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

Purinoreceptors and ectonucleotidases control ATP-induced calcium waveforms and calcium-dependent responses in microglia: Roles of P2 receptors and CD39 in ATP-stimulated microglia

Adenosine triphosphate (ATP) and its metabolites drive microglia migration and cytokine production by activating P2X- and P2Y- class purinergic receptors. Purinergic receptor activation gives rise to diverse intracellular calcium (Ca2+ signals, or waveforms, that differ in amplitude, duration, and frequency. Whether and how these characteristics of diverse waveforms influence microglia function is not well-established. We developed a computational model trained with data from published primary murine microglia studies. We simulate how purinoreceptors influence Ca2+ signaling and migration, as well as, how purinoreceptor expression modifies these processes. Our simulation confirmed that P2 receptors encode the amplitude and duration of the ATP-induced Ca2+ waveforms. Our simulations also implicate CD39, an ectonucleotidase that rapidly degrades ATP, as a regulator of purinergic receptor-induced Ca2+ responses. Namely, it was necessary to account for CD39 metabolism of ATP to align the model's predicted purinoreceptor responses with published experimental data. In addition, our modeling results indicate that small Ca2+ transients accompany migration, while large and sustained transients are needed for cytokine responses. Lastly, as a proof-of-principal, we predict Ca2+ transients and cell membrane displacements in a BV2 microglia cell line using published P2 receptor mRNA data to illustrate how our computer model may be extrapolated to other microglia subtypes. These findings provide important insights into how differences in purinergic receptor expression influence microglial responses to ATP.

P2X Receptor-Dependent Modulation of Mast Cell and Glial Cell Activities in Neuroinflammation

Localisation of mast cells (MCs) at the abluminal side of blood vessels in the brain favours their interaction with glial cells, neurons, and endothelial cells, resulting in the activation of these cells and the release of pro-inflammatory mediators. In turn, stimulation of glial cells, such as microglia, astrocytes, and oligodendrocytes may result in the modulation of MC activities. MCs, microglia, astrocytes, and oligodendrocytes all express P2X receptors (P2XRs) family members that are selectively engaged by ATP. As increased concentrations of extracellular adenosine 5′-triphosphate (ATP) are present in the brain in neuropathological conditions, P2XR activation in MCs and glial cells contributes to the control of their communication and amplification of the inflammatory response. In this review we discuss P2XR-mediated MC activation, its bi-directional effect on microglia, astrocytes and oligodendrocytes and role in neuroinflammation.

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.

Nucleotide P2Y13-stimulated phosphorylation of CREB is required for ADP-induced proliferation of late developing retinal glial progenitors in culture

Nucleotides stimulate phosphorylation of CREB to induce cell proliferation and survival in diverse cell types. We report here that ADP induces the phosphorylation of CREB in a time- and concentration-dependent manner in chick embryo retinal progenitors in culture. ADP-induced increase in phospho-CREB is mediated by P2 receptors as it is blocked by PPADS but not by the adenosine antagonists DPCPX or ZM241385. Incubation of the cultures with the CREB inhibitor KG-501 prevents ADP-induced incorporation of [³H]-thymidine, indicating that CREB is involved in retinal cell proliferation. No effect of this compound is observed on the viability of retinal progenitors. While no significant increase in CREB phosphorylation is observed with the P2Y1 receptor agonist MRS2365, ADP-induced phosphorylation of CREB is blocked by the P2Y13 receptor selective antagonist MRS2211, but not by MRS2179 or PSB0739, two antagonists of the P2Y1 and P2Y12 receptors, respectively, suggesting that ADP-induced CREB phosphorylation is mediated by P2Y13 receptors. ADP-induced increase in phospho-CREB is attenuated by the PI3K inhibitor LY294002 and completely prevented by the MEK inhibitor U0126, suggesting that at least ERK is involved in ADP-induced CREB phosphorylation. A pharmacological profile similar to the activation and inhibition of CREB phosphorylation is observed in the phosphorylation of ERK, suggesting that P2Y13 receptors mediate ADP induced ERK/CREB pathway in the cultures. While no increase in [³H]-thymidine incorporation is observed with the P2Y1 receptor agonist MRS2365, both MRS2179 and MRS2211 prevent ADP-mediated increase in [³H]-thymidine incorporation, but not progenitor's survival, suggesting that both P2Y1 and P2Y13 receptor subtypes are involved in ADP-induced cell proliferation. P2Y1 receptor-mediated increase in [Ca²⁺]_i is observed in glial cells only when cultures maintained for 9days are used. In glia from cultures cultivated for only 2days, no increase in [Ca²⁺]_i is detected with MRS2365 and no inhibition of ADP-mediated calcium response is observed with MRS2179. In contrast, MRS2211 attenuates ADP-mediated increase in [Ca²⁺]_i in glial cells from cultures at both stages, suggesting the presence of P2Y13 receptors coupled to calcium mobilization in proliferating retinal glial progenitors in culture.

