Molecular pharmacology of P2Y-receptors

P2 receptors in atherosclerosis and postangioplasty restenosis

Atherosclerosis is an immunoinflammatory process that involves complex interactions between the vessel wall and blood components and is thought to be initiated by endothelial dysfunction [1-3]. Extracellular nucleotides that are released from a variety of arterial and blood cells [4] can bind to P2 receptors and modulate proliferation and migration of smooth muscle cells (SMC), which is known to be involved in intimal hyperplasia that accompanies atherosclerosis and postangioplasty restenosis [5]. In addition, P2 receptors mediate many other functions, including platelet aggregation, leukocyte adherence, and arterial vasomotoricity. A direct pathological role of P2 receptors is reinforced by recent evidence showing that up-regulation and activation of P2Y(2) receptors in rabbit arteries mediates intimal hyperplasia [6]. In addition, up-regulation of functional P2Y receptors also has been demonstrated in the basilar artery of the rat double-hemorrhage model [7] and in coronary arteries of diabetic dyslipidemic pigs [8]. It has been proposed that up-regulation of P2Y receptors may be a potential diagnostic indicator for the early stages of atherosclerosis [9]. Therefore, particular effort must be made to understand the consequences of nucleotide release from cells in the cardiovascular system and the subsequent effects of P2 nucleotide receptor activation in blood vessels, which may reveal novel therapeutic strategies for atherosclerosis and restenosis after angioplasty.

Purinergic receptor expression and activation in first trimester and term human placenta

Intracellular calcium concentration ([Ca(2+)](i)) is an important signalling molecule in the human placenta and regulation of [Ca(2+)](i) must be tightly controlled to ensure normal cell function and in order to meet the changing demand for calcium with increased fetal growth over gestation. Little is known about the receptors and mechanisms involved in intracellular calcium signalling in the human placenta but in isolated cytotrophoblast cells members of the P2 purinergic receptor family have been shown to mediate an ATP-stimulated rise in [Ca(2+)](i). In this study we examined activation and expression of several of the purinergic receptor subtypes in human placental villous fragments at two stages of gestation, first trimester and term. We demonstrate mRNA and protein expression of the P2X(4), P2X(7) and P2Y(2) subtypes but found no evidence of P2Y(4) protein in the placenta. Using fluorescent calcium imaging we demonstrate that 300 microM ATP, 450 microM UTP and 300 microM BzATP significantly elevate [Ca(2+)](i) in villous fragments with a significant increase in agonist-induced response seen in the term compared to the first trimester fragments (ATP, P<0.0001; UTP, P=0.018; BzATP, P=0.015). The roles of the purinergic receptors within the human placenta are not known but it seems likely for this study that calcium handling through these receptors is altered with advancing gestation. This may be due to the need to meet increased fetal Ca(2+) requirements due to growth or as a secondary function to alterations in placental [Ca(2+)](i) signalling.

Schwann cells in rat vascular autonomic nerves activated via purinergic receptors

Schwann cells at the somatic neuromuscular junction possess adenosine receptors that when activated by the release of endogenous transmitter modulate quantal transmitter release. Recently, purinergic receptors have been shown to exist on Schwann cells of axon varicosities in visceral smooth muscle where they are activated by endogenous transmitters to give a calcium transient, although adenosine receptors were not identified. In the present work, we show that Schwann cells associated with axon varicosities of vascular smooth muscle, namely that of mesenteric blood vessels, possess both adenosine and purinergic receptors that when activated give rise to calcium transients in these cells. Then qualitative differences exist in the extent of adenosine and purine receptors that give rise to calcium transients in Schwann cells located in these different muscles.

Regulation of expression of early growth response transcription factors in rat primary cortical neurons by extracellular ATP

The zinc finger transcription factor early growth response-1 (Egr-1, NGFI-A, zif268, Krox 24, TIS8, ZENK) is upregulated immediately in the brain by cortical spreading depression (CSD) and other preconditioning stimuli and thus might participate in regulation of the overall genomic response to preconditioning. In the present study, the induction of expression of Egr-1 and other early growth response family members was characterized in rat primary cortical neuronal cultures. In neuronal cultures in vitro, depolarization or exposure to extracellular glutamate caused a 4-fold increase in egr-1 mRNA while exposure to extracellular ATP caused a 10-fold increase. The presence of mRNA encoding for multiple types of purinergic receptors was confirmed by RT-PCR. A number of nucleotide agonists proved effective in eliciting an increase in egr-1 mRNA. Over a limited range of concentration, the most effective agonists were ATP > ADP > alpha, beta-methylene ATP > UTP > cAMP > UDP > AMP > adenosine. Pertussis toxin, suramin, reactive blue 2, PPADS, DPCPX and inhibitors of Protein Kinase C, Protein Kinase A and PI3 kinase significantly reduced the upregulation of egr-1 by exposure to extracellular ATP. These findings suggest that neuronal metabotropic purinergic receptor activation contributes to the induction of early growth response transcription factors and may provide a target that can be manipulated to increase ischemic tolerance of the brain in vivo.

P2 receptors and neuronal injury

Extracellular adenosine 5'-triphosphate (ATP) was proposed to be an activity-dependent signaling molecule that regulates glia-glia and glia-neuron communications. ATP is a neurotransmitter of its own right and, in addition, a cotransmitter of other classical transmitters such as glutamate or GABA. The effects of ATP are mediated by two receptor families belonging either to the P2X (ligand-gated cationic channels) or P2Y (G protein-coupled receptors) types. P2X receptors are responsible for rapid synaptic responses, whereas P2Y receptors mediate slow synaptic responses and other types of purinergic signaling involved in neuronal damage/regeneration. ATP may act at pre- and postsynaptic sites and therefore, it may participate in the phenomena of long-term potentiation and long-term depression of excitatory synaptic transmission. The release of ATP into the extracellular space, e.g., by exocytosis, membrane transporters, and connexin hemichannels, is a widespread physiological process. However, ATP may also leave cells through their plasma membrane damaged by inflammation, ischemia, and mechanical injury. Functional responses to the activation of multiple P2 receptors were found in neurons and glial cells under normal and pathophysiological conditions. P2 receptor-activation could either be a cause or a consequence of neuronal cell death/glial activation and may be related to detrimental and/or beneficial effects. The present review aims at demonstrating that purinergic mechanisms correlate with the etiopathology of brain insults, especially because of the massive extracellular release of ATP, adenosine, and other neurotransmitters after brain injury. We will focus in this review on the most important P2 receptor-mediated neurodegenerative and neuroprotective processes and their beneficial modulation by possible therapeutic manipulations.

Nucleotides induce IL-6 release from human airway epithelia via P2Y2 and p38 MAPK-dependent pathways

Extracellular nucleotides can mediate a variety of cellular functions via interactions with purinergic receptors. We previously showed that mechanical ventilation (MV) induces airway IL-6 and ATP release, modifies luminal nucleotide composition, and alters lung purinoceptor expression. Here we hypothesize that extracellular nucleotides induce secretion of IL-6 by small airway epithelial cells (SAEC). Human SAEC were stimulated with nucleotides in the presence or absence of inhibitors. Supernatants were analyzed for IL-6 and lysates for p38 MAPK activity by ELISA. RNA was analyzed by real-time RT-PCR. Rats (n=51) were randomized to groups as follows: control, small-volume MV, large-volume MV, large-volume MV-intratracheal apyrase, or small-volume MV-intratracheal adenosine 5'-O-(3-thiotriphosphate) (ATPgammaS). After 1 h of MV, bronchoalveolar lavage fluid was analyzed for ATP and IL-6 by luminometry and ELISA. ATP and ATPgammaS increased SAEC IL-6 secretion in a time- and dose-dependent manner, an effect inhibited by apyrase. Agonists were ranked in the following order: ATPgammaS>ATP=UTP>ADP=adenosine>2-methylthio-ADP=control. SB-203580, but not U-0126 or JNK1 inhibitor, decreased nucleotide effects. Additionally, nucleotides induced p38 MAPK phosphorylation. Inhibitors of Ca2+ signaling, phospholipase C, transcription, and translation decreased IL-6 release. Furthermore, nucleotides increased IL-6 expression. In vivo, large-volume MV increased airway ATP and IL-6 concentrations. IL-6 release was decreased by apyrase and increased by ATPgammaS. Extracellular nucleotides induce P2Y2-mediated secretion of IL-6 by SAEC via Ca2+, phospholipase C, and p38 MAPK-dependent pathways. This effect is dependent on transcription and translation. Our findings were confirmed in an in vivo model, thus demonstrating a novel mechanism of nucleotide-induced IL-6 secretion by airway epithelia.

Direct excitation of deep dorsal horn neurones in the rat spinal cord by the activation of postsynaptic P2X receptors

ATP mediates somatosensory transmission in the spinal cord through the activation of P2X receptors. Nonetheless, the functional significance of postsynaptic P2X receptors in spinal deep dorsal horn neurones is still not yet well understood. Using the whole-cell patch-clamp technique, we investigated whether the activation of postsynaptic P2X receptors can modulate the synaptic transmission in lamina V neurones of postnatal day (P) 9-12 spinal cord slices. At a holding potential of -70 mV, ATPgammaS (100 microm), a nonhydrolysable ATP analogue, generated an inward current, which was resistant to tetrodotoxin (1 microm) in 61% of the lamina V neurones. The ATPgammaS-induced inward current was accompanied by a significant increase in the frequency of glutamatergic miniature excitatory postsynaptic currents (mEPSCs) in the majority of lamina V neurones. The ATPgammaS-induced inward current was not reproduced by P2Y receptor agonists, UTP (100 microm), UDP (100 microm), and 2-methylthio ADP (100 microm), and it was also not affected by the addition of guanosine-5'-O-(2-thiodiphosphate) (GDPbetaS) into the pipette solution, thus suggesting that ionotropic P2X receptors were activated by ATPgammaS instead of metabotropic P2Y receptors. On the other hand, alpha,beta-methylene ATP (100 microm) did not change any membrane current, but instead increased the mEPSC frequency in the majority of lamina V neurones. The ATPgammaS-induced inward current was suppressed by pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) (10 microm), but not by trinitrophenyl-ATP (TNP-ATP) (1 microm). Furthermore, we found that ATPgammaS (100 microm) produced a clear inward current which was observed in all lamina V neurones over P16 spinal cord slices, in contrast to P9-12. These results indicate that distinct subtypes of P2X receptors were functionally expressed at the post- and presynaptic sites in lamina V neurones, both of which may contribute to the hyperexcitability of lamina V in a different manner. In addition, the data relating to the developmental increase in the functional P2X receptors suggest that purinergic signalling may thus be more common in somatosensory transmission with maturation.

