Lack of run-down of smooth muscle P2X receptor currents recorded with the amphotericin permeabilized patch technique, physiological and pharmacological characterization of the properties of mesenteric artery P2X receptor ion channels

P2X3-selective mechanism of Gefapixant, a drug candidate for the treatment of refractory chronic cough

•

The mechanism by which Gefapixant/AF-219 selectively acts on the P2X3 receptor is unclear.

•

The negative allosteric site of AF-219 at P2X3 is also a potent allosteric site for other P2X subtypes.

•

The selectivity of AF-219 for P2X3 is determined by the accessibility of binding site and the internal shape of this pocket.

•

The finding will provide new perspectives for drug design against P2X3-mediated diseases such as RCC.

Gefapixant/AF-219, a selective inhibitor of the P2X3 receptor, is the first new drug other than dextromethorphan to be approved for the treatment of refractory chronic cough (RCC) in nearly 60 years. To date, seven P2X subtypes (P2X1-7) activated by extracellular ATP have been cloned, and subtype selectivity of P2X inhibitors is a prerequisite for reducing side effects. We previously identified the site and mechanism of action of Gefapixant/AF-219 on the P2X3 receptor, which occupies a pocket consisting of the left flipper (LF) and lower body (LB) domains. However, the mechanism by which AF-219 selectively acts on the P2X3 receptor is unknown. Here, we combined mutagenesis, chimera construction, molecular simulations, covalent occupation and chemical synthesis, and find that the negative allosteric site of AF-219 at P2X3 is also present in other P2X subtypes, at least for P2X1, P2X2 and P2X4. By constructing each chimera of AF-219 sensitive P2X3 and insensitive P2X2 subtypes, the insensitive P2X2 subtype was made to acquire the inhibitory properties of AF-219 and AF-353, an analog of AF-219 with higher affinity. Our results suggest that the selectivity of AF-219/AF-353 for P2X3 over the other P2X subtypes is determined by a combination of the accessibility of P2X3 binding site and the internal shape of this pocket, a finding that could provide new perspectives for drug design against P2X3-mediated diseases such as RCC, idiopathic pulmonary fibrosis, hypertension and overactive bladder disorder.

Aging Affects KV7 Channels and Perivascular Adipose Tissue-Mediated Vascular Tone

Aging is an independent risk factor for hypertension, cardiovascular morbidity, and mortality. However, detailed mechanisms linking aging to cardiovascular disease are unclear. We studied the aging effects on the role of perivascular adipose tissue and downstream vasoconstriction targets, voltage-dependent K_V7 channels, and their pharmacological modulators (flupirtine, retigabine, QO58, and QO58-lysine) in a murine model. We assessed vascular function of young and old mesenteric arteries in vitro using wire myography and membrane potential measurements with sharp electrodes. We also performed bulk RNA sequencing and quantitative reverse transcription-polymerase chain reaction tests in mesenteric arteries and perivascular adipose tissue to elucidate molecular underpinnings of age-related phenotypes. Results revealed impaired perivascular adipose tissue-mediated control of vascular tone particularly via K_V7.3–5 channels with increased age through metabolic and inflammatory processes and release of perivascular adipose tissue-derived relaxation factors. Moreover, QO58 was identified as novel pharmacological vasodilator to activate XE991-sensitive KCNQ channels in old mesenteric arteries. Our data suggest that targeting inflammation and metabolism in perivascular adipose tissue could represent novel approaches to restore vascular function during aging. Furthermore, K_V7.3–5 channels represent a promising target in cardiovascular aging.

Development of a P2X1-eYFP receptor knock-in mouse to track receptors in real time

A P2X1-eYFP knock-in mouse was generated to study receptor expression and mobility in smooth muscle and blood cells. eYFP was added to the C-terminus of the P2X1R and replaced the native P2X1R. Fluorescence corresponding to P2X1-eYFPR was detected in urinary bladder smooth muscle, platelets and megakaryocytes. ATP-evoked currents from wild type and P2X1-eYFP isolated urinary bladder smooth muscle cells had the same peak current amplitude and time-course showing that the eYFP addition had no obvious effect on properties. Fluorescence recovery after photobleaching (FRAP) in bladder smooth muscle cells demonstrated that surface P2X1Rs are mobile and their movement is reduced following cholesterol depletion. Compared to the platelet and megakaryocyte, P2X1-eYFP fluorescence was negligible in red blood cells and the majority of smaller marrow cells. The spatial pattern of P2X1-eYFP fluorescence in the megakaryocyte along with FRAP assessment of mobility suggested that P2X1Rs are expressed extensively throughout the membrane invagination system of this cell type. The current study highlights that the spatiotemporal properties of P2X1R expression can be monitored in real time in smooth muscle cells and megakaryocytes/platelets using the eYFP knock-in mouse model.

Organization of ATP-gated P2X1 receptor intracellular termini in apo and desensitized states

The human P2X1 receptor (hP2X1R) is a trimeric ligand-gated ion channel opened by extracellular ATP. The intracellular amino and carboxyl termini play significant roles in determining the time-course and regulation of channel gating-for example, the C terminus regulates recovery from the desensitized state following agonist washout. This suggests that the intracellular regions of the channel have distinct structural features. Studies on the hP2X3R have shown that the intracellular regions associate to form a cytoplasmic cap in the open state of the channel. However, intracellular features could not be resolved in the agonist-free apo and ATP-bound desensitized structures. Here we investigate the organization of the intracellular regions of hP2X1R in the apo and ATP-bound desensitized states following expression in HEK293 cells. We couple cysteine scanning mutagenesis of residues R25-G30 and H355-R360 with the use of bi-functional cysteine reactive cross-linking compounds of different lengths (MTS-2-MTS, BMB, and BM(PEG)2), which we use as molecular calipers. If two cysteine residues come into close proximity, we predict they will be cross-linked and result in ∼66% of the receptor subunits running on a Western blot as dimers. In the control construct (C349A) that removed the free cysteine C349, and some cysteine-containing mutants, cross-linker treatment does not result in dimerization. However, we detect efficient dimerization for R25C, G30C, P358C, K359C, and R360C. This selective pattern indicates that there is structural organization to these regions in the apo and desensitized states in a native membrane environment. The existence of such precap (apo) and postcap (desensitized) organization of the intracellular domains would facilitate efficient gating of the channel.

Gene expression profiles of ion channels and receptors in mouse resistance arteries: Effects of cell type, vascular bed, and age

OBJECTIVE

Receptors and ion channels of smooth muscle cells (SMCs) and endothelial cells (ECs) are integral to the regulation of vessel diameter and tissue blood flow. Physiological roles of ion channels and receptors in skeletal muscle and mesenteric arteries have been identified; however, their gene expression profiles are undefined. We tested the hypothesis that expression profiles for ion channels and receptors governing vascular reactivity vary with cell type, vascular bed, and age.

METHODS

Mesenteric and superior epigastric arteries were dissected from Old (24-26 months) and Young (3-6 months) C57BL/6J mice. ECs and SMCs were collected for analysis with custom qRT-PCR arrays to determine expression profiles of 80 ion channel and receptor genes. Bioinformatics analyses were applied to gain insight into functional interactions.

RESULTS

We identified 68 differences in gene expression with respect to cell type, vessel type, and age. Heat maps illustrate differential expression, and distance matrices predict patterns of coexpression. Gene networks based upon protein-protein interaction datasets and KEGG pathways illustrate biological processes affected by specific differences in gene expression.

CONCLUSIONS

Differences in gene expression profiles are most pronounced between microvascular ECs and SMCs with subtle variations between vascular beds and age groups.

