Renal ischemia-reperfusion induces a dysbalance of angiopoietins, accompanied by proliferation of pericytes and fibrosis

Hypoxic Inducible Factor Stabilization in Pericytes beyond Erythropoietin Production: The Good and the Bad

The paracrine signaling pathways for the crosstalk between pericytes and endothelial cells are essential for the coordination of cell responses to challenges such as hypoxia in both healthy individuals and pathological conditions. Ischemia-reperfusion injury (IRI), one of the causes of cellular dysfunction and death, is associated with increased expression of genes involved in cellular adaptation to a hypoxic environment. Hypoxic inducible factors (HIFs) have a central role in the response to processes initiated by IRI not only linked to erythropoietin production but also because of their participation in inflammation, angiogenesis, metabolic adaptation, and fibrosis. While pericytes have an essential physiological function in erythropoietin production, a lesser-known role of HIF stabilization during IRI is that pericytes' HIF expression could influence vascular remodeling, cell loss and organ fibrosis. Better knowledge of mechanisms that control functions and consequences of HIF stabilization in pericytes beyond erythropoietin production is advisable for the development of therapeutic strategies to influence disease progression and improve treatments. Thus, in this review, we discuss the dual roles-for good or bad-of HIF stabilization during IRI, focusing on pericytes, and consequences in particular for the kidneys.

Sirt3 deficiency promotes endothelial dysfunction and aggravates renal injury

Sirtuin 3 (SIRT3), the main deacetylase of mitochondria, modulates the acetylation levels of substrates governing metabolism and oxidative stress. In the kidney, we showed that SIRT3 affects the proper functioning of high energy-demanding cells, such as tubular cells and podocytes. Less is known about the role of SIRT3 in regulating endothelial cell function and its impact on the progression of kidney disease. Here, we found that whole body Sirt3-deficient mice exhibited reduced renal capillary density, reflecting endothelial dysfunction, and VEGFA expression compared to wild-type mice. This was paralleled by activation of hypoxia signaling, upregulation of HIF-1α and Angiopietin-2, and oxidative stress increase. These alterations did not result in kidney disease. However, when Sirt3-deficient mice were exposed to the nephrotoxic stimulus Adriamycin (ADR) they developed aggravated endothelial rarefaction, altered VEGFA signaling, and higher oxidative stress compared to wild-type mice receiving ADR. As a result, ADR-treated Sirt3-deficient mice experienced a more severe injury with exacerbated albuminuria, podocyte loss and fibrotic lesions. These data suggest that SIRT3 is a crucial regulator of renal vascular homeostasis and its dysregulation is a predisposing factor for kidney disease. By extension, our findings indicate SIRT3 as a pharmacologic target in progressive renal disease whose treatments are still imperfect.

Impact of Angiopoietin-2 on Kidney Diseases

Background

Angiopoietins (Ang) are essential angiogenic factors involved in angiogenesis, vascular maturation, and inflammation. The most studied angiopoietins, angiopoietin-1 (Ang-1) and angiopoietin-2 (Ang-2), behave antagonistically to each other in vivo to sustain vascular endothelium homeostasis. While Ang-1 typically acts as the endothelium-protective mediator, its context-dependent antagonist Ang-2 can promote endothelium permeability and vascular destabilization, hence contributing to a poor outcome in vascular diseases via endothelial injury, vascular dysfunction, and microinflammation. The pathogenesis of kidney diseases is associated with endothelial dysfunction and chronic inflammation in renal diseases.

Summary

Several preclinical studies report overexpression of Ang-2 in renal tissues of certain kidney disease models; additionally, clinical studies show increased levels of circulating Ang-2 in the course of chronic kidney disease, implying that Ang-2 may serve as a useful biomarker in these patients. However, the exact mechanisms of Ang-2 action in renal diseases remain unclear.

Key Messages

We summarized the recent findings on Ang-2 in kidney diseases, including preclinical studies and clinical studies, aiming to provide a systematic understanding of the role of Ang-2 in these diseases.

Pericyte-mediated constriction of renal capillaries evokes no-reflow and kidney injury following ischaemia

Acute kidney injury is common, with ~13 million cases and 1.7 million deaths/year worldwide. A major cause is renal ischaemia, typically following cardiac surgery, renal transplant or severe haemorrhage. We examined the cause of the sustained reduction in renal blood flow ('no-reflow'), which exacerbates kidney injury even after an initial cause of compromised blood supply is removed. Adult male Sprague-Dawley rats, or NG2-dsRed male mice were used in this study. After 60 min kidney ischaemia and 30-60 min reperfusion, renal blood flow remained reduced, especially in the medulla, and kidney tubule damage was detected as Kim-1 expression. Constriction of the medullary descending vasa recta and cortical peritubular capillaries occurred near pericyte somata, and led to capillary blockages, yet glomerular arterioles and perfusion were unaffected, implying that the long-lasting decrease of renal blood flow contributing to kidney damage was generated by pericytes. Blocking Rho kinase to decrease pericyte contractility from the start of reperfusion increased the post-ischaemic diameter of the descending vasa recta capillaries at pericytes, reduced the percentage of capillaries that remained blocked, increased medullary blood flow and reduced kidney injury. Thus, post-ischaemic renal no-reflow, contributing to acute kidney injury, reflects pericytes constricting the descending vasa recta and peritubular capillaries. Pericytes are therefore an important therapeutic target for treating acute kidney injury.

Angiopoietins as Prognostic Markers for Future Kidney Disease and Heart Failure Events after Acute Kidney Injury

BACKGROUND

The mechanisms underlying long-term sequelae after AKI remain unclear. Vessel instability, an early response to endothelial injury, may reflect a shared mechanism and early trigger for CKD and heart failure.

