Adora2b signaling on bone marrow derived cells dampens myocardial ischemia-reperfusion injury

ADORA2B, transcriptionally suppressing by MYC, promotes ferroptosis of chondrocytes via inhibition of the PI3K/Akt pathway in mice with osteoarthritis

Recent studies have shown that chondrocyte ferroptosis contributes importantly to the pathogenesis of osteoarthritis (OA). However, it is largely unknown how it is regulated. In this study, the data sets GSE167852 and GSE190184 were downloaded from the Gene Expression Omnibus (GEO) database, and 161 differentially expressed genes (DEGs) related to ferroptosis were screened by bioinformatics analysis. Subsequently, ADORA2B was screened as a candidate gene from DEGs, which was significantly upregulated in palmitic acid (PA) treated chondrocytes. CCK-8, EdU, Western blotting, and ferroptosis-related kits assays demonstrated that knockdown of ADORA2B constrained ferroptosis and promoted viability of chondrocytes. Overexpression of ADORA2B promoted ferroptosis, while the PI3K/Akt pathway inhibitor LY294002 reversed the promotion of ADORA2B on ferroptosis. Dual-luciferase reporter gene assay and chromatin immunoprecipitation (ChIP) assays indicated MYC was a transcription suppressor of ADORA2B, and overexpression of MYC promoted the viability, and inhibited the ferroptosis of chondrocytes, while ADORA2B overexpression abated the promotion of MYC on chondrocyte viability and the inhibition on ferroptosis. In vivo experiments showed that MYC overexpression alleviated cartilage tissue damage in OA mice, which was able to reversed by ADORA2B overexpression. In summary, ADORA2B, transcriptionally suppressing by MYC, promotes ferroptosis of chondrocytes via inhibition of the PI3K/Akt pathway. Thus, ADORA2B can be used as a potential treatment target for ferroptosis-related diseases.

Myeloid hypoxia-inducible factor HIF1A provides cardio-protection during ischemia and reperfusion via induction of netrin-1

The transcription factor hypoxia-inducible factor HIF1A induces cardioprotection from ischemia and reperfusion injury. Here, we investigate tissue-specific pathways that are critical for HIF1A-elicited tissue protection. Initial studies showed that mice with induced global Hif1a deletion (Hif1aloxP/loxP UbiquitinCre+) have exaggerated myocardial injury during in situ ischemia and reperfusion. Surprisingly, this phenotype was mirrored only in mice with myeloid-specific Hif1a deletion (Hif1a loxP/loxP LysM Cre+). In contrast, mice with myocardial specific (Hif1aloxP/loxP Myosin Cre+), or vascular Hif1a deletion (Hif1aloxP/loxP VEcadherin Cre+) experienced similar levels of injury as controls. Subsequent studies using adoptive transfer of Hif1a-deficient polymorphonuclear neutrophils (PMNs) prior to myocardial injury demonstrated increased reperfusion injury. On the contrary, the adoptive transfer of PMNs treated ex vivo with the hypoxia inducible factor (HIF) stabilizer dimethyloxalylglycine (DMOG) was associated with attenuated myocardial injury. Furthermore, DMOG-mediated cardioprotection was abolished in Hif1aloxP/loxP LysM Cre+ mice, but not in Hif2aloxP/loxP LysM Cre+ mice. Finally, studies of PMN-dependent HIF1A target genes implicated the neuronal guidance molecule netrin-1 in mediating the cardioprotective effects of myeloid HIF1A. Taken together, the present studies identified a functional role for myeloid-expressed HIF1A in providing cardioprotection during ischemia and reperfusion injury, which is mediated, at least in part, by the induction of the netrin-1 neuronal guidance molecule in neutrophils.

The Hypoxia-Adenosine Link during Myocardial Ischemia-Reperfusion Injury

Despite increasing availability and more successful interventional approaches to restore coronary reperfusion, myocardial ischemia-reperfusion injury is a substantial cause of morbidity and mortality worldwide. During myocardial ischemia, the myocardium becomes profoundly hypoxic, thus causing stabilization of hypoxia-inducible transcription factors (HIF). Stabilization of HIF leads to a transcriptional program that promotes adaptation to hypoxia and cellular survival. Transcriptional consequences of HIF stabilization include increases in extracellular production and signaling effects of adenosine. Extracellular adenosine functions as a signaling molecule via the activation of adenosine receptors. Several studies implicated adenosine signaling in cardioprotection, particularly through the activation of the Adora2a and Adora2b receptors. Adenosine receptor activation can lead to metabolic adaptation to enhance ischemia tolerance or dampen myocardial reperfusion injury via signaling events on immune cells. Many studies highlight that clinical strategies to target the hypoxia-adenosine link could be considered for clinical trials. This could be achieved by using pharmacologic HIF activators or by directly enhancing extracellular adenosine production or signaling as a therapy for patients with acute myocardial infarction, or undergoing cardiac surgery.

Remote Conditioning by Rhythmic Compression of Limbs Ameliorated Myocardial Infarction by Downregulation of Inflammation via A2 Adenosine Receptors

Background

Remote ischemic conditioning (RIC) is a cardioprotective phenomenon, yet transient ischemia is not a requisite trigger for remote cardioprotection. In fact, RIC is a stimulus compound containing interruption of the blood vessel and tissue compression. In this study, we evaluate the effects of remote tissue compression on infarct size after myocardial infarction and explore its preliminary mechanisms.

Methods and Results

We used a murine model of myocardial infarction to assess ischemia injury and identified remote conditioning by rhythmic compression on forelimb as a novel cardioprotective intervention. We show that the cardioprotective signal transduction of remote conditioning from the trigger limb to the heart involves the release of adenosine. Our results demonstrate that A2a and A2b receptors are indispensable parts for cardioprotection of remote conditioning, which is linked to its anti-inflammatory properties by the subsequent activation of cAMP/PKA/NF-κB axis.

Conclusion

Our results establish a new connection between remote tissue compression and cardiovascular diseases, which enhances our cognition about the role of tissue compression on RIC cardioprotection.

Decreased expression of adenosine receptor 2B confers cardiac protection against ischemia via restoring autophagic flux

Adora2B (adenosine receptor 2B) has been reported as one of the key modulators during cardiac remodeling after acute myocardial infarction (AMI). However, the molecular mechanism involved has not been well investigated. Thus, our study aims to investigate whether Adora2B contributes to cardiac remodeling after AMI and its underlying mechanisms. Adenovirus harboring Adora2B or shAdora2B was injected in the border zone in a mouse model of AMI experimentally produced by permanent ligation of left anterior descending (LAD) coronary artery. Decreased Adora2B expression protected the cardiomyocytes from MI-induced autophagic flux obstacle, improved cardiac function, and reduced fibrosis after MI. Adora2B downregulation attenuated the accumulation of LC3-II and p62, which are autophagy substrate proteins. An adenovirus containing mRFP-GFP-LC3 showed that decreased expression of Adora2B restored the autophagic flux by enhancing autophagosome conversion to autophagolysosome. Also, Adora2B knockdown improved cardiomyocytes' survival and protected mitochondrial function of cardiomyocytes insulted with hypoxia. Notably, the effect of Adora2B on autophagy flux and cardiomyocyte protection could be mitigated by autophagy inhibitor chloroquine. Our results demonstrate that decreased expression of Adora2B protected cardiomyocytes from impaired autophagy flux induced by MI. Modulation Adora2B expression plays a significant role in blunting the worsening of heart function and reducing scar formation, suggesting therapeutic potential by targeting Adora2B in AMI for the infarct healing.

