Feedback control of mitosis in budding yeast

Bub3 reads phosphorylated MELT repeats to promote spindle assembly checkpoint signaling

Regulation of macromolecular interactions by phosphorylation is crucial in signaling networks. In the spindle assembly checkpoint (SAC), which enables errorless chromosome segregation, phosphorylation promotes recruitment of SAC proteins to tensionless kinetochores. The SAC kinase Mps1 phosphorylates multiple Met-Glu-Leu-Thr (MELT) motifs on the kinetochore subunit Spc105/Knl1. The phosphorylated MELT motifs (MELT^P) then promote recruitment of downstream signaling components. How MELT^P motifs are recognized is unclear. In this study, we report that Bub3, a 7-bladed β-propeller, is the MELT^P reader. It contains an exceptionally well-conserved interface that docks the MELT^P sequence on the side of the β-propeller in a previously unknown binding mode. Mutations targeting the Bub3 interface prevent kinetochore recruitment of the SAC kinase Bub1. Crucially, they also cause a checkpoint defect, showing that recognition of phosphorylated targets by Bub3 is required for checkpoint signaling. Our data provide the first detailed mechanistic insight into how phosphorylation promotes recruitment of checkpoint proteins to kinetochores.

DOI:http://dx.doi.org/10.7554/eLife.01030.001

The cell cycle is the process by which a cell divides to produce two near-identical daughter cells. Two crucial parts of the cell cycle are the duplication of the chromosomes in the original cell, and the segregation of these chromosomes between the two daughter cells. These and other parts of the cell cycle are strictly regulated to prevent errors, which can lead to cancer and other diseases.

After chromosome duplication has taken place, the pairs of identical chromosomes, known as sister chromatids, remain tightly bound to each other. These sister chromatids line up in the middle of the cell, with protein filaments called microtubules connecting them to a bipolar structure called the spindle. For the cell to divide correctly, the sister chromatids in each pair must be connected to opposite poles of the spindle. A signalling network known as the spindle assembly checkpoint (SAC) ensures that the sister chromatids have enough time to line up correctly and to correct possible problems. Once everything is in place, the SAC releases its ‘break’, and the microtubules then pull the sister chromatids away from each other. This way, each daughter cell receives the same complement of chromosomes that was present in the mother cell.

The microtubules are not directly attached to the sister chromatids but to protein complexes called kinetochores that assemble on each sister chromatid. In particular, each microtubule binds to a very large protein complex called the KMN network. Knl1, which is part of this network, recruits two SAC proteins–Bub1 and Bub3–to the kinetochore. It is known that a phosphate group is added to Knl1 when the SAC is active, and that Knl1 can only recruit Bub1 and Bub3 after it has been phosphorylated. However, the details of the interactions between Knl1, Bub1 and Bub3 are not understood, and it is not clear whether these interactions are essential for the SAC.

Now Primorac et al. have shown that Bub3 binds directly to Knl1 through a region that contains multiple MELT motifs (where M, E, L and T are all amino acids), and that this interaction only happens if these ‘MELT repeats’ have been phosphorylated. Moreover, once bound to the Knl1, Bub3 then recruits Bub1 to the kinetochore. By showing that the recognition of phosphorylated Knl1 by the Bub1-Bub3 complex has a central role in the spindle assembly checkpoint, these results highlight the importance of phosphorylation as a way of regulating the timing of events during the cell cycle.

DOI:http://dx.doi.org/10.7554/eLife.01030.002

Nuclear PP2A-Cdc55 prevents APC-Cdc20 activation during the spindle assembly checkpoint

Cdc55, a regulatory B-subunit of protein phosphatase 2A (PP2A) complex, is essential for the spindle assembly checkpoint (SAC) in budding yeast, but the regulation and molecular targets of PP2A-Cdc55 have not been clearly defined or are controversial. Here, we show that an important target of Cdc55 in the SAC is the anaphase-promoting complex (APC) coupled with Cdc20 and that APC-Cdc20 is kept inactive by dephosphorylation by nuclear PP2A-Cdc55 when spindle is damaged. By isolating a new class of Cdc55 mutants specifically defective in the SAC and by artificially manipulating nucleocytoplasmic distribution of Cdc55, we further show that nuclear Cdc55 is essential for the SAC. Because the Cdc55-binding proteins Zds1 and Zds2 inhibit both nuclear accumulation of Cdc55 and SAC activity, we propose that spatial control of PP2A by Zds1 family proteins is important for tight control of SAC and mitotic progression.

Characterization of chromosome stability in diploid, polyploid and hybrid yeast cells

Chromosome instability is a key component of cancer progression and many heritable diseases. Understanding why some chromosomes are more unstable than others could provide insight into understanding genome integrity. Here we systematically investigate the spontaneous chromosome loss for all sixteen chromosomes in Saccharomyces cerevisiae in order to elucidate the mechanisms underlying chromosome instability. We observed that the stability of different chromosomes varied more than 100-fold. Consistent with previous studies on artificial chromosomes, chromosome loss frequency was negatively correlated to chromosome length in S. cerevisiae diploids, triploids and S. cerevisiae-S. bayanus hybrids. Chromosome III, an equivalent of sex chromosomes in budding yeast, was found to be the most unstable chromosome among all cases examined. Moreover, similar instability was observed in chromosome III of S. bayanus, a species that diverged from S. cerevisiae about 20 million years ago, suggesting that the instability is caused by a conserved mechanism. Chromosome III was found to have a highly relaxed spindle checkpoint response in the genome. Using a plasmid stability assay, we found that differences in the centromeric sequence may explain certain aspects of chromosome instability. Our results reveal that even under normal conditions, individual chromosomes in a genome are subject to different levels of pressure in chromosome loss (or gain).

Spatial rule-based modeling: a method and its application to the human mitotic kinetochore

A common problem in the analysis of biological systems is the combinatorial explosion that emerges from the complexity of multi-protein assemblies. Conventional formalisms, like differential equations, Boolean networks and Bayesian networks, are unsuitable for dealing with the combinatorial explosion, because they are designed for a restricted state space with fixed dimensionality. To overcome this problem, the rule-based modeling language, BioNetGen, and the spatial extension, SRSim, have been developed. Here, we describe how to apply rule-based modeling to integrate experimental data from different sources into a single spatial simulation model and how to analyze the output of that model. The starting point for this approach can be a combination of molecular interaction data, reaction network data, proximities, binding and diffusion kinetics and molecular geometries at different levels of detail. We describe the technique and then use it to construct a model of the human mitotic inner and outer kinetochore, including the spindle assembly checkpoint signaling pathway. This allows us to demonstrate the utility of the procedure, show how a novel perspective for understanding such complex systems becomes accessible and elaborate on challenges that arise in the formulation, simulation and analysis of spatial rule-based models.

