Xenopus: a prince among models for pronephric kidney development

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.

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.

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.

The enpp4 ectonucleotidase regulates kidney patterning signalling networks in Xenopus embryos

The enpp ectonucleotidases regulate lipidic and purinergic signalling pathways by controlling the extracellular concentrations of purines and bioactive lipids. Although both pathways are key regulators of kidney physiology and linked to human renal pathologies, their roles during nephrogenesis remain poorly understood. We previously showed that the pronephros was a major site of enpp expression and now demonstrate an unsuspected role for the conserved vertebrate enpp4 protein during kidney formation in Xenopus. Enpp4 over-expression results in ectopic renal tissues and, on rare occasion, complete mini-duplication of the entire kidney. Enpp4 is required and sufficient for pronephric markers expression and regulates the expression of RA, Notch and Wnt pathway members. Enpp4 is a membrane protein that binds, without hydrolyzing, phosphatidylserine and its effects are mediated by the receptor s1pr5, although not via the generation of S1P. Finally, we propose a novel and non-catalytic mechanism by which lipidic signalling regulates nephrogenesis.

Regeneration enhancers: A clue to reactivation of developmental genes

During tissue and organ regeneration, cells initially detect damage and then alter nuclear transcription in favor of tissue/organ reconstruction. Until recently, studies of tissue regeneration have focused on the identification of relevant genes. These studies show that many developmental genes are reused during regeneration. Concurrently, comparative genomics studies have shown that the total number of genes does not vastly differ among vertebrate taxa. Moreover, functional analyses of developmental genes using various knockout/knockdown techniques demonstrated that the functions of these genes are conserved among vertebrates. Despite these data, the ability to regenerate damaged body parts varies widely between animals. Thus, it is important to determine how regenerative transcriptional programs are triggered and why animals with low regenerative potential fail to express developmental genes after injury. Recently, we discovered relevant enhancers and named them regeneration signal-response enhancers (RSREs) after identifying their activation mechanisms in a Xenopus laevis transgenic system. In this review, we summarize recent studies of injury/regeneration-associated enhancers and then discuss their mechanisms of activation.

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.

Arid3a regulates nephric tubule regeneration via evolutionarily conserved regeneration signal-response enhancers

Amphibians and fish have the ability to regenerate numerous tissues, whereas mammals have a limited regenerative capacity. Despite numerous developmental genes becoming reactivated during regeneration, an extensive analysis is yet to be performed on whether highly regenerative animals utilize unique cis-regulatory elements for the reactivation of genes during regeneration and how such cis-regulatory elements become activated. Here, we screened regeneration signal-response enhancers at the lhx1 locus using Xenopus and found that the noncoding elements conserved from fish to human function as enhancers in the regenerating nephric tubules. A DNA-binding motif of Arid3a, a component of H3K9me3 demethylases, was commonly found in RSREs. Arid3a binds to RSREs and reduces the H3K9me3 levels. It promotes cell cycle progression and causes the outgrowth of nephric tubules, whereas the conditional knockdown of arid3a using photo-morpholino inhibits regeneration. These results suggest that Arid3a contributes to the regeneration of nephric tubules by decreasing H3K9me3 on RSREs.

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.

Proper Notch activity is necessary for the establishment of proximal cells and differentiation of intermediate, distal, and connecting tubule in Xenopus pronephros development

BACKGROUND

Notch signaling in pronephros development has been shown to regulate establishment of glomus and proximal tubule, but how Notch signal works on competency of pronephric anlagen during the generation of pronephric components remains to be understood.

RESULTS

We investigated how components of pronephros (glomus, proximal tubule, intermediate tubule, distal tubule, and connecting tubule) were generated in Xenopus embryos by timed overactivation and suppression of Notch signaling. Notch activation resulted in expansion of the glomus and disruption of the proximal tubule formation. Inhibition of Notch signaling reduced expression of wt1 and XSMP-30. In addition, when Notch signaling was overactivated at stage 20 on, intermediate, distal, and connecting tubule markers, gremlin and clcnkb, were decreased while Notch down-regulation increased gremlin and clcnkb. Similar changes were observed with segmental markers, cldn19, cldn14, and rhcg on activation or inhibition of Notch. Although Notch did not affect the expression of pan-pronephric progenitor marker, pax2, its activation inhibited lumen formation in the pronephros.

CONCLUSIONS

Notch signal is essential for glomus and proximal tubule development and inhibition of Notch is critical for the differentiation of the intermediate, distal, and connecting tubule.

