Adenosine triphosphate inhibits cytokine release from lipopolysaccharide-activated microglia via P2y receptors

Resident immune responses to spinal cord injury: role of astrocytes and microglia

Spinal cord injury can be traumatic or non-traumatic in origin, with the latter rising in incidence and prevalence with the aging demographics of our society. Moreover, as the global population ages, individuals with co-existent degenerative spinal pathology comprise a growing number of traumatic spinal cord injury cases, especially involving the cervical spinal cord. This makes recovery and treatment approaches particularly challenging as age and comorbidities may limit regenerative capacity. For these reasons, it is critical to better understand the complex milieu of spinal cord injury lesion pathobiology and the ensuing inflammatory response. This review discusses microglia-specific purinergic and cytokine signaling pathways, as well as microglial modulation of synaptic stability and plasticity after injury. Further, we evaluate the role of astrocytes in neurotransmission and calcium signaling, as well as their border-forming response to neural lesions. Both the inflammatory and reparative roles of these cells have eluded our complete understanding and remain key therapeutic targets due to their extensive structural and functional roles in the nervous system. Recent advances have shed light on the roles of glia in neurotransmission and reparative injury responses that will change how interventions are directed. Understanding key processes and existing knowledge gaps will allow future research to effectively target these cells and harness their regenerative potential.

Purinergic-Glycinergic Interaction in Neurodegenerative and Neuroinflammatory Disorders of the Retina

Neurodegenerative–neuroinflammatory disorders of the retina seriously hamper human vision. In searching for key factors that contribute to the development of these pathologies, we considered potential interactions among purinergic neuromodulation, glycinergic neurotransmission, and microglia activity in the retina. Energy deprivation at cellular levels is mainly due to impaired blood circulation leading to increased release of ATP and adenosine as well as glutamate and glycine. Interactions between these modulators and neurotransmitters are manifold. First, P2Y purinoceptor agonists facilitate reuptake of glycine by glycine transporter 1, while its inhibitors reduce reverse-mode operation; these events may lower extracellular glycine levels. The consequential changes in extracellular glycine concentration can lead to parallel changes in the activity of NR1/NR2B type NMDA receptors of which glycine is a mandatory agonist, and thereby may reduce neurodegenerative events in the retina. Second, P2Y purinoceptor agonists and glycine transporter 1 inhibitors may indirectly inhibit microglia activity by decreasing neuronal or glial glycine release in energy-compromised retina. These inhibitions may have a role in microglia activation, which is present during development and progression of neurodegenerative disorders such as glaucomatous and diabetic retinopathies and age-related macular degeneration or loss of retinal neurons caused by thromboembolic events. We have hypothesized that glycine transporter 1 inhibitors and P2Y purinoceptor agonists may have therapeutic importance in neurodegenerative–neuroinflammatory disorders of the retina by decreasing NR1/NR2B NMDA receptor activity and production and release of a series of proinflammatory cytokines from microglial cells.

Identification of P2Y receptors involved in oleamide-suppressing inflammatory responses in murine microglia and human dendritic cells

Microglia, a type of immune cell in the central nervous system, are involved in inflammation leading to neurodegenerative diseases. We previously identified oleamide from fermented dairy products as a neuroprotective compound suppressing microglial inflammation. Oleamide is an endocannabinoid and displays anti-inflammatory activity via the cannabinoid-2 (CB2) receptor; however, the mechanism underlying this anti-inflammatory activity has not been fully elucidated. Here, we found that the suppressive effect of oleamide on microglial tumor necrosis factor-α (TNF-α) production was canceled by inhibitors of G-protein-coupled receptor (GPCR) downstream signaling but not by a CB2 antagonist, suggesting that GPCRs other than CB2 are involved in the anti-inflammatory effects of oleamide. An extensive screen for GPCRs using a transforming growth factor-α shedding assay system identified P2Y1, P2Y4, P2Y6, P2Y10, and P2Y11 as candidates for the oleamide target. P2Y1 and P2Y10 agonists suppressed microglial TNF-α production, while a pan P2 receptor antagonist canceled the suppressive effect. Furthermore, we observed a relationship between the P2Y1 agonistic activities and the suppressive activities of oleamide and its analogs. Taken together, our results suggest that, in addition to CB2, P2Y type receptors are the potential targets of oleamide, and P2Y1 plays a role in the suppression of microglial inflammatory responses by oleamide. (200/200 words)

P2Y14 receptor activation decreases interleukin-6 production and glioma GL261 cell proliferation in microglial transwell cultures

Gliomas are rich in extracellular nucleotides that modulate glioma cell production of multiple cytokines including interleukin (IL)-6, which strongly contributes to glioma cell proliferation. However, little is known about how nucleotide signaling modulates microglial/macrophage (MG/MP) cytokine production in the context of gliomas, nor how MG/MP purinergic P2 receptor expression changes in the tumor micro-environment. We hypothesized that: (1) expression of key P2Y receptors will be augmented in glioma-derived MG/MP, and (2) selective activation of these receptors in vitro will regulate microglial production of IL-6 and glioma cell proliferation. We tested these hypotheses using the murine GL261 glioma model. Compared to MG/MP isolated from the normal brain tissue, CD11b⁺ cells isolated from GL261 tumors expressed higher levels of several P2 receptors, including P2Y14 receptors. To evaluate microglial P2Y14 receptor function in the context of tumor cells, we first cultured N9 microglia in transwells with GL261 cells and found that microglial P2Y14 mRNA levels were similarly increased in transwell cultures. GL261 cells did not express detectable P2Y14 levels either when they were cultured alone or in transwell cultures with N9 cells. Selective P2Y14 receptor activation with UDP-glucose (UDPG) did not affect IL-6 levels in either cell type cultured alone, but in transwell cultures, UDPG decreased IL-6 protein levels in the medium. Application of conditioned medium from UDPG-treated microglia reduced GL261 cell proliferation. Together, these data suggest that P2Y14 receptors may be a key a receptor involved in glioma cell-MG/MP communication in the tumor environment.

