MrgX2 is a high potency cortistatin receptor expressed in dorsal root ganglion

Mas-related G protein-coupled receptors in gastrointestinal dysfunction and inflammatory bowel disease: A review

Inflammatory bowel disease (IBD) is a chronic debilitating condition, hallmarked by persistent inflammation of the gastrointestinal tract. Despite recent advances in clinical treatments, the aetiology of IBD is unknown, and a large proportion of patients are refractory to pharmacotherapy. Understanding IBD immunopathogenesis is crucial to discern the cause of IBD and optimise treatments. Mas-related G protein-coupled receptors (Mrgprs) are a family of approximately 50 G protein-coupled receptors that were first identified over 20 years ago. Originally known for their expression in skin nociceptors and their role in transmitting the sensation of itch in the periphery, new reports have described the presence of Mrgprs in the gastrointestinal tract. In this review, we consider the impact of these findings and assess the evidence that suggests that Mrgprs may be involved in the disrupted homeostatic processes that contribute to gastrointestinal disorders and IBD. LINKED ARTICLES: This article is part of a themed issue Therapeutic Targeting of G Protein-Coupled Receptors: hot topics from the Australasian Society of Clinical and Experimental Pharmacologists and Toxicologists 2021 Virtual Annual Scientific Meeting. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.14/issuetoc.

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.

Cytotoxicity assessment of exfoliated MoS2 using primary human mast cells and the progenitor cell-derived mast cell line LAD2

Molybdenum disulfide is an emerging 2D material with several potential applications in medicine. Therefore, it is crucial to ascertain its biocompatibility. Mast cells are immune cells that are found in many organs and tissues in contact with the extracellular environment, and can be cultured from progenitor cells present in the bone marrow. Given the long period required for differentiation and proliferation of primary mast cells, human mast cell lines have emerged as a tractable model for biological and toxicological studies. Here, we compare two types of industrial MoS2 using CD34+-derived primary human mast cells and the LAD2 cell line. Minimal effects were observed on early-stage activation endpoints such as β-hexosaminidase release and expression of surface markers of mast cell activation. Transmission electron microscopy revealed limited uptake of the tested materials. Overall, MoS2 was found to be biocompatible, and the LAD2 cell line was validated as a useful in vitro model of mast cells.

A new MAS-related G protein-coupled receptor X2 cell membrane chromatography analysis model based on HALO-tag technology and its applications

Cell membrane chromatography (CMC) is an effective method for studying receptors with multiple transmembrane structure such as MAS-related G protein-coupled receptor X2 (MrgX2). CMC relies on the maintenance of the complete biological structure of a membrane receptor; however, it needs to be further improved to obtain a more convenient and stable CMC model. In the present study, the haloalkane dehalogenase protein tag (HALO-tag) technology was used to construct a new MrgX2/CMC model. The fusion receptors of MrgX2 with HALO-tag at the C terminus were expressed in HEK293 cells. The silica gel was modified with a substrate of HALO-tag (chloroalkanes) via one-step acylation for the rapid capture of fusion receptors. The new CMC model (MrgX2-HALO-tag/CMC model) was not only quicker to prepare but also more stable and had a longer lifespan than a previous MrgX2-SNAP-tag/CMC model. In combination with the high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) system, the MrgX2-HALO-tag/CMC model was used to screen and identify bioactive components in traditional Chinese medicine. Using this combination, sanggenon C and morusin were identified from Mori Cortex as anti-pseudo-allergic components. The MrgX2-HALO-tag/CMC model alone was also applied to analyze ligand-receptor interaction. The affinity order of four ligands to MrgX2 was as follows: desipramine < imipramine < amitriptyline < clomipramine. This was consistent with the results obtained using the MrgX2-SNAP-tag/CMC model. The MrgX2-HALO-tag/CMC model provides ideas and application prospects for the immobilization of cell membrane that contains receptors with more transmembrane structures.

The MRGPR family of receptors in immunity

The discovery of Mas-related G protein-coupled receptors (Mrgprs) has opened a compelling chapter in our understanding of immunity and sensory biology. This family of receptors, with their unique expression and diverse ligands, has emerged as key players in inflammatory states and hold the potential to alleviate human diseases. This review will focus on the members of this receptor family expressed on immune cells and how they govern immune and neuro-immune pathways underlying various physiological and pathological states. Immune cell-specific Mrgprs have been shown to control a variety of manifestations, including adverse drug reactions, inflammatory conditions, bacterial immunity, and the sensing of environmental exposures like allergens and irritants.

Proinflammatory chemokine CXCL14 activates MAS-related G protein-coupled receptor MRGPRX2 and its putative mouse ortholog MRGPRB2

Patients with idiopathic pulmonary fibrosis show a strongly upregulated expression of chemokine CXCL14, whose target is still unknown. Screening of CXCL14 in a panel of human G protein-coupled receptors (GPCRs) revealed its potent and selective activation of the orphan MAS-related GPCR X2 (MRGPRX2). This receptor is expressed on mast cells and - like CXCL14 - upregulated in bronchial inflammation. CXCL14 induces robust activation of MRGPRX2 and its putative mouse ortholog MRGPRB2 in G protein-dependent and β-arrestin recruitment assays that is blocked by a selective MRGPRX2/B2 antagonist. Truncation combined with mutagenesis and computational studies identified the pharmacophoric sequence of CXCL14 and its presumed interaction with the receptor. Intriguingly, C-terminal domain sequences of CXCL14 consisting of 4 to 11 amino acids display similar or increased potency and efficacy compared to the full CXCL14 sequence (77 amino acids). These results provide a rational basis for the future development of potential idiopathic pulmonary fibrosis therapies.

Molecular mechanisms of pruritus in prurigo nodularis

Pruritus is the most common symptom of dermatological disorders, and prurigo nodularis (PN) is notorious for intractable and severe itching. Conventional treatments often yield disappointing outcomes, significantly affecting patients' quality of life and psychological well-being. The pathogenesis of PN is associated with a self-sustained "itch-scratch" vicious cycle. Recent investigations of PN-related itch have partially revealed the intricate interactions within the cutaneous neuroimmune network; however, the underlying mechanism remains undetermined. Itch mediators play a key role in pruritus amplification in PN and understanding their action mechanism will undoubtedly lead to the development of novel targeted antipruritic agents. In this review, we describe a series of pruritogens and receptors involved in mediating itching in PN, including cytokines, neuropeptides, extracellular matrix proteins, vasculogenic substances, ion channels, and intracellular signaling pathways. Moreover, we provide a prospective outlook on potential therapies based on existing findings.