The P2Y purinergic receptor expressed by enteric neurones in guinea-pig intestine

Electrophysiological recording methods provided evidence for presynaptic release of ATP from enteric neurones and postganglionic sympathetic fibres in the enteric nervous system (ENS) of guinea-pig intestine (J Physiol Lond 2003; 550: 493-504). The released ATP acted at postsynaptic P2Y(1) receptors to evoke slow synaptic excitation in neurones in the submucosal division of the ENS. Here, we report the cloning and characterization of the P2Y(1) receptor, which was found in the guinea-pig submucosal layer. A 1178 bp cDNA clone was isolated from guinea-pig submucosal RNA by reverse transcription polymerase chain reaction (RT-PCR). The cDNA contained an open-reading frame of 1119 bp, encoding a 373 amino acid polypeptide of the same length and with 95% identity to the human P2Y(1) receptor. Stable expression of the guinea-pig cDNA in human embryonic kidney (HEK)293 cells was accompanied by a marked increase in sensitivity for elevation of free intracellular calcium evoked by ATP or related nucleotides. The potency order for ATP and its analogues was: 2-methio-adenosine diphosphate > 2-methio-adenosine triphosphate > ADP > ATP-gamma-S > ATP. The selective P2Y(1) receptor antagonist, MRS2179, was a competitive antagonist for the receptor with a pA(2) value of 6.5. The results add to existing evidence for expression of a functional P2Y(1) purinergic receptor in neurones of the submucosal division of the ENS.

Extracellular ATP activates ERK1/ERK2 via a metabotropic P2Y1 receptor in a Ca2+ independent manner in differentiated human skeletal muscle cells

ATP is released at the neuromuscular junction to regulate development and proliferation. The sequential expression of P2X and P2Y receptors has been correlated to these effects in many species and cell lines. We have therefore investigated ATP mediated signalling in differentiated primary human skeletal muscle cells. ATP was capable to trigger Ca2+ transients in these cells via P2Y receptors which were not attributable to Ca2+ influx via P2X receptors. Instead, ATP propagated the formation of inositol phosphate (IP) with an EC50 of 21.3 microM. The Ca2+ transient provoked by ATP was abrogated roughly 75% by the phospholipase C (PLC) inhibitor, U73122. Interestingly, the ryanodine sensitive Ca2+ pool was not involved in ATP triggered Ca2+ release. On mRNA level and by a pharmacological approach we confirmed the presence of the P2Y1, P2Y2, P2Y4 and P2Y6 receptors. Substantially, ATP activated IP formation via a P2Y1 receptor. In addition, ATP elicited extracellular signal regulated kinase (ERK)1/2 phosphorylation in a time and concentration dependent manner, again mainly via P2Y1 receptors. The ATP mediated ERK1/2 phosphorylation was strictly dependent on phospholipase C and PI3 kinase activity. Importantly, ATP mediated ERK1/2 phosphorylation was Ca2+ independent. This observation was corroborated by the finding that conventional protein kinase C inhibitors did not suppress ATP triggered ERK1/2 phosphorylation. Taken together, these observations highlight the importance of ATP as a co-neurotransmitter at the neuromuscular junction via dual signalling, i.e. IP3 receptor mediated Ca2+ transients and Ca2+ insensitive phosphorylation of ERK1/2.

The cAMP response to isoprenaline in mononuclear cells is markedly increased in the presence of platelets

We have previously shown that the addition of platelets to mononuclear cells (MNC) increases cAMP in MNC. This response may be of interest because the physical interaction between platelets and MNC plays an important role in the inflammatory process. We have now demonstrated that the addition of both isoprenaline and platelets to MNC resulted in a marked amplification of the cAMP response. Prostaglandins, ATP and adenosine and the P-selectin ligand PSGL-1 could not account for the response. No substance was found in the supernatant that could increase cAMP in MNC. W7, a Ca(2+)-calmodulin inhibitor and the addition of EDTA reduced the response to both platelets and isoprenaline. Furthermore, we have demonstrated that mRNA for type I adenylyl cyclase, which is sensitive to Ca(2+), is present in MNC. No increase in Ca(2+) in the cytoplasma in MNC was recorded, however, by quantitative fluorescence microscopy after addition of platelets to MNC. It is possible that there are small increments in Ca(2+) at the binding sites, which we were unable to detect by our technique. Alternatively the binding of platelets to MNC may induce intermolecular interactions in the cell membrane which may facilitate the synthesis of cAMP.

Extracellular nucleotides and adenosine independently activate AMP-activated protein kinase in endothelial cells: involvement of P2 receptors and adenosine transporters

AMP-activated protein kinase (AMPK) plays a key role in the regulation of energy homeostasis and is activated in response to cellular stress, including hypoxia/ischemia and hyperglycemia. The stress events are accompanied by rapid release of extracellular nucleotides from damaged tissues or activated endothelial cells (EC) and platelets. We demonstrate that extracellular nucleotides (ATP, ADP, and UTP, but not UDP) and adenosine independently induce phosphorylation and activation of AMPK in human umbilical vein EC (HUVEC) by the mechanism that is not linked to changes in AMP:ATP ratio. HUVEC express NTPDases, as well as 5'-nucleotidase; hence, nucleotides can be metabolized to adenosine. However, inhibition of 5'-nucleotidase had no effect on ATP/ADP/UTP-induced phospho- rylation of AMPK, indicating that AMPK activation occurred as a direct response to nucleotides. Nucleotide-evoked phosphorylation of AMPK in HUVEC was mediated by P2Y1, P2Y2, and/or P2Y4 receptors, whereas P2Y6, P2Y11, and P2X receptors were not involved. The nucleotide-induced phosphorylation of AMPK was affected by changes in the concentration of intracellular Ca2+ and by Ca2+/calmodulin-dependent kinase kinase (CaMKK), although most likely it was not dependent on LKB1 kinase. Adenosine-induced phosphorylation of AMPK was not mediated by P1 receptors but required adenosine uptake by equilibrative nucleoside transporters followed by its (intracellular) metabolism to AMP. Moreover, adenosine effect was Ca2+ and CaMKK independent, although probably associated with upstream LKB1. We hypothesize that P2 receptors and adenosine transporters could be novel targets for the pharmacological regulation of AMPK activity and its downstream effects on EC function.

The P2X7 receptor sustains the growth of human neuroblastoma cells through a substance P-dependent mechanism

P2X(7) is a receptor for extracellular nucleotides expressed by different normal cell types. P2X(7) triggering may result in stimulation of cell proliferation or induction of apoptosis depending on the level of activation. P2X(7) expression and function in B-cell chronic lymphocytic leukemia has been shown to correlate with disease severity. Here, we have asked the question of whether P2X(7) is expressed and functional in neuroblastoma, a pediatric tumor of neuroectodermal origin. P2X(7) was detected both in primary neuroblastoma tumors and in neuroblastoma cell lines. In the latter cells, P2X(7) stimulation by ATP was found to trigger (a) increased intracellular calcium fluxes, (b) plasma membrane depolarization, and (c) formation of a nonselective plasma membrane permeable pore. In contrast to the usual response typically observed in the majority of cell types, P2X(7) in vitro stimulation did not induce caspase-3 activation or apoptosis of neuroblastoma cells but rather supported their proliferation. Growth stimulation was partially due to substance P release from nucleotide-activated neuroblastoma cells. Therefore, neuroblastoma cells seem to have molded P2X(7) function to their advantage in two ways (i.e., by silencing P2X(7) proapoptotic activity and by coupling P2X(7) stimulation to release of locally acting trophic factors).

Ecto-nucleotidases of the CD39/NTPDase family modulate platelet activation and thrombus formation: Potential as therapeutic targets

Extracellular nucleotide P2-receptor-mediated effects on platelets, leukocytes and endothelium are modulated by ecto-nucleotidases. These ecto-enzymes hydrolyze extracellular nucleotides to the respective nucleosides. The dominant ecto-nucleotidase expressed by the endothelium, by monocytes and vascular smooth muscle cells is CD39/NTPDase1. Ecto-nucleotidase biochemical activity of CD39 is lost at sites of acute vascular injury, such as in ischemia reperfusion and immune graft rejection. CD39L(Like)1/NTPDase2, a related protein, is associated with the basolateral surface of endothelium, the adventitia of vessels and microvascular pericytes. CD39/NTPDase1 hydrolyzes both tri- and diphosphonucleosides and blocks platelet aggregation responses to ADP. In contrast, CD39L1/NTPDase2, a preferential nucleoside triphosphatase, activates platelets by preferentially converting ATP to ADP, the major agonist of platelet P2 receptors. Spatial and temporal expression of NTPDases in the vasculature appears to control platelet activation, thrombus size and stability by regulating phosphohydrolytic activity and consequent P2 receptor signaling. Constitutively circulating microparticles appear to be associated with functional NTPDases, and accumulation of these at sites of vascular injury might influence local thrombus formation and evolution. The phenotype of the cd39-null mouse is in keeping with disordered thromboregulation with heightened susceptibility to inflammatory vasculary reactions, increased permeability and high levels of tissue fibrin. Paradoxically, these mutant mice also exhibit a bleeding phenotype with differential platelet P2Y1 desensitization. Over-expression of CD39 at sites of vascular injury and inflammation by adenoviral vectors, by transgenesis or by the use of pharmacological modalities with soluble derivatives has been shown to have major potential in several animal models tested to date. Future clinical applications will involve the development of new therapeutic strategies to various inflammatory vascular diseases and in transplantation.

