Cohesin is dispensable for centromere cohesion in human cells

CCAR2 controls mitotic progression through spatiotemporal regulation of Aurora B

CCAR2 (cell cycle and apoptosis regulator 2) is a multifaceted protein involved in cell survival and death following cytotoxic stress. However, little is known about the physiological functions of CCAR2 in regulating cell proliferation in the absence of external stimuli. The present study shows that CCAR2-deficient cells possess multilobulated nuclei, suggesting a defect in cell division. In particular, the duration of mitotic phase was perturbed. This disturbance of mitotic progression resulted from premature loss of cohesion with the centromere, and inactivation of the spindle assembly checkpoint during prometaphase and metaphase. It resulted in the formation of lagging chromosomes during anaphase, leading ultimately to the activation of the abscission checkpoint to halt cytokinesis. The CCAR2-dependent mitotic progression was related to spatiotemporal regulation of active Aurora B. In conclusion, the results suggest that CCAR2 governs mitotic events, including proper chromosome segregation and cytokinetic division, to maintain chromosomal stability.

Meiosis-specific cohesin complexes display essential and distinct roles in mitotic embryonic stem cell chromosomes

BACKGROUND

Cohesin is a chromosome-associated SMC-kleisin complex that mediates sister chromatid cohesion, recombination, and most chromosomal processes during mitosis and meiosis. However, it remains unclear whether meiosis-specific cohesin complexes are functionally active in mitotic chromosomes.

RESULTS

Through high-resolution 3D-structured illumination microscopy (3D-SIM) and functional analyses, we report multiple biological processes associated with the meiosis-specific cohesin components, α-kleisin REC8 and STAG3, and the distinct loss of function of meiotic cohesin during the cell cycle of embryonic stem cells (ESCs). First, we show that STAG3 is required for the efficient localization of REC8 to the nucleus by interacting with REC8. REC8-STAG3-containing cohesin regulates topological properties of chromosomes and maintains sister chromatid cohesion. Second, REC8-cohesin has additional sister chromatid cohesion roles in concert with mitotic RAD21-cohesin on ESC chromosomes. SIM imaging of REC8 and RAD21 co-staining revealed that the two types of α-kleisin subunits exhibited distinct loading patterns along ESC chromosomes. Third, knockdown of REC8 or RAD21-cohesin not only leads to higher rates of premature sister chromatid separation and delayed replication fork progression, which can cause proliferation and developmental defects, but also enhances chromosome compaction by hyperloading of retinoblastoma protein-condensin complexes from the prophase onward.

CONCLUSIONS

Our findings indicate that the delicate balance between mitotic and meiotic cohesins may regulate ESC-specific chromosomal organization and the mitotic program.

Perturbing cohesin dynamics drives MRE11 nuclease-dependent replication fork slowing

Pds5 is required for sister chromatid cohesion, and somewhat paradoxically, to remove cohesin from chromosomes. We found that Pds5 plays a critical role during DNA replication that is distinct from its previously known functions. Loss of Pds5 hinders replication fork progression in unperturbed human and mouse cells. Inhibition of MRE11 nuclease activity restores fork progression, suggesting that Pds5 protects forks from MRE11-activity. Loss of Pds5 also leads to double-strand breaks, which are again reduced by MRE11 inhibition. The replication function of Pds5 is independent of its previously reported interaction with BRCA2. Unlike Pds5, BRCA2 protects forks from nucleolytic degradation only in the presence of genotoxic stress. Moreover, our iPOND analysis shows that the loading of Pds5 and other cohesion factors on replication forks is not affected by the BRCA2 status. Pds5 role in DNA replication is shared by the other cohesin-removal factor Wapl, but not by the cohesin complex component Rad21. Interestingly, depletion of Rad21 in a Pds5-deficient background rescues the phenotype observed upon Pds5 depletion alone. These findings support a model where loss of either component of the cohesin releasin complex perturbs cohesin dynamics on replication forks, hindering fork progression and promoting MRE11-dependent fork slowing.

