Distinct Roles of the Chromosomal Passenger Complex in the Detection of and Response to Errors in Kinetochore-Microtubule Attachment

A Comprehensive In Vitro Characterization of a New Class of Indole-Based Compounds Developed as Selective Haspin Inhibitors

Haspin is an emerging, but rather unexplored, divergent kinase involved in tumor growth by regulating the mitotic phase. In this paper, the in-silico design, synthesis, and biological characterization of a new series of substituted indoles acting as potent Haspin inhibitors are reported. The synthesized derivatives have been evaluated by FRET analysis, showing very potent Haspin inhibition. Then, a comprehensive in-cell investigation highlighted compounds 47 and 60 as the most promising inhibitors. These compounds were challenged for their synergic activity with paclitaxel in 2D and 3D cellular models, demonstrating a twofold improvement of the paclitaxel antitumor activity. Compound 60 also showed remarkable selectivity when tested in a panel of 70 diverse kinases. Finally, in-silico studies provided new insight about the chemical requirements useful to develop new Haspin inhibitors. Biological results, together with the drug-likeness profile of 47 and 60, make these derivatives deserving further studies.

Sororin is an evolutionary conserved antagonist of WAPL

Cohesin mediates sister chromatid cohesion to enable chromosome segregation and DNA damage repair. To perform these functions, cohesin needs to be protected from WAPL, which otherwise releases cohesin from DNA. It has been proposed that cohesin is protected from WAPL by SORORIN. However, in vivo evidence for this antagonism is missing and SORORIN is only known to exist in vertebrates and insects. It is therefore unknown how important and widespread SORORIN's functions are. Here we report the identification of SORORIN orthologs in Schizosaccharomyces pombe (Sor1) and Arabidopsis thaliana (AtSORORIN). sor1Δ mutants display cohesion defects, which are partially alleviated by wpl1Δ. Atsororin mutant plants display dwarfism, tissue specific cohesion defects and chromosome mis-segregation. Furthermore, Atsororin mutant plants are sterile and separate sister chromatids prematurely at anaphase I. The somatic, but not the meiotic deficiencies can be alleviated by loss of WAPL. These results provide in vivo evidence for SORORIN antagonizing WAPL, reveal that SORORIN is present in organisms beyond the animal kingdom and indicate that it has acquired tissue specific functions in plants.

Molecular density-accelerated binding-site maturation underlies CENP-T-dependent kinetochore assembly

Formation of macromolecular cellular structures relies on recruitment of multiple proteins, requiring the precisely controlled pairwise binding interactions. At human kinetochores, our recent work found that the high molecular density environment enables strong bonding between the Ndc80 complex and its two binding sites at the CENP-T receptor. However, the mechanistic basis for this unusual density-dependent facilitation remains unknown. Here, using quantitative single-molecule approaches, we reveal two distinct mechanisms that drive preferential recruitment of the Ndc80 complex to higher-order structures of CENP-T, as opposed to CENP-T monomers. First, the Ndc80 binding sites within the disordered tail of the CENP-T mature over time, leading to a stronger grip on the Spc24/25 heads of the Ndc80 complexes. Second, the maturation of Ndc80 binding sites is accelerated when CENP-T molecules are clustered in close proximity. The rates of the clustering-induced maturation are remarkably different for two binding sites within CENP-T, correlating with different interfaces formed by the corresponding CENP-T sequences as they wrap around the Spc24/25 heads. The differential clustering-dependent regulation of these sites is preserved in dividing human cells, suggesting a distinct regulatory entry point to control kinetochore-microtubule interactions. The tunable acceleration of slowly maturing binding sites by a high molecular-density environment may represent a fundamental physicochemical mechanism to assist the assembly of mitotic kinetochores and other macromolecular structures.

Low tension recruits the yeast Aurora B protein Ipl1 to centromeres in metaphase

Accurate genome segregation in mitosis requires that all chromosomes are bioriented on the spindle. Cells monitor biorientation by sensing tension across sister centromeres. Chromosomes that are not bioriented have low centromere tension, which allows Aurora B (yeast Ipl1) to perform error correction that locally loosens kinetochore-microtubule attachments to allow detachment of microtubules and fresh attempts at achieving biorientation. However, it is not known whether low tension recruits Aurora B to centromeres or, alternatively, whether low tension directly activates Aurora B already localized at centromeres. In this work, we experimentally induced low tension in metaphase Saccharomyces cerevisiae yeast cells, then monitored Ipl1 localization. We find low tension recruits Ipl1 to centromeres. Furthermore, low tension-induced Ipl1 recruitment depended on Bub1, which is known to provide a binding site for Ipl1. In contrast, Top2, which can also recruit Ipl1 to centromeres, was not required. Our results demonstrate cells are sensitive to low tension at centromeres and respond by actively recruiting Ip1l for error correction.

