The zebrafish pronephros: a model to study nephron segmentation

Zebrafish as a model for podocyte research

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

Xenopus Ssbp2 is required for embryonic pronephros morphogenesis and terminal differentiation

The nephron, functional unit of the vertebrate kidney, is specialized in metabolic wastes excretion and body fluids osmoregulation. Given the high evolutionary conservation of gene expression and segmentation patterning between mammalian and amphibian nephrons, the Xenopus laevis pronephric kidney offers a simplified model for studying nephrogenesis. The Lhx1 transcription factor plays several roles during embryogenesis, regulating target genes expression by forming multiprotein complexes with LIM binding protein 1 (Ldb1). However, few Lhx1-Ldb1 cofactors have been identified for kidney organogenesis. By tandem- affinity purification from kidney-induced Xenopus animal caps, we identified single-stranded DNA binding protein 2 (Ssbp2) interacts with the Ldb1-Lhx1 complex. Ssbp2 is expressed in the Xenopus pronephros, and knockdown prevents normal morphogenesis and differentiation of the glomus and the convoluted renal tubules. We demonstrate a role for a member of the Ssbp family in kidney organogenesis and provide evidence of a fundamental function for the Ldb1-Lhx1-Ssbp transcriptional complexes in embryonic development.

Adhesion GPCR Gpr126 (Adgrg6) Expression Profiling in Zebrafish, Mouse, and Human Kidney

Adhesion G protein-coupled receptors (aGPCRs) comprise the second-largest class of GPCRs, the most common target for approved pharmacological therapies. aGPCRs play an important role in development and disease and have recently been associated with the kidney. Several aGPCRs are expressed in the kidney and some aGPCRs are either required for kidney development or their expression level is altered in diseased kidneys. Yet, general aGPCR function and their physiological role in the kidney are poorly understood. Here, we characterize in detail Gpr126 (Adgrg6) expression based on RNAscope® technology in zebrafish, mice, and humans during kidney development in adults. Gpr126 expression is enriched in the epithelial linage during nephrogenesis and persists in the adult kidney in parietal epithelial cells, collecting ducts, and urothelium. Single-cell RNAseq analysis shows that gpr126 expression is detected in zebrafish in a distinct ionocyte sub-population. It is co-detected selectively with slc9a3.2, slc4a4a, and trpv6, known to be involved in apical acid secretion, buffering blood or intracellular pH, and to maintain high cytoplasmic Ca2+ concentration, respectively. Furthermore, gpr126-expressing cells were enriched in the expression of potassium transporter kcnj1a.1 and gcm2, which regulate the expression of a calcium sensor receptor. Notably, the expression patterns of Trpv6, Kcnj1a.1, and Gpr126 in mouse kidneys are highly similar. Collectively, our approach permits a detailed insight into the spatio-temporal expression of Gpr126 and provides a basis to elucidate a possible role of Gpr126 in kidney physiology.

IFT46 gene promoter-driven ciliopathy disease model in zebrafish

Ciliopathies are human genetic disorders caused by abnormal formation and dysfunction of cellular cilia. Cilia are microtubule-based organelles that project into the extracellular space and transduce molecular and chemical signals from the extracellular environment or neighboring cells. Intraflagellar transport (IFT) proteins are required for the assembly and maintenance of cilia by transporting proteins along the axoneme which consists of complexes A and B. IFT46, a core IFT-B protein complex, is required for cilium formation and maintenance during vertebrate embryonic development. Here, we introduce transgenic zebrafish lines under the control of ciliated cell-specific IFT46 promoter to recapitulate human ciliopathy-like phenotypes. We generated a Tg(IFT46:GAL4-VP16) line to temporo-spatially control the expression of effectors including fluorescent reporters or nitroreductase based on the GAL4/UAS system, which expresses GAL4-VP16 chimeric transcription factors in most ciliated tissues during embryonic development. To analyze the function of IFT46-expressing ciliated cells during zebrafish development, we generated the Tg(IFT46:GAL4-VP16;UAS;nfsb-mCherry) line, a ciliated cell-specific injury model induced by nitroreductase (NTR)/metrodinazole (MTZ). Conditionally, controlled ablation of ciliated cells in transgenic animals exhibited ciliopathy-like phenotypes including cystic kidneys and pericardial and periorbital edema. Altogether, we established a zebrafish NTR/MTZ-mediated ciliated cell injury model that recapitulates ciliopathy-like phenotypes and may be a vertebrate animal model to further investigate the etiology and therapeutic approaches to human ciliopathies.

Lateral thinking in syndromic congenital cardiovascular disease

Syndromic birth defects are rare diseases that can present with seemingly pleiotropic comorbidities. Prime examples are rare congenital heart and cardiovascular anomalies that can be accompanied by forelimb defects, kidney disorders and more. Whether such multi-organ defects share a developmental link remains a key question with relevance to the diagnosis, therapeutic intervention and long-term care of affected patients. The heart, endothelial and blood lineages develop together from the lateral plate mesoderm (LPM), which also harbors the progenitor cells for limb connective tissue, kidneys, mesothelia and smooth muscle. This developmental plasticity of the LPM, which founds on multi-lineage progenitor cells and shared transcription factor expression across different descendant lineages, has the potential to explain the seemingly disparate syndromic defects in rare congenital diseases. Combining patient genome-sequencing data with model organism studies has already provided a wealth of insights into complex LPM-associated birth defects, such as heart-hand syndromes. Here, we summarize developmental and known disease-causing mechanisms in early LPM patterning, address how defects in these processes drive multi-organ comorbidities, and outline how several cardiovascular and hematopoietic birth defects with complex comorbidities may be LPM-associated diseases. We also discuss strategies to integrate patient sequencing, data-aggregating resources and model organism studies to mechanistically decode congenital defects, including potentially LPM-associated orphan diseases. Eventually, linking complex congenital phenotypes to a common LPM origin provides a framework to discover developmental mechanisms and to anticipate comorbidities in congenital diseases affecting the cardiovascular system and beyond.

Xenopus Ssbp2 is required for embryonic pronephros morphogenesis and terminal differentiation

The nephron, functional unit of the vertebrate kidney, is specialized in metabolic wastes excretion and body fluids osmoregulation. Given the high evolutionary conservation of gene expression and segmentation patterning between mammalian and amphibian nephrons, the Xenopus laevis pronephric kidney offers a simplified model for studying nephrogenesis. The Lhx1 transcription factor plays several roles during embryogenesis, regulating target genes expression by forming multiprotein complexes with LIM binding protein 1 (Ldb1). However, few Lhx1-Ldb1 cofactors have been identified for kidney organogenesis. By tandem-affinity purification from kidney-induced Xenopus animal caps, we identified s ingle- s tranded DNA b inding p rotein 2 (Ssbp2) interacts with the Ldb1-Lhx1 complex. Ssbp2 is expressed in the Xenopus pronephros, and knockdown prevents normal morphogenesis and differentiation of the glomus and the convoluted renal tubules. We demonstrate a role for a member of the Ssbp family in kidney organogenesis and provide evidence of a fundamental function for the Ldb1-Lhx1-Ssbp transcriptional complexes in embryonic development.

