P2 purinoceptors: historical perspective and classification

Purinergic P2Y1 Receptors Control Rapid Expression of Plasma Membrane Processes in Hippocampal Astrocytes

Astrocytes regulate neuronal activity and blood brain barrier through tiny plasma membrane branches or astrocytic processes (APs) making contact with synapses and brain vessels. Several transmitters released by astrocytes and exerting their action on several receptor classes expressed by astrocytes themselves influence their physiology. Here we found that APs are dynamically modulated by purines. In live imaging experiments carried out in rat hippocampal astrocytes, Gq-coupled P2Y1 receptor blockade with the selective antagonist MRS2179 (1 μM) or inhibition of its effector phospholipase C using U73122 (3 μM) produced APs retraction, while stimulation of the same receptor with the selective agonist 2MeSADP (100 μM) increased their number. Since astrocytes, among other transmitters, release ATP by several mechanisms including connexin hemichannels, we used the connexin hemichannel inhibitor carbenoxolone (100 μM) and APs retraction was observed. In our system we then measured expression or function of channels important for modulation of volume transmission and K+ buffering, aquaporin-4, and K+ inward rectifying (Kir) channels, respectively. Aquaporin-4 expression level did not change whereas, in whole-cell patch-clamp recordings performed to measure Kir current, we observed an increase in K+ current in all conditions where APs number was reduced. These data are supporting the idea of a dynamic modulation of astrocytic processes by purinergic signal, strengthening the role of purines in brain homeostasis.

From one generation to the next: a comprehensive account of sympathetic receptor control in branching arteriolar trees

The effect of the sympathetic nervous system on blood flow distribution within skeletal muscle microvasculature is conditional upon regional activation of receptors for sympathetic neurotransmitters. Previous studies have shown that proximal arterioles are largely governed by adrenergic activation, whereas it is speculated that distal branches are controlled by peptidergic and purinergic activation. However, no study has systematically evaluated the activation of adrenergic, peptidergic and purinergic receptors in continuously branching arteriolar trees of an individual skeletal muscle model. Therefore, in the present study, sympathetic agonists were used to evaluate the constriction responses along first to fifth order arterioles in continuously branching arteriolar trees of a in vivo rat gluteus maximus muscle preparation with respect to specific activation of receptors for sympathetic neurotransmitters (α1R, α2R, NPY1R and P2X1R). Constriction responses were incorporated into a mathematical blood flow model to estimate the total flow, resistance and red blood cell flow heterogeneity within a computationally reconstructed gluteus maximus arteriolar network. For the first time, the effects of activating receptors for sympathetic neurotransmitters on vasoconstrictor responses and the ensuing haemodynamics in continuously branching arteriolar trees of skeletal muscle were characterized, where proximal arterioles responded most to α1R and α2R adrenergic activation, whereas distal arterioles responded most to Y1R and P2X1R activation. Total flow and resistance changed with activation of all receptors, whereas red blood cell flow heterogeneity was largely affected by peptidergic and purinergic activation in distal arterioles. The reported data highlight the functional consequences of topologically-dependent sympathetic control and may serve as novel input parameters in computational modelling of network flow.

Genomic Analysis of ATP Efflux in Saccharomyces cerevisiae

Adenosine triphosphate (ATP) plays an important role as a primary molecule for the transfer of chemical energy to drive biological processes. ATP also functions as an extracellular signaling molecule in a diverse array of eukaryotic taxa in a conserved process known as purinergic signaling. Given the important roles of extracellular ATP in cell signaling, we sought to comprehensively elucidate the pathways and mechanisms governing ATP efflux from eukaryotic cells. Here, we present results of a genomic analysis of ATP efflux from Saccharomyces cerevisiae by measuring extracellular ATP levels in cultures of 4609 deletion mutants. This screen revealed key cellular processes that regulate extracellular ATP levels, including mitochondrial translation and vesicle sorting in the late endosome, indicating that ATP production and transport through vesicles are required for efflux. We also observed evidence for altered ATP efflux in strains deleted for genes involved in amino acid signaling, and mitochondrial retrograde signaling. Based on these results, we propose a model in which the retrograde signaling pathway potentiates amino acid signaling to promote mitochondrial respiration. This study advances our understanding of the mechanism of ATP secretion in eukaryotes and implicates TOR complex 1 (TORC1) and nutrient signaling pathways in the regulation of ATP efflux. These results will facilitate analysis of ATP efflux mechanisms in higher eukaryotes.

Neurochemicals involved in medullary control of common carotid blood flow

The common carotid artery (CCA) supplies intra- and extra-cranial vascular beds. An area in the medulla controlling CCA blood flow is defined as the dorsal facial area (DFA) by Kuo et al. in 1987. In the DFA, presynaptic nitrergic and/or glutamatergic fibers innervate preganglionic nitrergic and/or cholinergic neurons which give rise to the preganglionic fibers of the parasympathetic 7th and 9th cranial nerves. Released glutamate from presynaptic nitrergic and/or glutamatergic fibers can activate N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA) receptors on preganglionic nitrergic and/or cholinergic neurons. By modulating this glutamate release, several neurochemicals including serotonin, arginine, nitric oxide, nicotine, choline and ATP in the DFA regulate CCA blood flow. Understanding the neurochemical regulatory mechanisms can provide important insights of the physiological roles of the DFA, and may help develop therapeutic strategies for diseases involving CCA blood flow, such as migraine, hypertensive disease, Alzheimer’s disease and cerebral ischemic stroke.

