Cysteine substitution mutagenesis and the effects of methanethiosulfonate reagents at P2X2 and P2X4 receptors support a core common mode of ATP action at P2X receptors

Engineering a human P2X2 receptor with altered ligand selectivity in yeast

P2X receptors are a family of ligand gated ion channels found in a range of eukaryotic species including humans but are not naturally present in the yeast Saccharomyces cerevisiae. We demonstrate the first recombinant expression and functional gating of the P2X2 receptor in baker's yeast. We leverage the yeast host for facile genetic screens of mutant P2X2 by performing site saturation mutagenesis at residues of interest, including SNPs implicated in deafness and at residues involved in native binding. Deep mutational analysis and rounds of genetic engineering yield mutant P2X2 F303Y A304W, which has altered ligand selectivity toward the ATP analog AMP-PNP. The F303Y A304W variant shows over 100-fold increased intracellular calcium amplitudes with AMP-PNP compared to the WT receptor and has a much lower desensitization rate. Since AMP-PNP does not naturally activate P2X receptors, the F303Y A304W P2X2 may be a starting point for downstream applications in chemogenetic cellular control. Interestingly, the A304W mutation selectively destabilizes the desensitized state, which may provide a mechanistic basis for receptor opening with suboptimal agonists. The yeast system represents an inexpensive, scalable platform for ion channel characterization and engineering by circumventing the more expensive and time-consuming methodologies involving mammalian hosts.

P2X2 receptor subunit interfaces are missense variant hotspots where mutations tend to increase apparent ATP affinity

Background and Purpose

P2X receptors are trimeric ligand‐gated ion channels that open a cation‐selective pore in response to ATP binding to their large extracellular domain. The seven known P2X subtypes can assemble as homotrimeric or heterotrimeric complexes and contribute to numerous physiological functions, including nociception, inflammation and hearing. The overall structure of P2X receptors is well established, but little is known about the range and prevalence of human genetic variations and the functional implications of specific domains.

Experimental Approach

Here, we examine the impact of P2X2 receptor inter‐subunit interface missense variants identified in the human population or by structural predictions. We test both single and double mutants through electrophysiological and biochemical approaches.

Key Results

We demonstrate that predicted extracellular domain inter‐subunit interfaces display a higher‐than‐expected density of missense variations and that the majority of mutations that disrupt putative inter‐subunit interactions result in channels with higher apparent ATP affinity. Lastly, we show that double mutants at the subunit interface show significant energetic coupling, especially if located in close proximity.

Conclusion and Implications

We provide the first structural mapping of the mutational distribution across the human population in a ligand‐gated ion channel and show that the density of missense mutations is constrained between protein domains, indicating evolutionary selection at the domain level. Our data may indicate that, unlike other ligand‐gated ion channels, P2X2 receptors have evolved an intrinsically high threshold for activation, possibly to allow for additional modulation or as a cellular protection mechanism against overstimulation.

LPEATs Tailor Plant Phospholipid Composition through Adjusting Substrate Preferences to Temperature

Acyl-CoA:lysophosphatidylethanolamine acyltransferases (LPEATs) are known as enzymes utilizing acyl-CoAs and lysophospholipids to produce phosphatidylethanolamine. Recently, it has been discovered that they are also involved in the growth regulation of Arabidopsis thaliana. In our study we investigated expression of each Camelina sativa LPEAT isoform and their behavior in response to temperature changes. In order to conduct a more extensive biochemical evaluation we focused both on LPEAT enzymes present in microsomal fractions from C. sativa plant tissues, and on cloned CsLPEAT isoforms expressed in yeast system. Phylogenetic analyses revealed that CsLPEAT1c and CsLPEAT2c originated from Camelina hispida, whereas other isoforms originated from Camelina neglecta. The expression ratio of all CsLPEAT1 isoforms to all CsLPEAT2 isoforms was higher in seeds than in other tissues. The isoforms also displayed divergent substrate specificities in utilization of LPE; CsLPEAT1 preferred 18:1-LPE, whereas CsLPEAT2 preferred 18:2-LPE. Unlike CsLPEAT1, CsLPEAT2 isoforms were specific towards very-long-chain fatty acids. Above all, we discovered that temperature strongly regulates LPEATs activity and substrate specificity towards different acyl donors, making LPEATs sort of a sensor of external thermal changes. We observed the presented findings not only for LPEAT activity in plant-derived microsomal fractions, but also for yeast-expressed individual CsLPEAT isoforms.

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.

Pharmacological and genetic characterisation of the canine P2X4 receptor

BACKGROUND AND PURPOSE

P2X4 receptors are emerging therapeutic targets for treating chronic pain and cardiovascular disease. Dogs are well-recognised natural models of human disease, but information regarding P2X4 receptors in dogs is lacking. To aid the development and validation of P2X4 receptor ligands, we have characterised and compared canine and human P2X4 receptors.

EXPERIMENTAL APPROACH

Genomic DNA was extracted from whole blood samples from 101 randomly selected dogs and sequenced across the P2RX4 gene to identify potential missense variants. Recombinant canine and human P2X4 receptors tagged with Emerald GFP were expressed in 1321N1 and HEK293 cells and analysed by immunoblotting and confocal microscopy. In these cells, receptor pharmacology was characterised using nucleotide-induced Fura-2 AM measurements of intracellular Ca²⁺ and known P2X4 receptor antagonists. P2X4 receptor-mediated inward currents in HEK293 cells were assessed by automated patch clamp.

KEY RESULTS

No P2RX4 missense variants were identified in any canine samples. Canine and human P2X4 receptors were localised primarily to lysosomal compartments. ATP was the primary agonist of canine P2X4 receptors with near identical efficacy and potency at human receptors. 2'(3')-O-(4-benzoylbenzoyl)-ATP, but not ADP, was a partial agonist with reduced potency for canine P2X4 receptors compared to the human orthologues. Five antagonists inhibited canine P2X4 receptors, with 1-(2,6-dibromo-4-isopropyl-phenyl)-3-(3-pyridyl)urea displaying reduced sensitivity and potency at canine P2X4 receptors.