TMED3 promotes the development of malignant melanoma by targeting CDCA8 and regulating PI3K/Akt pathway

BACKGROUND

Transmembrane emp24 domain containing (TMED) proteins are known to play pivotal roles in normal development, but have been reported to be implicated in pancreatic disease, immune system disorders, and cancers. As far as TMED3 is concerned, its roles in cancers are controversial. However, evidence describing TMED3 in the context of malignant melanoma (MM) is scarce.

RESULTS

In this study, we characterized the functional significance of TMED3 in MM and identified TMED3 as a tumor-promoting factor in MM development. Depletion of TMED3 arrested the development of MM in vitro and in vivo. Mechanistically, we found that TMED3 could interact with Cell division cycle associated 8 (CDCA8). Knocking down CDCA8 suppressed cell events associated with MM development. On the contrary, elevating CDCA8 augmented cell viability and motility and even reversed the inhibitory effects of TMED3 knockdown on MM development. On the other hand, we found that the levels of P-Akt and P-PI3K were decreased in response to TMED3 downregulation, which was partially abolished following SC79 treatment. Thus, our suspicion was that TMED3 exacerbates MM progression via PI3K/Akt pathway. More notably, previously decreased P-Akt and P-PI3K in TMED3-depleted cells were rescued after overexpressing CDCA8. Also, previously impaired cell events due to CDCA8 depletion were ameliorated after SC79 addition, implying that TMED3 regulates PI3K-AKT pathway via CDCA8, thereby promoting MM development.

CONCLUSIONS

Collectively, this study established the link between TMED3 and MM, and provides a potential therapeutic intervention for patients with MM harboring abundant TMED3.

Three interacting regions of the Ndc80 and Dam1 complexes support microtubule tip-coupling under load

Accurate mitosis requires kinetochores to make persistent, load-bearing attachments to dynamic microtubule tips, thereby coupling chromosome movements to tip growth and shortening. This tip-coupling behavior depends on the conserved Ndc80 complex and, in budding yeast, on the Dam1 complex, which bind each other directly via three distinct interacting regions. The functional relevance of these multiple interactions was mysterious. Here we show that interactions between two of these regions support the high rupture strengths that occur when applied force is rapidly increased and also support the stability of tip-coupling when force is held constant over longer durations. The contribution of either of these two regions to tip-coupling is reduced by phosphorylation by Aurora B kinase. The third interaction region makes no apparent contribution to rupture strength, but its phosphorylation by Aurora B kinase specifically decreases the long-term stability of tip-coupling. The specific reduction of long-term stability relative to short-term strength might have important implications for mitotic error correction.

The TFIIH complex is required to establish and maintain mitotic chromosome structure

Condensins compact chromosomes to promote their equal segregation during mitosis, but the mechanism of condensin engagement with and action on chromatin is incompletely understood. Here, we show that the general transcription factor TFIIH complex is continuously required to establish and maintain a compacted chromosome structure in transcriptionally silent Xenopus egg extracts. Inhibiting the DNA-dependent ATPase activity of the TFIIH complex subunit XPB rapidly and reversibly induces a complete loss of chromosome structure and prevents the enrichment of condensins I and II, but not topoisomerase II, on chromatin. In addition, inhibiting TFIIH prevents condensation of both mouse and Xenopus nuclei in Xenopus egg extracts, which suggests an evolutionarily conserved mechanism of TFIIH action. Reducing nucleosome density through partial histone depletion restores chromosome structure and condensin enrichment in the absence of TFIIH activity. We propose that the TFIIH complex promotes mitotic chromosome condensation by dynamically altering the chromatin environment to facilitate condensin loading and condensin-dependent loop extrusion.

Highway to hell-thy meiotic divisions: Chromosome passenger complex functions driven by microtubules: CPC interactions with both the chromosomes and microtubules are important for spindle assembly and function: CPC interactions with both the chromosomes and microtubules are important for spindle assembly and function

The chromosome passenger complex (CPC) localizes to chromosomes and microtubules, sometimes simultaneously. The CPC also has multiple domains for interacting with chromatin and microtubules. Interactions between the CPC and both the chromatin and microtubules is important for spindle assembly and error correction. Such dual chromatin-microtubule interactions may increase the concentration of the CPC necessary for efficient kinase activity while also making it responsive to specific conditions or structures in the cell. CPC-microtubule dependent functions are considered in the context of the first meiotic division. Acentrosomal spindle assembly is a process that depends on transfer of the CPC from the chromosomes to the microtubules. Furthermore, transfer to the microtubules is not only to position the CPC for a later role in cytokinesis; metaphase I error correction and subsequent bi-orientation of bivalents may depend on microtubule associated CPC interacting with the kinetochores.

