Antagonistic interaction between adenosine A2A receptors and Na+/K+-ATPase-α2 controlling glutamate uptake in astrocytes

Purinergic-associated immune responses in neurodegenerative diseases

The chronic activation of immune cells can participate in the development of pathological conditions such as neurodegenerative diseases including Alzheimer's disease (AD), Multiple Sclerosis (MS), Parkinson's disease (PD), Huntington's disease (HD) and Amyotrophic Lateral Sclerosis (ALS). In recent years, compelling evidence indicates that purinergic signaling plays a key role in neuro-immune cell functions. The extracellular release of adenosine 5'-triphosphate (ATP), and its breakdown products (ADP and adenosine) provide the versatile basis for complex purinergic signaling through the activation of several families of receptors. G-protein coupled adenosine A2A receptors, ionotropic P2X and G-protein coupled P2Y receptors for ATP and other nucleotides are abundant and widely distributed in neurons, microglia, and astrocytes of the central nervous system as well as in peripheral immune cells. These receptors are strongly linked to inflammation, with a functional interplay that may influence the intricate purinergic signaling involved in inflammatory responses. In the present review, we examine the roles of the purinergic receptors in neuro-immune cell functions with particular emphasis on A2AR, P2X4 and P2X7 and their possible relevance to specific neurodegenerative disorders. Understanding the molecular mechanisms governing purinergic receptor interaction will be crucial for advancing the development of effective immunotherapies targeting neurodegenerative diseases.

40 Hz light flickering facilitates the glymphatic flow via adenosine signaling in mice

The glymphatic-lymphatic system is increasingly recognized as fundamental for the homeostasis of the brain milieu since it defines cerebral spinal fluid flow in the brain parenchyma and eliminates metabolic waste. Animal and human studies have uncovered several important physiological factors regulating the glymphatic system including sleep, aquaporin-4, and hemodynamic factors. Yet, our understanding of the modulation of the glymphatic system is limited, which has hindered the development of glymphatic-based treatment for aging and neurodegenerative disorders. Here, we present the evidence from fluorescence tracing, two-photon recording, and dynamic contrast-enhanced magnetic resonance imaging analyses that 40 Hz light flickering enhanced glymphatic influx and efflux independently of anesthesia and sleep, an effect attributed to increased astrocytic aquaporin-4 polarization and enhanced vasomotion. Adenosine-A2A receptor (A2AR) signaling emerged as the neurochemical underpinning of 40 Hz flickering-induced enhancement of glymphatic flow, based on increased cerebrofluid adenosine levels, the abolishment of enhanced glymphatic flow by pharmacological or genetic inactivation of equilibrative nucleotide transporters-2 or of A2AR, and by the physical and functional A2AR-aquaporin-4 interaction in astrocytes. These findings establish 40 Hz light flickering as a novel non-invasive strategy of enhanced glymphatic flow, with translational potential to relieve brain disorders.

Adenosine and adenosine receptors in metabolic imbalance-related neurological issues

Metabolic syndromes (e.g., obesity) are characterized by insulin resistance, chronic inflammation, impaired glucose metabolism, and dyslipidemia. Recently, patients with metabolic syndromes have experienced not only metabolic problems but also neuropathological issues, including cognitive impairment. Several studies have reported blood-brain barrier (BBB) disruption and insulin resistance in the brain of patients with obesity and diabetes. Adenosine, a purine nucleoside, is known to regulate various cellular responses (e.g., the neuroinflammatory response) by binding with adenosine receptors in the central nervous system (CNS). Adenosine has four known receptors: A1R, A2AR, A2BR, and A3R. These receptors play distinct roles in various physiological and pathological processes in the brain, including endothelial cell homeostasis, insulin sensitivity, microglial activation, lipid metabolism, immune cell infiltration, and synaptic plasticity. Here, we review the recent findings on the role of adenosine receptor-mediated signaling in neuropathological issues related to metabolic imbalance. We highlight the importance of adenosine signaling in the development of therapeutic solutions for neuropathological issues in patients with metabolic syndromes.

Adora2A downregulation promotes caffeine neuroprotective effect against LPS-induced neuroinflammation in the hippocampus

Caffeine has been extensively studied in the context of CNS pathologies as many researchers have shown that consuming it reduces pro-inflammatory biomarkers, potentially delaying the progression of neurodegenerative pathologies. Several lines of evidence suggest that adenosine receptors, especially A1 and A2A receptors, are the main targets of its neuroprotective action. We found that caffeine pretreatment 15 min before LPS administration reduced the expression of Il1b in the hippocampus and striatum. The harmful modulation of caffeine-induced inflammatory response involved the downregulation of the expression of A2A receptors, especially in the hippocampus. Caffeine treatment alone promoted the downregulation of the adenosinergic receptor Adora2A; however, this promotion effect was reversed by LPS. Although administering caffeine increased the expression of the enzymes DNA methyltransferases 1 and 3A and decreased the expression of the demethylase enzyme Tet1, this effect was reversed by LPS in the hippocampus of mice that were administered Caffeine + LPS, relative to the basal condition; no significant differences were observed in the methylation status of the promoter regions of adenosine receptors. Finally, the bioinformatics analysis of the expanded network demonstrated the following results: the Adora2B gene connects the extended networks of the adenosine receptors Adora1 and Adora2A; the Mapk3 and Esr1 genes connect the extended Adora1 network; the Mapk4 and Arrb2 genes connect the extended Adora2A network with the extended network of the proinflammatory cytokine Il1β. These results indicated that the anti-inflammatory effects of acute caffeine administration in the hippocampus may be mediated by a complex network of interdependencies between the Adora2B and Adora2A genes.

Enhancement of the Evoked Excitatory Transmission in the Nucleus Tractus Solitarius Neurons after Sustained Hypoxia in Mice Depends on A2A Receptors

The first synapses of the afferents of peripheral chemoreceptors are located in the Nucleus Tractus Solitarius (NTS) and there is evidence that short-term sustained hypoxia (SH - 24 h, FiO2 0.1) facilitates glutamatergic transmission in NTS neurons of rats. Adenosine is an important neuromodulator of synaptic transmission and hypoxia contributes to increase its extracellular concentration. The A2A receptors mediate the excitatory actions of adenosine and are active players in the modulation of neuronal networks in the NTS. Herein, we used knockout mice for A2A receptors (A2AKO) and electrophysiological recordings of NTS neurons were performed to evaluate the contribution of these receptors in the changes in synaptic transmission in NTS neurons of mice submitted to SH. The membrane passive properties and excitability of NTS neurons were not affected by SH and were similar between A2AKO and wild-type mice. The overall amplitude of spontaneous glutamatergic currents in NTS neurons of A2AKO mice was lower than in Balb/c WT mice. SH increased the amplitude of evoked glutamatergic currents of NTS neurons from WT mice by a non-presynaptic mechanism, but this enhancement was not observed in NTS neurons of A2AKO mice. Under normoxia, the amplitude of evoked glutamatergic currents was similar between WT and A2AKO mice. The data indicate that A2A receptors (a) modulate spontaneous glutamatergic currents, (b) do not modulate the evoked glutamatergic transmission in the NTS neurons under control conditions, and (c) are required for the enhancement of glutamatergic transmission observed in the NTS neurons of mice submitted to SH.

