue growth and differentiation. TRPA1 appears to be implicated in mechanistic processes in various immunological inflammatory diseases and cancers of the skin, such as atopic and allergic contact dermatitis, psoriasis, bullous pemphigoid, cutaneous T-cell lymphoma, and melanoma. Here, we report recent findings on the implication of TRPA1 in skin physiology and pathophysiology. The potential use of TRPA1 antagonists in the treatment of inflammatory and immunological skin disorders will be also addressed.

Involvement of Neuro-Immune Interactions in Pruritus With Special Focus on Receptor Expressions

Pruritus is a common, but very challenging symptom with a wide diversity of underlying causes like dermatological, systemic, neurological and psychiatric diseases. In dermatology, pruritus is the most frequent symptom both in its acute and chronic form (over 6 weeks in duration). Treatment of chronic pruritus often remains challenging. Affected patients who suffer from moderate to severe pruritus have a significantly reduced quality of life. The underlying physiology of pruritus is very complex, involving a diverse network of components in the skin including resident cells such as keratinocytes and sensory neurons as well as transiently infiltrating cells such as certain immune cells. Previous research has established that there is a significant crosstalk among the stratum corneum, nerve fibers and various immune cells, such as keratinocytes, T cells, basophils, eosinophils and mast cells. In this regard, interactions between receptors on cutaneous and spinal neurons or on different immune cells play an important role in the processing of signals which are important for the transmission of pruritus. In this review, we discuss the role of various receptors involved in pruritus and inflammation, such as TRPV1 and TRPA1, IL-31RA and OSMR, TSLPR, PAR-2, NK1R, H1R and H4R, MRGPRs as well as TrkA, with a focus on interaction between nerve fibers and different immune cells. Emerging evidence shows that neuro-immune interactions play a pivotal role in mediating pruritus-associated inflammatory skin diseases such as atopic dermatitis, psoriasis or chronic spontaneous urticaria. Targeting these bidirectional neuro-immune interactions and the involved pruritus-specific receptors is likely to contribute to novel insights into the underlying pathogenesis and targeted treatment options of pruritus.

Inhibition of IL-13: A New Pathway for Atopic Dermatitis [Formula: see text]

Dupilumab, a monoclonal antibody against the common receptor of interleukin (IL)-4 and IL-13, was the first biologic therapy approved in Canada for treatment of moderate-to-severe atopic dermatitis (AD). While it is considered safe and effective, dupilumab is not universally effective and 8%-38% of patients develop conjunctivitis, while some patients develop head and neck dermatitis. Thus, new therapeutic options are warranted. While both IL-4 and IL-13 play important roles in the pathogenesis of AD, it has been recently demonstrated that IL-13 is the primary upregulated cytokine in AD skin biopsy samples. A placebo-controlled phase 2b clinical trial evaluating the efficacy and safety of lebrikizumab, an IL-13 inhibitor, in AD demonstrated that, at 16 weeks, Eczema Area and Severity Index (EASI) 75 and Investigator's Global Assessment (IGA) 0/1 were achieved by 60.6% and 44.6% of patients taking lebrikizumab at its highest dose (vs 24.3% and 15.3% of patients taking placebo, respectively). Moreover, treatment with lebrikizumab was associated with rapid improvement of pruritus and low rates of conjunctivitis (1.4%-3.8%). Another IL-13 monoclonal antibody, tralokinumab, was evaluated for safety and efficacy in moderate-to-severe AD. By week 12, among adults receiving 300 mg tralokinumab, 42.5% achieved EASI-75 and 26.7% achieved IGA 0/1 score (vs 15.5% and 11.8% in the placebo group, respectively). Both lebrikizumab and tralokinumab demonstrated acceptable safety profiles in AD (and non-AD) trials with adverse events often being comparable between treatment and control groups. Thus, IL-13 inhibitors may provide a safe and effective treatment alternative for patients with moderate-to-severe AD.

Does Cholinergic Stimulation Affect the P2X7 Receptor-Mediated Dye Uptake in Mast Cells and Macrophages?

Background: Extracellular ATP is a powerful trigger of neuroinflammation by activating immune cells via P2X7 receptors. Acetylcholine and nicotinic agonists inhibit ATP-triggered proinflammatory cytokines via the so-called “cholinergic anti-inflammatory pathway” (CAP). However, it remains unclear as to what stage of ATP-induced signaling cholinergic agents provide this anti-inflammatory effect. Using the specific property of P2X7 receptor to open a pathway permeable to large molecules, associated with activation of inflammasome, we studied the action of cholinergic agents on this key event in CAP activation.

Methods: Freshly isolated mouse peritoneal mast cells and primary human macrophages were used. To assess P2X7 channel opening, the permeability to the fluorescent dye YO-PRO1 or ethidium bromide (EtBr) was measured by flow cytometry. Expression of nicotinic receptors was probed in macrophages with the fluorescently labeled α-bungarotoxin or with patch-clamp recordings.

Results: ATP opened P2X7 ion channels in mast cells and macrophages permeable to YO-PRO1 or EtBr, respectively. This stimulatory effect in mast cells was inhibited by the specific P2X7 antagonist A839977 confirming that YO-PRO1 uptake was mediated via ATP-gated P2X7 ion channels. Cholinergic agents also slightly induced dye uptake to mast cells but not in macrophages, which expressed functional α7 nicotinic receptors. However, both in mast cells and in macrophages, acetylcholine and nicotine failed to inhibit the stimulatory effect of ATP on dye uptake.

Conclusion: These data suggest that in immune cells, cholinergic agents do not act on P2X7 receptor-coupled large pore formation but can mediate the anti-inflammatory effect underlying CAP downstream of ATP-driven signaling.

TRPA1 deficiency alleviates inflammation of atopic dermatitis by reducing macrophage infiltration

AIMS

The aim of this study was to investigate the role of TRPA1 in the pathogenesis of AD.

MAIN METHODS

The experimental atopic dermatitis (AD)-like skin lesions were established using 2,4-dinitrochlorobenzene (DNCB). Mice were divided into three groups: TRPA1^-/- and WT groups were treated with DNCB dissolved in a 3:1 mixture of acetone and olive oil; the negative control group was treated with 3:1 mixture of acetone and olive oil without DNCB. The treatment lasted for 21 days, after which the animals were sacrificed and their blood, ears and dorsal skin tissue samples were collected for analysis.

