CENP-E--dependent BubR1 autophosphorylation enhances chromosome alignment and the mitotic checkpoint

A kinetochore-associated kinesin-7 motor cooperates with BUB3.3 to regulate mitotic chromosome congression in Arabidopsis thaliana

Faithful genome partition during cell division relies on proper congression of chromosomes to the spindle equator before sister chromatid segregation. Here we uncover a kinesin-7 motor, kinetochore-associated kinesin 1 (KAK1), that is required for mitotic chromosome congression in Arabidopsis. KAK1 associates dynamically with kinetochores throughout mitosis. Loss of KAK1 results in severe defects in chromosome congression at metaphase, yet segregation errors at anaphase are rarely observed. KAK1 specifically interacts with the spindle assembly checkpoint protein BUB3.3 and both proteins show interdependent kinetochore localization. Chromosome misalignment in BUB3.3-depleted plants can be rescued by artificial tethering of KAK1 to kinetochores but not vice versa, demonstrating that KAK1 acts downstream of BUB3.3 to orchestrate microtubule-based chromosome transport at kinetochores. Moreover, we show that KAK1's motor activity is essential for driving chromosome congression to the metaphase plate. Thus, our findings reveal that plants have repurposed BUB3.3 to interface with a specialized kinesin adapted to integrate proper chromosome congression and checkpoint control through a distinct kinetochore design.

Kinesin-7 CENP-E mediates centrosome organization and spindle assembly to regulate chromosome alignment and genome stability

Chromosome congression and alignment are essential for cell cycle progression and genomic stability. Kinesin-7 CENP-E, a plus-end-directed kinesin motor, is required for chromosome biorientation, congression and alignment in cell division. However, it remains unclear how chromosomes are aligned and segregated in the absence of CENP-E in mitosis. In this study, we utilize the CRISPR-Cas9 gene editing method and high-throughput screening to establish CENP-E knockout cell lines and reveal that CENP-E deletion results in defects in chromosome congression, alignment and segregation, which further promotes aneuploidy and genomic instability in mitosis. Both CENP-E inhibition and deletion lead to the dispersion of spindle poles, the formation of the multipolar spindle and spindle disorganization, which indicates that CENP-E is necessary for the organization and maintenance of spindle poles. In addition, CENP-E heterozygous deletion in spleen tissues also leads to the accumulation of dividing lymphocytes and cell cycle arrest in vivo. Furthermore, CENP-E deletion also disrupts the localization of key kinetochore proteins and triggers the activation of the spindle assembly checkpoint. In summary, our findings demonstrate that CENP-E promotes kinetochore-microtubule attachment and spindle pole organization to regulate chromosome alignment and spindle assembly checkpoint during cell division.

Sensitization of cancer cells to paclitaxel-induced apoptosis by canagliflozin

Cancer cells consume more glucose and usually overexpress glucose transporters which have become potential targets for the development of anticancer drugs. It has been demonstrated that selective SGLT2 inhibitors, such as canagliflozin and dapagliflozin, display anticancer activity. Here we demonstrated that canagliflozin and dapagliflozin synergistically enhanced the growth inhibitory effect of paclitaxel in cancer cells including ovarian cancer and oral squamous cell carcinoma cells. Canagliflozin also inhibited glucose uptake via GLUTs. The combination of paclitaxel and WZB117, a GLUT inhibitor, exhibited a strong synergy, supporting the notion that inhibition of GLUTs by canagliflozin may also account for the synergy between canagliflozin and paclitaxel. Mechanistic studies in ES-2 ovarian cancer cells revealed that canagliflozin potentiated paclitaxel-induced apoptosis and DNA damaging effect. Paclitaxel in the nanomolar range elevated abnormal mitotic cells as well as aneuploid cells, and canagliflozin further enhanced this effect. Furthermore, canagliflozin downregulated cyclin B1 and phospho-BUBR1 upon spindle assembly checkpoint (SAC) activation by paclitaxel, and may consequently impair SAC. Thus, paclitaxel disturbed microtubule dynamics and canagliflozin compromised SAC activity, together they may induce premature mitotic exit, accumulation of aneuploid cells with DNA damage, and ultimately apoptosis.

Kinesin-7 CENP-E mediates chromosome alignment and spindle assembly checkpoint in meiosis I

In eukaryotes, meiosis is the genetic basis for sexual reproduction, which is important for chromosome stability and species evolution. The defects in meiosis usually lead to chromosome aneuploidy, reduced gamete number, and genetic diseases, but the pathogenic mechanisms are not well clarified. Kinesin-7 CENP-E is a key regulator in chromosome alignment and spindle assembly checkpoint in cell division. However, the functions and mechanisms of CENP-E in male meiosis remain largely unknown. In this study, we have revealed that the CENP-E gene was highly expressed in the rat testis. CENP-E inhibition influences chromosome alignment and spindle organization in metaphase I spermatocytes. We have found that a portion of misaligned homologous chromosomes is located at the spindle poles after CENP-E inhibition, which further activates the spindle assembly checkpoint during the metaphase-to-anaphase transition in rat spermatocytes. Furthermore, CENP-E depletion leads to abnormal spermatogenesis, reduced sperm count, and abnormal sperm head structure. Our findings have elucidated that CENP-E is essential for homologous chromosome alignment and spindle assembly checkpoint in spermatocytes, which further contribute to chromosome stability and sperm cell quality during spermatogenesis.

Kinesin-7 CENP-E in tumorigenesis: Chromosome instability, spindle assembly checkpoint, and applications

Kinesin motors are a large family of molecular motors that walk along microtubules to fulfill many roles in intracellular transport, microtubule organization, and chromosome alignment. Kinesin-7 CENP-E (Centromere protein E) is a chromosome scaffold-associated protein that is located in the corona layer of centromeres, which participates in kinetochore-microtubule attachment, chromosome alignment, and spindle assembly checkpoint. Over the past 3 decades, CENP-E has attracted great interest as a promising new mitotic target for cancer therapy and drug development. In this review, we describe expression patterns of CENP-E in multiple tumors and highlight the functions of CENP-E in cancer cell proliferation. We summarize recent advances in structural domains, roles, and functions of CENP-E in cell division. Notably, we describe the dual functions of CENP-E in inhibiting and promoting tumorigenesis. We summarize the mechanisms by which CENP-E affects tumorigenesis through chromosome instability and spindle assembly checkpoints. Finally, we overview and summarize the CENP-E-specific inhibitors, mechanisms of drug resistances and their applications.

Kinesin-7 CENP-E is essential for chromosome alignment and spindle assembly of mouse spermatocytes

Genome stability depends on chromosome congression and alignment during cell division. Kinesin-7 CENP-E is critical for kinetochore-microtubule attachment and chromosome alignment, which contribute to genome stability in mitosis. However, the functions and mechanisms of CENP-E in the meiotic division of male spermatocytes remain largely unknown. In this study, by combining the use of chemical inhibitors, siRNA-mediated gene knockdown, immunohistochemistry, and high-resolution microscopy, we have found that CENP-E inhibition results in chromosome misalignment and metaphase arrest in dividing spermatocyte during meiosis. Strikingly, we have revealed that CENP-E regulates spindle organization in metaphase I spermatocytes and cultured GC-2 spd cells. CENP-E depletion leads to spindle elongation, chromosome misalignment, and chromosome instability in spermatocytes. Together, these findings indicate that CENP-E mediates the kinetochore recruitment of BubR1, spindle assembly checkpoint and chromosome alignment in dividing spermatocytes, which finally contribute to faithful chromosome segregation and chromosome stability in the male meiotic division.

