Identification of a calcium permeable human acid-sensing ion channel 1 transcript variant

Acid-sensing ion channels and downstream signalling in cancer cells: is there a mechanistic link?

It is increasingly appreciated that the acidic microenvironment of a tumour contributes to its evolution and clinical outcomes. However, our understanding of the mechanisms by which tumour cells detect acidosis and the signalling cascades that it induces is still limited. Acid-sensing ion channels (ASICs) are sensitive receptors for protons; therefore, they are also candidates for proton sensors in tumour cells. Although in non-transformed tissue, their expression is mainly restricted to neurons, an increasing number of studies have reported ectopic expression of ASICs not only in brain cancer but also in different carcinomas, such as breast and pancreatic cancer. However, because ASICs are best known as desensitizing ionotropic receptors that mediate rapid but transient signalling, how they trigger intracellular signalling cascades is not well understood. In this review, we introduce the acidic microenvironment of tumours and the functional properties of ASICs, point out some conceptual problems, summarize reported roles of ASICs in different cancers, and highlight open questions on the mechanisms of their action in cancer cells. Finally, we propose guidelines to keep ASIC research in cancer on solid ground.

The funnel-web spider venom derived single knot peptide Hc3a modulates acid-sensing ion channel 1a desensitisation

Acid-sensing ion channel 1a (ASIC1a) is a proton-gated channel involved in synaptic transmission, pain signalling, and several ischemia-associated pathological conditions. The spider venom-derived peptides PcTx1 and Hi1a are two of the most potent ASIC1a inhibitors known and have been instrumental in furthering our understanding of the structure, function, and biological roles of ASICs. To date, homologous spider peptides with different pharmacological profiles at ASIC1a have yet to be discovered. Here we report the characterisation of Hc3a, a single inhibitor cystine knot peptide from the Australian funnel-web spider Hadronyche cerberea with sequence similarity to PcTx1. We show that Hc3a has complex pharmacology and binds different ASIC1a conformational states (closed, open, and desensitised) with different affinities, with the most prominent effect on desensitisation. Hc3a slows the desensitisation kinetics of proton-activated ASIC1a currents across multiple application pHs, and when bound directly to ASIC1a in the desensitised conformation promotes current inhibition. The solution structure of Hc3a was solved, and the peptide-channel interaction examined via mutagenesis studies to highlight how small differences in sequence between Hc3a and PcTx1 can lead to peptides with distinct pharmacology. The discovery of Hc3a expands the pharmacological diversity of spider venom peptides targeting ASIC1a and adds to the toolbox of compounds to study the intricacies of ASIC1 gating.

Mechanosensitive channels in the mechanical component of the exercise pressor reflex

The cardiovascular response is appropriately regulated during exercise to meet the metabolic demands of the active muscles. The exercise pressor reflex is a neural feedback mechanism through thin-fiber muscle afferents activated by mechanical and metabolic stimuli in the active skeletal muscles. The mechanical component of this reflex is referred to as skeletal muscle mechanoreflex. Its initial step requires mechanotransduction mediated by mechanosensors, which convert mechanical stimuli into biological signals. Recently, various mechanosensors have been identified, and their contributions to muscle mechanoreflex have been actively investigated. Nevertheless, the mechanosensitive channels responsible for this muscular reflex remain largely unknown. This review discusses progress in our understanding of muscle mechanoreflex under healthy conditions, focusing on mechanosensitive channels.

Acidosis-related pain and its receptors as targets for chronic pain

Sensing acidosis is an important somatosensory function in responses to ischemia, inflammation, and metabolic alteration. Accumulating evidence has shown that acidosis is an effective factor for pain induction and that many intractable chronic pain diseases are associated with acidosis signaling. Various receptors have been known to detect extracellular acidosis and all express in the somatosensory neurons, such as acid sensing ion channels (ASIC), transient receptor potential (TRP) channels and proton-sensing G-protein coupled receptors. In addition to sense noxious acidic stimulation, these proton-sensing receptors also play a vital role in pain processing. For example, ASICs and TRPs are involved in not only nociceptive activation but also anti-nociceptive effects as well as some other non-nociceptive pathways. Herein, we review recent progress in probing the roles of proton-sensing receptors in preclinical pain research and their clinical relevance. We also propose a new concept of sngception to address the specific somatosensory function of acid sensation. This review aims to connect these acid-sensing receptors with basic pain research and clinical pain diseases, thus helping with better understanding the acid-related pain pathogenesis and their potential therapeutic roles via the mechanism of acid-mediated antinociception.

The Role of Zinc in Modulating Acid-Sensing Ion Channel Function

Acid-sensing ion channels (ASICs) are proton-gated, voltage-independent sodium channels widely expressed throughout the central and peripheral nervous systems. They are involved in synaptic plasticity, learning/memory, fear conditioning and pain. Zinc, an important trace metal in the body, contributes to numerous physiological functions, with neurotransmission being of note. Zinc has been implicated in the modulation of ASICs by binding to specific sites on these channels and exerting either stimulatory or inhibitory effects depending on the ASIC subtype. ASICs have been linked to several neurological and psychological disorders, such as Alzheimer's disease, Parkinson's disease, ischemic stroke, epilepsy and cocaine addiction. Different ASIC isoforms contribute to the persistence of each of these neurological and psychological disorders. It is critical to understand how various zinc concentrations can modulate specific ASIC subtypes and how zinc regulation of ASICs can contribute to neurological and psychological diseases. This review elucidates zinc's structural interactions with ASICs and discusses the potential therapeutic implications zinc may have on neurological and psychological diseases through targeting ASICs.

The diverse functions of the DEG/ENaC family: linking genetic and physiological insights

The DEG/ENaC family of ion channels was defined based on the sequence similarity between degenerins (DEG) from the nematode Caenorhabditis elegans and subunits of the mammalian epithelial sodium channel (ENaC), and also includes a diverse array of non-voltage-gated cation channels from across animal phyla, including the mammalian acid-sensing ion channels (ASICs) and Drosophila pickpockets. ENaCs and ASICs have wide ranging medical importance; for example, ENaCs play an important role in respiratory and renal function, and ASICs in ischaemia and inflammatory pain, as well as being implicated in memory and learning. Electrophysiological approaches, both in vitro and in vivo, have played an essential role in establishing the physiological properties of this diverse family, identifying an array of modulators and implicating them in an extensive range of cellular functions, including mechanosensation, acid sensation and synaptic modulation. Likewise, genetic studies in both invertebrates and vertebrates have played an important role in linking our understanding of channel properties to function at the cellular and whole animal/behavioural level. Drawing together genetic and physiological evidence is essential to furthering our understanding of the precise cellular roles of DEG/ENaC channels, with the diversity among family members allowing comparative physiological studies to dissect the molecular basis of these diverse functions.

