Outlines of the pore in open and closed conformations describe the gating mechanism of ASIC1

Voltage-clamp fluorometry for advancing mechanistic understanding of ion channel mechanisms with a focus on acid-sensing ion channels

Voltage-clamp fluorometry (VCF) has revolutionized the study of ion channels by combining electrophysiology with fluorescence spectroscopy. VCF allows ion channel researchers to link dynamic structural changes, measured in real time, to function. Acid-sensing ion channels (ASICs) are Na+-permeable non-voltage-gated ion channels of the central and peripheral nervous system. They function as pH sensors, triggering neuronal excitation when pH decreases. Animal studies have shown the importance of ASICs for pain and fear sensation, learning, and neurodegeneration following ischaemic stroke. This review explores the technical bases and various developments of VCF, including fluorescence resonance energy transfer and the use of unnatural fluorescent amino acids. We provide an overview of VCF applications with a focus on ASICs, detailing how VCF has unveiled proton-induced conformational changes in key regions such as the acid pocket, wrist, and pore, crucial for understanding transitions between closed, open, and desensitized states.

Acid-sensing ion channel 1a modulation of apoptosis in acidosis-related diseases: implications for therapeutic intervention

Acid-sensing ion channel 1a (ASIC1a), a prominent member of the acid-sensing ion channel (ASIC) superfamily activated by extracellular protons, is ubiquitously expressed throughout the human body, including the nervous system and peripheral tissues. Excessive accumulation of Ca2+ ions via ASIC1a activation may occur in the acidified microenvironment of blood or local tissues. ASIC1a-mediated Ca2+‑induced apoptosis has been implicated in numerous pathologies, including neurological disorders, cancer, and rheumatoid arthritis. This review summarizes the role of ASIC1a in the modulation of apoptosis via various signaling pathways across different disease states to provide insights for future studies on the underlying mechanisms and development of therapeutic strategies.

Physiological insight into the conserved properties of Caenorhabditis elegans acid-sensing degenerin/epithelial sodium channels

Acid-sensing ion channels (ASICs) are members of the diverse family of degenerin/epithelial sodium channels (DEG/ENaCs). They perform a wide range of physiological roles in healthy organisms, including in gut function and synaptic transmission, but also play important roles in disease, as acidosis is a hallmark of painful inflammatory and ischaemic conditions. We performed a screen for acid sensitivity on all 30 subunits of the Caenorhabditis elegans DEG/ENaC family using two-electrode voltage clamp in Xenopus oocytes. We found two groups of acid-sensitive DEG/ENaCs characterised by being either inhibited or activated by increasing proton concentrations. Three of these acid-sensitive C. elegans DEG/ENaCs were activated by acidic pH, making them functionally similar to the vertebrate ASICs. We also identified three new members of the acid-inhibited DEG/ENaC group, giving a total of seven additional acid-sensitive channels. We observed sensitivity to the anti-hypertensive drug amiloride as well as modulation by the trace element zinc. Acid-sensitive DEG/ENaCs were found to be expressed in both neurons and non-neuronal tissue, highlighting the likely functional diversity of these channels. Our findings provide a framework to exploit the C. elegans channels as models to study the function of these acid-sensing channels in vivo, as well as to study them as potential targets for anti-helminthic drugs. KEY POINTS: Acidosis plays many roles in healthy physiology, including synaptic transmission and gut function, but is also a key feature of inflammatory pain, ischaemia and many other conditions. Cells monitor acidosis of their surroundings via pH-sensing channels, including the acid-sensing ion channels (ASICs). These are members of the degenerin/epithelial sodium channel (DEG/ENaC) family, along with, as the name suggests, vertebrate ENaCs and degenerins of the roundworm Caenorhabditis elegans. By screening all 30 C. elegans DEG/ENaCs for pH dependence, we describe, for the first time, three acid-activated members, as well as three additional acid-inhibited channels. We surveyed both groups for sensitivity to amiloride and zinc; like their mammalian counterparts, their currents can be blocked, enhanced or unaffected by these modulators. Likewise, they exhibit diverse ion selectivity. Our findings underline the diversity of acid-sensitive DEG/ENaCs across species and provide a comparative resource for better understanding the molecular basis of their function.

A flexible GAS belt responds to pore mutations changing the ion selectivity of proton-gated channels

Proton-gated ion channels conduct mainly Na+ to induce postsynaptic membrane depolarization. Finding the determinants of ion selectivity requires knowledge of the pore structure in the open conformation, but such information is not yet available. Here, the open conformation of the hASIC1a channel was computationally modeled, and functional effects of pore mutations were analyzed in light of the predicted structures. The open pore structure shows two constrictions of similar diameter formed by the backbone of the GAS belt and, right beneath it, by the side chains of H28 from the reentrant loop. Models of nonselective mutant channels, but not those that maintain ion selectivity, predict enlargement of the GAS belt, suggesting that this motif is quite flexible and that the loss of stabilizing interactions in the central pore leads to changes in size/shape of the belt. Our results are consistent with the "close-fit" mechanism governing selectivity of hASIC1a, wherein the backbone of the GAS substitutes at least part of the hydration shell of a permeant ion to enable crossing the pore constriction.

