Bub1 and CENP-U redundantly recruit Plk1 to stabilize kinetochore-microtubule attachments and ensure accurate chromosome segregation.

Bub1 is required for the kinetochore/centromere localization of two essential mitotic kinases Plk1 and Aurora B. Surprisingly, stable depletion of Bub1 by ∼95% in human cells marginally affects whole chromosome segregation fidelity. We show that CENP-U, which is recruited to kinetochores by the CENP-P and CENP-Q subunits of the CENP-O complex, is required to prevent chromosome mis-segregation in Bub1-depleted cells. Mechanistically, Bub1 and CENP-U redundantly recruit Plk1 to kinetochores to stabilize kinetochore-microtubule attachments, thereby ensuring accurate chromosome segregation. Furthermore, unlike its budding yeast homolog, the CENP-O complex does not regulate centromeric localization of Aurora B. Consistently, depletion of Bub1 or CENP-U sensitizes cells to the inhibition of Plk1 but not Aurora B kinase activity. Taken together, our findings provide mechanistic insight into the regulation of kinetochore function, which may have implications for targeted treatment of cancer cells with mutations perturbing kinetochore recruitment of Plk1 by Bub1 or the CENP-O complex.

Energy restriction causes metaphase delay and chromosome mis-segregation in cancer cells.

ATP metabolism during mitosis needs to be coordinated with numerous energy-demanding activities, especially in cancer cells whose metabolic pathways are reprogramed to sustain rapid proliferation in a nutrient-deficient environment. Although strategies targeting the energy metabolic pathways have shown therapeutic efficacy in preclinical cancer models, how normal cells and cancer cells differentially respond to energy shortage is unclear. In this study, using time-lapse microscopy, we found that cancer cells displayed unique mitotic phenotypes in a dose-dependent manner upon decreasing ATP (i.e. energy) supply. When reduction in ATP concentration was moderate, chromosome movements in mitosis were barely affected, while the metaphase-anaphase transition was significantly prolonged due to reduced tension between the sister-kinetochores, which delayed the satisfaction of the spindle assembly checkpoint. Further reduction in ATP concentration led to a decreased level of Aurora-B at the centromere, resulting in increased chromosome mis-segregation after metaphase delay. In contrast to cancer cells, ATP restriction in non-transformed cells induced cell cycle arrest in interphase, rather than causing mitotic defects. In addition, data mining of cancer patient database showed a correlation between signatures of energy production and chromosomal instability possibly resulted from mitotic defects. Together, these results reveal that energy restriction induces differential responses in normal and cancer cells, with chromosome mis-segregation only observed in cancer cells. This points to targeting energy metabolism as a potentially cancer-selective therapeutic strategy.

Aneuploidy renders cancer cells vulnerable to mitotic checkpoint inhibition.

Selective targeting of aneuploid cells is an attractive strategy for cancer treatment1. However, it is unclear whether aneuploidy generates any clinically relevant vulnerabilities in cancer cells. Here we mapped the aneuploidy landscapes of about 1,000 human cancer cell lines, and analysed genetic and chemical perturbation screens2-9 to identify cellular vulnerabilities associated with aneuploidy. We found that aneuploid cancer cells show increased sensitivity to genetic perturbation of core components of the spindle assembly checkpoint (SAC), which ensures the proper segregation of chromosomes during mitosis10. Unexpectedly, we also found that aneuploid cancer cells were less sensitive than diploid cells to short-term exposure to multiple SAC inhibitors. Indeed, aneuploid cancer cells became increasingly sensitive to inhibition of SAC over time. Aneuploid cells exhibited aberrant spindle geometry and dynamics, and kept dividing when the SAC was inhibited, resulting in the accumulation of mitotic defects, and in unstable and less-fit karyotypes. Therefore, although aneuploid cancer cells could overcome inhibition of SAC more readily than diploid cells, their long-term proliferation was jeopardized. We identified a specific mitotic kinesin, KIF18A, whose activity was perturbed in aneuploid cancer cells. Aneuploid cancer cells were particularly vulnerable to depletion of KIF18A, and KIF18A overexpression restored their response to SAC inhibition. Our results identify a therapeutically relevant, synthetic lethal interaction between aneuploidy and the SAC.

