Agonist trapped in ATP-binding sites of the P2X2 receptor

Subtype-Specific Ligand Binding and Activation Gating in Homomeric and Heteromeric P2X Receptors

P2X receptors are ATP-activated, non-specific cation channels involved in sensory signalling, inflammation, and certain forms of pain. Investigations of agonist binding and activation are essential for comprehending the fundamental mechanisms of receptor function. This encompasses the ligand recognition by the receptor, conformational changes following binding, and subsequent cellular signalling. The ATP-induced activation of P2X receptors is further influenced by the concentration of Mg2+ that forms a complex with ATP. To explore these intricate mechanisms, two new fluorescently labelled ATP derivatives have become commercially available: 2-[DY-547P1]-AHT-ATP (fATP) and 2-[DY-547P1]-AHT-α,βMe-ATP (α,βMe-fATP). We demonstrate a subtype-specific pattern of ligand potency and efficacy on human P2X2, P2X3, and P2X2/3 receptors with distinct relations between binding and gaiting. Given the high in vivo concentrations of Mg2+, the complex formed by Mg2+ and ATP emerges as an adequate ligand for P2X receptors. Utilising fluorescent ligands, we observed a Mg2+-dependent reduction in P2X2 receptor activation, while binding remained surprisingly robust. In contrast, P2X3 receptors initially exhibited decreased activation at high Mg2+ concentrations, concomitant with increased binding, while the P2X2/3 heteromer showed a hybrid effect. Hence, our new fluorescent ATP derivatives are powerful tools for further unravelling the mechanism underlying ligand binding and activation gating in P2X receptors.

Unlocking the therapeutic potential of P2X7 receptor: a comprehensive review of its role in neurodegenerative disorders

The P2X7 receptor (P2X7R), an ATP-gated ion channel, has emerged as a crucial player in neuroinflammation and a promising therapeutic target for neurodegenerative disorders. This review explores the current understanding of P2X7R's structure, activation, and physiological roles, focusing on its expression and function in microglial cells. The article examines the receptor's involvement in calcium signaling, microglial activation, and polarization, as well as its role in the pathogenesis of Alzheimer's disease, Parkinson's disease, multiple sclerosis, and amyotrophic lateral sclerosis. The review highlights the complex nature of P2X7R signaling, discussing its potential neuroprotective and neurotoxic effects depending on the disease stage and context. It also addresses the development of P2X7R antagonists and their progress in clinical trials, identifying key research gaps and future perspectives for P2X7R-targeted therapy development. By providing a comprehensive overview of the current state of knowledge and future directions, this review serves as a valuable resource for researchers and clinicians interested in exploring the therapeutic potential of targeting P2X7R for the treatment of neurodegenerative disorders.

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.

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.

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

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

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

P2X receptors

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

Tethering an N-Glycosylation Sequon-Containing Peptide Creates a Catalytically Competent Oligosaccharyltransferase Complex

Oligosaccharyltransferase (OST) transfers an oligosaccharide chain to the Asn residue in the Asn-X-Ser/Thr sequon in proteins, where X is not proline. A sequon was tethered to an archaeal OST enzyme via a disulfide bond. The positions of the cysteine residues in the OST protein and the sequon-containing acceptor peptide were selected by reference to the eubacterial OST structure in a noncovalent complex with an acceptor peptide. We determined the crystal structure of the cross-linked OST-sequon complex. The Ser/Thr-binding pocket recognizes the Thr residue in the sequon, and the catalytic structure termed the "carboxylate dyad" interacted with the Asn residue. Thus, the recognition and the catalytic mechanism of the sequon are conserved between the archaeal and eubacterial OSTs. We found that the tethered peptides in the complex were efficiently glycosylated in the presence of the oligosaccharide donor. The stringent requirements are greatly relaxed in the cross-linked state. The two conserved acidic residues in the catalytic structure were each dispensable, although the double mutation abolished the activity. A Gln residue at the Asn position in the sequon functioned as an acceptor, and the hydroxy group at position +2 was not required. In the standard assay using short free peptides, strong amino acid preferences were observed at the X position, but the preferences, except for Pro, completely disappeared in the cross-linked state. By skipping the initial binding process and stabilizing the complex state, the catalytically competent cross-linked complex offers a unique system for studying the oligosaccharyl transfer reaction.

