Persistent telomere cohesion triggers a prolonged anaphase

Telomeres use distinct mechanisms (not used by arms or centromeres) to mediate cohesion between sister chromatids. However, the motivation for a specialized mechanism at telomeres is not well understood. Here we show, using fluorescence in situ hybridization and live-cell imaging, that persistent sister chromatid cohesion at telomeres triggers a prolonged anaphase in normal human cells and cancer cells. Excess cohesion at telomeres can be induced by inhibition of tankyrase 1, a poly(ADP-ribose) polymerase that is required for resolution of telomere cohesion, or by overexpression of proteins required to establish telomere cohesion, the shelterin subunit TIN2 and the cohesin subunit SA1. Regardless of the method of induction, excess cohesion at telomeres in mitosis prevents a robust and efficient anaphase. SA1- or TIN2-induced excess cohesion and anaphase delay can be rescued by overexpression of tankyrase 1. Moreover, we show that primary fibroblasts, which accumulate excess telomere cohesion at mitosis naturally during replicative aging, undergo a similar delay in anaphase progression that can also be rescued by overexpression of tankyrase 1. Our study demonstrates that there are opposing forces that regulate telomere cohesion. The observation that cells respond to unresolved telomere cohesion by delaying (but not completely disrupting) anaphase progression suggests a mechanism for tolerating excess cohesion and maintaining telomere integrity. This attempt to deal with telomere damage may be ultimately futile for aging fibroblasts but useful for cancer cells.     

Roles and regulation of the Drosophila centromere cohesion protein MEI-S332 family

In meiosis, a physical attachment, or cohesion, between the centromeres of the sister chromatids is retained until their separation at anaphase II. This cohesion is essential for ensuring accurate segregation of the sister chromatids in meiosis II and avoiding aneuploidy, a condition that can lead to prenatal lethality or birth defects. The Drosophila MEI-S332 protein localizes to centromeres when sister chromatids are attached in mitosis and meiosis, and it is required to maintain cohesion at the centromeres after cohesion along the sister chromatid arms is lost at the metaphase I/anaphase I transition. MEI-S332 is the founding member of a family of proteins that protect centromeric cohesion but whose members also affect kinetochore behaviour and spindle microtubule dynamics. We compare the Drosophila MEI-S332 family members, evaluate the role of MEI-S332 in mitosis and meiosis I, and discuss the regulation of localization of MEI-S332 to the centromere and its dissociation at anaphase. We analyse the relationship between MEI-S332 and cohesin, a protein complex that is also necessary for sister-chromatid cohesion in mitosis and meiosis. In mitosis, centromere localization of 相似文献   

Centromere and telomere movements during early meiotic prophase of mouse and man are associated with the onset of chromosome pairing

The preconditions and early steps of meiotic chromosome pairing were studied by fluorescence in situ hybridization (FISH) with chromosome- specific DNA probes to mouse and human testis tissue sections. Premeiotic pairing of homologous chromosomes was not detected in spermatogonia of the two species. FISH with centromere- and telomere- specific DNA probes in combination with immunostaining (IS) of synaptonemal complex (SC) proteins to testis sections of prepuberal mice at days 4-12 post partum was performed to study sequentially the meiotic pairing process. Movements of centromeres and then telomeres to the nuclear envelope, and of telomeres along the nuclear envelope leading to the formation of a chromosomal bouquet were detected during mouse prophase. At the bouquet stage, pairing of a mouse chromosome-8- specific probe was observed. SC-IS and simultaneous telomere FISH revealed that axial element proteins appear as large aggregates in mouse meiocytes when telomeres are attached to the nuclear envelope. Axial element formation initiates during tight telomere clustering and transverse filament-IS indicated the initiation of synapsis during this stage. Comparison of telomere and centromere distribution patterns of mouse and human meiocytes revealed movements of centromeres and then telomeres to the nuclear envelope and subsequent bouquet formation as conserved motifs of the pairing process. Chromosome painting in human spermatogonia revealed compacted, largely mutually exclusive chromosome territories. The territories developed into long, thin threads at the onset of meiotic prophase. Based on these results a unified model of the pairing process is proposed.     

