Zyxin is important for the stability and function of podocytes, especially during mechanical stretch

Podocyte detachment due to mechanical stress is a common issue in hypertension-induced kidney disease. This study highlights the role of zyxin for podocyte stability and function. We have found that zyxin is significantly up-regulated in podocytes after mechanical stretch and relocalizes from focal adhesions to actin filaments. In zyxin knockout podocytes, we found that the loss of zyxin reduced the expression of vinculin and VASP as well as the expression of matrix proteins, such as fibronectin. This suggests that zyxin is a central player in the translation of mechanical forces in podocytes. In vivo, zyxin is highly up-regulated in patients suffering from diabetic nephropathy and in hypertensive DOCA-salt treated mice. Furthermore, zyxin loss in mice resulted in proteinuria and effacement of podocyte foot processes that was measured by super resolution microscopy. This highlights the essential role of zyxin for podocyte maintenance in vitro and in vivo, especially under mechanical stretch.

A Novel High-Content Screening Assay Identified Belinostat as Protective in a FSGS-Like Zebrafish Model

BACKGROUND

FSGS affects the complex three-dimensional morphology of podocytes, resulting in loss of filtration barrier function and the development of sclerotic lesions. Therapies to treat FSGS are limited, and podocyte-specific drugs are unavailable. To address the need for treatments to delay or stop FSGS progression, researchers are exploring the repurposing of drugs that have been approved by the US Food and Drug Administration (FDA) for other purposes.

METHODS

To identify drugs with potential to treat FSGS, we used a specific zebrafish screening strain to combine a high-content screening (HCS) approach with an in vivo model. This zebrafish screening strain expresses nitroreductase and the red fluorescent protein mCherry exclusively in podocytes (providing an indicator for podocyte depletion), as well as a circulating 78 kDa vitamin D-binding enhanced green fluorescent protein fusion protein (as a readout for proteinuria). To produce FSGS-like lesions in the zebrafish, we added 80 µ M metronidazole into the fish water. We used a specific screening microscope in conjunction with advanced image analysis methods to screen a library of 138 drugs and compounds (including some FDA-approved drugs) for podocyte-protective effects. Promising candidates were validated to be suitable for translational studies.

RESULTS

After establishing this novel in vivo HCS assay, we identified seven drugs or compounds that were protective in our FSGS-like model. Validation experiments confirmed that the FDA-approved drug belinostat was protective against larval FSGS. Similar pan-histone deacetylase inhibitors also showed potential to reproduce this effect.

CONCLUSIONS

Using an FSGS-like zebrafish model, we developed a novel in vivo HCS assay that identified belinostat and related pan-histone deacetylase inhibitors as potential candidates for treating FSGS.

Inhibition of the glucocorticoid receptor attenuates proteinuric kidney diseases in multiple species

BACKGROUND AND HYPOTHESIS

Glucocorticoids are the treatment of choice for proteinuric patients with minimal-change disease (MCD) and primary focal and segmental glomerulosclerosis (FSGS). Immunosuppressive as well as direct effects on podocytes are believed to mediate their actions. In this study, we analyzed the anti-proteinuric effects of inhibition of the glucocorticoid receptor (GR) in glomerular epithelial cells, including podocytes.

METHODS

We employed genetic and pharmacological approaches to inhibit the GR. Genetically, we used Pax8-Cre/GRfl/fl mice to specifically inactivate the GR in kidney epithelial cells. Pharmacologically, we utilized a glucocorticoid antagonist called mifepristone.

RESULTS

Genetic inactivation of GR, specifically in kidney epithelial cells, using Pax8-Cre/GRfl/fl mice, ameliorated proteinuria following protein overload. We further tested the effects of pharmacological GR inhibition in three models and species: the puromycin-aminonucleoside-induced nephrosis model in rats, the protein overload model in mice and the inducible transgenic NTR/MTZ zebrafish larvae with specific and reversible podocyte injury. In all three models, both pharmacological GR activation and inhibition consistently and significantly ameliorated proteinuria. Additionally, we translated our findings to humans, where three nephrotic adult patients with MCD or primary FSGS with contraindications or insufficient responses to corticosteroids, were treated with mifepristone. This treatment resulted in a clinically relevant reduction of proteinuria.

CONCLUSIONS

Thus, across multiple species and proteinuria models, both genetic and pharmacological GR inhibition was at least as effective as pronounced GR activation. While, the mechanism remains perplexing, GR inhibition may be a novel and targeted therapeutic approach to treat glomerular proteinuria potentially bypassing adverse actions of steroids.

Evaluation of the efficacy of cystinosin supplementation through CTNS mRNA delivery in experimental models for cystinosis

Messenger RNA (mRNA) therapies are emerging in different disease areas, but have not yet reached the kidney field. Our aim was to study the feasibility to treat the genetic defect in cystinosis using synthetic mRNA in cell models and ctns-/- zebrafish embryos. Cystinosis is a prototype lysosomal storage disorder caused by mutations in the CTNS gene, encoding the lysosomal cystine-H+ symporter cystinosin, and leading to cystine accumulation in all cells of the body. The kidneys are the first and the most severely affected organs, presenting glomerular and proximal tubular dysfunction, progressing to end-stage kidney failure. The current therapeutic standard cysteamine, reduces cystine levels, but has many side effects and does not restore kidney function. Here, we show that synthetic mRNA can restore lysosomal cystinosin expression following lipofection into CTNS-/- kidney cells and injection into ctns-/- zebrafish. A single CTNS mRNA administration decreases cellular cystine accumulation for up to 14 days in vitro. In the ctns-/- zebrafish, CTNS mRNA therapy improves proximal tubular reabsorption, reduces proteinuria, and restores brush border expression of the multi-ligand receptor megalin. Therefore, this proof-of-principle study takes the first steps in establishing an mRNA-based therapy to restore cystinosin expression, resulting in cystine reduction in vitro and in the ctns-/- larvae, and restoration of the zebrafish pronephros function.