DNA-Binding Anticancer Drugs: One Target, Two Actions.

Amsacrine, an anticancer drug first synthesised in 1970 by Professor Cain and colleagues, showed excellent preclinical activity and underwent clinical trial in 1978 under the auspices of the US National Cancer Institute, showing activity against acute lymphoblastic leukaemia. In 1984, the enzyme DNA topoisomerase II was identified as a molecular target for amsacrine, acting to poison this enzyme and to induce DNA double-strand breaks. One of the main challenges in the 1980s was to determine whether amsacrine analogues could be developed with activity against solid tumours. A multidisciplinary team was assembled in Auckland, and Professor Denny played a leading role in this approach. Among a large number of drugs developed in the programme, N-[2-(dimethylamino)-ethyl]-acridine-4-carboxamide (DACA), first synthesised by Professor Denny, showed excellent activity against a mouse lung adenocarcinoma. It underwent clinical trial, but dose escalation was prevented by ion channel toxicity. Subsequent work led to the DACA derivative SN 28049, which had increased potency and reduced ion channel toxicity. Mode of action studies suggested that both amsacrine and DACA target the enzyme DNA topoisomerase II but with a different balance of cellular consequences. As primarily a topoisomerase II poison, amsacrine acts to turn the enzyme into a DNA-damaging agent. As primarily topoisomerase II catalytic inhibitors, DACA and SN 28049 act to inhibit the segregation of daughter chromatids during anaphase. The balance between these two actions, one cell cycle phase specific and the other nonspecific, together with pharmacokinetic, cytokinetic and immunogenic considerations, provides links between the actions of acridine derivatives and anthracyclines such as doxorubicin. They also provide insights into the action of cytotoxic DNA-binding drugs.

Actinomycin D causes oocyte maturation failure by inhibiting chromosome separation and spindle assembly†.

Actinomycin D (ActD) has been considered as one of the most effective and safe chemotherapeutic medications for treating a number of cancers. Although ActD has been used in the treatment of gynecological tumors and pediatric tumors for more than 50 years, the toxic effects of ActD on mammalian oocytes remain unknown. In this study, the influence of ActD on mouse and human oocyte maturation and the possible mechanisms were investigated. Notably, ActD inhibited oocyte maturation and arrested oocytes at the metaphase I (MI) stage in a dose-dependent manner. In addition, ActD arrested oocyte maturation when the oocytes were treated at different successive stages, including the germinal vesicle (GV), germinal vesicle breakdown, and MI stages. In ActD-treated oocytes, disordered chromosome condensation and irregular spindle assembly occurred, resulting in incomplete chromosome segregation and oocytes arresting at the MI phase; these results possibly occurred because ActD triggered the formation of reactive oxygen species, resulting in DNA damage and decreased ATP in mouse GV oocytes. Besides, in vivo treatment with ActD also inhibited mouse oocyte maturation. Similar effects were seen in human oocytes. Collectively, our results indicated that ActD exposure disrupted oocyte maturation by increasing DNA damage, which is a finding that might help with optimizing future methods for female fertility preservation before undergoing chemotherapy.

Wnt10b-GSK3ß-dependent Wnt/STOP signaling prevents aneuploidy in human somatic cells.

Wnt signaling is crucial for proper development, tissue homeostasis and cell cycle regulation. A key role of Wnt signaling is the GSK3ß-mediated stabilization of ß-catenin, which mediates many of the critical roles of Wnt signaling. In addition, it was recently revealed that Wnt signaling can also act independently of ß-catenin. In fact, Wnt mediated stabilization of proteins (Wnt/STOP) that involves an LRP6-DVL-dependent signaling cascade is required for proper regulation of mitosis and for faithful chromosome segregation in human somatic cells. We show that inhibition of Wnt/LRP6 signaling causes whole chromosome missegregation and aneuploidy by triggering abnormally increased microtubule growth rates in mitotic spindles, and this is mediated by increased GSK3ß activity. We demonstrate that proper mitosis and maintenance of numerical chromosome stability requires continuous basal autocrine Wnt signaling that involves secretion of Wnts. Importantly, we identified Wnt10b as a Wnt ligand required for the maintenance of normal mitotic microtubule dynamics and for proper chromosome segregation. Thus, a self-maintaining Wnt10b-GSK3ß-driven cellular machinery ensures the proper execution of mitosis and karyotype stability in human somatic cells.

