Different roles of adenosine A1, A2A and A3 receptors in controlling kainate-induced toxicity in cortical cultured neurons

Adenosine Targeting as a New Strategy to Decrease Glioblastoma Aggressiveness

Simple Summary

Given the rising mortality rate caused by GBM, current therapies do not appear to be effective in counteracting tumor progression. The role of adenosine and its interaction with specific receptor subtypes in various physiological functions has been studied for years. Only recently, adenosine has been defined as a tumor-protective target because of its accumulation in the tumor microenvironment. Current knowledge of the adenosine pathway and its involvement in brain tumors would support research in the development of adenosine receptor antagonists that could represent alternative treatments for glioblastoma, used either alone and/or in combination with chemotherapy, immunotherapy, or both.

Abstract

Glioblastoma is the most commonly malignant and aggressive brain tumor, with a high mortality rate. The role of the purine nucleotide adenosine and its interaction with its four subtypes receptors coupled to the different G proteins, A1, A2A, A2B, and A3, and its different physiological functions in different systems and organs, depending on the active receptor subtype, has been studied for years. Recently, several works have defined extracellular adenosine as a tumoral protector because of its accumulation in the tumor microenvironment. Its presence is due to both the interaction with the A2A receptor subtype and the increase in CD39 and CD73 gene expression induced by the hypoxic state. This fact has fueled preclinical and clinical research into the development of efficacious molecules acting on the adenosine pathway and blocking its accumulation. Given the success of anti-cancer immunotherapy, the new strategy is to develop selective A2A receptor antagonists that could competitively inhibit binding to its endogenous ligand, making them reliable candidates for the therapeutic management of brain tumors. Here, we focused on the efficacy of adenosine receptor antagonists and their enhancement in anti-cancer immunotherapy.

Genetic tagging of the adenosine A2A receptor reveals its heterogeneous expression in brain regions

The adenosine A_2A receptor (A_2AR), a G protein-coupled receptor, is involved in numerous and varied physiological and pathological processes, including inflammation, immune responses, blood flow, and neurotransmission. Accordingly, it has become an important drug target for the treatment of neuropsychiatric disorders. However, the exact brain distribution of A_2AR in regions outside the striatum that display relatively low levels of endogenous A_2AR expression has hampered the exploration of A_2AR functions under both physiological and pathological conditions. To further study the detailed distribution of the A_2AR in low-expression regions, we have generated A_2AR knock-in mice in which the 3xHA-2xMyc epitope tag sequence was fused to the C-terminus of A_2AR (A_2AR-tag mice) via CRISPR/Cas9 technology. Here, using CRISPR/Cas9 technology, we have generated A_2AR knock-in mice in which the 3xHA-2xMyc epitope tag sequence was fused to the C-terminus of A_2AR (A_2AR-tag mice). The A_2AR-tag mice exhibited normal locomotor activity and emotional state. Consistent with previous studies, A_2AR fluorescence was widely detected in the striatum, nucleus accumbens, and olfactory tubercles, with numerous labeled cells being evident in these regions in the A_2AR-tag mouse. Importantly, we also identified the presence of a few but clearly labeled cells in heterogeneous brain regions where A_2AR expression has not previously been unambiguously detected, including the lateral septum, hippocampus, amygdala, cerebral cortex, and gigantocellular reticular nucleus. The A_2AR-tag mouse represents a novel useful genetic tool for monitoring the expression of A_2AR and dissecting its functions in brain regions other than the striatum.

Caffeine as a Factor Influencing the Functioning of the Human Body-Friend or Foe?

Nowadays, caffeine is one of the most commonly consumed substances, which presents in many plants and products. It has both positive and negative effects on the human body, and its activity concerns a variety of systems including the central nervous system, immune system, digestive system, respiratory system, urinary tract, etc. These effects are dependent on quantity, the type of product in which caffeine is contained, and also on the individual differences among people (sex, age, diet etc.). The main aim of this review was to collect, present, and analyze the available information including the latest discoveries on the impact of caffeine on human health and the functioning of human body systems, taking into account the role of caffeine in individual disease entities. We present both the positive and negative sides of caffeine consumption and the healing properties of this purine alkaloid in diseases such as asthma, Parkinson's disease, and others, not forgetting about the negative effects of excess caffeine (e.g., in people with hypertension, children, adolescents, and the elderly). In summary, we can conclude, however, that caffeine has a multi-directional influence on various organs of the human body, and because of its anti-oxidative properties, it was, and still is, an interesting topic for research studies including those aimed at developing new therapeutic strategies.

Gamma-decanolactone attenuates acute and chronic seizures in mice: a possible role of adenosine A1 receptors

This study aimed to investigate the possible gamma-decanolactone mechanisms of action in the GABAergic and adenosine systems using the aminophylline-induced acute crisis model and the pentylenetetrazole-induced kindling model. In the acute model, male mice received administration of bicuculline (GABAA receptor antagonist), 8-cyclopentyl-1,3-dipropylxanthine (A1 receptor antagonist) or ZM241385 (A2A receptor antagonist), 15 min before the treatment with gamma-decanolactone (300 mg/kg). After a single dose of aminophylline was administered, the animals were observed for 60 min. In the chronic model of seizure, 30 min after the treatment with gamma-decanolactone, mice received pentylenetetrazole once every third day. On the last day of kindling, the animals received the same GABA and adenosine antagonists used in the acute model, 15 min before gamma-decanolactone administration. The protein expression of GABAA α1 receptor and adenosine A1 receptor was detected using western blotting technique in hippocampal samples. The results showed that gamma-decanolactone increased the latency to first seizure and decreased seizure occurrence in the acute and chronic models. The adenosine A2A receptor antagonist and GABAA receptor antagonist were not able to change gamma-decanolactone behavioral seizure induced by aminophylline or pentylenetetrazole. The administration of adenosine A1 receptor antagonist reversed the protective effect of gamma-decanolactone in both models. In addition, gamma-decanolactone promoted an increase in the expression GABAA α1 receptor, in the hippocampus. The results suggest that the neuroprotective effect of gamma-decanolactone observed during the investigation could have a straight connection to its action on A1 adenosine receptors.