CONCLUSION AND IMPLICATIONS

P2X4 receptors are highly conserved across dog pedigrees and display expression patterns and pharmacological profiles similar to human receptors, supporting validation and use of therapeutic agents for P2X4 receptor-related disease onset and management in dogs and humans.

Homology Modeling of P2X Receptors

Since the X-ray structure of the zebra fish P2X4 receptor in the closed state was published in 2009 homology modeling has been used to generate structural models for P2X receptors. In this chapter, we outline how to use the MODELLER software to generate such structural models for P2X receptors whose structures have not been solved yet.

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.

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.

Regulation of Pannexin 1 Surface Expression by Extracellular ATP: Potential Implications for Nervous System Function in Health and Disease

Pannexin 1 (Panx1) channels are widely recognized for their role in ATP release, and as follows, their function is closely tied to that of ATP-activated P2X7 purinergic receptors (P2X7Rs). Our recent work has shown that extracellular ATP induces clustering of Panx1 with P2X7Rs and their subsequent internalization through a non-canonical cholesterol-dependent mechanism. In other words, we have demonstrated that extracellular ATP levels can regulate the cell surface expression of Panx1. Here we discuss two situations in which we hypothesize that ATP modulation of Panx1 surface expression could be relevant for central nervous system function. The first scenario involves the development of new neurons in the ventricular zone. We propose that ATP-induced Panx1 endocytosis could play an important role in regulating the balance of cell proliferation, survival, and differentiation within this neurogenic niche in the healthy brain. The second scenario relates to the spinal cord, in which we posit that an impairment of ATP-induced Panx1 endocytosis could contribute to pathological neuroplasticity. Together, the discussion of these hypotheses serves to highlight important outstanding questions regarding the interplay between extracellular ATP, Panx1, and P2X7Rs in the nervous system in health and disease.

Short- and long-term (trophic) purinergic signalling

There is long-term (trophic) purinergic signalling involving cell proliferation, differentiation, motility and death in the development and regeneration of most systems of the body, in addition to fast purinergic signalling in neurotransmission, neuromodulation and secretion. It is not always easy to distinguish between short- and long-term signalling. For example, adenosine triphosphate (ATP) can sometimes act as a short-term trigger for long-term trophic events that become evident days or even weeks after the original challenge. Examples of short-term purinergic signalling during sympathetic, parasympathetic and enteric neuromuscular transmission and in synaptic transmission in ganglia and in the central nervous system are described, as well as in neuromodulation and secretion. Long-term trophic signalling is described in the immune/defence system, stratified epithelia in visceral organs and skin, embryological development, bone formation and resorption and in cancer. It is likely that the increase in intracellular Ca(2+) in response to both P2X and P2Y purinoceptor activation participates in many short- and long-term physiological effects.This article is part of the themed issue 'Evolution brings Ca(2+) and ATP together to control life and death'.

Modeling Interactions among Individual P2 Receptors to Explain Complex Response Patterns over a Wide Range of ATP Concentrations

Extracellular ATP acts on the P2X family of ligand-gated ion channels and several members of the P2Y family of G protein-coupled receptors to mediate intercellular communication among many cell types including bone-forming osteoblasts. It is known that multiple P2 receptors are expressed on osteoblasts (P2X2,5,6,7 and P2Y1,2,4,6). In the current study, we investigated complex interactions within the P2 receptor network using mathematical modeling. To characterize individual P2 receptors, we extracted data from published studies of overexpressed human and rodent (rat and mouse) receptors and fit their dependencies on ATP concentration using the Hill equation. Next, we examined responses induced by an ensemble of endogenously expressed P2 receptors. Murine osteoblastic cells (MC3T3-E1 cells) were loaded with fluo-4 and stimulated with varying concentrations of extracellular ATP. Elevations in the concentration of cytosolic free calcium ([Ca²⁺]_i) were monitored by confocal microscopy. Dependence of the calcium response on ATP concentration exhibited a complex pattern that was not explained by the simple addition of individual receptor responses. Fitting the experimental data with a combination of Hill equations from individual receptors revealed that P2Y1 and P2X7 mediated the rise in [Ca²⁺]_i at very low and high ATP concentrations, respectively. Interestingly, to describe responses at intermediate ATP concentrations, we had to assume that a receptor with a K_1∕2 in that range (e.g. P2Y4 or P2X5) exerts an inhibitory effect. This study provides new insights into the interactions among individual P2 receptors in producing an ensemble response to extracellular ATP.

Characterization of ATPase Activity of P2RX2 Cation Channel

P2X purinergic receptors are plasma membrane ATP-dependent cation channels that are broadly distributed in the mammalian tissues. P2RX2 is a modulator of auditory sensory hair cell mechanotransduction and plays an important role in hair cell tolerance to noise. In this study, we demonstrate for the first time in vitro and in cochlear neuroepithelium, that P2RX2 possesses the ATPase activity. We observed that the P2RX2 V60L human deafness mutation alters its ability to bind ATP, while the G353R has no effect on ATP binding or hydrolysis. A non-hydrolysable ATP assay using HEK293 cells suggests that ATP hydrolysis plays a significant role in the opening and gating of the P2RX2 ion channel. Moreover, the results of structural modeling of the molecule was in agreement with our experimental observations. These novel findings suggest the intrinsic ATPase activity of P2RX2 and provide molecular insights into the channel opening.

Structural Insights into Divalent Cation Modulations of ATP-Gated P2X Receptor Channels

P2X receptors are trimeric ATP-gated cation channels involved in physiological processes ranging widely from neurotransmission to pain and taste signal transduction. The modulation of the channel gating, including that by divalent cations, contributes to these diverse physiological functions of P2X receptors. Here, we report the crystal structure of an invertebrate P2X receptor from the Gulf Coast tick Amblyomma maculatum in the presence of ATP and Zn(2+) ion, together with electrophysiological and computational analyses. The structure revealed two distinct metal binding sites, M1 and M2, in the extracellular region. The M1 site, located at the trimer interface, is responsible for Zn(2+) potentiation by facilitating the structural change of the extracellular domain for pore opening. In contrast, the M2 site, coupled with the ATP binding site, might contribute to regulation by Mg(2+). Overall, our work provides structural insights into the divalent cation modulations of P2X receptors.

