Podocin-green fluorescence protein allows visualization and functional analysis of podocytes

Zebrafish as a model for podocyte research

Podocytes, specialized postmitotic cells, are central players in various kidney-related diseases. Zebrafish have become a valuable model system for studying podocyte biology because they are genetically easy to manipulate, transparent, and their glomerular structure is similar to that of mammals. This review provides an overview of the knowledge of podocyte biology in zebrafish larvae, with particular focus on their essential contribution to understanding the mechanisms that underlie kidney diseases as well as supporting drug development. In addition, special attention is given to advances in live-imaging techniques allowing the observation of dynamic processes, including podocyte motility, podocyte process behavior, and glomerulus maturation. The review further addresses the functional aspects of podocytes in zebrafish larvae. This includes topics such as glomerular filtration, ultrastructural analyses, and evaluation of podocyte response to nephrotoxic insults. Studies presented in this context have provided important insights into the maintenance and resistance of the glomerular filtration barrier in zebrafish larvae and explored the potential transferability of these findings to mammals such as mice, rats, and most importantly, humans. The recent ability to identify potential therapeutic targets represents a promising new way to identify drugs that could effectively treat podocyte-associated glomerulopathies in humans. In summary, this review gives an overview about the importance of zebrafish as a model for podocyte-related disease and targeted drug development. It also highlights the key role of advanced imaging techniques in transparent zebrafish larvae, improving our understanding of glomerular diseases and the significant potential for translation of these findings to humans.

Effectiveness of zebrafish models in understanding human diseases-A review of models

Understanding the detailed mechanism behind every human disease, disorder, defect, and deficiency is a daunting task concerning the clinical diagnostic tools for patients. Hence, a closely resembling living or simulated model is of paramount interest for the development and testing of a probable novel drug for rectifying the conditions pertaining to the various ailments. The animal model that can be easily genetically manipulated to suit the study of the therapeutic motive is an indispensable asset and within the last few decades, the zebrafish models have proven their effectiveness by becoming such potent human disease models with their use being extended to various avenues of research to understand the underlying mechanisms of the diseases. As zebrafish are explored as model animals in understanding the molecular basis and genetics of many diseases owing to the 70% genetic homology between the human and zebrafish genes; new and fascinating facts about the diseases are being surfaced, establishing it as a very powerful tool for upcoming research. These prospective research areas can be explored in the near future using zebrafish as a model. In this review, the effectiveness of the zebrafish as an animal model against several human diseases such as osteoporosis, atrial fibrillation, Noonan syndrome, leukemia, autism spectrum disorders, etc. has been discussed.

Modeling Podocyte Ontogeny and Podocytopathies with the Zebrafish

Podocytes are exquisitely fashioned kidney cells that serve an essential role in the process of blood filtration. Congenital malformation or damage to podocytes has dire consequences and initiates a cascade of pathological changes leading to renal disease states known as podocytopathies. In addition, animal models have been integral to discovering the molecular pathways that direct the development of podocytes. In this review, we explore how researchers have used the zebrafish to illuminate new insights about the processes of podocyte ontogeny, model podocytopathies, and create opportunities to discover future therapies.

Interplay of immune and kidney resident cells in the formation of tertiary lymphoid structures in lupus nephritis

Kidney involvement confers significant morbidity and mortality in patients with systemic lupus erythematosus (SLE). The pathogenesis of lupus nephritis (LN) involves diverse mechanisms instigated by elements of the autoimmune response which alter the biology of kidney resident cells. Processes in the glomeruli and in the interstitium may proceed independently albeit crosstalk between the two is inevitable. Podocytes, mesangial cells, tubular epithelial cells, kidney resident macrophages and stromal cells with input from cytokines and autoantibodies present in the circulation alter the expression of enzymes, produce cytokines and chemokines which lead to their injury and damage of the kidney. Several of these molecules can be targeted independently to prevent and reverse kidney failure. Tertiary lymphoid structures with true germinal centers are present in the kidneys of patients with lupus nephritis and have been increasingly recognized to associate with poorer renal outcomes. Stromal cells, tubular epithelial cells, high endothelial vessel and lymphatic venule cells produce chemokines which enable the formation of structures composed of a T-cell-rich zone with mature dendritic cells next to a B-cell follicle with the characteristics of a germinal center surrounded by plasma cells. Following an overview on the interaction of the immune cells with kidney resident cells, we discuss the cellular and molecular events which lead to the formation of tertiary lymphoid structures in the interstitium of the kidneys of mice and patients with lupus nephritis. In parallel, molecules and processes that can be targeted therapeutically are presented.