Mechanisms for inhibition of P2 receptors signaling in neural cells

Trophic factors are required to ensure neuronal viability and regeneration after neural injury. Although abundant information is available on the factors that cause the activation of astrocytes, little is known about the molecular mechanisms underlying the regulation of this process. Nucleotides released into the extracellular space from injured or dying neural cells can activate astrocytes via P2 nucleotide receptors. After a brief historical review and update of novel P2 receptor antagonists, this article focuses on recent advancements toward understanding molecular mechanisms that regulate G protein-coupled P2Y receptor signaling. Among P2Y receptor subtypes, the heptahelical P2Y2 nucleotide receptor interacts with vitronectin receptors via an RGD sequence in the first extracellular loop, and this interaction is required for effective signal transduction to activate mitogen-activated protein kinases ERK1/2, to mobilize intracellular calcium stores via activation of phospholipase C, protein kinase C isoforms, and to activate focal adhesion kinase and other signaling events. Ligation of vitronectin receptors with specific antibodies caused an inhibition of P2Y2 receptor-induced ERK1/2 and p38 phosphorylation and P2Y2 receptor-induced cytoskeleton rearrangement and DNA synthesis. Structure-function studies have identified agonist-induced phosphorylation of the C-terminus of the P2Y2 receptor, an important mechanism for receptor desensitization. Understanding selective mechanisms for regulating P2Y2 receptor signaling could provide novel targets for therapeutic strategies in the management of brain injury, synaptogenesis, and neurological disorders.

P2 receptor-stimulation influences axonal outgrowth in the developing hippocampus in vitro

Extracellular ATP might act as a trophic factor on growing axons during development of the CNS via P2 receptors. In the present study the postnatal presence of selected P2 receptor subtypes was analyzed and their putative trophic capacity in entorhino-hippocampal slice co-cultures of mouse brain was tested. The effect of the P2 receptor ligands 2-methylthioadenosine-5'-triphosphate (P2X/Y receptor agonist) and pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (P2X/Y receptor antagonist) on axonal growth and fiber density of biocytin-labeled hippocampal projections was compared both with untreated cultures and with cultures treated with artificial cerebrospinal fluid. After 10 days in vitro, double immunofluorescence labeling revealed the expression of P2X(1), P2X(2), P2X(4) as well as P2Y(1) and P2Y(2) receptors in the examined regions of entorhinal fiber termination. Further, quantitative analysis of identified biocytin-traced entorhinal fibers showed a significant increase in fiber density in the dentate gyrus after incubation of the slices with the P2 receptor agonist 2-methylthioadenosine-5'-triphosphate. This neurite outgrowth promoting effect was completely abolished by the P2 receptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid. Our in vitro data indicate that ATP via its P2X and P2Y receptors can shape hippocampal connectivity during development.

Neurally released ATP mediates endothelium-dependent hyperpolarization in the circular smooth muscle cells of chicken anterior mesenteric artery

The object of the present study was to clarify the neurotransmitter(s) controlling membrane responses to electrical field stimulation (EFS) in the circular smooth muscle cells of first-order branches of chicken anterior mesenteric artery.EFS (five pulses at 20 Hz, 1 ms) evoked a hyperpolarization of amplitude--21.6+/-1.2 mV, total duration 21.8+/-1.2 s and latency 641.7+/-81.9 ms. The response was tetrodotoxin-sensitive and nonadrenergic noncholinergic (NANC) in nature. The NANC response was blocked by the nonspecific purinergic antagonist, suramin, indicating that the response is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). Either desensitization or blockade of P2Y receptor with its putative agonist 2-methylthioATP (1 microM for 30 min) or with its antagonist cibacron blue F3GA (10 microM), respectively, abolished the purinergic hyperpolarization. PPADS at concentrations up to 100 microM had no effect on the EFS-induced response, indicating that this response is mediated through P2Y, but not P2X, receptor. In addition, the response was completely abolished by two specific P2Y1 receptor antagonists, namely, MRS 2179 (300 nM) and A3P5PS (10 microM). Removal of the endothelium abolished the purinergic hyperpolarization, which was converted, in some preparations, to a small depolarization, indicating that the hyperpolarizing response is endothelium-dependent. The present study suggests that in first-order branches of chicken anterior mesenteric artery, ATP released from perivascular nerves may diffuse to the endothelium-activating P2Y1 receptor to induce release of an inhibitory substance that mediates hyperpolarization in the circular smooth muscle.

Internalization and desensitization of a green fluorescent protein-tagged P2Y nucleotide receptor are differently controlled by inhibition of calmodulin-dependent protein kinase II

De- and re-sensitization and trafficking of P2Y nucleotide receptors modulate physiological responses of these receptors. Here, we used the rat brain P2Y1 receptor tagged with green fluorescent protein (P2Y1-GFP receptor) expressed in HEK293 human embryonic kidney cells. Ca2+ release was used as a functional test to investigate ATP-induced receptor de- and re-sensitization. By confocal laser scanning microscopy (CLSM), endocytosis of P2Y1-GFP receptor was visualized in live cells. Stimulation of the cells with ATP induced complete receptor endocytosis within 30 min and appearance of the P2Y1 receptor in small vesicles. Removal of the agonist resulted in reappearance of the receptor after 60 min on the plasma membrane. Exposure of the cells to KN-62 and KN-93, inhibitors of the calmodulin dependent protein kinase II (CaMKII), prevented receptor internalization upon stimulation with ATP. However, the receptor which was still present on the plasma membrane was desensitized, seen by decreased Ca2+ response. The decreased Ca2+ response after 30-min exposure to ATP can be attributed to desensitization and is not as a result of depletion of internal stores, as the cells exposed to ATP for 30 min exhibited a normal Ca2+ response upon stimulation with thrombin. However, okadaic acid, an inhibitor of protein phosphatase 2A (PP2A), did not affect ATP-induced P2Y1 receptor endocytosis, but delayed the reappearance of the P2Y1 receptor on the plasma membrane after ATP withdrawal. Consistently, in okadaic acid-treated cells the ATP-induced Ca2+ response observed after the 30-min exposure to ATP recovered only partially. Thus, CaMKII seems to be involved in P2Y1 receptor internalization, but not desensitization, whereas protein phosphatase 2A might play a role in recycling of the receptor back to the plasma membrane.

Immune and inflammatory mechanisms in neuropathic pain

Tissue damage, inflammation or injury of the nervous system may result in chronic neuropathic pain characterised by increased sensitivity to painful stimuli (hyperalgesia), the perception of innocuous stimuli as painful (allodynia) and spontaneous pain. Neuropathic pain has been described in about 1% of the US population, is often severely debilitating and largely resistant to treatment. Animal models of peripheral neuropathic pain are now available in which the mechanisms underlying hyperalgesia and allodynia due to nerve injury or nerve inflammation can be analysed. Recently, it has become clear that inflammatory and immune mechanisms both in the periphery and the central nervous system play an important role in neuropathic pain. Infiltration of inflammatory cells, as well as activation of resident immune cells in response to nervous system damage, leads to subsequent production and secretion of various inflammatory mediators. These mediators promote neuroimmune activation and can sensitise primary afferent neurones and contribute to pain hypersensitivity. Inflammatory cells such as mast cells, neutrophils, macrophages and T lymphocytes have all been implicated, as have immune-like glial cells such as microglia and astrocytes. In addition, the immune response plays an important role in demyelinating neuropathies such as multiple sclerosis (MS), in which pain is a common symptom, and an animal model of MS-related pain has recently been demonstrated. Here, we will briefly review some of the milestones in research that have led to an increased awareness of the contribution of immune and inflammatory systems to neuropathic pain and then review in more detail the role of immune cells and inflammatory mediators.

Purinergic receptors in human placenta: evidence for functionally active P2X4, P2X7, P2Y2, and P2Y6

Appropriate regulation of ion transport by the human placental syncytiotrophoblast is important for fetal growth throughout pregnancy. In nonplacental tissues, ion transport can be modulated by extracellular nucleotides that raise intracellular calcium ([Ca2+]i) via activation of purinergic receptors. We tested the hypothesis that purinergic receptors are expressed by human placental cytotrophoblast cells and that their activation by extracellular nucleotides modulates ion (K+) efflux and [Ca2+]i. P2X/P2Y receptor agonists 5-bromouridine 5'-triphosphate (5-BrUTP), ADP, ATP, 2',3'-O-(4-benzoyl-benzoyl)adenosine 5'-triphosphate (BzATP), and UTP stimulated 86Rb (K+ tracer) efflux from cultured cytotrophoblast cells at early (mononuclear) or later (multinucleate syncytiotrophoblast-like) stages of differentiation, with ATP and UTP particularly potent. 2-Methylthioadenosine 5'-triphosphate (2-MeS-ATP), and UDP elevated 86Rb efflux only from multinucleated cells. All agonists caused a significant peak and plateau increase in [Ca2+]i, although the magnitude of responses was variable. The effect of BzATP, UTP, and UDP in multinucleated cells was unaffected, and that of ATP partially inhibited, by removal of extracellular Ca2+, implicating P2Y receptor activation. mRNA encoding P2X1, P2X2, P2X4, and P2X7 and P2Y1, P2Y2, P2Y4, P2Y6, and P2Y11 were identified in mono- and multinucleated cells, whereas P2X3 and P2X5 mRNA were absent from all samples. Western blot analysis revealed P2X4, P2X7, P2Y2, and P2Y6 protein in cytotrophoblast cells, but P2Y4 was not detected. On the basis of published agonist selectivity, the data indicate the presence of functionally active P2X4, P2X7, P2Y2, and P2Y6 receptors in cytotrophoblast cells. We propose that activation of these receptors, and subsequent elevation of [Ca2+]i, modulates syncytiotrophoblast homeostasis and/or maternofetal ion exchange in response to extracellular nucleotides.

