A role for ASIC3 in the modulation of high-intensity pain stimuli

Pathology and physiology of acid-sensitive ion channels in the bladder

Acid-sensitive ion channels (ASICs) are sodium-permeable channels activated by extracellular acidification. They can be activated and trigger the inward flow of Na+ when the extracellular environment is acidic, leading to membrane depolarization and thus inducing action potentials in neurons. There are four ASIC genes in mammals (ASIC1-4). ASIC is widely expressed in humans. It is closely associated with pain, neurological disorders, multiple sclerosis, epilepsy, migraines, and many other disorders. Bladder pain syndrome/interstitial cystitis (BPS/IC) is a specific syndrome characterized by bladder pain. Recent studies have shown that ASICs are closely associated with the development of BPS/IC. A study revealed that ASIC levels are significantly elevated in a BPS/IC model. Additionally, researchers have reported differential changes in ASICs in the bladders of patients with neurogenic lower urinary tract dysfunction (NLUTD) caused by spinal cord injury (SCI). In this review, we summarize the structure and physiological functions of ASICs and focus on the mechanisms by which ASICs mediate bladder disease.

Molecular Insights into Single Chain Lipid Modulation of Acid-Sensing Ion Channel 3

Polyunsaturated fatty acids (PUFAs) and their analogs play a significant role in modulating the activity of diverse ion channels, and recent studies show that these lipids potentiate acid-sensing ion channels (ASICs), leading to increased activity. The potentiation of the channel stems from multiple gating changes, but the exact mechanism of these effects remains uncertain. We posit a mechanistic explanation for one of these changes in channel function, the increase in the maximal current, by applying a combination of electrophysiology and all-atom molecular dynamics simulations on the open-state hASIC3. Microsecond-scale simulations were performed on open-state hASIC3 in the absence and presence of a PUFA, docosahexaenoic acid (DHA), and a PUFA analog, N-arachidonyl glycine (AG). Intriguingly, our simulations in the absence of PUFA or PUFA analogs reveal that a tail from the membrane phospholipid POPC inserts itself into the pore of the channel through lateral fenestrations on the sides of the transmembrane segments, obstructing ion permeation through the channel. The binding of either DHA or AG prevented POPC from accessing the pore in our simulations, relieving the block of ionic conduction by phospholipids. Finally, we use the single-channel recording to show that DHA increases the amplitude of the single-channel currents in ASIC3, which is consistent with our hypothesis that PUFAs relieve the pore block of the channel induced by POPCs. Together, these findings offer a potential mechanistic explanation of how PUFAs modulate ASIC maximal current, revealing a novel mechanism of action for PUFA-induced modulation of ion channels.

Insulin enhances acid-sensing ion channel currents in rat primary sensory neurons

Insulin has been shown to modulate neuronal processes through insulin receptors. The ion channels located on neurons may be important targets for insulin/insulin receptor signaling. Both insulin receptors and acid-sensing ion channels (ASICs) are expressed in dorsal root ganglia (DRG) neurons. However, it is still unclear whether there is an interaction between them. Therefore, the purpose of this investigation was to determine the effects of insulin on the functional activity of ASICs. A 5 min application of insulin rapidly enhanced acid-evoked ASIC currents in rat DRG neurons in a concentration-dependent manner. Insulin shifted the concentration-response plot for ASIC currents upward, with an increase of 46.2 ± 7.6% in the maximal current response. The insulin-induced increase in ASIC currents was eliminated by the insulin receptor antagonist GSK1838705, the tyrosine kinase inhibitor lavendustin A, and the phosphatidylinositol-3 kinase antagonist wortmannin. Moreover, insulin increased the number of acid-triggered action potentials by activating insulin receptors. Finally, local administration of insulin exacerbated the spontaneous nociceptive behaviors induced by intraplantar acid injection and the mechanical hyperalgesia induced by intramuscular acid injections through peripheral insulin receptors. These results suggested that insulin/insulin receptor signaling enhanced the functional activity of ASICs via tyrosine kinase and phosphatidylinositol-3 kinase pathways. Our findings revealed that ASICs were targets in primary sensory neurons for insulin receptor signaling, which may underlie insulin modulation of pain.

Sensory ASIC3 channel exacerbates psoriatic inflammation via a neurogenic pathway in female mice

Psoriasis is an immune-mediated skin disease associated with neurogenic inflammation, but the underlying molecular mechanism remains unclear. We demonstrate here that acid-sensing ion channel 3 (ASIC3) exacerbates psoriatic inflammation through a sensory neurogenic pathway. Global or nociceptor-specific Asic3 knockout (KO) in female mice alleviates imiquimod-induced psoriatic acanthosis and type 17 inflammation to the same extent as nociceptor ablation. However, ASIC3 is dispensable for IL-23-induced psoriatic inflammation that bypasses the need for nociceptors. Mechanistically, ASIC3 activation induces the activity-dependent release of calcitonin gene-related peptide (CGRP) from sensory neurons to promote neurogenic inflammation. Botulinum neurotoxin A and CGRP antagonists prevent sensory neuron-mediated exacerbation of psoriatic inflammation to similar extents as Asic3 KO. In contrast, replenishing CGRP in the skin of Asic3 KO mice restores the inflammatory response. These findings establish sensory ASIC3 as a critical constituent in psoriatic inflammation, and a promising target for neurogenic inflammation management.

ASICs mediate fast excitatory synaptic transmission for tactile discrimination

Tactile discrimination, the ability to differentiate objects' physical properties such as texture, shape, and edges, is essential for environmental exploration, social interaction, and early childhood development. This ability heavily relies on Merkel cell-neurite complexes (MNCs), the tactile end-organs enriched in the fingertips of humans and the whisker hair follicles of non-primate mammals. Although recent studies have advanced our knowledge on mechanical transduction in MNCs, it remains unknown how tactile signals are encoded at MNCs. Here, using rodent whisker hair follicles, we show that tactile signals are encoded at MNCs as fast excitatory synaptic transmission. This synaptic transmission is mediated by acid-sensing ion channels (ASICs) located on the neurites of MNCs, with protons as the principal transmitters. Pharmacological inhibition or genetic deletion of ASICs diminishes the tactile encoding at MNCs and impairs tactile discrimination in animals. Together, ASICs are required for tactile encoding at MNCs to enable tactile discrimination in mammals.

Molecular determinants of ASIC1 modulation by divalent cations

Acid-sensing ion channels (ASICs) are proton-gated cation channels widely expressed in the nervous system. ASIC gating is modulated by divalent cations as well as small molecules; however, the molecular determinants of gating modulation by divalent cations are not well understood. Previously, we identified two small molecules that bind to ASIC1a at a novel site in the acidic pocket and modulate ASIC1 gating in a manner broadly resembling divalent cations, raising the possibility that these small molecules may help to illuminate the molecular determinants of gating modulation by divalent cations. Here, we examined how these two groups of modulators might interact as well as mutational effects on ASIC1a gating and its modulation by divalent cations. Our results indicate that binding of divalent cations to an acidic pocket site plays a key role in gating modulation of the channel.

Role of proprioceptors in chronic musculoskeletal pain

Proprioceptors are non-nociceptive low-threshold mechanoreceptors. However, recent studies have shown that proprioceptors are acid-sensitive and express a variety of proton-sensing ion channels and receptors. Accordingly, although proprioceptors are commonly known as mechanosensing neurons that monitor muscle contraction status and body position, they may have a role in the development of pain associated with tissue acidosis. In clinical practice, proprioception training is beneficial for pain relief. Here we summarize the current evidence to sketch a different role of proprioceptors in 'non-nociceptive pain' with a focus on their acid-sensing properties.

CCK-8 enhances acid-sensing ion channel currents in rat primary sensory neurons

Cholecystokinin (CCK) is a peptide that has been implicated in pain modulation. Acid sensitive ion channels (ASICs) also play an important role in pain associated with tissue acidification. However, it is still unclear whether there is an interaction between CCK signaling and ASICs during pain process. Herein, we report that a functional link between them in rat dorsal root ganglion (DRG) neurons. Pretreatment with CCK-8 concentration-dependently increased acid-evoked ASIC currents. CCK-8 increased the maximum response of ASICs to acid, but did not changed their acid sensitivity. Enhancement of ASIC currents by CCK-8 was mediated by the stimulation of CCK2 receptor (CCK2R), rather than CCK1R. The enhancement of ASIC currents by CCK-8 was prevented by application of either G-protein inhibitor GDP-β-S or protein kinase C (PKC) inhibitor GF109203×, but not by protein kinase A (PKA) inhibitor H-89 or JNK inhibitor SP600125. Moreover, CCK-8 increased the number of action potentials triggered by acid stimuli by activating CCK2R. Finally, CCK-8 dose-dependently exacerbated acid-induced nociceptive behavior in rats through local CCK2R. Together, these results indicated that CCK-8/CCK2R activation enhanced ASIC-mediated electrophysiological activity in DRG neurons and nociception in rats. The enhancement effect depended on G-proteins and intracellular PKC signaling rather than PKA and JNK signaling pathway. These findings provided that CCK-8/CCK2R is an important therapeutic target for ASIC-mediated pain.