Considerations for studying phosphorylation of the mitotic checkpoint pseudokinase BUBR1

Budding uninhibited by benzimidazole 1-related protein 1 (BUBR1) is a mitotic checkpoint (better known as the spindle assembly checkpoint) protein that forms part of an inhibitory complex required to delay mitosis when errors occur in the attachment between chromosomes and the mitotic spindle. If these errors remain uncorrected, it could result in unequal distribution of genetic material to each of the nascent daughter cells, leading to potentially disastrous consequences at both the cellular and organismal level. In some higher eukaryotes including vertebrates, BUBR1 has a C-terminal kinase fold that is largely thought to be inactive, whereas in many species this domain has been lost through evolution and the truncated protein is known as mitotic arrest deficient 3 (MAD3). Here we present advice and practical considerations for the design of experiments, their analysis and interpretation to study the functions of the vertebrate BUBR1 during mitosis with emphasis on analysis implicating the pseudokinase domain.

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.

Transcriptomic analysis of granulosa cell populations proximal and distal to the germinal disc of chicken preovulatory follicles

Within the oocytes of chicken preovulatory follicles, the engulfed yolk constitutes 99% of the oocyte content, while the small germinal disc (GD) (which contains the nucleus and 99% ooplasm) occupies only less than 1%. Relative to the position of the GD, the single granulosa cell layer surrounding the oocyte can be sub-divided into two sub-populations: granulosa cells proximal (named Gp cells) and distal (Gd cells) to the GD. It was reported that Gp cells and Gd cells differ in their morphology, proliferative rate and steroidogenic capacity, however, the underlying mechanism controlling granulosa cell heterogeneity remains unclear. Here we analyzed the transcriptomes of Gd and Gp cells of preovulatory (F5 and F1) follicles in chicken ovaries. We found that: (1) genes associated with cell cycle and DNA replication (CDK1, CCNB3 etc.) have comparatively higher expression levels in Gp cells than in Gd cells, while genes associated with steroidogenesis (CYP51A1, DHCR24) are highly expressed in Gd cells, indicating that Gp cells are likely more mitotic and less steroidogenic than Gd cells; (2) genes associated with extracellular matrix remodeling, cell adhesion and sperm binding (ZP3, ZP2) are differentially expressed in Gp and Gd cells; (3) Furthermore, signaling molecules (WNT4/IHH) and receptors for NGF (NGFR), epidermal growth factor (EGFR), gonadotropins (FSHR/LHR) and prostaglandin (PTGER3) are abundantly but differentially expressed in Gp and Gd cells. Taken together, our data strongly supports the notion that Gp and Gd cells of preovulatory follicles differ in their proliferation rate, steroidogenic activity, ECM organization and sperm binding capacity, which are likely controlled by gonadotropins and local ovarian factors, such as GD-derived factors.

Leaving no-one behind: how CENP-E facilitates chromosome alignment

Chromosome alignment and biorientation is essential for mitotic progression and genomic stability. Most chromosomes align at the spindle equator in a motor-independent manner. However, a subset of polar kinetochores fail to bi-orient and require a microtubule motor-based transport mechanism to move to the cell equator. Centromere Protein E (CENP-E/KIF10) is a kinesin motor from the Kinesin-7 family, which localizes to unattached kinetochores during mitosis and utilizes plus-end directed microtubule motility to slide mono-oriented chromosomes to the spindle equator. Recent work has revealed how CENP-E cooperates with chromokinesins and dynein to mediate chromosome congression and highlighted its role at aligned chromosomes. Additionally, we have gained new mechanistic insights into the targeting and regulation of CENP-E motor activity at the kinetochore. Here, we will review the function of CENP-E in chromosome congression, the pathways that contribute to CENP-E loading at the kinetochore, and how CENP-E activity is regulated during mitosis.

Identification of CDK1 as a candidate marker in cutaneous squamous cell carcinoma by integrated bioinformatics analysis

BACKGROUND

Cutaneous squamous cell carcinoma (cSCC) is a relatively common cancer that accounts for nearly 50% of non-melanoma skin cancer cases. However, the genotypes that are linked with poor prognosis and/or high relapse rates and pathogenic mechanisms of cSCC are not fully understood. To address these points, three gene expression datasets were analyzed to identify candidate biomarker genes in cSCC.

METHODS

The GSE117247, GSE32979, and GSE98767 datasets comprising a total of 32 cSCC samples and 31 normal skin tissue samples were obtained from the National Center for Biotechnology Information Gene Expression Omnibus database. Differentially expressed genes (DEGs) were identified and underwent pathway enrichment analyses with the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes (KEGG). A putative DEG protein-protein interaction (PPI) network was also established that included hub genes. The expression of CDK1, MAD2L1, BUB1 ans CDC20 were examined in the study.

RESULTS

A total of 335 genes were identified, encompassing 219 found to be upregulated and 116 genes that were downregulated in cSCC, compared to normal tissue. Enriched functions of these DEGs were associated with Ephrin receptor signaling and cell division; cytosol, membrane, and extracellular exosomes; ATP-, poly(A) RNA-, and identical protein binding. We also established a PPI network comprising 332 nodes and identified KIF2C, CDC42, AURKA, MAD2L1, MYC, CDK1, FEN1, H2AFZ, BUB1, BUB1B, CKS2, CDC20, CCT2, ACTR2, ACTB, MAPK14, and HDAC1 as candidate hub genes. The expression of CDK1 are significantly higher in the cSCC tissues than that in normal skin.

CONCLUSIONS

The DEGs identified in this study are potential therapeutic targets and biomarkers for cSCC. CDK1 is a gene closely related to the occurrence and development of cSCC, which may play an important role. Bioinformatics analysis shows that it is involved in the important pathway of the pathogenesis of cSCC, and may be recognized and applied as a new biomarker in the future diagnosis and treatment of cSCC.

Identification of candidate biomarkers correlated with the pathogenesis and prognosis of breast cancer via integrated bioinformatics analysis

BACKGROUND

This study was carried out to identify potential key genes associated with the pathogenesis and prognosis of breast cancer (BC).

METHODS

Seven GEO datasets (GSE24124, GSE32641, GSE36295, GSE42568, GSE53752, GSE70947, GSE109169) were downloaded from the Gene Expression Omnibus (GEO) database. Differentially expressed genes (DEGs) between BC and normal breast tissue samples were screened by an integrated analysis of multiple gene expression profile datasets. Hub genes related to the pathogenesis and prognosis of BC were verified by employing protein-protein interaction (PPI) network.

RESULTS

Ten hub genes with high degree were identified, including CDK1, CDC20, CCNA2, CCNB1, CCNB2, BUB1, BUB1B, CDCA8, KIF11, and TOP2A. Lastly, the Kaplan-Meier plotter (KM plotter) online database demonstrated that higher expression levels of these genes were related to lower overall survival. Experimental validation showed that all 10 hub genes had the same expression trend as predicted.

CONCLUSION

The findings of this research would provide some directive significance for further investigating the diagnostic and prognostic biomarkers to facilitate the molecular targeting therapy of BC, which could be used as a new biomarker for diagnosis and to guide the combination medicine of BC.

BUBR1 Pseudokinase Domain Promotes Kinetochore PP2A-B56 Recruitment, Spindle Checkpoint Silencing, and Chromosome Alignment

The balance of phospho-signaling at the outer kinetochore is critical for forming accurate attachments between kinetochores and the mitotic spindle and timely exit from mitosis. A major player in determining this balance is the PP2A-B56 phosphatase, which is recruited to the kinase attachment regulatory domain (KARD) of budding uninhibited by benzimidazole 1-related 1 (BUBR1) in a phospho-dependent manner. This unleashes a rapid, switch-like phosphatase relay that reverses mitotic phosphorylation at the kinetochore, extinguishing the checkpoint and promoting anaphase. Here, we demonstrate that the C-terminal pseudokinase domain of human BUBR1 is required to promote KARD phosphorylation. Mutation or removal of the pseudokinase domain results in decreased PP2A-B56 recruitment to the outer kinetochore attenuated checkpoint silencing and errors in chromosome alignment as a result of imbalance in Aurora B activity. Our data, therefore, elucidate a function for the BUBR1 pseudokinase domain in ensuring accurate and timely exit from mitosis.

