Inhibition of human and mouse plasma membrane bound NTPDases by P2 receptor antagonists

Kinetic properties of E-NTPDase activity in lymphocytes isolated from bone marrow, thymus and mesenteric lymph nodes of Wistar rats

Plasma membrane anchored nucleotidases (E-ATPDases), as the E-NTPDase family, could hydrolyze and regulate the pericellular levels of nucleotides in lymphocytes. Each immune organ has a different microenvironment and display lymphocytes with different functions and phenotypes. Considering the different functions of each resident subpopulations of lymphocytes, the E-ATPDases activities in bone marrow (BML), thymus (TL) and mesenteric lymph node (MLL) lymphocytes of Wistar rats were characterized. The hydrolysis of extracellular nucleotides (eATP and eADP) showed linearity in time of reaction between 0 and 120 min, and concentration of lymphocytes expressed in proteins between 1 and 6 μg protein in the reaction medium. The optimal activity was attained at 37 °C in a pH value of 8.0. The necessity of the cofactors Ca2+ and Mg2+ for the enzymatic activity was confirmed through a curve of concentration of 0-1000 µM in the reaction medium, with both cofactors showing similar effects in the enzymatic activity. The Chevillard plot revealed that the hydrolysis of eATP and eADP occurred at the same active site of the enzyme. The analyses of E-ATPDases inhibitor and enzyme specificity showed possible involvement of E-NTPDase isoforms - 1 and - 2 in the isolated cells. Furthermore, different kinetic behavior of the nucleotide hydrolysis in each resident subpopulation lymphocyte was observed in this study, as MLL showed the higher Vmax with the lowest km values, while TL had the lowest Vmax and high km values. The kinetic values for E-NTPDase activity of each immune tissue lymphocytes can be an important therapeutic target for numeral immune-related diseases.

Suramin: Effectiveness of analogues reveals structural features that are important for the potent trypanocidal activity of the drug

Suramin was the first effective drug for the treatment of human African sleeping sickness. Structural analogues of the trypanocide have previously been shown to be potent inhibitors of several enzymes. Therefore, four suramin analogues lacking the methyl group on the intermediate rings and with different regiochemistry of the naphthalenetrisulphonic acid groups and the phenyl rings were tested to establish whether they exhibited improved antiproliferative activity against bloodstream forms of Trypanosomes brucei compared to the parent compound. The four analogues exhibited low trypanocidal activity and weak inhibition of the antitrypanosomal activity of suramin in competition experiments. This indicates that the strong trypanocidal activity of suramin is most likely due to the presence of methyl groups on its intermediate rings and to the specific regiochemistry of naphthalenetrisulphonic acid groups. These two structural features are also likely to be important for the inhibition mechanism of suramin because DNA distribution and nucleus/kinetoplast configuration analyses suggest that the analogues inhibit mitosis while suramin inhibits cytokinesis.

CD39 - A bright target for cancer immunotherapy

The ATP-adenosine pathway functions as a key modulator of innate and adaptive immunity within the tumor microenvironment, and cancer immune evasion largely involves the generation of high amounts of immunosuppressive extracellular adenosine (eADO). Consequently, inhibition of eADO-generating enzymes and/or eADO receptors can effectively restore the antitumor immunity of multiple immune cells. With several clinical strategies currently being explored to modulating the eADO pathway in patients with cancer, recent clinical data with antagonists targeting CD73 and A_2A receptor have demonstrated a promising therapeutic potential in cancer. Recent findings reveal that the ectonucleotidase CD39, the limiting enzyme been viewed as "immunological switch", converts ATP-driven pro-inflammatory milieu to an anti-inflammatory state mediated by adenosine. Owing to its superior feature of CD39 antagonism that rely not only on preventing the accumulation of adenosine but also on the stabilization of extracellular ATP to restore antitumor immunity, several inhibitors and clinical trials based on CD39 are being evaluated. Consequently, there is currently a focus on understanding the role of CD39 in governing immunity and how therapeutic strategies targeting this pathway alter the antitumor potential. We herein review the impact of CD39 on tumor microenvironment with a focus on treatment preference. Additionally, we also discuss the implication for rational combination therapies, molecular regulation, as well as potential limitations.

NTPDase8 protects mice from intestinal inflammation by limiting P2Y6 receptor activation: identification of a new pathway of inflammation for the potential treatment of IBD

OBJECTIVE

Nucleotides are danger signals that activate inflammatory responses via binding P2 receptors. The nucleoside triphosphate diphosphohydrolase-8 (NTPDase8) is an ectonucleotidase that hydrolyses P2 receptor ligands. We investigated the role of NTPDase8 in intestinal inflammation.

DESIGN

We generated NTPDase8-deficient (Entpd8-/-) mice to define the role of NTPDase8 in the dextran sodium sulfate (DSS) colitis model. To assess inflammation, colons were collected and analysed by histopathology, reverse transcriptase-quantitative real-time PCR (RT-qPCR) and immunohistochemistry. P2 receptor expression was analysed by RT-qPCR on primary intestinal epithelium and NTPDase8 activity by histochemistry. The role of intestinal P2Y6 receptors was assessed by bone marrow transplantation experiments and with a P2Y6 receptor antagonist.

RESULTS

NTPDase8 is the dominant enzyme responsible for the hydrolysis of nucleotides in the lumen of the colon. Compared with wild-type (WT) control mice, the colon of Entpd8-/- mice treated with DSS displayed significantly more histological damage, immune cell infiltration, apoptosis and increased expression of several proinflammatory cytokines. P2Y6 was the dominant P2Y receptor expressed at the mRNA level by the colonic epithelia. Irradiated P2ry6-/- mice transplanted with WT bone marrow were fully protected from DSS-induced intestinal inflammation. In agreement, the daily intrarectal injection of a P2Y6 antagonist protected mice from DSS-induced intestinal inflammation in a dose-dependent manner. Finally, human intestinal epithelial cells express NTPDase8 and P2Y6 similarly as in mice.

CONCLUSION

NTPDase8 protects the intestine from inflammation most probably by limiting the activation of P2Y6 receptors in colonic epithelial cells. This may provide a novel therapeutic strategy for the treatment of inflammatory bowel disease.

E-NTPDases: Possible Roles on Host-Parasite Interactions and Therapeutic Opportunities

Belonging to the GDA1/CD39 protein superfamily, nucleoside triphosphate diphosphohydrolases (NTPDases) catalyze the hydrolysis of ATP and ADP to the monophosphate form (AMP) and inorganic phosphate (Pi). Several NTPDase isoforms have been described in different cells, from pathogenic organisms to animals and plants. Biochemical characterization of nucleotidases/NTPDases has revealed the existence of isoforms with different specificities regarding divalent cations (such as calcium and magnesium) and substrates. In mammals, NTPDases have been implicated in the regulation of thrombosis and inflammation. In parasites, such as Trichomonas vaginalis, Trypanosoma spp., Leishmania spp., Schistosoma spp. and Toxoplasma gondii, NTPDases were found on the surface of the cell, and important processes like growth, infectivity, and virulence seem to depend on their activity. For instance, experimental evidence has indicated that parasite NTPDases can regulate the levels of ATP and Adenosine (Ado) of the host cell, leading to the modulation of the host immune response. In this work, we provide a comprehensive review showing the involvement of the nucleotidases/NTPDases in parasites infectivity and virulence, and how inhibition of NTPDases contributes to parasite clearance and the development of new antiparasitic drugs.

