P2Y2 receptors activate neuroprotective mechanisms in astrocytic cells

Purinergic receptor signaling regulates N-cadherin expression in primary astrocyte cultures

Extracellular ATP exerts both short-term and long-term effects in the CNS by stimulating cell-surface purinergic receptors. Here we have examined the effect of purinergic receptor activation on N-cadherin expression, a calcium-dependent cell adhesion molecule involved in many processes, including glia-glia and axon-glia interactions. When primary cultures of rat cortical astrocytes were treated with ATP, N-cadherin protein expression increased in a time- and concentration-dependent manner. In addition, ATP treatment caused an increase in N-cadherin immunoreactivity in both the cytoplasm and on the cell surface membrane. Interestingly, experiments with cycloheximide revealed that relocalization of N-cadherin to the cell surface membrane were independent of protein synthesis. The ATP-induced increase in N-cadherin protein expression was blocked by reactive blue 2 and 8-(p-sulfophenyl)-theophylline, suggesting involvement of both P2 and P1 purinergic receptors, respectively. In addition, N-cadherin expression was partially blocked when signaling from purinergic receptors to extracellular signal regulated protein kinase or Akt was inhibited by 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene or wortmannin, respectively. By using an in vitro model of traumatic CNS injury, we found that N-cadherin expression was increased when astrocytes were subjected to rapid and reversible mechanical strain. The findings presented here demonstrate a role for extracellular ATP, purinergic receptors and protein kinase signaling in regulating N-cadherin expression and suggest a role for this mechanism in cell-cell interactions.

Uracil nucleotides: from metabolic intermediates to neuroprotection and neuroinflammation

Uracil nucleotides (i.e., UTP and UDP) have been known for years as fundamental intermediates in the de novo synthesis of the other pyrimidine nucleotides, which altogether represent key building blocks for nucleic acid synthesis. In addition, their sugar conjugates (i.e., UDP-glucose and UDP-galactose) enter in several biochemical routes, for example leading to glycogen biosynthesis, and protein and lipid glycosylation, which in turn contribute to the synthesis of essential components of the cellular plasma membrane. More recently, the existence of a "pyrimidinergic transmission" has arisen from the discovery that several purinergic G protein-coupled P2Y receptors can be activated also or exclusively by uracil nucleotides and sugar conjugates. The number of these receptors is continuously growing over years with the discovery that previously "orphan" G protein-coupled receptors are actually responding to this class of molecules. Therefore, new unforeseen effects mediated by uracil derivatives have emerged, in particular in the nervous system, and previously unexplored avenues for the pharmacological manipulation of this system are currently under investigation. In this commentary we shall try to put together our current knowledge on the biochemical and receptor-mediated effects of uracil nucleotide derivatives with a specific focus on the nervous system in order to depict a clearer view of the importance of the pyrimidinergic system in both physiological and pathological conditions.

Caveolins in glial cell model systems: from detection to significance

Glial cells prevail in number and in diversity of cellular phenotypes in the nervous system. They have also gained prominence due to their multiple physiological and pathophysiological roles. Our current knowledge of the asymmetry and heterogeneity of the plasma membrane demands an in depth analysis of the diverse array of membrane microdomains postulated to exist in the context of glial cells. This review focuses and analyzes the studies reported to date on the detection of caveolae membrane rafts and the caveolin family members in glial cell model systems, the conditions leading to changes in their level of expression, and their functional and clinical significance. Outstanding in this work emerge the ubiquitous expression of caveolins, including the typically regarded 'muscle-specific' cav3, in diverse glial cell model systems, their participation in reactive astrogliosis, cancer, and their key relevance to calcium signaling. The knowledge obtained to date demands incorporation of the caveolins and caveolae membrane rafts in our current models on the role of glial cells in heath and neurological disease.

