Induction of Pronephric Tubules by Activin and Retinoic Acid in Presumptive Ectoderm of Xenopus laevis. (RA/kidney/mesoderm induction/Xenopus laevis)

Spemann-Mangold organizer and mesoderm induction

The discovery of the Spemann-Mangold organizer strongly influenced subsequent research on embryonic induction, with research aiming to elucidate the molecular characteristics of organizer activity being currently underway. Herein, we review the history of research on embryonic induction, and describe how the mechanisms of induction phenomena and developmental processes have been investigated. Classical experiments investigating the differentiation capacity and inductive activity of various embryonic regions were conducted by many researchers, and important theories of region-specific induction and the concept for chain of induction were proposed. The transition from experimental embryology to developmental biology has enabled us to understand the mechanisms of embryonic induction at the molecular level. Consequently, many inducing substances and molecules such as transcriptional factors and peptide growth factors involved in the organizer formation were identified. One of peptide growth factors, activin, acts as a mesoderm- and endoderm-inducing substance. Activin induces several tissues and organs from the undifferentiated cell mass of amphibian embryos in a concentration-dependent manner. We review the extent to which we can control in vitro organogenesis from undifferentiated cells, and discuss the application to stem cell-based regenerative medicine based on insights gained from animal experiments, such as in amphibians.

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 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.

HNF1B Alters an Evolutionarily Conserved Nephrogenic Program of Target Genes

SIGNIFICANCE STATEMENT

Mutations in hepatocyte nuclear factor-1 β ( HNF1B ) are the most common monogenic causes of congenital renal malformations. HNF1B is necessary to directly reprogram fibroblasts to induced renal tubule epithelial cells (iRECs) and, as we demonstrate, can induce ectopic pronephric tissue in Xenopus ectodermal organoids. Using these two systems, we analyzed the effect of HNF1B mutations found in patients with cystic dysplastic kidney disease. We found cross-species conserved targets of HNF1B, identified transcripts that are differentially regulated by the patient-specific mutant protein, and functionally validated novel HNF1B targets in vivo . These results highlight evolutionarily conserved transcriptional mechanisms and provide insights into the genetic circuitry of nephrogenesis.

BACKGROUND

Hepatocyte nuclear factor-1 β (HNF1B) is an essential transcription factor during embryogenesis. Mutations in HNF1B are the most common monogenic causes of congenital cystic dysplastic renal malformations. The direct functional consequences of mutations in HNF1B on its transcriptional activity are unknown.

METHODS

Direct reprogramming of mouse fibroblasts to induced renal tubular epithelial cells was conducted both with wild-type HNF1B and with patient mutations. HNF1B was expressed in Xenopus ectodermal explants. Transcriptomic analysis by bulk RNA-Seq identified conserved targets with differentially regulated expression by the wild-type or R295C mutant. CRISPR/Cas9 genome editing in Xenopus embryos evaluated transcriptional targets in vivo .

RESULTS

HNF1B is essential for reprogramming mouse fibroblasts to induced renal tubular epithelial cells and induces development of ectopic renal organoids from pluripotent Xenopus cells. The mutation R295C retains reprogramming and inductive capacity but alters the expression of specific sets of downstream target genes instead of diminishing overall transcriptional activity of HNF1B. Surprisingly, targets associated with polycystic kidney disease were less affected than genes affected in congenital renal anomalies. Cross-species-conserved transcriptional targets were dysregulated in hnf1b CRISPR-depleted Xenopus embryos, confirming their dependence on hnf1b .

CONCLUSIONS

HNF1B activates an evolutionarily conserved program of target genes that disease-causing mutations selectively disrupt. These findings provide insights into the renal transcriptional network that controls nephrogenesis.

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.

iPSC technology-based regenerative medicine for kidney diseases

With few curative treatments for kidney diseases, increasing attention has been paid to regenerative medicine as a new therapeutic option. Recent progress in kidney regeneration using human-induced pluripotent stem cells (hiPSCs) is noteworthy. Based on the knowledge of kidney development, the directed differentiation of hiPSCs into two embryonic kidney progenitors, nephron progenitor cells (NPCs) and ureteric bud (UB), has been established, enabling the generation of nephron and collecting duct organoids. Furthermore, human kidney tissues can be generated from these hiPSC-derived progenitors, in which NPC-derived glomeruli and renal tubules and UB-derived collecting ducts are interconnected. The induced kidney tissues are further vascularized when transplanted into immunodeficient mice. In addition to the kidney reconstruction for use in transplantation, it has been demonstrated that cell therapy using hiPSC-derived NPCs ameliorates acute kidney injury (AKI) in mice. Disease modeling and drug discovery research using disease-specific hiPSCs has also been vigorously conducted for intractable kidney disorders, such as autosomal dominant polycystic kidney disease (ADPKD). In an attempt to address the complications associated with kidney diseases, hiPSC-derived erythropoietin (EPO)-producing cells were successfully generated to discover drugs and develop cell therapy for renal anemia. This review summarizes the current status and future perspectives of developmental biology of kidney and iPSC technology-based regenerative medicine for kidney diseases.

