Blocking the immune response in ischemic acute kidney injury: the role of adenosine 2A agonists

Advances in pediatric acute kidney injury pathobiology: a report from the 26th Acute Disease Quality Initiative (ADQI) conference

BACKGROUND

In the past decade, there have been substantial advances in our understanding of the pathobiology of pediatric acute kidney injury (AKI). In particular, animal models and studies focused on the relationship between kidney development, nephron number, and kidney health have identified a number of heterogeneous pathophysiologies underlying AKI. Despite this progress, gaps remain in our understanding of the pathobiology of pediatric AKI.

METHODS

During the 26th Acute Disease Quality Initiative (ADQI) Consensus conference, a multidisciplinary group of experts discussed the evidence and used a modified Delphi process to achieve consensus on recommendations for opportunities to advance translational research in pediatric AKI. The current state of research understanding as well as gaps and opportunities for advancement in research was discussed, and recommendations were summarized.

RESULTS

Consensus was reached that to improve translational pediatric AKI advancements, diverse teams spanning pre-clinical to epidemiological scientists must work in concert together and that results must be shared with the community we serve with patient involvement. Public and private research support and meaningful partnerships with adult research efforts are required. Particular focus is warranted to investigate the pediatric nuances of AKI, including the effect of development as a biological variable on AKI incidence, severity, and outcomes.

CONCLUSIONS

Although AKI is common and associated with significant morbidity, the biologic basis of the disease spectrum throughout varying nephron developmental stages remains poorly understood. An incomplete understanding of factors contributing to kidney health, the diverse pathobiologies underlying AKI in children, and the historically siloed approach to research limit advances in the field. The recommendations outlined herein identify gaps and outline a strategic approach to advance the field of pediatric AKI via multidisciplinary translational research.

Connexin32 gap junction channels deliver miR155-3p to mediate pyroptosis in renal ischemia-reperfusion injury

OBJECTIVES

To explore whether the gap junction (GJ) composed by connexin32(Cx32) mediated pyroptosis in renal ischemia-reperfusion(I/R) injury via transmitting miR155-3p, with aim to provide new strategies for the prevention and treatment of acute kidney injury (AKI) after renal I/R.

METHODS

8-10 weeks of male C57BL/ 6 wild-type mice and Cx32 knockdown mice were divided into two groups respectively: control group and renal I/R group. MCC950 (50 mg/kg. ip.) was used to inhibit NLRP3 in vivo. Human kidney tubular epithelial cells (HK - 2) and rat kidney tubular epithelial cells (NRK-52E) were divided into high-density group and low-density group, and treated with hypoxia reoxygenation (H/R) to mimic I/R. The siRNA and plasmid of Cx32, mimic and inhibitor of miR155-3p were transfected into HK - 2 cells respectively. Kidney pathological and functional injuries were measured. Western Blot and immunofluorescent staining were used to observe the expression of NLRP3, GSDMD, GSDMD-N, IL - 18, and mature IL-18. The secretion of IL-18 and IL-1β in serum, kidney tissue and cells supernatant were detected by enzyme-linked immuno sorbent assay (ELISA) kit, and the expression of NLPR3 and miR155-3p were detected by RT-qPCR and fluorescence in situ hybridization (FISH).

RESULTS

Tubular pyroptosis were found to promote AKI after I/R in vivo and Cx32-GJ regulated pyroptosis by affecting the expression of miR155-3p after renal I/R injury. In vitro, H/R could lead to pyroptosis in HK-2 and NRK-52E cells. When the GJ channels were not formed, and Cx32 was inhibited or knockdown, the expression of miR155-3p was significantly reduced and the pyroptosis was obviously inhibited, leading to the reduction of injury and the increase of survival rate. Moreover, regulating the level of miR155-3p could affect survival rate and pyroptosis in vitro after H/R.

CONCLUSIONS

The GJ channels composed of Cx32 regulated tubular pyroptosis in renal I/R injury by transmitting miR155-3p. Inhibition of Cx32 could reduce the level of miR155-3p further to inhibit pyroptosis, leading to alleviation of renal I/R injury which provided a new strategy for preventing the occurrence of AKI. Video Abstract.

Dynamics of necroptosis in kidney ischemia-reperfusion injury

Necroptosis, a pathway of regulated necrosis, involves recruitment and activation of RIPK1, RIPK3 and MLKL, leading to cell membrane rupture, cell death and release of intracellular contents causing further injury and inflammation. Necroptosis is believed to play an important role in the pathogenesis of kidney ischemia-reperfusion injury (IRI). However, the dynamics of necroptosis in kidney IRI is poorly understood, in part due to difficulties in detecting phosphorylated MLKL (pMLKL), the executioner of the necroptosis pathway. Here, we investigated the temporal and spatial activation of necroptosis in a mouse model of unilateral warm kidney IRI, using a robust method to stain pMLKL. We identified the period 3-12 hrs after reperfusion as a critical phase for the activation of necroptosis in proximal tubular cells. After 12 hrs, the predominant pattern of pMLKL staining shifted from cytoplasmic to membrane, indicating progression to the terminal phase of necroptotic cell death. Mlkl-ko mice exhibited reduced kidney inflammation at 12 hrs and lower serum creatinine and tubular injury at 24 hrs compared to wild-type littermates. Interestingly, we observed increased apoptosis in the injured kidneys of Mlkl-ko mice, suggesting a relationship between necroptosis and apoptosis in kidney IRI. Together, our findings confirm the role of necroptosis and necroinflammation in kidney IRI, and identify the first 3 hrs following reperfusion as a potential window for targeted treatments.

Impaired microvascular circulation in distant organs following renal ischemia

Mortality in acute kidney injury (AKI) patients remains very high, although very important advances in understanding the pathophysiology and in diagnosis and supportive care have been made. Most commonly, adverse outcomes are related to extra-renal organ dysfunction and failure. We and others have documented inflammation in remote organs as well as microvascular dysfunction in the kidney after renal ischemia. We hypothesized that abnormal microvascular flow in AKI extends to distant organs. To test this hypothesis, we employed intravital multiphoton fluorescence imaging in a well-characterized rat model of renal ischemia/reperfusion. Marked abnormalities in microvascular flow were seen in every organ evaluated, with decreases up to 46% observed 48 hours postischemia (as compared to sham surgery, p = 0.002). Decreased microvascular plasma flow was found in areas of erythrocyte aggregation and leukocyte adherence to endothelia. Intravital microscopy allowed the characterization of the erythrocyte formations as rouleaux that flowed as one-dimensional aggregates. Observed microvascular abnormalities were associated with significantly elevated fibrinogen levels. Plasma flow within capillaries as well as microthrombi, but not adherent leukocytes, were significantly improved by treatment with the platelet aggregation inhibitor dipyridamole. These microvascular defects may, in part, explain known distant organ dysfunction associated with renal ischemia. The results of these studies are relevant to human acute kidney injury.