Aurora B Tension Sensing Mechanisms in the Kinetochore Ensure Accurate Chromosome Segregation

The accurate segregation of chromosomes is essential for the survival of organisms and cells. Mistakes can lead to aneuploidy, tumorigenesis and congenital birth defects. The spindle assembly checkpoint ensures that chromosomes properly align on the spindle, with sister chromatids attached to microtubules from opposite poles. Here, we review how tension is used to identify and selectively destabilize incorrect attachments, and thus serves as a trigger of the spindle assembly checkpoint to ensure fidelity in chromosome segregation. Tension is generated on properly attached chromosomes as sister chromatids are pulled in opposing directions but resisted by centromeric cohesin. We discuss the role of the Aurora B kinase in tension-sensing and explore the current models for translating mechanical force into Aurora B-mediated biochemical signals that regulate correction of chromosome attachments to the spindle.

A CLK1-KKT2 Signaling Pathway Regulating Kinetochore Assembly in Trypanosoma brucei

During mitosis, eukaryotic cells must duplicate and separate their chromosomes in a precise and timely manner. The apparatus responsible for this is the kinetochore, which is a large protein structure that links chromosomal DNA and spindle microtubules to facilitate chromosome alignment and segregation. The proteins that comprise the kinetochore in the protozoan parasite Trypanosoma brucei are divergent from yeast and mammals and comprise an inner kinetochore complex composed of 24 distinct proteins (KKT1 to KKT23, KKT25) that include four protein kinases, CLK1 (KKT10), CLK2 (KKT19), KKT2, and KKT3. We recently reported the identification of a specific trypanocidal inhibitor of T. brucei CLK1, an amidobenzimidazole, AB1. We now show that chemical inhibition of CLK1 with AB1 impairs inner kinetochore recruitment and compromises cell cycle progression, leading to cell death. Here, we show that KKT2 is a substrate for CLK1 and identify phosphorylation of S508 by CLK1 to be essential for KKT2 function and for kinetochore assembly. Additionally, KKT2 protein kinase activity is required for parasite proliferation but not for assembly of the inner kinetochore complex. We also show that chemical inhibition of the aurora kinase AUK1 does not affect CLK1 phosphorylation of KKT2, indicating that AUK1 and CLK1 are in separate regulatory pathways. We propose that CLK1 is part of a divergent signaling cascade that controls kinetochore function via phosphorylation of the inner kinetochore protein kinase KKT2. IMPORTANCE In eukaryotic cells, kinetochores are large protein complexes that link chromosomes to dynamic microtubule tips, ensuring proper segregation and genomic stability during cell division. Several proteins tightly coordinate kinetochore functions, including the protein kinase aurora kinase B. The kinetochore has diverse evolutionary roots. For example, trypanosomatids, single-cell parasitic protozoa that cause several neglected tropical diseases, possess a unique repertoire of kinetochore components whose regulation during the cell cycle remains unclear. Here, we shed light on trypanosomatid kinetochore biology by showing that the protein kinase CLK1 coordinates the assembly of the inner kinetochore by phosphorylating one of its components, KKT2, allowing the timely spatial recruitment of the rest of the kinetochore proteins and posterior attachment to microtubules in a process that is aurora kinase B independent.

Permitted and restricted steps of human kinetochore assembly in mitotic cell extracts

Mitotic kinetochores assemble via the hierarchical recruitment of numerous cytosolic components to the centromere region of each chromosome. However, how these orderly and localized interactions are achieved without spurious macromolecular assemblies forming from soluble kinetochore components in the cell cytosol remains poorly understood. We developed assembly assays to monitor the recruitment of green fluorescent protein-tagged recombinant proteins and native proteins from human cell extracts to inner kinetochore components immobilized on microbeads. In contrast to prior work in yeast and Xenopus egg extracts, we find that human mitotic cell extracts fail to support de novo assembly of microtubule-binding subcomplexes. A subset of interactions, such as those between CENP-A-containing nucleosomes and CENP-C, are permissive under these conditions. However, the subsequent phospho-dependent binding of the Mis12 complex is less efficient, whereas recruitment of the Ndc80 complex is blocked, leading to weak microtubule-binding activity of assembled particles. Using molecular variants of the Ndc80 complex, we show that auto-inhibition of native Ndc80 complex restricts its ability to bind to the CENP-T/W complex, whereas inhibition of the Ndc80 microtubule binding is driven by a different mechanism. Together, our work reveals regulatory mechanisms that guard against the spurious formation of cytosolic microtubule-binding kinetochore particles.

A degron-based strategy reveals new insights into Aurora B function in C. elegans

The widely conserved kinase Aurora B regulates important events during cell division. Surprisingly, recent work has uncovered a few functions of Aurora-family kinases that do not require kinase activity. Thus, understanding this important class of cell cycle regulators will require strategies to distinguish kinase-dependent from independent functions. Here, we address this need in C. elegans by combining germline-specific, auxin-induced Aurora B (AIR-2) degradation with the transgenic expression of kinase-inactive AIR-2. Through this approach, we find that kinase activity is essential for AIR-2’s major meiotic functions and also for mitotic chromosome segregation. Moreover, our analysis revealed insight into the assembly of the ring complex (RC), a structure that is essential for chromosome congression in C. elegans oocytes. AIR-2 localizes to chromosomes and recruits other components to form the RC. However, we found that while kinase-dead AIR-2 could load onto chromosomes, other components were not recruited. This failure in RC assembly appeared to be due to a loss of RC SUMOylation, suggesting that there is crosstalk between SUMOylation and phosphorylation in building the RC and implicating AIR-2 in regulating the SUMO pathway in oocytes. Similar conditional depletion approaches may reveal new insights into other cell cycle regulators.