LIM homeodomain proteins and associated partners: Then and now

The field of molecular embryology started around 1990 by identifying new genes and analyzing their functions in early vertebrate embryogenesis. Those genes encode transcription factors, signaling molecules, their regulators, etc. Most of those genes are relatively highly expressed in specific regions or exhibit dramatic phenotypes when ectopically expressed or mutated. This review focuses on one of those genes, Lim1/Lhx1, which encodes a transcription factor. Lim1/Lhx1 is a member of the LIM homeodomain (LIM-HD) protein family, and its intimate partner, Ldb1/NLI, binds to two tandem LIM domains of LIM-HDs. The most ancient LIM-HD protein and its partnership with Ldb1 were innovated in the metazoan ancestor by gene fusion combining LIM domains and a homeodomain and by creating the LIM domain-interacting domain (LID) in ancestral Ldb, respectively. The LIM domain has multiple interacting interphases, and Ldb1 has a dimerization domain (DD), the LID, and other interacting domains that bind to Ssbp2/3/4 and the boundary factor, CTCF. By means of these domains, LIM-HD-Ldb1 functions as a hub protein complex, enabling more intricate and elaborate gene regulation. The common, ancestral role of LIM-HD proteins is neuron cell-type specification. Additionally, Lim1/Lhx1 serves crucial roles in the gastrula organizer and in kidney development. Recent studies using Xenopus embryos have revealed Lim1/Lhx1 functions and regulatory mechanisms during development and regeneration, providing insight into evolutionary developmental biology, functional genomics, gene regulatory networks, and regenerative medicine. In this review, we also discuss recent progress at unraveling participation of Ldb1, Ssbp, and CTCF in enhanceosomes, long-distance enhancer-promoter interactions, and trans-interactions between chromosomes.

Therapeutic potential of human induced pluripotent stem cells and renal progenitor cells in experimental chronic kidney disease

BACKGROUND

Chronic kidney disease (CKD) is a global public health problem. Cell therapy using pluripotent stem cells represents an attractive therapeutic approach for the treatment of CKD.

METHODS

We transplanted mitomycin C (MMC)-treated human induced pluripotent stem cells (hiPSCs) and renal progenitor cells (RPCs) into a CKD rat model system. The RPC and hiPSC cells were characterized by immunofluorescence and qRT-PCR. Untreated 5/6 nephrectomized rats were compared to CKD animals receiving the same amount of MMC-treated hiPSCs or RPCs. Renal function, histology, and immunohistochemistry were evaluated 45 days post-surgery.

RESULTS

We successfully generated hiPSCs from peripheral blood and differentiated them into RPCs expressing renal progenitor genes (PAX2, WT1, SIX2, and SALL1) and podocyte-related genes (SYNPO, NPHS1). RPCs also exhibited reduced OCT4 expression, confirming the loss of pluripotency. After cell transplantation into CKD rats, the body weight change was significantly increased in both hiPSC and RPC groups, in comparison with the control group. Creatinine clearance (CCr) was preserved only in the hiPSC group. Similarly, the number of macrophages in the kidneys of the hiPSC group reached a statistically significant reduction, when compared to control rats. Both treatments reduced positive staining for the marker α-smooth muscle actin. Histological features showed decreased tubulointerstitial damage (interstitial fibrosis and tubular atrophy) as well as a reduction in glomerulosclerosis in both iPSC and RPC groups.

CONCLUSIONS

In conclusion, we describe that both MMC-treated hiPSCs and RPCs exert beneficial effects in attenuating CKD progression. Both cell types were equally efficient to reduce histological damage and weight loss caused by CKD. hiPSCs seem to be more efficient than RPCs, possibly due to a paracrine effect triggered by hiPSCs. These results demonstrate that the use of MMC-treated hiPSCs and RPCs improves clinical and histological CKD parameters, avoided tumor formation, and therefore may be a promising cell therapy strategy for CKD.

Advances in understanding vertebrate nephrogenesis

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

Differentiating Induced Pluripotent Stem Cells into Renal Cells: A New Approach to Treat Kidney Diseases

Renal disease is a major issue for global public health. Despite some progress in supportive care, the mortality rates among patients with this condition remain alarmingly high. Studies in pursuit of innovative strategies to treat renal diseases, especially stimulating kidney regeneration, have been developed. In this field, stem cell-based therapy has been a promising area. Induced pluripotent stem cell-derived renal cells (iPSC-RCs) represent an interesting source of cells for treating kidney diseases. Advances in regenerative medicine using iPSC-RCs and their application to the kidney are discussed in this review. Furthermore, the way differentiation protocols of induced pluripotent stem cells into renal cells may also be applied for the generation of kidney organoids is also described, contributing to studies in renal development, kidney diseases, and drug toxicity tests. The translation of the differentiation methodologies into animal model studies and the safety and feasibility of renal differentiated cells as a treatment for kidney injury are also highlighted. Although only few studies were published in this field, the results seem promising and support the use of iPSC-RCs as a potential therapy in the future.

