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

Migraine signaling pathways: purine metabolites that regulate migraine and predispose migraineurs to headache

Migraine is a debilitating disorder that afflicts over 1 billion people worldwide, involving attacks that result in a throbbing and pulsating headache. Migraine is thought to be a neurovascular event associated with vasoconstriction, vasodilation, and neuronal activation. Understanding signaling in migraine pathology is central to the development of therapeutics for migraine prophylaxis and for mitigation of migraine in the prodrome phase before pain sets in. The fact that both vasoactivity and neural sensitization are involved in migraine indicates that agonists which promote these phenomena may very well be involved in migraine pathology. One such group of agonists is the purines, in particular, adenosine phosphates and their metabolites. This manuscript explores what is known about the relationship between these metabolites and migraine pathology and explores the potential for such relationships through their known signaling pathways. Reported receptor involvement in vasoaction and nociception.

Expression profile of the zinc transporter ZnT3 in taste cells of rat circumvallate papillae and its role in zinc release, a potential mechanism for taste stimulation

Zinc is an essential trace element, and its deficiency causes taste dysfunction. Zinc accumulates in zinc transporter (ZnT)3-expressing presynaptic vesicles in hippocampal neurons and acts as a neurotransmitter in the central nervous system. However, the distribution of zinc and its role as a signal transmitter in taste buds remain unknown. Therefore, we examined the distribution of zinc and expression profiles of ZnT3 in taste cells and evaluated zinc release from isolated taste cells upon taste stimuli. Taste cells with a spindle or pyriform morphology were revealed by staining with the fluorescent zinc dye ZnAF-2DA and autometallography in the taste buds of rat circumvallate papillae. Znt3 mRNA levels were detected in isolated taste buds. ZnT3-immunoreactivity was found in phospholipase-β2-immunopositive type II taste cells and aromatic amino acid decarboxylase-immunopositive type III cells but not in nucleoside triphosphate diphosphohydrolase 2-immunopositive type I cells. Moreover, we examined zinc release from taste cells using human transient receptor potential A1-overexpressing HEK293 as zinc-sensor cells. These cells exhibited a clear response to isolated taste cells exposed to taste stimuli. However, pretreatment with magnesium-ethylenediaminetetraacetic acid, an extracellular zinc chelator - but not with zinc-ethylenediaminetetraacetic acid, used as a negative control - significantly decreased the response ratio of zinc-sensor cells. These findings suggest that taste cells release zinc to the intercellular area in response to taste stimuli and that zinc may affect signaling within taste buds.

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.

The Function and Regulation of Zinc in the Brain

Nearly sixty years ago Fredrich Timm developed a histochemical technique that revealed a rich reserve of free zinc in distinct regions of the brain. Subsequent electron microscopy studies in Timm- stained brain tissue found that this "labile" pool of cellular zinc was highly concentrated at synaptic boutons, hinting a possible role for the metal in synaptic transmission. Although evidence for activity-dependent synaptic release of zinc would not be reported for another twenty years, these initial findings spurred decades of research into zinc's role in neuronal function and revealed a diverse array of signaling cascades triggered or regulated by the metal. Here, we delve into our current understanding of the many roles zinc plays in the brain, from influencing neurotransmission and sensory processing, to activating both pro-survival and pro-death neuronal signaling pathways. Moreover, we detail the many mechanisms that tightly regulate cellular zinc levels, including metal binding proteins and a large array of zinc transporters.

Sperm ion channels and transporters in male fertility and infertility

Mammalian sperm cells must respond to cues originating from along the female reproductive tract and from the layers of the egg in order to complete their fertilization journey. Dynamic regulation of ion signalling is, therefore, essential for sperm cells to adapt to their constantly changing environment. Over the past 15 years, direct electrophysiological recordings together with genetically modified mouse models and human genetics have confirmed the importance of ion channels, including the principal Ca²⁺-selective plasma membrane ion channel CatSper, for sperm activity. Sperm ion channels and membrane receptors are attractive targets for both the development of contraceptives and infertility treatment drugs. Furthermore, in this era of assisted reproductive technologies, understanding the signalling processes implicated in defective sperm function, particularly those arising from genetic abnormalities, is of the utmost importance not only for the development of infertility treatments but also to assess the overall health of a patient and his children. Future studies to improve reproductive health care and overall health care as a function of the ability to reproduce should include identification and analyses of gene variants that underlie human infertility and research into fertility-related molecules.

New Insights of the Zn(II)-Induced P2 × 4R Positive Allosteric Modulation: Role of Head Receptor Domain SS2/SS3, E160 and D170

P2 × 4R is allosterically modulated by Zn(II), and despite the efforts to understand the mechanism, there is not a consensus proposal; C132 is a critical amino acid for the Zn(II) modulation, and this residue is located in the receptor head domain, forming disulfide SS3. To ascertain the role of the SS2/SS3 microenvironment on the rP2 × 4R Zn(II)-induced allosteric modulation, we investigated the contribution of each individual SS2/SS3 cysteine plus carboxylic acid residues E118, E160, and D170, located in the immediate vicinity of the SS2/SS3 disulfide bonds. To this aim, we combined electrophysiological recordings with protein chemical alkylation using thiol reagents such as N-ethylmaleimide or iodoacetamide, and a mutation of key amino acid residues together with P2 × 4 receptor bioinformatics. P2 × 4R alkylation in the presence of the metal obliterated the allosteric modulation, a finding supported by the site-directed mutagenesis of C132 and C149 by a corresponding alanine. In addition, while E118Q was sensitive to Zn(II) modulation, the wild type receptor, mutants E160Q and D170N, were not, suggesting that these acid residues participate in the modulatory mechanism. Poisson–Boltzmann analysis indicated that the E160Q and D170N mutants showed a shift towards more positive electrostatic potential in the SS2/SS3 microenvironment. Present results highlight the role of C132 and C149 as putative Zn(II) ligands; in addition, we infer that acid residues E160 and D170 play a role attracting Zn(II) to the head receptor domain.

