Michal Mokry

Ascl2 Acts as an R-spondin/Wnt-Responsive Switch to Control Stemness in Intestinal Crypts

The Wnt signaling pathway controls stem cell identity in the intestinal epithelium and in many other adult organs. The transcription factor Ascl2 (a Wnt target gene) is a master regulator of intestinal stem cell identity. It is unclear how the continuous Wnt gradient along the crypt axis is translated into discrete expression of Ascl2 and discrete specification of stem cells at crypt bottoms. We show that (1) Ascl2 is regulated in a direct autoactivatory loop, leading to a distinct on/off expression pattern, and (2) Wnt/R-spondin can activate this regulatory loop. This mechanism interprets the Wnt levels in the intestinal crypt and translates the continuous Wnt signal into a discrete Ascl2 on or off decision. In turn, Ascl2, together with β-catenin/Tcf, activates the genes fundamental to the stem cell state. In this manner, Ascl2 forms a transcriptional switch that is both Wnt responsive and Wnt dependent to define stem cell identity.

Wnt-induced transcriptional activation is exclusively mediated by TCF/LEF

Active canonical Wnt signaling results in recruitment of β‐catenin to DNA by TCF/LEF family members, leading to transcriptional activation of TCF target genes. However, additional transcription factors have been suggested to recruit β‐catenin and tether it to DNA. Here, we describe the genome‐wide pattern of β‐catenin DNA binding in murine intestinal epithelium, Wnt‐responsive colorectal cancer (CRC) cells and HEK293 embryonic kidney cells. We identify two classes of β‐catenin binding sites. The first class represents the majority of the DNA‐bound β‐catenin and co‐localizes with TCF4, the prominent TCF/LEF family member in these cells. The second class consists of β‐catenin binding sites that co‐localize with a minimal amount of TCF4. The latter consists of lower affinity β‐catenin binding events, does not drive transcription and often does not contain a consensus TCF binding motif. Surprisingly, a dominant‐negative form of TCF4 abrogates the β‐catenin/DNA interaction of both classes of binding sites, implying that the second class comprises low affinity TCF‐DNA complexes. Our results indicate that β‐catenin is tethered to chromatin overwhelmingly through the TCF/LEF transcription factors in these three systems.

FOXP1 acts through a negative feedback loop to suppress FOXO-induced apoptosis.

Transcriptional activity of Forkhead box transcription factor class O (FOXO) proteins can result in a variety of cellular outcomes depending on cell type and activating stimulus. These transcription factors are negatively regulated by the phosphoinositol 3-kinase (PI3K)–protein kinase B (PKB) signaling pathway, which is thought to have a pivotal role in regulating survival of tumor cells in a variety of cancers. Recently, it has become clear that FOXO proteins can promote resistance to anti-cancer therapeutics, designed to inhibit PI3K–PKB activity, by inducing the expression of proteins that provide feedback at different levels of this pathway. We questioned whether such a feedback mechanism may also exist directly at the level of FOXO-induced transcription. To identify critical modulators of FOXO transcriptional output, we performed gene expression analyses after conditional activation of key components of the PI3K–PKB–FOXO signaling pathway and identified FOXP1 as a direct FOXO transcriptional target. Using chromatin immunoprecipitation followed by next-generation sequencing, we show that FOXP1 binds enhancers that are pre-occupied by FOXO3. By sequencing the transcriptomes of cells in which FOXO is specifically activated in the absence of FOXP1, we demonstrate that FOXP1 can modulate the expression of a specific subset of FOXO target genes, including inhibiting expression of the pro-apoptotic gene BIK. FOXO activation in FOXP1-knockdown cells resulted in increased cell death, demonstrating that FOXP1 prevents FOXO-induced apoptosis. We therefore propose that FOXP1 represents an important modulator of FOXO-induced transcription, promoting cellular survival.

Enhancers reside in a unique epigenetic environment during early zebrafish development

BackgroundEnhancers, not promoters, are the most dynamic in their DNA methylation status throughout development and differentiation. Generally speaking, enhancers that are primed to or actually drive gene expression are characterized by relatively low levels of DNA methylation (hypo-methylation), while inactive enhancers display hyper-methylation of the underlying DNA. The direct functional significance of the DNA methylation state of enhancers is, however, unclear for most loci.ResultsIn contrast to conventional epigenetic interactions at enhancers, we find that DNA methylation status and enhancer activity during early zebrafish development display very unusual correlation characteristics: hypo-methylation is a unique feature of primed enhancers whereas active enhancers are generally hyper-methylated. The hypo-methylated enhancers that we identify (hypo-enhancers) are enriched close to important transcription factors that act later in development. Interestingly, hypo-enhancers are de-methylated shortly before the midblastula transition and reside in a unique epigenetic environment. Finally, we demonstrate that hypo-enhancers do become active at later developmental stages and that they are physically associated with the transcriptional start site of target genes, irrespective of target gene activity.ConclusionsWe demonstrate that early development in zebrafish embodies a time window characterized by non-canonical DNA methylation–enhancer relationships, including global DNA hypo-methylation of inactive enhancers and DNA hyper-methylation of active enhancers.

