Effects of neuroamines and divalent cations on cloned and mutated ATP-gated channels

Hybrid QM/MM Simulations Confirm Zn(II) Coordination Sphere That Includes Four Cysteines from the P2 × 4R Head Domain

To ascertain the role of Zn(II) as an allosteric modulator on P2X4R, QM/MM molecular dynamic simulations were performed on the WT and two P2X4R mutants suggested by previous electrophysiological data to affect Zn(II) binding. The Gibbs free energy for the reduction of the putative P2X4R Zn(II) binding site by glutathione was estimated at −22 kcal/mol. Simulations of the WT P2X4R head domain revealed a flexible coordination sphere dominated by an octahedral geometry encompassing C126, N127, C132, C149, C159 and a water molecule. The C132A mutation disrupted the metal binding site, leading to a coordination sphere with a majority of water ligands, and a displacement of the metal ion towards the solvent. The C132A/C159A mutant exhibited a tendency towards WT-like stability by incorporating the R148 backbone to the coordination sphere. Thus, the computational findings agree with previous experimental data showing Zn(II) modulation for the WT and C132A/C159A variants, but not for the C132A mutant. The results provide molecular insights into the nature of the Zn(II) modulation in P2X4R, and the effect of the C132A and C132A/C159A mutations, accounting for an elusive modulation mechanism possibly occurring in other extracellular or membrane protein.

Zinc as Allosteric Ion Channel Modulator: Ionotropic Receptors as Metalloproteins

Zinc is an essential metal to life. This transition metal is a structural component of many proteins and is actively involved in the catalytic activity of cell enzymes. In either case, these zinc-containing proteins are metalloproteins. However, the amino acid residues that serve as ligands for metal coordination are not necessarily the same in structural proteins compared to enzymes. While crystals of structural proteins that bind zinc reveal a higher preference for cysteine sulfhydryls rather than histidine imidazole rings, catalytic enzymes reveal the opposite, i.e., a greater preference for the histidines over cysteines for catalysis, plus the influence of carboxylic acids. Based on this paradigm, we reviewed the putative ligands of zinc in ionotropic receptors, where zinc has been described as an allosteric modulator of channel receptors. Although these receptors do not strictly qualify as metalloproteins since they do not normally bind zinc in structural domains, they do transitorily bind zinc at allosteric sites, modifying transiently the receptor channel's ion permeability. The present contribution summarizes current information showing that zinc allosteric modulation of receptor channels occurs by the preferential metal coordination to imidazole rings as well as to the sulfhydryl groups of cysteine in addition to the carboxyl group of acid residues, as with enzymes and catalysis. It is remarkable that most channels, either voltage-sensitive or transmitter-gated receptor channels, are susceptible to zinc modulation either as positive or negative regulators.

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

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

Solution structure of the rat P2X4 receptor head domain involved in inhibitory metal binding

The P2X receptor is an ATP-gated cation channel expressed on the plasma membrane. The head domain (Gln111-Val167 in the rat P2X4 receptor) regulates ATP-induced cation influx. In this study, we prepared a head domain with three disulfide bonds, such as the intact rat P2X4 receptor contains. NMR analysis showed that the head domain possessed the same fold as in the zebrafish P2X4 receptor previously determined by crystallography. Furthermore, the inhibitory, divalent, metal ion binding sites were determined by NMR techniques. These findings will be useful for the design of specific inhibitors for the P2X receptor family.

Antihyperalgesic effect of 5-HT7 receptor activation on the midbrain periaqueductal gray in a rat model of neuropathic pain

The 5-HT7 receptor is the most recently discovered receptor for 5-hydroxytryptamine (5-HT), and only little is known about the analgesic potential of this receptor. Adenosine triphosphate (ATP) modulates pain transmission by activating P2X/P2Y receptors, in which the P2X3 subtype is an important target for this effect. This study examined the antihyperalgesic effect of the 5-HT7 receptors in the ventrolateral midbrain periaqueductal gray (vlPAG), a crucial site for endogenous pain inhibition. This study also explored the importance of the interactions between the 5-HT7 and P2X3 receptors in this effect. To address this issue, neuropathic pain was induced through chronic constriction injury (CCI) of the sciatic nerve in Sprague-Dawley (SD) rats. The expression level and distribution of the 5-HT7 receptor were evaluated through Western blot and immunohistochemistry. The mechanical withdrawal threshold (MWT) was measured by using an electronic pressure meter test. Different doses (3, 6, and 12μmol) of AS-19, a selective agonist of the 5-HT7 receptor, were administered in the vlPAG of CCI rats. The effects of pretreatment with the selective 5-HT7 receptor antagonist SB-269970 or the selective P2X3 receptor antagonist A-317491 on the analgesic effect of AS-19 were observed. Results showed that CCI decreased the MWT values of the rats. The injury also increased the protein level of the 5-HT7 receptor in the vlPAG of neuropathic pain rats. AS-19 microinjection significantly elevated the MWT values in a dose-dependent manner, but SB-269970 pretreatment attenuated the antihyperalgesic effect of AS-19. Furthermore, the antihyperalgesic effect of the 5-HT7 receptor was partially but significantly blocked by A-317491 pretreatment. These data indicate that the 5-HT7 receptor in the vlPAG exerts an antihyperalgesic effect on rats with neuropathic pain. The 5-HT7 and P2X3 receptors interact in the vlPAG and exhibit an analgesic action through the enhanced function of the endogenous analgesic system.