KEY FINDINGS

Lower dermatitis score, ear thickness, pruritus score, and epidermal hyperplasia were observed in mice in TRPA1^-/- mice compared to the WT group. Besides, lower dermal mast cell infiltration, proinflammatory cytokines, Th2 cytokines and the infiltration of macrophages were observed in the TRPA1^-/- mice compared to the WT group. Furthermore, we demonstrated that TRPA1 antagonist HC-030031 could alleviate AD-like symptoms and reduce the degree of epidermal hyperplasia in mice.

SIGNIFICANCE

TRPA1 has a crucial role during the AD pathogenesis in mice, thus may be used as a potential new target for treating patients with chronic skin inflammatory disease.

MRGPRX2 sensing of cationic compounds-A bridge between nociception and skin diseases?

Mast cells are innate immune cells located at many barrier sites in the body and known to protect the host against environmental threats and to be involved in allergic diseases. More recently, new studies have investigated their roles in the regulation of skin inflammation and transmission of pain and itch sensations. Mast cell signalling through the Mas-related G protein-coupled receptor (MRGPR) X2 or its mouse orthologue MRGPRB2 has been reported to be one of the major mechanism by which mast cell can regulate such processes. MRGPRX2 and MRGPRB2 can induce mast cell degranulation upon binding to a broad panel of cationic molecules such as neuropeptides, bacteria-derived quorum sensing molecules, venom peptides, host defense peptides and, unfortunately, various FDA-approved drugs. Upon activation, mast cells release granule-associated proteases, lipids and multiple cytokines that can modulate vascular permeability, immune cells recruitment and activation status of tissue-projecting nociceptive sensory neurons (ie nociceptors). Here, we discuss the modality of MRGPRX2-dependent mast cell activation and its different consequences on the patterns of skin inflammation and associated diseases. We notably emphasize how MRGPRX2-dependent skin mast cell activation might trigger various pathological traits such as pruritus, pain and inflammation and therefore become a potential therapeutic target for inflammatory pain, itch, atopic dermatitis and drugs-induced injection site reactions.

Transgenic Chlamydomonas Expressing Human Transient Receptor Potential Ankyrin 1 (TRPA1) Channels to Assess the Effect of Agonists and Antagonists

Transient receptor potential ankyrin 1 (TRPA1) channel is an ion channel whose gating is controlled by agonists, such as allyl isothiocyanate (AITC), and temperature. Since TRPA1 is associated with various disease symptoms and chemotherapeutic side effects, it is a frequent target of drug development. To facilitate the screening of TRPA1 agonists and antagonists, this study aimed to develop a simple bioassay for TRPA1 activity. To this end, transgenic Chlamydomonas reinhardtii expressing human TRPA1 was constructed. The transformants exhibited positive phototaxis at high temperatures (≥20°C) but negative phototaxis at low temperatures (≤15°C); wild-type cells showed positive phototaxis at all temperatures examined. In the transgenic cells, negative phototaxis was inhibited by TRPA1 antagonists, such as HC030031, A-967079, and AP18, at low temperatures. Negative phototaxis was induced by TRPA1 agonists, such as icilin and AITC, at high temperatures. The effects of these agonists were blocked by TRPA1 antagonists. In wild-type cells, none of these substances had any effects on phototaxis. These results indicate that the action of TRPA1 agonists and antagonists can be readily assessed using the behavior of C. reinhardtii expressing human TRPA1 as an assessment tool.

TRPA1 as a therapeutic target for nociceptive pain

Introduction

Chronic pain affects approximatively 30–50% of the population globally. Pathologies such as migraine, diabetic neuropathy, nerve injury and treatment with chemotherapeutic agents, can induce chronic pain. Members of the transient receptor potential (TRP) channels, including the TRP ankyrin 1 (TRPA1), have a major role in pain.

Areas covered

We focus on TRPA1 as a therapeutic target for pain relief. The structure, localization, and activation of the channel and its implication in different pathways to signal pain are described. This paper underlines the role of pharmacological interventions on TRPA1 to reduce pain in numerous pain conditions. We conducted a literature search in PubMed up to and including July 2020.

Expert opinion

Our understanding of the molecular mechanisms underlying the sensitization of central and peripheral nociceptive pathways is limited. Preclinical evidence indicates that, in murine models of pain diseases, numerous mechanisms converge on the pathway that encompasses oxidative stress and Schwann cell TRPA1 to sustain chronic pain. Programs to identify and develop treatments to attenuate TRPA1-mediated chronic pain have emerged from this knowledge. Antagonists explored as a novel class of analgesics have a new and promising target in the TRPA1 expressed by peripheral glial cells.

Epidermal expression of human TRPM8, but not of TRPA1 ion channels, is associated with sensory responses to local skin cooling

Human cold perception and nociception play an important role in persisting pain. However, species differences in the target temperature of thermosensitive ion channels expressed in peripheral nerve endings have fueled discussions about the mechanism of cold nociception in humans. Most frequently implicated thermosensors are members of the transient receptor potential (TRP) ion channel family TRPM8 and TRPA1. Regularly observed, distinct cold pain phenotype groups suggested the existence of interindividually differing molecular bases. In 28 subjects displaying either high or medium sensitivity to local cooling of the skin, the density at epidermal nerve fibers of TRPM8, but not that of TRPA1 expression, correlated significantly with the cold pain threshold. Moreover, reproducible grouping of the subjects, based on high or medium sensitivity to cooling, was reflected in an analogous grouping based on high or low TRPM8 expression at epidermal nerve fibers. The distribution of TRPM8 expression in epidermal nerve fibers provided an explanation for the previously observed (bi)modal distribution of human cold pain thresholds which was reproduced in this study. In the light of current controversies on the role of human TRPA1 ion channels in cold pain perception, the present observations demonstrating a lack of association of TRPA1 channel expression with cold sensitivity-related measures reinforce doubts about involvement of this channel in cold pain in humans. Since TRP inhibitors targeting TRPM8 and TRPA1 are currently entering clinical phases of drug development, the existence of known species differences, in particular in the function of TRPA1, emphasizes the increasing importance of new methods to directly approach the roles of TRPs in humans.

