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

Purinergic signaling in the battlefield of viral infections

Purinergic signaling has been associated with immune defenses against pathogens such as bacteria, protozoa, fungi, and viruses, acting as a sentinel system that signals to the cells when a threat is present. This review focuses on the roles of purinergic signaling and its therapeutic potential for viral infections. In this context, the purinergic system may play potent antiviral roles by boosting interferon signaling. In other cases, though, it can contribute to a hyperinflammatory response and disease severity, resulting in poor outcomes, such as during flu and potentially COVID-19. Lastly, a third situation may occur since viruses are obligatory intracellular parasites that hijack the host cell machinery for their infection and replication. Viruses such as HIV-1 use the purinergic system to favor their infection and persistence within the host cell. Therefore, understanding the particular nuances of purinergic signaling in each viral infection may contribute to designing proper therapeutic strategies to treat viral diseases.

Platelets in Renal Disease

Kidney disease is a major global health concern, affecting millions of people. Nephrologists have shown interest in platelets because of coagulation disorders caused by renal diseases. With a better understanding of platelets, it has been found that these anucleate and abundant blood cells not only play a role in hemostasis, but also have important functions in inflammation and immunity. Platelets are not only affected by kidney disease, but may also contribute to kidney disease progression by mediating inflammation and immune effects. This review summarizes the current evidence regarding platelet abnormalities in renal disease, and the multiple effects of platelets on kidney disease progression. The relationship between platelets and kidney disease is still being explored, and further research can provide mechanistic insights into the relationship between thrombosis, bleeding, and inflammation related to kidney disease, and elucidate targeted therapies for patients with kidney disease.

Increased frequency of CD39+CD73+ regulatory T cells and Deltex-1 gene expression level in kidney transplant recipients with excellent long-term graft function

BACKGROUND

The ability of regulatory T cells (Tregs) to limit inflammatory responses has been demonstrated. However, different subpopulations of this cell have varying abilities to suppress alloreactive immune responses. The primary goal of this study was to assess the frequency of CD4⁺FOXP3⁺CD39⁺CD73⁺ Tregs and Deltex-1 gene expression on long-term renal transplant function.

METHODS

A total of 49 subjects were divided into 3 groups: (i) the excellent long-term graft function (ELTGF) group, (ii) the chronic graft dysfunction (CGD) group, and (iii) the healthy control (HC) group. Following sample collection, peripheral blood mononuclear cells (PBMCs) were isolated, and the Deltex-1 gene expression level and the frequency of CD4⁺FOXP3⁺CD39⁺CD73⁺ Tregs were evaluated.

RESULTS

The ELTGF group had more CD4⁺FOXP3⁺ Tregs than the CGD group, but the difference was not statistically significant (P = 0.07). However, the frequency of CD4⁺FOXP3⁺CD39⁺CD73⁺ Tregs and the ratio of these cells to total CD4⁺ lymphocytes significantly increased in the ELTGF group than in the CGD group (P = 0.04 and P = 0.02 respectively). In addition, the expression level of the Deltex-1 gene was significantly lower in the CGD group than in the other 2 groups (P = 0.01 and P = 0.04 respectively).

CONCLUSIONS

Given the increased frequency of CD4⁺FOXP3⁺CD39⁺CD73⁺ Tregs and the expression level of the Deltex-1 gene in the ELTGF group, it appears that these factors probably improved function and long-term survival of the transplanted organ through the suppression of alloreactive responses and reduction of inflammation. In other words, one of the immunological mechanisms involved in the CGD group may be a deficiency in Tregs.

Discovery of Hepatotoxic Equivalent Markers and Mechanism of Polygonum multiflorum Thunb. by Metabolomics Coupled with Molecular Docking

Polygonum multiflorum Thunb. (PMT), a commonly used Chinese herbal medicine for treating diseases such as poisoning and white hair, has attracted constant attention due to the frequent occurrence of liver injury incidents. To date, its hepatotoxic equivalent markers (HEMs) and potential hepatotoxic mechanisms are still unclear. In order to clarify the HEMs of PMT and further explore the potential mechanisms of hepatotoxicity, firstly, the chemical constituents in PMT extract were globally characterized, and the fingerprints of PMT extracts were established along with the detection of their hepatotoxicity in vivo. Then, the correlations between hepatotoxic features and component contents were modeled by chemometrics to screen HEMs of PMT, which were then further evaluated. Finally, the hepatotoxic mechanisms of PMT were investigated using liver metabolomics and molecular docking. The results show that the chemical combination of 2,3,5,4-tetrahydroxystilbene-2-O-β-D-glucoside (TSG) and emodin-8-O-glucoside (EG) was discovered as the HEMs of PMT through pre-screening and verifying process. Liver metabolomics revealed that PMT caused liver injury by interfering with purine metabolism, which might be related to mitochondrial function disorder and oxidative injury via the up-regulations of xanthosine and xanthine, and the down-regulation of 5' nucleotidase (NT5E) and adenylate kinase 2 (AK2). This study not only found that the HEMs of PMT were TSG and EG, but also clarified that PMT might affect purine metabolism to induce liver injury, which contributed to our understanding of the underlying mechanisms of PMT hepatotoxicity.