Inhibition of arterial medial calcification and bone mineralization by extracellular nucleotides: The same functional effect mediated by different cellular mechanisms

Arterial medial calcification (AMC) is thought to share some outward similarities to skeletal mineralization and has been associated with the transdifferentiation of vascular smooth muscle cells (VSMCs) to an osteoblast-like phenotype. ATP and UTP have previously been shown to inhibit bone mineralization. This investigation compared the effects of extracellular nucleotides on calcification in VSMCs with those seen in osteoblasts. ATP, UTP and the ubiquitous mineralization inhibitor, pyrophosphate (PP_i ), dose dependently inhibited VSMC calcification by ≤85%. Culture of VSMCs in calcifying conditions was associated with an increase in apoptosis; treatment with ATP, UTP, and PP_i reduced apoptosis to levels seen in non-calcifying cells. Extracellular nucleotides had no effect on osteoblast viability. Basal alkaline phosphatase (TNAP) activity was over 100-fold higher in osteoblasts than VSMCs. ATP and UTP reduced osteoblast TNAP activity (≤50%) but stimulated VSMC TNAP activity (≤88%). The effects of extracellular nucleotides on VSMC calcification, cell viability and TNAP activity were unchanged by deletion or inhibition of the P2Y₂ receptor. Conversely, the actions of ATP/UTP on bone mineralization and TNAP activity were attenuated in osteoblasts lacking the P2Y₂ receptor. Ecto-nucleotide pyrophosphatase/phosphodiesterase 1 (NPP1) hydrolyses ATP and UTP to produce PP_i . In both VSMCs and osteoblasts, deletion of NPP1 blunted the inhibitory effects of extracellular nucleotides suggesting involvement of P2 receptor independent pathways. Our results show that although the overall functional effect of extracellular nucleotides on AMC and bone mineralization is similar there are clear differences in the cellular mechanisms mediating these actions.

Structural insights into the nucleotide base specificity of P2X receptors

P2X receptors are trimeric ATP-gated cation channels involved in diverse physiological processes, ranging from muscle contraction to nociception. Despite the recent structure determination of the ATP-bound P2X receptors, the molecular mechanism of the nucleotide base specificity has remained elusive. Here, we present the crystal structure of zebrafish P2X4 in complex with a weak affinity agonist, CTP, together with structure-based electrophysiological and spectroscopic analyses. The CTP-bound structure revealed a hydrogen bond, between the cytosine base and the side chain of the basic residue in the agonist binding site, which mediates the weak but significant affinity for CTP. The cytosine base is further recognized by two main chain atoms, as in the ATP-bound structure, but their bond lengths seem to be extended in the CTP-bound structure, also possibly contributing to the weaker affinity for CTP over ATP. This work provides the structural insights for the nucleotide base specificity of P2X receptors.

Intersubunit physical couplings fostered by the left flipper domain facilitate channel opening of P2X4 receptors

P2X receptors are ATP-gated trimeric channels with important roles in diverse pathophysiological functions. A detailed understanding of the mechanism underlying the gating process of these receptors is thus fundamentally important and may open new therapeutic avenues. The left flipper (LF) domain of the P2X receptors is a flexible loop structure, and its coordinated motions together with the dorsal fin (DF) domain are crucial for the channel gating of the P2X receptors. However, the mechanism underlying the crucial role of the LF domain in the channel gating remains obscure. Here, we propose that the ATP-induced allosteric changes of the LF domain enable it to foster intersubunit physical couplings among the DF and two lower body domains, which are pivotal for the channel gating of P2X4 receptors. Metadynamics analysis indicated that these newly established intersubunit couplings correlate well with the ATP-bound open state of the receptors. Moreover, weakening or strengthening these physical interactions with engineered intersubunit metal bridges remarkably decreased or increased the open probability of the receptors, respectively. Further disulfide cross-linking and covalent modification confirmed that the intersubunit physical couplings among the DF and two lower body domains fostered by the LF domain at the open state act as an integrated structural element that is stringently required for the channel gating of P2X4 receptors. Our observations provide new mechanistic insights into P2X receptor activation and will stimulate development of new allosteric modulators of P2X receptors.

Impaired P2X1 Receptor-Mediated Adhesion in Eosinophils from Asthmatic Patients

Eosinophils play an important role in the pathogenesis of asthma and can be activated by extracellular nucleotides released following cell damage or inflammation. For example, increased ATP concentrations were reported in bronchoalveolar lavage fluids of asthmatic patients. Although eosinophils are known to express several subtypes of P2 receptors for extracellular nucleotides, their function and contribution to asthma remain unclear. In this article, we show that transcripts for P2X1, P2X4, and P2X5 receptors were expressed in healthy and asthmatic eosinophils. The P2X receptor agonist α,β-methylene ATP (α,β-meATP; 10 μM) evoked rapidly activating and desensitizing inward currents (peak 18 ± 3 pA/pF at -60 mV) in healthy eosinophils, typical of P2X1 homomeric receptors, which were abolished by the selective P2X1 antagonist NF449 (1 μM) (3 ± 2 pA/pF). α,β-meATP-evoked currents were smaller in eosinophils from asthmatic patients (8 ± 2 versus 27 ± 5 pA/pF for healthy) but were enhanced following treatment with a high concentration of the nucleotidase apyrase (17 ± 5 pA/pF for 10 IU/ml and 11 ± 3 pA/pF for 0.32 IU/ml), indicating that the channels are partially desensitized by extracellular nucleotides. α,β-meATP (10 μM) increased the expression of CD11b activated form in eosinophils from healthy, but not asthmatic, donors (143 ± 21% and 108 ± 11% of control response, respectively). Furthermore, α,β-meATP increased healthy (18 ± 2% compared with control 10 ± 1%) but not asthmatic (13 ± 1% versus 10 ± 0% for control) eosinophil adhesion. Healthy human eosinophils express functional P2X1 receptors whose activation leads to eosinophil αMβ2 integrin-dependent adhesion. P2X1 responses are constitutively reduced in asthmatic compared with healthy eosinophils, probably as the result of an increase in extracellular nucleotide concentration.

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.

Calcium Signalling through Ligand-Gated Ion Channels such as P2X1 Receptors in the Platelet and other Non-Excitable Cells

Ligand-gated ion channels on the cell surface are directly activated by the binding of an agonist to their extracellular domain and often referred to as ionotropic receptors. P2X receptors are ligand-gated non-selective cation channels with significant permeability to Ca(2+) whose principal physiological agonist is ATP. This chapter focuses on the mechanisms by which P2X1 receptors, a ubiquitously expressed member of the family of ATP-gated channels, can contribute to cellular responses in non-excitable cells. Much of the detailed information on the contribution of P2X1 to Ca(2+) signalling and downstream functional events has been derived from the platelet. The underlying primary P2X1-generated signalling event in non-excitable cells is principally due to Ca(2+) influx, although Na(+) entry will also occur along with membrane depolarization. P2X1 receptor stimulation can lead to additional Ca(2+) mobilization via a range of routes such as amplification of G-protein-coupled receptor-dependent Ca(2+) responses. This chapter also considers the mechanism by which cells generate extracellular ATP for autocrine or paracrine activation of P2X1 receptors. For example cytosolic ATP efflux can result from opening of pannexin anion-permeable channels or following damage to the cell membrane. Alternatively, ATP stored in specialised secretory vesicles can undergo quantal release via the process of exocytosis. Examples of physiological or pathophysiological roles of P2X1-dependent signalling in non-excitable cells are also discussed, such as thrombosis and immune responses.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