Synthesis, characterization and biological evaluation of thiadiazole amide derivatives as nucleoside triphosphate diphosphohydrolases (NTPDases) inhibitors

Importance of extracellular nucleotides is widely understood. These nucleotides act as ligand for P2X and P2Y receptors and modulate a variety of biological functions. However, their extracellular concentration is maintained by a chain of enzymes termed as ecto-nucleotidases. Amongst them, nucleoside triphosphate diphosphohydrolases (NTPDases) is an important enzyme family responsible for the dephosphorylation of these nucleotides. Overexpression of NTPDases leads to many pathological conditions such as cancer and thrombosis. So far, only a few NTPDase inhibitors have been reported. Considering this scarcity of (NTPDase) inhibitors, a number of thiadiazole amide derivatives were synthesized and screened against human (h)-NTPDases. Several compounds showed promising inhibitory activity; compound 5a (IC₅₀ (µM); 0.05 ± 0.008) and 5g (IC₅₀ (µM); 0.04 ± 0.006) appeared to be the most distinguished molecules corresponding to h-NTPDase1 and -2. However, h-NTPDase3 was the least susceptible isozyme and only three compounds (5d, 5e, 5j) strongly inhibited h-NTPDase3. Interestingly, compound 5e was recognized as the most active compound that showed dual inhibition against h-NTPDase3 as well as against h-NTPDase8. For better comprehension of binding mode of these inhibitors, most potent inhibitors were docked with their respective isozyme.

2-Substituted thienotetrahydropyridine derivatives: Allosteric ectonucleotidase inhibitors

The antithrombotic prodrugs ticlopidine and clopidogrel are thienotetrahydro-pyridine derivatives that are metabolized in the liver to produce thiols that irreversibly block adenosine diphosphate (ADP)-activated P2Y₁₂ receptors on thrombocytes. In their native, nonmetabolized form, both drugs were reported to act as inhibitors of ectonucleoside triphosphate diphosphohydrolase-1 (NTPDase1, CD39). CD39 catalyzes the extracellular hydrolysis of nucleoside tri- and diphosphates, mainly adenosine 5'-triphosphate (ATP) and ADP, yielding adenosine monophosphate, which is further hydrolyzed by ecto-5'-nucleotidase (CD73) to produce adenosine. While ATP has proinflammatory effects, adenosine is a potent anti-inflammatory, immunosuppressive agent. Inhibitors of CD39 and CD73 have potential as novel checkpoint inhibitors for the immunotherapy of cancer and infection. In the present study, we investigated 2-substituted thienotetrahydropyridine derivatives, structurally related to ticlopidine, as CD39 inhibitors. Due to their substituent on the 2-position, they will not be metabolically transformed into reactive thiols and can, therefore, be expected to be devoid of P2Y₁₂ receptor-antagonistic activity in vivo. Several of the investigated 2-substituted thienotetrahydropyridine derivatives showed concentration-dependent inhibition of CD39. The most potent derivative, 32, showed similar CD39-inhibitory potency to ticlopidine, both acting as allosteric inhibitors. Compound 32 showed an improved selectivity profile: While ticlopidine blocked several NTPDase isoenzymes, 32 was characterized as a novel dual inhibitor of CD39 and CD73.

Highly Potent and Selective Ectonucleoside Triphosphate Diphosphohydrolase (ENTPDase1, 2, 3 and 8) Inhibitors Having 2-substituted-7- trifluoromethyl-thiadiazolopyrimidones Scaffold

BACKGROUND

The ecto-nucleoside triphosphate diphosphohydrolases (NTPDases) terminate nucleotide signaling via the hydrolysis of extracellular nucleoside-5'-triphosphate and nucleoside- 5'-diphosphate, to nucleoside-5'-monophosphate and composed of eight Ca2+/Mg2+ dependent ectonucleotidases (NTPDase1-8). Extracellular nucleotides are involved in a variety of physiological mechanisms. However, they are rapidly inactivated by ectonucleotidases that are involved in the sequential removal of phosphate group from nucleotides with the release of inorganic phosphate and their respective nucleoside. Ectonucleoside triphosphate diphosphohydrolases (NTPDases) represent the key enzymes responsible for nucleotides hydrolysis and their overexpression has been related to certain pathological conditions. Therefore, the inhibitors of NTPDases are of particular importance in order to investigate their potential to treat various diseases e.g., cancer, ischemia and other disorders of the cardiovascular and immune system.

METHODS

Keeping in view the importance of NTPDase inhibitors, a series of thiadiazolopyrimidones were evaluated for their potential inhibitory activity towards NTPDases by the malachite green assay.

RESULTS

The results suggested that some of the compounds were found as non-selective inhibitors of isozyme of NTPDases, however, most of the compounds act as potent and selective inhibitors. In case of substituted amino derivatives (4c-m), the compounds 4m (IC50 = 1.13 ± 0.09 μM) and 4g (IC50 = 1.72 ± 0.08 μM) were found to be the most potent inhibitors of h-NTPDase1 and 2, respectively. Whereas, compound 4d showed the best inhibitory potential for both h-NTPDase3 (IC50 = 1.25 ± 0.06 μM) and h-NTPDase8 (0.21 ± 0.02 μM). Among 5a-t derivatives, compounds 5e (IC50 = 2.52 ± 0.15 μM), 5p (IC50 = 3.17 ± 0.05 μM), 5n (IC50 = 1.22 ± 0.06 μM) and 5b (IC50 = 0.35 ± 0.001 μM) were found to be the most potent inhibitors of h-NTPDase1, 2, 3 and 8, respectively. Interestingly, the inhibitory concentration values of above-mentioned inhibitors were several folds greater than suramin, a reference control. In order to determine the binding interactions, molecular docking studies of the most potent inhibitors were conducted into the homology models of NTPDases and the putative binding analysis further confirmed that selective and potent compounds bind deep inside the active pocket of the respective enzymes.

CONCLUSION

The docking analysis proposed that the inhibitory activity correlates with the hydrogen bonds inside the binding pocket. Thus, these derivatives are of interest and may further be investigated for their importance in medicinal chemistry.

Distinct kinetics for nucleotide hydrolysis in lymphocytes isolated from blood, spleen and cervical lymph nodes: Characterization of ectonucleotidase activity

Ectonucleotidases are a plasma membrane-bound enzyme that hydrolyses extracellular adenosine triphosphate (eATP) and adenosine diphosphate (eADP) to adenosine monophosphate (AMP). It regulates normal function of lymphocytes, acts as an inflammatory marker and represents a molecular target for new therapeutics. Thus, this study sought to isolate lymphocytes from blood (BL), spleen (SL) and cervical lymph node (CLL), and characterize the eATP and eADP enzymatic hydrolysis in Wistar rats. The hydrolysis of the nucleotides occurred primarily at pH 8.0, 37°C in the presence of Ca²⁺ or Mg²⁺ . Chevillard-plot showed the hydrolysis of eATP and eADP at the same active site. The inhibitors of some classical ATDPases did not cause any significant change on enzymatic activity. Inhibitors of E-NTPDase (-1, -2, -3 isoforms) and E-NPP-1 decrease the enzyme activity in all resident lymphocytes. Furthermore, kinetic parameters (Vmax and Km) revealed that SL had significantly (P < .001) higher enzymatic activity when compared to BL and CLL. In conclusion, this study standardized kinetic values for eATP and eADP hydrolysis for resident lymphocytes isolated from BL, SL and CLL.

Purinergic receptors in airway hydration

Airway epithelial purinergic receptors control key components of the mucociliary clearance (MCC), the dominant component of pulmonary host defense. In healthy airways, the periciliary liquid (PCL) is optimally hydrated, thus acting as an efficient lubricant layer over which the mucus layer moves by ciliary force. When the hydration of the airway surface decreases, the mucus becomes hyperconcentrated, the PCL collapses, and the "thickened" mucus layer adheres to cell surfaces, causing plaque/plug formation. Mucus accumulation is a major contributing factor to the progression of chronic obstructive lung diseases such as cystic fibrosis (CF) and chronic bronchitis (CB). Mucus hydration is regulated by finely tuned mechanisms of luminal Cl^- secretion and Na⁺ absorption with concomitant osmotically driven water flow. These activities are regulated by airway surface liquid (ASL) concentrations of adenosine and ATP, acting on airway epithelial A_2B and P2Y₂ receptors, respectively. The goal of this article is to provide an overview of our understanding of the role of purinergic receptors in the regulation of airway epithelial ion/fluid transport and the mechanisms of nucleotide release and metabolic activities that contribute to airway surface hydration in healthy and chronically obstructed airways.

