Transient Receptor Potential Vanilloid 4 Ion Channel Functions as a Pruriceptor in Epidermal Keratinocytes to Evoke Histaminergic Itch

Neuronal Mechanisms of Psoriatic Itch: Role of IL-17R/ERK/TRPV4 Signaling Pathway

Itch represents a major disease burden of psoriasis. Despite recent clinical studies showing the effectiveness of IL-17- and IL-17R-blocking antibodies in alleviating psoriatic itch, significant questions remain unanswered. Specifically, the crucial cellular site of action and the impacted signaling pathway of IL-17/IL-17R in psoriatic itch are elusive. Itch sensation relies on dorsal root ganglion (DRG) sensory neurons that transmit pruriceptive signals from the periphery to the CNS. IL-17RA and IL-17RC, 2 cognate receptors for IL-17, are expressed in DRG neurons. In this study, we demonstrated that IL-17RA and IL-17RC are upregulated in DRG neurons in a mouse model of psoriasis induced by imiquimod. Notably, conditional knockout of Il17ra or Il17rc in sensory neurons potently attenuated psoriasis-like itch. Furthermore, our in vitro assay with cultured neurons and in vivo experiment with animal model of psoriasis demonstrated that IL-17RA and IL-17RC upregulate the pruritic ion channel TRPV4 in DRG neurons through the extracellular signal-regulated kinase (ERK) signaling pathway. Specific deletion of Trpv4 or suppression of phosphorylation of ERK in DRG neurons mitigated psoriasis-like itch. These findings suggest that the IL-17R/ERK/TRPV4 signaling pathway in sensory neurons plays a significant role in psoriatic itch.

Keratinocytes: new perspectives in inflammatory skin diseases

Keratinocytes, the predominant cell type in the epidermis, are indispensable for maintaining skin barrier integrity, mediating host defense, and orchestrating immune responses. Beyond these well-established functions, emerging evidence reveals their dynamic interactions with the nervous system and their capacity to retain inflammatory memory. These discoveries position keratinocytes as key drivers of the onset, progression, and relapse of inflammatory skin diseases. In this review, we delve into the mechanisms underlying keratinocyte crosstalk with immune and neural cells, the metabolic reprogramming, including lactate and other metabolites, that may drive inflammatory memory, and the broader implications for disease pathogenesis and recurrence. Finally, we discuss the challenges to, and therapeutic potential of, targeting keratinocytes for the treatment of chronic inflammatory skin conditions.

S1P/S1PRs-TRPV4 axis is a novel therapeutic target for persistent pain and itch in chronic dermatitis

BACKGROUND AND PURPOSE

While pain and itch are both commonly associated with chronic dermatitis (CD), the molecular mechanisms underlying these debilitating symptoms is not well understood. This study aims to identify novel, endogenous compounds that mediate CD-associated pain and itch.

EXPERIMENTAL APPROACH

Lesional skin of CD model mice was examined using unbiased metabolomic analysis to identify candidate pain or itch inducing compounds in CD. Sphingosine-1-phosphate (S1P) concentration in CD model skin was analysed using UPLC/MS/MS. Behaviour, calcium imaging and immunofluorescence staining were used to determine the pain and itch effects and mechanisms of the identified CD-related compounds.

KEY RESULTS

In the lesional skin of CD model mice, 136 compounds were significantly changed. These compounds are predominately associated with the sphingolipids metabolism pathway. S1P is significantly increased in the lesional skin . The TRPV4 channel was critical for S1P induced itch and pain. Sphingosine kinase 2 (SPHK2), the key enzyme controlling S1P synthesis, was significantly increased in lesional skin. ABC294640, a SPHK2 inhibitor, significantly decreased S1P concentration in lesional CD model skin, as well as in model associated epidermal hyperplasia and chronic pain and itch. In CD patients, SPHK2 expression and S1P concentration were significantly elevated compared to healthy control skin.

CONCLUSION AND IMPLICATIONS

Our results indicate that, in CD, increased S1P induces chronic pain and itch partly through TRPV4. Inhibition of S1P synthesis or the S1P/S1P receptor-TRPV4 pathway are promising treatment strategies for CD-associated pain and itch.

The dual modulating effects of neuropeptide FF on morphine-induced analgesia at the spinal level

Increasing evidence indicates that neuropeptide FF (NPFF) produces analgesic effects and augments opioid-induced analgesia at the spinal level. However, our recent research demonstrated that NPFF exerted complex opioid-modulating effects in an inflammatory pain model after intrathecal (i.t.) injection. Consistent with previous findings, we found that i.t. NPFF dose-dependently attenuated complete Freund's adjuvant-induced pain hypersensitivity. Interestingly, pharmacological results illustrated that NPFF exhibited opposite opioid-modulating effects at the spinal level depending on its administration dosage, wherein i.t. NPFF potentiated morphine-induced anti-allodynia at the dose of 10 nmol, while attenuated morphine analgesia at an ultra-low-dose of 10 pmol. Behavioral results obtained from neuropeptide FF receptor 2 (NPFFR2) knockout animals suggested that both pro- and anti-opioid effects of NPFF were mediated by NPFFR2. Moreover, these modulating effects of spinal NPFFR2 were selectively targeting mu-opioid receptor, had no effect on delta- and kappa-opioid receptor agonist-induced analgesia. Finally, the opioid-modulating effects of NPFF were further verified using in vitro calcium imaging assay, demonstrating that pretreated with NPFF in primary-cultured spinal neurons significantly attenuated the inhibitory effects of morphine on high-K+-induced neuronal excitability. Taken together, our results suggested that NPFF exhibited dual modulating effects on morphine-induced analgesia after i.t. administration, which provides a possible mechanism to explain the complex opioid-modulating effects of endogenous NPFF systems.

Cough and itch: Common mechanisms of irritation in the throat and skin

Cough and itch are protective mechanisms in the body. Cough occurs as a reflex motor response to foreign body inhalation, while itch is a sensation that similarly evokes a scratch response to remove irritants from the skin. Both cough and itch can last for sustained periods, leading to debilitating chronic disorders that negatively impact quality of life. Understanding the parallels and differences between chronic cough and chronic itch may be paramount to developing novel therapeutic approaches. In this article, we identify connections in the mechanisms contributing to the complex cough and scratch reflexes and summarize potential shared therapeutic targets. An online search was performed using various search engines, including PubMed, Web of Science, Google Scholar, and ClinicalTrials.gov from 1983 to 2024. Articles were assessed for quality, and those relevant to the objective were analyzed and summarized. The literature demonstrated similarities in the triggers, peripheral and central nervous system processing, feedback mechanisms, immunologic mediators, and receptors involved in the cough and itch responses, with the neuronal sensitization processes exhibiting the greatest parallels between cough and itch. Given the substantial impact on quality of life, novel therapies targeting similar neuroimmune pathways may apply to both itch and cough and provide new avenues for enhancing their management.