Nonrandom DNA Segregation Detection under Replication Stress.

Nonrandom DNA segregation (NDS) is a mitotic event in which sister chromatids carrying the old (parent) DNA strands are distributed exclusively to one of the two daughter cells. Although this phenomenon occurs in multiple organisms, the low frequency poses an obstacle to observation. Here, we present an improved protocol to induce NDS under replication stress. This protocol can be modified to accommodate various cell lines. For complete details on the use and execution of this protocol, please refer to Xing et al. (2020).

CHK1 monitors spindle assembly checkpoint and DNA damage repair during the first cleavage of mouse early embryos.

OBJECTIVES: DNA damage and errors of accurate chromosome segregation lead to aneuploidy and foetal defects. DNA repair and the spindle assembly checkpoint (SAC) are the mechanisms developed to protect from these defects. Checkpoint kinase 1 (CHK1) is reported to be an important DNA damage response protein in multiple models, but its functions remain unclear in early mouse embryos. MATERIALS AND METHODS: Immunofluorescence staining, immunoblotting and real-time reverse transcription polymerase chain reaction were used to perform the analyses. Reactive oxygen species levels and Annexin-V were also detected. RESULTS: Loss of CHK1 activity accelerated progress of the cell cycle at the first cleavage; however, it disturbed the development of early embryos to the morula/blastocyst stages. Further analysis indicated that CHK1 participated in spindle assembly and chromosome alignment, possibly due to its regulation of kinetochore-microtubule attachment and recruitment of BubR1 and p-Aurora B to the kinetochores, indicating its role in SAC activity. Loss of CHK1 activity led to embryonic DNA damage and oxidative stress, which further induced early apoptosis and autophagy, indicating that CHK1 is responsible for interphase DNA damage repair. CONCLUSIONS: Our results indicate that CHK1 is a key regulator of the SAC and DNA damage repair during early embryonic development in mice.

Effects of malleable kinetochore morphology on measurements of intrakinetochore tension.

The distance between fluorescent spots formed by various kinetochore proteins (delta) is commonly interpreted as a manifestation of intrakinetochore tension (IKT) caused by microtubule-mediated forces. However, large-scale changes of the kinetochore architecture (such as its shape or dimensions) may also contribute to the value of delta. To assess contributions of these non-elastic changes, we compare behaviour of delta values in human kinetochores with small yet mechanically malleable kinetochores against compound kinetochores in Indian muntjac (IM) cells whose architecture remains constant. Due to the micrometre-scale length of kinetochore plates in IM, their shape and orientation are discernible in conventional light microscopy, which enables precise measurements of IKT independent of contributions from changes in overall architecture of the organelle. We find that delta in IM kinetochores remains relatively constant when microtubule-mediated forces are suppressed by Taxol, but it prominently decreases upon detachment of microtubules. By contrast, large decreases of delta observed in Taxol-treated human cells coincide with prominent changes in length and curvature of the kinetochore plate. These observations, supported by computational modelling, suggest that at least 50% of the decrease in delta in human cells reflects malleable reorganization of kinetochore architecture rather than elastic recoil due to IKT.

Cinobufagin Triggers Defects in Spindle Formation and Cap-Dependent Translation in Liver Cancer Cells by Inhibiting the AURKA-mTOR-eIF4E Axis.