During cell division, chromosomes must be accurately partitioned to ensure the proper distribution of genetic material. In mitosis, chromosomes are duplicated once and then divided once, generating daughter cells with the same amount of genetic material as the original cell. Conversely, during meiosis chromosomes are duplicated once and divided twice, to cut the chromosome number in half to generate eggs and sperm. One important protein that is required for both mitotic and meiotic chromosome segregation is the kinase Aurora B, which phosphorylates a variety of other cell division proteins. However, previous research has shown that some kinases have functions that are independent of their ability to phosphorylate other proteins. Thus, fully understanding how Aurora B regulates cell division requires methods to test whether its various functions require kinase activity. We designed and implemented such a strategy in the model organism C. elegans, by depleting Aurora B from meiotically and mitotically-dividing cells, leaving in place a kinase-inactive version. This work has lent insight into how Aurora B regulates cell division in C. elegans, and also serves as a proof of principle for our approach, which can now be applied to study other essential cell division kinases.

Borealin directs recruitment of the CPC to oocyte chromosomes and movement to the microtubules

The chromosomes in the oocytes of many animals appear to promote bipolar spindle assembly. In Drosophila oocytes, spindle assembly requires the chromosome passenger complex (CPC), which consists of INCENP, Borealin, Survivin, and Aurora B. To determine what recruits the CPC to the chromosomes and its role in spindle assembly, we developed a strategy to manipulate the function and localization of INCENP, which is critical for recruiting the Aurora B kinase. We found that an interaction between Borealin and the chromatin is crucial for the recruitment of the CPC to the chromosomes and is sufficient to build kinetochores and recruit spindle microtubules. HP1 colocalizes with the CPC on the chromosomes and together they move to the spindle microtubules. We propose that the Borealin interaction with HP1 promotes the movement of the CPC from the chromosomes to the microtubules. In addition, within the central spindle, rather than at the centromeres, the CPC and HP1 are required for homologous chromosome bi-orientation.

The right place at the right time: Aurora B kinase localization to centromeres and kinetochores

The fidelity of chromosome segregation during mitosis is intimately linked to the function of kinetochores, which are large protein complexes assembled at sites of centromeric heterochromatin on mitotic chromosomes. These key "orchestrators" of mitosis physically connect chromosomes to spindle microtubules and transduce forces through these connections to congress chromosomes and silence the spindle assembly checkpoint. Kinetochore-microtubule attachments are highly regulated to ensure that incorrect attachments are not prematurely stabilized, but instead released and corrected. The kinase activity of the centromeric protein Aurora B is required for kinetochore-microtubule destabilization during mitosis, but how the kinase acts on outer kinetochore substrates to selectively destabilize immature and erroneous attachments remains debated. Here, we review recent literature that sheds light on how Aurora B kinase is recruited to both centromeres and kinetochores and discuss possible mechanisms for how kinase interactions with substrates at distinct regions of mitotic chromosomes are regulated.

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.

Aurora B kinase is recruited to multiple discrete kinetochore and centromere regions in human cells

Aurora B kinase has a critical role in regulating attachments between kinetochores and spindle microtubules during mitosis. Early in mitosis, kinase activity at kinetochores is high to promote attachment turnover, and in later mitosis, activity decreases to ensure attachment stabilization. Aurora B localizes prominently to inner centromeres, and a population of the kinase is also detected at kinetochores. How Aurora B is recruited to and evicted from these regions to regulate kinetochore-microtubule attachments remains unclear. Here, we identified and investigated discrete populations of Aurora B at the centromere/kinetochore region. An inner centromere pool is recruited by Haspin phosphorylation of histone H3, and a kinetochore-proximal outer centromere pool is recruited by Bub1 phosphorylation of histone H2A. Finally, a third pool resides ~20 nm outside of the inner kinetochore protein CENP-C in early mitosis and does not require either the Bub1/pH2A/Sgo1 or Haspin/pH3 pathway for localization or activity. Our results suggest that distinct molecular pathways are responsible for Aurora B recruitment to centromeres and kinetochores.

