Single channel properties of P2X2 purinoceptors

The impact of the P2X7 receptor on the tumor immune microenvironment and its effects on tumor progression

Cancer is a significant global health concern, and finding effective methods to treat it has been a focus of scientific research. It has been discovered that the growth, invasion, and metastasis of tumors are closely related to the environment in which they exist, known as the tumor microenvironment (TME). The immune response interacting with the tumor occurring within the TME constitutes the tumor immune microenvironment, and the immune response can lead to anti-tumor and pro-tumor outcomes and has shown tremendous potential in immunotherapy. A channel called the P2X7 receptor (P2X7R) has been identified within the TME. It is an ion channel present in various immune cells and tumor cells, and its activation can lead to inflammation, immune responses, angiogenesis, immunogenic cell death, and promotion of tumor development. This article provides an overview of the structure, function, and pharmacological characteristics of P2X7R. We described the concept and components of tumor immune microenvironment and the influence immune components has on tumors. We also outlined the impact of P2X7R regulation and how it affects the development of tumors and summarized the effects of drugs targeting P2X7R on tumor progression, both past and current, assisting researchers in treating tumors using P2X7R as a target.

A New Model of Hemoglobin Oxygenation

The study of hemoglobin oxygenation, starting from the classical works of Hill, has laid the foundation for molecular biophysics. The cooperative nature of oxygen binding to hemoglobin has been variously described in different models. In the Adair model, which better fits the experimental data, the constants of oxygen binding at various stages differ. However, the physical meaning of the parameters in this model remains unclear. In this work, we applied Hill's approach, extending its interpretation; we obtained a good agreement between the theory and the experiment. The equation in which the Hill coefficient is modulated by the Lorentz distribution for oxygen partial pressure approximates the experimental data better than not only the classical Hill equation, but also the Adair equation.

Unveiling the Structure-Activity Relationships at the Orthosteric Binding Site of P2X Ion Channels: The Route to Selectivity

The orthosteric ATP-binding site of the P2X receptors is poorly understood. Only a few compounds were well characterized for their P2X receptor functional activity and subtype selectivity. This study represents the first fully functional characterization of various ATP derivatives combined with in silico studies to advance the understanding of SARs at the orthosteric binding sites of P2X receptors leading to the identification of 2-chloro-3-trifluoromethylbenzoyl ATP ester as a novel pan-P2X receptor agonist and several subtype-selective P2X receptor agonists. Furthermore, esterification of both hydroxyl functions of ATP using 1-naphthoic acid has led to compound 26 acting as an antagonist at P2X1-4 and P2X2/3 receptors and an agonist at P2X7 receptors. This particular ATP derivative will allow interrogating the P2X7 receptor function while antagonizing all other P2X receptor subtypes and therefore serve as a valuable pharmacological tool in the future.

Enlightening activation gating in P2X receptors

P2X receptors are trimeric nonselective cation channels gated by ATP. They assemble from seven distinct subunit isoforms as either homo- or heteromeric complexes and contain three extracellularly located binding sites for ATP. P2X receptors are expressed in nearly all tissues and are there involved in physiological processes like synaptic transmission, pain, and inflammation. Thus, they are a challenging pharmacological target. The determination of crystal and cryo-EM structures of several isoforms in the last decade in closed, open, and desensitized states has provided a firm basis for interpreting the huge amount of functional and biochemical data. Electrophysiological characterization in conjugation with optical approaches has generated significant insights into structure–function relationships of P2X receptors. This review focuses on novel optical and related approaches to better understand the conformational changes underlying the activation of these receptors.

Structural basis for the functional properties of the P2X7 receptor for extracellular ATP

The P2X7 receptor, originally known as the P2Z receptor due to its distinctive functional properties, has a structure characteristic of the ATP-gated ion channel P2X receptor family. The P2X7 receptor is an important mediator of ATP-induced purinergic signalling and is involved the pathogenesis of numerous conditions as well as in the regulation of diverse physiological functions. Functional characterisations, in conjunction with site-directed mutagenesis, molecular modelling, and, recently, structural determination, have provided significant insights into the structure–function relationships of the P2X7 receptor. This review discusses the current understanding of the structural basis for the functional properties of the P2X7 receptor.

Unravelling the intricate cooperativity of subunit gating in P2X2 ion channels

Ionotropic purinergic (P2X) receptors are trimeric channels that are activated by the binding of ATP. They are involved in multiple physiological functions, including synaptic transmission, pain and inflammation. The mechanism of activation is still elusive. Here we kinetically unraveled and quantified subunit activation in P2X2 receptors by an extensive global fit approach with four complex and intimately coupled kinetic schemes to currents obtained from wild type and mutated receptors using ATP and its fluorescent derivative 2-[DY-547P1]-AET-ATP (fATP). We show that the steep concentration-activation relationship in wild type channels is caused by a subunit flip reaction with strong positive cooperativity, overbalancing a pronounced negative cooperativity for the three ATP binding steps, that the net probability fluxes in the model generate a marked hysteresis in the activation-deactivation cycle, and that the predicted fATP binding matches the binding measured by fluorescence. Our results shed light into the intricate activation process of P2X channels.

Drosophila taste neurons as an agonist-screening platform for P2X receptors

The P2X receptor family of ATP-gated cation channels are attractive drug targets for pain and inflammatory disease, but no subtype-selective agonists, and few partially selective agonists have been described to date. As proof-of-concept for the discovery of novel P2X receptor agonists, here we demonstrate the use of Drosophila taste neurons heterologously expressing rat P2X2 receptors as a screening platform. We demonstrate that wild-type rat P2X2 expressed in Drosophila is fully functional (ATP EC₅₀ 8.7 µM), and that screening of small (2 µl) volumes of a library of 80 adenosine nucleotide analogues is rapid and straightforward. We have determined agonist potency and specificity profiles for rat P2X2 receptors; triphosphate-bearing analogues display broad activity, tolerating a number of substitutions, and diphosphate and monophosphate analogues display very little activity. While several ATP analogues gave responses of similar magnitude to ATP, including the previously identified agonists ATPγS and ATPαS, we were also able to identify a novel agonist, the synthetic analogue 2-fluoro-ATP, and to confirm its agonist activity on rat P2X2 receptors expressed in human cells. These data validate our Drosophila platform as a useful tool for the analysis of agonist structure-activity relationships, and for the screening and discovery of novel P2X receptor agonists.

Relating ligand binding to activation gating in P2X2 receptors using a novel fluorescent ATP derivative

Ionotropic purinergic receptors (P2X receptors) are non-specific cation channels that are activated by the binding of ATP at their extracellular side. P2X receptors contribute to multiple functions, including the generation of pain, inflammation, or synaptic transmission. The channels are trimers and structural information on several of their isoforms is available. In contrast, the cooperation of the subunits in the activation process is poorly understood. We synthesized a novel fluorescent ATP derivative, 2-[DY-547P1]-AET-ATP (fATP) to unravel the complex activation process in P2X2 and mutated P2X2 H319K channels with enhanced apparent affinity by characterizing the relation between ligand binding and activation gating. fATP is a full agonist with respect to ATP that reports the degree of binding by bright fluorescence. For quantifying the binding, a fast automated algorithm was employed on human embryonic kidney cell culture images. The concentrations of half maximum occupancy and activation as well as the respective Hill coefficients were determined. All Hill coefficients exceeded unity, even at an occupancy <10%, suggesting cooperativity of the binding even for the first and second binding step. fATP shows promise for continuative functional studies on other purinergic receptors and, beyond, any other ATP-binding proteins.