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.

Cloning and characterization of a P2X receptor expressed in the central nervous system of Lymnaea stagnalis

P2X receptors are membrane ion channels gated by extracellular ATP. Mammals possess seven distinct P2X subtypes (P2X1-7) that have important functions in a wide array of physiological processes including roles in the central nervous system (CNS) where they have been linked to modulation of neurotransmitter release. We report here the cloning and functional characterization of a P2X receptor from the mollusc Lymnaea stagnalis. This model organism has a relatively simple CNS consisting of large readily identifiable neurones, a feature which together with a well characterized neuronal circuitry for important physiological processes such as feeding and respiration makes it an attractive potential model to examine P2X function. Using CODEHOP PCR we identified a single P2X receptor (LymP2X) in Lymnaea CNS which was subsequently cloned by RT-PCR. When heterologously expressed in Xenopus oocytes, LymP2X exhibited ATP evoked inward currents (EC(50) 6.2 µM) which decayed during the continued presence of agonist. UTP and ADP did not activate the receptor whereas αβmeATP was a weak agonist. BzATP was a partial agonist with an EC(50) of 2.4 µM and a maximal response 33% smaller than that of ATP. The general P2 receptor antagonists PPADS and suramin both inhibited LymP2X currents with IC(50) values of 8.1 and 27.4 µM respectively. LymP2X is inhibited by acidic pH whereas Zn(2+) and Cu(2+) ions exhibited a biphasic effect, potentiating currents up to 100 µM and inhibiting at higher concentrations. Quantitative RT-PCR and in situ hybridization detected expression of LymP2X mRNA in neurones of all CNS ganglia suggesting this ion channel may have widespread roles in Lymnaea CNS function.

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.

Allosteric modulation of ATP-gated P2X receptor channels

Seven mammalian purinergic receptor subunits, denoted P2X1-P2X7, and several spliced forms of these subunits have been cloned. When heterologously expressed, these cDNAs encode ATP-gated non-selective cation channels organized as trimers. All activated receptors produce cell depolarization and promote Ca(2+) influx through their pores and indirectly by activating voltage-gated calcium channels. However, the biophysical and pharmacological properties of these receptors differ considerably, and the majority of these subunits are also capable of forming heterotrimers with other members of the P2X receptor family, which confers further different properties. These channels have three ATP binding domains, presumably located between neighboring subunits, and occupancy of at least two binding sites is needed for their activation. In addition to the orthosteric binding sites for ATP, these receptors have additional allosteric sites that modulate the agonist action at receptors, including sites for trace metals, protons, neurosteroids, reactive oxygen species and phosphoinositides. The allosteric regulation of P2X receptors is frequently receptor-specific and could be a useful tool to identify P2X members in native tissues and their roles in signaling. The focus of this review is on common and receptor-specific allosteric modulation of P2X receptors and the molecular base accounting for allosteric binding sites.

Pulsed electron spin resonance resolves the coordination site of Cu²(+) ions in α1-glycine receptor

Herein, we identify the coordination environment of Cu²(+) in the human α1-glycine receptor (GlyR). GlyRs are members of the pentameric ligand-gated ion channel superfamily (pLGIC) that mediate fast signaling at synapses. Metal ions like Zn²(+) and Cu²(+) significantly modulate the activity of pLGICs, and metal ion coordination is essential for proper physiological postsynaptic inhibition by GlyR in vivo. Zn²(+) can either potentiate or inhibit GlyR activity depending on its concentration, while Cu²(+) is inhibitory. To better understand the molecular basis of the inhibitory effect we have used electron spin resonance to directly examine Cu²(+) coordination and stoichiometry. We show that Cu²(+) has one binding site per α1 subunit, and that five Cu²(+) can be coordinated per GlyR. Cu²(+) binds to E192 and H215 in each subunit of GlyR with a 40 μM apparent dissociation constant, consistent with earlier functional measurements. However, the coordination site does not include several residues of the agonist/antagonist binding site that were previously suggested to have roles in Cu²(+) coordination by functional measurements. Intriguingly, the E192/H215 site has been proposed as the potentiating Zn²(+) site. The opposing modulatory actions of these cations at a shared binding site highlight the sensitive allosteric nature of GlyR.