Integrating neuroimmune systems in the neurobiology of depression

Data from clinical and preclinical studies indicate that immune dysregulation, specifically of inflammatory processes, is associated with symptoms of major depressive disorder (MDD). In particular, increased levels of circulating pro-inflammatory cytokines and concomitant activation of brain-resident microglia can lead to depressive behavioural symptoms. Repeated exposure to psychological stress has a profound impact on peripheral immune responses and perturbs the function of brain microglia, which may contribute to neurobiological changes underlying MDD. Here, we review these findings and discuss ongoing studies examining neuroimmune mechanisms that influence neuronal activity as well as synaptic plasticity. Interventions targeting immune-related cellular and molecular pathways may benefit subsets of MDD patients with immune dysregulation.

An introduction to the roles of purinergic signalling in neurodegeneration, neuroprotection and neuroregeneration

Purinergic signalling appears to play important roles in neurodegeneration, neuroprotection and neuroregeneration. Initially there is a brief summary of the background of purinergic signalling, including release of purines and pyrimidines from neural and non-neural cells and their ectoenzymatic degradation, and the current characterisation of P1 (adenosine), and P2X (ion channel) and P2Y (G protein-coupled) nucleotide receptor subtypes. There is also coverage of the localization and roles of purinoceptors in the healthy central nervous system. The focus is then on the roles of purinergic signalling in trauma, ischaemia, stroke and in neurodegenerative diseases, including Alzheimer's, Parkinson's and Huntington's diseases, as well as multiple sclerosis and amyotrophic lateral sclerosis. Neuroprotective mechanisms involving purinergic signalling are considered and its involvement in neuroregeneration, including the role of adult neural stem/progenitor cells. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

The two-hit hypothesis for neuroinflammation: role of exogenous ATP in modulating inflammation in the brain

Brain inflammation is a common occurrence following responses to varied insults such as bacterial infections, stroke, traumatic brain injury and neurodegenerative disorders. A common mediator for these varied inflammatory responses is prostaglandin E₂ (PGE₂), produced by the enzymatic activity of cyclooxygenases (COX) 1 and 2. Previous attempts to reduce neuronal inflammation through COX inhibition, by use of nonsteroidal anti-inflammatory drugs (NSAIDs), have met with limited success. We are proposing the two-hit model for neuronal injury—an initial localized inflammation mediated by PGE₂ (first hit) and the simultaneous release of adenosine triphosphate (ATP) by injured cells (second hit), which significantly enhances the inflammatory response through increased synthesis of PGE₂. Several evidences on the role of exogenous ATP in inflammation have been reported, including contrary instances where extracellular ATP reduces inflammatory events. In this review, we will examine the current literature on the role of P2 receptors, to which ATP binds, in modulating inflammatory reactions during neurodegeneration. Targeting the P2 receptors, therefore, provides a therapeutic alternative to reduce inflammation in the brain. P2 receptor-based anti-inflammatory drugs (PBAIDs) will retain the activities of essential COX enzymes, yet will significantly reduce neuroinflammation by decreasing the enhanced production of PGE₂ by extracellular ATP.

P2X7-dependent, but differentially regulated release of IL-6, CCL2, and TNF-α in cultured mouse microglia

ATP is an important regulator of microglia and its effects on microglial cytokine release are currently discussed as important contributors in a variety of brain diseases. We here analyzed the effects of ATP on the production of six inflammatory mediators (IL-6, IL-10, CCL2, IFN-γ, TNF-α, and IL-12p70) in cultured mouse primary microglia. Stimulation of P2X7 receptor by ATP (1 mM) or BzATP (500 µM) evoked the mRNA expression and release of proinflammatory cytokines IL-6, TNF-α, and the chemokine CCL2 in WT cells but not in P2X7(-/-) cells. The effects of ATP and BzATP were inhibited by the nonselective P2 receptor antagonists PPADs and suramin. Various selective P2X7 receptor antagonists blocked the P2X7-dependent release of IL-6 and CCL2, but, surprisingly, had no effect on BzATP-induced release of TNF-α in microglia. Calcium measurements confirmed that P2X7 is the main purine receptor activated by BzATP in microglia and showed that all P2X7 antagonists were functional. It is also presented that pannexin-1 hemichannel function and potential P2X4/P2X7 heterodimers are not involved in P2X7-dependent release of IL-6, CCL2, and TNF-α in microglia. How P2X7-specific antagonists only affect P2X7-dependent IL-6 and CCL2 release, but not TNF-α release is at the moment unclear, but indicates that the P2X7-dependent release of cytokines in microglia is differentially regulated.

