Bmp7 drives proximal tubule expansion and determines nephron number in the developing kidney.

The mammalian kidney is composed of thousands of nephrons that are formed through reiterative induction of a mesenchymal-to-epithelial transformation by a population of nephron progenitor cells. The number of nephrons in human kidneys ranges from several hundred thousand to nearly a million, and low nephron number has been implicated as a risk factor for kidney disease as an adult. Bmp7 is among a small number of growth factors required to support the proliferation and self-renewal of nephron progenitor cells, in a process that will largely determine the final nephron number. Once induced, each nephron begins as a simple tubule that undergoes extensive proliferation and segmental differentiation. Bmp7 is expressed both by nephron progenitor cells and the ureteric bud derivative branches that induce new nephrons. Here, we show that, in mice, Bmp7 expressed by progenitor cells has a major role in determining nephron number; nephron number is reduced to one tenth its normal value in its absence. Postnatally, Bmp7 also drives proliferation of the proximal tubule cells, and these ultimately constitute the largest segment of the nephron. Bmp7 appears to act through Smad 1,5,9(8), p38 and JNK MAP kinase. In the absence of Bmp7, nephrons undergo a hypertrophic process that involves p38. Following a global inactivation of Bmp7, we also see evidence for Bmp7-driven growth of the nephron postnatally. Thus, we identify a role for Bmp7 in supporting the progenitor population and driving expansion of nephrons to produce a mature kidney.

Epigenetic transcriptional reprogramming by WT1 mediates a repair response during podocyte injury.

In the context of human disease, the mechanisms whereby transcription factors reprogram gene expression in reparative responses to injury are not well understood. We have studied the mechanisms of transcriptional reprogramming in disease using murine kidney podocytes as a model for tissue injury. Podocytes are a crucial component of glomeruli, the filtration units of each nephron. Podocyte injury is the initial event in many processes that lead to end-stage kidney disease. Wilms tumor-1 (WT1) is a master regulator of gene expression in podocytes, binding nearly all genes known to be crucial for maintenance of the glomerular filtration barrier. Using murine models and human kidney organoids, we investigated WT1-mediated transcriptional reprogramming during the course of podocyte injury. Reprogramming the transcriptome involved highly dynamic changes in the binding of WT1 to target genes during a reparative injury response, affecting chromatin state and expression levels of target genes.

EED, a member of the polycomb group, is required for nephron differentiation and the maintenance of nephron progenitor cells.

Epigenetic regulation of gene expression has a crucial role allowing for the self-renewal and differentiation of stem and progenitor populations during organogenesis. The mammalian kidney maintains a population of self-renewing stem cells that differentiate to give rise to thousands of nephrons, which are the functional units that carry out filtration to maintain physiological homeostasis. The polycomb repressive complex 2 (PRC2) epigenetically represses gene expression during development by placing the H3K27me3 mark on histone H3 at promoter and enhancer sites, resulting in gene silencing. To understand the role of PRC2 in nephron differentiation, we conditionally inactivated the Eed gene, which encodes a nonredundant component of the PRC2 complex, in nephron progenitor cells. Resultant kidneys were smaller and showed premature loss of progenitor cells. The progenitors in Eed mutant mice that were induced to differentiate did not develop into properly formed nephrons. Lhx1, normally expressed in the renal vesicle, was overexpressed in kidneys of Eed mutant mice. Thus, PRC2 has a crucial role in suppressing the expression of genes that maintain the progenitor state, allowing nephron differentiation to proceed.

A homozygous missense variant in VWA2, encoding an interactor of the Fraser-complex, in a patient with vesicoureteral reflux.