Caffeine and adenosine A2A receptors rescue neuronal development in vitro of frontal cortical neurons in a rat model of attention deficit and hyperactivity disorder

Although some studies have supported the effects of caffeine for treatment of Attention deficit and hyperactivity disorder (ADHD), there were no evidences about its effects at the neuronal level. In this study, we sought to find morphological alterations during in vitro development of frontal cortical neurons from Spontaneoulsy hypertensive rats (SHR, an ADHD rat model) and Wistar-Kyoto rats (WKY, control strain). Further, we investigated the effects of caffeine and adenosine A₁ and A_2A receptors (A₁R and A_2AR) signaling. Cultured cortical neurons from WKY and SHR were analyzed by immunostaining of microtubule-associated protein 2 (MAP-2) and tau protein after treatment with either caffeine, or A₁R and A_2AR agonists or antagonists. Besides, the involvement of PI3K and not PKA signaling was also assessed. Neurons from ADHD model displayed less neurite branching, shorter maximal neurite length and decreased axonal outgrowth. While caffeine recovered neurite branching and elongation from ADHD neurons via both PKA and PI3K signaling, A_2AR agonist (CGS 21680) promoted more neurite branching via PKA signaling. The selective A_2AR antagonist (SCH 58261) was efficient in recovering axonal outgrowth from ADHD neurons through PI3K and not PKA signaling. For the first time, frontal cortical neurons were isolated from ADHD model and they presented disturbances in the differentiation and outgrowth. By showing that caffeine and A_2AR may act at neuronal level rescuing ADHD neurons outgrowth, our findings strengthen the potential of caffeine and A_2AR receptors as an adjuvant for ADHD treatment.

Coffee, caffeine, chlorogenic acid, and the purinergic system

Coffee is a drink prepared from roasted coffee beans and is lauded for its aroma and flavour. It is the third most popular beverage in the world. This beverage is known by its stimulant effect associated with the presence of methylxanthines. Caffeine, a purine-like molecule (1,3,7 trymetylxantine), is the most important bioactive compound in coffee, among others such as chlorogenic acid (CGA), diterpenes, and trigonelline. CGA is a phenolic acid with biological properties as antioxidant, anti-inflammatory, neuroprotector, hypolipidemic, and hypoglicemic. Purinergic system plays a key role inneuromodulation and homeostasis. Extracellular ATP, other nucleotides and adenosine are signalling molecules that act through their specific receptors, namely purinoceptors, P1 for nucleosides and P2 for nucleotides. They regulate many pathological processes, since adenosine, for instance, can limit the damage caused by ATP in the excitotoxicity from the neuronal cells. The primary purpose of this review is to discuss the effects of coffee, caffeine, and CGA on the purinergic system. This review focuses on the relationship/interplay between coffee, caffeine, CGA, and adenosine, and their effects on ectonucleotidases activities as well as on the modulation of P1 and P2 receptors from central nervous system and also in peripheral tissue.

Safety issues of compounds acting on adenosinergic signalling

OBJECTIVES

Much research has been performed on the field of identifying the roles of adenosine and adenosinergic signalling, but a relatively low number of marketing authorizations have been granted for adenosine receptor (AdR) ligands. In part, this could be related to their safety issues; therefore, our aim was to examine the toxicological and adverse effects data of different compounds acting on adenosinergic signalling, including different AdR ligands and compounds resembling the structure of adenosine. We also wanted to present recent pharmaceutical developments of experimental compounds that showed promising results in clinical trial setting.

KEY FINDINGS

Safety issues of compounds modulating adenosinergic signalling were investigated, and different mechanisms were presented. Structurally different classes of compounds act on AdRs, the most important being adenosine, adenosine derivatives and other non-nucleoside compounds. Many of them are either not selective enough or are targeting other targets of adenosinergic signalling such as metabolizing enzymes that regulate adenosine levels. Many other targets are also involved that are not part of adenosinergic signalling system such as GABA receptors, different channels, enzymes and others. Some synthetic AdR ligands even showed to be genotoxic.

SUMMARY

Current review presents safety data of adenosine, adenosine derivatives and other non-nucleoside compounds that modulate adenosinergic signalling. We have presented different mechanisms that participate to an adverse effect or toxic outcome. A separate section also deals with possible organ-specific toxic effects on different in-vitro and in-vivo models.

Adenosine production by brain cells

The early release of adenosine following traumatic brain injury (TBI) suppresses seizures and brain inflammation; thus, it is important to elucidate the cellular sources of adenosine following injurious stimuli triggered by TBI so that therapeutics for enhancing the early adenosine-release response can be optimized. Using mass spectrometry with ¹³ C-labeled standards, we investigated in cultured rat neurons, astrocytes, and microglia the effects of oxygen-glucose deprivation (OGD; models energy failure), H₂ O₂ (produces oxidative stress), and glutamate (induces excitotoxicity) on intracellular and extracellular levels of 5'-AMP (adenosine precursor), adenosine, and inosine and hypoxanthine (adenosine metabolites). In neurons, OGD triggered increases in intracellular 5'-AMP (2.8-fold), adenosine (2.6-fold), inosine (2.2-fold), and hypoxanthine (5.3-fold) and extracellular 5'-AMP (2.2-fold), adenosine (2.4-fold), and hypoxanthine (2.5-fold). In neurons, H₂ O₂ did not affect intracellular or extracellular purines; yet, glutamate increased intracellular adenosine, inosine, and hypoxanthine (1.7-fold, 1.7-fold, and 1.6-fold, respectively) and extracellular adenosine, inosine, and hypoxanthine (2.9-fold, 2.1-fold, and 1.6-fold, respectively). In astrocytes, neither H₂ O₂ nor glutamate affected intracellular or extracellular purines, and OGD only slightly increased intracellular and extracellular hypoxanthine. Microglia were unresponsive to OGD and glutamate, but were remarkably responsive to H₂ O₂ , which increased intracellular 5'-AMP (1.6-fold), adenosine (1.6-fold), inosine (2.1-fold), and hypoxanthine (1.6-fold) and extracellular 5'-AMP (5.9-fold), adenosine (4.0-fold), inosine (4.3-fold), and hypoxanthine (1.9-fold).

CONCLUSION

Under these particular experimental conditions, cultured neurons are the main contributors to adenosine production/release in response to OGD and glutamate, whereas cultured microglia are the main contributors upon oxidative stress. Developing therapeutics that recruit astrocytes to produce/release adenosine could have beneficial effects in TBI.

