Filtration Barrier Function Research Articles

A zebrafish model of conditional targeted podocyte ablation and regeneration

Podocytes are specialized cells that contribute critically to the normal structure and function of the glomerular filtration barrier. Their depletion plays an important role in the pathogenesis of glomerulosclerosis. Here, we report generation of a genetic model of conditional podocyte ablation and regeneration in zebrafish using a bacterial nitroreductase strategy to convert a prodrug, Metronidazole, into a cytotoxic metabolite. A transgenic zebrafish line was generated that expresses a green fluorescence protein (GFP) and the nitroreductase fusion protein under the control of the podocin promoter Tg(podocin:nitroreductase-GFP). Treatment of these transgenic zebrafish with Metronidazole results in podocyte apoptosis, a loss of nephrin and podocin expression, foot process effacement, and a leaky glomerular filtration barrier. Following Metronidazole washout, proliferating cells were detected in the glomeruli of recovering transgenic fish with a restoration of nitroreductase-GFP fluorescence, nephrin and podocin expression, a reestablishment of normal foot process architecture and glomerular barrier function. Thus, our studies show that zebrafish podocytes are capable of regenerating following depletion and establish the Tg(podocin:NTR-GFP) fish as a new model to study podocyte injury and repair.

The role of the podocyte in albumin filtration

In the past decade, our understanding of the role of podocytes in the function of the glomerular filtration barrier, and of the role of podocyte injury in the pathogenesis of proteinuric kidney disease, has substantially increased. Landmark genetic studies identified mutations in genes expressed by podocytes as a cause of albuminuria and nephrotic syndrome, leading to breakthrough discoveries from many laboratories. These discoveries contributed to a dramatic change in our view of the glomerular filtration barrier of the kidney and of the role of podocyte injury in the development of albuminuria and progressive kidney disease. In the past several years, studies have demonstrated that podocyte injury is a major cause of marked albuminuria and nephrotic syndrome, and have confirmed that podocytes are important for the maintenance of an intact glomerular filtration barrier. An essential role of loss of these cells in the pathogenesis of glomerulosclerosis and progressive proteinuric kidney disease has also been identified. In this Review, we discuss the importance of podocytes for the maintenance of an intact glomerular filtration barrier and their role in albumin handling.

Podocytes Are Nonhematopoietic Professional Antigen-Presenting Cells

Podocytes are essential to the structure and function of the glomerular filtration barrier; however, they also exhibit increased expression of MHC class II molecules under inflammatory conditions, and they remove Ig and immune complexes from the glomerular basement membrane (GBM). This finding suggests that podocytes may act as antigen-presenting cells, taking up and processing antigens to initiate specific T cell responses, similar to professional hematopoietic cells such as dendritic cells or macrophages. Here, MHC-antigen complexes expressed exclusively on podocytes of transgenic mice were sufficient to activate CD8+ T cells in vivo. In addition, deleting MHC class II exclusively on podocytes prevented the induction of experimental anti-GBM nephritis. Podocytes ingested soluble and particulate antigens, activated CD4+ T cells, and crosspresented exogenous antigen on MHC class I molecules to CD8+ T cells. In conclusion, podocytes participate in the antigen-specific activation of adaptive immune responses, providing a potential target for immunotherapies of inflammatory kidney diseases and transplant rejection.

“Zebrafishing” for Novel Genes Relevant to the Glomerular Filtration Barrier

Data for genes relevant to glomerular filtration barrier function or proteinuria is continually increasing in an era of microarrays, genome-wide association studies, and quantitative trait locus analysis. Researchers are limited by published literature searches to select the most relevant genes to investigate. High-throughput cell cultures and other in vitro systems ultimately need to demonstrate proof in an in vivo model. Generating mammalian models for the genes of interest is costly and time intensive, and yields only a small number of test subjects. These models also have many pitfalls such as possible embryonic mortality and failure to generate phenotypes or generate nonkidney specific phenotypes. Here we describe an in vivo zebrafish model as a simple vertebrate screening system to identify genes relevant to glomerular filtration barrier function. Using our technology, we are able to screen entirely novel genes in 4–6 weeks in hundreds of live test subjects at a fraction of the cost of a mammalian model. Our system produces consistent and reliable evidence for gene relevance in glomerular kidney disease; the results then provide merit for further analysis in mammalian models.

