The human homologue of Bub3 is required for kinetochore localization of Bub1 and a Mad3/Bub1-related protein kinase

Pan-cancer oncogenic properties and therapeutic potential of SF3B4

Splicing factor 3B (SF3B) subunit 4 (SF3B4), an SF3B complex component essential for spliceosome assembly and accurate splicing, plays a major role in cancer development. However, the precise mechanism through which SF3B4 contributes to tumor growth remains unclear. Here, we demonstrate that SF3B4 is strongly expressed in patients with various cancer types and correlated with their survival. By using hepatocellular carcinoma (HCC) as a model, we reveal that SF3B4's interactions with and regulatory influence on the checkpoint protein BUB1 are essential for appropriate cancer cell mitosis and proliferation. Our results thus demonstrate the roles of SF3B4 as both a cell-cycle regulator and an oncogenic factor in HCC, highlighting its potential as a pan-cancer therapeutic target and diagnostic biomarker.

Centromere Protein F in Tumor Biology: Cancer's Achilles Heel

BACKGROUND

Centromere protein F (CENP-F) is an important nuclear matrix protein that regulates mitosis and the cell cycle, and plays a crucial role in recruiting spindle checkpoint proteins to maintain the accuracy of chromosome segregation. Studies have shown that CENP-F is closely involved in the pathogenesis of various diseases, particularly in the development and progression of malignant tumors, where it exhibits significant oncogenic activity.

OBJECTIVE

This review aims to systematically summarize the molecular structure, subcellular localization, expression regulation, intracellular transport mechanisms, biological functions, and carcinogenic mechanisms of CENP-F, as well as explore its potential value in cancer diagnosis and therapy.

METHODS

A comprehensive review and analysis of domestic and international research literature related to CENP-F were conducted, focusing on its role in tumorigenesis, development, and as a therapeutic target.

RESULTS

CENP-F acts as an oncogene and can maintain or promote the malignant phenotype of tumor cells through multiple mechanisms, including regulating signaling pathways related to cell proliferation and apoptosis, promoting metabolic reprogramming, angiogenesis, and tumor cell invasion and metastasis. Additionally, it plays an important role in the immune microenvironment and drug resistance regulation.

CONCLUSION

CENP-F plays a key, multidimensional role in tumor biology and is a promising therapeutic target for cancer treatment. Further exploration of the core pathways through which CENP-F regulates tumorigenesis and its potential for clinical translation is needed.

Patulin and Xestoquinol are inhibitors of DNA topoisomerase 1

DNA topoisomerase 1 (TOP1) is essential for transcription, replication, and repair. Its function relies on two catalytic steps, DNA breakage and rejoining. Inhibitors of the second step prevent DNA rejoining and lead to persistent DNA breaks, acting as topoisomerase poisons, used as anticancer drugs. However, reliable inhibitors of the first step are not available. Here, we provide genetic and molecular evidence supporting that Patulin and, to a lesser extent, Xestoquinol inhibit the first catalytic step of TOP1 in vitro, in yeast and in human cells. Particularly, Patulin prevents the accumulation of TOP1 cleavage complexes caused by the TOP1 poison camptothecin (CPT) in human cells. Moreover, Patulin pretreatment of human or yeast cells reduces DNA damage and the accumulation of DNA breaks upon CPT exposure. Consistent with the protective role of TOP1 against harmful R-loops, Patulin treatment increases R-loops and R-loop-associated cytotoxicity, mimicking the effect of TOP1 silencing. Altogether our findings indicate that Patulin and Xestoquinol are nonpoisoning inhibitors of TOP1, which should potentiate new research approaches in molecular biology and medicine.

Role of spindle assembly checkpoint proteins in gametogenesis and embryogenesis

The spindle assembly checkpoint (SAC) is a surveillance mechanism that prevents uneven segregation of sister chromatids between daughter cells during anaphase. This essential regulatory checkpoint prevents aneuploidy which can lead to various congenital defects observed in newborns. Many studies have been carried out to elucidate the role of proteins involved in the SAC as well as the function of the checkpoint during gametogenesis and embryogenesis. In this review, we discuss the role of SAC proteins in regulating both meiotic and mitotic cell division along with several factors that influence the SAC strength in various species. Finally, we outline the role of SAC proteins and the consequences of their absence or insufficiency on proper gametogenesis and embryogenesis in vivo.

FRET-FLIM for the Study of Protein-Protein Interactions Underpinning Mitosis Checkpoints

Cell division is a key cellular process that ensures the continuation of life on Earth. In order to protect the genetic integrity of organisms, cell division must happen accurately, ensuring each daughter cell receives a complete copy of the original genome. The accuracy of this process is, in part, preserved by various cell cycle checkpoints. These checkpoints rely on the physical interactions of their components to ensure proper function. The spindle assembly checkpoint (SAC), for example, produces an inhibitory complex of BUBR1-BUB3 and MAD2 bound to CDC20. Many of these cell cycle checkpoint components have been identified in plants, but it has not yet been established whether plants have a mitotic checkpoint architecture that is similar to mammalian cells. To understand the function of plant cell cycle homologues, it is imperative to characterize their interactions in vivo. FRET-FLIM (Förster resonance energy transfer-fluorescence lifetime imaging microscopy), is a rapidly expanding technique that can be used to rapidly and simply characterize protein-protein interactions.

BUB1B monoallelic germline variants contribute to prostate cancer predisposition by triggering chromosomal instability

BACKGROUND

Prostate cancer (PrCa) is the most frequently diagnosed cancer in men. Variants in known moderate- to high-penetrance genes explain less than 5% of the cases arising at early-onset (< 56 years) and/or with familial aggregation of the disease. Considering that BubR1 is an essential component of the mitotic spindle assembly checkpoint, we hypothesized that monoallelic BUB1B variants could be sufficient to fuel chromosomal instability (CIN), potentially triggering (prostate) carcinogenesis.

METHODS

To unveil BUB1B as a new PrCa predisposing gene, we performed targeted next-generation sequencing in germline DNA from 462 early-onset/familial PrCa patients and 1,416 cancer patients fulfilling criteria for genetic testing for other hereditary cancer syndromes. To explore the pan-cancer role of BUB1B, we used in silico BubR1 molecular modeling, in vitro gene-editing, and ex vivo patients' tumors and peripheral blood lymphocytes.

RESULTS

Rare BUB1B variants were found in ~ 1.9% of the early-onset/familial PrCa cases and in ~ 0.6% of other cancer patients fulfilling criteria for hereditary disease. We further show that BUB1B variants lead to decreased BubR1 expression and/or stability, which promotes increased premature chromatid separation and, consequently, triggers CIN, driving resistance to Taxol-based therapies.

CONCLUSIONS

Our study shows that different BUB1B variants may uncover a trigger for CIN-driven carcinogenesis, supporting the role of BUB1B as a (pan)-cancer predisposing gene with potential impact on genetic counseling and treatment decision-making.