Reciprocal inhibition of Cd(2+) and Ca(2+) uptake in human intestinal crypt cells for voltage-independent Zn-activated pathways

Cadmium-Ca-Zn interactions for uptake have been studied in human intestinal crypt cells HIEC. Our results failed to demonstrate any significant cross-inhibition between Cd and Ca uptake under single metal exposure conditions. However, they revealed a strong reciprocal inhibition for a Zn-stimulated mechanism of transport. Optimal stimulation was observed under exposure conditions that favor an inward-directed Zn gradient, suggesting activation by extracellular rather than intracellular Zn. The effect of Zn on the uptake of Ca was concentration-dependent, and zinc-induced stimulation of Cd uptake resulted in a 3- and 5.8-fold increase in the K(m) and V(max) values, respectively. Neither basal nor Zn-stimulated Ca uptakes were sensitive to membrane depolarization. However, the stimulated component of uptake was inhibited by the trivalent cations Gd(3+), and La(3+) and to a lesser extent by Mg(2+) and Ba(2+). RT-PCR analysis as well as uptake measurement performed with extracellular ATP and/or suramin do not support the involvement of purinergic P2X receptor channels. Uptake and fluorescence data led to the conclusion that Zn is unlikely to trigger Ca influx in response to Ca release from thapsigargin-sensitive intracellular pools. Our data show that Zn may potentiate Cd accumulation in intestinal crypt cells through mechanism that still needs to be clarified.

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.

Comparison of P2X and TRPV1 receptors in ganglia or primary culture of trigeminal neurons and their modulation by NGF or serotonin

BACKGROUND

Cultured sensory neurons are a common experimental model to elucidate the molecular mechanisms of pain transduction typically involving activation of ATP-sensitive P2X or capsaicin-sensitive TRPV1 receptors. This applies also to trigeminal ganglion neurons that convey pain inputs from head tissues. Little is, however, known about the plasticity of these receptors on trigeminal neurons in culture, grown without adding the neurotrophin NGF which per se is a powerful algogen. The characteristics of such receptors after short-term culture were compared with those of ganglia. Furthermore, their modulation by chronically-applied serotonin or NGF was investigated.

RESULTS

Rat or mouse neurons in culture mainly belonged to small and medium diameter neurons as observed in sections of trigeminal ganglia. Real time RT-PCR, Western blot analysis and immunocytochemistry showed upregulation of P2X(3) and TRPV1 receptors after 1-4 days in culture (together with their more frequent co-localization), while P2X(2) ones were unchanged. TRPV1 immunoreactivity was, however, lower in mouse ganglia and cultures. Intracellular Ca(2+) imaging and whole-cell patch clamping showed functional P2X and TRPV1 receptors. Neurons exhibited a range of responses to the P2X agonist alpha, beta-methylene-adenosine-5'-triphosphate indicating the presence of homomeric P2X(3) receptors (selectively antagonized by A-317491) and heteromeric P2X(2/3) receptors. The latter were observed in 16 % mouse neurons only. Despite upregulation of receptors in culture, neurons retained the potential for further enhancement of P2X(3) receptors by 24 h NGF treatment. At this time point TRPV1 receptors had lost the facilitation observed after acute NGF application. Conversely, chronically-applied serotonin selectively upregulated TRPV1 receptors rather than P2X(3) receptors.

CONCLUSION

Comparing ganglia and cultures offered the advantage of understanding early adaptive changes of nociception-transducing receptors of trigeminal neurons. Culturing did not prevent differential receptor upregulation by algogenic substances like NGF or serotonin, indicating that chronic application led to distinct plastic changes in the molecular mechanisms mediating pain on trigeminal nociceptors.

Zinc and copper: pharmacological probes and endogenous modulators of neuronal excitability

As well as being key structural components of many proteins, increasing evidence suggests that zinc and copper ions function as signaling molecules in the nervous system and are released from the synaptic terminals of certain neurons. In this review, we consider the actions of these two ions on proteins that regulate neuronal excitability. In addition to the established actions of zinc, and to a lesser degree copper, on excitatory and inhibitory ligand-gated ion channels, we show that both ions have a number of actions on selected members of the voltage-gated-like ion channel superfamily. For example, zinc is a much more effective blocker of one subtype of tetrodotoxin (TTX)-insensitive sodium (Na+) channel (NaV1.5) than other Na+ channels, whereas a certain T-type calcium (Ca2+) channel subunit (CaV3.2) is particularly sensitive to zinc. For potassium (K+) channels, zinc can have profound effects on the gating of certain KV channels whereas zinc and copper have distinct actions on closely related members of the 2 pore domain potassium channel (K2P) channel family. In addition to direct actions on these proteins, zinc is able to permeate a number of membrane proteins such as (S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)/kainate receptors, Ca2+ channels and some transient receptor potential (trp) channels. There are a number of important physiological and pathophysiological consequences of these many actions of zinc and copper on membrane proteins, in terms of regulation of neuronal excitability and neurotoxicity. Furthermore, the concentration of free zinc and copper either in the synaptic cleft or neuronal cytoplasm may contribute to the etiology of certain disease states such as Alzheimer's disease (AD) and epilepsy.