A Role for the Adenosine ADORA2B Receptor in Midazolam Induced Cognitive Dysfunction

BACKGROUND

We recently reported a role for the circadian rhythm protein Period 2 (PER2) in midazolam induced cognitive dysfunction. Based on previous studies showing a critical role for the adenosine A2B receptor (ADORA2B) in PER2 regulation, we hypothesized that hippocampal ADORA2B is crucial for cognitive function.

METHODS

Midazolam treated C57BL/6J mice were analyzed for Adora2b hippocampal mRNA expression levels, and spontaneous T-maze alternation was determined in Adora2b-/- mice. Using the specific ADORA2B agonist BAY-60-6583 in midazolam treated C57BL/6J mice, we analyzed hippocampal Per2 mRNA expression levels and spontaneous T-maze alternation. Finally, Adora2b-/- mice were assessed for mRNA expression of markers for inflammation or cognitive function in the hippocampus.

RESULTS

Midazolam treatment significantly downregulated Adora2b or Per2 mRNA in the hippocampus of C57BL/6J mice, and hippocampal PER2 protein expression or T-maze alternation was significantly reduced in Adora2b-/- mice. ADORA2B agonist BAY-60-6583 restored midazolam mediated reduction in spontaneous alternation in C57BL/6J mice. Analysis of hippocampal Tnf-α or Il-6 mRNA levels in Adora2b-/- mice did not reveal an inflammatory phenotype. However, C-fos, a critical component of hippocampus-dependent learning and memory, was significantly downregulated in the hippocampus of Adora2b-/- mice.

CONCLUSION

These results suggest a role of ADORA2B in midazolam induced cognitive dysfunction. Further, our data demonstrate that BAY-60-6583 treatment restores midazolam induced cognitive dysfunction, possibly via increases of Per2. Additional mechanistic studies hint towards C-FOS as another potential underlying mechanism of memory impairment in Adora2b-/- mice. These findings suggest the ADORA2B agonist as a potential therapy in patients with midazolam induced cognitive dysfunction.

Dose-dependent Effects of Esmolol-epinephrine Combination Therapy in Myocardial Ischemia and Reperfusion Injury

BACKGROUND

Animal studies on cardiac arrest found that a combination of epinephrine with esmolol attenuates post-resuscitation myocardial dysfunction. Based on these findings, we hypothesized that esmololepinephrine combination therapy would be superior to a reported cardioprotective esmolol therapy alone in a mouse model of myocardial ischemia and reperfusion (IR) injury.

METHODS

C57BL/6J mice were subjected to 60 min of myocardial ischemia and 120 min of reperfusion. Mice received either saline, esmolol (0.4 mg/kg/h), epinephrine (0.05 mg/kg/h), or esmolol combined with epinephrine (esmolol: 0.4 mg/kg/h or 0.8 mg/kg/h and epinephrine: 0.05 mg/kg/h) during reperfusion. After reperfusion, infarct sizes in the area-at-risk and serum cardiac troponin-I levels were determined. Hemodynamic effects of drugs infused were determined by measurements of heart rate (HR) and mean arterial blood pressure (MAP) via a carotid artery catheter.

RESULTS

Esmolol during reperfusion resulted in robust cardioprotection (esmolol vs. saline: 24.3±8% vs. 40.6±3% infarct size), which was abolished by epinephrine co-administration (38.1±15% infarct size). Increasing the esmolol dose, however, was able to restore esmolol-cardioprotection in the epinephrine-esmolol (18.6±8% infarct size) co-treatment group with improved hemodynamics compared to the esmolol group (epinephrine-esmolol vs. esmolol: MAP 80 vs. 75 mmHg, HR 452 vs. 402 beats/min).

CONCLUSION

These results confirm earlier studies on esmolol-cardioprotection from myocardial IR-injury and demonstrate that a dose optimized epinephrine-esmolol co-treatment maintains esmolol-cardioprotection with improved hemodynamics compared to esmolol treatment alone. These findings might have implications for current clinical practice in hemodynamically unstable patients suffering from myocardial ischemia.

Adenosine and the Cardiovascular System

Adenosine is an endogenous nucleoside with a short half-life that regulates many physiological functions involving the heart and cardiovascular system. Among the cardioprotective properties of adenosine are its ability to improve cholesterol homeostasis, impact platelet aggregation and inhibit the inflammatory response. Through modulation of forward and reverse cholesterol transport pathways, adenosine can improve cholesterol balance and thereby protect macrophages from lipid overload and foam cell transformation. The function of adenosine is controlled through four G-protein coupled receptors: A₁, A_2A, A_2B and A₃. Of these four, it is the A_2A receptor that is in a large part responsible for the anti-inflammatory effects of adenosine as well as defense against excess cholesterol accumulation. A_2A receptor agonists are the focus of efforts by the pharmaceutical industry to develop new cardiovascular therapies, and pharmacological actions of the atheroprotective and anti-inflammatory drug methotrexate are mediated via release of adenosine and activation of the A_2A receptor. Also relevant are anti-platelet agents that decrease platelet activation and adhesion and reduce thrombotic occlusion of atherosclerotic arteries by antagonizing adenosine diphosphate-mediated effects on the P2Y12 receptor. The purpose of this review is to discuss the effects of adenosine on cell types found in the arterial wall that are involved in atherosclerosis, to describe use of adenosine and its receptor ligands to limit excess cholesterol accumulation and to explore clinically applied anti-platelet effects. Its impact on electrophysiology and use as a clinical treatment for myocardial preservation during infarct will also be covered. Results of cell culture studies, animal experiments and human clinical trials are presented. Finally, we highlight future directions of research in the application of adenosine as an approach to improving outcomes in persons with cardiovascular disease.