METHODS

To investigate whether plasma angiopoietins, markers of vessel homeostasis, are associated with CKD progression and heart failure admissions after hospitalization in patients with and without AKI, we conducted a prospective cohort study to analyze the balance between angiopoietin-1 (Angpt-1), which maintains vessel stability, and angiopoietin-2 (Angpt-2), which increases vessel destabilization. Three months after discharge, we evaluated the associations between angiopoietins and development of the primary outcomes of CKD progression and heart failure and the secondary outcome of all-cause mortality 3 months after discharge or later.

RESULTS

Median age for the 1503 participants was 65.8 years; 746 (50%) had AKI. Compared with the lowest quartile, the highest quartile of the Angpt-1:Angpt-2 ratio was associated with 72% lower risk of CKD progression (adjusted hazard ratio [aHR], 0.28; 95% confidence interval [CI], 0.15 to 0.51), 94% lower risk of heart failure (aHR, 0.06; 95% CI, 0.02 to 0.15), and 82% lower risk of mortality (aHR, 0.18; 95% CI, 0.09 to 0.35) for those with AKI. Among those without AKI, the highest quartile of Angpt-1:Angpt-2 ratio was associated with 71% lower risk of heart failure (aHR, 0.29; 95% CI, 0.12 to 0.69) and 68% less mortality (aHR, 0.32; 95% CI, 0.15 to 0.68). There were no associations with CKD progression.

CONCLUSIONS

A higher Angpt-1:Angpt-2 ratio was strongly associated with less CKD progression, heart failure, and mortality in the setting of AKI.

CU06-1004 enhances vascular integrity and improves cardiac remodeling by suppressing edema and inflammation in myocardial ischemia-reperfusion injury

Ischemia-reperfusion (I/R) injury accelerates the cardiomyocytes (CMs) death by oxidative stress, and thereby deteriorates cardiac function. There has been a paradigm shift in the therapeutic perspective more towards the prevention or amelioration of damage caused by reperfusion. Cardiac microvascular endothelial cells (CMECs) are more vulnerable to reperfusion injury and play the crucial roles more than CMs in the pathological process of early I/R injury. In this study, we investigate that CU06-1004, as a vascular leakage blocker, can improve cardiac function by inhibiting CMEC's hyperpermeability and subsequently reducing the neutrophil's plugging and infiltration in infarcted hearts. CU06-1004 was delivered intravenously 5 min before reperfusion and the rats were randomly divided into three groups: (1) vehicle, (2) low-CU06-1004 (1 mg/kg, twice at 24 h intervals), and (3) high-CU06-1004 (5 mg/kg, once before reperfusion). CU06-1004 treatment reduced necrotic size and cardiac edema by enhancing vascular integrity, as demonstrated by the presence of intact junction proteins on CMECs and surrounding pericytes in early I/R injury. It also decreased the expression of vascular cell adhesion molecule 1 (VCAM-1) on CMECs, resulting in reduced infiltration of neutrophils and macrophages. Echocardiography showed that the CU06-1004 treatment significantly improved cardiac function compared with the vehicle group. Interestingly, single high-dose treatment with CU06-1004 provided a greater functional improvement than repetitive low-dose treatment until 8 weeks post I/R. These findings demonstrate that CU06-1004 enhances vascular integrity and improves cardiac function by preventing lethal myocardial I/R injury. It can provide a promising therapeutic option, as potential adjunctive therapy to current reperfusion strategies.

Bone marrow-derived mesenchymal stem cells transplantation attenuates renal fibrosis following acute kidney injury by repairing the peritubular capillaries

After the severe initial insults of acute kidney injury, progressive kidney tubulointerstitial fibrosis may occur, the peritubular capillary (PTC) rarefaction plays a key role in the disease progression. However, the mechanisms of PTC damage were not fully understood and potential therapeutic interventions were not explored. Previous studies of our research team and others in this field suggested that bone marrow-derived mesenchymal stem cells (BMSCs) transplanted into the AKI rat model may preserve the kidney function and pathological changes. In the current study, with the ischemia/reperfusion AKI rat model, we revealed that BMSCs transplantation attenuated the renal function decrease in the AKI model through preserving the peritubular capillaries (PTCs) function. The density of PTCs is maintained by BMSCs transplantation in the AKI model, detachment and relocation of pericytes in the PTCs diminished. Then we established that BMSCs transplantation may attenuate the renal fibrosis and preserve the kidney function after AKI by repairing the PTCs. Improving the vitality of pericytes, suppressing the detachment and trans-differentiation of pericytes, directly differentiation of BMSCs into pericytes by BMSCs transplantation all participate in the PTC repair. Through these processes, BMSCs rescued the microvascular damage and improved the density of PTCs. As a result, a preliminary conclusion can be reached that BMSCs transplantation can be an effective therapy for delaying renal fibrosis after AKI.

Imaging the Renal Microcirculation in Cell Therapy

Renal microvascular rarefaction plays a pivotal role in progressive kidney disease. Therefore, modalities to visualize the microcirculation of the kidney will increase our understanding of disease mechanisms and consequently may provide new approaches for evaluating cell-based therapy. At the moment, however, clinical practice is lacking non-invasive, safe, and efficient imaging modalities to monitor renal microvascular changes over time in patients suffering from renal disease. To emphasize the importance, we summarize current knowledge of the renal microcirculation and discussed the involvement in progressive kidney disease. Moreover, an overview of available imaging techniques to uncover renal microvascular morphology, function, and behavior is presented with the associated benefits and limitations. Ultimately, the necessity to assess and investigate renal disease based on in vivo readouts with a resolution up to capillary level may provide a paradigm shift for diagnosis and therapy in the field of nephrology.

