Modulation of interleukin-1beta and tumor necrosis factor alpha signaling by P2 purinergic receptors in human fetal astrocytes

Notch signaling pathway: a new target for neuropathic pain therapy

The Notch gene, a highly evolutionarily conserved gene, was discovered approximately 110 years ago and has been found to play a crucial role in the development of multicellular organisms. Notch receptors and their ligands are single-pass transmembrane proteins that typically require cellular interactions and proteolytic processing to facilitate signal transduction. Recently, mounting evidence has shown that aberrant activation of the Notch is correlated with neuropathic pain. The activation of the Notch signaling pathway can cause the activation of neuroglia and the release of pro-inflammatory factors, a key mechanism in the development of neuropathic pain. Moreover, the Notch signaling pathway may contribute to the persistence of neuropathic pain by enhancing synaptic transmission and calcium inward flow. This paper reviews the structure and activation of the Notch signaling pathway, as well as its potential mechanisms of action, to provide novel insights for future treatments of neuropathic pain.

Consequences of the Lack of TNFR1 in Ouabain Response in the Hippocampus of C57BL/6J Mice

Ouabain is a cardiac glycoside that has a protective effect against neuroinflammation at low doses through Na⁺/K⁺-ATPase signaling and that can activate tumor necrosis factor (TNF) in the brain. TNF plays an essential role in neuroinflammation and regulates glutamate receptors by acting on two different receptors (tumor necrosis factor receptor 1 [TNFR1] and TNFR2) that have distinct functions and expression. The activation of constitutively and ubiquitously expressed TNFR1 leads to the expression of pro-inflammatory cytokines. Thus, this study aimed to elucidate the effects of ouabain in a TNFR1 knockout (KO) mouse model. Interestingly, the hippocampus of TNFR1 KO mice showed a basal increase in both TNFR2 membrane expression and brain-derived neurotrophic factor (BDNF) release, suggesting a compensatory mechanism. Moreover, ouabain activated TNF-α-converting enzyme/a disintegrin and metalloprotease 17 (TACE/ADAM17), decreased N-methyl-D-aspartate (NMDA) receptor subunit 2A (NR2A) expression, and induced anxiety-like behavior in both genotype animals, independent of the presence of TNFR1. However, ouabain induced an increase in interleukin (IL)-1β in the hippocampus, a decrease in IL-6 in serum, and an increase in NMDA receptor subunit 1 (NR1) only in wild-type (WT) mice, indicating that TNFR1 or TNFR2 expression may be important for some effects of ouabain. Collectively, our results indicate a connection between ouabain signaling and TNFR1, with the effect of ouabain partially dependent on TNFR1.

Altered astrocytic function in experimental neuroinflammation and multiple sclerosis

Multiple sclerosis (MS) is a chronic inflammatory disease of the central nervous system (CNS) that affects about 2.5 million people worldwide. In MS, the patients' immune system starts to attack the myelin sheath, leading to demyelination, neurodegeneration, and, ultimately, loss of vital neurological functions such as walking. There is currently no cure for MS and the available treatments only slow the initial phases of the disease. The later-disease mechanisms are poorly understood and do not directly correlate with the activity of immune system cells, the main target of the available treatments. Instead, evidence suggests that disease progression and disability are better correlated with the maintenance of a persistent low-grade inflammation inside the CNS, driven by local glial cells, like astrocytes and microglia. Depending on the context, astrocytes can (a) exacerbate inflammation or (b) promote immunosuppression and tissue repair. In this review, we will address the present knowledge that exists regarding the role of astrocytes in MS and experimental animal models of the disease.

Extracellular ATP Increases Glucose Metabolism in Skeletal Muscle Cells in a P2 Receptor Dependent Manner but Does Not Contribute to Palmitate-Induced Insulin Resistance

Saturated fatty acids such as palmitate contribute to the development of Type 2 Diabetes by reducing insulin sensitivity, increasing inflammation and potentially contributing to anabolic resistance. We hypothesized that palmitate-induced ATP release from skeletal muscle cells may increase inflammatory cytokine production and contribute to insulin/anabolic resistance in an autocrine/paracrine manner. In C2C12 myotubes differentiated at physiological glucose concentrations (5.5 mM), palmitate treatment (16 h) at concentrations greater than 250 μM increased release of ATP and inflammatory cytokines IL-6 and MIF, significantly blunted insulin and amino acid-induced signaling and reduced mitochondrial function. In contrast to our hypothesis, degradation of extracellular ATP using apyrase, did not alter palmitate-induced insulin resistance nor alter release of cytokines. Moreover, treatment with ATPγS (16 h), a non-hydrolysable ATP analog, in the absence of palmitate, did not diminish insulin sensitivity. Acute treatment with ATPγS produced insulin mimetic roles; increased phosphorylation of PKB (aka AKT), S6K1 and ERK and enhanced GLUT4-mediated glucose uptake in the absence of exogenous insulin. The increases in PKB and S6K1 phosphorylation were completely prevented by pre-incubation with broad spectrum purinergic receptor (P2R) blockers PPADs and suramin but not by P2 × 4 or P2 × 7 blockers 5-BDBD or A-438079, respectively. Moreover, ATPγS increased IL-6 yet decreased MIF release, similar to the cytokine profile produced by exercise. Acute and chronic treatment with ATPγS increased glycolytic rate in a manner that was differentially inhibited by PPADs and suramin, suggesting heterogeneous P2R activation in the control of cellular metabolism. In summary, our data suggest that the palmitate-induced increase in ATP does not contribute to insulin/anabolic resistance in a cell autonomous manner.

Targeting Cellular Stress Mechanisms and Metabolic Homeostasis by Chinese Herbal Drugs for Neuroprotection

Traditional Chinese medicine has been practiced for centuries in East Asia. Herbs are used to maintain health and cure disease. Certain Chinese herbs are known to protect and improve the brain, memory, and nervous system. To apply ancient knowledge to modern science, some major natural therapeutic compounds in herbs were extracted and evaluated in recent decades. Emerging studies have shown that herbal compounds have neuroprotective effects or can ameliorate neurodegenerative diseases. To understand the mechanisms of herbal compounds that protect against neurodegenerative diseases, we summarize studies that discovered neuroprotection by herbal compounds and compound-related mechanisms in neurodegenerative disease models. Those compounds discussed herein show neuroprotection through different mechanisms, such as cytokine regulation, autophagy, endoplasmic reticulum (ER) stress, glucose metabolism, and synaptic function. The interleukin (IL)-1β and tumor necrosis factor (TNF)-α signaling pathways are inhibited by some compounds, thus attenuating the inflammatory response and protecting neurons from cell death. As to autophagy regulation, herbal compounds show opposite regulatory effects in different neurodegenerative models. Herbal compounds that inhibit ER stress prevent neuronal death in neurodegenerative diseases. Moreover, there are compounds that protect against neuronal death by affecting glucose metabolism and synaptic function. Since the progression of neurodegenerative diseases is complicated, and compound-related mechanisms for neuroprotection differ, therapeutic strategies may need to involve multiple compounds and consider the type and stage of neurodegenerative diseases.

P2X7 purinergic receptors participate in the expression and extinction processes of contextual fear conditioning memory in mice

The purinergic system consists of two large receptor families - P2X and P2Y. Both are activated by adenosine triphosphate (ATP), although presenting different functions. These receptors are present in several brain regions, including those involved in emotion and stress-related behaviors. Hence, they seem to participate in fear- and anxiety-related responses. However, few studies have investigated the purinergic system in threatening situations, as observed in contextual fear conditioning (CFC). Therefore, this study investigated the involvement of purinergic receptors in the expression and extinction of aversive memories. C57Bl/6 background mice were submitted to the CFC protocol. Wildtype (WT) mice received i.p. injection of either a nonselective P2 receptor (P2R) antagonist, P178 (10 or 30 mg/kg); a selective P2X7 receptor (P2X7R) antagonist, A438079 (10 mg/kg); a selective P2Y1 receptor (P2Y1R) antagonist, MRS2179 (10 mg/kg); or vehicle 10 min prior to or immediately after the extinction session. Additionally, P2X7R KO mice were tested in the CFC protocol. After P2R antagonist treatment, contextual fear recall increased, while acquisition of extinction was impaired. Similar results were observed with the selective P2X7R antagonist, but not with the selective P2Y1R antagonist. Interestingly, P2X7R KO mice showed increased contextual fear recall, associated with impaired acquisition of extinction, in accordance with pharmacologic P2X7R antagonism. Our results suggest that specific pharmacological or genetic blockade of P2X7R promotes anxiogenic-like effects, along with deficits in extinction learning. Thus, these receptors could present an alternative treatment of stress-related psychiatric disorders.

