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

Retinoic acid control of pax8 during renal specification of Xenopus pronephros involves hox and meis3

Development of the Xenopus pronephros relies on renal precursors grouped at neurula stage into a specific region of dorso-lateral mesoderm called the kidney field. Formation of the kidney field at early neurula stage is dependent on retinoic (RA) signaling acting upstream of renal master transcriptional regulators such as pax8 or lhx1. Although lhx1 might be a direct target of RA-mediated transcriptional activation in the kidney field, how RA controls the emergence of the kidney field remains poorly understood. In order to better understand RA control of renal specification of the kidney field, we have performed a transcriptomic profiling of genes affected by RA disruption in lateral mesoderm explants isolated prior to the emergence of the kidney field and cultured at different time points until early neurula stage. Besides genes directly involved in pronephric development (pax8, lhx1, osr2, mecom), hox (hoxa1, a3, b3, b4, c5 and d1) and the hox co-factor meis3 appear as a prominent group of genes encoding transcription factors (TFs) downstream of RA. Supporting the idea of a role of meis3 in the kidney field, we have observed that meis3 depletion results in a severe inhibition of pax8 expression in the kidney field. Meis3 depletion only marginally affects expression of lhx1 and aldh1a2 suggesting that meis3 principally acts upstream of pax8. Further arguing for a role of meis3 and hox in the control of pax8, expression of a combination of meis3, hoxb4 and pbx1 in animal caps induces pax8 expression, but not that of lhx1. The same combination of TFs is also able to transactivate a previously identified pax8 enhancer, Pax8-CNS1. Mutagenesis of potential PBX-Hox binding motifs present in Pax8-CNS1 further allows to identify two of them that are necessary for transactivation. Finally, we have tested deletions of regulatory sequences in reporter assays with a previously characterized transgene encompassing 36.5 ​kb of the X. tropicalis pax8 gene that allows expression of a truncated pax8-GFP fusion protein recapitulating endogenous pax8 expression. This transgene includes three conserved pax8 enhancers, Pax8-CNS1, Pax8-CNS2 and Pax8-CNS3. Deletion of Pax8-CNS1 alone does not affect reporter expression, but deletion of a 3.5 ​kb region encompassing Pax8-CNS1 and Pax8-CNS2 results in a severe inhibition of reporter expression both in the otic placode and kidney field domains.

gldc Is Essential for Renal Progenitor Patterning during Kidney Development

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

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.

MECOM-associated syndrome: a heterogeneous inherited bone marrow failure syndrome with amegakaryocytic thrombocytopenia

Heterozygous mutations in MECOM (MDS1 and EVI1 complex locus) have been reported to be causative of a rare association of congenital amegakaryocytic thrombocytopenia and radioulnar synostosis. Here we report on 12 patients with congenital hypomegakaryocytic thrombocytopenia caused by MECOM mutations (including 10 novel mutations). The mutations affected different functional domains of the EVI1 protein. The spectrum of phenotypes was much broader than initially reported for the first 3 patients; we found familial as well as sporadic cases, and the clinical spectrum ranged from isolated radioulnar synostosis with no or mild hematological involvement to severe bone marrow failure without obvious skeletal abnormality. The clinical picture included radioulnar synostosis, bone marrow failure, clinodactyly, cardiac and renal malformations, B-cell deficiency, and presenile hearing loss. No single clinical manifestation was detected in all patients affected by MECOM mutations. Radioulnar synostosis and B-cell deficiency were observed only in patients with mutations affecting a short region in the C-terminal zinc finger domain of EVI1. We propose the term MECOM-associated syndrome for this heterogeneous hereditary disease and inclusion of MECOM sequencing in the diagnostic workup of congenital bone marrow failure.

The Transcription Factor Prdm16 Marks a Single Retinal Ganglion Cell Subtype in the Mouse Retina

Purpose

Retinal ganglion cells (RGC) can be categorized into roughly 30 distinct subtypes. How these subtypes develop is poorly understood, in part because few unique subtype markers have been characterized. We tested whether the Prdm16 transcription factor is expressed by RGCs as a class or within particular ganglion cell subtypes.

Methods

Embryonic and mature retinal sections and flatmount preparations were examined by immunohistochemistry for Prdm16 and several other cell type-specific markers. To visualize the morphology of Prdm16+ cells, we utilized Thy1-YFP-H transgenic mice, where a small random population of RGCs expresses yellow fluorescent protein (YFP) throughout the cytoplasm.