Increased vascular permeability and severe renal tubular damage after ischemia-reperfusion injury in mice lacking adiponectin or T-cadherin

Adiponectin (APN) is a circulating protein specifically produced by adipocytes. Native APN specifically binds to T-cadherin, a glycosylphosphatidylinositol-anchored protein, mediating the exosome-stimulating effects of APN in endothelial, muscle, and mesenchymal stem cells. It was previously reported that APN has beneficial effects on kidney diseases, but the role of T-cadherin has not been clarified yet. Here, our immunofluorescence study indicated the existence of both T-cadherin and APN protein in pericytes, subsets of tissue-resident mesenchymal stem/progenitor cells positive for platelet-derived growth factor receptor β (PDGFRβ), surrounding peritubular capillaries. In an acute renal ischemia-reperfusion (I/R) model, T-cadherin-knockout (Tcad-KO) mice, similar to APN-KO mice, exhibited the more progressive phenotype of renal tubular damage and increased vascular permeability than wild-type mice. In addition, in response to I/R-injury, the renal PDGFRβ-positive cell area increased in wild-type mice, but opposingly decreased in both Tcad-KO and APN-KO mice, suggesting severe pericyte loss. Mouse primary pericytes also expressed T-cadherin. APN promoted exosome secretion in a T-cadherin-dependent manner. Such exosome production from pericytes may play an important role in maintaining the capillary network and APN-mediated inhibition of renal tubular injury. In summary, our study suggested that APN protected the kidney in an acute renal injury model by binding to T-cadherin.NEW & NOTEWORTHY In the kidney, T-cadherin-associated adiponectin protein existed on peritubular capillary pericytes. In an acute renal ischemia-reperfusion model, deficiency of adiponectin or T-cadherin exhibited the more progressive phenotype of renal tubular damage and increased vascular permeability, accompanied by severe pericyte loss. In vitro, adiponectin promoted exosome secretion from mouse primary pericytes in a T-cadherin-dependent manner. Adiponectin plays an important role in maintaining the capillary network and amelioration of renal tubular injury by binding to T-cadherin.

Adenosine A1 Receptor Deficiency Aggravates Extracellular Matrix Accumulation in Diabetic Nephropathy through Disturbance of Peritubular Microenvironment

BACKGROUND

We previously observed that adenosine A1 receptor (A1AR) had a protective role in proximal tubular megalin loss associated with albuminuria in diabetic nephropathy (DN). In this study, we aimed to explore the role of A1AR in the fibrosis progression of DN.

METHODS

We collected DN patients' samples and established a streptozotocin-induced diabetes model in wild-type (WT) and A1AR-deficient (A1AR^-/-) mice. The location and expression of CD34, PDGFRβ, and A1AR were detected in kidney tissue samples from DN patients by immunofluorescent and immunohistochemical staining. We also analyzed the expression of TGFβ, collagen (I, III, and IV), α-SMA, and PDGFRβ using immunohistochemistry in WT and A1AR^-/- mice. CD34 and podoplanin expression were analyzed by Western blotting and immunohistochemical staining in mice, respectively. Human renal proximal tubular epithelial cells (HK2) were cultured in medium containing high glucose and A1AR agonist as well as antagonist.

RESULTS

In DN patients, the expression of PDGFRβ was higher with the loss of CD34. The location of PDGFRβ and TGFβ was near to each other. The A1AR, which was colocalized with CD34 partly, was also upregulated in DN patients. In WT-DN mice, obvious albuminuria and renal pathological leisure were observed. In A1AR^-/- DN mice, more severe renal tubular interstitial fibrosis and more extracellular matrix deposition were observed, with lower CD34 expression and pronounced increase of PDGFRβ. In HK2 cells, high glucose stimulated the epithelial-mesenchymal transition (EMT) process, which was inhibited by A1AR agonist.

CONCLUSION

A1AR played a critical role in protecting the tubulointerstitial fibrosis process in DN by regulation of the peritubular microenvironment.

Notch1 promotes the pericyte-myofibroblast transition in idiopathic pulmonary fibrosis through the PDGFR/ROCK1 signal pathway

The goals of this study were to investigate the role of the Notch1/PDGFRβ/ROCK1 signaling pathway in the pathogenesis of pulmonary fibrosis and to explore the possibility of treating fibrosis by targeting Notch1. Lung tissues from patients with pulmonary fibrosis were examined for the expression of Notch1/PDGFRβ/ROCK1 using RT-qPCR, western blotting, and immunostaining. Cultured mouse lung pericytes were transfected with Notch1-overexpressed vectors or shRNA targeting PDGFRβ/ROCK1 to examine cell behaviors, including proliferation, cell cycle arrest, and differentiation toward myofibroblasts. Finally, a mouse pulmonary fibrosis model was prepared, and a Notch1 inhibitor was administered to observe tissue morphology and pericyte cell behaviors. Human pulmonary fibrotic tissues presented with overexpression of Notch1, PDGFRβ, and ROCK1, in addition to a prominent transition of pericytes into myofibroblasts. In cultured mouse lung pericytes, overexpression of Notch1 led to the accelerated proliferation and differentiation of cells, and it also increased the expression of the PDGFRβ and ROCK1 proteins. The knockdown of PDGFRβ/ROCK1 in pericytes remarkably suppressed pericyte proliferation and differentiation. As further substantiation, the administration of a Notch1 inhibitor in a mouse model of lung fibrosis inhibited the PDGFRβ/ROCK1 pathway, suppressed pericyte proliferation and differentiation, and alleviated the severity of fibrosis. Our results showed that the Notch1 signaling pathway was aberrantly activated in pulmonary fibrosis, and this pathway may facilitate disease progression via mediating pericyte proliferation and differentiation. The inhibition of the Notch1 pathway may provide one promising treatment strategy for pulmonary fibrosis.