COVID-19, G protein-coupled receptor, and renin-angiotensin system autoantibodies: Systematic review and meta-analysis

INTRODUCTION

There are an increasing number of reports of autoantibodies (AAbs) against host proteins such as G-protein coupled receptors (GPCRs) and the renin-angiotensin system (RAS) in COVID-19 disease. Here we have undertaken a systematic review and meta-analysis of all reports of AAbs against GPCRs and RAS in COVID-19 patients including those with long-COVID or post-COVID symptoms.

METHODS

PubMed, Embase, Web of Science, and Scopus databases were searched to find papers on the role of GPCR and RAS AAbs in the presence and severity of COVID-19 or post- COVID symptoms available through March 21, 2023. Data on the prevalence of AngII or ACE, comparing AngII or ACE between COVID-19 and non-COVID-19, or comparing AngII or ACE between COVID-19 patients with different disease stages were pooled and a meta-analysed using random- or fixed-effects models were undertaken.

RESULTS

The search yielded a total of 1042 articles, of which 68 studies were included in this systematic review and nine in the meta-analysis. Among 18 studies that investigated GPCRs and COVID-19 severity, 18 distinct AAbs were detected. In addition, nine AAbs were found in case reports that assessed post- COVID, and 19 AAbs were found in other studies that assessed post- COVID or long- COVID symptoms. Meta-analysis revealed a significantly higher number of seropositive ACE2 AAbs in COVID-19 patients (odds ratio = 7.766 [2.056, 29.208], p = 0.002) and particularly in severe disease (odds ratio = 11.49 [1.04, 126.86], p = 0.046), whereas AngII-AAbs seropositivity was no different between COVID-19 and control subjects (odds ratio = 2.890 [0.546-15.283], p = 0.21).

CONCLUSIONS

GPCR and RAS AAbs may play an important role in COVID-19 severity, the development of disease progression, long-term symptoms COVID and post- COVID symptoms.

Basophils in pruritic skin diseases

Basophils are rare cells in the peripheral blood which have the capability to infiltrate into the skin. Invasion of basophils has been detected in pruritic skin diseases, including atopic dermatitis, bullous pemphigoid, chronic spontaneous urticaria and contact dermatitis. In the skin, basophils are important players of the inflammatory immune response, as they release Th2 cytokines, including interleukin (IL)-4 and IL-13, subsequently inducing the early activation of T-cells. Further, basophils release a multitude of mediators, such as histamine and IL-31, which both play an important role in the initiation of the pruritic response via activation of sensory nerves. Chronic pruritus significantly affects the quality of life and the working capability of patients, though its mechanisms are not fully elucidated yet. Since basophils and neurons share many receptors and channels, bidirectional interaction mechanisms, which drive the sensation of itch, are highlighted in this review.

G protein-coupled receptors as targets for transformative neuropsychiatric therapeutics

G protein-coupled receptors (GPCRs) constitute the largest family of druggable genes in the human genome. Even though perhaps 30% of approved medications target GPCRs, they interact with only a small number of them. Here, we consider whether there might be new opportunities for transformative therapeutics for neuropsychiatric disorders by specifically targeting both known and understudied GPCRs. Using psychedelic drugs that target serotonin receptors as an example, we show how recent insights into the structure, function, signaling, and cell biology of these receptors have led to potentially novel therapeutics. We next focus on the possibility that nonpsychedelic 5-HT2A receptor agonists might prove to be safe and rapidly acting antidepressants. Finally, we examine understudied and orphan GPCRs using the MRGPR family of receptors as an example.

Targeting active sites of inflammation using inherent properties of tissue-resident mast cells

Mast cells play a pivotal role in initiating and directing host's immune response. They reside in tissues that primarily interface with the external environment. Activated mast cells respond to environmental cues throughout acute and chronic inflammation through releasing immune mediators via rapid degranulation, or long-term de novo expression. Mast cell activation results in the rapid release of a variety of unique enzymes and reactive oxygen species. Furthermore, the increased density of mast cell unique receptors like mas related G protein-coupled receptor X2 also characterizes the inflamed tissues. The presence of these molecules (either released mediators or surface receptors) are particular to the sites of active inflammation, and are a result of mast cell activation. Herein, the molecular design principles for capitalizing on these novel mast cell properties is discussed with the goal of manipulating localized inflammation. STATEMENT OF SIGNIFICANCE: Mast cells are immune regulating cells that play a crucial role in both innate and adaptive immune responses. The activation of mast cells causes the release of multiple unique profiles of biomolecules, which are specific to both tissue and disease. These unique characteristics are tightly regulated and afford a localized stimulus for targeting inflammatory diseases. Herein, these important mast cell attributes are discussed in the frame of highlighting strategies for the design of bioresponsive functional materials to target regions of inflammations.

Rosmarinic acid ameliorates skin inflammation and pruritus in allergic contact dermatitis by inhibiting mast cell-mediated MRGPRX2/PLCγ1 signaling pathway

BACKGROUND

Allergic contact dermatitis (ACD) is one of the most common dermatoses, which has high disease burden and quality of life impairment. Anti-histamine is not effective in a part of the ACD patients. Thus, the discovery of novel antipruritic therapy is of highly demand.

OBJECTIVE

In this study, we investigated the anti-pruritic effects of rosmarinic acid (RA) and explored the underlying mechanism.

METHOD

SPF Balb/c mice were randomly divided into control group, ACD model group, RA group (1.0 mg/kg) and loratadine (LORA) group (1.5 mg/kg). Back epidermal thickness was recorded. H&E staining was used for pathological observation. Mast cell degranulation was assessed by toluidine blue staining. ELISA assay was employed to detect cytokines levels. Cortistatin-14 (CST-14) and Mas-related G protein-coupled receptor X2 (MRGPRX2) expression was detetcted by RT-PCR and western blot. Molecular docking assay was used to predict the affinity of RA and MRGPRX2. Surface plasmon resonance (SPR) assay was used to verify structure affinity of RA and MRGPRX2.

RESULTS

RA treatment significantly decreased epidermal keratinization and inflammatory cell infiltration in ACD mouse model. Administration of RA significantly reduced secretion of histamine, IL-13, and mRNA expression of CST-14. Furthermore, RA treatment increased mRNA expression of MRGPRX2. In addition, Molecular docking results predict that RA has a good affinity with MRGPRX2. RA displayed a structure affinity (K_D = 8.89 × 10^-4) with MRGPRX2 by SPR. RA inhibited CST-14 and Compound 48/80 (C48/80)-induced mast cell activation via MRGPRX2-PLCγ1-PKC-NF-κB signaling pathway.

