Purinoceptor function in the immune system

Functional characterization of purinergic P2Y2 and P2Y12 receptors involved in Japanese flounder (Paralichthys olivaceus) innate immune responses

G-protein-coupled P2Y receptors activated by extracellular nucleotides play important roles under different physiological and pathophysiological conditions in mammals. To investigate the immunological relevance of P2Y receptors in fish, we identified and characterized the P2Y₂ and P2Y₁₂ receptors in Japanese flounder Paralichthys olivaceus. The P. olivaceus P2Y₂ and P2Y₁₂ receptors harbor seven transmembrane domains but share only 24% sequence identity. Real-time PCR analysis revealed the constitutive but unequal mRNA expression pattern of P2Y₂R and P2Y₁₂R in normal Japanese flounder tissues with the dominant expression of P2Y₂R in head kidney and blood and P2Y₁₂R in hepatopancreas. In addition, the expression of P2Y₂ and P2Y₁₂ receptors was markedly modulated by PAMPs stimulation and Edwardsiella tarda infection. Furthermore, blockage of P2Y₁₂R potently increased ADP-activated pro-inflammatory cytokine IL-1beta gene expression in the head kidney macrophages (HKMs). Moreover, inhibition of P2Y₂ and P2Y₁₂ receptor activity with their respective potent antagonists significantly altered some of the LPS-induced pro-inflammatory cytokine gene expression in the HKMs. However, blockade of P2Y₁₂R did not affect the poly(I:C)-induced pro-inflammatory cytokine gene expression examined in the HKMs. Collectively, we have for the first time reported the role of purinergic P2Y₂ and P2Y₁₂ receptors in fish innate immunity. Our findings have also addressed the importance of extracellular ATP and its metabolites in fish innate immune responses.

P2X ion channel receptors and inflammation

Neuroinflammation limits tissue damage in response to pathogens or injury and promotes repair. There are two stages of inflammation, initiation and resolution. P2X receptors are gaining attention in relation to immunology and inflammation. The P2X7 receptor in particular appears to be an essential immunomodulatory receptor, although P2X1 and P2X4 receptors also appear to be involved. ATP released from damaged or infected cells causes inflammation by release of inflammatory cytokines via P2X7 receptors and acts as a danger signal by occupying upregulated P2X receptors on immune cells to increase immune responses. The purinergic involvement in inflammation is being explored for the development of novel therapeutic strategies.

Purinergic signalling and immune cells

This review article provides a historical perspective on the role of purinergic signalling in the regulation of various subsets of immune cells from early discoveries to current understanding. It is now recognised that adenosine 5'-triphosphate (ATP) and other nucleotides are released from cells following stress or injury. They can act on virtually all subsets of immune cells through a spectrum of P2X ligand-gated ion channels and G protein-coupled P2Y receptors. Furthermore, ATP is rapidly degraded into adenosine by ectonucleotidases such as CD39 and CD73, and adenosine exerts additional regulatory effects through its own receptors. The resulting effect ranges from stimulation to tolerance depending on the amount and time courses of nucleotides released, and the balance between ATP and adenosine. This review identifies the various receptors involved in the different subsets of immune cells and their effects on the function of these cells.

Effect of Adenosine on the Growth of Human T-Lymphocyte Leukemia Cell Line MOLT-4

Adenosine has been observed to suppress the growth of MOLT-4 human leukemia cells in vitro. Changes in the cell cycle, especially increased percentage of cells in S phase, prolonged generation time, and induction of apoptosis at higher adenosine concentrations have been found to be responsible for the growth suppression. Dipyridamole, a drug inhibiting the cellular uptake of adenosine, reversed partially but significantly the adenosine-induced growth suppression. It follows from these results that the action of adenosine on the MOLT-4 cells comprises its cellular uptake and intracellular operation. These findings present new data on anticancer efficacy of adenosine.

Role of TNF-alpha and extracellular ATP in THP-1 cell activation following allergen exposure

Dendritic cells (DCs), including Langerhans cells (LCs), play a critical role in the induction phase of allergic contact hypersensitivity. Following exposure to chemical allergens in the skin, LCs undergo a maturation process leading to the up-regulation of expression of co-stimulatory molecules, such as CD86, CD54 and CD40. Our previous study revealed that chemical allergens induce phenotype alterations (e.g., CD86, CD54 and CD40) and cytokine production (TNF-alpha and IL-8) in THP-1 cells that possibly reflect the maturation of dendritic cells during skin sensitization. However, the physiological signals for phenotypic alterations by chemical allergens are still not fully understood. Therefore, in this study, we investigated the effect of TNF-alpha and extracellular ATP on THP-1 cell activation induced by chemical allergens. Kinetic studies revealed that TNF-alpha and IL-8 release occurred in a time-dependent manner with release of two cytokines beginning at 3 hr post-exposure to well-known haptens, DNCB and NiSO(4). While recombinant human TNF-alpha augmented CD54 and CD40 expression in a dose-dependent manner, rhTNF-alpha did not increase CD86 expression. Furthermore, neutralization of TNF-alpha activity strongly inhibited CD54 and CD40 expression induced by allergens. On the contrary, extracellular ATP induced the up-regulation of both CD86 and CD54 expression. In the presence of the P2 receptor antagonist suramin, the up-regulation of CD86 and CD54 expression by allergens was in part suppressed. Therefore, we postulate that not only TNF-alpha but also extracellular ATP may contribute to cell activation following allergen stimulation, which might reflect the mechanism by which DCs respond to allergens.

