The conserved kinetochore protein shugoshin protects centromeric cohesion during meiosis

Random chromosome distribution in the first meiosis of F1 disomic substitution line 2R(2D) x rye hybrids (ABDR, 4× = 28) occurs without bipolar spindle assembly

The assembly of the microtubule-based spindle structure in plant meiosis remains poorly understood compared with our knowledge of mitotic spindle formation. One of the approaches in our understanding of microtubule dynamics is to study spindle assembly in meiosis of amphyhaploids. Using immunostaining with phH3Ser10, CENH3 and α-tubulin-specific antibodies, we studied the chromosome distribution and spindle organisation in meiosis of F₁ 2R(2D)xR wheat-rye hybrids (genome structure ABDR, 4× = 28), as well as in wheat and rye mitosis and meiosis. At the prometaphase of mitosis, spindle assembly was asymmetric; one half of the spindle assembled before the other, with simultaneous chromosome alignment in the spindle mid-zone. At diakinesis in wheat and rye, microtubules formed a pro-spindle which was subsequently disassembled followed by a bipolar spindle assembly. In the first meiosis of hybrids 2R(2D)xR, a bipolar spindle was not found and the kinetochore microtubules distributed the chromosomes. Univalent chromosomes are characterised by a monopolar orientation and maintenance of sister chromatid and centromere cohesion. Presence of bivalents did not affect the formation of a bipolar spindle. Since the central spindle was absent, phragmoplast originates from "interpolar" microtubules generated by kinetochores. Cell plate development occurred with a delay. However, meiocytes in meiosis II contained apparently normal bipolar spindles. Thus, we can conclude that: (1) cohesion maintenance in centromeres and between arms of sister chromatids may negatively affect bipolar spindle formation in the first meiosis; (2) 2R/2D rye/wheat chromosome substitution affects the regulation of the random chromosome distribution in the absence of a bipolar spindle.

Meiosis I Kinase Regulators: Conserved Orchestrators of Reductional Chromosome Segregation

Research over the last two decades has identified a group of meiosis-specific proteins, consisting of budding yeast Spo13, fission yeast Moa1, mouse MEIKIN, and Drosophila Mtrm, with essential functions in meiotic chromosome segregation. These proteins, which we call meiosis I kinase regulators (MOKIRs), mediate two major adaptations to the meiotic cell cycle to allow the generation of haploid gametes from diploid mother cells. Firstly, they promote the segregation of homologous chromosomes in meiosis I (reductional division) by ensuring that sister kinetochores face towards the same pole (mono-orientation). Secondly, they safeguard the timely separation of sister chromatids in meiosis II (equational division) by counteracting the premature removal of pericentromeric cohesin, and thus prevent the formation of aneuploid gametes. Although MOKIRs bear no obvious sequence similarity, they appear to play functionally conserved roles in regulating meiotic kinases. Here, the known functions of MOKIRs are reviewed and their possible mechanisms of action are discussed. Also see the video abstract here https://youtu.be/tLE9KL89bwk.

Adaptations for centromere function in meiosis

The aim of mitosis is to segregate duplicated chromosomes equally into daughter cells during cell division. Meiosis serves a similar purpose, but additionally separates homologous chromosomes to produce haploid gametes for sexual reproduction. Both mitosis and meiosis rely on centromeres for the segregation of chromosomes. Centromeres are the specialized regions of the chromosomes that are attached to microtubules during their segregation. In this review, we describe the adaptations and layers of regulation that are required for centromere function during meiosis, and their role in meiosis-specific processes such as homolog-pairing and recombination. Since female meiotic divisions are asymmetric, meiotic centromeres are hypothesized to evolve quickly in order to favor their own transmission to the offspring, resulting in the rapid evolution of many centromeric proteins. We discuss this observation using the example of the histone variant CENP-A, which marks the centromere and is essential for centromere function. Changes in both the size and the sequence of the CENP-A N-terminal tail have led to additional functions of the protein, which are likely related to its roles during meiosis. We highlight the importance of CENP-A in the inheritance of centromere identity, which is dependent on the stabilization, recycling, or re-establishment of CENP-A-containing chromatin during meiosis.

Establishing correct kinetochore-microtubule attachments in mitosis and meiosis

Faithful chromosome segregation in mitosis and meiosis requires that chromosomes properly attach to spindle microtubules. Initial kinetochore-microtubule attachments are often incorrect and rely on error correction mechanisms to release improper attachments, allowing the formation of new attachments. Aurora B kinase and, in mammalian germ cells, Aurora C kinase function as the enzymatic component of the Chromosomal Passenger Complex (CPC), which localizes to the inner centromere/kinetochore and phosphorylates kinetochore proteins for microtubule release during error correction. In this review, we discuss recent findings of the molecular pathways that regulate the chromosomal localization of Aurora B and C kinases in human cell lines, mice, fission yeast, and budding yeast. We also discuss differences in the importance of localization pathways between mitosis and meiosis.

Phospho-regulation of the Shugoshin - Condensin interaction at the centromere in budding yeast

Correct bioriented attachment of sister chromatids to the mitotic spindle is essential for chromosome segregation. In budding yeast, the conserved protein shugoshin (Sgo1) contributes to biorientation by recruiting the protein phosphatase PP2A-Rts1 and the condensin complex to centromeres. Using peptide prints, we identified a Serine-Rich Motif (SRM) of Sgo1 that mediates the interaction with condensin and is essential for centromeric condensin recruitment and the establishment of biorientation. We show that the interaction is regulated via phosphorylation within the SRM and we determined the phospho-sites using mass spectrometry. Analysis of the phosphomimic and phosphoresistant mutants revealed that SRM phosphorylation disrupts the shugoshin–condensin interaction. We present evidence that Mps1, a central kinase in the spindle assembly checkpoint, directly phosphorylates Sgo1 within the SRM to regulate the interaction with condensin and thereby condensin localization to centromeres. Our findings identify novel mechanisms that control shugoshin activity at the centromere in budding yeast.

Proper chromosome segregation in eukaryotes is ensured through correct attachment of the spindle microtubules to the centromeric chromosomal regions. The attachment is mediated via the multimolecular proteinaceous complex called the kinetochore. This enables the establishment of bioirentation, when each sister chromatid is attached to microtubules emanating from opposite spindle poles. Shugoshin (Sgo1) is a conserved centromeric protein that facilitates biorientation through its interactions with the protein phosphatase PP2A-Rts1, chromosome passenger complex and centromeric condensin. Here, we identified a serine-rich motif that is required for the interaction of shugoshin with the condensin complex. We show that loss of this region impairs condensin enrichment at the centromere, chromosome biorientation, segregation as well as the function of the chromosome passenger complex in the error correction. Moreover, the interaction is phosphoregulated, as phosphorylation of the serine-rich motif on Sgo1 disrupts its interaction with condensin. Finally, we show that the conserved spindle assembly checkpoint kinase Mps1 is responsible for this phosphorylation. Our findings uncover novel regulatory mechanisms that facilitate proper chromosome segregation.

Microsatellite frameshift variants in SGO1 of gastric cancer are not always associated with MSI status

AIMS

Although frameshift variants in the microsatellite area of shugoshin 1 (SGO1) have been reported in the context of microsatellite instability-high (MSI-H)/deficient mismatch repair gastrointestinal cancer, most have been evaluated only in early stage I-III patients, and only two of its five microsatellite regions have been evaluated. Therefore, we investigated the frequency and MSI status of microsatellite frameshift variants in gastric cancer cases, including stage IV.

METHODS

In a total of 55 cases, 30 gastric cancer resection and 25 non-resection cases, DNA was extracted from both tumour and normal parts and PCR was performed. The variant was confirmed by TA cloning, and MSI was evaluated using GeneMapper software.

