Pharmacology of acid-sensing ion channels – Physiological and therapeutical perspectives

Identification and analgesic activity study of analgesic protein Ⅶ-2 from Naja naja atra venom

Background

Acid-sensing ion channel 1a (ASIC1a) plays a critical role in physiological and pathological processes. To further elucidate the biological functions of ASICs and their relationships with disease occurrence and development, it is advantageous to investigate and develop additional regulatory factors for ASICs.

Methods

In this study, cation exchange chromatography was used to separate seven chromatographic components from Naja naja atra venom. Capillary electrophoresis was employed to detect that Ⅶ peak component containing a main protein Ⅶ-2, which could bind to ASIC1a. The analgesic effects of Ⅶ-2 protein were determined using hot plate methods, and ASIC1a expression in spinal cord tissue from rats with inflammatory pain was detected using western blot.

Results

The purified Ⅶ-2 protein named Naja naja atra venom-Ⅶ-2 (NNAV-Ⅶ-2) was obtained by Sephadex G-50 gel filtration, which exhibited a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with a molecular weight of 6.7 kD. Remarkably, the NNAV-Ⅶ-2 protein demonstrated a significant analgesic effect and downregulated ASIC1a expression in the spinal cord tissue of rats with inflammatory pain.

Conclusions

The analgesic mechanism of the NNAV-Ⅶ-2 protein may be associated with its binding to ASIC1a, consequently downregulating ASIC1a expression in neural tissues.

Withanicandrin Isolated from Datura Ferox Promotes Antinociception by Modulating the Asics and TRPS Channels and Anti-Inflammation in Adult Zebrafish

This is the first study to analyze the anti-inflammatory and antinociceptive effect of withanicandrin, isolated from Datura Ferox leaves, and the possible mechanism of action involved in adult zebrafish (ZFa). To this end, the animals were treated intraperitoneally (i. p.) with withanicandrin (4; 20 and 40 mg/kg; 20 μL) and subjected to locomotor activity and acute toxicity. Nociception tests were also carried out with chemical agents, in addition to tests to evaluate inflammatory processes induced by κ-Carrageenan 1.5 % and a Molecular Docking study. As a result, withanicandrin reduced nociceptive behavior by capsaicin at a dose of 40 mg/kg and by acid saline at doses of 4 and 40 mg/kg, through neuromodulation of TRPV1 channels and ASICs, identified through blocking the antinociceptive effect of withanicandrin by the antagonists capsazepine and naloxone. Furthermore, withanicandrin caused an anti-inflammatory effect through the reduction of abdominal edema, absence of leukocyte infiltrate in the liver tissue and reduction of ROS in thel liver tissue and presented better affinity energy compared to control morphine (TRPV1) and ibuprofen (COX-1 and COX-2).

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.

Lycocasine A, a Lycopodium Alkaloid from Lycopodiastrum casuarinoides and Its Acid-Sensing Ion Channel 1a Inhibitory Activity

A novel Lycopodium alkaloid, lycocasine A (1), and seven known Lycopodium alkaloids (2-8), were isolated from Lycopodiastrum casuarinoides. Their structures were determined through NMR, HRESIMS, and X-ray diffraction analysis. Compound 1 features an unprecedented 5/6/6 tricyclic skeleton, highlighted by a 5-aza-tricyclic[6,3,1,02,6]dodecane motif. In bioactivity assays, compound 1 demonstrated weak inhibitory activity against acid-sensing ion channel 1a.

4-(Azolyl)-Benzamidines as a Novel Chemotype for ASIC1a Inhibitors

Acid-sensing ion channels (ASICs) play a key role in the perception and response to extracellular acidification changes. These proton-gated cation channels are critical for neuronal functions, like learning and memory, fear, mechanosensation and internal adjustments like synaptic plasticity. Moreover, they play a key role in neuronal degeneration, ischemic neuronal injury, seizure termination, pain-sensing, etc. Functional ASICs are homo or heterotrimers formed with (ASIC1-ASIC3) homologous subunits. ASIC1a, a major ASIC isoform in the central nervous system (CNS), possesses an acidic pocket in the extracellular region, which is a key regulator of channel gating. Growing data suggest that ASIC1a channels are a potential therapeutic target for treating a variety of neurological disorders, including stroke, epilepsy and pain. Many studies were aimed at identifying allosteric modulators of ASIC channels. However, the regulation of ASICs remains poorly understood. Using all available crystal structures, which correspond to different functional states of ASIC1, and a molecular dynamics simulation (MD) protocol, we analyzed the process of channel inactivation. Then we applied a molecular docking procedure to predict the protein conformation suitable for the amiloride binding. To confirm the effect of its sole active blocker against the ASIC1 state transition route we studied the complex with another MD simulation run. Further experiments evaluated various compounds in the Enamine library that emerge with a detectable ASIC inhibitory activity. We performed a detailed analysis of the structural basis of ASIC1a inhibition by amiloride, using a combination of in silico approaches to visualize its interaction with the ion pore in the open state. An artificial activation (otherwise, expansion of the central pore) causes a complex modification of the channel structure, namely its transmembrane domain. The output protein conformations were used as a set of docking models, suitable for a high-throughput virtual screening of the Enamine chemical library. The outcome of the virtual screening was confirmed by electrophysiological assays with the best results shown for three hit compounds.

Opioid Analgesic as a Positive Allosteric Modulator of Acid-Sensing Ion Channels

Tafalgin (Taf) is a tetrapeptide opioid used in clinical practice in Russia as an analgesic drug for subcutaneous administration as a solution (4 mg/mL; concentration of 9 mM). We found that the acid-sensing ion channels (ASICs) are another molecular target for this molecule. ASICs are proton-gated sodium channels that mediate nociception in the peripheral nervous system and contribute to fear and learning in the central nervous system. Using electrophysiological methods, we demonstrated that Taf could increase the integral current through heterologically expressed ASIC with half-maximal effective concentration values of 0.09 mM and 0.3 mM for rat and human ASIC3, respectively, and 1 mM for ASIC1a. The molecular mechanism of Taf action was shown to be binding to the channel in the resting state and slowing down the rate of desensitization. Taf did not compete for binding sites with both protons and ASIC3 antagonists, such as APETx2 and amiloride (Ami). Moreover, Taf and Ami together caused an unusual synergistic effect, which was manifested itself as the development of a pronounced second desensitizing component. Thus, the ability of Taf to act as a positive allosteric modulator of these channels could potentially cause promiscuous effects in clinical practice. This fact must be considered in patients' treatment.

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.