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.

Retinoic Acid-Differentiated Neuroblastoma SH-SY5Y Is an Accessible In Vitro Model to Study Native Human Acid-Sensing Ion Channels 1a (ASIC1a)

Simple Summary

Human neuroblastoma SH-SY5Y is used in neurobiology for studying various neuropathophysiological processes. In this study, we differentiated neuroblastoma cells into a neuronal-like phenotype with retinoic acid and studied if functional acid-sensing, transient receptor potential vanilloid-1 and ankyrin-1 ion channels were expressed in it. We found that homomeric acid-sensing ion channels 1a were expressed predominantly and yielded large ionic currents that can be modulated with different ligands. This channel plays important roles in synaptic plasticity, neurodegeneration, and pain perception. Thus, retinoic acid-treated neuroblastoma is a suitable model system for pharmacological testing on native human acid-sensing ion channels 1a. This approach can facilitate the development of new drugs for neuroprotection and pain management.

Abstract

Human neuroblastoma SH-SY5Y is a prominent neurobiological tool used for studying neuropathophysiological processes. We investigated acid-sensing (ASIC) and transient receptor potential vanilloid-1 (TRPV1) and ankyrin-1 (TRPA1) ion channels present in untreated and differentiated neuroblastoma SH-SY5Y to propose a new means for their study in neuronal-like cells. Using a quantitative real-time PCR and a whole-cell patch-clamp technique, ion channel expression profiles, functionality, and the pharmacological actions of their ligands were characterized. A low-level expression of ASIC1a and ASIC2 was detected in untreated cells. The treatment with 10 μM of retinoic acid (RA) for 6 days resulted in neuronal differentiation that was accompanied by a remarkable increase in ASIC1a expression, while ASIC2 expression remained almost unaltered. In response to acid stimuli, differentiated cells showed prominent ASIC-like currents. Detailed kinetic and pharmacological characterization suggests that homomeric ASIC1a is a dominant isoform among the present ASIC channels. RA-treatment also reduced the expression of TRPV1 and TRPA1, and minor electrophysiological responses to their agonists were found in untreated cells. Neuroblastoma SH-SY5Y treated with RA can serve as a model system to study the effects of different ligands on native human ASIC1a in neuronal-like cells. This approach can improve the characterization of modulators for the development of new neuroprotective and analgesic drugs.

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.

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.

Signaling Pathways in Proton and Non-proton ASIC1a Activation

Acid-sensing ion channels (ASICs) regulate synaptic activities and play important roles in neurodegenerative diseases as well as pain conditions. Classically, ASICs are described as transiently activated by a reduced pH, followed by desensitization; the activation allows sodium influx, and in the case of ASIC1a-composed channels, also calcium to some degree. Several factors are emerging and extensively analyzed as modulators, activating, inhibiting, and potentiating specific channel subunits. However, the signaling pathways triggered by channel activation are only starting to be revealed.The channel has been recently shown to be activated through a mechanism other than proton-mediated. Indeed, the large extracellular loop of these channels opens the possibility that other non-proton ligands might exist. One such molecule discovered was a toxin present in the Texas coral snake venom. The finding was associated with the activation of the channel at neutral pH via the toxin and causing intense and unremitting pain.By using different pharmacological tools, we analyzed the downstream signaling pathway triggered either by the proton and non-proton activation for human, mouse, and rat ASIC1a-composed channels in in vitro models. We show that for all species analyzed, the non-protonic mode of activation determines the activation of the ERK signaling cascade at a higher level and duration compared to the proton mode.This study adds to the growing evidence of the important role ASIC1a channels play in different physiological and pathological conditions and also hints at a possible pathological mechanism for a sustained effect.

Acid-Sensing Ion Channels: Expression and Function in Resident and Infiltrating Immune Cells in the Central Nervous System

Peripheral and central immune cells are critical for fighting disease, but they can also play a pivotal role in the onset and/or progression of a variety of neurological conditions that affect the central nervous system (CNS). Tissue acidosis is often present in CNS pathologies such as multiple sclerosis, epileptic seizures, and depression, and local pH is also reduced during periods of ischemia following stroke, traumatic brain injury, and spinal cord injury. These pathological increases in extracellular acidity can activate a class of proton-gated channels known as acid-sensing ion channels (ASICs). ASICs have been primarily studied due to their ubiquitous expression throughout the nervous system, but it is less well recognized that they are also found in various types of immune cells. In this review, we explore what is currently known about the expression of ASICs in both peripheral and CNS-resident immune cells, and how channel activation during pathological tissue acidosis may lead to altered immune cell function that in turn modulates inflammatory pathology in the CNS. We identify gaps in the literature where ASICs and immune cell function has not been characterized, such as neurotrauma. Knowledge of the contribution of ASICs to immune cell function in neuropathology will be critical for determining whether the therapeutic benefits of ASIC inhibition might be due in part to an effect on immune cells.

Native expression of ASIC1a and ASIC1b human homologues in the HEK 293 cell line allows pharmacological evaluation of analgesics targeting acid sensation in humans

Nociceptors arising from the dorsal root ganglia (DRG) express acid-sensing ion channel-1 (ASIC1) subtypes to mediate the perception of inflammatory and neuropathic pain, and as such, these receptors are attractive targets for the development of analgesics for these painful conditions. Nevertheless, given that the human and rodent DRG differ considerably in subtype proportions of ASIC1 and that the pharmacological properties of rodent ASIC1 subtypes and their human homologues are distinct, ASIC1 inhibitors that demonstrate analgesic properties in rodents may not necessarily be effective in preventing pain in humans. In this study, we show that human embryonic kidney (HEK) 293 cells, which are routinely used as a cellular vehicle for the heterologous expression and pharmacological characterization of receptors and ion channels, natively transcribe the human homologues of ASIC1a and ASIC1b at similar proportions to those found in the human DRG. Importantly, HEK 293 ASIC1 is sensitive to inhibition by amiloride, ethylisopropyl amiloride, and the snake toxin mambalgin-1, but insensitive to inhibition by the ASIC1a inhibitor psalmotoxin-1 when applied at a physiological conditioning pH. Given that the human DRG transcribes the same set of ASIC1 subtypes as HEK 293 cells, our data support the notion that mambalgin-1 may be effective against acid pain sensation in humans. Moreover, our data suggest that the HEK 293 cell line may be a suitable tool for pharmacological screening and characterization of heteromeric human ASIC1.

