Molecular architecture of the kinetochore-microtubule interface

Centromeric localization of αKNL2 and CENP-C proteins in plants depends on their centromere-targeting domain and DNA-binding regions

In eukaryotes, accurate chromosome segregation during cell division relies on the centromeric histone H3 variant, CENH3. Our previous work identified KINETOCHORE NULL2 (αKNL2) as a plant CENH3 assembly factor, which contains a centromere-targeting motif, CENPC-k, analogous to the CENPC motif found in CENP-C. We also demonstrated that αKNL2 can bind DNA in vitro in a sequence-independent manner, without the involvement of its CENPC-k motif. In this study, we show that the CENPC-k and CENPC motifs alone are insufficient for centromere targeting in Nicotiana benthamiana and Arabidopsis thaliana. In silico analysis identified adjacent DNA-binding regions near the CENPC-k and CENPC motifs, suggesting their role in centromeric DNA interaction. We further demonstrated that protein fragments containing these motifs effectively target centromeres. Deletion of these DNA-binding domains reduced the centromeric localization of αKNL2-C, while fusing CENPC-k to the non-specific DNA-binding domain of histone-like nucleoid structuring protein from Escherichia coli successfully targeted it to centromeres. Our findings suggest that the centromeric targeting of αKNL2 and CENP-C proteins relies on the CENPC-k/CENPC motifs, and that their sequence-independent DNA-binding activity enhances their centromere anchoring. These insights into the mechanisms of αKNL2 and CENP-C targeting may facilitate the engineering of kinetochore structures by directing chromatin-modifying proteins to centromeres.

CENcyclopedia: Dynamic Landscape of Kinetochore Architecture Throughout the Cell Cycle

The kinetochore, an intricate macromolecular protein complex located on chromosomes, plays a pivotal role in orchestrating chromosome segregation. It functions as a versatile platform for microtubule assembly, diligently monitors microtubule binding fidelity, and acts as a force coupler. Comprising over 100 distinct proteins, many of which exist in multiple copies, the kinetochore's composition dynamically changes throughout the cell cycle, responding to specific timing and conditions. This dynamicity is important for establishing functional kinetochores, yet the regulatory mechanisms of these dynamics have largely remained elusive. In this study, we employed advanced quantitative immunofluorescence techniques to meticulously chart the dynamics of kinetochore protein levels across the cell cycle. These findings offer a comprehensive view of the dynamic landscape of kinetochore architecture, shedding light on the detailed mechanisms of microtubule interaction and the nuanced characteristics of kinetochore proteins. This study significantly advances our understanding of the molecular coordination underlying chromosome segregation.

Novel role of zinc-finger protein 518 in heterochromatin formation on α-satellite DNA

Aneuploidy is caused by chromosomal missegregation and is frequently observed in cancers and hematological diseases. Therefore, it is important to understand the molecular mechanisms underlying chromosomal segregation. The centromere's intricate structure is crucial for proper chromosome segregation, with heterochromatin at the pericentromeric α-satellites playing a key role. However, the mechanism targeting heterochromatin to pericentromeres remains elusive. This study identifies a novel mechanism involving two homologous zinc-finger proteins ZNF518A and ZNF518B in human pericentric heterochromatin formation. Our investigation demonstrated that ZNF518s localize to the centromere via centromere protein B (CENP-B). Moreover, ZNF518s interact with heterochromatin protein 1 (HP1) and H3K9 methyltransferase G9A, recruiting the heterochromatin components to pericentromeres. We found that centromeric histone H3K9 trimethylation was diminished in the absence of ZNF518s when another H3K9 methyltransferase, SUV39H1, was depleted. In somatic cells, the ZNF518s-G9a axis is not the principal pathway for heterochromatin formation but plays a supplementary role. Furthermore, ZNF518s are involved in histone H3K9 trimethylation at ectopic sites, indicating their broad role in heterochromatin establishment. Consequently, we propose that ZNF518s participate in the mechanism underlying heterochromatin establishment at pericentromeres. Our findings shed light on the novel mechanism underlying pericentromeric heterochromatin formation, highlighting the central role of ZNF518 in this process.

ImmunoCellCycle-ID - a high-precision immunofluorescence-based method for cell cycle identification

The cell cycle is a fundamental process essential for cell proliferation, differentiation and development. It consists of four major phases: G1, S, G2 and M. These phases collectively drive the reproductive cycle and are meticulously regulated by various proteins that play crucial roles in both the prevention and progression of cancer. Traditional methods for studying these functions, such as flow cytometry, require a substantial number of cells to ensure accuracy. In this study, we have developed a user-friendly immunofluorescence-based method for identifying cell cycle stages, providing single-cell resolution and precise identification of G1, early/mid S, late S, early/mid G2, late G2, and each sub-stage of the M phase using fluorescence microscopy called ImmunoCellCycle-ID. This method provides high-precision cell cycle identification and can serve as an alternative to, or in combination with, traditional flow cytometry to dissect detailed sub-stages of the cell cycle in a variety of cell lines.

Kinetochore dynein is sufficient to biorient chromosomes and remodel the outer kinetochore

Multiple microtubule-directed activities concentrate on mitotic chromosomes to ensure their faithful segregation. These include couplers and dynamics regulators localized at the kinetochore, the microtubule interface built on centromeric chromatin, as well as motor proteins recruited to kinetochores and chromatin. Here, we describe an in vivo approach in the C. elegans one-cell embryo in which removal of the major microtubule-directed activities on mitotic chromosomes is compared to the selective presence of individual activities. Our approach reveals that the kinetochore dynein module, comprised of cytoplasmic dynein and its kinetochore-specific adapters, is sufficient to biorient chromosomes; by contrast, this module is unable to support congression. In coordination with orientation, the dynein module directs removal of outermost kinetochore components, including dynein itself, independently of the other microtubule-directed activities and kinetochore-localized protein phosphatase 1. These observations indicate that the kinetochore dynein module is sufficient to biorient chromosomes and to direct remodeling of the outer kinetochore in a microtubule attachment state-sensitive manner.

Plant kinetochore complex: composition, function, and regulation

The kinetochore complex, an important protein assembly situated on the centromere, plays a pivotal role in chromosome segregation during cell division. Like in animals and fungi, the plant kinetochore complex is important for maintaining chromosome stability, regulating microtubule attachment, executing error correction mechanisms, and participating in signaling pathways to ensure accurate chromosome segregation. This review summarizes the composition, function, and regulation of the plant kinetochore complex, emphasizing the interactions of kinetochore proteins with centromeric DNAs (cenDNAs) and RNAs (cenRNAs). Additionally, the applications of the centromeric histone H3 variant (the core kinetochore protein CENH3, first identified as CENP-A in mammals) in the generation of ploidy-variable plants and synthesis of plant artificial chromosomes (PACs) are discussed. The review serves as a comprehensive roadmap for researchers delving into plant kinetochore exploration, highlighting the potential of kinetochore proteins in driving technological innovations in synthetic genomics and plant biotechnology.