Burnstock oration - purinergic signalling in kidney transplantation

Kidney transplantation is the preferred treatment for individuals with kidney failure offering improved quality and quantity of life. Despite significant advancements in short term graft survival, longer term survival rates have not improved greatly mediated in large by chronic antibody mediated rejection. Strategies to reduce the donor kidney antigenic load may translate to improved transplant survival. CD39 on the vascular endothelium and on circulating cells, in particular regulatory T cells (Treg), is upregulated in response to hypoxic stimuli and plays a critical role in regulating the immune response removing proinflammatory ATP and generating anti-inflammatory adenosine. Herein, the role of CD39 in reducing ischaemia-reperfusion injury (IRI) and on Treg within the context of kidney transplantation is reviewed.

CD73-Adenosinergic Axis Mediates the Protective Effect of Extracellular Vesicles Derived from Mesenchymal Stromal Cells on Ischemic Renal Damage in a Rat Model of Donation after Circulatory Death

We propose a new organ-conditioning strategy based on mesenchymal stromal cell (MSCs)/extracellular vesicle (EVs) delivery during hypothermic perfusion. MSCs/EVs marker CD73 is present on renal proximal tubular cells, and it protects against renal ischemia-reperfusion injury by converting adenosine monophosphate into adenosine (ADO). In this study, after checking if CD73-silenced EVs (EVsi) would impact in vitro tubular-cell proliferation, we perfused kidneys of a rat model of donation after circulatory death, with Belzer solution (BS) alone, BS supplemented with MSCs, EVs, or EVsi. The ADO and ATP levels were measured in the effluents and tissues. Global renal ischemic damage score (GRS), and tubular cell proliferation index (IPT) were evaluated in the tissue. EVsi did not induce cell proliferation in vitro. Ex vivo kidneys perfused with BS or BS + EVsi showed the worst GRS and higher effluent ADO levels than the MSC- and EV-perfused kidneys. In the EV-perfused kidneys, the tissue and effluent ATP levels and IPT were the highest, but not if CD73 was silenced. Tissue ATP content was positively correlated with tissue ADO content and negatively correlated with effluent ADO level in all groups. In conclusion, kidney conditioning with EVs protects against ischemic damage by activating the CD73/ADO system.

The Role of Purinergic Signaling in Heart Transplantation

Heart transplantation remains the optimal treatment option for patients with end-stage heart disease. Growing evidence demonstrates that purinergic signals mediated by purine nucleotides and nucleosides play vital roles in heart transplantation, especially in the era of ischemia-reperfusion injury (IRI) and allograft rejection. Purinergic signaling consists of extracellular nucleotides and nucleosides, ecto-enzymes, and cell surface receptors; it participates in the regulation of many physiological and pathological processes. During transplantation, excess adenosine triphosphate (ATP) levels are released from damaged cells, and driver detrimental inflammatory responses largely via purinergic P2 receptors. Ecto-nucleosidases sequentially dephosphorylate extracellular ATP to ADP, AMP, and finally adenosine. Adenosine exerts a cardioprotective effect by its anti-inflammatory, antiplatelet, and vasodilation properties. This review focused on the role of purinergic signaling in IRI and rejection after heart transplantation, as well as the clinical applications and prospects of purinergic signaling.

Mitochondrial Damage-associated Molecular Patterns as Potential Biomarkers in DCD Heart Transplantation: Lessons From Myocardial Infarction and Cardiac Arrest

Heart transplantation with donation after circulatory death (DCD) has become a real option to increase graft availability. However, given that DCD organs are exposed to the potentially damaging conditions of warm ischemia before procurement, new strategies for graft evaluation are of particular value for the safe expansion of DCD heart transplantation. Mitochondria-related parameters are very attractive as biomarkers because of their intimate association with cardiac ischemia-reperfusion injury. In this context, a group of mitochondrial components, called mitochondrial damage-associated molecular patterns (mtDAMPs), released by stressed cells, holds great promise. mtDAMPs may be released at different stages of DCD cardiac donation and may act as indicators of graft quality. Because of the lack of information available for DCD grafts, we consider that relevant information can be obtained from other acute cardiac ischemic conditions. Thus, we conducted a systematic review of original research articles in which mtDAMP levels were assessed in the circulation of patients with acute myocardial infarction and cardiac arrest. We conclude that 4 mtDAMPs, ATP, cytochrome c, mitochondrial DNA, and succinate, are rapidly released into the circulation after the onset of ischemia, and their concentrations increase with reperfusion. Importantly, circulating levels of mtDAMPs correlate with cardiac damage and may be used as prognostic markers for patient survival in these conditions. Taken together, these findings support the concept that mtDAMPs may be of use as biomarkers to assess the transplant suitability of procured DCD hearts, and ultimately aid in facilitating the safe, widespread adoption of DCD heart transplantation.

Peri-transplant aminophylline in pediatric kidney transplant recipients of donation after brain death: a double-blinded placebo-controlled randomized clinical trial

BACKGROUND

During kidney transplantation, the transplanted kidney undergoes ischemia reperfusion injury, with adenosine being a major mediator. This study aimed to assess whether aminophylline, an adenosine receptor antagonist, improves early graft function and reduces incidence of delayed graft function (DGF) and slow graft function (SGF).

METHODS

Single center, double-blinded, placebo-controlled randomized clinical trial. Pediatric patients admitted for renal transplantation from donation after brain death donors were randomized into a treatment arm receiving aminophylline and a placebo arm receiving normal saline infusions. Primary outcome was estimated glomerular filtration rate (eGFR) at 5 days post-transplant. Secondary outcomes were rates of DGF/SGF and urinary neutrophil gelatinase-associated lipocalin (NGAL) levels.