TRPV4-β-catenin axis is a novel therapeutic target for dry skin-induced chronic itch

Dry skin induced chronic pruritus is an increasingly common and debilitating problem, especially in the elderly. Although keratinocytes play important roles in innate and adaptive immunity and keratinocyte proliferation is a key feature of dry skin induced chronic pruritus, the exact contribution of keratinocytes to the pathogenesis of dry skin induced chronic pruritus is poorly understood. In this study, we generated the acetone-ether-water induced dry skin model in mice and found that epidermal hyperplasia induced by this model is partly dependent on the β-catenin signaling pathway. XAV939, an antagonist of β-catenin signaling pathway, inhibited epidermal hyperplasia in dry skin model mice. Importantly, dry skin induced chronic pruritus also dramatically reduced in XAV939 treated mice. Moreover, acetone-ether-water treatment-induced epidermal hyperplasia and chronic itch were decreased in Trpv4-/- mice. In vitro, XAV939 inhibited hypo-osmotic stress induced proliferation of HaCaT cells, and hypo-osmotic stress induced proliferation of in HaCaT cells and primary cultured keratinocytes were also significantly reduced by blocking TRPV4 function. Finally, thymic stromal lymphopoietin release was examined both in vivo and in vitro, which was significantly inhibited by XAV939 treatment and Trpv4 deficiency, and anti-TSLP antibody treatment significantly decreased AEW-induced scratching behavior. Overall, our study revealed a unique ability of TRPV4 expressing keratinocytes in the skin, which critically mediated dry skin induced epidermal hyperplasia and chronic pruritus, thus provided novel insights into the development of therapies for chronic pruritus in the elderly.

The Influence of Retinol Ointment on Rabbit Skin (Oryctolagus cuniculus) Ion Transport-An In Vitro Study

Retinoids are known to improve the condition of the skin. Transepithelial transport of sodium and chloride ions is important for proper skin function. So far, the effect of applying vitamin A preparations to the skin on ion transport has not been evaluated. In the study, electrophysiological parameters, including transepithelial electric potential (PD) and transepithelial resistance (R), of rabbit skin specimens after 24 h exposure to retinol ointment (800 mass units/g) were measured in a modified Ussing chamber. The R of the fragments incubated with retinol was significantly different than that of the control skin samples incubated in iso-osmotic Ringer solution. For the controls, the PD values were negative, whereas the retinol-treated specimens revealed positive PD values. Mechanical-chemical stimulation with the use of inhibitors of the transport of sodium (amiloride) or chloride (bumetanide) ions revealed specific changes in the maximal and minimal PD values measured for the retinol-treated samples. Retinol was shown to slightly modify the transport pathways of sodium and chloride ions. In particular, an intensification of the chloride ion secretion from keratinocytes was observed. The proposed action may contribute to deep hydration and increase skin tightness, limiting the action of other substances on its surface.

LPS exacerbates TRPV4-mediated itch through the intracellular TLR4-PI3K signalling

Pruritus is often accompanied with bacterial infections, but the underlying mechanism is not fully understood. Although previous studies revealed that lipopolysaccharides (LPS) could directly activate TRPV4 channel and TRPV4 is involved in the generation of both acute itch and chronic itch, whether and how LPS affects TRPV4-mediated itch sensation remains unclear. Here, we showed that LPS-mediated TRPV4 sensitization exacerbated GSK101-induced scratching behaviour in mice. Moreover, this effect was compromised in TLR4-knockout mice, suggesting LPS acted through a TLR4-dependent mechanism. Mechanistically, LPS enhanced GSK101-evoked calcium influx in mouse ear skin cells and HEK293T cells transfected with TRPV4. Further, LPS sensitized TRPV4 channel through the intracellular TLR4-PI3K-AKT signalling. In summary, our study found a modulatory role of LPS in TRPV4 function and highlighted the TLR4-TRPV4 interaction in itch signal amplification.

The involvement of keratinocytes in pruritus of chronic inflammatory dermatosis

Frequent itching and incessant scratching are commonly observed in various chronic inflammatory skin conditions, including atopic dermatitis and psoriasis. The persistent and prolonged nature of pruritus can worsen one's quality of life. Keratinocytes (KCs), the predominant cells of the epidermis, have been confirmed to interact with sensory neurons and immune cells and be involved in chronic skin inflammatory diseases associated with pruritus. Initially, KCs and sensory neurons form a unique synapse-like connection within the epidermis, serving as the structural foundation for their interaction. Additionally, several receptors, including toll-like receptors and protease-activated receptor 2, expressed on KCs, become activated in an inflammatory milieu. On the one hand, activated KCs are sources of pro-inflammatory cytokines and neurotrophic factors, such as adenosine triphosphate, thymic stromal lymphopoietin, and nerve growth factor, which directly or indirectly participate in stimulating sensory neurons, thereby contributing to the itch sensations. On the other hand, KCs also function as primary transducers alongside intraepidermal nerve endings, directly initiating pruritic responses. This review summarizes the current literature and highlights the critical role of KCs in the development and persistence of chronic itch in inflammatory skin disorders.

The blockade of neuropeptide FF receptor 1 and 2 differentially contributed to the modulating effects on fentanyl-induced analgesia and hyperalgesia in mice

Neuropeptide FF (NPFF) plays a critical role in various physiological processes through the activation of neuropeptide FF receptor 1 and 2 (NPFFR1 and NPFFR2). Numerous evidence has indicated that NPFF exhibits opposite opioid-modulating effects on opioid-induced analgesia after supraspinal and spinal administrations, while the detailed role of NPFFR1 and NPFFR2 remains unclear. In this study, we employed pharmacological and genetic inhibition of NPFFR to investigate the modulating roles of central NPFFR1 and NPFFR2 in opioid-induced analgesia and hyperalgesia, using a male mouse model of acute fentanyl-induced analgesia and secondary hyperalgesia. Our findings revealed that intrathecal (i.t.) injection of the nonselective NPFFR antagonist RF9 significantly enhanced fentanyl-induced analgesia, whereas intracerebroventricular (i.c.v.) injection did not show the same effect. Moreover, NPFFR2 deficient (npffr2-/-) mice exhibited stronger analgesic responses to fentanyl compared to wild type (WT) or NPFFR1 knockout (npffr1-/-) mice. Intrathecal injection of RF9 in npffr1-/- mice also significantly enhanced fentanyl-induced analgesia. These results indicate a crucial role of spinal NPFFR2 in the enhancement of opioid analgesia. Contrastingly, hyperalgesia induced by fentanyl was markedly reversed in npffr1-/- mice but remained unaffected in npffr2-/- mice. Similarly, i.c.v. injection of the selective NPFFR1 antagonist RF3286 effectively prevented fentanyl-induced hyperalgesia in WT or npffr2-/- mice. Notably, co-administration of i.c.v. RF3286 and i.t. RF9 augmented fentanyl-induced analgesia while reducing hyperalgesia. Collectively, these findings highlight the modulating effects of blocking spinal NPFFR2 and supraspinal NPFFR1 on fentanyl-induced analgesia and hyperalgesia, respectively, which shed a light on understanding the pharmacological function of NPFF system in future studies.