Relative Role of Akt, ERK and CREB in Alcohol-Induced Microglia P2X4R Receptor Expression

AIMS

Previously we have demonstrated altered microglia P2X4R expression in response to alcohol and pharmacological blockade with a selective P2X4R antagonist can reverse the action, suggesting that P2X4R play a role in mediating alcohol-induced effects on microglia. In the present study, we investigated the underlying signaling mediators, which may play a role in modulating P2X4R expression in microglia cells in response to alcohol.

METHODS

Embryonic stem cell-derived microglia (ESdM) cells were used to investigate the potential mechanisms involved in the regulation of P2X4R in response to alcohol. Selective P2X4R antagonist and kinase inhibitors were used to further corroborate the signal transduction pathway through which alcohol modulates P2X4R expression in microglia.

RESULTS

Alcohol (100 mM) suppressed phosphorylated AKT and ERK cascades in native ESdM cells. This alcohol-induced suppression was confirmed to be P2X4R-dependent through the use of a selective P2X4R antagonist and knockdown of P2XR4 by siRNA. Alcohol increased transcriptional activity of CREB. P2X4R antagonist blocked alcohol-induced effects on CREB, suggesting a P2X4R-mediated effect.

CONCLUSION

These findings provide important clues to the underlying mechanism of purinoceptors in alcohol-induced microglia immune suppression.

Microglial P2 Purinergic Receptor and Immunomodulatory Gene Transcripts Vary By Region, Sex, and Age in the Healthy Mouse CNS

Inflammatory damage in many neurodegenerative diseases is restricted to certain regions of the CNS, and while microglia have long been implicated in the pathology of many of these disorders, information comparing their gene expression in different CNS regions is lacking. Here we tested the hypothesis that the expression of purinergic receptors, estrogen receptors and other neuroprotective and pro-inflammatory genes differed among CNS regions in healthy mice. Because neurodegenerative diseases vary in incidence by sex and age, we also examined the regional distribution of these genes in male and female mice of four different ages between 21 days and 12 months. We postulated that pro-inflammatory gene expression would be higher in older animals, and lower in young adult females. We found that microglial gene expression differed across the CNS. Estrogen receptor alpha (Esr1) mRNA levels were often lower in microglia from the brainstem/spinal cord than from the cortex, whereas tumor necrosis factor alpha (Tnfα) expression was several times higher. In addition, the regional pattern of gene expression often changed with animal age; for example, no regional differences in P2X7 mRNA levels were detected in 21 day-old animals, but at 7 weeks and older, expression was highest in cerebellar microglia. Lastly, the expression of some genes was sexually dimorphic. In microglia from 12 month-old animals, mRNA levels of inducible nitric oxide synthase, but not Tnfα, were higher in females than males. These data suggest that microglial gene expression is not uniformly more pro-inflammatory in males or older animals. Moreover, microglia from CNS regions in which neuronal damage predominates in neurodegenerative disease do not generally express more pro-inflammatory genes than microglia from regions less frequently affected. This study provides an in-depth assessment of regional-, sex- and age-dependent differences in key microglial transcripts from the healthy mouse CNS.

Lack of a Functioning P2X7 Receptor Leads to Increased Susceptibility to Toxoplasmic Ileitis

BACKGROUND

Oral infection of C57BL/6J mice with the protozoan parasite Toxoplasma gondii leads to a lethal inflammatory ileitis.

PRINCIPAL FINDINGS

Mice lacking the purinergic receptor P2X7R are acutely susceptible to toxoplasmic ileitis, losing significantly more weight than C57BL/6J mice and exhibiting much greater intestinal inflammatory pathology in response to infection with only 10 cysts of T. gondii. This susceptibility is not dependent on the ability of P2X7R-deficient mice to control the parasite, which they accomplish just as efficiently as C57BL/6J mice. Rather, susceptibility is associated with elevated ileal concentrations of pro-inflammatory cytokines, reactive nitrogen intermediates and altered regulation of elements of NFκB activation in P2X7R-deficient mice.

CONCLUSIONS

Our data support the thesis that P2X7R, a well-documented activator of pro-inflammatory cytokine production, also plays an important role in the regulation of intestinal inflammation.

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.

