Distinct Expression of Mas1-Related G-Protein-Coupled Receptor B4 in Dorsal Root and Trigeminal Ganglia—Implications for Altered Behaviors in Acid-Sensing Ion Channel 3-Deficient Mice

MrgprB4 in trigeminal neurons expressing TRPA1 modulates unpleasant sensations

Gentle touch such as stroking of the skin produces a pleasant feeling, which is detected by a rare subset of sensory neurons that express Mas-related G protein-coupled receptor B4 (MrgprB4) in mice. We examined small populations of MrgprB4-positive neurons in the trigeminal ganglion and the dorsal root ganglion, and most of these were sensitive to transient receptor potential ankyrin 1 (TRPA1) agonist but not TRPV1, TRPM8, or TRPV4 agonists. Deficiency of MrgprB4 did not affect noxious pain or itch behaviors in the hairless plantar and hairy cheek. Although behavior related to acetone-induced cold sensing in the hind paw was not changed, unpleasant sensory behaviors in response to acetone application or sucrose splash to the cheek were significantly enhanced in Mrgprb4-knockout mice as well as in TRPA1-knockout mice. These results suggest that MrgprB4 in the trigeminal neurons produces pleasant sensations in cooperation with TRPA1, rather than noxious or cold sensations. Pleasant sensations may modulate unpleasant sensations on the cheek via MrgprB4.

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.

Deletion of Acid-Sensing Ion Channel 3 Relieves the Late Phase of Neuropathic Pain by Preventing Neuron Degeneration and Promoting Neuron Repair

Neuropathic pain is one type of chronic pain that occurs as a result of a lesion or disease to the somatosensory nervous system. Chronic excessive inflammatory response after nerve injury may contribute to the maintenance of persistent pain. Although the role of inflammatory mediators and cytokines in mediating allodynia and hyperalgesia has been extensively studied, the detailed mechanisms of persistent pain or whether the interactions between neurons, glia and immune cells are essential for maintenance of the chronic state have not been completely elucidated. ASIC3, a voltage-insensitive, proton-gated cation channel, is the most essential pH sensor for pain perception. ASIC3 gene expression is increased in dorsal root ganglion neurons after inflammation and nerve injury and ASIC3 is involved in macrophage maturation. ASIC currents are increased after nerve injury. However, whether prolonged hyperalgesia induced by the nerve injury requires ASIC3 and whether ASIC3 regulates neurons, immune cells or glial cells to modulate neuropathic pain remains unknown. We established a model of chronic constriction injury of the sciatic nerve (CCI) in mice. CCI mice showed long-lasting mechanical allodynia and thermal hyperalgesia. CCI also caused long-term inflammation at the sciatic nerve and primary sensory neuron degeneration as well as increased satellite glial expression and ATF3 expression. ASIC3 deficiency shortened mechanical allodynia and attenuated thermal hyperalgesia. ASIC3 gene deletion shifted ATF3 expression from large to small neurons and altered the M1/M2 macrophage ratio, thereby preventing small neuron degeneration and relieved pain.

Peripheral 5-HT3 mediates mirror-image pain by a cross-talk with acid-sensing ion channel 3

Mirror-image pain (MIP), which occurs along with complex regional pain syndrome, rheumatoid arthritis and chronic migraine, is characterized by increased pain sensitivity of healthy body regions other than the actual injured or inflamed sites. A high level of peripheral inflammation may activate central or peripheral glia, triggering mirror-image pain. However, which receptors mediate inflammatory signals to contribute glial activation remains unclear. Intraplantarly injecting mice with 5-hydroxytryptamine (5-HT) or acidic buffer (proton) caused only unilateral hyperalgesia, but co-injection of 5-HT/acid induced bilateral hyperalgesia (MIP). Blocking 5-HT₃ or acid-sensing ion channel 3 (ASIC3) abolished satellite glial activation, inhibiting MIP. Interestingly, intraplantar administration of a 5-HT₃ agonist induced MIP, and 5-HT₃-mediated MIP can be reversed by a 5-HT₃ antagonist or an ASIC3 blocker. Similar results were found using a ASIC3 agonist. Furthermore, 5-HT₃ was observed to co-localize with ASIC3 in DRG neurons; 5-HT₃ activation-induced an increase in intracellular calcium that was inhibited by an ASIC3 blocker and vice versa. A cross-talk between 5-HT₃ and ASIC3 mediates satellite glial activation, thereby triggering mirror-image pain.