Protein requirements for sister telomere association in human cells

Previous studies in human cells indicate that sister telomeres have distinct requirements for their separation at mitosis. In cells depleted for tankyrase 1, a telomeric poly(ADP-ribose) polymerase, sister chromatid arms and centromeres separate normally, but telomeres remain associated and cells arrest in mitosis. Here, we use biochemical and genetic approaches to identify proteins that might mediate the persistent association at sister telomeres. We use immunoprecipitation analysis to show that the telomeric proteins, TRF1 (an acceptor of PARsylation by tankyrase 1) and TIN2 (a TRF1 binding partner) each bind to the SA1 ortholog of the cohesin Scc3 subunit. Sucrose gradient sedimentation shows that TRF1 cosediments with the SA1-cohesin complex. Depletion of the SA1 cohesin subunit or the telomeric proteins (TRF1 and TIN2) restores the normal resolution of sister telomeres in mitosis in tankyrase 1-depleted cells. Moreover, depletion of TRF1 and TIN2 or SA1 abrogates the requirement for tankyrase 1 in mitotic progression. Our studies indicate that sister telomere association in human cells is mediated by a novel association between a cohesin subunit and components of telomeric chromatin.     

Separase is Required at Multiple Pre-Anaphase Cell Cycle Stages in Human Cells

The yeast separase proteins Esp1 and Cut1 are required for loss of sister chromatid cohesion that occurs at the moment of anaphase onset. Circumstantial evidence has linked human separase to centromere separation at anaphase, but a direct test that the role of this enzyme is functionally conserved with the yeast proteins is lacking. Here we describe the effects of separase depletion from human cells using RNA interference. Surprisingly, HeLa cells lacking separase are delayed or arrest at the G2/M phase transition. This arrest is not likely due to the activation of a known checkpoint control, but may be a result of a failure to construct a mitotic chromosome. Without separase, cells also have a prolonged prometaphase, perhaps resulting from defects in spindle assembly or dynamics. In cells that reach mitosis, sister arm resolution and separation are perturbed, whereas in anaphase cells sister centromeres do appear to separate. These data indicate that separase function is not restricted to anaphase initiation and that its role in promoting loss of sister chromatid cohesion might be preferentially at arms but not centromeres.     

Control of Shugoshin function during fission-yeast meiosis

Meiosis consists of a single round of DNA replication followed by two consecutive nuclear divisions. During the first division (MI), sister kinetochores must orient toward the same pole to favor reductional segregation. Correct chromosome segregation during the second division (MII) requires the retention of centromeric cohesion until anaphase II. The spindle checkpoint protein Bub1 is essential for both processes in fission yeast . When bub1 is deleted, the Shugoshin protein Sgo1 is not recruited to centromeres, cohesin Rec8 does not persist at centromeres, and sister-chromatid cohesion is lost by the end of MI. Deletion of bub1 also affects kinetochore orientation because sister centromeres can move to opposite spindle poles in approximately 30% of MI divisions. We show here that these two functions are separable within the Bub1 protein. The N terminus of Bub1 is necessary and sufficient for Sgo1 targeting to centromeres and the protection of cohesion, whereas the C-terminal kinase domain acts together with Sgo2, the second fission-yeast Shugoshin protein, to promote sister-kinetochore co-orientation during MI. Additional analyses suggest that the protection of centromeric cohesion does not operate when sister kinetochores attach to opposite spindle poles during MI. Sgo1-mediated protection of centromere cohesion might therefore be regulated by the mode of kinetochore attachment.     