Targeting Metabolism of Extracellular Nucleotides via Inhibition of Ectonucleotidases CD73 and CD39

In the tumor microenvironment, unusually high concentrations of extracellular adenosine promote tumor proliferation through various immunosuppressive mechanisms. Blocking adenosine production by inhibiting nucleotide-metabolizing enzymes, such as ectonucleotidases CD73 and CD39, represents a promising therapeutic strategy that may synergize with other immuno-oncology mechanisms and chemotherapies. Emerging small-molecule ectonucleotidase inhibitors have recently entered clinical trials. This Perspective will outline challenges, strategies, and recent advancements in targeting this class with small-molecule inhibitors, including AB680, the first small-molecule CD73 inhibitor to enter clinical development. Specific case studies, including structure-based drug design and lead optimization, will be outlined. Preclinical data on these molecules and their ability to enhance antitumor immunity will be discussed.

ATP-Diphosphohydrolases in Parasites: Localization, Functions and Recent Developments in Drug Discovery

ATP-diphosphohydrolases (EC 3.6.1.5), also known as ATPDases, NTPases, NTPDases, EATPases or apyrases, are enzymes that hydrolyze a variety of nucleoside tri- and diphosphates to their respective nucleosides, being their activities dependent on the presence of divalent cations, such as calcium and magnesium. Recently, ATP-diphosphohydrolases were identified on the surface of several parasites, such as Trypanosoma sp, Leishmania sp and Schistosoma sp. In parasites, the activity of ATPdiphosphohydrolases has been associated with the purine recuperation and/or as a protective mechanism against the host organism under conditions that involve ATP or ADP, such as immune responses and platelet activation. These proteins have been suggested as possible targets for the development of new antiparasitic drugs. In this review, we will comprehensively address the main aspects of the location and function of ATP-diphosphohydrolase in parasites. Also, we performed a detailed research in scientific database of recent developments in new natural and synthetic inhibitors of the ATPdiphosphohydrolases in parasites.

Inhibition of ATP hydrolysis restores airway surface liquid production in cystic fibrosis airway epithelia

Airway surface dehydration is a pathological feature of cystic fibrosis (CF) lung disease. CF is caused by mutations in the CF transmembrane conductance regulator (CFTR), a cyclic AMP-regulated Cl- channel controlled in part by the adenosine A2B receptor. An alternative CFTR-independent mechanism of fluid secretion is regulated by ATP via the P2Y2 receptor (P2Y2R) that activates Ca2+-regulated Cl- channels (CaCC/TMEM16) and inhibits Na+ absorption. However, due to rapid ATP hydrolysis, steady-state ATP levels in CF airway surface liquid (ASL) are inadequate to maintain P2Y2R-mediated fluid secretion. Therefore, inhibiting airway epithelial ecto-ATPases to increase ASL ATP levels constitutes a strategy to restore airway surface hydration in CF. Using [γ32P]ATP as radiotracer, we assessed the effect of a series of ATPase inhibitory compounds on the stability of physiologically occurring ATP concentrations. We identified the polyoxometalate [Co4(H2O)2(PW9O34)2]10- (POM-5) as the most potent and effective ecto-ATPase inhibitor in CF airway epithelial cells. POM-5 caused long-lasting inhibition of ATP hydrolysis in airway epithelia, which was reversible upon removal of the inhibitor. Importantly, POM-5 markedly enhanced steady-state levels of released ATP, promoting increased ASL volume in CF cell surfaces. These results provide proof of concept for ecto-ATPase inhibitors as therapeutic agents to restore hydration of CF airway surfaces. As a test of this notion, cell-free sputum supernatants from CF subjects were studied and found to have abnormally elevated ATPase activity, which was markedly inhibited by POM-5.

Molecular dynamic simulations reveal structural insights into substrate and inhibitor binding modes and functionality of Ecto-Nucleoside Triphosphate Diphosphohydrolases

Ecto-nucleotidase enzymes catalyze the hydrolysis of extracellular nucleotides to their respective nucleosides. Herein, we place the focus on the elucidation of structural features of the cell surface located ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDase1-3 and 8). The physiological role of these isozymes is crucially important as they control purinergic signaling by modulating the extracellular availability of nucleotides. Since, crystal or NMR structure of the human isozymes are not available – structures have been obtained by homology modeling. Refinement of the homology models with poor stereo-chemical quality is of utmost importance in order to derive reliable structures for subsequent studies. Therefore, the resultant models obtained by homology modelling were refined by running molecular dynamic simulation. Binding mode analysis of standard substrates and of competitive inhibitor was conducted to highlight important regions of the active site involved in hydrolysis of the substrates and possible mechanism of inhibition.

POM-1 inhibits P2 receptors and exhibits anti-inflammatory effects in macrophages

Extracellular nucleotides can modulate the immunological response by activating purinergic receptors (P2Rs) on the cell surface of macrophages, dendritic, and other immune cells. In particular, the activation of P2X7R can induce release of cytokines and cell death as well as the uptake of large molecules through the cell membrane by a mechanism still poorly understood. Polyoxotungstate-1 (POM-1) has been proposed as a potent inhibitor of ecto-nucleotidases, enzymes that hydrolyze extracellular nucleotides, regulating the activity of P2Rs. However, the potential impact of POM-1 on P2Rs has not been evaluated. Here, we used fluorescent dye uptake, cytoplasmic free Ca2+ concentration measurement, patch-clamp recordings, scanning electron microscopy, and quantification of inflammatory mediators to investigate the effects of POM-1 on P2Rs of murine macrophages. We observed that POM-1 blocks the P2YR-dependent cytoplasmic Ca2+ increase and has partial effects on the cytoplasmic Ca2+, increasing dependence on P2XRs. POM-1 can inhibit the events related with ATP-dependent inflammasome activation, anionic dye uptake, and also the opening of large conductance channels, which are associated with P2X7R-dependent pannexin-1 activation. On the other hand, this compound has no effects on cationic fluorescent dye uptake, apoptosis, and bleb formation, also dependent on P2X7R. Moreover, POM-1 can be considered an anti-inflammatory compound, because it prevents TNF-α and nitric oxide release from LPS-treated macrophages.

8-BuS-ATP derivatives as specific NTPDase1 inhibitors

BACKGROUND AND PURPOSE

Ectonucleotidases control extracellular nucleotide levels and consequently, their (patho)physiological responses. Among these enzymes, nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), -2, -3 and -8 are the major ectonucleotidases responsible for nucleotide hydrolysis at the cell surface under physiological conditions, and NTPDase1 is predominantly located at the surface of vascular endothelial cells and leukocytes. Efficacious inhibitors of NTPDase1 are required to modulate responses induced by nucleotides in a number of pathological situations such as thrombosis, inflammation and cancer.

EXPERIMENTAL APPROACH

Here, we present the synthesis and enzymatic characterization of five 8-BuS-adenine nucleotide derivatives as potent and selective inhibitors of NTPDase1.

KEY RESULTS

The compounds 8-BuS-AMP, 8-BuS-ADP and 8-BuS-ATP inhibit recombinant human and mouse NTPDase1 by mixed type inhibition, predominantly competitive with Ki values <1 μM. In contrast to 8-BuS-ATP which could be hydrolyzed by other NTPDases, the other BuS derivatives were resistant to hydrolysis by either NTPDase1, -2, -3 or -8. 8-BuS-AMP and 8-BuS-ADP were the most potent and selective inhibitors of NTPDase1 expressed in human umbilical vein endothelial cells as well as in situ in human and mouse tissues. As expected, as a result of their inhibition of recombinant human NTPDase1, 8-BuS-AMP and 8-BuS-ADP impaired the ability of this enzyme to block platelet aggregation. Importantly, neither of these two inhibitors triggered platelet aggregation nor prevented ADP-induced platelet aggregation, in support of their inactivity towards P2Y1 and P2Y12 receptors.

CONCLUSIONS AND IMPLICATIONS

The 8-BuS-AMP and 8-BuS-ADP have therefore potential to serve as drugs for the treatment of pathologies regulated by NTPDase1.