Xenopus: Driving the Discovery of Novel Genes in Patient Disease and Their Underlying Pathological Mechanisms Relevant for Organogenesis

Frog model organisms have been appreciated for their utility in exploring physiological phenomena for nearly a century. Now, a vibrant community of biologists that utilize this model organism has poised Xenopus to serve as a high throughput vertebrate organism to model patient-driven genetic diseases. This has facilitated the investigation of effects of patient mutations on specific organs and signaling pathways. This approach promises a rapid investigation into novel mechanisms that disrupt normal organ morphology and function. Considering that many disease states are still interrogated in vitro to determine relevant biological processes for further study, the prospect of interrogating genetic disease in Xenopus in vivo is an attractive alternative. This model may more closely capture important aspects of the pathology under investigation such as cellular micro environments and local forces relevant to a specific organ's development and homeostasis. This review aims to highlight recent methodological advances that allow investigation of genetic disease in organ-specific contexts in Xenopus as well as provide examples of how these methods have led to the identification of novel mechanisms and pathways important for understanding human disease.

Turning mesoderm into kidney

The intermediate mesoderm is located between the somites and the lateral plate mesoderm and gives rise to renal progenitors that contribute to the three mammalian kidney types (pronephros, mesonephros and metanephros). In this review, focusing largely on murine kidney development, we examine how the intermediate mesoderm forms during gastrulation/axis elongation and how it progressively gives rise to distinct renal progenitors along the rostro-caudal axis. We highlight some of the potential signalling cues and core transcription factor circuits that direct these processes, up to the point of early metanephric kidney formation.

Regenerating the kidney using human pluripotent stem cells and renal progenitors

INTRODUCTION

Chronic kidney disease is a major health-care problem worldwide and its cost is becoming no longer affordable. Indeed, restoring damaged renal structures or building a new kidney represents an ambitious and ideal alternative to renal replacement therapy. Streams of research have explored the possible application of pluripotent stem cells (SCs) (embryonic SCs and induced pluripotent SCs) in different strategies aimed at regenerate functioning nephrons and at understanding the mechanisms of kidney regeneration.

AREAS COVERED

In this review, we will focus on the main potential applications of human pluripotent SCs to kidney regeneration, including those leading to rebuilding new kidneys or part of them (organoids, scaffolds, biological microdevices) as well as those aimed at understanding the pathophysiological mechanisms of renal disease and regenerative processes (modeling of kidney disease, genome editing). Moreover, we will discuss the role of endogenous renal progenitors cells in order to understand and promote kidney regeneration, as an attractive alternative to pluripotent SCs.

EXPERT OPINION

Opportunities and pitfalls of all these strategies will be underlined, finally leading to the conclusion that a deeper knowledge of the biology of pluripotent SCs is mandatory, in order to allow us to hypothesize their clinical application.

The atypical mitogen-activated protein kinase ERK3 is essential for establishment of epithelial architecture

Epithelia contribute to physical barriers that protect internal tissues from the external environment and also support organ structure. Accordingly, establishment and maintenance of epithelial architecture are essential for both embryonic development and adult physiology. Here, using gene knockout and knockdown techniques along with gene profiling, we show that extracellular signal-regulated kinase 3 (ERK3), a poorly characterized atypical mitogen-activated protein kinase (MAPK), regulates the epithelial architecture in vertebrates. We found that in Xenopus embryonic epidermal epithelia, ERK3 knockdown impairs adherens and tight-junction protein distribution, as well as tight-junction barrier function, resulting in epidermal breakdown. Moreover, in human epithelial breast cancer cells, inhibition of ERK3 expression induced thickened epithelia with aberrant adherens and tight junctions. Results from microarray analyses suggested that transcription factor AP-2α (TFAP2A), a transcriptional regulator important for epithelial gene expression, is involved in ERK3-dependent changes in gene expression. Of note, TFAP2A knockdown phenocopied ERK3 knockdown in both Xenopus embryos and human cells, and ERK3 was required for full activation of TFAP2A-dependent transcription. Our findings reveal that ERK3 regulates epithelial architecture, possibly together with TFAP2A.

In Vitro Induction of Xenopus Embryonic Organs Using Animal Cap Cells

The animal cap-the presumptive ectoderm of the blastula embryo-can differentiate into a variety of tissues belonging to the three germ layers following exposure to specific inducers. The "animal cap assay" was devised based on the pluripotency of presumptive ectodermal cells and enabled many important discoveries in the field of embryonic induction and cell differentiation. Using this system, investigators can test multiple factors in solution simultaneously to determine their inducing activities qualitatively, quantitatively, and synergistically. Furthermore, after dissociation and induction, reaggregated animal cap cells can be induced to form higher-order organs. This protocol details preoperative preparations, followed by the basic animal cap assay. Advanced protocols for the induction of kidney, pancreas, and heart are also described.

