Pathways of survival induced by NGF and extracellular ATP after growth factor deprivation

ATP binding to Nerve Growth Factor (NGF) and pro-Nerve Growth Factor (proNGF): an endogenous molecular switch modulating neurotrophins activity

ATP has recently been reconsidered as a molecule with functional properties which go beyond its recognized role of the energetic driver of the cell. ATP has been described as an allosteric modulator as well as a biological hydrotrope with anti-aggregation properties in the crowded cellular environment. The role of ATP as a modulator of the homeostasis of the neurotrophins (NTs), a growth factor protein family whose most known member is the nerve growth factor (NGF), has been investigated. The modulation of NTs by small endogenous ligands is still a scarcely described area, with few papers reporting on the topic, and very few reports on the molecular determinants of these interactions. However, a detailed atomistic description of the NTs interaction landscape is of urgent need, aiming at the identification of novel molecules as potential therapeutics and considering the wide range of potential pharmacological applications for NGF and its family members. This mini-review will focus on the unique cartography casting the interactions of the endogenous ligand ATP, in the interaction with NGF as well as with its precursor proNGF. These interactions revealed interesting features of the ATP binding and distinct differences in the binding mode between the highly structured mature NGF and its precursor, proNGF, which is characterized by an intrinsically unstructured domain. The overview on the recent available data will be presented, together with the future perspectives on the field.

Effects of ATP and NGF on Proliferation and Migration of Neural Precursor Cells

Purinergic receptors belong to the most ancient neurotransmitter system. While their relevance in neurotransmission is well characterized, it has become clear that they have many other cellular functions. During development, they participate in regulation of proliferation and differentiation of stem cells. Here, we used rat embryonic telencephalon neurosphere cultures to detect purinergic P2 receptor subtype expression and possible synergistic actions of these receptors with NGF. Neurospheres proliferate in the presence of EGF and FGF-2; however, upon depletion of these growth factors, they migrate and differentiate into neurons and glial phenotypes. Expression patterns of P2X and P2Y receptors changed along neural differentiation. Gene expression of P2X2-7 and P2Y1,2,4,6,12,14 receptors was confirmed in undifferentiated and neural-differentiated neurospheres, with an up-regulation of P2X2 and P2X6 subtypes, together with a down-regulation of P2X4, P2X7 and P2Y subtypes upon induction to differentiation. BrdU-labeling and subsequent flow cytometry analysis was used to measure cell proliferation, which was increased by chronic exposure to NGF and increasing concentrations of ATP, in line with the expression levels of PCNA. Furthermore, a synergistic effect on proliferation was observed in conditions of co-incubation with ATP and NGF. While ATP and NGF independently promoted neural migration, no inter-relation between these factors was detected for this cellular process. As conclusion, an unknown synergism of ATP and NGF in proliferation is described. Future efforts may elucidate the underlying mechanisms of the interrelationship of ATP and NGF during neurogenesis.

P2Y(1) receptor mediated neuronal fibre outgrowth in organotypic brain slice co-cultures

Extracellular purines have multiple functional roles in development, plastic remodelling, and regeneration of the CNS by stimulating certain P2X/Y receptor (R) subtypes. In the present study we elucidated the involvement of P2YRs in neuronal fibre outgrowth in the developing nervous system. We particularly focused on the P2Y1R subtype and the dopaminergic system, respectively. For this purpose, we used organotypic slice co-cultures consisting of the ventral tegmental area/substantia nigra (VTA/SN) and the prefrontal cortex (PFC). After detecting the presence of the P2Y1R in VTA/SN, PFC, and on outgrowing fibres in the border region (e.g. on glial processes) connecting both brain slices, we could show that pharmacological modulation of the receptor influenced neuronal fibre outgrowth. Biocytin-tracing and tyrosine hydroxylase-immunolabelling together with quantitative image analysis revealed a significant increase in fibre growth in the border region of the co-cultures after treatment with ADPβS (P2Y1,12,13R agonist). The observed stimulatory potential of ADPβS was inhibited by pre-treatment with the P2X/YR antagonist PPADS. In P2Y1R knockout (P2Y1R(-/-)) mice, the ADPβS-induced stimulatory effect was absent, while growth was significantly enhanced in the co-cultures of the respective wild-type. This observation was confirmed in entorhino-hippocampal co-cultures, an example of a different projection system, expressing the P2Y1R. Using wortmannin and PD98059 we further showed that PI3K/Akt and MAPK/ERK cascades are involved in the mechanism underlying ADPβS-induced fibre growth. In conclusion, the data of this study clearly indicate that activation of the P2Y1R stimulates fibre growth and thereby emphasises the general role of this particular receptor subtype during development and regeneration.