Ion Selectivity in the ENaC/DEG Family: A Systematic Review with Supporting Analysis

The Epithelial Sodium Channel/Degenerin (ENaC/DEG) family is a superfamily of sodium-selective channels that play diverse and important physiological roles in a wide variety of animal species. Despite their differences, they share a high homology in the pore region in which the ion discrimination takes place. Although ion selectivity has been studied for decades, the mechanisms underlying this selectivity for trimeric channels, and particularly for the ENaC/DEG family, are still poorly understood. This systematic review follows PRISMA guidelines and aims to determine the main components that govern ion selectivity in the ENaC/DEG family. In total, 27 papers from three online databases were included according to specific exclusion and inclusion criteria. It was found that the G/SxS selectivity filter (glycine/serine, non-conserved residue, serine) and other well conserved residues play a crucial role in ion selectivity. Depending on the ion type, residues with different properties are involved in ion permeability. For lithium against sodium, aromatic residues upstream of the selectivity filter seem to be important, whereas for sodium against potassium, negatively charged residues downstream of the selectivity filter seem to be important. This review provides new perspectives for further studies to unravel the mechanisms of ion selectivity.

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

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

Determinants of ion selectivity in ASIC1a- and ASIC2a-containing acid-sensing ion channels

Trimeric acid-sensing ion channels (ASICs) contribute to neuronal signaling by converting extracellular acidification into excitatory sodium currents. Previous work with homomeric ASIC1a implicates conserved leucine (L7') and consecutive glycine-alanine-serine (GAS belt) residues near the middle, and conserved negatively charged (E18') residues at the bottom of the pore in ion permeation and/or selectivity. However, a conserved mechanism of ion selectivity throughout the ASIC family has not been established. We therefore explored the molecular determinants of ion selectivity in heteromeric ASIC1a/ASIC2a and homomeric ASIC2a channels using site-directed mutagenesis, electrophysiology, and molecular dynamics free energy simulations. Similar to ASIC1a, E18' residues create an energetic preference for sodium ions at the lower end of the pore in ASIC2a-containing channels. However, and in contrast to ASIC1a homomers, ion permeation through ASIC2a-containing channels is not determined by L7' side chains in the upper part of the channel. This may be, in part, due to ASIC2a-specific negatively charged residues (E59 and E62) that lower the energy of ions in the upper pore, thus making the GAS belt more important for selectivity. This is confirmed by experiments showing that the L7'A mutation has no effect in ASIC2a, in contrast to ASIC1a, where it eliminated selectivity. ASIC2a triple mutants eliminating both L7' and upper charges did not lead to large changes in selectivity, suggesting a different role for L7' in ASIC2a compared with ASIC1a channels. In contrast, we observed measurable changes in ion selectivity in ASIC2a-containing channels with GAS belt mutations. Our results suggest that ion conduction and selectivity in the upper part of the ASIC pore may differ between subtypes, whereas the essential role of E18' in ion selectivity is conserved. Furthermore, we demonstrate that heteromeric channels containing mutations in only one of two ASIC subtypes provide a means of functionally testing mutations that render homomeric channels nonfunctional.

Animal, Herb, and Microbial Toxins for Structural and Pharmacological Study of Acid-Sensing Ion Channels

Acid-sensing ion channels (ASICs) are of the most sensitive molecular sensors of extracellular pH change in mammals. Six isoforms of these channels are widely represented in membranes of neuronal and non-neuronal cells, where these molecules are involved in different important regulatory functions, such as synaptic plasticity, learning, memory, and nociception, as well as in various pathological states. Structural and functional studies of both wild-type and mutant ASICs are essential for human care and medicine for the efficient treatment of socially significant diseases and ensure a comfortable standard of life. Ligands of ASICs serve as indispensable tools for these studies. Such bioactive compounds can be synthesized artificially. However, to date, the search for such molecules has been most effective amongst natural sources, such as animal venoms or plants and microbial extracts. In this review, we provide a detailed and comprehensive structural and functional description of natural compounds acting on ASICs, as well as the latest information on structural aspects of their interaction with the channels. Many of the examples provided in the review demonstrate the undoubted fundamental and practical successes of using natural toxins. Without toxins, it would not be possible to obtain data on the mechanisms of ASICs' functioning, provide detailed study of their pharmacological properties, or assess the contribution of the channels to development of different pathologies. The selectivity to different isoforms and variety in the channel modulation mode allow for the appraisal of prospective candidates for the development of new drugs.

The His-Gly motif of acid-sensing ion channels resides in a reentrant 'loop' implicated in gating and ion selectivity

Acid-sensing ion channels (ASICs) are proton-gated members of the epithelial sodium channel/degenerin (ENaC/DEG) superfamily of ion channels and are expressed throughout the central and peripheral nervous systems. The homotrimeric splice variant ASIC1a has been implicated in nociception, fear memory, mood disorders and ischemia. Here, we extract full-length chicken ASIC1 (cASIC1) from cell membranes using styrene maleic acid (SMA) copolymer, elucidating structures of ASIC1 channels in both high pH resting and low pH desensitized conformations by single-particle cryo-electron microscopy (cryo-EM). The structures of resting and desensitized channels reveal a reentrant loop at the amino terminus of ASIC1 that includes the highly conserved 'His-Gly' (HG) motif. The reentrant loop lines the lower ion permeation pathway and buttresses the 'Gly-Ala-Ser' (GAS) constriction, thus providing a structural explanation for the role of the His-Gly dipeptide in the structure and function of ASICs.