TDAG8, TRPV1, and ASIC3 involved in establishing hyperalgesic priming in experimental rheumatoid arthritis

Rheumatoid arthritis (RA), characterized by chronic inflammation of synovial joints, is often associated with ongoing pain and increased pain sensitivity. High hydrogen ion concentration (acidosis) found in synovial fluid in RA patients is associated with disease severity. Acidosis signaling acting on proton-sensing receptors may contribute to inflammation and pain. Previous studies focused on the early phase of arthritis (<5 weeks) and used different arthritis models, so elucidating the roles of different proton-sensing receptors in the chronic phase of arthritis is difficult. We intra-articularly injected complete Freund’s adjuvant into mice once a week for 4 weeks to establish chronic RA pain. Mice with knockout of acid-sensing ion channel 3 (ASIC3) or transient receptor potential/vanilloid receptor subtype 1 (TRPV1) showed attenuated chronic phase (>6 weeks) of RA pain. Mice with T-cell death-associated gene 8 (TDAG8) knockout showed attenuated acute and chronic phases of RA pain. TDAG8 likely participates in the initiation of RA pain, but all three genes, TDAG8, TRPV1, and ASIC3, are essential to establish hyperalgesic priming to regulate the chronic phase of RA pain.

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.

Wogonin prevents rat dorsal root ganglion neurons death via inhibiting tunicamycin-induced ER stress in vitro

Wogonin is a natural flavonoid isolated from the root of Scutellaria baicalensis Georgi, which has been widely used in various research areas for its anti-oxidant, anti-inflammatory, and anti-cancer activities. It also presents a neuroprotective effect in the brain while encounters stress conditions, but the mechanisms controlling the neuroprotective effect of wogonin are not clear. In this study, we investigated the biomechanism underlying the neuroprotective effect of wogonin on rat dorsal root ganglion (DRG) neurons. Wogonin pre-treatment at 75 μM significantly increased the cell viability of DRG neurons and decreased the number of the propidium iodide-positive DRG neurons before the endoplasmic reticulum (ER) stress is being induced by tunicamycin (TUN) (0.75 μg/mL). In addition, Wogonin also inhibited the release of LDH and up-regulated the level of GSH. Furthermore, wogonin decreased the activation of ER stress-related molecules, including glucose-regulated protein 78 (GRP78), GRP94, C/EBP-homologous protein, active caspase12 and active caspase3, phosphorylation of pancreatic ER stress kinase, and eukaryotic initiation factor 2 alpha (eIF2α). In summary, our results indicated that wogonin could protect DRG neurons against TUN-induced ER stress.

Roles of ASIC3, TRPV1, and NaV1.8 in the transition from acute to chronic pain in a mouse model of fibromyalgia

Background

Tissue acidosis is effective in causing chronic muscle pain. However, how muscle nociceptors contribute to the transition from acute to chronic pain is largely unknown.

Results

Here we showed that a single intramuscular acid injection induced a priming effect on muscle nociceptors of mice. The primed muscle nociceptors were plastic and permitted the development of long-lasting chronic hyperalgesia induced by a second acid insult. The plastic changes of muscle nociceptors were modality-specific and required the activation of acid-sensing ion channel 3 (ASIC3) or transient receptor potential cation channel V1 (TRPV1). Activation of ASIC3 was associated with increased activity of tetrodotoxin (TTX)-sensitive voltage-gated sodium channels but not protein kinase Cϵ (PKCϵ) in isolectin B4 (IB4)-negative muscle nociceptors. In contrast, increased activity of TTX-resistant voltage-gated sodium channels with ASIC3 or TRPV1 activation in Na_V1.8-positive muscle nociceptors was required for the development of chronic hyperalgesia. Accordingly, compared to wild type mice, Na_V1.8-null mice showed briefer acid-induced hyperalgesia (5 days vs. >27 days).

Conclusion

ASIC3 activation may manifest a new type of nociceptor priming in IB4-negative muscle nociceptors. The activation of ASIC3 and TRPV1 as well as enhanced Na_V1.8 activity are essential for the development of long-lasting hyperalgesia in acid-induced, chronic, widespread muscle pain.