Orphan G protein-coupled receptors MrgA1 and MrgC11 are distinctively activated by RF-amide-related peptides through the Galpha q/11 pathway

Divergent sensory pathways of sneezing and coughing

Sneezing and coughing are primary symptoms of many respiratory viral infections and allergies. It is generally assumed that sneezing and coughing involve common sensory receptors and molecular neurotransmission mechanisms. Here, we show that the nasal mucosa is innervated by several discrete populations of sensory neurons, but only one population (MrgprC11+MrgprA3-) mediates sneezing responses to a multitude of nasal irritants, allergens, and viruses. Although this population also innervates the trachea, it does not mediate coughing, as revealed by our newly established cough model. Instead, a distinct sensory population (somatostatin [SST+]) mediates coughing but not sneezing, unraveling an unforeseen sensory difference between sneezing and coughing. At the circuit level, sneeze and cough signals are transmitted and modulated by divergent neuropathways. Together, our study reveals the difference in sensory receptors and neurotransmission/modulation mechanisms between sneezing and coughing, offering neuronal drug targets for symptom management in respiratory viral infections and allergies.

Ligand recognition and G protein coupling of the human itch receptor MRGPRX1

MRGPRX1, a Mas-related GPCR (MRGPR), is a key receptor for itch perception and targeting MRGPRX1 may have potential to treat both chronic itch and pain. Here we report cryo-EM structures of the MRGPRX1-Gi1 and MRGPRX1-Gq trimers in complex with two peptide ligands, BAM8-22 and CNF-Tx2. These structures reveal a shallow orthosteric pocket and its conformational plasticity for sensing multiple different peptidic itch allergens. Distinct from MRGPRX2, MRGPRX1 contains a unique pocket feature at the extracellular ends of TM3 and TM4 to accommodate the peptide C-terminal "RF/RY" motif, which could serve as key mechanisms for peptidic allergen recognition. Below the ligand binding pocket, the G6.48XP6.50F6.51G6.52X(2)F/W6.55 motif is essential for the inward tilting of the upper end of TM6 to induce receptor activation. Moreover, structural features inside the ligand pocket and on the cytoplasmic side of MRGPRX1 are identified as key elements for both Gi and Gq signaling. Collectively, our studies provide structural insights into understanding itch sensation, MRGPRX1 activation, and downstream G protein signaling.

Mechanism of agonist-induced activation of the human itch receptor MRGPRX1

Mas-related G-protein-coupled receptors X1-X4 (MRGPRX1-X4) are 4 primate-specific receptors that are recently reported to be responsible for many biological processes, including itch sensation, pain transmission, and inflammatory reactions. MRGPRX1 is the first identified human MRGPR, and its expression is restricted to primary sensory neurons. Due to its dual roles in itch and pain signaling pathways, MRGPRX1 has been regarded as a promising target for itch remission and pain inhibition. Here, we reported a cryo-electron microscopy (cryo-EM) structure of Gq-coupled MRGPRX1 in complex with a synthetic agonist compound 16 in an active conformation at an overall resolution of 3.0 Å via a NanoBiT tethering strategy. Compound 16 is a new pain-relieving compound with high potency and selectivity to MRGPRX1 over other MRGPRXs and opioid receptor. MRGPRX1 was revealed to share common structural features of the Gq-mediated receptor activation mechanism of MRGPRX family members, but the variable residues in orthosteric pocket of MRGPRX1 exhibit the unique agonist recognition pattern, potentially facilitating to design MRGPRX1-specific modulators. Together with receptor activation and itch behavior evaluation assays, our study provides a structural snapshot to modify therapeutic molecules for itch relieving and analgesia targeting MRGPRX1.

FSLLRY-NH2, a protease-activated receptor 2 (PAR2) antagonist, activates mas-related G protein-coupled receptor C11 (MrgprC11) to induce scratching behaviors in mice

AIMS

Protease-activated receptor 2 (PAR2), a type of G protein-coupled receptor (GPCR), plays a significant role in pathophysiological conditions such as inflammation. A synthetic peptide SLIGRL-NH₂ (SLIGRL) can activate PAR2, while FSLLRY-NH₂ (FSLLRY) is an antagonist. A previous study showed that SLIGRL activates both PAR2 and mas-related G protein-coupled receptor C11 (MrgprC11), a different type of GPCR expressed in sensory neurons. However, the impact of FSLLRY on MrgprC11 and its human ortholog MRGPRX1 was not verified. Hence, the present study aims to verify the effect of FSLLRY on MrgprC11 and MRGPRX1.

METHODS

The calcium imaging technique was applied to determine the effect of FSLLRY in HEK293T cells expressing MrgprC11/MRGPRX1 or dorsal root ganglia (DRG) neurons. Scratching behavior was also investigated in wild-type and PAR2 knockout mice after injecting FSLLRY.

KEY FINDINGS

It was surprisingly discovered that FSLLRY specifically activates MrgprC11 in a dose-dependent manner, but not other MRGPR subtypes. Furthermore, FSLLRY also moderately activated MRGPRX1. FSLLRY stimulates downstream pathways including Gα_q/11, phospholipase C, IP₃ receptor, and TRPC ion channels to evoke an increase in the intracellular calcium levels. The molecular docking analysis predicted that FSLLRY interacts with the orthosteric binding pocket of MrgprC11 and MRGPRX1. Finally, FSLLRY activated primary cultures of mouse sensory neurons, and induced scratching behaviors in mice.

SIGNIFICANCE

The present study has revealed that FSLLRY is capable of triggering itch sensation through activation of MrgprC11. This finding highlights the importance of considering the unexpected activation of MRGPRs in future therapeutic approaches aimed at the inhibition of PAR2.

Ligand recognition and allosteric modulation of the human MRGPRX1 receptor

The human MAS-related G protein-coupled receptor X1 (MRGPRX1) is preferentially expressed in the small-diameter primary sensory neurons and involved in the mediation of nociception and pruritus. Central activation of MRGPRX1 by the endogenous opioid peptide fragment BAM8-22 and its positive allosteric modulator ML382 has been shown to effectively inhibit persistent pain, making MRGPRX1 a promising target for non-opioid pain treatment. However, the activation mechanism of MRGPRX1 is still largely unknown. Here we report three high-resolution cryogenic electron microscopy structures of MRGPRX1-Gαq in complex with BAM8-22 alone, with BAM8-22 and ML382 simultaneously as well as with a synthetic agonist compound-16. These structures reveal the agonist binding mode for MRGPRX1 and illuminate the structural requirements for positive allosteric modulation. Collectively, our findings provide a molecular understanding of the activation and allosteric modulation of the MRGPRX1 receptor, which could facilitate the structure-based design of non-opioid pain-relieving drugs.

Neuro-immune interactions and the role of Mas-related G protein-coupled receptors in the gastrointestinal tract

Over the past decade, the research field dealing with the role of a new family of Rhodopsin A-like G protein-coupled receptors, that is, the family of Mas-related G protein-coupled receptors (Mrgprs) has expanded enormously. A plethora of recent studies have provided evidence that Mrgprs are key players in itch and pain, as well as in the initiation and modulation of inflammatory/allergic responses in the skin. Over the years, it has become clear that this role is not limited to the skin, but extends to other mucosal surfaces such as the respiratory tract and the gastrointestinal (GI) tract. In the GI tract, Mrgprs have emerged as novel interoceptive sensory pathways linked to health and disease, and are in close functional association with the gut's immune system. This review aims to provide an update of our current knowledge on the expression, distribution and function of members of this Mrgpr family in intrinsic and extrinsic neuro-immune pathways related to the GI system.

