Ectonucleotidases: Some recent developments and a note on nomenclature

2-MeS-beta,gamma-CCl2-ATP is a potent agent for reducing intraocular pressure

Extracellular nucleotides can modify the production or drainage of the aqueous humor via activation of P2 receptors and therefore affect the intraocular pressure (IOP). We have synthesized slowly hydrolyzable nucleoside di- and triphosphate analogues, 1, and 8-14. Analogues 8-14 were completely resistant to hydrolysis by alkaline phosphatase over 30 min at 37 degrees C. In human blood serum, analogues 8-14 exhibited high stability, e.g., analogues 9 and 10-14 were only 15% and 0% degraded after 24 h, respectively. Moreover, analogues 8-14 were highly stable at pH 1.4 (t(1/2) 1 h-30 days). Analogues 8-14 were agonists of the P2Y(1) receptor (EC(50) 0.57-9.54 muM). Ocular administration of most analogues into rabbits reduced IOP, e.g., analogue 9 reduced IOP by 32% (EC(50) 95.5 nM). Analogue 9 was more effective at reducing IOP than several common glaucoma drugs and represents a promising alternative to timolol maleate, which cannot be used for the treatment of patients suffering from asthma or cardiac problems.

Developmentally regulated expression of ectonucleotidases NTPDase5 and NTPDase6 and UDP-responsive P2Y receptors in the rat cochlea

Ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) regulate complex extracellular P2 receptor signalling pathways in mammalian tissues by hydrolysing extracellular nucleotides to the respective nucleosides. All enzymes from this family (NTPDase1-8) are expressed in the adult rat cochlea. This study reports the changes in expression of NTPDase5 and NTPDase6 in the developing rat cochlea. These two intracellular members of the E-NTPDase family can be released in a soluble form and show preference for nucleoside 5'-diphosphates, such as UDP and GDP. Here, we demonstrate differential spatial and temporal patterns for NTPDase5 and NTPDase6 expression during cochlear development, which are indicative of both cytosolic and extracellular action via pyrimidines. NTPDase5 is noted during the early postnatal period in developing sensory hair cells and supporting Deiters' cells of the organ of Corti, and primary auditory neurons located in the spiral ganglion. In contrast, NTPDase6 is confined to the embryonic and early postnatal hair cell bundles. NTPDase6 immunolocalisation in the developing cochlea underpins its putative role in hair cell bundle development, probably via cytosolic action, whilst NTPDase5 may have a broader extracellular role in the development of sensory and neural tissues in the rat cochlea. Both NTPDase5 and NTPDase6 colocalize with UDP-preferring P2Y(4), P2Y(6) and P2Y(14) receptors during cochlear development, but this strong association was lost in the adult cochlea. Spatiotemporal topographic expression of NTPDase5 and NTPDase6 and P2Y receptors in adult and developing cochlear tissues provide strong support for the role of pyrimidinergic signalling in cochlear development.

Trichomonas vaginalis: dehydroepiandrosterone sulfate and 17beta-estradiol alter NTPDase activity and gene expression

We investigated the effect of dehydroepiandrosterone sulfate (DHEAS) and 17beta-estradiol on NTPDase activity in fresh clinical (VP60) and long-term-grown (30236 ATCC) isolates of Trichomonas vaginalis followed by NTPDase gene transcriptional analysis. ATP hydrolysis was activated in vitro by 17beta-estradiol (0.01-1.0microM) in the VP60 isolate. Treatment for 2h with 17beta-estradiol (0.01-1microM) promoted an inhibition in nucleotide hydrolysis in the 30236 isolate whereas the 12h-treatment promoted an activation of nucleotide hydrolysis in both isolates. ADP hydrolysis was inhibited in vitro by 1.0-5.0microM DHEAS in the ATCC isolate. The treatment with DHEAS (0.01-1.0microM) for 2h inhibited ATP and ADP hydrolysis in VP60; however, during a 12h-treatment with DHEAS, nucleotide hydrolysis was inhibited in both isolates. Two NTPDase orthologous (NTPDaseA and NTPDaseB) were identified and the treatment with DHEAS for 12h was able to inhibit mRNA NTPDaseA transcript levels from the VP60. These findings demonstrate that NTPDase activity and gene expression pattern are modulated by exposure to steroids in T. vaginalis.

Changes in E-NTPDase 3 expression and extracellular nucleotide hydrolysis during the myofibroblast/lipocyte differentiation

Hepatic stellate cells (HSC) play a critical role in the development and maintenance of liver fibrosis. HSC are lipocytes that displayed the capacity to develop into myofibroblast-like cells. Ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) regulate the concentration of extracellular nucleotides, signaling molecules that play a role in the pathogenesis of hepatic fibrosis. In the present study, we identified and compared the expressions of E-NTPDase family members in two different phenotypes of the mouse hepatic stellate cell line (GRX) and evaluated the nucleotide hydrolysis by these cells. We show that both phenotypes of GRX cell line expressed NTPDase 3 and 5. However, only activated cells expressed NTPDase 6. In quiescent-like cells, the hydrolysis of triphosphonucleosides was significantly higher, and was related to an increase in Entpd3 mRNA expression. The diphosphonucleosides were hydrolyzed at a similar rate by two phenotypes of GRX cells. We suggest that up-regulation of Entpd3 mRNA expression modulates the extracellular concentration of nucleotides/nucleosides and affect P2-receptor signaling differently in quiescent-like cells and may play a role in the regulation of HSC functions.

A neuroprotective exercise protocol reduces the adenine nucleotide hydrolysis in hippocampal synaptosomes and serum of rats

Regular and moderate exercise has been considered as an interesting neuroprotective strategy. However, the molecular mechanisms by which physical exercise alters brain function are unclear. Purinergic signaling seems to modulate the pathophysiology of ischemic neuronal damage, since it has been described a neuroprotective activity of adenosine and a dual role of ATP. In the present study, we investigated the effect of daily moderate intensity exercise on ectonucleotidase activities in synaptosomes from hippocampus and the soluble nucleotidases from blood serum of rats. Adult male Wistar rats were assigned to non-exercised (sedentary) group and exercised during 20-min sessions on different programs. The effects of physical activity on hydrolysis of ATP, ADP and AMP were assayed in the synaptosomal fraction obtained from the hippocampus and serum approximately 16 h after the last training session. Our data demonstrated that a neuroprotective exercise protocol, daily 20 min of training in treadmill during 2 weeks, diminished significantly the ADP hydrolysis and there is a trend to reduce the ATP hydrolysis in both hippocampal synaptosomes and blood serum of rats. We suggest that the neuroprotective exercise protocol may modulate nucleotidase activities.

NTPDase family in zebrafish: Nucleotide hydrolysis, molecular identification and gene expression profiles in brain, liver and heart

The nucleoside triphosphate diphosphohydrolase (NTPDase) family cleaves tri- and diphosphonucleosides to monophosphonucleosides and is responsible for terminating purinergic transmission. Since the NTPDase family in zebrafish is poorly understood, here we evaluated the nucleotide hydrolysis in three tissues of adult zebrafish (brain, liver, and heart), confirmed the presence of distinct NTPDase members by a phylogenetic analysis and verified their relative gene expression profiles in the respective tissues. A different profile of ATP and ADP hydrolysis in the brain, liver, and heart as a function of time and protein concentration was observed. Sodium azide (20mM), ARL 67156 (300 microM) and Suramin (300 microM) differently altered the nucleotide hydrolysis in zebrafish tissues, suggesting the contribution of distinct NTPDase activities. Homology-based searches identified the presence of NTPDase1-6 and NTPDase8 orthologs and the phylogeny also grouped three NTPDase2 and two NTPDase5 paralogs. The deduced amino acid sequences share the apyrase conserved regions, conserved cysteine residues, putative N-glycosylation, phosphorylation, N-acetylation sites, and different numbers of transmembrane domains. RT-PCR experiments revealed the existence of a distinct relative entpd1-6 and entpd8 expression profile in brain, liver, and heart. Taken together, these results indicate that several NTPDase members might contribute to a tight regulation of nucleotide hydrolysis in zebrafish tissues.