Activation of P2X(7) receptors stimulates the expression of P2Y(2) receptor mRNA in astrocytes cultured from rat brain

Under pathological conditions brain cells release ATP at concentrations reported to activate P2X(7) ionotropic receptor subtypes expressed in both neuronal and glial cells. In the present study we report that the most potent P2X(7) receptor agonist BzATP stimulates the expression of the metabotropic ATP receptor P2Y(2) in cultured rat brain astrocytes. In other cell types several kinds of stimulation, including stress or injury, induce P2Y(2) expression that, in turn, is involved in different cell reactions. Similarly, it has recently been found that in astrocytes and astrocytoma cells P2Y(2) sites can trigger neuroprotective pathways through the activation of several mechanisms, including the induction of genes for antiapoptotic factors, neurotrophins, growth factors and neuropeptides. Here we present evidence that P2Y(2) mRNA expression in cultured astrocytes peaks 6 h after BzATP exposure and returns to basal levels after 24 h. This effect was mimicked by high ATP concentrations (1 mM) and was abolished by P2X(7)-antagonists oATP and BBG. The BzATP-evoked P2Y(2) receptor up-regulation in cultured astrocytes was coupled to an increased UTP-mediated intracellular calcium response. This effect was inhibited by oATP and BBG and by P2Y(2)siRNA, thus supporting evidence of increased P2Y(2) activity. To further investigate the mechanisms by which P2X(7) receptors mediated the P2Y(2) mRNA up-regulation, the cells were pre-treated with the chelating agent EGTA, or with inhibitors of mitogen-activated kinase (MAPK) (PD98059) or protein kinase C, (GF109203X). Each inhibitor significantly reduced the extent to which BzATP induced P2Y(2) mRNA. Both BzATP and ATP (1 mM) increased ERK1/2 activation. P2X(7)-induced ERK1/2 phosphorylation was unaffected by pre-treatment of astrocytes with EGTA whereas it was inhibited by GF109203X. Phorbol-12-myristate-13-acetate (PMA), an activator of PKCs, rapidly increased ERK1/2 activation. We conclude that activation of P2X(7) receptors in astrocytes enhances P2Y(2) mRNA expression by a mechanism involving both calcium influx and PKC/MAPK signalling pathways.

P2Y2 nucleotide receptors inhibit trauma-induced death of astrocytic cells

Nucleotides as well as other neurotransmitters are known to be released to the extracellular space upon injury. To determine whether nucleotides acting on P2Y(2) nucleotide receptors promote protective or degenerative events after trauma in astrocytic cells, a well-established model of in vitro brain trauma was applied to 1321N1 cells expressing recombinant P2Y(2) nucleotide receptors (P2Y(2)R-1321N1). Cellular death was examined by measuring DNA fragmentation and caspase activation. Fragmented DNA was observed 48 h post-injury in 1321N1 cells, while P2Y(2) nucleotide receptor expressing cells did not show DNA fragmentation. A laddering pattern of fragmented DNA following injury was observed upon inhibition of P2Y(2) nucleotide receptors with suramin. Time-dependent increases of cleaved caspase-9, a mitochondrial-associated caspase, correlated with injury-induced cellular death. A decreased bax/bcl-2 gene expression ratio was observed in P2Y(2)R-1321N1 cells after traumatic injury, while untransfected 1321N1 cells showed a significant time-dependent increase of the bax/bcl-2 gene expression ratio. Activation of protein kinases was assessed to determine the signaling pathways involved in cell death and survival responses following traumatic injury. In P2Y(2)R-1321N1 and 1321N1 cells p38 phosphorylation was stimulated in a time-dependent manner but the phosphatidylinositol 3-kinase-dependent activation of extracellular signal-regulated kinase 1/2 and protein kinase B (PKB)/Akt was only observed in P2Y(2)R-1321N1 cells after injury. The stress-activated protein kinase/c-Jun NH2-terminal kinase (SAPK/JNK) signaling pathway was not activated by traumatic injury in either astrocytic cell line. Inhibition of p38 kinase signaling pathway by treatment with PD1693, a MKK3/6 inhibitor, abolished the expression of cleaved caspase-9, the increase in the bax/bcl-2 gene expression ratio, as well as the fragmentation of DNA that followed injury of 1321N1 cells. Taken together, our results demonstrate a novel role for P2Y(2) nucleotide receptors and extracellular nucleotides in mediating survival responses to glial cells undergoing cellular death induced by trauma.