P2X(7) nucleotide receptors mediate caspase-8/9/3-dependent apoptosis in rat primary cortical neurons

Apoptosis is a major cause of cell death in the nervous system. It plays a role in embryonic and early postnatal brain development and contributes to the pathology of neurodegenerative diseases. Here, we report that activation of the P2X₇ nucleotide receptor (P2X₇R) in rat primary cortical neurons (rPCNs) causes biochemical (i.e., caspase activation) and morphological (i.e., nuclear condensation and DNA fragmentation) changes characteristic of apoptotic cell death. Caspase-3 activation and DNA fragmentation in rPCNs induced by the P2X₇R agonist BzATP were inhibited by the P2X₇R antagonist oxidized ATP (oATP) or by pre-treatment of cells with P2X₇R antisense oligonucleotide indicating a direct involvement of the P2X₇R in nucleotide-induced neuronal cell death. Moreover, Z-DEVD-FMK, a specific and irreversible cell permeable inhibitor of caspase-3, prevented BzATP-induced apoptosis in rPCNs. In addition, a specific caspase-8 inhibitor, Ac-IETD-CHO, significantly attenuated BzATP-induced caspase-9 and caspase-3 activation, suggesting that P2X₇R-mediated apoptosis in rPCNs occurs primarily through an intrinsic caspase-8/9/3 activation pathway. BzATP also induced the activation of C-jun N-terminal kinase 1 (JNK1) and extracellular signal-regulated kinases (ERK1/2) in rPCNs, and pharmacological inhibition of either JNK1 or ERK1/2 significantly reduced caspase activation by BzATP. Taken together, these data indicate that extracellular nucleotides mediate neuronal apoptosis through activation of P2X₇Rs and their downstream signaling pathways involving JNK1, ERK and caspases 8/9/3.

Human neutrophil peptides induce interleukin-8 production through the P2Y6 signaling pathway

Antimicrobial human neutrophil peptides (HNPs) play a pivotal role in innate host defense against a broad spectrum of prokaryotic pathogens. In addition, HNPs modulate cellular immune responses by producing the chemokine interleukin-8 (IL-8) in myeloid and epithelial cells and by exerting chemotaxis to T cells, immature dendritic cells, and monocytes. However, the mechanisms by which HNPs modulate the immune responses in the eukaryotic cells remain unclear. We demonstrated that, as with adenosine triphosphate (ATP) and uridine diphosphate (UDP), HNP stimulation of human lung epithelial cells selectively induced IL-8 production in 10 pro- and anti-inflammatory cytokines examined. HNP-induced IL-8 release was inhibited by treatment with the nucleotide receptor antagonists suramin and reactive blue. Transfection of lung epithelial cells with antisense oligonucleotides targeting specific purinergic P2Y receptors revealed that the P2Y6 (ligand of UDP) signaling pathway plays a predominant role in mediating HNP-induced IL-8 production.

Purinoceptors are involved in the induction of an osmolyte permeability in malaria-infected and oxidized human erythrocytes

In human erythrocytes, infection by the malaria parasite Plasmodium falciparum or oxidative stress induces a new organic osmolyte and anion permeability. To examine a role for autocrine purinoceptor signaling during this induction process, erythrocytic purinoceptor expression, and ATP release were determined. Furthermore, using pharmacological and genetic approaches the dependence on purinoceptor signaling of osmolyte permeability and Plasmodium development, both in vitro and in vivo, were assessed. Extracellular ATP did not induce an osmolyte permeability in non-infected or non-oxidized erythrocytes. ATP and other purinoceptor agonists increased the induction of osmolyte permeability during infection or oxidation as measured by isosmotic hemolysis and patch-clamp recording. Purinoceptor antagonists and apyrase decreased the induced permeability. The observed pharmacology suggested the involvement of P2Y purinoceptors. Accordingly, human erythrocytes expressed P2Y1 protein. Moreover, P2Y1-deficient mouse erythrocytes exhibited a delayed appearance of the osmolyte permeability during P. berghei infection- or oxidation compared with wild-type erythrocytes. Furthermore, the nonspecific purinoceptor antagonist suramin decreased in vitro growth and DNA/RNA amplification of P. falciparum in human erythrocytes and decreased in vivo growth of P. berghei. P. berghei developed slower in P2Y1-deficient mice in vivo compared with wild-type animals. In conclusion, induction of the osmolyte permeability in Plasmodium-infected erythrocytes involves autocrine purinoceptor signaling.

Purinergic P2Y6 receptors induce Ca2+ and CFTR dependent Cl- secretion in mouse trachea

In airways Cl- secretion is activated and Na+ absorption is inhibited when P2Y2 receptors are stimulated by ATP or UTP. Both nucleotides are subject to degradation to ADP and UDP by ecto-nucleotidases. Here we show that these metabolites change electrolyte transport by stimulation of P2Y6 receptors in mouse trachea. Immunohistochemistry confirmed luminal and basolateral expression of P2Y6 receptors. In Ussing chamber experiments luminal ADP, UDP or the P2Y6 receptor agonist INS48823 induced both transient and persistent increase in short circuit currents (ISC). Activation of ISC was inhibited by the P2Y6 receptor blocker PPADS. The transient response was inhibited by DIDS, whereas the persistent ISC was inhibited by glibenclamide and by the protein kinase A (PKA) blocker H-89. Moreover, sustained activation of ISC by luminal UDP was inhibited by blocking basolateral K+ channels with 293B. Possible effects of diphosphates on P2Y1 or adenosine receptors were excluded by the inhibitors MRS2179 and 8-SPT, respectively. Inhibition of amiloride sensitive Na+ absorption was only seen after blocking basolateral K+ channels with 293B. In contrast, Cl- secretion activated by basolateral ADP or UDP was only transient and was blocked by the sk4 K+ channel blocker clotrimazole. In summary, activation of luminal P2Y6 receptors in the airways shifts electrolyte transport towards secretion by increasing intracellular Ca+ and activation of PKA.

P2Y2 agonist induces mucin secretion via Ca2+- and inositol 1,4,5-triphosphate-dependent pathway in human middle ear epithelial cells

Purinergic agonists regulate mucin secretion in the airway epithelial cells. This study examined the effects of the apical application of purinergic agonists on Ca(2+) influx ([Ca(2+)](i)), and mucin secretion along with their underlying signaling pathway in normal human middle ear epithelial (NHMEE) cells. The apical membrane of NHMEE cells were stimulated with various purinergic agonists, including UTP, and the [Ca(2+)](i) was measured using a miniature Ussing double perfusion chamber. P2Y(2) receptor in NHMEE cells was also localized by immunohistochemistry. UTP-induced mucin secretion was quantified by an immunoblotting assay. The order of the purinergic agonist potency with respect to [Ca(2+)](i) determined in this study was ATP = UTP > 2-MeSATP > UDP >> adenosine which is consistent with that obtained from P2Y(2) receptor activation. The P2Y(2) receptor is expressed in the apical membrane of monolayered cultured NHMEE cells. Apical UTP-induced [Ca(2+)](i) was inhibited by 2-aminoethoxydiphenyl borate (2-APB) but not by ryanodine. UTP-induced mucin secretion was inhibited by a Ca(2+) chelating agent, BAPTA-AM, and was stimulated by ionomycin. UTP-induced mucin secretion was also suppressed by U73122 and 2-APB, while Calphostin C suppressed it to a small extent and PD98059 was ineffective. Caffeine also inhibited the UTP-induced [Ca(2+)](i) and mucin secretion. These results suggest that the P2Y(2) receptor is expressed in NHMEE cells, and plays a major role in modulating the [Ca(2+)](i) from the IP(3)-sensitive intracellular Ca(2+) store. UTP-induced mucin secretion in NHMEE cells is strongly dependent on Ca(2+)- and IP(3).

P2X2 receptors are essential for [Ca2+]i increases in response to ATP in cultured rat myenteric neurons

We characterized ATP-induced changes in intracellular Ca2+ concentration ([Ca2+]i) and membrane current in cultured rat myenteric neurons using ratiometric Ca2+ imaging with fura-2 and the whole cell patch-clamp technique, respectively. Neuronal cells were functionally identified by [Ca2+]i responses to high K+ and nicotine, which occurred only in cells positive for neuron-specific protein gene product 9.5 immunoreactivity. ATP evoked a dose-dependent increase of [Ca2+]i that was greatly decreased by the removal of extracellular Ca2+ concentration ([Ca2+]o). The amplitude of the [Ca2+]i response to ATP was reduced by half in the presence of voltage-dependent Ca2+ channel blockers. In [Ca2+]o-free solution, ATP produced a small transient rise in [Ca2+]i similar to that induced by P2Y agonists. At -60 mV, ATP evoked a slowly inactivating inward current that was suppressed by the removal of extracellular Na+ concentration. The current-voltage relation for ATP showed an inward rectification with the reversal potential of about 0 mV. The apparent rank order of potency for the purinoceptor agonist-induced increases of [Ca2+]i was ATP > or = adenosine 5'-O-3-triphosphate > or = CTP > or = 2-methylthio-ATP > benzoylbenzoyl-ATP. A similar potency order was obtained with current responses to these agonists. P2 antagonists inhibited inward currents induced by ATP. Ca2+ and Mg2+ suppressed the ATP-induced current, and Zn2+, Cu2+, and protons potentiated it. RT-PCR and immunocytochemical studies showed the expression of P2X2 receptors in cultured rat myenteric neurons. These results suggest that ATP mainly activates ionotropic P2X2 receptors, resulting in a [Ca2+]i increase dependent on [Ca2+]o in rat myenteric neurons. A small part of the ATP-induced [Ca2+]i increase may be also mediated via a P2Y receptor-related mechanism.