CD73 Rather Than CD39 Is Mainly Involved in Controlling Purinergic Signaling in Calcified Aortic Valve Disease

The study aimed to compare composition of peripheral blood T-cell subsets and assess their surface expression of CD39 and CD73 ectonucleotidases in patients with severe and moderate aortic stenosis (AS) as well as to evaluate involvement of T-cell-mediated immune processes in valve calcification. The study was performed with 38 patients suffering from severe calcified aortic stenosis (SAS), 33 patients with MAS, and 30 apparently healthy volunteers (HVs). The relative distribution and percentage of T-cell subsets expressing CD39 and CD73 were evaluated by flow cytometry. T helper (Th) and cytotoxic T-cell subsets (Tcyt) were identified by using CD3, CD4, and CD8 antibodies. Regulatory T cells (Tregs) were characterized by the expression of CD3, CD4, and high IL-2R alpha chain (CD25high) levels. CD45R0 and CD62L were used to assess differentiation stage of Th, Tcyt, and Treg subsets. It was found that MAS and SAS patients differed in terms of relative distribution of Tcyt and absolute number of Treg. Moreover, the absolute number of Tcyt and terminally differentiated CD45RA-positive effector T-cells (TEMRA) subset was significantly higher in SAS vs. MAS patients and HVs. However, the absolute and relative number of naïve Th and the absolute number of Treg were significantly higher in MAS vs. SAS patients; the relative number of naïve Tregs was significantly (p < 0.01) decreased in SAS patients. It was shown that CD73 expression was significantly higher in SAS vs. MAS patients noted in all EM, CM, TEMRA, and naïve Th cell subsets. However, only the latter were significantly increased (p = 0.003) in patients compared with HVs. SAS vs. MAS patients were noted to have significantly higher percentage of CD73+ EM Tcyt (p = 0.006) and CD73+ CM Tcyt (p = 0.002). The expression of CD73 in patients significantly differed in all three Treg populations such as EM (p = 0.049), CM (p = 0.044), and naïve (p < 0.001). No significant differences in CD39 expression level was found in MAS and SAS patients compared with the HV group. Overall, the data obtained demonstrated that purinergic signaling was involved in the pathogenesis of aortic stenosis and calcification potentially acting via various cell types, wherein among enzymes, degrading extracellular ATP CD73 rather than CD39 played a prominent role.

A2bR-dependent signaling alters immune cell composition and enhances IL-6 formation in the ischemic heart

Although the cardioprotective effect of adenosine is undisputed, the role of the adenosine A2breceptor (A2bR) in ischemic cardiac remodeling is not defined. In this study we aimed to unravel the role A2bR plays in modulating the immune response and the healing mechanisms after myocardial infarction. Genetic and pharmacological (PSB603) inactivation of A2bR as well as activation of A2bR with BAY60-6583 does not alter cardiac remodeling of the infarcted (50-min left anterior descending artery occlusion/reperfusion) murine heart. Flow cytometry of immune cell subsets identified a significant increase in B cells, NK cells, CD8 and CD4 T cells, as well as FoxP3-expressing regulatory T cells in the injured heart in A2bR-deficient mice. Analysis of T-cell function revealed that expression and secretion of interleukin (IL)-2, interferon (IFN)γ, and tumor necrosis factor (TNF)α by T cells is under A2bR control. In addition, we found substantial cellular heterogeneity in the response of immune cells and cardiomyocytes to A2bR deficiency: while in the absence of A2bR, expression of IL-6 was greatly reduced in cardiomyocytes and immune cells except T cells, and expression of IL-1β was strongly reduced in cardiomyocytes, granulocytes, and B cells as determined by quantitative PCR. Our findings indicate that A2bR signaling in the ischemic heart triggers substantial changes in cardiac immune cell composition of the lymphoid lineage and induces a profound cell type-specific downregulation of IL-6 and IL-1β. This suggests the presence of a targetable adenosine–A2bR–IL-6-axis triggered by adenosine formed by the ischemic heart.

NEW & NOTEWORTHY Genetic deletion and pharmacological inactivation/activation of A2bR does not alter cardiac remodeling after MI but is associated by compensatory upregulation of various pro- and anti-inflammatory immune cell subsets (B cells, NK cells, CD8 and CD4 T cells, regulatory T cells). In the inflamed heart, A2bR modulates the expression of IL-2, IFNγ, TNFα in T cells and of IL-6 in cardiomyocytes, monocytes, granulocytes and B cells. This suggests an important adenosine–IL-6 axis, which is controlled by A2bR via local adenosine.

Over-expression of JAZF1 promotes cardiac microvascular endothelial cell proliferation and angiogenesis via activation of the Akt signaling pathway in rats with myocardial ischemia-reperfusion

Myocardial ischemia-reperfusion (I/R) injury is caused by endothelial dysfunction and enhanced oxidative stress. The overexpression of JAZF1, a zinc finger protein, has been reported to promote cell proliferation and suppress myogenic differentiation in type 2 diabetes. However, the involvement of JAZF1 in myocardial I/R injury remains to be unclear. The current study aims to investigate the role by which JAZF1 influences cardiac microvascular endothelial cells (CMECs) in a rat model of myocardial I/R injury. A total of 50 rats were established as a myocardial I/R model to isolate CMECs, with alterations in JAZF1 expression. After that, the gain- or loss-function of JAZF1 on the proliferation, apoptosis and tube formation ability of CMECs were evaluated by a series of in vitro experiments. Results indicated that JAZF1 was down-regulated in CMECs of rats with myocardial I/R injury. After treatment with JAZF1, the levels of VEGF, Bcl-2, PDGF and p-Akt/Akt were all increased; however, the expression of Bax, caspase-3, caspase-9, p-Bad/Bad, c-caspase-3/caspase-3, c-caspase-9/caspase-9, and p-FKHR/FKHR exhibited decreased levels; CMEC proliferation and angiogenesis were increased, while cell apoptosis was attenuated. CMECs transfected with JAZF1 shRNA exhibited the contrary tendencies. The key findings of this study suggest that the over-expression of JAZF1 alleviates myocardial I/R injury by enhancing proliferation and angiogenesis of CMECs and in turn inhibiting apoptosis of CMECs via the activation of the Akt signaling pathway.

Immune cells as targets for cardioprotection: new players and novel therapeutic opportunities

New therapies are required to reduce myocardial infarct (MI) size and prevent the onset of heart failure in patients presenting with acute myocardial infarction (AMI), one of the leading causes of death and disability globally. In this regard, the immune cell response to AMI, which comprises an initial pro-inflammatory reaction followed by an anti-inflammatory phase, contributes to final MI size and post-AMI remodelling [changes in left ventricular (LV) size and function]. The transition between these two phases is critical in this regard, with a persistent and severe pro-inflammatory reaction leading to adverse LV remodelling and increased propensity for developing heart failure. In this review article, we provide an overview of the immune cells involved in orchestrating the complex and dynamic inflammatory response to AMI-these include neutrophils, monocytes/macrophages, and emerging players such as dendritic cells, lymphocytes, pericardial lymphoid cells, endothelial cells, and cardiac fibroblasts. We discuss potential reasons for past failures of anti-inflammatory cardioprotective therapies, and highlight new treatment targets for modulating the immune cell response to AMI, as a potential therapeutic strategy to improve clinical outcomes in AMI patients. This article is part of a Cardiovascular Research Spotlight Issue entitled 'Cardioprotection Beyond the Cardiomyocyte', and emerged as part of the discussions of the European Union (EU)-CARDIOPROTECTION Cooperation in Science and Technology (COST) Action, CA16225.