Molecular determinants for agonist recognition and discrimination in P2X2 receptors

P2X receptors (P2XRs) are ligand-gated cation channels involved in pain and inflammation. Gasparri et al. show that the backbone carbonyl atoms of amino acid residue Thr184 are involved in ligand discrimination, while those of Lys69 contribute mostly to ligand recognition by rat P2X2Rs.

P2X receptors (P2XRs) are trimeric ligand-gated ion channels that open a cation-selective pore in response to ATP binding. P2XRs contribute to synaptic transmission and are involved in pain and inflammation, thus representing valuable drug targets. Recent crystal structures have confirmed the findings of previous studies with regards to the amino acid chains involved in ligand recognition, but they have also suggested that backbone carbonyl atoms contribute to ATP recognition and discrimination. Here we use a combination of site-directed mutagenesis, amide-to-ester substitutions, and a range of ATP analogues with subtle alterations to either base or sugar component to investigate the contributions of backbone carbonyl atoms toward ligand recognition and discrimination in rat P2X2Rs. Our findings demonstrate that while the Lys69 backbone carbonyl makes an important contribution to ligand recognition, the discrimination between different ligands is mediated by both the side chain and the backbone carbonyl oxygen of Thr184. Together, our data demonstrate how conserved elements in P2X2Rs recognize and discriminate agonists.

Charge Discrimination in P2X4 Receptors Occurs in Two Consecutive Stages

P2X receptors are a group of trimeric cationic channels that are activated by adenosine 5'-triphosphate. They perform critical roles in the membranes of mammalian cells, and their improper functioning is associated with numerous diseases. Despite the vast amount of research devoted to them, several aspects of their operation are currently unclear, including the causes of their charge selectivity. We present the results of molecular dynamics simulation, which shed light on this issue for the case of P2X₄ channels. We examined in detail the behavior of Na⁺ and Cl^- ions inside the receptor. The examination reveals that charge discrimination occurs in two stages. First, cations bear precedence over anions to enter the extracellular vestibule. Then, cations at the extracellular vestibule are more likely to cross the pore than anions in an equivalent position. In this manner, a thorough but straightforward analysis of computational simulations suggests a stepwise mechanism, without a unique determinant factor.

ATPe Dynamics in Protozoan Parasites. Adapt or Perish

In most animals, transient increases of extracellular ATP (ATPe) are used for physiological signaling or as a danger signal in pathological conditions. ATPe dynamics are controlled by ATP release from viable cells and cell lysis, ATPe degradation and interconversion by ecto-nucleotidases, and interaction of ATPe and byproducts with cell surface purinergic receptors and purine salvage mechanisms. Infection by protozoan parasites may alter at least one of the mechanisms controlling ATPe concentration. Protozoan parasites display their own set of proteins directly altering ATPe dynamics, or control the activity of host proteins. Parasite dependent activation of ATPe conduits of the host may promote infection and systemic responses that are beneficial or detrimental to the parasite. For instance, activation of organic solute permeability at the host membrane can support the elevated metabolism of the parasite. On the other hand ecto-nucleotidases of protozoan parasites, by promoting ATPe degradation and purine/pyrimidine salvage, may be involved in parasite growth, infectivity, and virulence. In this review, we will describe the complex dynamics of ATPe regulation in the context of protozoan parasite⁻host interactions. Particular focus will be given to features of parasite membrane proteins strongly controlling ATPe dynamics. This includes evolutionary, genetic and cellular mechanisms, as well as structural-functional relationships.

ATP-Gated P2X Receptor Channels: Molecular Insights into Functional Roles

In the nervous system, ATP is co-stored in vesicles with classical transmitters and released in a regulated manner. ATP from the intracellular compartment can also exit the cell through hemichannels and following shear stress or membrane damage. In the past 30 years, the action of ATP as an extracellular transmitter at cell-surface receptors has evolved from somewhat of a novelty that was treated with skepticism to purinergic transmission being accepted as having widespread important functional roles mediated by ATP-gated ionotropic P2X receptors (P2XRs). This review focuses on work published in the last five years and provides an overview of ( a) structural studies, ( b) the molecular basis of channel properties and regulation of P2XRs, and ( c) the physiological and pathophysiological roles of ATP acting at defined P2XR subtypes.

New Insights Into Permeation of Large Cations Through ATP-Gated P2X Receptors

The permeability of large cations through the P2X pore has remained arguably the most controversial and complicated topic in P2X-related research, with the emergence of conflicting studies on the existence, mechanism and physiological relevance of a so-called “dilated” state. Due to the important role of several “dilating” P2X subtypes in numerous diseases, a clear and detailed understanding of this phenomenon represents a research priority. Recent advances, however, have challenged the existence of a progressive, ATP-induced pore dilation, by demonstrating that this phenomenon is an artifact of the method employed. Here, we discuss briefly the history of this controversial and enigmatic dilated state, from its initial discovery to its recent reconsideration. We will discuss the literature in which mechanistic pathways to a large cation-permeable state are proposed, as well as important advances in the methodology employed to study this elusive state. Considering recent literature, we will also open the discussion as to whether an intrinsically dilating P2X pore exists, as well as the physiological relevance of such a large cation-permeable pore and its potential use as therapeutic pathway.

P2X Receptor Activation

Extracellular ATP-gated P2X receptors are trimeric non-selective cation channels important for many physiological events including immune response and neural transmission. These receptors belong to a unique class of ligand-gated ion channels composed of only six transmembrane helices and a relatively small extracellular domain that harbors three ATP-binding pockets. The crystal structures of P2X receptors, including the recent P2X3 structures representing three different stages of the gating cycle, have provided a compelling structural foundation for understanding how this class of ligand-gated ion channels function. These structures, in combination with numerous functional studies ranging from classic mutagenesis and electrophysiology to modern optogenetic pharmacology, have uncovered unique molecular mechanisms of P2X receptor function. This review article summarizes the current knowledge in P2X receptor activation, especially focusing on the mechanisms underlying ATP-binding, conformational changes in the extracellular domain, and channel gating and desensitization.