In vitro study of neuroprotective properties of GK-2, a new original nerve growth factor mimetic

New nerve growth factor (NGF) mimetic GK-2, a substituted dimeric dipeptide, in a concentration of up to 10(-9)M produced a protective effect on the culture of immortalized mouse hippocampal neurons (line HT-22) after addition of H(2)O(2)and glutamate. GK-2 in a concentration of 10(-8)M protected rat PC-12 pheochromocytoma cells from the neurotoxin MPTP. The neuroprotective effect of this peptide on the model of oxidative stress was also observed in the primary culture of embryonic rat hippocampal neurons.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

ATP enhances neuronal differentiation of PC12 cells by activating PKCα interactions with cytoskeletal proteins

PKCα is a key mediator of the neuronal differentiation controlled by NGF and ATP. However, its downstream signaling pathways remain to be elucidated. To identify the signaling partners of PKCα, we analyzed proteins coimmunoprecipitated with this enzyme in PC12 cells differentiated with NGF and ATP and compared them with those obtained with NGF alone or growing media. Mass spectrometry analysis (LC-MS/MS) identified plectin, peripherin, filamin A, fascin, and β-actin as potential interacting proteins. The colocalization of PKCα and its interacting proteins increased when PC12 cells were differentiated with NGF and ATP. Peripherin and plectin organization and the cortical remodeling of β-actin were dramatically affected when PKCα was down-regulated, suggesting that all three proteins might be functional targets of ATP-dependent PKCα signaling. Taken together, these data demonstrate that PKCα is essential for controlling the neuronal development induced by NGF and ATP and interacts with the cytoskeletal components at two levels: assembly of the intermediate filament peripherin and organization of cortical actin.

Regulation of PC12 cell survival and differentiation by the new P2Y-like receptor GPR17

The P2Y-like receptor GPR17 has been reported to respond to both uracil nucleotides and cysteinyl-leukotrienes (cysLTs), such as UDP-glucose and LTD(4). Our previous data suggest a potential role for GPR17 in regulation of both cell viability and differentiation state of central nervous system cells. On this basis, in the present paper we investigated the effect of GPR17 receptor ligands on PC12 cell viability, following induction of morphological differentiation by nerve growth factor (NGF). In addition, the role of GPR17 ligands, either alone or in combination with growth factors, on the degree of PC12 cell differentiation was investigated. GPR17, which was not basally expressed in undifferentiated PC12 cells, was specifically induced by a 10day-treatment with NGF, suggesting a role in the control of neuronal specification. Both UDP-glucose and LTD(4), agonists at the nucleotide and cysLT GPR17 binding sites, respectively, induced a significant pro-survival effect on PC12 cells after priming with NGF. By in vitro silencing experiments with specific small interfering RNAs and by using receptor antagonists, we confirmed that the agonist effects are indeed mediated by the selective activation of GPR17. We also demonstrated that GPR17 agonists act, both alone and synergistically with NGF, to promote neurite outgrowth in PC12 cells. In addition, GPR17 ligands were able to confer an NGF-like activity to the epidermal growth factor (EGF), that, under these experimental conditions, also promoted cell differentiation and neurite elongation. Finally, we show that GPR17 ligands activate the intracellular phosphorylation of both ERK 1/2 and p38 kinases, that have been identified as important signalling pathways for neurotrophins in PC12 cells. Our results establish GPR17 as a neurotrophic regulator for neuronal-like cells and suggest a possible interplay between endogenous uracil derivatives, cysLTs and NGF in the signalling pathways involved in neuronal survival and differentiation. They also represent the first direct demonstration, in a native system, that GPR17 can indeed be activated by uracil nucleotides and cysLTs, in line with what previously demonstrated in recombinant expression systems.

Trophic functions of nucleotides in the central nervous system

In addition to short-term effects, one of the fundamental roles of extracellular nucleotides in the central nervous system involves long-term trophic effects. Physiological outcomes include neurogenesis, neuronal differentiation, glial proliferation, migration, growth arrest and apoptosis. Nucleotides exert these functions via P2-receptor-mediated mechanisms that can also interact with polypeptide-growth-factor-mediated or integrin-mediated signaling pathways. In addition, pathogenic roles for extracellular nucleotides in response to central nervous system injury including trauma and ischemia have been observed after the release of nucleotides by damaged and dying cells and in the development of neuropathic and inflammatory pain. Here, we illuminate the contribution of extracellular nucleotides to the development, growth, cellular plasticity and death of neural cells and the mechanisms regulating these trophic effects.

Do ATP and NO interact in the CNS?