Cell fate determinant Llgl1 is required for propagation of acute myeloid leukemia

Scribble complex proteins can influence cell fate decisions and self-renewal capacity of hematopoietic cells. While specific cellular functions of Scribble complex members are conserved in mammalian hematopoiesis, they appear to be highly context dependent. Using CRISPR/Cas9-based genetic screening, we have identified Scribble complex-related liabilities in AML including LLGL1. Despite its reported suppressive function in HSC self-renewal, inactivation of LLGL1 in AML confirms its relevant role for proliferative capacity and development of AML. Its function was conserved in human and murine models of AML and across various genetic backgrounds. Inactivation of LLGL1 results in loss of stemness-associated gene-expression including HoxA-genes and induces a GMP-like phenotype in the leukemia stem cell compartment. Re-expression of HoxA9 facilitates functional and phenotypic rescue. Collectively, these data establish LLGL1 as a specific dependency and putative target in AML and emphasizes its cell-type specific functions.

SuPAR mediates viral response proteinuria by rapidly changing podocyte function

Elevation in soluble urokinase receptor (suPAR) and proteinuria are common signs in patients with moderate to severe coronavirus disease 2019 (COVID-19). Here we characterize a new type of proteinuria originating as part of a viral response. Inoculation of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) causes increased suPAR levels and glomerulopathy in African green monkeys. Using an engineered mouse model with high suPAR expression, inhaled variants of SARS-CoV-2 spike S1 protein elicite proteinuria that could be blocked by either suPAR antibody or SARS-CoV-2 vaccination. In a cohort of 1991 COVID-19 patients, suPAR levels exhibit a stepwise association with proteinuria in non-Omicron, but not in Omicron infections, supporting our findings of biophysical and functional differences between variants of SARS-CoV-2 spike S1 protein and their binding to podocyte integrins. These insights are not limited to SARS-CoV-2 and define viral response proteinuria (VRP) as an innate immune mechanism and co-activation of podocyte integrins.

Morphological and behavioral analysis of Slc35f1-deficient mice revealed no neurodevelopmental phenotype

SLC35F1 is a member of the sugar-like carrier (SLC) superfamily that is expressed in the mammalian brain. Malfunction of SLC35F1 in humans is associated with neurodevelopmental disorders. To get insight into the possible roles of Slc35f1 in the brain, we generated Slc35f1-deficient mice. The Slc35f1-deficient mice are viable and survive into adulthood, which allowed examining adult Slc35f1-deficient mice on the anatomical as well as behavioral level. In humans, mutation in the SLC35F1 gene can induce a Rett syndrome-like phenotype accompanied by intellectual disability (Fede et al. Am J Med Genet A 185:2238-2240, 2021). The Slc35f1-deficient mice, however, display only a very mild phenotype and no obvious deficits in learning and memory as, e.g., monitored with the novel object recognition test or the Morris water maze test. Moreover, neuroanatomical parameters of neuronal plasticity (as dendritic spines and adult hippocampal neurogenesis) are also unaltered. Thus, Slc35f1-deficient mice display no major alterations that resemble a neurodevelopmental phenotype.

Super-Resolution Microscopy: A Technique to Revolutionize Research and Diagnosis of Glomerulopathies

Background

For decades, knowledge about glomerular (patho)physiology has been tightly linked with advances in microscopic imaging technology. For example, the invention of electron microscopy was required to hypothesize about the mode of glomerular filtration barrier function.

Summary

Super-resolution techniques, defined as fluorescence microscopy approaches that surpass the optical resolution limit of around 200 nm, have been made available to the scientific community. Several of these different techniques are currently in use in glomerular research. Using three-dimensional structured illumination microscopy, the exact morphology of the podocyte filtration slit can be morphometrically analyzed and quantitatively compared across samples originating from animal models or human biopsies.

Key Messages

Several quantitative image analysis approaches and their potential influence on glomerular research and diagnostics are discussed. By improving not only optical resolution but also information content and turnaround time, super-resolution microscopy has the potential to expand the diagnosis of glomerular disease. Soon, these approaches could be introduced into glomerular disease diagnosis.

ScoMorphoFISH: A deep learning enabled toolbox for single-cell single-mRNA quantification and correlative (ultra-)morphometry

Increasing the information depth of single kidney biopsies can improve diagnostic precision, personalized medicine and accelerate basic kidney research. Until now, information on mRNA abundance and morphologic analysis has been obtained from different samples, missing out on the spatial context and single‐cell correlation of findings. Herein, we present scoMorphoFISH, a modular toolbox to obtain spatial single‐cell single‐mRNA expression data from routinely generated kidney biopsies. Deep learning was used to virtually dissect tissue sections in tissue compartments and cell types to which single‐cell expression data were assigned. Furthermore, we show correlative and spatial single‐cell expression quantification with super‐resolved podocyte foot process morphometry. In contrast to bulk analysis methods, this approach will help to identify local transcription changes even in less frequent kidney cell types on a spatial single‐cell level with single‐mRNA resolution. Using this method, we demonstrate that ACE2 can be locally upregulated in podocytes upon injury. In a patient suffering from COVID‐19‐associated collapsing FSGS, ACE2 expression levels were correlated with intracellular SARS‐CoV‐2 abundance. As this method performs well with standard formalin‐fixed paraffin‐embedded samples and we provide pretrained deep learning networks embedded in a comprehensive image analysis workflow, this method can be applied immediately in a variety of settings.

FC076: Development of a High-Throughput in Vivo Drug Screening Assay Using a FSGS-Like Zebrafish Model

BACKGROUND AND AIMS

Focal segmental sclerosis (FSGS) is a severe histopathological condition which often results in end-stage renal disease. Loss of podocytes and severe changes of podocyte foot process morphology accompanied by proteinuria are the hallmarks of FSGS. Since no therapeutic agents are available until now, further efforts must be made to identify new drugs to treat this disease. The zebrafish larva is a widely used kidney research model as the morphology and function of zebrafish and mammalian glomeruli are very similar. Since our group has already established a FSGS-like zebrafish injury model based on the nitroreductase-metronidazole cell ablation system, we have developed a high-throughput based in vivo drug screening assay.