ASXL1 interacts with the cohesin complex to maintain chromatid separation and gene expression for normal hematopoiesis

ASXL1 is frequently mutated in a spectrum of myeloid malignancies with poor prognosis. Loss of Asxl1 leads to myelodysplastic syndrome-like disease in mice; however, the underlying molecular mechanisms remain unclear. We report that ASXL1 interacts with the cohesin complex, which has been shown to guide sister chromatid segregation and regulate gene expression. Loss of Asxl1 impairs the cohesin function, as reflected by an impaired telophase chromatid disjunction in hematopoietic cells. Chromatin immunoprecipitation followed by DNA sequencing data revealed that ASXL1, RAD21, and SMC1A share 93% of genomic binding sites at promoter regions in Lin-cKit+ (LK) cells. We have shown that loss of Asxl1 reduces the genome binding of RAD21 and SMC1A and alters the expression of ASXL1/cohesin target genes in LK cells. Our study underscores the ASXL1-cohesin interaction as a novel means to maintain normal sister chromatid separation and regulate gene expression in hematopoietic cells.

Investigating the Interplay between Sister Chromatid Cohesion and Homolog Pairing in Drosophila Nuclei

Following DNA replication, sister chromatids must stay connected for the remainder of the cell cycle in order to ensure accurate segregation in the subsequent cell division. This important function involves an evolutionarily conserved protein complex known as cohesin; any loss of cohesin causes premature sister chromatid separation in mitosis. Here, we examined the role of cohesin in sister chromatid cohesion prior to mitosis, using fluorescence in situ hybridization (FISH) to assay the alignment of sister chromatids in interphase Drosophila cells. Surprisingly, we found that sister chromatid cohesion can be maintained in G2 with little to no cohesin. This capacity to maintain cohesion is widespread in Drosophila, unlike in other systems where a reduced dependence on cohesin for sister chromatid segregation has been observed only at specific chromosomal regions, such as the rDNA locus in budding yeast. Additionally, we show that condensin II antagonizes the alignment of sister chromatids in interphase, supporting a model wherein cohesin and condensin II oppose each other’s functions in the alignment of sister chromatids. Finally, because the maternal and paternal homologs are paired in the somatic cells of Drosophila, and because condensin II has been shown to antagonize this pairing, we consider the possibility that condensin II-regulated mechanisms for aligning homologous chromosomes may also contribute to sister chromatid cohesion.

As cells grow, they replicate their DNA to give rise to two copies of each chromosome, known as sister chromatids, which separate from each other once the cell divides. To ensure that sister chromatids end up in different daughter cells, they are kept together from DNA replication until mitosis via a connection known as cohesion. A protein complex known as cohesin is essential for this process. Our work in Drosophila cells suggests that factors other than cohesin also contribute to sister chromatid cohesion in interphase. Additionally, we observed that the alignment of sister chromatids is regulated by condensin II, a protein complex involved in the compaction of chromosomes prior to division as well as the regulation of inter-chromosomal associations. These findings highlight that, in addition to their important individual functions, cohesin and condensin II proteins may interact to organize chromosomes over the course of the cell cycle. Finally, building on prior observations that condensin II is involved in the regulation of somatic homolog pairing in Drosophila, our work suggests that the mechanisms underlying homolog pairing may also contribute to sister chromatid cohesion.

3D-FISH analysis reveals chromatid cohesion defect during interphase in Roberts syndrome

Background

Roberts syndrome (RBS) is a rare autosomal recessive disorder mainly characterized by growth retardation, limb defects and craniofacial anomalies. Characteristic cytogenetic findings are “railroad track” appearance of chromatids and premature centromere separation in metaphase spreads. Mutations in the ESCO2 (establishment of cohesion 1 homolog 2) gene located in 8p21.1 have been found in several families. ESCO2, a member of the cohesion establishing complex, has a role in the effective cohesion between sister chromatids. In order to analyze sister chromatids topography during interphase, we performed 3D-FISH using pericentromeric heterochromatin probes of chromosomes 1, 4, 9 and 16, on preserved nuclei from a fetus with RBS carrying compound heterozygous null mutations in the ESCO2 gene.

Results

Along with the first observation of an abnormal separation between sister chromatids in heterochromatic regions, we observed a statistically significant change in the intranuclear localization of pericentromeric heterochromatin of chromosome 1 in cells of the fetus compared to normal cells, demonstrating for the first time a modification in the spatial arrangement of chromosome domains during interphase.