Transgenic fluorescent zebrafish lines that have revolutionized biomedical research

Since its debut in the biomedical research fields in 1981, zebrafish have been used as a vertebrate model organism in more than 40,000 biomedical research studies. Especially useful are zebrafish lines expressing fluorescent proteins in a molecule, intracellular organelle, cell or tissue specific manner because they allow the visualization and tracking of molecules, intracellular organelles, cells or tissues of interest in real time and in vivo. In this review, we summarize representative transgenic fluorescent zebrafish lines that have revolutionized biomedical research on signal transduction, the craniofacial skeletal system, the hematopoietic system, the nervous system, the urogenital system, the digestive system and intracellular organelles.

Visualizing gene expression during zebrafish pronephros development and regeneration

The vertebrate kidney is comprised of functional units known as nephrons. Defects in nephron development or activity are a common feature of kidney disease. Current medical treatments are unable to ameliorate the dire consequences of nephron deficit or injury. Although there have been tremendous advancements in our understanding of nephron ontogeny and the response to damage, many significant knowledge gaps still remain. The zebrafish embryo kidney, or pronephros, is an ideal model for many renal development and regeneration studies because it is comprised of nephrons that share conserved features with the nephron units that comprise the mammalian metanephric kidney. In this chapter, we provide an overview about the benefits of using the zebrafish pronephros to study the mechanisms underlying nephrogenesis as well as epithelial repair and regeneration. We subsequently detail methods for the spatiotemporal assessment of gene and protein expression in zebrafish embryos that can be used to extend the understanding of nephron development and disease, and thereby create new opportunities to identify therapeutic strategies for regenerative medicine.

A specific isoform of Pyd/ZO-1 mediates junctional remodeling and formation of slit diaphragms

Podocyte slit diaphragms are key components of the glomerular filtration barrier. Using Drosophila nephrocytes, Carrasco-Rando et al. propose a conserved role for Pyd/ZO-1 in triggering junctional remodeling leading to the formation of slit diaphragms.

The podocyte slit diaphragm (SD), responsible for blood filtration in vertebrates, is a major target of injury in chronic kidney disease. The damage includes severe morphological changes with destabilization of SDs and their replacement by junctional complexes between abnormally broadened foot processes. In Drosophila melanogaster, SDs are present in nephrocytes, which filter the fly's hemolymph. Here, we show that a specific isoform of Polychaetoid/ZO-1, Pyd-P, is essential for Drosophila SDs, since, in pyd mutants devoid of Pyd-P, SDs do not form and the SD component Dumbfounded accumulates at ectopic septate-like junctions between abnormally aggregated nephrocytes. Reintroduction of Pyd-P leads to junctional remodeling and their progressive normalization toward SDs. This transition requires the coiled-coil domain of Pyd-P and implies formation of nonclathrin vesicles containing SD components and their trafficking to the nephrocyte external membrane, where SDs assemble. Analyses in zebrafish suggest a conserved role for Tjp1a/ZO-1 in promoting junctional remodeling in podocytes.

The Efficacy of Puromycin and Adriamycin for Induction of Glomerular Failure in Larval Zebrafish Validated by an Assay of Glomerular Permeability Dynamics

Defects in the glomerular filtration barrier (GFB) play a major role in the onset of human renal diseases. Highly ramified glomerular cells named podocytes are a critical component of the GFB. Injury to podocytes results in abnormal excretion of plasma proteins, which can lead to chronic kidney disease. The conserved paired nephron of larval zebrafish is an excellent model for assessing glomerular function and injury. The efficacy of two known podocyte toxins was tested to refine models of acute podocyte injury in larval zebrafish. The validated compound was then used to test a novel assay of the dynamics of abnormal protein excretion. Injected adriamycin was found to be unsuitable for induction of glomerular injury due to off-target cardiovascular toxicity. In contrast, puromycin treatment resulted in a loss of discriminative filtration, measured by excretion of 70 kDa dextran, and podocyte effacement confirmed by electron microscopy. The dynamics of dextran excretion during puromycin injury modeled the onset of glomerular damage within 24 hours postinjection. These data validate puromycin for induction of acute podocyte injury in zebrafish larvae and describe a semihigh-throughput assay for quantifying the dynamics of abnormal protein excretion.