Platelet-derived growth factor triggers PKA-mediated signalling by a redox-dependent mechanism in rat renal mesangial cells

Inflammatory glomerular kidney diseases are often accompanied with a massive production of reactive oxygen species (ROS) that affect the function of the glomerular filtration barrier and contribute to mesangiolysis via the induction of cell death in mesangial cells. Intriguingly, ROS also trigger fine-tuned signalling processes that affect gene expression and cell proliferation or migration. To define such redox-driven signalling devices, a proteomics approach was performed to identify the formation of protein complexes induced by ROS. To this end, protein lysates of human podocytes were treated with or without hydrogen peroxide (250μM). Thereafter cell lysates were subjected to diagonal 2D gel electrophoresis and putative redox-affected proteins were analysed by MS/MS analysis. Among others, the regulatory subunit of protein kinase A (PKA) could be identified that forms homodimers under oxidative conditions. To evaluate whether ROS dependent dimerization of PKA also occurs in a more physiological setting, rat mesangial cells were treated with platelet-derived growth factor-BB (PDGF-BB) to induce ROS formation. This regimen resulted in a redox dependent dimerization of the R-subunits of PKA. To demonstrate whether PDGF-BB induced ROS formation affects PKA dependent pathways, the effects of PDGF-BB on phosphorylation of serine 157 of vasodilator stimulated protein (VASP) a classical target of PKA were analysed. Interestingly PDGF-BB induced VASP phosphorylation in a ROS dependent manner but independent of changes in cAMP levels. Taken together, we demonstrate a redox-mediated activation of PKA by PDGF-BB thus highlighting a physiological role of ROS as regulator of PKA activity in rat mesangial cells.

Abstract 516: Knockdown of the Hypertension Associated Gene NOSTRIN Alters Glomerular Barrier Function in Zebrafish (Danio rerio)

The bioavailability of nitric oxide (NO) has been associated with the development and progression of vascular and renal disease. NOSTRIN (for eNOS Traffic Inducer) has primarily been recognized as one important regulator of eNOS, the prime source of NO in the cardiovascular system, with a possible role in the pathogenesis of pre-eclampsia and the development of increased intrahepatic resistance in liver disease. Here, we identified NOSTRIN in the center of a QTL-overlap region in rat and human trait loci that are associated with hypertension. Glomerular NOSTRIN expression is detectable in podocytes in human and rat glomeruli and podocytic NOSTRIN expression is diminished in hypertensive kidney disease. We show that knockdown of NOSTRIN alters the glomerular filtration barrier function in larval zebrafish, inducing proteinuria and leading to ultrastructural morphological changes on the endothelial as well as epithelial side and the GBM of the glomerular capillary loop. We also demonstrate that NOSTRIN interacts with proteins associated with the podocyte slit membrane. We conclude that NOSTRIN expression is an important factor for the integrity of the glomerular filtration barrier. Disease related alteration of NOSTRIN expression may not only affect the vascular endothelium and therefore contribute to endothelial cell dysfunction but may also contribute to the development of podocyte disease and proteinuria.

VEGF165b overexpression restores normal glomerular water permeability in VEGF164-overexpressing adult mice

Vascular endothelial growth factor (VEGF)-A, a family of differentially spliced proteins produced by glomerular podocytes, maintains glomerular filtration barrier function. The expression of VEGF molecules is altered in human nephropathy. We aimed to determine the roles of the angiogenic VEGF(164) isoform, and the antiangiogenic VEGF(165)b isoform in mature, adult glomeruli in vivo using conditional, inducible transgenic overexpression systems in mice. Podocyte-specific VEGF(164) overexpression (up to 100 days) was induced by oral administration of doxycycline to adult podocin-rtTA/TetO-VEGF(164) double transgenic mice. The consequences of simultaneous overexpression of VEGF(164) and VEGF(165)b were assessed in triple-transgenic podocin-rtTA/TetO-VEGF(164)/nephrin-VEGF(165)b mice. Persistent VEGF(164) overexpression did not cause proteinuria but did increase glomerular ultrafiltration coefficient between days 3 and 7. Despite persistently increased VEGF(164) levels, glomerular ultrafiltration coefficient normalized by day 14 and remained normal up to 100 days. Decreased subpodocyte space (SPS) coverage of the glomerular capillary wall accompanied increased glomerular hydraulic conductivity in VEGF(164)-overexpressing mice. The changes in glomerular ultrafiltration coefficient and SPS coverage induced by 7 days of overexpression of VEGF(164) were not present in triple transgenic VEGF(164) and VEGF(165)b overexpressing mice. These results indicate that 1) the adult mouse glomerulus is relatively resistant to induced VEGF(164) overexpression. VEGF(164) overexpression altered glomerular permeability but did not cause proteinuria in these mature, adult animals; 2) the SPS is a dynamic VEGF-responsive modulator of glomerular function; and 3) the balance of VEGF isoforms plays a critical role in the regulation of glomerular permeability. VEGF(165)b is capable of preventing VEGF(164)-induced changes in glomerular permeability and ultrastructure in vivo.

