Visualization of the trimeric P2X2 receptor with a crown-capped extracellular domain

Multimeric Ionotropic Purinoceptor Detection by Protein Cross-Linking

P2X receptor subunits (P2X1 to P2X7) assemble to form trimeric homomers or heteromers. Here, we describe the use of protein cross-linking to study the composition of P2X receptor complexes. This simple protocol is useful for determining the stoichiometry of P2X heteromeric receptors as well as for assessing the effect of point mutation, truncation, or concatenation on the quaternary architecture of these receptors.

Deciphering the regulation of P2X4 receptor channel gating by ivermectin using Markov models

The P2X4 receptor (P2X4R) is a member of a family of purinergic channels activated by extracellular ATP through three orthosteric binding sites and allosterically regulated by ivermectin (IVM), a broad-spectrum antiparasitic agent. Treatment with IVM increases the efficacy of ATP to activate P2X4R, slows both receptor desensitization during sustained ATP application and receptor deactivation after ATP washout, and makes the receptor pore permeable to NMDG⁺, a large organic cation. Previously, we developed a Markov model based on the presence of one IVM binding site, which described some effects of IVM on rat P2X4R. Here we present two novel models, both with three IVM binding sites. The simpler one-layer model can reproduce many of the observed time series of evoked currents, but does not capture well the short time scales of activation, desensitization, and deactivation. A more complex two-layer model can reproduce the transient changes in desensitization observed upon IVM application, the significant increase in ATP-induced current amplitudes at low IVM concentrations, and the modest increase in the unitary conductance. In addition, the two-layer model suggests that this receptor can exist in a deeply inactivated state, not responsive to ATP, and that its desensitization rate can be altered by each of the three IVM binding sites. In summary, this study provides a detailed analysis of P2X4R kinetics and elucidates the orthosteric and allosteric mechanisms regulating its channel gating.

Ligand-gated ion channels play a crucial role in controlling many physiological and pathophysiological processes. Deciphering the gating kinetics of these channels is thus fundamental to understanding how these processes work. ATP-gated purinergic P2X receptors (P2XRs) are prototypic examples of such channels. They are ubiquitously expressed and play roles in numerous cellular processes, including neurotransmission, inflammation, and chronic pain. Seven P2X subunits, named P2X1 through P2X7, and several splice forms of these subunits have been identified in mammal. The receptors are organized as homo- or heterotrimers, each possessing three ATP-binding sites that, when occupied, lead to receptor activation and channel opening. The P2XRs are non-selective cation channels and the gating properties differ between the various receptors. Previously, we have used biophysical and mathematical modeling approaches to decipher the kinetics of homomeric P2X2aR, P2X2bR, P2X4R, and P2X7R. Here we extended our work on P2X4R gating. We developed two mathematical models that could capture the various patterns of ionic currents recorded experimentally and explain the particularly complex kinetics of the receptor during orthosteric activation and allosteric modulation. This was achieved by designing a computationally efficient, inference-based fitting algorithm that allowed for parameter optimization and model comparisons.

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.

Molecular structure and function of P2X receptors

ATP-gated P2X receptors are trimeric ion channels selective to cations. Recent progress in the molecular biophysics of these channels enables a better understanding of their function. In particular, data obtained from biochemical, electrophysiogical and molecular engineering in the light of recent X-ray structures now allow delineation of the principles of ligand binding, channel opening and allosteric modulation. However, although a picture emerges as to how ATP triggers channel opening, there are a number of intriguing questions that remain to be answered, in particular how the pore itself opens in response to ATP and how the intracellular domain, for which structural information is limited, moves during activation. In this review, we provide a summary of functional studies in the context of the post-structure era, aiming to clarify our understanding of the way in which P2X receptors function in response to ATP binding, as well as the mechanism by which allosteric modulators are able to regulate receptor function. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

P2X7 receptors: role in bone cell formation and function

The role of the P2X7 receptor (P2X7R) is being explored with intensive interest in the context of normal bone physiology, bone-related diseases and, to an extent, bone cancer. In this review, we cover the current understanding of P2X7R regulation of bone cell formation, function and survival. We will discuss how the P2X7R drives lineage commitment of undifferentiated bone cell progenitors, the vital role of P2X7R activation in bone mineralisation and its relatively unexplored role in osteocyte function. We also review how P2X7R activation is imperative for osteoclast formation and its role in bone resorption via orchestrating osteoclast apoptosis. Variations in the gene for the P2X7R (P2RX7) have implications for P2X7R-mediated processes and we review the relevance of these genetic variations in bone physiology. Finally, we highlight how targeting P2X7R may have therapeutic potential in bone disease and cancer.

