Organization of the pronephric filtration apparatus in zebrafish requires Nephrin, Podocin and the FERM domain protein Mosaic eyes

A brief overview on IRM function across evolution

The description of the Rst protein by Karl-Friedrich Fischbach and colleagues was a milestone in the discovery of the irre cell recognition module (IRM). IRM proteins represent a family of immunoglobulin superfamily cell adhesion proteins that orchestrate intercellular adhesion and signaling events necessary for the development of various tissues. This review briefly summarizes the fundamental role of IRM proteins for neuronal wiring and filtration in organisms spanning the evolutionary distance from Drosophila (nephrocyte diaphragm) to humans (slit diaphragm).

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.

Kidney injury and regeneration in zebrafish

Renal tubule epithelial cells can regenerate in response to acute injury. Although this process remains poorly understood, it appears to involve the reactivation of pathways that are operative during embryonic kidney formation. A better understanding of renal regeneration may lead to the development of new therapies that can attenuate acute kidney injury or expedite recovery. The zebrafish is being used as a model to understand renal regeneration. In this review, we summarize the current knowledge on zebrafish kidney formation, describe methods for inducing acute injury, and focus on the unique capacity of the zebrafish adult kidney to undergo de novo nephron formation in response to damage.

Mutations in EMP2 cause childhood-onset nephrotic syndrome

Nephrotic syndrome (NS) is a genetically heterogeneous group of diseases that are divided into steroid-sensitive NS (SSNS) and steroid-resistant NS (SRNS). SRNS inevitably leads to end-stage kidney disease, and no curative treatment is available. To date, mutations in more than 24 genes have been described in Mendelian forms of SRNS; however, no Mendelian form of SSNS has been described. To identify a genetic form of SSNS, we performed homozygosity mapping, whole-exome sequencing, and multiplex PCR followed by next-generation sequencing. We thereby detected biallelic mutations in EMP2 (epithelial membrane protein 2) in four individuals from three unrelated families affected by SRNS or SSNS. We showed that EMP2 exclusively localized to glomeruli in the kidney. Knockdown of emp2 in zebrafish resulted in pericardial effusion, supporting the pathogenic role of mutated EMP2 in human NS. At the cellular level, we showed that knockdown of EMP2 in podocytes and endothelial cells resulted in an increased amount of CAVEOLIN-1 and decreased cell proliferation. Our data therefore identify EMP2 mutations as causing a recessive Mendelian form of SSNS.

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.

osr1 is required for podocyte development downstream of wt1a

Odd-skipped related 1 (Osr1) encodes a zinc finger transcription factor required for kidney development. Osr1 deficiency in mice results in metanephric kidney agenesis, whereas knockdown or mutation studies in zebrafish revealed that pronephric nephrons require osr1 for proximal tubule and podocyte development. osr1-deficient pronephric podocyte progenitors express the Wilms' tumor suppressor wt1a but do not undergo glomerular morphogenesis or express the foot process junctional markers nephrin and podocin. The function of osr1 in podocyte differentiation remains unclear, however. Here, we found by double fluorescence in situ hybridization that podocyte progenitors coexpress osr1 and wt1a. Knockdown of wt1a disrupted podocyte differentiation and prevented expression of osr1. Blocking retinoic acid signaling, which regulates wt1a, also prevented osr1 expression in podocyte progenitors. Furthermore, unlike the osr1-deficient proximal tubule phenotype, which can be rescued by manipulation of endoderm development, podocyte differentiation was not affected by altered endoderm development, as assessed by nephrin and podocin expression in double osr1/sox32-deficient embryos. These results suggest a different, possibly cell- autonomous requirement for osr1 in podocyte differentiation downstream of wt1a. Indeed, osr1-deficient embryos did not exhibit podocyte progenitor expression of the transcription factor lhx1a, and forced expression of activated forms of the lhx1a gene product rescued nephrin expression in osr1-deficient podocytes. Our results place osr1 in a framework of transcriptional regulators that control the expression of podocin and nephrin and thereby mediate podocyte differentiation.

Slit diaphragm protein Neph1 and its signaling: a novel therapeutic target for protection of podocytes against glomerular injury

Podocytes are specialized epithelial cells that are critical components of the glomerular filtration barrier, and their dysfunction leads to proteinuria and renal failure. Therefore, preserving podocyte function is therapeutically significant. In this study, we identified Neph1 signaling as a therapeutic target that upon inhibition prevented podocyte damage from a glomerular injury-inducing agent puromycin aminonucleoside (PAN). To specifically inhibit Neph1 signaling, we used a protein transduction approach, where the cytoplasmic domain of Neph1 (Neph1CD) tagged with a protein transduction domain trans-activator of transcription was transduced in cultured podocytes prior to treatment with PAN. The PAN-induced Neph1 phosphorylation was significantly reduced in Neph1CD-transduced cells; in addition, these cells were resistant to PAN-induced cytoskeletal damage. The biochemical analysis using subfractionation studies showed that unlike control cells Neph1 was retained in the lipid raft fractions in the transduced cells following treatment with PAN, indicating that transduction of Neph1CD in podocytes prevented PAN-induced mislocalization of Neph1. In accordance, the immunofluorescence analysis further suggested that Neph1CD-transduced cells had increased ability to retain endogenous Neph1 at the membrane in response to PAN-induced injury. Similar results were obtained when angiotensin was used as an injury-inducing agent. Consistent with these observations, maintaining high levels of Neph1 at the membrane using a podocyte cell line overexpressing chimeric Neph1 increased the ability of podocytes to resist PAN-induced injury and PAN-induced albumin leakage. Using a zebrafish in vivo PAN and adriamycin injury models, we further demonstrated the ability of transduced Neph1CD to preserve glomerular function. Collectively, these results support the conclusion that inhibiting Neph1 signaling is therapeutically significant in preventing podocyte damage from glomerular injury.