Mambalgin-3 potentiates human acid-sensing ion channel 1b under mild to moderate acidosis: Implications as an analgesic lead

Acid-sensing ion channels (ASICs) are expressed in the nervous system, activated by acidosis, and implicated in pain pathways. Mambalgins are peptide inhibitors of ASIC1 and analgesic in rodents via inhibition of centrally expressed ASIC1a and peripheral ASIC1b. This activity has generated interest in mambalgins as potential therapeutics. However, most mechanism and structure–activity relationship work on mambalgins has focused on ASIC1a, and neglected the peripheral analgesic target ASIC1b. Here, we compare mambalgin potency and mechanism of action at heterologously expressed rat and human ASIC1 variants. Unlike the nanomolar inhibition at ASIC1a and rodent ASIC1b, we find mambalgin-3 only weakly inhibits human ASIC1b and ASIC1b/3 under severe acidosis, but potentiates currents under mild/moderate acidosis. Our data highlight the importance of understanding the activity of potential ASIC-targeting pharmaceuticals at human channels.

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.

Large Acid-Evoked Currents, Mediated by ASIC1a, Accompany Differentiation in Human Dopaminergic Neurons

Acid-sensing ion channels (ASICs) are proton-gated Na⁺ channels. They contribute to synaptic transmission, neuronal differentiation and neurodegeneration. ASICs have been mainly characterized in neurons from mice or rats and our knowledge of their properties in human neurons is scarce. Here, we functionally characterized ASICs in differentiating LUHMES cells, a human mesencephalic cell line with characteristics of dopaminergic neurons. We find that LUHMES cells express functional ASICs, predominantly homomeric ASIC1a. Expression starts early during differentiation with a striking surge in current amplitude at days 4-6 of differentiation, a time point where-based on published data-LUHMES cells start expressing synaptic markers. Peak ASIC expression therefore coincides with a critical period of LUHMES cell differentiation. It was associated with increased excitability, but not paralleled by an increase in ASIC1 mRNA or protein. In differentiating as well as in terminally differentiated LUHMES cells, ASIC activation by slight acidification elicited large currents, action potentials and a rise in cytosolic Ca²⁺. Applying the ASIC pore blocker diminazene during differentiation reduced the length of neurites, consistent with the hypothesis that ASICs play a critical role in LUHMES cell differentiation. In summary, our study establishes LUHMES cells as a valuable model to study the role of ASICs for neuronal differentiation and potentially also cell death in a human cell line.

Animal toxins - Nature's evolutionary-refined toolkit for basic research and drug discovery

Venomous animals have evolved toxins that interfere with specific components of their victim's core physiological systems, thereby causing biological dysfunction that aids in prey capture, defense against predators, or other roles such as intraspecific competition. Many animal lineages evolved venom systems independently, highlighting the success of this strategy. Over the course of evolution, toxins with exceptional specificity and high potency for their intended molecular targets have prevailed, making venoms an invaluable and almost inexhaustible source of bioactive molecules, some of which have found use as pharmacological tools, human therapeutics, and bioinsecticides. Current biomedically-focused research on venoms is directed towards their use in delineating the physiological role of toxin molecular targets such as ion channels and receptors, studying or treating human diseases, targeting vectors of human diseases, and treating microbial and parasitic infections. We provide examples of each of these areas of venom research, highlighting the potential that venom molecules hold for basic research and drug development.

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.

Mambalgin-2 Induces Cell Cycle Arrest and Apoptosis in Glioma Cells via Interaction with ASIC1a

Gliomas are fast growing and highly invasive brain tumors, characterized by tumor microenvironment acidification that drives glioma cell growth and migration. Channels containing Acid-sensing Ion Channel 1a subunit (ASIC1a) mediate amiloride-sensitive cation influx in late stage glioma cells, but not in normal astrocytes. Thus, selective targeting of ASIC1a can be a perspective strategy for glioma treatment. Here, ASIC1a expression in U251 MG and A172 glioma cells, but not in normal astrocytes, was demonstrated. Recombinant analog of mambalgin-2 from black mamba Dendroaspis polylepis inhibited amiloride-sensitive currents at ASIC1a both in Xenopus laevis oocytes and in U251 MG cells, while its mutants with impaired activity towards this channel did not. Mambalgin-2 inhibited U251 MG and A172 glioma cells growth with EC50 in the nanomolar range without affecting the proliferation of normal astrocytes. Notably, mambalgin-2 mutants did not affect glioma cell proliferation, pointing on ASIC1a as the main molecular target of mambalgin-2 in U251 MG and A172 cells. Mambalgin-2 induced a cell cycle arrest, inhibited Cyclin D1 and cyclin-dependent kinases (CDK) phosphorylation and caused apoptosis in U251 MG and A172 cells. Moreover, mambalgin-2 inhibited the growth of low-passage primary cells from a patient with glioblastoma. Altogether, our data point to mambalgin-2 as a useful hit for the development of new drugs for glioma treatment.

Synaptic signals mediated by protons and acid-sensing ion channels

Extracellular pH changes may constitute significant signals for neuronal communication. During synaptic transmission, changes in pH in the synaptic cleft take place. Its role in the regulation of presynaptic Ca²⁺ currents through multivesicular release in ribbon-type synapses is a proven phenomenon. In recent years, protons have been recognized as neurotransmitters that participate in neuronal communication in synapses of several regions of the CNS such as amygdala, nucleus accumbens, and brainstem. Protons are released by nerve stimulation and activate postsynaptic acid-sensing ion channels (ASICs). Several types of ASIC channels are expressed in the peripheral and central nervous system. The influx of Ca²⁺ through some subtypes of ASICs, as a result of synaptic transmission, agrees with the participation of ASICs in synaptic plasticity. Pharmacological and genetical inhibition of ASIC1a results in alterations in learning, memory, and phenomena like fear and cocaine-seeking behavior. The recognition of endogenous molecules, such as arachidonic acid, cytokines, histamine, spermine, lactate, and neuropeptides, capable of inhibiting or potentiating ASICs suggests the existence of mechanisms of synaptic modulation that have not yet been fully identified and that could be tuned by new emerging pharmacological compounds with potential therapeutic benefits.

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.

Selection of an ASIC1a-blocking combinatorial antibody that protects cells from ischemic death

Acid-sensing ion channels (ASICs) have emerged as important, albeit challenging therapeutic targets for pain, stroke, etc. One approach to developing therapeutic agents could involve the generation of functional antibodies against these channels. To select such antibodies, we used channels assembled in nanodiscs, such that the target ASIC1a has a configuration as close as possible to its natural state in the plasma membrane. This methodology allowed selection of functional antibodies that inhibit acid-induced opening of the channel in a dose-dependent way. In addition to regulation of pH, these antibodies block the transport of cations, including calcium, thereby preventing acid-induced cell death in vitro and in vivo. As proof of concept for the use of these antibodies to modulate ion channels in vivo, we showed that they potently protect brain cells from death after an ischemic stroke. Thus, the methodology described here should be general, thereby allowing selection of antibodies to other important ASICs, such as those involved in pain, neurodegeneration, and other conditions.

