A3 adenosine receptor agonist reduces brain ischemic injury and inhibits inflammatory cell migration in rats

Design, Synthesis and Biological Evaluation of N6-Substituted-C2-Alkynyl-4'-Thionucleoside and 4'-Trucatedthionucleoside Derivatives as Novel A3 Adenosine Receptor Ligands

Adenosine receptors (ARs) play crucial roles in various physiological processes, making them significant targets for therapeutic intervention. This study focuses on the design, synthesis, and evaluation of N6-substituted-C2-alkynyl-4'-thioadenosine and truncated 4'-thioadenosine derivatives as selective ligands for the human A3 adenosine receptor (hA3 AR). Binding affinity assays demonstrated that modifications at the C2-alkyne and N6-amine positions significantly influenced receptor selectivity and potency. Compound 3b showed high affinity for hA3 AR (Ki = 1.8 nM) with excellent subtype selectivity and acted as an agonist. In contrast, truncated derivative 6b was an antagonist, while 6s, which shares the same sugar structure as 6b, functioned as a partial agonist, highlighting the roles of sugar and base moieties in ligand function. Molecular docking studies provided further insight into distinct binding modes, demonstrating the impact of minor structural variations on ligand-receptor binding dynamics. These findings contribute to the development of potent and selective A3 AR modulators by emphasizing the importance of fine structural adjustments in ligand design.

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.

Gene regulation in activated microglia by adenosine A3 receptor agonists: a transcriptomics study

Most neurodegenerative disorders, including the two most common, Alzheimer's disease (AD) and Parkinson's disease (AD), course with activation of microglia, the resident innate immune cells of the central nervous system. A3 adenosine receptor (A3R) agonists have been proposed to be neuroprotective by regulating the phenotype of activated microglia. RNAseq was performed using samples isolated from lipopolysaccharide/interferon-γ activated microglia treated with 2-Cl-IB-MECA, a selective A3R agonist. The results showed that the number of negatively regulated genes in the presence of 2-Cl-IB-MECA was greater than the number of positively regulated genes. Gene ontology enrichment analysis showed regulation of genes participating in several cell processes, including those involved in immune-related events. Analysis of known and predicted protein-protein interactions showed that Smad3 and Sp1 are transcription factors whose genes are regulated by A3R activation. Under the conditions of cell activation and agonist treatment regimen, 2-Cl-IB-MECA did not lead to any tendency to favor the expression of genes related to neuroprotective microglia (M2).

Microglia at the blood brain barrier in health and disease

The blood brain barrier (BBB) plays a crucial role in maintaining brain homeostasis by selectively preventing the entry of substances from the peripheral blood into the central nervous system (CNS). Comprised of endothelial cells, pericytes, and astrocytes, this highly regulated barrier encompasses the majority of the brain's vasculature. In addition to its protective function, the BBB also engages in significant crosstalk with perivascular macrophages (MΦ) and microglia, the resident MΦ of the brain. These interactions play a pivotal role in modulating the activation state of cells comprising the BBB, as well as MΦs and microglia, themselves. Alterations in systemic metabolic and inflammatory states can promote endothelial cell dysfunction, reducing the integrity of the BBB and potentially allowing peripheral blood factors to leak into the CNS compartment. This may mediate activation of perivascular MΦs, microglia, and astrocytes, and initiate further immune responses within the brain parenchyma, suggesting neuroinflammation can be triggered by signaling from the periphery, without primary injury or disease originating within the CNS. The intricate interplay between the periphery and the CNS through the BBB highlights the importance of understanding the role of microglia in mediating responses to systemic challenges. Despite recent advancements, our understanding of the interactions between microglia and the BBB is still in its early stages, leaving a significant gap in knowledge. However, emerging research is shedding light on the involvement of microglia at the BBB in various conditions, including systemic infections, diabetes, and ischemic stroke. This review aims to provide a comprehensive overview of the current research investigating the intricate relationship between microglia and the BBB in health and disease. By exploring these connections, we hope to advance our understanding of the role of brain immune responses to systemic challenges and their impact on CNS health and pathology. Uncovering these interactions may hold promise for the development of novel therapeutic strategies for neurological conditions that involve immune and vascular mechanisms.

Role of Microglia in Stroke

Ischemic stroke is a complex brain pathology caused by an interruption of blood supply to the brain. It results in neurological deficits which that reflect the localization and the size of the compromised brain area and are the manifestation of complex pathogenic events triggered by energy depletion. Inflammation plays a prominent role, worsening the injury in the early phase and influencing poststroke recovery in the late phase. Activated microglia are one of the most important cellular components of poststroke inflammation, appearing from the first few hours and persisting for days and weeks after stroke injury. In this chapter, we will discuss the nature of the inflammatory response in brain ischemia, the contribution of microglia to injury and regeneration after stroke, and finally, how ischemic stroke directly affects microglia functions and survival.

First Potent Macrocyclic A3 Adenosine Receptor Agonists Reveal G-Protein and β-Arrestin2 Signaling Preferences

(N)-Methanocarba adenosine derivatives (A3 adenosine receptor (AR) agonists containing bicyclo[3.1.0]hexane replacing furanose) were chain-extended at N6 and C2 positions with terminal alkenes for ring closure. The resulting macrocycles of 17-20 atoms retained affinity, indicating a spatially proximal orientation of these receptor-bound chains, consistent with molecular modeling of 12. C2-Arylethynyl-linked macrocycle 19 was more A3AR-selective than 2-ether-linked macrocycle 12 (both 5'-methylamides, human (h) A3AR affinities (Ki): 22.1 and 25.8 nM, respectively), with lower mouse A3AR affinities. Functional hA3AR comparison of two sets of open/closed analogues in β-arrestin2 and Gi/o protein assays showed certain signaling preferences divergent from reference agonist Cl-IB-MECA 1. The potencies of 1 at all three Gαi isoforms were slightly less than its hA3AR binding affinity (Ki: 1.4 nM), while the Gαi1 and Gαi2 potencies of macrocycle 12 were roughly an order of magnitude higher than its radioligand binding affinity. Gαi2-coupling was enhanced in macrocycle 12 (EC50 2.56 nM, ∼40% greater maximal efficacy than 1). Di-O-allyl precursor 18 cyclized to form 19, increasing the Gαi1 potency by 7.5-fold. The macrocycles 12 and 19 and their open precursors 11 and 18 potently stimulated β-arrestin2 recruitment, with EC50 values (nM) of 5.17, 4.36, 1.30, and 4.35, respectively, and with nearly 50% greater efficacy compared to 1. This example of macrocyclization altering the coupling pathways of small-molecule (nonpeptide) GPCR agonists is the first for potent and selective macrocyclic AR agonists. These initial macrocyclic derivatives can serve as a guide for the future design of macrocyclic AR agonists displaying unanticipated pharmacology.

Species dependence of A3 adenosine receptor pharmacology and function

Efforts to fully understand pharmacological differences between G protein-coupled receptor (GPCR) species homologues are generally not pursued in detail during the drug development process. To date, many GPCRs that have been successfully targeted are relatively well-conserved across species in amino acid sequence and display minimal variability of biological effects. However, the A3 adenosine receptor (AR), an exciting drug target for a multitude of diseases associated with tissue injury, ischemia, and inflammation, displays as little as 70% sequence identity among mammalian species (e.g., rodent vs. primate) commonly used in drug development. Consequently, the pharmacological properties of synthetic A3AR ligands vary widely, not only in binding affinity, selectivity, and signaling efficacy, but to the extent that some function as agonists in some species and antagonists in others. Numerous heterocyclic antagonists that have nM affinity at the human A3AR are inactive or weakly active at the rat and mouse A3ARs. Positive allosteric modulators, including the imidazo [4,5-c]quinolin-4-amine derivative LUF6000, are only active at human and some larger animal species that have been evaluated (rabbit and dog), but not rodents. A3AR agonists evoke systemic degranulation of rodent, but not human mast cells. The rat A3AR undergoes desensitization faster than the human A3AR, but the human homologue can be completely re-sensitized and recycled back to the cell surface. Thus, comprehensive pharmacological evaluation and awareness of potential A3AR species differences are critical in studies to further understand the basic biological functions of this unique AR subtype. Recombinant A3ARs from eight different species have been pharmacologically characterized thus far. In this review, we describe in detail current knowledge of species differences in genetic identity, G protein-coupling, receptor regulation, and both orthosteric and allosteric A3AR pharmacology.