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.

A Highly Conserved Salt Bridge Stabilizes the Kinked Conformation of β2,3-Sheet Essential for Channel Function of P2X4 Receptors

Significant progress has been made in understanding the roles of crucial residues/motifs in the channel function of P2X receptors during the pre-structure era. The recent structural determination of P2X receptors allows us to reevaluate the role of those residues/motifs. Residues Arg-309 and Asp-85 (rat P2X4 numbering) are highly conserved throughout the P2X family and were involved in loss-of-function polymorphism in human P2X receptors. Previous studies proposed that they participated in direct ATP binding. However, the crystal structure of P2X demonstrated that those two residues form an intersubunit salt bridge located far away from the ATP-binding site. Therefore, it is necessary to reevaluate the role of this salt bridge in P2X receptors. Here, we suggest the crucial role of this structural element both in protein stability and in channel gating rather than direct ATP interaction and channel assembly. Combining mutagenesis, charge swap, and disulfide cross-linking, we revealed the stringent requirement of this salt bridge in normal P2X4 channel function. This salt bridge may contribute to stabilizing the bending conformation of the β2,3-sheet that is structurally coupled with this salt bridge and the α2-helix. Strongly kinked β2,3 is essential for domain-domain interactions between head domain, dorsal fin domain, right flipper domain, and loop β7,8 in P2X4 receptors. Disulfide cross-linking with directions opposing or along the bending angle of the β2,3-sheet toward the α2-helix led to loss-of-function and gain-of-function of P2X4 receptors, respectively. Further insertion of amino acids with bulky side chains into the linker between the β2,3-sheet or the conformational change of the α2-helix, interfering with the kinked conformation of β2,3, led to loss-of-function of P2X4 receptors. All these findings provided new insights in understanding the contribution of the salt bridge between Asp-85 and Arg-309 and its structurally coupled β2,3-sheet to the function of P2X receptors.

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

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

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

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

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

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

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

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

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

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'.

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.

Regulation of ATP-gated P2X channels: from redox signaling to interactions with other proteins

SIGNIFICANCE

The family of purinergic P2X receptors (P2XRs) is a part of ligand-gated superfamily of channels activated by extracellular adenosine-5'-triphosphate. P2XRs are present in virtually all mammalian tissues as well as in tissues of other vertebrate and nonvertebrate species and mediate a large variety of functions, including fast transmission at central synapses, contraction of smooth muscle cells, platelet aggregation, and macrophage activation to proliferation and cell death.

RECENT ADVANCES

The recent solving of crystal structure of the zebrafish P2X4.1R is a major advance in the understanding of structural correlates of channel activation and regulation. Combined with growing information obtained in the post-structure era and the reinterpretation of previous work within the context of the tridimensional structure, these data provide a better understanding of how the channel operates at the molecular levels.

CRITICAL ISSUES

This review focuses on the relationship between redox signaling and P2XR function. We also discuss other allosteric modulation of P2XR gating in the physiological/pathophysiological context. This includes the summary of extracellular actions of trace metals, which can be released to the synaptic cleft, pH decrease that happens during ischemia and inflammation, and calcium, an extracellular and intracellular messenger.

FUTURE DIRECTIONS

Our evolving understanding of activation and regulation of P2XRs is helpful in clarifying the mechanism by which these channels trigger and modulate cellular functions. Further research is required to identify the signaling pathways contributing to the regulation of the receptor activity and to develop novel and receptor-specific allosteric modulators, which could be used in vivo with therapeutic potential.