Role of nonsynaptic communication in regulating the immune response

The discovery of nonsynaptic communication in the 1960s and 1970s was an important milestone in investigating the function of the nervous system, and it revolutionized our view about information transmission between neurons. In addition, nonsynaptic communication has a practical importance not only within the nervous system, but in the communication between the peripheral nervous system and other organ systems. Nonsynaptic communication takes place in different immune organs, which are innervated by sympathetic nerve terminals. In addition, the function of microglia, one of the immunocompetent cell types of the brain, can also be affected by neurotransmitters released from axon varicosities. The various functions of immune cells are modulated by released neurotransmitters without any direct synaptic contact between nerve endings and targeted immune cells requiring only functional neurotransmitter receptors on immune cells. Here, we briefly overview the role of the various receptor subtypes mediating nonsynaptic modulation of the function of immunocompetent cells both in the periphery and in the central nervous system.

Role of interstitial ATP and adenosine in the regulation of renal hemodynamics and microvascular function

The role of adenosine in the regulation of renal hemodynamics and function has been studied extensively; however, another purine agent, ATP, is also gaining recognition for its paracrine role in the kidney. Adenosine and ATP bind to specific membrane-bound P1 and P2 purinoceptors, respectively, and initiate a variety of biological effects on renal microvascular tone, mesangial cell function, and renal epithelial transport. The purpose of this review is to summarize the potential roles of interstitial ATP and adenosine as regulators of renal hemodynamics and microcirculation. In vitro blood-perfused juxtamedullary nephron preparation was used to assess the roles of ATP and adenosine in the regulation of renal microvascular tone. This approach mimics the adventitial exposure of renal microvascular smooth muscle to ATP and adenosine synthesized locally and released into the interstitial fluid. ATP selectively vasoconstricts afferent but not efferent arterioles via P2X and P2Y receptors, whereas, adenosine vasoconstricts both vascular segments via activation of adenosine A(1) receptors. Furthermore, selective P2X and P2Y receptor stimulation increases intracellular calcium concentration in vascular smooth muscle cells that are freshly isolated from the preglomerular microvasculature. These data support the hypothesis that interstitial ATP plays a critical role in the control of renal microvascular function through mechanisms that are independent of adenosine receptors. We have recently developed a renal microdialysis method to determine the dynamics of ATP and adenosine levels in the renal cortical interstitium. In this review, we also summarize current knowledge pertaining to the alterations in renal interstitial ATP and adenosine in some pathophysiological conditions.

Ecto-ATPases of clinical and non-clinical isolates of Acanthamoeba

Acanthamoeba are opportunistic protozoan parasites that can cause fatal granulomatous amoebic encephalitis and eye keratitis, however the pathogenic mechanisms of Acanthamoeba remain unclear. In this study, we described the ability of live Acanthamoeba to hydrolyse extracellular ATP. Both clinical and non-clinical isolates belonging to genotypes, T1, T2, T3, T4 and T7 exhibited ecto-ATPase activities in vitro. Using non-denaturing polyacrylamide gel electrophoresis, ecto-ATPases were further characterized. All Acanthamoeba isolates tested, exhibited a single ecto-ATPase band (approximate molecular weight of 272 kDa). However, clinical isolates exhibited additional bands suggesting that ecto-ATPases may play a role in the pathogenesis of Acanthamoeba. This was supported using suramin (ecto-ATPase inhibitor), which inhibited Acanthamoeba-induced host cell cytotoxicity. Previously, we and others have shown that Acanthamoeba binds to host cells using their mannose-binding protein and binding can be blocked using exogenous alpha-mannose. In this study, we observed that alpha-mannose significantly increased ecto-ATPase activities of pathogenic Acanthamoeba belonging to T1, T2, T3 and T4 genotypes but had no effect on non-pathogenic Acanthamoeba (belonging to T7 genotype). Overall, we have shown, for the first time, that Acanthamoeba exhibit ecto-ATPase activities, which may play a role in the pathogenesis of Acanthamoeba as well as their potential role in the differentiation of pathogenic Acanthamoeba.

Functional expression of adenosine A2A and A3 receptors in the mouse dendritic cell line XS-106

