PMX-53 as a dual CD88 antagonist and an agonist for Mas-related gene 2 (MrgX2) in human mast cells

The Novel Receptor C5aR2 Is Required for C5a-Mediated Human Mast Cell Adhesion, Migration, and Proinflammatory Mediator Production

C5a generated during complement activation possesses proinflammatory and immunoregulatory properties critical for the development and modulation of allergic immune responses. In immune cells, C5a mediates its effects through binding to two G protein-coupled receptors, C5aR1 and C5aR2. Mast cells are key effectors in allergic reactions, and decades of research have suggested that the majority of C5a effects on mast cells are mediated through C5aR1, whereas the expression and function of C5aR2 have not been explored. We demonstrated that the human mast cell line Laboratory of Allergic Diseases 2 (LAD2) expresses surface C5aR2 but not C5aR1, whereas CD34(+) cell-derived primary mast cells do not express surface C5aR1 or C5aR2. Stem cell factor and IL-4 upregulated C5aR2 expression on LAD2 cells. Furthermore, C5a caused internalization of LAD2 cell-surface C5aR2. We therefore used LAD2 cells as a model to study C5a/C5aR2-induced biological responses and signaling in human mast cells. We found that whereas C5a was unable to induce degranulation, it stimulated GM-CSF, TNF, CXCL10, and CCL2 production. C5a caused ERK phosphorylation, a signaling molecule important in cytokine and chemokine generation. In addition, C5a stimulated adhesion and chemotaxis of mast cells. Wortmannin, an inhibitor of PI3K, and small interfering RNA against β-arrestin-2 blocked C5a-induced adhesion. Silencing of C5aR2 using lentiviral short hairpin RNA rendered the cells unresponsive to C5a-induced adhesion, chemotaxis, and mediator release, as well as ERK phosphorylation. Overall, this study reveals a novel role for C5aR2 in C5a-mediated activation of mast cells and demonstrates that C5aR2 ligation initiates a β-arrestin-2-, PI3K-, and ERK-dependent signaling pathway in these cells.

RNA-Seq Analysis of Human Trigeminal and Dorsal Root Ganglia with a Focus on Chemoreceptors

The chemosensory capacity of the somatosensory system relies on the appropriate expression of chemoreceptors, which detect chemical stimuli and transduce sensory information into cellular signals. Knowledge of the complete repertoire of the chemoreceptors expressed in human sensory ganglia is lacking. This study employed the next-generation sequencing technique (RNA-Seq) to conduct the first expression analysis of human trigeminal ganglia (TG) and dorsal root ganglia (DRG). We analyzed the data with a focus on G-protein coupled receptors (GPCRs) and ion channels, which are (potentially) involved in chemosensation by somatosensory neurons in the human TG and DRG. For years, transient receptor potential (TRP) channels have been considered the main group of receptors for chemosensation in the trigeminal system. Interestingly, we could show that sensory ganglia also express a panel of different olfactory receptors (ORs) with putative chemosensory function. To characterize OR expression in more detail, we performed microarray, semi-quantitative RT-PCR experiments, and immunohistochemical staining. Additionally, we analyzed the expression data to identify further known or putative classes of chemoreceptors in the human TG and DRG. Our results give an overview of the major classes of chemoreceptors expressed in the human TG and DRG and provide the basis for a broader understanding of the reception of chemical cues.