Dual actions of Psalmotoxin at ASIC1a and ASIC2a heteromeric channels (ASIC1a/2a)

Acid-Sensing Ion Channels (ASICs) are gated by extracellular protons and play important roles in physiological and pathological states, such as pain and stroke. ASIC1a and ASIC2a, two of the most highly expressed subunits in the brain, form functional homo- and hetero-meric (ASIC1a/2a) channels. The function of ASIC1a has been widely studied using psalmotoxin (PcTx1), a venom-derived peptide, as an ASIC1a-selective antagonist. Here, using whole-cell patch clamp, we show that PcTx1 has dual actions at ASIC1a/2a. It can either inhibit or potentiate the heteromeric channel, depending on the conditioning and stimulating pHs. Potent inhibition occurs only at conditioning pHs that begin to desensitize the channel (IC₅₀ = 2.9 nM at pH7.0, a threshold pH for desensitization of ASIC1a/2a). By contrast, potent potentiation can occur at the physiological pH in both CHO cells (EC₅₀ = 56.1 nM) and cortical neurons (threshold concentration < 10 nM). PcTx1 potentiates ASIC1a/2a by increasing the apparent affinity of channel activation for protons. As such, potentiation is the strongest at moderate pHs, diminishing with increasing proton concentrations. Our findings identify PcTx1 as a valuable tool for studying ASIC1a/2a function and contribute significantly to the understanding of the diverse and complex pharmacology of PcTx1.

Neuroprotective Effects of Psalmotoxin-1, an Acid-Sensing Ion Channel (ASIC) Inhibitor, in Ischemia Reperfusion in Mouse Eyes

PURPOSE

The purpose of the current study is to assess changes in the expression of Acid-Sensing Ion Channel (ASIC)1a and ASIC2 in retinal ganglion cells (RGCs) after retinal ischemia and reperfusion (I/R) injury and to test if inhibition of ASIC1a provides RGC neuroprotection.

METHODS

Transient ischemia was induced in one eye of C57BL/6 mice by raising intraocular pressure to 120 mmHg for 60 min followed by retinal reperfusion by restoring normal pressure. RGC function was measured by Pattern electroretinography (PERG). In addition, retinal ASIC1a and ASIC2 were observed by immunohistochemistry and western blot. Changes in calpain, fodrin, heat shock protein 70 (HSP70), Brn3a, super oxide dismutase-1 (SOD1), catalase, and glutathione perioxidase-4 (GPX4) protein levels were assessed by western blot. RGC numbers were measured by immunohistochemistry on whole retinal flat mounts using anti-RNA binding protein with multiple splicing (RBPMS) antibodies. Intravitreal injection of psalmotoxin-1, a selective ASIC1a blocker, was used to assess the neuroprotective effect of ASIC1a inhibition.

RESULTS

Levels of ASIC1a and ASIC2 after I/R increased in RGCs. Upregulation of ASIC1a but not ASIC2 was attenuated by intravitreal injection of psalmotoxin-1. I/R induced activation of calpain and degradation of fodrin, HSP70, and reduction in Brn3a. In contrast, while psalmotoxin-1 attenuated calpain activation and increased Brn3a levels, it failed to block HSP70 degradation. Unlike SOD1 protein which was reduced, catalase protein levels increased after I/R. Psalmotoxin-1, although not affecting SOD1 and GPX4, increased catalase levels significantly. Psalmotoxin-1 also increased RBPMS-labeled RGCs following I/R as judged by immunohistochemistry of retinal flat mounts. Finally, psalmotoxin-1 enhanced the amplitude of PERG following I/R, suggesting partial rescue of RGC function.

CONCLUSION

Psalmotoxin-1 appears to exert a neuroprotective effect under ischemic insults and targeting inhibition of ASICs may represent a new therapeutic approach in ischemic retinal diseases.

Glioblastoma cancer stem cell lines express functional acid sensing ion channels ASIC1a and ASIC3

Acidic microenvironment is commonly observed in tumour tissues, including glioblastoma (GBM), the most aggressive and lethal brain tumour in adults. Acid sensing ion channels (ASICs) are neuronal voltage-insensitive sodium channels, which are sensors of extracellular protons. Here we studied and functionally characterized ASICs in two primary glioblastoma stem cell lines as cell culture models. We detected transcripts of the ACCN2 and ACCN3 genes, coding for ASIC1 and ASIC3, respectively, but not transcripts of ACCN1 (coding for ASIC2). Available microarray data confirmed that ACCN1 is downregulated in glioma. Western blotting confirmed expression of ASIC1 and ASIC3, the most proton-sensitive ASICs, in both GBM cell lines. We characterized ASICs functionally using whole-cell patch clamp and detected different types of acid-sensitive currents. Some of these currents had kinetics typical for ASICs and were sensitive to specific toxin inhibitors of ASIC1a or ASIC3, demonstrating that the GBM cell lines express functional ASIC1a and ASIC3 that may enable GBM cells to sensitively detect extracellular pH in a tumour tissue. Microarray data revealed that expression of ACCN2 and ACCN3 is associated with improved survival of patients suffering from gliomas, suggesting that preserved susceptibility to extracellular pH may impair tumour growth.

Molecular dynamics simulations investigate the mechanism of Psalmotoxin 1 regulating gating process of an acid-sensing ion channel 1a at pH 5.5

Acid-sensing ion channel 1a (ASIC1a) is a cation channel activated by protons and causes neuronal death through central nervous system. Psalmotoxin1 (PcTx1) is a gating modifier for ASIC1a. The process of PcTx1 regulating the channel gating from the extracellular domain to the transmembrane domain is unclear. Here we used molecular dynamics (MD) simulations method to investigate how PcTx1 regulates the gating of the ASIC1a. Our results indicated that PcTx1can mainly regulate ASIC1a gating process through hydrogen bonds, which can affect their relative positions of several key domains in ASIC1a, further, a long-range conformational changes path was determined, which is composed of β1, β2, β10, α6, α7, β11, and β12 in ASIC1a.

