Susanne Mathia

Tubular von Hippel-Lindau Knockout Protects against Rhabdomyolysis-Induced AKI

Renal hypoxia occurs in AKI of various etiologies, but adaptation to hypoxia, mediated by hypoxia-inducible factor (HIF), is incomplete in these conditions. Preconditional HIF activation protects against renal ischemia-reperfusion injury, yet the mechanisms involved are largely unknown, and HIF-mediated renoprotection has not been examined in other causes of AKI. Here, we show that selective activation of HIF in renal tubules, through Pax8-rtTA-based inducible knockout of von Hippel-Lindau protein (VHL-KO), protects from rhabdomyolysis-induced AKI. In this model, HIF activation correlated inversely with tubular injury. Specifically, VHL deletion attenuated the increased levels of serum creatinine/urea, caspase-3 protein, and tubular necrosis induced by rhabdomyolysis in wild-type mice. Moreover, HIF activation in nephron segments at risk for injury occurred only in VHL-KO animals. At day 1 after rhabdomyolysis, when tubular injury may be reversible, the HIF-mediated renoprotection in VHL-KO mice was associated with activated glycolysis, cellular glucose uptake and utilization, autophagy, vasodilation, and proton removal, as demonstrated by quantitative PCR, pathway enrichment analysis, and immunohistochemistry. In conclusion, a HIF-mediated shift toward improved energy supply may protect against acute tubular injury in various forms of AKI.

Action of hypoxia-inducible factor in liver and kidney from mice with Pax8-rtTA-based deletion of von Hippel-Lindau protein.

Von Hippel‐Lindau protein (VHL) provides the degradation of hypoxia‐inducible factor (HIF). Tetracycline‐induced, Pax8‐rtTA‐based knockout of VHL (VHL‐KO) affects all renal tubules and periportal hepatocytes and leads to sustained upregulation of HIF. Here, we study the phenotype of VHL‐KO in both organs, the time course of changes, and long‐term morpho‐functional outcome.

Cyclosporin a induces renal episodic hypoxia

Cyclosporin A (CsA) causes renal toxicity. The underlying mechanisms are incompletely understood, but may involve renal hypoxia and hypoxia‐inducible factors (Hifs). We sought for hypoxia and Hif in mouse kidneys with CsA‐induced toxicity, assessed their time course, Hif‐mediated responses and the impact of interventional Hif upregulation.

Stabilization of hypoxia inducible factor-1α ameliorates acute renal neurogenic hypertension

Objectives: Renal neurogenic hypertension (RNH) contributes to cardiovascular morbidity. Renal hypoxia may cause RNH and vice versa, leading to a vicious circle. Hypoxia adaptation is conferred through hypoxia-inducible factors (HIFs). We hypothesized that acute RNH is accompanied by increased renal vascular resistance (RVR) and that hypertension and increased RVR are countered by increasing HIF-1&agr; by cobalt chloride (CoCl2) preconditioning. Methods: First, we studied mean arterial pressure (MAP) and RVR in innervated or denervated contralateral kidneys in anesthetized rats before and after unilateral intrarenal injection of phenol, a manoeuvre known to elicit acute RNH. Then HIF&agr; was induced by CoCl2 in drinking water (2 mM, 10 days) after which we compared intrarenal isotonic saline or phenol injection on MAP and RVR in CoCl2 preconditioned and control rats. HIF-1&agr; was determined by immunohistochemistry. Results: Unilateral intrarenal phenol induced immediate rise in MAP and contralateral RVR, and comparable HIF-1&agr; upregulation in both kidneys, consistent with bi-renal hypoxia. Removing the phenol-injected kidney immediately normalized MAP. Contralateral renal denervation had no effect on the rise in MAP, but abrogated the contralateral increase in RVR, suggesting mediation by increased efferent nerve activity. Strong renal staining for HIF-1&agr; confirmed efficacy of CoCl2 preconditioning, and time-dependent increase in heme oxygenase-1 gene expression stabilization of HIF&agr;. CoCl2 preconditioning prior to phenol reduced both &Dgr;MAP (+10 ± 2 vs. +20 ± 3%, P = 0.015) and &Dgr;RVR (+21 ± 11 vs. +90 ± 26%, P = 0.003). Conclusion: Acute RNH leads to renal vasoconstriction and increased renal HIF-1&agr;. Increasing HIF-1&agr; by CoCl2 preconditioning ameliorates intrarenal phenol-induced RNH and renal vasoconstriction.

Urinary Calprotectin Differentiates Between Prerenal and Intrinsic Acute Renal Allograft Failure.