Phylogenetic Analysis Provides Insight Into the Molecular Evolution of Nociception and Pain-Related Proteins

Nociception and pain sensation are important neural processes in humans to avoid injury. Many proteins are involved in nociception and pain sensation in humans; however, the evolution of these proteins in animals is unknown. Here, we chose nociception- and pain-related proteins, including G protein-coupled receptors (GPCRs), ion channels (ICs), and neuropeptides (NPs), which are reportedly associated with nociception and pain in humans, and identified their homologs in various animals by BLAST, phylogenetic analysis and protein architecture comparison to reveal their evolution from protozoans to humans. We found that the homologs of transient receptor potential channel A 1 (TRPA1), TRAPM, acid-sensing IC (ASIC), and voltage-dependent calcium channel (VDCC) first appear in Porifera. Substance-P receptor 1 (TACR1) emerged from Coelenterata. Somatostatin receptor type 2 (SSTR2), TRPV1 and voltage-dependent sodium channels (VDSC) appear in Platyhelminthes. Calcitonin gene-related peptide receptor (CGRPR) was first identified in Nematoda. However, opioid receptors (OPRs) and most NPs were discovered only in vertebrates and exist from agnatha to humans. The results demonstrated that homologs of nociception and pain-related ICs exist from lower animal phyla to high animal phyla, and that most of the GPCRs originate from low to high phyla sequentially, whereas OPRs and NPs are newly evolved in vertebrates, which provides hints of the evolution of nociception and pain-related proteins in animals and humans.

Ion Channel Genes in Painful Neuropathies

Neuropathic pain (NP) is a typical symptom of peripheral nerve disorders, including painful neuropathy. The biological mechanisms that control ion channels are important for many cell activities and are also therapeutic targets. Disruption of the cellular mechanisms that govern ion channel activity can contribute to pain pathophysiology. The voltage-gated sodium channel (VGSC) is the most researched ion channel in terms of NP; however, VGSC impairment is detected in only <20% of painful neuropathy patients. Here, we discuss the potential role of the other peripheral ion channels involved in sensory signaling (transient receptor potential cation channels), neuronal excitation regulation (potassium channels), involuntary action potential generation (hyperpolarization-activated cyclic nucleotide-gated channels), thermal pain (anoctamins), pH modulation (acid sensing ion channels), and neurotransmitter release (calcium channels) related to pain and their prospective role as therapeutic targets for painful neuropathy.

Small Extracellular Vesicles From Spared Nerve Injury Model and Sham Control Mice Differentially Regulate Gene Expression in Primary Microglia

Nerve injury outcomes might be predicted by examining small extracellular vesicles (sEVs) in circulation, as their biomolecular cargo facilitates cellular communication and can alter transcriptional state and behavior of recipient cells. We found that sEVs from the serum of spared nerve injury (SNI) model male mice had 7 differentially expressed miRNAs compared to sEVs from sham-operated control mice 4 weeks postsurgery. We investigated how these sEVs alter transcription in primary cortical microglia, a crucial mediator of neuropathic pain, using RNA sequencing. While the uptake of sEVs from both SNI model and sham groups changed gene expression in microglia compared to PBS treatment, sEVs from the sham group induced a more drastic change, particularly in genes involved in immune response. This was recapitulated by increased levels of pro-inflammatory cytokines and chemokines in microglia incubated with sEVs from sham control compared to sEVs from SNI model, naïve mice, or PBS. However, treating microglia with sEVs from female mice showed that serum sEVs derived from female SNI mice but not from female sham mice induced a more pronounced microglial secretion of pro-inflammatory mediators. Our data demonstrate that the molecular changes induced by sham surgery injuring skin and muscles are reflected in circulating sEVs in male mice 4 weeks later. Thus, when using sEVs from sham mice as control in comparative mechanistic studies after nerve injury, sex of mice should be taken into consideration. PERSPECTIVE: Microglial uptake of sEVs from male sham control mice induces higher pro-inflammatory responses compared to SNI sEVs but the reverse was observed upon treatment with sEVs from female mice. Wound healing may have a long-term impact on sEVs in male mice and should be considered for comparative studies using sEVs.

Regulation of pain neurotransmitters and chondrocytes metabolism mediated by voltage-gated ion channels: A narrative review

Osteoarthritis (OA) is one of the leading causes of chronic pain and dysfunction. It is essential to comprehend the nature of pain and cartilage degeneration and its influencing factors on OA treatment. Voltage-gated ion channels (VGICs) are essential in chondrocytes and extracellular matrix (ECM) metabolism and regulate the pain neurotransmitters between the cartilage and the central nervous system. This narrative review focused primarily on the effects of VGICs regulating pain neurotransmitters and chondrocytes metabolism, and most studies have focused on voltage-sensitive calcium channels (VSCCs), voltage-gated sodium channels (VGSCs), acid-sensing ion channels (ASICs), voltage-gated potassium channels (VGKCs), voltage-gated chloride channels (VGCCs). Various ion channels coordinate to maintain the intracellular environment's homeostasis and jointly regulate metabolic and pain under normal circumstances. In the OA model, the ion channel transport of chondrocytes is abnormal, and calcium influx is increased, which leads to increased neuronal excitability. The changes in ion channels are strongly associated with the OA disease process and individual OA risk factors. Future studies should explore how VGICs affect the metabolism of chondrocytes and their surrounding tissues, which will help clinicians and pharmacists to develop more effective targeted drugs to alleviate the progression of OA disease.

Genetic characterization of Macaca arctoides: A highlight of key genes and pathways

When compared to the approximately 22 other macaque species, Macaca arctoides has many unique phenotypes. These traits fall into various phenotypic categories, including genitalia, coloration, mating, and olfactory traits. Here we used a previously identified whole genome set of 690 outlier genes to look for possible genetic explanations of these unique traits. Of these, 279 genes were annotated miRNAs, which are non-coding. Patterns within the remaining outliers in coding genes were investigated using GO (n = 370) and String (n = 383) analysis, which showed many interconnected immune-related genes. Further, we compared the outliers to candidate pathways associated with M. arcotides' unique phenotypes, revealing 10/690 outlier genes that overlapped these four pathways: hedgehog signaling, WNT signaling, olfactory, and melanogenesis. Of these, genes in all pathways except olfactory had higher F_ST values than the rest of the genes in the genome based on permutation tests. Overall, our results point to many genes each having a small impact on phenotype, working in tandem to cause large systemic changes. Additionally, these results may indicate pleiotropy. This seems to be especially true with the development and coloration of M. arctoides. Our results highlight that development, melanogenesis, immune function, and miRNAs may be heavily involved in M. arctoides' evolutionary history.

Gene therapy for chronic pain: emerging opportunities in target-rich peripheral nociceptors

With sweeping advances in precision delivery systems and manipulation of the genomes and transcriptomes of various cell types, medical biotechnology offers unprecedented selectivity for and control of a wide variety of biological processes, forging new opportunities for therapeutic interventions. This perspective summarizes state-of-the-art gene therapies enabled by recent innovations, with an emphasis on the expanding universe of molecular targets that govern the activity and function of primary sensory neurons and which might be exploited to effectively treat chronic pain.

CGRP-dependent sensitization of PKC-δ positive neurons in central amygdala mediates chronic migraine

BACKGROUND

To investigate specific brain regions and neural circuits that are responsible for migraine chronification.

METHODS

We established a mouse model of chronic migraine with intermittent injections of clinically-relevant dose of nitroglycerin (0.1 mg/kg for 9 days) and validated the model with cephalic and extracephalic mechanical sensitivity, calcitonin gene-related peptide (CGRP) expression in trigeminal ganglion, and responsiveness to sumatriptan or central CGRP blockade. We explored the neurons that were sensitized along with migraine chronification and investigated their roles on migraine phenotypes with chemogenetics.