Microglia during development and aging

Microglia are critical nervous system-specific cells influencing brain development, maintenance of the neural environment, response to injury, and repair. They contribute to neuronal proliferation and differentiation, pruning of dying neurons, synaptic remodeling and clearance of debris and aberrant proteins. Colonization of the brain occurs during gestation with an expansion following birth with localization stimulated by programmed neuronal death, synaptic pruning, and axonal degeneration. Changes in microglia phenotype relate to cellular processes including specific neurotransmitter, pattern recognition, or immune-related receptor activation. Upon activation, microglia cells have the capacity to release a number of substances, e.g., cytokines, chemokines, nitric oxide, and reactive oxygen species, which could be detrimental or beneficial to the surrounding cells. With aging, microglia shift their morphology and may display diminished capacity for normal functions related to migration, clearance, and the ability to shift from a pro-inflammatory to an anti-inflammatory state to regulate injury and repair. This shift in microglia potentially contributes to increased susceptibility and neurodegeneration as a function of age. In the current review, information is provided on the colonization of the brain by microglia, the expression of various pattern recognition receptors to regulate migration and phagocytosis, and the shift in related functions that occur in normal aging.

Microglial phenotype and adaptation

Microglia are the prime innate immune cells of the central nervous system. They can transit from a (so-called) resting state under homeostatic conditions towards a pro-inflammatory activation state upon homeostatic disturbances. Under neurodegenerative conditions, microglia have been largely perceived as neurotoxic cells. It is now becoming clear that resting microglia are not inactive but that they serve house-keeping functions. Moreover, microglia activity is not limited to proinflammatory responses, but covers a spectrum of reactive profiles. Depending on the actual situation, activated microglia display specific effector functions supporting inflammation, tissue remodeling, synaptic plasticity and neurogenesis. Many of these functions not only relate to the current state of the local neural environment but also depend on previous experience. In this review, we address microglia functions with respect to determining factors, phenotypic presentations, adaptation to environmental signals and aging. Finally, we point out primary mechanisms of microglia activation, which may comprise therapeutic targets to control neuro-inflammatory and neurodegenerative activity.

The anti-inflammatory effects of the 5-HT₃ receptor antagonist tropisetron are mediated by the inhibition of p38 MAPK activation in primary human monocytes

BACKGROUND

There is evidence from human and animal research that 5-hydroxytryptamine (5-HT) 3 receptor antagonists, particularly tropisetron, exert analgesic and anti-inflammatory activity. We have demonstrated that tropisetron inhibited lipopolysaccharide (LPS)-stimulated tumor necrosis factor (TNF)alpha and interleukin-(IL-)1beta release in primary human monocytes. The underlying mechanisms of these effects have not been investigated in detail so far.

METHODS

The molecular mechanisms of the anti-inflammatory effects of tropisetron were investigated in human primary monocytes in vitro by studying IL-1beta and TNFalpha mRNA levels by PCR and reporter gene assay and by elucidating the phosphorylation of p38 mitogen activated kinase (MAPK) by Western blot.

RESULTS

The steady state levels of IL-1beta and TNFalpha mRNA in LPS-activated human peripheral monocytes and the transcriptional activity of the TNFalpha promoter were not inhibited by tropisetron, suggesting that the inhibitory activity of this 5-HT₃ receptor antagonist takes place at the post-transcriptional level. Additionally, we found that tropisetron prevents the phosphorylation and thus activation of the p38 MAPK, which is involved in post-transcriptional regulation of various cytokines.

CONCLUSION

Our data indicate that the anti-inflammatory effects of the 5-HT₃ receptor antagonist tropisetron, as shown in vivo, are possibly mediated by a selective inhibition of pro-inflammatory cytokines at the post-transcriptional level. 5-HT₃ receptor antagonists are therefore a new and promising therapeutic option. New and more selective--in respect to the 5-HT₃ subtypes--5-HT₃R antagonists might be a future perspective in the pharmacological treatment of inflammatory diseases.

P2Y receptors and pain transmission

It is widely accepted that the most important ATP receptors involved in pain transmission belong to the P2X₃ and P2X_2/3 subtypes, selectively expressed in small diameter dorsal root ganglion (DRG) neurons. However, several types of the metabotropic ATP (P2Y) receptors have also been found in primary afferent neurons; P2Y₁ and P2Y₂ receptors are typically expressed in small, nociceptive cells. Here we review the results available on the involvement of P2Y receptors in the modulation of pain transmission.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Uridine 5'-diphosphate induces chemokine expression in microglia and astrocytes through activation of the P2Y6 receptor

Chemokines play critical roles in inflammation by recruiting inflammatory cells to injury sites. In this study, we found that UDP induced expression of chemokines CCL2 (MCP-1) and CCL3 (MIP-1α) in microglia, astrocytes, and slice cultures by activation of P2Y(6). Interestingly, CCL2 was more highly expressed than CCL3. However, CCL2 synthesis kinetics in response to UDP differed in microglia and astrocytes; microglia rapidly produced small amounts of CCL2, whereas astrocytes continuously synthesized large amounts of CCL2, resulting in a high ultimate level of the chemokine. UDP-induced chemokine expression was reduced in the presence of a specific antagonist of P2Y(6) (MRS2578) or small interfering RNA directed against the P2Y(6) gene. Inhibition of phospholipase C and calcium increase, downstream signaling pathways of Gq-coupled P2Y(6), reduced UDP-induced chemokine expression. UDP activated two calcium-activated transcription factors, NFATc1 and c2. Furthermore, inhibitors of calcineurin (a phosphatase activating NFAT) and NFAT reduced UDP-induced chemokine synthesis. We also found, using a transmigration assay, that UDP-treated astrocytes recruited monocytes. These results suggest that UDP induces chemokine expression in microglia and astrocytes of the injured brain by activation of P2Y(6) receptors.