Neuroprotection afforded by targeting G protein-coupled receptors in heteromers and by heteromer-selective drugs

G protein-coupled receptors (GPCRs) are the target of hundreds of approved drugs. Although these drugs were designed to target individual receptors, it is becoming increasingly apparent that GPCRs interact with each other to form heteromers. Approved drug targets are often part of a GPCR heteromer, and therefore new drugs can be developed with heteromers in mind. This review presents several strategies to selectively target GPCRs in heteromeric contexts, namely, taking advantage of i) heteromer-mediated biased agonism/signalling, ii) discovery of drugs with higher affinity for the receptor if it is part of a heteromer (heteromer selective drugs), iii) allosteric compounds directed against the interacting transmembrane domains and, eventually, iv) antagonists that block both GPCRs in a heteromer. Heteromers provide unique allosteric sites that should help designing a new type of drug that by definition would be a heteromer selective drug. The review also provides examples of rhodopsin-like class A receptors in heteromers that could be targeted to neuroprotect and/or delay the progression of diseases such as Parkinson's and Alzheimer's. GPCRs in heteromers (GriH) with the potential to address dyskinesias, a common complication of dopaminergic replacement therapy in parkinsonian patients, are also described.

Abnormal expression of fission and fusion genes and the morphology of mitochondria in eutopic and ectopic endometrium

Mitochondria play a pivotal role in physiological and metabolic function of the cell. Mitochondrial dynamics orchestrate mitochondrial function and morphology, involving fission and fusion as well as ultrastructural remodeling. Mounting evidence unravels the close link between mitochondria and endometriosis. However, how mitochondrial architecture changes through fission and fusion in eutopic and ectopic tissues of women with ovarian endometriosis remains unknown. We detected the expression of fission and fusion genes and the morphology of mitochondria in eutopic and ectopic endometrium in ovarian endometriosis. The results showed that the expression of DRP1 and LCLAT1 was upregulated in eutopic endometrial stromal cells (ESCs), and the expression of DRP1, OPA1, MFN1, MFN2, and LCLAT1 was significantly downregulated in ectopic ESCs, and reduced number of mitochondria, wider cristae width and narrower cristae junction width was observed, but there was no difference in cell survival rate. The altered mitochondrial dynamics and morphology might, respectively, provide an advantage for migration and adhesion in eutopic ESCs and be the adaptive response in ectopic endometrial cells to survive under hypoxic and oxidative stress environment.

The adenosine A2A receptor in the basal ganglia: Expression, heteromerization, functional selectivity and signalling

Adenosine is a neuroregulatory nucleoside that acts through four G protein-coupled receptors (GPCRs), A1, A2A, A2B and A3, which are widely expressed in cells of the nervous system. The A2A receptor (A2AR), the GPCR with the highest expression in the striatum, has a similar role to that of receptors for dopamine, one of the main neurotransmitters. Neuronal and glial A2ARs participate in the modulation of dopaminergic transmission and act in almost any action in which the basal ganglia is involved. This chapter revisits the expression of the A2AR in the basal ganglia in health and disease, and describes the diversity of signalling depending on whether the receptors are expressed as monomer or as heteromer. The A2AR can interact with other receptors as adenosine A1, dopamine D2, or cannabinoid CB1 to form heteromers with relevant functions in the basal ganglia. Heteromerization, with these and other GPCRs, provides diversity to A2AR-mediated signalling and to the modulation of neurotransmission. Thus, selective A2AR antagonists have neuroprotective potential acting directly on neurons, but also through modulation of glial cell activation, for example, by decreasing neuroinflammatory events that accompany neurodegenerative diseases. In fact, A2AR antagonists are safe and their potential in the therapy of Parkinson's disease has already led to the approval of one of them, istradefylline, in Japan and United States. The receptor also has a key role in reward circuits and, again, heteromers with dopamine receptors, but also with cannabinoid CB1 receptors, participate in the events triggered by drugs of abuse.

The role of the ATP-adenosine axis in ischemic stroke

In ischemic stroke, the primary neuronal injury caused by the disruption of energy supply is further exacerbated by secondary sterile inflammation. The inflammatory cascade is largely initiated by the purine adenosine triphosphate (ATP) which is extensively released to the interstitial space during brain ischemia and functions as an extracellular danger signaling molecule. By engaging P2 receptors, extracellular ATP activates microglia leading to cytokine and chemokine production and subsequent immune cell recruitment from the periphery which further amplifies post-stroke inflammation. The ectonucleotidases CD39 and CD73 shape and balance the inflammatory environment by stepwise degrading extracellular ATP to adenosine which itself has neuroprotective and anti-inflammatory signaling properties. The neuroprotective effects of adenosine are mainly mediated through A₁ receptors and inhibition of glutamatergic excitotoxicity, while the anti-inflammatory capacities of adenosine have been primarily attributed to A_2A receptor activation on infiltrating immune cells in the subacute phase after stroke. In this review, we summarize the current state of knowledge on the ATP-adenosine axis in ischemic stroke, discuss contradictory results, and point out potential pitfalls towards translating therapeutic approaches from rodent stroke models to human patients.

Purinergic signaling: a potential therapeutic target for ischemic stroke

Pathogenesis of ischemic stroke is mainly characterized by thrombosis and neuroinflammation. Purinergic signaling pathway constitutes adenosine triphosphate (ATP), adenosine diphosphate (ADP), adenosine monophosphate (AMP), and adenosine (ADO). ATP is hydrolyzed to ADP and then to AMP by extracellular nucleotidase CD39; AMP is subsequently converted to adenosine by CD73. All these nucleotides and nucleosides act on purinergic receptors protecting against thrombosis and inhibit inflammation. In addition, many physical methods have been found to play a neuroprotective role through purinergic signaling. This review mainly introduces the role and potential mechanism of purinergic signalings in the treatment of ischemic stroke, so as to provide reference for seeking new treatment methods for stroke.

Adenosinergic Pathway in Parkinson's Disease: Recent Advances and Therapeutic Perspective

Parkinson's disease (PD) is a neurodegenerative disease characterized pathologically by α-synuclein (α-syn) aggregation. In PD, the current mainstay of symptomatic treatment is levodopa (L-DOPA)-based dopamine (DA) replacement therapy. However, the development of dyskinesia and/or motor fluctuations which is relevant to levodopa is restricting its long-term utility. Given that the ability of which is to modulate the striato-thalamo-cortical loops and function to modulate basal ganglia output, the adenosinergic pathway (AP) is qualified as a potential promising non-DA target. As an indispensable component of energy production pathways, AP modulates cellular metabolism and gene regulation in both neurons and neuroglia cells through the recognition and degradation of extracellular adenosine. In addition, AP is geared to the initiation, evolution, and resolution of inflammation as well. Besides the above-mentioned crosstalk between the adenosine and dopamine signaling pathways, the functions of adenosine receptors (A₁R, A_2AR, A_2BR, and A₃R) and metabolism enzymes in modulating PD pathological process have been extensively investigated in recent decades. Here we reviewed the emerging findings focused on the function of adenosine receptors, adenosine formation, and metabolism in the brain and discussed its potential roles in PD pathological process. We also recapitulated clinical studies and the preclinical evidence for the medical strategies targeting the Ado signaling pathway to improve motor dysfunction and alleviate pathogenic process in PD. We hope that further clinical studies should consider this pathway in their monotherapy and combination therapy, which would open new vistas to more targeted therapeutic approaches.

Adenosine A1R/A3R agonist AST-004 reduces brain infarction in mouse and rat models of acute ischemic stroke

Acute ischemic stroke (AIS) is the second leading cause of death globally. No Food and Drug Administration (FDA) approved therapies exist that target cerebroprotection following stroke. Our group recently reported significant cerebroprotection with the adenosine A1/A3 receptor agonist, AST-004, in a transient stroke model in non-human primates (NHP) and in a preclinical mouse model of traumatic brain injury (TBI). However, the specific receptor pathway activated was only inferred based on in vitro binding studies. The current study investigated the underlying mechanism of AST-004 cerebroprotection in two independent models of AIS: permanent photothrombotic stroke in mice and transient middle cerebral artery occlusion (MCAO) in rats. AST-004 treatments across a range of doses were cerebroprotective and efficacy could be blocked by A3R antagonism, indicating a mechanism of action that does not require A1R agonism. The high affinity A3R agonist MRS5698 was also cerebroprotective following stroke, but not the A3R agonist Cl-IB-MECA under our experimental conditions. AST-004 efficacy was blocked by the astrocyte specific mitochondrial toxin fluoroacetate, confirming an underlying mechanism of cerebroprotection that was dependent on astrocyte mitochondrial metabolism. An increase in A3R mRNA levels following stroke suggested an intrinsic cerebroprotective response that was mediated by A3R signaling. Together, these studies confirm that certain A3R agonists, such as AST-004, may be exciting new therapeutic avenues to develop for AIS.