Having a Coffee Break: The Impact of Caffeine Consumption on Microglia-Mediated Inflammation in Neurodegenerative Diseases

Caffeine is the major component of coffee and the most consumed psychostimulant in the world and at nontoxic doses acts as a nonselective adenosine receptor antagonist. Epidemiological evidence suggests that caffeine consumption reduces the risk of several neurological and neurodegenerative diseases. However, despite the beneficial effects of caffeine consumption in human health and behaviour, the mechanisms by which it impacts the pathophysiology of neurodegenerative diseases still remain to be clarified. A promising hypothesis is that caffeine controls microglia-mediated neuroinflammatory response associated with the majority of neurodegenerative conditions. Accordingly, it has been already described that the modulation of adenosine receptors, namely, the A2A receptor, affords neuroprotection through the control of microglia reactivity and neuroinflammation. In this review, we will summarize the main effects of caffeine in the modulation of neuroinflammation in neurodegenerative diseases.

Basal adenosine modulates the functional properties of AMPA receptors in mouse hippocampal neurons through the activation of A1R A2AR and A3R

Adenosine is a widespread neuromodulator within the CNS and its extracellular level is increased during hypoxia or intense synaptic activity, modulating pre- and postsynaptic sites. We studied the neuromodulatory action of adenosine on glutamatergic currents in the hippocampus, showing that activation of multiple adenosine receptors (ARs) by basal adenosine impacts postsynaptic site. Specifically, the stimulation of both A₁R and A₃R reduces AMPA currents, while A_2AR has an opposite potentiating effect. The effect of ARs stimulation on glutamatergic currents in hippocampal cultures was investigated using pharmacological and genetic approaches. A₃R inhibition by MRS1523 increased GluR1-Ser845 phosphorylation and potentiated AMPA current amplitude, increasing the apparent affinity for the agonist. A similar effect was observed blocking A₁R with DPCPX or by genetic deletion of either A₃R or A₁R. Conversely, impairment of A_2AR reduced AMPA currents, and decreased agonist sensitivity. Consistently, in hippocampal slices, ARs activation by AR agonist NECA modulated glutamatergic current amplitude evoked by AMPA application or afferent fiber stimulation. Opposite effects of AR subtypes stimulation are likely associated to changes in GluR1 phosphorylation and represent a novel mechanism of physiological modulation of glutamatergic transmission by adenosine, likely acting in normal conditions in the brain, depending on the level of extracellular adenosine and the distribution of AR subtypes.

Adenosine A3 receptor activation is neuroprotective against retinal neurodegeneration

Death of retinal neural cells, namely retinal ganglion cells (RGCs), is a characteristic of several retinal neurodegenerative diseases. Although the role of adenosine A3 receptor (A3R) in neuroprotection is controversial, A3R activation has been reported to afford protection against several brain insults, with few studies in the retina. In vitro models (retinal neural and organotypic cultures) and animal models [ischemia-reperfusion (I-R) and partial optic nerve transection (pONT)] were used to study the neuroprotective properties of A3R activation against retinal neurodegeneration. The A3R selective agonist (2-Cl-IB-MECA, 1 μM) prevented apoptosis (TUNEL(+)-cells) induced by kainate and cyclothiazide (KA + CTZ) in retinal neural cultures (86.5 ± 7.4 and 37.2 ± 6.1 TUNEL(+)-cells/field, in KA + CTZ and KA + CTZ + 2-Cl-IB-MECA, respectively). In retinal organotypic cultures, 2-Cl-IB-MECA attenuated NMDA-induced cell death, assessed by TUNEL (17.3 ± 2.3 and 8.3 ± 1.2 TUNEL(+)-cells/mm(2) in NMDA and NMDA+2-Cl-IB-MECA, respectively) and PI incorporation (ratio DIV4/DIV2 3.3 ± 0.3 and 1.3 ± 0.1 in NMDA and NMDA+2-Cl-IB-MECA, respectively) assays. Intravitreal 2-Cl-IB-MECA administration afforded protection against I-R injury decreasing the number of TUNEL(+) cells by 72%, and increased RGC survival by 57%. Also, intravitreal administration of 2-Cl-IB-MECA inhibited apoptosis (from 449.4 ± 37.8 to 207.6 ± 48.9 annexin-V(+)-cells) and RGC loss (from 1.2 ± 0.6 to 8.1 ± 1.7 cells/mm) induced by pONT. This study demonstrates that 2-Cl-IB-MECA is neuroprotective to the retina, both in vitro and in vivo. Activation of A3R may have great potential in the management of retinal neurodegenerative diseases characterized by RGC death, as glaucoma and diabetic retinopathy, and ischemic diseases.

The A3 adenosine receptor: history and perspectives

By general consensus, the omnipresent purine nucleoside adenosine is considered a major regulator of local tissue function, especially when energy supply fails to meet cellular energy demand. Adenosine mediation involves activation of a family of four G protein-coupled adenosine receptors (ARs): A(1), A(2)A, A(2)B, and A(3). The A(3) adenosine receptor (A(3)AR) is the only adenosine subtype to be overexpressed in inflammatory and cancer cells, thus making it a potential target for therapy. Originally isolated as an orphan receptor, A(3)AR presented a twofold nature under different pathophysiologic conditions: it appeared to be protective/harmful under ischemic conditions, pro/anti-inflammatory, and pro/antitumoral depending on the systems investigated. Until recently, the greatest and most intriguing challenge has been to understand whether, and in which cases, selective A(3) agonists or antagonists would be the best choice. Today, the choice has been made and A(3)AR agonists are now under clinical development for some disorders including rheumatoid arthritis, psoriasis, glaucoma, and hepatocellular carcinoma. More specifically, the interest and relevance of these new agents derives from clinical data demonstrating that A(3)AR agonists are both effective and safe. Thus, it will become apparent in the present review that purine scientists do seem to be getting closer to their goal: the incorporation of adenosine ligands into drugs with the ability to save lives and improve human health.