ATP and T-cell-mediated rejection

PURPOSE OF REVIEW

Purine nucleosides and nucleotides are released in the extracellular space following cell injury and act as paracrine mediators through a number of dedicated membrane receptors. In particular, extracellular ATP (eATP) significantly influences T-lymphocyte activation and phenotype. The purpose of this review is to discuss the role of ATP signaling in the T-cell-mediated alloimmune response.

RECENT FINDINGS

In various animal models of solid transplantation, the purinergic axis has been targeted to prevent acute rejection and to promote long-term graft tolerance. The inhibition of ATP-gated P2X receptors has been shown to halt lymphocyte activation, to downregulate both Th1 and Th17 responses and to promote T-regulatory (Treg) cell differentiation. Similarly, the inhibition of ATP signaling attenuated graft-versus-host disease in mice undergoing hematopoietic cell transplantation. Significantly, different drugs targeting the purinergic system have been recently approved for human use and may be a viable therapeutic option for transplant patients.

SUMMARY

The inhibition of eATP signaling downregulates the alloimmune response, expands Treg cells and promotes graft survival. This robust preclinical evidence and the recent advances in pharmacological research may lead to intriguing clinical applications.

The adenosine A2B G protein-coupled receptor: Recent advances and therapeutic implications

The adenosine A_2B receptor (A_2BAR) is one of four adenosine receptor subtypes belonging to the Class A family of G protein-coupled receptors (GPCRs). Until recently, the A_2BAR remained poorly characterised, in part due to its relatively low affinity for the endogenous agonist adenosine and therefore presumed minor physiological significance. However, the substantial increase in extracellular adenosine concentration, the sensitisation of the receptor and the upregulation of A_2BAR expression under conditions of hypoxia and inflammation, suggest the A_2BAR as an exciting therapeutic target in a variety of pathological disease states. Here we discuss the pharmacology of the A_2BAR and outline its role in pathophysiology including ischaemia-reperfusion injury, fibrosis, inflammation and cancer.

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.

Infarct-Sparing Effect of Adenosine A2B Receptor Agonist Is Primarily Due to Its Action on Splenic Leukocytes Via a PI3K/Akt/IL-10 Pathway

BACKGROUND

Adenosine A2B receptor (A_2BAR) agonist reduces myocardial reperfusion injury by acting on inflammatory cells. Recently, a cardiosplenic axis was shown to mediate the myocardial postischemic reperfusion injury. This study aimed to explore whether the infarct-squaring effect of A_2BAR agonist was primarily due to its action on splenic leukocytes.

METHODS

C57BL6 (wild type [WT]) mice underwent 40 min of left coronary artery occlusion followed by 60 min of reperfusion. A_2BAR knockout (KO) and interleukin (IL)-10KO mice served as donors for splenic leukocytes. Acute splenectomy was performed 30 min before ischemia. The acute splenic leukocyte adoptive transfer was performed by injecting 5 × 10⁶ live splenic leukocytes into splenectomized mice. BAY 60-6583, an A_2BAR agonist, was injected by i.v. 15 min before ischemia. The infarct size (IS) was determined using 2,3,5-triphenyltetrazolium chloride and Phthalo blue staining. The expression of p-Akt and IL-10 was estimated by Western blotting. Immunofluorescence staining assessed the localization of IL-10 expression.

RESULTS

BAY 60-6583 reduced the myocardial IS in intact mice but failed to reduce the same in splenectomized mice, which had a smaller IS than intact mice. BAY 60-6583 reduced the IS in splenectomized mice with the acute transfer of WT splenic leukocytes; however, it did not protect the heart of splenectomized mice with the acute transfer of A_2BRKO splenic leukocytes. Furthermore, BAY 60-6583 increased the levels of p-Akt and IL-10 in the WT spleen. Moreover, it did not exert any protective effect in IL-10KO mice.

CONCLUSIONS

A_2BAR activation before ischemia stimulated the IL-10 production in splenic leukocytes via a PI3K/Akt pathway, thereby exerting anti-inflammatory effects that limited the myocardial reperfusion injury.

Purinergic Regulation of Neutrophil Function

Purinergic signaling, which utilizes nucleotides (particularly ATP) and adenosine as transmitter molecules, plays an essential role in immune system. In the extracellular compartment, ATP predominantly functions as a pro-inflammatory molecule through activation of P2 receptors, whereas adenosine mostly functions as an anti-inflammatory molecule through activation of P1 receptors. Neutrophils are the most abundant immune cells in circulation and have emerged as an important component in orchestrating a complex series of events during inflammation. However, because of the destructive nature of neutrophil-derived inflammatory agents, neutrophil activation is fine-tuned, and purinergic signaling is intimately involved in this process. Indeed, shifting the balance between P2 and P1 signaling is critical for neutrophils to appropriately exert their immunologic activity. Here, we review the role of purinergic signaling in regulating neutrophil function, and discuss the potential of targeting purinergic signaling for the treatment of neutrophil-associated infectious and inflammatory diseases.

Hypoxia-inducible factor 2-alpha-dependent induction of amphiregulin dampens myocardial ischemia-reperfusion injury

Myocardial ischemia–reperfusion injury (IRI) leads to the stabilization of the transcription factors hypoxia-inducible factor 1-alpha (HIF1-alpha) and hypoxia-inducible factor 2-alpha (HIF2-alpha). While previous studies implicate HIF1-alpha in cardioprotection, the role of HIF2-alpha remains elusive. Here we show that HIF2-alpha induces the epithelial growth factor amphiregulin (AREG) to elicit cardioprotection in myocardial IRI. Comparing mice with inducible deletion of Hif1a or Hif2a in cardiac myocytes, we show that loss of Hif2-alpha increases infarct sizes. Microarray studies in genetic models or cultured human cardiac myocytes implicate HIF2-alpha in the myocardial induction of AREG. Likewise, AREG increases in myocardial tissues from patients with ischemic heart disease. Areg deficiency increases myocardial IRI, as does pharmacologic inhibition of Areg signaling. In contrast, treatment with recombinant Areg provides cardioprotection and reconstitutes mice with Hif2a deletion. These studies indicate that HIF2-alpha induces myocardial AREG expression in cardiac myocytes, which increases myocardial ischemia tolerance.

Myocardial ischemia–reperfusion injury stabilizes the hypoxia-inducible factor HIF2-alpha. Here, the authors show that HIF2-alpha protects the heart from injury via induction of the epidermal growth factor amphiregulin, and that amphiregulin administration is cardioprotective in mice.

The Circadian PER2 Enhancer Nobiletin Reverses the Deleterious Effects of Midazolam in Myocardial Ischemia and Reperfusion Injury

BACKGROUND

Recently, we identified the circadian rhythm protein Period 2 (PER2) in robust cardioprotection from myocardial ischemia (MI). Based on findings that perioperative MI is the most common major cardiovascular complication and that anesthetics can alter the expression of PER2, we hypothesized that an anesthesia mediated downregulation of PER2 could be detrimental if myocardial ischemia and reperfusion (IR) would occur.