Pharmacology and signaling of MAS-related G protein-coupled receptors

Signaling by heptahelical G protein-coupled receptors (GPCR) regulates many vital body functions. Consequently, dysfunction of GPCR signaling leads to pathologic states, and approximately 30% of all modern clinical drugs target GPCR. One decade ago, an entire new GPCR family was discovered, which was recently named MAS-related G protein-coupled receptors (MRGPR) by the HUGO Gene Nomenclature Committee. The MRGPR family consists of ∼40 members that are grouped into nine distinct subfamilies (MRGPRA to -H and -X) and are predominantly expressed in primary sensory neurons and mast cells. All members are formally still considered "orphan" by the Committee on Receptor Nomenclature and Drug Classification of the International Union of Basic and Clinical Pharmacology. However, several distinct peptides and amino acids are discussed as potential ligands, including β-alanine, angiotensin-(1-7), alamandine, GABA, cortistatin-14, and cleavage products of proenkephalin, pro-opiomelanocortin, prodynorphin, or proneuropeptide-FF-A. The full spectrum of biologic roles of all MRGPR is still ill-defined, but there is evidence pointing to a role of distinct MRGPR subtypes in nociception, pruritus, sleep, cell proliferation, circulation, and mast cell degranulation. This review article summarizes findings published in the last 10 years on the phylogenetic relationships, pharmacology, signaling, physiology, and agonist-promoted regulation of all MRGPR subfamilies. Furthermore, we highlight interactions between MRGPR and other hormonal systems, paying particular attention to receptor multimerization and morphine tolerance. Finally, we discuss the challenges the field faces presently and emphasize future directions of research.

Expression of Mas-related gene X2 on mast cells is upregulated in the skin of patients with severe chronic urticaria

BACKGROUND

Wheal reactions to intradermally injected neuropeptides, such as substance P (SP) and vasoactive intestinal peptide, are significantly larger and longer lasting in patients with chronic urticaria (CU) than in nonatopic control (NC) subjects. Mas-related gene X2 (MrgX2) has been identified as a receptor for basic neuropeptides, such as SP and vasoactive intestinal peptide. Mast cell (MC) responsiveness to eosinophil mediators contributes to the late-phase reaction of allergy.

OBJECTIVE

We sought to compare the frequency of MrgX2 expression in skin MCs from patients with CU and NC subjects and to identify the receptor for basic eosinophil granule proteins on human skin MCs.

METHODS

MrgX2 expression was investigated by using immunofluorescence in skin tissues from NC subjects and patients with severe CU and on skin-derived cultured MCs. MrgX2 expression in human MCs was reduced by using a lentiviral small hairpin RNA silencing technique. Ca(2+) influx was measured in CHO cells transfected with MrgX2 in response to eosinophil granule proteins. Histamine and prostaglandin D2 levels were measured by using enzyme immunoassays.

RESULTS

The number of MrgX2(+) skin MCs and the percentage of MrgX2(+) MCs in all MCs in patients with CU were significantly greater than those in NC subjects. Eosinophil infiltration in urticarial lesions was observed in 7 of 9 patients with CU. SP, major basic protein, and eosinophil peroxidase, but not eosinophil-derived neurotoxin, induced histamine release from human skin MCs through MrgX2.

CONCLUSION

MrgX2 might be a new target molecule for the treatment of wheal reactions in patients with severe CU.

International Union of Basic and Clinical Pharmacology. [corrected]. LXXXVII. Complement peptide C5a, C4a, and C3a receptors

The activation of the complement cascade, a cornerstone of the innate immune response, produces a number of small (74-77 amino acid) fragments, originally termed anaphylatoxins, that are potent chemoattractants and secretagogues that act on a wide variety of cell types. These fragments, C5a, C4a, and C3a, participate at all levels of the immune response and are also involved in other processes such as neural development and organ regeneration. Their primary function, however, is in inflammation, so they are important targets for the development of antiinflammatory therapies. Only three receptors for complement peptides have been found, but there are no satisfactory antagonists as yet, despite intensive investigation. In humans, there is a single receptor for C3a (C3a receptor), no known receptor for C4a, and two receptors for C5a (C5a₁ receptor and C5a₂ receptor). The most recently characterized receptor, the C5a₂ receptor (previously known as C5L2 or GPR77), has been regarded as a passive binding protein, but signaling activities are now ascribed to it, so we propose that it be formally identified as a receptor and be given a name to reflect this. Here, we describe the complex biology of the complement peptides, introduce a new suggested nomenclature, and review our current knowledge of receptor pharmacology.