Pharmacological profiles of cloned mammalian P2Y-receptor subtypes

Membrane-bound P2-receptors mediate the actions of extracellular nucleotides in cell-to-cell signalling. P2X-receptors are ligand-gated ion channels, whereas P2Y-receptors belong to the superfamily of G-protein-coupled receptors (GPCRs). So far, the P2Y family is composed out of 8 human subtypes that have been cloned and functionally defined; species orthologues have been found in many vertebrates. P2Y1-, P2Y2-, P2Y4-, P2Y6-, and P2Y11-receptors all couple to stimulation of phospholipase C. The P2Y11-receptor mediates in addition a stimulation of adenylate cyclase. In contrast, activation of the P2Y12-, P2Y13-, and P2Y14-receptors causes an inhibition of adenylate cyclase activity. The expression of P2Y1-receptors is widespread. The receptor is involved in blood platelet aggregation, vasodilatation and neuromodulation. It is activated by ADP and ADP analogues including 2-methylthio-ADP (2-MeSADP). 2'-Deoxy-N6-methyladenosine-3',5'-bisphosphate (MRS2179) and 2-chloro-N6-methyl-(N)-methanocarba-2'-deoxyadenosine 3',5'-bisphosphate (MRS2279) are potent and selective antagonists. P2Y2 transcripts are abundantly distributed. One important example for its functional role is the control of chloride ion fluxes in airway epithelia. The P2Y2-receptor is activated by UTP and ATP and blocked by suramin. The P2Y2-agonist diquafosol is used for the treatment of the dry eye disease. P2Y4-receptors are expressed in the placenta and in epithelia. The human P2Y4-receptor has a strong preference for UTP as agonist, whereas the rat P2Y4-receptor is activated about equally by UTP and ATP. The P2Y4-receptor is not blocked by suramin. The P2Y6-receptor has a widespread distribution including heart, blood vessels, and brain. The receptor prefers UDP as agonist and is selectively blocked by 1,2-di-(4-isothiocyanatophenyl)ethane (MRS2567). The P2Y11-receptor may play a role in the differentiation of immunocytes. The human P2Y11-receptor is activated by ATP as naturally occurring agonist and it is blocked by suramin and reactive blue 2 (RB2). The P2Y12-receptor plays a crucial role in platelet aggregation as well as in inhibition of neuronal cells. It is activated by ADP and very potently by 2-methylthio-ADP. Nucleotide antagonists including N6-(2-methylthioethyl)-2-(3,3,3-trifluoropropylthio)-beta,gamma-dichloromethylene-ATP (=cangrelor; AR-C69931MX), the nucleoside analogue AZD6140, as well as active metabolites of the thienopyridine compounds clopidogrel and prasugrel block the receptor. These P2Y12-antagonists are used in pharmacotherapy to inhibit platelet aggregation. The P2Y13-receptor is expressed in immunocytes and neuronal cells and is again activated by ADP and 2-methylthio-ADP. The 2-chloro-5-nitro pyridoxal-phosphate analogue 6-(2'-chloro-5'-nitro-azophenyl)-pyridoxal-alpha5-phosphate (MRS2211) is a selective antagonist. mRNA encoding for the human P2Y14-receptor is found in many tissues. However, a physiological role of the receptor has not yet been established. UDP-glucose and related analogues act as agonists; antagonists are not known. Finally, UDP has been reported to act on receptors for cysteinyl leukotrienes as an additional agonist--indicating a dual agonist specificity of these receptors.

Activation of human eosinophils via P2 receptors: novel findings and future perspectives

A growing body of information indicates that release of intracellular nucleotides represents an important way to modulate several cell pathways in physiological or pathological conditions. Nucleotides released as a consequence of cell damage, cell stress, bacterial infection, or other noxious stimuli signal at a class of plasma membrane receptors--P2 receptors--activating diverse intracellular pathways in many tissues and organs. For example, nucleotides secreted in the airway system control chloride/liquid secretion, goblet cell degranulation, and ciliary beat frequency. Several studies indicate that nucleotides play a role in airway diseases through their action on multiple cell types, including mast cells, dendritic cells, neurons, and eosinophils. Recent work by us and other groups led to the identification and characterization of P2 receptors expressed by human eosinophils. In this review, we will summarize recent developments in this field and put forward a hypothesis about the role of P2 receptors in pathophysiological conditions where eosinophils are major players.

Synthesis and preliminary evaluation of [3H]PSB-0413, a selective antagonist radioligand for platelet P2Y12 receptors

The selective antagonist radioligand [(3)H]2-propylthioadenosine-5'-adenylic acid (1,1-dichloro-1-phosphonomethyl-1-phosphonyl) anhydride ([(3)H]PSB-0413) was prepared by catalytic hydrogenation of its propargyl precursor with a high specific radioactivity of 74Ci/mmol. In preliminary saturation binding studies, [(3)H]PSB-0413 showed high affinity for platelet P2Y(12) receptors with a K(D) value of 4.57nM. Human platelets had a high density of P2Y(12) receptors exhibiting a B(max) value of 7.66pmol/mg of protein.

Differential Regulation of Ca2+ Signaling and Membrane Trafficking by Multiple P2 Receptors in Brown Adipocytes

Extracellular ATP triggers changes in intracellular Ca(2+), ion channel function, and membrane trafficking in adipocytes. The aim of the present study was to determine which P2 receptors might mediate the Ca(2+) signaling and membrane trafficking responses to ATP in brown fat cells. RT-PCR was used to determine which P2 receptors are expressed in brown fat cells. Responses to nucleotide agonists and antagonists were characterized using fura-2 fluorescence imaging of Ca(2+) responses, and FM 1-43 fluorescence imaging and membrane capacitance measurements to assess membrane trafficking. The pharmacology of the Ca(2+) responses fits the properties of the P2Y receptors for which mRNA is expressed, but the agonist and antagonist sensitivity of the membrane-trafficking response was not consistent with any P2 receptor described to date. Brown adipocytes expressed mRNA for P2Y(2), P2Y(6), and P2Y(12) metabotropic receptors and P2X(1), P2X(2), P2X(3), P2X(4), P2X(5), and P2X(7) ionotropic receptors. The agonists ATP, ADP, UTP, UDP and 2', 3'-(benzoylbenzoyl) ATP (BzATP) increased intracellular Ca(2+), while 100 microM: suramin, pyridoxal-phosphate-6-azophenyl-2' 4'-disulfonic acid (PPADS), or Reactive Blue 2 partially blocked Ca(2+) responses. ATP, but not ADP, UTP, UDP or BzATP activated membrane trafficking. The membrane response could be blocked completely with 1 microM: PPADS but not by the antagonist MRS2179. We conclude that multiple P2 receptors mediate the ATP responses of brown fat cells, and that membrane trafficking is regulated by a P2 receptor showing unusual properties.

Purinergic receptor modulation of BV-2 microglial cell activity: potential involvement of p38 MAP kinase and CREB

ATP is abundant in the extracellular fluid following brain injury, and it exerts potent modulatory effects on microglia, whose hyperactivation is thought to exacerbate neuronal damage. We show here that ATP decreases LPS-stimulated iNOS and COX-2 expression and reduces NO release in BV-2 microglia by a mechanism involving p38 MAP kinase. Further, we demonstrate that the inhibitory effects of ATP on NO production occur within 30 min of exposure and correlate with activation of the transcription factor CREB. Together, these data suggest that ATP may exert neuroprotective effects in the brain via a mechanism involving augmented activation of the p38/CREB pathway.

Metabotropic P2Y1 receptors inhibit P2X3 receptor-channels in rat dorsal root ganglion neurons

Whole-cell patch-clamp recordings from cultured rat dorsal root ganglion neurons demonstrated that the P2Y1 receptor agonists adenosine 5'-O-2-thiodiphosphate (ADP-beta-S) and 2-methylthio adenosine 5'-diphosphate (2-MeSADP) inhibit the alpha,beta-methylene adenosine 5'-triphosphate (alpha,beta-meATP)-induced P2X3 receptor-currents. This effect could be antagonized by the wide-spectrum G protein blocker GDP-beta-S and the P2Y(1) receptor antagonist MRS 2179. The P2Y12,13 receptor antagonist AR-C6993MX and pertussis toxin, a blocker of Galphai/o, did not interact with the effect of ADP-beta-S. Hence, the results indicate that ADP-sensitive P2Y1 receptors of rat dorsal root ganglion neurons inhibit ionotropic P2X3 receptors via G protein-activation.