Catalytic assembly of the mitotic checkpoint inhibitor BubR1-Cdc20 by a Mad2-induced functional switch in Cdc20

The mitotic checkpoint acts to maintain chromosome content by generation of a diffusible anaphase inhibitor. Unattached kinetochores catalyze a conformational shift in Mad2, converting an inactive open form into a closed form that can capture Cdc20, the mitotic activator of the APC/C ubiquitin ligase. Mad2 binding is now shown to promote a functional switch in Cdc20, exposing a previously inaccessible site for binding to BubR1's conserved Mad3 homology domain. BubR1, but not Mad2, binding to APC/C(Cdc20) is demonstrated to inhibit ubiquitination of cyclin B. Closed Mad2 is further shown to catalytically amplify production of BubR1-Cdc20 without necessarily being part of the complex. Thus, the mitotic checkpoint is produced by a cascade of two catalytic steps: an initial step acting at unattached kinetochores to produce a diffusible Mad2-Cdc20 intermediate and a diffusible step in which that intermediate amplifies production of BubR1-Cdc20, the inhibitor of cyclin B ubiquitination, by APC/C(Cdc20).

The tumor suppressor CDKN3 controls mitosis

Mitosis is controlled by a network of kinases and phosphatases. We screened a library of small interfering RNAs against a genome-wide set of phosphatases to comprehensively evaluate the role of human phosphatases in mitosis. We found four candidate spindle checkpoint phosphatases, including the tumor suppressor CDKN3. We show that CDKN3 is essential for normal mitosis and G1/S transition. We demonstrate that subcellular localization of CDKN3 changes throughout the cell cycle. We show that CDKN3 dephosphorylates threonine-161 of CDC2 during mitotic exit and we visualize CDC2(pThr-161) at kinetochores and centrosomes in early mitosis. We performed a phosphokinome-wide mass spectrometry screen to find effectors of the CDKN3-CDC2 signaling axis. We found that one of the identified downstream phosphotargets, CKβ phosphorylated at serine 209, localizes to mitotic centrosomes and controls the spindle checkpoint. Finally, we show that CDKN3 protein is down-regulated in brain tumors. Our findings indicate that CDKN3 controls mitosis through the CDC2 signaling axis. These results have implications for targeted anticancer therapeutics.

Bora regulates meiotic spindle assembly and cell cycle during mouse oocyte meiosis

Bora is the binding partner of Aurora A, which is required for its activation and phosphorylation of Polo like kinase 1 (Plk1), and is involved in the spindle assembly and progress of the cell cycle during mitosis. In this study, we examined the expression, localization, and function of Bora during mouse oocyte meiosis. The expression level of Bora was increased during oocyte meiotic maturation, with an elevated level at metaphase. Immunofluorescence analysis showed that Bora was concentrated as a dot shortly after germinal vesicle breakdown (GVBD), associating first with the surrounding chromosomes and then with the spindle throughout oocyte meiotic maturation. Further experiments confirmed that Bora co-localized with α-tubulin at prometaphase/metaphase, but dissociated from α-tubulin at anaphase/telophase. In metaphase-II-arrested oocytes, Bora was evenly distributed in the cytoplasm after treatment with a microtubule-depolymerizing agent, or recruited to the spindle after treatment with a microtubule-polymerizing agent, indicating that Bora was physically connected to the meiotic spindle and α-tubulin at metaphase. Furthermore, inhibition or depletion of Bora by either anti-Bora antibody or Bora siRNA microinjection significantly reduced the rates of GVBD and inhibited first polar body extrusion; caused morphologically defective spindles and misaligned chromosomes; arrested maturing oocytes at prometaphase/metaphase-I stage, or left oocytes and their first polar bodies with severely misaligned chromosomes and defective spindles; and/or caused the disappearance of Aurora A and Plk1 at the spindle. These results indicated that Bora acts as a critical regulator of Aurora A and Plk1, and is involved in microtubule organization during oocyte meiosis.

WEE1 inhibition and genomic instability in cancer

One of the hallmarks of cancer is genomic instability controlled by cell cycle checkpoints. The G1 and G2 checkpoints allow DNA damage responses, whereas the mitotic checkpoint enables correct seggregation of the sister chromosomes to prevent aneuploidy. Cancer cells often lack a functional G1 arrest and rely on G2 arrest for DNA damage responses. WEE1 kinase is an important regulator of the G2 checkpoint and is overexpressed in various cancer types. Inhibition of WEE1 is a promising strategy in cancer therapy in combination with DNA-damaging agents, especially when cancer cells harbor p53 mutations, as it causes mitotic catastrophy when DNA is not repaired during G2 arrest. Cancer cell response to WEE1 inhibition monotherapy has also been demonstrated in various types of cancer, including p53 wild-type cancers. We postulate that chromosomal instability can explain tumor response to WEE1 monotherapy. Therefore, chromosomal instability may need to be taken into account when determining the most effective strategy for the use of WEE1 inhibitors in cancer therapy.

The spindle assembly checkpoint

During mitosis and meiosis, the spindle assembly checkpoint acts to maintain genome stability by delaying cell division until accurate chromosome segregation can be guaranteed. Accuracy requires that chromosomes become correctly attached to the microtubule spindle apparatus via their kinetochores. When not correctly attached to the spindle, kinetochores activate the spindle assembly checkpoint network, which in turn blocks cell cycle progression. Once all kinetochores become stably attached to the spindle, the checkpoint is inactivated, which alleviates the cell cycle block and thus allows chromosome segregation and cell division to proceed. Here we review recent progress in our understanding of how the checkpoint signal is generated, how it blocks cell cycle progression and how it is extinguished.

Evolutionary cell biology of chromosome segregation: insights from trypanosomes

Faithful transmission of genetic material is essential for the survival of all organisms. Eukaryotic chromosome segregation is driven by the kinetochore that assembles onto centromeric DNA to capture spindle microtubules and govern the movement of chromosomes. Its molecular mechanism has been actively studied in conventional model eukaryotes, such as yeasts, worms, flies and human. However, these organisms are closely related in the evolutionary time scale and it therefore remains unclear whether all eukaryotes use a similar mechanism. The evolutionary origins of the segregation apparatus also remain enigmatic. To gain insights into these questions, it is critical to perform comparative studies. Here, we review our current understanding of the mitotic mechanism in Trypanosoma brucei, an experimentally tractable kinetoplastid parasite that branched early in eukaryotic history. No canonical kinetochore component has been identified, and the design principle of kinetochores might be fundamentally different in kinetoplastids. Furthermore, these organisms do not appear to possess a functional spindle checkpoint that monitors kinetochore-microtubule attachments. With these unique features and the long evolutionary distance from other eukaryotes, understanding the mechanism of chromosome segregation in T. brucei should reveal fundamental requirements for the eukaryotic segregation machinery, and may also provide hints about the origin and evolution of the segregation apparatus.