The impact of inosine on hippocampal synaptic transmission and plasticity involves the release of adenosine through equilibrative nucleoside transporters rather than the direct activation of adenosine receptors

Inosine has robust neuroprotective effects, but it is unclear if inosine acts as direct ligand of adenosine receptors or if it triggers metabolic effects indirectly modifying the activity of adenosine receptors. We now combined radioligand binding studies with electrophysiological recordings in hippocampal slices to test how inosine controls synaptic transmission and plasticity. Inosine was without effect at 30 μM and decreased field excitatory post-synaptic potentials by 14% and 33% at 100 and 300 μM, respectively. These effects were prevented by the adenosine A₁ receptor antagonist DPCPX. Inosine at 300 (but not 100) μM also decreased the magnitude of long-term potentiation (LTP), an effect prevented by DPCPX and by the adenosine A_2A receptor antagonist SCH58261. Inosine showed low affinity towards human and rat adenosine receptor subtypes with K_i values of > 300 µM; only at the human and rat A₁ receptor slightly higher affinities with K_i values of around 100 µM were observed. Affinity of inosine at the rat A₃ receptor was higher (K_i of 1.37 µM), while it showed no interaction with the human orthologue. Notably, the effects of inosine on synaptic transmission and plasticity were abrogated by adenosine deaminase and by inhibiting equilibrative nucleoside transporters (ENT) with dipyridamole and NBTI. This shows that the impact of inosine on hippocampal synaptic transmission and plasticity is not due to a direct activation of adenosine receptors but is instead due to an indirect modification of the tonic activation of these adenosine receptors through an ENT-mediated modification of the extracellular levels of adenosine.

Caffeine and Its Neuroprotective Role in Ischemic Events: A Mechanism Dependent on Adenosine Receptors

Ischemia is characterized by a transient, insufficient, or permanent interruption of blood flow to a tissue, which leads to an inadequate glucose and oxygen supply. The nervous tissue is highly active, and it closely depends on glucose and oxygen to satisfy its metabolic demand. Therefore, ischemic conditions promote cell death and lead to a secondary wave of cell damage that progressively spreads to the neighborhood areas, called penumbra. Brain ischemia is one of the main causes of deaths and summed with retinal ischemia comprises one of the principal reasons of disability. Although several studies have been performed to investigate the mechanisms of damage to find protective/preventive interventions, an effective treatment does not exist yet. Adenosine is a well-described neuromodulator in the central nervous system (CNS), and acts through four subtypes of G-protein-coupled receptors. Adenosine receptors, especially A1 and A2A receptors, are the main targets of caffeine in daily consumption doses. Accordingly, caffeine has been greatly studied in the context of CNS pathologies. In fact, adenosine system, as well as caffeine, is involved in neuroprotection effects in different pathological situations. Therefore, the present review focuses on the role of adenosine/caffeine in CNS, brain and retina, ischemic events.

Expression of the Adenosine A2A-A3 Receptor Heteromer in Different Brain Regions and Marked Upregulation in the Microglia of the Transgenic APPSw,Ind Alzheimer’s Disease Model

Adenosine (Ado) receptors have been instrumental in the detection of heteromers and other higher-order receptor structures, mainly via interactions with other cell surface G-protein-coupled receptors. Apart from the first report of the A₁ Ado receptor interacting with the A_2A Ado receptor, there has been more recent data on the possibility that every Ado receptor type, A₁, A_2A, A_2B, and A₃, may interact with each other. The aim of this paper was to look for the expression and function of the A_2A/A₃ receptor heteromer (A_2AA₃Het) in neurons and microglia. In situ proximity ligation assays (PLA), performed in primary cells, showed that A_2AA₃Het expression was markedly higher in striatal than in cortical and hippocampal neurons, whereas it was similar in resting and activated microglia. Signaling assays demonstrated that the effect of the A_2AR agonist, PSB 777, was reduced in the presence of the A₃R agonist, 2-Cl-IB-MECA, whereas the effect of the A₃R agonist was potentiated by the A_2AR antagonist, SCH 58261. Interestingly, the expression of the heteromer was markedly enhanced in microglia from the APP_Sw,Ind model of Alzheimer’s disease. The functionality of the heteromer in primary microglia from APP_Sw,Ind mice was more similar to that found in resting microglia from control mice.

Neuroprotective Roles of the Adenosine A3 Receptor Agonist AST-004 in Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) remains one of the greatest public health concerns with increasing morbidity and mortality rates worldwide. Our group reported that stimulation of astrocyte mitochondrial metabolism by P2Y1 receptor agonists significantly reduced cerebral edema and reactive gliosis in a TBI model. Subsequent data on the pharmacokinetics (PK) and rapid metabolism of these compounds suggested that neuroprotection was likely mediated by a metabolite, AST-004, which binding data indicated was an adenosine A3 receptor (A3R) agonist. The neuroprotective efficacy of AST-004 was tested in a control closed cortical injury (CCCI) model of TBI in mice. Twenty-four (24) hours post-injury, mice subjected to CCCI and treated with AST-004 (0.22 mg/kg, injected 30 min post-trauma) exhibited significantly less secondary brain injury. These effects were quantified with less cell death (PSVue794 fluorescence) and loss of blood brain barrier breakdown (Evans blue extravasation assay), compared to vehicle-treated TBI mice. TBI-treated mice also exhibited significantly reduced neuroinflammatory markers, glial-fibrillary acidic protein (GFAP, astrogliosis) and ionized Ca2+-binding adaptor molecule 1 (Iba1, microgliosis), both at the mRNA (qRT-PCR) and protein (Western blot and immunofluorescence) levels, respectively. Four (4) weeks post-injury, both male and female TBI mice presented a significant reduction in freezing behavior during contextual fear conditioning (after foot shock). AST-004 treatment prevented this TBI-induced impairment in male mice, but did not significantly affect impairment in female mice. Impairment of spatial memory, assessed 24 and 48 h after the initial fear conditioning, was also reduced in AST-004-treated TBI-male mice. Female TBI mice did not exhibit memory impairment 24 and 48 h after contextual fear conditioning and similarly, AST-004-treated female TBI mice were comparable to sham mice. Finally, AST-004 treatments were found to increase in vivo ATP production in astrocytes (GFAP-targeted luciferase activity), consistent with the proposed mechanism of action. These data reveal AST-004 as a novel A3R agonist that increases astrocyte energy production and enhances their neuroprotective efficacy after brain injury.

Pulsed Electromagnetic Fields: A Novel Attractive Therapeutic Opportunity for Neuroprotection After Acute Cerebral Ischemia

OBJECTIVES

Acute cerebral ischemia is characterized by several pathological processes evolving during time, which contribute to the final tissue damage. Secondary processes, such as prolonged inflammatory response, impaired mitochondrial function and oxidative stress, are responsible for the progression of brain injury to the peri-infarct area, called "penumbra." Adenosine has been shown to play a crucial role in regulating the inflammatory cascade following brain ischemia. Pulsed electromagnetic fields (PEMFs) act as modulators of adenosine receptors, increasing the functionality of the endogenous adenosine. In particular, PEMF exposure induces a significant upregulation of A_2A and A₃ adenosine receptors in different neuronal cell types. Several lines of evidence suggest that PEMF exposure might play a neuroprotective role after ischemic damage.

MATERIALS AND METHODS

This review summarizes the current knowledge on the mechanism of action of PEMFs and their biological effects on neuronal damage both in preclinical and clinical studies.

RESULTS

PEMFs counteract hypoxia-induced apoptosis and ROS production in neuronal-like cells and exert a strong anti-inflammatory effect on microglial cells. Data from stroke animal models showed that PEMFs exposure is able to reduce the size of the infarct area and decrease the levels of pro-inflammatory mediators. In clinical studies, PEMFs stimulation proved to be safe and well tolerated. Preliminary results on acute ischemic stroke patients showed a dose-dependent reduction in the lesion size.

