Transcription factor Tcf21 modulates urinary bladder size and differentiation.

Urinary bladder organogenesis requires coordinated cell growth, specification, and patterning of both mesenchymal and epithelial compartments. Tcf21, a gene that encodes a helix-loop-helix transcription factor, is specifically expressed in the mesenchyme of the bladder during development. Here we show that Tcf21 is required for normal development of the bladder. We found that the bladders of mice lacking Tcf21 were notably hypoplastic and that the Tcf21 mutant mesenchyme showed increased apoptosis. There was also a marked delay in the formation of visceral smooth muscle, accompanied by a defect in myocardin (Myocd) expression. Interestingly, there was also a marked delay in the formation of the basal cell layer of the urothelium, distinguished by diminished expression of Krt5 and Krt14. Our findings suggest that Tcf21 regulates the survival and differentiation of mesenchyme cell-autonomously and the maturation of the adjacent urothelium non-cell-autonomously during bladder development.

SpDamID: Marking DNA Bound by Protein Complexes Identifies Notch-Dimer Responsive Enhancers.

We developed Split DamID (SpDamID), a protein complementation version of DamID, to mark genomic DNA bound in vivo by interacting or juxtapositioned transcription factors. Inactive halves of DAM (DNA adenine methyltransferase) were fused to protein pairs to be queried. Either direct interaction between proteins or proximity enabled DAM reconstitution and methylation of adenine in GATC. Inducible SpDamID was used to analyze Notch-mediated transcriptional activation. We demonstrate that Notch complexes label RBP sites broadly across the genome and show that a subset of these complexes that recruit MAML and p300 undergo changes in chromatin accessibility in response to Notch signaling. SpDamID differentiates between monomeric and dimeric binding, thereby allowing for identification of half-site motifs used by Notch dimers. Motif enrichment of Notch enhancers coupled with SpDamID reveals co-targeting of regulatory sequences by Notch and Runx1. SpDamID represents a sensitive and powerful tool that enables dynamic analysis of combinatorial protein-DNA transactions at a genome-wide level.

TRIB2 acts downstream of Wnt/TCF in liver cancer cells to regulate YAP and C/EBPα function.

Dysregulation of Wnt signaling is closely associated with human liver tumorigenesis. However, liver cancer-specific Wnt transcriptional programs and downstream effectors remain poorly understood. Here, we identify tribbles homolog 2 (TRIB2) as a direct target of Wnt/TCF in liver cancer and demonstrate that transcription of Wnt target genes, including TRIB2, is coordinated by the TCF and FoxA transcription factors in liver cancer cells. We show that Wnt-TRIB2 activation is critical for cancer cell survival and transformation. Mechanistically, TRIB2 promotes protein stabilization of the YAP transcription coactivator through interaction with the ßTrCP ubiquitin ligase. Furthermore, we find that TRIB2 relieves the liver tumor suppressor protein C/EBPα-mediated inhibition of YAP/TEAD transcriptional activation in liver cancer cells. Altogether, our study uncovers a regulatory mechanism underlying liver cancer-specific Wnt transcriptional output, and suggests that TRIB2 functions as a signaling nexus to integrate the Wnt/ß-catenin, Hippo/YAP, and C/EBPα pathways in cancer cells.

Transcriptional regulatory control of mammalian nephron progenitors revealed by multi-factor cistromic analysis and genetic studies.