Advances in Understanding the Initial Steps of Pruritoceptive Itch: How the Itch Hits the Switch

Pruritoceptive (dermal) itch was long considered an accompanying symptom of diseases, a side effect of drug applications, or a temporary sensation induced by invading pruritogens, as produced by the stinging nettle. Due to extensive research in recent years, it was possible to provide detailed insights into the mechanism of itch mediation and modulation. Hence, it became apparent that pruritus is a complex symptom or disease in itself, which requires particular attention to improve patients’ health. Here, we summarize recent findings in pruritoceptive itch, including how this sensation is triggered and modulated by diverse endogenous and exogenous pruritogens and their receptors. A differentiation between mediating pruritogen and modulating pruritogen seems to be of great advantage to understand and decipher the molecular mechanism of itch perception. Only a comprehensive view on itch sensation will provide a solid basis for targeting this long-neglected adverse sensation accompanying numerous diseases and many drug side effects. Finally, we identify critical aspects of itch perception that require future investigation.

Voltage-dependent modulation of TRPA1 currents by diphenhydramine

Diphenhydramine (DPH) has been broadly used to treat allergy. When used as a topical medicine, DPH temporarily relieves itching and pain. Although transient receptor potential type A1 (TRPA1) channel is known to play roles in both acute and chronic itch and pain, whether DPH affects the activities of TRPA1 remains unclear. Using whole-cell patch clamp recordings, we demonstrated that DPH modulates the voltage-dependence of TRPA1. When co-applied with a TRPA1 agonist, DPH significantly enhanced the inward currents while suppressing the outward currents of TRPA1, converting the channel from outwardly rectifying to inwardly rectifying. This effect of DPH occurred no matter TRPA1 was activated by an electrophilic or non-electrophilic agonist and for both mouse and human TRPA1. The modulation of TRPA1 by DPH was maintained in the L906C mutant, which by itself also causes inward rectification of TRPA1, indicating that additional acting sites are present for the modulation of TRPA1 currents by DPH. Our recordings also revealed that DPH partially blocked capsaicin evoked TRPV1 currents. These data suggest that DPH may exert its therapeutic effects on itch and pain, through modulation of TRPA1 in a voltage-dependent fashion.

The formulae and biologically active ingredients of Chinese herbal medicines for the treatment of atopic dermatitis

Atopic dermatitis (AD) is a common relapsing inflammatory skin disease characterized by severe pruritus that seriously affects the quality of patients' life. There is an increasingly large amount of research demonstrating that traditional Chinese medicine (TCM) including herbal formulae and bioactive ingredients exerts pharmacological effects on atopic dermatitis. It has been a long history of TCM being used to treat atopic dermatitis, especially in preventing disease recurrence, maintaining long-term remission, and reducing disease burden. Nowadays, both of TCM monomer preparations and traditional formulae are still widely used. This review focuses on TCM as well as its bioactive ingredients for the treatment of AD, from the perspectives of animal model construction, pharmacodynamic mechanisms and clinical studies of formulae. To be more specific, the regulation and molecular mechanisms of the herbal formulae and bioactive ingredients of TCM are investigated, and the latest clinical research on TCM formulae is discussed. Furthermore, it provides a summary of the strengths and utilities of TCM, and will be useful for doctors who use Chinese medicine for treatment or researchers who select candidates for clinical treatments or further high-quality clinical studies.

Spinal cord NLRP1 inflammasome contributes to dry skin induced chronic itch in mice

BACKGROUND

Dry skin itch is one of the most common skin diseases and elderly people are believed to be particularly prone to it. The inflammasome has been suggested to play an important role in chronic inflammatory disorders including inflammatory skin diseases such as psoriasis. However, little is known about the role of NLRP1 inflammasome in dry skin-induced chronic itch.

METHODS

Dry skin-induced chronic itch model was established by acetone-ether-water (AEW) treatment. Spontaneous scratching behavior was recorded by video monitoring. The expression of nucleotide oligomerization domain (NOD)-like receptor protein 1 (NLRP1) inflammasome complexes, transient receptor potential vanilloid type 1 (TRPV1), and the level of inflammatory cytokines were determined by western blot, quantitative real-time PCR, and enzyme-linked immunosorbent assay (ELISA) kits. Nlrp1a knockdown was performed by an adeno-associated virus (AAV) vector containing Nlrp1a-shRNA-eGFP infusion. H.E. staining was used to evaluate skin lesion.

RESULTS

AEW treatment triggers spontaneous scratching and significantly increases the expression of NLRP1, ASC, and caspase-1 and the levels of IL-1β, IL-18, IL-6, and TNF-α in the spinal cord and the skin of mice. Spinal cord Nlrp1a knockdown prevents AEW-induced NLRP1 inflammasome assembly, TRPV1 channel activation, and spontaneous scratching behavior. Capsazepine, a specific antagonist of TRPV1, can also inhibit AEW-induced inflammatory response and scratching behavior. Furthermore, elderly mice and female mice exhibited more significant AEW-induced scratching behavior than young mice and male mice, respectively. Interestingly, AEW-induced increases in the expression of NLRP1 inflammasome complex and the levels of inflammatory cytokines were more remarkable in elderly mice and female mice than in young mice and male mice, respectively.

CONCLUSIONS

Spinal cord NLRP1 inflammasome-mediated inflammatory response contributes to dry skin-induced chronic itch by TRPV1 channel, and it is also involved in age and sex differences of chronic itch. Inhibition of NLRP1 inflammasome may offer a new therapy for dry skin itch.