Spindle assembly checkpoint gene BUB1B is essential in breast cancer cell survival

PURPOSE

The study aimed to investigate the role of spindle assembly checkpoint (SAC) in cancer cells with compromised genomic integrity. Chromosomal instability (CIN) gives cancer cells an adaptive advantage. However, maintaining the balance of this instability is crucial for the survival of cancer cells as it could lead them to the mitotic catastrophe. Therefore, cancer cells adapt to the detrimental effects of CIN. We hypothesized that changes in SAC might be one such adaptation mechanism. The focus of the study was BUB1B, an integral part of the checkpoint.

METHODS

Clinical datasets were analyzed to compare expression levels of SAC genes in normal tissue vs. breast carcinoma. The effects of the reduction of BUB1B expression was examined utilizing RNA interference method with siRNAs. In vitro viability, clonogenicity, apoptosis, and SAC activity levels of a variety of breast cancer (BrCa) cell lines, as well as in vivo tumorigenicity of the triple-negative breast cancer (TNBC) cell line MDA-MB-468, were tested. Additionally, the chromosomal stability of these cells was tested by immunofluorescence staining and flow cytometry.

RESULTS

In clinical breast cancer datasets, SAC genes were elevated in BrCa with BUB1B having the highest fold change. BUB1B overexpression was associated with a decreased probability of overall survival. The knockdown of BUB1B resulted in reduced viability and clonogenicity in BrCa cell lines and a significant increase in apoptosis and cell death. However, the viability and apoptosis levels of the normal breast epithelial cell line, MCF12A, were not affected. BUB1B knockdown also impaired chromosome alignment and resulted in acute chromosomal abnormalities. We also showed that BUB1B knockdown on the MDA-MB-468 cell line decreases tumor growth in mice.

CONCLUSIONS

A functional spindle assembly checkpoint is essential for the survival of BrCa cells. BUB1B is a critical factor in SAC, and therefore breast cancer cell survival. Impairment of BUB1B has damaging effects on cancer cell viability and tumorigenicity, especially on the more aggressive variants of BrCa.

Disruption of ruminal homeostasis by malnutrition involved in systemic ruminal microbiota-host interactions in a pregnant sheep model

BACKGROUND

Undernutrition is a prevalent and spontaneous condition in animal production which always affects microbiota-host interaction in gastrointestinal tract. However, how undernutrition affects crosstalk homeostasis is largely unknown. Here, we discover how undernutrition affects microbial profiles and subsequently how microbial metabolism affects the signal transduction and tissue renewal in ruminal epithelium, clarifying the detrimental effect of undernutrition on ruminal homeostasis in a pregnant sheep model.

RESULTS

Sixteen pregnant ewes (115 days of gestation) were randomly and equally assigned to the control (CON) and severe feed restriction (SFR) groups. Ewes on SFR treatment were restricted to a 30% level of ad libitum feed intake while the controls were fed normally. After 15 days, all ewes were slaughtered to collect ruminal digesta for 16S rRNA gene and metagenomic sequencing and ruminal epithelium for transcriptome sequencing. Results showed that SFR diminished the levels of ruminal volatile fatty acids and microbial proteins and repressed the length, width, and surface area of ruminal papillae. The 16S rRNA gene analysis indicated that SFR altered the relative abundance of ruminal bacterial community, showing decreased bacteria about saccharide degradation (Saccharofermentans and Ruminococcus) and propionate genesis (Succiniclasticum) but increased butyrate producers (Pseudobutyrivibrio and Papillibacter). Metagenome analysis displayed that genes related to amino acid metabolism, acetate genesis, and succinate-pathway propionate production were downregulated upon SFR, while genes involved in butyrate and methane genesis and acrylate-pathway propionate production were upregulated. Transcriptome and real-time PCR analysis of ruminal epithelium showed that downregulated collagen synthesis upon SFR lowered extracellular matrix-receptor interaction, inactivated JAK3-STAT2 signaling pathway, and inhibited DNA replication and cell cycle.

CONCLUSIONS

Generally, undernutrition altered rumen bacterial community and function profile to decrease ruminal energy retention, promoted epithelial glucose and fatty acid catabolism to elevate energy supply, and inhibited the proliferation of ruminal epithelial cells. These findings provide the first insight into the systemic microbiota-host interactions that are involved in disrupting the ruminal homeostasis under a malnutrition pattern. Video Abstract.

Discovery of Novel Agents on Spindle Assembly Checkpoint to Sensitize Vinorelbine-Induced Mitotic Cell Death Against Human Non-Small Cell Lung Cancers

Non-small cell lung cancer (NSCLC) accounts about 80% of all lung cancers. More than two-thirds of NSCLC patients have inoperable, locally advanced or metastatic tumors. Non-toxic agents that synergistically potentiate cancer-killing activities of chemotherapeutic drugs are in high demand. YL-9 was a novel and non-cytotoxic compound with the structure related to sildenafil but showing much less activity against phosphodiesterase type 5 (PDE5). NCI-H460, an NSCLC cell line with low PDE5 expression, was used as the cell model. YL-9 synergistically potentiated vinorelbine-induced anti-proliferative and apoptotic effects in NCI-H460 cells. Vinorelbine induced tubulin acetylation and Bub1-related kinase (BUBR1) phosphorylation, a necessary component in spindle assembly checkpoint. These effects, as well as BUBR1 cleavage, were substantially enhanced in co-treatment with YL-9. Several mitotic arrest signals were enhanced under combinatory treatment of vinorelbine and YL-9, including an increase of mitotic spindle abnormalities, increased cyclin B1 expression, B-cell lymphoma 2 (Bcl-2) phosphorylation and increased phosphoproteins. Moreover, YL-9 also displayed synergistic activity in combining with vinorelbine to induce apoptosis in A549 cells which express PDE5. In conclusion. the data suggest that YL-9 is a novel agent that synergistically amplifies vinorelbine-induced NSCLC apoptosis through activation of spindle assembly checkpoint and increased mitotic arrest of the cell cycle. YL-9 shows the potential for further development in combinatory treatment against NSCLC.

Kinesin-7 CENP-E regulates cell division, gastrulation and organogenesis in development

Kinesin-7 CENP-E motor protein is essential for chromosome alignment and kinetochore-microtubule attachment in cell division. Human CENP-E has recently identified to be linked with the microcephalic primordial dwarfism syndromes associated with a smaller head, brain malformations and a prominent nose. However, the roles of CENP-E in embryonic development remain largely unknown. In this study, we find that zebrafish CENP-E inhibition results in defects in early zygote cleavage, including asymmetric cell division, cell cycle arrest and the developmental abnormalities. We also demonstrate that CENP-E ablation in cultured cells leads to chromosome misalignment, spindle abnormalities and interruptions of the cell cycle. These observations suggest that CENP-E plays a key role in early cell division and cell cycle progression. Furthermore, we also find that CENP-E inhibition results in the defects in the epiboly, the developmental arrest, the smaller head and the abnormal embryo during zebrafish embryogenesis. Our data demonstrate new functions of CENP-E in development and provide insights into its essential roles in organogenesis.

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.