Centromere-localized Aurora B kinase is required for the fidelity of chromosome segregation

Aurora B kinase plays an essential role in chromosome bi-orientation, which is a prerequisite for equal segregation of chromosomes during mitosis. However, it remains largely unclear whether centromere-localized Aurora B is required for faithful chromosome segregation. Here we show that histone H3 Thr-3 phosphorylation (H3pT3) and H2A Thr-120 phosphorylation (H2ApT120) can independently recruit Aurora B. Disrupting H3pT3-mediated localization of Aurora B at the inner centromere impedes the decline in H2ApT120 during metaphase and causes H2ApT120-dependent accumulation of Aurora B at the kinetochore-proximal centromere. Consequently, silencing of the spindle assembly checkpoint (SAC) is delayed, whereas the fidelity of chromosome segregation is negligibly affected. Further eliminating an H2ApT120-dependent pool of Aurora B restores proper timing for SAC silencing but increases chromosome missegregation. Our data indicate that H2ApT120-mediated localization of Aurora B compensates for the loss of an H3pT3-dependent pool of Aurora B to correct improper kinetochore-microtubule attachments. This study provides important insights into how centromeric Aurora B regulates SAC and kinetochore attachment to microtubules to ensure error-free chromosome segregation.

The whole is greater than the sum of its parts: at the intersection of order, disorder, and kinetochore function

The kinetochore (KT) field has matured tremendously since Earnshaw first identified CENP-A, CENP-B, and CENP-C [1,2]. In the past 35 years, the accumulation of knowledge has included: defining the parts list, identifying epistatic networks of interdependence within the parts list, understanding the spatial organization of subcomplexes into a massive structure - hundreds of megadaltons in size, and dissecting the functions of the KT in its entirety as well as of its individual parts. Like nearly all cell and molecular biology fields, the structure-function paradigm has been foundational to advances in the KT field. A point nicely highlighted by the fact that we are at the precipice of the in vitro reconstitution of a functional KT holo complex. Yet conventional notions of structure cannot provide a complete picture of the KT especially since it contains an abundance of unstructured or intrinsically disordered constituents. The combination of structured and disordered proteins within the KT results in an assembled system that is functionally greater than the sum of its parts.

The Borealin dimerization domain interacts with Sgo1 to drive Aurora B-mediated spindle assembly

The chromosomal passenger complex (CPC), which includes the kinase Aurora B, is a master regulator of meiotic and mitotic processes that ensure the equal segregation of chromosomes. Sgo1 is thought to play a major role in the recruitment of the CPC to chromosomes, but the molecular mechanism and contribution of Sgo1-dependent CPC recruitment is currently unclear. Using Xenopus egg extracts and biochemical reconstitution, we found that Sgo1 interacts directly with the dimerization domain of the CPC subunit Borealin. Borealin and the PP2A phosphatase complex can bind simultaneously to the coiled-coil domain of Sgo1, suggesting that Sgo1 can integrate Aurora B and PP2A activities to modulate Aurora B substrate phosphorylation. A Borealin mutant that specifically disrupts the Sgo1–Borealin interaction results in defects in CPC chromosomal recruitment and Aurora B–dependent spindle assembly, but not in spindle assembly checkpoint signaling at unattached kinetochores. These findings establish a direct molecular connection between Sgo1 and the CPC and have major implications for the different functions of Aurora B, which promote the proper interaction between spindle microtubules and chromosomes.

Enrichment of Aurora B kinase at the inner kinetochore controls outer kinetochore assembly

Outer kinetochore assembly enables chromosome attachment to microtubules and spindle assembly checkpoint (SAC) signaling in mitosis. Aurora B kinase controls kinetochore assembly by phosphorylating the Mis12 complex (Mis12C) subunit Dsn1. Current models propose Dsn1 phosphorylation relieves autoinhibition, allowing Mis12C binding to inner kinetochore component CENP-C. Using Xenopus laevis egg extracts and biochemical reconstitution, we found that autoinhibition of the Mis12C by Dsn1 impedes its phosphorylation by Aurora B. Our data indicate that the INCENP central region increases Dsn1 phosphorylation by enriching Aurora B at inner kinetochores, close to CENP-C. Furthermore, centromere-bound CENP-C does not exchange in mitosis, and CENP-C binding to the Mis12C dramatically increases Dsn1 phosphorylation by Aurora B. We propose that the coincidence of Aurora B and CENP-C at inner kinetochores ensures the fidelity of kinetochore assembly. We also found that the central region is required for the SAC beyond its role in kinetochore assembly, suggesting that kinetochore enrichment of Aurora B promotes the phosphorylation of other kinetochore substrates.

Switching of INCENP paralogs controls transitions in mitotic chromosomal passenger complex functions