Sept7b is essential for pronephric function and development of left-right asymmetry in zebrafish embryogenesis

The conserved septin family of filamentous small GTPases plays important roles in mitosis, cell migration and cell morphogenesis by forming scaffolds and diffusion barriers. Recent studies in cultured cells in vitro indicate that a septin complex of septin 2, 7 and 9 is required for ciliogenesis and cilia function, but septin function in ciliogenesis in vertebrate organs in vivo is not understood. We show that sept7b is expressed in ciliated cells in different tissues during early zebrafish development. Knockdown of sept7b by using morpholino antisense oligonucleotides caused misorientation of basal bodies and cilia, reduction of apical actin and the shortening of motile cilia in Kupffer's vesicle and pronephric tubules. This resulted in pericardial and yolk sac edema, body axis curvature and hydrocephaly. Notably, in sept7b morphants we detected strong left-right asymmetry defects in the heart and lateral plate mesoderm (situs inversus), reduced fluid flow in the kidney, the formation of kidney cysts and loss of glomerular filtration barrier function. Thus, sept7b is essential during zebrafish development for pronephric function and ciliogenesis, and loss of expression of sept7b results in defects that resemble human ciliopathies.

Molecular Mechanisms of Podocyte Development Revealed by Zebrafish Kidney Research

Elucidating the gene regulatory networks that control kidney development can provide information about the origins of renal birth defects and kidney disease, as well as insights relevant to the design of clinical interventions for these conditions. The kidney is composed of functional units termed nephrons. Renal malfunction often arises from damage to cells known as podocytes, which are highly specialized epithelial cells that comprise the blood filter, or glomerulus, located on each nephron. Podocytes interact with the vasculature to create an elaborate sieve that collects circulatory fluid, and this filtrate enters the nephron where it is modified to produce urine and balance water homeostasis. Podocytes are an essential cellular component of the glomerular filtration barrier, helping to protect nephrons from the entry of large proteins and circulatory cells. Podocyte loss has catastrophic consequences for renal function and overall health, as podocyte destruction leads to nephron damage and pathological conditions like chronic kidney disease. Despite their importance, there is still a rather limited understanding about the molecular pathways that control podocyte formation. In recent years, however, studies of podocyte development using the zebrafish embryonic kidney, or pronephros, have been an expanding area of nephrology research. Zebrafish form an anatomically simple pronephros comprised of two nephrons that share a single blood filter, and podocyte progenitors can be easily visualized throughout the process of glomerular development. The zebrafish is an especially useful system for studying the mechanisms that are essential for formation of nephron cell types like podocytes due to the high genetic conservation between vertebrate species, including humans. In this review, we discuss how research using the zebrafish has provided new insights into the molecular regulation of the podocyte lineage during kidney ontogeny, complementing contemporary research in other animal models.

Nephrin and Podocin functions are highly conserved between the zebrafish pronephros and mammalian metanephros

The slit diaphragm (SD) is a highly specialized intercellular junction between podocyte foot processes and is crucial in the formation of the filtration barrier in the renal glomeruli. Zebrafish Nephrin and Podocin are important in the formation of the podocyte SD and mutations in NEPHRIN and PODOCIN genes cause human nephrotic syndrome. In the present study, the zebrafish Podocin protein was observed to be predominantly localized in the pronephric glomerular podocytes, as previously reported for Nephrin. To understand the function of Podocin and Nephrin in zebrafish, splice-blocking morpholino antisense oligonucleotides were used. Knockdown of Podocin or Nephrin by this method induced pronephric glomerular hypoplasia with pericardial edema. Human NEPHRIN and PODOCIN mRNA rescued this glomerular phenotype, however, the efficacy of the rescues was greatly reduced when mRNA-encoding human disease-causing NEPHRIN-R1109X and PODOCIN-R138Q were used. Furthermore, an association between zebrafish Nephrin and Podocin proteins was observed. Notably, Podocin-R150Q, corresponding to human PODOCIN-R138Q, markedly interacted with NEPHRIN compared with wild-type PODOCIN, suggesting that this strong binding capacity of mutated PODOCIN impairs the transport of NEPHRIN and PODOCIN out of the endoplasmic reticulum. The results suggest that the functions of Nephrin and Podocin are highly conserved between the zebrafish pronephros and mammalian metanephros. Accordingly, the zebrafish pronephros may provide a useful tool for analyzing disease-causing gene mutations in human kidney disorders.

Development of an automated imaging pipeline for the analysis of the zebrafish larval kidney

The analysis of kidney malformation caused by environmental influences during nephrogenesis or by hereditary nephropathies requires animal models allowing the in vivo observation of developmental processes. The zebrafish has emerged as a useful model system for the analysis of vertebrate organ development and function, and it is suitable for the identification of organotoxic or disease-modulating compounds on a larger scale. However, to fully exploit its potential in high content screening applications, dedicated protocols are required allowing the consistent visualization of inner organs such as the embryonic kidney. To this end, we developed a high content screening compatible pipeline for the automated imaging of standardized views of the developing pronephros in zebrafish larvae. Using a custom designed tool, cavities were generated in agarose coated microtiter plates allowing for accurate positioning and orientation of zebrafish larvae. This enabled the subsequent automated acquisition of stable and consistent dorsal views of pronephric kidneys. The established pipeline was applied in a pilot screen for the analysis of the impact of potentially nephrotoxic drugs on zebrafish pronephros development in the Tg(wt1b:EGFP) transgenic line in which the developing pronephros is highlighted by GFP expression. The consistent image data that was acquired allowed for quantification of gross morphological pronephric phenotypes, revealing concentration dependent effects of several compounds on nephrogenesis. In addition, applicability of the imaging pipeline was further confirmed in a morpholino based model for cilia-associated human genetic disorders associated with different intraflagellar transport genes. The developed tools and pipeline can be used to study various aspects in zebrafish kidney research, and can be readily adapted for the analysis of other organ systems.