Cimifugin Relieves Histamine-Independent Itch in Atopic Dermatitis via Targeting the CQ Receptor MrgprA3

Background: We previously found that cimifugin has a potent antiallergic inflammatory effect in atopic dermatitis (AD). However, whether cimifugin has an antipruritic effect in AD was unknown. Methods: Mouse scratching behavior tests were performed to verify the proposed antipruritic effect of cimifugin on DNFB- or FITC-mediated AD. Chloroquine (CQ)- and compound 48/80-evoked acute itch models were employed to clarify the effect of cimifugin on histamine-dependent or -independent itch. Intracellular calcium changes were assessed in a primary culture of mouse dorsal root ganglia (DRG) in response to pruritogen exposure with or without cimifugin treatment, including CQ, histamine, allyl-isothiocyanate (AITC), and capsaicin. Molecular docking and microscale thermophoresis (MST) assays were performed to predict and verify the binding ability and modes between cimifugin and the CQ receptor MrgprA3, respectively. Results: We found that cimifugin attenuates itch behaviors effectively in FITC-induced AD. Notably, cimifugin significantly alleviated acute itching behaviors induced by CQ but not compound 48/80 in vivo. Moreover, cimifugin remarkably inhibited CQ-evoked calcium influx in DRG cells but had no obvious effect on histamine-induced calcium influx. Nevertheless, cimifugin did not interfere with either AITC-stimulated TRPA1 activation- or capsaicin-stimulated TRPV1 activation-mediated calcium influx in DRG cells. Molecular docking predicted that CQ and cimifugin might share similar binding abilities and binding modes with MrgprA3. MST assay confirmed cimifugin directly targeting MrgprA3. Conclusion: The present study demonstrates that cimifugin has a potent antipruritic effect in AD with a histamine-independent mechanism via targeting the CQ receptor MrgprA3. Thus, cimifugin is a promising candidate antipruritic agent for AD.

Site-Specific Transient Receptor Potential Channel Mechanisms and Their Characteristics for Targeted Chronic Itch Treatment

Chronic itch is a debilitating condition with limited treatment options, severely affecting quality of life. The identification of pruriceptors has sparked a growing interest in the therapeutic potential of TRP channels in the context of itch. In this regard, we provided a comprehensive overview of the site-specific expression of TRP channels and their associated functions in response to a range of pruritogens. Although several potent antipruritic compounds that target specific TRP channels have been developed and have demonstrated efficacy in various chronic itch conditions through experimental means, a more thorough understanding of the potential for adverse effects or interactions with other TRP channels or GPCRs is necessary to develop novel and selective therapeutics that target TRP channels for treating chronic itch. This review focuses on the mechanism of itch associated with TRP channels at specific sites, from the skin to the sensory neuron, with the aim of suggesting specific therapeutic targets for treating this condition.

Involvement of the Transient Receptor Channels in Preclinical Models of Musculoskeletal Pain

BACKGROUND

Musculoskeletal pain is a condition that affects bones, muscles, and tendons and is present in various diseases and/or clinical conditions. This type of pain represents a growing problem with enormous socioeconomic impacts, highlighting the importance of developing treatments tailored to the patient's needs. TRP is a large family of non-selective cation channels involved in pain perception. Vanilloid (TRPV1 and TRPV4), ankyrin (TRPA1), and melastatin (TRPM8) are involved in physiological functions, including nociception, mediation of neuropeptide release, heat/cold sensing, and mechanical sensation.

OBJECTIVE

In this context, we provide an updated view of the most studied preclinical models of muscle hyperalgesia and the role of transient receptor potential (TRP) in these models.

METHODS

This review describes preclinical models of muscle hyperalgesia induced by intramuscular administration of algogenic substances and/or induction of muscle damage by physical exercise in the masseter, gastrocnemius, and tibial muscles.

RESULTS

The participation of TRPV1, TRPA1, and TRPV4 in different models of musculoskeletal pain was evaluated using pharmacological and genetic tools. All the studies detected the antinociceptive effect of respective antagonists or reduced nociception in knockout mice.

CONCLUSION

Hence, TRPV1, TRPV4, and TRPA1 blockers could potentially be utilized in the future for inducing analgesia in muscle hypersensitivity pathologies.

Regulation of Neural Functions by Brain Temperature and Thermo-TRP Channels

Body temperature is an important determinant in regulating the activities of animals. In humans, a mild 0.5 °C hyperthermia can cause headaches, demonstrating that the maintenance of normal body temperature is a key for our health. In a more extreme example, accidental acute hypothermia can lead to severe shivering, loss of consciousness, or death, although the details of these mechanisms are poorly understood. We previously found that the TRPV4 ion channel is constitutively activated by normal body temperature. The activation threshold of TRPV4 is >34 °C in the brain, which enables TRPV4 to convert thermal information into cellular signaling. Here we review the data that describe how the deletion of TRPV4 evokes abnormal behavior in mice. These studies demonstrate that the maintenance of body temperature and the sensory system for detecting body temperature, such as via TRPV4, are critical components for normal cellular function. Moreover, abnormal TRPV4 activation exacerbates cell death, epilepsy, stroke, or brain edema. Notably, TRPV4 can detect mechanical stimuli and contributes to various neural functions similar to the mechanosensitive characteristics of TRPV2. In this review, I summarize the findings related to TRPV2/TRPV4 and neural functions.

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.

