Expression level dependent changes in the properties of P2X2 receptors

Evidence for an effect of receptor density on ligand occupancy and agonist EC50

Drug-receptor interaction theory predicts that proportional receptor occupancy is a function of ligand concentration as defined by a ligand-receptor affinity constant, and is independent of receptor density. However, we previously observed that the EC₅₀ of 5-HT reduced as the density of 5-HT₃ receptors increased, suggesting an effect of receptor density on occupancy. The current study was designed to maximise variability in experimentally observed currents and confirm this apparent contradiction prospectively. Xenopus oocytes were injected with RNA encoding 5-HT₃A receptors under conditions designed to achieve varying receptor expression levels and 5-HT-evoked currents measured using two electrode voltage clamp. Results from 99 oocytes showed that as the maximal peak current increased from 0.05 µA to 12.1 µA there was a 3.7-fold reduction in EC₅₀. Since occupancy and conductance are directly related in this system, this indicates that for a given concentration of 5-HT, proportional occupancy increases with increased receptor density. We conclude that normalising data masks this correlation, and can result in reduced accuracy of pharmacological measurements. We propose a mechanistic explanation for our observations.

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

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

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

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

Identification and characterization of ATP-gated P2X2 receptor gene dominantly expressed in the Japanese flounder (Paralichthys olivaceus) head kidney macrophages

P2X2 receptor (P2X2R) belongs to the family of purinergic receptors that have been shown to play important roles in regulating host innate immune response. Although the immunologic significance of P2X2R has been studied in mammals, the presence and immune relevance of P2X2R in fish remains unclear. In this study we extended our previous observations by identifying and characterizing a P2X2R ortholog (termed PoP2X2R) from Japanese flounder (Paralichthys olivaceus). Quantitative real-time PCR analysis revealed that PoP2X2R mRNA transcripts are widely distributed in all examined normal tissues and are dominantly expressed in hepatopancreas tissue. In addition, we for the first time showed that multiple P2XR subtypes, including P2X2R, P2X4R and P2X7R are co-expressed in the Japanese flounder head kidney macrophages (HKMs) and peripheral blood lymphocytes (PBLs), indicating that they may assemble into hetero-receptor complex or interact in the form of homotrimers to trigger diverse purinergic signaling in the Japanese flounder immune cells. Compared with the known Japanese flounder P2X4 and P2X7 receptors, however, PoP2X2R is much more abundantly expressed in the Japanese flounder HKM cells, suggesting that PoP2X2R may play an important role in this type of immune cells. Glycosylation and immunohistochemistry analyses revealed that PoP2X2R is a glycoprotein expressed on the plasma membrane. Immune challenges experiments showed that PoP2X2R was significantly induced by LPS, poly(I:C) and zymosan stimulations in the HKM and PBL cells, and by Edwardsiella tarda infections in spleen and gill tissues as well. Taken together, we have identified and characterized a new P2X2R member that is involved in fish innate immune response.

Purinergic signalling in the enteric nervous system (An overview of current perspectives)

Purinergic Signalling in the Enteric Nervous System involves the regulated release of ATP (or a structurally-related nucleotide) which activates an extensive suite of membrane-inserted receptors (P2X and P2Y subtypes) on a variety of cell types in the gastrointestinal tract. P2X receptors are gated ion-channels permeable to sodium, potassium and calcium. They depolarise cells, act as a pathway for calcium influx to activate calcium-dependent processes and initiate gene transcription, interact at a molecular level as a form of self-regulation with lipids within the cell wall (e.g. PIP2) and cross-react with other membrane-inserted receptors to regulate their activity (e.g. nAChRs). P2Y receptors are metabotropic receptors that couple to G-proteins. They may release calcium ions from intracellular stores to activate calcium-dependent processes, but also may activate calcium-independent signalling pathways and influence gene transcription. Originally ATP was a candidate only for NANC neurotransmission, for inhibitory motoneurons supplying the muscularis externa of the gastrointestinal tract and bringing about the fast IJP. Purinergic signalling later included neuron-neuron signalling in the ENS, via the production of either fast or slow EPSPs. Later still, purinergic signalling included the neuro-epithelial synapse-for efferent signalling to epithelia cells participating in secretion and absorption, and afferent signalling for chemoreception and mechanoreception at the surface of the mucosa. Many aspects of purinergic signalling have since been addressed in a series of highly-focussed and authoritative reviews. In this overview however, the current focus is on key aspects of purinergic signalling where there remains uncertainty and ambiguity, with the view to stimulating further research in these areas.