Analysis of three μ1-AP1 subunits during zebrafish development

BACKGROUND

The family of AP-1 complexes mediates protein sorting in the late secretory pathway and it is essential for the development of mammals. The ubiquitously expressed AP-1A complex consists of four adaptins γ1, β1, μ1A, and σ1A. AP-1A mediates protein transport between the trans-Golgi network and early endosomes. The polarized epithelia AP-1B complex contains the μ1B-adaptin. AP-1B mediates specific transport of proteins from basolateral recycling endosomes to the basolateral plasma membrane of polarized epithelial cells.

RESULTS

Analysis of the zebrafish genome revealed the existence of three μ1-adaptin genes, encoding μ1A, μ1B, and the novel isoform μ1C, which is not found in mammals. μ1C shows 80% sequence identity with μ1A and μ1B. The μ1C expression pattern largely overlaps with that of μ1A, while μ1B is expressed in epithelial cells. By knocking-down the synthesis of μ1A, μ1B and μ1C with antisense morpholino techniques we demonstrate that each of these μ1 adaptins is essential for zebrafish development, with μ1A and μ1C being involved in central nervous system development and μ1B in kidney, gut and liver formation.

CONCLUSIONS

Zebrafish is unique in expressing three AP-1 complexes: AP-1A, AP-1B, and AP-1C. Our results demonstrate that they are not redundant and that each of them has specific functions, which cannot be fulfilled by one of the other isoforms. Each of the μ1 adaptins appears to mediate specific molecular mechanisms essential for early developmental processes, which depends on specific intracellular vesicular protein sorting pathways.

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.

odd-skipped related 2 is required for fin chondrogenesis in zebrafish

BACKGROUND

odd-skipped related 2 (osr2) encodes a vertebrate ortholog of the Drosophila odd-skipped zinc-finger transcription factor. Osr2 in mouse is required for proper palate, eyelid, and bone development. Zebrafish knock-down experiments have also suggested a role for osr2, along with its paralog osr1, in early pectoral fin specification and pronephric development.

RESULTS

We show here that osr2 has a specific function later in development, independent of osr1, in the regulation of sox9a expression and promoting fin chondrogenesis. mRNA in situ hybridization demonstrated osr2 expression in the developing floorplate and later during organogenesis in the pronephros and gut epithelium. In the pectoral fin buds, osr2 was specifically expressed in fin mesenchyme. osr2 knock down in zebrafish embryos disrupted both three and five zinc finger alternatively spliced osr2 isoforms and eliminated wild-type osr2 mRNA. osr2 morphants exhibited normal pectoral fin bud specification but exhibited defective fin chondrogenesis, with loss of differentiated chondrocytes. Defects in chondrogenesis were paralleled by loss of sox9a as well as subsequent col2a1 expression, linking osr2 function to essential regulators of chondrogenesis.

CONCLUSIONS

The zebrafish odd-skipped related 2 gene regulates sox9a and col2a1 expression in chondrocyte development and is specifically required for zebrafish fin morphogenesis.

The podocyte slit diaphragm--from a thin grey line to a complex signalling hub

The architectural design of our kidneys is amazingly complex, and culminates in the 3D structure of the glomerular filter. During filtration, plasma passes through a sieve consisting of a fenestrated endothelium and a broad basement membrane before it reaches the most unique part, the slit diaphragm, a specialized type of intercellular junction that connects neighbouring podocyte foot processes. When podocytes become stressed, irrespective of the causative stimulus, they undergo foot process effacement and loss of slit diaphragms--two key steps leading to proteinuria. Thus, proteinuria is the unifying denominator of a broad spectrum of podocytopathies. With the rising prevalence of chronic kidney disease and the fact that glomerular diseases account for the majority of patients with end-stage renal disease, further investigation and elucidation of this unique structure is of paramount importance. This Review recounts how perception of the slit diaphragm has changed over time as a result of intense research, from its first anatomical description as a thin intercellular connection, to an appreciation of its role as a dynamic signalling hub. These observations led to the introduction of novel concepts in podocyte biology, which could pave the way to development of highly desired, specific therapeutic strategies for glomerular diseases.

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.

The role of aquaporin and tight junction proteins in the regulation of water movement in larval zebrafish (Danio rerio)

Teleost fish living in freshwater are challenged by passive water influx; however the molecular mechanisms regulating water influx in fish are not well understood. The potential involvement of aquaporins (AQP) and epithelial tight junction proteins in the regulation of transcellular and paracellular water movement was investigated in larval zebrafish (Danio rerio). We observed that the half-time for saturation of water influx (K_u) was 4.3±0.9 min, and reached equilibrium at approximately 30 min. These findings suggest a high turnover rate of water between the fish and the environment. Water influx was reduced by the putative AQP inhibitor phloretin (100 or 500 μM). Immunohistochemistry and confocal microscopy revealed that AQP1a1 protein was expressed in cells on the yolk sac epithelium. A substantial number of these AQP1a1-positive cells were identified as ionocytes, either H⁺-ATPase-rich cells or Na⁺/K⁺-ATPase-rich cells. AQP1a1 appeared to be expressed predominantly on the basolateral membranes of ionocytes, suggesting its potential involvement in regulating ionocyte volume and/or water flux into the circulation. Additionally, translational gene knockdown of AQP1a1 protein reduced water influx by approximately 30%, further indicating a role for AQP1a1 in facilitating transcellular water uptake. On the other hand, incubation with the Ca²⁺-chelator EDTA or knockdown of the epithelial tight junction protein claudin-b significantly increased water influx. These findings indicate that the epithelial tight junctions normally act to restrict paracellular water influx. Together, the results of the present study provide direct in vivo evidence that water movement can occur through transcellular routes (via AQP); the paracellular routes may become significant when the paracellular permeability is increased.