An update on mechanisms of pruritus and their potential treatment in primary cutaneous T-cell lymphoma

Primary cutaneous T-cell lymphomas (CTCL), which include mycosis fungoides (MF) and Sézary syndrome (SS), are a group of lymphoproliferative disorders characterized by clonal accumulation of neoplastic T-lymphocytes in the skin. Severe pruritus, one of the most common and distressing symptoms in primary CTCL, can significantly impair emotional well-being, physical functioning, and interpersonal relationships, thus greatly reducing quality of life. Unfortunately, effectively managing pruritus remains challenging in CTCL patients as the underlying mechanisms are, as of yet, not fully understood. Previous studies investigating the mechanisms of itch in CTCL have identified several mediators and their corresponding antagonists used for treatment. However, a comprehensive overview of the mediators and receptors contributing to pruritus in primary CTCL is lacking in the current literature. Here, we summarize and review the mediators and receptors that may contribute to pruritus in primary CTCL to explore the mechanisms of CTCL pruritus and identify effective therapeutic targets using the PubMed and Web of Science databases. Studies were included if they described itch mediators and receptors in MF and SS. Overall, the available data suggest that proteases (mainly tryptase), and neuropeptides (particularly Substance P) may be of greatest interest. At the receptor level, cytokine receptors, MRGPRs, and TRP channels are most likely important. Future drug development efforts should concentrate on targeting these mediators and receptors for the treatment of CTCL pruritus.

TRPV4 Regulates the Development of Psoriasis by Controlling Adenosine Triphosphate Expression in Keratinocytes and the Neuroimmune System

TRPV4 is a calcium ion channel that is widely expressed in various cells. It is also involved in physiological and pathological processes. However, the role of TRPV4 in psoriasis remains unknown. We aimed to investigate the role of TRPV4 in psoriasis using human psoriasis skin samples and an imiquimod-induced psoriasis-like mouse model. Keratinocytes in human psoriasis skin had high TRPV4 expression. Trpv4-knockout mice had less severe dermatitis than wild-type mice in the imiquimod-induced mouse model. Knockout mice had significantly reduced epidermal thickness and a low number of infiltrated CD3+ T cells and CD68+ macrophages on the basis of histopathological studies and decreased mRNA expression of Il17a, Il17f, and Il23, as detected through qPCR. Furthermore, knockout mice had a significantly low expression of neuropeptides and the neuron marker PGP9.5. Adenosine triphosphate release was significantly suppressed by TRPV4 knockdown in both human and mouse keratinocytes in vitro. Finally, treatment with TRPV4 antagonist was significantly effective in preventing the progression of psoriasis-like dermatitis. In conclusion, TRPV4 mediates the expression of keratinocyte-derived adenosine triphosphate and increases the secretion of neuropeptides, resulting in the activation and amplification of IL-23/Th17 responses. Hence, TRPV4 can serve as a novel therapeutic target in psoriasis.

Transient receptor potential vanilloid 4 promotes cutaneous wound healing by regulating keratinocytes and fibroblasts migration and collagen production in fibroblasts in a mouse model

BACKGROUND

Transient receptor potential vanilloid 4 (TRPV4), a cation ion channel, is expressed in different cells, and it regulates the development of different diseases. We recently found a high TRPV4 expression in the wounded skin area. However, the role of TRPV4 in cutaneous wound healing is unknown.

OBJECTIVE

To investigate the role of TRPV4 in cutaneous wound healing in a mouse model.

METHODS

Skin wound healing experiment and histopathological studies were performed between WT and TRPV4 KO mice. The effect of TRPV4 antagonist and agonist on cell migration, proliferation, and differentiation were examined in vitro.

RESULTS

TRPV4 expression was enhanced in wounded area in the skin. TRPV4 KO mice had impaired cutaneous wound healing compared with the WT mice. Further, they had significantly suppressed re-epithelialization and formation of granulation tissue, amount of collagen deposition, and number of α-SMA-positive myofibroblasts in skin wounds. qPCR revealed that the KO mice had decreased mRNA expression of COL1A1 and ACTA2 in skin wounds. In vitro, treatment with selective TRPV4 antagonist suppressed migrating capacity, scratch stimulation enhanced the expression of phospho-ERK in keratinocytes, and TGF-β stimulation enhanced the mRNA expression of COL1A1 and ACTA2 in fibroblasts. Selective TRPV4 agonist suppressed cell migration in keratinocytes, and did not enhance proliferation and migration, but promoted differentiation in fibroblasts.

CONCLUSION

TRPV4 mediates keratinocytes and fibroblasts migration and increases collagen deposition in the wound area, thereby promoting cutaneous wound healing.

Alleviation of allergic conjunctivitis by (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid in mice

Background: Allergic conjunctivitis (AC) is a common ophthalmologic disorder that causes symptoms that often reduces a patient's quality of life (QOL). We investigated the effects of the eicosapentaenoic acid metabolite (±)5(6)-dihydroxy-8Z,11Z,14Z,17Z-eicosatetraenoic acid ((±)5(6)-DiHETE) on AC using a mouse model. Methods: BALB/c mice were sensitized with two injections of short ragweed pollen in alum, challenged fifth with pollen in eyedrops. The clinical signs and tear volume were evaluated at 15 min after the final challenge. Histamine-induced ocular inflammation model was prepared by instilling histamine onto the surface of the eye. Fifteen minutes after histamine application, tear volume was measured using the Schirmer tear test. Miles assay was performed to investigate vascular permeability. To cause scratching behavior 10 μg of serotonin was injected in the cheek. Results: Repeated topical application of pollen induced conjunctivitis, accompanied by eyelid edema and tearing in mice. Pollen application typically degranulates mast cells and recruits eosinophils to the conjunctiva. Intraperitoneal administration of 300 μg/kg of (±)5(6)-DiHETE significantly inhibited pollen-induced symptoms. The administration of (±)5(6)-DiHETE also attenuated mast cell degranulation and eosinophil infiltration into the conjunctiva. To assess the effects of (±)5(6)-DiHETE on the downstream pathway of mast cell activation in AC, we used a histamine-induced ocular inflammation model. Topical application of 4 μg/eye histamine caused eyelid edema and tearing and increased vascular permeability, as indicated by Evans blue dye extravasation. Intraperitoneal administration of 300 μg/kg or topical administration of 1 μg/eye (±)5(6)-DiHETE inhibited histamine-induced manifestations. Finally, we assessed the effects of (±)5(6)-DiHETE on itching. An intradermal injection of 10 μg serotonin in the cheek caused scratching behavior in mice. Intraperitoneal administration of 300 μg/kg (±)5(6)-DiHETE significantly inhibited serotonin-induced scratching. Conclusion: Thus, (±)5(6)-DiHETE treatment broadly suppressed AC pathology and could be a novel treatment option for AC.

Pruritogenic molecules in the skin of patients with dermatomyositis

Introduction

Pruritus is a common excruciating symptom in systemic autoimmune diseases such as dermatomyositis (DM) but the pathogenesis is not fully understood. We intended to investigate the targeted expression analysis of candidate molecules involved in the development of pruritus in lesional vs. non-lesional skin samples of patients affected with active DM. We looked for correlations between the investigated pruriceptive signaling molecules, disease activity, and itching sensation of DM patients.