Physiological Concentrations of Zinc Have Dual Effects on P2X Myenteric Receptors of Guinea Pig

We, hereby, characterize the pharmacological effects of physiological concentrations of Zinc on native myenteric P2X receptors from guinea-pig small intestine and on P2X2 isoforms present in most myenteric neurons. This is the first study describing opposite effects of Zinc on these P2X receptors. It was not possible to determine whether both effects were concentration dependent, yet the inhibitory effect was mediated by competitive antagonism and was concentration dependent. The potentiating effect appears to be mediated by allosteric changes induced by Zinc on P2X myenteric channels, which is more frequently observed in myenteric neurons with low zinc concentrations. In P2X2-1 and P2X2-2 variants, the inhibitory effect is more common than in P2X myenteric channels. However, in the variants, the potentiatory effect is of equal magnitude as the inhibitory effect. Inhibitory and potentiatory effects are likely mediated by different binding sites that appear to be present on both P2X2 variants. In conclusion, in myenteric native P2X receptors, Zinc has quantitatively different pharmacological effects compared to those observed on homomeric channels: P2X2-1 and P2X2-2. Potentiatory and inhibitory Zinc effects upon these receptors are mediated by two different binding sites. All our data suggest that myenteric P2X receptors have a more complex pharmacology than those of the recombinant P2X2 receptors, which is likely related to other subunits known to be expressed in myenteric neurons. Because these dual effects occur at Zinc physiological concentrations, we suggest that they could be involved in physiological and pathological processes.

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.

Molecular Structure and Regulation of P2X Receptors With a Special Emphasis on the Role of P2X2 in the Auditory System

The P2X purinergic receptors are cation-selective channels gated by extracellular adenosine 5'-triphosphate (ATP). These purinergic receptors are found in virtually all mammalian cell types and facilitate a number of important physiological processes. Within the past few years, the characterization of crystal structures of the zebrafish P2X4 receptor in its closed and open states has provided critical insights into the mechanisms of ligand binding and channel activation. Understanding of this gating mechanism has facilitated to design and interpret new modeling and structure-function experiments to better elucidate how different agonists and antagonists can affect the receptor with differing levels of potency. This review summarizes the current knowledge on the structure, activation, allosteric modulators, function, and location of the different P2X receptors. Moreover, an emphasis on the P2X2 receptors has been placed in respect to its role in the auditory system. In particular, the discovery of three missense mutations in P2X2 receptors could become important areas of study in the field of gene therapy to treat progressive and noise-induced hearing loss. J. Cell. Physiol. 231: 1656-1670, 2016. © 2015 Wiley Periodicals, Inc.

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'.

The role of purinergic signalling in the musculoskeletal system

Accumulating evidence now suggests that purinergic signalling exerts significant regulatory effects in the musculoskeletal system. In particular, it has emerged that extracellular nucleotides are key regulators of bone cell differentiation, survival and function. This review discusses our current understanding of the direct effects of purinergic signalling in bone, cartilage and muscle.

Ion channels and migraine

Migraine is one of the most common neurological disorders. Despite its prevalence, the basic physiology of the molecules and mechanisms that contribute to migraine headache is still poorly understood, making the discovery of more effective treatments extremely difficult. The consistent presence of head-specific pain during migraine suggests an important role for activation of the peripheral nociceptors localized to the head. Accordingly, this review will cover the current understanding of the biological mechanisms leading to episodic activation and sensitization of the trigeminovascular pain pathway, focusing on recent advances regarding activation and modulation of ion channels.

Purinergic signalling in the musculoskeletal system

It is now widely recognised that extracellular nucleotides, signalling via purinergic receptors, participate in numerous biological processes in most tissues. It has become evident that extracellular nucleotides have significant regulatory effects in the musculoskeletal system. In early development, ATP released from motor nerves along with acetylcholine acts as a cotransmitter in neuromuscular transmission; in mature animals, ATP functions as a neuromodulator. Purinergic receptors expressed by skeletal muscle and satellite cells play important pathophysiological roles in their development or repair. In many cell types, expression of purinergic receptors is often dependent on differentiation. For example, sequential expression of P2X5, P2Y1 and P2X2 receptors occurs during muscle regeneration in the mdx model of muscular dystrophy. In bone and cartilage cells, the functional effects of purinergic signalling appear to be largely negative. ATP stimulates the formation and activation of osteoclasts, the bone-destroying cells. Another role appears to be as a potent local inhibitor of mineralisation. In osteoblasts, the bone-forming cells, ATP acts via P2 receptors to limit bone mineralisation by inhibiting alkaline phosphatase expression and activity. Extracellular ATP additionally exerts significant effects on mineralisation via its hydrolysis product, pyrophosphate. Evidence now suggests that purinergic signalling is potentially important in several bone and joint disorders including osteoporosis, rheumatoid arthritis and cancers. Strategies for future musculoskeletal therapies might involve modulation of purinergic receptor function or of the ecto-nucleotidases responsible for ATP breakdown or ATP transport inhibitors.

Covalent modification of mutant rat P2X2 receptors with a thiol-reactive fluorophore allows channel activation by zinc or acidic pH without ATP

Rat P2X2 receptors open at an undetectably low rate in the absence of ATP. Furthermore, two allosteric modulators, zinc and acidic pH, cannot by themselves open these channels. We describe here the properties of a mutant receptor, K69C, before and after treatment with the thiol-reactive fluorophore Alexa Fluor 546 C(5)-maleimide (AM546). Xenopus oocytes expressing unmodified K69C were not activated under basal conditions nor by 1,000 µM ATP. AM546 treatment caused a small increase in the inward holding current which persisted on washout and control experiments demonstrated this current was due to ATP independent opening of the channels. Following AM546 treatment, zinc (100 µM) or acidic external solution (pH 6.5) elicited inward currents when applied without any exogenous ATP. In the double mutant K69C/H319K, zinc elicited much larger inward currents, while acidic pH generated outward currents. Suramin, which is an antagonist of wild type receptors, behaved as an agonist at AM546-treated K69C receptors. Several other cysteine-reactive fluorophores tested on K69C did not cause these changes. These modified receptors show promise as a tool for studying the mechanisms of P2X receptor activation.