Hydrogen-sulfide-mediated vasodilatory effect of nucleoside 5'-monophosphorothioates in perivascular adipose tissue

Hydrogen sulfide (H2S) is synthesized in perivascular adipose tissue (PVAT) and induces vasorelaxation. We examined whether the sulfur-containing AMP and GMP analogs AMPS and GMPS can serve as the H2S donors in PVAT. H2S production by isolated rat periaortic adipose tissue (PAT) was measured with a polarographic sensor. In addition, phenylephrine-induced contractility of aortic rings with (+) or without (-) PAT was examined. Isolated PAT produced H2S from AMPS or GMPS in the presence of the P2X7 receptor agonist BzATP. Phenylephrine-induced contractility of PAT(+) rings was lower than of PAT(-) rings. AMPS or GMPS had no effect on the contractility of PAT(-) rings, but used together with BzATP reduced the contractility of PAT(+) rings when endogenous H2S production was inhibited with propargylglycine. A high-fat diet reduced endogenous H2S production by PAT. Interestingly, AMPS and GMPS were converted to H2S by PAT of obese rats, and reduced contractility of PAT(+) aortic rings isolated from these animals even in the absence of BzATP. We conclude that (i) AMPS and GMPS can be hydrolyzed to H2S by PAT when P2X7 receptors are activated, (ii) a high-fat diet impairs endogenous H2S production by PAT, (iii) AMPS and GMPS restore the anticontractile effects of PAT in obese animals without P2X7 stimulation.

Relative motions between left flipper and dorsal fin domains favour P2X4 receptor activation

Channel gating in response to extracellular ATP is a fundamental process for the physiological functions of P2X receptors. Here we identify coordinated allosteric changes in the left flipper (LF) and dorsal fin (DF) domains that couple ATP-binding to channel gating. Engineered disulphide crosslinking or zinc bridges between the LF and DF domains that constrain their relative motions significantly influence channel gating of P2X4 receptors, confirming the essential role of these allosteric changes. ATP-binding-induced alterations in interdomain hydrophobic interactions among I208, L217, V291 and the aliphatic chain of K193 correlate well with these coordinated relative movements. Mutations on those four residues lead to impaired or fully abolished channel activations of P2X4 receptors. Our data reveal that ATP-binding-induced altered interdomain hydrophobic interactions and the concomitant coordinated motions of LF and DF domains are allosteric events essential for the channel gating of P2X4 receptors.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Desensitization properties of P2X3 receptors shaping pain signaling

ATP-gated P2X3 receptors are mostly expressed by nociceptive sensory neurons and participate in transduction of pain signals. P2X3 receptors show a combination of fast desensitization onset and slow recovery. Moreover, even low nanomolar agonist concentrations unable to evoke a response, can induce desensitization via a phenomenon called “high affinity desensitization.” We have also observed that recovery from desensitization is agonist-specific and can range from seconds to minutes. The recovery process displays unusually high temperature dependence. Likewise, recycling of P2X3 receptors in peri-membrane regions shows unexpectedly large temperature sensitivity. By applying kinetic modeling, we have previously shown that desensitization characteristics of P2X3 receptor are best explained with a cyclic model of receptor operation involving three agonist molecules binding a single receptor and that desensitization is primarily developing from the open receptor state. Mutagenesis experiments suggested that desensitization depends on a certain conformation of the ATP binding pocket and on the structure of the transmembrane domains forming the ion pore. Further molecular determinants of desensitization have been identified by mutating the intracellular N- and C-termini of P2X3 receptor. Unlike other P2X receptors, the P2X3 subtype is facilitated by extracellular calcium that acts via specific sites in the ectodomain neighboring the ATP binding pocket. Thus, substitution of serine275 in this region (called “left flipper”) converts the natural facilitation induced by extracellular calcium to receptor inhibition. Given their strategic location in nociceptive neurons and unique desensitization properties, P2X3 receptors represent an attractive target for development of new analgesic drugs via promotion of desensitization aimed at suppressing chronic pain.

Mass spectrometry analysis of human P2X1 receptors; insight into phosphorylation, modelling and conformational changes

Recombinant FlagHis₆ tagged Human P2X1 receptors expressed in HEK293 cells were purified, digested with trypsin and analysed by mass spectroscopy (96% coverage following de-glycosylation and reduction). The receptor was basally phosphorylated at residues S387, S388 and T389 in the carboxyl terminus, a triple alanine mutant of these residues had a modest ∼ 25% increase in current amplitude and recovery from desensitization. Chemical modification showed that intracellular lysine residues close to the transmembrane domains and the membrane stabilization motif are accessible to the aqueous environment. The membrane-impermeant cross-linking reagent 3,3′-Dithiobis (sulfosuccinimidylpropionate) (DTSSP) reduced agonist binding and P2X1 receptor currents by > 90%, and modified lysine residues were identified by mass spectroscopy. Mutation to remove reactive lysine residues around the ATP-binding pocket had no effect on inhibtion of agonist evoked currents following DTSSP. However, agonist evoked currents were ∼ 10-fold higher than for wild type following DTSSP treatment for mutants K199R, K221R and K199R-K221R. These mutations remove reactive residues distant from the agonist binding pocket that are close enough to cross-link adjacent subunits. These results suggest that conformational change in the P2X1 receptor is required for co-ordination of ATP action.

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

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

Purinergic receptors expressed in human skeletal muscle fibres

Purinergic receptors are present in most tissues and thought to be involved in various signalling pathways, including neural signalling, cell metabolism and local regulation of the microcirculation in skeletal muscles. The present study aims to determine the distribution and intracellular content of purinergic receptors in skeletal muscle fibres in patients with type 2 diabetes and age-matched controls. Muscle biopsies from vastus lateralis were obtained from six type 2 diabetic patients and seven age-matched controls. Purinergic receptors were analysed using light and confocal microscopy in immunolabelled transverse sections of muscle biopsies. The receptors P2Y(4), P2Y(11) and likely P2X(1) were present intracellularly or in the plasma membrane of muscle fibres and were thus selected for further detailed morphological analysis. P2X(1) receptors were expressed in intracellular vesicles and sarcolemma. P2Y(4) receptors were present in sarcolemma. P2Y(11) receptors were abundantly and diffusely expressed intracellularly and were more explicitly expressed in type I than in type II fibres, whereas P2X(1) and P2Y(4) showed no fibre-type specificity. Both diabetic patients and healthy controls showed similar distribution of receptors. The current study demonstrates that purinergic receptors are located intracellularly in human skeletal muscle fibres. The similar cellular localization of receptors in healthy and diabetic subjects suggests that diabetes is not associated with an altered distribution of purinergic receptors in skeletal muscle fibres. We speculate that the intracellular localization of purinergic receptors may reflect a role in regulation of muscle metabolism; further studies are nevertheless needed to determine the function of the purinergic system in skeletal muscle cells.