Identification of Subtypes and a Delayed Graft Function Predictive Signature Based on Ferroptosis in Renal Ischemia-Reperfusion Injury

Renal ischemia-reperfusion injury (IRI) is an inevitable process in kidney transplantation, leading to acute kidney injury, delayed graft function (DGF), and even graft loss. Ferroptosis is an iron-dependent regulated cell death in various diseases including IRI. We aimed to identify subtypes of renal IRI and construct a robust DGF predictive signature based on ferroptosis-related genes (FRGs). A consensus clustering analysis was applied to identify ferroptosis-associated subtypes of 203 renal IRI samples in the GSE43974 dataset. The FRG-associated DGF predictive signature was constructed using the Least Absolute Shrinkage and Selection Operator (LASSO), and its robustness was further verified in the validation set GSE37838. The present study revealed two ferroptosis-related patient clusters (pBECN1 and pNF2 cluster) in renal IRI samples based on distinct expression patterns of BECN1 and NF2 gene clusters. Cluster pBECN1 was metabolically active and closely correlated with less DGF, while pNF2 was regarded as the metabolic exhausted subtype with higher incidence of DGF. Additionally, a six-gene (ATF3, SLC2A3, CXCL2, DDIT3, and ZFP36) ferroptosis-associated signature was constructed to predict occurrence of DGF in renal IRI patients and exhibited robust efficacy in both the training and validation sets. High-risk patients tended to have more infiltration of dendritic cells, macrophages, and T cells, and they had significantly enriched chemokine-related pathway, WNT/β-catenin signaling pathway, and allograft rejection. Patients with low risks of DGF were associated with ferroptosis-related pathways such as glutathione and fatty acid metabolism pathways. In conclusion, patient stratification with distinct metabolic activities based on ferroptosis may help distinguish patients who may respond to metabolic therapeutics. Moreover, the DGF predictive signature based on FRGs may guide advanced strategies toward prevention of DGF in the early stage.

Acute Kidney Injury After Large Ventral Hernia Repair Requiring Transversus Abdominis Release

BACKGROUND

Acute kidney injury (AKI) is a known postoperative complication of open ventral hernia repair contributing to increased costs, hospital length of stay, and mortality. The aim of this study was to identify whether the muscle injury that occurs in a posterior separation of components via transversus abdominis release (TAR) contributes to a higher incidence of postoperative AKI.

METHODS

A retrospective cohort study of patients who underwent open retrorectus ventral hernia repair with and without TAR at a single institution between 2012 and 2019 was performed. Patients who underwent a separation of components via either unilateral or bilateral transversus abdominis release were compared to those who did not undergo TAR as part of their hernia repair (non-TAR). The outcome of interest was the development of postoperative AKI. Acute kidney injury was defined as an increase in creatinine of greater than 50% of the preoperative baseline. Univariate and multivariate analyses were performed to determine the influence of TAR on the development of AKI.

RESULTS

There were 523 patients who met inclusion criteria, of which 159 (30.4%) had a TAR as part of their retrorectus hernia repair. No differences were found in preoperative characteristics between the TAR and non-TAR group including age, gender, history of kidney disease, or history of diabetes. By contrast, the TAR group had significantly greater median estimated blood loss (100 mL vs 75 mL, P < .01), mean positive intraoperative fluid balance (2255 mL vs 1887 mL, P < .01), and operative duration (321 min vs 269 min, P < .001). The rate of AKI in the TAR group was 11% (n = 18) vs 6% (n = 23, P = .0503) in the non-TAR group. On multivariate analysis controlling for patient characteristics and intraoperative factors, TAR was the only factor with a significantly increased odds of AKI (OR 1.97, 95% CI 0.994-3.905, P = .0521).

CONCLUSIONS

In patients with large ventral hernias requiring retrorectus repair, performing a TAR is associated with a nearly 2-fold increase in the development of postoperative AKI. These findings suggest that these patients should be optimized perioperatively with emphasis on fluid resuscitation, limiting nephrotoxic medications and monitoring urine output.

[Therapeutics for acute tubular necrosis in 2020]

Acute kidney injury is a major cause of in-hospital morbidity and mortality because of the serious nature of the underlying illnesses and the high incidence of complications. The two major causes of acute kidney injury that occur in the hospital are prerenal disease and acute tubular necrosis. Acute tubular necrosis has a histological definition, even if a kidney biopsy is rarely performed. Kidney injuries occurring during acute tubular necrosis are underlined by different pathophysiological mechanisms that emphasize the role of hypoxia on the tubular cells such as apoptosis, cytoskeleton disruption, mitochondrial function and the inflammation mediated by innate immune cells. The microcirculation and the endothelial cells are also the targets of hypoxia-mediated impairment. Repair mechanisms are sometimes inadequate because of pro-fibrotic factors that will lead to chronic kidney disease. Despite all the potential therapeutic targets highlighted by the pathophysiological knowledge, further works remain necessary to find a way to prevent these injuries.

Mechanisms of haemolysis-induced kidney injury

Intravascular haemolysis is a fundamental feature of chronic hereditary and acquired haemolytic anaemias, including those associated with haemoglobinopathies, complement disorders and infectious diseases such as malaria. Destabilization of red blood cells (RBCs) within the vasculature results in systemic inflammation, vasomotor dysfunction, thrombophilia and proliferative vasculopathy. The haemoprotein scavengers haptoglobin and haemopexin act to limit circulating levels of free haemoglobin, haem and iron - potentially toxic species that are released from injured RBCs. However, these adaptive defence systems can fail owing to ongoing intravascular disintegration of RBCs. Induction of the haem-degrading enzyme haem oxygenase 1 (HO1) - and potentially HO2 - represents a response to, and endogenous defence against, large amounts of cellular haem; however, this system can also become saturated. A frequent adverse consequence of massive and/or chronic haemolysis is kidney injury, which contributes to the morbidity and mortality of chronic haemolytic diseases. Intravascular destruction of RBCs and the resulting accumulation of haemoproteins can induce kidney injury via a number of mechanisms, including oxidative stress and cytotoxicity pathways, through the formation of intratubular casts and through direct as well as indirect proinflammatory effects, the latter via the activation of neutrophils and monocytes. Understanding of the detailed pathophysiology of haemolysis-induced kidney injury offers opportunities for the design and implementation of new therapeutic strategies to counteract the unfavourable and potentially fatal effects of haemolysis on the kidney.

Regulated necrosis in kidney ischemia-reperfusion injury

Ischemia-reperfusion injury (IRI) is the outcome of an inflammatory process that is triggered when an organ undergoes a transient reduction or cessation of blood flow, followed by re-establishment of perfusion. In the clinical setting, IRI contributes to significant acute kidney injury, patient morbidity and mortality, and adverse outcomes in transplantation. Tubular cell death by necrosis and apoptosis is a central feature of renal IRI. Recent research has challenged traditional views of cell death by identifying new pathways in which cells die in a regulated manner but with the morphologic features of necrosis. This regulated necrosis (RN) takes several forms, with necroptosis and ferroptosis being the best described. The precise mechanisms and relationships between the RN pathways in renal IRI are currently the subject of active research. The common endpoint of RN is cell membrane rupture, resulting in the release of cytosolic components with subsequent inflammation and activation of the immune system. We review the evidence and mechanisms of RN in the kidney following renal IRI, and discuss the use of small molecule inhibitors and genetically modified mice to better understand this process and guide potentially novel therapeutic interventions.