Principles of Zebrafish Nephron Segment Development

Nephrons are the functional units which comprise the kidney. Each nephron contains a number of physiologically unique populations of specialized epithelial cells that are organized into discrete domains known as segments. The principles of nephron segment development have been the subject of many studies in recent years. Understanding the mechanisms of nephrogenesis has enormous potential to expand our knowledge about the basis of congenital anomalies of the kidney and urinary tract (CAKUT), and to contribute to ongoing regenerative medicine efforts aimed at identifying renal repair mechanisms and generating replacement kidney tissue. The study of the zebrafish embryonic kidney, or pronephros, provides many opportunities to identify the genes and signaling pathways that control nephron segment development. Here, we describe recent advances of nephron segment patterning and differentiation in the zebrafish, with a focus on distal segment formation.

Modeling Podocyte Ontogeny and Podocytopathies with the Zebrafish

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

The "3Ds" of Growing Kidney Organoids: Advances in Nephron Development, Disease Modeling, and Drug Screening

A kidney organoid is a three-dimensional (3D) cellular aggregate grown from stem cells in vitro that undergoes self-organization, recapitulating aspects of normal renal development to produce nephron structures that resemble the native kidney organ. These miniature kidney-like structures can also be derived from primary patient cells and thus provide simplified context to observe how mutations in kidney-disease-associated genes affect organogenesis and physiological function. In the past several years, advances in kidney organoid technologies have achieved the formation of renal organoids with enhanced numbers of specialized cell types, less heterogeneity, and more architectural complexity. Microfluidic bioreactor culture devices, single-cell transcriptomics, and bioinformatic analyses have accelerated the development of more sophisticated renal organoids and tailored them to become increasingly amenable to high-throughput experimentation. However, many significant challenges remain in realizing the use of kidney organoids for renal replacement therapies. This review presents an overview of the renal organoid field and selected highlights of recent cutting-edge kidney organoid research with a focus on embryonic development, modeling renal disease, and personalized drug screening.

Zebrafish Model to Study Podocyte Function Within the Glomerular Filtration Barrier

The zebrafish model has been used in many different fields of research because of its high homology to the human genome, its easy genetic manipulation, its high fecundity, and its rapid development. For glomerular diseases, zebrafish larvae have proven to be a versatile tool to study the contribution of different genes, because the zebrafish pronephros is very comparable to the human kidney in function and ultrastructure. Here we describe the principle and use of a simple screening assay based on the measurement of the fluorescence in the retinal vessel plexus of the Tg(l-fabp:DBP:eGFP) zebrafish line ("eye assay") to indirectly determine proteinuria as a hallmark of podocyte dysfunction. Furthermore, we illustrate how to analyze the obtained data and outline methods to attribute the findings to podocyte impairment.

A beautiful, complex simplicity: the origins of nephron segmentation uncovered by single-cell sequencing of the pronephros

Pronephric kidneys have a single large nephron that provides essential osmoregulation in amphibians and fish until the adult kidney forms. As mammalian kidneys evolved from the simple pronephric kidneys of the early vertebrates, understanding the structure and function of pronephroi gives insight into the blueprints underlying all nephrons. The article in this issue by Corkins et al. uses single-cell sequencing to demonstrate an extraordinary segmental complexity and the organizational roadmap that mammalian nephrons are based upon.

Visualizing multiciliated cells in the zebrafish

Ciliated cells serve vital functions in the body ranging from mechano- and chemo-sensing to fluid propulsion. Specialized cells with bundles dozens to hundreds of motile cilia known as multiciliated cells (MCCs) are essential as well, where they direct fluid movement in locations such as the respiratory, central nervous and reproductive systems. Intriguingly, the appearance of MCCs has been noted in the kidney in several disease conditions, but knowledge about their contributions to the pathobiology of these states has remained a mystery. As the mechanisms contributing to ciliopathic diseases are not yet fully understood, animal models serve as valuable tools for studying cilia development and how alterations in ciliated cell function impacts disease progression. Like other vertebrates, the zebrafish, Danio rerio, has numerous ciliated tissues. Among these, the embryonic kidney (or pronephros) is comprised of both monociliated cells and MCCs and therefore provides a setting to investigate both ciliated cell fate choice and ciliogenesis. Considering the zebrafish nephron resembles the segmentation and function of human nephrons, the zebrafish provide a tractable model for studying conserved ciliogenesis pathways in vivo. In this chapter, we provide an overview of ciliated cells with a special focus on MCCs, and present a suite of methods that can be used to visualize ciliated cells and their features in the developing zebrafish. Further, these methods enable precise quantification of ciliated cell number and various cilia-related characteristics.

Advances in Understanding the Genetic Mechanisms of Zebrafish Renal Multiciliated Cell Development

Cilia are microtubule-based organelles that project from the cell surface. In humans and other vertebrates, possession of a single cilium structure enables an assortment of cellular processes ranging from mechanosensation to fluid propulsion and locomotion. Interestingly, cells can possess a single cilium or many more, where so-called multiciliated cells (MCCs) possess apical membrane complexes with several dozen or even hundreds of motile cilia that beat in a coordinated fashion. Development of MCCs is, therefore, integral to control fluid flow and/or cellular movement in various physiological processes. As such, MCC dysfunction is associated with numerous pathological states. Understanding MCC ontogeny can be used to address congenital birth defects as well as acquired disease conditions. Today, researchers used both in vitro and in vivo experimental models to address our knowledge gaps about MCC specification and differentiation. In this review, we summarize recent discoveries from our lab and others that have illuminated new insights regarding the genetic pathways that direct MCC ontogeny in the embryonic kidney using the power of the zebrafish animal model.

gldc Is Essential for Renal Progenitor Patterning during Kidney Development

The glycine cleavage system (GCS) is a complex located on the mitochondrial membrane that is responsible for regulating glycine levels and contributing one-carbon units to folate metabolism. Congenital mutations in GCS components, such as glycine decarboxylase (gldc), cause an elevation in glycine levels and the rare disease, nonketotic hyperglycinemia (NKH). NKH patients suffer from pleiotropic symptoms including seizures, lethargy, mental retardation, and early death. Therefore, it is imperative to fully elucidate the pathological effects of gldc dysfunction and glycine accumulation during development. Here, we describe a zebrafish model of gldc deficiency that recapitulates phenotypes seen in humans and mice. gldc deficient embryos displayed impaired fluid homeostasis suggesting renal abnormalities, as well as aberrant craniofacial morphology and neural development defects. Whole mount in situ hybridization (WISH) revealed that gldc transcripts were highly expressed in the embryonic kidney, as seen in mouse and human repository data, and that formation of several nephron segments was disrupted in gldc deficient embryos, including proximal and distal tubule populations. These kidney defects were caused by alterations in renal progenitor populations, revealing that the proper function of Gldc is essential for the patterning of this organ. Additionally, further analysis of the urogenital tract revealed altered collecting duct and cloaca morphology in gldc deficient embryos. Finally, to gain insight into the molecular mechanisms underlying these disruptions, we examined the effects of exogenous glycine treatment and observed analogous renal and cloacal defects. Taken together, these studies indicate for the first time that gldc function serves an essential role in regulating renal progenitor development by modulating glycine levels.