Zebrafish as a Model for Drug Screening in Genetic Kidney Diseases

Genetic disorders account for a wide range of renal diseases emerging during childhood and adolescence. Due to the utilization of modern biochemical and biomedical techniques, the number of identified disease-associated genes is increasing rapidly. Modeling of congenital human disease in animals is key to our understanding of the biological mechanism underlying pathological processes and thus developing novel potential treatment options. The zebrafish (Danio rerio) has been established as a versatile small vertebrate organism that is widely used for studying human inherited diseases. Genetic accessibility in combination with elegant experimental methods in zebrafish permit modeling of human genetic diseases and dissecting the perturbation of underlying cellular networks and physiological processes. Beyond its utility for genetic analysis and pathophysiological and mechanistic studies, zebrafish embryos, and larvae are amenable for phenotypic screening approaches employing high-content and high-throughput experiments using automated microscopy. This includes large-scale chemical screening experiments using genetic models for searching for disease-modulating compounds. Phenotype-based approaches of drug discovery have been successfully performed in diverse zebrafish-based screening applications with various phenotypic readouts. As a result, these can lead to the identification of candidate substances that are further examined in preclinical and clinical trials. In this review, we discuss zebrafish models for inherited kidney disease as well as requirements and considerations for the technical realization of drug screening experiments in zebrafish.

High-content screening assay-based discovery of paullones as novel podocyte-protective agents

Podocyte dysfunction and loss is an early event and a hallmark of proteinuric kidney diseases. A podocyte's normal function is maintained via its unique cellular architecture that relies on an intracellular network of filaments, including filamentous actin (F-actin) and microtubules, that provides mechanical support. Damage to this filamentous network leads to changes in cellular morphology and results in podocyte injury, dysfunction, and death. Conversely, stabilization of this network protects podocytes and ameliorates proteinuria. This suggests that stabilization of podocyte architecture via its filamentous network could be a key therapeutic strategy for proteinuric kidney diseases. However, development of podocyte-directed therapeutics, especially those that target the cell's filamentous network, is still lacking, partly because of unavailability of appropriate cellular assays for use in a drug discovery environment. Here, we describe a new high-content screening-based methodology and its implementation on podocytes to identify paullone derivatives as a novel group of podocyte-protective compounds. We find that three compounds, i.e., kenpaullone, 1-azakenpaullone, and alsterpaullone, dose dependently protect podocytes from puromycin aminonucleoside (PAN)-mediated injury in vitro by reducing PAN-induced changes in both the filamentous actin and microtubules, with alsterpaullone providing maximal protection. Mechanistic studies further show that alsterpaullone suppressed PAN-induced activation of signaling downstream of GSK3β and p38 mitogen-activated protein kinase. In vivo it reduced ADR-induced glomerular injury in a zebrafish model. Together, these results identify paullone derivatives as novel podocyte-protective agents for future therapeutic development.

Disease modeling in genetic kidney diseases: zebrafish

Growing numbers of translational genomics studies are based on the highly efficient and versatile zebrafish (Danio rerio) vertebrate model. The increasing types of zebrafish models have improved our understanding of inherited kidney diseases, since they not only display pathophysiological changes but also give us the opportunity to develop and test novel treatment options in a high-throughput manner. New paradigms in inherited kidney diseases have been developed on the basis of the distinct genome conservation of approximately 70 % between zebrafish and humans in terms of existing gene orthologs. Several options are available to determine the functional role of a specific gene or gene sets. Permanent genome editing can be induced via complete gene knockout by using the CRISPR/Cas-system, among others, or via transient modification by using various morpholino techniques. Cross-species rescues succeeding knockdown techniques are employed to determine the functional significance of a target gene or a specific mutation. This article summarizes the current techniques and discusses their perspectives.

