Shugoshin is a Mad1/Cdc20-like interactor of Mad2

Mad2 is dispensable for accurate chromosome segregation but becomes essential when oocytes are subjected to environmental stress

Accurate chromosome segregation, monitored by the spindle assembly checkpoint (SAC), is crucial for the production of euploid cells. Previous in vitro studies by us and others showed that Mad2, a core member of the SAC, performs a checkpoint function in oocyte meiosis. Here, through an oocyte-specific knockout approach in mouse, we reconfirmed that Mad2-deficient oocytes exhibit an accelerated metaphase-to-anaphase transition caused by premature degradation of securin and cyclin B1 and subsequent activation of separase in meiosis I. However, it was surprising that the knockout mice were completely fertile and the resulting oocytes were euploid. In the absence of Mad2, other SAC proteins, including BubR1, Bub3 and Mad1, were normally recruited to the kinetochores, which likely explains the balanced chromosome separation. Further studies showed that the chromosome separation in Mad2-null oocytes was particularly sensitive to environmental changes and, when matured in vitro, showed chromosome misalignment, lagging chromosomes, and aneuploidy with premature separation of sister chromatids, which was exacerbated at a lower temperature. We reveal for the first time that Mad2 is dispensable for proper chromosome segregation but acts to mitigate environmental stress in meiotic oocytes.

Multi-omics analysis of kinesin family member 2C in human tumors: novel prognostic biomarker and tumor microenvironment regulator

Kinesin family member 2C (KIF2C) is the best-characterized member of the kinesin-13 family and is involved in accurately fine-tuned dynamics of mitotic spindles. As KIF2C is involved in both spindle formation and regulation of DNA double-strand breaks, precise regulation of KIF2C is essential to prevent malignant transformation associated with gains and losses of DNA content. In the present study, we initially reviewed The Cancer Genome Atlas database and observed that KIF2C is abundantly expressed in most tumor types. We then analyzed the gene alteration profile, protein expression, prognosis, and immune reactivities of KIF2C in more than 10,000 samples from several well-established databases. In addition, we conducted a gene enrichment set analysis to investigate the potential mechanisms underlying the role of KIF2C in tumorigenesis. Multi-omics analysis of KIF2C demonstrated significant statistical correlations between KIF2C expression and clinical prognosis, oncogenic signature gene sets, myeloid-derived suppressor cell infiltration, ImmunoScore, immune checkpoints, microsatellite instability, and tumor mutational burden across multiple tumors. Single-cell data showed that KIF2C is abundantly expressed in malignant cells. The experimental validation demonstrated that KIF2C is highly expressed in gastric cancer cell lines, gastric adenocarcinoma, and hepatocelluar carcinoma. The findings of this study provide important insight for understanding the role and mechanisms of KIF2C in tumorigenesis and immunotherapy in a variety of cancers.

SGOL2 is a novel prognostic marker and fosters disease progression via a MAD2-mediated pathway in hepatocellular carcinoma

Background

Shugoshin-like protein 2 (SGOL2) is a centromeric protein that ensures the correct and orderly process of mitosis by protecting and maintaining centripetal adhesions during meiosis and mitosis. Here, we examined the potential role of SGOL2 in cancers, especially in hepatocellular carcinoma (HCC).

Methods

One hundred ninety-nine normal adjacent tissues and 202 HCC samples were collected in this study. Human HCC cells (SK-HEP-1 and HEP-3B) were employed in the present study. Immunohistochemistry, immunofluorescence, western blot, Co-Immunoprecipitation technique, and bioinformatic analysis were utilized to assess the role of SGOL2 in HCC development process.

Results

Overexpression of SGOL2 predicted an unfavorable prognosis in HCC by The Cancer Genome Atlas database (TCGA), which were further validated in our two independent cohorts. Next, 47 differentially expressed genes positively related to both SGOL2 and MAD2 were identified to be associated with the cell cycle. Subsequently, we demonstrated that SGOL2 downregulation suppressed the malignant activities of HCC in vitro and in vivo. Further investigation showed that SGOL2 promoted tumor proliferation by regulating MAD2-induced cell-cycle dysregulation, which could be reversed by the MAD2 inhibitor M2I-1. Consistently, MAD2 upregulation reversed the knockdown effects of SGOL2-shRNA in HCC. Moreover, we demonstrated that SGOL2 regulated MAD2 expression level by forming a SGOL2-MAD2 complex, which led to cell cycle dysreuglation of HCC cells.

