Stephan Kellenberger

Changes in local proton concentrations within the body occur during synaptic transmission and metabolic activity and along the digestive tract. Perturbations to the strictly controlled physiological tissue pH are often associated with pathological processes. As such, many cell types require the ability to sense and respond to changes in local proton concentrations. Acid-sensing ion channels (ASICs) have been identified in all groups of Bilateria except for nematodes, arthropods, and mollusks. They are part of the epithelial Na⁺ channel/degenerin (ENaC/DEG) family, which encompasses ion channels across a wide range of tissues and organisms. ENaC/DEG channels are constitutively active or can be activated by various stimuli, such as mechanical stimuli or the binding of peptides or protons. ASICs are low pH-activated Na⁺-selective channels, formed by the assembly of three identical or homologous subunits around a central pore. In mammals, ASICs are mostly expressed in the nervous system. Extensive structure-function studies have identified residues involved in ion permeation and in the control and modulation of ASIC activity. The biological functions of ASICs are associated with situations of lowered pH, such as high neuronal activity, ischemia, and inflammation. Experiments with rodents showed roles of ASICs in the expression of fear, learning, pain sensation, and neurodegeneration. This review comprehensively examines ASICs, covering their evolutionary origins, biophysical properties, structure-function relationships, physiological and pathological roles, and their regulation and pharmacology.

The Concise Guide to Pharmacology 2025/26 marks the seventh edition in this series of biennial publications in the British Journal of Pharmacology. Presented in landscape format, the guide provides a comparative overview of the pharmacology of drug target families. The concise nature of the Concise Guide refers to the style of presentation, being clear, accessible, and well‐structured, rather than the scope of the content, which spans approximately 500 pages. The Concise Guide summarises the key pharmacological properties of around 1900 human drug targets, and nearly 7000 interactions, involving around 4400 ligands. While the content is a substantially condensed version of the more detailed information and links available at the www.guidetopharmacology.org website, the printed guide serves as a permanent, citable, point‐in‐time record, that remains stable despite ongoing updates to the online database. The full contents of this publication can be found at https://bpspubs.onlinelibrary.wiley.com/doi/10.1111/bph.70231 . The Concise Guides provide expert‐curated recommendations of ‘Gold Standard’ selective pharmacological tools, available either commercially or as donations, which enable the identification of individual drug targets or families of drug targets. While the Concise Guide offers a more streamlined overview, more comprehensive information, including detailed pharmacological profiles and links to multiple online databases, is available through the Guide to Pharmacology website. The 2025/26 edition of the Concise Guide is based on material current as of mid‐2025, and supersedes all previous editions, including the 2023/24 Guide, and earlier Guides to Receptors and Channels. It is produced in close conjunction with the Nomenclature and Standards Committee of the International Union of Basic and Clinical Pharmacology (NC‐IUPHAR), and as such provides official IUPHAR classification and nomenclature for human drug targets, where applicable. Ion channels are one of the six major pharmacological targets into which the Guide is divided, with the others being: G protein‐coupled receptors, nuclear hormone receptors, catalytic receptors, enzymes and transporters. Each section includes nomenclature guidance, concise summaries, information of the best available pharmacological tools, key references, and suggestions for further reading.