Conclusion

We hypothesize that the disorganization of nuclear architecture may result in multiple gene deregulations, either through disruption of DNA cis interaction –such as modification of chromatin loop formation and gene insulation - mediated by cohesin complex, or by relocation of chromosome territories. These changes may modify interactions between the chromatin and the proteins associated with the inner nuclear membrane or the pore complexes. This model offers a link between the molecular defect in cohesion and the complex phenotypic anomalies observed in RBS.

Electronic supplementary material

The online version of this article (doi:10.1186/s13039-014-0059-6) contains supplementary material, which is available to authorized users.

The roles of cohesins in mitosis, meiosis, and human health and disease

Mitosis and meiosis are essential processes that occur during development. Throughout these processes, cohesion is required to keep the sister chromatids together until their separation at anaphase. Cohesion is created by multiprotein subunit complexes called cohesins. Although the subunits differ slightly in mitosis and meiosis, the canonical cohesin complex is composed of four subunits that are quite diverse. The cohesin complexes are also important for DNA repair, gene expression, development, and genome integrity. Here we provide an overview of the roles of cohesins during these different events as well as their roles in human health and disease, including the cohesinopathies. Although the exact roles and mechanisms of these proteins are still being elucidated, this review serves as a guide for the current knowledge of cohesins.

Role of helicase-like transcription factor (hltf) in the G2/m transition and apoptosis in brain

HLTF participates in transcription, chromatin remodeling, DNA damage repair, and tumor suppression. Aside from being expressed in mouse brain during embryonic and postnatal development, little is known about Hltf's functional importance. Splice variant quantification of wild-type neonatal (6-8 hour postpartum) brain gave a ratio of 5:1 for Hltf isoform 1 (exons 1-25) to isoform 2 (exons 1-21 with exon 21 extended via a partial intron retention event). Western analysis showed a close correlation between mRNA and protein expression. Complete loss of Hltf caused encephalomalacia with increased apoptosis, and reduced viability. Sixty-four percent of Hltf null mice died, 48% within 12-24 hours of birth. An RNA-Seq snapshot of the neonatal brain transcriptome showed 341 of 20,000 transcripts were altered (p < 0.05) - 95 up regulated and 246 down regulated. MetaCore^TM enrichment pathway analysis revealed Hltf regulates cell cycle, cell adhesion, and TGF-beta receptor signaling. Hltf's most important role is in the G2/M transition of the cell cycle (p  =  4.672e-7) with an emphasis on transcript availability of major components in chromosome cohesion and condensation. Hltf null brains have reduced transcript levels for Rad21/Scc1, histone H3.3, Cap-E/Smc2, Cap-G/G2, and Aurora B kinase. The loss of Hltf in its yeast Rad5-like role in DNA damage repair is accompanied by down regulation of Cflar, a critical inhibitor of TNFRSF6-mediated apoptosis, and increased (p<0.0001) active caspase-3, an indicator of intrinsic triggering of apoptosis in null brains. Hltf also regulates Smad7/Bambi/Tgf-beta/Bmp5/Wnt10b signaling in brain. ChIP confirmed Hltf binding to consensus sequences in predicted (promoter Scgb3a1 gene) and previously unidentified (P-element on chromosome 7) targets. This study is the first to provide a comprehensive view of Hltf targets in brain. Moreover, it reveals how silencing Hltf disrupts cell cycle progression, and attenuates DNA damage repair.

Cohesin codes - interpreting chromatin architecture and the many facets of cohesin function

Sister chromatid tethering is maintained by cohesin complexes that minimally contain Smc1, Smc3, Mcd1 and Scc3. During S-phase, chromatin-associated cohesins are modified by the Eco1/Ctf7 family of acetyltransferases. Eco1 proteins function during S phase in the context of replicated sister chromatids to convert chromatin-bound cohesins to a tethering-competent state, but also during G2 and M phases in response to double-stranded breaks to promote error-free DNA repair. Cohesins regulate transcription and are essential for ribosome biogenesis and complete chromosome condensation. Little is known, however, regarding the mechanisms through which cohesin functions are directed. Recent findings reveal that Eco1-mediated acetylation of different lysine residues in Smc3 during S phase promote either cohesion or condensation. Phosphorylation and SUMOylation additionally impact cohesin functions. Here, we posit the existence of a cohesin code, analogous to the histone code introduced over a decade ago, and speculate that there is a symphony of post-translational modifications that direct cohesins to function across a myriad of cellular processes. We also discuss evidence that outdate the notion that cohesion defects are singularly responsible for cohesion-mutant-cell inviability. We conclude by proposing that cohesion establishment is linked to chromatin formation.