Recent insights into the causes and consequences of chromosome mis-segregation

Mitotic cells face the challenging task of ensuring accurate and equal segregation of their duplicated, condensed chromosomes between the nascent daughter cells. Errors in the process result in chromosome missegregation, a significant consequence of which is the emergence of aneuploidy-characterized by an imbalance in chromosome number-and the associated phenomenon of chromosome instability (CIN). Aneuploidy and CIN are common features of cancer, which leverages them to promote genome heterogeneity and plasticity, thereby facilitating rapid tumor evolution. Recent research has provided insights into how mitotic errors shape cancer genomes by inducing both numerical and structural chromosomal changes that drive tumor initiation and progression. In this review, we survey recent findings regarding the mitotic causes and consequences of aneuploidy. We discuss new findings into the types of chromosome segregation errors that lead to aneuploidy and novel pathways that protect genome integrity during mitosis. Finally, we describe new developments in our understanding of the immediate consequences of chromosome mis-segregation on the genome stability of daughter cells.

An egg-sabotaging mechanism drives non-Mendelian transmission in mice

Selfish genetic elements drive in meiosis to distort their transmission ratio and increase their representation in gametes, violating Mendel's law of segregation. The two established paradigms for meiotic drive, gamete killing and biased segregation, are fundamentally different. In gamete killing, typically observed with male meiosis, selfish elements sabotage gametes that do not contain them. By contrast, killing is predetermined in female meiosis, and selfish elements bias their segregation to the single surviving gamete (i.e., the egg in animal meiosis). Here, we show that a selfish element on mouse chromosome 2, Responder to drive 2 (R2d2), drives using a hybrid mechanism in female meiosis, incorporating elements of both killing and biased segregation. We propose that if R2d2 is destined for the polar body, it manipulates segregation to sabotage the egg by causing aneuploidy, which is subsequently lethal in the embryo, ensuring that surviving progeny preferentially contain R2d2. In heterozygous females, R2d2 orients randomly on the metaphase spindle but lags during anaphase and preferentially remains in the egg, regardless of its initial orientation. Thus, the egg genotype is either euploid with R2d2 or aneuploid with both homologs of chromosome 2, with only the former generating viable embryos. Consistent with this model, R2d2 heterozygous females produce eggs with increased aneuploidy for chromosome 2, increased embryonic lethality, and increased transmission of R2d2. In contrast to typical gamete killing of sisters produced as daughter cells in a single meiosis, R2d2 prevents production of any viable gametes from meiotic divisions in which it should have been excluded from the egg.

Protein lysine crotonylation in cellular processions and disease associations

Protein lysine crotonylation (Kcr) is one conserved form of posttranslational modifications of proteins, which plays an important role in a series of cellular physiological and pathological processes. Lysine ε-amino groups are the primary sites of such modification, resulting in four-carbon planar lysine crotonylation that is structurally and functionally distinct from the acetylation of these residues. High levels of Kcr modifications have been identified on both histone and non-histone proteins. The present review offers an update on the research progression regarding protein Kcr modifications in biomedical contexts and provides a discussion of the mechanisms whereby Kcr modification governs a range of biological processes. In addition, given the importance of protein Kcr modification in disease onset and progression, the potential viability of Kcr regulators as therapeutic targets is elucidated.

ImmunoCellCycle-ID: A high-precision immunofluorescence-based method for cell cycle identification

The cell cycle is a fundamental process essential for cell proliferation, differentiation, and development. It consists of four major phases: G1, S, G2, and M. These phases collectively drive the reproductive cycle and are meticulously regulated by various proteins that play critical roles in both the prevention and progression of cancer. Traditional methods for studying these functions, such as flow cytometry, require a substantial number of cells to ensure accuracy. In this study, we have developed a user-friendly, immunofluorescence-based method for identifying cell cycle stages, providing single-cell resolution and precise identification of G1, early S, late S, early G2, late G2, and each sub-stage of the M phase using fluorescence microscopy. This method provides high-precision cell cycle identification and can serve as an alternative to, or in combination with, traditional flow cytometry to dissect detailed substages of the cell cycle in a variety of cell lines.

β-TrCP-Mediated Proteolysis of Mis18β Prevents Mislocalization of CENP-A and Chromosomal Instability

Restricting the localization of evolutionarily conserved histone H3 variant CENP-A to the centromere is essential to prevent chromosomal instability (CIN), an important hallmark of cancers. Overexpressed CENP-A mislocalizes to non-centromeric regions and contributes to CIN in yeast, flies, and human cells. Centromeric localization of CENP-A is facilitated by the interaction of Mis18β with CENP-A specific chaperone HJURP. Cellular levels of Mis18β are regulated by β-transducin repeat containing protein (β-TrCP), an F-box protein of SCF (Skp1, Cullin, F-box) E3-ubiquitin ligase complex. Here, we show that defects in β-TrCP-mediated proteolysis of Mis18β contributes to the mislocalization of endogenous CENP-A and CIN in a triple-negative breast cancer (TNBC) cell line, MDA-MB-231. CENP-A mislocalization in β-TrCP depleted cells is dependent on high levels of Mis18β as depletion of Mis18β suppresses mislocalization of CENP-A in these cells. Consistent with these results, endogenous CENP-A is mislocalized in cells overexpressing Mis18β alone. In summary, our results show that β-TrCP-mediated degradation of Mis18β prevents mislocalization of CENP-A and CIN. We propose that deregulated expression of Mis18β may be one of the key mechanisms that contributes to chromosome segregation defects in cancers.

Organization of microtubule plus-end dynamics by phase separation in mitosis

In eukaryotes, microtubule polymers are essential for cellular plasticity and fate decisions. End-binding (EB) proteins serve as scaffolds for orchestrating microtubule polymer dynamics and are essential for cellular dynamics and chromosome segregation in mitosis. Here, we show that EB1 forms molecular condensates with TIP150 and MCAK through liquid-liquid phase separation to compartmentalize the kinetochore-microtubule plus-end machinery, ensuring accurate kinetochore-microtubule interactions during chromosome segregation in mitosis. Perturbation of EB1-TIP150 polymer formation by a competing peptide prevents phase separation of the EB1-mediated complex and chromosome alignment at the metaphase equator in both cultured cells and Drosophila embryos. Lys220 of EB1 is dynamically acetylated by p300/CBP-associated factor in early mitosis, and persistent acetylation at Lys220 attenuates phase separation of the EB1-mediated complex, dissolves droplets in vitro, and harnesses accurate chromosome segregation. Our data suggest a novel framework for understanding the organization and regulation of eukaryotic spindle for accurate chromosome segregation in mitosis.

Aurora B promotes the CENP-T-CENP-W interaction to guide accurate chromosome segregation in mitosis

Accurate chromosome segregation in mitosis depends on kinetochores that connect centromeric chromatin to spindle microtubules. Centromeres are captured by individual microtubules via a kinetochore constitutive centromere-associated network (CCAN) during chromosome segregation. CCAN contains 16 subunits, including CENP-W and CENP-T. However, the molecular recognition and mitotic regulation of the CCAN assembly remain elusive. Here, we revealed that CENP-W binds to the histone fold domain and an uncharacterized N-terminal region of CENP-T. Aurora B phosphorylates CENP-W at threonine 60, which enhances the interaction between CENP-W and CENP-T to ensure robust metaphase chromosome alignment and accurate chromosome segregation in mitosis. These findings delineate a conserved signaling cascade that integrates protein phosphorylation with CCAN integrity for the maintenance of genomic stability.