RESULTS

After repetitive nitroglycerin injections, mice displayed sustained supraorbital and hind paw mechanical hyperalgesia, which lasted beyond discontinuation of nitroglycerin infusion and could be transiently reversed by sumatriptan. The CGRP expression in trigeminal ganglion was also upregulated. We found the pERK positive cells were significantly increased in the central nucleus of the amygdala (CeA), and these sensitized cells in the CeA were predominantly protein kinase C-delta (PKC-δ) positive neurons co-expressing CGRP receptors. Remarkably, blockade of the parabrachial nucleus (PBN)-CeA CGRP neurotransmission by CGRP_8-37 microinjection to the CeA attenuated the sustained cephalic and extracephalic mechanical hyperalgesia. Furthermore, chemogenetic silencing of the sensitized CeA PKC-δ positive neurons reversed the mechanical hyperalgesia and CGRP expression in the trigeminal ganglion. In contrast, repetitive chemogenetic activation of the CeA PKC-δ positive neurons recapitulated chronic migraine-like phenotypes in naïve mice.

CONCLUSIONS

Our data suggest that CeA PKC-δ positive neurons innervated by PBN CGRP positive neurons might contribute to the chronification of migraine, which may serve as future therapeutic targets for chronic migraine.

Wireless neuromodulation in vitro and in vivo by intrinsic TRPC-mediated magnetomechanical stimulation

Various magnetic deep brain stimulation (DBS) methods have been developing rapidly in the last decade for minimizing the invasiveness of DBS. However, current magnetic DBS methods, such as magnetothermal and magnetomechanical stimulation, require overexpressing exogeneous ion channels in the central nervous system (CNS). It is unclear whether magnetomechanical stimulation can modulate non-transgenic CNS neurons or not. Here, we reveal that the torque of magnetic nanodiscs with weak and slow alternative magnetic field (50 mT at 10 Hz) could activate neurons through the intrinsic transient receptor potential canonical channels (TRPC), which are mechanosensitive ion channels widely expressed in the brain. The immunostaining with c-fos shows the increasement of neuronal activity by wireless DBS with magnetomechanical approach in vivo. Overall, this research demonstrates a magnetic nanodiscs-based magnetomechanical approach that can be used for wireless neuronal stimulation in vitro and untethered DBS in vivo without implants or genetic manipulation.

Mechanisms of Action of the Peptide Toxins Targeting Human and Rodent Acid-Sensing Ion Channels and Relevance to Their In Vivo Analgesic Effects

Acid-sensing ion channels (ASICs) are voltage-independent H⁺-gated cation channels largely expressed in the nervous system of rodents and humans. At least six isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) associate into homotrimers or heterotrimers to form functional channels with highly pH-dependent gating properties. This review provides an update on the pharmacological profiles of animal peptide toxins targeting ASICs, including PcTx1 from tarantula and related spider toxins, APETx2 and APETx-like peptides from sea anemone, and mambalgin from snake, as well as the dimeric protein snake toxin MitTx that have all been instrumental to understanding the structure and the pH-dependent gating of rodent and human cloned ASICs and to study the physiological and pathological roles of native ASICs in vitro and in vivo. ASICs are expressed all along the pain pathways and the pharmacological data clearly support a role for these channels in pain. ASIC-targeting peptide toxins interfere with ASIC gating by complex and pH-dependent mechanisms sometimes leading to opposite effects. However, these dual pH-dependent effects of ASIC-inhibiting toxins (PcTx1, mambalgin and APETx2) are fully compatible with, and even support, their analgesic effects in vivo, both in the central and the peripheral nervous system, as well as potential effects in humans.

Single Subcutaneous Injection of Lysophosphatidyl-Choline Evokes ASIC3-Dependent Increases of Spinal Dorsal Horn Neuron Activity

Lysophosphatidyl-choline (LPC), a member of the phospholipid family, is an emerging player in pain. It is known to modulate different pain-related ion channels, including Acid-Sensing Ion Channel 3 (ASIC3), a cationic channel mainly expressed in peripheral sensory neurons. LPC potentiates ASIC3 current evoked by mild acidifications, but can also activate the channel at physiological pH. Very recently, LPC has been associated to chronic pain in patients suffering from fibromyalgia or osteoarthritis. Accordingly, repetitive injections of LPC within mouse muscle or joint generate both persistent pain-like and anxiety-like behaviors in an ASIC3-dependent manner. LPC has also been reported to generate acute pain behaviors when injected intraplantarly in rodents. Here, we explore the mechanism of action of a single cutaneous injection of LPC by studying its effects on spinal dorsal horn neurons. We combine pharmacological, molecular and functional approaches including in vitro patch clamp recordings and in vivo recordings of spinal neuronal activity. We show that a single cutaneous injection of LPC exclusively affects the nociceptive pathway, inducing an ASIC3-dependent sensitization of nociceptive fibers that leads to hyperexcitabilities of both high threshold (HT) and wide dynamic range (WDR) spinal neurons. ASIC3 is involved in LPC-induced increase of WDR neuron’s windup as well as in WDR and HT neuron’s mechanical hypersensitivity, and it participates, together with TRPV1, to HT neuron’s thermal hypersensitivity. The nociceptive input induced by a single LPC cutaneous rather induces short-term sensitization, contrary to previously described injections in muscle and joint. If the effects of peripheral LPC on nociceptive pathways appear to mainly depend on peripheral ASIC3 channels, their consequences on pain may also depend on the tissue injected. Our findings contribute to a better understanding of the nociceptive signaling pathway activated by peripheral LPC via ASIC3 channels, which is an important step regarding the ASIC3-dependent roles of this phospholipid in acute and chronic pain conditions.

Pathology and physiology of acid‑sensitive ion channels in the digestive system (Review)

As a major proton-gated cation channel, acid-sensitive ion channels (ASICs) can perceive large extracellular pH changes. ASICs play an important role in the occurrence and development of diseases of various organs and tissues including in the heart, brain, and gastrointestinal tract, as well as in tumor proliferation, invasion, and metastasis in acidosis and regulation of an acidic microenvironment. The permeability of ASICs to sodium and calcium ions is the basis of their physiological and pathological roles in the body. This review summarizes the physiological and pathological mechanisms of ASICs in digestive system diseases, which plays an important role in the early diagnosis, treatment, and prognosis of digestive system diseases related to ASIC expression.

Upregulation of acid sensing ion channels is associated with esophageal hypersensitivity in GERD

Proton pump inhibitors (PPIs) are the mainstay of therapy for gastroesophageal reflux disease (GERD) but up to 60% of patients have inadequate response to therapy. Acid sensing ion channels (ASICs) play important roles in nociception. This study aimed to investigate whether the increased expression of ASICs results in neuronal hyperexcitability in GERD. Esophageal biopsies were taken from GERD patients and healthy subjects to compare expression of ASIC1 and 3. Next, gene and protein expression of ASIC1 and 3 from esophageal mucosa and dorsal root ganglia (DRG) neurons were measured by qPCR, Western‐blot and immunofluorescence in rodent models of reflux esophagitis (RE), non‐erosive reflux disease (NERD), and sham operated groups. Excitability of DRG neurons in the GERD and sham groups were also tested by whole‐cell patch‐clamp recordings. We demonstrated that ASIC1 and 3 expression were significantly increased in patients with RE compared with healthy controls. This correlated positively with symptom severity of heartburn and regurgitation (p < .001). Next, ASIC1 and 3 gene and protein expression in rodent models of RE and NERD were similarly increased in esophageal mucosa as well as T3–T5 DRG neurons compared with sham operation. DRG neurons from RE animals showed hyperexcitability compared with sham group. However, intrathecal injection of ASIC specific inhibitors, PcTx1 and APTEx‐2, as well as silencing ASIC1 and 3 genes with specific siRNAs prevented visceral hypersensitivity. Overall, upregulation of ASIC1 and 3 may lead to visceral hypersensitivity in RE and NERD and may be a potential therapeutic target for PPI non‐responsive patients.

Acid-sensing ion channel 3: An analgesic target

Acid-sensing ion channel 3 (ASIC3) belongs to the epithelial sodium channel/degenerin (ENaC/DEG) superfamily. There are 7 different ASIC subunits encoded by 5 different genes. Most ASIC subunits form trimeric ion channels that upon activation by extracellular protons mediate a transient inward current inducing cellular excitability. ASIC subunits exhibit differential tissue expression and biophysical properties, and the ability of subunits to form homo- and heteromeric trimers further increases the complexity of currents measured and their pharmacological properties. ASIC3 is of particular interest, not only because it exhibits high expression in sensory neurones, but also because upon activation it does not fully inactivate: a transient current is followed by a sustained current that persists during a period of extracellular acidity, i.e. ASIC3 can encode prolonged acidosis as a nociceptive signal. Furthermore, certain mediators sensitize ASIC3 enabling smaller proton concentrations to activate it and other mediators can directly activate the channel at neutral pH. Moreover, there is a plethora of evidence using transgenic mouse models and pharmacology, which supports ASIC3 as being a potential target for development of analgesics. This review will focus on current understanding of ASIC3 function to provide an overview of how ASIC3 contributes to physiology and pathophysiology, examining the mechanisms by which it can be modulated, and highlighting gaps in current understanding and future research directions.