CONCLUSION

RA exhibits anti-pruritic and anti-inflammatory effects in ACD mice by inhibiting MRGPRX2-PLCγ1-PKC-NF-κB signaling pathway. RA might emerge as a potential drug for the treatment of pruritus and skin inflammation in the setting of ACD.

Therapeutic Effect of a Latent Form of Cortistatin in Experimental Inflammatory and Fibrotic Disorders

Cortistatin is a cyclic neuropeptide that recently emerged as an attractive therapeutic factor for treating inflammatory, autoimmune, fibrotic, and pain disorders. Despite of its efficiency and apparent safety in experimental preclinical models, its short half-life in body fluids and its potential pleiotropic effects, due to its promiscuity for several receptors expressed in various cells and tissues, represent two major drawbacks for the clinical translation of cortistatin-based therapies. Therefore, the design of new strategies focused on increasing the stability, bioavailability, and target specificity of cortistatin are lately demanded by the industry. Here, we generated by molecular engineering a new cortistatin-based prodrug formulation that includes, beside the bioactive cortistatin, a molecular-shield provided by the latency-associated protein of the transforming growth factor-β1 and a cleavage site specifically recognized by metalloproteinases, which are abundant in inflammatory/fibrotic foci. Using different models of sepsis, inflammatory bowel disease, scleroderma, and pulmonary fibrosis, we demonstrated that this latent form of cortistatin was a highly effective protection against these severe disorders. Noteworthy, from a therapeutic point of view, is that latent cortistatin seems to require significantly lower doses and fewer administrations than naive cortistatin to reach the same efficacy. Finally, the metalloproteinase-cleavage site was essential for the latent molecule to exert its therapeutic action. In summary, latent cortistatin emerges as a promising innovative therapeutic tool for treating chronic diseases of different etiologies with difficult clinical solutions and as a starting point for a rational development of prodrugs based on the use of bioactive peptides.

Chronic Nodular Prurigo: An Update on the Pathogenesis and Treatment

Chronic nodular prurigo (CNPG) is a recalcitrant chronic itchy disorder that affects the quality of life. It can be triggered by multiple etiologies, such as atopic dermatitis, diabetes, and chronic renal diseases. The mechanisms of CNPG are complicated and involved the interaction of the cutaneous, immune, and nervous systems. Diverse immune cells, including eosinophils, neutrophils, T cells, macrophages, and mast cells infiltrated the lesional skin of CNPG, which initiated the inflammatory cytokines and pruritogens release. In addition, the interaction between the immune cells and activated peripheral sensory nerve fibers by neurotransmitters caused neuroinflammation in the skin and intractable itch. This itch-scratch vicious cycle of CNPG results in disease exacerbation. CNPG is difficult to treat with traditional therapies. Recently, great advances have been made in the pathophysiology of both inflammation and pruritus transmission in CNPG. In this review, we summarize the updated mechanisms and novel therapies for CNPG.

The Number of MRGPRX2-Expressing Cells Is Increased in Skin Lesions of Patients With Indolent Systemic Mastocytosis, But Is Not Linked to Symptom Severity

Background

Recently, the expression of the mast cell (MC) receptor Mas-related G protein–coupled receptor X2 (MRGPRX2) has been detected in lesional skin of adult patients with cutaneous mastocytosis. As of yet, little is known about the clinical relevance of MRGPRX2 and its agonists in patients with mastocytosis, including indolent systemic mastocytosis (ISM).

Methods

MRGPRX2 and MRGPRX2 agonists, cortistatin (CST), and major basic protein (MBP) were analyzed in lesional and non-lesional skin of patients with ISM and skin of healthy controls by immunohistochemistry. Co-localization of MRGPRX2 and MRGPRX2-mRNA with the MC marker tryptase was assessed by immunofluorescence microscopy and in situ hybridization, respectively. We assessed clinical, demographic, and laboratory data, including mastocytosis activity score (MAS), serum tryptase, and KIT D816V allele burden.

Results

The number of MRGPRX2-expressing (MRGPRX2+) cells, MRGPRX2-mRNA+ MCs, and CST-expressing (CST+) and MBP-expressing (MBP+) cells was significantly higher in lesional skin as compared to non-lesional skin and/or skin of healthy controls (all p < 0.05). Increased numbers of MRGPRX2+ cells, MRGPRX2-mRNA+ MCs, and CST+ and MBP+ cells were not associated with clinical and laboratory features of ISM, including disease burden, symptom severity, evidence of anaphylaxis, and tryptase levels.

Conclusions

Skin lesions of patients with ISM showed high numbers of MRGPRX2+ cells, although they were not linked to symptom severity. Clinical relevance of the MRGPRX2-mediated pathway of MC activation in ISM remains unclear and should be investigated in further studies.

Long COVID: Association of Functional Autoantibodies against G-Protein-Coupled Receptors with an Impaired Retinal Microcirculation

Long COVID (LC) describes the clinical phenotype of symptoms after infection with the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Diagnostic and therapeutic options are limited, as the pathomechanism of LC is elusive. As the number of acute SARS-CoV-2 infections was and is large, LC will be a challenge for the healthcare system. Previous studies revealed an impaired blood flow, the formation of microclots, and autoimmune mechanisms as potential factors in this complex interplay. Since functionally active autoantibodies against G-protein-coupled receptors (GPCR-AAbs) were observed in patients after SARS-CoV-2 infection, this study aimed to correlate the appearance of GPCR-AAbs with capillary microcirculation. The seropositivity of GPCR-AAbs was measured by an established cardiomyocyte bioassay in 42 patients with LC and 6 controls. Retinal microcirculation was measured by OCT-angiography and quantified as macula and peripapillary vessel density (VD) by the Erlangen-Angio Tool. A statistical analysis yielded impaired VD in patients with LC compared to the controls, which was accentuated in female persons. A significant decrease in macula and peripapillary VD for AAbs targeting adrenergic β2-receptor, MAS-receptor angiotensin-II-type-1 receptor, and adrenergic α1-receptor were observed. The present study might suggest that a seropositivity of GPCR-AAbs can be linked to an impaired retinal capillary microcirculation, potentially mirroring the systemic microcirculation with consecutive clinical symptoms.