Meiosis-specific decoupling of the pericentromere from the kinetochore

The primary constriction site of the M-phase chromosome is an established marker for the kinetochore position, often used to determine the karyotype of each species. Underlying this observation is the concept that the kinetochore is spatially linked with the pericentromere where sister-chromatids are most tightly cohered. Here, we found an unconventional pericentromere specification with sister chromatids mainly cohered at a chromosome end, spatially separated from the kinetochore in Peromyscus mouse oocytes. This distal locus enriched cohesin protectors, such as the Chromosomal Passenger Complex (CPC) and PP2A, at a higher level compared to its centromere/kinetochore region, acting as the primary site for sister-chromatid cohesion. Chromosomes with the distal cohesion site exhibited enhanced cohesin protection at anaphase I compared to those without it, implying that these distal cohesion sites may have evolved to ensure sister-chromatid cohesion during meiosis. In contrast, mitotic cells enriched CPC only near the kinetochore and the distal locus was not cohered between sister chromatids, suggesting a meiosis-specific mechanism to protect cohesin at this distal locus. We found that this distal locus corresponds to an additional centromeric satellite block, located far apart from the centromeric satellite block that builds the kinetochore. Several Peromyscus species carry chromosomes with two such centromeric satellite blocks. Analyses on three Peromyscus species revealed that the internal satellite consistently assembles the kinetochore in both mitosis and meiosis, whereas the distal satellite selectively enriches cohesin protectors in meiosis to promote sister-chromatid cohesion at that site. Thus, our study demonstrates that pericentromere specification is remarkably flexible and can control chromosome segregation in a cell-type and context dependent manner.

Disruption of Plasmodium falciparum kinetochore proteins destabilises the nexus between the centrosome equivalent and the mitotic apparatus

Plasmodium falciparum is the causative agent of malaria and remains a pathogen of global importance. Asexual blood stage replication, via a process called schizogony, is an important target for the development of new antimalarials. Here we use ultrastructure-expansion microscopy to probe the organisation of the chromosome-capturing kinetochores in relation to the mitotic spindle, the centriolar plaque, the centromeres and the apical organelles during schizont development. Conditional disruption of the kinetochore components, PfNDC80 and PfNuf2, is associated with aberrant mitotic spindle organisation, disruption of the centromere marker, CENH3 and impaired karyokinesis. Surprisingly, kinetochore disruption also leads to disengagement of the centrosome equivalent from the nuclear envelope. Severing the connection between the nucleus and the apical complex leads to the formation of merozoites lacking nuclei. Here, we show that correct assembly of the kinetochore/spindle complex plays a previously unrecognised role in positioning the nascent apical complex in developing P. falciparum merozoites.

Thermal Proteome Profiling Reveals Insight to Antiproliferative and Pro-Apoptotic Effects of Lagunamide A in the Modulation of DNA Damage Repair

Lagunamide A is a biologically active natural product with a yet unidentified molecular mode of action. Cellular studies revealed that lagunamide A is a potent inhibitor of cancer cell proliferation, promotes apoptosis and causes mitochondrial dysfunction. To decipher the cellular mechanism responsible for these effects, we utilized thermal protein profiling (TPP) and identified EYA3 as a stabilized protein in cells upon lagunamide A treatment. EYA3, involved in the DNA damage repair process, was functionally investigated via siRNA based knockdown studies and corresponding effects of lagunamide A on DNA repair were confirmed. Furthermore, we showed that lagunamide A sensitized tumor cells to treatment with the drug doxorubicin highlighting a putative therapeutic strategy.

Protein crotonylation: Basic research and clinical diseases

Crotonylation is an importantly conserved post-translational modification, which is completely different from acetylation. In recent years, it has been confirmed that crotonylation occurs on histone and non-histone. Crotonylated Histone primarily affects gene expression through transcriptional regulation, while non-histone Crotonylation mainly regulates protein functions including protein activity, localization, and stability, as well as protein-protein interactions. The change in protein expression and function will affect the physiological process of cells and even cause disease. Reviewing previous studies, this article summarizes the mechanisms of histone and non-histone crotonylation in regulating diseases and cellular physiological processes to explore the possibility of precise regulation of crotonylation sites as potential targets for disease treatment.

Molecular evolution of the mammalian kinetochore complex

Mammalian centromeres are satellite-rich chromatin domains that serve as sites for kinetochore complex assembly. Centromeres are highly variable in sequence and satellite organization across species, but the processes that govern the co-evolutionary dynamics between rapidly evolving centromeres and their associated kinetochore proteins remain poorly understood. Here, we pursue a course of phylogenetic analyses to investigate the molecular evolution of the complete kinetochore complex across primate and rodent species with divergent centromere repeat sequences and features. We show that many protein components of the core centromere associated network (CCAN) harbor signals of adaptive evolution, consistent with their intimate association with centromere satellite DNA and roles in the stability and recruitment of additional kinetochore proteins. Surprisingly, CCAN and outer kinetochore proteins exhibit comparable rates of adaptive divergence, suggesting that changes in centromere DNA can ripple across the kinetochore to drive adaptive protein evolution within distant domains of the complex. Our work further identifies kinetochore proteins subject to lineage-specific adaptive evolution, including rapidly evolving proteins in species with centromere satellites characterized by higher-order repeat structure and lacking CENP-B boxes. Thus, features of centromeric chromatin beyond the linear DNA sequence may drive selection on kinetochore proteins. Overall, our work spotlights adaptively evolving proteins with diverse centromere-associated functions, including centromere chromatin structure, kinetochore protein assembly, kinetochore-microtubule association, cohesion maintenance, and DNA damage response pathways. These adaptively evolving kinetochore protein candidates present compelling opportunities for future functional investigations exploring how their concerted changes with centromere DNA ensure the maintenance of genome stability.

DNAJC9 prevents CENP-A mislocalization and chromosomal instability by maintaining the fidelity of histone supply chains

The centromeric histone H3 variant CENP-A is overexpressed in many cancers. The mislocalization of CENP-A to noncentromeric regions contributes to chromosomal instability (CIN), a hallmark of cancer. However, pathways that promote or prevent CENP-A mislocalization remain poorly defined. Here, we performed a genome-wide RNAi screen for regulators of CENP-A localization which identified DNAJC9, a J-domain protein implicated in histone H3-H4 protein folding, as a factor restricting CENP-A mislocalization. Cells lacking DNAJC9 exhibit mislocalization of CENP-A throughout the genome, and CIN phenotypes. Global interactome analysis showed that DNAJC9 depletion promotes the interaction of CENP-A with the DNA-replication-associated histone chaperone MCM2. CENP-A mislocalization upon DNAJC9 depletion was dependent on MCM2, defining MCM2 as a driver of CENP-A deposition at ectopic sites when H3-H4 supply chains are disrupted. Cells depleted for histone H3.3, also exhibit CENP-A mislocalization. In summary, we have defined novel factors that prevent mislocalization of CENP-A, and demonstrated that the integrity of H3-H4 supply chains regulated by histone chaperones such as DNAJC9 restrict CENP-A mislocalization and CIN.