RESULTS

A frameshift variant of c.973delA was observed in 16 of the 45 evaluable cases. Its frequency was 35.6%. Of the 25 cases that could be assessed for MSI status, two cases of MSI-H were associated with the c.973delA SGO1 variant. However, c.973delA SGO1 variant was also observed in four cases of microsatellite stable.

CONCLUSION

Our study shows that SGO1 frameshift variants are not always associated with MSI status.

Atypical meiosis can be adaptive in outcrossed Schizosaccharomyces pombe due to wtf meiotic drivers

Killer meiotic drivers are genetic parasites that destroy ‘sibling’ gametes lacking the driver allele. The fitness costs of drive can lead to selection of unlinked suppressors. This suppression could involve evolutionary tradeoffs that compromise gametogenesis and contribute to infertility. Schizosaccharomyces pombe, an organism containing numerous gamete (spore)-killing wtf drivers, offers a tractable system to test this hypothesis. Here, we demonstrate that in scenarios analogous to outcrossing, wtf drivers generate a fitness landscape in which atypical spores, such as aneuploids and diploids, are advantageous. In this context, wtf drivers can decrease the fitness costs of mutations that disrupt meiotic fidelity and, in some circumstances, can even make such mutations beneficial. Moreover, we find that S. pombe isolates vary greatly in their ability to make haploid spores, with some isolates generating up to 46% aneuploid or diploid spores. This work empirically demonstrates the potential for meiotic drivers to shape the evolution of gametogenesis.

Inverted meiosis: an alternative way of chromosome segregation for reproduction

Canonical meiosis is characterized by two sequential rounds of nuclear divisions following one round of DNA replication-reductional segregation of homologous chromosomes during the first division and equational segregation of sister chromatids during the second division. Meiosis in an inverted order of two nuclear divisions-inverted meiosis has been observed in several species with holocentromeres as an adaptive strategy to overcome the obstacle in executing a canonical meiosis due to the holocentric chromosome structure. Recent findings of co-existence of inverted and canonical meiosis in two monocentric organisms, human and fission yeast, suggested that inverted meiosis could be common and also lead to the puzzle regarding the mechanistic feasibility for executing two meiosis programs simultaneously. Here, we discuss apparent conflicts for concurrent canonical meiosis and inverted meiosis. Furthermore, we attempt to provide a working model that may be compatible for both forms of meiosis.

Methotrexate impaired in-vivo matured mouse oocyte quality and the possible mechanisms

Background

Methotrexate (MTX) is an antifolate agent which is widely used in clinic for treating malignancies, rheumatoid arthritis and ectopic pregnancy. As reported, MTX has side effects on gastrointestinal system, nervous system and reproductive system, while its potential damages on oocyte quality are still unclear. It is known that oocyte quality is essential for healthy conception and the forthcoming embryo development. Thus, this work studied the effects of MTX on the oocyte quality.

Results

We established MTX model mice by single treatment with 5 mg/Kg MTX. Both morphological and molecular biology studies were performed to assess the in-vivo matured oocytes quality and to analyze the related mechanisms. The in-vivo matured oocytes from MTX-treated mice had poor in-vitro fertilization ability, and the resulting embryo formation rates and blastocyst quality were lower than the control group. We found that the in-vivo matured MTX-treated mouse oocytes displayed abnormal transcript expressions for genes of key enzymes in the folate cycles. MTX increased the rate of abnormal chromosome alignment and affected the regulation of chromosome separation via disrupting the spindle morphology and reducing the mRNA expressions of MAD2 and Sgo1. MTX reduced the DNA methylation levels in the in-vivo matured oocytes, and further studies showed that MTX altered the expressions of DNMT1 and DNMT 3b, and may also affect the levels of the methyl donor and its metabolite.

Conclusions

MTX impaired the in-vivo matured mouse oocyte quality by disturbing folate metabolism and affecting chromosome stability and methylation modification.

CDK Regulation of Meiosis: Lessons from S. cerevisiae and S. pombe

Meiotic progression requires precise orchestration, such that one round of DNA replication is followed by two meiotic divisions. The order and timing of meiotic events is controlled through the modulation of the phosphorylation state of proteins. Key components of this phospho-regulatory system include cyclin-dependent kinase (CDK) and its cyclin regulatory subunits. Over the past two decades, studies in budding and fission yeast have greatly informed our understanding of the role of CDK in meiotic regulation. In this review, we provide an overview of how CDK controls meiotic events in both budding and fission yeast. We discuss mechanisms of CDK regulation through post-translational modifications and changes in the levels of cyclins. Finally, we highlight the similarities and differences in CDK regulation between the two yeast species. Since CDK and many meiotic regulators are highly conserved, the findings in budding and fission yeasts have revealed conserved mechanisms of meiotic regulation among eukaryotes.

Kinesin-14 family proteins and microtubule dynamics define S. pombe mitotic and meiotic spindle assembly, and elongation

To segregate the chromosomes faithfully during cell division, cells assemble a spindle that captures the kinetochores and pulls them towards opposite poles. Proper spindle function requires correct interplay between microtubule motors and non-motor proteins. Defects in spindle assembly or changes in spindle dynamics are associated with diseases, such as cancer or developmental disorders. Here, we compared mitotic and meiotic spindles in fission yeast. We show that, even though mitotic and meiotic spindles underwent the typical three phases of spindle elongation, they have distinct features. We found that the relative concentration of the kinesin-14 family protein Pkl1 is decreased in meiosis I compared to mitosis, while the concentration of the kinesin-5 family protein Cut7 remains constant. We identified the second kinesin-14 family protein Klp2 and microtubule dynamics as factors necessary for proper meiotic spindle assembly. This work defines the differences between mitotic and meiotic spindles in fission yeast Schizosaccharomyces pombe, and provides prospect for future comparative studies.This article has an associated First Person interview with the first author of the paper.

Translesion synthesis polymerases contribute to meiotic chromosome segregation and cohesin dynamics in Schizosaccharomycespombe

Translesion synthesis polymerases (TLSPs) are non-essential error-prone enzymes that ensure cell survival by facilitating DNA replication in the presence of DNA damage. In addition to their role in bypassing lesions, TLSPs have been implicated in meiotic double-strand break repair in several systems. Here, we examine the joint contribution of four TLSPs to meiotic progression in the fission yeast Schizosaccharomyces pombe. We observed a dramatic loss of spore viability in fission yeast lacking all four TLSPs, which is accompanied by disruptions in chromosome segregation during meiosis I and II. Rec8 cohesin dynamics are altered in the absence of the TLSPs. These data suggest that the TLSPs contribute to multiple aspects of meiotic chromosome dynamics.

Analysis of the S. pombe Meiotic Proteome Reveals a Switch from Anabolic to Catabolic Processes and Extensive Post-transcriptional Regulation

Meiotic progression in S. pombe is regulated by stage-specific gene expression and translation, changes in RNA stability, expression of anti-sense transcripts, and targeted proteolysis of regulatory proteins. We have used SILAC labeling to examine the relative levels of proteins in diploid S. pombe cells during meiosis. Among the 3,268 proteins quantified at all time points, the levels of 880 proteins changed at least 2-fold; the majority of proteins showed stepwise increases or decreases during the meiotic divisions, while some changed transiently. Overall, we observed reductions in proteins involved in anabolism and increases in proteins involved in catabolism. We also observed increases in the levels of proteins of the ESCRT-III complex and revealed a role for ESCRT-III components in chromosome segregation and spore formation. Correlation with studies of meiotic gene expression and ribosome occupancy reveals that many of the changes in steady-state protein levels are post-transcriptional.

