Proton-binding sites of acid-sensing ion channel 1

Acid-sensing ion channels (ASICs) are proton-gated cation channels that exist throughout the mammalian central and peripheral nervous systems. ASIC1 is the most abundant of all the ASICs and is likely to modulate synaptic transmission. Identifying the proton-binding sites of ASCI1 is required to elucidate its pH-sensing mechanism. By using the crystal structure of ASIC1, the protonation states of each titratable site of ASIC1 were calculated by solving the Poisson-Boltzmann equation under conditions wherein the protonation states of all these sites are simultaneously in equilibrium. Four acidic-acidic residue pairs--Asp238-Asp350, Glu220-Asp408, Glu239-Asp346, and Glu80-Glu417--were found to be highly protonated. In particular, the Glu80-Glu417 pair in the inner pore was completely protonated and possessed 2 H(+), implying its possible importance as a proton-binding site. The pK(a) of Glu239, which forms a pair with a possible pH-sensing site Asp346, differs among each homo-trimer subunit due to the different H-bond pattern of Thr237 in the different protein conformations of the subunits. His74 possessed a pK(a) of ≈6-7. Conservation of His74 in the proton-sensitive ASIC3 that lacks a residue corresponding to Asp346 may suggest its possible pH-sensing role in proton-sensitive ASICs.     

Ion conduction and selectivity in acid-sensing ion channel 1

The ability of acid-sensing ion channels (ASICs) to discriminate among cations was assessed based on changes in conductance and reversal potential with ion substitution. Human ASIC1a was expressed in Xenopus laevis oocytes, and acid-induced currents were measured using two-electrode voltage clamp. Replacement of extracellular Na⁺ with Li⁺, K⁺, Rb⁺, or Cs⁺ altered inward conductance and shifted the reversal potentials consistent with a selectivity sequence of Li ∼ Na > K > Rb > Cs. Permeability decreased more rapidly than conductance as a function of atomic size, with P_K/P_Na = 0.1 and G_K/G_Na = 0.7 and P_Rb/P_Na = 0.03 and G_Rb/G_Na = 0.3. Stimulation of Cl⁻ currents when Na⁺ was replaced with Ca²⁺, Sr²⁺, or Ba²⁺ indicated a finite permeability to divalent cations. Inward conductance increased with extracellular Na⁺ in a hyperbolic manner, consistent with an apparent affinity (K_m) for Na⁺ conduction of 25 mM. Nitrogen-containing cations, including NH₄⁺, NH₃OH⁺, and guanidinium, were also permeant. In addition to passing through the channels, guanidinium blocked Na⁺ currents, implying competition for a site within the pore. The role of negative charges in an external vestibule of the pore was evaluated using the point mutation D434N. The mutant channel had a decreased single-channel conductance, measured in excised outside-out patches, and a macroscopic slope conductance that increased with hyperpolarization. It had a weakened interaction with Na⁺ (K_m = 72 mM) and a selectivity that was shifted toward larger atomic sizes. We conclude that the selectivity of ASIC1 is based at least in part on interactions with binding sites both within and internal to the outer vestibule.     

Insight toward epithelial Na+ channel mechanism revealed by the acid-sensing ion channel 1 structure