Homocysteine decreases extracellular nucleotide hydrolysis in rat platelets

Hyperhomocysteinemia is an independent risk factor for atherothrombotic disease. Platelets play an important role in cardiovascular disease and release pro-aggregates mediators when activated, such as ADP, a physiological agonist involved in normal hemostasis and thrombosis. NTPDases and 5'-nucleotidase are ecto-enzymes that hydrolyze ATP, ADP and AMP to adenosine playing an important role on blood flow and thrombogenesis by regulating ADP catabolism. The aim of the present study was evaluate extracellular adenine nucleotide hydrolysis of rat platelets exposed to homocysteine in vitro and in vivo. In vitro homocysteine (Hcy) in the concentration range of 20 to 500 microM caused a significant decrease on ATP (around 30%) and ADP (around 45%) hydrolysis, respectively, while AMP hydrolysis was not altered. Hcy was not able to inhibit the hydrolysis of ATP and ADP catalyzed by purified apyrase at the same concentrations tested in vitro on platelets, suggesting an indirect effect. The inhibitory effect of Hcy on platelets was prevented by antioxidants agents in vitro and in vivo. Furthermore homocysteine treatment increased platelet aggregation induced by ADP. Based on the results presented herein, we propose that inhibition of extracellular ATP and ADP hydrolysis caused by homocysteine was probably due oxidative stress, since antioxidants prevented such effects. These findings may contribute to an increase platelet response to ADP and consequence development of thrombotic risk attributed to hyperhomocysteinemia.

P2X receptors: dawn of the post-structure era

P2X receptors are non-selective cation channels gated by extracellular ATP. They play key roles in various physiological processes such as nerve transmission, pain sensation and the response to inflammation, making them attractive drug targets for the treatment of inflammatory pain. The recent report of the three-dimensional (3D) crystal structure of zebrafish P2X4.1 represents a step change in our understanding of these membrane ion channels, where previously only low-resolution structural data and inferences from indirect structure–function studies were available. The purpose of this review is to place previous work within the context of the new 3D structure, and to summarize the key questions and challenges which await P2X researchers as we move into the post-structure era.

Selective NTPDase2 expression modulates in vivo rat glioma growth

The ectonucleoside triphosphate diphosphohydrolases (E-NTPDases) are a family of ectoenzymes that hydrolyze extracellular nucleotides, thereby modulating purinergic signaling. Gliomas have low expression of all E-NTPDases, particularly NTPDase2, when compared to astrocytes in culture. Nucleotides induce glioma proliferation and ATP, although potentially neurotoxic, does not evoke cytotoxic action on the majority of glioma cultures. We have previously shown that the co-injection of apyrase with gliomas decreases glioma progression. Here, we tested whether selective re-establishment of NTPDase2 expression would affect glioma growth. NTPDase2 overexpression in C6 glioma cells had no effect on in vitro proliferation but dramatically increased tumor growth and malignant characteristics in vivo. Additionally, a sizable platelet sequestration in the tumor area and an increase in CD31 or platelet/endothelial cell adhesion molecule-1 (PECAM-1), vascular endothelial growth factor and OX-42 immunostaining were observed in C6-Enhanced Yellow Fluorescent Protein (EYFP)/NTPDase2-derived gliomas when compared to controls. Treatment with clopidogrel, a P2Y(12) antagonist with anti-platelet properties, decreased these parameters to control levels. These data suggest that the ADP derived from NTPDase2 activity stimulates platelet migration to the tumor area and that NTPDase2, by regulating angiogenesis and inflammation, seems to play an important role in tumor progression. In conclusion, our results point to the involvement of purinergic signaling in glioma progression.

Preservation of cochlear function in Cd39 deficient mice

Signalling actions of extracellular nucleotides via P2 receptors influence cellular function in most tissues. In the inner ear, P2 receptor signaling is involved in many processes including the regulation of hearing sensitivity and the cochlea's response to noise stress. CD39 (NTPDase1/ENTPD1) is an ectonucleotidase (ecto-nucleoside triphosphate diphosphohydrolase) that can hydrolyse purine and pyrimidine nucleoside tri- and di-phosphates to generate monophosphate nucleosides. Mice null for Cd39 exhibit major alterations in haemostasis and profound alterations in inflammatory and thrombotic reactions. Studies in the cochlea have suggested the involvement of purinergic-type signals that could be modulated by CD39 in regulation of cochlear blood flow and also auditory neurotransmission. This study aimed to determine the auditory phenotype of adult Cd39 null mice on the C57BL6 background. Auditory brainstem responses (ABR) and distortion product otoacoustic emissions (DPOAE) were unaffected in Cd39-deficient mice across the range of test frequencies, suggesting normal neural and outer hair cell function. Mutant mice also showed little difference to wild type mice in vulnerability to acoustic trauma. Gene expression analysis of other membrane-bound NTPDases with comparable hydrolytic activity demonstrated an up-regulation of Entpd2 and Entpd8 in the cochleae of Cd39 deficient mice. These findings suggest that Cd39 deletion alone does not adversely affect cochlear function, possibly as compensatory up-regulation of other surface located NTPDases may offset predicted alterations in cochlear homeostasis.

Influence of antidepressant drugs on Ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPPs) from salivary glands of rats

Xerostomia is commonly caused by antidepressant drugs and ATP can influence the saliva production. Adenosine is the product of extracellular hydrolysis of adenine nucleotides in submandibular gland cells, which occurs by the action of ectonucleotidases. In this study, we have evaluated the effect of three different antidepressants in ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP1-3) activities in cultured cells from salivary glands. Rats received imipramine (10mg/ml), fluoxetine (20mg/ml) or moclobemide (30mg/ml) by oral gavage. The drugs were administered once a day for 14 days. Our results have shown that the hydrolysis of p-nitrophenyl-5'-thymidine monophosphate increased in all treatments. These effects were not consequence of transcriptional control of E-NPP1-3 genes. The results reported here can highlight the importance of ectonucleotidases in the most common side effect caused by antidepressant therapy.

Metabolic biomarkers related to energy metabolism in Saudi autistic children

OBJECTIVES

Energy metabolism is usually manipulated in many neurodegenerative diseases. Autism is considered a definable systemic disorder resulting in a number of diverse factors that may affect the brain development and functions both pre and post natal. The increased prevalence of autism will have enormous future public implications and has stimulated intense research into potential etiologic factors. This study aims to establish a connection between autism and the deterioration accompanied it, especially in the brain cognitive areas through a postulation of energy manipulation.

MATERIALS AND METHODS

The biochemical changes in activities of enzymes and pathways that participate in the production of ATP as the most important high-energy compound needed by the human brain were measured in Saudi autistic children. Na(+)/K(+)ATPase, ectonucleotidases (NTPDases) (ADPase and ATPase) and creatine kinase (CK), were assessed in plasma of 30 Saudi autistic patients and compared to 30 age-matching control samples. In addition, adenosine mono, di and trinucleotides (ATP, ADP, and AMP) were measured calorimetrically in the red blood cells of both groups and the adenylate energy charge (AEC) was calculated. Moreover, lactate concentration in plasma of both groups was monitored.

RESULTS

The obtained data recorded 148.77% and 72.35% higher activities of Na(+)/K(+)ATPase and CK respectively in autistic patients which prove the impairment of energy metabolism in these children compared to age and sex matching healthy controls. While ADPase was significantly higher in autistic patients, ATPase were non-significantly elevated compared to control. In spite of the significant increase of Na(+)/K(+)ATPase activity in autistic patients, there was no significant difference in the levels of ATP, ADP, and AMP in both groups and the calculated AEC values were 0.814+/-0.094 and 0.806+/-0.081 for autistic and control groups respectively. The unchanged AEC value in autistic patients was easily correlated with the induced activity of CK and ADPase as two enzymes playing a critical role in the stabilization of AEC. Lactate as an important energy metabolite for the brain was significantly higher in autistic patients compared to control showing about 40% increase.