An injectable mesoporous silica-based analgesic delivery system prolongs the duration of sciatic nerve block in mice with minimal toxicity

The major limitation of traditional local anesthetics is the finite duration of a single injection. The present study developed two kinds of novel injectable anesthetic nanocomposites based on mesoporous silica, and evaluated their long-lasting analgesic effect and biosafety. The nanoparticulate carriers, mesoporous silica nanoparticles (MSNs) and mesoporous silica-coated gold nanorods (GNR@MSN), were firstly constructed using the oil-water biphase reaction approach and then ropivacaine (RPC), a local anesthetic, was loaded into the mesoporous carriers by vacuum suction. Transmission electron microscopic images showed the well-ordered mesoporous structure for drug loading. RPC-loaded MSNs and RPC-loaded GNR@MSN exhibited a sustained-release pattern in vitro, and the latter also showed a controlled-release manner triggered by near-infrared (NIR) irradiation. RPC-loaded MSNs and RPC-loaded GNR@MSN caused an initial sensory blockade in mice that lasted for 6 h, almost 2.5 folds of that from free RPC solution. Furthermore, upon NIR irradiation, the latter induced three additional periods of the blockade. Neither of them showed motor nerve block, which may be due to the sustained release manner. The low myotoxicity and low neurotoxicity of the two nanocomposites were presented both in vitro and in vivo. These results demonstrate the potential of the mesoporous silica-based analgesic nanocomposites in effectively controlling postoperative pain, maybe RPC-loaded MSNs for moderate pain and RPC-loaded GNR@MSN for severe pain. STATEMENT OF SIGNIFICANCE: Adequate postoperative analgesia helps early functional exercise after surgery and accelerates rapid recovery, while uncontrolled postoperative pain probably develops chronic post-surgical pain that impacts the life quality of patients for a long time. However, postoperative pain management is still a challenge. The current treatment drugs are always accompanied by some side effects due to their systemic effect. Opioids have risks of addiction and respiratory depression, and nonsteroidal anti-inflammatory drugs can lead to gastrointestinal reaction. Therefore, the long-lasting local anesthetic formulation with good biocompatibility is the most promising solution to manage post-surgical pain. The present study developed novel injectable anesthetic nanocomposites based on mesoporous silica, providing long-lasting pain relief in mice with minimal toxicity.

Acid-sensing ion channels modulate bladder nociception

Sensitization of neuronal pathways and persistent afferent drive are major contributors to somatic and visceral pain. However, the underlying mechanisms that govern whether afferent signaling will give rise to sensitization and pain are not fully understood. In the present report, we investigated the contribution of acid-sensing ion channels (ASICs) to bladder nociception in a model of chemical cystitis induced by cyclophosphamide (CYP). We found that the administration of CYP to mice lacking ASIC3, a subunit primarily expressed in sensory neurons, generates pelvic allodynia at a time point at which only modest changes in pelvic sensitivity are apparent in wild-type mice. The differences in mechanical pelvic sensitivity between wild-type and Asic3 knockout mice treated with CYP were ascribed to sensitized bladder C nociceptors. Deletion of Asic3 from bladder sensory neurons abolished their ability to discharge action potentials in response to extracellular acidification. Collectively, the results of our study support the notion that protons and their cognate ASIC receptors are part of a mechanism that operates at the nerve terminals to control nociceptor excitability and sensitization.NEW & NOTEWORTHY Our study indicates that protons and their cognate acid-sensing ion channel receptors are part of a mechanism that operates at bladder afferent terminals to control their function and that the loss of this regulatory mechanism results in hyperactivation of nociceptive pathways and the development of pain in the setting of chemical-induced cystitis.

The M1 and pre-M1 segments contribute differently to ion selectivity in ASICs and ENaCs

The ability to discriminate between different ionic species, termed ion selectivity, is a key feature of ion channels and forms the basis for their physiological function. Members of the degenerin/epithelial sodium channel (DEG/ENaC) superfamily of trimeric ion channels are typically sodium selective, but to a surprisingly variable degree. While acid-sensing ion channels (ASICs) are weakly sodium selective (sodium:potassium ratio ∼10:1), ENaCs show a remarkably high preference for sodium over potassium (>500:1). This discrepancy may be expected to originate from differences in the pore-lining second transmembrane segment (M2). However, these show a relatively high degree of sequence conservation between ASICs and ENaCs, and previous functional and structural studies could not unequivocally establish that differences in M2 alone can account for the disparate degrees of ion selectivity. By contrast, surprisingly little is known about the contributions of the first transmembrane segment (M1) and the preceding pre-M1 region. In this study, we used conventional and noncanonical amino acid-based mutagenesis in combination with a variety of electrophysiological approaches to show that the pre-M1 and M1 regions of mASIC1a channels are major determinants of ion selectivity. Mutational investigations of the corresponding regions in hENaC show that these regions contribute less to ion selectivity, despite affecting ion conductance. In conclusion, our work suggests that the remarkably different degrees of sodium selectivity in ASICs and ENaCs are achieved through different mechanisms. These results further highlight how M1 and pre-M1 are likely to differentially affect pore structure in these related channels.

Acid-Sensing Ion Channels in Zebrafish

Simple Summary

The present review collects data regarding the presence of ASICs (acid-sensing ion channels) in zebrafish, which have become, over several years, an important experimental model for the study of various diseases. ASICs are a family of ion channels involved in the perception of different types of stimuli. They are excitatory receptors for extracellular H⁺ involved in synaptic transmission, the peripheral perception of pain and in chemical or mechanosensation.

Abstract

The ASICs, in mammals as in fish, control deviations from the physiological values of extracellular pH, and are involved in mechanoreception, nociception, or taste receptions. They are widely expressed in the central and peripheral nervous system. In this review, we summarized the data about the presence and localization of ASICs in different organs of zebrafish that represent one of the most used experimental models for the study of several diseases. In particular, we analyzed the data obtained by immunohistochemical and molecular biology techniques concerning the presence and expression of ASICs in the sensory organs, such as the olfactory rosette, lateral line, inner ear, taste buds, and in the gut and brain of zebrafish.

ASIC3 inhibition modulates inflammation-induced changes in the activity and sensitivity of Aδ and C fiber sensory neurons that innervate bone

The Acid Sensing Ion Channel 3 (ASIC3) is a non-selective cation channel that is activated by acidification, and is known to have a role in regulating inflammatory pain. It has pro-algesic roles in a range of conditions that present with bone pain, but the mechanism for this has not yet been demonstrated. We aimed to determine if ASIC3 is expressed in Aδ and/or C fiber bone afferent neurons, and to explore its role in the activation and sensitization of bone afferent neurons after acute inflammation. A combination of retrograde tracing and immunohistochemistry was used to determine expression of ASIC3 in the soma of bone afferent neurons. A novel, in vivo, electrophysiological bone-nerve preparation was used to make recordings of the activity and sensitivity of bone afferent neurons in the presence of carrageenan-induced inflammation, with and without the selective ASIC3 inhibitor APET×2. A substantial proportion of bone afferent neurons express ASIC3, including unmyelinated (neurofilament poor) and small diameter myelinated (neurofilament rich) neurons that are likely to be C and Aδ nerve fibers respectively. Electrophysiological recordings revealed that application of APET×2 to the marrow cavity inhibited carrageenan-induced spontaneous activity of C and Aδ fiber bone afferent neurons. APET×2 also inhibited carrageenan-induced sensitization of Aδ and C fiber bone afferent neurons to mechanical stimulation, but had no effect on the sensitivity of bone afferent neurons in the absence of inflammation. This evidence supports a role for ASIC3 in the pathogenesis of pain associated with inflammation of the bone.

Esophageal afferent innervation and its role in gastro-esophageal reflux disease symptoms

PURPOSE OF REVIEW

Despite the wide prevalence of gastro-esophageal reflux disease (GERD), the neurophysiological mechanisms underlying heartburn perception in the esophagus of patients with GERD remains incompletely understood. Recent studies have highlighted the potential influence sensory afferent nerves innervating the oesophageal epithelium may have on heartburn pathogenesis. The purpose of this review is to consider the current understanding of esophageal afferent neuronal innervation, including the nociceptive role of acid-sensing receptors expressed on these sensory nerves, in relation to pain perception in the esophagus of GERD patients.