GLCCI1 single nucleotide polymorphisms in pediatric nephrotic syndrome

Empiric steroid therapy is the first-line therapy for pediatric nephrotic syndrome, but treatment response is variable. There are few predictors of steroid-responsiveness, although evidence for genetic factors does exist. Single nucleotide polymorphisms (SNPs) have been recently identified in the promoter region of glucocorticoid-induced transcript 1 gene (GLCCI1) which affect steroid-responsiveness in asthmatic patients. Independently, GLCCI1 was identified as a podocyte protein, the loss of which disrupts the function of the glomerular filtration barrier. We therefore examined whether SNPs associated with the steroid-responsive expression of GLCCI1 might predict steroid-responsiveness in nephrotic syndrome. A cohort of 211 pediatric patients with nephrotic syndrome and 102 controls were genotyped; among the cases, 117 were initial steroid responders, while 94 did not respond to oral steroids. No statistically significant differences were noted among the groups, although there was a trend based on the comparison of the small subgroups of steroid-responsive and non-responsive patients with biopsy-proven minimal change disease. While larger cohorts are needed to ascertain the possibility of a small effect of GLCCI1 SNPs on the steroid-responsiveness of nephrotic syndrome, the GLCCI1 SNPs associated with steroid-responsiveness in asthmatic patients are unlikely to have a clinically actionable impact in pediatric nephrotic syndrome.

A Photo Shoot of Proteinuria

Diseases of the renal glomerulus, the site of plasma filtration and the production of primary urine, account for a majority of chronic renal diseases.[1][1] The prevention of ESRD would require therapies that specifically interfere with the pathogenesis of the various underlying glomerular diseases

IL‐6 initiated cis‐signaling in cultured podocytes causes glomerular injury

The incidence of chronic kidney disease (CKD) parallels the global increase in obesity, diabetes and hypertension that characterize metabolic syndrome‐ a state of systemic inflammation. Glomerular dysfunction in CKD is marked by the loss of unique structure and function of podocytes in the glomerular filtration barrier. We hypothesized that IL‐6, an indicator of systemic inflammation, alters glomerular filtration barrier structure and function in a paracrine manner. Results show that IL‐6 (rIL‐6, 1–100pg/mL, 15min) alters glomerular barrier function demonstrated by increased glomerular albumin permeability (Palb) of isolated rat glomeruli using videomicroscopy. Increase in Palb (P<0.001, IL‐6 10pg/mL vs. Control) was blocked by α‐IL‐6 antibody. Expression of IL‐6 receptor components IL6‐Rα and gp130 in podocytes was established by immunoblotting and RT‐PCR. IL‐6 (100pg/mL, 15 min)‐induced signaling resulted in down‐regulation of ERK 1/2 phosphorylation (MAPK pathway) without an effect on Akt phosphorylation (PI3K‐Akt pathway). Finally, IL‐6 (1pg‐1ng/mL, 1hr)‐induced change in cell structure was demonstrated by confocal microscopy showing altered actin filaments and adhesion complexes in phalloidin (rhodamine) stained cells. We propose that increased IL‐6 contributes to podocyte injury observed in CKD through cellular IL‐6Rα and IL‐6Rα‐mediated cis‐signaling.