β-Defensins activate human mast cells via Mas-related gene X2

Human β-defensins (hBDs) stimulate degranulation in rat peritoneal mast cells in vitro and cause increased vascular permeability in rats in vivo. In this study, we sought to determine whether hBDs activate murine and human mast cells and to delineate the mechanisms of their regulation. hBD2 and hBD3 did not induce degranulation in murine peritoneal or bone marrow-derived mast cells (BMMC) in vitro and had no effect on vascular permeability in vivo. By contrast, these peptides induced sustained Ca(2+) mobilization and substantial degranulation in human mast cells, with hBD3 being more potent. Pertussis toxin (PTx) had no effect on hBD-induced Ca(2+) mobilization, but La(3+) and 2-aminoethoxydiphenyl borate (a dual inhibitor of inositol 1,4,5-triphosphate receptor and transient receptor potential channels) caused substantial inhibition of this response. Interestingly, degranulation induced by hBDs was substantially inhibited by PTx, La(3+), or 2-aminoethoxydiphenyl borate. Whereas human mast cells endogenously express G protein-coupled receptor, Mas-related gene X2 (MrgX2), rat basophilic leukemia, RBL-2H3 cells, and murine BMMCs do not. Silencing the expression of MrgX2 in human mast cells inhibited hBD-induced degranulation, but had no effect on anaphylatoxin C3a-induced response. Furthermore, ectopic expression of MrgX2 in RBL-2H3 and murine BMMCs rendered these cells responsive to hBDs for degranulation. This study demonstrates that hBDs activate human mast cells via MrgX2, which couples to both PTx-sensitive and insensitive signaling pathways most likely involving Gαq and Gαi to induce degranulation. Furthermore, murine mast cells are resistant to hBDs for degranulation, and this reflects the absence of MrgX2 in these cells.

Expression and distribution patterns of Mas-related gene receptor subtypes A-H in the mouse intestine: inflammation-induced changes

Mas-related gene (Mrg) receptors constitute a subfamily of G protein-coupled receptors that are implicated in nociception, and are as such considered potential targets for pain therapies. Furthermore, some Mrgs have been suggested to play roles in the regulation of inflammatory responses to non-immunological activation of mast cells and in mast cell-neuron communication. Except for MrgD, E and F, whose changed expression has been revealed during inflammation in the mouse intestine in our earlier studies, information concerning the remaining cloned mouse Mrg subtypes in the gastrointestinal tract during (patho) physiological conditions is lacking. Therefore, the present study aimed at identifying the presence and putative function of these remaining cloned Mrg subtypes (n = 19) in the (inflamed) mouse intestine. Using reverse transcriptase-PCR, quantitative-PCR and multiple immunofluorescence staining with commercial and newly custom-developed antibodies, we compared the ileum and the related dorsal root ganglia (DRG) of non-inflamed mice with those of two models of intestinal inflammation, i.e., intestinal schistosomiasis and 2,4,6-trinitrobenzene sulfonic acid-induced ileitis. In the non-inflamed ileum and DRG, the majority of the Mrg subtypes examined were sparsely expressed, showing a neuron-specific expression pattern. However, significant changes in the expression patterns of multiple Mrg subtypes were observed in the inflamed ileum; for instance, MrgA4, MrgB2and MrgB8 were expressed in a clearly increased number of enteric sensory neurons and in nerve fibers in the lamina propria, while de novo expression of MrgB10 was observed in enteric sensory neurons and in newly recruited mucosal mast cells (MMCs). The MrgB10 expressing MMCs were found to be in close contact with nerve fibers in the lamina propria. This is the first report on the expression of all cloned Mrg receptor subtypes in the (inflamed) mouse intestine. The observed changes in the expression and cellular localization of the Mrg subtypes suggest that these receptors are involved in the mediation of primary afferent responses, mast cell responses, and in neuroimmune communication during intestinal inflammation.

