The ion channel TRPA1 is required for chronic itch

Artemisia argyi volatile oil ameliorates allergic contact dermatitis via modulating TRPA1/CGRP signaling

ETHNOPHARMACOLOGICAL RELEVANCE

The leaves of Artemisia argyi Levl.et Vant. have a long history of being used to treat skin diseases such as pruritus and dermatitis in China, but the therapeutic effect on allergic contact dermatitis (ACD) is still unclear.

AIM OF THE STUDY

To investigate the effect and molecular mechanisms of the volatile oil of A. argyi leaves (abbreviated as 'AO') in the treatment of ACD.

MATERIALS AND METHODS

The main components in AO were analyzed using GC-MS. The effect of AO on channel currents in hTRPA1-transfected HEK293T cells was studied by whole-cell patch clamp. Subsequently, chloroquine-evoked acute itch and squaraine dibutyl ester (SADBE)-induced ACD chronic itch model was established to evaluate the antipruritic effect through counting scratching behavior, and the anti-inflammatory effects on ACD mice were measured using histological analysis. Meanwhile, the changes of CGRP, the infiltration of nerve fibers and the recruitment of dendritic cells, the expression of Il-23 and Il-17 mRNA in skin lesions, the phosphorylation of ERK and p38 in dorsal root ganglion (DRG), were evaluated by molecular biological methods. Then the inhibitory effect of AO on AITC- or SADBE-activated TRPA1 channels in primary DRG neurons of C57BL/6, Trpa1-/- or Trpv1-/- mice was elucidated by Ca2+ imaging and immunofluorescence.

RESULTS

AO treatment inhibited the activation of TRPA1 in HEK293T cells and alleviated acute itch caused by chloroquine, but this effect was lacking in Trpa1-/- mice. Furthermore, administration of AO attenuated scratching behavior in SADBE-induced ACD mice. AO also inhibited the increase of nerve fibers and recruitment of dendritic cells, and down-regulated the expression of CGRP and the levels of Il-23 and Il-17 mRNA. Meanwhile, AO reduced the expression of p-p38 and p-ERK in the lesioned skin and DRG of SADBE-induced ACD mice. Additionally, AO blocked the activation of TRPA1 channels and decreased the levels of CGRP, p-p38, and p-ERK in DRG neurons.

CONCLUSION

AO could inhibit TRPA1 channels in sensory neurons, thereby reducing the release of CGRP and exerting anti-pruritic and anti-inflammatory effect. These findings also provide a new strategy for exploring the role of A. argyi in treating ACD.

A thalamic nucleus reuniens-lateral septum-lateral hypothalamus circuit for comorbid anxiety-like behaviors in chronic itch

Chronic itch often clinically coexists with anxiety symptoms, creating a vicious cycle of itch-anxiety comorbidities that are difficult to treat. However, the neuronal circuit mechanisms underlying the comorbidity of anxiety in chronic itch remain elusive. Here, we report anxiety-like behaviors in mouse models of chronic itch and identify γ-aminobutyric acid-releasing (GABAergic) neurons in the lateral septum (LS) as the key player in chronic itch-induced anxiety. In addition, chronic itch is accompanied with enhanced activity and synaptic plasticity of excitatory projections from the thalamic nucleus reuniens (Re) onto LS GABAergic neurons. Selective chemogenetic inhibition of the Re → LS circuit notably alleviated chronic itch-induced anxiety, with no impact on anxiety induced by restraint stress. Last, GABAergic neurons in lateral hypothalamus (LH) receive monosynaptic inhibition from LS GABAergic neurons to mediate chronic itch-induced anxiety. These findings underscore the potential significance of the Re → LS → LH pathway in regulating anxiety-like comorbid symptoms associated with chronic itch.

Lysophosphatidic Acid Receptors LPAR5 and LPAR2 Inversely Control Hydroxychloroquine-Evoked Itch and Scratching in Mice

Lysophosphatidic acids (LPAs) evoke nociception and itch in mice and humans. In this study, we assessed the signaling paths. Hydroxychloroquine was injected intradermally to evoke itch in mice, which evoked an increase of LPAs in the skin and in the thalamus, suggesting that peripheral and central LPA receptors (LPARs) were involved in HCQ-evoked pruriception. To unravel the signaling paths, we assessed the localization of candidate genes and itching behavior in knockout models addressing LPAR5, LPAR2, autotaxin/ENPP2 and the lysophospholipid phosphatases, as well as the plasticity-related genes Prg1/LPPR4 and Prg2/LPPR3. LacZ reporter studies and RNAscope revealed LPAR5 in neurons of the dorsal root ganglia (DRGs) and in skin keratinocytes, LPAR2 in cortical and thalamic neurons, and Prg1 in neuronal structures of the dorsal horn, thalamus and SSC. HCQ-evoked scratching behavior was reduced in sensory neuron-specific Advillin-LPAR5-/- mice (peripheral) but increased in LPAR2-/- and Prg1-/- mice (central), and it was not affected by deficiency of glial autotaxin (GFAP-ENPP2-/-) or Prg2 (PRG2-/-). Heat and mechanical nociception were not affected by any of the genotypes. The behavior suggested that HCQ-mediated itch involves the activation of peripheral LPAR5, which was supported by reduced itch upon treatment with an LPAR5 antagonist and autotaxin inhibitor. Further, HCQ-evoked calcium fluxes were reduced in primary sensory neurons of Advillin-LPAR5-/- mice. The results suggest that LPA-mediated itch is primarily mediated via peripheral LPAR5, suggesting that a topical LPAR5 blocker might suppress "non-histaminergic" itch.

Visual analysis of global research on the transient receptor potential ankyrin 1 channel: A literature review from 2002 to 2022

Background and aims

The transient receptor potential ankyrin 1 (TRPA1) channel has become a focus in pain research. However, there are no bibliometric studies that systematically analyze the existing research in this area. This study aimed to provide a systematic review of the existing literature on TRPA1 using a bibliometric analysis.

Methods

Published literature in the field of TRPA1 was collected from the Web of Science Core Collection database. Quantitative and qualitative analyses of publications, countries, institutions, authors, journals, and other entries were conducted using Excel, VOSview, and Citespace software to provide insight into global research hotspots and trends in the TRPA1 field.

Results

This study included 1189 scientific products published in 398 journals from 52 countries. The United States of America (n = 367) had the most publications, ahead of Japan (n = 212) and China (n = 199). The University of Florence (n = 55) was the most productive institution and Pierangelo Geppetti (n = 46) was the most productive author. PLoS One (n = 40) published the most articles on TRPA1. Pain, cold, inflammation, covalent modification, hyperalgesia, and oxidative stress were the most common keywords used in the studies.

Conclusion

This study provides the first bibliometric analysis of TRPA1 publications. The physiological functions of TRPA1, TRPA1, and neuropathic pain, TRPA1 as a therapeutic target, and agonists of TRPA1 are trending in TRPA1 research. Neuropathic pain, apoptosis, and sensitization could be focus areas of future research. This study provides important insight in the field of TRPA1 research.

Current and emerging drugs for the treatment of pruritus: an update of the literature

INTRODUCTION

Pruritus, particularly in its chronic form, often imposes significant suffering and reductions in patients' quality of life. The pathophysiology of itch is varied depending on disease context, creating opportunities for unique drug development and multimodal therapy.

AREAS COVERED

The purpose of this article is to provide an update of the literature regarding current and emerging therapeutics in itch. We review the multitudes of drug targets available and corresponding drugs that have shown efficacy in clinical trials, with a particular emphasis on phase 2 and 3 trials and beyond. Broadly, these targets include therapies directed against type 2 inflammation (i.e. Th2 cytokines, JAK/STAT, lipid mediators, T-cell mediators, and other enzymes and receptors) and neural receptors and targets (i.e. PARs, TRP channels, opioid receptors, MRGPRs, GABA receptors, and cannabinoid receptors).

EXPERT OPINION

Therapeutics for itch are emerging at a remarkable pace, and we are entering an era with more and more specialized therapies. Increasingly, these treatments are able to relieve itch beyond their effect on inflammation by directly targeting the neurosensory system.

Borneol exerts its antipruritic effects by inhibiting TRPA1 and activating TRPM8

ETHNOPHARMACOLOGICAL RELEVANCE

Borneol is a long-established traditional Chinese medicine that has been found to be effective in treating pain and itchy skin. However, whether borneol has a therapeutic effect on chronic itch and its related mechanisms remain unclear.

AIM OF THE STUDY

To investigate the antipruritic effect of borneol and its molecular mechanism.