Engineering kidney cells: reprogramming and directed differentiation to renal tissues

Growing knowledge of how cell identity is determined at the molecular level has enabled the generation of diverse tissue types, including renal cells from pluripotent or somatic cells. Recently, several in vitro protocols involving either directed differentiation or transcription-factor-based reprogramming to kidney cells have been established. Embryonic stem cells or induced pluripotent stem cells can be guided towards a kidney fate by exposing them to combinations of growth factors or small molecules. Here, renal development is recapitulated in vitro resulting in kidney cells or organoids that show striking similarities to mammalian embryonic nephrons. In addition, culture conditions are also defined that allow the expansion of renal progenitor cells in vitro. Another route towards the generation of kidney cells is direct reprogramming. Key transcription factors are used to directly impose renal cell identity on somatic cells, thus circumventing the pluripotent stage. This complementary approach to stem-cell-based differentiation has been demonstrated to generate renal tubule cells and nephron progenitors. In-vitro-generated renal cells offer new opportunities for modelling inherited and acquired renal diseases on a patient-specific genetic background. These cells represent a potential source for developing novel models for kidney diseases, drug screening and nephrotoxicity testing and might represent the first steps towards kidney cell replacement therapies. In this review, we summarize current approaches for the generation of renal cells in vitro and discuss the advantages of each approach and their potential applications.

Toolbox in a tadpole: Xenopus for kidney research

Xenopus is a versatile model organism increasingly used to study organogenesis and genetic diseases. The rapid embryonic development, targeted injections, loss- and gain-of-function experiments and an increasing supply of tools for functional in vivo analysis are unique advantages of the Xenopus system. Here, we review the vast array of methods available that have facilitated its transition into a translational model. We will focus primarily on how these methods have been employed in the study of kidney development, renal function and kidney disease. Future advances in the fields of genome editing, imaging and quantitative 'omics approaches are likely to enable exciting and novel applications for Xenopus to deepen our understanding of core principles of renal development and molecular mechanisms of human kidney disease. Thus, using Xenopus in clinically relevant research diversifies the narrowing pool of "standard" model organisms and provides unique opportunities for translational research.

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.

Induction of nephron progenitors and glomeruli from human pluripotent stem cells

Studies of kidney regeneration using stem cells have progressed rapidly in recent years. Our group has developed a protocol to induce nephron progenitors from both mouse and human pluripotent stem cells which is based on a revised model of early stage kidney specification. The induced progenitors readily reconstitute three-dimensional nephron structures, including glomeruli and renal tubules, in vitro. We can further generate human induced pluripotent stem cells (iPSCs), in which nephrin-expressing glomerular podocytes are tagged with green fluorescent protein (GFP). The sorted GFP-positive cells retain the podocyte-specific molecular and structural features. Upon transplantation, mouse endothelial cells of the host animals are integrated into the human iPSC-derived glomeruli, and the podocytes show further maturation. Other laboratories have reported different protocols to induce nephron structures from human iPSCs in vitro. These findings will accelerate our understanding of kidney development and diseases in humans.

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.

The lymph node as a new site for kidney organogenesis

The shortage of organs for kidney transplantation has created the need to develop new strategies to restore renal structure and function. Given our recent finding that the lymph node (LN) can serve as an in vivo factory to generate or sustain complex structures like liver, pancreas, and thymus, we investigated whether it could also support kidney organogenesis from mouse renal embryonic tissue (metanephroi). Here we provide the first evidence that metanephroi acquired a mature phenotype upon injection into LN, and host cells likely contributed to this process. Urine-like fluid-containing cysts were observed in several grafts 12 weeks post-transplantation, indicating metanephroi transplants' ability to excrete products filtered from the blood. Importantly, the kidney graft adapted to a loss of host renal mass, speeding its development. Thus, the LN might provide a unique tool for studying the mechanisms of renal maturation, cell proliferation, and fluid secretion during cyst development. Moreover, we provide evidence that inside the LN, short-term cultured embryonic kidney cells stimulated with the Wnt agonist R-Spondin 2 gave rise to a monomorphic neuron-like cell population expressing the neuronal 200-kDa neurofilament heavy marker. This finding indicates that the LN might be used to validate the differentiation potential of candidate stem cells in regenerative nephrology.

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.

