A novel assay to assess the effect of pharmaceutical compounds on the differentiation of podocytes

A YAP/TAZ-ARHGAP29-RhoA Signaling Axis Regulates Podocyte Protrusions and Integrin Adhesions

Glomerular disease due to podocyte malfunction is a major factor in the pathogenesis of chronic kidney disease. Identification of podocyte-specific signaling pathways is therefore a prerequisite to characterizing relevant disease pathways and developing novel treatment approaches. Here, we employed loss of function studies for EPB41L5 (Yurt) as a central podocyte gene to generate a cell type-specific disease model. Loss of Yurt in fly nephrocytes caused protein uptake and slit diaphragm defects. Transcriptomic and proteomic analysis of human EPB41L5 knockout podocytes demonstrated impaired mechanotransduction via the YAP/TAZ signaling pathway. Further analysis of specific inhibition of the YAP/TAZ-TEAD transcription factor complex by TEADi led to the identification of ARGHAP29 as an EPB41L5 and YAP/TAZ-dependently expressed podocyte RhoGAP. Knockdown of ARHGAP29 caused increased RhoA activation, defective lamellipodia formation, and increased maturation of integrin adhesion complexes, explaining similar phenotypes caused by loss of EPB41L5 and TEADi expression in podocytes. Detection of increased levels of ARHGAP29 in early disease stages of human glomerular disease implies a novel negative feedback loop for mechanotransductive RhoA-YAP/TAZ signaling in podocyte physiology and disease.

The protective role of peroxisome proliferator-activated receptor gamma in lipotoxic podocytes

Podocytes are specialized epithelial cells that maintain the glomerular filtration barrier. These cells are susceptible to lipotoxicity in the obese state and irreversibly lost during kidney disease leading to proteinuria and renal injury. PPARγ is a nuclear receptor whose activation can be renoprotective. This study examined the role of PPARγ in the lipotoxic podocyte using a PPARγ knockout (PPARγKO) cell line and since the activation of PPARγ by Thiazolidinediones (TZD) is limited by their side effects, it explored other alternative therapies to prevent podocyte lipotoxic damage. Wild-type and PPARγKO podocytes were exposed to the fatty acid palmitic acid (PA) and treated with the TZD (Pioglitazone) and/or the Retinoid X receptor (RXR) agonist Bexarotene (BX). It revealed that podocyte PPARγ is essential for podocyte function. PPARγ deletion reduced key podocyte proteins including podocin and nephrin while increasing basal levels of oxidative and ER stress causing apoptosis and cell death. A combination therapy of low-dose TZD and BX activated both the PPARγ and RXR receptors reducing PA-induced podocyte damage. This study confirms the crucial role of PPARγ in podocyte biology and that their activation in combination therapy of TZD and BX may be beneficial in the treatment of obesity-related kidney disease.

The ShGlomAssay Combines High-Throughput Drug Screening With Downstream Analyses and Reveals the Protective Role of Vitamin D3 and Calcipotriol on Podocytes

Chronic kidney disease (CKD) is a major public health burden affecting more than 500 million people worldwide. Podocytopathies are the main cause for the majority of CKD cases due to pathogenic morphological as well as molecular biological alterations of postmitotic podocytes. Podocyte de-differentiation is associated with foot process effacement subsequently leading to proteinuria. Since currently no curative drugs are available, high throughput screening methods using a small number of animals are a promising and essential tool to identify potential drugs against CKD in the near future. Our study presents the implementation of the already established mouse GlomAssay as a semi-automated high-throughput screening method-shGlomAssay-allowing the analysis of several hundreds of FDA-verified compounds in combination with downstream pathway analysis like transcriptomic and proteomic analyses from the same samples, using a small number of animals. In an initial prescreening we have identified vitamin D3 and its analog calcipotriol to be protective on podocytes. Furthermore, by using RT-qPCR, Western blot, and RNA sequencing, we found that mRNA and protein expression of nephrin, the vitamin D receptor and specific podocyte markers were significantly up-regulated due to vitamin D3- and calcipotriol-treatment. In contrast, kidney injury markers were significantly down-regulated. Additionally, we found that vitamin D3 and calcipotriol have had neither influence on the expression of the miR-21 and miR-30a nor on miR-125a/b, a miRNA described to regulate the vitamin D receptor. In summary, we advanced the established mouse GlomAssay to a semi-automated high-throughput assay and combined it with downstream analysis techniques by using only a minimum number of animals. Hereby, we identified the vitamin D signaling pathway as podocyte protective and to be counteracting their de-differentiation.