Adenosine A2AR blockade prevents neuroinflammation-induced death of retinal ganglion cells caused by elevated pressure

BACKGROUND

Elevated intraocular pressure (IOP) is a major risk factor for glaucoma, a degenerative disease characterized by the loss of retinal ganglion cells (RGCs). There is clinical and experimental evidence that neuroinflammation is involved in the pathogenesis of glaucoma. Since the blockade of adenosine A2A receptor (A2AR) confers robust neuroprotection and controls microglia reactivity in the brain, we now investigated the ability of A2AR blockade to control the reactivity of microglia and neuroinflammation as well as RGC loss in retinal organotypic cultures exposed to elevated hydrostatic pressure (EHP) or lipopolysaccharide (LPS).

METHODS

Retinal organotypic cultures were either incubated with LPS (3 μg/mL), to elicit a pro-inflammatory response, or exposed to EHP (+70 mmHg), to mimic increased IOP, for 4 or 24 h, in the presence or absence of the A2AR antagonist SCH 58261 (50 nM). A2AR expression, microglial reactivity and neuroinflammatory response were evaluated by immunohistochemistry, quantitative PCR (qPCR) and enzyme-linked immunosorbent assay (ELISA). RGC loss was assessed by immunohistochemistry. In order to investigate the contribution of pro-inflammatory mediators to RGC loss, the organotypic retinal cultures were incubated with rabbit anti-tumour necrosis factor (TNF) (2 μg/mL) and goat anti-interleukin-1β (IL-1β) (1 μg/mL) antibodies.

RESULTS

We report that the A2AR antagonist (SCH 58261) prevented microglia reactivity, increase in pro-inflammatory mediators as well as RGC loss upon exposure to either LPS or EHP. Additionally, neutralization of TNF and IL-1β prevented RGC loss induced by LPS or EHP.

CONCLUSIONS

This work demonstrates that A2AR blockade confers neuroprotection to RGCs by controlling microglia-mediated retinal neuroinflammation and prompts the hypothesis that A2AR antagonists may be a novel therapeutic option to manage glaucomatous disorders.

Brain-derived neurotrophic factor mediates neuroprotection against Aβ-induced toxicity through a mechanism independent on adenosine 2A receptor activation

Brain-derived neurotrophic factor (BDNF) promotes neuronal survival through TrkB-FL activation. The activation of adenosine A2A receptors (A2AR) is essential for most of BDNF-mediated synaptic actions, such as synaptic plasticity, transmission and neurotransmitter release. We now aimed at evaluating the A2AR influence upon BDNF-mediated neuroprotection against Aβ25-35 toxicity in cultured neurons. Results showed that BDNF increases cell survival and reduces the caspase-3 and calpain activation induced by amyloid-β (Aβ) peptide, in a mechanism probably dependent on PLCγ pathway. This BDNF-mediated neuroprotection is not affected by A2AR activation or inhibition. Moreover neither activation nor inhibition of A2AR, per se, significantly influenced Aβ-induced neuronal death on calpain-mediated cleavage of TrkB induced by Aβ. In conclusion, these results suggest that, in opposition to the fast synaptic actions of BDNF, the neuroprotective actions of this neurotrophin against a strong Aβ insult do not require the activation of A2AR.

Expression, pharmacology and functional activity of adenosine A1 receptors in genetic models of Huntington's disease

Adenosine A1 receptor (A1R) stimulation exerts beneficial effects in response to various insults to the brain and, although it was found neuroprotective in a lesional model of Huntington's disease (HD), the features of this receptor in genetic models of HD have never been explored. In the present study we characterized the expression, affinity and functional effects of A1Rs in R6/2 mice (the most widely used transgenic model of HD) and in a cellular model of HD. Binding studies revealed that the density of A1Rs was significantly reduced in the cortex and the striatum of R6/2 mice compared to age-matched wild-type (WT), while receptor affinity was unchanged. The selective A1R agonist cyclopentyladenosine (CPA, 300nM) was significantly more effective in reducing synaptic transmission in corticostriatal slices from symptomatic R6/2 than in age-matched WT mice. Such an effect was due to a stronger inhibition of glutamate release from the pre-synaptic terminal. The different functional activities of A1Rs in HD mice were associated also to a different intracellular signaling pathway involved in the synaptic effect of CPA. In fact, while the PKA pathway was involved in both genotypes, p38 MAPK inhibitor SB203580 partially prevented synaptic effects of CPA in R6/2, but not in WT, mice; moreover, CPA differently modulated the phosphorylation status of p38 in the two genotypes. In vitro studies confirmed a different behavior of A1Rs in HD: CPA (100 nM for 5h) modulated cell viability in STHdh(Q111/Q111) (mhttHD cells), without affecting the viability of STHdh(Q7/Q7) (wthtt cells). This effect was prevented by the application of SB203580. Our results demonstrate that in the presence of the HD mutation A1Rs undergo profound changes in terms of expression, pharmacology and functional activity. These changes have to be taken in due account when considering A1Rs as a potential therapeutic target for this disease.

Adenosine receptors: expression, function and regulation

Adenosine receptors (ARs) comprise a group of G protein-coupled receptors (GPCR) which mediate the physiological actions of adenosine. To date, four AR subtypes have been cloned and identified in different tissues. These receptors have distinct localization, signal transduction pathways and different means of regulation upon exposure to agonists. This review will describe the biochemical characteristics and signaling cascade associated with each receptor and provide insight into how these receptors are regulated in response to agonists. A key property of some of these receptors is their ability to serve as sensors of cellular oxidative stress, which is transmitted by transcription factors, such as nuclear factor (NF)-κB, to regulate the expression of ARs. Recent observations of oligomerization of these receptors into homo- and heterodimers will be discussed. In addition, the importance of these receptors in the regulation of normal and pathological processes such as sleep, the development of cancers and in protection against hearing loss will be examined.