There is increasing evidence to suggest that adenosine receptors can modulate the function of cells involved in the immune system. For example, human dendritic cells derived from blood monocytes have recently been described to express functional adenosine A1, A2A and A3 receptors. Therefore, in the present study, we have investigated whether the recently established murine dendritic cell line XS-106 expresses functional adenosine receptors. The selective adenosine A3 receptor agonist 1-[2-chloro-6[[(3-iodophenyl)methyl]amino]-9H-purin-9-yl]-1-deoxy-N-methyl-beta-D-ribofuranuronamide (2-Cl-IB-MECA) inhibited forskolin-mediated [3H]cyclic AMP accumulation and stimulated concentration-dependent increases in p42/p44 mitogen-activated protein kinase (MAPK) phosphorylation. The selective adenosine A2A receptor agonist 4-[2-[[-6-amino-9-(N-ethyl-beta-D-ribofuranuronamidosyl)-9H-purin-2-yl]amino]ethyl]benzene-propanoic acid (CGS 21680) stimulated a robust increase in [3H]cyclic AMP accumulation and p42/p44 MAPK phosphorylation. In contrast, the selective adenosine A1 receptor agonist CPA (N6-cyclopentyladenosine) did not inhibit forskolin-mediated [3H]cyclic AMP accumulation or stimulate increases in p42/p44 MAPK phosphorylation. These observations suggest that XS-106 cells express functional adenosine A2A and A3 receptors. The non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine (NECA) inhibited lipopolysaccharide-induced tumour necrosis factor-alpha (TNF-alpha) release from XS-106 cells in a concentration-dependent fashion. Furthermore, treatment with Cl-IB-MECA (1 microM) or CGS 21680 (1 microM) alone produced a partial inhibition of lipopolysaccharide-induced TNF-alpha release (when compared to NECA), whereas a combination of both agonists resulted in the inhibition of TNF-alpha release comparable to that observed with NECA alone. Treatment of cells with the adenosine A2A receptor selective antagonists 4-(2-[7-amino-2-(2-furyl)[1,2,4]triazolo[2,3-a][1,3,5]triazin-5ylamino]ethyl)phenol (ZM 241385; 100 nM) and 5-amino-2-(2-furyl)-7-phenylethyl-pyrazolo[4,3-e]-1,2,4-triazolo[1,5c]pyrimidine (SCH 58261; 100 nM) and the adenosine A3 receptor selective antagonist N-[9-chloro-2-(2-furanyl)[1,2,4]-triazolo[1,5-c]quinazolin-5-benzeneacetamide (MRS 1220; 100 nM) partially blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. Combined addition of MRS 1220 and SCH 58261 completely blocked the inhibitory effects of NECA on lipopolysaccharide-induced TNF-alpha release. In conclusion, we have shown that the mouse dendritic cell line XS-106 expresses functional adenosine A2A and A3 receptors, which are capable of modulating TNF-alpha release.

Adenosine affects expression of membrane molecules, cytokine and chemokine release, and the T-cell stimulatory capacity of human dendritic cells

Dendritic cells (DCs) express functional purinergic type 1 receptors, but the effects of adenosine in these antigen-presenting cells have been only marginally investigated. Here, we further characterized the biologic activity of adenosine in immature DCs (iDCs) and lipopolysaccharide (LPS)-matured DCs (mDCs). Chronic stimulation with adenosine enhanced the macropinocytotic activity and the membrane expression of CD80, CD86, major histocompatibility complex (MHC) class I, and HLA-DR molecules on iDCs. Adenosine also increased LPS-induced CD54, CD80, MHC class I, and HLA-DR molecule expression in mDCs. In addition, adenosine dose-dependently inhibited tumor necrosis factor alpha and interleukin-12 (IL-12) release, whereas it enhanced the secretion of IL-10 from mDCs. The use of selective receptor agonists revealed that the modulation of the cytokine and cell-surface marker profile was due to activation of A(2) adenosine receptor. Functionally, adenosine reduced the allostimulatory capacity of iDCs, but not of mDCs. More important, DCs matured in the presence of adenosine had a reduced capacity to induce T helper 1 (Th1) polarization of naive CD4(+) T lymphocytes. Finally, adenosine augmented the release of the chemokine CCL17 and inhibited CXCL10 production by mDCs. In aggregate, the results provide initial evidence that adenosine diminishes the capacity of DCs to initiate and amplify Th1 immune responses.

Secretion of IL-2 and IFN-gamma, but not IL-4, by antigen-specific T cells requires extracellular ATP

Extracellular ATP and other nucleotides transmit signals to cells via surface-associated molecules whose binding sites face the extracellular milieu. Ecto-nucleoside triphosphate diphosphohydrolase is such an ATP-binding enzyme that is expressed by activated lymphocytes. We have previously shown that nonhydrolyzable ATP analogs block the lytic activity of NK cells and CD8(+) T cells as well as their E-NTPDase activity. These results suggest that the hydrolysis of ATP may play a role in lymphocyte function. Here we report that E-NTPDase activity is up-regulated within 15 min of T cell stimulation and that reversible and irreversible enzyme inhibitors profoundly reduce secretion of IL-2 and IFN-gamma, but not IL-4. TNF-alpha, IL-10, and IL-5 production showed intermediate sensitivity to these ATP analogs. Depletion of extracellular ATP also inhibited secretion of IFN-gamma, but not IL-4, supporting the interpretation that extracellular ATP is required for secretion of some, but not all, cytokines. E-NTPDase antagonists reduced transcription of IL-2 mRNA and inhibited TCR-mediated intracellular calcium flux. These results suggest that extracellular ATP plays an essential role in the TCR-mediated signal transduction cascade for expression of certain cytokine genes.