Purinergic signalling: from discovery to current developments

This lecture is about the history of the purinergic signalling concept. It begins with reference to the paper by Paton & Vane published in 1963, which identified non-cholinergic relaxation in response to vagal nerve stimulation in several species, although they suggested that it might be due to sympathetic adrenergic nerves in the vagal nerve trunk. Using the sucrose gap technique for simultaneous mechanical and electrical recordings in smooth muscle (developed while in Feldberg's department in the National Institute for Medical Research) of the guinea-pig taenia coli preparation (learned when working in Edith Bülbring's smooth muscle laboratory in Oxford Pharmacology), we showed that the hyperpolarizations recorded in the presence of antagonists to the classical autonomic neurotransmitters, acetylcholine and noradrenaline, were inhibitory junction potentials in response to non-adrenergic, non-cholinergic neurotransmission, mediated by intrinsic enteric nerves controlled by vagal and sacral parasympathetic nerves. We then showed that ATP satisfied the criteria needed to identify a neurotransmitter released by these nerves. Subsequently, it was shown that ATP is a cotransmitter in all nerves in the peripheral and central nervous systems. The receptors for purines and pyrimidines were cloned and characterized in the early 1990 s, and immunostaining showed that most non-neuronal cells as well as nerve cells expressed these receptors. The physiology and pathophysiology of purinergic signalling is discussed.

P2Y purinoceptors induce changes in intracellular calcium in acinar cells of rat lacrimal glands

Adenosine 5'-triphosphate (ATP) is an extracellular signal that regulates various cellular functions. Cellular secretory activities are enhanced by ATP as well as by cholinergic and adrenergic stimuli. The present study aimed to determine which purinoceptors play a role in ATP-induced changes in the intracellular concentration of calcium ions ([Ca²⁺](i)) and in the fine structure of acinar cells of rat lacrimal glands. ATP induced exocytotic structures, vacuolation and an increase in [Ca²⁺](i) in acinar cells. The removal of extracellular Ca²⁺ or the use of Ca²⁺ channel blockers partially inhibited the ATP-induced [Ca²⁺](i) increase. U73122 (an antagonist of PLC) and heparin (an antagonist of IP₃ receptors) did not completely inhibit the ATP-induced [Ca²⁺](i) increase. P1 purinoceptor agonists did not induce any changes in [Ca²⁺](i), whereas suramin (an antagonist of P2 receptors) completely inhibited ATP-induced changes in [Ca²⁺](i). A P2Y receptor agonist, 2-MeSATP, induced a strong increase in [Ca²⁺](i), although UTP (a P2Y₂,₄,₆ receptor agonist) had no effect, and reactive blue 2 (a P2Y receptor antagonist) resulted in partial inhibition. The potency order of ATP analogs (2-MeSATP > ATP >>> UTP) suggested that P2Y₁ played a significant role in the cellular response to ATP. BzATP (a P2X₇ receptor agonist) induced a small increase in [Ca²⁺](i), but α,β-meATP (a P2X₁,₃ receptor agonist) had no effect. RT-PCR indicated that P2X₂,₃,₄,₅,₆,₇ and P2Y₁,₂,₄,₁₂,₁₄ are expressed in acinar cells. In conclusion, the response of acinar cells to ATP is mediated by P2Y (especially P2Y₁) as well as by P2X purinoceptors.

Adenosine and immune imbalance in visceral leishmaniasis: the possible role of ectonucleotidases

Visceral leishmaniasis (VL) is the most severe form of leishmaniasis and is responsible for most Leishmania-associated deaths. VL represents a serious public health problem that affects many countries. The immune response in leishmaniasis is very complex and is poorly understood. The Th1 versus Th2 paradigm does not appear to be so clear in visceral leishmaniasis, suggesting that other immunosuppressive or immune-evasion mechanisms contribute to the pathogenesis of VL. It has been demonstrated that generation of adenosine, a potent endogenous immunosuppressant, by extracellular enzymes capable to hydrolyze adenosine tri-nucleotide (ATP) at the site of infection, can lead to immune impairment and contribute to leishmaniasis progression. In this regard, this paper discusses the unique features in VL immunopathogenesis, including a possible role for ectonucleotidases in leishmaniasis.

Purinergic signaling in embryonic and stem cell development

Nucleotides are of crucial importance as carriers of energy in all organisms. However, the concept that in addition to their intracellular roles, nucleotides act as extracellular ligands specifically on receptors of the plasma membrane took longer to be accepted. Purinergic signaling exerted by purines and pyrimidines, principally ATP and adenosine, occurs throughout embryologic development in a wide variety of organisms, including amphibians, birds, and mammals. Cellular signaling, mediated by ATP, is present in development at very early stages, e.g., gastrulation of Xenopus and germ layer definition of chick embryo cells. Purinergic receptor expression and functions have been studied in the development of many organs, including the heart, eye, skeletal muscle and the nervous system. In vitro studies with stem cells revealed that purinergic receptors are involved in the processes of proliferation, differentiation, and phenotype determination of differentiated cells. Thus, nucleotides are able to induce various intracellular signaling pathways via crosstalk with other bioactive molecules acting on growth factor and neurotransmitter receptors. Since normal development is disturbed by dysfunction of purinergic signaling in animal models, further studies are needed to elucidate the functions of purinoceptor subtypes in developmental processes.

Ectodermal P2X receptor function plays a pivotal role in craniofacial development of the zebrafish

P2X receptors are non-selective cation channels operated by extracellular ATP. Currently, little is known concerning the functions of these receptors during development. Previous work from our lab has shown that zebrafish have two paralogs of the mammalian P2X3 receptor subunit. One paralog, p2rx3.1, is expressed in subpopulations of neural and ectodermal cells in the embryonic head. To investigate the role of this subunit in early cranial development, we utilized morpholino oligonucleotides to disrupt its translation. Loss of this subunit resulted in craniofacial defects that included malformation of the pharyngeal skeleton. During formation of these structures, there was a marked increase in cell death within the branchial arches. In addition, the epibranchial (facial, glossopharyngeal, and vagal) cranial sensory ganglia and their circuits were perturbed. These data suggest that p2rx3.1 function in ectodermal cells is involved in purinergic signaling essential for proper craniofacial development and sensory circuit formation in the embryonic and larval zebrafish.