Cinobufagin is a Na+/K+-ATPase (NKA) inhibitor with excellent anticancer effects to prolong the survival of patients. The purpose of the present study was to clarify the underlying mechanism of the anticancer effects of cinobufagin using overexpression or inhibition of aurora kinase A (AURKA) signaling. First, high expression of Na+/K+-ATPase alpha 1 subunit (ATP1A1) and AURAK resulted in increased malignant transformation in hepatocellular carcinoma (HCC) patients using the cancer genome atlas (TCGA) data and tissue samples. After treatment with cinobufagin, we successfully screened 202, 249, and 335 changing expression proteins in Huh-7 cells under normal, overexpression, and inhibition of AURKA using tandem mass tags (TMT)-labeled quantitative proteomics coupled to 2D liquid chromatography-tandem mass spectrometry (LC-MS/MS). Bioinformatics analysis revealed that these molecules were closely associated with chromosome segregation, DNA damage, and regulation of translation processes. We further confirmed that cinobufagin induced DNA damage and chromosome segregation disorders and suppresses translational processing in oncogenes by decreasing the expression of AURKA, mechanistic target of rapamycin kinase (mTOR), p-mTOR, p-extracellular regulated protein kinases (ERK), eukaryotic translation initiation factor 4E (eIF4E), and p-eIF4E, while increasing the expression of p-eukaryotic translation initiation factor 4E binding protein 1 (4E-BP1) (S65, T37, T46, T45) and increasing the interaction between eIF4 and 4E-BP1. Our results suggested that cinobufagin performed an antitumor effects in liver cancer cells by inhibiting the AURKA-mTOR-eIF4E axis.

Janus Face of Drug-Induced Tetraploidy in Non-Hodgkin Lymphoma.

Anticancer agents often cause drug-induced tetraploidy (DIT) in cancer cells. DIT is not only a mechanism of inherited drug resistance, but proliferating DIT cells can produce progeny with increased ploidy or aneuploid genomes that drive aggressive disease. Here, we explore combinatorial therapeutic strategies for either preventing or eliminating DIT cells.

Overexpression of cyclin A1 promotes meiotic resumption but induces premature chromosome separation in mouse oocyte.

Mammalian cyclin A1 is prominently expressed in testis and essential for meiosis in the male mouse, however, it shows weak expression in ovary, especially during oocyte maturation. To understand why cyclin A1 behaves in this way in the oocyte, we investigated the effect of cyclin A1 overexpression on mouse oocyte meiotic maturation. Our results revealed that cyclin A1 overexpression triggered meiotic resumption even in the presence of germinal vesicle breakdown inhibitor, milrinone. Nevertheless, the cyclin A1-overexpressed oocytes failed to extrude the first polar body but were completely arrested at metaphase I. Consequently, cyclin A1 overexpression destroyed the spindle morphology and chromosome alignment by inducing premature separation of chromosomes and sister chromatids. Therefore, cyclin A1 overexpression will prevent oocyte maturation although it can promote meiotic resumption. All these results show that decreased expression of cyclin A1 in oocytes may have an evolutional significance to keep long-lasting prophase arrest and orderly chromosome separation during oocyte meiotic maturation.

Inhibition of HDAC6 by tubastatin A disrupts mouse oocyte meiosis via regulating histone modifications and mRNA expression.

Histone deacetylase 6 (HDAC6) participates in mouse oocyte maturation by deacetylating α-tubulin. However, how HDAC6 expression is regulated in oocytes remains unknown. In the present study, we discovered that mouse oocytes had a high level of HDAC6 expression and a low level of DNA methylation status in their promoter region. Then, a selective HDAC6 inhibitor, tubastatin A (Tub-A) was chosen to investigate the role of HDAC6 in oocyte maturation. Our results revealed that inhibition of HDAC6 caused meiotic progression arrest, disturbed spindle/chromosome organization, and kinetochore-microtubule attachments without impairing spindle assembly checkpoint function. Moreover, inhibition of HDAC6 not only increased the acetylation of α-tubulin but also elevated the acetylation status of H4K16 and decreased the phosphorylation level of H3T3 and H3S10. Conversely, depressed H3T3 phosphorylation by its kinase inhibitor increased the acetylation level of H4K16. Finally, single cell RNA-seq analysis revealed that the cell cycle-related genes CCNB1, CDK2, SMAD3, YWHAZ and the methylation-related genes DNMT1 and DNMT3B were strongly repressed in Tub-A treated oocytes. Taken together, our results indicate that HDAC6 plays important roles in chromosome condensation and kinetochore function via regulating several key histone modifications and messenger RNA transcription during oocyte meiosis.

Targets and mechanisms of chemically induced aneuploidy. Part 1 of the report of the 2017 IWGT workgroup on assessing the risk of aneugens for carcinogenesis and hereditary diseases.

