Store-Operated Calcium Entry via STIM1 Contributes to MRGPRX2 Induced Mast Cell Functions

MRGPRX2 antagonist GE1111 attenuated DNFB-induced atopic dermatitis in mice by reducing inflammatory cytokines and restoring skin integrity

Introduction

Atopic dermatitis (AD) is a chronic inflammatory skin disorder characterised by itching, erythema, and epidermal barrier dysfunction. The pathogenesis of AD is complex and multifactorial; however,mast cell (MC) activation has been reported to be one of the crucial mechanisms in the pathogenesis of AD. The MC receptor Mas related G protein-coupled receptor-X2 (MRGPRX2) has been identified as a prominent alternative receptor to the IgE receptor in causing MC activation and the subsequent release of inflammatory mediators. The current study aimed to evaluate the therapeutic effect of a novel small molecule MRGPRX2 antagonist GE1111 in AD using in vitro and in vivo approaches.

Methods

We developed an in vitro cell culture disease model by using LAD-2 MC, HaCaT keratinocytes and RAW 264.7 macrophage cell lines. We challenged keratinocytes and macrophage cells with CST-14 treated MC supernatant in the presence and absence of GE1111 and measured the expression of tight junction protein claudin 1, inflammatory cytokines and macrophage phagocytosis activity through immunohistochemistry, western blotting, RT-qPCR and fluorescence imaging techniques. In addition to this, we developed a DFNB-induced AD model in mice and evaluated the protective effect and underlying mechanism of GE1111.

Results and Discussion

Our in vitro findings demonstrated a potential therapeutic effect of GE1111, which inhibits the expression of TSLP, IL-13, MCP-1, TNF-a, and IL-1ß in MC and keratinocytes. In addition to this, GE1111 was able to preserve the expression of claudin 1 in keratinocytes and the phagocytotic activity of macrophage cells. The in vivo results demonstrated that GE1111 treatment significantly reduced phenotypic changes associated with AD (skin thickening, scaling, erythema and epidermal thickness). Furthermore, immunohistochemical analysis demonstrated that GE1111 treatment preserved the expression of the tight junction protein Involucrin and reduced the expression of the inflammatory mediator periostin in the mouse model of AD. These findings were supported by gene and protein expression analysis, where GE1111 treatment reduced the expression of TSLP, IL-13, and IL-1ß, as well as downstream signalling pathways of MRGPRX2 in AD skin lesions. In conclusion, our findings provide compelling in vitro and in vivo evidence supporting the contribution of MRGPRX2-MC interaction with keratinocytes and macrophages in the pathogenesis of AD.

MRGPRB2/X2 and the analogous effects of its agonist and antagonist in DSS-induced colitis in mice

The mast cell receptor Mrgprb2, a mouse orthologue of human Mrgprx2, is known as an inflammatory receptor and its elevated expression is associated with various diseases such as ulcerative colitis. We aimed to elucidate the role of Mrgprb2/x2 and the effect of its ligands on a chemically induced murine colitis model. We showed that in Mrgprb2-/- mice, there is a differential regulation of cytokine releases in the blood plasma and severe colonic damages after DSS treatment. Unexpectedly, we demonstrated that known Mrgprb2/x2 agonists (peptide P17, P17 analogues and CST-14) and antagonist (GE1111) similarly increased the survival rate of WT mice subjected to 4% DSS-induced colitis, ameliorated the colonic damages of 2.5% DSS-induced colitis, restored major protein mRNA expression involved in colon integrity, reduced CD68+ and F4/80+ immune cell infiltration and restored cytokine levels. Collectively, our findings highlight the eminent role of Mrpgrb2/x2 in conferring a beneficial effect in the colitis model, and this significance is demonstrated by the heightened severity of colitis with altered cytokine releases and inflammatory immune cell infiltration observed in the Mrgprb2 knockout mice. Elevated expression of Mrgprb2 in WT colitis murine models may represent the organism's adaptive protective mechanism since Mrgprb2 knockout results in severe colitis. On the other hand, both agonist and antagonist of Mrgprb2 analogously mitigated the severity of colitis in DSS-induced colitis model by altering Mrgprb2 expression, immune cell infiltration and inflammatory cytokine releases.

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.