Results

Prdm16 was expressed in the retina starting late in embryogenesis. Prdm16+ cells coexpressed the RGC marker Brn3a. These cells were arranged in an evenly spaced pattern and accounted for 2% of all ganglion cells. Prdm16+ cells coexpressed parvalbumin, but not calretinin, melanopsin, Smi32, or CART. This combination of marker expression and morphology data from Thy1-YFP-H mice suggested that the Prdm16+ cells represented a single ganglion cell subtype. Prdm16 also marked vascular endothelial cells and mural cells of retinal arterioles.

Conclusions

A single subtype of ganglion cell appears to be uniquely marked by Prdm16 expression. While the precise identity of these ganglion cells is unclear, they most resemble the G9 subtype described by Völgyi and colleagues in 2009. Future studies are needed to determine the function of these ganglion cells and whether Prdm16 regulates their development.

EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate development

The gene ectopic viral integration site 1 (EVI) and its variant myelodysplastic syndrome 1 (MDS)/EVI encode zinc-finger proteins that have been recognized as important oncogenes in various types of cancer. In contrast to the established role of EVI and MDS/EVI in cancer development, their potential function during vertebrate postembryonic development, especially in organ-specific adult stem cells, is unclear. Amphibian metamorphosis is strikingly similar to postembryonic development around birth in mammals, with both processes taking place when plasma thyroid hormone (T3) levels are high. Using the T3-dependent metamorphosis in Xenopus tropicalis as a model, we show here that high levels of EVI and MDS/EVI are expressed in the intestine at the climax of metamorphosis and are induced by T3. By using the transcription activator-like effector nuclease gene editing technology, we have knocked out both EVI and MDS/EVI and have shown that EVI and MDS/EVI are not essential for embryogenesis and premetamorphosis in X. tropicalis On the other hand, knocking out EVI and MDS/EVI causes severe retardation in the growth and development of the tadpoles during metamorphosis and leads to tadpole lethality at the climax of metamorphosis. Furthermore, the homozygous-knockout animals have reduced adult intestinal epithelial stem cell proliferation at the end of metamorphosis (for the few that survive through metamorphosis) or during T3-induced metamorphosis. These findings reveal a novel role of EVI and/or MDS/EVI in regulating the formation and/or proliferation of adult intestinal adult stem cells during postembryonic development in vertebrates.-Okada, M., Shi, Y.-B. EVI and MDS/EVI are required for adult intestinal stem cell formation during postembryonic vertebrate development.

Mammalian Glutamyl Aminopeptidase Genes (ENPEP) and Proteins: Comparative Studies of a Major Contributor to Arterial Hypertension

Glutamyl aminopeptidase (ENPEP) is a member of the M1 family of endopeptidases which are mammalian type II integral membrane zinc-containing endopeptidases. ENPEP is involved in the catabolic pathway of the renin-angiotensin system forming angiotensin III, which participates in blood pressure regulation and blood vessel formation. Comparative ENPEP amino acid sequences and structures and ENPEP gene locations were examined using data from several mammalian genome projects. Mammalian ENPEP sequences shared 71-98% identities. Five N-glycosylation sites were conserved for all mammalian ENPEP proteins examined although 9-18 sites were observed, in each case. Sequence alignments, key amino acid residues and predicted secondary and tertiary structures were also studied, including transmembrane and cytoplasmic sequences and active site residues. Highest levels of human ENPEP expression were observed in the terminal ileum of the small intestine and in the kidney cortex. Mammalian ENPEP genes contained 20 coding exons. The human ENPEP gene promoter and first coding exon contained a CpG island (CpG27) and at least 6 transcription factor binding sites, whereas the 3'-UTR region contained 7 miRNA target sites, which may contribute to the regulation of ENPEP gene expression in tissues of the body. Phylogenetic analyses examined the relationships of mammalian ENPEP genes and proteins, including primate, other eutherian, marsupial and monotreme sources, using chicken ENPEP as a primordial sequence for comparative purposes.