Understanding the Mechanisms of Proteinuria: Therapeutic Implications

A large body of evidence indicates that proteinuria is a strong predictor of morbidity, a cause of inflammation, oxidative stress and progression of chronic kidney disease, and development of cardiovascular disease. The processes that lead to proteinuria are complex and involve factors such as glomerular hemodynamic, tubular absorption, and diffusion gradients. Alterations in various different molecular pathways and interactions may lead to the identical clinical end points of proteinuria and chronic kidney disease. Glomerular diseases include a wide range of immune and nonimmune insults that may target and thus damage some components of the glomerular filtration barrier. In many of these conditions, the renal visceral epithelial cell (podocyte) responds to injury along defined pathways, which may explain the resultant clinical and histological changes. The recent discovery of the molecular components of the slit diaphragm, specialized structure of podocyte-podocyte interaction, has been a major breakthrough in understanding the crucial role of the epithelial layer of the glomerular barrier and the pathogenesis of proteinuria. This paper provides an overview and update on the structure and function of the glomerular filtration barrier and the pathogenesis of proteinuria, highlighting the role of the podocyte in this setting. In addition, current antiproteinuric therapeutic approaches are briefly commented.

Reversible cell-cycle entry in adult kidney podocytes through regulated control of telomerase and Wnt signaling.

Mechanisms of epithelial cell renewal remain poorly understood in the mammalian kidney, particularly in the glomerulus, a site of cellular damage in chronic kidney disease. Within the glomerulus, podocytes – differentiated epithelial cells critical for filtration – are thought to lack significant capacity for regeneration. Here, we show that podocytes rapidly lose differentiation markers and enter cell cycle in adult mice in which the telomerase protein component TERT is conditionally expressed. Transgenic TERT expression induces marked upregulation of Wnt signaling and disrupts glomerular structure resulting in a collapsing glomerulopathy resembling those in humans, including HIV-associated nephropathy (HIVAN). Human and mouse HIVAN kidneys show increased levels of TERT and activation of Wnt signaling, indicating that these are general features of collapsing glomerulopathies. Either silencing transgenic TERT expression or inhibition of Wnt signaling through systemic expression of the Wnt-inhibitor Dkk1 in TERT transgenic mice results in marked normalization of podocytes, including rapid cell cycle exit, re-expression of differentiation markers and improved filtration barrier function. These data reveal an unexpected property of podocytes to reversibly enter cell cycle, suggest that podocyte renewal may contribute to glomerular homeostasis and implicate the telomerase and Wnt/β-catenin pathways in podocyte proliferation and disease.

Purple corn anthocyanins retard diabetes-associated glomerulosclerosis in mesangial cells and db/db mice

Diabetic glomerulosclerosis is the hardening of the renal glomeruli that can lead to kidney failure. In the early stage of glomerulosclerosis occur renal mesangial expansion and renal filtration dysfunction. Purple corn has been classified as a functional food and is rich in anthocyanins exerting potential disease-preventive activities. The in vitro study using human renal mesangial cells examined that anthocyanin-rich purple corn butanol fraction (PCB) can attenuate high glucose (HG)-promoted mesangial cell proliferation and matrix accumulation. Cells were cultured for 3 days in media containing 33 mM glucose in the presence of 1-20 μg/mL PCB. In the in vivo animal study, db/db mice were treated with 10 mg/kg anthocyanin-rich polyphenolic extracts of purple corn (PCE) for 8 weeks. HG enhanced mesangial production of the fibrosis biomarkers of collagen IV and connective tissue growth factor (CTGF), which was markedly attenuated by adding PCB. Such mesangial fibrosis entailed interleukin-8 activation via eliciting Tyk2-STAT signaling pathway. PCB dampened HG-promoted mesangial hyperplasia that appeared to be attributed to increased expression of platelet-derived growth factor. The 8-week administration of PCE lowered plasma glucose level of db/db mice and ameliorated severe albuminuria. Moreover, PCE lessened collagen fiber accumulation in kidney glomeruli and CTGF expression via retarding TGF-β signaling. Protein expressions of nephrin and podocin, key proteins for filtration barrier function of the glomerular capillary wall, were repressed by treating mice with PCE. Purple corn may be a potent therapeutic agent for the treatment for diabetes-associated glomerulosclerosis accompanying proteinuria and kidney filtration dysfunction.