Inherent dynamics of head domain correlates with ATP-recognition of P2X4 receptors: insights gained from molecular simulations

P2X receptors are ATP-gated ion channels involved in many physiological functions, and determination of ATP-recognition (AR) of P2X receptors will promote the development of new therapeutic agents for pain, inflammation, bladder dysfunction and osteoporosis. Recent crystal structures of the zebrafish P2X4 (zfP2X4) receptor reveal a large ATP-binding pocket (ABP) located at the subunit interface of zfP2X4 receptors, which is occupied by a conspicuous cluster of basic residues to recognize triphosphate moiety of ATP. Using the engineered affinity labeling and molecular modeling, at least three sites (S1, S2 and S3) within ABP have been identified that are able to recognize the adenine ring of ATP, implying the existence of at least three distinct AR modes in ABP. The open crystal structure of zfP2X4 confirms one of three AR modes (named AR1), in which the adenine ring of ATP is buried into site S1 while the triphosphate moiety interacts with clustered basic residues. Why architecture of ABP favors AR1 not the other two AR modes still remains unexplored. Here, we examine the potential role of inherent dynamics of head domain, a domain involved in ABP formation, in AR determinant of P2X4 receptors. In silico docking and binding free energy calculation revealed comparable characters of three distinct AR modes. Inherent dynamics of head domain, especially the downward motion favors the preference of ABP for AR1 rather than AR2 and AR3. Along with the downward motion of head domain, the closing movement of loop139-146 and loop169-183, and structural rearrangements of K70, K72, R298 and R143 enabled ABP to discriminate AR1 from other AR modes. Our observations suggest the essential role of head domain dynamics in determining AR of P2X4 receptors, allowing evaluation of new strategies aimed at developing specific blockers/allosteric modulators by preventing the dynamics of head domain associated with both AR and channel activation of P2X4 receptors.

Structural and molecular modeling features of P2X receptors

Currently, adenosine 5'-triphosphate (ATP) is recognized as the extracellular messenger that acts through P2 receptors. P2 receptors are divided into two subtypes: P2Y metabotropic receptors and P2X ionotropic receptors, both of which are found in virtually all mammalian cell types studied. Due to the difficulty in studying membrane protein structures by X-ray crystallography or NMR techniques, there is little information about these structures available in the literature. Two structures of the P2X4 receptor in truncated form have been solved by crystallography. Molecular modeling has proven to be an excellent tool for studying ionotropic receptors. Recently, modeling studies carried out on P2X receptors have advanced our knowledge of the P2X receptor structure-function relationships. This review presents a brief history of ion channel structural studies and shows how modeling approaches can be used to address relevant questions about P2X receptors.

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.

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

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

Heteromeric assembly of P2X subunits

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

P2X receptor intermediate activation states have altered nucleotide selectivity

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

Two open states of P2X receptor channels

The occupancy of the orthosteric ligand binding sites of P2X receptor (P2XR) channels causes the rapid opening of a small cation-permeable pore, followed by a gradual dilation that renders the pore permeable to large organic cations. Electrophysiologically, this phenomenon was shown using whole-cell current recording on P2X2R-, P2X2/X5R-, P2X4R- and P2X7R-expressing cells that were bathed in N-methyl-D-glucamine (NMDG⁺)-containing buffers in the presence and/or absence of small monovalent and divalent cations. The pore dilation of P2X4R and P2X7R caused a secondary current growth, whereas that of P2X2R showed a sustained kinetic coupling of dilation and desensitization, leading to receptor channel closure. The pore size of the P2X7R open and dilated states was estimated to be approximately 0.85 nm and greater than 1 nm, respectively. The P2XR pore dilation was also observed in intact cells by measurement of fluorescent dye uptake/release, application of polyethylene glycols of different sizes, and atomic force microscopy. However, pore dilation was not observed at the single channel level. Structural data describing the dilated state are not available, and the relevance of orthosteric and allosteric ligand interactions to pore dilation was not studied.

