The Origin, Expression, Function and Future Research Focus of a G Protein-coupled Receptor, Mas-related Gene X2 (MrgX2)

Development of an environmentally sensitive fluorescent peptide probe for MrgX2 and application in ligand screening of peptide antibiotics

Mast cells (MCs) are primary effector cells involved in immediate allergic reactions. Mas-related G protein-coupled receptor-X2 (MrgX2), which is highly expressed on MCs, is involved in receptor-mediated drug-induced pseudo-anaphylaxis. Many small-molecule drugs and peptides activate MrgX2, resulting in MC activation and allergic reactions. Although small-molecule drugs can be identified using existing MrgX2 ligand-screening systems, there is still a lack of effective means to screen peptide ligands. In this study, to screen for peptide drugs, the MrgX2 high-affinity endogenous peptide ligand substance P (SP) was used as a recognition group to design a fluorescent peptide probe. Spectroscopic properties and fluorescence imaging of the probe were assessed. The probe was then used to screen for MrgX2 agonists among peptide antibiotics. In addition, the effects of peptide antibiotics on MrgX2 activation were investigated in vivo and in vitro. The environment-sensitive property of the probe was revealed by the dramatic increase in fluorescence intensity after binding to the hydrophobic ligand-binding domain of MrgX2. Based on these characteristics, it can be used for in situ selective visualization of MrgX2 in live cells. The probe was used to screen ten types of peptide antibiotics, and we found that caspofungin and bacitracin could compete with the probe and are hence potential ligands of MrgX2. Pharmacological experiments confirmed this hypothesis; caspofungin and bacitracin activated MCs via MrgX2 in vitro and induced local anaphylaxis in mice. Our research can be expected to provide new ideas for screening MrgX2 peptide ligands and reveal the mechanisms of adverse reactions caused by peptide drugs, thereby laying the foundation for improving their clinical safety.

MrgprF acts as a tumor suppressor in cutaneous melanoma by restraining PI3K/Akt signaling

The incidence of cutaneous melanoma (CM) has been increasing annually worldwide. In this study, we identify that MrgprF, a MAS related GPR family member, is decreased in cutaneous melanoma tissues and cell lines due to hypermethylation of its promoter region, and show that patients with CM expressing high levels of MrgprF exhibit an improved clinical outcome. We demonstrate that MrgprF forced expression inhibits tumor cell proliferation, migration, xenograft tumor growth, and metastasis. On the contrary, MrgprF knockdown promotes tumor cell proliferation and transformation of immortalized human keratinocyte-HaCaT cells, supporting the inhibitory role of MrgprF during tumor progression. Mechanistic studies reveal that MrgprF reduces the phosphoinositol‑3‑kinase (PI3K) complex formation between p101 and p110γ subunits, the critical step for phosphatidylinositol-(3, 4)-P2 (PIP2) conversion to phosphatidylinositol-(3, 4, 5)-P3 (PIP3), and then reduces the activation of PI3K/Akt signaling. This effect can be reversed by Akt specific agonist SC79. In addition, AMG 706, a previously documented inhibitor for endothelial cell proliferation, is identified as a potential agonist for MrgprF, and can impede tumor growth both in vitro and in vivo. Taken together, our findings suggest that MrgprF, a novel tumor suppressor in cutaneous melanoma, may be useful as a therapeutic target in the future.

Design and synthesis of first environment-sensitive coumarin fluorescent agonists for MrgX2