Ticlopidine in its prodrug form is a selective inhibitor of human NTPDase1

Nucleoside triphosphate diphosphohydrolase-1 (NTPDase1), like other ectonucleotidases, controls extracellular nucleotide levels and consequently their (patho)physiological responses such as in thrombosis, inflammation, and cancer. Selective NTPDase1 inhibitors would therefore be very useful. We previously observed that ticlopidine in its prodrug form, which does not affect P2 receptor activity, inhibited the recombinant form of human NTPDase1 (K_i = 14 μM). Here we tested whether ticlopidine can be used as a selective inhibitor of NTPDase1. We confirmed that ticlopidine inhibits NTPDase1 in different forms and in different assays. The ADPase activity of intact HUVEC as well as of COS-7 cells transfected with human NTPDase1 was strongly inhibited by 100 µM ticlopidine, 99 and 86%, respectively. Ticlopidine (100 µM) completely inhibited the ATPase activity of NTPDase1 in situ as shown by enzyme histochemistry with human liver and pancreas sections. Ticlopidine also inhibited the activity of rat and mouse NTPDase1 and of potato apyrase. At 100 µM ticlopidine did not affect the activity of human NTPDase2, NTPDase3, and NTPDase8, nor of NPP1 and NPP3. Weak inhibition (10–20%) of NTPDase3 and -8 was observed at 1 mM ticlopidine. These results show that ticlopidine is a specific inhibitor of NTPDase1 that can be used in enzymatic and histochemistry assays.

Mechanisms regulating airway nucleotides

In the respiratory system, extracellular nucleotides and nucleosides serve as signaling molecules for a wide spectrum of biological functions regulating airway defenses against infection and toxic material. Their concentrations are controlled by a complex network of cell surface enzymes named ectonucleotidases. This highly integrated metabolic network combines the activities of three dephosphorylating ectonucleotidases, namely nucleoside triphosphate diphosphohydrolases (NTPDases), nucleotide pyrophosphatase/phosphodiesterases (NPPs) and alkaline phosphatases (APs). Extracellular nucleotides are also inter-converted by the transphosphorylating activities of ecto adenylate kinase (ectoAK) and nucleoside diphosphokinase (NDPK). Different cell types use specific combinations of ectonucleotidases to regulate local concentrations of P2 receptor agonists (ATP, UTP, ADP and UDP). In addition, they provide AMP for the activity of ecto 5'-nucleotidase (ecto 5'-NT; CD73), which produces the P1 receptor agonist: adenosine (ADO). Finally, mechanisms are in place to prevent the accumulation of airway ADO, namely adenosine deaminases and nucleoside transporters. This chapter reviews the properties of each enzyme and transporter, and the current knowledge on their distribution and regulation in the airways.

Neuromodulation: purinergic signaling in respiratory control

The main functions of the respiratory neural network are to produce a coordinated, efficient, rhythmic motor behavior and maintain homeostatic control over blood oxygen and CO2/pH levels. Purinergic (ATP) signaling features prominently in these homeostatic reflexes. The signaling actions of ATP are produced through its binding to a diversity of ionotropic P2X and metabotropic P2Y receptors. However, its net effect on neuronal and network excitability is determined by the interaction between the three limbs of a complex system comprising the signaling actions of ATP at P2Rs, the distribution of multiple ectonucleotidases that differentially metabolize ATP into ADP, AMP, and adenosine (ADO), and the signaling actions of ATP metabolites, especially ADP at P2YRs and ADO at P1Rs. Understanding the significance of purinergic signaling is further complicated by the fact that neurons, glia, and the vasculature differentially express P2 and P1Rs, and that both neurons and glia release ATP. This article reviews at cellular, synaptic, and network levels, current understanding and emerging concepts about the diverse roles played by this three-part signaling system in: mediating the chemosensitivity of respiratory networks to hypoxia and CO2/pH; modulating the activity of rhythm generating networks and inspiratory motoneurons, and; controlling blood flow through the cerebral vasculature.

Nonhydrolyzable ATP analogues as selective inhibitors of human NPP1: a combined computational/experimental study

Elevated nucleotide pyrophosphatase/phosphodiesterase-1 (NPP1) activity is implicated in health disorders including pathological calcification. Specific NPP1 inhibitors would therefore be valuable for studying this enzyme and as potential therapeutic agents. Here we present a combined computational/experimental study characterizing 13 nonhydrolyzable ATP analogues as selective human NPP1 inhibitors. All analogues at 100 μM inhibited (66-99%) the hydrolysis of pnp-TMP by both recombinant NPP1 and cell surface NPP1 activity of osteocarcinoma (HTB-85) cells. These analogues only slightly altered the activity of other ectonucleotidases, NPP3 and NTPDases. The Ki,app values of the seven most potent and selective inhibitors were in the range of 0.5-56 μM, all with mixed type inhibition, predominantly competitive. Those molecules were docked into a newly developed homology model of human NPP1. All adopted ATP-like binding modes, suggesting competitive inhibition with the endogenous ligand. NPP1 selectivity versus NPP3 could be explained in terms of the electrostatic potential of the two proteins that of NPP1 favoring negatively charged ligands. Inhibitor 2 that had the lowest Ki,app (0.5 μM) was also inactive toward P2Y receptors. Overall, analogue 2 is the most potent and selective NPP1 inhibitor described so far.

Therapeutic potentials of ecto-nucleoside triphosphate diphosphohydrolase, ecto-nucleotide pyrophosphatase/phosphodiesterase, ecto-5'-nucleotidase, and alkaline phosphatase inhibitors

The modulatory role of extracellular nucleotides and adenosine in relevance to purinergic cell signaling mechanisms has long been known and is an object of much research worldwide. These extracellular nucleotides are released by a variety of cell types either innately or as a response to patho-physiological stress or injury. A variety of surface-located ecto-nucleotidases (of four major types; nucleoside triphosphate diphosphohydrolases or NTPDases, nucleotide pyrophosphatase/phosphodiesterases or NPPs, alkaline phosphatases APs or ALPs, and ecto-5'-nucleotidase or e5NT) are responsible for meticulously controlling the availability of these important signaling molecules (at their respective receptors) in extracellular environment and are therefore crucial for maintaining the integrity of normal cell functioning. Overexpression of many of these ubiquitous ecto-enzymes has been implicated in a variety of disorders including cell adhesion, activation, proliferation, apoptosis, and degenerative neurological and immunological responses. Selective inhibition of these ecto-enzymes is an area that is currently being explored with great interest and hopes remain high that development of selective ecto-nucleotidase inhibitors will prove to have many beneficial therapeutic implications. The aim of this review is to emphasize and focus on recent developments made in the field of inhibitors of ecto-nucleotidases and to highlight their structure activity relationships wherever possible. Most recent and significant advances in field of NTPDase, NPP, AP, and e5NT inhibitors is being discussed in detail in anticipation of providing prolific leads and relevant background for research groups interested in synthesis of selective ecto-nucleotidase inhibitors.

Reduced striatal ecto-nucleotidase activity in schizophrenia patients supports the "adenosine hypothesis"

Schizophrenia (SZ) is a major chronic neuropsychiatric disorder characterized by a hyperdopaminergic state. The hypoadenosinergic hypothesis proposes that reduced extracellular adenosine levels contribute to dopamine D2 receptor hyperactivity. ATP, through the action of ecto-nucleotidases, constitutes a main source of extracellular adenosine. In the present study, we examined the activity of ecto-nucleotidases (NTPDases, ecto-5'-nucleotidase, and alkaline phosphatase) in the postmortem putamen of SZ patients (n = 13) compared with aged-matched controls (n = 10). We firstly demonstrated, by means of artificial postmortem delay experiments, that ecto-nucleotidase activity in human brains was stable up to 24 h, indicating the reliability of this tissue for these enzyme determinations. Remarkably, NTPDase-attributable activity (both ATPase and ADPase) was found to be reduced in SZ patients, while ecto-5'-nucleotidase and alkaline phosphatase activity remained unchanged. In the present study, we also describe the localization of these ecto-enzymes in human putamen control samples, showing differential expression in blood vessels, neurons, and glial cells. In conclusion, reduced striatal NTPDase activity may contribute to the pathophysiology of SZ, and it represents a potential mechanism of adenosine signalling impairment in this illness.