METHOD

We used a zebrafish screening strain that expresses mCherry and the bacterial enzyme nitroreductase in podocytes. Additionally, a circulating vitamin-D-binding eGFP fusion protein with a size of 78 kDa is expressed and serves as a read-out for proteinuria. Incubation with 80 µM metronidazole for 24 h at 4 days post-fertilization (dpf) induces an FSGS-like phenotype in these larvae. A library of 138 epigenetic drugs were screened and the results are presented herein. Groups of 12 larvae received injury induction as well as drug treatment and were transferred individually into 96-well plates prepared with custom agarose molds. Larvae were oriented laterally and imaged with a high-content screening microscope (Acquifer Imaging Machine). The eGFP signal in the caudal vasculature as well as the glomerular mCherry signal served as readout systems. At 6 dpf, the same larvae and regions were imaged again. Custom written FIJI codes automatically segmented and measured the fluorescence intensities of both readouts and timepoints. For each larva, intensity ratios were determined and the ratios of each treatment group served as the input for statistical analysis.

RESULTS

Treatment for 24 hours with 80 µM metronidazole resulted in a measurable decline of the eGFP signal in the vasculature and of the podocyte mCherry signal compared to the vehicle (DMSO) group. Simultaneous treatment of metronidazole with the 138 drugs in the epigenetics library resulted in beneficial outcomes in varying degrees (9 drugs were positive, 36 compounds were lethal).

CONCLUSION

This new in vivo semi-automated high-throughput compatible drug screening assay allows rapid identification of FSGS protective drugs. Here we screened 138 epigenetic drugs, which led to the identification of 9 potential drugs which have had a beneficial effect on podocytes and/or the glomerular filtration barrier.

MO064: Expression Studies on Magi2 in Different FSGS Models

BACKGROUND AND AIMS

MAGI2 is a tight junction protein and member of the membrane-associated guanylate kinase (MAGUK) family. In the glomerulus, it is exclusively expressed in podocytes and interacts with the slit diaphragm protein nephrin. Recent studies and podocyte-specific MAGI2 knockout mice have demonstrated the essential role of this protein in the glomerulus. The aim of this work was to study the expression of MAGI2 in different focal segmental glomerulosclerosis (FSGS) mouse models, in the GlomAssay, in a FSGS-like zebrafish model, and in human biopsies of patients suffering from minimal change disease and FSGS. Additionally, using CRISPR/Cas9 knockout of magi2 and nephrin in zebrafish, we investigated the influence of both proteins on each other’s localization.

METHOD

We used nephrotoxic serum (NTS) as well as uninephrectomy DOCA/salt-induced hypertension as murine models. The podocyte-specific nitroreductase/metronidazole zebrafish model was used to induce FSGS-like disease in zebrafish upon partial podocyte-depletion. MAGI2 expression was evaluated in kidney biopsies of patients affected by minimal change disease (MCD), primary and secondary FSGS. Slides were imaged using confocal laser scanning and super-resolution 3D-structured illumination microscopy (3D-SIM). Filtration slit density (FSD) was measured on human biopsies by PEMP (podocyte exact morphology measurement procedure). MAGI2/nephrin ratio measurements were performed with a customized macro for the ImageJ-based platform FIJI. Magi2 expression was studied in the GlomAssay. To evaluate the interaction between magi2 and nephrin in the zebrafish model, the respective proteins were knocked-out using CRISPR/Cas9. The knockout was validated by RT–PCR and the expression of these two proteins in both lines was analyzed by immunofluorescence staining. Proteinuria was assessed using the excretion of eGFP-tagged vitamin D-binding protein from the vasculature and reuptake in proximal tubule cells.

RESULTS

MAGI2 was expressed in a linear pattern along the filtration slit and colocalized with nephrin in all three species investigated (human, mouse and zebrafish) (3D-SIM in Fig. 1). MAGI2 was significantly downregulated in NTS-induced nephritis, as well as in uninephrectomized and DOCA/salt-treated mice. In the GlomAssay, Magi2 was significantly downregulated in dedifferentiated podocytes after 6 days in cell culture due to podocyte dedifferentiation. A reduction of MAGI2 was found in human biopsies of patients suffering from primary FSGS, but not in MCD and secondary FSGS, respectively. In addition, we found that magi2 was downregulated in zebrafish larvae using our FSGS-like zebrafish model. When we examined the effect of nephrin on magi2 expression and vice versa, we found that magi2 expression was unchanged in nephrin-KO larvae, whereas nephrin expression was significantly reduced in MAGI2a-KO larvae.

CONCLUSION

Here, we have shown that the expression of MAGI2 in the glomerulus of different species such as zebrafish, mice and humans is associated with the expression of nephrin. By using knockout larvae, we showed that nephrin expression is dependent on Magi2 and not vice versa. Since the expression of Magi2 seems to be specifically regulated depending on the podocytopathy investigated, we propose a role for MAGI2 to distinguish between different glomerulopathies.

Pathogenesis of vaccine-induced immune thrombotic thrombocytopenia (VITT)

Vaccine-induced immune thrombotic thrombocytopenia (VITT; synonym, thrombosis with thrombocytopenia syndrome, is associated with high-titer immunoglobulin G antibodies directed against platelet factor 4 (PF4). These antibodies activate platelets via platelet FcγIIa receptors, with platelet activation greatly enhanced by PF4. Here we summarize the current concepts in the pathogenesis of VITT. We first address parallels between heparin-induced thrombocytopenia and VITT, and provide recent findings on binding of PF4 to adenovirus particles and non-assembled adenovirus proteins in the 2 adenovirus vector-based COVID-19 vaccines, ChAdOx1 nCoV-19 and Ad26.COV2.S. Further, we discuss the potential role of vaccine constituents such as glycosaminoglycans, EDTA, polysorbate 80, human cell-line proteins and nucleotides as potential binding partners of PF4. The immune response towards PF4 in VITT is likely triggered by a proinflammatory milieu. Human cell-line proteins, non-assembled virus proteins, and potentially EDTA may contribute to the proinflammatory state. The transient nature of the immune response towards PF4 in VITT makes it likely that-as in heparin-induced thrombocytopenia -marginal zone B cells are key for antibody production. Once high-titer anti-PF4 antibodies have been formed 5 to 20 days after vaccination, they activate platelets and granulocytes. Activated granulocytes undergo NETosis and the released DNA also forms complexes with PF4, which fuels the Fcγ receptor-dependent cell activation process, ultimately leading to massive thrombin generation. Finally, we summarize our initial observations indicating that VITT-like antibodies might also be present in rare patients with recurrent venous and arterial thrombotic complications, independent of vaccination.

