Biochemistry, localization and functional roles of ecto-nucleotidases in the nervous system

Spatiotemporally selective astrocytic ATP dynamics encode injury information sensed by microglia following brain injury in mice

Injuries to the brain result in tunable cell responses paired with stimulus properties, suggesting the existence of intrinsic processes that encode and transmit injury information; however, the molecular mechanism of injury information encoding is unclear. Here, using ATP fluorescent indicators, we identify injury-evoked spatiotemporally selective ATP dynamics, Inflares, in adult mice of both sexes. Inflares are actively released from astrocytes and act as the internal representations of injury. Inflares encode injury intensity and position at their population level through frequency changes and are further decoded by microglia, driving changes in their activation state. Mismatches between Inflares and injury severity lead to microglia dysfunction and worsening of injury outcome. Blocking Inflares in ischemic stroke in mice reduces secondary damage and improves recovery of function. Our results suggest that astrocytic ATP dynamics encode injury information and are sensed by microglia.

Deep Brain Stimulation Inhibits Epileptic Seizures via Increase of Adenosine Release and Inhibition of ENT1, CD39, and CD73 Expression

Deep brain stimulation (DBS) of the anterior nucleus of the thalamus is an efficacious treatment option for patients with refractory epilepsy. Our previous study demonstrates that adenosine is a potential target of DBS for the treatment of epilepsy. Equilibrative nucleoside transporters-1 (ENT1) and ectonucleotidases (CD39, CD73) function as regulators of extracellular adenosine in the brain. It is unclear whether ENT1, CD39, and CD73 are involved in the mechanism of DBS for epilepsy. A total of 48 SD male rats were divided into four groups: control (naïve rats), Pilo (pilocarpine induced rats with epilepsy), DBS (rats with epilepsy treated with DBS for 8 weeks), and sham. In the present study, video electroencephalogram monitoring, Morris water maze assays, in vivo measurements of adenosine using fiber photometry, histochemistry, and western blot were performed on the hippocampus. DBS markedly attenuated spontaneous recurrent seizures (SRSs) and enhanced spatial learning in rats with epilepsy, assessed through video-EEG and water maze assays. Fibred photometry measurements of an adenosine sensor revealed dynamic increase in extracellular adenosine during DBS. The expressions of ENT1, CD39, and CD73 in Pilo group and sham group increased compared with the control group, while the expressions of ENT1, CD39, and CD73 in DBS group decreased compared to that of Pilo group and sham group. The findings indicate that DBS reduces the number of SRSs and improves spatial memory in rats with epilepsy with concomitant decrease of ENT1, CD39, and CD73 expressions. Adenosine-modulating enzymes might be the potential targets of DBS for the treatment of epilepsy.

Caffeic acid attenuates memory dysfunction and restores the altered activity of cholinergic, monoaminergic and purinergic in brain of cadmium chloride exposure rats

OBJECTIVES

This study aims to evaluate the neuroprotective effect of caffeic acid (CAF) against cadmium chloride (CdCl2) in rats via its effect on memory index as well as on altered enzymatic activity in the brain of CdCl2-induced neurotoxicity.

METHODS

The experimental rats were divided into seven groups (n=6 rats per group) of healthy rats (group 1), CdCl2 -induced (CD) (3 mg/kg BW) rats (group 2), CD rats + Vitamin C (group 3), CD rats + CAF (10 and 20 mg/kg BW respectively) (group 4 & 5), and healthy rat + CAF (10 and 20 mg/kg BW respectively) (group 6 & 7). Thereafter, CdCl2 and CAF were administered orally to the experimental rats in group 2 to group 5 on daily basis for 14 days. Then, the Y-maze test was performed on the experimental rats to ascertain their memory index.

RESULTS

CdCl2 administration significantly altered cognitive function, the activity of cholinesterase, monoamine oxidase, arginase, purinergic enzymes, nitric oxide (NOx), and antioxidant status of Cd rats (untreated) when compared with healthy rats. Thereafter, CD rats treated with vitamin C and CAF (10 and 20 mg/kg BW) respectively exhibited an improved cognitive function, and the observed altered activity of cholinesterase, monoamine oxidase, arginase, purinergic were restored when compared with untreated CD rats. Also, the level of brain NOx and antioxidant status were significantly (p<0.05) enhanced when compared with untreated CD rats. In the same vein, CAF administration offers neuro-protective effect in healthy rats vis-à-vis improved cognitive function, reduction in the activity of some enzymes linked to the progression of cognitive dysfunction, and improved antioxidant status when compared to healthy rats devoid of CAF.

CONCLUSIONS

This study demonstrated the neuroprotective effect of CAF against CdCl2 exposure and in healthy rats.

Extracellular ATP Is a Homeostatic Messenger That Mediates Cell-Cell Communication in Physiological Processes and Psychiatric Diseases

Neuronal activity is the basis of information encoding and processing in the brain. During neuronal activation, intracellular ATP (adenosine triphosphate) is generated to meet the high-energy demands. Simultaneously, ATP is secreted, increasing the extracellular ATP concentration and acting as a homeostatic messenger that mediates cell-cell communication to prevent aberrant hyperexcitability of the nervous system. In addition to the confined release and fast synaptic signaling of classic neurotransmitters within synaptic clefts, ATP can be released by all brain cells, diffuses widely, and targets different types of purinergic receptors on neurons and glial cells, making it possible to orchestrate brain neuronal activity and participate in various physiological processes, such as sleep and wakefulness, learning and memory, and feeding. Dysregulation of extracellular ATP leads to a destabilizing effect on the neural network, as found in the etiopathology of many psychiatric diseases, including depression, anxiety, schizophrenia, and autism spectrum disorder. In this review, we summarize advances in the understanding of the mechanisms by which extracellular ATP serves as an intercellular signaling molecule to regulate neural activity, with a focus on how it maintains the homeostasis of neural networks. In particular, we also focus on neural activity issues that result from dysregulation of extracellular ATP and propose that aberrant levels of extracellular ATP may play a role in the etiopathology of some psychiatric diseases, highlighting the potential therapeutic targets of ATP signaling in the treatment of these psychiatric diseases. Finally, we suggest potential avenues to further elucidate the role of extracellular ATP in intercellular communication and psychiatric diseases.

Extracellular Purine Metabolism-Potential Target in Multiple Sclerosis

The purinergic signaling system comprises a complex network of extracellular purines and purine-metabolizing ectoenzymes, nucleotide and nucleoside receptors, ATP release channels, and nucleoside transporters. Because of its immunomodulatory function, this system is critically involved in the pathogenesis of multiple sclerosis (MS) and its best-characterized animal model, experimental autoimmune encephalomyelitis (EAE). MS is a chronic neuroinflammatory demyelinating and neurodegenerative disease with autoimmune etiology and great heterogeneity, mostly affecting young adults and leading to permanent disability. In MS/EAE, alterations were detected in almost all components of the purinergic signaling system in both peripheral immune cells and central nervous system (CNS) glial cells, which play an important role in the pathogenesis of the disease. A decrease in extracellular ATP levels and an increase in its downstream metabolites, particularly adenosine and inosine, were frequently observed at MS, indicating a shift in metabolism toward an anti-inflammatory environment. Accordingly, upregulation of the major ectonucleotidase tandem CD39/CD73 was detected in the blood cells and CNS of relapsing-remitting MS patients. Based on the postulated role of A2A receptors in the transition from acute to chronic neuroinflammation, the association of variants of the adenosine deaminase gene with the severity of MS, and the beneficial effects of inosine treatment in EAE, the adenosinergic system emerged as a promising target in neuroinflammation. More recently, several publications have identified ADP-dependent P2Y12 receptors and the major extracellular ADP producing enzyme nucleoside triphosphate diphosphohydrolase 2 (NTPDase2) as novel potential targets in MS.