The podocyte-specific knockout of palladin in mice with a 129 genetic background affects podocyte morphology and the expression of palladin interacting proteins

Proper and size selective blood filtration in the kidney depends on an intact morphology of podocyte foot processes. Effacement of interdigitating podocyte foot processes in the glomeruli causes a leaky filtration barrier resulting in proteinuria followed by the development of chronic kidney diseases. Since the function of the filtration barrier is depending on a proper actin cytoskeleton, we studied the role of the important actin-binding protein palladin for podocyte morphology. Podocyte-specific palladin knockout mice on a C57BL/6 genetic background (PodoPalldBL/6-/-) were back crossed to a 129 genetic background (PodoPalld129-/-) which is known to be more sensitive to kidney damage. Then we analyzed the morphological changes of glomeruli and podocytes as well as the expression of the palladin-binding partners Pdlim2, Lasp-1, Amotl1, ezrin and VASP in 6 and 12 months old mice. PodoPalld129-/- mice in 6 and 12 months showed a marked dilatation of the glomerular tuft and a reduced expression of the mesangial marker protein integrin α8 compared to controls of the same age. Furthermore, ultrastructural analysis showed significantly more podocytes with morphological deviations like an enlarged sub-podocyte space and regions with close contact to parietal epithelial cells. Moreover, PodoPalld129-/- of both age showed a severe effacement of podocyte foot processes, a significantly reduced expression of pLasp-1 and Pdlim2, and significantly reduced mRNA expression of Pdlim2 and VASP, three palladin-interacting proteins. Taken together, the results show that palladin is essential for proper podocyte morphology in mice with a 129 background.

EP300/CBP is crucial for cAMP-PKA pathway to alleviate podocyte dedifferentiation via targeting Notch3 signaling

Podocyte injury is the hallmark of proteinuric glomerular diseases. Notch3 is neo-activated simultaneously in damaged podocytes and podocyte's progenitor cells of FSGS, indicating a unique role of Notch3. We previously showed that activation of cAMP-PKA pathway alleviated podocyte injury possibly via inhibiting Notch3 expression. However, the mechanisms are unknown. In the present study, Notch3 signaling was significantly activated in ADR-induced podocytes in vitro and in PAN nephrosis rats and patients with idiopathic FSGS in vivo, concomitantly with podocyte dedifferentiation. In cultured podocytes, pCPT-cAMP, a selective cAMP-PKA activator, dramatically blocked ADR-induced activation of Notch3 signaling as well as inhibition of cAMP-PKA pathway, thus alleviating the decreased cell viability and podocyte dedifferentiation. Bioinformatics analysis revealed EP300/CBP, a transcriptional co-activator, as a central hub for the crosstalk between these two signaling pathways. Additionally, CREB/KLF15 in cAMP-PKA pathway competed with RBP-J the major transcriptional factor of Notch3 signaling for binding to EP300/CBP. EP300/CBP siRNA significantly inhibited these two signaling transduction pathways and disrupted the interactions between the above major transcriptional factors. These data indicate a crucial role of EP300/CBP in regulating the crosstalk between cAMP-PKA pathway and Notch3 signaling and modulating the phenotypic change of podocytes, and enrich the reno-protective mechanisms of cAMP-PKA pathway.

Specific domain V reduction of beta-2-glycoprotein I induces protein flexibility and alters pathogenic antibody binding

Beta-2-glycoprotein I (β2GPI) is a blood protein and the major antigen in the autoimmune disorder antiphospholipid syndrome (APS). β2GPI exists mainly in closed or open conformations and comprises of 11 disulfides distributed across five domains. The terminal Cys288/Cys326 disulfide bond at domain V has been associated with different cysteine redox states. The role of this disulfide bond in conformational dynamics of this protein has not been investigated so far. Here, we report on the enzymatic driven reduction by thioredoxin-1 (recycled by Tris(2-carboxyethyl)phosphine; TCEP) of β2GPI. Specific reduction was demonstrated by Western blot and mass spectrometry analyses confirming majority targeting to the fifth domain of β2GPI. Atomic force microscopy images suggested that reduced β2GPI shows a slightly higher proportion of open conformation and is more flexible compared to the untreated protein as confirmed by modelling studies. We have determined a strong increase in the binding of pathogenic APS autoantibodies to reduced β2GPI as demonstrated by ELISA. Our study is relevant for understanding the effect of β2GPI reduction on the protein structure and its implications for antibody binding in APS patients.