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.

Constructing an atomic-resolution model of human P2X7 receptor followed by pharmacophore modeling to identify potential inhibitors

The P2X purinoceptor 7 (P2X7R) is a trimeric ATP-activated ion channel gated by extracellular ATP. P2X7R has important role in numerous diseases including pain, neurodegeneration, and inflammatory diseases such as rheumatoid arthritis and osteoarthritis. In this prospective, the discovery of small-molecule inhibitors for P2X7R as a novel therapeutic target has received considerable attention in recent years. At first, 3D structure of P2X7R was built by using homology modeling (HM) and a 50ns molecular dynamics simulation (MDS). Ligand-based quantitative pharmacophore modeling methodology of P2X7R antagonists were developed based on training set of 49 compounds. The best four-feature pharmacophore model, includes two hydrophobic aromatic, one hydrophobic and one aromatic ring features, has the highest correlation coefficient (0.874), cost difference (368.677), low RMSD (2.876), as well as it shows a high goodness of fit and enrichment factor. Consequently, some hit compounds were introduced as final candidates by employing virtual screening and molecular docking procedure simultaneously. Among these compounds, six potential molecule were identified as potential virtual leads which, as such or upon further optimization, can be used to design novel P2X7R inhibitors.

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'.

Identification of functionally important residues of the rat P2X4 receptor by alanine scanning mutagenesis of the dorsal fin and left flipper domains

Crystallization of the zebrafish P2X4 receptor in both open and closed states revealed conformational differences in the ectodomain structures, including the dorsal fin and left flipper domains. Here, we focused on the role of these domains in receptor activation, responsiveness to orthosteric ATP analogue agonists, and desensitization. Alanine scanning mutagenesis of the R203-L214 (dorsal fin) and the D280-N293 (left flipper) sequences of the rat P2X4 receptor showed that ATP potency/efficacy was reduced in 15 out of 26 alanine mutants. The R203A, N204A, and N293A mutants were essentially non-functional, but receptor function was restored by ivermectin, an allosteric modulator. The I205A, T210A, L214A, P290A, G291A, and Y292A mutants exhibited significant changes in the responsiveness to orthosteric analog agonists 2-(methylthio)adenosine 5′-triphosphate, adenosine 5′-(γ-thio)triphosphate, 2′(3′-O-(4-benzoylbenzoyl)adenosine 5′-triphosphate, and α,β-methyleneadenosine 5′-triphosphate. In contrast, the responsiveness of L206A, N208A, D280A, T281A, R282A, and H286A mutants to analog agonists was comparable to that of the wild type receptor. Among these mutants, D280A, T281A, R282A, H286A, G291A, and Y292A also exhibited increased time-constant of the desensitizing current response. These experiments, together with homology modeling, indicate that residues located in the upper part of the dorsal fin and left flipper domains, relative to distance from the channel pore, contribute to the organization of the ATP binding pocket and to the initiation of signal transmission towards residues in the lower part of both domains. The R203 and N204 residues, deeply buried in the protein, may integrate the output signal from these two domains towards the gate. In addition, the left flipper residues predominantly account for the control of transition of channels from an open to a desensitized state.

Purinergic P2X receptors: structural models and analysis of ligand-target interaction

The purinergic P2X receptors are ligand-gated cation channels activated by the endogenous ligand ATP. They assemble as homo- or heterotrimers from seven cloned subtypes (P2X1-7) and all trimer subunits present a common topology consisting in intracellular N- and C- termini, two transmembrane domains and a large extracellular domain. These membrane proteins are present in virtually all mammalian tissues and regulate a large variety of responses in physio- and pathological conditions. The development of ligands that selectively activate or block specific P2X receptor subtypes hence represents a promising strategy to obtain novel pharmacological tools for the treatment of pain, cancer, inflammation, and neurological, cardiovascular, and endocrine diseases. The publication of the crystal structures of zebrafish P2X4 receptor in inactive and ATP-bound active forms provided structural data for the analysis of the receptor structure, the interpretation of mutagenesis data, and the depiction of ligand binding and receptor activation mechanism. In addition, the availability of ATP-competitive ligands presenting selectivity for P2X receptor subtypes supports the design of new potent and selective ligands with possibly improved pharmacokinetic profiles, with the final aim to obtain new drugs. This study describes molecular modelling studies performed to develop structural models of the human and rat P2X receptors in inactive and active states. These models allowed to analyse the role of some non-conserved residues at ATP binding site and to study the receptor interaction with some non-specific or subtype selective agonists and antagonists.

Conformational flexibility of the agonist binding jaw of the human P2X3 receptor is a prerequisite for channel opening

BACKGROUND AND PURPOSE

It is assumed that ATP induces closure of the binding jaw of ligand-gated P2X receptors, which eventually results in the opening of the membrane channel and the flux of cations. Immobilization by cysteine mutagenesis of the binding jaw inhibited ATP-induced current responses, but did not allow discrimination between disturbances of binding, gating, subunit assembly or trafficking to the plasma membrane.

EXPERIMENTAL APPROACH

A molecular model of the pain-relevant human (h)P2X3 receptor was used to identify amino acid pairs, which were located at the lips of the binding jaw and did not participate in agonist binding but strongly approached each other even in the absence of ATP.

KEY RESULTS

A series of cysteine double mutant hP2X3 receptors, expressed in HEK293 cells or Xenopus laevis oocytes, exhibited depressed current responses to α,β-methylene ATP (α,β-meATP) due to the formation of spontaneous inter-subunit disulfide bonds. Reducing these bonds with dithiothreitol reversed the blockade of the α,β-meATP transmembrane current. Amino-reactive fluorescence labelling of the His-tagged hP2X3 receptor and its mutants expressed in HEK293 or X. laevis oocytes demonstrated the formation of inter-subunit cross links in cysteine double mutants and, in addition, confirmed their correct trimeric assembly and cell surface expression.

CONCLUSIONS AND IMPLICATIONS

In conclusion, spontaneous tightening of the binding jaw of the hP2X3 receptor by inter-subunit cross-linking of cysteine residues substituted at positions not directly involved in agonist binding inhibited agonist-evoked currents without interfering with binding, subunit assembly or trafficking.