An emerging role for prdm family genes in dorsoventral patterning of the vertebrate nervous system

The embryonic vertebrate neural tube is divided along its dorsoventral (DV) axis into eleven molecularly discrete progenitor domains. Each of these domains gives rise to distinct neuronal cell types; the ventral-most six domains contribute to motor circuits, while the five dorsal domains contribute to sensory circuits. Following the initial neurogenesis step, these domains also generate glial cell types—either astrocytes or oligodendrocytes. This DV pattern is initiated by two morphogens—Sonic Hedgehog released from notochord and floor plate and Bone Morphogenetic Protein produced in the roof plate—that act in concentration gradients to induce expression of genes along the DV axis. Subsequently, these DV-restricted genes cooperate to define progenitor domains and to control neuronal cell fate specification and differentiation in each domain. Many genes involved in this process have been identified, but significant gaps remain in our understanding of the underlying genetic program. Here we review recent work identifying members of the Prdm gene family as novel regulators of DV patterning in the neural tube. Many Prdm proteins regulate transcription by controlling histone modifications (either via intrinsic histone methyltransferase activity, or by recruiting histone modifying enzymes). Prdm genes are expressed in spatially restricted domains along the DV axis of the neural tube and play important roles in the specification of progenitor domains, as well as in the subsequent differentiation of motor neurons and various types of interneurons. Strikingly, Prdm proteins appear to function by binding to, and modulating the activity of, other transcription factors (particularly bHLH proteins). The identity of key transcription factors in DV patterning of the neural tube has been elucidated previously (e.g. the nkx, bHLH and pax families), but it now appears that an additional family is also required and that it acts in a potentially novel manner.

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.

Spatiotemporal expression of Prdm genes during Xenopus development

Epigenetic regulation is known to be important in embryonic development, cell differentiation and regulation of cancer cells. Molecular mechanisms of epigenetic modification have DNA methylation and histone tail modification such as acetylation, phosphorylation and ubiquitination. Until now, many kinds of enzymes that modify histone tail with various functional groups have been reported and regulate the epigenetic state of genes. Among them, Prdm genes were identified as histone methyltransferase. Prdm genes are characterized by an N-terminal PR/SET domain and C-terminal some zinc finger domains and therefore they are considered to have both DNA-binding ability and methylation activity. Among vertebrate, fifteen members are estimated to belong to Prdm genes family. Even though Prdm genes are thought to play important roles for cell fate determination and cell differentiation, there is an incomplete understanding of their expression and functions in early development. Here, we report that Prdm genes exhibit dynamic expression pattern in Xenopus embryogenesis. By whole mount in situ hybridization analysis, we show that Prdm genes are expressed in spatially localized manners in embryo and all of Prdm genes are expressed in neural cells in developing central nervous systems. Our study suggests that Prdm genes may be new candidates to function in neural cell differentiation.

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 Prdm13 histone methyltransferase encoding gene is a Ptf1a-Rbpj downstream target that suppresses glutamatergic and promotes GABAergic neuronal fate in the dorsal neural tube

The basic helix-loop-helix (bHLH) transcriptional activator Ptf1a determines inhibitory GABAergic over excitatory glutamatergic neuronal cell fate in progenitors of the vertebrate dorsal spinal cord, cerebellum and retina. In an in situ hybridization expression survey of PR domain containing genes encoding putative chromatin-remodeling zinc finger transcription factors in Xenopus embryos, we identified Prdm13 as a histone methyltransferase belonging to the Ptf1a synexpression group. Gain and loss of Ptf1a function analyses in both frog and mice indicates that Prdm13 is positively regulated by Ptf1a and likely constitutes a direct transcriptional target. We also showed that this regulation requires the formation of the Ptf1a-Rbp-j complex. Prdm13 knockdown in Xenopus embryos and in Ptf1a overexpressing ectodermal explants lead to an upregulation of Tlx3/Hox11L2, which specifies a glutamatergic lineage and a reduction of the GABAergic neuronal marker Pax2. It also leads to an upregulation of Prdm13 transcription, suggesting an autonegative regulation. Conversely, in animal caps, Prdm13 blocks the ability of the bHLH factor Neurog2 to activate Tlx3. Additional gain of function experiments in the chick neural tube confirm that Prdm13 suppresses Tlx3(+)/glutamatergic and induces Pax2(+)/GABAergic neuronal fate. Thus, Prdm13 is a novel crucial component of the Ptf1a regulatory pathway that, by modulating the transcriptional activity of bHLH factors such as Neurog2, controls the balance between GABAergic and glutamatergic neuronal fate in the dorsal and caudal part of the vertebrate neural tube.

