The cloning of a G protein-coupled extracellular Ca(2+) (Ca(o)(2+))-sensing receptor (CaR) has elucidated the molecular basis for many of the previously recognized effects of Ca(o)(2+) on tissues that maintain systemic Ca(o)(2+) homeostasis, especially parathyroid chief cells and several cells in the kidney. The availability of the cloned CaR enabled the development of DNA and antibody probes for identifying the CaR's mRNA and protein, respectively, within these and other tissues. It also permitted the identification of human diseases resulting from inactivating or activating mutations of the CaR gene and the subsequent generation of mice with targeted disruption of the CaR gene. The characteristic alterations in parathyroid and renal function in these patients and in the mice with "knockout" of the CaR gene have provided valuable information on the CaR's physiological roles in these tissues participating in mineral ion homeostasis. Nevertheless, relatively little is known about how the CaR regulates other tissues involved in systemic Ca(o)(2+) homeostasis, particularly bone and intestine. Moreover, there is evidence that additional Ca(o)(2+) sensors may exist in bone cells that mediate some or even all of the known effects of Ca(o)(2+) on these cells. Even more remains to be learned about the CaR's function in the rapidly growing list of cells that express it but are uninvolved in systemic Ca(o)(2+) metabolism. Available data suggest that the receptor serves numerous roles outside of systemic mineral ion homeostasis, ranging from the regulation of hormonal secretion and the activities of various ion channels to the longer term control of gene expression, programmed cell death (apoptosis), and cellular proliferation. In some cases, the CaR on these "nonhomeostatic" cells responds to local changes in Ca(o)(2+) taking place within compartments of the extracellular fluid (ECF) that communicate with the outside environment (e.g., the gastrointestinal tract). In others, localized changes in Ca(o)(2+) within the ECF can originate from several mechanisms, including fluxes of calcium ions into or out of cellular or extracellular stores or across epithelium that absorb or secrete Ca(2+). In any event, the CaR and other receptors/sensors for Ca(o)(2+) and probably for other extracellular ions represent versatile regulators of numerous cellular functions and may serve as important therapeutic targets.

Extracellular Calcium Sensing and Extracellular Calcium Signaling

Mast cells are primary effectors in allergic reactions, and may have important roles in disease by secreting histamine and various inflammatory and immunomodulatory substances. Although they are classically activated by immunoglobulin (Ig)E antibodies, a unique property of mast cells is their antibody-independent responsiveness to a range of cationic substances, collectively called basic secretagogues, including inflammatory peptides and drugs associated with allergic-type reactions. The pathogenic roles of these substances have prompted a decades-long search for their receptor(s). Here we report that basic secretagogues activate mouse mast cells in vitro and in vivo through a single receptor, Mrgprb2, the orthologue of the human G-protein-coupled receptor MRGPRX2. Secretagogue-induced histamine release, inflammation and airway contraction are abolished in Mrgprb2-null mutant mice. Furthermore, we show that most classes of US Food and Drug Administration (FDA)-approved peptidergic drugs associated with allergic-type injection-site reactions also activate Mrgprb2 and MRGPRX2, and that injection-site inflammation is absent in mutant mice. Finally, we determine that Mrgprb2 and MRGPRX2 are targets of many small-molecule drugs associated with systemic pseudo-allergic, or anaphylactoid, reactions; we show that drug-induced symptoms of anaphylactoid responses are significantly reduced in knockout mice; and we identify a common chemical motif in several of these molecules that may help predict side effects of other compounds. These discoveries introduce a mouse model to study mast cell activation by basic secretagogues and identify MRGPRX2 as a potential therapeutic target to reduce a subset of drug-induced adverse effects.

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Identification of a mast-cell-specific receptor crucial for pseudo-allergic drug reactions

Putative Mammalian Taste Receptors: A Class of Taste-Specific GPCRs with Distinct Topographic Selectivity

Evolution, structure, and activation mechanism of family 3/C G-protein-coupled receptors.