METHODS AND RESULTS

We exposed mice to pentobarbital, fentanyl, ketamine, propofol, midazolam or isoflurane and determined cardiac Per2 mRNA levels. Unexpectedly, only midazolam treatment resulted in an immediate and significant downregulation of Per2 transcript levels. Subsequent studies in mice pretreated with midazolam using an in-situ mouse model for myocardial (IR)-injury revealed a significant and dramatic increase in infarct sizes or Troponin-I serum levels in the midazolam treated group when compared to controls. Using the recently identified flavonoid, nobiletin, as a PER2 enhancer completely abolished the deleterious effects of midazolam during myocardial IR-injury. Moreover, nobiletin treatment alone significantly reduced infarct sizes or Troponin I levels in wildtype but not in Per2-/- mice. Pharmacological studies on nobiletin like flavonoids revealed that only nobiletin and tangeritin, both found to enhance PER2, were cardioprotective in our murine model for myocardial IR-injury.

CONCLUSION

We identified midazolam mediated downregulation of cardiac PER2 as an underlying mechanism for a deleterious effect of midazolam pretreatment in myocardial IR-injury. These findings highlight PER2 as a cardioprotective mechanism and suggest the PER2 enhancers nobiletin or tangeritin as a preventative therapy for myocardial IR-injury in the perioperative setting where midazolam pretreatment occurs frequently.

Leukocyte diversity in resolving and nonresolving mechanisms of cardiac remodeling

In response to myocardial infarction (MI), time-dependent leukocyte infiltration is critical to program the acute inflammatory response. Post-MI leukocyte density, residence time in the infarcted area, and exit from the infarcted injury predict resolving or nonresolving inflammation. Overactive or unresolved inflammation is the primary determinant in heart failure pathology post-MI. Here, our review describes supporting evidence that the acute inflammatory response also guides the generation of healing and regenerative mediators after cardiac damage. Time-dependent leukocyte density and diversity and the magnitude of myocardial injury is responsible for the resolving and nonresolving pathway in myocardial healing. Post MI, the diversity of leukocytes, such as neutrophils, macrophages, and lymphocytes, has been explored that regulate the clearance of deceased cardiomyocytes by using the classic and reparative pathways. Among the innovative factors and intermediates that have been recognized as essential in acute the self-healing and clearance mechanism, we highlight specialized proresolving mediators as the emerging factor for post-MI reparative mechanisms-translational leukocyte modifiers, such as aging, the source of leukocytes, and the milieu around the leukocytes. In the clinical setting, it is possible that leukocyte diversity is more prominent as a result of risk factors, such as obesity, diabetes, and hypertension. Pharmacologic agents are critical modifiers of leukocyte diversity in healing mechanisms that may impair or stimulate the clearance mechanism. Future research is needed, with a focused approach to understand the molecular targets, cellular effectors, and receptors. A clear understanding of resolving and nonresolving inflammation in myocardial healing will help to develop novel targets with major emphasis on the resolution of inflammation in heart failure pathology.-Tourki, B., Halade, G. Leukocyte diversity in resolving and nonresolving mechanisms of cardiac remodeling.

Intense light-elicited upregulation of miR-21 facilitates glycolysis and cardioprotection through Per2-dependent mechanisms

A wide search for ischemic preconditioning (IPC) mechanisms of cardioprotection identified the light elicited circadian rhythm protein Period 2 (Per2) to be cardioprotective. Studies on cardiac metabolism found a key role for light elicited Per2 in mediating metabolic dependence on carbohydrate metabolism. To profile Per2 mediated pathways following IPC of the mouse heart, we performed a genome array and identified 352 abundantly expressed and well-characterized Per2 dependent micro RNAs. One prominent result of our in silico analysis for cardiac Per2 dependent micro RNAs revealed a selective role for miR-21 in the regulation of hypoxia and metabolic pathways. Based on this Per2 dependency, we subsequently found a diurnal expression pattern for miR-21 with higher miR-21 expression levels at Zeitgeber time (ZT) 15 compared to ZT3. Gain or loss of function studies for miR-21 using miRNA mimics or miRNA inhibitors and a Seahorse Bioanalyzer uncovered a critical role of miR-21 for cellular glycolysis, glycolytic capacity, and glycolytic reserve. Exposing mice to intense light, a strategy to induce Per2, led to a robust induction of cardiac miR-21 tissue levels and decreased infarct sizes, which was abolished in miR-21-/- mice. Similarly, first translational studies in humans using intense blue light exposure for 5 days in healthy volunteers resulted in increased plasma miR-21 levels which was associated with increased phosphofructokinase activity, the rate-limiting enzyme in glycolysis. Together, we identified miR-21 as cardioprotective downstream target of Per2 and suggest intense light therapy as a potential strategy to enhance miR-21 activity and subsequent carbohydrate metabolism in humans.

CD73 on T Cells Orchestrates Cardiac Wound Healing After Myocardial Infarction by Purinergic Metabolic Reprogramming

BACKGROUND

T cells are required for proper healing after myocardial infarction. The mechanism of their beneficial action, however, is unknown. The proinflammatory danger signal ATP, released from damaged cells, is degraded by the ectonucleotidases CD39 and CD73 to the anti-inflammatory mediator adenosine. Here, we investigate the contribution of CD73-derived adenosine produced by T cells to cardiac remodeling after ischemia/reperfusion and define its mechanism of action.

METHODS

Myocardial ischemia (50 minutes followed by reperfusion) was induced in global CD73^-/- and CD4-CD73^-/- mice. Tissue injury, T-cell purinergic signaling, cytokines, and cardiac function (magnetic resonance tomography at 9.4 T over 4 weeks) were analyzed.

RESULTS

Changes in functional parameters of CD4-CD73^-/- mice were identical to those in global CD73 knockouts (KOs). T cells infiltrating the injured heart significantly upregulated at the gene (quantitative polymerase chain reaction) and protein (enzymatic activity) levels critical transporters and enzymes (connexin43, connexin37, pannexin-1, equilibrative nucleoside transporter 1, CD39, CD73, ecto-nucleotide pyrophosphatase/phosphodiesterases 1 and 3, CD157, CD38) for the accelerated release and hydrolysis of ATP, cAMP, AMP, and NAD to adenosine. It is surprising that a lack of CD39 on T cells (from CD39^-/- mice) did not alter ATP hydrolysis and very likely involves pyrophosphatases (ecto-nucleotide pyrophosphatase/phosphodiesterases 1 and 3). Circulating T cells predominantly expressed A_2a receptor (A_2aR) transcripts. After myocardial infarction, A_2b receptor (A_2bR) transcription was induced in both T cells and myeloid cells in the heart. Thus, A_2aR and A_2bR signaling may contribute to myocardial responses after myocardial infarction. In the case of T cells, this was associated with an accelerated secretion of proinflammatory and profibrotic cytokines (interleukin-2, interferon-γ, and interleukin-17) when CD73 was lacking. Cytokine production by T cells from peripheral lymph nodes was inhibited by A_2aR activation (CGS-21680). The A_2bR agonist BAY 60-6583 showed off-target effects. The adenosine receptor agonist NECA inhibited interferon-γ and stimulated interleukin-6 production, each of which was antagonized by a specific A_2bR antagonist (PSB-603).