Zebrafish nephrogenesis is regulated by interactions between retinoic acid, mecom, and Notch signaling

The zebrafish pronephros provides a conserved model to study kidney development, in particular to delineate the poorly understood processes of how nephron segment pattern and cell type choice are established. Zebrafish nephrons are divided into distinct epithelial regions that include a series of proximal and distal tubule segments, which are comprised of intercalated transporting epithelial cells and multiciliated cells (MCC). Previous studies have shown that retinoic acid (RA) regionalizes the renal progenitor field into proximal and distal domains and that Notch signaling later represses MCC differentiation, but further understanding of these pathways has remained unknown. The transcription factor mecom (mds1/evi1 complex) is broadly expressed in renal progenitors, and then subsequently marks the distal tubule. Here, we show that mecom is necessary to form the distal tubule and to restrict both proximal tubule formation and MCC fate choice. We found that mecom and RA have opposing roles in patterning discrete proximal and distal segments. Further, we discovered that RA is required for MCC formation, and that one mechanism by which RA promotes MCC fate choice is to inhibit mecom. Next, we determined the epistatic relationship between mecom and Notch signaling, which limits MCC fate choice by lateral inhibition. Abrogation of Notch signaling with the γ-secretase inhibitor DAPT revealed that Notch and mecom did not have additive effects in blocking MCC formation, suggesting that they function in the same pathway. Ectopic expression of the Notch signaling effector, Notch intracellular domain (NICD), rescued the expansion of MCCs in mecom morphants, indicating that mecom acts upstream to induce Notch signaling. These findings suggest a model in which mecom and RA arbitrate proximodistal segment domains, while MCC fate is modulated by a complex interplay in which RA inhibition of mecom, and mecom promotion of Notch, titrates MCC number. Taken together, our studies have revealed several essential and novel mechanisms that control pronephros development in the zebrafish.

Tissue-specific upregulation of MDS/EVI gene transcripts in the intestine by thyroid hormone during Xenopus metamorphosis

Background

Intestinal remodeling during amphibian metamorphosis resembles the maturation of the adult intestine during mammalian postembryonic development when the adult epithelial self-renewing system is established under the influence of high concentrations of plasma thyroid hormone (T3). This process involves de novo formation and subsequent proliferation and differentiation of the adult stem cells.

Methodology/Principal Findings

The T3-dependence of the formation of adult intestinal stem cell during Xenopus laevis metamorphosis offers a unique opportunity to identify genes likely important for adult organ-specific stem cell development. We have cloned and characterized the ectopic viral integration site 1 (EVI) and its variant myelodysplastic syndrome 1 (MDS)/EVI generated via transcription from the upstream MDS promoter and alternative splicing. EVI and MDS/EVI have been implicated in a number of cancers including breast, leukemia, ovarian, and intestinal cancers. We show that EVI and MDS/EVI transcripts are upregulated by T3 in the epithelium but not the rest of the intestine in Xenopus laevis when adult stem cells are forming in the epithelium.

Conclusions/Significance

Our results suggest that EVI and MDS/EVI are likely involved in the development and/or proliferation of newly forming adult intestinal epithelial cells.

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.

HNF1β is essential for nephron segmentation during nephrogenesis

Nephrons comprise a blood filter and an epithelial tubule that is subdivided into proximal and distal segments, but what directs this patterning during kidney organogenesis is not well understood. Using zebrafish, we found that the HNF1β paralogues hnf1ba and hnf1bb, which encode homeodomain transcription factors, are essential for normal segmentation of nephrons. Embryos deficient in hnf1ba and hnf1bb did not express proximal and distal segment markers, yet still developed an epithelial tubule. Initiating hnf1ba/b expression required Pax2a and Pax8, but hnf1ba/b-deficient embryos did not exhibit the expected downregulation of pax2a and pax8 at later stages of development, suggesting complex regulatory loops involving these molecules. Embryos deficient in hnf1ba/b also did not express the irx3b transcription factor, which is responsible for differentiation of the first distal tubule segment. Reciprocally, embryos deficient in irx3b exhibited downregulation of hnf1ba/b transcripts in the distal early segment, suggesting a segment-specific regulatory circuit. Deficiency of hnf1ba/b also led to ectopic expansion of podocytes into the proximal tubule domain. Epistasis experiments showed that the formation of podocytes required wt1a, which encodes the Wilms' tumor suppressor-1 transcription factor, and rbpj, which encodes a mediator of canonical Notch signaling, downstream or parallel to hnf1ba/b. Taken together, these results suggest that Hnf1β factors are essential for normal segmentation of nephrons during kidney organogenesis.