P2-Purinoceptors: Advances and therapeutic opportunities

The recent cloning of a number of distinct receptors belonging to the P₂-purinoceptor superfamily has provided conclusive evidence for a pivotal role for ATP and other nucleotides as effector molecules involved in cell-to-cell communication and the modulation of many basic aspects of tissue function. ATP itself is being clinically evaluated as a cytotoxic agent for the treatment of cancer and as an adjunct to inhalation anaesthetic use. The pyrimidine nucleotide, UTP, is in clinical trials for the treatment of cystic fibrosis. The stable ATP bioisostere, ARL 67085, is being developed as a novel antithrombotic agent, blocking with a superior safety profile and increased efficacy as compared to other agents. The diversity of P₂ receptors, with eleven having been defined using both pharmacological and molecular cloning criteria, indicates considerable additional potential and subtlety in regard to the effects of ATP on tissue function and pathophysiology.

Histamine-induced myosin light chain phosphorylation breaks down the barrier integrity of cultured corneal epithelial cells

PURPOSE

To investigate changes in the phosphorylation of myosin light chain (MLC) in response to histamine and its effect on the barrier integrity of corneal epithelial cells.

MATERIALS AND METHODS

Experiments were performed in bovine corneal epithelial cells (BCEC). RT-PCR and Western blotting were employed to characterize expression of H1 receptors and MLC kinase (MLCK). Phosphorylation of MLC was assessed by urea-glycerol gel electrophoresis and Western blotting. Barrier integrity was determined as permeability to horseradish peroxidase (HRP; 44 kDa) across monolayers grown on porous filters.

RESULTS

Expression of both H1 receptors and MLCK was found in BCEC. Exposure to histamine induced significant MLC phosphorylation concomitant with an increase in HRP permeability. In addition, organization of the cortical actin found in resting cells was disrupted. In contrast to histamine, ATP (a P2Y receptor agonist) induced dephosphorylation of MLC. Pre-exposure to ATP reduced the effect of histamine on HRP permeability and disruption of cortical actin.

CONCLUSION

MLC phosphorylation, a biochemical pre-requisite for increased contractility of the actin cytoskeleton, led to histamine-induced breakdown of the barrier integrity in the corneal epithelial cells. This is attributed to weakening of the tethering forces at the tight junctions by the centripetal forces produced by increased actin contractility.

[32P]2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine-3',5'-bisphosphate ([32P]MRS2500), a novel radioligand for quantification of native P2Y1 receptors

Analysis of the P2Y family of nucleotide-activated G-protein-coupled receptors has been compromised by the lack of selective high-affinity, high-specific-radioactivity radioligands. We have pursued quantification of the P2Y1 receptor through the development of a series of selective P2Y1 receptor antagonists. Recently, we synthesized 2-iodo-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2500), a selective, competitive antagonist that exhibits a Ki of 0.8 nM in competition-binding assays with [3H]MRS2279. A 3'-monophosphate precursor molecule, MRS2608, was radiolabeled at the 5' position with 32P using polynucleotide kinase and [gamma32P]ATP to yield [32P]MRS2500. [32P]MRS2500 bound selectively to Sf9 insect cell membranes expressing the human P2Y1 receptor (Sf9-P2Y1), but did not detectably bind membranes expressing other P2Y receptors. P2Y1 receptor binding to [32P]MRS2500 was saturable with a KD of 1.2 nM. Agonists and antagonists of the P2Y1 receptor inhibited [32P]MRS2500 binding in Sf9-P2Y1 membranes with values in agreement with those observed in functional assays of the P2Y1 receptor. A high-affinity binding site for [32P]MRS2500 (KD=0.33 nM) was identified in rat brain, which exhibited the pharmacological selectivity of the P2Y1 receptor. Distribution of this binding site varied among rat tissues, with the highest amount of binding appearing in lung, liver, and brain. Among brain regions, distribution of the [32P]MRS2500 binding site varied by six-fold, with the highest and lowest amounts of sites detected in cerebellum and cortex, respectively. Taken together, these data illustrate the synthesis and characterization of a novel P2Y1 receptor radioligand and its utility for examining P2Y1 receptor expression in native mammalian tissues.

Noradrenaline and extracellular nucleotide cotransmission involves activation of vasoconstrictive P2X(1,3)- and P2Y6-like receptors in mouse perfused kidney

Nucleotides like ATP and UTP act as potent extracellular signalling molecules. Released from sympathetic nerve endings as cotransmitters of noradrenaline or paracrine from nonexcitatory cells, they activate specific receptors (ion-gated P2X(1-7) and G-protein-coupled P2Y(1,2,4,6,11-15)). Which of these subtypes, however, are able to modulate vasoconstriction in the kidney is unclear. Wild-type- and P2Y4-receptor-deficient mice kidneys were isolated and perfused with Krebs-Henseleit solution. Pressor responses to renal nerve stimulations (RNS) and added drugs were recorded. Release of endogenous noradrenaline was measured by HPLC. RNS (1-15 Hz) induced a frequency-dependent increase in the perfusion pressor (14.2+/-5.1-67.3+/-6.9 mmHg) and noradrenaline release (1.4+/-0.3-24.2+/-3.4 ng g(-1) kidney). Pressor responses to RNS were not (1-2 Hz) or only partially (5-15 Hz) blocked by the alpha-adrenoceptor antagonist phentolamine (1 microM). Combination of phentolamine and the P2-receptor blocker PPADS (5 microM) prevented RNS-induced pressor responses. The P2X(1,3)-receptor selective antagonist NF279 (10 microM) reduced RNS-induced pressor responses in a frequency-dependent manner. Perfusion of ATP, ADP, UTP, UDP and alpha,beta-meATP concentration dependently increased perfusion pressor with the following rank order of potency alpha,beta-meATP>ADP approximately ATP approximately UDP > or = UTP. NF279 (10 microM) reduced alpha,beta-meATP- (0.1 microM) (21.7+/-3.9% of control) but not UTP- (0.3 microM) (102.6+/-15.3% of control) induced pressor responses. No differences in nucleotide-induced effects were detected among wild-type and P2Y4-receptor knockout mice. Continuous perfusion of alpha,beta-meATP (0.01 microM) potentiated UTP-, UDP- and ATP-gamma S-induced pressor responses. Neuronally and paracrine-released nucleotides evoked renal vasoconstriction by activation of P2X(1,3)- and P2Y6-like receptors in mice. Pretreatment with the P2X(1,3)-receptor agonist alpha,beta-meATP potentiated P2Y6-like receptor-mediated vasoconstrictions.