RESULTS

Twenty-three patients were randomized to aminophylline and 27 to placebo. There was no difference in day 5 eGFR, rate of DGF/SGF, or urine NGAL/Creatinine level between aminophylline vs. placebo arm (eGFR 67.39 ± 38.9 ml/min/1.73m² vs. 80.48 ± 52.1 ml/min/1.73m²p = 0.32; DGF/SGF 5/23 (21.7%) vs. 3/27 (11.1%) p = 0.31; urine NGAL/creatinine 300.5 ng/mg IQR 105.5-1464.5 ng/mg vs. 425.4 ng/mg IQR 140.3-1126.2 ng/mg, p = 0.95; respectively). At 12 months, there was 100% patient survival and 98% graft survival. eGFR at 12 months was similar between the two arms.

CONCLUSIONS

There was no benefit in peri-transplant aminophylline administration. Our results are limited by small sample size, since sample calculations were based on primary outcome of day 5 eGFR and low rate of DGF/SGF, which may have precluded us from demonstrating efficacy. Further clinical studies are necessary to determine any benefit of aminophylline in kidney transplant recipients, particularly from high-risk donors.

The role of the CD39-CD73-adenosine pathway in liver disease

Extracellular adenosine triphosphate (ATP) is a danger signal released by dying and damaged cells, and it functions as an immunostimulatory signal that promotes inflammation. The ectonucleotidases CD39/ectonucleoside triphosphate diphosphohydrolase-1 and CD73/ecto-5'-nucleotidase are cell-surface enzymes that breakdown extracellular ATP into adenosine. This drives a shift from an ATP-driven proinflammatory environment to an anti-inflammatory milieu induced by adenosine. The CD39-CD73-adenosine pathway changes dynamically with the pathophysiological context in which it is embedded. Accumulating evidence suggests that CD39 and CD73 play important roles in liver disease as critical components of the extracellular adenosinergic pathway. Recent studies have shown that the modification of the CD39-CD73-adenosine pathway alters the liver's response to injury. Moreover, adenosine exerts different effects on the pathophysiology of the liver through different receptors. In this review, we aim to describe the role of the CD39-CD73-adenosine pathway and adenosine receptors in liver disease, highlighting potential therapeutic targets in this pathway, which will facilitate the development of therapeutic strategies for the treatment of liver disease.

Conversion of extracellular ATP into adenosine: a master switch in renal health and disease

ATP and its ultimate degradation product adenosine are potent extracellular signalling molecules that elicit a variety of pathophysiological functions in the kidney through the activation of P2 and P1 purinergic receptors, respectively. Extracellular purines can modulate immune responses, balancing inflammatory processes and immunosuppression; indeed, alterations in extracellular nucleotide and adenosine signalling determine outcomes of inflammation and healing processes. The functional activities of ectonucleotidases such as CD39 and CD73, which hydrolyse pro-inflammatory ATP to generate immunosuppressive adenosine, are therefore pivotal in acute inflammation. Protracted inflammation may result in aberrant adenosinergic signalling, which serves to sustain inflammasome activation and worsen fibrotic reactions. Alterations in the expression of ectonucleotidases on various immune cells, such as regulatory T cells and macrophages, as well as components of the renal vasculature, control purinergic receptor-mediated effects on target tissues within the kidney. The role of CD39 as a rheostat that can have an impact on purinergic signalling in both acute and chronic inflammation is increasingly supported by the literature, as detailed in this Review. Better understanding of these purinergic processes and development of novel drugs targeting these pathways could lead to effective therapies for the management of acute and chronic kidney disease.

Plasma Adenylate Levels are Elevated in Cardiopulmonary Arrest Patients and May Predict Mortality

Cerebral and cardiac dysfunction cause morbidity and mortality in postcardiac arrest syndrome (PCAS) patients. Predicting clinical outcome is necessary to provide the optimal level of life support for these patients. In this pilot study, we examined whether plasma ATP and adenylate levels have value in predicting clinical outcome in PCAS patients. In total, 15 patients who experienced cardiac arrest outside the hospital setting and who could be reanimated were enrolled in this study. Healthy volunteers (n = 8) served as controls. Of the 15 PCAS patients, 8 died within 4 days after resuscitation. Of the 7 survivors, 2 lapsed into vegetative states, 1 survived with moderate disabilities, and 4 showed good recoveries. Arterial blood samples were drawn immediately after successful resuscitation and return of spontaneous circulation (ROSC). The concentrations of ATP and other adenylates in plasma were assessed with high-performance liquid chromatography. PCAS patients had significantly higher ATP levels than healthy controls. Plasma ATP levels correlated with lactate levels, Acute Physiology and Chronic Health Evaluation II scores, and the time it took to ROSC (time-to-ROSC). Plasma adenylate levels in patients who died after resuscitation were significantly higher than in survivors. Based on our results and receiver-operating characteristic curve analysis, we conclude that plasma adenylate levels may help predict outcome in PCAS patients.

Extracellular nucleotide signaling in solid organ transplantation

The role of extracellular purine nucleotides, including adenosine triphosphate (ATP) and adenosine, as modulators of posttransplantation outcome and ischemia-reperfusion injury is becoming increasingly evident. Upon pathological release of ATP, binding and activation of P2 purinergic surface receptors promote tissue injury and inflammation, while the expression and activation of P1 receptors for adenosine have been shown to attenuate inflammation and limit ischemia-induced damage, which are central to the viability and long-term success of allografts. Here we review the current state of the transplant field with respect to the role of extracellular nucleotide signaling, with a focus on the sources and functions of extracellular ATP. The connection between ischemia reperfusion, purinergic signaling, and graft preservation, as well as the role of ATP and adenosine as driving factors in the promotion and suppression of posttransplant inflammation and allograft rejection, are discussed. We also examine novel therapeutic approaches that take advantage of the ischemia-reperfusion-responsive and immunomodulatory roles for purinergic signaling with the goal of enhancing graft viability, attenuating posttransplant inflammation, and minimizing complications including rejection, graft failure, and associated comorbidities.

