mMrgprA3 / mMrgprC11 / hMrgprX1 : potential therapeutic targets for allergic contact dermatitis induced pruritus in mice and human

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.

Anti-atopic dermatitis effect of fraxinellone via inhibiting IL-31 in vivo and in vitro

Chronic recurrent itch and skin inflammation are prominent features of atopic dermatitis (AD), which is closely related to the immune response driven by T-helper type 2 (Th2) cells. The expression of interleukin 31 (IL-31) is positively correlated with the severity of dermatitis. Anti-IL-31 receptor α (IL-31RA) targeted drugs have been used to treat AD, however, they are expensive and have side effects. Fraxinellone (FRA) is one of the main limonoid components in the dried root bark of Dictamnus dasycarpus Turcz.; however, its anti-inflammatory and antipruritic effects on atopic dermatitis (AD) have not been previously reported. In this study, we investigated the anti-dermatitis effect of FRA and its potential mechanism of action using a 2,4-dinitrofluorobenzene (DNFB)-induced AD-like mouse model and lipopolysaccharide (LPS)-stimulated HaCaT cells. FRA significantly inhibited chronic pruritus, epidermal thickening, and inflammatory infiltration in AD mice. FRA not only inhibited the levels of IL-31 in the serum and lesioned skin of AD mice but also significantly downregulated the mRNA expression and protein levels of IL-31, IL-31RA, transient receptor potential (TRP) V1, and TRPA1 in the lesioned skin and dorsal root ganglion (DRG) of AD mice. In LPS-stimulated HaCaT cells, FRA inhibited the production of iNOS and COX2, as well as the protein levels of IL-31, IL-31RA, TRPV1 and TRPA1, showing significant anti-inflammatory effects. In summary, our findings suggest that FRA exerts antipruritic and anti-inflammatory effects in AD by regulating the IL-31 pathway, and may hold promise for the clinical treatment of AD.

MrgprA3+ Primary Sensory Neurons Mediate Acute Allergic Itch Responses in Atopic Dermatitis Model Mice

Itch is a prominent symptom of atopic dermatitis (AD). However, the underlying mechanism remains complex and has not yet been fully elucidated. Mas-related G protein-coupled receptor A3 (MrgprA3) has emerged attention as a marker of primary sensory neurons that specifically transmit itch signals; however, its involvement in AD-related itch has not been extensively explored. In this study, we developed an AD itch mouse model by repeatedly applying house dust mite (HDM) extract to barrier-impaired skin via a special diet. To clarify the role of MrgprA3+ neurons in itch behavior in our AD model, we adopted a toxin receptor-mediated cell knockout strategy using transgenic mice in which the diphtheria toxin receptor (DTR) gene was placed under the control of the Mrgpra3 promoter. When the HDM extract was repeatedly applied to the face and back skin of special diet-fed mice, the mice exhibited AD-like dry and eczematous skin lesions accompanied by three types of itch-related behaviors:1) spontaneous scratching, 2) acute scratching after antigen challenge, and 3) light touch-evoked scratching. Upon diphtheria toxin administration, substantial depletion of DTR+/MrgprA3+ neurons was observed in the dorsal root ganglion. Ablation of MrgprA3+ neurons suppressed acute itch responses after HDM application, whereas spontaneous and touch-evoked itch behaviors remained unaffected. Our findings unequivocally demonstrate that in our AD model, MrgprA3+ primary sensory neurons mediate acute allergic itch responses, whereas these neurons are not involved in spontaneous itch or alloknesis.

