Model systems for the study of kidney development: use of the pronephros in the analysis of organ induction and patterning

Generating kidney organoids based on developmental nephrology

Over the past decade, the induction protocols for the two types of kidney organoids (nephron organoids and ureteric bud organoids) from pluripotent stem cells (PSCs) have been established based on the knowledge gained in developmental nephrology. Kidney organoids are now used for disease modeling and drug screening, but they also have potential as tools for clinical transplantation therapy. One of the options to achieve this goal would be to assemble multiple renal progenitor cells (nephron progenitor, ureteric bud, stromal progenitor) to reproduce the organotypic kidney structure from PSCs. At least from mouse PSCs, all the three progenitors have been induced and assembled into such "higher order" kidney organoids. We will provide an overview of the developmental nephrology required for the induction of renal progenitors and discuss recent advances and remaining challenges of kidney organoids for clinical transplantation therapy.

Xenopus laevis (Daudin, 1802) as a Model Organism for Bioscience: A Historic Review and Perspective

In vitro systems have been mainly promoted by authorities to sustain research by following the 3Rs principle, but continuously increasing amounts of evidence point out that in vivo experimentation is also of extreme relevance. Xenopus laevis, an anuran amphibian, is a significant model organism in the study of evolutionary developmental biology, toxicology, ethology, neurobiology, endocrinology, immunology and tumor biology; thanks to the recent development of genome editing, it has also acquired a relevant position in the field of genetics. For these reasons, X. laevis appears to be a powerful and alternative model to the zebrafish for environmental and biomedical studies. Its life cycle, as well as the possibility to obtain gametes from adults during the whole year and embryos by in vitro fertilization, allows experimental studies of several biological endpoints, such as gametogenesis, embryogenesis, larval growth, metamorphosis and, of course, the young and adult stages. Moreover, with respect to alternative invertebrate and even vertebrate animal models, the X. laevis genome displays a higher degree of similarity with that of mammals. Here, we have reviewed the main available literature on the use of X. laevis in the biosciences and, inspired by Feymann's revised view, "Plenty of room for biology at the bottom", suggest that X. laevis is a very useful model for all possible studies.

A comparative study of cellular diversity between the Xenopus pronephric and mouse metanephric nephron

The kidney is an essential organ that ensures bodily fluid homeostasis and removes soluble waste products from the organism. Nephrons, the functional units of the kidney, comprise a blood filter, the glomerulus or glomus, and an epithelial tubule that processes the filtrate from the blood or coelom and selectively reabsorbs solutes, such as sugars, proteins, ions, and water, leaving waste products to be eliminated in the urine. Genes coding for transporters are segmentally expressed, enabling the nephron to sequentially process the filtrate. The Xenopus embryonic kidney, the pronephros, which consists of a single large nephron, has served as a valuable model to identify genes involved in nephron formation and patterning. Therefore, the developmental patterning program that generates these segments is of great interest. Prior work has defined the gene expression profiles of Xenopus nephron segments via in situ hybridization strategies, but a comprehensive understanding of the cellular makeup of the pronephric kidney remains incomplete. Here, we carried out single-cell mRNA sequencing of the functional Xenopus pronephric nephron and evaluated its cellular composition through comparative analyses with previous Xenopus studies and single-cell mRNA sequencing of the adult mouse kidney. This study reconstructs the cellular makeup of the pronephric kidney and identifies conserved cells, segments, and associated gene expression profiles. Thus, our data highlight significant conservation in podocytes, proximal and distal tubule cells, and divergence in cellular composition underlying the capacity of each nephron to remove wastes in the form of urine, while emphasizing the Xenopus pronephros as a model for physiology and disease.

Evolutionary Transition in the Regulation of Vertebrate Pronephros Development: A New Role for Retinoic Acid

The anterior-posterior (AP) axis in chordates is regulated by a conserved set of genes and signaling pathways, including Hox genes and retinoic acid (RA), which play well-characterized roles in the organization of the chordate body plan. The intermediate mesoderm (IM), which gives rise to all vertebrate kidneys, is an example of a tissue that differentiates sequentially along this axis. Yet, the conservation of the spatiotemporal regulation of the IM across vertebrates remains poorly understood. In this study, we used a comparative developmental approach focusing on non-conventional model organisms, a chondrichthyan (catshark), a cyclostome (lamprey), and a cephalochordate (amphioxus), to assess the involvement of RA in the regulation of chordate and vertebrate pronephros formation. We report that the anterior expression boundary of early pronephric markers (Pax2 and Lim1), positioned at the level of somite 6 in amniotes, is conserved in the catshark and the lamprey. Furthermore, RA, driving the expression of Hox4 genes like in amniotes, regulates the anterior pronephros boundary in the catshark. We find no evidence for the involvement of this regulatory hierarchy in the AP positioning of the lamprey pronephros and the amphioxus pronephros homolog, Hatschek's nephridium. This suggests that despite the conservation of Pax2 and Lim1 expressions in chordate pronephros homologs, the responsiveness of the IM, and hence of pronephric genes, to RA- and Hox-dependent regulation is a gnathostome novelty.

The Pronephros; a Fresh Perspective

Contemporary papers and book chapters on nephrology open with the assumption that human kidney development passes through three morphological stages: pronephros, mesonephros, and metanephros. Current knowledge of the human pronephros, however, appears to be based on only a hand full of human specimens. The ongoing use of variations in the definition of a pronephros hampers the interpretation of study results. Because of the increased interest in the anamniote pronephros as a genetic model for kidney organogenesis we aimed to provide an overview of the literature concerning kidney development and to clarify the existence of a pronephros in human embryos. We performed an extensive literature survey regarding vertebrate renal morphology and we investigated histological sections of human embryos between 2 and 8 weeks of development. To facilitate better understanding of the literature about kidney development, a referenced glossary with short definitions was composed. The most striking difference between pronephros versus meso- and metanephros is found in nephron architecture. The pronephros consists exclusively of non-integrated nephrons with external glomeruli, whereas meso- and metanephros are composed of integrated nephrons with internal glomeruli. Animals whose embryos have comparatively little yolk at their disposal and hence have a free-swimming larval stage do develop a pronephros that is dedicated to survival in aquatic environments. Species in which embryos do not have a free-swimming larval stage have embryos that are supplied with a large amount of yolk or that develop within the body of the parent. In those species the pronephros is usually absent, incompletely developed, and apparently functionless. Non-integrated nephrons were not identified in histological sections of human embryos. Therefore, we conclude that a true pronephros is not detectable in human embryos although the most cranial part of the amniote excretory organ is often confusingly referred to as pronephros. The term pronephros should be avoided in amniotes unless all elements for a functional pronephros are undeniably present.