Cell type- and tissue-specific functions of ecto-5'-nucleotidase (CD73)

Ecto-5'-nucleotidase [cluster of differentiation 73 (CD73)] is a ubiquitously expressed glycosylphosphatidylinositol-anchored glycoprotein that converts extracellular adenosine 5'-monophosphate to adenosine. Anti-CD73 inhibitory antibodies are currently undergoing clinical testing for cancer immunotherapy. However, many protective physiological functions of CD73 need to be taken into account for new targeted therapies. This review examines CD73 functions in multiple organ systems and cell types, with a particular focus on novel findings from the last 5 years. Missense loss-of-function mutations in the CD73-encoding gene NT5E cause the rare disease "arterial calcifications due to deficiency of CD73." Aside from direct human disease involvement, cellular and animal model studies have revealed key functions of CD73 in tissue homeostasis and pathology across multiple organ systems. In the context of the central nervous system, CD73 is antinociceptive and protects against inflammatory damage, while also contributing to age-dependent decline in cortical plasticity. CD73 preserves barrier function in multiple tissues, a role that is most evident in the respiratory system, where it inhibits endothelial permeability in an adenosine-dependent manner. CD73 has important cardioprotective functions during myocardial infarction and heart failure. Under ischemia-reperfusion injury conditions, rapid and sustained induction of CD73 confers protection in the liver and kidney. In some cases, the mechanism by which CD73 mediates tissue injury is less clear. For example, CD73 has a promoting role in liver fibrosis but is protective in lung fibrosis. Future studies that integrate CD73 regulation and function at the cellular level with physiological responses will improve its utility as a disease target.

The Role of Hydrogen Peroxide in Redox-Dependent Signaling: Homeostatic and Pathological Responses in Mammalian Cells

Hydrogen peroxide (H₂O₂) is an important metabolite involved in most of the redox metabolism reactions and processes of the cells. H₂O₂ is recognized as one of the main molecules in the sensing, modulation and signaling of redox metabolism, and it is acting as a second messenger together with hydrogen sulfide (H₂S) and nitric oxide (NO). These second messengers activate in turn a cascade of downstream proteins via specific oxidations leading to a metabolic response of the cell. This metabolic response can determine proliferation, survival or death of the cell depending on which downstream pathways (homeostatic, pathological, or protective) have been activated. The cells have several sources of H₂O₂ and cellular systems strictly control its concentration in different subcellular compartments. This review summarizes research on the role played by H₂O₂ in signaling pathways of eukaryotic cells and how this signaling leads to homeostatic or pathological responses.

Comparison of the expression of cluster of differentiation (CD)39 and CD73 between propofol- and sevoflurane-based anaesthesia during open heart surgery

High expression of cluster of differentiation (CD)39 and CD73 has cardio-protective effects. We hypothesised that the expression of CD39 and CD73 would differ between propofol- and volatile anaesthetic-based anaesthesia in patients undergoing open heart surgery (OHS). The objective of this prospective randomized trial was to compare the changes in CD39 and CD73 levels in CD4+ T cells between propofol- and sevoflurane-based anaesthesia during OHS. The study randomly allocated 156 patients undergoing OHS to a propofol or sevoflurane group. Blood was obtained preoperatively and up to 48 hours after weaning from cardiopulmonary bypass (CPB). The expression levels of CD39 and CD73 in circulating CD4+ T cells, serum cytokines and other laboratory parameters were analysed. The primary outcome was the expression of CD39 and CD73 on CD4+ T cells. Demographic data and perioperative haemodynamic changes did not show significant differences between the two groups. The expression of CD39 and CD73 in the sevoflurane group was significantly lower than in the propofol group (P < 0.001). Other laboratory findings including cardiac enzymes and cytokine levels, did not show significant intergroup differences. Propofol attenuated the decrease in CD39 and CD73 in circulating CD4+ T cells compared to sevoflurane-based anaesthesia during OHS.

Microarray analysis after adipose derived mesenchymal stem cells injection in monosodium iodoacetate-induced osteoarthritis rats

Osteoarthritis (OA) is a degenerative joint disease characterized by abrasion, and ultimately, destruction of the articular cartilage and trabecular bone loss. OA is still considered a devastating disease, which requires an aggressive therapeutic approach. Despite the therapeutic potential of human adipose-derived mesenchymal stem cells (AD-MSCs), the molecular parameters needed to define the "stemness" remain largely unknown. Using high-density oligonucleotide microarrays, the differential gene expression profiles between a fraction of human adipose-derived (AD) mononuclear cells and its MSC subpopulation were obtained. Of particular interest was a subset of 58 genes preferentially expressed at sevenfold or higher in the group treated with human AD-MSCs. This subset contained numerous genes involved in the inflammatory response, immune response, lipid metabolism, cell death, cell proliferation, and DNA repair. Additionally, four protein networks were constructed. The interaction network consisted of 46 proteins encoded by up-regulated genes. However, the interaction network also consisted of 38 proteins encoded by down-regulated genes. My results provide a basis for a more reproducible and reliable quality control using genotypic analysis for the definition of human AD-MSCs. Therefore, these results will provide a basis for studies on molecular mechanisms controlling the core properties of human MSCs.