The epithelial Na(+) channel/degenerin (ENaC/DEG) protein family includes a diverse group of ion channels, including nonvoltage-gated Na(+) channels of epithelia and neurons, and the acid-sensing ion channel 1 (ASIC1). In mammalian epithelia, ENaC helps regulate Na(+) and associated water transport, making it a critical determinant of systemic blood pressure and pulmonary mucosal fluidity. In the nervous system, ENaC/DEG proteins are related to sensory transduction. While the importance and physiological function of these ion channels are established, less is known about their structure. One hallmark of the ENaC/DEG channel family is that each channel subunit has only two transmembrane domains connected by an exceedingly large extracellular loop. This subunit structure was recently confirmed when Jasti and colleagues determined the crystal structure of chicken ASIC1, a neuronal acid-sensing ENaC/DEG channel. By mapping ENaC to the structural coordinates of cASIC1, as we do here, we hope to provide insight toward ENaC structure. ENaC, like ASIC1, appears to be a trimeric channel containing 1alpha, 1beta, and 1gamma subunit. Heterotrimeric ENaC and monomeric ENaC subunits within the trimer possibly contain many of the major secondary, tertiary, and quaternary features identified in cASIC1 with a few subtle but critical differences. These differences are expected to have profound effects on channel behavior. In particular, they may contribute to ENaC insensitivity to acid and to its constitutive activity in the absence of time- and ligand-dependent inactivation. Experiments resulting from this comparison of cASIC1 and ENaC may help clarify unresolved issues related to ENaC architecture, and may help identify secondary structures and residues critical to ENaC function.     

Insights into the mechanism of pore opening of acid-sensing ion channel 1a

Acid-sensing ion channels (ASICs) are trimeric cation channels that undergo activation and desensitization in response to extracellular acidification. The underlying mechanism coupling proton binding in the extracellular region to pore gating is unknown. Here we probed the reactivity toward methanethiosulfonate (MTS) reagents of channels with cysteine-substituted residues in the outer vestibule of the pore of ASIC1a. We found that positively-charged MTS reagents trigger pore opening of G428C. Scanning mutagenesis of residues in the region preceding the second transmembrane spanning domain indicated that the MTSET-modified side chain of Cys at position 428 interacts with Tyr-424. This interaction was confirmed by double-mutant cycle analysis. Strikingly, Y424C-G428C monomers were associated by intersubunit disulfide bonds and were insensitive to MTSET. Despite the spatial constraints introduced by these intersubunit disulfide bonds in the outer vestibule of the pore, Y424C-G428C transitions between the resting, open, and desensitized states in response to extracellular acidification. This finding suggests that the opening of the ion conductive pathway involves coordinated rotation of the second transmembrane-spanning domains.     

A nonproton ligand sensor in the acid-sensing ion channel

Acid-sensing ion channels (ASICs) have long been considered as extracellular proton (H(+))-gated cation channels, and peripheral ASIC3 channels seem to be a natural sensor of acidic pain. Here, we report the identification of a nonproton sensor on ASIC3. We show first that 2-guanidine-4-methylquinazoline (GMQ) causes persistent ASIC3 channel activation at the normal pH. Using GMQ as a probe and combining mutagenesis and covalent modification analysis, we then uncovered a ligand sensor lined by residues around E423 and E79 of the extracellular "palm" domain of the ASIC3 channel that is crucial for activation by nonproton activators. Furthermore, we show that GMQ activates sensory neurons and causes pain-related behaviors in an ASIC3-dependent manner, indicating the functional significance of ASIC activation by nonproton ligands. Thus, natural ligands beyond protons may activate ASICs under physiological and pathological conditions through the nonproton ligand sensor, serving for channel activation independent of abrupt and marked acidosis.     

Effects of autophagy on acid-sensing ion channel 1a-mediated apoptosis in rat articular chondrocytes

The incidence of spinal cord injuries (SCI) is high every year. As the spinal cord is the highway that allows for the brain to control the rest of the body, spinal cord injuries greatly impact the quality of life of the patients. The SCI include the primary response consisting of the initial accident-induced damage and the secondary response that is characterized by damage due to inflammation and biological responses. Astrocytes are the first to act at the site of the injury, forming a glial scar and attracting immune cells. The immune system plays a role in cleaning out the debris caused by the injury, as well as preventing neurons to grow and heal. The secondary injury caused by the inflammatory response is the major target to combat SCI. This article critically reviews the key players in the inflammatory SCI response and potential therapies, specifically targeting astrocytes, neutrophils, and macrophages. These cells are both beneficial and detrimental following SCI, depending on the released molecules and the types of cells infiltrated to the site of injury. Indeed, depending on the subtype of macrophages, M1 or M2, beneficial or detrimental response could be incited. Therapeutic strategies to regulate and manipulate the immune cells via increasing or decreasing their recruitment to the site of injury could be developed together with upregulating and downregulating the release of certain chemicals from the infiltrated cells.     

