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

Basement membrane diseases in zebrafish

Basement membranes (BMs) are a complex, sheet-like network of specialized extracellular matrix that underlies epithelial cells and surrounds muscle cells. They provide adherence between neighboring tissues, permit some flexibility of these adherent structures, and can act as a store for growth factors and as a guide for cell migration. The BM is not just a static structure; its deposition and remodeling are important for many processes including embryonic development, immune response, and wound healing. To date, dysfunction in BM deposition or remodeling has been linked to many human congenital disorders and diseases, affecting many different tissues in the body, including malformations, dystrophies, and cancer. However, many questions remain to be answered on the role BM proteins, and their mutations, play in the pathogenesis of human disease. In recent years, the zebrafish (Danio rerio) has emerged as a powerful animal model for human development and disease. In the first part of this chapter, we provide an overview of described defects caused by BM dysfunction in zebrafish, including development and function of notochord, muscle, central nervous system, skin, cardiovascular system, and kidney. In the second part, we will describe details of methods used to visualize and assess the structure of the BM in zebrafish, and to functionally analyze its different components.

Microdomain-forming proteins and the role of the reggies/flotillins during axon regeneration in zebrafish

The two proteins reggie-1 and reggie-2 (flotillins) were identified in axon-regenerating neurons in the central nervous system and shown to be essential for neurite growth and regeneration in fish and mammals. Reggies/flotillins are microdomain scaffolding proteins sharing biochemical properties with lipid raft molecules, form clusters at the cytoplasmic face of the plasma membrane and interact with signaling molecules in a cell type specific manner. In this review, reggie microdomains, lipid rafts, related scaffolding proteins and caveolin-which, however, are responsible for their own microdomains and functions-are introduced. Moreover, the function of the reggies in axon growth is demonstrated: neurons fail to extend axons after reggie knockdown. Furthermore, our current concept of the molecular mechanism underlying reggie function is presented: the association of glycosyl-phophatidyl inositol (GPJ)-anchored surface proteins with reggie microdomains elicits signals which activate src tyrosine and mitogen-activated protein kinases, as well as small guanosine 5'-triphosphate-hydrolyzing enzymes. This leads to the mobilization of intracellular vesicles and to the recruitment of bulk membrane and specific cargo proteins, such as cadherin, to specific sites of the plasma membrane such as the growth cone of elongating axons. Thus, reggies regulate the targeted delivery of cargo-a process which is required for process extension and growth. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Deficiency of a transmembrane prolyl 4-hydroxylase in the zebrafish leads to basement membrane defects and compromised kidney function

Prolyl 4-hydroxylases (P4Hs) catalyze the hydroxylation of collagens and hypoxia-inducible factor (HIF)-α subunits. We studied the zebrafish homologue of the recently characterized human transmembrane P4H (P4H-TM) that can hydroxylate HIF-α, but not collagens, in vitro and influence HIF-α levels in cellulo. The zebrafish P4H-TM mRNA had its highest expression in the eye and brain and lower levels in other tissues, including the kidney. Morpholino knockdown of P4H-TM in embryos resulted in a reduction in the size of the eye and head and morphological alterations in the head from 2 days postfertilization onward. In addition, pericardial edema, regarded as a sign of kidney dysfunction, developed from 3 days postfertilization onward. The phenotype was dependent on the P4H-TM catalytic activity because similar results were obtained with morpholinos targeting either translation initiation or catalytic residues of the enzyme. Structural and functional analyses of the morphant pronephric kidneys revealed fragmented glomerular basement membranes (BMs), disorganized podocyte foot processes, and severely compromised pronephric kidney function leading to proteinuria. The opacity of the eye lens was increased due to the presence of extra nuclei and deposits, and the structure of the lens capsule BM was altered. Our data suggest that P4H-TM catalytic activity is required for the proper development of the glomerular and lens capsule BMs. Many HIF target genes were induced in the P4H-TM-deficient morphants, but the observed phenotype is not likely to be mediated at least solely via the HIF pathway, and thus P4H-TM probably has additional, as yet unknown, substrates.