Prognostic significance of NT5E/CD73 in neuroblastoma and its function in CSC stemness maintenance

Cluster of differentiation 73 (CD73), a cell surface enzyme that catalyzes adenosine monophosphate (AMP) breakdown to adenosine, is differentially expressed in cancers and has prognostic significance. We investigated its expression profile in neuroblastoma (NB), its association with NB clinical outcomes, and its influence in the regulation of cancer stem cells' (CSCs) stemness maintenance. RNA-Seq data mining (22 independent study cohorts, total n = 3836) indicated that high CD73 can predict good NB prognosis. CD73 expression (immunohistochemistry) gauged in an NB patient cohort (n = 87) showed a positive correlation with longer overall survival (OS, P = 0.0239) and relapse-free survival (RFS, P = 0.0242). Similarly, high CD73 correlated with longer OS and RFS in advanced disease stages, MYCN non-amplified (MYCN-na), and Stage-4-MYCN-na subsets. Despite no definite association in children < 2 years old (2Y), high CD73 correlated with longer OS (P = 0.0294) and RFS (P = 0.0315) in children > 2Y. Consistently, high CD73 was associated with better OS in MYCN-na, high-risk, and stage-4 subsets of children > 2Y. Multivariate analysis identified CD73 as an independent (P = 0.001) prognostic factor for NB. Silencing CD73 in patient-derived (stage 4, progressive disease) CHLA-171 and CHLA-172 cells revealed cell-line-independent activation of 58 CSC stemness maintenance molecules (QPCR profiling). Overexpressing CD73 in CHLA-20 and CHLA-90 cells with low CD73 and silencing in CHLA-171 and CHLA-172 cells with high CD73 showed that CD73 regulates epithelial to mesenchymal transition (E-Cadherin, N-Cadherin, Vimentin), stemness maintenance (Sox2, Nanog, Oct3/4), self-renewal capacity (Notch), and differentiation inhibition (leukemia inhibitory factor, LIF) proteins (confocal-immunofluorescence). These results demonstrate that high CD73 can predict good prognosis in NB, and further suggest that CD73 regulates stemness maintenance in cells that defy clinical therapy.

P2X7 receptor-mediated depression-like reactions arising in the mouse medial prefrontal cortex

Major depressive disorder is a frequent and debilitating psychiatric disease. We have shown in some of the acute animal models of major depressive disorder (tail suspension test and forced swim test) that depression-like behavior can be aggravated in mice by the microinjection into the medial prefrontal cortex of the P2X7R agonistic adenosine 5'-triphosphate or its structural analog dibenzoyl-ATP, and these effects can be reversed by the P2X7R antagonistic JNJ-47965567. When measuring tail suspension test, the prolongation of immobility time by the P2YR agonist adenosine 5'-[β-thio]diphosphate and the reduction of the adenosine 5'-(γ-thio)triphosphate effect by P2Y1R (MRS 2179) or P2Y12R (PSB 0739) antagonists, but not by JNJ-47965567, all suggest the involvement of P2YRs. In order to elucidate the localization of the modulatory P2X7Rs in the brain, we recorded current responses to dibenzoyl-ATP in layer V astrocytes and pyramidal neurons of medial prefrontal cortex brain slices by the whole-cell patch-clamp procedure; the current amplitudes were not altered in preparations taken from tail suspension test or foot shock-treated mice. The release of adenosine 5'-triphosphate was decreased by foot shock, although not by tail suspension test both in the hippocampus and PFC. In conclusion, we suggest, that in the medial prefrontal cortex, acute stressful stimuli cause supersensitivity of P2X7Rs facilitating the learned helplessness reaction.

Increased Synaptic ATP Release and CD73-Mediated Formation of Extracellular Adenosine in the Control of Behavioral and Electrophysiological Modifications Caused by Chronic Stress

Increased ATP release and its extracellular catabolism through CD73 (ecto-5'-nucleotidase) lead to the overactivation of adenosine A_2A receptors (A_2AR), which occurs in different brain disorders. A_2AR blockade blunts mood and memory dysfunction caused by repeated stress, but it is unknown if increased ATP release coupled to CD73-mediated formation of extracellular adenosine is responsible for A_2AR overactivation upon repeated stress. This was now investigated in adult rats subject to repeated stress for 14 consecutive days. Frontocortical and hippocampal synaptosomes from stressed rats displayed an increased release of ATP upon depolarization, coupled to an increased density of vesicular nucleotide transporters and of CD73. The continuous intracerebroventricular delivery of the CD73 inhibitor α,β-methylene ADP (AOPCP, 100 μM) during restraint stress attenuated mood and memory dysfunction. Slice electrophysiological recordings showed that restraint stress decreased long-term potentiation both in prefrontocortical layer II/III-layer V synapses and in hippocampal Schaffer fibers-CA1 pyramid synapses, which was prevented by AOPCP, an effect occluded by adenosine deaminase and by the A_2AR antagonist SCH58261. These results indicate that increased synaptic ATP release coupled to CD73-mediated formation of extracellular adenosine contributes to mood and memory dysfunction triggered by repeated restraint stress. This prompts considering interventions decreasing ATP release and CD73 activity as novel strategies to mitigate the burden of repeated stress.

Adenosine-independent regulation of the sleep-wake cycle by astrocyte activity

Astrocytes play a crucial role in regulating sleep-wake behavior, and adenosine signaling is generally thought to be involved. Here we show multiple lines of evidence supporting that modulation of the sleep-wake behavior by astrocyte Ca²⁺ activity could occur without adenosine signaling. In the basal forebrain and the brainstem, two brain regions that are known to be essential for sleep-wake regulation, chemogenetically-induced astrocyte Ca²⁺ elevation significantly modulated the sleep-wake cycle. Although astrocyte Ca²⁺ level positively correlated with the amount of extracellular adenosine, as revealed by a genetically encoded adenosine sensor, we found no detectable change in adenosine level after suppressing astrocyte Ca²⁺ elevation, and transgenic mice lacking one of the major extracellular ATP-adenosine conversion enzymes showed similar extracellular adenosine level and astrocyte Ca²⁺-induced sleep modulation. Furthermore, astrocyte Ca²⁺ is dependent primarily on local neuronal activity, causing brain region-specific regulation of the sleep-wake cycle. Thus, neural activity-dependent astrocyte activity could regulate the sleep-wake behavior independent of adenosine signaling.

NT5E gene and CD38 protein as potential prognostic biomarkers for childhood B-acute lymphoblastic leukemia

The risk stratification of B-acute lymphoblastic leukemia (B-ALL) is based on clinical and biological factors. However, B-ALL has significant biological and clinical heterogeneity and 50% of B-ALL patients do not have defined prognostic markers. In this sense, the identification of new prognostic biomarkers is necessary. Considering different cohorts of childhood B-ALL patients, gene (DPP4/CD38/ENTPD1/NT5E) and protein (CD38/CD39/CD73) expressions of ectonucleotidases were analyzed in silico and ex vivo and the association with prognosis was established. In univariate analyses, expression of NT5E was significantly associated with worse progression-free survival (PFS) in bone marrow (BM) samples. In multivariate analyses, Kaplan-Meier analysis, and log-rank test, higher NT5E expression predicted unfavorable PFS in BM samples. Considering minimal residual disease (MRD), higher levels of cellularity were associated with the high NT5E expression at day 8 of induction therapy. In addition, we observed that white blood cells (WBC) of childhood B-ALL patients had more CD38 compared to the same cell population of healthy donors (HD). In fact, MRD > 0.1% patients had higher CD38 protein expression on WBC in comparison to HD. Noteworthy, we observed higher CD38 expression on WBC than blasts in MRD > 0.1% patients. We suggest that NT5E gene and CD38 protein expression, of the ectonucleotidases family, could provide interesting prognostic biomarkers for childhood B-ALL.

Ectonucleotidases in the hippocampus: Spatial distribution and expression after ovariectomy and estradiol replacement

Extracellular purine nucleotides, such as adenosine 5'-triphosphate (ATP), are important modulators of hippocampal function and plasticity. In the extracellular space, ATP is inherently short-lived molecule, which undergoes rapid enzymatic degradation to adenosine by ectonucleotidases. Given that ectonucleotidases have distinct and overlapping distribution in the hippocampus, and as ovarian hormones participate in a formation, maturation, and a refinement of synaptic contacts, both during development and in adulthood, the present chapter summarizes known data about spatial distribution of selected ecto-enzymes and estradiol-induced effects on ectonucleotidases in the rat hippocampus.

Ectonucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase in purinergic signaling: how the field developed and where we are now

Geoffrey Burnstock will be remembered as the scientist who set up an entirely new field of intercellular communication, signaling via nucleotides. The signaling cascades involved in purinergic signaling include intracellular storage of nucleotides, nucleotide release, extracellular hydrolysis, and the effect of the released compounds or their hydrolysis products on target tissues via specific receptor systems. In this context ectonucleotidases play several roles. They inactivate released and physiologically active nucleotides, produce physiologically active hydrolysis products, and facilitate nucleoside recycling. This review briefly highlights the development of our knowledge of two types of enzymes involved in extracellular nucleotide hydrolysis and thus purinergic signaling, the ectonucleoside triphosphate diphosphohydrolases, and ecto-5'-nucleotidase.