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.

ATP-activated P2X2 current in mouse spermatozoa

Sperm cells acquire hyperactivated motility as they ascend the female reproductive tract, which enables them to overcome barriers and penetrate the cumulus and zona pellucida surrounding the egg. This enhanced motility requires Ca(2+) entry via cation channel of sperm (CatSper) Ca(2+)-selective ion channels in the sperm tail. Ca(2+) entry via CatSper is enhanced by the membrane hyperpolarization mediated by Slo3, a K(+) channel also present in the sperm tail. To date, no transmitter-mediated currents have been reported in sperm and no currents have been detected in the head or midpiece of mature spermatozoa. We screened a number of neurotransmitters and biomolecules to examine their ability to induce ion channel currents in the whole spermatozoa. Surprisingly, we find that none of the previously reported neurotransmitter receptors detected by antibodies alone are functional in mouse spermatozoa. Instead, we find that mouse spermatozoa have a cation-nonselective current in the midpiece of spermatozoa that is activated by external ATP, consistent with an ATP-mediated increase in intracellular Ca(2+) as previously reported. The ATP-dependent current is not detected in mice lacking the P2X2 receptor gene (P2rx2(-/-)). Furthermore, the slowly desensitizing and strongly outwardly rectifying ATP-gated current has the biophysical and pharmacological properties that mimic heterologously expressed mouse P2X2. We conclude that the ATP-induced current on mouse spermatozoa is mediated by the P2X2 purinergic receptor/channel. Despite the loss of ATP-gated current, P2rx2(-/-) spermatozoa have normal progressive motility, hyperactivated motility, and acrosome reactions. However, fertility of P2rx2(-/-) males declines with frequent mating over days, suggesting that P2X2 receptor adds a selection advantage under these conditions.

Discovery of potent competitive antagonists and positive modulators of the P2X2 receptor

Evaluation and optimization of anthraquinone derivatives related to Reactive Blue 2 at P2X2 receptors yielded the first potent and selective P2X2 receptor antagonists. The compounds were tested for inhibition of ATP (10 μM) mediated currents in Xenopus oocytes expressing the rat P2X2 receptor. The most potent antagonists were sodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)phenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (63, PSB-10211, IC(50) 86 nM) and disodium 1-amino-4-[3-(4,6-dichloro[1,3,5]triazine-2-ylamino)-4-sulfophenylamino]-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (57, PSB-1011, IC(50) 79 nM). Compound 57 exhibited a competitive mechanism of action (pA(2) 7.49). It was >100-fold selective versus P2X4, P2X7, and several investigated P2Y receptor subtypes (P2Y(2,4,6,12)); selectivity versus P2X1 and P2X3 receptors was moderate (>5-fold). Compound 57 was >13-fold more potent at the homomeric P2X2 than at the heteromeric P2X2/3 receptor. Several anthraquinone derivatives were found to act as positive modulators of ATP effects at P2X2 receptors, for example, sodium 1-amino-4-(3-phenoxyphenylamino)-9,10-dioxo-9,10-dihydroanthracene-2-sulfonate (51, PSB-10129, EC(50) 489 nM), which led to about a 3-fold increase in the ATP-elicited current.

Allosteric modulation of purine and pyrimidine receptors

Among the purine and pyrimidine receptors, the discovery of small molecular allosteric modulators has been most highly advanced for the A(1) and A(3) adenosine receptors (ARs). These AR modulators have allosteric effects that are structurally separated from the orthosteric effects in SAR studies. The benzoylthiophene derivatives tend to act as allosteric agonists as well as selective positive allosteric modulators (PAMs) of the A(1) AR. A 2-amino-3-aroylthiophene derivative T-62 has been under development as a PAM of the A(1) AR for the treatment of chronic pain. Several structurally distinct classes of allosteric modulators of the human A(3) AR have been reported: 3-(2-pyridinyl)isoquinolines, 2,4-disubstituted quinolines, 1H-imidazo-[4,5-c]quinolin-4-amines, endocannabinoid 2-arachidonylglycerol, and the food dye Brilliant Black BN. Site-directed mutagenesis of A(1) and A(3) ARs has identified residues associated with the allosteric effect, distinct from those that affect orthosteric binding. A few small molecular allosteric modulators have been reported for several of the P2X ligand-gated ion channels and the G protein-coupled P2Y receptor nucleotides. Metal ion modulation of the P2X receptors has been extensively explored. The allosteric approach to modulation of purine and pyrimidine receptors looks promising for development of drugs that are event and site specific in action.

Electrophysiological classification of P2X7 receptors in rat cultured neocortical astroglia

BACKGROUND AND PURPOSE

P2X7 receptors are ATP-gated cation channels mediating important functions in microglial cells, such as the release of cytokines and phagocytosis. Electrophysiological evidence that these receptors also occur in CNS astroglia is rare and rather incomplete.

EXPERIMENTAL APPROACH

We used whole-cell patch-clamp recordings to search for P2X7 receptors in astroglial-neuronal co-cultures prepared from the cerebral cortex of rats.

KEY RESULTS

All the astroglial cells investigated responded to ATP with membrane currents, reversing around 0 mV. These currents could be also detected in isolated outside-out patch vesicles. The results of the experiments with the P2X [alpha,beta-methylene ATP and 2'-3'-O-(4-benzoyl) ATP] and P2Y receptor agonists [adenosine 5'-O-(2-thiodiphosphate), uridine 5'-diphosphate, uridine 5'-triphosphate (UTP) and UDP-glucose] suggested the involvement of P2X receptors in this response. The potentiation of ATP responses in a low divalent cation or alkaline bath, but not by ivermectin, made it likely that a P2X7 receptor is operational. Blockade of the ATP effect by the P2X7 antagonists Brilliant Blue G, calmidazolium and oxidized ATP corroborated this assumption.