Ca(2+) coding and decoding strategies for the specification of neural and renal precursor cells during development

During embryogenesis, a rise in intracellular Ca(2+) is known to be a widespread trigger for directing stem cells towards a specific tissue fate, but the precise Ca(2+) signalling mechanisms involved in achieving these pleiotropic effects are still poorly understood. In this review, we compare the Ca(2+) signalling events that appear to be one of the first steps in initiating and regulating both neural determination (neural induction) and kidney development (nephrogenesis). We have highlighted the necessary and sufficient role played by Ca(2+) influx and by Ca(2+) transients in the determination and differentiation of pools of neural or renal precursors. We have identified new Ca(2+) target genes involved in neural induction and we showed that the same Ca(2+) early target genes studied are not restricted to neural tissue but are also present in other tissues, principally in the pronephros. In this review, we also described a mechanism whereby the transcriptional control of gene expression during neurogenesis and nephrogenesis might be directly controlled by Ca(2+) signalling. This mechanism involves members of the Kcnip family such that a change in their binding properties to specific DNA sites is a result of Ca(2+) binding to EF-hand motifs. The different functions of Ca(2+) signalling during these two events illustrate the versatility of Ca(2+) as a second messenger.

Proteomic analysis of embryonic kidney development: Heterochromatin proteins as epigenetic regulators of nephrogenesis

Elucidation of the mechanisms underlying the nephrogenesis will boost enormously the regenerative medicine. Here we performed 2-D gel-based comparative proteome analyses of rat embryonic kidney from different developmental stages. Out of 288 non-redundant identified proteins, 102 were common in all developmental stages. 86% of the proteins found in E14 and E16 were identical, in contrast only 37% of the identified proteins overlap between E14 and P1. Bioinformatics analysis suggests developmental stage-specific pathway activation and highlighted heterochromatin protein 1 (Cbx1, Cbx3, Cbx5) and Trim28 as potential key players in nephrogenesis. These are involved in the epigenetic regulation of gene silencing and were down-regulated in the course of kidney development. Trim28 is a potential epigenetic regulator of the branching inhibitor Bmp4. Silencing of Trim28 in cultured kidneys resulted in branching arrest. In contrast knockdown of Cbx5 was associated with abnormal ureteric bud growth and slight impairment of branching. ChIP analysis showed that the H3K9me3 distribution on Bmp4 promoters at E14 and E19 inversely correlate with mRNA expression levels. The concentrated expression-pattern of heterochromatin proteins and the negative impact of their silencing on kidney development, suggest an important role in reciprocal and inductive signaling between the ureteric bud and the metanephric mesenchyme.

Huntingtin is required for ciliogenesis and neurogenesis during early Xenopus development

Huntington's Disease (HD) is a neurodegenerative disorder that results from the abnormal expansion of poly-glutamine (polyQ) repeats in the Huntingtin (HTT) gene. Although HTT has been linked to a variety of cellular events, it is still not clear what the physiological functions of the protein are. Because of its critical role during mouse embryonic mouse development, we investigated the functions of Htt during early Xenopus embryogenesis. We find that reduction of Htt levels affects cilia polarity and function and causes whole body paralysis. Moreover, Htt loss of function leads to abnormal development of trigeminal and motor neurons. Interestingly, these phenotypes are partially rescued by either wild-type or expanded HTT. These results show that the Htt activity is required for normal embryonic development, and highlight the usefulness of the Xenopus system for investigating proteins involved in human diseases.

TRPP2-dependent Ca2+ signaling in dorso-lateral mesoderm is required for kidney field establishment in Xenopus

In Xenopus laevis embryos, kidney field specification is dependent on retinoic acid (RA) and coincides with a dramatic increase of Ca(2+) transients, but the role of Ca(2+) signaling in the kidney field is unknown. Here, we identify TRPP2, a member of the transient receptor potential (TRP) superfamily of channel proteins encoded by the pkd2 gene, as a central component of Ca(2+) signaling in the kidney field. TRPP2 is strongly expressed at the plasma membrane where it might regulate extracellular Ca(2+) entry. Knockdown of pkd2 in the kidney field results in the downregulation of pax8, but not of other kidney field genes (lhx1, osr1 and osr2). We further show that inhibition of Ca(2+) signaling with an inducible Ca(2+) chelator also causes downregulation of pax8, and that pkd2 knockdown results in a severe inhibition of Ca(2+) transients in kidney field explants. Finally, we show that disruption of RA results both in an inhibition of intracellular Ca(2+) signaling and of TRPP2 incorporation into the plasma membrane of kidney field cells. We propose that TRPP2-dependent Ca(2+) signaling is a key component of pax8 regulation in the kidney field downstream of RA-mediated non-transcriptional control of TRPP2.