Nephron progenitor number determines nephron endowment; a reduced nephron count is linked to the onset of kidney disease. Several transcriptional regulators including Six2, Wt1, Osr1, Sall1, Eya1, Pax2, and Hox11 paralogues are required for specification and/or maintenance of nephron progenitors. However, little is known about the regulatory intersection of these players. Here, we have mapped nephron progenitor-specific transcriptional networks of Six2, Hoxd11, Osr1, and Wt1. We identified 373 multi-factor associated 'regulatory hotspots' around genes closely associated with progenitor programs. To examine their functional significance, we deleted 'hotspot' enhancer elements for Six2 and Wnt4. Removal of the distal enhancer for Six2 leads to a ~40% reduction in Six2 expression. When combined with a Six2 null allele, progeny display a premature depletion of nephron progenitors. Loss of the Wnt4 enhancer led to a significant reduction of Wnt4 expression in renal vesicles and a mildly hypoplastic kidney, a phenotype also enhanced in combination with a Wnt4 null mutation. To explore the regulatory landscape that supports proper target gene expression, we performed CTCF ChIP-seq to identify insulator-boundary regions. One such putative boundary lies between the Six2 and Six3 loci. Evidence for the functional significance of this boundary was obtained by deep sequencing of the radiation-induced Brachyrrhine (Br) mutant allele. We identified an inversion of the Six2/Six3 locus around the CTCF-bound boundary, removing Six2 from its distal enhancer regulation, but placed next to Six3 enhancer elements which support ectopic Six2 expression in the lens where Six3 is normally expressed. Six3 is now predicted to fall under control of the Six2 distal enhancer. Consistent with this view, we observed ectopic Six3 in nephron progenitors. 4C-seq supports the model for Six2 distal enhancer interactions in wild-type and Br/+ mouse kidneys. Together, these data expand our view of the regulatory genome and regulatory landscape underpinning mammalian nephrogenesis.

Hnf4a Is Required for the Development of Cdh6-Expressing Progenitors into Proximal Tubules in the Mouse Kidney.

BACKGROUND: Hepatocyte NF 4α (Hnf4a) is a major regulator of renal proximal tubule (PT) development. In humans, a mutation in HNF4A impairs PT functions and is associated with Fanconi renotubular syndrome (FRTS). In mice, mosaic deletion of Hnf4a in the developing kidney reduces the population of PT cells, leading to FRTS-like symptoms. The molecular mechanisms underlying the role of Hnf4a in PT development remain unclear. METHODS: The gene deletion tool Osr2Cre removed Hnf4a in developing nephrons in mice, generating a novel model for FRTS. Immunofluorescence analysis characterized the mutant phenotype, and lineage analysis tested whether Cadherin-6 (Cdh6)-expressing cells are PT progenitors. Genome-wide mapping of Hnf4a binding sites and differential gene analysis of Hnf4a mutant kidneys identified direct target genes of Hnf4a. RESULTS: Deletion of Hnf4a with Osr2Cre led to the complete loss of mature PT cells, lethal to the Hnf4a mutant mice. Cdh6high, lotus tetragonolobus lectin-low (LTLlow) cells serve as PT progenitors and demonstrate higher proliferation than Cdh6low, LTLhigh differentiated PT cells. Additionally, Hnf4a is required for PT progenitors to differentiate into mature PT cells. Genomic analyses revealed that Hnf4a directly regulates the expression of genes involved in transmembrane transport and metabolism. CONCLUSIONS: Hnf4a promotes the differentiation of PT progenitors into mature PT cells by regulating the expression of genes associated with reabsorption, the major function of PT cells.

Notch is required for the formation of all nephron segments and primes nephron progenitors for differentiation.

Notch signaling plays important roles during mammalian nephrogenesis. To investigate whether Notch regulates nephron segmentation, we performed Notch loss-of-function and gain-of-function studies in developing nephrons in mice. Contrary to the previous notion that Notch signaling promotes the formation of proximal tubules and represses the formation of distal tubules in the mammalian nephron, we show that inhibition of Notch blocks the formation of all nephron segments and that constitutive activation of Notch in developing nephrons does not promote or repress the formation of a specific segment. Cells lacking Notch fail to form the S-shaped body and show reduced expression of Lhx1 and Hnf1b Consistent with this, we find that constitutive activation of Notch in mesenchymal nephron progenitors causes ectopic expression of Lhx1 and Hnf1b and that these cells eventually form a heterogeneous population that includes proximal tubules and other types of cells. Our data suggest that Notch signaling is required for the formation of all nephron segments and that it primes nephron progenitors for differentiation rather than directing their cell fates into a specific nephron segment.

Repression of hepatocyte nuclear factor 4 alpha by AP-1 underlies dyslipidemia associated with retinoic acid.