Conclusion

SGOL2 acts as an oncogene in HCC cells by regulating MAD2 and then dysregulating the cell cycle, providing a potential therapeutic target in HCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40364-022-00422-z.

Mad2 promotes Cyclin B2 recruitment to the kinetochore for guiding accurate mitotic checkpoint

Accurate mitotic progression relies on the dynamic phosphorylation of multiple substrates by key mitotic kinases. Cyclin-dependent kinase 1 is a master kinase that coordinates mitotic progression and requires its regulatory subunit Cyclin B to ensure full kinase activity and substrate specificity. The function of Cyclin B2, which is a closely related family member of Cyclin B1, remains largely elusive. Here, we show that Mad2 promotes the kinetochore localization of Cyclin B2 and that their interaction at the kinetochores guides accurate chromosome segregation. Our biochemical analyses have characterized the Mad2-Cyclin B2 interaction and delineated a novel Mad2-interacting motif (MIM) on Cyclin B2. The functional importance of the Cyclin B2-Mad2 interaction was demonstrated by real-time imaging in which MIM-deficient mutant Cyclin B2 failed to rescue the chromosomal segregation defects. Taken together, we have delineated a previously undefined function of Cyclin B2 at the kinetochore and have established, in human cells, a mechanism of action by which Mad2 contributes to the spindle checkpoint.

Functions and Regulation of Meiotic HORMA-Domain Proteins

During meiosis, homologous chromosomes must recognize, pair, and recombine with one another to ensure the formation of inter-homologue crossover events, which, together with sister chromatid cohesion, promote correct chromosome orientation on the first meiotic spindle. Crossover formation requires the assembly of axial elements, proteinaceous structures that assemble along the length of each chromosome during early meiosis, as well as checkpoint mechanisms that control meiotic progression by monitoring pairing and recombination intermediates. A conserved family of proteins defined by the presence of a HORMA (HOp1, Rev7, MAd2) domain, referred to as HORMADs, associate with axial elements to control key events of meiotic prophase. The highly conserved HORMA domain comprises a flexible safety belt sequence, enabling it to adopt at least two of the following protein conformations: one closed, where the safety belt encircles a small peptide motif present within an interacting protein, causing its topological entrapment, and the other open, where the safety belt is reorganized and no interactor is trapped. Although functional studies in multiple organisms have revealed that HORMADs are crucial regulators of meiosis, the mechanisms by which HORMADs implement key meiotic events remain poorly understood. In this review, we summarize protein complexes formed by HORMADs, discuss their roles during meiosis in different organisms, draw comparisons to better characterize non-meiotic HORMADs (MAD2 and REV7), and highlight possible areas for future research.

Rice Cell Division Cycle 20s are required for faithful chromosome segregation and cytokinesis during meiosis

Chromosome segregation must be under strict regulation to maintain chromosome euploidy and stability. Cell Division Cycle 20 (CDC20) is an essential cell cycle regulator that promotes the metaphase-to-anaphase transition and functions in the spindle assembly checkpoint, a surveillance pathway that ensures the fidelity of chromosome segregation. Plant CDC20 genes are present in multiple copies, and whether CDC20s have the same functions in plants as in yeast and animals is unclear, given the potential for divergence or redundancy among the multiple copies. Here, we studied all three CDC20 genes in rice (Oryza sativa) and constructed two triple mutants by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated genome editing to explore their roles in development. Knocking out all three CDC20 genes led to total sterility but did not affect vegetative development. Loss of the three CDC20 proteins did not alter mitotic division but severely disrupted meiosis as a result of asynchronous and unequal chromosome segregation, chromosome lagging, and premature separation of chromatids. Immunofluorescence of tubulin revealed malformed meiotic spindles in microsporocytes of the triple mutants. Furthermore, cytokinesis of meiosis I was absent or abnormal, and cytokinesis II was completely prevented in all mutant microsporocytes; thus, no tetrads or pollen formed in either cdc20 triple mutant. Finally, the subcellular structures and functions of the tapetum were disturbed by the lack of CDC20 proteins. These findings demonstrate that the three rice CDC20s play redundant roles but are indispensable for faithful meiotic chromosome segregation and cytokinesis, which are required for the production of fertile microspores.