Conditional KCa3.1-transgene induction in murine skin produces pruritic eczematous dermatitis with severe epidermal hyperplasia and hyperkeratosis

Ion channels have recently attracted attention as potential mediators of skin disease. Here, we explored the consequences of genetically encoded induction of the cell volume-regulating Ca2+-activated KCa3.1 channel (Kcnn4) for murine epidermal homeostasis. Doxycycline-treated mice harboring the KCa3.1+-transgene under the control of the reverse tetracycline-sensitive transactivator (rtTA) showed 800-fold channel overexpression above basal levels in the skin and solid KCa3.1-currents in keratinocytes. This overexpression resulted in epidermal spongiosis, progressive epidermal hyperplasia and hyperkeratosis, itch and ulcers. The condition was accompanied by production of the pro-proliferative and pro-inflammatory cytokines, IL-β1 (60-fold), IL-6 (33-fold), and TNFα (26-fold) in the skin. Treatment of mice with the KCa3.1-selective blocker, Senicapoc, significantly suppressed spongiosis and hyperplasia, as well as induction of IL-β1 (-88%) and IL-6 (-90%). In conclusion, KCa3.1-induction in the epidermis caused expression of pro-proliferative cytokines leading to spongiosis, hyperplasia and hyperkeratosis. This skin condition resembles pathological features of eczematous dermatitis and identifies KCa3.1 as a regulator of epidermal homeostasis and spongiosis, and as a potential therapeutic target.

PAR2 Mediates Itch via TRPV3 Signaling in Keratinocytes

Animal studies have suggested that transient receptor potential ion channels and G-protein coupled receptors play important roles in itch transmission. TRPV3 gain-of-function mutations have been identified in patients with Olmsted syndrome, which is associated with severe pruritus. However, the mechanisms causing itch remain poorly understood. Here, we show that keratinocytes lacking TRPV3 impair the function of protease-activated receptor 2 (PAR2), resulting in reduced neuronal activation and scratching behavior in response to PAR2 agonists. Moreover, we show that TRPV3 and PAR2 were upregulated in skin biopsies from patients and mice with atopic dermatitis, whereas their inhibition attenuated scratching and inflammatory responses in mouse atopic dermatitis models. These results reveal a previously unrecognized link between TRPV3 and PAR2 in keratinocytes to convey itch information and suggest that a blockade of PAR2 or TRPV3 individually or both may serve as a potential approach for antipruritic therapy in atopic dermatitis.

Interleukin-13: Targeting an underestimated cytokine in atopic dermatitis

Atopic dermatitis (AD) is a common inflammatory skin condition that has traditionally been considered a paradigmatic type 2 immunity (T2)-driven disease. Interleukin (IL)-4 and IL-13 are both pivotal cytokines involved in the generation of allergic diseases. Currently, besides dupilumab, which blocks the binding of both cytokines to their receptors, a number of new pharmacologic entities have been designed to target both T2 cytokines and/or their receptors and/or receptor-associated signal transduction machinery such as Janus kinases. Recently, IL-13 has been suggested to be the key T2 cytokine driving inflammation in the periphery, while IL-4 may merely have a central effect. There is increasing evidence that this concept holds true for the inflammatory reaction underlying AD, where IL-13 is overexpressed locally and has a significant impact on skin biology, including the recruitment of inflammatory cells, the alteration of the skin microbiome, and the decrease in the epidermal barrier function. This review provides an update on the role of IL-13 in AD and discusses the different strategies aimed at interfering with its biologic activity as well as their potential in a precision medicine approach in the management of AD.

Validation of antibodies for the specific detection of human TRPA1

The transient receptor potential cation channel family member ankyrin 1 (TRPA1) is a potential target for several diseases, but detection of human TRPA1 (hTRPA1) protein in cells and tissues is problematic as rigorous antibody validation is lacking. We expressed hTRPA1 in a TRPA1-negative cell line to evaluate 5 commercially available antibodies by western blotting, immunofluorescence, immunocytochemistry and flow cytometry. The three most cited anti-TRPA1 antibodies lacked sensitivity and/or specificity, but two mouse monoclonal anti-TRPA1 antibodies detected hTRPA1 specifically in the above assays. This enabled the development of a flow cytometry assay, which demonstrated strong expression of TRPA1 in human lung myofibroblasts, human airway smooth muscle cells but not lung mast cells. The most cited anti-TRPA1 antibodies lack sensitivity and/or specificity for hTRPA1. We have identified two anti-TRPA1 antibodies which detect hTRPA1 specifically. Previously published data regarding human TRPA1 protein expression may need revisiting.

Mammalian Transient Receptor Potential TRPA1 Channels: From Structure to Disease

The transient receptor potential ankyrin (TRPA) channels are Ca²⁺-permeable nonselective cation channels remarkably conserved through the animal kingdom. Mammals have only one member, TRPA1, which is widely expressed in sensory neurons and in non-neuronal cells (such as epithelial cells and hair cells). TRPA1 owes its name to the presence of 14 ankyrin repeats located in the NH₂ terminus of the channel, an unusual structural feature that may be relevant to its interactions with intracellular components. TRPA1 is primarily involved in the detection of an extremely wide variety of exogenous stimuli that may produce cellular damage. This includes a plethora of electrophilic compounds that interact with nucleophilic amino acid residues in the channel and many other chemically unrelated compounds whose only common feature seems to be their ability to partition in the plasma membrane. TRPA1 has been reported to be activated by cold, heat, and mechanical stimuli, and its function is modulated by multiple factors, including Ca²⁺, trace metals, pH, and reactive oxygen, nitrogen, and carbonyl species. TRPA1 is involved in acute and chronic pain as well as inflammation, plays key roles in the pathophysiology of nearly all organ systems, and is an attractive target for the treatment of related diseases. Here we review the current knowledge about the mammalian TRPA1 channel, linking its unique structure, widely tuned sensory properties, and complex regulation to its roles in multiple pathophysiological conditions.

Non-Analgesic Symptomatic or Disease-Modifying Potential of TRPA1

TRPA1, a versatile ion channel of the Transient Receptor Potential (TRP) channel family, detects a large variety of chemicals and can contribute to signal processing of other stimuli, e.g., due to its sensitivity to cytosolic calcium elevation or phosphoinositolphosphate modulation. At first, TRPA1 was found on sensory neurons, where it can act as a sensor for potential or actual tissue damage that ultimately may elicit pain or itch as warning symptoms. This review provides an update regarding the analgesic and antipruritic potential of TRPA1 modulation and the respective clinical trials. Furthermore, TRPA1 has been found in an increasing amount of other cell types. Therefore, the main focus of the review is to discuss the non-analgesic and particularly the disease-modifying potential of TRPA1. This includes diseases of the respiratory system, cancer, ischemia, allergy, diabetes, and the gastrointestinal system. The involvement of TRPA1 in the respective pathophysiological cascades is so far mainly based on pre-clinical data.

