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

Hnf1b renal expression directed by a distal enhancer responsive to Pax8

Xenopus provides a simple and efficient model system to study nephrogenesis and explore the mechanisms causing renal developmental defects in human. Hnf1b (hepatocyte nuclear factor 1 homeobox b), a gene whose mutations are the most commonly identified genetic cause of developmental kidney disease, is required for the acquisition of a proximo-intermediate nephron segment in Xenopus as well as in mouse. Genetic networks involved in Hnf1b expression during kidney development remain poorly understood. We decided to explore the transcriptional regulation of Hnf1b in the developing Xenopus pronephros and mammalian renal cells. Using phylogenetic footprinting, we identified an evolutionary conserved sequence (CNS1) located several kilobases (kb) upstream the Hnf1b transcription start and harboring epigenomic marks characteristics of a distal enhancer in embryonic and adult renal cells in mammals. By means of functional expression assays in Xenopus and mammalian renal cell lines we showed that CNS1 displays enhancer activity in renal tissue. Using CRISPR/cas9 editing in Xenopus tropicalis, we demonstrated the in vivo functional relevance of CNS1 in driving hnf1b expression in the pronephros. We further showed the importance of Pax8-CNS1 interaction for CNS1 enhancer activity allowing us to conclude that Hnf1b is a direct target of Pax8. Our work identified for the first time a Hnf1b renal specific enhancer and may open important perspectives into the diagnosis for congenital kidney anomalies in human, as well as modeling HNF1B-related diseases.

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.

Activation of podocyte Notch mediates early Wt1 glomerulopathy

The Wilms' tumor suppressor gene, WT1, encodes a zinc finger protein that regulates podocyte development and is highly expressed in mature podocytes. Mutations in the WT1 gene are associated with the development of renal failure due to the formation of scar tissue within glomeruli, the mechanisms of which are poorly understood. Here, we used a tamoxifen-based CRE-LoxP system to induce deletion of Wt1 in adult mice to investigate the mechanisms underlying evolution of glomerulosclerosis. Podocyte apoptosis was evident as early as the fourth day post-induction and increased during disease progression, supporting a role for Wt1 in mature podocyte survival. Podocyte Notch activation was evident at disease onset with upregulation of Notch1 and its transcriptional targets, including Nrarp. There was repression of podocyte FoxC2 and upregulation of Hey2 supporting a role for a Wt1/FoxC2/Notch transcriptional network in mature podocyte injury. The expression of cleaved Notch1 and HES1 proteins in podocytes of mutant mice was confirmed in early disease. Furthermore, induction of podocyte HES1 expression was associated with upregulation of genes implicated in epithelial mesenchymal transition, thereby suggesting that HES1 mediates podocyte EMT. Lastly, early pharmacological inhibition of Notch signaling ameliorated glomerular scarring and albuminuria. Thus, loss of Wt1 in mature podocytes modulates podocyte Notch activation, which could mediate early events in WT1-related glomerulosclerosis.

EphA7 regulates claudin6 and pronephros development in Xenopus

Eph/ephrin molecules are widely expressed during embryonic development, and function in a variety of developmental processes. Here we studied the roles of the Eph receptor EphA7 and its soluble form in Xenopus pronephros development. EphA7 is specifically expressed in pronephric tubules at tadpole stages and knockdown of EphA7 by a translation blocking morpholino led to defects in tubule cell differentiation and morphogenesis. A soluble form of EphA7 (sEphA7) was also identified. Interestingly, the membrane level of claudin6 (CLDN6), a tetraspan transmembrane tight junction protein, was dramatically reduced in the translation blocking morpholino injected embryos, but not when a splicing morpholino was used, which blocks only the full length EphA7. In cultured cells, EphA7 binds and phosphorylates CLDN6, and reduces its distribution at the cell surface. Our work suggests a role of EphA7 in the regulation of cell adhesion during pronephros development, whereas sEphA7 works as an antagonist.