Nephron reconstitution from pluripotent stem cells

It has been a challenge in developmental biology and regenerative medicine to generate nephron progenitors that reconstitute the three-dimensional (3D) nephron structure in vitro. Many studies have tried to induce nephron progenitors from pluripotent stem cells by mimicking the developmental processes in vivo. However, the current developmental model does not precisely address the spatiotemporal origin of nephron progenitors, hampering our understanding of cell fate decisions in the kidney. Here, we present a revised model of early-stage kidney specification, suggesting distinct origins of the two major kidney components: the ureteric bud and metanephric mesenchyme. This model enables the induction of metanephric nephron progenitors from both mouse and human pluripotent stem cells. The induced cells self-organize in the presence of Wnt signaling and reconstitute 3D nephron structures including both nephric tubules with a clear lumina and glomeruli with podocytes. The engrafted kidney tissue develops vascularized glomeruli and nephric tubules, but it does not produce urine, suggesting the requirement for further maturation. Nevertheless, the generation of nephron components from human-induced pluripotent stem cells will be useful for future application in regenerative therapy and modeling of congenital kidney diseases in vitro. This review discusses the possibility of de novo organogenesis of a functional kidney from pluripotent stem cells and the future direction toward clinical applications.

Kidney regeneration: common themes from the embryo to the adult

The vertebrate kidney has an inherent ability to regenerate following acute damage. Successful regeneration of the injured kidney requires the rapid replacement of damaged tubular epithelial cells and reconstitution of normal tubular function. Identifying the cells that participate in the regeneration process as well as the molecular mechanisms involved may reveal therapeutic targets for the treatment of kidney disease. Renal regeneration is associated with the expression of genetic pathways that are necessary for kidney organogenesis, suggesting that the regenerating tubular epithelium may be "reprogrammed" to a less-differentiated, progenitor state. This review will highlight data from various vertebrate models supporting the hypothesis that nephrogenic genes are reactivated as part of the process of kidney regeneration following acute kidney injury (AKI). Emphasis will be placed on the reactivation of developmental pathways and how our understanding of the resulting regeneration process may be enhanced by lessons learned in the embryonic kidney.

Gene regulatory network of renal primordium development

Animal development progresses through the stepwise deployment of gene regulatory networks (GRN) encoded in the genome. Comparative analyses in different species and organ systems have revealed that GRN blueprints are composed of subcircuits with stereotypical architectures that are often reused as modular units. In this review, we report the evidence for the GRN underlying renal primordium development. In vertebrates, renal development is initiated by the induction of a field of intermediate mesoderm cells competent to undergo lineage specification and nephric (Wolffian) duct formation. Definition of the renal field leads to the activation of a core regulatory subcircuit composed of the transcription factors Pax2/8, Gata3 and Lim1. These transcription factors turn on a second layer of transcriptional regulators while also activating effectors of tissue morphogenesis and cellular specialization. Elongation and connection of the nephric duct to the cloaca (bladder/urethra primordium) is followed by metanephric kidney induction through signals emanating from the metanephric mesenchyme. Central to this process is the activation and positioning of the glial cell line-derived neurotrophic factor (Gdnf)-Ret signaling pathway by network subcircuits located in the mesenchyme and epithelial tissues of the caudal trunk. Evidence shows that each step of the renal primordium developmental program is regulated by structured GRN subunits organized in a hierarchical manner. Understanding the structure and dynamics of the renal GRN will help us understand the intrinsic phenotypical variability of congenital anomalies of the kidney and urinary tract and guide our approaches to regenerative medicine.

Redefining the in vivo origin of metanephric nephron progenitors enables generation of complex kidney structures from pluripotent stem cells

Recapitulating three-dimensional (3D) structures of complex organs, such as the kidney, from pluripotent stem cells (PSCs) is a major challenge. Here, we define the developmental origins of the metanephric mesenchyme (MM), which generates most kidney components. Unexpectedly, we find that posteriorly located T(+) MM precursors are developmentally distinct from Osr1(+) ureteric bud progenitors during the postgastrulation stage, and we identify phasic Wnt stimulation and stage-specific growth factor addition as molecular cues that promote their development into the MM. We then use this information to derive MM from PSCs. These progenitors reconstitute the 3D structures of the kidney in vitro, including glomeruli with podocytes and renal tubules with proximal and distal regions and clear lumina. Furthermore, the glomeruli are efficiently vascularized upon transplantation. Thus, by reevaluating the developmental origins of metanephric progenitors, we have provided key insights into kidney specification in vivo and taken important steps toward kidney organogenesis in vitro.

Activin A regulates growth of gastro-intestinal epithelial cells by mediating epithelial-mesenchymal interaction

The importance of epithelial-mesenchymal interaction on the development of gastro-intestinal (GI) organs has been repeatedly reported, but its molecular mechanism has not been fully understood though several factors including hepatocyte growth factor and endothelin-3 have been shown to mediate it. Activins have been demonstrated to play important roles in the regulation of organogenesis in vertebrates, but their roles in the regulation of growth and differentiation of GI organs remain to be solved. In the present study, we examined expression of activins in developing rat GI tract, and found that inhibin bA encoding activin A was specifically expressed by GI mesenchymes, while inhibin bB encoding activin B was expressed by both epithelial and mesenchymal components. We then examined the effect of activin A on the growth of fetal rat GI epithelial cells in primary culture. We found that activin A inhibited the growth of forestomach and glandular stomach epithelial cells while it stimulated the growth of colonic epithelial cells. These results suggest that activin A secreted from GI mesenchymes region-specifically regulates the growth of attaching epithelial cells. We thus conclude that activin A mediates epithelial-mesenchymal interaction in the developing GI tract.