The novel suramin analogue NF864 selectively blocks P2X1 receptors in human platelets with potency in the low nanomolar range

The role of ATP-stimulated P2X1 receptors in human platelets is still unclear. They may act alone or in synergy with other pathways, such as P2Y1 or P2Y12 receptors, to accelerate and enhance calcium mobilisation, shape change and aggregation. To date very few pharmacological means of selectively inhibiting platelet P2X1 receptors have been described, although recent work has shown that suramin is a useful lead compound for the development of high-affinity P2X1 antagonists. We therefore investigated the effects of a series of bivalent and tetravalent suramin analogues on alphabeta meATP (P2X1 receptors)-induced or ADP (P2Y1 receptors)-induced intracellular calcium increases and shape change, as well as on ADP-induced aggregation (P2Y1 & P2Y12 receptors) in human platelets. Changes in intracellular calcium were measured using standard fluorescence techniques, while shape change and aggregation were determined by turbidimetry. The novel tetravalent compound NF864 (8,8',8'',8'''-(carbonylbis(imino-5,1,3-benzenetriyl-bis(carbonylimino)))tetrakis-naphthalene-1,3,5-trisulfonic acid-dodecasodium salt) proved to be the most potent platelet P2X1 antagonist reported to date, blocking alphabeta meATP-induced Ca2+ increases and shape change in a concentration-dependent manner, with a pA2 of 8.17 and 8.49, respectively. The ability to inhibit the platelet P2X1 receptor displayed the following order : NF864 > NF449 > or = NF110 > NF023 = MK-HU1 = suramin. A different antagonistic profile was observed for ADP-induced Ca2+ increases, shape change and aggregation; however, overall four compounds showed sufficient ability to selectively inhibit P2X1 responses, with the order NF110 > NF449 > or = NF864 > or = MK-HU1. Therefore, these compounds should prove useful tools for investigating the functional significance of platelet P2X1 receptors in thrombosis and haemostasis, NF864 being the most promising compound.

Structure-activity relationships of novel P2-receptor antagonists structurally related to Reactive Blue 2

P2 membrane receptors for nucleotides represent significant targets for experimental pharmacology and drug research. In earlier publications, we have shown that Reactive Blue 2 (RB 2), one of the most widely used P2-receptor antagonists, displays only moderate affinity and does not discriminate between native P2X- and P2Y-receptor subtypes. In the present study we have pharmacologically evaluated a series of 15 synthesized and re-evaluated four commercially obtained and chromatographically purified RB 2 type anthraquinone derivatives on contractions of the rat vas deferens (RVD) elicited by alpha,beta-methylene ATP (alpha,beta-meATP), mediated by P2X1-receptors, and relaxations of the carbachol-precontracted guinea-pig taenia coli (GPTC) elicited by adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), mediated by P2Y1-like receptors. Based on the structure-activity relationships (SAR) it is concluded that hydrophobic interactions of aromatic pi-electron systems, hydrogen bonds with nitrogen as donor and acceptor atoms, and, particularly, position, conformational distance and number of anionic sulfonate groups are of great importance for the blockade of the two native P2-receptor subtypes. We have also identified novel, for the most part reversible antagonists that bind with higher affinity and improved subtype selectivity in comparison to RB 2. In particular, 1-amino-4-{4-[4-chloro-6-(2-sulfonatophenylamino)-[1,3,5]triazine-2-ylamino]-2-sulfonatophenylamino}-9,10-dioxo-9,10-dihydroanthracene-2-sulfonic acid trisodium salt (MG 50-3-1) is the most potent antagonist at the P2Y1-like-receptors of the GPTC reported so far (IC50=4.6 nM). It is significantly less potent as reversible antagonist at the P2X1-receptors of the RVD (IC50=2.8 microM). Thus, MG 50-3-1 represents a selective pharmacological tool and may be a lead compound for future investigations.

Involvement of P2 receptors in the growth and survival of neurons in the CNS

Extracellular adenosine 5'-triphosphate (ATP) has been recognized as a ubiquitous, unstable signalling molecule, acting as a fast neurotransmitter and modulator of transmitter release and neuronal excitability. Recent findings have demonstrated that ATP is a growth factor participating in differentiation, cell proliferation, and survival, as well as a toxic agent that mediates cellular degeneration and death. Potential sources of extracellular purines in the nervous system include neurons, glia, endothelium, and blood. A complex family of ectoenzymes rapidly hydrolyzes or interconverts extracellular nucleotides, thereby either terminating their signalling action or producing an active metabolite of altered purinoceptor selectivity. Most effects are mediated through the 2 main subclasses of specific cell surface receptors, P2X and P2Y. Members of these P2X/Y receptor families are widely expressed in the central nervous system (CNS) and are involved in glia-glia and glia-neuron communications, whereby they play important physiological and pathophysiological roles in a variety of biological processes. After different kinds of "acute" CNS injury (e.g., ischemia, hypoxia, mechanical stress, axotomy), extracellular ATP can reach high concentrations, up to the millimolar range, flowing out from cells into the extracellular space, exocytotically, via transmembrane transport, or as a result of cell damage. In this review, P2 receptor activation as a cause or a consequence of neuronal cell activation or death and/or glial activation is described. The involvement of P2 receptors is also described under different "chronic" pathological conditions, such as pain, epilepsia, toxic influence of ethanol or amphetamine, retinal diseases, Alzheimer's disease (AD), and possibly, Parkinson's disease. The relationship between changes in P2 receptor expression and the specific response of different cell types to injury is extremely complex and can be related to detrimental and/or beneficial effects. The present review therefore considers ATP acting via P2 receptors as a potent regulator of normal physiological and pathological processes in the brain, with a focus on pathophysiological implications of P2 receptor functions.

The extracellular zinc-sensing receptor mediates intercellular communication by inducing ATP release

Taste and salivary secretion disorders have been linked to zinc deficiency, indeed zinc is found in secretory granules in the salivary gland. The signaling role for the zinc release in this tissue, however, is poorly understood. Here, we address the signaling pathways and physiological role of the zinc-sensing receptor, ZnR, in the ductal salivary gland cell line, HSY. Exposure of these cells to zinc triggered intracellular Ca2+ release from thapsigargin-sensitive stores. The G alpha q inhibitor, YM-254890 (1 microM), eliminated the Zn2+-dependent Ca2+ response, demonstrating that ZnR is a G alpha q-coupled receptor. Dose-response curves yielded an apparent K0.5 of 36 microM and a Hill coefficient of 7 in the absence of extracellular Ca2+, and K0.5 of 55 microM with a Hill coefficient of 3 in its presence. This indicates that although Zn2+ is essential for ZnR activation, Ca2+ may affect the receptor co-operativity. The homologous desensitization pattern of ZnR was characterized by pre-exposure of cells to Zn2+ at concentrations found to activate the receptor. Re-exposure of cells to Zn2+ elicited an attenuated Zn2+-dependent Ca2+ response for at least 3 h, indicating that the ZnR is strongly desensitized by Zn2+. Finally, we studied the paracrine affects of ZnR using a co-culture consisting of the HSY cells and vascular smooth muscle cells (VSMCs). While no Zn2+-dependent Ca2+ release was observed in VSMC alone, application of Zn2+ to the co-culture induced a Ca2+ rise in both HSY cells and VSMC. This Ca2+ rise was inhibited by the ATP scavenger, apyrase. Taken together, our results demonstrate that ZnR activity is monitored in salivary cells and is modulated by extracellular Ca2+. We further show that ZnR enhances secretion of ATP, thereby linking zinc to key signaling pathways involved in modification of salivary secretions by the ductal cells.

Disruption of a new X linked gene highly expressed in brain in a family with two mentally retarded males

BACKGROUND

Mental retardation (MR) affects 2-3% of the human population and some of these cases are genetically determined. Although several genes responsible for MR have been identified, many cases have still not been explained.

METHODS

We have identified a pericentric inversion of the X chromosome inv(X)(p22.3;q13.2) segregating in a family where two male carriers have severe MR while female carriers are not affected.

RESULTS

The molecular characterisation of this inversion led us to identify two new genes which are disrupted by the breakpoints: KIAA2022 in Xq13.2 and P2RY8 in Xp22.3. These genes were not previously fully characterised in humans. KIAA2022 encodes a protein which lacks significant homology to any other known protein and is highly expressed in the brain. P2RY8 is a member of the purine nucleotide G-protein coupled receptor gene family. It is located in the pseudo-autosomal region of the X chromosome and is not expressed in brain.

CONCLUSIONS

Because the haploinsufficiency of P2RY8 in carrier mothers does not have a phenotypic consequence, we propose that the severe MR of the affected males in this family is due to the absence of the KIAA2022 gene product. However, screening 20 probands from X linked MR families did not reveal mutations in KIAA2022. Nonetheless, the high expression of this gene in fetal brain and in the adult cerebral cortex could be consistent with a role in brain development and/or cognitive function.

Activation of APE1/Ref-1 is dependent on reactive oxygen species generated after purinergic receptor stimulation by ATP

Apurinic apyrimidinic endonuclease redox effector factor-1 (APE1/Ref-1) is involved both in the base excision repair (BER) of DNA lesions and in the eukaryotic transcriptional regulation. APE1/Ref-1 is regulated at both the transcriptional and post-translational levels, through control of subcellular localization and post-translational modification. In response to stress conditions, several cell types release ATP, which exerts stimulatory effects on eukaryotic cells via the purinergic receptors (P2) family. By using western blot and immunofluorescence analysis on a human tumour thyroid cell line (ARO), we demonstrate that purinergic stimulation by extracellular ATP induces quick cytoplasm to nucleus translocation of the protein at early times and its neosynthesis at later times. Continuous purinergic triggering by extracellular ATP released by ARO cells is responsible for the control of APE1/Ref-1 intracellular level. Interference with intracellular pathways activated by P2 triggering demonstrates that Ca²⁺ mobilization and intracellular reactive oxygen species (ROS) production are responsible for APE1/Ref-1 translocation. The APE1/Ref-1 activities on activator protein-1 (AP-1) DNA binding and DNA repair perfectly match its nuclear enrichment upon ATP stimulation. The biological relevance of our data is reinforced by the observation that APE1/Ref-1 stimulation by ATP protects ARO cells by H₂O₂-induced cell death. Our data provide new insights into the complex mechanisms regulating APE1/Ref-1 functions.