Ecto-nucleoside triphosphate diphosphohydrolase 2 modulates local ATP-induced calcium signaling in human HaCaT keratinocytes

Keratinocytes are the major building blocks of the human epidermis. In many physiological and pathophysiological conditions, keratinocytes release adenosine triphosphate (ATP) as an autocrine/paracrine mediator that regulates cell proliferation, differentiation, and migration. ATP receptors have been identified in various epidermal cell types; therefore, extracellular ATP homeostasis likely determines its long-term, trophic effects on skin health. We investigated the possibility that human keratinocytes express surface-located enzymes that modulate ATP concentration, as well as the corresponding receptor activation, in the pericellular microenvironment. We observed that the human keratinocyte cell line HaCaT released ATP and hydrolyzed extracellular ATP. Interestingly, ATP hydrolysis resulted in adenosine diphosphate (ADP) accumulation in the extracellular space. Pharmacological inhibition by ARL 67156 or gene silencing of the endogenous ecto-nucleoside triphosphate diphosphohydrolase (NTPDase) isoform 2 resulted in a 25% reduction in both ATP hydrolysis and ADP formation. Using intracellular calcium as a reporter, we found that although NTPDase2 hydrolyzed ATP and generated sustainable ADP levels, only ATP contributed to increased intracellular calcium via P2Y2 receptor activation. Furthermore, knocking down NTPDase2 potentiated the nanomolar ATP-induced intracellular calcium increase, suggesting that NTPDase2 globally attenuates nucleotide concentration in the pericellular microenvironment as well as locally shields receptors in the vicinity from being activated by extracellular ATP. Our findings reveal an important role of human keratinocyte NTPDase2 in modulating nucleotide signaling in the extracellular milieu of human epidermis.

Inhibition of ecto-ATPase activity by curcumin in hepatocellular carcinoma HepG2 cells

Effects of curcumin, a major constituent of turmeric, on ecto-nucleotidases have not been clarified. Here, we investigated whether curcumin affects ecto-nucleotidase activities in human hepatocellular carcinoma HepG2 cells. In the cells, high levels of Mg(2+)-dependent activity of ecto-nucleotidases were observed in the presence of 1 mM adenosine triphosphate (ATP). The activity was inhibited by ecto-ATPase inhibitors such as suramin, ZnCl(2) and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. On the other hand, the activity was significantly decreased at alkaline pH (pH 9) and was not inhibited by levamisole, an inhibitor of alkaline phosphatase. In the presence of ATP, curcumin inhibited the activity in a concentration-dependent manner (IC(50) = 6.2 μM). In contrast, curcumin had no effects on ecto-nucleotidase activity in the presence of ADP (1 mM) or AMP (1 mM). The K (m) value for ATP hydrolysis of curcumin-sensitive ecto-ATPase was similar to the value of NTPDase2, an isoform of ecto-nucleoside triphosphate diphosphohydrolase. These results suggest that curcumin is a potent inhibitor of ecto-ATPase and may affect extracellular ATP-dependent responses.

Impact of ectoenzymes on p2 and p1 receptor signaling

P2 receptors that are activated by extracellular nucleotides (e.g., ATP, ADP, UTP, UDP, Ap(n)A) and P1 receptors activated by adenosine control a diversity of biological processes. The activation of these receptors is tightly regulated by ectoenzymes that metabolize their ligands. This review presents these enzymes as well as their roles in the regulation of P2 and P1 receptor activation. We focus specifically on the role of ectoenzymes in processes of our interest, that is, inflammation, vascular tone, and neurotransmission. An update on the development of ectonucleotidase inhibitors is also presented.

NTPDase1 controls IL-8 production by human neutrophils

The ectonucleotidase NTPDase1 (CD39) terminates P2 receptor activation by the hydrolysis of extracellular nucleotides (i.e., the P2 receptor ligands). In agreement with that role, exacerbated inflammation has been observed in NTPDase1-deficient mice. In this study, we extend these observations by showing that inhibition of NTPDase1 markedly increases IL-8 production by TLR-stimulated human neutrophils. First, immunolabeling of human blood neutrophils and neutrophil-like HL60 cells displayed the expression of NTPDase1 protein, which correlated with the hydrolysis of ATP at their surface. NTPDase1 inhibitors (e.g., NF279 and ARL 67156) as well as NTPDase1-specific small interfering RNAs markedly increased IL-8 production in neutrophils stimulated with LPS and Pam(3)CSK(4) (agonists of TLR4 and TLR1/2, respectively) but not with flagellin (TLR5) and gardiquimod (TLR7 and 8). This increase in IL-8 release was due to the synergy between TLRs and P2 receptors. Indeed, ATP was released from neutrophils constitutively and accumulated in the medium upon NTPDase1 inhibition by NF279. Likewise, both human blood neutrophils and neutrophil-like HL60 cells produced IL-8 in response to exogenous nucleotides, ATP being the most potent inducer. In agreement, P2Y(2) receptor knockdown in neutrophil-like HL60 cells markedly decreased LPS- and Pam(3)CSK(4)-induced IL-8 production. In line with these in vitro results, injection of LPS in the air pouches of NTPDase1-deficient mice triggered an increased production of the chemokines MIP-2 and keratinocyte-derived chemokine (i.e., the rodent counterparts of human IL-8) compared with that in wild-type mice. In summary, NTPDase1 controls IL-8 production by human neutrophils via the regulation of P2Y(2) activation.

The P2X1 receptor and platelet function

Extracellular nucleotides are ubiquitous signalling molecules, acting via the P2 class of surface receptors. Platelets express three P2 receptor subtypes, ADP-dependent P2Y1 and P2Y12 G-protein-coupled receptors and the ATP-gated P2X1 non-selective cation channel. Platelet P2X1 receptors can generate significant increases in intracellular Ca(2+), leading to shape change, movement of secretory granules and low levels of α(IIb)β(3) integrin activation. P2X1 can also synergise with several other receptors to amplify signalling and functional events in the platelet. In particular, activation of P2X1 receptors by ATP released from dense granules amplifies the aggregation responses to low levels of the major agonists, collagen and thrombin. In vivo studies using transgenic murine models show that P2X1 receptors amplify localised thrombosis following damage of small arteries and arterioles and also contribute to thromboembolism induced by intravenous co-injection of collagen and adrenaline. In vitro, under flow conditions, P2X1 receptors contribute more to aggregate formation on collagen-coated surfaces as the shear rate is increased, which may explain their greater contribution to localised thrombosis in arterioles compared to venules within in vivo models. Since shear increases substantially near sites of stenosis, anti-P2X1 therapy represents a potential means of reducing thrombotic events at atherosclerotic plaques.