Aedes aegypti salivary gland extract alleviates acute itching by blocking TRPA1 channels

Aedes aegypti (Ae. aegypti) saliva induces a variety of anti-inflammatory and immunomodulatory activities. Interestingly, although it is known that mosquito bites cause allergic reactions in sensitised hosts, the primary exposure of humans to Ae. aegypti does not evoke significant itching. Whether active components in the saliva of Ae. aegypti can counteract the normal itch reaction to injury produced by a histaminergic or non-histaminergic pathway in vertebrate hosts is unknown. This study investigated the effects of Ae. aegypti mosquito salivary gland extract (SGE) on sensitive reactions such as itching and associated skin inflammation. Acute pruritus and plasma extravasation were induced in mice by the intradermal injection of either compound 48/80 (C48/80), the Mas-related G protein-coupled receptor (Mrgpr) agonist chloroquine (CQ), or the transient receptor potential ankyrin 1 (TRPA1) agonist allyl isothiocyanate (AITC). The i.d. co-injection of Ae. aegypti SGE inhibited itching, plasma extravasation, and neutrophil influx evoked by C48/80, but it did not significantly affect mast cell degranulation in situ or in vitro. Additionally, SGE partially reduced CQ- and AITC-induced pruritus in vivo, suggesting that SGE affects pruriceptive nerve firing independently of the histaminergic pathway. Activation of TRPA1 significantly increased intracellular Ca²⁺ in TRPA-1-transfected HEK293t lineage, which was attenuated by SGE addition. We showed for the first time that Ae. aegypti SGE exerts anti-pruriceptive effects, which are partially regulated by the histamine-independent itch TRPA1 pathway. Thus, SGE may possess bioactive molecules with therapeutic potential for treating nonhistaminergic itch.

Neuroimmune interactions in atopic and allergic contact dermatitis

The skin is a barrier organ populated by many types of skin-resident immune cells and sensory neurons. It has become increasingly appreciated that neuroimmune interactions are an important component of inflammatory diseases such as atopic dermatitis and allergic contact dermatitis. Neuropeptides secreted from nerve terminals play an important role in mediating cutaneous immune cell function, and soluble mediators derived from immune cells interact with neurons to induce itch. In this review article, we will explore emerging research describing neuronal effector functions on skin immune cells in mouse models of atopic and contact dermatitis. We will also discuss the contributions of both specific neuronal subsets and secreted immune factors to itch induction and the associated inflammatory processes. Finally, we will explore how treatment strategies have emerged around these findings and discuss the relationship between scratching and dermatitis.

Osthole relieves skin damage and inhibits chronic itch through modulation of Akt/ZO-3 pathway in atopic dermatitis

Atopic dermatitis (AD) is the most prevalent chronic inflammatory skin condition and significantly reduces quality of life. Tight junction (TJ), which is located directly beneath the stratum corneum, maintains skin barrier function and aids in the identification of the cell's "territory". We evaluated seventeen TJ related genes to explore AD related alterations of TJ. Remarkably, we found that the expression of ZO-3, a gene that had not been linked to the development of TJ in AD, was significantly down-regulated in the skin of AD mice and patients. siRNA mediated knock-down of ZO-3 significantly decreased transepithelial electrical resistance in HaCaT cells, demonstrating that ZO-3 is essential to epidermal barrier function. In addition to ZO-3 downregulation, protein kinase B (Akt) phosphorylation was increased in the skin of AD mice. We further confirmed an inverse relationship between Akt phosphorylation and ZO-3 expression in AD using HaCaT cells and mouse model. Finally, we tested the efficacy of osthole as a treatment for AD in mice and HaCaT cells. Osthole inhibits Akt phosphorylation, and thereby enhances ZO-3 expression in mouse models of AD, resulting in greatly lessened AD associated skin damage and chronic itch, and osthole also increased the expression of ZO-3 in HaCaT cells by inhibiting the phosphorylation of Akt. Together, we established that ZO-3 is essential for the development of TJ in AD skin and HaCaT cells, and our findings provide fresh support for osthole's ability to protect ZO-3 expression and the epidermal barrier in AD.

Aconitine - A promising candidate for treating cold and mechanical allodynia in cancer induced bone pain

BACKGROUND AND AIMS

Patients suffering from cancer induced bone pain (CIBP) have a poor quality of life that is exacerbated by the lack of effective therapeutic drugs. Monkshood is a flowering plant that has been used in traditional Chinese medicine where it has been used to relieve cold pain. Aconitine is the active component of monkshood, but the molecular mechanism for how this compound reduces pain is unclear.