Moderne Anwendungsbeispiele aus der Elektrophysiologie

Alle Verfahren, die wir bisher diskutiert haben, Flussmessungen, Messungen von stationären und transienten Strömen und die entsprechenden Auswert¬verfahren können genutzt werden, um Funktion, Regulation und Struktur-Funktionsbeziehungen zu analysieren. Solche Informationen können wir gewinnen, indem wir die Funktionen von chemisch oder genetisch modifizierten Carriern oder Kanälen charakterisieren und miteinander vergleichen. Letzteres gilt auch für natürlich auftretende Mutationen, die Ursache für verschiedene Krankheiten sein können; die mit den genannten Methoden gewonnenen Erkenntnisse sind wichtiger Bestandteil für das Verständnis und die Behandlung solcher Krankheiten. Für viele der Transportproteine konnte die Aminosäuresequenz und die mögliche Orientierung des Proteins in der Membran ermittelt werden, oder es konnte sogar die dreidimensionale Struktur bestimmt werden. Im Folgenden wollen wir die Vorgehensweise eines Elektrophysiologen an Beispielen illustrieren, um Struktur, Funktion und Regulation von Membrantransport zu untersuchen. Für das Verständnis der Wirkung chemischer Stoffe sowie die Entwicklung neuer Medikamente zur Behandlung von Krankheiten stellt die Elektrophysiologie eine leistungsfähige Methode bereit, um die Wechselwirkung solcher Substanzen mit ihren Rezeptoren zu erforschen. Dieses soll beispielhaft an viralen Ionenkanälen illustriert werden.

P2X2 Dominant Deafness Mutations Have No Negative Effect on Wild-Type Isoform: Implications for Functional Rescue and in Deafness Mechanism

The P2X2 receptor is an ATP-gated ion channel, assembled by three subunits. Recently, it has been found that heterozygous mutations of P2X2 V60L and G353R can cause autosomal dominant nonsyndromic hearing loss. However, the underlying mechanism remains unclear. The fact that heterozygous mutations cause deafness suggests that the mutations may have dominant-negative effect (DNE) on wild-type (WT) P2X2 isoforms and/or other partners leading to hearing loss. In this study, the effect of these dominant deafness P2X2 mutations on WT P2X2 was investigated. We found that sole transfection of both V60L and G353R deafness mutants could efficiently target to the plasma membrane, like WT P2X2, but exhibit a significantly reduced response to ATP stimulation. Both mutants reduced the channel conductance, but G353R mutation also altered the voltage dependency. Co-expression with WT P2X2 could restore the response to ATP. As the ratio of WT P2X2 vs. mutants increased, the response to ATP was also increased. Computer modeling confirmed that both V60L and G353R dominant-deafness mutant subunits do not have any negative effect on WT P2X2 subunit, when assembled as a heterotrimer. Improper docking or defective gating is the more likely mechanism for impaired channel function by these P2X2 deafness mutations. These results suggest that P2X2 dominant deafness mutations do not have negative effects on WT P2X2 isoforms, and that adding additional WT P2X2 could rescue the lost channel function caused by the deafness mutations. These P2X2 dominant deafness mutations may have negative-effects on other partners leading to hearing loss.

Novel channel-mediated choline transport in cholinergic neurons of the mouse retina

Choline uptake into the presynaptic terminal of cholinergic neurons is mediated by the high-affinity choline transporter and is essential for acetylcholine synthesis. In a previous study, we reported that P2X₂ purinoceptors are selectively expressed in OFF-cholinergic amacrine cells of the mouse retina. Under specific conditions, P2X₂ purinoceptors acquire permeability to large cations, such as N-methyl-d-glucamine, and therefore potentially could act as a noncanonical pathway for choline entry into neurons. We tested this hypothesis in OFF-cholinergic amacrine cells of the mouse retina. ATP-induced choline currents were observed in OFF-cholinergic amacrine cells, but not in ON-cholinergic amacrine cells, in mouse retinal slice preparations. High-affinity choline transporters are expressed at higher levels in ON-cholinergic amacrine cells than in OFF-cholinergic amacrine cells. In dissociated preparations of cholinergic amacrine cells, ATP-activated cation currents arose from permeation of extracellular choline. We also examined the pharmacological properties of choline currents. Pharmacologically, α,β-methylene ATP did not produce a cation current, whereas ATPγS and benzoyl-benzoyl-ATP (BzATP) activated choline currents. However, the amplitude of the choline current activated by BzATP was very small. The choline current activated by ATP was strongly inhibited by pyridoxalphosphate-6-azophenyl-2',4'-sulfonic acid. Accordingly, P2X₂ purinoceptors expressed in HEK-293T cells were permeable to choline and similarly functioned as a choline uptake pathway. Our physiological and pharmacological findings support the hypothesis that P2 purinoceptors, including P2X₂ purinoceptors, function as a novel choline transport pathway and may provide a new regulatory mechanism for cholinergic signaling transmission at synapses in OFF-cholinergic amacrine cells of the mouse retina.NEW & NOTEWORTHY Choline transport across the membrane is exerted by both the high-affinity and low-affinity choline transporters. We found that choline can permeate P2 purinergic receptors, including P2X₂ purinoceptors, in cholinergic neurons of the retina. Our findings show the presence of a novel choline transport pathway in cholinergic neurons. Our findings also indicate that the permeability of P2X₂ purinergic receptors to choline observed in the heterologous expression system may have a physiological relevance in vivo.

The Dynamic Behavior of the P2X4 Ion Channel in the Closed Conformation

We present the results of a detailed molecular dynamics study of the closed form of the P2X₄ receptor. The fluctuations observed in the simulations were compared with the changes that occur in the transition from the closed to the open structure. To get further insight on the opening mechanism, the actual displacements were decomposed into interchain motions and intrachain deformations. This analysis revealed that the iris-like expansion of the transmembrane helices mainly results from interchain motions that already take place in the closed conformation. However, these movements cannot reach the amplitude required for the opening of the channel because they are impeded by interactions occurring around the ATP binding pocket. This suggests that the union of ATP produces distortions in the chains that eliminate the restrictions on the interchain displacements, leading to the opening of the pore.

On the permeation of large organic cations through the pore of ATP-gated P2X receptors

Pore dilation is thought to be a hallmark of purinergic P2X receptors. The most commonly held view of this unusual process posits that under prolonged ATP exposure the ion pore expands in a striking manner from an initial small-cation conductive state to a dilated state, which allows the passage of larger synthetic cations, such as N-methyl-d-glucamine (NMDG⁺). However, this mechanism is controversial, and the identity of the natural large permeating cations remains elusive. Here, we provide evidence that, contrary to the time-dependent pore dilation model, ATP binding opens an NMDG⁺-permeable channel within milliseconds, with a conductance that remains stable over time. We show that the time course of NMDG⁺ permeability superimposes that of Na⁺ and demonstrate that the molecular motions leading to the permeation of NMDG⁺ are very similar to those that drive Na⁺ flow. We found, however, that NMDG⁺ "percolates" 10 times slower than Na⁺ in the open state, likely due to a conformational and orientational selection of permeating molecules. We further uncover that several P2X receptors, including those able to desensitize, are permeable not only to NMDG⁺ but also to spermidine, a large natural cation involved in ion channel modulation, revealing a previously unrecognized P2X-mediated signaling. Altogether, our data do not support a time-dependent dilation of the pore on its own but rather reveal that the open pore of P2X receptors is wide enough to allow the permeation of large organic cations, including natural ones. This permeation mechanism has considerable physiological significance.