Reconstructing the Origins of the Somatostatin and Allatostatin-C Signaling Systems Using the Accelerated Evolution of Biodiverse Cone Snail Toxins

Somatostatin and its related peptides (SSRPs) form an important family of hormones with diverse physiological roles. The ubiquitous presence of SSRPs in vertebrates and several invertebrate deuterostomes suggests an ancient origin of the SSRP signaling system. However, the existence of SSRP genes outside of deuterostomes has not been established, and the evolutionary history of this signaling system remains poorly understood. Our recent discovery of SSRP-like toxins (consomatins) in venomous marine cone snails (Conus) suggested the presence of a related signaling system in mollusks and potentially other protostomes. Here, we identify the molluscan SSRP-like signaling gene that gave rise to the consomatin family. Following recruitment into venom, consomatin genes experienced strong positive selection and repeated gene duplications resulting in the formation of a hyperdiverse family of venom peptides. Intriguingly, the largest number of consomatins was found in worm-hunting species (>400 sequences), indicating a homologous system in annelids, another large protostome phylum. Consistent with this, comprehensive sequence mining enabled the identification of SSRP-like sequences (and their corresponding orphan receptor) in annelids and several other protostome phyla. These results established the existence of SSRP-like peptides in many major branches of bilaterians and challenge the prevailing hypothesis that deuterostome SSRPs and protostome allatostatin-C are orthologous peptide families. Finally, having a large set of predator-prey SSRP sequences available, we show that although the cone snail's signaling SSRP-like genes are under purifying selection, the venom consomatin genes experience rapid directional selection to target receptors in a changing mix of prey.

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.

P17 induces chemotaxis and differentiation of monocytes via MRGPRX2-mediated mast cell-line activation

BACKGROUND

P17, a peptide isolated from Tetramorium bicarinatum ant venom, is known to induce an alternative phenotype of human monocyte-derived macrophages via activation of an unknown G protein-coupled receptor (GPCR).

OBJECTIVE

We sought to investigate the mechanism of action and the immunomodulatory effects of P17 mediated through MRGPRX2 (Mas-related G protein-coupled receptor X2).

METHODS

To identify the GPCR for P17, we screened 314 GPCRs. Upon identification of MRGPRX2, a battery of in silico, in vitro, ex vivo, and in vivo assays along with the receptor mutation studies were performed. In particular, to investigate the immunomodulatory actions, we used β-hexosaminidase release assay, cytokine releases, quantification of mRNA expression, cell migration and differentiation assays, immunohistochemical labeling, hematoxylin and eosin, and immunofluorescence staining.

RESULTS

P17 activated MRGPRX2 in a dose-dependent manner in β-arrestin recruitment assay. In LAD2 cells, P17 induced calcium and β-hexosaminidase release. Quercetin- and short hairpin RNA-mediated knockdown of MRGPRX2 reduced P17-evoked β-hexosaminidase release. In silico and in vitro mutagenesis studies showed that residue Lys8 of P17 formed a cation-π interaction with the Phe172 of MRGPRX2 and [Ala8]P17 lost its activity partially. P17 activated LAD2 cells to recruit THP-1 and human monocytes in Transwell migration assay, whereas MRGPRX2-impaired LAD2 cells cannot. In addition, P17-treated LAD2 cells stimulated differentiation of THP-1 and human monocytes, as indicated by the enhanced expression of macrophage markers cluster of differentiation 11b and TNF-α by quantitative RT-PCR. Immunohistochemical and immunofluorescent staining suggested monocyte recruitment in mice ears injected with P17.

CONCLUSIONS

Our data provide novel structural information regarding the interaction of P17 with MRGPRX2 and intracellular pathways for its immunomodulatory action.

Identification of short peptide sequences that activate human mast cells via Mas-related G-protein coupled receptor member X2

Peptide based therapeutics are desirable owing to their high biological specificity. However, a number of these fail in clinical testing due to an adverse inflammatory response. Mast cells play a key role in directing the host response to drugs and related products. Although the role of FcεRI receptor is well known, Mas-related G-protein coupled receptor X2 (MRGPRX2) binding of endogenous peptides, and drugs will activate mast cells independent of FcεRI. Identifying peptides that activate mast cells through MRGPRX2, and their respective activation potency, can be used to reduce the failure rate of peptide therapeutics at clinical trial. Moreover, it will allow for peptide design where mast cell activation is actually desired. It was found that FRKKW and WNKWAL are two motifs that activate human LAD2 cells similar to PAMP-12 controls. Peptide activators of MRGPRX2 could be reduced to X_a-(Y)_{(n ≥ 3)}-X_b where: X_a is an aromatic residue; X_b is a hydrophobic residue; and Y is a minimum 3 residue long sequence, containing a minimum of one positively charged residue with the remainder being uncharged residues. Artificial peptides WKKKW and FKKKF were constructed to test this structural functionality and were similar to PAMP-12 controls. Peptides with different activation potentials were found where FRKKW = WKKKW = FKKKF > PAMP-12 = WNKWAL > YKKKY > FRKKANKWALSR = FRKKWNKAALSR > KWKWK > FRKK = WNKWA > KYKYK > NKWALSR = YKKY = WNK. These sequences should be considered when designing peptide-based therapeutics. STATEMENT OF SIGNIFICANCE: Mast cells release immune regulating molecules upon activation that direct host's immune response. MRGPRX2 receptor provides an alternate pathway for mast cell activation that is independent of FcεRI receptor. It is thought that mast cell activation through MRGPRX2 plays a critical role in high failure rates of drugs in clinical trials. Identifying peptide sequences that activate mast cells through MRGPRX2 can serve two important purposes, namely, sequences to avoid when designing peptide therapeutics, and artificial peptides with different activation potentials for mast cells. Herein, we have identified a general amino acid sequence that induces mast cell activation through MRGPRX2. Furthermore, by modulating the identified sequence, artificial peptides have been designed which activate mast cells by varying degrees for therapeutic applications.

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.

Atypical opioid receptors: unconventional biology and therapeutic opportunities

Endogenous opioid peptides and prescription opioid drugs modulate pain, anxiety and stress by activating four opioid receptors, namely μ (mu, MOP), δ (delta, DOP), κ (kappa, KOP) and the nociceptin/orphanin FQ receptor (NOP). Interestingly, several other receptors are also activated by endogenous opioid peptides and influence opioid-driven signaling and biology. However, they do not meet the criteria to be recognized as classical opioid receptors, as they are phylogenetically distant from them and are insensitive to classical non-selective opioid receptor antagonists (e.g. naloxone). Nevertheless, accumulating reports suggest that these receptors may be interesting alternative targets, especially for the development of safer analgesics. Five of these opioid peptide-binding receptors belong to the family of G protein-coupled receptors (GPCRs)-two are members of the Mas-related G protein-coupled receptor X family (MrgX1, MrgX2), two of the bradykinin receptor family (B_1, B₂), and one is an atypical chemokine receptor (ACKR3). Additionally, the ion channel N-methyl-d-aspartate receptors (NMDARs) are also activated by opioid peptides. In this review, we recapitulate the implication of these alternative receptors in opioid-related disorders and discuss their unconventional biology, with members displaying signaling to scavenging properties. We provide an overview of their established and emerging roles and pharmacology in the context of pain management, as well as their clinical relevance as alternative targets to overcome the hurdles of chronic opioid use. Given the involvement of these receptors in a wide variety of functions, including inflammation, chemotaxis, anaphylaxis or synaptic transmission and plasticity, we also discuss the challenges associated with the modulation of both their canonical and opioid-driven signaling.