CHARMM-GUI PDB manipulator for advanced modeling and simulations of proteins containing nonstandard residues

CHARMM-GUI, http://www.charmm-gui.org, is a web-based graphical user interface to prepare molecular simulation systems and input files to facilitate the usage of common and advanced simulation techniques. Since it is originally developed in 2006, CHARMM-GUI has been widely adopted for various purposes and now contains a number of different modules designed to setup a broad range of simulations including free energy calculation and large-scale coarse-grained representation. Here, we describe functionalities that have recently been integrated into CHARMM-GUI PDB Manipulator, such as ligand force field generation, incorporation of methanethiosulfonate spin labels and chemical modifiers, and substitution of amino acids with unnatural amino acids. These new features are expected to be useful in advanced biomolecular modeling and simulation of proteins.

Involvement of ectodomain Leu 214 in ATP binding and channel desensitization of the P2X4 receptor

P2X receptors are trimeric ATP-gated cation permeable ion channels. When ATP binds, the extracellular head and dorsal fin domains are predicted to move closer to each other. However, there are scant functional data corroborating the role of the dorsal fin in ligand binding. Here using site-directed mutagenesis and electrophysiology, we show that a dorsal fin leucine, L214, contributes to ATP binding. Mutant receptors containing a single substitution of alanine, serine, glutamic acid, or phenylalanine at L214 of the rat P2X4 receptor exhibited markedly reduced sensitivities to ATP. Mutation of other dorsal fin side chains, S216, T223, and D224, did not significantly alter ATP sensitivity. Exposure of L214C to sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES(-)) or (2-aminoethyl) methanethiosulfonate hydrobromide in the absence of ATP blocked responses evoked by subsequent ATP application. In contrast, when MTSES(-) was applied in the presence of ATP, no current inhibition was observed. Furthermore, L214A also slightly reduced the inhibitory effect of the antagonist 2',3'-O-(2,4,6-trinitrophenyl)-ATP, and the blockade was more rapidly reversible after washout. Certain L214 mutants also showed effects on current desensitization in the continued presence of ATP. L214I exhibited an accelerated current decline, whereas L214M exhibited a slower rate. Taken together, these data reveal that position L214 participates in both ATP binding and conformational changes accompanying channel opening and desensitization, providing compelling evidence that the dorsal fin domain indeed has functional properties that are similar to those previously reported for the body domains.

Ectodomain movements of an ATP-gated ion channel (P2X2 receptor) probed by disulfide locking

The ectodomain of the P2X receptor is formed mainly from two- or three-stranded β-sheets provided symmetrically by each of the three subunits. These enclose a central cavity that is closed off furthest from the plasma membrane (the turret) and that joins with the transmembrane helices to form the ion permeation pathway. Comparison of closed and open crystal structures indicates that ATP binds in a pocket positioned between strands provided by different subunits and that this flexes the β-sheets of the lower body and enlarges the central cavity: this pulls apart the outer ends of the transmembrane helices and thereby opens an aperture, or gate, where they intersect within the membrane bilayer. In the present work, we examined this opening model by introducing pairs of cysteines into the rat P2X2 receptor that might form disulfide bonds within or between subunits. Receptors were expressed in human embryonic kidney cells, and disulfide formation was assessed by observing the effect of dithiothreitol on currents evoked by ATP. Substitutions in the turret (P90C, P89C/S97C), body wall (S65C/S190C, S65C/D315C) and the transmembrane domains (V48C/I328C, V51C/I328C, S54C/I328C) strongly inhibited ATP-evoked currents prior to reduction with dithiothreitol. Western blotting showed that these channels also formed predominately as dimers and/or trimers rather than monomers. The results strongly support the channel opening mechanism proposed on the basis of available crystal structures.

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.

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.

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.

Salt bridge switching from Arg290/Glu167 to Arg290/ATP promotes the closed-to-open transition of the P2X2 receptor

P2X receptors are trimeric adenosine-5'-triphosphate (ATP)-gated cation channels involved in fast signal transduction in many cell types. In this study, we used homology modeling of the rat P2X2 receptor with the zebrafish P2X4 X-ray template to determine that the side chains of the Glu167 and Arg290 residues are in close spatial vicinity within the ATP-binding pocket when the rat P2X2 channel is closed. Through charge reversal mutation analysis and mutant cycle analysis, we obtained evidence that Glu167 and Arg290 form an electrostatic interaction. In addition, disulfide trapping indicated the close proximity of Glu167 and Arg290 when the channel is in the closed state, but not in the ATP-bound open state. Consistent with a gating-induced movement that disrupts the Glu167/Arg290 salt bridge, a comparison of the closed and open rat P2X2 receptor models revealed a significant rearrangement of the protein backbone and the side chains of the Glu167 and Arg290 residues during the closed-to-open transition. The associated release of the Glu167/Arg290 salt bridge during channel opening allows a strong ionic interaction between Arg290 and a γ-phosphate oxygen of ATP. We conclude from these results that the state-dependent salt bridge switching from Arg290/Glu167 to Arg290/ATP fulfills a dual role: to destabilize the closed state of the receptor and to promote the ionic coordination of ATP in the ATP-binding pocket.

Heat shock protein 90 inhibitors reduce trafficking of ATP-gated P2X1 receptors and human platelet responsiveness

We have used selective inhibitors to determine whether the molecular chaperone heat shock protein 90 (HSP90) has an effect on both recombinant and native human P2X1 receptors. P2X1 receptor currents in HEK293 cells were reduced by ∼70–85% by the selective HSP90 inhibitor geldanamycin (2 μm, 20 min). This was associated with a speeding in the time course of desensitization as well as a reduction in cell surface expression. Imaging in real time of photoactivatable GFP-tagged P2X receptors showed that they are highly mobile. Geldanamycin almost abolished this movement for P2X1 receptors but had no effect on P2X2 receptor trafficking. P2X1/2 receptor chimeras showed that the intracellular N and C termini were involved in geldanamycin sensitivity. Geldanamycin also inhibited native P2X1 receptor-mediated responses. Platelet P2X1 receptors play an important role in hemostasis, contribute to amplification of signaling to a range of stimuli including collagen, and are novel targets for antithrombotic therapies. Platelet P2X1 receptor-, but not P2Y1 receptor-, mediated increases in intracellular calcium were reduced by 40–45% following HSP90 inhibition with geldanamycin or radicicol. Collagen stimulation leads to ATP release from platelets, and calcium increases to low doses of collagen were also reduced by ∼40% by the HSP90 inhibitors consistent with an effect on P2X1 receptors. These studies suggest that HSP90 inhibitors may be as effective as selective antagonists in regulating platelet P2X1 receptors, and their potential effects on hemostasis should be considered in clinical studies.