Methods

Interleukins (IL-33 and IL-6), tumor necrosis factor α (TNF-α), peroxisome proliferator-activated receptor γ (PPAR-γ), and ion channels belonging to the transient receptor potential (TRP) family were analyzed. The expression of TNF-α, PPAR-γ, IL-33, IL-6, and TRP channels in lesional DM skin was evaluated by RT-qPCR and immunohistochemistry and was compared with non-lesional DM skin samples. Pruritus, disease activity, and damage of DM were evaluated by the 5-D itch scale and Cutaneous Dermatomyositis Disease Area and Severity Index (CDASI), respectively. Statistical analysis was performed with IBM SPSS 28 software.

Results

A total of 17 active DM patients participated in the study. We could show that the itching score was positively correlated with the CDASI activity score (Kendall's tau-b = 0.571; p = 0.003). TNF-α gene expression was significantly higher in lesional DM skin than in non-lesional DM skin (p = 0.009) and differed in the subgroups of patients with different itch intensities (p = 0.038). The mRNA expression of lesional IL-6 correlated positively with 5-D itch and CDASI activity score (Kendall's tau-b = 0.585; p = 0.008 and 0.45; p = 0.013, respectively). TRPV4 expressions were positively correlated with CDASI damage score (Kendall's tau-b = 0.626; p < 0.001), but the mRNA expressions of the TRP family, PPAR-γ, IL-6, and IL-33 were not different in lesional and non-lesional samples. Immunohistochemistry analysis did not find significant alterations in the expressions of TNF-α, PPAR-γ, IL-6, and IL-33 in lesional and non-lesional regions.

Discussion

Our results argue that cutaneous disease activity, TNF-α, and IL-6 might play a central role in DM-associated itch, while TRPV4 plays a central role in tissue regeneration.

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.

Mechanisms of pruritus in cholestasis: understanding and treating the itch

Pruritus in cholestatic liver diseases can be a major burden and dramatically impair the quality of life of those affected. Here, we provide an update on the latest insights into the molecular pathogenesis of and novel therapeutic approaches for cholestasis-associated itch. Endogenous and exogenous small-molecule pruritogen candidates bind to their receptors on unmyelinated itch C-fibres in the skin. Candidate pruritogens in cholestasis include certain lysophospholipids and sulfated progesterone metabolites, among others, whereas total bile acid or bilirubin conjugates seem unlikely to have a dominant role in the pathogenesis of cholestasis-associated pruritus. Transmission of itch signals via primary, secondary and tertiary itch neurons to the postcentral gyrus and activation of scratch responses offer various targets for therapeutic intervention. At present, evidence-based treatment options for pruritus in fibrosing cholangiopathies, such as primary biliary cholangitis and primary sclerosing cholangitis, are the peroxisome proliferator-associated receptor (PPAR) agonist bezafibrate and the pregnane X receptor (PXR) agonist rifampicin. In pruritus of intrahepatic cholestasis of pregnancy, ursodeoxycholic acid is recommended and might be supported in the third trimester by rifampicin if needed. Alternatively, non-absorbable anion exchange resins, such as cholestyramine, can be administered, albeit with poor trial evidence. Liver transplantation for intolerable refractory pruritus has become an extremely rare therapeutic strategy.

TRPV4 Activation Increases the Expression of CD207 (Langerin) of Monocyte-Derived Langerhans Cells without Affecting their Maturation

Langerhans cells (LCs) are the sole professional antigen-presenting cell normally found in the human epidermal compartment. Research into their physiological role is hindered by the fact that they are invariably activated during isolation from the skin. To overcome this challenge, we turned to a monocyte-derived LC (moLC) model, which we characterized with RNA sequencing, and compared the transcriptome of moLCs with that of donor-matched immature dendritic cells. We found that moLCs express markers characteristic of LC2 cells as well as TRPV4. TRPV4 is especially important in the skin because it has been linked to the conservation of the skin barrier, immunological responses, as well as acute and chronic itch, but we know little about its function on LCs. Our results show that TRPV4 activation increased the expression of Langerin and led to increased intracellular calcium concentration in moLCs. Regarding the functionality of moLCs, we found that TRPV4 agonism had a mitigating effect on their inflammatory responses because it decreased their cytokine production and T-cell activating capability. Because TRPV4 has emerged as a potential therapeutic target in dermatological conditions, it is important to highlight LCs as, to our knowledge, a previously unreported target of these therapies.

The temperature-sensitive receptors TRPV4 and TRPM8 have important roles in the pruritus of rosacea

BACKGROUND

Certain sensations are the secondary phenotypes of rosacea and affect patients' quality of life. Transient receptor potential (TRP) channels may be involved in its occurrence. However, there is a lack of research independently discussing itch in rosacea.

OBJECTIVES

Our study aimed to investigate risk factors for pruritus in rosacea patients and to discover the molecular mechanism of pruritus.

METHODS

A binary logistic regression model was used to identify significant variables affecting pruritus in 782 rosacea patients. The LL-37 was injected intradermally into the face of mice to establish the animal model. qRT-PCR, immunohistochemistry and immunofluorescence were used to analyse the expression differences in pruritus-related molecules in mouse skin and the corresponding trigeminal ganglion (TG) between pruritus and nonpruritus groups.

RESULTS

The incidence of pruritus in rosacea was 42.46%, and the incidence of other symptoms increased with pruritus. Temperature effects were prominently related to the itch sensation of rosacea. Intradermal injection of LL-37 not only caused rosacea-like facial lesions but also induced a behavioural pattern indicative of pruritus. Increased expression of the temperature-sensitive receptors TRPV4 and TRPM8 was found in pruritic mouse skin and TG and human skin samples.

CONCLUSIONS

In rosacea patients, pruritus occurs frequently along with burning, flushing and sensitivity, most likely due to changes in temperature. The temperature-sensitive receptors TRPV4 and TRPM8 are both involved in the mechanism of pruritus in rosacea.