Inter- and intrasubunit interactions between transmembrane helices in the open state of P2X receptor channels

P2X receptor channels open in response to the binding of extracellular ATP, a property that is essential for purinergic sensory signaling. Apo and ATP-bound X-ray structures of the detergent-solubilized zebrafish P2X4 receptor provide a blueprint for receptor mechanisms but unexpectedly showed large crevices between subunits within the transmembrane (TM) domain of the ATP-bound structure. Here we investigate both intersubunit and intrasubunit interactions between TM helices of P2X receptors in membranes using both computational and functional approaches. Our results suggest that intersubunit crevices found in the TM domain of the ATP-bound crystal structure are not present in membrane-embedded receptors but substantiate helix interactions within individual subunits and identify a hot spot at the internal end of the pore where both the gating and permeation properties of P2X receptors can be tuned. We propose a model for the structure of the open state that has stabilizing intersubunit interactions and that is compatible with available structural constraints from functional channels in membrane environments.

Novel approaches to drug design for the treatment of schizophrenia

INTRODUCTION

Schizophrenia is an important health issue affecting almost 1% of the population with significant unmet medical needs. The classical drug targets for the treatment of schizophrenia are dopamine D2 receptors. Second-generation ('atypical') drugs block more receptors of the G-protein-coupled receptor (GPCR) class 1 (e.g., clozapine is a D(2)-5HT(2) antagonist).

AREAS COVERED

In this article, the author presents the new targets for GPCR as well as ligand-gated ion. Furthermore, the author reviews the opportunities for drug design offered by the structures solved recently.

EXPERT OPINION

For drug design, the availability of these protein structures, or the possibility to build high quality models, allows to shift the paradigm from ligand-based to target-based drug design. The analysis of the drugs, both on the market and in development, shows that numerous targets are being considered which may reveal an ambiguity on the ideal drug target. This situation may be simplified, in the future, owing to recent integrative projects: the 'Human Brain Project' and the 'Brain Activity Map' that aim at modeling the brain as well as the Allen Atlas.

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

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

Allosteric nature of P2X receptor activation probed by photoaffinity labelling

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

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.

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

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

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.

Involvement of the cysteine-rich head domain in activation and desensitization of the P2X1 receptor

P2X receptors (P2XRs) are ligand-gated ion channels activated by extracellular ATP. Although the crystal structure of the zebrafish P2X4R has been solved, the exact mode of ATP binding and the conformational changes governing channel opening and desensitization remain unknown. Here, we used voltage clamp fluorometry to investigate movements in the cysteine-rich head domain of the rat P2X1R (A118-I125) that projects over the proposed ATP binding site. On substitution with cysteine residues, six of these residues (N120-I125) were specifically labeled by tetramethyl-rhodamine-maleimide and showed significant changes in the emission of the fluorescence probe on application of the agonists ATP and benzoyl-benzoyl-ATP. Mutants N120C and G123C showed fast fluorescence decreases with similar kinetics as the current increases. In contrast, mutants P121C and I125C showed slow fluorescence increases that seemed to correlate with the current decline during desensitization. Mutant E122C showed a slow fluorescence increase and fast decrease with ATP and benzoyl-benzoyl-ATP, respectively. Application of the competitive antagonist 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) resulted in large fluorescence changes with the N120C, E122C, and G123C mutants and minor or no changes with the other mutants. Likewise, TNP-ATP-induced changes in control mutants distant from the proposed ATP binding site were comparably small or absent. Combined with molecular modeling studies, our data confirm the proposed ATP binding site and provide evidence that ATP orients in its binding site with the ribose moiety facing the solution. We also conclude that P2XR activation and desensitization involve movements of the cysteine-rich head domain.

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.

Effects of nucleotide analogs at the P2X3 receptor and its mutants identify the agonist binding pouch

In this study, we investigated the effects of single alanine substitutions of amino acid residues in the supposed ATP binding site of the human P2X3 receptor on the agonistic effect of nucleotide analogs. The wild-type and mutant receptors were expressed in HEK293 cells, and the nucleotide effects were measured by means of the whole-cell patch-clamp method. Modifications in the receptor binding site changed the concentration-response relationship, the current kinetics, and the recovery from desensitization during fast, pulsed, local agonist applications. On the basis of this fact, we were able to distinguish binding from other effects, such as gating/desensitization, by using a hidden Markov model that describes the complete channel behavior with a matrix of rate constants. The binding energies of the nucleotide analogs were calculated and compared with the binding energies of ATP at both the wild-type receptor and its alanine mutants. Changes in the binding energies caused by alterations in the receptor and/or agonist structures were concluded to be attributable to the preferential binding of certain structural constituents of ATP to certain amino acid moieties of the receptor. The results were also checked for consistency with a P2X3 homology model that we developed from the known zebrafish P2X4 crystal structure in the closed state. The functional data correlated well with the predictions of the agonist dockings to the structural model.