Chronic administration of docosahexaenoic acid or eicosapentaenoic acid, but not arachidonic acid, alone or in combination with uridine, increases brain phosphatide and synaptic protein levels in gerbils

Synthesis of phosphatidylcholine, the most abundant brain membrane phosphatide, requires three circulating precursors: choline; a pyrimidine (e.g. uridine); and a polyunsaturated fatty acid. Supplementing a choline-containing diet with the uridine source uridine-5'-monophosphate (UMP) or, especially, with UMP plus the omega-3 fatty acid docosahexaenoic acid (given by gavage), produces substantial increases in membrane phosphatide and synaptic protein levels within gerbil brain. We now compare the effects of various polyunsaturated fatty acids, given alone or with UMP, on these synaptic membrane constituents. Gerbils received, daily for 4 weeks, a diet containing choline chloride with or without UMP and/or, by gavage, an omega-3 (docosahexaenoic or eicosapentaenoic acid) or omega-6 (arachidonic acid) fatty acid. Both of the omega-3 fatty acids elevated major brain phosphatide levels (by 18-28%, and 21-27%) and giving UMP along with them enhanced their effects significantly. Arachidonic acid, given alone or with UMP, was without effect. After UMP plus docosahexaenoic acid treatment, total brain phospholipid levels and those of each individual phosphatide increased significantly in all brain regions examined (cortex, striatum, hippocampus, brain stem, and cerebellum). The increases in brain phosphatides in gerbils receiving an omega-3 (but not omega-6) fatty acid, with or without UMP, were accompanied by parallel elevations in levels of pre- and post-synaptic proteins (syntaxin-3, PSD-95 and synapsin-1) but not in those of a ubiquitous structural protein, beta-tubulin. Hence administering omega-3 polyunsaturated fatty acids can enhance synaptic membrane levels in gerbils, and may do so in patients with neurodegenerative diseases, especially when given with a uridine source, while the omega-6 polyunsaturated fatty acid arachidonic acid is ineffective.

Cortical spreading depression releases ATP into the extracellular space and purinergic receptor activation contributes to the induction of ischemic tolerance

Cortical Spreading Depression (CSD) is a well-studied model of preconditioning that provides a high degree of tolerance to a subsequent ischemic event in the brain. The present study was undertaken in order to determine whether the release of ATP during CSD could contribute to the induction of ischemic tolerance. Direct measurement of ATP levels during CSD indicates that with each CSD wave ATP is released into the extracellular space at levels exceeding 100 microM. Cultures of rat primary cortical neurons exposed to low levels of extracellular ATP developed tolerance to subsequent oxygen-glucose deprivation (OGD) or metabolic hypoxia. The preconditioning effect requires new protein synthesis and develops with time, suggesting that a complex genomic response is required for the induction of tolerance. Multiple purinergic receptors are involved in mediating tolerance, with P2Y receptor activation having the greatest effect. Although extracellular adenosine or glutamate may make a small contribution, most of the tolerance was found to be induced independently of adenosine or glutamate receptor activation. Multiple signal transduction pathways mediate the response to extracellular ATP with the protein kinase A pathway and activation of phospholipase C contributing the most. The results are consistent with the proposal that CSD releases ATP into the extracellular space and the subsequent activation of P2Y receptors makes a major contribution to the induction of ischemic tolerance in the brain.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

P2Y2 receptors induced cell surface redistribution of alpha(v) integrin is required for activation of ERK 1/2 in U937 cells

Nucleotides released from cells due to stress, injury or inflammation, induce mitogenic effects in monocytes via activation of P2Y(2) nucleotide receptors (P2Y(2)Rs). Here we show that P2Y(2) nucleotide receptors in U937 monocytic cells regulate the activation of extracellular signal-regulated kinases 1 and 2 (ERK 1/2) by inducing the clustering of alpha(v) integrins. The activation of phosphatidylinositol 3-kinase by P2Y(2)R ligands was required for alpha(v) clustering, suggesting a means whereby two different classes of receptors communicate to induce mitogenic responses in monocytic cells. P2Y(2)R-induced alpha(v) clustering was also associated with a flattened phenotype of the U937 cells, consistent with the role of the P2Y(2)R in regulating early events in cell migration.