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.

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.

High potency zinc modulation of human P2X2 receptors and low potency zinc modulation of rat P2X2 receptors share a common molecular mechanism

Human P2X2 receptors (hP2X2) are strongly inhibited by zinc over the range of 2-100 μM, whereas rat P2X2 receptors (rP2X2) are strongly potentiated over the same range, and then inhibited by zinc over 100 μM. However, the biological role of zinc modulation is unknown in either species. To identify candidate regions controlling zinc inhibition in hP2X2 a homology model based on the crystal structure of zebrafish P2X4.1 was made. In this model, His-204 and His-209 of one subunit were near His-330 of the adjacent subunit. Cross-linking studies confirmed that these residues are within 8 Å of each other. Simultaneous mutation of these three histidines to alanines decreased the zinc potency of hP2X2 nearly 100-fold. In rP2X2, one of these histidines is replaced by a lysine, and in a background in which zinc potentiation was eliminated, mutation of Lys-197 to histidine converted rP2X2 from low potency to high potency inhibition. We explored whether the zinc-binding site lies within the vestibules running down the central axis of the receptor. Elimination of all negatively charged residues from the upper vestibule had no effect on zinc inhibition. In contrast, mutation of several residues in the hP2X2 middle vestibule resulted in dramatic changes in the potency of zinc inhibition. In particular, the zinc potency of P206C could be reversibly shifted from extremely high (∼10 nM) to very low (>100 μM) by binding and unbinding MTSET. These results suggest that the cluster of histidines at the subunit interface controls access of zinc to its binding site.

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

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

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

BACKGROUND AND PURPOSE

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

EXPERIMENTAL APPROACH

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

KEY RESULTS

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

CONCLUSIONS AND IMPLICATIONS

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

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.

Polar residues in the second transmembrane domain of the rat P2X2 receptor that affect spontaneous gating, unitary conductance, and rectification

Membrane ion channels activated by extracellular ATP (P2X receptors) are widely distributed in the nervous system. Their molecular architecture is fundamentally distinct from that of the nicotinic or glutamate receptor families. We have measured single-channel currents, spontaneous gating, and rectification of rat P2X2 receptor in which polar and charged residues of the second transmembrane domain (TM2) were systematically probed by mutagenesis. The results suggest that Asn(333) and Asp(349) lie respectively in external and internal vestibules. Substitutions at Asn(333), Thr(336), and Ser(340) were particularly likely to cause spontaneously active channels. At Thr(336), Thr(339), and Ser(340), the introduction of positive charge (Arg, Lys, or His, or Cys followed by treatment with 2-aminoethyl methanethiosulphonate) greatly enhanced outward currents, suggesting that side-chains of these three residues are exposed in the permeation pathway of the open channel. These functional findings are interpreted in the context of the recently reported 3.1 A crystal structure of the zebrafish P2X4.1 receptor in the closed state. They imply that the gate is formed by residues Asn(333) to Thr(339) and that channel opening involves a counter-clockwise rotation and separation of the TM2 helices.

Opposite effects of zinc on human and rat P2X2 receptors

P2X(2) receptors from rats show potentiation when a submaximal concentration of ATP is combined with zinc in the range of 10-100 microM. Alignment of the amino acid sequences of human P2X(2) (hP2X(2)) and rat P2X(2) (rP2X(2)) indicated that only one of two histidines essential for zinc potentiation in rP2X(2) is present at the homologous position in hP2X(2) (H132), with the position homologous to rat H213 instead having an arginine (R225). When expressed in Xenopus oocytes, mouse P2X(2a) and P2X(2b) receptors showed zinc potentiation indistinguishable from rat P2X(2a), but hP2X(2b) receptors were inhibited by zinc. The extent of zinc inhibition of hP2X(2b) varied with the ATP concentration, with an IC(50) of 8.4 microM zinc when ATP was applied at 10% of maximal and 87 microM zinc when ATP was applied at 99% of maximal. Site-directed mutagenesis showed that none of the nine histidines in the extracellular domain of hP2X(2b) were required for zinc inhibition, although inhibition was attenuated in the H204A and H209A mutations. Mutating R225 to a cysteine was sufficient to confer zinc potentiation onto hP2X(2b), and zinc potentiation was absent in the hP2X(2b)H132A/R225C double mutant. This suggests that zinc potentiation in the mutant hP2X(2b) uses the same mechanism as zinc potentiation in wild-type rP2X(2a). Because of the species-specific modulation by zinc, evidence for an in vivo role of P2X(2) receptors based on studies conducted on genetically modified mice needs to be viewed with caution when extrapolations are made to the function of the human nervous system.