Evolutionary Dynamics and Molecular Mechanisms of HORMA Domain Protein Signaling

Controlled assembly and disassembly of multi-protein complexes is central to cellular signaling. Proteins of the widespread and functionally diverse HORMA family nucleate assembly of signaling complexes by binding short peptide motifs through a distinctive safety-belt mechanism. HORMA proteins are now understood as key signaling proteins across kingdoms, serving as infection sensors in a bacterial immune system and playing central roles in eukaryotic cell cycle, genome stability, sexual reproduction, and cellular homeostasis pathways. Here, we describe how HORMA proteins' unique ability to adopt multiple conformational states underlies their functions in these diverse contexts. We also outline how a dedicated AAA+ ATPase regulator, Pch2/TRIP13, manipulates HORMA proteins' conformational states to activate or inactivate signaling in different cellular contexts. The emergence of Pch2/TRIP13 as a lynchpin for HORMA protein action in multiple genome-maintenance pathways accounts for its frequent misregulation in human cancers and highlights TRIP13 as a novel therapeutic target. Expected final online publication date for the Annual Review of Biochemistry, Volume 91 is June 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Shugoshin ensures maintenance of the spindle assembly checkpoint response and efficient spindle disassembly

Shugoshin proteins are evolutionarily conserved across eukaryotes, with some species-specific cellular functions, ensuring the fidelity of chromosome segregation. They act as adaptors at various subcellular locales to mediate several protein-protein interactions in a spatio-temporal manner. Here, we characterize shugoshin (Sgo1) in the human fungal pathogen Candida albicans. We observe that Sgo1 retains its centromeric localization and performs its conserved functions of regulating the sister chromatid biorientation, centromeric condensin localization, and maintenance of chromosomal passenger complex (CPC). We identify novel roles of Sgo1 as a spindle assembly checkpoint (SAC) component with functions in maintaining a prolonged SAC response by retaining Mad2 and Bub1 at the kinetochores in response to improper kinetochore-microtubule attachments. Strikingly, we discover the in vivo localization of Sgo1 along the length of the mitotic spindle. Our results indicate that Sgo1 performs a hitherto unknown function of facilitating timely disassembly of the mitotic spindle in C. albicans. To summarize, this study unravels a unique functional adaptation of shugoshin in maintaining genomic stability.

A highly conserved pocket on PP2A-B56 is required for hSgo1 binding and cohesion protection during mitosis

The shugoshin proteins are universal protectors of centromeric cohesin during mitosis and meiosis. The binding of human hSgo1 to the PP2A-B56 phosphatase through a coiled-coil (CC) region mediates cohesion protection during mitosis. Here we undertook a structure function analysis of the PP2A-B56-hSgo1 complex, revealing unanticipated aspects of complex formation and function. We establish that a highly conserved pocket on the B56 regulatory subunit is required for hSgo1 binding and cohesion protection during mitosis in human somatic cells. Consistent with this, we show that hSgo1 blocks the binding of PP2A-B56 substrates containing a canonical B56 binding motif. We find that PP2A-B56 bound to hSgo1 dephosphorylates Cdk1 sites on hSgo1 itself to modulate cohesin interactions. Collectively our work provides important insight into cohesion protection during mitosis.