Tightening of the ATP-binding sites induces the opening of P2X receptor channels

The opening of ligand-gated ion channels in response to agonist binding is a fundamental process in biology. In ATP-gated P2X receptors, little is known about the molecular events that couple ATP binding to channel opening. In this paper, we identify structural changes of the ATP site accompanying the P2X2 receptor activation by engineering extracellular zinc bridges at putative mobile regions as revealed by normal mode analysis. We provide evidence that tightening of the ATP sites shaped like open 'jaws' induces opening of the P2X ion channel. We show that ATP binding favours jaw tightening, whereas binding of a competitive antagonist prevents gating induced by this movement. Our data reveal the inherent dynamic of the binding jaw, and provide new structural insights into the mechanism of P2X receptor activation.

Agonist binding evokes extensive conformational changes in the extracellular domain of the ATP-gated human P2X1 receptor ion channel

P2X receptors for ATP have a wide range of physiological roles and comprise a structurally distinct family of ligand-gated trimeric ion channels. The crystal structure of a P2X4 receptor, in combination with mutagenesis studies, has provided a model of the intersubunit ATP-binding sites and identified an extracellular lateral portal, adjacent to the membrane, that funnels ions to the channel pore. However, little is known about the extent of ATP-induced conformational changes in the extracellular domain of the receptor. To address this issue, we have used MTSEA-biotinylation (N-Biotinoylaminoethyl methanethiosulfonate) to show ATP-sensitive accessibility of cysteine mutants at the human P2X1 receptor. Mapping these data to a P2X1 receptor homology model identifies significant conformational rearrangement. Electron microscopy of purified P2X1 receptors showed marked changes in structure on ATP binding, and introducing disulphide bonds between adjacent subunits to restrict intersubunit movements inhibited channel function. These results are consistent with agonist-induced rotation of the propeller-head domain of the receptor, sliding of adjacent subunits leading to restricted access to the upper vestibule, movement in the ion conducting lateral portals, and gating of the channel pore.

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.

Role of the ectodomain serine 275 in shaping the binding pocket of the ATP-gated P2X3 receptor

ATP-activated P2X3 receptors expressed in nociceptive sensory neurons play an important role in pain signaling. Basic properties of this receptor subtype, including very strong desensitization, depend on the rate of dissociation of the agonist from the binding site. Even though the rough structure of the ATP binding site has been proposed on the basis of the X-ray structure of the zebrafish P2X4 receptor and mutagenesis studies, the fine subunit-specific structural properties predisposing the receptor to tight capture of the agonist inside the binding pocket have not been elucidated. In this work, by exploring in silico the functional role for the left flipper located in the ectodomain region, we identified within this loop a candidate residue S275, which could contribute to the closure of the agonist-binding pocket. Testing of the S275 mutants using the patch-clamp technique revealed a crucial role for S275 in agonist binding and receptor desensitization. The S275A mutant showed a reduced rate of onset of desensitization and accelerated resensitization and was weakly inhibited by nanomolar agonist. Extracellular calcium application produced inhibition instead of facilitation of membrane currents. Moreover, some full agonists became only partial agonists when applied to the S275A receptor. These effects were stronger with the more hydrophobic mutants S275C and S275V. Taken together, our data suggest that S275 contributes to the closure of the agonist-binding pocket and that effective capture of the agonist provided by the left flipper in calcium-dependent manner determines the high rate of desensitization, slow recovery, and sensitivity to nanomolar agonist of the P2X3 receptor.

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.

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.