Human Mas-related G protein-coupled receptors-X1 induce chemokine receptor 2 expression in rat dorsal root ganglia neurons and release of chemokine ligand 2 from the human LAD-2 mast cell line

Primate-specific Mas-related G protein-coupled receptors-X1 (MRGPR-X1) are highly enriched in dorsal root ganglia (DRG) neurons and induce acute pain. Herein, we analyzed effects of MRGPR-X1 on serum response factors (SRF) or nuclear factors of activated T cells (NFAT), which control expression of various markers of chronic pain. Using HEK293, DRG neuron-derived F11 cells and cultured rat DRG neurons recombinantly expressing human MRGPR-X1, we found activation of a SRF reporter gene construct and induction of the early growth response protein-1 via extracellular signal-regulated kinases-1/2 known to play a significant role in the development of inflammatory pain. Furthermore, we observed MRGPR-X1-induced up-regulation of the chemokine receptor 2 (CCR2) via NFAT, which is considered as a key event in the onset of neuropathic pain and, so far, has not yet been described for any endogenous neuropeptide. Up-regulation of CCR2 is often associated with increased release of its endogenous agonist chemokine ligand 2 (CCL2). We also found MRGPR-X1-promoted release of CCL2 in a human connective tissue mast cell line endogenously expressing MRGPR-X1. Thus, we provide first evidence to suggest that MRGPR-X1 induce expression of chronic pain markers in DRG neurons and propose a so far unidentified signaling circuit that enhances chemokine signaling by acting on two distinct yet functionally co-operating cell types. Given the important role of chemokine signaling in pain chronification, we propose that interruption of this signaling circuit might be a promising new strategy to alleviate chemokine-promoted pain.

Roles for NHERF1 and NHERF2 on the regulation of C3a receptor signaling in human mast cells

Background

The anaphylatoxin C3a binds to the G protein coupled receptor (GPCR, C3aR) and activates divergent signaling pathways to induce degranulation and cytokine production in human mast cells. Adapter proteins such as the Na⁺/H⁺ exchange regulatory factor (NHERF1 and NHERF2) have been implicated in regulating functions of certain GPCRs by binding to the class I PDZ (PSD-95/Dlg/Zo1) motifs present on their cytoplasmic tails. Although C3aR possesses a class I PDZ motif, the possibility that it interacts with NHERF proteins to modulate signaling in human mast cells has not been determined.

Methodology/Principal Findings

Using reverse transcription PCR and Western blotting, we found that NHERF1 and NHERF2 are expressed in human mast cell lines (HMC-1, LAD2) and CD34⁺-derived primary human mast cells. Surprisingly, however, C3aR did not associate with these adapter proteins. To assess the roles of NHERFs on signaling downstream of C3aR, we used lentiviral shRNA to stably knockdown the expression of these proteins in human mast cells. Silencing the expression of NHERF1 and NHERF2 had no effect on C3aR desensitization, agonist-induced receptor internalization, ERK/Akt phosphorylation or chemotaxis. However, loss of NHERF1 and NHERF2 resulted in significant inhibition of C3a-induced mast cell degranulation, NF-κB activation and chemokine production.

Conclusion/Significance

This study demonstrates that although C3aR possesses a class I PDZ motif, it does not associate with NHERF1 and NHERF2. Surprisingly, these proteins provide stimulatory signals for C3a-induced degranulation, NF-κB activation and chemokine generation in human mast cells. These findings reveal a new level of complexity for the functional regulation of C3aR by NHERFs in human mast cells.

New developments in C5a receptor signaling

Complement activation usually results in the formation of complement fragment 5a (C5a) that interacts with its two receptors, C5aR and C5L2. These receptors belong to the rhodopsin family of G protein-coupled seven transmembrane-containing receptors. C5aR and C5L2 are expressed on/in a wide variety of cells and tissues. Interaction of C5a with C5aR leads to many pleiotropic effects, including the release of cytokines and chemokines and recruitment of inflammatory cells. In certain circumstances, C5a-C5aR interactions can also result in pathophysiological changes as seen in sepsis, rheumatoid arthritis, asthma, acute lung injury and ischemia-reperfusion injury. This overview of the C5a-C5aR interactions describes how such interactions facilitate the pivotal role the complement system plays in the host's innate and adaptive responses.