CONCLUSION

The present study confirmed the impairment of energy metabolism in Saudi autistic patients which could be correlated to the oxidative stress previously recorded in the same investigated samples. The identification of biochemical markers related to autism would be advantageous for earlier clinical diagnosis and intervention.

Differential ectonucleotidase expression in human bladder cancer cell lines

Bladder cancer is the most prevalent tumor in the genitourinary tract. Nucleotides are important molecules that regulate many pathophysiological functions in the extracellular space. Studies have revealed evidence of a relationship between purinergic signaling and urothelial malignancies. Nucleotide-mediated signaling is controlled by a highly efficient enzymatic cascade, which includes the members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDases), ectonucleotide pyrophosphatase/phosphodiesterase (E-NPPs), ecto-alkaline phosphatases, and ecto-5'-nucleotidase/CD73. In an attempt to identify possible differential expression of ectonucleotidases during bladder cancer progression, a comparative analysis between RT4 (grade 1) and T24 (grade 3) bladder cancer cell lines was performed. In RT4 cells, the hydrolysis of tri- and diphosphate nucleosides was higher than monophosphonucleosides. T24 cells, however, presented the opposite profile, a low level of hydrolysis of tri- and diphosphate nucleosides and a high level of hydrolysis of monophosphates. Phosphodiesterase activity was negligible in both cell lines at physiological pH, indicating that these enzymes are not active under our assay conditions, although they are expressed in both cell lines. The T24 cells expressed NTPDase5 mRNA, while the RT4 cells expressed NTPDase3 and NTPDase5 mRNA. Both cell lines expressed ecto-5'-nucleotidase/CD73 mRNA. The present work describes, for the first time, the differential pattern of ectonucleotidases in the more malignant bladder cancer cells compared with cells derived from an early stage of bladder cancer. Our results open new avenues for research into the physiological roles of this family of enzymes and their possible therapeutic potential in bladder cancer.

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.

Endotoxemia alters nucleotide hydrolysis in platelets of rats

Platelets play a critical role in homeostasis and blood clotting at sites of vascular injury, and also in various ways in innate immunity and inflammation. Platelets are one of the first cells to accumulate at an injured site, and local release of their secretome at some point initiate an inflammatory cascade that attracts leukocytes, activates target cells, stimulates vessel growth and repair. The level of exogenous ATP in the body may be increased in various inflammatory and shock conditions, primarily as a consequence of nucleotide release from platelets, endothelium and blood vessel cells. An increase of ATP release has been described during inflammation and this compound presents proinflammatory properties. ADP is a nucleotide known to induce changes in platelets shape and aggregation, to promote the exposure of fibrinogen-binding sites and to inhibit the stimulation of adenylate cyclase. Adenosine, the final product of the nucleotide hydrolysis, is a vasodilator and an inhibitor of platelet aggregation. There is a group of ecto-enzymes responsible for extracellular nucleotide hydrolysis named ectonucleotidases, which includes the NTPDase (nucleoside triphosphate diphosphohydrolase) family, the NPP (nucleoside pyrophosphatase/phosphodiesterase) family and an ecto-5'-nucleotidase. Therefore, we have aimed to investigate the effect of lipopolysaccharide endotoxin from Escherichia coli on ectonucleotidases in platelets from adult rats in order to better understand the role of extracellular adenine nucleotides and nucleosides in the maintenance of blood homeostasis in inflammatory processes. LPS administered in vitro was not able to alter the ATP, ADP, AMP and rho-Nph-5'-TMP hydrolysis of platelets from untreated rats in all concentrations tested (25-100 microg/ml). There was a significant decrease in ATP, ADP, AMP and rho-Nph-5'-TMP hydrolysis in rat platelets after 48 hours of LPS exposure (2 mg/Kg, i.p.). ATP and ADP hydrolysis has been reduced about 28% whereas it has been observed a significant 30% and 26% decrease on AMP and rho-Nph-5'-TMP hydrolysis. Platelet aggregation and platelet number have shown a significant decrease in LPS-treated rats (40% and 55%, respectively) when compared to control group. These results suggest that changes observed in platelet count and, consequently, in nucleotidase activities from circulatory system could alter extracellular nucleotide and nucleoside levels, which might modulate the inflammatory process.

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.

Postictal serum nucleotidases activities in patients with epilepsy

Adenosine, a potent anticonvulsant, can be produced in the body by the hydrolysis of adenine nucleotides through the action of ecto- or soluble nucleotidases. Changes in nucleotide hydrolysis occur after pentylenetetrazol-induced epileptic events. We evaluated serum ATP, ADP and AMP hydrolysis rates and soluble nucleotide phosphodiesterase (PDEase) activity at 5, 10, 15, 30 and 60 min, and 12h following an epileptic event. Fifteen patients (seven female, eight male; mean age 15.5 years) were included in the study. The type of seizure was generalized in four patients and was localization related in the remaining 11. There were no differences in adenine nucleotide hydrolysis rates between patients and healthy subjects in the interictal stage. In comparison with controls, ATP, ADP and AMP hydrolysis rates were significantly increased at 5 min (53+/-1.4%, 79.2+/-2.8% and 37.0+/-2.6%, respectively) and up to 30 min following the epileptic event. In contrast to ADP and AMP, ATP hydrolysis remained significantly increased at 60 min (71.4+/-1.6%), returning to the basal level after 12h. Serum PDEase activity was also significantly higher in the patients than in healthy subjects, peaking at 15 min (61+/-2.9%) and remaining significantly increased up to 60 min (4.6+/-1.2%) following the epileptic episode. Globally, the variations in the postictal serum ADP hydrolysis rate almost overlapped those of AMP hydrolysis, whereas changes in the ATP hydrolysis rate overlapped those of PDEase activity. The clinical significance of this elevation in postictal soluble serum nucleotidase activity remains to be clarified. However, it is possible to hypothesize that the higher nucleotidase activity might play a role in the modulation of epileptic events.

Diminution in adenine nucleotide hydrolysis by platelets and serum from rats submitted to Walker 256 tumour

Extracellular adenine nucleotide hydrolysis in the circulation is mediated by the action of an NTPDase (CD39, apyrase) and of a 5'-nucleotidase (CD73), presenting as a final product, adenosine. Among other properties described for adenine nucleotides, an anti-cancer activity is suggested, since ATP is considered a cytotoxic molecule in several tumour cell systems. Conversely, some studies demonstrate that adenosine presents a tumour-promoting activity. In this study, we evaluated the pattern of adenine nucleotide hydrolysis by serum and platelets from rats submitted to the Walker 256 tumour model. Extracellular adenine nucleotide hydrolysis by blood serum and platelets obtained from rats at, 6, 10 and 15 days after the subcutaneous Walker 256 tumour inoculation, was evaluated. Our results demonstrate a significant reduction in ATP, ADP and AMP hydrolysis in blood serum at 6, 10 and 15 days after tumour induction. In platelets, a significant reduction in ATP and AMP hydrolysis was observed at 10 and 15 days after tumour induction, while an inhibition of ADP hydrolysis was observed at all times studied. Based on these results, it is possible to suggest a physiologic protection mechanism against the tumoral process in circulation. The inhibition in nucleotide hydrolysis observed probably maintains ATP levels elevated (cytotoxic compound) and, at the same time, reduces the adenosine production (tumour-promoting molecule) in the circulation.