The intracellular amino terminus plays a dominant role in desensitization of ATP-gated P2X receptor ion channels

P2X receptors show marked variations in the time-course of response to ATP application from rapidly desensitizing P2X1 receptors to relatively sustained P2X2 receptors. In this study we have used chimeras between human P2X1 and P2X2 receptors in combination with mutagenesis to address the contribution of the extracellular ligand binding loop, the transmembrane channel, and the intracellular regions to receptor time-course. Swapping either the extracellular loop or both transmembrane domains (TM1 and -2) between the P2X1 and P2X2 receptors had no effect on the time-course of ATP currents in the recipient receptor. These results suggest that the agonist binding and channel-forming portions of the receptor do not play a major role in the control of the time-course. In contrast replacing the amino terminus of the P2X1 receptor with that from the non-desensitizing P2X2 receptor (P2X1-2N) slowed desensitization, and the mirror chimera induced rapid desensitization in the P2X2-1N chimera. These reciprocal effects on time-course can be replicated by changing four variant amino acids just before the first transmembrane (TM1) segment. These pre-TM1 residues also had a dominant effect on chimeras where both TMs had been transferred; mutating the variant amino acids 21-23 to those found in the P2X2 receptor removed desensitization from the P2X1-2TM1/-2 chimera, and the reciprocal mutants induced rapid desensitization in the non-desensitizing P2X2-1TM1/-2 chimera. These results suggest that the intracellular amino terminus, in particular the region just before TM1, plays a dominant role in the regulation of the time-course of ATP evoked P2X receptor currents.

Alterations in vasoconstrictor responses to the endothelium-derived contracting factor uridine adenosine tetraphosphate are region specific in DOCA-salt hypertensive rats

Uridine adenosine tetraphosphate (Up(4)A) has been recently identified as a novel and potent endothelium-derived contracting factor and contains both purine and pyrimidine moieties, which activate purinergic P2X and P2Y receptors. The present study was designed to compare contractile responses to Up(4)A and other nucleotides such as ATP (P2X/P2Y agonist), UTP (P2Y(2)/P2Y(4) agonist), UDP (P2Y(6) agonist), and α,β-methylene ATP (P2X(1) agonist) in different vascular regions [thoracic aorta, basilar, small mesenteric, and femoral arteries] from deoxycorticosterone acetate-salt (DOCA-salt) and control rats. In DOCA-salt rats [vs. control uninephrectomized (Uni) rats]: (1) in thoracic aorta, Up(4)A-, ATP-, and UTP-induced contractions were unchanged; (2) in basilar artery, Up(4)A-, ATP-, UTP- and UDP-induced contractions were increased, and expression for P2X(1), but not P2Y(2) or P2Y(6) was decreased; (3) in small mesenteric artery, Up(4)A-induced contraction was decreased and UDP-induced contraction was increased; expression of P2Y(2) and P2X(1) was decreased whereas P2Y(6) expression was increased; (4) in femoral artery, Up(4)A-, UTP-, and UDP-induced contractions were increased, but expression of P2Y(2), P2Y(6) and P2X(1) was unchanged. The α,β-methylene ATP-induced contraction was bell-shaped and the maximal contraction was reached at a lower concentration in basilar and mesenteric arteries from Uni rats, compared to arteries from DOCA-salt rats. These results suggest that Up(4)A-induced contraction is heterogenously affected among various vascular beds in arterial hypertension. P2Y receptor activation may contribute to enhancement of Up(4)A-induced contraction in basilar and femoral arteries. These changes in vascular reactivity to Up(4)A may be adaptive to the vascular alterations produced by hypertension.

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.

Identification of human P2X1 receptor-interacting proteins reveals a role of the cytoskeleton in receptor regulation

P2X1 receptors are ATP-gated ion channels expressed by smooth muscle and blood cells. Carboxyl-terminally His-FLAG-tagged human P2X1 receptors were stably expressed in HEK293 cells and co-purified with cytoskeletal proteins including actin. Disruption of the actin cytoskeleton with cytochalasin D inhibited P2X1 receptor currents with no effect on the time course of the response or surface expression of the receptor. Stabilization of the cytoskeleton with jasplakinolide had no effect on P2X1 receptor currents but decreased receptor mobility. P2X2 receptor currents were unaffected by cytochalasin, and P2X1/2 receptor chimeras were used to identify the molecular basis of actin sensitivity. These studies showed that the intracellular amino terminus accounts for the inhibitory effects of cytoskeletal disruption similar to that shown for lipid raft/cholesterol sensitivity. Stabilization of the cytoskeleton with jasplakinolide abolished the inhibitory effects of cholesterol depletion on P2X1 receptor currents, suggesting that lipid rafts may regulate the receptor through stabilization of the cytoskeleton. These studies show that the cytoskeleton plays an important role in P2X1 receptor regulation.

The P2X1 receptor and platelet function

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

Dual signaling pathways of arterial constriction by extracellular uridine 5'-triphosphate in the rat

We investigated actions of uridine 5'-triphosphate (UTP) in rat aorta, cerebral and mesenteric arteries, and their single myocytes. UTP (≥10 µM) elicited an inward-rectifying current strongly reminiscent of activation of P2X(1) receptor, and a similar current was also induced by α,β-methylene adenosine 5'-triphosphate (ATP) (≥100 nM). UTP desensitized α,β-methylene ATP-evoked current, and vice versa. The UTP-activated current was insensitive to G-protein modulators, TRPC3 inhibitors, or TRPC3 antibody, but was sensitive to P2-receptor inhibitors or P2X(1)-receptor antibody. Both UTP (1 mM) and α,β-methylene ATP (10 µM) elicited similar conductance single channel activities. UTP (≥10 µM) provoked a dose-dependent contraction of de-endothelialized aortic ring preparation consisting of phasic and tonic components. Removal of extracellular Ca(2+) or bath-applied 2',3'-O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP) (30 µM) or nifedipine (10 µM) completely inhibited the phasic contraction while only partially reducing the tonic one. The tonic contraction was almost completely abolished by additional application of thapsigargin (2 µM). Similar biphasic rises in [Ca(2+)](i) were also evoked by UTP in rat aortic myocytes. In contrast to the low expression of TRPC3, significant expression of P2X(1) receptor was detected in all arteries by RT-PCR and immunoblotting, and its localization was limited to plasma membrane of myocytes as indicated by immunohistochemistry. These results suggest that UTP dually activates P2X(1)-like and P2Y receptors, but not TRPC3.

Purinoreceptor-mediated current in myocytes from renal resistance arteries

BACKGROUND AND PURPOSE

Ionotropic purinoreceptors (P2X) in renal vascular smooth muscle cells (RVSMCs) are involved in mediating the sympathetic control and paracrine regulation of renal blood flow (RBF). Activation of P2X receptors elevates [Ca(2+)](i) in RVSMCs triggering their contraction, leading to renal vasoconstriction and decrease of RBF. The goal of the present work was to characterize the P2X receptor-mediated ionic current (I(P2X)) and to identify the types of P2X receptors expressed in myocytes isolated from interlobar and arcuate arteries of rat kidney.

EXPERIMENTAL APPROACH

The expression of P2X receptors in isolated RVSMCs was analysed by reverse transcription (RT)-PCR. I(P2X) and membrane potential were recorded using the amphotericin B-perforated patch method.

KEY RESULTS

RT-PCR analysis on single RVSMCs showed the presence of genes encoding P2X1 and P2X4 receptors. Under voltage clamp conditions, the selective P2X receptor agonist alphabeta-methylene ATP (alphabeta-meATP) evoked I(P2X) similar to that induced by ATP. Under current clamp conditions, both ATP and alphabeta-meATP evoked a spike-like membrane depolarization followed by a sustained depolarization, linking P2X receptors in RVSMCs to sympathetic control of renal vascular tone. A selective antagonist of P2X1 receptors, NF279, reduced I(P2X) amplitude by approximately 65% concentration-dependently manner within the nanomolar to sub-micromolar range. The residual current was resistant to micromolar concentrations of NF279, but was inhibited by sub-millimolar to millimolar concentrations of NF279.

CONCLUSIONS AND IMPLICATIONS

Two types of functional P2X receptors, monomeric P2X1 and heteromeric P2X1/4 receptors, are expressed in RVSMCs. Our study has identified important targets for possible pharmacological intervention in the sympathetic control of renal circulation.