Acid-sensing ion channels in sensory signaling

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

Identification of TMEM206 proteins as pore of PAORAC/ASOR acid-sensitive chloride channels

Acid-sensing ion channels have important functions in physiology and pathology, but the molecular composition of acid-activated chloride channels had remained unclear. We now used a genome-wide siRNA screen to molecularly identify the widely expressed acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR. ASOR is formed by TMEM206 proteins which display two transmembrane domains (TMs) and are expressed at the plasma membrane. Ion permeation-changing mutations along the length of TM2 and at the end of TM1 suggest that these segments line ASOR’s pore. While not belonging to a gene family, TMEM206 has orthologs in probably all vertebrates. Currents from evolutionarily distant orthologs share activation by protons, a feature essential for ASOR’s role in acid-induced cell death. TMEM206 defines a novel class of ion channels. Its identification will help to understand its physiological roles and the diverse ways by which anion-selective pores can be formed.

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

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

The mechanosensitive Piezo1 channel: structural features and molecular bases underlying its ion permeation and mechanotransduction

The evolutionarily conserved Piezo family of proteins, including Piezo1 and Piezo2, encodes the long-sought-after mammalian mechanosensitive cation channels that play critical roles in various mechanotransduction processes such as touch, pain, proprioception, vascular development and blood pressure regulation. Mammalian Piezo proteins contain over 2500 amino acids with numerous predicted transmembrane segments, and do not bear sequence homology with any known class of ion channels. Thus, it is imperative, but challenging, to understand how they serve as effective mechanotransducers for converting mechanical force into electrochemical signals. Here, we review the recent major breakthroughs in determining the three-bladed, propeller-shaped structure of mouse Piezo1 using the state-of-the-art cryo-electron microscopy (cryo-EM) and functionally dissecting out the molecular bases that define its ion permeation and mechanotransduction properties, which provide key insights into clarifying its oligomeric status and pore-forming region. We also discuss the hypothesis that the complex Piezo proteins can be deduced into discrete mechanotransduction and ion-conducting pore modules, which coordinate to fulfil their specialized function in mechanical sensing and transduction, ion permeation and selection.

Diminazene Is a Slow Pore Blocker of Acid-Sensing Ion Channel 1a (ASIC1a)

Acid-sensing ion channels (ASICs) are neuronal receptors for extracellular protons. They contribute to the excitatory postsynaptic current and to the detection of painful acidosis. Moreover, they are activated during peripheral inflammation and acidosis associated with various neuronal disorders, such as stroke and neuroinflammation, rendering them interesting drug targets. Diminazene aceturate is a small-molecule inhibitor of ASICs with a reported apparent affinity in the low micromolar range, making it an interesting lead compound. It was reported that diminazene accelerates desensitization of ASICs, which was, however, not explained mechanistically. Furthermore, a binding site in a groove of the extracellular domain was proposed but not experimentally verified. In this study, we revisited the mechanism of inhibition by diminazene and its binding site on ASIC1a, the ASIC subunit with the greatest importance in the central nervous system. We show that diminazene slowly blocks ASIC1a, leading to the apparent acceleration of desensitization and underestimating its potency; we show that diminazene indeed has a submicromolar potency at ASIC1a (IC₅₀ 0.3 μM). Moreover, we show that the inhibition is voltage-dependent and competes with that by amiloride, a pore blocker of ASICs. Finally, we identify by molecular docking a binding site in the ion pore that we confirm by site-directed mutagenesis. In summary, our results show that diminazene blocks ASIC1a by a slow open-channel block and suggest that diminazene is an interesting lead compound for high-affinity blockers of ASICs.

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.

Modulation of the FMRFamide-gated Na+ channel by external Ca2

FMRFamide-gated Na⁺ channel (FaNaC) is a member of the DEG/ENaC family. Amino acid sequence of the second transmembrane region (TM2) of FaNaC is quite similar to that of the acid-sensing ion channels (ASIC) of the same family. In the upper part of TM2, there are two aspartate residues (D552 and D556 in Aplysia FaNaC, AkFaNaC) which construct two negative rings in the external vestibule. In the present study, we examined the function of D552/D556 mutants of AkFaNaC in Xenopus oocytes with special interest in Ca²⁺ sensitivity of FaNaC. The FMRFamide-evoked current through AkFaNaC was depressed by submillimolar Ca²⁺ such that the current in Ca²⁺-free condition was 2-3-fold larger than that in the control solution which contained 1.8 mM CaCl ₂. Both D552 and D556 were found to be indispensable for the sensitivity of FaNaC to submillimolar Ca²⁺. Unexpectedly, however, both acidic residues were not essential for the inhibition by millimolar Ca²⁺ concentrations. The Ca²⁺-sensitive gating of FaNaC was recapitulated by an allosteric model in which Ca²⁺-bound channels are more difficult to open. The desensitization of FaNaC was also inhibited by Ca²⁺, which was abolished in some D552/D556 mutants. Structural models of FaNaC made by homology modeling showed that the distance between oxygen atoms of D552 and D556 on the adjacent subunits is close enough to coordinate Ca²⁺ in the nonconducting desensitized channel but not in the open channel. The results suggest that Ca²⁺ coordination between oxygen atoms of D552 and D556 disturbs the opening transition as well as the desensitization of FaNaC.