Divalent cation modulation of a-type potassium channels in acutely dissociated central neurons from wide-type and mutant Drosophila

Drosophila mutants provide an ideal model to study channel-type specificity of ion channel regulation in situ. In this study, the effects of divalent cations on voltage-gated K+ currents were investigated in acutely dissociated central neurons of Drosophila third instar larvae using the whole-cell patch-clamp recording. Our data showed that micromolar Cd2+ enhanced the peak inactivating current (I(A)) without affecting the delayed component (I(K)). The same results were obtained in Ca(2+)-free external solution, and from slo1 mutation, which eliminates transient Ca(2+)-activated K+ current. Micromolar Cd2+ and Zn2+, and millimolar Ca2+ and Mg2+ all shifted the steady-state inactivation curve of I(A) without affecting the voltage-dependence of I(A) activation, whereas millimolar Cd2+ markedly affected both the activation and steady-state inactivation curves for I(A). Divalent cations affected I(A) with different potency; the sequence was: Zn2+ > Cd2+ > Ca2+ > Mg2+. The modulation of I(A) by Cd2+ was partially inhibited in Sh(M), a null Shaker (one of I(A)-encoding genes) mutation. Taken together, the channel-type specificity, the asymmetric effects on I(A) activation and inactivation kinetics, and the diverse potency of divalent cations all strongly support the idea that physiological divalent cations modulate A-type K+ channels through specific binding to extracellular sites of the channels.

Characterization of voltage-dependent gating of P2X2 receptor/channel

The role of a voltage-dependent gate of recombinant P2X2 receptor/channel was investigated in Xenopus oocytes. When a voltage step to -110 mV was applied from a holding potential of -50 mV, a gradual increase was observed in current evoked by 30 microM ATP. Contribution of this voltage-dependent component to total ATP-evoked current was greater when the current was evoked by lower concentrations of ATP. The voltage-dependent gate closed upon depolarization, and half the gates were closed at -80 mV. On the other hand, a potential at which half the gates opened was about -30 mV or more positive, which was determined using a series of hyperpolarization steps. The results of the present study suggest that the voltage-dependent gate behavior of P2X2 receptor is not due to simple activation and deactivation of a single gate, but rather due to transition from a low to a high ATP affinity state.

Functional regulation of P2X6 receptors by N-linked glycosylation: identification of a novel alpha beta-methylene ATP-sensitive phenotype

Investigation of rat recombinant P2X(6) receptors has been limited because of the difficulty in obtaining functional expression in heterologous systems. In this study, we demonstrate glycosylation-dependent regulation of recombinant P2X(6) receptor function and associated conferral of a novel phenotype that is sensitive to the P2X(1) and P2X(3) receptor agonist, alphabeta-methylene ATP. In cells functionally expressing P2X(6) receptors, ATP and alphabeta-methylene ATP evoked slowly desensitizing inward currents (EC(50) values, 0.5 and 0.6 microM, respectively) with slow kinetics of current decay on agonist washout. 2',3'-O-(2,4,6-trinitrophenyl ATP) and iso-pyridoxalphosphate-6-azophenyl-2'-5'-disulfonate were effective antagonists (IC(50) values, 0.8 and 22 microM, respectively); however, suramin was relatively ineffective. Reverse transcription-polymerase chain reaction analysis confirmed the absence of other P2X receptor subunits. Western analysis of membrane fractions from functional and nonfunctional clones confirmed the presence of P2X(6) at the cell membrane but revealed a difference in apparent molecular mass of immunoreactive products ( approximately 70 and approximately 60 kDa, respectively). N-glycosidase F treatment of both functional and nonfunctional receptor cell membranes increased the electrophoretic mobilities of immunoreactive products, with both proteins migrating at approximately 55 kDa, demonstrating an increased level of glycosylation of the P2X(6) receptor in functional compared with nonfunctional cells. This study demonstrates that nonfunctional rat recombinant P2X(6) receptors 1) are expressed on the membrane surface of human embryonic kidney cells and 2) are glycosylated. Expression of the novel functional receptor phenotype is associated with further glycosylation, resulting in an apparently larger molecular mass. These results suggest that P2X(6) receptor subunits contribute to alphabeta-methylene ATP sensitivity.

Allosteric modulation of native cochlear P2X receptors: insights from comparison with recombinant P2X2 receptors

Extracellular adenosine 5'-triphosphate (ATP)-gated ion channels assembled from P2X receptor subunits exhibit subunit-selective allosteric modulation by protons and divalent cations. In voltage-clamped guinea-pig cochlear outer hair cells (OHC) and Deiters' cells (DC), H(+) and Cu(2+), but not Zn(2+), enhanced the P2X receptor-mediated inward currents. Acid pH (6.5) potentiated OHC ATP-gated currents by 45%. Co-application of Cu(2+) (1-40 microM) with ATP increased the response by 20%. In DCs, ATP-gated currents were potentiated 85% by acid pH, and 70% by Cu(2+). Alkaline pH inhibited ATP-gated inward currents by 73% in OHCs and 85% in DCs. Zn(2+) was either ineffective (1-10 microM) or inhibitory (40-400 microM). Recombinant rat P2X(2) receptor-mediated inward currents in XENOPUS oocytes displayed allosteric modulation that was different from the native guinea-pig cochlear P2X receptors. The oocyte ATP-gated inward current was potentiated 450% by shifting from pH 7.5 to pH 6.5, and 130% with 40 microM Cu(2+). The enhanced response to ATP with acid pH and Cu(2+) is a signature of the P2X(2) subunit. In contrast to native guinea-pig cochlear cells, extracellular Zn(2+) (40 microM) increased the recombinant ATP-gated inward current by 200% in oocytes. These results suggest that the positive allosteric modulation of cochlear OHC and DC ATP-gated ion channels by protons and Cu(2+) arises in part from the P2X(2) receptor subunit, with additional regulatory elements.