Kinesin-7 CENP-E regulates chromosome alignment and genome stability of spermatogenic cells

Kinesin-7 CENP-E is an essential kinetochore motor required for chromosome alignment and congression. However, the specific functions of CENP-E in the spermatogenic cells during spermatogenesis remain unknown. In this study, we find that CENP-E proteins are expressed in the spermatogonia, spermatocytes, and the elongating spermatids. CENP-E inhibition by specific inhibitor GSK923295 results in the disruption of spermatogenesis and cell cycle arrest of spermatogenic cells. Both spermatogonia and spermatocytes are arrested in metaphase and several chromosomes are not aligned at the equatorial plate. We find that CENP-E inhibition leads to chromosome misalignment, the spindle disorganization, and the formation of the aneuploidy cells. Furthermore, the inhibition of CENP-E results in the defects in the formation of spermatids, including the sperm head condensation and the sperm tail formation. We have revealed that kinesin-7 CENP-E is essential for chromosome alignment and genome stability of the spermatogenic cells.

The Mitotic Apparatus and Kinetochores in Microcephaly and Neurodevelopmental Diseases

Regulators of mitotic division, when dysfunctional or expressed in a deregulated manner (over- or underexpressed) in somatic cells, cause chromosome instability, which is a predisposing condition to cancer that is associated with unrestricted proliferation. Genes encoding mitotic regulators are growingly implicated in neurodevelopmental diseases. Here, we briefly summarize existing knowledge on how microcephaly-related mitotic genes operate in the control of chromosome segregation during mitosis in somatic cells, with a special focus on the role of kinetochore factors. Then, we review evidence implicating mitotic apparatus- and kinetochore-resident factors in the origin of congenital microcephaly. We discuss data emerging from these works, which suggest a critical role of correct mitotic division in controlling neuronal cell proliferation and shaping the architecture of the central nervous system.

Cellular Senescence: Defining a Path Forward

Cellular senescence is a cell state implicated in various physiological processes and a wide spectrum of age-related diseases. Recently, interest in therapeutically targeting senescence to improve healthy aging and age-related disease, otherwise known as senotherapy, has been growing rapidly. Thus, the accurate detection of senescent cells, especially in vivo, is essential. Here, we present a consensus from the International Cell Senescence Association (ICSA), defining and discussing key cellular and molecular features of senescence and offering recommendations on how to use them as biomarkers. We also present a resource tool to facilitate the identification of genes linked with senescence, SeneQuest (available at http://Senequest.net). Lastly, we propose an algorithm to accurately assess and quantify senescence, both in cultured cells and in vivo.

Mouse Models of Accelerated Cellular Senescence

Senescent cells accumulate in multiple tissues as virtually all vertebrate organisms age. Senescence is a highly conserved response to many forms of cellular stress intended to block the propagation of damaged cells. Senescent cells have been demonstrated to play a causal role in aging via their senescence-associated secretory phenotype and by impeding tissue regeneration. Depletion of senescent cells either through genetic or pharmacologic methods has been demonstrated to extend murine lifespan and delay the onset of age-related diseases. Measuring the burden and location of senescent cells in vivo remains challenging, as there is no marker unique to senescent cells. Here, we describe multiple methods to detect the presence and extent of cellular senescence in preclinical models, with a special emphasis on murine models of accelerated aging that exhibit a more rapid onset of cellular senescence.

Identification of candidate genes associated with the pathogenesis of small cell lung cancer via integrated bioinformatics analysis

The pathogenesis of small cell lung cancer (SCLC), a highly metastatic malignant tumor, remains unclear. In the present study, important genes and pathways that are involved in the pathogenesis of SCLC were identified. The following four datasets were downloaded from the Gene Expression Omnibus: GSE60052, GSE43346, GSE15240 and GSE6044. The differentially expressed genes (DEGs) between the SCLC samples and the normal samples were analyzed using R software. The limma package was used for every dataset. The RobustRankAggreg package was used to integrate the DEGs from the four datasets. Functional and pathway enrichment analyses were conducted using the Gene Ontology and Kyoto Encyclopedia of Genes and Genomes databases with FunRich software and R software, respectively. In addition, the protein-protein interaction (PPI) network of the DEGs was constructed using the STRING database and Cytoscape software. Hub genes and significant modules were identified using Molecular Complex Detection in Cytoscape software. Finally, the expression values of hub genes were determined using the Oncomine online database. In total, 412 DEGs were identified following the integration of the four datasets, with 146 upregulated genes and 266 downregulated genes. The upregulated DEGs were primarily enriched in the cell cycle, cell division and microtubule binding. The downregulated DEGs were primarily enriched in the complement and coagulation cascades, the cytokine-mediated signaling pathway and protein binding. Eight hub genes and 1 significant module correlated to the cell cycle pathway were identified based on a subset of the PPI network. Finally, five hub genes were identified as highly expressed in SCLC tissue compared with normal tissue. The cell cycle pathway may be the pathway most closely associated with the pathogenesis of SCLC. NDC80, BUB1B, PLK1, CDC20 and MAD2L1 should be the focus of follow-up studies regarding the diagnosis and treatment of SCLC.

BubR1 phosphorylates CENP-E as a switch enabling the transition from lateral association to end-on capture of spindle microtubules

Error-free mitosis depends on accurate chromosome attachment to spindle microtubules, powered congression of those chromosomes, their segregation in anaphase, and assembly of a spindle midzone at mitotic exit. The centromere-associated kinesin motor CENP-E, whose binding partner is BubR1, has been implicated in congression of misaligned chromosomes and the transition from lateral kinetochore-microtubule association to end-on capture. Although previously proposed to be a pseudokinase, here we report the structure of the kinase domain of Drosophila melanogaster BubR1, revealing its folding into a conformation predicted to be catalytically active. BubR1 is shown to be a bona fide kinase whose phosphorylation of CENP-E switches it from a laterally attached microtubule motor to a plus-end microtubule tip tracker. Computational modeling is used to identify bubristatin as a selective BubR1 kinase antagonist that targets the αN1 helix of N-terminal extension and αC helix of the BubR1 kinase domain. Inhibition of CENP-E phosphorylation is shown to prevent proper microtubule capture at kinetochores and, surprisingly, proper assembly of the central spindle at mitotic exit. Thus, BubR1-mediated CENP-E phosphorylation produces a temporal switch that enables transition from lateral to end-on microtubule capture and organization of microtubules into stable midzone arrays.

A tension-independent mechanism reduces Aurora B-mediated phosphorylation upon microtubule capture by CENP-E at the kinetochore

During mitosis, Aurora B kinase is required for forming proper bi-oriented kinetochore-microtubule attachments. Current models suggest that tension exerted between a pair of sister-kinetochores (inter-kinetochore stretch) produces a spatial separation of Aurora B kinase from kinetochore-associated microtubule binding substrates, such as the Knl1-Mis12-Ndc80 (KMN) network, resulting in a decrease of phosphorylation and, thus, an increase of affinity for microtubules. Using Single-Molecule High-Resolution Colocalization (SHREC) microscopy analysis of the kinetochore-associated motor CENP-E, we now show that CENP-E undergoes structural rearrangements prior to and after tension generation at the kinetochore, and displays a bi-modal Gaussian distribution on a pair of bi-oriented sister kinetochores. The conformational change of CENP-E depends on its microtubule-stimulated motor motility and the highly flexible coiled-coil between its motor and kinetochore-binding tail domains. Chemical inhibition of the motor motility or perturbations of the coiled-coil domain of CENP-E increases Aurora B-mediated Ndc80 phosphorylation in a tension-independent manner. Metaphase chromosome misalignment caused by CENP-E inhibition can be rescued by chemical inhibition of Aurora B kinase. Furthermore, a pair of monotelic sister-kinetochores shows asymmetric levels of Aurora B-mediated phosphorylation in mono-polar spindles depending on CENP-E motor activity. These results collectively suggest a tension-independent mechanism to reduce Aurora B-mediated phosphorylation of outer kinetochore components in response to microtubule capture by CENP-E.