Structure of the human KMN complex and implications for regulation of its assembly

Biorientation of chromosomes during cell division is necessary for precise dispatching of a mother cell's chromosomes into its two daughters. Kinetochores, large layered structures built on specialized chromosome loci named centromeres, promote biorientation by binding and sensing spindle microtubules. One of the outer layer main components is a ten-subunit assembly comprising Knl1C, Mis12C and Ndc80C (KMN) subcomplexes. The KMN is highly elongated and docks on kinetochores and microtubules through interfaces at its opposite extremes. Here, we combine cryogenic electron microscopy reconstructions and AlphaFold2 predictions to generate a model of the human KMN that reveals all intra-KMN interfaces. We identify and functionally validate two interaction interfaces that link Mis12C to Ndc80C and Knl1C. Through targeted interference experiments, we demonstrate that this mutual organization strongly stabilizes the KMN assembly. Our work thus reports a comprehensive structural and functional analysis of this part of the kinetochore microtubule-binding machinery and elucidates the path of connections from the chromatin-bound components to the force-generating components.

NMR study of the structure and dynamics of the BRCT domain from the kinetochore protein KKT4

KKT4 is a multi-domain kinetochore protein specific to kinetoplastids, such as Trypanosoma brucei. It lacks significant sequence similarity to known kinetochore proteins in other eukaryotes. Our recent X-ray structure of the C-terminal region of KKT4 shows that it has a tandem BRCT (BRCA1 C Terminus) domain fold with a sulfate ion bound in a typical binding site for a phosphorylated serine or threonine. Here we present the 1H, 13C and 15N resonance assignments for the BRCT domain of KKT4 (KKT4463-645) from T. brucei. We show that the BRCT domain can bind phosphate ions in solution using residues involved in sulfate ion binding in the X-ray structure. We have used these assignments to characterise the secondary structure and backbone dynamics of the BRCT domain in solution. Mutating the residues involved in phosphate ion binding in T. brucei KKT4 BRCT results in growth defects confirming the importance of the BRCT phosphopeptide-binding activity in vivo. These results may facilitate rational drug design efforts in the future to combat diseases caused by kinetoplastid parasites.

Contribution of CENP-F to FOXM1-Mediated Discordant Centromere and Kinetochore Transcriptional Regulation

Proper chromosome segregation is required to ensure chromosomal stability. The centromere (CEN) is a unique chromatin domain defined by CENP-A and is responsible for recruiting the kinetochore (KT) during mitosis, ultimately regulating microtubule spindle attachment and mitotic checkpoint function. Upregulation of many CEN/KT genes is commonly observed in cancer. Here, we show that although FOXM1 occupies promoters of many CEN/KT genes with MYBL2, FOXM1 overexpression alone is insufficient to drive the FOXM1-correlated transcriptional program. CENP-F is canonically an outer kinetochore component; however, it functions with FOXM1 to coregulate G2/M transcription and proper chromosome segregation. Loss of CENP-F results in altered chromatin accessibility at G2/M genes and reduced FOXM1-MBB complex formation. We show that coordinated CENP-FFOXM1 transcriptional regulation is a cancer-specific function. We observe a small subset of CEN/KT genes including CENP-C, that are not regulated by FOXM1. Upregulation of CENP-C in the context of CENP-A overexpression leads to increased chromosome missegregation and cell death suggesting that escape of CENP-C from FOXM1 regulation is a cancer survival mechanism. Together, we show that FOXM1 and CENP-F coordinately regulate G2/M genes, and this coordination is specific to a subset of genes to allow for maintenance of chromosome instability levels and subsequent cell survival.

Mitosis detection, fast and slow: Robust and efficient detection of mitotic figures

Counting of mitotic figures is a fundamental step in grading and prognostication of several cancers. However, manual mitosis counting is tedious and time-consuming. In addition, variation in the appearance of mitotic figures causes a high degree of discordance among pathologists. With advances in deep learning models, several automatic mitosis detection algorithms have been proposed but they are sensitive to domain shift often seen in histology images. We propose a robust and efficient two-stage mitosis detection framework, which comprises mitosis candidate segmentation (Detecting Fast) and candidate refinement (Detecting Slow) stages. The proposed candidate segmentation model, termed EUNet, is fast and accurate due to its architectural design. EUNet can precisely segment candidates at a lower resolution to considerably speed up candidate detection. Candidates are then refined using a deeper classifier network, EfficientNet-B7, in the second stage. We make sure both stages are robust against domain shift by incorporating domain generalization methods. We demonstrate state-of-the-art performance and generalizability of the proposed model on the three largest publicly available mitosis datasets, winning the two mitosis domain generalization challenge contests (MIDOG21 and MIDOG22). Finally, we showcase the utility of the proposed algorithm by processing the TCGA breast cancer cohort (1,124 whole-slide images) to generate and release a repository of more than 620K potential mitotic figures (not exhaustively validated).

Sgo1 interacts with CENP-A to guide accurate chromosome segregation in mitosis

Shugoshin-1 (Sgo1) is necessary for maintaining sister centromere cohesion and ensuring accurate chromosome segregation during mitosis. It has been reported that the localization of Sgo1 at the centromere is dependent on Bub1-mediated phosphorylation of histone H2A at T120. However, it remains uncertain whether other centromeric proteins play a role in regulating the localization and function of Sgo1 during mitosis. Here, we show that CENP-A interacts with Sgo1 and determines the localization of Sgo1 to the centromere during mitosis. Further biochemical characterization revealed that lysine and arginine residues in the C-terminal domain of Sgo1 are critical for binding CENP-A. Interestingly, the replacement of these basic amino acids with acidic amino acids perturbed the localization of Sgo1 and Aurora B to the centromere, resulting in aberrant chromosome segregation and premature chromatid separation. Taken together, these findings reveal a previously unrecognized but direct link between Sgo1 and CENP-A in centromere plasticity control and illustrate how the Sgo1-CENP-A interaction guides accurate cell division.

Fluorescence complementation-based FRET imaging reveals centromere assembly dynamics

Visualization of specific molecules and their assembly in real time and space is essential to delineate how cellular dynamics and signaling circuit are orchestrated during cell division cycle. Our recent studies reveal structural insights into human centromere-kinetochore core CCAN complex. Here we introduce a method for optically imaging trimeric and tetrameric protein interactions at nanometer spatial resolution in live cells using fluorescence complementation-based Förster resonance energy transfer (FC-FRET). Complementary fluorescent protein molecules were first used to visualize dimerization followed by FRET measurements. Using FC-FRET, we visualized centromere CENP-SXTW tetramer assembly dynamics in live cells, and dimeric interactions between CENP-TW dimer and kinetochore protein Spc24/25 dimer in dividing cells. We further delineated the interactions of monomeric CENP-T with Spc24/25 dimer in dividing cells. Surprisingly, our analyses revealed critical role of CDK1 kinase activity in the initial recruitment of Spc24/25 by CENP-T. However, interactions between CENP-T and Spc24/25 during chromosome segregation is independent of CDK1. Thus, FC-FRET provides a unique approach to delineate spatiotemporal dynamics of trimerized and tetramerized proteins at nanometer scale and establishes a platform to report the precise regulation of multimeric protein interactions in space and time in live cells.