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.

Synaptic effects induced by alcohol

Ethanol (EtOH) has effects on numerous cellular molecular targets, and alterations in synaptic function are prominent among these effects. Acute exposure to EtOH activates or inhibits the function of proteins involved in synaptic transmission, while chronic exposure often produces opposing and/or compensatory/homeostatic effects on the expression, localization, and function of these proteins. Interactions between different neurotransmitters (e.g., neuropeptide effects on release of small molecule transmitters) can also influence both acute and chronic EtOH actions. Studies in intact animals indicate that the proteins affected by EtOH also play roles in the neural actions of the drug, including acute intoxication, tolerance, dependence, and the seeking and drinking of EtOH. This chapter reviews the literature describing these acute and chronic synaptic effects of EtOH and their relevance for synaptic transmission, plasticity, and behavior.

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.

Molecular and functional properties of P2X receptors--recent progress and persisting challenges

ATP-gated P2X receptors are trimeric ion channels that assemble as homo- or heteromers from seven cloned subunits. Transcripts and/or proteins of P2X subunits have been found in most, if not all, mammalian tissues and are being discovered in an increasing number of non-vertebrates. Both the first crystal structure of a P2X receptor and the generation of knockout (KO) mice for five of the seven cloned subtypes greatly advanced our understanding of their molecular and physiological function and their validation as drug targets. This review summarizes the current understanding of the structure and function of P2X receptors and gives an update on recent developments in the search for P2X subtype-selective ligands. It also provides an overview about the current knowledge of the regulation and modulation of P2X receptors on the cellular level and finally on their physiological roles as inferred from studies on KO mice.

P2X4 receptors interact with both P2X2 and P2X7 receptors in the form of homotrimers

BACKGROUND AND PURPOSE

The P2X receptor family consists of seven subunit types - P2X1-P2X7. All but P2X6 are able to assemble as homotrimers. In addition, various subunit permutations have been reported to form heterotrimers. Evidence for heterotrimer formation includes co-localization, co-immunoprecipitation and the generation of receptors with novel functional properties; however, direct structural evidence for heteromer formation, such as chemical cross-linking and single-molecule imaging, is available in only a few cases. Here we examined the nature of the interaction between two pairs of subunits - P2X2 and P2X4, and P2X4 and P2X7.

EXPERIMENTAL APPROACH

We used several experimental approaches, including in situ proximity ligation, co-immunoprecipitation, co-isolation on affinity beads, chemical cross-linking and atomic force microscopy (AFM) imaging.

KEY RESULTS

Both pairs of subunits co-localize upon co-transfection, interact intimately within cells, and can be co-immunoprecipitated and co-isolated from cell extracts. Despite this, chemical cross-linking failed to show evidence for heteromer formation. AFM imaging of isolated receptors showed that all three subunits had the propensity to form receptor dimers. This self-association is likely to account for the observed close interaction between the subunit pairs, in the absence of true heteromer formation.

CONCLUSIONS AND IMPLICATIONS

We conclude that both pairs of receptors interact in the form of distinct homomers. We urge caution in the interpretation of biochemical evidence indicating heteromer formation in other cases.

A consensus segment in the M2 domain of the hP2X(7) receptor shows ion channel activity in planar lipid bilayers and in biological membranes

The P2X(7) receptor (P2X(7)R) is an ATP-gated, cation-selective channel permeable to Na(+), K(+) and Ca(2+). This channel has also been associated with the opening of a non-selective pore that allows the flow of large organic ions. However, the biophysical properties of the P2X(7)R have yet to be characterized unequivocally. We investigated a region named ADSEG, which is conserved among all subtypes of P2X receptors (P2XRs). It is located in the M2 domain of hP2X(7)R, which aligns with the H5 signature sequence of potassium channels. We investigated the channel forming ability of ADSEG in artificial planar lipid bilayers and in biological membranes using the cell-attached patch-clamp techniques. ADSEG forms channels, which exhibit a preference for cations. They are voltage independent and show long-term stability in planar lipid bilayers as well as under patch-clamping conditions. The open probability of the ADSEG was similar to that of native P2X(7)R. The conserved part of the M2 domain of P2X(7)R forms ionic channels in planar lipid bilayers and in biological membranes. Its electrophysiological characteristics are similar to those of the whole receptor. Conserved and hydrophobic part of the M2 domain forms ion channels.