Tension sensing by Aurora B kinase is independent of survivin-based centromere localization

Accurate segregation of the replicated genome requires chromosome biorientation on the spindle. Biorientation is ensured by Aurora B kinase, a member of the 4-subunit chromosomal passenger complex (CPC)^1,2. Localization of the CPC to the inner centromere is central to the current model for how tension ensures chromosome biorientation—kinetochore-spindle attachments not under tension remain close to the inner centromere and are destabilized by Aurora B phosphorylation, whereas kinetochores under tension are pulled away from the influence of Aurora B, stabilizing their microtubule attachments^3–5. Here we show that an engineered truncation of the INCENP/Sli15 subunit of budding yeast CPC that eliminates association with the inner centromere nevertheless supports proper chromosome segregation during both mitosis and meiosis. Truncated INCENP/Sli15 suppresses the deletion phenotypes of the inner centromere-targeting proteins Survivin/Bir1, Borealin/Nbl1, Bub1 and Sgo1⁶. Unlike wildtype INCENP/Sli15, truncated INCENP/Sli15 localizes to pre-anaphase spindle microtubules. Premature targeting of full-length INCENP/Sli15 to microtubules by preventing Cdk1 phosphorylation also suppresses inviability of Survivin/Bir1 deletion. These results suggest that activation of Aurora B/Ipl1 by clustering either on chromatin or on microtubules is sufficient for chromosome biorientation.

A minimal anaphase promoting complex/cyclosome (APC/C) in Trypanosoma brucei

The anaphase-promoting complex/cyclosome (APC/C) is a multi-subunit E3 ubiquitin ligase that initiates chromosome segregation and mitotic exit by targeting critical cell-cycle regulators for proteolytic destruction. Previously, seven APC/C subunit homologues were identified in the genome of Trypanosoma brucei. In the present study, we tested five of them in yeast complementation studies and found none of them capable of complementing the yeast mutants lacking the corresponding subunits, suggesting significant discrepancies between the two APC/C’s. Subunit homologues of mitotic checkpoint complex (MCC) have not yet been identified in T. brucei, raising the possibility that a MCC-APC/C complex equivalent may not exist in T. brucei. We performed tandem affinity purification of the protein complex containing a APC1 fusion protein expressed in the cells enriched in different phases of the cell cycle of procyclic form T. brucei, and compared their protein profiles using LC-MS/MS analyses. The seven putative APC/C subunits were identified in the protein complex throughout the cell cycle together with three additional proteins designated the associated proteins (AP) AP1, AP2 and AP3. Abundance of the 10 proteins remained relatively unchanged throughout the cell cycle, suggesting that they are the core subunits of APC/C. AP1 turned out to be a homologue of APC4. An RNAi knockdown of APC4 and AP3 showed no detectable cellular phenotype, whereas an AP2 knockdown enriched the cells in G2/M phase. The AP2-depleted cells showed stabilized mitotic cyclin B. An accumulation of poly-ubiquitinated cyclin B was indicated in the cells treated with the proteasome inhibitor MG132, demonstrating the involvement of proteasome in degrading poly-ubiquitinated cyclin B. In all, a 10-subunit APC/C machinery with a conserved function is identified in T. brucei without linking to a MCC-like complex, thus indicating a unique T. brucei APC/C.

Microtubule attachment and spindle assembly checkpoint signalling at the kinetochore

In eukaryotes, chromosome segregation during cell division is facilitated by the kinetochore, a multiprotein structure that is assembled on centromeric DNA. The kinetochore attaches chromosomes to spindle microtubules, modulates the stability of these attachments and relays the microtubule-binding status to the spindle assembly checkpoint (SAC), a cell cycle surveillance pathway that delays chromosome segregation in response to unattached kinetochores. Recent studies are shaping current thinking on how each of these kinetochore-centred processes is achieved, and how their integration ensures faithful chromosome segregation, focusing on the essential roles of kinase-phosphatase signalling and the microtubule-binding KMN protein network.

The budding yeast Cdc48(Shp1) complex promotes cell cycle progression by positive regulation of protein phosphatase 1 (Glc7)

The conserved, ubiquitin-selective AAA ATPase Cdc48 regulates numerous cellular processes including protein quality control, DNA repair and the cell cycle. Cdc48 function is tightly controlled by a multitude of cofactors mediating substrate specificity and processing. The UBX domain protein Shp1 is a bona fide substrate-recruiting cofactor of Cdc48 in the budding yeast S. cerevisiae. Even though Shp1 has been proposed to be a positive regulator of Glc7, the catalytic subunit of protein phosphatase 1 in S. cerevisiae, its cellular functions in complex with Cdc48 remain largely unknown. Here we show that deletion of the SHP1 gene results in severe growth defects and a cell cycle delay at the metaphase to anaphase transition caused by reduced Glc7 activity. Using an engineered Cdc48 binding-deficient variant of Shp1, we establish the Cdc48^Shp1 complex as a critical regulator of mitotic Glc7 activity. We demonstrate that shp1 mutants possess a perturbed balance of Glc7 phosphatase and Ipl1 (Aurora B) kinase activities and show that hyper-phosphorylation of the kinetochore protein Dam1, a key mitotic substrate of Glc7 and Ipl1, is a critical defect in shp1. We also show for the first time a physical interaction between Glc7 and Shp1 in vivo. Whereas loss of Shp1 does not significantly affect Glc7 protein levels or localization, it causes reduced binding of the activator protein Glc8 to Glc7. Our data suggest that the Cdc48^Shp1 complex controls Glc7 activity by regulating its interaction with Glc8 and possibly further regulatory subunits.

The Mub1/Ubr2 ubiquitin ligase complex regulates the conserved Dsn1 kinetochore protein

The kinetochore is the macromolecular complex that assembles onto centromeric DNA and orchestrates the segregation of duplicated chromosomes. More than 60 components make up the budding yeast kinetochore, including inner kinetochore proteins that bind to centromeric chromatin and outer proteins that directly interact with microtubules. However, little is known about how these components assemble into a functional kinetochore and whether there are quality control mechanisms that monitor kinetochore integrity. We previously developed a method to isolate kinetochore particles via purification of the conserved Dsn1 kinetochore protein. We find that the Mub1/Ubr2 ubiquitin ligase complex associates with kinetochore particles through the CENP-C^Mif2 protein. Although Mub1/Ubr2 are not stable kinetochore components in vivo, they regulate the levels of the conserved outer kinetochore protein Dsn1 via ubiquitylation. Strikingly, a deletion of Mub1/Ubr2 restores the levels and viability of a mutant Dsn1 protein, reminiscent of quality control systems that target aberrant proteins for degradation. Consistent with this, Mub1/Ubr2 help to maintain viability when kinetochores are defective. Together, our data identify a previously unknown regulatory mechanism for the conserved Dsn1 kinetochore protein. We propose that Mub1/Ubr2 are part of a quality control system that monitors kinetochore integrity, thus ensuring genomic stability.