Probing the Proton-Gated ASIC Channels Using Tetraalkylammonium Ions

The action of tetraalkylammonium ions, from tetrametylammonium (TMA) to tetrapentylammonium (TPtA), on the recombinant and native acid-sensing ion channels (ASICs) was studied using the patch-clamp approach. The responses of ASIC1a, ASIC2a, and native heteromeric ASICs were inhibited by TPtA. The peak currents through ASIC3 were unaffected, whereas the steady-state currents were significantly potentiated. This effect was characterized by an EC50 value of 1.22 ± 0.12 mM and a maximal effect of 3.2 ± 0.5. The effects of TPtA were voltage-independent but significantly decreased under conditions of strong acidification, which caused saturation of ASIC responses. Molecular modeling predicted TPtA binding in the acidic pocket of closed ASICs. Bound TPtA can prevent acidic pocket collapse through a process involving ASIC activation and desensitization. Tetraethylammonium (TEA) inhibited ASIC1a and native ASICs. The effect was independent of the activating pH but decreased with depolarization, suggesting a pore-blocking mechanism.

Enhanced harm detection following maternal separation: Transgenerational transmission and reversibility by inhaled amiloride

Early-life adversities are associated with altered defensive responses. Here, we demonstrate that the repeated cross-fostering (RCF) paradigm of early maternal separation is associated with enhancements of distinct homeostatic reactions: hyperventilation in response to hypercapnia and nociceptive sensitivity, among the first generation of RCF-exposed animals, as well as among two successive generations of their normally reared offspring, through matrilineal transmission. Parallel enhancements of acid-sensing ion channel 1 (ASIC1), ASIC2, and ASIC3 messenger RNA transcripts were detected transgenerationally in central neurons, in the medulla oblongata, and in periaqueductal gray matter of RCF-lineage animals. A single, nebulized dose of the ASIC-antagonist amiloride renormalized respiratory and nociceptive responsiveness across the entire RCF lineage. These findings reveal how, following an early-life adversity, a biological memory reducible to a molecular sensor unfolds, shaping adaptation mechanisms over three generations. Our findings are entwined with multiple correlates of human anxiety and pain conditions and suggest nebulized amiloride as a therapeutic avenue.

Triggering of Major Brain Disorders by Protons and ATP: The Role of ASICs and P2X Receptors

Adenosine triphosphate (ATP) is well-known as a universal source of energy in living cells. Less known is that this molecule has a variety of important signaling functions: it activates a variety of specific metabotropic (P2Y) and ionotropic (P2X) receptors in neuronal and non-neuronal cell membranes. So, a wide variety of signaling functions well fits the ubiquitous presence of ATP in the tissues. Even more ubiquitous are protons. Apart from the unspecific interaction of protons with any protein, many physiological processes are affected by protons acting on specific ionotropic receptors-acid-sensing ion channels (ASICs). Both protons (acidification) and ATP are locally elevated in various pathological states. Using these fundamentally important molecules as agonists, ASICs and P2X receptors signal a variety of major brain pathologies. Here we briefly outline the physiological roles of ASICs and P2X receptors, focusing on the brain pathologies involving these receptors.

Dual contribution of ASIC1a channels in the spinal processing of pain information by deep projection neurons revealed by computational modeling

Dorsal horn of the spinal cord is an important crossroad of pain neuraxis, especially for the neuronal plasticity mechanisms that can lead to chronic pain states. Windup is a well-known spinal pain facilitation process initially described several decades ago, but its exact mechanism is still not fully understood. Here, we combine both ex vivo and in vivo electrophysiological recordings of rat spinal neurons with computational modeling to demonstrate a role for ASIC1a-containing channels in the windup process. Spinal application of the ASIC1a inhibitory venom peptides mambalgin-1 and psalmotoxin-1 (PcTx1) significantly reduces the ability of deep wide dynamic range (WDR) neurons to develop windup in vivo. All deep WDR-like neurons recorded from spinal slices exhibit an ASIC current with biophysical and pharmacological characteristics consistent with functional expression of ASIC1a homomeric channels. A computational model of WDR neuron supplemented with different ASIC1a channel parameters accurately reproduces the experimental data, further supporting a positive contribution of these channels to windup. It also predicts a calcium-dependent windup decrease for elevated ASIC conductances, a phenomenon that was experimentally validated using the Texas coral snake ASIC-activating toxin (MitTx) and calcium-activated potassium channel inhibitory peptides (apamin and iberiotoxin). This study supports a dual contribution to windup of calcium permeable ASIC1a channels in deep laminae projecting neurons, promoting it upon moderate channel activity, but ultimately leading to calcium-dependent windup inhibition associated to potassium channels when activity increases.

Modulation of Slow Desensitization (Tachyphylaxis) of Acid-Sensing Ion Channel (ASIC)1a

Among the proton-activated channels of the ASIC family, ASIC1a exhibits a specific tachyphylaxis phenomenon in the form of a progressive decrease in the response amplitude during a series of activations. This process is well known, but its mechanism is poorly understood. Here, we demonstrated a partial reversibility of this effect using long-term whole-cell recording of CHO cells transfected with rASIC1a cDNA. Thus, tachyphylaxis represents a slow desensitization of ASIC1a. Prolonged acidifications provided the same recovery from slow desensitization as short acidifications of the same frequency. Slow desensitization and steady-state desensitization are independent processes although the latter attenuates the development of the former. We found that drugs which facilitate ASIC1a activation (e.g., amitriptyline) cause an enhancement of slow desensitization, while inhibition of ASIC1a by 9-aminoacridine attenuates this process. Overall, for a broad variety of exposures, including increased calcium concentration, different pH conditions, and modulating drugs, we found a correlation between their effects on ASIC1a response amplitude and the development of slow desensitization. Thus, our results demonstrate that slow desensitization occurs only when ASIC1a is in the open state.

Analysis of residue-residue interactions in the structures of ASIC1a suggests possible gating mechanisms

The gating mechanism of acid-sensitive ion channels (ASICs) remains unclear, despite the availability of atomic-scale structures in various functional states. The collapse of the acidic pocket and structural changes in the low-palm region are assumed to trigger activation. For the acidic pocket, protonation of some residues can minimize repulsion in the collapsed conformation. The relationship between low-palm rearrangements and gating is unknown. In this work, we performed a Monte Carlo energy optimization of known ASIC1a structures and determined the residue-residue interactions in different functional states. For rearrangements in the acidic pocket, our results are consistent with previously proposed mechanisms, although significant complexity was revealed for the residue-residue interactions. The data support the proposal of a gating mechanism in the low-palm region, in which residues E80 and E417 share a proton to activate the channel.

Aquaporins and Ion Channels as Dual Targets in the Design of Novel Glioblastoma Therapeutics to Limit Invasiveness

Current therapies for Glioblastoma multiforme (GBM) focus on eradicating primary tumors using radiotherapy, chemotherapy and surgical resection, but have limited success in controlling the invasive spread of glioma cells into a healthy brain, the major factor driving short survival times for patients post-diagnosis. Transcriptomic analyses of GBM biopsies reveal clusters of membrane signaling proteins that in combination serve as robust prognostic indicators, including aquaporins and ion channels, which are upregulated in GBM and implicated in enhanced glioblastoma motility. Accumulating evidence supports our proposal that the concurrent pharmacological targeting of selected subclasses of aquaporins and ion channels could impede glioblastoma invasiveness by impairing key cellular motility pathways. Optimal sets of channels to be selected as targets for combined therapies could be tailored to the GBM cancer subtype, taking advantage of differences in patterns of expression between channels that are characteristic of GBM subtypes, as well as distinguishing them from non-cancerous brain cells such as neurons and glia. Focusing agents on a unique channel fingerprint in GBM would further allow combined agents to be administered at near threshold doses, potentially reducing off-target toxicity. Adjunct therapies which confine GBM tumors to their primary sites during clinical treatments would offer profound advantages for treatment efficacy.