CONCLUSIONS

Altogether, these data demonstrate the efficacy of PEMFs against several mechanisms underlying ischemic damage and suggest that PEMFs might represent a novel noninvasive adjunctive treatment for acute ischemic stroke, providing neuroprotection and reducing functional deficits following ischemia.

G-Protein-Coupled Receptors and Ischemic Stroke: a Focus on Molecular Function and Therapeutic Potential

In ischemic stroke, there is only one approved drug, tissue plasminogen activator, to be used in clinical conditions for thrombolysis. New neuroprotective therapies for ischemic stroke are desperately needed. Several targets and pathways have been shown to confer neuroprotective effects in ischemic stroke. G-protein-coupled receptors (GPCRs) are one of the most frequently targeted receptors for developing novel therapeutics for central nervous system disorders. GPCRs are a large family of cell surface receptors that response to a wide variety of extracellular stimuli. GPCRs are involved in a wide range of physiological and pathological processes. More than 90% of the identified non-sensory GPCRs are expressed in the brain, where they play important roles in regulating mood, pain, vision, immune responses, cognition, and synaptic transmission. There is also good evidence that GPCRs are implicated in the pathogenesis of stroke. This review narrates the pathophysiological role and possible targeted therapy of GPCRs in ischemic stroke.

LJ529 attenuates mast cell-related inflammation via A3R-PKCε-ALDH2 pathway after subarachnoid hemorrhage in rats

BACKGROUND AND PURPOSE

Mast cells (MCs) has been recognized as an effector of inflammation or a trigger of inflammatory factors during stroke. LJ529 was reported to attenuate inflammation through a Gi protein-coupled Adenosine A3 receptor (A₃R) after ischemia. Here, we aim to study the protective effect and its mechanism of LJ529 in subarachnoid hemorrhage (SAH) rat model for mast cell-related inflammation.

METHODS

155 Sprague-Dawley adult male rats were used in experiments. Endovascular perforation was used for SAH model. Intraperitoneal LJ529 was performed 1 h after SAH. Neurological scores were measured 24 h after SAH. Rotarod and morris water maze tests were evaluated for 21 days after SAH. Mast cell degranulation was assessed with Toluidine blue staining and Chymase/Typtase protein expressions. Mast cell-related inflammation was evaluated using IL-6, TNF-α and MCP-1 protein expressions. MRS1523, inhibitor of GPR18 and ε-V1-2, inhibitor of PKCε were respectively given intraperitoneally (i.p.) 1 h and 30 min before SAH for mechanism studies. Pathway related proteins were investigated with western blot and immunofluorescence staining.

RESULTS

Expression of A₃R, PKCε increased after SAH. LJ529 treatment attenuated mast cell degranulation and inflammation. Meanwhile, both short-term and long-term neurological functions were improved after LJ529 treatment. Administration of LJ529 resulted in increased expressions of A₃R, PKCε, ALDH2 proteins and decreased expressions of Chymase, Typtase, IL-6, TNF-α and MCP-1 proteins. MRS1523 abolished the treatment effects of LJ529 on neurobehavior and protein levels. ε-V1-2 also reversed the outcomes of LJ529 administration through reduction in protein expressions downstream of PKCε.

CONCLUSIONS

LJ529 attenuated mast cell-related inflammation through inhibiting degranulation via A3R-PKCε-ALDH2 pathway after SAH. LJ529 may serve as a potential treatment strategy to relieve post-SAH brain injury.

Activation of adenosine A3 receptor reduces early brain injury by alleviating neuroinflammation after subarachnoid hemorrhage in elderly rats

The incidence of subarachnoid hemorrhage (SAH) and hazard ratio of death increase with age. Overactivation of microglia contributes to brain damage. This study aimed to investigate the effects of A3 adenosine receptors (A3R) activation on neurofunction and microglial phenotype polarization in the context of SAH in aged rats. The A3R agonist (CI-IB-MECA) and antagonist (MRS1523) were used in the SAH model. Microglia were cultured to mimic SAH in the presence or absence of CI-IB-MECA and/or siRNA for A3R. The neurofunction and status of the microglial phenotype were evaluated. The P38 inhibitor SB202190 and the STAT6 inhibitor AS1517499 were used to explore the signaling pathway. The results showed that SAH induced microglia to polarize to the M(LPS) phenotype both in vivo and in vitro. CI-IB-MECA distinctly skewed microglia towards the M(IL-4) phenotype and ameliorated neurological dysfunction, along with the downregulation of inflammatory cytokines. Knockdown of A3R or inhibition of P38 and/or STAT6 weakened the effects of CI-IB-MECA on microglial phenotypic shifting. Collectively, our findings suggest that activation of A3R exerted anti-inflammatory and neuroprotective effects by regulating microglial phenotype polarization through P38/STAT6 pathway and indicated that A3R agonists may be a promising therapeutic options for the treatment of brain injury after SAH.

Adenosine A3 receptor as a novel therapeutic target to reduce secondary events and improve neurocognitive functions following traumatic brain injury

Background

Traumatic brain injury (TBI) is a common pathological condition that presently lacks a specific pharmacological treatment. Adenosine levels rise following TBI, which is thought to be neuroprotective against secondary brain injury. Evidence from stroke and inflammatory disease models suggests that adenosine signaling through the G protein-coupled A₃ adenosine receptor (A₃AR) can provide antiinflammatory and neuroprotective effects. However, the role of A₃AR in TBI has not been investigated.

Methods

Using the selective A₃AR agonist, MRS5980, we evaluated the effects of A₃AR activation on the pathological outcomes and cognitive function in CD1 male mouse models of TBI.

Results

When measured 24 h after controlled cortical impact (CCI) TBI, male mice treated with intraperitoneal injections of MRS5980 (1 mg/kg) had reduced secondary tissue injury and brain infarction than vehicle-treated mice with TBI. These effects were associated with attenuated neuroinflammation marked by reduced activation of nuclear factor of kappa light polypeptide gene enhancer in B cells (NFκB) and MAPK (p38 and extracellular signal-regulated kinase (ERK)) pathways and downstream NOD-like receptor pyrin domain-containing 3 inflammasome activation. MRS5980 also attenuated TBI-induced CD4⁺ and CD8⁺ T cell influx. Moreover, when measured 4–5 weeks after closed head weight-drop TBI, male mice treated with MRS5980 (1 mg/kg) performed significantly better in novel object-placement retention tests (NOPRT) and T maze trials than untreated mice with TBI without altered locomotor activity or increased anxiety.

Conclusion

Our results provide support for the beneficial effects of small molecule A₃AR agonists to mitigate secondary tissue injury and cognitive impairment following TBI.

Activation of Adenosine A3 Receptor Inhibits Microglia Reactivity Elicited by Elevated Pressure

Glaucoma is a progressive chronic retinal degenerative disease and a leading cause of global irreversible blindness, characterized by optic nerve damage and retinal ganglion cell (RGC) death. Elevated intraocular pressure (IOP) is a main risk factor of glaucoma. Neuroinflammation plays an important role in glaucoma. We have been demonstrating that elevated pressure triggers microglia reactivity that contribute to the loss of RGCs. Adenosine, acting on adenosine receptors, is a crucial modulator of microglia phenotype. Microglia express all adenosine receptors. Previously, we demonstrated that the activation of adenosine A₃ receptor (A₃R) affords protection to the retina, including RGCs, unveiling the possibility for a new strategy for glaucoma treatment. Since microglial cells express A₃R, we now studied the ability of a selective A₃R agonist (2-Cl-IB-MECA) in controlling microglia reactivity induced by elevated hydrostatic pressure (EHP), used to mimic elevated IOP. The activation of A₃R reduced EHP-induced inducible nitric oxide synthase (iNOS) expression, microglia migration and phagocytosis in BV-2 cells. In retinal microglia, proliferation and phagocytosis elicited by EHP were also decreased by A₃R activation. This work demonstrates that 2-Cl-IB-MECA, the selective agonist of A₃R, is able to hinder microglia reactivity, suggesting that A₃R agonists could afford protection against glaucomatous degeneration through the control of neuroinflammation.