Phospho-KNL-1 recognition by a TPR domain targets the BUB-1-BUB-3 complex to C. elegans kinetochores

During mitosis, the Bub1-Bub3 complex concentrates at kinetochores, the microtubule-coupling interfaces on chromosomes, where it contributes to spindle checkpoint activation, kinetochore-spindle microtubule interactions, and protection of centromeric cohesion. Bub1 has a conserved N-terminal tetratricopeptide repeat (TPR) domain followed by a binding motif for its conserved interactor Bub3. The current model for Bub1-Bub3 localization to kinetochores is that Bub3, along with its bound motif from Bub1, recognizes phosphorylated "MELT" motifs in the kinetochore scaffold protein Knl1. Motivated by the greater phenotypic severity of BUB-1 versus BUB-3 loss in C. elegans, we show that the BUB-1 TPR domain directly recognizes a distinct class of phosphorylated motifs in KNL-1 and that this interaction is essential for BUB-1-BUB-3 localization and function. BUB-3 recognition of phospho-MELT motifs additively contributes to drive super-stoichiometric accumulation of BUB-1-BUB-3 on its KNL-1 scaffold during mitotic entry. Bub1's TPR domain interacts with Knl1 in other species, suggesting that collaboration of TPR-dependent and Bub3-dependent interfaces in Bub1-Bub3 localization and functions may be conserved.

Kinetoplastid kinetochore proteins KKT14-KKT15 are divergent Bub1/BubR1-Bub3 proteins

Faithful transmission of genetic material is crucial for the survival of all organisms. In many eukaryotes, a feedback control mechanism called the spindle checkpoint ensures chromosome segregation fidelity by delaying cell cycle progression until all chromosomes achieve proper attachment to the mitotic spindle. Kinetochores are the macromolecular complexes that act as the interface between chromosomes and spindle microtubules. While most eukaryotes have canonical kinetochore proteins that are widely conserved, kinetoplastids such as Trypanosoma brucei have a seemingly unique set of kinetochore proteins including KKT1-25. It remains poorly understood how kinetoplastids regulate cell cycle progression or ensure chromosome segregation fidelity. Here, we report a crystal structure of the C-terminal domain of KKT14 from Apiculatamorpha spiralis and uncover that it is a pseudokinase. Its structure is most similar to the kinase domain of a spindle checkpoint protein Bub1. In addition, KKT14 has a putative ABBA motif that is present in Bub1 and its paralogue BubR1. We also find that the N-terminal part of KKT14 interacts with KKT15, whose WD40 repeat beta-propeller is phylogenetically closely related to a direct interactor of Bub1/BubR1 called Bub3. Our findings indicate that KKT14-KKT15 are divergent orthologues of Bub1/BubR1-Bub3, which promote accurate chromosome segregation in trypanosomes.

The two sides of chromosomal instability: drivers and brakes in cancer

Chromosomal instability (CIN) is a hallmark of cancer and is associated with tumor cell malignancy. CIN triggers a chain reaction in cells leading to chromosomal abnormalities, including deviations from the normal chromosome number or structural changes in chromosomes. CIN arises from errors in DNA replication and chromosome segregation during cell division, leading to the formation of cells with abnormal number and/or structure of chromosomes. Errors in DNA replication result from abnormal replication licensing as well as replication stress, such as double-strand breaks and stalled replication forks; meanwhile, errors in chromosome segregation stem from defects in chromosome segregation machinery, including centrosome amplification, erroneous microtubule-kinetochore attachments, spindle assembly checkpoint, or defective sister chromatids cohesion. In normal cells, CIN is deleterious and is associated with DNA damage, proteotoxic stress, metabolic alteration, cell cycle arrest, and senescence. Paradoxically, despite these negative consequences, CIN is one of the hallmarks of cancer found in over 90% of solid tumors and in blood cancers. Furthermore, CIN could endow tumors with enhanced adaptation capabilities due to increased intratumor heterogeneity, thereby facilitating adaptive resistance to therapies; however, excessive CIN could induce tumor cells death, leading to the "just-right" model for CIN in tumors. Elucidating the complex nature of CIN is crucial for understanding the dynamics of tumorigenesis and for developing effective anti-tumor treatments. This review provides an overview of causes and consequences of CIN, as well as the paradox of CIN, a phenomenon that continues to perplex researchers. Finally, this review explores the potential of CIN-based anti-tumor therapy.

The Arabidopsis BUB1/MAD3 family protein BMF3 requires BUB3.3 to recruit CDC20 to kinetochores in spindle assembly checkpoint signaling

The spindle assembly checkpoint (SAC) ensures faithful chromosome segregation during cell division by monitoring kinetochore-microtubule attachment. Plants produce both sequence-conserved and diverged SAC components, and it has been largely unknown how SAC activation leads to the assembly of these proteins at unattached kinetochores to prevent cells from entering anaphase. In Arabidopsis thaliana, the noncanonical BUB3.3 protein was detected at kinetochores throughout mitosis, unlike MAD1 and the plant-specific BUB1/MAD3 family protein BMF3 that associated with unattached chromosomes only. When BUB3.3 was lost by a genetic mutation, mitotic cells often entered anaphase with misaligned chromosomes and presented lagging chromosomes after they were challenged by low doses of the microtubule depolymerizing agent oryzalin, resulting in the formation of micronuclei. Surprisingly, BUB3.3 was not required for the kinetochore localization of other SAC proteins or vice versa. Instead, BUB3.3 specifically bound to BMF3 through two internal repeat motifs that were not required for BMF3 kinetochore localization. This interaction enabled BMF3 to recruit CDC20, a downstream SAC target, to unattached kinetochores. Taken together, our findings demonstrate that plant SAC utilizes unconventional protein interactions for arresting mitosis, with BUB3.3 directing BMF3's role in CDC20 recruitment, rather than the recruitment of BUB1/MAD3 proteins observed in fungi and animals. This distinct mechanism highlights how plants adapted divergent versions of conserved cell cycle machinery to achieve specialized SAC control.

Multimerization of a disordered kinetochore protein promotes accurate chromosome segregation by localizing a core dynein module

Kinetochores connect chromosomes and spindle microtubules to maintain genomic integrity through cell division. Crosstalk between the minus-end directed motor dynein and kinetochore-microtubule attachment factors promotes accurate chromosome segregation by a poorly understood pathway. Here, we identify a linkage between the intrinsically disordered protein Spc105 (KNL1 orthologue) and dynein using an optogenetic oligomerization assay. Core pools of the checkpoint protein BubR1 and the adaptor complex RZZ contribute to the linkage. Furthermore, a minimal segment of Spc105 with a propensity to multimerize and which contains protein binding motifs is sufficient to link Spc105 to RZZ/dynein. Deletion of the minimal region from Spc105 compromises the recruitment of its binding partners to kinetochores and elevates chromosome missegregation due to merotelic attachments. Restoration of normal chromosome segregation and localization of BubR1 and RZZ requires both protein binding motifs and oligomerization of Spc105. Together, our results reveal that higher-order multimerization of Spc105 contributes to localizing a core pool of RZZ that promotes accurate chromosome segregation.

Phospho-KNL-1 recognition by a TPR domain targets the BUB-1-BUB-3 complex to C. elegans kinetochores

During mitosis, the Bub1-Bub3 complex concentrates at kinetochores, the microtubule-coupling interfaces on chromosomes, where it contributes to spindle checkpoint activation, kinetochore-spindle microtubule interactions, and protection of centromeric cohesion. Bub1 has a conserved N-terminal tetratricopeptide (TPR) domain followed by a binding motif for its conserved interactor Bub3. The current model for Bub1-Bub3 localization to kinetochores is that Bub3, along with its bound motif from Bub1, recognizes phosphorylated "MELT" motifs in the kinetochore scaffold protein Knl1. Motivated by the greater phenotypic severity of BUB-1 versus BUB-3 loss in C. elegans, we show that the BUB-1 TPR domain directly recognizes a distinct class of phosphorylated motifs in KNL-1 and that this interaction is essential for BUB-1-BUB-3 localization and function. BUB-3 recognition of phospho-MELT motifs additively contributes to drive super-stoichiometric accumulation of BUB-1-BUB-3 on its KNL-1 scaffold during mitotic entry. Bub1's TPR domain interacts with Knl1 in other species, suggesting that collaboration of TPR-dependent and Bub3-dependent interfaces in Bub1-Bub3 localization and functions may be conserved.