Comparative analysis of ChAdOx1 nCoV-19 and Ad26.COV2.S SARS-CoV-2 vector vaccines

Vector-based SARS-CoV-2 vaccines have been associated with vaccine- induced thrombosis with thrombocytopenia syndrome (VITT/TTS), but the causative factors are still unresolved. We comprehensively analyzed the ChAdOx1 nCoV-19 (AstraZeneca) and Ad26.COV2.S (Johnson & Johnson) vaccines. ChAdOx1 nCoV-19 contains significant amounts of host cell protein impurities, including functionally active proteasomes, and adenoviral proteins. A much smaller amount of impurities was found in Ad26.COV2.S. Platelet factor 4 formed complexes with ChAdOx1 nCoV-19 constituents, but not with purified virions from ChAdOx1 nCoV-19 or with Ad26.COV2.S. Vascular hyperpermeability was induced by ChAdOx nCoV-19 but not by Ad26.COV2.S. These differences in impurities together with EDTAinduced capillary leakage might contribute to the higher incidence rate of VITT associated with ChAdOx1 nCoV-19 compared to Ad26.COV2.S.

Studies on the Role of the Transcription Factor Tcf21 in the Transdifferentiation of Parietal Epithelial Cells into Podocyte-Like Cells

BACKGROUND/AIMS

Podocyte differentiation is essential for proper blood filtration in the kidney. It is well known that transcription factors play an essential role to maintain the differentiation of podocytes. The present study is focused on the basic helix-loop-helix (bHLH) transcription factor Tcf21 (Pod1) which is essential for the development of podocytes in vivo. Since parietal epithelial cells (PECs) are still under debate to be progenitor cells which can differentiate into podocytes, we wanted to find out whether the expression of Tcf21 induces a transition of PECs into podocytes.

METHODS

We transfected PECs with Tcf21-GFP and analyzed the expression of PEC- and podocyte-specific markers. Furthermore, we performed ChIP-Seq analysis to identify new putative interaction partners and target genes of Tcf21.

RESULTS

By gene arrays analysis, we found that podocytes express high levels of Tcf21 in vivo in contrast to cultured podocytes and parietal epithelial cells (PECs) in vitro. After the expression of Tcf21 in PECs, we observed a downregulation of specific PEC markers like caveolin‑1, β-catenin and Pax2. Additionally, we found that the upregulation of Tcf21 induced multi-lobulation of cell nuclei, budding and a formation of micronuclei (MBM). Furthermore, a high number of PECs showed a tetraploid set of chromosomes. By qRT-PCR and Western blot analysis, we revealed that the transcription factor YY1 is downregulated by Tcf21. Interestingly, co-expression of YY1 and Tcf21 rescues MBM and reduced tetraploidy. By ChIP-Seq analysis, we identified a genome-wide Tcf21-binding site (CAGCTG), which matched the CANNTG sequence, a common E-box binding motif used by bHLH transcription factors. Using this technique, we identified additional Tcf21 targets genes that are involved in the regulation of the cell cycle (e.g. Mdm2, Cdc45, Cyclin D1, Cyclin D2), on the stability of microtubules (e.g. Mapt) as well as chromosome segregation.

CONCLUSION

Taken together, we demonstrate that Tcf21 inhibits the expression of PEC-specific markers and of the transcription factor YY1, induces MBM as well as regulates the cell cycle suggesting that Tcf21 might be important for PEC differentiation into podocyte-like cells.

Super-resolved local recruitment of CLDN5 to filtration slits implicates a direct relationship with podocyte foot process effacement

Under healthy conditions, foot processes of neighbouring podocytes are interdigitating and connected by an electron‐dense slit diaphragm. Besides slit diaphragm proteins, typical adherens junction proteins are also found to be expressed at this cell‐cell junction. It is therefore considered as a highly specialized type of adherens junction. During podocyte injury, podocyte foot processes lose their characteristic 3D structure and the filtration slits typical meandering structure gets linearized. It is still under debate how this change of structure leads to the phenomenon of proteinuria. Using super‐resolution 3D‐structured illumination microscopy, we observed a spatially restricted up‐regulation of the tight junction protein claudin‐5 (CLDN5) in areas where podocyte processes of patients suffering from minimal change disease (MCD), focal and segmental glomerulosclerosis (FSGS) as well as in murine nephrotoxic serum (NTS) nephritis and uninephrectomy DOCA‐salt hypertension models, were locally injured. CLDN5/nephrin ratios in human glomerulopathies and NTS‐treated mice were significantly higher compared to controls. In patients, the CLDN5/nephrin ratio is significantly correlated with the filtration slit density as a foot process effacement marker, confirming a direct association of local CLDN5 up‐regulation in injured foot processes. Moreover, CLDN5 up‐regulation was observed in some areas of high filtration slit density, suggesting that CLND5 up‐regulation preceded the changes of foot processes. Therefore, CLDN5 could serve as a biomarker predicting early foot process effacement.

FC 017DEEP-LEARNING ENABLED QUANTIFICATION OF SINGLE-CELL SINGLE-MRNA TRANSCRIPTS AND CORRELATIVE SUPER-RESOLVED PODOCYTE FOOT PROCESS MORPHOMETRY IN ROUTINE KIDNEY BIOPSY SPECIMEN

Background and Aims

Although high-throughput single-cell transcriptomic analysis, super-resolution light microscopy and deep-learning methods are broadly used, the gold-standard to evaluate kidney biopsies is still the histologic assessment of formalin-fixed and paraffin embedded (FFPE) samples with parallel ultrastructural evaluation. Recently, we and others have shown that super-resolution fluorescence microscopy can be used to study glomerular ultrastructure in human biopsy samples. Additionally, in the last years mRNA in situ hybridization techniques have been improved to increase specificity and sensitivity to enable transcriptomic analysis with single-mRNA resolution (smFISH).

Method

For smFISH, we used the fluorescent multiplex RNAscope kit with probes targeting ACE2, WT1, PPIB, UBC and POLR2A. To find an on-slide reference gene, the normfinder algorithm was used. The smFISH protocol was combined with a single-step anti-podocin immunofluorescence enabled by VHH nanobodies. Podocytes were labeled by tyramide-signal amplified immunofluorescence using recombinant anti-WT1 antibodies. Slides were imaged using confocal laser scanning, as well as 3D structured illumination microscopy. Deep-learning networks to segment glomeruli and cell nuclei (UNet and StarDist) were trained using the ZeroCostDL4Mic approach. Scripts to automate analysis were developed in the ImageJ1 macro language.

