Unanticipated parallels in architecture and mechanism between ATP-gated P2X receptors and acid sensing ion channels

Purinergic signaling: decoding its role in COVID-19 pathogenesis and promising treatment strategies

The pathogenesis of coronavirus disease (COVID-19), caused by severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2), is complex and involves dysregulated immune responses, inflammation, and coagulopathy. Purinergic signaling, mediated by extracellular nucleotides and nucleosides, has emerged as a significant player in the pathogenesis of COVID-19. Extracellular adenosine triphosphate (ATP), released from damaged or infected cells, is a danger signal triggering immune responses. It activates immune cells, releasing pro-inflammatory cytokines, contributing to the cytokine storm observed in severe COVID-19 cases. ATP also promotes platelet activation and thrombus formation, contributing to the hypercoagulability seen in COVID-19 patients. On the other hand, adenosine, an immunosuppressive nucleoside, can impair anti-viral immune responses and promote tissue damage through its anti-inflammatory effects. Modulating purinergic receptors represents a promising therapeutic strategy for COVID-19. Understanding the role of purinergic signaling in COVID-19 pathogenesis and developing targeted therapeutic approaches can potentially improve patient outcomes. This review focuses on the part of purinergic signaling in COVID-19 pathogenesis and highlights potential therapeutic approaches targeting purinergic receptors.

The role of P2X7 receptor in infection and metabolism: Based on inflammation and immunity

The P2X7 receptor (P2X7R) is a ligand-gated receptor belonging to the P2 receptor family. It is distributed in various tissues of the human body and is involved in regulating the physiological functions of tissues and cells to affect the occurrence and development of diseases. Unlike all other P2 receptors, the P2X7 receptor is mainly expressed in immune cells and can be activated not only by extracellular nucleotides but also by non-nucleotide substances which act as positive allosteric modulators. In this review, we comprehensively describe the role of the P2X7 receptor in infection and metabolism based on its role as an important regulator of inflammation and immunity, and briefly introduce the structure and general function of the P2X7 receptor. These provide a clear knowledge framework for the study of the P2X7 receptor in human health. Targeting the P2X7 receptor may be an effective method for the treatment of inflammatory and immune diseases. And its role in microbial infection and metabolism may be the main direction for in-depth research on the P2X7 receptor in the future.

ATP signaling in the integrative neural center of Aplysia californica

ATP and its ionotropic P2X receptors are components of the most ancient signaling system. However, little is known about the distribution and function of purinergic transmission in invertebrates. Here, we cloned, expressed, and pharmacologically characterized the P2X receptors in the sea slug Aplysia californica—a prominent neuroscience model. AcP2X receptors were successfully expressed in Xenopus oocytes and displayed activation by ATP with two-phased kinetics and Na⁺-dependence. Pharmacologically, they were different from other P2X receptors. The ATP analog, Bz-ATP, was a less effective agonist than ATP, and PPADS was a more potent inhibitor of the AcP2X receptors than the suramin. AcP2X were uniquely expressed within the cerebral F-cluster, the multifunctional integrative neurosecretory center. AcP2X receptors were also detected in the chemosensory structures and the early cleavage stages. Therefore, in molluscs, rapid ATP-dependent signaling can be implicated both in development and diverse homeostatic functions. Furthermore, this study illuminates novel cellular and systemic features of P2X-type ligand-gated ion channels for deciphering the evolution of neurotransmitters.

High glucose inhibits osteogenic differentiation and proliferation of MC3T3‑E1 cells by regulating P2X7