Mechanisms of kinesin-7 CENP-E in kinetochore-microtubule capture and chromosome alignment during cell division

Chromosome congression is essential for faithful chromosome segregation and genomic stability in cell division. Centromere-associated protein E (CENP-E), a plus-end-directed kinesin motor, is required for congression of pole-proximal chromosomes in metaphase. CENP-E accumulates at the outer plate of kinetochores and mediates the kinetochore-microtubule capture. CENP-E also transports the chromosomes along spindle microtubules towards the equatorial plate. CENP-E interacts with Bub1-related kinase, Aurora B and core kinetochore components during kinetochore-microtubule attachment. In this review, we introduce the structures and mechanochemistry of kinesin-7 CENP-E. We highlight the complicated interactions between CENP-E and partner proteins during chromosome congression. We summarise the detailed roles and mechanisms of CENP-E in mitosis and meiosis, including the kinetochore-microtubule capture, chromosome congression/alignment in metaphase and the regulation of spindle assembly checkpoint. We also shed a light on the roles of CENP-E in tumourigenesis and CENP-E's specific inhibitors.

The microtubule-associated protein EML3 regulates mitotic spindle assembly by recruiting the Augmin complex to spindle microtubules

In all eukaryotes, a functional mitotic spindle is essential for distributing duplicated chromosomes into daughter cells. Mitotic spindle assembly involves highly ordered arrangement of microtubules (MTs). The Augmin protein complex recruits γ-tubulin ring complex (γ-TuRC) to MTs and thereby promotes MT-based MT nucleation and mitotic spindle assembly. However, several factors that may promote Augmin recruitment to MTs remain unknown. Here, we show that echinoderm microtubule-associated protein-like 3 (EML3), an MT-associated protein, facilitates binding between MTs and Augmin/γ-TuRC and recruiting the latter to MTs for proper mitotic spindle assembly and kinetochore-MT connections. Using immunofluorescence microscopy, live-cell imaging, and immunoprecipitation assays, we found that EML3 recruits Augmin/γ-TuRC to the MTs to enhance MT-based MT nucleation in both spindle and small acentrosomal asters. We also noted that the EML3-mediated recruitment is controlled by cyclin-dependent kinase 1 (CDK1), which phosphorylated EML3 at Thr-881 and promoted its binding to Augmin/γ-TuRC. RNAi-mediated EML3 knockdown in HeLa cells reduced spindle localization of Augmin/γ-TuRC, which resulted in abnormal spindle assembly and caused kinetochore-MT misconnection. The introduction of exogenous WT or a Thr-881 phosphorylation mimic EML3 variant into the EML3 knockdown cells restored normal Augmin/γ-TuRC localization and spindle assembly. The EML3 knockdown also affected the spindle assembly checkpoint, delaying chromosome congression and cell division. Taken together, our results indicate that EML3 regulates mitotic spindle assembly and the kinetochore-MT connection by regulating MT-based MT nucleation and recruiting Augmin/γ-TuRC to MTs.

CENPE expression is associated with its DNA methylation status in esophageal adenocarcinoma and independently predicts unfavorable overall survival

Centrosome-associated protein E (CENPE) is a plus end-directed kinetochore motor protein, which plays a critical role in mitosis. In this in silico study, using data from the Cancer Genome Atlas-Esophageal Carcinoma (TCGA-ESCA), we analyzed the expression profile of CENPE mRNA in esophageal squamous cell carcinoma (ESCC) and adenocarcinoma (EA), its independent prognostic value and the potential mechanisms of its dysregulation in EA. Results showed that both ESCC and EA tissues had significantly elevated CENPE expression compared with their respective adjacent normal tissues. However, Kaplan-Meier survival curves showed that high CENPE was associated with unfavorable OS in EA. Univariate and multivariate analysis confirmed that CENPE expression was an independent indicator of unfavorable OS in EA patients, as a continuous variable (HR: 1.861, 95%CI: 1.235–2.806, p = 0.003) or as categorical variables (HR: 2.550, 95%CI: 1.294–5.025, p = 0.007). However, CENPE expression had no prognostic value in ESCC. Compared with the methylation status in normal samples, 3 CpG sites were hypomethylated (cg27388036, cg27443373, and cg24651824) in EA, among which two sites (cg27443373 and cg24651824) showed moderately negative correlation with CENPE expression. In addition, we also found that although heterozygous loss (-1) was frequent in EA (50/88, 56.8%), it was not necessarily associated with decreased CENPE expression compared with the copy neutral (0) cases. The methylation of the -1 group was significantly lower than that of the +1/0 group (p = 0.04). Based on these findings, we infer that CENPE upregulation in EA might serve as a valuable indicator of unfavorable OS. The methylation status of cg27443373 and cg24651824 might play a critical role in modulating CENPE expression.

BubR1 depletion delays apoptosis in the microtubule-depolymerized cells

We investigated the role of a spindle assembly checkpoint protein, BubR1, in determining the mechanism of cell killing of an anti-microtubule agent CXI-benzo-84. CXI-benzo-84 dampened microtubule dynamics in live MCF-7 cells. The compound arrested MCF-7 cells in mitosis and induced apoptosis in these cells. Though CXI-benzo-84 efficiently depolymerized microtubules in the BubR1-depleted MCF-7 cells, it did not arrest the BubR1-depleted cells at mitosis. Interestingly, apoptosis occurred in the BubR1-depleted MCF-7 cells in the absence of a mitotic block suggesting that the mitotic block is not a prerequisite for the induction of apoptosis by anti-microtubule agents. In the presence of CXI-Benzo-84, the level of apoptosis was initially found to be lesser in the BubR1-depleted MCF-7 cells than the control cells; however, the BubR1-depleted cells displayed a similar level of apoptosis as the control cells at 72 h of drug treatment. The depletion of BubR1 enhanced DNA damage in MCF-7 cells upon microtubule depolymerization. In addition, CXI-benzo-84 in combination with cisplatin induced more cell death in BubR1-depleted cells than the BubR1-expressing MCF-7 cells. The results indicated a possibility that the BubR1-compromised cancer patients can be treated with combination therapy.

Environmental stresses induce karyotypic instability in colorectal cancer cells

Understanding how cells acquire genetic mutations is a fundamental biological question with implications for many different areas of biomedical research, ranging from tumor evolution to drug resistance. While karyotypic heterogeneity is a hallmark of cancer cells, few mutations causing chromosome instability have been identified in cancer genomes, suggesting a nongenetic origin of this phenomenon. We found that in vitro exposure of karyotypically stable human colorectal cancer cell lines to environmental stress conditions triggered a wide variety of chromosomal changes and karyotypic heterogeneity. At the molecular level, hyperthermia induced polyploidization by perturbing centrosome function, preventing chromosome segregation, and attenuating the spindle assembly checkpoint. The combination of these effects resulted in mitotic exit without chromosome segregation. Finally, heat-induced tetraploid cells were on the average more resistant to chemotherapeutic agents. Our studies suggest that environmental perturbations promote karyotypic heterogeneity and could contribute to the emergence of drug resistance.

Dual gene deficient models of ApcMin/+ mouse in assessing molecular mechanisms of intestinal carcinogenesis

The Apc^Min/+ mouse, carrying an inactivated allele of the adenomatous polyposis coli (Apc) gene, is a widely used animal model of human colorectal tumorigenesis. While crossed with other gene knockout or knock-in mice, these mice possess advantages in investigation of human intestinal tumorigenesis. Intestinal tumor pathogenesis involves multiple gene alterations; thus, various double gene deficiency models could provide novel insights into molecular mechanisms of tumor biology, as well as gene-gene interactions involved in intestinal tumor development and assessment of novel strategies for preventing and treating intestinal cancer. This review discusses approximately 100 double gene deficient mice and their associated intestinal tumor development and progression phenotypes. The dual gene knockouts based on the Apc mutation background consist of inflammation and immune-related, cell cycle-related, Wnt/β-catenin signaling-related, tumor growth factor (TGF)-signaling-related, drug metabolism-related, and transcription factor genes, as well as some oncogenes and tumor suppressors. Future studies should focus on conditional or inducible dual or multiple mouse gene knockout models to investigate the molecular mechanisms underlying intestinal tumor development, as well as potential drug targets.