The Prdm family: expanding roles in stem cells and development

Members of the Prdm family are characterized by an N-terminal PR domain that is related to the SET methyltransferase domain, and multiple zinc fingers that mediate sequence-specific DNA binding and protein-protein interactions. Prdm factors either act as direct histone methyltransferases or recruit a suite of histone-modifying enzymes to target promoters. In this way, they function in many developmental contexts to drive and maintain cell state transitions and to modify the activity of developmental signalling pathways. Here, we provide an overview of the structure and function of Prdm family members and discuss the roles played by these proteins in stem cells and throughout development.

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.

PRDM16 expression in the developing mouse embryo

PRDM16 is a member of the PR domain-containing protein family and is associated with various disease states including myelodysplastic syndrome and adult T-cell leukemia, as well as developmental abnormalities such as cleft palate. It is also known to act as a regulator of cell differentiation. Expression analysis of PRDM16 is limited, especially within the developing embryo. The current study evaluated the temporal and spatial localization of PRDM16 during early mouse development (embryonic days 8.5-14.5). PRDM16 was first detected on E9.5 in a limited number of tissues and by E14.5, was expressed in a broad range of developing tissues including those of the brain, lung, kidney, and gastrointestinal tract. The expression pattern is consistent with a role for PRDM16 in the development of multiple tissues. Collectively, these studies are the first to characterize the expression of the PRDM16 gene during early murine development.

Serotonin 2B receptor signaling is required for craniofacial morphogenesis and jaw joint formation in Xenopus

Serotonin (5-HT) is a neuromodulator that plays many different roles in adult and embryonic life. Among the 5-HT receptors, 5-HT2B is one of the key mediators of 5-HT functions during development. We used Xenopus laevis as a model system to further investigate the role of 5-HT2B in embryogenesis, focusing on craniofacial development. By means of gene gain- and loss-of-function approaches and tissue transplantation assays, we demonstrated that 5-HT2B modulates, in a cell-autonomous manner, postmigratory skeletogenic cranial neural crest cell (NCC) behavior without altering early steps of cranial NCC development and migration. 5-HT2B overexpression induced the formation of an ectopic visceral skeletal element and altered the dorsoventral patterning of the branchial arches. Loss-of-function experiments revealed that 5-HT2B signaling is necessary for jaw joint formation and for shaping the mandibular arch skeletal elements. In particular, 5-HT2B signaling is required to define and sustain the Xbap expression necessary for jaw joint formation. To shed light on the molecular identity of the transduction pathway acting downstream of 5-HT2B, we analyzed the function of phospholipase C beta 3 (PLC) in Xenopus development and showed that PLC is the effector of 5-HT2B during craniofacial development. Our results unveiled an unsuspected role of 5-HT2B in craniofacial development and contribute to our understanding of the interactive network of patterning signals that is involved in the development and evolution of the vertebrate mandibular arch.

Zinc finger transcription factor ecotropic viral integration site 1 is induced by all-trans retinoic acid (ATRA) and acts as a dual modulator of the ATRA response

Ecotropic viral integration site 1 (EVI1) plays important roles in leukaemia and development, and its expression is temporally and spatially highly restricted during the latter process. Nevertheless, the only physiological agent that to date has been shown to regulate transcription of this gene in mammalian cells is all-trans retinoic acid. Here we describe the identification of a retinoic acid response element that was located in the most distal of several alternative first exons of the human EVI1 gene and was constitutively bound by canonical retinoid receptors in NTERA-2 teratocarcinoma cells. Furthermore, it was the target of negative feedback by EVI1 on the induction of its own promoter by retinoic acid. This process required a previously described transcription repression domain of EVI1. Extending its role as a modulator of the retinoic acid response, EVI1 had the opposite effect on the RARβ retinoic acid response element, whose induction by all-trans retinoic acid it enhanced through a mechanism that involved almost all of its known functional domains. Augmentation of the retinoic acid response by EVI1 was also observed for the endogenous RARβ gene. Thus, we have established EVI1 as a novel type of modulator of the retinoic acid response, which can both enhance and repress induction by this agent in a promoter-specific manner.