Expression, purification, electron microscopy, N-glycosylation mutagenesis and molecular modeling of human P2X4 and Dictyostelium discoideum P2XA

The recent publication of the apo-, closed-state 3D crystal structure of zebrafish (zf) P2X4.1 has not only revolutionized the P2X research field, but also highlighted the need for further crystal structures, of receptors in different activation states, so that we can gain a complete molecular understanding of ion channel function. zfP2X4.1 was selected as a 3D-crystallization candidate because of its ability to form stable trimers in detergent solution, and purified from over-expression in baculovirus-infected Spodoptera frugiperda (Sf9) insect cells. In this work, we have used a similar approach to express both human P2X4 (hP2X4) and Dictyostelium discoideum P2XA (DdP2XA) in Sf9 cells. Although hP2X4 did not form stable trimers in detergent solution, both receptors bound to ATP-coupled resins, indicating that their extracellular domains were folded correctly. DdP2XA formed strong trimers in detergent solution, and we were able to selectively purify trimers using preparative electrophoresis, and build a 21Å-resolution 3D structure using transmission electron microscopy and single particle analysis. Although the structure of DdP2XA possessed similar dimensions to those of the previously determined low-resolution hP2X4 structure and the zfP2X4.1 crystal structure, N-glycosylation mutagenesis and molecular modeling indicated differences between N-glycan usage and predicted accessibility in models of DdP2XA based on the zfP2X4.1 crystal structure. Our data demonstrate that DdP2XA expressed in insect cells retains ATP-binding capacity after detergent solubilization, is an ideal candidate for structural study, and possesses a significantly different 3D structure to that of both hP2X4 and zfP2X4.1.

Bioinformatic characterization of the trimeric intracellular cation-specific channel protein family

Trimeric intracellular cation-specific (TRIC) channels are integral to muscle excitation-contraction coupling. TRIC channels provide counter-ionic flux when calcium is rapidly transported from intracellular stores to the cell cytoplasm. Until recently, knowledge of the presence of these proteins was limited to animals. We analyzed the TRIC family and identified a profusion of prokaryotic family members with topologies and motifs similar to those of their eukaryotic counterparts. Prokaryotic members far outnumber eukaryotic members, and although none has been functionally characterized, the evidence suggests that they function as secondary carriers. The presence of fused N- or C-terminal domains of known biochemical functions as well as genomic context analyses provide clues about the functions of these prokaryotic homologs. They are proposed to function in metabolite (e.g., amino acid/nucleotide) efflux. Phylogenetic analysis revealed that TRIC channel homologs diverged relatively early during evolutionary history and that horizontal gene transfer was frequent in prokaryotes but not in eukaryotes. Topological analyses of TRIC channels revealed that these proteins possess seven putative transmembrane segments (TMSs), which arose by intragenic duplication of a three-TMS polypeptide-encoding genetic element followed by addition of a seventh TMS at the C terminus to give the precursor of all current TRIC family homologs. We propose that this family arose in prokaryotes.

Structural interpretation of P2X receptor mutagenesis studies on drug action

P2X receptors for ATP are ligand gated cation channels that form from the trimeric assembly of subunits with two transmembrane segments, a large extracellular ligand binding loop, and intracellular amino and carboxy termini. The receptors are expressed throughout the body, involved in functions ranging from blood clotting to inflammation, and may provide important targets for novel therapeutics. Mutagenesis based studies have been used to develop an understanding of the molecular basis of their pharmacology with the aim of developing models of the ligand binding site. A crystal structure for the zebra fish P2X4 receptor in the closed agonist unbound state has been published recently, which provides a major advance in our understanding of the receptors. This review gives an overview of mutagenesis studies that have led to the development of a model of the ATP binding site, as well as identifying residues contributing to allosteric regulation and antagonism. These studies are discussed with reference to the crystal to provide a structural interpretation of the molecular basis of drug action.

P2X receptors in health and disease

Seven P2X receptor subunits have been cloned which form functional homo- and heterotrimers. These are cation-selective channels, equally permeable to Na(+) and K(+) and with significant Ca(2+) permeability. The three-dimensional structure of the P2X receptor is described. The channel pore is formed by the α-helical transmembrane spanning region 2 of each subunit. When ATP binds to a P2X receptor, the pore opens within milliseconds, allowing the cations to flow. P2X receptors are expressed on both central and peripheral neurons, where they are involved in neuromuscular and synaptic neurotransmission and neuromodulation. They are also expressed in most types of nonneuronal cells and mediate a wide range of actions, such as contraction of smooth muscle, secretion, and immunomodulation. Changes in the expression of P2X receptors have been characterized in many pathological conditions of the cardiovascular, gastrointestinal, respiratory, and urinogenital systems and in the brain and special senses. The therapeutic potential of P2X receptor agonists and antagonists is currently being investigated in a range of disorders, including chronic neuropathic and inflammatory pain, depression, cystic fibrosis, dry eye, irritable bowel syndrome, interstitial cystitis, dysfunctional urinary bladder, and cancer.