Dynamin Binding Protein Is Required for Xenopus laevis Kidney Development

The adult human kidney contains over one million nephrons, with each nephron consisting of a tube containing segments that have specialized functions in nutrient and water absorption and waste excretion. The embryonic kidney of Xenopus laevis consists of a single functional nephron composed of regions that are analogous to those found in the human nephron, making it a simple model for the study of nephrogenesis. The exocyst complex, which traffics proteins to the cell membrane in vesicles via CDC42, is essential for normal kidney development. Here, we show that the CDC42-GEF, dynamin binding protein (Dnmbp/Tuba), is essential for nephrogenesis in Xenopus. dnmbp is expressed in Xenopus embryo kidneys during development, and knockdown of Dnmbp using two separate morpholino antisense oligonucleotides results in reduced expression of late pronephric markers, whereas the expression of early markers of nephrogenesis remains unchanged. A greater reduction in expression of markers of differentiated distal and connecting tubules was seen in comparison to proximal tubule markers, indicating that Dnmbp reduction may have a greater impact on distal and connecting tubule differentiation. Additionally, Dnmbp reduction results in glomus and ciliary defects. dnmbp knockout using CRISPR results in a similar reduction of late markers of pronephric tubulogenesis and also results in edema formation in later stage embryos. Overexpression of dnmbp in the kidney also resulted in disrupted pronephric tubules, suggesting that dnmbp levels in the developing kidney are tightly regulated, with either increased or decreased levels leading to developmental defects. Together, these data suggest that Dnmbp is required for nephrogenesis.

Xenbase: Facilitating the Use of Xenopus to Model Human Disease

At a fundamental level most genes, signaling pathways, biological functions and organ systems are highly conserved between man and all vertebrate species. Leveraging this conservation, researchers are increasingly using the experimental advantages of the amphibian Xenopus to model human disease. The online Xenopus resource, Xenbase, enables human disease modeling by curating the Xenopus literature published in PubMed and integrating these Xenopus data with orthologous human genes, anatomy, and more recently with links to the Online Mendelian Inheritance in Man resource (OMIM) and the Human Disease Ontology (DO). Here we review how Xenbase supports disease modeling and report on a meta-analysis of the published Xenopus research providing an overview of the different types of diseases being modeled in Xenopus and the variety of experimental approaches being used. Text mining of over 50,000 Xenopus research articles imported into Xenbase from PubMed identified approximately 1,000 putative disease- modeling articles. These articles were manually assessed and annotated with disease ontologies, which were then used to classify papers based on disease type. We found that Xenopus is being used to study a diverse array of disease with three main experimental approaches: cell-free egg extracts to study fundamental aspects of cellular and molecular biology, oocytes to study ion transport and channel physiology and embryo experiments focused on congenital diseases. We integrated these data into Xenbase Disease Pages to allow easy navigation to disease information on external databases. Results of this analysis will equip Xenopus researchers with a suite of experimental approaches available to model or dissect a pathological process. Ideally clinicians and basic researchers will use this information to foster collaborations necessary to interrogate the development and treatment of human diseases.

Stereological study of developing glomerular forms during human fetal kidney development

BACKGROUND

Human fetal kidney development is a complex and stepwise process. The number, shape, size and distribution of glomeruli provide important information on kidney organization. The aim of this study was to quantify glomerular developing forms during human fetal kidney development using stereological methods.

METHODS

Kidney tissue specimens of 40 human fetuses with gestational ages ranging from 9 to 40 weeks were analyzed. Specimens were divided into eight groups based on gestational age, each corresponding to 1 lunar month. Stereological methods were used at the light microscopy level to estimate volume, surface and numerical density of the glomerular developing forms.

RESULTS

During gestation, nephrogenesis continually advanced, and the number of nephrons increased. Volume, surface and numerical densities of vesicular forms and S-shaped bodies decreased gradually in parallel with gradual increases in estimated stereological parameters for vascularized glomeruli. Volume density and surface density of vascularized glomeruli increased gradually during fetal kidney development, and numerical density increased until the seventh lunar month. A relative decrease in vascularized glomeruli per unit volume of cortex occurred during the last 3 lunar months. Nephrogenesis began to taper off by 32 weeks and was completed by 36 weeks of gestation. The last sample in which we observed vesicles was from a fetus aged 32 weeks, and the last sample with S-shaped bodies was from a fetus aged 36 weeks.

CONCLUSIONS

The present study is one of few quantitative studies conducted on human kidney development. Knowledge of normal human kidney morphogenesis during development could be important for future medical practice. Events occurring during fetal life may have significant consequences later in life.

Transgenic Xenopus laevis Line for In Vivo Labeling of Nephrons within the Kidney

Xenopus laevis embryos are an established model for studying kidney development. The nephron structure and genetic pathways that regulate nephrogenesis are conserved between Xenopus and humans, allowing for the study of human disease-causing genes. Xenopus embryos are also amenable to large-scale screening, but studies of kidney disease-related genes have been impeded because assessment of kidney development has largely been limited to examining fixed embryos. To overcome this problem, we have generated a transgenic line that labels the kidney. We characterize this cdh17:eGFP line, showing green fluorescent protein (GFP) expression in the pronephric and mesonephric kidneys and colocalization with known kidney markers. We also demonstrate the feasibility of live imaging of embryonic kidney development and the use of cdh17:eGFP as a kidney marker for secretion assays. Additionally, we develop a new methodology to isolate and identify kidney cells for primary culture. We also use morpholino knockdown of essential kidney development genes to establish that GFP expression enables observation of phenotypes, previously only described in fixed embryos. Taken together, this transgenic line will enable primary kidney cell culture and live imaging of pronephric and mesonephric kidney development. It will also provide a simple means for high-throughput screening of putative human kidney disease-causing genes.

Wilms' Tumor Protein 1 and Enzymatic Oxidation of 5-Methylcytosine in Brain Tumors: Potential Perspectives

The patterns of 5-methylcytosine (5mC) and its oxidized derivatives, 5-hydroxymethylcytosine, 5-formylcytosine, and 5-carboxylcytosine (5caC) are reportedly altered in a range of cancers. Likewise, Wilms' Tumor protein 1 (WT1), a transcription factor essential for urogenital, epicardium, and kidney development exhibits aberrant expression in multiple tumors. Interestingly, WT1 directly interacts with TET proteins that catalyze the enzymatic oxidation of 5mC and exhibits high affinity for 5caC-containing DNA substrates in vitro. Here we review recent developments in the fields of Tet-dependent 5mC oxidation and WT1 biology and explore potential perspectives for studying the interplay between TETs and WT1 in brain tumors.

Dissecting hematopoietic and renal cell heterogeneity in adult zebrafish at single-cell resolution using RNA sequencing

The work by Tang et al. provides a comprehensive, single-cell, transcriptomic analysis of kidney and blood cells from the adult zebrafish, identifying novel cell types, including two classes of NK immune cells, classically defined and erythroid-primed hematopoietic stem and progenitor cells, mucin-secreting kidney cells, and kidney stem/progenitor cells.