Heat shock 70-kDa protein 5 (Hspa5) is essential for pronephros formation by mediating retinoic acid signaling

Heat shock 70-kDa protein 5 (Hspa5), also known as binding immunoglobulin protein (Bip) or glucose-regulated protein 78 (Grp78), belongs to the heat shock protein 70 kDa family. As a multifunctional protein, it participates in protein folding and calcium homeostasis and serves as an essential regulator of the endoplasmic reticulum (ER) stress response. It has also been implicated in signal transduction by acting as a receptor or co-receptor residing at the plasma membrane. Its function during embryonic development, however, remains largely elusive. In this study, we used morpholino antisense oligonucleotides (MOs) to knock down Hspa5 activity in Xenopus embryos. In Hspa5 morphants, pronephros formation was strongly inhibited with the reduction of pronephric marker genes Lim homeobox protein 1 (lhx1), pax2, and β1 subunit of Na/K-ATPase (atp1b1). Pronephros tissue was induced in vitro by treating animal caps with all-trans-retinoic acid and activin. Depletion of Hspa5 in animal caps, however, blocked the induction of pronephros as well as reduced the expression of retinoic acid (RA)-responsive genes, suggesting that knockdown of Hspa5 attenuated RA signaling. Knockdown of Hspa5 in animal caps resulted in decreased expression of lhx1, a transcription factor directly regulated by RA signaling and essential for pronephros specification. Co-injection of Hspa5MO with lhx1 mRNA partially rescued the phenotype induced by Hspa5MO. These results suggest that the RA-Lhx1 signaling cascade is involved in Hspa5MO-induced pronephros malformation. This study shows that Hspa5, a key regulator of the unfolded protein response, plays an essential role in pronephros formation, which is mediated in part through RA signaling during early embryonic development.

Heparanase 2, mutated in urofacial syndrome, mediates peripheral neural development in Xenopus

Urofacial syndrome (UFS; previously Ochoa syndrome) is an autosomal recessive disease characterized by incomplete bladder emptying during micturition. This is associated with a dyssynergia in which the urethral walls contract at the same time as the detrusor smooth muscle in the body of the bladder. UFS is also characterized by an abnormal facial expression upon smiling, and bilateral weakness in the distribution of the facial nerve has been reported. Biallelic mutations in HPSE2 occur in UFS. This gene encodes heparanase 2, a protein which inhibits the activity of heparanase. Here, we demonstrate, for the first time, an in vivo developmental role for heparanase 2. We identified the Xenopus orthologue of heparanase 2 and showed that the protein is localized to the embryonic ventrolateral neural tube where motor neurons arise. Morpholino-induced loss of heparanase 2 caused embryonic skeletal muscle paralysis, and morphant motor neurons had aberrant morphology including less linear paths and less compactly-bundled axons than normal. Biochemical analyses demonstrated that loss of heparanase 2 led to upregulation of fibroblast growth factor 2/phosphorylated extracellular signal-related kinase signalling and to alterations in levels of transcripts encoding neural- and muscle-associated molecules. Thus, a key role of heparanase 2 is to buffer growth factor signalling in motor neuron development. These results shed light on the pathogenic mechanisms underpinning the clinical features of UFS and support the contention that congenital peripheral neuropathy is a key feature of this disorder.

The bigger the better: determining nephron size in kidney

The main functions of the kidney are to excrete metabolic waste products and actively reabsorb essential molecules such as amino acids, ions, glucose and water. In humans, a wide range of genetic disorders exist characterized by wasting of metabolically important compounds. At the cellular level, more than 20 highly specialized renal epithelial cell types located in different segments of the nephron contribute to the reabsorption process. In particular, proximal tubular cells play a crucial role and are uniquely adapted to maximize reabsorption efficiency. They accommodate high numbers of transporters and channels by increasing the apical surface area in contact with the primary filtrate by forming a brush border as well as undergoing hypertrophy and hyperplasia. This adaptation is evolutionarily conserved and is detected in the primitive pronephric kidney of fish and amphibians as well as the metanephric kidney of higher vertebrates. Surprisingly, signaling pathways regulating these three processes have remained largely unknown. Here we summarize recent studies that highlight the early phases of kidney development as a critical juncture in establishing proximal tubule size.

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.

Vertebrate kidney tubules elongate using a planar cell polarity-dependent, rosette-based mechanism of convergent extension

Cystic kidney diseases are a global public health burden, affecting over 12 million people. Although much is known about the genetics of kidney development and disease, the cellular mechanisms driving normal kidney tubule elongation remain unclear. Here, we used in vivo imaging to show for the first time that mediolaterally oriented cell intercalation is fundamental to vertebrate kidney morphogenesis. Unexpectedly, we found that kidney tubule elongation is driven in large part by a myosin-dependent, multicellular rosette-based mechanism, previously only described in Drosophila melanogaster. In contrast to findings in Drosophila, however, non-canonical Wnt and planar cell polarity (PCP) signaling is required to control rosette topology and orientation during vertebrate kidney tubule elongation. These data resolve long-standing questions concerning the role of PCP signaling in the developing kidney and, moreover, establish rosette-based intercalation as a deeply conserved cellular engine for epithelial morphogenesis.

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.

Pescadillo homologue 1 and Peter Pan function during Xenopus laevis pronephros development

BACKGROUND INFORMATION

pes1 (pescadillo homologue 1) and ppan (Peter Pan) are multifunctional proteins involved in ribosome biogenesis, cell proliferation, apoptosis, cell migration and regulation of gene expression. Both proteins are required for early neural development in Xenopus laevis, as previously demonstrated.