From Infancy to Fancy: A Glimpse into the Evolutionary Journey of Podocytes in Culture

Podocytes are critical components of the filtration barrier and responsible for maintaining healthy kidney function. An assault on podocytes is generally associated with progression of chronic glomerular diseases. Therefore, podocyte pathophysiology is a favorite research subject for nephrologists. Despite this, podocyte research has lagged because of the unavailability of techniques for culturing such specialized cells ex vivo in quantities that are adequate for mechanistic studies. In recent years, this problem was circumvented by the efforts of researchers, who successfully developed several in vitro podocyte cell culture model systems that paved the way for incredible discoveries in the field of nephrology. This review sets us on a journey that provides a comprehensive insight into the groundbreaking breakthroughs and novel technologic advances made in the field of podocyte cell culture so far, beginning from its inception, evolution, and progression. In this study, we also describe in detail the pros and cons of different models that are being used to culture podocytes. Our extensive and exhaustive deliberation on the status of podocyte cell culture will facilitate researchers to choose wisely an appropriate model for their own research to avoid potential pitfalls in the future.

Comparative Analysis of Podocyte Foot Process Morphology in Three Species by 3D Super-Resolution Microscopy

Since the size selectivity of the filtration barrier and kidney function are highly dependent on podocyte foot process morphology, visualization of foot processes is important. However, the size of foot processes is below the optical resolution of light microscopy. Therefore, electron microcopy has been indispensable to detect changes in foot process morphology so far, but it is a sophisticated and time-consuming technique. Recently, our group has shown that 3D structured illumination microscopy (3D-SIM), a super-resolution microscopy (SRM) technique, can visualize individual foot processes in human biopsies. Moreover, we have developed a software-based approach to directly quantify the structure of podocyte foot processes named Podocyte Exact Morphology Measurement Procedure (PEMP). As shown in patients suffering from minimal change disease (MCD), PEMP allows the quantification of changes of the foot process morphology by measuring the filtration slit density (FSD). Since rodents are frequently used in basic research, we have applied PEMP to quantify foot processes of mice and rats. Comparative analysis of nephrin-stained kidneys from humans, rats, and mice showed significant differences of the FSD. The highest FSD was measured in mice (3.83 ± 0.37 μm⁻¹; mean ± SD) followed by rats (3.36 ± 0.42 μm⁻¹) and humans (3.11 ± 0.26 μm⁻¹). To demonstrate that PEMP can be used to determine foot process morphology also in affected animals, we measured the FSD in palladin-knockout mice on a 129S1 genetic background compared to wild-type littermates. Taken together, we established a method for the quick and exact quantification of podocyte foot process morphology which can be applied to diagnosis and basic research.

BDNF: mRNA expression in urine cells of patients with chronic kidney disease and its role in kidney function

Podocyte loss and changes to the complex morphology are major causes of chronic kidney disease (CKD). As the incidence is continuously increasing over the last decades without sufficient treatment, it is important to find predicting biomarkers. Therefore, we measured urinary mRNA levels of podocyte genes NPHS1, NPHS2, PODXL and BDNF, KIM‐1, CTSL by qRT‐PCR of 120 CKD patients. We showed a strong correlation between BDNF and the kidney injury marker KIM‐1, which were also correlated with NPHS1, suggesting podocytes as a contributing source. In human biopsies, BDNF was localized in the cell body and major processes of podocytes. In glomeruli of diabetic nephropathy patients, we found a strong BDNF signal in the remaining podocytes. An inhibition of the BDNF receptor TrkB resulted in enhanced podocyte dedifferentiation. The knockdown of the orthologue resulted in pericardial oedema formation and lowered viability of zebrafish larvae. We found an enlarged Bowman's space, dilated glomerular capillaries, podocyte loss and an impaired glomerular filtration. We demonstrated that BDNF is essential for glomerular development, morphology and function and the expression of BDNF and KIM‐1 is highly correlated in urine cells of CKD patients. Therefore, BDNF mRNA in urine cells could serve as a potential CKD biomarker.