Mutational analysis of the conserved cysteines of the rat P2X2 purinoceptor

P2X receptors are ATP-gated cation channels that are widely expressed in the brain. The extracellular domains of all seven P2X receptors contain 10 conserved cysteines, which could form disulfide bonds or binding sites for transition metals that modulate P2X receptors. To test whether these cysteines are critical for receptor function, we studied wild-type rat P2X(2) receptors and 10 mutant P2X(2) receptors, each containing an alanine substituted for a cysteine. Nine mutants were functional but had reduced maximum currents compared with wild-type P2X(2) expressed in either Xenopus oocytes or human embryonic kidney (HEK) 293 cells. The 10th mutant (C224A) did not respond to ATP when expressed in oocytes and gave very small currents in HEK 293 cells. Seven mutants (C113A, C124A, C130A, C147A, C158A, C164A, and C214A) showed rightward shifts (9- to 30-fold) in their ATP concentration-response relationships and very little potentiation by zinc. In contrast, C258A and C267A had EC(50) values similar to those of wild-type P2X(2) and were potentiated by zinc. Acidic pH potentiated wild-type and all mutant receptor currents. Despite the loss of zinc potentiation in seven mutants, these cysteines are unlikely to be exposed in the zinc-binding site, because [2-(trimethylammonium)ethyl] methanethiosulfonate bromide did not prevent zinc potentiation of wild-type receptor currents. On the basis of correlations in the maximum current, EC(50), zinc potentiation, and pH potentiation, we suggest that the following cysteine pairs form disulfide bonds: C113-C164, C214-C224, and C258-C267. We also suggest that C124, C130, C147, and C158 form two disulfide bonds, but we are unable to assign specific cysteine pairs to these two bonds.

The role of histidine residues in modulation of the rat P2X(2) purinoceptor by zinc and pH

P2X(2) receptor currents are potentiated by acidic pH and zinc. To identify residues necessary for proton and zinc modulation, alanines were singly substituted for each of the nine histidines in the extracellular domain of the rat P2X(2) receptor. Wild-type and mutant receptors were expressed in Xenopus oocytes and analysed with two-electrode voltage clamp. All mutations caused less than a 2-fold change in the EC(50) of the ATP concentration-response relation. Decreasing the extracellular pH from 7.5 to 6.5 potentiated the responses to 10 microM ATP of wild-type P2X(2) and eight mutant receptors more than 4-fold, but the response of the mutant receptor H319A was potentiated only 1.4-fold. The H319A mutation greatly attenuated the maximal potentiation that could be produced by a drop in pH, shifted the pK(a) (-log of dissociation constant) of the potentiation to a more basic pH as compared with P2X(2) and revealed a substantial pH-dependent decrease in the maximum response with a pK(a) near 6.0. Substituting a lysine for H319 reduced the EC(50) for ATP 40-fold. Zinc (20 microM) potentiated the responses to 10 microM ATP of wild-type P2X(2) and seven histidine mutants by approximately 8-fold but had virtually no effect on the responses of two mutants, H120A and H213A. Neither H120A nor H213A removed the voltage-independent inhibition caused by high concentrations of zinc. The observation that different mutations selectively eliminated pH or zinc potentiation implies that there are two independent sites of action, even though the mechanisms of pH and zinc potentiation appear similar.

Properties of native P2X receptors in rat trigeminal mesencephalic nucleus neurones: lack of correlation with known, heterologously expressed P2X receptors

Trigeminal mesencephalic nucleus (MNV) neurones express functional P2X receptors. In order to determine the molecular identity of the P2X receptors in this nucleus we have used whole cell patch clamp recording of P2X receptor-mediated currents to determine the pharmacological properties of the receptors, and have compared them with those of cloned P2X receptor subunits. The purine nucleotides ATP (300 microM), ATP-gamma-S (30 microM) and alphabetameATP (300 microM) evoked inward currents in all MNV neurones whereas alphabetameADP (300 microM) did not. betagammame-L-ATP (300 microM) evoked only a small ( approximately 20 pA) current in 3 out of 6 MNV neurones. The P2X receptor antagonist TNP-ATP (10 nM-10 microM) and raised extracellular Ca(2+) (8 and 30 mM) reduced, but did not abolish, the current evoked by ATP-gamma-S. The current remaining in TNP-ATP was insensitive to blockade by raised Ca(2+). These properties suggest that MNV neurones do not express homomeric P2X(3), P2X(4) or P2X(6) receptors. Whilst the TNP-ATP-insensitive ATP-gamma-S-evoked current has many characteristics similar to both homomeric P2X(2) and P2X(5) receptors, its insensitivity to blockade by raised Ca(2+) is difficult to reconcile with the receptor being a P2X(2) or P2X(5) homomeric channel. More likely, the receptor is a heteromer that comprises either or both of these subunits. The TNP-ATP-sensitive component of the ATP-gamma-S-evoked current is dissimilar to known cloned homomeric or heteromeric P2X receptors.