P2X receptors: dawn of the post-structure era

P2X receptors are non-selective cation channels gated by extracellular ATP. They play key roles in various physiological processes such as nerve transmission, pain sensation and the response to inflammation, making them attractive drug targets for the treatment of inflammatory pain. The recent report of the three-dimensional (3D) crystal structure of zebrafish P2X4.1 represents a step change in our understanding of these membrane ion channels, where previously only low-resolution structural data and inferences from indirect structure–function studies were available. The purpose of this review is to place previous work within the context of the new 3D structure, and to summarize the key questions and challenges which await P2X researchers as we move into the post-structure era.

Signaling at purinergic P2X receptors

P2X receptors are membrane cation channels gated by extracellular ATP. Seven P2X receptor subunits (P2X(1-7)) are widely distributed in excitable and nonexcitable cells of vertebrates. They play key roles in inter alia afferent signaling (including pain), regulation of renal blood flow, vascular endothelium, and inflammatory responses. We summarize the evidence for these and other roles, emphasizing experimental work with selective receptor antagonists or with knockout mice. The receptors are trimeric membrane proteins: Studies of the biophysical properties of mutated subunits expressed in heterologous cells have indicated parts of the subunits involved in ATP binding, ion permeation (including calcium permeability), and membrane trafficking. We review our current understanding of the molecular properties of P2X receptors, including how this understanding is informed by the identification of distantly related P2X receptors in simple eukaryotes.

Dynamic aspects of functional regulation of the ATP receptor channel P2X2

The P2X(2) channel is a ligand-gated channel activated by ATP. Functional features that reflect the dynamic flexibility of the channel include time-dependent pore dilatation following ATP application and direct inhibitory interaction with activated nicotinic acetylcholine receptors on the membrane. We have been studying the mechanisms by which P2X(2) channel functionality is dynamically regulated. Using a Xenopus oocyte expression system, we observed that the pore properties, including ion selectivity and rectification, depend on the open channel density on the membrane. Pore dilatation was apparent when the open channel density was high and inward rectification was modest. We also observed that P2X(2) channels show voltage dependence, despite the absence of a canonical voltage sensor. At a semi-steady state after ATP application, P2X(2) channels were activated upon membrane hyperpolarization. This voltage-dependent activation was also [ATP] dependent. With increases in [ATP], the speed of hyperpolarization-induced activation was increased and the conductance-voltage relationship was shifted towards depolarized potentials. Based on analyses of experimental data and various simulations, we propose that these phenomena can be explained by assuming a fast ATP binding step and a rate-limiting voltage-dependent gating step. Complete elucidation of these regulatory mechanisms awaits dynamic imaging of functioning P2X(2) channels.

Voltage- and [ATP]-dependent gating of the P2X(2) ATP receptor channel

P2X receptors are ligand-gated cation channels activated by extracellular adenosine triphosphate (ATP). Nonetheless, P2X₂ channel currents observed during the steady-state after ATP application are known to exhibit voltage dependence; there is a gradual increase in the inward current upon hyperpolarization. We used a Xenopus oocyte expression system and two-electrode voltage clamp to analyze this “activation” phase quantitatively. We characterized the conductance–voltage relationship in the presence of various [ATP], and observed that it shifted toward more depolarized potentials with increases in [ATP]. By analyzing the rate constants for the channel's transition between a closed and an open state, we showed that the gating of P2X₂ is determined in a complex way that involves both membrane voltage and ATP binding. The activation phase was similarly recorded in HEK293 cells expressing P2X₂ even by inside-out patch clamp after intensive perfusion, excluding a possibility that the gating is due to block/unblock by endogenous blocker(s) of oocytes. We investigated its structural basis by substituting a glycine residue (G344) in the second transmembrane (TM) helix, which may provide a kink that could mediate “gating.” We found that, instead of a gradual increase, the inward current through the G344A mutant increased instantaneously upon hyperpolarization, whereas a G344P mutant retained an activation phase that was slower than the wild type (WT). Using glycine-scanning mutagenesis in the background of G344A, we could recover the activation phase by introducing a glycine residue into the middle of second TM. These results demonstrate that the flexibility of G344 contributes to the voltage-dependent gating. Finally, we assumed a three-state model consisting of a fast ATP-binding step and a following gating step and estimated the rate constants for the latter in P2X₂-WT. We then executed simulation analyses using the calculated rate constants and successfully reproduced the results observed experimentally, voltage-dependent activation that is accelerated by increases in [ATP].