All-trans retinoic acid (atRA) is used to treat certain cancers and dermatologic diseases. A common adverse effect of atRA is hypercholesterolemia; cytochrome P450 (CYP) 7A repression is suggested as a driver. However, the underlying molecular mechanisms remain unclear. We investigated CYP7A1 expression in the presence of atRA in human hepatocytes and hepatic cell lines. In HepaRG cells, atRA increased cholesterol levels dose-dependently alongside dramatic decreases in CYP7A1 expression. Lentiviral-mediated CYP7A1 overexpression reversed atRA-induced cholesterol accumulation, suggesting that CYP7A1 repression mediated cholesterol accumulation. In CYP7A1 promoter reporter assays and gene-knockdown studies, altered binding of hepatocyte nuclear factor 4 α (HNF4α) to the proximal promoter was essential for atRA-mediated CYP7A1 repression. Pharmacologic inhibition of c-Jun N-terminal kinase (JNK) and ERK pathways attenuated atRA-mediated CYP7A1 repression and cholesterol accumulation. Overexpression of AP-1 (c-Jun/c-Fos), a downstream target of JNK and ERK, repressed CYP7A1 expression. In DNA pull-down and chromatin immunoprecipitation assays, AP-1 exhibited sequence-specific binding to the proximal CYP7A1 promoter region overlapping the HNF4α binding site, and atRA increased AP-1 but decreased HNF4α recruitment to the promoter. Collectively, these results indicate that atRA activates JNK and ERK pathways and the downstream target AP-1 represses HNF4α transactivation of the CYP7A1 promoter, potentially responsible for hypercholesterolemia.

Notch signaling promotes nephrogenesis by downregulating Six2.

During nephrogenesis, multipotent mesenchymal nephron progenitors develop into distinct epithelial segments. Each nephron segment has distinct cell types and physiological function. In the current model of kidney development, Notch signaling promotes the formation of proximal tubules and represses the formation of distal tubules. Here, we present a novel role of Notch in nephrogenesis. We show in mice that differentiation of nephron progenitors requires downregulation of Six2, a transcription factor required for progenitor maintenance, and that Notch signaling is necessary and sufficient for Six2 downregulation. Furthermore, we find that nephron progenitors lacking Notch signaling fail to differentiate into any nephron segments, not just proximal tubules. Our results demonstrate how cell fates of progenitors are regulated by a transcription factor governing progenitor status and by a differentiation signal in nephrogenesis.

Osr1 acts downstream of and interacts synergistically with Six2 to maintain nephron progenitor cells during kidney organogenesis.

Mammalian kidney organogenesis involves reciprocal epithelial-mesenchymal interactions that drive iterative cycles of nephron formation. Recent studies have demonstrated that the Six2 transcription factor acts cell autonomously to maintain nephron progenitor cells, whereas canonical Wnt signaling induces nephron differentiation. How Six2 maintains the nephron progenitor cells against Wnt-directed commitment is not well understood, however. We report here that Six2 is required to maintain expression of Osr1, a homolog of the Drosophila odd-skipped zinc-finger transcription factor, in the undifferentiated cap mesenchyme. Tissue-specific inactivation of Osr1 in the cap mesenchyme caused premature depletion of nephron progenitor cells and severe renal hypoplasia. We show that Osr1 and Six2 act synergistically to prevent premature differentiation of the cap mesenchyme. Furthermore, although both Six2 and Osr1 could form protein interaction complexes with TCF proteins, Osr1, but not Six2, enhances TCF interaction with the Groucho family transcriptional co-repressors. Moreover, we demonstrate that loss of Osr1 results in ß-catenin/TCF-mediated ectopic activation of Wnt4 enhancer-driven reporter gene expression in the undifferentiated nephron progenitor cells in vivo. Together, these data indicate that Osr1 plays crucial roles in Six2-dependent maintenance of nephron progenitors during mammalian nephrogenesis by stabilizing TCF-Groucho transcriptional repressor complexes to antagonize Wnt-directed nephrogenic differentiation.

Single cell dissection of early kidney development: multilineage priming.