P2X7 Receptor Agonist 2'(3')-O-(4-Benzoylbenzoyl)ATP Differently Modulates Cell Viability and Corticostriatal Synaptic Transmission in Experimental Models of Huntington's Disease

Huntington’s disease (HD) is a life-threatening neurodegenerative disorder. Altered levels and functions of the purinergic ionotropic P2X7 receptors (P2X7Rs) have been found in animal and cellular models of HD, suggesting their possible role in the pathogenesis of the disease; accordingly, the therapeutic potential of P2X7R antagonists in HD has been proposed. Here we further investigated the effects of P2X7R ligands in in vitro and ex vivo HD experimental models. In ST14A/Q120 rat striatal cells, we found a reduction of P2X7R expression; however, the P2X7R agonist 2′(3′)-O-(4-benzoylbenzoyl)adenosine-5′-triphosphate (BzATP) induced cellular death, and this effect was fully reversed by the antagonist periodate-oxidized adenosine 5′-triphosphate (OxATP). Moreover, in corticostriatal slices from symptomatic R6/2 mice, BzATP reduced the synaptic transmission to a larger extent than in wild-type (WT) mice. Such an effect was accompanied by a concomitant increase of the paired-pulse ratio, suggesting a presynaptic inhibitory action. This was confirmed to be the case, since while the effects of BzATP were unaffected by the P2X7R antagonist OxATP, they were blocked by the adenosine A₁ receptor (A₁R) antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), suggesting possible BzATP hydrolysis to 2′(3′)-O-(4-benzoylbenzoyl)adenosine (Bz-adenosine) and consequent activation of A₁Rs as a mechanism. Taken together, these data point out that 1) P2X7R expression and activity are confirmed to be altered in the presence of HD mutation; 2) in some experimental settings, such an abnormal functioning can be ascribed to presynaptic A₁Rs activation.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

Influence of purinergic signaling on glucose transporters: A possible mechanism against insulin resistance?

Metabolic disorders, such as insulin resistance, affect many people worldwide due to the prevalence of obesity and type 2 diabetes, which are pathologies that impair glycemic metabolism. Glucose is the primary energetic substrate of the body and is essential for cellular function. As the cell membrane is not permeable to glucose molecules, there are two distinct groups of glucose transporters: sodium-glucose-linked transporters (SGLTs) and the glucose transporter (GLUT) family. These transporters facilitate the entry of glucose into the bloodstream or cytoplasm where it functions in the production of adenosine 5 ́-triphosphate (ATP). This nucleotide acts in several cellular mechanisms, such as protein phosphorylation and cellular immune processes. ATP directly and indirectly acts as an agonist for purinergic receptors in high concentrations in the extracellular environment. Composed by P1 and P2 groups, the purinoreceptors cover several cellular mechanisms involving cytokines, tumors, and metabolic signaling pathways. Previous publications have indicated that the purinergic signaling activity in insulin resistance and glucose transporters modulates relevant actions on the deregulations that can affect glycemic homeostasis. Thus, this review focuses on the pharmacological influence of purinergic signaling on the modulation of glucose transporters, aiming for a new way to combat insulin resistance and other metabolic disorders.

History of ectonucleotidases and their role in purinergic signaling

Ectonucleotidases are key for purinergic signaling. They control the duration of activity of purinergic receptor agonists. At the same time, they produce hydrolysis products as additional ligands of purinergic receptors. Due to the considerable diversity of enzymes, purinergic receptor ligands and purinergic receptors, deciphering the impact of extracellular purinergic receptor control has become a challenge. The first group of enzymes described were the alkaline phosphatases - at the time not as nucleotide-metabolizing but as nonspecific phosphatases. Enzymes now referred to as nucleoside triphosphate diphosphohydrolases and ecto-5'-nucleotidase were the first and only nucleotide-specific ectonucleotidases identified. And they were the first group of enzymes related to purinergic signaling. Additional research brought to light a surprising number of ectoenzymes with broad substrate specificity, which can also hydrolyze nucleotides. This short overview traces the development of the field and briefly highlights important results and benefits for therapies of human diseases achieved within nearly a century of investigations.

Real-time measurement of adenosine and ATP release in the central nervous system

This brief review recounts how, stimulated by the work of Geoff Burnstock, I developed biosensors that allowed direct real-time measurement of ATP and adenosine during neural function. The initial impetus to create an adenosine biosensor came from trying to understand how ATP and adenosine-modulated motor pattern generation in the frog embryo spinal cord. Early biosensor measurements demonstrated slow accumulation of adenosine during motor activity. Subsequent application of these biosensors characterized real-time release of adenosine in in vitro models of brain ischaemia, and this line of work has recently led to clinical measurements of whole blood purine levels in patients undergoing carotid artery surgery or stroke. In parallel, the wish to understand the role of ATP signalling in the chemosensory regulation of breathing stimulated the development of ATP biosensors. This revealed that release of ATP from the chemosensory areas of the medulla oblongata preceded adaptive changes in breathing, triggered adaptive changes in breathing via activation of P2 receptors, and ultimately led to the discovery of connexin26 as a channel that mediates CO₂-gated release of ATP from cells.

Co-localization and confinement of ecto-nucleotidases modulate extracellular adenosine nucleotide distributions

Nucleotides comprise small molecules that perform critical signaling roles in biological systems. Adenosine-based nucleotides, including adenosine tri-, di-, and mono-phosphate, are controlled through their rapid degradation by diphosphohydrolases and ecto-nucleotidases (NDAs). The interplay between nucleotide signaling and degradation is especially important in synapses formed between cells, which create signaling 'nanodomains'. Within these 'nanodomains', charged nucleotides interact with densely-packed membranes and biomolecules. While the contributions of electrostatic and steric interactions within such nanodomains are known to shape diffusion-limited reaction rates, less is understood about how these factors control the kinetics of nucleotidase activity. To quantify these factors, we utilized reaction-diffusion numerical simulations of 1) adenosine triphosphate (ATP) hydrolysis into adenosine monophosphate (AMP) and 2) AMP into adenosine (Ado) via two representative nucleotidases, CD39 and CD73. We evaluate these sequentially-coupled reactions in nanodomain geometries representative of extracellular synapses, within which we localize the nucleotidases. With this model, we find that 1) nucleotidase confinement reduces reaction rates relative to an open (bulk) system, 2) the rates of AMP and ADO formation are accelerated by restricting the diffusion of substrates away from the enzymes, and 3) nucleotidase co-localization and the presence of complementary (positive) charges to ATP enhance reaction rates, though the impact of these contributions on nucleotide pools depends on the degree to which the membrane competes for substrates. As a result, these contributions integratively control the relative concentrations and distributions of ATP and its metabolites within the junctional space. Altogether, our studies suggest that CD39 and CD73 nucleotidase activity within junctional spaces can exploit their confinement and favorable electrostatic interactions to finely control nucleotide signaling.

The Role of Purinergic Receptors in the Circadian System

The circadian system is an internal time-keeping system that synchronizes the behavior and physiology of an organism to the 24 h solar day. The master circadian clock, the suprachiasmatic nucleus (SCN), resides in the hypothalamus. It receives information about the environmental light/dark conditions through the eyes and orchestrates peripheral oscillators. Purinergic signaling is mediated by extracellular purines and pyrimidines that bind to purinergic receptors and regulate multiple body functions. In this review, we highlight the interaction between the circadian system and purinergic signaling to provide a better understanding of rhythmic body functions under physiological and pathological conditions.

Focusing on Adenosine Receptors as a Potential Targeted Therapy in Human Diseases

Adenosine is involved in a range of physiological and pathological effects through membrane-bound receptors linked to G proteins. There are four subtypes of adenosine receptors, described as A1AR, A2AAR, A2BAR, and A3AR, which are the center of cAMP signal pathway-based drug development. Several types of agonists, partial agonists or antagonists, and allosteric substances have been synthesized from these receptors as new therapeutic drug candidates. Research efforts surrounding A1AR and A2AAR are perhaps the most enticing because of their concentration and affinity; however, as a consequence of distressing conditions, both A2BAR and A3AR levels might accumulate. This review focuses on the biological features of each adenosine receptor as the basis of ligand production and describes clinical studies of adenosine receptor-associated pharmaceuticals in human diseases.

Adenosine-5'-triphosphate suppresses proliferation and migration capacity of human endometrial stem cells

Extracellular ATP through the activation of the P2X and P2Y purinergic receptors affects the migration, proliferation and differentiation of many types of cells, including stem cells. High plasticity, low immunogenicity and immunomodulation ability of mesenchymal stem cells derived from human endometrium (eMSCs) allow them to be considered a prominent tool for regenerative medicine. Here, we examined the role of ATP in the proliferation and migration of human eMSCs. Using a wound healing assay, we showed that ATP-induced activation of purinergic receptors suppressed the migration ability of eMSCs. We found the expression of one of the ATP receptors, the P2X₇ receptor in eMSCs. In spite of this, cell activation with specific P2X₇ receptor agonist, BzATP did not significantly affect the cell migration. The allosteric P2X₇ receptor inhibitor, AZ10606120 also did not prevent ATP-induced inhibition of cell migration, confirming that inhibition occurs without P2X₇ receptor involvement. Flow cytometry analysis showed that high concentrations of ATP did not have a cytotoxic effect on eMSCs. At the same time, ATP induced the cell cycle arrest, suppressed the proliferative and migration capacity of eMSCs and therefore could affect the regenerative potential of these cells.