Extracellular ATP regulates cell death of lymphocytes and monocytes induced by membrane-bound lipoproteins of Mycoplasma fermentans and Mycoplasma salivarium

The cytotoxicities of lipoproteins of Mycoplasma fermentans and Mycoplasma salivarium to a lymphocytic cell line, MOLT-4, and a monocytic cell line, HL-60, was upregulated by ATP added extracellularly in a dose-dependent manner. These lipoproteins induced ATP release and plasma membrane permeability increase in these cell lines. In addition, periodate-oxidized ATP, an antagonist for P2X purinergic receptors, suppressed the cytotoxicity of the lipoproteins, suggesting the possibility that P2X receptors for ATP play crucial roles in the cytotoxicity. Activation of caspase-3 induced by the lipoproteins, which was assessed by the cleavage of the synthetic substrate DEVD-pNA and the endogenous substrate poly(ADP-ribose) polymerase, was also upregulated and downregulated by extracellular ATP and periodate-oxidized ATP, respectively. On the basis of these results, this study suggests that mycoplasmal lipoproteins induce the permeability increase in lymphocytes and monocytes, by which ATP is released, and the ATP regulates the cytotoxicities of the lipoproteins to the cells, possibly by interaction with ATP receptors such as P2X purinergic receptors.

Expression and function of adenosine receptors in human dendritic cells

Dendritic cells (DCs) are specialized antigen-presenting cells characterized by their ability to migrate into target sites, process antigens, and activate naive T cells. In this study, we analyzed the biological activity and intracellular signaling of adenosine by using reverse transcriptase-polymerase chain reaction assays to investigate mRNA expression of A(1), A(2a) and A(3) adenosine receptors in immature and mature human DCs. Functional experiments on adenosine stimulation showed chemotaxis, intracellular calcium transients, and actin polymerization, but no activation of adenylate cyclase in immature DCs. Experiments with receptor isotype-selective agonists and antagonists as well as pertussis toxin revealed that chemotaxis, calcium transients, and actin polymerization were mediated via G(i-) or G(0-)protein-coupled A(1) and A(3) receptors. Maturation of DCs induced by lipopolysaccharide (LPS) resulted in down-regulation of A(1) and A(3) receptor mRNAs, although A(2a) receptor mRNA was still expressed. However, in LPS-differentiated DCs, adenosine and an A(2a) receptor agonist stimulated adenylate cyclase activity, enhanced intracellular cAMP levels, and inhibited interleukin 12 (IL-12) production. These effects could be completely prevented by pretreatment with A(2) receptor antagonist. These findings strongly suggest that adenosine has important but distinct biological effects in DCs activity as a chemotaxin for immature DCs and as a modulator of IL-12 production in mature DCs. These effects can be explained by differential expression of adenosine receptor subtypes.

Inhibition of NF-kappaB-mediated gene transcription by the human A2B adenosine receptor in Chinese hamster ovary cells

NF-kappaB is a transcription factor that plays a vital role in regulating inducible gene expression in immune and inflammatory responses. In view of the well documented effects of adenosine on immune and inflammatory responses, we have explored whether adenosine A1, A2B and A3 receptors regulate NF-kappaB activity in transfected Chinese hamster ovary (CHO) cells using a luciferase reporter gene construct. No increases in NF-kappaB activity were observed in CHO-A1, -A2B and -A3 cells stimulated with the non-selective adenosine receptor agonist 5'-N-ethylcarboxamidoadenosine. Elevating intracellular cyclic AMP (cAMP) levels using forskolin (direct activator of adenylyl cyclase) and rolipram (type IV, cAMP-specific phosphodiesterase inhibitor), inhibited NF-kappaB activity in CHO cells. Adenosine A2B receptor stimulation also inhibited NF-kappaB activity, whereas adenosine A1 and A3 receptor activation had no effect. These data reflect the known coupling of adenosine A2B receptors to increases in cAMP. In conclusion, adenosine A1, A2B and A3 receptors do not directly activate NF-kappaB in CHO cells. However, adenosine A2B receptor activation significantly inhibited NF-kappaB activity. Inhibition of NF-kappaB activity by the adenosine A2B receptor may contribute to the anti-inflammatory effects of adenosine.

P2Y(2) nucleotide receptor signaling in human monocytic cells: activation, desensitization and coupling to mitogen-activated protein kinases

Activation of P2Y(2) receptors by extracellular nucleotides has been shown to induce phenotypic differentiation of human promonocytic U937 cells that is associated with the inflammatory response. The P2Y(2) receptor agonist, UTP, induced the phosphorylation of the MAP kinases MEK1/2 and ERK1/2 in a sequential manner, since ERK1/2 phosphorylation was abolished by the MEK1/2 inhibitor PD 098059. Other results indicated that P2Y(2) receptors can couple to MAP kinases via phosphatidylinositol 3-kinase (PI3K) and c-src. Accordingly, ERK1/2 phosphorylation induced by UTP was inhibited by the PI3K inhibitors, wortmannin and LY294002, and the c-src inhibitors, radicicol and PP2, but not by inhibitors of protein kinase C (PKC). The phosphorylation of ERK1/2 was independent of the ability of P2Y(2) receptors to increase the concentration of intracellular free calcium, since chelation of intracellular calcium by BAPTA did not diminish the phosphorylation of ERK1/2 induced by UTP. A 5-minute treatment with UTP reduced U937 cell responsiveness to a subsequent UTP challenge. UTP-induced desensitization was characterized by an increase in the EC(50) for receptor activation (from 0.44 to 9.3 microM) and a dramatic ( approximately 75%) decrease in the maximal calcium mobilization induced by a supramaximal dose of UTP. Phorbol ester treatment also caused P2Y(2) receptor desensitization (EC(50) = 12.3 microM UTP and maximal calcium mobilization reduced by approximately 33%). The protein kinase C inhibitor GF 109203X failed to significantly inhibit the UTP-induced desensitization of the P2Y(2) receptor, whereas the protein phosphatase inhibitor okadaic acid blocked receptor resensitization. Recovery of receptor activity after UTP-induced desensitization was evident in cells treated with agonist for 5 or 30 min. However, P2Y(2) receptor activity remained partially desensitized 30 min after pretreatment of cells with UTP for 1 h or longer. This sustained desensitized state correlated with a decrease in P2Y(2) receptor mRNA levels. Desensitization of ERK1/2 phosphorylation was induced by a 5-minute pretreatment with UTP, and cell responsiveness did not return even after a 30-minute incubation of cells in the absence of an agonist. Results suggest that desensitization of the P2Y(2) receptor may involve covalent modifications (i.e., receptor phosphorylation) that functionally uncouple the receptor from the calcium signaling pathway, and that transcriptional regulation may play a role in long-term desensitization. Our results indicate that calcium mobilization and ERK1/2 phosphorylation induced by P2Y(2) receptor activation are independent events in U937 monocytes.