We used a single cell RNA-seq strategy to create an atlas of gene expression patterns in the developing kidney. At several stages of kidney development, histologically uniform populations of cells give rise to multiple distinct lineages. We performed single cell RNA-seq analysis of total mouse kidneys at E11.5 and E12.5, as well as the renal vesicles at P4. We define an early stage of progenitor cell induction driven primarily by gene repression. Surprising stochastic expression of marker genes associated with differentiated cell types was observed in E11.5 progenitors. We provide a global view of the polarized gene expression already present in the renal vesicle, the first epithelial precursor of the nephron. We show that Hox gene read-through transcripts can be spliced to produce intergenic homeobox swaps. We also identify a surprising number of genes with partially degraded noncoding RNA. Perhaps most interesting, at early developmental times single cells often expressed genes related to several developmental pathways. This provides powerful evidence that initial organogenesis involves a process of multilineage priming. This is followed by a combination of gene repression, which turns off the genes associated with most possible lineages, and the activation of increasing numbers of genes driving the chosen developmental direction.

Etv2 and fli1b function together as key regulators of vasculogenesis and angiogenesis.

OBJECTIVE: The E26 transformation-specific domain transcription factor Etv2/Etsrp/ER71 is a master regulator of vascular endothelial differentiation during vasculogenesis, although its later role in sprouting angiogenesis remains unknown. Here, we investigated in the zebrafish model a role for Etv2 and related E26 transformation-specific factors, Fli1a and Fli1b in developmental angiogenesis. APPROACH AND RESULTS: Zebrafish fli1a and fli1b mutants were obtained using transposon-mediated gene trap approach. Individual fli1a and fli1b homozygous mutant embryos display normal vascular patterning, yet the angiogenic recovery observed in older etv2 mutant embryos does not occur in embryos lacking both etv2 and fli1b. Etv2 and fli1b double-deficient embryos fail to form any angiogenic sprouts and show greatly increased apoptosis throughout the axial vasculature. In contrast, fli1a mutation did not affect the recovery of etv2 mutant phenotype. Overexpression analyses indicate that both etv2 and fli1b, but not fli1a, induce the expression of multiple vascular markers and of each other. Temporal inhibition of Etv2 function using photoactivatable morpholinos indicates that the function of Etv2 and Fli1b during angiogenesis is independent from the early requirement of Etv2 during vasculogenesis. RNA-Seq analysis and chromatin immunoprecipitation suggest that Etv2 and Fli1b share the same transcriptional targets and bind to the same E26 transformation-specific sites. CONCLUSIONS: Our data argue that there are 2 phases of early vascular development with distinct requirements of E26 transformation-specific transcription factors. Etv2 alone is required for early vasculogenesis, whereas Etv2 and Fli1b function redundantly during late vasculogenesis and early embryonic angiogenesis.

Hedgehog signaling is required for the maintenance of mesenchymal nephron progenitors.

Background: Mesenchymal nephron progenitors (mNPs) give rise to all nephron tubules in the mammalian kidney. Since premature depletion of these cells leads to low nephron numbers, high blood pressure, and various renal diseases, it is critical to understand how mNPs are maintained. While Fgf, Bmp, and Wnt signaling pathways are known to be required for the maintenance of these cells, it is unclear if any other signaling pathways also play roles. Methods: To test the potential role of Hedgehog signaling in mNPs, we conditionally deleted Shh from the collecting duct and Smo from the nephron lineage. To identify the genes regulated by Hedgehog signaling in mNPs, we performed RNA-seq analysis from mNPs with different Smo doses. To test if the upregulation of Notch signaling mimics loss of Hedgehog signaling, we performed Jag1 gain-of-function study in mNPs. Results: We found that loss of either Shh or Smo resulted in premature depletion of mNPs. Our transcriptional profiling data from Smo loss- and gain-of-function mutant mNPs suggested that Hedgehog signaling inhibited the activation of Notch signaling and upregulated the expression of Fox transcription factors such as Foxc1 and Foxp4. Consistent with these observations, we found that ectopic expression of Jag1 caused the premature depletion of mNPs as seen in the Smo mutant kidney. We also found that Foxc1 was capable of binding to mitotic condensed chromatin, a feature of a mitotic bookmarking factor. Conclusions: Our study demonstrates a previously unappreciated role of Hedgehog signaling in preventing premature depletion of mNPs by repressing Notch signaling and likely by activating the expression of Fox factors.