Hypoxia Attenuates Purinergic P2X Receptor-Induced Inflammatory Gene Expression in Brainstem Microglia

Hypoxia and increased extracellular nucleotides are frequently coincident in the brainstem. Extracellular nucleotides are potent modulators of microglial inflammatory gene expression via P2X purinergic receptor activation. Although hypoxia is also known to modulate inflammatory gene expression, little is known about how hypoxia or P2X receptor activation alone affects inflammatory molecule production in brainstem microglia, nor how hypoxia and P2X receptor signaling interact when they occur together. In the study reported here, we investigated the ability of a brief episode of hypoxia (2 hours) in the presence and absence of the nonselective P2X receptor agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) to promote inflammatory gene expression in brainstem microglia in adult rats. We evaluated inducible nitric oxide synthase (iNOS), tumor necrosis factor alpha (TNFα), and interleukin (IL)-6 messenger RNA levels in immunomagnetically isolated brainstem microglia. While iNOS and IL-6 gene expression increased with hypoxia and BzATP alone, TNFα expression was unaffected. Surprisingly, BzATP-induced inflammatory effects were lost after hypoxia, suggesting that hypoxia impairs proinflammatory P2X-receptor signaling. We also evaluated the expression of key P2X receptors activated by BzATP, namely P2X1, P2X4, and P2X7. While hypoxia did not alter their expression, BzATP upregulated P2X4 and P2X7 mRNAs; these effects were ablated in hypoxia. Although both P2X4 and P2X7 receptor expression correlated with increased microglial iNOS and IL-6 levels in microglia from normoxic rats, in hypoxia, P2X7 only correlated with IL-6, and P2X4 correlated only with iNOS. In addition, correlations between P2X7 and P2X4 were lost following hypoxia, suggesting that P2X4 and P2X7 receptor signaling differs in normoxia and hypoxia. Together, these data suggest that hypoxia suppresses P2X receptor-induced inflammatory gene expression, indicating a potentially immunosuppressive role of extracellular nucleotides in brainstem microglia following exposure to hypoxia.

Neurochemical Changes in the Mouse Hippocampus Underlying the Antidepressant Effect of Genetic Deletion of P2X7 Receptors

Recent investigations have revealed that the genetic deletion of P2X7 receptors (P2rx7) results in an antidepressant phenotype in mice. However, the link between the deficiency of P2rx7 and changes in behavior has not yet been explored. In the present study, we studied the effect of genetic deletion of P2rx7 on neurochemical changes in the hippocampus that might underlie the antidepressant phenotype. P2X7 receptor deficient mice (P2rx7−/−) displayed decreased immobility in the tail suspension test (TST) and an attenuated anhedonia response in the sucrose preference test (SPT) following bacterial endotoxin (LPS) challenge. The attenuated anhedonia was reproduced through systemic treatments with P2rx7 antagonists. The activation of P2rx7 resulted in the concentration-dependent release of [³H]glutamate in P2rx7+/+ but not P2rx7−/− mice, and the NR2B subunit mRNA and protein was upregulated in the hippocampus of P2rx7−/− mice. The brain-derived neurotrophic factor (BDNF) expression was higher in saline but not LPS-treated P2rx7−/− mice; the P2rx7 antagonist Brilliant blue G elevated and the P2rx7 agonist benzoylbenzoyl ATP (BzATP) reduced BDNF level. This effect was dependent on the activation of NMDA and non-NMDA receptors but not on Group I metabotropic glutamate receptors (mGluR_1,5). An increased 5-bromo-2-deoxyuridine (BrdU) incorporation was also observed in the dentate gyrus derived from P2rx7−/− mice. Basal level of 5-HT was increased, whereas the 5HIAA/5-HT ratio was lower in the hippocampus of P2rx7−/− mice, which accompanied the increased uptake of [³H]5-HT and an elevated number of [³H]citalopram binding sites. The LPS-induced elevation of 5-HT level was absent in P2rx7−/− mice. In conclusion there are several potential mechanisms for the antidepressant phenotype of P2rx7−/− mice, such as the absence of P2rx7-mediated glutamate release, elevated basal BDNF production, enhanced neurogenesis and increased 5-HT bioavailability in the hippocampus.