Evaluation of itch and pain induced by bovine adrenal medulla (BAM)8-22, a new human model of non-histaminergic itch

Chronic itch is a socioeconomic burden with limited management options. Non-histaminergic itch, involved in problematic pathological itch conditions, is transmitted by a subgroup of polymodal C-fibers. Cowhage is traditionally used for studying experimentally induced non-histaminergic itch in humans, but encounter some limitations. The present study therefore aims to design a new human, experimental model of non-histaminergic itch based on the application of bovine adrenal medulla (BAM)8-22, an endogenous peptide that activates MrgprX1 receptor. 22 healthy subjects were recruited. Different concentrations (0.5, 1, and 2 mg/ml) of BAM8-22 solution and vehicle, applied by a single skin prick test (SPT), were tested in the first session. In the second session, the BAM8-22 solution (1 mg/ml) was applied by different number of SPTs (1, 5, and 25) and by heat-inactivated cowhage spicules coated with BAM8-22. Provoked itch and pain intensities were monitored for 9 minutes followed by the measurement of superficial blood perfusion (SBP), mechanical and thermal sensitivity. BAM8-22 induced itch at the concentration of 1 mg/ml, 2 mg/ml (p<0.05), and with the significantly highest intensity when applied through BAM8-22 spicules (p<0.001). No concomitant pain sensation nor increased SBP were observed. SBP increased only in the 25 SPTs area probably due to micro-trauma from the multiple skin penetrations. Mechanical and thermal sensitivities were not affected by any of the applications. BAM8-22 applied through heat-inactivated spicules was the most efficient method to induce itch (without pain nor changes in SBP, mechanical and thermal sensitivity) suggesting BAM8-22 as a novel non-histaminergic, human, experimental itch model.

Mas-Related G Protein-Coupled Receptors (Mrgprs) as Mediators of Gut Neuro-Immune Signaling

Over the past 15 years, the research field on Mas-related G protein-coupled receptors (Mrgprs), a relatively new family of rhodopsin A-like G protein-coupled receptors, has expanded enormously, and a plethora of recent studies have provided evidence that several of these Mrgpr family members play an important role in the underlying mechanisms of itch and pain, as well as in the initiation and modulation of inflammatory/allergic responses. Initial studies mainly focused on the skin, but more recently also visceral organs such as the respiratory and gastrointestinal (GI) tracts emerged as sites for Mrgpr involvement. It has become clear that the gastrointestinal tract and its innervation in close association with the immune system represent a novel expression site for Mrgprs where they contribute to the interoceptive mechanisms maintaining homeostasis and might constitute promising targets in chronic abdominal pain disorders. In this short review, we provide an update of our current knowledge on the expression, distribution, and function of members of this Mrgpr family in intrinsic and extrinsic neuro-immune pathways related to the gastrointestinal tract, their mediatory role(s) in gut neuro-immune signaling, and their involvement in visceral afferent (nociceptive) pathways.

Therapeutic Potential of MRGPRX2 Inhibitors on Mast Cells

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

Lithocholic Acid Activates Mas-Related G Protein-Coupled Receptors, Contributing to Itch in Mice

The present study focused on lithocholic acid (LCA), a secondary bile acid that contributes to cholestatic pruritus. Although recent studies have found that LCA acts on MAS-related G protein-coupled receptor family member X4 (MRGPRX4) in humans, it is unclear which subtypes of MRGPRs are activated by LCA in mice since there is no precise ortholog of human MRGPRX4 in the mouse genome. Using calcium imaging, we found that LCA could activate mouse Mrgpra1 when transiently expressed in HEK293T cells. Moreover, LCA similarly activates mouse Mrgprb2. Importantly, LCA-induced responses showed dose-dependent effects through Mrgpra1 and Mrgprb2. Moreover, treatment with QWF (an antagonist of Mrgpra1 and Mrgprb2), YM254890 (Gα_q inhibitor), and U73122 (an inhibitor of phospholipase C) significantly suppressed the LCA-induced responses, implying that the LCA-induced responses are indeed mediated by Mrgpra1 and Mrgprb2. Furthermore, LCA activated primary cultures of mouse sensory neurons and peritoneal mast cells, suggesting that Mrgpra1 and Mrgprb2 contribute to LCA-induced pruritus. However, acute injection of LCA did not induce noticeable differences in scratching behavior, implying that the pruritogenic role of LCA may be marginal in non-cholestatic conditions. In summary, the present study identified for the first time that LCA can activate Mrgpra1 and Mrgprb2. The current findings provide further insight into the similarities and differences between human and mouse MRGPR families, paving a way to understand the complex roles of these pruriceptors.

Potential and Realized Impact of Astroglia Ca2 + Dynamics on Circuit Function and Behavior

Astroglia display a wide range of spontaneous and behavioral state-dependent Ca²⁺ dynamics. During heightened vigilance, noradrenergic signaling leads to quasi-synchronous Ca²⁺ elevations encompassing soma and processes across the brain-wide astroglia network. Distinct from this vigilance-associated global Ca²⁺ rise are apparently spontaneous fluctuations within spatially restricted microdomains. Over the years, several strategies have been pursued to shed light on the physiological impact of these signals including deletion of endogenous ion channels or receptors and reduction of intracellular Ca²⁺ through buffering, extrusion or inhibition of release. Some experiments that revealed the most compelling behavioral alterations employed chemogenetic and optogenetic manipulations to modify astroglia Ca²⁺ signaling. However, there is considerable contrast between these findings and the comparatively modest effects of inhibiting endogenous sources of Ca²⁺. In this review, we describe the underlying mechanisms of various forms of astroglia Ca²⁺ signaling as well as the functional consequences of their inhibition. We then discuss how the effects of exogenous astroglia Ca²⁺ modification combined with our knowledge of physiological mechanisms of astroglia Ca²⁺ activation could guide further refinement of behavioral paradigms that will help elucidate the natural Ca²⁺-dependent function of astroglia.

Pruriception and neuronal coding in nociceptor subtypes in human and nonhuman primates

In humans, intradermal administration of β-alanine (ALA) and bovine adrenal medulla peptide 8–22 (BAM8-22) evokes the sensation of itch. Currently, it is unknown which human dorsal root ganglion (DRG) neurons express the receptors of these pruritogens, MRGPRD and MRGPRX1, respectively, and which cutaneous afferents these pruritogens activate in primate. In situ hybridization studies revealed that MRGPRD and MRGPRX1 are co-expressed in a subpopulation of TRPV1+ human DRG neurons. In electrophysiological recordings in nonhuman primates (Macaca nemestrina), subtypes of polymodal C-fiber nociceptors are preferentially activated by ALA and BAM8-22, with significant overlap. When pruritogens ALA, BAM8-22, and histamine, which activate different subclasses of C-fiber afferents, are administered in combination, human volunteers report itch and nociceptive sensations similar to those induced by a single pruritogen. Our results provide evidence for differences in pruriceptive processing between primates and rodents, and do not support the spatial contrast theory of coding of itch and pain.