Size matters in activation/inhibition of ligand-gated ion channels

Cys loop, glutamate, and P2X receptors are ligand-gated ion channels (LGICs) with 5, 4, and 3 protomers, respectively. There is now growing atomic level understanding of their gating mechanisms. Although each family is unique in the architecture of the ligand-binding pocket, the pathway for motions to propagate from ligand-binding domain to transmembrane domain, and the gating motions of the transmembrane domain, there are common features among the LGICs, which are the focus of the present review. In particular, agonists and competitive antagonists apparently induce opposite motions of the binding pocket. A simple way to control the motional direction is ligand size. Agonists, usually small, induce closure of the binding pocket, leading to opening of the channel pore, whereas antagonists, usually large, induce opening of the binding pocket, thereby stabilizing the closed pore. A cross-family comparison of the gating mechanisms of the LGICs, focusing in particular on the role played by ligand size, provides new insight on channel activation/inhibition and design of pharmacological compounds.

Molecular mechanism of ATP binding and ion channel activation in P2X receptors

P2X receptors are trimeric ATP-activated ion channels permeable to Na+, K+ and Ca2+. The seven P2X receptor subtypes are implicated in physiological processes that include modulation of synaptic transmission, contraction of smooth muscle, secretion of chemical transmitters and regulation of immune responses. Despite the importance of P2X receptors in cellular physiology, the three-dimensional composition of the ATP-binding site, the structural mechanism of ATP-dependent ion channel gating and the architecture of the open ion channel pore are unknown. Here we report the crystal structure of the zebrafish P2X4 receptor in complex with ATP and a new structure of the apo receptor. The agonist-bound structure reveals a previously unseen ATP-binding motif and an open ion channel pore. ATP binding induces cleft closure of the nucleotide-binding pocket, flexing of the lower body β-sheet and a radial expansion of the extracellular vestibule. The structural widening of the extracellular vestibule is directly coupled to the opening of the ion channel pore by way of an iris-like expansion of the transmembrane helices. The structural delineation of the ATP-binding site and the ion channel pore, together with the conformational changes associated with ion channel gating, will stimulate development of new pharmacological agents.

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.

Molecular basis of selective antagonism of the P2X1 receptor for ATP by NF449 and suramin: contribution of basic amino acids in the cysteine-rich loop

BACKGROUND AND PURPOSE

The cysteine-rich head region, which is adjacent to the proposed ATP-binding pocket in the extracellular ligand-binding loop of P2X receptors for ATP, is absent in the antagonist-insensitive Dictyostelium receptors. In this study we have determined the contribution of the head region to the antagonist action of NF449 and suramin at the human P2X1 receptor.

EXPERIMENTAL APPROACH

Chimeras and point mutations in the cysteine-rich head region were made between human P2X1 and P2X2 receptors. Mutant receptors were expressed in Xenopus oocytes and P2X receptor currents characterized using two-electrode voltage clamp.

KEY RESULTS

The chimera replacing the region between the third and fourth conserved cysteine residues of the P2X1 receptor with the corresponding part of P2X2 reduced NF449 sensitivity a thousand fold from an IC₅₀ of ∼1 nM at the P2X1 receptor to that of the P2X2 receptor (IC₅₀∼1 µM). A similar decrease in sensitivity resulted from mutation of four positively charged P2X1 receptor residues in this region that are absent from the P2X2 receptor. These chimeras and mutations were also involved in determining sensitivity to the antagonist suramin. Reciprocal chimeras and mutations in the P2X2 receptor produced modest increases in antagonist sensitivity.

CONCLUSIONS AND IMPLICATIONS

These data indicate that a cluster of positively charged residues at the base of the cysteine-rich head region can account for the highly selective antagonism of the P2X1 receptor by the suramin derivative NF449.

Gating mechanism of a P2X4 receptor developed from normal mode analysis and molecular dynamics simulations

P2X receptors are trimeric ATP-gated cation channels participating in diverse physiological processes. How ATP binding triggers channel opening remains unclear. Here the gating mechanism of a P2X receptor was studied by normal mode analysis and molecular dynamics (MD) simulations. Based on the resting-state crystal structure, a normal mode involving coupled motions of three β-strands (β1, β13, and β14) at the trimeric interface of the ligand-binding ectodomain and the pore-lining helix (TM2) in the transmembrane domain (TMD) was identified. The resulting widening of the fenestrations above the TMD and opening of the transmembrane pore produce known signatures of channel activation. In MD simulations, ATP was initially placed in the putative binding pocket (defined by four charged residues located in β1, β13 and β14) in two opposite orientations, with the adenine either proximal or distal to the TMD. In the proximal orientation, the triphosphate group extends outward to draw in the four charged residues, leading to closure of β13/β14 toward β1. The adenine ring, wedged between β1 and β13, acts as a fulcrum for the β14 lever, turning a modest closure around the triphosphate group into significant opening of the pre-TM2 loop. The motions of these β-strands are similar to those in the putative channel-activation normal mode. In the distal orientation, the ATP stabilizes the trimeric interface and the closure of the pre-TM2 loop, possibly representing desensitization. Our computational studies produced the first complete model, supported by experimental data, for how ATP binding triggers activation of a P2X receptor.