A bifunctional kinase-phosphatase module balances mitotic checkpoint strength and kinetochore-microtubule attachment stability

Two major mechanisms safeguard genome stability during mitosis: the mitotic checkpoint delays mitosis until all chromosomes have attached to microtubules, and the kinetochore-microtubule error-correction pathway keeps this attachment process free from errors. We demonstrate here that the optimal strength and dynamics of these processes are set by a kinase-phosphatase pair (PLK1-PP2A) that engage in negative feedback from adjacent phospho-binding motifs on the BUB complex. Uncoupling this feedback to skew the balance towards PLK1 produces a strong checkpoint, hypostable microtubule attachments and mitotic delays. Conversely, skewing the balance towards PP2A causes a weak checkpoint, hyperstable microtubule attachments and chromosome segregation errors. These phenotypes are associated with altered BUB complex recruitment to KNL1-MELT motifs, implicating PLK1-PP2A in controlling auto-amplification of MELT phosphorylation. In support, KNL1-BUB disassembly becomes contingent on PLK1 inhibition when KNL1 is engineered to contain excess MELT motifs. This elevates BUB-PLK1/PP2A complex levels on metaphase kinetochores, stabilises kinetochore-microtubule attachments, induces chromosome segregation defects and prevents KNL1-BUB disassembly at anaphase. Together, these data demonstrate how a bifunctional PLK1/PP2A module has evolved together with the MELT motifs to optimise BUB complex dynamics and ensure accurate chromosome segregation.

Analysis of Bub3 and Nup75 in the Drosophila male germline lineage

Extensive communication at the stem cell-niche interface and asymmetric stem cell division is key for the homeostasis of the Drosophila male germline stem cell system. To improve our understanding of these processes, we analysed the function of the mitotic checkpoint complex (MCC) component Bub3 and the nucleoporin Nup75, a component of the nuclear pore complex realizing the transport of signalling effector molecules to the nucleus, in the Drosophila testis. By lineage-specific interference, we found that the two genes control germline development and maintenance. Bub3 is continuously required in the germline, as its loss results in the beginning in an over-proliferation of early germ cells and later on in loss of the germline. The absence of the germline lineage in such testes has dramatic cell non-autonomous consequences, as cells co-expressing markers of hub and somatic cyst cell fates accumulate and populate in extreme cases the whole testis. Our analysis of Nups showed that some of them are critical for lineage maintenance, as their depletion results in the loss of the affected lineage. In contrast, Nup75 plays a role in controlling proliferation of early germ cells but not differentiating spermatogonia and seems to be involved in keeping hub cells quiescent. In sum, our analysis shows that Bub3 and Nup75 are required for male germline development and maintenance.

An intrinsically disordered kinetochore protein coordinates mechanical regulation of chromosome segregation by dynein

Kinetochores connect chromosomes and spindle microtubules to maintain genomic integrity through cell division. Crosstalk between the minus-end directed motor dynein and kinetochore-microtubule attachment factors promotes accurate chromosome segregation through a poorly understood pathway. Here we identify a physical linkage between the intrinsically disordered protein Spc105 (KNL1 orthologue) and dynein using an optogenetic oligomerization assay. Core pools of the checkpoint protein BubR1 and the adaptor complex RZZ mediate the connection of Spc105 to dynein. Furthermore, a minimal segment of Spc105 that contains regions with a propensity to multimerize and binding motifs for Bub1 and BubR1 is sufficient to functionally link Spc105 to RZZ and dynein. Deletion of the minimal region from Spc105 reduces recruitment of its binding partners to bioriented kinetochores and causes chromosome mis-segregation. Restoration of normal chromosome segregation and localization of BubR1 and RZZ requires both protein binding motifs and higher-order oligomerization of Spc105. Together, our results reveal that higher-order multimerization of Spc105 is required to recruit a core pool of RZZ that modulates microtubule attachment stability to promote accurate chromosome segregation.

Hyper-active RAS/MAPK introduces cancer-specific mitotic vulnerabilities

Aneuploidy, the incorrect number of whole chromosomes, is a common feature of tumors that contributes to their initiation and evolution. Preventing aneuploidy requires properly functioning kinetochores, which are large protein complexes assembled on centromeric DNA that link mitotic chromosomes to dynamic spindle microtubules and facilitate chromosome segregation. The kinetochore leverages at least two mechanisms to prevent aneuploidy: error correction and the spindle assembly checkpoint (SAC). BubR1, a factor involved in both processes, was identified as a cancer dependency and therapeutic target in multiple tumor types; however, it remains unclear what specific oncogenic pressures drive this enhanced dependency on BubR1 and whether it arises from BubR1's regulation of the SAC or error-correction pathways. Here, we use a genetically controlled transformation model and glioblastoma tumor isolates to show that constitutive signaling by RAS or MAPK is necessary for cancer-specific BubR1 vulnerability. The MAPK pathway enzymatically hyperstimulates a network of kinetochore kinases that compromises chromosome segregation, rendering cells more dependent on two BubR1 activities: counteracting excessive kinetochore-microtubule turnover for error correction and maintaining the SAC. This work expands our understanding of how chromosome segregation adapts to different cellular states and reveals an oncogenic trigger of a cancer-specific defect.

BubR1 recruitment to the kinetochore via Bub1 enhances spindle assembly checkpoint signaling

During mitosis, unattached kinetochores in a dividing cell activate the spindle assembly checkpoint (SAC) and delay anaphase onset by generating the anaphase-inhibitory mitotic checkpoint complex (MCC). These kinetochores generate the MCC by recruiting its constituent proteins, including BubR1. In principle, BubR1 recruitment to signaling kinetochores should increase its local concentration and promote MCC formation. However, in human cells BubR1 is mainly thought to sensitize the SAC to silencing. Whether BubR1 localization to signaling kinetochores by itself enhances SAC signaling remains unknown. Therefore, we used ectopic SAC activation (eSAC) systems to isolate two molecules that recruit BubR1 to the kinetochore, the checkpoint protein Bub1 and the KI and MELT motifs in the kinetochore protein KNL1, and observed their contribution to eSAC signaling. Our quantitative analyses and mathematical modeling show that Bub1-mediated BubR1 recruitment to the human kinetochore promotes SAC signaling and highlight BubR1's dual role of strengthening the SAC directly and silencing it indirectly.

BUB3, beyond the Simple Role of Partner

The BUB3 protein plays a key role in the activation of the spindle assembly checkpoint (SAC), a ubiquitous surveillance mechanism that ensures the fidelity of chromosome segregation in mitosis and, consequently, prevents chromosome mis-segregation and aneuploidy. Besides its role in SAC signaling, BUB3 regulates chromosome attachment to the spindle microtubules. It is also involved in telomere replication and maintenance. Deficiency of the BUB3 gene has been closely linked to premature aging. Upregulation of the BUB3 gene has been found in a variety of human cancers and is associated with poor prognoses. Here, we review the structure and functions of BUB3 in mitosis, its expression in cancer and association with survival prognoses, and its potential as an anticancer target.