MATERIALS AND METHODS

DrugBAN framework and molecular docking were applied to predict the targets of borneol, and the calcium imaging or patch-clamp recording analysis were used to detect the effects of borneol on TRPA1, TRPM8 or TRPV3 channels in HEK293T cells. In addition, various mouse models of acute itch and chronic itch were established to evaluate the antipruritic effects of borneol on C57BL/6J mice. Then, the borneol-induced pruritic relief was further investigated in Trpa1-/-, Trpm8-/-, or Trpa1-/-/Trpm8-/- mice. The effects of borneol on the activation of TRPM8 and the inhibition of TRPA1 were also measured in dorsal root ganglia neurons of wild-type (WT), Trpm8-/- and Trpv1-/- mice. Lastly, a randomized, double-blind study of adult patients was conducted to evaluate the clinical antipruritic effect of borneol.

RESULTS

TRPA1, TRPV3 and TRPM8 are the potential targets of borneol according to the results of DrugBAN algorithm and molecular docking. Calcium imaging and patch-clamp recording analysis demonstrated that borneol activates TRPM8 channel-induced cell excitability and inhibits TRPA1 channel-mediated cell excitability in transfected HEK293T cells. Animal behavior analysis showed that borneol can significantly reduce acute and chronic itch behavior in C57BL/6J mice, but this effect was eliminated in Trpa1-/-, Trpm8-/- mice, or at least in Trpa1-/-/Trpm8-/- mice. Borneol elicits TRPM8 channel induced [Ca2+]i responses but inhibits AITC or SADBE-induced activation of TRPA1 channels in dorsal root ganglia neurons of WT and Trpv1-/- mice, respectively. Furthermore, the clinical results indicated that borneol could reduce itching symptoms in patients and its efficacy is similar to that of menthol.

CONCLUSION

Borneol has therapeutic effects on multiple pruritus models in mice and patients with chronic itch, and the mechanism may be through inhibiting TRPA1 and activating TRPM8.

Staphylococcus aureus exacerbates dermal IL-33/ILC2 axis activation through evoking RIPK3/MLKL-mediated necroptosis of dry skin

Atopic dermatitis (AD) is a persistent skin disease typified by symptoms of dry skin and recurrent eczema. Patients with AD are at heightened risk for Staphylococcus aureus infection. Group 2 innate lymphoid cells (ILC2s) are mainly activated by epithelial cell-derived cytokines IL-33 and involved in the pathogenesis of AD. However, little is known about the effect of skin delipidization on the epithelial cell-derived cytokines and dermal ILC2s in AD. In our study, we investigated the mechanism by which S. aureus infection modulates and exacerbates the pathogenesis of dry skin, leading to type 2 inflammation in the context of innate immunity. In vivo, we found that S. aureus infection aggravated delipidization-induced dermal IL-33 release and dermal ILC2 accumulation, which exacerbated skin inflammation. We also noticed that Il33fl/fl K14cre mice and Tlr2-/- mice exhibited attenuated skin inflammation. In vitro, treatment with necroptosis inhibitors reduced IL-33 release from S. aureus-infected keratinocytes. Mechanistically, we observed an increase in the necroptosis-associated kinases, MLKL and RIPK3, in S. aureus-infected mice, indicating that IL-33 release was associated with necroptotic cell death responses. Our results reveal that S. aureus infection-elicited keratinocyte necroptosis contributes to IL-33-mediated type 2 inflammation, which exacerbates the pathogenesis of dry skin.

The G protein-coupled estrogen receptor of the trigeminal ganglion regulates acute and chronic itch in mice

AIMS

Itch is an unpleasant sensation that severely impacts the patient's quality of life. Recent studies revealed that the G protein-coupled estrogen receptor (GPER) may play a crucial role in the regulation of pain and itch perception. However, the contribution of the GPER in primary sensory neurons to the regulation of itch perception remains elusive. This study aimed to investigate whether and how the GPER participates in the regulation of itch perception in the trigeminal ganglion (TG).

METHODS AND RESULTS

Immunofluorescence staining results showed that GPER-positive (GPER+ ) neurons of the TG were activated in both acute and chronic itch. Behavioral data indicated that the chemogenetic activation of GPER+ neurons of the TG of Gper-Cre mice abrogated scratching behaviors evoked by acute and chronic itch. Conversely, the chemogenetic inhibition of GPER+ neurons resulted in increased itch responses. Furthermore, the GPER expression and function were both upregulated in the TG of the dry skin-induced chronic itch mouse model. Pharmacological inhibition of GPER (or Gper deficiency) markedly increased acute and chronic itch-related scratching behaviors in mouse. Calcium imaging assays further revealed that Gper deficiency in TG neurons led to a marked increase in the calcium responses evoked by agonists of the transient receptor potential ankyrin A1 (TRPA1) and transient receptor potential vanilloid V1 (TRPV1).

CONCLUSION

Our findings demonstrated that the GPER of TG neurons is involved in the regulation of acute and chronic itch perception, by modulating the function of TRPA1 and TRPV1. This study provides new insights into peripheral itch sensory signal processing mechanisms and offers new targets for future clinical antipruritic therapy.

Targeting Transient Receptor Potential (TRP) Channels, Mas-Related G-Protein-Coupled Receptors (Mrgprs), and Protease-Activated Receptors (PARs) to Relieve Itch

Itch (pruritus) is a sensation in the skin that provokes the desire to scratch. The sensation of itch is mediated through a subclass of primary afferent sensory neurons, termed pruriceptors, which express molecular receptors that are activated by itch-evoking ligands. Also expressed in pruriceptors are several types of Transient Receptor Potential (TRP) channels. TRP channels are a diverse class of cation channels that are responsive to various somatosensory stimuli like touch, pain, itch, and temperature. In pruriceptors, TRP channels can be activated through intracellular signaling cascades initiated by pruritogen receptors and underly neuronal activation. In this review, we discuss the role of TRP channels TRPA1, TRPV1, TRPV2, TRPV3, TRPV4, TRPM8, and TRPC3/4 in acute and chronic pruritus. Since these channels often mediate itch in association with pruritogen receptors, we also discuss Mas-related G-protein-coupled receptors (Mrgprs) and protease-activated receptors (PARs). Additionally, we cover the exciting therapeutic targets amongst the TRP family, as well as Mrgprs and PARs for the treatment of pruritus.

A Novel Flp Reporter Mouse Shows That TRPA1 Expression Is Largely Limited to Sensory Neuron Subsets

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal cation channel that is activated by electrophilic irritants, oxidative stress, cold temperature, and GPCR signaling. TRPA1 expression has been primarily identified in subsets of nociceptive sensory afferents and is considered a target for future analgesics. Nevertheless, TRPA1 has been implicated in other cell types including keratinocytes, epithelium, enterochromaffin cells, endothelium, astrocytes, and CNS neurons. Here, we developed a knock-in mouse that expresses the recombinase FlpO in TRPA1-expressing cells. We crossed the TRPA1Flp mouse with the R26ai65f mouse that expresses tdTomato in a Flp-sensitive manner. We found tdTomato expression correlated well with TRPA1 mRNA expression and sensitivity to TRPA1 agonists in subsets of TRPV1 (transient receptor potential vanilloid receptor type 1)-expressing neurons in the vagal ganglia and dorsal root ganglia (DRGs), although tdTomato expression efficiency was limited in DRG. We observed tdTomato-expressing afferent fibers centrally (in the medulla and spinal cord) and peripherally in the esophagus, gut, airways, bladder, and skin. Furthermore, chemogenetic activation of TRPA1-expressing nerves in the paw evoked flinching behavior. tdTomato expression was very limited in other cell types. We found tdTomato in subepithelial cells in the gut mucosa but not in enterochromaffin cells. tdTomato was also observed in supporting cells within the cochlea, but not in hair cells. Lastly, tdTomato was occasionally observed in neurons in the somatomotor cortex and the piriform area, but not in astrocytes or vascular endothelium. Thus, this novel mouse strain may be useful for mapping and manipulating TRPA1-expressing cells and deciphering the role of TRPA1 in physiological and pathophysiological processes.

Prolonged Antipruritic Effect of Botulinum Toxin Type A on Cowhage-induced Itch: A Randomized, Single-blind, Placebo-controlled Trial

Botulinum toxin type A (Botox) is thought to have antipruritic effects through inhibition of pruritic factors, including acetylcholine, substance P, and glutamate. The aim of this randomized, single-blind, placebo-controlled trial was to test the effect of botulinum toxin type A on cowhage, a non-histaminergic model for chronic itch. Botulinum toxin type A was injected into the arm of 35 healthy subjects, with a saline control injected into the contralateral arm. Thermal sensory parameters (warmth and heat thresholds and heat pain intensity) and itch intensity after cowhage application were examined on test areas. Botulinum toxin type A reduced itch intensity, overall perceived itch (area under the curve (AUC); percentage change from baseline), and peak itch intensity compared with the control at 1 week, 1 month, and 3 months. Botulinum toxin type A had no effect on thermal thresholds or heat pain intensity. In conclusion, botulinum toxin type A reduced cowhage itch for at least 3 months, which suggests that botulinum toxin type A is a potential long-lasting treatment for localized, non-histaminergic itch.