Nerve growth factor-stimulated neuronal differentiation induces changes in P2 receptor expression and nucleotide-stimulated catecholamine release

Extracellular nucleotides modulate synaptic transmission and neuronal communication by activating purinergic 2 (P2) (nucleotide) receptors. Here, we assessed changes in the regulation by nucleotides and their receptors of an important physiological response - release and uptake of catecholamines - that accompanies sympathoadrenal neuronal differentiation. Nerve growth factor (NGF)-promoted differentiation of pheochromocytoma 12 (PC12) cells enhanced the ability of the non-hydrolyzable ATP analog, ATPgammaS, to stimulate catecholamine (norepinephrine, NE) release and this enhancement occurred without a significant alteration in NE uptake. In addition to ATPgammaS, 2-MeSATP and alphabetaMeATP, P2X receptor-selective agonists, caused greater NE release from NGF-differentiated than from undifferentiated PC12 cells. NGF-differentiated PC12 cells had altered mRNA expression of several P2Y and P2X receptors but protein expression was only increased for P2X, in particular P2X(1-4,) receptors and P2X, but not P2Y, receptor inhibitors blunted the NGF-promoted enhancement in nucleotide-regulated catecholamine release. Surprisingly, siRNA directed against P2X(2), the receptor with the highest expression, failed to alter NE release by ATPgammaS. These findings indicate that sympathetic neuronal differentiation by NGF increases both the expression of P2X receptor sub-types and their regulation of catecholamine release. NGF-promoted increased expression of P2X receptors thus appears to be a physiologically important response that characterizes sympathetic neuronal differentiation.

Astroglia: important mediators of traumatic brain injury

Traumatic brain injury (TBI) research to date has focused almost exclusively on the pathophysiology of injured neurons with very little attention paid to non-neuronal cells. However in the past decade, exciting discoveries have challenged this century-old view of passive glial cells and have led to a reinterpretation of the role of glial cells in central nervous system (CNS) biology and pathology. In this chapter we review several lines of evidence, indicating that glial cells, particularly astrocytes, are active partners to neurons in the brain, and summarize recent findings that detail the significance of astrocyte pathology in traumatic brain injury.

P2Y2 receptor up-regulation induced by guanosine or UTP in rat brain cultured astrocytes

Among P2 metabotropic ATP receptors, P2Y2 subtype seems to be peculiar as its upregulation triggers important biological events in different cells types. In non-stimulated cells including astrocytes, P2Y2 receptors are usually expressed at levels lower than P2Y1 sites, however the promoter region of the P2Y2 receptors has not yet been studied and little is known about the mechanisms underlying the regulation of the expression of this ATP receptor. We showed that not only UTP and ATP are the most potent and naturally occurring agonist for P2Y2 sites, but also guanosine induced an up-regulation of astrocyte P2Y2 receptor mRNA evaluated by Northern blot analysis. We also focused our attention on this nucleoside since in our previous studies it was reported to be released by cultured astrocytes and to exert different neuroprotective effects. UTP and guanosine-evoked P2Y2 receptor up-regulation in rat brain cultured astrocytes was linked to an increased P2Y2-mediated intracellular calcium response, thus suggesting an increased P2Y2 activity. Actinomycin D, a RNA polymerase inhibitor, abrogated both UTP and guanosine-mediated P2Y2 up-regulation, thus indicating that de novo transcription was required. The effect of UTP and guanosine was also evaluated in astrocytes pretreated with different inhibitors of signal transduction pathways including ERK, PKC and PKA reported to be involved in the regulation of other cell surface receptor mRNAs. The results show that ERK1-2/MAPK pathway play a key role in the P2Y2 receptor up-regulation mediated by either UTP or guanosine. Moreover, our data suggest that PKA is also involved in guanosine-induced transcriptional activation of P2Y2 mRNA and that increased intracellular calcium levels and PKC activation may also mediate P2Y2 receptor up-regulation triggered by UTP. The extracellular release of ATP under physiological and pathological conditions has been widely studied. On the contrary, little is known about the release of pyrimidines and in particular of UTP. Here we show that astrocytes are able to release UTP, either at rest or during and following hypoxia/hypoglycemia obtained by submitting the cells to glucose-oxygen deprivation (OGD). Interestingly, also P2Y2 receptor mRNA increased by about two-fold the control values when the cultures were submitted to OGD. It has been recently reported that P2Y2 receptors can play a protective role in astrocytes, thus either guanosine administration or increased extracellular concentrations of guanosine and UTP reached locally following CNS injury may increase P2Y2-mediated biological events aimed at promoting a protective astrocyte response.