Discontinuation of anti-VEGF cancer therapy promotes metastasis through a liver revascularization mechanism

The impact of discontinuation of anti-VEGF cancer therapy in promoting cancer metastasis is unknown. Here we show discontinuation of anti-VEGF treatment creates a time-window of profound structural changes of liver sinusoidal vasculatures, exhibiting hyper-permeability and enlarged open-pore sizes of the fenestrated endothelium and loss of VE-cadherin. The drug cessation caused highly leaky hepatic vasculatures permit tumour cell intravasation and extravasation. Discontinuation of an anti-VEGF antibody-based drug and sunitinib markedly promotes liver metastasis. Mechanistically, host hepatocyte, but not tumour cell-derived vascular endothelial growth factor (VEGF), is responsible for cancer metastasis. Deletion of hepatocyte VEGF markedly ablates the ‘off-drug'-induced metastasis. These findings provide mechanistic insights on anti-VEGF cessation-induced metastasis and raise a new challenge for uninterrupted and sustained antiangiogenic therapy for treatment of human cancers.

Anti-VEGF therapy often produces limited beneficial effects in cancer patients. Here, the authors show that discontinuation of anti-VEGF cancer therapy in xenografts-bearing mice increases cancer cells extravasation and intravasation in liver through the host-derived VEGF.

Zebrafish kidney development

The kidney of the zebrafish shares many features with other vertebrate kidneys including the human kidney. Similar cell types and shared developmental and patterning mechanisms make the zebrafish pronephros a valuable model for kidney organogenesis. Here we review recent advances in studies of zebrafish pronephric development and provide experimental protocols to analyze kidney cell types and structures, measure nephron function, live image kidney cells in vivo, and probe mechanisms of kidney regeneration after injury.

Little fish, big catch: zebrafish as a model for kidney disease

The zebrafish, Danio rerio, is a relevant vertebrate model for biomedical research and translational studies because of its broad genetic conservation with humans. In recent years, scientists have formulated a growing list of zebrafish kidney disease paradigms, the study of which has contributed a multitude of insights into the basic biology of human conditions and even identified potential therapeutic agents. Conversely, there are also distinctive aspects of zebrafish biology lacking in higher vertebrates, such as the capacity to heal without lasting scar formation after tissue damage and the ability to generate nephrons throughout their lifespan, which makes the zebrafish uniquely suited to study regeneration in the context of the kidney. Here, we review several informative zebrafish models of kidney disease and discuss their future applications in nephrology.

Knockdown of Tmem234 in zebrafish results in proteinuria

Podocytes are highly specialized epithelial cells located at the outer aspects of the glomerular capillary tuft and critical components of the kidney filtration barrier. To maintain their unique features, podocytes express a number of proteins that are only sparsely found elsewhere in the body. In this study, we have identified four (Tmem234, Znf185, Lrrc49, and Slfn5) new highly podocyte-enriched proteins. The proteins are strongly expressed by podocytes, while other parts of the kidney show only weak or no expression. Tmem234, Slfn5, and Lrrc49 are located in foot processes, whereas Znf185 is found in both foot and major processes. Expressional studies in developing kidneys show that these proteins are first expressed at the capillary stage glomerulus, the same stage when the formation of major and foot processes begins. We identified zebrafish orthologs for Tmem234 and Znf185 genes and knocked down their expression using morpholino technology. Studies in zebrafish larvae indicate that Tmem234 is essential for the organization and functional integrity of the pronephric glomerulus filtration barrier, as inactivation of Tmem234 expression results in foot process effacement and proteinuria. In summary, we have identified four novel highly podocyte-enriched proteins and show that one of them, Tmem234, is essential for the normal filtration barrier in the zebrafish pronephric glomerulus. Identification of new molecular components of the kidney filtration barrier opens up possibilities to study their role in glomerulus biology and diseases.

Zebrafish Models of Kidney Damage and Repair

The vertebrate kidney possesses the capacity to repair damaged nephrons, and this potential is conserved regardless of the complexity of species-specific kidneys. However, many aquatic vertebrates possess the ability to not only repair existing nephrons, but also generate new nephrons after injury. Adult zebrafish have the ability to recover from acute renal injury not only by replacing lost injured epithelial cells of endogenous nephrons, but by also generating de novo nephrons. This strong regeneration potential, along with other unique characteristics such as the high degree of genetic conservation with humans, the ease of harvesting externally fertilized, transparent embryos, the accessibility to larval and adult kidneys, and the ability to perform whole organism phenotypic small molecule screens, has positioned zebrafish as a unique vertebrate model to study kidney injury. In this review, we provide an overview of the contribution of zebrafish larvae/adult studies to the understanding of renal regeneration, diseases, and therapeutic discovery.

