MrgprX2 regulates mast cell degranulation through PI3K/AKT and PLCγ signaling in pseudo-allergic reactions

Flow cytometry-based basophil and mast cell activation tests in allergology: State of the art

The major challenge in allergy diagnosis is development of accessible and reliable diagnostics that can predict the clinical outcome following exposure to culprit allergen(s) or cross-reactive molecules and identification of safer alternatives than the current state-of-the-art methods. There is accumulating evidence that flow-based analyses for the quantification of activated basophils and mast cells subsequent to in vitro challenge (the basophil and mast cell activation test [BAT/MAT] or basophil activation test [BAT] and mast cell activation test [MAT]) could meet the diagnostic requirements for IgE-dependent allergies, drug hypersensitivities, and subsets of autoimmune urticaria. Furthermore, the BAT and MAT have found application in research and other nondiagnostic fields. However, appropriate use of the BAT and MAT requires understanding of the diversity of the source materials used and degranulation metrics to ensure correct test performance and interpretation of results. In this review, we provide the main applications and limitations of the BAT and MAT, as performed thus far.

Ursolic acid attenuates pseudo-allergic reactions via reducing MRGPRX2-mediated mast cell degranulation

Mas-related G protein-coupled receptor X2 (MRGPRX2) is a newly identified receptor on mast cells that contribute to IgE-independent pseudo-allergy. Ursolic acid (UA), a pentacyclic triterpenoid, has been reported for its anti-allergy effects. However, the protective mechanism against pseudo-allergic reactions remains unclear. This study aims to investigate the effects of UA on pseudo-allergic reactions both in vivo and in vitro, focusing on the therapeutical mechanism underlying its effect on mast cells. In present study, UA reduced degranulation and chemokines production induced by MRGPRX2 agonists, including compound 48/80 (C48/80) and substance P (SP), in LAD2 cells in vitro. UA also alleviated C48/80 and SP-induced systemic anaphylaxis and passive cutaneous anaphylaxis (PCA) in vivo. Furthermore, UA demonstrated strong binding affinity to the MRGPRX2 protein, leading to a decrease in calcium influx in both LAD2 cells and MRGPRX2-HEK293 cells stimulated with C48/80 and SP. Moreover, UA effectively suppressed phosphorylation levels within phospholipase C-γ (PLCγ) pathway and nuclear factor kappa-B (NF-κB) pathway of MRGPRX2 downstream proteins. Our findings indicated that UA exerts an attenuating effect in pseudo-allergic reactions by suppressing MRGPRX2-mediated mast cell activation, targeting PLCγ pathway and NF-κB pathway. These results suggest that UA may serve as a promising therapeutic agent for MRGPRX2-dependent pseudo-allergic reactions.

Amphotericin B for injection triggers degranulation of human LAD2 mast cells by MRGPRX2 and pseudo-allergic reactions in mice via MRGPRB2 activation

Amphotericin B, a polyene macrolide antifungal agent, still plays an important role in the management of serious systemic fungal infections. Amphotericin B deoxycholate (AmBd) has been used to treat invasive fungal infections for over 60 years and remains the primary clinical formulation currently available. Anaphylactoid reactions triggered by AmBd in the clinic have been documented. However, the molecular and cellular events contributing to these reactions have not been clearly elucidated to date. This study demonstrates that the human Mas-related G protein-coupled receptor X2 (MRGPRX2) is the receptor that mediates these anaphylactoid responses. Molecular docking and cellular thermal shift assay (CETSA) indicate that AmBd exhibits potential affinity with MRGPRX2. In vitro, exposure to AmBd results in significant release of LAD2 mast cell granules and induces intracellular Ca2+ mobilization as well as activation of PLC-γ/IP3R and PI3K/AKT signaling pathways. However, these phenomena are reduced in MRGPRX2-knockdown LAD2 cells. In vivo, AmBd triggers paw swelling and a rapid drop in core body temperature in wild-type (WT) mice. However, these reactions are almost absent in MRGPRB2 (the mouse homolog of MRGPRX2) knockout mice (MRGPRB2MUT, MUT). The above results suggest that AmBd activates PLC-γ/IP3R and PI3K/AKT signaling via MRGPRX2 (in human LAD2 mast cells) or MRGPRB2 (in mice), leading to the release of mast cell granules and subsequent triggering of pseudo-allergic reactions. Taken together, this study clarifies the role of MRGPRX2 in triggering pseudo-allergic reactions to AmBd and suggests that MRGPRX2 could be a potential therapeutic target for controlling these reactions.