Model organisms for functional validation in genetic renal disease

Functional validation is key for establishing new disease genes in human genetics. Over the years, model organisms have been utilized in a very effective manner to prove causality of genes or genetic variants for a wide variety of diseases. Also in hereditary renal disease, model organisms are very helpful for functional validation of candidate genes and variants identified by next-generation sequencing strategies and for obtaining insights into the pathophysiology. Due to high genetic conservation as well as high anatomical and physiological similarities with the human kidney, almost all genetic kidney diseases can be studied in the mouse. However, mouse work is time consuming and expensive, so there is a need for alternative models. In this review, we will provide an overview of model organisms used in renal research, focusing on mouse, zebrafish, frog, and fruit flies.

Selecting the right therapeutic target for kidney disease

Kidney disease is a complex disease with several different etiologies and underlying associated pathophysiology. This is reflected by the lack of effective treatment therapies in chronic kidney disease (CKD) that stop disease progression. However, novel strategies, recent scientific breakthroughs, and technological advances have revealed new possibilities for finding novel disease drivers in CKD. This review describes some of the latest advances in the field and brings them together in a more holistic framework as applied to identification and validation of disease drivers in CKD. It uses high-resolution ‘patient-centric’ omics data sets, advanced in silico tools (systems biology, connectivity mapping, and machine learning) and ‘state-of-the-art‘ experimental systems (complex 3D systems in vitro, CRISPR gene editing, and various model biological systems in vivo). Application of such a framework is expected to increase the likelihood of successful identification of novel drug candidates based on strong human target validation and a better scientific understanding of underlying mechanisms.

Exposure to Sri Lanka's local groundwater in a CKDu prevalent area causes kidney damage in zebrafish

How local groundwater induces chronic kidney disease of unknown etiology (CKDu) in Sri Lanka is still elusive. This study aims to elucidate the impacts of Sri Lanka's local groundwater in a CKDu prevalent area and reveal the possible pathogenic mechanism of CKDu using zebrafish models. The drinking water from the local underground well in Vavuniya was sampled and the water quality parameters including Na⁺, Mg²⁺, K⁺, Ca²⁺, Cl^-, NO^3-, SO₄^2-, and F^- were analyzed. Then, local groundwater exposure to zebrafish larvae and 293T cells was performed, and water with high hardness and fluoride was prepared as parallel groups. Our result showed that exposure to Sri Lanka's local groundwater caused developmental toxicity, kidney damage, and pronephric duct obstruction as well as abnormal behavior in zebrafish. Similar results were also found after exposure to water with high hardness and fluoride in zebrafish. Further, the expression levels of marker genes related to renal development and functions (foxj1a, dync2h1, pkd2, gata3, and slc20a1) were significantly altered, which is also confirmed in the 293T cells. Taken together, those results indicated that Sri Lanka's local groundwater in a CKDu prevalent area could cause kidney damage, implying that high water hardness and fluorine might be the inducible environmental factors for the etiological cause of CKDu.

Thiosulfate sulfurtransferase prevents hyperglycemic damage to the zebrafish pronephros in an experimental model for diabetes

Thiosulfate sulfurtransferase (TST, EC 2.8.1.1), also known as Rhodanese, was initially discovered as a cyanide detoxification enzyme. However, it was recently also found to be a genetic predictor of resistance to obesity-related type 2 diabetes. Diabetes type 2 is characterized by progressive loss of adequate β-cell insulin secretion and onset of insulin resistance with increased insulin demand, which contributes to the development of hyperglycemia. Diabetic complications have been replicated in adult hyperglycemic zebrafish, including retinopathy, nephropathy, impaired wound healing, metabolic memory, and sensory axonal degeneration. Pancreatic and duodenal homeobox 1 (Pdx1) is a key component in pancreas development and mature beta cell function and survival. Pdx1 knockdown or knockout in zebrafish induces hyperglycemia and is accompanied by organ alterations similar to clinical diabetic retinopathy and diabetic nephropathy. Here we show that pdx1-knockdown zebrafish embryos and larvae survived after incubation with thiosulfate and no obvious morphological alterations were observed. Importantly, incubation with hTST and thiosulfate rescued the hyperglycemic phenotype in pdx1-knockdown zebrafish pronephros. Activation of the mitochondrial TST pathway might be a promising option for therapeutic intervention in diabetes and its organ complications.

Insights Gained From Zebrafish Models for the Ciliopathy Joubert Syndrome

Cilia are quasi-ubiquitous microtubule-based sensory organelles, which play vital roles in signal transduction during development and cell homeostasis. Dysfunction of cilia leads to a group of Mendelian disorders called ciliopathies, divided into different diagnoses according to clinical phenotype constellation and genetic causes. Joubert syndrome (JBTS) is a prototypical ciliopathy defined by a diagnostic cerebellar and brain stem malformation termed the “Molar Tooth Sign” (MTS), in addition to which patients display variable combinations of typical ciliopathy phenotypes such as retinal dystrophy, fibrocystic renal disease, polydactyly or skeletal dystrophy. Like most ciliopathies, JBTS is genetically highly heterogeneous with ∼40 associated genes. Zebrafish are widely used to model ciliopathies given the high conservation of ciliary genes and the variety of specialized cilia types similar to humans. In this review, we compare different existing JBTS zebrafish models with each other and describe their contributions to our understanding of JBTS pathomechanism. We find that retinal dystrophy, which is the most investigated ciliopathy phenotype in zebrafish ciliopathy models, is caused by distinct mechanisms according to the affected gene. Beyond this, differences in phenotypes in other organs observed between different JBTS-mutant models suggest tissue-specific roles for proteins implicated in JBTS. Unfortunately, the lack of systematic assessment of ciliopathy phenotypes in the mutants described in the literature currently limits the conclusions that can be drawn from these comparisons. In the future, the numerous existing JBTS zebrafish models represent a valuable resource that can be leveraged in order to gain further insights into ciliary function, pathomechanisms underlying ciliopathy phenotypes and to develop treatment strategies using small molecules.