MRGPRX2 and Adverse Drug Reactions

Many adverse reactions to therapeutic drugs appear to be allergic in nature, and are thought to be triggered by patient-specific Immunoglobulin E (IgE) antibodies that recognize the drug molecules and form complexes with them that activate mast cells. However, in recent years another mechanism has been proposed, in which some drugs closely associated with allergic-type events can bypass the antibody-mediated pathway and trigger mast cell degranulation directly by activating a mast cell-specific receptor called Mas-related G protein-coupled receptor X2 (MRGPRX2). This would result in symptoms similar to IgE-mediated events, but would not require immune priming. This review will cover the frequency, severity, and dose-responsiveness of allergic-type events for several drugs shown to have MRGPRX2 agonist activity. Surprisingly, the analysis shows that mild-to-moderate events are far more common than currently appreciated. A comparison with plasma drug levels suggests that MRGPRX2 mediates many of these mild-to-moderate events. For some of these drugs, then, MRGPRX2 activation may be considered a regular and predictable feature after administration of high doses.

The Neuropeptide Cortistatin Alleviates Neuropathic Pain in Experimental Models of Peripheral Nerve Injury

Neuropathic pain is one of the most severe forms of chronic pain caused by the direct injury of the somatosensory system. The current drugs for treating neuropathies have limited efficacies or show important side effects, and the development of analgesics with novel modes of action is critical. The identification of endogenous anti-nociceptive factors has emerged as an attractive strategy for designing new pharmacological approaches to treat neuropathic pain. Cortistatin is a neuropeptide with potent anti-inflammatory activity, recently identified as a natural analgesic peptide in several models of pain evoked by inflammatory conditions. Here, we investigated the potential analgesic effect of cortistatin in neuropathic pain using a variety of experimental models of peripheral nerve injury caused by chronic constriction or partial transection of the sciatic nerve or by diabetic neuropathy. We found that the peripheral and central injection of cortistatin ameliorated hyperalgesia and allodynia, two of the dominant clinical manifestations of chronic neuropathic pain. Cortistatin-induced analgesia was multitargeted, as it regulated the nerve damage-induced hypersensitization of primary nociceptors, inhibited neuroinflammatory responses, and enhanced the production of neurotrophic factors both at the peripheral and central levels. We also demonstrated the neuroregenerative/protective capacity of cortistatin in a model of severe peripheral nerve transection. Interestingly, the nociceptive system responded to nerve injury by secreting cortistatin, and a deficiency in cortistatin exacerbated the neuropathic pain responses and peripheral nerve dysfunction. Therefore, cortistatin-based therapies emerge as attractive alternatives for treating chronic neuropathic pain of different etiologies.

MRGPRX2 activation as a rapid, high-throughput mechanistic-based approach for detecting peptide-mediated human mast cell degranulation liabilities

Mast cells play key roles in allergy, anaphylaxis/anaphylactoid reactions, and defense against pathogens/toxins. These cells contain cytoplasmic granules with a wide spectrum of pleotropic mediators that are released upon activation. While mast cell degranulation (MCD) occurs upon clustering of the IgE receptor bound to IgE and antigen, MCD is also triggered through non-IgE-mediated mechanisms, one of which is via Mas-related G protein-coupled receptor X2 (MRGPRX2). MRGPRX2 can be activated by many basic biogenic amines and peptides. Consequently, MRGPRX2-mediated MCD is an important potential safety liability for peptide therapeutics. To facilitate peptide screening for this liability in early preclinical drug development, a rapid, high-throughput engineered CHO-K1 cell-based MRGPRX2 activation assay was evaluated and compared to histamine release in CD34⁺ stem cell-derived mature human mast cells as a reference assay, using 30 positive control and 29 negative control peptides for MCD. Both G protein-dependent (Ca²⁺ endpoint) and -independent (β-arrestin endpoint) pathways were assessed in the MRGPRX2 activation assay. The MRGPRX2 activation assay had a sensitivity of 100% for both Ca²⁺ and β-arrestin endpoints and a specificity of 93% (β-arrestin endpoint) and 83% (Ca²⁺ endpoint) compared to histamine release in CD34⁺ stem cell-derived mature human mast cells. These findings suggest that assessing MRGPRX2 activation in an engineered cell model can provide value as a rapid, high-throughput, economical mechanism-based screening tool for early MCD hazard identification during preclinical safety evaluation of peptide-based therapeutics.

Ligands and Signaling of Mas-Related G Protein-Coupled Receptor-X2 in Mast Cell Activation

Mas-related G protein-coupled receptor-X2 (MRGPRX2) is known as a novel receptor to activate mast cells (MCs). MRGPRX2 plays a dual role in promoting MC-dependent host defense and immunomodulation and contributing to the pathogenesis of pseudo-allergic drug reactions, pain, itching, and inflammatory diseases. In this article, we discuss the possible signaling pathways of MCs activation mediated by MRGPRX2 and summarize and classify agonists and inhibitors of MRGPRX2 in MCs activation. MRGPRX2 is a low-affinity and low-selectivity receptor, which allows it to interact with a diverse group of ligands. Diverse MRGPRX2 ligands utilize conserved residues in its transmembrane (TM) domains and carboxyl-terminus Ser/Thr residues to undergo ligand binding and G protein coupling. The coupling likely initiates phosphorylation cascades, induces Ca²⁺ mobilization, and causes degranulation and generation of cytokines and chemokines via MAPK and NF-κB pathways, resulting in MCs activation. Agonists of MRGPRX2 on MCs are divided into peptides (including antimicrobial peptides, neuropeptides, MC degranulating peptides, peptide hormones) and nonpeptides (including FDA-approved drugs). Inhibitors of MRGPRX2 include non-selective GPCR inhibitors, herbal extracts, small-molecule MRGPRX2 antagonists, and DNA aptamer drugs. Screening and classifying MRGPRX2 ligands and summarizing their signaling pathways would improve our understanding of MRGPRX2-mediated physiological and pathological effects on MCs.