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.

Immunosensation: Neuroimmune Cross Talk in the Skin

Classically, skin was considered a mere structural barrier protecting organisms from a diversity of environmental insults. In recent decades, the cutaneous immune system has become recognized as a complex immunologic barrier involved in both antimicrobial immunity and homeostatic processes like wound healing. To sense a variety of chemical, mechanical, and thermal stimuli, the skin harbors one of the most sophisticated sensory networks in the body. However, recent studies suggest that the cutaneous nervous system is highly integrated with the immune system to encode specific sensations into evolutionarily conserved protective behaviors. In addition to directly sensing pathogens, neurons employ novel neuroimmune mechanisms to provide host immunity. Therefore, given that sensation underlies various physiologies through increasingly complex reflex arcs, a much more dynamic picture is emerging of the skin as a truly systemic organ with highly coordinated physical, immunologic, and neural functions in barrier immunology.

The role of pruriceptors in enhancing sensitivity to pruritogens in a murine chronic compression model of dorsal root ganglion

Chronic pruritus is a symptom that commonly observed in neurological diseases. It has been hypothesized that the chronic pruritus may result from sensitization of itch-signaling pathways but the mechanisms remain obscure. In this study, we established a mouse model of chronic compression of dorsal root ganglion (CCD) and injected various pruritogenic and algogenic agents intradermally to the calf skin ipsilateral to the compressed dorsal root ganglion (DRG). Compared to the naïve mice, a significant increase in itch-related behaviors was observed in the CCD mice after the injection of pruritogens including histamine and BAM8-22, but not after the injection of capsaicin, although all the above agents evoked enhanced pain-related behaviors toward the injected site. In addition, we investigated if pruritogen-evoked activities of DRG neurons were enhanced in this model. In vivo calcium imaging revealed that compressed DRG neurons exhibited enhanced responses to histamine and BAM8-22. Immunoflorescent staining also showed that the histamine receptor H1 and the capsaicin receptor TRPV1 were significantly upregulated in DRG neurons. Our findings indicated that the sensitization of primary pruriceptive neurons may underlie the enhanced itch sensation after chronic compression of DRG in the mice, and may play a role in chronic pruritus in neurological diseases.

The signaling pathway and polymorphisms of Mrgprs

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

Tick peptides evoke itch by activating MrgprC11/MRGPRX1 to sensitize TRPV1 in pruriceptors

BACKGROUND

Tick bites severely threaten human health because they allow the transmission of many deadly pathogens, including viruses, bacteria, protozoa, and helminths. Pruritus is a leading symptom of tick bites, but its molecular and neural bases remain elusive.

OBJECTIVES

This study sought to discover potent drugs and targets for the specific prevention and treatment of tick bite-induced pruritus and arthropod-related itch.

METHODS

We used live-cell calcium imaging, patch-clamp recordings, and genetic ablation and evaluated mouse behavior to investigate the molecular and neural bases of tick bite-induced pruritus.

RESULTS

We found that 2 tick salivary peptides, IP defensin 1 (IPDef1) and IR defensin 2 (IRDef2), induced itch in mice. IPDef1 was further revealed to have a stronger pruritogenic potential than IRDef2 and to induce pruritus in a histamine-independent manner. IPDef1 evoked itch by activating mouse MrgprC11 and human MRGPRX1 on dorsal root ganglion neurons. IPDef1-activated MrgprC11/X1 signaling sensitized downstream ion channel TRPV1 on dorsal root ganglion neurons. Moreover, IPDef1 also activated mouse MrgprB2 and its ortholog human MRGPRX2 selectively expressed on mast cells, inducing the release of inflammatory cytokines and driving acute inflammation in mice, although mast cell activation did not contribute to oxidated IPDef1-induced itch.

CONCLUSIONS

Our study identifies tick salivary peptides as a new class of pruritogens that initiate itch through MrgprC11/X1-TRPV1 signaling in pruritoceptors. Our work will provide potential drug targets for the prevention and treatment of pruritus induced by the bites or stings of tick and maybe other arthropods.

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.

Itch: A Paradigm of Neuroimmune Crosstalk

Although the medical definition of itch has been in existence for 360 years, only in the last 20 years have we begun to understand the basic mechanisms that underlie this unique sensation. Therapeutics that specifically target chronic itch as a pathologic entity are currently still not available. Recent seminal advances in itch circuitry within the nervous system have intersected with discoveries in immunology in unexpected ways to rapidly inform emerging treatment strategies. The current review aims to introduce these basic concepts in itch biology and highlight how distinct immunologic pathways integrate with recently identified itch-sensory circuits in the nervous system to inform a major new paradigm of neuroimmunology and therapeutic development for chronic itch.

Recent advances in understanding the molecular mechanisms of cholestatic pruritus: A review

Cholestasis, a condition characterized by an abnormal decrease in bile flow, is accompanied by various symptoms such as pruritus. Although cholestatic pruritus is a prominent condition, its precise mechanisms have largely been elusive. Recently, advancements have been made for understanding the etiology and pathogenesis of cholestatic pruritus. The current review therefore focuses on summarizing the overall progress made in the elucidation of its molecular mechanisms. We have reviewed the available animal models on cholestasis to compare the differences between them, characterized potential pruritogens involved in cholestatic pruritus, and have summarized the receptor and ion channels implicated in the condition. Finally, we have discussed the available treatment options for alleviation of cholestatic pruritus. As our understanding of the mechanisms of cholestatic pruritus deepens, novel strategies to cure this condition are awaited.

Advances in Understanding the Initial Steps of Pruritoceptive Itch: How the Itch Hits the Switch

Pruritoceptive (dermal) itch was long considered an accompanying symptom of diseases, a side effect of drug applications, or a temporary sensation induced by invading pruritogens, as produced by the stinging nettle. Due to extensive research in recent years, it was possible to provide detailed insights into the mechanism of itch mediation and modulation. Hence, it became apparent that pruritus is a complex symptom or disease in itself, which requires particular attention to improve patients’ health. Here, we summarize recent findings in pruritoceptive itch, including how this sensation is triggered and modulated by diverse endogenous and exogenous pruritogens and their receptors. A differentiation between mediating pruritogen and modulating pruritogen seems to be of great advantage to understand and decipher the molecular mechanism of itch perception. Only a comprehensive view on itch sensation will provide a solid basis for targeting this long-neglected adverse sensation accompanying numerous diseases and many drug side effects. Finally, we identify critical aspects of itch perception that require future investigation.

Neural Mechanisms of Itch

Itch is a unique sensation that helps organisms scratch away external threats; scratching itself induces an immune response that can contribute to more itchiness. Itch is induced chemically in the peripheral nervous system via a wide array of receptors. Given the superficial localization of itch neuron terminals, cells that dwell close to the skin contribute significantly to itch. Certain mechanical stimuli mediated by recently discovered circuits also contribute to the itch sensation. Ultimately, in the spinal cord, and likely in the brain, circuits that mediate touch, pain, and itch engage in cross modulation. Much of itch perception is still a mystery, but we present in this review the known ligands and receptors associated with itch. We also describe experiments and findings from investigations into the spinal and supraspinal circuitry responsible for the sensation of itch.