Getting through anaphase: splitting the sisters and beyond

Sister-chromatid cohesion, thought to be primarily mediated by the cohesin complex, is essential for chromosome segregation. The forces holding the two sisters resist the tendency of microtubules to prematurely pull sister DNAs apart and thereby prevent random segregation of the genome during mitosis, and consequent aneuploidy. By counteracting the spindle pulling forces, cohesion between the two sisters generates the tension necessary to stabilize microtubule–kinetochore attachments. Upon entry into anaphase, however, the linkages that hold the two sister DNAs must be rapidly destroyed to allow physical separation of chromatids. Anaphase cells must therefore possess mechanisms that ensure faithful segregation of single chromatids that are now attached stably to the spindle in a manner no longer dependent on tension. In the present review, we discuss the nature of the cohesive forces that hold sister chromatids together, the mechanisms that trigger their physical separation, and the anaphase-specific changes that ensure proper segregation of single chromatids during the later stages of mitosis.

Rec8-containing cohesin maintains bivalents without turnover during the growing phase of mouse oocytes

During female meiosis, bivalent chromosomes are thought to be held together from birth until ovulation by sister chromatid cohesion mediated by cohesin complexes whose ring structure depends on kleisin subunits, either Rec8 or Scc1. Because cohesion is established at DNA replication in the embryo, its maintenance for such a long time may require cohesin turnover. To address whether Rec8- or Scc1-containing cohesin holds bivalents together and whether it turns over, we created mice whose kleisin subunits can be cleaved by TEV protease. We show by microinjection experiments and confocal live-cell imaging that Rec8 cleavage triggers chiasmata resolution during meiosis I and sister centromere disjunction during meiosis II, while Scc1 cleavage triggers sister chromatid disjunction in the first embryonic mitosis, demonstrating a dramatic transition from Rec8- to Scc1-containing cohesin at fertilization. Crucially, activation of an ectopic Rec8 transgene during the growing phase of Rec8(TEV)(/TEV) oocytes does not prevent TEV-mediated bivalent destruction, implying little or no cohesin turnover for ≥2 wk during oocyte growth. We suggest that the inability of oocytes to regenerate cohesion may contribute to age-related meiosis I errors.

Studies of haspin-depleted cells reveal that spindle-pole integrity in mitosis requires chromosome cohesion

Cohesins and their regulators are vital for normal chromosome cohesion and segregation. A number of cohesion proteins have also been localized to centrosomes and proposed to function there. We show that RNAi-mediated depletion of factors required for cohesion, including haspin, Sgo1 and Scc1, leads to the generation of multiple acentriolar centrosome-like foci and disruption of spindle structure in mitosis. Live-cell imaging reveals that, in haspin-depleted cells, these effects occur only as defects in chromosome cohesion become manifest, and they require ongoing microtubule dynamics and kinesin-5 (also known as Eg5) activity. Inhibition of topoisomerase II in mitosis, which prevents decatenation and separation of chromatids, circumvents the loss of cohesion and restores integrity of the spindle poles. Although these results do not rule out roles for cohesin proteins at centrosomes, they suggest that when cohesion is compromised, spindle-pole integrity can be disrupted as an indirect consequence of the failure to properly integrate chromosome- and centrosome-initiated pathways for spindle formation.