Single-cell sequencing dissects the transcriptional identity of activated fibroblasts and identifies novel persistent distal tubular injury patterns in kidney fibrosis.

Examining kidney fibrosis is crucial for mechanistic understanding and developing targeted strategies against chronic kidney disease (CKD). Persistent fibroblast activation and tubular epithelial cell (TEC) injury are key CKD contributors. However, cellular and transcriptional landscapes of CKD and specific activated kidney fibroblast clusters remain elusive. Here, we analyzed single cell transcriptomic profiles of two clinically relevant kidney fibrosis models which induced robust kidney parenchymal remodeling. We dissected the molecular and cellular landscapes of kidney stroma and newly identified three distinctive fibroblast clusters with "secretory", "contractile" and "vascular" transcriptional enrichments. Also, both injuries generated failed repair TECs (frTECs) characterized by decline of mature epithelial markers and elevation of stromal and injury markers. Notably, frTECs shared transcriptional identity with distal nephron segments of the embryonic kidney. Moreover, we identified that both models exhibited robust and previously unrecognized distal spatial pattern of TEC injury, outlined by persistent elevation of renal TEC injury markers including Krt8 and Vcam1, while the surviving proximal tubules (PTs) showed restored transcriptional signature. We also found that long-term kidney injuries activated a prominent nephrogenic signature, including Sox4 and Hox gene elevation, which prevailed in the distal tubular segments. Our findings might advance understanding of and targeted intervention in fibrotic kidney disease.

Wnt9b plays a central role in the regulation of mesenchymal to epithelial transitions underlying organogenesis of the mammalian urogenital system.

The vertebrate urogenital system forms due to inductive interactions between the Wolffian duct, its derivative the ureteric bud, and their adjacent mesenchymes. These establish epithelial primordia within the mesonephric (embryonic) and metanephric (adult) kidneys and the Müllerian duct, the anlage of much of the female reproductive tract. We show that Wnt9b is expressed in the inductive epithelia and is essential for the development of mesonephric and metanephric tubules and caudal extension of the Müllerian duct. Wnt9b is required for the earliest inductive response in metanephric mesenchyme. Further, Wnt9b-expressing cells can functionally substitute for the ureteric bud in these interactions. Wnt9b acts upstream of another Wnt, Wnt4, in this process, and our data implicate canonical Wnt signaling as one of the major pathways in the organization of the mammalian urogenital system. Together these findings suggest that Wnt9b is a common organizing signal regulating diverse components of the mammalian urogenital system.

ß-catenin regulates the formation of multiple nephron segments in the mouse kidney.

The nephron is composed of distinct segments that perform unique physiological functions. Little is known about how multipotent nephron progenitor cells differentiate into different nephron segments. It is well known that ß-catenin signaling regulates the maintenance and commitment of mesenchymal nephron progenitors during kidney development. However, it is not fully understood how it regulates nephron segmentation after nephron progenitors undergo mesenchymal-to-epithelial transition. To address this, we performed ß-catenin loss-of-function and gain-of-function studies in epithelial nephron progenitors in the mouse kidney. Consistent with a previous report, the formation of the renal corpuscle was defective in the absence of ß-catenin. Interestingly, we found that epithelial nephron progenitors lacking ß-catenin were able to form presumptive proximal tubules but that they failed to further develop into differentiated proximal tubules, suggesting that ß-catenin signaling plays a critical role in proximal tubule development. We also found that epithelial nephron progenitors lacking ß-catenin failed to form the distal tubules. Expression of a stable form of ß-catenin in epithelial nephron progenitors blocked the proper formation of all nephron segments, suggesting tight regulation of ß-catenin signaling during nephron segmentation. This work shows that ß-catenin regulates the formation of multiple nephron segments along the proximo-distal axis of the mammalian nephron.