Diabetes mellitus adversely affects human bones and increases the risk of developing osteoporosis. In the present study, treatment with 30 mmol/l glucose was used to establish a high glucose (HG) cell model in vitro. Plasmids were used to overexpress the P2X purinoceptor 7 (P2X7) gene. Brilliant blue G and (4‑benzoyl‑benzoyl)‑ATP were used as a P2X7 antagonist and agonist, respectively. Proliferation of osteogenic MC3T3‑E1 cells and alkaline phosphatase (ALP) activity were determined using MTT and colorimetric assays, respectively. Alizarin Red S was used to assess calcification of MC3T3‑E1 cells. Western blotting and reverse transcription‑quantitative PCR were performed to determine protein and mRNA expression levels. The results demonstrated that HG inhibited MC3T3‑E1 cell proliferation and P2X7 expression, reduced calcification, and downregulated the expression levels of ALP and osteocalcin (Ocn) in MC3T3‑E1 cells. Overexpression of P2X7 in HG conditions increased calcification and proliferation, and upregulated the levels of ALP and Ocn in MC3T3‑E1 cells. Inhibition of P2X7 downregulated the expressions of ALP and Ocn in MC3T3‑E1 cells under HG conditions. Therefore, the present results indicated that HG caused damage to osteogenic MC3T3‑E1 cells. Thus, P2X7 may be a regulatory factor that may be used to counteract the effects of HG on osteogenesis.

Receptor-Receptor Interactions as a Widespread Phenomenon: Novel Targets for Drug Development?

The discovery of receptor-receptor interactions (RRI) has expanded our understanding of the role that G protein-coupled receptors (GPCRs) play in intercellular communication. The finding that GPCRs can operate as receptor complexes, and not only as monomers, suggests that several different incoming signals could already be integrated at the plasma membrane level via direct allosteric interactions between the protomers that form the complex. Most research in this field has focused on neuronal populations and has led to the identification of a large number of RRI. However, RRI have been seen to occur not only in neurons but also in astrocytes and, outside the central nervous system, in cells of the cardiovascular and endocrine systems and in cancer cells. Furthermore, RRI involving the formation of macromolecular complexes are not limited to GPCRs, being also observed in other families of receptors. Thus, RRI appear as a widespread phenomenon and oligomerization as a common mechanism for receptor function and regulation. The discovery of these macromolecular assemblies may well have a major impact on pharmacology. Indeed, the formation of receptor complexes significantly broadens the spectrum of mechanisms available to receptors for recognition and signaling, which may be implemented through modulation of the binding sites of the adjacent protomers and of their signal transduction features. In this context, the possible appearance of novel allosteric sites in the receptor complex structure may be of particular relevance. Thus, the existence of RRI offers the possibility of new therapeutic approaches, and novel pharmacological strategies for disease treatment have already been proposed. Several challenges, however, remain. These include the accurate characterization of the role that the receptor complexes identified so far play in pathological conditions and the development of ligands specific to given receptor complexes, in order to efficiently exploit the pharmacological properties of these complexes.

The ASIC3/P2X3 cognate receptor is a pain-relevant and ligand-gated cationic channel

Two subclasses of acid-sensing ion channels (ASIC3) and of ATP-sensitive P2X receptors (P2X3Rs) show a partially overlapping expression in sensory neurons. Here we report that both recombinant and native receptors interact with each other in multiple ways. Current measurements with the patch-clamp technique prove that ASIC3 stimulation strongly inhibits the P2X3R current partly by a Ca²⁺-dependent mechanism. The proton-binding site is critical for this effect and the two receptor channels appear to switch their ionic permeabilities during activation. Co-immunoprecipation proves the close association of the two protein structures. BN-PAGE and SDS-PAGE analysis is also best reconciled with the view that ASIC3 and P2X3Rs form a multiprotein structure. Finally, in vivo measurements in rats reveal the summation of pH and purinergically induced pain. In conclusion, the receptor subunits do not appear to form a heteromeric channel, but tightly associate with each other to form a protein complex, mediating unidirectional inhibition.

Two subclasses of ligand-gated ion channels (ASIC3 and P2X3) are both present at sensory neurons and might be therefore subject to receptor crosstalk. Here authors use electrophysiology, biochemistry and co-immunoprecipitation to show that the two ion channels interact and affect P2X3 currents.