PICH and cotargeted Plk1 coordinately maintain prometaphase chromosome arm architecture

To maintain genomic stability, chromosome architecture needs to be tightly regulated as chromosomes undergo condensation during prophase and separation during anaphase, but the mechanisms remain poorly understood. Here, we show that the Plk1-binding protein PICH and Plk1 kinase coordinately maintain chromosome architecture during prometaphase. PICH knockdown results in a loss of Plk1 from the chromosome arm and an increase in highly disorganized "wavy" chromosomes that exhibit an "open" or "X-shaped" configuration, consistent with a loss of chromosome arm cohesion. Such chromosome disorganization occurs with essentially no change in the localization of condensin or cohesin on chromosomes. Interestingly, the chromosome disorganization could be prevented by treatment with a topoisomerase II inhibitor ICRF-193, suggesting that the PICH-Plk1 complex normally maintains chromosome architecture in a manner that involves topoisomerase II activity. PICH knockdown does not affect initial chromosome compaction at prophase but causes anaphase DNA bridge formation and failed abscission. Our studies suggest that the PICH-Plk1 complex plays a critical role in maintaining prometaphase chromosome architecture.

Centromere DNA decatenation depends on cohesin removal and is required for mammalian cell division

Sister chromatid cohesion is mediated by DNA catenation and proteinaceous cohesin complexes. The recent visualization of PICH (Plk1-interacting checkpoint helicase)-coated DNA threads in anaphase cells raises new questions as to the role of DNA catenation and its regulation in time and space. In the present study we show that persistent DNA catenation induced by inhibition of Topoisomerase-IIalpha can contribute to sister chromatid cohesion in the absence of cohesin complexes and that resolution of catenation is essential for abscission. Furthermore, we use an in vitro chromatid separation assay to investigate the temporal and functional relationship between cohesin removal and Topoisomerase-IIalpha-mediated decatenation. Our data suggest that centromere decatenation can occur only after separase activation and cohesin removal, providing a plausible explanation for the persistence of centromere threads after anaphase onset.

Cohesin promotes the repair of ionizing radiation-induced DNA double-strand breaks in replicated chromatin

The cohesin protein complex holds sister chromatids together after synthesis until mitosis. It also contributes to post-replicative DNA repair in yeast and higher eukaryotes and accumulates at sites of laser-induced damage in human cells. Our goal was to determine whether the cohesin subunits SMC1 and Rad21 contribute to DNA double-strand break repair in X-irradiated human cells in the G2 phase of the cell cycle. RNA interference-mediated depletion of SMC1 sensitized HeLa cells to X-rays. Repair of radiation-induced DNA double-strand breaks, measured by gammaH2AX/53BP1 foci analysis, was slower in SMC1- or Rad21-depleted cells than in controls in G2 but not in G1. Inhibition of the DNA damage kinase DNA-PK, but not ATM, further inhibited foci loss in cohesin-depleted cells in G2. SMC1 depletion had no effect on DNA single-strand break repair in either G1 or late S/G2. Rad21 and SMC1 were recruited to sites of X-ray-induced DNA damage in G2-phase cells, but not in G1, and only when DNA damage was concentrated in subnuclear stripes, generated by partially shielded ultrasoft X-rays. Our results suggest that the cohesin complex contributes to cell survival by promoting the repair of radiation-induced DNA double-strand breaks in G2-phase cells in an ATM-dependent pathway.

SUMO modification of DNA topoisomerase II: trying to get a CENse of it all

DNA topoisomerase II (topo II) is an essential determinant of chromosome structure and function, acting to resolve topological problems inherent in recombining, transcribing, replicating and segregating DNA. In particular, the unique decatenating activity of topo II is required for sister chromatids to disjoin and separate in mitosis. Topo II exhibits a dynamic localization pattern on mitotic chromosomes, accumulating at centromeres and axial chromosome cores prior to anaphase. In organisms ranging from yeast to humans, a fraction of topo II is targeted for SUMO conjugation in mitotic cells, and here we review our current understanding of the significance of this modification. As we shall see, an emerging consensus is that in metazoans SUMO modification is required for topo II to accumulate at centromeres, and that in the absence of this regulation there is an elevated frequency of chromosome non-disjunction, segregation errors, and aneuploidy. The underlying molecular mechanisms for how SUMO controls topo II are as yet unclear. In closing, however, we will evaluate two possible interpretations: one in which SUMO promotes enzyme turnover, and a second in which SUMO acts as a localization tag for topo II chromosome trafficking.