Targeted ASO-mediated Atp1a2 knockdown in astrocytes reduces SOD1 aggregation and accelerates disease onset in mutant SOD1 mice

Astrocyte-specific ion pump α2-Na+/K+-ATPase plays a critical role in the pathogenesis of amyotrophic lateral sclerosis (ALS). Here, we test the effect of Atp1a2 mRNA-specific antisense oligonucleotides (ASOs) to induce α2-Na+/K+-ATPase knockdown in the widely used ALS animal model, SOD1*G93A mice. Two ASOs led to efficient Atp1a2 knockdown and significantly reduced SOD1 aggregation in vivo. Although Atp1a2 ASO-treated mice displayed no off-target or systemic toxicity, the ASO-treated mice exhibited an accelerated disease onset and shorter lifespan than control mice. Transcriptomics studies reveal downregulation of genes involved in oxidative response, metabolic pathways, trans-synaptic signaling, and upregulation of genes involved in glutamate receptor signaling and complement activation, suggesting a potential role for these molecular pathways in de-coupling SOD1 aggregation from survival in Atp1a2 ASO-treated mice. Together, these results reveal a role for α2-Na+/K+-ATPase in SOD1 aggregation and highlight the critical effect of temporal modulation of genetically validated therapeutic targets in neurodegenerative diseases.

Natural brain state change with E/I balance shifting toward inhibition is associated with vigilance impairment

The delicate balance between cortical excitation and inhibition (E/I) plays a pivotal role in brain state changes. While previous studies have associated cortical hyperexcitability with brain state changes induced by sleep deprivation, whether cortical hypoexcitability is also linked to brain state changes and, if so, how it could affect cognitive performance remain unknown. Here, we address these questions by examining the brain state change occurring after meals, i.e., postprandial somnolence, and comparing it with that induced by sleep deprivation. By analyzing features representing network excitability based on electroencephalogram (EEG) signals, we confirmed cortical hyperexcitability under sleep deprivation but revealed hypoexcitability under postprandial somnolence. In addition, we found that both sleep deprivation and postprandial somnolence adversely affected the level of vigilance. These results indicate that cortical E/I balance toward inhibition is associated with brain state changes, and deviation from the balanced state, regardless of its direction, could impair cognitive performance.

A2AR and traumatic brain injury

Accumulating evidence has revealed the adenosine 2A receptor is a key tuner for neuropathological and neurobehavioral changes following traumatic brain injury by experimental animal models and a few clinical trials. Here, we highlight recent data involving acute/sub-acute and chronic alterations of adenosine and adenosine 2A receptor-associated signaling in pathological conditions after trauma, with an emphasis of traumatic brain injury, including neuroinflammation, cognitive and psychiatric disorders, and other severe consequences. We expect this would lead to the development of therapeutic strategies for trauma-related disorders with novel mechanisms of action.

Aberrant adenosine signaling in patients with focal cortical dysplasia

Focal cortical dysplasia (FCD), a common malformation of cortical development, is frequently associated with pharmacoresistant epilepsy in both children and adults. Adenosine is an inhibitory modulator of brain activity and a prospective anti-seizure agent with potential for clinical translation. Our previous results demonstrated that the major adenosine-metabolizing enzyme adenosine kinase (ADK) was upregulated in balloon cells (BCs) within FCD type IIB lesions, suggesting that dysfunction of the adenosine system is implicated in the pathophysiology of FCD. In our current study, we therefore performed a comprehensive analysis of adenosine signaling in surgically resected cortical specimens from patients with FCD type I and type II via immunohistochemistry and immunoblot analysis. Adenosine enzyme signaling was assessed by quantifying the levels of the key enzymes of adenosine metabolism, i.e., ADK, adenosine deaminase (ADA), and ecto-5'-nucleotidase (CD73). Adenosine receptor signaling was assessed by quantifying the levels of adenosine A2A receptor (A2AR) and putative downstream mediators of adenosine, namely, glutamate transporter-1 (GLT-1) and mammalian target of rapamycin (mTOR). Within lesions in FCD specimens, we found that the adenosine-metabolizing enzymes ADK and ADA, as well as the adenosine-producing enzyme CD73, were upregulated. We also observed an increase in A2AR density, as well as a decrease in GLT-1 levels and an increase in mTOR levels, in FCD specimens compared with control tissue. These results suggest that dysregulation of the adenosine system is a common pathologic feature of both FCD type I and type II. The adenosine system might therefore be a therapeutic target for the treatment of epilepsy associated with FCD.

Adenosine A2A receptor availability in cerebral gray and white matter of patients with Parkinson's disease

OBJECTIVE

Atrophic changes in cerebral gray matter of patients with PD have been reported extensively. There is evidence suggesting an association between cortical gyrification changes and white matter abnormalities. Adenosine A2A receptors have been shown to be upregulated in cerebral white matter and on reactive astrocytes in preclinical models of neurodegenerative diseases. We, therefore, sought to investigate in vivo changes in A2A receptor availability in cerebral gray and white matter of PD patients and its association with gray matter atrophy.

METHODS

Eighteen patients with PD without dyskinesia and seven healthy controls were enrolled for this study. Brain MRI and dynamic PET scan was acquired with [11C]TMSX radioligand which binds selectively to A2A receptors. FreeSurfer software was used to segment cerebral gray and white matter structures. The resulting masks were used to calculate region specific volumes and to derive distribution volume ratios (DVRs), after co-registration with PET images, for the quantification of specific [11C]TMSX binding.

RESULTS

We showed an increase in A2A receptor availability in frontal (P < 0.001) and parietal (P < 0.001) white matter and a decrease in occipital (P = 0.02) gray matter of PD patients as compared to healthy controls. A decrease in gray matter volume ratios was observed in frontal (P < 0.01), parietal (P < 0.001), temporal (P < 0.01) and occipital (P < 0.01) ROIs in patients with PD versus healthy controls.

CONCLUSIONS

Our results suggest a role of A2A receptor-based signaling in the neurodegenerative changes seen in the cerebral gray and white matter of patients with PD.

Adenosine A2A Receptor Up-Regulation Pre-Dates Deficits of Synaptic Plasticity and of Memory in Mice Exposed to Aβ1-42 to Model Early Alzheimer's Disease

The intracerebroventricular (icv) injection of amyloid peptides (Aβ) models Alzheimer's disease (AD) in mice, as typified by the onset within 15 days of deficits of memory and of hippocampal long-term potentiation (LTP) that are prevented by the blockade of adenosine A2A receptors (A2AR). Since A2AR overfunction is sufficient to trigger memory deficits, we tested if A2AR were upregulated in hippocampal synapses before the onset of memory deficits to support the hypothesis that A2AR overfunction could be a trigger of AD. Six to eight days after Aβ-icv injection, mice displayed no alterations of hippocampal dependent memory; however, they presented an increased excitability of hippocampal synapses, a slight increase in LTP magnitude in Schaffer fiber-CA1 pyramid synapses and an increased density of A2AR in hippocampal synapses. A2AR blockade with SCH58261 (50 nM) normalized excitability and LTP in hippocampal slices from mice sacrificed 7-8 days after Aβ-icv injection. Fifteen days after Aβ-icv injection, mice displayed evident deficits of hippocampal-dependent memory deterioration, with reduced hippocampal CA1 LTP but no hyperexcitability and a sustained increase in synaptic A2AR, which blockade restored LTP magnitude. This shows that the upregulation of synaptic A2AR precedes the onset of deterioration of memory and of hippocampal synaptic plasticity, supporting the hypothesis that the overfunction of synaptic A2AR could be a trigger of memory deterioration in AD.