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

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

The evolution and formation of centromeric repeats analysis in Vitis vinifera

MAIN CONCLUSION

Six grape centromere-specific markers for cytogenetics were mined by combining genetic and immunological assays, and the possible evolution mechanism of centromeric repeats was analyzed. Centromeric histone proteins are functionally conserved; however, centromeric repetitive DNA sequences may represent considerable diversity in related species. Therefore, studying the characteristics and structure of grape centromere repeat sequences contributes to a deeper understanding of the evolutionary process of grape plants, including their origin and mechanisms of polyploidization. Plant centromeric regions are mainly composed of repetitive sequences, including SatDNA and transposable elements (TE). In this research, the characterization of centromere sequences in the whole genome of grapevine (Vitis vinifera L.) has been conducted. Five centromeric tandem repeat sequences (Vv1, Vv2, Vv5, Vv6, and Vv8) and one long terminal repeat (LTR) sequence Vv24 were isolated. These sequences had different centromeric distributions, which indicates that grape centromeric sequences may undergo rapid evolution. The existence of extrachromosomal circular DNA (eccDNA) and gene expression in CenH3 subdomain region may provide various potential mechanisms for the generation of new centromeric regions.

DNA strand breaks at centromeres: Friend or foe?

Centromeres are large structural regions in the genomic DNA, which are essential for accurately transmitting a complete set of chromosomes to daughter cells during cell division. In humans, centromeres consist of highly repetitive α-satellite DNA sequences and unique epigenetic components, forming large proteinaceous structures required for chromosome segregation. Despite their biological importance, there is a growing body of evidence for centromere breakage across the cell cycle, including periods of quiescence. In this review, we provide an up-to-date examination of the distinct centromere environments at different stages of the cell cycle, highlighting their plausible contribution to centromere breakage. Additionally, we explore the implications of these breaks on centromere function, both in terms of negative consequences and potential positive effects.

Emerging roles of DNA repair factors in the stability of centromeres

Satellite DNA sequences are an integral part of centromeres, regions critical for faithful segregation of chromosomes during cell division. Because of their complex repetitive structure, satellite DNA may act as a barrier to DNA replication and other DNA based transactions ultimately resulting in chromosome breakage. Over the past two decades, several DNA repair proteins have been shown to bind and function at centromeres. While the importance of these repair factors is highlighted by various structural and numerical chromosome aberrations resulting from their inactivation, their roles in helping to maintain genome stability by solving the intrinsic difficulties of satellite DNA replication or promoting their repair are just starting to emerge. In this review, we summarize the current knowledge on the role of DNA repair and DNA damage response proteins in maintaining the structure and function of centromeres in different contexts. We also report the recent connection between the roles of specific DNA repair factors at these genomic loci with age-related increase of chromosomal instability under physiological and pathological conditions.

An egg sabotaging mechanism drives non-Mendelian transmission in mice

During meiosis, homologous chromosomes segregate so that alleles are transmitted equally to haploid gametes, following Mendel's Law of Segregation. However, some selfish genetic elements drive in meiosis to distort the transmission ratio and increase their representation in gametes. The established paradigms for drive are fundamentally different for female vs male meiosis. In male meiosis, selfish elements typically kill gametes that do not contain them. In female meiosis, killing is predetermined, and selfish elements bias their segregation to the single surviving gamete (i.e., the egg in animal meiosis). Here we show that a selfish element on mouse chromosome 2, R2d2, drives using a hybrid mechanism in female meiosis, incorporating elements of both male and female drivers. If R2d2 is destined for the polar body, it manipulates segregation to sabotage the egg by causing aneuploidy that is subsequently lethal in the embryo, so that surviving progeny preferentially contain R2d2. In heterozygous females, R2d2 orients randomly on the metaphase spindle but lags during anaphase and preferentially remains in the egg, regardless of its initial orientation. Thus, the egg genotype is either euploid with R2d2 or aneuploid with both homologs of chromosome 2, with only the former generating viable embryos. Consistent with this model, R2d2 heterozygous females produce eggs with increased aneuploidy for chromosome 2, increased embryonic lethality, and increased transmission of R2d2. In contrast to a male meiotic driver, which kills its sister gametes produced as daughter cells in the same meiosis, R2d2 eliminates "cousins" produced from meioses in which it should have been excluded from the egg.

Pentachloronitrobenzene disturbed murine ventricular wall development by inhibiting cardiomyocyte proliferation via Hec1 downregulation

Exposure to the organochlorine fungicide pentachloronitrobenzene (PCNB) causes developmental abnormalities, including cardiac malformation. However, the molecular mechanism of PCNB cardiotoxicity remains elusive. We found that oral administration of PCNB to pregnant mice induced a hypoplastic wall with significant thinning of the compact myocardium in the developing hearts. PCNB significantly downregulates the expression of Hec1, a member of the NDC80 kinetochore complex, resulting in aberrant spindles, chromosome missegregation and an arrest in cardiomyocyte proliferation. Cardiac-specific ablation of Hec1 sharply inhibits cardiomyocyte proliferation, leading to thinning of the compact myocardium and embryonic lethality. Mechanistically, we found that activating transcription factor 3 (ATF3) transactivates Hec1 expression. Either HEC1 or ATF3 overexpression significantly rescues mitotic defects and restore the decreased proliferative ability of cardiomyocytes caused by PCNB exposure. Our findings highlight that maternal PCNB exposure disrupts embryonic cardiac function by inhibiting cardiomyocyte proliferation and interfering with ventricular wall development, partially attributed to the downregulation of the Atf3-Hec1 axis.

NuSAP regulates microtubule flux and Kif2A localization to ensure accurate chromosome congression

Precise chromosome congression and segregation requires the proper assembly of a steady-state metaphase spindle, which is dynamic and maintained by continuous microtubule flux. NuSAP is a microtubule-stabilizing and -bundling protein that promotes chromosome-dependent spindle assembly. However, its function in spindle dynamics remains unclear. Here, we demonstrate that NuSAP regulates the metaphase spindle length control. Mechanistically, NuSAP facilitates kinetochore capture and spindle assembly by promoting Eg5 binding to microtubules. It also prevents excessive microtubule depolymerization through interaction with Kif2A, which reduces Kif2A spindle-pole localization. NuSAP is phosphorylated by Aurora A at Ser-240 during mitosis, and this phosphorylation promotes its interaction with Kif2A on the spindle body and reduces its localization with the spindle poles, thus maintaining proper spindle microtubule flux. NuSAP knockout resulted in the formation of shorter spindles with faster microtubule flux and chromosome misalignment. Taken together, we uncover that NuSAP participates in spindle assembly, dynamics, and metaphase spindle length control through the regulation of microtubule flux and Kif2A localization.