Localization of plasma membrane bound NTPDases in the murine reproductive tract

Extracellular nucleotides might influence aspects of the biology of reproduction in that ATP affects smooth muscle contraction, participates in steroidogenesis and spermatogenesis, and also regulates transepithelial transport, as in oviducts. Activation of cellular nucleotide purinergic receptors is influenced by four plasma membrane-bound members of the ectonucleoside triphosphate diphosphohydrolase (E-NTPDase) family, namely NTPDase1, NTPDase2, NTPDase3, and NTPDase8 that differ in their ecto-enzymatic properties. The purpose of this study was to characterize the expression profile of the membrane-bound NTPDases in the murine female and male reproductive tracts by immunological techniques (immunolabelling, Western blotting) and by enzymatic assays, in situ and on tissue homogenates. Other than the expected expression on vascular endothelial and smooth muscle cells, NTPDase1 was also detected in Sertoli cells and interstitial macrophages in testes, in ovarian granulosa cells, and in apical cells from epididymal epithelium. NTPDase2 was largely expressed by cells in the connective tissue; NTPDase3 in secretory epithelia, and finally, NTPDase8 was not detected in any of the tissues studied here. In addition, NTPDase6 was putatively detected in Golgi-phase acrosome vesicles of round spermatids. This descriptive study suggests close regulation of extracellular nucleotide levels in the genital tract by NTPDases that may impact specific biological functions.

Membrane compartments and purinergic signalling: the purinome, a complex interplay among ligands, degrading enzymes, receptors and transporters

Receptors should be properly analysed in view of the microenvironment in which they are embedded. Therefore, the concept of 'receptosome' was formulated to the complex interactions taking place between receptors and other proteins at the plasma membrane level, and to explain very heterogeneous or divergent cellular responses to common epigenetic factors and modifications to the extracellular environment. The receptosome thus becomes a molecular network connecting transmitters, hormones or growth factors, to both their specific receptors and unique downstream effector proteins. As an example of receptosome, we introduce here the 'purinome' as molecular complex responsible for the biological effects of extracellular purine and pyrimidine ligands. In addition to a vast heterogeneity of purinergic ligands, the purinome thus consists of ectonucleotide-metabolizing enzymes hydrolysing nucleoside phosphates, purinergic receptors classified as P1 for adenosine/AMP and P2 for nucleosides tri-/diphosphates, nucleoside transporters with both equilibrative and concentrative properties and finally, nucleotide channels and transporters. Notably, these purinergic elements are not independent, but they play tightly concerted actions under physiological conditions. As a whole and not singularly, they trigger, maintain and terminate the purinergic signalling. This signifies that the purinome is not a new, mere definition of juxtaposed purinergic units, but rather the experimental evidence of complex and dynamic molecular cross-talk and cooperation networks. Alteration of this dynamic equilibrium may even participate in many pathological states. As a consequence, to be successful against pathological conditions, the genetic/pharmacological manipulation of purinergic mechanisms must go well beyond single proteins, and be more holistically oriented.

A comparative study of ectonucleotidase and P2 receptor mRNA profiles in C6 cell line cultures and C6 ex vivo glioma model

Glioblastoma multiforme is the most common type of primary brain tumour and has the worst clinical outcome. Nucleotides represent an important class of extracellular molecules involved in cell proliferation, differentiation and apoptosis. Alterations in purinergic signalling have been implicated in pathological processes, such as cancer, and glioma cell lines are widely employed as a model to study the biology of brain tumours. Increasing evidence, however, suggests that glioma cell lines may not present all the phenotypic and genetic characteristics of the primary tumours. We have compared the biological characteristics of C6 rat glioma cells in culture and the same cells after their implantation in the rat brain and growth in culture (denominated as the C6 ex vivo culture model). Parameters evaluated included cell morphology, differentiation, angiogenic markers, purinergic receptors and ecto-nucleotidase mRNA profile/enzymatic activity. Analysis of the C6 glioma cell line and C6 ex vivo glioma cultures revealed distinct cell morphologies, although cell differentiation and angiogenic marker expressions were similar. Both glioma models co-expressed multiple P2X and P2Y receptor subtypes with some differences. In addition, the C6 glioma cell line and C6 ex vivo glioma cultures exhibited similar extracellular ATP metabolism and cell proliferation behaviour when exposed to cytotoxic ATP concentrations. Thus, the disruption of purinergic signalling is a feature shown not only by glioma cell lineages, but also by primary glioma cultures. Our results therefore suggest the participation of the purinergic system in glioma malignancy.

Trypanosoma brucei brucei: effects of ferrous iron and heme on ecto-nucleoside triphosphate diphosphohydrolase activity

Trypanosoma brucei brucei is the causative agent of animal African trypanosomiasis, also called nagana. Procyclic vector form resides in the midgut of the tsetse fly, which feeds exclusively on blood. Hemoglobin digestion occurs in the midgut resulting in an intense release of free heme. In the present study we show that the magnesium-dependent ecto-nucleoside triphosphate diphosphohydrolase (E-NTPDase) activity of procyclic T. brucei brucei is inhibited by ferrous iron and heme. The inhibition of E-NTPDase activity by ferrous iron, but not by heme, was prevented by pre-incubation of cells with catalase. However, antioxidants that permeate cells, such as PEG-catalase and N-acetyl-cysteine prevented the inhibition of E-NTPDase by heme. Ferrous iron was able to induce an increase in lipid peroxidation, while heme did not. Therefore, both ferrous iron and heme can inhibit E-NTPDase activity of T. brucei brucei by means of formation of reactive oxygen species, but apparently acting through distinct mechanisms.

Antipsychotic drugs inhibit nucleotide hydrolysis in zebrafish (Danio rerio) brain membranes

Haloperidol (HAL), olanzapine (OLZ), and sulpiride (SULP) are antipsychotic drugs widely used in the pharmacotherapy of psychopathological symptoms observed in schizophrenia or mood-related psychotic symptoms in affective disorders. Here, we tested the in vitro effects of different concentrations of a typical (HAL) and two atypical (OLZ and SULP) antipsychotic drugs on ectonucleotidase activities from zebrafish brain membranes. HAL inhibited ATP (28.9%) and ADP (26.5%) hydrolysis only at 250 microM. OLZ decreased ATPase activity at all concentrations tested (23.8-60.7%). SULP did not promote significant changes on ATP hydrolysis but inhibited ADP hydrolysis at 250 microM (25.6%). All drugs tested, HAL, OLZ, and SULP, did not promote any significant changes on 5'-nucleotidase activity in the brain membranes of zebrafish. These findings demonstrated that antipsychotic drugs could inhibit NTPDase activities whereas did not change 5'-nucleotidase. Such modulation can alter the adenosine levels, since the ectonucleotidase pathway is an important source of extracellular adenosine. Thus, it is possible to suggest that changes promoted by antipsychotic drugs in the bilayer membrane could alter the NTPDase activities, modulating extracellular ATP and adenosine levels.

Coordinated regulation of esterification and lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in rat adipocytes

Inhibition of lipolysis by palmitate, H2O2 and the anti-diabetic sulfonylurea drug, glimepiride, in isolated rat adipocytes has previously been shown to rely on the degradation of cyclic adenosine monophosphate by the phosphodiesterase, Gce1, and the 5'-nucleotidase, CD73. These glycosylphosphatidylinositol (GPI)-anchored proteins are translocated from plasma membrane lipid rafts to intracellular lipid droplets upon H2O2-induced activation of a GPI-specific phospholipase C (GPI-PLC) in response to palmitate and glimepiride in intact adipocytes and, as demonstrated here, in cell-free systems as well. The same agents are also known to stimulate the incorporation of fatty acids into triacylglycerol. Here the involvement of H2O2 production, GPI-PLC activation and translocation of Gce1 and CD73 in the agent-induced esterification and accompanying lipid droplet formation was tested in rat adipocytes using relevant inhibitors. The results demonstrate that upregulation of the esterification and accumulation of triacylglycerol by glimepiride depends on the sequential H2O2-induced GPI-PLC activation and GPI-protein translocation as does inhibition of lipolysis. In contrast, stimulation of the esterification and triacylglycerol accumulation by palmitate relies on insulin-independent tyrosine phosphorylation and thus differs from its anti-lipolytic mechanism. As expected, insulin regulates lipid metabolism via typical insulin signalling independent of H2O2 production, GPI-PLC activation and GPI-protein translocation, albeit these processes are moderately stimulated by insulin. In conclusion, triacylglycerol and lipid droplet formation in response to glimepiride and H2O2 may involve the hydrolysis of cyclic adenosine monophosphate by lipid droplet-associated Gce1 and CD73 which may regulate lipid droplet-associated triacylglycerol-synthesizing and hydrolyzing enzymes in coordinated and inverse fashion.