Derivatives of 2-aminobenzimidazole potentiate ASIC open state with slow kinetics of activation and desensitization

The pharmacology of acid-sensitive ion channels (ASICs) is diverse, but potent and selective modulators, for instance for ASIC2a, are still lacking. In the present work we studied the effect of five 2-aminobenzimidazole derivatives on native ASICs in rat brain neurons and recombinant receptors expressed in CHO cells using the whole-cell patch clamp method. 2-aminobenzimidazole selectively potentiated ASIC3. Compound Ru-1355 strongly enhanced responses of ASIC2a and caused moderate potentiation of native ASICs and heteromeric ASIC1a/ASIC2a. The most active compound, Ru-1199, caused the strongest potentiation of ASIC2a, but also potentiated native ASICs, ASIC1a and ASIC3. The potentiating effects depended on the pH and was most pronounced with intermediate acidifications. In the presence of high concentrations of Ru-1355 and Ru-1199, the ASIC2a responses were biphasic, the initial transient currents were followed by slow component. These slow additional currents were weakly sensitive to the acid-sensitive ion channels pore blocker diminazene. We also found that sustained currents mediated by ASIC2a and ASIC3 are less sensitive to diminazene than the peak currents. Different sensitivities of peak and sustained components to the pore-blocking drug suggest that they are mediated by different open states. We propose that the main mechanism of action of 2-aminobenzimidazole derivatives is potentiation of the open state with slow kinetics of activation and desensitization.

Global Epidemiology, Clinical Features, Diagnosis and Current Therapeutic Novelties in Migraine Therapy and their Prevention: A Narrative Review

INTRODUCTION

The current article reviews the latest information on epidemiology, clinical features, diagnosis, recent advancements in clinical management, current therapeutic novelties, and the prevention of migraines. In a narrative review, all studies as per developed MeSH terms published until February 2023, excluding those irrelevant, were identified through a PubMed literature search.

METHODS

Overall, migraine affects more than a billion people annually and is one of the most common neurological illnesses. A wide range of comorbidities is associated with migraines, including stress and sleep disturbances. To lower the worldwide burden of migraine, comprehensive efforts are required to develop and enhance migraine treatment, which is supported by informed healthcare policy. Numerous migraine therapies have been successful, but not all patients benefit from them.

RESULTS

CGRP pathway-targeted therapy demonstrates the importance of translating mechanistic understanding into effective treatment. In this review, we discuss clinical features, diagnosis, and recently approved drugs, as well as a number of potential therapeutic targets, including pituitary adenylate cyclase-activating polypeptide (PACAP), adenosine, opioid receptors, potassium channels, transient receptor potential ion channels (TRP), and acid-sensing ion channels (ASIC).

CONCLUSION

In addition to providing more treatment options for improved clinical care, a better understanding of these mechanisms facilitates the discovery of novel therapeutic targets.

Advances in Antibody-Based Therapeutics for Cerebral Ischemia

Cerebral ischemia is an acute disorder characterized by an abrupt reduction in blood flow that results in immediate deprivation of both glucose and oxygen. The main types of cerebral ischemia are ischemic and hemorrhagic stroke. When a stroke occurs, several signaling pathways are activated, comprising necrosis, apoptosis, and autophagy as well as glial activation and white matter injury, which leads to neuronal cell death. Current treatments for strokes include challenging mechanical thrombectomy or tissue plasminogen activator, which increase the danger of cerebral bleeding, brain edema, and cerebral damage, limiting their usage in clinical settings. Monoclonal antibody therapy has proven to be effective and safe in the treatment of a variety of neurological disorders. In contrast, the evidence for stroke therapy is minimal. Recently, Clone MTS510 antibody targeting toll-like receptor-4 (TLR4) protein, ASC06-IgG1 antibody targeting acid sensing ion channel-1a (ASIC1a) protein, Anti-GluN1 antibodies targeting N-methyl-D-aspartate (NMDA) receptor associated calcium influx, GSK249320 antibody targeting myelin-associated glycoprotein (MAG), anti-High Mobility Group Box-1 antibody targeting high mobility group box-1 (HMGB1) are currently under clinical trials for cerebral ischemia treatment. In this article, we review the current antibody-based pharmaceuticals for neurological diseases, the use of antibody drugs in stroke, strategies to improve the efficacy of antibody therapeutics in cerebral ischemia, and the recent advancement of antibody drugs in clinical practice. Overall, we highlight the need of enhancing blood-brain barrier (BBB) penetration for the improvement of antibody-based therapeutics in the brain, which could greatly enhance the antibody medications for cerebral ischemia in clinical practice.

Insights into the Cellular Interactions and Molecular Mechanisms of Ketogenic Diet for Comprehensive Management of Epilepsy

A high-fat diet with appropriate protein and low carbohydrate content, widely known as the ketogenic diet (KD), is considered as an effective non-pharmacotherapeutic treatment option for certain types of epilepsies. Several preclinical and clinical studies have been carried out to elucidate its mechanism of antiepileptic action. Ketone bodies produced after KD's breakdown interact with cellular excito-inhibitory processes and inhibit abnormal neuronal firing. The generated ketone bodies decrease glutamate release by inhibiting the vesicular glutamate transporter 1 and alter the transmembrane potential by hyperpolarization. Apart from their effect on the well-known pathogenic mechanisms of epilepsy, some recent studies have shown the interaction of KD metabolites with novel neuronal targets, particularly adenosine receptors, adenosine triphosphate-sensitive potassium channel, mammalian target of rapamycin, histone deacetylase, hydroxycarboxylic acid receptors, and the NLR family pyrin domain containing 3 inflammasomes to suppress seizures. The role of KD in augmenting gut microbiota as a potential mechanism for epileptic seizure suppression has been established. Furthermore, some recent findings also support the beneficial effect of KD against epilepsy- associated comorbidities. Despite several advantages of the KD in epilepsy management, its use is also associated with a wide range of side effects. Hypoglycemia, excessive ketosis, acidosis, renal stones, cardiomyopathies, and other metabolic disturbances are the primary adverse effects observed with the use of KD. However, in some recent studies, modified KD has been tested with lesser side effects and better tolerability. The present review discusses the molecular mechanism of KD and its role in managing epilepsy and its associated comorbidities.

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.