Zebrafish: A Model to Study and Understand the Diabetic Nephropathy and Other Microvascular Complications of Type 2 Diabetes Mellitus

Simple Summary

Diabetes is a chronic metabolic disease characterized by high blood glucose levels (hyperglycemia). Type 2 diabetes mellitus (T2DM) and its complications are a worldwide public health problem, affecting people from all developed and developing countries. Hyperglycemia can cause damage to the vascular system and dysfunction of organs, such as the kidneys, heart, retina of the eyes, and nerves. Diabetic nephropathy (DN) is one of the most severe micro-vascular complications, which can lead to ESRD (end-stage renal disease). Zebrafish are ideal for wide-scale analysis or screening, due to their small size, quick growth, transparent embryos, vast number of offspring, and gene similarity with humans, which combine to make zebrafish an ideal model for diabetes. The readily available tools for gene editing using morpholinos or CRISPR/Cas9, as well as chemical/drug therapy by microinjection or skin absorption, enable zebrafish diabetes mellitus models to be established in a number of ways. In this review, we emphasize the physiological and pathological processes relating to micro-vascular problems in zebrafish, as well as the many experimental zebrafish models used to research DN, and the DN-related outcomes and mechanisms observed in zebrafish. This study specifies the benefits and drawbacks and future perspective of using zebrafish as a disease model.

Abstract

Diabetes mellitus (DM) is a complicated metabolic illness that has had a worldwide impact and placed an unsustainable load on both developed and developing countries’ health care systems. According to the International Diabetes Federation, roughly 537 million individuals had diabetes in 2021, with type 2 diabetes mellitus accounting for the majority of cases (T2DM). T2DM is a chronic illness defined by insufficient insulin production from pancreatic islet cells. T2DM generates various micro and macrovascular problems, with diabetic nephropathy (DN) being one of the most serious microvascular consequences, and which can lead to end-stage renal disease. The zebrafish (Danio rerio) has set the way for its future as a disease model organism. As numerous essential developmental processes, such as glucose metabolism and reactive metabolite production pathways, have been identified in zebrafish that are comparable to those seen in humans, it is a good model for studying diabetes and its consequences. It also has many benefits over other vertebrate models, including the permeability of its embryos to small compounds, disease-driven therapeutic target selection, in vivo validation, and deconstruction of biological networks. The organism can also be utilized to investigate and understand the genetic abnormalities linked to the onset of diabetes problems. Zebrafish may be used to examine and visualize the growth, morphology, and function of organs under normal physiological and diabetic settings. The zebrafish has become one of the most useful models for studying DN, especially when combined with genetic alterations and/or mutant or transgenic fish lines. The significant advancements of CRISPR and next-generation sequencing technology for disease modelling in zebrafish, as well as developments in molecular and nano technologies, have advanced the understanding of the molecular mechanisms of several human diseases, including DN. In this review, we emphasize the physiological and pathological processes relating to microvascular problems in zebrafish, as well as the many experimental zebrafish models used to research DN, and the DN-related outcomes and mechanisms observed in zebrafish.

Embryonic and aglomerular kidney development in the bay pipefish, Syngnathus leptorhynchus

In this report we describe the embryogenesis of the bay pipefish, Syngnathus leptorhynchus, and the organogenesis of its aglomerular kidney. Early development was analyzed via a series of montages and images documenting embryos collected from the brood pouches of pregnant males. Despite differences in terminal morphology between pipefish and common teleost models such as medaka and zebrafish, the embryogenesis of these highly advanced fishes is generally similar to that of other fishes. One of the unique features of these fishes is their utilization of an aglomerular kidney. Histological analysis revealed a single long, unbranched kidney tubule in late embryos. The development and structure of this organ was further investigated by cloning the sodium potassium ATPase alpha subunit, atp1a, from S. leptorhynchus and developing whole mount fluorescent in situ hybridization protocols for embryos of this species. Fluorescent stereoscopic and confocal visualization techniques were then used to characterize the 3D morphology of aglomerular kidneys in intact embryos. In all embryonic stages characterized, the aglomerular kidney is a single unbranched tube extending from just behind the head to the cloaca.

Zebrafish (Danio rerio) larva as an in vivo vertebrate model to study renal function

There is an increasing interest in using zebrafish (Danio rerio) larva as a vertebrate screening model to study drug disposition. As the pronephric kidney of zebrafish larvae shares high similarity with the anatomy of nephrons in higher vertebrates including humans, we explored in this study whether 3- to 4-day-old zebrafish larvae have a fully functional pronephron. Intravenous injection of fluorescent polyethylene glycol and dextran derivatives of different molecular weight revealed a cutoff of 4.4-7.6 nm in hydrodynamic diameter for passive glomerular filtration, which is in agreement with corresponding values in rodents and humans. Distal tubular reabsorption of a FITC-folate conjugate, covalently modified with PEG2000, via folate receptor 1 was shown. Transport experiments of fluorescent substrates were assessed in the presence and absence of specific inhibitors in the blood systems. Thereby, functional expression in the proximal tubule of organic anion transporter oat (slc22) multidrug resistance-associated protein mrp1 (abcc1), mrp2 (abcc2), mrp4 (abcc4), and zebrafish larva p-glycoprotein analog abcb4 was shown. In addition, nonrenal clearance of fluorescent substrates and plasma protein binding characteristics were assessed in vivo. The results of transporter experiments were confirmed by extrapolation to ex vivo experiments in killifish (Fundulus heteroclitus) proximal kidney tubules. We conclude that the zebrafish larva has a fully functional pronephron at 96 h postfertilization and is therefore an attractive translational vertebrate screening model to bridge the gap between cell culture-based test systems and pharmacokinetic experiments in higher vertebrates.NEW & NOTEWORTHY The study of renal function remains a challenge. In vitro cell-based assays are approved to study, e.g., ABC/SLC-mediated drug transport but do not cover other renal functions such as glomerular filtration. Here, in vivo studies combined with in vitro assays are needed, which are time consuming and expensive. In view of these limitations, our proof-of-concept study demonstrates that the zebrafish larva is a translational in vivo test model that allows for mechanistic investigations to study renal function.