Effect of Lead (Pb) on Inflammatory Processes in the Brain

That the nervous system is the main target of lead (Pb) has long been considered an established fact until recent evidence has linked the Pb effect on the immune system to the toxic effects of Pb on the nervous system. In this paper, we present recent literature reports on the effect of Pb on the inflammatory processes in the brain, particularly the expression of selected cytokines in the brain (interleukin 6, TGF-β1, interleukin 16, interleukin 18, and interleukin 10); expression and activity of enzymes participating in the inflammatory processes, such as cyclooxygenase 2, caspase 1, nitrogen oxide synthase (NOS 2) and proteases (carboxypeptidases, metalloproteinases and chymotrypsin); and the expression of purine receptors P2X4 and P2X7. A significant role in the development of inflammatory processes in the brain is also played by microglia (residual macrophages in the brain and the spinal cord), which act as the first line of defense in the central nervous system, and astrocytes—Whose most important function is to maintain homeostasis for the proper functioning of neurons. In this paper, we also present evidence that exposure to Pb may result in micro and astrogliosis by triggering TLR4-MyD88-NF-κB signaling cascade and the production of pro-inflammatory cytokines.

Extracellular ATP induces graded reactive response of astrocytes and strengthens their antioxidative defense in vitro

It is widely accepted that adenosine triphosphate (ATP) acts as a universal danger-associated molecular pattern with several known mechanisms for immune cell activation. In the central nervous system, ATP activates microglia and astrocytes and induces a neuroinflammatory response. The aim of the present study was to describe responses of isolated astrocytes to increasing concentrations of ATP (5 µM to 1 mM), which were intended to mimic graded intensity of the extracellular stimulus. The results show that ATP induces graded activation response of astrocytes in terms of the cell proliferation, stellation, shape remodeling, and underlying actin and GFAP filament rearrangement, although the changes occurred without an apparent increase in GFAP and actin protein expression. On the other hand, ATP in the range of applied concentrations did not evoke IL-1β release from cultured astrocytes, nor did it modify the release from LPS and LPS+IFN-γ-primed astrocytes. ATP did not promote astrocyte migration in the wound-healing assay, nor did it increase production of reactive oxygen and nitrogen species and lipid peroxidation. Instead, ATP strengthened the antioxidative defense of astrocytes by inducing Cu/ZnSOD and MnSOD activities and by increasing their glutathione content. Our current results suggest that although ATP triggers several attributes of activated astrocytic phenotype with a magnitude that increases with the concentration, it is not sufficient to induce full-blown reactive phenotype of astrocytes in vitro. © 2016 Wiley Periodicals, Inc.

Analgesic effect of total flavonoids from Sanguis draxonis on spared nerve injury rat model of neuropathic pain

BACKGROUND

Sanguis draxonis (SD) is a kind of red resin obtained from the wood of Dracaena cochinchinensis (Lour.) S. C. Chen (D. cochinchinensis). The active components of total flavonoids from SD (SDF) have analgesic effect.

AIM

The aim of this study is to evaluate the analgesic effects and potential mechanism of SDF on mechanical hypersensitivity induced by spared nerve injury (SNI) model of neuropathic pain in the rat.

METHODS

SNI model in rats was established and then the rats were treated with SDF intragastric administration for 14 days. Paw withdrawal mechanical threshold (PMWT) in response to mechanical stimulation was measured by von Frey filaments on day 1 before operation and days 1, 3, 5, 7, 9, 11, 14 after operation, respectively. After 14 days, we measured the levels of nitric oxide (NO), nitric oxide synthase (NOS), tumor necrosis factor-α (TNF-α), interleukin-1β (IL-1β) and interleukin-10 (IL-10) in the spinal dorsal horn. In addition, the expression of fibroblast growth factor receptor 3 (FGFR3), phosphorylated cyclic AMP response element-binding protein (p-CREB) and glial fibrillary acidic protein (GFAP) of the spinal dorsal horn was evaluated by western blotting and an immunofluorescence histochemical method, respectively.

RESULTS

Intragastric administration of SDF (100, 200, 400 mg/kg) alleviated significantly SNI-induced mechanical hypersensitivity, as PMWT increased in a dose-dependent manner. Moreover, SDF not only reduced the level of NO, NOS, TNF-α and IL-1β, but also upregulated the level of IL-10 in the spinal dorsal horn of SNI rats. At the same time, SDF (100, 200, 400 mg/kg) could inhibit the expression of FGFR3, GFAP and p-CREB in the spinal dorsal horn.

CONCLUSION

SDF has potentially reduced mechanical hypersensitivity induced by SNI model of neuropathic pain which may be attributed to inhibition of astrocytic function (like release pro-inflammatory cytokines) and NO release as well as p-CREB activation in the spinal dorsal horn.

Purines in neurite growth and astroglia activation

The mammalian nervous system is a complex, functional network of neurons, consisting of local and long-range connections. Neuronal growth is highly coordinated by a variety of extracellular and intracellular signaling molecules. Purines turned out to be an essential component of these processes. Here, we review the current knowledge about the involvement of purinergic signaling in the regulation of neuronal development. We particularly focus on its role in neuritogenesis: the formation and extension of neurites. In the course of maturation mammals generally lose their ability to regenerate the central nervous system (CNS) e.g. after traumatic brain injury; although, spontaneous regeneration still occurs in the peripheral nervous system (PNS). Thus, it is crucial to translate the knowledge about CNS development and PNS regeneration into novel approaches to enable neurons of the mature CNS to regenerate. In this context we give a general overview of growth-inhibitory and growth-stimulatory factors and mechanisms involved in neurite growth. With regard to neuronal growth, astrocytes are an important cell population. They provide structural and metabolic support to neurons and actively participate in brain signaling. Astrocytes respond to injury with beneficial or detrimental reactions with regard to axonal growth. In this review we present the current knowledge of purines in these glial functions. Moreover, we discuss organotypic brain slice co-cultures as a model which retains neuron-glia interactions, and further presents at once a model for CNS development and regeneration. In summary, the purinergic system is a pivotal factor in neuronal development and in the response to injury. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Extracellular ATP Selectively Upregulates Ecto-Nucleoside Triphosphate Diphosphohydrolase 2 and Ecto-5'-Nucleotidase by Rat Cortical Astrocytes In Vitro

Extracellular ATP (eATP) acts as a danger-associated molecular pattern which induces reactive response of astrocytes after brain insult, including morphological remodeling of astrocytes, proliferation, chemotaxis, and release of proinflammatory cytokines. The responses induced by eATP are under control of ecto-nucleotidases, which catalyze sequential hydrolysis of ATP to adenosine. In the mammalian brain, ecto-nucleotidases comprise three enzyme families: ecto-nucleoside triphosphate diphosphohydrolases 1-3 (NTPDase1-3), ecto-nucleotide pyrophosphatase/phospodiesterases 1-3 (NPP1-3), and ecto-5'-nucleotidase (eN), which crucially determine ATP/adenosine ratio in the pericellular milieu. Altered expression of ecto-nucleotidases has been demonstrated in several experimental models of human brain dysfunctions. In the present study, we have explored the pattern of NTPDase1-3, NPP1-3, and eN expression by cultured cortical astrocytes challenged with 1 mmol/L ATP (eATP). At the transcriptional level, eATP upregulated expression of NTPDase1, NTPDase2, NPP2, and eN, while, at translational and functional levels, these were paralleled only by the induction of NTPDase2 and eN. Additionally, eATP altered membrane topology of eN, from clusters localized in membrane domains to continuous distribution along the cell membrane. Our results suggest that eATP, by upregulating NTPDase2 and eN and altering the enzyme membrane topology, affects local kinetics of ATP metabolism and signal transduction that may have important roles in the process related to inflammation and reactive gliosis.