Bergapten inhibits airway inflammation and MRGPRX2-mediated mast cells activation by targeting NR4A1

Asthma is a serious global health problem affecting 300 million persons around the world. Mast cells (MCs) play a major role in airway hyperresponsiveness (AHR) and inflammation in asthma, their exact effector mechanisms remain unclear. Here, we aim to investigate the inhibitory effect of Bergapten (BER) on MRGPRX2-mediated MCs activation through asthma model. Mouse model of asthma was established to examine the anti-asthmatic effects of BER. Calcium (Ca2+) influx, β-hexosaminidase and histamine release were used to assess MCs degranulation in vitro. RNA-Seq technique was conducted to study the gene expression profile. RT-PCR and Western Blotting were performed to examine targeting molecules expression. BER inhibited AHR, inflammation, mucous secretion, collagen deposition and lung MCs activation in asthma model. BER dramatically reduced levels of IL4, IL-5, and IL-13 in bronchoalveolar lavage fluid (BALF), as well as inflammatory cells. BER also reduced serum IgE levels. Pretreatment MCs with BER inhibited substance P (SP)-induced Ca2+ influx, degranulation and cytokines release from MCs. BER also reduced the phosphorylation levels of PKC, PLC, IP3R, AKT and ERK, which were induced by SP. Furthermore, RNA-seq analysis showed that SP up-regulated 68 genes in MCs, while were reversed by BER. Among these 68 genes, SP up-regulated NR4A1 expression, and this effect was inhibited by BER. Meanwhile, knockdown of NR4A1 significantly attenuated SP-induced MCs degranulation. In conclusion, NR4A1 plays a major role in MRGPRX2-mediated MCs activation, BER inhibited AHR and inflammation in asthmatic model by inhibiting MCs activation through MRGPRX2-NR4A1 pathway.

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.

Synovial microenvironment-influenced mast cells promote the progression of rheumatoid arthritis

Mast cells are phenotypically and functionally heterogeneous, and their state is possibly controlled by local microenvironment. Therefore, specific analyses are needed to understand whether mast cells function as powerful participants or dispensable bystanders in specific diseases. Here, we show that degranulation of mast cells in inflammatory synovial tissues of patients with rheumatoid arthritis (RA) is induced via MAS-related G protein-coupled receptor X2 (MRGPRX2), and the expression of MHC class II and costimulatory molecules on mast cells are upregulated. Collagen-induced arthritis mice treated with a combination of anti-IL-17A and cromolyn sodium, a mast cell membrane stabilizer, show significantly reduced clinical severity and decreased bone erosion. The findings of the present study suggest that synovial microenvironment-influenced mast cells contribute to disease progression and may provide a further mast cell-targeting therapy for RA.

Secreted phosphoprotein 1 regulates natural compound 3',4',5,7-tetrahydroxyflavone to inhibit mast cell-mediated allergic inflammation

BACKGROUND

Mast cells (MCs) are important effector cells in anaphylaxis and anaphylactic disease. 3',4',5,7-tetrahydroxyflavone (THF) presents in many medicinal plants and exerts a variety of pharmacological effects. In this study, we evaluated the effect of THF on C48/80-induced anaphylaxis and the mechanisms underlying its effects, including the role of secreted phosphoprotein 1 (SPP1), which has not been reported to IgE-independent MC activation.

RESULTS

THF inhibited C48/80-induced Ca2+ flow and degranulation via the PLCγ/PKC/IP3 pathway in vitro. RNA-seq showed that THF inhibited the expression of SPP1 and downstream molecules. SPP1 is involved in pseudo-anaphylaxis reactions. Silencing SPP1 affects the phosphorylation of AKT and P38. THF suppressed C48/80-induced paw edema, hypothermia and serum histamine, and chemokines release in vivo.

CONCLUSIONS

Our results validated SPP1 is involved in IgE-independent MC activation anaphylactoid reactions. THF inhibited C48/80-mediated anaphylactoid reactions both in vivo and in vitro, suppressed calcium mobilization and inhibited SPP1-related pathways.

Activation of ryanodine-sensitive calcium store drives pseudo-allergic dermatitis via Mas-related G protein-coupled receptor X2 in mast cells

Mast cell (MC) activation is implicated in the pathogenesis of multiple immunodysregulatory skin disorders. Activation of an IgE-independent pseudo-allergic route has been recently found to be mainly mediated via Mas-Related G protein-coupled receptor X2 (MRGPRX2). Ryanodine receptor (RYR) regulates intracellular calcium liberation. Calcium mobilization is critical in the regulation of MC functional programs. However, the role of RYR in MRGPRX2-mediated pseudo-allergic skin reaction has not been fully addressed. To study the role of RYR in vivo, we established a murine skin pseudo-allergic reaction model. RYR inhibitor attenuated MRGPRX2 ligand substance P (SP)-induced vascular permeability and neutrophil recruitment. Then, we confirmed the role of RYR in an MC line (LAD2 cells) and primary human skin-derived MCs. In LAD2 cells, RYR inhibitor pretreatment dampened MC degranulation (detected by β-hexosaminidase retlease), calcium mobilization, IL-13, TNF-α, CCL-1, CCL-2 mRNA, and protein expression activated by MRGPRX2 ligands, namely, compound 48/80 (c48/80) and SP. Moreover, the inhibition effect of c48/80 by RYR inhibitor was verified in skin MCs. After the confirmation of RYR2 and RYR3 expression, the isoforms were silenced by siRNA-mediated knockdown. MRGPRX2-induced LAD2 cell exocytosis and cytokine generation were substantially inhibited by RYR3 knockdown, while RYR2 had less contribution. Collectively, our finding suggests that RYR activation contributes to MRGPRX2-triggered pseudo-allergic dermatitis, and provides a potential approach for MRGPRX2-mediated disorders.