The flawless execution of cell division is essential to the survival of all organisms. The loss or gain of a single chromosome, the state called aneuploidy, is a hallmark of cancer cells and is the leading cause of spontaneous miscarriages and hereditary birth defects. Segregation is mediated by the kinetochore, the macromolecular complex that assembles on each chromosome and attaches to spindle microtubules to pull chromosomes to opposite poles when cells divide. It is therefore critical to understand how kinetochores are assembled and maintained. Here, we find that the levels of a conserved kinetochore protein are regulated by proteolysis. We propose that cells have quality control systems that ensure kinetochore integrity and thus genome stability.

The Oxytricha trifallax macronuclear genome: a complex eukaryotic genome with 16,000 tiny chromosomes

With more chromosomes than any other sequenced genome, the macronuclear genome of Oxytricha trifallax has a unique and complex architecture, including alternative fragmentation and predominantly single-gene chromosomes.

The macronuclear genome of the ciliate Oxytricha trifallax displays an extreme and unique eukaryotic genome architecture with extensive genomic variation. During sexual genome development, the expressed, somatic macronuclear genome is whittled down to the genic portion of a small fraction (∼5%) of its precursor “silent” germline micronuclear genome by a process of “unscrambling” and fragmentation. The tiny macronuclear “nanochromosomes” typically encode single, protein-coding genes (a small portion, 10%, encode 2–8 genes), have minimal noncoding regions, and are differentially amplified to an average of ∼2,000 copies. We report the high-quality genome assembly of ∼16,000 complete nanochromosomes (∼50 Mb haploid genome size) that vary from 469 bp to 66 kb long (mean ∼3.2 kb) and encode ∼18,500 genes. Alternative DNA fragmentation processes ∼10% of the nanochromosomes into multiple isoforms that usually encode complete genes. Nucleotide diversity in the macronucleus is very high (SNP heterozygosity is ∼4.0%), suggesting that Oxytricha trifallax may have one of the largest known effective population sizes of eukaryotes. Comparison to other ciliates with nonscrambled genomes and long macronuclear chromosomes (on the order of 100 kb) suggests several candidate proteins that could be involved in genome rearrangement, including domesticated MULE and IS1595-like DDE transposases. The assembly of the highly fragmented Oxytricha macronuclear genome is the first completed genome with such an unusual architecture. This genome sequence provides tantalizing glimpses into novel molecular biology and evolution. For example, Oxytricha maintains tens of millions of telomeres per cell and has also evolved an intriguing expansion of telomere end-binding proteins. In conjunction with the micronuclear genome in progress, the O. trifallax macronuclear genome will provide an invaluable resource for investigating programmed genome rearrangements, complementing studies of rearrangements arising during evolution and disease.

The macronuclear genome of the ciliate Oxytricha trifallax, contained in its somatic nucleus, has a unique genome architecture. Unlike its diploid germline genome, which is transcriptionally inactive during normal cellular growth, the macronuclear genome is fragmented into at least 16,000 tiny (∼3.2 kb mean length) chromosomes, most of which encode single actively transcribed genes and are differentially amplified to a few thousand copies each. The smallest chromosome is just 469 bp, while the largest is 66 kb and encodes a single enormous protein. We found considerable variation in the genome, including frequent alternative fragmentation patterns, generating chromosome isoforms with shared sequence. We also found limited variation in chromosome amplification levels, though insufficient to explain mRNA transcript level variation. Another remarkable feature of Oxytricha's macronuclear genome is its inordinate fondness for telomeres. In conjunction with its possession of tens of millions of chromosome-ending telomeres per macronucleus, we show that Oxytricha has evolved multiple putative telomere-binding proteins. In addition, we identified two new domesticated transposase-like protein classes that we propose may participate in the process of genome rearrangement. The macronuclear genome now provides a crucial resource for ongoing studies of genome rearrangement processes that use Oxytricha as an experimental or comparative model.

Combination analysis of Bub1 and Mad2 expression in endometrial cancer: act as a prognostic factor in endometrial cancer

INTRODUCTION

This study aimed to evaluate the diagnostic value of Bub1 and Mad2 together in endometrial cancer.

MATERIALS AND METHODS

Sixty-three patients with endometrial cancer, including EECs and NEECs, were examined statistically. Bub1 and Mad2 in patient tissues were detected by indirect streptavidin-biotin-peroxidase complex method.

RESULTS

39 cases (61.9 %) were in clinical stage I, 17 cases (27 %) in stage II, 5 (7.9 %) in stage III and 2 cases (3.2 %) in stage IV. Bub1 positive and Mad2 positive rate were identified in 18 and 54 patients. The lower positive of Bub1 and higher positive rate of Mad2 were related to clinical stage and histological grade of endometrial cancer (P = 0.009, 0.002, respectively), especially in NEECs. The expression of Bub1 was negatively correlated with Mad2 and Mtp53 (both P < 0.05). The expression of Mad2 with Mtp53 was positively correlated (P < 0.05). Statistically significant difference between Bub1/Mad2 expression and survival was discovered in endometrial cancer. Specificity and sensitivity of combination between Bub1 negative and Mad2 positive cases were higher than those alone in NEEC subtype. Bub1 and Mad2 are associated with histological differentiation, clinical stage and decreased postoperative survival in endometrial cancer.

CONCLUSIONS

The combination analysis of Bub1 and Mad2 might be the valuable prognostic, even diagnostic clinicopathologic factor which can be applicable in routine examination.

BRK1, a Bub1-related kinase, is essential for generating proper tension between homologous kinetochores at metaphase I of rice meiosis

Bub1 (for budding uninhibited by benzimidazole 1), one of the main spindle checkpoint kinases, acts as a kinetochore scaffold for assembling other checkpoint proteins. Here, we identify a plant Bub1-related kinase 1 (BRK1) in rice (Oryza sativa). The brk1 mutants are sterile due to the precocious separation of sister chromatids at the onset of anaphase I. The centromeric recruitment of SHUGOSHIN1 and phosphorylation of histone H2A at Thr-134 (H2A-pT134) depend on BRK1. Although the homologs can faithfully separate from each other at the end of meiosis I, the uncorrected merotelic attachment of paired sister kinetochores at the early stage of metaphase I in brk1 reduces the tension across homologous kinetochores, causes the metaphase I spindle to be aberrantly shaped, and subsequently affects the synchronicity of homolog separation at the onset of anaphase I. In addition, the phosphorylation of inner centromeric histone H3 at Ser-10 (H3-pS10) during diakinesis depends on BRK1. Therefore, we speculate that BRK1 may be required for normal localization of Aurora kinase before the onset of metaphase I, which is responsible for correcting the merotelic attachment.