Histone H2A phosphorylation recruits topoisomerase IIα to centromeres to safeguard genomic stability

Chromosome segregation in mitosis requires the removal of catenation between sister chromatids. Timely decatenation of sister DNAs at mitotic centromeres by topoisomerase IIα (TOP2A) is crucial to maintain genomic stability. The chromatin factors that recruit TOP2A to centromeres during mitosis remain unknown. Here, we show that histone H2A Thr-120 phosphorylation (H2ApT120), a modification generated by the mitotic kinase Bub1, is necessary and sufficient for the centromeric localization of TOP2A. Phosphorylation at residue-120 enhances histone H2A binding to TOP2A in vitro. The C-gate and the extreme C-terminal region are important for H2ApT120-dependent localization of TOP2A at centromeres. Preventing H2ApT120-mediated accumulation of TOP2A at mitotic centromeres interferes with sister chromatid disjunction, as evidenced by increased frequency of anaphase ultra-fine bridges (UFBs) that contain catenated DNA. Tethering TOP2A to centromeres bypasses the requirement for H2ApT120 in suppressing anaphase UFBs. These results demonstrate that H2ApT120 acts as a landmark that recruits TOP2A to mitotic centromeres to decatenate sister DNAs. Our study reveals a fundamental role for histone phosphorylation in resolving centromere DNA entanglements and safeguarding genomic stability during mitosis.

The Major Factors Causing the Microspore Abortion of Genic Male Sterile Mutant NWMS1 in Wheat (Triticum aestivum L.)

Male sterility is a valuable trait for genetic research and production application of wheat (Triticum aestivum L.). NWMS1, a novel typical genic male sterility mutant, was obtained from Shengnong 1, mutagenized with ethyl methane sulfonate (EMS). Microstructure and ultrastructure observations of the anthers and microspores indicated that the pollen abortion of NWMS1 started at the early uninucleate microspore stage. Pollen grain collapse, plasmolysis, and absent starch grains were the three typical characteristics of the abnormal microspores. The anther transcriptomes of NWMS1 and its wild type Shengnong 1 were compared at the early anther development stage, pollen mother cell meiotic stage, and binucleate microspore stage. Several biological pathways clearly involved in abnormal anther development were identified, including protein processing in endoplasmic reticulum, starch and sucrose metabolism, lipid metabolism, and plant hormone signal transduction. There were 20 key genes involved in the abnormal anther development, screened out by weighted gene co-expression network analysis (WGCNA), including SKP1B, BIP5, KCS11, ADH3, BGLU6, and TIFY10B. The results indicated that the defect in starch and sucrose metabolism was the most important factor causing male sterility in NWMS1. Based on the experimental data, a primary molecular regulation model of abnormal anther and pollen developments in mutant NWMS1 was established. These results laid a solid foundation for further research on the molecular mechanism of wheat male sterility.

The Opposing Functions of Protein Kinases and Phosphatases in Chromosome Bipolar Attachment

Accurate chromosome segregation during cell division is essential to maintain genome integrity in all eukaryotic cells, and chromosome missegregation leads to aneuploidy and therefore represents a hallmark of many cancers. Accurate segregation requires sister kinetochores to attach to microtubules emanating from opposite spindle poles, known as bipolar attachment or biorientation. Recent studies have uncovered several mechanisms critical to chromosome bipolar attachment. First, a mechanism exists to ensure that the conformation of sister centromeres is biased toward bipolar attachment. Second, the phosphorylation of some kinetochore proteins destabilizes kinetochore attachment to facilitate error correction, but a protein phosphatase reverses this phosphorylation. Moreover, the activity of the spindle assembly checkpoint is regulated by kinases and phosphatases at the kinetochore, and this checkpoint prevents anaphase entry in response to faulty kinetochore attachment. The fine-tuned kinase/phosphatase balance at kinetochores is crucial for faithful chromosome segregation during both mitosis and meiosis. Here, we discuss the function and regulation of protein phosphatases in the establishment of chromosome bipolar attachment with a focus on the model organism budding yeast.

The pericentromeric protein shugoshin 2 cooperates with HSF1 in heat shock response and RNA Pol II recruitment

The recruitment of RNA polymerase II (Pol II) to core promoters is highly regulated during rapid induction of genes. In response to heat shock, heat shock transcription factor 1 (HSF1) is activated and occupies heat shock gene promoters. Promoter-bound HSF1 recruits general transcription factors and Mediator, which interact with Pol II, but stress-specific mechanisms of Pol II recruitment are unclear. Here, we show in comparative analyses of HSF1 paralogs and their mutants that HSF1 interacts with the pericentromeric adaptor protein shugoshin 2 (SGO2) during heat shock in mouse cells, in a manner dependent on inducible phosphorylation of HSF1 at serine 326, and recruits SGO2 to the HSP70 promoter. SGO2-mediated binding and recruitment of Pol II with a hypophosphorylated C-terminal domain promote expression of HSP70, implicating SGO2 as one of the coactivators that facilitate Pol II recruitment by HSF1. Furthermore, the HSF1-SGO2 complex supports cell survival and maintenance of proteostasis in heat shock conditions. These results exemplify a proteotoxic stress-specific mechanism of Pol II recruitment, which is triggered by phosphorylation of HSF1 during the heat shock response.

Construction of strains to identify novel factors for regulation of centromeric cohesion protection (CCP) and sister kinetochore mono-orientation (SKM)

Background

Meiosis-I is a unique type of chromosome segregation where each chromosome aligns and segregates from its homolog. The mechanism of meiosis I homolog separation in different eukaryotes depends on their centromere and kinetochore architecture which in turn relies mainly on two processes, first on a specialized four protein complex known as monopolin and second, the centromeric cohesion protection (CCP). However, in mammals the complex has not been identified. Furthermore, in budding yeast, there could be additional factors in this process which includes some meiosis specific and some non meiosis specific factors.

Result

We constructed two strains. In the first strain we expressed Mam1 and Cdc5 which leads to sister kinetochore monoorientation (SKM) and in the second case we expressed Rec8 and Spo13 which enhanced CCP even in mitosis. The expression of these proteins in mitotically dividing cells caused co-orientation of the chromosomes, which lead to the cell death followed by miss-segregation of chromosomes. Then we utilized these strains to screen the cDNA libraries from yeast and mammals to identify the novel factors which participate in CCP and SKM. Finally, SGY4119 strain expressing Spo13 and Rec8 was transformed with pRS316 gal cDNA library and transformants were screened for lethality on galactose. We screened ~ 10⁵ transformants colonies. Out of these ~ 3000 colonies were able to survive on galactose plate which was narrow down to 6 on the basis of desired phenotype.

Conclusion

So far, meiosis specific kinetochore proteins have been identified only in two yeasts. Recently, in mammals a meiosis specific kinetochore protein (MEIKIN) has been identified with similar function. Till now a single protein in mammals and four proteins monopolin complex in budding yeast has been identified to coorient the centromere. Many more novel factors have to be identified yet. That is why we wished to device genetic screen using a functional genomics approach. Since the list of proteins already identified in yeast is not exhaustive as the circumstantial evidence suggests, we wish to use the same yeast strains to identify additional novel yeast proteins that may be involved in the execution of meiosis.

Role of E2Fs and mitotic regulators controlled by E2Fs in the epithelial to mesenchymal transition

The epithelial-to-mesenchymal transition (EMT) is a complex cellular process in which epithelial cells acquire mesenchymal properties. EMT occurs in three biological settings: development, wound healing and fibrosis, and tumor progression. Despite occurring in three independent biological settings, EMT signaling shares some molecular mechanisms that allow epithelial cells to de-differentiate and acquire mesenchymal characteristics that confer cells invasive and migratory capacity to distant sites. Here we summarize the molecular mechanism that delineates EMT and we will focus on the role of E2 promoter binding factors (E2Fs) in EMT during tumor progression. Since the E2Fs are presently undruggable due to their control in numerous pivotal cellular functions and due to the lack of selectivity against individual E2Fs, we will also discuss the role of three mitotic regulators and/or mitotic kinases controlled by the E2Fs (NEK2, Mps1/TTK, and SGO1) in EMT that can be useful as drug targets.