Basic mechanisms of itch

Pruritus (or itch) is an unpleasant sensation leading to a desire to scratch. In the epidermis, there are selective C or Aδ epidermal nerve endings that are pruriceptors. At their other ends, peripheral neurons form synapses with spinal neurons and interneurons. Many areas in the central nervous system are involved in itch processing. Although itch does not occur solely because of parasitic, allergic, or immunologic diseases, it is usually the consequence of neuroimmune interactions. Histamine is involved in a minority of itchy conditions, and many other mediators play a role: cytokines (eg, IL-4, IL-13, IL-31, IL-33, and thymic stromal lymphopoietin), neurotransmitters (eg, substance P, calcitonin gene-related peptide, vasoactive intestinal peptide, neuropeptide Y, NBNP, endothelin 1, and gastrin-releasing peptide), and neurotrophins (eg, nerve growth factor and brain-derived neurotrophic factor). Moreover, ion channels such as voltage-gated sodium channels, transient receptor potential vanilloid 1, transient receptor ankyrin, and transient receptor potential cation channel subfamily M (melastatin) member 8 play a crucial role. The main markers of nonhistaminergic pruriceptors are PAR-2 and MrgprX2. A notable phenomenon is the sensitization to pruritus, in which regardless of the initial cause of pruritus, there is an increased responsiveness of peripheral and central pruriceptive neurons to their normal or subthreshold afferent input in the context of chronic itch.

Topical borneol relieves nonhistaminergic pruritus via targeting TRPA1 and TRPM8 channels in peripheral nerve terminals of mice

Borneol has been used successfully for the treatment of itchy skin in traditional Chinese medicine. However, the antipruritic effect of borneol has rarely been studied, and the mechanism is unclear. Here, we showed that topical application of borneol on skin substantially suppressed pruritogen chloroquine- and compound 48/80-induced itching in mice. The potential targets of borneol, including transient receptor potential cation channel subfamily V member 3 (TRPV3), transient receptor potential cation channel subfamily A member 1 (TRPA1), transient receptor potential cation channel subfamily M member 8 (TRPM8), and gamma-aminobutyric acid type A (GABA_A) receptor were pharmacologically inhibited or genetically knocked out one by one in mouse. Itching behavior studies demonstrated that the antipruritic effect of borneol is largely independent of TRPV3 and GABA_A receptor, and TRPA1 and TRPM8 channels are responsible for a major portion of the effect of borneol on chloroquine-induced nonhistaminergic itching. Borneol activates TRPM8 and inhibits TRPA1 in sensory neurons of mice. Topical co-application of TRPA1 antagonist and TRPM8 agonist mimicked the effect of borneol on chloroquine-induced itching. Intrathecal injection of a group II metabotropic glutamate receptor antagonist partially attenuated the effect of borneol and completely abolished the effect of TRPM8 agonist on chloroquine-induced itching, suggesting that a spinal glutamatergic mechanism is involved. In contrast, the effect of borneol on compound 48/80-induced histaminergic itching occurs through TRPA1-and TRPM8-independent mechanisms. Our work demonstrates that borneol is an effective topical itch reliever, and TRPA1 inhibition and TRPM8 activation in peripheral nerve terminals account for its antipruritic effect.

Novel proresolving lipid mediator mimetic 3-oxa-PD1n-3 docosapentaenoic acid reduces acute and chronic itch by modulating excitatory and inhibitory synaptic transmission and astroglial secretion of lipocalin-2 in mice

ABSTRACT

Specialized proresolving mediators (SPMs) have demonstrated potent analgesic actions in animal models of pathological pain. The actions of SPMs in acute and chronic itch are currently unknown. Recently, n-3 docosapentaenoic acid (DPA) was found to be a substrate for the biosynthesis of several novel families of SPMs and 3-oxa-PD1 n-3 DPA (3-oxa-PD1) is an oxidation-resistant metabolic stable analogue of the n-3 DPA-derived protectin D1 (PD1). In this article, we demonstrate that 3-oxa-PD1 effectively reduces both acute and chronic itch in mouse models. Intrathecal injection of 3-oxa-PD1 (100 ng) reduced acute itch induced by histamine, chloroquine, or morphine. Furthermore, intrathecal 3-oxa-PD1 effectively reduced chronic itch, induced by cutaneous T-cell lymphoma (CTCL), allergic contact dermatitis with dinitrofluorobenzene, and psoriasis by imiquimod. Intratumoral injection of 3-oxa-PD1 also suppressed CTCL-induced chronic itch. Strikingly, the antipruritic effect lasted for several weeks after 1-week intrathecal 3-oxa-PD1 treatment. Whole-cell recordings revealed significant increase in excitatory postsynaptic currents in spinal dorsal horn (SDH) neurons of CTCL mice, but this increase was blocked by 3-oxa-PD1. 3-oxa-PD1 further increased inhibitory postsynaptic currents in SDH neurons of CTCL mice. Cutaneous T-cell lymphoma increased the spinal levels of lipocalin-2 (LCN2), an itch mediator produced by astrocytes. 3-oxa-PD1 suppressed LCN2 production in CTCL mice and LCN2 secretion in astrocytes. Finally, CTCL-induced anxiety was alleviated by intrathecal 3-oxa-PD1. Our findings suggest that 3-oxa-PD1 potently inhibits acute and chronic itch through the regulation of excitatory or inhibitory synaptic transmission and astroglial LCN2 production. Therefore, stable SPM analogs such as 3-oxa-PD1 could be useful to treat pruritus associated with different skin injuries.

Butyrate inhibits Staphylococcus aureus-aggravated dermal IL-33 expression and skin inflammation through histone deacetylase inhibition

Atopic dermatitis (AD) is an inflammatory skin disease caused by the disruption of skin barrier, and is dominated by the type 2 immune responses. Patients with AD have a high risk of developing Staphylococcus aureus infection. Interleukin-33 (IL-33), an alarmin, has been implicated in the pathophysiology of AD development. Butyrate, a short chain fatty acid known to be produced from the fermentation of glycerol by the commensal skin bacterium, Staphylococcus epidermidis, has been reported to possess antimicrobial and anti-inflammatory properties that suppress inflammatory dermatoses. However, little is known about the effects of butyrate on dermal IL-33 expression and associated immune response in S. aureus-aggravated skin inflammation in the context of AD. To decipher the underlying mechanism, we established an AD-like mouse model with epidermal barrier disruption by delipidizing the dorsal skin to induce AD-like pathophysiology, followed by the epicutaneous application of S. aureus and butyrate. We discovered that S. aureus infection exacerbated IL-33 release from keratinocytes and aggravated dermal leukocyte infiltration and IL-13 expression. Moreover, we showed that butyrate could attenuate S. aureus-aggravated skin inflammation with decreased IL-33, IL-13, and leukocyte infiltration in the skin. Mechanistically, we demonstrated that butyrate suppressed IL-33 expression and ameliorated skin inflammation through histone deacetylase 3 (HDAC3) inhibition. Overall, our findings revealed the potential positive effect of butyrate in controlling inflammatory skin conditions in AD aggravated by S. aureus infection.

Nerve growth factor mediates activation of transient receptor potential vanilloid 1 in neurogenic pruritus of psoriasis

Pruritus is a common and painful symptom in psoriasis with profoundly negative impacts on quality of life. The underlying mechanisms of pruritus are complex and multifactorial, and accumulating evidence suggests that pruritus induced by neurogenic inflammation predominates in psoriasis. Nerve growth factor (NGF) -mediated transient receptor potential vanilloid receptor 1(TRPV1) pathway has emerged as a crucial node in the regulation of neurogenic pruritus. TRPV1 appears coupled to most pruritus-specific molecules via the neuro-immune axis. While the modes of regulation differ for each axis, TRPV1 is involved in substantial biochemical crosstalk-causing feedback loops with significant effects on neurogenic pruritus. Therefore, TRPV1 has emerged as a target molecular in drug development for pruritus in psoriasis. However, no significant clinical progress occurred in the development of systemic TRPV1 antagonists due to elevated core temperature. Thus, topical application of TRPV1 antagonists and interference with mediators linked to the TRPV1 activation pathway may be promising therapeutic options to ameliorate pruritus. This Review focuses on recent advances in complicated regulation of NGF-mediated TRPV1 pathway in psoriatic neurogenic pruritus, as well as the therapeutic options that arise from the dissection of the pathway.