Neocortical neurogenesis in development and evolution-Human-specific features

In this review, we focus on human-specific features of neocortical neurogenesis in development and evolution. Two distinct topics will be addressed. In the first section, we discuss the expansion of the neocortex during human evolution and concentrate on the human-specific gene ARHGAP11B. We review the ability of ARHGAP11B to amplify basal progenitors and to expand a primate neocortex. We discuss the contribution of ARHGAP11B to neocortex expansion during human evolution and its potential implications for neurodevelopmental disorders and brain tumors. We then review the action of ARHGAP11B in mitochondria as a regulator of basal progenitor metabolism, and how it promotes glutaminolysis and basal progenitor proliferation. Finally, we discuss the increase in cognitive performance due to the ARHGAP11B-induced neocortical expansion. In the second section, we focus on neocortical development in modern humans versus Neanderthals. Specifically, we discuss two recent findings pointing to differences in neocortical neurogenesis between these two hominins that are due to a small number of amino acid substitutions in certain key proteins. One set of such proteins are the kinetochore-associated proteins KIF18a and KNL1, where three modern human-specific amino acid substitutions underlie the prolongation of metaphase during apical progenitor mitosis. This prolongation in turn is associated with an increased fidelity of chromosome segregation to the apical progenitor progeny during modern human neocortical development, with implications for the proper formation of radial units. Another such key protein is transketolase-like 1 (TKTL1), where a single modern human-specific amino acid substitution endows TKTL1 with the ability to amplify basal radial glia, resulting in an increase in upper-layer neuron generation. TKTL1's ability is based on its action in the pentose phosphate pathway, resulting in increased fatty acid synthesis. The data imply greater neurogenesis during neocortical development in modern humans than Neanderthals due to TKTL1, in particular in the developing frontal lobe.

Higher-order protein assembly controls kinetochore formation

To faithfully segregate chromosomes during vertebrate mitosis, kinetochore-microtubule interactions must be restricted to a single site on each chromosome. Prior work on pair-wise kinetochore protein interactions has been unable to identify the mechanisms that prevent outer kinetochore formation in regions with a low density of CENP-A nucleosomes. To investigate the impact of higher-order assembly on kinetochore formation, we generated oligomers of the inner kinetochore protein CENP-T using two distinct, genetically engineered systems in human cells. Although individual CENP-T molecules interact poorly with outer kinetochore proteins, oligomers that mimic centromeric CENP-T density trigger the robust formation of functional, cytoplasmic kinetochore-like particles. Both in cells and in vitro, each molecule of oligomerized CENP-T recruits substantially higher levels of outer kinetochore components than monomeric CENP-T molecules. Our work suggests that the density dependence of CENP-T restricts outer kinetochore recruitment to centromeres, where densely packed CENP-A recruits a high local concentration of inner kinetochore proteins.

Contribution of CENP-F to FOXM1-mediated discordant centromere and kinetochore transcriptional regulation

Proper chromosome segregation is required to ensure genomic and chromosomal stability. The centromere is a unique chromatin domain present throughout the cell cycle on each chromosome defined by the CENP-A nucleosome. Centromeres (CEN) are responsible for recruiting the kinetochore (KT) during mitosis, ultimately regulating spindle attachment and mitotic checkpoint function. Upregulation of many genes that encode the CEN/KT proteins is commonly observed in cancer. Here, we show although that FOXM1 occupies the promoters of many CEN/KT genes with MYBL2, occupancy is insufficient alone to drive the FOXM1 correlated transcriptional program. We show that CENP-F, a component of the outer kinetochore, functions with FOXM1 to coregulate G2/M transcription and proper chromosome segregation. Loss of CENP-F results in alteration of chromatin accessibility at G2/M genes, including CENP-A, and leads to reduced FOXM1-MBB complex formation. The FOXM1-CENP-F transcriptional coordination is a cancer-specific function. We observed that a few CEN/KT genes escape FOXM1 regulation such as CENP-C which when upregulated with CENP-A, leads to increased chromosome misegregation and cell death. Together, we show that the FOXM1 and CENP-F coordinately regulate G2/M gene expression, and this coordination is specific to a subset of genes to allow for proliferation and maintenance of chromosome stability for cancer cell survival.

CENP-A and CENP-B collaborate to create an open centromeric chromatin state

Centromeres are epigenetically defined via the presence of the histone H3 variant CENP-A. Contacting CENP-A nucleosomes, the constitutive centromere associated network (CCAN) and the kinetochore assemble, connecting the centromere to spindle microtubules during cell division. The DNA-binding centromeric protein CENP-B is involved in maintaining centromere stability and, together with CENP-A, shapes the centromeric chromatin state. The nanoscale organization of centromeric chromatin is not well understood. Here, we use single-molecule fluorescence and cryoelectron microscopy (cryoEM) to show that CENP-A incorporation establishes a dynamic and open chromatin state. The increased dynamics of CENP-A chromatin create an opening for CENP-B DNA access. In turn, bound CENP-B further opens the chromatin fiber structure and induces nucleosomal DNA unwrapping. Finally, removal of CENP-A increases CENP-B mobility in cells. Together, our studies show that the two centromere-specific proteins collaborate to reshape chromatin structure, enabling the binding of centromeric factors and establishing a centromeric chromatin state.

RZZ-Spindly and CENP-E form an integrated platform to recruit dynein to the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin-7) and dynein-dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we show that when corona assembly is prevented through MPS1 inhibition, CENP-E is absolutely required to retain RZZS at kinetochores. An RZZS phosphomimetic mutant bypasses this requirement, demonstrating the existence of a second receptor for polymeric RZZS. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

Dynamic phosphorylation of CENP-N by CDK1 guides accurate chromosome segregation in mitosis

In mitosis, accurate chromosome segregation depends on the kinetochore, a supermolecular machinery that couples dynamic spindle microtubules to centromeric chromatin. However, the structure-activity relationship of the constitutive centromere-associated network (CCAN) during mitosis remains uncharacterized. Building on our recent cryo-electron microscopic analyses of human CCAN structure, we investigated how dynamic phosphorylation of human CENP-N regulates accurate chromosome segregation. Our mass spectrometric analyses revealed mitotic phosphorylation of CENP-N by CDK1, which modulates the CENP-L-CENP-N interaction for accurate chromosome segregation and CCAN organization. Perturbation of CENP-N phosphorylation is shown to prevent proper chromosome alignment and activate the spindle assembly checkpoint. These analyses provide mechanistic insight into a previously undefined link between the centromere-kinetochore network and accurate chromosome segregation.

Quantitative Super-Resolution Microscopy Reveals the Relationship between CENP-A Stoichiometry and Centromere Physical Size

Centromeric chromatin is thought to play a critical role in ensuring the faithful segregation of chromosomes during mitosis. However, our understanding of this role is presently limited by our poor understanding of the structure and composition of this unique chromatin. The nucleosomal variant, CENP-A, localizes to narrow regions within the centromere, where it plays a major role in centromeric function, effectively serving as a platform on which the kinetochore is assembled. Previous work found that, within a given cell, the number of microtubules within kinetochores is essentially unchanged between CENP-A-localized regions of different physical sizes. However, it is unknown if the amount of CENP-A is also unchanged between these regions of different sizes, which would reflect a strict structural correspondence between these two key characteristics of the centromere/kinetochore assembly. Here, we used super-resolution optical microscopy to image and quantify the amount of CENP-A and DNA within human centromere chromatin. We found that the amount of CENP-A within CENP-A domains of different physical sizes is indeed the same. Further, our measurements suggest that the ratio of CENP-A- to H3-containing nucleosomes within these domains is between 8:1 and 11:1. Thus, our results not only identify an unexpectedly strict relationship between CENP-A and microtubules stoichiometries but also that the CENP-A centromeric domain is almost exclusively composed of CENP-A nucleosomes.

