PARP1- and CTCF-Mediated Interactions between Active and Repressed Chromatin at the Lamina Promote Oscillating Transcription.

Transcriptionally active and inactive chromatin domains tend to segregate into separate sub-nuclear compartments to maintain stable expression patterns. However, here we uncovered an inter-chromosomal network connecting active loci enriched in circadian genes to repressed lamina-associated domains (LADs). The interactome is regulated by PARP1 and its co-factor CTCF. They not only mediate chromatin fiber interactions but also promote the recruitment of circadian genes to the lamina. Synchronization of the circadian rhythm by serum shock induces oscillations in PARP1-CTCF interactions, which is accompanied by oscillating recruitment of circadian loci to the lamina, followed by the acquisition of repressive H3K9me2 marks and transcriptional attenuation. Furthermore, depletion of H3K9me2/3, inhibition of PARP activity by olaparib, or downregulation of PARP1 or CTCF expression counteracts both recruitment to the envelope and circadian transcription. PARP1- and CTCF-regulated contacts between circadian loci and the repressive chromatin environment at the lamina therefore mediate circadian transcriptional plasticity.

The visualization of large organized chromatin domains enriched in the H3K9me2 mark within a single chromosome in a single cell.

Despite considerable efforts, our understanding of the organization of higher order chromatin conformations in single cells and how these relate to chromatin marks remains poor. We have earlier invented the Chromatin In Situ Proximity (ChrISP) technique to determine proximities between chromatin fibers within a single chromosome. Here we used ChrISP to identify chromosome 11-specific hubs that are enriched in the H3K9me2 mark and that project toward the nuclear membrane in finger-like structures. Conversely, chromosome 11-specfic chromatin hubs, visualized by the presence of either H3K9me1 or H3K9me3 marks, are chromosome-wide and largely absent at the nuclear periphery. As the nuclear periphery-specific chromatin hubs were lost in the induced reduction of H3K9me2 levels, they likely represent Large Organization Chromatin in Lysine Methylation (LOCK) domains, previously identified by ChIP-seq analysis. Strikingly, the downregulation of the H3K9me2/3 marks also led to the chromosome-wide compaction of chromosome 11, suggesting a pleiotropic function of these features not recognized before. The ChrISP-mediated visualization of dynamic chromatin states in single cells thus provides an analysis of chromatin structures with a resolution far exceeding that of any other light microscopic technique.

Chromatin in situ proximity (ChrISP): single-cell analysis of chromatin proximities at a high resolution.

Current techniques for analyzing chromatin structures are hampered by either poor resolution at the individual cell level or the need for a large number of cells to obtain higher resolution. This is a major problem as it hampers our understanding of chromatin conformation in single cells and how these respond to environmental cues. Here we describe a new method, chromatin in situ proximity (ChrISP), which reproducibly scores for proximities between two different chromatin fibers in 3-D with a resolution of ~170Å in single cells. The technique is based on the in situ proximity ligation assay (ISPLA), but ChrISP omits the rolling circle amplification step (RCA). Instead, the proximities between chromatin fibers are visualized by a fluorescent connector oligonucleotide DNA, here termed splinter, forming a circular DNA with another circle-forming oligonucleotide, here termed backbone, upon ligation. In contrast to the regular ISPLA technique, our modification enables detection of chromatin fiber proximities independent of steric hindrances from nuclear structures. We use this method to identify higher order structures of individual chromosomes in relation to structural hallmarks of interphase nuclei and beyond the resolution of the light microscope.

Mutational analysis of the poly(ADP-ribosyl)ation sites of the transcription factor CTCF provides an insight into the mechanism of its regulation by poly(ADP-ribosyl)ation.

Poly(ADP-ribosyl)ation of the conserved multifunctional transcription factor CTCF was previously identified as important to maintain CTCF insulator and chromatin barrier functions. However, the molecular mechanism of this regulation and also the necessity of this modification for other CTCF functions remain unknown. In this study, we identified potential sites of poly(ADP-ribosyl)ation within the N-terminal domain of CTCF and generated a mutant deficient in poly(ADP-ribosyl)ation. Using this CTCF mutant, we demonstrated the requirement of poly(ADP-ribosyl)ation for optimal CTCF function in transcriptional activation of the p19ARF promoter and inhibition of cell proliferation. By using a newly generated isogenic insulator reporter cell line, the CTCF insulator function at the mouse Igf2-H19 imprinting control region (ICR) was found to be compromised by the CTCF mutation. The association and simultaneous presence of PARP-1 and CTCF at the ICR, confirmed by single and serial chromatin immunoprecipitation assays, were found to be independent of CTCF poly(ADP-ribosyl)ation. These results suggest a model of CTCF regulation by poly(ADP-ribosyl)ation whereby CTCF and PARP-1 form functional complexes at sites along the DNA, producing a dynamic reversible modification of CTCF. By using bioinformatics tools, numerous sites of CTCF and PARP-1 colocalization were demonstrated, suggesting that such regulation of CTCF may take place at the genome level.