Anatomical integration of the sacral-hindlimb unit coordinated by GDF11 underlies variation in hindlimb positioning in tetrapods

Elucidating how body parts from different primordia are integrated during development is essential for understanding the nature of morphological evolution. In tetrapod evolution, while the position of the hindlimb has diversified along with the vertebral formula, the mechanism responsible for this coordination has not been well understood. However, this synchronization suggests the presence of an evolutionarily conserved developmental mechanism that coordinates the positioning of the hindlimb skeleton derived from the lateral plate mesoderm with that of the sacral vertebrae derived from the somites. Here we show that GDF11 secreted from the posterior axial mesoderm is a key factor in the integration of sacral vertebrae and hindlimb positioning by inducing Hox gene expression in two different primordia. Manipulating the onset of GDF11 activity altered the position of the hindlimb in chicken embryos, indicating that the onset of Gdf11 expression is responsible for the coordinated positioning of the sacral vertebrae and hindlimbs. Through comparative analysis with other vertebrate embryos, we also show that each tetrapod species has a unique onset timing of Gdf11 expression, which is tightly correlated with the anteroposterior levels of the hindlimb bud. We conclude that the evolutionary diversity of hindlimb positioning resulted from heterochronic shifts in Gdf11 expression, which led to coordinated shifts in the sacral-hindlimb unit along the anteroposterior axis.

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.

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.

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

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

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.

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

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

The genetics and epigenetics of kidney development

The development of the mammalian kidney has been studied at the genetic, biochemical, and cell biological level for more than 40 years. As such, detailed mechanisms governing early patterning, cell lineages, and inductive interactions have been well described. How genes interact to specify the renal epithelial cells of the nephrons and how this specification is relevant to maintaining normal renal function is discussed. Implicit in the development of the kidney are epigenetic mechanisms that mark renal cell types and connect certain developmental regulatory factors to chromatin modifications that control gene expression patterns and cellular physiology. In adults, such regulatory factors and their epigenetic pathways may function in regeneration and may be disturbed in disease processes.

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

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

Hes4 controls proliferative properties of neural stem cells during retinal ontogenesis

The retina of fish and amphibian contains genuine neural stem cells located at the most peripheral edge of the ciliary marginal zone (CMZ). However, their cell-of-origin as well as the mechanisms that sustain their maintenance during development are presently unknown. We identified Hes4 (previously named XHairy2), a gene encoding a bHLH-O transcriptional repressor, as a stem cell-specific marker of the Xenopus CMZ that is positively regulated by the canonical Wnt pathway and negatively by Hedgehog signaling. We found that during retinogenesis, Hes4 labels a small territory, located first at the pigmented epithelium (RPE)/neural retina (NR) border and later in the retinal margin, that likely gives rise to adult retinal stem cells. We next addressed whether Hes4 might impart this cell subpopulation with retinal stem cell features: inhibited RPE or NR differentiation programs, continuous proliferation, and slow cell cycle speed. We could indeed show that Hes4 overexpression cell autonomously prevents retinal precursor cells from commitment toward retinal fates and maintains them in a proliferative state. Besides, our data highlight for the first time that Hes4 may also constitute a crucial regulator of cell cycle kinetics. Hes4 gain of function indeed significantly slows down cell division, mainly through the lengthening of G1 phase. As a whole, we propose that Hes4 maintains particular stemness features in a cellular cohort dedicated to constitute the adult retinal stem cell pool, by keeping it in an undifferentiated and slowly proliferative state along embryonic retinogenesis. Stem Cells 2012;30:2784–2795

HNF1B controls proximal-intermediate nephron segment identity in vertebrates by regulating Notch signalling components and Irx1/2