Acetazolamide: Old drug, new evidence?

Acetazolamide is an old drug used as an antiepileptic agent, amongst other indications. The drug is seldom used, primarily due to perceived poor efficacy and adverse events. Acetazolamide acts as a noncompetitive inhibitor of carbonic anhydrase, of which there are several subtypes in humans. Acetazolamide causes an acidification of the intracellular and extracellular environments activating acid‐sensing ion channels, and these may account for the anti‐seizure effects of acetazolamide. Other potential mechanisms are modulation of neuroinflammation and attenuation of high‐frequency oscillations. The overall effect increases the seizure threshold in critical structures such as the hippocampus. The evidence for its clinical efficacy was from 12 observational studies of 941 patients. The 50% responder rate was 49%, 20% of patients were rendered seizure‐free, and 30% were noted to have had at least one adverse event. We conclude that the evidence from several observational studies may overestimate efficacy because they lack a comparator; hence, this drug would need further randomized placebo‐controlled trials to assess effectiveness and harm.

Pharmacological Validation of ASIC1a as a Druggable Target for Neuroprotection in Cerebral Ischemia Using an Intravenously Available Small Molecule Inhibitor

Acidosis is a hallmark of ischemic stroke and a promising neuroprotective target for preventing neuronal injury. Previously, genetic manipulations showed that blockade of acid-sensing ion channel 1a (ASIC1a)-mediated acidotoxicity could dramatically alleviate the volume of brain infarct and restore neurological function after cerebral ischemia. However, few pharmacological candidates have been identified to exhibit efficacy on ischemic stroke through inhibition of ASIC1a. In this work, we examined the ability of a toxin-inspired compound 5b (C5b), previously found to effectively inhibit ASIC1a in vitro, to exert protective effects in animal models of ischemic stroke in vivo. We found that C5b exerts significant neuroprotective effects not only in acid-induced neuronal death in vitro but also ischemic brain injury in vivo, suggesting that ASIC1a is a druggable target for therapeutic development. More importantly, C5b is able to cross the blood brain barrier and significantly reduce brain infarct volume when administered intravenously in the ischemic animal model, highlighting its systemic availability for therapies against neurodegeneration due to acidotoxicity. Together, our data demonstrate that C5b is a promising lead compound for neuroprotection through inhibiting ASIC1a, which warrants further translational studies.

Acid-Sensing Ion Channels in Glial Cells

Acid-sensing ion channels (ASICs) are proton-gated cation channels and key mediators of responses to neuronal injury. ASICs exhibit unique patterns of distribution in the brain, with high expression in neurons and low expression in glial cells. While there has been a lot of focus on ASIC in neurons, less is known about the roles of ASICs in glial cells. ASIC1a is expressed in astrocytes and might contribute to synaptic transmission and long-term potentiation. In oligodendrocytes, constitutive activation of ASIC1a participates in demyelinating diseases. ASIC1a, ASIC2a, and ASIC3, found in microglial cells, could mediate the inflammatory response. Under pathological conditions, ASIC dysregulation in glial cells can contribute to disease states. For example, activation of astrocytic ASIC1a may worsen neurodegeneration and glioma staging, activation of microglial ASIC1a and ASIC2a may perpetuate ischemia and inflammation, while oligodendrocytic ASIC1a might be involved in multiple sclerosis. This review concentrates on the unique ASIC components in each of the glial cells and integrates these glial-specific ASICs with their physiological and pathological conditions. Such knowledge provides promising evidence for targeting of ASICs in individual glial cells as a therapeutic strategy for a diverse range of conditions.

Acid-Sensing Ion Channel 2: Function and Modulation

Acid-sensing ion channels (ASICs) have an important influence on human physiology and pathology. They are members of the degenerin/epithelial sodium channel family. Four genes encode at least six subunits, which combine to form a variety of homotrimers and heterotrimers. Of these, ASIC1a homotrimers and ASIC1a/2 heterotrimers are most widely expressed in the central nervous system (CNS). Investigations into the function of ASIC1a in the CNS have revealed a wealth of information, culminating in multiple contemporary reviews. The lesser-studied ASIC2 subunits are in need of examination. This review will focus on ASIC2 in health and disease, with discussions of its role in modulating ASIC function, synaptic targeting, cardiovascular responses, and pharmacology, while exploring evidence of its influence in pathologies such as ischemic brain injury, multiple sclerosis, epilepsy, migraines, drug addiction, etc. This information substantiates the ASIC2 protein as a potential therapeutic target for various neurological, psychological, and cerebrovascular diseases.

Development of ASIC1a ligand-gated ion channel drug screening assays across multiple automated patch clamp platforms

Human acid-sensing ion channels (ASIC) are ligand-gated ionotropic receptors expressed widely in peripheral tissues as well as sensory and central neurons and implicated in detection of inflammation, tissue injury, and hypoxia-induced acidosis. This makes ASIC channels promising targets for drug discovery in oncology, pain and ischemia, and several modulators have progressed into clinical trials. We describe the use of hASIC1a as a case study for the development and validation of low, medium and high throughput automated patch clamp (APC) assays suitable for the screening and mechanistic profiling of new ligands for this important class of ligand-gated ion channel. Initial efforts to expand on previous manual patch work describing an endogenous hASIC1a response in HEK cells were thwarted by low current expression and unusual pharmacology, so subsequent work utilized stable hASIC1a CHO cell lines. Ligand-gated application protocols and screening assays on the Patchliner, QPatch 48, and SyncroPatch 384 were optimized and validated based on pH activation and nM-μM potency of reference antagonists (e.g., Amiloride, Benzamil, Memantine, Mambalgin-3, A-317567, PcTx1). By optimizing single and stacked pipette tip applications available on each APC platform, stable pH-evoked currents during multiple ligand applications enabled cumulative EC₅₀ and IC₅₀ determinations with minimized receptor desensitization. Finally, we successfully demonstrated for the first time on an APC platform the ability to use current clamp to implement the historical technique of input resistance tracking to measure ligand-gated changes in membrane conductance on the Patchliner platform.

Pain-related toxins in scorpion and spider venoms: a face to face with ion channels

Pain is a common symptom induced during envenomation by spiders and scorpions. Toxins isolated from their venom have become essential tools for studying the functioning and physiopathological role of ion channels, as they modulate their activity. In particular, toxins that induce pain relief effects can serve as a molecular basis for the development of future analgesics in humans. This review provides a summary of the different scorpion and spider toxins that directly interact with pain-related ion channels, with inhibitory or stimulatory effects. Some of these toxins were shown to affect pain modalities in different animal models providing information on the role played by these channels in the pain process. The close interaction of certain gating-modifier toxins with membrane phospholipids close to ion channels is examined along with molecular approaches to improve selectivity, affinity or bioavailability in vivo for therapeutic purposes.