Control of myeloid cell functions by nociceptors

The immune system has evolved to protect the host from infectious agents, parasites, and tumor growth, and to ensure the maintenance of homeostasis. Similarly, the primary function of the somatosensory branch of the peripheral nervous system is to collect and interpret sensory information about the environment, allowing the organism to react to or avoid situations that could otherwise have deleterious effects. Consequently, a teleological argument can be made that it is of advantage for the two systems to cooperate and form an “integrated defense system” that benefits from the unique strengths of both subsystems. Indeed, nociceptors, sensory neurons that detect noxious stimuli and elicit the sensation of pain or itch, exhibit potent immunomodulatory capabilities. Depending on the context and the cellular identity of their communication partners, nociceptors can play both pro- or anti-inflammatory roles, promote tissue repair or aggravate inflammatory damage, improve resistance to pathogens or impair their clearance. In light of such variability, it is not surprising that the full extent of interactions between nociceptors and the immune system remains to be established. Nonetheless, the field of peripheral neuroimmunology is advancing at a rapid pace, and general rules that appear to govern the outcomes of such neuroimmune interactions are beginning to emerge. Thus, in this review, we summarize our current understanding of the interaction between nociceptors and, specifically, the myeloid cells of the innate immune system, while pointing out some of the outstanding questions and unresolved controversies in the field. We focus on such interactions within the densely innervated barrier tissues, which can serve as points of entry for infectious agents and, where known, highlight the molecular mechanisms underlying these interactions.

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.

Role of MrgprB2 in Rosacea-Like Inflammation in Mice: Modulation by β-Arrestin 2

Cathelicidin LL-37‒mediated activation of mast cells (MCs) has been implicated in the pathogenesis of rosacea, but the receptor involved and the mechanism of its activation and regulation remain unknown. We found that skin biopsies from patients with rosacea display higher frequencies of MCs expressing MRGPRX2 (mouse counterpart MrgprB2) than normal skin. Intradermal injection of LL-37 in wild-type mice resulted in MC recruitment, expression of inflammatory mediators, and development of rosacea-like inflammation. These responses were substantially reduced in MrgprB2‒/‒ mice and abolished in MC deficient Wsh/Wsh mice. β-arrestin 2 is an adaptor protein that regulates G protein-coupled receptor function by receptor desensitization and also by activation of downstream signaling. We found that LL-37‒induced rosacea-like inflammation was significantly reduced in mice with MC-specific deletion of β-arrestin 2 compared with that in control mice. Interestingly, the absence of β-arrestin 2 resulted in enhanced cofilin phosphorylation and substantial inhibition of LL-37‒induced chemotaxis of mouse peritoneal MCs. Furthermore, LL-37‒induced extracellular signal‒regulated kinase 1/2 phosphorylation, NF-κB activation, and proinflammatory cytokine/chemokine production were reduced in β-arrestin 2‒/‒ peritoneal MCs compared with those in wild-type cells. These findings suggest that MRGPRX2/B2 participates in rosacea and that β-arrestin 2 contributes to its pathogenesis by promoting cofilin dephosphorylation, extracellular signal‒regulated kinase 1/2 and NF-κB phosphorylation, MC chemotaxis, and chemokine/cytokine generation.

Licochalcone A inhibits IgE-mediated allergic reaction through PLC/ERK/STAT3 pathway

Licochalcone (LicA) is a flavonoid commonly derived from the licorice plant that is reported to have a variety of pharmacological activities. However, few studies have focused on its anti-allergic properties. IgE-mediated passive and systemic anaphylaxis mice models were used to assess the in vivo anti-allergic effect of LicA and its underlying mechanism, while degranulation, cytokines, and chemokines released from laboratory of allergic disease (LAD2) cells were used to assess its in vitro anti-allergic effect. We used western blot analysis to explore the downstream signaling pathway of its anti-allergic effect. We found that in the mouse model, LicA attenuated IgE-mediated paw inflammation, recovered the allergy-induced drop in body temperature, and reduced the concentrations of tumor necrosis factor-alpha and monocyte chemo-attractant protein-1 in mouse serum in a dose-dependent manner. LicA inhibited the allergic reaction via inhibition of IgE-mediated LAD2 cell activation through the PLC/ERK/STAT3 pathway.