A single inner centromere protein (INCENP) found throughout eukaryotes modulates Aurora B kinase activity and chromosomal passenger complex (CPC) localization, which is essential for timely mitotic progression. It has been proposed that INCENP might act as a rheostat to regulate Aurora B activity through mitosis, with successively higher activity threshold levels for chromosome alignment, the spindle checkpoint, anaphase spindle transfer and finally spindle elongation and cytokinesis. It remains mechanistically unclear how this would be achieved. Here, we reveal that the urochordate, Oikopleura dioica, possesses two INCENP paralogs, which display distinct localizations and subfunctionalization in order to complete M-phase. INCENPa was localized on chromosome arms and centromeres by prometaphase, and modulated Aurora B activity to mediate H3S10/S28 phosphorylation, chromosome condensation, spindle assembly and transfer of the CPC to the central spindle. Polo-like kinase (Plk1) recruitment to CDK1 phosphorylated INCENPa was crucial for INCENPa-Aurora B enrichment on centromeres. The second paralog, INCENPb was enriched on centromeres from prometaphase, and relocated to the central spindle at anaphase onset. In the absence of INCENPa, meiotic spindles failed to form, and homologous chromosomes did not segregate. INCENPb was not required for early to mid M-phase events but became essential for the activity and localization of Aurora B on the central spindle and midbody during cytokinesis in order to allow abscission to occur. Together, our results demonstrate that INCENP paralog switching on centromeres modulates Aurora B kinase localization, thus chronologically regulating CPC functions during fast embryonic divisions in the urochordate O. dioica. Abbreviations: CCAN: constitutive centromere-associated network; CENPs: centromere proteins; cmRNA: capped messenger RNA; CPC: chromosomal passenger complex; INCENP: inner centromere protein; Plk1: polo-like kinase 1; PP1: protein phosphatase 1; PP2A: protein phosphatase 2A; SAC: spindle assembly checkpoint; SAH: single α-helix domain.

Microtubule-interfering agents, spindle defects, and interkinetochore tension

Microtubule-interfering agents have been very useful both as biological tools in studying mitosis and as chemotherapeutic agents against cancer. It remains poorly understood how these agents converge on the spindle assembly checkpoint (SAC) to halt mitotic progression, while inhibiting microtubule dynamics by different mechanisms. Cells arrested at mitosis by various microtubule-interfering agents exhibit strikingly different defects in the mitotic spindle. However, all the arrested cells possess the 3F3/2 phosphoepitope at the sister kinetochores of chromosomes, indicating the decrease of tension across the paired kinetochores. In addition, microtubule-interfering agents result in a comparable reduction in the distance between sister kinetochores, suggesting that these agents decrease interkinetochore tension to similar degrees. Here, we discuss recent progress that suggests impairment of kinetochore-microtubule attachment and reduction of interkinetochore tension as common mechanisms underlying the persistent SAC activation in response to diverse microtubule-interfering agents.

Cell Division Cycle Associated 8 Is a Key Regulator of Tamoxifen Resistance in Breast Cancer

Purpose

Breast cancer (BC) is one of the most common malignancies globally, and millions of women worldwide are diagnosed with BC every year. Up to 70% of BC patients are estrogen receptor (ER)-positive. Numerous studies have shown that tamoxifen has a significant therapeutic effect on both primary and metastatic ER-positive BC patients. Although tamoxifen is currently one of the most successful therapeutic agents for BC, a significant proportion of patients will eventually become resistant to tamoxifen, leading to tumor recurrence and metastasis. Knowledge about the development of tamoxifen resistance in BC patients is still limited.

Methods

We applied a loss-and-gain method to study the biological functional role of cell division cycle associated 8 (CDCA8) in tamoxifen resistance in BC cells.

Results

We found that CDCA8 was significantly elevated in tamoxifen-resistant BC cells. Knockdown of CDCA8 expression significantly inhibited the proliferation of tamoxifen-resistant BC cells and reduced their resistance to tamoxifen. In contrast, overexpression of CDCA8 promoted the growth of tamoxifen-sensitive BC cells and induced their resistance to tamoxifen.

Conclusion

In this study, we reported that CDCA8 is a key regulator of tamoxifen resistance in BC, suggesting that CDCA8 may serve as a potential therapeutic target for BC treatment.

The COMA complex interacts with Cse4 and positions Sli15/Ipl1 at the budding yeast inner kinetochore

Kinetochores are macromolecular protein complexes at centromeres that ensure accurate chromosome segregation by attaching chromosomes to spindle microtubules and integrating safeguard mechanisms. The inner kinetochore is assembled on CENP-A nucleosomes and has been implicated in establishing a kinetochore-associated pool of Aurora B kinase, a chromosomal passenger complex (CPC) subunit, which is essential for chromosome biorientation. By performing crosslink-guided in vitro reconstitution of budding yeast kinetochore complexes we showed that the Ame1/Okp1^CENP-U/Q heterodimer, which forms the COMA complex with Ctf19/Mcm21^CENP-P/O, selectively bound Cse4^CENP-A nucleosomes through the Cse4 N-terminus. The Sli15/Ipl1^{INCENP/Aurora-B} core-CPC interacted with COMA in vitro through the Ctf19 C-terminus whose deletion affected chromosome segregation fidelity in Sli15 wild-type cells. Tethering Sli15 to Ame1/Okp1 rescued synthetic lethality upon Ctf19 depletion in a Sli15 centromere-targeting deficient mutant. This study shows molecular characteristics of the point-centromere kinetochore architecture and suggests a role for the Ctf19 C-terminus in mediating CPC-binding and accurate chromosome segregation.