Pathological potential of astroglial purinergic receptors

Acute brain injury and neurodegenerative disorders may result in astroglial activation. Astrocytes are able to determine the progression and outcome of these neuropathologies in a beneficial or detrimental way. Nucleotides, e.g. adenosine 5'-triphosphate (ATP), released after acute or chronic neuronal injury, are important mediators of glial activation and astrogliosis.Acute injury may cause significant changes in ATP balance, resulting in (1) a decline of intracellular ATP levels and (2) an increase in extracellular ATP concentrations via efflux from the intracellular space. The released ATP may have trophic effects, but can also act as a proinflammatory mediator or cytotoxic factor, inducing necrosis/apoptosis as a universal "danger" signal. Furthermore, ATP, primarily released from astrocytes, is a means of communication between neurons, glial cells, and intracerebral blood vessels.Astrocytes express a heterogeneous battery of purinergic ionotropic and metabotropic receptors (P2XRs and P2YRs, respectively) to respond to extracellular nucleotides.In this chapter, we summarize the contemporary knowledge on the pathological potential of P2Rs in relation to changes of astrocytic functions, determined by distinct molecular signaling cascades, in a variety of diseases. We discuss specific aspects of reactive astrogliosis, with respect to the involvement of prominent receptor subtypes, such as the P2X7 and P2Y1/2Rs. Examples of purinergic signaling of microglia, oligodendrocytes, and blood vessels under pathophysiological conditions will also be presented.The understanding of the pathological potential of purinergic signaling in "controlling and fine-tuning" of astrocytic responses is important for identifying possible therapeutic principles to treat acute and chronic central nervous system diseases.

Importance of glial activation in neuropathic pain

Glia plays a crucial role in the maintenance of neuronal homeostasis in the central nervous system. The microglial production of immune factors is believed to play an important role in nociceptive transmission. Pain may now be considered a neuro-immune disorder, since it is known that the activation of immune and immune-like glial cells in the dorsal root ganglia and spinal cord results in the release of both pro- and anti-inflammatory cytokines, as well as algesic and analgesic mediators. In this review we presented an important role of cytokines (IL-1alfa, IL-1beta, IL-2, IL-4, IL-6, IL-10, IL-15, IL-18, TNFalpha, IFNgamma, TGF-beta 1, fractalkine and CCL2); complement components (C1q, C3, C5); metaloproteinases (MMP-2,-9) and many other factors, which become activated on spinal cord and DRG level under neuropathic pain. We discussed the role of the immune system in modulating chronic pain. At present, unsatisfactory treatment of neuropathic pain will seek alternative targets for new drugs and it is possible that anti-inflammatory factors like IL-10, IL-4, IL-1alpha, TGF-beta 1 would fulfill this role. Another novel approach for controlling neuropathic pain can be pharmacological attenuation of glial and immune cell activation. It has been found that propentofylline, pentoxifylline, minocycline and fluorocitrate suppress the development of neuropathic pain. The other way of pain control can be the decrease of pro-nociceptive agents like transcription factor synthesis (NF-kappaB, AP-1); kinase synthesis (MEK, p38MAPK, JNK) and protease activation (cathepsin S, MMP9, MMP2). Additionally, since it is known that the opioid-induced glial activation opposes opioid analgesia, some glial inhibitors, which are safe and clinically well tolerated, are proposed as potential useful ko-analgesic agents for opioid treatment of neuropathic pain. This review pointed to some important mechanisms underlying the development of neuropathic pain, which led to identify some possible new approaches to the treatment of neuropathic pain, based on the more comprehensive knowledge of the interaction between the nervous system and glial and immune cells.

Pathophysiology of astroglial purinergic signalling

Astrocytes are fundamental for central nervous system (CNS) physiology and are the fulcrum of neurological diseases. Astroglial cells control development of the nervous system, regulate synaptogenesis, maturation, maintenance and plasticity of synapses and are central for nervous system homeostasis. Astroglial reactions determine progression and outcome of many neuropathologies and are critical for regeneration and remodelling of neural circuits following trauma, stroke, ischaemia or neurodegenerative disorders. They secrete multiple neurotransmitters and neurohormones to communicate with neurones, microglia and the vascular walls of capillaries. Signalling through release of ATP is the most widespread mean of communication between astrocytes and other types of neural cells. ATP serves as a fast excitatory neurotransmitter and has pronounced long-term (trophic) roles in cell proliferation, growth, and development. During pathology, ATP is released from damaged cells and acts both as a cytotoxic factor and a proinflammatory mediator, being a universal "danger" signal. In this review, we summarise contemporary knowledge on the role of purinergic receptors (P2Rs) in a variety of diseases in relation to changes of astrocytic functions and nucleotide signalling. We have focussed on the role of the ionotropic P2X and metabotropic P2YRs working alone or in concert to modify the release of neurotransmitters, to activate signalling cascades and to change the expression levels of ion channels and protein kinases. All these effects are of great importance for the initiation, progression and maintenance of astrogliosis-the conserved and ubiquitous glial defensive reaction to CNS pathologies. We highlighted specific aspects of reactive astrogliosis, especially with respect to the involvement of the P2X(7) and P2Y(1)R subtypes. Reactive astrogliosis exerts both beneficial and detrimental effects in a context-specific manner determined by distinct molecular signalling cascades. Understanding the role of purinergic signalling in astrocytes is critical to identifying new therapeutic principles to treat acute and chronic neurological diseases.

Astrocyte immune responses in epilepsy

Astrocytes, the major glial cell type of the central nervous system (CNS), are known to play a major role in the regulation of the immune/inflammatory response in several human CNS diseases. In epilepsy-associated pathologies, the presence of astrogliosis has stimulated extensive research focused on the role of reactive astrocytes in the pathophysiological processes that underlie the development of epilepsy. In brain tissue from patients with epilepsy, astrocytes undergo significant changes in their physiological properties, including the activation of inflammatory pathways. Accumulating experimental evidence suggests that proinflammatory molecules can alter glio-neuronal communications contributing to the generation of seizures and seizure-related neuronal damage. In particular, both in vitro and in vivo data point to the role of astrocytes as both major source and target of epileptogenic inflammatory signaling. In this context, understanding the astroglial inflammatory response occurring in epileptic brain tissue may provide new strategies for targeting astrocyte-mediated epileptogenesis. This article reviews current evidence regarding the role of astrocytes in the regulation of the innate immune responses in epilepsy. Both clinical observations in drug-resistant human epilepsies and experimental findings in clinically relevant models will be discussed and elaborated, highlighting specific inflammatory pathways (such as interleukin-1β/toll-like receptor 4) that could be potential targets for antiepileptic, disease-modifying therapeutic strategies.

P2Y receptors on astrocytes and microglia mediate opposite effects in astroglial proliferation

Nucleotides released upon brain injury signal to astrocytes and microglia playing an important role in astrogliosis, but the participation of microglia in the purinergic modulation of astrogliosis is still unclear. Highly enriched astroglial cultures and co-cultures of astrocytes and microglia were used to investigate the influence of microglia in the modulation of astroglial proliferation mediated by nucleotides. In highly enriched astroglial cultures, adenosine-5'-triphosphate (ATP), adenosine 5'-O-(3-thio)-triphosphate (ATPγS), adenosine 5'-O-(3-thio)-diphosphate (ADPβS; 0.01-1 mM), and adenosine-5'-diphosphate (ADP; 0.1-1 mM) increased proliferation up to 382%, an effect abolished in co-cultures containing 8% of microglia. The loss of ATP proliferative effect in co-cultures is supported by its fast metabolism and reduced ADP accumulation, an agonist of P2Y(1,12) receptors that mediate astroglial proliferation. No differences in ADPβS and ATPγS metabolism or P2Y(1,12) receptors expression were found in co-cultures that could explain the loss of their proliferative effect. However, conditioned medium from microglia cultures or co-cultures treated with ADPβS, when tested in highly enriched astroglial cultures, also prevented ADPβS proliferative effect. None of the uracil nucleotides tested had any effect in proliferation of highly enriched astroglial cultures, but uridine-5'-triphosphate (UTP; 0.1-1 mM) inhibited proliferation up to 66% in co-cultures, an effect that was dependent on uridine-5'-diphosphate (UDP) accumulation, coincident with a co-localization of P2Y(6) receptors in microglia and due to cell apoptosis. The results indicate that microglia control astroglial proliferation by preventing the proliferative response to adenine nucleotides and favouring an inhibitory effect of UTP/UDP. Several microglial P2Y receptors may be involved by inducing the release of messengers that restrain astrogliosis, a beneficial effect for neuronal repair mechanisms following brain injury.