Systematic comparisons of various markers for mast cell activation in RBL-2H3 cells

Mast cell activation plays a key role in various allergic diseases and anaphylaxis. Several methods/techniques can be used for detection of mast cell activation. However, there was no previous systematic evaluation to compare the efficacy of each method/technique. The present study thus systematically compared various markers for mast cell activation induced by IgE cross-linking. The widely used RBL-2H3 mast cells were sensitized with anti-DNP (dinitrophenyl) IgE overnight and activated with DNP-BSA (bovine serum albumin) for up to 4 h. The untreated cells and those with anti-DNP IgE sensitization but without DNP-BSA activation served as the controls. Intracellular calcium level gradually increased to ~2-fold at 1 h, reached its peak (~5-fold) at 2 h, and returned to the basal level at 3-h post-activation. The increases in cellular tryptase level (by Western blotting) (~0.3- to 0.4-fold) and average cell size (~2.5-fold) and decrease of nucleus/cytoplasm ratio (~0.4- to 0.5-fold) were marginal at all time-points. By contrast, β-hexosaminidase release and CD63 expression (by both flow cytometry and immunofluorescence detection/localization), secreted tryptase level (by Western blotting), and tryptase expression (by immunofluorescence detection/localization) stably and obviously increased (~10-fold as compared with the untreated control and sensitized-only cells or detectable only after activation). Based on these data, the stably obvious increases (by ≥ 10-fold) in β-hexosaminidase release, CD63 expression (by both flow cytometry and immunofluorescence staining), secreted tryptase level (by Western blotting), and tryptase expression (by immunofluorescence staining) are recommended as the markers of choice for the in vitro study of mast cell activation using RBL-2H3 cells.

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.

Inhibition of Orai Channel Function Regulates Mas-Related G Protein-Coupled Receptor-Mediated Responses in Mast Cells

Mast cells (MCs) are tissue resident immune cells that play important roles in the pathogenesis of allergic disorders. These responses are mediated via the cross-linking of cell surface high affinity IgE receptor (FcϵRI) by antigen resulting in calcium (Ca2+) mobilization, followed by degranulation and release of proinflammatory mediators. In addition to FcϵRI, cutaneous MCs express Mas-related G protein-coupled receptor X2 (MRGPRX2; mouse ortholog MrgprB2). Activation of MRGPRX2/B2 by the neuropeptide substance P (SP) is implicated in neurogenic inflammation, chronic urticaria, mastocytosis and atopic dermatitis. Although Ca2+ entry is required for MRGPRX2/B2-mediated MC responses, the possibility that calcium release-activated calcium (CRAC/Orai) channels participate in these responses has not been tested. Lentiviral shRNA-mediated silencing of Orai1, Orai2 or Orai3 in a human MC line (LAD2 cells) resulted in partial inhibition of SP-induced Ca2+ mobilization, degranulation and cytokine/chemokine generation (TNF-α, IL-8, and CCL-3). Synta66, which blocks homo and hetero-dimerization of Orai channels, caused a more robust inhibition of SP-induced responses than knockdown of individual Orai channels. Synta66 also blocked SP-induced extracellular signal-regulated kinase 1/2 (ERK1/2) and Akt phosphorylation and abrogated cytokine/chemokine production. It also inhibited SP-induced Ca2+ mobilization and degranulation in primary human skin MCs and mouse peritoneal MCs. Furthermore, Synta66 attenuated both SP-induced cutaneous vascular permeability and leukocyte recruitment in mouse peritoneum. These findings demonstrate that Orai channels contribute to MRGPRX2/B2-mediated MC activation and suggest that their inhibition could provide a novel approach for the modulation of SP-induced MC/MRGPRX2-mediated disorders.