Human kidney cell reprogramming: applications for disease modeling and personalized medicine

The ability to reprogram fully differentiated cells into a pluripotent embryonic state, termed induced pluripotent stem cells (iPSCs), has been met with great excitement. iPSC technology has advanced the fundamental study of disease modeling with the potential for cell-replacement therapy, especially in the neuronal and cardiac fields. However, renal medicine as of yet has not benefited from similar advancements. This review summarizes the unique characteristics of iPSCs and their potential applications for modeling kidney disease. Pioneering such endeavors could yield constructs that recapitulate disease phenotypes, open avenues for more targeted drug development, and potentially serve as replenishable sources for replacement of kidney cells in the setting of human disease.

A role for Vg1/Nodal signaling in specification of the intermediate mesoderm

The intermediate mesoderm (IM) is the embryonic source of all kidney tissue in vertebrates. The factors that regulate the formation of the IM are not yet well understood. Through investigations in the chick embryo, the current study identifies and characterizes Vg1/Nodal signaling (henceforth referred to as 'Nodal-like signaling') as a novel regulator of IM formation. Excess Nodal-like signaling at gastrulation stages resulted in expansion of the IM at the expense of the adjacent paraxial mesoderm, whereas inhibition of Nodal-like signaling caused repression of IM gene expression. IM formation was sensitive to levels of the Nodal-like pathway co-receptor Cripto and was inhibited by a truncated form of the secreted molecule cerberus, which specifically blocks Nodal, indicating that the observed effects are specific to the Nodal-like branch of the TGFβ signaling pathway. The IM-promoting effects of Nodal-like signaling were distinct from the known effects of this pathway on mesoderm formation and left-right patterning, a finding that can be attributed to specific time windows for the activities of these Nodal-like functions. Finally, a link was observed between Nodal-like and BMP signaling in the induction of IM. Activation of IM genes by Nodal-like signaling required an active BMP signaling pathway, and Nodal-like signals induced phosphorylation of Smad1/5/8, which is normally associated with activation of BMP signaling pathways. We postulate that Nodal-like signaling regulates IM formation by modulating the IM-inducing effects of BMP signaling.

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.

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.

Programming pluripotent precursor cells derived from Xenopus embryos to generate specific tissues and organs

Xenopus embryos provide a rich source of pluripotent cells that can be differentiated into functional organs. Since the molecular principles of vertebrate organogenesis appear to be conserved between Xenopus and mammals, this system can provide useful guidelines for the directional manipulation of human embryonic stem cells. Pluripotent Xenopus cells can be easily isolated from the animal pole of blastula stage Xenopus embryos. These so called "animal cap" cells represent prospective ectodermal cells, but give rise to endodermal, mesodermal and neuro-ectodermal derivatives if treated with the appropriate factors. These factors include evolutionary conserved modulators of the key developmental signal transduction pathways that can be supplied either by mRNA microinjection or direct application of recombinant proteins. This relatively simple system has added to our understanding of pancreas, liver, kidney, eye and heart development. In particular, recent studies have used animal cap cells to generate ectopic eyes and hearts, setting the stage for future work aimed at programming pluripotent cells for regenerative medicine.

In vitro organogenesis using multipotent cells

Abstract The establishment of efficient methods for promoting stem cell differentiation into target cells is important not only in regenerative medicine, but also in drug discovery. In addition to embryonic stem (ES) cells and various somatic stem cells, such as mesenchymal stem cells derived from bone marrow, adipose tissue, and umbilical cord blood, a novel dedifferentiation technology that allows the generation of induced pluripotent stem (iPS) cells has been recently developed. Although an increasing number of stem cell populations are being described, there remains a lack of protocols for driving the differentiation of these cells. Regeneration of organs from stem cells in vitro requires precise blueprints for each differentiation step. To date, studies using various model organisms, such as zebrafish, Xenopus laevis, and gene-targeted mice, have uncovered several factors that are critical for the development of organs. We have been using X. laevis, the African clawed frog, which has developmental patterns similar to those seen in humans. Moreover, Xenopus embryos are excellent research tools for the development of differentiation protocols, since they are available in high numbers and are sufficiently large and robust for culturing after simple microsurgery. In addition, Xenopus eggs are fertilized externally, and all stages of the embryo are easily accessible, making it relatively easy to study the functions of individual gene products during organogenesis using microinjection into embryonic cells. In the present review, we provide examples of methods for in vitro organ formation that use undifferentiated Xenopus cells. We also describe the application of amphibian differentiation protocols to mammalian stem cells, so as to facilitate the development of efficient methodologies for in vitro differentiation.