Interaction of Purinergic P2X4 and P2X7 Receptor Subunits

P2X4 and P2X7 are members of the P2X receptor family, comprising seven isoforms (P2X1–P2X7) that form homo- and heterotrimeric non-specific cation channels gated by extracellular ATP. P2X4 and P2X7 are widely coexpressed, particularly in secretory epithelial cells and immune and inflammatory cells, and regulate inflammation and nociception. Although functional heteromerization has been established for P2X2 and P2X3 subunits expressed in sensory neurons, there are contradictory reports regarding a functional interaction between P2X4 and P2X7 subunits. To resolve this issue, we coexpressed P2X4 and P2X7 receptor subunits labeled with green (EGFP) and red (TagRFP) fluorescent proteins in Xenopus laevis oocytes and investigated a putative physical interaction between the fusion proteins by Förster resonance energy transfer (FRET). Coexpression of P2X4 and P2X7 subunits with EGFP and TagRFP located in the extracellular receptor domains led to significant FRET signals. Significant FRET signals were also measured between C-terminally fluorophore-labeled full-length P2X4^1-384 and C-terminally truncated fluorescent P2X7^1-408 subunits. We furthermore used the two-electrode voltage clamp technique to investigate whether human P2X4 and P2X7 receptors (hP2X4, hP2X7) functionally interact at the level of ATP-induced whole-cell currents. Concentration–response curves and effects of ivermectin (P2X4-potentiating drug) or BzATP (P2X7-specific agonist) were consistent with a model in which coexpressed hP2X4 and hP2X7 do not interact. Similarly, the effect of adding specific inhibitors of P2X4 (PSB-15417) or P2X7 (oATP, A438079) could be explained by a model in which only homomers exist, and that these are blocked by the respective antagonist. In conclusion, we show that P2X4 and P2X7 subunits can form heterotrimeric P2X4/P2X7 receptors. However, unlike observations for P2X2 and P2X3, coexpression of P2X4 and P2X7 subunits does not result in a novel electrophysiologically discriminable P2X receptor phenotype.

P2X receptors

Extracellular adenosine 5'-triphosphate (ATP) activates cell surface P2X and P2Y receptors. P2X receptors are membrane ion channels preferably permeable to sodium, potassium and calcium that open within milliseconds of the binding of ATP. In molecular architecture, they form a unique structural family. The receptor is a trimer, the binding of ATP between subunits causes them to flex together within the ectodomain and separate in the membrane-spanning region so as to open a central channel. P2X receptors have a widespread tissue distribution. On some smooth muscle cells, P2X receptors mediate the fast excitatory junction potential that leads to depolarization and contraction. In the central nervous system, activation of P2X receptors allows calcium to enter neurons and this can evoke slower neuromodulatory responses such as the trafficking of receptors for the neurotransmitter glutamate. In primary afferent nerves, P2X receptors are critical for the initiation of action potentials when they respond to ATP released from sensory cells such as taste buds, chemoreceptors or urothelium. In immune cells, activation of P2X receptors triggers the release of pro-inflammatory cytokines such as interleukin 1β. The development of selective blockers of different P2X receptors has led to clinical trials of their effectiveness in the management of cough, pain, inflammation and certain neurodegenerative diseases.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

The P2X7 Receptor in Infection and Inflammation

Adenosine triphosphate (ATP) accumulates at sites of tissue injury and inflammation. Effects of extracellular ATP are mediated by plasma membrane receptors named P2 receptors (P2Rs). The P2R most involved in inflammation and immunity is the P2X7 receptor (P2X7R), expressed by virtually all cells of innate and adaptive immunity. P2X7R mediates NLRP3 inflammasome activation, cytokine and chemokine release, T lymphocyte survival and differentiation, transcription factor activation, and cell death. Ten human P2RX7 gene splice variants and several SNPs that produce complex haplotypes are known. The P2X7R is a potent stimulant of inflammation and immunity and a promoter of cancer cell growth. This makes P2X7R an appealing target for anti-inflammatory and anti-cancer therapy. However, an in-depth knowledge of its structure and of the associated signal transduction mechanisms is needed for an effective therapeutic development.

Allosteric modulation as a unifying mechanism for receptor function and regulation

Four major receptor families enable cells to respond to chemical and physical signals from their proximal environment. The ligand- and voltage-gated ion channels, G-protein-coupled receptors, nuclear hormone receptors and receptor tyrosine kinases are all allosteric proteins that carry multiple, spatially distinct, yet conformationally linked ligand-binding sites. Recent studies point to common mechanisms governing the allosteric transitions of these receptors, including the impact of oligomerization, pre-existing and functionally distinct conformational ensembles, intrinsically disordered regions, and the occurrence of allosteric modulatory sites. Importantly, synthetic allosteric modulators are being discovered for these receptors, providing an enriched, yet challenging, landscape for novel therapeutics.