Recent advances in single-cell, transcriptomic profiling have provided unprecedented access to investigate cell heterogeneity during tissue and organ development. In this study, we used massively parallel, single-cell RNA sequencing to define cell heterogeneity within the zebrafish kidney marrow, constructing a comprehensive molecular atlas of definitive hematopoiesis and functionally distinct renal cells found in adult zebrafish. Because our method analyzed blood and kidney cells in an unbiased manner, our approach was useful in characterizing immune-cell deficiencies within DNA–protein kinase catalytic subunit (prkdc), interleukin-2 receptor γ a (il2rga), and double-homozygous–mutant fish, identifying blood cell losses in T, B, and natural killer cells within specific genetic mutants. Our analysis also uncovered novel cell types, including two classes of natural killer immune cells, classically defined and erythroid-primed hematopoietic stem and progenitor cells, mucin-secreting kidney cells, and kidney stem/progenitor cells. In total, our work provides the first, comprehensive, single-cell, transcriptomic analysis of kidney and marrow cells in the adult zebrafish.

Peroxiredoxin1, a novel regulator of pronephros development, influences retinoic acid and Wnt signaling by controlling ROS levels

Peroxiredoxin1 (Prdx1) is an antioxidant enzyme belonging to the peroxiredoxin family of proteins. Prdx1 catalyzes the reduction of H₂O₂ and alkyl hydroperoxide and plays an important role in different biological processes. Prdx1 also participates in various age-related diseases and cancers. In this study, we investigated the role of Prdx1 in pronephros development during embryogenesis. Prdx1 knockdown markedly inhibited proximal tubule formation in the pronephros and significantly increased the cellular levels of reactive oxygen species (ROS), which impaired primary cilia formation. Additionally, treatment with ROS (H₂O₂) severely disrupted proximal tubule formation, whereas Prdx1 overexpression reversed the ROS-mediated inhibition in proximal tubule formation. Epistatic analysis revealed that Prdx1 has a crucial role in retinoic acid and Wnt signaling pathways during pronephrogenesis. In conclusion, Prdx1 facilitates proximal tubule formation during pronephrogenesis by regulating ROS levels.

A Gene Implicated in Activation of Retinoic Acid Receptor Targets Is a Novel Renal Agenesis Gene in Humans

Renal agenesis (RA) is one of the more extreme examples of congenital anomalies of the kidney and urinary tract (CAKUT). Bilateral renal agenesis is almost invariably fatal at birth, and unilateral renal agenesis can lead to future health issues including end-stage renal disease. Genetic investigations have identified several gene variants that cause RA, including EYA1, LHX1, and WT1 However, whereas compound null mutations of genes encoding α and γ retinoic acid receptors (RARs) cause RA in mice, to date there have been no reports of variants in RAR genes causing RA in humans. In this study, we carried out whole exome sequence analysis of two families showing inheritance of an RA phenotype, and in both identified a single candidate gene, GREB1L Analysis of a zebrafish greb1l loss-of-function mutant revealed defects in the pronephric kidney just prior to death, and F0 CRISPR/Cas9 mutagenesis of Greb1l in the mouse revealed kidney agenesis phenotypes, implicating Greb1l in this disorder. GREB1L resides in a chromatin complex with RAR members, and our data implicate GREB1L as a coactivator for RARs. This study is the first to associate a component of the RAR pathway with renal agenesis in humans.

Xenopus: leaping forward in kidney organogenesis

While kidney donations stagnate, the number of people in need of kidney transplants continues to grow. Although transplanting culture-grown organs is years away, pursuing the engineering of the kidney de novo is a valid means of closing the gap between the supply and demand of kidneys for transplantation. The structural organization of a mouse kidney is similar to that of humans. Therefore, mice have traditionally served as the primary model system for the study of kidney development. The mouse is an ideal model organism for understanding the complexity of the human kidney. Nonetheless, the elaborate structure of the mammalian kidney makes the discovery of new therapies based on de novo engineered kidneys more challenging. In contrast to mammals, amphibians have a kidney that is anatomically less complex and develops faster. Given that analogous genetic networks regulate the development of mammalian and amphibian nephric organs, using embryonic kidneys of Xenopus laevis (African clawed frog) to analyze inductive cell signaling events and morphogenesis has many advantages. Pioneering work that led to the ability to generate kidney organoids from embryonic cells was carried out in Xenopus. In this review, we discuss how Xenopus can be utilized to compliment the work performed in mammalian systems to understand kidney development.

Use of Xenopus Frogs to Study Renal Development/Repair

The Xenopus genus includes several members of aquatic frogs native to Africa but is perhaps best known for the species Xenopus laevis and Xenopus tropicalis. These species were popularized as model organisms from as early as the 1800s and have been instrumental in expanding several biological fields including cell biology, environmental toxicology, regenerative biology, and developmental biology. In fact, much of what we know about the formation and maturation of the vertebrate renal system has been acquired by examining the intricate genetic and morphological patterns that epitomize nephrogenesis in Xenopus. From these numerous reports, we have learned that the process of kidney development is as unique among organs as it is conserved among vertebrates. While development of most organs involves increases in size at a single location, development of the kidney occurs through a series of three increasingly complex nephric structures that are temporally distinct from one another and which occupy discrete spatial locales within the body. These three renal systems all serve to provide homeostatic, osmoregulatory, and excretory functions in animals. Importantly, the kidneys in amphibians, such as Xenopus, are less complex and more easily accessed than those in mammals, and thus tadpoles and frogs provide useful models for understanding our own kidney development. Several descriptive and mechanistic studies conducted with the Xenopus model system have allowed us to elucidate the cellular and molecular mediators of renal patterning and have also laid the foundation for our current understanding of kidney repair mechanisms in vertebrates. While some species-specific responses to renal injury have been observed, we still recognize the advantage of the Xenopus system due to its distinctive similarity to mammalian wound healing, reparative, and regenerative responses. In addition, the first evidence of renal regeneration in an amphibian system was recently demonstrated in Xenopus laevis. As genetic and molecular tools continue to advance, our appreciation for and utilization of this amphibian model organism can only intensify and will certainly provide ample opportunities to further our understanding of renal development and repair.

Conserved and novel functions of programmed cellular senescence during vertebrate development

Cellular senescence, a form of stable cell cycle arrest that is traditionally associated with tumour suppression, has been recently found to occur during mammalian development. Here, we show that cell senescence is an intrinsic part of the developmental programme in amphibians. Programmed senescence occurs in specific structures during defined time windows during amphibian development. It contributes to the physiological degeneration of the amphibian pronephros and to the development of the cement gland and oral cavity. In both contexts, senescence depends on TGFβ but is independent of ERK/MAPK activation. Furthermore, elimination of senescent cells through temporary TGFβ inhibition leads to developmental defects. Our findings uncover conserved and new roles of senescence in vertebrate organogenesis and support the view that cellular senescence may have arisen in evolution as a developmental mechanism.