CONCLUSIONS

This work demonstrates that CD73 on T cells plays a crucial role in the cardiac wound healing process after myocardial infarction. The underlying mechanism involves a profound increase in the hydrolysis of ATP/NAD and AMP, resulting primarily from the upregulation of pyrophosphatases and CD73. We also define A_2bR/A_2aR-mediated autacoid feedback inhibition of proinflammatory/profibrotic cytokines by T cell-derived CD73.

ADORA2b Signaling in Cardioprotection

Cardiovascular disease is the number one cause of death worldwide. A powerful strategy for cardioprotection would be to identify specific molecules or targets that mimic ischemic preconditioning (IP), where short non-lethal episodes of ischemia and reperfusion prior to myocardial infarction result in dramatic reduction of infarct sizes. Since 1960 researchers believed that adenosine has a strong cardio-protective potential. In fact, with the discovery of cardiac IP in 1986 by Murry et al., adenosine was the first identified molecule that was used in studying the underlying mechanism of IP. Today we know, based on genetic studies, that adenosine is crucial for IP mediated cardio-protection and that the adenosine receptors ADORA1, ADORA2a and ADORA2b play an important role. However, the ADORA2b receptor is the only receptor so far which has been found to play a role in human and murine myocardial ischemia. With recent advances using tissue specific mice for the ADORA2b, we were able to uncover cardiomyocytes and endothelia as the responsible cell type for cardiac IP. Using a wide search for ADORA2b downstream targets, our group identified the circadian rhythm protein, Period 2 (PER2), as a novel target for IP mediated cardioprotection. Mechanistic studies on PER2 mediated cardioprotection revealed an important role for PER2 in optimizing cardiac metabolism through activation of oxygen saving pathways. Thus, cardiomyocyte or endothelial expressed ADORA2b or the downstream circadian rhythm protein PER2 are key targets for cardiac IP and could represent novel strategies to treat or prevent MI.

The Many Faces of the A2b Adenosine Receptor in Cardiovascular and Metabolic Diseases

Modulation of the low affinity adenosine receptor subtype, the A2b adenosine receptor (A2bAR), has gained interest as a therapeutic target in various pathologic areas associated with cardiovascular disease. The actions of the A2bAR are diverse and at times conflicting depending on cell and tissue type and the timing of activation or inhibition of the receptor. The A2bAR is a promising and exciting pharmacologic target, however, a thorough understanding of A2bAR action is necessary to reach the therapeutic potential of this receptor. This review will focus on the role of the A2bAR in various cardiovascular and metabolic pathologies in which the receptor is currently being studied. We will illustrate the complexities of A2bAR signaling and highlight areas of research with potential for therapeutic development.

Epithelial-specific A2B adenosine receptor signaling protects the colonic epithelial barrier during acute colitis

Central to inflammatory bowel disease (IBD) pathogenesis is loss of mucosal barrier function. Emerging evidence implicates extracellular adenosine signaling in attenuating mucosal inflammation. We hypothesized that adenosine-mediated protection from intestinal barrier dysfunction involves tissue-specific signaling through the A2B adenosine receptor (Adora2b) at the intestinal mucosal surface. To address this hypothesis, we combined pharmacologic studies and studies in mice with global or tissue-specific deletion of the Adora2b receptor. Adora2b(-/-) mice experienced a significantly heightened severity of colitis, associated with a more acute onset of disease and loss of intestinal epithelial barrier function. Comparison of mice with Adora2b deletion on vascular endothelial cells (Adora2b(fl/fl)VeCadCre(+)) or intestinal epithelia (Adora2b(fl/fl)VillinCre(+)) revealed a selective role for epithelial Adora2b signaling in attenuating colonic inflammation. In vitro studies with Adora2b knockdown in intestinal epithelial cultures or pharmacologic studies highlighted Adora2b-driven phosphorylation of vasodilator-stimulated phosphoprotein (VASP) as a specific barrier repair response. Similarly, in vivo studies in genetic mouse models or treatment studies with an Adora2b agonist (BAY 60-6583) recapitulate these findings. Taken together, our results suggest that intestinal epithelial Adora2b signaling provides protection during intestinal inflammation via enhancing mucosal barrier responses.

Differential Tissue-Specific Function of Adora2b in Cardioprotection

The adenosine A2b receptor (Adora2b) has been implicated in cardioprotection from myocardial ischemia. As such, Adora2b was found to be critical in ischemic preconditioning (IP) or ischemia/reperfusion (IR) injury of the heart. Whereas Adora2b is present on various cells types, the tissue-specific role of Adora2b in cardioprotection is still unknown. To study the tissue-specific role of Adora2b signaling on inflammatory cells, endothelia, or myocytes during myocardial ischemia in vivo, we intercrossed floxed Adora2b mice with Lyz2-Cre(+), VE-cadherin-Cre(+), or myosin-Cre(+) transgenic mice, respectively. Mice were exposed to 60 min of myocardial ischemia with or without IP (four times for 5 min) followed by 120 min of reperfusion. Cardioprotection by IP was abolished in Adora2b(f/f)-VE-cadherin-Cre(+) or Adora2b(f/f)-myosin-Cre(+), indicating that Adora2b signaling on endothelia or myocytes mediates IP. In contrast, primarily Adora2b signaling on inflammatory cells was necessary to provide cardioprotection in IR injury, indicated by significantly larger infarcts and higher troponin levels in Adora2b(f/f)-Lyz2-Cre(+) mice only. Cytokine profiling of IR injury in Adora2b(f/f)-Lyz2-Cre(+) mice pointed toward polymorphonuclear neutrophils (PMNs). Analysis of PMNs from Adora2b(f/f)-Lyz2-Cre(+) confirmed PMNs as one source of identified tissue cytokines. Finally, adoptive transfer of Adora2b(-/-) PMNs revealed a critical role of Adora2b on PMNs in cardioprotection from IR injury. Adora2b signaling mediates different types of cardioprotection in a tissue-specific manner. These findings have implications for the use of Adora2b agonists in the treatment or prevention of myocardial injury by ischemia.

Adenosine 2B Receptor Activation Reduces Myocardial Reperfusion Injury by Promoting Anti-Inflammatory Macrophages Differentiation via PI3K/Akt Pathway

BACKGROUND

Activation of the adenosine A2B receptor (A2BR) can reduce myocardial ischemia/reperfusion (IR) injury. However, the mechanism underlying the A2BR-mediated cardioprotection is less clear. The present study was designed to investigate the potential mechanisms of cardioprotection mediated by A2BR.