The P2X7-Egr pathway regulates nucleotide-dependent inflammatory gene expression in microglia

Microglial hyperactivity contributes to neuronal damage resulting from CNS injury and disease. Therefore, a better understanding of endogenous microglial receptor systems that can be exploited to modulate their inflammatory functions is important if better, neuroprotective therapeutics are to be designed. Previous studies from our lab and others have demonstrated that the P2X7 purinergic receptor agonist BzATP attenuates microglial inflammatory mediator production stimulated by lipopolysaccharide (LPS), suggesting that purinergic receptors may be one such receptor system that can be used for manipulating microglial activation. However, although P2X7 receptor activation is well recognized to regulate processing and release of cytokines, little is known concerning its role in regulating the transcription of inflammatory genes, nor the molecular mechanisms underlying these transcriptional effects. In the present studies, we identify that the transcription factors early growth response (Egr)-1, -2 and -3 are downstream signaling targets of P2X7 receptors in microglia, and that their activation is sensitive to MEK and p38 mitogen-activated protein kinase (MAPK) inhibitors. Moreover, using RNAi, we demonstrate that Egr factors and P2X7 receptors are necessary for BzATP-mediated attenuation of iNOS, and stimulation of TNF-α and IL-6 gene expression. BzATP also attenuates neuronal death induced by LPS conditioned medium, and P2X7 receptors are required for this effect. These studies are the first to identify Egr factors as regulators of inflammatory gene expression following P2X7 receptor activation, and suggest that P2X7 receptors may utilize the MAPK-Egr pathway to exert differential effects on microglial inflammatory activities which are beneficial to neuron survival.

ATP stimulates chemokine production via a store-operated calcium entry pathway in C6 glioma cells

Background

Glioma present as one of the most challenging cancers to treat, however, understanding of tumor cell biology is not well understood. Extracellular adenosine triphosphate (ATP) could serve as a critical signaling molecule regulating tumor development. This study has examined pharmacological modulation of calcium (Ca²⁺) entry through store-operated channels (SOC) on cellular expression and production of immune-cell mobilizing chemokines in ATP-stimulated C6 glioma cells.

Methods

Calcium spectrofluorometry was carried out to measure mobilization of intracellular Ca²⁺ [Ca²⁺]i following ATP stimulation of rat C6 glioma cells. Pretreatment with two inhibitors of SOC, SKF96365 or gadolinium, was used to examine for effects on [Ca²⁺]i. RT-PCR was performed to determine effects of purinergic stimulation on C6 cell expression of metabotropic P2Y receptors (P2YR) and the chemokines, monocyte chemoattractant protein-1 (MCP-1) and interleukin-8 (IL-8). ELISA was carried out to measure production of MCP-1 and IL-8 with ATP stimulation of glioma cells.

Results

Application of ATP (at 100 μM) to C6 glioma induced an increase in [Ca²⁺]i with the response exhibiting two components of decay. In the presence of the SOC inhibitors, SKF96365 or gadolinium, or with Ca²⁺-free solution, ATP responses lacked a slow phase suggesting the secondary component was due to SOC-mediated influx of Ca²⁺. RT-PCR confirmed expression of purinergic P2Y-subtype receptors in C6 cells which would serve as a precursor to activation of SOC. In addition, ATP-stimulated C6 cells showed enhanced expression of the chemokines, MCP-1 and IL-8, with SKF96365 or gadolinium effective in reducing chemokine expression. Gadolinium treatment of ATP-stimulated C6 cells was also found to inhibit the production of MCP-1 and IL-8.

Conclusion

These results suggest ATP-induced Ca²⁺ entry, mediated by activation of SOC in C6 glioma, as a mechanism leading to increased cellular expression and release of chemokines. Elevated levels of MCP-1 and IL-8 are predicted to enhance the mobility of tumor cells and promote recruitment of microglia into developing tumors thereby supporting tumor growth.

Expression of P2 nucleotide receptors varies with age and sex in murine brain microglia

Microglia are implicated in multiple neurodegenerative disorders, many of which display sexual dimorphisms and have symptom onsets at different ages. P2 purinergic receptors are critical for regulating various microglial functions, but little is known about how their expression varies with age or sex. Therefore, comprehensive information about purinergic receptor expression in normal microglia, in both sexes, over age is necessary if we are to better understand their roles in the healthy and diseased CNS. We analyzed the expression of all fourteen rodent P2X and P2Y receptors in CD11b⁺ cells freshly-isolated from the brains of C57Bl/6 mice at five different ages ranging from postnatal day 3 to 12 months, in males and females, using quantitative RT-PCR. We also compared purinergic receptor expression in microglia freshly-isolated from 3 day-old pups to that in primary neonatal microglial cultures created from mice of the same age. We observed patterns in P2 receptor expression with age, most notably increased expression with age and age-restricted expression. There were also several receptors that showed sexually dimorphic expression. Lastly, we noted that in vitro culturing of neonatal microglia greatly changed their P2 receptor expression profiles. These data represent the first complete and systematic report of changes in purinergic receptor expression of microglia with age and sex, and provide important information necessary for accurate in vitro modeling of healthy animals.