METHODS AND RESULTS

In this study, we employed molecular and behavioral experiments to explore the analgesic effect of aconitine. We observed aconitine alleviated cold hyperalgesia and AITC (allyl-isothiocyanate, TRPA1 agonist) induced pain. Interestingly, we found aconitine directly inhibits TRPA1 activity in calcium imaging studies. More importantly, we found aconitine alleviated cold and mechanical allodynia in CIBP mice. Both the activity and expression of TRPA1 in L4 and L5 DRG (Dorsal Root Ganglion) neurons were reduced with the treatment of aconitine in the CIBP model. Moreover, we observed aconiti radix (AR) and aconiti kusnezoffii radix (AKR), both components of monkshood that contain aconitine, alleviated cold hyperalgesia and AITC induced pain. Furthermore, both AR and AKR alleviated CIBP induced cold allodynia and mechanical allodynia.

CONCLUSIONS

Taken together, aconitine alleviates both cold and mechanical allodynia in cancer induced bone pain via the regulation of TRPA1. This research on the analgesic effect of aconitine in cancer induced bone pain highlights a component of a traditional Chinese medicine may have clinical applications for pain.

Dictamnine ameliorates chronic itch in DNFB-induced atopic dermatitis mice via inhibiting MrgprA3

Chronic itch is the most prominent feature of atopic dermatitis (AD), and antihistamine treatment is often less effective in reducing clinical pruritus severity in AD. Multiple studies have shown that histamine-independent itch pathway is thought to predominate in AD-induced chronic itch. Mas-related G-protein-coupled receptor (Mrgpr) A3⁺ sensory neurons have been identified as one of the major itch-sensing neuron populations, and transient receptor potential (TRP) channel A1 is the key downstream of MrgprA3-mediated histamine-independent itch. MrgprA3-TRPA1 signal pathway is necessary for the development of chronic itch and may be the potentially promising target of chronic itch in AD. Dictamnine is one of the main quinoline alkaloid components of Cortex Dictamni (a traditional Chinese medicine widely used in clinical treatment of skin diseases). However, the anti-inflammatory and anti-pruritic effect of dictamnine on AD have not been reported. In this study, we used the 2,4-dinitrofluorobenzene (DNFB)-induced AD mouse model to observe the scratching behavior, inflammatory manifestations, and to detect the expression of MrgprA3 and TRPA1 in skin and DRG. The data demonstrated that dictamnine effectively inhibited AD-induced chronic itch, inflammation symptoms, epidermal thickening, inflammatory cell infiltration, and downregulated the expression of MrgprA3 and TRPA1. Furthermore, dictamnine restrained the excitability of MrgprA3⁺ and TRPA1⁺ neurons. Molecular docking also indicated that dictamnine has better binding affinity with MrgprA3. These results suggest that dictamnine may inhibit chronic itch caused by AD through the MrgprA3-TRPA1 mediated histamine-independent itch pathway, and may have a potential utility in AD treatment.

Berbamine Reduces Chloroquine-Induced Itch in Mice through Inhibition of MrgprX1

Chloroquine (CQ) is an antimalaria drug that has been widely used for decades. However, CQ-induced pruritus remains one of the major obstacles in CQ treatment for uncomplicated malaria. Recent studies have revealed that MrgprX1 plays an essential role in CQ-induced itch. To date, a few MrgprX1 antagonists have been discovered, but they are clinically unavailable or lack selectivity. Here, a cell-based high-throughput screening was performed to identify novel antagonists of MrgprX1, and the screening of 2543 compounds revealed two novel MrgprX1 inhibitors, berbamine and closantel. Notably, berbamine potently inhibited CQ-mediated MrgprX1 activation (IC₅₀ = 1.6 μM) but did not alter the activity of other pruritogenic GPCRs. In addition, berbamine suppressed the CQ-mediated phosphorylation of ERK1/2. Interestingly, CQ-induced pruritus was significantly reduced by berbamine in a dose-dependent manner, but berbamine had no effect on histamine-induced, protease-activated receptors 2-activating peptide-induced, and deoxycholic acid-induced itch in mice. These results suggest that berbamine is a novel, potent, and selective antagonist of MrgprX1 and may be a potential drug candidate for the development of therapeutic agents to treat CQ-induced pruritus.