Histone methyltransferases MES-4 and MET-1 promote meiotic checkpoint activation in Caenorhabditis elegans

Chromosomes that fail to synapse during meiosis become enriched for chromatin marks associated with heterochromatin assembly. This response, called meiotic silencing of unsynapsed or unpaired chromatin (MSUC), is conserved from fungi to mammals. In Caenorhabditis elegans, unsynapsed chromosomes also activate a meiotic checkpoint that monitors synapsis. The synapsis checkpoint signal is dependent on cis-acting loci called Pairing Centers (PCs). How PCs signal to activate the synapsis checkpoint is currently unknown. We show that a chromosomal duplication with PC activity is sufficient to activate the synapsis checkpoint and that it undergoes heterochromatin assembly less readily than a duplication of a non-PC region, suggesting that the chromatin state of these loci is important for checkpoint function. Consistent with this hypothesis, MES-4 and MET-1, chromatin-modifying enzymes associated with transcriptional activity, are required for the synapsis checkpoint. In addition, a duplication with PC activity undergoes heterochromatin assembly when mes-4 activity is reduced. MES-4 function is required specifically for the X chromosome, while MES-4 and MET-1 act redundantly to monitor autosomal synapsis. We propose that MES-4 and MET-1 antagonize heterochromatin assembly at PCs of unsynapsed chromosomes by promoting a transcriptionally permissive chromatin environment that is required for meiotic checkpoint function. Moreover, we suggest that different genetic requirements to monitor the behavior of sex chromosomes and autosomes allow for the lone unsynapsed X present in male germlines to be shielded from inappropriate checkpoint activation.

Novel checkpoint pathway organization promotes genome stability in stationary-phase yeast cells

Most DNA alterations occur during DNA replication in the S phase of the cell cycle. However, the majority of eukaryotic cells exist in a nondividing, quiescent state. Little is known about the factors involved in preventing DNA instability within this stationary-phase cell population. Previously, we utilized a unique assay system to identify mutations that increased minisatellite alterations specifically in quiescent cells in Saccharomyces cerevisiae. Here we conducted a modified version of synthetic genetic array analysis to determine if checkpoint signaling components play a role in stabilizing minisatellites in stationary-phase yeast cells. Our results revealed that a subset of checkpoint components, specifically MRC1, CSM3, TOF1, DDC1, RAD17, MEC3, TEL1, MEC1, and RAD53, prevent stationary-phase minisatellite alterations within the quiescent cell subpopulation of stationary-phase cells. Pathway analysis revealed at least three pathways, with MRC1, CSM3, and TOF1 acting in a pathway independent of MEC1 and RAD53. Overall, our data indicate that some well-characterized checkpoint components maintain minisatellite stability in stationary-phase cells but are regulated differently in those cells than in actively growing cells. For the MRC1-dependent pathway, the checkpoint itself may not be the important element; rather, it may be loss of the checkpoint proteins' other functions that contributes to DNA instability.

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.

Mad2 and the APC/C compete for the same site on Cdc20 to ensure proper chromosome segregation

The spindle assembly checkpoint (SAC) is essential to ensure proper chromosome segregation and thereby maintain genomic stability. The SAC monitors chromosome attachment, and any unattached chromosomes generate a "wait anaphase" signal that blocks chromosome segregation. The target of the SAC is Cdc20, which activates the anaphase-promoting complex/cyclosome (APC/C) that triggers anaphase and mitotic exit by ubiquitylating securin and cyclin B1. The inhibitory complex formed by the SAC has recently been shown to inhibit Cdc20 by acting as a pseudosubstrate inhibitor, but in this paper, we show that Mad2 also inhibits Cdc20 by binding directly to a site required to bind the APC/C. Mad2 and the APC/C competed for Cdc20 in vitro, and a Cdc20 mutant that does not bind stably to Mad2 abrogated the SAC in vivo. Thus, we provide insights into how Cdc20 binds the APC/C and uncover a second mechanism by which the SAC inhibits the APC/C.

Mitosis: short-circuiting spindle checkpoint signaling

The spindle checkpoint forms an intricate signaling circuit to sense unattached kinetochores, to inhibit the anaphase-promoting complex/cyclosome (APC/C), and to delay anaphase onset. Using clever genetic experiments in the budding yeast, Lau and Murray define the endpoint of checkpoint signaling and provide key mechanistic insights into checkpoint inhibition of APC/C.

Is germline transmission of MAD2 gene deletion associated with human fetal loss?

The spindle assembly checkpoint (SAC) monitors proper attachment of spindles to the kinetochore during mitotic and meiotic cell divisions and thus prevents aneuploidy. Chromosomal aneuploidy has been found to be associated with pregnancy loss and birth defects. Mad2 is one of the critical molecules of SAC. Deregulated Mad2 expression has been found to be associated with defective SAC-mediated abnormal meiotic progression in cell studies using animal models. Whether mutation in MAD2L1 is associated with the loss of Mad2 expression in aborted human fetuses is unknown. In this study, a correlation between aneuploidy and MAD2 defect was examined in primary fibroblast cultures obtained from abortuses. We report three trisomic abortuses with undetectable Mad2 expression. Further, quantitative real-time PCR revealed copy number deletion of MAD2 gene in these fetuses. Analysis of parental DNA samples available from two families revealed copy number loss of the same gene, suggesting Mendelian inheritance of MAD2 deletion. This germline transmission of exonic deletion of MAD2 is possibly associated with its loss of expression resulting in abnormal SAC function, subsequent aneuploidy and pregnancy loss.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

Human papillomavirus type 16 E7 oncoprotein engages but does not abrogate the mitotic spindle assembly checkpoint

The mitotic spindle assembly checkpoint (SAC) ensures faithful chromosome segregation during mitosis by censoring kinetochore-microtubule interactions. It is frequently rendered dysfunctional during carcinogenesis causing chromosome missegregation and genomic instability. There are conflicting reports whether the HPV16 E7 oncoprotein drives chromosomal instability by abolishing the SAC. Here we report that degradation of mitotic cyclins is impaired in cells with HPV16 E7 expression. RNAi-mediated depletion of Mad2 or BubR1 indicated the involvement of the SAC, suggesting that HPV16 E7 expression causes sustained SAC engagement. Mutational analyses revealed that HPV16 E7 sequences that are necessary for retinoblastoma tumor suppressor protein binding as well as sequences previously implicated in binding the nuclear and mitotic apparatus (NuMA) protein and in delocalizing dynein from the mitotic spindle contribute to SAC engagement. Importantly, however, HPV16 E7 does not markedly compromise the SAC response to microtubule poisons.

