What is the Role of Soluble Urokinase-Type Plasminogen Activator in Renal Disease?

&lt;b&gt;&lt;i&gt;Context:&lt;/i&gt;&lt;/b&gt; Soluble urokinase-type plasminogen activator ­(suPAR) is an inflammatory signal with pleiotropic biological effects depending on context and post-translational modifications. Recently, [Hayek, et al: Nat Med 2017; 23: 945–953] it has been found that there is a link between suPAR and renal disease in several guises, and a key question is whether it is a driver or a marker of renal disease, and if so of which types of kidney damage. &lt;b&gt;&lt;i&gt;Subject of Review:&lt;/i&gt;&lt;/b&gt; Circulating suPAR has been postulated to cause acute proteinuric kidney disease, specifically focal segmental glomerulosclerosis (FSGS), though both the animal models and clinical data in the original reports have been challenged. More recently, suPAR levels are linked to the risk of progression of chronic kidney disease (CKD) by directly activating signaling pathways in the podocyte or as a sensitive early biomarker. &lt;b&gt;&lt;i&gt;Second Opinion:&lt;/i&gt;&lt;/b&gt; The evidence for suPAR as a driver of FSGS proteinuria is not yet established, and more needs to be done, particularly by measuring different circulating suPAR fragments and identifying “second hits,” to clarify if any role exists. The evidence for suPAR as a sensitive biomarker for CKD progression is currently stronger, and needs further independent validation as well as prospective serial suPAR measurements in CKD cohorts. Whether it is a surrogate marker for ongoing inflammatory drivers of kidney disease, or has direct mechanistic effects in podocytes will depend on further studies, as well as trials of suPAR removal or direct blockade of downstream pathways.</jats:p

Cell biology and genetics of minimal change disease

Minimal change disease (MCD) is an important cause of nephrotic syndrome and is characterized by massive proteinuria and hypoalbuminemia, resulting in edema and hypercholesterolemia. The podocyte plays a key role in filtration and its disruption results in a dramatic loss of function leading to proteinuria. Immunologic disturbance has been suggested in the pathogenesis of MCD. Because of its clinical features, such as recurrent relapse/remission course, steroid response in most patients, and rare familial cases, a genetic defect has been thought to be less likely in MCD. Recent progress in whole-exome sequencing reveals pathogenic mutations in familial cases in steroid-sensitive nephrotic syndrome (SSNS) and sheds light on possible mechanisms and key molecules in podocytes in MCD. On the other hand, in the majority of cases, the existence of circulating permeability factors has been implicated along with T lymphocyte dysfunction. Observations of benefit with rituximab added B cell involvement to the disease. Animal models are unsatisfactory, and the humanized mouse may be a good model that well reflects MCD pathophysiology to investigate suggested “T cell dysfunction” directly related to podocytes in vivo. Several candidate circulating factors and their effects on podocytes have been proposed but are still not sufficient to explain whole mechanisms and clinical features in MCD. Another circulating factor disease is focal segmental glomerulosclerosis (FSGS), and it is not clear if this is a distinct entity, or on the same spectrum, implicating the same circulating factor(s). These patients are mostly steroid resistant and often have a rapid relapse after transplantation. In clinical practice, predicting relapse or disease activity and response to steroids is important and is an area where novel biomarkers can be developed based on our growing knowledge of podocyte signaling pathways. In this review, we discuss recent findings in genetics and podocyte biology in MCD

Insulin induces bioenergetic changes and alters mitochondrial dynamics in podocytes

Diabetic nephropathy (DN) is one of the most frequent complications of diabetes. Early stages of DN are associated with hyperinsulinemia and progressive insulin resistance in insulin-sensitive cells, including podocytes. The diabetic environment induces pathological changes, especially in podocyte bioenergetics, which is tightly linked with mitochondrial dynamics. The regulatory role of insulin in mitochondrial morphology in podocytes has not been fully elucidated. Therefore, the main goal of the present study was to investigate effects of insulin on the regulation of mitochondrial dynamics and bioenergetics in human podocytes. Biochemical analyses were performed to assess oxidative phosphorylation efficiency by measuring the oxygen consumption rate (OCR) and glycolysis by measuring the extracellular acidification rate (ECAR). mRNA and protein expression were determined by real-time polymerase chain reaction and Western blot. The intracellular mitochondrial network was visualized by MitoTracker staining. All calculations were conducted using CellProfiler software. Short-term insulin exposure exerted inhibitory effects on various parameters of oxidative respiration and adenosine triphosphate production, and glycolysis flux was elevated. After a longer time of treating cells with insulin, an increase in mitochondrial size was observed, accompanied by a reduction of expression of the mitochondrial fission markers DRP1 and FIS1 and an increase in mitophagy. Overall, we identified a previously unknown role for insulin in the regulation of oxidative respiration and glycolysis and elucidated mitochondrial dynamics in human podocytes. The present results emphasize the importance of the duration of insulin stimulation for its metabolic and molecular effects, which should be considered in clinical and experimental studies of DN.</p