Purinergic signalling and disorders of the central nervous system

Purines have key roles in neurotransmission and neuromodulation, with their effects being mediated by the purine and pyrimidine receptor subfamilies, P1, P2X and P2Y. Recently, purinergic mechanisms and specific receptor subtypes have been shown to be involved in various pathological conditions including brain trauma and ischaemia, neurodegenerative diseases involving neuroimmune and neuroinflammatory reactions, as well as in neuropsychiatric diseases, including depression and schizophrenia. This article reviews the role of purinergic signalling in CNS disorders, highlighting specific purinergic receptor subtypes, most notably A(2A), P2X(4) and P2X(7), that might be therapeutically targeted for the treatment of these conditions.

P2X(7) receptor stimulation in primary cultures of rat spinal microglia induces downregulation of the activity for glutamate transport

It has been shown that spinal microglia expressing certain types of glutamate transporters function in the modulation of neuropathogenesis. In this study, the effect of ATP, potentially able to mediate the communication between neurons and glial cells in the spinal cord on the transport of glutamate in cultured spinal microglia, was investigated. Both GLAST and GLT-1 were detected in the cells. Preincubation with ATP or 2'-3'-O-(4-benzoyl-benzoyl) ATP (BzATP), a selective agonist for the P2X(7) receptor, significantly blocked the uptake of glutamate. The effect of BzATP was reversed by pretreatment with brilliant blue G or oxidized ATP, each a selective antagonist for P2X(7). The inhibitory effect of P2X(7) receptor activation also occurred in the absence of extracellular Na(+) or Ca(2+), suggesting that the receptor regulates glutamate transport by a metabotropic pathway. Furthermore, pretreatment with inhibitors of mitogen-activated protein kinase kinase, or antioxidants, significantly reversed the inhibitory effect of BzATP on the uptake of glutamate. Incubation with BzATP led to a marked decrease in the V(max), but not the K(m), of glutamate transport. However, treatment with BzATP did not induce the trafficking of glutamate transporters. These results suggest that the activation of P2X(7) receptors in spinal microglia is important in the regulation of glutamate transport via activation of the extracellular signal-regulated kinase cascade and production of oxidants.

Neurotransmitter receptors on microglia

Microglia are the intrinsic immune cells of the brain and express chemokine and cytokine receptors that interact with the peripheral immune cells. Recent studies have indicated that microglia also respond to the brain's classical signalling substances, the neurotransmitters. Here, we review the evidence for the expression of neurotransmitter receptors on microglia and the consequences of this receptor activation for microglial behaviour. It is evident that neurotransmitters instruct microglia to perform distinct types of responses, such as triggering an inflammatory cascade or acquiring a neuroprotective phenotype. Understanding how microglia respond to different neurotransmitters will thus have important implications for controlling the reactivity of these cells in acute injury, as well as for treating chronic neurodegenerative diseases.

Release of ATP in the central nervous system during systemic inflammation: real-time measurement in the hypothalamus of conscious rabbits

Receptors for extracellular ATP (both ionotropic and metabotropic) are widely expressed in the CNS both in neurones and glia. ATP can modulate neuronal activity in many parts of the brain and contributes to the central nervous control of several physiological functions. Here we show that during the systemic inflammatory response the extracellular concentrations of ATP increase in the anterior hypothalamus and this has a profound effect on the development of the thermoregulatory febrile response. In conscious rabbits we measured ATP release in real time with novel amperometric biosensors and monitored a marked increase in the concentration of ATP (4.0 ± 0.7 μm) in the anterior hypothalamus in response to intravenous injection of bacterial endotoxin – lipopolysaccharide (LPS). No ATP release was observed in the posterior hypothalamus. The release of ATP coincided with the development of the initial phase of the febrile response, starting 18 ± 2 min and reaching its peak 45 ± 2 min after LPS injection. Application of the ATP receptor antagonists pyridoxal-5′-phosphate-6-azophenyl-2′,4′-disulphonic acid, Brilliant Blue G or periodate oxidized ATP dialdehyde to the site of ATP release in the anterior hypothalamus markedly augmented and prolonged the febrile response. These data indicate that during the development of the systemic inflammation, ATP is released in the anterior hypothalamus to limit the magnitude and duration of fever. This release may also have a profound effect on the hypothalamic control of other physiological functions in which ATP and related purines have been implicated to play modulatory roles, such as food intake, hormone secretion, cardiovascular activity and sleep.

Physiology and pathophysiology of purinergic neurotransmission

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

Downregulation of TLR4-dependent ATP production is critical for estrogen-mediated immunoprotection in Kupffer cells following trauma-hemorrhage

Toll-like receptor 4 (TLR4) mediates mitochondrial DNA (mtDNA) damage and biogenic responses. Mitochondrial transcription factor A (Tfam) is an essential regulator for mtDNA transcription and ATP production. Increased ATP levels were associated with normalization of immune function following trauma-hemorrhage. Moreover, administration of 17beta-estradiol following trauma-hemorrhage upregulates cardiac Tfam and ATP levels. We therefore hypothesized that the salutary effect of 17beta-estradiol on Kupffer cell function following trauma-hemorrhage is mediated via negative regulation of TLR4, which downregulates iNOS, upregulates Tfam and mtDNA-encoded gene cytochrome c oxidase I (mtCOI), and consequently increases cellular ATP levels. Male C3H/HeN, C3H/HeOuJ (intact TLR4), and C3H/HeJ (TLR4 mutant) mice were subjected to trauma-hemorrhage (mean BP 35 +/- 5 mmHg approximately 90 min, then resuscitation) or sham operation. At the beginning of resuscitation, mice received 17beta-estradiol (25 microg/25 g) or vehicle intravenously and were sacrificed 2 h thereafter. Kupffer cell TLR4, iNOS, IL-6 and TNF-alpha production capacities were increased, and ATP, Tfam, and mtCOI levels were decreased following trauma-hemorrhage. Administration of 17beta-estradiol following trauma-hemorrhage prevented the increase in Kupffer cell TLR4, iNOS, and cytokine production. This was accompanied by normalized ATP, Tfam, and mtCOI levels. Furthermore, the decreased Kupffer cell ATP and mtCOI levels were not observed in TLR4 mutant mice following trauma-hemorrhage. Taken together, these findings suggest that downregulation of TLR4-dependent ATP production is critical to 17beta-estradiol-mediated immunoprotection in Kupffer cells following trauma-hemorrhage.