Results

First, we show robust functionality of threeplex smFISH in archived routine FFPE kidney biopsy samples with single-mRNA resolution. As variations in sample preparation can negatively influence mRNA-abundance, we established PPIB as an ideal on-slide reference gene to account for different RNA-integrities present in biopsy samples. PPIB was chosen for its most stable expression in microarray dataset of various glomerular diseases determined by the Normfinder algorithm as well as its smFISH performance.

To segment glomeruli and to label glomerular and tubulointerstitial cell subsets, we established a combination of smFISH and immunofluorescence. As smFISH requires intense tissue digestion to liberate cross-linked RNAs, immunofluorescence protocols had to be adapted: For podocin, a small-sized single-step label approach enabled by small nanobodies and for WT1, tyramide signal amplification was used.

For enhanced segmentation performance, we used deep learning: First, a network was customized to recognize DAPI+ cell nuclei and WT1/DAPI+ podocyte nuclei. Second, a UNet was trained to segment glomeruli in podocin-stained tissue sections. Using these segmentation masks, we could annotate PPIB-normalized single mRNA transcripts to individual cells. We established an ImageJ script to automatize transcript quantification.

As a proof-of-principle, we demonstrate inverse expression of WT1 and ACE2 in glomerular vs. tubulointerstitial single cells. Furthermore, in the podocyte subset, WT1 highly clustered whereas no significant ACE2 expression was found under baseline conditions.

Additionally, when imaged with super-resolution microscopy, podocyte filtration slit morphology could be visualized The optical resolution was around 125 nm and therefore small enough to resolve individual foot processes. The filtration slit density as a podocyte-integrity marker did not differ significantly from undigested tissue sections proving the suitability for correlative podocyte foot process morphometry with single-podocyte transcript analysis.

Conclusion

Here we present a modular toolbox which combines algorithms for multiplexed, normalized single-cell gene expression with single mRNA resolution in cellular subsets (glomerular, tubulointerstitial and podocytes). Additionally, this approach enables correlation with podocyte filtration slit ultrastructure and gross glomerular morphometry.

MO030IDENTIFICATION OF REGULATED MRNAS AND MIRNAS IN GLOMERULI ISOLATED FROM AN FSGS-LIKE ZEBRAFISH MODEL

Background and Aims

The zebrafish (Danio rerio) is a powerful animal model to study glomerular morphology and the function of the permselectivity of the glomerular filtration barrier.

Since zebrafish larvae develop quickly and can be bred to become transparent, in vivo observation of these animals is possible. At 48 hours post fertilization, zebrafish larvae develop a single glomerulus which is attached to a pair of tubules. Like in mammals, the glomerular filtration barrier consists of a fenestrated endothelium, the glomerular basement membrane and interdigitating podocyte foot processes bridged by a slit diaphragm. By using genetically modified zebrafish strains with fluorescently labeled podocytes, it is possible to study alterations of the glomerulus during the development of renal disease like focal segmental glomerulosclerosis (FSGS) directly in vivo. FSGS is characterized by podocyte loss, the effacement of their foot processes as well as scarring of the glomerulus. To study FSGS in zebrafish larvae, we induced podocyte detachment by the use of a zebrafish strain expressing the enzyme nitroreductase converting metronidazole into a toxic substance specifically in podocytes. The aim of our study was to collect glomeruli for the identification of mRNAs as well as miRNAs by RNA_Seq that are up- and down-regulated in the glomeruli of this FSGS-like disease model.

Method

The transgenic zebrafish strain Cherry (Tg(nphs2:GAL4); Tg(UAS:Eco.nfsB-mCherry); mitfaw2/w2; mpv17a9/a9) which expresses the prokaryotic enzyme nitroreductase (NTR) fused to mCherry, a red fluorescent protein, under the control of the podocyte-specific podocin (nphs2) promoter in a transparent zebrafish strain, was utilized. After addition of metronidazole (MTZ) into the tank water, MTZ is converted into a cytotoxin by NTR leading to dose-dependent apoptosis exclusively in podocytes. Cherry larvae were treated at 4 days post fertilization (dpf) for 48 h with 80 µM MTZ. MTZ-treated and control larvae were homogenized at 6 dpf. The cell suspension was diluted, and red-fluorescent glomeruli were collected using a micropipette and a microscope. Total RNA was isolated, and integrity was checked by a Bioanalyzer. Libraries were generated with a MACE kit and True Quant small RNA seq kit by GenXPro. Constructs were amplified by PCR and sequenced on an Illumina Hiseq 2000. Normalization and statistical analysis for differential gene expression were done using DESeq2.

Results

Zebrafish larvae showed severe whole-body edema, proteinuria, loss of podocytes and an increased mortality rate after MTZ-treatment. The glomerular histology resembled mammalian FSGS. We found that only the RNA of manually collected glomeruli had an excellent quality. Using RNA_Seq, we identified a total of 16941 genes. DESeq2 analysis showed 494 up-regulated and 473 down-regulated genes. Gene ontology (GO) enrichment analysis of up-regulated genes revealed a total of 167 that are significantly enriched in GO terms (e.g. metabolic processes, immune response and ion transport). Down-regulated genes were enriched in 14 GO terms and most of them are linked to normal glomerular function and the slit diaphragm. DESeq2 analysis identified 200 miRNAs of 777 small RNAs. Some of these miRNA are already described to be regulated in different glomerular diseases like FSGS, lupus nephritis, IgA nephropathy and diabetic nephropathy.

Conclusion

We analyzed isolated glomeruli from transgenic zebrafish larvae that developed a FSGS-like disease. By sequencing, we have found mRNAs and miRNAs that were significantly regulated after the onset of disease. Detailed knowledge of these mRNAs and miRNA-based gene regulation will help to uncover the pathomechanism as well as to develop therapeutics for the treatment of FSGS.