Sequential effects of spadetail, one-eyed pinhead and no tail on midline convergence of nephric primordia during zebrafish embryogenesis

Midline convergence of organ primordia is an important mechanism that shapes the vertebrate body plan. Here, we focus on the morphogenetic movements of pronephric glomerular primordia (PGP) occurring during zebrafish embryonic kidney development. To characterize the process of PGP midline convergence, we used Wilms' tumour 1a (wt1a) as a marker to label kidney primordia, and performed quantitative analyses of the migration of the bilateral PGP. The PGP initially are approximately 350 μm apart in a wild type embryo at 10h post fertilization (hpf). The inter-PGP distance decreases exponentially between 10 and 48 hpf, while the anterior-posterior (A-P) dimension of each PGP increases linearly between 10 and 12 hpf, then decreases substantially between 12 and 24 hpf. Using mutants in the Nodal receptor cofactor one-eyed pinhead (oep) and the T-box transcription factors spadetail (spt) and no tail (ntl), we were able to define distinctive regulation underlying these sequential phases of PGP midline migration. Zygotic oep mutants (Zoep(-/-)) exhibited defects in midline convergence after 16 hpf. Spt is necessary for PGP convergence from 10 hpf, whereas ntl's effect on convergence does not begin until 24 hpf. Notably, we observed normal cardiac convergence in spt(-/-) and ntl(-/-) embryos implying that these novel roles of spt and ntl in PGP migration cannot be explained simply by generalised effects on midline convergence. These findings demonstrate that quantitative approaches to developmental migration allow the parsing of early patterning events, and in this instance suggest that the zebrafish may offer insights into midline urogenital migration anomalies in humans.

Myo1c is an unconventional myosin required for zebrafish glomerular development

The targeting and organization of podocyte slit diaphragm proteins nephrin and neph1 is critical for development and maintenance of a functional glomerular filtration barrier. Myo1c is a non-muscle myosin motor protein that interacts directly with nephrin and neph1 and mediates their intracellular transport to the podocyte intercellular junction. Here we investigated the necessity of Myo1c in podocyte development using zebrafish as a model system. Immunofluorescence microscopy and in situ RNA hybridization analysis of zebrafish embryos showed that Myo1c is widely expressed in various tissues including the zebrafish glomerulus. Knockdown of the Myo1c gene in zebrafish using antisense morpholino derivatives resulted in an abnormal developmental phenotype that included pericardial edema and dilated renal tubules. Ultra-structural analysis of the glomerulus in Myo1c depleted zebrafish showed abnormal podocyte morphology and absence of the slit diaphragm. Consistent with these observations, the glomerular filter permeability appeared altered in zebrafish in which Myo1c expression was attenuated. The specificity of Myo1c knockdown was confirmed by a rescue experiment in which co-injection of Myo1c morpholino derivatives with orthologous Myo1c mRNA prepared from mouse cDNA lessened phenotypic abnormalities including edema in Myo1c morphants. Thus, our results demonstrate that Myo1c is necessary for podocyte morphogenesis.

The role of the podocyte in albumin filtration

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

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.

Developmental localization of nephrin in zebrafish and medaka pronephric glomerulus

Slit diaphragm (SD) is a highly specialized intercellular junction between podocyte foot processes and plays a crucial role in the formation of the filtration barrier. In this study, we examined the developmental localization of Nephrin, an essential component of SD, in the pronephric glomerulus of zebrafish and medaka. In the mature glomerulus of both fish, Nephrin is localized along the glomerular basement membrane as seen in mammals, indicating that Nephrin is localized at the SD. Interestingly, Nephrin was detected already in immature podocytes before the SD and foot processes started to form in both fish. Nephrin was localized along the cell surface of immature podocytes but as different localization patterns. In zebrafish, Nephrin signal bordered the lateral membrane of podocytes, which were columnar in shape, as in rat immature podocytes. However, in medaka immature podocytes, Nephrin was localized in a punctate pattern among podocyte cell bodies. These findings suggest that Nephrin needs to be integrated to the membrane before the formation of the SD and then moves to the proper site to form the SD. Furthermore, a podocyte-specific marker, such as Nephrin, should be a useful tool for the future analysis of pronephric glomerular development in fish mutants and morphants.

HNF1β is essential for nephron segmentation during nephrogenesis

Nephrons comprise a blood filter and an epithelial tubule that is subdivided into proximal and distal segments, but what directs this patterning during kidney organogenesis is not well understood. Using zebrafish, we found that the HNF1β paralogues hnf1ba and hnf1bb, which encode homeodomain transcription factors, are essential for normal segmentation of nephrons. Embryos deficient in hnf1ba and hnf1bb did not express proximal and distal segment markers, yet still developed an epithelial tubule. Initiating hnf1ba/b expression required Pax2a and Pax8, but hnf1ba/b-deficient embryos did not exhibit the expected downregulation of pax2a and pax8 at later stages of development, suggesting complex regulatory loops involving these molecules. Embryos deficient in hnf1ba/b also did not express the irx3b transcription factor, which is responsible for differentiation of the first distal tubule segment. Reciprocally, embryos deficient in irx3b exhibited downregulation of hnf1ba/b transcripts in the distal early segment, suggesting a segment-specific regulatory circuit. Deficiency of hnf1ba/b also led to ectopic expansion of podocytes into the proximal tubule domain. Epistasis experiments showed that the formation of podocytes required wt1a, which encodes the Wilms' tumor suppressor-1 transcription factor, and rbpj, which encodes a mediator of canonical Notch signaling, downstream or parallel to hnf1ba/b. Taken together, these results suggest that Hnf1β factors are essential for normal segmentation of nephrons during kidney organogenesis.

Structural disorganization of pronephric glomerulus in zebrafish mpp5a/nagie oko mutant

BACKGROUND

The podocyte slit diaphragm (SD) is an essential component of the selective filtration barrier in the glomerulus. Several structural proteins required for formation and maintenance of SD have been identified; however, molecular mechanisms regulating these proteins are still limited.