Changes in H+, K+, and Ca2+ Concentrations, as Observed in Seizures, Induce Action Potential Signaling in Cortical Neurons by a Mechanism That Depends Partially on Acid-Sensing Ion Channels

Acid-sensing ion channels (ASICs) are activated by extracellular acidification. Because ASIC currents are transient, these channels appear to be ideal sensors for detecting the onset of rapid pH changes. ASICs are involved in neuronal death after ischemic stroke, and in the sensation of inflammatory pain. Ischemia and inflammation are associated with a slowly developing, long-lasting acidification. Recent studies indicate however that ASICs are unable to induce an electrical signaling activity under standard experimental conditions if pH changes are slow. In situations associated with slow and sustained pH drops such as high neuronal signaling activity and ischemia, the extracellular K⁺ concentration increases, and the Ca²⁺ concentration decreases. We hypothesized that the concomitant changes in H⁺, K⁺, and Ca²⁺ concentrations may allow a long-lasting ASIC-dependent induction of action potential (AP) signaling. We show that for acidification from pH7.4 to pH7.0 or 6.8 on cultured cortical neurons, the number of action potentials and the firing time increased strongly if the acidification was accompanied by a change to higher K⁺ and lower Ca²⁺ concentrations. Under these conditions, APs were also induced in neurons from ASIC1a^-/- mice, in which a pH of ≤ 5.0 would be required to activate ASICs, indicating that ASIC activation was not required for the AP induction. Comparison between neurons of different ASIC genotypes indicated that the ASICs modulate the AP induction under such changed ionic conditions. Voltage-clamp measurements of the Na⁺ and K⁺ currents in cultured cortical neurons showed that the lowering of the pH inhibited Na⁺ and K⁺ currents. In contrast, the lowering of the Ca²⁺ together with the increase in the K⁺ concentration led to a hyperpolarizing shift of the activation voltage dependence of voltage-gated Na⁺ channels. We conclude that the ionic changes observed during high neuronal activity mediate a sustained AP induction caused by the potentiation of Na⁺ currents, a membrane depolarization due to the changed K⁺ reversal potential, the activation of ASICs, and possibly effects on other ion channels. Our study describes therefore conditions under which slow pH changes induce neuronal signaling by a mechanism involving ASICs.

Paradoxical Potentiation of Acid-Sensing Ion Channel 3 (ASIC3) by Amiloride via Multiple Mechanisms and Sites Within the Channel

Acid-Sensing Ion Channels (ASICs) are proton-gated sodium-selective cation channels that have emerged as metabolic and pain sensors in peripheral sensory neurons and contribute to neurotransmission in the CNS. These channels and their related degenerin/epithelial sodium channel (DEG/ENaC) family are often characterized by their sensitivity to amiloride inhibition. However, amiloride can also cause paradoxical potentiation of ASIC currents under certain conditions. Here we characterized and investigated the determinants of paradoxical potentiation by amiloride on ASIC3 channels. While inhibiting currents induced by acidic pH, amiloride potentiated sustained currents at neutral pH activation. These effects were accompanied by alterations in gating properties including (1) an alkaline shift of pH-dependent activation, (2) inhibition of pH-dependent steady-state desensitization (SSD), (3) prolongation of desensitization kinetics, and (4) speeding of recovery from desensitization. Interestingly, extracellular Ca²⁺ was required for paradoxical potentiation and it diminishes the amiloride-induced inhibition of SSD. Site-directed mutagenesis within the extracellular non-proton ligand-sensing domain (E79A, E423A) demonstrated that these residues were critical in mediating the amiloride-induced inhibition of SSD. However, disruption of the purported amiloride binding site (G445C) within the channel pore blunted both the inhibition and potentiation of amiloride. Together, our results suggest that the myriad of modulatory and blocking effects of amiloride are the result of a complex competitive interaction between amiloride, Ca²⁺, and protons at probably more than one site in the channel.

Acid-sensing ion channels as potential therapeutic targets

Tissue acidification is associated with a variety of disease states, and acid-sensing ion channels (ASICs) that can sense changes in pH have gained traction as possible pharmaceutical targets. An array of modulators, ranging from small molecules to large biopharmaceuticals, are known to inhibit ASICs. Here, we summarize recent insights from animal studies to assess the therapeutic potential of ASICs in disorders such as ischemic stroke, various pain-related processes, anxiety, and cardiac pathologies. We also review the factors that present a challenge in the pharmacological targeting of ASICs, and which need to be taken into careful consideration when developing potent and selective modulators in the future.

Investigation of Hemodynamic Receptors of the Internal Carotid Artery Segments

Objectives

Internal carotid artery (ICA), the main artery of the brain, passes through the cavernous sinus (CS) which forms one of these venous pools. During this transition, while there is arterial blood in the lumen of ICA, its outer surface is in contact with venous blood from the brain. Herein, we aimed to detect the receptor differences of ICA in this highly specialized anatomical region of the skull base.

Methods

We performed the study on 10 human cadavers and searched CGRPR, TRP12, ASIC3 and ACTHR receptors via immunostaining using laser scanning confocal microscopy.

Results

We determined TRP12 receptor positive in the tunica media and tunica adventitia layers of the cavernous segment of ICA. We did not detect similar positivity in the cervical part of the ICA. In the receptor scan we made in terms of CGRPR, while we detected positivity in the tunica media layer of the cavernous segment, we found positivity in the tunica intima layer of the cervicalis segment of the ICA. We did not detect any positivity for ASIC3 and ACTHR receptors in both parts of the ICA.

Conclusions

As a result, we observed various differences in receptors between ICA segments. While the outer surface of the ICA in the cervical region did not show any receptor positivity, we detected TRP12 receptor positivity along the tissue contour of vessel in the CS. We assume that it may provide a new perspective on pathologies of the CS/ICA and preservation of brain hemodynamics for clinicians.

ASIC1a is required for neuronal activation via low-intensity ultrasound stimulation in mouse brain

Accumulating evidence has shown transcranial low-intensity ultrasound can be potentially a non-invasive neural modulation tool to treat brain diseases. However, the underlying mechanism remains elusive and the majority of studies on animal models applying rather high-intensity ultrasound that cannot be safely used in humans. Here, we showed low-intensity ultrasound was able to activate neurons in the mouse brain and repeated ultrasound stimulation resulted in adult neurogenesis in specific brain regions. In vitro calcium imaging studies showed that a specific ultrasound stimulation mode, which combined with both ultrasound-induced pressure and acoustic streaming mechanotransduction, is required to activate cultured cortical neurons. ASIC1a and cytoskeletal proteins were involved in the low-intensity ultrasound-mediated mechanotransduction and cultured neuron activation, which was inhibited by ASIC1a blockade and cytoskeleton-modified agents. In contrast, the inhibition of mechanical-sensitive channels involved in bilayer-model mechanotransduction like Piezo or TRP proteins did not repress the ultrasound-mediated neuronal activation as efficiently. The ASIC1a-mediated ultrasound effects in mouse brain such as immediate response of ERK phosphorylation and DCX marked neurogenesis were statistically significantly compromised by ASIC1a gene deletion. Collated data suggest that ASIC1a is the molecular determinant involved in the mechano-signaling of low-intensity ultrasound that modulates neural activation in mouse brain.