Zombies Never Die: The Double Life Bub1 Lives in Mitosis

When eukaryotic cells enter mitosis, dispersed chromosomes move to the cell center along microtubules to form a metaphase plate which facilitates the accurate chromosome segregation. Meanwhile, kinetochores not stably attached by microtubules activate the spindle assembly checkpoint and generate a wait signal to delay the initiation of anaphase. These events are highly coordinated. Disruption of the coordination will cause severe problems like chromosome gain or loss. Bub1, a conserved serine/threonine kinase, plays important roles in mitosis. After extensive studies in the last three decades, the role of Bub1 on checkpoint has achieved a comprehensive understanding; its role on chromosome alignment also starts to emerge. In this review, we summarize the latest development of Bub1 on supporting the two mitotic events. The essentiality of Bub1 in higher eukaryotic cells is also discussed. At the end, some undissolved questions are raised for future study.

Functional dissection of human mitotic genes using CRISPR-Cas9 tiling screens

In this Resource/Methodology, Herman et al. developed a method that leverages CRISPR–Cas9-induced mutations across protein-coding genes for the a priori identification of functional regions at the sequence level. As a test case, they applied this method to 48 human mitotic genes, revealing hundreds of regions required for cell proliferation, including domains that were experimentally characterized, ones that were predicted based on homology, and novel ones.

The identity of human protein-coding genes is well known, yet our in-depth knowledge of their molecular functions and domain architecture remains limited by shortcomings in homology-based predictions and experimental approaches focused on whole-gene depletion. To bridge this knowledge gap, we developed a method that leverages CRISPR–Cas9-induced mutations across protein-coding genes for the a priori identification of functional regions at the sequence level. As a test case, we applied this method to 48 human mitotic genes, revealing hundreds of regions required for cell proliferation, including domains that were experimentally characterized, ones that were predicted based on homology, and novel ones. We validated screen outcomes for 15 regions, including amino acids 387–402 of Mad1, which were previously uncharacterized but contribute to Mad1 kinetochore localization and chromosome segregation fidelity. Altogether, we demonstrate that CRISPR–Cas9-based tiling mutagenesis identifies key functional domains in protein-coding genes de novo, which elucidates separation of function mutants and allows functional annotation across the human proteome.

Role of ubiquitin-protein ligase UBR5 in the disassembly of mitotic checkpoint complexes

The mitotic checkpoint system is essential for the prevention of mistakes in the segregation of chromosomes in mitosis. As long as chromosomes are not attached correctly to the mitotic spindle, a mitotic checkpoint complex (MCC) is assembled and inhibits the action of ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) to initiate anaphase. When the checkpoint is turned off, MCC is disassembled, allowing anaphase initiation. The mechanisms of MCC disassembly have been studied, but the regulation of this process remained obscure. We found that a second ubiquitin ligase, UBR5 (ubiquitin-protein ligase N-recognin 5), ubiquitylates MCC components and stimulates the disassembly of MCC from APC/C, as well as the dissociation of a subcomplex of MCC.

The mitotic (or spindle assembly) checkpoint system ensures accurate chromosome segregation in mitosis by preventing the onset of anaphase until correct bipolar attachment of sister chromosomes to the mitotic spindle is attained. It acts by promoting the assembly of a mitotic checkpoint complex (MCC), composed of mitotic checkpoint proteins BubR1, Bub3, Mad2, and Cdc20. MCC binds to and inhibits the action of ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome), which targets for degradation regulators of anaphase initiation. When the checkpoint system is satisfied, MCCs are disassembled, allowing the recovery of APC/C activity and initiation of anaphase. Many of the pathways of the disassembly of the different MCCs have been elucidated, but the mode of their regulation remained unknown. We find that UBR5 (ubiquitin-protein ligase N-recognin 5) is associated with the APC/C*MCC complex immunopurified from extracts of nocodazole-arrested HeLa cells. UBR5 binds to mitotic checkpoint proteins BubR1, Bub3, and Cdc20 and promotes their polyubiquitylation in vitro. The dissociation of a Bub3*BubR1 subcomplex of MCC is stimulated by UBR5-dependent ubiquitylation, as suggested by observations that this process in mitotic extracts requires UBR5 and α−β bond hydrolysis of adenosine triphosphate. Furthermore, a system reconstituted from purified recombinant components carries out UBR5- and ubiquitylation-dependent dissociation of Bub3*BubR1. Immunodepletion of UBR5 from mitotic extracts slows down the release of MCC components from APC/C and prolongs the lag period in the recovery of APC/C activity in the exit from mitotic checkpoint arrest. We suggest that UBR5 may be involved in the regulation of the inactivation of the mitotic checkpoint.

Inhibitors of the Bub1 spindle assembly checkpoint kinase: synthesis of BAY-320 and comparison with 2OH-BNPP1

Bub1 is a serine/threonine kinase proposed to function centrally in mitotic chromosome alignment and the spindle assembly checkpoint (SAC); however, its role remains controversial. Although it is well documented that Bub1 phosphorylation of Histone 2A at T120 (H2ApT120) recruits Sgo1/2 to kinetochores, the requirement of its kinase activity for chromosome alignment and the SAC is debated. As small-molecule inhibitors are invaluable tools for investigating kinase function, we evaluated two potential Bub1 inhibitors: 2OH-BNPPI and BAY-320. After confirming that both inhibit Bub1 in vitro, we developed a cell-based assay for Bub1 inhibition. We overexpressed a fusion of Histone 2B and Bub1 kinase region, tethering it in proximity to H2A to generate a strong ectopic H2ApT120 signal along chromosome arms. Ectopic signal was effectively inhibited by BAY-320, but not 2OH-BNPP1 at concentrations tested. In addition, only BAY-320 was able to inhibit endogenous Bub1-mediated Sgo1 localization. Preliminary experiments using BAY-320 suggest a minor role for Bub1 kinase activity in chromosome alignment and the SAC; however, BAY-320 may exhibit off-target effects at the concentration required. Thus, 2OH-BNPP1 may not be an effective Bub1 inhibitor in cellulo, and while BAY-320 can inhibit Bub1 in cells, off-target effects highlight the need for improved Bub1 inhibitors.

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.

Spindle assembly checkpoint activation and silencing at kinetochores

The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.

Mapping Proximity Associations of Core Spindle Assembly Checkpoint Proteins

The spindle assembly checkpoint (SAC) is critical for sensing defective microtubule-kinetochore attachments and tension across the kinetochore and functions to arrest cells in prometaphase to allow time to repair any errors before proceeding into anaphase. Dysregulation of the SAC leads to chromosome segregation errors that have been linked to human diseases like cancer. Although much has been learned about the composition of the SAC and the factors that regulate its activity, the proximity associations of core SAC components have not been explored in a systematic manner. Here, we have taken a BioID2-proximity-labeling proteomic approach to define the proximity protein environment for each of the five core SAC proteins BUB1, BUB3, BUBR1, MAD1L1, and MAD2L1 in mitotic-enriched populations of cells where the SAC is active. These five protein association maps were integrated to generate a SAC proximity protein network that contains multiple layers of information related to core SAC protein complexes, protein-protein interactions, and proximity associations. Our analysis validated many known SAC complexes and protein-protein interactions. Additionally, it uncovered new protein associations, including the ELYS-MAD1L1 interaction that we have validated, which lend insight into the functioning of core SAC proteins and highlight future areas of investigation to better understand the SAC.

BUB1B Promotes Proliferation of Prostate Cancer via Transcriptional Regulation of MELK

BACKGROUND

Prostate cancer remains one of the most common and deadliest forms of cancer, generally respond well to radical prostatectomy and associated interventions, up to 30% of individuals will suffer disease relapse. Although BUB1B was found to be essential for cell growth and proliferation, even in several kinds of tumor cells, the specific importance and mechanistic role of BUB1B in prostate cancer remain unclear.