Intersubunit physical couplings fostered by the left flipper domain facilitate channel opening of P2X4 receptors

P2X receptors are ATP-gated trimeric channels with important roles in diverse pathophysiological functions. A detailed understanding of the mechanism underlying the gating process of these receptors is thus fundamentally important and may open new therapeutic avenues. The left flipper (LF) domain of the P2X receptors is a flexible loop structure, and its coordinated motions together with the dorsal fin (DF) domain are crucial for the channel gating of the P2X receptors. However, the mechanism underlying the crucial role of the LF domain in the channel gating remains obscure. Here, we propose that the ATP-induced allosteric changes of the LF domain enable it to foster intersubunit physical couplings among the DF and two lower body domains, which are pivotal for the channel gating of P2X4 receptors. Metadynamics analysis indicated that these newly established intersubunit couplings correlate well with the ATP-bound open state of the receptors. Moreover, weakening or strengthening these physical interactions with engineered intersubunit metal bridges remarkably decreased or increased the open probability of the receptors, respectively. Further disulfide cross-linking and covalent modification confirmed that the intersubunit physical couplings among the DF and two lower body domains fostered by the LF domain at the open state act as an integrated structural element that is stringently required for the channel gating of P2X4 receptors. Our observations provide new mechanistic insights into P2X receptor activation and will stimulate development of new allosteric modulators of P2X receptors.

Cryo-EM Structures of the Magnesium Channel CorA Reveal Symmetry Break upon Gating

CorA, the major Mg(2+) uptake system in prokaryotes, is gated by intracellular Mg(2+) (KD ∼ 1-2 mM). X-ray crystallographic studies of CorA show similar conformations under Mg(2+)-bound and Mg(2+)-free conditions, but EPR spectroscopic studies reveal large Mg(2+)-driven quaternary conformational changes. Here, we determined cryo-EM structures of CorA in the Mg(2+)-bound closed conformation and in two open Mg(2+)-free states at resolutions of 3.8, 7.1, and 7.1 Å, respectively. In the absence of bound Mg(2+), four of the five subunits are displaced to variable extents (∼ 10-25 Å) by hinge-like motions as large as ∼ 35° at the stalk helix. The transition between a single 5-fold symmetric closed state and an ensemble of low Mg(2+), open, asymmetric conformational states is, thus, the key structural signature of CorA gating. This mechanism is likely to apply to other structurally similar divalent ion channels.

Structural Changes Fundamental to Gating of the Cystic Fibrosis Transmembrane Conductance Regulator Anion Channel Pore

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial cell anion channel. Potentiator drugs used in the treatment of cystic fibrosis act on the channel to increase overall channel function, by increasing the stability of its open state and/or decreasing the stability of its closed state. The structure of the channel in either the open state or the closed state is not currently known. However, changes in the conformation of the protein as it transitions between these two states have been studied using functional investigation and molecular modeling techniques. This review summarizes our current understanding of the architecture of the transmembrane channel pore that controls the movement of chloride and other small anions, both in the open state and in the closed state. Evidence for different kinds of changes in the conformation of the pore as it transitions between open and closed states is described, as well as the mechanisms by which these conformational changes might be controlled to regulate normal channel gating. The ways that key conformational changes might be targeted by small compounds to influence overall CFTR activity are also discussed. Understanding the changes in pore structure that might be manipulated by such small compounds is key to the development of novel therapeutic strategies for the treatment of cystic fibrosis.

Insights into the channel gating of P2X receptors from structures, dynamics and small molecules

P2X receptors, as ATP-gated non-selective trimeric ion channels, are permeable to Na(+), K(+) and Ca(2+). Comparing with other ligand-gated ion channel families, P2X receptors are distinct in their unique gating properties and pathophysiological roles, and have attracted attention as promising drug targets for a variety of diseases, such as neuropathic pain, multiple sclerosis, rheumatoid arthritis and thrombus. Several small molecule inhibitors for distinct P2X subtypes have entered into clinical trials. However, many questions regarding the gating mechanism of P2X remain unsolved. The structural determinations of P2X receptors at the resting and ATP-bound open states revealed that P2X receptor gating is a cooperative allosteric process involving multiple domains, which marks the beginning of the post-structure era of P2X research at atomic level. Here, we review the current knowledge on the structure-function relationship of P2X receptors, depict the whole picture of allosteric changes during the channel gating, and summarize the active sites that may contribute to new strategies for developing novel allosteric drugs targeting P2X receptors.