P2X1 receptor mobility and trafficking; regulation by receptor insertion and activation

P2X1 receptors for ATP contribute to signalling in a variety of cell types and following stimulation undergo rapid desensitisation (within 1 s), and require ∼5 min to recover. In HEK293 cells P2X1 receptors C-terminally tagged with enhanced green fluorescent protein (P2X1-eGFP) were predominantly expressed at the cell surface. Following > 90% photo-bleaching of P2X1-eGFP within a 6 μm² circle at the cell surface fluorescence recovery after photo-bleaching (FRAP) was fit with a time constant of ∼60 s and recovered to ∼75% of pre-bleach levels. Following activation of the P2X1 receptor with α,β-methylene ATP the associated calcium influx doubled the FRAP recovery rate. The protein synthesis inhibitor cycloheximide had only a small effect on repeated FRAP and indicated a limited contribution of new P2X1 receptors to the FRAP. Inhibition of trafficking with brefeldin A reduced recovery and this effect could be reversed following receptor activation. In contrast, the dynamin inhibitor dynasore had no effect on FRAP under unstimulated conditions but reduced the level of recovery following agonist stimulation. In functional studies both brefeldin A and dynasore increased the recovery time from desensitisation. Taken together these studies demonstrate for the first time an important role of receptor recycling on P2X1 receptor responsiveness.

Dinucleoside polyphosphates: strong endogenous agonists of the purinergic system

The purinergic system is composed of mononucleosides, mononucleoside polyphosphates and dinucleoside polyphosphates as agonists, as well as the respective purinergic receptors. Interest in the role of the purinergic system in cardiovascular physiology and pathophysiology is on the rise. This review focuses on the overall impact of dinucleoside polyphosphates in the purinergic system. Platelets, adrenal glands, endothelial cells, cardiomyocytes and tubular cells release dinucleoside polyphosphates. Plasma concentrations of dinucleoside polyphosphates are sufficient to cause direct vasoregulatory effects and to induce proliferative effects on vascular smooth muscle cells and mesangial cells. In addition, increased plasma concentrations of a dinucleoside polyphosphate were recently demonstrated in juvenile hypertensive patients. In conclusion, the current literature accentuates the strong physiological and pathophysiological impact of dinucleoside polyphosphates on the cardiovascular system.

Functional evidence for the expression of P2X1, P2X4 and P2X7 receptors in human lung mast cells

BACKGROUND AND PURPOSE

P2X receptors are widely expressed in cells of the immune system with varying functions. This study sought to characterize P2X receptor expression in the LAD2 human mast cell line and human lung mast cells (HLMCs).

EXPERIMENTAL APPROACH

Reverse transcriptase polymerase chain reaction (RT-PCR) and patch clamp studies were used to characterize P2X expression in mast cells using a range of pharmacological tools.

KEY RESULTS

RT-PCR revealed P2X1, P2X4 and P2X7 transcripts in both cell types; mRNA for P2X6 was also detected in LAD2 cells. Under whole-cell patch clamp conditions, rapid application of ATP (1-1000 microM) to cells clamped at -60 mV consistently evoked inward currents in both types of cells. Brief application of ATP (1 s) evoked a rapidly desensitizing P2X1-like current in both cell types. This current was also elicited by alphabetamethylene ATP (10 microM, 94% cells, n= 31) and was antagonized in LAD2 cells by NF 449 (1 microM) and pyridoxal phosphate-6-azo(benzene-2,4-disulphonic acid) (1-10 microM). A P2X7-like non-desensitizing current in response to high concentrations of ATP (1-5 mM) was also seen in both cell types (96% LAD2, n= 24; 54% HLMCs, n= 24) which was antagonized by AZ11645373 (1 microM). P2X7-like responses were also evoked in LAD2 cells by 2'(3')-0-(4-benzoylbenzoyl)ATP (300 microM). A P2X4-like current was evoked by 100 microM ATP (80% LAD2, n= 10; 21% HLMCs, n= 29), the amplitude and duration of which was potentiated by ivermectin (3 microM).

CONCLUSION AND IMPLICATIONS

Our data confirmed the presence of functional P2X1, P2X4 and P2X7 receptors in LAD2 cells and HLMCs.

Membrane compartments and purinergic signalling: occurrence and function of P2X receptors in lung

P2X receptors are cation-selective ion channels activated by extracellular ATP. They form homo- and heterotrimeric complexes that differ in their functional properties and subcellular localization. These membrane ion channels are also expressed in pulmonary epithelial cells. Recent work indicates that alveolar epithelial type I cells selectively express P2X(4) and P2X(7) receptor subtypes in addition to a large number of other ion channels present in the alveolar epithelium. Up- or downregulation of their expression is associated with several disease states. This minireview analyses the role of P2X receptors and of extracellular ATP and adenosine in lung disease, the relationship of P2X receptors to other ion channels in the alveolar epithelium and their distribution in lipid rafts/caveolae.

Molecular properties of P2X receptors

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

A C-terminal lysine that controls human P2X4 receptor desensitization

Receptor desensitization can determine the time course of transmitter action and profoundly alter sensitivity to drugs. Among P2X receptors, ion currents through homomeric P2X4 receptors exhibit intermediate desensitization when compared with P2X1 and P2X3 (much faster) and P2X2 and P2X7 (slower). We recorded membrane currents in HEK293 cells transfected to express the human P2X4 receptor. The decline in current during a 4-s application of ATP (100 microm) was about 30%; this was not different during whole-cell or perforated patch recording. Alanine-scanning mutagenesis of the intracellular C terminus identified two positions with much accelerated desensitization kinetics (Lys373: 92% and Tyr374: 74%). At position 373, substitution of Arg or Cys also strongly accelerated desensitization: however, in the case of K373C the wild-type phenotype was fully restored by adding ethylammonium methanethiosulfonate. At position 374, phenylalanine could replace tyrosine. These results indicate that wild-type desensitization properties requires an aromatic moiety at position 374 and an amino rather than a guanidino group at position 373. These residues lie between previously identified motifs involved in membrane trafficking (YXXXK and YXXGL) and implicates the C-terminal also in rearrangements leading to channel closing during the presence of agonist.

Disruption of lipid rafts inhibits P2X1 receptor-mediated currents and arterial vasoconstriction

P2X1 receptors for ATP are ligand-gated cation channels expressed on a range of smooth muscle preparations and blood platelets. The receptors appear to be clustered close to sympathetic nerve varicosities and mediate the underlying membrane potential changes and constriction following nerve stimulation in a range of arteries and resistance arterioles. In this study we have used discontinuous sucrose density gradients, Western blot analysis, and cholesterol measurements to show that recombinant and smooth muscle (rat tail artery, vas deferens, and bladder) P2X1 receptors are present in cholesterol-rich lipid rafts and co-localize with the lipid raft markers flotillin-1 and -2. Lipid rafts are specialized lipid membrane microdomains involved in signaling and trafficking. To determine whether lipid raft association was essential for P2X1 receptor channel function we used the cholesterol-depleting agent methyl-beta-cyclodextrin (10 mm for 1 h). This led to a redistribution of the P2X1 receptor throughout the sucrose gradient and reduced P2X1 receptor-mediated (alpha,beta-methylene ATP, 10 microm) currents in HEK293 cells by >90% and contractions of the rat tail artery by approximately 50%. However contractions evoked by potassium chloride (60 mm) were unaffected by methyl-beta-cyclodextrin and the inactive analogue alpha-cyclodextrin had no effect on P2X1 receptor-mediated currents or contractions. P2X1 receptors are subject to ongoing regulation by receptors and kinases, and the present results suggest that lipid rafts are an essential component in the maintenance of these localized signaling domains and play an important role in P2X1 receptor-mediated control of arteries.