Enhanced maximal exercise capacity, vasodilation to electrical muscle contraction, and hind limb vascular density in ASIC1a null mice

Acid-sensing ion channel (ASIC) proteins form extracellular proton-gated, cation-selective channels in neurons and vascular smooth muscle cells and are proposed to act as extracellular proton sensors. However, their importance to vascular responses under conditions associated with extracellular acidosis, such as strenuous exercise, is unclear. Therefore, the purpose of this study was to determine if one ASIC protein, ASIC1a, contributes to extracellular proton-gated vascular responses and exercise tolerance. To determine if ASIC1a contributes to exercise tolerance, we determined peak oxygen (O2) uptake in conscious ASIC1a-/- mice during exhaustive treadmill running. Loss of ASIC1a was associated with a greater peak running speed (60 ± 2 vs. 53 ± 3 m·min-1, P = 0.049) and peak oxygen (O2) uptake during exhaustive treadmill running (9563 ± 120 vs. 8836 ± 276 mL·kg-1·h-1, n = 6-7, P = 0.0082). There were no differences in absolute or relative lean body mass, as determined by EchoMRI. To determine if ASIC1a contributes to vascular responses during muscle contraction, we measured femoral vascular conductance (FVC) during a stepwise electrical stimulation (0.5-5.0 Hz at 3 V for 60 sec) of the left major hind limb muscles. FVC increased to a greater extent in ASIC1a-/- versus ASIC1a+/+ mice (0.44 ± 0.03 vs. 0.30 ± 0.04 mL·min-1·100 g hind limb mass-1 · mmHg-1, n = 5 each, P = 0.0009). Vasodilation following local application of external protons in the spinotrapezius muscle increased the duration, but not the magnitude, of the vasodilatory response in ASIC1a-/- mice. Finally, we examined hind limb vascular density using micro-CT and found increased density of 0-80 μm vessels (P < 0.05). Our findings suggest an increased vascular density and an enhanced vasodilatory response to local protons, to a lesser degree, may contribute to the enhanced vascular conductance and increased peak exercise capacity in ASIC1a-/- mice.

Acid-sensing ion channel (ASIC) structure and function: Insights from spider, snake and sea anemone venoms

Acid-sensing ion channels (ASICs) are proton-activated cation channels that are expressed in a variety of neuronal and non-neuronal tissues. As proton-gated channels, they have been implicated in many pathophysiological conditions where pH is perturbed. Venom derived compounds represent the most potent and selective modulators of ASICs described to date, and thus have been invaluable as pharmacological tools to study ASIC structure, function, and biological roles. There are now ten ASIC modulators described from animal venoms, with those from snakes and spiders favouring ASIC1, while the sea anemones preferentially target ASIC3. Some modulators, such as the prototypical ASIC1 modulator PcTx1 have been studied in great detail, while some of the newer members of the club remain largely unstudied. Here we review the current state of knowledge on venom derived ASIC modulators, with a particular focus on their molecular interaction with ASICs, what they have taught us about channel structure, and what they might still reveal about ASIC function and pathophysiological roles. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Multiple H+ sensors mediate the extracellular acidification-induced [Ca2+]i elevation in cultured rat ventricular cardiomyocytes

Acidosis has been known to cause “Ca²⁺ transients”, however, the mechanism is still uncertain. Here, we demonstrated that multiple H⁺ sensors, such as ASICs, TRPV1 and proton-sensing G protein coupled receptors (GPCRs) are involved in extracellular acidification-induced intracellular calcium ([Ca²⁺]_i) elevation. By using calcium imaging measures, we observed that both ASIC and TRPV1 channels inhibitors suppressed the [Ca²⁺]_i elevation induced by extracellular acidosis in cultured rat cardiac myocytes. Then, both channels mRNA and proteins were identified by RT-PCR, western blotting and immunofluorescence. ASIC-like and TRPV1-like currents were induced by extracellular acidification, suggesting that functional ASIC and TRPV1 channels jointly mediated extracellular calcium entry. Furthermore, either pre-exhaustion of sarcoplasmic reticulum (SR) Ca²⁺ with thapsigargin or IP₃ receptor blocker 2-APB or PLC inhibitor U73122 significantly attenuated the elevation of [Ca²⁺]_i, indicating that the intracellular Ca²⁺ stores and the PLC-IP₃ signaling also contributed to the acidosis-induced elevation of [Ca²⁺]_i. By using genetic and pharmacological approaches, we identified that ovarian cancer G protein-coupled receptor 1 (OGR1) might be another main component in acidosis-induced release of [Ca²⁺]_i. These results suggest that multiple H⁺-sensitive receptors are involved in “Ca²⁺ transients” induced by acidosis in the heart.

Acid-Sensing Ion Channel Pharmacology, Past, Present, and Future …

pH is one of the most strictly controlled parameters in mammalian physiology. An extracellular pH of ~7.4 is crucial for normal physiological processes, and perturbations to this have profound effects on cell function. Acidic microenvironments occur in many physiological and pathological conditions, including inflammation, bone remodeling, ischemia, trauma, and intense synaptic activity. Cells exposed to these conditions respond in different ways, from tumor cells that thrive to neurons that are either suppressed or hyperactivated, often fatally. Acid-sensing ion channels (ASICs) are primary pH sensors in mammals and are expressed widely in neuronal and nonneuronal cells. There are six main subtypes of ASICs in rodents that can form homo- or heteromeric channels resulting in many potential combinations. ASICs are present and activated under all of the conditions mentioned earlier, suggesting that they play an important role in how cells respond to acidosis. Compared to many other ion channel families, ASICs were relatively recently discovered-1997-and there is a substantial lack of potent, subtype-selective ligands that can be used to elucidate their structural and functional properties. In this chapter I cover the history of ASIC channel pharmacology, which began before the proteins were even identified, and describe the current arsenal of tools available, their limitations, and take a glance into the future to predict from where new tools are likely to emerge.

Discovery and molecular interaction studies of a highly stable, tarantula peptide modulator of acid-sensing ion channel 1

Acute pharmacological inhibition of acid-sensing ion channel 1a (ASIC1a) is efficacious in rodent models in alleviating symptoms of neurological diseases such as stroke and multiple sclerosis. Thus, ASIC1a is a promising therapeutic target and selective ligands that modulate it are invaluable research tools and potential therapeutic leads. Spider venoms have provided an abundance of voltage-gated ion channel modulators, however, only one ASIC modulator (PcTx1) has so far been isolated from this source. Here we report the discovery, characterization, and chemical stability of a second spider venom peptide that potently modulates ASIC1a and ASIC1b, and investigate the molecular basis for its subtype selectivity. π-TRTX-Hm3a (Hm3a) is a 37-amino acid peptide isolated from Togo starburst tarantula (Heteroscodra maculata) venom with five amino acid substitutions compared to PcTx1, and is also three residues shorter at the C-terminus. Hm3a pH-dependently inhibited ASIC1a with an IC₅₀ of 1-2 nM and potentiated ASIC1b with an EC₅₀ of 46.5 nM, similar to PcTx1. Using ASIC1a to ASIC1b point mutants in rat ASIC1a revealed that Glu177 and Arg175 in the palm region opposite α-helix 5 play an important role in the Hm3a-ASIC1 interaction and contribute to the subtype-dependent effects of the peptide. Despite its high sequence similarity with PcTx1, Hm3a showed higher levels of stability over 48 h. Overall, Hm3a represents a potent, highly stable tool for the study of ASICs and will be particularly useful when stability in biological fluids is required, for example in long term in vitro cell-based assays and in vivo experiments. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