Experimental models of acute kidney injury for translational research

Preclinical models of human disease provide powerful tools for therapeutic discovery but have limitations. This problem is especially apparent in the field of acute kidney injury (AKI), in which clinical trial failures have been attributed to inaccurate modelling performed largely in rodents. Multidisciplinary efforts such as the Kidney Precision Medicine Project are now starting to identify molecular subtypes of human AKI. In addition, over the past decade, there have been developments in human pluripotent stem cell-derived kidney organoids as well as zebrafish, rodent and large animal models of AKI. These organoid and AKI models are being deployed at different stages of preclinical therapeutic development. However, the traditionally siloed, preclinical investigator-driven approaches that have been used to evaluate AKI therapeutics to date rarely account for the limitations of the model systems used and have given rise to false expectations of clinical efficacy in patients with different AKI pathophysiologies. To address this problem, there is a need to develop more flexible and integrated approaches, involving teams of investigators with expertise in a range of different model systems, working closely with clinical investigators, to develop robust preclinical evidence to support more focused interventions in patients with AKI.

Prostaglandin signaling in ciliogenesis and development

Prostaglandin (PG) signaling regulates a wide variety of physiological and pathological processes, including body temperature, cardiovascular homeostasis, reproduction, and inflammation. Recent studies have revealed that PGs play pivotal roles in embryo development, ciliogenesis, and organ formation. Prostaglandin E2 (PGE2) and its receptor EP4 modulate ciliogenesis by increasing the anterograde intraflagellar transport. Many G-protein-coupled receptors (GPCRs) including EP4 are localized in cilia for modulating cAMP signaling under various conditions. During development, PGE2 signaling regulates embryogenesis, hepatocyte differentiation, hematopoiesis, and kidney formation. Prostaglandins are also essential for skeletal muscle repair. This review outlines recent advances in understanding the functions and mechanisms of prostaglandin signaling in ciliogenesis, embryo development, and organ formation.

Zebrafish Embryos and Larvae as Alternative Animal Models for Toxicity Testing

Prerequisite to any biological laboratory assay employing living animals is consideration about its necessity, feasibility, ethics and the potential harm caused during an experiment. The imperative of these thoughts has led to the formulation of the 3R-principle, which today is a pivotal scientific standard of animal experimentation worldwide. The rising amount of laboratory investigations utilizing living animals throughout the last decades, either for regulatory concerns or for basic science, demands the development of alternative methods in accordance with 3R to help reduce experiments in mammals. This demand has resulted in investigation of additional vertebrate species displaying favourable biological properties. One prominent species among these is the zebrafish (Danio rerio), as these small laboratory ray-finned fish are well established in science today and feature outstanding biological characteristics. In this review, we highlight the advantages and general prerequisites of zebrafish embryos and larvae before free-feeding stages for toxicological testing, with a particular focus on cardio-, neuro, hepato- and nephrotoxicity. Furthermore, we discuss toxicokinetics, current advances in utilizing zebrafish for organ toxicity testing and highlight how advanced laboratory methods (such as automation, advanced imaging and genetic techniques) can refine future toxicological studies in this species.

Innovations and Emerging Therapies to Combat Renal Cell Damage: NAD+ As a Drug Target

Significance: Acute kidney injury (AKI) is a common and life-threatening complication in hospitalized and critically ill patients. It is defined by an abrupt deterioration in renal function, clinically manifested by increased serum creatinine levels, decreased urine output, or both. To execute all its functions, namely excretion of waste products, fluid/electrolyte balance, and hormone synthesis, the kidney requires incredible amounts of energy in the form of adenosine triphosphate. Recent Advances: Adequate mitochondrial functioning and nicotinamide adenine dinucleotide (NAD⁺) homeostasis are essential to meet these high energetic demands. NAD⁺ is a ubiquitous essential coenzyme to many cellular functions. NAD⁺ as an electron acceptor mediates metabolic pathways such as oxidative phosphorylation (OXPHOS) and glycolysis, serves as a cosubstrate of aging molecules (i.e., sirtuins), participates in DNA repair mechanisms, and mediates mitochondrial biogenesis. Critical Issues: In many forms of AKI and chronic kidney disease, renal function deterioration has been associated with mitochondrial dysfunction and NAD⁺ depletion. Based on this, therapies aiming to restore mitochondrial function and increase NAD⁺ availability have gained special attention in the last two decades. Future Directions: Experimental and clinical studies have shown that by restoring mitochondrial homeostasis and increasing renal tubulo-epithelial cells, NAD⁺ availability, AKI incidence, and chronic long-term complications are significantly decreased. This review covers some general epidemiological and pathophysiological concepts; describes the role of mitochondrial homeostasis and NAD⁺ metabolism; and analyzes the underlying rationale and role of NAD⁺ aiming therapies as promising preventive and therapeutic strategies for AKI. Antioxid. Redox Signal. 35, 1449-1466.

The individual and combined effects of ochratoxin A with citrinin and their metabolites (ochratoxin B, ochratoxin C, and dihydrocitrinone) on 2D/3D cell cultures, and zebrafish embryo models

Ochratoxin A and citrinin are nephrotoxic mycotoxins produced by Aspergillus, Penicillium, and/or Monascus species. The combined effects of ochratoxin A and citrinin have been examined in more studies; however, only limited data are available regarding the co-exposure to their metabolites. In this investigation, the individual toxic effects of ochratoxin A, ochratoxin B, ochratoxin C, citrinin, and dihydrocitrinone were tested as well as the combinations of ochratoxin A with the latter mycotoxins were examined on 2D and 3D cell cultures, and on zebrafish embryos. Our results demonstrate that even subtoxic concentrations of certain mycotoxins can increase the toxic impact of ochratoxin A. In addition, typically additive effects or synergism were observed as the combined effects of mycotoxins tested. These observations highlight that different cell lines (e.g. MDBK vs. MDCK), cell cultures (e.g. 2D vs. 3D), and models (e.g. in vitro vs. in vivo) can show different (sometimes opposite) impacts. Mycotoxin combinations considerably increased miR-731 levels in zebrafish embryos, which is an early marker of the toxicity on kidney development. These results underline that the co-exposure to mycotoxins (and/or mycotoxin metabolites) should be seriously considered, since even the barely toxic mycotoxins (or metabolites) in combinations can cause significant toxicity.

IPIP27A cooperates with OCRL to support endocytic traffic in the zebrafish pronephric tubule

Endocytosis is a fundamentally important process through which material is internalized into cells from the extracellular environment. In the renal proximal tubule, endocytosis of the abundant scavenger receptor megalin and its co-receptor cubilin play a vital role in retrieving low molecular weight proteins from the renal filtrate. Although we know much about megalin and its ligands, the machinery and mechanisms by which the receptor is trafficked through the endosomal system remain poorly defined. In this study, we show that Ipip27A, an interacting partner of the Lowe syndrome protein OCRL, is required for endocytic traffic of megalin within the proximal renal tubule of zebrafish larvae. Knockout of Ipip27A phenocopies the endocytic phenotype seen upon loss of OCRL, with a deficit in uptake of both fluid-phase and protein cargo, which is accompanied by a reduction in megalin abundance and altered endosome morphology. Rescue and co-depletion experiments indicate that Ipip27A functions together with OCRL to support proximal tubule endocytosis. The results therefore identify Ipip27A as a new player in endocytic traffic in the proximal tubule in vivo and support the view that defective endocytosis underlies the renal tubulopathy in Lowe syndrome and Dent-2 disease.