MrgprA3-expressing pruriceptors drive pruritogen-induced alloknesis through mechanosensitive Piezo2 channel

Although touch and itch are coded by distinct neuronal populations, light touch also provokes itch in the presence of exogenous pruritogens, resulting in a phenomenon called alloknesis. However, the cellular and molecular mechanisms underlying the initiation of pruritogen-induced mechanical itch sensitization are poorly understood. Here, we show that intradermal injections of histamine or chloroquine (CQ) provoke alloknesis through activation of TRPV1- and MrgprA3-expressing prurioceptors, and functional ablation of these neurons reverses pruritogen-induced alloknesis. Moreover, genetic ablation of mechanosensitive Piezo2 channel function from MrgprA3-expressing prurioceptors also dampens pruritogen-induced alloknesis. Mechanistically, histamine and CQ sensitize Piezo2 channel function, at least in part, through activation of the phospholipase C (PLC) and protein kinase C-δ (PKCδ) signaling. Collectively, our data find a TRPV1+/MrgprA3+ prurioceptor-Piezo2 signaling axis in the initiation of pruritogen-induced mechanical itch sensitization in the skin.

Sensitization of colonic nociceptors by IL-13 is dependent on JAK and p38 MAPK activity

The effective management of visceral pain is a significant unmet clinical need for those affected by gastrointestinal diseases, such as inflammatory bowel disease (IBD). The rational design of novel analgesics requires a greater understanding of the mediators and mechanisms underpinning visceral pain. Interleukin-13 (IL-13) production by immune cells residing in the gut is elevated in IBD, and IL-13 appears to be important in the development of experimental colitis. Furthermore, receptors for IL-13 are expressed by neurons innervating the colon, though it is not known whether IL-13 plays any role in visceral nociception per se. To resolve this, we used Ca²⁺ imaging of cultured sensory neurons and ex vivo electrophysiological recording from the lumbar splanchnic nerve innervating the distal colon. Ca²⁺ imaging revealed the stimulation of small-diameter, capsaicin-sensitive sensory neurons by IL-13, indicating that IL-13 likely stimulates nociceptors. IL-13-evoked Ca²⁺ signals were attenuated by inhibition of Janus (JAK) and p38 kinases. In the lumbar splanchnic nerve, IL-13 did not elevate baseline firing, nor sensitize the response to capsaicin application, but did enhance the response to distention of the colon. In line with Ca²⁺ imaging experiments, IL-13-mediated sensitization of the afferent response to colon distention was blocked by inhibition of either JAK or p38 kinase signaling. Together, these data highlight a potential role for IL-13 in visceral nociception and implicate JAK and p38 kinases in pronociceptive signaling downstream of IL-13.

Rg3-enriched Korean red ginseng alleviates chloroquine-induced itch and dry skin pruritus in an MrgprA3-dependent manner in mice

Background

Previous studies have found that Korean red ginseng extract (KRG) has antipruritic effects, which can be attributed to the presence of Rg3, one of the most potent ginsenosides. Therefore, Rg3-enriched KRG extract (Rg3EKRG) is anticipated to have enhanced antipruritic effects. The present study was conducted to examine the effects of Rg3EKRG in acute chloroquine (CQ)-induced and chronic dry skin pruritus.

Methods

Calcium imaging technique was used in HE293T cells expressing MrgprA3 and TRPA1 ("MrgprA3/TRPA1") and in primary cultures of mouse dorsal root ganglia (DRG) neurons. Mouse scratching behavior tests were performed on dry skin models. To verify the altered expression of itch-related genes, real-time RNA sequencing analysis and PCR were performed on DRG sections obtained from dry skin models.

Results

Rg3EKRG suppressed CQ-induced intracellular calcium changes to a greater degree than KRG. Rg3EKRG dose-dependently inhibited CQ-induced responses in MrgprA3/TRPA1 cells. Rg3EKRG likely targeted MrgprA3 rather than TRPA1 to exert its inhibitory effect. Further, Rg3EKRG strongly inhibited the scratching behavior in mice induced by acute CQ injection. Importantly, DRG neurons obtained from dry skin mice models showed increased mRNA levels of MrgprA3, and treatment with Rg3EKRG alleviated chronic dry skin conditions and suppressed spontaneous scratching behaviors.

Conclusion

The results of the present study imply that Rg3EKRG has a stronger antipruritic effect than KRG, inhibiting both acute CQ-induced and chronic dry skin pruritus in an MrgprA3-dependent manner. Therefore, Rg3EKRG is a potential antipruritic agent that can suppress acute and chronic itching at the peripheral sensory neuronal level.

CYP1B1-derived epoxides modulate the TRPA1 channel in chronic pain

Pain is often debilitating, and current treatments are neither universally efficacious nor without risks. Transient receptor potential (TRP) ion channels offer alternative targets for pain relief, but little is known about the regulation or identities of endogenous TRP ligands that affect inflammation and pain. Here, transcriptomic and targeted lipidomic analysis of damaged tissue from the mouse spinal nerve ligation (SNL)-induced chronic pain model revealed a time-dependent increase in Cyp1b1 mRNA and a concurrent accumulation of 8,9-epoxyeicosatrienoic acid (EET) and 19,20-EpDPA post injury. Production of 8,9-EET and 19,20-EpDPA by human/mouse CYP1B1 was confirmed in vitro, and 8,9-EET and 19,20-EpDPA selectively and dose-dependently sensitized and activated TRPA1 in overexpressing HEK-293 cells and Trpa1-expressing/AITC-responsive cultured mouse peptidergic dorsal root ganglia (DRG) neurons. TRPA1 activation by 8,9-EET and 19,20-EpDPA was attenuated by the antagonist A967079, and mouse TRPA1 was more responsive to 8,9-EET and 19,20-EpDPA than human TRPA1. This latter effect mapped to residues Y933, G939, and S921 of TRPA1. Intra-plantar injection of 19,20-EpDPA induced acute mechanical, but not thermal hypersensitivity in mice, which was also blocked by A967079. Similarly, Cyp1b1-knockout mice displayed a reduced chronic pain phenotype following SNL injury. These data suggest that manipulation of the CYP1B1-oxylipin-TRPA1 axis might have therapeutic benefit.

Sensory neuron-expressed TRPC3 mediates acute and chronic itch

ABSTRACT

Chronic pruritus is a prominent symptom of allergic contact dermatitis (ACD) and represents a huge unmet health problem. However, its underlying cellular and molecular mechanisms remain largely unexplored. TRPC3 is highly expressed in primary sensory neurons and has been implicated in peripheral sensitization induced by proinflammatory mediators. Yet, the role of TRPC3 in acute and chronic itch is still not well defined. Here, we show that, among mouse trigeminal ganglion (TG) neurons, Trpc3 mRNA is predominantly expressed in nonpeptidergic small diameter TG neurons of mice. Moreover, Trpc3 mRNA signal was present in most presumptively itch sensing neurons. TRPC3 agonism induced TG neuronal activation and acute nonhistaminergic itch-like and pain-like behaviors in naive mice. In addition, genetic deletion of Trpc3 attenuated acute itch evoked by certain common nonhistaminergic pruritogens, including endothelin-1 and SLIGRL-NH2. In a murine model of contact hypersensitivity (CHS), the Trpc3 mRNA expression level and function were upregulated in the TG after CHS. Pharmacological inhibition and global knockout of Trpc3 significantly alleviated spontaneous scratching behaviors without affecting concurrent cutaneous inflammation in the CHS model. Furthermore, conditional deletion of Trpc3 in primary sensory neurons but not in keratinocytes produced similar antipruritic effects in this model. These findings suggest that TRPC3 expressed in primary sensory neurons may contribute to acute and chronic itch through a histamine independent mechanism and that targeting neuronal TRPC3 might benefit the treatment of chronic itch associated with ACD and other inflammatory skin disorders.

miRNA-203b-3p Induces Acute and Chronic Pruritus through 5-HTR2B and TRPV4

Growing evidence indicates that transient receptor potential (TRP) channels contribute to different forms of pruritus. However, the endogenous mediators that cause itch through transient receptor potential channels signaling are poorly understood. In this study, we show that genetic deletion or pharmacological antagonism of TRPV4 attenuated itch in a mouse model of psoriasis induced by topical application of imiquimod. Human psoriatic lesions showed increased expression of several microRNAs, including the miR-203b-3p, which induced a calcium ion response in rodent dorsal root ganglion neurons and scratching behavior in mice through 5-HTR2B activation and the protein kinase C‒dependent phosphorylation of TRPV4. Computer simulation revealed that the miR-203b-3p core sequence (GUUAAGAA) that causes 5-HTR2B/TRPV4-dependent itch targets the extracellular side of 5-HTR2B by interacting with a portion of the receptor pocket consistent with its activation. Overall, we reveal the unconventional pathophysiological role of an extracellular microRNA that can behave as an itch promoter through 5-HTR2B and TRPV4.