Understanding mast cell heterogeneity at single cell resolution

Mast cells (MC)s are evolutionarily conserved, tissue-resident immune cells with diverse roles in allergy, cancer, and protection from infection by helminths and microorganisms. The significant diversity in MC development and tissue-specific functional characteristics has recently begun to be understood. Exciting developments in single-cell-based RNA, protein, and chromatin profiling technologies offer new opportunities to characterize MC heterogeneity and to uncover novel MC functions and subtypes; these developments might lead to new and clinically effective therapies for certain pathologies. In this review, we provide an overview of the current understanding of MC development and heterogeneity and discuss new insights gained from single-cell-based studies that may lead to future research directions and therapeutic opportunities.

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.

A group of cationic amphiphilic drugs activates MRGPRX2 and induces scratching behavior in mice

BACKGROUND

Mas gene-related G protein-coupled receptors (MRGPRs) are a G protein-coupled receptor family responsive to various exogenous and endogenous agonists, playing a fundamental role in pain and itch sensation. The primate-specific family member MRGPRX2 and its murine orthologue MRGPRB2 are expressed by mast cells mediating IgE-independent signaling and pseudoallergic drug reactions.

OBJECTIVES

Our aim was to increase knowledge about the function and regulation of MRGPRX2/MRGPRB2, which is of major importance in prevention of drug hypersensitivity reactions and drug-induced pruritus.

METHODS

To identify novel MRGPR (ant)agonists, we screened a library of pharmacologically active compounds by utilizing a high-throughput calcium mobilization assay. The identified hit compounds were analyzed for their pseudoallergic and pruritogenic effects in mice and human.

RESULTS

We found a class of commonly used drugs activating MRGPRX2 that, to a large extent, consists of antidepressants, antiallergic drugs, and antipsychotics. Three-dimensional pharmacophore modeling revealed structural similarities of the identified agonists, classifying them as cationic amphiphilic drugs. Mast cell activation was investigated by using the 3 representatively selected antidepressants clomipramine, paroxetine, and desipramine. Indeed, we were able to show a concentration-dependent activation and MRGPRX2-dependent degranulation of the human mast cell line LAD2 (Laboratory of Allergic Diseases-2). Furthermore, clomipramine, paroxetine, and desipramine were able to induce degranulation of human skin and murine peritoneal mast cells. These substances elicited dose-dependent scratching behavior following intradermal injection into C57BL/6 mice but less so in MRGPRB2-mutant mice, as well as wheal-and-flare reactions following intradermal injections in humans.

CONCLUSION

Our results contribute to the characterization of structure-activity relationships and functionality of MRGPRX2 ligands and facilitate prediction of adverse reactions such as drug-induced pruritus to prevent severe drug hypersensitivity reactions.

Minireview: Mas-related G protein-coupled receptor X2 activation by therapeutic drugs

Symptoms that resemble allergic reactions, such as pruritus, flushing, and hypotension, are common side effects of therapeutic drugs. In a true allergic reaction, Immunoglobulin E (IgE) antibodies recognize the drug and trigger mediator release from mast cells through cross-linking of IgE receptors. However, many drugs can bypass this pathway and can activate mast cells directly through MRGPRX2, a G protein-coupled receptor that responds to a wide range of small molecules, peptides, and proteins that have little in common except for a net positive charge. This review will provide an overview of MRGPRX2, including its expression pattern, studies of its pharmacology, and its orthologs. It also will review evidence for MRGPRX2 activation by many drugs closely associated with these reactions.

Mas-related G protein-coupled receptors (Mrgprs) - Key regulators of neuroimmune interactions

Interplay between physiological systems in the body plays a prominent role in health and disease. At the cellular level, such interplay is orchestrated through the binding of specific ligands to their receptors expressed on cell surface. G protein-coupled receptors (GPCR) are seven-transmembrane domain receptors that initiate various cellular responses and regulate homeostasis. In this review, we focus on particular GPCRs named Mas-related G protein-coupled receptors (Mrgprs) mainly expressed by sensory neurons and specialized immune cells. We describe the different subfamilies of Mrgprs and their specific ligands, as well as recent advances in the field that illustrate the role played by these receptors in neuro-immune biological processes, including itch, pain and inflammation in diverse organs.

Beyond somatosensation: Mrgprs in mucosal tissues

Mas-related G coupled receptors (Mrgprs) are a superfamily of receptors expressed in sensory neurons that are known to transmit somatic sensations from the skin to the central nervous system. Interestingly, Mrgprs have recently been implicated in sensory and motor functions of mucosal-associated neuronal circuits. The gastrointestinal and pulmonary tracts are constantly exposed to noxious stimuli. Therefore, it is likely that neuronal Mrgpr signaling pathways in mucosal tissues, akin to their family members expressed in the skin, might relay messages that alert the host when mucosal tissues are affected by damaging signals. Further, Mrgprs have been proposed to mediate the cross-talk between sensory neurons and immune cells that promotes host-protective functions at barrier sites. Although the mechanisms by which Mrgprs are activated in mucosal tissues are not completely understood, these exciting studies implicate Mrgprs as potential therapeutic targets for conditions affecting the intestinal and airway mucosa. This review will highlight the central role of Mrgpr signaling pathways in the regulation of homeostasis at mucosal tissues.

The role of Mrgprs in pain

Owing to their functional diversity, the Mas-related G-protein-coupled receptor (Mrgpr) family has a role in both itch and pain modulation. While primarily linked to pruritis, Mrgprs were originally characterized in small-diameter nociceptive neurons of dorsal root ganglia (DRG) and trigeminal ganglia. This review will focus on the role Mrgpr's have in pain physiology, discussing recent discoveries as well as how Mrgpr's may provide a new target for the treatment of pathological pain.

The Possible Uses and Challenges of Nanomaterials in Mast Cell Research

Mast cells are tissue-resident immune cells that are involved in inflammation and fibrosis but also serve beneficial roles, including tissue maintenance, angiogenesis, pathogen clearance, and immunoregulation. Their multifaceted response and the ability of their mediators to target multiple organs and tissues means that mast cells play important roles in numerous conditions, including asthma, atopic dermatitis, drug sensitivities, ischemic heart disease, Alzheimer disease, arthritis, irritable bowel syndrome, infections (parasites, bacteria and viruses), and cancer. As a result, mast cells have become an important target for drug discovery and diagnostic research. Recent work has focused on applying novel nanotechnologies to explore cell biology. In this brief review, we will highlight the use of nanomaterials to modify mast cell functions and will discuss the potential of these technologies as research tools for understanding mast cell biology.