Local regulation of [(3)H]-noradrenaline release from the isolated guinea-pig right atrium by P(2X)-receptors located on axon terminals

In this study the regulation of cardiac sympathetic outflow by presynaptic P(2X) receptor-gated ion channels was examined. ATP (30 microM - 1 mM) and other P2-receptor agonists elicited [(3)H]-noradrenaline ([(3)H]-NA) outflow from the isolated guinea-pig right atrium with the potency order of ATP>2-methyl-thioATP>alpha,beta-methylene-ATP=ADP, whereas ss, gamma-methylene-L-ATP was inactive. Ca(2+)-free conditions abolished both electrical field stimulation (EFS)- and ATP-evoked release of tritium. Unlike from EFS-induced outflow, ATP-induced [(3)H]-NA outflow was not reduced by omega-Conotoxin-GVIA (100 nM), Cd(2+) (100 microM) and tetrodotoxin (1 microM). The rapid extracellular decomposition of ATP was revealed by HPLC analysis. However, the effect of ATP to promote [(3)H]-NA release was not prevented by 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 250 nM), 3, 7-dimethyl-1-propargylxanthine (DMPX, 250 nM), or by reactive blue 2 (RB2, 10 microM), antagonists of A(1)-, A(2)- and inhibitory P(2) receptors. Zn(2+) (50 microM), the P(2X)-receptor modulator potentiated, and P(2X) receptor antagonists, i.e. suramin (300 microM), pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS, 30 microM) and 2'-o-(trinitrophenyl)-adenosine 5'-triphosphate (TNP-ATP, 30 microM) antagonized the ATP (1 mM)-evoked response. RT - PCR study revealed the expression of P(2X2) and P(2X3) receptor mRNAs in guinea-pig superior cervical ganglion. PPADS (30 microM) significantly reduced the EFS-induced [(3)H]-NA outflow in the presence DPCPX (250 nM) and RB2 (10 microM). In summary a P(2X)-type purinoceptor regulates noradrenaline release from the isolated right atrium of the guinea-pig. The pharmacological profile of the receptor resemble to homo-oligomeric P(2X3) or hetero-oligomeric P(2X2)/P(2X3) complexes, and provide a new target to intervene on sympathetic neuroeffector transmission at the presynaptic site.

Unique functional properties of a sensory neuronal P2X ATP-gated channel from zebrafish

We report here the structural and functional characterization of an ionotropic P2X ATP receptor from the lower vertebrate zebrafish (Danio rerio). The full-length cDNA encodes a 410-amino acid-long channel subunit zP2X(3), which shares only 54% identity with closest mammalian P2X subunits. When expressed in Xenopus oocytes in homomeric form, ATP-gated zP2X(3) channels evoked a unique nonselective cationic current with faster rise time, faster kinetics of desensitization, and slower recovery than any other known P2X channel. Interestingly, the order of agonist potency for this P2X receptor was found similar to that of distantly related P2X(7) receptors, with benzoylbenzoyl ATP (EC(50) = 5 microM) >> ATP (EC(50) = 350 microM) = ADP > alpha,beta-methylene ATP (EC(50) = 480 microM). zP2X(3) receptors are highly sensitive to blockade by the antagonist trinitrophenyl ATP (IC(50) < 5 nM) but are weakly sensitive to the noncompetitive antagonist pyridoxal phosphate-6-azophenyl-2',4'-disulfonic acid. zP2X(3) subunit mRNA is exclusively expressed at high levels in trigeminal neurons and Rohon-Beard cells during embryonic development, suggesting that neuronal P2X receptors mediating fast ATP responses were selected early in the vertebrate phylogeny to play an important role in sensory pathways.

Pharmacology of cloned P2X receptors

There are seven P2X receptor cDNAs currently known. Six homomeric (P2X1, P2X2, P2X3, P2X4, P2X5, P2X7) and three heteromeric (P2X2/P2X3, P2X4/P2X6, P2X1/P2X5) P2X receptor channels have been characterized in heterologous expression systems. Homomeric P2X1 and P2X3 receptors are readily distinguishable by their rapid desensitization, the agonist action of alpha beta methyleneATP, and the block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. P2X2 receptors are unique among homomeric forms in their potentiation by low pH. Homomeric P2X4 receptors are much less sensitive to antagonism by suramin and pyridoxal 5-phosphate-6-azo-2',4'-disulfonic acid. Homomeric P2X7 receptors are the only form in which 2',3'-O-(4-benzoylbenzoyl)-ATP is more potent than ATP. The heteromeric P2X2/P2X3 receptor resembles P2X2 in slow desensitization kinetics and potentiation by low pH and is similar to P2X3 with respect to agonism by alpha beta methyleneATP and block by 2',3'-O-(2,4,6-trinitrophenyl)-ATP. Other agonists, antagonists, and ions that can be used to differentiate among the receptors are discussed.