Adenosine 5'-triphosphate: a P2-purinergic agonist in the myocardium

ATP, besides an intracellular energy source, is an agonist when applied to a variety of different cells including cardiomyocytes. Sources of ATP in the extracellular milieu are multiple. Extracellular ATP is rapidly degraded by ectonucleotidases. Today ionotropic P2X(1--7) receptors and metabotropic P2Y(1,2,4,6,11) receptors have been cloned and their mRNA found in cardiomyocytes. On a single cardiomyocyte, micromolar ATP induces nonspecific cationic and Cl(-) currents that depolarize the cells. ATP both increases directly via a G(s) protein and decreases Ca(2+) current. ATP activates the inward-rectifying currents (ACh- and ATP-activated K(+) currents) and outward K(+) currents. P2-purinergic stimulation increases cAMP by activating adenylyl cyclase isoform V. It also involves tyrosine kinases to activate phospholipase C-gamma to produce inositol 1,4,5-trisphosphate and Cl(-)/HCO(3)(-) exchange to induce a large transient acidosis. No clear correlation is presently possible between an effect and the activation of a given P2-receptor subtype in cardiomyocytes. ATP itself is generally a positive inotropic agent. Upon rapid application to cells, ATP induces various forms of arrhythmia. At the tissue level, arrhythmia could be due to slowing of electrical spread after both Na(+) current decrease and cell-to-cell uncoupling as well as cell depolarization and Ca(2+) current increase. In as much as the information is available, this review also reports analog effects of UTP and diadenosine polyphosphates.

Extracellular ATP induces a distorted maturation of dendritic cells and inhibits their capacity to initiate Th1 responses

Dendritic cells (DCs) express functional purinergic receptors, but the effects of purine nucleotides on DC functions have been marginally investigated. In this study, we report on the ability of micromolar concentrations of ATP to affect the maturation and Ag-presenting function of monocyte-derived DCs in vitro. Chronic stimulation (24 h) of DCs with low, noncytotoxic ATP doses increased membrane expression of CD54, CD80, CD86, and CD83, slightly reduced the endocytic activity of DCs, and augmented their capacity to promote proliferation of allogeneic naive T lymphocytes. Moreover, ATP enhanced LPS- and soluble CD40 ligand-induced CD54, CD86, and CD83 expression. On the other hand, ATP markedly and dose-dependently inhibited LPS- and soluble CD40 ligand-dependent production of IL-1alpha, IL-1beta, TNF-alpha, IL-6, and IL-12, whereas IL-1 receptor antagonist and IL-10 production was not affected. As a result, T cell lines generated from allogeneic naive CD45RA(+) T cells primed with DCs matured in the presence of ATP produced lower amounts of IFN-gamma and higher levels of IL-4, IL-5, and IL-10 compared with T cell lines obtained with LPS-stimulated DCs. ATP inhibition of TNF-alpha and IL-12 production by mature DCs was not mediated by PGs or elevation of intracellular cAMP and did not require ATP degradation. The inability of UTP and the similar potency of ADP to reproduce ATP effects indicated that ATP could function through the P2X receptor family. These results suggest that extracellular ATP may serve as an important regulatory signal to dampen IL-12 production by DCs and thus prevent exaggerated and harmful immune responses.

Ischemic-like condition releases norepinephrine and purines from different sources in superfused rat spleen strips

Transmitters and cotransmitters of the sympathetic nervous system are involved in the regulation of a variety of immune cell functions. However, it is not entirely clear what stimuli lead to the release of these molecules in immune organs. In this study, we investigated whether local ischemia can cause the parallel release of norepinephrine and its cotransmitter, ATP, in the spleen. Ischemic-like conditions, simulated by transient (15 min) O(2) and glucose deprivation, elicited a reversible increase in the release of both norepinephrine and purines from superfused spleen strips preloaded with [3H]norepinephrine or [3H]adenosine. HPLC analysis of the released tritium label revealed a net increase in the amount of ATP, ADP, AMP, adenosine, inosine, hypoxanthine and xanthine in response to ischemic-like condition. Selective O(2) or glucose deprivation, and Ca(2+)-free conditions differentially affected the outflow of [3H]norepinephrine and [3H]purines, indicating that they derived from different sources. The ABC transporter inhibitors glibenclamide (100 microM) and verapamil (100 microM) as well as low-temperature inhibited [3H]purine release evoked by ischemic-like conditions. Surgical denervation of the spleen reduced endogenous catecholamine content and [3H]norepinephrine uptake of the spleen, but not that of [3H]adenosine. In summary, these results demonstrate the release of norepinephrine and purines in response to an ischemic-like condition in an immune organ. Although both could provide an important source of extracellular catecholamines and purines involved at various levels of immunomodulation, the source and mechanism of norepinephrine and purine efflux seem different.