MRGPRX2 sensing of cationic compounds-A bridge between nociception and skin diseases?

Mast cells are innate immune cells located at many barrier sites in the body and known to protect the host against environmental threats and to be involved in allergic diseases. More recently, new studies have investigated their roles in the regulation of skin inflammation and transmission of pain and itch sensations. Mast cell signalling through the Mas-related G protein-coupled receptor (MRGPR) X2 or its mouse orthologue MRGPRB2 has been reported to be one of the major mechanism by which mast cell can regulate such processes. MRGPRX2 and MRGPRB2 can induce mast cell degranulation upon binding to a broad panel of cationic molecules such as neuropeptides, bacteria-derived quorum sensing molecules, venom peptides, host defense peptides and, unfortunately, various FDA-approved drugs. Upon activation, mast cells release granule-associated proteases, lipids and multiple cytokines that can modulate vascular permeability, immune cells recruitment and activation status of tissue-projecting nociceptive sensory neurons (ie nociceptors). Here, we discuss the modality of MRGPRX2-dependent mast cell activation and its different consequences on the patterns of skin inflammation and associated diseases. We notably emphasize how MRGPRX2-dependent skin mast cell activation might trigger various pathological traits such as pruritus, pain and inflammation and therefore become a potential therapeutic target for inflammatory pain, itch, atopic dermatitis and drugs-induced injection site reactions.

Convergent Cortistatin losses parallel modifications in circadian rhythmicity and energy homeostasis in Cetacea and other mammalian lineages

The ancestors of Cetacea underwent profound morpho-physiological alterations. By displaying an exclusive aquatic existence, cetaceans evolved unique patterns of locomotor activity, vigilant behaviour, thermoregulation and circadian rhythmicity. Deciphering the molecular landscape governing many of these adaptations is key to understand the evolution of phenotypes. Here, we investigate Cortistatin (CORT), a neuropeptide displaying an important role in mammalian biorhythm regulation. This neuropeptide is a known neuroendocrine factor, stimulating slow-wave sleep, but also involved in the regulation of energy metabolism and hypomotility inducement. We assessed the functional status of CORT in 359 mammalian genomes (25 orders), including 30 species of Cetacea. Our findings indicate that cetaceans and other mammals with atypical biorhythms, thermal constraints and/or energy metabolism, have accumulated deleterious mutations in CORT. In light of the pleiotropic action of this neuropeptide, we suggest that this inactivation contributed to a plethora of phenotypic adjustments to accommodate adaptive solutions to specific ecological niches.

Mas-related G protein-coupled receptor X2 and its activators in dermatologic allergies

The Mas-related G protein-coupled receptor X2 (MRGPRX2) is a multiligand receptor responding to various exogenous and endogenous stimuli. Being highly expressed on skin mast cells, MRGPRX2 triggers their degranulation and release of proinflammatory mediators, and it promotes multicellular signaling cascades, such as itch induction and transmission in sensory neurons. The expression of MRGPRX2 by skin mast cells and the levels of the MRGPRX2 agonists (eg, substance P, major basic protein, eosinophil peroxidase) are upregulated in the serum and/or skin of patients with inflammatory and pruritic skin diseases, such as chronic spontaneous urticaria or atopic dermatitis. Therefore, MRGPRX2 and its agonists might be potential biomarkers for the progression of cutaneous inflammatory diseases and the response to treatment. In addition, they may represent promising targets for prevention and treatment of signs and symptoms in patients with skin diseases or drug reactions. To assess this possibility, this review explores the role and relevance of MRGPRX2 and its activators in cutaneous inflammatory disorders and chronic pruritus.

Physiology and Pathophysiology of Itch

Itch is a topic to which everyone can relate. The physiological roles of itch are increasingly understood and appreciated. The pathophysiological consequences of itch impact quality of life as much as pain. These dynamics have led to increasingly deep dives into the mechanisms that underlie and contribute to the sensation of itch. When the prior review on the physiology of itching was published in this journal in 1941, itch was a black box of interest to a small number of neuroscientists and dermatologists. Itch is now appreciated as a complex and colorful Rubik's cube. Acute and chronic itch are being carefully scratched apart and reassembled by puzzle solvers across the biomedical spectrum. New mediators are being identified. Mechanisms blur boundaries of the circuitry that blend neuroscience and immunology. Measures involve psychophysics and behavioral psychology. The efforts associated with these approaches are positively impacting the care of itchy patients. There is now the potential to markedly alleviate chronic itch, a condition that does not end life, but often ruins it. We review the itch field and provide a current understanding of the pathophysiology of itch. Itch is a disease, not only a symptom of disease.

Tachykinin-1 receptor antagonism suppresses substance-P- and compound 48/80-induced mast cell activation from rat mast cells expressing functional mas-related GPCR B3

OBJECTIVE

Mice and rats are important animal models for mast cell (MC) study. However, rat Mas-related-GPCR-B3 receptor (MRGPRB3) has been less studied than its mouse counterpart. Therefore, we aimed to characterize rat MRGPRB3.

METHODS

Mrgprb3 mRNA expression was assessed in peritoneal cells (RPCs) and peritoneal MCs (RPMCs) of wild-type rats, RPCs of MC-deficient rats, and RBL-2H3 cells by reverse-transcriptase polymerase chain reaction (RT-PCR). RPMCs, MRGPRX2-transfected and non-transfected RBL-2H3 cells were activated by 15-30 min incubation with DNP-BSA, substance-P (SP), or compound-48/80. L732138 or CP96344 was used as a tachykinin/neurokinin-1-receptor antagonist. Histamine release from MCs was measured by HPLC fluorometry.

RESULTS

Mrgprb3 mRNA expression was found in all cells, with the highest level in wild-type RPCs. All cells responded to DNP-BSA, but only MRGPRX2-transfected-RBL-2H3 cells and RPMCs responded to all activators. L732138 (0.1-10 μM) and CP96344 (1-100 μM) suppressed SP (10 μM)-induced RPMC activation. L732138 inhibition was dose independent, whereas CP96344 inhibition occurred in a dose-dependent manner. Additionally, only CP96344 suppressed SP (100 μM)- and compound-48/80 (10 μg/mL)-induced RPMC activation.