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.

Evidence for the existence of P2Y1,2,4 receptor subtypes in HEK-293 cells: reactivation of P2Y1 receptors after repetitive agonist application

ATP, ADPbetaS and UTP induced a comparable rise in the intracellular Ca2+ concentration ([Ca2+]i) in HEK-293 cells using fura-2 microfluorimetry. The responses persisted in Ca2+-free medium, but were abolished following depletion of intracellular Ca2+ stores by cyclopiazonic acid. Cross-desensitisation experiments demonstrated that exposure to ADPbetaS has no marked effect on UTP-induced [Ca2+]i transients and vice versa. Whereas the P2Y1 receptor-selective antagonist 2'-deoxy-N6-methyladenosine 3',5'-diphosphate (MRS 2179) abolished the responses to ADPbetaS, it decreased and did not alter the responses to ATP and UTP respectively. Although the P2Y1/P2Y4 receptor-preferential antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulphonic acid (PPADS) abolished the responses to ADPbetaS, and decreased those to ATP, it also depressed the UTP-induced [Ca2+]i transients. Suramin, an antagonist with preference for P2Y2 receptors decreased both the ATP- and UTP-induced [Ca2+]i reactions. After numerous splittings, HEK-293 cells failed to react to ADPbetaS; however, repeated superfusion with this P2Y1 receptor agonist restored the [Ca2+]i signals. In agreement with the functional data, real-time polymerase chain reaction and immunocytochemical studies indicated the presence of P2Y1, P2Y2 and P2Y4 receptors. Our findings raise doubt with respect to the reliability of HEK-293 cells as expression systems for recombinant P2X receptors, because of a possible functional interaction with endogenous P2Y receptors.

Synthesis of pyridoxal phosphate derivatives with antagonist activity at the P2Y13 receptor

We have synthesized a series of derivatives of the known P2 receptor antagonist PPADS (pyridoxal-5'-phosphate-6-azo-phenyl-2,4-disulfonate) and examined their ability to inhibit functional activity of the recombinant human P2Y13 nucleotide receptor expressed in 1321N1 human astrocytoma cells co-expressing G(alpha)16 protein (AG32). Analogues of PPADS modified through substitution of the phenylazo ring, including halo and nitro substitution, and 5'-alkyl phosphonate analogues were synthesized and tested. A 6-benzyl-5'-methyl phosphonate analogue was prepared to examine the effect of stable replacement of the azo linkage. The highest antagonistic potency was observed for 6-(3-nitrophenylazo) derivatives of pyridoxal-5'-phosphate. The 2-chloro-5-nitro analogue (MRS 2211) and 4-chloro-3-nitro analogue (MRS 2603) inhibited ADP (100 nM)-induced inositol trisphosphate (IP3) formation with pIC50 values of 5.97 and 6.18, respectively, being 45- and 74-fold more potent than PPADS. The antagonism of MRS 2211 was competitive with a pA2 value of 6.3. MRS2211 and MRS2603 inhibited phospholipase C (PLC) responses to 30 nM 2-methylthio-ADP in human P2Y1 receptor-mediated 1321N1 astrocytoma cells with IC50 values of >10 and 0.245 microM, respectively. Both analogues were inactive (IC50>10 microM) as antagonists of human P2Y12 receptor-mediated PLC responses in 1321N1 astrocytoma cells. Thus, MRS2211 displayed >20-fold selectivity as antagonist of the P2Y13 receptor in comparison to P2Y1 and P2Y12 receptors, while MRS2603 antagonized both P2Y1 and P2Y13 receptors.

An electrophysiological study of excitatory purinergic neuromuscular transmission in longitudinal smooth muscle of chicken anterior mesenteric artery

1. The object of the present study was to clarify the neurotransmitters controlling membrane responses to electrical field stimulation (EFS) in the longitudinal smooth muscle cells of the chicken anterior mesenteric artery. 2. EFS (5 pulses at 20 Hz) evoked a depolarization of amplitude 19.7+/-2.1 mV, total duration 29.6+/-3.1 s and latency 413.0+/-67.8 ms. This depolarization was tetrodotoxin (TTX)-sensitive and its amplitude was partially decreased by atropine (0.5 microM); however, its duration was shortened by further addition of prazosin (10 microM). 3. Atropine/prazosin-resistant component was blocked by the nonspecific purinergic antagonist, suramin, in a dose-dependent manner, indicating that this component is mediated by the neurotransmitter adenosine 5'-triphosphate (ATP). 4. Neither desensitization nor blocking of P2X receptor with its putative receptor agonist alpha,beta-methylene ATP (alpha,beta-MeATP, 1 microM) and its antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic (PPADS, up to 50 microM), had significant effect on the purinergic depolarization. In contrast, either desensitization or blocking of P2Y receptor with its putative agonist 2-methylthioATP (2-MeSATP, 1 microM) and its antagonist Cibacron blue F3GA (CBF3GA, 10 microM) abolished the purinergic depolarization, indicating that this response is mediated through P2Y but not P2X receptor. 5. The purinergic depolarization was inhibited by pertussis toxin (PTX, 600 ng ml(-1)). Furthermore, it was significantly inhibited by a phospholipase C (PLC) inhibitor, U-73122 (10 microM), indicating that the receptors involved in mediating the purinergic depolarization are linked to a PTX-sensitive G-protein, which is involved in a PLC-mediated signaling pathway. 6. Data of the present study suggest that the EFS-induced excitatory membrane response occurring in the longitudinal smooth muscle of the chicken anterior mesenteric artery is mainly purinergic in nature and is mediated via P2Y purinoceptors.

Leukotriene and purinergic receptors are involved in the hyperpolarizing effect of glucagon in liver cells

The pancreatic hormone glucagon hyperpolarizes the liver cell membrane. In the present study, we investigated the cellular signalling pathway of glucagon-induced hyperpolarization of liver cells by using the conventional microelectrode method. The membrane potential was recorded in superficial liver cells of superfused mouse liver slices. In the presence of the K+ channel blockers tetraethylammonium (TEA, 1 mmol/l) and Ba2+ (BaCl2, 5 mmol/l) and the blocker of the Na+/K+ ATPase, ouabain (1 mmol/l), no glucagon-induced hyperpolarization was observed confirming previous findings. The hyperpolarizing effect of glucagon was abolished by the leukotriene B4 receptor antagonist CP 195543 (0.1 mmol/l) and the purinergic receptor antagonist PPADS (5 micromol/l). ATPgammaS (10 micromol/l), a non-hydrolyzable ATP analogue, induced a hyperpolarization of the liver cell membrane similar to glucagon. U 73122 (1 micromol/l), a blocker of phospholipase C, prevented both the glucagon- and ATPgammaS-induced hyperpolarization. These findings suggest that glucagon affects the hepatic membrane potential partly by inducing the formation and release of leukotrienes and release of ATP acting on purinergic receptors of the liver cell membrane.

Direct excitation of inhibitory interneurons by extracellular ATP mediated by P2Y1 receptors in the hippocampal slice

ATP is an important cell-to-cell signaling molecule mediating the interactions between astrocytes and neurons in the CNS. In the hippocampal slices, ATP suppresses excitatory transmission mostly through activation of adenosine A1 receptors, because the ectoenzyme activity for the extracellular breakdown of ATP to adenosine is high in slice preparations in contrast to culture environments. Because the hippocampus is also rich in the expression of P2 receptors activated specifically by ATP, we examined whether ATP modulates neuronal excitability in the acute slice preparations independently of adenosine receptors. Although ATP decreased the frequency of spontaneously occurring EPSCs in the CA3 pyramidal neurons through activation of adenosine A1 receptors, ATP concurrently increased the frequency of IPSCs in a manner dependent on action potential generation. This effect was mediated by P2Y1 receptors because (1) 2-methylthio-ATP (2meSATP) was the most potent agonist, (2) 2'-deoxy-N6-methyladenosine-3',5'-bisphosphate diammonium (MRS2179) abolished this effect, and (3) this increase in IPSC frequency was not observed in the transgenic mice lacking P2Y1 receptor proteins. Application of 2meSATP elicited MRS2179-sensitive time- and voltage-dependent inward currents in the interneurons, which depolarized the cell to firing threshold. Also, it increased [Ca2+]i in both astrocytes and interneurons, but, unlike the former effect, the latter was entirely dependent on Ca2+ entry. Thus, in hippocampal slices, in addition to activating A1 receptors of the excitatory terminals after being converted to adenosine, ATP activates P2Y1 receptors in the interneurons, which is linked to activation of unidentified excitatory conductance, through mechanisms distinct from those in the astrocytes.

Interplay between ryanodine and IP3 receptors in ATP-stimulated mouse luteinized-granulosa cells

In mouse luteinized-granulosa cells (MGLC), ATP induces an increase in intracellular Ca2+ concentration by stimulating phospholipase C (PLC) associated with purinergic receptors, leading to production of inositol 1,4,5-trisphosphate (IP3) and subsequent release of Ca2+ from intracellular stores. In this study, we examined the cross-talk between the ryanodine receptors (RyR) and IP3 receptors (IP3R) in response to ATP in MGLC. Specifically, the effect of RyR modulators on ATP response was examined. The results showed that ATP-induced intracellular calcium elevation was abolished by inhibitors of the RyR, such as dantrolene (25 microM) and ryanodine (80 microM). When the MGLC were stimulated with activators of RyR, 2 microM ryanodine and 10 mM caffeine, the ATP-elicited response was decreased. These actions were independent of IP3 production stimulated by ATP. Hence, ATP-induced intracellular Ca2+ mobilization involves the coordinated action of both types of calcium release channels (CRCs). Using fluorescent probes, it was shown that IP3R is uniformly distributed throughout the cell; in contrast, RyR is mainly found around the nuclei. It is concluded that the IP3R and the RyR are functionally associated, and both play a role in the pattern of Ca2+ increase observed during purinergic stimulation of MGLC. This coupling may provide a highly efficient amplification mechanism for ATP stimulation of Ca2+ mobilization.