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.

Therapeutic Inhibition of Acid Sensing Ion Channel 1a Recovers Heart Function After Ischemia-Reperfusion Injury

Background: Ischemia-reperfusion injury (IRI) is one of the major risk factors implicated in morbidity and mortality associated with cardiovascular disease. During cardiac ischemia, the build-up of acidic metabolites results in decreased intracellular and extracellular pH that can reach as low as 6.0-6.5. The resulting tissue acidosis exacerbates ischemic injury and significantly impacts cardiac function. Methods: We used genetic and pharmacological methods to investigate the role of acid sensing ion channel 1a (ASIC1a) in cardiac IRI at the cellular and whole organ level. Human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CMs) as well as ex vivo and in vivo models of IRI were used to test the efficacy of ASIC1a inhibitors as pre- and post-conditioning therapeutic agents. Results: Analysis of human complex trait genetics indicate that variants in the ASIC1 genetic locus are significantly associated with cardiac and cerebrovascular ischemic injuries. Using hiPSC-CMs in vitro and murine ex vivo heart models, we demonstrate that genetic ablation of ASIC1a improves cardiomyocyte viability after acute IRI. Therapeutic blockade of ASIC1a using specific and potent pharmacological inhibitors recapitulates this cardioprotective effect. We used an in vivo model of myocardial infarction (MI) and two models of ex vivo donor heart procurement and storage as clinical models to show that ASIC1a inhibition improves post-IRI cardiac viability. Use of ASIC1a inhibitors as pre- or post-conditioning agents provided equivalent cardioprotection to benchmark drugs, including the sodium-hydrogen exchange inhibitor zoniporide. At the cellular and whole organ level, we show that acute exposure to ASIC1a inhibitors has no impact on cardiac ion channels regulating baseline electromechanical coupling and physiological performance. Conclusions: Collectively, our data provide compelling evidence for a novel pharmacological strategy involving ASIC1a blockade as a cardioprotective therapy to improve the viability of hearts subjected to IRI.

Synthesis and Characterization of Novel Mono- and Bis-Guanyl Hydrazones as Potent and Selective ASIC1 Inhibitors Able to Reduce Brain Ischemic Insult

Acid-sensitive ion channels (ASICs) are sodium channels partially permeable to Ca²⁺ ions, listed among putative targets in central nervous system (CNS) diseases in which a pH modification occurs. We targeted novel compounds able to modulate ASIC1 and to reduce the progression of ischemic brain injury. We rationally designed and synthesized several diminazene-inspired diaryl mono- and bis-guanyl hydrazones. A correlation between their predicted docking affinities for the acidic pocket (AcP site) in chicken ASIC1 and their inhibition of homo- and heteromeric hASIC1 channels in HEK-293 cells was found. Their activity on murine ASIC1a currents and their selectivity vs mASIC2a were assessed in engineered CHO-K1 cells, highlighting a limited isoform selectivity. Neuroprotective effects were confirmed in vitro, on primary rat cortical neurons exposed to oxygen-glucose deprivation followed by reoxygenation, and in vivo, in ischemic mice. Early lead 3b, showing a good selectivity for hASIC1 in human neurons, was neuroprotective against focal ischemia induced in mice.

Histamine and Corticosterone Modulate Acid Sensing Ion Channels (ASICs) Dependent Long-term Potentiation at the Mouse Anterior Cingulate Cortex

Increase in proton concentration [H⁺] or decrease in local and global extracellular pH occurs in both physiological and pathological conditions. Acid-sensing ion channels (ASICs), belonging to the ENaC/Deg superfamily, play an important role in signal transduction as proton sensor. ASICs and in particular ASIC1a (one of the six ASICs subunits) which is permeable to Ca²⁺, are involved in many physiological processes including synaptic plasticity and neurodegenerative diseases. Activity-dependent long-term potentiation (LTP) is a major type of long-lasting synaptic plasticity in the CNS, associated with learning, memory, development, fear and persistent pain. Neurons in the anterior cingulate cortex (ACC) play critical roles in pain perception and chronic pain and express ASIC1a channels. During synaptic transmission, acidification of the synaptic cleft presumably due to the co-release of neurotransmitter and H⁺ from synaptic vesicles activates postsynaptic ASIC1a channels in ACC of mice. This generates ASIC1a synaptic currents that add to the glutamatergic excitatory postsynaptic currents (EPSCs). Here we report that modulators like histamine and corticosterone, acting through ASIC1a regulate synaptic plasticity, reducing the threshold for LTP induction of glutamatergic EPSCs. Our findings suggest a new role for ASIC1a mediating the neuromodulator action of histamine and corticosterone regulating specific forms of synaptic plasticity in the mouse ACC.

Overexpression of acid-sensing ion channel 1a (ASIC1a) promotes breast cancer cell proliferation, migration and invasion

Background

The microenvironment of various tumor tissues is acidic. Acid-sensing ion channels (ASICs) are a class of ligand-gated ion channels which are sensitive to extracellular protons and are often highly expressed in tumor tissues. Breast cancer, whose extracellular microenvironment is thought to be acidic, is the most common cancer type among females in the world.

Methods

Thirty breast cancer tissues and adjacent normal tissues of patients were collected from 2009 to 2015 at the Xinhua hospital affiliated to Shanghai Jiao Tong University School of Medicine. The expression of acid-sensing ion channel 1a (ASIC1a), a subtype of ASICs family, was detected by immunohistochemistry in breast cancer tissues, and the effect of ASIC1a on the physiological activity of tumor cells was analyzed in vitro and in vivo experiments.

Results

In this study, it was found that ASIC1a is highly expressed in the tissues of breast cancer patients. In vitro experiments revealed that down-regulation of ASIC1a by its antagonist PcTx-1 or ASIC1a siRNA could significantly weaken the migration, proliferation and invasion of tumor cells. In vivo studies, down-regulation or inhibition of the ASIC1a could inhibit breast tumor growth.

Conclusions

The high expression of ASIC1a might be related to the enhanced biological activity of breast cancer cells. Whether ASIC1a is a potential therapeutic target for some types of breast cancer deserves further study.

Structural insights into human acid-sensing ion channel 1a inhibition by snake toxin mambalgin1

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are involved in diverse neuronal processes including pain sensing. The peptide toxin Mambalgin1 (Mamba1) from black mamba snake venom can reversibly inhibit the conductance of ASICs, causing an analgesic effect. However, the detailed mechanism by which Mamba1 inhibits ASIC1s, especially how Mamba1 binding to the extracellular domain affects the conformational changes of the transmembrane domain of ASICs remains elusive. Here, we present single-particle cryo-EM structures of human ASIC1a (hASIC1a) and the hASIC1a-Mamba1 complex at resolutions of 3.56 and 3.90 Å, respectively. The structures revealed the inhibited conformation of hASIC1a upon Mamba1 binding. The combination of the structural and physiological data indicates that Mamba1 preferentially binds hASIC1a in a closed state and reduces the proton sensitivity of the channel, representing a closed-state trapping mechanism.