Purinergic modulation of microglial cell activation

Microglial cells are resident macrophages in the brain and their activation is an important part of the brain immune response and the pathology of the major CNS diseases. Microglial activation is triggered by pathological signals and is characterized by morphological changes, proliferation, phagocytosis and the secretion of various cytokines and inflammatory mediators, which could be both destructive and protective for the nervous tissue. Purines are one of the most important mediators which regulate different aspects of microglial function. They could be released to the extracellular space from neurons, astrocytes and from the microglia itself, upon physiological neuronal activity and in response to pathological stimuli and cellular damage. Microglial activation is regulated by various subtypes of nucleotide (P2X, P2Y) and adenosine (A₁, A(₂A) and A₃) receptors, which control ionic conductances, membrane potential, gene transcription, the production of inflammatory mediators and cell survival. Among them, the role of P2X₇ receptors is especially well delineated, but P2X₄, various P2Y, A₁, A(₂A) and A₃ receptors also powerfully participate in the microglial response. The pathological role of microglial purine receptors has also been demonstrated in disease models; e.g., in ischemia, sclerosis multiplex and neuropathic pain. Due to their upregulation and selective activation under pathological conditions, they provide new avenues in the treatment of neurodegenerative and neuroinflammatory illnesses.

Different patterns of Ca²⁺ signals are induced by low compared to high concentrations of P2Y agonists in microglia

Brain-resident macrophages (microglia) are key cellular elements in the preservation of tissue integrity. On the other hand, they can also contribute to the development of pathological events by causing an extensive and inappropriate inflammatory response. A growing number of reports indicate the involvement of nucleotides in the control of microglial functions. With this study on P2Y receptors in rat microglia, we want to contribute to the definition of their expression profile and to the characterisation of their signalling mechanisms leading to Ca²⁺ movements. Endogenous nucleotides, when applied at a concentration of 100 μM, elicited robust Ca²⁺ transients, thanks to a panel of metabotropic receptors comprising mainly P2Y₂, P2Y₆ and P2Y₁₂ subtypes. The involvement of P2Y₁₂ receptors in Ca²⁺ responses induced by adenine nucleotides was confirmed by the pharmacological and pertussis toxin sensitivity of the response induced by adenosine diphosphate (ADP). Beside the G protein involved, Gi and Gq respectively, adenine and uracil nucleotides differed also for induction by the latter of a capacitative Ca²⁺ plateau. Moreover, when applied at low (sub-micromolar) concentrations with a long-lasting challenge, uracil nucleotides elicited oscillatory Ca²⁺ changes with low frequency of occurrence (≤ 1 min⁻), sometimes superimposed to an extracellular Ca²⁺-dependent sustained Ca²⁺ rise. We conclude that different patterns of Ca²⁺ transients are induced by low (i.e., oscillatory Ca²⁺ activity) compared to high (i.e., fast release followed by sustained raise) concentrations of nucleotides, which can suggest different roles played by receptor stimulation depending not only on the type but also on the concentration of nucleotides.

Purinergic signaling and microglia

Microglial cells are considered as the pathologic sensors of the brain. In this paper, we review mechanisms of purinergic signaling in microglia. As ATP is not only considered as a physiological signaling substance but is also elevated in pathology, it is not surprising that microglia express a variety of P2X, P2Y and adenosin receptors. As a rapid physiological event, ATP triggers a cationic conductance, increases the potassium conductance and also elicits a calcium response. As a long-term effect, purinergic receptor activation is linked to the movement of microglial processes and, in the context of pathology, to chemotaxis. The purinoreceptors also modulate the release of substances from microglia, such as cytokines, nitric oxide, or superoxide, which are important in the context of a pathologic response.

Adenosine 5'-triphosphate and adenosine as endogenous signaling molecules in immunity and inflammation

Human health is under constant threat of a wide variety of dangers, both self and nonself. The immune system is occupied with protecting the host against such dangers in order to preserve human health. For that purpose, the immune system is equipped with a diverse array of both cellular and non-cellular effectors that are in continuous communication with each other. The naturally occurring nucleotide adenosine 5'-triphosphate (ATP) and its metabolite adenosine (Ado) probably constitute an intrinsic part of this extensive immunological network through purinergic signaling by their cognate receptors, which are widely expressed throughout the body. This review provides a thorough overview of the effects of ATP and Ado on major immune cell types. The overwhelming evidence indicates that ATP and Ado are important endogenous signaling molecules in immunity and inflammation. Although the role of ATP and Ado during the course of inflammatory and immune responses in vivo appears to be extremely complex, we propose that their immunological role is both interdependent and multifaceted, meaning that the nature of their effects may shift from immunostimulatory to immunoregulatory or vice versa depending on extracellular concentrations as well as on expression patterns of purinergic receptors and ecto-enzymes. Purinergic signaling thus contributes to the fine-tuning of inflammatory and immune responses in such a way that the danger to the host is eliminated efficiently with minimal damage to healthy tissues.