RESULTS

We show that the expression of both genes in the developing pronephros depends on wnt4 and fzd3 (frizzled homologue 3) function. Loss of pes1 or ppan by MO (morpholino oligonucleotide)-based knockdown approaches resulted in strong malformations during pronephric tubule formation. Defects were already notable during specification of pronephric progenitor cells, as shown by lhx1 expression. Moreover, we demonstrated that Xenopus pes1 and ppan interact physically and functionally and that pes1 and ppan can cross-rescue the loss of function phenotype of one another. Interference with rRNA synthesis, however, did not result in a similar early pronephros phenotype.

CONCLUSION

These results demonstrate that pes1 and ppan are required for Xenopus pronephros development and indicate that their function in the pronephros is independent of their role in ribosome biosynthesis.

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.

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.

The nephrogenic potential of the transcription factors osr1, osr2, hnf1b, lhx1 and pax8 assessed in Xenopus animal caps

BACKGROUND

The three distinct types of kidneys, pronephros, mesonephros and metanephros, develop consecutively in vertebrates. The earliest form of embryonic kidney, the pronephros, is derived from intermediate mesoderm and the first expressed genes localized in the pronephros anlage are the transcription factors osr1, osr2, hnf1b, lhx1 and pax8, here referred to as the early nephrogenic transcription factors. However, the pathway inducing nephrogenesis and the network of theses factors are poorly understood. Treatment of the undifferentiated animal pole explant (animal cap) of Xenopus with activin A and retinoic acid induces pronephros formation providing a powerful tool to analyze key molecular events in nephrogenesis.

RESULTS

We have investigated the expression kinetics of the early nephrogenic transcription factors in activin A and retinoic acid treated animal caps and their potential to induce pronephric differentiation. In treated animal caps, expression of osr1, osr2, hnf1b and lhx1 are induced early, whereas pax8 expression occurs later implying an indirect activation. Activin A alone is able to induce osr2 and lhx1 after three hours treatment in animal caps while retinoic acid fails to induce any of these nephrogenic transcription factors. The early expression of the five transcription factors and their interference with pronephros development when overexpressed in embryos suggest that these factors potentially induce nephrogenesis upon expression in animal caps. But no pronephros development is achieved by either overexpression of OSR1, by HNF1B injection with activin A treatment, or the combined application of LHX1 and PAX8, although they influenced the expression of several early nephrogenic transcription factors in some cases. In an additional approach we could show that HNF1B induces several genes important in nephrogenesis and regulates lhx1 expression by an HNF1 binding site in the lhx1 promoter.

CONCLUSIONS

The early nephrogenic transcription factors play an important role in nephrogenesis, but have no pronephros induction potential upon overexpression in animal caps. They activate transcriptional cascades that partially reflect the gene activation initiated by activin A and retinoic acid. Significantly, HNF1B activates the lhx1 promoter directly, thus extending the known activin A regulation of the lhx1 gene via an activin A responsive element.

Inversin relays Frizzled-8 signals to promote proximal pronephros development

Mutations of inversin cause type II nephronophthisis, an infantile autosomal recessive disease characterized by cystic kidney disease and developmental defects. Inversin regulates Wnt signaling and is required for convergent extension movements during early embryogenesis. We now show that Inversin is essential for Xenopus pronephros formation, involving two distinct and opposing forms of cell movements. Knockdown of Inversin abrogated both proximal pronephros extension and distal tubule differentiation, phenotypes similar to that of Xenopus deficient in Frizzled-8. Exogenous Inversin rescued the pronephric defects caused by lack of Frizzled-8, indicating that Inversin acts downstream of Frizzled-8 in pronephros morphogenesis. Depletion of Inversin prevents the recruitment of Dishevelled in response to Frizzled-8 and impeded the accumulation of Dishevelled at the apical membrane of tubular epithelial cells in vivo. Thus, defective tubule morphogenesis seems to contribute to the renal pathology observed in patients with nephronophthisis type II.

Molecular anatomy of the kidney: what have we learned from gene expression and functional genomics?

The discipline of paediatric nephrology encompasses the congenital nephritic syndromes, renal dysplasias, neonatal renal tumours, early onset cystic disease, tubulopathies and vesicoureteric reflux, all of which arise due to defects in normal kidney development. Indeed, congenital anomalies of the kidney and urinary tract (CAKUT) represent 20-30% of prenatal anomalies, occurring in 1 in 500 births. Developmental biologists have studied the anatomical and morphogenetic processes involved in kidney development for the last five decades. However, with the advent of transgenic mice, the sequencing of the genome, improvements in mutation detection and the advent of functional genomics, our understanding of the molecular basis of kidney development has grown significantly. Here we discuss how the advent of new genetic and genomics approaches has added to our understanding of kidney development and paediatric renal disease, as well as identifying areas in which we are still lacking knowledge.