P2X7 receptor activation increases expression of caveolin-1 and formation of macrophage lipid rafts, thereby boosting CD39 activity

Macrophages are tissue-resident immune cells that are crucial for the initiation and maintenance of immune responses. Purinergic signaling modulates macrophage activity and impacts cellular plasticity. The ATP-activated purinergic receptor P2X7 (also known as P2RX7) has pro-inflammatory properties, which contribute to macrophage activation. P2X7 receptor signaling is, in turn, modulated by ectonucleotidases, such as CD39 (also known as ENTPD1), expressed in caveolae and lipid rafts. Here, we examined P2X7 receptor activity and determined impacts on ectonucleotidase localization and function in macrophages primed with lipopolysaccharide (LPS). First, we verified that ATP boosts CD39 activity and caveolin-1 protein expression in LPS-primed macrophages. Drugs that disrupt cholesterol-enriched domains - such as nystatin and methyl-β-cyclodextrin - decreased CD39 enzymatic activity in all circumstances. We noted that CD39 colocalized with lipid raft markers (flotillin-2 and caveolin-1) in macrophages that had been primed with LPS followed by treatment with ATP. P2X7 receptor inhibition blocked these ATP-mediated increases in caveolin-1 expression and inhibited the colocalization with CD39. Further, we found that STAT3 activation is significantly attenuated caveolin-1-deficient macrophages treated with LPS or LPS+BzATP. Taken together, our data suggest that P2X7 receptor triggers the initiation of lipid raft-dependent mechanisms that upregulates CD39 activity and could contribute to limit macrophage responses restoring homeostasis.

Purinergic Signaling in Glioma Progression

Among the pathological alterations that give tumor cells invasive potential, purinergic signaling is emerging as an important component. Studies performed in in vitro, in vivo and ex vivo glioma models indicate that alterations in the purinergic signaling are involved in the progression of these tumors. Gliomas have low expression of all E-NTPDases, 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 cells in culture. The importance of extracellular ATP for glioma pathobiology was confirmed by the reduction in glioma tumor size by apyrase, which degrades extracellular ATP to AMP, and the striking increase in tumor size by over-expression of an ecto-enzyme that degrades ATP to ADP, suggesting the effect of extracellular ATP on the tumor growth depends on the nucleotide produced by its degradation. The participation of purinergic receptors on glioma progression, particularly P2X₇, is involved in the resistance to ATP-induced cell death. Although more studies are necessary, the purinergic signaling, including ectonucleotidases and receptors, may be considered as future target for glioma pharmacological or gene therapy.

Tinkering with targeting nucleotide signaling for control of intracellular Leishmania parasites

Nucleotides are one of the most primitive extracellular signalling molecules across all phyla and regulate a multitude of responses. The biological effects of extracellular nucleotides/sides are mediated via the specific purinergic receptors present on the cell surface. In mammalian system, adenine nucleotides are the predominant nucleotides found in the extracellular milieu and mediate a constellation of physiological functions. In the context of host-pathogen interaction, extracellular ATP is recognized as a danger signal and potentiates the release of pro-inflammatory mediators from activated immune cells, on the other hand, its breakdown product adenosine exerts potential anti-inflammatory and immunosuppressive actions. Therefore, it is increasingly apparent that the interplay between extracellular ATP/adenosine ratios has a significant role in coordinating the regulation of the immune system in health and diseases. Several pathogens express ectonucleotidases on their surface and exploit the purinergic signalling as one of the mechanisms to modulate the host immune response. Leishmania pathogens are one of the most successful intracellular pathogens which survive within host macrophages and manipulate protective Th1 response into disease promoting Th2 response. In this review, we discuss the regulation of extracellular ATP and adenosine levels, the role of ATP/adenosine counter signalling in regulating the inflammation and immune responses during infection and how Leishmania parasites exploit the purinergic signalling to manipulate host response. We also discuss the challenges and opportunities in targeting purinergic signalling and the future prospects.

Role of Ectonucleotidases in Synapse Formation During Brain Development: Physiological and Pathological Implications

BACKGROUND

Extracellular adenine nucleotides and nucleosides, such as ATP and adenosine, are among the most recently identified and least investigated diffusible signaling factors that contribute to the structural and functional remodeling of the brain, both during embryonic and postnatal development. Their levels in the extracellular milieu are tightly controlled by various ectonucleotidases: ecto-nucleotide pyrophosphatase/phosphodiesterases (E-NPP), alkaline phosphatases (AP), ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDases) and ecto-5'- nucleotidase (eN).

METHODS

Studies related to the expression patterns of ectonucleotidases and their known features during brain development are reviewed, highlighting involvement of these enzymes in synapse formation and maturation in physiological as well as in pathological states.

RESULTS

During brain development and in adulthood all ectonucleotidases have diverse expression pattern, cell specific localization and function. NPPs are expressed at early embryonic days, but the expression of NPP3 is reduced and restricted to ependymal area in adult brain. NTPDase2 is dominant ectonucleotidase existing in the progenitor cells as well as main astrocytic NTPDase in the adult brain, while NTPDase3 is fully expressed after third postnatal week, almost exclusively on varicose fibers. Specific brain AP is functionally associated with synapse formation and this enzyme is sufficient for adenosine production during neurite growth and peak of synaptogenesis. eN is transiently associated with synapses during synaptogenesis, however in adult brain it is more glial than neuronal enzyme.

CONCLUSION

Control of extracellular adenine nucleotide levels by ectonucleotidases are important for understanding the role of purinergic signaling in developing tissues and potential targets in developmental disorders such as autism.

Purinergic receptors in neurogenic processes

Neurogenesis is a process of generating functional neurons, which occurs during embryonic and adult stages in mammals. While neurogenesis during development phase is characterized by intensive proliferation activity in all regions of the brain to form the architecture and neural function of the nervous system, adult neurogenesis occurs with less intensity in two brain regions and is involved in the maintenance of neurogenic niches, local repair, memory and cognitive functions in the hippocampus. Taking such differences into account, the understanding of molecular mechanisms involved in cell differentiation in developmental stages and maintenance of the nervous system is an important research target. Although embryonic and adult neurogenesis presents several differences, signaling through purinergic receptors participates in this process throughout life. For instance, while embryonic neurogenesis involves P2X7 receptor down-regulation and calcium waves triggered by P2Y1 receptor stimulation, adult neurogenesis may be enhanced by increased activity of A_2A and P2Y1 receptors and impaired by A₁, P2Y13 and P2X7 receptor stimulation.

Neuromodulatory Effects of Guanine-Based Purines in Health and Disease

The function of guanine-based purines (GBPs) is mostly attributed to the intracellular modulation of heteromeric and monomeric G proteins. However, extracellular effects of guanine derivatives have also been recognized. Thus, in the central nervous system (CNS), a guanine-based purinergic system that exerts neuromodulator effects, has been postulated. The thesis that GBPs are neuromodulators emerged from in vivo and in vitro studies, in which neurotrophic and neuroprotective effects of these kinds of molecules (i.e., guanosine) were demonstrated. GBPs induce several important biological effects in rodent models and have been shown to reduce seizures and pain, stabilize mood disorder behavior and protect against gliomas and diseases related with aging, such as ischemia or Parkinson and Alzheimer diseases. In vitro studies to evaluate the protective and trophic effects of guanosine, and of the nitrogenous base guanine, have been fundamental for understanding the mechanisms of action of GBPs, as well as the signaling pathways involved in their biological roles. Conversely, although selective binding sites for guanosine have been identified in the rat brain, GBP receptors have not been still described. In addition, GBP neuromodulation may depend on the capacity of GBPs to interact with well-known membrane proteins in glutamatergic and adenosinergic systems. Overall, in this review article, we present up-to-date GBP biology, focusing mainly on the mechanisms of action that may lead to the neuromodulator role of GBPs observed in neurological disorders.

Cooperation between NMDA-Type Glutamate and P2 Receptors for Neuroprotection during Stroke: Combining Astrocyte and Neuronal Protection

Excitotoxicity is the principle mechanism of acute injury during stroke. It is defined as the unregulated accumulation of excitatory neurotransmitters such as glutamate within the extracellular space, leading to over-activation of receptors, ionic disruption, cell swelling, cytotoxic Ca2+ elevation and a feed-forward loop where membrane depolarisation evokes further neurotransmitter release. Glutamate-mediated excitotoxicity is well documented in neurons and oligodendrocytes but drugs targeting glutamate excitotoxicity have failed clinically which may be due to their inability to protect astrocytes. Astrocytes make up ~50% of the brain volume and express high levels of P2 adenosine triphosphate (ATP)-receptors which have excitotoxic potential, suggesting that glutamate and ATP may mediate parallel excitotoxic cascades in neurons and astrocytes, respectively. Mono-cultures of astrocytes expressed an array of P2X and P2Y receptors can produce large rises in [Ca2+]i; mono-cultured neurons showed lower levels of functional P2 receptors. Using high-density 1:1 neuron:astrocyte co-cultures, ischemia (modelled as oxygen-glucose deprivation: OGD) evoked a rise in extracellular ATP, while P2 blockers were highly protective of both cell types. GluR blockers were only protective of neurons. Neither astrocyte nor neuronal mono-cultures showed significant ATP release during OGD, showing that cell type interactions are required for ischemic release. P2 blockers were also protective in normal-density co-cultures, while low doses of combined P2/GluR blockers where highly protective. These results highlight the potential of combined P2/GluR block for protection of neurons and glia.