Pseudohypoaldosteronism type 1 and Liddle's syndrome mutations that affect the single-channel properties of the epithelial Na+ channel

These studies test whether three disease-causing mutations in genes (SCNN1A and SCNN1G) encoding subunits of the epithelial Na⁺ channel, ENaC, affect the biophysical and gating properties of this important renal ion channel. The S562P missense mutation in αENaC and the K106_S108delinsN mutation in γENaC are associated with pseudohypoaldosteronism type 1 (PHA1). The N530S missense mutation in γENaC causes Liddle’s syndrome. Incorporation of S562P into αENaC and K106_S108N into γENaC resulted in significant decreases in macroscopic ENaC currents. Conversely, incorporation of N530S into γENaC increased macroscopic ENaC current. The S562P substitution resulted in a nonfunctional channel. The K106_S108N mutation produced a functional channel having a normal macroscopic current–voltage relation, there was a slight but significant decrease in unitary conductance and a marked decrease in single-channel open probability. The N530S substitution increased single-channel open probability having no effect on the macroscopic current–voltage relation or unitary conductance of the channel. These findings are consistent with mutation of residues at 562 in αENaC and 530 in γENaC, and a 3′ splice site in SCNN1G (318-1 G→A; K106_108SdelinsN) resulting in aberrant ENaC activity due to changes in the biophysical and gating properties of the channel. Such changes likely contribute to the cellular mechanism underpinning the PHA1 and Liddle’s syndrome caused by these mutations in ENaC subunits.

Key sites for P2X receptor function and multimerization: overview of mutagenesis studies on a structural basis

P2X receptors constitute a seven-member family (P2X1-7) of extracellular ATP-gated cation channels of widespread expression. Because P2X receptors have been implicated in neurological, inflammatory and cardiovascular diseases, they constitute promising drug targets. Since the first P2X cDNA sequences became available in 1994, numerous site-directed mutagenesis studies have been conducted to disclose key sites of P2X receptor function and oligomerization. The publication of the 3-Å crystal structures of the zebrafish P2X4 (zfP2X4) receptor in the homotrimeric apo-closed and ATP-bound open states in 2009 and 2012, respectively, has ushered a new era by allowing for the interpretation of the wealth of molecular data in terms of specific three-dimensional models and by paving the way for designing more-decisive experiments. Thanks to these structures, the last five years have provided invaluable insight into our understanding of the structure and function of the P2X receptor class of ligandgated ion channels. In this review, we provide an overview of mutagenesis studies of the pre- and post-crystal structure eras that identified amino acid residues of key importance for ligand binding, channel gating, ion flow, formation of the pore and the channel gate, and desensitization. In addition, the sites that are involved in the trimerization of P2X receptors are reviewed based on mutagenesis studies and interface contacts that were predicted by the zfP2X4 crystal structures.

ASICs as therapeutic targets for migraine

Migraine is the most common neurological disorder and one of the most common chronic pain conditions. Despite its prevalence, the pathophysiology leading to migraine is poorly understood and the identification of new therapeutic targets has been slow. Several processes are currently thought to contribute to migraine including altered activity in the hypothalamus, cortical-spreading depression (CSD), and afferent sensory input from the cranial meninges. Decreased extracellular pH and subsequent activation of acid-sensing ion channels (ASICs) may contribute to each of these processes and may thus play a role in migraine pathophysiology. Although few studies have directly examined a role of ASICs in migraine, studies directly examining a connection have generated promising results including efficacy of ASIC blockers in both preclinical migraine models and in human migraine patients. The purpose of this review is to discuss the pathophysiology thought to contribute to migraine and findings that implicate decreased pH and/or ASICs in these events, as well as propose issues to be resolved in future studies of ASICs and migraine. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Quantifying Ca2+ current and permeability in ATP-gated P2X7 receptors