Calpain-mediated cleavage of Fbxw7 during excitotoxicity

Neuronal cell death induced by ischemic injury has been attributed to glutamate receptor-mediated excitotoxicity, which is known to be accompanied by Ca²⁺ overload in the cytoplasm with concomitant activation of calcium-dependent mechanisms. More specifically, the overactivation of calpains, calcium-dependent cysteine proteases, have been associated with neuronal cell death following glutamate treatment. Previously, we observed decreased expression levels of F-box/WD repeat domain-containing protein 7 (Fbxw7) after the hyperactivation of cyclin-dependent kinase 5 (Cdk5) in cortical neurons challenged with glutamate. As determined using in vitro calpain cleavage assays, we demonstrated that the cleavage of Fbxw7 was mediated by activated calpain and attenuated in the presence of the calpain inhibitor, calpeptin. Using the rat middle cerebral artery occlusion model, we confirmed that Fbxw7 was indeed cleaved by activated calpain in the ipsilateral cortex. Based on our data, we hypothesize that the negative regulation of Fbxw7 by calpain may contribute to neuronal cell death and that the preservation of Fbxw7 by the inhibition of calpain, Cdk5, or both composes a novel protective mechanism following excitotoxicity.

Role and Function of Adenosine and its Receptors in Inflammation, Neuroinflammation, IBS, Autoimmune Inflammatory Disorders, Rheumatoid Arthritis and Psoriasis

The physiological effects of endogenous adenosine on various organ systems are very complex and numerous which are elicited upon activation of any of the four G-protein-coupled receptors (GPCRs) denoted as A1, A2A, A2B and A3 adenosine receptors (ARs). Several fused heterocyclic and non-xanthine derivatives are reported as a possible target for these receptors due to physiological problems and lack of selectivity of xanthine derivatives. In the present review, we have discussed the development of various new chemical entities as a target for these receptors. In addition, compounds acting on adenosine receptors can be utilized in treating diseases like inflammation, neuroinflammation, autoimmune and related diseases.

Cyclic adenosine monophosphate in acute ischemic stroke: some to update, more to explore

The development of effective treatment for ischemic stroke, which is a common cause of morbidity and mortality worldwide, remains an unmet goal because the current first-line treatment management interventional therapy has a strict time window and serious complications. In recent years, a growing body of evidence has shown that the elevation of intracellular and extracellular cyclic adenosine monophosphate (cAMP) alleviates brain damage after ischemic stroke by attenuating neuroinflammation in the central nervous system and peripheral immune system. In the central nervous system, upregulated intracellular cAMP signaling can alleviate immune-mediated damage by restoring neuronal morphology and function, inhibiting microglia migration and activation, stabilizing the membrane potential of astrocytes and improving the cellular functions of endothelial cells and oligodendrocytes. Enhancement of the extracellular cAMP signaling pathway can improve neurological function by activating the cAMP-adenosine pathway to reduce immune-mediated damage. In the peripheral immune system, cAMP can act on various immune cells to suppress peripheral immune function, which can alleviate the inflammatory response in the central nervous system and improve the prognosis of acute cerebral ischemic injury. Therefore, cAMP may play key roles in reducing post-stroke neuroinflammatory damage. The protective roles of the cAMP indicate that the cAMP enhancing drugs such as cAMP supplements, phosphodiesterase inhibitors, adenylate cyclase agonists, which are currently used in the treatment of heart and lung diseases. They are potentially able to be applied as a new therapeutic strategy in ischemic stroke. This review focuses on the immune-regulating roles and the clinical implication of cAMP in acute ischemic stroke.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body’s response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

Historical and Current Adenosine Receptor Agonists in Preclinical and Clinical Development

Adenosine receptors (ARs) function in the body's response to conditions of pathology and stress associated with a functional imbalance, such as in the supply and demand of energy/oxygen/nutrients. Extracellular adenosine concentrations vary widely to raise or lower the basal activation of four subtypes of ARs. Endogenous adenosine can correct an energy imbalance during hypoxia and other stress, for example, by slowing the heart rate by A₁AR activation or increasing the blood supply to heart muscle by the A_2AAR. Moreover, exogenous AR agonists, antagonists, or allosteric modulators can be applied for therapeutic benefit, and medicinal chemists working toward that goal have reported thousands of such agents. Thus, numerous clinical trials have ensued, using promising agents to modulate adenosinergic signaling, most of which have not succeeded. Currently, short-acting, parenteral agonists, adenosine and Regadenoson, are the only AR agonists approved for human use. However, new concepts and compounds are currently being developed and applied toward preclinical and clinical evaluation, and initial results are encouraging. This review focuses on key compounds as AR agonists and positive allosteric modulators (PAMs) for disease treatment or diagnosis. AR agonists for treating inflammation, pain, cancer, non-alcoholic steatohepatitis, angina, sickle cell disease, ischemic conditions and diabetes have been under development. Multiple clinical trials with two A₃AR agonists are ongoing.

Pharmacology of Adenosine Receptors: The State of the Art

Adenosine is a ubiquitous endogenous autacoid whose effects are triggered through the enrollment of four G protein-coupled receptors: A₁, A_2A, A_2B, and A₃. Due to the rapid generation of adenosine from cellular metabolism, and the widespread distribution of its receptor subtypes in almost all organs and tissues, this nucleoside induces a multitude of physiopathological effects, regulating central nervous, cardiovascular, peripheral, and immune systems. It is becoming clear that the expression patterns of adenosine receptors vary among cell types, lending weight to the idea that they may be both markers of pathologies and useful targets for novel drugs. This review offers an overview of current knowledge on adenosine receptors, including their characteristic structural features, molecular interactions and cellular functions, as well as their essential roles in pain, cancer, and neurodegenerative, inflammatory, and autoimmune diseases. Finally, we highlight the latest findings on molecules capable of targeting adenosine receptors and report which stage of drug development they have reached.

A3 adenosine receptor agonist attenuates neuropathic pain by suppressing activation of microglia and convergence of nociceptive inputs in the spinal dorsal horn

Peripheral nerve injuries cause glial activation and neuronal hyperactivity in the spinal dorsal horn. These changes have been considered to be involved in the underlying mechanisms for the development and maintenance of neuropathic pain. Using double immunofluorescence labeling, we previously demonstrated that spinal microglial activation induced by nerve injury enhanced convergence of nociceptive inputs in the spinal dorsal horn from uninjured afferents. The adenosine A₃ receptor (A₃AR) agonists have been shown to have antinociceptive activities in several experimental neuropathic pain models. However, the mechanisms underlying these antinociceptive actions of the A₃AR agonist are still not fully explored. In this study, the effects of the A₃AR agonist (i.e., IB-MECA) on microglial activation, enhancement of convergent nociceptive inputs, and nocifensive behaviors were examined after tibial nerve injury. Injury to the tibial nerve initially caused hyposensitivity to touch stimulus at 3 days, and then resulted in tactile allodynia at 14-day post-injury. The daily systemic administration of IB-MECA (0.1 mg/kg/day) for 8 days in a row starting on the day of nerve injury or 7 days after nerve injury prevented the development of behaviorally assessed hypersensitivities, and spinal microglial activation induced by nerve injury. These treatments also suppressed anomalous convergence of nociceptive primary inputs in the spinal dorsal horn. The present findings indicate that the A₃AR agonist attenuates neuropathic pain states by suppressing enhanced microglial activation, and anomalous convergence of nociceptive inputs in the spinal dorsal horn from uninjured afferents after injury to the peripheral nerve.

Inflammation in stroke: the role of cholinergic, purinergic and glutamatergic signaling

The inflammatory response is a major factor in stroke pathophysiology and contributes to secondary neuronal damage in both acute and chronic stages of the ischemic injury. Recent work in experimental cerebral ischemia has demonstrated the involvement of neurotransmitter signaling in the modulation of neuroinflammation. The present review discusses recent findings on the therapeutic potential and diagnostic perspectives of cholinergic, purinergic and glutamatergic receptors and transporters in experimental stroke. It provides evidence of the role of neurotransmission signaling as a promising inflammatory biomarker in stroke. Finally, recent molecular imaging studies using positron emission tomography of cholinergic receptors and glutamatergic transporters are outlined along with their potential as novel anti-inflammatory therapy to reduce the outcome of cerebral ischemia.