Histidine Residues Are Responsible for Bidirectional Effects of Zinc on Acid-Sensing Ion Channel 1a/3 Heteromeric Channels

Acid-sensing ion channel (ASIC) subunits 1a and 3 are highly expressed in central and peripheral sensory neurons, respectively. Endogenous biomolecule zinc plays a critical role in physiological and pathophysiological conditions. Here, we found that currents recorded from heterologously expressed ASIC1a/3 channels using the whole-cell patch-clamp technique were regulated by zinc with dual effects. Co-application of zinc dose-dependently potentiated both peak amplitude and the sustained component of heteromeric ASIC1a/3 currents; pretreatment with zinc between 3 to 100 µM exerted the same potentiation as co-application. However, pretreatment with zinc induced a significant inhibition of heteromeric ASIC1a/3 channels when zinc concentrations were over 250 µM. The potentiation of heteromeric ASIC1a/3 channels by zinc was pH dependent, as zinc shifted the pH dependence of ASIC1a/3 currents from a pH₅₀ of 6.54 to 6.77; whereas the inhibition of ASIC1a/3 currents by zinc was also pH dependent. Furthermore, we systematically mutated histidine residues in the extracellular domain of ASIC1a or ASIC3 and found that histidine residues 72 and 73 in both ASIC1a and ASIC3, and histidine residue 83 in the ASIC3 were responsible for bidirectional effects on heteromeric ASIC1a/3 channels by zinc. These findings suggest that histidine residues in the extracellular domain of heteromeric ASIC1a/3 channels are critical for zinc-mediated effects.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

Heterologous expression of biologically active Mambalgin-1 peptide as a new potential anticancer, using a PVX-based viral vector in Nicotiana benthamiana

Mambalgin-1 is a peptide that acts as a potent analgesic through inhibiting acid-sensing ion channels (ASIC) in nerve cells. Research has shown that ASIC channels are involved in the proliferation and growth of cancer cells; therefore, Mambalgin-1 can be a potential anti-cancer by inhibiting these channels. In the present study, the Nicotiana benthamiana codon optimized Mambalgin-1 gene was synthesized and cloned in PVX (potato virus X) viral vector. The two cultures of Agrobacterium containing Mambalgin-1 and P19 silencing suppressor genes were co-agroinfiltrated into N. benthamiana leaves. Five days post infiltration, the production of recombinant Mambalgin-1 was determined by western blotting. For biological activity, MTT (3(4, 5-dimethylthiazole-2-yl)-2, 5-diphenyltetrazolium bromide) was analyzed for the cytotoxicity recombinant Mambalgin-1 from the transformed plants on nervous (SH-SY5Y) and breast (MCF7) cancer cells. The results showed that the plants expressing open reading frame of Mambalgin-1 showed recombinant 7.4 kDa proteins in the entire plant extract. In the MTT test, it was found that Mambalgin-1 had cytotoxic effects on SH-SY5Y cancer cells, yet no effects on MCF7 cancer cells were observed. According to the results, the expression of the biologically active recombinant Mambalgin-1 in the transformed plant leaves was confirmed and Mambalgin-1 can also have anti-cancer (inhibition of ASIC channels) potential along with its already known analgesic effect.

Slowing of the Time Course of Acidification Decreases the Acid-Sensing Ion Channel 1a Current Amplitude and Modulates Action Potential Firing in Neurons

Acid-sensing ion channels (ASICs) are H⁺-activated neuronal Na⁺ channels. They are involved in fear behavior, learning, neurodegeneration after ischemic stroke and in pain sensation. ASIC activation has so far been studied only with fast pH changes, although the pH changes associated with many roles of ASICs are slow. It is currently not known whether slow pH changes can open ASICs at all. Here, we investigated to which extent slow pH changes can activate ASIC1a channels and induce action potential signaling. To this end, ASIC1a current amplitudes and charge transport in transfected Chinese hamster ovary cells, and ASIC-mediated action potential signaling in cultured cortical neurons were measured in response to defined pH ramps of 1-40 s duration from pH 7.4 to pH 6.6 or 6.0. A kinetic model of the ASIC1a current was developed and integrated into the Hodgkin-Huxley action potential model. Interestingly, whereas the ASIC1a current amplitude decreased with slower pH ramps, action potential firing was higher upon intermediate than fast acidification in cortical neurons. Indeed, fast pH changes (<4 s) induced short action potential bursts, while pH changes of intermediate speed (4-10 s) induced longer bursts. Slower pH changes (>10 s) did in many experiments not generate action potentials. Computer simulations corroborated these observations. We provide here the first description of ASIC function in response to defined slow pH changes. Our study shows that ASIC1a currents, and neuronal activity induced by ASIC1a currents, strongly depend on the speed of pH changes. Importantly, with pH changes that take >10 s to complete, ASIC1a activation is inefficient. Therefore, it is likely that currently unknown modulatory mechanisms allow ASIC activity in situations such as ischemia and inflammation.

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.

Involvement of Acid-Sensing Ion Channel 1b in the Development of Acid-Induced Chronic Muscle Pain

Acid-sensing ion channels (ASICs) are important acid sensors involved in neural modulation in the central nervous system and pain-associated tissue acidosis in the peripheral system. Among ASIC subtypes, ASIC1b is the most selectively expressed in peripheral sensory neurons. However, the role of ASIC1b is still elusive in terms of its functions and expression profile. In this study, we probed the role of ASIC1b in acid-induced muscle pain in Asic1b-knockout (Asic1b^–/–) and Asic1b-Cre transgenic (Asic1b^Cre) mice. We tested the effect of ASIC1b knockout in a mouse model of fibromyalgia induced by dual intramuscular acid injections. In this model, a unilateral acid injection to the gastrocnemius muscle induced transient bilateral hyperalgesia in wild-type (Asic1b^+/+) but not Asic1b^–/– mice; a second acid injection, spaced 1 or 5 days apart, to the same muscle induced chronic hyperalgesia lasting for 4 weeks in Asic1b^+/+ mice, but the duration of hyperalgesia was significantly shortened in Asic1b^–/– mice. Mambalgin-1, an ASIC1b-containing channel inhibitor that was mixed with acid saline at the first injection, dose-dependently blocked the acid-induced transient and chronic hyperalgesia in Asic1b^+/+ mice. In contrast, psalmotoxin 1 (PcTx1), an ASIC1a-selective antagonist, had no effect on acid-induced transient or chronic hyperalgesia. We used whole-cell patch clamp recording to study the properties of acid-induced currents in ASIC1b-expressing dorsal root ganglia (DRG) neurons from Asic1b^Cre-TdTomato reporter mice. Medium- to large-sized ASIC1b-expressing DRG neurons mainly exhibited an amiloride-sensitive ASIC-like biphasic current (I_ASIC) in response to acid stimulation, whereas small- to medium-sized ASIC1b-expressing DRG neurons predominantly exhibited an amiloride-insensitive sustained current. Specifically, mambalgin-1 selectively inhibited the I_ASIC in most ASIC1b-expressing DRG neurons. However, PcTx1 or APETx2 (an ASIC3-selective antagonist) had only a mild inhibitory effect on I_ASIC in about half of the ASIC1b-expressing DRG neurons. In situ hybridization revealed that ASIC1b-positive DRG neurons co-expressed highly with ASIC1a and ASIC2a mRNA and partially with ASIC3 and ASIC2b. Thus, ASIC1b might form a wide variety of heteromeric channels. ASIC1b-containing heteromeric channels might be promising targets for the therapeutic treatment of acid-induced chronic muscle pain.