Congenital anomalies of the kidney and urinary tract (CAKUT) are the most common cause (40-50%) of chronic kidney disease (CKD) in children. About 40 monogenic causes of CAKUT have so far been discovered. To date less than 20% of CAKUT cases can be explained by mutations in these 40 genes. To identify additional monogenic causes of CAKUT, we performed whole exome sequencing (WES) and homozygosity mapping (HM) in a patient with CAKUT from Indian origin and consanguineous descent. We identified a homozygous missense mutation (c.1336C>T, p.Arg446Cys) in the gene Von Willebrand factor A domain containing 2 (VWA2). With immunohistochemistry studies on kidneys of newborn (P1) mice, we show that Vwa2 and Fraser extracellular matrix complex subunit 1 (Fras1) co-localize in the nephrogenic zone of the renal cortex. We identified a pronounced expression of Vwa2 in the basement membrane of the ureteric bud (UB) and derivatives of the metanephric mesenchyme (MM). By applying in vitro assays, we demonstrate that the Arg446Cys mutation decreases translocation of monomeric VWA2 protein and increases translocation of aggregated VWA2 protein into the extracellular space. This is potentially due to the additional, unpaired cysteine residue in the mutated protein that is used for intermolecular disulfide bond formation. VWA2 is a known, direct interactor of FRAS1 of the Fraser-Complex (FC). FC-encoding genes and interacting proteins have previously been implicated in the pathogenesis of syndromic and/or isolated CAKUT phenotypes in humans. VWA2 therefore constitutes a very strong candidate in the search for novel CAKUT-causing genes. Our results from in vitro experiments indicate a dose-dependent neomorphic effect of the Arg446Cys homozygous mutation in VWA2.

Ectopic Phosphorylated Creb Marks Dedifferentiated Proximal Tubules in Cystic Kidney Disease.

Ectopic cAMP signaling is pathologic in polycystic kidney disease; however, its spatiotemporal actions are unclear. We characterized the expression of phosphorylated Creb (p-Creb), a target and mediator of cAMP signaling, in developing and cystic kidney models. We also examined tubule-specific effects of cAMP analogs in cystogenesis in embryonic kidney explants. In wild-type mice, p-Creb marked nephron progenitors (NP), early epithelial NP derivatives, ureteric bud, and cortical stroma; p-Creb was present in differentiated thick ascending limb of Henle, collecting duct, and stroma; however, it disappeared in mature NP-derived proximal tubules. In Six2cre;Frs2αFl/Fl mice, a renal cystic model, ectopic p-Creb stained proximal tubule-derived cystic segments that lost the differentiation marker lotus tetragonolobus lectin. Furthermore, lotus tetragonolobus lectin-negative/p-Creb-positive cyst segments (re)-expressed Ncam1, Pax2, and Sox9 markers of immature nephron structures and dedifferentiated proximal tubules after acute kidney injury. These dedifferentiation markers were co-expressed with p-Creb in renal cysts in Itf88 knockout mice subjected to ischemia and Six2cre;Pkd1Fl/Fl mice, other renal cystogenesis models. 8-Br-cAMP addition to wild-type embryonic kidney explants induced proximal tubular cystogenesis and p-Creb expression; these effects were blocked by co-addition of protein kinase A inhibitor. Thus p-Creb/cAMP signaling is appropriate in NP and early nephron derivatives, but disappears in mature proximal tubules. Moreover, ectopic p-Creb expression/cAMP signaling marks dedifferentiated proximal tubular cystic segments. Furthermore, proximal tubules are predisposed to become cystic after cAMP stimulation.

WT1 targets Gas1 to maintain nephron progenitor cells by modulating FGF signals.

Development of the metanephric kidney depends on tightly regulated interplay between self-renewal and differentiation of a nephron progenitor cell (NPC) pool. Several key factors required for the survival of NPCs have been identified, including fibroblast growth factor (FGF) signaling and the transcription factor Wilms' tumor suppressor 1 (WT1). Here, we present evidence that WT1 modulates FGF signaling by activating the expression of growth arrest-specific 1 (Gas1), a novel WT1 target gene and novel modulator of FGF signaling. We show that WT1 directly binds to a conserved DNA binding motif within the Gas1 promoter and activates Gas1 mRNA transcription in NPCs. We confirm that WT1 is required for Gas1 expression in kidneys in vivo. Loss of function of GAS1 in vivo results in hypoplastic kidneys with reduced nephron mass due to premature depletion of NPCs. Although kidney development in Gas1 knockout mice progresses normally until E15.5, NPCs show decreased rates of proliferation at this stage and are depleted as of E17.5. Lastly, we show that Gas1 is selectively required for FGF-stimulated AKT signaling in vitro. In summary, our data suggest a model in which WT1 modulates receptor tyrosine kinase signaling in NPCs by directing the expression of Gas1.

Genome-Wide Analysis of Wilms' Tumor 1-Controlled Gene Expression in Podocytes Reveals Key Regulatory Mechanisms.