Mast cell signaling and its role in urticaria

Chronic urticaria is a mast cell (MC)-driven disease characterized by the development of itching wheals and/or angioedema. In the last decades, outstanding progress has been made in defining the mechanisms involved in MC activation, and novel activating and inhibitory receptors expressed in MC surface were identified and characterized. Besides an IgE-mediated activation through high-affinity IgE receptor cross-linking, other activating receptors, including Mas-related G-protein-coupled receptor-X2, C5a receptor, and protease-activated receptors 1 and 2 are responsible for MC activation. This would partly explain the reason some subgroups of chronic spontaneous urticaria (CSU), the most frequent form of urticaria in the general population, do not respond to IgE target therapies, requiring other therapeutic approaches for improving the management of the disease. In this review, we shed some light on the current knowledge of the immunologic and nonimmunologic mechanisms regulating MC activation in CSU, considering the complex inflammatory scenario underlying CSU pathogenesis, and novel potential MC-targeted therapies, including surface receptors and cytoplasmic signaling proteins.

Oxytocin Attenuates Sympathetic Innervation with Inhibition of Cardiac Mast Cell Degranulation in Rats after Myocardial Infarction

Sympathetic hyperinnervation is the leading cause of fatal ventricular arrhythmia (VA) after myocardial infarction (MI). Cardiac mast cells cause arrhythmias directly through degranulation. However, the role and mechanism of mast cell degranulation in sympathetic remodeling remain unknown. We investigated the role of oxytocin (OT) in stabilizing cardiac mast cells and improving sympathetic innervation in rats. MI was induced by coronary artery ligation. Western blotting, immunofluorescence, and toluidine staining of mast cells were performed to determine the expression and location of target protein. Mast cells accumulated significantly in peri-infarcted tissues and were present in a degranulated state. They expressed OT receptor (OTR), and OT infusion reduced the number of degranulated cardiac mast cells post-MI. Sympathetic hyperinnervation was attenuated as assessed by immunofluorescence for tyrosine hydroxylase (TH). Seven days post-MI, the arrhythmia score of programmed electrical stimulation was higher in vehicle-treated rats with MI than in rats treated with OT. An in vitro study showed that OT stabilized mast cells via the phosphoinositide 3-kinase/protein kinase B (PI3K/Akt) signaling pathway. Further in vivo studies on OTR-deficient mice showed worsening mast cell degranulation and worsening sympathetic innervation. OT pretreatment inhibited cardiac mast cell degranulation post-MI and prevented sympathetic hyperinnervation, along with mast cell stabilization via the PI3K/Akt pathway. SIGNIFICANCE STATEMENT: This is the first study to elucidate the role and mechanism of oxytocin (OT) in inflammatory-sympathetic communication mediated sympathetic hyperinnervation after myocardial infarction (MI), providing new approaches to prevent fatal arrhythmias.

Knowledge mapping and global trends of drug hypersensitivity from 2013 to 2023: A bibliometric analysis

BACKGROUND

Drug hypersensitivity is a major global public health issue with a significant increase in prevalence in populations. Here, we provide a deep insight into the frontier hotspot and future direction in the field of drug hypersensitivity.

METHODS

A knowledge map is portrayed based on publications related to drug hypersensitivity from Web of Science Core Collection using CiteSpace. Co-occurrence relationships of countries, institutes, authors, journals, references, and keywords are constructed. According to the co-occurrence relationships, hotspots and future trends are overviewed.

RESULTS

The United States ranked first in the world and China with the second highest publications was the only developing country. Torres, Mayorga, and Blanca were highly productive authors. Harvard University was the institution with the most research publications. Keywords co-occurrence analysis suggested applications in emerging causes, potential mechanisms, and clinical diagnosis as the research hotspots and development frontiers.

CONCLUSION

Research on drug hypersensitivity is in a rapid development stage and an emerging trend in reports of anaphylaxis to polyethylene glycols is identified. Developing algorithms for understanding the standardization process of culprit drugs, clinical manifestations, and diagnostic methods will be the focus of future direction. In addition, a better understanding of the mechanisms to culprit drugs with immunological precise phenotypic definitions and high-throughput platforms is needed.