Effect of high glucose levels in human platelet NTPDase and 5'-nucleotidase activities

OBJECTIVES

The objective of this work was to evaluate the effect of different glucose levels on the ATP, ADP and AMP hydrolysis in the platelets of diabetic, hypertensive and diabetic/hypertensive participants.

METHODS

The activities of the enzymes NTPDase (ATP and ADP hydrolysis) and 5'-nucleotidase (AMP hydrolysis), and CD39 expression were analyzed in human blood platelets of diabetic (DM-2), hypertensive (HT) and diabetic/hypertensive (DM-2/HT) patients. To evaluate the interference of glucose and fructose in NTPDase and 5'-nucleotidase activities, experiments were performed with glucose, fructose and mannitol concentrations ranging from 5 to 30 mM in platelet-rich plasma (PRP). Pre-incubation times of 10, 120 min and 24h were used.

RESULTS

NTPDase and 5'-nucleotidase activities increased with increasing glucose and fructose concentrations (P<0.001) and the different times of pre-incubation did not interfere in ectonucleotidases activities (P>0.5). NTPDase and 5'-nucleotidase activities demonstrated a positive correlation between serum glucose levels and ATP and ADP hydrolysis in DM-2 and DM-2/HT patients. CD39 expression demonstrated that DM-2, HT and DM-2/HT groups presented a significant increase when compared to the control group (P<0.004).

CONCLUSION

The hydrolysis of adenine nucleotides is enhanced in platelets of patients with diabetes and hypertension. We observed that an increasing glucose concentration had a direct effect on ATP, ADP and AMP hydrolysis. Furthermore, CD39 expression was enhanced in all patients groups, indicating that these enzyme activities are related with diabetes and hypertension.

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.

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.

Protein cooperation: from neurons to networks

A constant pattern through the development of cellular life is that not only cells but also subcellular components such as proteins, either being enzymes, receptors, signaling or structural proteins, strictly cooperate. Discerning how protein cooperation originated and propagates over evolutionary time, how proteins work together to a shared outcome far beyond mere interaction, thus represents a theoretical and experimental challenge for evolutionary, molecular, and computational biology, and a timely fruition also for biotechnology. In this review, we describe some basic principles sustaining not only cellular but especially protein cooperative behavior, with particular emphasis on neurobiological systems. We illustrate experimental results and numerical models substantiating that bench research, as well as computer analysis, indeed concurs in recognizing the natural propensity of proteins to cooperate. At the cellular level, we exemplify network connectivity in the thalamus, hippocampus and basal ganglia. At the protein level, we depict numerical models about the receptosome, the protein machinery connecting neurotransmitters or growth factors to specific, unique downstream effector proteins. We primarily focus on the purinergic P2/P1 receptor systems for extracellular purine and pyrimidine nucleotides/nucleosides. By spanning concepts such as single-molecule biology to membrane computing, we seek to stimulate a scientific debate on the implications of protein cooperation in neurobiological systems.

Hydrolysis of adenine nucleotides in platelets from patients with acute myocardial infarction

OBJECTIVES

To investigate the rate of ATP, ADP and AMP hydrolysis on the surface of platelets from acute myocardial infarction (AMI) patients.

DESIGN AND METHODS

Twenty-five patients diagnosed with AMI, through clinical criteria, electrocardiographic changes and increase of cardiac biomarkers, as well as 25 healthy patients were selected. The hydrolysis of ATP, ADP and AMP was verified in isolated platelets of these patients.

RESULTS

The results demonstrated that an increase in ATP (54%) and ADP (45%) hydrolysis occurred in AMI patients when compared to the control group. The hydrolysis of AMP also increased by 46% in AMI patients probably leading to an enhancement in the adenosine level.

CONCLUSIONS

Our results suggest an increase in nucleotide hydrolysis in platelets from AMI patients, which could be related to a compensatory organic response to thrombotic events that occur in AMI.

Extracellular ATP and cancer: an overview with special reference to P2 purinergic receptors

Purinergic signal transduction mechanisms have been appreciated as a complex intercellular signalling network that plays an important regulatory role in both short- and long-term processes in practically every living cell. One of the most intriguing aspects of the field is the participation of ATP and other purine nucleotides in the determination of cell fate and the way they direct cells towards proliferation, differentiation or apoptosis, thereby possibly taking part in promoting or preventing malignant transformation. In this review, following a very brief introduction to the historical aspects of purinergic signalling and a concise overview of the structure of and signal transduction pathways coupled to P2 purinergic receptors, the current theories concerning the possible ways how extracellular ATP can alter the function of tumour cells and the effectiveness of anticancer therapies are discussed, including pharmacological, nutritional, vasoactive and 'anti-antioxidant' actions of the nucleotide. The effects of ATP on animals inoculated with human tumours and on patients with cancer are looked over next, and then an overview of the literature regarding the expression and presumed functions of P2 purinoceptors on tumour cells in vitro is presented, sorted out according to the relevant special clinical fields. The article is closed by reviewing the latest developments in the diagnostic use of P2 purinergic receptors as tumour markers and prognostic factors, while discussing some of the difficulties and pitfalls of the therapeutic use of ATP analogues.

Ectonucleotidase activities are altered in serum and platelets of L-NAME-treated rats

It is well known that hypertension is closely associated to the development of vascular diseases and that the inhibition of nitric oxide biosynthesis by administration of Nomega-Nitro-L-arginine methyl ester hydrochloride(L-NAME) leads to arterial hypertension. In the vascular system, extracellular purines mediate several effects;thus, ADP is the most important platelet agonist and recruiting ag ent, while adenosine, an end product of nucleotide metabolism, is a vasodilator and inhibitor of platelet activation and recruitment. Members of several families of enzymes, known as ectonucleotidases, including E-NTPDases (ecto-nucleoside triphosphate diphosphohydrolase), E-NPP (ecto-nucleotide pyrophosphatase/phosphodiesterase) and 5'-nucleotidase are able to hydrolyze extracellular nucleotides until their respective nucleosides. We investigated the ectonucleotidase activities of serum and platelets from rats made hypertensive by oral administration of L-NAME (30 mg/kg/day for 14 days or 30 mg/kg/day for 14 days plus 7 days of L-NAME washout, in the drinking water) in comparison to normotensive control rats. L-NAME promoted a significant rise in systolic blood pressure from 112 +/- 9.8 to 158 +/- 23 mmHg. The left ventricle weight index (LVWI) was increased in rats treated with L-NAME for 14 days when compared to control animals. In serum samples, ATP, ADP and AMP hydrolysis were reduced by about 27%, 36% and 27%, respectively. In platelets, the decrease in ATP, ADP and AMP hydrolysis was approximately 27%, 24% and 32%, respectively. All parameters recovered after 7 days of L-NAME washout. HPLC demonstrated a reduction in ADP, AMP and hypoxanthine levels by about 64%, 69% and 87%,respectively. In this study, we showed that ectonucleotidase activities are decreased in serum and platelets from L-NAME-treated rats, which should represent an additional risk for the development of hypertension. The modulation of ectonucleotidase activities may represent an approach to antihypertensive therapy via inhibition of spontaneous platelet activation and recruitment, as well as thrombus formation.