Extracellular ATP and TNF-alpha synergize in the activation and maturation of human dendritic cells

Extracellular ATP mediates numerous biological activities by interacting with plasma membrane P2 purinergic receptors. Recently, P2 receptors have been described on dendritic cells (DC), but their functional role remains unclear. Proposed functions include improved Ag presentation, cytokine production, chemotaxis, and induction of apoptosis. We investigated the effects of ATP and of other P2 receptor agonists on endocytosis, phenotype, IL-12 secretion, and T cell stimulatory capacity of human monocyte-derived DC. We found that in the presence of extracellular ATP, DC transiently increase their endocytotic activity. Subsequently, DC up-regulate CD86, CD54, and MHC-II; secrete IL-12; and exhibit an improved stimulatory capacity for allogeneic T cells. These effects were more pronounced when chemically modified ATP derivatives with agonistic activity on P2 receptors, which are resistent to degradation by ectonucleotidases, were applied. Furthermore, ATP and TNF-alpha synergized in the activation of DC. Stimulated with a combination of ATP and TNF-alpha, DC expressed the maturation marker CD83, secreted large amounts of IL-12, and were potent stimulators of T cells. In the presence of the P2 receptor antagonist suramin, the effects of ATP were completely abolished. Our results suggest that extracellular ATP may play an important immunomodulatory role by activating DC and by skewing the immune reaction toward a Th1 response through the induction of IL-12 secretion.

Kinetics and mechanism of ATP-dependent IL-1 beta release from microglial cells

Endotoxin-dependent release of IL-1 beta from mouse microglial cells is a very inefficient process, as it is slow and leads to accumulation of a modest amount of extracellular cytokine. Furthermore, secreted IL-1 beta is mostly in the procytokine unprocessed form. Addition of extracellular ATP to LPS-primed microglia caused a burst of release of a large amount of processed IL-1 beta. ATP had no effect on the accumulation of intracellular pro-IL-1 beta in the absence of LPS. In LPS-treated cells, ATP slightly increased the synthesis of pro-IL-1 beta. Optimal ATP concentration for IL-1 beta secretion was between 3 and 5 mM, but significant release could be observed at concentrations as low as 1 mM. At all ATP concentrations IL-1 beta release could be inhibited by increasing the extracellular K+ concentration. ATP-dependent IL-1 beta release was also inhibited by 90 and 60% by the caspase inhibitors YVAD and DEVD, respectively. Accordingly, in ATP-stimulated microglia, the p20 proteolytic fragment derived from activation of the IL-1-beta-converting enzyme could be detected by immunoblot analysis. These experiments show that in mouse microglial cells extracellular ATP triggers fast maturation and release of intracellularly accumulated IL-beta by activating the IL-1-beta-converting enzyme/caspase 1.

Mouse Dendritic Cells Express the P2X7 Purinergic Receptor: Characterization and Possible Participation in Antigen Presentation

Immune cells express P2 purinoceptors of the P2Y and P2X subtypes. In the present work, we show that three dendritic cell (DC) lines, D2SC/1, CB1, and FSDC, representative of immature DCs, express the P2X7 (formerly P2Z) receptor, as judged from RT-PCR amplification, reactivity to a specific antiserum, and pharmacological and functional evidence. Receptor expression is higher in FSDC cells, a cell line that is functionally more mature than D2SC/1 and CB1. From the wild-type DC population, we selected cell clones lacking the P2X7R (P2X7less). We also used a P2XR blocker, oxidized ATP, to irreversibly inhibit the P2X7R. Ability of P2X7less FSDCs or of oxidized ATP-inhibited FSDCs to stimulate Ag-specific TH lymphocytes was severely decreased although Ag endocytosis was minimally affected. During coculture with TH lymphocytes, wild-type FSDC secreted large amounts of IL-1β. Release of this cytokine was reduced in P2X7less DCs. These data show that DCs express the P2X7 purinoceptor and suggest a correlation between P2X7R expression and Ag-presenting activity.