Differential adenosine uptake in mixed neuronal/glial or purified glial cultures of avian retinal cells: modulation by adenosine metabolism and the ERK cascade

Adenosine is an important modulator of neuronal survival and differentiation in the CNS. Our previous work showed that nucleoside transporters (NTs) are present in cultures of chick retinal cells, but little is known about the mechanisms regulating adenosine transport in these cultures. Our aim in the present work was to study the participation of the adenosine metabolism as well as the ERK pathway on adenosine uptake in different types of retinal cultures (mixed and purified glial cultures). Kinetic analysis in both cultures revealed that the uptake reached equilibrium after 30 min and presented two components. Incubation of cultures with S-(p-nitrobenzyl)-6-thioinosine (NBTI) or dipyridamole, different inhibitors of equilibrative nucleoside transporters (ENTs), produced a significant and concentration-dependent uptake reduction in both cultures. However, while dipyridamole presented similar maximal inhibitory effects in both cultures (although in different concentrations), the inhibition by NBTI was smaller in glial cultures than in mixed cultures, suggesting the presence of different transporters. Moreover, pre-incubation of [(3)H]-adenosine with adenosine deaminase (ADA) or adenosine kinase (ADK) inhibition with iodotubercidin promoted significant uptake inhibition in both cultures, indicating that the uptake is predominantly for adenosine and not inosine, and that taken up adenosine is preferentially directed to the synthesis of adenine nucleotides. In both cultures, the MEK inhibitors PD98059 or UO126, but not the inactive analog U0124, induced a significant and concentration-dependent uptake decrease. We have not observed any change in adenosine metabolism induced by MEK inhibitors, suggesting that this pathway is mediating a direct effect on NTs. Our results show the expression of different NTs in retinal cells in culture and that the activity of these transporters can be regulated by the ERK pathway or metabolic enzymes such as ADK which are then potential targets for regulation of Ado levels in normal or pathological conditions.

Activation of adenosine A2A receptor up-regulates BDNF expression in rat primary cortical neurons

As a member of neurotrophin family, brain derived neurotrophic factor (BDNF) plays critical roles in neuronal development, differentiation, synaptogenesis, and neural protection from the harmful stimuli. There have been reported that adenosine A2(A) receptor subtype is widely distributed in the brain regions, such as hippocampus, striatum, and cortex. Adenosine A2(A) receptor is colocalized with BDNF in brain regions and the functional interaction between A2(A) receptor stimulation and BDNF action has been suggested. In this study, we investigated the possibility that the activation of A2(A) receptor modulates BDNF production in rat primary cortical neuron. CGS21680, an adenosine A2(A) receptor agonist, induced BDNF expression and release. An antagonist against A2(A) receptor, ZM241385, prevented CGS21680-induced increase in BDNF production. A2(A) receptor stimulation induced the activation of Akt-GSK-3β signaling pathway and the blockade of the signaling pathway with specific inhibitors abolished the increase in BDNF production, possibly via modulation of ERK1/2-CREB pathway. The physiological roles of A2(A) receptor-induced BDNF production was demonstrated by the protection of neurons from the excitotoxicity and increased neurite extension as well as synapse formation from immature and mature neurons. Taken together, activation of A2(A) receptor regulates BDNF production in rat cortical neuron, which provides neuro-protective action.

Cell signaling in NMDA preconditioning and neuroprotection in convulsions induced by quinolinic acid

The search for novel, less invasive therapeutic strategies to treat neurodegenerative diseases has stimulated scientists to investigate the mechanisms involved in preconditioning. Preconditioning has been report to occur in many organs and tissues. In the brain, the modulation of glutamatergic transmission is an important and promising target to the use of effective neuroprotective agents. The glutamatergic excitotoxicity is a factor common to neurodegenerative diseases and acute events such as cerebral ischemia, traumatic brain injury and epilepsy. In this review we focus on the neuroprotection and preconditioning by chemical agents. Specially, chemical preconditioning models using N-methyl-d-aspartate (NMDA) pre-treatment, which has demonstrated to lead to neuroprotection against seizures and damage to neuronal tissue induced by quinolinic acid (QA). Here we attempted to gather important results obtained in the study of cellular and molecular mechanisms involved in NMDA preconditioning and neuroprotection.

Astrocytic activation and an inhibition of MAP kinases are required for proteinase-activated receptor-2-mediated protection from neurotoxicity

Proteinase-activated receptor-2 (PAR-2) expression levels are altered in several CNS disorders with these changes being proposed to either exacerbate or diminish the disease state depending on the cell type in which this occurs. Here we present data investigating the consequence of PAR-2 activation on kainate (KA)-induced neurotoxicity in organotypic hippocampal slices cultures (OHSC). Exposure of OHSC to the PAR-2 activators trypsin or Ser-Leu-Ile-Gly-Arg-Leu (SLIGRL) induced no neurotoxicity when applied alone but was neuroprotective against KA-induced neurotoxicity. SLIGRL-mediated neuroprotection involved astrocytic activation as the neuroprotective effect was abolished following OHSC pre-treatment with fluoroacetate. Moreover, co-application of either reparixin or LY341495, antagonists of the CXCR2 chemokine receptor and metabotropic glutamate receptors respectively, inhibited the SLIGRL-mediated neuroprotection. SLIGRL application inhibited both p38 MAPK and ERK activity in OHSC, but not the JNK 1/2 signalling pathway. Accordingly, the co-application of the p38 MAPK and ERK inhibitors SB203580 and UO126 reduced KA-induced cell death, mimicking PAR-2-mediated neuroprotection. These data indicate that PAR-2 activation is neuroprotective and involves astrocytic activation, gliotransmitter release, and the subsequent inhibition of MAPK signalling cascades, providing further evidence for PAR-2 as an interesting therapeutic target in certain CNS disorders.

The A3 adenosine receptor attenuates the calcium rise triggered by NMDA receptors in retinal ganglion cells

The A(3) adenosine receptor is emerging as an important regulator of neuronal signaling, and in some situations receptor stimulation can limit excitability. As the NMDA receptor frequently contributes to neuronal excitability, this study examined whether A(3) receptor activation could alter the calcium rise accompanying NMDA receptor stimulation. Calcium levels were determined from fura-2 imaging of isolated rat retinal ganglion cells as these neurons possess both receptor types. Brief application of glutamate or NMDA led to repeatable and reversible elevations of intracellular calcium. The A(3) agonist Cl-IB-MECA reduced the response to both glutamate and NMDA. While adenosine mimicked the effect of Cl-IB-MECA, the A(3) receptor antagonist MRS 1191 impeded the block by adenosine, implicating a role for the A(3) receptor in response to the natural agonist. The A(1) receptor antagonist DPCPX provided additional inhibition, implying a contribution from both A(1) and A(3) adenosine receptors. The novel A(3) agonist MRS 3558 (1'S,2'R,3'S,4'R,5'S)-4-(2-chloro-6-(3-chlorobenzylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo [3.1.0] hexane-1-carboxamide and mixed A(1)/A(3) agonist MRS 3630 (1'S,2'R,3'S,4'R,5'S)-4-(2-chloro-6-(cyclopentylamino)-9H-purin-9-yl)-2,3-dihydroxy-N-methylbicyclo [3.1.0] hexane-1-carboxamide also inhibited the calcium rise induced by NMDA. Low levels of MRS 3558 were particularly effective, with an IC(50) of 400 pM. In all cases, A(3) receptor stimulation inhibited only 30-50% of the calcium rise. In summary, stimulation of the A(3) adenosine receptor by either endogenous or synthesized agonists can limit the calcium rise accompanying NMDA receptor activation. It remains to be determined if partial block of the calcium rise by A(3) agonists can modify downstream responses to NMDA receptor stimulation.