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

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

In vitro regeneration of kidney from pluripotent stem cells

Although renal transplantation has proved a successful treatment for the patients with end-stage renal failure, the therapy is hampered by the problem of serious shortage of donor organs. Regenerative medicine using stem cells, including cell transplantation therapy, needs to be developed to solve the problem. We previously identified the multipotent progenitor cells in the embryonic mouse kidney that can give rise to several kinds of epithelial cells found in adult kidney, such as glomerular podocytes and renal tubular epithelia. Establishing the method to generate the progenitors from human pluripotent stem cells that have the capacity to indefinitely proliferate in vitro is required for the development of kidney regeneration strategy. We review the current status of the research on the differentiation of pluripotent stem cells into renal lineages and describe cues to promote this research field.

XPteg (Xenopus proximal tubules-expressed gene) is essential for pronephric mesoderm specification and tubulogenesis

Retinoic acid (RA) signaling is important for the early steps of nephrogenic cell fate specification. Here, we report a novel target gene of RA signaling named XPteg (Xenopus proximal tubules-expressed gene) which is critical for pronephric development. XPteg starts to be expressed at the earliest stage of embryonic kidney specification and was restricted to the pronephric proximal tubules during kidney development. Anti-sense morpholino (MO)-mediated knockdown of XPteg perturbed formation of pronephros as demonstrated by reduced expression of pronephric tubule markers. Conversely, overexpression of XPteg promoted endogenous and ectopic expression of those markers and expanded pronephric tubules. Treatment of retinoic acid induced the expression of XPteg in the pronephric field without protein synthesis. Furthermore, we found that the pronephric defects caused by a dominant negative RA receptor could be rescued by coexpression of XPteg. Taken together, these results suggest that XPteg functions as a direct transcriptional target of RA signaling to regulate pronephric tubulogenesis in Xenopus early development.

Isolation and differentiation of Xenopus animal cap cells

Xenopus is used as a model animal for investigating the inductive events and organogenesis that occur during early vertebrate development. Given that they are easy to obtain in high numbers and are relatively large in size, Xenopus embryos are excellent specimens for performing manipulations such as microinjection and microsurgery. The animal cap, which is the area around the animal pole of the blastula, is destined to form the ectoderm during normal development. However, these cells retain pluripotentiality and upon exposure to specific inducers, the animal cap can differentiate into neural, mesodermal, and endodermal tissues. In this sense, the cells of the animal cap are equivalent to mammalian embryonic stem cells. In this unit, the isolation and differentiation of animal cap cells, the so-called animal cap assay, is described. Useful methods for analyzing the mechanism of animal cap differentiation at the molecular level are also described.

Interplay between activin and Hox genes determines the formation of the kidney morphogenetic field

The kidney develops in a specific position along the anterior-posterior axis. All vertebrate kidney tissues are derived from the intermediate mesoderm (IM), and early kidney genes such as Lim1 and Pax2 are expressed in amniotes posterior to the sixth somite axial level. IM cells anterior to this level do not express kidney genes owing to changes in their competence to respond to kidney-inductive signals present along the entire axis. We aimed to understand the molecular mechanisms governing the loss of competence of anterior IM cells and the formation of the anterior border of the kidney morphogenetic field. We identified the dorsal neural tube as the potential kidney-inductive tissue and showed that activin, a secreted morphogen, is necessary but insufficient for Lim1 induction and establishment of the kidney field. Activin or activin-like and BMP signaling cascades are activated along the entire axis, including in anterior non-kidney IM, suggesting that competence to respond to these signals involves downstream or other components. Detailed expression pattern analysis of Hox genes during early chick development revealed that paralogous group four genes share the same anterior border as the kidney genes. Ectopic expression of Hoxb4 in anterior non-kidney IM, either by retinoic acid (RA) administration or plasmid-mediated overexpression, resulted in ectopic kidney gene expression. The anterior expansion of Lim1 expression was restrained when Hoxb4 was co-expressed with a truncated form of activin receptor. We suggest a model in which the competence of IM cells to respond to TGFbeta signaling and express kidney genes is driven by RA and mediated by Hoxb4.

An increase in intracellular Ca2+ is involved in pronephric tubule differentiation in the amphibian Xenopus laevis

The pronephros is the first kidney to develop and is the functional embryonic kidney in lower vertebrates. It has previously been shown that pronephric tubules can be induced to form ex vivo in ectodermal tissue by treatment with activin A and retinoic acid. In this study, we investigated the role of Ca(2+) signaling in the formation of the pronephric tubules both in intact Xenopus embryos and ex vivo. In the ex vivo system, retinoic acid but not activin A stimulated the generation of Ca(2+) transients during tubule formation. Furthermore, tubule differentiation could be induced by agents that increase the concentration of intracellular Ca(2+) in activin A-treated ectoderm. In addition, tubule formation was inhibited by loading the ectodermal tissue with the Ca(2+) chelator, BAPTA-AM prior to activin A/retinoic acid treatment. In intact embryos, Ca(2+) transients were also recorded during tubule formation, and photo-activation of the caged Ca(2+) chelator, diazo-2, localized to the pronephric domain, produced embryos with a shortened and widened tubule phenotype. In addition, the location of the Ca(2+) transients observed, correlated with the expression pattern of the specific pronephric tubule gene, XSMP-30. These data indicate that Ca(2+) might be a necessary signal in the process of tubulogenesis both ex vivo and in intact embryos.