Regulation of P2X2 receptors by the neuronal calcium sensor VILIP1

Extracellular adenosine triphosphate (ATP) activates P2X receptors, which are involved in diverse physiological functions. Using a proteomic approach, we identified the neuronal calcium sensor VILIP1 as interacting with P2X2 receptors. We found that VILIP1 forms a signaling complex in vitro and in vivo with P2X2 receptors and regulates P2X2 receptor sensitivity to ATP, peak response, surface expression, and diffusion. VILIP1 constitutively binds to P2X2 receptors and displays enhanced interactions in an activation- and calcium-dependent manner owing to exposure of its binding segment in P2X2 receptors. VILIP1-P2X2 interactions are also enhanced in hippocampal neurons during conditions of action potential firing known to trigger P2X2 receptor activation. Our data thus reveal a previously unrecognized function for the neuronal calcium sensor protein VILIP1 and a mechanism for regulation of ATP-dependent P2X receptor signaling by neuronal calcium sensors.

Molecular shape, architecture, and size of P2X4 receptors determined using fluorescence resonance energy transfer and electron microscopy

P2X receptors are ATP-gated nonselective cation channels with important physiological roles. However, their structures are poorly understood. Here, we analyzed the architecture of P2X receptors using fluorescence resonance energy transfer (FRET) microscopy and direct structure determination using electron microscopy. FRET efficiency measurements indicated that the distance between the C-terminal tails of P2X₄ receptors was 5.6 nm. Single particle analysis of purified P2X₄ receptors was used to determine the three-dimensional structure at a resolution of 21Å_; the orientation of the particle with respect to the membrane was assigned by labeling the intracellular C termini with 1.8-nm gold particles and the carbohydrate-rich ectodomain with lectin. We found that human P2X₄ is a globular torpedo-like molecule with an approximate volume of 270 nm³ and a compact propeller-shaped ectodomain. In this structure, the distance between the centers of the gold particles was 6.1 nm, which closely matches FRET data. Thus, our data provide the first views of the architecture, shape, and size of single P2X receptors, furthering our understanding of this important family of ligand-gated ion channels.

Potential therapeutic targets for ATP-gated P2X receptor ion channels

P2X receptors make up a novel family of ligand-gated ion channels that are activated by binding of extracellular ATP. These receptors can form a number of homomeric and heteromeric ion channels, which are widely distributed throughout the human body. They are thought to play an important role in many cellular processes, including synaptic transmission and thrombocyte aggregation. These ion channels are also involved in the pathology of several disease states, including chronic inflammation and neuropathic pain, and thus are the potential targets for drug development. The recent discovery of potent and highly selective antagonists for P2X(7) receptors, through the use of high-throughput screening, has helped to further understand the P2X receptor pharmacology and provided new evidence that P2X(7) receptors play a specific role in chronic pain states. In this review, we discuss how the P2X family of ion channels has distinguished itself as a potential new drug target. We are optimistic that safe and effective candidate drugs will be suitable for progression into clinical development.

ATP-gated P2X cation-channels

P2X receptors are ATP-gated cation channels with important roles in diverse pathophysiological processes. Substantial progress has been made in the last few years with the discovery of both subunit selective antagonists and modulators. The purpose of this brief review is to summarize the advances in the pharmacology of P2X receptors, with key properties presented in an easy to access format. Ligand-gated ion channels consist of three families in mammals; the ionotropic glutamate receptors, the Cys-loop receptors (for GABA, ACh, glycine and serotonin) and the P2X receptors for ATP. The first two of these are considered in articles accompanying this Special Issue. Here we consider the pharmacological properties of P2X receptors. We do not present a detailed discussion of P2X receptor physiological roles or structure-function studies. Moreover, the pharmacological basis for discriminating between the main subtypes of P2X receptor and their nomenclature has been published by the Nomenclature Committee of the International Union of Pharmacology (NC-IUPHAR) P2X Receptor Subcommittee, and so these aspects are not revisited here. Instead in this brief article we seek to present a summary of the pharmacology of recombinant homomeric and heteromeric P2X receptors, with particular emphasis on new antagonists. In this article we have tried to present as much information as possible in two tables in the hope this will be useful as a day-to-day resource, and also because an excellent and detailed review has recently been published.

