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

CD4+CD25+ regulatory T cells suppress contact hypersensitivity reactions through a CD39, adenosine-dependent mechanism

BACKGROUND

Injection of regulatory T (Treg) cells into sensitized mice abrogates the elicitation phase of contact hypersensitivity (CHS) reactions by blocking the adherence of leukocytes to vascular endothelium.

OBJECTIVE

We set out to analyze whether adenosine, a suppressive factor recently described as produced by Treg cells, can account for the suppression of the effector T-cell-endothelial cell (EC) interaction.

METHODS

T cells and ECs were cultured in the presence of adenosine, and expression of adhesion molecules and adhesion of T cells to ECs under shear stress were assessed. Furthermore, we injected Treg cells derived from ectonucleotidase-deficient (CD39-/-) mice into sensitized mice and analyzed the sticking and rolling of leukocytes during a CHS response using intravital microscopy.

RESULTS

Adenosine or Treg cells, respectively, abrogated the adherence of effector T cells to ECs in vitro. Likewise, injection of adenosine and Treg cells abrogated the ear-swelling reaction, indicating a role of adenosine during Treg cell-induced suppression of CHS responses. As a source for Treg cell-derived adenosine, we identified the ectonucleotidase CD39 because CD39-deficient Treg cells did not prevent adhesion of leukocytes to the endothelium. Furthermore, we show that the impaired adhesion of effector T cells to inflamed endothelium was induced by adenosine-mediated downregulation of expression of E- and P-selectin on the vascular endothelium.

CONCLUSION

Adenosine release by Treg cells is essential to block leukocyte adhesion to endothelium, providing a novel mechanism by which Treg cells mediate immune suppression in vivo.

Purinergic signaling and immune modulation at the schistosome surface?

After tissue stress or injury, intracellular ATP can be released into the extracellular environment. This signals cell damage because extracellular ATP acts as a danger-associated molecular pattern (DAMP) that is potently proinflammatory. Vertebrates temper this effect by catabolizing ATP to adenosine - a strongly anti-inflammatory molecule - using a set of characterized ecto-enzymes (notably alkaline phosphatase, phosphodiesterase and ATP diphosphohydrolase). Strikingly, schistosomes in the bloodstream have this same set of ATP-catabolizing enzymes on their tegumental surfaces. It is our opinion that these function to remove the DAMP (ATP) released by host cells in response to schistosome intravascular migration. We propose this as one mechanism by which schistosomes prevent their hosts from focusing immunological mediators in their vicinity.

Functional interactions between P2X4 and P2X7 receptors from mouse salivary epithelia

Mouse parotid acinar cells express P2X4 and P2X7 receptors (mP2X4R and mP2X7R) whose physiological function remains undetermined. Here we show that mP2X4R expressed in HEK-293 cells do not allow the passage of tetraethylammonium (TEA+) and promote little, if any, ethidium bromide (EtBr) uptake when stimulated with ATP or BzATP. In contrast, mP2X7R generates slowly decaying TEA+ current, sustained Na+ current and promotes robust EtBr uptake. However, ATP-activated TEA+ current from acinar cells was unlike that generated by mP2X7R or mP2X4R. Functional interactions between mP2X4R and mP2X7R were investigated in HEK cells co-transfected with different mP2X4 : mP2X7 cDNA ratios and using solutions containing either TEA+ or Na+ ions. Co-expressed channels generated a TEA+ current that displayed faster decay during ATP stimulation than mP2X7R alone. Moreover, cells transfected with a 2 : 1 cDNA ratio displayed decaying kinetics similar to those observed in acinar cells. Concentration-response curves in Na+-containing solutions were constructed for heterologously expressed mP2X4R, mP2X7R and mP2X4R:mP2X7R co-expressions as well as acinar cells. The EC50 values determined were 11, 220, 434 and 442 microM, respectively. Na+ currents generated by expressing mP2X4R or mP2X7R alone were potentiated by ivermectin (IVM). In contrast, IVM potentiation in acinar cells and HEK cells co-expressing P2X4 and P2X7 (1 : 1 or 2 : 1 cDNA ratios) was seen only when the ATP concentration was lowered from 5 to 0.03 mM. Taken together our observations indicate a functional interaction between murine P2X7 and P2X4 receptors. Such interaction might occur in acinar cells to shape the response to extracellular ATP in salivary epithelia.

Activation of microglia by amyloid {beta} requires P2X7 receptor expression

Extracellular ATP is a mediator of intercellular communication and a danger signal. Release of this and other nucleotides modulates microglia responses via P2Y and P2X receptors, among which the P2X(7) subtype stands out for its proinflammatory activity and for up-regulation in a transgenic model of Alzheimer disease and in brains from Alzheimer disease patients. Here we show that amyloid beta (Abeta) triggered increases in intracellular Ca(2+) ([Ca(2+)](i)), ATP release, IL-1beta secretion, and plasma membrane permeabilization in microglia from wild-type but not from P2X(7)-deleted mice. Likewise, intra-hippocampal injection of Abeta caused a large accumulation of IL-1beta in wild-type but not in P2X(7)(-/-) mice. These observations suggest that Abeta activates a purinergic autocrine/paracrine stimulatory loop of which the P2X(7) receptor is an obligate component. Identification of the P2X(7) receptor as a non-dispensable factor of Abeta-mediated microglia stimulation may open new avenues for the treatment of Alzheimer disease.

Expression of purinergic receptors and modulation of P2X7 function by the inflammatory cytokine IFNgamma in human epithelial cells

The cervical epithelial cell line, HeLa, is one of the oldest and most commonly used cell lines in cell biology laboratories. Although a truncated P2X(7) receptor has recently been identified in HeLa cells, the expression of other purinergic receptors or the function of the P2X(7) protein has not been characterized. We here show that HeLa cells express transcripts for most P2X and P2Y purinergic receptors. Treatment of cells with ATP or other P2X(7) agonists does not stimulate cell death, but can induce atypical calcium fluxes and ion currents. Cervical epithelial cells represent an important target for sexually-transmitted pathogens and are commonly exposed to pro-inflammatory cytokines such as IFNgamma. Stimulation of HeLa cells with IFNgamma upregulates expression of P2X(7) mRNA and full-length protein, modifies ATP-dependent calcium fluxes, and renders the cells sensitive to ATP-induced apoptosis, which can be blocked by a P2X(7) antagonist. IFNgamma treatment also increased dramatically the sensitivity of the intestinal epithelial cell line, HCT8, to ATP-induced apoptosis. Significantly, IFNgamma also stimulated P2X(7) expression on human intestinal tissues. Responses to other purinergic receptor ligands suggest that HeLa cells may also express functional P2Y(1), P2Y(2) and P2Y(6) receptors, which could be relevant for modulating ion homeostasis in the cells.

Extracellular ATP and P2 receptors are required for IL-8 to induce neutrophil migration

The chemokine interleukin 8 (IL-8) is a major chemoattractant for human neutrophils. Here, we demonstrate novel evidence that IL-8-induced neutrophil chemotaxis requires a concurrent activation of P2 receptors, most likely the P2Y(2) which is dominantly expressed in these cells. Indeed, the migration of human neutrophils towards IL-8 was significantly inhibited by the P2Y receptor antagonists, suramin and reactive blue 2 (RB-2) and potentiated by a P2Y(2) ligand, ATP, but insensitive to specific antagonists of P2Y(1), P2Y(6) and P2Y(11) receptors. Adenosine had no effect on neutrophil migration towards IL-8 which contrasted with the stimulatory effect of this molecule on neutrophil chemotaxis caused by formyl-Met-Leu-Phe (fMLP or fMLF). Taken together, these data suggest that extracellular ATP is necessary for IL-8 to exert its chemotactic effect on neutrophils.