RESULTS

Here, we demonstrate that MAGUK p55 subfamily member 5a (Mpp5a)/Nagie oko, a component of the Crb multi-protein complex, was colocalized with an SD-associated protein ZO-1 in the zebrafish pronephric glomerulus. To characterize the function of Mpp5a, zebrafish mpp5a(m520) mutant embryos, which are known to have defects in cardiac and neuronal morphogenesis, were analyzed. These mutants failed to merge the bilateral glomerular primordia and to form the glomerular capillary and mesangium, but the foot processes and SD showed normal appearance. The structural disorganization in the mpp5a(m520) mutant glomerulus was quite similar to that of a cardiac troponin T2a/tnnt2a/silent heart knockdown zebrafish, which exhibited circulatory failure due to lack of heart beating.

CONCLUSIONS

Mpp5a is not prerequisite to form podocyte slit diaphragm in the pronephric glomerular development in zebrafish. The structural disorganization of the pronephric glomerulus in the mpp5a(m520) mutant is likely to result from circulatory failure, rather than the anomaly of Mpp5a protein in the glomerulus.

Kidney organogenesis in the zebrafish: insights into vertebrate nephrogenesis and regeneration

Vertebrates form a progressive series of up to three kidney organs during development-the pronephros, mesonephros, and metanephros. Each kidney derives from the intermediate mesoderm and is comprised of conserved excretory units called nephrons. The zebrafish is a powerful model for vertebrate developmental genetics, and recent studies have illustrated that zebrafish and mammals share numerous similarities in nephron composition and physiology. The zebrafish embryo forms an architecturally simple pronephros that has two nephrons, and these eventually become a scaffold onto which a mesonephros of several hundred nephrons is constructed during larval stages. In adult zebrafish, the mesonephros exhibits ongoing nephrogenesis, generating new nephrons from a local pool of renal progenitors during periods of growth or following kidney injury. The characteristics of the zebrafish pronephros and mesonephros make them genetically tractable kidney systems in which to study the functions of renal genes and address outstanding questions about the mechanisms of nephrogenesis. Here, we provide an overview of the formation and composition of these zebrafish kidney organs, and discuss how various zebrafish mutants, gene knockdowns, and transgenic models have created frameworks in which to further delineate nephrogenesis pathways.

Live imaging kidney development in zebrafish

The zebrafish has emerged as a powerful model to study organ development and regeneration. It has a number of advantages over other vertebrate model systems. The embryo can be kept transparent throughout embryonic development, which allows direct visualization of the developing organs. In addition, embryos can be easily manipulated surgically, genetically, or chemically. Furthermore, because nephron shape and function are remarkably conserved among vertebrates, zebrafish findings can directly inform human studies. Here, we describe a simple procedure that can be used by laboratories to investigate the development of zebrafish kidney and other organs by time-lapse microscopy.

A comparative analysis of glomerulus development in the pronephros of medaka and zebrafish

The glomerulus of the vertebrate kidney links the vasculature to the excretory system and produces the primary urine. It is a component of every single nephron in the complex mammalian metanephros and also in the primitive pronephros of fish and amphibian larvae. This systematic work highlights the benefits of using teleost models to understand the pronephric glomerulus development. The morphological processes forming the pronephric glomerulus are astoundingly different between medaka and zebrafish. (1) The glomerular primordium of medaka - unlike the one of zebrafish - exhibits a C-shaped epithelial layer. (2) The C-shaped primordium contains a characteristic balloon-like capillary, which is subsequently divided into several smaller capillaries. (3) In zebrafish, the bilateral pair of pronephric glomeruli is fused at the midline to form a glomerulus, while in medaka the two parts remain unmerged due to the interposition of the interglomerular mesangium. (4) Throughout pronephric development the interglomerular mesangial cells exhibit numerous cytoplasmic granules, which are reminiscent of renin-producing (juxtaglomerular) cells in the mammalian afferent arterioles. Our systematic analysis of medaka and zebrafish demonstrates that in fish, the morphogenesis of the pronephric glomerulus is not stereotypical. These differences need be taken into account in future analyses of medaka mutants with glomerulus defects.

Expression of fgf23 and αklotho in developing embryonic tissues and adult kidney of the zebrafish, Danio rerio

Fibroblast growth factor 23 (FGF23) is an endocrine hormone that is secreted by bone and acts on the kidney and parathyroid glands to regulate phosphate homeostasis. The effects of FGF23 on phosphate homeostasis are mediated by binding to FGF receptors and their coreceptor, αklotho, which are abundantly expressed in the kidney and parathyroid glands. However, the mechanisms of how FGF23 regulates phosphate handling in the proximal tubule are unclear because αklotho is primarily expressed in the distal nephron in humans and rodents. The purpose of this study was to gain additional insight into the FGF23-αklotho system by investigating the spatial and temporal aspects of the expression of fgf23 and αklotho in the zebrafish, Danio rerio. Here, we report that zebrafish fgf23 begins to be expressed after organogenesis and is continually expressed into adulthood in the corpuscles of Stannius, which are endocrine glands that lie in close proximity to the nephron and are thought to contribute to calcium and phosphate homeostasis in fish. Zebrafish αklotho expression can be detected by 24-h postfertilization in the brain, pancreas and the distal pronephros, and by 56-h postfertilization in liver. Expression in the distal pronephros persists throughout development, and by Day 5, there is also strong expression in the proximal pronephros. αklotho continues to be expressed in the tubules of the metanephros of the adult kidney. These data indicate conservation of the FGF23-αklotho system across species and suggest a likely role for αklotho in the proximal and distal tubules.

Genome-wide association and functional follow-up reveals new loci for kidney function

Chronic kidney disease (CKD) is an important public health problem with a genetic component. We performed genome-wide association studies in up to 130,600 European ancestry participants overall, and stratified for key CKD risk factors. We uncovered 6 new loci in association with estimated glomerular filtration rate (eGFR), the primary clinical measure of CKD, in or near MPPED2, DDX1, SLC47A1, CDK12, CASP9, and INO80. Morpholino knockdown of mpped2 and casp9 in zebrafish embryos revealed podocyte and tubular abnormalities with altered dextran clearance, suggesting a role for these genes in renal function. By providing new insights into genes that regulate renal function, these results could further our understanding of the pathogenesis of CKD.