Adenosine A2A receptor blocks the A 1 receptor inhibition of renal Na + transport and oxygen consumption

A high renal oxygen (O₂ ) need is primarily associated with the renal tubular O₂ consumption (VO₂ ) necessary for a high rate of sodium (Na⁺ ) transport. Limited O₂ availability leads to increased levels of adenosine, which regulates the kidney via activation of both A₁ and A_2A adenosine receptors (A1R and A2AR, respectively). The relative contributions of A1R and A2AR to the regulation of renal Na⁺ transport and VO₂ have not been determined. We demonstrated that A1R activation has a dose-dependent biphasic effect on both renal Na⁺ /H⁺ exchanger-3 (NHE3), a major player in Na⁺ transport, and VO₂ . Here, we report concentration-dependent effects of adenosine: less than 5 × 10^-7  M adenosine-stimulated NHE3 activity; between 5 × 10^-7  M and 10^-5  M adenosine-inhibited NHE3 activity; and greater than 10^-5  M adenosine reversed the change in NHE3 activity (returned to baseline). A1R activation mediated the activation and inhibition of NHE3 activity, whereas 10^-4  M adenosine had no effect on the NHE3 activity due to A2AR activation. The following occurred when A1R and A2AR were activated: (a) Blockade of the A2AR receptor restored the NHE3 inhibition mediated by A1R activation, (b) the NHE-dependent effect on VO₂ mediated by A1R activation became NHE independent, and (c) A2AR bound to A1R. In summary, A1R affects VO₂ via NHE-dependent mechanisms, whereas A2AR acts via NHE-independent mechanisms. When both A1R and A2AR are activated, the A2AR effect on NHE3 and VO₂ predominates, possibly via an A1R-A2AR protein interaction. A2AR-A1R heterodimerization is proposed as the molecular mechanism enabling the NHE-independent control of renal VO₂ .

Ischemia-Reperfusion Injury Reduces Long Term Renal Graft Survival: Mechanism and Beyond

Ischemia-reperfusion injury (IRI) during renal transplantation often initiates non-specific inflammatory responses that can result in the loss of kidney graft viability. However, the long-term consequence of IRI on renal grafts survival is uncertain. Here we review clinical evidence and laboratory studies, and elucidate the association between early IRI and later graft loss. Our critical analysis of previous publications indicates that early IRI does contribute to later graft loss through reduction of renal functional mass, graft vascular injury, and chronic hypoxia, as well as subsequent fibrosis. IRI is also known to induce kidney allograft dysfunction and acute rejection, reducing graft survival. Therefore, attempts have been made to substitute traditional preserving solutions with novel agents, yielding promising results.

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Ischaemia reperfusion injury (IRI) potentiates delayed renal graft function and causes reduction in renal graft survival

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IRI causes innate immune system activation, hypoxic injury, inflammation and graft vascular disease

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Reducing prolonged cold ischaemic time improves graft survival

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Novel protective strategies include mesenchymal stem cells, machine perfusion, and ex vivo preservation solution saturated with gas.

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Further studies are needed to investigate the long-term effects of novel ex vivo preservation agents

Valproic Acid Prevents Renal Dysfunction and Inflammation in the Ischemia-Reperfusion Injury Model

Ischemia-reperfusion injury (IRI) is a major contributor to acute kidney injury (AKI). At present, there are no effective therapies to prevent AKI. The aim of this study was to analyse whether valproic acid (VPA), a histone deacetylase inhibitor with anti-inflammatory properties, prevents renal IRI. Male Wistar rats were divided into three groups: SHAM rats were subjected to a SHAM surgery, IRI rats underwent bilateral renal ischemia for 45 min, and IRI + VPA rats were treated with VPA at 300 mg/kg twice daily 2 days before bilateral IRI. Animals were euthanized at 48 hours after IRI. VPA attenuated renal dysfunction after ischemia, which was characterized by a decrease in BUN (mg/dL), serum creatinine (mg/dL), and FENa (%) in the IRI + VPA group (39 ± 11, 0.5 ± 0.05, and 0.5 ± 0.06, resp.) compared with the IRI group (145 ± 35, 2.7 ± 0.05, and 4.9 ± 1, resp.; p < 0.001). Additionally, significantly lower acute tubular necrosis grade and number of apoptotic cells were found in the IRI + VPA group compared to the IRI group (p < 0.001). Furthermore, VPA treatment reduced inflammatory cellular infiltration and expression of proinflammatory cytokines. These data suggest that VPA prevents the renal dysfunction and inflammation that is associated with renal IRI.

Recombinant N-Terminal Slit2 Inhibits TGF-β-Induced Fibroblast Activation and Renal Fibrosis

Fibrosis and inflammation are closely intertwined injury pathways present in nearly all forms of CKD for which few safe and effective therapies exist. Slit glycoproteins signaling through Roundabout (Robo) receptors have been described to have anti-inflammatory effects through regulation of leukocyte cytoskeletal organization. Notably, cytoskeletal reorganization is also required for fibroblast responses to TGF-β Here, we examined whether Slit2 also controls TGF-β-induced renal fibrosis. In cultured renal fibroblasts, which we found to express Slit2 and Robo-1, the bioactive N-terminal fragment of Slit2 inhibited TGF-β-induced collagen synthesis, actin cytoskeletal reorganization, and Smad2/3 transcriptional activity, but the inactive C-terminal fragment of Slit2 did not. In mouse models of postischemic renal fibrosis and obstructive uropathy, treatment with N-terminal Slit2 before or after injury inhibited the development of renal fibrosis and preserved renal function, whereas the C-terminal Slit2 had no effect. Our data suggest that administration of recombinant Slit2 may be a new treatment strategy to arrest chronic injury progression after ischemic and obstructive renal insults by not only attenuating inflammation but also, directly inhibiting renal fibrosis.

Dipyridamole attenuates ischemia reperfusion induced acute kidney injury through adenosinergic A1 and A2A receptor agonism in rats

Dipyridamole (DYP) is an anti-platelet agent with marked vasodilator, anti-oxidant, and anti-inflammatory activity. The present study investigated the role of adenosine receptors in DYP-mediated protection against ischemia reperfusion-induced acute kidney injury (AKI) in rats. The rats were subjected to bilateral renal ischemia for 40 min followed by reperfusion for 24 h. The renal damage induced by ischemia reperfusion injury (IRI) was assessed by measuring creatinine clearance, blood urea nitrogen, uric acid, plasma potassium, fractional excretion of sodium, and microproteinuria in rats. The oxidative stress in renal tissues was assessed by quantification of thiobarbituric acid-reactive substances, superoxide anion generation, and reduced glutathione level. The hematoxylin-eosin staining was carried out to observe histopathological changes in renal tissues. DYP (10 and 30 mg/kg, intraperitoneal, i.p.) was administered 30 min before subjecting the rats to renal IRI. In separate groups, caffeine (50 mg/kg, i.p.), an adenosinergic A1 and A2A receptor antagonist was administered with and without DYP treatment before subjecting the rats to renal IRI. The ischemia reperfusion-induced AKI was demonstrated by significant changes in serum as well as urinary parameters, enhanced oxidative stress, and histopathological changes in renal tissues. The administration of DYP demonstrated protection against AKI. The prior treatment with caffeine abolished DYP-mediated reno-protection suggesting role of A1 and A2A adenosine receptors in DYP-mediated reno-protection in rats. It is concluded that adenosine receptors find their definite involvement in DYP-mediated anti-oxidative and reno-protective effect against ischemia reperfusion-induced AKI.