Epigenetics in Turner syndrome

Background

Monosomy of the X chromosome is the most frequent genetic abnormality in human as it is present in approximately 2% of all conceptions, although 99% of these embryos are spontaneously miscarried. In postnatal life, clinical features of Turner syndrome may include typical dysmorphic stigmata, short stature, sexual infantilism, and renal, cardiac, skeletal, endocrine and metabolic abnormalities.

Main text

Turner syndrome is due to a partial or total loss of the second sexual chromosome, resulting in the development of highly variable clinical features. This phenotype may not merely be due to genomic imbalance from deleted genes but may also result from additive influences on associated genes within a given gene network, with an altered regulation of gene expression triggered by the absence of the second sex chromosome. Current studies in human and mouse models have demonstrated that this chromosomal abnormality leads to epigenetic changes, including differential DNA methylation in specific groups of downstream target genes in pathways associated with several clinical and metabolic features, mostly on autosomal chromosomes. In this article, we begin exploring the potential involvement of both genetic and epigenetic factors in the origin of X chromosome monosomy. We review the dispute between the meiotic and post-zygotic origins of 45,X monosomy, by mainly analyzing the findings from several studies that compare gene expression of the 45,X monosomy to their euploid and/or 47,XXX trisomic cell counterparts on peripheral blood mononuclear cells, amniotic fluid, human fibroblast cells, and induced pluripotent human cell lines. From these studies, a profile of epigenetic changes seems to emerge in response to chromosomal imbalance. An interesting finding of all these studies is that methylation-based and expression-based pathway analyses are complementary, rather than overlapping, and are correlated with the clinical picture displayed by TS subjects.

Conclusions

The clarification of these possible causal pathways may have future implications in increasing the life expectancy of these patients and may provide informative targets for early pharmaceutical intervention.

Extract of bulbus Fritillaria cirrhosa perturbs spindle assembly checkpoint, induces mitotic aberrations and genomic instability in human colon epithelial cell line

BACKGROUND

Bulbus Fritillaria cirrhosa D. Don (BFC) has been used in China as a folk medicine for the treatment of cough and asthma for more than 2000 years. The antitussive and antiasthmatic effects of BFC have been reported before, nevertheless its toxicity and safety have not been documented. This study investigated the possible effects of BFC on spindle assembly checkpoint (SAC), mitotic fidelity and genomic stability in human NCM460 colon epithelial cells.

METHODS

Cells were treated with BFC (0, 20, 40, 80 and 160μg/ml) for 24, 48 and 72h and harvested differently according to the biomarkers observed. Mitotic aberrations were assessed by the biomarkers of chromosome misalignment (CMA), chromosome lagging (CL) and chromatin bridge (CB). Frequencies of micronuclei (MN), nucleoplasmic bridge and nuclear bud (NB) in cytokinesis-block micronucleus assay were used as indicators of genomic instability (GIN). SAC activity was determined by anaphase to metaphase ratio (AMR) and the expression of several SAC genes, including CENP-E, Mps1, Bub1, Mad-1, BubR1 and Mad-2.

RESULTS

Compared with the control, cells in BFC treated groups (80 and 160μg/ml) showed: 1) increased AMR (p<0.05), up-regulated expression of Mps1, Bub1 and Mad-1 (p<0.05) and down-regulated expression of CENP-E, BubR1 and Mad-2 (p<0.05); 2) increased frequencies of CMA, CL and CB (p<0.01); 3) increased incidences of MN and NB (p<0.01).

CONCLUSIONS

This study revealed for the first time that BFC causes mitotic aberrations and GIN in human colon epithelial cells and these effects maybe the result of SAC dysfunction.

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

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

A dynein independent role of Tctex-1 at the kinetochore

Dynein light chains are accessory subunits of the cytoplasmic dynein complex, a minus-end directed microtubule motor. Here, we demonstrate that the dynein light chain Tctex-1 associates with unattached kinetochores and is essential for accurate chromosome segregation. Tctex-1 knockdown in cells does not affect the localization and function of dynein at the kinetochore, but produces a prolonged mitotic arrest with a few misaligned chromosomes, which are subsequently missegregated during anaphase. This function is independent of Tctex-1's association with dynein. The kinetochore localization of Tctex-1 is independent of the ZW10-dynein pathway, but requires the Ndc80 complex. Thus, our findings reveal a dynein independent role of Tctex-1 at the kinetochore to enhance the stability of kinetochore-microtubule attachment.

Role of Intrinsic and Extrinsic Factors in the Regulation of the Mitotic Checkpoint Kinase Bub1

The spindle assembly checkpoint (SAC) monitors microtubule attachment to kinetochores to ensure accurate sister chromatid segregation during mitosis. The SAC members Bub1 and BubR1 are paralogs that underwent significant functional specializations during evolution. We report an in-depth characterization of the kinase domains of Bub1 and BubR1. BubR1 kinase domain binds nucleotides but is unable to deliver catalytic activity in vitro. Conversely, Bub1 is an active kinase regulated by intra-molecular phosphorylation at the P+1 loop. The crystal structure of the phosphorylated Bub1 kinase domain illustrates a hitherto unknown conformation of the P+1 loop docked into the active site of the Bub1 kinase. Both Bub1 and BubR1 bind Bub3 constitutively. A hydrodynamic characterization of Bub1:Bub3 and BubR1:Bub3 demonstrates both complexes to have 1:1 stoichiometry, with no additional oligomerization. Conversely, Bub1:Bub3 and BubR1:Bub3 combine to form a heterotetramer. Neither BubR1:Bub3 nor Knl1, the kinetochore receptor of Bub1:Bub3, modulate the kinase activity of Bub1 in vitro, suggesting autonomous regulation of the Bub1 kinase domain. We complement our study with an analysis of the Bub1 substrates. Our results contribute to the mechanistic characterization of a crucial cell cycle checkpoint.

Zwint-1 is required for spindle assembly checkpoint function and kinetochore-microtubule attachment during oocyte meiosis

The key step for faithful chromosome segregation during meiosis is kinetochore assembly. Defects in this process result in aneuploidy, leading to miscarriages, infertility and various birth defects. However, the roles of kinetochores in homologous chromosome segregation during meiosis are ill-defined. Here we found that Zwint-1 is required for homologous chromosome segregation during meiosis. Knockdown of Zwint-1 accelerated the first meiosis by abrogating the kinetochore recruitment of Mad2, leading to chromosome misalignment and a high incidence of aneuploidy. Although Zwint-1 knockdown did not affect Aurora C kinase activity, the meiotic defects following Zwint-1 knockdown were similar to those observed with ZM447439 treatment. Importantly, the chromosome misalignment following Aurora C kinase inhibition was not restored after removing the inhibitor in Zwint-1-knockdown oocytes, whereas the defect was rescued after the inhibitor washout in the control oocytes. These results suggest that Aurora C kinase-mediated correction of erroneous kinetochore-microtubule attachment is primarily regulated by Zwint-1. Our results provide the first evidence that Zwint-1 is required to correct erroneous kinetochore-microtubule attachment and regulate spindle checkpoint function during meiosis.