CONCLUSIONS AND IMPLICATIONS

Rat cultured cortical astroglia possesses functional P2X7 receptors. It is suggested that astrocytic P2X7 receptors respond to high local ATP concentrations during neuronal injury.

Pharmacological properties and physiological function of a P2X-like current in single proximal tubule cells isolated from frog kidney

Although previous studies have provided evidence for the expression of P2X receptors in renal proximal tubule, only one cell line study has provided functional evidence. The current study investigated the pharmacological properties and physiological role of native P2X-like currents in single frog proximal tubule cells using the whole-cell patch-clamp technique. Extracellular ATP activated a cation conductance (P2X_f) that was also Ca²⁺-permeable. The agonist sequence for activation was ATP = αβ-MeATP > BzATP = 2-MeSATP, and P2X_f was inhibited by suramin, PPADS and TNP-ATP. Activation of P2X_f attenuated the rundown of a quinidine-sensitive K⁺ conductance, suggesting that P2X_f plays a role in K⁺ channel regulation. In addition, ATP/ADP apyrase and inhibitors of P2X_f inhibited regulatory volume decrease (RVD). These data are consistent with the presence of a P2X receptor that plays a role in the regulation of cell volume and K⁺ channels in frog renal proximal tubule cells.

Zinc: the brain's dark horse

Zinc is a life-sustaining trace element, serving structural, catalytic, and regulatory roles in cellular biology. It is required for normal mammalian brain development and physiology, such that deficiency or excess of zinc has been shown to contribute to alterations in behavior, abnormal central nervous system development, and neurological disease. In this light, it is not surprising that zinc ions have now been shown to play a role in the neuromodulation of synaptic transmission as well as in cortical plasticity. Zinc is stored in specific synaptic vesicles by a class of glutamatergic or "gluzinergic" neurons and is released in an activity-dependent manner. Because gluzinergic neurons are found almost exclusively in the cerebral cortex and limbic structures, zinc may be critical for normal cognitive and emotional functioning. Conversely, direct evidence shows that zinc might be a relatively potent neurotoxin. Neuronal injury secondary to in vivo zinc mobilization and release occurs in several neurological disorders such as Alzheimer's disease and amyotrophic lateral sclerosis, in addition to epilepsy and ischemia. Thus, zinc homeostasis is integral to normal central nervous system functioning, and in fact its role may be underappreciated. This article provides an overview of zinc neurobiology and reviews the experimental evidence that implicates zinc signals in the pathophysiology of neuropsychiatric diseases. A greater understanding of zinc's role in the central nervous system may therefore allow for the development of therapeutic approaches where aberrant metal homeostasis is implicated in disease pathogenesis.

Measuring ion channels on solid supported membranes

Application of solid supported membranes (SSMs) for the functional investigation of ion channels is presented. SSM-based electrophysiology, which has been introduced previously for the investigation of active transport systems, is expanded for the analysis of ion channels. Membranes or liposomes containing ion channels are adsorbed to an SSM and a concentration gradient of a permeant ion is applied. Transient currents representing ion channel transport activity are recorded via capacitive coupling. We demonstrate the application of the technique to liposomes reconstituted with the peptide cation channel gramicidin, vesicles from native tissue containing the nicotinic acetylcholine receptor, and membranes from a recombinant cell line expressing the ionotropic P2X2 receptor. It is shown that stable ion gradients, both inside as well as outside directed, can be applied and currents are recorded with an excellent signal/noise ratio. For the nicotinic acetylcholine receptor and the P2X2 receptor excellent assay quality factors of Z' = 0.55 and Z' = 0.67, respectively, are obtained. This technique opens up new possibilities in cases where conventional electrophysiology fails like the functional characterization of ion channels from intracellular compartments. It also allows for robust fully automatic assays for drug screening.

Carbon monoxide is a rapid modulator of recombinant and native P2X(2) ligand-gated ion channels

BACKGROUND AND PURPOSE

Carbon monoxide (CO) is a potent modulator of a wide variety of physiological processes, including sensory signal transduction. Many afferent sensory pathways are dependent upon purinergic neurotransmission, but direct modulation of the P2X purinoceptors by this important, endogenously produced gas has never been investigated.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp experiments were used to measure ATP-elicited currents in human embryonic kidney 293 cells heterologously expressing P2X(2), P2X(3), P2X(2/3) and P2X(4) receptors and in rat pheochromocytoma (PC12) cells known to express native P2X(2) receptors. Modulation was investigated using solutions containing CO gas and the CO donor molecule, tricarbonyldichlororuthenium (II) dimer (CORM-2).

KEY RESULTS

CO was a potent and selective modulator of native P2X(2) receptors, and these effects were mimicked by a CO donor (CORM-2). Neither pre-incubation with 8-bromoguanosine-3',5'-cyclomonophosphate nor 1H-[1,2,4]Oxadiazolo[4,3-a]quinoxalin-1-one (a potent blocker of soluble guanylyl cyclase) affected the ability of the CO donor to enhance the ATP-evoked P2X(2) currents. The CO donor caused a small, but significant inhibition of currents evoked by P2X(2/3) and P2X(4) receptors, but was without effect on P2X(3) receptors.

CONCLUSIONS AND IMPLICATIONS

These data provided an explanation for how CO might regulate sensory neuronal traffic in physiological reflexes such as systemic oxygen sensing but also showed that CO could be used as a selective pharmacological tool to assess the involvement of homomeric P2X(2) receptors in physiological systems.