Time-course changes in ectonucleotidase activities during experimental autoimmune encephalomyelitis

The aim of the present study was to analyze the activities of extracellular purine metabolizing enzymes, CD39 (apyrase, EC 3.6.1.5) and CD73 (ecto-5' nucleotidase, EC 3.1.3.5) in experimental autoimmune encephalomyelitis (EAE). The levels of ATP, ADP and AMP hydrolysis were analyzed in the blood serum and in the rat spinal cord plasma membrane preparation 8, 15 and 25 days after induction of EAE. The animals were divided in three groups: control (saline), CFA (adjuvant-only) and EAE (CFA and homogenate of spinal cords). Eight days after immunization, ATP, ADP and AMP hydrolysis in the blood serum and spinal cord membrane preparations were unaffected in EAE compared to both, control and CFA group. In the peak of disease, ATP, ADP and AMP hydrolysis in EAE group showed significant decrease in the blood serum and prominent increase in the spinal cord membrane preparation compared to CFA and control group. At the end of illness, as judged by disappearance of clinical manifestation of EAE, ATP, ADP and AMP hydrolysis, although closer to CFA levels, were still significantly different in respect to the CFA group. Modulation of ATP, ADP and AMP hydrolysis suggests that they operate during EAE and might represent the basis of novel therapeutic strategies in immune-mediated diseases, such as MS.

A P2X ion channel-triggered NF-kappaB pathway enhances TNF-alpha-induced IL-8 expression in airway epithelial cells

Extracellular ATP, acting at P2Y and P2X receptors, has recently been shown to contribute to airway inflammation. The aim of our study was to investigate the molecular mechanisms involved in the ATP-dependent regulation of IL-8 production by airway epithelial cells. Treatment of human normal tracheal (NT)-1 cells with ATP or its two analogs, alpha,beta-methylene ATP (alpha,beta-meATP) and 2'- and 3'-O-(4-benzoyl-benzoyl)-ATP (BzATP) activated NF-kappaB through the IkappaB kinase (IKK) complex, a process requiring Ca(2+), calmodulin (CaM), and Ca(2+)/CaM-dependent kinase (CaMK), but independent from phospholipase C. alpha,beta-meATP-induced IKK activation also occurred in the alveolar A549 cell line. Real-time RT-PCR revealed that NT-1 and A549 cells expressed P2X(4), P2X(5),and P2X(6) subtype mRNAs, whereas P2X(7) mRNAs were only detected in NT-1 cells. Polarized human primary nasal epithelial cells expressed all four P2X subtypes. Both alpha,beta-meATP and BzATP caused Ca(2+)-dependent binding of phosphorylated p65 (S536) NF-kappaB subunit to the endogenous IL-8 gene promoter in NT-1 cells. Although these agonists did not induce significant IL-8 gene expression by these cells, they markedly enhanced TNF-alpha-induced NF-kappaB activation, resulting in increased IL-8 expression and release. Application of alpha,beta-meATP or BzATP at the apical side of polarized human primary nasal epithelial cells sufficed to cause CaMK-dependent IL-8 release by these cells. Thus, ATP promotes TNF-alpha-elicited IL-8 expression through P2X ion channel-triggered Ca(2+) entry, leading to CaMK-dependent IKK activation and binding of active p65 to IL-8 gene promoter.

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

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

Possible effects of microbial ecto-nucleoside triphosphate diphosphohydrolases on host-pathogen interactions

In humans, purinergic signaling plays an important role in the modulation of immune responses through specific receptors that recognize nucleoside tri- and diphosphates as signaling molecules. Ecto-nucleoside triphosphate diphosphohydrolases (ecto-NTPDases) have important roles in the regulation of purinergic signaling by controlling levels of extracellular nucleotides. This process is key to pathophysiological protective responses such as hemostasis and inflammation. Ecto-NTPDases are found in all higher eukaryotes, and recently it has become apparent that a number of important parasitic pathogens of humans express surface-located NTPDases that have been linked to virulence. For those parasites that are purine auxotrophs, these enzymes may play an important role in purine scavenging, although they may also influence the host response to infection. Although ecto-NTPDases are rare in bacteria, expression of a secreted NTPDase in Legionella pneumophila was recently described. This ecto-enzyme enhances intracellular growth of the bacterium and potentially affects virulence. This discovery represents an important advance in the understanding of the contribution of other microbial NTPDases to host-pathogen interactions. Here we review other progress made to date in the characterization of ecto-NTPDases from microbial pathogens, how they differ from mammalian enzymes, and their association with organism viability and virulence. In addition, we postulate how ecto-NTPDases may contribute to the host-pathogen interaction by reviewing the effect of selected microbial pathogens on purinergic signaling. Finally, we raise the possibility of targeting ecto-NTPDases in the development of novel anti-infective agents based on potential structural and clear enzymatic differences from the mammalian ecto-NTPDases.

Purinergic signalling in inflammation of the central nervous system

Inflammation is the most fundamental body reaction to noxious stimuli. No vascularized tissue, organ or apparatus is free from this response. Several mediators of inflammation, originating from outside (exogenous) or inside (endogenous) the body, are known. Among the endogenous factors, extracellular nucleotides and nucleosides are attracting interest for their ubiquity and striking ability to modulate diverse immune responses. Until recently, it was doubted that the central nervous system (CNS), reportedly an 'immunoprivileged organ', could be the site of immune reactions. Nowadays, it is acknowledged that inflammation and immunity have a key role in a vast range of CNS diseases. Likewise, it is clear that purinergic signalling profoundly affects neuroinflammation. Here, we provide a brief update of the state of the art in this expanding field.

Adenosine receptors and inflammation

Extracellular adenosine is produced in a coordinated manner from cells following cellular challenge or tissue injury. Once produced, it serves as an autocrine- and paracrine-signaling molecule through its interactions with seven-membrane-spanning G-protein-coupled adenosine receptors. These signaling pathways have widespread physiological and pathophysiological functions. Immune cells express adenosine receptors and respond to adenosine or adenosine agonists in diverse manners. Extensive in vitro and in vivo studies have identified potent anti-inflammatory functions for all of the adenosine receptors on many different inflammatory cells and in various inflammatory disease processes. In addition, specific proinflammatory functions have also been ascribed to adenosine receptor activation. The potent effects of adenosine signaling on the regulation of inflammation suggest that targeting specific adenosine receptor activation or inactivation using selective agonists and antagonists could have important therapeutic implications in numerous diseases. This review is designed to summarize the current status of adenosine receptor signaling in various inflammatory cells and in models of inflammation, with an emphasis on the advancement of adenosine-based therapeutics to treat inflammatory disorders.