Striatal astrocytic A2A-D2 receptor-receptor interactions and their role in neuropsychiatric disorders

It is now generally accepted that astrocytes are active players in synaptic transmission, so that a neurocentric perspective of the integrative signal communication in the central nervous system is shifting towards a neuro-astrocentric perspective. Astrocytes respond to synaptic activity, release chemical signals (gliotransmitters) and express neurotransmitter receptors (G protein-coupled and ionotropic receptors), thus behaving as co-actors with neurons in signal communication in the central nervous system. The ability of G protein-coupled receptors to physically interact through heteromerization, forming heteromers and receptor mosaics with new distinct signal recognition and transduction pathways, has been intensively studied at neuronal plasma membrane, and has changed the view of the integrative signal communication in the central nervous system. One of the best-known examples of receptor-receptor interaction through heteromerization, with relevant consequences for both the physiological and the pharmacological points of view, is given by adenosine A2A and dopamine D2 receptors on the plasma membrane of striatal neurons. Here we review evidence that native A2A and D2 receptors can interact through heteromerization at the plasma membrane of astrocytes as well. Astrocytic A2A-D2 heteromers were found able to control the release of glutamate from the striatal astrocyte processes. A2A-D2 heteromers on striatal astrocytes and astrocyte processes are discussed as far as their potential relevance in the control of glutamatergic transmission in striatum is concerned, including potential roles in glutamatergic transmission dysregulation in pathological conditions including schizophrenia or the Parkinson's disease.

Pharmacological Characterization of P626, a Novel Dual Adenosine A2A/A2B Receptor Antagonist, on Synaptic Plasticity and during an Ischemic-like Insult in CA1 Rat Hippocampus

In recent years, the use of multi-target compounds has become an increasingly pursued strategy to treat complex pathologies, including cerebral ischemia. Adenosine and its receptors (A₁AR, A_2AAR, A_2BAR, A₃AR) are known to play a crucial role in synaptic transmission either in normoxic or ischemic-like conditions. Previous data demonstrate that the selective antagonism of A_2AAR or A_2BAR delays anoxic depolarization (AD) appearance, an unequivocal sign of neuronal injury induced by a severe oxygen-glucose deprivation (OGD) insult in the hippocampus. Furthermore, the stimulation of A_2AARs or A_2BARs by respective selective agonists, CGS21680 and BAY60-6583, increases pre-synaptic neurotransmitter release, as shown by the decrease in paired-pulse facilitation (PPF) at Schaffer collateral-CA1 synapses. In the present research, we investigated the effect/s of the newly synthesized dual A_2AAR/A_2BAR antagonist, P626, in preventing A_2AAR- and/or A_2BAR-mediated effects by extracellular recordings of synaptic potentials in the CA1 rat hippocampal slices. We demonstrated that P626 prevented PPF reduction induced by CGS21680 or BAY60-6583 and delayed, in a concentration-dependent manner, AD appearance during a severe OGD. In conclusion, P626 may represent a putative neuroprotective compound for stroke treatment with the possible translational advantage of reducing side effects and bypassing differences in pharmacokinetics due to combined treatment.

Astrocytic A2A receptors silencing negatively impacts hippocampal synaptic plasticity and memory of adult mice

Astrocytes are wired to bidirectionally communicate with neurons namely with synapses, thus shaping synaptic plasticity, which in the hippocampus is considered to underlie learning and memory. Adenosine A_2A receptors (A_2A R) are a potential candidate to modulate this bidirectional communication, since A_2A R regulate synaptic plasticity and memory and also control key astrocytic functions. Nonetheless, little is known about the role of astrocytic A_2A R in synaptic plasticity and hippocampal-dependent memory. Here, we investigated the impact of genetic silencing astrocytic A_2A R on hippocampal synaptic plasticity and memory of adult mice. The genetic A_2A R silencing in astrocytes was accomplished by a bilateral injection into the CA1 hippocampal area of a viral construct (AAV5-GFAP-GFP-Cre) that inactivate A_2A R expression in astrocytes of male adult mice carrying "floxed" A_2A R gene, as confirmed by A_2A R binding assays. Astrocytic A_2A R silencing alters astrocytic morphology, typified by an increment of astrocytic arbor complexity, and led to deficits in spatial reference memory and compromised hippocampal synaptic plasticity, typified by a reduction of LTP magnitude and a shift of synaptic long-term depression (LTD) toward LTP. These data indicate that astrocytic A_2A R control astrocytic morphology and influence hippocampal synaptic plasticity and memory of adult mice in a manner different from neuronal A_2A R.

Effects of adenosine A2A receptors on cognitive function in health and disease

Adenosine A2A receptors have been studied extensively in the context of motor function and movement disorders such as Parkinson's disease. In addition to these roles, A2A receptors have also been increasingly implicated in cognitive function and cognitive impairments in diverse conditions, including Alzheimer's disease, schizophrenia, acute brain injury, and stress. We review the roles of A2A receptors in cognitive processes in health and disease, focusing primarily on the effects of reducing or enhancing A2A expression levels or activities in animal models. Studies reveal that A2A receptors in neurons and astrocytes modulate multiple aspects of cognitive function, including memory and motivation. Converging evidence also indicates that A2A receptor levels and activities are aberrantly increased in aging, acute brain injury, and chronic disorders, and these increases contribute to neurocognitive impairments. Therapeutically targeting A2A receptors with selective modulators may alleviate cognitive deficits in diverse neurological and neuropsychiatric conditions. Further research on the exact neural mechanisms of these effects as well as the efficacy of selective A2A modulators on cognitive alterations in humans are important areas for future investigation.

The role of adenosine A2A receptors in Alzheimer's disease and tauopathies

Adenosine signals through four distinct G protein-coupled receptors that are located at various synapses, cell types and brain areas. Through them, adenosine regulates neuromodulation, neuronal signaling, learning and cognition as well as the sleep-wake cycle, all strongly impacted in neurogenerative disorders, among which Alzheimer's Disease (AD). AD is a complex form of cognitive deficits characterized by two pathological hallmarks: extracellular deposits of aggregated β-amyloid peptides and intraneuronal fibrillar aggregates of hyper- and abnormally phosphorylated Tau proteins. Both lesions contribute to the early dysfunction and loss of synapses which are strongly associated to the development of cognitive decline in AD patients. The present review focuses on the pathophysiological impact of the A_2ARs dysregulation observed in cognitive area from AD patients. We are reviewing not only evidence of the cellular changes in A_2AR levels in pathological conditions but also describe what is currently known about their consequences in term of synaptic plasticity, neuro-glial miscommunication and memory abilities. We finally summarize the proof-of-concept studies that support A_2AR as credible targets and the clinical interest to repurpose adenosine drugs for the treatment of AD and related disorders. This article is part of the Special Issue on "Purinergic Signaling: 50 years".

Cyclic AMP response element-binding protein (CREB) transcription factor in astrocytic synaptic communication

Astrocytes are known to actively participate in synaptic communication by forming structures called tripartite synapses. These synapses consist of presynaptic axon terminals, postsynaptic dendritic spines, and astrocytic processes where astrocytes release and receive transmitters. Although the transcription factor cyclic AMP response element (CRE)-binding protein (CREB) has been actively studied as an important factor for mediating synaptic activity-induced responses in neurons, its role in astrocytes is relatively unknown. Synaptic signals are known to activate various downstream pathways in astrocytes, which can activate the CREB transcription factor. Therefore, there is a need to summarize studies on astrocytic intracellular pathways that are induced by synaptic communication resulting in activation of the CREB pathway. In this review, we discuss the various neurotransmitter receptors and intracellular pathways that can induce CREB activation and CREB-induced gene regulation in astrocytes.