Nephron progenitor commitment is a stochastic process influenced by cell migration.

Progenitor self-renewal and differentiation is often regulated by spatially restricted cues within a tissue microenvironment. Here, we examine how progenitor cell migration impacts regionally induced commitment within the nephrogenic niche in mice. We identify a subset of cells that express Wnt4, an early marker of nephron commitment, but migrate back into the progenitor population where they accumulate over time. Single cell RNA-seq and computational modelling of returning cells reveals that nephron progenitors can traverse the transcriptional hierarchy between self-renewal and commitment in either direction. This plasticity may enable robust regulation of nephrogenesis as niches remodel and grow during organogenesis.

Hnf4a deletion in the mouse kidney phenocopies Fanconi renotubular syndrome.

Different nephron tubule segments perform distinct physiological functions, collectively acting as a blood filtration unit. Dysfunction of the proximal tubule segment can lead to Fanconi renotubular syndrome (FRTS), with major symptoms such as excess excretion of water, glucose, and phosphate in the urine. It has been shown that a mutation in HNF4A is associated with FRTS in humans and that Hnf4a is expressed specifically in proximal tubules in adult rat nephrons. However, little is known about the role of Hnf4a in nephrogenesis. Here, we found that Hnf4a is expressed in both presumptive and differentiated proximal tubules in the developing mouse kidney. We show that Hnf4a is required for the formation of differentiated proximal tubules but is dispensable for the formation of presumptive proximal tubules. Furthermore, we show that loss of Hnf4a decreased the expression of proximal tubule-specific genes. Adult Hnf4a mutant mice presented with FRTS-like symptoms, including polyuria, polydipsia, glycosuria, and phosphaturia. Analysis of the adult Hnf4a mutant kidney also showed proximal tubule dysgenesis and nephrocalcinosis. Our results demonstrate the critical role of Hnf4a in proximal tubule development and provide mechanistic insight into the etiology of FRTS.

Defective transcription elongation in a subset of cancers confers immunotherapy resistance.

The nature and role of global transcriptional deregulations in cancers are not fully understood. We report that a large proportion of cancers have widespread defects in mRNA transcription elongation (TE). Cancers with TE defects (TEdeff) display spurious transcription and defective mRNA processing of genes characterized by long genomic length, poised promoters and inducible expression. Signaling pathways regulated by such genes, such as pro-inflammatory response pathways, are consistently suppressed in TEdeff tumors. Remarkably, TEdeff correlates with the poor response and outcome in immunotherapy, but not chemo- or targeted therapy, -treated renal cell carcinoma and metastatic melanoma patients. Forced pharmacologic or genetic induction of TEdeff in tumor cells impairs pro-inflammatory response signaling, and imposes resistance to the innate and adaptive anti-tumor immune responses and checkpoint inhibitor therapy in vivo. Therefore, defective TE is a previously unknown mechanism of tumor immune resistance, and should be assessed in cancer patients undergoing immunotherapy.

YAP/TAZ and Hedgehog Coordinate Growth and Patterning in Gastrointestinal Mesenchyme.

YAP/TAZ are the major mediators of mammalian Hippo signaling; however, their precise function in the gastrointestinal tract remains poorly understood. Here we dissect the distinct roles of YAP/TAZ in endodermal epithelium and mesenchyme and find that, although dispensable for gastrointestinal epithelial development and homeostasis, YAP/TAZ function as the critical molecular switch to coordinate growth and patterning in gut mesenchyme. Our genetic analyses reveal that Lats1/2 kinases suppress expansion of the primitive mesenchymal progenitors, where YAP activation also prevents induction of the smooth muscle lineage through transcriptional repression of Myocardin. During later development, zone-restricted downregulation of YAP/TAZ provides the positional cue and allows smooth muscle cell differentiation induced by Hedgehog signaling. Taken together, our studies identify the mesenchymal requirement of YAP/TAZ in the gastrointestinal tract and highlight the functional interplays between Hippo and Hedgehog signaling underlying temporal and spatial control of tissue growth and specification in developing gut.