The role of reactive-oxygen-species in microbial persistence and inflammation

The mechanisms of chronic infections caused by opportunistic pathogens are of keen interest to both researchers and health professionals globally. Typically, chronic infectious disease can be characterized by an elevation in immune response, a process that can often lead to further destruction. Reactive-Oxygen-Species (ROS) have been strongly implicated in the aforementioned detrimental response by host that results in self-damage. Unlike excessive ROS production resulting in robust cellular death typically induced by acute infection or inflammation, lower levels of ROS produced by host cells are increasingly recognized to play a critical physiological role for regulating a variety of homeostatic cellular functions including growth, apoptosis, immune response, and microbial colonization. Sources of cellular ROS stimulation can include "danger-signal-molecules" such as extracellular ATP (eATP) released by stressed, infected, or dying cells. Particularly, eATP-P2X(7) receptor mediated ROS production has been lately found to be a key modulator for controlling chronic infection and inflammation. There is growing evidence that persistent microbes can alter host cell ROS production and modulate eATP-induced ROS for maintaining long-term carriage. Though these processes have yet to be fully understood, exploring potential positive traits of these "injurious" molecules could illuminate how opportunistic pathogens maintain persistence through physiological regulation of ROS signaling.

Lipopolysaccharide enhances asymmetrical production of cytokines and nitric oxide by left and right cerebral cortical microglial cells in BALB/C mice

Lipopolysaccharide (LPS)-induced inflammatory factors production by the cerebral cortical glial cells in two sides of the murine brain are different. To determine if microglial cells, a subset of glial cells, are involved in asymmetric production, interleukin-6 (IL-6), interleukin-1β (IL-1β) and nitric oxide (NO) responses to LPS by microglial cells in the right and left cerebral cortices were examined. Primary microglial cells were isolated from BALB/C neonatal mice, treated with LPS (10 µg ml(-1) ) for 24 h and examined for IL-6, IL-1β and NO production. At untreated state, the levels of IL-6, IL-1β and NO showed no statistical difference between left and right. However, after LPS treatment, the levels of IL-6, IL-1β and NO for the right microglial cells was statistically significant higher than the left (P < 0·05). Our results denote that enhanced production of IL-6, IL-1β and NO after LPS treatment in microglia is directly proportional to their basal-state levels, and right cortical microglia produce higher levels of IL-6, IL-1β and NO than left cortical microglia.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

Cell signaling via the P2X(7) nucleotide receptor: linkage to ROS production, gene transcription, and receptor trafficking

Extracellular nucleotides can act as important intercellular signals in diverse biological processes, including the enhanced production of factors that are key to immune response regulation. One receptor that binds extracellular adenosine triphosphate released at sites of infection and injury is P2X(7), which is an ionotrophic receptor that can also lead to the formation of a non-specific pore, activate multiple mitogen-activated protein kinases (MAPKs), and stimulate the production of immune mediators including interleukin family members and reactive oxygen species (ROS). In the present report, we have investigated the signaling mechanisms by which P2X(7) promotes monocytic cell mediator production and induces transcription factor expression/phosphorylation, as well as how receptor-associated pore activity is regulated by intracellular trafficking. We report that P2X(7) stimulates ROS production in macrophages through the MAPKs ERK1/2 and the nicotinamide adenine dinucleotide phosphate oxidase complex, activates several transcription factors including cyclic-AMP response element-binding protein and components of the activating protein-1 complex, and contains specific sequences within its intracellular C-terminus that appear critical for its activity. Altogether, these data further implicate P2X(7) activation and signaling as a fundamental modulator of macrophage immune responses.

P2Y1 receptor signaling enhances neuroprotection by astrocytes against oxidative stress via IL-6 release in hippocampal cultures

Cell survival is a critical issue in the onset and progression of neurodegenerative diseases and following pathological events including ischemia and traumatic brain injury. Oxidative stress is the main cause of cell damage in such pathological conditions. Here, we report that adenosine 5'-triphosphate (ATP) protects hippocampal astrocytes from hydrogen peroxide (H(2)O(2))-evoked oxidative injury in astrocyte monocultures. The effect of ATP was prevented by a selective antagonist of or siRNAs against P2Y(1)R. Interestingly, in astrocyte-neuron cocultures, ATP also produced neuroprotective effects against H(2)O(2)-evoked neuronal cell death, whereas ATP did not produce any neuroprotective effects in monocultures. The ATP-induced neuroprotection in cocultures was completely inhibited by silencing of astrocytic P2Y(1)R expression, indicating that ATP acts on astrocytes and enhances their neuroprotective functions by activating P2Y(1)R. Furthermore, this neuroprotective effect was mimicked by applying conditioned medium from astrocytes that had been stimulated by ATP, implying an involvement of diffusible factors from astrocytes. We found that, in both purified astrocyte cultures and astrocyte-neuronal cocultures, ATP and the P2Y(1)R agonist 2-methylthioadenosine 5' diphosphate (2MeSADP) induced the release of interleukin-6 (IL-6), but this did not occur in neuron monocultures. Moreover, exogenous IL-6 produced a neuroprotective effect, and the neuroprotection induced by P2Y(1)R-stimulated astrocytes was prevented in the presence of an anti-IL-6 antibody. Taken together, these results suggest that P2Y(1)R-stimulated astrocytes protect against neuronal damage induced by oxidative stress, and that IL-6 is a crucial signaling molecule released from astrocytes. Thus, activation of P2Y(1)R in astrocytes may rescue neurons from secondary cell death under pathological conditions.

Extracellular inosine participates in tumor necrosis factor-alpha induced nitric oxide production in cultured Sertoli cells

Recent reports have described purinergic modulation of tumor necrosis factor-alpha (TNF-alpha) signaling in neutrophils and astrocytes. In Sertoli cells, both TNF-R1 and TNF-R2 TNF-alpha receptors are present and this cytokine modulates many functions of these cells related to the maintenance of spermatogenesis. Sertoli cells express distinct purinoreceptors and previous work has shown that these cells secrete extracellular nucleotides and their metabolites. In this work, we studied the possible role of extracellular purines in TNF-alpha signaling in cultured Sertoli cells. This cytokine increased inosine concentration from 30 min to 6 h, with no effect at 24 h. Both TNF-alpha and inosine increased nitrite accumulation and nitric oxide synthase activity. Erythro-9-(2-hydroxy-3-nonyl)adenine (EHNA), an adenosine deaminase inhibitor, abolished the TNF-alpha induced inosine increase, nitrite accumulation and nitric oxide synthase activity. These results suggest that extracellular inosine acts as intermediary in TNF-alpha stimulated nitric oxide production in cultured Sertoli cells.

Cell-specific and concentration-dependent actions of interleukin-1 in acute brain inflammation

Interleukin (IL)-1 is a pivotal pro-inflammatory cytokine and an important mediator of both acute and chronic central nervous system (CNS) injuries. Despite intense research in CNS IL-1 biology over the past two decades, its precise mechanism of action in inflammatory responses to acute brain disorders remains largely unknown. In particular, much effort has been focussed on using in vitro approaches to better understand the cellular and signalling mechanisms of actions of IL-1, yet some discrepancies in the literature regarding the effects produced by IL-1beta in in vitro paradigms of injury still exist, particularly as to whether IL-1 exerts neurotoxic or neuroprotective effects. Here we aim to review the cell-specific and concentration-dependent actions of IL-1 in brain cells, to depict the mechanism by which this cytokine induces neurotoxicity or neuroprotection in acute brain injury.