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).

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.

Receptor webs: can the chunking theory tell us more about it?

Fundamental concepts shared by several classes of ionotropic and metabotropic cell surface receptors, such as receptor mosaic, cooperation, clustering, propensity to oligomerize, all finding expression in the dynamically structured mosaic membrane, will be revisited here in the light of the "combinatorial receptor web model" and the unifying information-processing mechanism defined as "chunking theory". Particularly the ubiquitous and phylogenetically most ancient P2 receptors for extracellular nucleotides will be regarded here as a prototype of receptor family. Whereas up to now we have mainly studied single receptors with the aim to make intelligible their participation to putative functions into wider biological contexts, from now on we should revise our perspective and look more thoroughly at the entire repertoire of expressed cellular receptors, in order to explain complex receptor-function relationships. A way of doing this, is to group the overall receptor web carried by a cell into patterned combinatorial clusters, the "chunks". We deem that the chunk, originally considered an information measure for cognitive systems, from computer science to linguistics, with applications into broad cognitive skills from pianists' finger tapping to chess players' memory retrieval, will rightly become an information measure for receptor webs, thus explaining the numerous receptor subtypes within the same receptor family that are simultaneously expressed on a single cell, as well as the plethora of different, even opposite, biological outputs often triggered by a single ligand. We are confident that the chunking theory will prove to be useful with receptor systems, and it will not be simply a mere speculative exercise.

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.

Effects in vitro of guanidinoacetate on adenine nucleotide hydrolysis and acetylcholinesterase activity in tissues from adult rats

Guanidinoacetate methyltransferase (GAMT) deficiency is a disorder of creatine metabolism characterized by low plasma creatine concentrations in combination with elevated guanidinoacetate (GAA) concentrations. The aim of this work was to investigate the in vitro effect of guanidinoacetate in NTPDase, 5'-nucleotidase and acetylcholinesterase activities in the synaptosomes, platelets and blood of rats. The results showed that in synaptosomes the NTPDase and 5'-nucleotidase activities were inhibited significantly in the presence of GAA at concentrations of 50, 100, 150 and 200 microM (P < 0.05). However, in platelets GAA at the same concentrations caused a significant increase in the activities of these two enzymes (P < 0.05). In relation to the acetylcholinesterase activity, GAA caused a significant inhibition in the activity of this enzyme in blood at concentrations of 150 and 200 microM (P < 0.05), but did not alter the acetylcholinesterase activity in synaptosomes from the cerebral cortex. Our results suggest that alterations caused by GAA in the activities of these enzymes may contribute to the understanding of the neurological dysfunction of GAMT-deficient patients.

Ecto-nucleotidase pathway is altered by different treatments with fluoxetine and nortriptyline

Depression is one of the most disabling diseases and causes a significant burden to both individual and society. Selective serotonin reuptake inhibitors and tricyclic antidepressants, such as fluoxetine and nortriptyline, respectively, are commonly used in treatment for depression. These antidepressants were tested on cerebral cortex and hippocampal synaptosomes after acute and chronic in vivo and in vitro treatments. In chronic treatment, fluoxetine and nortriptyline decreased ATP hydrolysis (17.8% and 16.3%, respectively) in hippocampus. In cerebral cortex, nortriptyline increased ATP (32.3%), ADP (51.8%), and AMP (59.5%) hydrolysis. However, fluoxetine decreased ATP (25.5%) hydrolysis and increased ADP (80.1%) and AMP (33.3%) hydrolysis. Significant activation of ADP hydrolysis was also observed in acute treatment with nortriptyline (49.8%) in cerebral cortex. However, in hippocampus, ATP (24.7%) and ADP (46.1%) hydrolysis were inhibited. Fluoxetine did not alter enzyme activities in acute treatment for both structures. In addition, there were significant changes in NTPDase activities when fluoxetine and nortriptyline (100, 250, and 500 microM) were tested in vitro. There was no inhibitory effect of fluoxetine and nortriptyline on AMP hydrolysis in cerebral cortex and hippocampus. The findings showed that these antidepressant drugs can affect the ecto-nucleotidase pathway, suggesting that the extracellular adenosine levels could be modulated by these drugs.

E-NTPDases and ecto-5'-nucleotidase expression profile in rat heart left ventricle and the extracellular nucleotide hydrolysis by their nerve terminal endings

In this study, we have identified the E-NTPDase family members and ecto-5'-nucleotidase/CD73 in rat heart left ventricle. Moreover, we characterize the biochemical properties and enzyme activities from synaptosomes of the nerve terminal endings of heart left ventricle. We observe divalent cation-dependent enzymes that presented optimum pH of 8.0 for ATP and ADP hydrolysis, and 9.5 for AMP hydrolysis. The apparent K(M) values are 40 microM, 90 microM and 39 microM and apparent V(max) values are 537, 219 and 111 nmol Pi released/min/mg of protein for ATP, ADP and AMP hydrolysis, respectively. Ouabain, orthovanadate, NEM, lanthanum and levamisole do not affect ATP and ADP hydrolysis in rat cardiac synaptosomes. Oligomycin (2 microg/mL) and sodium azide (0.1 mM), both mitochondrial ATPase inhibitors, inhibit only the ATP hydrolysis. High concentrations of sodium azide and gadolinium chloride show an inhibition on both, ATP and ADP hydrolysis. Suramin inhibit more strongly ATP hydrolysis than ADP hydrolysis whereas Evans blue almost abolish both hydrolysis. AMP hydrolysis is not affected by levamisole and tetramisole, whereas 0.1 mM ammonium molybdate practically abolish the ecto-5'-nucleotidase activity. RT-PCR analysis from left ventricle tissue demonstrate different levels of expression of Entpd1 (Cd39), Entpd2 (Cd39L1), Entpd3 (Cd39L3), Entpd5 (Cd39L4) Entpd6, (Cd39L2) and 5'-NT/CD73. By quantitative real-time PCR we identify the Entpd2 as the enzyme with the highest expression in rat left ventricle. Our results contribute to the understanding about the control of the extracellular nucleotide levels in and cardiac system.

Purinergic signaling in the inner ear

Epithelial cells of the inner ear coordinate their ion transport activity through a number of mechanisms. One important mechanism is the autocrine and paracrine signaling among neighboring cells in the ear via nucleotides, such as adenosine, ATP and UTP. This review summarizes observations on the release, detection and degradation of nucleotides by epithelial cells of the inner ear. Purinergic signaling is thought to be important for endolymph ion homeostasis and for protection from acoustic over-stimulation.

Nucleotidase and DNase activities in Brazilian snake venoms

Among the myriad of enzymes present in animal venoms, nucleotidases and nucleases are poorly investigated. Herein, we studied such enzymes in 28 crude venoms of animals found in Brazil. Higher levels of ATPase, 5'-nucleotidase, ADPase, phosphodiesterase and DNase activities were observed in snake venoms belonging to Bothrops, Crotalus and Lachesis genera than to Micrurus genus. The venom of Bothrops brazili snake showed the highest nucleotidase and DNase activities, whereas that of Micrurus frontalis snake the highest alkaline phosphatase activity. On the other hand, the venoms of the snake Philodryas olfersii and the spider Loxosceles gaucho were devoid of most nucleotidase and DNase activities. Species that exhibited similar nucleotidase activities by colorimetric assays showed different banding pattern by zymography, suggesting the occurrence of structural differences among them. Hydrolysis of nucleotides showed that 1 mol of ATP is cleaved in 1 mol of pyrophosphate and 1 mol of orthophosphate, whereas 1 mol of ADP is cleaved exclusively in 2 mol of orthophosphates. Pyrophosphate is barely hydrolyzed by snake venoms. Phosphodiesterase activity was better correlated with 5'-nucleotidase, ADPase and ATPase activities than with DNase activity, evidencing that phosphodiesterases are not the main agent of DNA hydrolysis in animal venoms. The omnipresence of nucleotidase and DNase activities in viperid venoms implies a role for them within the repertoire of enzymes involved in immobilization and death of preys.