Mast Cells and Sensory Nerves Contribute to Neurogenic Inflammation and Pruritus in Chronic Skin Inflammation

The intimate interaction between mast cells and sensory nerves can be illustrated by the wheal and surrounding flare in an urticarial reaction in human skin. This reaction is typically associated with an intense itch at the reaction site. Upon activation, cutaneous mast cells release powerful mediators, such as histamine, tryptase, cytokines, and growth factors that can directly stimulate corresponding receptors on itch-mediating sensory nerves. These include, e.g., H1- and H4-receptors, protease-activated receptor-2, IL-31 receptor, and the high-affinity receptor of nerve growth factor (TrkA). On the other hand, sensory nerves can release neuropeptides, including substance P and vasoactive intestinal peptide, that are able to stimulate mast cells to release mediators leading to potentiation of the reciprocal interaction, inflammation, and itch. Even though mast cells are well recognized for their role in allergic skin whealing and urticaria, increasing evidence supports the reciprocal function between mast cells and sensory nerves in neurogenic inflammation in chronic skin diseases, such as psoriasis and atopic dermatitis, which are often characterized by distressing itch, and exacerbated by psychological stress. Increased morphological contacts between mast cells and sensory nerves in the lesional skin in psoriasis and atopic dermatitis as well as experimental models in mice and rats support the essential role for mast cell-sensory nerve communication in consequent pruritus. Therefore, we summarize here the present literature pointing to a close association between mast cells and sensory nerves in pruritic skin diseases as well as review the essential supporting findings on pruritic models in mice and rats.

TRPA1 Channel as a Regulator of Neurogenic Inflammation and Pain: Structure, Function, Role in Pathophysiology, and Therapeutic Potential of Ligands

TRPA1 is a cation channel located on the plasma membrane of many types of human and animal cells, including skin sensory neurons and epithelial cells of the intestine, lungs, urinary bladder, etc. TRPA1 is the major chemosensor that also responds to thermal and mechanical stimuli. Substances that activate TRPA1, e.g., allyl isothiocyanates (pungent components of mustard, horseradish, and wasabi), cinnamaldehyde from cinnamon, organosulfur compounds from garlic and onion, tear gas, acrolein and crotonaldehyde from cigarette smoke, etc., cause burning, mechanical and thermal hypersensitivity, cough, eye irritation, sneezing, mucus secretion, and neurogenic inflammation. An increased activity of TRPA1 leads to the emergence of chronic pruritus and allergic dermatitis and is associated with episodic pain syndrome, a hereditary disease characterized by episodes of debilitating pain triggered by stress. TRPA1 is now considered as one of the targets for developing new anti-inflammatory and analgesic drugs. This review summarizes information on the structure, function, and physiological role of this channel, as well as describes known TRPA1 ligands and their significance as therapeutic agents in the treatment of inflammation-associated pain.

Role of TRPA1 receptors in skin inflammation induced by volatile chemical irritants in mice

Contact dermatitis is a very common inflammatory reaction in the skin, causing not only aesthetic problems but also loss functionality at work. The molecular mechanisms of contact dermatitis induced by chemical irritants are still unclear. Considering that transient receptor potential channels (TRP) may induce neurogenic inflammation and the exacerbation of inflammatory responses, here we investigated the role of transient receptor potential channel ankyrin type-1 (TRPA1) in skin inflammation evoked by chemical irritants. Ear oedema and nociceptive responses elicited by the topical application of xylene and toluene were measured in Swiss mice, wild type and TRPA1 knockout (Trpa1^-/-) C57BL/6 mice. Histological analyses were performed in mice subjected to the ear oedema assay. Topical application of xylene and toluene in the mouse ear induced an edematogenic response (0.113 ± 0.008 mm and 0.067 ± 0.011 mm), compared to vehicle (0.008 ± 0.008 mm), assessed by ear thickness measurements and histological analyses. These responses were prevented by topical pretreatment with a selective TRPA1 antagonist, HC-030031 (% inhibition: xylene 36.8 ± 9.4% and toluene 50.7 ± 11.0%), and by the genetic deletion of TRPA1 ((% inhibition: xylene 66.6 ± 16.7% and toluene 75 ± 0%). In addition, the topical application of xylene and toluene to the mouse paw elicited nociceptive responses, which were significantly reduced by oral treatment with HC-030031 ((% of inhibition: 84.9 ± 1.3% and 27.1 ± 8.0%, respectively); nociceptive responses were almost completely abolished in Trpa1^-/-mice. Our data suggest that the activation of TRPA1 could be involved in some of the symptoms of irritant-mediated contact dermatitis, such as oedema, pain and neurogenic inflammation.

Is TRPA1 Burning Down TRPV1 as Druggable Target for the Treatment of Chronic Pain?

Over the last decades, a great array of molecular mediators have been identified as potential targets for the treatment of chronic pain. Among these mediators, transient receptor potential (TRP) channel superfamily members have been thoroughly studied. Namely, the nonselective cationic channel, transient receptor potential ankyrin subtype 1 (TRPA1), has been described as a chemical nocisensor involved in noxious cold and mechanical sensation and as rivalling TRPV1, which traditionally has been considered as the most important TRP channel involved in nociceptive transduction. However, few TRPA1-related drugs have succeeded in clinical trials. In the present review, we attempt to discuss the latest data on the topic and future directions for pharmacological intervention.