METHODS

Quantitative Real-Time PCR and Western-blot were used in the detection of mRNA and protein expression. Lentivirus infection was used to overexpression or knock down the target gene. Flow cytometry analysis was performed to test protein expression and apoptosis level. Immunohistochemistry was used to identify protein expression in tissue. Statistical differences between the two groups are evaluated by two-tailed t-tests. The comparison among multiple groups is performed by one-way Analysis of Variance (ANOVA) followed by Dunnett's posttest. The statistical significance of the Kaplan-Meier survival plot is determined by log-rank analysis.

RESULTS

In the present report, we found BUB1B expression to be highly increased in prostate cancer tissues relative to normal controls. We further found BUB1B to be essential for efficient tumor cell proliferation, and to correlate with poorer prostate cancer patient outcomes. From a mechanistic perspective, the ability of BUB1B to regulate MELK was found to be essential for its ability to promote prostate cancer cell proliferation.

CONCLUSION

Altogether, our data suggest that BUB1B is up-regulated in prostate cancer, suggesting that the growth of cancer cells may depend on BUB1B-dependent regulation of MELK transcription. BUB1B may serve as a clinical prognostic factor and a druggable target for prostate cancer.

Yeast Fin1-PP1 dephosphorylates an Ipl1 substrate, Ndc80, to remove Bub1-Bub3 checkpoint proteins from the kinetochore during anaphase

The spindle assembly checkpoint (SAC) prevents anaphase onset in response to chromosome attachment defects, and SAC silencing is essential for anaphase onset. Following anaphase onset, activated Cdc14 phosphatase dephosphorylates the substrates of cyclin-dependent kinase to facilitate anaphase progression and mitotic exit. In budding yeast, Cdc14 dephosphorylates Fin1, a regulatory subunit of protein phosphatase 1 (PP1), to enable kinetochore localization of Fin1-PP1. We previously showed that kinetochore-localized Fin1-PP1 promotes the removal of the SAC protein Bub1 from the kinetochore during anaphase. We report here that Fin1-PP1 also promotes kinetochore removal of Bub3, the Bub1 partner, but has no effect on another SAC protein Mad1. Moreover, the kinetochore localization of Bub1-Bub3 during anaphase requires Aurora B/Ipl1 kinase activity. We further showed that Fin1-PP1 facilitates the dephosphorylation of kinetochore protein Ndc80, a known Ipl1 substrate. This dephosphorylation reduces kinetochore association of Bub1-Bub3 during anaphase. In addition, we found that untimely Ndc80 dephosphorylation causes viability loss in response to tensionless chromosome attachments. These results suggest that timely localization of Fin1-PP1 to the kinetochore controls the functional window of SAC and is therefore critical for faithful chromosome segregation.

Physiological relevance of post-translational regulation of the spindle assembly checkpoint protein BubR1

BubR1 is an essential component of the spindle assembly checkpoint (SAC) during mitosis where it functions to prevent anaphase onset to ensure proper chromosome alignment and kinetochore-microtubule attachment. Loss or mutation of BubR1 results in aneuploidy that precedes various potential pathologies, including cancer and mosaic variegated aneuploidy (MVA). BubR1 is also progressively downregulated with age and has been shown to be directly involved in the aging process through suppression of cellular senescence. Post-translational modifications, including but not limited to phosphorylation, acetylation, and ubiquitination, play a critical role in the temporal and spatial regulation of BubR1 function. In this review, we discuss the currently characterized post-translational modifications to BubR1, the enzymes involved, and the biological consequences to BubR1 functionality and implications in diseases associated with BubR1. Understanding the molecular mechanisms promoting these modifications and their roles in regulating BubR1 is important for our current understanding and future studies of BubR1 in maintaining genomic integrity as well as in aging and cancer.

Bub1 kinase in the regulation of mitosis

Accurate chromosome segregation is required for cell survival and organismal development. During mitosis, the spindle assembly checkpoint acts as a safeguard to maintain the high fidelity of mitotic chromosome segregation by monitoring the attachment of kinetochores to the mitotic spindle. Bub1 is a conserved kinase critical for the spindle assembly checkpoint. Bub1 also facilitates chromosome alignment and contributes to the regulation of mitotic duration. Here, focusing on the spindle assembly checkpoint and on chromosome alignment, we summarize the primary literature on Bub1, discussing its structure and functional domains, as well its regulation and roles in mitosis. In addition, we discuss recent evidence for roles of Bub1 beyond mitosis regulation in TGFβ signaling and telomere replication. Finally, we discuss the involvement of Bub1 in human diseases, especially in cancer, and the potential of using Bub1 as a drug target for therapeutic applications.

Genes Regulating Spermatogenesis and Sperm Function Associated With Rare Disorders

Spermatogenesis is a cell differentiation process that ensures the production of fertilizing sperm, which ultimately fuse with an egg to form a zygote. Normal spermatogenesis relies on Sertoli cells, which preserve cell junctions while providing nutrients for mitosis and meiosis of male germ cells. Several genes regulate normal spermatogenesis, some of which are not exclusively expressed in the testis and control multiple physiological processes in an organism. Loss-of-function mutations in some of these genes result in spermatogenesis and sperm functionality defects, potentially leading to the insurgence of rare genetic disorders. To identify genetic intersections between spermatogenesis and rare diseases, we screened public archives of human genetic conditions available on the Genetic and Rare Diseases Information Center (GARD), the Online Mendelian Inheritance in Man (OMIM), and the Clinical Variant (ClinVar), and after an extensive literature search, we identified 22 distinct genes associated with 21 rare genetic conditions and defective spermatogenesis or sperm function. These protein-coding genes regulate Sertoli cell development and function during spermatogenesis, checkpoint signaling pathways at meiosis, cellular organization and shape definition during spermiogenesis, sperm motility, and capacitation at fertilization. A number of these genes regulate folliculogenesis and oogenesis as well. For each gene, we review the genotype–phenotype association together with associative or causative polymorphisms in humans, and provide a description of the shared molecular mechanisms that regulate gametogenesis and fertilization obtained in transgenic animal models.

Fabrication of polylactic acid/paclitaxel nano fibers by electrospinning for cancer therapeutics

Polylactic acid (PLA) is a thermoplastic and biodegradable polyester, largely derived from renewable resources such as corn starch, cassava starch and sugarcane. However, PLA is only soluble in a narrow range of solvents such as tetrahydrofuran, dioxane, chlorinated solvents and heated benzene. The limited choices of solvent for PLA dissolution have imposed significant challenges in the development of specifically engineered PLA nanofibers with electrospinning techniques. Generally, the electrospun polymeric materials have been rendered with unique properties such as high porosity and complex geometry while maintaining its biodegradability and biocompatibility for emerging biomedical applications. In this study, a new anticancer drug delivery system composed of PLA nanofibers with encapsulated paclitaxel was developed by the electrospinning of the respective nanofibers on top of a spin-coated thin film with the same chemical compositions. Our unique approach is meant for promoting strong bonding between PLA-based nanofibers and their respective films in order to improve the prolonged release properties and composite film stability within a fluctuative physiochemical environment during cell culture. PLA/paclitaxel nanofiber supported on respective polymeric films were probed by scanning electronic microscope, Fourier transform infrared spectrometer and water contact measurement for determining their surface morphologies, fibers’ diameters, molecular vibrational modes, and wettability, respectively. Moreover, PLA/paclitaxel nanofibers supported on respective spin-coated films at different loadings of paclitaxel were evaluated for their abilities in killing human colorectal carcinoma cells (HCT-116). More importantly, MTT assays showed that regardless of the concentrations of paclitaxel, the growth of HCT-116 was effectively inhibited by the prolonged release of paclitaxel from PLA/paclitaxel nanofibers. An effective prolonged delivery system of paclitaxel based on PLA nanofiber-based film has demonstrated exciting potentials for emerging applications as implantable drug delivery patch in post-surgical cancer eradication.