Adenosine receptors and neurological disease: neuroprotection and neurodegeneration

Adenosine receptors modulate neuronal and synaptic function in a range of ways that may make them relevant to the occurrence, development and treatment of brain ischemic damage and degenerative disorders. A(1) adenosine receptors tend to suppress neural activity by a predominantly presynaptic action, while A(2A) adenosine receptors are more likely to promote transmitter release and postsynaptic depolarization. A variety of interactions have also been described in which adenosine A(1) or A(2) adenosine receptors can modify cellular responses to conventional neurotransmitters or receptor agonists such as glutamate, NMDA, nitric oxide and P2 purine receptors. Part of the role of adenosine receptors seems to be in the regulation of inflammatory processes that often occur in the aftermath of a major insult or disease process. All of the adenosine receptors can modulate the release of cytokines such as interleukins and tumor necrosis factor-alpha from immune-competent leukocytes and glia. When examined directly as modifiers of brain damage, A(1) adenosine receptor (AR) agonists, A(2A)AR agonists and antagonists, as well as A(3)AR antagonists, can protect against a range of insults, both in vitro and in vivo. Intriguingly, acute and chronic treatments with these ligands can often produce diametrically opposite effects on damage outcome, probably resulting from adaptational changes in receptor number or properties. In some cases molecular approaches have identified the involvement of ERK and GSK-3beta pathways in the protection from damage. Much evidence argues for a role of adenosine receptors in neurological disease. Receptor densities are altered in patients with Alzheimer's disease, while many studies have demonstrated effects of adenosine and its antagonists on synaptic plasticity in vitro, or on learning adequacy in vivo. The combined effects of adenosine on neuronal viability and inflammatory processes have also led to considerations of their roles in Lesch-Nyhan syndrome, Creutzfeldt-Jakob disease, Huntington's disease and multiple sclerosis, as well as the brain damage associated with stroke. In addition to the potential pathological relevance of adenosine receptors, there are earnest attempts in progress to generate ligands that will target adenosine receptors as therapeutic agents to treat some of these disorders.

Wound healing essentials: let there be oxygen

The state of wound oxygenation is a key determinant of healing outcomes. From a diagnostic standpoint, measurements of wound oxygenation are commonly used to guide treatment planning such as amputation decision. In preventive applications, optimizing wound perfusion and providing supplemental O(2) in the perioperative period reduces the incidence of postoperative infections. Correction of wound pO(2) may, by itself, trigger some healing responses. Importantly, approaches to correct wound pO(2) favorably influence outcomes of other therapies such as responsiveness to growth factors and acceptance of grafts. Chronic ischemic wounds are essentially hypoxic. Primarily based on the tumor literature, hypoxia is generally viewed as being angiogenic. This is true with the condition that hypoxia be acute and mild to modest in magnitude. Extreme near-anoxic hypoxia, as commonly noted in problem wounds, is not compatible with tissue repair. Adequate wound tissue oxygenation is required but may not be sufficient to favorably influence healing outcomes. Success in wound care may be improved by a personalized health care approach. The key lies in our ability to specifically identify the key limitations of a given wound and in developing a multifaceted strategy to specifically address those limitations. In considering approaches to oxygenate the wound tissue it is important to recognize that both too little as well as too much may impede the healing process. Oxygen dosing based on the specific need of a wound therefore seems prudent. Therapeutic approaches targeting the oxygen sensing and redox signaling pathways are promising.

Adenosine induces loss of actin stress fibers and inhibits contraction in hepatic stellate cells via Rho inhibition

The Rho/ROCK pathway is activated in differentiated hepatic stellate cells (HSCs) and is necessary for assembly of actin stress fibers, contractility, and chemotaxis. Despite the importance of this pathway in HSC biology, physiological inhibitors of the Rho/ROCK pathway in HSCs are not known. We demonstrate that adenosine induces loss of actin stress fibers in the LX-2 cell line and primary HSCs in a manner indistinguishable from Rho/ROCK inhibition. Loss of actin stress fibers occurs via the A2a receptor at adenosine concentrations above 10 muM, which are present during tissue injury. We further demonstrate that loss of actin stress fibers is due to a cyclic adenosine monophosphate, protein kinase A-mediated pathway that results in Rho inhibition. Furthermore, a constitutively active Rho construct can inhibit the ability of adenosine to induce loss of actin stress fibers. Actin stress fibers are required for HSC contraction, and we demonstrate that adenosine inhibits endothelin-1 and lysophosphatidic acid-mediated HSC contraction. We propose that adenosine is a physiological inhibitor of the Rho pathway in HSCs with functional consequences, including loss of HSC contraction.

Mechanisms of induction of adenosine receptor genes and its functional significance

Adenosine is a metabolite generated and released from cells, particularly under injury or stress. It elicits protective or damaging responses via signaling through the adenosine receptors, including the adenylyl cyclase inhibitory A(1) and A(3), and the adenylyl cyclase stimulatory A(2A) and A(2B). Multiple adenosine receptor types, including stimulatory and inhibitory, can be found in the same cell, suggesting that a careful balance of adenosine receptor expression in a particular cell is necessary for a specific adenosine-induced response. This balance could be controlled by differential expression of the adenosine receptor genes under different stimuli. Here, we have reviewed an array of studies that have characterized basal or induced expression of the adenosine receptors and common as well as distinct mechanisms of effect, in hopes that ongoing studies on this topic will further elucidate detailed mechanisms of adenosine receptor regulation, leading to potential therapeutic applications.

Clinical pharmacology of exogenously administered alkaline phosphatase

PURPOSE

To evaluate the clinical pharmacology of exogenous alkaline phosphatase (AP).

METHODS

Randomized, double-blind, placebo-controlled sequential protocols of (1) ascending doses and infusion duration (volunteers) and (2) fixed dose and duration (patients) were conducted at clinical pharmacology and intensive care units. A total of 103 subjects (67 male volunteers and 36 patients with severe sepsis) were administered exogenous, 10-min IV infusions (three ascending doses) or 24-72 h continuous (132.5-200 U kg(-1) 24 h(-1)) IV infusion with/without preceding loading dose and experimental endotoxemia for evaluations of pharmacokinetics, pharmacodynamics, safety parameters, antigenicity, inflammatory markers, and outcomes.

RESULTS

Linearity and dose-proportionality were shown during 10-min infusions. The relatively short elimination half-life necessitated a loading dose to achieve stable enzyme levels. Pharmacokinetic parameters in volunteers and patients were similar. Innate immunity response was not significantly influenced by AP, while renal function significantly improved in sepsis patients.

CONCLUSIONS

The pharmacokinetics of exogenous AP is linear, dose-proportional, exhibit a short half-life, and are not influenced by renal impairment or dialysis.

Dynamics of shear-induced ATP release from red blood cells

Adenosine triphosphate (ATP) is a regulatory molecule for many cell functions, both for intracellular and, perhaps less well known, extracellular functions. An important example of the latter involves red blood cells (RBCs), which help regulate blood pressure by releasing ATP as a vasodilatory signaling molecule in response to the increased shear stress inside arterial constrictions. Although shear-induced ATP release has been observed widely and is believed to be triggered by deformation of the cell membrane, the underlying mechanosensing mechanism inside RBCs is still controversial. Here, we use an in vitro microfluidic approach to investigate the dynamics of shear-induced ATP release from human RBCs with millisecond resolution. We demonstrate that there is a sizable delay time between the onset of increased shear stress and the release of ATP. This response time decreases with shear stress, but surprisingly does not depend significantly on membrane rigidity. Furthermore, we show that even though the RBCs deform significantly in short constrictions (duration of increased stress <3 ms), no measurable ATP is released. This critical timescale is commensurate with a characteristic membrane relaxation time determined from observations of the cell deformation by using high-speed video. Taken together our results suggest a model wherein the retraction of the spectrin-actin cytoskeleton network triggers the mechanosensitive ATP release and a shear-dependent membrane viscosity controls the rate of release.