Technique to Target Microinjection to the Developing Xenopus Kidney

The embryonic kidney of Xenopus laevis (frog), the pronephros, consists of a single nephron, and can be used as a model for kidney disease. Xenopus embryos are large, develop externally, and can be easily manipulated by microinjection or surgical procedures. In addition, fate maps have been established for early Xenopus embryos. Targeted microinjection into the individual blastomere that will eventually give rise to an organ or tissue of interest can be used to selectively overexpress or knock down gene expression within this restricted region, decreasing secondary effects in the rest of the developing embryo. In this protocol, we describe how to utilize established Xenopus fate maps to target the developing Xenopus kidney (the pronephros), through microinjection into specific blastomere of 4- and 8-cell embryos. Injection of lineage tracers allows verification of the specific targeting of the injection. After embryos have developed to stage 38 - 40, whole-mount immunostaining is used to visualize pronephric development, and the contribution by targeted cells to the pronephros can be assessed. The same technique can be adapted to target other tissue types in addition to the pronephros.

Zebrafish kidney development

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

Epithelial cell fate in the nephron tubule is mediated by the ETS transcription factors etv5a and etv4 during zebrafish kidney development

Kidney development requires the differentiation and organization of discrete nephron epithelial lineages, yet the genetic and molecular pathways involved in these events remain poorly understood. The embryonic zebrafish kidney, or pronephros, provides a simple and useful model to study nephrogenesis. The pronephros is primarily comprised of two types of epithelial cells: transportive and multiciliated cells (MCCs). Transportive cells occupy distinct tubule segments and are characterized by the expression of various solute transporters, while MCCs function in fluid propulsion and are dispersed in a "salt-and-pepper" fashion within the tubule. Epithelial cell identity is reliant on interplay between the Notch signaling pathway and retinoic acid (RA) signaling, where RA promotes MCC fate by inhibiting Notch activity in renal progenitors, while Notch acts downstream to trigger transportive cell formation and block adoption of an MCC identity. Previous research has shown that the transcription factor ets variant 5a (etv5a), and its closely related ETS family members, are required for ciliogenesis in other zebrafish tissues. Here, we mapped etv5a expression to renal progenitors that occupy domains where MCCs later emerge. Thus, we hypothesized that etv5a is required for normal development of MCCs in the nephron. etv5a loss of function caused a decline of MCC number as indicated by the reduced frequency of cells that expressed the MCC-specific markers outer dense fiber of sperm tails 3b (odf3b) and centrin 4 (cetn4), where rescue experiments partially restored MCC incidence. Interestingly, deficiency of ets variant 4 (etv4), a related gene that is broadly expressed in the posterior mesoderm during somitogenesis stages, also led to reduced MCC numbers, which were further reduced by dual etv5a/4 deficiency, suggesting that both of these ETS factors are essential for MCC formation and that they also might have redundant activities. In epistatic studies, exogenous RA treatment expanded the etv5a domain within the renal progenitor field and RA inhibition blocked etv5a in this populace, indicating that etv5a acts downstream of RA. Additionally, treatment with exogenous RA partially rescued the reduced MCC phenotype after loss of etv5a. Further, abrogation of Notch with the small molecule inhibitor DAPT increased the renal progenitor etv5a expression domain as well as MCC density in etv5a deficient embryos, suggesting Notch acts upstream to inhibit etv5a. In contrast, etv4 levels in renal progenitors were unaffected by changes in RA or Notch signaling levels, suggesting a possible non-cell autonomous role during pronephros formation. Taken together, these findings have revealed new insights about the genetic mechanisms of epithelial cell development during nephrogenesis.

Evaluation of nephrotoxic effects of aristolochic acid on zebrafish (Danio rerio) larvae

To analyze the toxic effects of aristolochic acid (AA) on developed kidneys in zebrafish larvae, zebrafish at 3 days postfertilization were treated with various concentrations of AA for 24 h before the status of kidney injury was investigated from several points of view. It was found that 21% of the larvae treated with 10 µmoL/L AA exhibited evident periocular edema. When the concentrations of AA were increased to 20 and 40 µmoL/L, defect in the cardiovascular system characterized by slow heart beat and blood flow was seen coupled with periocular edema. Creatinine in the whole larval tissue determined by liquid chromatography-mass spectrometry/mass spectrometry exhibited dramatic increase in the treated groups in a dose-dependent manner within a certain range of doses. Several evident protein bands were detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in supernatant of the treated larvae, indicating leakage of glomerular filtration barrier. Results of quantitative polymerase chain reaction show that the messenger RNA expression of nephrin in the 20 and 40 µmoL/L AA-treated groups decreased to 0.58 ± 0.062 and 0.37 ± 0.075-folds of the control, respectively. Kidney damage was further confirmed by the histological changes in paraffin sections of treated larvae, for example, cystic glomeruli and disorganized epithelia cells of pronephric tubules. Our results revealed that AA exerted toxic effects on developed kidney of zebrafish larvae in a dose-dependent manner and podocyte dysfunction may be involved in the kidney injury and proteinuria.

Nephron Patterning: Lessons from Xenopus, Zebrafish, and Mouse Studies

The nephron is the basic structural and functional unit of the vertebrate kidney. To ensure kidney functions, the nephrons possess a highly segmental organization where each segment is specialized for the secretion and reabsorption of particular solutes. During embryogenesis, nephron progenitors undergo a mesenchymal-to-epithelial transition (MET) and acquire different segment-specific cell fates along the proximo-distal axis of the nephron. Even if the morphological changes occurring during nephrogenesis are characterized, the regulatory networks driving nephron segmentation are still poorly understood. Interestingly, several studies have shown that the pronephric nephrons in Xenopus and zebrafish are segmented in a similar fashion as the mouse metanephric nephrons. Here we review functional and molecular aspects of nephron segmentation with a particular interest on the signaling molecules and transcription factors recently implicated in kidney development in these three different vertebrate model organisms. A complete understanding of the mechanisms underlying nephrogenesis in different model organisms will provide novel insights on the etiology of several human renal diseases.

Pax8 and Pax2 are specifically required at different steps of Xenopus pronephros development

The respective role of Pax2 and Pax8 in early kidney development in vertebrates is poorly understood. In this report, we have studied the roles of Pax8 and Pax2 in Xenopus pronephros development using a loss-of-function approach. Our results highlight a differential requirement of these two transcription factors for proper pronephros formation. Pax8 is necessary for the earliest steps of pronephric development and its depletion leads to a complete absence of pronephric tubule. Pax2 is required after the establishment of the tubule pronephric anlage, for the expression of several terminal differentiation markers of the pronephric tubule. Neither Pax2 nor Pax8 is essential to glomus development. We further show that Pax8 controls hnf1b, but not lhx1 and Osr2, expression in the kidney field as soon as the mid-neurula stage. Pax8 is also required for cell proliferation of pronephric precursors in the kidney field. It may exert its action through the wnt/beta-catenin pathway since activation of this pathway can rescue MoPax8 induced proliferation defect and Pax8 regulates expression of the wnt pathway components, dvl1 and sfrp3. Finally, we observed that loss of pronephros in Pax8 morphants correlates with an expanded vascular/blood gene expression domain indicating that Pax8 function is important to delimit the blood/endothelial genes expression domain in the anterior part of the dorso-lateral plate.