Adenosine prevents the death of mesencephalic dopaminergic neurons by a mechanism that involves astrocytes

The purinergic nucleoside adenosine effectively prevented the death of dopaminergic neurons that occurs spontaneously and progressively in cultures of rat mesencephalon. Adenosine also significantly increased dopamine uptake, attesting to the state of differentiation and functional integrity of the neurons that were rescued. The effects of adenosine were (a) specific to the dopaminergic neurons in these cultures, (b) long-lived, (c) still observed when the treatment was delayed after plating, (d) potentiated by inhibition of adenosine deaminase, and (e) abolished when this enzyme was added in excess to the culture medium. The action of adenosine was mimicked by 5'-(N-ethylcarboxamido)adenosine and dibutyryl-cyclic AMP, but not by CGS-21680, suggesting the possible involvement of A2B subtype purinergic receptors. ATP was also neuroprotective, but its action resulted principally from conversion to adenosine by ectonucleotidases. Several anticancer drugs, including cytosine arabinoside, have been shown previously to prevent apoptosis in cultured dopaminergic neurons by a mechanism that requires the inhibition of proliferating astrocytes. In the presence of adenosine, astrocytes were more differentiated, and their proliferation rate was significantly reduced, suggesting that the neurotrophic effect of the adenine nucleoside resulted also from the repression of the astroglial cells. We did not find evidence of a trophic intermediary in adenosine-treated cultures, however, leading to the hypothesis that limitation of astrocyte replication in itself and/or ensuing changes in the glial phenotype were crucial. Our results suggest that molecules that modulate adenine nucleotide/nucleoside release may be useful for the treatment of chronic neurodegenerative conditions affecting dopaminergic neurons, such as Parkinson's disease.

Functional and molecular diversity of purinergic ion channel receptors

P2X receptors are membrane ion channels gated by extracellular adenosine 5'-triphosphate (ATP); nucleotides also activate a family of seven transmembrane G protein-coupled receptors (P2Y). P2X receptors are widely expressed on mammalian cells, where they can be broadly differentiated into three groups. The first group is almost equally well activated by ATP and its analog alpha beta methyleneATP (alpha beta meATP), whereas a second group is not activated by alpha beta meATP. A third-group type of receptor (termed P2Z) is distinguished by the fact that the channel opening is followed by cell permeabilization and lysis if the agonist application is continued for more than a few seconds. Seven cDNAs have been cloned that encode P2X receptor subunits. When expressed individually in heterologous systems, P2X1 and P2X3 subunits form channels activated by ATP or alpha beta meATP; whereas P2X2, P2X4, and P2X5 form channels activated by ATP but not alpha beta meATP. P2X6 receptors do not express readily, and P2X7 receptors correspond closely in their properties to P2Z. Further phenotypes can be produced when two subunits are coexpressed, indicating hetero-multimerization. This chapter compares the properties of the native P2X receptors with those of the cloned and expressed subunits.

Human primary fibroblasts in vitro express a purinergic P2X7 receptor coupled to ion fluxes, microvesicle formation and IL-6 release

We have investigated reponses to extracellular ATP in human fibroblasts obtained by skin biopsies. Our data show that these cells express a P2X7 purinergic receptor, as judged by (1) RT-PCR with specific primers, (2) reactivity with a specific anti-P2X7 antiserum, (3) activation by the selective P2X agonist benzoylbenzoylATP and (4) stimulation of transmembrane ion fluxes. Stimulation with benzoylbenzoylATP, and to a lesser extent with ATP, also caused striking morphological changes and increased formation of cytoplasmic microvesicles. These changes were fully reversible upon nucleotide removal. Two known blockers of P2X receptors, oxidised ATP and pyridoxalphosphate-6-azophenyl-2′,4′-disulfonic acid, inhibited the morphological changes fully and the ion fluxes partially. The residual rise in intracellular Ca2+ levels and membrane depolarization observed in the presence of the inhibitors were dependent upon activation of a P2Y-type receptor exhibiting a peculiar pharmacological profile, in that CTP was the preferred agonist. ATP stimulation triggered release of the pro-inflammatory cytokine IL-6 in fibroblasts pre-treated with PMA and bacterial endotoxin. These observations reveal a novel pathway for fibroblast activation and for their recruitment in the inflammatory response.

Purinergic signalling: pathophysiological roles

In this review, after a summary of the history and current status of the receptors involved in purinergic signalling, we focus on the distribution and physiological roles of purines and pyrimidines in both short-term events such as neurotransmission, exocrine and endocrine secretion and regulation of immune cell function, and long-term events such as cell growth, differentiation and proliferation in development and regeneration. Finally, the protective roles of nucleosides and nucleotides in events such as cancer, ischemia, wound healing, drug toxicity, inflammation and pain are explored and some suggestions made for future developments in this rapidly expanding field, with particular emphasis on the involvement of selective agonists and antagonists for purinergic receptor subtypes in therapeutic strategies.

Modulation of apoptosis in human lymphocytes by adenosine analogues

We previously described that 2-chloroadenosine (2CA) and 2-chloro-2'-deoxyadenosine (2CdA) induced apoptosis in human peripheral blood mononuclear cells (PBMC). In this study we tested different adenosine analogues on PBMC and we found that the modifications introduced in the 2CA structure prevented the molecule from exerting its apoptotic effect. On the other hand, substitutions on 2CdA are tolerated, although with a significant decrease in activity.