P2Y receptor-mediated excitation in the posterior hypothalamus

Histaminergic neurons located in the posterior hypothalamus (tuberomamillary nucleus, TMN) project widely through the whole brain controlling arousal and attention. They are tonically active during wakefulness but cease firing during sleep. As a homeostatic theory of sleep involves ATP depletion and adenosine accumulation in the brain, we investigated the role of ATP and its analogues as well as adenosine on neuronal activity in the TMN. We show increased firing of rat TMN neurons by ATP, ADP, UTP and 2meSATP, indicating activation of receptors belonging to the P2Y family. Adenosine affected neither membrane potential nor firing of these cells. Single-cell reverse transcriptase-polymerase chain reaction revealed that P2Y1 and P2Y4 are prevailing receptors in TMN neurons. P2Y1 receptor mRNA was detected with a higher frequency in 2-week-old than in 4-week-old rats; in accordance, 2meSATP was more potent than ATP. Semi-quantitative real-time polymerase chain reaction revealed a developmental downregulation of mRNA levels for P2Y1 and P2Y4 receptors. Immunocytochemistry demonstrated neuronal and glial localization of the P2Y1 receptor protein. Network activity measured with multielectrode arrays in primary cultures made from the posterior hypothalamus was enhanced by UTP and 2meSATP (P2Y4 and P2Y1 agonists, respectively). ATP caused an inhibition of firing, which was reversed in the presence of suramin or gabazine [gamma-aminobutyric acid (GABA)A receptor antagonist], indicating that GABAergic neurons are preferentially activated by ATP in this network. Excitation of the wake-active TMN neurons by nucleotides and the lack of adenosine action may be important factors in sleep-wake regulation.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Roles of P2 receptors in glial cells: focus on astrocytes

Central nervous system glial cells release and respond to nucleotides under both physiological and pathological conditions, suggesting that these molecules play key roles in both normal brain function and in repair after damage. In particular, ATP released from astrocytes activates P2 receptors on astrocytes and other brain cells, allowing a form of homotypic and heterotypic signalling, which also involves microglia, neurons and oligodendrocytes. Multiple P2X and P2Y receptors are expressed by both astrocytes and microglia; however, these receptors are differentially recruited by nucleotides, depending upon specific pathophysiological conditions, and also mediate the long-term trophic changes of these cells during inflammatory gliosis. In astrocytes, P2-receptor-induced gliosis occurs via activation of the extracellular-regulated kinases (ERK) and protein kinase B/Akt pathways and involves induction of inflammatory and anti-inflammatory genes, cyclins, adhesion and antiapoptotic molecules. While astrocytic P2Y₁ and P2Y_2,4 are primarily involved in short-term calcium-dependent signalling, multiple P2 receptor subtypes seem to cooperate to astrocytic long-term changes. Conversely, in microglia, exposure to inflammatory and immunological stimuli results in differential functional changes of distinct P2 receptors, suggesting highly specific roles in acquisition of the activated phenotype. We believe that nucleotide-induced activation of astrocytes and microglia may originally start as a defence mechanism to protect neurons from cytotoxic and ischaemic insults; dysregulation of this process in chronic inflammatory diseases eventually results in neuronal cell damage and loss. On this basis, full elucidation of the specific roles of P2 receptors in these cells may help exploit the beneficial neuroprotective features of activated glia while attenuating their harmful properties and thus provide the basis for novel neuroprotective strategies that specifically target the purinergic system.