RECENT FINDINGS

Central and peripheral pathways of sensitization following noxious stimulation of nociceptive receptors expressed on afferent nerves can regulate the strength of sensory nerve activation in the esophagus, which can result in the amplification or suppression of afferent signal transmission. The localization and characterization of mucosal sensory afferent nerves vary between GERD phenotypes and may explain the heterogeneity of symptom perception in patients with apparently similar levels of reflux.

SUMMARY

In this review, we discuss the relevance of afferent esophageal innervation in heartburn perception, with a particular focus on the pathways of reflux-induced activation of nociceptive nerves.

Citric Acid in Drug Formulations Causes Pain by Potentiating Acid-Sensing Ion Channel 1

Pain at the injection site is a common complaint of patients receiving therapeutic formulations containing citric acid. Despite the widely acknowledged role of acid-sensing ion channels (ASICs) in acid-related perception, the specific ASIC subtype mediating pain caused by subcutaneous acid injection and the mechanism by which citrate affects this process are less clear. Here, male mice subjected to intraplantar acid injection responded by executing a withdrawal reflex, and this response was abolished by ASIC1 but not ASIC2 knockout. Although intraplantar injection of neutral citrate solution did not produce this response, intraplantar injection of acidic citrate solution produced a withdrawal reflex greater than that produced by acidity alone. Consistent with the behavioral data, neutral citrate failed to produce an electrophysiological response in HEK293 cells, which express ASIC1, but acidic citrate produced a whole-cell inward current greater than that produced by acidity alone. Saturating the intracellular solution with citrate had no effect on the potentiating effect of extracellular citrate, suggesting that citrate acted extracellularly to potentiate ASIC1. Moreover, exposure to citrate immediately before acid stimulation failed to potentiate ASIC1 currents, which ruled out the involvement of a metabotropic receptor gated by a citrate metabolite. Finally, removal of calcium ions from the extracellular solution mimicked the potentiating effect of citrate and prevented citrate from further potentiating ASIC1. Our data demonstrate that ASIC1 is necessary for the nociceptive response caused by subcutaneous acid infusion and that neutral citrate, despite not inducing ASIC1 currents or nociceptive behavior on its own, potentiates acid nociception by removing the inhibitory effect of extracellular calcium ions on ASIC1.

SIGNIFICANCE STATEMENT Citric acid is a common ingredient used in pharmaceutical formulations. Despite the widespread clinical use of these formulations, it remains unclear how citric acid causes pain when injected into patients. We identified ASIC1 as the key receptor used to detect injection-site pain caused by acid, and we showed that neutral citrate does not stimulate ASIC1; instead, citrate substantially potentiates ASIC1 activation when injected simultaneously with acid. In addition, we demonstrated that citrate potentiates ASIC1 by removing the inhibitory action of calcium on the extracellular side of the receptor. Given that injection-site pain is the primary complaint of patients receiving citrate-containing medical products, our data provide mechanistic insight into a common medical complaint and suggest a means of avoiding injection pain.

Macrophage regulator of G-protein signaling 12 contributes to inflammatory pain hypersensitivity

Background

Pain is a predominant symptom in rheumatoid arthritis (RA) patients that results from joint inflammation and is augmented by central sensitization. Regulator of G-protein signaling 12 (RGS12) is the largest protein in the RGS protein family and plays a key role in the development of inflammation. This study investigated the regulation of RGS12 in inflammatory pain and explored the underlying mechanisms and potential RA pain targets.

Methods

Macrophage-specific RGS12-deficient (LysM-Cre⁺;RGS12^fl/fl) mice were generated by mating RGS12^fl/fl mice with LysM-Cre⁺ transgenic mice. Collagen antibody-induced arthritis (CAIA) models were induced in LysM-Cre⁺;RGS12^fl/fl mice by the administration of a cocktail of five monoclonal antibodies and LPS. Mouse nociception was examined using the von Frey and heat plate tests. Primary macrophages and RAW264.7 cells were used to analyze the regulatory function and mechanism of RGS12 in vitro. The expression and function of RGS12 and COX2 (cyclooxygenase 2) were determined by real-time PCR, ELISA, and luciferase assays.

Results

Ablation of RGS12 in macrophages decreased pain-related phenotypes, such as paw swelling, the clinical score, and the inflammatory score, in the CAIA model. LysM-Cre⁺;RGS12^fl/fl mice displayed increased resistance to thermal and mechanical stimulation from day 3 to day 9 during CAIA, indicating the inhibition of inflammatory pain. Overexpression of COX2 and PGE2 in macrophages enhanced RGS12 expression, and PGE2 regulated RGS12 expression through the G-protein-coupled receptors EP2 and EP4. Furthermore, RGS12 or the RGS12 PTB domain strengthened the transcriptional regulation of COX2 by NF-κB, whereas inhibiting NF-κB suppressed RGS12-mediated regulation of COX2 in macrophages.

Conclusions

Our results demonstrate that the deletion of RGS12 in macrophages attenuates inflammatory pain, which is likely due to impaired regulation of the COX2/PGE2 signaling pathway.

Hydrogen Sulfide Upregulates Acid-sensing Ion Channels via the MAPK-Erk1/2 Signaling Pathway

Hydrogen sulfide (H₂S) emerged recently as a new gasotransmitter and was shown to exert cellular effects by interacting with proteins, among them many ion channels. Acid-sensing ion channels (ASICs) are neuronal voltage-insensitive Na⁺ channels activated by extracellular protons. ASICs are involved in many physiological and pathological processes, such as fear conditioning, pain sensation, and seizures. We characterize here the regulation of ASICs by H₂S. In transfected mammalian cells, the H₂S donor NaHS increased the acid-induced ASIC1a peak currents in a time- and concentration-dependent manner. Similarly, NaHS potentiated also the acid-induced currents of ASIC1b, ASIC2a, and ASIC3. An upregulation induced by the H₂S donors NaHS and GYY4137 was also observed with the endogenous ASIC currents of cultured hypothalamus neurons. In parallel with the effect on function, the total and plasma membrane expression of ASIC1a was increased by GYY4137, as determined in cultured cortical neurons. H₂S also enhanced the phosphorylation of the extracellular signal-regulated kinase (pErk1/2), which belongs to the family of mitogen-activated protein kinases (MAPKs). Pharmacological blockade of the MAPK signaling pathway prevented the GYY4137-induced increase of ASIC function and expression, indicating that this pathway is required for ASIC regulation by H₂S. Our study demonstrates that H₂S regulates ASIC expression and function, and identifies the involved signaling mechanism. Since H₂S shares several roles with ASICs, as for example facilitation of learning and memory, protection during seizure activity, and modulation of nociception, it may be possible that H₂S exerts some of these effects via a regulation of ASIC function.

Upregulation of Spinal ASIC1 and NKCC1 Expression Contributes to Chronic Visceral Pain in Rats

Aims: To determine whether acid-sensing ion channel 1 (ASIC1)–sodium-potassium-chloride cotransporter 1 (NKCC1) signaling pathway participates in chronic visceral pain of adult rats with neonatal maternal deprivation (NMD).

Methods: Chronic visceral pain was detected by colorectal distension (CRD). Western blotting and Immunofluorescence were performed to detect the expression and location of ASIC1 and NKCC1. Whole-cell patch-clamp recordings were performed to record spinal synaptic transmission.

Results: The excitatory synaptic transmission was enhanced and the inhibitory synaptic transmission was weakened in the spinal dorsal horn of NMD rats. ASIC1 and NKCC1 protein expression in the spinal dorsal horn was significantly up-regulated in NMD rats. Incubation of Amiloride reduced the amplitude of mEPSCs. Incubation of Bumetanide (BMT) increased the amplitude of mIPSCs. Intrathecal injection of ASIC1 or NKCC1 inhibitors reversed the threshold of CRD in NMD rats. Also, Amiloride treatment significantly reversed the expression of NKCC1 in the spinal dorsal horn of NMD rats.

Conclusion: Our data suggest that the ASIC1-NKCC1 signaling pathway is involved in chronic visceral pain in NMD rats.

