Spinal GRPR and NPRA Contribute to Chronic Itch in a Murine Model of Allergic Contact Dermatitis

Factor XII and prekallikrein promote microvascular inflammation and psoriasis in mice

BACKGROUND AND PURPOSE

Psoriasis is an autoimmune inflammatory skin disease, featuring microvascular abnormalities and elevated levels of bradykinin. Contact activation of Factor XII can initiate the plasma kallikrein-kinin cascade, producing inflammation and angioedema. The role of Factor XII in psoriasis is unknown.

EXPERIMENTAL APPROACH

The effects of deficiency of Factor XII or its enzymatic substrate, prekallikrein, were examined in the imiquimod-induced mouse model of psoriasis. Skin microcirculation was assessed using intravital confocal microscopy and laser Doppler flowmeter. A novel antibody blocking Factor XII activation was evaluated for psoriasis prevention.

KEY RESULTS

Expression of Factor XII was markedly up-regulated in human and mouse psoriatic skin. Genetic deletion of Factor XII or prekallikrein, attenuated imiquimod-induced psoriatic lesions in mice. Psoriatic induction increased skin microvascular blood perfusion, causing vasodilation, hyperpermeability and angiogenesis. It also promoted neutrophil-vascular interaction, inflammatory cytokine release and enhanced Factor XII / prekallikrein enzymatic activity with elevated bradykinin. Factor XII or prekallikrein deficiency ameliorated these microvascular abnormalities and abolished bradykinin increase. Antagonism of bradykinin B2 receptors reproduced the microvascular protection of Factor XII / prekallikrein deficiency, attenuated psoriatic lesions, and prevented protection by Factor XII / prekallikrein deficiency against psoriasis. Furthermore, treatment of mice with Factor XII antibody alleviated experimentally induced psoriasis and suppressed microvascular inflammation.

CONCLUSION AND IMPLICATIONS

Activation of Factor XII promoted psoriasis via prekallikrein-dependent formation of bradykinin, which critically mediated psoriatic microvascular inflammation. Inhibition of contact activation represents a novel therapeutic strategy for psoriasis.

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.

Xanthotoxol relieves itch in mice via suppressing spinal GRP/GRPR signaling

Although pruritus, commonly known as itch, is a common and debilitating symptom associated with various skin conditions, there is a lack of effective therapies available. Xanthotoxol (XAN), a biologically active linear furocoumarin, shows potential in the treatment of various neurological disorders. In this study, we discovered that administering XAN either through intraperitoneal or intrathecal injections effectively reduced scratching behavior induced by compound 48/80 or chloroquine. Importantly, XAN also substantially alleviates chronic itch in dry skin and allergic contact dermatitis mice. Substantial progress has highlighted the crucial role of gastrin-releasing peptide (GRP)-gastrin-releasing peptide receptor (GRPR) signaling in the dorsal spinal cord in transmitting various types of itch. Our behavior tests revealed that XAN significantly alleviated scratching behaviors induced by intrathecal administration of GRP or GRPR agonist bombesin. Furthermore, XAN reduced the activation of neurons in the spinal cord caused by intrathecal administration of GRP in mice. Moreover, XAN attenuates the activation of spinal GRPR-positive neurons in itchy mice. These findings suggest that XAN mitigates itch in mice by suppressing spinal GRP/GRPR signaling, thereby establishing XAN as a promising therapeutic option for treating pruritus.

Microglia are involved in regulating histamine-dependent and non-dependent itch transmissions with distinguished signal pathways

Although itch and pain have many similarities, they are completely different in perceptual experience and behavioral response. In recent years, we have a deep understanding of the neural pathways of itch sensation transmission. However, there are few reports on the role of non-neuronal cells in itch. Microglia are known to play a key role in chronic neuropathic pain and acute inflammatory pain. It is still unknown whether microglia are also involved in regulating the transmission of itch sensation. In the present study, we used several kinds of transgenic mice to specifically deplete CX3CR1+ microglia and peripheral macrophages together (whole depletion), or selectively deplete microglia alone (central depletion). We observed that the acute itch responses to histamine, compound 48/80 and chloroquine were all significantly reduced in mice with either whole or central depletion. Spinal c-fos mRNA assay and further studies revealed that histamine and compound 48/80, but not chloroquine elicited primary itch signal transmission from DRG to spinal Npr1- and somatostatin-positive neurons relied on microglial CX3CL1-CX3CR1 pathway. Our results suggested that microglia were involved in multiple types of acute chemical itch transmission, while the underlying mechanisms for histamine-dependent and non-dependent itch transmission were different that the former required the CX3CL1-CX3CR1 signal pathway.