Activation of Src/kinase/phospholipase C/mitogen-activated protein kinase and induction of neurite expression by ATP, independent of nerve growth factor

Extracellular ATP has been reported to potentiate the neurite outgrowth induced by nerve growth factor. In the present study the neurotrophic effect of ATP and other nucleotides was examined in mouse neuroblastoma neuro2a cells which lack nerve growth factor receptor. Exposure of neuro2a cells to ATP resulted in a dramatic increase in neurite bearing cells as compared with untreated control cells. Experiments performed with purinergic receptor agonists and antagonists suggest that the ATP stimulates neurite outgrowth via P2 receptors. Neurite outgrowth was completely blocked by P2 receptor antagonist suramin whereas the P1 receptor antagonist CGS15943 was ineffective. P1 receptor agonist 5'-(N-ethylcarboxamido)adenosine failed to induce neurite outgrowth. The potency order of different P2 receptor agonists was ATP=ATPgammaS>ADP>>2Me-S-ATP. It was insensitive to UTP and antagonist pyridoxal phosphate-6-azo (benzene-2,4-disulfonic acid) suggesting the involvement of P2Y11 receptor in the observed neuritogenic effect. The signaling pathway leading to ATP-induced neuritogenesis was investigated. The neuritogenic effect of ATP is independent of rise in intracellular Ca(2+) as pharmacological profile of neuritogenic P2Y receptor does not match with that of P2Y2 receptor associated with [Ca(2+)](i) signaling cascade. Exposure of cells to ATP caused activation of Src kinase, phospholipase Cgamma and extracellular signal-regulated kinases ERK1/2. Mitogen-activated protein kinase (MAPK) inhibitor U0126 drastically reduced the number of neurite bearing cells in ATP-treated cultures implying that the neurotrophic effect of ATP is mediated by MAPK. Our results demonstrate that ATP can stimulate neurite outgrowth independent of other neurotrophic factors and can be an effective trophic agent.

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.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.

Agonist-induced phosphorylation and desensitization of the P2Y2 nucleotide receptor

Purification of HA-tagged P2Y2 receptors from transfected human 1321N1 astrocytoma cells yielded a protein with a molecular size determined by SDS-PAGE to be in the range of 57-76 kDa, which is typical of membrane glycoproteins with heterogeneous complex glycosylation. The protein phosphatase inhibitor, okadaic acid, attenuated the recovery of receptor activity from the agonist-induced desensitized state, suggesting a role for P2Y2 receptor phosphorylation in desensitization. Isolation of HA-tagged P2Y2 nucleotide receptors from metabolically [32P]-labelled cells indicated a (3.8 +/- 0.2)-fold increase in the [32P]-content of the receptor after 15 min of treatment with 100 microM UTP, as compared to immunoprecipitated receptors from untreated control cells. Receptor sequestration studies indicated that approximately 40% of the surface receptors were internalized after a 15-min stimulation with 100 microM UTP. Point mutation of three potential GRK and PKC phosphorylation sites in the third intracellular loop and C-terminal tail of the P2Y2 receptor (namely, S243A, T344A, and S356A) extinguished agonist-induced receptor phosphorylation, caused a marked reduction in the efficacy of UTP to desensitize P2Y2 receptor signalling to intracellular calcium mobilization, and impaired agonist-induced receptor internalization. Activation of PKC isoforms with phorbol 12-myristate 13-acetate that caused heterologous receptor desensitization did not increase the level of P2Y2 receptor phosphorylation. Our results indicate a role for receptor phosphorylation by phorbol-insensitive protein kinases in agonist-induced desensitization of the P2Y2 nucleotide receptor.