The zebrafish pronephros: a model to study nephron segmentation

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

The MDS1-EVI1 gene complex as a retrovirus integration site: impact on behavior of hematopoietic cells and implications for gene therapy

Gene therapy trials have been performed with virus-based vectors that have the ability to integrate permanently into genomic DNA and thus allow prolonged expression of corrective genes after transduction of hematopoietic stem and progenitor cells. Adverse events observed during the X-linked severe combined immunodeficiency gene therapy trial revealed a significant risk of genotoxicity related to retrovirus vector integration and activation of adjacent proto-oncogenes, with several cases of T-cell leukemia linked to vector activation of the LMO2 gene. In patients with chronic granulomatous disease (CGD), rhesus macaques, and mice receiving hematopoietic stem and progenitor cells transduced with retrovirus vectors, a highly non-random pattern of vector integration has been reported. The most striking finding has been overrepresentation of integrations in one specific genomic locus, a complex containing the MDS1 and the EVI1 genes. Most evidence suggests that this overrepresentation is primarily due to a modification of primitive myeloid cell behavior by overexpression of EVI1 or MDS1-EVI1, as opposed to a specific predilection for integration at this site. Three different proteins can be produced from this complex locus: MDS1, MDS1-EVI1, and EVI1. This review will summarize current knowledge regarding this locus and its gene products, with specific focus on issues with relevance to gene therapy, leukemogenesis, and hematopoiesis. Insights into the mechanisms that result in altered hematopoiesis and leukemogenesis when this locus is dysregulated could improve the safety of gene therapy in the future.

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.

Zebrafish Hoxb1a regulates multiple downstream genes including prickle1b

Despite 30 years of Hox gene study, we have a remarkably limited knowledge of the downstream target genes that Hox transcription factors regulate to confer regional identity. Here, we have used a microarray approach to identify genes that function downstream of a single vertebrate Hox gene, zebrafish hoxb1a. This gene plays a critical and conserved role in vertebrate hindbrain development, conferring identity to hindbrain rhombomere (r) 4. For example, zebrafish Hoxb1a, similar to mouse Hoxb1, is required for the migration of r4-derived facial branchiomotor neurons into the posterior hindbrain. We have screened microarrays carrying more than 16,000 expressed sequence tags (ESTs) for genes that are differentially regulated in normal versus Hoxb1a-deficient r4 tissue. Using this approach, we have identified both positively and negatively regulated candidate Hoxb1a target genes. We have used in situ hybridization to validate twelve positively regulated Hoxb1a targets. These downstream targets are expressed in a variety of subdomains within r4, with one gene, a novel prickle homolog (pk1b), expressed specifically within the facial branchiomotor neurons. Using morpholino knock-down and cell transplantation, we demonstrate that the Hoxb1a target Prickle1b functions cell-autonomously to control facial neuron migration, a single aspect of r4 identity.

The oncogene and developmental regulator EVI1: expression, biochemical properties, and biological functions

The EVI1 gene codes for a zinc finger transcription factor with important roles both in normal development and in leukemogenesis. Transcriptional activation of this gene through chromosome rearrangements or other, yet to be identified mechanisms leads to particularly aggressive forms of human myeloid leukemia. In vitro as well as in animal model systems, EVI1 affected cellular proliferation, differentiation, and apoptosis in cell type specific ways. Retroviral integrations into the EVI1 locus provided cells with increased abilities to engraft, survive, and proliferate in bone marrow transplantation experiments. Experimental overexpression of EVI1 by itself was insufficient to cause leukemia in animal model systems, but it cooperated with other genes in this process. This review summarizes the currently available experimental evidence for the proposed biochemical and biological functions of this important oncogene.

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.

PRDM16/MEL1: a novel Smad binding protein expressed in murine embryonic orofacial tissue

TGFbeta signaling regulates central cellular processes such as proliferation and extracellular matrix production during development of the orofacial region. Extracellular TGFbeta binds to cell surface receptors to activate the nucleocytoplasmic Smad proteins that, along with other transcription factors and cofactors, bind specific DNA sequences in the promoters of target genes to regulate their expression. To determine the identity of Smad binding proteins that regulate TGFbeta signaling in developing murine orofacial tissue, a yeast two-hybrid screening approach was employed. The PR-domain containing protein, PRDM16/MEL1 was identified as a novel Smad binding protein. The interaction between PRDM16/MEL1 and Smad 3 was confirmed by GST pull-down assays. The expression of PRDM16/MEL1 was detected in developing orofacial tissue by both Northern blot and in situ hybridization. PRDM16/MEL1 was constitutively expressed in orofacial tissue on E12.5-E14.5 as well as other embryonic tissues such as heart, brain, liver, and limb buds. Taken together, these results demonstrate that PRDM16/MEL1 is a Smad binding protein that may be important for development of orofacial structures through modulation of the TGFbeta signaling pathway.