MicroRNAs in kidney development: lessons from the frog

Kidney development is a paradigm of how multiple cell types are integrated into highly specialized epithelial structures via various inductive events. A network of transcription factors and signaling pathways have been identified as crucial regulators. The recent discovery of a group of small, non-coding RNAs, microRNAs (miRNAs), has added a new layer of complexity. Studies using the pronephric kidney of Xenopus and the metanephric kidney of mouse have demonstrated that a tight regulation of mRNA stability and translation efficiency by miRNAs is very important as well. The interplay between miRNAs and the transcriptional network provides plasticity and robustness to the system. Importantly, miRNAs are not only necessary for early aspects of kidney development, but also later in life. As such they may provide a mean to maintain/modulate kidney function during homeostasis and injury.

Normal levels of p27 are necessary for somite segmentation and determining pronephric organ size

The Xenopus laevis cyclin dependent kinase inhibitor p27(Xic1) has been shown to be involved in exit from the cell cycle and differentiation of cells into a quiescent state in the nervous system, muscle tissue, heart and retina. We show that p27(Xic1) is expressed in the developing kidney in the nephrostomal regions. Using overexpression and morpholino oligonucleotide (MO) knock-down approaches we show normal levels of p27(Xic1) regulate pronephros organ size by regulating cell cycle exit. Knock-down of p27(Xic1) expression using a MO prevented myogenesis, as previously reported; an effect that subsequently inhibits pronephrogenesis. Furthermore, we show that normal levels of p27(Xic1) are required for somite segmentation also through its cell cycle control function. Finally, we provide evidence to suggest correct paraxial mesoderm segmentation is not necessary for pronephric induction in the intermediate mesoderm. These results indicate novel developmental roles for p27(Xic1), and reveal its differentiation function is not universally utilised in all developing tissues.

Notch activates Wnt-4 signalling to control medio-lateral patterning of the pronephros

Previous studies have highlighted a role for the Notch signalling pathway during pronephrogenesis in the amphibian Xenopus laevis, and in nephron development in the mammalian metanephros, yet a mechanism for this function remains elusive. Here, we further the understanding of how Notch signalling patterns the early X. laevis pronephros anlagen, a function that might be conserved in mammalian nephron segmentation. Our results indicate that early phase pronephric Notch signalling patterns the medio-lateral axis of the dorso-anterior pronephros anlagen, permitting the glomus and tubules to develop in isolation. We show that this novel function acts through the Notch effector gene hrt1 by upregulating expression of wnt4. Wnt-4 then patterns the proximal pronephric anlagen to establish the specific compartments that span the medio-lateral axis. We also identified pronephric expression of lunatic fringe and radical fringe that is temporally and spatially appropriate for a role in regulating Notch signalling in the dorso-anterior region of the pronephros anlagen. On the basis of these results, along with data from previous publications, we propose a mechanism by which the Notch signalling pathway regulates a Wnt-4 function that patterns the proximal pronephric anlagen.

Organization of the pronephric kidney revealed by large-scale gene expression mapping

BACKGROUND

The pronephros, the simplest form of a vertebrate excretory organ, has recently become an important model of vertebrate kidney organogenesis. Here, we elucidated the nephron organization of the Xenopus pronephros and determined the similarities in segmentation with the metanephros, the adult kidney of mammals.

RESULTS

We performed large-scale gene expression mapping of terminal differentiation markers to identify gene expression patterns that define distinct domains of the pronephric kidney. We analyzed the expression of over 240 genes, which included members of the solute carrier, claudin, and aquaporin gene families, as well as selected ion channels. The obtained expression patterns were deposited in the searchable European Renal Genome Project Xenopus Gene Expression Database. We found that 112 genes exhibited highly regionalized expression patterns that were adequate to define the segmental organization of the pronephric nephron. Eight functionally distinct domains were discovered that shared significant analogies in gene expression with the mammalian metanephric nephron. We therefore propose a new nomenclature, which is in line with the mammalian one. The Xenopus pronephric nephron is composed of four basic domains: proximal tubule, intermediate tubule, distal tubule, and connecting tubule. Each tubule may be further subdivided into distinct segments. Finally, we also provide compelling evidence that the expression of key genes underlying inherited renal diseases in humans has been evolutionarily conserved down to the level of the pronephric kidney.

CONCLUSION

The present study validates the Xenopus pronephros as a genuine model that may be used to elucidate the molecular basis of nephron segmentation and human renal disease.