Bullatine A, a diterpenoid alkaloid of the genus Aconitum, could attenuate ATP-induced BV-2 microglia death/apoptosis via P2X receptor pathways

Bullatine A (BLA), a diterpenoid alkaloid of the genus Aconitum, possesses anti-rheumatic, anti-inflammatory and anti-nociceptive effects. The mechanism underlying the effects was examined in the present study. The effect of BLA on extracellular ATP induced cell death/apoptosis and pro-inflammatory cytokines release were investigated using BV-2 microglia cell line. The mediation/efficacy of inflammatory cytokines and P2X receptors was evaluated by detecting the mRNA levels of iNOS, IL-6, IL-1β and P2X receptors, respectively. The results demonstrated that BV-2 cells could be damaged after incubation with higher dose of ATP, leading to activation of pro-inflammatory cytokines, transcriptional activation of iNOS and overproduction of NO via activation of P2X receptor. The BLA (1-50μM) potently inhibits ATP-induced BV-2 cell death/apoptosis and P2X receptor-mediated inflammatory responses via selectively suppressing the up-regulation of P2X7 receptor mRNA. Since P2X7 receptors have an important role in immune and pain response, inflammation and inflammatory disease, this discovery of BLA as a potent P2X7 antagonist indicated that BLA may be a potential useful candidate for the treatment of neurodegenerative diseases such as arthritis.

P2X7 receptor activation regulates microglial cell death during oxygen-glucose deprivation

Brain-resident microglia may promote tissue repair following stroke but, like other cells, they are vulnerable to ischemia. Here we identify mechanisms involved in microglial ischemic vulnerability. Using time-lapse imaging of cultured BV2 microglia, we show that simulated ischemia (oxygen-glucose deprivation; OGD) induces BV2 microglial cell death. Removal of extracellular Ca(2+) or application of Brilliant Blue G (BBG), a potent P2X7 receptor (P2X7R) antagonist, protected BV2 microglia from death. To validate and extend these in vitro findings, we assessed parenchymal microglia in freshly isolated hippocampal tissue slices from GFP-reporter mice (CX3CR1(GFP/+)). We confirmed that calcium removal or application of apyrase, an ATP-degrading enzyme, abolished OGD-induced microglial cell death in situ, consistent with involvement of ionotropic purinergic receptors. Indeed, whole cell recordings identified P2X7R-like currents in tissue microglia, and OGD-induced microglial cell death was inhibited by BBG. These pharmacological results were complemented by studies in tissue slices from P2X7R null mice, in which OGD-induced microglia cell death was reduced by nearly half. Together, these results indicate that stroke-like conditions induce calcium-dependent microglial cell death that is mediated in part by P2X7R. This is the first identification of a purinergic receptor regulating microglial survival in living brain tissues. From a therapeutic standpoint, these findings could help direct novel approaches to enhance microglial survival and function following stroke and other neuropathological conditions.

Acute resveratrol treatment modulates multiple signaling pathways in the ischemic brain

Resveratrol has several beneficial effects, including reductions of oxidative stress, inflammatory responses and apoptosis. It has been known that resveratrol is a sirtuin 1 (SIRT1) activator and protective effects of resveratrol are mediated by Akt and mitogen-activated protein kinases. However, it is not examined whether these pathways are regulated by resveratrol in the ischemic brain. Previously, we found that acute resveratrol treatment reduces brain injury induced by transient focal ischemic stroke. In the present study, we defined the signaling pathways modulated by resveratrol in ischemia by examining SIRT1 expression and phosphorylation of Akt, ERK1/2 and p38 in the ischemic cortex. Resveratrol increased expression of SIRT1 and phosphorylation of Akt and p38 but inhibited the increase in phosphorylation of ERK1/2. Gene and protein levels of peroxisome proliferator-activated receptor γ coactivator 1α, a downstream molecule of SIRT1, and mRNA levels of its target genes antioxidative superoxide dismutase 2 and uncoupling protein 2 were elevated. Resveratrol also increased phosphorylation of cyclic AMP-response-element-binding protein and transcription of the anti-apoptotic gene Bcl-2. These results suggest that various neuroprotective actions of resveratrol, including anti-oxidative, anti-apoptotic and inflammatory effects, are mediated via modulation of multiple signaling pathways in the ischemic brain.

UTP affects the Schwannoma cell line proteome through P2Y receptors leading to cytoskeletal reorganisation

Glial cells in the peripheral nervous system, such as Schwann cells, respond to nucleotides, which play an important role in axonal regeneration and myelination. Metabotropic P2Y receptor agonists are promising therapeutic molecules for peripheral neuropathies. Nevertheless, the proteomic mechanisms involved in nucleotide action on Schwann cells remain unknown. Here, we studied intracellular protein changes in RT4-D6P2T Schwann cells after treatment with nucleotides and Nucleo CMP Forte (CMPF), a nucleotide-based drug. After treatment with CMPF, 2-D DIGE revealed 11 differential gel spots, which were all upregulated. Among these, six different proteins were identified by MS. Some of these proteins are involved in actin remodelling (actin-related protein, Arp3), membrane vesicle transport (Rab GDP dissociation inhibitor β, Rab GDI), and the endoplasmic reticulum stress response (protein disulfide isomerase A3, PDI), which are hallmarks of a possible P2Y receptor signalling pathway. Expression of P2Y receptors in RT4-D6P2T cells was demonstrated by RT-PCR and a transient elevation of intracellular calcium measured in response to UTP. Actin reorganisation was visualized after UTP treatment using phalloidin-FITC staining and was blocked by the P2Y antagonist suramin, which also inhibited Arp3, Rab GDI, and PDI protein upregulation. Our data indicate that extracellular UTP interacts with Schwann P2Y receptors and activates molecular machinery that induces changes in the glial cell cytoskeleton.