Preferential use of unobstructed lateral portals as the access route to the pore of human ATP-gated ion channels (P2X receptors)

P2X receptors are trimeric cation channels with widespread roles in health and disease. The recent crystal structure of a P2X4 receptor provides a 3D view of their topology and architecture. A key unresolved issue is how ions gain access to the pore, because the structure reveals two different pathways within the extracellular domain. One of these is the central pathway spanning the entire length of the extracellular domain and covering a distance of ≈70 Å. The second consists of three lateral portals, adjacent to the membrane and connected to the transmembrane pore by short tunnels. Here, we demonstrate the preferential use of the lateral portals. Owing to their favorable diameters and equivalent spacing, the lateral portals split the task of ion supply threefold and minimize an ion's diffusive path before it succumbs to transmembrane electrochemical gradients.

Activation and regulation of purinergic P2X receptor channels

Mammalian ATP-gated nonselective cation channels (P2XRs) can be composed of seven possible subunits, denoted P2X1 to P2X7. Each subunit contains a large ectodomain, two transmembrane domains, and intracellular N and C termini. Functional P2XRs are organized as homomeric and heteromeric trimers. This review focuses on the binding sites involved in the activation (orthosteric) and regulation (allosteric) of P2XRs. The ectodomains contain three ATP binding sites, presumably located between neighboring subunits and formed by highly conserved residues. The detection and coordination of three ATP phosphate residues by positively charged amino acids are likely to play a dominant role in determining agonist potency, whereas an AsnPheArg motif may contribute to binding by coordinating the adenine ring. Nonconserved ectodomain histidines provide the binding sites for trace metals, divalent cations, and protons. The transmembrane domains account not only for the formation of the channel pore but also for the binding of ivermectin (a specific P2X4R allosteric regulator) and alcohols. The N- and C- domains provide the structures that determine the kinetics of receptor desensitization and/or pore dilation and are critical for the regulation of receptor functions by intracellular messengers, kinases, reactive oxygen species and mercury. The recent publication of the crystal structure of the zebrafish P2X4.1R in a closed state provides a major advance in the understanding of this family of receptor channels. We will discuss data obtained from numerous site-directed mutagenesis experiments accumulated during the last 15 years with reference to the crystal structure, allowing a structural interpretation of the molecular basis of orthosteric and allosteric ligand actions.

Cysteine scanning mutagenesis (residues Glu52-Gly96) of the human P2X1 receptor for ATP: mapping agonist binding and channel gating

P2X receptors are ATP-gated cation channels. The x-ray structure of a P2X4 receptor provided a major advance in understanding the molecular basis of receptor properties. However, how agonists are coordinated, the extent of the binding site, and the contribution of the vestibules in the extracellular domain to ionic permeation have not been addressed. We have used cysteine-scanning mutagenesis to determine the contribution of residues Glu⁵²–Gly⁹⁶ to human P2X1 receptor properties. ATP potency was reduced for the mutants K68C, K70C, and F92C. The efficacy of the partial agonist BzATP was also reduced for several mutants forming the back of the proposed agonist binding site. Molecular docking in silico of both ATP and BzATP provided models of the agonist binding site consistent with these data. Individual cysteine mutants had no effect or slightly increased antagonism by suramin or pyridoxal-phosphate-6-azophenyl-2′,4′-disulfonate. Mutants at the entrance to and lining the upper vestibule were unaffected by cysteine-reactive methanethiosulfonate (MTS) reagents, suggesting that it does not contribute to ionic permeation. Mutants that were sensitive to modification by MTS reagents were predominantly found either around the proposed ATP binding pocket or on the strands connecting the binding pocket to the transmembrane region and lining the central vestibule. In particular, ATP sensitivity and currents were increased by a positively charged MTS reagent at the G60C mutant at the interface between the central and extracellular vestibule. This suggests that dilation of the base of the central vestibule contributes to gating of the receptor.

Agonist trapped in ATP-binding sites of the P2X2 receptor

ATP-gated P2X receptors are trimeric ion channels, as recently confirmed by X-ray crystallography. However, the structure was solved without ATP and even though extracellular intersubunit cavities surrounded by conserved amino acid residues previously shown to be important for ATP function were proposed to house ATP, the localization of the ATP sites remains elusive. Here we localize the ATP-binding sites by creating, through a proximity-dependent "tethering" reaction, covalent bonds between a synthesized ATP-derived thiol-reactive P2X2 agonist (NCS-ATP) and single cysteine mutants engineered in the putative binding cavities of the P2X2 receptor. By combining whole-cell and single-channel recordings, we report that NCS-ATP covalently and specifically labels two previously unidentified positions N140 and L186 from two adjacent subunits separated by about 18 Å in a P2X2 closed state homology model, suggesting the existence of at least two binding modes. Tethering reaction at both positions primes subsequent agonist binding, yet with distinct functional consequences. Labeling of one position impedes subsequent ATP function, which results in inefficient gating, whereas tethering of the other position, although failing to produce gating by itself, enhances subsequent ATP function. Our results thus define a large and dynamic intersubunit ATP-binding pocket and suggest that receptors trapped in covalently agonist-bound states differ in their ability to gate the ion channel.

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.

Molecular determinants of potent P2X2 antagonism identified by functional analysis, mutagenesis, and homology docking

P2X2 receptors are members of the ATP-gated P2X family of cation channels, and they participate in neurotransmission in sympathetic ganglia and interneurons. Here, we identified 7,7'-(carbonylbis(imino-3,1-phenylenecarbonylimino-3,1-(4-methyl-phenylene)carbonylimino))bis(1-methoxy-naphthalene-3,6-disulfonic acid) tetrasodium salt (NF770) as a nanomolar-potent competitive P2X2 receptor antagonist within a series of 139 suramin derivatives. Three structural determinants contributed to the inhibition of P2X2 receptors by NF770: 1) a "large urea" structure with two symmetric phenylenecarbonylimino groups; 2) attachment of the naphthalene moiety in position 7,7'; and 3) the specific position of two sulfonic acid groups (3,3'; 6,6') and of one methoxy group (1,1') at the naphthalene moiety. This structure-activity relationship was interpreted using a rat P2X2 homology model based on the crystal structure of the closed zebrafish P2X4 receptor. Docking of the suramin derivatives into the modeled ATP-binding pocket provides a uniform explanation for the observed differences in inhibitory potencies. Changes in the chemical structure that increase the inhibitory potency of the suramin derivatives improved the spatial orientation within the ATP-binding pocket to allow for stronger polar interactions of functional groups with Gly72, Glu167, or Arg290. Gly72 is responsible for the orientation of the methoxy group close to Arg290 or Glu167. Combined mutational and functional analysis confirmed that residues Gly72 and Glu167 are as important for ATP binding as Arg290, the ATP-binding role of which has been shown in previous studies. The in silico prediction of Gly72 and Glu167 as ATP-binding residues strongly supports the validity of our homology docking.