Comparative analyses of various IgE-mediated and non-IgE-mediated inducers of mast cell degranulation for in vitro study

In vitro investigations of mast cell (MC) degranulation are essential for studying many diseases, particularly allergy and urticaria. Many MC-degranulation inducers are currently available. However, there is no previous systematic comparative analysis of these available inducers in term of their efficacies to induce MC degranulation. Herein, we performed systematic comparisons of efficacies of five well-known and commonly used MC-degranulation inducers. RBL-2H3 cells were sensitized with 50 ng/ml anti-DNP IgE or biotinylated IgE followed by stimulation with 100 ng/ml DNP-BSA or streptavidin, respectively. For non-IgE-mediated inducers, the cells were treated with 5 µg/ml substance P, compound 48/80, or A23187. At 15-, 30-, 45- and 60-min post-induction, several common MC-degranulation markers (including intracellular [Ca2+], β-hexosaminidase release, tryptase expression by immunofluorescence staining, cellular tryptase level by immunoblotting, secretory tryptase level by immunoblotting, CD63 expression by immunofluorescence staining, and CD63 expression by flow cytometry) were evaluated. The data showed that all these markers significantly increased after activation by all inducers. Among them, A23187 provided the greatest degrees of increases in intracellular [Ca2+] and β-hexosaminidase release at all time-points and upregulation of CD63 at one time-point. These data indicate that all these IgE-mediated (anti-DNP IgE/DNP-BSA and biotinylated IgE/streptavidin) and non-IgE-mediated (substance P, compound 48/80, and A23187) inducers effectively induce MC degranulation, while A23187 seems to be the most effective inducer for MC degranulation.

Beyond Allergies-Updates on The Role of Mas-Related G-Protein-Coupled Receptor X2 in Chronic Urticaria and Atopic Dermatitis

Mast cells (MCs) are an important part of the immune system, responding both to pathogens and toxins, but they also play an important role in allergic diseases, where recent data show that non-IgE-mediated activation is also of relevance, especially in chronic urticaria (CU) and atopic dermatitis (AD). Skin MCs express Mas-related G-protein-coupled receptor X2 (MRGPRX2), a key protein in non-IgE-dependent MC degranulation, and its overactivity is one of the triggering factors for the above-mentioned diseases, making MRGPRX2 a potential therapeutic target. Reviewing the latest literature revealed our need to focus on the discovery of MRGPRX2 activators as well as the ongoing vast research towards finding specific MRGPRX2 inhibitors for potential therapeutic approaches. Most of these studies are in their preliminary stages, with one drug currently being investigated in a clinical trial. Future studies and improved model systems are needed to verify whether any of these inhibitors may have the potential to be the next therapeutic treatment for CU, AD, and other pseudo-allergic reactions.

A novel method for evaluating pseudoallergy based on β-hexosaminidase and its application for traditional Chinese medicine injections

Pseudoallergy is a typical and common adverse drug reaction to injections, especially in traditional Chinese medicine injections (TCMIs). At present, the evaluation methods for pseudoallergy include cell methods in vitro and animal methods in vivo. The mast cell evaluation method based on the β-hexosaminidase (β-Hex)-catalyzed substrate, 4-nitrophenyl-β-N-acetyl-D-glucosaminide (4-NPG), is an important method for the evaluation of drug-induced pseudoallergy, but it is prone to false positive results and has insufficient sensitivity. In this study, a novel β-Hex evaluation system with rat basophilic leukemia-2H3 cells based on high-performance liquid chromatography-fluorescence detection (HPLC-FLD) was established, which effectively increased the sensitivity and avoided false positive results. Cell viabilities were measured by 3-(4,5-dimethyl-2-thiazolyl)-2,5-diphenyl tetrazolium bromide assay. In addition, a method for the determination of histamine, which is another indicator in the development of pseudoallergy, was established to validate the above method. The results of this novel method indicated that two TCMIs (Shuxuening injection and Shenqi Fuzheng injection), which were considered to be pseudoallergenic using 4-NPG, were not pseudoallergenic. Overall, the novel β-Hex/HPLC-FLD evaluation system using Rat basophilic leukemia-2H3 cells established was effective and precise. It could be used for the evaluation of pseudoallergic reactions caused by TCMIs and other injections.