Regulated exocytosis from astrocytes physiological and pathological related aspects

Astrocytes have traditionally been considered ancillary, satellite cells of the nervous system. However, it is a very recent acquisition that glial cells generate signaling loops which are integral to the brain circuitry and participate, interactively with neuronal networks, in the processing of information. Such a conceptual breakthrough makes this field of investigation one of the hottest in neuroscience, as it calls for a revision of past theories of brain function as well as for new strategies of experimental exploration of brain function. Glial cells are electrically not excitable, and it was only the use of optical recording techniques together with calcium sensitive dyes, that allowed the chemical excitability of glial cells to become apparent. Studies using these new techniques have shown for the first time that glial cells are activated by surrounding synaptic activity and translate neuronal signals into their own calcium code. Intracellular calcium concentration([Ca2+]i) elevations in glial cells have then shown to underlie spatial transfer of information in the glial network, accompanied by release of chemical transmitters (gliotransmitters) such as glutamate and back-signaling to neurons. As a consequence, optical imaging techniques applied to cell cultures or intact tissue have become a state-of-the-art technology for studying glial cell signaling. The molecular mechanisms leading to release of "gliotransmitters," especially glutamate, from glia are under debate. Accumulating evidence clearly indicates that astrocytes secrete numerous transmitters by Ca(2+)-dependent exocytosis. This review will discuss the mechanisms underlying the release of chemical transmitters from astrocytes with a particular emphasis to the regulated exocytosis processes.

Cytokine mRNA induction by interleukin-1beta or tumor necrosis factor alpha in vitro and in vivo

Hypothalamic and cortical mRNA levels for cytokines such as interleukin-1beta (IL1beta), tumor necrosis factor alpha (TNFalpha), nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) are impacted by systemic treatments of IL1beta and TNFalpha. To investigate the time course of the effects of IL1beta and TNFalpha on hypothalamic and cortical cytokine gene expression, we measured mRNA levels for IL1beta, TNFalpha, interleukin-6 (IL-6), interleukin-10 (IL-10), IL1 receptor 1, BDNF, NGF, and glutamate decarboxylase-67 in vitro using hypothalamic and cortical primary cultures. IL1beta and TNFalpha mRNA levels increased significantly in a dose-dependent fashion after exposure to either IL1beta or TNFalpha. IL1beta increased IL1beta mRNA in both the hypothalamic and cortical cultures after 2-6 h while TNFalpha mRNA increased significantly within 30 min and continued to rise up to 2-6 h. Most of the other mRNAs showed significant changes independent of dose in vitro. In vivo, intracerebroventricular (icv) injection of IL1beta or TNFalpha also significantly increased IL1beta, TNFalpha and IL6 mRNA levels in the hypothalamus and cortex. IL1beta icv, but not TNFalpha, increased NGF mRNA levels in both these areas. Results support the hypothesis that centrally active doses of IL1beta and TNFalpha enhance their own mRNA levels as well as affect mRNA levels for other neuronal growth factors.

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.

The correlation between cytokine production by cerebral cortical glial cells and brain lateralization in mice

Objectives.   This study aims to explore the relationship among the levels of interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α) produced by cortical glial cells, and identify any correlation between neuromodulation and brain lateralization. Material and Methods.   Cortical glial cells from Balb/c neonatal mice were cultured in vitro and the effects of treating or not treating these cells from both hemispheres with lipopolysaccharide (LPS) (10 µg/mL) for 24 hours were tested. The levels of IL-1β, IL-6, and TNF-α in left and right cortical glial cell cultures and the time course of any changes were compared. Rusults.   The production of IL-1β and TNF-α had no significant difference between right and left cortex in the untreated group within 24 hours. IL-6 was significantly higher in the right than the left cortical glial cells. In the LPS-treated group, increased levels of IL-1β, TNF-α, and IL-6 were found, particularly for IL-6, and all were significantly increased in cortical glia cells from the right side. The time course shows that the expression of IL-1β in right cortex and IL-6 in both sides is time-dependent (p < 0.05). Conclusion.   Lipopolysaccharide increases cytokine production in both cerebral cortices, three cytokines have different expression time course within 72 hours, but only IL-1β in right cortex and IL-6 releasing is time-dependent, and more so on the right side than the left in 24 hours. We proposed the increased immunosuppressive activity of right cortex was due to the higher expression of IL-1β, TNF-α, and IL-6 in the right cortical glial cells, whereas there would be more immunoenhancement activity of the left cortex due to the lower levels of these three kinds of cytokines, this being a less pronounced effect than that on the right side. One of the reasons for the brain lateralization may be the different production of cytokines by the cortical glial cells on either side.

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.

IL-1beta regulates blood-brain barrier permeability via reactivation of the hypoxia-angiogenesis program

Loss of blood-brain barrier (BBB) integrity is believed to be an early and significant event in lesion pathogenesis in the inflammatory demyelinating disease multiple sclerosis (MS), and understanding mechanisms involved may lead to novel therapeutic avenues for this disorder. Well-differentiated endothelium forms the basis of the BBB, while astrocytes control the balance between barrier stability and permeability via production of factors that restrict or promote vessel plasticity. In this study, we report that the proinflammatory cytokine IL-1beta, which is prominently expressed in active MS lesions, causes a shift in the expression of these factors to favor plasticity and permeability. The transcription factor, hypoxia inducible factor-1 (HIF-1), plays a significant role in this switch. Using a microarray-based approach, we found that in human astrocytes, IL-1beta induced the expression of genes favoring vessel plasticity, including HIF-1alpha and its target, vascular endothelial growth factor-A (VEGF-A). Demonstrating relevance to MS, we showed that HIF-1alpha and VEGF-A were expressed by reactive astrocytes in active MS lesions, while the VEGF receptor VEGFR2/flk-1 localized to endothelium and IL-1 to microglia/macrophages. Suggesting functional significance, we found that expression of IL-1beta in the brain induced astrocytic expression of HIF-1alpha, VEGF-A, and BBB permeability. In addition, we confirmed VEGF-A to be a potent inducer of BBB permeability and angiogenesis, and demonstrated the importance of IL-1beta-induced HIF-1alpha in its regulation. These results suggest that IL-1beta contributes to BBB permeability in MS via reactivation of the HIF-VEGF axis. This pathway may represent a potential therapeutic target to restrict lesion formation.

Inflammation-Like Glial Response in Lead-Exposed Immature Rat Brain

Numerous studies on lead (Pb) neurotoxicity have indicated this metal to be a dangerous toxin, particularly during developmental stages of higher organisms. Astrocytes are responsible for sequestration of this metal in brain tissue. Activation of astroglia may often lead to loss of the buffering function and contribute to pathological processes. This phenomenon is accompanied by death of neuronal cells and may be connected with inflammatory events arising from the production of a wide range of cytokines and chemokines. The effects of prolonged exposure to Pb upon glial activation are examined in immature rats to investigate this potential proinflammatory effect. When analyzed at the protein level, glial activation is observed after Pb exposure, as reflected by the increased level of glial fibrillary acidic protein and S-100beta proteins in all parts of the brain examined. These changes are associated with elevation of proinflammatory cytokines. Production of interleukin (IL)-1beta and tumor necrosis factor-alpha is observed in hippocampus, and production of IL-6 is seen in forebrain. The expression of fractalkine is observed in both hippocampus and forebrain but inconsiderably in the cerebellum. In parallel with cytokine expression, signs of synaptic damage in hippocampus are seen after Pb exposure, as indicated by decreased levels of the axonal markers synapsin I and synaptophysin. Obtained results indicate chronic glial activation with coexisting inflammatory and neurodegenerative features as a new mechanism of Pb neurotoxicity in immature rat brain.