A selectivity filter at the intracellular end of the acid-sensing ion channel pore

Increased extracellular proton concentrations during neurotransmission are converted to excitatory sodium influx by acid-sensing ion channels (ASICs). 10-fold sodium/potassium selectivity in ASICs has long been attributed to a central constriction in the channel pore, but experimental verification is lacking due to the sensitivity of this structure to conventional manipulations. Here, we explored the basis for ion selectivity by incorporating unnatural amino acids into the channel, engineering channel stoichiometry and performing free energy simulations. We observed no preference for sodium at the “GAS belt” in the central constriction. Instead, we identified a band of glutamate and aspartate side chains at the lower end of the pore that enables preferential sodium conduction.

DOI:http://dx.doi.org/10.7554/eLife.24630.001

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.'

In Touch With the Mechanosensitive Piezo Channels: Structure, Ion Permeation, and Mechanotransduction

Mechanotransduction, the conversion of mechanical forces into biological signals, plays critical roles in various physiological and pathophysiological processes in mammals, such as conscious sensing of touch, pain, and sound, as well as unconscious sensing of blood flow-associated shear stress, urine flow, and bladder distention. Among the various molecules involved in mechanotransduction, mechanosensitive (MS) cation channels have long been postulated to represent one critical class of mechanotransducers that directly and rapidly converts mechanical force into electrochemical signals. Despite the awareness of their functional significance, the molecular identities of MS cation channels in mammals had remained elusive for decades till the groundbreaking finding that the Piezo family of genes, including Piezo1 and Piezo2, constitutes their essential components. Since their identification about 6years ago, tremendous progress has been made in understanding their physiological and pathophysiological importance in mechanotransduction and their structure-function relationships of being the prototypic class of mammalian MS cation channels. On the one hand, Piezo proteins have been demonstrated to serve as physiologically and pathophysiologically important mechanotransducers for most, if not all, mechanotransduction processes. On the other hand, they have been shown to form a remarkable three-bladed, propeller-shaped homotrimeric channel complex comprising a separable ion-conducting pore module and mechanotransduction modules. In this chapter, we review the major advancements, with a particular focus on the structural and biophysical features that enable Piezo proteins to serve as sophisticated MS cation channels for force sensing, transduction, and ion conduction.

Ca2+ permeability and Na+ conductance in cellular toxicity caused by hyperactive DEG/ENaC channels

Hyperactivated DEG/ENaC channels cause neuronal death mediated by intracellular Ca²⁺ overload. Mammalian ASIC1a channels and MEC-4(d) neurotoxic channels in Caenorhabditis elegans both conduct Na⁺ and Ca²⁺, raising the possibility that direct Ca²⁺ influx through these channels contributes to intracellular Ca²⁺ overload. However, we showed that the homologous C. elegans DEG/ENaC channel UNC-8(d) is not Ca²⁺ permeable, yet it is neurotoxic, suggesting that Na⁺ influx is sufficient to induce cell death. Interestingly, UNC-8(d) shows small currents due to extracellular Ca²⁺ block in the Xenopus oocyte expression system. Thus, MEC-4(d) and UNC-8(d) differ both in current amplitude and Ca²⁺ permeability. Given that these two channels show a striking difference in toxicity, we wondered how Na⁺ conductance vs. Ca²⁺ permeability contributes to cell death. To address this question, we built an UNC-8/MEC-4 chimeric channel that retains the calcium permeability of MEC-4 and characterized its properties in Xenopus oocytes. Our data support the hypothesis that for Ca²⁺-permeable DEG/ENaC channels, both Ca²⁺ permeability and Na⁺ conductance contribute to toxicity. However, for Ca²⁺-impermeable DEG/ENaCs (e.g., UNC-8), our evidence shows that constitutive Na⁺ conductance is sufficient to induce toxicity, and that this effect is enhanced as current amplitude increases. Our work further refines the contribution of different channel properties to cellular toxicity induced by hyperactive DEG/ENaC channels.

pH-Regulated Nonelectrogenic Anion Transport by Phenylthiosemicarbazones

Gated ion transport across biological membranes is an intrinsic process regulated by protein channels. Synthetic anion carriers (anionophores) have potential applications in biological research; however, previously reported examples are mostly nonspecific, capable of mediating both electrogenic and electroneutral (nonelectrogenic) transport processes. Here we show the transmembrane Cl(-) transport studies of synthetic phenylthiosemicarbazones mimicking the function of acid-sensing (proton-gated) ion channels. These anionophores have remarkable pH-switchable transport properties with up to 640-fold increase in transport efficacy on going from pH 7.2 to 4.0. This "gated" process is triggered by protonation of the imino nitrogen and concomitant conformational change of the anion-binding thiourea moiety from anti to syn. By using a combination of two cationophore-coupled transport assays, with either monensin or valinomycin, we have elucidated the fundamental transport mechanism of phenylthiosemicarbazones which is shown to be nonelectrogenic, inseparable H(+)/Cl(-) cotransport. This study demonstrates the first examples of pH-switchable nonelectrogenic anion transporters.