Aurora B-INCENP Localization at Centromeres/Inner Kinetochores Is Required for Chromosome Bi-orientation in Budding Yeast

For proper chromosome segregation in mitosis, sister kinetochores must interact with microtubules from opposite spindle poles (chromosome bi-orientation) [1, 2]. To promote bi-orientation, Aurora B kinase disrupts aberrant kinetochore-microtubule interactions [3, 4, 5, 6]. It has long been debated how Aurora B halts this action when bi-orientation is established and tension is applied across sister kinetochores. A popular explanation for it is that, upon bi-orientation, sister kinetochores are pulled in opposite directions, stretching the outer kinetochores [7, 8] and moving Aurora B substrates away from Aurora-B-localizing sites at centromeres (spatial separation model) [3, 5, 9]. This model predicts that Aurora B localization at centromeres is required for bi-orientation. However, this notion was challenged by the observation that Bir1 (yeast survivin), which recruits Ipl1-Sli15 (yeast Aurora B-INCENP) to centromeres, can become dispensable for bi-orientation [10]. This raised the possibility that Aurora B localization at centromeres is dispensable for bi-orientation. Alternatively, there might be a Bir1-independent mechanism for recruiting Ipl1-Sli15 to centromeres or inner kinetochores [5, 9]. Here, we show that the COMA inner kinetochore sub-complex physically interacts with Sli15, recruits Ipl1-Sli15 to the inner kinetochore, and promotes chromosome bi-orientation, independently of Bir1, in budding yeast. Moreover, using an engineered recruitment of Ipl1-Sli15 to the inner kinetochore when both Bir1 and COMA are defective, we show that localization of Ipl1-Sli15 at centromeres or inner kinetochores is required for bi-orientation. Our results give important insight into how Aurora B disrupts kinetochore-microtubule interaction in a tension-dependent manner to promote chromosome bi-orientation.

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The COMA inner kinetochore sub-complex facilitates chromosome bi-orientation

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COMA physically interacts with Sli15 and recruits Ipl1-Sli15 to the inner kinetochore

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This function of COMA is independent of Bir1 and its role supporting robust cohesion

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Localizing Ipl1-Sli15 at centromeres/inner kinetochores is crucial for bi-orientation

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.

Aurora-PLK1 cascades as key signaling modules in the regulation of mitosis

Mitosis is controlled by reversible protein phosphorylation involving specific kinases and phosphatases. A handful of major mitotic protein kinases, such as the cyclin B-CDK1 complex, the Aurora kinases, and Polo-like kinase 1 (PLK1), cooperatively regulate distinct mitotic processes. Research has identified proteins and mechanisms that integrate these kinases into signaling cascades that guide essential mitotic events. These findings have important implications for our understanding of the mechanisms of mitotic regulation and may advance the development of novel antimitotic drugs. We review collected evidence that in vertebrates, the Aurora kinases serve as catalytic subunits of distinct complexes formed with the four scaffold proteins Bora, CEP192, INCENP, and TPX2, which we deem "core" Aurora cofactors. These complexes and the Aurora-PLK1 cascades organized by Bora, CEP192, and INCENP control crucial aspects of mitosis and all pathways of spindle assembly. We compare the mechanisms of Aurora activation in relation to the different spindle assembly pathways and draw a functional analogy between the CEP192 complex and the chromosomal passenger complex that may reflect the coevolution of centrosomes, kinetochores, and the actomyosin cleavage apparatus. We also analyze the roles and mechanisms of Aurora-PLK1 signaling in the cell and centrosome cycles and in the DNA damage response.

CDCA8 expression and its clinical relevance in patients with bladder cancer

Cell division cycle associated 8 (CDCA8) overexpression is detected in various malignant tumors and closely associated with tumor growth. However, the correlations of CDCA8 expression with clinicopathological factors and prognosis of bladder cancer (BC) remain unclear. The purpose of this study was to identify the expression of CDCA8 and its clinical relevance in BC patients.GEO datasets were employed to obtain CDCA8 expression data and its clinical information in BC samples. Real-time PCR (RT-PCR) was performed to detect the expression of CDCA8 in BC and the adjacent normal tissues. Nonpaired t test was used to statistically analyze the difference between the 2 groups. Cox univariable and multivariable analyses of overall survival (OS) and cancer specific survival (CSS) among BC patients were performed. Biological processes or signaling pathways that might mediate the activity of CDCA8 in BC were analyzed.CDCA8 levels were significantly higher in BC (8.870 ± 0.08281 vs 7.472 ± 0.07035, P < .0001). CDCA8 expression was significantly associated with tumor progression (P = .001), T stage (P < .0001), N stage (P = .013), and grade (P < .0001). Higher expression of CDCA8 predicted poor cancer-specific survival (P < .0001, HR = 0.2752, 95% CI:0.1364-0.5554) and overall survival (P < .0001, HR = 0.4270, 95% CI: 0.2630-0.6930) in patients with BC. Cox univariable and multivariable analyses showed that intravesical therapy, N stage and progression were the independent influence factors of overall survival among bladder cancer patients, CDCA8 expression, tumor grade and progression were the independent influence factors of cancer specific survival among bladder cancer patients. The results of GSEA indicated that CDCA8-regulated gene sets associated with spermatogenesis, G2M checkpoint, E2F targets, Myc targets, mTORC1 signaling, mitotic spindle angiogenesis, PI3K/AKT/mTOR signaling, cholesterol homeostasis and glycolysis. Finally, RT-PCR results confirmed that CDCA8 expression was upregulated in BC (P = .0039).CDCA8 is overexpressed in BC and its high levels are correlated with poor clinicopathological features of BC patients. Therefore, CDCA8 may act as a novel prognostic marker and therapeutical target in the diagnosis and treatment of patients with BC.