Opposite modulation of astroglial proliferation by adenosine 5'-O-(2-thio)-diphosphate and 2-methylthioadenosine-5'-diphosphate: mechanisms involved

The contribution of P2Y(12,13) receptors to astroglial proliferation was investigated by testing the effects of two agonists with high affinity for these receptors, adenosine 5'-O-(2-thio)-diphosphate (ADPβS) and 2-methylthioadenosine-5'-diphosphate (2-MeSADP), in the incorporation of [(3)H]-thymidine. The effect of ATP, an endogenous inducer of astroglial proliferation, was also investigated. ADPβS and ATP (0.01-1 mM) increased astroglial proliferation up to 282%, an effect inhibited by the P2Y(1) receptor antagonist MRS 2179 (30 μM). The P2Y(12) receptor antagonists MRS 2395 (10 μM) and AR-C 66096 (10 μM) also reduced ADPβS proliferative effect, whereas the effect of ATP was attenuated by the A(2A) and A(2B) receptor antagonists SCH 58261 (30 nM) and MRS 1706 (10 nM), respectively. Studies of the signalling pathway activated showed that ADPβS effect was attenuated by pertussis toxin and by inhibition of phopholipase C (PLC), protein kinase C (PKC) and extracellular signal-regulated kinase1/2 (ERK1/2). The effect of ATP was also attenuated by inhibition of protein kinase A (PKA). The agonist 2-MeSADP (0.001-10 μM) had no effect in astroglial proliferation, but at higher concentrations (0.1-1 mM) it inhibited up to 63%, by mechanisms independent of P2Y(1,12,13) receptors activation. It was metabolised into 2-methylthioadenosine (2-MeSADO), the metabolite responsible for inhibition of astroglial proliferation. The effect of 2-MeSADO (0.1 mM) was attenuated by the A(3) receptors antagonist MRS 1523 (10 μM) and by the inhibitor of nucleoside transporters uridine (0.3 mM). 2-MeSADO did not induce apoptosis but increased lactate dehydrogenase release, an indicator of necrotic cell death. Astroglial proliferation induced by ADPβS was mediated by P2Y(1) and P2Y(12) receptors, leading to activation of PLC-PKC-ERK1/2 signalling pathway. The ATP proliferative effect was also mediated by PKA, supporting the contribution of the A(2) receptors. 2-MeSADP inhibition of astroglial proliferation depended on its conversion into 2-MeSADO, which activated A(3) receptors, blocked [(3)H]-thymidine uptake by astrocytes and led to cell death.

Diadenosine 5',5''-(boranated)polyphosphonate analogues as selective nucleotide pyrophosphatase/phosphodiesterase inhibitors

Nucleotide pyrophosphatase/phosphodiesterases (NPPs) hydrolyze extracellular nucleotides and dinucleotides and thus control purinergic signaling. Enhanced NPP activity is implicated in health disorders such as osteoarthritis and cancer. We designed novel diadenosine polyphosphonate derivatives as potential NPP inhibitors. Analogues 1-4 bear a phosphonate and/or boranophosphate group and/or a 2'-H atom instead of a 2'-OH group. In comparison to ATP, analogues 1-4 were barely hydrolyzed by human NTPDase1, -2, -3, and -8 (<5% hydrolysis) and NPP1 and -3 (≤ 13%) and were not hydrolyzed by ecto-5'-nucleotidase, unlike AMP. These derivatives did not affect NTPDase activity, and analogues 1 and 2 did not inhibit ecto-5'-nucleotidase. All analogues blocked ∼80% of the NPP2-dependent hydrolysis of pnp-TMP, a specific NPP substrate, and inhibited the catabolism of pnp-TMP (K(i) and IC₅₀ both found to be between 10 and 60 μM), Ap₅A, and ATP by NPP1. The activity of NPP3 was inhibited to a lesser extent by the new analogues, with compounds 1 and 4 being the most effective in that respect. The analogues dramatically reduced the level of hydrolysis of pnp-TMP at the cell surface of both osteocarcinoma and colon cancer cells. Importantly, analogues 1-4 exhibited significantly reduced agonistic activity toward human P2Y₁,₁₁) receptors (except for analogue 1) and no activity with human P2Y₂ receptor. Our data provide strong evidence that analogue 2 is the first specific NPP inhibitor to be described.

Identification of the ectonucleotidases expressed in mouse, rat, and human Langerhans islets: potential role of NTPDase3 in insulin secretion

Extracellular nucleotides and adenosine regulate endocrine pancreatic functions such as insulin secretion by Langerhans islet β-cells via the activation of specific P2 and P1 receptors. Membrane-bound ectonucleotidases regulate the local concentration of these ligands and consequently control the activation of their receptors. The objective of this study was to identify and localize the major ectonucleotidases, namely NTPDases and ecto-5'-nucleotidase, present in the endocrine pancreas. In addition, the potential implication of ecto-ATPase activity on insulin secretion was investigated in the rat β-cell line INS-1 (832/13). The localization of ectonucleotidase activity and protein was carried out in situ by enzyme histochemistry and immunolocalization in mouse, rat, and human pancreas sections. NTPDase1 was localized in all blood vessels and acini, and NTPDase2 was localized in capillaries of Langerhans islets and in peripheral conjunctive tissue, whereas NTPDase3 was detected in all Langerhans islet cell types. Interestingly, among the mammalian species tested, ecto-5'-nucleotidase was present only in rat Langerhans islet cells, where it was coexpressed with NTPDase3. Notably, the inhibition of NTPDase3 activity by BG0136 and NF279 facilitated insulin release from INS-1 (832/13) cells under conditions of low glycemia, probably by affecting P2 receptor activation. NTPDase3 activity also regulated the inhibitory effect of exogenous ATP in the presence of a high glucose concentration most likely by controlling adenosine production. In conclusion, all pancreatic endocrine cells express NTPDase3 that was shown to modulate insulin secretion in rat INS-1 (832/13) β-cells. Ecto-5'-nucleotidase is expressed in rat Langerhans islet cells but absent in human and mouse endocrine cells.

Activity-dependent release of adenosine: a critical re-evaluation of mechanism

Adenosine is perhaps the most important and universal modulator in the brain. The current consensus is that it is primarily produced in the extracellular space from the breakdown of previously released ATP. It is also accepted that it can be released directly, as adenosine, during pathological events primarily by equilibrative transport. Nevertheless, there is a growing realization that adenosine can be rapidly released from the nervous system in a manner that is dependent upon the activity of neurons. We consider three competing classes of mechanism that could explain neuronal activity dependent adenosine release (exocytosis of ATP followed by extracellular conversion to adenosine; exocytotic release of an unspecified transmitter followed by direct non-exocytotic adenosine release from an interposed cell; and direct exocytotic release of adenosine) and outline discriminatory experimental tests to decide between them. We review several examples of activity dependent adenosine release and explore their underlying mechanisms where these are known. We discuss the limits of current experimental techniques in definitively discriminating between the competing models of release, and identify key areas where technologies need to advance to enable definitive discriminatory tests. Nevertheless, within the current limits, we conclude that there is evidence for a mechanism that strongly resembles direct exocytosis of adenosine underlying at least some examples of neuronal activity dependent adenosine release.

Epitope mapping in cell surface proteins by site-directed masking: defining the structural elements of NTPDase3 inhibition by a monoclonal antibody

We adapted the method of epitope mapping by site-directed masking, which was described for purified soluble antigens [Paus,D. and Winter,G. (2006) Proc. Natl Acad. Sci. USA, 103, 9172-9177.], to map the binding site of an inhibitory monoclonal antibody on the cell surface protein ecto-nucleotidase NTPDase3. Using homology modeling, we built a 3D structure of NTPDase3 and designed 21 single cysteine mutations distributed over the surface of the enzyme. The mutant proteins were expressed in cells, biotinylated with a cysteine-specific reagent, and then extracted with detergent and immobilized on streptavidin-coated plates. Tethering NTPDase3 via cysteine residues located in a surface patch near the active site cleft masked the epitope and blocked antibody binding, as evaluated by enzyme inhibition assay and by ELISA. We then constructed 18 single alanine substitution mutations within the defined patch and found that W403A, D414A, E415A and R419A decreased the inhibitory effect of the antibody, whereas the double mutation W403A/R419A abolished both antibody binding and enzyme inhibition, suggesting the critical role of these residues for interaction with the antibody. Lack of competition between the antibody and a non-hydrolyzable substrate analog AMPPCP, as well as location of the epitope adjacent to the active site, suggest a noncompetitive mechanism of inhibition by steric hindrance. The described technique should be useful for systematic epitope mapping in cell membrane proteins for which either a 3D structure is available, or a sufficiently accurate 3D model can be obtained by homology modeling.