Optogenetic Control of Ca2+ and Voltage-Dependent Large Conductance (BK) Potassium Channels

Ca²⁺ concentration jumps for the activation of Ca²⁺-dependent ion channels or transporters can be obtained either by fast solution exchange or by the use of caged Ca²⁺. Here, we report on an alternate optogenetic method for the activation of Ca²⁺ and voltage-dependent large conductance (BK) potassium channels. This was achieved through the use of the light-gated channelrhodopsin 2 variant, CatCh(Calcium translocating Channelrhodopsin) with enhanced Ca, which produces locally [Ca²⁺] in the μM range on the inner side of the membrane, without significant [Ca²⁺] increase in the cytosol. BK channel subunits α and β1 were expressed together with CatCh in HEK293 cells, and voltage and Ca²⁺ dependence were analyzed. Light activation of endogenous BK channels under native conditions in astrocytes and glioma cells was also investigated. Additionally, BK channels were used as sensors for the calibration of the [Ca²⁺] on the inner surface of the cell membrane.

Mechanistic insights from resolving ligand-dependent kinetics of conformational changes at ATP-gated P2X1R ion channels

Structural studies of P2X receptors show a novel U shaped ATP orientation following binding. We used voltage clamp fluorometry (VCF) and molecular dynamics (MD) simulations to investigate agonist action. For VCF the P2X1 receptor (P2X1R) K190C mutant (adjacent to the agonist binding pocket) was labelled with the fluorophore MTS-TAMRA and changes in fluorescence on agonist treatment provided a real time measure of conformational changes. Studies with heteromeric channels incorporating a key lysine mutation (K68A) in the ATP binding site demonstrate that normally three molecules of ATP activate the receptor. The time-course of VCF responses to ATP, 2'-deoxy ATP, 3'-deoxy ATP, Ap5A and αβmeATP were agonist dependent. Comparing the properties of the deoxy forms of ATP demonstrated the importance of the 2' hydroxyl group on the ribose ring in determining agonist efficacy consistent with MD simulations showing that it forms a hydrogen bond with the γ-phosphate oxygen stabilizing the U-shaped conformation. Comparison of the recovery of fluorescence on agonist washout, with channel activation to a second agonist application for the partial agonists Ap5A and αβmeATP, showed a complex relationship between conformational change and desensitization. These results highlight that different agonists induce distinct conformational changes, kinetics and recovery from desensitization at P2X1Rs.

Fast-Scan Cyclic Voltammetry (FSCV) Detection of Endogenous Octopamine in Drosophila melanogaster Ventral Nerve Cord

Octopamine is an endogenous biogenic amine neurotransmitter, neurohormone, and neuromodulator in invertebrates and has functional analogy with norepinephrine in vertebrates. Fast-scan cyclic voltammetry (FSCV) can detect rapid changes in neurotransmitters, but FSCV has not been optimized for octopamine detection in situ. The goal of this study was to characterize octopamine release in the ventral nerve cord of Drosophila larvae for the first time. A FSCV waveform was optimized so that the potential for octopamine oxidation would not be near the switching potential where interferences can occur. Endogenous octopamine release was stimulated by genetically inserting either the ATP sensitive channel, P2X2, or the red-light sensitive channelrhodopsin, CsChrimson, into cells expressing tyrosine decarboxylase (TDC), an octopamine synthesis enzyme. To ensure that release is due to octopamine and not the precursor tyramine, the octopamine synthesis inhibitor disulfiram was applied, and the signal decreased by 80%. Stimulated release was vesicular, and a 2 s continuous light stimulation of CsChrimson evoked 0.22 ± 0.03 μM of octopamine release in the larval ventral nerve cord. Repeated stimulations were stable with 2 or 5 min interstimulation times. With pulsed stimulations, the release was dependent on the frequency of applied light pulse. An octopamine transporter has not been identified, and blockers of the dopamine transporter and serotonin transporter had no significant effect on the clearance time of octopamine, suggesting that they do not take up octopamine. This study shows that octopamine can be monitored in Drosophila, facilitating future studies of how octopamine release functions in the insect brain.

Dynamic Regulation of Cell Volume and Extracellular ATP of Human Erythrocytes

Introduction

The peptide mastoparan 7 (MST7) triggered in human erythrocytes (rbcs) the release of ATP and swelling. Since swelling is a well-known inducer of ATP release, and extracellular (ATPe), interacting with P (purinergic) receptors, can affect cell volume (Vr), we explored the dynamic regulation between Vr and ATPe.

Methods and Treatments

We made a quantitative assessment of MST7-dependent kinetics of Vr and of [ATPe], both in the absence and presence of blockers of ATP efflux, swelling and P receptors.

Results

In rbcs 10 μM MST7 promoted acute, strongly correlated changes in [ATPe] and Vr. Whereas MST7 induced increases of 10% in Vr and 190 nM in [ATPe], blocking swelling in a hyperosmotic medium + MST7 reduced [ATPe] by 40%. Pre-incubation of rbcs with 10 μM of either carbenoxolone or probenecid, two inhibitors of the ATP conduit pannexin 1, reduced [ATPe] by 40–50% and swelling by 40–60%, while in the presence of 80 U/mL apyrase, an ATPe scavenger, cell swelling was prevented. While exposure to 10 μM NF110, a blocker of ATP-P2X receptors mediating sodium influx, reduced [ATPe] by 48%, and swelling by 80%, incubation of cells in sodium free medium reduced swelling by 92%.

Analysis and Discussion

Results were analyzed by means of a mathematical model where ATPe kinetics and Vr kinetics were mutually regulated. Model dependent fit to experimental data showed that, upon MST7 exposure, ATP efflux required a fast 1960-fold increase of ATP permeability, mediated by two kinetically different conduits, both of which were activated by swelling and inactivated by time. Both experimental and theoretical results suggest that, following MST7 exposure, ATP is released via two conduits, one of which is mediated by pannexin 1. The accumulated ATPe activates P2X receptors, followed by sodium influx, resulting in cell swelling, which in turn further activates ATP release. Thus swelling and P2X receptors constitute essential components of a positive feedback loop underlying ATP-induced ATP release of rbcs.

Photo-switchable tweezers illuminate pore-opening motions of an ATP-gated P2X ion channel

P2X receptors function by opening a transmembrane pore in response to extracellular ATP. Recent crystal structures solved in apo and ATP-bound states revealed molecular motions of the extracellular domain following agonist binding. However, the mechanism of pore opening still remains controversial. Here we use photo-switchable cross-linkers as ‘molecular tweezers’ to monitor a series of inter-residue distances in the transmembrane domain of the P2X2 receptor during activation. These experimentally based structural constraints combined with computational studies provide high-resolution models of the channel in the open and closed states. We show that the extent of the outer pore expansion is significantly reduced compared to the ATP-bound structure. Our data further reveal that the inner and outer ends of adjacent pore-lining helices come closer during opening, likely through a hinge-bending motion. These results provide new insight into the gating mechanism of P2X receptors and establish a versatile strategy applicable to other membrane proteins.