CD39-adenosinergic axis in renal pathophysiology and therapeutics

Extracellular ATP interacts with purinergic type 2 (P2) receptors and elicits many crucial biological functions. Extracellular ATP is sequentially hydrolyzed to ADP and AMP by the actions of defined nucleotidases, such as CD39, and AMP is converted to adenosine, largely by CD73, an ecto-5'-nucleotidase. Extracellular adenosine interacts with P1 receptors and often opposes the effects of P2 receptor activation. The balance between extracellular ATP and adenosine in the blood and extracellular fluid is regulated chiefly by the activities of CD39 and CD73, which constitute the CD39-adenosinergic axis. In recent years, several studies have shown this axis to play critical roles in transport of water/sodium, tubuloglomerular feedback, renin secretion, ischemia reperfusion injury, renal fibrosis, hypertension, diabetic nephropathy, transplantation, inflammation, and macrophage transformation. Important developments include global and targeted gene knockout and/or transgenic mouse models of CD39 or CD73, biological or small molecule inhibitors, and soluble engineered ectonucleotidases to directly impact the CD39-adenosinergic axis. This review presents a comprehensive picture of the multiple roles of CD39-adenosinergic axis in renal physiology, pathophysiology, and therapeutics. Scientific advances and greater understanding of the role of this axis in the kidney, in both health and illness, will direct development of innovative therapies for renal diseases.

Extracellular ATP signaling and clinical relevance

Since purinergic signaling was discovered in the early 1970s, it has been shown that extracellular nucleotides, and their derivative nucleosides, are released in a regulated or unregulated manner by cells in various challenging settings and then bind defined purinergic receptors to activate intricate signaling networks. Extracellular ATP plays a role based on different P2 receptor subtypes expressed on specific cell types. Sequential hydrolysis of extracellular ATP catalyzed by ectonucleotidases (e.g. CD39, CD73) is the main pathway for the generation of adenosine, which in turn activates P1 receptors. Many studies have demonstrated that extracellular ATP signaling functions as an important dynamic regulatory pathway to coordinate appropriate immune responses in various pathological processes, including intracellular infection, host-tumor interaction, pro-inflammation vascular injury, and transplant immunity. ATP receptors and CD39 also participate in related clinical settings. Here, we review the latest research in to the development of promising clinical treatment strategies.

AMP and adenosine are both ligands for adenosine 2B receptor signaling

Adenosine is considered the canonical ligand for the adenosine 2B receptor (A_2BR). A_2BR is upregulated following kidney ischemia augmenting post ischemic blood flow and limiting tubular injury. In this context the beneficial effect of A_2BR signaling has been attributed to an increase in the pericellular concentration of adenosine. However, following renal ischemia both kidney adenosine monophosphate (AMP) and adenosine levels are substantially increased. Using computational modeling and calcium mobilization assays, we investigated whether AMP could also be a ligand for A_2BR. The computational modeling suggested that AMP interacts with more favorable energy to A_2BR compared with adenosine. Furthermore, AMPαS, a non-hydrolyzable form of AMP, increased calcium uptake by Chinese hamster ovary (CHO) cells expressing the human A_2BR, indicating preferential signaling via the G_q pathway. Therefore, a putative AMP-A_2BR interaction is supported by the computational modeling data and the biological results suggest this interaction involves preferential G_q activation. These data provide further insights into the role of purinergic signaling in the pathophysiology of renal IRI.

Purinergic Signalling: Therapeutic Developments

Purinergic signalling, i.e., the role of nucleotides as extracellular signalling molecules, was proposed in 1972. However, this concept was not well accepted until the early 1990's when receptor subtypes for purines and pyrimidines were cloned and characterised, which includes four subtypes of the P1 (adenosine) receptor, seven subtypes of P2X ion channel receptors and 8 subtypes of the P2Y G protein-coupled receptor. Early studies were largely concerned with the physiology, pharmacology and biochemistry of purinergic signalling. More recently, the focus has been on the pathophysiology and therapeutic potential. There was early recognition of the use of P1 receptor agonists for the treatment of supraventricular tachycardia and A2A receptor antagonists are promising for the treatment of Parkinson's disease. Clopidogrel, a P2Y12 antagonist, is widely used for the treatment of thrombosis and stroke, blocking P2Y12 receptor-mediated platelet aggregation. Diquafosol, a long acting P2Y2 receptor agonist, is being used for the treatment of dry eye. P2X3 receptor antagonists have been developed that are orally bioavailable and stable in vivo and are currently in clinical trials for the treatment of chronic cough, bladder incontinence, visceral pain and hypertension. Antagonists to P2X7 receptors are being investigated for the treatment of inflammatory disorders, including neurodegenerative diseases. Other investigations are in progress for the use of purinergic agents for the treatment of osteoporosis, myocardial infarction, irritable bowel syndrome, epilepsy, atherosclerosis, depression, autism, diabetes, and cancer.

The Differential Effect of Apyrase Treatment and hCD39 Overexpression on Chronic Renal Fibrosis After Ischemia-Reperfusion Injury

BACKGROUND

Renal ischemia-reperfusion injury (IRI) leads to acute kidney injury and renal fibrosis. CD39 is a key purinergic enzyme in the hydrolysis of adenosine triphosphate (ATP) and increased CD39 enzymatic activity protects from acute IRI but its effect on renal fibrosis is not known.

METHODS

Using a mouse model of unilateral renal IRI, the effects of increased CD39 activity (using soluble apyrase and mice expressing human CD39 transgene) on acute and chronic renal outcomes were examined. Nucleotide (ATP, adenosine diphosphate, adenosine monophosphate) and nucleoside (adenosine and inosine) levels were quantified by high-performance liquid chromatography. Soluble apyrase reduced acute renal injury at 24 hours and renal fibrosis at 4 weeks post-IRI, compared with vehicle-treated mice.