GRPR down-regulation inhibits spermatogenesis through Ca2+ mediated by PLCβ/IP3R signaling pathway in long-term formaldehyde-exposed rats

Formaldehyde (FA), which is known as an air pollutant, has been proven to induce male infertility. However, the underlying mechanism of FA-induced male infertility remains elusive. In this study, 24 male SD rats were exposed to different levels of FA (0, 0.5, 2.46, and 5 mg/m3) for eight consecutive weeks. Through HE staining and sperm smear, we observed that FA exposure resulted in spermatogenic injury and the sperm quality decreased in rats. The qRT-PCR and Western blot analysis further revealed that GRPR was down-regulated in testicular tissues of FA-exposed rats as well as primary spermatogenic cells. Meanwhile, ZDOCK uncovered an interaction between GRPR and PLCβ. In addition, the CCK8, Fluo 3-AM and Flow cytometry results showed that FA exposure suppressed the expression of GRPR, PLCβ and IP3R, consequently reducing the Ca2+ concentration in spermatogenic cells, inducing apoptosis and inhibiting proliferation of spermatogenic cells. Moreover, rescue experiments confirmed that promoting GRPR could improve intracellular Ca2+ concentration by upregulating PLCβ and IP3R, partially reducing the apoptosis and promoting the proliferation of FA-treated spermatogenic cells. These findings revealed that GRPR participates in spermatogenesis through Ca2+ mediated by the PLCβ/IP3R signaling pathway in FA-exposed rats.

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.

TRPA1 is involved in the inhibitory effect of Ke-teng-zi on allergic contact dermatitis via MAPK and JAK/STAT3 signaling pathways

ETHNOPHARMACOLOGICAL RELEVANCE

The seeds of Entada phaseoloides (Linn.) Merr. commonly named "Ke-teng-zi" is a traditional Chinese folk medicine and reported to treat dermatitis, spasm, and headache. However, the exact effect and the mechanism of Ke-teng-zi on the treatment of dermatitis is unclear.

AIM OF THE STUDY

To elucidate the antipruritic effect and molecular mechanisms of Ke-teng-zi on the treatment of allergic contact dermatitis (ACD).

MATERIALS AND METHODS

The main components of the n-butanol fraction of 70% ethanol extract from Ke-teng-zi (abbreviated as KB) were analyzed by HPLC. The chloroquine (CQ)-induced acute itch and squaraine dibutyl ester (SADBE)-induced ACD chronic itch in mice was established, and the TNF-α/IFN-γ stimulated Human keratinocytes (HaCaT) were used to evaluate the antipruritic and anti-inflammatory effects of KB. Behavioral tests, lesion scoring, and histology were also examined. The expression levels of molecules in MAPK and JAK/STAT3 pathways, the mRNA levels of chemokines and cytokines in both the skin of ACD mice and the HaCaT cells were detected by western blot and qPCR. Furthermore, whole-cell patch-clamp recordings in TRPA1-tranfected HEK293T cells were used to elucidate the effect of KB on TRPA1 channels. TRPA1 siRNA was used to evaluate the role of TRPA1 in the anti-inflammatory effect of KB in keratinocytes.

RESULTS

The main compounds in KB could bind to the active sites of TRPA1 mainly through hydrogen bond and hydrophobic bond interactions. KB could inhibit the scratching behavior in CQ-induced acute itch, and the inhibitory effect of KB was blocked by TRPA1 inhibitor HC-030031. In addition, KB significantly decreased the scratching bouts of ACD mice, reduced the skin lesion scores, mast cells degranulation, and epidermal thickening, inhibited the production of inflammatory chemokines/cytokines and CGRP, and down-regulated the levels of p-ERK1/2, p-p38, and p-STAT3, compared to the ACD mice. Moreover, continuous application of KB induced the desensitization of TRPA1 channels. Also, KB inhibited the expression of p-ERK1/2, p-p38, and p-STAT3, and down-regulated the expression of inflammatory chemokines and cytokines in vitro, which were reversed by the TRPA1 siRNA.