DOI:http://dx.doi.org/10.7554/eLife.11050.001

Protein receptors in the cell membrane play an important role transmitting signals from outside to inside the cell. Members of the P2X family of receptors are ion channels that form pores through the membrane. When a molecule of ATP binds to the external region of the receptor, it activates it and causes the receptor to change from a closed to an open shape. Once opened, ions flow through the channel’s pore and trigger a response inside the cell. P2X receptors are found on most animal cells (including nerve cells) and are involved in both normal cellular activity and processes linked to disease, including inflammation and chronic pain.

The P2X receptor has three parts or subunits, and each contributes to the channel’s pore. Recent research using a technique called X-ray crystallography has revealed how ATP binding causes shape changes in the external region of the receptor. But these three-dimensional structures did not reveal details of how the subunits move to open or close the channel’s pore.

Habermacher et al. have now added light-sensitive linkers onto the P2X receptor in a way that meant that different colors of light could be used to force parts of the receptor to come closer together or move apart. This allowed the pore to be opened and closed in response to changes in light. Habermacher et al. then studied the behavior of these modified receptors within a natural membrane and found that the light stimulated movements were similar to those seen with ATP. When the behavior of the receptor and light-sensitive linkers was studied using computer simulations, it led to new models of the P2X pore in the open and closed state. In these models, the open channel was more tightly packed than in the previous structure and an unexpected hinge-bending movement was seen to accompany the opening of the channel. It is hoped that this new approach will also be useful for probing how other membrane proteins change their shape when activated.

DOI:http://dx.doi.org/10.7554/eLife.11050.002

Examples of Applications of Electrophysiology

All the techniques that we have discussed, flux measurements, steady-state and transient current measurements, as well as single-channel recordings and the corresponding analysis can be applied to the analysis of structure-function relationships. Such structure-function information can be obtained if we characterize and compare the function of wild-type and chemically or genetically modified transporters by using these techniques. The latter also includes naturally occurring mutations that are the source of various diseases; this is an important feature for the understanding and curing of such diseases.

For many of the transporters the amino acid sequence and the possible orientation of the protein in the membrane, or even the three-dimensional structures have been determined. In this chapter on Examples of Applications of Electrophysiology, we will illustrate the strategy of an electrophysiologist in investigating structure, function and regulation of membrane transport using as an example the Na⁺,K⁺-ATPase, the neurotransmitter transporter GAT (Na⁺-dependent GABA transporter), and the nucleotide receptors that form channels in the presence of extracellular ATP.

For an understanding of drug action as well as the development of new drugs for the treatment of diseases, electrophysiology is a powerful method to elucidate drug receptor interaction. As an example, this will be illustrated for viral ion channels that are essential for virus reproduction.

Molecular Structure and Regulation of P2X Receptors With a Special Emphasis on the Role of P2X2 in the Auditory System

The P2X purinergic receptors are cation-selective channels gated by extracellular adenosine 5'-triphosphate (ATP). These purinergic receptors are found in virtually all mammalian cell types and facilitate a number of important physiological processes. Within the past few years, the characterization of crystal structures of the zebrafish P2X4 receptor in its closed and open states has provided critical insights into the mechanisms of ligand binding and channel activation. Understanding of this gating mechanism has facilitated to design and interpret new modeling and structure-function experiments to better elucidate how different agonists and antagonists can affect the receptor with differing levels of potency. This review summarizes the current knowledge on the structure, activation, allosteric modulators, function, and location of the different P2X receptors. Moreover, an emphasis on the P2X2 receptors has been placed in respect to its role in the auditory system. In particular, the discovery of three missense mutations in P2X2 receptors could become important areas of study in the field of gene therapy to treat progressive and noise-induced hearing loss. J. Cell. Physiol. 231: 1656-1670, 2016. © 2015 Wiley Periodicals, Inc.

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.

Physical basis of apparent pore dilation of ATP-activated P2X receptor channels

The selectivity of ion channels is fundamental for their roles in electrical and chemical signaling and in ion homeostasis. Although most ion channels exhibit stable ion selectivity, the prevailing view of purinergic P2X receptor channels, transient receptor potential V1 (TRPV1) channels and acid-sensing ion channels (ASICs) is that their ion conduction pores dilate upon prolonged activation. We investigated this mechanism in P2X receptors and found that the hallmark shift in equilibrium potential observed with prolonged channel activation does not result from pore dilation, but from time-dependent alterations in the concentration of intracellular ions. We derived a physical model to calculate ion concentration changes during patch-clamp recordings, which validated our experimental findings and provides a quantitative guideline for effectively controlling ion concentration. Our results have fundamental implications for understanding ion permeation and gating in P2X receptor channels, as well as more broadly for using patch-clamp techniques to study ion channels and neuronal excitability.

Characterization of dopamine releasable and reserve pools in Drosophila larvae using ATP/P2X2 -mediated stimulation

Dopaminergic signaling pathways are conserved between mammals and Drosophila, but the factors important for maintaining the functional pool of synaptic dopamine are not fully understood in Drosophila. In this study, we characterized the releasable and reserve dopamine pools in Drosophila larvae using ATP/P2X2 -mediated stimulation. Dopamine release was stable with stimulations performed at least every 5 min but decayed with stimulations performed 2 min apart or less, indicating the replenishment of the releasable pool occurred on a time scale between 2 and 5 min. Dopamine synthesis or uptake was pharmacologically inhibited with 3-iodotyrosine and cocaine, respectively, to evaluate their contributions to maintain the releasable dopamine pool. We found that both synthesis and uptake were needed to maintain the releasable dopamine pool, with synthesis playing a major part in long-term replenishment and uptake being more important for short-term replenishment. These effects of synthesis and uptake on different time scales in Drosophila are analogous to mammals. However, unlike in mammals, cocaine did not activate a reserve pool of dopamine in Drosophila when using P2X2 stimulations. Our study shows that both synthesis and uptake replenish the releasable pool, providing a better understanding of dopamine regulation in Drosophila. The maintenance of the releasable dopamine pool was examined in Drosophila larva. Both synthesis and uptake were needed to maintain the releasable dopamine pool, with synthesis being most important on a longer time scale and uptake on a shorter time scale. Dopamine release was stimulated by applying ATP which activated P2X2 channels specifically expressed in dopaminergic neurons.

High-frequency voltage oscillations in cultured astrocytes

Because of their close interaction with neuronal physiology, astrocytes can modulate brain function in multiple ways. Here, we demonstrate a yet unknown astrocytic phenomenon: Astrocytes cultured on microelectrode arrays (MEAs) exhibited extracellular voltage fluctuations in a broad frequency spectrum (100–600 Hz) after electrical stimulation. These aperiodic high-frequency oscillations (HFOs) could last several seconds and did not spread across the MEA. The voltage-gated calcium channel antagonist cilnidipine dose-dependently decreased the power of the oscillations. While intracellular calcium was pivotal, incubation with bafilomycin A1 showed that vesicular release of transmitters played only a minor role in the emergence of HFOs. Gap junctions and volume-regulated anionic channels had just as little functional impact, which was demonstrated by the addition of carbenoxolone (100 μmol/L) and NPPB (100 μmol/L). Hyperpolarization with low potassium in the extracellular solution (2 mmol/L) dramatically raised oscillation power. A similar effect was seen when we added extra sodium (+50 mmol/L) or if we replaced it with NMDG⁺ (50 mmol/L). The purinergic receptor antagonist PPADS suppressed the oscillation power, while the agonist ATP (100 μmol/L) had only an increasing effect when the bath solution pH was slightly lowered to pH 7.2. From these observations, we conclude that astrocytic voltage oscillations are triggered by activation of voltage-gated calcium channels and driven by a downstream influx of cations through channels that are permeable for large ions such as NMDG⁺. Most likely candidates are subtypes of pore-forming P2X channels with a low affinity for ATP.