Transient receptor potential ankyrin 1 (TRPA1) positively regulates imiquimod-induced, psoriasiform dermal inflammation in mice

Transient receptor potential ankyrin 1 (TRPA1), a membrane protein ion channel, is known to mediate itch and pain in skin. The function of TRPA1, however, in psoriasiform dermatitis (PsD) is uncertain. Herein, we found that expression of TRPA1 is highly up-regulated in human psoriatic lesional skin. To study the role of TRPA1 in PsD, we assessed Psoriasis Severity Index (PSI) scores, transepidermal water loss (TEWL), skin thickness and pathology, and examined dermal inflammatory infiltrates, Th17-related genes and itch-related genes in c57BL/6 as wild-type (WT) and TRPA1 gene knockout (KO) mice following daily application of topical IMQ cream for 5 days. Compared with WT mice, clinical scores, skin thickness change and TEWL scores were similar on day 3, but were significantly decreased on day 5 in IMQ-treated TRPA1 KO mice (vs WT mice), suggesting reduced inflammation and skin barrier defects. Additionally, the relative area of epidermal Munro's microabscesses and mRNA levels of neutrophil inducible chemokines (S100A8, S100A9 and CXCL1) were decreased in the treated skin of TRPA1 KO mice, suggesting that neutrophil recruitment was impaired in the KO mice. Furthermore, mast cells, CD31⁺ blood vascular cells, CD45⁺ leukocytes and CD3⁺ T cells were all reduced in the treated skin of TRPA1 KO mice. Lastly, mRNA expression levels of IL-1β, IL-6, IL-23, IL-17A, IL-17F and IL-22 were decreased in TRPA1 KO mice. In summary, these results suggest a key role for TRPA1 in psoriasiform inflammation and raising its potential as a target for therapeutic intervention.

Spinal IL-33/ST2 signaling mediates chronic itch in mice through the astrocytic JAK2-STAT3 cascade

Interleukin-33 (IL-33) and its receptor ST2 contribute to spinal glial activation and chronic pain. A recent study showed that peripheral IL-33 plays a pivotal role in the pathogenesis of chronic itch induced by poison ivy. However, how IL-33/ST2 signaling in the spinal cord potentially mediates chronic itch remains elusive. Here, we determined that St2^-/- substantially reduced scratching behaviors in 2,4-dinitrofluorobenzene (DNFB)-induced allergic contact dermatitis (ACD) as well as acetone and diethylether followed by water-induced dry skin in mice. Intrathecal administration of the neutralizing anti-ST2 or anti-IL-33 antibody remarkably decreased the scratching response in DNFB-induced ACD mice. Expression of spinal IL-33 and ST2 significantly increased in ACD mice, as evidenced by increased mRNA and protein levels. Immunofluorescence and in situ hybridization demonstrated that increased expression of spinal IL-33 was predominant in oligodendrocytes and astrocytes, whereas ST2 was mainly expressed in astrocytes. Further studies showed that in ACD mice, the activation of astrocytes and increased phosphorylation of signal transducer and activator of transcription 3 (STAT3) were markedly attenuated by St2^-/- . Intrathecal injection of Janus Kinase 2 Inhibitor AG490 significantly alleviated scratching behaviors in ACD mice. rIL-33 pretreatment exacerbated gastrin-releasing peptide (GRP)-evoked scratching behaviors. This increased gastrin-releasing peptide receptor (GRPR) expression was abolished by St2^-/- . Tnf-α upregulation was suppressed by St2^-/- . Our results indicate that the spinal IL-33/ST2 signaling pathway contributes to chronic itch via astrocytic JAK2-STAT3 cascade activation, promoting TNF-α release to regulate the GRP/GRPR signaling-related itch response. Thus, these findings provide a potential therapeutic option for treating chronic pruritus.

Understanding the Burden of Atopic Dermatitis in Africa and the Middle East

Atopic dermatitis (AD) is a common inflammatory skin disease characterized by intensely pruritic lesions. The prevalence of atopic dermatitis is increasing in developing regions, including Africa and the Middle East. However, these regions are underrepresented in the dermatology literature, and a better understanding of the growing burden of atopic dermatitis in Africa and the Middle East is necessary. Herein, we summarize current knowledge on atopic dermatitis epidemiology, disease burden, and treatment options in Africa and the Middle East, highlighting the unmet needs of patients in these regions. With these needs in mind, we provide clinical recommendations for appropriate management of atopic dermatitis in Africa and the Middle East. FUNDING: Pfizer Inc. Plain language summary available for this article.

Psychophysical and vasomotor evidence for interdependency of TRPA1 and TRPV1-evoked nociceptive responses in human skin: an experimental study

The TRPA1 and TRPV1 receptors are important pharmaceutical targets for antipruritic and analgesic therapy. Obtaining further knowledge on their roles and interrelationship in humans is therefore crucial. Preclinical results are contradictory concerning coexpression and functional interdependency of TRPV1 and TRPA1, but no human evidence exists. This human experimental study investigated whether functional responses from the subpopulation of TRPA1 nociceptors could be evoked after defunctionalization of TRPV1 nociceptors by cutaneous application of high-concentration capsaicin. Two quadratic areas on each forearm were randomized to pretreatment with an 8% topical capsaicin patch or vehicle for 24 hours. Subsequently, areas were provoked by transdermal 1% topical capsaicin (TRPV1 agonist) or 10% topical allyl isothiocyanate ("AITC," a TRPA1 agonist), delivered by 12 mm Finn chambers. Evoked pain intensities were recorded during pretreatments and chemical provocations. Quantitative sensory tests were performed before and after provocations to assess changes of heat pain sensitivity. Imaging of vasomotor responses was used to assess neurogenic inflammation after the chemical provocations. In the capsaicin-pretreated areas, both the subsequent 1% capsaicin- and 10% AITC-provoked pain was inhibited by 92.9 ± 2.5% and 86.9 ± 5.0% (both: P < 0.001), respectively. The capsaicin-ablated skin areas showed significant heat hypoalgesia at baseline (P < 0.001) as well as heat antihyperalgesia, and inhibition of neurogenic inflammation evoked by both 1% capsaicin and 10% AITC provocations (both: P < 0.001). Ablation of cutaneous capsaicin-sensitive afferents caused consistent and equal inhibition of both TRPV1- and TRPA1-provoked responses assessed psychophysically and by imaging of vasomotor responses. This study suggests that TRPA1 nociceptive responses in human skin strongly depend on intact capsaicin-sensitive, TRPV1 fibers.