Trypsin cleaves acid-sensing ion channel 1a in a domain that is critical for channel gating

Acid-sensing ion channels (ASICs) are neuronal Na(+) channels that are members of the epithelial Na(+) channel/degenerin family and are transiently activated by extracellular acidification. ASICs in the central nervous system have a modulatory role in synaptic transmission and are involved in cell injury induced by acidosis. We have recently demonstrated that ASIC function is regulated by serine proteases. We provide here evidence that this regulation of ASIC function is tightly linked to channel cleavage. Trypsin cleaves ASIC1a with a similar time course as it changes ASIC1a function, whereas ASIC1b, whose function is not modified by trypsin, is not cleaved. Trypsin cleaves ASIC1a at Arg-145, in the N-terminal part of the extracellular loop, between a highly conserved sequence and a sequence that is critical for ASIC1a inhibition by the venom of the tarantula Psalmopoeus cambridgei. This channel domain controls the inactivation kinetics and co-determines the pH dependence of ASIC gating. It undergoes a conformational change during inactivation, which renders the cleavage site inaccessible to trypsin in inactivated channels.     

Molecular and functional characterization of acid-sensing ion channel (ASIC) 1b.

Acid-sensing ion channels (ASICs) are activated by extracellular protons and are involved in neurotransmission in the central nervous system, in pain perception, as well as in mechanotransduction. Six different ASIC subunits have been cloned to date, which are encoded by four genes (ASIC1-ASIC4). Proton-gated currents have been described in isolated neurons from sensory ganglia as well as from central nervous system. However, it is largely unclear which of the cloned ASIC subunits underlie these native proton-gated currents. Recently, a splice variant, ASIC-beta, has been described for ASIC1a. In this variant about one-third of the protein is exchanged at the N terminus. Here we show that ASIC-beta has a longer N terminus than previously reported, extending the sequence divergence between ASIC1a and this new variant (ASIC1b). We investigated in detail kinetic and selectivity properties of ASIC1b in comparison to ASIC1a. Kinetics is similar for ASIC1b and ASIC1a. Ca(2+) permeability of ASIC1a is low, whereas ASIC1b is impermeable to Ca(2+). Currents through ASIC1a resemble currents, which have been described in sensory and central neurons, whereas the significance of ASIC1b remains to be established. Moreover, we show that a pre-transmembrane 1 domain controls the permeability to divalent cations in ASIC1, contributing to our understanding of the pore structure of these channels.     

Acidosis induces RIPK1-dependent death of glioblastoma stem cells via acid-sensing ion channel 1a

Eliciting regulated cell death, like necroptosis, is a potential cancer treatment. However, pathways eliciting necroptosis are poorly understood. It has been reported that prolonged activation of acid-sensing ion channel 1a (ASIC1a) induces necroptosis in mouse neurons. Glioblastoma stem cells (GSCs) also express functional ASIC1a, but whether prolonged activation of ASIC1a induces necroptosis in GSCs is unknown. Here we used a tumorsphere formation assay to show that slight acidosis (pH 6.6) induces necrotic cell death in a manner that was sensitive to the necroptosis inhibitor Nec-1 and to the ASIC1a antagonist PcTx1. In addition, genetic knockout of ASIC1a rendered GSCs resistant to acid-induced reduction in tumorsphere formation, while the ASIC1 agonist MitTx1 reduced tumorsphere formation also at neutral pH. Finally, a 20 amino acid fragment of the ASIC1 C-terminus, thought to interact with the necroptosis kinase RIPK1, was sufficient to reduce the formation of tumorspheres. Meanwhile, the genetic knockout of MLKL, the executive protein in the necroptosis cascade, did not prevent a reduction in tumor sphere formation, suggesting that ASIC1a induced an alternative cell death pathway. These findings demonstrate that ASIC1a is a death receptor on GSCs that induces cell death during prolonged acidosis. We propose that this pathway shapes the evolution of a tumor in its acidic microenvironment and that pharmacological activation of ASIC1a might be a potential new strategy in tumor therapy.Subject terms: Cancer stem cells, Cancer microenvironment, CNS cancer     