Complementary roles of murine NaV1.7, NaV1.8 and NaV1.9 in acute itch signalling

Acute pruritus occurs in various disorders. Despite severe repercussions on quality of life treatment options remain limited. Voltage-gated sodium channels (Na_V) are indispensable for transformation and propagation of sensory signals implicating them as drug targets. Here, Na_V1.7, 1.8 and 1.9 were compared for their contribution to itch by analysing Na_V-specific knockout mice. Acute pruritus was induced by a comprehensive panel of pruritogens (C48/80, endothelin, 5-HT, chloroquine, histamine, lysophosphatidic acid, trypsin, SLIGRL, β-alanine, BAM8-22), and scratching was assessed using a magnet-based recording technology. We report an unexpected stimulus-dependent diversity in Na_V channel-mediated itch signalling. Na_V1.7^−/− showed substantial scratch reduction mainly towards strong pruritogens. Na_V1.8^−/− impaired histamine and 5-HT-induced scratching while Na_V1.9 was involved in itch signalling towards 5-HT, C48/80 and SLIGRL. Furthermore, similar microfluorimetric calcium responses of sensory neurons and expression of itch-related TRP channels suggest no change in sensory transduction but in action potential transformation and conduction. The cumulative sum of scratching over all pruritogens confirmed a leading role of Na_V1.7 and indicated an overall contribution of Na_V1.9. Beside the proposed general role of Na_V1.7 and 1.9 in itch signalling, scrutiny of time courses suggested Na_V1.8 to sustain prolonged itching. Therefore, Na_V1.7 and 1.9 may represent targets in pruritus therapy.

Mas-related G protein-coupled receptor C11 (Mrgprc11) induces visceral hypersensitivity in the mouse colon: A novel target in gut nociception?

BACKGROUND

Visceral hypersensitivity, an important cause of abdominal pain in disorders such as IBD and IBS, presents with a poorly understood pathophysiology and limited treatment options. Several members of the Mas-related G protein-coupled receptor family (Mrgprs) have become promising targets in pain research. The potential link between the murine Mrgpr C11 (Mrgprc11) and gut nociception is currently uninvestigated. Therefore, we explored the expression and functional role of Mrgprc11 in the gut nociceptive innervation.

METHODS

Mrgprc11 expression was evaluated in DRG neurons innervating the mouse colon using in situ hybridization and immunohistochemistry. Visceromotor responses to colorectal distension (CRD) assessed the effect of the Mrgprc11 agonist, BAM(8-22), on colonic pain sensitivity in healthy mice. Moreover, we determined pERK1/2-immunoreactivity in the thoracolumbar spinal cord after noxious CRD. Finally, from a translational point of view, we looked for expression of the human counterpart of Mrgprc11, MRGPRX1, in human thoracolumbar DRGs.

KEY RESULTS

In situ hybridization and immunohistochemistry revealed Mrgprc11 expression in colonic DRG neurons. Intracolonic administration of BAM(8-22) significantly increased colonic pain sensitivity in an Mrgprc11-dependent manner, and led to a significantly increased degree of neuronal activation in the splanchnic spinal cord upon noxious stimulation. Furthermore, MRGPRX1 expression was also detected in human thoracolumbar DRG neurons. CONCLUSIONS & INFERENCES: Our findings established a novel function for Mrgprc11 in the gut nociceptive innervation and propose the receptor as a new player in visceral hypersensitivity. Given the presence of MRGPRX1 in human DRG neurons, our study warrants future research on its therapeutic potential in abdominal pain disorders.

BAM8-22 and its receptor MRGPRX1 may attribute to cholestatic pruritus

Pruritus is an unexpected symptom observed in cholestasis and its mechanism is still unclear. Here, we show that bovine adrenal medulla (BAM) 8-22, an endogenous itch-inducing peptide, could be involved in cholestatic pruritus. It was found that bile duct ligation (BDL) mice, an obstructive cholestasis model, showed increased spontaneous scratching behaviour. Importantly, the mRNA level of proenkephalin, a precursor polypeptide of BAM8-22, was significantly increased in the skin of BDL mice. Furthermore, the mRNA level of Mrgprx1, which encodes a receptor for BAM8-22, was significantly increased in the dorsal root ganglia (DRG) of BDL mice. This was further confirmed by elevation of intracellular calcium levels upon BAM8-22 treatment in primarily-cultured DRG neurons. In addition, BDL mice showed augmented scratching behaviour by BAM8-22, indicating enhanced activity of MRGPRX1. Moreover, the skin homogenate of BDL mice induced elevation of intracellular calcium levels through MRGPRX1. Finally, among the various bile acids, chenodeoxycholic acid significantly increased proenkephalin transcription in a human keratinocyte cell line (HaCaT). In conclusion, cholestatic pruritus could be attributed in part to enhanced action of both BAM8-22 in the skin and its receptor MRGPRX1 in sensory neurons.

Peripheral and Central Mechanisms of Itch

Itch is a unique sensory experience that is encoded by genetically distinguishable neurons both in the peripheral nervous system (PNS) and central nervous system (CNS) to elicit a characteristic behavioral response (scratching). Itch interacts with the other sensory modalities at multiple locations, from its initiation in a particular dermatome to its transmission to the brain where it is finally perceived. In this review, we summarize the current understanding of the molecular and neural mechanisms of itch by starting in the periphery, where itch is initiated, and discussing the circuits involved in itch processing in the CNS.

Protease Activated Receptor-2 Induces Immune Activation and Visceral Hypersensitivity in Post-infectious Irritable Bowel Syndrome Mice

BACKGROUND

The role of protease activated receptor-2 (PAR-2) in the pathogenesis of abdominal pain in irritable bowel syndrome (IBS) is not well defined.

AIMS

To investigate the role of PAR-2-mediated visceral hypersensitivity in a post-infectious IBS (PI-IBS) mouse model.

METHODS

T. spiralis-infected PI-IBS mouse model was used. Fecal serine protease activity and intestinal mast cells were evaluated. Intestinal permeability was assessed by urine lactulose/mannitol ratio, and colonic expressions of PAR-2 and tight junction (TJ) proteins were examined by Western blot. Intestinal immune profile was assessed by measuring Th (T helper) 1/Th2 cytokine expression. Visceral sensitivity was evaluated by abdominal withdrawal reflex in response to colorectal distention.

RESULTS

Colonic PAR-2 expression as well as fecal serine protease activity and intestinal mast cell counts were elevated in PI-IBS compared to the control mice. Decreased colonic TJ proteins expression, increased lactulose/mannitol ratio, elevated colonic Th1/Th2 cytokine ratio, and visceral hypersensitivity were observed in PI-IBS compared to the control mice. Administration of PAR-2 agonist in control mice demonstrated similar changes observed in PI-IBS mice, while PAR-2 antagonist normalized the increased intestinal permeability and reduced visceral hypersensitivity observed in PI-IBS mice.

CONCLUSIONS

PAR-2 activation increases intestinal permeability leading to immune activation and visceral hypersensitivity in PI-IBS mouse model.