Mas related G-protein-coupled receptor member X2 (MrgX2) has been identified as the crucial receptor in drug induced pseudo-allergic reactions and allergic diseases. In this research, the first type of fluorescent agonists (ZX1, ZX2 and ZX3) for MrgX2 were developed by conjugating environment-sensitive fluorophore coumarin to MrgX2 selective agonists (R)-ZINC-3573. Their environment-sensitive property was confirmed by the dramatically increase of fluorescent intensity after binding to the hydrophobic ligand binding domain MrgX2, which help to overcome the high background signal. Based on these characteristics, they can be used for selective visualization of MrgX2 in living cells even with their own background interference. Among these fluorescent agonists, compound ZX2 possessed splendid spectroscopic properties, outstanding pharmacological activities (EC₅₀ = 0.93 μM, K_D = 1.97 μM). And a competitive binding assay was established with ZX2 to analysis the binding affinity of MrgX2 agonists, which shown high coherence with the results of cell membrane chromatography. To our knowledge, these probes are the first fluorescent ligands of MrgX2 with agonistic activity and environment-sensitive property, which is expected to use for the development of MrgX2 molecular pharmacology and serve as a convenient high-throughput screening tool for the drug candidates targeting MrgX2.

Establishment of substance P modified affinity chromatography for specific detection and enrichment of Mas-related G protein-coupled receptor X2

Mas-related G protein-coupled receptor X2 (MrgX2) has been identified to be critical in drug-induced pseudo-allergic reactions and allergic diseases. Herein, an affinity high-performance liquid chromatography was established for the specific detection and enrichment of MrgX2. Substance P was used as an affinity ligand and immobilized on a glutaraldehyde-modified amino silica gel. The successful grafting of substance P was characterized by infrared spectroscopy, elemental analysis, X-ray photoelectron spectroscopy, thermogravimetric analysis, and nitrogen adsorption and desorption analyzes. The prepared materials were then used as the stationary phase to investigate the retention behavior of MrgX2 recombinant protein on the affinity column. The results obtained with the analytical techniques show the specificity and selectivity of the MrgX2 recombinant protein on the affinity column. The repeatability and reproducibility for the analysis of MrgX2 on the NH₂-Silico@GD@SP column show relative standard deviation (RSD) values lower than the acceptance criteria of 2 and 5% of retention time, and RSD of peak areas < 7%. The RSD value of the results obtained for the control of the activity of the prepared columns respond to the acceptance criteria of 5% and proves that the NH2-Silico@GD@SP column are stable until 48 h. The suitability of the NH₂-Silico@GD@SP column offline SEC system has been tested by using MrgX2 as positive control. The results of this experiment indicate that the offline system may be used to analyze the retention fraction. MrgX2 extracted from human mast cells LAD2 was also verified. An obvious retention can be observed and the natural MrgX2 was concentrated 114.6 times compared with the original complex components by using the affinity column. These results may provide a new approach for the specific detection and enrichment of G-protein-coupled receptors.

Targeted Therapy for Chronıc Spontaneous Urtıcarıa: Ratıonale and Recent Progress

Chronic spontaneous urticaria (CSU) is characterized by the presence of wheals, angioedema, or both for at least 6 weeks. It may persist for a long time-up to 50% of the patients have been reported to be symptomatic 5 years after the onset. Some patients can suffer more than one episode of CSU during their lifetime. Considering the recurrences, disabling symptoms, and significant impact on quality of life, proper and effective treatment of CSU is critical. The use of antihistamines (AHs) is still the mainstay of treatment. However, given the low rates of response to AHs (38.6% and 63.2% to standard doses and higher doses, respectively), the complete control of symptoms seems difficult to attain. The use of omalizumab for CSU has been a major breakthrough in the care of patients with CSU. However, the partial response and lack of response to omalizumab in a subgroup of patients, as high as 70% in some studies, make the development of alternative treatments desirable. Ever-increasing knowledge on the pathogenesis is making new target molecules available and enabling drug development for CSU. In addition to drug repurposing as in anti-IL-4/13, IL-5, and IL-17 antibodies, novel targeted therapy options such as ligelizumab and Bruton's tyrosine kinase inhibitors are currently undergoing clinical trials and will be available in the near future. This article reviews the current challenges in the treatment of CSU, the pathogenesis and potential target molecules, and the rationale for novel treatments and their rapidly developing status.

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.

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.