Transcription factor HNF1beta and novel partners affect nephrogenesis

Heterozygous mutations of the tissue-specific transcription factor hepatocyte nuclear factor (HNF)1beta, cause maturity onset diabetes of the young (MODY5) and kidney anomalies including agenesis, hypoplasia, dysplasia and cysts. Because of these renal anomalies, HNF1beta is classified as a CAKUT (congenital anomalies of the kidney and urinary tract) gene. We searched for human fetal kidney proteins interacting with the N-terminal region of HNF1beta using a bacterial two-hybrid system and identified five novel proteins along with the known partner DCoH. The interactions were confirmed for four of these proteins by GST pull-down assays. Overexpression of two proteins, E4F1 and ZFP36L1, in Xenopus embryos interfered with pronephros formation. Further, in situ hybridization showed overlapping expression of HNF1beta, E4F1 and ZFP36L1 in the developing pronephros. HNF1beta is present largely in the nucleus where it colocalized with E4F1. However, ZFP36L1 was located predominantly in the cytoplasm. A nuclear function for ZFP36L1 was shown as it was able to reduce HNF1beta transactivation in a luciferase reporter system. Our studies show novel proteins may cooperate with HNF1beta in human metanephric development and propose that E4F1 and ZFP36L1 are CAKUT genes. We searched for mutations in the open reading frame of the ZFP36L1 gene in 58 patients with renal anomalies but found none.

A functional screen for genes involved in Xenopus pronephros development

In Xenopus, the pronephros is the functional larval kidney and consists of two identifiable components; the glomus, the pronephric tubules, which can be divided into four separate segments, based on marker gene expression. The simplicity of this organ, coupled with the fact that it displays the same basic organization and function as more complex mesonephros and metanephros, makes this an attractive model to study vertebrate kidney formation. In this study, we have performed a functional screen specifically to identify genes involved in pronephros development in Xenopus. Gain-of-function screens are performed by injecting mRNA pools made from a non-redundant X. tropicalis full-length plasmid cDNA library into X. laevis eggs, followed by sib-selection to identify the single clone that caused abnormal phenotypes in the pronephros. Out of 768 egg and gastrula stage cDNA clones, 31 genes, approximately 4% of the screened clones, affected pronephric marker expression examined by whole mount in situ hybridization or antibody staining. Most of the positive clones had clear expression patterns in pronephros and predicted/established functions highly likely to be involved in developmental processes. In order to carry out a more detailed study, we selected Sox7, Cpeb3, P53csv, Mecr and Dnajc15, which had highly specific expression patterns in the pronephric region. The over-expression of these five selected clones indicated that they caused pronephric abnormalities with different temporal and spatial effects. These results suggest that our strategy to identify novel genes involved in pronephros development was highly successful, and that this strategy is effective for the identification of novel genes involved in late developmental events.

The zebrafish pronephros: a model to study nephron segmentation

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

Amphibians used in research and teaching

Amphibians have long been utilized in scientific research and in education. Historically, investigators have accumulated a wealth of information on the natural history and biology of amphibians, and this body of information is continually expanding as researchers describe new species and study the behaviors of these animals. Amphibians evolved as models for a variety of developmental and physiological processes, largely due to their unique ability to undergo metamorphosis. Scientists have used amphibian embryos to evaluate the effects of toxins, mutagens, and teratogens. Likewise, the animals are invaluable in research due to the ability of some species to regenerate limbs. Certain species of amphibians have short generation times and genetic constructs that make them desirable for transgenic and knockout technology, and there is a current national focus on developing these species for genetic and genomic research. This group of vertebrates is also critically important in the investigation of the inter-relationship of humans and the environment based on their sensitivity to climatic and habitat changes and environmental contamination.

Xenopus Bicaudal-C is required for the differentiation of the amphibian pronephros

The RNA-binding molecule Bicaudal-C regulates embryonic development in Drosophila and Xenopus. Interestingly, mouse mutants of Bicaudal-C do not show early patterning defects, but instead develop polycystic kidney disease (PKD). To further investigate the molecular mechanism of Bicaudal-C in kidney development, we analyzed its function in the developing amphibian pronephros. Bicaudal-C mRNA was present in the epithelial structures of the Xenopus pronephros, the tubules and the duct, but not the glomus. Inhibition of the translation of endogenous Bicaudal-C with antisense morpholino oligomers (xBic-C-MO) led to a PKD-like phenotype in Xenopus. Embryos lacking Bicaudal-C developed generalized edemas and dilated pronephric tubules and ducts. This phenotype was caused by impaired differentiation of the pronephros. Molecular markers specifically expressed in the late distal tubule were absent in xBic-C-MO-injected embryos. Furthermore, Bicaudal-C was not required for primary cilia formation, an important organelle affected in PKD. These data support the idea that Bicaudal-C functions downstream or parallel of a cilia-regulated signaling pathway. This pathway is required for terminal differentiation of the late distal tubule of the Xenopus pronephros and regulates renal epithelial cell differentiation, which--when disrupted--results in PKD.