Emerging therapeutic targets of sepsis-associated acute kidney injury

Sepsis-associated acute kidney injury (SA-AKI) is linked to high morbidity and mortality. To date, singular approaches to target specific pathways known to contribute to the pathogenesis of SA-AKI have failed. Because of the complexity of the pathogenesis of SA-AKI, a reassessment necessitates integrative approaches to therapeutics of SA-AKI that include general supportive therapies such as the use of vasopressors, fluids, antimicrobials, and target-specific and time-dependent therapeutics. There has been recent progress in our understanding of the pathogenesis and treatment of SA-AKI including the temporal nature of proinflammatory and anti-inflammatory processes. In this review, we discuss the clinical and experimental basis of emerging therapeutic approaches that focus on targeting early proinflammatory and late anti-inflammatory processes, as well as therapeutics that may enhance cellular survival and recovery. Finally, we include ongoing clinical trials in sepsis.

Biliary tract external drainage alleviates kidney injury in shock

BACKGROUND

Kidney injury is common in hemorrhagic shock (HS). Kidney injury leads to a systemic increase in serum chemokines and cytokines and causes injuries to other vital organs. Our previous studies showed that vitamin C led to organ protection and inflammation inhibitory effects in rat models of HS via induction heme oxygenase-1 (HO-1). We also found that biliary tract external drainage (BTED) increased the expression levels of HO-1 in rat livers. We investigated roles of BTED in kidney injury and its relationship with the HO-1 pathway in HS in this research.

METHODS

Rat models of HS were induced by drawing blood from the femoral artery. BTED was performed by inserting a catheter into the bile duct. Thirty-six Sprague-Dawley rats were randomized to sham group; HS group; zinc protoporphyrin IX (Znpp) group; BTED group; BTED + Znpp group, and BTED + bile infusion group. The expression levels of HO-1 in the kidney were analyzed by Western blotting. The expression levels of occludin messenger RNA in the kidney were analyzed by real-time reverse transcription-polymerase chain reaction. The expression levels of occludin in the kidney were analyzed by immunohistochemistry. Histology of renal was performed by hematoxylin and eosin staining.

RESULTS

Occludin messenger RNA and protein levels in the kidney increased markedly after BTED under HS conditions. Renal histopathologic scores decreased significantly after BTED under HS conditions. Znpp significantly inhibited all mentioned effects.

CONCLUSIONS

BTED alleviates kidney injury in rats of HS via the HO-1 pathway.

Preconditioning strategies for kidney ischemia reperfusion injury: implications of the "time-window" in remote ischemic preconditioning

Remote ischemic preconditioning (IP) is a potential renoprotective strategy. However, there has been no demonstrated result in large animals and the role of time window in remote IP remains to be defined. Using a single-kidney porcine model, we evaluated organ protective function of remote IP in renal ischemia reperfusion injury. Fifteen Yorkshire pigs, 20 weeks old and weighing 35-38 kg were used. One week after left nephrectomy, we performed remote IP (clamping right external iliac artery, 2 cycles of 10 minutes) and right renal artery clamping (warm ischemia; 90 minutes). The animals were randomly divided into three groups: control group, warm ischemia without IP; group 1 (remote IP with early window [IP-E]), IP followed by warm ischemia with a 10-minute time window; and group 2 (remote IP with late window [IP-L]), IP followed by warm ischemia after a 24-hour time window. There were no differences in serum creatinine changes between groups. The IP-L group had lower urinary neutrophil gelatinase-associated lipocalin than control and IP-E at 72 hours post-ischemia. At 72 hours post-ischemia, the urinary kidney injury molecule-1 (KIM-1) was lower in the IP-L group than in the control and IP-E groups, and the IP-L group KIM-1 was near pre-ischemic levels, whereas the control and IP-E group KIM-1 levels were rising. Microalbumin also tended to be lower in the IP-L group. Taken together, remote IP showed a significant reduction in renal injury biomarkers from ischemia reperfusion injury. To effectively provide kidney protection, remote IP might require a considerable, rather than short, time window of ischemia.

Advances in the pathogenesis of cardiorenal syndrome type 3

Cardiorenal syndrome (CRS) type 3 is a subclassification of the CRS whereby an episode of acute kidney injury (AKI) leads to the development of acute cardiac injury or dysfunction. In general, there is limited understanding of the pathophysiologic mechanisms involved in CRS type 3. An episode of AKI may have effects that depend on the severity and duration of AKI and that both directly and indirectly predispose to an acute cardiac event. Experimental data suggest that cardiac dysfunction may be related to immune system activation, inflammatory mediators release, oxidative stress, and cellular apoptosis which are well documented in the setting of AKI. Moreover, significant derangements, such as fluid and electrolyte imbalance, metabolic acidosis, and uremia, which are typical features of acute kidney injury, may impair cardiac function. In this review, we will focus on multiple factors possibly involved in the pathogenesis issues regarding CRS type 3.

Bis maltolato oxovanadium (BMOV) and ischemia/reperfusion-induced acute kidney injury in rats

Background

The aim of the present study was to test the potential protective effects of the organic vanadium salt bis (maltolato) oxovanadium (BMOV; 15 mg/kg) in the context of renal ischemia/reperfusion (30 min of ischemia) and its effects on renal oxygenation and renal function in the acute phase of reperfusion (up to 90 min post-ischemia).

Methods

Ischemia was established in anesthetized and mechanically ventilated male Wistar rats by renal artery clamping. Renal microvascular and venous oxygenation were measured using phosphorimetry. Creatinine clearance rate, sodium reabsorption, and renal oxygen handling efficiency were considered markers for renal function.

Results

The main findings were that BMOV did not affect the systemic and renal hemodynamic and oxygenation variables and partially protected renal sodium reabsorption.

Conclusions

Pretreatment with the organic vanadium compound BMOV did not protect the kidney from I/R injury.

Biology of renal recovery: molecules, mechanisms, and pathways

Acute kidney injury (AKI) contributes to progressive kidney disease. Although significant advances have been made in the understanding of mechanisms of AKI, less is known about the biological basis that links the initial injury to progressive interstitial fibrosis, tubular dysfunction, and capillary rarefaction. The round table discussion focused on mechanisms of renal recovery and fibrosis following AKI. The knowledge gained by understanding these pathways will serve to identify novel therapeutic targets in the future.