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

The G protein-coupled receptor MrgprX2 in mast cells is known to be a crucial receptor for pseudo-allergic reactions. MrgprX2 activation leads to elevated intracellular calcium levels and mast cell degranulation, but the underlying mechanism remains to be elucidated. Herein, we investigated the role of the phosphatidylinositol 3 kinase (PI3K)/serum-threonine kinase (AKT) signaling pathway and phospholipase C gamma (PLCγ) in mast cell degranulation mediated by MrgprX2 in LAD2 human-derived mast cells. The results showed that phosphorylated AKT (p-AKT) and PLCγ up-regulation were accompanied by an increase in intracellular calcium following activation of MrgprX2 by Compound 48/80, an inducer of mast cell degranulation. In contrast, p-AKT and PLCγ were down-regulated and intracellular calcium levels decreased after MrgprX2 knockdown. Mast cell degranulation was clearly suppressed; however, inhibiting PI3K and PLCγ phosphorylation did not influence MrgprX2 expression. The increase in calcium concentration was suppressed and mast cell degranulation was weakened. Furthermore, by inhibiting PI3K and PLCγ phosphorylation in animals, the allergic symptoms caused by C48/80 were obviously reduced. We deduced that during the mast cell degranulation observed in pseudoallergic reactions, MrgprX2 regulated intracellular calcium levels via the PI3K/AKT and PLCγ pathways.

Therapeutic Potential of MRGPRX2 Inhibitors on Mast Cells

Mast cells (MCs) act as primary effectors in inflammatory and allergic reactions by releasing intracellularly-stored inflammatory mediators in diseases. The two major pathways for MC activation are known to be immunoglobulin E (IgE)-dependent and -independent. Although IgE-dependent signaling is the main pathway to MC activation, IgE-independent pathways have also been found to serve pivotal roles in the pathophysiology of various inflammatory conditions. Recent studies have shown that human and mouse MCs express several regulatory receptors such as toll-like receptors (TLRs), CD48, C300a, and GPCRs, including mas-related GPCR-X2 (MRGPRX2). MRGPRX2 has been reported as a novel GPCR that is expressed in MCs activated by basic secretagogues, neurokinin peptides, host defense antimicrobial peptides, and small molecule compounds (e.g., neuromuscular blocking agents) and leads to MC degranulation and eicosanoids release under in vitro experimental condition. Functional analyses of MRGPRX2 and Mrgprb2 (mouse ortholog) indicate that MRGPRX2 is involved in MC hypersensitivity reactions causing neuroinflammation such as postoperative pain, type 2 inflammation, non-histaminergic itch, and drug-induced anaphylactic-like reactions. In this review, we discuss the roles in innate immunity through functional studies on MRGPRX2-mediated IgE-independent MC activation and also the therapeutic potential of MRGPRX2 inhibitors on allergic and inflammatory diseases.

An integrated strategy for the identification and screening of anti-allergy components from natural products based on calcium fluctuations and cell extraction coupled with HPLC-Q-TOF-MS

Allergic diseases are a significant public health problem worldwide. Traditional Chinese medicines (TCMs) with reported anti-allergy effects may be important sources for the development of new anti-allergy drugs. Thus, establishing an analytical method that can simultaneously identify and screen anti-allergic compounds in TCMs is important. The increased concentrations of intracellular calcium ions resulting in mast cell degranulation releasing active mediators play a key role in allergic diseases, which can be used as a potential index to identify anti-allergic herbs and compounds. In this study, we provide a new strategy that was applied to screening natural anti-allergic compounds based on fluorescence calcium ion (Ca²⁺) fluctuation integrated with cell extract and high-performance liquid chromatography-mass spectrometry (HPLC-MS). A low-cost, convenient fluorescence detection Ca²⁺ signaling method was established and successfully applied to identify three herbs. Then, the method was integrated with biospecific cell fishing and HPLC-MS to screen potential active components that have the effect of stabilizing the cell membrane of rat basophilic leukemia granulocytes (RBL-2H3). Seven components, namely, albiflorin and paeoniflorin from Radix Paeoniae Alba, ononin and formononetin from Radix Astragali, cimifugin, 4'-O-β-D-glucosyl-5-O-methylvisamminol, and prim-O-glucosylcimifugin from Radix Saposhnikoviae were fished. These seven compounds have the effect of inhibiting cell Ca²⁺ influx. 4'-O-β-D-Glucosyl-5-O-methylvisamminol, prim-O-glucosylcimifugin, paeoniflorin, ononin, and formononetin significantly inhibit the release of β-hexosaminidase, which is equivalent to the positive drug. In conclusion, the integrated strategy of fluorescence detection calcium ion kinetic method binding with biospecific cell fishing was an effective mode to identify and screen natural anti-allergic compounds.