Notch signaling, wt1 and foxc2 are key regulators of the podocyte gene regulatory network in Xenopus

Podocytes are highly specialized cells in the vertebrate kidney. They participate in the formation of the size-exclusion barrier of the glomerulus/glomus and recruit mesangial and endothelial cells to form a mature glomerulus. At least six transcription factors (wt1, foxc2, hey1, tcf21, lmx1b and mafb) are known to be involved in podocyte specification, but how they interact to drive the differentiation program is unknown. The Xenopus pronephros was used as a paradigm to address this question. All six podocyte transcription factors were systematically eliminated by antisense morpholino oligomers. Changes in the expression of the podocyte transcription factors and of four selected markers of terminal differentiation (nphs1, kirrel, ptpru and nphs2) were analyzed by in situ hybridization. The data were assembled into a transcriptional regulatory network for podocyte development. Although eliminating the six transcription factors individually interfered with aspects of podocyte development, no single gene regulated the entire differentiation program. Only the combined knockdown of wt1 and foxc2 resulted in a loss of all podocyte marker gene expression. Gain-of-function studies showed that wt1 and foxc2 were sufficient to increase podocyte gene expression within the glomus proper. However, the combination of wt1, foxc2 and Notch signaling was required for ectopic expression in ventral marginal zone explants. Together, this approach demonstrates how complex interactions are required for the correct spatiotemporal execution of the podocyte gene expression program.

A model organism approach: defining the role of Neph proteins as regulators of neuron and kidney morphogenesis

Mutations of the immunoglobulin superfamily proteins nephrin and Neph1 lead to congenital nephrotic syndrome in humans or mice. Neph proteins are three closely related molecules that are evolutionarily conserved and mediate cell recognition. Their importance for morphogenetic processes including the formation of the kidney filtration barrier in vertebrates and synaptogenesis in Caenorhabditis elegans has recently been uncovered. However, the individual morphogenetic function of mammalian Neph1-3 isoforms remained elusive. We demonstrate now that the Neph/nephrin family proteins can form cell-cell adhesion modules across species. Expression of all three mammalian Neph isoforms partially rescued mutant C. elegans lacking their Neph homolog syg-1 and restored synapse formation, suggesting a functional redundancy between the three isoforms. Strikingly, the rescue of defective synaptic connectivity was prevented by deletion of the highly conserved cytoplasmic PSD95/Dlg/ZO-1-binding motif of SYG-1/Neph proteins, indicating the critical role of this intracellular signaling motif for SYG-1/Neph-dependent morphogenetic events. To determine the significance of Neph isoform redundancy for vertebrate kidney development, we analyzed the expression pattern and the functional role of Neph proteins in zebrafish. In situ hybridizations identified zNeph1 and zNeph2 as glomerular proteins. Morpholino knockdown of either zNeph1 or zNeph2 resulted in loss of slit diaphragms and leakiness of the glomerular filtration barrier. This is the first report utilizing C. elegans to study mammalian Neph/nephrin protein function and to demonstrate a functional overlap of Neph1-3 proteins. Furthermore, we identify Neph2 as a novel critical regulator of glomerular function, indicating that both Neph1 and Neph2 are required for glomerular maintenance and development.

Zebrafish kidney development

The zebrafish pronephric kidney provides a useful and relevant model of kidney development and function. It is composed of cell types common to all vertebrate kidneys and pronephric organogenesis is regulated by transcription factors that have highly conserved functions in mammalian kidney development. Pronephric nephrons are a good model of tubule segmentation and differentiation of epithelial cell types. The pronephric glomerulus provides a simple model to assay gene function in regulating cell structure and cell interactions that form the blood filtration apparatus. The relative simplicity of the pronephric kidney combined with the ease of genetic manipulation in zebrafish makes it well suited for mutation analysis and gene discovery, in vivo imaging, functional screens of candidate genes from other species, and cell isolation by FACS . In addition, the larval and adult zebrafish kidneys have emerged as systems to study kidney regeneration after injury. This chapter provides a review of pronephric structure and development as well as current methods to study the pronephros.