N6-Substituted 5'-N-Methylcarbamoyl-4'-selenoadenosines as Potent and Selective A3 Adenosine Receptor Agonists with Unusual Sugar Puckering and Nucleobase Orientation

Potent and selective A₃ adenosine receptor (AR) agonists were identified by the replacement of 4'-oxo- or 4'-thionucleosides with bioisosteric selenium. Unlike previous agonists, 4'-seleno analogues preferred a glycosidic syn conformation and South sugar puckering, as shown in the X-ray crystal structure of 5'-N-methylcarbamoyl derivative 3p. Among the compounds tested, N⁶-3-iodobenzyl analogue 3d was found to be the most potent A₃AR full agonist (K_i = 0.57 nM), which was ≥800- and 1900-fold selective for A₁AR and A_2AAR, respectively. In the N⁶-cycloalkyl series, 2-Cl analogues generally exhibited better hA₃AR affinity than 2-H analogues, whereas 2-H > 2-Cl in the N⁶-3-halobenzyl series. N⁷ isomers 3t and 3u were much weaker in binding than corresponding N⁹ isomers, but compound 3t lacked A₃AR activation, appearing to be a weak antagonist. 2-Cl-N⁶-3-iodobenzyl analogue 3p inhibited chemoattractant-induced migration of microglia/monocytes without inducing cell death at ≤50 μM. This suggests the potential for the development of 4'-selenonucleoside A₃AR agonists as novel antistroke agents.

A2B adenosine receptors stimulate IL-6 production in primary murine microglia through p38 MAPK kinase pathway

The hallmark of neuroinflammation is the activation of microglia, the immunocompetent cells of the CNS, releasing a number of proinflammatory mediators implicated in the pathogenesis of neuronal diseases. Adenosine is an ubiquitous autacoid regulating several microglia functions through four receptor subtypes named A₁, A_2A, A_2B and A₃ (ARs), that represent good targets to suppress inflammation occurring in CNS. Here we investigated the potential role of ARs in the modulation of IL-6 secretion and cell proliferation in primary microglial cells. The A_2BAR agonist 2-[[6-Amino-3,5-dicyano-4-[4-(cyclopropylmethoxy)phenyl]-2-pyridinyl]thio]-acetamide (BAY60-6583) stimulated IL-6 increase under normoxia and hypoxia, in a dose- and time-dependent way. In cells incubated with the blockers of phospholipase C (PLC), protein kinase C epsilon (PKC-ε) and PKC delta (PKC-δ) the IL-6 increase due to A_2BAR activation was strongly reduced, whilst it was not affected by the inhibitor of adenylyl cyclase (AC). Investigation of cellular signalling involved in the A_2BAR effect revealed that only the inhibitor of p38 mitogen activated protein kinase (MAPK) was able to block the agonist's effect on IL-6 secretion, whilst inhibitors of pERK1/2, JNK1/2 MAPKs and Akt were not. Stimulation of p38 by BAY60-6583 was A_2BAR-dependent, through a pathway affecting PLC, PKC-ε and PKC-δ but not AC, in both normoxia and hypoxia. Finally, BAY60-6583 increased microglial cell proliferation involving A_2BAR, PLC, PKC-ε, PKC-δ and p38 signalling. In conclusion, A_2BARs activation increased IL-6 secretion and cell proliferation in murine primary microglial cells, through PLC, PKC-ε, PKC-δ and p38 pathways, thus suggesting their involvement in microglial activation and neuroinflammation.

How does adenosine control neuronal dysfunction and neurodegeneration?

The adenosine modulation system mostly operates through inhibitory A₁ (A₁ R) and facilitatory A_2A receptors (A_2A R) in the brain. The activity-dependent release of adenosine acts as a brake of excitatory transmission through A₁ R, which are enriched in glutamatergic terminals. Adenosine sharpens salience of information encoding in neuronal circuits: high-frequency stimulation triggers ATP release in the 'activated' synapse, which is locally converted by ecto-nucleotidases into adenosine to selectively activate A_2A R; A_2A R switch off A₁ R and CB1 receptors, bolster glutamate release and NMDA receptors to assist increasing synaptic plasticity in the 'activated' synapse; the parallel engagement of the astrocytic syncytium releases adenosine further inhibiting neighboring synapses, thus sharpening the encoded plastic change. Brain insults trigger a large outflow of adenosine and ATP, as a danger signal. A₁ R are a hurdle for damage initiation, but they desensitize upon prolonged activation. However, if the insult is near-threshold and/or of short-duration, A₁ R trigger preconditioning, which may limit the spread of damage. Brain insults also up-regulate A_2A R, probably to bolster adaptive changes, but this heightens brain damage since A_2A R blockade affords neuroprotection in models of epilepsy, depression, Alzheimer's, or Parkinson's disease. This initially involves a control of synaptotoxicity by neuronal A_2A R, whereas astrocytic and microglia A_2A R might control the spread of damage. The A_2A R signaling mechanisms are largely unknown since A_2A R are pleiotropic, coupling to different G proteins and non-canonical pathways to control the viability of glutamatergic synapses, neuroinflammation, mitochondria function, and cytoskeleton dynamics. Thus, simultaneously bolstering A₁ R preconditioning and preventing excessive A_2A R function might afford maximal neuroprotection. The main physiological role of the adenosine modulation system is to sharp the salience of information encoding through a combined action of adenosine A_2A receptors (A_2A R) in the synapse undergoing an alteration of synaptic efficiency with an increased inhibitory action of A₁ R in all surrounding synapses. Brain insults trigger an up-regulation of A_2A R in an attempt to bolster adaptive plasticity together with adenosine release and A₁ R desensitization; this favors synaptotocity (increased A_2A R) and decreases the hurdle to undergo degeneration (decreased A₁ R). Maximal neuroprotection is expected to result from a combined A_2A R blockade and increased A₁ R activation. This article is part of a mini review series: "Synaptic Function and Dysfunction in Brain Diseases".

Adenosine-generating ovarian cancer cells attract myeloid cells which differentiate into adenosine-generating tumor associated macrophages - a self-amplifying, CD39- and CD73-dependent mechanism for tumor immune escape

BACKGROUND

Ovarian cancer (OvCA) tissues show abundant expression of the ectonucleotidases CD39 and CD73 which generate immunomodulatory adenosine, thereby inhibiting cytotoxic lymphocytes. Little, however, is known about the effect of adenosine on myeloid cells. Considering that tumor associated macrophages (TAM) and myeloid-derived suppressor cells (MDSC) constitute up to 20 % of OvCA tissue, we investigated the effect of adenosine on myeloid cells and explored a possible contribution of myeloid cells to adenosine generation in vitro and ex vivo.

METHODS

Monocytes were used as human blood-derived myeloid cells. After co-incubation with SK-OV-3 or OAW-42 OvCA cells, monocyte migration was determined in transwell assays. For conversion into M2-polarized "TAM-like" macrophages, monocytes were co-incubated with OAW-42 cells. Ex vivo TAMs were obtained from OvCA ascites. Macrophage phenotypes were investigated by intracellular staining for IL-10 and IL-12. CD39 and CD73 expression were assessed by FACS analysis both on in vitro-induced TAM-like macrophages and on ascites-derived ex situ-TAMs. Myeloid cells in solid tumor tissue were analyzed by immunohistochemistry. Generation of biologically active adenosine by TAM-like macrophages was measured in luciferase-based reporter assays. Functional effects of adenosine were investigated in proliferation-experiments with CD4(+) T cells and specific inhibitors.

RESULTS

When CD39 or CD73 activity on OvCA cells were blocked, the migration of monocytes towards OvCA cells was significantly decreased. In vivo, myeloid cells in solid ovarian cancer tissue were found to express CD39 whereas CD73 was mainly detected on stromal fibroblasts. Ex situ-TAMs and in vitro differentiated TAM-like cells, however, upregulated the expression of CD39 and CD73 compared to monocytes or M1 macrophages. Expression of ectonucleotidases also translated into increased levels of biologically active adenosine. Accordingly, co-incubation with these TAMs suppressed CD4(+) T cell proliferation which could be rescued via blockade of CD39 or CD73.

CONCLUSION

Adenosine generated by OvCA cells likely contributes to the recruitment of TAMs which further amplify adenosine-dependent immunosuppression via additional ectonucleotidase activity. In solid ovarian cancer tissue, TAMs express CD39 while CD73 is found on stromal fibroblasts. Accordingly, small molecule inhibitors of CD39 or CD73 could improve immune responses in ovarian cancer.