Comparative transcriptome profiling of the human and mouse dorsal root ganglia: an RNA-seq-based resource for pain and sensory neuroscience research

Molecular neurobiological insight into human nervous tissues is needed to generate next-generation therapeutics for neurological disorders such as chronic pain. We obtained human dorsal root ganglia (hDRG) samples from organ donors and performed RNA-sequencing (RNA-seq) to study the hDRG transcriptional landscape, systematically comparing it with publicly available data from a variety of human and orthologous mouse tissues, including mouse DRG (mDRG). We characterized the hDRG transcriptional profile in terms of tissue-restricted gene coexpression patterns and putative transcriptional regulators, and formulated an information-theoretic framework to quantify DRG enrichment. Relevant gene families and pathways were also analyzed, including transcription factors, G-protein-coupled receptors, and ion channels. Our analyses reveal an hDRG-enriched protein-coding gene set (∼140), some of which have not been described in the context of DRG or pain signaling. Most of these show conserved enrichment in mDRG and were mined for known drug-gene product interactions. Conserved enrichment of the vast majority of transcription factors suggests that the mDRG is a faithful model system for studying hDRG, because of evolutionarily conserved regulatory programs. Comparison of hDRG and tibial nerve transcriptomes suggests trafficking of neuronal mRNA to axons in adult hDRG, and are consistent with studies of axonal transport in rodent sensory neurons. We present our work as an online, searchable repository (https://www.utdallas.edu/bbs/painneurosciencelab/sensoryomics/drgtxome), creating a valuable resource for the community. Our analyses provide insight into DRG biology for guiding development of novel therapeutics and a blueprint for cross-species transcriptomic analyses.

Conopeptides promote itch through human itch receptor hMgprX1

Members of Mas related G-protein coupled receptors (Mrgpr) are known to mediate itch. To date, several compounds have been shown to activate these receptors, including chloroquine, a common antimalarial drug, and peptides of the RF-amide family. However, specific ligands for these receptors are still lacking and there is a need for novel compounds that can be used to modulate the receptors in order to understand the cellular and molecular mechanism in which they mediate itch. Some cone snail venoms were previously shown to induce itch in mice. Here, we show that the venom of Conus textile induces itch through activation of itch-sensing sensory neurons, marked by their sensitivity to chloroquine. Two RF-amide peptides, CNF-Tx1 and CNF-Tx2, were identified in a C. textile venom gland transcriptome. These belong to the conorfamide family of peptides which includes previously described peptides from the venoms of Conus victoriae (CNF-Vc1) and Conus spurius (CNF-Sr1 and CNF-Sr2). We show that CNF-Vc1 and CNF-Sr1 activate MrgprC11 whereas CNF-Vc1 and CNF-Tx2 activate the human MrgprX1 (hMrgprX1). The peptides CNF-Tx1 and CNF-Sr2 do not activate MrgprC11 or hMrgprX1. Intradermal injection of CNF-Vc1 and CNF-Tx2 into the cheek of a transgenic mouse expressing hMrgprX1 instead of endogenous mouse Mrgprs resulted in itch-related scratching thus demonstrating the in vivo activity of these peptides. Using truncated analogues of CNF-Vc1, we identified amino acids at positions 7-14 as important for activity against hMrgprX1. The conopeptides reported here are tools that can be used to advance our understanding of the cellular and molecular mechanism of itch mediated by Mrgprs.

Mas-Related G Protein-Coupled Receptors and the Biology of Itch Sensation

Chronic, persistent itch is a devastating symptom that causes much suffering. In recent years, there has been great progress made in understanding the molecules, cells, and circuits underlying itch sensation. Once thought to be carried by pain-sensing neurons, itch is now believed to be capable of being transmitted by dedicated sensory labeled lines. Members of the Mas-related G protein-coupled receptor (Mrgpr) family demarcate an itch-specific labeled line in the peripheral nervous system. In the spinal cord, the expression of other proteins identifies additional populations of itch-dedicated sensory neurons. However, as evidence for labeled-line coding has mounted, studies promoting alternative itch-coding strategies have emerged, complicating our understanding of the neural basis of itch. In this review, we cover the molecules, cells, and circuits related to understanding the neural basis of itch, with a focus on the role of Mrgprs in mediating itch sensation.

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

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.

Substitution with a Single Cysteine in the Green Fluorescent Protein-Based Calcium Indicator GCaMP3 Enhances Calcium Sensitivity

Genetically encoded calcium indicators (GECI) such as GCaMP3 are attracting significant attention as a good option for measuring intracellular calcium levels. Recently, a modified GCaMP3 called dCys-GCaMP3 was developed by replacing two threonine residues with cysteines. dCys-GCaMP3 proved to be a better calcium indicator, but it was not clear how and why the two cysteine residues were able to enhance the protein's calcium sensitivity. The aim of the present study was to investigate the possible roles of these cysteine residues in dCys-GCaMP3. dCys-GCaMP3 (Thr330Cys;Thr364Cys) exhibited enhanced fluorescence intensity compared to the canonical GCaMP3 in calcium imaging experiments. However, substitution of a single residue at position 330 with cysteine (Thr330Cys) also afforded comparable sensitivity to GCaMP3. In contrast, the other single residue substitution at position 364 with cysteine (Thr364Cys) failed to enhance calcium sensitivity, showing that cysteine at position 330 is essential to improve calcium sensitivity. Thr330Cys substitution in the GCaMP3 or "Cys³³⁰-GCaMP3" showed significantly reduced background fluorescence, and the fluorescence intensity was proportional to the amount of DNA used to transfect the cells used in the study. The substitute had to be cysteine, because replacement with other amino acids such as alanine, valine, and aspartate did not improve GCaMP3's calcium sensitivity. Cys³³⁰-GCaMP3 outperformed a synthetic calcium-specific indicator, Fluo-3, in various calcium imaging experiments. Thus, the present study asserts that substituting the threonine at position 330 in GCaMP3 with cysteine is essential to enhance calcium sensitivity, and suggests that Cys³³⁰-GCaMP3 can be used as a potent fluorescent calcium indicator to measure intracellular calcium levels.

Emerging Roles for MAS-Related G Protein-Coupled Receptor-X2 in Host Defense Peptide, Opioid, and Neuropeptide-Mediated Inflammatory Reactions

Mast cells (MCs) are tissue-resident immune cells that contribute to host defense but are best known for their roles in allergic and inflammatory diseases. In humans, MCs are divided into two subtypes based on the protease content of their secretory granules. Thus, human lung MCs contain only tryptase and are known as MC_T, whereas skin MCs contain both tryptase and chymase and are known as MC_TC. Patients with severe asthma display elevated MCs in the lung, which undergo phenotypic change from MC_T to MC_TC. Although the human genome contains four Mas related G protein coupled receptor X (MRGPRX) genes, an important feature of MC_TC is that they selectively express MRGPRX2. It is activated by antimicrobial host defense peptides such as human β-defensins and the cathelicidin LL-37 and likely contributes to host defense. MRGPRX2 is also a receptor for the neuropeptide substance P, major basic protein, eosinophil peroxidase, opioids, and many FDA-approved cationic drugs. Increased expression of MRGPRX2 or enhanced downstream signaling likely contributes to chronic inflammatory diseases such as rosacea, atopic dermatitis, chronic urticaria, and severe asthma. In this chapter, I will discuss the expression profile and function of MRGPRX1-4 and review the emerging roles of MRGPRX2 on host defense, chronic inflammatory diseases, and drug-induced pseudoallergic reactions. I will also examine the novel aspects of MRGPRX2 signaling in MCs as it related to degranulation and review the mechanisms of its regulation.