Securin-independent regulation of separase by checkpoint-induced shugoshin-MAD2

Separation of eukaryotic sister chromatids during the cell cycle is timed by the spindle assembly checkpoint (SAC) and ultimately triggered when separase cleaves cohesion-mediating cohesin1-3. Silencing of the SAC during metaphase activates the ubiquitin ligase APC/C (anaphase-promoting complex, also known as the cyclosome) and results in the proteasomal destruction of the separase inhibitor securin1. In the absence of securin, mammalian chromosomes still segregate on schedule, but it is unclear how separase is regulated under these conditions4,5. Here we show that human shugoshin 2 (SGO2), an essential protector of meiotic cohesin with unknown functions in the soma6,7, is turned into a separase inhibitor upon association with SAC-activated MAD2. SGO2-MAD2 can functionally replace securin and sequesters most separase in securin-knockout cells. Acute loss of securin and SGO2, but not of either protein individually, resulted in separase deregulation associated with premature cohesin cleavage and cytotoxicity. Similar to securin8,9, SGO2 is a competitive inhibitor that uses a pseudo-substrate sequence to block the active site of separase. APC/C-dependent ubiquitylation and action of the AAA-ATPase TRIP13 in conjunction with the MAD2-specific adaptor p31comet liberate separase from SGO2-MAD2 in vitro. The latter mechanism facilitates a considerable degree of sister chromatid separation in securin-knockout cells that lack APC/C activity. Thus, our results identify an unexpected function of SGO2 in mitotically dividing cells and a mechanism of separase regulation that is independent of securin but still supervised by the SAC.

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.

Aurora B kinase activity is regulated by SET/TAF1 on Sgo2 at the inner centromere

Phosphorylation of kinetochore proteins destabilizes improper kinetochore–microtubule attachments. Asai et al. find that SET/TAF1, an inhibitor of the PP2A phosphatase, binds shugoshin 2 and corrects erroneous kinetochore–microtubule attachment by maintaining Aurora B kinase activity. Therefore, SET has a key role in establishing chromosome bi-orientation by balancing Aurora B and PP2A activity.

The accurate regulation of phosphorylation at the kinetochore is essential for establishing chromosome bi-orientation. Phosphorylation of kinetochore proteins by the Aurora B kinase destabilizes improper kinetochore–microtubule attachments, whereas the phosphatase PP2A has a counteracting role. Imbalanced phosphoregulation leads to error-prone chromosome segregation and aneuploidy, a hallmark of cancer cells. However, little is known about the molecular events that control the balance of phosphorylation at the kinetochore. Here, we show that localization of SET/TAF1, an oncogene product, to centromeres maintains Aurora B kinase activity by inhibiting PP2A, thereby correcting erroneous kinetochore–microtubule attachment. SET localizes at the inner centromere by interacting directly with shugoshin 2, with SET levels declining at increased distances between kinetochore pairs, leading to establishment of chromosome bi-orientation. Moreover, SET overexpression induces chromosomal instability by disrupting kinetochore–microtubule attachment. Thus, our findings reveal the novel role of SET in fine-tuning the phosphorylation level at the kinetochore by balancing the activities of Aurora B and PP2A.

Shugoshin protects centromere pairing and promotes segregation of nonexchange partner chromosomes in meiosis

Faithful chromosome segregation during meiosis I depends upon the formation of connections between homologous chromosomes. Crossovers between homologs connect the partners, allowing them to attach to the meiotic spindle as a unit, such that they migrate away from one another at anaphase I. Homologous partners also become connected by pairing of their centromeres in meiotic prophase. This centromere pairing can promote proper segregation at anaphase I of partners that have failed to become joined by a crossover. Centromere pairing is mediated by synaptonemal complex (SC) proteins that persist at the centromere when the SC disassembles. Here, using mouse spermatocyte and yeast model systems, we tested the role of shugoshin in promoting meiotic centromere pairing by protecting centromeric synaptonemal components from disassembly. The results show that shugoshin protects the centromeric SC in meiotic prophase and, in anaphase, promotes the proper segregation of partner chromosomes that are not linked by a crossover.

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.

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.