The conundrum of protection from AKI by adenosine in rodent clamp ischemia models

Kim et al. show that isoflurane uses a tubule-based transforming growth factor-β/CD73-dependent process that generates adenosine to protect mice from ischemic acute kidney injury (AKI) with effects to prevent the 'no-reflow phenomenon' and decrease inflammation. While direct cytoprotection occurred in culture, extensive research suggests that in vivo adenosine protection from rodent ischemic AKI is mediated by a mutually cooperative mechanism involving blood flow, inflammation, and innate immunity through multiple adenosine receptors with promiscuous actions on diverse cell types.

Role of Toll-like receptor-4 in renal graft ischemia-reperfusion injury

Toll-like receptor-4 (TLR-4) has been increasingly recognized as playing a critical role in the pathogenesis of ischemia-reperfusion injury (IRI) of renal grafts. This review provides a detailed overview of the new understanding of the involvement of TLR-4 in ischemia-reperfusion injury of renal grafts and its clinical significance in renal transplantation. TLR-4 not only responds to exogenous microbial motifs but can also recognize molecules which are released by stressed and necrotic cells, as well as degraded products of endogenous macromolecules. Upregulation of TLR-4 is found in tubular epithelial cells, vascular endothelial cells, and infiltrating leukocytes during renal ischemia-reperfusion injury, which is induced by massive release of endogenous damage-associated molecular pattern molecules such as high-mobility group box chromosomal protein 1. Activation of TLR-4 promotes the release of proinflammatory mediators, facilitates leukocyte migration and infiltration, activates the innate and adaptive immune system, and potentiates renal fibrosis. TLR-4 inhibition serves as the target of pharmacological agents, which could attenuate ischemia-reperfusion injury and associated delayed graft function and allograft rejection. There is evidence in the literature showing that targeting TLR-4 could improve long-term transplantation outcomes. Given the pivotal role of TLR-4 in ischemia-reperfusion injury and associated delayed graft function and allograft rejection, inhibition of TLR-4 using pharmacological agents could be beneficial for long-term graft survival.

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.

Slit2 prevents neutrophil recruitment and renal ischemia-reperfusion injury

Neutrophils recruited to the postischemic kidney contribute to the pathogenesis of ischemia-reperfusion injury (IRI), which is the most common cause of renal failure among hospitalized patients. The Slit family of secreted proteins inhibits chemotaxis of leukocytes by preventing activation of Rho-family GTPases, suggesting that members of this family might modulate the recruitment of neutrophils and the resulting IRI. Here, in static and microfluidic shear assays, Slit2 inhibited multiple steps required for the infiltration of neutrophils into tissue. Specifically, Slit2 blocked the capture and firm adhesion of human neutrophils to inflamed vascular endothelial barriers as well as their subsequent transmigration. To examine whether these observations were relevant to renal IRI, we administered Slit2 to mice before bilateral clamping of the renal pedicles. Assessed at 18 hours after reperfusion, Slit2 significantly inhibited renal tubular necrosis, neutrophil and macrophage infiltration, and rise in plasma creatinine. In vitro, Slit2 did not impair the protective functions of neutrophils, including phagocytosis and superoxide production, and did not inhibit neutrophils from killing the extracellular pathogen Staphylococcus aureus. In vivo, administration of Slit2 did not attenuate neutrophil recruitment or bacterial clearance in mice with ascending Escherichia coli urinary tract infections and did not increase the bacterial load in the livers of mice infected with the intracellular pathogen Listeria monocytogenes. Collectively, these results suggest that Slit2 may hold promise as a strategy to combat renal IRI without compromising the protective innate immune response.

Therapeutic role of toll-like receptor modification in cardiovascular dysfunction

Toll-like receptors (TLR) are key pattern recognition receptors in the innate immune system. The TLR-mediated immune response against pathogens is usually protective however inappropriate TLR activation may lead to excessive tissue damage. It is well recognised that TLRs respond to a variety of endogenous as well as exogenous ligands. By responding to endogenous ligands that are exposed during cellular damage, TLRs have been implicated in a range of pathological conditions associated with cardiovascular dysfunction. Increasing knowledge on the mechanisms involved in TLR signalling has encouraged the exploration of therapeutic pharmacological modulation of TLR activation in conditions such as atherosclerosis, ischaemic heart disease, heart failure and ischaemic reperfusion injury. The aim of this review is to explore the translational potentials of TLR modification in cardiovascular dysfunction, where these agents have been studied.

The vitamin E derivative, ESeroS-GS, attenuates renal ischemia-reperfusion injury in rats

BACKGROUND

Acute kidney injury (AKI), which occurs during renal transplantation and cardiovascular surgery, is a major clinical problem associated with high mortality, and has limited treatment options. Anti-inflammation therapy has been suggested to improve the course and outcome of AKI. In this study, we hypothesized that ESeroS-GS, a vitamin E derivative, inhibits cytokine production and prevents renal ischemia-reperfusion (I/R) injury in rats.

METHODS

Rats received an intravenous infusion of ESeroS-GS or saline, and underwent experimentally-induced renal I/R injury or sham treatment. Rats were sacrificed after 60 min of ischemia and 24 h of reperfusion. To evaluate the renal protective effects of ESero-GS, renal function was examined, kidneys were histologically assessed, levels of myeloperoxidase (MPO) and serum cytokines were measured, and caspase 3/7 activity was determined.

RESULTS

ESeroS-GS attenuated I/R-induced histologic alterations, reduced levels of MPO and serum BUN, Cre, TNF-α, and IL-6, and decreased caspase 3/7 activity in kidneys of rats subjected to renal I/R injury.

CONCLUSIONS

ESeroS-GS attenuated renal injury after I/R by reducing serum cytokine levels. Our findings suggest that ESeroS-GS may have therapeutic potential against various human I/R conditions.

Pathogenesis of acute kidney injury: foundation for clinical practice

The pathogenesis of acute kidney injury (AKI) is complex, involving such factors as vasoconstriction, leukostasis, vascular congestion, cell death, and abnormal immune modulators and growth factors. Many targeted clinical therapies have failed, are inconclusive, or have yet to be tested. Given the complexity of the pathogenesis of AKI, it may be naive to expect that one therapeutic intervention would have success. Some examples of detrimental processes that can be blocked in preclinical models to improve kidney function and survival are apoptotic cell death in tubular epithelial cells, complement-mediated immune system activation, and impairment of cellular homeostasis and metabolism. Modalities with the potential to decrease morbidity and mortality in patients with AKI include vasodilators, growth factors, anti-inflammatory agents, and cell-based therapies. Pharmacologic agents that target these diverse pathways are being used clinically for other indications. Using combinatorial approaches in future clinical trials may improve our ability to prevent and treat AKI.