A cell membrane protein called Notch1, which binds to signaling molecules outside cells and then alters the activity of genes inside the cells, might be a promising target for drugs to treat the lung damage of pulmonary fibrosis. This condition, generally of unknown cause, involves thickening, stiffening and scarring of lung tissue. It can lead to serious breathing difficulties and eventually death, especially in people aged over 70. Hui Wang and colleagues at Central South University, Changsha, investigated the significance of the Notch1 signaling pathway by examining lung tissue from patients and manipulating the activity of the pathway in mouse cells. They conclude that Notch1 signaling is activated in pulmonary fibrosis. Drugs that could inhibit the pathway, for example by binding to the Notch1 protein, might open a promising new avenue toward treatment.

Ginsenoside Rg1 Regulates SIRT1 to Ameliorate Sepsis-Induced Lung Inflammation and Injury via Inhibiting Endoplasmic Reticulum Stress and Inflammation

Objectives

To investigate the protective effect of ginsenoside Rg1 on relieving sepsis-induced lung inflammation and injury in vivo and in vitro.

Methods

Cultured human pulmonary epithelial cell line A549 was challenged with LPS to induce cell injury, and CLP mouse model was generated to mimic clinical condition of systemic sepsis. Rg1 was applied to cells or animals at indicated dosage. Apoptosis of cultured cells was quantified by flow cytometry, along with ELISA for inflammatory cytokines in supernatant. For septic mice, lung tissue pathology was examined, plus ELISA assay for serum cytokines. Western blotting was used to examine the activation of inflammatory pathways and ER stress marker proteins in both cells and mouse lung tissues. Reactive oxygen species (ROS) level was quantified by DCFDA kit.

Results

Ginsenoside Rg1 treatment remarkably suppressed apoptosis rate of LPS-induced A549 cells, relieved mouse lung tissue damage, and elevated survival rate. Rg1 treatment also rescued cells from LPS-induced intracellular ROS. In both A549 cells and mouse lung tissues, further study showed that Rg1 perfusion significantly suppressed the secretion of inflammatory cytokines including tumor necrosis factor- (TNF-) alpha and interleukin- (IL-) 6 and relieved cells from ER stress as supported by decreased expression of marker proteins via upregulating sirtuin 1 (SIRT1).

Conclusion

Our results showed that ginsenoside Rg1 treatment effectively relieved sepsis-induced lung injury in vitro and in vivo, mainly via upregulating SIRT1 to relieve ER stress and inflammation. These findings provide new insights for unrevealing potential candidate for severe sepsis accompanied with lung injury.

Serum angiopoietin-2 concentrations of post-PCI are correlated with the parameters of renal function in patients with coronary artery disease

Patients with coronary artery disease (CAD) frequently have comorbidity of chronic kidney disease (CKD). Their renal function may deteriorate because of the use of contrast agent after percutaneous coronary intervention (PCI). Angiopoietin-2 (Ang-2), which is highly expressed in the site of angiogenesis, plays an important role in both CAD and CKD. This study aimed to investigate the relation of serum Ang-2 concentrations with the renal function after PCI.This study enrolled 57 patients with CAD undergoing PCI. Blood samples for Ang-2 were collected in the first morning after admission and within 24 to 48 h after PCI. The parameters of renal function (serum creatinine, cystatin C and eGFR) were tested on the first day after admission and within 72 h after PCI.Overall, serum Ang-2 levels of post-PCI were significantly lower than those of pre-PCI [median, 1733 (IQR, 1100-2568) vs median, 2523 (IQR, 1702-3640) pg/mL; P < .001]. However, in patients with CKD (eGFR < 60 mL/min/1.73 m), there was no significant difference between serum Ang-2 levels of post-PCI and those of pre-PCI [median, 2851 (IQR, 1720-4286) vs. median, 2492 (IQR, 1434-4994) pg/mL; P = .925]. In addition, serum Ang-2 levels of post-PCI, but not pre-PCI, were significantly correlated with the post-PCI parameters of renal function.Serum Ang-2 concentrations of post-PCI are closely related to renal function in patients with CAD. It may have potential to be the early biomarker of contrast-induced nephropathy in the future.

Kidney injury molecule-1 staining in renal allograft biopsies 10 days after transplantation is inversely correlated with functioning proximal tubular epithelial cells

Background

Kidney injury molecule-1 (KIM-1) and neutrophil gelatinase-associated lipocalin (NGAL) are promising biomarkers for monitoring delayed graft function (DGF) after kidney transplantation. Here we investigated localization and distribution of KIM-1 and NGAL staining in renal allograft biopsies and studied their association with histological features, functional DGF (fDGF) and the tubular function slope (TFS), a functioning proximal tubular epithelial cell (PTEC) marker.

Methods

Day 10 protocol biopsies of 64 donation after circulatory death recipients were stained for KIM-1 and NGAL and the positive area was quantified using ImageJ software. Biopsies were scored according to Banff and acute tubular necrosis (ATN) criteria. A 99mtechnetium-mercaptoacetyltriglycine (99mTc-MAG3)-renography was performed to calculate TFS.

Results

KIM-1 staining was located on the brush border of tubular epithelial cells (TECs) and correlated with denudation, while NGAL was present more focally in a cytoplasmic distribution. KIM-1 and NGAL staining were not correlated and no co-localization was observed. Quantitative stainings were not associated with fDGF, but KIM-1 tended to be higher in patients with prolonged fDGF (≥21 days; P = 0.062). No correlation was observed between the quantitative tissue stainings and urinary KIM-1 or NGAL. Quantitative KIM-1 staining was inversely correlated with the TFS (Spearman's ρ = -0.53; P < 0.001), whereas NGAL was not. The latter finding might be because cortical NGAL staining is dependent on filtration and subsequent reabsorption by functioning PTECs. Staining of NGAL was indeed restricted to PTECs, as shown by co-localization with a PTEC-specific lectin.

Conclusions

KIM-1 and NGAL staining showed different localization and distribution. Quantitative KIM-1 staining was inversely correlated with functioning PTECs.