Region-specific neuroprotective effect of ZM 241385 towards glutamate uptake inhibition in cultured neurons

Active uptake by neurons and glial cells is the main mechanism for maintaining extracellular glutamate at low, non-toxic concentrations. Adenosine A(2A) receptors regulate extracellular glutamate levels by acting on both the release and the uptake of glutamate. The aim of this study was to evaluate whether the inhibition of the effects of glutamate uptake blockers by adenosine A(2A) receptor antagonists resulted in neuroprotection. In cortical and striatal neuronal cultures, the application of l-trans-pyrrolidine-2,4-dicarboxylic acid (PDC, a transportable competitive inhibitor of glutamate uptake), induced a dose-dependent increase in lactate dehydrogenase (LDH) levels, an index of cytotoxicity. Such an effect of PDC was significantly reduced by pre-treatment with the adenosine A(2A) receptor antagonist ZM 241385 (50 nM) in striatal, but not cortical, cultures. The protective effects of ZM 241385 were specifically due to a counteraction of PDC effects, since ZM 241385 was totally ineffective in preventing the cytotoxicity induced by direct application of glutamate to cultures. These results indicate that adenosine A(2A) receptor antagonists prevent the toxic effects induced by a transportable competitive inhibitor of glutamate uptake, that such an effect specifically occurs in the striatum and that it does not depend on a direct blockade of glutamate-induced toxicity.

Does exposure to extremely low frequency magnetic fields produce functional changes in human brain?

Behavioral and neurophysiological changes have been reported after exposure to extremely low frequency magnetic fields (ELF-MF) both in animals and in humans. The physiological bases of these effects are still poorly understood. In vitro studies analyzed the effect of ELF-MF applied in pulsed mode (PEMFs) on neuronal cultures showing an increase in excitatory neurotransmission. Using transcranial brain stimulation, we studied noninvasively the effect of PEMFs on several measures of cortical excitability in 22 healthy volunteers, in 14 of the subjects we also evaluated the effects of sham field exposure. After 45 min of PEMF exposure, intracortical facilitation produced by paired pulse brain stimulation was significantly enhanced with an increase of about 20%, while other parameters of cortical excitability remained unchanged. Sham field exposure produced no effects. The increase in paired-pulse facilitation, a physiological parameter related to cortical glutamatergic activity, suggests that PEMFs exposure may produce an enhancement in cortical excitatory neurotransmission. This study suggests that PEMFs may produce functional changes in human brain.

Adenosine receptor ligands protect against a combination of apoptotic and necrotic cell death in cerebellar granule neurons

Agonists at A(1) receptors and antagonists at A(2A) receptors are known to be neuroprotective against excitotoxicity. We set out to clarify the mechanisms involved by studying interactions between adenosine receptor ligands and endogenous glutamate in cultures of rat cerebellar granule neurons (CGNs). Glutamate and the selective agonist N-methyl-D: -aspartate (NMDA), applied to CGNs at 9 div (days in vitro), both induced cell death in a concentration-dependent manner, which was attenuated by treatment with the NMDA receptor antagonists dizocilpine, D: -2-amino-5-phosphono-pentanoic acid (D: -AP5) or kynurenic acid (KYA), but not by the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). Glutamate toxicity was reduced in the presence of all of the following: cyclosporin A (CsA), a blocker of the membrane permeability transition pore, the caspase-3 inhibitor, benzyloxycarbonyl-Asp(OMe)-Glu(OMe)-Val-Asp(OMe)-fluoromethylketone (Z-DEVD-fmk), the poly (ADP-ribose) polymerase (PARP-1) inhibitor 3,4-dihydro-5-[4-(1-piperidinyl)butoxyl]-1(2H)-isoquinolinone (DPQ), and nicotinamide. This is indicative of involvement of both apoptotic and necrotic processes. The A(1) receptor agonist, N (6)-cyclopentyladenosine (CPA), and the A(2A) receptor antagonist 4-(2-[7-amino-2-[2-furyl][1,2,4]triazolo[2,3-a][1,3,5]triazo-5-yl-amino]ethyl)phenol (ZM241385) afforded significant protection, while the A(1) receptor blocker 8-cyclopentyl-1,3-dipropylxanthine (DPCPX) and the A(2A) receptor agonist 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxyamidoadenosine (CGS21680) had no effect. These results confirm that glutamate-induced neurotoxicity in CGNs is mainly via the NMDA receptor, but show that a form of cell death which exhibits aspects of both apoptosis and necrosis is involved. The protective activity of A(1) receptor activation or A(2A) receptor blockade occurs against this mixed profile of cell death, and appears not to involve the selective inhibition of classical apoptotic or necrotic cascades.