Ectonucleotidases in the kidney

Members of all four families of ectonucleotidases, namely ectonucleoside triphosphate diphosphohydrolases (NTPDases), ectonucleotide pyrophosphatase/phosphodiesterases (NPPs), ecto-5'-nucleotidase and alkaline phosphatases, have been identified in the renal vasculature and/or tubular structures. In rats and mice, NTPDase1, which hydrolyses ATP through to AMP, is prominent throughout most of the renal vasculature and is also present in the thin ascending limb of Henle and medullary collecting duct. NTPDase2 and NTPDase3, which both prefer ATP over ADP as a substrate, are found in most nephron segments beyond the proximal tubule. NPPs catalyse not only the hydrolysis of ATP and ADP, but also of diadenosine polyphosphates. NPP1 has been identified in proximal and distal tubules of the mouse, while NPP3 is expressed in the rat glomerulus and pars recta, but not in more distal segments. Ecto-5'-nucleotidase, which catalyses the conversion of AMP to adenosine, is found in apical membranes of rat proximal convoluted tubule and intercalated cells of the distal nephron, as well as in the peritubular space. Finally, an alkaline phosphatase, which can theoretically catalyse the entire hydrolysis chain from nucleoside triphosphate to nucleoside, has been identified in apical membranes of rat proximal tubules; however, this enzyme exhibits relatively high K (m) values for adenine nucleotides. Although information on renal ectonucleotidases is still incomplete, the enzymes' varied distribution in the vasculature and along the nephron suggests that they can profoundly influence purinoceptor activity through the hydrolysis, and generation, of agonists of the various purinoceptor subtypes. This review provides an update on renal ectonucleotidases and speculates on the functional significance of these enzymes in terms of glomerular and tubular physiology and pathophysiology.

ATP in central respiratory control: a three-part signaling system

The landmark demonstrations in 2005 that ATP released centrally during hypoxia and hypercapnia contributes to the respective ventilatory responses validated a decade-old hypothesis and ignited interest in the potential significance of P2 receptor signaling in central respiratory control. Our objective in this review is to provide a non-specialist overview of ATP signaling from the perspective that it is a three-part system where the net effects are determined by an interaction between the signaling actions of ATP and adenosine at P2 and P1 receptors, respectively, and a family of enzymes (ectonucleotidases) that breakdown ATP into adenosine. We review the rationale for the original interest in P2 signaling in respiratory control, the evolution of this hypothesis, and the mechanisms by which ATP might affect respiratory behaviour. The potential significance of P2 receptor, P1 receptor and ectonucleotidase diversity for the different compartments of the respiratory control system is also considered. We conclude with a look to future questions and technical challenges.

Immunocytochemical localization of NTPDases1 and 2 in the neural retina of mouse and zebrafish

Ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) are a family of membrane-bound enzymes that hydrolyze extracellular di- and triphosphate nucleosides. E-NTPDases have been proposed to control extracellular nucleotide levels that mediate intercellular communication by binding to specific membrane receptors. Here we show a detailed immunocytochemical localization of two enzymes of the E-NTPDase family in the retinal layers of two vertebrate species, namely, the mouse and the zebrafish. In the mouse retina, NTPDase2 was chiefly localized in Müller glia and ganglion cell processes. NTPDase1 was located on neurons as well, since it was expressed by horizontal and ganglion cell processes, suggesting that nucleotides such as ATP and ADP can be hydrolyzed at the surface of these cells. NTPDase1 was also detected in intraretinal blood vessels of the mouse. Regarding zebrafish, NTPDases1 and 2 seem to be differentially localized in horizontal cell processes, photoreceptor segments, and ganglion cell dendrites and axons, but absent from Müller glia. Moreover, NTPDases1 and 2 appear to be expressed within the germinal margin of the zebrafish retina that contains proliferative and differentiating cells. Retinal homogenates from both species exhibited ecto-ATPase activity which might be attributed at least to NTPDases1 and 2, whose expression is described in this report. Our results suggest a compartmentalized regulation of extracellular nucleotide/nucleoside concentration in the retinal layers, supporting a relevant role for extracellular nucleotide mediated-signaling in vertebrate retinas.

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.

Characterization of a monoclonal antibody as the first specific inhibitor of human NTP diphosphohydrolase-3 : partial characterization of the inhibitory epitope and potential applications

The study and therapeutic modulation of purinergic signaling is hindered by a lack of specific inhibitors for NTP diphosphohydrolases (NTPDases),which are the terminating enzymes for these processes. In addition, little is known of the NTPDase protein structural elements that affect enzymatic activity and which could be used as targets for inhibitor design. In the present study, we report the first inhibitory monoclonal antibodies specific for an NTPDase, namely human NTPDase3 (EC 3.6.1.5), as assessed by ELISA, western blotting, flow cytometry, immunohistochemistry and inhibition assays. Antibody recognition of NTPDase3 is greatly attenuated by denaturation with SDS, and abolished by reducing agents, indicating the significance of the native conformation and the disulfide bonds for epitope recognition. Using site-directed chemical cleavage, the SDS-resistant parts of the epitope were located in two fragments of the C-terminal lobe ofNTPDase3 (i.e. Leu220-Cys347 and Cys347-Pro485), which are both required for antibody binding. Additional site-directed mutagenesis revealed the importance of Ser297 and the fifth disulfide bond (Cys399-Cys422) for antibody binding, indicating that the discontinuous inhibitory epitope is located on the extracellular C-terminal lobe of NTPDase3. These antibodies inhibit recombinant NTPDase3 by 60-90%, depending on the conditions. More importantly, they also efficiently inhibit the NTPDase3expressed in insulin secreting human pancreatic islet cells in situ. Because insulin secretion is modulated by extracellular ATP and purinergic receptors, this finding suggests the potential application of these inhibitory antibodies for the study and control of insulin secretion.

Relative contribution of ecto-ATPase and ecto-ATPDase pathways to the biphasic effect of ATP on acetylcholine release from myenteric motoneurons

BACKGROUND AND PURPOSE

The relative contribution of distinct ecto-nucleotidases to the modulation of purinergic signalling may depend on differential tissue distribution and substrate preference.

EXPERIMENTAL APPROACH

Extracellular ATP catabolism (assessed by high-performance liquid chromatography) and its influence on [(3)H]acetylcholine ([(3)H]ACh) release were investigated in the myenteric plexus of rat ileum in vitro.

KEY RESULTS

ATP was primarily metabolized via ecto-ATPDase (adenosine 5'-triphosphate diphosphohydrolase) into AMP, which was then dephosphorylated into adenosine by ecto-5'-nucleotidase. Alternative conversion of ATP into ADP by ecto-ATPase (adenosine 5'-triphosphatase) was more relevant at high ATP concentrations. ATP transiently increased basal [(3)H]ACh outflow in a 2',3'-O-(2,4,6-trinitrophenyl)adenosine-5'-triphosphate (TNP-ATP)-dependent, tetrodotoxin-independent manner. ATP and ATPgammaS (adenosine 5'-[gamma-thio]triphosphate), but not alpha,beta-methyleneATP, decreased [(3)H]ACh release induced by electrical stimulation. ADP and ADPbetaS (adenosine 5'[beta-thio]diphosphate) only decreased evoked [(3)H]ACh release. Inhibition by ADPbetaS was prevented by MRS 2179 (2'-deoxy-N(6)-methyl adenosine 3',5'-diphosphate diammonium salt, a selective P2Y(1) antagonist); blockade of ADP inhibition required co-application of MRS 2179 plus adenosine deaminase (which inactivates endogenous adenosine). Blockade of adenosine A(1) receptors with 1,3-dipropyl-8-cyclopentyl xanthine enhanced ADPbetaS inhibition, indicating that P2Y(1) stimulation is cut short by tonic adenosine A(1) receptor activation. MRS 2179 facilitated evoked [(3)H]ACh release, an effect reversed by the ecto-ATPase inhibitor, ARL67156, which delayed ATP conversion into ADP without affecting adenosine levels.

CONCLUSIONS AND IMPLICATIONS

ATP transiently facilitated [(3)H]ACh release from non-stimulated nerve terminals via prejunctional P2X (probably P2X(2)) receptors. Hydrolysis of ATP directly into AMP by ecto-ATPDase and subsequent formation of adenosine by ecto-5'-nucleotidase reduced [(3)H]ACh release via inhibitory adenosine A(1) receptors. Stimulation of inhibitory P2Y(1) receptors by ADP generated alternatively via ecto-ATPase might be relevant in restraining ACh exocytosis when ATP saturates ecto-ATPDase activity.