Identification of molecular targets for toxic action by persulfate, an industrial sulfur compound

Persulfate salts are broadly used as industrial chemicals and exposure to them causes occupational asthma, occupational rhinitis and contact dermatitis. However, the mechanisms underlying these toxic actions are not fully elucidated. Transient receptor potential (TRP) vanilloid 1 (V1), ankyrin 1 (A1) and melastatin 8 (M8) are non-selective cation channels preferentially expressing sensory neurons. These channels are known to be involved in respiratory and skin diseases. In the present study, we investigated the effects of sodium persulfate on these TRP channels. In wild-type mouse sensory neurons, persulfate evoked [Ca²⁺]_i increases that were inhibited by removal of extracellular Ca²⁺ or blockers of TRPA1 but not by those of TRPV1 and TRPM8. Persulfate failed to evoke [Ca²⁺]_i responses in neurons from TRPA1(-/-) mice, but did evoke them in neurons from TRPV1(-/-) mice. In HEK 293 cells expressing mouse TRPA1 (mTRPA1-HEK), persulfate induced [Ca²⁺]_i increases. Moreover, in HEK 293 cells expressing mouse TRPV1 (mTRPV1-HEK), a high concentration of persulfate also evoked [Ca²⁺]_i increases. Similar [Ca²⁺]_i responses were observed in HEK 293 cells expressing human TRPA1 and human TRPV1. Current responses were also elicited by persulfate in mTRPA1- and mTRPV1-HEK. Analysis using mutated channels revealed that persulfate acted on electrophilic agonist-sensitive cysteine residues of TRPA1, and it indirectly activated TRPV1 due to the external acidification, because of the disappearance of [Ca²⁺]_i responses in acid-insensitive mTRPV1 mutant. These results demonstrate that persulfate activates nociceptive TRPA1 and TRPV1 channels. It is suggested that activation of these nociceptive channels may be involved in respiratory and skin injuries caused by exposure to this industrial sulfur compound. Thus, selective TRPA1 and TRPV1 channel blockers may be effective to remedy persulfate-induced toxic actions.

Mast cell tetrahydrobiopterin contributes to itch in mice

GTP cyclohydrolase (GCH1) governs de novo synthesis of the enzyme cofactor, tetrahydrobiopterin (BH4), which is essential for biogenic amine production, bioactive lipid metabolism and redox coupling of nitric oxide synthases. Overproduction of BH4 via upregulation of GCH1 in sensory neurons is associated with nociceptive hypersensitivity in rodents, and neuron-specific GCH1 deletion normalizes nociception. The translational relevance is revealed by protective polymorphisms of GCH1 in humans, which are associated with a reduced chronic pain. Because myeloid cells constitute a major non-neuronal source of BH4 that may contribute to BH4-dependent phenotypes, we studied here the contribution of myeloid-derived BH4 to pain and itch in lysozyme M Cre-mediated GCH1 knockout (LysM-GCH1-/- ) and overexpressing mice (LysM-GCH1-HA). Unexpectedly, knockout or overexpression in myeloid cells had no effect on nociceptive behaviour, but LysM-driven GCH1 knockout reduced, and its overexpression increased the scratching response in Compound 48/80 and hydroxychloroquine-evoked itch models, which involve histamine and non-histamine dependent signalling pathways. Mechanistically, GCH1 overexpression increased BH4, nitric oxide and hydrogen peroxide, and these changes were associated with increased release of histamine and serotonin and degranulation of mast cells. LysM-driven GCH1 knockout had opposite effects, and pharmacologic inhibition of GCH1 provided even stronger itch suppression. Inversely, intradermal BH4 provoked scratching behaviour in vivo and BH4 evoked an influx of calcium in sensory neurons. Together, these loss- and gain-of-function experiments suggest that itch in mice is contributed by BH4 release plus BH4-driven mediator release from myeloid immune cells, which leads to activation of itch-responsive sensory neurons.

Transient Receptor Potential Ankyrin 1 Enhances Ovalbumin-Induced Acute Allergic Inflammation in Murine Models

BACKGROUND

Transient receptor potential ankyrin 1 (TRPA1) is an ion channel known to mediate nociception and neurogenic inflammation, and to be activated by reactive oxygen and nitrogen species (ROS and RNS) produced at the sites of inflammation. Because neurogenic inflammation as well as the release of ROS and RNS are typical features of early stages of allergic responses, we hypothesized that TRPA1 may be involved in triggering and/or amplifying allergic inflammation.

OBJECTIVE

This study aims at exploring the role of TRPA1 ion channel in acute ovalbumin-induced allergic inflammation in applicable murine models.

METHODS

The effects of pharmacological blockade and genetic deletion of TRPA1 in ovalbumin-induced allergic conjunctivitis and acute paw inflammation were studied in mice sensitized to ovalbumin.

RESULTS

Ovalbumin-induced allergic conjunctivitis was milder in TRPA1-deficient mice and alleviated in wild-type mice treated with the TRPA1 antagonist TCS 5861528. Subcutaneous challenge with ovalbumin caused a significant paw edema and interleukin (IL)-4 production in sensitized mice; these responses were attenuated in animals treated with the TRPA1 antagonist and in TRPA1-deficient mice. Interestingly, blockade of the major secondary effector of TRPA1, substance P, also resulted in attenuated ovalbumin-induced paw edema and IL-4 production. However, the splenocytes' responses to ovalbumin were similar in cells from wild-type and TRPA1-deficient mice sensitized to ovalbumin.

CONCLUSION

These results introduce a novel concept that TRPA1 mediates early events in allergic inflammation, but does not seem to affect allergic sensitization, and could therefore be a novel drug target to treat conditions associated with allergic inflammation.

Mast cell-neural interactions contribute to pain and itch

Mast cells are best recognized for their role in allergy and anaphylaxis, but increasing evidence supports their role in neurogenic inflammation leading to pain and itch. Mast cells act as a "power house" by releasing algogenic and pruritogenic mediators, which initiate a reciprocal communication with specific nociceptors on sensory nerve fibers. Consequently, nerve fibers release inflammatory and vasoactive neuropeptides, which in turn activate mast cells in a feedback mechanism, thus promoting a vicious cycle of mast cell and nociceptor activation leading to neurogenic inflammation and pain/pruritus. Mechanisms underlying mast cell differentiation, activation, and intercellular interactions with inflammatory, vascular, and neural systems are deeply influenced by their microenvironment, imparting enormous heterogeneity and complexity in understanding their contribution to pain and pruritus. Neurogenic inflammation is central to both pain and pruritus, but specific mediators released by mast cells to promote this process may vary depending upon their location, stimuli, underlying pathology, gender, and species. Therefore, in this review, we present the contribution of mast cells in pathological conditions, including distressing pruritus exacerbated by psychologic stress and experienced by the majority of patients with psoriasis and atopic dermatitis and in different pain syndromes due to mastocytosis, sickle cell disease, and cancer.