The role of the P2X₇ receptor in infectious diseases

ATP is an extracellular signal for the immune system, particularly during an inflammatory response. It is sensed by the P2X₇ receptor, the expression of which is upregulated by pro-inflammatory cytokines. Activation of the P2X₇ receptor opens a cation-specific channel that alters the ionic environment of the cell, activating several pathways, including (i) the inflammasome, leading to production of IL-1β and IL-18; (ii) the stress-activated protein kinase pathway, resulting in apoptosis; (iii) the mitogen-activated protein kinase pathway, leading to generation of reactive oxygen and nitrogen intermediates; and (iv) phospholipase D, stimulating phagosome-lysosome fusion. The P2X₇ receptor can initiate host mechanisms to remove pathogens, most particularly those that parasitise macrophages. At the same time, the P2X₇ receptor may be subverted by pathogens to modulate host responses. Moreover, recent genetic studies have demonstrated significant associations between susceptibility or resistance to parasites and bacteria, and loss-of-function or gain-of-function polymorphisms in the P2X₇ receptor, underscoring its importance in infectious disease.

Lack of neuroprotection in the absence of P2X7 receptors in toxin-induced animal models of Parkinson's disease

Background

Previous studies indicate a role of P2X₇ receptors in processes that lead to neuronal death. The main objective of our study was to examine whether genetic deletion or pharmacological blockade of P2X₇ receptors influenced dopaminergic cell death in various models of Parkinson's disease (PD).

Results

mRNA encoding P2X₇ and P2X₄ receptors was up-regulated after treatment of PC12 cells with 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP). P2X₇ antagonists protected against MPTP and rotenone induced toxicity in the LDH assay, but failed to protect after rotenone treatment in the MTT assay in PC12 cells and in primary midbrain culture. In vivo MPTP and in vitro rotenone pretreatments increased the mRNA expression of P2X₇ receptors in the striatum and substantia nigra of wild-type mice. Basal mRNA expression of P2X₄ receptors was higher in P2X₇ knockout mice and was further up-regulated by MPTP treatment. Genetic deletion or pharmacological inhibition of P2X₇ receptors did not change survival rate or depletion of striatal endogenous dopamine (DA) content after in vivo MPTP or in vitro rotenone treatment. However, depletion of norepinephrine was significant after MPTP treatment only in P2X₇ knockout mice. The basal ATP content was higher in the substantia nigra of wild-type mice, but the ADP level was lower. Rotenone treatment elicited a similar reduction in ATP content in the substantia nigra of both genotypes, whereas reduction of ATP was more pronounced after rotenone treatment in striatal slices of P2X₇ deficient mice. Although the endogenous amino acid content remained unchanged, the level of the endocannabinoid, 2-AG, was elevated by rotenone in the striatum of wild-type mice, an effect that was absent in mice deficient in P2X₇ receptors.

Conclusions

We conclude that P2X₇ receptor deficiency or inhibition does not support the survival of dopaminergic neurons in an in vivo or in vitro models of PD.

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-P2X₇ 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.

The role of the transcription factor CREB in immune function

CREB is a transcription factor that regulates diverse cellular responses, including proliferation, survival, and differentiation. CREB is induced by a variety of growth factors and inflammatory signals and subsequently mediates the transcription of genes containing a cAMP-responsive element. Several immune-related genes possess this cAMP-responsive element, including IL-2, IL-6, IL-10, and TNF-α. In addition, phosphorylated CREB has been proposed to directly inhibit NF-κB activation by blocking the binding of CREB binding protein to the NF-κB complex, thereby limiting proinflammatory responses. CREB also induces an antiapoptotic survival signal in monocytes and macrophages. In T and B cells, CREB activation promotes proliferation and survival and differentially regulates Th1, Th2, and Th17 responses. Finally, CREB activation is required for the generation and maintenance of regulatory T cells. This review summarizes current advances involving CREB in immune function--a role that is continually being defined.