CONCLUSIONS

RPMCs expressing functional MRGPRB3 response upon MRGPRX2 ligands to regulated MC-mediated activities. It`s provide novel insights for future pseudo-allergic studies in rodents.

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

Mast cells are inflammatory immune cells that play an essential role in mediating allergic reactions in humans. It is well-known that mast cell activation is critically regulated by intracellular calcium ion (Ca²⁺) concentrations. MAS-related G-protein coupled receptor-X2 (MRGPRX2) is a G-protein coupled receptor (GPCR) expressed on mast cells that is activated by various ligands, including several FDA approved drugs; consequently, this receptor has been implicated in causing pseudo-allergic reactions in humans. MRGPRX2 activation leads to an increase in intracellular Ca²⁺ levels; however, the Ca²⁺ mobilizing mechanisms utilized by this receptor are largely unknown. Previous reports showed that store-operated Ca²⁺ entry (SOCE) via the calcium sensor, stromal interaction molecule 1 (STIM1), regulates mast cell response induced by the high-affinity IgE receptor (FcεRI). In this study, using complementary pharmacologic and genetic ablation approaches we demonstrate that SOCE through STIM1 promotes MRGPRX2-induced human mast cell response in vitro. Importantly, SOCE also critically modulates MrgprB2 (mouse ortholog of human MRGPRX2) dependent inflammation in in vivo mouse models of pseudo-allergy. Collectively, our data suggests that MRGPRX2/MrgprB2 activation of mast cells is dependent on SOCE via STIM1, and further characterization of the MRGPRX2-SOCE-STIM1 pathway will lead to the identification of novel targets for the treatment of pseudo-allergic reactions in humans.

Peptides of major basic protein and eosinophil cationic protein activate human mast cells

The eosinophil granule proteins, major basic protein (MBP) and eosinophil cationic protein (ECP), activate mast cells during inflammation; however the mechanism responsible for this activity is poorly understood. We found that some theoretical tryptase-digested fragments of MBP and ECP induced degranulation of human cord blood-derived mast cells (HCMCs). The spectrum of activities of these peptides in HCMCs coincided with intracellular Ca²⁺ mobilization activities in Mas-related G-protein coupled receptor family member X2 (MRGPRX2)-expressing HEK293 cells. Two peptides corresponding to MBP residues 99–110 (MBP (99–110)) and ECP residues 29–45 (ECP (29–45)), respectively, induced degranulation of HCMCs and intracellular Ca²⁺ mobilization in MRGPRX2-expressing HEK293 cells in a concentration-dependent manner. Stimulation with MBP (99–110) or ECP (29–45) induced the production of prostaglandin D2 by HCMCs. The activities of MBP (99–110) and ECP (29–45) in both HCMCs and MRGPRX2-expressing HEK293 cells were inhibited by MRGPRX2-specific antagonists. In conclusion, these results indicated that MBP and ECP fragments activate HCMCs, and it may occur via MRGPRX2. Our findings suggest that tryptase-digested fragments of eosinophil cationic proteins acting via the MRGPRX2 pathway may further our understanding of mast cell/eosinophil communication.

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Mast cell activators were found in tryptase-fragments from MBP and ECP.

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These fragments activated MRGPRX2-expressing cells.

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MRGPRX2 antagonists inhibited mast cell activation caused by active fragments.

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Active fragments may work as mediators in mast cell/eosinophil communication.

Cortistatin, a novel cardiovascular protective peptide

Cortistatin (CST) is a small molecule bioactive peptide containing an FWKT tetramer. It is widely distributed in nervous, immune and endocrine systems. Many studies have shown that CST can exert many biological effects, for example: regulating sleep, learning and memory processes, inducing immune tolerance, inhibiting inflammatory responses, and regulating endocrine metabolism. Notably, it is found that CST and its receptors are also widely distributed in the cardiovascular system, such as the aorta, coronary arteries and heart. In recent years, increasing studies have shown that CST played an important role in the development of cardiovascular diseases, such as reducing myocardial damage, inhibiting autoimmune myocarditis, alleviating vascular smooth muscle cell (VSMC) proliferation and migration, reducing vascular calcification (VC), and inhibiting atherosclerosis and aneurysm formation. Therefore, we reviewed the cardiovascular effects of CST in the heart and blood vessels, which will help to understand the role of CST and its receptors in the pathogenesis of cardiovascular diseases, and highlight novel strategies and targets for the prevention and treatment of cardiovascular diseases.

Substance P and neurokinin 1 receptor are new targets for the treatment of chronic pruritus

BACKGROUND

Chronic pruritus is a distressing symptom associated with various dermatological conditions and systemic diseases. Current treatment options are often inadequate, resulting in impaired quality of life for many patients. An understanding of the underlying mechanisms of itch across pruritic conditions is important for development of effective, targeted treatments for chronic pruritus.

OBJECTIVES

To provide an overview of the pathogenesis of chronic pruritus, focusing on the role of substance P (SP) and neurokinin 1 receptor (NK₁ R) in itch signalling, and to describe data supporting NK₁ R antagonism as a potential strategy for the treatment of chronic pruritus.

METHODS

A PubMed search was conducted to determine what data were available that investigated the role of SP and NK₁ R in itch signalling.

RESULTS

SP is a neuropeptide that is a mediator of itch signalling. One of the target receptors for SP is NK₁ R, which is expressed in the central nervous system and on multiple cell types involved in the initiation and transmission of itch. Studies demonstrating that SP and NK₁ R are overexpressed across multiple chronic itch-inducing conditions and that NK₁ R antagonism disrupts itch signalling and reduces itch provide a rationale for targeting this pathway as a potential treatment of chronic pruritus across multiple diseases.

CONCLUSIONS

A large and growing body of evidence, including recent phase II clinical studies of NK₁ R antagonists, demonstrate that SP and NK₁ R play an important role in itch signalling. Additional studies are ongoing to further evaluate the use of NK₁ R antagonists for the treatment of chronic pruritus. What's already known about this topic? Chronic pruritus has a significant impact on quality of life. Current treatment options for chronic pruritus are inadequate. Substance P (SP) and neurokinin 1 receptor (NK₁ R) have been shown to play a role in itch signalling, and may be a rational target for addressing chronic pruritus. NK₁ R antagonists are being evaluated as potential treatment for chronic pruritus. What does this study add? This review provides a compilation of the most up-to-date data elucidating the role of SP and NK₁ R in itch signalling, which supports targeting this pathway as a potential treatment of chronic pruritus. NK₁ R antagonism disrupts itch signalling and reduces itch. A summary of the latest data on NK₁ R antagonists in the treatment of pruritus is provided.