P2 receptors: intracellular signaling

P2 receptors for extracellular nucleotides are divided into two categories: the ion channel receptors (P2X) and the G-protein-coupled receptors (P2Y). For the P2X receptors, signal transduction appears to be relatively simple. Upon activation by extracellular ATP, a channel comprised of P2X receptor subunits opens and allows cations to move across the plasma membrane, resulting in changes in the electrical potential of the cell that, in turn, propagates a signal. This regulated flux of ions across the plasma membrane has important signaling functions, especially in impulse propagation in the nervous system and in muscle contractility. In addition, P2X receptor activation causes the accumulation of calcium ions in the cytoplasm, which is responsible for activating numerous signaling molecules. For the P2Y receptors, signal transduction is more complex. Intracellular signaling cascades are the main routes of communication between G-protein-coupled receptors and regulatory targets within the cell. These signaling cascades operate mainly by the sequential activation or deactivation of heterotrimeric and monomeric G proteins, phospholipases, protein kinases, adenylyl and guanylyl cyclases, and phosphodiesterases that regulate many cellular processes, including proliferation, differentiation, apoptosis, metabolism, secretion, and cell migration. In addition, there are numerous ion channels, cell adhesion molecules and receptor tyrosine kinases that are modulated by P2Y receptors and operate to transmit an extracellular signal to an intracellular response. These intracellular signaling pathways and their regulation by P2 receptors are discussed in this review.

Pathophysiology and therapeutic potential of purinergic signaling

The concept of a purinergic signaling system, using purine nucleotides and nucleosides as extracellular messengers, was first proposed over 30 years ago. After a brief introduction and update of purinoceptor subtypes, this article focuses on the diverse pathophysiological roles of purines and pyrimidines as signaling molecules. These molecules mediate short-term (acute) signaling functions in neurotransmission, mechanosensory transduction, secretion and vasodilatation, and long-term (chronic) signaling functions in cell proliferation, differentiation, and death involved in development and regeneration. Plasticity of purinoceptor expression in pathological conditions is frequently observed, including an increase in the purinergic component of autonomic cotransmission. Recent advances in therapies using purinergic-related drugs in a wide range of pathological conditions will be addressed with speculation on future developments in the field.

Role of cAMP-response element-binding protein phosphorylation in hepatic apoptosis under protein kinase C alpha suppression during sepsis

Previous studies have shown that a decrease in protein kinase C (PKC) alpha levels contributes to hepatic failure and/or apoptosis during sepsis, and suppression of PKCalpha plays a critical role in triggering caspase-dependent apoptosis, which can modulate expression of Bcl-xL. However, the underlying molecular mechanism remains uncertain. In the present study, we examined whether a decrease in the nuclear PKCalpha levels causes hepatic apoptosis via modulation of cAMP-response element-binding protein (CREB) or nuclear factor-kappaB (NFkappaB), the crucial factors regulating the expression of prosurvival Bcl-xL. For polymicrobial sepsis induction, a cecal ligation and puncture model was used; at 9 or 18 h after CLP, experiments were terminated, referring as early or late sepsis, respectively. Additionally, PKCalpha was suppressed by stable transfection of antisense PKCalpha plasmid into a Clone-9 rat hepatic epithelial cell. The results showed that the nuclear PKCalpha was significantly decreased in the liver during sepsis, which was accompanied by decreases in phospho-CREB content, DNA-binding activity of CREB, and Bcl-xL expression. Likewise, the binding activity of NFkappaB increased significantly, which was associated with a decrease in cytosolic inhibitory-kappaBalpha content. The in vitro suppression of PKCalpha also resulted in decreases in the phospho-CREB content and DNA-binding activity, which were accompanied by down-regulation of Bcl-xL and apoptosis, but no significant alteration in NFkappaB-binding activity. The in vivo and in vitro results suggest that the suppression of PKCalpha results in a decreased CREB phosphorylation and subsequent down-regulation of Bcl-xL, which may contribute to the hepatic apoptosis during sepsis.

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.

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.

P2Y nucleotide receptor interaction with alpha integrin mediates astrocyte migration