Inducible podocyte injury and proteinuria in transgenic zebrafish

Damage or loss of podocytes causes glomerulosclerosis in murine models, and mutations in podocyte-specific genes cause nephrotic syndrome in humans. Zebrafish provide a valuable model for kidney research, but disruption of pronephroi leads to death within a few days, thereby preventing the study of CKD. In this study, we generated an inducible model of podocyte injury in zebrafish (pod::NTR-mCherry) by expressing a bacterial nitroreductase, which converts metronidazole to a cytotoxin, specifically in podocytes under the control of the zebrafish nphs2/podocin promoter. Application of the prodrug metronidazole to the transgenic fish induces acute damage to the podocytes in pronephroi of larval zebrafish and the mesonephroi of adult zebrafish, resulting in foot-process effacement and podocyte loss. We also developed a functional assay of the glomerular filtration barrier by creating transgenic zebrafish expressing green fluorescent protein (GFP)-tagged vitamin D-binding protein (VDBP) as a tracer for proteinuria. In the VDBP-GFP and pod::NTR-mCherry double-transgenic fish, induction of podocyte damage led to whole-body edema, and the proximal tubules reabsorbed and accumulated VDBP-GFP that leaked through the glomeruli, mimicking the phenotype of human nephrotic syndrome. Moreover, expression of wt1b::GFP, a marker for the developing nephron, extended into the Bowman capsule in response to podocyte injury, suggesting that zebrafish have a podocyte-specific repair process known to occur in mammalian metanephros. These data support the use of these transgenic zebrafish as a model system for studies of glomerular pathogenesis and podocyte regeneration.

Inducible podocyte injury and proteinuria in transgenic zebrafish

Damage or loss of podocytes causes glomerulosclerosis in murine models, and mutations in podocyte-specific genes cause nephrotic syndrome in humans. Zebrafish provide a valuable model for kidney research, but disruption of pronephroi leads to death within a few days, thereby preventing the study of CKD. In this study, we generated an inducible model of podocyte injury in zebrafish (pod::NTR-mCherry) by expressing a bacterial nitroreductase, which converts metronidazole to a cytotoxin, specifically in podocytes under the control of the zebrafish nphs2/podocin promoter. Application of the prodrug metronidazole to the transgenic fish induces acute damage to the podocytes in pronephroi of larval zebrafish and the mesonephroi of adult zebrafish, resulting in foot-process effacement and podocyte loss. We also developed a functional assay of the glomerular filtration barrier by creating transgenic zebrafish expressing green fluorescent protein (GFP)-tagged vitamin D-binding protein (VDBP) as a tracer for proteinuria. In the VDBP-GFP and pod::NTR-mCherry double-transgenic fish, induction of podocyte damage led to whole-body edema, and the proximal tubules reabsorbed and accumulated VDBP-GFP that leaked through the glomeruli, mimicking the phenotype of human nephrotic syndrome. Moreover, expression of wt1b::GFP, a marker for the developing nephron, extended into the Bowman capsule in response to podocyte injury, suggesting that zebrafish have a podocyte-specific repair process known to occur in mammalian metanephros. These data support the use of these transgenic zebrafish as a model system for studies of glomerular pathogenesis and podocyte regeneration.

The lineage-specific gene ponzr1 is essential for zebrafish pronephric and pharyngeal arch development

The Homeobox (Hox) and Paired box (Pax) gene families are key determinants of animal body plans and organ structure. In particular, they function within regulatory networks that control organogenesis. How these conserved genes elicit differences in organ form and function in response to evolutionary pressures is incompletely understood. We molecularly and functionally characterized one member of an evolutionarily dynamic gene family, plac8 onzin related protein 1 (ponzr1), in the zebrafish. ponzr1 mRNA is expressed early in the developing kidney and pharyngeal arches. Using ponzr1-targeting morpholinos, we show that ponzr1 is required for formation of the glomerulus. Loss of ponzr1 results in a nonfunctional glomerulus but retention of a functional pronephros, an arrangement similar to the aglomerular kidneys found in a subset of marine fish. ponzr1 is integrated into the pax2a pathway, with ponzr1 expression requiring pax2a gene function, and proper pax2a expression requiring normal ponzr1 expression. In addition to pronephric function, ponzr1 is required for pharyngeal arch formation. We functionally demonstrate that ponzr1 can act as a transcription factor or co-factor, providing the first molecular mode of action for this newly described gene family. Together, this work provides experimental evidence of an additional mechanism that incorporates evolutionarily dynamic, lineage-specific gene families into conserved regulatory gene networks to create functional organ diversity.

Techniques for the in vivo assessment of cardio-renal function in zebrafish (Danio rerio) larvae

Zebrafish, a well-established vertebrate model, offer unique advantages for assessing renal function and physiology. Assays determining renal glomerular function based on cardiovascular erythrocyte flow and reduction of injected FITC-inulin were developed, each validated using the nephrotoxin gentamicin. Bland–Atlman analysis showed a strong association between measurements of the rate of inulin excretion and that of fluorescent reduction from the arterial vasculature. Reduced renal clearance of inulin, resulting from gentamicin or NaCl loading, was concurrent with reduced erythrocyte velocity, and yolk sac and pericardium oedema. These techniques, assessing pronephric function, highlight the potential for in vivo physiological study in this genetically tractable model.