BackgroundUrinary calprotectin has recently been identified as a promising biomarker for the differentiation between prerenal and intrinsic acute kidney injury (AKI) in the nontransplant population. The present study investigates whether calprotectin is able to differentiate between these 2 entities in transplant recipients as well. MethodsUrinary calprotectin was assessed by enzyme-linked immunosorbent assay in 328 subjects including 125 cases of intrinsic acute allograft failure, 27 prerenal graft failures, 118 patients with stable graft function, and 58 healthy controls. Acute graft failure was defined as AKI stages 1 to 3 (Acute Kidney Injury Network criteria), exclusion criteria were obstructive uropathy, urothelial carcinoma, and metastatic cancer. The clinical differentiation of prerenal and intrinsic graft failure was performed either by biopsy or by a clinical algorithm including response to fluid repletion, history, physical examination, and urine dipstick examination. ResultsReasons for intrinsic graft failure comprised rejection, acute tubular necrosis, urinary tract infection/pyelonephritis, viral nephritis, and interstitial nephritis. Calprotectin concentrations of patients with stable graft function (50.4 ng/mL) were comparable to healthy controls (54.8 ng/mL, P = 0.70) and prerenal graft failure (53.8 ng/mL, P = 0.62). Median urinary calprotectin was 36 times higher in intrinsic AKI (1955 ng/mL) than in prerenal AKI (P < 0.001). Receiver-operating characteristic curve analysis revealed a high accuracy of calprotectin (area under the curve, 0.94) in the differentiation of intrinsic versus prerenal AKI. A cutoff level of 134.5 ng/mL provided a sensitivity of 90.4% and a specificity of 74.1%. Immunohistochemical stainings for calprotectin in renal allograft biopsy specimens confirmed the serological results. ConclusionsUrinary calprotectin is a promising biomarker for the differentiation of prerenal and intrinsic acute renal allograft failure.

Myoglobin facilitates angiotensin II induced constriction of renal afferent arterioles.

Vasoconstriction plays an important role in the development of acute kidney injury in rhabdomyolysis. We hypothesized that myoglobin enhances the angiotensin II (ANG II) response in afferent arterioles by increasing superoxide and reducing nitric oxide (NO) bioavailability. Afferent arterioles of C57Bl6 mice were isolated perfused, and vasoreactivity was analyzed using video microscopy. NO bioavailability, superoxide concentration in the vessel wall, and changes in cytosolic calcium were measured using fluorescence techniques. Myoglobin treatment (10-5 M) did not change the basal arteriolar diameter during a 20-min period compared with control conditions. NG-nitro-l-arginine methyl ester (l-NAME, 10-4 M) and l-NAME + myoglobin reduced diameters to 94.7 and 97.9% of the initial diameter, respectively. Myoglobin or l-NAME enhanced the ANG II-induced constriction of arterioles compared with control (36.6 and 34.2%, respectively, vs. 65.9%). Norepinephrine responses were not influenced by myoglobin. Combined application of myoglobin and l-NAME further facilitated the ANG II response (7.0%). Myoglobin or l-NAME decreased the NO-related fluorescence in arterioles similarly. Myoglobin enhanced the superoxide-related fluorescence, and tempol prevented this enhancement. Tempol also partly prevented the myoglobin effect on the ANG II response. Myoglobin increased the fura 2 fluorescence ratio (cytosolic calcium) during ANG II application (10-12 to 10-6 M). The results suggest that the enhanced afferent arteriolar reactivity to ANG II is mainly due to a myoglobin-induced increase in superoxide and associated reduction in the NO bioavailability. Signaling pathways for the augmented ANG II response include enhanced cytosolic calcium transients. In conclusion, myoglobin may contribute to the afferent arteriolar vasoconstriction in this rhabdomyolysis model.

Cyclosporine A induces endothelin‐converting enzyme‐1: Studies in vivo and in vitro

Cyclosporine A (CsA) induces renal vasoconstriction and hypoxia and enhances the expression of endothelin‐1 (ET‐1) pro‐hormone (pre‐pro‐ET‐1), plausibly leading to a feed‐forward loop of renal vasoconstriction, hypoxia and enhanced synthesis of the potent vasoconstrictor ET‐1. Endothelin‐converting enzyme (ECE)‐1 cleaves big endothelin to generate endothelin (ET)‐1 and is upregulated by hypoxia via hypoxia‐inducible factor (HIF). We hypothesized that in addition to the direct induction of ET‐1 synthesis, CsA might also intensify renal ECE‐1 expression, thus contributing to enhanced ET‐1 synthesis following CsA.