Silibinin attenuated pseudo-allergic reactions and mast cell degranulation via PLCγ and PI3K/Akt signaling pathway

Anaphylaxis is a type of potentially fatal hypersensitivity reaction resulting from the activation of mast cells. Many endogenous or exogenous factors could cause this reaction. Silibinin is the main chemical component of silymarin and has been reported to have pharmacological activities. However, the anti-allergic reaction effect of silibinin has not yet been investigated. This study aimed to evaluate the effect of silibinin to attenuate pseudo-allergic reactions in vivo and to investigate the underlying mechanism in vitro. In this study, calcium imaging was used to assess Ca2+ mobilization. The levels of cytokines and chemokines, released by stimulated mast cells, were measured using enzyme immunoassay kits. The activity of silibinin was evaluated in a mouse model of passive cutaneous anaphylaxis (PCA). Western blotting was used to explore the related molecular signaling pathways. In results, silibinin markedly inhibited mast cell degranulation, calcium mobilization, and preventing the release of cytokines and chemokines in a dose-dependent manner via the PLCγ and PI3K/Akt signaling pathway. Silibinin also attenuated PCA in a dose-dependent manner. In summary, silibinin has an anti-pseudo-allergic pharmacological activity, which makes it a potential candidate for the development of a novel agent to arrest pseudo-allergic reactions.

Sophorolipid inhibits histamine-induced itch by decreasing PLC/IP3R signaling pathway activation and modulating TRPV1 activity

Biosurfactants are attracting much interest due to their potential application as therapeutic agents in the medical and cosmetic field. Previous studies have demonstrated that biosurfactant such as sophorolipid (SL) exhibits immunomodulatory effects. In this article, we found the potential of sophorolipid for inhibiting histamine-induced itch and preliminarily explored its molecular basis. First, behavioral tests indicated that SL can remit the histamine-induced scratching behaviors of mice. Second, SL can suppress the the calcium influx induced by histamine, HTMT and VUF8430 in HaCaT cells. RT-PCR analysis showed that the histamine-induced upregulation of mRNA levels of phospholipase Cγ1, 1,4,5-trisphosphate receptor (IP3R), and protein kinase Cα can be inhibted by SL, suggesting that SL may impede the PLC/IP3R signaling pathway activated by histamine. In further tests, the capsaicin-induced calcium influx can also be inhibited by SL. The immunofluorescence and molecular docking analysis indicated that SL acts as an inhibitor of transient receptor potential vanilloid-1 (TRPV1) activation to decrease calcium influx against stimuli. In summary, these results revealed that SL may inhibit histamine-induced itch by decreasing PLC/IP3R signaling pathway activation and modulating TRPV1 activity. This paper indicates that SL can be a useful treatment for histamine-dependent itch.

Liquiritin inhibits MRGPRX2-mediated pseudo-allergy through the PI3K/AKT and PLCγ signaling pathways

Liquiritin is a natural flavone with a variety of pharmacological effects derived from the medicinal food homology plant Glycyrrhiza uralensis Fisch. As a kind of lethal allergic reactions, pseudo-allergic reactions (PARs) arise from the Mas-related G protein coupled receptor X2 (MRGPRX2)-triggered fast degranulation of mast cells (MCs). In the current work, the anti-pseudo-allergy action and potential mechanisms of liquiritin were explored in vivo and in vitro. Liquiritin suppressed the calcium influx and degranulation elicited by Compound 48/80 (C48/80) in mouse peritoneal mast cells (MPMCs). In mice, liquiritin also inhibited the C48/80-elicited hind paw extravasation, as well as the elevations in TNF-α and histamine levels. Molecular docking combined with detection of HEK293T cells expressing human MRGPRX2 showed that liquiritin was a potential MRGPRX2 antagonist and inhibited PARs through the PI3K/AKT and PLCγ signaling pathways downstream of MRGPRX2. The present work opens a new avenue for the PARs management.

Inhibition of Mast Cell Degranulation in Atopic Dermatitis by Celastrol through Suppressing MRGPRX2

Background

Atopic dermatitis is a common dermatological disease, and mast cell degranulation is believed to be related with the progression of atopic dermatitis. Mas-related G protein-coupled receptor-X2 (MRGPRX2), and calcium release-activated calcium channel protein 1-2 (ORAI-1, ORAI-2) are involved in mast cell degranulation. Celastrol is an active monomer of Tripterygium wilfordii, and it presents an antiatopic role.