Schip1 is a novel podocyte foot process protein that mediates actin cytoskeleton rearrangements and forms a complex with Nherf2 and ezrin

Background

Podocyte foot process effacement accompanied by actin cytoskeleton rearrangements is a cardinal feature of many progressive human proteinuric diseases.

Results

By microarray profiling of mouse glomerulus, SCHIP1 emerged as one of the most highly enriched transcripts. We detected Schip1 protein in the kidney glomerulus, specifically in podocytes foot processes. Functionally, Schip1 inactivation in zebrafish by morpholino knock-down results in foot process disorganization and podocyte loss leading to proteinuria. In cultured podocytes Schip1 localizes to cortical actin-rich regions of lamellipodia, where it forms a complex with Nherf2 and ezrin, proteins known to participate in actin remodeling stimulated by PDGFβ signaling. Mechanistically, overexpression of Schip1 in vitro causes accumulation of cortical F-actin with dissolution of transversal stress fibers and promotes cell migration in response to PDGF-BB stimulation. Upon actin disassembly by latrunculin A treatment, Schip1 remains associated with the residual F-actin-containing structures, suggesting a functional connection with actin cytoskeleton possibly via its interaction partners. A similar assay with cytochalasin D points to stabilization of cortical actin cytoskeleton in Schip1 overexpressing cells by attenuation of actin depolymerisation.

Conclusions

Schip1 is a novel glomerular protein predominantly expressed in podocytes, necessary for the zebrafish pronephros development and function. Schip1 associates with the cortical actin cytoskeleton network and modulates its dynamics in response to PDGF signaling via interaction with the Nherf2/ezrin complex. Its implication in proteinuric diseases remains to be further investigated.

Using zebrafish to study podocyte genesis during kidney development and regeneration

During development, vertebrates form a progression of up to three different kidneys that are comprised of functional units termed nephrons. Nephron composition is highly conserved across species, and an increasing appreciation of the similarities between zebrafish and mammalian nephron cell types has positioned the zebrafish as a relevant genetic system for nephrogenesis studies. A key component of the nephron blood filter is a specialized epithelial cell known as the podocyte. Podocyte research is of the utmost importance as a vast majority of renal diseases initiate with the dysfunction or loss of podocytes, resulting in a condition known as proteinuria that causes nephron degeneration and eventually leads to kidney failure. Understanding how podocytes develop during organogenesis may elucidate new ways to promote nephron health by stimulating podocyte replacement in kidney disease patients. In this review, we discuss how the zebrafish model can be used to study kidney development, and how zebrafish research has provided new insights into podocyte lineage specification and differentiation. Further, we discuss the recent discovery of podocyte regeneration in adult zebrafish, and explore how continued basic research using zebrafish can provide important knowledge about podocyte genesis in embryonic and adult environments. genesis 52:771-792, 2014. © 2014 Wiley Periodicals, Inc.

Grouper tshβ promoter-driven transgenic zebrafish marks proximal kidney tubule development

Kidney tubule plays a critical role in recovering or secreting solutes, but the detailed morphogenesis remains unclear. Our previous studies have found that grouper tshβ (gtshβ) is also expressed in kidney, however, the distribution significance is still unknown. To understand the gtshβ role and kidney tubule morphogenesis, here, we have generated a transgenic zebrafish line Tg(gtshβ:GFP) with green fluorescent protein driven by the gtshβ promoter. Similar to the endogenous tshβ in zebrafish or in grouper, the gtshβ promoter-driven GFP is expressed in pituitary and kidney, and the developing details of proximal kidney tubule are marked in the transgenic zebrafish line. The gfp initially transcribes at 16 hours post fertilization (hpf) above the dorsal mesentery, and partially co-localizes with pronephric tubular markers slc20a1a and cdh17. Significantly, the GFP specifically localizes in proximal pronephric segments during embryogenesis and resides at kidney duct epithelium in adult fish. To test whether the gtshβ promoter-driven GFP may serve as a readout signal of the tubular development, we have treated the embryos with retinoic acid signaing (RA) reagents, in which exogenous RA addition results in a distal extension of the proximal segments, while RA inhibition induces a weakness and shortness of the proximal segments. Therefore, this transgenic line provides a useful tool for genetic or chemical analysis of kidney tubule.