Neph3 associates with regulation of glomerular and neural development in zebrafish

Neph3 (filtrin) is a membrane protein expressed in the glomerular epithelial cells (podocytes), but its role in the glomerulus is still largely unknown. To characterize the function of Neph3 in the glomerulus, we employed the zebrafish as a model system. Here we show that the expression of neph3 in pronephros starts before the onset of nephrin and podocin expression, peaks when the nephron primordium differentiates into glomerulus and tubulus, and is then downregulated upon glomerular maturation. By histology, we found that neph3 is specifically expressed in pronephric podocytes at 36hpf. Furthermore, disruption of neph3 expression by antisense morpholino oligonucleotides results in distorted body curvature and transient pericardial edema, the latter likely reflecting perturbation of glomerular osmoregulatory function. Histological analysis of neph3 morphants reveals altered glomerular morphology and dilated pronephric tubules. The phenotype of neph3 morphants, curved body and pericardial edema, is rescued by wild-type zebrafish neph3 mRNA. In addition to glomerulus, neph3 is highly expressed in the developing brain and specific regions of mature midbrain and hindbrain. In line with this, neph3 morphants show aberrant brain morphology. Collectively, the expression of neph3 in glomerulus and brain together with the morphant phenotype imply that neph3 is a pleiotropic gene active during distinct stages of tissue differentiation and associates directly in the regulation of both glomerular and neural development.

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.

Genetic association for renal traits among participants of African ancestry reveals new loci for renal function

Chronic kidney disease (CKD) is an increasing global public health concern, particularly among populations of African ancestry. We performed an interrogation of known renal loci, genome-wide association (GWA), and IBC candidate-gene SNP association analyses in African Americans from the CARe Renal Consortium. In up to 8,110 participants, we performed meta-analyses of GWA and IBC array data for estimated glomerular filtration rate (eGFR), CKD (eGFR <60 mL/min/1.73 m(2)), urinary albumin-to-creatinine ratio (UACR), and microalbuminuria (UACR >30 mg/g) and interrogated the 250 kb flanking region around 24 SNPs previously identified in European Ancestry renal GWAS analyses. Findings were replicated in up to 4,358 African Americans. To assess function, individually identified genes were knocked down in zebrafish embryos by morpholino antisense oligonucleotides. Expression of kidney-specific genes was assessed by in situ hybridization, and glomerular filtration was evaluated by dextran clearance. Overall, 23 of 24 previously identified SNPs had direction-consistent associations with eGFR in African Americans, 2 of which achieved nominal significance (UMOD, PIP5K1B). Interrogation of the flanking regions uncovered 24 new index SNPs in African Americans, 12 of which were replicated (UMOD, ANXA9, GCKR, TFDP2, DAB2, VEGFA, ATXN2, GATM, SLC22A2, TMEM60, SLC6A13, and BCAS3). In addition, we identified 3 suggestive loci at DOK6 (p-value = 5.3×10(-7)) and FNDC1 (p-value = 3.0×10(-7)) for UACR, and KCNQ1 with eGFR (p = 3.6×10(-6)). Morpholino knockdown of kcnq1 in the zebrafish resulted in abnormal kidney development and filtration capacity. We identified several SNPs in association with eGFR in African Ancestry individuals, as well as 3 suggestive loci for UACR and eGFR. Functional genetic studies support a role for kcnq1 in glomerular development in zebrafish.

Uncharted waters: nephrogenesis and renal regeneration in fish and mammals

The major functions of the vertebrate kidney are the removal of metabolic waste and the balance of salt and water. These roles are fulfilled by nephrons, which generally comprise a blood filter (glomerulus) attached to an epithelial tubule. The number of nephrons in the mammalian kidney is set at the end of kidney organogenesis in the late fetal or neonatal period. Subsequent increases in nephron size and functionality then occur during postnatal growth to match increases in body mass/fluid. Because of this strategy of renal development, injuries or birth defects that reduce nephron number lead to a permanent nephron deficit and increase the risk of kidney disease. In contrast to mammals, fish kidneys continue to add nephrons throughout their lifespan. In response to renal injury, fish increase the rate of nephrogenesis, effectively replacing lost nephrons and maintaining their nephron endowment. A better understanding of the remarkable nephrogenic abilities of fish kidneys may lead to innovative ways to restore nephrogenesis in the adult mammalian kidney. This review examines our current understanding of nephrogenesis in mammals and fish and explores possible explanations for why fish, but not mammals, utilize a perpetual nephrogenesis strategy to grow and maintain their kidneys.

Non-muscle myosin IIA is required for the development of the zebrafish glomerulus

Mutations in the MYH9 gene, coding for the non-muscle myosin heavy chain IIA (NMHC-IIA), are responsible for syndromes characterized by macrothrombocytopenia associated with deafness, cataracts, and severe glomerular disease. Electron microscopy of renal biopsies from these patients found glomerular abnormalities characterized by alterations in mesangial cells, podocytes, and thickening of the glomerular basement membrane. Knockout of NMHC-IIA in mice is lethal, and therefore little is known about the glomerular-related functions of Myh9. Here, we use zebrafish as a model to study the role and function of zNMHC-IIA in the glomerulus. Knockdown of zNMHC-IIA resulted in malformation of the glomerular capillary tuft characterized by few and dilated capillaries of the pronephros. In zNMHC-IIA morphants, endothelial cells failed to develop fenestrations, mesangial cells were absent or reduced, and the glomerular basement membrane appeared nonuniformly thickened. Knockdown of zNMHC-IIA did not impair the formation of podocyte foot processes or slit diaphragms; however, podocyte processes were less uniform in these morphants compared to controls. In vivo clearance of fluorescent dextran indicated that the glomerular barrier function was not compromised by zNMHC-IIA knockdown; however, glomerular filtration was significantly reduced. Thus, our results demonstrate an important role of zNMHC-IIA for the proper formation and function of the glomerulus in zebrafish.