Transgenic fluorescent zebrafish lines that have revolutionized biomedical research

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

The Zebrafish Model to Understand Epigenetics in Renal Diseases

Epigenetic modifications are able to alter gene expression and include DNA methylation, different histone variants, and post-transcriptional modifications (PTMs), such as acetylation or phosphorylation, and through short/long RNAs, respectively. In this review, we focus on current knowledge concerning epigenetic modifications in gene regulation. We describe different forms of epigenetic modifications and explain how epigenetic changes can be detected. The relevance of epigenetics in renal diseases is highlighted with multiple examples and the use of the zebrafish model to study glomerular diseases in general and epigenetics in renal diseases in particular is discussed. We end with an outlook on how to use epigenetic modifications as a therapeutic target for different diseases. Here, the zebrafish model can be employed as a high-throughput screening tool not only to discover epigenetic alterations contributing to disease, but also to test novel substances that change epigenetic signatures in vivo. Therefore, the zebrafish model harbors the opportunity to find novel pathogenic pathways allowing a pre-selection of potential targets and compounds to be tested for renal diseases.

Blood Pressure Regulation Evolved from Basic Homeostatic Components

Blood pressure (BP) is determined by several physiological factors that are regulated by a range of complex neural, endocrine, and paracrine mechanisms. This study examined a collection of 198 human genes related to BP regulation, in the biological processes and functional prisms, as well as gene expression in organs and tissues. This was made in conjunction with an orthology analysis performed in 19 target organisms along the phylogenetic tree. We have demonstrated that transport and signaling, as well as homeostasis in general, are the most prevalent biological processes associated with BP gene orthologs across the examined species. We showed that these genes and their orthologs are expressed primarily in the kidney and adrenals of complex organisms (e.g., high order vertebrates) and in the nervous system of low complexity organisms (e.g., flies, nematodes). Furthermore, we have determined that basic functions such as ion transport are ancient and appear in all organisms, while more complex regulatory functions, such as control of extracellular volume emerged in high order organisms. Thus, we conclude that the complex system of BP regulation evolved from simpler components that were utilized to maintain specific homeostatic functions that play key roles in existence and survival of organisms.

Exploration of zebrafish larvae as an alternative whole-animal model for nephrotoxicity testing

Due to an increasing demand for testing of new and existing chemicals and legal restrictions for the use of animals, there is a strong need for alternative approaches to assess systemic toxicity. Embryonic and larval zebrafish (Danio rerio) are increasingly recognized as a promising alternative whole-animal model that may be able to overcome limitations of cell-based in vitro assays and bridge the gap between high-throughput in vitro screening and low-throughput in vivo tests in animals. Despite the relatively simple anatomical structure of the zebrafish larval kidney (pronephros) - composed of only two nephrons - the pronephros shares major functions and cell types with mammalian nephrons. Glomerular filtration begins at 48 h post fertilization. The aim of the present study was to investigate if early zebrafish larvae might be a suitable model for nephrotoxicity testing. On day 3 post fertilization, larval zebrafish were treated with selected nephrotoxins (aristolochic acid, cadmium chloride, potassium bromate, ochratoxin A, gentamicin) for 48 h. Histological evaluation of zebrafish larvae exposed to model nephrotoxins revealed tubule injury as evidenced by dilated tubules with loss of the brush border, tubule cell necrosis and disorganization of the tubular epithelium. These changes were most severe after treatment with gentamicin, which also impaired pronephros function as evidenced by reduced clearance of FITC-dextran. Whole-mount in situ hybridization showing loss of cdh17 expression revealed site-specific injury to the proximal tubule segment. Analysis of genes previously identified as novel biomarkers of kidney injury in mammals showed upregulation of the kidney injury marker genes heme oxygenase 1 (hmox1), clusterin (clu), secreted phosphoprotein/osteopontin (spp1), connective tissue growth factor (ctgf) and kim-1 (havcr-1) in response to nephrotoxin treatment, although the response of individual genes varied across compounds. Consistent with the severity of lesions and impaired kidney function, the most prominent gene expression changes occurred in larvae exposed to gentamicin. Overall, our results suggest that larval zebrafish may be a suitable alternative model organism for nephrotoxicity screening, yet further improvements and integration with quantitative in vitro to in vivo extrapolation will be needed to predict human toxicity.

New reporter zebrafish line unveils heterogeneity among lymphatic endothelial cells during development

BACKGROUND

In zebrafish, lymphatic endothelial cells (LECs) originate from multiple/several distinct progenitor populations and generate organ-specific lymphatic vasculatures. Cell fate and tissue specificities were determined using a combination of genetically engineered transgenic lines in which the promoter of a LEC-specific gene drives expression of a fluorescent reporter protein.

RESULTS

We established a novel zebrafish transgenic line expressing eGFP under the control of part of the zebrafish batf3 promoter (Basic Leucine Zipper ATF-Like Transcription Factor 3). Spatiotemporal examination of Tg(batf3MIN:eGFP) transgenic fish revealed a typical lymphatic expression pattern, which does not perfectly recapitulate the expression pattern of existing LEC transgenic lines. eGFP⁺ cells constitute a heterogeneous endothelial cell population, which expressed LEC and/or blood endothelial cells (BEC) markers in different tissues. In addition, we characterize the renal eGFP⁺ cell as a population of interest to study kidney diseases and regeneration.

CONCLUSION

Our Tg(batf3MIN:eGFP) reporter zebrafish line provides a useful system to study LEC populations, of which heterogeneity depends on origin of progenitors, tissue environment and physiological conditions. We further developed a novel fish-adapted tissue clearing method, which allows deep imaging and 3D-visualization of vascular and lymphatic networks in the whole organism.

Returning to kidney development to deliver synthetic kidneys

There is no doubt that the development of transplantable synthetic kidneys could improve the outcome for the many millions of people worldwide suffering from chronic kidney disease. Substantial progress has been made in the last 6 years in the generation of kidney tissue from stem cells. However, the limited scale, incomplete cellular complexity and functional immaturity of such structures suggests we are some way from this goal. While developmental biology has successfully guided advances to date, these human kidney models are limited in their capacity for ongoing nephrogenesis and lack corticomedullary definition, a unified vasculature and a coordinated exit path for urinary filtrate. This review will reassess our developmental understanding of how the mammalian embryo manages to create kidneys, how this has informed our progress to date and how both engineering and developmental biology can continue to guide us towards a synthetic kidney.