Myoblast fusion: when it takes more to make one

Cell-cell fusion is a crucial and highly regulated event in the genesis of both form and function of many tissues. One particular type of cell fusion, myoblast fusion, is a key cellular process that shapes the formation and repair of muscle. Despite its importance for human health, the mechanisms underlying this process are still not well understood. The purpose of this review is to highlight the recent literature pertaining to myoblast fusion and to focus on a comparison of these studies across several model systems, particularly the fly, zebrafish and mouse. Advances in technical analysis and imaging have allowed identification of new fusion genes and propelled further characterization of previously identified genes in each of these systems. Among the cellular steps identified as critical for myoblast fusion are migration, recognition, adhesion, membrane alignment and membrane pore formation and resolution. Importantly, striking new evidence indicates that orthologous genes govern several of these steps across these species. Taken together, comparisons across three model systems are illuminating a once elusive process, providing exciting new insights and a useful framework of genes and mechanisms.

Bringing together components of the fly renal system

The function of all animal excretory systems is to rid the body of toxins and to maintain homeostatic balance. Although excretory organs in diverse animal species appear superficially different they are often built on two common principals: filtration and tubular secretion/reabsorbtion. The Drosophila excretory system is composed of filtration nephrocytes and Malpighian (renal) tubules. Here we review recent molecular genetic data on the development and differentiation of nephrocytes and renal tubules. We focus in particular on the molecular mechanisms that underpin key cell and tissue behaviours during morphogenesis, drawing parallels with other species where they exist. Finally we assess the implications of patterned tissue differentiation for the subsequent regulation of renal function. These studies highlight the continuing usefulness of the fly to provide fundamental insights into the complexities of organ formation.

FERM proteins in animal morphogenesis

Proteins containing a FERM domain are ubiquitous components of the cytocortex of animal cells where they are engaged in structural, transport, and signaling functions. Recent years have seen a wealth of genetic studies in model organisms that explore FERM protein function in development and tissue organization. In addition, mutations in several FERM protein-encoding genes have been associated with human diseases. This review will provide a brief overview of the FERM domain structure and the FERM protein superfamily and then discuss recent advances in our understanding of the mechanism of function and developmental requirement of several FERM proteins including Moesin, Myosin-VIIA, Myosin-XV, Coracle/Band4.1 as well as Yurt and its vertebrate homologs Mosaic Eyes and EPB41L5/YMO1/Limulus.

A role for nephrin, a renal protein, in vertebrate skeletal muscle cell fusion

Skeletal muscle is formed via fusion of myoblasts, a well-studied process in Drosophila. In vertebrates however, this process is less well understood, and whether there is evolutionary conservation with the proteins studied in flies is under investigation. Sticks and stones (Sns), a cell surface protein found on Drosophila myoblasts, has structural homology to nephrin. Nephrin is a protein expressed in kidney that is part of the filtration barrier formed by podocytes. No previous study has established any role for nephrin in skeletal muscle. We show, using two models, zebrafish and mice, that the absence of nephrin results in poorly developed muscles and incompletely fused myotubes, respectively. Although nephrin-knockout (nephrin(KO)) myoblasts exhibit prolonged activation of MAPK/ERK pathway during myogenic differentiation, expression of myogenin does not seem to be altered. Nevertheless, MAPK pathway blockade does not rescue myoblast fusion. Co-cultures of unaffected human fetal myoblasts with nephrin(KO) myoblasts or myotubes restore the formation of mature myotubes; however, the contribution of nephrin(KO) myoblasts is minimal. These studies suggest that nephrin plays a role in secondary fusion of myoblasts into nascent myotubes, thus establishing a possible functional conservation with Drosophila Sns.