Lactic acid suppresses MRGPRX2 mediated mast cell responses

MAS related G-protein coupled receptor X2 (MRGPRX2) is a G-protein coupled receptor (GPCR) expressed in human mast cells that has been implicated to play an important role in causing pseudo-allergic reactions as well as exacerbating inflammation during asthma and other allergic diseases. Lactic acid, a byproduct of glucose metabolism, is abundantly present in inflamed tissues and has been shown to regulate functions of several immune cells. Because the endogenous ligands for MRGPRX2 (substance P and LL-37) are elevated during pathologic conditions, such as cancer and asthma, and given that lactic acid levels are also enhanced in these patients, we explored the role of lactic acid in regulating mast cells response via MRGPRX2 and MrgprB2, the mouse orthologue of the human receptor. We found that lactic acid suppressed both the early (Ca²⁺ mobilization and degranulation) and late (chemokine/cytokine release) phases of mast cell activation; this data was confirmed in LAD2, human skin and mouse peritoneal mast cells. In LAD2 cells, the reduction in degranulation and chemokine/cytokine production mediated by lactic acid was dependent on pH. In agreement with our in vitro studies, lactic acid also reduced passive systemic anaphylaxis to compound 48/80 (a known MRGPRX2/MrgprB2 ligand) and skin inflammation in a mouse model of rosacea that is dependent on MrgprB2 expression on skin mast cells. Our data thus suggest that lactic acid may serve to inhibit mast cell-mediated inflammation during asthma and reduce immune response during cancer by affecting mast cell activation through MRGPRX2.

Multifaceted MRGPRX2: New insight into the role of mast cells in health and disease

Cutaneous mast cells (MCs) express Mas-related G protein-coupled receptor-X2 (MRGPRX2; mouse ortholog MrgprB2), which is activated by an ever-increasing number of cationic ligands. Antimicrobial host defense peptides (HDPs) generated by keratinocytes contribute to host defense likely by 2 mechanisms, one involving direct killing of microbes and the other via MC activation through MRGPRX2. However, its inappropriate activation may cause pseudoallergy and likely contribute to the pathogenesis of rosacea, atopic dermatitis, allergic contact dermatitis, urticaria, and mastocytosis. Gain- and loss-of-function missense single nucleotide polymorphisms in MRGPRX2 have been identified. The ability of certain ligands to serve as balanced or G protein-biased agonists has been defined. Small-molecule HDP mimetics that display both direct antimicrobial activity and activate MCs via MRGPRX2 have been developed. In addition, antibodies and reagents that modulate MRGPRX2 expression and signaling have been generated. In this article, we provide a comprehensive update on MrgprB2 and MRGPRX2 biology. We propose that harnessing MRGPRX2's host defense function by small-molecule HDP mimetics may provide a novel approach for the treatment of antibiotic-resistant cutaneous infections. In contrast, MRGPRX2-specific antibodies and inhibitors could be used for the modulation of allergic and inflammatory diseases that are mediated via this receptor.

Store-operated Calcium Entry-associated Regulatory Factor Regulates Airway Inflammation and Airway Remodeling in Asthma Mice Models

BACKGROUND

Store-operated calcium entry (SOCE) is involved in the pathogenesis of airway inflammation and remodeling in asthma. Store-operated calcium entry-associated regulatory factor (SARAF) can down-regulate SOCE.

OBJECTIVE

We sought to investigate the role of SARAF in the regulation of airway inflammation and remodeling in asthma mice models, as well as in the functional regulation of human airway smooth muscle cells (hASMCs).

METHODS

Balb/c mice were sensitized and challenged with ovalbumin to establish the asthma mice models. Mice were transfected with lentivirus, which expressed the SARAF gene + GFP or the negative control gene + GFP. Airway resistance was measured with the animal pulmonary function system. Airway inflammation and remodeling were evaluated via histological staining. In vitro cultured hASMCs were transfected with scrambled small interfering RNA(siRNA) or SARAF-specific siRNA respecitvely. The proliferation, migration rate, hypertrophy and SOCE activity of hASMCs were examined with cell counting kit 8, wound healing test, bright field imaging and Ca²⁺ fluorescence imaging, respectively. SARAF expression was measured by quantitative real-time-PCR.

RESULTS

Asthma mice models showed decreased SARAF mRNA expression in the lungs. SARAF overexpression attenuated airway inflammation, resistance and also remodeling. Downregulation of SARAF expression with siRNA promoted the proliferation, migration, hypertrophy and SOCE activity in hASMCs.

CONCLUSIONS

SARAF plays a protective role against airway inflammation and remodeling in asthma mice models by blunting SOCE; SARAF may also be a functional regulating factor of hASMCs.