Mucosal pathogenesis in gastro-esophageal reflux disease

BACKGROUND

Despite gastro-esophageal reflux disease affecting up to 20% of Western populations, relatively little is known about the molecular mechanisms underlying its most troublesome symptom: heartburn. Recent findings have unveiled the role of components of the esophageal mucosa in the pathogenesis of GERD including sensory nociceptive nerves and inflammatory mediators. Erosive esophagitis was long believed to develop as a result of acid injury at the esophageal lumen, but novel concepts suggest the generation of reflux-induced esophageal injury as a result of cytokine-mediated inflammation. Moreover, the localization and characterization of mucosal afferent nerves vary between GERD phenotypes and could explain the heterogeneity of symptom perception between patients who experience similar levels of acid reflux.

PURPOSE

The purpose of this review is to consider the crosstalk of different factors of the esophageal mucosa in the pathogenesis of GERD, with a particular focus on mucosal innervation and molecular basis of acid-induced cytokine response. We discuss the current understanding of the mucosal response to acid injury, the nociceptive role of acid-sensitive receptors expressed in the esophageal mucosa, and the role of esophageal epithelial cells in initiating the onset of erosive esophagitis.

Chinese Medicine Huzhen Tongfeng Formula Effectively Attenuates Gouty Arthritis by Inhibiting Arachidonic Acid Metabolism and Inflammatory Mediators

The Chinese herbal medicine, Huzhen Tongfeng Formula (HZTF), derived from traditional Chinese medicine (TCM) practice, has recognized therapeutic benefits for gouty arthritis (GA). HZTF is currently in the late stage of approval process as a new anti-GA drug application. However, the underlying mechanism of HZTF as an antigout medication is unclear. In this study, we combined network pharmacology and experimental validation approaches to elucidate the mechanism of action of HZTF. First, the relative drug-disease target networks were constructed and analyzed for pathway enrichment. Potential pathways were then validated by in vitro and in vivo experiments. We found that 34 compounds from HZTF matched 181 potential drug targets. Topology analysis revealed 77 core targets of HZTF, which were highly related to gout, following screening of KEGG pathway enrichment. Further analysis demonstrated that the arachidonic acid metabolic pathway was the most relevant pathway involved in the mechanism of HZTF. Validation experiments showed that HZTF significantly inhibited the inflammatory cell infiltration into gouty joints, improved the swelling of affected joints, and increased the pain threshold. HZTF significantly reduced the transcription and production of various cytokines and inflammatory mediators in vitro. In particular, cyclooxygenase (COX)-1, COX-2, and 5-lipoxygenase were simultaneously downregulated. In conclusion, our study suggests that the antigout mechanism of HZTF is associated with the inhibition of the arachidonic acid pathway, resulting in the suppression of inflammatory cytokines and mediators. These findings extend our understanding of the pharmacological action of HZTF, rationalizing the application HZTF as an effective herbal therapy for GA.

The Role of Acid-sensing Ion Channel 3 in the Modulation of Tooth Mechanical Hyperalgesia Induced by Orthodontic Tooth Movement

This study aimed to explore the role of acid-sensing ion channel 3 (ASIC3) in the modulation of tooth mechanical hyperalgesia induced by orthodontic tooth movement. In male Sprague-Dawley rats, closed coil springs were ligated between mandibular incisors and molars to mimic orthodontic tooth movement. Bite force was assessed to evaluate tooth mechanical hyperalgesia. The alveolar bone, trigeminal ganglia, and trigeminal nucleus caudalis underwent immunohistochemical staining and immunoblotting for ASIC3. The inferior alveolar nerves were transected to explore the interaction between the periodontal sensory endings and trigeminal ganglia. The role of ASIC3 in trigeminal ganglia was further explored with lentivirus-mediated ASIC3 ribonucleic acid interference. Results showed that ASIC3 was expressed in the periodontal Ruffini endings and expression of ASIC3 protein was elevated in periodontal tissues, trigeminal ganglia, and trigeminal nucleus caudalis, following orthodontic tooth movement. ASIC3 agonists and antagonists significantly aggravated and mitigated tooth mechanical hyperalgesia, respectively. ASIC3 expression decreased after inferior alveolar nerve transection in periodontal tissues. Both in vitro and vivo, the lentivirus vector carrying ASIC3 shRNA inhibited ASIC3 expression and relieved tooth mechanical hyperalgesia. To conclude, ASIC3 is important in the modulation of tooth mechanical hyperalgesia induced by orthodontic tooth movement. Further, the role of ASIC3 in the modulation of pain in periodontal tissues is regulated by trigeminal ganglia. An adjuvant analgesic therapy targeting ASIC3 could alleviate orthodontic movement-associated mechanical hyperalgesia in rats.

In silico screening of GMQ-like compounds reveals guanabenz and sephin1 as new allosteric modulators of acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) are voltage-independent cation channels that detect decreases in extracellular pH. Dysregulation of ASICs underpins a number of pathologies. Of particular interest is ASIC3, which is recognised as a key sensor of acid-induced pain and is important in the establishment of pain arising from inflammatory conditions, such as rheumatoid arthritis. Thus, the identification of new ASIC3 modulators and the mechanistic understanding of how these compounds modulate ASIC3 could be important for the development of new strategies to counteract the detrimental effects of dysregulated ASIC3 activity in inflammation. Here, we report the identification of novel ASIC3 modulators based on the ASIC3 agonist, 2-guanidine-4-methylquinazoline (GMQ). Through a GMQ-guided in silico screening of Food and Drug administration (FDA)-approved drugs, 5 compounds were selected and tested for their modulation of rat ASIC3 (rASIC3) using whole-cell patch-clamp electrophysiology. Of the chosen drugs, guanabenz (GBZ), an α₂-adrenoceptor agonist, produced similar effects to GMQ on rASIC3, activating the channel at physiological pH (pH 7.4) and potentiating its response to mild acidic (pH 7) stimuli. Sephin1, a GBZ derivative that lacks α₂-adrenoceptor activity, has been proposed to act as a selective inhibitor of a regulatory subunit of the stress-induced protein phosphatase 1 (PPP1R15A) with promising therapeutic potential for the treatment of multiple sclerosis. However, we found that like GBZ, sephin1 activates rASIC3 at pH 7.4 and potentiates its response to acidic stimulation (pH 7), i.e. sephin1 is a novel modulator of rASIC3. Furthermore, docking experiments showed that, like GMQ, GBZ and sephin1 likely interact with the nonproton ligand sensor domain of rASIC3. Overall, these data demonstrate the utility of computational analysis for identifying novel ASIC3 modulators, which can be validated with electrophysiological analysis and may lead to the development of better compounds for targeting ASIC3 in the treatment of inflammatory conditions.

Acid-sensing ion channels in sensory signaling

Acid-sensing ion channels (ASICs) are cation-permeable channels that in the periphery are primarily expressed in sensory neurons that innervate tissues and organs. Soon after the cloning of the ASIC subunits, almost 20 yr ago, investigators began to use genetically modified mice to assess the role of these channels in physiological processes. These studies provide critical insights about the participation of ASICs in sensory processes, including mechanotransduction, chemoreception, and nociception. Here, we provide an extensive assessment of these findings and discuss the current gaps in knowledge with regard to the functions of ASICs in the peripheral nervous system.

Acid-sensing ion channels (ASICs) influence excitability of stellate neurons in the mouse cochlear nucleus

Acid-sensing ion channels (ASICs) are voltage-independent and proton-gated channels. In this study, we aimed to test the hypothesis whether ASICs might be involved in modifying the excitability of stellate cells in the cochlear nucleus (CN). We determined gene expressions of ASIC1, ASIC2 and ASIC3 in the CN of BALB/mice. ASIC currents in stellate cells were characterized by using whole-cell patch-clamp technique. In the voltage-clamp experiments, inward currents were recorded upon application of 2-[N-Morpholino ethanesulfonic acid]-normal artificial cerebrospinal fluid (MES-aCSF), whose pH 50 was 5.84. Amiloride inhibited the acid-induced currents in a dose-dependent manner. Inhibition of the ASIC currents by extracellular Ca²⁺ and Pb²⁺ (10 μM) was significant evidence for the existence of homomeric ASIC1a subunits. ASIC currents were increased by 20% upon extracellular application of Zn²⁺ (300 μM) (p < 0.05, n = 13). In current-clamp experiments, application of MES-aCSF resulted in the depolarization of stellate cells. The results show that the ASIC currents in stellate cells of the cochlear nucleus are carried largely by the ASIC1a and ASIC2a channels. ASIC channels affect the excitability of the stellate cells and therefore they appear to have a role in the processing of auditory information.