Characterization of Rat NTPDase1, -2, and -3 ectodomains refolded from bacterial inclusion bodies

The ecto-nucleoside triphosphate diphosphohydrolases or NTPDases are a family of membrane-bound enzymes that catalyze the sequential removal of gamma- and beta-phosphate from ATP, ADP, and other nucleotides. NTPDase1, -2, -3, and -8 are the enzymes responsible for signal conversion and termination in purinergic signaling. They are anchored to the cytoplasmic membrane by two transmembrane helices with a large catalytic domain pointing toward the extracellular space. Here we report the first successful expression and purification of the soluble extracellular domains of rat NTPDase1, -2, and -3 from bacterial inclusion bodies. The refolded proteins show characteristics similar to the wild type enzymes, for example in that they are dependent on divalent metal ions for catalysis and hydrolyze a wide variety of nucleoside tri- and diphosphates, whereas the monophosphate AMP is not further degraded. Nucleoside triphosphates are hydrolyzed at a higher rate than the corresponding diphosphates. Other characteristics of the recombinant enzymes however reflect the absence of transmembrane regions and side chain glycosylation. For example all three enzymes are monomeric and only subtly activated by Mg2+ ions as compared to Ca2+ ions. Although having a considerably higher specificity constant kcat/Km for ADP as for ATP, the bacterially expressed variant of NTPDase1 in contrast to its wild type counterpart releases intermediate ADP to a substantial amount. The presented expression system will allow large scale production of active protein suitable for structural studies, development of inhibitors, and even clinical application.

ATP and acetylcholine, equal brethren

Acetylcholine was the first neurotransmitter identified and ATP is the hitherto final compound added to the list of small molecule neurotransmitters. Despite the wealth of evidence assigning a signaling role to extracellular ATP and other nucleotides in neural and non-neural tissues, the significance of this signaling pathway was accepted very reluctantly. In view of this, this short commentary contrasts the principal molecular and functional components of the cholinergic signaling pathway with those of ATP and other nucleotides. It highlights pathways of their discovery and analyses tissue distribution, synthesis, uptake, vesicular storage, receptors, release, extracellular hydrolysis as well as pathophysiological significance. There are differences but also striking similarities. Comparable to ACh, ATP is taken up and stored in synaptic vesicles, released in a Ca(2+)-dependent manner, acts on nearby ligand-gated or metabotropic receptors and is hydrolyzed extracellularly. ATP and acetylcholine are also costored and coreleased. In addition, ATP is coreleased from biogenic amine storing nerve terminals as well as from at least subpopulations of glutamatergic and GABAergic terminals. Both ACh and ATP fulfill the criteria postulated for neurotransmitters. More recent evidence reveals that the two messengers are not confined to neural functions, exerting a considerable variety of non-neural functions in non-innervated tissues. While it has long been known that a substantial number of pathologies originate from malfunctions of the cholinergic system there is now ample evidence that numerous pathological conditions have a purinergic component.

H(2)O(2) modulates purinergic-dependent calcium signalling in osteoblast-like cells

Reactive oxygen species (ROS) have long been considered as toxic by-products of aerobic metabolism and appear involved in the pathogenesis of degenerative diseases. The physiological role of ROS as second messengers in cell signal transduction is, on the other hand, increasingly recognized. Here we investigated the effects of H(2)O(2) and extracellular nucleotides on calcium signalling in four osteoblastic cell lines. In the highly differentiated HOBIT cells, sensitive to nanomolar concentrations of ADP and UTP, millimolar H(2)O(2) induced oscillatory increases of the cytosolic calcium concentration followed by a steady and sustained calcium increase. Long lasting rhythmic calcium activity was induced by micromolar H(2)O(2) doses. The H(2)O(2)-induced calcium signals, due to both release from intracellular stores and influx from the extracellular milieu, were totally prevented by incubating the cells with the P2 receptor antagonist suramin or with the ATP/ADP hydrolyzing enzyme apyrase. In the osteosarcoma SaOS-2 cells micromolar H(2)O(2) failed to evoke calcium signals and millimolar H(2)O(2) induced a slowly developing calcium influx which was unaffected by suramin and apyrase. These cells responded to micromolar concentrations of ATP and ADP, but were largely insensitive to UTP. ROS 17/2.8 osteosarcoma cells were totally insensitive to ATP, ADP and UTP in keeping with the evidence that these cells lack functional purinergic receptors. In these cells, H(2)O(2) up to 1mM did not increase the cytosolic calcium concentration. In ROS/P2Y(2) cells, stably expressing the P2Y(2) receptor, spontaneous calcium oscillations were observed in 38% of the population and nanomolar concentration of extracellular ATP or UTP activated oscillations in quiescent cells. Spontaneous calcium signals were inhibited by suramin and apyrase. In these cells H(2)O(2) induced oscillatory calcium activity that was blocked by suramin and apyrase. The sensitivity of ROS/P2Y(2) cells to UTP decreased significantly in the presence of DTT, which was effective also in inhibiting spontaneous calcium oscillations. On the other hand, the membrane-impermeant thiol oxidant DTNB induced calcium oscillations that were inhibited by incubating the cells with suramin or apyrase. Since peroxide did not increase extracellular ATP in these cell lines, we propose that, in osteoblasts, mild oxidative conditions could activate purinergic signalling through the sensitization of P2Y(2) receptor.

Biochemical characterization of ecto-nucleotide pyrophosphatase/phosphodiesterase (E-NPP, E.C. 3.1.4.1) from rat heart left ventricle

In the present study we investigate the biochemical properties of the members of NPP family in synaptosomes prepared from rat heart left ventricles. Using p-nitrophenyl-5'-thymidine monophosphate (p-Nph-5'-TMP) as substrate for E-NPPs in rat cardiac synaptosomes, we observed an alkaline pH dependence, divalent cation dependence and the K ( M ) value corresponded to 91.42 +/- 13.97 microM and the maximal velocity (V ( max )) value calculated was 63.79 +/- 3.59 nmol p-nitrophenol released/min/mg of protein (mean +/- SD, n = 4). Levamisole (1 mM), was ineffective as inhibitor of p-Nph-5'-TMP hydrolysis in pH 8.9 (optimum pH for the enzyme characterized). Suramin (0.25 mM) strongly reduced the hydrolysis of p-Nph-5'-TMP by about 46%. Sodium azide (10 and 20 mM) and gadolinium chloride (0.3 and 0.5 mM), E-NTPases inhibitors, had no effects on p-Nph-5'-TMP hydrolysis. RT-PCR analysis of left ventricle demonstrated the expression of NPP2 and NPP3 enzymes, but excluded the presence of NPP1 member. By quantitative real-time PCR we identified the NPP3 as the enzyme with the highest expression in rat left ventricle. The demonstration of the presence of the E-NPP family in cardiac system, suggest that these enzymes could contribute with the fine-tuning control of the nucleotide levels at the nerve terminal endings of left ventricles that are involved in several cardiac pathologies.