An aneuploidy workgroup was established as part of the 7th International Workshops on Genotoxicity Testing. The workgroup conducted a review of the scientific literature on the biological mechanisms of aneuploidy in mammalian cells and methods used to detect chemical aneugens. In addition, the current regulatory framework was discussed, with the objective to arrive at consensus statements on the ramifications of exposure to chemical aneugens for human health risk assessment. As part of these efforts, the workgroup explored the use of adverse outcome pathways (AOPs) to document mechanisms of chemically induced aneuploidy in mammalian somatic cells. The group worked on two molecular initiating events (MIEs), tubulin binding and binding to the catalytic domain of aurora kinase B, which result in several adverse outcomes, including aneuploidy. The workgroup agreed that the AOP framework provides a useful approach to link evidence for MIEs with aneuploidy on a cellular level. The evidence linking chemically induced aneuploidy with carcinogenicity and hereditary disease was also reviewed and is presented in two companion papers. In addition, the group came to the consensus that the current regulatory test batteries, while not ideal, are sufficient for the identification of aneugens and human risk assessment. While it is obvious that there are many different MIEs that could lead to the induction of aneuploidy, the most commonly observed mechanisms involving chemical aneugens are related to tubulin binding and, to a lesser extent, inhibition of mitotic kinases. The comprehensive review presented here should help with the identification and risk management of aneugenic agents.

High Proliferation Rate and a Compromised Spindle Assembly Checkpoint Confers Sensitivity to the MPS1 Inhibitor BOS172722 in Triple-Negative Breast Cancers.

BOS172722 (CCT289346) is a highly potent, selective, and orally bioavailable inhibitor of spindle assembly checkpoint kinase MPS1. BOS172722 treatment alone induces significant sensitization to death, particularly in highly proliferative triple-negative breast cancer (TNBC) cell lines with compromised spindle assembly checkpoint activity. BOS172722 synergizes with paclitaxel to induce gross chromosomal segregation defects caused by MPS1 inhibitor-mediated abrogation of the mitotic delay induced by paclitaxel treatment. In in vivo pharmacodynamic experiments, BOS172722 potently inhibits the spindle assembly checkpoint induced by paclitaxel in human tumor xenograft models of TNBC, as measured by inhibition of the phosphorylation of histone H3 and the phosphorylation of the MPS1 substrate, KNL1. This mechanistic synergy results in significant in vivo efficacy, with robust tumor regressions observed for the combination of BOS172722 and paclitaxel versus either agent alone in long-term efficacy studies in multiple human tumor xenograft TNBC models, including a patient-derived xenograft and a systemic metastasis model. The current target indication for BOS172722 is TNBC, based on their high sensitivity to MPS1 inhibition, the well-defined clinical patient population with high unmet need, and the synergy observed with paclitaxel.

PAK4 Regulates Actin and Microtubule Dynamics during Meiotic Maturation in Mouse Oocyte.

Meiotic maturation of oocyte is an important process for successful fertilization, in which cytoskeletal integrality takes a significant role. The p-21 activated kinases (PAKs) belong to serine/threonine kinases that affect wide range of processes that are crucial for cell motility, survival, cell cycle, and proliferation. In this study, we used a highly selective inhibitor of PAK4, PF-3758309, to investigate the functions of PAK4 during meiotic maturation of mouse oocytes. We found that PAK4 inhibition resulted in meiotic arrest by inducing abnormal microfilament and microtubule dynamics. PAK4 inhibition impaired the microtubule stability and led to the defective kinetochore-microtubule (K-M) attachment which inevitably resulted in aneuploidy. Also, PAK4 inhibition induced abnormal acentriolar centrosome assembly during meiotic maturation. In conclusion, all these combined results suggest that PAK4 is necessary for the oocyte meiosis maturation as a regulator of cytoskeleton.

Inhibition of spindle extension through the yeast S phase checkpoint is coupled to replication fork stability and the integrity of centromeric DNA.