Response to ATP is accompanied by a Ca2+ influx via P2X purinoceptors in the coronary arterioles of golden hamsters

In the vascular wall, adenosine-5'-triphosphate (ATP) released along with noradrenaline from sympathetic nerve terminals is considered to play an important role in controlling intracellular calcium ion ([Ca2+]i) levels in arteries. The present study examined how vascular smooth muscle cells in coronary arterioles respond to ATP in relation to [Ca2+]i dynamics. For this purpose, the dynamics of [Ca2+]i in the coronary arterioles of golden hamsters was examined by real-time laser scanning confocal microscopy. This technique enabled the visualization of [Ca2+]i changes in response to ATP in the intact coronary arterioles, the ultrastructure of which was well preserved. It was shown that an increase in [Ca2+]i in the arteriole smooth muscle cells was elicited by ATP. While P1 purinoceptor agonists have no effect on this process, P2 purinoceptor agonists were found to induce a [Ca2+]i increase in the smooth muscle cells. Suramin (an antagonist of P2X and P2Y receptors) completely inhibited ATP-induced [Ca2+]i dynamics, but reactive blue 2 (a P2Y receptor antagonist) did not. Uridine-5'-triphosphate (a P2Y receptor agonist) had no effect on [Ca2+]i, but alpha,beta-methylene ATP (a P2X receptor agonist) caused a strong increase in [Ca2+]i. We conclude that smooth muscle cells of the hamster coronary arterioles possess P2X, but not P1 or P2Y purinoceptors. The smooth muscle cells probably respond to extracellular ATP via P2X purinoceptors, resulting in the contraction of the coronary arterioles.

ATP modulates intracellular Ca2+ and firing rate through a P2Y1 purinoceptor in cane toad pacemaker cells

The effect of extracellular ATP (10-100 microM) on intracellular Ca2+ concentration ([Ca2+]i) and firing rate has been studied in single pacemaker cells isolated from the sinus venosus of cane toads. In spontaneously firing cells, ATP initially increased peak [Ca2+]i by 43 +/- 5 %, increased diastolic [Ca2+]i by 20 + 3 % and increased the firing rate by 58 +/- 8 %. These early effects were followed by a late phase in which both the peak [Ca2+]i and the firing rate declined. Adenosine, and UTP (respectively, P1- and P2Y2,4,6-selective agonists) caused no significant change in [Ca2+]i or firing rate, while alphabeta-methylene ATP (a P2X1,3 agonist) caused a small increase in firing rate but no changes in [Ca2+]i. In contrast the P2Y1-selective agonist 2-MesADP (1 microM) mimicked the biphasic effects of ATP and these effects were inhibited by the purinoceptor antagonists suramin and PPADS and by the P2Y1-selective antagonist MRS 2179. Immunohistochemistry established that P2Y1 purinoceptors were present on the cell surface. Western blotting analysis demonstrated that the P2Y1 antibody recognised a 57 kDa protein. After sarcoplasmic reticulum Ca2+ release was prevented with caffeine or ryanodine, ATP no longer had any effect on [Ca2+]i or firing rate. Furthermore, the SR Ca2+ store content was decreased during the late phase of 2-MesADP application. The effect of ATP was coupled to phospholipase C (PLC) activity because the PLC inhibitor U-73122 eliminated the effects of ATP. Our study shows that in toad pacemaker cells, the biphasic effects of ATP on pacemaker activity are mainly through P2Y1 purinoceptors, which are able to modulate Ca2+ release from the SR Ca2+ store.

ADP stimulation of inositol phosphates in hepatocytes: role of conversion to ATP and stimulation of P2Y2 receptors

1 Accumulation of inositol (poly)phosphates (InsP(x)) has been studied in rat hepatocytes labelled with [(3)H]inositol. Stimulation with ADP resulted in a significant increase in total [(3)H]InsP(x), whereas 2-MeSADP had only a small effect and ADPbetaS was ineffective. UTP and ITP also stimulated substantial increases in [(3)H]InsP(x). 2 The dose-response curve to ADP was largely unaltered by the presence of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P). Similarly, inclusion of MRS 2179, a more selective P2Y(1) antagonist, had no effect on the dose-response curve to ADP. 3 The inclusion of hexokinase in the assay reduced, but did not abolish, the response to ADP. 4 HPLC analysis revealed that ADP in the medium was rapidly converted to AMP and ATP. The inclusion of hexokinase removed ATP, but exacerbated the decline in ADP concentration, leading to increased levels of AMP. 2-MeSADP was stable in the medium and ATP was largely unaffected. 5 The addition of the adenylate kinase inhibitor, diadenosine pentaphosphate (Ap(5)A) significantly reduced the ADP response. HPLC analysis conducted in parallel demonstrated that this treatment inhibited conversion of ADP to ATP and AMP. 6 Inclusion of the P1 antagonist CGS 15943 had no effect on the dose-response curve to ADP. 7 These observations indicate that hepatocytes respond to ADP with an increase in inositol (poly)phosphates following conversion to ATP. P2Y(1) activation in hepatocytes does not appear to be coupled to inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) production.

Peripheral and central sensitization in fibromyalgia: pathogenetic role

Characteristic symptoms of fibromyalgia syndrome include widespread pain, fatigue, sleep abnormalities, and distress. Patients with fibromyalgia show psychophysical evidence of mechanical, thermal, and electrical hyperalgesia. Peripheral and central abnormalities of nociception have been described in fibromyalgia. Important nociceptor systems in the skin and muscles seem to undergo profound changes in patients with fibromyalgia through unknown mechanisms. They include sensitization of vanilloid receptor, acid-sensing ion channel receptors, and purino-receptors. Tissue mediators of inflammation and nerve growth factors can excite these receptors and cause extensive changes in pain sensitivity, but patients with fibromyalgia lack consistent evidence for inflammatory soft tissue abnormalities. Therefore, recent investigations have focused on central nervous system mechanisms of pain in fibromyalgia.