Identification of the amino acid residues in the extracellular domain of rat P2X(7) receptor involved in functional inhibition by acidic pH

BACKGROUND AND PURPOSE

P2X(7), receptors are potently inhibited by extracellular acidification. The underlying molecular basis remains unknown. This study aimed to examine the role of extracellular histidine, lysine, aspartic acid and glutamic acid residues in the functional inhibition of rat P2X(7) receptors by acidic pH.

EXPERIMENTAL APPROACH

We introduced point mutations into rat P2X(7) receptor by site-directed mutagenesis, expressed wild type (WT) and mutant receptors in human embryonic kidney (HEK293) cells and, using patch clamp recording, characterized the effects of acidic pH on BzATP [2'-3'O-(4-benzoylbenzoyl) adenosine 5'-triphosphate]-evoked ionic currents.

KEY RESULTS

Reducing extracellular pH, that is, increasing extracellular proton concentrations, inhibited BzATP-evoked currents in cells expressing WT P2X(7) receptors, with IC(50) value (half-maximal antagonist or inhibitor concentration) for protons of 0.2 mumol.L(-1). The major effect of acidification was suppression of the maximal current response without altering the agonist sensitivity. five residues in the receptor extracellular domain (His(85), Lys(110), Lys(137), Asp(197) and His(219)) were mutated to alanine and current inhibition by protons assessed. Compared with WT, the H85A, H219A, K137A mutants were two- to threefold more sensitive, whereas the K110A and D197A mutants were 2.5- and 9-fold less sensitive. Double-alanine substitution of Lys(110) and Asp(197) resulted in 23-fold decreased sensitivity to inhibition by protons. Furthermore, charge neutralization (K110M, K110F, D197N and D197F), but not charge conserving mutation (K110R and D197E), attenuated the inhibition of currents by protons.

CONCLUSIONS AND IMPLICATIONS

Functional inhibition of rat P2X(7) receptors by acidic pH was variably affected by the extra-cellular His(85), Lys(110), Lys(137), Asp(197) and His(219) residues, with the Asp(197) residue being most critical for this inhibition.

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.

Distribution of P2X(3) receptor immunoreactivity in myenteric ganglia of the mouse esophagus

Intraganglionic laminar endings (IGLEs) represent the major vagal afferent terminals throughout the gut. Electrophysiological experiments revealed a modulatory role of ATP in the IGLE-mechanotransduction process and the P2X(2)-receptor has been described in IGLEs of mouse, rat and guinea pig. Another purinoceptor, the P2X(3)-receptor, was found in IGLEs of the rat esophagus. These findings prompted us to investigate occurrence and distribution of the P2X(3)-receptor in the mouse esophagus. Using multichannel immunofluorescence and confocal microscopy, P2X(3)-immunoreactivity (-iry) was found colocalized with the vesicular glutamate transporter 2 (VGLUT2), a specific marker for IGLEs, on average in three-fourths of esophageal IGLEs. The distribution of P2X(3) immunoreactive (-ir) IGLEs was similar to that of P2X(2)-iry and showed increasing numbers towards the abdominal esophagus. P2X(3)/P2X(2)-colocalization within IGLEs suggested the occurrence of heteromeric P2X(2/3) receptors. In contrast to the rat, where only a few P2X(3)-ir perikarya were described, P2X(3) stained perikarya in ~80% of myenteric ganglia in the mouse. Detailed analysis revealed P2X(3)-iry in subpopulations of nitrergic (nNOS) and cholinergic (ChAT) myenteric neurons and ganglionic neuropil of the mouse esophagus. We conclude that ATP might act as a neuromodulator in IGLEs via a (P2X(2))-P2X(3) receptor-mediated pathway especially in the abdominal portion of the mouse esophagus.

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.

Orthosteric and allosteric binding sites of P2X receptors

P2X receptors for ATP comprise a distinct family of ligand gated ion channels with a range of properties. They have been shown to be involved in a variety of physiological processes including blood clotting, sensory perception, pain sensation, bone formation as well as inflammation and may provide a number of novel drug targets. In addition to the orthosteric site for ATP binding it has been suggested that there may be additional allosteric sites that regulate agonist action at the receptor. There is currently no crystal structure available for P2X receptors and the lack of sequence similarity to other ATP binding proteins has meant that a mutagenesis-based approach has been used primarily to investigate receptor structure-function. This review aims to provide an overview of recent work that gives an insight into residues involved in ATP action and allosteric regulation.

Identification of key residues coordinating functional inhibition of P2X7 receptors by zinc and copper

P2X(7) receptors are distinct from other ATP-gated P2X receptors in that they are potently inhibited by submicromolar concentrations of zinc and copper. The molecular basis for the strong functional inhibition by zinc and copper at this purinergic ionotropic receptor is controversial. We hypothesized that it involves a direct interaction of zinc and copper with residues in the ectodomain of the P2X(7) receptor. Fourteen potential metal interacting residues are conserved in the ectodomain of all mammalian P2X(7) receptors, none of which is homologous to previously identified sites in other P2X receptors shown to be important for functional potentiation by zinc. We introduced alanine substitutions into each of these residues, expressed wild-type and mutated receptors in human embryonic kidney 293 cells, and recorded resulting ATP and BzATP-evoked membrane currents. Agonist concentration-response curves were similar for all 12 functional mutant receptors. Alanine substitution at His(62) or Asp(197) strongly attenuated both zinc and copper inhibition, and the double mutant [H62A/D197A] mutant receptor was virtually insensitive to inhibition by zinc or copper. Thus, we conclude that zinc and copper inhibition is due to a direct interaction of these divalent cations with ectodomain residues of the P2X(7) receptor, primarily involving combined interaction with His(62) and Asp(197) residues.

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.

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.

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.