Budding yeast treated with hydroxyurea (HU) activate the S phase checkpoint kinase Rad53, which prevents DNA replication forks from undergoing aberrant structural transitions and nuclease processing. Rad53 is also required to prevent premature extension of the mitotic spindle that assembles during a HU-extended S phase. Here we present evidence that checkpoint restraint of spindle extension is directly coupled to Rad53 control of replication fork stability. In budding yeast, centromeres are flanked by replication origins that fire in early S phase. Mutations affecting the Zn2+-finger of Dbf4, an origin activator, preferentially reduce centromere-proximal origin firing in HU, corresponding with suppression of rad53 spindle extension. Inactivating Exo1 nuclease or displacing centromeres from origins provides a similar suppression. Conversely, short-circuiting Rad53 targeting of Dbf4, Sld3, and Dun1, substrates contributing to fork stability, induces spindle extension. These results reveal spindle extension in HU-treated rad53 mutants is a consequence of replication fork catastrophes at centromeres. When such catastrophes occur, centromeres become susceptible to nucleases, disrupting kinetochore function and spindle force balancing mechanisms. At the same time, our data indicate centromere duplication is not required to stabilize S phase spindle structure, leading us to propose a model for how monopolar kinetochore-spindle attachments may contribute to spindle force balance in HU.

Identification of Bioactive Small Molecule Inhibitors of Separase.

Separase, a cysteine protease of the CD clan, triggers chromosome segregation during mitosis by cleaving the cohesin ring entrapping the two sister chromatids. Deregulated separase activity is associated with aneuploidy, a hallmark of most human cancers. In fact, separase is highly overexpressed in many solid cancers, making it an attractive chemotherapeutic target. To identify small molecules capable of inhibiting separase in its complex cellular environment, we established a highly sensitive assay to quantify separase activity in cells and screened a 51 009-member library for separase inhibitors. In vitro assays confirmed that the identified compounds efficiently inhibited separase, while not affecting caspase-1, another CD-clan protease structurally related to separase. Importantly, HeLa cells with compromised separase activity displayed severe chromosome segregation defects upon compound treatment, confirming that the identified inhibitors are bioactive in tumor tissue culture cells. Structure-activity relationship studies succeeded in the optimization of the most promising inhibitor. Overall, this study demonstrates the feasibility of identifying separase-specific inhibitors, which serve as promising lead compounds for the development of clinically relevant separase inhibiting drugs.

Watching DNA Replication Inhibitors in Action: Exploiting Time-Lapse Microfluidic Microscopy as a Tool for Target-Drug Interaction Studies in Mycobacterium.

Spreading resistance to antibiotics and the emergence of multidrug-resistant strains have become frequent in many bacterial species, including mycobacteria, which are the causative agents of severe diseases and which have profound impacts on global health. Here, we used a system of microfluidics, fluorescence microscopy, and target-tagged fluorescent reporter strains of Mycobacterium smegmatis to perform real-time monitoring of replisome and chromosome dynamics following the addition of replication-altering drugs (novobiocin, nalidixic acid, and griselimycin) at the single-cell level. We found that novobiocin stalled replication forks and caused relaxation of the nucleoid and that nalidixic acid triggered rapid replisome collapse and compaction of the nucleoid, while griselimycin caused replisome instability, with the subsequent overinitiation of chromosome replication and overrelaxation of the nucleoid. In addition to study target-drug interactions, our system also enabled us to observe how the tested antibiotics affected the physiology of mycobacterial cells (i.e., growth, chromosome segregation, etc.).

Mild replication stress causes aneuploidy by deregulating microtubule dynamics in mitosis.

Chromosomal instability (CIN) causes structural and numerical chromosome aberrations and represents a hallmark of cancer. Replication stress (RS) has emerged as a driver for structural chromosome aberrations while mitotic defects can cause whole chromosome missegregation and aneuploidy. Recently, first evidence indicated that RS can also influence chromosome segregation in cancer cells exhibiting CIN, but the underlying mechanisms remain unknown. Here, we show that chromosomally unstable cancer cells suffer from very mild RS, which allows efficient proliferation and which can be mimicked by treatment with very low concentrations of aphidicolin. Both, endogenous RS and aphidicolin-induced very mild RS cause chromosome missegregation during mitosis leading to the induction of aneuploidy. Moreover, RS triggers an increase in microtubule plus end growth rates in mitosis, an abnormality previously identified to cause chromosome missegregation in cancer cells. In fact, RS-induced chromosome missegregation is mediated by increased mitotic microtubule growth rates and is suppressed after restoration of proper microtubule growth rates and upon rescue of replication stress. Hence, very mild and cancer-relevant RS triggers aneuploidy by deregulating microtubule dynamics in mitosis.