Impact statement

The study of the epithelial to mesenchymal transition (EMT) is an active area of research since it is one of the early intermediates to invasion and metastasis—a state of the cancer cells that ultimately kills many cancer patients. We will present in this review that besides their canonical roles as regulators of proliferation, unregulated expression of the E2F transcription factors may contribute to cancer initiation and progression to metastasis by signaling centrosome amplification, chromosome instability, and EMT. Since our discovery that the E2F activators control centrosome amplification and mitosis in cancer cells, we have identified centrosome and mitotic regulators that may represent actionable targets against EMT and metastasis in cancer cells. This is impactful to all of the cancer patients in which the Cdk/Rb/E2F pathway is deregulated, which has been estimated to be most cancer patients with solid tumors.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN. Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach. Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13 Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13 Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8. Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13 Δ cells.

Spo13 prevents premature cohesin cleavage during meiosis

Background: Meiosis produces gametes through two successive nuclear divisions, meiosis I and meiosis II. In contrast to mitosis and meiosis II, where sister chromatids are segregated, during meiosis I, homologous chromosomes are segregated. This requires the monopolar attachment of sister kinetochores and the loss of cohesion from chromosome arms, but not centromeres, during meiosis I. The establishment of both sister kinetochore mono-orientation and cohesion protection rely on the budding yeast meiosis I-specific Spo13 protein, the functional homolog of fission yeast Moa1 and mouse MEIKIN.

Methods: Here we investigate the effects of loss of SPO13 on cohesion during meiosis I using a live-cell imaging approach.

Results: Unlike wild type, cells lacking SPO13 fail to maintain the meiosis-specific cohesin subunit, Rec8, at centromeres and segregate sister chromatids to opposite poles during anaphase I. We show that the cohesin-destabilizing factor, Wpl1, is not primarily responsible for the loss of cohesion during meiosis I. Instead, premature loss of centromeric cohesin during anaphase I in spo13Δ cells relies on separase-dependent cohesin cleavage. Further, cohesin loss in spo13Δ anaphase I cells is blocked by forcibly tethering the regulatory subunit of protein phosphatase 2A, Rts1, to Rec8.

Conclusions: Our findings indicate that separase-dependent cleavage of phosphorylated Rec8 causes premature cohesin loss in spo13Δ cells.

Distributing meiotic crossovers for optimal fertility and evolution

During meiosis, homologous chromosomes of a diploid cell are replicated and, without a second replication, are segregated during two nuclear divisions to produce four haploid cells (including discarded polar bodies in females of many species). Proper segregation of chromosomes at the first division requires in most species that homologous chromosomes be physically connected. Tension generated by connected chromosomes moving to opposite sides of the cell signals proper segregation. In the absence of the required connections, called crossovers, chromosomes often segregate randomly and produce aneuploid gametes and, thus, dead or disabled progeny. To be effective, crossovers must be properly distributed along chromosomes. Crossovers within or too near the centromere interfere with proper segregation; crossovers too near each other can ablate the required tension; and crossovers too concentrated in only one or a few regions would not re-assort most genetic characters important for evolution. Here, we discuss current knowledge of how the optimal distribution of crossovers is achieved in the fission yeast Schizosaccharomyces pombe, with reference to other well-studied species for comparison and illustration of the diversity of biology.

Implementation of the CRISPR-Cas13a system in fission yeast and its repurposing for precise RNA editing

In contrast to genome editing, which introduces genetic changes at the DNA level, disrupting or editing gene transcripts provides a distinct approach to perturbing a genetic system, offering benefits complementary to classic genetic approaches. To develop a new toolset for manipulating RNA, we first implemented a member of the type VI CRISPR systems, Cas13a from Leptotrichia shahii (LshCas13a), in Schizosaccharomyces pombe, an important model organism employed by biologists to study key cellular mechanisms conserved from yeast to humans. This approach was shown to knock down targeted endogenous gene transcripts with different efficiencies. Second, we engineered an RNA editing system by tethering an inactive form of LshCas13a (dCas13) to the catalytic domain of human adenosine deaminase acting on RNA type 2 (hADAR2d), which was shown to be programmable with crRNA to target messenger RNAs and precisely edit specific nucleotide residues. We optimized system parameters using a dual-fluorescence reporter and demonstrated the utility of the system in editing randomly selected endogenous gene transcripts. We further used it to restore the transposition of retrotransposon Tf1 mutants in fission yeast, providing a potential novel toolset for retrovirus manipulation and interference.

Interplay between Phosphatases and the Anaphase-Promoting Complex/Cyclosome in Mitosis

Accurate division of cells into two daughters is a process that is vital to propagation of life. Protein phosphorylation and selective degradation have emerged as two important mechanisms safeguarding the delicate choreography of mitosis. Protein phosphatases catalyze dephosphorylation of thousands of sites on proteins, steering the cells through establishment of the mitotic phase and exit from it. A large E3 ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C) becomes active during latter stages of mitosis through G1 and marks hundreds of proteins for destruction. Recent studies have revealed the complex interregulation between these two classes of enzymes. In this review, we highlight the direct and indirect mechanisms by which phosphatases and the APC/C mutually influence each other to ensure accurate spatiotemporal and orderly progression through mitosis, with a particular focus on recent insights and conceptual advances.

OsMTOPVIB is required for meiotic bipolar spindle assembly

The organization of microtubules into a bipolar spindle is essential for chromosome segregation. Both centrosome and chromatin-dependent spindle assembly mechanisms are well studied in mouse, Drosophila melanogaster, and Xenopus oocytes; however, the mechanism of bipolar spindle assembly in plant meiosis remains elusive. According to our observations of microtubule assembly in Oryza sativa, Zea mays, Arabidopsis thaliana, and Solanum lycopersicum, we propose that a key step of plant bipolar spindle assembly is the correction of the multipolar spindle into a bipolar spindle at metaphase I. The multipolar spindles failed to transition into bipolar ones in OsmtopVIB with the defect in double-strand break (DSB) formation. However, bipolar spindles were normally assembled in several other mutants lacking DSB formation, such as Osspo11-1, pair2, and crc1, indicating that bipolar spindle assembly is independent of DSB formation. We further revealed that the mono-orientation of sister kinetochores was prevalent in OsmtopVIB, whereas biorientation of sister kinetochores was frequently observed in Osspo11-1, pair2, and crc1 In addition, mutations of the cohesion subunit OsREC8 resulted in biorientation of sister kinetochores as well as bipolar spindles even in the background of OsmtopVIB Therefore, we propose that biorientation of the kinetochore is required for bipolar spindle assembly in the absence of homologous recombination.

Patronus is the elusive plant securin, preventing chromosome separation by antagonizing separase

Accurate chromosome segregation at mitosis and meiosis is crucial to prevent genome instability, birth defect, and cancer. Accordingly, separase, the protease that triggers chromosome distribution, is tightly regulated by a direct inhibitor, the securin. However, securin has not been identified, neither functionnally nor by sequence similarity, in other clades that fungi and animals. This raised doubts about the conservation of this mechanism in other branches of eukaryotes. Here, we identify and characterize the securin in plants. Despite extreme sequence divergence, the securin kept the same core function and is likely a universal regulator of cell division in eukaryotes.