The synaptic action of Degenerin/Epithelial sodium channels

Degenerin/Epithelial Sodium Channels (DEG/ENaCs) are a large family of animal-specific non-voltage gated ion channels, with enriched expression in neuronal and epithelial tissues. While neuronal DEG/ENaCs were originally characterized as sensory receptor channels, recent studies indicate that several DEG/ENaC family members are also expressed throughout the central nervous system. Human genome-wide association studies have linked DEG/ENaC-coding genes with several neurologic and psychiatric disorders, including epilepsy and panic disorder. In addition, studies in rodent models further indicate that DEG/ENaC activity in the brain contributes to many behaviors, including those related to anxiety and long-term memory. Although the exact neurophysiological functions of DEG/ENaCs remain mostly unknown, several key studies now suggest that multiple family members might exert their neuronal function via the direct modulation of synaptic processes. Here, we review and discuss recent findings on the synaptic functions of DEG/ENaCs in both vertebrate and invertebrate species, and propose models for their possible roles in synaptic physiology.

Marine Toxins and Nociception: Potential Therapeutic Use in the Treatment of Visceral Pain Associated with Gastrointestinal Disorders

Visceral pain, of which the pathogenic basis is currently largely unknown, is a hallmark symptom of both functional disorders, such as irritable bowel syndrome, and inflammatory bowel disease. Intrinsic sensory neurons in the enteric nervous system and afferent sensory neurons of the dorsal root ganglia, connecting with the central nervous system, represent the primary neuronal pathways transducing gut visceral pain. Current pharmacological therapies have several limitations, owing to their partial efficacy and the generation of severe adverse effects. Numerous cellular targets of visceral nociception have been recognized, including, among others, channels (i.e., voltage-gated sodium channels, VGSCs, voltage-gated calcium channels, VGCCs, Transient Receptor Potential, TRP, and Acid-sensing ion channels, ASICs) and neurotransmitter pathways (i.e., GABAergic pathways), which represent attractive targets for the discovery of novel drugs. Natural biologically active compounds, such as marine toxins, able to bind with high affinity and selectivity to different visceral pain molecular mediators, may represent a useful tool (1) to improve our knowledge of the physiological and pathological relevance of each nociceptive target, and (2) to discover therapeutically valuable molecules. In this review we report the most recent literature describing the effects of marine toxin on gastrointestinal visceral pain pathways and the possible clinical implications in the treatment of chronic pain associated with gut diseases.

Roles of ASICs in Nociception and Proprioception

Acid-sensing ion channels (ASICs) are a group of proton-gated ion channels belonging to the degenerin/epithelial sodium channel (DED/ENaC) family. There are at least six ASIC subtypes - ASIC1a, ASIC1b, ASIC2a, ASIC2b, ASIC3, and ASIC4 - all expressed in somatosensory neurons. ASIC3 is the most abundant in dorsal root ganglia (DRG) and the most sensitive to extracellular acidification. ASICs were found as the major player involved in acid-induced pain in humans. Accumulating evidence has further shown ASIC3 as the molecular determinant involved in pain-associated tissue acidosis in rodent models. Besides having a role in nociception, members of the DEG/ENaC family have been demonstrated as essential mechanotransducers in the nematode Caenorhabditis elegans and fly Drosophila melanogaster. ASICs are mammalian homologues of DEG/ENaC and therefore may play a role in mechanotransduction. However, the role of ASICs in neurosensory mechanotransduction is disputed. Here we review recent studies to probe the roles of ASICs in acid nociception and neurosensory mechanotransduction. In reviewing genetic models and delicate electrophysiology approaches, we show ASIC3 as a dual-function protein for both acid-sensing and mechano-sensing in somatosensory nerves and therefore involved in regulating both nociception and proprioception.

Shear force modulates the activity of acid-sensing ion channels at low pH or in the presence of non-proton ligands

Acid-sensing ion channels (ASICs) belong to the degenerin/epithelial sodium channel protein family that form mechanosensitive ion channels. Evidence as to whether or not ASICs activity is directly modulated by mechanical force is lacking. Human ASICs (hASIC1_V3, hASIC2a and hASIC3a) were heterologously expressed as homomeric channels in Xenopus oocytes and two-electrode voltage-clamp recordings were performed. hASIC3a was expressed in HEK-293 cells and currents measured by whole-cell patch-clamp recordings. ASIC currents in response to shear force (SF) were measured at pH 7.4, acidic pH, or in the presence of non-proton ligands at pH 7.4. SF was applied via a fluid stream generated through a pressurized perfusion system. No effect was observed at pH 7.4. Increased transient currents for each homomeric channel were observed when elevated SF was applied in conjunction with acidic pH (6.0-4.0). The sustained current was not (hASIC2a) or only slightly increased (hASIC1_V3 and hASIC3a). SF-induced effects were not seen in water injected oocytes and were blocked by amiloride. Non-proton ligands activated a persistent current in hASIC1_V3 and cASIC1 (MitTx) and hASIC3a (GMQ) at pH 7.4. Here SF caused a further current increase. Results suggest that ASICs do have an intrinsic ability to respond to mechanical force, supporting their role as mechanosensors in certain local environments.

Modulation of DEG/ENaCs by Amphiphiles Suggests Sensitivity to Membrane Alterations

The bile acid-sensitive ion channel is activated by amphiphilic substances such as bile acids or artificial detergents via membrane alterations; however, the mechanism of membrane sensitivity of the bile acid-sensitive ion channel is not known. It has also not been systematically investigated whether other members of the degenerin/epithelial Na⁺ channel (DEG/ENaC) gene family are affected by amphiphilic compounds. Here, we show that DEG/ENaCs ASIC1a, ASIC3, ENaC, and the purinergic receptor P2X2 are modulated by a large number of different, structurally unrelated amphiphilic substances, namely the detergents N-lauroylsarcosine, Triton X-100, and β-octylglucoside; the fenamate flufenamic acid; the antipsychotic drug chlorpromazine; the natural phenol resveratrol; the chili pepper compound capsaicin; the loop diuretic furosemide; and the antiarrythmic agent verapamil. We determined the modification of membrane properties using large-angle x-ray diffraction experiments on model lipid bilayers, revealing that the amphiphilic compounds are positioned in a characteristic fashion either in the lipid tail group region or in the lipid head group region, demonstrating that they perturbed the membrane structure. Collectively, our results show that DEG/ENaCs and structurally related P2X receptors are modulated by diverse amphiphilic molecules. Furthermore, they suggest alterations of membrane properties by amphiphilic compounds as a mechanism contributing to modulation.

The Effect of ASIC3 Knockout on Corticostriatal Circuit and Mouse Self-grooming Behavior

Stereotypic and/or repetitive behavior is one of the major symptoms of autism spectrum disorder (ASD). Increase of self-grooming behavior is a behavioral phenotype commonly observed in the mouse models for ASD. Previously, we have shown that knockout of acid-sensing ion channel 3 (ASIC3) led to the increased self-grooming behavior in resident-intruder test. Given the facts that ASIC3 is mainly expressed in the peripheral dorsal root ganglion (DRG) and conditional knockout of ASIC3 in the proprioceptors induced proprioception deficits. We speculate a hypothesis that stereotypic phenotype related to ASD, pararalled with striatal dysfunction, might be caused by proprioception defect in the peripheral sensory neuron origin. Herein, we investigate in depth whether and how ASIC3 is involved in the regulation of self-grooming behavior. First, we observed that Asic3 null mutant mice exhibited increased self-grooming in social interaction during juvenile stage. Similarly, they displayed increased self-grooming behavior in a novel cage in the absence of cagemate. To further understand the mechanism by which ASIC3 affects grooming behavior, we analyzed neurochemical, neuropathological and electrophysiological features in the dorsal striatum of Asic3 null mutant mice. Knockout of Asic3 increased dopamine (DA) activity and phospho-ERK immunoreactivities in the dorsal striatum. Furthermore, we detected a lower paired-pulse ratio (PPR) and impaired long-term potentiation (LTP) in corticostriatal circuits in Asic3 null mutant mice as compared with wild-type (WT) littermates. Moreover, knockout of Asic3 altered the medial spiny neurons in the striatum with defects in presynaptic function and decrease of dendritic spines. Lastly, genetic ablation of Asic3 specifically in parvalbumin-positive (PV+) cells resulted in the increase of self-grooming behavior in mice. These findings suggest knockout of Asic3 in the PV+ neurons alters grooming behavior by co-opting corticostriatal circuits.