Ontogeny and osmoregulatory function of the urinary system in the Persian sturgeon, Acipenser persicus (Borodin, 1897)

The structure of the kidney and the localization of Na(+), K(+)-ATPase (NKA) immunopositive cells were examined throughout the postembryonic development of the Persian sturgeon, Acipenser persicus, from newly hatched prelarvae (10mm) to 20 days post hatch (20 DPH) larvae (31mm). Investigations were conducted through histology and immunohistochemistry by using the light and immunofluorescence microscopy. The pronephros was observed in newly hatched prelarvae. The cells lining the distal pronephric tubules and their collecting ducts showed laterally expressed NKA immunofluorescence that later extended throughout the whole cytoplasm. Mesonephrogenous placodes and pre-glomeruli were distinguished at 2 DPH along the collecting ducts posteriorly. Their tubules were formed and present in kidney mesenchyma, differentiated into neck, proximal, distal and collecting segments at 7 DPH when NKA immunopositive cells were observed. Their distal and collecting tubules showed an increasing immunofluorescence throughout their cytoplasm while the glomeruli remained unstained. From D 9 to D 17, the epithelial layer of pronephric collecting duct changed along the mesonephros to form ureters. Ureters, possessing isolated strong NKA immunopositive cells, appeared as two sac-like structures hanging under the trunk kidney. Since NKA immunopositive cells were not observed on the tegument or along the digestive tract of newly hatched prelarva, and also the gills are not formed yet, the pronephros is the only osmoregulatory organ until 4 DPH. At the larval stage, the pronephros and mesonephros are functional osmoregulatory organs and actively reabsorb necessary ions from the filtrate.

Loss of a kidney during fetal life: long-term consequences and lessons learned

Epidemiological studies reveal that children born with a solitary functioning kidney (SFK) have a greater predisposition to develop renal insufficiency and hypertension in early adulthood. A congenital SFK is present in patients with unilateral renal agenesis or unilateral multicystic kidney dysplasia, leading to both structural and functional adaptations in the remaining kidney, which act to mitigate the reductions in glomerular filtration rate and sodium excretion that would otherwise ensue. To understand the mechanisms underlying the early development of renal insufficiency in children born with a SFK, we established a model of fetal uninephrectomy (uni-x) in sheep, a species that similar to humans complete nephrogenesis before birth. This model results in a 30% reduction in nephron number rather than 50%, due to compensatory nephrogenesis in the remaining kidney. Similar to children with a congenital SFK, uni-x sheep demonstrate a progressive increase in arterial pressure and a loss of renal function with aging. This review summarizes the compensatory changes in renal hemodynamics and tubular sodium handling that drive impairments in renal function and highlights the existence of sex differences in the functional adaptations following the loss of a kidney during fetal life.

New tides: using zebrafish to study renal regeneration

Over the past several decades, the zebrafish has become one of the major vertebrate model organisms used in biomedical research. In this arena, the zebrafish has emerged as an applicable system for the study of kidney diseases and renal regeneration. The relevance of the zebrafish model for nephrology research has been increasingly appreciated as the understanding of zebrafish kidney structure, ontogeny, and the response to damage has steadily expanded. Recent studies have documented the amazing regenerative characteristics of the zebrafish kidney, which include the ability to replace epithelial populations after acute injury and to grow new renal functional units, termed nephrons. Here we discuss how nephron composition is conserved between zebrafish and mammals, and highlight how recent findings from zebrafish studies utilizing transgenic technologies and chemical genetics can complement traditional murine approaches in the effort to dissect how the kidney responds to acute damage and identify therapeutics that enhance human renal regeneration.

Impaired ability to modulate glomerular filtration rate in aged female sheep following fetal uninephrectomy

Fetal uninephrectomy (uni-x) results in hypertension at a later age in female than male sheep. We hypothesized that dysregulation of tubular sodium handling contributes to the reduced ability to regulate extracellular fluid (ECF) homeostasis in older females born with a congenital nephron deficit. Following renal excretory balance studies, the response to inhibition of the Na(+)K(+)2Cl(-) cotransporter with furosemide (0.5 mg/kg bolus + 1 mg/kg per hour, i.v) or vehicle treatment was examined in conscious 5-year-old female uni-x (n = 7) and sham (n = 7) sheep. Balance studies in meal-fed sheep demonstrated that while average 24 h sodium excretion over 6 days was not different between the groups, the daily variation in sodium excretion was significantly greater in uni-x compared to sham sheep (31 ± 4% vs. 12 ± 2%; P < 0.001). Basal plasma renin activity (PRA) and renal cortical cyclooxygenase-2 (COX-2) gene expression were lower in uni-x sheep (both, P < 0.01). The increases in glomerular filtration rate (GFR) and renal blood flow observed in sham sheep in response to furosemide were significantly attenuated in uni-x sheep (both P GROUP×TREAT < 0.05). However, fractional sodium excretion increased by a greater extent in the uni-x (4.4 ± 1.0%) as compared to the sham sheep (2.0 ± 0.4%; P GROUP×TIME < 0.05) in response to furosemide. In conclusion, fetal uni-x was associated with altered renal sodium handling and hypertension in aged females. The impaired ability to modulate PRA and GFR in the adults with a congenital nephron deficit may reduce the capacity of the kidney to respond to gains or losses in ECF to maintain a stable internal environment.

Roles of Iroquois Transcription Factors in Kidney Development

Congenital anomalies of the kidney and urinary tract (CAKUT) affect 1/500 live births. CAKUT lead to end stage renal failure in children, and are associated with high morbidity rates. Understanding the mechanisms of kidney development, and that of other associated urogenital tissues, is crucial to the prevention and treatment of CAKUT. The kidney arises from self-renewing mesenchymal renal stem cells that produce nephrons, which are the principal functional units of the organ. To date, the genetic and cellular mechanisms that control nephrogenesis have remained poorly understood. In recent years, developmental studies using amphibians and zebrafish have revealed that their simple embryonic kidney, known as the pronephros, is a useful paradigm for comparative studies of nephron ontogeny. Here, we discuss the new found roles for Iroquois transcription factors in pronephric nephron patterning, and explore the relevance of these findings for kidney development in humans.