Antimicrobial effects of H4-(86-100), histogranin and related compounds--possible involvement of DNA gyrase

Histone-derived antimicrobial peptides have been identified in various organisms from plants to humans. The rat histone H4 mRNA variants, H4-v.1 and rat histogranin (HNr) mRNAs, were recently reported to be involved in the synthesis of H4-(86-100) and its related peptide HNr, respectively. Herein, the two peptides were investigated for putative antimicrobial activity and found to inhibit growth of gram-negative (Escherichia coli, Pseudomonas aeruginosa) and gram-positive (Bacillus subtilis, Staphylococcus aureus) bacteria. Their inhibitory potencies in E. coli (LD(50): 3.48 and 4.34 microg x mL(-1)) are comparable to that of the antimicrobial peptide LL-37 (LD(50): 4.10 microg x mL(-1)). The antimicrobial activities of H4-(86-100) and HNr depend upon the integrity of the molecules, as precursors [H4-(84-102), pro-HNr] and fragments [bovine histogranin (HNb)-(1-13), HNb-(3-13), H4-(89-102) or OGP] are at least five times less potent than the parent peptides. Among various HN-like compounds, cyclo-(-Gly-pCl-Phe-Tyr-D-Arg) (compound 3) and N-5-guanidino pentanamide-(2R)-yl-2-N-(p-hydroxyphenylacetyl)-4-(p-chlorobenzoyl)-phenylene diamine (compound 8) display antimicrobial activities comparable to that of HNr. Interestingly, the antimicrobial activities of H4-(86-100), HNr and compound 3, like those of quinolone antibiotics acting as DNA gyrase poisons, are potentiated by ATP (1 mM) and coumermycin A1 (a DNA gyrase-linked ATPase inhibitor) and blocked by 2,4-dinitrophenol (DNP, an uncoupler of oxidative phosphorylation) and fluoroacetic acid (a metabolic poison). Finally, in vitro experiments indicate that H4-(86-100), HNr, compound 3 and compound 8, but not HNb-(1-13) or HNb-(3-13), inhibit DNA gyrase-mediated supercoiling of pBR322 DNA. These data indicate that the naturally occurring H4-(86-100) and HNr display antimicrobial effects that involve a modulation of ATP-dependent DNA gyrase.

Wounding sheets of epithelial cells activates the epidermal growth factor receptor through distinct short- and long-range mechanisms

Wounding epithelia induces activation of the epidermal growth factor receptor (EGFR), which is absolutely required for induction of motility. ATP is released from cells after wounding; it binds to purinergic receptors on the cell surface, and the EGFR is subsequently activated. Exogenous ATP activates phospholipase D, and we show here that ATP activates the EGFR through the phospholipase D2 isoform. The EGFR is activated in cells far (>0.3 cm) from wounds, which is mediated by diffusion of extracellular ATP because activation at a distance from wounds is abrogated by eliminating ATP in the medium with apyrase. In sharp contrast, activation of the EGFR near wounds is not sensitive to apyrase. Time-lapse microscopy revealed that cells exhibit increased motilities near edges of wounds; this increase in motility is not sensitive to apyrase, and apyrase does not detectably inhibit healing of wounds in epithelial sheets. This novel ATP/PLD2-independent pathway activates the EGFR by a transactivation process through ligand release, and it involves signaling by a member of the Src family of kinases. We conclude that wounding activates two distinct signaling pathways that induce EGFR activation and promote healing of wounds in epithelial cells. One pathway signals at a distance from wounds through release of ATP, and another pathway acts locally and is independent on ATP signaling.

Regulation of enteric functions by adenosine: pathophysiological and pharmacological implications

The wide distribution of ATP and adenosine receptors as well as enzymes for purine metabolism in different gut regions suggests a complex role for these mediators in the regulation of gastrointestinal functions. Studies in rodents have shown a significant involvement of adenosine in the control of intestinal secretion, motility and sensation, via activation of A1, A2A, A2B or A3 purinergic receptors, as well as the participation of ATP in the regulation of enteric functions, through the recruitment of P2X and P2Y receptors. Increasing interest is being focused on the involvement of ATP and adenosine in the pathophysiology of intestinal disorders, with particular regard for inflammatory bowel diseases (IBDs), intestinal ischemia, post-operative ileus and related dysfunctions, such as gut dysmotility, diarrhoea and abdominal discomfort/pain. Current knowledge suggests that adenosine contributes to the modulation of enteric immune and inflammatory responses, leading to anti-inflammatory actions. There is evidence supporting a role of adenosine in the alterations of enteric motor and secretory activity associated with bowel inflammation. In particular, several studies have highlighted the importance of adenosine in diarrhoea, since this nucleoside participates actively in the cross-talk between immune and epithelial cells in the presence of diarrhoeogenic stimuli. In addition, adenosine exerts complex regulatory actions on pain transmission at peripheral and spinal sites. The present review illustrates current information on the role played by adenosine in the regulation of enteric functions, under normal or pathological conditions, and discusses pharmacological interventions on adenosine pathways as novel therapeutic options for the management of gut disorders and related abdominal symptoms.

A 2A adenosine receptor regulates glia proliferation and pain after peripheral nerve injury

Peripheral nerve injury produces a persistent neuropathic pain state characterized by spontaneous pain, allodynia and hyperalgesia. In this study, we evaluated the possible involvement of A 2ARs in the development of neuropathic pain and the expression of microglia and astrocytes in the spinal cord after sciatic nerve injury. For this purpose, partial ligation of the sciatic nerve was performed in A 2A knockout mice and wild-type littermates. The development of mechanical and thermal allodynia, as well as thermal hyperalgesia was evaluated by using the von Frey filament model, the cold-plate test and the plantar test, respectively. In wild-type animals, sciatic nerve injury led to a neuropathic pain syndrome that was revealed in these three nociceptive behavioural tests. However, a significant decrease of the mechanical allodynia and a suppression of thermal hyperalgesia and allodynia were observed in A 2AR deficient mice. The expression of microglia and astrocytes was enhanced in wild-type mice exposed to sciatic nerve injury and this response was attenuated in knockout animals. Taken together, our results demonstrate the involvement of A 2ARs in the control of neuropathic pain and propose this receptor as an interesting target for the development of new drugs for the management of this clinical syndrome.

Deletion of cd39/entpd1 results in hepatic insulin resistance

OBJECTIVE

Extracellular nucleotides are important mediators of inflammatory responses and could also impact metabolic homeostasis. Type 2 purinergic (P2) receptors bind extracellular nucleotides and are expressed by major peripheral tissues responsible for glucose homeostasis. CD39/ENTPD1 is the dominant vascular and immune cell ectoenzyme that hydrolyzes extracellular nucleotides to regulate purinergic signaling.

RESEARCH DESIGN AND METHODS

We have studied Cd39/Entpd1-null mice to determine whether any associated changes in extracellular nucleotide concentrations influence glucose homeostasis.

RESULTS

Cd39/Entpd1-null mice have impaired glucose tolerance and decreased insulin sensitivity with significantly higher plasma insulin levels. Hyperinsulinemic-euglycemic clamp studies indicate altered hepatic glucose metabolism. These effects are mimicked in vivo by injection into wild-type mice of either exogenous ATP or an ecto-ATPase inhibitor, ARL-67156, and by exposure of hepatocytes to extracellular nucleotides in vitro. Increased serum interleukin-1beta, interleukin-6, interferon-gamma, and tumor necrosis factor-alpha levels are observed in Cd39/Entpd1-null mice in keeping with a proinflammatory phenotype. Impaired insulin sensitivity is accompanied by increased activation of hepatic c-Jun NH(2)-terminal kinase/stress-activated protein kinase in Cd39/Entpd1 mice after injection of ATP in vivo. This results in decreased tyrosine phosphorylation of insulin receptor substrate-2 with impeded insulin signaling.