Mas-related gene X2 (MrgX2) is a novel G protein-coupled receptor for the antimicrobial peptide LL-37 in human mast cells: resistance to receptor phosphorylation, desensitization, and internalization

Human LL-37 is a multifunctional antimicrobial peptide that promotes inflammation, angiogenesis, wound healing, and tumor metastasis. Most effects of LL-37 are mediated via the activation of the cell surface G protein-coupled receptor FPR2 on leukocytes and endothelial cells. Although LL-37 induces chemotaxis, degranulation, and chemokine production in mast cells, the receptor involved and the mechanism of its regulation remain unknown. MrgX2 is a member of Mas-related genes that is primarily expressed in human dorsal root ganglia and mast cells. We found that a human mast cell line LAD2 and CD34(+) cell-derived primary mast cells, which natively express MrgX2, responded to LL-37 for sustained Ca(2+) mobilization and substantial degranulation. However, an immature human mast cell line, HMC-1, that lacks functional MrgX2 did not respond to LL-37. shRNA-mediated knockdown of MrgX2 in LAD2 mast cell line and primary CD34(+) cell-derived mast cells caused a substantial reduction in LL-37-induced degranulation. Furthermore, mast cell lines stably expressing MrgX2 responded to LL-37 for chemotaxis, degranulation, and CCL4 production. Surprisingly, MrgX2 was resistant to LL-37-induced phosphorylation, desensitization, and internalization. In addition, shRNA-mediated knockdown of the G protein-coupled receptor kinases (GRK2 and GRK3) had no effect on LL-37-induced mast cell degranulation. This study identified MrgX2 as a novel G protein-coupled receptor for the antibacterial peptide LL-37 and demonstrated that unlike most G protein-coupled receptors it is resistant to agonist-induced receptor phosphorylation, desensitization, and internalization.

Regulation of C3a receptor signaling in human mast cells by G protein coupled receptor kinases

Background

The complement component C3a activates human mast cells via its cell surface G protein coupled receptor (GPCR) C3aR. For most GPCRs, agonist-induced receptor phosphorylation leads to receptor desensitization, internalization as well as activation of downstream signaling pathways such as ERK1/2 phosphorylation. Previous studies in transfected COS cells overexpressing G protein coupled receptor kinases (GRKs) demonstrated that GRK2, GRK3, GRK5 and GRK6 participate in agonist-induced C3aR phosphorylation. However, the roles of these GRKs on the regulation of C3aR signaling and mediator release in human mast cells remain unknown.

Methodology/Principal Findings

We utilized lentivirus short hairpin (sh)RNA to stably knockdown the expression of GRK2, GRK3, GRK5 and GRK6 in human mast cell lines, HMC-1 and LAD2, that endogenously express C3aR. Silencing GRK2 or GRK3 expression caused a more sustained Ca²⁺ mobilization, attenuated C3aR desensitization, and enhanced degranulation as well as ERK1/2 phosphorylation when compared to shRNA control cells. By contrast, GRK5 or GRK6 knockdown had no effect on C3aR desensitization, but caused a significant decrease in C3a-induced mast cell degranulation. Interestingly, GRK5 or GRK6 knockdown rendered mast cells more responsive to C3a for ERK1/2 phosphorylation.

Conclusion/Significance

This study demonstrates that GRK2 and GRK3 are involved in C3aR desensitization. Furthermore, it reveals the novel finding that GRK5 and GRK6 promote C3a-induced mast cell degranulation but inhibit ERK1/2 phosphorylation via C3aR desensitization-independent mechanisms. These findings thus reveal a new level of complexity for C3aR regulation by GRKs in human mast cells.

G protein coupled receptor specificity for C3a and compound 48/80-induced degranulation in human mast cells: roles of Mas-related genes MrgX1 and MrgX2