Chromosome distribution at anaphase of mitosis and meiosis is triggered by separase, an evolutionarily conserved protease. Separase must be tightly regulated to prevent the untimely release of chromatid cohesion and disastrous chromosome distribution defects. Securin is the key inhibitor of separase in animals and fungi, but has not been identified in other eukaryotic lineages. Here, we identified PATRONUS1 and PATRONUS2 (PANS1 and PANS2) as the Arabidopsis homologs of securin. Disruption of PANS1 is known to lead to the premature separation of chromosomes at meiosis, and the simultaneous disruption of PANS1 and PANS2 is lethal. Here, we show that PANS1 targeting by the anaphase-promoting complex is required to trigger chromosome separation, mirroring the regulation of securin. We showed that PANS1 acts independently from Shugosins. In a genetic screen for pans1 suppressors, we identified SEPARASE mutants, showing that PANS1 and SEPARASE have antagonistic functions in vivo. Finally, we showed that the PANS1 and PANS2 proteins interact directly with SEPARASE. Altogether, our results show that PANS1 and PANS2 act as a plant securin. Remote sequence similarity was identified between the plant patronus family and animal securins, suggesting that they indeed derive from a common ancestor. Identification of patronus as the elusive plant securin illustrates the extreme sequence divergence of this central regulator of mitosis and meiosis.

Meiotic spindle formation in mammalian oocytes: implications for human infertility

In the final stage of oogenesis, mammalian oocytes generate a meiotic spindle and undergo chromosome segregation to yield an egg that is ready for fertilization. Herein, we describe the recent advances in understanding the mechanisms controlling formation of the meiotic spindle in metaphase I (MI) and metaphase II (MII) in mammalian oocytes, and focus on the differences between mouse and human oocytes. Unlike mitotic cells, mammalian oocytes lack typical centrosomes that consist of two centrioles and the surrounding pericentriolar matrix proteins, which serve as microtubule-organizing centers (MTOCs) in most somatic cells. Instead, oocytes rely on different mechanisms for the formation of microtubules in MI spindles. Two different mechanisms have been described for MI spindle formation in mammalian oocytes. Chromosome-mediated microtubule formation, including RAN-mediated spindle formation and chromosomal passenger complex-mediated spindle elongation, controls the growth of microtubules from chromatin, while acentriolar MTOC-mediated microtubule formation contributes to spindle formation. Mouse oocytes utilize both chromatin- and MTOC-mediated pathways for microtubule formation. The existence of both pathways may provide a fail-safe mechanism to ensure high fidelity of chromosome segregation during meiosis. Unlike mouse oocytes, human oocytes considered unsuitable for clinical in vitro fertilization procedures, lack MTOCs; this may explain why meiosis in human oocytes is often error-prone. Understanding the mechanisms of MI/MII spindle formation, spindle assembly checkpoint, and chromosome segregation, in mammalian oocytes, will provide valuable insights into the molecular mechanisms of human infertility.

Centromeric Cohesin: Molecular Glue and Much More

Sister chromatid cohesion, mediated by the cohesin complex, is a prerequisite for faithful chromosome segregation during mitosis. Premature release of sister chromatid cohesion leads to random segregation of the genetic material and consequent aneuploidy. Multiple regulatory mechanisms ensure proper timing for cohesion establishment, concomitant with DNA replication, and cohesion release during the subsequent mitosis. Here we summarize the most important phases of the cohesin cycle and the coordination of cohesion release with the progression through mitosis. We further discuss recent evidence that has revealed additional functions for centromeric localization of cohesin in the fidelity of mitosis in metazoans. Beyond its well-established role as "molecular glue", centromeric cohesin complexes are now emerging as a scaffold for multiple fundamental processes during mitosis, including the formation of correct chromosome and kinetochore architecture, force balance with the mitotic spindle, and the association with key molecules that regulate mitotic fidelity, particularly at the chromosomal inner centromere. Centromeric chromatin may be thus seen as a dynamic place where cohesin ensures mitotic fidelity by multiple means.

Reductional Meiosis I Chromosome Segregation Is Established by Coordination of Key Meiotic Kinases

Meiosis produces gametes through a specialized, two-step cell division, which is highly error prone in humans. Reductional meiosis I, where maternal and paternal chromosomes (homologs) segregate, is followed by equational meiosis II, where sister chromatids separate. Uniquely during meiosis I, sister kinetochores are monooriented and pericentromeric cohesin is protected. Here, we demonstrate that these key adaptations for reductional chromosome segregation are achieved through separable control of multiple kinases by the meiosis-I-specific budding yeast Spo13 protein. Recruitment of Polo kinase to kinetochores directs monoorientation, while independently, cohesin protection is achieved by containing the effects of cohesin kinases. Therefore, reductional chromosome segregation, the defining feature of meiosis, is established by multifaceted kinase control by a master regulator. The recent identification of Spo13 orthologs, fission yeast Moa1 and mouse MEIKIN, suggests that kinase coordination by a meiosis I regulator may be a general feature in the establishment of reductional chromosome segregation.

A genome-wide RNAi screen identifies the SMC5/6 complex as a non-redundant regulator of a Topo2a-dependent G2 arrest

The Topo2a-dependent arrest is associated with faithful segregation of sister chromatids and has been identified as dysfunctional in numerous tumour cell lines. This genome-protecting pathway is poorly understood and its characterization is of significant interest, potentially offering interventional opportunities in relation to synthetic lethal behaviours in arrest-defective tumours. Using the catalytic Topo2a inhibitor ICRF193, we have performed a genome-wide siRNA screen in arrest-competent, non-transformed cells, to identify genes essential for this arrest mechanism. In addition, we have counter-screened several DNA-damaging agents and demonstrate that the Topo2a-dependent arrest is genetically distinct from DNA damage checkpoints. We identify the components of the SMC5/6 complex, including the activity of the E3 SUMO ligase NSE2, as non-redundant players that control the timing of the Topo2a-dependent arrest in G2. We have independently verified the NSE2 requirement in fibroblasts from patients with germline mutations that cause severely reduced levels of NSE2. Through imaging Topo2a-dependent G2 arrested cells, an increased interaction between Topo2a and NSE2 is observed at PML bodies, which are known SUMOylation hotspots. We demonstrate that Topo2a is SUMOylated in an ICRF193-dependent manner by NSE2 at a novel non-canonical site (K1520) and that K1520 sumoylation is required for chromosome segregation but not the G2 arrest.

Bub1 kinase- and H2A phosphorylation-independent regulation of Shugoshin proteins under glucose-restricted conditions

Shugoshin family proteins are involved in various aspects of chromatin regulations, such as chromosome segregation, chromatin structure, and gene expression. In growing yeast and mammalian cells, C-terminal phosphorylation of histone H2A by Bub1 kinase is essential for the localization of Shugoshin proteins to chromatin. Here, we show that in stationary-phase cells, Bub1-mediated H2A phosphorylation is not necessary for chromatin localization of the Shugoshin paralog Sgo2 in Schizosaccharomyces pombe, or for Sgo2-dependent suppression of gene expression in subtelomeric regions. The conserved C-terminal basic domain of Sgo2, which directly binds with phosphorylated H2A, is also dispensable for the localization of Sgo2 to chromatin at stationary phase. Instead, we found that the conserved N-terminal coiled-coil domain and the uncharacterized medial region of Sgo2 are required for Bub1-independent localization of Sgo2. Moreover, Set2-mediated H3K36 methylation was important for the regulation. Intriguingly, the chromatin localization of Sgo2 in the absence of Bub1 was also observed when cells were grown in low-glucose medium. These findings suggest a novel mechanism between nutrient availability and regulation of chromatin by Shugoshin proteins.