P2X1 receptors and the endothelium

Adenosine triphosphate (ATP) is now established as a principle vaso-active mediator in the vasculature. Its actions on arteries are complex, and are mediated by the P2X and P2Y receptor families. It is generally accepted that ATP induces a bi-phasic response in arteries, inducing contraction via the P2X and P2Y receptors on the smooth muscle cells, and vasodilation via the actions of P2Y receptors located on the endothelium. However, a number of recent studies have placed P2X1 receptors on the endothelium of some arteries. The use of a specific P2X1 receptor ligand, alpha, beta methylene ATP has demonstrated that P2X1 receptors also have a bi-functional role. The actions of ATP on P2X1 receptors is therefore dependant on its location, inducing contraction when located on the smooth muscle cells, and dilation when expressed on the endothelium, comparable to that of P2Y receptors.

G-protein-coupled receptor regulation of P2X1 receptors does not involve direct channel phosphorylation

P2X1 receptors for ATP are ligand-gated cation channels, which mediate smooth muscle contraction, contribute to blood clotting and are co-expressed with a range of GPCRs (G-protein-coupled receptors). Stimulation of Galpha(q)-coupled mGluR1alpha (metabotropic glutamate receptor 1alpha), P2Y1 or P2Y2 receptors co-expressed with P2X(1) receptors in Xenopus oocytes evoked calcium-activated chloride currents (I(ClCa)) and potentiated subsequent P2X1-receptor-mediated currents by up to 250%. The mGluR1alpha-receptor-mediated effects were blocked by the phospholipase C inhibitor U-73122. Potentiation was mimicked by treatment with the phor-bol ester PMA. P2X receptors have a conserved intracellular PKC (protein kinase C) site; however, GPCR- and PMA-mediated potentiation was still observed with point mutants in which this site was disrupted. Similarly, the potentiation by GPCRs or PMA was unaffected by chelating the intracellular calcium rise with BAPTA/AM [bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid tetrakis-(acetoxymethyl ester)] or the PKC inhibitors Ro-32-0432 and bisindolylmaleimide I, suggesting that the regulation does not involve a calcium-sensitive form of PKC. However, both GPCR and PMA potentiation were blocked by the kinase inhibitor staurosporine. Potentiation by phorbol esters was recorded in HEK-293 cells expressing P2X1 receptors, and radiolabelling of phosphorylated proteins in these cells demonstrated that P2X1 receptors are basally phosphorylated and that this level of phosphorylation is unaffected by phorbol ester treatment. This demonstrates that P2X1 regulation does not result directly from phosphorylation of the channel, but more likely by a staurosporine-sensitive phosphorylation of an accessory protein in the P2X1 receptor complex and suggests that in vivo fine-tuning of P2X1 receptors by GPCRs may contribute to cardiovascular control and haemostasis.

Emerging roles for P2X1 receptors in platelet activation

The platelet surface membrane possesses three P2 receptors activated by extracellular adenosine nucleotides; one member of the ionotropic receptor family (P2X(1)) and two members of the G-protein-coupled receptor family (P2Y(1) and P2Y(12)). P2Y(1) and P2Y(12) receptors have firmly established roles in platelet activation during thrombosis and haemostasis, whereas the importance of the P2X(1) receptor has been more controversial. However, recent studies have demonstrated that P2X(1) receptors can generate significant functional platelet responses alone and in synergy with other receptor pathways. In addition, studies in transgenic animals indicate an important role for P2X(1) receptors in platelet activation, particularly under conditions of shear stress and thus during arterial thrombosis. This review discusses the background behind discovery of P2X(1) receptors in platelets and their precursor cell, the megakaryocyte, and how signalling via these ion channels may participate in platelet activation.

Novel role for P2X receptor activation in endothelium-dependent vasodilation

1 ATP is an important vasoactive mediator, which acts via two receptor classes: P2X and P2Y. Activation of P2X receptors has traditionally been associated with the well-characterised vasoconstrictor properties of ATP. 2 In the current study, we have shown that the P2X(1 & 3) receptor ligand, alpha, beta methylene ATP, induces vasodilation of rat isolated mesenteric arteries and that P2X1 receptors are abundantly expressed in the endothelium of these vessels. 3 Second-order rat mesenteric arteries were mounted in myographs and vasomotor responses recorded. Both ATP and alpha, beta methylene ATP induced a constriction followed by a vasodilation. The dilator effects of either ATP or alpha, beta methylene ATP were slower in onset than those induced by acetylcholine. By contrast, the traditional vasodilator P2Y ligand, ADP, induced vasodilation without contraction. 4 Vasodilation induced by alpha, beta methylene ATP was endothelial dependent, but was not affected by treatment of the vessels with L-NAME plus indomethacin alone. Dilation was, however, partially inhibited by the combination of apamin plus charybdotoxin and blocked by treating vessels with all four drugs. 5 Using confocal microscopy, P2X1 receptor immunoreactivity was localised to the endothelial, smooth muscle and adventitial layers of mesenteric vessels. P2X1 protein migrated as a primary band at around 50-60 kDa in vascular tissue. 6 These results show for the first time that P2X1 receptors are expressed on the endothelium and that a selective ligand of this receptor results in vasoconstriction followed by vasodilation. These observations have important implications for our understanding of the role of purines in biological responses.

P2X1 receptor currents after disruption of the PKC site and its surroundings by dominant negative mutations in HEK293 cells

It has been suggested that phosphorylation at the T18P19R20 PKC sites of the P2X1 receptor regulates its functions. Here, we show that mutation at T18 (T18A and T18N) almost abolishes P2X1 current in response to ATP and that mutations of R20T but not of P19V also decrease the P2X1 current. Immunoblotting with anti-Thr(P)-Pro monoclonal antibody of membrane proteins from HEK293 cells transfected with P2X1R20T indicate the absence of Thr(P)18 which is present in HEK293 cells transfected with WT P2X1. We conclude that T18P19R20 is phosphorylated following P2X1 binding of ligand but that the three PKC sites function to different degree.

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

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

Cellular Distribution and Functions of P2 Receptor Subtypes in Different Systems

This review is aimed at providing readers with a comprehensive reference article about the distribution and function of P2 receptors in all the organs, tissues, and cells in the body. Each section provides an account of the early history of purinergic signaling in the organ?cell up to 1994, then summarizes subsequent evidence for the presence of P2X and P2Y receptor subtype mRNA and proteins as well as functional data, all fully referenced. A section is included describing the plasticity of expression of P2 receptors during development and aging as well as in various pathophysiological conditions. Finally, there is some discussion of possible future developments in the purinergic signaling field.

Lack of evidence for functional ADP-activated human P2X1 receptors supports a role for ATP during hemostasis and thrombosis

Purine nucleotides acting through P2 receptors play key roles in platelet signaling. The P2X1 receptor is an adenosine triphosphate (ATP)-gated ion channel that mediates a rapid calcium influx signal, but can also synergize with subsequent adenosine diphosphate (ADP)-evoked P2Y1 receptor-mediated responses and thus may contribute to platelet activation during hemostasis. Recent studies have shown that P2X1 receptors contribute to the formation of platelet thrombi, particularly under conditions of high shear stress. Based on intracellular Ca2+ measurements a previous report has suggested that a splice variant of the P2X1 receptor, P2X1del, is expressed in platelets and, in contrast to the full-length P2X1WT receptor, is activated by ADP. In the present study we show that the P2X1del receptor fails to form functional ion channels and is below the limit of detection in human platelets. Furthermore, ADP does not contribute to the rapid ionotropic P2X receptor-mediated response in platelets. These results support the notion that ATP is the principal physiologic agonist at P2X1 receptors and that it plays a role in the activation of platelets.