CONCLUSIONS

KB alleviated the pruritus and skin inflammation in ACD mice through TRPA1 channels desensitization and down-regulation of intracellular MAPK and JAK/STAT3 signaling pathways. Our results suggested that Ke-teng-zi is a potential drug for the treatment of inflammatory skin diseases such as ACD.

Role of GRPR in Acupuncture Intervention in the "Itch-scratch Vicious Cycle" Spinal Circuit of Chronic Pruritus

Many previous studies have shown the potential antipruritic effect of acupuncture. This paper reviews the antipruritic mechanisms of acupuncture according to these aspects: sample characteristics, detail of intervention, and effects evaluation. The majority of research on acupuncture's antipruritic effect has focused on primary afferents of the peripheral mechanism. Relatively few studies, however, have addressed the central mechanisms. Combination the latest research achievements of chronic itch, gastrin-releasing peptide receptor (GRPR) in the dorsal horn of the spinal cord may represent the first molecule identified that is dedicated to mediating the itch response and may provide an important therapeutic target for the treatment of chronic pruritic conditions. Therefore, GRPR may be a new target for acupuncture to relieve itch in the future and provide new ideas for acupuncture intervention in the mechanisms of the spinal level of the "itch-scratch vicious cycle" of chronic itch.

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.

Anti-inflammatory and anti-pruritic effects of Chi-Huang Solution in a murine model of allergic contact dermatitis

ETHNOPHARMACOLOGICAL RELEVANCE

In treating atopic dermatitis, multi-mode management is adopted, including trying to avoid the allergens, controlling and preventing secondary infections, and using drugs to control itching. At present, most of the commonly used anti-pruritic drugs in the clinic are single-target and lead to serious side effects. Many studies have shown that a variety of traditional Chinese medicines have significant anti-inflammatory and anti-pruritic effects, and have the characteristics of multiple components, multiple targets, and multiple effects.

AIM OF THE STUDY

The study aimed to explore the anti-inflammatory and anti-pruritic effects of the Chi-Huang Solution in a murine model of Allergic contact dermatitis (ACD). This study considers the effectiveness of the Chi-Huang Solution for external use on skin to provide an experimental basis for the clinical development and application of Chinese medicine and related preparations for Canine atopic dermatitis (CAD).

MATERIALS AND METHODS

Forty-two male SPF C57BL/6 mice were randomly divided into control group (n = 6), ACD model group (n = 6), HAC control group (n = 6), and 4 Chi-Huang Solution groups (n = 6 in each group). With SADBE induce the murine model of ACD chronic pruritus, and initially evaluate whether the model is successful by counting scratching behavior, measuring the skin fold thickness and skin lesion score within 1 h. After treating the ACD model mice with deionized water, HAC, 1CH, 2CH, 3CH, and 4CH for 7 days, behavioral changes were used to evaluate the anti-pruritic effect. The skin fold thickness, skin lesion score, and spleen index were used to evaluate the anti-inflammatory effect of the Chi-Huang Solution. H.E. staining was used for the epidermal thickness measurement and pathological evaluation. RT-qPCR was used to analyze the mRNA expression of related inflammatory factors such as IL-1β, TNF-α, IL-33, IL-4, IL-17A, CXCL10, and its receptor CXCR3 in the skin of the lesion site, as well as to detect the mRNA expression of pruritus-related genes such as TRPV1, TRPA1, and GRP in DRG.

RESULTS

After the treatment of low-dose (0.1 g/mL) and medium-dose (0.2 g/mL) Chi-Huang Solution, the scratching times both decreased significantly (P < 0.05), meanwhile the medium-dose Chi-Huang Solution had an obvious effect on reducing scratches/scab score (P < 0.05). Moreover, no matter what dose it takes, all Chi-Huang Solution can alleviate the epidermal thickening (P < 0.05) and the infiltration of mast cells in the ACD murine model of ACD. It is worth mentioning that the count of mast cells in the dermis was significantly down-regulated after the treatment of medium-dose Chi-Huang Solution (P < 0.005). Furthermore, Chi-Huang Solution can significantly down-regulate the mRNA expression of related inflammatory factors in the skin, and reduce the mRNA expression of pruritus-related genes, such as TRPA1, TRPV1, and GRP in the spinal cord.

CONCLUSIONS

The results indicated that Chi-Huang Solution for external use exhibits significant anti-inflammatory and anti-pruritic effects on SADBE-induced ACD chronic pruritus murine models. Chi-Huang Solution might emerge as an effective drug for the treatment of CAD and high-dose Chi-Huang Solution (0.4 g/ml) has better comprehensive effects.