The mitotic checkpoint complex (MCC): looking back and forth after 15 years

The mitotic checkpoint is a specialized signal transduction pathway that contributes to the fidelity of chromosome segregation. The signaling of the checkpoint originates from defective kinetochore-microtubule interactions and leads to formation of the mitotic checkpoint complex (MCC), a highly potent inhibitor of the Anaphase Promoting Complex/Cyclosome (APC/C)—the E3 ubiquitin ligase essential for anaphase onset. Many important questions concerning the MCC and its interaction with APC/C have been intensively investigated and debated in the past 15 years, such as the exact composition of the MCC, how it is assembled during a cell cycle, how it inhibits APC/C, and how the MCC is disassembled to allow APC/C activation. These efforts have culminated in recently reported structure models for human MCC:APC/C supra-complexes at near-atomic resolution that shed light on multiple aspects of the mitotic checkpoint mechanisms. However, confusing statements regarding the MCC are still scattered in the literature, making it difficult for students and scientists alike to obtain a clear picture of MCC composition, structure, function and dynamics. This review will comb through some of the most popular concepts or misconceptions about the MCC, discuss our current understandings, present a synthesized model on regulation of CDC20 ubiquitination, and suggest a few future endeavors and cautions for next phase of MCC research.

MAD2γ, a novel MAD2 isoform, reduces mitotic arrest and is associated with resistance in testicular germ cell tumors

Background: Prolonged mitotic arrest in response to anti-cancer chemotherapeutics, such as DNA-damaging agents, induces apoptosis, mitotic catastrophe, and senescence. Disruptions in mitotic checkpoints contribute resistance to DNA-damaging agents in cancer. MAD2 has been associated with checkpoint failure and chemotherapy response. In this study, a novel splice variant of MAD2, designated MAD2γ, was identified, and its association with the DNA damage response was investigated.

Methods: Endogenous expression of MAD2γ and full-length MAD2 (MAD2α) was measured using RT-PCR in cancer cell lines, normal foreskin fibroblasts, and tumor samples collected from patients with testicular germ cell tumors (TGCTs). A plasmid expressing MAD2γ was transfected into HCT116 cells, and its intracellular localization and checkpoint function were evaluated according to immunofluorescence and mitotic index.

Results: MAD2γ was expressed in several cancer cell lines and non-cancerous fibroblasts. Ectopically expressed MAD2γ localized to the nucleus and reduced the mitotic index, suggesting checkpoint impairment. In patients with TGCTs, the overexpression of endogenous MAD2γ, but not MAD2α, was associated with resistance to cisplatin-based chemotherapy. Likewise, cisplatin induced the overexpression of endogenous MAD2γ, but not MAD2α, in HCT116 cells.

Conclusions: Overexpression of MAD2γ may play a role in checkpoint disruption and is associated with resistance to cisplatin-based chemotherapy in TGCTs.

Shugoshins: tension-sensitive pericentromeric adaptors safeguarding chromosome segregation

The shugoshin/Mei-S332 family are proteins that associate with the chromosomal region surrounding the centromere (the pericentromere) and that play multiple and distinct roles in ensuring the accuracy of chromosome segregation during both mitosis and meiosis. The underlying role of shugoshins appears to be to serve as pericentromeric adaptor proteins that recruit several different effectors to this region of the chromosome to regulate processes critical for chromosome segregation. Crucially, shugoshins undergo changes in their localization in response to the tension that is exerted on sister chromosomes by the forces of the spindle that will pull them apart. This has led to the idea that shugoshins provide a platform for activities required at the pericentromere only when sister chromosomes lack tension. Conversely, disassembly of the shugoshin pericentromeric platform may provide a signal that sister chromosomes are under tension. Here the functions and regulation of these important tension-sensitive pericentromeric proteins are discussed.

The chromosome axis controls meiotic events through a hierarchical assembly of HORMA domain proteins

Proteins of the HORMA domain family play central, but poorly understood, roles in chromosome organization and dynamics during meiosis. In Caenorhabditis elegans, four such proteins (HIM-3, HTP-1, HTP-2, and HTP-3) have distinct but overlapping functions. Through combined biochemical, structural, and in vivo analysis, we find that these proteins form hierarchical complexes through binding of their HORMA domains to cognate peptides within their partners' C-terminal tails, analogous to the "safety belt" binding mechanism of Mad2. These interactions are critical for recruitment of HIM-3, HTP-1, and HTP-2 to chromosome axes. HTP-3, in addition to recruiting the other HORMA domain proteins to the axis, plays an independent role in sister chromatid cohesion and double-strand break formation. Finally, we find that mammalian HORMAD1 binds a motif found both at its own C terminus and at that of HORMAD2, indicating that this mode of intermolecular association is a conserved feature of meiotic chromosome structure in eukaryotes.