Water Extract of Senecio scandens Buch.-Ham Ameliorates Pruritus by Inhibiting MrgprB2 Receptor

Background

Senecio scandens Buch.-Ham (S. scandens) belongs to the Compositae family. As a Traditional Chinese medicine, S. scandens has been used in China to treat conjunctivitis, mastitis and vaginitis, it also has the function of antibacterial and relieving itching.

Methods

Water extract of S. scandens (WSS) was prepared and its quality was controlled by HPLC. The antipruritic effects of WSS were evaluated by itch behavioral experiments. The oxazolone and compound 48/80 were induced to mice scratch behavior, scratch was recorded 30 min after sensitization. The relationship between the antipruritic mechanism and MrgprB2 on mast cell was studied by using mast cell-deficient Kit (W-sh) “Sash” mice and MrgprB2^−/− mice. The mast cells were observed by toluidine blue staining. In vitro, the effects of WSS on MrgprB2 were studied by calcium imaging; The whole-cell patch clamp method recorded the MrgprB2 mediate voltage-dependent currents in mast cells.

Results

The content of rutin (0.012%) and hyperin (0.014%) in the WSS were determined. WSS could ameliorate the pruritus induced by Oxazolone (inhibition was 41.19%, p = 0.004) and compound 48/80 (inhibition was 50.29%, p = 0.001). Meanwhile, WSS could reduce the number of mast cells in mice skin tissue with allergic contact dermatitis (ACD) (p = 0.002) or compound 48/80 (p = 0.013). In addition, WSS could inhibit the calcium influx (1 mg/mL: p = 0.001, 3 mg/mL: p < 0.0001) and the voltage-dependent currents induced by activation of MrgprB2 on mast cell. WSS also attenuated the calcium influx induced by compound 48/80 in HEK293 cells overexpressing MrgprB2/X2.

Conclusion

These results showed that WSS could ameliorate pruritus by inhibiting MrgprB2 receptor on mast cells.

In vitro models for investigating itch

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

Inhibitory effects of antibiotic-induced gut microbiota depletion on acute itch behavior in mice

BACKGROUND

The gut microbiota is known to be associated with the regulation of many neurological diseases and behaviors, including chronic pain. However, it is unclear whether the gut microbiota is critical to the itch sensation. In this study, we investigated the effects of gut microbiota depletion on acute itch.

METHODS

First, an antibiotic cocktail was orally administered to deplete the gut microbiota in male C57BL/6 mice. Then, pruritogens were intradermally injected to induce acute itch behavior. In addition, antibiotic-treated mice received transplantation of fecal microbiota from untreated mice, followed by tests for acute itch. The changes in c-Fos expression in trigeminal ganglia (TG) neurons were also investigated by immunofluorescence staining.

RESULTS

Our results indicated that chronic antibiotic treatment significantly reduced the diversity and richness of the gut microbiota of mice. Compared to vehicle-treated mice, antibiotic-treated mice showed reductions in acute itch behavior induced by compound 48/80, chloroquine (CQ), and serotonin (5-HT), respectively. Moreover, repositioning of microbiota reversed the reductions in acute itch behavior in antibiotic-treated mice. In addition, immunofluorescence staining revealed that antibiotic-treated mice displayed decreased c-Fos expression in ipsilateral TG compared to controls.

CONCLUSIONS

Our study, for the first time, discovered that antibiotic-induced gut microbiota depletion could reduce acute itch behavior, which may be connected with decreased TG neuronal activity.