A reverse genetic screen in the zebrafish identifies crb2b as a regulator of the glomerular filtration barrier

The glomerular filtration barrier is necessary for the selective passage of low molecular weight waste products and the retention of blood plasma proteins. Damage to the filter results in proteinuria. The filtration barrier is the major pathogenic site in almost all glomerular diseases and its study is therefore of clinical significance. We have taken advantage of the zebrafish pronephros as a system for studying glomerular filtration. In order to identify new regulators of filtration barrier assembly, we have performed a reverse genetic screen in the zebrafish testing a group of genes which are enriched in their expression within the mammalian glomerulus. In this novel screen, we have coupled gene knockdown using morpholinos with a physiological glomerular dye filtration assay to test for selective glomerular permeability in living zebrafish larvae. Screening 20 genes resulted in the identification of ralgps1, rapgef2, rabgef1, and crb2b. The crumbs (crb) genes encode a family of evolutionarily conserved proteins important for apical-basal polarity within epithelia. The crb2b gene is expressed in zebrafish podocytes. Electron microscopic analysis of crb2b morphants reveals a gross disorganization of podocyte foot process architecture and loss of slit diaphragms while overall polarity is maintained. Nephrin, a major component of the slit diaphragm, is apically mis-localized in podocytes from crb2b morphants suggesting that crb2b is required for the proper protein trafficking of Nephrin. This report is the first to show a role for crb function in podocyte differentiation. Furthermore, these results suggest a novel link between epithelial polarization and the maintenance of a functional filtration barrier.

FGF-dependent left-right asymmetry patterning in zebrafish is mediated by Ier2 and Fibp1

Establishment of left-right asymmetry in vertebrates requires nodal, Wnt-PCP and FGF signaling and involves ciliogenesis in a laterality organ. Effector genes through which FGF signaling affects laterality have not been described. We isolated the zebrafish ier2 and fibp1 genes as FGF target genes and show that their protein products interact. Knock down of these factors interferes with establishment of organ laterality and causes defective cilia formation in Kupffer's Vesicle, the zebrafish laterality organ. Cilia are also lost after suppression of FGF8, but can be rescued by injection of ier2 and fibp1 mRNA. We conclude that Ier2 and Fibp1 mediate FGF signaling in ciliogenesis in Kupffer's Vesicle and in the establishment of laterality in the zebrafish embryo.

The insect nephrocyte is a podocyte-like cell with a filtration slit diaphragm

The nephron is the basic structural and functional unit of the vertebrate kidney. It is composed of a glomerulus, the site of ultrafiltration, and a renal tubule, along which the filtrate is modified. Although widely regarded as a vertebrate adaptation1 ‘nephron-like’ features can be found in the excretory systems of many invertebrates, raising the possibility that components of the vertebrate excretory system were inherited from their invertebrate ancestors2. Here we show that the insect nephrocyte has remarkable anatomical, molecular and functional similarity with the glomerular podocyte, a cell in the vertebrate kidney that forms the main size-selective barrier as blood is ultrafiltered to make urine. In particular, both cell types possess a specialised filtration diaphragm, known as the slit diaphragm in podocytes or the nephrocyte diaphragm in nephrocytes. We find that fly orthologues of the major constituents of the slit diaphragm, including nephrin, neph1, CD2AP, ZO-1 and podocin are expressed in the nephrocyte and form a complex of interacting proteins that closely mirrors the vertebrate slit diaphragm complex. Furthermore, we find the nephrocyte diaphragm is completely lost in flies mutant for nephrin or neph1 orthologues, a phenotype resembling loss of the slit diaphragm in the absence of either nephrin (as in the human kidney disease NPHS1) or neph1. These changes drastically impair filtration function in the nephrocyte. The similarities we describe between invertebrate nephrocytes and vertebrate podocytes provide evidence suggesting the two cell types are evolutionarily related and establish the nephrocyte as a simple model in which to study podocyte biology and podocyte-associated diseases.

odd skipped related1 reveals a novel role for endoderm in regulating kidney versus vascular cell fate

The kidney and vasculature are intimately linked both functionally and during development, when nephric and blood/vascular progenitor cells occupy adjacent bands of mesoderm in zebrafish and frog embryos. Developmental mechanisms that underlie the differentiation of kidney versus blood/vascular lineages remain unknown. The odd skipped related1 (osr1) gene encodes a zinc-finger transcription factor that is expressed in the germ ring mesendoderm and subsequently in the endoderm and intermediate mesoderm, prior to the expression of definitive kidney or blood/vascular markers. Knockdown of osr1 in zebrafish embryos resulted in a complete, segment-specific loss of anterior kidney progenitors and a compensatory increase in the number of angioblast cells in the same trunk region. Histology revealed a subsequent absence of kidney tubules, an enlarged cardinal vein and expansion of the posterior venous plexus. Altered kidney versus vascular development correlated with expanded endoderm development in osr1 knockdowns. Combined osr1 loss of function and blockade of endoderm development by knockdown of sox32/casanova rescued anterior kidney development. The results indicate that osr1 activity is required to limit endoderm differentiation from mesendoderm; in the absence of osr1, excess endoderm alters mesoderm differentiation, shifting the balance from kidney towards vascular development.