Extracellular histidine residues identify common structural determinants in the copper/zinc P2X2 receptor modulation

To assess the mechanism of P2X2 receptor modulation by transition metals, the cDNA for the wild-type receptor was injected to Xenopus laevis oocytes and examined 48-72 h later by the two-electrode voltage-clamp technique. Copper was the most potent of the trace metals examined; at 10 microm it evoked a 25-fold potentiation of the 10 microm ATP-gated currents. Zinc, nickel or mercury required 10-fold larger concentrations to cause comparable potentiations, while palladium, cobalt or cadmium averaged only 12- and 3-fold potentiations, respectively. Platinum was inactive. The non-additive effect of copper and zinc at 10-100 microm suggests a common site of action; these metals also shifted to the left the ATP concentration-response curves. To define residues necessary for trace metal modulation, alanines were singly substituted for each of the nine histidines in the extracellular domain of the rat P2X2 receptor. The H120A and H213A mutants were resistant to the modulator action of copper, zinc and other metals with the exception of mercury. Mutant H192A showed a reduction but not an abrogation of the copper or zinc potentiation. H245A showed less affinity for copper while this mutant flattened the zinc-induced potentiation. Mutant H319A reduced the copper but not the zinc-induced potentiation. In contrast, mutants H125A, H146A, H152A and H174A conserved the wild-type receptor sensitivity to trace metal modulation. We propose that His120, His192, His213 and His245 form part of a common allosteric metal-binding site of the P2X2 receptor, which for the specific coordination of copper, but not zinc, additionally involves His319.

Differential expression of P2X receptors on neurons from different parasympathetic ganglia

Whole-cell patch clamp recording and immunohistochemistry were used to investigate the expression of P2X receptors on rat parasympathetic ganglion neurons of the otic, sphenopalatine, submandibular, intracardiac and paratracheal ganglia. Neurons from all five ganglia responded to ATP with a rapidly activating, sustained inward current. Neurons of intracardiac and paratracheal ganglia were insensitive to alphabeta-meATP, while all neurons in the otic and some neurons of sphenopalatine and submandibular ganglia responded. Lowering pH potentiated ATP responses in neurons from all five ganglia. Co-application of Zn(2+) potentiated ATP responses in intracardiac, paratracheal and submandibular ganglion neurons. Immunohistochemistry revealed strong and specific staining for the P2X(2) subunit in all five ganglia and strong P2X(3) staining in otic, sphenopalatine and submandibular ganglia. In conclusion, there is heterogeneity in P2X receptor expression in different parasympathetic ganglia of the rat, but the predominant receptor subtypes involved appear to be homomeric P2X(2) and heteromeric P2X(2/3).

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.

High zinc sensitivity and pore formation in an invertebrate P2X receptor

To investigate fast purinergic signaling in invertebrates, we examined the functional properties of a P2X receptor subunit cloned from the parasitic platyhelminth Schistosoma mansoni. This purinoceptor (SmP2X) displays unambiguous homology of primary sequence with vertebrate P2X subunits. SmP2X subunits assemble into homomeric ATP-gated channels that exhibit slow activation kinetics and are blocked by suramin and PPADS but not TNP-ATP. SmP2X mediates the uptake of the dye YO-PRO-1 through the formation of large pores and can be blocked by submicromolar concentrations of extracellular Zn2+ ions (IC50 = 0.4 microM). The unique receptor phenotype defined by SmP2X suggests that slow kinetics, modulation by zinc and the ability to form large pores are ancestral properties of P2X receptors. The high sensitivity of SmP2X to zinc further reveals a zinc regulation requirement for the parasite's physiology that could potentially be exploited for therapeutic purposes.

Purinergic Signalling to Rat Ovarian Smooth Muscle: Changes in P2X Receptor Expression during Pregnancy

The expression of P2X and P2Y receptor subtypes in the smooth muscle of the rat ovary during the oestrus cycle and pregnancy was examined using immunohistochemistry. RT-PCR studies of P2X receptor mRNA were also carried out. In the non-pregnant rats, P2X2 receptor protein was dominant in the smooth muscle of perifollicular rings and blood vessels. P2X1 protein expression was seen on vascular smooth muscle too, but little, if any, was present on perifollicular smooth muscle. No changes in P2X1 or P2X2 receptor expression were seen during the oestrous cycle. During early and mid-late pregnancy, there was a switch from P2X2 to P2X1 receptor protein expression in the smooth muscle of the perifollicular ring; P2X1 receptors were also more prominently expressed than P2X2 receptors on ovarian vascular smooth muscle in non-pregnant animals, but during late pregnancy the expression of P2X2 receptors was found to equal that of the P2X1 receptors. There was a return to non-pregnant P2 receptor subtype distribution 2 days after birth. Ovarian vascular and perifollicular smooth muscle showed immunoreactivity for P2Y1, but not for P2X3-7, P2Y2 or P2Y4 receptors. P2Y1 receptor expression in ovarian smooth muscle of both blood vessels and follicular rings did not show significant changes during the oestrus cycle or pregnancy. RT-PCR studies indicated that P2X1 and P2X2 receptor mRNA was present in the ovary during pregnant and non-pregnant conditions. P2X4-6 receptor mRNA was also present in all stages studied, however no immunostaining showing receptor protein for these subtypes was seen on the ovarian sections examined. In summary, purinergic signalling to ovarian perifollicular smooth muscle changed from P2X2 to P2X1 receptors during pregnancy, while there was an increase in P2X2 receptor expression on vascular smooth muscle.