Astrocytes and major depression: The purinergic avenue

Major depressive disorder (MDD) is one of the most prevalent psychiatric illnesses worldwide which impairs the social functioning of the afflicted patients. Astrocytes promote homeostasis of the CNS and provide defense against various types of harmful influences. Increasing evidence suggests that the number, morphology and function of astrocytes are deteriorated in the depressed brain and the malfunction of the astrocytic purinergic system appears to participate in the pathophysiology of MDD. Adenosine 5'-triphosphate (ATP) released from astrocytes modulates depressive-like behavior in animal models and probably also clinical depression in patients. Astrocytes possess purinergic receptors, such as adenosine A_2A receptors (Rs), and P2X7, P2Y₁, and P2Y₁₁Rs, which mediate neuroinflammation, neuro(glio)transmission, and synaptic plasticity in depression-relevant areas of the brain (e.g. medial prefrontal cortex, hippocampus, amygdala nuclei). By contrast, astrocytic A₁Rs are neuroprotective and immunosuppressive. In the present review, we shall discuss the release of purines from astrocytes, and the expression/function of astrocytic purinergic receptors. Subsequently, we shall review in more detail novel evidence indicating that the dysregulation of astrocytic purinergic signaling actively contributes to the pathophysiology of depression and shall discuss possible therapeutic options based on knowledge recently acquired in this field.

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.

Aβ1-42 peptides blunt the adenosine A2A receptor-mediated control of the interplay between P2X7 and P2Y1 receptors mediated calcium responses in astrocytes

The contribution of astrocytes to Alzheimer's disease (AD) is still ill defined. AD involves an abnormal accumulation of amyloid-β peptides (Aβ) and increased production of danger signals such as ATP. ATP can direct or indirectly, through its metabolism into adenosine, trigger adaptive astrocytic responses resulting from intracellular Ca2+ oscillations. AD also triggers an upregulation of astrocytic adenosine A2A receptors (A2AR), which blockade prevents memory dysfunction in AD. We now investigated how Aβ peptides affect ATP-mediated Ca2+ responses in astrocytes measured by fluorescence live-cell imaging and whether A2AR control astrocytic Ca2+ responses mediated by ATP receptors, mainly P2X7R and P2Y1R. In primary cultures of rat astrocytes exposed to Aβ1-42, ATP-evoked Ca2+ responses had a lower amplitude but a longer duration than in control astrocytes and involved P2X7R and P2Y1R, the former potentiating the later. Moreover, Aβ1-42 exposure increased protein levels of P2Y1R in astrocytes. A2AR antagonism with SCH58261 controlled in a protein kinase A-dependent manner both P2X7R- and P2Y1R-mediated Ca2+ responses in astrocytes. The interplay between these purinoceptors in astrocytes was blunted upon exposure to Aβ1-42. These findings uncover the ability of A2AR to regulate the inter-twinned P2X7R- and P2Y1R-mediated Ca2+ dynamics in astrocytes, which is disrupted in conditions of early AD.

The Role of Astrocytes in Synapse Loss in Alzheimer's Disease: A Systematic Review

Alzheimer's disease (AD) is the most common cause of dementia, affecting 35 million people worldwide. One pathological feature of progressing AD is the loss of synapses. This is the strongest correlate of cognitive decline. Astrocytes, as an essential part of the tripartite synapse, play a role in synapse formation, maintenance, and elimination. During AD, astrocytes get a reactive phenotype with an altered gene expression profile and changed function compared to healthy astrocytes. This process likely affects their interaction with synapses. This systematic review aims to provide an overview of the scientific literature including information on how astrocytes affect synapse formation and elimination in the brain of AD patients and in animal models of the disease. We review molecular and cellular changes in AD astrocytes and conclude that these predominantly result in lower synapse numbers, indicative of decreased synapse support or even synaptotoxicity, or increased elimination, resulting in synapse loss, and consequential cognitive decline, as associated with AD. Preventing AD induced changes in astrocytes might therefore be a potential therapeutic target for dementia.

Systematic Review Registration:https://www.crd.york.ac.uk/prospero/display_record.php?RecordID=148278, identifier [CRD148278].

Adenosinergic Signaling as a Key Modulator of the Glioma Microenvironment and Reactive Astrocytes

Astrocytes are numerous glial cells of the central nervous system (CNS) and play important roles in brain homeostasis. These cells can directly communicate with neurons by releasing gliotransmitters, such as adenosine triphosphate (ATP) and glutamate, into the multipartite synapse. Moreover, astrocytes respond to tissue injury in the CNS environment. Recently, astrocytic heterogeneity and plasticity have been discussed by several authors, with studies proposing a spectrum of astrocytic activation characterized by A1/neurotoxic and A2/neuroprotective polarization extremes. The fundamental roles of astrocytes in communicating with other cells and sustaining homeostasis are regulated by purinergic signaling. In the CNS environment, the gliotransmitter ATP acts cooperatively with other glial signaling molecules, such as cytokines, which may impact CNS functions by facilitating/inhibiting neurotransmitter release. Adenosine (ADO), the main product of extracellular ATP metabolism, is an important homeostatic modulator and acts as a neuromodulator in synaptic transmission via P1 receptor sensitization. Furthermore, purinergic signaling is a key factor in the tumor microenvironment (TME), as damaged cells release ATP, leading to ADO accumulation in the TME through the ectonucleotidase cascade. Indeed, the enzyme CD73, which converts AMP to ADO, is overexpressed in glioblastoma cells; this upregulation is associated with tumor aggressiveness. Because of the crucial activity of CD73 in these cells, extracellular ADO accumulation in the TME contributes to sustaining glioblastoma immune escape while promoting A2-like activation. The present review describes the importance of ADO in modulating astrocyte polarization and simultaneously promoting tumor growth. We also discuss whether targeting of CD73 to block ADO production can be used as an alternative cancer therapy.

The Pathophysiology and Treatment of Essential Tremor: The Role of Adenosine and Dopamine Receptors in Animal Models

Essential tremor (ET) is one of the most common neurological disorders that often affects people in the prime of their lives, leading to a significant reduction in their quality of life, gradually making them unable to independently perform the simplest activities. Here we show that current ET pharmacotherapy often does not sufficiently alleviate disease symptoms and is completely ineffective in more than 30% of patients. At present, deep brain stimulation of the motor thalamus is the most effective ET treatment. However, like any brain surgery, it can cause many undesirable side effects; thus, it is only performed in patients with an advanced disease who are not responsive to drugs. Therefore, it seems extremely important to look for new strategies for treating ET. The purpose of this review is to summarize the current knowledge on the pathomechanism of ET based on studies in animal models of the disease, as well as to present and discuss the results of research available to date on various substances affecting dopamine (mainly D3) or adenosine A1 receptors, which, due to their ability to modulate harmaline-induced tremor, may provide the basis for the development of new potential therapies for ET in the future.