Complement Activation During Ischemia/Reperfusion Injury Induces Pericyte-to-Myofibroblast Transdifferentiation Regulating Peritubular Capillary Lumen Reduction Through pERK Signaling

Pericytes are one of the principal sources of scar-forming myofibroblasts in chronic kidneys disease. However, the modulation of pericyte-to-myofibroblast transdifferentiation (PMT) in the early phases of acute kidney injury is poorly understood. Here, we investigated the role of complement in inducing PMT after transplantation. Using a swine model of renal ischemia/reperfusion (I/R) injury, we found the occurrence of PMT after 24 h of I/R injury as demonstrated by reduction of PDGFRβ⁺/NG2⁺ cells with increase in myofibroblasts marker αSMA. In addition, PMT was associated with significant reduction in peritubular capillary luminal diameter. Treatment by C1-inhibitor (C1-INH) significantly preserved the phenotype of pericytes maintaining microvascular density and capillary lumen area at tubulointerstitial level. In vitro, C5a transdifferentiated human pericytes in myofibroblasts, with increased αSMA expression in stress fibers, collagen I production, and decreased antifibrotic protein Id2. The C5a-induced PMT was driven by extracellular signal-regulated kinases phosphorylation leading to increase in collagen I release that required both non-canonical and canonical TGFβ pathways. These results showed that pericytes are a pivotal target of complement activation leading to a profibrotic maladaptive cellular response. Our studies suggest that C1-INH may be a potential therapeutic strategy to counteract the development of PMT and capillary lumen reduction in I/R injury.

Cytoresistance after acute kidney injury is limited to the recovery period of proximal tubule integrity and possibly involves Hippo-YAP signaling

Rat proximal tubule (PT) cells that have recovered from severe acute kidney injury induced by uranyl acetate (UA) develop cytoresistance to subsequent UA treatments. We reported that enhanced G1 arrest might contribute to cytoresistance. Herein, we examined these mechanisms by investigating Yes-associated protein (YAP), a regulator of cell number, and survivin, a downstream mediator of YAP that inhibits apoptosis. Rats pretreated with saline (vehicle group) or UA (AKI group) were injected with UA 2 weeks, 2 months, or 6 months after treatment. Cytoresistance, evaluated by serum creatinine, was observed at 2 weeks, was attenuated at 2 months, and was lost at 6 months in the AKI group. Based on immunohistochemistry, overexpressed YAP/survivin in PT cells and an increased number of PT cells was found before the second insult at 2 weeks, regressed gradually, and returned to a normal value by 6 months in the AKI group. Cell cycle status, assessed by flow cytometry, was equivalent in all groups before the second insult. However, early G1 phase (cyclin D1-) and p27+ PT cells increased in the AKI group compared to those in the vehicle group until 2 months, but were comparable to those in the vehicle group at 6 months. p21+ PT cells increased at 2 weeks, but normalized by 2 months. Thus, PT cells that have recovered from AKI transiently overexpress YAP/survivin, probably inhibiting apoptosis and resulting in acquired cytoresistance. This effect occurs until PT remodeling is complete, subceullular PT integrity is restored, and cell numbers are normalized.

Ablation of endothelial prolyl hydroxylase domain protein-2 promotes renal vascular remodelling and fibrosis in mice

Accumulating evidence demonstrates that hypoxia-inducible factor (HIF-α) hydroxylase system has a critical role in vascular remodelling. Using an endothelial-specific prolyl hydroxylase domain protein-2 (PHD2) knockout (PHD2^EC KO) mouse model, this study investigates the regulatory role of endothelial HIF-α hydroxylase system in the development of renal fibrosis. Knockout of PHD2 in EC up-regulated the expression of HIF-1α and HIF-2α, resulting in a significant decline of renal function as evidenced by elevated levels of serum creatinine. Deletion of PHD2 increased the expression of Notch3 and transforming growth factor (TGF-β1) in EC, thus further causing glomerular arteriolar remodelling with an increased pericyte and pericyte coverage. This was accompanied by a significant elevation of renal resistive index (RI). Moreover, knockout of PHD2 in EC up-regulated the expression of fibroblast-specific protein-1 (FSP-1) and increased interstitial fibrosis in the kidney. These alterations were strongly associated with up-regulation of Notch3 and TGF-β1. We concluded that the expression of PHD2 in endothelial cells plays a critical role in renal fibrosis and vascular remodelling in adult mice. Furthermore, these changes were strongly associated with up-regulation of Notch3/TGF-β1 signalling and excessive pericyte coverage.

Transplantation of Kidneys From Donors With Acute Kidney Injury: Friend or Foe?

The gap between supply and demand in kidney transplantation has led to increased use of marginal kidneys; however, kidneys with acute kidney injury are often declined/discarded. To determine whether this policy is justified, we analyzed outcomes of donor kidneys with acute kidney injury (AKI) in a large UK cohort. A retrospective analysis of the UK Transplant Registry evaluated deceased donors between 2003 and 2013. Donors were classified as no AKI, or AKI stage 1-3 according to Acute Kidney Injury Network (AKIN) criteria. Relationship of AKI with delayed graft function/primary nonfunction (DGF/PNF), estimated glomerular filtration rate (eGFR), and graft-survival at 90 days and 1 year was analyzed. There were 11 219 kidneys (1869 [17%] with AKI) included. Graft failure at 1 year is greater for donors with AKI than for those without (graft survival 89% vs. 91%, p = 0.02; odds ratio (OR) 1.20 [95% confidence interval (CI): 1.03-1.41]). DGF rates increase with donor AKI stage (p < 0.005), and PNF rates are significantly higher for AKIN stage 3 kidneys (9% vs. 4%, p = 0.04) Analysis of association between AKI and recipient eGFR suggests a risk of inferior eGFR with AKI versus no AKI (p < 0.005; OR 1.25 [95% CI: 1.08-1.31]). We report a small reduction in 1-year graft-survival of kidneys from donors with AKI. We conclude that AKI stage 1 or 2 kidneys should be used; however, caution is advised for AKI stage 3 donors.