Cohesin: its roles and mechanisms

The cohesin complex is a major constituent of interphase and mitotic chromosomes. Apart from its role in mediating sister chromatid cohesion, it is also important for DNA double-strand-break repair and transcriptional control. The functions of cohesin are regulated by phosphorylation, acetylation, ATP hydrolysis, and site-specific proteolysis. Recent evidence suggests that cohesin acts as a novel topological device that traps chromosomal DNA within a large tripartite ring formed by its core subunits.

The cohesin complex and its roles in chromosome biology

Cohesin is a chromosome-associated multisubunit protein complex that is highly conserved in eukaryotes and has close homologs in bacteria. Cohesin mediates cohesion between replicated sister chromatids and is therefore essential for chromosome segregation in dividing cells. Cohesin is also required for efficient repair of damaged DNA and has important functions in regulating gene expression in both proliferating and post-mitotic cells. Here we discuss how cohesin associates with DNA, how these interactions are controlled during the cell cycle; how binding of cohesin to DNA may mediate sister chromatid cohesion, DNA repair, and gene regulation; and how defects in these processes can lead to human disease.

Chromosome cohesion - rings, knots, orcs and fellowship

Sister-chromatid cohesion is essential for accurate chromosome segregation. A key discovery towards our understanding of sister-chromatid cohesion was made 10 years ago with the identification of cohesins. Since then, cohesins have been shown to be involved in cohesion in numerous organisms, from yeast to mammals. Studies of the composition, regulation and structure of the cohesin complex led to a model in which cohesin loading during S-phase establishes cohesion, and cohesin cleavage at the onset of anaphase allows sister-chromatid separation. However, recent studies have revealed activities that provide cohesion in the absence of cohesin. Here we review these advances and propose an integrative model in which chromatid cohesion is a result of the combined activities of multiple cohesion mechanisms.

Persistence of DNA threads in human anaphase cells suggests late completion of sister chromatid decatenation

PICH (Plk1-interacting checkpoint helicase) was recently identified as an essential component of the spindle assembly checkpoint and shown to localize to kinetochores, inner centromeres, and thin threads connecting separating chromosomes even during anaphase. In this paper, we have used immuno-fiber fluorescence in situ hybridization and chromatin-immunoprecipitation to demonstrate that PICH associates with centromeric chromatin during anaphase. Furthermore, by careful analysis of PICH-positive anaphase threads through FISH as well as bromo-deoxyurdine and CREST labeling, we strengthen the evidence that these threads comprise mainly alphoid centromere deoxyribonucleic acid. Finally, by timing the addition of ICRF-193 (a specific inhibitor of topoisomerase-II alpha) to cells synchronized in anaphase, we demonstrate that topoisomerase activity is required specifically to resolve PICH-positive threads during anaphase (as opposed to being required to prevent the formation of such threads during earlier cell cycle stages). These data indicate that PICH associates with centromeres during anaphase and that most PICH-positive threads evolve from inner centromeres as these stretch in response to tension. Moreover, they show that topoisomerase activity is required during anaphase for the resolution of PICH-positive threads, implying that the complete separation of sister chromatids occurs later than previously assumed.

Sister chromatid cohesion: the cohesin cleavage model does not ring true

Sister chromatid cohesion is important for high fidelity chromosome segregation during anaphase. Gene products that provide structural components (cohesin complex or cohesin) and regulatory components responsible for cohesion are conserved through eukaryotes. A simple model where cohesion establishment occurs by replication through static cohesin rings and cohesion dissolution occurs by Esp1p/separase mediated cleavage of the cohesin rings (Mcd1p/Rad21p/Scc1p sub-unit cleavage) has become widespread. A growing body of evidence is inconsistent with this ring cleavage model. This review will summarize the evidence showing that cohesin complex is not static but is regulated at multiple cell cycle stages before anaphase in a separase independent manner. Separase is indeed required at anaphase for complete chromosome segregation. However, multiple mechanisms for cohesion dissolution appear to act concurrently during anaphase. Separase is only one such mechanism and its importance varies from organism to organism. The idea that cohesin is a dynamic complex subjected to regulation at various cell cycle stages by multiple mechanisms makes sense in light of the myriad functions in which it has been implicated, such as DNA damage repair, gene silencing and chromosome condensation.