Expression of acid-sensing ion channels and selection of reference genes in mouse and naked mole rat

Acid-sensing ion channels (ASICs) are a family of ion channels comprised of six subunits encoded by four genes and they are expressed throughout the peripheral and central nervous systems. ASICs have been implicated in a wide range of physiological and pathophysiological processes: pain, breathing, synaptic plasticity and excitotoxicity. Unlike mice and humans, naked mole-rats do not perceive acid as a noxious stimulus, even though their sensory neurons express functional ASICs, likely an adaptation to living in a hypercapnic subterranean environment. Previous studies of ASIC expression in the mammalian nervous system have often not examined all subunits, or have failed to adequately quantify expression between tissues; to date there has been no attempt to determine ASIC expression in the central nervous system of the naked mole-rat. Here we perform a geNorm study to identify reliable housekeeping genes in both mouse and naked mole-rat and then use quantitative real-time PCR to estimate the relative amounts of ASIC transcripts in different tissues of both species. We identify RPL13A (ribosomal protein L13A) and CANX (calnexin), and β-ACTIN and EIF4A (eukaryotic initiation factor 4a) as being the most stably expressed housekeeping genes in mouse and naked mole-rat, respectively. In both species, ASIC3 was most highly expressed in dorsal root ganglia (DRG), and ASIC1a, ASIC2b and ASIC3 were more highly expressed across all brain regions compared to the other subunits. We also show that ASIC4, a proton-insensitive subunit of relatively unknown function, was highly expressed in all mouse tissues apart from DRG and hippocampus, but was by contrast the lowliest expressed ASIC in all naked mole-rat tissues.

Molecular dynamics and functional studies define a hot spot of crystal contacts essential for PcTx1 inhibition of acid-sensing ion channel 1a

BACKGROUND AND PURPOSE

The spider-venom peptide PcTx1 is the most potent and selective inhibitor of acid-sensing ion channel (ASIC) 1a. It has centrally acting analgesic activity and is neuroprotective in rodent models of ischaemic stroke. Understanding the molecular details of the PcTx1 : ASIC1a interaction should facilitate development of therapeutically useful ASIC1a modulators. Previously, we showed that several key pharmacophore residues of PcTx1 reside in a dynamic β-hairpin loop; conclusions confirmed by recent crystal structures of the complex formed between PcTx1 and chicken ASIC1 (cASIC1). Numerous peptide : channel contacts were observed in these crystal structures, but it remains unclear which of these are functionally important.

EXPERIMENTAL APPROACH

We combined molecular dynamics (MD) simulations of the PcTx1 : cASIC1 complex with mutagenesis of PcTx1 and rat ASIC1a.

KEY RESULTS

Crystal structures of the PcTx1 : cASIC1 complex indicated that 15 PcTx1 residues form a total of 57 pairwise intermolecular contacts (<5 Å) with 32 channel residues. MD simulations, however, suggested that about half of these interactions do not persist in solution. Mutation to alanine of only eight of 15 PcTx1 contact residues substantially altered ASIC1a inhibition by PcTx1. Our data reveal that many of the peptide-channel interactions observed in the PcTx1 : cASIC1 crystal structures are not important for PcTx1 inhibition of rat ASIC1a.

CONCLUSIONS AND IMPLICATIONS

We identified the atomic interactions that are critical for PcTx1 inhibition of ASIC1a. Our data highlight the value of combining structural information, MD and functional experiments to obtain detailed insight into the molecular basis of protein : protein interactions.

Knockdown of acid-sensing ion channel 1a (ASIC1a) suppresses disease phenotype in SCA1 mouse model

The mutated ataxin-1 protein in spinocerebellar ataxia 1 (SCA1) targets Purkinje cells (PCs) of the cerebellum and causes progressive ataxia due to loss of PCs and neurons of the brainstem. The exact mechanism of this cellular loss is still not clear. Currently, there are no treatments for SCA1; however, understanding of the mechanisms that regulate SCA1 pathology is essential for devising new therapies for SCA1 patients. We previously established a connection between the loss of intracellular calcium-buffering and calcium-signalling proteins with initiation of neurodegeneration in SCA1 transgenic (Tg) mice. Recently, acid-sensing ion channel 1a (ASIC1a) have been implicated in calcium-mediated toxicity in many brain disorders. Here, we report generating SCA1 Tg mice in the ASIC1a knockout (KO) background and demonstrate that the deletion of ASIC1a gene expression causes suppression of the SCA1 disease phenotype. Loss of the ASIC1a channel in SCA1/ASIC1a KO mice resulted in the improvement of motor deficit and decreased PC degeneration. Interestingly, the expression of the ASIC1 variant, ASIC1b, was upregulated in the cerebellum of both SCA1/ASIC1a KO and ASIC1a KO animals as compared to the wild-type (WT) and SCA1 Tg mice. Further, these SCA1/ASIC1a KO mice exhibited translocation of PC calcium-binding protein calbindin-D28k from the nucleus to the cytosol in young animals, which otherwise have both cytosolic and nuclear localization. Furthermore, in addition to higher expression of calcium-buffering protein parvalbumin, PCs of the older SCA1/ASIC1a KO mice showed a decrease in morphologic abnormalities as compared to the age-matched SCA1 animals. Our data suggest that ASIC1a may be a mediator of SCA1 pathogenesis and targeting ASIC1a could be a novel approach to treat SCA1.

Biophysical properties of acid-sensing ion channels (ASICs)

Acid-sensing ion channels (ASICs) are ligand-gated ion channels that are exquisitely sensitive to extracellular protons and can sense transient as well as sustained acidification. In this review, we will discuss activation and desensitization of ASICs by protons. We show that a linear reaction scheme can reproduce the basic electrophysiological properties of ASICs, including steady-state desensitization. Moreover, we will discuss how a desensitizing receptor can sense sustained acidosis and what we know about the putative proton sensor. We will briefly discuss modulation of proton gating by neuropeptides and small positively charged ligands. Finally, we will review the pore properties of ASICs and their relation to the recently reported crystal structure of the open ASIC pore. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Proton-sensitive cation channels and ion exchangers in ischemic brain injury: new therapeutic targets for stroke?