Acid-sensing ion channels (ASICs, nomenclature as agreed by NC-IUPHAR [52, 2, 3]) are members of a Na+ channel superfamily that includes the epithelial Na+ channel (ENaC), the FMRF-amide activated channel (FaNaC) of invertebrates, the degenerins (DEG) of Caenorhabditis elegans, channels in Drosophila melanogaster and the mammalian bile acid-activated ion channel BASIC [94], previously known as BLINaC [75] and INaC [77]. ASIC subunits contain 2 TM domains and a large extracellular part whose shape resembles that of a hand, as shown by high-resolution structures of chicken and human ASIC1a [49, 43, 7, 101, 100, 81]. They assemble as homo- or heterotrimers to form proton-gated, voltage-insensitive, Na+ permeable, channels that are activated by levels of acidosis occurring in both physiological and pathophysiological conditions with ASIC3 also playing a role in mechanosensation (reviewed in [48, 93, 52, 74, 23]). Splice variants of ASIC1 [termed ASIC1a (ASIC, ASICα, BNaC2α) [88], ASIC1b (ASICβ, BNaC2β) [19] and ASIC1b2 (ASICβ2) [83]; note that ASIC1a is also permeable to Ca2+], ASIC2 [termed ASIC2a (MDEG1, BNaC1α, BNC1α) [70, 89, 42] and ASIC2b (MDEG2, BNaC1β) [60]] differ in the first third of the protein. Unlike ASIC2a (listed in table), heterologous expression of ASIC2b alone does not support H+-gated currents. A third member, ASIC3 (DRASIC, TNaC1) [87] is one of the most pH-sensitive isoforms (along with ASIC1a) and has the fastest activation and desensitisation kinetics, however can also carry small sustained currents. ASIC4 (SPASIC) evolved as a proton-sensitive channel but seems to have lost this function in mammals [62]. Mammalian ASIC4 does not support a proton-gated channel in heterologous expression systems but is reported to downregulate the expression of ASIC1a and ASIC3 [1, 47, 35, 58, 24]. ASICs channels are primarily expressed in central (ASIC1a, -2a, 2b and -4) and peripheral neurons including nociceptors (ASIC1-3) where they participate in neuronal sensitivity to acidosis. Humans express, in contrast to rodents, ASIC3 also in the brain [28]. ASICs have also been detected in photoreceptors and retinal cells (ASIC1-3), cochlear hair cells (ASIC1b), testis (hASIC3), pituitary gland (ASIC4), lung epithelial cells (ASIC1a and -3), urothelial cells, adipose cells (ASIC3), vascular smooth muscle cells (ASIC1-3), immune cells (ASIC1,-3 and -4) and bone (ASIC1-3) (ASIC distribution is reviewed in [59, 29, 46]). A neurotransmitter-like function of protons has been suggested, involving postsynaptically located ASICs of the CNS in functions such as learning and fear perception [36, 54, 104] and of the PNS in mechanoreceptor-neurite transmission [98, 97]. ASIC activation also contributes to cell damage in focal ischemia [95, 73] and autoimmune inflammation (arthritis and multiple sclerosis) [41, 96], as well as neuron activation during seizures and pain [93, 30, 31, 13, 33]. Heterologously expressed heteromultimers form ion channels with differences in kinetics, ion selectivity, pH- sensitivity and sensitivity to blockers that resemble some of the native proton activated currents recorded from neurones [60, 5, 39, 11]. In general, the known small molecule inhibitors of ASICs are non-selective or partially selective, whereas the venom peptide inhibitors have substantially higher selectivity and potency. Several clinically used drugs are known to inhibit ASICs, however they are generally more potent at other targets (e.g. amiloride at ENaCs, ibuprofen at COX enzymes) [72, 67]. The information in the tables below are for the effects of inhibitors on homomeric channels, for information of known effects on heteromeric channels see the comments below.

Extracellular acidification opens the Na ⁺ -selective acid-sensing ion channels (ASICs), which are key players in neuronal excitation, learning, pain perception, fear conditioning, and ischemic stroke-induced cell death. Their agonists – protons – can theoretically bind to almost any titratable residue, complicating the study of ASIC’s activation mechanism. To identify proton binding sites, we developed an ensemble-based computational approach combining Poisson–Boltzmann electrostatics with multiple short MD simulations to calculate pKa values across ASIC1a structures of closed, open, and desensitized states. This method preserves local ion environments and captures solvation-sensitive conformational variability. Predicted pH-sensing residues were conservatively mutated and functionally tested. Residues whose mutation altered pH sensitivity were further analyzed via a palette of substitutions and mathematical modeling, revealing the key side-chain features governing their role. This computational and experimental strategy identified H73 of the wrist, K211 and E242 of the acidic pocket and E375, E413 and E418 of the palm as pH sensors for activation, and E242, E375 and E413 as pH sensors for steady-state desensitization, and revealed how residues of these clusters contribute to channel function. Our findings offer a comprehensive map of pH sensing in ASIC1a and introduce a robust method for investigating pH-dependent mechanisms in other proteins.