Pathophysiology and therapeutic potential of purinergic signaling

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

Unilateral cortical application of tumor necrosis factor alpha induces asymmetry in Fos- and interleukin-1beta-immunoreactive cells within the corticothalamic projection

A unilateral microinjection of tumor necrosis factor alpha (TNFalpha) (150 ng) onto the primary somatosensory cortex induces state-dependent asymmetries in electroencephalographic (EEG) slow wave activity during non-rapid eye movement sleep in rats [H. Yoshida, Z. Peterfi, F. Garcia-Garcia, R. Kirkpatrick, T. Yasuda, J.M. Krueger, State-specific asymmetries in EEG slow wave activity induced by local application of TNF alpha, Brain Res. 1009 (2004) 129-136]. In the current study, analogous TNFalpha injections were performed to determine Fos- and interleukin-1beta (IL1beta) immunoreactivity (IR). A unilateral microinjection of TNFalpha increased the number of Fos- and IL1beta-IR cells in the primary somatosensory cortex relative to the contralateral side that received heat-inactivated TNFalpha. These asymmetric TNFalpha-induced increases in the number of Fos- and IL1beta-IR cells were evident along the outside surface of the cortex (mainly layers II and III) in a restricted rostral to caudal zone. Asymmetrical increases in the number of Fos-IR cells were also observed in the subcortical region that receives the main cortical projection from the somatosensory cortex, the somatic region of the reticular nucleus of the thalamus (reticular thalamus). The IL1beta-IR cells double-labeled with glial fibrillary acidic protein (GFAP), suggesting that many of the IL1beta-IR cells were astrocytes. The number of the IL1beta-IR cells in the reticular thalamus increased significantly ipsilateral to the TNFalpha injection. Current results indicated that Fos- and IL1beta-IR may be utilized to study the functional neuroanatomy involved in the TNFalpha-mediated state-dependent enhancement of EEG slow wave activity.

Gene expression profile of activated microglia under conditions associated with dopamine neuronal damage

Microglia are the resident antigen-presenting cells within the central nervous system (CNS), and they serve immune-like functions in protecting the brain against injury and invading pathogens. By contrast, activated microglia can secrete numerous reactants that damage neurons. The pathogenesis of various neurodegenerative diseases has been associated with microglial activation, but the signaling pathways that program a neuronally protective or destructive phenotype in microglia are not known. To increase the understanding of microglial activation, microarray analysis was used to profile the transcriptome of BV-2 microglial cells after activation. Microglia were activated by lipopolysaccharide, the HIV neurotoxic protein TAT, and dopamine quinone, each of which has been linked to dopamine neuronal damage. We identified 210 of 9882 genes whose expression was differentially regulated by all activators (116 increased and 94 decreased in expression). Gene ontology analysis assigned up-regulated genes to a number of specific biological processes and molecular functions, including immune response, inflammation, and cytokine/chemokine activity. Genes down-regulated in expression contribute to conditions that are permissive of microglial migration, lowered adhesion to matrix, lessened phagocytosis, and reduction in receptors that oppose chemotaxis and inflammation. These results elaborate a broad profile of microglial genes whose expression is altered by conditions associated with both neurodegenerative diseases and microglial activation.

Purinergic mediated changes in Ca2+ mobilization and functional responses in microglia: effects of low levels of ATP

Microglia, the immune effector cells of the brain, are stimulated by a diversity of agents to transiently increase levels of intracellular calcium ([Ca2+]i). Changes in [Ca2+]i induced by compounds such as adenosine triphosphate (ATP) serve important roles in cellular signal transduction linking stimuli with cellular functional responses. Purinergic responses in microglia, like that in other cells, are mediated by two families of receptors classified as P2Y and P2X. Activation of metabotropic receptors (P2YR) leads to increased [Ca2+]i due to depletion of intracellular stores, a process that can trigger activation of Ca2+ entry through plasmalemmal store-operated channels (SOC). Activation of ionotropic receptors (P2XR) is associated with influx of Na+ and Ca2+ and efflux of K+ through nonselective cationic channels, leading to cellular depolarization. An intriguing property of purinergic stimulation of microglia is the dependence of cellular responses on agonist concentration. As one example, activation of the subtype P2X7R by higher levels of ATP (millimolar range), leads to a marked enhancement in microglial secretion of inflammatory mediators. Other members of the ionotropic P2XR family sensitive to lower levels of ATP, however, are also important in mediating microglial inflammatory responses in brain. At lower concentrations of ATP (100 microM), activation of SOC in human microglia is not only coupled to P2YR-dependent depletion of internal stores, but is also modulated by ATP binding to a P2XR (not P2X7R). The modulation is consistent with a P2XR-mediated influx of Na+ and inhibition of SOC by depolarization. In this review, a primary focus is placed on the effects of low concentrations of ATP (< or =100 microM) to induce changes in [Ca2+]i and modify functional processes in microglia. In essence, responses mediated by purinergic receptors other than P2X7R are considered.

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.