Inhibitory effect of paeoniflorin on IgE-dependent and IgE-independent mast cell degranulation in vitro and vivo

The incidence of allergic diseases has increased to such a point that they have become common and have reached epidemic levels. However, their pathogenesis is not fully understood. Paeoniae Radix Rubra is a traditional Chinese medicine that is also used as a dietary supplement. Its main active ingredient is paeoniflorin. Paeoniflorin has good anti-inflammatory, immunomodulation, and antitumor effects. It is utilized in the treatment of various diseases in clinical settings. However, its effects on type I allergies and pseudoallergic reactions have not been comprehensively studied. In this study, we aimed to use DNP-IgE/DNP-BSA and C48/80 to simulate type I allergies and pseudoallergic reactions to evaluate the therapeutic effects of paeoniflorin to these diseases and identify its molecular mechanisms in cell degranulation both in vivo and in vitro. Results showed that paeoniflorin inhibited the degranulation of RBL-2H3 cells induced by these two stimuli (IgE-dependent and IgE-independent stimuli) in a dose-dependent manner. Moreover, qPCR and western blot analyses indicated that paeoniflorin may regulate the IgE/FcεR I, MRGPRB3, and downstream signal transduction pathways to exert its therapeutic effects on type I allergies and pseudoallergic reactions. In addition, DNP-IgE/DNP-BSA and compound 48/80 were used to induce the establishment of a passive cutaneous anaphylaxis mouse model. Paeoniflorin was found to suppress the extravasation of Evans Blue and tissue edema in the ears, back skin, and paws of the mice. This result further confirmed that paeoniflorin has a notable therapeutic effect on type I allergies and pseudoallergic reactions. Therefore, paeoniflorin could potentially be used as a drug for the treatment of type I allergies and pseudoallergic reactions. This study provides new insights into expanding the treatment range of paeoniflorin and its pharmacological mechanism.

Unlocking the Non-IgE-Mediated Pseudo-Allergic Reaction Puzzle with Mas-Related G-Protein Coupled Receptor Member X2 (MRGPRX2)

Mas-related G-protein coupled receptor member X2 (MRGPRX2) is a class A GPCR expressed on mast cells. Mast cells are granulated tissue-resident cells known for host cell response, allergic response, and vascular homeostasis. Immunoglobulin E receptor (FcεRI)-mediated mast cell activation is a well-studied and recognized mechanism of allergy and hypersensitivity reactions. However, non-IgE-mediated mast cell activation is less explored and is not well recognized. After decades of uncertainty, MRGPRX2 was discovered as the receptor responsible for non-IgE-mediated mast cells activation. The puzzle of non-IgE-mediated pseudo-allergic reaction is unlocked by MRGPRX2, evidenced by a plethora of reported endogenous and exogenous MRGPRX2 agonists. MRGPRX2 is exclusively expressed on mast cells and exhibits varying affinity for many molecules such as antimicrobial host defense peptides, neuropeptides, and even US Food and Drug Administration-approved drugs. The discovery of MRGPRX2 has changed our understanding of mast cell biology and filled the missing link of the underlying mechanism of drug-induced MC degranulation and pseudo-allergic reactions. These non-canonical characteristics render MRGPRX2 an intriguing player in allergic diseases. In the present article, we reviewed the emerging role of MRGPRX2 as a non-IgE-mediated mechanism of mast cell activation in pseudo-allergic reactions. We have presented an overview of mast cells, their receptors, structural insight into MRGPRX2, MRGPRX2 agonists and antagonists, the crucial role of MRGPRX2 in pseudo-allergic reactions, current challenges, and the future research direction.

The Multifaceted Mas-Related G Protein-Coupled Receptor Member X2 in Allergic Diseases and Beyond

Recent research on mast cell biology has turned its focus on MRGPRX2, a new member of the Mas-related G protein-coupled subfamily of receptors (Mrgprs), originally described in nociceptive neurons of the dorsal root ganglia. MRGPRX2, a member of this group, is present not only in neurons but also in mast cells (MCs), specifically, and potentially in other cells of the immune system, such as basophils and eosinophils. As emerging new functions for this receptor are studied, a variety of both natural and pharmacologic ligands are being uncovered, linked to the ability to induce receptor-mediated MC activation and degranulation. The diversity of these ligands, characterized in their human, mice, or rat homologues, seems to match that of the receptor's interactions. Natural ligands include host defense peptides, basic molecules, and key neuropeptides such as substance P and vasointestinal peptide (known for their role in the transmission of pain and itch) as well as eosinophil granule-derived proteins. Exogenous ligands include MC secretagogues such as compound 48/80 and mastoparan, a component of bee wasp venom, and several peptidergic drugs, among which are members of the quinolone family, neuromuscular blocking agents, morphine, and vancomycin. These discoveries shed light on its capacity as a multifaceted participant in naturally occurring responses within immunity and neural stimulus perception, as in responses at the center of immune pathology. In host defense, the mice Mrgprb2 has been proven to aid mast cells in the detection of peptidic molecules from bacteria and in the release of peptides with antimicrobial activities and other immune mediators. There are several potential actions described for it in tissue homeostasis and repair. In the realm of pathologic response, there is evidence to suggest that this receptor is also involved in chronic inflammation. Furthermore, MRGPRX2 has been linked to the pathophysiology of non-IgE-mediated immediate hypersensitivity drug reactions. Different studies have shown its possible role in other allergic diseases as well, such as asthma, atopic dermatitis, contact dermatitis, and chronic spontaneous urticaria. In this review, we sought to cover its function in physiologic processes and responses, as well as in allergic and nonallergic immune disease.