Acid-sensing ion channels (ASICs, nomenclature as agreed by NC-IUPHAR [52, 2, 3]) are members of a Na+ channel superfamily that includes the epithelial Na+ channel (ENaC), the FMRF-amide activated channel (FaNaC) of invertebrates, the degenerins (DEG) of Caenorhabitis elegans, channels in Drosophila melanogaster and the mammalian bile acid-activated ion channel BASIC [94], previously known as BLINaC [75] and INaC [77]. ASIC subunits contain 2 TM domains and a large extracellular part whose shape resembles that of a hand, as shown by high-resolution structures of chicken and human ASIC1a [49, 43, 7, 101, 100, 81]. They assemble as homo- or heterotrimers to form proton-gated, voltage-insensitive, Na+ permeable, channels that are activated by levels of acidosis occurring in both physiological and pathophysiological conditions with ASIC3 also playing a role in mechanosensation (reviewed in [48, 93, 52, 74, 23]). Splice variants of ASIC1 [termed ASIC1a (ASIC, ASICα, BNaC2α) [88], ASIC1b (ASICβ, BNaC2β) [19] and ASIC1b2 (ASICβ2) [83]; note that ASIC1a is also permeable to Ca2+], ASIC2 [termed ASIC2a (MDEG1, BNaC1α, BNC1α) [70, 89, 42] and ASIC2b (MDEG2, BNaC1β) [60]] differ in the first third of the protein. Unlike ASIC2a (listed in table), heterologous expression of ASIC2b alone does not support H+-gated currents. A third member, ASIC3 (DRASIC, TNaC1) [87] is one of the most pH-sensitive isoforms (along with ASIC1a) and has the fastest activation and desensitisation kinetics, however can also carry small sustained currents. ASIC4 (SPASIC) evolved as a proton-sensitive channel but seems to have lost this function in mammals [62]. Mammalian ASIC4 does not support a proton-gated channel in heterologous expression systems but is reported to downregulate the expression of ASIC1a and ASIC3 [1, 47, 35, 58, 24]. ASICs channels are primarily expressed in central (ASIC1a, -2a, 2b and -4) and peripheral neurons including nociceptors (ASIC1-3) where they participate in neuronal sensitivity to acidosis. Humans express, in contrast to rodents, ASIC3 also in the brain [28]. ASICs have also been detected in photoreceptors and retinal cells (ASIC1-3), cochlear hair cells (ASIC1b), testis (hASIC3), pituitary gland (ASIC4), lung epithelial cells (ASIC1a and -3), urothelial cells, adipose cells (ASIC3), vascular smooth muscle cells (ASIC1-3), immune cells (ASIC1,-3 and -4) and bone (ASIC1-3) (ASIC distribution is reviewed in [59, 29, 46]). A neurotransmitter-like function of protons has been suggested, involving postsynaptically located ASICs of the CNS in functions such as learning and fear perception [36, 54, 104] and of the PNS in mechanoreceptor-neurite transmission [98, 97]. ASIC activation also contributes to cell damage in focal ischemia [95, 73] and autoimmune inflammation (arthritis and multiple sclerosis) [41, 96], as well as neuron activation during seizures and pain [93, 30, 31, 13, 33]. Heterologously expressed heteromultimers form ion channels with differences in kinetics, ion selectivity, pH- sensitivity and sensitivity to blockers that resemble some of the native proton activated currents recorded from neurones [60, 5, 39, 11]. In general, the known small molecule inhibitors of ASICs are non-selective or partially selective, whereas the venom peptide inhibitors have substantially higher selectivity and potency. Several clinically used drugs are known to inhibit ASICs, however they are generally more potent at other targets (e.g. amiloride at ENaCs, ibuprofen at COX enzymes) [72, 67]. The information in the tables below are for the effects of inhibitors on homomeric channels, for information of known effects on heteromeric channels see the comments below.

Ligand-gated ion channels (LGICs) are integral membrane proteins that contain a pore which allows the regulated flow of selected ions across the plasma membrane. Ion flux is passive and driven by the electrochemical gradient for the permeant ions. These channels are open, or gated, by the binding of a neurotransmitter to an orthosteric site(s) that triggers a conformational change that results in the conducting state. Modulation of gating can occur by the binding of endogenous, or exogenous, modulators to allosteric sites. LGICs mediate fast synaptic transmission, on a millisecond time scale, in the nervous system and at the somatic neuromuscular junction. Such transmission involves the release of a neurotransmitter from a pre-synaptic neurone and the subsequent activation of post-synaptically located receptors that mediate a rapid, phasic, electrical signal (the excitatory, or inhibitory, post-synaptic potential). However, in addition to their traditional role in phasic neurotransmission, it is now established that some LGICs mediate a tonic form of neuronal regulation that results from the activation of extra-synaptic receptors by ambient levels of neurotransmitter. The expression of some LGICs by non-excitable cells is suggestive of additional functions.By convention, the LGICs comprise the excitatory, cation-selective, nicotinic acetylcholine [959, 210], 5-HT3 [68, 1441], ionotropic glutamate [856, 1375] and P2X receptors [659, 1330] and the inhibitory, anion-selective, GABAA [1066, 83] and glycine receptors [878, 1539]. The nicotinic acetylcholine, 5-HT3, GABAA and glycine receptors (and an additional zinc-activated channel) are pentameric structures and are frequently referred to as the Cys-loop receptors due to the presence of a defining loop of residues formed by a disulphide bond in the extracellular domain of their constituent subunits [966, 1357]. However, the prokaryotic ancestors of these receptors contain no such loop and the term pentameric ligand-gated ion channel (pLGIC) is gaining acceptance in the literature [573]. The ionotropic glutamate and P2X receptors are tetrameric and trimeric structures, respectively. Multiple genes encode the subunits of LGICs and the majority of these receptors are heteromultimers. Such combinational diversity results, within each class of LGIC, in a wide range of receptors with differing pharmacological and biophysical properties and varying patterns of expression within the nervous system and other tissues. The LGICs thus present attractive targets for new therapeutic agents with improved discrimination between receptor isoforms and a reduced propensity for off-target effects. The development of novel, faster screening techniques for compounds acting on LGICs [359] will greatly aid in the development of such agents.