Zinc and copper modulate differentially the P2X4 receptor

The rat ATP P2X4 receptor was expressed in Xenopus laevis oocytes to assess the effect of zinc and copper as possible regulators of purinergic mechanisms. ATP applied for 20 s evoked an inward cationic current with a median effective concentration (EC50) of 21.4+/-2.8 microM and a Hill coefficient (nH) of 1.5+/-0.1. Coapplication of ATP plus 10 microM zinc displaced leftward, in a parallel fashion, the ATP concentration-response curve, reducing the EC50 to 8.4+/-1.8 microM (p < 0.01) without altering the receptor nH. The zinc potentiation was fast in onset, easily reversible, and voltage-independent and did not require metal preexposure. The zinc EC50 was 2-5 microM, with a bell-shaped curve. At concentrations of 100-300 microM, zinc produced less potentiation, and at 1 mM, it inhibited 50% the ATP current. The effect of zinc was mimicked by cadmium. In contrast, copper inhibited the ATP-evoked currents in a time- and concentration-dependent fashion, reducing the maximal current (Imax) without altering the EC50. The copper-induced inhibition was slow in onset, slowly reversible, and voltage-independent. Whereas coapplication of 300 microM copper plus ATP reduced Imax to 36.2+/-5%, the coapplication of, or 60-s preexposure by, 10 microM copper reduced Imax to 79+/-9.2% (p < 0.05) and 39.6+/-8.7% (p < 0.01), respectively. The inhibition was noncompetitive in nature and mimicked by mercury. Cobalt, barium, and manganese did not modify significantly the ATP-evoked current, demonstrating metal specificity. The simultaneous 1-min preapplication of both metals revealed that the 10 microM zinc-induced potentiation was obliterated by 10 microM copper, whereas 30 microM copper not only reduced the potentiation, but inhibited the ATP response. Following coapplication of both metals for 20 s with ATP, at least 100 microM copper was required to counteract the 10 microM zinc-induced potentiation. The simultaneous preincubation with both metals provided evidence for a noncompetitive interaction. We hypothesize the existence of metal binding site(s), which are most likely localized in the extracellular domain of the P2X4 receptor structure. These sites are selective and accessible to extracellular metal applications and bind micromolar concentrations of metals. The present results are compatible with the working hypothesis that trace metals, such as copper and zinc, are physiological modulators of the P2X4 receptor. The modulation of brain purinergic transmission by physiologically and toxicologically relevant trace metal cations is highlighted.

Block and unblock by imipramine of cloned and mutated P2X2 receptor/channel expressed in Xenopus oocytes

Effects of imipramine on the cloned P2X2 receptor/channel and its mutants expressed in Xenopus oocytes were examined. Imipramine (100 microM) partially blocked an ionic current mediated through the wild-type P2X2 receptor/channel. With a higher concentration (300 microM) of imipramine, the current block was attenuated, suggesting that the second, lower affinity, 'unblocking' binding-site for imipramine exists in addition to the 'blocking' binding-site. These profiles of the modulation by imipramine were influenced by the substitution of negatively charged or polarized amino acid residues near the outer mouth of the channel pore (Asp315, Thr330 and Asn333) with neutral amino acid residues (Val or Ile). With the neutralization of Asp315, the current 'block' by 100 microM imipramine was attenuated. With the neutralization of Thr330, the current 'block' by 100 microM imipramine was enhanced. With the neutralization of Asn333, the 'unblock' by 300 microM imipramine disappeared. The results suggest that imipramine modulates P2X2 receptor/channels by interacting these amino acid residues.

Modulation of ATP-responses at recombinant rP2X4 receptors by extracellular pH and zinc

The modulatory effects of extracellular H+ and Zn2+ were tested against ATP-responses at rat P2X4 (rP2X4) receptors expressed in Xenopus oocytes under voltage-clamp conditions. ATP (0.1-100 microM, at pH 7.5), evoked inward currents via rP2X4 receptors (EC50 value, 4.1+/-0.98 microM; nH, 1.2+/-0.1). ATP potency was reduced 2 fold, at pH 6.5, without altering maximal activity. ATP potency was reduced by a further 4 fold, at pH 5.5, and the maximal activity of ATP was also reduced. Alkaline conditions (pH 8.0) had no effect on ATP-responses. Zn2+ (100 nM - 10 microM) potentiated ATP-responses at the rP2X4 receptor by 2 fold, whereas higher concentrations (30 microM - 1 mM) inhibited ATP-responses. Zn2+ potentiation was due to an increase in ATP potency, whereas its inhibitory action was due to a reduction in ATP efficacy. Zn2+ modulation of ATP-responses was pH-dependent. At pH 6.5, the bell-shaped curve for Zn2+ was shifted to the right by 1 log unit. At pH 5.5, Zn2+ potentiation was abolished and its inhibitory effect reduced considerably. Suramin (50 microM) also potentiated ATP-responses at rP2X4 receptors. Neither H+ (pH 6.5 and 5.5), Zn2+ (10-100 microM) or a combination of both failed to reveal an inhibitory action of suramin at rP2X4 receptors. In conclusion, H+ and Zn2+ exerted opposite effects on the rP2X4 receptor by lowering and raising agonist potency, respectively. H+ (> or = 3 microM) and Zn2+ (> or = 30 microM) also reduces agonist efficacy by lowering the number of rP2X4 receptors available for activation. The striking differences between the modulatory actions of H+ and Zn2+ at rP2X4 and rP2X2 receptors are discussed.