Emerging roles of keratinocytes in nociceptive transduction and regulation

Keratinocytes are the predominant block-building cells in the epidermis. Emerging evidence has elucidated the roles of keratinocytes in a wide range of pathophysiological processes including cutaneous nociception, pruritus, and inflammation. Intraepidermal free nerve endings are entirely enwrapped within the gutters of keratinocyte cytoplasm and form en passant synaptic-like contacts with keratinocytes. Keratinocytes can detect thermal, mechanical, and chemical stimuli through transient receptor potential ion channels and other sensory receptors. The activated keratinocytes elicit calcium influx and release ATP, which binds to P2 receptors on free nerve endings and excites sensory neurons. This process is modulated by the endogenous opioid system and endothelin. Keratinocytes also express neurotransmitter receptors of adrenaline, acetylcholine, glutamate, and γ-aminobutyric acid, which are involved in regulating the activation and migration, of keratinocytes. Furthermore, keratinocytes serve as both sources and targets of neurotrophic factors, pro-inflammatory cytokines, and neuropeptides. The autocrine and/or paracrine mechanisms of these mediators create a bidirectional feedback loop that amplifies neuroinflammation and contributes to peripheral sensitization.

The role of TRPV4 channels in cutaneous epithelia

Transient receptor potential vanilloid 4 (TRPV4) channels are multi-modally activated cation permeable channels that are expressed most organ tissues including the skin. TRPV4 is highly expressed in the skin and functions in skin resident cells such as epidermal keratinocytes, melanocytes, immune mast cells and macrophages, and cutaneous neurons. TRPV4 plays many crucial roles in skin homeostasis to affect an extensive range of processes such as temperature sensation, osmo-sensation, hair growth, cell apoptosis, skin barrier integrity, differentiation, nociception and itch. Since TRPV4 functions in a plenitude of pathological states, TRPV4 can become a versatile therapeutic target for diseases such as chronic pain, itch and skin cancer.

TRPV4: A trigger of pathological RhoA activation in neurological disease

Transient receptor potential vanilloid 4 (TRPV4), a member of the TRP superfamily, is a broadly expressed, cell surface-localized cation channel that is activated by a variety of environmental stimuli. Importantly, TRPV4 has been increasingly implicated in the regulation of cellular morphology. Here we propose that TRPV4 and the cytoskeletal remodeling small GTPase RhoA together constitute an environmentally sensitive signaling complex that contributes to pathological cell cytoskeletal alterations during neurological injury and disease. Supporting this hypothesis is our recent work demonstrating direct physical and bidirectional functional interactions of TRPV4 with RhoA, which can lead to activation of RhoA and reorganization of the actin cytoskeleton. Furthermore, a confluence of evidence implicates TRPV4 and/or RhoA in pathological responses triggered by a range of acute neurological insults ranging from stroke to traumatic injury. While initiated by a variety of insults, TRPV4-RhoA signaling may represent a common pathway that disrupts axonal regeneration and blood-brain barrier integrity. These insights also suggest that TRPV4 inhibition may represent a safe, feasible, and precise therapeutic strategy for limiting pathological TRPV4-RhoA activation in a range of neurological diseases.

Physiological and Pathological Significance of Esophageal TRP Channels: Special Focus on TRPV4 in Esophageal Epithelial Cells

Transient receptor potential vanilloid 4 (TRPV4) is a non-selective cation channel that is broadly expressed in different human tissues, including the digestive system, where it acts as a molecular sensor and a transducer that regulates a variety of functional activities. Despite the extensive research to determine the role of this channel in the physiology and pathophysiology of different organs, the unique morphological and functional features of TRPV4 in the esophagus remain largely unknown. Ten years ago, TRPV4 was shown to be highly expressed in esophageal epithelial cells where its activation induces Ca²⁺-dependent ATP release, which, in turn, mediates several functions, ranging from mechanosensation to wound healing. This review summarizes the research progress on TRPV4, and focuses on the functional expression of TRPV4 in esophageal epithelium and its possible role in different esophageal diseases that would support TRPV4 as a candidate target for future therapeutic approaches to treat patients with these conditions.

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

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

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.

Involvement of Sensory Neurone-TRPV4 in Acute and Chronic Itch Behaviours

Abstract is missing (Short communication)

Mild Skin Heating Evokes Warmth Hyperknesis Selectively for Histaminergic and Serotoninergic Itch in Humans

Chronic itch can severely affect quality of life. Patients report that their chronic itch can be exacerbated by exposure to warm conditions (“warmth hyperknesis”). The aim of this mechanistic study was to investigate the effect of mild heating of the skin in humans on various experimental models of itch. A total of 18 healthy subjects were recruited to the study. Itch was provoked by histamine, serotonin, or cowhage in 3 different sessions. The provoked area was heated with an infrared lamp, and the skin temperature was either not altered, or was increased by 4°C or 7°C. Subsequent to induction of itch, the itch intensity was recorded for 10 min while the skin was heated continuously throughout the entire period of itch induction. Heating the skin resulted in a significant increase in itch intensity when provoked by histamine or serotonin. It is possible that thermoception and pruriception interact and selectively produce a higher itch intensity in histaminergic and serotoninergic itch.

Role of TRP ion channels in pruritus

The transient receptor potential (TRP) channel superfamily responds to various physical, chemical, and environmental stimuli including the detection of sensations both harmful and non-harmful. Among these sensations is pruritus, or itch. There are at least 27 different TRP channels and about six of them are involved in pruriception. The function of these six receptors is primarily seen in the skin and the dorsal root ganglia. Identification and biological insights provided by these receptors in pruriception is important for human health as mutations and activations of many of these channels cause discomfort and disease. This review will focus on involvement of TRP channels in pruriception that may render these channels as the targets of many antagonistic topical medications, which may help patients' better cope with the pruritus that results from various cutaneous and systemic diseases.

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.

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.

Mini-review: Dissecting receptor-mediated stimulation of TRPV4 in nociceptive and inflammatory pathways

Transient Receptor Potential Vanilloid 4 (TRPV4) is a polymodal, non-selective cation channel that detects thermal, mechanical, and environmental cues and contributes to a range of diverse physiological processes. The effects of chronic TRPV4 stimulation and gain-of-function genetic mutations suggest that TRPV4 may also be a valuable therapeutic target for pathophysiological events including neurogenic inflammation, peripheral neuropathies, and impaired wound healing. There has been significant interest in defining how and where TRPV4 may promote inflammation and pain. Endogenous stimuli such as osmotic stress and lipid binding are established TRPV4 activators. The TRP channel family is also well-known to be controlled by 'receptor-operated' pathways. For example, G protein-coupled receptors (GPCRs) expressed by primary afferent neurons or other cells in inflammatory pathways utilize TRPV4 as an effector protein to amplify nociceptive and inflammatory signaling. Contributing to disorders including arthritis, neuropathies, and pulmonary edema, GPCRs such as the protease-activated receptor PAR2 mediate activation of kinase signaling cascades to increase TRPV4 phosphorylation, resulting in sensitization and enhanced neuronal excitability. Phospholipase activity also leads to production of polyunsaturated fatty acid lipid mediators that directly activate TRPV4. Consistent with the contribution of TRPV4 to disease, pharmacological inhibition or genetic ablation of TRPV4 can diminish receptor-mediated inflammatory events. This review outlines how receptor-mediated signaling is a major endogenous driver of TRPV4 gating and discusses key signaling pathways and emerging TRPV4 modulators such as the mechanosensitive Piezo1 ion channel. A collective understanding of how endogenous stimuli can influence TRPV4 function is critical for future therapeutic endeavors to modulate this channel.