Association Between Adenosine A2A Receptors and Connexin 43 Regulates Hemichannels Activity and ATP Release in Astrocytes Exposed to Amyloid-β Peptides

Increasing evidence implicates astrocytes and the associated purinergic modulation in Alzheimer's disease (AD), characterized by cognitive deficits involving the extracellular amyloid-β peptides (Aβ) accumulation. Aβ can affect astrocytic gliotransmitters release, namely ATP, which is rapidly metabolized into adenosine by ecto-5'-nucleotidase, CD73, resulting in adenosine A_2A receptors (A_2AR) activation that bolsters neurodegeneration. AD's brains exhibit an upregulation of A_2AR and of connexin 43 (Cx43), which in astrocytes forms hemichannels that can mediate ATP release. However, a coupling between astrocytic A_2AR and Cx43 remains to be established. This was now investigated using astrocytic primary cultures exposed to Aβ_1-42 peptides. Aβ triggered ATP release through Cx43 hemichannels, a process blocked by A_2AR antagonists and mimicked by selective A_2AR activation. A_2AR directly regulated hemichannels activity and prevented Cx43 upregulation and phosphorylation observed in Aβ_1-42-exposed astrocytes. Moreover, a proximity ligand assay revealed a physical association between astrocytic A_2AR and Cx43. Finally, the blockade of CD73-mediated extracellular formation of ATP-derived adenosine prevented the Aβ-induced increase of Cx43 hemichannel activity and of ATP release. Overall, the data identify a feed-forward loop involving astrocytic A_2AR and Cx43 hemichannels, whereby A_2AR increase Cx43 hemichannel activity leading to increased ATP release, which is converted into adenosine by CD73, sustaining the increased astrocytic A_2AR activity in AD-like conditions.

Astrocytes and Adenosine A2A Receptors: Active Players in Alzheimer's Disease

Astrocytes, through their numerous processes, establish a bidirectional communication with neurons that is crucial to regulate synaptic plasticity, the purported neurophysiological basis of memory. This evidence contributed to change the classic “neurocentric” view of Alzheimer’s disease (AD), being astrocytes increasingly considered a key player in this neurodegenerative disease. AD, the most common form of dementia in the elderly, is characterized by a deterioration of memory and of other cognitive functions. Although, early cognitive deficits have been associated with synaptic loss and dysfunction caused by amyloid-β peptides (Aβ), accumulating evidences support a role of astrocytes in AD. Astrocyte atrophy and reactivity occurring at early and later stages of AD, respectively, involve morphological alterations that translate into functional changes. However, the main signals responsible for astrocytic alterations in AD and their impact on synaptic function remain to be defined. One possible candidate is adenosine, which can be formed upon extracellular catabolism of ATP released by astrocytes. Adenosine can act as a homeostatic modulator and also as a neuromodulator at the synaptic level, through the activation of adenosine receptors, mainly of A₁R and A_2AR subtypes. These receptors are also present in astrocytes, being particularly relevant in pathological conditions, to control the morphofunctional responses of astrocytes. Here, we will focus on the role of A_2AR, since they are particularly associated with neurodegeneration and also with memory processes. Furthermore, A_2AR levels are increased in the AD brain, namely in astrocytes where they can control key astrocytic functions. Thus, unveiling the role of A_2AR in astrocytes function might shed light on novel therapeutic strategies for AD.

Exogenous Adenosine Antagonizes Excitatory Amino Acid Toxicity in Primary Astrocytes

Excitatory toxicity is still a hot topic in the study of ischemic stroke, and related research has focused mainly on neurons. Adenosine is an important neuromodulator that is known as a "biosignature" in the central nervous system (CNS). The protective effect of exogenous adenosine on neurons has been confirmed, but its mechanism remains elusive. In this study, astrocytes were pretreated with adenosine, and the effects of an A2a receptor (A2aR) inhibitor (SCH58261) and A2b receptor (A2bR) inhibitor (PSB1115) on excitatory glutamate were investigated. An oxygen glucose deprivation/reoxygenation (OGD/R) and glutamate model was generated in vitro. Post-model assessment included expression levels of glutamate transporters (glt-1), gap junction protein (Cx43) and glutamate receptor (AMPAR), Na+-K+-ATPase activity, and diffusion distance of dyes. Glutamate and glutamine contents were determined at different time points. The results showed that (1) adenosine could improve the function of Na+-K+-ATPase, upregulate the expression of glt-1, and enhance the synthesis of glutamine in astrocytes. This effect was associated with A2aR activation but not with A2bR activation. (2) Adenosine could inhibit the expression of gap junction protein (Cx43) and reduce glutamate diffusion. Inhibition of A2aR attenuated adenosine inhibition of gap junction intercellular communication (GJIC) in the OGD/R model, while it enhanced adenosine inhibition of GJIC in the glutamate model, depending on the glutamate concentration. (3) Adenosine could cause AMPAR gradually entered the nucleus from the cytoplasm, thereby reducing the expression of AMPAR on the cell membrane. Taken together, the results indicate that adenosine plays a role of anti-excitatory toxicity effect in protection against neuronal death and the functional recovery of ischemic stroke mainly by targeting astrocytes, which are closely related to A2aR. The present study provided a scientific basis for adenosine prevention and ischemic stroke treatment, thereby providing a new approach for alleviating ischemic stroke.

Purinergic transmission in depressive disorders

Purinergic signaling involves the actions of purine nucleotides and nucleosides (such as adenosine) at P1 (adenosine), P2X, and P2Y receptors. Here, we present recent data contributing to a comprehensive overview of the association between purinergic signaling and depression. We start with background information on adenosine production and metabolism, followed by a detailed characterization of P1 and P2 receptors, with an emphasis on their expression and function in the brain as well as on their ligands. We provide data suggestive of altered metabolism of adenosine in depressed patients, which might be regarded as a disease biomarker. We then turn to considerable amount of preclinical/behavioral data obtained with the aid of the forced swim test, tail suspension test, learned helplessness model, or unpredictable chronic mild stress model and genetic activation/inactivation of P1 or P2 receptors as well as nonselective or selective ligands of P1 or P2 receptors. We also aimed to discuss the reason underlying discrepancies observed in such studies.

Allosteric Interactions between Adenosine A2A and Dopamine D2 Receptors in Heteromeric Complexes: Biochemical and Pharmacological Characteristics, and Opportunities for PET Imaging

Adenosine and dopamine interact antagonistically in living mammals. These interactions are mediated via adenosine A_2A and dopamine D₂ receptors (R). Stimulation of A_2AR inhibits and blockade of A_2AR enhances D₂R-mediated locomotor activation and goal-directed behavior in rodents. In striatal membrane preparations, adenosine decreases both the affinity and the signal transduction of D₂R via its interaction with A_2AR. Reciprocal A_2AR/D₂R interactions occur mainly in striatopallidal GABAergic medium spiny neurons (MSNs) of the indirect pathway that are involved in motor control, and in striatal astrocytes. In the nucleus accumbens, they also take place in MSNs involved in reward-related behavior. A_2AR and D₂R co-aggregate, co-internalize, and co-desensitize. They are at very close distance in biomembranes and form heteromers. Antagonistic interactions between adenosine and dopamine are (at least partially) caused by allosteric receptor–receptor interactions within A_2AR/D₂R heteromeric complexes. Such interactions may be exploited in novel strategies for the treatment of Parkinson’s disease, schizophrenia, substance abuse, and perhaps also attention deficit-hyperactivity disorder. Little is known about shifting A_2AR/D₂R heteromer/homodimer equilibria in the brain. Positron emission tomography with suitable ligands may provide in vivo information about receptor crosstalk in the living organism. Some experimental approaches, and strategies for the design of novel imaging agents (e.g., heterobivalent ligands) are proposed in this review.