Precocious acquisition of neuroepithelial character in the eye field underlies the onset of eye morphogenesis

Using high-resolution live imaging in zebrafish, we show that presumptive eye cells acquire apicobasal polarity and adopt neuroepithelial character prior to other regions of the neural plate. Neuroepithelial organization is first apparent at the margin of the eye field, whereas cells at its core have mesenchymal morphology. These core cells subsequently intercalate between the marginal cells contributing to the bilateral expansion of the optic vesicles. During later evagination, optic vesicle cells shorten, drawing their apical surfaces laterally relative to the basal lamina, resulting in further laterally directed evagination. The early neuroepithelial organization of the eye field requires Laminin1, and ectopic Laminin1 can redirect the apicobasal orientation of eye field cells. Furthermore, disrupting cell polarity through combined abrogation of the polarity protein Pard6γb and Laminin1 severely compromises optic vesicle evagination. Our studies elucidate the cellular events underlying early eye morphogenesis and provide a framework for understanding epithelialization and complex tissue formation.

Generation of the podocyte and tubular components of an amniote kidney: timing of specification and a role for Wnt signaling

Kidneys remove unwanted substances from the body and regulate the internal body environment. These functions are carried out by specialized cells (podocytes) that act as a filtration barrier between the internal milieu and the outside world, and by a series of tubules and ducts that process the filtrate and convey it to the outside. In the kidneys of amniote vertebrates, the filtration (podocyte) and tubular functions are tightly integrated into functional units called nephrons. The specification of the podocyte and tubular components of amniote nephrons is currently not well understood. The present study investigates podocyte and tubule differentiation in the avian mesonephric kidney, and presents several findings that refine our understanding of the initial events of nephron formation. First, well before the first morphological or molecular signs of nephron formation, mesonephric mesenchyme can be separated on the basis of morphology and the expression of the transcription factor Pod1 into dorsal and ventral components, which can independently differentiate in culture along tubule and podocyte pathways, respectively. Second, canonical Wnt signals, which are found in the nephric duct adjacent to the dorsal mesonephric mesenchyme and later in portions of the differentiating nephron, strongly inhibit podocyte but not tubule differentiation, suggesting that Wnt signaling plays an important role in the segmentation of the mesonephric mesenchyme into tubular and glomerular segments. The results are discussed in terms of their broader implications for models of nephron segmentation.

Enhanced XAO: the ontology of Xenopus anatomy and development underpins more accurate annotation of gene expression and queries on Xenbase

BACKGROUND

The African clawed frogs Xenopus laevis and Xenopus tropicalis are prominent animal model organisms. Xenopus research contributes to the understanding of genetic, developmental and molecular mechanisms underlying human disease. The Xenopus Anatomy Ontology (XAO) reflects the anatomy and embryological development of Xenopus. The XAO provides consistent terminology that can be applied to anatomical feature descriptions along with a set of relationships that indicate how each anatomical entity is related to others in the embryo, tadpole, or adult frog. The XAO is integral to the functionality of Xenbase (http://www.xenbase.org), the Xenopus model organism database.

RESULTS

We significantly expanded the XAO in the last five years by adding 612 anatomical terms, 2934 relationships between them, 640 synonyms, and 547 ontology cross-references. Each term now has a definition, so database users and curators can be certain they are selecting the correct term when specifying an anatomical entity. With developmental timing information now asserted for every anatomical term, the ontology provides internal checks that ensure high-quality gene expression and phenotype data annotation. The XAO, now with 1313 defined anatomical and developmental stage terms, has been integrated with Xenbase expression and anatomy term searches and it enables links between various data types including images, clones, and publications. Improvements to the XAO structure and anatomical definitions have also enhanced cross-references to anatomy ontologies of other model organisms and humans, providing a bridge between Xenopus data and other vertebrates. The ontology is free and open to all users.

CONCLUSIONS

The expanded and improved XAO allows enhanced capture of Xenopus research data and aids mechanisms for performing complex retrieval and analysis of gene expression, phenotypes, and antibodies through text-matching and manual curation. Its comprehensive references to ontologies across taxa help integrate these data for human disease modeling.

Improvement in renal hemodynamics following combined angiotensin II infusion and AT1R blockade in aged female sheep following fetal unilateral nephrectomy

Renin-angiotensin system (RAS) is a powerful modulator of renal hemodynamic and fluid homeostasis. Up-regulation in components of intra-renal RAS occurs with ageing. Recently we reported that 2 year old uninephrectomised (uni-x) female sheep have low renin hypertension and reduced renal function. By 5 years of age, these uni-x sheep had augmented decrease in renal blood flow (RBF) compared to sham. We hypothesised that this decrease in RBF in 5 year old uni-x sheep was due to an up-regulation in components of the intra-renal RAS. In this study, renal responses to angiotensin II (AngII) infusion and AngII type 1 receptor (AT1R) blockade were examined in the same 5 year old sheep. We also administered AngII in the presence of losartan to increase AngII bioavailability to the AT2R in order to understand AT2R contribution to renal function in this model. Uni-x animals had significantly lower renal cortical content of renin, AngII (∼40%) and Ang 1-7 (∼60%) and reduced cortical expression of AT1R gene than sham animals. In response to both AngII infusion and AT1R blockade via losartan, renal hemodynamic responses and tubular sodium excretion were significantly attenuated in uni-x animals compared to sham. However, AngII infusion in the presence of losartan caused ∼33% increase in RBF in uni-x sheep compared to ∼14% in sham (P<0.05). This was associated with a significant decrease in renal vascular resistance in the uni-x animals (22% vs 15%, P<0.05) without any changes in systemic blood pressure. The present study shows that majority of the intra-renal RAS components are suppressed in this model of low renin hypertension. However, increasing the availability of AngII to AT2R by AT1R blockade improved renal blood flow in uni-x sheep. This suggests that manipulation of the AT2R maybe a potential therapeutic target for treatment of renal dysfunction associated with a congenital nephron deficit.

Regeneration of functional pronephric proximal tubules after partial nephrectomy in Xenopus laevis

BACKGROUND

While the renal system is critical for maintaining homeostatic equilibrium within the body, it is also susceptible to various kinds of damage. Tubule dysfunction in particular contributes to acute renal injury and chronic kidney disease in millions of patients worldwide. Because current treatments are highly invasive and often unavailable, gaining a better understanding of the regenerative capacity of renal structures is vital. Although the effects of various types of acute damage have been previously studied, the ability of the excretory system to repair itself after dramatic tissue loss due to mechanical damage is less well characterized.

RESULTS

A novel unilateral nephrectomy technique was developed to excise pronephric proximal tubules from Xenopus laevis tadpoles to study tubule repair after injury. Immunohistochemical detection of protein expression and renal uptake assays demonstrated that X. laevis larvae have the capacity to regenerate functional proximal tubules following resection.