Effects of the abused solvent toluene on recombinant P2X receptors expressed in HEK293 cells

ATP acts as a neurotransmitter in both the peripheral and central nervous systems by activating receptors in the P2Y and P2X families. P2Y receptors are coupled to intracellular signaling pathways, while P2X receptors contain an integral membrane-spanning pore and act as ion channels. Previous studies have established that certain abused drugs such as alcohol inhibit P2X receptors. In this study, we have examined the sensitivity of both homomeric and heteromeric P2X receptors to toluene, a commercial solvent widely used as a drug of abuse. P2X receptors were transiently expressed in HEK293 cells, and agonist-gated currents were measured using whole-cell patch clamp electrophysiology. Toluene potentiated currents in cells expressing homomeric P2X2 or P2X4 subunits when ATP concentrations were near or below the EC50 concentration. This potentiation was rapid in onset, voltage independent and was readily reversed upon washout of the toluene-containing solution. The toluene-induced potentiation of P2X2 currents was not altered by lowering the pH of the recording media to 5.5 or by including the heavy-metal chelator EDTA in the recording solution. At maximal ATP concentrations, toluene did not affect ATP-gated currents in cells expressing P2X2 or P2X4 receptors. ATP-gated currents were also markedly potentiated by toluene in cells transfected with both P2X4 and P2X6 subunits. In contrast, P2X3 receptor currents were inhibited by toluene at both low and high ATP concentrations. HEK293 cells transfected with both P2X2 and P2X3 subunits showed non-desensitizing currents when stimulated with alpha, beta-methylene ATP. In these cells, toluene potentiated currents only at sub-maximal concentrations of alpha, beta-methylene ATP. The results of this study suggest that the abused solvent toluene affects the function of P2X receptors in a subunit-dependent and agonist-dependent fashion.

Molecular physiology of P2 receptors in the central nervous system

Neurons of the central nervous system (CNS) are endowed with ATP-sensitive receptors belonging to the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. Whereas a number of P2X receptors mediate fast synaptic responses to the transmitter ATP, P2Y receptors mediate either slow changes of the membrane potential in response to non-synaptically released ATP or the interaction with receptors for other transmitters. To date seven P2X and seven P2Y receptors of human origin have been molecularly identified and functionally characterized. P2X subunits may occur as homooligomers or as heterooligomeric assemblies of more than one subunit. P2X(7) subunits do not form heterooligomeric assemblies and are unique in mediating apoptosis and necrosis of glial cells and possibly also of neurons. The P2X(2), P2X(4), P2X(4)/P2X(6) and P2Y(1) receptors appear to be the predominant neuronal types. The localisation of these receptors may be at the somato-dendritic region (postsynaptic) or at the nerve terminals (presynaptic). Postsynaptic P2 receptors appear to be mostly excitatory, while presynaptic P2 receptors may be either excitatory (P2X) or inhibitory (P2Y). Since in the CNS the stimulation of a single neuron may activate multiple networks, a concomitant stimulation of facilitatory and inhibitory circuits as a result of ATP release is also possible. Finally, the enzymatic degradation of ATP may lead to the local generation of adenosine which can modulate via A(1) or A(2A) receptor-activation the ATP effect.

Quantal release of ATP from clusters of PC12 cells

Although ATP is important for intercellular communication, little is known about the mechanism of endogenous ATP release due to a dearth of suitable models. Using PC12 cells known to express the P2X2 subtype of ATP receptors and to store ATP with catecholamines inside dense-core vesicles, we found that clusters of PC12 cells cultured for 3-7 days generated small transient inward currents (STICs) after an inward current elicited by exogenous ATP. The amplitude of STICs in individual cells correlated with the peak amplitude of ATP-induced currents. STICs appeared as asynchronous responses (approximately 20 pA average amplitude) for 1-20 s and were investigated with a combination of patch clamping, Ca2+ imaging, biochemistry and electron microscopy. Comparable STICs were produced by focal KCl pulses and were dependent on extracellular Ca2+. STICs were abolished by the P2X antagonist PPADS and potentiated by Zn2+, suggesting they were mediated by P2X2 receptor activation. The highest probability of observing STICs was after the peak of intracellular Ca2+ increase caused by KCl. Biochemical measurements indicated that KCl application induced a significant release of ATP from PC12 cells. Electron microscopy studies showed narrow clefts without 'synaptic-like' densities between clustered cells. Our data suggest that STICs were caused by quantal release of endogenous ATP by depolarized PC12 cells in close juxtaposition to the recorded cell. Thus, STICs may be a new experimental model to characterize the physiology of vesicular release of ATP and to study the kinetics and pharmacology of P2X2 receptor-mediated quantal currents.

Identification of P2X receptors in cultured mouse and rat parasympathetic otic ganglion neurones including P2X knockout studies

We have used patch-clamp recording from cultured neurones, immunohistochemistry and gene deletion techniques to characterize the P2X receptors present in mouse otic ganglion neurones, and demonstrated the presence of similar receptors in rat neurones. All neurones from wild-type (WT) mice responded to ATP (EC(50) 109 microM), but only 38% also responded to alpha beta-meATP (EC(50) 39 microM). The response to alpha beta-meATP was blocked by TNP-ATP with an IC(50) of 38.6 nM. Lowering extracellular pH and co-application of Zn(2+) potentiated responses to ATP and alpha beta-meATP. In P2X(3)(-/-) mouse otic ganglion, all neurones tested responded to 100 microM ATP with a sustained current, but none responded to alpha beta-meATP. In P2X(2)(-/-) mice, no sustained currents were observed, but 36% of neurones responded to both ATP and alpha beta-meATP with transient currents. In P2X(2)/P2X(3)(Dbl-/-) mice, no responses to ATP or alpha beta-meATP were detected, suggesting that other P2X subunits were not involved. In rat otic ganglia, 96% of neurones responded to both ATP and alpha beta-meATP with sustained currents, suggesting a greater proportion of neurones expressing P2X(2/3) receptors. The maximum response to alpha beta-meATP was 40-60% of that evoked by ATP in the same cell. Immunohistochemistry revealed staining for P2X(2) and P2X(3) subunits in WT mouse otic ganglion neurones, which was absent in knockout animals. In conclusion, we have shown for the first time that at least two distinct P2X receptors are present in mouse and rat otic neurones, probably homomeric P2X(2) and heteromeric P2X(2/3) receptors.