Na+ , K+ -ATPase participates in the protective mechanism of rat cerebral ischemia-reperfusion through the interaction with glutamate transporter-1

Glutamate excitotoxicity in cerebral ischemia/reperfusion is an important cause of neurological damage. The aim of this study was to investigate the mechanism of Na+, K+-ATPase (NKA) involved in l ow concentration of ouabain (Oua, activating NKA)-induced protection of rat cerebral ischemia-reperfusion injury. The 2,3,5-triphenyltetrazolium chloride (TTC) staining and neurological deficit scores (NDS) were performed to evaluate rat cerebral injury degree respectively at 2 h, 6 h, 1 d and 3 d after reperfusion of middle cerebral artery occlusion (MCAO) 2 h in rats. NKA α1/α2 subunits and glutamate transporter-1 (GLT-1) protein expression were investigated by Western blotting. The cerebral infarct volume ratio were evidently decreased in Oua group vs MCAO/R group at 1 d and 3 d after reperfusion of 2 h MCAO in rats (*p ＜ 0.05 ). Moreover, NDS were not significantly different (p ＞ 0.05 ). NKA α1 was decreased at 6 h and 1 d after reperfusion of 2 h MCAO in rats, and was improved in Oua group. However, NKA α1 and α2 were increased at 3 d after reperfusion of 2 h MCAO in rats, and was decreased in Oua group. GLT-1 was decreased at 6 h, 1 d and 3 d after reperfusion of 2 h MCAO in rats, and was improved in Oua group. These data indicated that l ow concentration of Oua could improve MCAO/R injury through probably changing NKA α1/α2 and GLT-1 protein expression, then increasing GLT-1 function and promoting Glu transport and absorption, which could be useful to determine potential therapeutic strategies for patients with stroke. Low concentration of Oua improved rat MCAO/R injury via NKA α1/α2 and GLT-1.

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.

Maternal separation with early weaning impairs neuron-glia integrity: non-invasive evaluation and substructure demonstration

Astrocytes and oligodendrocytes play essential roles in regulating neural signal transduction along neural circuits in CNS. The perfect coordination of neuron/astrocyte and neuron/oligodendrocyte entities was termed as neuron-glia integrity recently. Here we monitored the status of neuron-glia integrity via non-invasive neuroimaging methods and demonstrated the substructures of it using other approaches in an animal model of maternal separation with early weaning (MSEW), which mimics early life neglect and abuse in humans. Compared to controls, MSEW rats showed higher glutamate level, but lower GABA in prefrontal cortex (PFC) detected by chemical exchange saturation transfer and ¹H-MRS methods, lower levels of glial glutamate transporter-1 and ATP-α, but increased levels of glutamate decarboxylase-65 and glutamine synthetase in PFC; reduced fractional anisotropy in various brain regions revealed by diffusion tensor imaging, along with increased levels of N-acetyl-aspartate measured by ¹H-MRS; and hypomyelination in PFC as evidenced by relevant cellular and molecular changes.

Purinergic signaling orchestrating neuron-glia communication

This review discusses the evidence supporting a role for ATP signaling (operated by P₂X and P₂Y receptors) and adenosine signaling (mainly operated by A₁ and A_2A receptors) in the crosstalk between neurons, astrocytes, microglia and oligodendrocytes. An initial emphasis will be given to the cooperation between adenosine receptors to sharpen information salience encoding across synapses. The interplay between ATP and adenosine signaling in the communication between astrocytes and neurons will then be presented in context of the integrative properties of the astrocytic syncytium, allowing to implement heterosynaptic depression processes in neuronal networks. The process of microglia 'activation' and its control by astrocytes and neurons will then be analyzed under the perspective of an interplay between different P₂ receptors and adenosine A_2A receptors. In spite of these indications of a prominent role of purinergic signaling in the bidirectional communication between neurons and glia, its therapeutical exploitation still awaits obtaining an integrated view of the spatio-temporal action of ATP signaling and adenosine signaling, clearly distinguishing the involvement of both purinergic signaling systems in the regulation of physiological processes and in the control of pathogenic-like responses upon brain dysfunction or damage.

Effects of Chronic Caffeine Administration on Behavioral and Molecular Adaptations to Sensory Contact Model Induced Stress in Adolescent Male Mice

Previous studies have shown that caffeine attenuates stress-induced mood dysfunction and memory deterioration through neuronal adenosine A2A receptors antagonism. However, whether caffeine exerts this effect through modulating other molecular targets, which interfere with the resilience to social defeat stress in adolescent male mice is unknown. This study was conducted to investigate the role of caffeine in the behavioral responses to social stress induced by the sensory contact model (SCM) and the possible alteration of the gene expression level of Na/K ATPase pump. Adolescent male mice were exposed to SCM for 12 days. Caffeine was administered intraperitoneal daily for 14 days after SCM. The time spent in interaction zone, social interaction ratio, preference index to novel objects, time spent in the open arms and immobility time in forced swimming test were used to measure the locomotor activity, social avoidance, short-term memory, anxiety and depression in mice. The results showed that chronic treatment with caffeine for 14 days improved locomotor activity, reversed the avoidance of social behavior, improved preference to novel objects, and reversed depression induced by social defeat stress in adolescent male mice, suggesting the enhancement of the resilience to social defeat stress induced by caffeine. Moreover, caffeine treatment did alter gene expression levels of Na/K ATPase isoforms in both prefrontal cortex and hippocampus. Altered gene expression was significant in most cases and correlates with the observed behavioral changes. Taken together, our findings provide new insight into the effects of chronic caffeine administration on locomotor activity, social avoidance, short-term memory and depression in adolescent male mice exposed to SCM.

Adenosine A2AReceptors in Substance Use Disorders: A Focus on Cocaine

Several psychoactive drugs can evoke substance use disorders (SUD) in humans and animals, and these include psychostimulants, opioids, cannabinoids (CB), nicotine, and alcohol. The etiology, mechanistic processes, and the therapeutic options to deal with SUD are not well understood. The common feature of all abused drugs is that they increase dopamine (DA) neurotransmission within the mesocorticolimbic circuitry of the brain followed by the activation of DA receptors. D₂ receptors were proposed as important molecular targets for SUD. The findings showed that D₂ receptors formed heteromeric complexes with other GPCRs, which forced the addiction research area in new directions. In this review, we updated the view on the brain D₂ receptor complexes with adenosine (A)2A receptors (A_2AR) and discussed the role of A_2AR in different aspects of addiction phenotypes in laboratory animal procedures that permit the highly complex syndrome of human drug addiction. We presented the current knowledge on the neurochemical in vivo and ex vivo mechanisms related to cocaine use disorder (CUD) and discussed future research directions for A_2AR heteromeric complexes in SUD.

Hippocampal synaptic dysfunction in the SOD1G93A mouse model of Amyotrophic Lateral Sclerosis: Reversal by adenosine A2AR blockade

Amyotrophic Lateral Sclerosis (ALS) mostly affects motor neurons, but non-motor neural and cognitive alterations have been reported in ALS mouse models and patients. Here, we evaluated if time-dependent biphasic changes in synaptic transmission and plasticity occur in hippocampal synapses of ALS SOD1^G93A mice. Recordings were performed in hippocampal slices of SOD1^G93A and age-matched WT mice, in the pre-symptomatic and symptomatic stages. We found an enhancement of pre-synaptic function and increased adenosine A_2A receptor levels in the hippocampus of pre-symptomatic mice. In contrast, in symptomatic mice, there was an impairment of long-term potentiation (LTP) and a decrease in NMDA receptor-mediated synaptic currents, with A_2AR levels also being increased. Chronic treatment with the A_2AR antagonist KW-6002, rescued LTP and A_2AR values. Altogether, these findings suggest an increase in synaptic function during the pre-symptomatic stage, followed by a decrease in synaptic plasticity in the symptomatic stage, which involves over-activation of A_2AR from early disease stages.