Absence of yKu/Hdf1 but not myosin-like proteins alters chromosome dynamics during prophase I in yeast

Abstract Meiosis is central to the formation of haploid gametes or spores in that it segregates homologous chromosomes and halves the chromosome number. A prerequisite of this genome bisection is the pairing of homologous chromosomes during the first meiotic prophase. When budding yeast cells are induced to undergo meiosis, this has profound consequences for nuclear structure: after premeiotic DNA replication centromeres disperse, while telomeres move about the nuclear periphery and temporarily cluster during the leptotene/zygotene transition (bouquet stage) of the prophase to first meiotic division. In vegetative cells, Hdf1p (yKu) and the myosin-like proteins Mlp1p and Mlp2p have been suggested to contribute to the organization of silent chromatin, tethering of telomeres to the nuclear periphery, DNA repair, and telomere maintenance. Here, we investigated by molecular cytology whether yKu and Mlp proteins contribute to telomere and chromosome dynamics in meiosis. It was found that mlp1 Δ mlp2 Δ double-mutant cells undergo centromere dispersion, telomere clustering, homologue pairing, and sporulation like wild type. On the other hand, cells deficient for yKu underwent meiosis-specific chromosomal events with a delay, while they eventually sporulated like wild type. These results suggest that the absence of yKu not only affects vegetative nuclear architecture ( Laroche et al., 1998 ) but also interferes with the ordered occurrence of chromosome dynamics during first meiotic prophase.     

Chl1p, a DNA helicase-like protein in budding yeast, functions in sister-chromatid cohesion

From the time of DNA replication until anaphase onset, sister chromatids remain tightly paired along their length. Ctf7p/Eco1p is essential to establish sister-chromatid pairing during S-phase and associates with DNA replication components. DNA helicases precede the DNA replication fork and thus will first encounter chromatin sites destined for cohesion. In this study, I provide the first evidence that a DNA helicase is required for proper sister-chromatid cohesion. Characterizations of chl1 mutant cells reveal that CHL1 interacts genetically with both CTF7/ECO1 and CTF18/CHL12, two genes that function in sister-chromatid cohesion. Consistent with genetic interactions, Chl1p physically associates with Ctf7p/Eco1p both in vivo and in vitro. Finally, a functional assay reveals that Chl1p is critical for sister-chromatid cohesion. Within the budding yeast genome, Chl1p exhibits the highest degree of sequence similarity to human CHL1 isoforms and BACH1. Previous studies revealed that human CHLR1 exhibits DNA helicase-like activities and that BACH1 is a helicase-like protein that associates with the tumor suppressor BRCA1 to maintain genome integrity. Our findings document a novel role for Chl1p in sister-chromatid cohesion and provide new insights into the possible mechanisms through which DNA helicases may contribute to cancer progression when mutated.     

Regulated Separation of Sister Centromeres depends on the Spindle Assembly Checkpoint but not on the Anaphase Promoting Complex/Cyclosome

Key to faithful genetic inheritance is the cohesion between sister centromeres that physically links replicated sister chromatids and is then abruptly lost at the onset of anaphase. Misregulated cohesion causes aneuploidy, birth defects and perhaps initiates cancers. Loss of centromere cohesion is controlled by the spindle checkpoint and is thought to depend on a ubiquitin ligase, the Anaphase Promoting Complex/Cyclosome (APC). But here we present evidence that the APC pathway is dispensable for centromere separation at anaphase in mammals, and that anaphase proceeds in the presence of cyclin B and securin. Arm separation is perturbed in the absence of APC, compromising the fidelity of segregation, but full sister chromatid separation is achieved after a delayed anaphase. Thereafter, cells arrest terminally in telophase with high levels of cyclin B. Extending these findings we provide evidence that the spindle checkpoint regulates centromere cohesion through an APC-independent pathway. We propose that this Centromere Linkage Pathway (CLiP) is a second branch that stems from the spindle checkpoint to regulate cohesion preferentially at the centromeres and that Sgo1 is one of its components.