Methods

2,4-Dinitrofluorobenzene (DNFB) and compound 48/80 (C 48/80) were used to establish a slow and acute scratching animal model, respectively. Hematoxylin-eosin and toluidine blue staining was used to investigate tissue injury. Inflammatory factor concentration was measured with ELISA. The expression of MRGPRX2, ORAI-1, and ORAI-2 was detected with immunohistochemistry (IHC) staining. Gene expression profiling and microRNA array were performed to investigate gene differential expression.

Results

Celastrol greatly inhibited atopic dermatitis-related tissues injury, mast cell production, histamine release, scratching level, inflammatory factor expression, and activation of MRGPRX2/ORAI axis in the DNFB-induced atopic dermatitis model. The influence of Celastrol on atopic dermatitis was remarkably reversed by overexpression of MRGPRX2.

Conclusion

We found that the improvements of atopic dermatitis caused by Celastrol were reversed by treatment with MRGPRX2^OE, indicating that Celastrol might affect atopic dermatitis through MRGPRX2. This study might provide a novel thought for the prevention and treatment of atopic dermatitis by regulating MRGPRX2.

Mas-related G protein-coupled receptor MRGPRX2 in human basophils: Expression and functional studies

Background

Occupancy of MRGPRX2 heralds a new era in our understandings of immediate drug hypersensitivity reactions (IDHRs), but a constitutive expression of this receptor by basophils is debated.

Objective

To explore the expression and functionality of MRGPRX2 in and on basophils.

Methods

Basophils from patients with birch pollen allergy, IDHRs to moxifloxacin, and healthy controls were studied in different conditions, that is, in rest, after stimulation with anti-IgE, recombinant major birch pollen allergen (rBet v 1), moxifloxacin, fMLP, substance P (SP), or other potential basophil secretagogues. In a separate set of experiments, basophils were studied after purification and resuspension in different media.

Results

Resting whole blood basophils barely express MRGPRX2 on their surface and are unresponsive to SP or moxifloxacin. However, surface MRGPRX2 is quickly upregulated upon incubation with anti-IgE or fMLP. Pre-stimulation with anti-IgE can induce a synergic effect on basophil degranulation in IgE-responsive subjects after incubation with SP or moxifloxacin, provided that basophils have been obtained from patients who experienced an IDHR to moxifloxacin. Cell purification can trigger a "spontaneous" and functional upregulation of MRGPRX2 on basophils, not seen in whole blood cells, and its surface density can be influenced by distinct culture media.

Conclusion

Basophils barely express MRGPRX2 in resting conditions. However, the receptor can be quickly upregulated after stimulation with anti-IgE, fMLP, or after purification, making cells responsive to MRGPRX2 occupation. We anticipate that such "conditioned" basophils constitute a model to explore MRGPRX2 agonism or antagonism, including IDHRs originating from the occupation of this receptor.

A Novel Mast Cell Stabilizer JM25-1 Rehabilitates Impaired Gut Barrier by Targeting the Corticotropin-Releasing Hormone Receptors

Mast cell (MC) plays a central role in intestinal permeability; however, few MC-targeting drugs are currently available for protection of the intestinal barrier in clinical practice. A nonfluorinated Lidocaine analog 2-diethylamino-N-2,5-dimethylphenyl acetamide (JM25-1) displays anti-allergic effect, but its impact on MC remains elusive. In this study, we explored whether JM25-1 has therapeutic potential on intestinal barrier defect through stabilizing MC. JM25-1 alleviated release of β-hexosaminidase and cytokine production of MC. The paracellular permeability was redressed by JM25-1 in intestinal epithelial cell monolayers co-cultured with activated MC. In vivo, JM25-1 diminished intestinal mucosal MC amount and cytokine production, especially downregulating the expression of CRHR1, accompanied by an increase of CRHR2. Protective effects appeared in JM25-1-treated stress rats with a recovery of weight and intestinal barrier integrity. Through network pharmacology analysis, JM25-1 showed a therapeutic possibility for irritable bowel syndrome (IBS) with predictive targeting on PI3K/AKT/mTOR signaling. As expected, JM25-1 reinforced p-PI3K, p-AKT, p-mTOR signaling in MC, while the mTOR inhibitor Rapamycin reversed the action of JM25-1 on the expression of CRHR1 and CRHR2. Moreover, JM25-1 successfully remedied intestinal defect and declined MC and CRHR1 expression in rat colon caused by colonic mucus of IBS patients. Our data implied that JM25-1 possessed therapeutic capacity against intestinal barrier defects by targeting the CRH receptors of MC through PI3K/AKT/mTOR signaling.