Structure of the mitotic checkpoint complex

In mitosis, the spindle assembly checkpoint (SAC) ensures genome stability by delaying chromosome segregation until all sister chromatids have achieved bipolar attachment to the mitotic spindle. The SAC is imposed by the mitotic checkpoint complex (MCC), whose assembly is catalysed by unattached chromosomes and which binds and inhibits the anaphase-promoting complex/cyclosome (APC/C), the E3 ubiquitin ligase that initiates chromosome segregation. Here, using the crystal structure of Schizosaccharomyces pombe MCC (a complex of mitotic spindle assembly checkpoint proteins Mad2, Mad3 and APC/C co-activator protein Cdc20), we reveal the molecular basis of MCC-mediated APC/C inhibition and the regulation of MCC assembly. The MCC inhibits the APC/C by obstructing degron recognition sites on Cdc20 (the substrate recruitment subunit of the APC/C) and displacing Cdc20 to disrupt formation of a bipartite D-box receptor with the APC/C subunit Apc10. Mad2, in the closed conformation (C-Mad2), stabilizes the complex by optimally positioning the Mad3 KEN-box degron to bind Cdc20. Mad3 and p31(comet) (also known as MAD2L1-binding protein) compete for the same C-Mad2 interface, which explains how p31(comet) disrupts MCC assembly to antagonize the SAC. This study shows how APC/C inhibition is coupled to degron recognition by co-activators.

A stochastic model of kinetochore-microtubule attachment accurately describes fission yeast chromosome segregation

In fission yeast, erroneous attachments of spindle microtubules to kinetochores are frequent in early mitosis. Most are corrected before anaphase onset by a mechanism involving the protein kinase Aurora B, which destabilizes kinetochore microtubules (ktMTs) in the absence of tension between sister chromatids. In this paper, we describe a minimal mathematical model of fission yeast chromosome segregation based on the stochastic attachment and detachment of ktMTs. The model accurately reproduces the timing of correct chromosome biorientation and segregation seen in fission yeast. Prevention of attachment defects requires both appropriate kinetochore orientation and an Aurora B-like activity. The model also reproduces abnormal chromosome segregation behavior (caused by, for example, inhibition of Aurora B). It predicts that, in metaphase, merotelic attachment is prevented by a kinetochore orientation effect and corrected by an Aurora B-like activity, whereas in anaphase, it is corrected through unbalanced forces applied to the kinetochore. These unbalanced forces are sufficient to prevent aneuploidy.

A quantitative model for cyclin-dependent kinase control of the cell cycle: revisited

The eukaryotic cell division cycle encompasses an ordered series of events. Chromosomal DNA is replicated during S phase of the cell cycle before being distributed to daughter cells in mitosis. Both S phase and mitosis in turn consist of an intricately ordered sequence of molecular events. How cell cycle ordering is achieved, to promote healthy cell proliferation and avert insults on genomic integrity, has been a theme of Paul Nurse's research. To explain a key aspect of cell cycle ordering, sequential S phase and mitosis, Stern & Nurse proposed 'A quantitative model for cdc2 control of S phase and mitosis in fission yeast'. In this model, S phase and mitosis are ordered by their dependence on increasing levels of cyclin-dependent kinase (Cdk) activity. Alternative mechanisms for ordering have been proposed that rely on checkpoint controls or on sequential waves of cyclins with distinct substrate specificities. Here, we review these ideas in the light of experimental evidence that has meanwhile accumulated. Quantitative Cdk control emerges as the basis for cell cycle ordering, fine-tuned by cyclin specificity and checkpoints. We propose a molecular explanation for quantitative Cdk control, based on thresholds imposed by Cdk-counteracting phosphatases, and discuss its implications.

Spindle assembly checkpoint: the third decade

The spindle assembly checkpoint controls cell cycle progression during mitosis, synchronizing it with the attachment of chromosomes to spindle microtubules. After the discovery of the mitotic arrest deficient (MAD) and budding uninhibited by benzymidazole (BUB) genes as crucial checkpoint components in 1991, the second decade of checkpoint studies (2001–2010) witnessed crucial advances in the elucidation of the mechanism through which the checkpoint effector, the mitotic checkpoint complex, targets the anaphase-promoting complex (APC/C) to prevent progression into anaphase. Concomitantly, the discovery that the Ndc80 complex and other components of the microtubule-binding interface of kinetochores are essential for the checkpoint response finally asserted that kinetochores are crucial for the checkpoint response. Nevertheless, the relationship between kinetochores and checkpoint control remains poorly understood. Crucial advances in this area in the third decade of checkpoint studies (2011–2020) are likely to be brought about by the characterization of the mechanism of kinetochore recruitment, activation and inactivation of checkpoint proteins, which remains elusive for the majority of checkpoint components. Here, we take a molecular view on the main challenges hampering this task.

Expression profiles of cohesins, shugoshins and spindle assembly checkpoint genes in rhesus macaque oocytes predict their susceptibility for aneuploidy during embryonic development

High frequencies of chromosomal anomalies are reported in human and non-human primate in vitro-produced preimplantation embryos. It is unclear why certain embryos develop aneuploidies while others remain euploid. A differential susceptibility to aneuploidy is most likely a consequence of events that occur before oocyte collection. One hypothesis is that the relative transcript levels of cohesins, shugoshins and spindle assembly checkpoint genes are correlated with the occurrence of chromosomal anomalies. Transcript levels of these genes were quantified in individual oocytes that were either mature (group 1, low aneuploidy rate) or immature (group 2, high aneuploidy rate) at retrieval, utilizing TaqMan-based real-time PCR. The transcript level in each oocyte was categorized as absent, below the median or above the median in order to conduct comparisons. Statistically significant differences were observed between group 1 and group 2 for SGOL1 and BUB1. There were more oocytes with SGOL1 expression levels above the median in group 1, while oocytes lacking BUB1 were only observed in group 1. These findings suggest that higher SGOL1 levels in group 1 oocytes could better protect against a premature separation of sister chromatids than in embryos derived from group 2 oocytes. The absence of BUB1 transcripts in group 1 was frequently associated with reduced expression of either mitotic cohesins or shugoshins. We hypothesize that ablation of BUB1 could induce mitotic arrest in oocytes that fail to express a complete complement of cohesins and shugoshins, thereby reducing the number of developing aneuploid preimplantation embryos.

Structural analysis reveals features of the spindle checkpoint kinase Bub1-kinetochore subunit Knl1 interaction

The function of the essential checkpoint kinases Bub1 and BubR1 requires their recruitment to mitotic kinetochores. Kinetochore recruitment of Bub1 and BubR1 is proposed to rely on the interaction of the tetratricopeptide repeats (TPRs) of Bub1 and BubR1 with two KI motifs in the outer kinetochore protein Knl1. We determined the crystal structure of the Bub1 TPRs in complex with the cognate Knl1 KI motif and compared it with the structure of the equivalent BubR1TPR-KI motif complex. The interaction developed along the convex surface of the TPR assembly. Point mutations on this surface impaired the interaction of Bub1 and BubR1 with Knl1 in vitro and in vivo but did not cause significant displacement of Bub1 and BubR1 from kinetochores. Conversely, a 62-residue segment of Bub1 that includes a binding domain for the checkpoint protein Bub3 and is C terminal to the TPRs was necessary and largely sufficient for kinetochore recruitment of Bub1. These results shed light on the determinants of kinetochore recruitment of Bub1.