Supplemental Figures     

Centromere and telomere redistribution precedes homologue pairing and terminal synapsis initiation during prophase I of cattle spermatogenesis

Alterations in nuclear topology associated with meiotic chromosome pairing were studied in premeiotic cells and spermatocytes I of adult bovine males. To this end, we performed FISH with chromosome, pericentromeric satellite-DNA and telomere-specific probes in combination with immunostaining of synaptonemal complex proteins (SCP3, SCP1) on testis tissue sections. Nuclei of premeiotic cells (spermatogonia) exhibited a scattered telomere distribution while pericentromeres formed a few intranuclear clusters. We observed that the chromosome pairing process in cattle prophase I is preceded by repositioning of centromeres and telomeres to the nuclear periphery during preleptotene. Clustering of chromosome ends (bouquet formation) was observed during the transition from leptonema to zygonema and coincided with pairing of a sub-centromeric marker of bovine chromosomes 7. Dissolution of bouquet topology during zygonema left perinuclear telomeres scattered over the nuclear periphery at pachynema. SCP3 staining in frozen tissue sections revealed the appearance of this axial element protein in intranuclear aggregates during preleptotene, followed by extensive axial element formation during leptotene. Synapsis as revealed by SCP1 staining initiated peripherally at earliest zygotene, at this stage nuclei still contained numerous SCP3 clusters. Our observations reveal prominent non-homologous satellite-DNA associations in spermatogonia and indicate the conservation of topological features of the meiotic chromosome pairing process among mammals. The comparison of telomere dynamics in mouse and cattle prophase I suggests that a larger number of chromosomes prolongs the duration of the bouquet stage.     

Mutant telomere sequences lead to impaired chromosome separation and a unique checkpoint response

Mutation of the template region in the RNA component of telomerase can cause incorporation of mutant DNA sequences at telomeres. We made all 63 mutant sequence combinations at template positions 474-476 of the yeast telomerase RNA, TLC1. Mutants contained faithfully incorporated template mutations, as well as misincorporated sequences in telomeres, a phenotype not previously reported for Saccharomyces cerevisiae telomerase template mutants. Although growth rates and telomere profiles varied widely among the tlc1 mutants, chromosome separation and segregation were always aberrant. The mutants showed defects in sister chromatid separation at centromeres as well as telomeres, suggesting activation of a cell cycle checkpoint. Deletion of the DNA damage response genes DDC1, MEC3, or DDC2/SML1 failed to restore chromosome separation in the tlc1 template mutants. These results suggest that mutant telomere sequences elicit a checkpoint that is genetically distinct from those activated by deletion of telomerase or DNA damage.     

The mitotic centromeric protein MEI-S332 and its role in sister-chromatid cohesion

Faithful segregation of sister chromatids during cell division requires properly regulated cohesion between the sister centromeres. The sister chromatids are attached along their lengths, but particularly tightly in the centromeric regions. Therefore specific cohesion proteins may be needed at the centromere. Here we show that Drosophila MEI-S332 protein localizes to mitotic metaphase centromeres. Both overexpression and mutation of MEI-S332 increase the number of apoptotic cells. In mei-S332 mutants the ratio of metaphase to anaphase figures is lower than wild type, but it is higher if MEI-S332 is overexpressed. In chromosomal squashes centromeric attachments appear weaker in mei-S332 mutants than wild type and tighter when MEI-S332 is overexpressed. These results are consistent with MEI-S332 contributing to centromeric sister-chromatid cohesion in a dose-dependent manner. MEI-S332 is the first member identified of a predicted class of centromeric proteins that maintain centromeric cohesion. Received: 11 December 1998; in revised form: 4 August 1999 / Accepted: 13 August 1999     

Telomeres act autonomously in maize to organize the meiotic bouquet from a semipolarized chromosome orientation