Mathematical model of nucleotide regulation on airway epithelia. Implications for airway homeostasis

In the airways, adenine nucleotides support a complex signaling network mediating host defenses. Released by the epithelium into the airway surface liquid (ASL) layer, they regulate mucus clearance through P2 (ATP) receptors, and following surface metabolism through P1 (adenosine; Ado) receptors. The complexity of ASL nucleotide regulation provides an ideal subject for biochemical network modeling. A mathematical model was developed to integrate nucleotide release, the ectoenzymes supporting the dephosphorylation of ATP into Ado, Ado deamination into inosine (Ino), and nucleoside uptake. The model also includes ecto-adenylate kinase activity and feed-forward inhibition of Ado production by ATP and ADP. The parameters were optimized by fitting the model to experimental data for the steady-state and transient concentration profiles generated by adding ATP to polarized primary cultures of human bronchial epithelial (HBE) cells. The model captures major aspects of ATP and Ado regulation, including their >4-fold increase in concentration induced by mechanical stress mimicking normal breathing. The model also confirmed the independence of steady-state nucleotide concentrations on the ASL volume, an important regulator of airway clearance. An interactive approach between simulations and assays revealed that feed-forward inhibition is mediated by selective inhibition of ecto-5'-nucleotidase. Importantly, the model identifies ecto-adenylate kinase as a key regulator of ASL ATP and proposes novel strategies for the treatment of airway diseases characterized by impaired nucleotide-mediated clearance. These new insights into the biochemical processes supporting ASL nucleotide regulation illustrate the potential of this mathematical model for fundamental and clinical research.

Selective nucleoside triphosphate diphosphohydrolase-2 (NTPDase2) inhibitors: nucleotide mimetics derived from uridine-5'-carboxamide

Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases, subtypes 1, 2, 3, 8 of NTPDases) dephosphorylate nucleoside tri- and diphosphates to the corresponding di- and monophosphates. In the present study we synthesized adenine and uracil nucleotide mimetics, in which the phosphate residues were replaced by phosphonic acid esters attached to the nucleoside at the 5'-position by amide linkers. Among the synthesized uridine derivatives, we identified the first potent and selective inhibitors of human NTPDase2. The most potent compound was 19a (PSB-6426), which was a competitive inhibitor of NTPDase2 exhibiting a K i value of 8.2 microM and selectivity versus other NTPDases. It was inactive toward uracil nucleotide-activated P2Y 2, P2Y 4, and P2Y 6 receptor subtypes. Compound 19a was chemically and metabolically highly stable. In contrast to the few known (unselective) NTPDase inhibitors, 19a is an uncharged molecule and may be perorally bioavailable. NTPDase2 inhibitors have potential as novel cardioprotective drugs for the treatment of stroke and for cancer therapy.

Structure-activity relationships of anthraquinone derivatives derived from bromaminic acid as inhibitors of ectonucleoside triphosphate diphosphohydrolases (E-NTPDases)

Reactive blue 2 (RB-2) had been characterized as a relatively potent ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) inhibitor with some selectivity for NTPDase3. In search for the pharmacophore and to analyze structure-activity relationships we synthesized a series of truncated derivatives and analogs of RB-2, including 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinones, 1-amino-2-methyl-4-arylaminoanthraquinones, 1-amino-4-bromoanthraquinone 2-sulfonic acid esters and sulfonamides, and bis-(1-amino-4-bromoanthraquinone) sulfonamides, and investigated them in preparations of rat NTPDase1, 2, and 3 using a capillary electrophoresis assay. Several 1-amino-2-sulfo-4-ar(alk)ylaminoanthraquinone derivatives inhibited E-NTPDases in a concentration-dependent manner. The 2-sulfonate group was found to be required for inhibitory activity, since 2-methyl-substituted derivatives were inactive. 1-Amino-2-sulfo-4-p-chloroanilinoanthraquinone (18) was identified as a nonselective competitive blocker of NTPDases1, 2, and 3 (K_i 16–18 μM), while 1-amino-2-sulfo-4-(2-naphthylamino)anthraquinone (21) was a potent inhibitor with preference for NTPDase1 (K_i 0.328 μM) and NTPDase3 (K_i 2.22 μM). Its isomer, 1-amino-2-sulfo-4-(1-naphthylamino)anthraquinone (20), was a potent and selective inhibitor of rat NTPDase3 (K_i 1.5 μM).

A positive inotropic effect of ATP in the human cardiac atrium

We studied contractile effects in isolated electrically driven (1 Hz) atrial preparations from patients undergoing cardiac bypass surgery. ATP concentration dependently (10, 30, and 100 microM) and rapidly decreased force of contraction (negative inotropic effect, NIE) and thereafter more slowly increased force of contraction. The maximum positive inotropic effect (PIE) at 100 microM ATP amounted to 152% of the predrug value (n = 9) and was stable and could be washed out fast and completely. The PIE did not affect time parameters of contraction (time to peak tension and time of relaxation). Moreover, a similar NIE and PIE were noted with adenosine 5'-O-(2-thiotriphosphate) (100 microM). In contrast 2-methyl-thio-ATP did not exert a NIE but only a PIE. In a second set of experiments, preparations were first incubated for 30 min with purinoreceptor antagonists and, in their continuous presence, 100 microM ATP was applied. However, the PIE and NIE of ATP could neither be blocked with suramin (100 and 500 microM), pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (50 microM), nor reactive blue 2 (30, 100, and 500 microM), which are known blockers for subtypes of P(2) receptors, or 1,3-dipropyl-cyclopentvl-xanthine (1 and 10 microM), a subtype (A(1) adenosine) P(1) receptor blocker. Likewise, the inhibitor of phospholipase C (PLC) activity (U-73122) and the inhibitor of adenylate cyclase activity (SQ-022563) (10 microM each) failed to affect the NIE and the PIE of ATP. We tentatively suggest that the PIE of ATP might be mediated via P(2X4)-like receptors. In summary, we describe a novel biphasic effect of ATP on force contraction in the isolated human atrium. It is conceivable that ATP plays a physiological role in the human heart, for instance, after cardiac injury to sustain contractility.

NTPDase and 5'-nucleotidase activities in physiological and disease conditions: new perspectives for human health

Extracellular nucleotides and nucleosides act as signaling molecules involved in a wide spectrum of biological effects. Their levels are controlled by a complex cell surface-located group of enzymes called ectonucleotidases. There are four major families of ectonucleotidases, nucleoside triphosphate diphosphohydrolases (NTPDases/CD39), ectonucleotide pyrophosphatase/phosphodiesterases (E-NPPs), alkaline phosphatases and ecto-5'-nucleotidase. In the last few years, substantial progress has been made toward the molecular identification of members of the ectonucleotidase families and their enzyme structures and functions. In this review, there is an emphasis on the involvement of NTPDase and 5'-nucleotidase activities in disease processes in several tissues and cell types. Brief background information is given about the general characteristics of these enzymes, followed by a discussion of their roles in thromboregulatory events in diabetes, hypertension, hypercholesterolemia and cancer, as well as in pathological conditions where platelets are less responsive, such as in chronic renal failure. In addition, immunomodulation and cell-cell interactions involving these enzymes are considered, as well as ATP and ADP hydrolysis under different clinical conditions related with alterations in the immune system, such as acute lymphoblastic leukemia (ALL), B-chronic lymphocytic leukemia (B-CLL) and infections associated with human immunodeficiency virus (HIV). Finally, changes in ATP, ADP and AMP hydrolysis induced by inborn errors of metabolism, seizures and epilepsy are discussed in order to highlight the importance of these enzymes in the control of neuronal activity in pathological conditions. Despite advances made toward understanding the molecular structure of ectonucleotidases, much more investigation will be necessary to entirely grasp their role in physiological and pathological conditions.