Overexpression of transcription factor FOXC2 in cultured human podocytes upregulates injury markers and increases motility

Obesity and diabetes-related kidney diseases associate with renal failure and cardiovascular morbidity, and represent a major health issue worldwide. However, the molecular mechanisms leading to their development remain poorly understood. We observed increased expression of transcription factor FoxC2 in the podocytes of obese Zucker rats that are insulin resistant and albuminuric. We also found that depletion of adiponectin, an adipocyte-derived hormone whose secretion is decreased in obesity, up regulated FOXC2 in differentiated human podocytes in vitro. Overexpression of FOXC2 in cultured human podocytes led to increased nuclear expression of FOXC2 associated with a change of cellular morphology. This was accompanied by upregulation of vimentin, a key mesenchymal marker, and active beta-catenin, associated with podocyte injury. We also observed re-organization of the actin cytoskeleton, disrupted localization of the tight junction protein ZO-1, and increased motility of podocytes overexpressing FOXC2. These data indicate that the expression of FOXC2 in podocytes needs to be tightly regulated, and that its overexpression induces a chain of cellular events leading to podocyte dysfunction. These changes may lead to podocyte detachment and depletion ultimately contributing to albuminuria. We also suggest a novel molecular mechanism linking obesity-induced decrease in adiponectin to podocyte dysfunction via upregulation of FOXC2. (C) 2015 Elsevier Inc. All rights reserved.Peer reviewe

Data supporting the regulation of FOXC2 in podocyte dysfunction

Abstract This data article shows the expression levels of specific podocyte injury markers and podocyte slit diaphragm protein nephrin in obese and lean Zucker rat glomeruli. It also contains information on the effect of the overexpression of transcription factor FOXC2 on the ratio of F- and G-actin and the expression level of ZO-1 in differentiated human podocytes. The article also shows data on the effect of treatments of differentiated podocytes with various factors associated with obesity and diabetes on the expression level of FOXC2. The detailed interpretation of these data and other aspects of podocyte injury mediated by upregulation of FOXC2 can be found in “Overexpression of transcription factor FOXC2 in cultured human podocytes upregulates injury markers and increases motility [1].Peer reviewe

Albumin uptake in human podocytes: a possible role for the cubilin-amnionless (CUBAM) complex

Abstract Albumin re-uptake is a receptor-mediated pathway located in renal proximal tubuli. There is increasing evidence of glomerular protein handling by podocytes, but little is known about the mechanism behind this process. In this study, we found that human podocytes in vitro are committed to internalizing albumin through a receptor-mediated mechanism even after exposure to low doses of albumin. We show that these cells express cubilin, megalin, ClC-5, amnionless and Dab2, which are partners in the tubular machinery. Exposing human podocytes to albumin overload prompted an increase in CUBILIN, AMNIONLESS and CLCN5 gene expression. Inhibiting cubilin led to a reduction in albumin uptake, highlighting its importance in this mechanism. We demonstrated that human podocytes are committed to performing endocytosis via a receptor-mediated mechanism even in the presence of low doses of albumin. We also disclosed that protein overload first acts on the expression of the cubilin-amnionless (CUBAM) complex in these cells, then involves the ClC-5 channel, providing the first evidence for a possible role of the CUBAM complex in albumin endocytosis in human podocytes

Disease causing mutations in inverted formin 2 regulate its binding to G-actin, F-actin capping protein (CapZ α-1) and profilin 2

Focal segmental glomerulosclerosis (FSGS) is a devastating form of nephrotic syndrome which ultimately leads to end stage renal failure (ESRF). Mutations in inverted formin 2 (INF2), a member of the formin family of actin-regulating proteins, have recently been associated with a familial cause of nephrotic syndrome characterized by FSGS. INF2 is a unique formin that can both polymerize and depolymerize actin filaments. How mutations in INF2 lead to disease is unknown. In the present study, we show that three mutations associated with FSGS, E184K, S186P and R218Q, reduce INF2 auto-inhibition and increase association with monomeric actin. Furthermore using a combination of GFP–INF2 expression in human podocytes and GFP-Trap purification coupled with MS we demonstrate that INF2 interacts with profilin 2 and the F-actin capping protein, CapZ α-1. These interactions are increased by the presence of the disease causing mutations. Since both these proteins are involved in the dynamic turnover and restructuring of the actin cytoskeleton these changes strengthen the evidence that aberrant regulation of actin dynamics underlies the pathogenesis of disease