Distinct macrophage phenotypes contribute to kidney injury and repair

The ischemically injured kidney undergoes tubular cell necrosis and apoptosis, accompanied by an interstitial inflammatory cell infiltrate. In this study, we show that iNos-positive proinflammatory (M1) macrophages are recruited into the kidney in the first 48 hours after ischemia/reperfusion injury, whereas arginase 1- and mannose receptor-positive, noninflammatory (M2) macrophages predominate at later time points. Furthermore, depletion of macrophages before ischemia/reperfusion diminishes kidney injury, whereas depletion at 3 to 5 days after injury slows tubular cell proliferation and repair. Infusion of Ifnγ-stimulated, bone marrow-derived macrophages into macrophage-depleted mice at the time of kidney reperfusion restored injury to the level seen without macrophage depletion, suggesting that proinflammatory macrophages worsen kidney damage. In contrast, the appearance of macrophages with the M2 phenotype correlated with the proliferative phase of kidney repair. In vitro studies showed that IFNγ-stimulated, proinflammatory macrophages begin to express markers of M2 macrophages when cocultured with renal tubular cells. Moreover, IL-4-stimulated macrophages with an M2 phenotype, but not IFNγ-stimulated proinflammatory macrophages, promoted renal tubular cell proliferation. Finally, tracking fluorescently labeled, IFNγ-stimulated macrophages that were injected after injury showed that inflammatory macrophages can switch to an M2 phenotype in the kidney at the onset of kidney repair. Taken together, these studies show that macrophages undergo a switch from a proinflammatory to a trophic phenotype that supports the transition from tubule injury to tubule repair.

Dangers within: DAMP responses to damage and cell death in kidney disease

The response to exogenous pathogens leads to activation of innate immunity through the release of pathogen-associated molecular patterns (PAMPs) and their binding to pattern recognition receptors. A classic example is septic shock where Toll receptor 4 recognizes PAMPs. Although well accepted, this concept does not explain the activation of innate immunity and inflammation occurs with transplantation, autoimmunity, or trauma. Increasingly recognized is that endogenous molecules released by dying cells (damage-associated molecular patterns; DAMPs) activate cellular receptors leading to downstream inflammation. Thus endogenous danger signals and exogenous PAMPs elicit similar responses through seemingly similar mechanisms. Also emerging is our understanding that normal repair processes benefit from dampening the immune response to these endogenous danger molecules. Here we focus on the role of DAMPs and their putative receptors in the pathogenesis of acute and chronic kidney diseases.

Macrophages, dendritic cells, and kidney ischemia-reperfusion injury

Dendritic cells and macrophages are critical early initiators of innate immunity in the kidney and orchestrate inflammation subsequent to ischemia-reperfusion injury. They are the most abundant leukocytes present in the kidney, and they represent a heterogeneous population of cells that are capable of inducing sterile inflammation after reperfusion directly through the production of proinflammatory cytokines and other soluble inflammatory mediators or indirectly through activation of effector T lymphocytes and natural killer T cells. In addition, recent studies have indicated that kidney and immune cell micro-RNAs control gene expression and have the ability to regulate the initial inflammatory response to injury. Although dendritic cells and macrophages contribute to both innate and adaptive immunity and to injury and repair, this review focuses on the initial innate response to kidney ischemia-reperfusion injury.

Targeting of adenosine receptors in ischemia-reperfusion injury

IMPORTANCE OF THE FIELD

Ischemia-reperfusion (IR) injury is a common problem after transplantation as well as myocardial infarction and stroke. IR initiates an inflammatory response leading to rapid tissue damage. Adenosine, produced in response to IR, is generally considered a protective signaling molecule and elicits its physiological responses through four distinct adenosine receptors. The short half-life, lack of specificity and rapid metabolism limits the use of adenosine as a therapeutic agent. Thus, intense research efforts have focused on the synthesis and implementation of specific adenosine receptor agonists and antagonists as potential therapeutic agents for a variety of inflammatory conditions including IR injury.

AREAS COVERED IN THIS REVIEW

Current knowledge on IR injury with a focus on lung, heart and kidney and studies that have advanced our understanding of the role of adenosine receptors and the therapeutic potential of adenosine receptor agonists and antagonists for the prevention of IR injury.

WHAT THE READER WILL GAIN

Insight into the role of adenosine receptor signaling in IR injury.

TAKE HOME MESSAGE

No therapies are currently available that specifically target IR injury; however, targeting of specific adenosine receptors may offer therapeutic strategies in this regard.

Regulatory T cells in transplantation: does extracellular adenosine triphosphate metabolism through CD39 play a crucial role?

Despite tremendous improvements in short-term renal allograft survival, many patients still have chronic rejection or side effects of nonspecific immunosuppression. The discovery of Foxp3(+) regulatory T cells (Tregs) has revolutionized the concepts in immunoregulation and offers perspectives for overcoming rejection. Recently, a subset of Foxp3(+)CD39(+) effector/memory-like Tregs (T(REM)) was identified. The role of CD39(+) Tregs in immunoregulation is supported by the occurrence of alopecia areata and experimental autoimmune encephalomyelitis in CD39-deficient mice and by the failure of CD39(-) Tregs to suppress contact hypersensitivity. In humans, CD39 polymorphisms have been associated with diabetes and nephropathy, and multiple sclerosis patients have reduced numbers of blood CD39(+) Tregs. Preliminary experiments in a murine transplantation model showed that CD39(+) Tregs can determine allograft outcome. CD39 degrades the extracellular adenosine triphosphate (ATP) released during tissue injury, which otherwise would trigger inflammation. Currently, our groups are assessing the role of CD39(+) Tregs and extracellular ATP metabolism in clinical transplantation and whether tolerogenic Treg profiles possess immunopredictive value, envisioning the development of clinical trials using CD39(+) Treg-based vaccination for autoimmunity or transplantation. This is a comprehensive review on the fundamentals of Treg biology, the potential role of ATP metabolism in immunoregulation, and the potential use of Treg-based immunotherapy in transplantation.

Adenosine A(2A) agonist administration improves islet transplant outcome: Evidence for the role of innate immunity in islet graft rejection

Activation of adenosine A(2A) receptors inhibits inflammation in ischemia/reperfusion injury, and protects against cell damage at the injury site. Following transplantation 50% of islets die due to inflammation and apoptosis. This study investigated the effects of adenosine A(2A) receptor agonists (ATL146e and ATL313) on glucose-stimulated insulin secretion (GSIS) in vitro and transplanted murine syngeneic islet function in vivo. Compared to vehicle controls, ATL146e (100 nM) decreased insulin stimulation index [SI, (insulin)(high glucose)/(insulin)(low glucose)] (2.36 +/- 0.22 vs. 3.75 +/- 0.45; n = 9; p < 0.05). Coculture of islets with syngeneic leukocytes reduced SI (1.41 +/- 0.17; p < 0.05), and this was restored by ATL treatment (2.57 +/- 0.18; NS). Addition of a selective A(2A)AR antagonist abrogated ATL's protective effect, reducing SI (1.11 +/- 0.42). ATL treatment of A(2A)AR(+/+) islet/A(2A)AR(-/-) leukocyte cocultures failed to protect islet function (SI), implicating leukocytes as likely targets of A(2A)AR agonists. Diabetic recipient C57BL/6 mice (streptozotocin; 250 mg/kg, IP) received islet transplants to either the renal subcapsular or hepatic-intraportal site. Recipient mice receiving ATL therapy (ATL 146e or ATL313, 60 ng/kg/min, IP) achieved normoglycemia more rapidly than untreated recipients. Histological examination of grafts suggested reduced cellular necrosis, fibrosis, and lymphocyte infiltration in agonist-treated animals. Administration of adenosine A(2A) receptor agonists (ATL146e or ATL313) improves in vitro GSIS by an effect on leukocytes, and improves survival and functional engraftment of transplanted islets by inhibiting inflammatory islet damage in the peritransplant period, suggesting a potentially significant new strategy for reducing inflammatory islet loss in clinical transplantation.