Amphibians as Animal Models for Laboratory Research in Physiology

The concept of animal models is well honored, and amphibians have played a prominent part in the success of using key species to discover new information about all animals. As animal models, amphibians offer several advantages that include a well-understood basic physiology, a taxonomic diversity well suited to comparative studies, tolerance to temperature and oxygen variation, and a greater similarity to humans than many other currently popular animal models. Amphibians now account for approximately 1/4 to 1/3 of lower vertebrate and invertebrate research, and this proportion is especially true in physiological research, as evident from the high profile of amphibians as animal models in Nobel Prize research. Currently, amphibians play prominent roles in research in the physiology of musculoskeletal, cardiovascular, renal, respiratory, reproductive, and sensory systems. Amphibians are also used extensively in physiological studies aimed at generating new insights in evolutionary biology, especially in the investigation of the evolution of air breathing and terrestriality. Environmental physiology also utilizes amphibians, ranging from studies of cryoprotectants for tissue preservation to physiological reactions to hypergravity and space exploration. Amphibians are also playing a key role in studies of environmental endocrine disruptors that are having disproportionately large effects on amphibian populations and where specific species can serve as sentinel species for environmental pollution. Finally, amphibian genera such as Xenopus, a genus relatively well understood metabolically and physiologically, will continue to contribute increasingly in this new era of systems biology and "X-omics."

The cellular basis of kidney development

Mammalian kidney development has helped elucidate the general concepts of mesenchymal-epithelial interactions, inductive signaling, epithelial cell polarization, and branching morphogenesis. Through the use of genetically engineered mouse models, the manipulation of Xenopus and chick embryos, and the identification of human renal disease genes, the molecular bases for many of the early events in the developing kidney are becoming increasingly clear. Early patterning of the kidney region depends on interactions between Pax/Eya/Six genes, with essential roles for lim1 and Odd1. Ureteric bud outgrowth and branching morphogenesis are controlled by the Ret/Gdnf pathway, which is subject to positive and negative regulation by a variety of factors. A clear role for Wnt proteins in induction of the kidney mesenchyme is now well established and complements the classic literature nicely. Patterning along the proximal distal axis as the nephron develops is now being investigated and must involve aspects of Notch signaling. The development of a glomerulus requires interactions between epithelial cells and infiltrating endothelial cells to generate a unique basement membrane. The integrity of the glomerular filter depends in large part on the proteins of the nephrin complex, localized to the slit diaphragm. Despite the kidney's architectural complexity, with the advent of genomics and expression arrays, it is becoming one of the best-characterized organ systems in developmental biology.

Progression of chronic kidney disease: insights from animal models

PURPOSE OF REVIEW

Chronic kidney diseases are emerging as a worldwide public health problem. Clarification of the mechanisms underlying progression of proteinuric nephropathies received significant input from the generation of transgenic and knockout animals and from novel approaches to block mediators of injury. Reviewed here are advances in animal models used as a tool to address some relevant questions to the pathophysiology of human chronic nephropathies.

RECENT FINDINGS

Gene targeting in rodents identified podocyte loss as central event in the development of glomerulosclerosis. The trigger is dysfunction or absence of podocyte molecules that stabilize the slit diaphragm or anchor foot processes to the basement membrane. Sustained injury of the glomerular barrier to proteins is transmitted to the tubulointerstitial compartment leading to inflammation and fibrosis. Blocking NF-kappaB activity and chemokine signals in the kidney effectively interrupts such process. Growth factors produced by tubular cells and inflammatory cells contribute to interstitial fibrogenesis via myofibroblast activation.

SUMMARY

Development of genetically engineered animals and techniques to specifically manipulate cellular mediators has highlighted the determinants of glomerulosclerosis and tubulointerstitial injury. This knowledge will provide basis for novel interventions to protect the podocyte in chronic progressive glomerulopathies and to halt renal scarring and loss of function.

The Na+/PO4 cotransporter SLC20A1 gene labels distinct restricted subdomains of the developing pronephros in Xenopus and zebrafish embryos

The embryonic pronephric kidneys of Xenopus and zebrafish serve as models to study vertebrate nephrogenesis. Recently, multiple subdomains within the Xenopus pronephros have been defined based on the expression of several transport proteins. In contrast, very few studies on the expression of renal transporters have been conducted in zebrafish. We have recently shown that the anterior and posterior segments of the zebrafish pronephric duct may correspond to the proximal tubule and distal tubule/duct compartments of the Xenopus and higher vertebrate pronephros, respectively. Here, we report the embryonic expression pattern of the Na(+)/PO(4) cotransporter SLC20A1 (PiT1/Glvr-1) gene encoding a type III sodium-dependent phosphate cotransporter in Xenopus and zebrafish. In Xenopus, SLC20A1 mRNA is expressed in the somitic mesoderm and lower level of expression is detected in the neural tube, eye, and neural crest cells. From stage 25, SLC20A1 is also detectable in the developing pronephros where expression is restricted to the late portion of the distal pronephric tubules. In zebrafish, SLC20A1 is transcribed from mid-somitogenesis in the anterior part of the pronephros where its expression corresponds to the rostral portion of the expression of other proximal tubule-specific markers. Outside the pronephros, lower level of SLC20A1 expression is also observed in the posterior cardinal and caudal veins. Based on the SLC20A1 expression domain and that of other transporters, four segments have been defined within the zebrafish pronephros. Together, our data reveal that the zebrafish and Xenopus pronephros have non-identical proximo-distal organizations.