Diisocyanates influence models of atopic dermatitis through direct activation of TRPA1

We recently used EPA databases to identify that isocyanates, most notably toluene diisocyanate (TDI), were the pollutant class with the strongest spatiotemporal and epidemiologic association with atopic dermatitis (AD). Our findings demonstrated that isocyanates like TDI disrupted lipid homeostasis and modeled benefit in commensal bacteria like Roseomonas mucosa through disrupting nitrogen fixation. However, TDI has also been established to activate transient receptor potential ankyrin 1 (TRPA1) in mice and thus could directly contribute to AD through induction of itch, rash, and psychological stress. Using cell culture and mouse models, we now demonstrate that TDI induced skin inflammation in mice as well as calcium influx in human neurons; each of these findings were dependent on TRPA1. Furthermore, TRPA1 blockade synergized with R. mucosa treatment in mice to improve TDI-independent models of AD. Finally, we show that the cellular effects of TRPA1 are related to shifting the balance of the tyrosine metabolites epinephrine and dopamine. This work provides added insight into the potential role, and therapeutic potential, or TRPA1 in the pathogenesis of AD.

Upregulation of DRG protein TMEM100 facilitates dry-skin-induced pruritus by enhancing TRPA1 channel function

The dry skin tortures numerous patients with severe itch. The transient receptor potential cation channel V member 1 (TRPV1) and A member 1 (TRPA1) are two essential receptors for peripheral neural coding of itch sensory, mediating histaminergic and nonhistaminergic itch separately. In the dorsal root ganglion, transmembrane protein 100 (TMEM100) is structurally related to both TRPV1 and TRPA1 receptors, but the exact role of TMEM100 in itch sensory coding is still unknown. Here, in this study, we find that TMEM100 ⁺ DRG neurons account for the majority of activated neurons in an acetone-ether-water (AEW)-induced dry skin itch model, and some TMEM100 ⁺ DRG neurons are colocalized with both TRPA1 and the chloroquine-related Mrgpr itch receptor family. Both the expression and function of TRPA1 channels, but not TRPV1 channels, are upregulated in the AEW model, and specific DRG Tmem100 gene knockdown alleviates AEW-induced itch and rescues the expression and functional changes of TRPA1. Our results strongly suggest that TMEM100 protein in DRG is the main facilitating factor for dry-skin-related chronic itch, and specific suppression of TMEM100 in DRG could be a novel effective treatment strategy for patients who suffer from dry skin-induced itch.

An Anterior Cingulate Cortex-to-Midbrain Projection Controls Chronic Itch in Mice

Itch is an unpleasant sensation that provokes the desire to scratch. While acute itch serves as a protective system to warn the body of external irritating agents, chronic itch is a debilitating but poorly-treated clinical disease leading to repetitive scratching and skin lesions. However, the neural mechanisms underlying the pathophysiology of chronic itch remain mysterious. Here, we identified a cell type-dependent role of the anterior cingulate cortex (ACC) in controlling chronic itch-related excessive scratching behaviors in mice. Moreover, we delineated a neural circuit originating from excitatory neurons of the ACC to the ventral tegmental area (VTA) that was critically involved in chronic itch. Furthermore, we demonstrate that the ACC→VTA circuit also selectively modulated histaminergic acute itch. Finally, the ACC neurons were shown to predominantly innervate the non-dopaminergic neurons of the VTA. Taken together, our findings uncover a cortex–midbrain circuit for chronic itch-evoked scratching behaviors and shed novel insights on therapeutic intervention.

The role of TRPA1 channels in thermosensation

Transient receptor potential ankyrin 1 (TRPA1) is a polymodal nonselective cation channel sensitive to different physical and chemical stimuli. TRPA1 is associated with many important physiological functions in different species and thus is involved in different degrees of evolution. TRPA1 acts as a polymodal receptor for the perceiving of irritating chemicals, cold, heat, and mechanical sensations in various animal species. Numerous studies have supported many functions of TRPA1, but its temperature-sensing function remains controversial. Although TRPA1 is widely distributed in both invertebrates and vertebrates, and plays a crucial role in tempreture sensing, the role of TRPA1 thermosensation and molecular temperature sensitivity are species-specific. In this review, we summarize the temperature-sensing role of TRPA1 orthologues in terms of molecular, cellular, and behavioural levels.

Isosorbide Fatty Acid Diesters Have Synergistic Anti-Inflammatory Effects in Cytokine-Induced Tissue Culture Models of Atopic Dermatitis

Atopic dermatitis (AD) is a chronic disease in which epidermal barrier disruption triggers Th2-mediated eruption of eczematous lesions. Topical emollients are a cornerstone of chronic management. This study evaluated efficacy of two plant-derived oil derivatives, isosorbide di-(linoleate/oleate) (IDL) and isosorbide dicaprylate (IDC), using AD-like tissue culture models. Treatment of reconstituted human epidermis with cytokine cocktail (IL-4 + IL-13 + TNF-α + IL-31) compromised the epidermal barrier, but this was prevented by co-treatment with IDL and IDC. Cytokine stimulation also dysregulated expression of keratinocyte (KC) differentiation genes whereas treatment with IDC or IDL + IDC up-regulated genes associated with early (but not late) KC differentiation. Although neither IDL nor IDC inhibited Th2 cytokine responses, both compounds repressed TNF-α-induced genes and IDL + IDC led to synergistic down-regulation of inflammatory (IL1B, ITGA5) and neurogenic pruritus (TRPA1) mediators. Treatment of cytokine-stimulated skin explants with IDC decreased lactate dehydrogenase (LDH) secretion by more than 50% (more than observed with cyclosporine) and in vitro LDH activity was inhibited by IDL and IDC. These results demonstrate anti-inflammatory mechanisms of isosorbide fatty acid diesters in AD-like skin models. Our findings highlight the multifunctional potential of plant oil derivatives as topical ingredients and support studies of IDL and IDC as therapeutic candidates.

In vitro models for investigating itch

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

Miswiring of Merkel cell and pruriceptive C fiber drives the itch-scratch cycle

Itch sensation provokes the scratch reflex to protect us from harmful stimuli in the skin. Although scratching transiently relieves acute itch through activation of mechanoreceptors, it propagates the vicious itch-scratch cycle in chronic itch by further aggravating itch over time. Although well recognized clinically, the peripheral mechanisms underlying the itch-scratch cycle remain poorly understood. Here, we show that mechanical stimulation of the skin results in activation of the Piezo2 channels on Merkel cells that pathologically promotes spontaneous itch in experimental dry skin. Three-dimensional reconstruction and immunoelectron microscopy revealed structural alteration of MRGPRA3⁺ pruriceptor nerve endings directed toward Merkel cells in the setting of dry skin. Our results uncover a functional miswiring mechanism under pathologic conditions, resulting in touch receptors triggering the firing of pruriceptors in the skin to drive the itch-scratch cycle.

Distribution of Cannabinoid Receptors in Keratinocytes of Healthy Dogs and Dogs With Atopic Dermatitis

It is commonly accepted that some form of skin barrier dysfunction is present in canine atopic dermatitis (AD), one of the most common cutaneous pruritic inflammatory diseases of dogs. The impaired skin barrier function facilitates the penetration of allergens and subsequently stronger sensitization responses. The role of the endocannabinoid system (ECS) in the physiology and pathology of the skin is becoming increasingly established. It has been demonstrated that cannabinoid receptors are expressed in healthy and diseased skin and, based on current knowledge, it could be stated that cannabinoids are important mediators in the skin. The present study has been designed to immunohistochemically investigate the expression of the cannabinoid receptors type 1 (CB1R) and 2 (CB2R) and the cannabinoid-related receptors G protein-coupled receptor 55 (GPR55), transient receptor potential vanilloid 1 (TRPV1) and ankyrin 1 (TRPA1), peroxisome proliferator-activated receptors alpha (PPARα), and serotoninergic receptor 1a (5-HT1aR) in keratinocytes of healthy dogs and of dogs with AD. Samples of skin tissues were collected from 7 healthy controls (CTRL-dogs) and from 8 dogs with AD (AD-dogs). The tissue samples were processed using an immunofluorescence assay with commercially available antibodies, and the immunolabelling of the receptors studied was quantitatively evaluated. The keratinocytes of the CTRL- and the AD-dogs showed immunoreactivity for all the receptors investigated with a significant upregulation of CB2R, TRPA1, and 5-HT1aR in the epidermis of the AD-dogs. The presence of cannabinoid and cannabinoid-related receptors in healthy keratinocytes suggested the possible role of the ECS in canine epidermal homeostasis while their overexpression in the inflamed tissues of the AD-dogs suggested the involvement of the ECS in the pathogenesis of this disease, having a possible role in the related skin inflammation and itching. Based on the present findings, the ECS could be considered a potential therapeutic target for dogs with AD.