An asparagine residue regulating conductance through P2X2 receptor/channels

Single channel currents were recorded from Xenopus oocytes expressing wild-type and mutated P2X2 receptors. When 100 mM Na+ was used as the permeant cation, unitary currents of about 80 pS were recorded from the oocyte expressing the wild-type channels. The single channel conductance was roughly halved when Asn333 was replaced by Ile (N333I). A similar decrease in single channel currents was also observed when 100 mM Li+ or Cs+ was used as the permeant cation. With two other mutants, in which Asp315 was replaced by Val (D315V) or Tyr330 was replaced by lie (T333I), single channel conductance was almost the same as that of the wild-type channels. The results suggest that Asn333, which is believed to be involved in the channel pore, plays an essential role in ion transport through P2X2 receptor/channels.

An aspartic acid residue near the second transmembrane segment of ATP receptor/channel regulates agonist sensitivity

Charged or polarized amino acid residues near or within the second transmembrane (M2) segment of neuronal ATP receptor/channels (P2X2 receptors) were neutralized by site-directed mutagenesis, and the properties of the mutants were electrophysiologically characterized using Xenopus oocytes. When Asp315 was substituted with Val (D315V), the sensitivity to ATP was reduced by about 60-fold. The sensitivity to ATP was not affected by the neutralization of Lys324, which is involved in a Walker type A ATP-binding sequence, Lys366, Tyr330, or Asn333. With D315V channels, the sensitivities to other agonists (ADP, ATP gamma S, and 2-methylthio ATP) were also reduced. The sensitivities to antagonists (suramin and Cibacron Blue F3GA) were, however, not affected by this neutralization. The results suggest that Asp315, which is assumed to be present in the extracellular region near the M2 segment of P2X2 receptor/channels, serves to maintain agonist sensitivity.

Zn2+ modulation of ATP-responses at recombinant P2X2 receptors and its dependence on extracellular pH

1. Using recombinant P2X2 receptors expressed in Xenopus oocytes, the modulatory effects of zinc (Zn2+) on ATP-responses were studied under voltage-clamp conditions and at different levels of extracellular pH. 2. Zn2+ (0.3-300 microM) added to the bathing medium potentiated ATP-activated membrane currents, increasing ATP-responses by up to 20 fold. This potentiating effect was reversed on washout. Zn2+-potentiation was reduced in an exponential manner (decaying 1/e in 42 s) as the interval was lengthened between adding Zn2+ then ATP to the superfusate. 3. The potentiating effect of Zn2+ was progressively diminished by acidic shifts in extracellular pH (pHe) which, of itself, also potentiated ATP-responses at P2X2 receptors. The maximal potentiating effects of Zn2+ and H+ were not additive. 4. Neither Zn2+ nor H+ potentiation of ATP-responses was abolished by diethylpyrocarbonate (DEPC, 0.3-3 mM), which irreversibly denatures histidyl residues. Nine histidyl residues are present in the extracellular loop of P2X2 receptors. 5. Zn2+ also enhanced the blocking activity of the P2 receptor antagonist suramin at P2X2 receptors. Therefore, Zn2+ also mimics H+ in increasing suramin-activity at P2X2 receptors. 6. In summary, Zn2+ and H+ potentiate agonist and antagonist activity at P2X2 receptors but their effects are not wholly alike for receptor agonism. There, the potentiating effects of Zn2+ are time-dependent and gradually convert to inhibition while those of H+ are time-independent, persistent and more potent, suggesting that either these modulators interact in a different way with a single allosteric site or with different allosteric sites.

pH dependence of facilitation by neurotransmitters and divalent cations of P2X2 purinoceptor/channels

The pH dependence of the facilitation by dopamine (10 microM), 5-hydroxytryptamine (10 microM), adenosine (1 and 100 microM), Zn2+ (10 microM) and Cd2+ (1 mM) of P2X2 purinoceptor/channels was tested by expressing these channels in Xenopus oocytes. In a pH range between 6.0 and 8.5, concentration-response curves for an inward current activated by ATP were shifted toward a lower concentration range at a more acidic pH, indicating that the sensitivity to ATP is pH-dependent. Comparison of the effects of the neurotransmitters and the divalent cations on the ATP-activated current was made using a concentration of ATP which activated 40-50% of the maximal current at each pH value. The current facilitation by dopamine was obvious at pH 7.1 and 7.7, but was not observed at pH 8.5. At pH 6.0, the current was inhibited upon first trials of dopamine, but it was facilitated upon second trials. With 5-hydroxytryptamine and adenosine, the current facilitation was most remarkable at pH 6.0, less remarkable at pH 7.1 and 7.7, and the facilitation was almost abolished at pH 8.5. On the other hand, the current facilitation by Zn2+ and Cd2+ was more remarkable at alkaline pH values (7.7 and 8.5), and the facilitation was almost abolished at pH 6.0. The results suggest that the facilitation of P2X2 purinoceptors depends on pH, and the pH dependence was different between the neurotransmitters and the divalent cations.