Predictive and prognostic values of BubR1 and synuclein-gamma expression in breast cancer

The aim of this study is to determine the expression level of spindle assembly checkpoint (SAC) proteins-BubR1 and synuclein-gamma (SNCG) in human breast cancer tissues and to test whether there is a relationship between their expression levels and clinicopathologic parameters including respons to taxanes, tumor grade, estrogen receptor (ER) pozitivity, HER2 status, and overall survival (OS). We analyzed retrospectively paraffin-embedded tissue sections from 55 breast cancer patients whose clinical outcomes had been tracked after taxane treatment in neoadjuvan and metastatic setting. The expression status of BubR1 and SNCG was defined by immunohistochemistry (IHC) using the anti-BubR1 and anti-SNCG antibody. The BubR1 and SNCG was overexpressed in 38% and 62% of the study group, respectively. There was borderline significant correlation between low BubR1 expression and increased taxane sensitivity (P=0.05). In contrast, high SNCG expression was significantly associated with decreased taxane sensitivity (P=0.01). There was no association between the clinicopathologic parameters including histologic grade, ER positivity and HER2 status and the level of these proteins. However, triple negative tumors showed significantly more high BubR1 expression than those other molecular subtypes (P=0.04). Kaplan-Meier survival analysis failed to show a significant correlation between expression levels of BubR1 and SNCG and overall survival although patients with low levels of both proteins had a marginally longer survival time compared to those with high levels. In summary, our data suggest that both BubR1 and SNCG may be promising predictive marker rather than prognostic marker in patients with breast cancer.

Overexpression of the ∆Np73 isoform is associated with centrosome amplification in brain tumor cell lines

The p73 protein is a member of the p53 family, and this protein is known to be essential for the maintenance of genomic stability, DNA repair, and apoptosis regulation. Transcription from two promoters leads to two main N-terminal isoforms: the TAp73 isoform is reported to have tumor suppressor function, whereas the ΔNp73 isoform likely has oncogenic potential. The present study is focused on the investigation of a possible role of both these p73 N-terminal isoforms in the process of centrosome amplification. HGG-02 and GM7 glioblastoma cell lines and the Daoy medulloblastoma cell line were used in this study. The cells were analyzed using indirect immunofluorescence to determine TAp73 and ΔNp73 expression patterns and possible co-localization with the BubR1 protein, as well as the number of centrosomes. A transiently transfected GM7 cell line was used to verify the results concerning the N-terminal isoforms in relation to centrosome amplification. We found that increased immunoreactivity for the ΔNp73 isoform is associated with the occurrence of an abnormal number of centrosomes in particular cells. Using the transiently transfected GM7 cell line, we confirmed that centrosome amplification is present in cells with overexpression of the ΔNp73 isoform. In contrast, the immunoreactivity for the TAp73 isoform was weak or medium in most of the cells with an aberrant number of centrosomes. To determine the putative counterpart of the p73 N-terminal isoforms among spindle assembly checkpoint (SAC) proteins, we also evaluated possible interactions between the N-terminal isoforms and BubR1 protein, but no co-localization of these proteins was observed.

The spindle checkpoint and chromosome segregation in meiosis

The spindle checkpoint is a key regulator of chromosome segregation in mitosis and meiosis. Its function is to prevent precocious anaphase onset before chromosomes have achieved bipolar attachment to the spindle. The spindle checkpoint comprises a complex set of signaling pathways that integrate microtubule dynamics, biomechanical forces at the kinetochores, and intricate regulation of protein interactions and post-translational modifications. Historically, many key observations that gave rise to the initial concepts of the spindle checkpoint were made in meiotic systems. In contrast with mitosis, the two distinct chromosome segregation events of meiosis present a special challenge for the regulation of checkpoint signaling. Preservation of fidelity in chromosome segregation in meiosis, controlled by the spindle checkpoint, also has a significant impact in human health. This review highlights the contributions from meiotic systems in understanding the spindle checkpoint as well as the role of checkpoint signaling in controlling the complex divisions of meiosis.

Bimodal activation of BubR1 by Bub3 sustains mitotic checkpoint signaling

The mitotic checkpoint (also known as the spindle assembly checkpoint) prevents premature anaphase onset through generation of an inhibitor of the E3 ubiquitin ligase APC/C, whose ubiquitination of cyclin B and securin targets them for degradation. Combining in vitro reconstitution and cell-based assays, we now identify dual mechanisms through which Bub3 promotes mitotic checkpoint signaling. Bub3 enhances signaling at unattached kinetochores not only by facilitating binding of BubR1 but also by enhancing Cdc20 recruitment to kinetochores mediated by BubR1's internal Cdc20 binding site. Downstream of kinetochore-produced complexes, Bub3 promotes binding of BubR1's conserved, amino terminal Cdc20 binding domain to a site in Cdc20 that becomes exposed by initial Mad2 binding. This latter Bub3-stimulated event generates the final mitotic checkpoint complex of Bub3-BubR1-Cdc20 that selectively inhibits ubiquitination of securin and cyclin B by APC/C(Cdc20). Thus, Bub3 promotes two distinct BubR1-Cdc20 interactions, involving each of the two Cdc20 binding sites of BubR1 and acting at unattached kinetochores or cytoplasmically, respectively, to facilitate production of the mitotic checkpoint inhibitor.

Mutations in CENPE define a novel kinetochore-centromeric mechanism for microcephalic primordial dwarfism

Defects in centrosome, centrosomal-associated and spindle-associated proteins are the most frequent cause of primary microcephaly (PM) and microcephalic primordial dwarfism (MPD) syndromes in humans. Mitotic progression and segregation defects, microtubule spindle abnormalities and impaired DNA damage-induced G2-M cell cycle checkpoint proficiency have been documented in cell lines from these patients. This suggests that impaired mitotic entry, progression and exit strongly contribute to PM and MPD. Considering the vast protein networks involved in coordinating this cell cycle stage, the list of potential target genes that could underlie novel developmental disorders is large. One such complex network, with a direct microtubule-mediated physical connection to the centrosome, is the kinetochore. This centromeric-associated structure nucleates microtubule attachments onto mitotic chromosomes. Here, we described novel compound heterozygous variants in CENPE in two siblings who exhibit a profound MPD associated with developmental delay, simplified gyri and other isolated abnormalities. CENPE encodes centromere-associated protein E (CENP-E), a core kinetochore component functioning to mediate chromosome congression initially of misaligned chromosomes and in subsequent spindle microtubule capture during mitosis. Firstly, we present a comprehensive clinical description of these patients. Then, using patient cells we document abnormalities in spindle microtubule organization, mitotic progression and segregation, before modeling the cellular pathogenicity of these variants in an independent cell system. Our cellular analysis shows that a pathogenic defect in CENP-E, a kinetochore-core protein, largely phenocopies PCNT-mutated microcephalic osteodysplastic primordial dwarfism-type II patient cells. PCNT encodes a centrosome-associated protein. These results highlight a common underlying pathomechanism. Our findings provide the first evidence for a kinetochore-based route to MPD in humans.

Chemogenetic evaluation of the mitotic kinesin CENP-E reveals a critical role in triple-negative breast cancer

Breast cancer patients with tumors lacking the three diagnostic markers (ER, PR, and HER2) are classified as triple-negative (primarily basal-like) and have poor prognosis because there is no disease-specific therapy available. To address this unmet medical need, gene expression analyses using more than a thousand breast cancer samples were conducted, which identified elevated centromere protein E (CENP-E) expression in the basal-a molecular subtype relative to other subtypes. CENP-E, a mitotic kinesin component of the spindle assembly checkpoint, is shown to be induced in basal-a tumor cell lines by the mitotic spindle inhibitor drug docetaxel. CENP-E knockdown by inducible shRNA reduces basal-a breast cancer cell viability. A potent, selective CENP-E inhibitor (PF-2771) was used to define the contribution of CENP-E motor function to basal-like breast cancer. Mechanistic evaluation of PF-2771 in basal-a tumor cells links CENP-E-dependent molecular events (e.g., phosphorylation of histone H3 Ser-10; phospho-HH3-Ser10) to functional outcomes (e.g., chromosomal congression defects). Across a diverse panel of breast cell lines, CENP-E inhibition by PF-2771 selectively inhibits proliferation of basal breast cancer cell lines relative to premalignant ones and its response correlates with the degree of chromosomal instability. Pharmacokinetic-pharmacodynamic efficacy analysis in a basal-a xenograft tumor model shows that PF-2771 exposure is well correlated with increased phospho-HH3-Ser10 levels and tumor growth regression. Complete tumor regression is observed in a patient-derived, basal-a breast cancer xenograft tumor model treated with PF-2771. Tumor regression is also observed with PF-2771 in a taxane-resistant basal-a model. Taken together, CENP-E may be an effective therapeutic target for patients with triple-negative/basal-a breast cancer.