Structure and function of the vomeronasal system: an update

Immunoglobulin E-independent activation of mast cell is mediated by Mrg receptors.

By a combination of PCR with degenerate primers and low stringency probing, we have isolated a large family of genes related to the Ca2+-sensing receptor from the genome of Fugu rubripes. One of the genes (type I) is the Fugu homologue of the Ca2+-sensing receptor. The remaining genes can be divided into five classes (type II–VI) on the bases of gene structure. In several types, the genes occur in clusters as tandem arrays. These genes appear to be the homologues of the vomeronasal pheromone receptors recently described in rodents. The Fugu genes are expressed in the tissues of the nose, suggesting that they may have a similar physiological role.

Putative pheromone receptors related to the Ca2+-sensing receptor in Fugu.

Mast cells play a central role in inflammatory and allergic reactions by releasing inflammatory mediators through 2 main pathways, immunoglobulin E-dependent and E-independent activation. In the latter pathway, mast cells are activated by a diverse range of basic molecules (collectively known as basic secretagogues) through Mas-related G protein-coupled receptors (MRGPRs). In addition to the known basic secretagogues, here, we discovered several endogenous protein and enzyme fragments (such as chaperonin-10 fragment) that act as bioactive peptides and induce immunoglobulin E-independent mast cell activation via MRGPRX2 (previously known as MrgX2), leading to the degranulation of mast cells. We discuss the possibility that MRGPRX2 responds various as-yet-unidentified endogenous ligands that have specific characteristics, and propose that MRGPRX2 plays an important role in regulating inflammatory responses to endogenous harmful stimuli, such as protein breakdown products released from damaged or dying cells.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/sji.12655

Endogenous protein and enzyme fragments induce immunoglobulin E-independent activation of mast cells via a G protein-coupled receptor, MRGPRX2

The object is to provide: a method for screening a substance capable of regulating degranulation or inhibiting prostaglandin D2 production; and a method for identification of a substance capable of inhibiting the degranulation or prostaglandin D2 production in a mast cell, these methods being useful for developing a therapeutic agent for an allergic disease or an autoimmune disease based on a new mechanism through a G-protein conjugated receptor Rec168. Disclosed is a method for screening of a substance capable of inhibiting the signal transduction capability or a substance capable of regulating the degranulation or inhibiting the prostaglandin D2 production in a mast cell through Rec168. The method comprises the steps of: determining the signal transduction capability of a ligand having a Rec168-agonistic activity toward Rec168 in the presence or absence of a substance to be tested; and comparing a measurement value obtained in the presence of the substance with a measurement value obtained in the absence of the substance. Also disclosed is a method for identification of a substance capable of inhibiting the degranulation or prostaglandin D2 production in a mast cell.

SCREENING METHOD AND IDENTIFICATION METHOD FOR SUBSTANCE CAPABLE OF INHIBITING DEGRANULATION REACTION OR PROSTAGLANDIN D2 PRODUCTION IN MAST CELL THROUGH Rec168, AND THERAPEUTIC AGENT FOR INFLAMMATORY DISEASE COMPRISING THE SUBSTANCE

Yuko Nozaki

Papers

Immunoglobulin E-independent activation of mast cell is mediated by Mrg receptors.

Putative pheromone receptors related to the Ca2+-sensing receptor in Fugu.

Endogenous protein and enzyme fragments induce immunoglobulin E-independent activation of mast cells via a G protein-coupled receptor, MRGPRX2

SCREENING METHOD AND IDENTIFICATION METHOD FOR SUBSTANCE CAPABLE OF INHIBITING DEGRANULATION REACTION OR PROSTAGLANDIN D2 PRODUCTION IN MAST CELL THROUGH Rec168, AND THERAPEUTIC AGENT FOR INFLAMMATORY DISEASE COMPRISING THE SUBSTANCE