Ischemic brain injury results from complicated cellular mechanisms. The present therapy for acute ischemic stroke is limited to thrombolysis with the recombinant tissue plasminogen activator (rtPA) and mechanical recanalization. Therefore, a better understanding of ischemic brain injury is needed for the development of more effective therapies. Disruption of ionic homeostasis plays an important role in cell death following cerebral ischemia. Glutamate receptor-mediated ionic imbalance and neurotoxicity have been well established in cerebral ischemia after stroke. However, non-NMDA receptor-dependent mechanisms, involving acid-sensing ion channel 1a (ASIC1a), transient receptor potential melastatin 7 (TRPM7), and Na(+)/H(+) exchanger isoform 1 (NHE1), have recently emerged as important players in the dysregulation of ionic homeostasis in the CNS under ischemic conditions. These H(+)-sensitive channels and/or exchangers are expressed in the majority of cell types of the neurovascular unit. Sustained activation of these proteins causes excessive influx of cations, such as Ca(2+), Na(+), and Zn(2+), and leads to ischemic reperfusion brain injury. In this review, we summarize recent pre-clinical experimental research findings on how these channels/exchangers are regulated in both in vitro and in vivo models of cerebral ischemia. The blockade or transgenic knockdown of these proteins was shown to be neuroprotective in these ischemia models. Taken together, these non-NMDA receptor-dependent mechanisms may serve as novel therapeutic targets for stroke intervention.

Acid-sensing ion channels (ASICs): therapeutic targets for neurological diseases and their regulation

Extracellular acidification occurs not only in pathological conditions such as inflammation and brain ischemia, but also in normal physiological conditions such as synaptic transmission. Acid-sensing ion channels (ASICs) can detect a broad range of physiological pH changes during pathological and synaptic cellular activities. ASICs are voltage-independent, proton-gated cation channels widely expressed throughout the central and peripheral nervous system. Activation of ASICs is involved in pain perception, synaptic plasticity, learning and memory, fear, ischemic neuronal injury, seizure termination, neuronal degeneration, and mechanosensation. Therefore, ASICs emerge as potential therapeutic targets for manipulating pain and neurological diseases. The activity of these channels can be regulated by many factors such as lactate, Zn²⁺, and Phe-Met-Arg-Phe amide (FMRFamide)-like neuropeptides by interacting with the channel’s large extracellular loop. ASICs are also modulated by G protein-coupled receptors such as CB₁ cannabinoid receptors and 5-HT₂. This review focuses on the physiological roles of ASICs and the molecular mechanisms by which these channels are regulated. [BMB Reports 2013; 46(6): 295-304]

Acid-sensing ion channels contribute to neurotoxicity

Acidosis that occurs under pathological conditions not only affects intracellular signaling molecules, but also directly activates a unique family of ligand-gated ion channels: acid-sensing ion channels (ASICs). ASICs are widely expressed throughout the central and peripheral nervous systems and play roles in pain sensation, learning and memory, and fear conditioning. Overactivation of ASICs contributes to neurodegenerative diseases such as ischemic brain/spinal cord injury, multiple sclerosis, Parkinson's disease, and Huntington's disease. Thus, targeting ASICs might be a potential therapeutic strategy for these conditions. This mini-review focuses on the electrophysiology and pharmacology of ASICs and roles of ASICs in neuronal toxicity.

Conformational changes in the lower palm domain of ASIC1a contribute to desensitization and RFamide modulation

Acid-sensing ion channel 1a (ASIC1a) is a proton-gated cation channel that contributes to fear and pain as well as neuronal damage following persistent cerebral acidosis. Neuropeptides can affect acid-induced neuronal injury by altering ASIC1a inactivation and/or steady-state desensitization. Yet, exactly how ASIC1a inactivation and desensitization occur or are modulated by peptides is not completely understood. We found that regions of the extracellular palm domain and the β_11-12 linker are important for inactivation and steady-state desensitization of ASIC1a. The single amino acid substitutions L280C and L415C dramatically enhanced the rate of inactivation and altered the pH-dependence of steady-state desensitization. Further, the use of methanethiosulfonate (MTS) reagents suggests that the lower palm region (L280C) undergoes a conformational change when ASIC1a transitions from closed to desensitized. We determined that L280C also displays an altered response to the RFamide peptide, FRRFamide. Further, the presence of FRRFamide limited MTS modification of L280C. Together, these results indicate a potential role of the lower palm domain in peptide modulation and suggest RFamide-related peptides promote conformational changes within this region. These data provide empirical support for the idea that L280, and likely this region of the central vestibule, is intimately involved in channel inactivation and desensitization.

Venom toxins in the exploration of molecular, physiological and pathophysiological functions of acid-sensing ion channels

Acid-sensing ion channels (ASICs) are voltage-independent proton-gated cation channels that are largely expressed in the nervous system as well as in some non-neuronal tissues. In rodents, six different isoforms (ASIC1a, 1b, 2a, 2b, 3 and 4) can associate into homo- or hetero-trimers to form a functional channel. Specific polypeptide toxins targeting ASIC channels have been isolated from the venoms of spider (PcTx1), sea anemone (APETx2) and snakes (MitTx and mambalgins). They exhibit different and sometimes partially overlapping pharmacological profiles and are usually blockers of ASIC channels, except for MitTx, which is a potent activator. This review focuses on the use of these toxins to explore the structure-function relationships, the physiological and the pathophysiological roles of ASIC channels, illustrating at the same time the therapeutic potential of some of these natural compounds.

Acid-sensing ion channels: trafficking and synaptic function

Extracellular acidification occurs in the brain with elevated neural activity, increased metabolism, and neuronal injury. This reduction in pH can have profound effects on brain function because pH regulates essentially every single biochemical reaction. Therefore, it is not surprising to see that Nature evolves a family of proteins, the acid-sensing ion channels (ASICs), to sense extracellular pH reduction. ASICs are proton-gated cation channels that are mainly expressed in the nervous system. In recent years, a growing body of literature has shown that acidosis, through activating ASICs, contributes to multiple diseases, including ischemia, multiple sclerosis, and seizures. In addition, ASICs play a key role in fear and anxiety related psychiatric disorders. Several recent reviews have summarized the importance and therapeutic potential of ASICs in neurological diseases, as well as the structure-function relationship of ASICs. However, there is little focused coverage on either the basic biology of ASICs or their contribution to neural plasticity. This review will center on these topics, with an emphasis on the synaptic role of ASICs and molecular mechanisms regulating the spatial distribution and function of these ion channels.