Signal transmission within the P2X2 trimeric receptor

The ATP activation signal in trimeric P2X₂ receptors propagates down individual subunits before spreading to all three at the level of the pore.

P2X₂ receptor channel, a homotrimer activated by the binding of extracellular adenosine triphosphate (ATP) to three intersubunit ATP-binding sites (each located ∼50 Å from the ion permeation pore), also shows voltage-dependent activation upon hyperpolarization. Here, we used tandem trimeric constructs (TTCs) harboring critical mutations at the ATP-binding, linker, and pore regions to investigate how the ATP activation signal is transmitted within the trimer and how signals generated by ATP and hyperpolarization converge. Analysis of voltage- and [ATP]-dependent gating in these TTCs showed that: (a) Voltage- and [ATP]-dependent gating of P2X₂ requires binding of at least two ATP molecules. (b) D315A mutation in the β-14 strand of the linker region connecting the ATP-binding domains to the pore-forming helices induces two different gating modes; this requires the presence of the D315A mutation in at least two subunits. (c) The T339S mutation in the pore domains of all three subunits abolishes the voltage dependence of P2X₂ gating in saturating [ATP], making P2X₂ equally active at all membrane potentials. Increasing the number of T339S mutations in the TTC results in gradual changes in the voltage dependence of gating from that of the wild-type channel, suggesting equal and independent contributions of the subunits at the pore level. (d) Voltage- and [ATP]-dependent gating in TTCs differs depending on the location of one D315A relative to one K308A that blocks the ATP binding and downstream signal transmission. (e) Voltage- and [ATP]-dependent gating does not depend on where one T339S is located relative to K308A (or D315A). Our results suggest that each intersubunit ATP-binding signal is directly transmitted on the same subunit to the level of D315 via the domain that contributes K308 to the β-14 strand. The signal subsequently spreads equally to all three subunits at the level of the pore, resulting in symmetric and independent contributions of the three subunits to pore opening.

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.

Conductance simulation of the purinergic P2X2, P2X4, and P2X7 ionic channels using a combined Brownian dynamics and molecular dynamics approach

This paper investigates the application of an original combined approach of molecular and Brownian dynamic methods with quantum chemistry calculations for modeling the process of conductance of ion channels using purinergic P2X family receptors P2X2, P2X4, and P2X7 as a case study. A simplified model of the ionic channel in the lipid bilayer has been developed. A high level of conductance (30 pS) of P2X2 ionic channel together with the key role of Asp349 in forming the selectivity filter of P2X2 has been shown by using this approach. Calculated P2X2 permeability to monovalent cations Li(+), Na(+), and K(+) conforms to the free diffusion coefficient of these ions, which shows the low selectivity of P2X2 ionic channel.

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.

Principles and properties of ion flow in P2X receptors

P2X receptors are a family of trimeric ion channels that are gated by extracellular adenosine 5′-triphosphate (ATP). These receptors have long been a subject of intense research interest by virtue of their vital role in mediating the rapid and direct effects of extracellular ATP on membrane potential and cytosolic Ca²⁺ concentration, which in turn underpin the ability of ATP to regulate a diverse range of clinically significant physiological functions, including those associated with the cardiovascular, sensory, and immune systems. An important aspect of an ion channel's function is, of course, the means by which it transports ions across the biological membrane. A concerted effort by investigators over the last two decades has culminated in significant advances in our understanding of how P2X receptors conduct the inward flux of Na⁺ and Ca²⁺ in response to binding by ATP. However, this work has relied heavily on results from current recordings of P2X receptors altered by site-directed mutagenesis. In the absence of a 3-dimensional channel structure, this prior work provided only a vague and indirect appreciation of the relationship between structure, ion selectivity and flux. The recent publication of the crystal structures for both the closed and open channel conformations of the zebrafish P2X4 receptor has thus proved a significant boon, and has provided an important opportunity to overview the amassed functional data in the context of a working 3-dimensional model of a P2X receptor. In this paper, we will attempt to reconcile the existing functional data regarding ion permeation through P2X receptors with the available crystal structure data, highlighting areas of concordance and discordance as appropriate.

Exocytosis of ATP from astrocytes modulates phasic and tonic inhibition in the neocortex

Astrocytes secrete ATP by exocytosis from synaptic-like vesicles, activating neuronal P2X receptors, which contribute to postsynaptic GABA receptor down-regulation, ultimately mediating the communication between astrocytes and neurons required for brain function.

Communication between neuronal and glial cells is important for many brain functions. Astrocytes can modulate synaptic strength via Ca²⁺-stimulated release of various gliotransmitters, including glutamate and ATP. A physiological role of ATP release from astrocytes was suggested by its contribution to glial Ca²⁺-waves and purinergic modulation of neuronal activity and sleep homeostasis. The mechanisms underlying release of gliotransmitters remain uncertain, and exocytosis is the most intriguing and debated pathway. We investigated release of ATP from acutely dissociated cortical astrocytes using “sniff-cell” approach and demonstrated that release is vesicular in nature and can be triggered by elevation of intracellular Ca²⁺ via metabotropic and ionotropic receptors or direct UV-uncaging. The exocytosis of ATP from neocortical astrocytes occurred in the millisecond time scale contrasting with much slower nonvesicular release of gliotransmitters via Best1 and TREK-1 channels, reported recently in hippocampus. Furthermore, we discovered that elevation of cytosolic Ca²⁺ in cortical astrocytes triggered the release of ATP that directly activated quantal purinergic currents in the pyramidal neurons. The glia-driven burst of purinergic currents in neurons was followed by significant attenuation of both synaptic and tonic inhibition. The Ca²⁺-entry through the neuronal P2X purinoreceptors led to phosphorylation-dependent down-regulation of GABAA receptors. The negative purinergic modulation of postsynaptic GABA receptors was accompanied by small presynaptic enhancement of GABA release. Glia-driven purinergic modulation of inhibitory transmission was not observed in neurons when astrocytes expressed dn-SNARE to impair exocytosis. The astrocyte-driven purinergic currents and glia-driven modulation of GABA receptors were significantly reduced in the P2X4 KO mice. Our data provide a key evidence to support the physiological importance of exocytosis of ATP from astrocytes in the neocortex.