Acute Lipopolysaccharide-Mediated Injury in Neonatal White Matter Glia: Role of TNF-α, IL-1β, and Calcium

Bacterial infection is implicated in the selective CNS white matter injury associated with cerebral palsy, a common birth disorder. Exposure to the bacterial endotoxin LPS produced death of white matter glial cells in isolated neonatal rat optic nerve (RON) (a model white matter tract), over a 180-min time course. A delayed intracellular Ca(2+) concentration ([Ca(2+)](i)) rise preceded cell death and both events were prevented by removing extracellular Ca(2+). The cytokines TNF-alpha or IL-1beta, but not IL-6, mimicked the cytotoxic effect of LPS, whereas blocking either TNF-alpha with a neutralizing Ab or IL-1 with recombinant antagonist prevented LPS cytotoxicity. Ultrastructural examination showed wide-scale oligodendroglial cell death in LPS-treated rat optic nerves, with preservation of astrocytes and axons. Fluorescently conjugated LPS revealed LPS binding on microglia and astrocytes in neonatal white and gray matter. Astrocyte binding predominated, and was particularly intense around blood vessels. LPS can therefore bind directly to developing white matter astrocytes and microglia to evoke rapid cell death in neighboring oligodendroglia via a calcium- and cytokine-mediated pathway. In addition to direct toxicity, LPS increased the degree of acute cell death evoked by ischemia in a calcium-dependent manner.

Immunoregulatory effects of adenosine 5'-triphosphate on cytokine release from stimulated whole blood

In vitro studies suggest that extracellular nucleotides and nucleosides may be important regulators of inflammatory and immune responses. Most studies with adenosine 5'-triphosphate (ATP) have been performed in cell lines, which are remote from the human situation. The purpose of the present study was to determine the effects of ATP on TNF-alpha, IL-6 and IL-10 release in stimulated whole blood. Blood samples were drawn from healthy volunteers and incubated with ATP and lipopolysaccharide (LPS) + phytohemagglutinin (PHA) for 24 h. Contrary to expectations, ATP at 100 microM and 300 microM induced a reduction in TNF-alpha secretion by 32+/-8% (mean +/- SEM) and 65+/-4%, respectively. Furthermore, these ATP concentrations induced an increase in IL-10 secretion by 48+/-5% and 62+/-7% in whole blood. The ATP analogue adenosine 5'-O-(3-thiotriphosphate) (ATP-gamma-S) and adenosine 5'-diphosphate (ADP) also inhibited TNF-alpha release, but only ADP showed a stimulatory effect on IL-10. Co-treatment with adenosine deaminase did not reverse the ATP effect on TNF-alpha and IL-10. These results show, for the first time, that ATP inhibits the inflammatory response in stimulated whole blood as indicated by inhibition of TNF-alpha and stimulation of IL-10 release and that this effect is predominantly mediated by ATP and not by adenosine.

Pathophysiological roles of extracellular nucleotides in glial cells: differential expression of purinergic receptors in resting and activated microglia

Microglial cells are the major cellular elements with immune function inside the CNS and play important roles in orchestrating inflammatory brain response to hypoxia and trauma. Although a complete knowledge of the endogenous factors leading to a prompt activation of microglia is not yet available, activation of P2 purinoreceptors by extracellular ATP has been indicated as a primary factor in microglial response. A still unresolved question, however, is which subtype(s) of P2 receptors mediate(s) the response to ATP. By a combination of RT-PCR, Western blotting, and single-cell calcium imaging, we assessed the presence and the activity of P2 receptor subtypes in the mouse microglial cell line N9. All members of the P2 receptor family, including the recently reported receptor for sugar nucleotides (P2Y(14)), were found to be present in these cells at mRNA and/or protein level. The functionality of the receptors was assessed by analysis of the calcium responses evoked by specific agonists both in N9 cells and in primary microglia from rat brain. Interestingly, a different functional profile of P2 receptors was observed in resting or in LPS-activated N9 cells. Overnight exposure to LPS increased P2Y(6) and P2Y(14), decreased P2X(7), and left unchanged P2Y(1) and P2Y(2,4) receptor activity. The change in the P2 receptor profile in activated cells suggests selective roles for specific P2 receptor subtypes in microglial activation triggered by LPS. We speculate that modulation of microglial cell function via subtype-selective P2 receptor ligands may open up new strategies in the therapeutic management of inflammatory neurological diseases characterized by abnormal microglia response.

Role of microglia in central nervous system infections

The nature of microglia fascinated many prominent researchers in the 19th and early 20th centuries, and in a classic treatise in 1932, Pio del Rio-Hortega formulated a number of concepts regarding the function of these resident macrophages of the brain parenchyma that remain relevant to this day. However, a renaissance of interest in microglia occurred toward the end of the 20th century, fueled by the recognition of their role in neuropathogenesis of infectious agents, such as human immunodeficiency virus type 1, and by what appears to be their participation in other neurodegenerative and neuroinflammatory disorders. During the same period, insights into the physiological and pathological properties of microglia were gained from in vivo and in vitro studies of neurotropic viruses, bacteria, fungi, parasites, and prions, which are reviewed in this article. New concepts that have emerged from these studies include the importance of cytokines and chemokines produced by activated microglia in neurodegenerative and neuroprotective processes and the elegant but astonishingly complex interactions between microglia, astrocytes, lymphocytes, and neurons that underlie these processes. It is proposed that an enhanced understanding of microglia will yield improved therapies of central nervous system infections, since such therapies are, by and large, sorely needed.