Mechanisms for the control of local tissue blood flow during thermal interventions: influence of temperature-dependent ATP release from human blood and endothelial cells

New Findings

What is the central question of this study?

Skin and muscle blood flow increases with heating and decreases with cooling, but the temperature‐sensitive mechanisms underlying these responses are not fully elucidated.

What is the main finding and its importance?

We found that local tissue hyperaemia was related to elevations in ATP release from erythrocytes. Increasing intravascular ATP augmented skin and tissue perfusion to levels equal or above thermal hyperaemia. ATP release from isolated erythrocytes was altered by heating and cooling. Our findings suggest that erythrocytes are involved in thermal regulation of blood flow via modulation of ATP release.

Local tissue perfusion changes with alterations in temperature during heating and cooling, but the thermosensitivity of the vascular ATP signalling mechanisms for control of blood flow during thermal interventions remains unknown. Here, we tested the hypotheses that the release of the vasodilator mediator ATP from human erythrocytes, but not from endothelial cells or other blood constituents, is sensitive to both increases and reductions in temperature and that increasing intravascular ATP availability with ATP infusion would potentiate thermal hyperaemia in limb tissues. We first measured blood temperature, brachial artery blood flow and plasma [ATP] during passive arm heating and cooling in healthy men and found that they increased by 3.0 ± 1.2°C, 105 ± 25 ml min⁻¹ °C⁻¹ and twofold, respectively, (all P < 0.05) with heating, but decreased or remained unchanged with cooling. In additional men, infusion of ATP into the brachial artery increased skin and deep tissue perfusion to levels equal or above thermal hyperaemia. In isolated erythrocyte samples exposed to different temperatures, ATP release increased 1.9‐fold from 33 to 39°C (P < 0.05) and declined by ∼50% at 20°C (P < 0.05), but no changes were observed in cultured human endothelial cells, plasma or serum samples. In conclusion, increases in plasma [ATP] and skin and deep tissue perfusion with limb heating are associated with elevations in ATP release from erythrocytes, but not from endothelial cells or other blood constituents. Erythrocyte ATP release is also sensitive to temperature reductions, suggesting that erythrocytes may function as thermal sensors and ATP signalling generators for control of tissue perfusion during thermal interventions.

Targeting ENT1 and adenosine tone for the treatment of Huntington's disease

Huntington's disease (HD) is caused by an abnormal CAG expansion in the exon 1 of huntingtin gene. The treatment of HD is an unmet medical need. Given the important role of adenosine in modulating brain activity, in this study, levels of adenosine and adenine nucleotides in the cerebral spinal fluid of patients with HD and in the brain of two mouse models of HD (R6/2 and Hdh150Q) were analysed. The expression and activity of ENT1 in the striatum of mice with HD were measured. Targeting adenosine tone for treating HD was examined in R6/2 mice by genetic removal of ENT1 and by giving an ENT1 inhibitor, respectively. The results showed that the adenosine homeostasis is dysregulated in the brain of patients and mice with HD. In patients, the ratio of adenosine/ATP in the cerebral spinal fluid was negatively correlated with the disease duration, and tended to have a positive correlation with independence scale and functional capacity. In comparison to controls, mRNA level of ENT1 was higher in the striatum of R6/2 and Hdh150Q mice. Intrastriatal administration of ENT1 inhibitors increased extracellular level of adenosine in the striatum of R6/2 mice to a much higher level than controls. Chronic inhibition of ENT1 or by genetic removal of ENT1 enhanced the survival of R6/2 mice. Collectively, adenosine homeostasis and ENT1 expression are altered in HD. The inhibition of ENT1 can enhance extracellular adenosine level and be a potential therapeutic approach for treating HD.

Occurrence of oxidative stress in dairy cows seropositives for Brucella abortus

Bovine brucellosis is an important zoonotic disease caused by the bacterium Brucella abortus that leads to economic losses due to animal discard and commercial restrictions. Since positive animals for brucellosis are culled, little is known about the pathogenesis of this disease. Therefore, the aims of this study were to evaluate possible changes in the activity of deaminase adenosine (ADA) and the oxidative stress in cows seropositives for brucellosis (Experiment I), and to evaluate the seroprevalence of B. abortus in dairy cows from the Western state of Santa Catarina, Southern Brazil (Experiment II). The Experiment I evaluated 20 pregnant cows: ten seropositives for B. abortus and ten seronegatives that were used as controls. The ADA activity and markers of oxidative stress (TBARS, catalase (CAT) and superoxide dismutase (SOD)) were evaluated in these animals. A reduction in the activity of ADA and catalase enzymes in seropositive animals was observed (p < 0.001). Conversely, there was an increase in TBARS levels and superoxide dismutase activity in cows infected by B. abortus (p < 0.001). The presence of oxidative stress and a reduction of ADA might be related to the modulation of the inflammatory response. The experiment II was performed due to a high number of herds with restrictions imposed by cases of brucellosis in the state of Santa Catarina in the last two years, and thus, the seroprevalence for B. abortus was evaluated in 1242 serum samples of cows of 69 herds. The serodiagnosis was performed using two tests: buffered acidified antigen and 2-mercaptoethanol. However, none of the serum samples were positive for B. abortus. Although we did not find seropositive animals for brucellosis in our study, the disease still requires continued surveillance, due to its economic impact, and to the oxidative stress caused by it, which may have contributed to cases of abortion in three seropositive cows (Experiment I) in the final third of the gestation.

Blockade of ATP P2X7 receptor enhances ischiatic nerve regeneration in mice following a crush injury

Preventing damage caused by nerve degeneration is a great challenge. There is a growing body of evidence implicating extracellular nucleotides and their P2 receptors in many pathophysiological mechanisms. In this work we aimed to investigate the effects of the administration of Brilliant Blue G (BBG) and Pyridoxalphosphate-6-azophenyl-2', 4'- disulphonic acid (PPADS), P2X7 and P2 non-selective receptor antagonists, respectively, on sciatic nerve regeneration. Four groups of mice that underwent nerve crush lesion were used: two control groups treated with vehicle (saline), a group treated with BBG and a group treated with PPADS during 28days. Gastrocnemius muscle weight was evaluated. For functional evaluation we used the Sciatic Functional Index (SFI) and the horizontal ladder walking test. Nerves, dorsal root ganglia and spinal cords were processed for light and electron microscopy. Antinoceptive effects of BBG and PPADS were evaluated through von Frey E, and the levels of IL-1β and TNF-α were analyzed by ELISA. BBG promoted an increase in the number of myelinated fibers and on axon, fiber and myelin areas. BBG and PPADS led to an increase of TNF-α and IL-1β in the nerve on day 1 and PPADS caused a decrease of IL-1β on day 7. Mechanical allodynia was reversed on day 7 in the groups treated with BBG and PPADS. We concluded that BBG promoted a better morphological regeneration after ischiatic crush injury, but this was not followed by anticipation of functional improvement. In addition, both PPADS and BBG presented anti-inflammatory as well as antinociceptive effects.

Manganese(II) Chloride Alters Nucleotide and Nucleoside Catabolism in Zebrafish (Danio rerio) Adult Brain

ATP and adenosine, the main signaling molecules of purinergic system, are involved in toxicological effects induced by metals. The manganese (Mn) exposure induces several cellular changes, which could interfere with signaling pathways, such as the purinergic system. In this study, we evaluated the effects of exposure to manganese(II) chloride (MnCl₂) during 96 h on nucleoside triphosphate diphosphohydrolase (NTPDase), ecto-5'-nucleotidase, and adenosine deaminase (ADA) activities, followed by analyzing the gene expression patterns of NTPDases (entpd1, entpd2a.1, entpd2a.2, entpd2-like, entpd3) and ADA (ADA ₁ , ADA _2.1 , ADA _2.2 , ADAasi, ADAL) families in zebrafish brain. In addition, the brain metabolism of nucleotides and nucleosides was evaluated after MnCl₂ exposure. The results showed that MnCl₂ exposure during 96 h inhibited the NTPDase (1.0 and 1.5 mM) and ecto-ADA (0.5, 1.0, and 1.5 mM) activities, further decreasing ADA2.1 expression at all MnCl₂ concentrations analyzed. Purine metabolism was also altered by the action of MnCl₂. An increased amount of ADP appeared at all MnCl₂ concentrations analyzed; however, AMP and adenosine levels are decreased at the concentrations of 1.0 and 1.5 mM MnCl₂, whereas decreased inosine (INO) levels were observed at all concentrations tested. The findings of this study demonstrated that MnCl₂ may inhibit NTPDase and ecto-ADA activities, consequently modulating nucleotide and nucleoside levels, which may contribute for the toxicological effects induced by this metal.