The nephron is a highly specialised segmented structure that provides essential filtration and resorption renal functions. It arises by formation of a polarised renal vesicle that differentiates into a comma-shaped body and then a regionalised S-shaped body (SSB), with the main prospective segments mapped to discrete domains. The regulatory circuits involved in initial nephron patterning are poorly understood. We report here that HNF1B, a transcription factor known to be involved in ureteric bud branching and initiation of nephrogenesis, has an additional role in segment fate acquisition. Hnf1b conditional inactivation in murine nephron progenitors results in rudimentary nephrons comprising a glomerulus connected to the collecting system by a short tubule displaying distal fates. Renal vesicles develop and polarise normally but fail to progress to correctly patterned SSBs. Major defects are evident at late SSBs, with altered morphology, reduction of a proximo-medial subdomain and increased apoptosis. This is preceded by strong downregulation of the Notch pathway components Lfng, Dll1 and Jag1 and the Irx1/2 factors, which are potential regulators of proximal and Henle's loop segment fates. Moreover, HNF1B is recruited to the regulatory sequences of most of these genes. Overexpression of a HNF1B dominant-negative construct in Xenopus embryos causes downregulation specifically of proximal and intermediate pronephric segment markers. These results show that HNF1B is required for the acquisition of a proximo-intermediate segment fate in vertebrates, thus uncovering a previously unappreciated function of a novel SSB subcompartment in global nephron segmentation and further differentiation.

Comparative functional analysis of ZFP36 genes during Xenopus development

ZFP36 constitutes a small family of RNA binding proteins (formerly known as the TIS11 family) that target mRNA and promote their degradation. In mammals, ZFP36 proteins are encoded by four genes and, although they show similar activities in a cellular RNA destabilization assay, there is still a limited knowledge of their mRNA targets and it is not known whether or not they have redundant functions. In the present work, we have used the Xenopus embryo, a model system allowing gain- and loss-of-function studies, to investigate, whether individual ZFP36 proteins had distinct or redundant functions. We show that overexpression of individual amphibian zfp36 proteins leads to embryos having the same defects, with alteration in somites segmentation and pronephros formation. In these embryos, members of the Notch signalling pathway such as hairy2a or esr5 mRNA are down-regulated, suggesting common targets for the different proteins. We also show that mouse Zfp36 protein overexpression gives the same phenotype, indicating an evolutionary conserved property among ZFP36 vertebrate proteins. Morpholino oligonucleotide-induced loss-of-function leads to defects in pronephros formation, reduction in tubule size and duct coiling alterations for both zfp36 and zfp36l1, indicating no functional redundancy between these two genes. Given the conservation in gene structure and function between the amphibian and mammalian proteins and the conserved mechanisms for pronephros development, our study highlights a potential and hitherto unreported role of ZFP36 gene in kidney 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.

Xenopus pronephros development--past, present, and future

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

Wt1a, Foxc1a, and the Notch mediator Rbpj physically interact and regulate the formation of podocytes in zebrafish

Podocytes help form the glomerular blood filtration barrier in the kidney and their injury or loss leads to renal disease. The Wilms' tumor suppressor-1 (Wt1) and the FoxC1/2 transcription factors, as well as Notch signaling, have been implicated as important regulators of podocyte fate. It is not known whether these factors work in parallel or sequentially on different gene targets, or as higher-order transcriptional complexes on common genes. Here, we use the zebrafish to demonstrate that embryos treated with morpholinos against wt1a, foxc1a, or the Notch transcriptional mediator rbpj develop fewer podocytes, as determined by wt1b, hey1 and nephrin expression, while embryos deficient in any two of these factors completely lack podocytes. From GST-pull-downs and co-immunoprecipitation experiments we show that Wt1a, Foxc1a, and Rbpj can physically interact with each other, whereas only Rbpj binds to the Notch intracellular domain (NICD). In transactivation assays, combinations of Wt1, FoxC1/2, and NICD synergistically induce the Hey1 promoter, and have additive or repressive effects on the Podocalyxin promoter, depending on dosage. Taken together, these data suggest that Wt1, FoxC1/2, and Notch signaling converge on common target genes where they physically interact to regulate a podocyte-specific gene program. These findings further our understanding of the transcriptional circuitry responsible for podocyte formation and differentiation during kidney development.

Role of Tbx2 in defining the territory of the pronephric nephron

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

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.