Epithelial sodium channel (ENaC) family: Phylogeny, structure-function, tissue distribution, and associated inherited diseases

The epithelial sodium channel (ENaC) is composed of three homologous subunits and allows the flow of Na(+) ions across high resistance epithelia, maintaining body salt and water homeostasis. ENaC dependent reabsorption of Na(+) in the kidney tubules regulates extracellular fluid (ECF) volume and blood pressure by modulating osmolarity. In multi-ciliated cells, ENaC is located in cilia and plays an essential role in the regulation of epithelial surface liquid volume necessary for cilial transport of mucus and gametes in the respiratory and reproductive tracts respectively. The subunits that form ENaC (named as alpha, beta, gamma and delta, encoded by genes SCNN1A, SCNN1B, SCNN1G, and SCNN1D) are members of the ENaC/Degenerin superfamily. The earliest appearance of ENaC orthologs is in the genomes of the most ancient vertebrate taxon, Cyclostomata (jawless vertebrates) including lampreys, followed by earliest representatives of Gnathostomata (jawed vertebrates) including cartilaginous sharks. Among Euteleostomi (bony vertebrates), Actinopterygii (ray finned-fishes) branch has lost ENaC genes. Yet, most animals in the Sarcopterygii (lobe-finned fish) branch including Tetrapoda, amphibians and amniotes (lizards, crocodiles, birds, and mammals), have four ENaC paralogs. We compared the sequences of ENaC orthologs from 20 species and established criteria for the identification of ENaC orthologs and paralogs, and their distinction from other members of the ENaC/Degenerin superfamily, especially ASIC family. Differences between ENaCs and ASICs are summarized in view of their physiological functions and tissue distributions. Structural motifs that are conserved throughout vertebrate ENaCs are highlighted. We also present a comparative overview of the genotype-phenotype relationships in inherited diseases associated with ENaC mutations, including multisystem pseudohypoaldosteronism (PHA1B), Liddle syndrome, cystic fibrosis-like disease and essential hypertension.

Ion Permeation and Mechanotransduction Mechanisms of Mechanosensitive Piezo Channels

Piezo proteins have been proposed as the long-sought-after mechanosensitive cation channels in mammals that play critical roles in various mechanotransduction processes. However, the molecular bases that underlie their ion permeation and mechanotransduction have remained functionally undefined. Here we report our finding of the miniature pore-forming module of Piezo1 that resembles the pore architecture of other trimeric channels and encodes the essential pore properties. We further identified specific residues within the pore module that determine unitary conductance, pore blockage and ion selectivity for divalent and monovalent cations and anions. The non-pore-containing region of Piezo1 confers mechanosensitivity to mechano-insensitive trimeric acid-sensing ion channels, demonstrating that Piezo1 channels possess intrinsic mechanotransduction modules separate from their pore modules. In conclusion, this is the first report on the bona fide pore module and mechanotransduction components of Piezo channels, which define their ion-conducting properties and gating by mechanical stimuli, respectively.

Acid-sensing ion channels and their modulators

According to a modern look acid-sensing ion channels (ASICs) are one of the most important receptors that perceive pH change in the body. ASICs represent proton-gated Na+-selective channels, which are expressed in neurons of the central and peripheral nervous system. These channels are attracting attention of researchers around the world, as they are involved in various physiological processes in the body. Drop of pH may occur in tissues in norm (e.g. the accumulation of lactic acid, the release of protons upon ATP hydrolysis) and pathology (inflammation, ischemic stroke, tissue damage and seizure). These processes are accompanied by unpleasant pain sensations, which may be short-lived or can lead to chronic inflammatory diseases. Modulators of ASIC channels activity are potential candidates for new effective analgesic and neuroprotection drugs. This review summarizes available information about structure, function, and physiological role of ASIC channels. In addition a description of all known ligands of these channels and their practical relevance is provided.

The Human Acid-Sensing Ion Channel ASIC1a: Evidence for a Homotetrameric Assembly State at the Cell Surface

The chicken acid-sensing ion channel ASIC1 has been crystallized as a homotrimer. We address here the oligomeric state of the functional ASIC1 in situ at the cell surface. The oligomeric states of functional ASIC1a and mutants with additional cysteines introduced in the extracellular pore vestibule were resolved on SDS-PAGE. The functional ASIC1 complexes were stabilized at the cell surface of Xenopus laevis oocytes or CHO cells either using the sulfhydryl crosslinker BMOE, or sodium tetrathionate (NaTT). Under these different crosslinking conditions ASIC1a migrates as four distinct oligomeric states that correspond by mass to multiples of a single ASIC1a subunit. The relative importance of each of the four ASIC1a oligomers was critically dependent on the availability of cysteines in the transmembrane domain for crosslinking, consistent with the presence of ASIC1a homo-oligomers. The expression of ASIC1a monomers, trimeric or tetrameric concatemeric cDNA constructs resulted in functional channels. The resulting ASIC1a complexes are resolved as a predominant tetramer over the other oligomeric forms, after stabilization with BMOE or NaTT and SDS-PAGE/western blot analysis. Our data identify a major ASIC1a homotetramer at the surface membrane of the cell expressing functional ASIC1a channel.