Assembly and trafficking of P2X purinergic receptors (Review)

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

Contribution of extracellular negatively charged residues to ATP action and zinc modulation of rat P2X2 receptors

Two histidines are known to be essential for zinc potentiation of rat P2X2 receptors, but the chemistry of zinc coordination would suggest that other residues also participate in this zinc-binding site. There is also a second lower affinity zinc-binding site in P2X2 receptors whose constituents are unknown. To assess whether the extracellular acidic residues of the P2X2 receptor contribute to zinc potentiation or inhibition, site-directed mutagenesis was used to produce alanine substitutions at each extracellular glutamate or aspartate. Two electrode voltage clamp recordings from Xenopus oocytes indicated that 7 of the 34 mutants (D82A, E85A, E91A, E115A, D136A, D209A, and D281A) were deficient in zinc potentiation and one mutant (E84A) was deficient in zinc inhibition. Additional tests on cysteine mutants at these eight positions indicated that D136 is the only residue that is a strong candidate to be at the potentiating zinc-binding site, and that E84 is unlikely to be at the inhibitory zinc-binding site.

Responses of rat P2X2 receptors to ultrashort pulses of ATP provide insights into ATP binding and channel gating

To gain insight into the way that P2X(2) receptors localized at synapses might function, we explored the properties of outside-out patches containing many of these channels as ATP was very rapidly applied and removed. Using a new method to calibrate the speed of exchange of solution over intact patches, we were able to reliably produce applications of ATP lasting <200 micros. For all concentrations of ATP, there was a delay of at least 80 micros between the time when ATP arrived at the receptor and the first detectable flow of inward current. In response to 200-micros pulses of ATP, the time constant of the rising phase of the current was approximately 600 micros. Thus, most channel openings occurred when no free ATP was present. The current deactivated with a time constant of approximately 60 ms. The amplitude of the peak response to a brief pulse of a saturating concentration of ATP was approximately 70% of that obtained during a long application of the same concentration of ATP. Thus, ATP leaves fully liganded channels without producing an opening at least 30% of the time. Extensive kinetic modeling revealed three different schemes that fit the data well, a sequential model and two allosteric models. To account for the delay in opening at saturating ATP, it was necessary to incorporate an intermediate closed state into all three schemes. These kinetic properties indicate that responses to ATP at synapses that use homomeric P2X(2) receptors would be expected to greatly outlast the duration of the synaptic ATP transient produced by a single presynaptic spike. Like NMDA receptors, P2X(2) receptors provide the potential for complex patterns of synaptic integration over a time scale of hundreds of milliseconds.

A histidine scan to probe the flexibility of the rat P2X2 receptor zinc-binding site

The response of P2X(2) receptors to submaximal concentrations of ATP is potentiated by low levels of extracellular zinc. Histidines 120 and 213 have previously been shown to be essential in binding zinc across an intersubunit binding site. We tested the flexibility of the zinc-binding site by making mutations that had the effect of shifting the two essential histidines up to 13 residues upstream or downstream from their original positions and then testing the ability of the mutated receptors to respond to zinc. Using this method, we were able to explore potential orientations of the two regions relative to one another. Our data are consistent with a moderately flexible zinc-binding site and inconsistent with parallel and anti-parallel orientations of the regions surrounding histidines 120 and 213.

Participation of the Lys313-Ile333 Sequence of the Purinergic P2X4 Receptor in Agonist Binding and Transduction of Signals to the Channel Gate