Amino acid residues constituting the agonist binding site of the human P2X3 receptor

Homomeric P2X3 receptors are present in sensory ganglia and participate in pain perception. Amino acid (AA) residues were replaced in the four supposed nucleotide binding segments (NBSs) of the human (h) P2X3 receptor by alanine, and these mutants were expressed in HEK293 cells and Xenopus laevis oocytes. Patch clamp and two-electrode voltage clamp measurements as well as the Ca(2+) imaging technique were used to compare the concentration-response curves of the selective P2X1,3 agonist α,β-methylene ATP obtained at the wild-type P2X3 receptor and its NBS mutants. Within these NBSs, certain Gly (Gly-66), Lys (Lys-63, Lys-176, Lys-284, Lys-299), Asn (Asn-177, Asn-279), Arg (Arg-281, Arg-295), and Thr (Thr-172) residues were of great importance for a full agonist response. However, the replacement of further AAs in the NBSs by Ala also appeared to modify the amplitude of the current and/or [Ca(2+)](i) responses, although sometimes to a minor degree. The agonist potency decrease was additive after the simultaneous replacement of two adjacent AAs by Ala (K65A/G66A, F171A/T172A, N279A/F280A, F280A/R281A) but was not altered after Ala substitution of two non-adjacent AAs within the same NBS (F171A/N177A). SDS-PAGE in the Cy5 cell surface-labeled form demonstrated that the mutants appeared at the cell surface in oocytes. Thus, groups of AAs organized in NBSs rather than individual amino acids appear to be responsible for agonist binding at the hP2X3 receptor. These NBSs are located at the interface of the three subunits forming a functional receptor.

A putative extracellular salt bridge at the subunit interface contributes to the ion channel function of the ATP-gated P2X2 receptor

The recent crystal structure of the ATP-gated P2X4 receptor revealed a static view of its architecture, but the molecular mechanisms underlying the P2X channels activation are still unknown. By using a P2X2 model based on the x-ray structure, we sought salt bridges formed between charged residues located in a region that directly connects putative ATP-binding sites to the ion channel. To reveal their significance for ion channel activation, we made systematic charge exchanges and measured the effects on ATP sensitivity. We found that charge reversals at the interfacial residues Glu(63) and Arg(274) produced gain-of-function phenotypes that were cancelled upon paired charge swapping. These results suggest that a putative intersubunit salt bridge formed between Glu(63) and Arg(274) contributes to the ion channel function. Engineered cysteines E63C and R274C formed redox-dependent cross-links in the absence of ATP. By contrast, the presence of ATP reduced the rate of disulfide bond formation, indicating that ATP binding might trigger relative movement of adjacent subunits at the level of Glu(63) and Arg(274), allowing the transmembrane helices to open the channel.

New structure enlivens interest in P2X receptors

P2X receptors are ATP-gated membrane ion channels with multifarious roles, including afferent sensation, autocrine feedback loops, and inflammation. Their molecular operation has been less well elucidated compared with other ligand-gated channels (nicotinic acetylcholine receptors, ionotropic glutamate receptors). This will change with the recent publication of the crystal structure of a closed P2X receptor. Here we re-interpret results from 15 years of experiments using site-directed mutagenesis with a model based on the new structure. Previous predictions of receptor stoichiometry, the extracellular ATP binding site, inter-subunit contacts, and many details of the permeation pathway fall into place in three dimensions. We can therefore quickly understand how the channel operates at the molecular level. This is important not only for ion- channel aficionados, but also those engaged in developing effective antagonists at P2X receptors for potential therapeutic use.

Identification and characterization of a novel variant of the human P2X(7) receptor resulting in gain of function

The P2X(7) receptor exhibits significant allelic polymorphism in humans, with both loss and gain of function variants potentially impacting on a variety of infectious and inflammatory disorders. At least five loss-of-function polymorphisms (G150R, R307Q, T357S, E496A, and I568N) and two gain-of-function polymorphisms (H155Y and Q460R) have been identified and characterized to date. In this study, we used RT-PCR cloning to isolate and characterize P2X(7) cDNA clones from human PBMCs and THP-1 cells. A previously unreported variant with substitutions of V80M and A166G was identified. When expressed in HEK293 cells, this variant exhibited heightened sensitivity to the P2X(7) agonist (BzATP) relative to the most frequent allele, as shown by pore formation measured by fluorescent dye uptake into cells. Mutational analyses showed that A166G alteration was critical for the gain-of-function change, while V80M was not. Full-length variants with multiple previously identified nonsynonymous SNPs (H155Y, H270R, A348T, and E496A) were also identified. Distinct functional phenotypes of the P2X(7) variants or mutants constructed with multiple polymorphisms were observed. Gain-of-function variations (A166G or H155Y) could not rescue the loss-of-function E496A polymorphism. Synergistic effects of the gain-of-function variations were also observed. We also identified the A348T alteration as a weak gain-of-function variant. Thus, these results identify the new gain-of-function variant A166G and demonstrate that multiple-gene polymorphisms contribute to functional phenotypes of the human P2X(7) receptor. Furthermore, the results demonstrate that the C-terminal of the cysteine-rich domain 1 of P2X(7) is critical for regulation of P2X(7)-mediated pore formation.