Regulation of neurocoel morphogenesis by Pard6 gamma b

The Par3/Par6/aPKC protein complex plays a key role in the establishment and maintenance of apicobasal polarity, a cellular characteristic essential for tissue and organ morphogenesis, differentiation and homeostasis. During a forward genetic screen for liver and pancreas mutants, we identified a pard6gammab mutant, representing the first known pard6 mutant in a vertebrate organism. pard6gammab mutants exhibit defects in epithelial tissue development as well as multiple lumens in the neural tube. Analyses of the cells lining the neural tube cavity, or neurocoel, in wildtype and pard6gammab mutant embryos show that lack of Pard6gammab function leads to defects in mitotic spindle orientation during neurulation. We also found that the PB1 (aPKC-binding) and CRIB (Cdc-42-binding) domains and the KPLG amino acid sequence within the PDZ domain (Pals1-and Crumbs binding) are not required for Pard6gammab localization but are essential for its function in neurocoel morphogenesis. Apical membranes are reduced, but not completely absent, in mutants lacking the zygotic, or both the maternal and zygotic, function of pard6gammab, leading us to examine the localization and function of the three additional zebrafish Pard6 proteins. We found that Pard6alpha, but not Pard6beta or Pard6gammaa, could partially rescue the pard6gammab(s441) mutant phenotypes. Altogether, these data indicate a previously unappreciated functional diversity and complexity within the vertebrate pard6 gene family.

Disease-dependent mechanisms of albuminuria

The mechanism of albuminuria is perhaps one of the most complex yet important questions in renal physiology today. Recent studies have directly demonstrated that the normal glomerulus filters substantial amounts of albumin and that charge selectivity plays little or no role in preventing this process. This filtered albumin is then processed by proximal tubular cells by two distinct pathways; dysfunction in either one of these pathways gives rise to discrete forms of albuminuria. Most of the filtered albumin is returned to the peritubular blood supply by a retrieval pathway. Albuminuria in the nephrotic range would arise from retrieval pathway dysfunction. The small quantities of filtered albumin that are not retrieved undergo obligatory lysosomal degradation before urinary excretion as small peptide fragments. This degradation pathway is sensitive to metabolic factors responsible for hypertrophy and fibrosis, particularly molecules such as angiotensin II and transforming growth factor-beta1, whose production is stimulated by hyperglycemic and hypertensive environments. Dysfunction in this degradation pathway leads to albuminuria below the nephrotic range. These new insights into albumin filtration and processing argue for a reassessment of the role of podocytes and the slit diaphragm as major direct determinants governing albuminuria, provide information on how glomerular morphology and "tubular" albuminuria may be interrelated, and offer a new rationale for drug development.

The zebrafish pronephros: a model to study nephron segmentation

Nephrons possess a segmental organization where each segment is specialized for the secretion and reabsorption of particular solutes. The developmental control of nephron segment patterning remains one of the enigmas within the field of renal biology. Achieving an understanding of the mechanisms that direct nephron segmentation has the potential to shed light on the causes of kidney birth defects and renal diseases in humans. Researchers studying embryonic kidney development in zebrafish and Xenopus have recently demonstrated that the pronephric nephrons in these vertebrates are segmented in a similar fashion as their mammalian counterparts. Further, it has been shown that retinoic acid signaling establishes proximodistal segment identities in the zebrafish pronephros by modulating the expression of renal transcription factors and components of signaling pathways that are known to direct segment fates during mammalian nephrogenesis. These findings present the zebrafish model as an excellent genetic system in which to interrogate the conserved developmental pathways that control nephron segmentation in both lower vertebrates and mammals.