P2 receptor-mediated Ca2+ transients in rat cerebral artery smooth muscle cells

Significant Ca(2+) release was previously noted with the activation of L-type Ca(2+) current in rat superior cerebral artery smooth muscle cells. Here we examined whether the P(2X) current that is partly carried by Ca(2+) also triggers Ca(2+) release in this preparation. Application of P(2X) agonists evoked membrane currents and concomitant Ca(2+) transients in whole cell voltage-clamped single cells. The expected increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) was calculated from the time-integrated P(2X) current by assuming Ca(2+) is the only charge carrier. The measured increase in [Ca(2+)](i) was plotted as a function of the expected increase in [Ca(2+)](i), and Ca(2+)-buffering power was obtained as a reciprocal of the linear fit to this relationship. Both ryanodine, a Ca(2+)-induced Ca(2+)-release inhibitor, and cADP ribose, a putative activator of Ca(2+)-induced Ca(2+) release, had no significant effects on Ca(2+)-buffering power. These results suggest that Ca(2+) influx through P(2X) receptors does not trigger significant Ca(2+) release. We then examined whether P(2X) responses influence the subsequent P(2Y) response. P(2Y) responses were characterized by measuring the rate of [Ca(2+)](i) increase obtained as the slope of the linear regression to the rising phase of the Ca(2+) transient. During simultaneous application of the P(2X) and P(2Y) agonist, the rate of [Ca(2+)](i) increase was facilitated or suppressed depending on the size of the P(2X) receptor-mediated [Ca(2+)](i) increase. Membrane depolarization close to the Ca(2+) equilibrium potential significantly promoted the rate of [Ca(2+)](i) increase. Our results suggest that the [Ca(2+)](i) increase and membrane depolarization caused by the P(2X) current may regulate the subsequent P(2Y) response.

Endothelium-dependent relaxation and endothelial hyperpolarization by P2Y receptor agonists in rat-isolated mesenteric artery

(1) Vasorelaxation and hyperpolarization of endothelial cells by adenosine 5'-[beta-thio]diphosphate (ADPbetaS) and adenosine 5'-[gamma-thio]triphosphate (ATPgammaS) were studied in rat-isolated mesenteric artery. Effects from stimulation of P2X receptors were avoided by desensitization with alpha,beta-methylene adenosine triphosphate. (2) ADPbetaS caused concentration- and endothelium-dependent relaxations of methoxamine-precontracted small (third generation) and main mesenteric artery. These were inhibited by N(omega)-nitro-L-arginine methyl ester (L-NAME) or a combination of apamin plus charybdotoxin (inhibitors of Ca(2+)-activated K(+) channels); L-NAME, apamin and charybdotoxin applied together abolished the response. (3) ATPgammaS induced limited relaxation (35% of methoxamine-induced tone at 10 micro M) of small mesenteric artery, which was sensitive to L-NAME or endothelium denudation. However, it almost completely relaxed the main mesenteric artery over an extended concentration range (>6 orders of magnitude) in an endothelium-dependent manner. This relaxation was inhibited by either L-NAME or a combination of apamin with charybdotoxin, and abolished by a combination of all the three inhibitors. (4) The P2Y(1) receptor antagonist MRS 2179 (2'-deoxy-N(6)-methyladenosine 3',5'-bisphosphate; 0.3-3 micro M) caused parallel rightward shifts of the concentration/relaxation curve to ADPbetaS (pA(2)=7.1). However, MRS 2179 did not inhibit, but potentiated, relaxant responses to ATPgammaS. MRS 2179 did not affect the contractile responses ATPgammaS in small mesenteric artery; ATPgammaS did not contract the main mesenteric artery. (5) ADPbetaS hyperpolarized the endothelium of the main mesenteric artery in a concentration-dependent manner. This was unaffected by L-NAME but antagonized by MRS 2179. ATPgammaS also hyperpolarized the mesenteric artery endothelium in a concentration-dependent manner but, when ATPgammaS was applied at 10 micro M, its effect was potentiated by MRS 2179 (3 micro M). (6) It is concluded that both relaxation and hyperpolarization to ADPbetaS are mediated by P2Y(1) receptors and that the endothelial hyperpolarization is related to the L-NAME-resistant relaxation. Relaxation to the P2Y(2) agonist ATPgammaS shows regional variation along the mesenteric vasculature. The mechanisms for potentiation of relaxation and hyperpolarization by ATPgammaS are unknown, but may indicate interactions between P2Y receptor subtypes.

Cloning, tissue distribution and functional characterization of the chicken P2X1 receptor

We describe a new chicken P2X subunit that is an orthologue of the mammalian P2X1 receptor. Functional characterization of chicken P2X1 receptors was performed using the amphotericin B perforated patch configuration to avoid the current run-down observed under whole-cell patch-clamp conditions. Responses to agonists and to the antagonist PPADS (pyridoxal 5-phosphate 6-azophenyl-2',4'-disulfonic acid) were similar to what has been described for mammalian orthologues. However, the antagonists suramin and NF023 were much less potent at chicken P2X1 receptors than at human P2X1 receptors. In embryonic tissues, transcript expression is predominant in lung, liver and skeletal muscle. Overlapping expression with cP2X4 and cP2X5 subunits in several embryonic tissues, including skeletal muscle, indicates that the native embryonic P2X receptors could be heteromultimeric.

P2X(1) receptor-deficient mice establish the native P2X receptor and a P2Y6-like receptor in arteries

The contribution of P2 receptors to vasoconstriction of mouse mesenteric arteries was determined using wild-type (WT) and P2X(1) receptor-deficient (KO) animals. alpha,beta-methylene ATP (alpha,beta-meATP) and ATP evoked transient inward currents and constrictions of WT mesenteric arteries. In contrast, alpha,beta-meATP (100 microM) and ATP (100 microM) failed to evoke responses in KO arteries from a range of vascular beds. Nerve stimulation (100 pulses at 10 Hz) evoked constrictions of mesenteric arteries. For WT arteries, the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2'-5'-disulfonate (PPADS) (30 microM) reduced the amplitude of response by approximately 50%; the residual constriction was abolished by prazosin (0.1 microM). In KO mice, vasoconstriction induced by nerve stimulation was reduced in amplitude by approximately 50%, unaffected by PPADS, but was abolished by prazosin. ADP (1 mM) (a P2Y(1), P2Y(12), and P2Y(13) receptor agonist) was ineffective. Because ATP had no effect on mesenteric artery tone from KO mice, this rules out the contribution of P2Y(2) receptors. The P2Y(4) receptor agonist ITP also failed to contract mesenteric arteries. However, UTP and UDP evoked sustained contractions of mesenteric arteries with similar potency (EC(50) approximately 10 microM). Complementary studies using reverse-transcriptase polymerase chain reaction showed that mesenteric arteries express P2Y(1), P2Y(2), and P2Y(6) receptors. These results demonstrate that homomeric P2X(1) receptors underlie the artery smooth muscle P2X receptor phenotype and contribute approximately 50% to sympathetic neurogenic vasoconstriction and indicate the presence of a UTP- and UDP-sensitive P2Y(6)-like receptor, but not vasoconstrictor P2Y(2) or P2Y(4) receptors, on mouse mesenteric arteries.