Protons and Psalmotoxin-1 reveal nonproton ligand stimulatory sites in chicken acid-sensing ion channel

Acid-sensing ion channels (ASICs) are proton-sensitive, sodium-selective channels expressed in the nervous system that sense changes in extracellular pH. These ion channels are sensitive to an increasing number of nonproton ligands that include natural venom peptides and guanidine compounds. In the case of chicken ASIC1, the spider toxin Psalmotoxin-1 (PcTx1) activates the channel, resulting in an inward current. Furthermore, a growing class of ligands containing a guanidine group has been identified that stimulate peripheral ASICs (ASIC3), but exert subtle influence on other ASIC subtypes. The effects of the guanidine compounds on cASIC1 have not been the focus of previous study. Here, we investigated the interaction of the guanidine compound 2-guanidine-4-methylquinazoline (GMQ) on cASIC1 proton activation and PcTx1 stimulation. Exposure of expressed cASIC1 to PcTx1 resulted in biphasic currents consisting of a transient peak followed by an irreversible cASIC1 PcTx1 persistent current. This cASIC1 PcTx1 persistent current may be the result of locking the cASIC1 protein into a desensitized transition state. The guanidine compound GMQ increased the apparent affinity of protons on cASIC1 and decreased the half-maximal constant of the cASIC1 steady-state desensitization profile. Furthermore, GMQ stimulated the cASIC1 PcTx1 persistent current in a concentration-dependent manner, which resulted in a non-desensitizing inward current. Our data suggests that GMQ may have multiple sites within cASIC1 and may act as a “molecular wedge” that forces the PcTx1-desensitized ASIC into an open state. Our findings indicate that guanidine compounds, such as GMQ, may alter acid-sensing ion channel activity in combination with other stimuli, and that additional ASIC subtypes (along with ASIC3) may serve to sense and mediate signals from multiple stimuli.     

Candidate amino acids involved in H+ gating of acid-sensing ion channel 1a

Acid-sensing ion channels are ligand-gated cation channels, gated by extracellular H(+). H(+) is the simplest ligand possible, and whereas for larger ligands that gate ion channels complex binding sites in the three-dimensional structure of the proteins have to be assumed, H(+) could in principle gate a channel by titration of a single amino acid. Experimental evidence suggests a more complex situation, however. For example, it has been shown that extracellular Ca(2+) ions compete with H(+); probably Ca(2+) ions bound to the extracellular loop of ASICs stabilize the closed state of the channel and have to be displaced before the channel can open. In such a scheme, amino acids contributing to Ca(2+) binding would also be candidates contributing to H(+) gating. In this study we systematically screened more than 40 conserved, charged amino acids in the extracellular region of ASIC1a for a possible contribution to H(+) gating. We identified four amino acids where substitution strongly affects H(+) gating: Glu(63), His(72)/His(73), and Asp(78). These amino acids are highly conserved among H(+)-sensitive ASICs and are candidates for the "H(+) sensor" of ASICs.     