Activation of the transcription factor FosB/activating protein-1 (AP-1) is a prominent downstream signal of the extracellular nucleotide receptor P2RX7 in monocytic and osteoblastic cells

Activation of the ionotropic P2RX7 nucleotide receptor by extracellular ATP has been implicated in modulating inflammatory disease progression. Continuous exposure of P2RX7 to ligand can result in apoptosis in many cell types, including monocytic cells, whereas transient activation of P2RX7 is linked to inflammatory mediator production and the promotion of cell growth. Given the rapid hydrolysis of ATP in the circulation and interstitial space, transient activation of P2RX7 appears critically important for its action, yet its effects on gene expression are unclear. The present study demonstrates that short-term stimulation of human and mouse monocytic cells as well as mouse osteoblasts with P2RX7 agonists substantially induces the expression of several activating protein-1 (AP-1) members, particularly FosB. The potent activation of FosB after P2RX7 stimulation is especially noteworthy considering that little is known concerning the role of FosB in immunological regulation. Interestingly, the magnitude of FosB activation induced by P2RX7 stimulation appears greater than that observed with other known inducers of FosB expression. In addition, we have identified a previously unrecognized role for FosB in osteoblasts with respect to nucleotide-induced expression of cyclooxygenase-2 (COX-2), which is the rate-limiting enzyme in prostaglandin biosynthesis from arachidonic acid and is critical for osteoblastic differentiation and immune behavior. The present studies are the first to link P2RX7 action to FosB/AP-1 regulation in multiple cell types, including a role in nucleotide-induced COX-2 expression, and support a role for FosB in the control of immune and osteogenic function by P2RX7.

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 modulates transcription factors through P2Y2 and P2Y4 receptors via PKC/MAPKs and PKC/Src pathways in MCF-7 cells

In this work, we studied the involvement of PKC and Src in the phosphorylation of ERK1/2, p38 and JNK1 MAPKs and in the modulation of ATF-1, c-Fos, c-Jun and Jun D transcription factors by ATP in MCF-7 breast cancer cells. RT-PCR studies and nucleotide sequence analysis confirmed first the expression of P2Y(2)- and P2Y(4)-receptor subtypes. The use of specific inhibitors and Src antisense oligonucleotides showed that PKC, but not Src, plays a role in the phosphorylation of MAPKs by ATP. ATP stimulated the expression of c-Fos and the phosphorylation c-Jun, Jun D and ATF-1. PKC and Src only participated in c-Fos induction and in ATF-1 phosphorylation. Pharmacological inhibition of MAPKs demonstrated that c-Fos induction and phosphorylation of c-Jun and Jun D, but not of ATF-1, depend on MAPK activation. These results suggest that stimulation of P2Y(2) and P2Y(4) receptors by ATP modulates transcription factors through PKC/MAPKs and PKC/Src pathways in MCF-7 cells.

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.

Tumor necrosis factor-alpha upregulates 11beta-hydroxysteroid dehydrogenase type 1 expression by CCAAT/enhancer binding protein-beta in HepG2 cells

The enzyme 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) catalyzes the conversion of inactive to active glucocorticoids. 11beta-HSD1 plays a crucial role in the pathogenesis of obesity and controls glucocorticoid actions in inflammation. Several studies have demonstrated that TNF-alpha increases 11beta-HSD1 mRNA and activity in various cell models. Here, we demonstrate that mRNA and activity of 11beta-HSD1 is increased in liver tissue from transgenic mice overexpressing TNF-alpha, indicating that this effect also occurs in vivo. To dissect the molecular mechanism of this increase, we investigated basal and TNF-alpha-induced transcription of the 11beta-HSD1 gene (HSD11B1) in HepG2 cells. We found that TNF-alpha acts via p38 MAPK pathway. Transient transfections with variable lengths of human HSD11B1 promoter revealed highest activity with or without TNF-alpha in the proximal promoter region (-180 to +74). Cotransfection with human CCAAT/enhancer binding protein-alpha (C/EBPalpha) and C/EBPbeta-LAP expression vectors activated the HSD11B1 promoter with the strongest effect within the same region. Gel shift and RNA interference assays revealed the involvement of mainly C/EBPalpha, but also C/EBPbeta, in basal and only of C/EBPbeta in the TNF-alpha-induced HSD11B1 expression. Chromatin immunoprecipitation assay confirmed in vivo the increased abundance of C/EBPbeta on the proximal HSD11B1 promoter upon TNF-alpha treatment. In conclusion, C/EBPalpha and C/EBPbeta control basal transcription, and TNF-alpha upregulates 11beta-HSD1, most likely by p38 MAPK-mediated increased binding of C/EBPbeta to the human HSD11B1 promoter. To our knowledge, this is the first study showing involvement of p38 MAPK in the TNF-alpha-mediated 11beta-HSD1 regulation, and that TNF-alpha stimulates enzyme activity in vivo.