Differential effects of Th1, monocyte/macrophage and Th2 cytokine mixtures on early gene expression for immune-related molecules by central nervous system mixed glial cell cultures

Cytokines secreted within the central nervous system (CNS) are important in the development of multiple sclerosis (MS) lesions. The balance between Th1, monocyte/macrophage (M/M) and Th2 cytokines in the CNS may be pivotal in determining the outcome of lesion development. We examined the effects of mixtures of cytokines on gene expression by CNS glial cells, as mixtures of cytokines are present in MS lesions, which in turn contain mixtures of glial cells. In this initial analysis by gene array, we examined changes at 6 hours to identify early changes in gene expression that represent primary responses to the cytokines. Rat glial cells were incubated with mixtures of Th1, M/M and Th2 cytokines for 6 hours and examined for changes in early gene expression employing microarray gene chip technology. A minimum of 814 genes were differentially regulated by one or more of the cytokine mixtures in comparison to controls, including changes in expression in a large number of genes for immune system-related proteins. Expression of the proteins for these genes likely influences development and inhibition of MS lesions as well as protective and regenerative processes. Analysing gene expression for the effects of various combinations of exogenous cytokines on glial cells in the absence of the confounding effects of inflammatory cells themselves should increase our understanding of cytokine-induced pathways in the CNS.

Diadenosine polyphosphate analog controls postsynaptic excitation in CA3-CA1 synapses via a nitric oxide-dependent mechanism

Previously, we have described the modulatory effect of diadenosine polyphosphates Ap4A and Ap5A on synaptic transmission in the rat hippocampal slices mediated by presynaptic receptors (Klishin et al., 1994). In contrast, we now describe how nonhydrolyzable Ap4A analog diadenosine-5',5'''-P1,P4-[beta,beta'-methylene]tetraphosphate (AppCH2ppA) at low micromolar concentrations exerts strong nondesensitizing inhibition of orthodromically evoked field potentials (OFPs) without affecting the amplitude of excitatory postsynaptic currents and antidromically evoked field potentials, as recorded in hippocampal CA1 zone. The effects of AppCH2ppA on OFPs are eliminated by a P2 receptor antagonist pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) but not mimicked by purinoceptor agonists alpha,beta-methylene-ATP and adenosine 5'-O-(3-thio)-triphosphate, indicating that a P2-like receptor is involved but not one belonging to the conventional P2X/P2Y receptor classes. Diadenosine polyphosphate receptor (P4) antagonist Ip4I (diinosine tetraphosphate) was unable to modulate AppCH2ppA effects. Thus, the PPADS-sensitive P2-like receptor for AppCH2ppA seems to control selectively dendritic excitation of the CA1 neurons. The specific nitric oxide (NO)-scavenger 2-phenyl-4,4,5,5-tetramethyl-imidazoline-1-oxyl-3-oxide is shown to significantly attenuate AppCH2ppA-mediated inhibitory effects, indicating that NO is involved in the cascade of events initiated by AppCH2ppA. Further downstream mediation by adenosine A1 receptors is also demonstrated. Hence, AppCH2ppA-mediated effects involve PPADS-sensitive P2-like receptor activation leading to the production of NO that stimulates intracellular synthesis of adenosine, causing in turn postsynaptic A1 receptor activation and subsequent postsynaptic CA1 dendritic inhibition. Such spatially selective postsynaptic dendritic inhibition may influence dendritic electrogenesis in pyramidal neurons and consequently mediate control of neuronal network activity.

Uridine enhances neurite outgrowth in nerve growth factor-differentiated PC12 [corrected]

During rapid cell growth the availability of phospholipid precursors like cytidine triphosphate and diacylglycerol can become limiting in the formation of key membrane constituents like phosphatidylcholine. Uridine, a normal plasma constituent, can be converted to cytidine triphosphate in PC12 [corrected] cells and intact brain, and has been shown to produce a resulting increase in phosphatidylcholine synthesis. To determine whether treatments that elevate uridine availability also thereby augment membrane production, we exposed PC12 [corrected] cells which had been differentiated by nerve growth factor to various concentrations of uridine, and measured the numbers of neurites the cells produced. After 4 but not 2 days uridine significantly and dose-dependently increased the number of neurites per cell. This increase was accompanied by increases in neurite branching and in levels of the neurite proteins neurofilament M [corrected] and neurofilament 70. Uridine treatment also increased intracellular levels of cytidine triphosphate, which suggests that uridine may affect neurite outgrowth by enhancing phosphatidylcholine synthesis. Uridine may also stimulate neuritogenesis by a second mechanism, since the increase in neurite outgrowth was mimicked by exposing the cells to uridine triphosphate, and could be blocked by various drugs known to antagonize P2Y receptors (suramin; Reactive Blue 2; pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid). Treatment of the cells with uridine or uridine triphosphate stimulated their accumulation of inositol phosphates, and this effect was also blocked by pyridoxal-phosphate-6-azophenyl-2',4' disulfonic acid. Moreover, degradation of nucleotides by apyrase blocked the stimulatory effect of uridine on neuritogenesis. Taken together these data indicate that uridine can regulate the output of neurites from differentiating PC12 [corrected] cells, and suggest that it does so in two ways, i.e. both by acting through cytidine triphosphate as a precursor for phosphatidylcholine biosynthesis and through uridine triphosphate as an agonist for P2Y receptors.

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.

Bi-functional effects of ATP/P2 receptor activation on tumor necrosis factor-alpha release in lipopolysaccharide-stimulated astrocytes

Neuroinflammation is associated with a variety of CNS pathologies. Levels of tumor necrosis factor-alpha (TNF-alpha), a major proinflammatory cytokine, as well as extracellular ATP, are increased following various CNS insults. Here we report on the relationship between ATP/P2 purinergic receptor activation and lipopolysaccharide (LPS)-induced TNF-alpha release from primary cultures of rat cortical astrocytes. Using ELISA, we confirmed that treatment with LPS stimulated the release of TNF-alpha in a concentration and time dependent manner. ATP treatment alone had no effect on TNF-alpha release. LPS-induced TNF-alpha release was attenuated by 1 mm ATP, a concentration known to activate P2X7 receptors. Consistent with this, 3'-O-(4-Benzoyl)benzoyl-ATP (BzATP), a P2X7 receptor agonist, also attenuated LPS-induced TNF-alpha release. This reduction in TNF-alpha release was not due to loss of cell viability. Adenosine and 2-chloroadenosine were ineffective, suggesting that attenuation of LPS-induced TNF-alpha release by ATP was not due to ATP breakdown and subsequent activation of adenosine/P1 receptors. Interestingly, treatment of astrocyte cultures with 10 microm or 100 microm ATP potentiated TNF-alpha release induced by a submaximal concentration of LPS. UTP and 2methylthioADP (2-MeSADP), P2Y receptor agonists, also enhanced this LPS-induced TNF-alpha release. Our observations demonstrate opposing effects of ATP/P2 receptor activation on TNF-alpha release, i.e. P2X receptor activation attenuates, whereas P2Y receptor activation potentiates TNF-alpha release in LPS-stimulated astrocytes. These observations suggest a mechanism whereby astrocytes can sense the severity of damage in the CNS via ATP release from damaged cells and can modulate the TNF-alpha mediated inflammatory response depending on the extracellular ATP concentration and corresponding type of astrocyte ATP/P2 receptor activated.

Immune and glial cell factors as pain mediators and modulators

A decade ago the attention of pain scientists was focused on a small number of molecules such as prostaglandin and bradykinin as peripheral pain mediators or modulators. These factors were known to be produced by tissue damage or inflammation, and considered responsible for the activation and sensitization of peripheral pain signaling sensory neurons. A small number of molecules were also identified as central pain mediators, most notably glutamate and substance P released from central nociceptive nerve terminals, and, starting at that time, appreciation that nitric oxide might be produced by dorsal horn neurons and act as a diffusible transmitter to increase excitability of central pain circuits. During the last decade evidence has emerged for many novel pain mediators. The old ones have not disappeared, although their roles have been redefined in some cases. Prostaglandin E2 (PGE2), for instance, is now recognized as playing a prominent role in CNS as well as peripheral tissues. The newly identified mediators include a variety of factors produced and released from nonneuronal cells-predominantly immune and glial cells. The evidence is now growing apace that these are important mediators of persistent pain states and can act at a number of loci. Here we review the actions of several of these factors-the pro-inflammatory cytokines, some chemokines, and some neurotrophic factors, which, in addition to their traditionally recognized roles, are all capable of changing the response properties of peripheral and central pain signaling neurons. We review these actions, first in periphery, where a substantial literature has accumulated, and then in spinal cord, where the role of factors from nonneuronal cells has only recently been identified as of considerable importance.