BuGZ facilitates loading of spindle assembly checkpoint proteins to kinetochores in early mitosis

BuGZ is a kinetochore component that binds to and stabilizes Bub3, a key player in mitotic spindle assembly checkpoint signaling. Bub3 is required for kinetochore recruitment of Bub1 and BubR1, two proteins that have essential and distinct roles in the checkpoint. Both Bub1 and BubR1 localize to kinetochores through interactions with Bub3, which are mediated through conserved GLEBS domains in both Bub1 and BubR1. BuGZ also has a GLEBS domain, which is required for its kinetochore localization as well, presumably mediated through Bub3 binding. Although much is understood about the requirements for Bub1 and BubR1 interaction with Bub3 and kinetochores, much less is known regarding BuGZ's requirements. Here, we used a series of mutants to demonstrate that BuGZ kinetochore localization requires only its core GLEBS domain, which is distinct from the requirements for both Bub1 and BubR1. Furthermore, we found that the kinetics of Bub1, BubR1, and BuGZ loading to kinetochores differ, with BuGZ localizing prior to BubR1 and Bub1. To better understand how complexes containing Bub3 and its binding partners are loaded to kinetochores, we carried out size-exclusion chromatography and analyzed Bub3-containing complexes from cells under different spindle assembly checkpoint signaling conditions. We found that prior to kinetochore formation, Bub3 is complexed with BuGZ but not Bub1 or BubR1. Our results point to a model in which BuGZ stabilizes Bub3 and promotes Bub3 loading onto kinetochores in early mitosis, which, in turn, facilitates Bub1 and BubR1 kinetochore recruitment and spindle assembly checkpoint signaling.

Untangling the contribution of Haspin and Bub1 to Aurora B function during mitosis

Aurora B kinase is essential for faithful chromosome segregation during mitosis. During (pro)metaphase, Aurora B is concentrated at the inner centromere by the kinases Haspin and Bub1. However, how Haspin and Bub1 collaborate to control Aurora B activity at centromeres remains unclear. Here, we show that either Haspin or Bub1 activity is sufficient to recruit Aurora B to a distinct chromosomal locus. Moreover, we identified a small, Bub1 kinase–dependent Aurora B pool that supported faithful chromosome segregation in otherwise unchallenged cells. Joined inhibition of Haspin and Bub1 activities fully abolished Aurora B accumulation at centromeres. While this impaired the correction of erroneous KT–MT attachments, it did not compromise the mitotic checkpoint, nor the phosphorylation of the Aurora B kinetochore substrates Hec1, Dsn1, and Knl1. This suggests that Aurora B substrates at the kinetochore are not phosphorylated by centromere-localized pools of Aurora B, and calls for a reevaluation of the current spatial models for how tension affects Aurora B–dependent kinetochore phosphorylation.

BUB1B promotes hepatocellular carcinoma progression via activation of the mTORC1 signaling pathway

Background and Aims

Accumulating studies identified that BUB1 mitotic checkpoint serine/threonine kinase B (BUB1B) is integrally involved in the initiation and development of tumors. Nevertheless, the precise biological role and underlying mechanisms of BUB1B in hepatocellular carcinoma (HCC) remain indistinct.

Method

To figure out the role of BUB1B in HCC, we first assessed its expression using The Cancer Genome Atlas (TCGA) and Gene Expression Profiling Interactive Analysis (GEPIA) databases. We then verified BUB1B expression in HCC tissues, nontumor tissues, and HCC cell lines through western blotting, quantitative reverse transcription‐polymerase chain reaction, and immunohistochemistry. To explore the specific function of BUB1B in HCC in vivo and in vitro, we performed the flow cytometry, Cell Counting Kit‐8, 5‐ethynyl‐2′‐deoxyuridine incorporation, colony formation, Transwell, wound‐healing, subcutaneous tumor growth, and metastasis assays. Additionally, we identified the BUB1B‐regulated pathways involved in HCC by using gene set enrichment analysis.

Results

Our data displayed that higher BUB1B expression was detected in HCC tissues and HCC cell lines. The overexpression of BUB1B was positively correlated with adverse clinicopathological characteristics. Survival analyses showed that lower recurrence‐free and overall survival rates were correlated with the overexpression of BUB1B in patients with HCC. Moreover, the malignancy of HCC was facilitated by BUB1B both in vivo and in vitro. Lastly, the results were confirmed by western blots, which showed that BUB1B upregulated mTORC1 signaling pathway in HCC. Meanwhile, the oncogenic effect of BUB1B will be impaired when the mTORC1 signaling pathway was inhibited by rapamycin.

Conclusion

We highlighted that BUB1B played an oncogenic role in HCC and was identified as a possible clinical prognostic factor and a potential novel therapeutic target for HCC.

A Biosensor for the Mitotic Kinase MPS1 Reveals Spatiotemporal Activity Dynamics and Regulation

Accurate chromosome segregation during cell division critically depends on error correction of chromosome-spindle interactions and the spindle assembly checkpoint (SAC) [1-3]. The kinase MPS1 is an essential regulator of both processes, ensuring full chromosome biorientation before anaphase onset [3, 4]. To understand when and where MPS1 activation occurs and how MPS1 signaling is modulated during mitosis, we developed MPS1sen, a sensitive and specific FRET-based biosensor for MPS1 activity. By placing MPS1sen at different subcellular locations, we show that MPS1 activity initiates in the nucleus ∼9-12 min prior to nuclear envelope breakdown (NEB) in a kinetochore-dependent manner and reaches the cytoplasm at the start of NEB. Soon after initiation, MPS1 activity increases with switch-like kinetics, peaking at completion of NEB. We further show that timing and extent of pre-NEB MPS1 activity is regulated by Aurora B and PP2A-B56. MPS1sen phosphorylation declines in prometaphase as a result of formation of kinetochore-microtubule attachments, reaching low but still detectable levels at metaphase. Finally, leveraging the sensitivity and dynamic range of MPS1sen, we show deregulated MPS1 signaling dynamics in colorectal cancer cell lines and tumor organoids with diverse genomic instability phenotypes.

The Kinase Activity of Drosophila BubR1 Is Required for Insulin Signaling-Dependent Stem Cell Maintenance

As a core component of the mitotic checkpoint complex, BubR1 has a modular organization of molecular functions, with KEN box and other motifs at the N terminus inhibiting the anaphase-promoting complex/cyclosome, and a kinase domain at the C terminus, whose function remains unsettled, especially at organismal levels. We generate knock-in BubR1 mutations in the Drosophila genome to separately disrupt the KEN box and the kinase domain. All of the mutants are homozygously viable and fertile and show no defects in mitotic progression. The mutants without kinase activity have an increased lifespan and phenotypic changes associated with attenuated insulin signaling, including reduced InR on the cell membrane, weakened PI3K and AKT activity, and elevated expression of dFoxO targets. The BubR1 kinase-dead mutants have a reduced cap cell number in female germaria, which can be rescued by expressing a constitutively active InR. We conclude that one major physiological role of BubR1 kinase in Drosophila is to modulate insulin signaling.