Structure and activity of the acid-sensing ion channels

The acid-sensing ion channels (ASICs) are a family of proton-sensing channels expressed throughout the nervous system. Their activity is linked to a variety of complex behaviors including fear, anxiety, pain, depression, learning, and memory. ASICs have also been implicated in neuronal degeneration accompanying ischemia and multiple sclerosis. As a whole, ASICs represent novel therapeutic targets for several clinically important disorders. An understanding of the correlation between ASIC structure and function will help to elucidate their mechanism of action and identify potential therapeutics that specifically target these ion channels. Despite the seemingly simple nature of proton binding, multiple studies have shown that proton-dependent gating of ASICs is quite complex, leading to activation and desensitization through distinct structural components. This review will focus on the structural aspects of ASIC gating in response to both protons and the newly discovered activators GMQ and MitTx. ASIC modulatory compounds and their action on proton-dependent gating will also be discussed. This review is dedicated to the memory of Dale Benos, who made a substantial contribution to our understanding of ASIC activity.

Inhibition of human acid-sensing ion channel 1b by zinc

Acid-sensing ion channel 1b (ASIC1b) is expressed in peripheral sensory neurons and has been implicated in nociception. Understanding the modulation of ASIC1b will provide important insight into how ASIC1b contributes to pain sensation. In our previous study, we showed that zinc, an important modulator of pain sensation, reduces rat ASIC1b current. However, rat ASIC1b shows several important differences from its recently identified human homolog. Most noticeably, human ASIC1b (hASIC1b) has a sustained component, which may play a role in persistent pain. Therefore, we tested here the hypothesis that zinc modulates the current properties of hASIC1b. Bath application of zinc suppressed the peak amplitude of hASIC1b currents, with a half-maximum inhibitory concentration of 37 μM. However, zinc did not affect the sustained component of hASIC1b currents. The effect of zinc was independent of pH-dependent activation, steady-state desensitization, and extracellular Ca(2+), suggesting noncompetitive mechanisms. Further, we found that extracellular site(s) of the hASIC1b subunit is important for the effect of zinc. Mutating cysteine 196, but not cysteine 309, in the extracellular domain of the hASIC1b abolished the zinc inhibition. These results suggest that, through modulating cysteine196, zinc may have a modulatory role in acute pain.

Heteromeric acid-sensing ion channels (ASICs) composed of ASIC2b and ASIC1a display novel channel properties and contribute to acidosis-induced neuronal death

Acid-sensing ion channel (ASIC) subunits associate to form homomeric or heteromeric proton-gated ion channels in neurons throughout the nervous system. The ASIC1a subunit plays an important role in establishing the kinetics of proton-gated currents in the CNS, and activation of ASIC1a homomeric channels induces neuronal death after local acidosis that accompanies cerebral ischemia. The ASIC2b subunit is expressed in the brain in a pattern that overlaps ASIC1a, yet the contribution of ASIC2b has remained elusive. We find that coexpression of ASIC2b with ASIC1a in Xenopus oocytes results in novel proton-gated currents with properties distinct from ASIC1a homomeric channels. In particular, ASIC2b/1a heteromeric channels are inhibited by the nonselective potassium channel blockers tetraethylammonium and barium. In addition, steady-state desensitization is induced at more basic pH values, and Big Dynorphin sensitivity is enhanced in these unique heteromeric channels. Cultured hippocampal neurons show proton-gated currents consistent with ASIC2b contribution, and these currents are lacking in neurons from mice with an ACCN1 (ASIC2) gene disruption. Finally, we find that these ASIC2b/1a heteromeric channels contribute to acidosis-induced neuronal death. Together, our results show that ASIC2b confers unique properties to heteromeric channels in central neurons. Furthermore, these data indicate that ASIC2, like ASIC1, plays a role in acidosis-induced neuronal death and implicate the ASIC2b/1a subtype as a novel pharmacological target to prevent neuronal injury after stroke.

Cysteine 149 in the extracellular finger domain of acid-sensing ion channel 1b subunit is critical for zinc-mediated inhibition

Acid-sensing ion channel 1b (ASIC1b) is a proton-gated Na(+) channel mostly expressed in peripheral sensory neurons. To date, the functional significance of ASIC1b in these cells is unclear due to the lack of a specific inhibitor/blocker. A better understanding of the regulation of ASIC1b may provide a clue for future investigation of its functional importance. One important regulator of acid-sensing ion channels (ASICs) is zinc. In this study, we examined the detailed zinc inhibition of ASIC1b currents and specific amino acid(s) involved in the inhibition. In Chinese hamster ovary (CHO) cells expressing rat ASIC1b subunit, pretreatment with zinc concentration-dependently inhibited the ASIC1b currents triggered by pH dropping from 7.4 to 6.0 with a half-maximum inhibitory concentration of 26 μM. The inhibition of ASIC1b currents by pre-applied zinc was independent of pH, voltage, or extracellular Ca(2+). Further, we showed that the effect of zinc is dependent on the extracellular cysteine, but not histidine residue. Mutating cysteine 149, but not cysteine 58 or cysteine 162, located in the extracellular domain of the ASIC1b subunit abolished the zinc inhibition. These findings suggest that cysteine 149 in the extracellular finger domain of ASIC1b subunit is critical for zinc-mediated inhibition and provide the basis for future mechanistic studies addressing the functional significance of zinc inhibition of ASIC1b.

ASIC3 channels integrate agmatine and multiple inflammatory signals through the nonproton ligand sensing domain

Background

Acid-sensing ion channels (ASICs) have long been known to sense extracellular protons and contribute to sensory perception. Peripheral ASIC3 channels represent natural sensors of acidic and inflammatory pain. We recently reported the use of a synthetic compound, 2-guanidine-4-methylquinazoline (GMQ), to identify a novel nonproton sensing domain in the ASIC3 channel, and proposed that, based on its structural similarity with GMQ, the arginine metabolite agmatine (AGM) may be an endogenous nonproton ligand for ASIC3 channels.

Results

Here, we present further evidence for the physiological correlation between AGM and ASIC3. Among arginine metabolites, only AGM and its analog arcaine (ARC) activated ASIC3 channels at neutral pH in a sustained manner similar to GMQ. In addition to the homomeric ASIC3 channels, AGM also activated heteromeric ASIC3 plus ASIC1b channels, extending its potential physiological relevance. Importantly, the process of activation by AGM was highly sensitive to mild acidosis, hyperosmolarity, arachidonic acid (AA), lactic acid and reduced extracellular Ca²⁺. AGM-induced ASIC3 channel activation was not through the chelation of extracellular Ca²⁺ as occurs with increased lactate, but rather through a direct interaction with the newly identified nonproton ligand sensing domain. Finally, AGM cooperated with the multiple inflammatory signals to cause pain-related behaviors in an ASIC3-dependent manner.

Conclusions

Nonproton ligand sensing domain might represent a novel mechanism for activation or sensitization of ASIC3 channels underlying inflammatory pain-sensing under in vivo conditions.