Endothelial Dysfunction in Renal Interstitial Fibrosis

Kidney disease affects millions of people worldwide and it is now widely accepted that many pathological processes may persist after acute kidney injury that can cause the progression to CKD. Tubulointerstitial fibrosis manifests soon after injury and while many cellular and molecular components of kidney fibrosis have been discovered, largely in animal models, new therapeutic strategies are still desperately needed. The renal endothelium has emerged as important in progression of fibrosis through regulation of hypoxia, inflammation and cellular crosstalk. This review aims to highlight our current understanding of the role of the endothelium in interstitial fibrosis and to identify potential therapeutic targets. © 2016 S. Karger AG, Basel.

Hypercoagulability causes atrial fibrosis and promotes atrial fibrillation

AIMS

Atrial fibrillation (AF) produces a hypercoagulable state. Stimulation of protease-activated receptors by coagulation factors provokes pro-fibrotic, pro-hypertrophic, and pro-inflammatory responses in a variety of tissues. We studied the effects of thrombin on atrial fibroblasts and tested the hypothesis that hypercoagulability contributes to the development of a substrate for AF.

METHODS AND RESULTS

In isolated rat atrial fibroblasts, thrombin enhanced the phosphorylation of the pro-fibrotic signalling molecules Akt and Erk and increased the expression of transforming growth factor β1 (2.7-fold) and the pro-inflammatory factor monocyte chemoattractant protein-1 (6.1-fold). Thrombin also increased the incorporation of ³H-proline, suggesting enhanced collagen synthesis by fibroblasts (2.5-fold). All effects could be attenuated by the thrombin inhibitor dabigatran. In transgenic mice with a pro-coagulant phenotype (TM^pro/pro), the inducibility of AF episodes lasting >1 s was higher (7 out of 12 vs. 1 out of 10 in wild type) and duration of AF episodes was longer compared with wild type mice (maximum episode duration 42.8 ± 68.4 vs. 0.23 ± 0.39 s). In six goats with persistent AF treated with nadroparin, targeting Factor Xa-mediated thrombin generation, the complexity of the AF substrate was less pronounced than in control animals (LA maximal activation time differences 23.3 ± 3.1 ms in control vs. 15.7 ± 2.1 ms in nadroparin, P < 0.05). In the treated animals, AF-induced α-smooth muscle actin expression was lower and endomysial fibrosis was less pronounced.

CONCLUSION

The hypercoagulable state during AF causes pro-fibrotic and pro-inflammatory responses in adult atrial fibroblasts. Hypercoagulability promotes the development of a substrate for AF in transgenic mice and in goats with persistent AF. In AF goats, nadroparin attenuates atrial fibrosis and the complexity of the AF substrate. Inhibition of coagulation may not only prevent strokes but also inhibit the development of a substrate for AF.

The Synthetic Tie2 Agonist Peptide Vasculotide Protects Renal Vascular Barrier Function In Experimental Acute Kidney Injury

Microvascular barrier dysfunction plays a major role in the pathophysiology of acute kidney injury (AKI). Angiopoietin-1, the natural agonist ligand for the endothelial-specific Tie2 receptor, is a non-redundant endothelial survival and vascular stabilization factor. Here we evaluate the efficacy of a polyethylene glycol-clustered Tie2 agonist peptide, vasculotide (VT), to protect against endothelial-cell activation with subsequent microvascular dysfunction in a murine model of ischemic AKI. Renal ischemia reperfusion injury (IRI) was induced by clamping of the renal arteries for 35 minutes. Mice were treated with VT or PEGylated cysteine before IRI. Sham-operated animals served as time-matched controls. Treatment with VT significantly reduced transcapillary albumin flux and renal tissue edema after IRI. The protective effects of VT were associated with activation of Tie2 and stabilization of its downstream effector, VE-cadherin in renal vasculature. VT abolished the decline in renal tissue blood flow, attenuated the increase of serum creatinine and blood urea nitrogen after IRI, improved recovery of renal function and markedly reduced mortality compared to PEG [HR 0.14 (95% CI 0.05-0.78) P < 0.05]. VT is inexpensive to produce, chemically stable and unrelated to any Tie2 ligands. Thus, VT may represent a novel therapy to prevent AKI in patients.

Early systemic microvascular damage in pigs with atherogenic diabetes mellitus coincides with renal angiopoietin dysbalance

Background

Diabetes mellitus (DM) is associated with a range of microvascular complications including diabetic nephropathy (DN). Microvascular abnormalities in the kidneys are common histopathologic findings in DN, which represent one manifestation of ongoing systemic microvascular damage. Recently, sidestream dark-field (SDF) imaging has emerged as a noninvasive tool that enables one to visualize the microcirculation. In this study, we investigated whether changes in the systemic microvasculature induced by DM and an atherogenic diet correlated spatiotemporally with renal damage.

Methods

Atherosclerotic lesion development was triggered in streptozotocin-induced DM pigs (140 mg/kg body weight) by administering an atherogenic diet for approximately 11 months. Fifteen months following induction of DM, microvascular morphology was visualized in control pigs (n = 7), non-diabetic pigs fed an atherogenic diet (ATH, n = 5), and DM pigs fed an atherogenic diet (DM+ATH, n = 5) using SDF imaging of oral mucosal tissue. Subsequently, kidneys were harvested from anethesized pigs and the expression levels of well-established markers for microvascular integrity, such as Angiopoietin-1 (Angpt1) and Angiopoietin-2 (Angpt2) were determined immunohistochemically, while endothelial cell (EC) abundance was determined by immunostaining for von Willebrand factor (vWF).