Neutrophils in acute kidney injury: not neutral any more

Awad and colleagues elucidate the spatiotemporal profile of neutrophil infiltration in the kidney following ischemia-reperfusion injury. Using elegant in vivo labeling techniques, they demonstrate increased neutrophil content in the kidney following ischemia-reperfusion, which is largely due to transmigration from the circulation into the interstitial compartment. The authors also provide mechanistic insights into this phenomenon and show that adenosine 2A receptor agonists reduce interstitial neutrophil infiltration and improve renal function.

Compartmentalization of neutrophils in the kidney and lung following acute ischemic kidney injury

During renal ischemia-reperfusion, local and distant tissue injury is caused by an influx of neutrophils into the affected tissues. Here we measured the kinetics of margination and transmigration of neutrophils in vivo in the kidney and lungs following renal ischemia-reperfusion. After bilateral renal injury, kidney neutrophil content increased threefold at 24 h. The neutrophils were found primarily in the interstitium and to a lesser degree marginated to the vascular endothelium. These interstitial neutrophils had significantly lower levels of intracellular IFN-gamma, IL-4, IL-6, and IL-10 a tendency for decreased amounts of IL-4 and TNF-alpha compared to the marginated neutrophils. Localization of the neutrophils to the kidney interstitium was confirmed by high resolution microscopy and these sites of transmigration were directly associated with areas of increased vascular permeability. Activation of the adenosine 2A receptor significantly decreased both kidney neutrophil transmigration by about half and vascular permeability by about a third. After unilateral renal ischemia-reperfusion, the unclipped kidney and lungs did not accumulate interstitial neutrophils or have increased vascular permeability despite a marked increase of neutrophil margination in the lungs. Our findings suggest there is a sequential recruitment and transmigration of neutrophils from the vasculature into the kidney interstitium at the site of tissue injury following renal ischemia-reperfusion.

Adenosine and renal tubular function

PURPOSE OF REVIEW

Intrarenal adenosine is present in the cytoplasm of renal epithelial cells and in the extracellular space. Adenosine is generated at high levels in response to imbalance between energy demand and supply (e.g. increased tubular sodium chloride transport or hypoxia) and activates cell membrane adenosine receptors to affect renal vascular and tubular functions. Adenosine regulates renal sodium and water excretion via a myriad of effects on renal hemodynamic, glomerular filtration rate, renin secretion and direct effects on the renal tubule epithelium. This review examines the direct effects of adenosine on renal tubular epithelial transport in light of the most recent evidence and discusses some physiologic and pathophysiologic implications.

RECENT FINDINGS

Intrarenal adenosine affects proximal fluid and solute transport in a biphasic fashion. Under physiological conditions adenosine stimulates proximal tubular re-absorption, thus reducing the load delivered to the distal nephron. A supra-physiologic increase in adenosine such as in ischemia reduces reabsorption in the proximal tubule, thus reducing renal oxygen consumption.

SUMMARY

Intrarenal adenosine and its receptors have important regulatory functions in the renal epithelium. A complete understanding of this autocrine/paracrine system holds great potential for novel therapeutic strategies, such as the use of nucleoside analogues for reno-protection in renal ischemia.

Divergent roles of sphingosine kinases in kidney ischemia-reperfusion injury

Sphingosine-1-phosphate (S1P), produced by sphingosine kinase 1 (SphK1) or kinase 2 (SphK2), mediates biological effects through intracellular and/or extracellular mechanisms. Here we determined a role for these kinases in kidney injury of wild-type mice following ischemia-reperfusion. SphK1 but not SphK2 mRNA expression and activity increased in the kidney following injury relative to sham-operated animals. Although SphK1(-/-) mice had no alteration in renal function following injury, mice with a disrupted SphK2 gene (SphK2(tr/tr)) had histological damage and impaired function. The immune-modulating pro-drug, FTY720, an S1P agonist failed to provide protection in SphK2(tr/tr) mice. Injured kidneys of these mice showed increased neutrophil infiltration and neutrophil chemokine expression along with a 3- to 5-fold increase in expression of the G-protein-coupled receptor S1P(3) compared to heterozygous SphK2(+/tr) mice. Kidney function and reduced vascular permeability were preserved in S1P(3)(-/-) compared to S1P(3)(+/-) mice after ischemia-reperfusion injury, suggesting increased S1P(3) mRNA may play a role in the injury of SphK2(tr/tr) mice. Our study suggests that constitutive expression of SphK2 may contribute to reduced ischemia-reperfusion injury of the kidney, and its absence may enhance injury due to increased neutrophil infiltration and S1P(3) activation. We also confirm that SphK2 is necessary to mediate the protective effects of FTY720.

The chemokine receptors CCR2 and CX3CR1 mediate monocyte/macrophage trafficking in kidney ischemia-reperfusion injury

Chemokines and their receptors such as CCR2 and CX3CR1 mediate leukocyte adhesion and migration into injured tissue. To further define mechanisms of monocyte trafficking during kidney injury we identified two groups of F4/80-positive cells (F4/80(low) and F4/80(high)) in the normal mouse kidney that phenotypically correspond to macrophages and dendritic cells, respectively. Following ischemia and 3 h of reperfusion, there was a large influx of F4/80(low) inflamed monocytes, but not dendritic cells, into the kidney. These monocytes produced TNF-alpha, IL-6, IL-1alpha and IL-12. Ischemic injury induced in CCR2(-/-) mice or in CCR2(+/+) mice, made chimeric with CCR2(-/-) bone marrow, resulted in lower plasma creatinine levels and their kidneys had fewer infiltrated F4/80(low) macrophages compared to control mice. CX3CR1 expression contributed to monocyte recruitment into inflamed kidneys, as ischemic injury in CX3CR1(-/-) mice was reduced, with fewer F4/80(low) macrophages than controls. Monocytes transferred from CCR2(+/+) or CX3CR1(+/-) mice migrated into reperfused kidneys better than monocytes from either CCR2(-/-) or CX3CR1(-/-) mice. Adoptive transfer of monocytes from CCR2(+/+) mice, but not CCR2(-/-) mice, reversed the protective effect in CCR2(-/-) mice following ischemia-reperfusion. Egress of CD11b(+)Ly6C(high) monocytes from blood into inflamed kidneys was CCR2- and CX3CR1-dependent. Our study shows that inflamed monocyte migration, through CCR2- and CX3CR1-dependent mechanisms, plays a critical role in kidney injury following ischemia reperfusion.