Reconstructing disease dynamics for mechanistic insights and clinical benefit

Diseases change over time, both phenotypically and in their underlying molecular processes. Though understanding disease progression dynamics is critical for diagnostics and treatment, capturing these dynamics is difficult due to their complexity and the high heterogeneity in disease development between individuals. We present TimeAx, an algorithm which builds a comparative framework for capturing disease dynamics using high-dimensional, short time-series data. We demonstrate the utility of TimeAx by studying disease progression dynamics for multiple diseases and data types. Notably, for urothelial bladder cancer tumorigenesis, we identify a stromal pro-invasion point on the disease progression axis, characterized by massive immune cell infiltration to the tumor microenvironment and increased mortality. Moreover, the continuous TimeAx model differentiates between early and late tumors within the same tumor subtype, uncovering molecular transitions and potential targetable pathways. Overall, we present a powerful approach for studying disease progression dynamics-providing improved molecular interpretability and clinical benefits for patient stratification and outcome prediction.

Nuclear lamin A-associated proteins are required for centromere assembly

Many Lamin A-associated proteins (LAAP's) that are key constituents of the nuclear envelope (NE), assemble at the "core" domains of chromosomes during NE reformation and mitotic exit. However, the identity and function of the chromosomal core domains remain ill-defined. Here, we show that a distinct section of the core domain overlaps with the centromeres/kinetochores of chromosomes during mitotic telophase. The core domain can thus be demarcated into a kinetochore proximal core (KPC) on one side of the segregated chromosomes and the kinetochore distal core (KDC) on the opposite side, close to the central spindle. We next tested if centromere assembly is connected to NE re-formation. We find that centromere assembly is markedly perturbed after inhibiting the function of LMNA and the core-localized LAAPs, BANF1 and Emerin. We also find that the LAAPs exhibit multiple biochemical interactions with the centromere and inner kinetochore proteins. Consistent with this, normal mitotic progression and chromosome segregation was severely impeded after inhibiting LAAP function. Intriguingly, the inhibition of centromere function also interferes with the assembly of LAAP components at the core domain, suggesting a mutual dependence of LAAP and centromeres for their assembly at the core domains. Finally, we find that the localization of key proteins involved in the centromeric loading of CENP-A, including the Mis18 complex and HJURP were markedly affected in LAAP-inhibited cells. Our evidence points to a model where LAAP assembly at the core domain serves a key function in loading new copies of centromeric proteins during or immediately after mitotic exit.

The Ndc80-Cdt1-Ska1 complex is a central processive kinetochore-microtubule coupling unit

It is known that microtubule-binding proteins including the Ska1 complex and the DNA replication licensing factor, Cdt1, enable the kinetochore-localized Ndc80 complex to form robust kinetochore-microtubule attachments. However, it is not clear how the Ndc80 complex is stably coupled to dynamic spindle microtubule plus-ends. Here, we have developed a conditional auxin-inducible degron approach to reveal a function for Cdt1 in chromosome segregation and kinetochore-microtubule interactions that is separable from its role in DNA replication licensing. Further, we demonstrate that a direct interaction between Cdt1 and Ska1 is required for recruiting Cdt1 to kinetochores and spindle microtubules. Cdt1 phosphorylation by Cdk1 kinase is critical for Ska1 binding, kinetochore-microtubule attachments, and mitotic progression. Furthermore, we show that Cdt1 synergizes with Ndc80 and Ska1 for microtubule binding, including forming a diffusive, tripartite Ndc80-Cdt1-Ska1 complex that can processively track dynamic microtubule plus-ends in vitro. Taken together, our data identify the Ndc80-Cdt1-Ska1 complex as a central molecular unit that can promote processive bidirectional tip-tracking of microtubules by kinetochores.

Conserved and divergent mechanisms of inner kinetochore assembly onto centromeric chromatin

Kinetochores are large protein complexes built on centromeric chromatin that mediate chromosome segregation. The inner kinetochore, or constitutive centromere-associated network (CCAN), assembles onto centromeres defined by centromere protein A (CENP-A) nucleosomes (CENP-ANuc), and acts as a platform for the regulated assembly of the microtubule-binding outer kinetochore. Recent cryo-EM work revealed structural conservation of CCAN, from the repeating human regional centromeres to the point centromere of budding yeast. Centromere recognition is determined mainly through engagement of duplex DNA proximal to the CENP-A nucleosome by a DNA-binding CENP-LN channel located at the core of CCAN. Additional DNA interactions formed by other CCAN modules create an enclosed DNA-binding chamber. This configuration explains how kinetochores maintain their tight grip on centromeric DNA to withstand the forces of chromosome segregation. Defining the higher-order architecture of complete kinetochore assemblies with implications for understanding the 3D organisation of regional centromeres and mechanisms of kinetochore dynamics, including how kinetochores sense and respond to tension, are important future directions.

Coordination between the Ndc80 complex and dynein is essential for microtubule plus-end capture by kinetochores during early mitosis

Mitotic kinetochores are initially captured by dynamic microtubules via a "search-and-capture" mechanism. The microtubule motor, dynein, is critical for kinetochore capture as it has been shown to transport microtubule-attached chromosomes toward the spindle pole during prometaphase. The microtubule-binding nuclear division cycle 80 (Ndc80) complex that is recruited to kinetochores in prophase is known to play a central role in forming kinetochore-microtubule (kMT) attachments in metaphase. It is not yet clear, however, how Ndc80 contributes to initial kMT capture during prometaphase. Here, by combining CRISPR/Cas9-mediated knockout and RNAi technology with assays specific to study kMT capture, we show that mitotic cells lacking Ndc80 exhibit substantial defects in this function during prometaphase. Rescue experiments show that Ndc80 mutants deficient in microtubule-binding are unable to execute proper kMT capture. While cells inhibited of dynein alone are predominantly able to make initial kMT attachments, cells co-depleted of Ndc80 and dynein show severe defects in kMT capture. Further, we use an in vitro total internal reflection fluorescence microscopy assay to reconstitute microtubule capture events, which suggest that Ndc80 and dynein coordinate with each other for microtubule plus-end capture and that the phosphorylation status of Ndc80 is critical for productive kMT capture. A novel interaction between Ndc80 and dynein that we identify in prometaphase extracts might be critical for efficient plus-end capture. Thus, our studies, for the first time, identify a distinct event in the formation of initial kMT attachments, which is directly mediated by Ndc80 and in coordination with dynein is required for efficient kMT capture and chromosome alignment.

ATR protects centromere identity by promoting DAXX association with PML nuclear bodies

Centromere protein A (CENP-A) defines centromere identity and nucleates kinetochore formation for mitotic chromosome segregation. Here, we show that ataxia telangiectasia and Rad3-related (ATR) kinase, a master regulator of the DNA damage response, protects CENP-A occupancy at interphase centromeres in a DNA damage-independent manner. In unperturbed cells, ATR localizes to promyelocytic leukemia nuclear bodies (PML NBs), which house the histone H3.3 chaperone DAXX (death domain-associated protein 6). We find that ATR inhibition reduces DAXX association with PML NBs, resulting in the DAXX-dependent loss of CENP-A and an aberrant increase in H3.3 at interphase centromeres. Additionally, we show that ATR-dependent phosphorylation within the C terminus of DAXX regulates CENP-A occupancy at centromeres and DAXX localization. Lastly, we demonstrate that acute ATR inhibition during interphase leads to kinetochore formation defects and an increased rate of lagging chromosomes. These findings highlight a mechanism by which ATR protects centromere identity and genome stability.