The transcription factor Wilms' tumor suppressor 1 (WT1) is key to podocyte development and viability; however, WT1 transcriptional networks in podocytes remain elusive. We provide a comprehensive analysis of the genome-wide WT1 transcriptional network in podocytes in vivo using chromatin immunoprecipitation followed by sequencing (ChIPseq) and RNA sequencing techniques. Our data show a specific role for WT1 in regulating the podocyte-specific transcriptome through binding to both promoters and enhancers of target genes. Furthermore, we inferred a podocyte transcription factor network consisting of WT1, LMX1B, TCF21, Fox-class and TEAD family transcription factors, and MAFB that uses tissue-specific enhancers to control podocyte gene expression. In addition to previously described WT1-dependent target genes, ChIPseq identified novel WT1-dependent signaling systems. These targets included components of the Hippo signaling system, underscoring the power of genome-wide transcriptional-network analyses. Together, our data elucidate a comprehensive gene regulatory network in podocytes suggesting that WT1 gene regulatory function and podocyte cell-type specification can best be understood in the context of transcription factor-regulatory element network interplay.

The transcription factor Sry-related HMG box-4 (SOX4) is required for normal renal development in vivo.

BACKGROUND: The DNA-binding transcription factor Wilms' Tumor Suppressor-1 (WT1) plays an essential role in nephron progenitor differentiation during renal development. We previously used Wt1 chromatin-immunoprecipitation coupled to microarray (ChIP-chip) to identify novel Wt1 target genes that may regulate nephrogenesis in vivo. We discovered that all three members of the SoxC subfamily, namely, Sox4, Sox11, and Sox12, are bound by Wt1 in mouse embryonic kidneys in vivo. SoxC genes play master roles in determining neuronal and mesenchymal progenitor cell fate in a multitude of developmental processes, but their function in the developing kidney is largely unknown. RESULTS: Here we show that all three SoxC genes are expressed in the nephrogenic lineages during renal development. Conditional ablation of Sox4 in nephron progenitors and their cellular descendants (Sox4(nephron-) mice) results in a significant reduction in nephron endowment. By postnatal day (P)7, Sox4(nephron-) renal corpuscles exhibit reduced numbers of Wt1+ podocytes together with loss of expression of the slit diaphragm protein nephrin. Sox4(nephron-) mice develop early-onset proteinacious glomerular injury within 2 weeks of birth progressing to end-stage renal failure within 5-9 months. CONCLUSIONS: Collectively, our results demonstrate an essential requirement of Sox4 for normal renal development in vivo.

c-Met and NF-κB-dependent overexpression of Wnt7a and -7b and Pax2 promotes cystogenesis in polycystic kidney disease.

The mechanisms of cystogenesis in autosomal dominant polycystic kidney disease (ADPKD) are not fully understood. Hyperactivation of the tyrosine kinase c-Met contributes to cyst formation, but we do not know the downstream mediators. Here, we found that hyperactivated c-Met led to increased NF-κB signaling, which in turn, drove de novo expression of Wnt7a and overexpression of Wnt7b in Pkd1(-/-) mouse kidneys. Hyperactivated Wnt signaling increased expression of the transcription factor Pax2 in the cells lining cysts. Furthermore, blocking Wnt signaling with DKK1 decreased cyst formation in an organ culture model of ADPKD. In summary, these results suggest that the c-Met/NF-κB/Wnt/Pax2 signaling transduction axis may provide pharmacological targets for the treatment of ADPKD.

WT1-dependent sulfatase expression maintains the normal glomerular filtration barrier.

Paracrine signaling between podocytes and glomerular endothelial cells through vascular endothelial growth factor A (VEGFA) maintains a functional glomerular filtration barrier. Heparan sulfate proteoglycans (HSPGs), located on the cell surface or in the extracellular matrix, bind signaling molecules such as VEGFA and affect their local concentrations, but whether modulation of these moieties promotes normal crosstalk between podocytes and endothelial cells is unknown. Here, we found that the transcription factor Wilms' Tumor 1 (WT1) modulates VEGFA and FGF2 signaling by increasing the expression of the 6-O-endosulfatases Sulf1 and Sulf2, which remodel the heparan sulfate 6-O-sulfation pattern in the extracellular matrix. Mice deficient in both Sulf1 and Sulf2 developed age-dependent proteinuria as a result of ultrastructural abnormalities in podocytes and endothelial cells, a phenotype similar to that observed in children with WT1 mutations and in Wt1(+/-) mice. These kidney defects associated with a decreased distribution of VEGFA in the glomerular basement membrane and on endothelial cells. Collectively, these data suggest that WT1-dependent sulfatase expression plays a critical role in maintaining the glomerular filtration barrier by modulating the bioavailability of growth factors, thereby promoting normal crosstalk between podocytes and endothelial cells.