Evidence for a functional vasoconstrictor role for ATP in the human cutaneous microvasculature

NEW FINDINGS

What is the central question of this study? In young adults, about half of the cold-related reduction in skin blood flow during cold exposure is mediated by noradrenaline, while the remainder is attributable to other substances co-released with noradrenaline that have yet to be identified. What is the main finding and its importance? Purinergic receptor blockade blunted the vasoconstriction response to whole-body cooling and to intradermal administration of tyramine. These results indicate that ATP is necessary to vasoconstrict blood vessels in the skin adequately and prevent heat loss in a cold environment. Noradrenaline is responsible for eliciting ∼60% of the reflex cutaneous vasoconstriction (VC) response in young adults, while the remainder is attributable to one or more unidentified co-released sympathetic adrenergic neurotransmitter(s). Inconsistent evidence has placed neuropeptide Y in this role; however, other putative cotransmitters have yet to be tested. We hypothesize that ATP contributes to the reflex cutaneous VC response. Two protocols were conducted in young adults (n = 10); both involved the placement of three microdialysis probes in forearm skin and whole-body cooling (skin temperature = 30.5°C). In protocol 1, the following solutions were infused: (i) lactated Ringer solution (control); (ii) 10 mm l-NAME; and (iii) purinergic receptor blockade with 1 mm suramin plus l-NAME. In protocol 2, the following solutions were infused: (i) lactated Ringer solution; (ii) suramin plus l-NAME; and (iii) suramin plus l-NAME plus adrenoreceptor blockade with 5 mm yohimbine plus 1 mm propranolol. Laser Doppler flux (LDF) was measured over each microdialysis site, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP) and expressed as percentage changes from baseline (%ΔCVC_BASELINE ). l-NAME was used to block the vasodilatory influence of ATP and unmask the P₂ X-mediated VC response to exogenous ATP infusion (-21 ± 6%ΔCVC_BASELINE ). During cooling, the VC response (control, -39 ± 8%ΔCVC_BASELINE ) was attenuated at the suramin site (-21 ± 4%ΔCVC_BASELINE ) and further blunted with combined adrenoreceptor blockade (-9 ± 3%ΔCVC_BASELINE ; P < 0.05). Compared with the control site (-22 ± 5%ΔCVC_BASELINE ), suramin inhibited pharmacologically induced VC to tyramine (-12 ± 6%ΔCVC_BASELINE ; P < 0.05), which displaces adrenergic neurotransmitters from axon terminals. These data indicate that ATP contributes to the cutaneous VC response in humans.

Guanosine: a Neuromodulator with Therapeutic Potential in Brain Disorders

Guanosine is a purine nucleoside with important functions in cell metabolism and a protective role in response to degenerative diseases or injury. The past decade has seen major advances in identifying the modulatory role of extracellular action of guanosine in the central nervous system (CNS). Evidence from rodent and cell models show a number of neurotrophic and neuroprotective effects of guanosine preventing deleterious consequences of seizures, spinal cord injury, pain, mood disorders and aging-related diseases, such as ischemia, Parkinson’s and Alzheimer’s diseases. The present review describes the findings of in vivo and in vitro studies and offers an update of guanosine effects in the CNS. We address the protein targets for guanosine action and its interaction with glutamatergic and adenosinergic systems and with calcium-activated potassium channels. We also discuss the intracellular mechanisms modulated by guanosine preventing oxidative damage, mitochondrial dysfunction, inflammatory burden and modulation of glutamate transport. New and exciting avenues for future investigation into the protective effects of guanosine include characterization of a selective guanosine receptor. A better understanding of the neuromodulatory action of guanosine will allow the development of therapeutic approach to brain diseases.

Memory deficit, toxic effects and activity of Na(+), K(+)-ATPase and NTPDase in brain of Wistar rats submitted to orally treatment with alpha-terpinene

The neurotoxic effects and activity of Na(+), K(+)-ATPase and NTPDase in Wistar rats after treatment with α-terpinene (daily oral administration of 0.5, 0.75 and 1.0mLkg(-1) for 10days) were examined. Results of the inhibitory avoidance task showed a memory deficit (p<0.05) in rats treated with all doses of α-terpinene. The evaluation of DNA damage in brain tissue revealed an increase (p<0.05) on frequency of damage and damage index in all concentrations. According to the cytotoxicity assay, doses of 0.5, 0.75 and 1.0mLkg(-1) increase the lactate dehydrogenase levels, and doses of 1.0mLkg(-1) also decrease (p<0.05) cell viability in brain cells. A decrease (p<0.05) on Na(+), K(+)-ATPase activity in brain tissue and on NTPDase activity in serum were observed in all concentrations of α-terpinene. These results suggest that the α-terpinene was cytotoxic and genotoxic to the brain cells by inducing loss of cell viability and DNA damage, as well as causing alterations in Na(+), K(+)-ATPase and NTPDase activity, what may contribute to the memory deficit of treated animals. Thus, α-terpinene cannot be consumed by the population at the doses studied.

Signaling mechanism for modulation by ATP of glycine receptors on rat retinal ganglion cells

ATP modulates voltage- and ligand-gated channels in the CNS via the activation of ionotropic P2X and metabotropic P2Y receptors. While P2Y receptors are expressed in retinal neurons, the function of these receptors in the retina is largely unknown. Using whole-cell patch-clamp techniques in rat retinal slice preparations, we demonstrated that ATP suppressed glycine receptor-mediated currents of OFF type ganglion cells (OFF-GCs) dose-dependently, and the effect was in part mediated by P2Y1 and P2Y11, but not by P2X. The ATP effect was abolished by intracellular dialysis of a Gq/11 protein inhibitor and phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor, but not phosphatidylcholine (PC)-PLC inhibitor. The ATP effect was accompanied by an increase in [Ca(2+)]i through the IP3-sensitive pathway and was blocked by intracellular Ca(2+)-free solution. Furthermore, the ATP effect was eliminated in the presence of PKC inhibitors. Neither PKA nor PKG system was involved. These results suggest that the ATP-induced suppression may be mediated by a distinct Gq/11/PI-PLC/IP3/Ca(2+)/PKC signaling pathway, following the activation of P2Y1,11 and other P2Y subtypes. Consistently, ATP suppressed glycine receptor-mediated light-evoked inhibitory postsynaptic currents of OFF-GCs. These results suggest that ATP may modify the ON-to-OFF crossover inhibition, thus changing action potential patterns of OFF-GCs.

ATP as a Messenger in Astrocyte-Neuronal Communication

Astrocytes are activated during both excitatory and inhibitory synaptic transmission and respond with intracellular Ca2+i elevations. Ca2+i oscillations and waves in astrocytes now appear to represent the glial arm of a dynamic neuronal-glial signaling process. Advances within the last year have shown that stimuli that elevate Ca2+i in astrocytes have the potential to modulate synaptic function. Recent studies have shown that astrocytic calcium waves, initially believed to depend on the integrity of functional gap junction channels for the passage of intercellular signals, are actually mediated by release of ATP and subsequent activation of purinergic receptors on neighboring cells. ATP release is in turn regulated by the expression of gap junction proteins, establishing a novel dimension between gap junctions and extracellular-mediated signaling events. The role of ATP and its breakdown product, adenosine, on synaptic transmission are discussed.

The Guanine-Based Purinergic System: The Tale of An Orphan Neuromodulation

Guanine-based purines (GBPs) have been recently proposed to be not only metabolic agents but also extracellular signaling molecules that regulate important functions in the central nervous system. In such way, GBPs-mediated neuroprotection, behavioral responses and neuronal plasticity have been broadly described in the literature. However, while a number of these functions (i.e., GBPs neurothophic effects) have been well-established, the molecular mechanisms behind these GBPs-dependent effects are still unknown. Furthermore, no plasma membrane receptors for GBPs have been described so far, thus GBPs are still considered orphan neuromodulators. Interestingly, an intricate and controversial functional interplay between GBPs effects and adenosine receptors activity has been recently described, thus triggering the hypothesis that GBPs mechanism of action might somehow involve adenosine receptors. Here, we review recent data describing the GBPs role in the brain. We focus on the involvement of GBPs regulating neuronal plasticity, and on the new hypothesis based on putative GBPs receptors. Overall, we expect to shed some light on the GBPs world since although these molecules might represent excellent candidates for certain neurological diseases management, the lack of putative GBPs receptors precludes any high throughput screening intent for the search of effective GBPs-based drugs.