Brain function depends on the interaction between two major types of cells: neurons transmitting electrical signals and glial cells, which control cerebral circulation and neuronal homeostasis. There is a growing evidence of the participation of astrocytes in regulating neuronal excitability and synaptic plasticity via the release of “gliotransmitters,” which include glutamate and ATP. The importance of ATP release from astrocytes was suggested by studies that demonstrated its contribution to neuronal activity and sleep homeostasis via modulation of known “purinergic” receptors. But the mechanisms underlying gliotransmitter release and the physiological significance of direct glia-to-neuron communication remain unknown and intensively debated. Here, we investigate the release of ATP from astrocytes of brain neocortex and demonstrate that astrocytes can release ATP by Ca²⁺-dependent exocytosis, most likely from synaptic-like microvesicles. We also find that vesicular release of ATP from astrocytes can directly activate excitatory signaling in the neighboring neurons, operating through purinergic P2X receptors. We saw that activation of these P2X receptors by astrocyte-driven ATP down-regulated the inhibitory synaptic signaling in the neocortical neurons. Our results imply that exocytosis of gliotransmitters is important for the communication between astrocytes and neurons in the neocortex.

Exploring the ATP-binding site of P2X receptors

P2X receptors are ATP-gated non-selective cation channels involved in many different physiological processes, such as synaptic transmission, inflammation, and neuropathic pain. They form homo- or heterotrimeric complexes and contain three ATP-binding sites in their extracellular domain. The recent determination of X-ray structures of a P2X receptor solved in two states, a resting closed state and an ATP-bound, open-channel state, has provided unprecedented information not only regarding the three-dimensional shape of the receptor, but also on putative conformational changes that couple ATP binding to channel opening. These data provide a structural template for interpreting the huge amount of functional, mutagenesis, and biochemical data collected during more than fifteen years. In particular, the interfacial location of the ATP binding site and ATP orientation have been successfully confirmed by these structural studies. It appears that ATP binds to inter-subunit cavities shaped like open jaws, whose tightening induces the opening of the ion channel. These structural data thus represent a firm basis for understanding the activation mechanism of P2X receptors.

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.

Heteromeric assembly of P2X subunits

Transcripts and/or proteins of P2X receptor (P2XR) subunits have been found in virtually all mammalian tissues. Generally more than one of the seven known P2X subunits have been identified in a given cell type. Six of the seven cloned P2X subunits can efficiently form functional homotrimeric ion channels in recombinant expression systems. This is in contrast to other ligand-gated ion channel families, such as the Cys-loop or glutamate receptors, where homomeric assemblies seem to represent the exception rather than the rule. P2XR mediated responses recorded from native tissues rarely match exactly the biophysical and pharmacological properties of heterologously expressed homomeric P2XRs. Heterotrimerization of P2X subunits is likely to account for this observed diversity. While the existence of heterotrimeric P2X2/3Rs and their role in physiological processes is well established, the composition of most other P2XR heteromers and/or the interplay between distinct trimeric receptor complexes in native tissues is not clear. After a description of P2XR assembly and the structure of the intersubunit ATP-binding site, this review summarizes the distribution of P2XR subunits in selected mammalian cell types and the biochemically and/or functionally characterized heteromeric P2XRs that have been observed upon heterologous co-expression of P2XR subunits. We further provide examples where the postulated heteromeric P2XRs have been suggested to occur in native tissues and an overview of the currently available pharmacological tools that have been used to discriminate between homo- and heteromeric P2XRs.

P2X receptor intermediate activation states have altered nucleotide selectivity

Purinergic P2X receptors are widely distributed in the nervous system and are known to play roles in primary afferent transmission and central respiratory regulation. They are trimeric membrane proteins, with the extracellular domain that provides three intersubunit ATP binding sites. We expressed the rat P2X7 receptor in human embryonic kidney cells and measured membrane currents before and after photo-affinity labeling with the agonist 2'(3')-O-(4-benzoylbenzoyl)-ATP (BzATP). After tethering BzATP with ultraviolet light, a persistent current remained after washing out the agonist. Additional current could now be elicited by other nucleotides (CTP and ADP) that are not normally effective as P2X receptor agonists. Similar results were obtained at P2X2 receptors even without previous agonist tethering: exposure to low concentrations of ATP caused the receptor to become sensitive to activation by CTP and ADP. The results show that ATP binding to the first of the three binding sites causes a conformational change to an intermediate closed state that shows increased effectiveness of pyrimidine and diphosphate nucleotide analogs.

Validation of Alexa-647-ATP as a powerful tool to study P2X receptor ligand binding and desensitization

Ion channel opening and desensitization is a fundamental process in neurotransmission. The ATP-gated P2X1 receptor (P2X1R) shows rapid and long-lasting desensitization upon agonist binding. This makes the electrophysiological investigation of its desensitization process, agonist unbinding, and recovery from desensitization a challenging task. Here, we show that the fluorescent agonist Alexa-647-ATP is a potent agonist at the P2X1R and a versatile tool to directly visualize agonist binding and unbinding. We demonstrate that the long-lasting desensitization of the P2X1R is due to both slow unbinding of agonist from the desensitized receptor and agonist mediated receptor internalization. Furthermore, the unbinding of the agonist Alexa-647-ATP from the desensitized receptor is accelerated in the continuous presence of competitive ligand. Modeling of our data indicates that three agonist molecules are required to drive the receptor into desensitization. Direct visualization of ligand unbinding from the desensitized receptor demonstrates the cooperativity of this process.

Insights into the Molecular Mechanisms Underlying Mammalian P2X7 Receptor Functions and Contributions in Diseases, Revealed by Structural Modeling and Single Nucleotide Polymorphisms

The mammalian P2X7 receptors (P2X7Rs), a member of the ionotropic P2X receptor family with distinctive functional properties, play an important part in mediating extracellular ATP signaling in health and disease. A clear delineation of the molecular mechanisms underlying the key receptor properties, such as ATP-binding, ion permeation, and large pore formation of the mammalian P2X7Rs, is still lacking, but such knowledge is crucial for a better understanding of their physiological functions and contributions in diseases and for development of therapeutics. The recent breakthroughs in determining the atomic structures of the zebrafish P2X4.1R in the closed and ATP-bound open states have provided the long-awaited structural information. The human P2RX7 gene is abundant with non-synonymous single nucleotide polymorphisms (NS-SNPs), which generate a repertoire of human P2X7Rs with point mutations. Characterizations of the NS-SNPs identified in patients of various disease conditions and the resulting mutations have informed previously unknown molecular mechanisms determining the mammalian P2X7R functions and diseases. In this review, we will discuss the new insights into such mechanisms provided by structural modeling and recent functional and genetic linkage studies of NS-SNPs.

Allosteric nature of P2X receptor activation probed by photoaffinity labelling

BACKGROUND AND PURPOSE

In P2X receptors, agonist binding at the interface between neighbouring subunits is efficiently transduced to ion channel gating. However, the relationship between binding and gating is difficult to study because agonists continuously bind and unbind. Here, we covalently incorporated agonists in the binding pocket of P2X receptors and examined how binding site occupancy affects the ability of the channel to gate.