Involvement of Neuro-Immune Interactions in Pruritus With Special Focus on Receptor Expressions

Pruritus is a common, but very challenging symptom with a wide diversity of underlying causes like dermatological, systemic, neurological and psychiatric diseases. In dermatology, pruritus is the most frequent symptom both in its acute and chronic form (over 6 weeks in duration). Treatment of chronic pruritus often remains challenging. Affected patients who suffer from moderate to severe pruritus have a significantly reduced quality of life. The underlying physiology of pruritus is very complex, involving a diverse network of components in the skin including resident cells such as keratinocytes and sensory neurons as well as transiently infiltrating cells such as certain immune cells. Previous research has established that there is a significant crosstalk among the stratum corneum, nerve fibers and various immune cells, such as keratinocytes, T cells, basophils, eosinophils and mast cells. In this regard, interactions between receptors on cutaneous and spinal neurons or on different immune cells play an important role in the processing of signals which are important for the transmission of pruritus. In this review, we discuss the role of various receptors involved in pruritus and inflammation, such as TRPV1 and TRPA1, IL-31RA and OSMR, TSLPR, PAR-2, NK1R, H1R and H4R, MRGPRs as well as TrkA, with a focus on interaction between nerve fibers and different immune cells. Emerging evidence shows that neuro-immune interactions play a pivotal role in mediating pruritus-associated inflammatory skin diseases such as atopic dermatitis, psoriasis or chronic spontaneous urticaria. Targeting these bidirectional neuro-immune interactions and the involved pruritus-specific receptors is likely to contribute to novel insights into the underlying pathogenesis and targeted treatment options of pruritus.

Harnessing the Power of Mast Cells in unconventional Immunotherapy Strategies and Vaccine Adjuvants

Mast cells are long-lived, granular, myeloid-derived leukocytes that have significant protective and repair functions in tissues. Mast cells sense disruptions in the local microenvironment and are first responders to physical, chemical and biological insults. When activated, mast cells release growth factors, proteases, chemotactic proteins and cytokines thereby mobilizing and amplifying the reactions of the innate and adaptive immune system. Mast cells are therefore significant regulators of homeostatic functions and may be essential in microenvironmental changes during pathogen invasion and disease. During infection by helminths, bacteria and viruses, mast cells release antimicrobial factors to facilitate pathogen expulsion and eradication. Mast cell-derived proteases and growth factors protect tissues from insect/snake bites and exposure to ultraviolet radiation. Finally, mast cells release mediators that promote wound healing in the inflammatory, proliferative and remodelling stages. Since mast cells have such a powerful repertoire of functions, targeting mast cells may be an effective new strategy for immunotherapy of disease and design of novel vaccine adjuvants. In this review, we will examine how certain strategies that specifically target and activate mast cells can be used to treat and resolve infections, augment vaccines and heal wounds. Although these strategies may be protective in certain circumstances, mast cells activation may be deleterious if not carefully controlled and any therapeutic strategy using mast cell activators must be carefully explored.

LL-37 restored glucocorticoid sensitivity impaired by virus dsRNA in lung

Glucocorticoids play a key role in treatment of inflammatory lung diseases including both airway and parenchymal lung diseases. RNA viral infections are major causes of chronic lung disease exacerbations and can determine glucocorticoid resistance. The antibacterial peptide LL-37, the only member of human cathelicidin family, also functions as antiviral-activity enhancer. However, whether it can alleviate the glucocorticoid resistance caused by RNA viruses remains unclear. Here, we used type I (BEAS-2B) and type II (A549) lung epithelial cells to assess the effect of LL-37 on dsRNA-induced glucocorticoid resistance. We verified that LL-37 and polyinosinic-polycytidylic acid (poly I:C, a mimic of viral dsRNA) interact and enter both cell lines. Co-treatment with LL-37 and poly I:C increased glucocorticoid-induced expression of promyelocytic leukemia zinc finger (PLZF), an anti-inflammatory protein, compared to poly I:C alone. Pre-treatment with LL-37 also restored transactivation of the glucocorticoid response element (GRE). Moreover, LL-37 rescued poly I:C-induced glucocorticoid resistance by increasing phosphorylation and nuclear translocation of glucocorticoid receptor. Importantly, LL-37 downregulated poly I:C-induced Erk and Akt signaling pathways in lung epithelial cells. Finally, we verified our data in vivo, showing that mCRAMP, the mouse LL-37 ortholog, can alleviate poly I:C-induced glucocorticoid insensitivity in a murine asthma model. In summary, this study showed that LL-37 restored glucocorticoid sensitivity impaired by dsRNA possibly by inhibiting Akt pathway, in addition to Erk1/2 pathway. These findings suggest LL-37 as a therapeutic agent for treatment of viral infections in inflammatory pulmonary diseases.