During meiosis, chromosomes undergo large-scale reorganization to allow pairing between homologues, which is necessary for recombination and segregation. In many organisms, pairing of homologous chromosomes is accompanied, and possibly facilitated, by the bouquet, the clustering of telomeres in a small region of the nuclear periphery. Taking advantage of the cytological accessibility of meiosis in maize, we have characterized the organization of centromeres and telomeres throughout meiotic prophase. Our results demonstrate that meiotic centromeres are polarized prior to the bouquet stage, but that this polarization does not contribute to bouquet formation. By examining telocentric and ring chromosomes, we have tested the cis-acting requirements for participation in the bouquet. We find that: (a) the healed ends of broken chromosomes, which contain telomere repeats, can enter the bouquet; (b) ring chromosomes enter the bouquet, indicating that terminal position on a chromosome is not necessary for telomere sequences to localize to the bouquet; and (c) beginning at zygotene, the behavior of telomeres is dominant over any centromere-mediated chromosome behavior. The results of this study indicate that specific chromosome regions are acted upon to determine the organization of meiotic chromosomes, enabling the bouquet to form despite large-scale changes in chromosome architecture.     

Telomere length as a quantitative trait: genome-wide survey and genetic mapping of telomere length-control genes in yeast

Telomere length-variation in deletion strains of Saccharomyces cerevisiae was used to identify genes and pathways that regulate telomere length. We found 72 genes that when deleted confer short telomeres, and 80 genes that confer long telomeres relative to those of wild-type yeast. Among identified genes, 88 have not been previously implicated in telomere length control. Genes that regulate telomere length span a variety of functions that can be broadly separated into telomerase-dependent and telomerase-independent pathways. We also found 39 genes that have an important role in telomere maintenance or cell proliferation in the absence of telomerase, including genes that participate in deoxyribonucleotide biosynthesis, sister chromatid cohesion, and vacuolar protein sorting. Given the large number of loci identified, we investigated telomere lengths in 13 wild yeast strains and found substantial natural variation in telomere length among the isolates. Furthermore, we crossed a wild isolate to a laboratory strain and analyzed telomere length in 122 progeny. Genome-wide linkage analysis among these segregants revealed two loci that account for 30%–35% of telomere length-variation between the strains. These findings support a general model of telomere length-variation in outbred populations that results from polymorphisms at a large number of loci. Furthermore, our results laid the foundation for studying genetic determinants of telomere length-variation and their roles in human disease.     

Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction

The segregation of centromeres and telomeres at mitosis is coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. Mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during fission yeast mitosis. Telomere dispersion initiates in metaphase, whereas disjunction takes place in anaphase. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres, whereas disjunction occurs at anaphase after the phosphorylation of condensin subunit Cnd2. Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescued cell death after Aurora inhibition by promoting the loading of condensin on chromosome arms. Our findings reveal an essential role for telomeres in chromosome arm segregation.     

Calpain-1 cleaves Rad21 to promote sister chromatid separation

Defining the mechanisms of chromosomal cohesion and dissolution of the cohesin complex from chromatids is important for understanding the chromosomal missegregation seen in many tumor cells. Here we report the identification of a novel cohesin-resolving protease and describe its role in chromosomal segregation. Sister chromatids are held together by cohesin, a multiprotein ring-like complex comprised of Rad21, Smc1, Smc3, and SA2 (or SA1). Cohesin is known to be removed from vertebrate chromosomes by two distinct mechanisms, namely, the prophase and anaphase pathways. First, PLK1-mediated phosphorylation of SA2 in prophase leads to release of cohesin from chromosome arms, leaving behind centromeric cohesins that continue to hold the sisters together. Then, at the onset of anaphase, activated separase cleaves the centromeric cohesin Rad21, thereby opening the cohesin ring and allowing the sister chromatids to separate. We report here that the calcium-dependent cysteine endopeptidase calpain-1 is a Rad21 peptidase and normally localizes to the interphase nuclei and chromatin. Calpain-1 cleaves Rad21 at L192, in a calcium-dependent manner. We further show that Rad21 cleavage by calpain-1 promotes separation of chromosome arms, which coincides with a calcium-induced partial loss of cohesin at several chromosomal loci. Engineered cleavage of Rad21 at the calpain-cleavable site without activation of calpain-1 can lead to a loss of sister chromatid cohesion. Collectively, our work reveals a novel function of calpain-1 and describes an additional pathway for sister chromatid separation in humans.     