Extracellular ATP and nerve growth factor intensify hypoglycemia-induced cell death in primary neurons: role of P2 and NGFRp75 receptors

In this study, we monitored the direct expression of P2 receptors for extracellular ATP in cerebellar granule neurons undergoing metabolism impairment. Glucose deprivation for 30-60 min inhibited P2Y1 receptor protein, only weakly modulated P2X1, P2X2 and P2X3, and up-regulated by about two-fold P2X4, P2X7 and P2Y4. The P2X/Y antagonist basilen blue, protecting cerebellar neurons from hypoglycemic cell death, maintained within basal levels only the expression of P2X7 and P2Y4 proteins, but not P2X4 or P2Y1. Glucose starvation transiently increased (up to three-fold) the expression of NGFRp75 receptor protein and strongly stimulated the extracellular release of nerve growth factor (NGF; about 10-fold). Exogenously added NGF then augmented hypoglycemic neuronal death by about 60%, increasing the percentage of Höechst-positive nuclei (from approximately 62 to 95%), reducing lactate dehydrogenase (LDH) release (from about 50 to 14%) and significantly overstimulating the hypoglycemia-induced expression of P2X7 and P2Y4. Conversely, extracellular ATP augmented hypoglycemic neuronal death by about 80%, reducing the number of Höechst-positive nuclei (from approximately 62% to 14%), augmenting LDH outflow (by about 30%) and further increasing the hypoglycemia-induced expression of NGFRp75. Our results indicate that P2 and NGFRp75 receptors are modulated during glucose starvation and that extracellular ATP and NGF drive features of, respectively, necrotic and apoptotic hypoglycemic cell death, aggravating the consequences of metabolism impairment in cerebellar primary neurons.

Molecular physiology of P2X receptors

P2X receptors are membrane ion channels that open in response to the binding of extracellular ATP. Seven genes in vertebrates encode P2X receptor subunits, which are 40-50% identical in amino acid sequence. Each subunit has two transmembrane domains, separated by an extracellular domain (approximately 280 amino acids). Channels form as multimers of several subunits. Homomeric P2X1, P2X2, P2X3, P2X4, P2X5, and P2X7 channels and heteromeric P2X2/3 and P2X1/5 channels have been most fully characterized following heterologous expression. Some agonists (e.g., alphabeta-methylene ATP) and antagonists [e.g., 2',3'-O-(2,4,6-trinitrophenyl)-ATP] are strongly selective for receptors containing P2X1 and P2X3 subunits. All P2X receptors are permeable to small monovalent cations; some have significant calcium or anion permeability. In many cells, activation of homomeric P2X7 receptors induces a permeability increase to larger organic cations including some fluorescent dyes and also signals to the cytoskeleton; these changes probably involve additional interacting proteins. P2X receptors are abundantly distributed, and functional responses are seen in neurons, glia, epithelia, endothelia, bone, muscle, and hemopoietic tissues. The molecular composition of native receptors is becoming understood, and some cells express more than one type of P2X receptor. On smooth muscles, P2X receptors respond to ATP released from sympathetic motor nerves (e.g., in ejaculation). On sensory nerves, they are involved in the initiation of afferent signals in several viscera (e.g., bladder, intestine) and play a key role in sensing tissue-damaging and inflammatory stimuli. Paracrine roles for ATP signaling through P2X receptors are likely in neurohypophysis, ducted glands, airway epithelia, kidney, bone, and hemopoietic tissues. In the last case, P2X7 receptor activation stimulates cytokine release by engaging intracellular signaling pathways.

ATP-induced endothelium-independent enhancement of lymphatic vasomotion in guinea-pig mesentery involves P2X and P2Y receptors

1. The present study has investigated mechanisms underlying ATP-induced endothelium-independent enhancement of vasomotion in guinea-pig mesenteric lymphatic vessels. 2. Lymphatic vasomotion, vessel tone and smooth muscle [Ca(2+)](i) showed similar ATP concentration-response curves. 3. ATP, at 0.1 mM, caused a biphasic increase in tonic [Ca(2+)](i) and superimposed vasomotion-associated Ca(2+) transients. All ATP-induced [Ca(2+)](i) changes were abolished by incubating the smooth muscle with suramin (0.1 mM). 4. alpha,beta-MeATP (0.1 mM) and UTP (0.1 mM) caused similar changes in [Ca(2+)](i) but the responses to these agonists were smaller than to ATP. 5. The actions of alpha,beta-MeATP (0.1 mM) were inhibited by suramin (0.1 mM) and PPADS (30 micro M) but not by reactive blue 2 (30 micro M). 6. In the presence of alpha,beta-MeATP (0.1 mM), the increases in tonic [Ca(2+)](i) and vasomotion-associated Ca(2+) transients induced by ATP (0.1 mM) were inhibited by U73122 (5 micro M), CPA (20 micro M) and heparin, whereas U73343 (5 micro M) and pre-treatment with PTx (100 ng ml(-1)) had no significant effects. 7. Depletion of the intracellular stores with CPA (20 micro M) caused an increase in [Ca(2+)](i), which was not blocked by desensitization of P(2X) receptors with alpha,beta-MeATP. 8. The data indicate that ATP, at relatively high concentrations increases lymphatic smooth muscle [Ca(2+)](i) and vasomotion through activation of P(2X1) and P(2Y2) purinoceptors present on lymphatic smooth muscle. The increase in [Ca(2+)](i) is likely to result from Ca(2+) release from inositol-1,4,5-trisphosphate-sensitive stores as well as Ca(2+) influx through store-operated channels and P(2X)-gated channels.

The involvement of smooth muscle P2X receptors in the prolonged vasorelaxation response to purine nucleotides in the rat mesenteric arterial bed

1. ATP and adenine dinucleotides can elicit three different types of vasomotor response in the rat mesenteric arterial bed; vasocontraction, rapid relaxation (which may be masked by contraction) and slow and prolonged vasorelaxation. Contraction is mediated by smooth muscle P2X receptors and rapid relaxation by endothelial P2Y receptors. The mechanism of prolonged relaxation is, however, controversial. 2. In the present study, bolus injection of doses of alpha,beta-methylene ATP (alpha,beta-meATP; 5 pmol - 0.5 micromol; P2X receptor agonist) in methoxamine-preconstricted rat isolated mesenteric arterial beds, mimicked the action of ATP, causing contraction (R(max) 76+/-9 mmHg) followed by prolonged relaxation (78+/-11%; t(1/2) 14.6+/-1.5 min). KCl also elicited a biphasic response (R(max) contraction 73+/-8 mmHg; R(max) prolonged relaxation 70+/-6%; t(1/2) 7.7+/-1.9 min). 3. P2X receptor desensitization caused by perfusion with alpha,beta-meATP (10 microM) abolished contraction and prolonged relaxation to doses of alpha,beta-meATP (50 nmol). Rapid relaxation (32+/-7%; t(1/2) 32+/-2 s) was revealed, which was abolished by removal of the endothelium using distilled water. 4. Sodium deoxycholate treatment blocked contractile and prolonged relaxation responses to alpha,beta-meATP, ATP and KCl, whilst distilled water treatment had no significant effect on either phase of the biphasic responses. 5. These data indicate that smooth muscle P2X receptors are involved in both phases of the biphasic response (contraction followed by prolonged relaxation) to purine nucleotides in the rat isolated mesenteric arterial bed. Caution should be applied when using sodium deoxycholate to remove the endothelium because of possible damage caused by the detergent to receptors and/or the vascular smooth muscle.