SUMO control of centromere homeostasis

Centromeres are unique chromosomal loci that form the anchorage point for the mitotic spindle during mitosis and meiosis. Their position and function are specified by a unique chromatin domain featuring the histone H3 variant CENP-A. While typically formed on centromeric satellite arrays, CENP-A nucleosomes are maintained and assembled by a strong self-templated feedback mechanism that can propagate centromeres even at non-canonical sites. Central to the epigenetic chromatin-based transmission of centromeres is the stable inheritance of CENP-A nucleosomes. While long-lived at centromeres, CENP-A can turn over rapidly at non-centromeric sites and even erode from centromeres in non-dividing cells. Recently, SUMO modification of the centromere complex has come to the forefront as a mediator of centromere complex stability, including CENP-A chromatin. We review evidence from different models and discuss the emerging view that limited SUMOylation appears to play a constructive role in centromere complex formation, while polySUMOylation drives complex turnover. The deSUMOylase SENP6/Ulp2 and the proteins segregase p97/Cdc48 constitute the dominant opposing forces that balance CENP-A chromatin stability. This balance may be key to ensuring proper kinetochore strength at the centromere while preventing ectopic centromere formation.

A mechanism that integrates microtubule motors of opposite polarity at the kinetochore corona

Chromosome biorientation on the mitotic spindle is prerequisite to errorless genome inheritance. CENP-E (kinesin 7) and Dynein-Dynactin (DD), microtubule motors with opposite polarity, promote biorientation from the kinetochore corona, a polymeric structure whose assembly requires MPS1 kinase. The corona's building block consists of ROD, Zwilch, ZW10, and the DD adaptor Spindly (RZZS). How CENP-E and DD are scaffolded and mutually coordinated in the corona remains unclear. Here, we report near-complete depletion of RZZS and DD from kinetochores after depletion of CENP-E and the outer kinetochore protein KNL1. With inhibited MPS1, CENP-E, which we show binds directly to RZZS, is required to retain kinetochore RZZS. An RZZS phosphomimetic mutant bypasses this requirement. With active MPS1, CENP-E is dispensable for corona expansion, but strictly required for physiological kinetochore accumulation of DD. Thus, we identify the corona as an integrated scaffold where CENP-E kinesin controls DD kinetochore loading for coordinated bidirectional transport of chromosome cargo.

Distinct dynein complexes defined by DYNLRB1 and DYNLRB2 regulate mitotic and male meiotic spindle bipolarity

Spindle formation in male meiosis relies on the canonical centrosome system, which is distinct from acentrosomal oocyte meiosis, but its specific regulatory mechanisms remain unknown. Herein, we report that DYNLRB2 (Dynein light chain roadblock-type-2) is a male meiosis-upregulated dynein light chain that is indispensable for spindle formation in meiosis I. In Dynlrb2 KO mouse testes, meiosis progression is arrested in metaphase I due to the formation of multipolar spindles with fragmented pericentriolar material (PCM). DYNLRB2 inhibits PCM fragmentation through two distinct pathways; suppressing premature centriole disengagement and targeting NuMA (nuclear mitotic apparatus) to spindle poles. The ubiquitously expressed mitotic counterpart, DYNLRB1, has similar roles in mitotic cells and maintains spindle bipolarity by targeting NuMA and suppressing centriole overduplication. Our work demonstrates that two distinct dynein complexes containing DYNLRB1 or DYNLRB2 are separately used in mitotic and meiotic spindle formations, respectively, and that both have NuMA as a common target.

Kinetochore dynein is sufficient to biorient chromosomes and remodel the outer kinetochore

Multiple microtubule-directed activities concentrate on chromosomes during mitosis to ensure their accurate distribution to daughter cells. These activities include couplers and dynamics regulators localized at the kinetochore, the specialized microtubule interface built on centromeric chromatin, as well as motor proteins recruited to kinetochores and to mitotic chromatin. Here, we describe an in vivo reconstruction approach in which the effect of removing the major microtubule-directed activities on mitotic chromosomes is compared to the selective presence of individual activities. This approach revealed that the kinetochore dynein module, comprised of the minus end-directed motor cytoplasmic dynein and its kinetochore-specific adapters, is sufficient to biorient chromosomes and to remodel outer kinetochore composition following microtubule attachment; by contrast, the kinetochore dynein module is unable to support chromosome congression. The chromosome-autonomous action of kinetochore dynein, in the absence of the other major microtubule-directed factors on chromosomes, rotates and orients a substantial proportion of chromosomes such that their sister chromatids attach to opposite spindle poles. In tight coupling with orientation, the kinetochore dynein module drives removal of outermost kinetochore components, including the dynein motor itself and spindle checkpoint activators. The removal is independent of the other major microtubule-directed activities and kinetochore-localized protein phosphatase 1, suggesting that it is intrinsic to the kinetochore dynein module. These observations indicate that the kinetochore dynein module has the ability coordinate chromosome biorientation with attachment state-sensitive remodeling of the outer kinetochore that facilitates cell cycle progression.

High expression of centromere protein N as novel biomarkers for gastric adenocarcinoma

BACKGROUND

The role and mechanism of centromeric protein N (CENP-N), which has been associated with the development of various cancer types, are yet unclear in stomach adenocarcinoma (STAD).

METHODS

Data from the Cancer Genome Atlas and Genotype-Tissue Expression were used to determine whether CENP-N expression was altered in STAD tumors compared to normal tissues. Xiantao was used to perform Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes(KEGG) enrichment analysis on CENP-N. The relationship between CENP-N expression and immune cell infiltration was assessed using TCGA database. The expression of CENP-N in STAD and surrounding tissues was confirmed using immunohistochemical staining and the correlation between CENP-N expression and clinicopathological characteristics was examined. The effects of CENP-N knockdown by siRNA on proliferation were measured by CCK-8 and EdU assays in AGS cells. Following siRNA transfection, flow cytometry was performed to evaluate cell cycle and apoptotic alterations in AGS cells. The effect of CENP-N knockdown on the expression level of related proteins was detected by Westren blot.

RESULTS

CENP-N was highly expressed in STAD tissues, which was confirmed by our immunohistochemistry results. The degree of invasion, TNM stage, and lymph node metastases were all strongly associated with CENP-N expression. CENP-N was essential for the cell cycle, DNA replication, chromosomal segregation, and nuclear division; there was a positive correlation between CENP-N expression and infiltrating Th2 and NK CD56dim cells and a negative correlation between CENP-N expression and mast, pDC, NK, and B cell infiltration. When CENP-N expression in AGS cells was knocked down, cell proliferation dramatically reduced (p < .05) and the percentage of cells in the S and G2-M phases decreased significantly (p < .05). Silencing CENP-N significantly promoted the apoptosis of AGS cells (p < .05). Mechanistic investigations showed that silencing CENP-N expression may inhibit STAD proliferation through the Cyclin E1 and promote STAD apoptosis through the Bcl-2/Bax.

CONCLUSION

According to our data, CENP-N acts as an oncogene in STAD and may be a viable therapeutic target.