Two subtypes of G protein-coupled nucleotide receptors, P2Y(1) and P2Y(2) are involved in calcium signalling in glioma C6 cells

1. In glioma C6 cells, the stimulation of P2Y receptors by ADP, ATP and UTP initiated an increase in the intracellular Ca2+ concentration, in a process that involved the release of Ca2+ from InsP(3)-sensitive store and the capacitative, extracellular Ca2+ entry. The presence of external Ca2+ was not necessary to elevate Ca(2+). 2. The rank order of potencies of nucleotide analogues in stimulating [Ca2+](i) was: 2MeSADP > ADP > 2MeSATP = 2ClATP > ATP > UTP. alpha,beta-Methylene ATP, adenosine and AMP were ineffective. 3. ADP and UTP effects were additive, while actions of ATP and UTP were not additive on [Ca2+](i) increase. Similarly, cross-desensitization between ATP and UTP but not between ADP and UTP occurred. 4. Suramin, a non-specific nucleotide receptors inhibitor, antagonized ATP-, UTP- and ADP-evoked Ca2+ responses. PPADS, a selective antagonist of the P2Y(1) receptor-generated InsP(3) accumulation, decreased ADP-initiated Ca2+ response with no effect on ATP and UTP. 5. Pertussis toxin (PTX) reduced ADP- and ATP-induced Ca2+ increases. Short-term treatment with TPA, inhibited both ATP and ADP stimulatory effects on [Ca2+](i). 6. ADP inhibited isoproterenol-induced cyclic AMP accumulation. PTX blocked this effect, but PPADS did not. 7. RT - PCR analysis revealed the molecular identity of P2Y receptors expressed by glioma C6 cells to be both P2Y(1) and P2Y(2). 8. It is concluded that both P2Y(1) and P2Y(2) receptors co-exist in glioma C6 cells. ADP acts as agonist of the first, and ATP and UTP of the second one. Both receptors are linked to phospholipase C (PLC).

Released ATP is an extracellular cytotoxic mediator in salivary histatin 5-induced killing of Candida albicans

Salivary histatins (Hsts) are antifungal peptides with promise as therapeutic agents against candidiasis. Hst 5 kills the fungal pathogen Candida albicans via a mechanism that involves release of cellular ATP in the absence of cytolysis. Here we demonstrate that released ATP has a further role in Hst 5 killing. Incubation of the cells with ATP analogues induced cell death, and addition of the ATP scavenger apyrase to remove extracellular ATP released during Hst 5 treatment resulted in a reduction in cell killing. Experiments using anaerobically grown C. albicans with decreased susceptibility to Hst 5 confirmed that depletion of cellular ATP as a result of ATP efflux was not sufficient to cause cell death. In contrast to Hst-susceptible aerobic cultures, anaerobically grown cells were not killed by exogenously applied ATP. These findings established that Hst binding, subsequent entry into the cells, and ATP release precede the signal for cytotoxicity, which is mediated by extracellular ATP. In a higher-eukaryote paradigm, released ATP acts as a cytotoxic mediator by binding to membrane nucleotide P2X receptors. Based on a pharmacological profile and detection of a C. albicans 60-kDa membrane protein immunoreactive with antibody to P2X(7) receptor, we propose that released ATP in response to Hst 5 activates candidal P2X(7)-like receptors to cause cell death.

Evidence that 2-methylthioATP and 2-methylthioADP are both agonists at the rat hepatocyte P2Y(1) receptor

In the absence of selective antagonists, pharmacological characterization of P2Y receptor subtypes has relied heavily upon their distinct agonist profiles. 2-methylthioADP (2-MeSADP) is a selective agonist for the P2Y(1) receptor. The agonist action of 2-MeSATP at the P2Y(1) receptor has recently been questioned. The effects of both 2-MeSADP and 2-MeSATP have been studied on rat hepatocytes injected with the bioluminescent Ca(2+) indicator, aequorin. Single hepatocytes generate series of repetitive transients in cytosolic free calcium concentration ([Ca(2+)](i)) when stimulated with agonists acting through the phosphoinositide signalling pathway. The transients induced by 2-MeSADP and 2-MeSATP in the same cell were indistinguishable, indicating that they act at a common receptor. In contrast the transients evoked by ATP and UTP had very different profiles. Treatment of 2-MeSATP with an ATP-regenerating system to remove contaminating 2-MeSADP did not abolish its agonist activity. Application of the P2Y(1) antagonist, adenosine-3'-phosphate-5'-phosphate (A3P5P) inhibited the transients induced by both 2-MeSADP and 2-MeSATP. In contrast the transients induced by ATP and UTP were enhanced by the addition of A3P5P. These results indicate that both 2-MeSADP and 2-MeSATP are agonists at the rat hepatocyte P2Y(1) receptor.

Activity of Novel Adenine Nucleotide Derivatives as Agonists and Antagonists at Recombinant Rat P2X Receptors