WT1 interacts with MAD2 and regulates mitotic checkpoint function

Tumour suppressors safeguard the fidelity of the mitotic checkpoint by transcriptional regulation of genes that encode components of the mitotic checkpoint complex (MCC). Here we report a new role for the tumour suppressor and transcription factor, WT1, in the mitotic checkpoint. We show that WT1 regulates the MCC by directly interacting with the spindle assembly checkpoint protein, MAD2. WT1 colocalizes with MAD2 during mitosis and preferentially binds to the functionally active, closed-conformer, C-MAD2. Furthermore, WT1 associates with the MCC containing MAD2, BUBR1 and CDC20, resulting in prolonged inhibition of the anaphase-promoting complex/cyclosome (APC/C) and delayed degradation of its substrates SECURIN and CYCLIN B1. Strikingly, RNA interference-mediated depletion of WT1 leads to enhanced turnover of SECURIN, decreased lag time to anaphase and defects in chromosome segregation. Our findings identify WT1 as a regulator of the mitotic checkpoint and chromosomal stability.

Sgol2 provides a regulatory platform that coordinates essential cell cycle processes during meiosis I in oocytes

Accurate chromosome segregation depends on coordination between cohesion resolution and kinetochore-microtubule interactions (K-fibers), a process regulated by the spindle assembly checkpoint (SAC). How these diverse processes are coordinated remains unclear. We show that in mammalian oocytes Shugoshin-like protein 2 (Sgol2) in addition to protecting cohesin, plays an important role in turning off the SAC, in promoting the congression and bi-orientation of bivalents on meiosis I spindles, in facilitating formation of K-fibers and in limiting bivalent stretching. Sgol2’s ability to protect cohesin depends on its interaction with PP2A, as is its ability to silence the SAC, with the latter being mediated by direct binding to Mad2. In contrast, its effect on bivalent stretching and K-fiber formation is independent of PP2A and mediated by recruitment of MCAK and inhibition of Aurora C kinase activity respectively. By virtue of its multiple interactions, Sgol2 links many of the processes essential for faithful chromosome segregation.

DOI:http://dx.doi.org/10.7554/eLife.01133.001

Human reproductive cells—eggs and sperm—are produced through a process called meiosis. This means a ‘parent’ cell in the ovaries or testes undergoes two stages of cell division: it first divides into two cells, which then divide again to produce four ‘daughter’ cells. A crucial part of meiosis is ensuring that each daughter cell has half the number of chromosomes that the parent cell did.

Before the first round of meiosis, the chromosomes in the parent cell are copied to produce enough chromosomes for the four daughter cells. The distribution of these chromosomes between the daughter cells is determined by the opposing forces acting on them. The pairs of identical chromosomes produced during the copying process are held together by a proteinaceous glue, while microtubules attached to a structure called the spindle—which has poles at opposite ends of the cell—try to pull these pairs of chromosomes apart.

Rattani et al. now show that a protein called shugoshin-like 2 (Sgol2), which is involved in holding pairs of identical chromosomes together after the first round of division, has a bigger role than was previously realised. Sgol2 performs three other functions: it helps to align the chromosomes prior to division by, it is thought, facilitating the formation of the K-fibers that attach the microtubules to the chromosomes; it turns off the checkpoint that monitors the alignment of the chromosomes and the attachment of the microtubules; and it regulates a number of the enzymes involved in the process. The specific interactions that allow Sgol2 to perform these diverse functions in meiosis were also identified. Thus, Rattani et al. show that in linking so many essential processes throughout meiosis, Sgol2 appears to have a key, if not unique, role in determining the fate of chromosomes as cells divide.