Critical Players and Therapeutic Targets in Chronic Itch

Chronic itch is one of the most prominent clinical characteristics of diverse systematic diseases. It is a devastating sensation in pathological diseases. Despite its importance, there are no FDA-labelled drugs specifically geared toward chronic itch. The associated complex pathogenesis and diverse causes escalate chronic itch to being one of the top challenges in healthcare. Humanized antibodies against IL-13, IL-4, and IL-31 proved effective in treatment of itch-associated atopic dermatitis but remain to be validated in chronic itch. There are still no satisfactory anti-itch therapeutics available toward itch-related neuropeptides including GRP, BNP, SST, CGRP, and SP. The newly identified potential itch targets including OSM, NMB, glutamate, periostin, and Serpin E1 have opened new avenues for therapeutic development. Proof-of-principle studies have been successfully performed on antagonists against these proteins and their receptors in itch treatment in animal models. Their translational interventions in humans need to be evaluated. It is of great importance to summarize and compare the newly emerging knowledge on chronic itch and its pathways to promote the development of novel anti-itch therapeutics. The goal of this review is to analyze the different physiologies and pathophysiologies of itch mediators, whilst assessing their suitability as new targets and discussing future therapeutic development.

Functional roles of neuromedin B and gastrin-releasing peptide in regulating itch and pain in the spinal cord of non-human primates

Despite accumulating evidence in rodents, the functional role of neuromedin B (NMB) in regulating somatosensory systems in primate spinal cord is unknown. We aimed to compare the expression patterns of NMB and its receptor (NMBR) and the behavioral effects of intrathecal (i.t.) NMB with gastrin-releasing peptide (GRP) on itch or pain in non-human primates (NHPs). We used six adult rhesus monkeys. The mRNA or protein expressions of NMB, GRP, and their receptors were evaluated by quantitative reverse transcription polymerase chain reaction, immunohistochemistry, or in situ hybridization. We determined the behavioral effects of NMB or GRP via acute thermal nociception, capsaicin-induced thermal allodynia, and itch scratching response assays. NMB expression levels were greater than those of GRP in the dorsal root ganglia and spinal dorsal horn. Conversely, NMBR expression was significantly lower than GRP receptor (GRPR). I.t. NMB elicited only mild scratching responses, whereas GRP caused robust scratching responses. GRP- and NMB-elicited scratching responses were attenuated by GRPR (RC-3095) and NMBR (PD168368) antagonists, respectively. Moreover, i.t. NMB and GRP did not induce thermal hypersensitivity and GRPR and NMBR antagonists did not affect peripherally elicited thermal allodynia. Consistently, NMBR expression was low in both itch- and pain-responsive neurons in the spinal dorsal horn. Spinal NMB-NMBR system plays a minimal functional role in the neurotransmission of itch and pain in primates. Unlike the functional significance of the GRP-GRPR system in itch, drugs targeting the spinal NMB-NMBR system may not effectively alleviate non-NMBR-mediated itch.

BNP facilitates NMB-encoded histaminergic itch via NPRC-NMBR crosstalk

Histamine-dependent and -independent itch is conveyed by parallel peripheral neural pathways that express gastrin-releasing peptide (GRP) and neuromedin B (NMB), respectively, to the spinal cord of mice. B-type natriuretic peptide (BNP) has been proposed to transmit both types of itch via its receptor NPRA encoded by Npr1. However, BNP also binds to its cognate receptor, NPRC encoded by Npr3 with equal potency. Moreover, natriuretic peptides (NP) signal through the G_i-couped inhibitory cGMP pathway that is supposed to inhibit neuronal activity, raising the question of how BNP may transmit itch information. Here, we report that Npr3 expression in laminae I-II of the dorsal horn partially overlaps with NMB receptor (NMBR) that transmits histaminergic itch via G_q-couped PLCβ-Ca²⁺ signaling pathway. Functional studies indicate that NPRC is required for itch evoked by histamine but not chloroquine (CQ), a nonhistaminergic pruritogen. Importantly, BNP significantly facilitates scratching behaviors mediated by NMB, but not GRP. Consistently, BNP evoked Ca²⁺ responses in NMBR/NPRC HEK 293 cells and NMBR/NPRC dorsal horn neurons. These results reveal a previously unknown mechanism by which BNP facilitates NMB-encoded itch through a novel NPRC-NMBR cross-signaling in mice. Our studies uncover distinct modes of action for neuropeptides in transmission and modulation of itch in mice.