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'.

Architectural and functional similarities between trimeric ATP-gated P2X receptors and acid-sensing ion channels

ATP-gated P2X receptors and acid-sensing ion channels are two distinct ligand-gated ion channels that assemble into trimers. They are involved in many important physiological functions such as pain sensation and are recognized as important therapeutic targets. They have unrelated primary structures and respond to different ligands (ATP and protons) and are thus considered as two different ion channels. As a consequence, comparisons of the biophysical properties and underlying mechanisms have only been rarely made between these two channels. However, the recent determination of their molecular structures by X-ray crystallography has revealed unexpected parallels in the architecture of the two pores, providing a basis for possible functional analogies. In this review, we analyze the structural and functional similarities that are shared by these trimeric ion channels, and we outline key unanswered questions that, if addressed experimentally, may help us to elucidate how two unrelated ion channels have adopted a similar fold of the pore.

Conserved charged residues at the surface and interface of epithelial sodium channel subunits--roles in cell surface expression and the sodium self-inhibition response

The epithelial sodium channel (ENaC) is composed of three homologous subunits that form a triangular pyramid-shaped funnel, anchored in the membrane with a stem of six transmembrane domains. We examined the structure-function relationships of 17 conserved charged residues on the surface of the ectodomain of human γ-ENaC subunit by alanine mutagenesis and co-expression with α- and β-ENaC subunits in Xenopus oocytes. The results showed that Na(+) conductance of cells expressing these mutants can be accounted for by two parameters: (a) the ENaC density on the cell surface as measured by the fluorescence of an α-EnaC-yellow fluorescent protein hybrid and (b) the sodium self-inhibition (SSI) response that reflects the open probability of the channel (Po). Overall, the activity of all 17 mutants was correlated with surface levels of ENaC. There was no significant correlation between these parameters measured for α- and γ-ENaC subunit mutants at nine homologous positions. Thus, the functions of most of the homologous surface residues examined differ between the two subunits. Only four mutants (K328, D510, R514 and E518) significantly reduced the SSI response. The α-ENaC homologs of three of these (R350, E530 and E538) also severely affected the SSI response. The cASIC1 homologs of these (K247, E417, Q421) are located at the interface between subunits, on or about the ion pathway at the rotational symmetry axis in the center of the trimer. Thus, it is likely that these residues are involved in conformational changes that lead to channel constriction and the SSI response upon Na(+) ion flooding.

X-ray structure of acid-sensing ion channel 1-snake toxin complex reveals open state of a Na(+)-selective channel

Acid-sensing ion channels (ASICs) detect extracellular protons produced during inflammation or ischemic injury and belong to the superfamily of degenerin/epithelial sodium channels. Here, we determine the cocrystal structure of chicken ASIC1a with MitTx, a pain-inducing toxin from the Texas coral snake, to define the structure of the open state of ASIC1a. In the MitTx-bound open state and in the previously determined low-pH desensitized state, TM2 is a discontinuous α helix in which the Gly-Ala-Ser selectivity filter adopts an extended, belt-like conformation, swapping the cytoplasmic one-third of TM2 with an adjacent subunit. Gly 443 residues of the selectivity filter provide a ring of three carbonyl oxygen atoms with a radius of ∼3.6 Å, presenting an energetic barrier for hydrated ions. The ASIC1a-MitTx complex illuminates the mechanism of MitTx action, defines the structure of the selectivity filter of voltage-independent, sodium-selective ion channels, and captures the open state of an ASIC.

Protonation controls ASIC1a activity via coordinated movements in multiple domains

Acid-sensing ion channels (ASICs) are neuronal Na(+)-conducting channels activated by extracellular acidification. ASICs are involved in pain sensation, expression of fear, and neurodegeneration after ischemic stroke. Functional ASICs are composed of three identical or homologous subunits, whose extracellular part has a handlike structure. Currently, it is unclear how protonation of residues in extracellular domains controls ASIC activity. Knowledge of these mechanisms would allow a rational development of drugs acting on ASICs. Protonation may induce conformational changes that control the position of the channel gate. We used voltage-clamp fluorometry with fluorophores attached to residues in different domains of ASIC1a to detect conformational changes. Comparison of the timing of fluorescence and current signals identified residues involved in movements that preceded desensitization and may therefore be associated with channel opening or early steps leading to desensitization. Other residues participated in movements intimately linked to desensitization and recovery from desensitization. Fluorescence signals of all mutants were detected at more alkaline pH than ionic currents. Their midpoint of pH dependence was close to that of steady-state desensitization, whereas the steepness of the pH fluorescence relationship was closer to that of current activation. A sequence of movements was observed upon acidification, and its backward movements during recovery from desensitization occurred in the reverse order, indicating that the individual steps are interdependent. Furthermore, the fluorescence signal of some labeled residues in the finger domain was strongly quenched by a Trp residue in the neighboring β-ball domain. Upon channel activation, their fluorescence intensity increased, indicating that the finger moved away from the β ball. This extensive analysis of activity-dependent conformational changes in ASICs sheds new light on the mechanisms by which protonation controls ASIC activity.