Novel small molecule MRGPRX2 antagonists inhibit a murine model of allergic reaction

BACKGROUND

Activation of Mas-related G protein-coupled receptor X2 (MRGPRX2) is a crucial non-IgE pathway for mast cell activation associated with allergic reactions and inflammation. Only a few peptides and small compounds targeting MRGPRX2 have been reported, with limited information on their pharmacologic activity.

OBJECTIVE

We sought to develop novel small molecule MRGPRX2 antagonists to treat MRGPRX2-mediated allergies and inflammation.

METHODS

A computational approach was used to design novel small molecules as MRGPRX2 antagonists. The short-listed molecules were synthesized and characterized by liquid chromatography and mass spectrometry as well as nuclear magnetic resonance. Inhibitory activity on MRGPRX2 signaling was assessed in vitro by using functional bioassays (β-hexosaminidase, calcium flux, and chemokine synthesis) and receptor activation assays (β-arrestin recruitment and Western blot analysis) in human LAD-2 mast cells and HTLA cells. In vivo effects of the novel MRGPRX2 antagonists were assessed using a mouse model of acute allergy and systemic anaphylaxis.

RESULTS

The novel small molecules demonstrated higher binding affinity with MRGPRX2 in the docking study. The half-maximal inhibitory concentration is in the low micromolar range (5-21 μM). The small molecules inhibited not only the early phase of mast cell activation but also the late phase, associated with chemokine and prostaglandin release. Further, Western blot analysis revealed inhibition of downstream phospholipase C-γ, extracellular signal-regulated protein kinase 1/2, and Akt signaling pathway. Moreover, in the mouse models of allergies, small molecule administration effectively blocks acute, systemic allergic reactions and inflammation and prevents systemic anaphylaxis.

CONCLUSION

The small molecules might hold a significant therapeutic promise to treat MRGPRX2-mediated allergies and inflammation.

Hypersensitivity reactions to small molecule drugs

Drug hypersensitivity reactions induced by small molecule drugs encompass a broad spectrum of adverse drug reactions with heterogeneous clinical presentations and mechanisms. These reactions are classified into allergic drug hypersensitivity reactions and non-allergic drug hypersensitivity reactions. At present, the hapten theory, pharmacological interaction with immune receptors (p-i) concept, altered peptide repertoire model, and altered T-cell receptor (TCR) repertoire model have been proposed to explain how small molecule drugs or their metabolites induce allergic drug hypersensitivity reactions. Meanwhile, direct activation of mast cells, provoking the complement system, stimulating or inhibiting inflammatory reaction-related enzymes, accumulating bradykinin, and/or triggering vascular hyperpermeability are considered as the main factors causing non-allergic drug hypersensitivity reactions. To date, many investigations have been performed to explore the underlying mechanisms involved in drug hypersensitivity reactions and to search for predictive and preventive methods in both clinical and non-clinical trials. However, validated methods for predicting and diagnosing hypersensitivity reactions to small molecule drugs and deeper insight into the relevant underlying mechanisms are still limited.

P2X4 receptor stimulation enhances MrgprB2-mediated mast cell activation and pseudoallergic reactions in mice