Results

Our study revealed an increase in the capillary tortuosity index in DM+ATH pigs (2.31±0.17) as compared to the control groups (Controls 0.89±0.08 and ATH 1.55±0.11; p<0.05). Kidney biopsies showed marked glomerular lesions consisting of mesangial expansion and podocyte lesions. Furthermore, we observed a disturbed Angpt2/ Angpt1balance in the cortex of the kidney, as evidenced by increased expression of Angpt2 in DM+ATH pigs as compared to Control pigs (p<0.05).

Conclusion

In the setting of DM, atherogenesis leads to the augmentation of mucosal capillary tortuosity, indicative of systemic microvascular damage. Concomitantly, a dysbalance in renal angiopoietins was correlated with the development of diabetic nephropathy. As such, our studies strongly suggest that defects in the systemic microvasculature mirror the accumulation of microvascular damage in the kidney.

Circulating microRNAs associate with diabetic nephropathy and systemic microvascular damage and normalize after simultaneous pancreas-kidney transplantation

Because microvascular disease is one of the most important drivers of diabetic complications, early monitoring of microvascular integrity may be of clinical value. By assessing profiles of circulating microRNAs (miRNAs), known regulators of microvascular pathophysiology, in healthy controls and diabetic nephropathy (DN) patients before and after simultaneous pancreas-kidney transplantation (SPK), we aimed to identify differentially expressed miRNAs that associate with microvascular impairment. Following a pilot study, we selected 13 candidate miRNAs and determined their circulating levels in DN (n = 21), SPK-patients (n = 37), healthy controls (n = 19), type 1 diabetes mellitus patients (n = 15) and DN patients with a kidney transplant (n = 15). For validation of selected miRNAs, 14 DN patients were studied longitudinally up to 12 months after SPK. We demonstrated a direct association of miR-25, -27a, -126, -130b, -132, -152, -181a, -223, -320, -326, -340, -574-3p and -660 with DN. Of those, miR-25, -27a, -130b, -132, -152, -320, -326, -340, -574-3p and -660 normalized after SPK. Importantly, circulating levels of some of these miRNAs tightly associate with microvascular impairment as they relate to aberrant capillary tortuosity, angiopoietin-2/angiopoietin-1 ratios, circulating levels of soluble-thrombomodulin and insulin-like growth factor. Taken together, circulating miRNA profiles associate with DN and systemic microvascular damage, and might serve to identify individuals at risk of experiencing microvascular complications, as well as give insight into underlying pathologies.

Diverse origins of the myofibroblast—implications for kidney fibrosis

Fibrosis is the common end point of chronic kidney disease. The persistent production of inflammatory cytokines and growth factors leads to an ongoing process of extracellular matrix production that eventually disrupts the normal functioning of the organ. During fibrosis, the myofibroblast is commonly regarded as the predominant effector cell. Accumulating evidence has demonstrated a diverse origin of myofibroblasts in kidney fibrosis. Proposed major contributors of myofibroblasts include bone marrow-derived fibroblasts, tubular epithelial cells, endothelial cells, pericytes and interstitial fibroblasts; the published data, however, have not yet clearly defined the relative contribution of these different cellular sources. Myofibroblasts have been reported to originate from various sources, irrespective of the nature of the initial damage responsible for the induction of kidney fibrosis. Here, we review the possible relevance of the diversity of myofibroblast progenitors in kidney fibrosis and the implications for the development of novel therapeutic approaches. Specifically, we discuss the current status of preclinical and clinical antifibrotic therapy and describe targeting strategies that might help support resident and circulating cells to maintain or regain their original functional differentiation state. Such strategies might help these cells resist their transition to a myofibroblast phenotype to prevent, or even reverse, the fibrotic state.

The angiopoietin/TIE receptor system: Focusing its role for ischemia-reperfusion injury

Ischemia and reperfusion (I/R) are of fatal consequence for the affected organs, as they provoke a profound inflammatory reaction. This thoroughly destroys cells and tissues, inducing functional failure or even complete loss of organ function. Since I/R is primarily a vascular problem, the interaction between the endothelium and the surrounding environment is of great significance. The angiopoietins (ANG) and the TIE receptors are key players for the vascular homeostasis. This review summarizes biochemical and cellular mechanisms leading to I/R injury. After a brief introduction to the ANG/TIE system, a comprehensive overview of its role for the development of I/R syndrome is given. Finally, current therapeutic approaches to mitigate the consequences of I/R by modulating ANG/TIE signaling are reviewed in detail.

Platelet-derived growth factor receptor-β expression in human peritoneum

INTRODUCTION

Simple peritoneal fibrosis and encapsulating peritoneal sclerosis (EPS) are important lesions in the peritoneum of patients on peritoneal dialysis (PD). We have previously described a population of podoplanin-positive myofibroblasts in peritoneal biopsies from patients with EPS. Platelet-derived growth factor receptor-β (PDGFRβ) is a marker of pericytes, and PDGFs might be involved in the fibrotic response of the peritoneum. This study aimed to describe PDGFRβ in the human peritoneum.

METHODS

In this retrospective analysis, we localized PDGFRβ in peritoneal biopsies from patients with EPS (n = 6) and patients on PD without signs of EPS (n = 5), and compared them with normal peritoneum (n = 4) and peritoneum from uremic patients (n = 5). Consecutive sections were stained for smooth-muscle actin (SMA) and podoplanin. Slides were scored semiquantitatively by 2 observers blinded to the diagnosis.

RESULTS

PDGFRβ was expressed by cells of arterial walls in all biopsies. A prominent population of PDGFRβ-positive cells was present in the normal peritoneum, which were SMA negative on consecutive sections. In patients on PD, a high number of PDGFRβ were also positive for SMA. In EPS, the majority of podoplanin-positive cells were positive for PDGFRβ. In peritoneal biopsies from normal and uremic patients, the expression of SMA was mainly restricted to cells of arterial walls. Podoplanin expression was restricted to lymphatic vessels in normal peritoneum, in uremic patients, and in patients on PD without EPS.