Beyond the neuron-cellular interactions early in Alzheimer disease pathogenesis

The symptoms of Alzheimer disease reflect a loss of neural circuit integrity in the brain, but neurons do not work in isolation. Emerging evidence suggests that the intricate balance of interactions between neurons, astrocytes, microglia and vascular cells required for healthy brain function becomes perturbed during the disease, with early changes likely protecting neural circuits from damage, followed later by harmful effects when the balance cannot be restored. Moving beyond a neuronal focus to understand the complex cellular interactions in Alzheimer disease and how these change throughout the course of the disease may provide important insight into developing effective therapeutics.

Adenosine A2A-dopamine D2 receptor-receptor interaction in neurons and astrocytes: Evidence and perspectives

The discovery of receptor-receptor interactions in the early 1980s, together with a more accurate focusing of allosteric mechanisms in proteins, expanded the knowledge on the G protein-coupled receptor (GPCR)-mediated signaling processes. GPCRs were seen to operate not only as monomers, but also as quaternary structures shaped by allosteric interactions. These integrative mechanisms can change the function of the GPCRs involved, leading to a sophisticated dynamic of the receptor assembly in terms of modulation of recognition and signaling. In this context, the heterodimeric complex formed by the adenosine A_2A and the dopamine D₂ receptors likely represents a prototypical example. The pharmacological evidence obtained, together with the tissue distribution of the A_2A-D₂ heteromeric complexes, suggested they could represent a target for new therapeutic strategies addressing significant disorders of the central nervous system. The research findings and the perspectives they offer from the therapeutic standpoint are the focus of the here presented discussion.

Novel Players in the Aging Synapse: Impact on Cognition

While neuronal loss has long been considered as the main contributor to age-related cognitive decline, these alterations are currently attributed to gradual synaptic dysfunction driven by calcium dyshomeostasis and alterations in ionotropic/metabotropic receptors. Given the key role of the hippocampus in encoding, storage, and retrieval of memory, the morpho- and electrophysiological alterations that occur in the major synapse of this network-the glutamatergic-deserve special attention. We guide you through the hippocampal anatomy, circuitry, and function in physiological context and focus on alterations in neuronal morphology, calcium dynamics, and plasticity induced by aging and Alzheimer's disease (AD). We provide state-of-the art knowledge on glutamatergic transmission and discuss implications of these novel players for intervention. A link between regular consumption of caffeine-an adenosine receptor blocker-to decreased risk of AD in humans is well established, while the mechanisms responsible have only now been uncovered. We review compelling evidence from humans and animal models that implicate adenosine A2A receptors (A2AR) upsurge as a crucial mediator of age-related synaptic dysfunction. The relevance of this mechanism in patients was very recently demonstrated in the form of a significant association of the A2AR-encoding gene with hippocampal volume (synaptic loss) in mild cognitive impairment and AD. Novel pathways implicate A2AR in the control of mGluR5-dependent NMDAR activation and subsequent Ca2+ dysfunction upon aging. The nature of this receptor makes it particularly suited for long-term therapies, as an alternative for regulating aberrant mGluR5/NMDAR signaling in aging and disease, without disrupting their crucial constitutive activity.

Upregulation of adenosine A2A receptor and downregulation of GLT1 is associated with neuronal cell death in Rasmussen's encephalitis

Rasmussen encephalitis (RE) is a severe pediatric inflammatory brain disease characterized by unilateral inflammation and atrophy of the cerebral cortex, drug-resistant focal epilepsy and progressive neurological and cognitive deterioration. The etiology and pathogenesis of RE remain unclear. Our previous results demonstrated that the adenosine A1 receptor (A1R) and the major adenosine-removing enzyme adenosine kinase play an important role in the etiology of RE. Because the downstream pathways of inhibitory A1R signaling are modulated by stimulatory A2AR signaling, which by itself controls neuro-inflammation, glial activation and glial glutamate homeostasis through interaction with glutamate transporter GLT-1, we hypothesized that maladaptive changes in adenosine A2A receptor (A2AR) expression are associated with RE. We used immunohistochemistry and Western blot analysis to examine the expression of A2ARs, glutamate transporter-I (GLT-1) and the apoptotic marker Bcl-2 in surgically resected cortical specimens from RE patients (n = 18) in comparison with control cortical tissue. In lesions of the RE specimen we found upregulation of A2ARs, downregulation of GLT-1 and increased apoptosis of both neurons and astroglia. Double staining revealed colocalization of A2ARs and Bcl-2 in RE lesions. These results suggest that maladaptive changes in A2AR expression are associated with a decrease in GLT-I expression as a possible precipitator for apoptotic cell loss in RE. Because A2AR antagonists are already under clinical evaluation for Parkinson's disease, the A2AR might likewise be a tractable target for the treatment of RE.

Carbamazepine Attenuates Astroglial L-Glutamate Release Induced by Pro-Inflammatory Cytokines via Chronically Activation of Adenosine A2A Receptor

Carbamazepine (CBZ) binds adenosine receptors, but detailed effects of CBZ on astroglial transmission associated with adenosine receptor still need to be clarified. To clarify adenosinergic action of CBZ on astroglial transmission, primary cultured astrocytes were acutely or chronically treated with CBZ, proinflammatory cytokines (interferon γ (IFNγ) and tumor necrosis factor α (TNFα)), and adenosine A2A receptor (A2AR) agonist (CGS21680). IFNγ and TNFα increased basal, adenophostin-A (AdA)-evoked, and 2-amino-3-(3-hydroxy-5-methyl-isoxazol-4-yl)propanoic acid (AMPA)-evoked astroglial L-glutamate releases. In physiological condition, CGS21680 increased basal astroglial L-glutamate release but glutamate transporter inhibition prevented this CGS21680 action. CBZ did not affect basal release, whereas glutamate transporter inhibition generated CBZ-induced glutamate release. Furthermore, AdA-evoked and AMPA-evoked releases were inhibited by CBZ but were unaffected by CGS21680. Contrary to physiological condition, chronic administrations of IFNγ and TNFα enhanced basal, AdA-, and AMPA-evoked releases, whereas IFNγ and TNFα decreased and increased CGS21680-evoked releases via modulation A2AR expression. Both chronic administration of CGS21680 and CBZ suppressed astroglial L-glutamate release responses induced by chronic cytokine exposer. Especifically, chronic administration of CBZ and CGS21680 prevented the reduction and elevation of A2AR expression by respective IFNγ and TNFα. These findings suggest that A2AR agonistic effects of CBZ contribute to chronic prevention of pathomechanisms developments of several neuropsychiatric disorders associated with proinflammatory cytokines.

Feedback adaptation of synaptic excitability via Glu:Na+ symport driven astrocytic GABA and Gln release

Glutamatergic transmission composed of the arriving of action potential at the axon terminal, fast vesicular Glu release, postsynaptic Glu receptor activation, astrocytic Glu clearance and Glu→Gln shuttle is an abundantly investigated phenomenon. Despite its essential role, however, much less is known about the consequences of the mechanistic connotations of Glu:Na⁺ symport. Due to the coupled Na⁺ transport, Glu uptake results in significantly elevated intracellular astrocytic [Na⁺] that markedly alters the driving force of other Na⁺-coupled astrocytic transporters. The resulting GABA and Gln release by reverse transport through the respective GAT-3 and SNAT3 transporters help to re-establish the physiological Na⁺ homeostasis without ATP dissipation and consequently leads to enhanced tonic inhibition and replenishment of axonal glutamate pool. Here, we place this emerging astrocytic adjustment of synaptic excitability into the centre of future perspectives. This article is part of the issue entitled 'Special Issue on Neurotransmitter Transporters'.

Receptor-Receptor Interactions as a Widespread Phenomenon: Novel Targets for Drug Development?