Strategy, Management and Health PolicyVenture Capital Enabling TechnologyPreclinical ResearchPreclinical Development Toxicology, Formulation Drug Delivery, PharmacokineticsClinical Development Phases I-III Regulatory, Quality, ManufacturingPostmarketing Phase IVThe effects of structural modifications of adenine nucleotides previously shown to enhance either agonist (2-thioether groups) or antagonist (additional phosphate moieties at the 3'- or 2'-position) properties at P2Y(1) receptors were examined at recombinant rat P2X(1), P2X(2), P2X(3), and P2X(4) receptors expressed in Xenopus oocytes. The potency of P2Y(1) agonists HT-AMP (2-(hexylthio)adenosine-5'-monophosphate) and PAPET (2-[2-(4-aminophenyl)ethylthio]adenosine-5'-triphosphate) was examined at P2X receptors. Both nucleotides showed a preference for the Group I (α,β-meATP-sensitive, fast-inactivating) P2X sub-units. HT-AMP was 5-fold more potent than ATP at P2X(3) receptors and a partial agonist at all except P2X(2) receptors, at which it was a full agonist. The efficacy of HT-AMP was as low as 23% at P2X(4) receptors. PAPET was a weak partial agonist at rat P2X(4) receptors and a nearly full agonist at the other subtypes. At rat P2X(3) receptors, PAPET was more potent than any other known agonist (EC(50) = 17 ± 3 nM). MRS 2179 (N(6)-methyl-2'-deoxyadenosine 3', 5-bisphosphate, a potent P2Y(1) receptor antagonist) inhibited ATP-evoked responses at rat P2X(1) receptors with an IC(50) value of 1.15 ± 0.21 μM. MRS 2179 was a weak antagonist at rat P2X(3) receptors, with an IC(50) value of 12.9 ± 0.1 μM, and was inactive at rat P2X(2) and P2X(4) receptors. Thus, MRS 2179 was 11-fold and 130-fold selective for P2Y(1) receptors vs. P2X(1) and P2X(3) receptors, respectively. MRS 2209, the corresponding 3'-deoxy-2'-phosphate isomer, was inactive at rat P2X(1) receptors, thus demonstrating its greater selectivity as a P2Y(1) receptor antagonist. Various adenine bisphosphates in the family of MRS 2179 containing modifications of either the adenine (P2Y(1) antagonists with 2- and 6-substitutions), the phosphate (a 3',5'-cyclic diphosphate, inactive at P2Y(1) receptors), or the ribose moieties (antagonist carbocyclic analogue), were inactive at both rat P2X(1) and P2X(3) receptors. An anhydrohexitol derivative (MRS 2269) and an acyclic derivative (MRS 2286), proved to be selective antagonists at P2Y(1) receptors, since they were inactive as agonist or antagonist at P2X(1) and P2X(3) receptors.

Evidence that rat hepatocytes co-express functional P2Y1 and P2Y2 receptors

Previous studies have indicated the expression of multiple P2Y receptors by rat hepatocytes although they have not been identified. Here we show by reverse transcriptase-polymerase chain reaction (RT - PCR) that rat hepatocytes express mRNA encoding all of the four cloned rat P2Y receptors (P2Y(1), P2Y(2), P2Y(4) and P2Y(6)). The effects of UTP have been examined on single aequorin-injected rat hepatocytes. The [Ca(2+)](i) transients induced by UTP were indistinguishable from those induced by ATP in the same cell. The modulatory effects of elevated intracellular cyclic AMP concentration were the same on both UTP- and ATP-induced [Ca(2+)](i) transients. UDP, an agonist at the P2Y(6) receptor, failed to induce transients in hepatocytes, indicating that functional P2Y(6) receptors coupled to increased [Ca(2+)](i) are not expressed. The transients evoked by ADP were more sensitive to inhibition by suramin than those induced by either ATP or UTP. Within an individual cell, the transients induced by ATP and UTP were inhibited by the same concentration of suramin. This sensitivity of ATP and UTP responses to suramin suggests action through P2Y(2) rather than P2Y(4) receptors. Co-application of 30 microM pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) caused a decrease in frequency and amplitude of transients induced by ADP. ATP- and UTP-induced transients also displayed a decrease in amplitude in response to addition of PPADS, but this was accompanied by an increase in frequency of transients. In conclusion the data presented here are consistent with the co-expression of P2Y(1) and P2Y(2) receptors by rat hepatocytes.

Inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes

1. Modulation of intracellular free Ca2+ concentration ([Ca2+]i) by extracellular ATP was investigated in cultured adult rat brown adipocytes using the fluorescent Ca2+ indicator fura-2. 2. Bath application of ATP in micromolar concentrations caused a large increase in [Ca2+]i in cells previously stimulated with noradrenaline. This ATP-induced [Ca2+]i increase exhibited a monotonic decline to near the resting levels within approximately 2 min, even in the continued presence of the agonist. 3. The magnitude and time course of the [Ca2+]i increase in response to ATP were not significantly affected by removal of extracellular Ca2+, suggesting that a mobilization of intracellular Ca2+ primarily contributes to the increase. 4. The [Ca2+]i increase in response to ATP was sensitive to inhibition by suramin, suggesting the involvement of P2 purinoceptors in the response. 5. Thapsigargin (100 nM) evoked a gradual and irreversible increase in [Ca2+]i which was entirely dependent upon extracellular Ca2+, providing functional evidence for the expression of store-operated Ca2+ entry in these brown adipocytes. 6. Extracellular ATP at a concentration of 10 microM depressed this thapsigargin (100 nM)-induced [Ca2+]i increase by 92 +/- 3 % (n = 8 cells), strongly suggesting that ATP inhibits an influx of Ca2+ across the plasma membrane through the store-operated pathway. Bath application of phorbol 12-myristate 13-acetate (PMA, 5 microM) did not affect the thapsigargin-induced [Ca2+]i increase, indicating that the inhibitory action of ATP is not mediated by activation of protein kinase C (PKC). 7. These results indicate that extracellular ATP not only mobilizes Ca2+ from the intracellular stores but also exerts a potent inhibitory effect on the store-operated Ca2+ entry process in adult rat brown adipocytes.

Control of apical membrane chloride permeability in the renal A6 cell line by nucleotides