South (S)- and North (N)-Methanocarba-7-Deazaadenosine Analogues as Inhibitors of Human Adenosine Kinase

Adenosine kinase (AdK) inhibitors raise endogenous adenosine levels, particularly in disease states, and have potential for treatment of seizures, neurodegeneration, and inflammation. On the basis of the South (S) ribose conformation and molecular dynamics (MD) analysis of nucleoside inhibitors bound in AdK X-ray crystallographic structures, (S)- and North (N)-methanocarba (bicyclo[3.1.0]hexane) derivatives of known inhibitors were prepared and compared as human (h) AdK inhibitors. 5'-Hydroxy (34, MRS4202 (S); 55, MRS4380 (N)) and 5'-deoxy 38a (MRS4203 (S)) analogues, containing 7- and N(6)-NH phenyl groups in 7-deazaadenine, robustly inhibited AdK activity (IC50 ∼ 100 nM), while the 5'-hydroxy derivative 30 lacking the phenyl substituents was weak. Docking in the hAdK X-ray structure and MD simulation suggested a mode of binding similar to 5'-deoxy-5-iodotubercidin and other known inhibitors. Thus, a structure-based design approach for further potency enhancement is possible. The potent AdK inhibitors in this study are ready to be further tested in animal models of epilepsy.

Post-ischemic treatment of WIB801C, standardized Cordyceps extract, reduces cerebral ischemic injury via inhibition of inflammatory cell migration

ETHNOPHARMACOLOGICAL RELEVANCE

Anti-inflammatory therapy has been intensively investigated as a potential strategy for treatment of cerebral stroke. However, despite many positive outcomes reported in animal studies, anti-inflammatory treatments have not proven successful in humans as yet. Although immunomodulatory activity and safety of Cordyceps species (Chinese caterpillar fungi) have been proven in clinical trials and traditional Asian prescriptions for inflammatory diseases, its anti-ischemic effect remains elusive.

AIM OF THE STUDY

In the present study, therefore, we investigated the potential therapeutic efficacy of WIB801C, the standardized extract of Cordyceps militaris, for treatment of cerebral ischemic stroke.

MATERIALS AND METHODS

The anti-chemotactic activity of WIB801C was assayed in cultured rat microglia/macrophages. Sprague-Dawley rats were subjected to ischemic stroke via either transient (1.5-h tMCAO and subsequent 24-h reperfusion) or permanent middle cerebral artery occlusion (pMCAO for 24-h without reperfusion). WIB801C was orally administered twice at 3- and 8-h (50mg/kg each) after the onset of MCAO. Infarct volume, edema, blood brain barrier and white matter damages, neurological deficits, and long-term survival rates were investigated. The infiltration of inflammatory cells into ischemic lesions was assayed by immunostaining.

RESULTS

WIB801C significantly decreased migration of cultured microglia/macrophages. This anti-chemotactic activity of WIB-801C was not mediated via adenosine A3 receptors, although cordycepin, the major ingredient of WIB801C, is known as an adenosine receptor agonist. Post-ischemic treatment with WIB801C significantly reduced the infiltration of ED-1-and MPO-positive inflammatory cells into ischemic lesions in tMCAO rats. WIB801C-treated rats exhibited significantly decreased infarct volume and cerebral edema, less white matter and blood-brain barrier damages, and improved neurological deficits. WIB801C also improved survival rates over 34 days after ischemia onset. A significant reduction in infarct volume and neurobehavioral deficits by WIB801C was also observed in rats subjected to pMCAO.

CONCLUSIONS

In summary, post-ischemic treatment of WIB801C reduced infiltration of inflammatory cells into ischemic lesions via inhibition of chemotaxis, which confers long-lasting histological and neurological protection in ischemic brain. WIB801C may be a promising anti-ischemic drug candidate with clinically relevant therapeutic time window and safety.

Identification of A3 adenosine receptor agonists as novel non-narcotic analgesics

Chronic pain negatively impacts the quality of life in a variety of patient populations. The current therapeutic repertoire is inadequate in managing patient pain and warrants the development of new therapeutics. Adenosine and its four cognate receptors (A1 , A2A , A2B and A3 ) have important roles in physiological and pathophysiological states, including chronic pain. Preclinical and clinical studies have revealed that while adenosine and agonists of the A1 and A2A receptors have antinociceptive properties, their therapeutic utility is limited by adverse cardiovascular side effects. In contrast, our understanding of the A3 receptor is only in its infancy, but exciting preclinical observations of A3 receptor antinociception, which have been bolstered by clinical trials of A3 receptor agonists in other disease states, suggest pain relief without cardiovascular side effects and with sufficient tolerability. Our goal herein is to briefly discuss adenosine and its receptors in the context of pathological pain and to consider the current data regarding A3 receptor-mediated antinociception. We will highlight recent findings regarding the impact of the A3 receptor on pain pathways and examine the current state of selective A3 receptor agonists used for these studies. The adenosine-to-A3 receptor pathway represents an important endogenous system that can be targeted to provide safe, effective pain relief from chronic pain.

Selective A3 adenosine receptor agonist protects against doxorubicin-induced cardiotoxicity

PURPOSE

Doxorubicin (DOX) is an effective anticancer drug; however, its clinical use is limited by its cardiotoxic effect. Adenosine was proved to mediate anti-inflammatory effects and protected from myocardial ischemia/reperfusion injury. So the present work was designed to examine the effectiveness of a selective A3 adenosine receptor agonist (Cl-IB-MECA) in DOX-induced cardiotoxicity and to elucidate the underlying mechanisms via studying its effect on different oxidative stress, inflammatory and apoptotic markers.

METHODS

Firstly the potential cardioprotective dose of Cl-IB-MECA was screened in male Wistar rats at different doses (20, 40 and 80 µg/kg; i.v) against a single dose of DOX (15 mg/kg; i.p). Secondly, the dose of 40 µg/kg Cl-IB-MECA was selected for further assessment of the cardioprotective mechanisms.

RESULTS

Cl-IB-MECA at a dose 40 µg/kg (i.v) protects against DOX-induced bradycardia, elevated creatine kinase isoenzyme-MB and histopathological changes. Also, it significantly ameliorates oxidative stress injury evoked by DOX as evidenced by inhibition of reduced glutathione depletion and lipid peroxidation as well as elevation of antioxidant enzyme activities. Additionally, DOX provoked inflammatory responses by increasing the expressions of nuclear factor kappa B and the levels of tumor necrosis factor alpha. Cl-IB-MECA pretreatment significantly inhibited these inflammatory responses. Furthermore, DOX induced apoptotic tissue damage by increasing cytochrome c expressions which was suppressed by Cl-IB-MECA pretreatment.

CONCLUSION

Cl-IB-MECA protects against DOX-induced cardiotoxicity through restoration of the oxidant/antioxidant status and consequential suppression of DOX-induced inflammatory responses and abrogation of the resultant apoptotic signals.

Treadmill exercise promotes neuroprotection against cerebral ischemia-reperfusion injury via downregulation of pro-inflammatory mediators

BACKGROUND

Stroke is one of the major causes of morbidity and mortality worldwide, which is associated with serious physical deficits that affect daily living and quality of life and produces immense public health and economic burdens. Both clinical and experimental data suggest that early physical training after ischemic brain injury may reduce the extent of motor dysfunction. However, the exact mechanisms have not been fully elucidated. The aim of this study was to investigate the effects of aerobic exercise on neuroprotection and understand the underlying mechanisms.

MATERIALS AND METHODS

Middle cerebral artery occlusion (MCAO) was conducted to establish a rat model of cerebral ischemia-reperfusion injury to mimic ischemic stroke. Experimental animals were divided into the following three groups: sham (n=34), MCAO (n=39), and MCAO plus treadmill exercise (n=28). The effects of aerobic exercise intervention on ischemic brain injury were evaluated using functional scoring, histological analysis, and Bio-Plex Protein Assays.

RESULTS

Early aerobic exercise intervention was found to improve motor function, prevent death of neuronal cells, and suppress the activation of microglial cells and astrocytes. Furthermore, it was observed that aerobic exercise downregulated the expression of the cytokine interleukin-1β and the chemokine monocyte chemotactic protein-1 after transient MCAO in experimental rats.

CONCLUSION

This study demonstrates that treadmill exercise rehabilitation promotes neuroprotection against cerebral ischemia-reperfusion injury via the downregulation of proinflammatory mediators.