An itch is a common sensation that makes us want to scratch. Most short-term itches are caused by histamine, a chemical that is released by immune cells following an infection or in response to an allergic reaction. Chronic itching, on the other hand, is not usually triggered by histamine, and is typically the result of neurological or skin disorders, such as atopic dermatitis.

The sensation of itching is generated by signals that travel from the skin to nerve cells in the spinal cord. Studies in mice have shown that the neuropeptides responsible for delivering these signals differ depending on whether or not the itch involves histamine: GRPs (short for gastrin-releasing proteins) convey histamine-independent itches, while NMBs (short for neuromedin B) convey histamine-dependent itches.

It has been proposed that another neuropeptide called BNP (short for B-type natriuretic peptide) is able to transmit both types of itch signals to the spinal cord. But it remains unclear how this signaling molecule is able to do this.

To investigate, Meng, Liu, Liu, Liu et al. carried out a combination of behavioral, molecular and pharmacological experiments in mice and nerve cells cultured in a laboratory. The experiments showed that BNP alone cannot transmit the sensation of itching, but it can boost itching signals that are triggered by histamine.

It is widely believed that BNP activates a receptor protein called NPRA. However, Meng et al. found that the BNP actually binds to another protein which alters the function of the receptor activated by NMBs. These findings suggest that BNP modulates rather than initiates histamine-dependent itching by enhancing the interaction between NMBs and their receptor.

Understanding how itch signals travel from the skin to neurons in the spinal cord is crucial for designing new treatments for chronic itching. The work by Meng et al. suggests that treatments targeting NPRA, which was thought to be a key itch receptor, may not be effective against chronic itching, and that other drug targets need to be explored.

Central, but not peripheral, nervous system ERK2 is essential for itch signals in murine allergic skin inflammation

BACKGROUND

Itch is a common cutaneous symptom in a variety of dermatological diseases, but detailed neuropathological mechanisms remain to be fully elucidated. This study aimed to assess in vivo ERK2 functions in the nervous system for itch responses.

METHODS

We generated conditional knockout mice deficient in ERK2 of the central nervous system (CNS) or peripheral nervous system (PNS), respectively, and assessed chemical and mechanical itch responses in vivo.

RESULTS

Chemical itch responses to histamine, but not to BAM8-22, were alleviated in CNS Erk2-deficient mice. In contrast, both histamine- and BAM8-22-induced mechanical itch (alloknesis) were alleviated in CNS Erk2-deficient mice. Neither chemical itch nor mechanical itch induced by these pruritogens was affected by PNS ERK2 deficiency. Spontaneous scratching behaviors during acute and chronic contact hypersensitivity were impaired in CNS Erk2-deficient mice, but not PNS Erk2-deficient mice. In addition, CNS ERK2 deficiency attenuated mechanical itch responses during chronic contact hypersensitivity. Again, PNS Erk2-deficient mice showed comparable responses of mechanical itch to control mice. In addition, alleviated mechanical itch in CNS Erk2-deficient mice was observed in IgE-mediated prurigo-like allergic skin inflammation. Mechanical itch induced by IL-31 was also alleviated by CNS ERK2 deficiency. Phosphorylated ERK1/2 was detected in neurokinin B-expressing cells of the spinal dorsal horn of control mice; these cells accumulated during the induction of chronic contact hypersensitivity. Notably, phosphorylated ERK1/2 was also localized in spinal urocortin3-expressing neurons that are known to transmit mechanical itch.

CONCLUSIONS

Spinal cord ERK2 could be a potential therapeutic target for intractable itch in pruritic skin diseases.

A neuropeptide code for itch

Itch is one of the most primal sensations, being both ubiquitous and important for the well-being of animals. For more than a century, a desire to understand how itch is encoded by the nervous system has prompted the advancement of many theories. Within the past 15 years, our understanding of the molecular and neural mechanisms of itch has undergone a major transformation, and this remarkable progress continues today without any sign of abating. Here I describe accumulating evidence that indicates that itch is distinguished from pain through the actions of itch-specific neuropeptides that relay itch information to the spinal cord. According to this model, classical neurotransmitters transmit, inhibit and modulate itch information in a context-, space- and time-dependent manner but do not encode itch specificity. Gastrin-releasing peptide (GRP) is proposed to be a key itch-specific neuropeptide, with spinal neurons expressing GRP receptor (GRPR) functioning as a key part of a convergent circuit for the conveyance of peripheral itch information to the brain.