The discovery of receptor-receptor interactions (RRI) has expanded our understanding of the role that G protein-coupled receptors (GPCRs) play in intercellular communication. The finding that GPCRs can operate as receptor complexes, and not only as monomers, suggests that several different incoming signals could already be integrated at the plasma membrane level via direct allosteric interactions between the protomers that form the complex. Most research in this field has focused on neuronal populations and has led to the identification of a large number of RRI. However, RRI have been seen to occur not only in neurons but also in astrocytes and, outside the central nervous system, in cells of the cardiovascular and endocrine systems and in cancer cells. Furthermore, RRI involving the formation of macromolecular complexes are not limited to GPCRs, being also observed in other families of receptors. Thus, RRI appear as a widespread phenomenon and oligomerization as a common mechanism for receptor function and regulation. The discovery of these macromolecular assemblies may well have a major impact on pharmacology. Indeed, the formation of receptor complexes significantly broadens the spectrum of mechanisms available to receptors for recognition and signaling, which may be implemented through modulation of the binding sites of the adjacent protomers and of their signal transduction features. In this context, the possible appearance of novel allosteric sites in the receptor complex structure may be of particular relevance. Thus, the existence of RRI offers the possibility of new therapeutic approaches, and novel pharmacological strategies for disease treatment have already been proposed. Several challenges, however, remain. These include the accurate characterization of the role that the receptor complexes identified so far play in pathological conditions and the development of ligands specific to given receptor complexes, in order to efficiently exploit the pharmacological properties of these complexes.

Adenosine Actions on Oligodendroglia and Myelination in Autism Spectrum Disorder

Autism spectrum disorder (ASD) is the most commonly diagnosed neurodevelopmental disorder. Independent of neuronal dysfunction, ASD and its associated comorbidities have been linked to hypomyelination and oligodendroglial dysfunction. Additionally, the neuromodulator adenosine has been shown to affect certain ASD comorbidities and symptoms, such as epilepsy, impairment of cognitive function, and anxiety. Adenosine is both directly and indirectly responsible for regulating the development of oligodendroglia and myelination through its interaction with, and modulation of, several neurotransmitters, including glutamate, dopamine, and serotonin. In this review, we will focus on the recent discoveries in adenosine interaction with physiological and pathophysiological activities of oligodendroglia and myelination, as well as ASD-related aspects of adenosine actions on neuroprotection and neuroinflammation. Moreover, we will discuss the potential therapeutic value and clinical approaches of adenosine manipulation against hypomyelination in ASD.

Microglia in Alzheimer's Disease: Risk Factors and Inflammation

Microglia are resident immune cells in the central nervous system (CNS) that originate from myeloid progenitor cells in the embryonic yolk sac and are maintained independently of circulating monocytes throughout life. In the healthy state, microglia are highly dynamic and control the environment by rapidly extending and retracting their processes. When the CNS is inflamed, microglia can give rise to macrophages, but the regulatory mechanisms underlying this process have not been fully elucidated. Recent genetic studies have suggested that microglial function is compromised in Alzheimer's disease (AD), and that environmental factors such as diet and brain injury also affect microglial activation. In addition, studies of triggering receptor expressed on myeloid cells 2-deficiency in AD mice revealed heterogeneous microglial reactions at different disease stages, complicating the therapeutic strategy for AD. In this paper, we describe the relationship between genetic and environmental risk factors and the roles of microglia in AD pathogenesis, based on studies performed in human patients and animal models. We also discuss the mechanisms of inflammasomes and neurotransmitters in microglia, which accelerate the development of amyloid-β and tau pathology.

Na+ , K+ -ATPase activity in children with autism spectrum disorder: Searching for the reason(s) of its decrease in blood cells

Na⁺, K⁺‐ATPase (NKA) activity, which establishes the sodium and potassium gradient across the cell membrane and is instrumental in the propagation of the nerve impulses, is altered in a number of neurological and neuropsychiatric disorders, including autism spectrum disorders (ASD). In the present work, we examined a wide range of biochemical and cellular parameters in the attempt to understand the reason(s) for the severe decrease in NKA activity in erythrocytes of ASD children that we reported previously. NKA activity in leukocytes was found to be decreased independently from alteration in plasma membrane fluidity. The different subunits were evaluated for gene expression in leukocytes and for protein expression in erythrocytes: small differences in gene expression between ASD and typically developing children were not apparently paralleled by differences in protein expression. Moreover, no gross difference in erythrocyte plasma membrane oxidative modifications was detectable, although oxidative stress in blood samples from ASD children was confirmed by increased expression of NRF2 mRNA. Interestingly, gene expression of some NKA subunits correlated with clinical features. Excess inhibitory metals or ouabain‐like activities, which might account for NKA activity decrease, were ruled out. Plasma membrane cholesterol, but not phosphatidylcholine and phosphatidlserine, was slighty decreased in erythrocytes from ASD children. Although no compelling results were obtained, our data suggest that alteration in the erytrocyte lipid moiety or subtle oxidative modifications in NKA structure are likely candidates for the observed decrease in NKA activity. These findings are discussed in the light of the relevance of NKA in ASD. Autism Res2018, 11: 1388–1403. © 2018 International Society for Autism Research, Wiley Periodicals, Inc.

Lay Summary

The activity of the cell membrane enzyme NKA, which is instrumental in the propagation of the nerve impulses, is severely decreased in erythrocytes from ASD children and in other brain disorders, yet no explanation has been provided for this observation. We strived to find a biological/biochemical cause of such alteration, but most queries went unsolved because of the complexity of NKA regulation. As NKA activity is altered in many brain disorders, we stress the relevance of studies aimed at understanding its regulation in ASD.

Guanosine Attenuates Behavioral Deficits After Traumatic Brain Injury by Modulation of Adenosinergic Receptors

Traumatic brain injury (TBI) is a leading cause of disability worldwide, triggering chronic neurodegeneration underlying cognitive and mood disorder still without therapeutic prospects. Based on our previous observations that guanosine (GUO) attenuates short-term neurochemical alterations caused by TBI, this study investigated the effects of chronical GUO treatment in behavioral, molecular, and morphological disturbances 21 days after trauma. Rats subject to TBI displayed mood (anxiety-like) and memory dysfunction. This was accompanied by a decreased expression of both synaptic (synaptophysin) and plasticity proteins (BDNF and CREB), a loss of cresyl violet-stained neurons, and increased astrogliosis and microgliosis in the hippocampus. Notably, chronic GUO treatment (7.5 mg/kg i.p. daily starting 1 h after TBI) prevented all these TBI-induced long-term behavioral, neurochemical, and morphological modifications. This neuroprotective effect of GUO was abrogated in the presence of the adenosine A₁ receptor antagonist DPCPX (1 mg/kg) but unaltered by the adenosine A_2A receptor antagonist SCH58261 (0.05 mg/kg). These findings show that a chronic GUO treatment prevents the long-term mood and memory dysfunction triggered by TBI, which involves adenosinergic receptors.

The Adenosinergic Signaling: A Complex but Promising Therapeutic Target for Alzheimer's Disease

Alzheimer’s disease (AD) is the most common neurodegenerative disorder in elderly people. AD is characterized by a progressive cognitive decline and it is neuropathologically defined by two hallmarks: extracellular deposits of aggregated β-amyloid (Aβ) peptides and intraneuronal fibrillar aggregates of hyper- and abnormally phosphorylated Tau proteins. AD results from multiple genetic and environmental risk factors. Epidemiological studies reported beneficial effects of caffeine, a non-selective adenosine receptors antagonist. In the present review, we discuss the impact of caffeine and of adenosinergic system modulation on AD, in terms of pathology and therapeutics.