Kinetochore-microtubule attachment throughout mitosis potentiated by the elongated stalk of the kinetochore kinesin CENP-E

Centromere protein E (CENP-E) is a highly elongated kinesin that transports pole-proximal chromosomes during congression in prometaphase. During metaphase, it facilitates kinetochore-microtubule end-on attachment required to achieve and maintain chromosome alignment. In vitro CENP-E can walk processively along microtubule tracks and follow both growing and shrinking microtubule plus ends. Neither the CENP-E-dependent transport along microtubules nor its tip-tracking activity requires the unusually long coiled-coil stalk of CENP-E. The biological role for the CENP-E stalk has now been identified through creation of "Bonsai" CENP-E with significantly shortened stalk but wild-type motor and tail domains. We demonstrate that Bonsai CENP-E fails to bind microtubules in vitro unless a cargo is contemporaneously bound via its C-terminal tail. In contrast, both full-length and truncated CENP-E that has no stalk and tail exhibit robust motility with and without cargo binding, highlighting the importance of CENP-E stalk for its activity. Correspondingly, kinetochore attachment to microtubule ends is shown to be disrupted in cells whose CENP-E has a shortened stalk, thereby producing chromosome misalignment in metaphase and lagging chromosomes during anaphase. Together these findings establish an unexpected role of CENP-E elongated stalk in ensuring stability of kinetochore-microtubule attachments during chromosome congression and segregation.

Cenp-meta is required for sustained spindle checkpoint

Cenp-E is a kinesin-like motor protein required for efficient end-on attachment of kinetochores to the spindle microtubules. Cenp-E immunodepletion in Xenopus mitotic extracts results in the loss of mitotic arrest and massive chromosome missegregation, whereas its depletion in mammalian cells leads to chromosome segregation defects despite the presence of a functional spindle assembly checkpoint (SAC). Cenp-meta has previously been reported to be the Drosophila homolog of vertebrate Cenp-E. In this study, we show that cenp-metaΔ mutant neuroblasts arrest in mitosis when treated with colchicine. cenp-metaΔ mutant cells display a mitotic delay. Yet, despite the persistence of the two checkpoint proteins Mad2 and BubR1 on unattached kinetochores, these cells eventually enter anaphase and give rise to highly aneuploid daughter cells. Indeed, we find that cenp-metaΔ mutant cells display a slow but continuous degradation of cyclin B, which eventually triggers the mitotic exit observed. Thus, our data provide evidence for a role of Cenp-meta in sustaining the SAC response.

Simultaneous reduction of MAD2 and BUBR1 expression induces mitotic spindle alterations associated with p53 dependent cell cycle arrest and death

Most human tumors are characterized by aneuploidy that is believed to be the consequence of chromosomal instability (CIN). The mechanism(s) leading to aneuploidy and the pathways that allow its tolerance are not completely understood. The Spindle Assembly Checkpoint (SAC) is a cellular surveillance mechanism working during mitosis, and alterations of genes that encode components of the SAC weakening the mitotic checkpoint, induce aneuploidy by chromosome mis-segregation. We induced aneuploidy in near-diploid tumor cells by simultaneous depletion of the SAC proteins MAD2 and BUBR1 by RNA interference in the attempt to gain further insight on the cellular responses to aneuploidy. Individual reduction of MAD2 and BUBR1 protein levels caused defective mitosis and aneuploidy, while co-depletion of MAD2 and BUBR1 caused cell cycle arrest and cell death in addition to aneuploidy. The simultaneous reduction of the two SAC proteins induced high percentage of hyperdiploid cells and p53 stabilization suggesting that hyperdiploidy could activate a p53 controlled pathway. The results indicate that p53 is required to induce cell cycle arrest and cell death when the mitotic checkpoint is strongly perturbed, thereby preventing aneuploid cell propagation.

A microtubule-associated zinc finger protein, BuGZ, regulates mitotic chromosome alignment by ensuring Bub3 stability and kinetochore targeting

Equal chromosome segregation requires proper assembly of many proteins, including Bub3, onto kinetochores to promote kinetochore-microtubule interactions. By screening for mitotic regulators in the spindle envelope and matrix (Spemix), we identify a conserved Bub3 interacting and GLE-2-binding sequence (GLEBS) containing ZNF207 (BuGZ) that associates with spindle microtubules and regulates chromosome alignment. Using its conserved GLEBS, BuGZ directly binds and stabilizes Bub3. BuGZ also uses its microtubule-binding domain to enhance the loading of Bub3 to kinetochores that have assumed initial interactions with microtubules in prometaphase. This enhanced Bub3 loading is required for proper chromosome alignment and metaphase to anaphase progression. Interestingly, we show that microtubules are required for the highest kinetochore loading of Bub3, BubR1, and CENP-E during prometaphase. These findings suggest that BuGZ not only serves as a molecular chaperone for Bub3 but also enhances its loading onto kinetochores during prometaphase in a microtubule-dependent manner to promote chromosome alignment.

Mitosis as an anti-cancer drug target

Suppression of cell proliferation by targeting mitosis is one potential cancer intervention. A number of existing chemotherapy drugs disrupt mitosis by targeting microtubule dynamics. While efficacious, these drugs have limitations, i.e. neuropathy, unpredictability and development of resistance. In order to overcome these issues, a great deal of effort has been spent exploring novel mitotic targets including Polo-like kinase 1, Aurora kinases, Mps1, Cenp-E and KSP/Eg5. Here we summarize the latest developments in the discovery and clinical evaluation of new mitotic drug targets.

Loss of BubR1 acetylation causes defects in spindle assembly checkpoint signaling and promotes tumor formation

Failure of chromosome–spindle attachment and a weakened spindle assembly checkpoint lead to genetic instability and cancer in mice expressing acetylation-deficient BubR1.

BubR1 acetylation is essential in mitosis. Mice heterozygous for the acetylation-deficient BubR1 allele (K243R/+) spontaneously developed tumors with massive chromosome missegregations. K243R/+ mouse embryonic fibroblasts (MEFs) exhibited a weakened spindle assembly checkpoint (SAC) with shortened mitotic timing. The generation of the SAC signal was intact, as Mad2 localization to the unattached kinetochore (KT) was unaltered; however, because of the premature degradation of K243R-BubR1, the mitotic checkpoint complex disassociated prematurely in the nocodazole-treated condition, suggesting that maintenance of the SAC is compromised. BubR1 acetylation was also required to counteract excessive Aurora B activity at the KT for stable chromosome–spindle attachments. The association of acetylation-deficient BubR1 with PP2A-B56α phosphatase was reduced, and the phosphorylated Ndc80 at the KT was elevated in K243R/+ MEFs. In relation, there was a marked increase of micronuclei and p53 mutation was frequently detected in primary tumors of K243R/+ mice. Collectively, the combined effects of failure in chromosome–spindle attachment and weakened SAC cause genetic instability and cancer in K243R/+ mice.