METHODS AND RESULTS

C57BL/6 mice underwent 40-minute ischemia and 60-minute reperfusion. ATL-801, a potent selective A2BR antagonist, could not block ischemic preconditioning induced protection. BAY 60-6583, a highly selective A2BR agonist, significantly reduced myocardial infarct size, and its protective effect could be blocked by either ATL-801 or wortmannin. BAY 60-6583 increased phosphorylated Akt (p-Akt) levels in the heart at 10 min of reperfusion, and this phosphorylation could also be blocked by ATL-801 or wortmannin. Furthermore, BAY 60-6583 significantly increased M2 macrophages and decreased M1 macrophage and neutrophils infiltration in reperfused hearts, which also could be blocked by wortmannin. Meanwhile, confocal imaging studies showed that the majority of Akt phosphorylation in the heart was colocalized to CD206+ cells in both control and BAY 60-6583 pretreated hearts.

CONCLUSION

Our results indicated that pretreatment with BAY 60-6583 protects the heart against myocardial IR injury by its anti-inflammatory effects, probably by modulating macrophages phenotype switching via a PI3K/Akt pathway.

Extracellular nucleotide and nucleoside signaling in vascular and blood disease

Nucleotides and nucleosides-such as adenosine triphosphate (ATP) and adenosine-are famous for their intracellular roles as building blocks for the genetic code or cellular energy currencies. In contrast, their function in the extracellular space is different. Here, they are primarily known as signaling molecules via activation of purinergic receptors, classified as P1 receptors for adenosine or P2 receptors for ATP. Because extracellular ATP is rapidly converted to adenosine by ectonucleotidase, nucleotide-phosphohydrolysis is important for controlling the balance between P2 and P1 signaling. Gene-targeted mice for P1, P2 receptors, or ectonucleotidase exhibit only very mild phenotypic manifestations at baseline. However, they demonstrate alterations in disease susceptibilities when exposed to a variety of vascular or blood diseases. Examples of phenotypic manifestations include vascular barrier dysfunction, graft-vs-host disease, platelet activation, ischemia, and reperfusion injury or sickle cell disease. Many of these studies highlight that purinergic signaling events can be targeted therapeutically.

Netrin-1 guides inflammatory cell migration to control mucosal immune responses during intestinal inflammation

The intestinal epithelium is a dynamic barrier playing an active role in intestinal homeostasis and inflammation. Intestinal barrier function is dysregulated during inflammatory bowel disease (IBD), with epithelial cells playing a significant part in generating an inflammatory milieu through the release of signals that attract leukocytes to the intestinal lamina propria. However, it is increasingly appreciated that the intestinal epithelium mediates a counterbalancing response to drive resolution. Drawing analogies with neuronal development, where the balance of chemoattractive and chemorepellent signals is key to directed neuronal movement it has been postulated that such secreted cues play a role in leukocyte migration. Netrin-1 is one of the best-described neuronal guidance molecules, which has been shown to play a significant role in directed migration of leukocytes. Prior to our study the potential role of netrin-1 in IBD was poorly characterized. We defined netrin-1 as an intestinal epithelial-derived protein capable of limiting neutrophil recruitment to attenuate acute colitis. Our study highlights that the intestinal epithelium releases factors during acute inflammation that are responsible for fine-tuning the immune response. Exploration of these epithelial-mediated protective mechanisms will shed light on the complexity of the intestinal epithelial barrier in health and disease.

The Role of Ectonucleotidases CD39 and CD73 and Adenosine Signaling in Solid Organ Transplantation

Extracellular adenosine is a potent immunomodulatory molecule that accumulates in states of inflammation. Nucleotides such as adenosine triphosphate and adenosine diphosphate are release from injured and necrotic cells and hydrolyzed to adenosine monophosphate and adenosine by the concerted action of the ectonucleotidases CD39 and CD73. Accumulating evidence suggest that purinergic signaling is involved in the inflammatory response that accompanies acute rejection and chronic allograft dysfunction. Modification of the purinergic pathway has been shown to alter graft survival in a number of solid organ transplant models and the response to ischemia–reperfusion injury (IRI). Furthermore, the purinergic pathway is intrinsically involved in B and T cell biology and function. Although T cells have traditionally been considered the orchestrators of acute allograft rejection, a role for B cells in chronic allograft loss is being increasingly appreciated. This review focuses on the role of the ectonucleotidases CD39 and CD73 and adenosine signaling in solid organ transplantation including the effects on IRI and T and B cell biology.

Cardiac Per2 functions as novel link between fatty acid metabolism and myocardial inflammation during ischemia and reperfusion injury of the heart

Disruption of peripheral circadian rhyme pathways dominantly leads to metabolic disorders. Studies on circadian rhythm proteins in the heart indicated a role for Clock or Per2 in cardiac metabolism. In contrast to Clock^−/−, Per2^−/− mice have larger infarct sizes with deficient lactate production during myocardial ischemia. To test the hypothesis that cardiac Per2 represents an important regulator of cardiac metabolism during myocardial ischemia, we measured lactate during reperfusion in Per1^−/−, Per2^−/− or wildtype mice. As lactate measurements in whole blood indicated an exclusive role of Per2 in controlling lactate production during myocardial ischemia, we next performed gene array studies using various ischemia-reperfusion protocols comparing wildtype and Per2^−/− mice. Surprisingly, high-throughput gene array analysis revealed dominantly lipid metabolism as the differentially regulated pathway in wildtype mice when compared to Per2^−/−. In all ischemia-reperfusion protocols used, the enzyme enoyl-CoA hydratase, which is essential in fatty acid beta-oxidation, was regulated in wildtype animals only. Studies using nuclear magnet resonance imaging (NMRI) confirmed altered fatty acid populations with higher mono-unsaturated fatty acid levels in hearts from Per2^−/− mice. Unexpectedly, studies on gene regulation during reperfusion revealed solely pro inflammatory genes as differentially regulated ‘Per2-genes’. Subsequent studies on inflammatory markers showed increasing IL-6 or TNFα levels during reperfusion in Per2^−/− mice. In summary, these studies reveal an important role of cardiac Per2 for fatty acid metabolism and inflammation during myocardial ischemia and reperfusion, respectively.

Detrimental role of the airway mucin Muc5ac during ventilator-induced lung injury

Acute lung injury (ALI) is associated with high morbidity and mortality in critically ill patients. At present, the functional contribution of airway mucins to ALI is unknown. We hypothesized that excessive mucus production could be detrimental during lung injury. Initial transcriptional profiling of airway mucins revealed a selective and robust induction of MUC5AC upon cyclic mechanical stretch exposure of pulmonary epithelia (Calu-3). Additional studies confirmed time- and stretch-dose-dependent induction of MUC5AC transcript or protein during cyclic mechanical stretch exposure in vitro or during ventilator-induced lung injury in vivo. Patients suffering from ALI showed a 58-fold increase in MUC5AC protein in their bronchoalveolar lavage. Studies of the MUC5AC promoter implicated nuclear factor κB in Muc5ac induction during ALI. Moreover, mice with gene-targeted deletion of Muc5ac⁻/⁻ experience attenuated lung inflammation and pulmonary edema during injurious ventilation. We observed that neutrophil trafficking into the lungs of Muc5ac⁻/⁻ mice was selectively attenuated. This implicates that endogenous Muc5ac production enhances pulmonary neutrophil trafficking during lung injury. Together, these studies reveal a detrimental role for endogenous Muc5ac production during ALI and suggest pharmacological strategies to dampen mucin production in the treatment of lung injury.