Chemogenetic activation of central gastrin-releasing peptide-expressing neurons elicits itch-related scratching behavior in male and female mice

Several lines of evidence have clarified that the key transmission pathways of itching sensation travel from the periphery to the central nervous system (CNS). Despite the functional significance of gastrin-releasing peptide (GRP) and its cognate receptor in the itch processing mechanism in the spinal dorsal horn (SDH), the roles of GRP-expressing (GRP+ ) neurons in different regions remain unclear. This study aimed to determine whether GRP+ neurons in the CNS directly modulated itch processing. To specifically activate spinal and supraspinal GRP neurons by the designer receptors exclusively activated by designer drugs (DREADDs) system, CAG-LSL-Gq-DREADD mice were crossed with GRP-Cre mice, resulting in the development of GRP-hM3Dq mice. Immunohistochemistry showed that hM3Dq was highly expressed in the SDH and brainstem closely related to sensory processing. The intraperitoneal, intrathecal, or intracerebroventricular administration of clozapine-N-oxide, an agonist of hM3Dq, strongly elicited dermatome-dependent itch-related scratching behavior, but did not change pain sensitivity. Importantly, GRP-Gq-DREADD-mediated scratching behavior in GRP-hM3Dq mice was not affected by the ablation of transient receptor potential vanilloid 1+ sensory C-fibers, and it was also observed to a similar degree under chronic itch conditions. Furthermore, there were no significant sex differences in the scratching behavior elicited by GRP-Gq-DREADD, suggesting that itch-dominant roles of central GRP+ neurons might be common in both sexes, at least under normal physiological conditions. These novel findings not only contribute to understanding the functional roles of central GRP+ neurons further, but also propose the development of future effective therapeutics for intractable itching.

Genistein suppresses allergic contact dermatitis through regulating the MAP2K2/ERK pathway

Genistein is one of the main components of soybeans and has been reported to be a potential candidate for the treatment of obesity, cancer, osteoporosis and cardiovascular diseases. Recently, genistein has been shown to have therapeutic effects on some chronic skin diseases, but its underlying mechanisms remain unclear. In this study, we evaluated the role of genistein in alleviating squaric acid dibutylester (SADBE)-induced allergic contact dermatitis (ACD) in mice, and elucidated the potential molecular mechanisms in human keratinocyte (HaCaT) cell line. The impacts of genistein on the production of pro-inflammatory chemokines and cytokines including CXCL9, TSLP, TNF-α, IL-1β and IL-6 in the skin and serum of ACD mice were assessed, as well as the phosphorylation of components in the MAPK and JAK-STAT3 signaling pathways in the skin and dorsal root ganglions (DRGs). The results showed that genistein exerted protective effects on skin damage and inflammatory cell infiltration. Moreover, genistein significantly inhibited the increased expressions of pro-inflammatory factors in skin and peripheral blood, and down-regulated the levels of p-ERK, p-p38 and p-STAT3 in skin and DRGs. Furthermore, genistein inhibited the phosphorylation of ERK and STAT3 to downregulate the expression of cytokines and chemokines, and feedback downregulate phospho-p38 in TNF-α/IFN-γ-induced HaCaT cells. The genistein-mediated inhibitory effect on the MAPK pathway can be reversed by siMAP2K2 but not by siMAP2K4. Altogether, our findings demonstrated that genistein exhibits strong antipruritic and anti-inflammatory effects in ACD mice by inhibiting the production of pro-inflammatory cytokines and intracellular MAP2K2/ERK cell signaling, which makes genistein a potentially valuable candidate for the treatment of skin conditions and systemic syndromes in the setting of contact dermatitis.

Bombesin Receptor Family Activation and CNS/Neural Tumors: Review of Evidence Supporting Possible Role for Novel Targeted Therapy

G-protein-coupled receptors (GPCRs) are increasingly being considered as possible therapeutic targets in cancers. Activation of GPCR on tumors can have prominent growth effects, and GPCRs are frequently over-/ectopically expressed on tumors and thus can be used for targeted therapy. CNS/neural tumors are receiving increasing attention using this approach. Gliomas are the most frequent primary malignant brain/CNS tumor with glioblastoma having a 10-year survival <1%; neuroblastomas are the most common extracranial solid tumor in children with long-term survival<40%, and medulloblastomas are less common, but one subgroup has a 5-year survival <60%. Thus, there is an increased need for more effective treatments of these tumors. The Bombesin-receptor family (BnRs) is one of the GPCRs that are most frequently over/ectopically expressed by common tumors and is receiving particular attention as a possible therapeutic target in several tumors, particularly in prostate, breast, and lung cancer. We review in this paper evidence suggesting why a similar approach in some CNS/neural tumors (gliomas, neuroblastomas, medulloblastomas) should also be considered.