Lmx1b and FoxC combinatorially regulate podocin expression in podocytes

Podocin is a key protein of the kidney podocyte slit diaphragm protein complex, an important part of the glomerular filtration barrier. Mutations in the human podocin gene NPHS2 cause familial or sporadic forms of renal disease owing to the disruption of filtration barrier integrity. The exclusive expression of NPHS2 in podocytes reflects its unique function and raises interesting questions about its transcriptional regulation. Here, we further define a 2.5-kb zebrafish nphs2 promoter fragment previously described and identify a 49-bp podocyte-specific transcriptional enhancer using Tol2-mediated G0 transgenesis in zebrafish. Within this enhancer, we identified a cis-acting element composed of two adjacent DNA-binding sites (FLAT-E and forkhead) bound by transcription factors Lmx1b and FoxC. In zebrafish, double knockdown of Lmx1b and FoxC orthologs using morpholino doses that caused no or minimal phenotypic changes upon individual knockdown completely disrupted podocyte development in 40% of injected embryos. Co-overexpression of the two genes potently induced endogenous nphs2 expression in zebrafish podocytes. We found that the NPHS2 promoter also contains a cis-acting Lmx1b-FoxC motif that binds LMX1B and FoxC2. Furthermore, a genome-wide search identified several genes that carry the Lmx1b-FoxC motif in their promoter regions. Among these candidates, motif-driven podocyte enhancer activity of CCNC and MEIS2 was functionally analyzed in vivo. Our results show that podocyte expression of some genes is combinatorially regulated by two transcription factors interacting synergistically with a common enhancer. This finding provides insights into transcriptional mechanisms required for normal and pathologic podocyte functions.

Podocyte developmental defects caused by adriamycin in zebrafish embryos and larvae: a novel model of glomerular damage

The zebrafish pronephros is gaining popularity in the nephrology community, because embryos are easy to cultivate in multiwell plates, allowing large number of experiments to be conducted in an in vivo model. In a few days, glomeruli reach complete development, with a structure that is similar to that of the mammalian counterpart, showing a fenestrated endothelium and a basement membrane covered by the multiple ramifications of mature podocytes. As a further advantage, zebrafish embryos are permeable to low molecular compounds, and this explains their extensive use in drug efficacy and toxicity experiments. Here we show that low concentrations of adriamycin (i.e. 10 and 20 µM), when dissolved in the medium of zebrafish embryos at 9 hours post-fertilization and removed after 48 hours (57 hpf), alter the development of podocytes with subsequent functional impairment, demonstrated by onset of pericardial edema and reduction of expression of the podocyte proteins nephrin and wt1. Podocyte damage is morphologically confirmed by electron microscopy and functionally supported by increased clearance of microinjected 70 kDa fluorescent dextran. Importantly, besides pericardial edema and glomerular damage, which persist and worsen after adriamycin removal from the medium, larvae exposed to adriamycin 10 and 20 µM do not show any myocardiocyte alterations nor vascular changes. The only extra-renal effect is a transient delay of cartilage formation that rapidly recovers once adriamycin is removed. In summary, this low dose adriamycin model can be applied to analyze podocyte developmental defects, such as those observed in congenital nephrotic syndrome, and can be taken in consideration for pharmacological studies of severe early podocyte injury.

Visualization of cytoskeletal dynamics in podocytes using adenoviral vectors

Glomerular visceral epithelial cells (podocytes) play a key role in maintaining selective protein filtration in the kidney. Podocytes have a complex cell shape characterized by the presence of numerous actin-rich processes, which cover the surface of glomerular capillaries and are connected by specialized cell-cell adhesion complexes (slit diaphragms). Human genetic studies and experiments in knockout mouse models show that actin filaments and actin-associated proteins are indispensable for the maintenance of podocyte shape, slit diaphragm integrity, and normal glomerular filtration. The ability to examine cytoskeletal protein organization and dynamics in podocytes and to test the effects of disease-associated mutations on protein localization provides valuable information for researchers aiming to dissect the molecular mechanisms of podocyte dysfunction. We describe how adenovirus-mediated transduction of cultured podocytes with DNA constructs can be used to reliably introduce fluorescently tagged cytoskeletal markers for live cell imaging with high efficiency and low toxicity. This technique can be used to study the dynamic reorganization of the podocyte cytoskeleton and to test the effects of novel mutations on podocyte cytoskeletal dynamics.