CONCLUSIONS

CD39/Entpd1 is a modulator of extracellular nucleotide signaling and also influences metabolism. Deletion of Cd39/Entpd1 both directly and indirectly impacts insulin regulation and hepatic glucose metabolism. Extracellular nucleotides serve as "metabolokines," indicating further links between inflammation and associated metabolic derangements.

Adenine nucleotide hydrolysis in patients with aseptic and bacterial meningitis

The meningitis is a disease with high mortality rates capable to cause neurologic sequelae. The adenosine (the final product of ATP hydrolysis by ectonucleotidases), have a recognized neuroprotective actions in the central nervous system (CNS) in pathological conditions. The aim of the present study was evaluate the adenine nucleotides hydrolysis for to verify one possible role of ATP, ADP and AMP hydrolysis in inflammatory process such as meningitis. The hydrolysis was verified in cerebrospinal fluid (CSF) from human patients with aseptic and bacterial meningitis. Our results showed that the ATP hydrolysis was reduced 12.28% (P < 0.05) in bacterial meningitis and 22% (P < 0.05) in aseptic meningitis. ADP and AMP hydrolysis increased 79.13% (P < 0.05) and 26.37% (P < 0.05) in bacterial meningitis, respectively, and 57.39% (P < 0.05) and 42.64% (P < 0.05) in aseptic meningitis, respectively. This may be an important protective mechanism in order to increase adenosine production.

Modulation of murine dendritic cell function by adenine nucleotides and adenosine: involvement of the A(2B) receptor

Adenosine triphosphate has previously been shown to induce semi-mature human monocyte-derived dendritic cells (DC). These are characterized by the up-regulation of co-stimulatory molecules, the inhibition of IL-12 and the up-regulation of some genes involved in immune tolerance, such as thrombospondin-1 and indoleamine 2,3-dioxygenase. The actions of adenosine triphosphate are mediated by the P2Y(11) receptor; since there is no functional P2Y(11) gene in the murine genome, we investigated the action of adenine nucleotides on murine DC. Adenosine 5'-(3-thiotriphosphate) and adenosine inhibited the production of IL-12p70 by bone marrow-derived DC (BMDC). These inhibitions were relieved by 8-p-sulfophenyltheophylline, an adenosine receptor antagonist. The use of selective ligands and A(2B) (-/-) BMDC indicated the involvement of the A(2B) receptor. A microarray experiment, confirmed by quantitative PCR, showed that, in presence of LPS, 5'-(N-ethylcarboxamido) adenosine (NECA, the most potent A(2B) receptor agonist) regulated the expression of several genes: arginase I and II, thrombospondin-1 and vascular endothelial growth factor were up-regulated whereas CCL2 and CCL12 were down-regulated. We further showed that NECA, in combination with LPS, increased the arginase I enzymatic activity. In conclusion, the described actions of adenine nucleotides on BMDC are mediated by their degradation product, adenosine, acting on the A(2B) receptor, and will possibly lead to an impairment of Th1 response or tolerance.

Purinergic regulation of neutrophil chemotaxis

Chemotaxis allows polymorphonuclear neutrophils (PMN) to rapidly reach infected and inflamed sites. However, excessive influx of PMN damages host tissues. Better knowledge of the mechanisms that control PMN chemotaxis may lead to improved treatments of inflammatory diseases. Recent findings suggest that ATP and adenosine are involved in PMN chemotaxis. Therefore, these purinergic signaling processes may be suitable targets for novel therapeutic approaches to ameliorate host tissue damage.

Purinergic signalling and disorders of the central nervous system

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

Adenosine A2A receptor activation inhibits T helper 1 and T helper 2 cell development and effector function

Adenosine is an immunosuppressive nucleoside, and adenosine A(2A) receptors inhibit T-cell activation. We investigated the role of A(2A) receptors in regulating T helper (Th)1- and Th2-cell development and effector function. A(2A)-receptor stimulation suppressed the development of T-cell receptor (TCR) -stimulated naive T cells into both Th1 and Th2 cells, as indicated by decreased IFN-gamma production by cells developed under Th1-skewing conditions and decreased interleukin (IL) -4, IL-5, and IL-10 production by cells developed under Th2-skewing conditions. Using A(2A) receptor-deficient mice, we demonstrate that A(2A) receptor activation inhibits Th1- and Th2-cell development by decreasing the proliferation and IL-2 production of naive T cells, irrespective of whether the cells are expanded under Th1- or Th2-skewing environment. Using in vivo established Th1 and Th2 cells, we further demonstrate the nonselective nature of A(2A) receptor-mediated immunosuppressive effects, because A(2A) receptor activation decreased IFN-gamma and IL-4 secretion and mRNA level of TCR-stimulated effector Th1 and Th2 cells, respectively. A(2A) receptor mRNA expression in both Th1 and Th2 effector cells increased following TCR stimulation. In summary, these data demonstrate that A(2A) receptor activation has strong inhibitory actions during early developmental, as well as late effector, stages of Th1- and Th2-cell responses.

Allergen-induced airway hyperresponsiveness is absent in ecto-5'-nucleotidase (CD73)-deficient mice

Adenosine is formed from extracellular purines by ecto-5'-nucleotidase (CD73) and is an essential player in allergic airway inflammation. The contribution of adenosine and other purines to electrolyte transport and mucociliary clearance was studied in airways of allergen challenged mice. No signs for allergen-induced inflammation were found in CD73-/- mice, and adenosine monophosphate (AMP) was unable to elicit airway Cl(-) secretion in these animals. Tracheas of ovalbumin (OVA)-treated BALB/c and CD73+/+ mice were hyperresponsive towards methacholine when assessed by Penh and direct optical measurement of contraction. In addition Cl(-) secretion activated by ATP and ADP was enhanced. These changes were not observed in CD73-/- mice. Expression of CFTR or CLCA was unchanged upon OVA treatment of CD73 mice, suggesting enhanced Cl(-) secretion due to upregulated purinergic pathways. Mucociliary clearance was determined by measuring particle transport in excised mouse tracheas and was strongly enhanced in OVA-challenged CD73+/+ mice, but remained unchanged in CD73-/- mice. While mucociliary clearance is activated by allergen exposure independent of functional ecto-5'-nucleotidase, airway inflammation is largely dependent on CD73. Thus, ecto-5'-nucleotidase may provide a novel target for therapeutic intervention, probably by local application of ecto-5'-nucleotidase inhibitors through inhalation.