Measuring NDC80 binding reveals the molecular basis of tension-dependent kinetochore-microtubule attachments

Proper kinetochore-microtubule attachments, mediated by the NDC80 complex, are required for error-free chromosome segregation. Erroneous attachments are corrected by the tension dependence of kinetochore-microtubule interactions. Here, we present a method, based on fluorescence lifetime imaging microscopy and Förster resonance energy transfer, to quantitatively measure the fraction of NDC80 complexes bound to microtubules at individual kinetochores in living human cells. We found that NDC80 binding is modulated in a chromosome autonomous fashion over prometaphase and metaphase, and is predominantly regulated by centromere tension. We show that this tension dependency requires phosphorylation of the N-terminal tail of Hec1, a component of the NDC80 complex, and the proper localization of Aurora B kinase, which modulates NDC80 binding. Our results lead to a mathematical model of the molecular basis of tension-dependent NDC80 binding to kinetochore microtubules in vivo.

When a cell divides, each new cell that forms needs to contain a complete set of DNA, which is stored in structures called chromosomes. So first, the chromosomes duplicate, and the two copies are held together. A protein structure known as a kinetochore then forms on each copy of the chromosome. The kinetochores act as a pair of hands that pull the chromosome copies apart and toward opposite sides of the dividing cell. They do this by grabbing protein ‘ropes’ called microtubules that extend toward the chromosomes from each side of the cell.

Kinetochores grip the microtubule ropes more tightly when the connection is under greater tension. This helps the kinetochores to remain attached to the microtubules that will separate the chromosome copies while releasing the microtubules that would pull both copies to the same side. Previous research has shown that hundreds of finger-like structures made out of a protein group called NDC80 extend from each kinetochore ‘hand’ and attach to the microtubules. What remains a mystery is whether and how the NDC80 fingers grip the microtubules more tightly when tension is greater in cells.

Yoo et al. developed a technique for counting how many of the available NDC80 fingers of a single kinetochore are attached to microtubules within a living human cell. The new technique combines genetic engineering, fluorescence imaging and statistical methods to quantify the attachment of NDC80 to microtubules over time and space.

Yoo et al. found that more NDC80 bound to microtubules when there was greater tension. This relationship between binding and tension depends on an enzyme called Aurora B, which modifies the tip of each NDC80 finger and consequently changes the binding of NDC80 to microtubules. Yoo et al. further showed that Aurora B needs to be properly placed between two kinetochore hands to make NDC80-microtubule binding dependent on tension. Without this tension dependency, chromosomes could segregate unevenly into the newly formed cells – a problem that can lead to cancer, infertility and birth defects. The results presented by Yoo et al. therefore expand our understanding of how these diseases originate and may eventually help researchers to develop new treatments for them.

Understanding eukaryotic chromosome segregation from a comparative biology perspective

A long-appreciated variation in fundamental cell biological processes between different species is becoming increasingly tractable due to recent breakthroughs in whole-genome analyses and genome editing techniques. However, the bulk of our mechanistic understanding in cell biology continues to come from just a few well-established models. In this Review, I use the highly diverse strategies of chromosome segregation in eukaryotes as an instrument for a more general discussion on phenotypic variation, possible rules underlying its emergence and its utility in understanding conserved functional relationships underlying this process. Such a comparative approach, supported by modern molecular biology tools, might provide a wider, holistic view of biology that is difficult to achieve when concentrating on a single experimental system.

Aurora A Kinase Function at Kinetochores

One of the most important regulatory aspects of chromosome segregation is the ability of kinetochores to precisely control their attachment strength to spindle microtubules. Central to this regulation is Aurora B, a mitotic kinase that phosphorylates kinetochore substrates to promote microtubule turnover. A critical target of Aurora B is the kinetochore protein Ndc80/Hec1, which is a component of the NDC80 complex, the primary force-transducing link between kinetochores and microtubules. Although Aurora B is regarded as the "master regulator" of kinetochore-microtubule attachment, it is becoming clear that this kinase is not solely responsible for phosphorylating Hec1 and other kinetochore substrates to facilitate microtubule turnover. In particular, there is growing evidence that Aurora A kinase, whose activities at spindle poles have been extensively described, has additional roles at kinetochores in regulating the kinetochore-microtubule interface.

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.