ATP-gated P2X7 receptors are prominently expressed in inflammatory cells and play a key role in the immune response. A major consequence of receptor activation is the regulated influx of Ca(2+) through the self-contained cation non-selective channel. Although the physiological importance of the resulting rise in intracellular Ca(2+) is universally acknowledged, the biophysics of the Ca(2+) flux responsible for the effects are poorly understood, largely because traditional methods of measuring Ca(2+) permeability are difficult to apply to P2X7 receptors. Here we use an alternative approach, called dye-overload patch-clamp photometry, to quantify the agonist-gated Ca(2+) flux of recombinant P2X7 receptors of dog, guinea pig, human, monkey, mouse, rat, and zebrafish. We find that the magnitude of the Ca(2+) component of the ATP-gated current depends on the species of origin, the splice variant, and the concentration of the purinergic agonist. We also measured a significant contribution of Ca(2+) to the agonist-gated current of the native P2X7Rs of mouse and human immune cells. Our results provide cross-species quantitative measures of the Ca(2+) current of the P2X7 receptor for the first time, and suggest that the cytoplasmic N terminus plays a meaningful role in regulating the flow of Ca(2+) through the channel.

General rules for the arrangements and gating motions of pore-lining helices in homomeric ion channels

The pore-lining helix (PLH) bundles are central to the function of all ion channels, as their conformational rearrangements dictate channel gating. Here we explore all plausible oligomeric arrangements of the PLH bundles within homomeric ion channels by building models using generic restraints. In particular, the distance between two neighbouring PLHs was bounded both below and above in order to avoid steric clash and allow proper packing. The resulting models provide a theoretical representation of the accessible space for oligomeric arrangements. While the represented space is confined, it encompasses nearly all the ion channel PLH bundles for which the structures are currently known. For a multitude of channels, gating models suggested by paths within the confined accessible space are in qualitative agreement with those established in previous structural and computational studies.

Rearrangements of the pore-lining helix (PLH) bundles of ion channels are central to their gating mechanisms. Here, Dai et al. use a modelling approach to define the general rules that govern the arrangements and gating motions of the PLHs in homomeric ion channels.

Relative motions between left flipper and dorsal fin domains favour P2X4 receptor activation

Channel gating in response to extracellular ATP is a fundamental process for the physiological functions of P2X receptors. Here we identify coordinated allosteric changes in the left flipper (LF) and dorsal fin (DF) domains that couple ATP-binding to channel gating. Engineered disulphide crosslinking or zinc bridges between the LF and DF domains that constrain their relative motions significantly influence channel gating of P2X4 receptors, confirming the essential role of these allosteric changes. ATP-binding-induced alterations in interdomain hydrophobic interactions among I208, L217, V291 and the aliphatic chain of K193 correlate well with these coordinated relative movements. Mutations on those four residues lead to impaired or fully abolished channel activations of P2X4 receptors. Our data reveal that ATP-binding-induced altered interdomain hydrophobic interactions and the concomitant coordinated motions of LF and DF domains are allosteric events essential for the channel gating of P2X4 receptors.

Imaging P2X4 receptor subcellular distribution, trafficking, and regulation using P2X4-pHluorin

A P2X4 receptor labeled with the pH-sensitive GFP superecliptic pHluorin represents a useful probe to investigate P2X4 receptor distribution, trafficking, and up-regulation.