Prolonged podocyte depletion in larval zebrafish resembles mammalian focal and segmental glomerulosclerosis

Focal and segmental glomerulosclerosis (FSGS) is a histological pattern frequently found in patients with nephrotic syndrome that often progress to end-stage kidney disease. The initial step in development of this histologically defined entity is injury and ultimately depletion of podocytes, highly arborized interdigitating cells on the glomerular capillaries with important function for the glomerular filtration barrier. Since there are still no causal therapeutic options, animal models are needed to develop new treatment strategies. Here, we present an FSGS-like model in zebrafish larvae, an eligible vertebrate model for kidney research. In a transgenic zebrafish strain, podocytes were depleted, and the glomerular response was investigated by histological and morphometrical analysis combined with immunofluorescence staining and ultrastructural analysis by transmission electron microscopy. By intravenous injection of fluorescent high-molecular weight dextran, we confirmed leakage of the size selective filtration barrier. Additionally, we observed severe podocyte foot process effacement of remaining podocytes, activation of proximal tubule-like parietal epithelial cells identified by ultrastructural cytomorphology, and expression of proximal tubule markers. These activated cells deposited extracellular matrix on the glomerular tuft which are all hallmarks of FSGS. Our findings indicate that glomerular response to podocyte depletion in larval zebrafish resembles human FSGS in several important characteristics. Therefore, this model will help to investigate the disease development and the effects of potential drugs in a living organism.

P0420PARTIAL PHARMACOGENETIC PODOCYTE DEPLETION IN A HIGH-THROUGHPUT ZEBRAFISH MODEL RESEMBLES HUMAN FOCAL AND SEGMENTAL GLOMERULOSCLEROSIS

Background and Aims

Although focal and segmental glomerulosclerosis (FSGS) has been in the scientific focus for many years, it is still a massive burden for patients with no causal therapeutic option. In FSGS, glomerular podocytes are injured, parietal epithelial cells (PECs) are activated and engage in the formation of cellular lesions leading to progressive glomerular scarring. Therefore, novel drug-screening assays are needed. Unfortunately, simple cellular in vitro-based screening assays are not ideal as glomerular architecture and crosstalk between glomerular cells is insufficiently modelled. Therefore, reliable animal models are still required for drug development, which unfortunately are not ideal for high-throughput applications. To date, due to its size, easy maintenance and breeding, zebrafish larvae are the simplest vertebrate model that are used in high-content screenings. Until today, it was unclear whether zebrafish can be used as a model for human FSGS. We therefore aimed to investigate whether partial podocyte-depletion in larval zebrafish leads to formation of FSGS-like disease and if the model can be used for screening purposes.

Method

We used a transgenic zebrafish model of pharmacogenetic podocyte depletion: In the Tg(nphs2:GAL4), Tg(UAS:Eco.nfsb-mCherry) strain, podocytes express the bacterial nitroreductase under control of the podocin promotor and can be dose-dependently ablated upon administration of metronidazole. Proteinuria was quantified using in vivo confocal laser scanning microscopy of intravenously administered high-molecular-weight fluorescent dextran. Plastic-embedded larvae where histologically and morphometrically assessed using HE, PAS and Jone’s silver staining after metronidazole washout. Glomerular ultrastructure was assessed using transmission electron microscopy of ultrathin sections. Immunofluorescence staining was carried out on kryosections to investigate extracellular matrix deposition (collagen-1, laminin), cellular proliferation (pcna) as well as parietal cell origin and activation (pax2a).

Results

To partially deplete podocytes, larvae where treated with 80 µM metronidazole from 4-6 days post fertilization, so that a subset of podocytes was depleted. In contrast to controls, podocyte-depleted larvae developed severe whole-body edema (Fig. A). Dynamic in vivo imaging of intravascular 500 kDa fluorescent dextran revealed massive leakage of the glomerular filtration barrier. Ultrastructural and immunofluorescent evaluation showed broad foot process effacement of remaining podocytes (Fig. D) and massive decrease of the slit diaphragm component podocin. Moreover, we found numerous sub-podocyte space pseudocysts (asterisk in Fig. D), microvillous transformation and formation of podocytic tight junctions as well as parietovisceral adhesions of the two layers of Bowman’s capsule. Parietal epithelial cells where activated, changed their phenotype towards a cuboidal shape, began to proliferate as demonstrated by pcna immunofluorescence and where recruited to cellular lesions on the glomerular tuft as demonstrated by the presence of cuboidal pax2a+ cells on the glomerular tuft (arrowheads Fig. B). Moreover, we found significant extracellular matrix deposition by the pax2a+ cells as demonstrated by Jone’s silver staining and laminin immunofluorescence (Fig. C).

Conclusion

Herein we show that upon podocyte-depletion, zebrafish larvae develop important functional and morphological features of human FSGS such as severe proteinuria and edema, podocyte foot process effacement, activation of parietal epithelial cells which contribute to cellular lesions and deposit extracellular matrix on the glomerular tuft. We conclude that this model resembles the human disease in important features and therefore propose its applicability for a high-throughput drug screening assay for FSGS.

P0053A METHOD FOR THE ISOLATION OF FLUORESCENT GLOMERULI FROM ZEBRAFISH LARVAE

Background and Aims

The zebrafish is a powerful animal model to study the glomerular morphology and the function of the permselectivity of the glomerular filtration barrier. Since zebrafish larvae develop quickly and can be bred to transparency, in vivo observation of these animals is possible. At 48 hours post fertilization (dpf), zebrafish develop a single filtering glomerulus which is attached to a pair of renal tubules. Like in mammals, the glomerular filtration barrier consists of a fenestrated endothelium, the glomerular basement membrane (GBM) and interdigitating podocyte foot processes bridged by a molecularly conserved slit diaphragm. By the use of genetically modified zebrafish strains with fluorescently labeled podocytes, it is possible to study alterations of the glomerulus during the development of renal disease directly in vivo and in vitro. As an injury model we used the nitroreductase/metronidazole (NTR/MTZ) zebrafish line to induce podocyte apoptosis and detachment from the GBM. Moreover, treatment of these larvae with MTZ induces glomerular injury that mimics focal segmental glomerulosclerosis (FSGS). The aim of our study was to establish a glomeruli isolation method which allows us to identify deregulation of miRNAs and mRNAs in the injured glomeruli by sequencing.