Astrocytes become activated in response to brain injury, as characterized by increased expression of glial fibrillary acidic protein (GFAP) and increased rates of cell migration and proliferation. Damage to brain cells causes the release of cytoplasmic nucleotides, such as ATP and uridine 5'-triphosphate (UTP), ligands for P2 nucleotide receptors. Results in this study with primary rat astrocytes indicate that activation of a G protein-coupled P2Y(2) receptor for ATP and UTP increases GFAP expression and both chemotactic and chemokinetic cell migration. UTP-induced astrocyte migration was inhibited by silencing of P2Y(2) nucleotide receptor (P2Y(2)R) expression with siRNA of P2Y(2)R (P2Y(2)R siRNA). UTP also increased the expression in astrocytes of alpha(V)beta(3/5) integrins that are known to interact directly with the P2Y(2)R to modulate its function. Anti-alpha(V) integrin antibodies prevented UTP-stimulated astrocyte migration, suggesting that P2Y(2)R/alpha(V) interactions mediate the activation of astrocytes by UTP. P2Y(2)R-mediated astrocyte migration required the activation of the phosphatidylinositol-3-kinase (PI3-K)/protein kinase B (Akt) and the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK/ERK) signaling pathways, responses that also were inhibited by anti-alpha(V) integrin antibody. These results suggest that P2Y(2)Rs and their associated signaling pathways may be important factors regulating astrogliosis in brain disorders.

Traumatic injury activates protein kinase B/Akt in cultured astrocytes: role of extracellular ATP and P2 purinergic receptors

Protein kinase B/Akt is a key signaling molecule that regulates cell survival, growth, and metabolism, and inhibits apoptosis. Traumatic brain injury (TBI) activates Akt, and Akt has been implicated in neuronal survival after TBI, but little is known about injury-induced Akt activation in astrocytes, cells that exhibit hypertrophic and hyperplastic responses to CNS injury. Here we have investigated the effect of mechanical strain on Akt activation in primary cultures of rat cortical astrocytes growing on deformable Silastic membranes. When astrocytes were subjected to mechanical strain (50 msec; 5-7.5 mm displacement), we observed an increase in phosphorylation of serine 473, a key indicator of Akt activation. Akt phosphorylation was increased at 3 min postinjury, was maximal from 5 to 10 min, and declined gradually thereafter. Akt activation was also dependent on the severity of the injury. Stretch-induced Akt phosphorylation was attenuated by blocking calcium influx and phosphoinositide 3-kinase (PI3K), an upstream activator of Akt. In addition, we found that ATP is rapidly released after mechanical strain and that the P2 purinergic receptor antagonist iso-pyridoxal-5'-phosphate-6-azophenyl-2',5'disulfonate (PPADS) attenuated trauma-induced Akt activation. We conclude that mechanical strain causes activation of Akt in astrocytes via stimulation of P2 receptors. This suggests that P2 receptor/Akt signaling promotes astrocyte survival and growth, and this process may play a role in the generation of reactive gliosis after TBI.

Lung epithelial cells release ATP during ozone exposure: signaling for cell survival

The common air pollutant ozone causes acute toxicity to human airways. In primary and transformed epithelial cells from all levels of human or rat airways, ozone levels relevant to air pollution (50-200 ppb) increased extracellular [ATP] within 7-30 min. A human bronchial epithelial cell line (16HBE14o(-)) that forms electrically resistant polarized monolayers had up to 10-fold greater apical than basolateral surface extracellular [ATP] within 7 min of ozone exposure. Increased extracellular [ATP] appeared due to ATP secretion or release because (1) inhibition of ectonucleotidase (cell surface enzyme(s) which degrade ATP) by ozone did not occur until >120 min of ozone exposure and (2) brefeldin A, a secretory inhibitor, eliminated elevation of extracellular [ATP] without affecting intracellular ATP. Extracellular ATP protected against ozone toxicity in a P2Y receptor-dependent manner as (1) removal of ATP and adenosine by apyrase and adenosine deaminase, respectively, potentiated ozone toxicity, (2) extracellular supplementation with ATP, a poorly hydrolyzable ATP analog ATPgammaS, or UTP inhibited apoptotic and necrotic ozone-mediated cell death, and (3) ATP-mediated protection was eliminated by P2 and P2Y receptor inhibitors suramin and Cibacron blue (reactive blue 2), respectively. The decline in glucose uptake caused by prolonged ozone exposure was prevented by supplemental extracellular ATP, an effect blocked by suramin. Further, Akt and ERK phosphorylation resulted from exposure to supplemental extracellular ATP. Thus, extracellularly released ATP signals to prevent ozone-induced death and supplementation with ATP or its analogs can augment protection, at least in part via Akt and /or ERK signaling pathways and their metabolic effects.