Kctd15 regulates nephron segment development by repressing Tfap2a activity

A functional vertebrate kidney relies on structural units called nephrons, which are epithelial tubules with a sequence of segments each expressing a distinct repertoire of solute transporters. The transcriptiona`l codes driving regional specification, solute transporter program activation and terminal differentiation of segment populations remain poorly understood. Here, we demonstrate that the KCTD15 paralogs kctd15a and kctd15b function in concert to restrict distal early (DE)/thick ascending limb (TAL) segment lineage assignment in the developing zebrafish pronephros by repressing Tfap2a activity. During renal ontogeny, expression of these factors colocalized with tfap2a in distal tubule precursors. kctd15a/b loss primed nephron cells to adopt distal fates by driving slc12a1, kcnj1a.1 and stc1 expression. These phenotypes were the result of Tfap2a hyperactivity, where kctd15a/b-deficient embryos exhibited increased abundance of this transcription factor. Interestingly, tfap2a reciprocally promoted kctd15a and kctd15b transcription, unveiling a circuit of autoregulation operating in nephron progenitors. Concomitant kctd15b knockdown with tfap2a overexpression further expanded the DE population. Our study reveals that a transcription factor-repressor feedback module employs tight regulation of Tfap2a and Kctd15 kinetics to control nephron segment fate choice and differentiation during kidney development.

Sequence analysis and spatiotemporal developmental distribution of the Cat-1-type transporter slc7a1a in zebrafish (Danio rerio)

Cationic amino acid transporter 1 (Cat-1 alias Slc7a1) is a Na⁺-independent carrier system involved in transport and absorption of the cationic amino acids lysine, arginine, histidine, and ornithine and has also been shown to be indispensable in a large variety of biological processes. Starting from isolated full-length zebrafish (Danio rerio) cDNA for slc7a1a, we performed comparative and phylogenetic sequence analysis, investigated the conservation of the gene during vertebrate evolution, and defined tissue expression during zebrafish development. Whole mount in situ hybridization first detected slc7a1a transcripts in somites, eyes, and brain at 14 h post-fertilization (hpf) with additional expression in the distal nephron at 24 hpf and in branchial arches at 3 days post-fertilization (dpf), with significant increase by 5 dpf. Taken together, the expression analysis of the zebrafish Cat-1 system gene slc7a1a suggests a functional role(s) during the early development of the central nervous system, muscle, gills, and kidney. Graphical abstract.

Advances in understanding vertebrate nephrogenesis

The kidney is a complex organ that performs essential functions such as blood filtration and fluid homeostasis, among others. Recent years have heralded significant advancements in our knowledge of the mechanisms that control kidney formation. Here, we provide an overview of vertebrate renal development with a focus on nephrogenesis, the process of generating the epithelialized functional units of the kidney. These steps begin with intermediate mesoderm specification and proceed all the way to the terminally differentiated nephron cell, with many detailed stages in between. The establishment of nephron architecture with proper cellular barriers is vital throughout these processes. Continuously striving to gain further insights into nephrogenesis can ultimately lead to a better understanding and potential treatments for developmental maladies such as Congenital Anomalies of the Kidney and Urinary Tract (CAKUT).

Application of zebrafish to safety evaluation in drug discovery

Traditionally, safety evaluation at the early stage of drug discovery research has been done using in silico, in vitro, and in vivo systems in this order because of limitations on the amount of compounds available and the throughput ability of the assay systems. While these in vitro assays are very effective tools for detecting particular tissue-specific toxicity phenotypes, it is difficult to detect toxicity based on complex mechanisms involving multiple organs and tissues. Therefore, the development of novel high throughput in vivo evaluation systems has been expected for a long time. The zebrafish (Danio rerio) is a vertebrate with many attractive characteristics for use in drug discovery, such as a small size, transparency, gene and protein similarity with mammals (80% or more), and ease of genetic modification to establish human disease models. Actually, in recent years, the zebrafish has attracted interest as a novel experimental animal. In this article, the author summarized the features of zebrafish that make it a suitable laboratory animal, and introduced and discussed the applications of zebrafish to preclinical toxicity testing, including evaluations of teratogenicity, hepatotoxicity, and nephrotoxicity based on morphological findings, evaluation of cardiotoxicity using functional endpoints, and assessment of seizure and drug abuse liability.

A Technique for Studying Glomerular Filtration Integrity in the Zebrafish Pronephros

With the advances in next-generation sequencing and rapid filtering of candidate variants in diseased patients, it has been increasingly important to develop translatable in vivo models to study genetic changes. This allows for functional validation of pathogenic mutations and establishes a system to understand the etiology of disease. Due to the ease of genetic manipulation and rapid ex utero development, the zebrafish has become a valuable resource to study important biological processes, including nephrogenesis. The development and function of the zebrafish pronephros are akin to that of mammals. As such, they offer a tractable model to study kidney disease, especially diabetic nephropathy. However, in order to study kidney dysfunction in zebrafish it is imperative that an appropriate readout is available. The appearance of macro-proteins in patient's urine is indicative of defective kidney function. In this technical chapter, we describe the in vivo use of fluorescently tagged dextrans of different molecular weights to reveal the integrity of the zebrafish glomerular filtration barrier.

Genetic compensation by epob in pronephros development in epoa mutant zebrafish

Zebrafish erythropoietin a (epoa) is a well characterized regulator of red blood cell formation. Recent morpholino mediated knockdown data have also identified epoa being essential for physiological pronephros development in zebrafish, which is driven by blocking apoptosis in developing kidneys. Yet, zebrafish mutants for epoa have not been described so far. In order to compare a transient knockdown vs. permanent knockout for epoa in zebrafish on pronephros development, we used CRISPR/Cas9 technology to generate epoa knockout zebrafish mutants and we performed structural and functional studies on pronephros development. In contrast to epoa morphants, epoa^-/- zebrafish mutants showed normal pronephros structure; however, a previously uncharacterized gene in zebrafish, named epob, was identified and upregulated in epoa^-/- mutants. epob knockdown altered pronephros development, which was further aggravated in epoa^-/- mutants. Likewise, epoa and epob morphants regulated similar and differential gene signatures related to kidney development in zebrafish. In conclusion, stable loss of epoa during embryonic development can be compensated by epob leading to phenotypical discrepancies in epoa knockdown and knockout zebrafish embryos.