Human chromatid cohesin component hRad21 is phosphorylated in M phase and associated with metaphase centromeres

Sister chromatids duplicated in S phase are connected with each other during G(2) and M phase until the onset of anaphase. This chromatid cohesion is essential for correct segregation of genetic material to daughter cells. Recently, understanding of the molecular mechanisms governing chromatid cohesion in yeast has been greatly advanced, whereas these processes in mammalian cells remain unclear. We report here biochemical and cytological analyses of human Rad21, a homologue of the yeast cohesin subunit, Scc1p/Mcd1p. hRad21 is a nuclear phosphorylated protein. Its abundance does not change during the cell cycle, and it becomes hyperyphosphorylated in M phase. Most hRad21 is not associated with chromatin when the nuclear envelope breakdown takes place in prophase. However, a detailed analysis of the spread chromosomes indicated that hRad21 remains associated with prometaphase-like chromosomes along their entire lengths. The mitotic chromatin-bound hRad21 becomes dissociated in a highly regulated manner because hRad21 remains specifically at the centromeres but disappears from the arm regions on metaphase-like chromosomes. Interestingly, hRad21 at the metaphase centromeres appears to be present at the inner pairing domain where the two sister chromatids are supposed to be in intimate contact. These results suggest that hRad21 has a critical role in chromatid cohesion in human mitotic cells.     

Meiotic cohesion requires accumulation of ORD on chromosomes before condensation

Cohesion between sister chromatids is a prerequisite for accurate chromosome segregation during mitosis and meiosis. To allow chromosome condensation during prophase, the connections that hold sister chromatids together must be maintained but still permit extensive chromatin compaction. In Drosophila, null mutations in the orientation disruptor (ord) gene lead to meiotic nondisjunction in males and females because cohesion is absent by the time that sister kinetochores make stable microtubule attachments. We provide evidence that ORD is concentrated within the extrachromosomal domains of the nuclei of Drosophila primary spermatocytes during early G2, but accumulates on the meiotic chromosomes by mid to late G2. Moreover, using fluorescence in situ hybridization to monitor cohesion directly, we show that cohesion defects first become detectable in ord(null) spermatocytes shortly after the time when wild-type ORD associates with the chromosomes. After condensation, ORD remains bound at the centromeres of wild-type spermatocytes and persists there until centromeric cohesion is released during anaphase II. Our results suggest that association of ORD with meiotic chromosomes during mid to late G2 is required to maintain sister-chromatid cohesion during prophase condensation and that retention of ORD at the centromeres after condensation ensures the maintenance of centromeric cohesion until anaphase II.     

Vertebrate shugoshin links sister centromere cohesion and kinetochore microtubule stability in mitosis

Drosophila MEI-S332 and fungal Sgo1 genes are essential for sister centromere cohesion in meiosis I. We demonstrate that the related vertebrate Sgo localizes to kinetochores and is required to prevent premature sister centromere separation in mitosis, thus providing an explanation for the differential cohesion observed between the arms and the centromeres of mitotic sister chromatids. Sgo is degraded by the anaphase-promoting complex, allowing the separation of sister centromeres in anaphase. Intriguingly, we show that Sgo interacts strongly with microtubules in vitro and that it regulates kinetochore microtubule stability in vivo, consistent with a direct microtubule interaction. Sgo is thus critical for mitotic progression and chromosome segregation and provides an unexpected link between sister centromere cohesion and microtubule interactions at kinetochores.