TRPA1: a molecular view

The transient receptor potential ankyrin 1 (TRPA1) ion channel is expressed in pain-sensing neurons and other tissues and has become a major target in the development of novel pharmaceuticals. A remarkable feature of the channel is its long list of activators, many of which we are exposed to in daily life. Many of these agonists induce pain and inflammation, making TRPA1 a major target for anti-inflammatory and analgesic therapies. Studies in human patients and in experimental animals have confirmed an important role for TRPA1 in a number of pain conditions. Over the recent years, much progress has been made in elucidating the molecular structure of TRPA1 and in discovering binding sites and modulatory sites of the channel. Because the list of published mutations and important molecular sites is steadily growing and because it has become difficult to see the forest for the trees, this review aims at summarizing the current knowledge about TRPA1, with a special focus on the molecular structure and the known binding or gating sites of the channel.

Itching, chloroquine, and malaria: a review of recent molecular and neuroscience advances and their contribution to mechanistic understanding and therapeutics of chronic non-histaminergic pruritus

Chloroquine (CQ) is an antimalarial drug that elicits severe pruritus in black Africans with malaria fever. This acute itching (2-7 days duration) exhibits age dependency and a racial and genetic predilection. CQ itch is non-histaminergic, which makes it both a good model and a tool to probe the mechanisms of chronic itch. This review focuses on recently discovered mechanisms, neuroscience, mediators, and receptors that are implicated in molecular studies of CQ pruritus. CQ pruritus mechanisms are also compared to that of itching following other systemic diseases, such as chronic kidney disease, chronic liver disease, skin disorders, and burns. There are striking similarities between CQ itching pathways and other chronic itch secondary to systemic disease with or without skin lesions, which have not been previously highlighted. Prominent among these are the shared roles of skin, neural and spinal μ opiate receptors, kappa opiate receptor, nitric oxide, serotonin via 5HT1B/D receptors, cytokines, especially interleukins, and tumor necrosis factor. There is elaborate "cross talk" among the diverse mediators and receptors involved in CQ-induced pruritus. CQ also binds to the mas-related G protein coupled receptors MrgprA3/MrgprX1 present in a small proportion (4-5%) of dorsal root ganglion neurons and skin. The mrgprA3 CQ receptors are coupled to PLC-β3 and a chloride channel to initiate skin itch action potentials in C nerve fibers. Mrgpra3/X1 couples to TRPA1 for calcium influx into neuronal cells at noncutaneous sites. Central CQ itch occurs via gastrin-related peptide (GRP) and its receptor (GRPR) in the dorsal spinothalamic tracts, as well as glutamic mediated GRP projection to parabrachial nucleus. The possibility of chronic itch therapy based on personalized medicine, genetics, and transcriptomics or the use of itch "polypill/polycream" are discussed.

TRP Channels as Drug Targets to Relieve Itch

Although acute itch has a protective role by removing irritants to avoid further damage, chronic itch is debilitating, significantly impacting quality of life. Over the past two decades, a considerable amount of stimulating research has been carried out to delineate mechanisms of itch at the molecular, cellular, and circuit levels. There is growing evidence that transient receptor potential (TRP) channels play important roles in itch signaling. The purpose of this review is to summarize our current knowledge about the role of TRP channels in the generation of itch under both physiological and pathological conditions, thereby identifying them as potential drug targets for effective anti-itch therapies.

Itching and its related factors in subtypes of eczema: a cross-sectional multicenter study in tertiary hospitals of China

Itching is a leading symptom of eczema or dermatitis and has a great impact on patients' lives. Previous studies on itching have focused mostly on atopic dermatitis (AD). A cross-sectional multicenter study was conducted among outpatients with eczema from 39 tertiary hospitals in mainland China from July 1 to September 30, 2014. This work elaborates on itching in different types of eczema. Itching was very common (97%, 8499/8758) in outpatients with eczema. The severity of the itch increased with age and disease duration (P < 0.001). The top three subtypes of dermatitis with severe itching were atopic dermatitis (30.4%), widespread eczema (30.1%), and asteatotic eczema (27.9%). Widespread eczema refers to the involvement of more than three body parts, without clinical features of other specific types of eczema. The proportion of outpatients without itching was highest in hand eczema (6.8%). Positive correlations were observed between the severity of itching and the proportions of different diseases based on trend tests, including atopic dermatitis (P < 0.001), widespread eczema (P < 0.001), asteatotic eczema (P < 0.001), and autosensitization dermatitis (P < 0.001). Eczema outpatients with older age, longer disease duration, and, especially, a history of allergic diseases might be more prone to itching.

Atopic dermatitis

Atopic dermatitis (AD) is the most common chronic inflammatory skin disease, with a lifetime prevalence of up to 20% and substantial effects on quality of life. AD is characterized by intense itch, recurrent eczematous lesions and a fluctuating course. AD has a strong heritability component and is closely related to and commonly co-occurs with other atopic diseases (such as asthma and allergic rhinitis). Several pathophysiological mechanisms contribute to AD aetiology and clinical manifestations. Impairment of epidermal barrier function, for example, owing to deficiency in the structural protein filaggrin, can promote inflammation and T cell infiltration. The immune response in AD is skewed towards T helper 2 cell-mediated pathways and can in turn favour epidermal barrier disruption. Other contributing factors to AD onset include dysbiosis of the skin microbiota (in particular overgrowth of Staphylococcus aureus), systemic immune responses (including immunoglobulin E (IgE)-mediated sensitization) and neuroinflammation, which is involved in itch. Current treatments for AD include topical moisturizers and anti-inflammatory agents (such as corticosteroids, calcineurin inhibitors and cAMP-specific 3',5'-cyclic phosphodiesterase 4 (PDE4) inhibitors), phototherapy and systemic immunosuppressants. Translational research has fostered the development of targeted small molecules and biologic therapies, especially for moderate-to-severe disease.