XSip1 neuralizing activity involves the co-repressor CtBP and occurs through BMP dependent and independent mechanisms

The DNA-binding transcription factor Smad-interacting protein-1 (Sip1) (also named Zfhx1b/ZEB2) plays essential roles in vertebrate embryogenesis. In Xenopus, XSip1 is essential at the gastrula stage for neural tissue formation, but the precise molecular mechanisms that underlie this process have not been fully identified yet. Here we show that XSip1 functions as a transcriptional repressor during neural induction. We observed that constitutive activation of BMP signaling prevents neural induction by XSip1 but not the inhibition of several epidermal genes. We provide evidence that XSip1 binds directly to the BMP4 proximal promoter and modulates its activity. Finally, by deletion and mutational analysis, we show that XSip1 possesses multiple repression domains and that CtBPs contribute to its repression activity. Consistent with this, interference with XCtBP function reduced XSip1 neuralizing activity. These results suggest that Sip1 acts in neural tissue formation through direct repression of BMP4 but that BMP-independent mechanisms are involved as well. Our data also provide the first demonstration of the importance of CtBP binding in Sip1 transcriptional activity in vivo.

Notch signaling controls the differentiation of transporting epithelia and multiciliated cells in the zebrafish pronephros

Epithelial tubules consist of multiple cell types that are specialized for specific aspects of organ function. In the zebrafish pronephros, multiciliated cells (MCCs) are specialized for fluid propulsion, whereas transporting epithelial cells recover filtered-blood solutes. These cell types are distributed in a ;salt-and-pepper' fashion in the pronephros, suggesting that a lateral inhibition mechanism may play a role in their differentiation. We find that the Notch ligand Jagged 2 is expressed in MCCs and that notch3 is expressed in pronephric epithelial cells. Morpholino knockdown of either jagged 2 or notch3, or mutation in mind bomb (in which Notch signaling is impaired), dramatically expands ciliogenic gene expression, whereas ion transporter expression is lost, indicating that pronephric cells are transfated to MCCs. Conversely, ectopic expression of the Notch1a intracellular domain represses MCC differentiation. Gamma-secretase inhibition using DAPT demonstrated a requirement for Notch signaling early in pronephric development, before the pattern of MCC differentiation is apparent. Strikingly, we find that jagged 2 knockdown generates extra cilia and is sufficient to rescue the kidney cilia mutant double bubble. Our results indicate that Jagged 2/Notch signaling modulates the number of multiciliated versus transporting epithelial cells in the pronephros by way of a genetic pathway involving repression of rfx2, a key transcriptional regulator of the ciliogenesis program.

Conservation and expression of a novel alternatively spliced Evi1 exon

The Evi1 transcriptional repressor protein is expressed in a developmentally regulated manner, is essential for normal development, participates in regulating cell proliferation and differentiation of cells of haemopoietic and neuronal origin and contributes to the progression of leukaemia. In this report we describe a new murine Evi1 gene transcript (Delta105) that contains two alternatively spliced regions encoding a 9 amino acid insertion (Rp+9) within the repressor domain (Rp) and a 105 amino acid C-terminal truncation. Abundant levels of the 105 amino acid truncated protein are observed in murine leukaemia cells. The combined primary sequence alterations do not affect the DNA binding, transcriptional repressor or CtBP2 protein binding properties of Evi1 but they do reduce its transforming and cell proliferation stimulating activities. Reduced transforming activity is most likely due to the C-terminal truncation as the activity of Evi1 containing either Rp or Rp+9 is indistinguishable. Both isoforms exist in all murine tissues and cell lines examined. However, only the Rp+9 alternative splice variant is also found in humans and other vertebrates. Murine and human forms of Evi1 with Rp or Rp+9 exist. The additional 9 amino acids are encoded by a conserved 27 nucleotide exon, the overall structural organisation of the gene being preserved in the two species. The function of the Rp+9 and Delta105 splice variants is unknown although the conservation of Rp+9 throughout evolution in vertebrate species suggests it is essential to the broad spectrum of biological activities attributed to this developmentally essential protein.

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.

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

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