To study the roles of the Lys(313)-Ile(333) ectodomain sequence of the rat P2X(4) receptor in ATP binding and transduction of signals to the channel gate, the conserved Lys(313), Tyr(315), Gly(316), Ike(317), Arg(318), Asp(320), Val(323), Lys(329), Phe(330), and Ile(333) residues were mutated. Current recordings were done on lifted cells and ATP was applied using an ultrafast solution-switching system. The rates of wild type channel opening and closing in the presence of ATP, but not the rate of washout-induced closing, were dependent on agonist concentration. All mutants other than I317A were expressed in the plasma membrane at comparable levels. The majority of mutants showed significant changes in the peak amplitude of responses and the EC(50) values for ATP. When stimulated with the supramaximal (1.4 mm) ATP concentration, mutants also differed in the kinetics of their activation, deactivation, and/or desensitization. The results suggest a critical role of the Lys(313) residue in receptor function other than coordination of the phosphate group of ATP and possible contribution of the Tyr(315) residue to the agonist binding module. The pattern of changes of receptor function by mutation of other residues was consistent with the operation of the Gly(316)-Ile(333) sequence as a signal transduction module between the ligand binding domain and the channel gate in the second transmembrane domain.

Pharmacology of P2X channels

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

Molecular properties of P2X receptors

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

An intersubunit zinc binding site in rat P2X2 receptors

P2X receptors are ATP-gated ion channels made up of three similar or identical subunits. It is unknown whether ligand binding is intersubunit or intrasubunit, either for agonists or for allosteric modulators. Zinc binds to rat P2X2 receptors and acts as an allosteric modulator, potentiating channel opening. To probe the location of this zinc binding site, P2X2 receptors bearing mutations of the histidines at positions 120 and 213 were expressed in Xenopus oocytes. Studies of H120C and H213C mutants produced five lines of evidence consistent with the hypothesis that the residues in these positions bind zinc. Mixing of subunits containing the H120A or H213A mutation generated receptors that showed zinc potentiation, even though neither of these mutant receptors showed zinc potentiation on its own. Furthermore, expression of trimeric concatamers with His --> Ala mutations at some but not all six positions showed that zinc potentiation correlated with the number of intersubunit histidine pairs. These results indicate that zinc potentiation requires an interaction across a subunit interface. Expression of the H120C/H213C double mutant resulted in the formation of ectopic disulfide bonds that could be detected by changes in the physiological properties of the receptors after treatment with reducing and oxidizing agents. Immunoblot analysis of H120C/H213C protein separated under nonreducing conditions demonstrated that the ectopic bonds were between adjacent subunits. Taken together, these data indicate that His120 and His213 sit close to each other across the interface between subunits and are likely to be key components of the zinc binding site in P2X2 receptors.

Density-dependent changes of the pore properties of the P2X2 receptor channel

Ligand-gated ion channels underlie and play important roles in synaptic transmission, and it is generally accepted that the ion channel pores have a rigid structure that enables strict regulation of ion permeation. One exception is the P2X ATP-gated channel. After application of ATP, the ion selectivity of the P2X2 channel time-dependently changes, i.e. permeability to large cations gradually increases, and there is significant cell-to-cell variation in the intensity of inward rectification. Here we show P2X2 channel properties are correlated with the expression level: increasing P2X2 expression level in oocytes increases permeability to large cations, decreases inward rectification and increases ligand sensitivity. We also observed that the inward rectification changed in a dose-dependent manner, i.e. when low concentration of ATP was applied to an oocyte with a high expression level, the intensity of inward rectification of the evoked current was weak. Taken together, these results show that the pore properties of P2X2 channel are not static but change dynamically depending on the open channel density. Furthermore, we identified by mutagenesis study that Ile328 located at the outer mouth of the pore is critical for the density-dependent changes of P2X2. Our findings suggest synaptic transmission can be modulated by the local density-dependent changes of channel properties caused, for example, by the presence of clustering molecules.

Heterogeneity of P2X Receptors in Sympathetic Neurons: Contribution of Neuronal P2X1 Receptors Revealed Using Knockout Mice

P2X receptors are highly expressed throughout the nervous system, where ATP has been shown to be a neurotransmitter. The aim of this study was to characterize P2X receptor expression within sympathetic postganglionic neurons from the superior cervical ganglia. Reverse transcription-polymerase chain reaction showed the presence of mRNA for all P2X receptors, raising the possibility of multiple subunit expression within these ganglia. Whole-cell patch-clamp and calcium imaging studies revealed a heterogeneous population of P2X receptors in approximately 70% of neurons. We propose that the heterogeneity in properties could be caused by differential expression and/or subunit composition of the P2X receptor. The dominant phenotype was P2X2-like; neurons showed slow desensitization, sensitivity to antagonists, and a profile of ionic modulation that is characteristic of P2X2 receptors: potentiation by acidification and extracellular Zn2+ and attenuation by high extracellular Ca2+ and pH. A subpopulation of neurons (10-15%) were alpha,beta-methylene ATP (alpha,beta-meATP) sensitive, and in neurons from P2X1 receptor-deficient mice the alpha,beta-meATP response was reduced to 2% of all neurons, demonstrating a direct role for P2X1 subunits. Control alpha,beta-meATP responses were eliminated by high extracellular Ca(2+) and pH, indicating the presence of heteromeric channels incorporating the properties of P2X1 and P2X2 receptors. This study demonstrates that in neurons, the P2X1 receptor can contribute to the properties of heteromeric P2X receptors. Given the expression of P2X1 receptors in a range of neurons, it seems likely that regulation of the properties of P2X receptors by this subunit is more widespread.