Proton-mediated conformational changes in an acid-sensing ion channel

Acid-sensing ion channels are cation channels activated by external protons and play roles in nociception, synaptic transmission, and the physiopathology of ischemic stroke. Using luminescence resonance energy transfer (LRET), we show that upon proton binding, there is a conformational change that increases LRET efficiency between the probes at the thumb and finger subdomains in the extracellular domain of acid-sensing ion channels. Additionally, we show that this conformational change is lost upon mutating Asp-238, Glu-239, and Asp-260, which line the finger domains, to alanines. Electrophysiological studies showed that the single mutant D260A shifted the EC50 by 0.2 pH units, the double mutant D238A/E239A shifted the EC50 by 2.5 pH units, and the triple mutant D238A/E239A/D260A exhibited no response to protons despite surface expression. The LRET experiments on D238A/E239A/D260A showed no changes in LRET efficiency upon reduction in pH from 8 to 6. The LRET and electrophysiological studies thus suggest that the three carboxylates, two of which are involved in carboxyl/carboxylate interactions, are essential for proton-induced conformational changes in the extracellular domain, which in turn are necessary for receptor activation.

Acid-sensing ion channels under hypoxia

Hypoxia represents the lack of oxygen below the basic level, and the range of known channels related to hypoxia is continually increasing. Since abnormal hypoxia initiates pathological processes in numerous diseases via, to a great degree, producing acidic microenvironment, the significance of these channels in this environment has, until now, remained completely unknown. However, recent discovery of acid-sensing ion channels (ASICs) have enhanced our understanding of the hypoxic channelome. They belong to the degenerin/epithelial Na (+) channel family and function once extracellular pH decreases to a certain level. So does the ratiocination emerge that ASICs participate in many hypoxia-induced pathological processes, including pain, apoptosis, malignancy, which all appear to involve them. Since evidence suggests that activity of ASICs is altered under pathological hypoxia, future studies are needed to deeply explore the relationship between ASICs and hypoxia, which may provide a progressive understanding of hypoxic effects in cancer, arthritis, intervertebral disc degeneration, ischemic brain injury and so on.

High Ca(2+) permeability of a peptide-gated DEG/ENaC from Hydra

Degenerin/epithelial Na⁺ channels (DEG/ENaCs) are Na⁺ channels that are blocked by the diuretic amiloride. In general, they are impermeable for Ca²⁺ or have a very low permeability for Ca²⁺. We describe here, however, that a DEG/ENaC from the cnidarian Hydra magnipapillata, the Hydra Na⁺ channel (HyNaC), is highly permeable for Ca²⁺ (P_Ca/P_Na = 3.8). HyNaC is directly gated by Hydra neuropeptides, and in Xenopus laevis oocytes expressing HyNaCs, RFamides elicit currents with biphasic kinetics, with a fast transient component and a slower sustained component. Although it was previously reported that the sustained component is unselective for monovalent cations, the selectivity of the transient component had remained unknown. Here, we show that the transient current component arises from secondary activation of the Ca²⁺-activated Cl⁻ channel (CaCC) of Xenopus oocytes. Inhibiting the activation of the CaCC leads to a simple on–off response of peptide-activated currents with no apparent desensitization. In addition, we identify a conserved ring of negative charges at the outer entrance of the HyNaC pore that is crucial for the high Ca²⁺ permeability, presumably by attracting divalent cations to the pore. At more positive membrane potentials, the binding of Ca²⁺ to the ring of negative charges increasingly blocks HyNaC currents. Thus, HyNaC is the first member of the DEG/ENaC gene family with a high Ca²⁺ permeability.

Insight into DEG/ENaC channel gating from genetics and structure

The founding members of the superfamily of DEG/ENaC ion channel proteins are C. elegans proteins that form mechanosensitive channels in touch and pain receptors. For more than a decade, the research community has used mutagenesis to identify motifs that regulate gating. This review integrates insight derived from unbiased in vivo mutagenesis screens with recent crystal structures to develop new models for activation of mechanically gated DEGs.

Toxin binding reveals two open state structures for one acid-sensing ion channel

Of the three principal conformations of acid-sensing ion channels (ASICs)—closed, open and desensitized—only the atomic structure of the desensitized conformation had been known. Two recent papers report the crystal structure of chicken ASIC1 in complex with the spider toxin psalmotoxin 1, and one of these studies finds that, depending on the pH, channels are in two different open conformations. Compared with the desensitized conformation, toxin binding induces only subtle structural changes in the lower part of the large extracellular domain but a complete rearrangement of the two transmembrane domains (TMDs), suggesting that desensitization gating (the transition from open to desensitized) is mainly associated with conformational rearrangements of the TMDs. Moreover, the study reveals how two different arrangements of the TMDs in the open state give rise to ion pores with different selectivity for monovalent cations.