Expression of trpC1 and trpC6 orthologs in zebrafish

Transient receptor potential (TRP) genes encode subunits that form cation-selective ion channels in a variety of organisms and cell types. TRP channels serve diverse functions ranging from thermal, tactile, taste, and osmolar sensing to fluid flow sensing. TRPC1 and TRPC6 belong to the TRPC subfamily, members of which are thought to contribute to several cellular events such as regulated migration of neuronal dendrites, contractile responses of smooth muscle cells and maintenance of the structural integrity of kidney podocytes. Pathogenic roles have been suggested for TRPC1 in asthma and chronic obstructive pulmonary disease, and TRPC6 dysfunction was recently linked to proteinuric kidney disease. To explore the potential roles for TRPC channels in zebrafish organ function, we cloned zebrafish trpC1 and trpC6 cDNAs, and investigated their expression during zebrafish development. We detected trpC1 expression in the head, in cells surrounding the outflow tract of the heart, and in the ganglion cells as well as the inner nuclear layer of the eye. trpC6 expression was detected in the head, pectoral fins, aortic endothelial cells, and gastrointestinal smooth muscle cells. Our results point to roles of TRPC channels in several tissues during zebrafish development, and suggest that the zebrafish may be a suitable model system to study the pathophysiology of TRPC1 and TRPC6 in specific cell types.

Modeling ciliopathies: Primary cilia in development and disease

Primary (nonmotile) cilia are currently enjoying a renaissance in light of novel ascribed functions ranging from mechanosensory to signal transduction. Their importance for key developmental pathways such as Sonic Hedgehog (Shh) and Wnt is beginning to emerge. The function of nodal cilia, for example, is vital for breaking early embryonic symmetry, Shh signaling is important for tissue morphogenesis and successful Wnt signaling for organ growth and differentiation. When ciliary function is perturbed, photoreceptors may die, kidney tubules develop cysts, limb digits multiply and brains form improperly. The etiology of several uncommon disorders has recently been associated with cilia dysfunction. The causative genes are often similar and their cognate proteins certainly share cellular locations and/or pathways. Animal models of ciliary gene ablation such as Ift88, Kif3a, and Bbs have been invaluable for understanding the broad function of the cilium. Herein, we describe the wealth of information derived from the study of the ciliopathies and their animal models.

Fish and frogs: models for vertebrate cilia signaling

The presence of cilia in many vertebrate cell types and its function has been ignored for many years. Only in the past few years has its importance been rediscovered. In part, this was triggered by the realization that many gene products mutated in polycystic kidney diseases are localized to cilia and dysfunctional cilia result in kidney disease. Another breakthrough was the observation that the establishment of the left-right body axis is dependent on cilia function. Since then, many other developmental paradigms have been shown to rely on cilia-dependent signaling. In addition to mouse and Chlamydomonas, lower vertebrate model systems such as zebrafish, medaka and Xenopus have provided important new insights into cilia signaling and its role during embryonic development. This review will summarize those studies. We will also illustrate how these lower vertebrates are promising model systems for future studies defining the physiological function of cilia during organogenesis and disease pathophysiology.

Teaching molecular genetics: chapter 4-positional cloning of genetic disorders

Positional cloning is the approach of choice for the identification of genetic mutations underlying the pathological development of diseases with simple Mendelian inheritance. It consists of different consecutive steps, starting with recruitment of patients and DNA collection, that are critical to the overall process. A genetic analysis of the enrolled patients and their families is performed, based on genetic recombination frequencies generated by meiotic cross-overs and on genome-wide molecular studies, to define a critical DNA region of interest. This analysis culminates in a statistical estimate of the probability that disease features may segregate in the families independently or in association with specific molecular markers located in known regions. In this latter case, a marker can be defined as being linked to the disease manifestations. The genetic markers define an interval that is a function of their recombination frequencies with the disease, in which the disease gene is localised. The identification and characterisation of chromosome abnormalities as translocations, deletions and duplications by classical cytogenetic methods or by the newly developed microarray-based comparative genomic hybridisation (array CGH) technique may define extensions and borders of the genomic regions involved. The step following the definition of a critical genomic region is the identification of candidate genes that is based on the analysis of available databases from genome browsers. Positional cloning culminates in the identification of the causative gene mutation, and the definition of its functional role in the pathogenesis of the disorder, by the use of cell-based or animal-based experiments. More often, positional cloning ends with the generation of mice with homologous mutations reproducing the human clinical phenotype. Altogether, positional cloning has represented a fundamental step in the research on genetic renal disorders, leading to the definition of several disease mechanisms and allowing a proper diagnostic approach to many conditions.