Sa12b Peptide from Solitary Wasp Inhibits ASIC Currents in Rat Dorsal Root Ganglion Neurons

In this work, we evaluate the effect of two peptides Sa12b (EDVDHVFLRF) and Sh5b (DVDHVFLRF-NH₂) on Acid-Sensing Ion Channels (ASIC). These peptides were purified from the venom of solitary wasps Sphex argentatus argentatus and Isodontia harmandi, respectively. Voltage clamp recordings of ASIC currents were performed in whole cell configuration in primary culture of dorsal root ganglion (DRG) neurons from (P7-P10) CII Long-Evans rats. The peptides were applied by preincubation for 25 s (20 s in pH 7.4 solution and 5 s in pH 6.1 solution) or by co-application (5 s in pH 6.1 solution). Sa12b inhibits ASIC current with an IC₅₀ of 81 nM, in a concentration-dependent manner when preincubation application was used. While Sh5b did not show consistent results having both excitatory and inhibitory effects on the maximum ASIC currents, its complex effect suggests that it presents a selective action on some ASIC subunits. Despite the similarity in their sequences, the action of these peptides differs significantly. Sa12b is the first discovered wasp peptide with a significant ASIC inhibitory effect.

Alkaloid Lindoldhamine Inhibits Acid-Sensing Ion Channel 1a and Reveals Anti-Inflammatory Properties

Acid-sensing ion channels (ASICs), which are present in almost all types of neurons, play an important role in physiological and pathological processes. The ASIC1a subtype is the most sensitive channel to the medium's acidification, and it plays an important role in the excitation of neurons in the central nervous system. Ligands of the ASIC1a channel are of great interest, both fundamentally and pharmaceutically. Using a two-electrode voltage-clamp electrophysiological approach, we characterized lindoldhamine (a bisbenzylisoquinoline alkaloid extracted from the leaves of Laurus nobilis L.) as a novel inhibitor of the ASIC1a channel. Lindoldhamine significantly inhibited the ASIC1a channel's response to physiologically-relevant stimuli of pH 6.5-6.85 with IC50 range 150-9 μM, but produced only partial inhibition of that response to more acidic stimuli. In mice, the intravenous administration of lindoldhamine at a dose of 1 mg/kg significantly reversed complete Freund's adjuvant-induced thermal hyperalgesia and inflammation; however, this administration did not affect the pain response to an intraperitoneal injection of acetic acid (which correlated well with the function of ASIC1a in the peripheral nervous system). Thus, we describe lindoldhamine as a novel antagonist of the ASIC1a channel that could provide new approaches to drug design and structural studies regarding the determinants of ASIC1a activation.

ASIC3-dependent metabolomics profiling of serum and urine in a mouse model of fibromyalgia

Fibromyalgia (FM) is characterized by chronic widespread pain. The pathogenesis of FM remains unclear. No specific biomarkers are available. Animal models of FM may provide an opportunity to explore potential biomarkers in a relative homogenous disease condition. Here, we probed the metabolomics profiles of serum and urine in a mouse model of FM induced by intermittent cold stress (ICS). We focused on the role of acid-sensing ion channel 3 (ASIC3) in the metabolomics profiling because ICS treatment induced chronic widespread muscle pain lasting for 1 month in wild-type (Asic3^+/+) but not Asic3-knockout (Asic3^−/−) mice. Serum and urine samples were collected from both genotypes at different ICS stages, including before ICS (basal level) and post-ICS at days 10 (middle phase, P10) and 40 (recovery phase, P40). Control naïve mice and ICS-induced FM mice differed in ¹H-NMR- and LC-MS-based metabolomics profiling. On pathway analysis, the leading regulated pathways in Asic3^+/+ mice were taurine and hypotaurine, cysteine and methionine, glycerophospholipid, and ascorbate and aldarate metabolisms, and the major pathways in Asic3^−/− mice involved amino acid-related metabolism. Finally, we developed an algorithm for the impactful metabolites in the FM model including cis-aconitate, kynurenate, taurine, pyroglutamic acid, pyrrolidonecarboxylic acid, and 4-methoxyphenylacetic acid in urine as well as carnitine, deoxycholic acid, lysoPC(16:0), lysoPC(20:3), oleoyl-L-carnitine, and trimethylamine N-oxide in serum. Asic3^−/− mice were impaired in only muscle allodynia development but not other pain symptoms in the ICS model, so the ASIC3-dependent metabolomics changes could be useful for developing diagnostic biomarkers specific to chronic widespread muscle pain, the core symptom of FM. Further pharmacological validations are needed to validate these metabolomics changes as potential biomarkers for FM diagnosis and/or treatment responses.

Multiple Modulation of Acid-Sensing Ion Channel 1a by the Alkaloid Daurisoline

Acid-sensing ion channels (ASICs) are proton-gated sodium-selective channels that are expressed in the peripheral and central nervous systems. ASIC1a is one of the most intensively studied isoforms due to its importance and wide representation in organisms, but it is still largely unexplored as a target for therapy. In this study, we demonstrated response of the ASIC1a to acidification in the presence of the daurisoline (DAU) ligand. DAU alone did not activate the channel, but in combination with protons, it produced the second peak component of the ASIC1a current. This second peak differs from the sustained component (which is induced by RF-amide peptides), as the second (DAU-induced) peak is completely desensitized, with the same kinetics as the main peak. The co-application of DAU and mambalgin-2 indicated that their binding sites do not overlap. Additionally, we found an asymmetry in the pH activation curve of the channel, which was well-described by a mathematical model based on the multiplied probabilities of protons binding with a pool of high-cooperative sites and a single proton binding with a non-cooperative site. In this model, DAU targeted the pool of high-cooperative sites and, when applied with protons, acted as an inhibitor of ASIC1a activation. Moreover, DAU's occupation of the same binding site most probably reverses the channel from steady-state desensitization in the pH 6.9-7.3 range. DAU features disclose new opportunities in studies of ASIC structure and function.

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.