Project description:Heterochromatin is integral to cell identity maintenance by impeding the activation of genes for alternate cell fates. Heterochromatic regions are associated with histone 3 lysine 9 trimethylation (H3K9me3) or H3K27me3, but these modifications are also found in euchromatic regions that permit transcription. We discovered that resistance to sonication is a reliable indicator of the heterochromatin state, and we developed a biophysical method (gradient-seq) to discriminate subtypes of H3K9me3 and H3K27me3 domains in sonication-resistant heterochromatin (srHC) versus euchromatin. These classifications are more accurate than the histone marks alone in predicting transcriptional silence and resistance of alternate fate genes to activation during direct cell conversion. Our proteomics of H3K9me3-marked srHC and functional screens revealed diverse proteins, including RBMX and RBMXL1, that impede gene induction during cellular reprogramming. Isolation of srHC with gradient-seq provides a genome-wide map of chromatin structure, elucidating subtypes of repressed domains that are uniquely predictive of diverse other chromatin properties.
Project description:Heterochromatic regions in mammalian cells suppress recombination, silence transcription, and are crucial for maintaining cell differentiation. Genomic and biochemical characterization of heterochromatin has relied on the associated histone modifications H3K9me3 and H3K27me3, yet these marks are also found in euchromatic regions that permit transcription. We employed a biophysical method to isolate sonication resistant heterochromatin from human somatic cells, mapped its genomic organization compared to histone modifications, and used proteomics to reveal an extensive number of heterochromatin-bound proteins. We discriminate subtypes of H3K9me3- and H3K27me3-marked domains, in sonication-resistant heterochromatin versus euchromatin, and we present a resource of hundreds of proteins that preferentially bind heterochromatin, a set enriched for RNA-binding proteins and proteins that oppose iPS reprogramming. The sonication-resistant heterochromatin landscape includes repressed genes for alternative lineages that are resistant to activation by introduced transcription factors. Depletion of identified heterochromatin-associated proteins reduces this barrier, rendering alternative-lineage genes more competent for transcriptional activation. Overall design: This study was designed to map the organization of sonication resistant heterochromatin in the human genome. Crosslinked, sonicated chromatin from human BJ fibroblasts was fractionated using sucrose gradients, to yield a Euchromatin fraction (fraction #2, containing highly sonicated fragments) and a Sonication-Resistant Heterochromatin fraction (fractions #10-17, containing larger, sonication-resistant fragments). In parallel, we performed H3K9me3-directed chromatin immunoprecipitation (ChIP), as well as H3K9me3-directed IPs performed from the Euchromatin fraction and from the Sonication-Resistant Heterochromatin fraction. Corresponding samples for both gradient analysis and ChIP were also sequenced. Purified DNAs containing larger DNA fragments were sonicated (after protein removal) to produce shorter fragments for library preparation. In total there are 7 sample types (5 purified chromatin samples and 2 types of input), with 2 biological replicates for each.
Project description:Heterochromatic regions in mammalian cells suppress recombination, silence transcription, and are crucial for maintaining cell differentiation. Genomic and biochemical characterization of heterochromatin has relied on the associated histone modifications H3K9me3 and H3K27me3, yet these marks are also found in euchromatic regions that permit transcription. We employed a biophysical method to isolate sonication resistant heterochromatin from human somatic cells, mapped its genomic organization compared to histone modifications, and used proteomics to reveal an extensive number of heterochromatin-bound proteins. We discriminate subtypes of H3K9me3- and H3K27me3-marked domains, in sonication-resistant heterochromatin versus euchromatin, and we present a resource of hundreds of proteins that preferentially bind heterochromatin, a set enriched for RNA-binding proteins and proteins that oppose iPS reprogramming. The sonication-resistant heterochromatin landscape includes repressed genes for alternative lineages that are resistant to activation by introduced transcription factors. Depletion of identified heterochromatin-associated proteins reduces this barrier, rendering alternative-lineage genes more competent for transcriptional activation. Overall design: This study was designed to assess the role of human SUV39H1 and RBMX/RBMXL1 in impeding the activation of heterochromatic genes by transcription factors. RNA-seq was performed in human BJ foreskin fibroblasts treated with siRNAs in two main experimental protocols: cells expressing the hepatic transcription factors FOXA3/HNF1A/HNF4A ("hepatic TF" protocol) and cells expressing no ectopic transcription factors ("no TF" protocol). There are a total of 12 RNA-seq samples: two experimental protocols, three siRNAs per protocol, and two biological replicates per siRNA/protocol. The three siRNAs are: non-targeting control siRNA, siRNA against SUV39H1, and siRNA co-targeting RBMX and RBMX/L1
Project description:We use transposon-based clonal analysis to identify the lineage classes that make the adult zebrafish caudal fin. We identify nine distinct lineage classes, including epidermis, melanocyte/xanthophore, iridophore, intraray glia, lateral line, osteoblast, dermal fibroblast, vascular endothelium, and resident blood. These lineage classes argue for distinct progenitors, or organ founding stem cells (FSCs), for each lineage, which retain fate restriction throughout growth of the fin. Thus, distinct FSCs exist for the four neuroectoderm lineages, and dermal fibroblasts are not progenitors for fin ray osteoblasts; however, artery and vein cells derive from a shared lineage in the fin. Transdifferentiation of cells or lineages in the regeneration blastema is often postulated. However, our studies of single progenitors or FSCs reveal no transfating or transdifferentiation between these lineages in the regenerating fin. This result shows that, the same as in growth, lineages retain fate restriction when passed through the regeneration blastema.