Animal models of nephrotic syndrome

Animal models of proteinuria and nephrotic syndrome are essential tools for studying the mechanisms of action of abnormalities in individual components of the podocyte and glomerular basement membrane. In recent years a variety of in vivo models have been developed to elucidate the function of specific podocyte proteins and their role in the pathogenesis of proteinuria and glomerulosclerosis. In this overview of the animal models currently available we discuss their contribution to our mechanistic understanding and their potential use in screening for novel targeted therapies of steroid-resistant nephrotic syndrome.

Myo1e impairment results in actin reorganization, podocyte dysfunction, and proteinuria in zebrafish and cultured podocytes

Background

Podocytes serve as an important constituent of the glomerular filtration barrier. Recently, we and others identified Myo1e as a key molecular component of the podocyte cytoskeleton.

Results

Myo1e mRNA and protein was expressed in human and mouse kidney sections as determined by Northern blot and reverse transcriptase PCR, and its expression was more evident in podocytes by immunofluorescence. By specific knock-down of MYO1E in zebrafish, the injected larvae exhibited pericardial edema and pronephric cysts, consistent with the appearance of protein in condensed incubation supernate. Furthermore, specific inhibition of Myo1e expression in a conditionally immortalized podocyte cell line induced morphological changes, actin cytoskeleton rearrangement, and dysfunction in cell proliferation, migration, endocytosis, and adhesion with the glomerular basement membrane.

Conclusions

Our results revealed that Myo1e is a key component contributing to the functional integrity of podocytes. Its impairment may cause actin cytoskeleton re-organization, alteration of cell shape, and membrane transport, and podocyte drop-out from the glomerular basement membrane, which might eventually lead to an impaired glomerular filtration barrier and proteinuria.

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.

A remote cis-acting variant at 3q links glomerular NCK1 to diabetic nephropathy

We have previously reported genetic association of a single nucleotide polymorphism (SNP), rs1866813, at 3q locus with increased risk of diabetic nephropathy (DN). The SNP is located approximately 70 kb downstream of a cluster of four genes. This raises a question how the remote noncoding polymorphism affects the risk of DN. In this study, we tested a long-range regulatory potential of this variant by a series of experiments. In a luciferase assay, two alleles of the SNP showed differential effects on the luciferase activity in transfected cells in vitro. Using transgenic zebrafish, we further demonstrated in vivo that two alleles of the SNP differentially regulated GFP expression in zebrafish podocytes. Immunofluorescence staining and Western blotting verified that only Nck1 of the four nearby genes was predominantly expressed in mouse glomeruli as well as in podocytes. Furthermore, genotypes of the SNP rs1866813 were correlated with NCK1 expression in immortalized lymphocytes from diabetic patients. The risk allele was associated with increased NCK1 expression compared to the non-risk allele, consistent with the results of the reporter-based studies. Interestingly, differential expression of glomerular Nck1 between mouse strains carrying the nephropathy-prone 129/Sv allele and nephropathy-resistant C57BL/6 allele was also observed. Our results show that the DN-associated SNP rs1866813 is a remote cis-acting variant differentially regulating glomerular NCK1 expression. This finding implicates an important role for glomerular NCK1 in DN pathogenesis under hyperglycemia.

Glcci1 deficiency leads to proteinuria

Unbiased transcriptome profiling and functional genomics approaches identified glucocorticoid-induced transcript 1 (GLCCI1) as being a transcript highly specific for the glomerulus, but its role in glomerular development and disease is unknown. Here, we report that mouse glomeruli express far greater amounts of Glcci1 protein compared with the rest of the kidney. RT-PCR and Western blotting demonstrated that mouse glomerular Glcci1 is approximately 60 kD and localizes to the cytoplasm of podocytes in mature glomeruli. In the fetal kidney, intense Glcci1 expression occurs at the capillary-loop stage of glomerular development. Using gene knockdown in zebrafish with morpholinos, morphants lacking Glcci1 function had collapsed glomeruli with foot-process effacement. Permeability studies of the glomerular filtration barrier in these zebrafish morphants demonstrated a disruption of the selective glomerular permeability filter. Taken together, these data suggest that Glcci1 promotes the normal development and maintenance of podocyte structure and function.