Calcium Mediates Glomerular Filtration through Calcineurin and mTORC2/Akt Signaling

Alterations to the structure of the glomerular filtration barrier lead to effacement of podocyte foot processes, leakage of albumin, and the development of proteinuria. To better understand the signaling pathways involved in the response of the glomerular filtration barrier to injury, we studied freshly isolated rat glomeruli, which allows for the monitoring and pharmacologic manipulation of early signaling events. Administration of protamine sulfate rapidly damaged the isolated glomeruli, resulting in foot process effacement and albumin leakage. Inhibition of calcium channels and chelation of extracellular calcium reduced protamine sulfate-induced damage, suggesting that calcium signaling plays a critical role in the initial stages of glomerular injury. Calcineurin inhibitors (FK506 and cyclosporine A) and the cathepsin L inhibitor E64 all inhibited protamine sulfate-mediated barrier changes, which suggests that calcium signaling acts, in part, through calcineurin- and cathepsin L-dependent cleavage of synaptopodin, a regulator of actin dynamics. The mTOR inhibitor rapamycin also protected glomeruli, demonstrating that calcium signaling has additional calcineurin-independent components. Furthermore, activation of Akt through mTOR had a direct role on glomerular barrier integrity, and activation of calcium channels mediated this process, likely independent of phosphoinositide 3-kinase. Taken together, these results demonstrate the importance of calcium and related signaling pathways in the structure and function of the glomerular filtration barrier.

An in vitro model of the glomerular capillary wall using electrospun collagen nanofibres in a bioartificial composite basement membrane.

The filtering unit of the kidney, the glomerulus, contains capillaries whose walls function as a biological sieve, the glomerular filtration barrier. This comprises layers of two specialised cells, glomerular endothelial cells (GEnC) and podocytes, separated by a basement membrane. Glomerular filtration barrier function, and dysfunction in disease, remains incompletely understood, partly due to difficulties in studying the relevant cell types in vitro. We have addressed this by generation of unique conditionally immortalised human GEnC and podocytes. However, because the glomerular filtration barrier functions as a whole, it is necessary to develop three dimensional co-culture models to maximise the benefit of the availability of these cells. Here we have developed the first two tri-layer models of the glomerular capillary wall. The first is based on tissue culture inserts and provides evidence of cell-cell interaction via soluble mediators. In the second model the synthetic support of the tissue culture insert is replaced with a novel composite bioartificial membrane. This consists of a nanofibre membrane containing collagen I, electrospun directly onto a micro-photoelectroformed fine nickel supporting mesh. GEnC and podocytes grew in monolayers on either side of the insert support or the novel membrane to form a tri-layer model recapitulating the human glomerular capillary in vitro. These models will advance the study of both the physiology of normal glomerular filtration and of its disruption in glomerular disease.

Nephrin expression in adult rodent central nervous system and its interaction with glutamate receptors

Nephrin is an immunoglobulin-like adhesion molecule first discovered as a major component of the podocyte slit diaphragm, where its integrity is essential to the function of the glomerular filtration barrier. Outside the kidney, nephrin has been shown in other restricted locations, most notably in the central nervous system (CNS) of embryonic and newborn rodents. With the aim of better characterizing nephrin expression and its role in the CNS of adult rodents, we studied its expression pattern and possible binding partners in CNS tissues and cultured neuronal cells and compared these data to those obtained in control renal tissues and podocyte cell cultures. Our results show that, besides a number of locations already found in embryos and newborns, endogenous nephrin in adult rodent CNS extends to the pons and corpus callosum and is expressed by granule cells and Purkinje cells of the cerebellum, with a characteristic alternating expression pattern. In primary neuronal cells we find nephrin expression close to synaptic proteins and demonstrate that nephrin co-immunoprecipitates with Fyn kinase, glutamate receptors and the scaffolding molecule PSD95, an assembly that is reminiscent of those made by synaptic adhesion molecules. This role seems to be confirmed by our findings of impaired maturation and reduced glutamate exocytosis occurring in Neuro2A cells upon nephrin silencing. Of note, we disclose that the very same nephrin interactions occur in renal glomeruli and cultured podocytes, supporting our hypothesis that podocytes organize and use similar molecular intercellular signalling modules to those used by neuronal cells.