Project description:The progressive restriction of cell fate during lineage differentiation is a poorly understood phenomenon despite its ubiquity in multicellular organisms. We recently used a cell fusion assay to define a mode of epigenetic silencing that we termed "occlusion", wherein affected genes are silenced by cis-acting chromatin mechanisms irrespective of whether trans-acting transcriptional activators are present. We hypothesized that occlusion of lineage-inappropriate genes could contribute to cell fate restriction. Here, we test this hypothesis by introducing bacterial artificial chromosomes (BACs), which are devoid of chromatin modifications necessary for occlusion, into mouse fibroblasts. We found that BAC transgenes corresponding to occluded endogenous genes are expressed in most cases, whereas BAC transgenes corresponding to silent but non-occluded endogenous genes are not expressed. This indicates that the cellular milieu in trans supports the expression of most occluded genes in fibroblasts, and that the silent state of these genes is solely the consequence of occlusion in cis. For the BAC corresponding to the occluded myogenic master regulator Myf5, expression of the Myf5 transgene on the BAC triggered fibroblasts to acquire a muscle-like phenotype. These results provide compelling evidence for a critical role of gene occlusion in cell fate restriction.
Project description:Eukaryotic genomes can be organized into distinct domains of heterochromatin or euchromatin. In the fission yeast Schizosaccharomyces pombe, assembly of heterochromatin at the silent mating-type region is critical for cell fate determination in the form of mating-type switching. Here, we report that the ubiquitin ligase, Msc1, is a critical factor required for proper cell fate determination in S. pombe. In the absence of Msc1, the in vivo mobility of Swi6 at heterochromatic foci is compromised, and centromere heterochromatin becomes hyperenriched with the heterochromatin binding protein Swi6/HP1. However, at the mating-type locus, Swi6 recruitment is defective in the absence of Msc1. Therefore, Msc1 links maintaining dynamic heterochromatin with proper heterochromatin assembly and cell fate determination. These findings have implications for understanding mechanisms of differentiation in other organisms.
Project description:Heterochromatin domains are stably repressed chromatin structures composed of a core assembly of silencing proteins that condense adjacent nucleosomes. The minimal heterochromatin structure can serve as a platform for recruitment of complementary regulatory factors. We find that a reconstituted budding yeast heterochromatin domain can act as a platform to recruit multiple factors that play a role in regulating heterochromatin function. We uncover the direct interaction between the SIR heterochromatin complex and a chromosomal boundary protein that restricts the spread of heterochromatin. We find that the SIR complex relieves a mechanism of auto-inhibition within the boundary protein Yta7, allowing the Yta7 bromodomain to engage chromatin. Our results suggest that budding yeast shares with other eukaryotes the ability to establish complex heterochromatin domains that coordinate multiple mechanisms of silencing regulation through physical interactions.
Project description:Dynamic organization of chromatin within the three-dimensional nuclear space has been postulated to regulate gene expression and cell fate. Here, we define the genome-wide distribution of nuclear peripheral heterochromatin as a multipotent P19 cell adopts either a neural or a cardiac fate. We demonstrate that H3K9me2-marked nuclear peripheral heterochromatin undergoes lineage-specific reorganization during cell-fate determination. This is associated with spatial repositioning of genomic loci away from the nuclear periphery as shown by 3D immuno-FISH. Locus repositioning is not always associated with transcriptional changes, but a subset of genes is upregulated. Mef2c is specifically repositioned away from the nuclear periphery during early neurogenic differentiation, but not during early cardiogenic differentiation, with associated transcript upregulation. Myocd is specifically repositioned during early cardiogenic differentiation, but not during early neurogenic differentiation, and is transcriptionally upregulated at later stages of cardiac differentiation. We provide experimental evidence for lineage-specific regulation of nuclear architecture during cell-fate determination in a mouse cell line.
Project description:Protection of euchromatin from invasion by gene-repressive heterochromatin is critical for cellular health and viability. In addition to constitutive loci such as pericentromeres and subtelomeres, heterochromatin can be found interspersed in gene-rich euchromatin, where it regulates gene expression pertinent to cell fate. While heterochromatin and euchromatin are globally poised for mutual antagonism, the mechanisms underlying precise spatial encoding of heterochromatin containment within euchromatic sites remain opaque. We investigated ectopic heterochromatin invasion by manipulating the fission yeast mating type locus boundary using a single-cell spreading reporter system. We found that heterochromatin repulsion is locally encoded by Set1/COMPASS on certain actively transcribed genes and that this protective role is most prominent at heterochromatin islands, small domains interspersed in euchromatin that regulate cell fate specifiers. Sensitivity to invasion by heterochromatin, surprisingly, is not dependent on Set1 altering overall gene expression levels. Rather, the gene-protective effect is strictly dependent on Set1's catalytic activity. H3K4 methylation, the Set1 product, antagonizes spreading in two ways: directly inhibiting catalysis by Suv39/Clr4 and locally disrupting nucleosome stability. Taken together, these results describe a mechanism for spatial encoding of euchromatic signals that repel heterochromatin invasion.