Differential migration, LPS-induced cytokine, chemokine, and NO expression in immortalized BV-2 and HAPI cell lines and primary microglial cultures

Microglial cells are hematopoietically derived monocytes of the CNS and serve important neuromodulatory, neurotrophic, and neuroimmune roles. Following insult to the CNS, microglia develop a reactive phenotype, migrate to the site of injury, proliferate, and release a range of proinflammatory, anti-inflammatory, and neurotrophic factors. Isolation of primary microglial cell cultures has been an integral step in elucidating the many roles of these cells. In addition to primary microglial cells, several immortalized cell lines have been created to model primary microglia in vitro, including murine-derived BV-2 cells and rat derived highly aggressive proliferating immortalized (HAPI) cells. Here, we compare rat primary microglial, BV-2, and HAPI cells in experiments assessing migration, expression of activation markers, and production and release of nitric oxide, cytokines, and chemokines. BV-2 and HAPI cells responded similarly to primary microglia in experiments assessing migration, ionized calcium binding adaptor molecule 1 expression, and nitric oxide release. However, BV-2 and HAPI cells did not model primary microglia in experiments assessing tumor necrosis factor-alpha, interleukin-1beta, interleukin-6, and monocyte chemoattractant protein-1 expression and release and phospho-extracellular signal-regulated kinase 44/42 expression following lipopolysaccharide treatment. These results indicate that BV-2 and HAPI cell cultures only partially model primary microglia and that their use should therefore be carefully considered.

The inflammatory effects of UDP-glucose in N9 microglia are not mediated by P2Y14 receptor activation

In this study we evaluated the functionality and inflammatory effects of P2Y14 receptors in murine N9 microglia. The selective P2Y14 receptor agonist UDP-glucose (UDPG) derived from microbial sources dose dependently stimulated expression of cyclooxygenase-2 and inducible nitric oxide synthase, and potentiated the effects of bacterial lipopolysaccharide on nitric oxide production. However, another selective P2Y14 receptor agonist, UDP-galactose, did not affect these endpoints either alone or in combination with lipopolysaccharide. Interestingly, synthetic UDPG also had no detectable pro-inflammatory effects, although P2Y14 receptors are both expressed and functional in N9 microglia. While synthetic UDPG decreased levels of phosphorylated cyclic AMP response element binding protein, an effect that was blocked by pertussis toxin, the pro-inflammatory effects of microbial-derived UDPG were insensitive to pertussis toxin. These data suggest that the pro-inflammatory effects of microbial-derived UDPG are independent of P2Y14 receptors and imply that microbial-derived contaminants in the UDPG preparation may be involved in the observed inflammatory effects.

Simvastatin prevents the inflammatory process and the dopaminergic degeneration induced by the intranigral injection of lipopolysaccharide

Anti-inflammatory strategies have attracted much interest for their potential to prevent further deterioration of Parkinson's disease. Recent experimental and clinical evidence indicate that statins - extensively used in medical practice as effective lipid-lowering agents - have also anti-inflammatory effects. In this study, we investigated the influence of simvastatin on the degenerative process of the dopaminergic neurons of the rat following intranigral injection of lipopolysaccharide (LPS), a potent inductor of inflammation that we have previously used as an animal model of Parkinson's disease. We evaluated TH positive neurons, astroglial, and microglial populations and found that simvastatin prevented the inflammatory processes, as the induction of interleukin-1beta, tumor necrosis factor-alpha, and iNOS and the consequent dopaminergic degeneration induced by LPS. Moreover, simvastatin produced the activation of the neurotrophic factor BDNF, along with the prevention of the oxidative damage to proteins. Moreover, it also prevents the main changes produced by LPS on different mitogen-activated protein kinases, featured as increases of P-c-Jun N-terminal protein kinase, P-extracellular signal-regulated kinase, p-38, and P-glycogen synthase kinase and the decrease of the promotion of cell survival signals such as cAMP response element-binding protein and Akt. Our results suggest that statins could delay the progression of dopaminergic degeneration in disorders involving inflammatory processes.

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.