The coxsackie and adenovirus receptor (CAR) is required for renal epithelial differentiation within the zebrafish pronephros

The coxsackie and adenovirus receptor (CAR) is a member of the immunoglobulin superfamily and a component of vertebrate tight junctions. CAR protein is widely expressed in fish and mammals in organs of epithelial origin suggesting possible functions in epithelial biology. In order to gain insight into its function, we knocked the CAR gene down in zebrafish using antisense morpholinos. We identified a requirement for CAR in the terminal differentiation of glomerular podocytes and pronephric tubular epithelia. Podocytes differentiate in CAR morphants but are not able to elaborate a regularly patterned architecture of foot processes. In the tubules, CAR was required for the apposition of plasma membranes from adjacent epithelial cells but did not appear to be necessary for the formation of tight junctions. Additionally, tubular epithelia lacking CAR were not able to elaborate apical brush border microvilli. These results establish a requirement for CAR in the terminal differentiation of renal glomerular and tubular cell types.

Rapid screening of glomerular slit diaphragm integrity in larval zebrafish

Gene array-type experiments have identified large numbers of genes thought to be important for the integrity of the glomerular slit diaphragm. Confirmation of individual proteins has been limited by the expenses and time involved in generating transgenic or knockout mice for each candidate. We present a functional screening assay based on the clearance of a 70-kDa fluorescent dextran in another vertebrate system that is rapid and low in cost. In the pronephric glomerulus of larval zebrafish, we have demonstrated quantifiable loss of slit diaphragm integrity in a zebrafish model of puromycin aminonucleoside (PA) toxicity. In addition, after knockdown of CD2-associated protein (CD2AP) and podocin, two well-characterized genetic contributors to podocyte differentiation in mammals, we observed glomerular loss of serum macromolecules similar to that seen in mammalian kidneys with inborn mutations in these genes. Increased filtration of 70-kDa FITC-labeled dextran correlates with effacement of podocyte foot processes in ultrastructural analysis. These findings document the value of the zebrafish model in genomics and pharmacological screening applications.

The Wilms tumor genes wt1a and wt1b control different steps during formation of the zebrafish pronephros

The Wilms tumor protein WT1 is an essential factor for kidney development. In humans, mutations in WT1 lead to Wilms tumor, a pediatric kidney cancer as well as to developmental anomalies concerning the urogenital tract. Inactivation of Wt1 in mice causes multiple organ defects most notably agenesis of the kidneys. In zebrafish, two paralogous wt1 genes exist, wt1a and wt1b. The wt1 genes are expressed in a similar and overlapping but not identical pattern. Here, we have examined the role of both wt1 genes in early kidney development employing a transgenic line with pronephros specific GFP expression and morpholino knockdown experiments. Inactivation of wt1a led to failure of glomerular differentiation and morphogenesis resulting in a rapidly expanding general body edema. In contrast, knockdown of wt1b was compatible with early glomerular development. After 48 h, however, wt1b morphant embryos developed cysts in the region of the glomeruli and tubules and subsequent pericardial edema at 4 days post-fertilization. Thus, our data suggest different functions for wt1a and wt1b in zebrafish nephrogenesis. While wt1a has a more fundamental and early role in pronephros development and is essential for the formation of glomerular structures, wt1b functions at later stages of nephrogenesis.