RESULTS

Soluble apyrase reduced renal ATP, adenosine diphosphate, and adenosine monophosphate, but not adenosine levels, during ischemia. In comparison with wild-type littermates, hCD39 transgenic mice were protected from acute renal injury at 24 hours, but had increased renal fibrosis at 4 weeks post-IRI. hCD39 transgene expression was localized to the vascular endothelium at baseline and did not affect total renal nucleotide and nucleoside levels during ischemia. However, hCD39 transgene was more widespread at 4 weeks post-IRI and was associated with higher renal adenosine levels at 4 weeks post-IRI compared with wild-type littermates.

CONCLUSIONS

A single dose of apyrase administration before IRI protects from both acute and chronic renal injuries and may have clinical application in protection from ischemic-induced renal injury. Furthermore, transgenic expression of hCD39 is associated with increased renal fibrosis after ischemia.

Novel Therapies for Acute Kidney Injury

Acute kidney injury (AKI) is a common disease with a complex pathophysiology. The old paradigm of identifying renal injury based on location-prerenal, intrarenal, and postrenal-is now being supplanted with a new paradigm based on observable kidney injury patterns. The pathophysiology of AKI on a molecular and microanatomical level includes inflammation, immune dysregulation, oxidative injury, and impaired microcirculation. Treatment has traditionally been supportive, including the avoidance of nephrotoxins, judicious volume and blood pressure management, hemodynamic monitoring, and renal replacement therapy. Fluid overload and chloride-rich fluids are now implicated in the development of AKI, and resuscitation with a balanced, buffered solution at a conservative rate will mitigate risk. Novel therapies, which address specific observable kidney injury patterns include direct oxygen-free radical scavengers such as α-lipoic acid, curcumin, sodium-2-mercaptoethane sulphonate, propofol, and selenium. In addition, angiotensin II and adenosine receptor antagonists hope to ameliorate kidney injury via manipulation of renal hemodynamics and tubulo-glomerular feedback. Alkaline phosphatase, sphingosine 1 phosphate analogues, and dipeptidylpeptidase-4 inhibitors counteract kidney injury via manipulation of inflammatory pathways. Finally, genetic modifiers such as 5INP may mitigate AKI via transcriptive processes.

The P2X7 receptor antagonist, oxidized adenosine triphosphate, ameliorates renal ischemia-reperfusion injury by expansion of regulatory T cells

Extracellular adenosine triphosphate (ATP) binds to purinergic receptors and, as a danger molecule, promotes inflammatory responses. Here we tested whether periodate-oxidized ATP (oATP), a P2X7 receptor (P2X7R) antagonist can attenuate renal ischemia-reperfusion injury and clarify the related cellular mechanisms. Treatment with oATP prior to ischemia-reperfusion injury decreased blood urea nitrogen, serum creatinine, the tubular injury score, and tubular epithelial cell apoptosis after injury. The infiltration of dendritic cells, neutrophils, macrophages, CD69⁺CD4⁺, and CD44⁺CD4⁺ T cells was attenuated, but renal Foxp3⁺CD4⁺ Treg infiltration was increased by oATP. The levels of IL-6 and CCL2 were reduced in the oATP group. Additionally, oATP treatment following injury improved renal function, decreased the infiltration of innate and adaptive effector cells, and increased the renal infiltration of Foxp3⁺CD4⁺ Tregs. Post-ischemia-reperfusion injury oATP treatment increased tubular cell proliferation and reduced renal fibrosis. oATP treatment attenuated renal functional deterioration after ischemia-reperfusion injury in RAG-1 knockout mice; however, Treg depletion using PC61 abrogated the beneficial effects of oATP in wild-type mice. Furthermore, oATP treatment after transfer of Tregs from wild-type mice improved the beneficial effects of Tregs on ischemia-reperfusion injury, but treatment after transfer of Tregs from P2X7R knockout mice did not. Renal ischemia-reperfusion injury was also attenuated in P2X7R knockout mice. Experiments using bone marrow chimeras established that P2X7R expression on hematopoietic cells rather than non-hematopoietic cells, such as tubular epithelial cells, plays a major role in ischemia-reperfusion injury. Thus, oATP attenuated acute renal damage and facilitated renal recovery in ischemia-reperfusion injury by expansion of Tregs.

Development of a novel strategy to target CD39 antithrombotic activity to the endothelial-platelet microenvironment in kidney ischemia-reperfusion injury

Kidney ischemia-reperfusion injury (IRI) is common during transplantation. IRI is characterised by inflammation and thrombosis and associated with acute and chronic graft dysfunction. P-selectin and its ligand PSGL-1 are cell adhesion molecules that control leukocyte-endothelial and leukocyte-platelet interactions under inflammatory conditions. CD39 is the dominant vascular nucleotidase that facilitates adenosine generation via extracellular ATP/ADP-phosphohydrolysis. Adenosine signalling is protective in renal IRI, but CD39 catalytic activity is lost with exposure to oxidant stress. We designed a P-selectin targeted CD39 molecule (rsol.CD39-PSGL-1) consisting of recombinant soluble CD39 that incorporates 20 residues of PSGL-1 that bind P-selectin. We hypothesised that rsol.CD39-PSGL-1 would maintain endothelial integrity by focusing the ectonucleotidase platelet-inhibitory activity and reducing leukocyte adhesion at the injury site. The rsol.CD39-PSGL-1 displayed ADPase activity and inhibited platelet aggregation ex vivo, as well as bound with high specificity to soluble P-selectin and platelet surface P-selectin. Importantly, mice injected with rsol.CD39-PSGL-1 and subjected to renal IRI showed significantly less kidney damage both biochemically and histologically, compared to those injected with solCD39. Furthermore, the equivalent dose of rsol.CD39-PSGL-1 had no effect on tail template bleeding times. Hence, targeting recombinant CD39 to the injured vessel wall via PSGL-1 binding resulted in substantial preservation of renal function and morphology after IRI without toxicity. These studies indicate potential translational importance to clinical transplantation and nephrology.