Sphingosine-1-phosphate receptors: biology and therapeutic potential in kidney disease

The major sphingolipid metabolite, sphingosine-1-phosphate (S1P), has important biological functions. S1P is the ligand for a family of five G-protein-coupled receptors with distinct signaling pathways that regulate angiogenesis, vascular maturation, immunity, chemotaxis, and other important biological pathways. Recently, clinical trials have targeted S1P receptors (S1PRs) for autoimmune diseases and transplantation and have generated considerable interest in developing additional, more selective compounds. This review summarizes current knowledge on the biology of S1P and S1PRs that forms the basis for future drug development and the treatment of kidney disease.

Activation of adenosine 2A receptors preserves structure and function of podocytes

Adenosine 2A receptor (A(2A)R) activation was recently shown to be renoprotective in diabetic nephropathy. A(2A)R are found in glomeruli and have been shown to associate with the podocyte cytoskeletal protein alpha-actinin-4, but the effect of their activation on podocyte structure and function is unknown. Podocyte injury was induced in C57BL/6 mice with puromycin aminonucleoside, and the selective A(2A)R agonist ATL313 was found to attenuate the resulting albuminuria and foot process fusion. The selective A(2A)R antagonist ZM241385 reversed the effects of ATL313. In vitro, A(2A)R mRNA and protein were expressed in a conditionally immortalized podocyte cell line, and A(2A)R-like immunoreactivity co-localized with the actin cytoskeleton. Treatment with ATL313 also blocked the increased podocyte permeability to albumin and disruption of the actin cytoskeleton that accompanied puromycin aminonucleoside-induced injury in vitro. ATL313 was ineffective, however, in the presence of the A(2A)R antagonist and in A(2A)R-deficient podocytes. It was concluded that A(2A)R activation reduces glomerular proteinuria, at least in part, by preserving the normal structure of podocyte foot processes, slit diaphragms, and actin cytoskeleton.

Novel pharmacological approaches to the treatment of renal ischemia-reperfusion injury: a comprehensive review

Renal ischemia-reperfusion (I-R) contributes to the development of ischemic acute renal failure (ARF). Multi-factorial processes are involved in the development and progression of renal I-R injury with the generation of reactive oxygen species, nitric oxide and peroxynitrite, and the decline of antioxidant protection playing major roles, leading to dysfunction, injury, and death of the cells of the kidney. Renal inflammation, involving cytokine/adhesion molecule cascades with recruitment, activation, and diapedesis of circulating leukocytes is also implicated. Clinically, renal I-R occurs in a variety of medical and surgical settings and is responsible for the development of acute tubular necrosis (a characteristic feature of ischemic ARF), e.g., in renal transplantation where I-R of the kidney directly influences graft and patient survival. The cellular mechanisms involved in the development of renal I-R injury have been targeted by several pharmacological interventions. However, although showing promise in experimental models of renal I-R injury and ischemic ARF, they have not proved successful in the clinical setting (e.g., atrial natriuretic peptide, low-dose dopamine). This review highlights recent pharmacological developments, which have shown particular promise against experimental renal I-R injury and ischemic ARF, including novel antioxidants and antioxidant enzyme mimetics, nitric oxide and nitric oxide synthase inhibitors, erythropoietin, peroxisome-proliferator-activated receptor agonists, inhibitors of poly(ADP-ribose) polymerase, carbon monoxide-releasing molecules, statins, and adenosine. Novel approaches such as recent research involving combination therapies and the potential of non-pharmacological strategies are also considered.

Deficiency of heme oxygenase-1 impairs renal hemodynamics and exaggerates systemic inflammatory responses to renal ischemia

Heme oxygenase-1 may exert cytoprotective effects. In this study we examined the sensitivity of heme oxygenase-1 knockout (HO-1(-/-)) mice to renal ischemia by assessing glomerular filtration rate (GFR) and cytokine expression in the kidney, and inflammatory responses in the systemic circulation and in vital extrarenal organs. Four hours after renal ischemia, the GFR of HO-1(-/-) mice was much lower than that of wild-type mice in the absence of changes in renal blood flow or cardiac output. Eight hours after renal ischemia, there was a marked induction of interleukin-6 (IL-6) mRNA and its downstream signaling effector, phosphorylated signal transducer and activator of transcription 3 (pSTAT3), in the kidney, lung, and heart in HO-1(-/-) mice. Systemic levels of IL-6 were markedly and uniquely increased in HO-1(-/-) mice after ischemia as compared to wild-type mice. The administration of an antibody to IL-6 protected against the renal dysfunction and mortality observed in HO-1(-/-) mice following ischemia. We suggest that the exaggerated production of IL-6, occurring regionally and systemically following localized renal ischemia, in an HO-1-deficient state may underlie the heightened sensitivity observed in this setting.

NKT Cell Activation Mediates Neutrophil IFN-γ Production and Renal Ischemia-Reperfusion Injury

Previous work has shown that ischemia-reperfusion (IR) injury (IRI) is dependent on CD4(+) T cells from naive mice acting within 24 h. We hypothesize that NKT cells are key participants in the early innate response in IRI. Kidneys from C57BL/6 mice were subjected to IRI (0.5, 1, 3, and 24 h of reperfusion). After 30 min of reperfusion, we observed a significant increase in CD4(+) cells (145% of control) from single-cell kidney suspensions as measured by flow cytometry. A significant fraction of CD4(+) T cells expressed the activation marker, CD69(+), and adhesion molecule, LFA-1(high). Three hours after reperfusion, kidney IFN-gamma-producing cells were comprised largely of GR-1(+)CD11b(+) neutrophils, but also contained CD1d-restricted NKT cells. Kidney IRI in mice administered Abs to block CD1d, or deplete NKT cells or in mice deficient of NKT cells (Jalpha18(-/-)), was markedly attenuated. These effects were associated with a significant decrease in renal infiltration and, in activation of NKT cells, and a decrease in IFN-gamma-producing neutrophils. The results support the essential role of NKT cells and neutrophils in the innate immune response of renal IRI by mediating neutrophil infiltration and production of IFN-gamma.

Targeting sphingosine 1 phosphate receptor type 1 receptors in acute kidney injury

Sphingosine 1-phosphate analogs have a multitude of effects with the best characterized one being mediated through sphingosine 1-phosphate type 1 receptors (S1P1 receptor). Currently, S1P1 receptor agonists are being developed and tested for human disease. Because of the potent effect of S1P1 agonists to modulate the immune system, these compounds are ideal for blocking immune mechanisms that mediate acute kidney injury (AKI). This disorder continues to remain an important disease that is characterized by high morbidity and mortality. Currently there are no FDA approved drugs for the treatment of AKI. This review summarizes current knowledge on the mechanism of AKI due to ischemia-reperfusion and early studies that target S1P1 receptors for the treatment and prevention of AKI.

Pharmacologic treatment of acute kidney injury: why drugs haven't worked and what is on the horizon

Current strategies to limit the extent of injury in acute renal failure are based on extensive studies that identified cellular and molecular mechanisms of acute kidney injury. Despite successes in various animal models, translation to human studies has failed or studies are inconclusive. This review describes past failures and barriers to successful clinical trials. It also focuses on promising preclinical studies using novel compounds that currently are in or close to human investigation. Implementation of previous or novel compounds in well-designed clinical trials provides hope for the successful treatment of this devastating disorder.