Adenosine deaminase-related genes: molecular identification, tissue expression pattern and truncated alternative splice isoform in adult zebrafish (Danio rerio)

Adenosine deaminase (ADA) is responsible for cleaving the neuromodulator adenosine to inosine. Two members of ADA subfamilies, known as ADA1 and ADA2, were described and evidence demonstrated another similar protein group named ADAL (adenosine deaminase "like"). Although the identification of ADA members seems to be consistent, the expression profile of ADA1, ADA2 and ADAL genes in zebrafish has not yet been reported. The aim of the present study was to map the expression pattern of ADA-related genes in various tissues of adult zebrafish (Danio rerio). An extensive search on zebrafish genome followed by a phylogenetic analysis confirmed the presence of distinct ADA-related genes (ADA1, ADAL and two orthologous genes of ADA2). Specific primers for each ADA member were designed, optimized semi-quantitative RT-PCR experiments were conducted and the relative amount of transcripts was determined. The tissue samples (brain, gills, heart, liver, skeletal muscle and kidney) were collected and the expression of ADA1, ADAL and ADA2 genes was characterized. ADA1 had a similar expression pattern, whereas ADAL was less expressed in the heart. The highest relative amount of ADA2-1 transcripts was observed in the brain, liver and gills and it was less expressed in the heart. RT-PCR assays revealed that the other ADA2 form (ADA2-2) was expressed ubiquitously and at comparable levels in zebrafish tissues. The strategy adopted also allowed the identification of an ADA2-1 truncated alternative splice isoform (ADA2-1/T), which was expressed at different intensities. These findings demonstrated the existence of different ADA-related genes, their distinct expression pattern and a truncated ADA2-1 isoform, which suggest a high degree of complexity in zebrafish adenosinergic system.

The A3 adenosine receptor: an enigmatic player in cell biology

Adenosine is a primordial signaling molecule present in every cell of the human body that mediates its physiological functions by interacting with 4 subtypes of G-protein-coupled receptors, termed A1, A2A, A2B and A3. The A3 subtype is perhaps the most enigmatic among adenosine receptors since, although several studies have been performed in the years to elucidate its physiological function, it still presents in several cases a double nature in different pathophysiological conditions. The 2 personalities of A3 often come into direct conflict, e.g., in ischemia, inflammation and cancer, rendering this receptor as a single entity behaving in 2 different ways. This review focuses on the most relevant aspects of A3 adenosine subtype activation and summarizes the pharmacological evidence as the basis of the dichotomy of this receptor in different therapeutic fields. Although much is still to be learned about the function of the A3 receptor and in spite of its duality, at the present time it can be speculated that A3 receptor selective ligands might show utility in the treatment of ischemic conditions, glaucoma, asthma, arthritis, cancer and other disorders in which inflammation is a feature. The biggest and most intriguing challenge for the future is therefore to understand whether and where selective A3 agonists or antagonists are the best choice.

Blockade of adenosine A(2A) receptors prevents staurosporine-induced apoptosis of rat hippocampal neurons

Since adenosine A(2A) receptor (A(2A)Rs) blockade protects against noxious brain insults involving apoptosis, we directly tested if A(2A)R blockade prevents apoptosis induced by staurosporine (STS). Exposure of rat hippocampal neurons to STS (30 nM, 24 h) decreased neuronal viability while increasing the number apoptotic-like neurons and de-localizing mitochondria and cytochrome c immunoreactivities. This was prevented by the selective A(2A)R antagonists, SCH58261 and ZM241385 (50 nM). Shorter incubation periods (6 h) with STS caused no neuronal loss but decreased synaptophysin and MAP-2 immunoreactivities, which was prevented by SCH58261. Furthermore, STS (100 nM) decreased MTT reduction and increased caspase-3 activity in rat hippocampal nerve terminals, which was prevented by SCH58261. These results show that A(2A)R blockade inhibits STS-induced apoptotic-like neuronal cell death. This begins with an apoptotic-like synaptotoxicity, which later evolved into an overt neurotoxicity, and A(2A)Rs effectively control this initial synaptotoxicity, in agreement with their predominant synaptic localization in the hippocampus.

Differential Expression of Cerebellar Metabotropic Glutamate Receptors mGLUR2/3 and mGLUR4a after the Administration of a Convulsant Drug and the Adenosine Analogue Cyclopentyladenosine

Metabotropic glutamate receptors (mGluR) play a role in synaptic transmission, neuronal modulation and plasticity but their action in epileptic activity is still controversial. On the other hand adenosine acts as a neuromodulator with endogenous anticonvulsive properties. Since cerebellum from epileptic patients has shown neuronal damage, sometimes associated with Purkinje cells loss, we have explored the effect of repetitive seizures on two types of mGluR in the cerebellum. Seizures were induced by the convulsant drug 3-mercaptopropionic acid (MP) and the effect of the adenosine analogue cyclopentyladenosine (CPA) alone or before MP administration (CPA+MP) were also evaluated. The expression of the receptors subtypes 2/3 (mGluR2/3) and 4a (mGluR4a) was assessed by immunocitochemistry. Granular cell layer was labeled with mGluR2/3 antibody and increased immunoreactivity was observed after MP (60%), CPA (53%) and CPA + MP (85%) treatments. Control cerebellum slices showed mGluR4a reactivity around Purkinje cells, while MP, CPA and CPA+MP treatment decreased this immunostaining. Repetitive administration of MP and CPA induces an increased cerebellar mGluR2/3 and a decreased mGluR4a immunostaining, suggesting a distinct participation of both receptors that may be related to the type of cell involved. A protective action and /or an apoptotic effect may not be discarded. CPA repetitive administration although increase seizure latency, cannot prevent seizure activity.

Endogenous Expression of Adenosine A1, A2 and A3 Receptors in Rat C6 Glioma Cells

Inhibitory and stimulatory adenosine receptors have been identified and characterized in both membranes and intact rat C6 glioma cells. In membranes, saturation experiment performed with [(3)H]DPCPX, selective A(1)R antagonist, revealed a single binding site with a K (D) = 9.4 +/- 1.4 nM and B (max) = 62.7 +/- 8.6 fmol/mg protein. Binding of [(3)H]DPCPX in intact cell revealed a K (D) = 17.7 +/- 1.3 nM and B (max )= 567.1 +/- 26.5 fmol/mg protein. On the other hand, [(3)H]ZM241385 binding experiments revealed a single binding site population of receptors with K (D) = 16.5 +/- 1.3 nM and B (max) = 358.9 +/- 52.4 fmol/mg protein in intact cells, and K (D) = 4.7 +/- 0.6 nM and B (max) = 74.3 +/- 7.9 fmol/mg protein in plasma membranes, suggesting the presence of A(2A) receptor in C6 cells. A(1), A(2A), A(2B) and A(3 )adenosine receptors were detected by Western-blotting and immunocytochemistry, and their mRNAs quantified by real time PCR assays. Gialpha and Gsalpha proteins were also detected by Western-blotting and RT-PCR assays. Furthermore, selective A(1)R agonists inhibited forskolin- and GTP-stimulated adenylyl cyclase activity and CGS 21680 and NECA stimulated this enzymatic activity in C6 cells. These results suggest that C6 glioma cells endogenously express A(1) and A(2) receptors functionally coupled to adenylyl cyclase inhibition and stimulation, respectively, and suggest these cells as a model to study the role of adenosine receptors in tumoral cells.