CONCLUSIONS

As podoplanin-positive myofibroblasts express PDGFRβ, these cells might be related to pericytes (rather than other sources of fibroblasts). PDGFRβ might turn out to be a therapeutic target in EPS.

Kidney pericytes: roles in regeneration and fibrosis

Renal pericytes have been neglected for many years, but recently they have become an intensively studied cell population in renal biology and pathophysiology. Pericytes are stromal cells that support vasculature, and a subset of pericytes are mesenchymal stem cells. In kidney, pericytes have been reported to play critical roles in angiogenesis, regulation of renal medullary and cortical blood flow, and serve as progenitors of interstitial myofibroblasts in renal fibrogenesis. They interact with endothelial cells through distinct signaling pathways and their activation and detachment from capillaries after acute or chronic kidney injury may be critical for driving chronic kidney disease progression. By contrast, during kidney homeostasis it is likely that pericytes serve as a local stem cell population that replenishes differentiated interstitial and vascular cells lost during aging. This review describes both the regenerative properties of pericytes as well as involvement in pathophysiologic conditions such as fibrogenesis.

Mesenchymal stromal cells to prevent fibrosis in kidney transplantation

PURPOSE OF REVIEW

Kidney transplantation has improved the life expectancy and quality of life for patients with end-stage renal failure. However, despite the impressive improvements in short-term outcome parameters because of better and more potent immunosuppressive drugs, the long-term survival of renal allografts has changed little over the last decades. Sustained inflammation in the areas of interstitial fibrosis and tubular atrophy (IFTA) is a strong predictor of allograft failure. Mesenchymal stromal cells (MSCs) have potent anti-inflammatory and reparative properties, and could thus play a role in controlling these processes.

RECENT FINDINGS

Local resident MSCs and exogenous MSCs have been implicated in the repair of the injured kidney, mostly by their paracrine functions. In the experimental models and clinical trials, first results with MSCs for the treatment of inflammation and IFTA suggest beneficial effects.

SUMMARY

Endogenously and exogenously administered MSCs might enhance the intrinsic reparative capabilities of the kidney in transplant recipients and maybe developed as a tool to control both inflammation and fibrosis.

Cell-specific translational profiling in acute kidney injury

Acute kidney injury (AKI) promotes an abrupt loss of kidney function that results in substantial morbidity and mortality. Considerable effort has gone toward identification of diagnostic biomarkers and analysis of AKI-associated molecular events; however, most studies have adopted organ-wide approaches and have not elucidated the interplay among different cell types involved in AKI pathophysiology. To better characterize AKI-associated molecular and cellular events, we developed a mouse line that enables the identification of translational profiles in specific cell types. This strategy relies on CRE recombinase-dependent activation of an EGFP-tagged L10a ribosomal protein subunit, which allows translating ribosome affinity purification (TRAP) of mRNA populations in CRE-expressing cells. Combining this mouse line with cell type-specific CRE-driver lines, we identified distinct cellular responses in an ischemia reperfusion injury (IRI) model of AKI. Twenty-four hours following IRI, distinct translational signatures were identified in the nephron, kidney interstitial cell populations, vascular endothelium, and macrophages/monocytes. Furthermore, TRAP captured known IRI-associated markers, validating this approach. Biological function annotation, canonical pathway analysis, and in situ analysis of identified response genes provided insight into cell-specific injury signatures. Our study provides a deep, cell-based view of early injury-associated molecular events in AKI and documents a versatile, genetic tool to monitor cell-specific and temporal-specific biological processes in disease modeling.

Association of kidney function with changes in the endothelial surface layer

BACKGROUND AND OBJECTIVES

ESRD is accompanied by endothelial dysfunction. Because the endothelial glycocalyx (endothelial surface layer) governs interactions between flowing blood and the vessel wall, perturbation could influence disease progression. This study used a novel noninvasive sidestream-darkfield imaging method, which measures the accessibility of red blood cells to the endothelial surface layer in the microcirculation (perfused boundary region), to investigate whether renal function is associated with endothelial surface layer dimensions.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

Perfused boundary region was measured in control participants (n=10), patients with ESRD (n=23), participants with normal kidney function after successful living donor kidney transplantation (n=12), and patients who developed interstitial fibrosis/tubular atrophy after kidney transplantation (n=10). In addition, the endothelial activation marker angiopoietin-2 and shed endothelial surface layer components syndecan-1 and soluble thrombomodulin were measured using ELISA.

RESULTS

Compared with healthy controls (1.82 ± 0.16 µm), ESRD patients had a larger perfused boundary region (+0.23; 95% confidence interval, 0.46 to <0.01; P<0.05), which signifies loss of endothelial surface layer dimensions. This large perfused boundary region was accompanied by higher circulating levels of syndecan-1 (+57.71; 95% confidence interval, 17.38 to 98.04; P<0.01) and soluble thrombomodulin (+12.88; 95% confidence interval, 0.29 to 25.46; P<0.001). After successful transplantation, the perfused boundary region was indistinguishable from healthy controls (without elevated levels of soluble thrombomodulin or syndecan-1). In contrast, however, patients who developed interstitial fibrosis and tubular atrophy showed a large perfused boundary region (+0.36; 95% confidence interval, 0.09 to 0.63; P<0.01) and higher levels of endothelial activation markers. In addition, a significant correlation between perfused boundary region, angiopoietin-2, and eGFR was observed (perfused boundary region versus GFR: Spearman's ρ=0.31; P<0.05; perfused boundary region versus angiopoietin-2: Spearman's ρ=-0.33; P<0.05).

CONCLUSION

Reduced renal function is strongly associated with low endothelial surface layer dimensions. After successful kidney transplantation, the endothelial surface layer is indistinguishable from control.