Enzymatically derived NO and extracellular ATP are receiving greater attention due to their role as messengers in the CNS during different physiological and pathological processes. Ionotropic (P2XR) and metabotropic (P2YR) purinergic receptors mediate ATP effects and are present throughout the body. Particularly P2XR are crucial for brain plasticity mechanisms, and are involved in the pathogenesis of different CNS illnesses. NO does not have a specific receptor and its actions are directly dependent on the production on demand by different nitric oxide synthase isoforms. NO synthesizing enzymes are present virtually in all tissues, and NO influences multifarious physiological and pathological functions. Interestingly, various are the tissue and organs modulated by both ATP and NO, such as the immune, brain and vascular systems. Moreover, direct interactions between purinergic and nitrergic mechanisms outside the CNS are well documented, with several studies also indicating that ATP and NO do participate to the same CNS functions. In the past few years, further experimental evidence supported the physiological and pathological relevance of ATP and NO direct interactions in the CNS. The aim of the present review is to provide an account of the available information on the interplay between purinergic and nitrergic systems, focussing on the CNS. The already established relevance of ATP and NO in different pathological processes would predict that the knowledge of ATP/NO cross-talk mechanisms would support pharmacological approaches toward the development of novel ATP/NO combined pharmacological agents.

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.

Hyperbaric oxygenation in peripheral nerve repair and regeneration

Peripheral nerves are essential connections between the central nervous system and muscles, autonomic structures and sensory organs. Their injury is one of the major causes for severe and longstanding impairment in limb function. Acute peripheral nerve lesion has an important inflammatory component and is considered as ischemia-reperfusion (IR) injury. Surgical repair has been the standard of care in peripheral nerve lesion. It has reached optimal technical development but the end results still remain unpredictable and complete functional recovery is rare. Nevertheless, nerve repair is not primarily a mechanical problem and microsurgery is not the only key to success. Lately, there have been efforts to develop alternatives to nerve graft. Work has been carried out in basal lamina scaffolds, biologic and non-biologic structures in combination with neurotrophic factors and/or Schwann cells, tissues, immunosuppressive agents, growth factors, cell transplantation, principles of artificial sensory function, gene technology, gangliosides, implantation of microchips, hormones, electromagnetic fields and hyperbaric oxygenation (HBO). HBO appears to be a beneficial adjunctive treatment for surgical repair in the acute peripheral nerve lesion, when used at lower pressures and in a timely fashion (<6 hours).

Comparative analysis of P2Y4 and P2Y6 receptor architecture in native and transfected neuronal systems

Although extensive studies provided molecular and pharmacological characterization of metabotropic P2Y receptors for extracellular nucleotides, little is still known about their quaternary structure. By the use of transfected cellular systems and SDS-PAGE, in our previous work we established the propensity of P2Y(4) receptor to form dimeric interactions. Here we focused on endogenously expressed P2Y(4) and P2Y(6) subtypes, comparing their oligomeric complexes under Blue Native (BN) gel electrophoresis. We provided evidence that P2Y(4) and P2Y(6) receptors form high order complexes in native neuronal phenotypes and that the oligomers can be disaggregated down to the dimeric P2Y(4) or to the dimeric and monomeric P2Y(6) receptor. Moreover, dimeric P2Y(4) and monomeric P2Y(6) proteins display selective microdomain partitioning in lipid rafts from specialized subcellular compartments such as synaptosomes. Ligand activation by UTP shifted the oligomerization of P2Y(6) but not of P2Y(4) receptor, as analysed by BN electrophoresis. Finally, whereas transfected P2Y(4) and P2Y(6) proteins homo-interact and posses the appropriate domains to associate with all P2Y(1,2,4,6,11) subtypes, in naive PC12 cells the endogenous P2Y(4) forms hetero-oligomers only with the P2Y(6) subunit. In conclusion, our results indicate that quaternary structure distinguishing P2Y(4) from P2Y(6) receptors might be crucial for specific ligand activation, membrane partitioning and consequent functional regulation.