Immediate visualization of recombination events and chromosome segregation defects in fission yeast meiosis

Schizosaccharomyces pombe, also known as fission yeast, is an established model for studying chromosome biological processes. Over the years, research employing fission yeast has made important contributions to our knowledge about chromosome segregation during meiosis, as well as meiotic recombination and its regulation. Quantification of meiotic recombination frequency is not a straightforward undertaking, either requiring viable progeny for a genetic plating assay, or relying on laborious Southern blot analysis of recombination intermediates. Neither of these methods lends itself to high-throughput screens to identify novel meiotic factors. Here, we establish visual assays novel to Sz. pombe for characterizing chromosome segregation and meiotic recombination phenotypes. Genes expressing red, yellow, and/or cyan fluorophores from spore-autonomous promoters have been integrated into the fission yeast genomes, either close to the centromere of chromosome 1 to monitor chromosome segregation, or on the arm of chromosome 3 to form a genetic interval at which recombination frequency can be determined. The visual recombination assay allows straightforward and immediate assessment of the genetic outcome of a single meiosis by epi-fluorescence microscopy without requiring tetrad dissection. We also demonstrate that the recombination frequency analysis can be automatized by utilizing imaging flow cytometry to enable high-throughput screens. These assays have several advantages over traditional methods for analyzing meiotic phenotypes.

Destabilization of spindle assembly checkpoint causes aneuploidy during meiosis II in murine post-ovulatory aged oocytes

Mammalian oocyte quality degrades over time after ovulation in vitro, which can cause fatal defects such as chromosomal aneuploidy. As various oocyte manipulations employed in assisted reproductive technology are time consuming, post-ovulatory aging is a serious problem to overcome in reproductive medicine or ova research. In this study, we investigated the effects of postovulatory aging on the incidence of chromosome aneuploidy during meiosis II, with a focus on the expression of functional proteins from the spindle assembly checkpoint (SAC). Chromosome analysis was used to assess the rate of aneuploidy in in vitro aged oocytes, or in early embryos derived from aged oocytes. Immunofluorescent staining was used to detect the localization of MAD2, which is a SAC signal that monitors the correct segregation of sister chromatids. Immunoblotting was used to quantify cohesin subunits, which are adhesion factors connecting sister chromatids at the metaphase II (MII) centromere. It was shown that post-ovulatory oocyte aging inhibits MAD2 localization to the sister kinetochore. Furthermore, oocyte aging prevented cohesin subunits from being maintained or degraded at the appropriate time. These data suggest that the destabilization of SAC signaling causes sister chromatid segregation errors in MII oocytes, and consequently increases the incidence of aneuploidy in early embryos. Our findings have provided distinct evidence that the post-ovulatory aging of oocytes might also be a risk factor for aneuploidy, irrespective of maternal age.

Meiosis-specific prophase-like pathway controls cleavage-independent release of cohesin by Wapl phosphorylation

Sister chromatid cohesion on chromosome arms is essential for the segregation of homologous chromosomes during meiosis I while it is dispensable for sister chromatid separation during mitosis. It was assumed that, unlike the situation in mitosis, chromosome arms retain cohesion prior to onset of anaphase-I. Paradoxically, reduced immunostaining signals of meiosis-specific cohesin, including the kleisin Rec8, were observed on chromosomes during late prophase-I of budding yeast. This decrease is seen in the absence of Rec8 cleavage and depends on condensin-mediated recruitment of Polo-like kinase (PLK/Cdc5). In this study, we confirmed that this release indeed accompanies the dissociation of acetylated Smc3 as well as Rec8 from meiotic chromosomes during late prophase-I. This release requires, in addition to PLK, the cohesin regulator, Wapl (Rad61/Wpl1 in yeast), and Dbf4-dependent Cdc7 kinase (DDK). Meiosis-specific phosphorylation of Rad61/Wpl1 and Rec8 by PLK and DDK collaboratively promote this release. This process is similar to the vertebrate “prophase” pathway for cohesin release during G2 phase and pro-metaphase. In yeast, meiotic cohesin release coincides with PLK-dependent compaction of chromosomes in late meiotic prophase-I. We suggest that yeast uses this highly regulated cleavage-independent pathway to remove cohesin during late prophase-I to facilitate morphogenesis of condensed metaphase-I chromosomes.

In meiosis the life and health of future generations is decided upon. Any failure in chromosome segregation has a detrimental impact. Therefore, it is currently believed that the physical connections between homologous chromosomes are maintained by meiotic cohesin with exceptional stability. Indeed, it was shown that cohesive cohesin does not show an appreciable turnover during long periods in oocyte development. In this context, it was long assumed but not properly investigated, that the prophase pathway for cohesin release would be specific to mitosis and would be safely suppressed during meiosis so as not to endanger essential connections between chromosomes. However, a previous study on budding yeast meiosis suggests the presence of cleavage-independent pathway of cohesin release during late prophase-I. In the work presented here we confirmed that the prophase pathway is not suppressed during meiosis, at least in budding yeast and showed that this cleavage-independent release is regulated by meiosis-specific phosphorylation of two cohesin subunits, Rec8 and Rad61(Wapl) by two cell-cycle regulators, PLK and DDK. Our results suggest that late meiotic prophase-I actively controls cohesin dynamics on meiotic chromosomes for chromosome segregation.

The cohesin complex in mammalian meiosis

Cohesin is an evolutionary conserved multi-protein complex that plays a pivotal role in chromosome dynamics. It plays a role both in sister chromatid cohesion and in establishing higher order chromosome architecture, in somatic and germ cells. Notably, the cohesin complex in meiosis differs from that in mitosis. In mammalian meiosis, distinct types of cohesin complexes are produced by altering the combination of meiosis-specific subunits. The meiosis-specific subunits endow the cohesin complex with specific functions for numerous meiosis-associated chromosomal events, such as chromosome axis formation, homologue association, meiotic recombination and centromeric cohesion for sister kinetochore geometry. This review mainly focuses on the cohesin complex in mammalian meiosis, pointing out the differences in its roles from those in mitosis. Further, common and divergent aspects of the meiosis-specific cohesin complex between mammals and other organisms are discussed.

Aurora B kinase activity-dependent and -independent functions of the chromosomal passenger complex in regulating sister chromatid cohesion

The chromosomal passenger complex (CPC) is a master regulator of mitosis. CPC consists of inner centromere protein (INCENP), Survivin, Borealin, and the kinase Aurora B and plays key roles in regulating kinetochore-microtubule attachments and spindle assembly checkpoint signaling. However, the role of CPC in sister chromatid cohesion, mediated by the cohesin complex, remains incompletely understood. Here, we show that Aurora B kinase activity contributes to centromeric cohesion protection partly through promoting kinetochore localization of the kinase Bub1. Interestingly, disrupting the interaction of INCENP with heterochromatin protein 1 (HP1) in HeLa cells selectively weakens cohesion at mitotic centromeres without detectably reducing the kinase activity of Aurora B. Thus, through this INCENP-HP1 interaction, the CPC also protects centromeric cohesion independently of Aurora B kinase activity. Moreover, the requirement for the INCENP-HP1 interaction in centromeric cohesion protection can be bypassed by tethering HP1 to centromeres or by depleting the cohesin release factor Wapl. We provide further evidence suggesting that the INCENP-HP1 interaction protects centromeric cohesion by promoting the centromere localization of Haspin, a protein kinase that antagonizes Wapl activity at centromeres. Taken together, this study identifies Aurora B kinase activity-dependent and -independent roles for the CPC in regulating centromeric cohesion during mitosis in human cells.