PIEZO1 transduces mechanical itch in mice

Itch triggers scratching, a behavioural defence mechanism that aids in the removal of harmful irritants and parasites¹. Chemical itch is triggered by many endogenous and exogenous cues, such as pro-inflammatory histamine, which is released during an allergic reaction¹. Mechanical itch can be triggered by light sensations such as wool fibres or a crawling insect². In contrast to chemical itch pathways, which have been extensively studied, the mechanisms that underlie the transduction of mechanical itch are largely unknown. Here we show that the mechanically activated ion channel PIEZO1 (ref. ³) is selectively expressed by itch-specific sensory neurons and is required for their mechanically activated currents. Loss of PIEZO1 function in peripheral neurons greatly reduces mechanically evoked scratching behaviours and both acute and chronic itch-evoked sensitization. Finally, mice expressing a gain-of-function Piezo1 allele⁴ exhibit enhanced mechanical itch behaviours. Our studies reveal the polymodal nature of itch sensory neurons and identify a role for PIEZO1 in the sensation of itch.

Experiments in mice show that the mechanically activated ion channel PIEZO1 is expressed in itch-specific sensory neurons and has a role in transducing mechanical itch.

Are TRPA1 and TRPV1 channel-mediated signalling cascades involved in UVB radiation-induced sunburn?

Burn injuries are underappreciated injuries associated with substantial morbidity and mortality. Overexposure to ultraviolet (UV) radiation has dramatic clinical effects in humans and is a significant public health concern. Although the mechanisms underlying UVB exposure are not fully understood, many studies have made substantial progress in the pathophysiology of sunburn in terms of its molecular aspects in the last few years. It is well established that the transient receptor potential ankyrin 1 (TRPA1), and vanilloid 1 (TRPV1) channels modulate the inflammatory, oxidative, and proliferative processes underlying UVB radiation exposure. However, it is still unknown which mechanisms underlying TRPV1/A1 channel activation are elicited in sunburn induced by UVB radiation. Therefore, in this review, we give an overview of the TRPV1/A1 channel-mediated signalling cascades that may be involved in the pathophysiology of sunburn induced by UVB radiation. These data will undoubtedly help to explain the various features of sunburn and contribute to the development of novel therapeutic approaches to better treat it.

Neuro-immune communication regulating pruritus in atopic dermatitis

Atopic dermatitis (AD) is a common, chronic-relapsing inflammatory skin disease with significant disease burden. Genetic and environmental trigger factors contribute to AD, activating two of our largest organs, the nervous and immune system. Dysregulation of neuro-immune circuits plays a key role in the pathophysiology of AD causing inflammation, pruritus, pain, and barrier dysfunction. Sensory nerves can be activated by environmental or endogenous trigger factors transmitting itch stimuli to the brain. Upon stimulation, sensory nerve endings also release neuromediators into the skin contributing again to inflammation, barrier dysfunction and itch. Additionally, dysfunctional peripheral and central neuronal structures contribute to neuroinflammation, sensitization, nerve elongation, neuropathic itch, thus chronification and therapy-resistance. Consequently, neuro-immune circuits in skin and central nervous system may be targets to treat pruritus in AD. Cytokines, chemokines, proteases, lipids, opioids, ions excite/sensitize sensory nerve endings not only induce itch but further aggravate/perpetuate inflammation, skin barrier disruption, and pruritus. Thus, targeted therapies for neuro-immune circuits as well as pathway inhibitors (e.g., kinase inhibitors) may be beneficial to control pruritus in AD either in systemic and/or topical form. Understanding neuro-immune circuits and neuronal signaling will optimize our approach to control all pathological mechanisms in AD, inflammation, barrier dysfunction and pruritus.

Mitochondrial Reactive Oxygen Species Elicit Acute and Chronic Itch via Transient Receptor Potential Canonical 3 Activation in Mice

Mitochondrial reactive oxygen species (mROS) that are overproduced by mitochondrial dysfunction are linked to pathological conditions including sensory abnormalities. Here, we explored whether mROS overproduction induces itch through transient receptor potential canonical 3 (TRPC3), which is sensitive to ROS. Intradermal injection of antimycin A (AA), a selective inhibitor of mitochondrial electron transport chain complex III for mROS overproduction, produced robust scratching behavior in naïve mice, which was suppressed by MitoTEMPO, a mitochondria-selective ROS scavenger, and Pyr10, a TRPC3-specific blocker, but not by blockers of TRPA1 or TRPV1. AA activated subsets of trigeminal ganglion neurons and also induced inward currents, which were blocked by MitoTEMPO and Pyr10. Besides, dry skin-induced chronic scratching was relieved by MitoTEMPO and Pyr10, and also by resveratrol, an antioxidant. Taken together, our results suggest that mROS elicit itch through TRPC3, which may underlie chronic itch, representing a potential therapeutic target for chronic itch.

Histamine Sensitization of the Voltage-Gated Sodium Channel Nav1.7 Contributes to Histaminergic Itch in Mice

Itch, a common clinical symptom of many skin diseases, severely impairs the life quality of patients. Nav1.7, a subtype of voltage-gated sodium channels mainly expressed in primary sensory neurons, is responsible for the amplification of threshold currents that trigger action potential (AP) generation. Gain-of-function mutation of Nav1.7 leads to paroxysmal itch, while pharmacological inhibition of Nav1.7 alleviates histamine-dependent itch. However, the crosstalk between histamine and Nav1.7 that leads to itch is unclear. In the present study, we demonstrated that in the dorsal root ganglion (DRG) neurons from histamine-dependent itch model mice induced by compound 48/80, tetrodotoxin-sensitive (TTX-S) but not TTX-resistant Na⁺ currents were activated at more hyperpolarized membrane potentials compared to those on DRG neurons from vehicle-treated mice. Meanwhile, bath application of histamine shifted the activation voltages of TTX-S Na⁺ currents to the hyperpolarized direction, increased the AP frequency, and reduced the current threshold required to elicit APs. Further mechanistic studies demonstrated that selective activation of H1 but not H2 and H4 receptors mimicked histamine effect on TTX-S Na⁺ channels in DRG neurons. The protein kinase C (PKC) inhibitor GO 8963, but not the PKA inhibitor H89, normalized histamine-sensitized TTX-S Na⁺ channels. We also demonstrated that histamine shifted the activation voltages of Na⁺ currents to the hyperpolarized direction in Chinese hamster ovary (CHO) cells expressing Nav1.7. Importantly, selective inhibition of Nav1.7 by PF-05089771 significantly relieved the scratching frequency in a histamine-dependent itch model induced by compound 48/80. Taken together, these data suggest that activation of H1 receptors by histamine sensitizes Nav1.7 channels through the PKC pathway in DRG neurons that contributes to histamine-dependent itch.

Roles of Epithelial and Mesenchymal TRP Channels in Mediating Inflammatory Fibrosis

The maintenance of normal vision is dependent on preserving corneal transparency. For this to occur, this tissue must remain avascular and its stromal architecture needs to be retained. Epithelial transparency is maintained provided the uppermost stratified layers of this tissue are composed of terminally differentiated non-keratinizing cells. In addition, it is essential that the underlying stromal connective tissue remains avascular and scar-free. Keratocytes are the source of fibroblasts that are interspersed within the collagenous framework and the extracellular matrix. In addition, there are sensory nerve fibers whose lineage is possibly either neural crest or mesenchymal. Corneal wound healing studies have been undertaken to delineate the underlying pathogenic responses that result in the development of opacification following chemical injury. An alkali burn is one type of injury that can result in severe and long- lasting losses in ocular transparency. During the subsequent wound healing process, numerous different proinflammatory cytokines and proteolytic enzymes undergo upregulation. Such increases in their expression levels induce maladaptive expression of sustained stromal inflammatory fibrosis, neovascularization, and losses in the smooth optical properties of the corneal outer surface. It is becoming apparent that different transient receptor potential channel (TRP) isoforms are important players in mediating these different events underlying the wound healing process since injury upregulates both their expression levels and functional involvement. In this review, we focus on the involvement of TRPV1, TRPA1 and TRPV4 in mediating some of the responses that underlie the control of anterior ocular tissue homeostasis under normal and pathological conditions. They are expressed on both different cell types throughout this tissue and also on corneal sensory nerve endings. Their roles have been extensively studied as sensors and transducers of environmental stimuli resulting from exposure to intrinsic modulators and extrinsic ligands. These triggers include alteration of the ambient temperature and mechanical stress, etc., that can induce pathophysiological responses underlying losses in tissue transparency activated by wound healing in mice losses in tissue transparency. In this article, experimental findings are reviewed about the role of injury-induced TRP channel activation in mediating inflammatory fibrotic responses during wound healing in mice.