Expression regulation and function of heparan sulfate 6-O-endosulfatases in the spermatogonial stem cell niche.

Glial cell line-derived neurotrophic factor (GDNF) is a heparan sulfate (HS)-binding factor. GDNF is produced by somatic Sertoli cells, where it signals to maintain spermatogonial stem cells (SSCs) and reproduction. Here, we investigate the roles of extracellular HS 6-O-endosulfatases (Sulfs), Sulf1 and Sulf2, in the matrix transmission of GDNF from Sertoli cells to SSCs. Although Sulfs are not required for testis formation, Sulf deficiency leads to the accelerated depletion of SSCs, a testis phenotype similar to that of GDNF+/- mice. Mechanistically, we show that Sulfs are expressed in GDNF-producing Sertoli cells. In addition, reduced Sulf activity profoundly worsens haplo-deficient GDNF phenotypes in our genetic studies. These findings establish a critical role of Sulfs in promoting GDNF signaling and support a model in which Sulfs regulate the bioavailability of GDNF by enzymatically remodeling HS 6-O-desulfation to release GDNF from matrix sequestration. Further, Sertoli cell-specific transcriptional factor Wilm's tumor 1 (WT1) directly activates the transcription of both Sulf1 and Sulf2 genes. Together, our studies not only identify Sulfs as essential regulators of GDNF signaling in the SSC niche, but also as direct downstream targets of WT1, thus establishing a physiological role of WT1 in Sertoli cells.

Failure to ubiquitinate c-Met leads to hyperactivation of mTOR signaling in a mouse model of autosomal dominant polycystic kidney disease.

Autosomal dominant polycystic kidney disease (ADPKD) is a common inherited disorder that is caused by mutations at two loci, polycystin 1 (PKD1) and polycystin 2 (PKD2). It is characterized by the formation of multiple cysts in the kidneys that can lead to chronic renal failure. Previous studies have suggested a role for hyperactivation of mammalian target of rapamycin (mTOR) in cystogenesis, but the etiology of mTOR hyperactivation has not been fully elucidated. In this report we have shown that mTOR is hyperactivated in Pkd1-null mouse cells due to failure of the HGF receptor c-Met to be properly ubiquitinated and subsequently degraded after stimulation by HGF. In Pkd1-null cells, Casitas B-lineage lymphoma (c-Cbl), an E3-ubiquitin ligase for c-Met, was sequestered in the Golgi apparatus with α3ß1 integrin, resulting in the inability to ubiquitinate c-Met. Treatment of mouse Pkd1-null cystic kidneys in organ culture with a c-Met pharmacological inhibitor resulted in inhibition of mTOR activity and blocked cystogenesis in this mouse model of ADPKD. We therefore suggest that blockade of c-Met is a potential novel therapeutic approach to the treatment of ADPKD.

Genomic characterization of Wilms' tumor suppressor 1 targets in nephron progenitor cells during kidney development.

The Wilms' tumor suppressor 1 (WT1) gene encodes a DNA- and RNA-binding protein that plays an essential role in nephron progenitor differentiation during renal development. To identify WT1 target genes that might regulate nephron progenitor differentiation in vivo, we performed chromatin immunoprecipitation (ChIP) coupled to mouse promoter microarray (ChIP-chip) using chromatin prepared from embryonic mouse kidney tissue. We identified 1663 genes bound by WT1, 86% of which contain a previously identified, conserved, high-affinity WT1 binding site. To investigate functional interactions between WT1 and candidate target genes in nephron progenitors, we used a novel, modified WT1 morpholino loss-of-function model in embryonic mouse kidney explants to knock down WT1 expression in nephron progenitors ex vivo. Low doses of WT1 morpholino resulted in reduced WT1 target gene expression specifically in nephron progenitors, whereas high doses of WT1 morpholino arrested kidney explant development and were associated with increased nephron progenitor cell apoptosis, reminiscent of the phenotype observed in Wt1(-/-) embryos. Collectively, our results provide a comprehensive description of endogenous WT1 target genes in nephron progenitor cells in vivo, as well as insights into the transcriptional signaling networks controlled by WT1 that might direct nephron progenitor fate during renal development.