DOI:http://dx.doi.org/10.7554/eLife.01133.002

Chromatin reorganization through mitosis

Chromosome condensation is one of the major chromatin-remodeling events that occur during cell division. The changes in chromatin compaction and higher-order structure organization are essential requisites for ensuring a faithful transmission of the replicated genome to daughter cells. Although the observation of mitotic chromosome condensation has fascinated Scientists for a century, we are still far away from understanding how the process works from a molecular point of view. In this chapter, I will analyze our current understanding of chromatin condensation during mitosis with particular attention to the major molecular players that trigger and maintain this particular chromatin conformation. However, within the chromosome, not all regions of the chromatin are organized in the same manner. I will address separately the structure and functions of particular chromatin domains such as the centromere. Finally, the transition of the chromatin through mitosis represents just an interlude for gene expression between two cell cycles. How the transcriptional information that governs cell linage identity is transmitted from mother to daughter represents a big and interesting question. I will present how cells take care of the aspect ensuring that mitotic chromosome condensation and the block of transcription does not wipe out the cell identity.

Phosphorylation-enabled binding of SGO1-PP2A to cohesin protects sororin and centromeric cohesion during mitosis

Timely dissolution of sister-chromatid cohesion in mitosis ensures accurate chromosome segregation to guard against aneuploidy and tumorigenesis. The complex of shugoshin and protein phosphatase 2A (SGO1-PP2A) protects cohesin at centromeres from premature removal by mitotic kinases and WAPL in prophase. Here we address the regulation and mechanism of human SGO1 in centromeric cohesion protection, and show that cyclin-dependent kinase (CDK)-mediated, mitosis-specific phosphorylation of SGO1 activates its cohesion-protection function and enables its direct binding to cohesin. The phospho-SGO1-bound cohesin complex contains PP2A, PDS5 and hypophosphorylated sororin, but lacks WAPL. Expression of non-phosphorylatable sororin bypasses the requirement for SGO1-PP2A in centromeric cohesion. Thus, mitotic phosphorylation of SGO1 targets SGO1-PP2A to cohesin, promotes dephosphorylation of PDS5-bound sororin and protects centromeric cohesin from WAPL. PP2A-orchestrated, site-selective dephosphorylation of cohesin and its regulators underlies centromeric cohesion protection.

The Mad1-Mad2 balancing act--a damaged spindle checkpoint in chromosome instability and cancer

Cancer cells are commonly aneuploid. The spindle checkpoint ensures accurate chromosome segregation by controlling cell cycle progression in response to aberrant microtubule-kinetochore attachment. Damage to the checkpoint, which is a partial loss or gain of checkpoint function, leads to aneuploidy during tumorigenesis. One form of damage is a change in levels of the checkpoint proteins mitotic arrest deficient 1 and 2 (Mad1 and Mad2), or in the Mad1:Mad2 ratio. Changes in Mad1 and Mad2 levels occur in human cancers, where their expression is regulated by the tumor suppressors p53 and retinoblastoma 1 (RB1). By employing a standard assay, namely the addition of a mitotic poison at mitotic entry, it has been shown that checkpoint function is normal in many cancer cells. However, in several experimental systems, it has been observed that this standard assay does not always reveal checkpoint aberrations induced by changes in Mad1 or Mad2, where excess Mad1 relative to Mad2 can lead to premature anaphase entry, and excess Mad2 can lead to a delay in entering anaphase. This Commentary highlights how changes in the levels of Mad1 and Mad2 result in a damaged spindle checkpoint, and explores how these changes cause chromosome instability that can lead to aneuploidy during tumorigenesis.