Chromatin Conformation Links Distal Target Genes to CKD Loci

Genome-wide association studies (GWASs) have identified many genetic risk factors for CKD. However, linking common variants to genes that are causal for CKD etiology remains challenging. By adapting self-transcribing active regulatory region sequencing, we evaluated the effect of genetic variation on DNA regulatory elements (DREs). Variants in linkage with the CKD-associated single-nucleotide polymorphism rs11959928 were shown to affect DRE function, illustrating that genes regulated by DREs colocalizing with CKD-associated variation can be dysregulated and therefore, considered as CKD candidate genes. To identify target genes of these DREs, we used circular chromosome conformation capture (4C) sequencing on glomerular endothelial cells and renal tubular epithelial cells. Our 4C analyses revealed interactions of CKD-associated susceptibility regions with the transcriptional start sites of 304 target genes. Overlap with multiple databases confirmed that many of these target genes are involved in kidney homeostasis. Expression quantitative trait loci analysis revealed that mRNA levels of many target genes are genotype dependent. Pathway analyses showed that target genes were enriched in processes crucial for renal function, identifying dysregulated geranylgeranyl diphosphate biosynthesis as a potential disease mechanism. Overall, our data annotated multiple genes to previously reported CKD-associated single-nucleotide polymorphisms and provided evidence for interaction between these loci and target genes. This pipeline provides a novel technique for hypothesis generation and complements classic GWAS interpretation. Future studies are required to specify the implications of our dataset and further reveal the complex roles that common variants have in complex diseases, such as CKD.

P-292 The Use of Intestinal Organoids for a Detailed Delineation of Epithelial Responses to Microbial Antigens

Background: The development of a recent stem cell based organoid model can be used to culture intestinal epithelium in vitro. Intestinal organoids have a crypt-villus architecture and consist of all cell types that are present in the native intestinal epithelium. The organoid system enables us to address one of the enigmas of the pathogenesis of IBD: How do the various subsets of epithelial cells regulate responsiveness upon chronic microbial exposure? Methods: The epithelial responsiveness to microbial antigens was systematically studied by using human intestinal organoids and exposing them to bacterial lysates. We have profiled transcriptomes and active DNA regulatory elements, thereby elucidating the processes that are involved in the epithelial response to microbes and their kinetics. Furthermore, we determined the location specific responses (i.e., duodenum, ileum and colon) and we discriminated between the effect of apical and basolateral exposure. The responses of the different epithelial cell types were determined by the use of both single cell RNA-sequencing and culturing methods that alter cell type composition of the organoids. Results: Expression profiling of the kinetics of the epithelial response shows that the exposure of organoids to bacterial lysate results in an acute inflammatory response including upregulation of the NF-kB pathway and downregulation of cell cycle processes. Furthermore, the upregulated genes are highly enriched for genes that are associated with IBD. The inflammatory response is downregulated upon chronic exposure, resulting in a state that is featured by the upregulation of a distinctive set of genes including HNF4&agr;. Duodenum-, ileum- and colon-derived organoids show location specific responses upon basolateral versus apical exposure. Furthermore, we show that responsiveness decreases upon differentiation of the epithelium and that the cell types involved in the response to microbial antigens are Wnt-dependent. Conclusions: We used a human intestinal organoid model to characterize the responsiveness of the intestinal epithelium. The expression patterns and DNA regulatory elements were profiled upon acute and chronic exposure to microbial antigens. Finally, we delineated the anatomy of the response by identifying cell types that are involved and we characterized differences in responsiveness between different parts of the intestine.