GRPR/Extracellular Signal-Regulated Kinase and NPRA/Extracellular Signal-Regulated Kinase Signaling Pathways Play a Critical Role in Spinal Transmission of Chronic Itch

Intractable or recurrent chronic itch greatly reduces the patients' QOL and impairs their daily activities. In this study, we investigated whether there are certain key signaling molecules downstream of the recently identified peptides mediating itch in the spinal cord. RNA sequencing analysis of mouse spinal cord in chronic itch models induced by squaric acid dibutylester and imiquimod showed that extracellular signal-regulated kinase (ERK) 1/2 cascade is the most significantly upregulated gene cluster in both models. In four different mouse models of chronic itch, sustained ERK phosphorylation was detected mainly in spinal neurons, and MAPK/ERK kinase inhibitors significantly inhibited chronic itch in these models. Phosphorylated ERK was observed in the interneurons expressing the receptors of different neuropeptides for itch, including gastrin-releasing peptide receptor, natriuretic peptide receptor A, neuromedin B receptor, and sst2A. Blocking gastrin-releasing peptide receptor and natriuretic peptide receptor A by genetic approaches or toxins in mice significantly attenuated or ablated spinal phosphorylated ERK. When human embryonic kidney 293T cells transfected with these receptors were exposed to their respective agonists, ERK was the most significantly activated intracellular signaling molecule. Together, our work showed that phosphorylated ERK is a unique marker for itch signal transmission in the spinal cord and an attractive target for the treatment of chronic itch.

A spinal neural circuitry for converting touch to itch sensation

Touch and itch sensations are crucial for evoking defensive and emotional responses, and light tactile touch may induce unpleasant itch sensations (mechanical itch or alloknesis). The neural substrate for touch-to-itch conversion in the spinal cord remains elusive. We report that spinal interneurons expressing Tachykinin 2-Cre (Tac2^Cre) receive direct Aβ low threshold mechanoreceptor (LTMR) input and form monosynaptic connections with GRPR neurons. Ablation or inhibition markedly reduces mechanical but not acute chemical itch nor noxious touch information. Chemogenetic inhibition of Tac2^Cre neurons also displays pronounced deficit in chronic dry skin itch, a type of chemical itch in mice. Consistently, ablation of gastrin-releasing peptide receptor (GRPR) neurons, which are essential for transmitting chemical itch, also abolishes mechanical itch. Together, these results suggest that innocuous touch and chemical itch information converge on GRPR neurons and thus map an exquisite spinal circuitry hard-wired for converting innocuous touch to irritating itch.

Thymol activates TRPM8-mediated Ca2+ influx for its antipruritic effects and alleviates inflammatory response in Imiquimod-induced mice

Psoriasis is a highly prevalent chronic dermatitis, characterized by widespread skin inflammation and spontaneous itch. Given the adverse reactions and drug dependence of current treatment, new drugs for psoriasis therapy are urgently needed. This study aims to explore the anti-psoriatic effects of thymol in imiquimod (IMQ) induced mice, and elucidate the potential mechanisms for its therapeutic activities. Thymol reduced the scratching behavior in IMQ mice, and activated Ca²⁺ response in cervical DRG neurons via TRPM8 channel. Also, thymol alleviated psoriasis-like skin lesions, and attenuated the enhanced infiltration of dermal neutrophils, dendritic cells (DCs) and Th17 cells. In addition, it reversed the upregulated expression of pro-inflammatory cytokines in the skin (TNF-α, IL-22, IL-23, IL-17A, IL-17F, IL-17C, IL-6, IL-1β and IFN-γ) and serum (TNF-α, IL-6, IL-1β, IL-17A and IFN-γ). Our results indicated that thymol can effectively ameliorate pruritus and the symptoms of psoriasis-like inflammation induced by IMQ, which makes it a promising drug for the treatment of psoriasis.