Assembly and trafficking of P2X purinergic receptors (Review)

P2X receptors are cation selective ion channels gated by the binding of extracellular ATP. Seven subtypes have been identified and they have widespread and overlapping distributions throughout the body. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. They form part of larger signalling complexes, interacting with unrelated ion channels and other membrane and cytosolic proteins. Up- or down-regulation of their expression is associated with several disease states. This review aims to summarize recent work on the assembly and trafficking of this family of receptors.

Immuno-proteomic approach to excitation--contraction coupling in skeletal and cardiac muscle: molecular insights revealed by the mitsugumins

A comprehensive understanding of excitation-contraction (E-C) coupling in skeletal and cardiac muscle requires that all the major components of the Ca(2+) release machinery be resolved. We utilized a unique immuno-proteomic approach to generate a monoclonal antibody library that targets proteins localized to the skeletal muscle triad junction, which provides a structural context to allow efficient E-C coupling. Screening of this library has identified several mitsugumins (MG); proteins that can be localized to the triad junction in mammalian skeletal muscle. Many of these proteins, including MG29 and junctophilin, are important components in maintaining the structural integrity of the triad junction. Other triad proteins, such as calumin, play a more direct role in regulation of muscle Ca(2+) homeostasis. We have recently identified a family of trimeric intracellular cation-selective (TRIC) channels that allow for K(+) movement into the endoplasmic or sarcoplasmic reticulum to counter a portion of the transient negative charge produced by Ca(2+) release into the cytosol. Further study of TRIC channel function and other novel mitsugumins will increase our understanding of E-C coupling and Ca(2+) homoeostasis in muscle physiology and pathophysiology.

Determination of the architecture of ionotropic receptors using AFM imaging

Fast neurotransmission in the nervous system is mediated by ionotropic receptors, all of which contain several subunits surrounding an integral ion channel. There are three major families of ionotropic receptors: the 'Cys-loop' receptors (including the nicotinic receptor for acetylcholine, the 5-HT(3) receptor, the GABA(A) receptor and the glycine receptor), the glutamate receptors (including the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid, kainate and N-methyl-D: -aspartic acid receptors) and the P2X receptors for adenosine triphosphate. These receptors are often built from multiple types of subunit, raising the question of the stoichiometry and subunit arrangement within the receptors. This question is of therapeutic significance because in some cases drug-binding sites are located at subunit-subunit interfaces. In this paper, we describe a general method, based on atomic force microscopy imaging, to solve the architecture of multi-subunit proteins, such as the ionotropic receptors. Specific epitope tags are engineered onto each receptor subunit. The subunits are then expressed exogenously in cultured cells, and the receptors are isolated from detergent extracts of membrane fractions by affinity chromatography. The receptors are imaged both alone and in complex with anti-epitope antibodies. The size of the imaged particles provides an estimate of the subunit stoichiometry, whereas the geometry of the receptor-antibody complexes produces more detailed information about the receptor architecture. We use an automated, unbiased system to identify receptors and receptor-antibody complexes and to determine the geometry of the complexes. We are also able to determine the orientation of the receptors on the mica substrate, which will allow us to solve the subunit arrangement within receptors, such as the GABA(A) receptor, which contain three types of subunits.

The motor protein prestin is a bullet-shaped molecule with inner cavities

Prestin is a transmembrane motor protein localized at the outer hair cells (OHCs) of the mammalian inner ear. Voltage-dependent conformational changes in prestin generate changes in the length of OHCs. A loss of prestin function is reported to induce severe auditory deficiencies, suggesting prestin-dependent changes of OHC length may be at least a part of cochlear amplification. Here we expressed the recombinant FLAG-fused prestin proteins in Sf9 cells and purified to particles of a uniform size in EM. The square-shaped top view of purified prestin, the binding of multiple anti-FLAG antibodies to each prestin particle, the native-PAGE analysis, and the much larger molecular weight obtained from size exclusion chromatography than the estimation for the monomer all support that prestin is a tetramer (Zheng, J., Du, G. G., Anderson, C. T., Keller, J. P., Orem, A., Dallos, P., and Cheatham, M. (2006) J. Biol. Chem. 281, 19916-19924). From negatively stained prestin particles, the three-dimensional structure was reconstructed at 2 nm resolution assuming 4-fold symmetry. Prestin is shown to be a bullet-shaped particle with a large cytoplasmic domain. The surface representation demonstrates indentations on the molecule, and the slice images indicate the inner cavities of sparse densities. The dimensions, 77 x 77 x 115 A, are consistent with the previously reported sizes of motor proteins on the surface of OHCs.