Targeted deletion of ectonucleoside triphosphate diphosphohydrolase 1/CD39 leads to desensitization of pre- and postsynaptic purinergic P2 receptors

We previously reported that ATP coreleased with norepinephrine from cardiac sympathetic nerves activates presynaptic P2X purinoceptors (P2XR), thereby enhancing norepinephrine exocytosis. Blockade of ectonucleoside triphosphate diphosphohydrolase 1 (E-NTPDase1/CD39) potentiates norepinephrine exocytosis, whereas recombinant soluble CD39 (solCD39) in-hibits it. This suggested that CD39 gene (Entpd1) deletion would enhance purinergic and adrenergic signaling by preserving ATP and its norepinephrine-releasing activity. However, we found that the neurogenic contractile response of vasa deferentia from Entpd1-null (CD39(-/-)) mice was attenuated and accompanied by reduced activity of pre- and postsynaptic P2XR, whereas contractile responses to K(+) or norepinephrine remained intact. In addition, the magnitude of ATP and norepinephrine exocytosis from cardiac synaptosomes was decreased in CD39(-/-) mice. Inhibition of E-NTPDase1/CD39, or solCD39 administration, did not affect the attenuated contractile response of vasa deferentia from CD39(-/-) mice. Notably, Entpd1 deletion and pharmacological P2XR desensitization in control mice similarly attenuated vasa deferentia responses. Thus, excessive and prolonged ATP exposure resulting from CD39 deletion desensitizes pre- and postjunctional P2XR at the sympathetic neuromuscular junction. This diminishes purinergic activity directly and adrenergic activity indirectly. It remains to be determined whether this desensitization results from receptor internalization, changes in receptor conformation or phosphorylation. Shutdown of ATP signaling in CD39(-/-) mice may represent a defense mechanism for the prevention of purinergic overstimulation. Our findings emphasize the cardioprotective role of neuronal CD39: by reducing presynaptic facilitatory effects of neurotransmitter ATP, CD39 attenuates norepinephrine release and its dysfunctional consequences. Moreover, by virtue of its antithrombotic action CD39 can potentially prevent the transition from myocardial ischemia to infarction.

Fluoxetine and nortriptyline affect NTPDase and 5′-nucleotidase activities in rat blood serum

Depression is a serious condition associated with considerable morbidity and mortality. Selective serotonin reuptake inhibitors and tricyclic antidepressants, such as fluoxetine and nortriptyline, respectively, were commonly used in treatment for depression. Selective serotonin reuptake inhibitors have been associated with increased risk of bleeding complications, possibly as a result of inhibition of platelet aggregation. ATP, ADP and adenosine are signaling molecules in the vascular system and nucleotidases activities are considered an important thromboregulatory system which functions in the maintenance of blood fluidity. Therefore, here we investigate the effect of in vivo (acute and chronic) and in vitro treatments with the antidepressant drugs on nucleotidases activities in rat blood serum. In acute treatment, nortriptyline decreased ATP hydrolysis (41%), but not altered ADP and AMP hydrolysis. In contrast, fluoxetine did not alter NTPDase and ecto-5'-nucleotidase activities. A significant inhibition of ATP, ADP, and AMP hydrolysis were observed in chronic treatment with fluoxetine (60%, 32%, and 42% for ATP, ADP, and AMP hydrolysis, respectively). Similar effects were shown in chronic treatment with nortriptyline (37%, 41%, and 30% for ATP, ADP, and AMP hydrolysis, respectively). In addition, there were no significant changes in NTPDase and ecto-5'-nucleotidase activities when fluoxetine and nortriptyline (100, 250, and 500 microM) were tested in vitro. Our results have shown that fluoxetine and nortriptyline changed the nucleotide catabolism, suggesting that homeostasis of vascular system can be altered by antidepressant treatments.

Proposal of a guanine-based purinergic system in the mammalian central nervous system

Guanine-based purines have been traditionally studied as modulators of intracellular processes, mainly G-protein activity. However, they also exert several extracellular effects not related to G proteins, including modulation of glutamatergic activity, trophic effects on neural cells, and behavioral effects. In this article, the putative roles of guanine-based purines on the nervous system are reviewed, and we propose a specific guanine-based purinergic system in addition to the well-characterized adenine-based purinergic system. Current evidence suggest that guanine-based purines modulate glutamatergic parameters, such as glutamate uptake by astrocytes and synaptic vesicles, seizures induced by glutamatergic agents, response to ischemia and excitotoxicity, and are able to affect learning, memory and anxiety. Additionally, guanine-based purines have important trophic functions affecting the development, structure, or maintenance of neural cells. Although studies addressing the mechanism of action (receptors and second messenger systems) of guanine-based purines are still insufficient, these findings point to the guanine-based purines (nucleotides and guanosine) as potential new targets for neuroprotection and neuromodulation.

Ectonucleotidases in Müller glial cells of the rodent retina: Involvement in inhibition of osmotic cell swelling

Extracellular nucleotides mediate glia-to-neuron signalling in the retina and are implicated in the volume regulation of retinal glial (Müller) cells under osmotic stress conditions. We investigated the expression and functional role of ectonucleotidases in Müller cells of the rodent retina by cell-swelling experiments, calcium imaging, and immuno- and enzyme histochemistry. The swelling of Müller cells under hypoosmotic stress was inhibited by activation of an autocrine purinergic signalling cascade. This cascade is initiated by exogenous glutamate and involves the consecutive activation of P2Y₁ and adenosine A1 receptors, the action of ectoadenosine 5′-triphosphate (ATP)ases, and a nucleoside-transporter-mediated release of adenosine. Inhibition of ectoapyrases increased the ATP-evoked calcium responses in Müller cell endfeet. Müller cells were immunoreactive for nucleoside triphosphate diphosphohydrolases (NTPDase)2 (but not NTPDase1), ecto-5′-nucleotidase, P2Y₁, and A1 receptors. Enzyme histochemistry revealed that ATP but not adenosine 5′-diphosphate (ADP) is extracellularly metabolised in retinal slices of NTPDase1 knockout mice. NTPDase1 activity and protein is restricted to blood vessels, whereas activity of alkaline phosphatase is essentially absent at physiological pH. The data suggest that NTPDase2 is the major ATP-degrading ectonucleotidase of the retinal parenchyma. NTPDase2 expressed by Müller cells can be implicated in the regulation of purinergic calcium responses and cellular volume.

The effect of aluminium on NTPDase and 5'-nucleotidase activities from rat synaptosomes and platelets

Aluminium (Al), a neurotoxic compound, has been investigated in a large number of studies both in vivo and in vitro. In this study, we investigated the effect in vivo of long-term exposure to Al on NTPDase (nucleoside triphosphate diphosphohydrolase) and 5'-nucleotidase activities in the synaptosomes (obtained from the cerebral cortex and hippocampus) and platelets of rats. Here, we investigated a possible role of platelets as peripheral markers in rats. Rats were loaded by gavage with AlCl(3) 50 mg/(kg day), 5 days per week, totalizing 60 administrations. The animals were divided into four groups: (1) control (C), (2) 50 mg/kg of citrate solution (Ci), (3) 50 mg/kg of Al plus citrate (Al+Ci) solution and (4) 50 mg/kg of Al (Al). ATP hydrolysis was increased in the synaptosomes from the cerebral cortex by 42.9% for Al+Ci and 39.39% for Al, when compared to their respective control (p<0.05). ADP hydrolysis was increased by 13.15% for both Al and Al+Ci, and AMP hydrolysis increased by 32.7% for Al and 27.25% for Al+Ci (p<0.05). In hippocampal synaptosomes, the hydrolysis of ATP, ADP and AMP, was increased by 58.5%, 28.5% and 25.92%, respectively, for Al (p<0.05) and 36.7%, 22.5% and 37.64% for Al+Ci, both when compared to their respective controls. ATP, ADP and AMP hydrolysis, in platelets, was increased by 172.3%, 188.52% and 92.1%, respectively in Al+Ci, and 317.9%, 342.8% and 177.9%, respectively, for Al, when compared to their respective controls (p<0.05). Together, these results indicate that Al increases NTPDase and 5'-nucleotidase activities, in synaptosomal fractions and platelets. Thus, we suggest that platelets could be sensitive peripheral markers of Al toxicity of the central nervous system.