The signaling pathway and polymorphisms of Mrgprs

Mas-related G protein-coupled receptors (Mrgprs) are a family of receptors implicated in a diverse array of human diseases. Since their discovery in 2001, great progress has been made in determining their relation to human disease. Vital for Mrgprs therapeutic efforts across all disease disciplines is a thorough understanding of Mrgprs signal transduction pathways and polymorphisms, as these offer insights into new drug candidates, existing discrepancies in drug response, and differences in disease susceptibility. In this review, we discuss the current state of knowledge regarding Mrgprs signaling pathways and polymorphisms.

A Critical Role for Na+/H+ Exchanger Regulatory Factor 1 in Modulating FcεRI-Mediated Mast Cell Activation

Mast cells are tissue-resident immune cells that play pivotal roles in initiating and amplifying allergic/anaphylactic reactions in humans. Their activation occurs via multiple mechanisms, which include cross-linking of the IgE-bound, high-affinity IgE receptors (FcεRI) by allergens or Ags and the binding of anaphylatoxins such as C3a to its receptor, C3aR. We have previously demonstrated that the Na⁺/H⁺ exchanger regulatory factor 1 (NHERF1) promotes C3aR functions in human mast cells. In the current study, we show that NHERF1 regulates mast cell response following FcεRI stimulation. Specifically, intracellular Ca²⁺ mobilization, activation of the MAPKs (ERK1/2 and P38), and production of cytokines (IL-13 and IL-6) following exposure to IgE/Ag were significantly reduced in mast cells from NHERF1^+/‒ mice. In agreement with our in vitro data, mast cell-mediated passive cutaneous anaphylaxis and passive systemic anaphylaxis were reduced in NHERF1^+/‒ mice and mast cell-deficient Kit^W-sh/W-sh mice engrafted with NHERF1^+/‒ mast cells. Mechanistically, the levels of microRNAs (miRNAs) that regulate mast cell responses, miRNA 155-3p and miRNA 155-5p, were altered in mast cells from NHERF1^+/‒ mice. Moreover, NHERF1 rapidly localized to the nucleus of mast cells following FcεRI stimulation. In summary, our results suggest that the NHERF1 acts as an adapter molecule and promotes IgE/Ag-induced mast cell activation. Further elucidating the mechanisms through which NHERF1 modulates mast cell responses will lend insights into the development of new therapeutic strategies to target mast cells during anaphylaxis or other allergic diseases.

Emerging mechanisms contributing to mast cell-mediated pathophysiology with therapeutic implications

Mast cells are tissue-resident immune cells that play key roles in the initiation and perpetuation of allergic inflammation, usually through IgE-mediated mechanisms. Mast cells are, however, evolutionary ancient immune cells that can be traced back to urochordates and before the emergence of IgE antibodies, suggesting their involvement in antibody-independent biological functions, many of which are still being characterized. Herein, we summarize recent advances in understanding the roles of mast cells in health and disease, partly through the study of emerging non-IgE receptors such as the Mas-related G protein-coupled receptor X2, implicated in pseudo-allergic reactions as well as in innate defense and neuronal sensing; the mechano-sensing adhesion G protein-coupled receptor E2, variants of which are associated with familial vibratory urticaria; and purinergic receptors, which orchestrate tissue damage responses similarly to the IL-33 receptor. Recent evidence also points toward novel mechanisms that contribute to mast cell-mediated pathophysiology. Thus, in addition to releasing preformed mediators contained in granules and synthesizing mediators de novo, mast cells also secrete extracellular vesicles, which convey biological functions. Understanding their release, composition and uptake within a variety of clinical conditions will contribute to the understanding of disease specific pathology and likely lead the way to novel therapeutic approaches. We also discuss recent advances in the development of therapies targeting mast cell activity, including the ligation of inhibitory ITIM-containing receptors, and other strategies that suppress mast cells or responses to mediators for the management of mast cell-related diseases.