Purinergic signaling in kidney disease

Nucleotides are key subunits for nucleic acids and provide energy for intracellular metabolism. They can also be released from cells to act physiologically as extracellular messengers or pathologically as danger signals. Extracellular nucleotides stimulate membrane receptors in the P2 and P1 family. P2X are ATP-activated cation channels; P2Y and P1 are G-protein coupled receptors activated by ATP, ADP, UTP, and UDP in the case of P2 or adenosine for P1. Renal P2 receptors influence both vascular contractility and tubular function. Renal cells also express ectonucleotidases that rapidly hydrolyze extracellular nucleotides. These enzymes integrate this multireceptor purinergic-signaling complex by determining the nucleotide milieu to titrate receptor activation. Purinergic signaling also regulates immune cell function by modulating the synthesis and release of various cytokines such as IL1-β and IL-18 as part of inflammasome activation. Abnormal or excessive stimulation of this intricate paracrine system can be pro- or anti-inflammatory, and is also linked to necrosis and apoptosis. Kidney tissue injury causes a localized increase in ATP concentration, and sustained activation of P2 receptors can lead to renal glomerular, tubular, and vascular cell damage. Purinergic receptors also regulate the activity and proliferation of fibroblasts, promoting both inflammation and fibrosis in chronic disease. In this short review we summarize some of the recent findings related to purinergic signaling in the kidney. We focus predominantly on the P2X7 receptor, discussing why antagonists have so far disappointed in clinical trials and how advances in our understanding of purinergic signaling might help to reposition these compounds as potential treatments for renal disease.

Unlocking the Potential of Purinergic Signaling in Transplantation

Purinergic signaling has been recognized as playing an important role in inflammation, angiogenesis, malignancy, diabetes and neural transmission. Activation of signaling pathways downstream from purinergic receptors may also be implicated in transplantation and related vascular injury. Following transplantation, the proinflammatory "danger signal" adenosine triphosphate (ATP) is released from damaged cells and promotes proliferation and activation of a variety of immune cells. Targeting purinergic signaling pathways may promote immunosuppression and ameliorate inflammation. Under pathophysiological conditions, nucleotide-scavenging ectonucleotidases CD39 and CD73 hydrolyze ATP, ultimately, to the anti-inflammatory mediator adenosine. Adenosine suppresses proinflammatory cytokine production and is associated with improved graft survival and decreased severity of graft-versus-host disease. Furthermore, purinergic signaling is involved both directly and indirectly in the mechanism of action of several existing immunosuppressive drugs, such as calcineurin inhibitors and mammalian target of rapamycin inhibitors. Targeting of purinergic receptor pathways, particularly in the setting of combination therapies, could become a valuable immunosuppressive strategy in transplantation. This review focuses on the role of the purinergic signaling pathway in transplantation and immunosuppression and explores possible future applications in clinical practice.

Sesamin protects against renal ischemia reperfusion injury by promoting CD39-adenosine-A2AR signal pathway in mice

Ischemia reperfusion injury (IRI) is a leading cause of acute kidney injury with high morbidity and mortality due to limited therapy. Here, we examine whether sesamin attenuates renal IRI in an animal model and explore the underlying mechanisms. Male mice were subjected to right renal ischemia for 30 min followed by reperfusion for 24 h with sesamin (100 mg/kg) during which the left kidney was removed. Renal damage and function were assessed subsequently. The results showed that sesamin reduced kidney ischemia reperfusion injury, as assessed by decreased serum creatinine (Scr) and Blood urea nitrogen (BUN), alleviated tubular damage and apoptosis. In addition, sesamin inhibited neutrophils infiltration and pro-inflammatory cytokines tumor necrosis factor (TNF)-α and interleukin (IL)-1β production in IR-preformed kidney. Notably, sesamin promoted the expression of CD39, A2A adenosine receptor (A2AAR), and A2BAR mRNA and protein as well as adenosine production. Furthermore, CD39 inhibitor or A2AR antagonist abolished partly the protection of sesamin in kidney IRI. In conclusion, sesamin could effectively protect kidney from IRI by inhibiting inflammatory responses, which might be associated with promoting the adenosine-CD39-A2AR signaling pathway.