CONCLUSIONS

We have validated the renal identity of the restored tubules and demonstrated their ability to functional normally providing the first evidence of regeneration of renal tissue in an amphibian system. Importantly, this tubule restoration occurs by means of a process involving an early apoptotic event and the biphasic expression of the matrix metalloproteinase, Xmmp-9.

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

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

Xenopus as a model system for the study of GOLPH2/GP73 function: Xenopus GOLPH2 is required for pronephros development

GOLPH2 is a highly conserved protein. It is upregulated in a number of tumors and is being considered as an emerging biomarker for related diseases. However, the function of GOLPH2 remains unknown. The Xenopus model is used to study the function of human proteins. We describe the isolation and characterization of Xenopus golph2, which dimerizes and localizes to the Golgi in a manner similar to human GOLPH2. Xenopus golph2 is expressed in the pronephros during early development. The morpholino-mediated knockdown of golph2 results in edema formation. Additionally, Nephrin expression is enhanced in the glomus, and the expression of pronephric marker genes, such as atp1b1, ClC-K, NKCC2, and NBC1, is diminished in the tubules and duct. Expression patterns of the transcription factors WT1, Pax2, Pax8, Lim1, GATA3, and HNF1β are also examined in the golph2 knockdown embryos, the expression of WT1 is increased in the glomus and expanded laterally in the pronephric region. We conclude that the deletion of golph2 causes an increase in the expression of WT1, which may promote glomus formation and inhibit pronephric tubule differentiation.

Renal responses to furosemide are significantly attenuated in male sheep at 6 months of age following fetal uninephrectomy

We have previously shown that fetal uninephrectomy (uni-x) at 100 days of gestation (term = 150 days) in male sheep results in a 30% nephron deficit, reduction in glomerular filtration rate (GFR) and renal blood flow, and elevation in arterial pressure at 6 mo of age. Furthermore, in response to an acute 0.9% saline load, sodium excretion was significantly delayed in uni-x animals leading us to speculate that tubuloglomerular feedback (TGF) activity was reset in uni-x animals. In the present study, we induced TGF blockade by furosemide administration (1.5 mg/kg iv over 90 min) and determined GFR, effective renal plasma flow, and urine and sodium excretion responses in 6-mo-old male sheep. In response to furosemide, a significant diuresis and natriuresis was observed in the sham group; however, the response was significantly delayed and reduced in uni-x animals (both, Ptreatment×time < 0.001). Cummulative urinary and sodium output was significantly less in the uni-x compared with the sham sheep (both, Ptreatment×time < 0.001). GFR was increased in the sham but not the uni-x sheep ( Ptreatment×time < 0.0001). In conclusion, the excretory response to furosemide was attenuated in the uni-x sheep, and this suggests a rightward resetting of the TGF operating point. The TGF mechanism is important in the fine tuning of sodium homeostasis and is likely a contributing factor for the dysfunction in sodium regulation we have previously observed in the uni-x animals.

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

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

Urine-concentrating defects exacerbate with age in male offspring with a low-nephron endowment

Fetal uninephrectomy (uni-x) in male sheep at 100 days of gestation (term = 150 days) reduces overall nephron endowment without affecting birth weight. Offspring have a lower glomerular filtration rate (GFR) and elevated mean arterial pressure (MAP) at 6 mo of age. This study investigated whether this reduction in renal function was associated with impaired urine-concentrating ability at 6 mo of age and exacerbated with ageing (4 yr) and examined response to 1) nonpressor dose of exogenous arginine vasopressin (AVP; 0.2 μg·kg−1·h−1 iv) and 2) 30 h of water deprivation. Basal MAP was higher in uni-x animals at both ages, and became further elevated with age compared with the sham group (elevation in MAP with age; sham: ∼4 mmHg, uni-x: 9 mmHg, Pgroup × age < 0.01). GFR declined with ageing in both groups with the decrease being greater with age in the uni-x group (further 26%, Pgroup × age < 0.001). In response to AVP infusion, urine osmolality increased in both treatment groups; this response was significantly lower in the uni-x animals and became further reduced with ageing. Uni-x animals had reduced renal expression of vasopressin-2 receptor and aquaporin-2 at both ages ( P < 0.01). The increase in plasma AVP levels in response to dehydration was similar between the treatment groups, suggesting the urine-concentrating defect was associated with these renal gene changes rather than defects in AVP secretion. Renal insufficiency due to a low-nephron endowment increases the risk of hypertension and chronic renal disease and may incur greater vulnerability to physiological challenges such as water deprivation as observed in the uni-x animals.

Xenopus pronephros development--past, present, and future

Kidney development is a multi-step process where undifferentiated mesenchyme is converted into a highly complex organ through several inductive events. The general principles regulating these events have been under intense investigation and despite extensive progress, many open questions remain. While the metanephric kidneys of mouse and rat have served as the primary model, other organisms also significantly contribute to the field. In particular, the more primitive pronephric kidney has emerged as an alternative model due to its simplicity and experimental accessibility. Many aspects of nephron development such as the patterning along its proximo-distal axis are evolutionarily conserved and are therefore directly applicable to higher vertebrates. This review will focus on the current understanding of pronephros development in Xenopus. It summarizes how signaling, transcriptional regulation, as well as post-transcriptional mechanisms contribute to the differentiation of renal epithelial cells. The data show that even in the simple pronephros the mechanisms regulating kidney organogenesis are highly complex. It also illustrates that a multifaceted analysis embracing modern genome-wide approaches combined with single gene analysis will be required to fully understand all the intricacies.

Fetal uninephrectomy in male sheep alters the systemic and renal responses to angiotensin II infusion and AT1R blockade

Fetal uninephrectomy (uni-x) at 100 days of gestation results in compensatory nephrogenesis in the remaining kidney, resulting in a 30% reduction in total nephron number in male sheep. Recently, we showed that uni-x males at 6 mo of age have elevated arterial pressure, reduced renal blood flow (RBF), glomerular filtration rate (GFR), and low plasma renin levels (Singh R, Denton K, Bertram J, Jefferies A, Head G, Lombardo P, Schneider-Kolsky M, Moritz K. J Hypertens 27: 386–396, 2009; Singh R, Denton K, Jefferies A, Bertram J, Moritz K. Clin Sci (Lond) 118: 669–680, 2010). We hypothesized this was due to upregulation of the intrarenal renin-angiotensin system (RAS). In this study, renal responses to ANG II infusion and ANG II type 1 receptor (AT1R) blockade were examined in the same 6-mo-old male sheep. Uni-x animals had reduced levels of renal tissue and plasma renin and ANG II. Renal gene expression of renin, and gene and protein levels of AT1R and AT2R, were significantly lower in uni-x animals. In response to graded ANG II infusion, sham animals had the expected decrease in conscious RBF and GFR. Interestingly, the response was biphasic in uni-x sheep, with GFR initially decreasing, but then increasing at higher ANG II doses (34 ± 7%; Pgroup × treatment < 0.001), due to a paradoxical decrease in renal vascular resistance ( Pgroup × treatment < 0.001). In response to AT1R blockade, while GFR and RBF responded similarly between groups, there was a marked increase in sodium excretion in uni-x compared with sham sheep (209 ± 35 vs. 25 ± 12%; P < 0.001). In conclusion, in 6-mo-old male sheep born with a single kidney, these studies demonstrate that this is a low-renin form of hypertension, in which responses to ANG II are perturbed and the intrarenal RAS is downregulated.