LL-37-induced human mast cell activation through G protein-coupled receptor MrgX2

Human LL-37 is an important class of cationic antimicrobial peptide (CAP) that is known to stimulate mast cell activation. While many studies have been conducted on LL-37, to date little is known about the functional receptors for LL-37-induced human mast cell activation, in particular in terms of the release of de novo synthesized mediators. Thus, the aim of the present study is to identify the functional receptors for LL-37-induced human mast cell activation in terms of the degranulation and release of de novo synthesized mediators and investigate the downstream signalling pathways involved in mast cell activation. Overall, our study importantly demonstrates that LL-37-induced human mast cell degranulation and release of de novo synthesized mediators function primarily through the activation of MrgX2. We furthermore show that LL-37-induced human mast cell line LAD2 cells are involved in the degranulation and release of IL-8, and that FPRL1 and P2X7 have only a partial effect on IL-8 release, and no effect on mast cell degranulation triggered by LL-37. Instead, we find that silencing the expression of MrgX2 in human mast cell significantly inhibits the LL-37-induced degranulation and release of IL-8. Overall, this effect is associated with the activation of the G_i protein, PLC/PKC/Calcium/NFAT, PI3K/Akt and MAPKs signalling pathways.

The dual role of cathelicidins in systemic inflammation

Antimicrobial peptides are key components of the innate immune system. They act as broad-spectrum antimicrobial agents against Gram-positive and -negative bacteria, viruses, and fungi. More recently, antimicrobial peptides have been ascribed immunomodulatory functions, including roles in wound healing, induction of cytokines, and altering host gene expression. Cathelicidins are a class of antimicrobial peptide found in humans, mice, and rats, among others. Known as LL-37 in humans and cathelin-related antimicrobial peptide (CRAMP) in rodents, cathelicidins are produced by many different cells, including macrophages, neutrophils, and epithelial cells. The role of cathelicidins is somewhat confounding, as they exhibit both pro-and anti-inflammatory activity. A major obstacle in the study of cathelicidins is the inability of exogenous LL-37 or CRAMP to mimic the activity of their endogenous counterparts. Nevertheless, studies have shown that LL-37 is recognized by multiple receptors, and may stabilize or modulate Toll-like receptor signaling. In addition, cathelicidins play a role in apoptosis, inflammasome activation, and phagocytosis. However, many studies are revealing the dual effects of cathelicidins. For example, CRAMP appears to be protective in models of group A Streptococcus skin infection, pneumonia, and meningitis, but detrimental in cases of severe bacterial infection, such as septic shock. It is becoming increasingly clear that the activity of cathelicidins is modulated by complex interactions with the microenvironment, as well as the disease background. This article reviews what is currently known about the activity of cathelicidins in an attempt to understand their complex roles in systemic diseases.

Molecular mechanisms of LL-37-induced receptor activation: An overview

The human cathelicidin peptide LL-37 plays a crucial role in the immune system on many levels, from the first line of defense in epithelial cells to restoring the tissue after infection. On host cells, the majority of the LL-37-induced effects are mediated via the direct or indirect activation of several structurally unrelated cell surface receptors or intracellular targets. How LL-37 is able to affect multiple receptors is currently not well understood. So far, the mechanistic details underlying receptor activation are poorly investigated and evidence for a conventional ligand/receptor interaction is scarce. Over the past few decades, a large number of studies have reported on the activation of a receptor and/or components of the downstream signal transduction pathway induced by LL-37. This review summarizes the current knowledge on molecular mechanisms underlying LL-37-induced receptor activation.