Advances in early kidney specification, development and patterning

The kidney is a model developmental system for understanding mesodermal patterning and organogenesis, a process that requires regional specification along multiple body axes, the proliferation and differentiation of progenitor cells, and integration with other tissues. Recent progress in the field has highlighted the essential roles of intrinsic nuclear factors and secreted signaling molecules in specifying renal epithelial stem cells and their self-renewal, in driving the complex dynamics of epithelial cell branching morphogenesis, and in nephron patterning. How these developments influence and advance our understanding of kidney development is discussed.

The Notch effector Hey1 associates with myogenic target genes to repress myogenesis

Members of the Hey family of transcriptional repressors are basic helix-loop-helix proteins that are thought to act downstream of Notch in diverse tissues. Although forced expression of Hey1, a target of Notch in myoblasts, is sufficient to recapitulate inhibitory effects of the pathway on differentiation, how Hey1 interferes with myogenic transcription has not been fully elucidated. We provide multiple lines of evidence that Hey1 does not target the intrinsic transcriptional activity of the skeletal muscle master regulator MyoD. Our results indicate instead that Hey1 is recruited to the promoter regions of myogenin and Mef2C, two genes whose induction is critical for myogenesis. Expression of Hey1 in C2C12 myoblasts correlates with reduced recruitment of MyoD to these promoters, arguing that Hey1 inhibits myogenesis by associating with and repressing expression of key myogenic targets.

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.

A dual requirement for Iroquois genes during Xenopus kidney development

The Iroquois (Irx) genes encode evolutionary conserved homeoproteins. We report that Xenopus genes Irx1 and Irx3 are expressed and required during different stages of Xenopus pronephros development. They are initially expressed during mid-neurulation in domains extending over most of the prospective pronephric territory. Expression onset takes place after kidney anlage specification, but before pronephric organogenesis occurs. Later, during nephron segmentation, expression becomes restricted to the intermediate tubule region of the proximal-distal axis. Loss- and gain-of-function analyses, performed with specific morpholinos and inducible wild-type and dominant-negative constructs, reveal a dual requirement for Irx1 and Irx3 during pronephros development. During neurula stages, these genes maintain the specification of the pronephric territory and define its size. This seems to occur, at least in part, through positive regulation of Bmp signalling. Subsequently, Irx genes are required for proper formation of the intermediate tubule. Finally, we find that retinoic acid signalling activates both Irx1 and Irx3 genes in the pronephros.

The prepattern transcription factor Irx3 directs nephron segment identity

The nephron, the basic structural and functional unit of the vertebrate kidney, is organized into discrete segments, which are composed of distinct renal epithelial cell types. Each cell type carries out highly specific physiological functions to regulate fluid balance, osmolarity, and metabolic waste excretion. To date, the genetic basis of regionalization of the nephron has remained largely unknown. Here we show that Irx3, a member of the Iroquois (Irx) gene family, acts as a master regulator of intermediate tubule fate. Comparative studies in Xenopus and mouse have identified Irx1, Irx2, and Irx3 as an evolutionary conserved subset of Irx genes, whose expression represents the earliest manifestation of intermediate compartment patterning in the developing vertebrate nephron discovered to date. Intermediate tubule progenitors will give rise to epithelia of Henle's loop in mammals. Loss-of-function studies indicate that irx1 and irx2 are dispensable, whereas irx3 is necessary for intermediate tubule formation in Xenopus. Furthermore, we demonstrate that misexpression of irx3 is sufficient to direct ectopic development of intermediate tubules in the Xenopus mesoderm. Taken together, irx3 is the first gene known to be necessary and sufficient to specify nephron segment fate in vivo.

Molecular insights into segmentation along the proximal-distal axis of the nephron

The structure of a mammalian kidney is parsed into large collections of polarized nephrons, and each segment is home to a diverse community of cells that specialize in renal endocrine and excretory functions. Early developmental lengthening and diversification of nephron segments along a proximal--distal axis initiate all subsequent facets of tubular growth and function. Morphogenic cues and biochemical interactions that are critical to this process are starting to emerge. The underlying principles of regional cell signaling and transcriptional control organizing early segmentation are the subject of this review.