Pseudohypoaldosteronism type 1 (PHA-1) is a rare genetic disease caused by aldosterone resistance, characterized by severe sodium loss, hyperkalemia, dehydration, and vomiting. The Epithelial Na⁺ Channel (ENaC) is a cation channel that constitutes the rate-limiting step of transepithelial Na⁺ transport in many tissues and regulates blood volume and pressure. Mutations in any of its subunits (α, β, or γ) have been shown to cause PHA-1B. The present investigation is a case study of a 4-month-old female born to consanguineous parents with symptoms suggestive of a form of PHA-1. The child presented with failure to thrive, accompanied by mild hyponatremia and hyperkalemia, together with a normal anion gap metabolic acidosis. Whole exome sequencing, conducted to identify genetic variants, revealed a variant of uncertain significance, the homozygous missense mutation c.1594G > A, p. Gly532Ser in the SCNN1G gene, associated with PHA-1B3. To investigate the functional impact of this mutation, in vitro electrophysiological and biochemical studies were performed with wild type αβγ and mutant αβγG532S-ENaC. This analysis showed that the γG532S mutation reduced, but did not suppress ENaC expression and activity. The functional observation explains the mild phenotype of this novel SCNN1G mutation, which contrasts with the typically severe presentation of autosomal recessive PHA-1B. In our case, the patient showed a positive clinical response to sodium chloride supplementation alone. These findings suggest that certain missense mutations in SCNN1G may result in a milder disease course, underscoring the importance of functional studies in understanding genotype–phenotype correlations in PHA-1.

Volume-regulated anion channels (VRACs) are multimeric proteins composed of different paralogs of the LRRC8 family. They are activated in response to hypotonic swelling, but little is known about their specific functions. We studied two human individuals with the same congenital syndrome affecting blood vessels, brain, eyes, and bones. The LRRC8C gene harbored de novo variants in both patients, located in a region of the gene encoding the boundary between the pore and a cytoplasmic domain, which is depleted of sequence variations in control subjects. When studied by cryo-EM, both LRRC8C mutant proteins assembled as their wild-type counterparts, but showed increased flexibility, suggesting a destabilization of subunit interactions. When co-expressed with the obligatory LRRC8A subunit, the mutants exhibited enhanced activation, resulting in channel activity even at isotonic conditions in which wild-type channels are closed. We conclude that structural perturbations of LRRC8C impair channel gating and constitute the mechanistic basis of the dominant gain-of-function effect of these pathogenic variants. The pleiotropic phenotype of this novel clinical entity associated with monoallelic LRRC8C variants indicates the fundamental roles of VRACs in different tissues and organs.

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 channels (ASICs) are neuronal Na ⁺ -permeable ion channels activated by extracellular acidification. ASICs are involved in learning, fear sensing, pain sensation and neurodegeneration. Increasing the extracellular Ca ²⁺ concentration decreases the H ⁺ sensitivity of ASIC1a, suggesting a competition for binding sites between H ⁺ and Ca ²⁺ ions. Here, we predicted candidate residues for Ca ²⁺ binding on ASIC1a, based on available structural information and our molecular dynamics simulations. With functional measurements, we identified several residues in cavities previously associated with pH-dependent gating, whose mutation reduced the modulation by extracellular Ca ²⁺ of the ASIC1a pH dependence of activation and desensitization. This occurred probably owing to a disruption of Ca ²⁺ binding. Our results link one of the two predicted Ca ²⁺ -binding sites in each ASIC1a acidic pocket to the modulation of channel activation. Mg ²⁺ regulates ASICs in a similar way as does Ca ²⁺ . We show that Mg ²⁺ shares some of the binding sites with Ca ²⁺ . Finally, we provide evidence that some of the ASIC1a Ca ²⁺ -binding sites are functionally conserved in the splice variant ASIC1b. Our identification of divalent cation-binding sites in ASIC1a shows how Ca ²⁺ affects ASIC1a gating, elucidating a regulatory mechanism present in many ion channels.