Lack of evidence for direct phosphorylation of recombinantly expressed P2X(2) and P2X (3) receptors by protein kinase C

P2X₃ and P2X₂₊₃ receptors are present on sensory neurons, where they contribute not only to transient nociceptive responses, but also to hypersensitivity underlying pathological pain states elicited by nerve injuries. Increased signalling through P2X₃ and P2X₂₊₃ receptors may arise from an increased routing to the plasma membrane and/or gain of function of pre-existing receptors. An obvious effector mechanism for functional modulation is protein kinase C (PKC)-mediated phosphorylation, since all P2X family members share a conserved consensus sequence for PKC, TXR/K, within the intracellularly located N-terminal domain. Contradictory reports have been published regarding the exact role of this motif. In the present study, we confirm that site-directed elimination of the potential phosphor-acceptor threonine or the basic residue in the P+2 position of the TXR/K sequence accelerates desensitization of P2X₂ receptors and abolishes P2X₃ receptor function. Moreover, the PKC activator phorbol 12-myristate 13-acetate increased P2X₃ (but not P2X₂) receptor-mediated currents. Biochemically, however, we were unable to demonstrate by various experimental approaches a direct phosphorylation of wild-type P2X₂ and P2X₃ receptors expressed in both Xenopus laevis oocytes and HEK293 cells. In conclusion, our data support the view that the TXR/K motif plays an important role in P2X function and that phorbol 12-myristate 13-acetate is capable of modulating some P2X receptor subtypes. The underlying mechanism, however, is unlikely to involve direct PKC-mediated P2X receptor phosphorylation.

TRIC channels are essential for Ca2+ handling in intracellular stores

Cell signalling requires efficient Ca2+ mobilization from intracellular stores through Ca2+ release channels, as well as predicted counter-movement of ions across the sarcoplasmic/endoplasmic reticulum membrane to balance the transient negative potential generated by Ca2+ release. Ca2+ release channels were cloned more than 15 years ago, whereas the molecular identity of putative counter-ion channels remains unknown. Here we report two TRIC (trimeric intracellular cation) channel subtypes that are differentially expressed on intracellular stores in animal cell types. TRIC subtypes contain three proposed transmembrane segments, and form homo-trimers with a bullet-like structure. Electrophysiological measurements with purified TRIC preparations identify a monovalent cation-selective channel. In TRIC-knockout mice suffering embryonic cardiac failure, mutant cardiac myocytes show severe dysfunction in intracellular Ca2+ handling. The TRIC-deficient skeletal muscle sarcoplasmic reticulum shows reduced K+ permeability, as well as altered Ca2+ 'spark' signalling and voltage-induced Ca2+ release. Therefore, TRIC channels are likely to act as counter-ion channels that function in synchronization with Ca2+ release from intracellular stores.

Acidic amino acids impart enhanced Ca2+ permeability and flux in two members of the ATP-gated P2X receptor family

P2X receptors are ATP-gated cation channels expressed in nerve, muscle, bone, glands, and the immune system. The seven family members display variable Ca²⁺ permeabilities that are amongst the highest of all ligand-gated channels (Egan and Khakh, 2004). We previously reported that polar residues regulate the Ca²⁺ permeability of the P2X₂ receptor (Migita et al., 2001). Here, we test the hypothesis that the formal charge of acidic amino acids underlies the higher fractional Ca²⁺ currents (Pf%) of the rat and human P2X₁ and P2X₄ subtypes. We used patch-clamp photometry to measure the Pf% of HEK-293 cells transiently expressing a range of wild-type and genetically altered receptors. Lowering the pH of the extracellular solution reduced the higher Pf% of the P2X₁ receptor but had no effect on the lower Pf% of the P2X₂ receptor, suggesting that ionized side chains regulate the Ca²⁺ flux of some family members. Removing the fixed negative charges found at the extracellular ends of the transmembrane domains also reduced the higher Pf% of P2X₁ and P2X₄ receptors, and introducing these charges at homologous positions increased the lower Pf% of the P2X₂ receptor. Taken together, the data suggest that COO⁻ side chains provide an electrostatic force that interacts with Ca²⁺ in the mouth of the pore. Surprisingly, the glutamate residue that is partly responsible for the higher Pf% of the P2X₁ and P2X₄ receptors is conserved in the P2X₃ receptor that has the lowest Pf% of all family members. We found that neutralizing an upstream His⁴⁵ increased Pf% of the P2X₃ channel, suggesting that this positive charge masks the facilitation of Ca²⁺ flux by the neighboring Glu⁴⁶. The data support the hypothesis that formal charges near the extracellular ends of transmembrane domains contribute to the high Ca²⁺ permeability and flux of some P2X receptors.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Pharmacology of P2X channels