Coordinate integrin and c-Met signaling regulate Wnt gene expression during epithelial morphogenesis.

Integrin receptors for the extracellular matrix and receptor tyrosine kinase growth factor receptors represent two of the major families of receptors that transduce into cells information about the surrounding environment. Wnt proteins are a major family of signaling molecules that regulate morphogenetic events. There is presently little understanding of how the expression of Wnt genes themselves is regulated. In this study, we demonstrate that alpha3beta1 integrin, a major laminin receptor involved in the development of the kidney, and c-Met, the receptor for hepatocyte growth factor, signal coordinately to regulate the expression of Wnt7b in the mouse. Wnt signals in turn appear to regulate epithelial cell survival in the papilla of the developing kidney, allowing for the elongation of epithelial tubules to form a mature papilla. Together, these results demonstrate how signals from integrins and growth factor receptors can be integrated to regulate the expression of an important family of signaling molecules so as to regulate morphogenetic events.

Angioblast-mesenchyme induction of early kidney development is mediated by Wt1 and Vegfa.

Most studies on kidney development have considered the interaction of the metanephric mesenchyme and the ureteric bud to be the major inductive event that maintains tubular differentiation and branching morphogenesis. The mesenchyme produces Gdnf, which stimulates branching, and the ureteric bud stimulates continued growth of the mesenchyme and differentiation of nephrons from the induced mesenchyme. Null mutation of the Wt1 gene eliminates outgrowth of the ureteric bud, but Gdnf has been identified as a target of Pax2, but not of Wt1. Using a novel system for microinjecting and electroporating plasmid expression constructs into murine organ cultures, it has been demonstrated that Vegfa expression in the mesenchyme is regulated by Wt1. Previous studies had identified a population of Flk1-expressing cells in the periphery of the induced mesenchyme, and adjacent to the stalk of the ureteric bud, and that Vegfa was able to stimulate growth of kidneys in organ culture. Here it is demonstrated that signaling through Flk1 is required to maintain expression of Pax2 in the mesenchyme of the early kidney, and for Pax2 to stimulate expression of Gdnf. However, once Gdnf stimulates branching of the ureteric bud, the Flk1-dependent angioblast signal is no longer required to maintain branching morphogenesis and induction of nephrons. Thus, this work demonstrates the presence of a second set of inductive events, involving the mesenchymal and angioblast populations, whereby Wt1-stimulated expression of Vegfa elicits an as-yet-unidentified signal from the angioblasts, which is required to stimulate the expression of Pax2 and Gdnf, which in turn elicits an inductive signal from the ureteric bud.

Wt1 functions in the development of germ cells in addition to somatic cell lineages of the testis.

The Wilms' tumor suppressor gene, Wt1, encodes a transcription factor critical for development of the urogenital system. To identify lineages within the developing urogenital system that have a cell-autonomous requirement for Wt1, chimeric mice were generated from Wt1-null ES cells. Males with large contributions of Wt1-/- cells showed hypoplastic and dysgenic testes, with seminiferous tubules lacking spermatogonia. Wt1-null cells contributed poorly to both somatic and germ cell lineages within the developing gonad, suggesting an unexpected role for Wt1 in germ cell development in addition to a role in the development of the somatic lineages of the gonad. Wt1 expression was detected in embryonic germ cells beginning at embryonic day 11.5 after migrating primordial germ cells (PGCs) have entered the gonad. Germ cells isolated from Wt1-null embryos showed impaired growth in culture, further demonstrating a role for Wt1 in germ cell proliferation or survival. Therefore, Wt1 plays important, and in some cases previously unrecognized, roles in multiple lineages during urogenital development.

A mammal-specific exon of WT1 is not required for development or fertility.

The WT1 tumor suppressor gene is a zinc finger-containing transcription factor which is required for development of the kidney and gonads. A mammal-specific alternative splicing event within this gene results in the presence or absence of a 17-amino-acid sequence within the WT1 protein. To determine the function of this sequence in vivo, gene targeting was utilized to specifically eliminate the exon encoding this sequence in mice. Mice lacking WT1 exon 5 develop normally. Adult mice lacking this exon are viable and fertile, and females are capable of lactation.