Shugoshins function as a guardian for chromosomal stability in nuclear division

Accurate chromosome segregation during mitosis and meiosis is regulated and secured by several distinctly different yet intricately connected regulatory mechanisms. As chromosomal instability is a hallmark of a majority of tumors as well as a cause of infertility for germ cells, extensive research in the past has focused on the identification and characterization of molecular components that are crucial for faithful chromosome segregation during cell division. Shugoshins, including Sgo1 and Sgo2, are evolutionarily conserved proteins that function to protect sister chromatid cohesion, thus ensuring chromosomal stability during mitosis and meiosis in eukaryotes. Recent studies reveal that Shugoshins in higher animals play an essential role not only in protecting centromeric cohesion of sister chromatids and assisting bi-orientation attachment at the kinetochores, but also in safeguarding centriole cohesion/engagement during early mitosis. Many molecular components have been identified that play essential roles in modulating/mediating Sgo functions. This review primarily summarizes recent advances on the mechanisms of action of Shugoshins in suppressing chromosomal instability during nuclear division in eukaryotic organisms.

Shugoshins: from protectors of cohesion to versatile adaptors at the centromere

Sister chromatids are held together by a protein complex named cohesin. Shugoshin proteins protect cohesin from cleavage by separase during meiosis I in eukaryotes and from phosphorylation-mediated removal during mitosis in vertebrates. This protection is crucial for chromosome segregation during mitosis and meiosis. Mechanistically, shugoshins shield cohesin by forming a complex with the phosphatase PP2A, which dephosphorylates cohesin, leading to its retention at centromeres during the onset of meiotic anaphase and vertebrate mitotic prophase I. In addition to this canonical function, shugoshins have evolved novel, species-specific cellular functions, the mechanisms of which remain a subject of intense debate, but are likely to involve spatio-temporally coordinated interactions with the chromosome passenger complex, the spindle checkpoint and the anaphase promoting complex. Here, we compare and contrast these remarkable features of shugoshins in model organisms and humans.

Xenopus Shugoshin 2 regulates the spindle assembly pathway mediated by the chromosomal passenger complex

Shugoshins (Sgo) are conserved proteins that act as protectors of centromeric cohesion and as sensors of tension for the machinery that eliminates improper kinetochore-microtubule attachments. Most vertebrates contain two Sgo proteins, but their specific functions are not always clear. Xenopus laevis Sgo1, XSgo1, protects centromeric cohesin from the prophase dissociation pathway. Here, we report the identification of XSgo2 and show that it does not regulate cohesion. Instead, we find that it participates in bipolar spindle assembly. Both Sgo proteins interact physically with the Chromosomal Passenger Complex (CPC) containing Aurora B, a key regulator of mitosis, but the functional consequences of such interaction are distinct. XSgo1 is required for proper localization of the CPC while XSgo2 positively contributes to its activation and the subsequent phosphorylation of at least one key substrate for bipolar spindle assembly, the microtubule depolymerizing kinesin MCAK (Mitotic Centromere-Associated Kinesin). Thus, the two Xenopus Sgo proteins have non-overlapping functions in chromosome segregation. Our results further suggest that this functional specificity could rely on the association of XSgo1 and XSgo2 with different regulatory subunits of the PP2A complex.

Checkpoint-independent stabilization of kinetochore-microtubule attachments by Mad2 in human cells

Faithful chromosome segregation is required for cell and organism viability and relies on both the mitotic checkpoint and the machinery that corrects kinetochore-microtubule (k-MT) attachment errors. Most solid tumors have aneuploid karyotypes and many missegregate chromosomes at high rates in a phenomenon called chromosomal instability (CIN). Mad2 is essential for mitotic checkpoint function and is frequently overexpressed in human tumors that are CIN. For unknown reasons, cells overexpressing Mad2 display high rates of lagging chromosomes. Here, we explore this phenomenon and show that k-MT attachments are hyperstabilized by Mad2 overexpression and that this undermines the efficiency of correction of k-MT attachment errors. Mad2 affects k-MT attachment stability independently of the mitotic checkpoint because k-MT attachments are unaltered upon Mad1 depletion and Mad2 overexpression hyperstabilizes k-MT attachments in Mad1-deficient cells. Mad2 mediates these effects with Cdc20 by altering the centromeric localization and activity of Aurora B kinase, a known regulator of k-MT attachment stability. These data reveal a new function for Mad2 to stabilize k-MT attachments independent of the checkpoint and explain why Mad2 overexpression increases chromosome missegregation to cause chromosomal instability in human tumors.