Activation of adenosine A3 receptors reduces ischemic brain injury in rodents

Adenosine A3 receptor (A3R) agonists have been shown to reduce cardiac and lung injury, but the protective roles of A3R agonists in the CNS are not well characterized. The protective effect of selective A3R agonist chloro-N(6)-(3-iodo-benzyl)-adenosine-5'-N-methyluronamide (Cl-IB-MECA) was first examined in primary cortical cultures. In cortical culture, Cl-IB-MECA pretreatment antagonized the hypoxia-mediated decrease in cell viability. In vivo, Cl-IB-MECA or vehicle was given intracerebroventricularly or intravenously to anesthetized rats. Animals were subjected to focal cerebral ischemia induced by transient middle cerebral artery (MCA) ligation. Intracerebroventricular or repeated intravenous administration (i.e., at 165 min and 15 min before MCA ligation) of Cl-IB-MECA did not alter blood pressure during ischemia but increased locomotor activity and decreased cerebral infarction 2 days after. In these animals, Cl-IB-MECA also reduced the density of TUNEL labeling in the lesioned cortex. The possibility of endogeneous neuroprotection was further examined in A3R knockout mice. After MCA ligation, an increase in cerebral infarction was found in the A3R knockouts compared with the A3R wild-type controls, suggesting that A3Rs are tonically activated during ischemia. Additionally, intracerebroventricular pretreatment with Cl-IB-MECA decreased the size of infarction in the wild-type controls, but not in the A3R knockout animals, suggesting that Cl-IB-MECA-induced protection was mediated through the A3 receptors. Collectively, these data suggest that Cl-IB-MECA reduced cerebral infarction through the activation of A3Rs and suppression of apoptosis.

Association of adenosine deaminase polymorphism with mild mental retardation

The etiology of mild mental retardation remains undefined in about 60% of cases. Even though the causes of mild mental retardation are likely to be heterogeneous, the evidence for genetic involvement is increasing, along with the development of specific diagnostic techniques. To improve our understanding of the genetic basis of mild mental retardation, we explored the role of polymorphisms of adenosine deaminase, an enzyme that is supposed to act as a neuroregulatory protein. To this end, we conducted an association study comparing children with mild mental retardation of unknown origin with two groups of controls: (1) apparently healthy children and (2) children with moderate or severe mental retardation of known etiology. Overall, 338 participants were enrolled in the study. Cases (ie, 80 children) were more likely than controls (ie, 153 healthy children and 105 children with moderate or severe mental retardation) to have the low-activity ADA-Asn 8 (ADA(1) *2) polymorphism (P < .05) and to present the ADA(1) *2/ ADA(2) *1 haplotype. No significant differences were found with respect to adenosine deaminase polymorphisms when comparing the group with moderate or severe mental retardation of known causes and healthy controls. In conclusion, our findings suggest a possible role for a low-activity genotype (ADA-8Asn) (ADA(1) *2) of adenosine deaminase in the pathogenesis of mild mental retardation.

Regulation of expression of early growth response transcription factors in rat primary cortical neurons by extracellular ATP

The zinc finger transcription factor early growth response-1 (Egr-1, NGFI-A, zif268, Krox 24, TIS8, ZENK) is upregulated immediately in the brain by cortical spreading depression (CSD) and other preconditioning stimuli and thus might participate in regulation of the overall genomic response to preconditioning. In the present study, the induction of expression of Egr-1 and other early growth response family members was characterized in rat primary cortical neuronal cultures. In neuronal cultures in vitro, depolarization or exposure to extracellular glutamate caused a 4-fold increase in egr-1 mRNA while exposure to extracellular ATP caused a 10-fold increase. The presence of mRNA encoding for multiple types of purinergic receptors was confirmed by RT-PCR. A number of nucleotide agonists proved effective in eliciting an increase in egr-1 mRNA. Over a limited range of concentration, the most effective agonists were ATP > ADP > alpha, beta-methylene ATP > UTP > cAMP > UDP > AMP > adenosine. Pertussis toxin, suramin, reactive blue 2, PPADS, DPCPX and inhibitors of Protein Kinase C, Protein Kinase A and PI3 kinase significantly reduced the upregulation of egr-1 by exposure to extracellular ATP. These findings suggest that neuronal metabotropic purinergic receptor activation contributes to the induction of early growth response transcription factors and may provide a target that can be manipulated to increase ischemic tolerance of the brain in vivo.

Modification of adenosine A1 and A2A receptor density in the hippocampus of streptozotocin-induced diabetic rats

Adenosine A(1) and A(2A) receptors are neuromodulatory systems that can control mnemonic behavior, which is modified by diabetes. Since the density of these adenosine receptors can change upon chronic noxious brain conditions, we now tested if the density of A(1) and A(2A) receptors was modified in the hippocampus of streptozotocin-induced diabetic rats. The binding density of the selective A(1) receptor antagonist, (3)H-DPCPX, was decreased by 36% in total hippocampal membranes 7 days after induction of diabetes and this down-regulation was maintained after 30 and 90 days, which was also confirmed by Western blot analysis of A(1) receptor immunoreactivity. In contrast, the binding density of the selective A(2A) receptor antagonist, (3)H-SCH 58261, was enhanced by 83% in total hippocampal membranes 7 days after induction of diabetes and this up-regulation persisted after 30 and 90 days. These results show that the balance between inhibitory A(1) and facilitatory A(2A) adenosine receptors is modified in the hippocampus of streptozotocin-induced diabetic rats. Thus, the most abundant A(1) receptors are down-regulated and there is an up-regulation of A(2A) receptors, suggesting a gain of function of hippocampal A(2A) receptors compared to A(1) receptors in diabetes, as has been observed in other chronic noxious brain conditions where A(2A) receptor blockade affords robust neuroprotection.