The cdx genes and retinoic acid control the positioning and segmentation of the zebrafish pronephros

Kidney function depends on the nephron, which comprises a blood filter, a tubule that is subdivided into functionally distinct segments, and a collecting duct. How these regions arise during development is poorly understood. The zebrafish pronephros consists of two linear nephrons that develop from the intermediate mesoderm along the length of the trunk. Here we show that, contrary to current dogma, these nephrons possess multiple proximal and distal tubule domains that resemble the organization of the mammalian nephron. We examined whether pronephric segmentation is mediated by retinoic acid (RA) and the caudal (cdx) transcription factors, which are known regulators of segmental identity during development. Inhibition of RA signaling resulted in a loss of the proximal segments and an expansion of the distal segments, while exogenous RA treatment induced proximal segment fates at the expense of distal fates. Loss of cdx function caused abrogation of distal segments, a posterior shift in the position of the pronephros, and alterations in the expression boundaries of raldh2 and cyp26a1, which encode enzymes that synthesize and degrade RA, respectively. These results suggest that the cdx genes act to localize the activity of RA along the axis, thereby determining where the pronephros forms. Consistent with this, the pronephric-positioning defect and the loss of distal tubule fate were rescued in embryos doubly-deficient for cdx and RA. These findings reveal a novel link between the RA and cdx pathways and provide a model for how pronephric nephrons are segmented and positioned along the embryonic axis.

In Vitro organogenesis using amphibian pluripotent cells

Mesoderm induction as a result of the interaction between endoderm and ectoderm is one of the most crucial events in vertebrate development. We identified activin as a strong mesoderm-inducing factor in an animal cap assay, an in vitro assay system using amphibian pluripotential cell mass. Activin induces mesodermal tisswes including most dorsal mesodermal tissue, notochord (which has important roles in neural induction, somite segmentation, and endodermal organogenesis), and its effects are concentration-dependent. Animal cap cells treated with high concentrations of activin differentiate into anterior endoderm, which can act as an organizer, or center of body patterning. We have established an in vitro induction system for 22 different organs and tissues using animal cap cells, and have isolated many organ-specific genes. With these useful methods, and analysis of newly isolated tissue- and organ-specific genes, the molecular biological "road map" for organogenesis is being established.

Retinoic acid signalling is required for specification of pronephric cell fate

The mechanisms by which a subset of mesodermal cells are committed to a nephrogenic fate are largely unknown. In this study, we have investigated the role of retinoic acid (RA) signalling in this process using Xenopus laevis as a model system and Raldh2 knockout mice. Pronephros formation in Xenopus embryo is severely impaired when RA signalling is inhibited either through expression of a dominant-negative RA receptor, or by expressing the RA-catabolizing enzyme XCyp26 or through treatment with chemical inhibitors. Conversely, ectopic RA signalling expands the size of the pronephros. Using a transplantation assay that inhibits RA signalling specifically in pronephric precursors, we demonstrate that this signalling is required within this cell population. Timed antagonist treatments show that RA signalling is required during gastrulation for expression of Xlim-1 and XPax-8 in pronephric precursors. Moreover, experiments conducted with a protein synthesis inhibitor indicate that RA may directly regulate Xlim-1. Raldh2 knockout mouse embryos fail to initiate the expression of early kidney-specific genes, suggesting that implication of RA signalling in the early steps of kidney formation is evolutionary conserved in vertebrates.

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.

Cloning and expression of the amphibian homologue of the human PKD1 gene

PKD1 is the gene responsible for autosomal dominant polycystic kidney disease (ADPKD) type 1 in humans. The PKD1 gene product is likely to be a calcium channel regulator. In this paper, we describe the isolation and characterization of the Xenopus homologue of the human PKD1 gene. We isolated and cloned genomic fragments corresponding to the amphibian homologue of PKD1 from a BAC library, and after sequencing the clones, we designed primers for the amplification of the transcript and sequenced 10 kb of ORF. The sequence of the putative protein clearly demonstrated that this gene is the homologue of human PKD1. Analysis of the tissue expression patterns of xPKD1 demonstrated a high level of expression in the kidney. A similar analysis in developing embryos and in an in vitro nephrogenic system suggests that xPKD1 is associated with, and probably involved in, the development of the amphibian pronephros.