Biophysics of mitosis

Mitosis is the process by which eukaryotic cells organize and segregate their chromosomes in preparation for cell division. It is accomplished by a cellular machine composed largely of microtubules (MTs) and their associated proteins. This article reviews literature on mitosis from a biophysical point of view, drawing attention to the assembly and motility processes required to do this complex job with precision. Work from both the recent and the older literature is integrated into a description of relevant biological events and the experiments that probe their mechanisms. Theoretical work on specific subprocesses is also reviewed. Our goal is to provide a document that will expose biophysicists to the fascination of this quite amazing process and provide them with a good background from which they can pursue their own research interests in the subject.

Loss of function of the Cik1/Kar3 motor complex results in chromosomes with syntelic attachment that are sensed by the tension checkpoint

The attachment of sister kinetochores by microtubules emanating from opposite spindle poles establishes chromosome bipolar attachment, which generates tension on chromosomes and is essential for sister-chromatid segregation. Syntelic attachment occurs when both sister kinetochores are attached by microtubules from the same spindle pole and this attachment is unable to generate tension on chromosomes, but a reliable method to induce syntelic attachments is not available in budding yeast. The spindle checkpoint can sense the lack of tension on chromosomes as well as detached kinetochores to prevent anaphase onset. In budding yeast Saccharomyces cerevisiae, tension checkpoint proteins Aurora/Ipl1 kinase and centromere-localized Sgo1 are required to sense the absence of tension but are dispensable for the checkpoint response to detached kinetochores. We have found that the loss of function of a motor protein complex Cik1/Kar3 in budding yeast leads to syntelic attachments. Inactivation of either the spindle or tension checkpoint enables premature anaphase entry in cells with dysfunctional Cik1/Kar3, resulting in co-segregation of sister chromatids. Moreover, the abolished Kar3-kinetochore interaction in cik1 mutants suggests that the Cik1/Kar3 complex mediates chromosome movement along microtubules, which could facilitate bipolar attachment. Therefore, we can induce syntelic attachments in budding yeast by inactivating the Cik1/Kar3 complex, and this approach will be very useful to study the checkpoint response to syntelic attachments.

Chromosome bipolar attachment occurs when sister chromatids are attached by microtubules emanating from opposite spindle poles and is essential for faithful sister-chromatid segregation. Chromosomes are under tension once bipolar attachment is established. The absence of tension is sensed by the tension checkpoint that prevents chromosome segregation. The attachment of sister chromatids by microtubules from the same spindle pole generates syntelic attachment, which fails to generate tension on chromosomes. However, a reliable method to induce syntelic attachment is not available. Our findings indicate that the inactivation of the motor complex, Cik1/Kar3, results in chromosomes with syntelic attachment in budding yeast. In the absence of the tension checkpoint, yeast cells with dysfunctional Cik1/Kar3 enter anaphase, resulting in co-segregation of sister chromatids. Therefore, with this method we can experimentally induce syntelic attachment in yeast and investigate how cells respond to this incorrect attachment.

How mitotic errors contribute to karyotypic diversity in cancer

Aneuploidy is a common feature of cancer cells, and is believed to play a critical role in tumorigenesis and cancer progression. Most cancer cells also exhibit high rates of mitotic chromosome mis-segregation, a phenomenon known as chromosomal instability, which leads to high variability of the karyotype. Here, we describe the nature, nuances, and implications of cancer karyotypic diversity. Moreover, we summarize recent studies aimed at identifying the mitotic defects that may be responsible for inducing chromosome mis-segregation in cancer cells. These include kinetochore attachment errors, spindle assembly checkpoint dysfunction, mitotic spindle defects, and other cell division inaccuracies. Finally, we discuss how such mitotic errors generate karyotypic diversity in cancer cells.

Mad2 and Mad3 cooperate to arrest budding yeast in mitosis

BACKGROUND

The spindle checkpoint ensures accurate chromosome transmission by delaying chromosome segregation until all chromosomes are correctly aligned on the mitotic spindle. The checkpoint is activated by kinetochores that are not attached to microtubules or are attached but not under tension and arrests cells at metaphase by inhibiting the anaphase-promoting complex (APC) and its coactivator Cdc20. Despite numerous studies, we still do not understand how the checkpoint proteins coordinate with each other to inhibit APC(Cdc20) activity.

RESULTS

To ask how the checkpoint components induce metaphase arrest, we constructed fusions of checkpoint proteins and expressed them in the budding yeast Saccharomyces cerevisiae to mimic possible protein interactions during checkpoint activation. We found that expression of a Mad2-Mad3 protein fusion or noncovalently linked Mad2 and Mad3, but not the overexpression of the two separate proteins, induces metaphase arrest that is independent of functional kinetochores or other checkpoint proteins. We further showed that artificially tethering Mad2 to Cdc20 also arrests cells in metaphase independently of other checkpoint components.

CONCLUSION

Our results suggest that Mad3 is required for the stable binding of Mad2 to Cdc20 in vivo, which is sufficient to inhibit APC activity and is the most downstream event in spindle checkpoint activation.

Rong Li: tipping the balance in cell biology. Interview by Caitlin Sedwick

Li takes multiple approaches to understand how signaling pathways really work.

Centromere-tethered Mps1 pombe homolog (Mph1) kinase is a sufficient marker for recruitment of the spindle checkpoint protein Bub1, but not Mad1

The spindle checkpoint delays the onset of anaphase until all of the chromosomes properly achieve bipolar attachment to the spindle. It has been shown that unattached kinetochores are the site that emits a signal for activation of the checkpoint. Although the components of the checkpoint such as Bub1, Mad1 and Mad2 selectively accumulate at unattached kinetochores, the answer to how they recognize unattached kinetochores has remained elusive. Mps1 pombe homolog (Mph1) kinase has been shown to function upstream of most of the components of the checkpoint and thus it is thought to recognize unattached kinetochores by itself and recruit other components. In this study we have expressed a fusion protein of Mph1 and Ndc80 (a kinetochore protein of the outer plate) and shown that the fusion protein arrests cell cycle progression in a spindle-checkpoint\x{2013}dependent manner in fission yeast. When expression of Mad2 is turned off, the cells grow normally with Mph1 constitutively localized at centromeres/kinetochores. Under this condition, Bub1 can be found with Mph1 throughout the cell cycle, indicating that localization of Mph1 at centromeres/kinetochores is sufficient to recruit Bub1. In contrast, Mad1 is found to transiently localize at kinetochores, which are presumably unattached to the spindle, but soon it dissociates from kinetochores. We propose that Mph1 is a sufficient marker for recruitment of Bub1. Mad1, in contrast, requires an additional condition/component for stable association with kinetochores.