Mechanisms of pruritogen-induced activation of itch nerves in isolated mouse skin

KEY POINTS

Chloroquine (CQ) stimulates itch nerves and causes intense scratching in mice by activating the G-protein coupled receptor (GPCR) MrgprA3; it is not known how stimulation of MrgprA3 (or other GPCRs) leads to activation of the itch nerve terminals in the skin, but previous studies have found that transient receptor potential A1 (TRPA1) gene deletion blocks CQ-induced scratching. In the present study we used a novel dorsal skin-nerve preparation to evaluate mechanisms underlying CQ- and histamine-induced action potential discharge in itch nerve terminals. We found that CQ activation of the nerves requires the beta3 isoform of phospholipase C, but TRPA1 or other TRP channel are not required. Evidence is provided for a role for calcium-activated chloride channels such as TMEM16a in GPCR-activation of itch nerve terminals. The mechanism by which TRP channels participate in pruritogen-induced scratching may involve sites of action other than the primary afferent terminals.

ABSTRACT

Chloroquine (CQ) and histamine are pruritogens commonly used to study itch in the mouse. A novel skin-nerve preparation was used to evaluate chloroquine (CQ)- and histamine-induced activation of afferent nerves in the dorsal thoracic skin of the mouse. All CQ sensitive nerves were C-fibres, and were also sensitive to histamine. The response to CQ, but not histamine, was largely absent in mrgpr-cluster Δ^-/- mice, supporting the hypothesis that CQ evokes itch largely via stimulation of MrgprA3 receptors. The CQ-induced action potential discharge was largely absent in phospholipase Cβ3 knockout animals. The CQ and histamine responses were not influenced by removal of TRPA1, TRPV1, TRPC3 or TRPC6, nor by the TRP channel blocker Ruthenium Red. The bouts of scratching in response to CQ were not different between wild-type and TRPA1-deficient mice. A selective inhibitor of the calcium-activated chloride channel TMEM16A, N-((4-methoxy)-2-naphthyl)-5-nitroanthranilic acid (MONNA), inhibited CQ-induced action potential discharge at itch nerve terminals and bouts of scratching by about 50%. Although TRPA1 and TRPV1 channels may be involved in the scratching responses to intradermal pruritogens, this is unlikely to be due to an effect at the nerve terminals, where chloride channels may play a more important role.

Targeting human Mas-related G protein-coupled receptor X1 to inhibit persistent pain

Human Mas-related G protein-coupled receptor X1 (MRGPRX1) is a promising target for pain inhibition, mainly because of its restricted expression in nociceptors within the peripheral nervous system. However, constrained by species differences across Mrgprs, drug candidates that activate MRGPRX1 do not activate rodent receptors, leaving no responsive animal model to test the effect on pain in vivo. Here, we generated a transgenic mouse line in which we replaced mouse Mrgprs with human MrgprX1 This humanized mouse allowed us to characterize an agonist [bovine adrenal medulla 8-22 (BAM8-22)] and a positive allosteric modulator (PAM), ML382, of MRGPRX1. Cellular studies suggested that ML382 enhances the ability of BAM8-22 to inhibit high-voltage-activated Ca²⁺ channels and attenuate spinal nociceptive transmission. Importantly, both BAM8-22 and ML382 effectively attenuated evoked, persistent, and spontaneous pain without causing obvious side effects. Notably, ML382 by itself attenuated both evoked pain hypersensitivity and spontaneous pain in MrgprX1 mice after nerve injury without acquiring coadministration of an exogenous agonist. Our findings suggest that humanized MrgprX1 mice provide a promising preclinical model and that activating MRGPRX1 is an effective way to treat persistent pain.

The molecular and cellular mechanisms of itch and the involvement of TRP channels in the peripheral sensory nervous system and skin

Itch is an unpleasant cutaneous sensation that can arise following insect bites, exposure to plant ingredients, and some diseases. Itch can also have idiopathic causes. Itch sensations are thought to protect against external insults and toxic substances. Although itch is not directly lethal, chronic and long lasting itch in certain diseases can worsen quality of life. Therefore, the mechanisms responsible for chronic itch require careful investigation. There is a significant amount of basic research concerning itch, and the effect of various itch mediators on primary sensory neurons have been studied. Interestingly, many mediators of itch involve signaling related to transient receptor potential (TRP) channels. TRP channels, especially thermosensitive TRP channels, are expressed by primary sensory neurons and skin keratinocytes, which receive multimodal stimuli, including those that cause itch sensations. Here we review the molecular and cellular mechanisms of itch and the involvement of TRP channels in mediating itch sensations.

Optogenetic and Chemogenetic Approaches for Studying Astrocytes and Gliotransmitters

The brain consists of heterogeneous populations of neuronal and non-neuronal cells. The revelation of their connections and interactions is fundamental to understanding normal brain functions as well as abnormal changes in pathological conditions. Optogenetics and chemogenetics have been developed to allow functional manipulations both in vitro and in vivo to examine causal relationships between cellular changes and functional outcomes. These techniques are based on genetically encoded effector molecules that respond exclusively to exogenous stimuli, such as a certain wavelength of light or a synthetic ligand. Activation of effector molecules provokes diverse intracellular changes, such as an influx or efflux of ions, depolarization or hyperpolarization of membranes, and activation of intracellular signaling cascades. Optogenetics and chemogenetics have been applied mainly to the study of neuronal circuits, but their use in studying non-neuronal cells has been gradually increasing. Here we introduce recent studies that have employed optogenetics and chemogenetics to reveal the function of astrocytes and gliotransmitters.

Investigating the role of MRGPRC11 and capsaicin-sensitive afferent nerves in the anti-influenza effects exerted by SLIGRL-amide in murine airways

BACKGROUND

The hexapeptide SLIGRL-amide activates protease-activated receptor-2 (PAR-2) and mas-related G protein-coupled receptor C11 (MRGPRC11), both of which are known to be expressed on populations of sensory nerves. SLIGRL-amide has recently been reported to inhibit influenza A (IAV) infection in mice independently of PAR-2 activation, however the explicit roles of MRGPRC11 and sensory nerves in this process are unknown. Thus, the principal aim of this study was to determine whether SLIGRL-amide-induced inhibition of influenza infection is mediated by MRGPRC11 and/or by capsaicin-sensitive sensory nerves.

METHODS

The inhibitory effect of SLIGRL-amide on IAV infection observed in control mice in vivo was compared to effects produced in mice that did not express MRGPRC11 (mrgpr-cluster∆ (-/-) mice) or had impaired sensory nerve function (induced by chronic pre-treatment with capsaicin). Complementary mechanistic studies using both in vivo and ex vivo approaches investigated whether the anti-IAV activity of SLIGRL-amide was (1) mimicked by either activators of MRGPRC11 (BAM8-22) or by activators (acute capsaicin) or selected mediators (substance P, CGRP) of sensory nerve function, or (2) suppressed by inhibitors of sensory nerve function (e.g. NK1 receptor antagonists).