Cell biology of membrane trafficking in human disease

Understanding the molecular and cellular mechanisms underlying membrane traffic pathways is crucial to the treatment and cure of human disease. Various human diseases caused by changes in cellular homeostasis arise through a single gene mutation(s) resulting in compromised membrane trafficking. Many pathogenic agents such as viruses, bacteria, or parasites have evolved mechanisms to subvert the host cell response to infection, or have hijacked cellular mechanisms to proliferate and ensure pathogen survival. Understanding the consequence of genetic mutations or pathogenic infection on membrane traffic has also enabled greater understanding of the interactions between organisms and the surrounding environment. This review focuses on human genetic defects and molecular mechanisms that underlie eukaryote exocytosis and endocytosis and current and future prospects for alleviation of a variety of human diseases.

Slit Diaphragm Junctional Complex and Regulation of the Cytoskeleton

In recent years, identification of proteins found at the slit diaphragm has greatly improved our understanding of the molecular mechanisms responsible for forming and maintaining the glomerular filtration barrier. This review examines the function of proteins identified at the podocyte intercellular junction in coordinating podocyte intercellular junction dynamics or cytoskeletal dynamics.

Positional cloning uncovers mutations in PLCE1 responsible for a nephrotic syndrome variant that may be reversible

Nephrotic syndrome, a malfunction of the kidney glomerular filter, leads to proteinuria, edema and, in steroid-resistant nephrotic syndrome, end-stage kidney disease. Using positional cloning, we identified mutations in the phospholipase C epsilon gene (PLCE1) as causing early-onset nephrotic syndrome with end-stage kidney disease. Kidney histology of affected individuals showed diffuse mesangial sclerosis (DMS). Using immunofluorescence, we found PLCepsilon1 expression in developing and mature glomerular podocytes and showed that DMS represents an arrest of normal glomerular development. We identified IQ motif-containing GTPase-activating protein 1 as a new interaction partner of PLCepsilon1. Two siblings with a missense mutation in an exon encoding the PLCepsilon1 catalytic domain showed histology characteristic of focal segmental glomerulosclerosis. Notably, two other affected individuals responded to therapy, making this the first report of a molecular cause of nephrotic syndrome that may resolve after therapy. These findings, together with the zebrafish model of human nephrotic syndrome generated by plce1 knockdown, open new inroads into pathophysiology and treatment mechanisms of nephrotic syndrome.

Nephrin ectodomain engagement results in Src kinase activation, nephrin phosphorylation, Nck recruitment, and actin polymerization

A properly established and maintained podocyte intercellular junction, or slit diaphragm, is a necessary component of the selective permeability barrier of the kidney glomerulus. The observation that mutation or deletion of the slit diaphragm transmembrane protein nephrin results in failure of podocyte foot process morphogenesis and concomitant proteinuria first suggested the hypothesis that nephrin serves as a component of a signaling complex that directly integrates podocyte junctional integrity with cytoskeletal dynamics. The observations made herein provide the first direct evidence to our knowledge for a phosphorylation-mediated signaling mechanism by which this integrative function is derived. Our data support the model that during podocyte intercellular junction formation, engagement of the nephrin ectodomain induces transient Fyn catalytic activity that results in nephrin phosphorylation on specific nephrin cytoplasmic domain tyrosine residues. We found that this nephrin phosphorylation event resulted in recruitment of the SH2-SH3 domain-containing adapter protein Nck and assembly of actin filaments in an Nck-dependent fashion. Considered in the context of the role of nephrin family proteins in other organisms and the integral relationship of actin dynamics and junction formation, these observations establish a function for nephrin in regulating actin cytoskeletal dynamics.

The zebrafish kohtalo/trap230 gene is required for the development of the brain, neural crest, and pronephric kidney

Mutation of the gene encoding the Mediator component thyroid hormone receptor-associated protein (TRAP)230/MED12 affects the development of multiple systems in zebrafish embryogenesis. We isolated two ethylnitrosourea-induced alleles in the gene encoding this protein and named the locus kohtalo (kto) after the homologous locus in Drosophila. Homozygous kto mutant zebrafish embryos show defects in brain, neural crest, and kidney development and die at approximately 6 days postfertilization. In the affected tissues, differentiation is initiated and many cell type-specific genes are expressed, but there is a failure of morphogenesis and failure to complete differentiation. These results suggest that critical targets of TRAP230 function may include proteins important for cell mobility, cell sorting, and tissue assembly.