Regulation of Vascular and Renal Function by Metabolite Receptors

To maintain metabolic homeostasis, the body must be able to monitor the concentration of a large number of substances, including metabolites, in real time and to use that information to regulate the activities of different metabolic pathways. Such regulation is achieved by the presence of sensors, termed metabolite receptors, in various tissues and cells of the body, which in turn convey the information to appropriate regulatory or positive or negative feedback systems. In this review, we cover the unique roles of metabolite receptors in renal and vascular function. These receptors play a wide variety of important roles in maintaining various aspects of homeostasis-from salt and water balance to metabolism-by sensing metabolites from a wide variety of sources. We discuss the role of metabolite sensors in sensing metabolites generated locally, metabolites generated at distant tissues or organs, or even metabolites generated by resident microbes. Metabolite receptors are also involved in various pathophysiological conditions and are being recognized as potential targets for new drugs. By highlighting three receptor families-(a) citric acid cycle intermediate receptors, (b) purinergic receptors, and

Adenosine receptor expression in the development of renal fibrosis following ischemic injury

Long-term renal allograft survival has not improved despite improvements in short term outcomes. Graft loss is characterized histologically by the development of interstitial fibrosis and tubular atrophy (IFTA). Mechanisms underlying the development of IFTA are multifactorial and include ischemia-reperfusion injury (IRI). Therapeutic options to reduce IFTA include management of immunologic causes, such as rejection, but despite these efforts IFTA can still occur and leads to the inexorable destruction of the transplanted kidney. The adenosine A2B receptor (A2BR) has recently been implicated in the development of renal fibrosis. We performed an observational study to examine the mRNA expression of the adenosine receptors after renal ischemia up to the development of renal fibrosis in a mouse model of unilateral IRI. A2BR was the only adenosine receptor that showed elevated expression following ischemia until the development of renal fibrosis 4 weeks after injury. At 2 weeks after ischemia, increased expression of the fibrotic markers transforming growth factor β and Collagen-1α was observed. Expression of hypoxia inducible factor 1α and endothelin-1, which lie downstream of A2BR activation and have been recognized to promote renal fibrosis, were also significantly up-regulated at 2 weeks after ischemia. Expression of fibrotic markers returned to baseline by 4 weeks after ischemia, indicating resolution of injury with the concurrent development of renal fibrosis and reduced renal function. Our data suggest that A2BR may be a therapeutic target in reducing the development of renal fibrosis after ischemia.

Alternative splicing of human NT5E in cirrhosis and hepatocellular carcinoma produces a negative regulator of ecto-5'-nucleotidase (CD73)

Alternative splicing of human NT5E generates CD73S, an endoplasmic reticulum–associated and dimerization-deficient glycoprotein that lacks enzymatic activity. CD73S functions as a negative regulator of canonical CD73 by promoting its proteasomal degradation, which may have significance in chronic liver disease and liver cancer.

Ecto-5′-nucleotidase (CD73), encoded by NT5E, is the major enzymatic source of extracellular adenosine. CD73 controls numerous pathophysiological responses and is a potential disease target, but its regulation is poorly understood. We examined NT5E regulation by alternative splicing. Genomic database analysis of human transcripts led us to identify NT5E-2, a novel splice variant that was expressed at low abundance in normal human tissues but was significantly up-regulated in cirrhosis and hepatocellular carcinoma (HCC). NT5E-2 encodes a shorter CD73 isoform we named CD73S. The presence of CD73S protein, which lacks 50 amino acids, was detected in HCC using an isoform-specific antibody. A noncanonical mouse mRNA, similar to human CD73S, was observed, but the corresponding protein was undetectable. The two human isoforms exhibited functional differences, such that ectopic expression of canonical CD73 (CD73L) in human HepG2 cells was associated with decreased expression of the proliferation marker Ki67, whereas CD73S expression did not have an effect on Ki67 expression. CD73S was glycosylated, catalytically inactive, unable to dimerize, and complexed intracellularly with the endoplasmic reticulum chaperone calnexin. Furthermore, CD73S complexed with CD73L and promoted proteasome-dependent CD73L degradation. The findings reveal species-specific CD73 regulation, with potential significance to cancer, fibrosis, and other diseases characterized by changes in CD73 expression and function.

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.

Alkaline phosphatase: a possible treatment for sepsis-associated acute kidney injury in critically ill patients

Acute kidney injury (AKI) is a common disease in the intensive care unit and accounts for high morbidity and mortality. Sepsis, the predominant cause of AKI in this setting, involves a complex pathogenesis in which renal inflammation and hypoxia are believed to play an important role. A new therapy should be aimed at targeting both these processes, and the enzyme alkaline phosphatase, with its dual mode of action, might be a promising candidate. First, alkaline phosphatase is able to reduce inflammation through dephosphorylation and thereby detoxification of endotoxin (lipopolysaccharide), which is an important mediator of sepsis. Second, adenosine triphosphate, released during cellular stress caused by inflammation and hypoxia, has detrimental effects but can be converted by alkaline phosphatase into adenosine with anti-inflammatory and tissue-protective effects. These postulated beneficial effects of alkaline phosphatase have been confirmed in animal experiments and two phase 2a clinical trials showing that kidney function improved in critically ill patients with sepsis-associated AKI. Because renal inflammation and hypoxia also are observed commonly in AKI induced by other causes, it would be of interest to investigate the therapeutic effect of alkaline phosphatase in these nephropathies as well.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

The importance of C4d in biopsies of kidney transplant recipients

Antibody-mediated rejection (AMR) is highly detrimental to the prolonged survival of transplanted kidneys. C4d has been regarded as a footprint of AMR tissue damage, and the introduction of C4d staining in daily clinical practice aroused an ever-increasing interest in the role of antibody-mediated mechanisms in allograft rejection. Despite the general acceptance of the usefulness of C4d in the identification of acute AMR, the data for C4d staining in chronic AMR is variable. The presence of C4d in the majority of the biopsies with features of chronic antibody-mediated rejection is reported, but this rejection without C4d staining is observed as well, suggesting that C4d is specific but not sensitive. Further studies on AMR with positive C4d staining in biopsy specimens are really important, as well as the study of novel routine markers that may participate in the pathogenesis of this process.