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BackgroundEnhancers, not promoters, are the most dynamic in their DNA methylation status throughout development and differentiation. Generally speaking, enhancers that are primed to or actually drive gene expression are characterized by relatively low levels of DNA methylation (hypo-methylation), while inactive enhancers display hyper-methylation of the underlying DNA. The direct functional significance of the DNA methylation state of enhancers is, however, unclear for most loci.ResultsIn contrast to conventional epigenetic interactions at enhancers, we find that DNA methylation status and enhancer activity during early zebrafish development display very unusual correlation characteristics: hypo-methylation is a unique feature of primed enhancers whereas active enhancers are generally hyper-methylated. The hypo-methylated enhancers that we identify (hypo-enhancers) are enriched close to important transcription factors that act later in development. Interestingly, hypo-enhancers are de-methylated shortly before the midblastula transition and reside in a unique epigenetic environment. Finally, we demonstrate that hypo-enhancers do become active at later developmental stages and that they are physically associated with the transcriptional start site of target genes, irrespective of target gene activity.ConclusionsWe demonstrate that early development in zebrafish embodies a time window characterized by non-canonical DNA methylation–enhancer relationships, including global DNA hypo-methylation of inactive enhancers and DNA hyper-methylation of active enhancers.

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BackgroundEnhancers, not promoters, are the most dynamic in their DNA methylation status throughout development and differentiation. Generally speaking, enhancers that are primed to or actually drive gene expression are characterized by relatively low levels of DNA methylation (hypo-methylation), while inactive enhancers display hyper-methylation of the underlying DNA. The direct functional significance of the DNA methylation state of enhancers is, however, unclear for most loci.ResultsIn contrast to conventional epigenetic interactions at enhancers, we find that DNA methylation status and enhancer activity during early zebrafish development display very unusual correlation characteristics: hypo-methylation is a unique feature of primed enhancers whereas active enhancers are generally hyper-methylated. The hypo-methylated enhancers that we identify (hypo-enhancers) are enriched close to important transcription factors that act later in development. Interestingly, hypo-enhancers are de-methylated shortly before the midblastula transition and reside in a unique epigenetic environment. Finally, we demonstrate that hypo-enhancers do become active at later developmental stages and that they are physically associated with the transcriptional start site of target genes, irrespective of target gene activity.ConclusionsWe demonstrate that early development in zebrafish embodies a time window characterized by non-canonical DNA methylation–enhancer relationships, including global DNA hypo-methylation of inactive enhancers and DNA hyper-methylation of active enhancers.

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BackgroundEnhancers, not promoters, are the most dynamic in their DNA methylation status throughout development and differentiation. Generally speaking, enhancers that are primed to or actually drive gene expression are characterized by relatively low levels of DNA methylation (hypo-methylation), while inactive enhancers display hyper-methylation of the underlying DNA. The direct functional significance of the DNA methylation state of enhancers is, however, unclear for most loci.ResultsIn contrast to conventional epigenetic interactions at enhancers, we find that DNA methylation status and enhancer activity during early zebrafish development display very unusual correlation characteristics: hypo-methylation is a unique feature of primed enhancers whereas active enhancers are generally hyper-methylated. The hypo-methylated enhancers that we identify (hypo-enhancers) are enriched close to important transcription factors that act later in development. Interestingly, hypo-enhancers are de-methylated shortly before the midblastula transition and reside in a unique epigenetic environment. Finally, we demonstrate that hypo-enhancers do become active at later developmental stages and that they are physically associated with the transcriptional start site of target genes, irrespective of target gene activity.ConclusionsWe demonstrate that early development in zebrafish embodies a time window characterized by non-canonical DNA methylation–enhancer relationships, including global DNA hypo-methylation of inactive enhancers and DNA hyper-methylation of active enhancers.

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BackgroundEnhancers, not promoters, are the most dynamic in their DNA methylation status throughout development and differentiation. Generally speaking, enhancers that are primed to or actually drive gene expression are characterized by relatively low levels of DNA methylation (hypo-methylation), while inactive enhancers display hyper-methylation of the underlying DNA. The direct functional significance of the DNA methylation state of enhancers is, however, unclear for most loci.ResultsIn contrast to conventional epigenetic interactions at enhancers, we find that DNA methylation status and enhancer activity during early zebrafish development display very unusual correlation characteristics: hypo-methylation is a unique feature of primed enhancers whereas active enhancers are generally hyper-methylated. The hypo-methylated enhancers that we identify (hypo-enhancers) are enriched close to important transcription factors that act later in development. Interestingly, hypo-enhancers are de-methylated shortly before the midblastula transition and reside in a unique epigenetic environment. Finally, we demonstrate that hypo-enhancers do become active at later developmental stages and that they are physically associated with the transcriptional start site of target genes, irrespective of target gene activity.ConclusionsWe demonstrate that early development in zebrafish embodies a time window characterized by non-canonical DNA methylation–enhancer relationships, including global DNA hypo-methylation of inactive enhancers and DNA hyper-methylation of active enhancers.