Green tea polyphenols reduce autoimmune symptoms in a murine model for human Sjogren's syndrome and protect human salivary acinar cells from TNF-alpha-induced cytotoxicity

Sjogren's syndrome (SS) is a relatively common autoimmune disorder. A key feature of SS is lymphocytic infiltration of the salivary and lacrimal glands, associated with the destruction of secretory functions of these glands. Current treatment of SS targets the symptoms but is unable to reduce or prevent the damage to the glands. We reported previously that the major green tea polyphenol (GTP) epigallocatechin-3-gallate (EGCG) inhibits autoantigen expression in normal human keratinocytes and immortalized normal human salivary acinar cells (Hsu et al. 2005). However, it is not known whether GTPs have this effect in vivo, if they can reduce lymphocytic infiltration, or protect salivary acinar cells from tumor necrosis factor-alpha (TNF-alpha)-induced cytotoxicity. Here, we demonstrate that in the NOD mouse, a model for human SS, oral administration of green tea extract reduced the serum total autoantibody levels and the autoimmune-induced lymphocytic infiltration of the submandibular glands. Further, we show that EGCG protected normal human salivary acinar cells from TNF-alpha-induced cytotoxicity. This protection was associated with specific phosphorylation of p38 MAPK, and inhibitors of the p38 MAPK pathway blocked the protective effect. In conclusion, GTPs may provide a degree of protection against autoimmune-induced tissue damage in SS, mediated in part through activation of MAPK elements.

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.

Activation of transcription factors by extracellular nucleotides in immune and related cell types

Extracellular nucleotides, acting through P2 receptors, can regulate gene expression via intracellular signaling pathways that control the activity of transcription factors. Relatively little is known about the activation of transcription factors by nucleotides in immune cells. The NF-kappaB family of transcription factors is critical for many immune and inflammatory responses. Nucleotides released from damaged or stressed cells can act alone through certain P2 receptors to alter NF-kappaB activity or they can enhance responses induced by pathogen-associated molecules such as LPS. Nucleotides have also been shown to regulate the activity of other transcription factors (AP-1, NFAT, CREB and STAT) in immune and related cell types. Here, we provide an overview of transcription factors shown to be activated by nucleotides in immune cells, and describe what is known about their mechanisms of activation and potential functions. Furthermore, we propose areas for future work in this new and expanding field.

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.

A New Approach to Managing Oral Manifestations of Sjogren's Syndrome and Skin Manifestations of Lupus

Sjogren's syndrome (SS) is an autoimmune disorder that affects the salivary glands, leading to xerostomia, and the lacrimal glands, resulting in xerophthalmia. Secondary SS is associated with other autoimmune disorders such as systemic rheumatic diseases and systemic lupus erythematosis (SLE), which can affect multiple organs, including the epidermis. Recent studies have demonstrated that green tea polyphenols (GTPs) possess both anti-inflammatory and anti-apoptotic properties in normal human cells. Epidemiological evidence has indicated that, in comparison to the United States, the incidence of SS, clinical xerostomia and lupus is considerably lower in China and Japan, the two leading green tea-consuming countries.Thus, GTPs might be responsible, in part, for geographical differences in the incidence of xerostomia by reducing the initiation or severity of SS and lupus. Consistent with this, molecular, cellular and animal studies indicate that GTPs could provide protective effects against autoimmune reactions in salivary glands and skin. Therefore, salivary tissues and epidermal keratinocytes could be primary targets for novel therapies using GTPs. This review article evaluates the currently available research data on GTPs, focusing on their potential application in the treatment of the oral manifestations of SS and skin manifestations of SLE.

Nucleotide signaling in nervous system development

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

Upregulation of CCAAT/enhancer binding protein β in activated astrocytes and microglia

The transcription factor CCAAT/enhancer binding protein beta (C/EBPbeta) regulates the expression of key genes in inflammation but little is known about the involvement of C/EBPbeta in glial activation. In this report, we have studied the patterns of astroglial and microglial C/EBPbeta expression in primary mouse cortical cultures. We show that both astrocytes and microglia express C/EBPbeta in untreated mixed glial cultures. C/EBPbeta is upregulated when glial activation is induced by lipopolysaccharide (LPS). The LPS-induced upregulation of glial C/EBPbeta is rapid (2 h at mRNA level, 4 h at protein level). It is elicited by low concentrations of LPS (almost maximal effect at 1 ng/mL) and it is reversed by the protein synthesis inhibitor cycloheximide. C/EBPbeta nuclear levels increase both in astrocytes and microglia after LPS treatment, and the response is more marked in microglia. The LPS-induced increase in microglial C/EBPbeta is prevented by coadministration of the MAP kinase inhibitors SB203580 (p38 inhibitor) + SP600125 (JNK inhibitor) or SB203580 + U0126 (ERK inhibitor). Systemic injection of LPS also increases brain nuclear levels of C/EBPbeta as shown by Western blot, and this increase is localized in microglial cells as shown by double immunofluorescence, in the first report to our knowledge of C/EBPbeta expression in activated glial cells in vivo. These findings support a role for C/EBPbeta in the activation of astrocytes and, particularly, microglia. Given the nature of the C/EBPbeta-regulated genes, we hypothesize that this factor participates in neurotoxic effects associated with glial activation. (c) 2006 Wiley-Liss, Inc.