Uracil nucleotides stimulate human neural precursor cell proliferation and dopaminergic differentiation: involvement of MEK/ERK signalling

Isolation and propagation of neural stem cells derived from human brain tissue uniquely enables the study of human neurogenesis in vitro. In addition, ex vivo-expanded human neural stem/precursor cells (NPCs) may offer novel therapeutic strategies. We investigated the effects of extracellular nucleotides on the proliferation and differentiation of human mesencephalic neural stem/precursor cells (hmNPCs). When combined with the mitogens epidermal growth factor and fibroblast growth factor 2, UTP (1 microm) boosted proliferation of hmNPCs as shown by increased expression of the proliferation marker proliferating cell nuclear antigen (330%). UTP-induced proliferation was abrogated by the preferential P2Y receptor blocker pyridoxal phosphate-6-azophenyl-2',4'-disulphonic acid (PPADS). UTP also stimulated dopaminergic differentiation. Treatment with UTP (100 microm) increased the number of tyrosine hydroxylase (TH)-positive cells and TH protein by 267 and 319% respectively. UTP-stimulated dopaminergic differentiation of hmNPCs was blocked by the P2 receptor antagonists suramin (10 microm) and PPADS (100 microm). In addition, UDP (1 microm) enhanced TH protein expression by 194%. During differentiation, treatment with UTP stimulated the extracellular signal-regulated kinase (ERK) pathway. Both ERK1/2 phosphorylation and dopaminergic differentiation were inhibited by U0126, a selective ERK kinase inhibitor, as well as by suramin. When other P2 receptor agonists (ATP, ADP and adenosine 5'-O-(2-thiophosphate) (ADPbetaS); all 100 microm) were applied, both proliferation and dopaminergic differentiation of NPCs were compromised. We conclude that uracil nucleotides exert specific P2 receptor-mediated effects on midbrain-derived human NPCs, and may be used to enhance both proliferation and dopaminergic differentiation.

Functions of neuronal P2Y receptors

Within the last 15 years, at least eight different G protein-coupled nucleotide receptors, i.e., P2Y receptors, have been characterized by molecular means. While ionotropic P2X receptors are mainly involved in fast synaptic neurotransmission, P2Y receptors rather mediate slower neuromodulatory effects. This P2Y receptor-dependent neuromodulation relies on changes in synaptic transmission via either pre- or postsynaptic sites of action. At both sites, the regulation of voltage-gated or transmitter-gated ion channels via G protein-linked signaling cascades has been identified as the predominant underlying mechanisms. In addition, neuronal P2Y receptors have been found to be involved in neurotoxic and neurotrophic effects of extracellular adenosine 5-triphosphate. This review provides an overview of the most prominent actions mediated by neuronal P2Y receptors and describes the signaling cascades involved.

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.

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.

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.

Differences in the neurotoxicity profile induced by ATP and ATPgammaS in cultured cerebellar granule neurons

Extracellular ATP and P2 receptors may play a crucial role in the neurodegeneration of the CNS. Here, we investigated in neuronal cerebellar granule cultures the biological effect of the quite stable P2 receptor agonist ATPgammaS and compare it to the cytotoxic action of ATP. Time-course experiments showed that 500 microM ATPgammaS causes 50-100% cell death in 15-24 h. As proved by pharmacological means, ATPgammaS toxicity apparently involves neither indirect activation of NMDA receptors, nor ectonucleotidase activities, nor nucleoside transport and intracellular purine metabolism. Moreover, ATPgammaS induces detrimental effects without modifying the expression of several P2X and P2Y receptor proteins. Cell death instead occurs after extracellular release of the cytosolic enzyme lactic dehydrogenase and inhibition of the overall activity of the intracellular dehydrogenases. Moreover, ATPgammaS causes transient outflow of cytochrome c from mitochondria (maximal 2.5-fold stimulation in 4 h), it raises the intracellular reactive oxygen species (about four-fold in 1 h) and cAMP levels (about 40% in 15 min-4 h). Among several P2 receptor antagonists, only pyridoxal-phosphate-6-azophenyl-2',4'-disulphonic acid 4-sodium promotes 80-100% neuroprotection.

Addicted to death: invasive cancer and the immune response to unscheduled cell death

The development of an invasive cancer involves a progressive switch from predominantly apoptotic (scheduled) to necrotic (unscheduled) tumor cell death. This switch is associated with chronic and increasing release of intracellular factors that in turn promote reactive angiogenesis and stromal proliferation and mediates the disordered tumor microenvironment associated with local immune suppression. The authors review the relevant immunobiology of these factors, including the nuclear protein HMGB1; the products of purine metabolism (uric acid, ATP, and adenosine); the S100 family members; and the heat shock proteins, which we believe drive futile cycles of cell death followed by reparative cell growth. The authors also present a novel and provocative hypothesis that suggests that most of the derangements that we associate with progression of cancer and the associated immunologic consequences can indeed be ascribed to the consequences of disordered tumor cell death rather than cell growth. Thus the fundamental defect in invasive human cancers, in the authors' view, is not one of cell growth but rather one of disordered cell death, resulting in turn in a tumor microenvironment that encourages tumor growth, progression, and local immunosuppression, a condition the authors have termed "addicted to death." This new understanding could inform and drive the development of more effective biologic therapies for patients with cancer.

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.