Cutaneous pain in disorders affecting peripheral nerves

Our ability to quickly detect and respond to harmful environmental stimuli is vital for our safety and survival. This inherent acute pain detection is a "gift" because it both protects our body from harm and allows healing of damaged tissues [1]. Damage to tissues from trauma or disease can result in distorted or amplified nociceptor signaling and sensitization of the spinal cord and brain (Central Nervous System; CNS) pathways to normal input from light touch mechanoreceptors. Together, these processes can result in nagging to unbearable chronic pain and extreme sensitivity to light skin touch (allodynia). Unlike acute protective pain, chronic pain and allodynia serve no useful purpose and can severely reduce the quality of life of an affected person. Chronic pain can arise from impairment to peripheral neurons, a phenomenon called "peripheral neuropathic pain." Peripheral neuropathic pain can be caused by many insults that directly affect peripheral sensory neurons, including mechanical trauma, metabolic imbalance (e.g., diabetes), autoimmune diseases, chemotherapeutic agents, viral infections (e.g., shingles). These insults cause "acquired" neuropathies such as small-fiber neuropathies, diabetic neuropathy, chemotherapy-induced peripheral neuropathy, and post herpetic neuralgia. Peripheral neuropathic pain can also be caused by genetic factors and result in hereditary neuropathies that include Charcot-Marie-Tooth disease, rare channelopathies and Fabry disease. Many acquired and hereditary neuropathies affect the skin, our largest organ and protector of nearly our entire body. Here we review how cutaneous nociception (pain perceived from the skin) is altered following diseases that affect peripheral nerves that innervate the skin. We provide an overview of how noxious stimuli are detected and encoded by molecular transducers on subtypes of cutaneous afferent endings and conveyed to the CNS. Next, we discuss several acquired and hereditary diseases and disorders that cause painful or insensate (lack of sensation) cutaneous peripheral neuropathies, the symptoms and percepts patients experience, and how cutaneous afferents and other peripheral cell types are altered in function in these disorders. We highlight exciting new research areas that implicate non-neuronal skin cells, particularly keratinocytes, in cutaneous nociception and peripheral neuropathies. Finally, we conclude with ideas for innovative new directions, areas of unmet need, and potential opportunities for novel cutaneous therapeutics that may avoid CNS side effects, as well as ideas for improved translation of mechanisms identified in preclinical models to patients.

Irradiation of the Normal Murine Tongue Causes Upregulation and Activation of Transient Receptor Potential (TRP) Ion Channels

Signal transduction at sensory neurons occurs via transmembrane flux of cations, which is largely governed by the transient receptor potential (TRP) family of ion channels. It is unknown whether TRP channel activation contributes to the pain that accompanies radiation-induced oral mucositis. This study sought to characterize changes in TRP channel expression and function that occur in the locally irradiated tissues and afferent neurons of mice. Female CD-1 mice received single high-dose (27 Gy) tongue irradiation, or sham irradiation. Animals were euthanized either before overt glossitis developed (days 1 and 5 postirradiation), when glossitis was severe (day 11), or after mice had recovered (days 21 and 45). Tongue irradiation caused upregulation of the Trpv1 gene in trigeminal ganglia (TG) neurons. Other TRP genes (Trpv2, Trpv4, Trpa1, Trpm8) and Gfrα3 (which acts upstream of several TRP channels) were also upregulated in TGs and/or tongue tissue, in response to radiation. Ex vivo calcium imaging experiments demonstrated that the proportions of TG neurons responding to histamine (an activator of TRPV1, TRPV4 and TRPA1), TNF-α (an activator of TRPV1, TRPV2 and TRPV4), and capsaicin (a TRPV1 agonist), were increased as early as one day after tongue irradiation; these changes persisted for at least 21 days. In a subsequent experiment, we found that genetic deletion of TRPV1 mitigated weight loss (a surrogate marker of pain severity) in mice with severe glossitis. The results intimate that various TRP channels, and TRPV1 in particular, should be explored as analgesic targets for patients experiencing pain after oral irradiation.

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.

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.

Multiple versus solitary giant cell lesions of the jaw: Similar or distinct entities?

The majority of giant cell lesions of the jaw present as a solitary focus of disease in bones of the maxillofacial skeleton. Less frequently they occur as multifocal lesions. This raises the clinical dilemma if these should be considered distinct entities and therefore each need a specific therapeutic approach. Solitary giant cell lesions of the jaw present with a great diversity of symptoms. Recent molecular analysis revealed that these are associated with somatic gain-of-function mutations in KRAS, FGFR1 or TRPV4 in a large component of the mononuclear stromal cells which all act on the RAS/MAPK pathway. For multifocal lesions, a small group of neoplastic multifocal giant cell lesions of the jaw remain after ruling out hyperparathyroidism. Strikingly, most of these patients are diagnosed with jaw lesions before the age of 20 years, thus before the completion of dental and jaw development. These multifocal lesions are often accompanied by a diagnosis or strong clinical suspicion of a syndrome. Many of the frequently reported syndromes belong to the so-called RASopathies, with germline or mosaic mutations leading to downstream upregulation of the RAS/MAPK pathway. The other frequently reported syndrome is cherubism, with gain-of-function mutations in the SH3BP2 gene leading through assumed and unknown signaling to an autoinflammatory bone disorder with hyperactive osteoclasts and defective osteoblastogenesis. Based on this extensive literature review, a RAS/MAPK pathway activation is hypothesized in all giant cell lesions of the jaw. The different interaction between and contribution of deregulated signaling in individual cell lineages and crosstalk with other pathways among the different germline- and non-germline-based alterations causing giant cell lesions of the jaw can be explanatory for the characteristic clinical features. As such, this might also aid in the understanding of the age-dependent symptomatology of syndrome associated giant cell lesions of the jaw; hopefully guiding ideal timing when installing treatment strategies in the future.