Subtype-specific modulation of acid-sensing ion channel (ASIC) function by 2-guanidine-4-methylquinazoline

Acid-sensing ion channels (ASICs) are neuronal Na(+)-selective channels that are transiently activated by extracellular acidification. ASICs are involved in fear and anxiety, learning, neurodegeneration after ischemic stroke, and pain sensation. The small molecule 2-guanidine-4-methylquinazoline (GMQ) was recently shown to open ASIC3 at physiological pH. We have investigated the mechanisms underlying this effect and the possibility that GMQ may alter the function of other ASICs besides ASIC3. GMQ shifts the pH dependence of activation to more acidic pH in ASIC1a and ASIC1b, whereas in ASIC3 this shift goes in the opposite direction and is accompanied by a decrease in its steepness. GMQ also induces an acidic shift of the pH dependence of inactivation of ASIC1a, -1b, -2a, and -3. As a consequence, the activation and inactivation curves of ASIC3 but not other ASICs overlap in the presence of GMQ at pH 7.4, thereby creating a window current. At concentrations >1 mM, GMQ decreases maximal peak currents by reducing the unitary current amplitude. Mutation of residue Glu-79 in the palm domain of ASIC3, previously shown to be critical for channel opening by GMQ, disrupted the GMQ effects on inactivation but not activation. This suggests that this residue is involved in the consequences of GMQ binding rather than in the binding interaction itself. This study describes the mechanisms underlying the effects of a novel class of ligands that modulate the function of all ASICs as well as activate ASIC3 at physiological pH.

Structural plasticity and dynamic selectivity of acid-sensing ion channel-spider toxin complexes

Acid-sensing ion channels (ASICs) are voltage-independent, amiloride-sensitive channels involved in diverse physiological processes ranging from nociception to taste. Despite the importance of ASICs in physiology, we know little about the mechanism of channel activation. Here we show that psalmotoxin activates non-selective and Na(+)-selective currents in chicken ASIC1a at pH 7.25 and 5.5, respectively. Crystal structures of ASIC1a-psalmotoxin complexes map the toxin binding site to the extracellular domain and show how toxin binding triggers an expansion of the extracellular vestibule and stabilization of the open channel pore. At pH 7.25 the pore is approximately 10 Å in diameter, whereas at pH 5.5 the pore is largely hydrophobic and elliptical in cross-section with dimensions of approximately 5 by 7 Å, consistent with a barrier mechanism for ion selectivity. These studies define mechanisms for activation of ASICs, illuminate the basis for dynamic ion selectivity and provide the blueprints for new therapeutic agents.

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.

Gating mechanism of a P2X4 receptor developed from normal mode analysis and molecular dynamics simulations

P2X receptors are trimeric ATP-gated cation channels participating in diverse physiological processes. How ATP binding triggers channel opening remains unclear. Here the gating mechanism of a P2X receptor was studied by normal mode analysis and molecular dynamics (MD) simulations. Based on the resting-state crystal structure, a normal mode involving coupled motions of three β-strands (β1, β13, and β14) at the trimeric interface of the ligand-binding ectodomain and the pore-lining helix (TM2) in the transmembrane domain (TMD) was identified. The resulting widening of the fenestrations above the TMD and opening of the transmembrane pore produce known signatures of channel activation. In MD simulations, ATP was initially placed in the putative binding pocket (defined by four charged residues located in β1, β13 and β14) in two opposite orientations, with the adenine either proximal or distal to the TMD. In the proximal orientation, the triphosphate group extends outward to draw in the four charged residues, leading to closure of β13/β14 toward β1. The adenine ring, wedged between β1 and β13, acts as a fulcrum for the β14 lever, turning a modest closure around the triphosphate group into significant opening of the pre-TM2 loop. The motions of these β-strands are similar to those in the putative channel-activation normal mode. In the distal orientation, the ATP stabilizes the trimeric interface and the closure of the pre-TM2 loop, possibly representing desensitization. Our computational studies produced the first complete model, supported by experimental data, for how ATP binding triggers activation of a P2X receptor.

Contribution of residues in second transmembrane domain of ASIC1a protein to ion selectivity

Acid-sensing ion channels (ASICs) are proton-gated cation-selective channels expressed in the peripheral and central nervous systems. The ion permeation pathway of ASIC1a is defined by residues 426-450 in the second transmembrane (TM2) segment. The gate, formed by the intersection of the TM2 segments, localizes near the extracellular boundary of the plasma membrane. We explored the contribution to ion permeation and selectivity of residues in the TM2 segment of ASIC1a. Studies of accessibility with positively charged methanethiosulfonate reagents suggest that the permeation pathway in the open state constricts below the gate, restricting the passage to large ions. Substitution of residues in the intracellular vestibule at positions 437, 438, 443, or 446 significantly increased the permeability to K(+) versus Na(+). ASIC1a shows a selectivity sequence for alkali metals of Na(+)>Li(+)>K(+)≫Rb(+)>Cs(+). Alanine and cysteine substitutions at position 438 increased, to different extents, the relative permeability to Li(+), K(+), Rb(+), and Cs(+). For these mutants, ion permeation was not a function of the diameter of the nonhydrated ion, suggesting that Gly-438 encompasses an ion coordination site that is essential for ion selectivity. M437C and A443C mutants showed slightly increased permeability to K(+), Rb(+), and Cs(+), suggesting that substitutions at these positions influence ion discrimination by altering molecular sieving. Our results indicate that ion selectivity is accomplished by the contribution of multiple sites in the pore of ASIC1a.