P2X4 receptors are adenosine triphosphate (ATP)-gated cation channels present on the plasma membrane (PM) and also within intracellular compartments such as vesicles, vacuoles, lamellar bodies (LBs), and lysosomes. P2X4 receptors in microglia are up-regulated in epilepsy and in neuropathic pain; that is to say, their total and/or PM expression levels increase. However, the mechanisms underlying up-regulation of microglial P2X4 receptors remain unclear, in part because it has not been possible to image P2X4 receptor distribution within, or trafficking between, cellular compartments. Here, we report the generation of pH-sensitive fluorescently tagged P2X4 receptors that permit evaluations of cell surface and total receptor pools. Capitalizing on information gained from zebrafish P2X4.1 crystal structures, we designed a series of mouse P2X4 constructs in which a pH-sensitive green fluorescent protein, superecliptic pHluorin (pHluorin), was inserted into nonconserved regions located within flexible loops of the P2X4 receptor extracellular domain. One of these constructs, in which pHluorin was inserted after lysine 122 (P2X4-pHluorin123), functioned like wild-type P2X4 in terms of its peak ATP-evoked responses, macroscopic kinetics, calcium flux, current–voltage relationship, and sensitivity to ATP. P2X4-pHluorin123 also showed pH-dependent fluorescence changes, and was robustly expressed on the membrane and within intracellular compartments. P2X4-pHluorin123 identified cell surface and intracellular fractions of receptors in HEK-293 cells, hippocampal neurons, C8-B4 microglia, and alveolar type II (ATII) cells. Furthermore, it showed that the subcellular fractions of P2X4-pHluorin123 receptors were cell and compartment specific, for example, being larger in hippocampal neuron somata than in C8-B4 cell somata, and larger in C8-B4 microglial processes than in their somata. In ATII cells, P2X4-pHluorin123 showed that P2X4 receptors were secreted onto the PM when LBs undergo exocytosis. Finally, the use of P2X4-pHluorin123 showed that the modulator ivermectin did not increase the PM fraction of P2X4 receptors and acted allosterically to potentiate P2X4 receptor responses. Collectively, our data suggest that P2X4-pHluorin123 represents a useful optical probe to quantitatively explore P2X4 receptor distribution, trafficking, and up-regulation.

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.

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.

Optical control of trimeric P2X receptors and acid-sensing ion channels

P2X receptors are trimeric membrane proteins that function as ion channels gated by extracellular ATP. We have engineered a P2X2 receptor that opens within milliseconds by irradiation at 440 nm, and rapidly closes at 360 nm. This requires bridging receptor subunits via covalent attachment of 4,4'-bis(maleimido)azobenzene to a cysteine residue (P329C) introduced into each second transmembrane domain. The cis-trans isomerization of the azobenzene pushes apart the outer ends of the transmembrane helices and opens the channel in a light-dependent manner. Light-activated channels exhibited similar unitary currents, rectification, calcium permeability, and dye uptake as P2X2 receptors activated by ATP. P2X3 receptors with an equivalent mutation (P320C) were also light sensitive after chemical modification. They showed typical rapid desensitization, and they could coassemble with native P2X2 subunits in pheochromocytoma cells to form light-activated heteromeric P2X2/3 receptors. A similar approach was used to open and close human acid-sensing ion channels (ASICs), which are also trimers but are unrelated in sequence to P2X receptors. The experiments indicate that the opening of the permeation pathway requires similar and substantial movements of the transmembrane helices in both P2X receptors and ASICs, and the method will allow precise optical control of P2X receptors or ASICs in intact tissues.

Functional architecture of the CFTR chloride channel

Cystic fibrosis is caused by mutations in the cystic fibrosis transmembrane conductance regulator (CFTR), a member of the ATP-binding cassette (ABC) family of membrane transport proteins. CFTR is unique among ABC proteins in that it functions not as an active transporter but as an ATP-gated Cl(-) channel. As an ion channel, the function of the CFTR transmembrane channel pore that mediates Cl(-) movement has been studied in great detail. On the other hand, only low resolution structural data is available on the transmembrane parts of the protein. The structure of the channel pore has, however, been modeled on the known structure of active transporter ABC proteins. Currently, significant barriers exist to building a unified view of CFTR pore structure and function. Reconciling functional data on the channel with indirect structural data based on other proteins with very different transport functions and substrates has proven problematic. This review summarizes current structural and functional models of the CFTR Cl(-) channel pore, including a comprehensive review of previous electrophysiological investigations of channel structure and function. In addition, functional data on the three-dimensional arrangement of pore-lining helices, as well as contemporary hypotheses concerning conformational changes in the pore that occur during channel opening and closing, are discussed. Important similarities and differences between different models of the pore highlight current gaps in our knowledge of CFTR structure and function. In order to fill these gaps, structural and functional models of the membrane-spanning pore need to become better integrated.

Protons and Psalmotoxin-1 reveal nonproton ligand stimulatory sites in chicken acid-sensing ion channel: Implication for simultaneous modulation in ASICs

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.