CD73 is required for efficient entry of lymphocytes into the central nervous system during experimental autoimmune encephalomyelitis

CD73 is a cell surface enzyme of the purine catabolic pathway that catalyzes the breakdown of AMP to adenosine. Because of the strong immunosuppressive and antiinflammatory properties of adenosine, we predicted that cd73(-/-) mice would develop severe experimental autoimmune encephalomyelitis (EAE), an animal model for the central nervous system (CNS) inflammatory disease, multiple sclerosis. Surprisingly, cd73(-/-) mice were resistant to EAE. However, CD4 T cells from cd73(-/-) mice secreted more proinflammatory cytokines than wild-type (WT) mice and were able to induce EAE when transferred into naïve cd73(+/+) T cell-deficient recipients. Therefore, the protection from EAE observed in cd73(-/-) mice was not caused by a deficiency in T cell responsiveness. Immunohistochemistry showed that cd73(-/-) mice had fewer infiltrating lymphocytes in their CNS compared with WT mice. Importantly, susceptibility to EAE could be induced in cd73(-/-) mice after the transfer of WT CD73(+)CD4(+) T cells, suggesting that CD73 must be expressed either on T cells or in the CNS for disease induction. In the search for the source of CD73 in the CNS that might facilitate lymphocyte migration, immunohistochemistry revealed a lack of CD73 expression on brain endothelial cells and high expression in the choroid plexus epithelium which regulates lymphocyte immunosurveillance between the blood and cerebrospinal fluid. Because blockade of adenosine receptor signaling with the A(2a) adenosine receptor-specific antagonist SCH58261 protected WT mice from EAE induction, we conclude that CD73 expression and adenosine receptor signaling are required for the efficient entry of lymphocytes into the CNS during EAE development.

Activation of the A(3) adenosine receptor suppresses superoxide production and chemotaxis of mouse bone marrow neutrophils

Adenosine is formed in injured/ischemic tissues, where it suppresses the actions of essentially all cells of the immune system. Most of the anti-inflammatory actions of adenosine have been attributed to signaling through the G(s) protein-coupled A(2A) adenosine receptor (AR). Here, we report that the A(3)AR is highly expressed in murine neutrophils isolated from bone marrow. Selective activation of the A(3)AR with (2S,3S,4R,5R)-3-amino-5-[6-(2,5-dichlorobenzylamino)purin-9-yl]-4-hydroxytetrahydrofuran-2-carboxylic acid methylamide (CP-532,903) potently inhibited mouse bone marrow neutrophil superoxide generation and chemotaxis induced by various activating agents. The selectivity of CP-532,903 was confirmed in assays using neutrophils obtained from A(2A)AR and A(3)AR gene "knockout" mice. In a model of thioglycollate-induced inflammation, treating mice with CP-532,903 inhibited recruitment of leukocytes into the peritoneum by specifically activating the A(3)AR. Collectively, our findings support the theory that the A(3)AR contributes to the anti-inflammatory actions of adenosine on neutrophils and provide a potential mechanistic explanation for the efficacy of A(3)AR agonists in animal models of inflammation (i.e., inhibition of neutrophil-mediated tissue injury).

"Host tissue damage" signal ATP impairs IL-12 and IFNgamma secretion in LPS stimulated whole human blood

OBJECTIVE

Critical illness is associated with tissue damage, inflammation and the development of immune dysfunction. Leukocyte reprogramming occurs leading to insufficient production of pro-inflammatory cytokines upon subsequent stimulation. Cellular nucleotides released during tissue damage act via purinergic receptors to modulate immune function. We hypothesized that extracellular nucleotides in concentrations similar to those found near injured and ischemic tissues will modulate cytokine secretion.

DESIGN AND PARTICIPANTS

Bench study in 28 healthy human volunteers using standardized lipopolysaccharide (LPS) stimulated ex vivo whole blood cultures (ILCS).

SETTING

The Nepean Hospital Laboratories, University of Sydney.

INTERVENTIONS

Nucleotides ATP, ADP and other P2 purinergic receptor agonists ATPgammaS, BzATP, UTP and P1 agonist CV1808 were injected into the ILCS, and cultured for 6, 12 and 24 h as indicated.

MEASUREMENTS AND RESULTS

ATP (100 muM) reduced the LPS stimulated secretion of TNFalpha at 6 and 12 h, as well as IL-12(p70) and IFNgamma at 24 h of incubation. ADP, ATPgammaS, BzATP, and CV1808, but not UTP displayed IL-12(p70) and IFNgamma reducing effect similar to ATP. Higher ATP concentration (500 muM) had even more pronounced immunosuppressive effect. Nucleotides had variable effect on stimulated IL-10 secretion.

CONCLUSIONS

ATP and ADP at high-micromolar concentrations reduce secretion of the main Th1 cytokines TNFalpha, IL-12(p70) and IFNgamma in LPS stimulated human blood. As immune dysfunction associated with critical illness is characterized by decreased TNFalpha, IL-12 and IFNgamma production by leukocytes, extracellular nucleotides might contribute to its pathogenesis [corrected]

Structural insight into signal conversion and inactivation by NTPDase2 in purinergic signaling

Cell surface-located nucleoside triphosphate diphosphohydrolases (NTPDase1, -2, -3, and -8) are oligomeric integral membrane proteins responsible for signal conversion and inactivation in extracellular nucleotide-mediated "purinergic" signaling. They catalyze the sequential hydrolysis of the signaling molecule ATP via ADP to AMP. Here we present the structure of the extracellular domain of Rattus norvegicus NTPDase2 in an active state at resolutions between 1.7 A and 2.1 A in four different forms: (i) apo form, (ii) ternary complex with the nonhydrolyzable ATP analog AMPPNP and cofactor Ca(2+), (iii) quaternary complex with Ca(2+) and bound products AMP and phosphate, and (iv) binary product complex with AMP only. Analysis of the ATP (analog) binding mode explains the importance of several residues for activity and allows suggestion of a catalytic mechanism. The carboxylate group of E165 serves as a catalytic base and activates a water molecule, which is well positioned for nucleophilic attack on the terminal phosphate. Based on analysis of the two product complex structures in which AMP adopts different conformations, a substrate binding mode for ADP hydrolysis is proposed. This allows for an understanding of how the same hydrolytic site can be engaged in ATP and ADP but not AMP hydrolysis.

Extracellular nucleotide metabolism in Leishmania: influence of adenosine in the establishment of infection

Leishmaniasis is a parasitic disease with a variety of clinical forms, which are related to the Leishmania species involved. In the murine model, Leishmania amazonensis causes chronic non-healing lesions in Leishmania braziliensis- or Leishmania major-resistant mouse strains. In this study, we investigated the involvement of the pathway of extracellular nucleotide hydrolysis, with special focus on the role of extracellular adenosine, in the establishment of Leishmania infection. Our results show that the more virulent parasite--L. amazonensis--hydrolyzes higher amounts of ATP, ADP and AMP than the two other species, probably due to the higher expression of membrane NTPDase. Corroborating the idea that increased production of adenosine is important to lesion development and establishment of tissue parasitism, we observed that increased 5'-nucleotidase activity in L. braziliensis or addition of adenosine at the moment of infection with this parasite resulted in an increase in lesion size and parasitism as well as a delay in lesion healing. Furthermore, inhibition of adenosine receptor A2B led to decreased lesion size and parasitism. Thus, our results suggest that the conversion of ATP, a molecule with pro-inflammatory activity, into adenosine, which possesses immunomodulatory properties, may contribute to the establishment of infection by Leishmania.