1. The effect of extracellular nucleotides applied on the apical side of polarised A6 cells grown on permeant filters was investigated by measuring the changes in (i) the 36Cl efflux through the apical membranes, (ii) the intracellular chloride concentrations (aCli, measured with N-(6-methoxyquinolyl) acetoethyl ester, MQAE), (iii) ICl, the short-circuit current in the absence of Na+ transport and (iv) the characteristics of the apical chloride channels using a patch-clamp approach. 2. ATP or UTP (0.1-500 microM) transiently stimulated ICl. The sequence of purinergic agonist potencies was UTP = ATP > ADP >> the P2X-selective agonist beta,gamma-methylene ATP = the P2Y-selective agonist 2-methylthioATP. Suramin (100 microM) as the P2Y antagonist Reactive Blue 2 (10 microM) had no effect on the UTP (or ATP)-stimulated current. These findings are consistent with the presence of P2Y2-like receptors located on the apical membranes of A6 cells. Apical application of adenosine also transiently increased ICl. This effect was blocked by theophylline while the UTP-stimulated ICl was not. The existence of a second receptor, of the P1 type is proposed. 3. ATP (or UTP)-stimulated ICl was blocked by apical application of 200 microM N-phenylanthranilic acid (DPC) or 100 microM niflumic acid while 100 microM glibenclamide was ineffective. 4. Ionomycin and thapsigargin both transiently stimulated ICl; the nucleotide stimulation of ICl was not suppressed by pre-treatment with these agents. Chlorpromazin (50 microM), a Ca2+-calmodulin inhibitor strongly inhibited the stimulation of ICl induced either by apical UTP or by ionomycin application. BAPTA-AM pre-treatment of A6 cells blocked the UTP-stimulated ICl. Niflumic acid also blocked the ionomycin stimulated ICl. 5. A fourfold increase in 36Cl effluxes through the apical membranes was observed after ATP or UTP application. These increases of the apical chloride permeability could also be observed when following aCli changes. Apical application of DPC (1 mM) or 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB; 500 microM) produced an incomplete inhibition of 36Cl effluxes through the apical membranes in ATP-stimulated and in untreated monolayers. 6. In single channel patch-clamp experiments, an apical chloride channel with a unitary single channel conductance of 7.3 +/- 0.6 pS (n = 12) was usually observed. ATP application induced the activation of one or more of these channels within a few minutes. 7. These results indicate that multiple purinergic receptor subtypes are present in the apical membranes of A6 cells and that nucleotides can act as modulators of Cl- secretion in renal cells.

Evidence for two different P2X-receptors mediating vasoconstriction of Ap5A and Ap6A in the isolated perfused rat kidney

The activation of various P2-receptor subtypes in rat renal vasculature by P1, P5-diadenosine pentaphosphate (ApsA) and P1, P6-diadenosine hexaphosphate (Ap6A) were studied by measuring their effects on perfusion pressure during continuous perfusion in a rat isolated perfused kidney. Permanent perfusion with Ap5A and Ap6A elicited both a transient and sustained vasoconstriction with both vasoconstrictions to be different: the transient vasoconstriction can be elicited with concentrations > or = 10 nM, whereas the sustained vasoconstriction is observed with concentrations > or = 1 nM. ApsA and Ap6A act via the same receptors as alpha,beta-methylene ATP (alpha,beta-meATP). The rank order of potency for transient vasconstriction was alpha,beta-meATP = ApsA>Ap6A>B,gamma-meATP, and for sustained vasoconstriction alpha,beta-meATP = Ap5A > beta,gamma-meATP > or = Ap6A. Suramin, a non-selective P2-receptor antagonist, and pyridoxal-phosphate-6-azophenyl-2;4-disulphonic acid (PPADS) a highly selective P2X-receptor antagonist antagonized both the transient and the sustained vasoconstriction. Taken together the results of the agonist profile of Ap5A and Ap6A and comparing its findings to literature it can be demonstrated that the transient but not the sustained vasoconstriction is mediated via the P2X1-receptor which is present in rat renal vasculature. It is demonstrated that the agonist profile of the sustained vasoconstriction induced by ApsA and Ap6A does not fit to any currently known P2X- or P2Y-receptor subtype. We conclude a yet unidentified P2X-receptor or chimeric P2X-receptor may contribute to the effects on rat renal vasculature produced by Ap5A and Ap6A and which may play an important role in glomerular perfusion pressure and blood pressure control.

Presence of dinucleotide and ATP receptors in human cerebrocortical synaptic terminals

Human cerebrocortical synaptic terminals elicited concentration-dependent Ca2+ transients after Ap5A (diadenosine pentaphosphate) and ATP stimulation, with EC50 values of 23.44 +/- 3.70 microM and 11.48 +/- 2.12 microM, respectively. The lack of cross-desensitisation and the selective antagonism by Ip5I (diinosine pentaphosphate), suggests the activation of a dinucleotide receptor by Ap5A, and a P2X receptor by ATP. Ap5A Ca2+ transients were partially abolished by omega-conotoxin GVI-A (53%), suggesting the participation of a N-type Ca2+ channel in the dinucleotide response. ATP effect on Ca2+ entry was abolished by nicardipine (44%) and by omega-conotoxin GVI-A (52%), suggesting the participation of L- and N-type Ca2+ channels. These data suggest that Ap5A and ATP activate dinucleotide and P2X receptors, respectively, in human brain synaptic terminals.

Antagonists of the platelet P2T receptor: a novel approach to antithrombotic therapy

The platelet P2T receptor plays a major role in platelet aggregation, and its antagonists are predicted to have significant therapeutic potential as antithrombotic agents. We have explored analogues of adenosine triphosphate (ATP), which is a weak, nonselective but competitive P2T receptor antagonist. Modification of the polyphosphate side chain to prevent breakdown to the agonist adenosine diphosphate (ADP) and substitution of the adenine moiety to enhance affinity and selectivity for the P2T receptor led to the identification of 10e (AR-C67085MX), having an IC50 of 2.5 nM against ADP-induced aggregation of human platelets. Compound 10e was the first very potent antagonist of the P2T receptor, with a selectivity for that subtype of the P2 receptor family of >1000-fold. Further modification of the structure produced compound 10l (AR-C69931MX) having an IC50 of 0.4 nM. In vivo, at maximally effective antithrombotic doses, there is little prolongation of bleeding time (1.4-fold), which is in marked contrast to the 5-6-fold found with GPIIb/IIIa antagonists.

Tenidap enhances P2Z/P2X7 receptor signalling in macrophages

Tenidap is an anti-inflammatory drug whose mechanism of action is not fully understood. It has been shown to block plasma membrane anion transport and to decrease release of interleukin-1beta, probably via the inhibition of interleukin-1beta converting enzyme. In the present study we showed that: (a) tenidap increases the sensitivity of mouse macrophages to cytotoxic effects mediated by extracellular ATP; (b) tenidap increases lucifer yellow uptake through the macrophage ATP receptor; (c) pretreatment with oxidised ATP, a blocker of the P2Z/P2X7 receptor, inhibits cytotoxicity and lucifer yellow uptake due to the combined effects of ATP and tenidap; (d) macrophages lacking the P2Z/P2X7 receptor are resistant to the synergistic effect of tenidap and ATP. The results suggest that tenidap synergises with extracellular ATP for activation of the P2Z/P2X7 receptor.