Purinergic signalling in brain ischemia

Ischemia is a multifactorial pathology characterized by different events evolving in the time. After ischemia a primary damage due to the early massive increase of extracellular glutamate is followed by activation of resident immune cells, i.e microglia, and production or activation of inflammation mediators. Protracted neuroinflammation is now recognized as the predominant mechanism of secondary brain injury progression. Extracellular concentrations of ATP and adenosine in the brain increase dramatically during ischemia in concentrations able to stimulate their respective specific P2 and P1 receptors. Both ATP P2 and adenosine P1 receptor subtypes exert important roles in ischemia. Although adenosine exerts a clear neuroprotective effect through A1 receptors during ischemia, the use of selective A1 agonists is hampered by undesirable peripheral effects. Evidence up to now in literature indicate that A2A receptor antagonists provide protection centrally by reducing excitotoxicity, while agonists at A2A (and possibly also A2B) and A3 receptors provide protection by controlling massive infiltration and neuroinflammation in the hours and days after brain ischemia. Among P2X receptors most evidence indicate that P2X7 receptor contribute to the damage induced by the ischemic insult due to intracellular Ca(2+) loading in central cells and facilitation of glutamate release. Antagonism of P2X7 receptors might represent a new treatment to attenuate brain damage and to promote proliferation and maturation of brain immature resident cells that can promote tissue repair following cerebral ischemia. Among P2Y receptors, antagonists of P2Y12 receptors are of value because of their antiplatelet activity and possibly because of additional anti-inflammatory effects. Moreover strategies that modify adenosine or ATP concentrations at injury sites might be of value to limit damage after ischemia. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Non-Selective Cannabinoid Receptor Antagonists, Hinokiresinols Reduce Infiltration of Microglia/Macrophages into Ischemic Brain Lesions in Rat via Modulating 2-Arachidonolyglycerol-Induced Migration and Mitochondrial Activity

Growing evidence suggests that therapeutic strategies to modulate the post-ischemic inflammatory responses are promising approaches to improve stroke outcome. Although the endocannabinoid system has been emerged as an endogenous therapeutic target to regulate inflammation after stroke insult, the downstream mechanisms and their potentials for therapeutic intervention remain controversial. Here we identified trans- and cis-hinokiresinols as novel non-selective antagonists for two G-protein-coupled cannabinoid receptors, cannabinoid receptor type 1 and type 2. The Electric Cell-substrate Impedance Sensing and Boyden chamber migration assays using primary microglial cultures revealed that both hinokiresinols significantly inhibited an endocannabinoid, 2-arachidonoylglycerol-induced migration. Hinokiresinols modulated 2-arachidonoylglycerol-induced mitochondrial bioenergetics in microglia as evidenced by inhibition of ATP turnover and reduction in respiratory capacity, thereby resulting in impaired migration activity. In rats subjected to transient middle cerebral artery occlusion (1.5-h) followed by 24-h reperfusion, post-ischemic treatment with hinokiresinols (2 and 7-h after the onset of ischemia, 10 mg/kg) significantly reduced cerebral infarct and infiltration of ED1-positive microglial/macrophage cells into cerebral ischemic lesions in vivo. Co-administration of exogenous 2-AG (1 mg/kg, i.v., single dose at 2 h after starting MCAO) abolished the protective effect of trans-hinokiresionol. These results suggest that hinokiresinols may serve as stroke treatment by targeting the endocannabinoid system. Alteration of mitochondrial bioenergetics and consequent inhibition of inflammatory cells migration may be a novel mechanism underlying anti-ischemic effects conferred by cannabinoid receptor antagonists.

Purinergic mechanisms in neuroinflammation: An update from molecules to behavior

The principle functions of neuroinflammation are to limit tissue damage and promote tissue repair in response to pathogens or injury. While neuroinflammation has utility, pathophysiological inflammatory responses, to some extent, underlie almost all neuropathology. Understanding the mechanisms that control the three stages of inflammation (initiation, propagation and resolution) is therefore of critical importance for developing treatments for diseases of the central nervous system. The purinergic signaling system, involving adenosine, ATP and other purines, plus a host of P1 and P2 receptor subtypes, controls inflammatory responses in complex ways. Activation of the inflammasome, leading to release of pro-inflammatory cytokines, activation and migration of microglia and altered astroglial function are key regulators of the neuroinflammatory response. Here, we review the role of P1 and P2 receptors in mediating these processes and examine their contribution to disorders of the nervous system. Firstly, we give an overview of the concept of neuroinflammation. We then discuss the contribution of P2X, P2Y and P1 receptors to the underlying processes, including a discussion of cross-talk between these different pathways. Finally, we give an overview of the current understanding of purinergic contributions to neuroinflammation in the context of specific disorders of the central nervous system, with special emphasis on neuropsychiatric disorders, characterized by chronic low grade inflammation or maternal inflammation. An understanding of the important purinergic contribution to neuroinflammation underlying neuropathology is likely to be a necessary step towards the development of effective interventions. This article is part of the Special Issue entitled 'Purines in Neurodegeneration and Neuroregeneration'.

Engagement of the GABA to KCC2 signaling pathway contributes to the analgesic effects of A3AR agonists in neuropathic pain

More than 1.5 billion people worldwide suffer from chronic pain, yet current treatment strategies often lack efficacy or have deleterious side effects in patients. Adenosine is an inhibitory neuromodulator that was previously thought to mediate antinociception through the A1 and A2A receptor subtypes. We have since demonstrated that A3AR agonists have potent analgesic actions in preclinical rodent models of neuropathic pain and that A3AR analgesia is independent of adenosine A1 or A2A unwanted effects. Herein, we explored the contribution of the GABA inhibitory system to A3AR-mediated analgesia using well-characterized mouse and rat models of chronic constriction injury (CCI)-induced neuropathic pain. The deregulation of GABA signaling in pathophysiological pain states is well established: GABA signaling can be hampered by a reduction in extracellular GABA synthesis by GAD65 and enhanced extracellular GABA reuptake via the GABA transporter, GAT-1. In neuropathic pain, GABAAR-mediated signaling can be further disrupted by the loss of the KCC2 chloride anion gradient. Here, we demonstrate that A3AR agonists (IB-MECA and MRS5698) reverse neuropathic pain via a spinal mechanism of action that modulates GABA activity. Spinal administration of the GABAA antagonist, bicuculline, disrupted A3AR-mediated analgesia. Furthermore, A3AR-mediated analgesia was associated with reductions in CCI-related GAD65 and GAT-1 serine dephosphorylation as well as an enhancement of KCC2 serine phosphorylation and activity. Our results suggest that A3AR-mediated reversal of neuropathic pain increases modulation of GABA inhibitory neurotransmission both directly and indirectly through protection of KCC2 function, underscoring the unique utility of A3AR agonists in chronic pain.

Amelioration of Cerebral Ischemic Injury by a Synthetic Seco-nucleoside LMT497

Recently, we reported that the A3 adenosine receptor (A3AR) agonist LJ529 (2-chloro-N(6)-(3-iodobnzyl)-5'-N-methylcarbamoyl-4'-thioadenosine) reduces cerebral ischemic injury via inhibition of recruitment of peripheral inflammatory cells into ischemic brain lesion. A3AR agonists, however, are known to possess anti-platelet activity, which may deter the combination therapy with tissue plasminogen activator for the therapy of cerebral ischemic stroke. Thus, the present study investigates the neuroprotective/anti-ischemic effect of a synthetic seco-nucleoside, LMT497 ((S)-2-((R)-1-(2-chloro-6-(3-iodobenzylamino)-9H-purin-9-yl)-2-hydroxyethoxy)-3-hydroxy-N-methylpropanamide) with little anti-platelet activity. LMT497 neither showed A3AR binding activity nor anti-platelet activity. In our present study LMT497 significantly attenuated the injury/death of cortical neurons exposed to oxygen-glucose deprivation (OGD) followed by re-oxygenation (R). LMT497 significantly reduced the ascending cellular level of reactive oxygen species under ischemic conditions by increasing the superoxide dismutase (SOD) levels. LMT497 also inhibited the migration of microglia which mediates inflammatory responses in ischemia. In rats subjected to middle cerebral artery occlusion (MCAO, 1.5 h) followed by reperfusion, LMT497 largely reduced brain infarction volume, and edema, and improved neurological score. Therapeutic efficacy of LMT497 was obtained by twice treatments even at 10 h and 18 h after the onset of ischemia. Collectively, LMT497 could be a therapeutic drug candidate with a wide therapeutic time window for the treatment of cerebral ischemic stroke.