Crosstalk between the equilibrative nucleoside transporter ENT2 and alveolar Adora2b adenosine receptors dampens acute lung injury

The signaling molecule adenosine has been implicated in attenuating acute lung injury (ALI). Adenosine signaling is terminated by its uptake through equilibrative nucleoside transporters (ENTs). We hypothesized that ENT-dependent adenosine uptake could be targeted to enhance adenosine-mediated lung protection. To address this hypothesis, we exposed mice to high-pressure mechanical ventilation to induce ALI. Initial studies demonstrated time-dependent repression of ENT1 and ENT2 transcript and protein levels during ALI. To examine the contention that ENT repression represents an endogenous adaptive response, we performed functional studies with the ENT inhibitor dipyridamole. Dipyridamole treatment (1 mg/kg; EC50=10 μM) was associated with significant increases in ALI survival time (277 vs. 395 min; P<0.05). Subsequent studies in gene-targeted mice for Ent1 or Ent2 revealed a selective phenotype in Ent2(-/-) mice, including attenuated pulmonary edema and improved gas exchange during ALI in conjunction with elevated adenosine levels in the bronchoalveolar fluid. Furthermore, studies in genetic models for adenosine receptors implicated the A2B adenosine receptor (Adora2b) in mediating ENT-dependent lung protection. Notably, dipyridamole-dependent attenuation of lung inflammation was abolished in mice with alveolar epithelial Adora2b gene deletion. Our newly identified crosstalk pathway between ENT2 and alveolar epithelial Adora2b in lung protection during ALI opens possibilities for combined therapies targeted to this protein set.

Attenuating myocardial ischemia by targeting A2B adenosine receptors

Myocardial ischemia is associated with profound tissue hypoxia due to an imbalance in oxygen supply and demand, and studies of hypoxia-elicited adaptive responses during myocardial ischemia revealed a cardioprotective role for the signaling molecule adenosine. In ischemic human hearts, the A2B adenosine receptor (ADORA2B) is selectively induced. Functional studies in genetic models show that ADORA2B signaling attenuates myocardial infarction by adapting metabolism towards more oxygen efficient utilization of carbohydrates. This adenosine-mediated cardio-adaptive response involves the transcription factor hypoxia-inducible factor HIF1α and the circadian rhythm protein PER2. In this article, we discuss advances in the understanding of adenosine-elicited cardioprotection with particular emphasis on ADORA2B, its downstream targets, and the implications for novel strategies to prevent or treat myocardial ischemia.

Ligand-specific binding and activation of the human adenosine A(2B) receptor

Adenosine A(2B) receptors, which play a role in inflammation and cancer, are of considerable interest as novel drug targets. To gain deeper insights into ligand binding and receptor activation, we exchanged amino acids predicted to be close to the binding pocket. The alanine mutants were stably expressed in CHO cells and characterized by radioligand binding and cAMP assays using three structural classes of ligands: xanthine (antagonist), adenosine, and aminopyridine derivatives (agonists). Asn282(7.45) and His280(7.43) were found to stabilize the binding site by intramolecular hydrogen bond formation as in the related A(2A) receptor subtype. Trp247(6.48), Val250(6.51), and particularly Ser279(7.42) were shown to be important for binding of nucleosidic agonists. Leu81(3.28), Asn186(5.42), and Val250(6.51) were discovered to be crucial for binding of the xanthine-derived antagonist PSB-603. Leu81(3.28), which is not conserved among adenosine receptor subtypes, may be important for the high selectivity of PSB-603. The N186(5.42)A mutant resulted in an increased potency for agonists. The interactions of the non-nucleosidic agonist BAY60-6583 were different from those of the nucleosides: while BAY60-6583 appeared not to interact with Ser279(7.42), its interactions with Trp247(6.48) and Val250(6.51) were significantly weaker compared to those of NECA. Moreover, our results discount the hypothesis of Trp247(6.48) serving as a "toogle switch" because BAY60-6583 was able to activate the corresponding mutant. This study reveals distinct interactions of structurally diverse ligands with the human A(2B) receptor and differences between closely related receptor subtypes (A(2B) and A(2A)). It will contribute to the understanding of G protein-coupled receptor function and advance A(2B) receptor ligand design.

The CD39-adenosinergic axis in the pathogenesis of renal ischemia-reperfusion injury

Hypoxic injury occurs when the blood supply to an organ is interrupted; subsequent reperfusion halts ongoing ischemic damage but paradoxically leads to further inflammation. Together this is termed ischemia-reperfusion injury (IRI). IRI is inherent to organ transplantation and impacts both the short- and long-term outcomes of the transplanted organ. Activation of the purinergic signalling pathway is intrinsic to the pathogenesis of, and endogenous response to IRI. Therapies targeting the purinergic pathway in IRI are an attractive avenue for the improvement of transplant outcomes and the basis of ongoing research. This review aims to examine the role of adenosine receptor signalling and the ecto-nucleotidases, CD39 and CD73, in IRI, with a particular focus on renal IRI.

Adora2b adenosine receptor signaling protects during acute kidney injury via inhibition of neutrophil-dependent TNF-α release

Renal ischemia is among the leading causes of acute kidney injury (AKI). Previous studies have shown that extracellular adenosine is a prominent tissue-protective cue elicited during ischemia, including signaling events through the adenosine receptor 2b (Adora2b). To investigate the functional role of Adora2b signaling in cytokine-mediated inflammatory pathways, we screened wild-type and Adora2b-deficient mice undergoing renal ischemia for expression of a range of inflammatory cytokines. These studies demonstrated a selective and robust increase of TNF-α levels in Adora2b-deficient mice following renal ischemia and reperfusion. Based on these findings, we next sought to understand the contribution of TNF-α on ischemic AKI through a combination of loss- and gain-of-function studies. Loss of TNF-α, through either Ab blockade or study of Tnf-α-deficient animals, resulted in significantly attenuated tissue injury and improved kidney function following renal ischemia. Conversely, transgenic mice with overexpression of TNF-α had significantly pronounced susceptibility to AKI. Furthermore, neutrophil depletion or reconstitution of Adora2b(-/-) mice with Tnf-α-deficient neutrophils rescued their phenotype. In total, these data demonstrate a critical role of adenosine signaling in constraining neutrophil-dependent production of TNF-α and implicate therapies targeting TNF-α in the treatment of ischemic AKI.