RESULTS

SLIGRL-amide and BAM8-22 dose-dependently inhibited IAV infection in mrgpr-cluster∆ (-/-) mice that do not express MRGPRC11. In addition, SLIGRL-amide and BAM8-22 each inhibited IAV infection in capsaicin-pre-treated mice that lack functional sensory nerves. Furthermore, the anti-IAV activity of SLIGRL-amide was not mimicked by the sensory neuropeptides substance P or CGRP, nor blocked by either NK1 (L-703,606, RP67580) and CGRP receptor (CGRP8-37) antagonists. Direct stimulation of airway sensory nerves through acute exposure to the TRPV1 activator capsaicin also failed to mimic SLIGRL-amide-induced inhibition of IAV infectivity. The anti-IAV activity of SLIGRL-amide was mimicked by the purinoceptor agonist ATP, a direct activator of mucus secretion from airway epithelial cells. Additionally, both SLIGRL-amide and ATP stimulated mucus secretion and inhibited IAV infectivity in mouse isolated tracheal segments.

CONCLUSIONS

SLIGRL-amide inhibits IAV infection independently of MRGPRC11 and independently of capsaicin-sensitive, neuropeptide-releasing sensory nerves, and its secretory action on epithelial cells warrants further investigation.

Roads Less Traveled: Sexual Dimorphism and Mast Cell Contributions to Migraine Pathology

Migraine is a common, little understood, and debilitating disease. It is much more prominent in women than in men (~2/3 are women) but the reasons for female preponderance are not clear. Migraineurs frequently experience severe comorbidities, such as allergies, depression, irritable bowel syndrome, and others; many of the comorbidities are more common in females. Current treatments for migraine are not gender specific, and rarely are migraine and its comorbidities considered and treated by the same specialist. Thus, migraine treatments represent a huge unmet medical need, which will only be addressed with greater understanding of its underlying pathophysiology. We discuss the current knowledge about sex differences in migraine and its comorbidities, and focus on the potential role of mast cells (MCs) in both. Sex-based differences in pain recognition and drug responses, fluid balance, and the blood–brain barrier are recognized but their impact on migraine is not well studied. Furthermore, MCs are well recognized for their prominent role in allergies but much less is known about their contributions to pain pathways in general and migraine specifically. MC-neuron bidirectional communication uniquely positions these cells as potential initiators and/or perpetuators of pain. MCs can secrete nociceptor sensitizing and activating agents, such as serotonin, prostaglandins, histamine, and proteolytic enzymes that can also activate the pain-mediating transient receptor potential vanilloid channels. MCs express receptors for both estrogen and progesterone that induce degranulation upon binding. Furthermore, environmental estrogens, such as Bisphenol A, activate MCs in preclinical models but their impact on pain pathways or migraine is understudied. We hope that this discussion will encourage scientists and physicians alike to bridge the knowledge gaps linking sex, MCs, and migraine to develop better, more comprehensive treatments for migraine patients.

Mas-related gene (Mrg) C receptors inhibit mechanical allodynia and spinal microglia activation in the early phase of neuropathic pain in rats

Mas-related gene (Mrg) C receptors are exclusively expressed in the trigeminal and dorsal root ganglia (DRG). However, their functional roles are poorly understood. This study was aimed to determine the effect of MrgC receptors on pain hypersensitivity in the early phase of neuropathic pain and its underlying mechanisms. Intrathecal (i.t.) administration of the selective MrgC receptor agonist bovine adrenal medulla 8-22 (BAM8-22) at 1 or 10nmol attenuated mechanical allodynia one day after L5 spinal nerve ligation (SNL) surgery. I.t. BAM8-22 (10 nmol) inhibited SNL-induced microglia activation in the spinal dorsal horn on day 2 post-SNL. The BAM8-22 treatment also abolished SNL-induced upregulation of neuronal nitric oxide synthesis (nNOS) in the dorsal root ganglia (DRG). On the other hand, SNL, but not sham, surgery reduced the expression of MrgC receptor mRNA in the injured L5 DRG without changing thier levels in the adjacent uninjured L4 or L6 DRG on day 2 following the surgery. These results suggest that the activation of MrgC receptors can relieve pain hypersensitivity by the inhibition of nNOS increase in DRG neurons and microglia activation in the spinal dorsal horn in the early time following peripheral nerve injury. This study provides evidence that MrgC receptors could be targeted as a novel therapy for neuropathic pain with limited unwanted effects.

Activation of spinal MrgC-Gi-NR2B-nNOS signaling pathway by Mas oncogene-related gene C receptor agonist bovine adrenal medulla 8-22 attenuates bone cancer pain in mice

OBJECTIVES

In the present study, we investigate the effects of Mas oncogene-related gene (Mrg) C receptors (MrgC) on the expression and activation of spinal Gi protein, N-methyl-D-aspartate receptor subunit 2B (NR2B), and neuronal nitric oxide synthase (nNOS) in mouse model of bone cancer pain.

METHODS

The number of spontaneous foot lift (NSF) and paw withdrawal mechanical threshold (PWMT) were measured after inoculation of tumor cells and intrathecal injection of MrgC agonist bovine adrenal medulla 8-22 (BAM8-22) or MrgC antagonist anti-MrgC for 14 days after operation. Expression of spinal MrgC, Gi protein, NR2B and nNOS and their phosphorylated forms after inoculation was examined by immunohistochemistry and Western blotting. Double labeling was used to identify the co-localization of NR2B or nNOS with MrgC in spinal cord dorsal horn (SCDH) neurons. The effects of intrathecal injection of BAM8-22 or anti-MrgC on nociceptive behaviors and the corresponding expression of spinal MrgC, Gi protein, NR2B and nNOS were also investigated.

RESULTS

The expression of spinal MrgC, Gi protein, NR2B, and nNOS was higher in tumor-bearing mice in comparison to sham mice or normal mice. Intrathecal injection of MrgC agonist BAM8-22 significantly alleviated bone cancer pain, up-regulated MrgC and Gi protein expression, and down-regulated the expression of spinal p-NR2B, t-nNOS and p-nNOS in SCDH on day 14 after operation, whereas administration of anti-MrgC produced the opposite effect. Meanwhile, MrgC-like immunoreactivity (IR) co-localizes with NR2B-IR or nNOS-IR in SCDH neurons.

CONCLUSIONS

The present study demonstrates that MrgC-activated spinal Gi-NR2B-nNOS signaling pathway plays important roles in the development of bone cancer pain. These findings may provide a novel strategy for the treatment of bone cancer pain.

Mas-Related G Protein-Coupled Receptors Offer Potential New Targets for Pain Therapy

The founding member of the Mas-related G-protein-coupled receptor (Mrgpr) family was discovered in 1986. Since then, many more members of this receptor family have been identified in multiple species, and their physiologic functions have been investigated widely. Because they are expressed exclusively in small-diameter primary sensory neurons, the roles of Mrgpr proteins in pain and itch have been best studied. This review will focus specifically on the current knowledge of their roles in pathological pain and the potential development of new pharmacotherapies targeted at some Mrgprs for the treatment of chronic pain. We will also discuss the limitations and future scope of this receptor family in pain treatment.