Intestinal inflammation increases the expression of the P2Y6 receptor on epithelial cells and the release of CXC chemokine ligand 8 by UDP

Epithelial cells participate in the immune response of the intestinal mucosa. Extracellular nucleotides have been recognized as inflammatory molecules. We investigated the role of extracellular nucleotides and their associated P2Y receptors in the secretion of cytokines by epithelial cells. The effect of intestinal inflammation on P2Y(6) receptor expression was determined by PCR in the mouse, rat, and human. Localization of the P2Y(6) receptor was determined by immunofluorescence microscopy in the colon of normal and dextran sulfate sodium-treated mice. The effect of P2Y(6) activation by UDP on cytokine expression and release by epithelial cells was determined using a combination of Western blots, luciferase assays, RT-PCR, cytokine Ab arrays, and ELISA. Inflammation up-regulates P2Y(2) as well as P2Y(6) receptor expression in the mucosa of the colon of colitic mice. In vitro, we demonstrated that UDP could be released by Caco-2/15 cells. We have confirmed the increased expression of P2Y(6) by challenging intestinal epithelial cell-6 and Caco-2/15 cells with TNF-alpha and IFN-gamma and showing that stimulation of epithelial cells by UDP results in an increased expression and release of CXCL8 by an ERK1/2-dependent mechanism. The increase in CXCL8 expression was associated with a transcriptional activation by the P2Y(6) receptor. This study is the first report demonstrating the implication of P2Y receptors in the inflammatory response of intestinal epithelial cells. We show for the first time that P2Y(6), as well as P2Y(2), expression is increased by the stress associated with intestinal inflammation. These results demonstrate the emergence of extracellular nucleotide signaling in the orchestration of intestinal inflammation.

Pharmacological modulation of adenosine system: novel options for treatment of inflammatory bowel diseases

Inflammatory bowel diseases (IBDs) are chronic disorders resulting from abnormal and persistent immune responses which lead to severe tissue injury and disturbances in digestive motor/secretory functions. At present, pharmacotherapy represents the cornerstone for the management of IBDs, and recent advances in understanding the immunopathogenesis of intestinal inflammation suggest the adenosine system as an attractive target for development of novel drugs against gut inflammatory disorders. Consistent evidence indicates that adenosine plays a relevant role in the regulation of immune system via interaction with specific cell-membrane G-protein-coupled receptors (A(1), A(2a), A(2b), and A(3)). Moreover, this nucleoside is implicated in the control of enteric neurotransmission and gut motor functions. In the presence of inflammation, the adenosine system acts as a sensible sensor apparatus, which, through dynamic modifications in the expression of ecto-enzymes and purinergic receptors, adapts its metabolism to tissue health status and contributes to the mechanisms deputed to the protection of tissues against inflammatory injuries. In keeping with these concepts, it is becoming increasingly appreciated that drugs targeted on adenosine receptors or enzymes responsible for adenosine catabolism can exert beneficial effects on experimental models of intestinal inflammation. This review aims to discuss the role of adenosine in the regulation of enteric immune responses and gut neuromuscular functions in the presence of inflammation, as well as to highlight the mechanisms through which the pharmacological modulation of adenosine pathways may have potential applications for the therapeutic management of IBDs.

The signaling role of extracellular ATP and its dependence on Ca2+ flux in elicitation of Salvia miltiorrhiza hairy root cultures

The application of a polysaccharide elicitor from yeast extract, YE, to Salvia miltiorrhiza hairy root cultures induced transient release of ATP from the roots to the medium, leading to a dose-dependent increase in the extracellular ATP (eATP) level. The eATP level rose to a peak (about 6.5 nM with 100 mg l(-1) YE) at about 10 h after YE treatment, but dropped to the control level 6 h later. The elicitor-induced ATP release was dependent on membrane Ca2+ influx, and abolished by the Ca2+ chelator EGTA or the channel blocker La3+. The YE-induced H2O2 production was strongly inhibited by reactive blue (RB), a specific inhibitor of membrane purinoceptors. On the other hand, the application of exogenous ATP at 10-100 microM to the cultures also induced rapid and dose-dependent increases in H2O2 production and medium pH, both of which were effectively blocked by RB and EGTA. The non-hydrolyzable ATP analog ATPgammaS was as effective as ATP, but the hydrolyzed derivatives ADP or AMP were not so effective in inducing the pH and H2O2 increases. Our results suggest that ATP release is an early event and that eATP plays a signaling role in the elicitation of plant cell responses; Ca2+ is required for activation of the elicitor-induced ATP release and the eATP signal transduction. This is the first report on ATP release induced by a fungal elicitor and its involvement in the elicitor-induced responses in plant cells.

How dying cells alert the immune system to danger

When a cell dies in vivo, the event does not go unnoticed. The host has evolved mechanisms to detect the death of cells and rapidly investigate the nature of their demise. If cell death is a result of natural causes - that is, it is part of normal physiological processes - then there is little threat to the organism. In this situation, little else is done other than to remove the corpse. However, if cells have died as the consequence of some violence or disease, then both defence and repair mechanisms are mobilized in the host. The importance of these processes to host defence and disease pathogenesis has only been appreciated relatively recently. This article reviews our current knowledge of these processes.

Enzymatic properties of an ecto-nucleoside triphosphate diphosphohydrolase from Legionella pneumophila: substrate specificity and requirement for virulence

Legionella pneumophila is the predominant cause of Legionnaires disease, a severe and potentially fatal form of pneumonia. Recently, we identified an ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) from L. pneumophila, termed Lpg1905, which enhances intracellular replication of L. pneumophila in eukaryotic cells. Lpg1905 is the first prokaryotic member of the CD39/NTPDase1 family of enzymes, which are characterized by the presence of five apyrase conserved regions and the ability to hydrolyze nucleoside tri- and diphosphates. Here we examined the substrate specificity of Lpg1905 and showed that apart from ATP and ADP, the enzyme catalyzed the hydrolysis of GTP and GDP but had limited activity against CTP, CDP, UTP, and UDP. Based on amino acid residues conserved in the apyrase conserved regions of eukaryotic NTPDases, we generated five site-directed mutants, Lpg1905E159A, R122A, N168A, Q193A, and W384A. Although the mutations E159A, R122A, Q193A, and W384A abrogated activity completely, N168A resulted in decreased activity caused by reduced affinity for nucleotides. When introduced into the lpg1905 mutant strain of L. pneumophila, only N168A partially restored the ability of L. pneumophila to replicate in THP-1 macrophages. Following intratracheal inoculation of A/J mice, none of the Lpg1905 mutants was able to restore virulence to an lpg1905 mutant during lung infection, thereby demonstrating the importance of NTPDase activity to L. pneumophila infection. Overall, the kinetic studies undertaken here demonstrated important differences to mammalian NTPDases and different sensitivities to NTPDase inhibitors that may reflect underlying structural variations.

Adenosine receptors and asthma

The accumulation of evidence implicating a role for adenosine in the pathogenesis of asthma has led to investigations into all adenosine receptor subtypes as potential therapeutic targets for the treatment of asthma. Selective A(1) receptor antagonists are currently in preclinical development since adenosine has been shown experimentally to mediate various features of asthma through this receptor such as bronchoconstriction, mucus secretion and inflammation. The A(2A) receptor is expressed on most inflammatory cells implicated in asthma, and as A(2A) stimulation activates adenylate cyclase and consequently elevates cAMP, selective A(2A) receptor agonists have now reached clinical development. However, initial reports concerning their efficacy are inconclusive. A(2B) receptor antagonists are also under investigation based on the rationale that inhibiting the effects of adenosine on mast cells would be beneficial, in addition to other reported pro-inflammatory effects mediated by the A(2B) receptor on cells such as airway smooth muscle, epithelial cells and fibroblasts. Whilst the effects in pre-clinical models are promising, their efficacy in the clinical setting has also yet to be reported. Finally, adenosine A(3) receptor stimulation has been demonstrated to mediate inhibitory effects on eosinophils since it also elevates cAMP. However, some experimental reports suggest that A(3) antagonists mediate anti-inflammatory effects, thus the rationale for A(3) receptor ligands as therapeutic agents remains to be determined. In conclusion, establishing the precise role of adenosine in the pathogenesis of asthma and developing appropriate subtype selective agonists/antagonists represents an exciting opportunity for the development of novel therapeutics for the treatment of asthma.