Although human mast cells express G protein coupled receptors for the anaphylatoxin C3a, previous studies indicated that C3a causes mast cell degranulation, at least in part, via a C3a receptor-independent mechanism similar to that proposed for polycationic molecules such as compound 48/80. The purpose of the present study was to delineate the receptor specificity of C3a-induced degranulation in human mast cells. We found that C3a, a C3a receptor "superagonist" (E7) and compound 48/80 induced Ca(2+) mobilization and degranulation in a differentiated human mast cell line, LAD2. However, C3a and E7 caused Ca(2+) mobilization in an immature mast cell line, HMC-1 but compound 48/80 did not. We have previously shown that LAD2 cells express MrgX1 and MrgX2 but HMC-1 cells do not. To delineate the receptor specificity for C3a and compound 48/80 further, we generated stable transfectants expressing MrgX1 and MrgX2 in a rodent mast cell line, RBL-2H3 cells. We found that compound 48/80 caused degranulation in RBL-2H3 cells expressing MrgX1 and MrgX2 but C3a did not. By contrast, E7 activated RBL-2H3 cells expressing MrgX2 but not MrgX1. These findings demonstrate that in contrast to previous reports, C3a and compound 48/80 do not use a shared mechanism for mast cell degranulation. It shows that while compound 48/80 utilizes MrgX1 and MrgX2 for mast cell degranulation C3a does not. It further reveals the novel finding that the previously characterized synthetic peptide, C3a receptor "superagonist" E7 activates human mast cells via two mechanisms; one involving the C3a receptor and the other MrgX2.

Inhibiting the C5-C5a receptor axis

Activation of the complement system is a major pathogenic event that drives various inflammatory responses in numerous diseases. All pathways of complement activation lead to cleavage of the C5 molecule generating the anaphylatoxin C5a and, C5b that subsequently forms the terminal complement complex (C5b-9). C5a exerts a predominant pro-inflammatory activity through interactions with the classical G-protein coupled receptor C5aR (CD88) as well as with the non-G protein coupled receptor C5L2 (GPR77), expressed on various immune and non-immune cells. C5b-9 causes cytolysis through the formation of the membrane attack complex (MAC), and sub-lytic MAC and soluble C5b-9 also possess a multitude of non-cytolytic immune functions. These two complement effectors, C5a and C5b-9, generated from C5 cleavage, are key components of the complement system responsible for propagating and/or initiating pathology in different diseases, including paroxysmal nocturnal hemoglobinuria, rheumatoid arthritis, ischemia-reperfusion injuries and neurodegenerative diseases. Thus, the C5-C5a receptor axis represents an attractive target for drug development. This review provides a comprehensive analysis of different methods of inhibiting the generation of C5a and C5b-9 as well as the signalling cascade of C5a via its receptors. These include the inhibition of C5 cleavage through targeting of C5 convertases or via the C5 molecule itself, as well as blocking the activity of C5a by neutralizing antibodies and pharmacological inhibitors, or by targeting C5a receptors per se. Examples of drugs and naturally occurring compounds used are discussed in relation to disease models and clinical trials. To date, only one such compound has thus far made it to clinical medicine: the anti-C5 antibody eculizumab, for treating paroxysmal nocturnal hemoglobinuria. However, a number of drug candidates are rapidly emerging that are currently in early-phase clinical trials. The C5-C5a axis as a target for drug development is highly promising for the treatment of currently intractable major human diseases.

A complement receptor C5a antagonist regulates epithelial to mesenchymal transition and crystallin expression after lens cataract surgery in mice

PURPOSE

To evaluate the effects of complement employing a mouse model for secondary cataract.

METHODS

The role of complement receptor C5a (CD88) was evaluated after cataract surgery in mice. An antagonist specific to C5a receptor was administered intraperitoneally to mice. Epithelial to mesenchymal transition (EMT) was evaluated by alpha-smooth muscle actin (α-SMA) staining and proliferation by bromodeoxyuridine (5-bromo-2'-deoxyuridine, BrdU) incorporation. Gene expression patterns was examined by microarray analysis and quantitative polymerase chain reaction (QPCR).

RESULTS

We found that administration of a C5aR antagonist in C57BL/6J mice decreases EMT, as evidenced by α-SMA expression, and cell proliferation. Gene expression by microarray analysis reveals discreet steps of gene regulation in the two major stages that of EMT and lens fiber differentiation in vivo. A hallmark of the microarray analysis is that the antagonist seems to be a novel stage-specific regulator of crystallin genes. At week two, which is marked by lens fiber differentiation genes encoding 12 crystallins and 3 lens-specific structural proteins were severely down-regulated.

CONCLUSIONS

These results suggest a possible therapeutic role of an antagonist to C5aR in preventing secondary cataracts after surgery. Also these results suggest that crystallin gene expression can be regulated by pro-inflammatory events in the eye.