An integrin antagonist (MK-0429) decreases proteinuria and renal fibrosis in the ZSF1 rat diabetic nephropathy model

Multiple integrins have been implicated in modulating renal function. Modulation of integrin function can lead to pathophysiological processes associated with diabetic nephropathy such as alterations in the glomerular filtration barrier and kidney fibrosis. The complexity of these pathophysiological changes implies that multiple integrin subtypes might need to be targeted to ameliorate the progression of renal disease. To address this hypothesis, we investigated the effects of MK‐0429, a compound that was originally developed as an αvβ3 inhibitor for the treatment of osteoporosis, on renal function and fibrosis. We demonstrated that MK‐0429 is an equipotent pan‐inhibitor of multiple av integrins. MK‐0429 dose‐dependently inhibited podocyte motility and also suppressed TGF‐β‐induced fibrosis marker gene expression in kidney fibroblasts. Moreover, in the obese ZSF1 rat model of diabetic nephropathy, chronic treatment with MK‐0429 resulted in significant reduction in proteinuria, kidney fibrosis, and collagen accumulation. In summary, our results suggest that inhibition of multiple integrin subtypes might lead to meaningful impact on proteinuria and renal fibrosis in diabetic nephropathy.

The Role of Palladin in Podocytes

Background Podocyte loss and effacement of interdigitating podocyte foot processes are the major cause of a leaky filtration barrier and ESRD. Because the complex three-dimensional morphology of podocytes depends on the actin cytoskeleton, we studied the role in podocytes of the actin bundling protein palladin, which is highly expressed therein.Methods We knocked down palladin in cultured podocytes by siRNA transfection or in zebrafish embryos by morpholino injection and studied the effects by immunofluorescence and live imaging. We also investigated kidneys of mice with podocyte-specific knockout of palladin (PodoPalld-/- mice) by immunofluorescence and ultrastructural analysis and kidney biopsy specimens from patients by immunostaining for palladin.Results Compared with control-treated podocytes, palladin-knockdown podocytes had reduced actin filament staining, smaller focal adhesions, and downregulation of the podocyte-specific proteins synaptopodin and α-actinin-4. Furthermore, palladin-knockdown podocytes were more susceptible to disruption of the actin cytoskeleton with cytochalasin D, latrunculin A, or jasplakinolide and showed altered migration dynamics. In zebrafish embryos, palladin knockdown compromised the morphology and dynamics of epithelial cells at an early developmental stage. Compared with PodoPalld+/+ controls, PodoPalld-/- mice developed glomeruli with a disturbed morphology, an enlarged subpodocyte space, mild effacement, and significantly reduced expression of nephrin and vinculin. Furthermore, nephrotoxic serum injection led to significantly higher levels of proteinuria in PodoPalld-/- mice than in controls. Kidney biopsy specimens from patients with diabetic nephropathy and FSGS showed downregulation of palladin in podocytes as well.Conclusions Palladin has an important role in podocyte function in vitro and in vivo.

The transcription factor Dach1 is essential for podocyte function

Dedifferentiation and loss of podocytes are the major cause of chronic kidney disease. Dach1, a transcription factor that is essential for cell fate, was found in genome‐wide association studies to be associated with the glomerular filtration rate. We found that podocytes express high levels of Dach1 in vivo and to a much lower extent in vitro. Parietal epithelial cells (PECs) that are still under debate to be a type of progenitor cell for podocytes expressed Dach1 only at low levels. The transfection of PECs with a plasmid encoding for Dach1 induced the expression of synaptopodin, a podocyte‐specific protein, demonstrated by immunocytochemistry and Western blot. Furthermore, synaptopodin was located along actin fibres in a punctate pattern in Dach1‐expressing PECs comparable with differentiated podocytes. Moreover, dedifferentiating podocytes of isolated glomeruli showed a significant reduction in the expression of Dach1 together with synaptopodin after 9 days in cell culture. To study the role of Dach1 in vivo, we used the zebrafish larva as an animal model. Knockdown of the zebrafish ortholog Dachd by morpholino injection into fertilized eggs resulted in a severe renal phenotype. The glomeruli of the zebrafish larvae showed morphological changes of the glomerulus accompanied by down‐regulation of nephrin and leakage of the filtration barrier. Interestingly, glomeruli of biopsies from patients suffering from diabetic nephropathy showed also a significant reduction of Dach1 and synaptopodin in contrast to control biopsies. Taken together, Dach1 is a transcription factor that is important for podocyte differentiation and proper kidney function.