Emerging Role of Transient Receptor Potential Vanilloid 4 (TRPV4) Ion Channel in Acute and Chronic Itch

Itch is a clinical problem that leaves many sufferers insufficiently treated, with over 20 million cases in the United States. This is due to incomplete understanding of its molecular, cellular, and cell-to-cell signaling mechanisms. Transient receptor potential (TRP) ion channels are involved in several sensory modalities including pain, vision, taste, olfaction, hearing, touch, and thermosensation, as well as itch. Relative to the extensive studies on TRPV1 and TRPA1 ion channels in itch modulation, TRPV4 has received relatively little research attention and its mechanisms have remained poorly understood until recently. TRPV4 is expressed in ganglion sensory neurons and a variety of skin cells. Growing evidence in the past few years strongly suggests that TRPV4 in these cells contributes to acute and chronic disease-associated itch. This review focuses on the current experimental evidence involving TRPV4 in itch under pathophysiological conditions and discusses its possible cellular and molecular mechanisms.

Epithelia-Sensory Neuron Cross Talk Underlies Cholestatic Itch Induced by Lysophosphatidylcholine

BACKGROUND & AIMS

Limited understanding of pruritus mechanisms in cholestatic liver diseases hinders development of antipruritic treatments. Previous studies implicated lysophosphatidic acid (LPA) as a potential mediator of cholestatic pruritus.

METHODS

Pruritogenicity of lysophosphatidylcholine (LPC), LPA's precursor, was examined in naïve mice, cholestatic mice, and nonhuman primates. LPC's pruritogenicity involving keratinocyte TRPV4 was studied using genetic and pharmacologic approaches, cultured keratinocytes, ion channel physiology, and structural computational modeling. Activation of pruriceptor sensory neurons by microRNA-146a (miR-146a), secreted from keratinocytes, was identified by in vitro and ex vivo Ca2+ imaging assays. Sera from patients with primary biliary cholangitis were used for measuring the levels of LPC and miR-146a.

RESULTS

LPC was robustly pruritic in mice. TRPV4 in skin keratinocytes was essential for LPC-induced itch and itch in mice with cholestasis. Three-dimensional structural modeling, site-directed mutagenesis, and channel function analysis suggested a TRPV4 C-terminal motif for LPC binding and channel activation. In keratinocytes, TRPV4 activation by LPC induced extracellular release of miR-146a, which activated TRPV1+ sensory neurons to cause itch. LPC and miR-146a levels were both elevated in sera of patients with primary biliary cholangitis with itch and correlated with itch intensity. Moreover, LPC and miR-146a were also increased in sera of cholestatic mice and elicited itch in nonhuman primates.

CONCLUSIONS

We identified LPC as a novel cholestatic pruritogen that induces itch through epithelia-sensory neuron cross talk, whereby it directly activates skin keratinocyte TRPV4, which rapidly releases miR-146a to activate skin-innervating TRPV1+ pruriceptor sensory neurons. Our findings support the new concept of the skin, as a sensory organ, playing a critical role in cholestatic itch, beyond liver, peripheral sensory neurons, and central neural pathways supporting pruriception.

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.

Mechanisms of Broad-Band UVB Irradiation‒Induced Itch in Mice

Although sunburn can produce severe uncontrollable itching, the underlying mechanisms of UV irradiation‒induced itch are poorly understood because of a lack of experimental animal models of sunburn itch. In this study, we established a sunburn-related mouse model and found that broad-band UVB irradiation elicited scratching but not wiping behavior in mice. Using a combination of live-cell calcium ion imaging and quantitative RT-PCR on dorsal root ganglion neurons, H&E staining, immunofluorescence staining of skin preparations, and behavioral testing, in combination with genetic and pharmacological approaches, we showed that TRPV1-positive dorsal root ganglion neurons but not mast cells are involved in broad-band UVB irradiation‒induced itch. Moreover, both genetic and pharmacological inhibition of TRPV1 function significantly alleviated the broad-band UVB irradiation‒induced itch response. Collectively, our results suggest that broad-band UVB irradiation evokes itch sensation in mice by promoting TRPV1 channel function in dorsal root ganglion neurons and provide potential therapeutic targets for sunburn-related itch.

TRPV4 antagonists: a patent review (2015-2020)

Introduction: Transient receptor potential vanilloid 4 (TRPV4) is an ion channel that is widely expressed and is activated by numerous chemical, osmotic and mechanical stimuli. By modulating Ca²⁺ entry, TRPV4 regulates cellular signaling associated with a variety of (patho)physiological processes and is a target of interest for treatment of human diseases including heart failure, respiratory diseases, gastrointestinal disorders, dermatological conditions, pain and cancer, among others.Areas covered: This article reviews small molecule TRPV4 antagonists and new therapeutic use claims disclosed in the patent literature from 2015 to 2020, including applications covering the first potent and selective TRPV4 clinical candidate and other advanced chemotypes.Expert opinion: TRPV4 has proven to be a tractable target and significant progress in discovery of TRPV4 antagonists has been realized in recent years. Several unique chemical templates with drug-like properties inhibit the channel and show efficacy in models that suggest their potential for treatment of a variety of diseases. While compelling clinical efficacy has not yet been seen in the limited early studies conducted with GSK2798745, evaluation of TRPV4 antagonists in larger trials across several indications is warranted given the availability of high-quality candidates and the promise of therapeutic benefit based on pre-clinical evidence.

Neuropathy-causing TRPV4 mutations disrupt TRPV4-RhoA interactions and impair neurite extension

TRPV4 is a cell surface-expressed calcium-permeable cation channel that mediates cell-specific effects on cellular morphology and function. Dominant missense mutations of TRPV4 cause distinct, tissue-specific diseases, but the pathogenic mechanisms are unknown. Mutations causing peripheral neuropathy localize to the intracellular N-terminal domain whereas skeletal dysplasia mutations are in multiple domains. Using an unbiased screen, we identified the cytoskeletal remodeling GTPase RhoA as a TRPV4 interactor. TRPV4-RhoA binding occurs via the TRPV4 N-terminal domain, resulting in suppression of TRPV4 channel activity, inhibition of RhoA activation, and extension of neurites in vitro. Neuropathy but not skeletal dysplasia mutations disrupt TRPV4-RhoA binding and cytoskeletal outgrowth. However, inhibition of RhoA restores neurite length in vitro and in a fly model of TRPV4 neuropathy. Together these results identify RhoA as a critical mediator of TRPV4-induced cell structure changes and suggest that disruption of TRPV4-RhoA binding may contribute to tissue-specific toxicity of TRPV4 neuropathy mutations.

TRPV4 dominant mutations cause neuropathy. Here, the authors show that TRPV4 binds and interacts with RhoA, modulating the actin cytoskeleton. Neuropathy-causing mutations of TRPV4 disrupt this complex, leading to RhoA activation and impairment of neurite extension in cultured cells and flies.

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

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