Significant progress in understanding the pharmacological characteristics and physiological importance of homomeric and heteromeric P2X channels has been achieved in recent years. P2X channels, gated by ATP and most likely trimerically assembled from seven known P2X subunits, are present in a broad distribution of tissues and are thought to play an important role in a variety of physiological functions, including peripheral and central neuronal transmission, smooth muscle contraction, and inflammation. The known homomeric and heteromeric P2X channels can be distinguished from each other on the basis of pharmacological differences when expressed recombinantly in cell lines, but whether this pharmacological classification holds true in native cells and in vivo is less well-established. Nevertheless, several potent and selective P2X antagonists have been discovered in recent years and shown to be efficacious in various animal models including those for visceral organ function, chronic inflammatory and neuropathic pain, and inflammation. The recent advancement of drug candidates targeting P2X channels into human trials, confirms the medicinal exploitability of this novel target family and provides hope that safe and effective medicines for the treatment of disorders involving P2X channels may be identified in the near future.

Molecular properties of P2X receptors

P2X receptors for adenosine tri-phosphate (ATP) are a distinct family of ligand-gated cation channels with two transmembrane domains, intracellular amino and carboxy termini and a large extracellular ligand binding loop. Seven genes (P2X(1-7)) have been cloned and the channels form as either homo or heterotrimeric channels giving rise to a wide range of phenotypes. This review aims to give an account of recent work on the molecular basis of the properties of P2X receptors. In particular, to consider emerging information on the assembly of P2X receptor subunits, channel regulation and desensitisation, targeting, the molecular basis of drug action and the functional contribution of P2X receptors to physiological processes.

The suramin analog 4,4',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis (carbonylimino)))tetra-kis-benzenesulfonic acid (NF110) potently blocks P2X3 receptors: subtype selectivity is determined by location of sulfonic acid groups

We have previously identified the suramin analog 4,4',4'',4'''-(carbonylbis(imino-5,1,3-benzenetriylbis(carbonylimino)))tetrakis-benzene-1,3-disulfonic acid (NF449) as a low nanomolar potency antagonist of recombinant P2X(1) receptors. Here, we characterize, by two-electrode voltage-clamp electrophysiology, three isomeric suramin analogs designated para-4,4',4'',4''''-(carbonylbis(imino-5,1,3-benzenetriylbis (carbonylimino)))tetrakis-benzenesulfonic acid (NF110), meta-(3,3',3'',3''''-(carbonylbis(imino-5,1,3-benzenetriylbis (carbonylimino)))tetra-kis-benzenesulfonic acid (NF448), and ortho-(2,2',2'',2''''-(carbonylbis(imino-5,1,3-benzenetriylbis (carbonylimino)))tetra-kis-benzenesulfonic acid (MK3) with respect to their potency in antagonizing rat P2X receptor-mediated inward currents in Xenopus laevis oocytes. Meta, para, and ortho refer to the position of the single sulfonic acid group relative to the amide bond linking the four symmetrically oriented benzenesulfonic acid moieties to the central, invariant suramin core. NF448, NF110, and MK3 were >200-fold less potent in blocking P2X(1) receptors than NF449, from which they differ structurally only by having one instead of two sulfonic acid residues per benzene ring. Although the meta- and ortho-isomers retained P2X(1) receptor selectivity, the para-isomer NF110 exhibited a significantly increased activity at P2X(3) receptors (K(i) approximately 36 nM) and displayed the following unique selectivity profile among suramin derivatives: P2X(2+3) = P2X(3) > P2X(1) > P2X(2) >> P2X(4) > P2X(7). The usefulness of NF110 as a P2X(3) receptor antagonist in native tissues could be demonstrated by showing that NF110 blocks alphabeta-methylene-ATP-induced currents in rat dorsal root ganglia neurons with similar potency as recombinant rat P2X(3) receptors. Together, these data highlight the importance of both the number and exact location of negatively charged groups for P2X subtype potency and selectivity.