The cytokine IL-1beta transiently enhances P2X7 receptor expression and function in human astrocytes

Extracellular nucleotide di- and triphosphates such as ATP and ADP mediate their effects through purinergic P2 receptors belonging to either the metabotropic P2Y or the ionotropic P2X receptor family. The P2X7R is a unique member of the P2X family, which forms a pore in response to ligand stimulation, regulating cell permeability, cytokine release, and/or apoptosis. This receptor is also unique in that its affinity for the ligand benzoyl-benzoyl ATP (BzATP) is at least 10-fold greater than that of ATP. Primary human fetal astrocytes in culture express low-levels of P2X7R mRNA and protein, and BzATP induces only a slight influx in intracellular calcium [Ca2+]i, with little demonstrable effect on gene expression or pore formation in these cells. We now show that, following treatment with the proinflammatory cytokine IL-1beta, BzATP induces a robust rise in [Ca2+]i with agonist and antagonist profiles indicative of the P2X7R. IL-1beta also induced the formation of membrane pores as evidenced by the uptake of YO-PRO-1 (375 Da). Quantitative real-time PCR demonstrated transient upregulation of P2X7R mRNA in IL-1beta-treated cells, while FACS analysis indicated a similar upregulation of P2X7R protein at the cell membrane. In multiple sclerosis lesions, immunoreactivity for the P2X7R was demonstrated on reactive astrocytes in autopsy brain tissues. In turn, P2X7R stimulation increased the production of IL-1-induced nitric oxide synthase activity by astrocytes in culture. These studies suggest that signaling via the P2X7R may modulate the astrocytic response to inflammation in the human central nervous system.

15-Deoxy-Δ (12,14)-PGJ2 inhibits astrocyte IL-1 signaling: inhibition of NF-κB and MAP kinase pathways and suppression of cytokine and chemokine expression

We studied the role of 15-deoxy-delta (12,14)-PGJ2 (15d-PGJ2), a macrophage inhibitor with reported therapeutic effects on experimental allergic encephalomyelitis, in human astrocyte activation in vitro. 15d-PGJ2 inhibited a broad range of astrocyte inflammatory gene expression induced by IL-1, including cytokines (TNFalpha and IL-6), chemokines (RANTES/CCL5 and IP-10/CXCL10) and inducible nitric oxide synthase. 15d-PGJ2 inhibited transactivation of NF-kappaB-dependent promoters, as well as p38 and JNK MAPK phosphorylation induced by IL-1, while having no inhibitory effect on IFN-induced Stat signaling pathways. Our results demonstrating 15d-PGJ2-mediated astrocyte deactivation through inhibition of NF-kappaB are similar to those described for macrophages, and add astrocytes as additional targets for this prostaglandin (PG).

ATP-induced inhibition of gap junctional communication is enhanced by interleukin-1 beta treatment in cultured astrocytes

Nucleotides are signaling molecules involved in variety of interactions between neurons, between glial cells as well as between neurons and glial cells. In addition, ATP and other nucleotides are massively released following brain insults, including inflammation, and may thereby be involved in mechanisms of cerebral injury. Recent concepts have shown that in astrocytes intercellular communication through gap junctions may play an important role in neuroprotection. Therefore, we have studied the effects of nucleotides on gap junction communication in astrocytes. Based on measurement of intercellular dye coupling and recording of junctional currents, the present study shows that ATP (10-100 microM) induces a rapid and a concentration-dependent inhibition of gap junction communication in cultured cortical astrocytes from newborn mice. Effects of agonists and antagonists of purinergic receptors indicate that the inhibition of gap junctional communication by ATP mainly involves the stimulation of metabotropic purinergic 1 (P2Y(1)) receptors. Pretreatment with the pro-inflammatory cytokine interleukin-1beta (10 ng/ml, 24 h), which has no effect by itself on gap junctional communication, increases the inhibitory effect of ATP and astrocytes become sensitive to uridine 5'-triphosphate (UTP). As indicated by the enhanced expression of P2Y(2) receptor mRNA, P2Y(2) receptors are responsible for the increased responses evoked by ATP and UTP in interleukin-1beta-treated cells. In addition, the effect of endothelin-1, a well-known inhibitor of gap junctional communication in astrocytes was also exacerbated following interleukin-1beta treatment. We conclude that ATP decreases intercellular communication through gap junctions in astrocytes and that the increased sensitivity of gap junction channels to nucleotides and endothelin-1 is a characteristic feature of astrocytes exposed to pro-inflammatory treatments.

Effects of neuroinflammation on glia-glia gap junctional intercellular communication: a perspective

Gap junctions serve as intercellular conduits that allow for the direct transfer of small molecular weight molecules (up to 1 kDa) including ions involved in cellular excitability, metabolic precursors, and second messengers. The observation of extensive intercellular coupling and large numbers of gap junctions in the central nervous system (CNS) suggests a syncytium-like organization of glial compartments. Inflammation is a hallmark of various CNS diseases such as bacterial and viral infections, multiple sclerosis, Alzheimer's disease, and cerebral ischemia. A general consequence of brain inflammation is reactive gliosis typified by astrocyte hypertrophy and proliferation of astrocytes and microglia. Changes in gap junction intercellular communication as reflected by alterations in dye coupling and connexin expression have been associated with numerous CNS inflammatory diseases, which may have dramatic implications on the survival of neuronal and glial populations in the context of neuroinflammation. A review of the effects of inflammatory products on glia-glia gap junctional communication and glial glutamate release is presented. In addition, the hypothesis of a "syncytial switch" based upon differential regulation of gap junction expression in astrocytes and microglia during normal CNS homeostasis and neuroinflammation is proposed.

Oxidized ATP (oATP) attenuates proinflammatory signaling via P2 receptor-independent mechanisms

Periodate-oxidized ATP (oATP), which covalently modifies nucleotide-binding proteins, can significantly attenuate proinflammatory signaling. Although the P2X7 nucleotide receptor (P2X7R) is irreversibly antagonized by oATP, it is unclear whether anti-inflammatory actions of oATP are predominantly mediated via its actions on P2X7R. Here, we describe inhibitory effects of oATP on proinflammatory responses in three human cell types that lack expression of P2X7R: human umbilical vein endothelial cells (HUVEC), HEK293 cells, and 1321N1 astrocytes. oATP decreased by 40-70% the secretion of interleukin (IL)-8 stimulated by tumor necrosis factor-alpha (TNF-alpha) in all three cell types, by IL-1beta in HUVEC and 1321N1 cells, and by endotoxin in HUVEC. Attenuation of TNF-alpha-stimulated IL-8 secretion by oATP was similar in wild-type HEK cells or HEK cells stably expressing recombinant P2X7R. oATP also attenuated cytokine-stimulated expression of nuclear factor-kappaB-luciferase reporter genes expressed in HEK or 1321N1 cells, but did not affect the rapid downregulation of IkappaB. oATP had no effect on uridine triphosphate-induced activation of native P2Y2 receptors in HEK cells, but reduced the potency and efficacy of ADP as an agonist of native P2Y1 receptors. However, inhibition of P2Y1 receptors with the specific antagonist MRS2216 did not mimic the effects of oATP on TNF-alpha-stimulated IL-8 secretion. Although 1321N1 astrocytes lack expression of any known P2 receptor subtypes, oATP markedly inhibited ecto-ATPase activity in these cells, resulting in a significant accumulation of extracellular ATP. In summary, oATP can attenuate proinflammatory signaling by mechanisms independent of the expression or activation of known P2 receptor subtypes.