Steady-state binding of [3H]ATP to rat liver plasma membranes and competition by various purinergic agonists and antagonists

Steady-state analysis of nucleotide-binding sites on rat liver plasma membranes was carried out using 3H-labelled ATP as radioligand under complete inhibition of ecto-ATPase activity by excess EDTA. Binding of [3H]ATP to the membranes is saturable, reversible and apparently involves one population of specific binding sites with Kd of about 90 nM and binding capacity (Bmax) of 15 pmol/mg protein. A broad spectrum of purinergic agonists and antagonists was examined as potential inhibitors of the measured binding. The displacement studies showed the following rank order of inhibitory potency for [3H]ATP-binding sites (pIC50 values in parentheses): ATPgammaS (7.49)>2-MeSATP (7.18)>ATP (6.91)>ADPbetaS (6.64)>/=ADP (6.56)>>RB2 (6.14)>>suramin (5.40)>>Ap4A (4. 57)>alpha,beta-MeATP (4.19)>/=beta,gamma-MeATP (3.97). AMP, adenosine, Ap5A, PPADS, beta-glycerophosphate as well as non-adenine nucleoside triphosphates GTP, UTP and CTP did not exert any effect on the measured binding at concentration ranges of 10-6-10-4 M. In order to ascertain whether ATP and its analogues are capable of interacting with the same binding domain, 2-MeSATP and ADP were treated as alternative ligands that could compete with unlabelled ATP for its binding sites. A 2-fold increase of Kd value for ATP-receptor interaction was observed in the presence of 2-MeSATP (60 nM) or ADP (250 nM) without any modulation of Bmax value, confirming that inhibitory effects of these compounds are competitive in nature. These studies demonstrate that ATP and its analogues are able to interact with a single binding domain on liver plasma membranes, which may be identified as ligand-binding component of P2 purinoceptors of the P2Y1 subtype.

ATP receptor-mediated enhancement of fast excitatory neurotransmitter release in the brain

ATP-gated cation channels (P2X receptors) exist on the soma of proprioceptive neurons in the trigeminal mesencephalic nucleus (MNV) in the brain stem. However, these pseudomonopolar neurons seem to receive no synaptic input to their soma; we therefore hypothesized that in MNV neurons, the P2X receptors of importance may be those located on their central terminal projections. Here, we show in trigeminal mesencephalic motor nucleus neurons, which receive their major input from the MNV, that both exogenous ATP (1 mM) and high frequency focal stimulation to evoke endogenous ATP release enhanced the frequency of spontaneous fast excitatory postsynaptic currents (EPSCs) with no change in their amplitude. The enhancement was reduced by the antagonists suramin (300 microM) and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (30 microM) and persisted when action potential conduction was blocked with tetrodotoxin (1 microM). Thus, functional P2X receptors are expressed on nerve terminals in the brain stem, where they increase the spontaneous release of glutamate onto trigeminal mesencephalic motor nucleus neurons.

A domain contributing to the ion channel of ATP-gated P2X2 receptors identified by the substituted cysteine accessibility method

P2X receptors are a family of ATP-gated ion channels thought to have intracellular N and C termini and two transmembrane segments separating a large extracellular domain. We examined the involvement of the second putative transmembrane domain (TM2) of the P2X2 subunit in ion conduction, using the substituted cysteine accessibility method (SCAM). This method tests the ability of hydrophilic reagents such as Ag+ or the methanethiosulfonates to modify covalently the sulfhydryl side chains exposed to aqueous environments. ATP-gated current was measured in HEK293 cells transiently expressing either wild-type or functional mutant P2X2 receptors containing a cysteine substitution in or around TM2. Application of Ag+ to gating channels had no sustained effect on wild-type P2X2 (WT) but irreversibly altered whole-cell currents in 15 mutants. By contrast, bath application of (2-aminoethyl)methanethiosulfonate (MTSEA) to closed channels inhibited 8 of the 15 residues affected by Ag+ when the channel was gating. Inhibition of the closed channel was prevented in seven of eight mutants when membrane-permeant MTSEA was scavenged by 20 mM intracellular cysteine, indicating that these seven mutants lie on the intracellular side of the channel gate. Further, MTSEA inhibited current through G342C in the absence of intracellular cysteine but augmented the current when cysteine was present, suggesting that this residue may be part of the gate. Taken together, the data help to the identify a functional domain of the channel pore by mapping residues on either side of the channel gate.

Spontaneous cell fusion in macrophage cultures expressing high levels of the P2Z/P2X7 receptor

Mouse and human macrophages express a plasma membrane receptor for extracellular ATP named P2Z/P2X7. This molecule, recently cloned, is endowed with the intriguing property of forming an aqueous pore that allows transmembrane fluxes of hydrophylic molecules of molecular weight below 900. The physiological function of this receptor is unknown. In a previous study we reported experiments suggesting that the P2Z/P2X7 receptor is involved in the formation of macrophage-derived multinucleated giant cells (MGCs; Falzoni, S., M. Munerati, D. Ferrari, S. Spisani, S. Moretti, and F. Di Virgilio. 1995. J. Clin. Invest. 95:1207- 1216). We have selected several clones of mouse J774 macrophages that are characterized by either high or low expression of the P2Z/P2X7 receptor and named these clones P2Zhyper or P2Zhypo, respectively. P2Zhyper, but not P2Zhypo, cells grown to confluence in culture spontaneously fuse to form MGCs. As previously shown for human macrophages, fusion is inhibited by the P2Z/P2X7 blocker oxidized ATP. MGCs die shortly after fusion through a dramatic process of cytoplasmic sepimentation followed by fragmentation. These observations support our previous hypothesis that the P2Z/P2X7 receptor is involved in macrophage fusion.