Exploring neuronal mechanisms involved in the scratching behavior of a mouse model of allergic contact dermatitis by transcriptomics

Background

Allergic contact dermatitis (ACD) is a common skin condition characterized by contact hypersensitivity to allergens, accompanied with skin inflammation and a mixed itch and pain sensation. The itch and pain dramatically affects patients’ quality of life. However, still little is known about the mechanisms triggering pain and itch sensations in ACD.

Methods

We established a mouse model of ACD by sensitization and repetitive challenge with the hapten oxazolone. Skin pathological analysis, transcriptome RNA sequencing (RNA-seq), qPCR, Ca²⁺ imaging, immunostaining, and behavioral assay were used for identifying gene expression changes in dorsal root ganglion innervating the inflamed skin of ACD model mice and for further functional validations.

Results

The model mice developed typical ACD symptoms, including skin dryness, erythema, excoriation, edema, epidermal hyperplasia, inflammatory cell infiltration, and scratching behavior, accompanied with development of eczematous lesions. Transcriptome RNA-seq revealed a number of differentially expressed genes (DEGs), including 1436-DEG mRNAs and 374-DEG-long noncoding RNAs (lncRNAs). We identified a number of DEGs specifically related to sensory neuron signal transduction, pain, itch, and neuroinflammation. Comparison of our dataset with another published dataset of atopic dermatitis mouse model identified a core set of genes in peripheral sensory neurons that are exclusively affected by local skin inflammation. We further found that the expression of the pain and itch receptor MrgprD was functionally upregulated in dorsal root ganglia (DRG) neurons innervating the inflamed skin of ACD model mice. MrgprD activation induced by its agonist β-alanine resulted in exaggerated scratching responses in ACD model mice compared with naïve mice.

Conclusions

We identified the molecular changes and cellular pathways in peripheral sensory ganglia during ACD that might participate in neurogenic inflammation, pain, and itch. We further revealed that the pain and itch receptor MrgprD is functionally upregulated in DRG neurons, which might contribute to peripheral pain and itch sensitization during ACD. Thus, targeting MrgprD may be an effective method for alleviating itch and pain in ACD.

Supplementary Information

The online version contains supplementary material available at 10.1186/s11658-022-00316-w.

Paeonol Ameliorates Chronic Itch and Spinal Astrocytic Activation via CXCR3 in an Experimental Dry Skin Model in Mice

Paeonol is a bioactive phenol presents mainly in Paeonia suffruticosa Andr. (Paeoniaceae), Paeonia lactiflora Pall., and Dioscorea japonica Thunb. (Dioscoreaceae), harboring various pharmacological activities including anti-inflammatory, antioxidant, immune regulatory activity and reverse chemoresistance. Recent reports revealed paeonol exhibited good effects on chronic dermatitis, such as atopic dermatitis (AD) and psoriasis. However, whether paeonol is effective for dry skin disease and its mechanism of action still remain unclear. In this study, we analysed the effects of paeonol on a mouse model of dry skin treated with acetone-ether-water (AEW), which showed impressive activities in reducing scratching behavior and skin inflammation. To elucidate the underlying molecular targets for the anti-pruritic ability of paeonol, we screened the expression of possible chemokine pathways in the spinal cord. The expression of CXCR3 was significantly alleviated by paeonol, which increased greatly in the spinal neurons of AEW mice. In addition, treatment of paeonol significantly inhibited AEW-induced expression of astrocyte activity-dependent genes including Tlr4, Lcn2 and Hspb1 et al. The inhibitory effects of paeonol on scratching behavior and astrocytic activation in the spinal cord induced by AEW were abolished when CXCR3 was antagonized or genetically ablated. Taken together, our results indicated that paeonol can ameliorate AEW-induced inflammatory response and itching behavior, and reduce the expression of spinal astrocyte activity-dependent genes induced by AEW, which are driven by CXCR3.

The Role of Transient Receptor Potential A1 and G Protein-Coupled Receptor 39 in Zinc-Mediated Acute and Chronic Itch in Mice

Itching is a common symptom of many skin or systemic diseases and has a negative impact on the quality of life. Zinc, one of the most important trace elements in an organism, plays an important role in the regulation of pain. Whether and how zinc regulates itching is largely unclear. Herein, we explored the role of Zn²⁺ in the regulation of acute and chronic itch in mice. It is found that intradermal injection (i.d.) of Zn²⁺ dose-dependently induced acute itch and transient receptor potential A1 (TRPA1) participated in Zn²⁺-induced acute itch in mice. Moreover, the pharmacological analysis showed the involvement of histamine, mast cells, opioid receptors, and capsaicin-sensitive C-fibers in Zn²⁺-induced acute itch in mice. Systemic administration of Zn²⁺ chelators, such as N,N,N′,N′-Tetrakis(2-pyridylmethyl)ethylenediamine (TPEN), pyrithione, and clioquinol were able to attenuate both acute itch and dry skin-induced chronic itch in mice. Quantitative polymerase chain reaction (Q-PCR) analysis showed that the messenger RNA (mRNA) expression levels of zinc transporters (ZIPs and ZnTs) significantly changed in the dorsal root ganglia (DRG) under dry skin-induced chronic itch condition in mice. Activation of extracellular signal-regulated kinase (ERK) pathway was induced in the DRG and skin by the administration of zinc or under dry skin condition, which was inhibited by systemic administration of Zn²⁺ chelators. Finally, we found that the expression of GPR39 (a zinc-sensing GPCR) was significantly upregulated in the dry skin mice model and involved in the pathogenesis of chronic itch. Together, these results indicated that the TRPA1/GPR39/ERK axis mediated the zinc-induced itch and, thus, targeting zinc signaling may be a promising strategy for anti-itch therapy.

Selective activation of TRPA1 ion channels by nitrobenzene skin sensitizers DNFB and DNCB

2, 4-dinitrofluorobenzene (DNFB) and 2, 4-dinitrochlorobenzene (DNCB) are well known as skin sensitizers that can cause dermatitis. DNFB has shown to more potently sensitize skin; however, how DNFB and DNCB cause skin inflammation at a molecular level and why this difference in their sensitization ability is observed remains unknown. In this study, we aimed to identify the molecular targets and mechanisms on which DNFB and DNCB act. We used a fluorescent calcium imaging plate reader in an initial screening assay before patch-clamp recordings for validation. Molecular docking in combination with site-directed mutagenesis was then carried out to investigate DNFB and DNCB binding sites in the TRPA1 ion channel that may be selectively activated by these tow sensitizers. We found that DNFB and DNCB selectively activated TRPA1 channel with EC₅₀ values of 2.3 ± 0.7 μM μM and 42.4 ± 20.9 μM, respectively. Single-channel recordings revealed that DNFB and DNCB increase the probability of channel opening and acts on three residues (C621, E625 and Y658) critical for TRPA1 activation. Our findings may not only help explain the molecular mechanism underlying the dermatitis and pruritus caused by chemicals like DNFB and DNCB, but also provide a molecular tool 7.5-fold more potent than the current TRPA1 activator allyl isothiocyanate (AITC) used for investigating TRPA1 channel pharmacology and pathology.

Proton Sensing on the Ocular Surface: Implications in Eye Pain

Protons reaching the eyeball from exogenous acidic substances or released from damaged cells during inflammation, immune cells, after tissue injury or during chronic ophthalmic conditions, activate or modulate ion channels present in sensory nerve fibers that innervate the ocular anterior surface. Their identification as well as their role during disease is critical for the understanding of sensory ocular pathophysiology. They are likely to mediate some of the discomfort sensations accompanying several ophthalmic formulations and may represent novel targets for the development of new therapeutics for ocular pathologies. Among the ion channels expressed in trigeminal nociceptors innervating the anterior surface of the eye (cornea and conjunctiva) and annex ocular structures (eyelids), members of the TRP and ASIC families play a critical role in ocular acidic pain. Low pH (pH 6) activates TRPV1, a polymodal ion channel also activated by heat, capsaicin and hyperosmolar conditions. ASIC1, ASIC3 and heteromeric ASIC1/ASIC3 channels present in ocular nerve terminals are activated at pH 7.2–6.5, inducing pain by moderate acidifications of the ocular surface. These channels, together with TRPA1, are involved in acute ocular pain, as well as in painful sensations during allergic keratoconjunctivitis or other ophthalmic conditions, as blocking or reducing channel expression ameliorates ocular pain. TRPV1, TRPA1 and other ion channels are also present in corneal and conjunctival cells, promoting inflammation of the ocular surface after injury. In addition to the above-mentioned ion channels, members of the K_2P and P2X ion channel families are also expressed in trigeminal neurons, however, their role in ocular pain remains unclear to date. In this report, these and other ion channels and receptors involved in acid sensing during ocular pathologies and pain are reviewed.