EXPERIMENTAL APPROACH

We used a strategy for tethering agonists to their ATP-binding pocket, while simultaneously probing ion channel gating using electrophysiology. The agonist 2',3'-O-(4-benzoylbenzoyl)-ATP (BzATP), a photoaffinity analogue of ATP, enabled us to trap rat homomeric P2X2 receptor and a P2X2/1 receptor chimera in different agonist-bound states. UV light was used to control the degree of covalent occupancy of the receptors.

KEY RESULTS

Irradiation of the P2X2/1 receptor chimera - BzATP complex resulted in a persistent current that lasted even after extensive washout, consistent with photochemical tethering of the agonist BzATP and trapping of the receptors in an open state. Partial labelling with BzATP primed subsequent agonist binding and modulated gating efficiency for both full and partial agonists.

CONCLUSIONS AND IMPLICATIONS

Our photolabelling strategy provides new molecular insights into the activation mechanism of the P2X receptor. We show here that priming with full agonist molecules leads to an increase in gating efficiency after subsequent agonist binding.

Pannexin: from discovery to bedside in 11±4 years?

Pannexin1 (Panx1) originally was discovered as a gap junction related protein. However, rather than forming the cell-to-cell channels of gap junctions, Panx1 forms a mechanosensitive and highly ATP permeable channel in the cell membrane allowing the exchange of molecules between the cytoplasm and the extracellular space. The list of arguments for Panx1 representing the major ATP release channel includes: (1) Panx1 is expressed in (all?) cells releasing ATP in a non-vesicular fashion, such as erythrocytes; (2) in cells with polar release of ATP, Panx1 is expressed at the ATP release site, such as the apical membrane in airway epithelial cells; (3) the pharmacology of Panx1 channels matches that of ATP release; (4) mutation of Panx1 in strategic positions in the protein modifies ATP release; and (5) knockdown or knockout of Panx1 attenuates or abolishes ATP release. Panx1, in association with the purinergic receptor P2X7, is involved in the innate immune response and in apoptotic/pyroptotic cell death. Inflammatory processes are responsible for amplification of the primary lesion in CNS trauma and stroke. Panx1, as an early signal event and as a signal amplifier in these processes, is an obvious target for the prevention of secondary cell death due to inflammasome activity. Since Panx1 inhibitors such as probenecid are already clinically tested in different settings they should be considered for therapy in stroke and CNS trauma.

Moving through the gate in ATP-activated P2X receptors

P2X receptors are nonselective cation channels gated by extracellular ATP. They represent new therapeutic targets, and they form channels with a unique trimeric architecture. In 2009, the first crystal structure of a P2X receptor was reported, in which the receptor was in an ATP-free, closed channel state. However, our view recently changed when a second crystal structure was reported, in which a P2X receptor was bound to ATP and resolved in an open channel conformation. This remarkable structure not only confirms many key experimental data, including the recent mechanisms of ATP binding and ion permeation, but also reveals unanticipated mechanisms. Certainly, this new information will accelerate our understanding of P2X receptor function and pharmacology at the atomic level.

Covalent modification of mutant rat P2X2 receptors with a thiol-reactive fluorophore allows channel activation by zinc or acidic pH without ATP

Rat P2X2 receptors open at an undetectably low rate in the absence of ATP. Furthermore, two allosteric modulators, zinc and acidic pH, cannot by themselves open these channels. We describe here the properties of a mutant receptor, K69C, before and after treatment with the thiol-reactive fluorophore Alexa Fluor 546 C(5)-maleimide (AM546). Xenopus oocytes expressing unmodified K69C were not activated under basal conditions nor by 1,000 µM ATP. AM546 treatment caused a small increase in the inward holding current which persisted on washout and control experiments demonstrated this current was due to ATP independent opening of the channels. Following AM546 treatment, zinc (100 µM) or acidic external solution (pH 6.5) elicited inward currents when applied without any exogenous ATP. In the double mutant K69C/H319K, zinc elicited much larger inward currents, while acidic pH generated outward currents. Suramin, which is an antagonist of wild type receptors, behaved as an agonist at AM546-treated K69C receptors. Several other cysteine-reactive fluorophores tested on K69C did not cause these changes. These modified receptors show promise as a tool for studying the mechanisms of P2X receptor activation.

Neuromodulation by extracellular ATP and P2X receptors in the CNS

Extracellular adenosine 5' triphosphate (ATP) is a widespread cell-to-cell signaling molecule in the brain, where it activates cell surface P2X and P2Y receptors. P2X receptors define a protein family unlike other neurotransmitter-gated ion channels in terms of sequence, subunit topology, assembly, and architecture. Within milliseconds of binding ATP, they catalyze the opening of a cation-selective pore. However, recent data show that P2X receptors often underlie neuromodulatory responses on slower time scales of seconds or longer. Herein, we review these findings at molecular, cellular and systems levels. We propose that, while P2X receptors are fast ligand-gated cation channels, they are most adept at mediating slow neuromodulatory functions that are more widespread and more physiologically utilized than fast ATP synaptic transmission in the CNS.

Intermediate closed channel state(s) precede(s) activation in the ATP-gated P2X2 receptor

The molecular mechanism underlying channel opening in response to agonist binding remains a challenging issue in neuroscience. In this regard, many efforts have been recently undertaken in ATP-gated P2X receptors. Among those efforts, we have provided evidence in the P2X2 receptor that tightening of ATP sites upon agonist binding induces opening of the ion channel. Here we extend our analysis to show that the sulfhydryl-reactive ATP analog 8-thiocyano-ATP (NCS-ATP), a potent P2X2 agonist, when covalently labeled in the ATP-binding site at position Leu186 likely favors the tightening mechanism, but not the channel opening mechanism. Our data predict the existence of intermediate or preactivation state(s) trapped by NCS-ATP, in which tightening of the binding site is favored while the channel is still closed. We propose that this (these) intermediate ATP-bound state(s) prime(s) channel gating in the P2X2 receptor.

Activation of trimeric P2X2 receptors by fewer than three ATP molecules

P2X receptors are trimeric membrane proteins. When they bind extracellular ATP, a conformational change occurs that opens a transmembrane ion channel. The ATP-binding pocket is formed in a cleft between two subunits, and a critical amino acid residue for ATP contact is Lys⁶⁹ (P2X2 numbering). In the present work, we sought to determine whether the binding of fewer than three ATP molecules could open the ion channel. We expressed eight concatenated cDNAs in human embryonic kidney cells, which encoded three serially joined, epitope-tagged, subunits with either Lys or Ala at position 69 (denoted as KKK, KKA, KAK, AKK, KAA, AKA, AAK, and AAA). Western blotting of surface-biotinylated proteins indicated that breakdown of concatemers to individual subunits was minimal. Recording of membrane currents in response to ATP (whole cell and excised outside-out patch) showed that all formed functional channels except AAK, AKA, and AAA. There was no difference in the kinetics of activation and deactivation among KKK, KKA, KAK, and AKK channels, and amplitude of the unitary conductances was in all cases not different from that found after expression of a single wild-type subunit. Currents through KKA and KAK receptors were larger than those observed for AKK receptors. The results indicate that trimeric P2X receptors containing only two intact binding sites can be readily activated by ATP.