Functional characterization of intracellular Dictyostelium discoideum P2X receptors

Indicative of cell surface P2X ion channel activation, extracellular ATP evokes a rapid and transient calcium influx in the model eukaryote Dictyostelium discoideum. Five P2X-like proteins (dP2XA-E) are present in this organism. However, their roles in purinergic signaling are unclear, because dP2XA proved to have an intracellular localization on the contractile vacuole where it is thought to be required for osmoregulation. To determine functional properties of the remaining four dP2X-like proteins and to assess their cellular roles, we recorded membrane currents from expressed cloned receptors and generated a quintuple knock-out Dictyostelium strain devoid of dP2X receptors. ATP evoked inward currents at dP2XB and dP2XE receptors but not at dP2XC or dP2XD. beta,gamma-Imido-ATP was more potent than ATP at dP2XB but a weak partial agonist at dP2XE. Currents in dP2XB and dP2XE were strongly inhibited by Na(+) but insensitive to copper and the P2 receptor antagonists pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid and suramin. Unusual for P2X channels, dP2XA and dP2XB were also Cl(-)-permeable. The extracellular purinergic response to ATP persisted in p2xA/B/C/D/E quintuple knock-out Dictyostelium demonstrating that dP2X channels are not responsible. dP2XB, -C, -D, and -E were found to be intracellularly localized to the contractile vacuole with the ligand binding domain facing the lumen. However, quintuple p2xA/B/C/D/E null cells were still capable of regulating cell volume in water demonstrating that, contrary to previous findings, dP2X receptors are not required for osmoregulation. Responses to the calmodulin antagonist calmidazolium, however, were reduced in p2xA/B/C/D/E null cells suggesting that dP2X receptors play a role in intracellular calcium signaling.

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.

Contribution of the region Glu181 to Val200 of the extracellular loop of the human P2X1 receptor to agonist binding and gating revealed using cysteine scanning mutagenesis

At the majority of mutants in the region Glu181-Val200 incorporating a conserved AsnPheThrΦΦxLys motif cysteine substitution had no effect on sensitivity to ATP, partial agonists, or methanethiosulfonate (MTS) compounds. For the F185C mutant the efficacy of partial agonists was reduced by ∼ 90% but there was no effect on ATP potency or the actions of MTS reagents. At T186C, F188C and K190C mutants ATP potency and partial agonists responses were reduced. The ATP sensitivity of the K190C mutant was rescued towards WT levels by positively charged (2-aminoethyl)methanethiosulfonate hydrobromide and reduced by negatively charged sodium (2-sulfonatoethyl) methanethiosulfonate. Both MTS reagents decreased ATP potency at the T186C mutant, and abolished responses at the F195C mutant. ³²P-2-azido ATP binding to the mutants T186C and K190C was sensitive to MTS reagents consistent with an effect on binding, however binding at F195C was unaffected indicating an effect on gating. The accessibility of the introduced cysteines was probed with (2-aminoethyl)methanethiosulfonate hydrobromide-biotin, this showed that the region Thr186-Ser192 is likely to form a beta sheet and that accessibility is blocked by ATP. Taken together these results suggest that Thr186, Phe188 and Lys190 are involved in ATP binding to the receptor and Phe185 and Phe195 contribute to agonist evoked conformational changes.

A P2X receptor from the tardigrade species Hypsibius dujardini with fast kinetics and sensitivity to zinc and copper

Background

Orthologs of the vertebrate ATP gated P2X channels have been identified in Dictyostelium and green algae, demonstrating that the emergence of ionotropic purinergic signalling was an early event in eukaryotic evolution. However, the genomes of a number of animals including Drosophila melanogaster and Caenorhabditis elegans, both members of the Ecdysozoa superphylum, lack P2X-like proteins, whilst other species such as the flatworm Schistosoma mansoni have P2X proteins making it unclear as to what stages in evolution P2X receptors were lost. Here we describe the functional characterisation of a P2X receptor (HdP2X) from the tardigrade Hypsibius dujardini demonstrating that purinergic signalling is preserved in some ecdysozoa.

Results

ATP (EC₅₀ ~44.5 μM) evoked transient inward currents in HdP2X with millisecond rates of activation and desensitisation. HdP2X is antagonised by pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid (IC₅₀ 15.0 μM) and suramin (IC₅₀ 22.6 μM) and zinc and copper inhibit ATP-evoked currents with IC₅₀ values of 62.8 μM and 19.9 μM respectively. Site-directed mutagenesis showed that unlike vertebrate P2X receptors, extracellular histidines do not play a major role in coordinating metal binding in HdP2X. However, H306 was identified as playing a minor role in the actions of copper but not zinc. Ivermectin potentiated responses to ATP with no effect on the rates of current activation or decay.

Conclusion

The presence of a P2X receptor in a tardigrade species suggests that both nematodes and arthropods lost their P2X genes independently, as both traditional and molecular phylogenies place the divergence between Nematoda and Arthropoda before their divergence from Tardigrada. The phylogenetic analysis performed in our study also clearly demonstrates that the emergence of the family of seven P2X channels in human and other mammalian species was a relatively recent evolutionary event that occurred subsequent to the split between vertebrates and invertebrates. Furthermore, several characteristics of HdP2X including fast kinetics with low ATP sensitivity, potentiation by ivermectin in a channel with fast kinetics and distinct copper and zinc binding sites not dependent on histidines make HdP2X a useful model for comparative structure-function studies allowing a better understanding of P2X receptors in higher organisms.

Amino acid variations resulting in functional and nonfunctional zebrafish P2X(1) and P2X (5.1) receptors

Several zebrafish P2X receptors (zP2X(1), zP2X(2), and zP2X(5.1)) have been reported to produce little or no current although their mammalian orthologs produce functional homomeric receptors. We isolated new cDNA clones for these P2X receptors that revealed sequence variations in each. The new variants of zP2X(1) and zP2X(5.1) produced substantial currents when expressed by Xenopus oocytes, however the new variant of zP2X(2) was still nonfunctional. zP2X(2) lacks two lysine residues essential for ATP responsiveness in other P2X receptors; however introduction of these two lysines was insufficient to allow this receptor to function as a homotrimer. We also tested whether P2X signaling is required for myogenesis or synaptic communication at the zebrafish neuromuscular junction. We found that embryonic skeletal muscle expressed only one P2X receptor, P2X(5.1). Antisense knockdown of P2X(5.1) eliminated skeletal muscle responsiveness to ATP but did not prevent myogenesis or behaviors that require functional transmission at the neuromuscular junction.