Dehydroepiandrosterone potentiates native ionotropic ATP receptors containing the P2X2 subunit in rat sensory neurones

We have studied the modulatory effect of dehydroepiandrosterone (DHEA), the most abundant neurosteroid produced by glial cells and neurones, on membrane currents induced by the activation of ionotropic ATP (P2X) receptors in neonatal rat dorsal root ganglion neurones. ATP (1 microM) induced three types of currents/responses termed F (fast and transient), S (slowly desensitizing) and M (mixed, sum of F- and S-type responses). DHEA (10 nM to 100 microM) concentration-dependently increased the amplitude of plateau-like currents of S- and M-type responses evoked by submaximal (1 microM) but not saturating (100 microM or 1 mM) concentrations of ATP. Alphabeta-methylene ATP (alphabetame-ATP, 5 microM) also evoked F-, S- and M-type responses, the plateau phases of which were potentiated by lowering external pH (6.3) and by ivermectin (IVM, 3 microM), indicating the presence heteromeric P2X2-containing receptors and possibly of functional native P2X4/6 receptors. There was a strict correlation between the potentiating effects of low pH and DHEA on alphabetame-ATP responses but not between that of IVM and DHEA, suggesting that DHEA selectively modulated P2X2-containing receptors. DHEA also potentiated putative homomeric P2X2 receptor responses recorded in the continuous presence of 1 microM 2'-(or 3')-O-(2,4,6-trinitrophenyl) adenosine 5'-triphosphate (TNP-ATP). Our results constitute the first demonstration of a fast potentiation of P2X receptors by a neurosteroid and suggest that DHEA could be an endogenous modulator of P2X2-containing receptors thereby contributing to the facilitation of the detection and/or the transmission of nociceptive messages, particularly under conditions of inflammatory pain where the P2X receptor signalling pathway appears to be upregulated.

Intracellular calcium measurements as a method in studies on activity of purinergic P2X receptor channels

Extracellular nucleotide-activated purinergic receptors (P2XRs) are a family of cation-permeable channels that conduct small cations, including Ca2+, leading to the depolarization of cells and subsequent stimulation of voltage-gated Ca2+ influx in excitable cells. Here, we studied the spatiotemporal characteristics of intracellular Ca2+ signaling and its dependence on current signaling in excitable mouse immortalized gonadotropin-releasing hormone-secreting cells (GT1) and nonexcitable human embryonic kidney cells (HEK-293) cells expressing wild-type and chimeric P2XRs. In both cell types, P2XR generated depolarizing currents during the sustained ATP stimulation, which desensitized in order (from rapidly desensitizing to nondesensitizing): P2X3R > P2X2b + X4R > P2X2bR > P2X2a + X4R > P2X4R > P2X2aR > P2X7R. HEK-293 cells were not suitable for studies on P2XR-mediated Ca2+ influx because of the coactivation of endogenously expressed Ca2+-mobilizing purinergic P2Y receptors. However, when expressed in GT1 cells, all wild-type and chimeric P2XRs responded to agonist binding with global Ca2+ signals, which desensitized in the same order as current signals but in a significantly slower manner. The global distribution of Ca2+ signals was present independently of the rate of current desensitization. The temporal characteristics of Ca2+ signals were not affected by voltage-gated Ca2+ influx and removal of extracellular sodium. Ca2+ signals reflected well the receptor-specific EC50 values for ATP and the extracellular Zn2+ and pH sensitivities of P2XRs. These results indicate that intracellular Ca2+ measurements are useful for characterizing the pharmacological properties and messenger functions of P2XRs, as well as the kinetics of channel activity, when the host cells do not express other members of purinergic receptors.