Pseudoallergies caused by drugs make disease treatment difficult. Mas-relate G protein-coupled receptor X2 (MRGPRX2), which is specifically expressed in mast cells (MCs), has been implicated in pseudoallergies. High concentrations of therapeutic agents are typically required to stimulate MRGPRX2. Although regulatory mechanisms may enhance this response, the factors involved in this regulation are not well-understood. In this study, the effects of extracellular ATP on MC activation induced by MrgprB2, the mouse ortholog of human MRGPRX2, were examined in mouse peritoneal MCs (PMCs). ATP alone induced minimal PMC degranulation but markedly enhanced degranulation induced by the MrgprB2 agonist compound 48/80 (CP48/80), substance P, PAMP-12, and vancomycin. ATP promoted CP48/80-induced increase in intracellular Ca²⁺ in PMCs. This enhancement effect of ATP was absent in PMCs prepared from P2X4 receptor (P2X4R)-deficient mice and inhibited by the PI3K inhibitor wortmannin. In addition, P2X4R deficiency reduced the skin-specific and systemic anaphylactic responses to CP48/80 in vivo. In MC-deficient Kit^W-sh/W-sh mice, reconstitution with MCs obtained from wild-type mice led to a more severe anaphylactic response to CP48/80 compared to that from P2X4R-deficient mice. P2X4R-mediated effect may be involved in MrgprB2-mediated MC activation in vivo and is a potential target for alleviating pseudoallergic reactions.

β-arrestin-1 and β-arrestin-2 Restrain MRGPRX2-Triggered Degranulation and ERK1/2 Activation in Human Skin Mast Cells

As a novel receptor that efficiently elicits degranulation upon binding to one of its numerous ligands, MRGPRX2 has moved to the center of attention in mast cell (MC) research. Indeed, MRGPRX2 is believed to be a major component of pseudo-allergic reactions to drugs and of neuropeptide-elicited MC activation in skin diseases alike. MRGPRX2 signals via G proteins which organize downstream events ultimately leading to granule discharge. Skin MCs require both PI3K and ERK1/2 cascades for efficient exocytosis. β-arrestins act as opponents of G proteins and lead to signal termination with or without subsequent internalization. We recently demonstrated that ligand-induced internalization of MRGPRX2 requires the action of β-arrestin-1, but not of β-arrestin-2. Here, by using RNA interference, we find that both isoforms counter skin MC degranulation elicited by three MRGPRX2 agonists but not by FcεRI-aggregation. Analyzing whether this occurs through MRGPRX2 stabilization under β-arrestin attenuation, we find that reduction of β-arrestin-1 indeed leads to increased MRGPRX2 abundance, while this is not observed for β-arrestin-2. This led us speculate that β-arrestin-2 is involved in signal termination without cellular uptake of MRGPRX2. This was indeed found to be the case, whereby interference with β-arrestin-2 has an even stronger positive effect on ERK1/2 phosphorylation compared to β-arrestin-1 perturbation. Neither β-arrestin-1 nor β-arrestin-2 had an impact on AKT phosphorylation nor affected signaling via the canonical FcεRI-dependent route. We conclude that in skin MCs, β-arrestin-2 is chiefly involved in signal termination, whereas β-arrestin-1 exerts its effects by controlling MRGPRX2 abundance.

MRGPRX2-Mediated Degranulation of Human Skin Mast Cells Requires the Operation of Gαi, Gαq, Ca++ Channels, ERK1/2 and PI3K—Interconnection between Early and Late Signaling

The recent discovery of MRGPRX2 explains mast cell (MC)-dependent symptoms independently of FcεRI-activation. Because of its novelty, signaling cascades triggered by MRGPRX2 are rudimentarily understood, especially in cutaneous MCs, by which MRGPRX2 is chiefly expressed. Here, MCs purified from human skin were used following preculture or ex vivo and stimulated by FcεRI-aggregation or MRGPRX2 agonists (compound 48/80, Substance P) in the presence/absence of inhibitors. Degranulation was assessed by β-hexosaminidase or histamine release. Phosphorylation events were studied by immunoblotting. As a G protein-coupled receptor, MRGPRX2 signals by activating G proteins; however, their nature has remained controversial. In skin MCs, G_αi and G_αq were required for degranulation, but G_αi was clearly more relevant. Ca++ channels were likewise crucial. Downstream, PI3K was essential for granule discharge initiated by MRGPRX2 or FcεRI. ERK1/2 and JNK were additional participants, especially in the allergic route. Addressing possible points of intersection between early and later events, pERK1/2 and pAKT were found to depend on G_αi, further highlighting its significance. G_αq and Ca++ channels made some contributions to the phosphorylation of ERK. Ca++ differentially affected PI3K activation in FcεRI- vis-à-vis MRGPRX2-signaling, as channel inhibition increased pAKT only when triggered via FcεRI. Collectively, our study significantly extends our understanding of the molecular framework behind granule secretion from skin MCs.