Wt1a, Foxc1a, and the Notch mediator Rbpj physically interact and regulate the formation of podocytes in zebrafish

Podocytes help form the glomerular blood filtration barrier in the kidney and their injury or loss leads to renal disease. The Wilms' tumor suppressor-1 (Wt1) and the FoxC1/2 transcription factors, as well as Notch signaling, have been implicated as important regulators of podocyte fate. It is not known whether these factors work in parallel or sequentially on different gene targets, or as higher-order transcriptional complexes on common genes. Here, we use the zebrafish to demonstrate that embryos treated with morpholinos against wt1a, foxc1a, or the Notch transcriptional mediator rbpj develop fewer podocytes, as determined by wt1b, hey1 and nephrin expression, while embryos deficient in any two of these factors completely lack podocytes. From GST-pull-downs and co-immunoprecipitation experiments we show that Wt1a, Foxc1a, and Rbpj can physically interact with each other, whereas only Rbpj binds to the Notch intracellular domain (NICD). In transactivation assays, combinations of Wt1, FoxC1/2, and NICD synergistically induce the Hey1 promoter, and have additive or repressive effects on the Podocalyxin promoter, depending on dosage. Taken together, these data suggest that Wt1, FoxC1/2, and Notch signaling converge on common target genes where they physically interact to regulate a podocyte-specific gene program. These findings further our understanding of the transcriptional circuitry responsible for podocyte formation and differentiation during kidney development.

Nephrocystin-4 is required for pronephric duct-dependent cloaca formation in zebrafish

NPHP4 mutations cause nephronophthisis, an autosomal recessive cystic kidney disease associated with renal fibrosis and kidney failure. The NPHP4 gene product nephrocystin-4 interacts with other nephrocystins, cytoskeletal and ciliary proteins; however, the molecular and cellular functions of nephrocystin-4 have remained elusive. Here we demonstrate that nephrocystin-4 is required for normal cloaca formation during zebrafish embryogenesis. Time-lapse imaging of the developing zebrafish pronephros revealed that tubular epithelial cells at the distal pronephros actively migrate between the yolk sac extension and the blood island towards the ventral fin fold to join the proctodeum and to form the cloaca. Nphp4-deficient pronephric duct cells failed to connect with their ectodermal counterparts, and instead formed a vesicle at the obstructed end of the pronephric duct. Nephrocystin-4 interacts with nephrocystin-1 and Par6. Depletion of zebrafish NPHP1 (nphp1) increased the incidence of cyst formation and randomization of the normal body axis, but did not augment cloaca malformation in nphp4-deficient zebrafish embryos. However, simultaneous depletion of zebrafish Par6 (pard6) aggravated cloaca formation defects in nphp4-depleted embryos, suggesting that nphp4 orchestrates directed cell migration and cloaca formation through interaction with the Par protein complex.

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

Podocytes do not remain fully differentiated when cultured, and they are difficult to image in vivo, making the study of podocyte biology challenging. Zebrafish embryos are transparent and develop a single, midline, pronephric glomerulus accessible for imaging and systematic functional analysis. Here, we describe a transgenic zebrafish line that expresses green fluorescence protein (GFP) from the zebrafish podocin promoter. The line recapitulates the endogenous pronephric podocin expression pattern, showing GFP expression exclusively in podocytes starting 2 days postfertilization. Using the podocyte GFP signal as a guide for dissection, we examined the pronephric glomerulus by scanning electron microscopy; the surface ultrastructure exhibited fine, interdigitating podocyte foot processes surrounding glomerular capillaries. To determine whether the GFP signal could serve as a direct readout of developmental abnormalities or injury to the glomerulus, we knocked down the podocyte-associated protein crb2b; this led to a loss of GFP signal. Thus, podocin-GFP zebrafish provide a model for ultrastructural studies and in vivo visualization and functional analysis of glomerular podocytes. This model should also be useful for high-throughput genetic or chemical analysis of glomerular development and function.

Expression and function of the Ets transcription factor pea3 during formation of zebrafish pronephros

Polyomavirus enhancer activator 3 (Pea3), belonging to the PEA3 subfamily of Ets transcription factors, is essential for certain organogenesis in mammals. Previously, we found that pea3 correlated with wt1 expression and may contribute to nephrogenesis in rats. Here, we observed that pea3 was mainly expressed in the zebrafish pronephric glomerulus. We further performed functional analyses by in situ hybridization of pea3 in zebrafish embryos after pea3 messenger RNA (mRNA) overexpression and inhibition of double-target genes (pea3 and erm, another member of the PEA3 subfamily) by antisense morpholino-oligonucleotides (MO). Overexpression of pea3 induced abnormal pronephrogenesis. However, MO-pea3 coinjected with MO-erm, but not alone, inhibited zebrafish pronephros development, and these defects were rescued by overexpression of the zebrafish wt1a gene. Thus, pea3 and erm are required for zebrafish pronephrogenesis and can functionally complement each other, and the wt1a gene may be one of their downstream targets.

Identification of adult nephron progenitors capable of kidney regeneration in zebrafish

Loss of kidney function underlies many renal diseases. Mammals can partly repair their nephrons (the functional units of the kidney), but cannot form new ones. By contrast, fish add nephrons throughout their lifespan and regenerate nephrons de novo after injury, providing a model for understanding how mammalian renal regeneration may be therapeutically activated. Here we trace the source of new nephrons in the adult zebrafish to small cellular aggregates containing nephron progenitors. Transplantation of single aggregates comprising 10-30 cells is sufficient to engraft adults and generate multiple nephrons. Serial transplantation experiments to test self-renewal revealed that nephron progenitors are long-lived and possess significant replicative potential, consistent with stem-cell activity. Transplantation of mixed nephron progenitors tagged with either green or red fluorescent proteins yielded some mosaic nephrons, indicating that multiple nephron progenitors contribute to a single nephron. Consistent with this, live imaging of nephron formation in transparent larvae showed that nephrogenic aggregates form by the coalescence of multiple cells and then differentiate into nephrons. Taken together, these data demonstrate that the zebrafish kidney probably contains self-renewing nephron stem/progenitor cells. The identification of these cells paves the way to isolating or engineering the equivalent cells in mammals and developing novel renal regenerative therapies.