The interaction between two extracellular linker regions controls sustained opening of acid-sensing ion channel 1

Activation of acid-sensing ion channels (ASICs) contributes to neuronal death during stroke, to axonal degeneration during neuroinflammation, and to pain during inflammation. Although understanding ASIC gating may help to modulate ASIC activity during these pathologic situations, at present it is poorly understood. The ligand, H(+), probably binds to several sites, among them amino acids within the large extracellular domain. The extracellular domain is linked to the two transmembrane domains by the wrist region that is connected to two anti-parallel β-strands, β1 and β12. Thus, the wrist region together with those β-strands may have a crucial role in transmitting ligand binding to pore opening and closing. Here we show that amino acids in the β1-β2 linker determine constitutive opening of ASIC1b from shark. The most crucial residue within the β1-β2 linker (Asp(110)), when mutated from aspartate to cysteine, can be altered by cysteine-modifying reagents much more readily when channels are closed than when they are desensitized. Finally, engineering of a cysteine at position 110 and at an adjacent position in the β11-β12 linker leads to spontaneous formation of a disulfide bond that traps the channel in the desensitized conformation. Collectively, our results suggest that the β1-β2 and β11-β12 linkers are dynamic during gating and tightly appose to each other during desensitization gating. Hindrance of this tight apposition leads to reopening of the channel. It follows that the β1-β2 and β11-β12 linkers modulate gating movements of ASIC1 and may thus be drug targets to modulate ASIC activity.     

Calcium-permeable acid-sensing ion channel is a molecular target of the neurotoxic metal ion lead

Acid-sensing ion channels (ASICs) are emerging as fundamental players in the regulation of neural plasticity and in pathological conditions. Here we showed that lead (Pb2+), a well known neurotoxic metal ion, reversibly and concentration-dependently inhibited ASIC currents in the acutely dissociated spinal dorsal horn and hippocampal CA1 neurons of rats. In vitro expression of ASIC subunits in combination demonstrated that both ASIC1 and -3 subunits were sensitive to Pb2+. Mechanistically, Pb2+ reduced the pH sensitivity of ASICs independent of membrane voltage change. Moreover, Pb2+ inhibited the ASIC-mediated membrane depolarization and the elevation of intracellular Ca2+ concentration. In addition, we compared the effect of Pb2+ with that of Ca2+ or amiloride to explore the possible interactions of Pb2+ and Ca2+ in regulating ASICs, and we found that Pb2+ inhibited ASIC currents independent of the amiloride/Ca2+ blockade. Because ASIC1b and -3 subunits are mainly expressed in peripheral neurons, our data identified ASIC1a-containing Ca2+-permeable ASIC as a novel central target of Pb2+ action, which may contribute to Pb2+ neurotoxicity.     

Regulation of the epithelial sodium channel by serine proteases in human airways.

The epithelial sodium channel (ENaC) constitutes the rate-limiting step for sodium absorption across airway epithelia, which in turn regulates airway surface liquid (ASL) volume and the efficiency of mucociliary clearance. This role in ASL volume regulation suggests that ENaC activity is influenced by local factors rather than systemic signals indicative of total body volume homeostasis. Based on reports that ENaC may be regulated by extracellular serine protease activity in Xenopus and mouse renal epithelia, we sought to identify proteases that serve similar functions in human airway epithelia. Homology screening of a human airway epithelial cDNA library identified two trypsin-like serine proteases (prostasin and TMPRSS2) that, as revealed by in situ hybridization, are expressed in airway epithelia. Functional studies in the Xenopus oocyte expression system demonstrated that prostasin increased ENaC currents 60--80%, whereas TMPRSS2 markedly decreased ENaC currents and protein levels. Studies of primary nasal epithelial cultures in Ussing chambers revealed that inhibition of endogenous serine protease activity with aprotinin markedly decreased ENaC-mediated currents and sensitized the epithelia to subsequent channel activation by exogenous trypsin. These data, therefore, suggest that protease-mediated regulation of sodium absorption is a function of human airway epithelia, and prostasin is a likely candidate for this activity.     