The Notch-effector HRT1 gene plays a role in glomerular development and patterning of the Xenopus pronephros anlagen

Notch signaling has been shown to play a role in cell fate decisions in the Xenopus pronephros anlagen. Here, we show that the Xenopus Hairy-related transcription factor (HRT) gene XHRT1, and the Hairy/Enhancer of split (HES) genes Xhairy1, Xhairy2b, esr9 and esr10, have distinct restricted dynamic expression patterns during pronephros development, and that their expression is regulated by Notch. XHRT1, which is the earliest and strongest gene expressed in the pronephric region, is initially transcribed predominantly in the forming glomus, where it is downregulated by antisense morpholino oligonucleotide inhibition of xWT1. Later, it is activated in the most dorsoanterior part of the pronephros anlagen that gives rise to the proximal tubules. In agreement with this dynamic expression profile, we found that early activation of Notch favors glomus, whereas only later activation promotes proximal tubule formation. We show that, among the bHLH-O factors tested, only XHRT1 efficiently inhibits distal tubule and duct formation, and that only its translational inhibition causes a reduction of the expression of proximal tubule and glomus markers. Using domain swap experiments, we found that the XHRT1 C-terminal region is crucial for its activity. Together, our results provide evidence that XHRT1 plays an important role in glomerular development and early proximodistal patterning that is distinct from those of the other pronephric bHLH repressors.

Evi1 is specifically expressed in the distal tubule and duct of the Xenopus pronephros and plays a role in its formation

The ecotropic viral integration site 1 (Evi1) and related MEL1 (MDS1/Evi1-like gene 1) genes are zinc finger oncogenic transcription factors involved in myeloid leukaemia. Here, we show that in Xenopus, Evi1 and MEL1 have partially overlapping restricted embryonic expression profiles. Within the pronephros, Evi1 and MEL1 are sequentially expressed within the distal tubule and duct compartments, Evi1 transcription being detected prior to any sign of pronephric morphogenesis. In the pronephros of zebrafish embryos, Evi1 expression is restricted to the posterior portion of the duct, the anterior portion having characteristics of proximal tubules. In the Xenopus pronephros, Evi1 expression is upregulated by retinoid signaling and repressed by overexpression of xWT1 and by Notch signaling. Overexpression of Evi1 from late neurula stage specifically inhibits the expression of proximal tubule and glomus pronephric markers. We show that the first zinc finger and CtBP interaction domains are required for this activity. Overexpression of a hormone-inducible Evi1-VP16 antimorphic fusion with activation at neurula stage disrupts distal tubule and duct formation and expands the expression of glomus markers. Although overexpression of this construct also causes in many embryos a reduction of proximal tubule markers, embryos with expanded and ectopic staining have been also observed. Together, these data indicate that Evi1 plays a role in the proximo-distal patterning of the pronephros and suggest that it may do so by functioning as a CtBP dependent repressor.

Germ layers to organs: Using Xenopus to study “later” development

The amphibian embryo is a highly successful model system with great promise for organogenesis research. Since the late 1800s, amphibians have been employed to understand vertebrate development and since the 1950s, the African clawed frog Xenopus laevis has been the amphibian of choice. In the past two decades, Xenopus has led the way forward in, among other things, identifying transcription factors, gene regulatory networks and inter- and intracellular signaling pathways that control early development (from fertilization through gastrulation and neurulation). Perhaps the best measure of how successful Xenopus has been as a model for early mammalian development is the observation that much of the knowledge gleaned from Xenopus studies has subsequently directly translated to discoveries of similar mechanisms operating in mouse development. Despite this great success in early development, research on organogenesis in Xenopus has lagged behind the mouse. However, recent technical advances now make Xenopus amenable for studies on later development, including organogenesis. Here, we discuss why Xenopus is well suited for such research and, we believe, permits addressing questions that have been difficult to approach using other model systems. We also highlight how Xenopus researchers have already begun studying a number of major organs, pancreas, liver, kidney and heart, and suggest how Xenopus might contribute more to these areas in the near future.

Xenopus as a model organism in developmental chemical genetic screens

Chemical genetics is a potentially powerful tool for studying developmental processes in vertebrate systems. We present data showing Xenopus laevis as a model organism in which systematic chemical genetic screens can be carried out. Previous forward chemical genetic screens, including those with developing zebrafish embryos, have demonstrated the nature and value of biological information gained with this approach. We show how amenable Xenopus is to chemical genetics by investigating a series of compounds either with known biochemical effects, or previously identified to give developmental phenotypes, on a range of biological functions, including the development of pigmentation, the heart and the central nervous system in zebrafish. We have found that the compounds give comparable phenotypes when applied to developing Xenopus embryos. We have also studied the penetrance and expressivity of these chemical genetic phenotypes in relation to genetic variation and the developmental window during which the compound is present. Finally, we assess the feasibility and the potential throughput of a screen in this vertebrate species.