Transcriptional profiling of the zebrafish proximal tubule

The hepatocyte nuclear factor-1β (Hnf1b) transcription factor is a key regulator of kidney tubule formation and is associated with a syndrome of renal cysts and early onset diabetes. To further our understanding of Hnf1b in the developing zebrafish kidney, we performed RNA sequencing analysis of proximal tubules from hnf1b-deficient larvae. This analysis revealed an enrichment of gene transcripts encoding transporters of the solute carrier (SLC) superfamily, including multiple members of slc2 and slc5 glucose transporters. An investigation of expression of slc2a1a, slc2a2, and slc5a2 as well as a poorly studied glucose/mannose transporter encoded by slc5a9 revealed that these genes undergo dynamic spatiotemporal changes during tubule formation and maturation. A comparative analysis of zebrafish SLC genes with those expressed in mouse proximal tubules showed a substantial overlap at the level of gene families, indicating a high degree of functional conservation between zebrafish and mammalian proximal tubules. Taken together, our findings are consistent with a role for Hnf1b as a critical determinant of proximal tubule transport function by acting upstream of a large number of SLC genes and validate the zebrafish as a physiologically relevant model of the mammalian proximal tubule.

Visualizing gene expression during zebrafish pronephros development and regeneration

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

Tfap2a is a novel gatekeeper of nephron differentiation during kidney development

Renal functional units known as nephrons undergo patterning events during development that create a segmental array of cellular compartments with discrete physiological identities. Here, from a forward genetic screen using zebrafish, we report the discovery that transcription factor AP-2 alpha (tfap2a) coordinates a gene regulatory network that activates the terminal differentiation program of distal segments in the pronephros. We found that tfap2a acts downstream of Iroquois homeobox 3b (irx3b), a distal lineage transcription factor, to operate a circuit consisting of tfap2b, irx1a and genes encoding solute transporters that dictate the specialized metabolic functions of distal nephron segments. Interestingly, this regulatory node is distinct from other checkpoints of differentiation, such as polarity establishment and ciliogenesis. Thus, our studies reveal insights into the genetic control of differentiation, where tfap2a is essential for regulating a suite of segment transporter traits at the final tier of zebrafish pronephros ontogeny. These findings have relevance for understanding renal birth defects, as well as efforts to recapitulate nephrogenesis in vivo to facilitate drug discovery and regenerative therapies.

Mechanisms of Nephrogenesis Revealed by Zebrafish Chemical Screen: Prostaglandin Signaling Modulates Nephron Progenitor Fate

Nephron development involves the creation of discrete segment populations that are specialized to fulfill unique physiological roles. As such, renal function is reliant on the proper execution of segment patterning programs. Despite the central importance of nephron segmentation, the genetic mechanisms that regulate this process are far from understood, in large part due to the experimental complexities and cost of interrogating these events in the mammalian metanephros. For this reason, forward genetics utilizing phenotypic screening in the zebrafish pronephros provides an avenue to gain novel insights about the mechanisms of nephron segmentation in the vertebrate kidney. Discoveries from zebrafish can highlight possible conserved pathways and provide a useful starting point for reverse genetic analyses with other animal models or in vitro approaches. In this review, we discuss the results of a novel chemical screen using the zebrafish to identify segmentation regulators. Through this screen, we identified for the first time that prostaglandin signaling can modulate nephron segmentation, and that it is normally requisite during development to mitigate segment fate choice in the embryonic kidney. We briefly discuss how these discoveries relate to current knowledge about nephron segmentation. Finally, we explore the possible implications of these findings for understanding renal ontogeny and disease, and how this knowledge may be useful for ongoing research initiatives that are aimed at deciphering how to build or rebuild the human kidney.

In vivo analysis of renal epithelial cells in zebrafish

The zebrafish kidney has been used effectively for studying kidney development, repair and disease. New gene editing capability makes it a more versatile in vivo vertebrate model system to investigate renal epithelial cells in their native environment. In this chapter we focus on dissecting gene function in basic cellular biology of renal epithelial cells, including lumen formation and cell polarity, in intact zebrafish embryos.

SLC41A1 is essential for magnesium homeostasis in vivo

Solute carrier family 41 member A1 (SLC41A1) has been suggested to mediate magnesium (Mg2+) transport by several in vitro studies. However, the physiological function of SLC41A1 remains to be elucidated. In this study, cellular Mg2+ transport assays combined with zebrafish slc41a1 knockdown experiments were performed to disclose SLC41A1 function and its physiological relevance. The gene slc41a1 is ubiquitously expressed in zebrafish tissues and is regulated by water and dietary Mg2+ availability. Knockdown of slc41a1 in zebrafish larvae grown in a Mg2+-free medium resulted in a unique phenotype characterized by a decrease in zebrafish Mg content. This decrease shows that SLC41A1 is required to maintain Mg2+ balance and its dysfunction results in renal Mg2+ wasting in zebrafish larvae. Importantly, the Mg content of the larvae is rescued when mouse SLC41A1 is expressed in slc41a1-knockdown zebrafish. Conversely, expression of mammalian SLC41A1-p.Asp262Ala, harboring a mutation in the ion-conducting SLC41A1 pore, did not reverse the renal Mg2+ wasting. 25Mg2+ transport assays in human embryonic kidney 293 (HEK293) cells overexpressing SLC41A1 demonstrated that SLC41A1 mediates cellular Mg2+ extrusion independently of sodium (Na+). In contrast, SLC41A1-p.Asp262Ala expressing HEK293 cells displayed similar Mg2+ extrusion activities than control (mock) cells. In polarized Madin-Darby canine kidney cells, SLC41A1 localized to the basolateral cell membrane. Our results demonstrate that SLC41A1 facilitates renal Mg2+ reabsorption in the zebrafish model. Furthermore, our data suggest that SLC41A1 mediates both Mg2+ uptake and extrusion.

Iroquois transcription factor irx2a is required for multiciliated and transporter cell fate decisions during zebrafish pronephros development

The genetic regulation of nephron patterning during kidney organogenesis remains poorly understood. Nephron tubules in zebrafish are composed of segment populations that have unique absorptive and secretory roles, as well as multiciliated cells (MCCs) that govern fluid flow. Here, we report that the transcription factor iroquois 2a (irx2a) is requisite for zebrafish nephrogenesis. irx2a transcripts localized to the developing pronephros and maturing MCCs, and loss of function altered formation of two segment populations and reduced MCC number. Interestingly, irx2a deficient embryos had reduced expression of an essential MCC gene ets variant 5a (etv5a), and were rescued by etv5a overexpression, supporting the conclusion that etv5a acts downstream of irx2a to control MCC ontogeny. Finally, we found that retinoic acid (RA) signaling affects the irx2a expression domain in renal progenitors, positioning irx2a downstream of RA. In sum, this work reveals new roles for irx2a during nephrogenesis, identifying irx2a as a crucial connection between RA signaling, segmentation, and the control of etv5a mediated MCC formation. Further investigation of the genetic players involved in these events will enhance our understanding of the molecular pathways that govern renal development, which can be used help create therapeutics to treat congenital and acquired kidney diseases.