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

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

The RepID-CRL4 ubiquitin ligase complex regulates metaphase to anaphase transition via BUB3 degradation

The spindle assembly checkpoint (SAC) prevents premature chromosome segregation by inactivating the anaphase promoting complex/cyclosome (APC/C) until all chromosomes are properly attached to mitotic spindles. Here we identify a role for Cullin–RING ubiquitin ligase complex 4 (CRL4), known for modulating DNA replication, as a crucial mitotic regulator that triggers the termination of the SAC and enables chromosome segregation. CRL4 is recruited to chromatin by the replication origin binding protein RepID/DCAF14/PHIP. During mitosis, CRL4 dissociates from RepID and replaces it with RB Binding Protein 7 (RBBP7), which ubiquitinates the SAC mediator BUB3 to enable mitotic exit. During interphase, BUB3 is protected from CRL4-mediated degradation by associating with promyelocytic leukemia (PML) nuclear bodies, ensuring its availability upon mitotic onset. Deficiencies in RepID, CRL4 or RBBP7 delay mitotic exit, increase genomic instability and enhance sensitivity to paclitaxel, a microtubule stabilizer and anti-tumor drug.

The spindle assembly checkpoint (SAC) safeguards chromosome segregation by regulating the anaphase promoting complex/cyclosome (APC/C), allowing chromosomes to correctly attach to mitotic spindles. Here the authors reveal a role for Cullin–RING ubiquitin ligase complex 4 (CRL4) in regulating metaphase to anaphase transition via BUB3 degradation.

Orchestration of the spindle assembly checkpoint by CDK1-cyclin B1

In mitosis, the spindle assembly checkpoint (SAC) monitors the formation of microtubule-kinetochore attachments during capture of chromosomes by the mitotic spindle. Spindle assembly is complete once there are no longer any unattached kinetochores. Here, we will discuss the mechanism and key components of spindle checkpoint signalling. Unattached kinetochores bind the principal spindle checkpoint kinase monopolar spindle 1 (MPS1). MPS1 triggers the recruitment of other spindle checkpoint proteins and the formation of a soluble inhibitor of anaphase, thus preventing exit from mitosis. On microtubule attachment, kinetochores become checkpoint silent due to the actions of PP2A-B56 and PP1. This SAC responsive period has to be coordinated with mitotic spindle formation to ensure timely mitotic exit and accurate chromosome segregation. We focus on the molecular mechanisms by which the SAC permissive state is created, describing a central role for CDK1-cyclin B1 and its counteracting phosphatase PP2A-B55. Furthermore, we discuss how CDK1-cyclin B1, through its interaction with MAD1, acts as an integral component of the SAC, and actively orchestrates checkpoint signalling and thus contributes to the faithful execution of mitosis.

The Bub1-TPR Domain Interacts Directly with Mad3 to Generate Robust Spindle Checkpoint Arrest

The spindle checkpoint monitors kinetochore-microtubule interactions and generates a “wait anaphase” delay when any defects are apparent [1, 2, 3]. This provides time for cells to correct chromosome attachment errors and ensure high-fidelity chromosome segregation. Checkpoint signals are generated at unattached chromosomes during mitosis. To activate the checkpoint, Mps1^Mph1 kinase phosphorylates the kinetochore component KNL1^Spc105/Spc7 on conserved MELT motifs to recruit Bub3-Bub1 complexes [4, 5, 6] via a direct Bub3 interaction with phospho-MELT motifs [7, 8]. Mps1^Mph1 then phosphorylates Bub1, which strengthens its interaction with Mad1-Mad2 complexes to produce a signaling platform [9, 10]. The Bub1-Mad1 platform is thought to recruit Mad3, Cdc20, and Mad2 to produce the mitotic checkpoint complex (MCC), which is the diffusible wait anaphase signal [9, 11, 12]. The MCC binds and inhibits the mitotic E3 ubiquitin ligase, known as Cdc20-anaphase promoting complex/cyclosome (APC/C), and stabilizes securin and cyclin to delay anaphase onset [13, 14, 15, 16, 17]. Here we demonstrate, in both budding and fission yeast, that kinetochores and KNL1^Spc105/Spc7 can be bypassed; simply inducing heterodimers of Mps1^Mph1 kinase and Bub1 is sufficient to trigger metaphase arrest that is dependent on Mad1, Mad2, and Mad3. We use this to dissect the domains of Bub1 necessary for arrest, highlighting the need for Bub1-CD1, which binds Mad1 [9], and Bub1’s highly conserved N-terminal tetratricopeptide repeat (TPR) domain [18, 19]. We demonstrate that the Bub1 TPR domain is both necessary and sufficient to bind and recruit Mad3. We propose that this brings Mad3 into close proximity to Mad1-Mad2 and Mps1^Mph1 kinase, enabling efficient generation of MCC complexes.

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Heterodimers of Mps1 and Bub1 generate robust spindle checkpoint arrest in yeasts

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This arrest is independent of kinetochores but requires Bub1-CD1 and the Bub1-TPR

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The Bub1-TPR is both necessary and sufficient for Mad3 interaction and recruitment

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Recombinant fission yeast Bub1-TPR and Mad3 form a stable complex

Defective mitosis-linked DNA damage response and chromosomal instability in liver cancer

Hepatocellular carcinoma (HCC), the most common form of liver cancer, represents a health problem in hepatic viruses-eradicating era because obesity, type 2 diabetes, and nonalcoholic steatohepatitis (NASH) are considered emerging pathogenic factors. Metabolic disorders underpin mitotic errors that lead to numerical and structural chromosome aberrations in a significant proportion of cell divisions. Here, we review that genomically unstable HCCs show evidence for a paradoxically DNA damage response (DDR) which leads to ongoing chromosome segregation errors. The understanding of DDR induced by defective mitoses is crucial to our ability to develop or improve liver cancer therapeutic strategies.

Identification of significant genes with poor prognosis in ovarian cancer via bioinformatical analysis

Ovarian cancer (OC) is the highest frequent malignant gynecologic tumor with very complicated pathogenesis. The purpose of the present academic work was to identify significant genes with poor outcome and their underlying mechanisms. Gene expression profiles of GSE36668, GSE14407 and GSE18520 were available from GEO database. There are 69 OC tissues and 26 normal tissues in the three profile datasets. Differentially expressed genes (DEGs) between OC tissues and normal ovarian (OV) tissues were picked out by GEO2R tool and Venn diagram software. Next, we made use of the Database for Annotation, Visualization and Integrated Discovery (DAVID) to analyze Kyoto Encyclopedia of Gene and Genome (KEGG) pathway and gene ontology (GO). Then protein-protein interaction (PPI) of these DEGs was visualized by Cytoscape with Search Tool for the Retrieval of Interacting Genes (STRING). There were total of 216 consistently expressed genes in the three datasets, including 110 up-regulated genes enriched in cell division, sister chromatid cohesion, mitotic nuclear division, regulation of cell cycle, protein localization to kinetochore, cell proliferation and Cell cycle, progesterone-mediated oocyte maturation and p53 signaling pathway, while 106 down-regulated genes enriched in palate development, blood coagulation, positive regulation of transcription from RNA polymerase II promoter, axonogenesis, receptor internalization, negative regulation of transcription from RNA polymerase II promoter and no significant signaling pathways. Of PPI network analyzed by Molecular Complex Detection (MCODE) plug-in, all 33 up-regulated genes were selected. Furthermore, for the analysis of overall survival among those genes, Kaplan–Meier analysis was implemented and 20 of 33 genes had a significantly worse prognosis. For validation in Gene Expression Profiling Interactive Analysis (GEPIA), 15 of 20 genes were discovered highly expressed in OC tissues compared to normal OV tissues. Furthermore, four genes (BUB1B, BUB1, TTK and CCNB1) were found to significantly enrich in the cell cycle pathway via re-analysis of DAVID. In conclusion, we have identified four significant up-regulated DEGs with poor prognosis in OC on the basis of integrated bioinformatical methods, which could be potential therapeutic targets for OC patients.