Expression of mitotic-arrest deficiency 2 predicts the efficacy of neoadjuvant chemotherapy for locally advanced uterine cervical cancer

We previously reported satisfactory therapeutic results when using cisplatin-based cyclic balloon-occluded arterial infusion chemotherapy as neoadjuvant chemotherapy (NAC), which enabled hysterectomy to be performed for patients with locally advanced cervical cancer. Mitotic arrest deficiency 2 (MAD2) is a key component of the mitotic spindle checkpoint pathway. The expression of MAD2 is associated with tumor progression and resistance to chemotherapy. Therefore, the aim of the present study was to examine whether the expression of MAD2 is related to the efficacy of NAC for locally advanced uterine cervical cancer. We reviewed 53 cases of locally advanced uterine cervical cancer (stage IIIa-IIIb; based on the International Federation of Gynecology and Obstetrics criteria). These patients were initially treated at Osaka City University Medical School Hospital, Japan, from 1995 to 2008 and were under 70 years old. Tumor samples were obtained by biopsy prior to NAC. Cases were divided into two groups: one group in which NAC was effective, surgery was possible and radiotherapy was performed (NAC+OP+R group; n=33), and another group in which NAC was ineffective and radiation therapy was performed (NAC+R group; n=20). MAD2 expression was examined in paraffin-embedded sections using the avidin-biotin peroxidase complex method. The results showed that MAD2 expression was significantly higher in the NAC+R group compared to the NAC+OP+R group (P<0.001). There was no significant difference in overall survival between the two groups, although the prognosis for the NAC+OP+R group tended to be slightly better (P=0.064). Taken together, these results suggest that the expression of MAD2 may predict the efficacy of NAC as a treatment for locally advanced uterine cervical cancer.

Mitotic arrest deficiency 2 induces carcinogenesis in mucinous ovarian tumors

Mitotic arrest deficiency 2 (MAD2) is a key component of the mitotic spindle checkpoint pathway. A compromised mitotic spindle checkpoint results in an abnormal number of chromosomes. This is referred to as chromosomal instability, and has been reported in most types of human cancer. The aim of this study was to examine the expression of MAD2 in mucinous ovarian tumors exhibiting varying degrees of malignancy. We reviewed 128 cases of mucinous ovarian tumors initially treated at Osaka City University Medical School Hospital, Japan. Tumor samples were obtained following surgery. The cases were divided into three groups: benign (group B; n=30), borderline malignant (group BM; n=55) and malignant (group M; n=43). MAD2 expression was examined in paraffin-embedded sections using the avidin-biotin peroxidase complex method. Results showed MAD2 expression to be significantly greater in group M compared to groups B and BM (P<0.05). In addition, there was a moderate correlation between MAD2 expression and the degree of malignancy (r=0.51, P<0.05). However, when the samples in group M were classified according to a low or high expression of MAD2, no difference was observed in terms of overall survival. These findings suggest that the overexpression of MAD2 may be correlated to carcinogenesis in mucinous ovarian tumors.

Defining genome maintenance pathways using functional genomic approaches

Genome maintenance activities including DNA repair, cell division cycle control, and checkpoint signaling pathways preserve genome integrity and prevent disease. Defects in these pathways cause birth defects, neurodegeneration, premature aging, and cancer. Recent technical advances in functional genomic approaches such as expression profiling, proteomics, and RNA interference (RNAi) technologies have rapidly expanded our knowledge of the proteins that work in these pathways. In this review, we examine the use of these high-throughput methodologies in higher eukaryotic organisms for the interrogation of genome maintenance activities.

Deletion of Cryptococcus neoformans AIF ortholog promotes chromosome aneuploidy and fluconazole-resistance in a metacaspase-independent manner

Apoptosis is a form of programmed cell death critical for development and homeostasis in multicellular organisms. Apoptosis-like cell death (ALCD) has been described in several fungi, including the opportunistic human pathogen Cryptococcus neoformans. In addition, capsular polysaccharides of C. neoformans are known to induce apoptosis in host immune cells, thereby contributing to its virulence. Our goals were to characterize the apoptotic signaling cascade in C. neoformans as well as its unique features compared to the host machinery to exploit the endogenous fungal apoptotic pathways as a novel antifungal strategy in the future. The dissection of apoptotic pathways revealed that apoptosis-inducing factor (Aif1) and metacaspases (Mca1 and Mca2) are independently required for ALCD in C. neoformans. We show that the apoptotic pathways are required for cell fusion and sporulation during mating, indicating that apoptosis may occur during sexual development. Previous studies showed that antifungal drugs induce ALCD in fungi and that C. neoformans adapts to high concentrations of the antifungal fluconazole (FLC) by acquisition of aneuploidy, especially duplication of chromosome 1 (Chr1). Disruption of aif1, but not the metacaspases, stimulates the emergence of aneuploid subpopulations with Chr1 disomy that are resistant to fluconazole (FLC(R)) in vitro and in vivo. FLC(R) isolates in the aif1 background are stable in the absence of the drug, while those in the wild-type background readily revert to FLC sensitivity. We propose that apoptosis orchestrated by Aif1 might eliminate aneuploid cells from the population and defects in this pathway contribute to the selection of aneuploid FLC(R) subpopulations during treatment. Aneuploid clinical isolates with disomies for chromosomes other than Chr1 exhibit reduced AIF1 expression, suggesting that inactivation of Aif1 might be a novel aneuploidy-tolerating mechanism in fungi that facilitates the selection of antifungal drug resistance.

p31(comet) acts to ensure timely spindle checkpoint silencing subsequent to kinetochore attachment

The spindle assembly checkpoint links the onset of anaphase to completion of chromosome-microtubule attachment and is mediated by the binding of Mad and Bub proteins to kinetochores of unattached or maloriented chromosomes. Mad2 and BubR1 traffic between kinetochores and the cytosol, thereby transmitting a "wait anaphase" signal to the anaphase-promoting complex. It is generally assumed that this signal dissipates automatically upon kinetochore-microtubule binding, but it has been shown that under conditions of nocodazole-induced arrest p31(comet), a Mad2-binding protein, is required for mitotic progression. In this article we investigate the localization and function of p31(comet) during normal, unperturbed mitosis in human and marsupial cells. We find that, like Mad2, p31(comet) traffics on and off kinetochores and is also present in the cytosol. Cells depleted of p31(comet) arrest in metaphase with mature bipolar kinetochore-microtubule attachments, a satisfied checkpoint, and high cyclin B levels. Thus p31(comet) is required for timely mitotic exit. We propose that p31(comet) is an essential component of the machinery that silences the checkpoint during each cell cycle.