Adenosine A1 Receptor-Mediated Endocytosis of AMPA Receptors Contributes to Impairments in Long-Term Potentiation (LTP) in the Middle-Aged Rat Hippocampus

Aging causes multiple changes in the mammalian brain, including changes in synaptic signaling. Previous reports have shown increased extracellular adenosine in the aging brain, and we recently reported that activation of adenosine A1 receptors (A1Rs) induces AMPA receptor (AMPAR) internalization in rat hippocampus. This study investigated whether aging-related changes in the rat hippocampus include altered surface expression of adenosine A1 and A2A receptors, and whether these changes correspond to changes in AMPAR surface expression and altered synaptic plasticity. We found reduced A1R surface expression in middle-aged rat hippocampus, and also reduced GluA1 and GluA2 AMPAR subunit surface expression. Using a chemically-induced LTP (cLTP) experimental protocol, we recorded fEPSPs in young (1 month old) and middle-aged (7-12 month old) rat hippocampal slices. There were significant impairments in cLTP in middle-aged slices, suggesting impaired synaptic plasticity. Since we previously showed that the A1R agonist N(6)-cyclopentyladenosine (CPA) reduced both A1Rs and GluA2/GluA1 AMPARs, we hypothesized that the observed impaired synaptic plasticity in middle-aged brains is regulated by A1R-mediated AMPAR internalization by clathrin-mediated endocytosis. Following cLTP, we found a significant increase in GluA1 and GluA2 surface expression in young rats, which was blunted in middle-aged brains or in young brains pretreated with CPA. Blocking A1Rs with 8-cyclopentyl-1,3-dipropylxanthine or AMPAR endocytosis with either Tat-GluA2-3Y peptide or dynasore (dynamin inhibitor) similarly enhanced AMPAR surface expression following cLTP. These data suggest that age-dependent alteration in adenosine receptor expression contributes to increased AMPAR endocytosis and impaired synaptic plasticity in aged brains.

ATPergic signalling during seizures and epilepsy

Much progress has been made over the last few decades in the identification of new anti-epileptic drugs (AEDs). However, 30% of epilepsy patients suffer poor seizure control. This underscores the need to identify alternative druggable neurotransmitter systems and drugs with novel mechanisms of action. An emerging concept is that seizure generation involves a complex interplay between neurons and glial cells at the tripartite synapse and neuroinflammation has been proposed as one of the main drivers of epileptogenesis. The ATP-gated purinergic receptor family is expressed throughout the brain and is functional on neurons and glial cells. ATP is released in high amounts into the extracellular space after increased neuronal activity and during chronic inflammation and cell death to act as a neuro- and gliotransmitter. Emerging work shows pharmacological targeting of ATP-gated purinergic P2 receptors can potently modulate seizure generation, inflammatory processes and seizure-induced brain damage. To date, work showing the functional contribution of P2 receptors has been mainly performed in animal models of acute seizures, in particular, by targeting the ionotropic P2X7 receptor subtype. Other ionotropic P2X and metabotropic P2Y receptor family members have also been implicated in pathological processes following seizures such as the P2X4 receptor and the P2Y12 receptor. However, during epilepsy, the characterization of P2 receptors was mostly restricted to the study of expressional changes of the different receptor subtypes. This review summarizes the work to date on ATP-mediated signalling during seizures and the functional impact of targeting the ATP-gated purinergic receptors on seizures and seizure-induced pathology. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Effects of Oral Administration of Chitin Nanofiber on Plasma Metabolites and Gut Microorganisms

The aim of this study was to examine the effects of oral administration of chitin nanofibers (CNFs) and surface-deacetylated (SDA) CNFs on plasma metabolites using metabolome analysis. Furthermore, we determined the changes in gut microbiota and fecal organic acid concentrations following oral administrations of CNFs and SDACNFs. Healthy female mice (six-week-old) were fed a normal diet and administered tap water with 0.1% (v/v) CNFs or SDACNFs for 28 days. Oral administration of CNFs increased plasma levels of adenosine triphosphate (ATP), adenosine diphosphate (ADP), and serotonin (5-hydroxytryptamine, 5-HT). Oral administration of SDACNFs affected the metabolisms of acyl-carnitines and fatty acids. The fecal organic level analysis indicated that oral administration of CNFs stimulated and activated the functions of microbiota. These results indicate that oral administration of CNFs increases plasma levels of ATP and 5-HT via activation of gut microbiota.

Enzymes involved in metabolism of extracellular nucleotides and nucleosides: functional implications and measurement of activities

Extracellular nucleotides and nucleosides mediate diverse signaling effects in virtually all organs and tissues. Most models of purinergic signaling depend on functional interactions between distinct processes, including (i) the release of endogenous ATP and other nucleotides, (ii) triggering of signaling events via a series of nucleotide-selective ligand-gated P2X and metabotropic P2Y receptors as well as adenosine receptors and (iii) ectoenzymatic interconversion of purinergic agonists. The duration and magnitude of purinergic signaling is governed by a network of ectoenzymes, including the enzymes of the nucleoside triphosphate diphosphohydrolase (NTPDase) family, the nucleotide pyrophosphatase/phosphodiesterase (NPP) family, ecto-5'-nucleotidase/CD73, tissue-nonspecific alkaline phosphatase (TNAP), prostatic acid phosphatase (PAP) and other alkaline and acid phosphatases, adenosine deaminase (ADA) and purine nucleoside phosphorylase (PNP). Along with "classical" inactivating ectoenzymes, recent data provide evidence for the co-existence of a counteracting ATP-regenerating pathway comprising the enzymes of the adenylate kinase (AK) and nucleoside diphosphate kinase (NDPK/NME/NM23) families and ATP synthase. This review describes recent advances in this field, with special emphasis on purine-converting ectoenzymes as a complex and integrated network regulating purinergic signaling in such (patho)physiological states as immunomodulation, inflammation, tumorigenesis, arterial calcification and other diseases. The second part of this review provides a comprehensive overview and basic principles of major approaches employed for studying purinergic activities, including spectrophotometric Pi-liberating assays, high-performance liquid chromatographic (HPLC) and thin-layer chromatographic (TLC) analyses of purine substrates and metabolites, capillary electrophoresis, bioluminescent, fluorometric and electrochemical enzyme-coupled assays, histochemical staining, and further emphasizes their advantages, drawbacks and suitability for assaying a particular catalytic reaction.

Intradermal administration of ATP augments methacholine-induced cutaneous vasodilation but not sweating in young males and females

Acetylcholine released from cholinergic nerves is a key neurotransmitter contributing to heat stress-induced cutaneous vasodilation and sweating. Given that sympathetic cholinergic nerves also release ATP, ATP may play an important role in modulating cholinergic cutaneous vasodilation and sweating. However, the pattern of response may differ between males and females given reports of sex-related differences in the peripheral mechanisms governing these heat loss responses. Cutaneous vascular conductance (CVC, laser-Doppler perfusion units/mean arterial pressure) and sweat rate (ventilated capsule) were evaluated in 17 young adults (8 males, 9 females) at four intradermal microdialysis skin sites continuously perfused with: 1) lactated Ringer (Control), 2) 0.3 mM ATP, 3) 3 mM ATP, or 4) 30 mM ATP. At all skin sites, methacholine was coadministered in a concentration-dependent manner (0.0125, 0.25, 5, 100, 2,000 mM, each for 25 min). In both males and females, CVC was elevated with the lone infusion of 30 mM ATP (both P < 0.05), but not with 0.3 and 3 mM ATP compared with control (all P >0.27). However, 0.3 mM ATP induced a greater increase in CVC compared with control in response to 100 mM methacholine infusion in males (P < 0.05). In females, 0.3 mM ATP infusion resulted in a lower concentration of methacholine required to elicit a half-maximal response (EC50) (P < 0.05). In both males and females, methacholine-induced sweating was unaffected by any concentration of ATP (all P > 0.44). We demonstrate that ATP enhances cholinergic cutaneous vasodilation albeit the pattern of response differs between males and females. Furthermore, we show that ATP does not modulate cholinergic sweating.