Recent Progress on the Localization of the Spindle Assembly Checkpoint Machinery to Kinetochores

Faithful chromosome segregation during mitosis is crucial for maintaining genome stability. The spindle assembly checkpoint (SAC) is a surveillance mechanism that ensures accurate mitotic progression. Defective SAC signaling leads to premature sister chromatid separation and aneuploid daughter cells. Mechanistically, the SAC couples the kinetochore microtubule attachment status to the cell cycle progression machinery. In the presence of abnormal kinetochore microtubule attachments, the SAC prevents the metaphase-to-anaphase transition through a complex kinase-phosphatase signaling cascade which results in the correct balance of SAC components recruited to the kinetochore. The correct kinetochore localization of SAC proteins is a prerequisite for robust SAC signaling and, hence, accurate chromosome segregation. Here, we review recent progresses on the kinetochore recruitment of core SAC factors.

The BUB3-BUB1 Complex Promotes Telomere DNA Replication

Telomeres and telomere-binding proteins form complex secondary nucleoprotein structures that are critical for genome integrity but can present serious challenges during telomere DNA replication. It remains unclear how telomere replication stress is resolved during S phase. Here, we show that the BUB3-BUB1 complex, a component in spindle assembly checkpoint, binds to telomeres during S phase and promotes telomere DNA replication. Loss of the BUB3-BUB1 complex results in telomere replication defects, including fragile and shortened telomeres. We also demonstrate that the telomere-binding ability of BUB3 and kinase activity of BUB1 are indispensable to BUB3-BUB1 function at telomeres. TRF2 targets BUB1-BUB3 to telomeres, and BUB1 can directly phosphorylate TRF1 and promote TRF1 recruitment of BLM helicase to overcome replication stress. Our findings have uncovered previously unknown roles for the BUB3-BUB1 complex in S phase and shed light on how proteins from diverse pathways function coordinately to ensure proper telomere replication and maintenance.

RAE1 promotes BMAL1 shuttling and regulates degradation and activity of CLOCK: BMAL1 heterodimer

Circadian rhythm is an autoregulatory rhythm, which is sustained by various mechanisms. The nucleocytoplasmic shuttling of BMAL1 is essential for CLOCK translocation between cytoplasm and nucleus and maintenance of the correct pace of the circadian clock. Here we showed that RAE1 and NUP98 can promote the degradation of BMAL1 and CLOCK. Knockdown of RAE1 and NUP98 suppressed BMAL1 shuttling, leading to cytoplasm accumulation of CLOCK. Furthermore, Chip assay showed that knockdown of RAE1 and NUP98 can enhance the interaction between CLOCK: BMAL1 and E-box region in the promoters of Per2 and Cry1 while reducing its transcription activation activity. Our present study firstly revealed that RAE1 and NUP98 are critical regulators for BMAL1 shuttling.

Coordination of Protein Kinase and Phosphoprotein Phosphatase Activities in Mitosis

Dynamic changes in protein phosphorylation govern the transitions between different phases of the cell division cycle. A "tug of war" between highly conserved protein kinases and the family of phosphoprotein phosphatases (PPP) establishes the phosphorylation state of proteins, which controls their function. More than three-quarters of all proteins are phosphorylated at one or more sites in human cells, with the highest occupancy of phosphorylation sites seen in mitosis. Spatial and temporal regulation of opposing kinase and PPP activities is crucial for accurate execution of the mitotic program. The role of mitotic kinases has been the focus of many studies, while the contribution of PPPs was for a long time underappreciated and is just emerging. Misconceptions regarding the specificity and activity of protein phosphatases led to the belief that protein kinases are the primary determinants of mitotic regulation, leaving PPPs out of the limelight. Recent studies have shown that protein phosphatases are specific and selective enzymes, and that their activity is tightly regulated. In this review, we discuss the emerging roles of PPPs in mitosis and their regulation of and by mitotic kinases, as well as mechanisms that determine PPP substrate recognition and specificity.

Pellino 1 inactivates mitotic spindle checkpoint by targeting BubR1 for ubiquitinational degradation

Aberrant constitutive activation of receptor-mediated downstream signalling plays an active role in the deregulation of cell cycle control. The mitotic spindle checkpoint is important in preventing abnormal mitotic cell cycle with chromosome missegregation from achieving neoplastic aneuploidy. However, mechanisms coupling receptor-mediated signalling to mitotic spindle checkpoint regulation remain unclear. Pellino 1 is a receptor signal-responsive E3 ubiquitin ligase, and the application of certain receptor-mediated signalling regulates the expression and activity of Pellino 1. In the present study, Pellino 1 expression induced extensive chromosome aneuploidy and allowed abnormal mitotic cells to adapt and become aneuploid in vitro and in vivo. Pellino 1 directly interacted with BubR1, a key component of mitotic spindle checkpoint, in a mitotic cell-cycle dependent manner, and down-regulated the stability of BubR1 by ubiquitination-mediated degradation and induced mitotic dysfunction. In summary, Pellino 1 expression acts as an inhibitory signal of the homeostatic regulation of mitotic cell cycle and checkpoint, and thus contributes to the initiation and progression of neoplastic chromosome aneuploidy.

The Ins and Outs of Aurora B Inner Centromere Localization

Error-free chromosome segregation is essential for the maintenance of genomic integrity during cell division. Aurora B, the enzymatic subunit of the Chromosomal Passenger Complex (CPC), plays a crucial role in this process. In early mitosis Aurora B localizes predominantly to the inner centromere, a specialized region of chromatin that lies at the crossroads between the inter-kinetochore and inter-sister chromatid axes. Two evolutionarily conserved histone kinases, Haspin and Bub1, control the positioning of the CPC at the inner centromere and this location is thought to be crucial for the CPC to function. However, recent studies sketch a subtler picture, in which not all functions of the CPC require strict confinement to the inner centromere. In this review we discuss the molecular pathways that direct Aurora B to the inner centromere and deliberate if and why this specific localization is important for Aurora B function.

Zfp207 is a Bub3 binding protein regulating meiotic chromosome alignment in mouse oocytes

Zinc finger proteins are a massive, diverse family of proteins that serve a wide variety of biological functions. However, the roles of them during meiosis are not yet clearly defined. Here, we report that Zfp207 localizes at the kinetochores during mouse oocyte meiotic maturation. Depletion of Zfp207 leads to a significantly higher proportion of impaired spindle organization and misaligned chromosomes in oocytes. This is coupled with the defective kinetochore-microtubule attachments, and resultantly increasing incidence of aneuploid metaphase II eggs. The precocious polar body extrusion and escape of metaphase I arrest induced by nocodazole treatment in Zfp207-depleted oocytes indicates that Zfp207 is essential for activation of SAC (Spindle Assembly Checkpoint) activity. Notably, we find that Zfp207 binds to Bub3 to form a complex and maintains its protein level in oocytes, and that overexpression of Bub3 is able to partially rescue the occurrence of aneuploid eggs in Zfp207-depleted oocytes. Collectively, we identify Zfp207 as a novel Bub3 binding protein in oocytes which plays an important role in controlling meiotic chromosome alignment and SAC function.