Genetic Interactions between the Aurora Kinases Reveal New Requirements for AURKB and AURKC during Oocyte Meiosis

Errors in chromosome segregation during female meiosis I occur frequently, and aneuploid embryos account for 1/3 of all miscarriages in humans [1]. Unlike mitotic cells that require two Aurora kinase (AURK) homologs to help prevent aneuploidy (AURKA and AURKB), mammalian germ cells also require a third (AURKC) [2, 3]. AURKA is the spindle-pole-associated homolog, and AURKB/C are the chromosome-localized homologs. In mitosis, AURKB has essential roles as the catalytic subunit of the chromosomal passenger complex (CPC), regulating chromosome alignment, kinetochore-microtubule attachments, cohesion, the spindle assembly checkpoint, and cytokinesis [4, 5]. In mouse oocyte meiosis, AURKC takes over as the predominant CPC kinase [6], although the requirement for AURKB remains elusive [7]. In the absence of AURKC, AURKB compensates, making defining potential non-overlapping functions difficult [6, 8]. To investigate the role(s) of AURKB and AURKC in oocytes, we analyzed oocyte-specific Aurkb and Aurkc single- and double-knockout (KO) mice. Surprisingly, we find that double KO female mice are fertile. We demonstrate that, in the absence of AURKC, AURKA localizes to chromosomes in a CPC-dependent manner. These data suggest that AURKC prevents AURKA from localizing to chromosomes by competing for CPC binding. This competition is important for adequate spindle length to support meiosis I. We also describe a unique requirement for AURKB to negatively regulate AURKC to prevent aneuploidy. Together, our work reveals oocyte-specific roles for the AURKs in regulating each other's localization and activity. This inter-kinase regulation is critical to support wild-type levels of fecundity in female mice.

Shugoshin Is Essential for Meiotic Prophase Checkpoints in C. elegans

The conserved factor Shugoshin is dispensable in C. elegans for the two-step loss of sister chromatid cohesion that directs the proper segregation of meiotic chromosomes. We show that the C. elegans ortholog of Shugoshin, SGO-1, is required for checkpoint activity in meiotic prophase. This role in checkpoint function is similar to that of conserved proteins that structure meiotic chromosome axes. Indeed, null sgo-1 mutants exhibit additional phenotypes similar to that of a partial loss-of-function allele of the axis component, HTP-3: premature synaptonemal complex disassembly, the activation of alternate DNA repair pathways, and an inability to recruit a conserved effector of the DNA damage pathway, HUS-1. SGO-1 localizes to pre-meiotic nuclei when HTP-3 is present but not yet loaded onto chromosome axes and genetically interacts with a central component of the cohesin complex, SMC-3, suggesting that it contributes to meiotic chromosome metabolism early in meiosis by regulating cohesin. We propose that SGO-1 acts during pre-meiotic replication to ensure fully functional meiotic chromosome architecture, rendering these chromosomes competent for checkpoint activity and normal progression of meiotic recombination. Given that most research on Shugoshin has focused on its regulation of sister chromatid cohesion during chromosome segregation, this novel role may be conserved but previously uncharacterized in other organisms. Further, our findings expand the repertoire of Shugoshin's functions beyond coordinating regulatory activities at the centromere.

An interplay between Shugoshin and Spo13 for centromeric cohesin protection and sister kinetochore mono-orientation during meiosis I in Saccharomyces cerevisiae

Meiosis is a specialized cell division process by which haploid gametes are produced from a diploid mother cell. Reductional chromosome segregation during meiosis I (MI) is achieved by two unique and conserved events: centromeric cohesin protection (CCP) and sister kinetochore mono-orientation (SKM). In Saccharomyces cerevisiae, a meiosis-specific protein Spo13 plays a role in both these centromere-specific events. Despite genome-wide association of Spo13, we failed to detect its function in global processes such as cohesin loading, cohesion establishment and homologs pairing. While Shugoshin (Sgo1) and protein phosphatase 2A (PP2ARts1) play a central role in CCP, it is not fully understood whether Spo13 functions in the process through a Sgo1- PP2ARts1-dependent or -independent mechanism. To delineate this and to find the relative contribution of each of these proteins in CCP and SKM, we meticulously observed the sister chromatid segregation pattern in the wild type, sgo1Δ, rts1Δ and spo13Δ single mutants and in their respective double mutants. We found that Spo13 protects centromeric cohesin through a Sgo1- PP2ARts1-independent mechanism. To our surprise, we observed a hitherto unknown role of Sgo1 in SKM. Further investigation revealed that Sgo1-mediated recruitment of aurora kinase Ipl1 to the centromere facilitates monopolin loading at the kinetochore during MI. Hence, this study uncovers the role of Sgo1 in SKM and demonstartes how the regulators (Sgo1, PP2ARts1, Spo13) work in a coordinated manner to achieve faithful chromosome segregation during meiosis, the failure of which leads to aneuploidy and birth defects.

Tripartite Chromatin Localization of Budding Yeast Shugoshin Involves Higher-Ordered Architecture of Mitotic Chromosomes

The spindle assembly checkpoint (SAC) is key to faithful segregation of chromosomes. One requirement that satisfies SAC is appropriate tension between sister chromatids at the metaphase-anaphase juncture. Proper tension generated by poleward pulling of mitotic spindles signals biorientation of the underlying chromosome. In the budding yeast, the tension status is monitored by the conserved Shugoshin protein, Sgo1p, and the tension sensing motif (TSM) of histone H3. ChIP-seq reveals a unique TSM-dependent, tripartite domain of Sgo1p in each mitotic chromosome. This domain consists of one centromeric and two flanking peaks 3 - 4 kb away, present exclusively in mitosis. Strikingly, this trident motif coincides with cohesin localization, but only at the centromere and the two immediate adjacent loci, despite that cohesin is enriched at numerous regions throughout mitotic chromosomes. Chromosome conformation capture assays reveal apparent looping at the centromeric and pericentric regions. The TSM-Sgo1p-cohesin triad is therefore at the center stage of higher-ordered chromatin architecture for error-free segregation.

Protein phosphatase 2A as therapeutic targets in various disease models

There are a large number of signalling pathways responsible for transmitting information within the cell. Although cellular signalling is thought to be majorly governed by protein kinases 'cascade effects'; their antagonists protein phosphatases also play a crucial dual role in signal transduction. By dephosphorylating the proteins involved in signalling pathways, phosphatases may lead to their activation and sometimes they may terminate a signal generated by kinases activity. Due to counterbalancing the function of phosphorylation, the protein phosphatases are very important to signal transduction processes and thus the control of phosphatase activity is as significant as kinases, in the regulation of a plethora of cellular processes. In general, the protein phosphatases are comprised of a catalytic subunit with one or more regulatory and/or targeting subunits associated with it. The Protein Phosphatase 2A (PP2A), a member of serine/threonine phosphatases family, is ubiquitously expressed a remarkably conserved enzyme in the cell. Its catalytic activity has been highly regulated and may have enormous therapeutic potential which is still untapped. It has specificities for a number of substrates which witnessed its involvement in various signalling modules of cell cycle regulation, cell morphology and development. Thus it can be an appropriate target for studying different diseases associated with abnormal signal transduction pathways such as neurodegenerative diseases and malignancies. This review will focus on the structure and regulatory pathways of PP2A. The de-regulation of PP2A in some specific pathology such as Cancer, Heart diseases, Neurodegenerative disorders and Diabetes will also be touched upon.

Pericentromere-Specific Cohesin Complex Prevents Meiotic Pericentric DNA Double-Strand Breaks and Lethal Crossovers

In most eukaryotes, meiotic crossovers are essential for error-free chromosome segregation but are specifically repressed near centromeres to prevent missegregation. Recognized for >85 years, the molecular mechanism of this repression has remained unknown. Meiotic chromosomes contain two distinct cohesin complexes: pericentric complex (for segregation) and chromosomal arm complex (for crossing over). We show that the pericentric-specific complex also actively represses pericentric meiotic double-strand break (DSB) formation and, consequently, crossovers. We uncover the mechanism by which fission yeast heterochromatin protein Swi6 (mammalian HP1-homolog) prevents recruitment of activators of meiotic DSB formation. Localizing missing activators to wild-type pericentromeres bypasses repression and generates abundant crossovers but reduces gamete viability. The molecular mechanism elucidated here likely extends to other species, including humans, where pericentric crossovers can result in disorders, such as Down syndrome. These mechanistic insights provide new clues to understand the roles played by multiple cohesin complexes, especially in human infertility and birth defects.