Association Between Acid-Sensing Ion Channel 3 Gene Variants and Balance Impairment in People With Mild Traumatic Brain Injury

Introduction: Dizziness and balance impairment are common symptoms of mild traumatic brain injury (mTBI). Acid-sensing ion channel 3 (ASIC3) is expressed in the vestibular and proprioceptive systems and associated with balance functions. However, whether the genetic variants of ASIC3 are associated with people who suffer dizziness and balance impairment after mTBI remained unknown. Materials and methods: A total of 200 people with mTBI and 109 non-mTBI controls were recruited. Dizziness, balance functions, and the ability to perform daily activities were assessed by Dizziness Handicap Inventory (DHI), and objective balance functions were investigated by the postural stability test. Three diseases-related genetic variants of ASIC3 were determined through polymerase chain reaction and followed by restriction fragment length polymorphism. The Student's t-test and Mann-Whitney U-test were used for normal and abnormal distributed data, respectively. The regression was applied to adjust gender and age. The normality of continuous data was evaluated by Shapiro-Wilk test. Results: In the mTBI people, the rs2288645-A allele carriers exhibited a significantly worse physical domain DHI score (A-allele carriers: 11.39 ± 8.42, non-A carriers: 8.76 ± 7.87, p = 0.03). The rs4148855-GTC deletion carriers an exhibited significantly worse overall postural stability (GTC deletion carriers: 0.53 ± 0.33, non-carriers: 0.46 ± 0.20, p = 0.03). In the controls, rs2288646-A allele carriers were significant worse in the medial-to-lateral postural stability (A-allele carriers: 0.31 ± 0.17, non-A carriers: 0.21 ± 0.10, p = 0.01). Conclusion: The present study demonstrated that ASIC3 genetic variants were associated with certain aspects of balance functions and dizziness questionnaires in people of mTBI and non-mTBI. It provides a possible evidence that ASIC3 could be a new target for the management of the balancing disorders. However, further investigations are warranted to elucidate the underlying mechanisms and clinical significance.

Identification of a unique Ca2+-binding site in rat acid-sensing ion channel 3

Acid-sensing ion channels (ASICs) evolved to sense changes in extracellular acidity with the divalent cation calcium (Ca²⁺) as an allosteric modulator and channel blocker. The channel-blocking activity is most apparent in ASIC3, as removing Ca²⁺ results in channel opening, with the site's location remaining unresolved. Here we show that a ring of rat ASIC3 (rASIC3) glutamates (Glu435), located above the channel gate, modulates proton sensitivity and contributes to the formation of the elusive Ca²⁺ block site. Mutation of this residue to glycine, the equivalent residue in chicken ASIC1, diminished the rASIC3 Ca²⁺ block effect. Atomistic molecular dynamic simulations corroborate the involvement of this acidic residue in forming a high-affinity Ca²⁺ site atop the channel pore. Furthermore, the reported observations provide clarity for past controversies regarding ASIC channel gating. Our findings enhance understanding of ASIC gating mechanisms and provide structural and energetic insights into this unique calcium-binding site.

Acid-sensing ion channels: dual function proteins for chemo-sensing and mechano-sensing

Background

Acid-sensing ion channels (ASICs) are a group of amiloride-sensitive ligand-gated ion channels belonging to the family of degenerin/epithelial sodium channels. ASICs are predominantly expressed in both the peripheral and central nervous system and have been characterized as potent proton sensors to detect extracellular acidification in the periphery and brain.

Main body

Here we review the recent studies focusing on the physiological roles of ASICs in the nervous system. As the major acid-sensing membrane proteins in the nervous system, ASICs detect tissue acidosis occurring at tissue injury, inflammation, ischemia, stroke, and tumors as well as fatiguing muscle to activate pain-sensing nerves in the periphery and transmit pain signals to the brain. Arachidonic acid and lysophosphocholine have been identified as endogenous non-proton ligands activating ASICs in a neutral pH environment. On the other hand, ASICs are found involved in the tether model mechanotransduction, in which the extracellular matrix and cytoplasmic cytoskeletons act like a gating-spring to tether the mechanically activated ion channels and thus transmit the stimulus force to the channels. Accordingly, accumulating evidence has shown ASICs play important roles in mechanotransduction of proprioceptors, mechanoreceptors and nociceptors to monitor the homoeostatic status of muscle contraction, blood volume, and blood pressure as well as pain stimuli.

Conclusion

Together, ASICs are dual-function proteins for both chemosensation and mechanosensation involved in monitoring physiological homoeostasis and pathological signals.

Molecular basis of inhibition of acid sensing ion channel 1A by diminazene

Acid-sensing ion channels (ASICs) are trimeric proton-gated cation permeable ion channels expressed primarily in neurons. Here we employed site-directed mutagenesis and electrophysiology to investigate the mechanism of inhibition of ASIC1a by diminazene. This compound inhibits mouse ASIC1a with a half-maximal inhibitory concentration (IC50) of 2.4 μM. At first, we examined whether neutralizing mutations of Glu79 and Glu416 alter diminazene block. These residues form a hexagonal array in the lower palm domain that was previously shown to contribute to pore opening in response to extracellular acidification. Significantly, single Gln substitutions at positions 79 and 416 in ASIC1a reduced diminazene apparent affinity by 6-7 fold. This result suggests that diminazene inhibits ASIC1a in part by limiting conformational rearrangement in the lower palm domain. Because diminazene is charged at physiological pHs, we assessed whether it inhibits ASIC1a by blocking the ion channel pore. Consistent with the notion that diminazene binds to a site within the membrane electric field, diminazene block showed a strong dependence with the membrane potential. Moreover, a Gly to Ala mutation at position 438, in the ion conduction pathway of ASIC1a, increased diminazene IC50 by one order of magnitude and eliminated the voltage dependence of block. Taken together, our results indicate that the inhibition of ASIC1a by diminazene involves both allosteric modulation and blocking of ion flow through the conduction pathway. Our findings provide a foundation for the development of more selective and potent ASIC pore blockers.

Subtype-selective inhibition of acid-sensing ion channel 3 by a natural flavonoid

AIMS

Acid-sensing ion channels (ASICs) are extracellular proton-gated cation channels that have been implicated in multiple physiological and pathological processes, and peripheral ASIC3 prominently participate into the pathogenesis of chronic pain, itch, and neuroinflammation, which necessitates the need for discovery and development of novel modulators in a subtype-specific manner.

METHODS

Whole-cell patch clamp recordings and behavioral assays were used to examine the effect of several natural compounds on the ASIC-mediated currents and acid-induced nocifensive behavior, respectively.

RESULTS

We identified a natural flavonoid compound, (-)-epigallocatechin gallate (EGCG, compound 11), that acts as a potent inhibitor for the ASIC3 channel in an isoform-specific way. The compound 11 inhibited ASIC3 currents with an apparent half maximal inhibitory concentration of 13.2 μmol/L when measured at pH 5.0. However, at the concentration up to 100 μmol/L, the compound 11 had no significant impacts on the homomeric ASIC1a, 1b, and 2a channels. In contrast to most of the known ASIC inhibitors that usually bear either basic or carboxylic groups, the compound 11 belongs to the polyphenolic family. In compound 11, both the chirality and the 3-hydroxyl group of its pyrogallol part, in addition to the integrity of the gallate part, are crucial for the inhibitory efficacy. Finally, EGCG was found significantly to decrease the acid-induced nocifensive behavior in mice.

CONCLUSION

Taken together, these results thus defined a novel backbone structure for small molecule drug design targeting ASIC3 channels to treat pain-related diseases.

Acidosis counteracts itch tachyphylaxis to consecutive pruritogen exposure dependent on acid-sensing ion channel 3

Tachyphylaxis of itch refers to a markedly reduced scratching response to consecutive exposures of a pruritogen, a process thought to protect against tissue damage by incessant scratching and to become disrupted in chronic itch. Here, we report that a strong stimulation of the Mas-related G-protein-coupled receptor C11 by its agonist, Ser-Leu-Ile-Gly-Arg-Leu-NH₂ (SL-NH₂) or bovine adrenal medulla 8-22 peptide, via subcutaneous injection in mice induces tachyphylaxis to the subsequent application of SL-NH₂ to the same site. Notably, co-application of acid and SL-NH₂ following the initial injection of the pruritogen alone counteracted itch tachyphylaxis by augmenting the scratching behaviors in wild-type but not in acid-sensing ion channel 3-null, animals. Using an activity-dependent silencing strategy, we identified that acid-sensing ion channel 3-mediated itch enhancement mainly occurred via the Mas-related G-protein-coupled receptor C11-responsive sensory neurons. Together, our results indicate that acid-sensing ion channel 3, activated by concomitant acid and certain pruritogens, constitute a novel signaling pathway that counteracts itch tachyphylaxis to successive pruritogenic stimulation, which likely contributes to chronic itch associated with tissue acidosis.