P2X receptor-mediated excitation of nociceptive afferents in the normal and arthritic rat knee joint

1. We tested the hypothesis that functional P2X receptors are present on peripheral terminals of primary afferent articular nociceptors in the rat knee joint. Neural activity was recorded extracellularly from the medial articular nerve innervating the knee joint in rats anaesthetized with pentobarbitone. 2. The selective P2X receptor agonist, alphabeta methylene ATP (alphabetameATP), and the endogenous ligand, ATP, caused a rapid short-lasting excitation of a sub-population of C and Adelta nociceptive afferent nerves innervating normal knee joints when injected intra-arterially or intra-articularly, and this effect was antagonized by the non-selective P2 receptor antagonist PPADS. 3. Induction of a chronic (14-21 days) unilateral inflammatory arthritis of the knee joint using locally injected Freund's adjuvant neither increased or decreased responsiveness of joint nociceptors to alphabetameATP or ATP. 4. Our results support the hypothesis that alphabetameATP-sensitive P2X receptors are expressed on peripheral nociceptive afferents in the rat knee joint suggesting that they may be involved in the initiation of nociception and pain.

P2Y2 receptors are expressed by human osteoclasts of giant cell tumor but do not mediate ATP-induced bone resorption

Extracellular nucleotides acting through P2 receptors elicit a range of responses in many cell types. Previously, we have cloned the G-protein coupled P2Y2 receptor from a human osteoclastoma complementary deoxyribonucleic acid (cDNA) library and demonstrated its expression by reverse transcription linked (RT)-PCR and Southern analysis in a number of skeletal tissues, including a purified population of giant cells. In this study we have localized the expression of P2Y2 receptor transcripts to osteoclasts of giant cell tumor of bone by in situ hybridization. In osteoblasts and other cell types, the P2Y2 receptor is coupled to Ins(1,4,5)P3-mediated Ca2+ release from intracellular stores. In this study, the P2Y2 receptor agonists adenosine triphosphate (ATP) and uridine triphosphate (UTP) did not increase cytosolic free calcium concentration ([Ca2+]i) in giant cells isolated from osteoclastoma, while the G-protein coupled calcium sensing receptor agonist, Ni2+, elevated [Ca2+]i in the same cells. These data indicate that P2Y2 receptor transcripts expressed by giant cells are not presented at the surface of cells as functional receptors, or alternatively, functional receptors are coupled to an effector other than [Ca2+]i. ATPgammaS (10 micromol/L), but not UTP (10 micromol/L), significantly stimulated resorption by an enriched giant cell population. These results indicate that ATP-induced effects on resorption, following direct osteoclastic activation, are mediated by a P2 receptor other than the P2Y2 subtype. Nucleotides, released locally in the bone microenvironment in response to acute trauma or transient physical stress, will interact with a complement of P2 receptors expressed by both osteoclasts and osteoblasts to influence the remodeling process.

Nucleotide receptors

Adenosine 5'-triphosphate (ATP) and/or related nucleotides act at both ionotropic (P2X) and metabotropic (P2Y) receptors. P2X receptor subunits (P2X1-P2X7) form ligand-gated cation channels, as homomultimers or heteromultimers. Recent work indicates that P2X3 subunits participate in channels expressed by nociceptive sensory neurons, and that the second of the two transmembrane domains of each subunit contributes to the ion permeation pathway. P2X7 subunits form large cytolytic pores in addition to cation channels; they have been found in macrophages and brain microglia. P2Y receptors form a distinct subset of G-protein-coupled receptors; most couple through G proteins to phospholipase C, but inhibition of adenylate cyclase and N-type Ca2+ channels, and activation of K+ channels also occurs. Expressed P2Y receptors have generally been distinguished pharmacologically by the rank order of effectiveness of agonists; some prefer pyrimidines to purines. Recent studies suggest that it is important to use purified nucleotides in such classifications. Several P2Y receptors have a very widespread tissue distribution.

Purinoceptors in the Central Nervous System

New exciting developments on the occurrence and functional role of purinoceptors in mammalian brain were presented at the session "Purinoceptors in the central nervous system" chaired by Flaminio Cattabeni and Tom Dunwiddie at the Purines '96 international conference. The focus of the session were topics of recent interest, including the sources and mechanisms involved in ATP and adenosine release during physiological neurotransmission in hippocampus, the brain expression of the recently cloned P2 receptors, and the role of the various adenosine receptor subtypes in brain protection from neurodegeneration associated with trauma-, ischemia-and excessive excitatory amino acid neurotransmission. New important insights into the mechanisms responsible for the formation and release of adenosine into the extracellular space were provided by data obtained by Dunwiddie and coworkers in hippocampal pyramidal neurons. These data may have functional implications for the role of purines in modulation of synaptic plasticity and long-term potentiation in this brain area, and hence in cognitive functions. Buell provided an updated overview on the cloning, molecular characteristics and brain expression of various ligand-gated P2X purinoceptors; although the functional role of these receptors in mammalian brain still awaits elucidation, their widespread distribution in the nervous system strongly suggests that ATP-mediated events are more prevalent and important in brain than expected. Pedata presented data on the functional interrelationships between adenosine and glutamate in the brain, and also provided evidence for alterations of the reciprocal regulation between these two systems in aged brain, which may have important implications for both ischemia-and trauma-associated neurodegenerative events and senescence-associated cognitive impairment. Finally, von Lubitz provided novel data on the molecular mechanisms likely to be at the basis of the brain protective effects associated with the chronic stimulation of the adenosine A3 receptor, further confirming that this receptor represents a crucial target for the development of new antiischemic and antineurodegenerative therapeutic agents.