Properties of native P2X receptors in large multipolar neurons dissociated from rat hypothalamic arcuate nucleus

ATP, the ligand of P2X receptors, is a candidate of neurotransmitter or co-transmitter in the peripheral and the central nervous systems. Anatomical studies have revealed the wide distribution of P2X receptors in the brain. So far, P2X-mediated small synaptic responses have been recorded in some brain regions. To determine the physiological significance of postsynaptic ATP receptors in the brain, we have investigated the P2X responses in rat dissociated hypothalamic arcuate neurons by using the patch-clamp technique. ATP evoked inward currents in a concentration-dependent manner (EC(50)=42 microM) at a holding potential of -70 mV. The current-voltage relationship showed a marked inward rectification starting around -10 mV. Although neither 300 microM alphabeta-methylene-ATP nor 300 microM betagamma-methylene-ATP induced any currents, 100 microM ATPgammaS and 100 microM 2-methylthio-ATP evoked inward currents of which amplitude was about 60% of the control currents evoked by 100 microM ATP. PPADS, one of P2 receptor antagonists, inhibited the ATP-evoked currents in a time- and a concentration-dependent manners (IC(50)=19 microM at 2 min). Permeant Ca(2+) inhibited the ATP-evoked currents in the range of millimolars (IC(50)=7 mM); however, Cd(2+) (1-300 microM), a broad cation channel blocker, facilitated the currents with slow off-response. Zn(2+) in the range of 1-100 microM facilitated the currents whereas Zn(2+) at the concentrations over 100 microM inhibited the currents. These observations suggest that functional P2X receptors are expressed in the hypothalamic arcuate nucleus. The most likely subunit combinations of the P2X receptors are P2X(2)-homomultimer and P2X(2)/P2X(6)-heteromultimer.

Extended pharmacological profiles of rat P2Y2 and rat P2Y4 receptors and their sensitivity to extracellular H+ and Zn2+ ions

1. Two molecularly distinct rat P2Y receptors activated equally by adenosine-5'-triphosphate (ATP) and uridine-5'-triphosphate (UTP) (rP2Y2 and rP2Y4 receptors) were expressed in Xenopus oocytes and studied extensively to find ways to pharmacologically distinguish one from the other. 2. Both P2Y subtypes were activated fully by a number of nucleotides. Tested nucleotides were equipotent at rP2Y4 (ATP=UTP=CTP=GTP=ITP), but not at rP2Y2 (ATP=UTP>CTP>GTP>ITP). For dinucleotides (ApnA, n=2-6), rP2Y4 was only fully activated by Ap4A, which was as potent as ATP. All tested dinucleotides, except for Ap2A, fully activated rP2Y2, but none were as potent as ATP. ATP gamma S and BzATP fully activated rP2Y2, whereas ATP gamma S was a weak agonist and BzATP was inactive (as an agonist) at rP2Y4 receptors. 3. Each P2Y subtype showed different sensitivities to known P2 receptor antagonists. For rP2Y2, the potency order was suramin>>PPADS= RB-2>TNP-ATP and suramin was a competitive antagonist (pA2, 5.40). For rP2Y4, the order was RB-2>>suramin>PPADS> TNP-ATP and RB-2 was a competitive antagonist (pA2, 6.43). Also, BzATP was an antagonist at rP2Y4 receptors. 4. Extracellular acidification (from pH 8.0 to pH 5.5) enhanced the potency of ATP and UTP by 8-10-fold at rP2Y4 but did not affect agonist responses at rP2Y2 receptors. 5. Extracellular Zn2+ ions (0.1-300 microM) coapplied with ATP inhibited agonist responses at rP2Y4 but not at rP2Y2 receptors. 6. These two P2Y receptors differ significantly in terms of agonist and antagonist profiles, and the modulatory activities of extracellular H+ and Zn2+ ions. These pharmacological differences will help to distinguish between rP2Y2 and rP2Y4 receptors, in vivo.

Unusual absence of endothelium-dependent or -independent vasodilatation to purines or pyrimidines in the rat renal artery

BACKGROUND

Adenosine triphosphate (ATP) is a cotransmitter with noradrenaline (NA) in sympathetic perivascular nerves. It has a dual role in the maintenance of vascular tone as ATP, released from endothelial cells during shear stress or hypoxia, induces vasodilatation via endothelial P2Y receptors or by direct action on smooth muscle. The role and distribution of P2 receptors is well characterized for many blood vessels but not for the rat renal artery. This study aims to determine whether ATP is a vasoconstrictor cotransmitter with NA and whether ATP induces vasodilatation via the endothelium or smooth muscle.

METHODS

On isolated rat renal arteries, electrical field stimulation (EFS) in the absence and presence of antagonists to P2X receptors and alpha1-adrenoceptors was examined. Concentration-response curves were constructed to NA, ATP, alpha,beta-methylene ATP (alpha,beta-meATP), uridine triphosphate (UTP), and 2-methylthio ADP (2-MeSADP) on low tone. Curves to acetylcholine (ACh), 2-MeSADP, and UTP were constructed on raised tone. Immunofluorescent localization of P2X and P2Y receptor subtypes was performed.

RESULTS

Electrical field stimulation induced vasoconstriction, partially inhibited by the P2X receptor antagonist, pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid, and predominantly by prazosin. Exogenous NA and ATP mimicked EFS; immunostaining for P2X1 and P2X2 receptors was expressed on vascular smooth muscle. Unusually, ATP, 2-MeSADP, and UTP failed to induce vasodilatation. Acetylcholine induced vasodilatation. alpha,beta-meATP, 2-MeSADP, and UTP induced vasoconstriction via P2X1, P2Y1, and P2Y2 receptors, respectively. Immunostaining for P2X1, P2Y1, and P2Y2 receptors was expressed on the vascular smooth muscle.

CONCLUSION

Adenosine triphosphate and NA are cotransmitters in sympathetic nerves supplying the rat renal artery, NA being the dominant partner. The novel feature of this vessel is that purines and pyrimidines do not produce either endothelium-dependent or -independent vasodilatation; P2X1, P2Y1, and P2Y2 receptors on the smooth muscle all mediate vasoconstriction.