NTPDase and 5'-nucleotidase activities in synaptosomes of rabbits experimentally infected with BoHV-5

Bovine herpesvirus type 5 (BoHV-5) is the causative agent of herpetic meningoencephalitis in cattle. The purinergic system is described as a modulator of the immune response and neuroinflammation. These functions are related to the extracellular nucleotides concentration. NTPDase and 5'-nucleotidase are enzymes responsible for controlling the extracellular concentration of adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine (ADO). The aim of this study is to determinate the ectonucleotidase activity in cortical synaptosomes and synaptosomes from the hippocampus of rabbits experimentally infected with BoHV-5. Rabbits were divided into four groups, two control groups (non-inoculated animals), and two infected groups (inoculated with BoHV-5). The infected groups received 0.5 ml of BoHV-5 suspension with 10(7.5)TCID50 of viral strain SV-507/99, per paranasal sinuses, and the control groups received 0.5 ml of minimum essential media per paranasal sinuses. Animals were submitted to euthanasia on days 7 and 12 post-inoculation (p.i.); cerebral cortex and hippocampus were collected for the synaptosomes isolation and posterior determination of the ectonucleotidase activities. The results showed a decrease (P < 0.05) in ectonucleotidase activity in synaptosomes from the cerebral cortex of infected rabbits, whereas an increased (P < 0.05) ectonucleotidase activity was observed in synaptosomes from the hippocampus. These differences may be related with the heterogeneous distribution of ectonucleotidases in the different brain regions and also with the viral infectivity. Therefore, it is possible to speculate that BoHV-5 replication results in changes in ectonucleotidase activity in the brain, which may contribute to the neurological signs commonly observed in this disease.

Absence epileptic activity changing effects of non-adenosine nucleoside inosine, guanosine and uridine in Wistar Albino Glaxo Rijswijk rats

Adenosine (Ado) and non-adenosine (non-Ado) nucleosides such as inosine (Ino), guanosine (Guo) and uridine (Urd) may have regionally different roles in the regulation of physiological and pathophysiological processes in the central nervous system (CNS) such as epilepsy. It was demonstrated previously that Ino and Guo decreased quinolinic acid (QA)-induced seizures and Urd reduced penicillin-, bicuculline- and pentylenetetrazole (PTZ)-induced seizures. It has also been demonstrated that Ino and Urd may exert their effects through GABAergic system by altering the function of GABA(A) type of gamma-aminobutyric acid receptors (GABAA receptors) whereas Guo decreases glutamate-induced excitability through glutamatergic system, which systems (GABAergic and glutamatergic) are involved in pathomechanisms of absence epilepsy. Thus, we hypothesized that Ino and Guo, similarly to the previously described effect of Urd, might also decrease absence epileptic activity. We investigated in the present study whether intraperitoneal (i.p.) application of Ino (500 and 1000mg/kg), Guo (20 and 50mg/kg), Urd (500 and 1000mg/kg), GABA(A) receptor agonist muscimol (1 and 3mg/kg), GABA(A) receptor antagonist bicuculline (2 and 4mg/kg), non-selective Ado receptor antagonist theophylline (5 and 10mg/kg) and non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist (+)-5-methyl-10,11-dihydro-5H-dibenzo (a,d) cyclohepten-5,10-imine maleate (MK-801, 0.0625 and 0.1250mg/kg) alone and in combination have modulatory effects on absence epileptic activity in Wistar Albino Glaxo Rijswijk (WAG/Rij) rats. We found that Guo decreased the number of spike-wave discharges (SWDs) whereas Ino increased it dose-dependently. We strengthened that Urd can decrease absence epileptic activity. Our results suggest that Guo, Urd and their analogs could be potentially effective drugs for treatment of human absence epilepsy.

Extracellular ATP metabolism on vascular endothelial cells: A pathway with pro-thrombotic and anti-thrombotic molecules

Vascular endothelial contributes to the metabolism and interconversion of extracellular adenine nucleotides via ecto-ATPase/ADPase (CD39) and ecto-5'nucleotidase (CD73) activities. These enzymes collectively dephosphorylate ATP, ADP, and AMP with the production of additional adenosine. In the vascular system, adenine nucleotides (ATP and ADP) and nucleoside adenosine represent an important class of extracellular molecules involved in modulating the processes linked to vascular thrombosis exerting various effects in platelets. Yet, the mechanisms by which the extracellular ATP metabolism in the local environment trigger pro-thrombotic and anti-thrombotic states are yet to be fully elucidated. In this article, the relative contribution of extracellular ATP metabolism in platelet regulation is explored.

NTS adenosine A2a receptors inhibit the cardiopulmonary chemoreflex control of regional sympathetic outputs via a GABAergic mechanism

Adenosine is a powerful central neuromodulator acting via opposing A1 (inhibitor) and A2a (activator) receptors. However, in the nucleus of the solitary tract (NTS), both adenosine receptor subtypes attenuate cardiopulmonary chemoreflex (CCR) sympathoinhibition of renal, adrenal, and lumbar sympathetic nerve activity and attenuate reflex decreases in arterial pressure and heart rate. Adenosine A1 receptors inhibit glutamatergic transmission in the CCR pathway, whereas adenosine A2a receptors most likely facilitate release of an unknown inhibitory neurotransmitter, which, in turn, inhibits the CCR. We hypothesized that adenosine A2a receptors inhibit the CCR via facilitation of GABA release in the NTS. In urethane-chloralose-anesthetized rats (n = 51), we compared regional sympathetic responses evoked by stimulation of the CCR with right atrial injections of the 5-HT3 receptor agonist phenylbiguanide (1-8 μg/kg) before and after selective stimulation of NTS adenosine A2a receptors [microinjections into the NTS of CGS-21680 (20 pmol/50 nl)] preceded by blockade of GABAA or GABAB receptors in the NTS [bicuculline (10 pmol/100 nl) or SCH-50911 (1 nmol/100 nl)]. Blockade of GABAA receptors virtually abolished adenosine A2a receptor-mediated inhibition of the CCR. GABAB receptors had much weaker but significant effects. These effects were similar for the different sympathetic outputs. We conclude that stimulation of NTS adenosine A2a receptors inhibits CCR-evoked hemodynamic and regional sympathetic reflex responses via a GABA-ergic mechanism.

Cutaneous vascular and sweating responses to intradermal administration of ATP: a role for nitric oxide synthase and cyclooxygenase?

KEY POINTS

In humans in vivo, the mechanisms behind ATP-mediated cutaneous vasodilatation along with whether and how ATP increases sweating remains uncertain. Recent work has implicated nitric oxide synthase (NOS), cyclooxygenase (COX) and/or adenosine in the modulation of cutaneous vasodilatation and sweat production during both local (i.e. localized heating) and whole-body heat stress (i.e. exercise-induced heat stress). We evaluated whether ATP-mediated cutaneous vasodilatation and sweating is mediated via NOS, COX and/or adenosine. We show that in humans in vivo, intradermal administration of ATP induces pronounced vasodilatation which is partially mediated by NOS, but neither COX nor adenosine influences ATP-mediated vasodilatation, and ATP alone does not induce an increase in sweating. These findings advance our basic physiological knowledge regarding control of skin blood flow and sweating, and provide insight into the mechanisms governing thermoeffector activity, which has major implications for whole-body heat exchange and therefore core temperature regulation in humans during heat stress.

ABSTRACT

In humans in vivo, the mechanisms behind ATP-mediated cutaneous vasodilatation and whether and how ATP increases sweating remain uncertain. We evaluated whether ATP-mediated cutaneous vasodilatation and sweating is mediated via nitric oxide synthase (NOS), cyclooxygenase (COX) and/or adenosine-dependent mechanisms. Cutaneous vascular conductance (CVC, laser Doppler perfusion units/mean arterial pressure) and sweat rate (ventilated capsule) were evaluated at intradermal microdialysis forearm skin sites, each receiving pharmacological agents (two separate protocols). In Protocol 1 (n = 12), sites were perfused with: (1) lactated Ringer solution (Control), (2) 10 mm N(ω) -nitro-l-arginine (l-NNA, a NOS inhibitor), (3) 10 mm ketorolac (Ketorolac, a COX inhibitor) or (4) a combination of 10 mm l-NNA + 10 mm ketorolac (l-NNA + Ketorolac). In Protocol 2 (n = 8), sites were perfused with: (1) lactated Ringer solution (Control) or (2) 4 mm theophylline (Theophylline, an adenosine receptor inhibitor). At all sites, ATP was simultaneously perfused at 0.12, 1.2, 12, 120 and 1200 nm min(-1) (each for 20 min). Relative to CVC at the Control site with ATP infused at 120 nm min(-1) (71 ± 9% of max CVC), CVC at the Ketorolac site was comparable (64 ± 13% of max CVC, P = 0.407), but lower at l-NNA (51 ± 15% of max CVC, P = 0.040) and l-NNA + Ketorolac (51 ± 13% of max CVC, P = 0.049) sites. Conversely, across the four skin sites at any other ATP infusion rate (all P > 0.174), no differences in CVC were observed. Theophylline did not influence CVC at any ATP infusion rate (all P > 0.234). Furthermore, no ATP infusion rate elicited an increase in sweating from baseline at any skin site (all P > 0.235). We show that NOS, but neither COX nor adenosine receptors, modulates ATP-mediated cutaneous vasodilatation, whereas ATP does not directly increase sweating.