Synthesis of FMRFaNV, a Photoreleasable Caged Transmitter Designed to Study Neuron-Glia Interactions in the Central Nervous System

Neuroscience studies require technologies able to deliver compounds with both scale and timing compatibility with morphological and physiological synaptic properties. In this light, two-photon flash photolysis has been extensively used to successfully apply glutamate or other neurotransmitters at the synaptic level. However, the set of commercially available caged compounds is restricted and incompatible with studies demanding high cell specificity. The gain in cell specificity is especially relevant and challenging when studying neuron-glia interactions in the central nervous system. Here we develop a system to mimic the metabotropic glutamate receptor-dependent response of astrocytes, a glial cell type, following synaptic glutamate release. For this, we expressed an exogeneous orphan Gq-coupled protein of the Mas-related-gene (Mrg) family in glial cells and generated an MrgR's agonist peptide (FMRFa) that was chemically caged with a nitroveratryl photolabile protecting group (NV). NV has an appropriate quantum yield and a high absorption maximum that makes it very adapted to experiments with very short irradiation time. This novel caged compound allowed the activation of MrgR with both single- and two-photon light sources. Indeed, MrgR activation induced calcium transients and morphological changes in astrocytes as described previously. Thus, FMRFaNV is a very promising tool to study neuron-glia interactions.

MAS and its related G protein-coupled receptors, Mrgprs

The Mas-related G protein-coupled receptors (Mrgprs or Mas-related genes) comprise a subfamily of receptors named after the first discovered member, Mas. For most Mrgprs, pruriception seems to be the major function based on the following observations: 1) they are relatively promiscuous in their ligand specificity with best affinities for itch-inducing substances; 2) they are expressed in sensory neurons and mast cells in the skin, the main cellular components of pruriception; and 3) they appear in evolution first in tetrapods, which have arms and legs necessary for scratching to remove parasites or other noxious substances from the skin before they create harm. Because parasites coevolved with hosts, each species faced different parasitic challenges, which may explain another striking observation, the multiple independent duplication and expansion events of Mrgpr genes in different species as a consequence of parallel adaptive evolution. Their predominant expression in dorsal root ganglia anticipates additional functions of Mrgprs in nociception. Some Mrgprs have endogenous ligands, such as β-alanine, alamandine, adenine, RF-amide peptides, or salusin-β. However, because the functions of these agonists are still elusive, the physiologic role of the respective Mrgprs needs to be clarified. The best studied Mrgpr is Mas itself. It was shown to be a receptor for angiotensin-1-7 and to exert mainly protective actions in cardiovascular and metabolic diseases. This review summarizes the current knowledge about Mrgprs, their evolution, their ligands, their possible physiologic functions, and their therapeutic potential.

Interaction of endokinin A/B and (Mpa(6))-γ2-MSH-6-12 in pain regulation in mice

The present study focused on the interactive effects of (Mpa(6))-γ2-MSH-6-12 (Mpa, spinal level) and endokinin A/B (EKA/B, supraspinal level) on pain regulation in mice. EKA/B (30 pmol) only weakened 100 pmol Mpa-induced hyperalgesia at 5 min, but could enhance it during 20-30 min. However, EKA/B (100 pmol) antagonized all dose levels of Mpa significantly at 5 min and blocked them completely at 10 min. EKA/B (3 nmol) co-injected with Mpa presented marked analgesia at 5 min and enduring hyperalgesia within 20-60 min. To investigate the underlying mechanisms between Mpa and EKA/B, SR140333B and SR142801 (NK1 and NK3 receptor antagonists, respectively) were utilized. SR140333B had no influence on Mpa, while SR142801 potentiated it during 20-30 min. Whereas, SR140333B and SR142801 could block the co-administration of Mpa and EKA/B (30 pmol) separately at 5 min and 30 min. These phenomena might attribute to that these two antagonists promoted the antagonism of EKA/B (30 pmol) at the early stage, while antagonized EKA/B preferentially in the latter period. SR140333B weakened the analgesia of EKA/B (3 nmol), but produced no effect on Mpa. However, SR140333B failed to affect the co-injection of Mpa and EKA/B, which implied that EKA/B cooperated with Mpa prior to SR140333B. These results could potentially help to better understand the interaction of NK and MrgC receptors in pain regulation in mice.

ASIC3 Mediates Itch Sensation in Response to Coincident Stimulation by Acid and Nonproton Ligand

The regulation and mechanisms underlying itch sensation are complex. Here, we report a role for acid-sensing ion channel 3 (ASIC3) in mediating itch evoked by certain pruritogens during tissue acidosis. Co-administration of acid with Ser-Leu-Ile-Gly-Arg-Leu-NH2 (SL-NH2) increased scratching behavior in wild-type, but not ASIC3-null, mice, implicating the channel in coincident detection of acidosis and pruritogens. Mechanistically, SL-NH2 slowed desensitization of proton-evoked currents by targeting the previously identified nonproton ligand-sensing domain located in the extracellular region of ASIC3 channels in primary sensory neurons. Ablation of the ASIC3 gene reduced dry-skin-induced scratching behavior and pathological changes under conditions with concomitant inflammation. Taken together, our data suggest that ASIC3 mediates itch sensation via coincident detection of acidosis and nonproton ligands that act at the nonproton ligand-sensing domain of the channel.

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.

The inhibition of high-voltage-activated calcium current by activation of MrgC11 involves phospholipase C-dependent mechanisms

High-voltage-activated (HVA) calcium channels play an important role in synaptic transmission. Activation of Mas-related G-protein-coupled receptor subtype C (MrgC; mouse MrgC11, rat homolog rMrgC) inhibits HVA calcium current (ICa) in small-diameter dorsal root ganglion (DRG) neurons, but the intracellular signaling cascade underlying MrgC agonist-induced inhibition of HVA ICa in native DRG neurons remains unclear. To address this question, we conducted patch-clamp recordings in MrgA3-eGFP-wild-type mice, in which most MrgA3-eGFP(+) DRG neurons co-express MrgC11 and can be identified for recording. We found that the inhibition of HVA ICa by JHU58 (0.001-100nM, a dipeptide, MrgC-selective agonist) was significantly reduced by pretreatment with a phospholipase C (PLC) inhibitor (U73122, 1μM), but not by its inactive analog (U73343) or vehicle. Further, in rats that had undergone spinal nerve injury, pretreatment with intrathecal U73122 nearly abolished the inhibition of mechanical hypersensitivity by intrathecal JHU58. The inhibition of HVA ICa in MrgA3-eGFP(+) neurons by JHU58 (100nM) was partially reduced by pretreatment with a Gβγ blocker (gallein, 100μM). However, applying a depolarizing prepulse and blocking the Gαi and Gαs pathways with pertussis toxin (PTX) (0.5μg/mL) and cholera toxin (CTX) (0.5μg/mL), respectively, had no effect. These findings suggest that activation of MrgC11 may inhibit HVA ICa in mouse DRG neurons through a voltage-independent mechanism that involves activation of the PLC, but not Gαi or Gαs, pathway.