Cutaneous Neuroimmune Interactions of TSLP and TRPV4 Play Pivotal Roles in Dry Skin-Induced Pruritus

Dry skin is a symptom of skin barrier dysfunction that evokes pruritus; however, the cutaneous neuroimmune interactions underlying dry skin-induced pruritus remain unclear. Therefore, we aimed to elucidate the mechanisms underlying dry skin-induced pruritus. To this end, an acetone/ethanol/water (AEW)-induced mouse model of dry skin was used in this study. We observed that the production of thymic stromal lymphopoietin (TSLP) significantly increased in the keratinocytes of AEW mice. Importantly, treatment with an antagonist of transient receptor potential cation channel subfamily V member 4 (TRPV4), HC067047, ameliorated dry skin conditions in AEW mice. The symptoms of dry skin were significantly reduced in Trpv4 knockout (KO) mice following treatment with AEW. The increase in the intracellular calcium levels by TSLP in the dorsal root ganglia (DRG) of Trpv4 KO mice was also significantly attenuated. The spontaneous scratching bouts were significantly decreased in both the HC067047-treated and Trpv4 KO AEW mice. Importantly, the TSLP-dependent release of tryptase from the mast cells was significantly reduced in both the HC067047-treated mice and Trpv4 KO AEW mice. Notably, inhibition of the TSLP-induced signaling pathway in DRG selectively reduced the spontaneous scratching bouts in AEW mice. Overall, the results demonstrated that the cutaneous neuroimmune interactions of TSLP and TRPV4 play pivotal roles in dry skin-induced pruritus.

WAO-ARIA consensus on chronic cough - Part 1: Role of TRP channels in neurogenic inflammation of cough neuronal pathways

BACKGROUND

Cough features a complex peripheral and central neuronal network. The function of the chemosensitive and stretch (afferent) cough receptors is well described but partly understood. It is speculated that chronic cough reflects a neurogenic inflammation of the cough reflex, which becomes hypersensitive. This is mediated by neuromediators, cytokines, inflammatory cells, and a differential expression of neuronal (chemo/stretch) receptors, such as transient receptor potential (TRP) and purinergic P2X ion channels; yet the overall interaction of these mediators in neurogenic inflammation of cough pathways remains unclear.

OBJECTIVES

The World Allergy Organization/Allergic Rhinitis and its Impact on Asthma (WAO/ARIA) Joint Committee on Chronic Cough reviewed the current literature on neuroanatomy and pathophysiology of chronic cough. The role of TRP ion channels in pathogenic mechanisms of the hypersensitive cough reflex was also examined.

OUTCOMES

Chemoreceptors are better studied in cough neuronal pathways compared to stretch receptors, likely due to their anatomical overabundance in the respiratory tract, but also their distinctive functional properties. Central pathways are important in suppressive mechanisms and behavioral/affective aspects of chronic cough. Current evidence strongly suggests neurogenic inflammation induces a hypersensitive cough reflex marked by increased expression of neuromediators, mast cells, and eosinophils, among others. TRP ion channels, mainly TRP V1/A1, are important in the pathogenesis of chronic cough due to their role in mediating chemosensitivity to various endogenous and exogenous triggers, as well as a crosstalk between neurogenic and inflammatory pathways in cough-associated airways diseases.

Neuron‒Mast Cell Cross-Talk in the Skin

Skin-resident mast cells (MCs) and cutaneous sensory neurons both play crucial roles in microbial‒host defense and inflammatory diseases. MCs can be directly activated by pathogens or their products, resulting in the release of numerous mediators that promote innate immune responses and also activate sensory neurons. Cutaneous sensory neurons can also directly detect the presence of pathogens, resulting in the release of neuropeptides that modulate MC function. In this review, we will focus on the reciprocal interactions between cutaneous sensory neurons and MCs and the importance of this cross-talk in skin diseases.

Dissecting the precise nature of itch-evoked scratching

Itch is a discrete and irritating sensation tightly coupled to a drive to scratch. Acute scratching developed evolutionarily as an adaptive defense against skin irritants, pathogens, or parasites. In contrast, the itch-scratch cycle in chronic itch is harmful, inducing escalating itch and skin damage. Clinically and preclinically, scratching incidence is currently evaluated as a unidimensional motor parameter and believed to reflect itch severity. We propose that scratching, when appreciated as a complex, multidimensional motor behavior, will yield greater insight into the nature of itch and the organization of neural circuits driving repetitive motor patterns. We outline the limitations of standard measurements of scratching in rodent models and present new approaches to observe and quantify itch-evoked scratching. We argue that accurate quantitative measurements of scratching are critical for dissecting the molecular, cellular, and circuit mechanisms underlying itch and for preclinical development of therapeutic interventions for acute and chronic itch disorders.

IL-33 signaling in sensory neurons promotes dry skin itch

BACKGROUND

Chronic pruritus, or itch, is common and debilitating, but the neuroimmune mechanisms that drive chronic itch are only starting to be elucidated. Recent studies demonstrate that the IL-33 receptor (IL-33R) is expressed by sensory neurons. However, whether sensory neuron-restricted activity of IL-33 is necessary for chronic itch remains poorly understood.

OBJECTIVES

We sought to determine if IL-33 signaling in sensory neurons is critical for the development of chronic itch in 2 divergent pruritic disease models.

METHODS

Plasma levels of IL-33 were assessed in patients with atopic dermatitis (AD) and chronic pruritus of unknown origin (CPUO). Mice were generated to conditionally delete IL-33R from sensory neurons. The contribution of neuronal IL-33R signaling to chronic itch development was tested in mouse models that recapitulate key pathologic features of AD and CPUO, respectively.

RESULTS

IL-33 was elevated in both AD and CPUO as well as their respective mouse models. While neuron-restricted IL-33R signaling was dispensable for itch in AD-like disease, it was required for the development of dry skin itch in a mouse model that mirrors key aspects of CPUO pathology.

CONCLUSIONS

These data highlight how IL-33 may be a predominant mediator of itch in certain contexts, depending on the tissue microenvironment. Further, this study provides insight into future therapeutic strategies targeting the IL-33 pathway for chronic itch.

Chronic itch: emerging treatments following new research concepts

Until recently, itch pathophysiology was poorly understood and treatments were poorly effective in relieving itch. Current progress in our knowledge of the itch processing, the numerous mediators and receptors involved has led to a large variety of possible therapeutic pathways. Currently, inhibitors of IL-31, IL-4/13, NK₁ receptors, opioids and cannabinoids, JAK, PDE4 or TRP are the main compounds involved in clinical trials. However, many new targets, such as Mas-related GPCRs and unexpected new pathways need to be also explored.

Cinnamaldehyde elicits itch behavior via TRPV1 and TRPV4 but not TRPA1

Introduction:

Cinnamaldehyde (CA) elicits itch sensation in humans. We investigated if CA elicits scratching behavior in mice and determined the roles for TRPV1, TRPA1, and TRPV4.

Materials and Methods:

Scratching behavior elicited by intradermal injection of CA was assessed in wildtype (WT) mice and knockout (KO) mice lacking TRPV1, TRPA1, TRPV4, or deficient in mast cells. We also assessed scratching and wet dog shakes elicited by low-threshold mechanical stimulation of skin treated topically with CA or vehicle. Using calcium imaging we tested if CA activates dorsal root ganglion (DRG) neurons of each genotype.

Results:

Intradermal cheek injection of CA elicited dose-dependent hindlimb scratch bouts, with fewer forelimb wipes and facial groom bouts that were not dose-dependent. CA elicited significantly fewer scratch bouts in TRPV1 and TRPV4 KO mice, but not TRPA1KOs, compared with WTs. There were no sex differences across genotypes. The histamine H1 antagonist cetirizine did not affect CA-evoked scratching, which was normal in mast cell deficient mice, indicating lack of histamine involvement. Scores for alloknesis were significantly greater following topical application of CA compared with vehicle. Post-CA alloknesis scores were significantly higher in TRPV4KOs of both sexes and in female TRPV1 and TRPA1KOs, compared with WTs. Low threshold mechanical stimuli also elicited significantly more wet dog shakes in mice treated topically with 20% CA, with significantly fewer in TRPV1, TRPA1, and TRPV4KOs compared with WTs. In calcium imaging studies, CA excited 24% of WT DRG cells, significantly fewer (11.5%) in cells from TRPV4KOs, and none in TRPA1KOs. Responses of cells of all genotypes exhibited significant sensitization to repeated CA stimulation. Sensitization was significantly enhanced by IL-4, which itself excited 16% of WT DRG cells and none from TRPA1KOs.

Discussion:

The results indicate that TRPA1 is dispensable for CA-evoked scratching, which depends partly on TRPV1 and TRPV4.