Understanding Podocyte Biology to Develop Novel Kidney Therapeutics

Over the past two decades it has become increasing clear that injury and loss of podocytes is an early and common clinical observation presented in many forms of glomerulopathy and chronic kidney disease. Identification of disease-causing monogenic mutations in numerous podocyte-expressed genes as well as studies conducted using preclinical animal models have shown that the podocyte plays a central role in establishing kidney dysfunction. In this review, we summarize current knowledge regarding the potential for podocyte-targeted therapies and give our view on how a deeper understanding of the molecular makeup of the podocyte will enable future therapeutic interventions. Specifically, we recount some of the currently described podocentric strategies for therapy and summarize the status and evolution of various model systems used to facilitate our understanding of the molecular and functional underpinnings of podocyte biology.

The impact of α-Lipoic acid on cell viability and expression of nephrin and ZNF580 in normal human podocytes

Human podocytes (hPC) are essential for maintaining normal kidney function and dysfunction or loss of hPC play a pivotal role in the manifestation and progression of chronic kidney diseases including diabetic nephropathy. Previously, α-Lipoic acid (α-LA), a licensed drug for treatment of diabetic neuropathy, was shown to exhibit protective effects on diabetic nephropathy in vivo. However, the effect of α-LA on hPC under non-diabetic conditions is unknown. Therefore, we analyzed the impact of α-LA on cell viability and expression of nephrin and zinc finger protein 580 (ZNF580) in normal hPC in vitro. Protein analyses were done via Western blot techniques. Cell viability was determined using a functional assay. hPC viability was dynamically modulated via α-LA stimulation in a concentration-dependent manner. This was associated with reduced nephrin and ZNF580 expression and increased nephrin phosphorylation in normal hPC. Moreover, α-LA reduced nephrin and ZNF580 protein expression via 'kappa-light-chain-enhancer' of activated B-cells (NF-κB) inhibition. These data demonstrate that low α-LA had no negative influence on hPC viability, whereas, high α-LA concentrations induced cytotoxic effects on normal hPC and reduced nephrin and ZNF580 expression via NF-κB inhibition. These data provide first novel information about potential cytotoxic effects of α-LA on hPC under non-diabetic conditions.

A novel assay to assess the effect of pharmaceutical compounds on the differentiation of podocytes

BACKGROUND AND PURPOSE

Therapeutic options for treating glomerulopathies, the main cause of chronic kidney disease, are limited. Podocyte dedifferentiation is a major event in the pathogenesis of glomerulopathies. The goal of the present study was, therefore, to develop an assay to monitor podocyte differentiation suitable for compound screening.

EXPERIMENTAL APPROACH

We isolated and cultured glomeruli from transgenic mice, expressing cyan fluorescent protein (CFP) under the control of the promoter of nephrin, a marker of podocyte differentiation. Mean CFP fluorescence intensity per glomerulus (MFG) was determined by summation of all glomerular voxels from confocal z-stacks in the absence and presence of pharmaceutical compounds.

KEY RESULTS

In untreated cultured glomeruli, MFG remained fairly stable during the first 5 days, when foot processes were already effaced, and the level of many podocyte-specific proteins was only mildly affected, as revealed by proteomics. Between day 6 and 9, MFG decreased to almost zero. The decrease in MFG was paralleled by a decrease in CFP and nephrin expression, as determined by RT-PCR, western blots and proteomics. Puromycin aminonucleoside (PAN), which damages podocytes, concentration-dependently induced a complete loss of MFG. Dexamethasone (25 μM) and pioglitazone (10 μM) markedly attenuated the effect of 0.6 μg·mL^-1 PAN on MFG.

CONCLUSION AND IMPLICATIONS

In summary, we established a novel assay to assess the effect of pharmaceutical compounds on the differentiation of podocytes in situ. Our assay is suitable for compound screening to identify drugs for the treatment of glomerulopathies.