ATP- and UTP-activated P2Y receptors differently regulate proliferation of human lung epithelial tumor cells

The involvement of P2Y receptors, which are activated by extracellular nucleotides, in proliferative regulation of human lung epithelial cells is unclear. Here we show that extracellular ATP and UTP stimulate bromodeoxyuridine (BrdU) incorporation into epithelial cell lines. The nucleotide efficacy profile [ATP = ADP > UDP >or= UTP > adenosine >or= 2-methylthioadenosine-5'-diphosphate, with alpha,beta-methylene adenosine 5'-triphosphate, 2',3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate, AMP, UMP, and ATPalphaS inactive] and PCR analysis indicate involvement of P2Y2 and P2Y6 receptors. The signal transduction pathway, which, via the P2Y2 receptor, transmits the proliferative activity of ATP or UTP in A549 cells downstream of phospholipase C, depends on Ca2+/calmodulin-dependent protein kinase II and nuclear factor-kappaB, but not on protein kinase C. Signaling does not involve the mitogen-activated protein kinases extracellular signal-regulated kinases-1 and -2, the phosphatidylinositol 3-kinase pathway, or Src kinases. Thus nucleotides regulate proliferation of human lung epithelial cells by a novel pathway. The stimulatory effect of UTP, but not ATP, in A549 cells is attenuated by preincubation with interleukin-1beta and interleukin-6, but not tumor necrosis factor-alpha. This indicates an important role for the pyrimidine-activated P2Y receptor in the inflammatory response of lung epithelia. ATP antagonizes the antiproliferative effect of the anticancer drugs paclitaxel and etoposide, whereas it enhances the activity of cisplatin about fourfold. Thus pathways activated by extracellular nucleotides differentially control proliferation of lung epithelial tumor cells.

Potentiation by ATP of lipopolysaccharide-stimulated nitric oxide production in cultured astrocytes

The functional changes of astrocytes are deeply involved in neurodegenerating processes of various CNS diseases. ATP is released during various neuronal damages such as brain ischemia and may control astrocyte functions. We examined the effect of ATP on the production of nitric oxide in the cultured astrocytes from rat embryo. The astrocytes were stimulated by lipopolysaccharide instead of pathological activation in vivo. Nitric oxide production was evaluated by the fluorometric assay of nitrite accumulated in the medium. The expression of inducible nitric oxide synthase was analyzed by Western blotting. Nitric oxide production induced by 1 ng/ml lipopolysaccharide was enhanced by ATP with maximal enhancement of three- to four-fold; a half-effective concentration was about 0.3 mM. In the absence of ATP, half-effective concentration of lipopolysaccharide on nitric oxide production was about 3 ng/ml; however, half-effective concentration shifted to 0.3 ng/ml in the presence of 1.5-mM ATP. Several other P2 receptor agonists (uridine triphosphate, ADP, adenosine monophosphate, 2'- and 3'-O - (4-benzoylbenzoyl)-ATP, and 2-methylthioATP) showed a similar enhancing effect, and an antagonist, ATP-2',3'-dialdehyde, showed an inhibiting effect. Western blotting analysis revealed that the extent of lipopolysaccharide-induced expression of nitric oxide synthase increased several-fold by the addition of ATP; half-effective concentration was about 0.5 mM. These results suggest that the extracellular ATP plays an important role as a transmitter and regulates astrocyte functions via a certain P2 receptor and that such a change in astrocyte function is involved in either protection or aggravation in neurodegenerative processes.

Stimulation of P2Y1 receptors causes anxiolytic-like effects in the rat elevated plus-maze: implications for the involvement of P2Y1 receptor-mediated nitric oxide production

The widespread and abundant distribution of P2Y receptors in the mammalian brain suggests important functions for these receptors in the CNS. To study a possible involvement of the P2Y receptors in the regulation of fear and anxiety, the influences of the P2Y(1,11,12) receptor-specific agonist adenosine 5'-O-(2-thiodiphosphate) (ADPbetaS), the P2X(1,3) receptor agonist alpha,beta-methylene ATP (alpha,betameATP), the unspecific P2 receptor antagonist pyridoxalphosphate-6-azopheny l-2',4'-disulfonic acid (PPADS), and the specific P2Y(1) receptor antagonist N(6)-methyl-2'-deoxyadenosine-3',5'-bisphosphate (MRS 2179) on the elevated plus-maze behavior of the rat were investigated. All tested compounds were given intracerebroventricularly (0.5 microl). ADPbetaS (50 and 500 fmol) produced an anxiolytic-like behavioral profile reflected by an increase of the open arm exploration. The anxiolytic-like effects were antagonized by pretreatment with PPADS (5 pmol) or MRS 2179 (5 pmol). Both compounds caused anxiogenic-like effects when given alone. Furthermore, the anxiolytic-like effects of ADPbetaS could be antagonized by pretreatment with the nitric oxide synthase (NOS) inhibitor N(w)-nitro-L-arginine methyl ester (L-NAME). In addition, the anxiogenic-like effects of PPADS were reversed by the pretreatment with L-arginine (500 pmol), which is the natural substrate for NOS, but not by D-arginine (500 pmol), which is not. Immunofluorescence staining revealed the presence of P2Y(1) receptors on neurons in different brain regions such as hypothalamus, amygdala, hippocampus and the periaqueductal gray. Furthermore, the colocalization of P2Y(1) receptors and neuronal NOS (nNOS) on some neurons in these regions could be demonstrated. The highest density of P2Y(1)- and nNOS-immunoreactivity was detected in the dorsomedial hypothalamic nucleus. Taken together, the present results suggest that P2Y(1) receptors are involved in the modulation of anxiety in the rat. The anxiolytic-like effects after stimulation of P2Y(1) receptors seem to be in close connection with the related nitric oxide production.

Adenosine suppresses activation of nuclear factor-kappaB selectively induced by tumor necrosis factor in different cell types

Adenosine is an endogenous immunomodulator that has been shown to exhibit anti-inflammatory and immunosuppressive properties through a mechanism that is not fully established. Owing to the pivotal role of nuclear factor (NF)-kappaB in these responses, we tested the hypothesis that adenosine mediates its effects through suppression of NF-kappaB activation. We investigated the effects of adenosine on NF-kappaB activation induced by various inflammatory agents in human myeloid KBM-5 cells. The treatment of these cells with adenosine suppressed TNF-induced NF-kappaB activation, but had no effect on activation of another redox-sensitive transcription factor, AP-1. These effects were not restricted to myeloid cells, as NF-kappaB activation in other lymphocytic and epithelial cell types was also inhibited. The effect on TNF-induced NF-kappaB activation was selective as adenosine had minimal effect on NF-kappaB activation induced by H(2)O(2), PMA, LPS, okadaic acid, or ceramide, suggesting differences in the pathway leading to NF-kappaB activation by different agents. Adenosine also suppressed NF-kappaB-dependent reporter gene expression activated by TNF or by overexpression of TNFR1, TRAF 2, NIK, and p65 subunit of NF-kappaB. The suppression of TNF-induced NF-kappaB activation by adenosine was found not to be because of inhibition of TNF-induced IkappaBalpha phosphorylation and degradation or IkappaBalpha kinase activation. The suppression of TNF-induced NF-kappaB activation was unique to adenosine, as neither its metabolites (inosine, AMP, and ATP) nor pyrimidines (thymidine and uridine) had any effect. Overall, our results clearly demonstrate that adenosine selectively suppresses TNF-induced NF-kappaB activation, which may contribute to its role in suppression of inflammation and of the immune system.

Mechanisms of P2X7 receptor-mediated ERK1/2 phosphorylation in human astrocytoma cells

Astrocytes are involved in normal and pathological brain functions, where they become activated and undergo reactive gliosis. Astrocytes have been shown to respond to extracellular nucleotides via the activation of P2 receptors, either G protein-coupled P2Y receptors or P2X receptors that are ligand-gated ion channels. In this study, we have examined the manner in which activation of the P2X(7) nucleotide receptor, an extracellular ATP-gated ion channel expressed in astrocytes, can lead to the phosphorylation of ERK1/2. Results showed that the P2X(7) receptor agonist 2',3'-O-(4-benzoyl)benzoyl-ATP induced ERK1/2 phosphorylation in human astrocytoma cells overexpressing the recombinant rat P2X(7) receptor (rP2X(7)-R), a response that was inhibited by the P2X(7) receptor antagonist, oxidized ATP. Other results suggest that rP2X(7)-R-mediated ERK1/2 phosphorylation was linked to the phosphorylation of the proline-rich/Ca(2+)-activated tyrosine kinase Pyk2, c-Src, phosphatidylinositol 3'-kinase, and protein kinase Cdelta activities and was dependent on the presence of extracellular Ca(2+). These results support the hypothesis that the P2X(7) receptor and its signaling pathways play a role in astrocyte-mediated inflammation and neurodegenerative disease.