Nucleotide- and nucleoside-converting ectoenzymes: Important modulators of purinergic signalling cascade

The involvement of extracellular nucleotides and adenosine in an array of cell-specific responses has long been known and appreciated, but the integrative view of purinergic signalling as a multistep coordinated cascade has emerged recently. Current models of nucleotide turnover include: (i) transient release of nanomolar concentrations of ATP and ADP; (ii) triggering of signalling events via a series of ligand-gated (P2X) and metabotropic (P2Y) receptors; (iii) nucleotide breakdown by membrane-bound and soluble nucleotidases, including the enzymes of ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) family, ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP) family, ecto-5'-nucleotidase/CD73, and alkaline phosphatases; (iv) interaction of the resulting adenosine with own nucleoside-selective receptors; and finally, (v) extracellular adenosine inactivation via adenosine deaminase and purine nucleoside phosphorylase reactions and/or nucleoside uptake by the cells. In contrast to traditional paradigms that focus on purine-inactivating mechanisms, it has now become clear that "classical" intracellular ATP-regenerating enzymes, adenylate kinase, nucleoside diphosphate (NDP) kinase and ATP synthase can also be co-expressed on the cell surface. Furthermore, data on the ability of various cells to retain micromolar ATP levels in their pericellular space, as well as to release other related compounds (adenosine, UTP, dinucleotide polyphosphates and nucleotide sugars) gain another important insight into our understanding of mechanisms regulating a signalling cascade. This review summarizes recent advances in this rapidly evolving field, with particular emphasis on the nucleotide-releasing and purine-converting pathways in the vasculature.

Extracellular purines are biomarkers of neutrophilic airway inflammation

Purinergic signalling regulates airway defence mechanisms, suggesting that extracellular purines could serve as airway inflammation biomarkers in cystic fibrosis (CF). The purines adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP) and adenosine were measured in sputum from 21 adults (spontaneously expectorated from seven CF patients, induced from 14 healthy controls) to assess normal values and CF-associated changes. Subsequently, purine levels were measured in bronchoalveolar lavage fluid (BALF) from 37 children (25 CF patients, 12 disease controls) and compared with neutrophil counts, presence of airway infection and lung function. To noninvasively assess airway purines, ATP levels were measured using luminometry in exhaled breath condensate (EBC) from 14 children with CF and 14 healthy controls, then 14 CF children during a pulmonary exacerbation. Both ATP and AMP were elevated in sputum and BALF from CF subjects compared with controls. In BALF, ATP and AMP levels were inversely related to lung function and strongly correlated with neutrophil counts. In EBC, ATP levels were increased in CF relative to controls and decreased after treatment of CF pulmonary exacerbation. The purines adenosine triphosphate and adenosine monophosphate are candidate biomarkers of neutrophilic airways inflammation. Measurement of purines in sputum or exhaled breath condensate may provide a relatively simple and noninvasive method to track this inflammation.

Physiological and pathophysiological roles of extracellular ATP in chemosensory control of breathing

The purine nucleotide ATP mediates several distinct forms of sensory transduction in both the peripheral and central nervous systems. These processes share common mechanisms that involve the release of ATP to activate ionotropic P2X and/or metabotropic P2Y receptors. Extracellular ATP signalling plays an important role in ventilatory control, mediating both peripheral and central chemosensory transduction to changes in arterial levels of oxygen and carbon dioxide. New data also suggest that extracellular ATP may play an important role in mediating certain neurophysiological responses to systemic inflammation. Here, we propose the novel concept that both peripheral and central neurophysiological effects of ATP may contribute to alterations in ventilatory control during inflammatory pathophysiological states.

Purinergic receptors in gastrointestinal inflammation

Purinergic receptors comprise a family of transmembrane receptors that are activated by extracellular nucleosides and nucleotides. The two major classes of purinergic receptors, P1 and P2, are expressed widely in the gastrointestinal tract as well as immune cells. The purinergic receptors serve a variety of functions from acting as neurotransmitters, to autocoid and paracrine signaling, to cell activation and immune response. Nucleosides and nucleotide agonist of purinergic receptors are released by many cell types in response to specific physiological signals, and their levels are increased during inflammation. In the past decade, the advent of genetic knockout mice and the development of highly potent and selective agonists and antagonists for the purinergic receptors have significantly advanced the understanding of purinergic receptor signaling in health and inflammation. In fact, agonist/antagonists of purinergic receptors are emerging as therapeutic modalities to treat intestinal inflammation. In this article, the distribution of the purinergic receptors in the gastrointestinal tract and their physiological and pathophysiological role in intestinal inflammation will be reviewed.

Modulation of P2X7 receptor expression in macrophages from mineral oil-injected mice

P2X7 receptor activation is involved in a number of pro-inflammatory responses in macrophages and other immune cells. Their expression can be positively modulated with lipopolysaccharide (LPS) and TNFalpha, reinforcing their role during inflammation. We investigated the effect of substances capable of recruiting macrophages into the peritoneal cavity of mice (mineral oil and thioglycolate) on P2X7 receptor expression and function, addressing whether these stimuli can interfere with multinucleated giant cell (MGC) formation, ATP-induced apoptosis, plasma membrane permeabilization and nitric oxide production. It was demonstrated that mineral oil treatment reduces P2X7-dependent MGC formation, whereas thioglycolate treatment does not. Mineral oil treatment reduced P2X7 receptor expression, down-modulating ATP-induced apoptosis, permeabilization and nitric oxide production. In conclusion, mineral oil down modulated P2X7 expression and consequently P2X7-associated phenomena, but thioglycolate did not. These effects might be associated with the unpleasant side effects already described during long-term administration of mineral oil for cosmetic purposes or as a laxative and could be useful in understanding the mechanism of recycling and modulation of P2 receptors present in other situations of immunopathological interest.

Extracellular ATP-induced NO production and its dependence on membrane Ca2+ flux in Salvia miltiorrhiza hairy roots

Extracellular ATP (eATP) is a novel signalling agent, and nitric oxide (NO) is a well-established signal molecule with diverse functions in plant growth and development. This study characterizes NO production induced by exogenous ATP and examines its relationship with other important signalling agents, Ca(2+) and H(2)O(2) in Salvia miltiorrhiza hairy root culture. Exogenous ATP was applied at 10-500 microM to the hairy root cultures and stimulated NO production was detectable within 30 min. The NO level increased with ATP dose from 10-100 microM but decreased from 100-200 muM or higher. The ATP-induced NO production was mimicked by a non-hydrolysable ATP analogue ATPgammaS, but only weakly by ADP, AMP or adenosine. The ATP-induced NO production was blocked by Ca(2+) antagonists, but not affected by a protein kinase inhibitor. ATP also induced H(2)O(2) production, which was dependent on both Ca(2+) and protein kinases, and also on NO biosynthesis. On the other hand, ATP induced a rapid increase in the intracellular Ca(2+) level, which was dependent on NO but not H(2)O(2). The results suggest that NO is implicated in ATP-induced responses and signal transduction in plant cells, and ATP signalling is closely related to Ca(2+) and ROS signalling.