Method

The transgenic zebrafish strain Cherry (Tg(nphs2:Eco.nfsB-mCherry); mitfaw2/w2; mpv17a9/a9) which expresses the prokaryotic enzyme nitroreductase (NTR) fused to mCherry, a red fluorescent protein, under the control of the podocyte-specific podocin (nphs2) promoter in a transparent zebrafish strain, was used. The NTR/MTZ is a model of cell ablation to mimic podocyte injury. The prodrug MTZ (80 µM) is converted into a cytotoxin by NTR leading to a dose-dependent apoptosis exclusively in NTR-expressing podocytes. To induce podocyte injury, we treated Cherry larvae at 4 days post fertilization with MTZ (80 µM) freshly dissolved in 0.1% DMSO-E3 medium for 48 hours. Control larvae were treated with 0.1% DMSO-E3 medium. The treatment was stopped by a MTZ washout at 6 dpf.

In order to perform the miRNA and mRNA sequencing on glomeruli isolated from MTZ-treated and control larvae we tried to establish a method to obtain total RNA samples of good quality. For this purpose, three different approaches were tested and validated: 1) Sieving method, 2) Fluorescence-Activated Cell Sorting method (FACS), and 3) manual isolation of glomeruli by using a micropipette.

Results

Zebrafish larvae developed a glomerular damage similar to FSGS after MTZ-treatment. MTZ-treated larvae showed severe pericardial edema, a reduction of the nephrin and podocin expression, proteinuria and an increased mortality rate at 8 dpf.

After many tests we showed that glomeruli isolation using the sieving method and FACS were not efficient due to contaminations with other organs (sieving) and a loss of a large amount of cells per sample (FACS), respectively. Samples of the required quality for sequencing resulted only from the manual glomeruli isolation.

Conclusion

Here we describe methods to isolate fluorescent glomeruli from transgenic zebrafish larvae. For our studies, we used the NTZ/MTR kidney disease model in order to identify mRNAs and miRNAs regulated in response to glomerular damage. This technique will further allow to screen for healing drugs in high-throughput experiments.

Circulating factors cause proteinuria in parabiotic zebrafish

Proteinuria can be induced by impairment of any component of the glomerular filtration barrier (GFB). To determine the role of circulating permeability factors on glomerular damage, we developed a parabiosis-based zebrafish model to generate a common circulation between zebrafish larvae. A morpholino-mediated knockdown of a podocyte specific gene (nephronectin) was induced in one zebrafish larva which was then fused to an un-manipulated fish. Notably, proteinuria and glomerular damage were present in the manipulated fish and in the parabiotically-fused partner. Thus, circulating permeability factors may be induced by proteinuria even when an induced podocyte gene dysregulation is the initiating cause.

ARP3 Controls the Podocyte Architecture at the Kidney Filtration Barrier

Podocytes, highly specialized epithelial cells, build the outer part of the kidney filtration barrier and withstand high mechanical forces through a complex network of cellular protrusions. Here, we show that Arp2/3-dependent actin polymerization controls actomyosin contractility and focal adhesion maturation of podocyte protrusions and thereby regulates formation, maintenance, and capacity to adapt to mechanical requirements of the filtration barrier. We find that N-WASP-Arp2/3 define the development of complex arborized podocyte protrusions in vitro and in vivo. Loss of dendritic actin networks results in a pronounced activation of the actomyosin cytoskeleton and the generation of over-maturated but less efficient adhesion, leading to detachment of podocytes. Our data provide a model to explain podocyte protrusion morphology and their mechanical stability based on a tripartite relationship between actin polymerization, contractility, and adhesion.

•

ARP3-dependent actin assembly is required for podocyte process formation

•

Arp2/3 thereby links process formation, podocyte adhesion and mechano-adaptation

•

Arp2/3 function is regulated by a reciprocal interplay with actomyosin

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.

Novel Microscopic Techniques for Podocyte Research

Together with endothelial cells and the glomerular basement membrane, podocytes form the size-specific filtration barrier of the glomerulus with their interdigitating foot processes. Since glomerulopathies are associated with so-called foot process effacement-a severe change of well-formed foot processes into flat and broadened processes-visualization of the three-dimensional podocyte morphology is a crucial part for diagnosis of nephrotic diseases. However, interdigitating podocyte foot processes are too narrow to be resolved by classic light microscopy due to Ernst Abbe's law making electron microscopy necessary. Although three dimensional electron microscopy approaches like serial block face and focused ion beam scanning electron microscopy and electron tomography allow volumetric reconstruction of podocytes, these techniques are very time-consuming and too specialized for routine use or screening purposes. During the last few years, different super-resolution microscopic techniques were developed to overcome the optical resolution limit enabling new insights into podocyte morphology. Super-resolution microscopy approaches like three dimensional structured illumination microscopy (3D-SIM), stimulated emission depletion microscopy (STED) and localization microscopy [stochastic optical reconstruction microscopy (STORM), photoactivated localization microscopy (PALM)] reach resolutions down to 80-20 nm and can be used to image and further quantify podocyte foot process morphology. Furthermore, in vivo imaging of podocytes is essential to study the behavior of these cells in situ. Therefore, multiphoton laser microscopy was a breakthrough for in vivo studies of podocytes in transgenic animal models like rodents and zebrafish larvae because it allows imaging structures up to several hundred micrometer in depth within the tissue. Additionally, along with multiphoton microscopy, lightsheet microscopy is currently used to visualize larger tissue volumes and therefore image complete glomeruli in their native tissue context. Alongside plain visualization of cellular structures, atomic force microscopy has been used to study the change of mechanical properties of podocytes in diseased states which has been shown to be a culprit in podocyte maintenance. This review discusses recent advances in the field of microscopic imaging and demonstrates their currently used and other possible applications for podocyte research.

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.

Structured illumination microscopy and automatized image processing as a rapid diagnostic tool for podocyte effacement

The morphology of podocyte foot processes is obligatory for renal function. Here we describe a method for the superresolution-visualization of podocyte foot processes using structured illumination microscopy of the slit diaphragm, which before has only been achieved by electron microscopy. As a proof of principle, we measured a mean foot process width of 0.249 ± 0.068 µm in healthy kidneys and a significant higher mean foot process width of 0.675 ± 0.256 µm in minimal change disease patients indicating effacement of foot processes. We then hypothesized that the slit length per glomerular capillary surface area (slit diaphragm density) could be used as an equivalent for the diagnosis of effacement. Using custom-made software we measured a mean value of 3.10 ± 0.27 µm⁻¹ in healthy subjects and 1.83 ± 0.49 µm⁻¹ in the minimal change disease patients. As foot process width was highly correlated with slit diaphragm density (R² = 0.91), we concluded that our approach is a valid method for the diagnosis of foot process effacement. In summary, we present a new technique to quantify podocyte damage, which combines superresolution microscopy with automatized image processing. Due to its diverse advantages, we propose this technique to be included into routine diagnostics of glomerular histopathology.