Pronephric tubulogenesis requires Daam1-mediated planar cell polarity signaling

Canonical β-catenin-mediated Wnt signaling is essential for the induction of nephron development. Noncanonical Wnt/planar cell polarity (PCP) pathways contribute to processes such as cell polarization and cytoskeletal modulation in several tissues. Although PCP components likely establish the plane of polarization in kidney tubulogenesis, whether PCP effectors directly modulate the actin cytoskeleton in tubulogenesis is unknown. Here, we investigated the roles of Wnt PCP components in cytoskeletal assembly during kidney tubule morphogenesis in Xenopus laevis and zebrafish. We found that during tubulogenesis, the developing pronephric anlagen expresses Daam1 and its interacting Rho-GEF (WGEF), which compose one PCP/noncanonical Wnt pathway branch. Knockdown of Daam1 resulted in reduced expression of late pronephric epithelial markers with no apparent effect upon early markers of patterning and determination. Inhibiting various points in the Daam1 signaling pathway significantly reduced pronephric tubulogenesis. These data indicate that pronephric tubulogenesis requires the Daam1/WGEF/Rho PCP pathway.

Genetic approach to elucidation of sasang constitutional medicine

Sasang Constitutional Medicine (SCM) offers a medical principle that classifies humans into four constitution groups and guides their treatment with constitution-matched medical assistance. The principle of this traditional medicine, although requires significant scientific support, appears to suggest a genetic influence on constitution type. The relative frequency of constitution types in a population, for instance, has remained relatively constant since Jema Lee first described them from his observations. In addition, the body compartment concept of SCM appears to be related to the anterio-posterior patterning of the embryonic gut and associated internal organs. This study describes the attributes of the constitution concept of SCM that can be interpreted in the language of genetics and current approaches to identity the genetic factors that make up the constitution. These efforts should make it possible to interpret the principle of this traditional medicine scientifically. Considering the recent trend in medicine that pursues individualized or tailored medical offerings, once SCM is proven to be explainable with scientific evidence, it will be able to contribute to and take a place in the rapidly evolving medicine environment.

Nephric duct insertion is a crucial step in urinary tract maturation that is regulated by a Gata3-Raldh2-Ret molecular network in mice

Urinary tract development depends on a complex series of events in which the ureter moves from its initial branch point on the nephric duct (ND) to its final insertion site in the cloaca (the primitive bladder and urethra). Defects in this maturation process can result in malpositioned ureters and hydronephrosis, a common cause of renal disease in children. Here, we report that insertion of the ND into the cloaca is an unrecognized but crucial step that is required for proper positioning of the ureter and that depends on Ret signaling. Analysis of Ret mutant mice at birth reveals hydronephrosis and defective ureter maturation, abnormalities that our results suggest are caused, at least in part, by delayed insertion of the ND. We find a similar set of malformations in mutants lacking either Gata3 or Raldh2. We show that these factors act in parallel to regulate ND insertion via Ret. Morphological analysis of ND extension in wild-type embryos reveals elaborate cellular protrusions at ND tips that are not detected in Ret, Gata3 or Raldh2 mutant embryos, suggesting that these protrusions may normally be important for fusion with the cloaca. Together, our studies reveal a novel Ret-dependent event, ND insertion, that, when abnormal, can cause obstruction and hydronephrosis at birth; whether ND defects underlie similar types of urinary tract abnormalities in humans is an interesting possibility.

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

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

Lhx1 is required for specification of the renal progenitor cell field

In the vertebrate embryo, the kidney is derived from the intermediate mesoderm. The LIM-class homeobox transcription factor lhx1 is expressed early in the intermediate mesoderm and is one of the first genes to be expressed in the nephric mesenchyme. In this study, we investigated the role of Lhx1 in specification of the kidney field by either overexpressing or depleting lhx1 in Xenopus embryos or depleting lhx1 in an explant culture system. By overexpressing a constitutively-active form of Lhx1, we established its capacity to expand the kidney field during the specification stage of kidney organogenesis. In addition, the ability of Lhx1 to expand the kidney field diminishes as kidney organogenesis transitions to the morphogenesis stage. In a complimentary set of experiments, we determined that embryos depleted of lhx1, show an almost complete loss of the kidney field. Using an explant culture system to induce kidney tissue, we confirmed that expression of genes from both proximal and distal kidney structures is affected by the absence of lhx1. Taken together our results demonstrate an essential role for Lhx1 in driving specification of the entire kidney field from the intermediate mesoderm.

Non-canonical wnt signals antagonize and canonical wnt signals promote cell proliferation in early kidney development

Canonical and non-canonical wnt signals often have opposed roles. In this report, we use developing Xenopus embryos to demonstrate a novel anti-proliferative role for non-canonical wnt signals in the very earliest stages of kidney development. Non-canonical wnt signals were down-regulated using PDZ domain mutants of dishevelled 2 and up-regulated using wild-type vang-like 2, while canonical signals were manipulated using dominant-negative forms of lef1 or treatment with lithium. When non-canonical signals are down-regulated in the developing Xenopus pronephros, cell proliferation rates increased and when canonical signals were shutdown the opposite occurred. Treatment with lithium chloride has a powerful pro-proliferative effect on the forming nephric primordium. Together these data show that in addition to previously documented antagonisms between these distinct wnt signaling pathways, they also have opposing effects on cell division.

Role of Tbx2 in defining the territory of the pronephric nephron

Despite extensive study of the development of the nephron, which is the functional unit of the kidney, the molecular mechanisms underlying the determination of nephron size remain largely unknown. Using the Xenopus pronephros, we demonstrate here that Tbx2, a T-box transcriptional repressor, functions to demarcate the territory of the pronephric nephron. Tbx2 is specifically expressed around three distinct components of the pronephric nephron: the tubule, duct and glomus. Gain of function of Tbx2 inhibits nephric mesoderm formation. Conversely, Tbx2 loss of function expands the boundary of each component of the pronephric nephron, resulting in an enlarged pronephros. BMP signals induce Tbx2 in the non-nephric mesoderm, which inhibits the expression of the nephric markers Hey1 and Gremlin. Importantly, these pronephric molecules repress Tbx2 expression by antagonizing BMP signals in the nephric mesoderm. These results suggest that the negative regulatory loops between BMP/Tbx2 and Gremlin or Hey1 are responsible for defining the territory of the pronephric nephron.