Tyrosine Kinase Signaling in Kidney Glomerular Podocytes

During the last decade, several key molecules have been identified as essential components for the filtration barrier function of kidney glomerular podocytes. Mutations in genes encoding these molecules severely impair the podocyte architecture in the affected patients, leading to the development of proteinuria. Extensive investigations have been performed on the function of these molecules, which highlights the importance of tyrosine kinase signaling in the podocytes. An Src family tyrosine kinase, Fyn, plays a major role in this signaling pathway. Here, we review the current understanding of this important signal transduction system and its role in the development and the maintenance of podocytes.

Anion exchanger 1 in red blood cells and kidney: Band 3’s in a podThis paper is one of a selection of papers published in a Special Issue entitled CSBMCB 53rd Annual Meeting — Membrane Proteins in Health and Disease, and has undergone the Journal’s usual peer review process.

The bicarbonate/chloride exchanger 1 (AE1, Band 3) is abundantly expressed in the red blood cell membrane, where it is involved in gas exchange and functions as a major site of cytoskeletal attachment to the erythrocyte membrane. A truncated kidney isoform (kAE1) is highly expressed in type A intercalated cells of the distal tubules, where it is vital for urinary acidification. Recently, kAE1 has emerged as a novel physiologically significant protein in the kidney glomerulus. This minireview will discuss the known interactions of kAE1 in the podocytes and the possible mechanisms whereby this important multispanning membrane protein may contribute to the function of the glomerular filtration barrier and prevent proteinuria.

Over-expressing transient receptor potential cation channel 6 in podocytes induces cytoskeleton rearrangement through increases of intracellular Ca2+ and RhoA activation

Transient receptor potential cation channel 6 (TRPC6) is one of the key molecules for filtration barrier function of podocytes. Over-expression of TRPC6 in podocytes is frequently found in acquired or inherited proteinuric kidney diseases, and animal model over-expression of TRPC6 may lead to proteinuria. To investigate the impact of TRPC6 over-expression in podocytes on its function and its relation to proteinuria in kidney diseases, we over-expressed TRPC6 in mouse podocytes by transient transfection of TRPC6 cDNA plasmid, and observed their changes in foot processes, intracellular F-actin distribution, nephrin and synaptopodin expression, electrophysiology, RhoA activity and intracellular Ca(2+). In podocytes over-expressing TRPC6, cell processes were reduced remarkably in association with the derangement of cytoskeleton demonstrated by the abnormal distribution of intracellular F-actin. These cells also displayed a higher increase of intracellular Ca(2+) ion to the TRPC6 agonist 1-oleoyl-acetyl-sn-glycerol and a higher current in the patch-clamp experiment, down-regulation of nephrin and synaptopodin expression and increase of activated RhoA. These changes could be rescued by the treatment of the cells with U73122 to block TRPC6 channel or BAPTA-AM to chelate intracellular Ca(2+) ion. Additionally, the podocytes over-expressing TRPC6 treated with RhoA inhibitor Y-27632 showed an improvement in F-actin arrangement in the cells and increase of nephrin and synaptopodin expression. From these results, we therefore propose that over-expression of TRPC6 in podocytes may be one of the fundamental changes relating to the dysfunction of the slit diaphragm and proteinuria. Podocytes over-expressing TRPC6 may lead to higher intracellular Ca(2+) ion concentration in the presence of stimuli. The increase of intracellular Ca(2+) down-regulates the expression of two important molecules, nephrin on slit diaphragm and synaptopodin in cytoskeleton, and stimulates RhoA activity, which in turn causes F-actin derangement and the decrease of foot processes.

Molecular and Genetic Basis of Inherited Nephrotic Syndrome

Nephrotic syndrome is an heterogeneous disease characterized by increased permeability of the glomerular filtration barrier for macromolecules. Podocytes, the visceral epithelial cells of glomerulus, play critical role in ultrafiltration of plasma and are involved in a wide number of inherited and acquired glomerular diseases. The identification of mutations in nephrin and other podocyte genes as causes of genetic forms of nephrotic syndrome has revealed new important aspects of the pathogenesis of proteinuric kidney diseases and expanded our knowledge of the glomerular biology. Moreover, a novel concept of a highly dynamic slit diaphragm proteins is emerging. The most significant discoveries in our understanding of the structure and function of the glomerular filtration barrier are reviewed in this paper.