Studying meiotic cohesin in somatic cells reveals that Rec8-containing cohesin requires Stag3 to function and is regulated by Wapl and sororin

The DNA-embracing, ring-shaped multiprotein complex cohesin mediates sister chromatid cohesion and is stepwise displaced in mitosis by Wapl and separase (also known as ESPL1) to facilitate anaphase. Proper regulation of chromosome cohesion throughout meiosis is critical for preventing formation of aneuploid gametes, which are associated with trisomies and infertility in humans. Studying cohesion in meiocytes is complicated by their difficult experimental amenability and the absence of cohesin turnover. Here, we use cultured somatic cells to unravel fundamental aspects of meiotic cohesin. When expressed in Hek293 cells, the kleisin Rec8 displays no affinity for the peripheral cohesin subunits Stag1 or Stag2 and remains cytoplasmic. However, co-expression of Stag3 is sufficient for Rec8 to enter the nucleus, load onto chromatin, and functionally replace its mitotic counterpart Scc1 (also known as RAD21) during sister chromatid cohesion and dissolution. Rec8-Stag3 cohesin physically interacts with Pds5, Wapl and sororin (also known as CDCA5). Importantly, Rec8-Stag3 cohesin is shown to be susceptible to Wapl-dependent ring opening and sororin-mediated protection. These findings exemplify that our model system is suitable to rapidly generate testable predictions for important unresolved issues of meiotic cohesion regulation.

Meiosis: SUMO Keeps a Tight Grip on Sister Chromatids

Maintaining cohesion between sister chromatids during the first meiotic cell division is crucial for preventing oocyte aneuploidy. In a new paper in Current Biology, Yi and colleagues present evidence that the Small Ubiquitin-related Modifier (SUMO) pathway protects centromeric sister cohesion during the meiosis I-II transition in mouse oocytes.

Critical roles of Shugoshin and histones as tension sensors during mitosis

Biorientation of paired sister chromosomes is required to maintain mitotic fidelity. A critical signal indicative of bipolar attachment is tension between cohesion-linked sister chromatids. Key components of the tension signaling apparatus include the Shugoshin family of proteins and the tension sensing motif of histone H3. Shugoshin proteins are recruited to chromatin to create discrete domains integral to tension sensing. Many factors involved in the chromatin association of Shugoshin proteins are well established, most strikingly through modifications found directly on centromeric and pericentric chromatin. It has been well established that phosphorylation at the centromere is essential to nucleating Shugoshin recruitment, but recent evidence revealed a role for pericentric histones and acetylation in modulating Shugoshin recruitment and activity. These data demonstrate that chromatins are not simply passive cargo during mitosis, but are instead actively involved in their segregation.

Malonylation of histone H2A at lysine 119 inhibits Bub1-dependent H2A phosphorylation and chromosomal localization of shugoshin proteins

Post-translational modifications of histones, such as acetylation and phosphorylation, are highly conserved in eukaryotes and their combination enables precise regulation of many cellular functions. Recent studies using mass spectrometry have revealed various non-acetyl acylations in histones, including malonylation and succinylation, which change the positive charge of lysine into a negative one. However, the molecular function of histone malonylation or succinylation is poorly understood. Here, we discovered the functions of malonylation in histone H2A at lysine 119 (H2A-K119) in chromosome segregation during mitosis and meiosis. Analyses of H2A-K119 mutants in Saccharomyces cerevisiae and Schizosaccharomyces pombe showed that anionic mutations, specifically to aspartate (K119D) and glutamate (K119E), showed mis-segregation of the chromosomes and sensitivity to microtubule-destabilizing reagents in mitosis and meiosis. We found that the chromosomal localization of shugoshin proteins, which depends on Bub1-catalyzed phosphorylation of H2A at serine 121 (H2A-S121), was significantly reduced in the H2A-K119D and the H2A-K119E mutants. Biochemical analyses using K119-unmodified or -malonylated H2A-C-tail peptides showed that H2A-K119 malonylation inhibited the interaction between Bub1 and H2A, leading to a decrease in Bub1-dependent H2A-S121 phosphorylation. Our results indicate a novel crosstalk between lysine malonylation and serine/threonine phosphorylation, which may be important for fine-tuning chromatin functions such as chromosome segregation.

The Post-anaphase SUMO Pathway Ensures the Maintenance of Centromeric Cohesion through Meiosis I-II Transition in Mammalian Oocytes

The production of haploid gametes requires the maintenance of centromeric cohesion between sister chromatids through the transition between two successive meiotic divisions, meiosis I and meiosis II. One mechanism for the cohesion maintenance is shugoshin-dependent protection of centromeric cohesin at anaphase I onset [1-3]. However, how centromeric cohesion is maintained during late anaphase I and telophase I, when centromeric shugoshin is undetectable [1-3], remains largely unexplored. Here we show that the centromeric small ubiquitin-related modifier (SUMO) pathway is critical for the maintenance of centromeric cohesion during post-anaphase-I periods in mouse oocytes. SUMO2/3 and E3 ligase PIAS are enriched near centromeres during late anaphase I and telophase I. Specific perturbation of the centromeric SUMO pathway results in precocious loss of centromeric cohesin at telophase I, although shugoshin-dependent centromeric protection at anaphase I onset remains largely intact. Prevention of the SUMO perturbation during post-anaphase-I periods restores the maintenance of centromeric cohesion through the meiosis I-II transition. Thus, the post-anaphase-I centromeric SUMO pathway ensures continuous maintenance of centromeric cohesion through the meiosis I-II transition.

Condensin I protects meiotic cohesin from WAPL-1 mediated removal

Condensin complexes are key determinants of higher-order chromatin structure and are required for mitotic and meiotic chromosome compaction and segregation. We identified a new role for condensin in the maintenance of sister chromatid cohesion during C. elegans meiosis. Using conventional and stimulated emission depletion (STED) microscopy we show that levels of chromosomally-bound cohesin were significantly reduced in dpy-28 mutants, which lack a subunit of condensin I. SYP-1, a component of the synaptonemal complex central region, was also diminished, but no decrease in the axial element protein HTP-3 was observed. Surprisingly, the two key meiotic cohesin complexes of C. elegans were both depleted from meiotic chromosomes following the loss of condensin I, and disrupting condensin I in cohesin mutants increased the frequency of detached sister chromatids. During mitosis and meiosis in many organisms, establishment of cohesion is antagonized by cohesin removal by Wapl, and we found that condensin I binds to C. elegans WAPL-1 and counteracts WAPL-1-dependent cohesin removal. Our data suggest that condensin I opposes WAPL-1 to promote stable binding of cohesin to meiotic chromosomes, thereby ensuring linkages between sister chromatids in early meiosis.

Structural biology of the separase-securin complex with crucial roles in chromosome segregation

The cysteine protease separase opens the cohesin ring by cleaving its kleisin subunit and is a pivotal cell cycle factor for the transition from metaphase to anaphase. It is inhibited by forming a complex with the chaperone securin, and in vertebrates, also by the Cdk1-cyclin B1 complex. Separase is activated upon the destruction of securin or cyclin B1 by the proteasome, after ubiquitination by the anaphase-promoting complex/cyclosome (APC/C). Here we review recent structures of the active protease segment of Chaetomium thermophilum separase in complex with a substrate-mimic inhibitor and full-length Saccharomyces cerevisiae and Caenorhabditis elegans separase in complex with securin. These structures define the mechanism for substrate recognition and catalysis by separase, and show that securin has extensive contacts with separase, consistent with its chaperone function. They confirm that securin inhibits separase by binding as a pseudo substrate.