Mutation of a conserved glutamine residue does not abolish desensitization of acid-sensing ion channel 1

Desensitization is a common feature of ligand-gated ion channels, although the molecular cause varies widely between channel types. Mutations that greatly reduce or nearly abolish desensitization have been described for many ligand-gated ion channels, including glutamate, GABA, glycine, and nicotinic receptors, but not for acid-sensing ion channels (ASICs) until recently. Mutating Gln276 to a glycine (Q276G) in human ASIC1a was reported to mostly abolish desensitization at both the macroscopic and the single channel levels, potentially providing a valuable tool for subsequent studies. However, we find that in both human and chicken ASIC1, the effect of Q276G is modest. In chicken ASIC1, the equivalent Q277G slightly reduces desensitization when using pH 6.5 as a stimulus but desensitizes, essentially like wild-type, when using more acidic pH values. In addition, steady-state desensitization is intact, albeit right-shifted, and recovery from desensitization is accelerated. Molecular dynamics simulations indicate that the Gln277 side chain participates in a hydrogen bond network that might stabilize the desensitized conformation. Consistent with this, destabilizing this network with the Q277N or Q277L mutations largely mimics the Q277G phenotype. In human ASIC1a, the Q276G mutation also reduces desensitization, but not to the extent reported previously. Interestingly, the kinetic consequences of Q276G depend on the human variant used. In the common G212 variant, Q276G slows desensitization, while in the rare D212 variant desensitization accelerates. Our data reveal that while the Q/G mutation does not abolish or substantially impair desensitization as previously reported, it does point to unexpected differences between chicken and human ASICs and the need for careful scrutiny before using this mutation in future studies.     

Modeling ion channel regulation

Remarkable recent successes in structure determinations of voltage-gated channels, ligand-gated channels, mechanosensitive channels and proton channels have advanced our understanding of the molecular basis of ion channel gating substantially. Models have helped to clarify aspects of this process and are now being designed as sophisticated biomimetics for various technological applications.     

Annexin II light chain p11 promotes functional expression of acid-sensing ion channel ASIC1a

Acid-sensing ion channels (ASICs) have been implicated in a wide variety of physiological functions. We have used a rat dorsal root ganglion cDNA library in a yeast two-hybrid assay to identify sensory neuron proteins that interact with ASICs. We found that annexin II light chain p11 physically interacts with the N terminus of ASIC1a, but not other ASIC isoforms. Immunoprecipitation studies confirmed an interaction between p11 and ASIC1 in rat dorsal root ganglion neurons in vivo. Coexpression of p11 and ASIC1a in CHO-K1 cells led to a 2-fold increase in expression of the ion channel at the cell membrane as determined by membrane-associated immunoreactivity and cell-surface biotinylation. Consistent with these findings, peak ASIC1a currents in transfected CHO-K1 cells were up-regulated 2-fold in the presence of p11, whereas ASIC3-mediated currents were unaffected by p11 expression. Neither the pH dependence of activation nor the rates of desensitization were altered by p11, suggesting that its primary role in regulating ASIC1a activity is to enhance cell-surface expression of ASIC1a. These data demonstrate that p11, already known to traffic members of the voltage-gated sodium and potassium channel families as well as transient receptor potential and chloride channels, also plays a selective role in enhancing ASIC1a functional expression.     

ASIC2b-dependent regulation of ASIC3, an essential acid-sensing ion channel subunit in sensory neurons via the partner protein PICK-1

ASIC3, an acid-sensing ion channel subunit expressed essentially in sensory neurons, has been proposed to be involved in pain. We show here for the first time that native ASIC3-like currents were increased in cultured dorsal root ganglion (DRG) neurons following protein kinase C (PKC) stimulation. This increase was induced by the phorbol ester PDBu and by pain mediators, such as serotonin, which are known to activate the PKC pathway through their binding to G protein-coupled receptors. We demonstrate that this regulation involves the silent ASIC2b subunit, an ASIC subunit also expressed in sensory neurons. Indeed, heteromultimeric ASIC3/ASIC2b channels, but not homomeric ASIC3 channels, are positively regulated by PKC. The increase of ASIC3/ASIC2b current is accompanied by a shift in its pH dependence toward more physiological pH values and may lead to an increase of sensory neuron excitability. This regulation by PKC requires PICK-1 (protein interacting with C kinase), a PDZ domain-containing protein, which interacts with the ASIC2b C terminus.