Are podocytes motile?

Podocytes, the postmitotic and highly branched epithelial cells of the glomerulus, play a pivotal role for the function of the glomerular filtration barrier and the development of chronic kidney disease. It has long been discussed whether podocytes in vivo are motile and can laterally migrate in a coordinated way along the capillaries until they reach the position of naked glomerular basement membrane often found in podocytopathies. Such motility would also be the prerequisite for the replacement of lost podocytes by progenitor cells. Additionally, the change of the podocyte foot processes from a normal to an effaced morphology, like it is found in many kidney diseases, would require a dynamic behavior of podocytes. Since the actin cytoskeleton is expressed in podocytes in vitro and in vivo and the morphology of podocytes is highly dependent on actin, actin-associated, and actin-regulating proteins, it was assumed that podocytes are dynamic and motile. After earlier technical limitations had been overcome and novel microscopic techniques like multiphoton microscopy had been developed, it became possible to continuously study the behavior of podocytes in living rodents and zebrafish larvae under physiological and pathological conditions. Recent in vivo microscopic studies in different model organisms suggest that lateral migration of podocytes in situ is a very unlikely event and only dynamic apical cell protrusions can be observed under pathological conditions. This review discusses recent findings concerning different forms of motility (like lateral translocative (LTM), apical translocative (ATM), and stationary motility (SM)) and their role for podocytopathies.

4D in vivo imaging of glomerular barrier function in a zebrafish podocyte injury model

AIM

Zebrafish larvae with their simplified pronephros are an ideal model to study glomerular physiology. Although several groups use zebrafish larvae to assess glomerular barrier function, temporary or slight changes are still difficult to measure. The aim of this study was to investigate the potential of in vivo two-photon microscopy (2-PM) for long-term imaging of glomerular barrier function in zebrafish larvae.

METHODS

As a proof of principle, we adapted the nitroreductase/metronidazole model of targeted podocyte ablation for 2-PM. Combination with a strain, which expresses eGFP-vitamin D-binding protein in the blood plasma, led to a strain that allowed induction of podocyte injury with parallel assessment of glomerular barrier function. We used four-dimensional (4D) 2-PM to assess eGFP fluorescence over 26 h in the vasculature and in tubules of multiple zebrafish larvae (5 days post-fertilization) simultaneously.

RESULTS

By 4D 2-PM, we observed that, under physiological conditions, eGFP fluorescence was retained in the vasculature and rarely detected in proximal tubule cells. Application of metronidazole induced podocyte injury and cell death as shown by TUNEL staining. Induction of podocyte injury resulted in a dramatic decrease of eGFP fluorescence in the vasculature over time (about 50% and 90% after 2 and 12 h respectively). Loss of vascular eGFP fluorescence was paralleled by an endocytosis-mediated accumulation of eGFP fluorescence in proximal tubule cells, indicating proteinuria.

CONCLUSION

We established a microscopy-based method to monitor the dynamics of glomerular barrier function during induction of podocyte injury in multiple zebrafish larvae simultaneously over 26 h.

Acute podocyte injury is not a stimulus for podocytes to migrate along the glomerular basement membrane in zebrafish larvae

Podocytes have a unique 3D structure of major and interdigitating foot processes which is the prerequisite for renal blood filtration. Loss of podocytes leads to chronic kidney disease ending in end stage renal disease. Until now, the question if podocytes can be replaced by immigration of cells along the glomerular basement membrane (GBM) is under debate. We recently showed that in contrast to former theories, podocytes are stationary in the zebrafish pronephros and neither migrate nor change their branching pattern of major processes over 23 hours. However, it was still unclear whether podocytes are able to migrate during acute injury. To investigate this, we applied the nitroreductase/metronidazole zebrafish model of podocyte injury to in vivo two-photon microscopy. The application of metronidazole led to retractions of major processes associated with a reduced expression of podocyte-specific proteins and a formation of subpodocyte pseudocyst. Electron microscopy showed that broad areas of the capillaries became denuded. By 4D in vivo observation of single podocytes, we could show that the remaining podocytes did not walk along GBM during 24 h. This in vivo study reveals that podocytes are very stationary cells making regenerative processes by podocyte walking along the GBM very unlikely.

MDM2 prevents spontaneous tubular epithelial cell death and acute kidney injury

Murine double minute-2 (MDM2) is an E3-ubiquitin ligase and the main negative regulator of tumor suppressor gene p53. MDM2 has also a non-redundant function as a modulator of NF-kB signaling. As such it promotes proliferation and inflammation. MDM2 is highly expressed in the unchallenged tubular epithelial cells and we hypothesized that MDM2 is necessary for their survival and homeostasis. MDM2 knockdown by siRNA or by genetic depletion resulted in demise of tubular cells in vitro. This phenotype was completely rescued by concomitant knockdown of p53, thus suggesting p53 dependency. In vivo experiments in the zebrafish model demonstrated that the tubulus cells of the larvae undergo cell death after the knockdown of mdm2. Doxycycline-induced deletion of MDM2 in tubular cell-specific MDM2-knockout mice Pax8rtTa-cre; MDM2f/f caused acute kidney injury with increased plasma creatinine and blood urea nitrogen and sharp decline of glomerular filtration rate. Histological analysis showed massive swelling of renal tubular cells and later their loss and extensive tubular dilation, markedly in proximal tubules. Ultrastructural changes of tubular epithelial cells included swelling of the cytoplasm and mitochondria with the loss of cristae and their transformation in the vacuoles. The pathological phenotype of the tubular cell-specific MDM2-knockout mouse model was completely rescued by co-deletion of p53. Tubular epithelium compensates only partially for the cell loss caused by MDM2 depletion by proliferation of surviving tubular cells, with incomplete MDM2 deletion, but rather mesenchymal healing occurs. We conclude that MDM2 is a non-redundant survival factor for proximal tubular cells by protecting them from spontaneous p53 overexpression-related cell death.