The pyrimidinergic P2Y6 receptor mediates a novel release of proinflammatory cytokines and chemokines in monocytic cells stimulated with UDP

The human P2Y6 receptor (hP2Y6) is a member of the G protein-coupled pyrimidinergic P2 receptor family that responds specifically to the extracellular nucleotide uridine diphosphate (UDP). Recently, the hP2Y6 receptor has been reported to mediate monocyte IL-8 production in response to UDP or lipopolysaccharide (LPS), but the role of hP2Y6 in regulating other pro-inflammatory cytokines or mediators is largely unknown. We demonstrate here that UDP specifically induces soluble TNF-alpha and IL-8 production in a promonocytic U937 cell line stably transfected with hP2Y6. However, we did not detect IL-1alpha, IL-1beta, IL-6, IL-10, IL-18, and PGE2 in the conditioned media from the same cell line. These results distinguish UDP/P2Y6 signaling from LPS signaling. Interestingly, UDP induces the production of IL-8, but not TNF-alpha, in human astrocytoma 1321N1 cell lines stably transfected with hP2Y6. Therefore, the immune effect of UDP/P2Y6 signaling on the production of proinflammatory cytokines is selective and dependent on cell types. We further identify that UDP can also induce the production of proinflammatory chemokines MCP-1 and IP-10 in hP2Y6 transfected promonocytic U937 cell lines, but not astrocytoma 1321N1 cell lines stably transfected with hP2Y6. From the Taqman analysis, UDP stimulation significantly upregulates the mRNA levels of IL-8, IP-10, and IL-1beta, but not TNF-alpha. Taken together, these new findings expand the pro-inflammatory biology of UDP mediated by the P2Y6 receptor.

Purine- and pyrimidine-induced responses and P2Y receptor characterization in the hamster proximal urethra

1. Purine and pyrimidine compounds were investigated on hamster proximal urethral circular smooth muscle preparations. In situ hybridization studies were carried out to localize P2Y(1), P2Y(2), P2Y(4) and P2Y(6) mRNA. Protein expression was studied using Western blotting analysis with antibodies against P2Y(1) and P2Y(2) receptors. 2. The hamster urethra relaxed with an agonist potency order of: 2-MeSADP>beta,gamma-meATP=ATP=adenosine=ADP>2-MeSATP>alpha,beta-meATP>TTP>CTP=UTP>GTP=UDP. The high potency of 2-MeSADP is suggestive of an action via P2Y(1) receptors. Although the order is not characteristic for any known single P2Y receptor subtype, it may represent a combination of P2Y receptor subtypes. 4. The selective P2Y(1) receptor antagonist MRS2179 inhibited ATP-, 2-MeSADP-, 2-MeSATP-, beta,gamma-meATP-, and to a lesser degree alpha,beta-meATP-induced responses. 3. Adenosine, but not ATP, was inhibited by the adenosine receptor antagonist 8-phenyltheophylline, indicating that ATP was not acting via adenosine following enzymatic breakdown. 5. Western blotting analysis showed the expression of both P2Y(1) and P2Y(2) receptors, confirming the results obtained with in situ hybridization that showed the expression of both P2Y(1) and P2Y(2), but not P2Y(4) or P2Y(6) mRNA, in smooth muscle layers of the hamster proximal urethra. 6. It is proposed that the relaxant response of the urethra to ATP may be evoked through the activation of the combination of receptors for P2Y(1) and to a lesser extent P2Y(2) receptors, which may mediate a trophic effect in addition. A P2Y subtype responsive to alpha,beta-meATP and P1 receptors may contribute to urethral smooth muscle relaxation.

Satellite glial cells in sensory ganglia: from form to function

Current information indicates that glial cells participate in all the normal and pathological processes of the central nervous system. Although much less is known about satellite glial cells (SGCs) in sensory ganglia, it appears that these cells share many characteristics with their central counterparts. This review presents information that has been accumulated recently on the physiology and pharmacology of SGCs. It appears that SGCs carry receptors for numerous neuroactive agents (e.g., ATP, bradykinin) and can therefore receive signals from other cells and respond to changes in their environment. Activation of SGCs might in turn influence neighboring neurons. Thus SGCs are likely to participate in signal processing and transmission in sensory ganglia. Damage to the axons of sensory ganglia is known to contribute to neuropathic pain. Such damage also affects SGCs, and it can be proposed that these cells have a role in pathological changes in the ganglia.

Metabotropic P2 receptor activation regulates oligodendrocyte progenitor migration and development

To gain insights into the role of purinergic receptors in oligodendrocyte development, we characterized the expression and functional activity of P2 receptors in cultured rat oligodendrocyte progenitors and investigated the effects of ATP and its breakdown products on the migration and proliferation of this immature glial cell population. Using Western blot analysis, we show that oligodendrocyte progenitors express several P2X (P2X(1,2,3,4,7)) and P2Y (P2Y(1,2,4)) receptors. Intracellular Ca(2+) recording by Fura-2 video imaging allowed to determine the rank potency order of the P2 agonists tested: ADPbetaS = ADP = Benzoyl ATP > ATP > ATPgammaS > UTP, alpha,beta-meATP ineffective. Based on the above findings, on pharmacological inhibition by the antagonists oxATP and MRS2179, and on the absence of alpha,betameATP-induced inward current in whole-cell recording, P2X(7) and P2Y(1) were identified as the main ionotropic and metabotropic P2 receptors active in OPs. As a functional correlate of these findings, we show that ATP and, among metabotropic agonists, ADP and the P2Y(1)-specific agonist ADPbetaS, but not UTP, induce oligodendrocyte progenitor migration. Moreover, ATP and ADP inhibited the proliferation of oligodendrocyte progenitors induced by platelet-derived growth factor, both in purified cultures and in cerebellar tissue slices. The effects of ATP and ADP on cell migration and proliferation were prevented by the P2Y(1) antagonist MRS2179. By confocal laser scanning microscopy, P2Y(1) receptors were localized in NG2-labeled oligodendrocyte progenitors in the developing rat brain. These data indicate that ATP and ADP may regulate oligodendrocyte progenitor functions by a mechanism that involves mainly activation of P2Y(1) receptors.

P2X and P2Y purinergic receptors on human intestinal epithelial carcinoma cells: effects of extracellular nucleotides on apoptosis and cell proliferation

Extracellular nucleotides interact with purinergic receptors, which regulate ion transport in a variety of epithelia. With the use of two different human epithelial carcinoma cell lines (HCT8 and Caco-2), we have shown by RT-PCR that the cells express mRNA for P2X1, P2X3, P2X4, P2X5, P2X6, P2X7, P2Y1, P2Y2, P2Y4, P2Y6, P2Y11, and P2Y12 receptors. Protein expression for P2Y1 and P2Y2 receptors was also demonstrated immunohistochemically, and P2X receptor subtype protein was present in the following decreasing order: P2X4 > P2X7 > P2X1 > P2X3 > P2X6 > P2X5 >> P2X2. The functional presence of P2X7, P2Y1, P2Y2, and P2Y4 receptors was shown based on the effect of extracellular nucleotides on apoptosis or cell proliferation, and measurement of nucleotide-dependent calcium fluxes using a fluorometric imaging plate reader in the presence of different selective agonists and antagonists. ATP, at high concentrations, induced apoptosis through ligation of P2X7 and P2Y1 receptors; conversely, ATP, at lower concentrations, and UTP stimulated proliferation, probably acting via P2Y2 receptors. We therefore propose that stimulation or dysfunction of purinergic receptors may contribute at least partially to modulation of epithelial carcinoma cell proliferation and apoptosis.

Degradation of extracellular ATP by the retinal pigment epithelium

Stimulation of ATP or adenosine receptors causes important physiological changes in retinal pigment epithelial (RPE) cells that may influence their relationship to the adjacent photoreceptors. While RPE cells have been shown to release ATP, the regulation of extracellular ATP levels and the production of dephosphorylated purines is not clear. This study examined the degradation of ATP by RPE cells and the physiological effects of the adenosine diphosphate (ADP) that result. ATP was readily broken down by both cultured human ARPE-19 cells and the apical membrane of fresh bovine RPE cells. The compounds ARL67156 and betagamma-mATP inhibited this degradation in both cell types. RT-PCR analysis of ARPE-19 cells found mRNA message for multiple extracellular degradative enzymes; ectonucleotide pyrophosphatase/phosphodiesterase eNPP1, eNPP2, and eNPP3; the ectoATPase ectonucleoside triphosphate diphosphohydrolase NTPDase2, NTPDase3, and some message for NTPDase1. Considerable levels of ADP bathed RPE cells, consistent with a role for NTPDase2. ADP and ATP increased levels of intracellular Ca(2+). Both responses were inhibited by thapsigargin and P2Y(1) receptor inhibitor MRS 2179. Message for both P2Y(1) and P2Y(12) receptors was detected in ARPE-19 cells. These results suggest that extracellular degradation of ATP in subretinal space can result in the production of ADP. This ADP can stimulate P2Y receptors and augment Ca(2+) signaling in the RPE.