Establishing correct kinetochore-microtubule attachments in mitosis and meiosis

Aurora B and Aurora C pools at two chromosomal regions collaboratively maintain chromosome alignment and prevent aneuploidy at the second meiotic division in mammalian oocytes

Correct chromosome segregation is essential to preserve genetic integrity. The two protein kinases, Aurora B and its meiotic homolog Aurora C, regulate attachments between chromosomal kinetochores and microtubules, thereby contributing to the accuracy of the chromosome segregation process. Here we performed a detailed examination of the localization and activity of Aurora B/C kinases, their partner Incenp and the kinetochore target Hec1, during the second meiotic division in mouse oocytes. We found that a majority of Aurora B and C changed their localization from the outer kinetochore region of chromosomes at prometaphase II to an inner central region localized between sister centromeres at metaphase II. Depletion of the Aurora B/C pool at the inner central region using the haspin kinase inhibitor 5-iodotubercidin resulted in chromosome misalignments at the metaphase II stage. To further understand the role of the Aurora B/C pool at the central region, we examined the behaviour of single chromatids, that lack a central Aurora B/C pool but retain Aurora B/C at the outer kinetochores. We found that kinetochore-microtubule attachments at single chromatids were corrected at both prometaphase II and metaphase II stages, but that single chromatids compared to paired chromatids were more prone to misalignments following treatment of oocytes with the Aurora B/C inhibitory drugs AZD1152 and GSK1070916. We conclude that the Aurora B/C pool at the inner central region stabilizes chromosome alignment during metaphase II arrest, while Aurora B/C localized at the kinetochore assist in re-establishing chromosome positioning at the metaphase plate if alignment is lost. Collaboratively these two pools prevent missegregation and aneuploidy at the second meiotic division in mammalian oocytes.

Aneuploidy of Specific Chromosomes is Beneficial to Cells Lacking Spindle Checkpoint Protein Bub3

Aneuploidy typically poses challenges for cell survival and growth. However, recent studies have identified exceptions where aneuploidy is beneficial for cells with mutations in certain regulatory genes. Our research reveals that cells lacking the spindle checkpoint gene BUB3 exhibit aneuploidy of select chromosomes. While the spindle checkpoint is not essential in budding yeast, the loss of BUB3 and BUB1 increases the probability of chromosome missegregation compared to wildtype cells. Contrary to the prevailing assumption that the aneuploid cells would be outcompeted due to growth defects, our findings demonstrate that bub3 Δ cells consistently maintained aneuploidy of specific chromosomes over many generations. We investigated whether the persistence of these additional chromosomes in bub3 Δ cells resulted from the beneficial elevated expression of certain genes, or mere tolerance. We identified several genes involved in chromosome segregation and cell cycle regulation that confer an advantage to Bub3-depleted cells. Overall, our results suggest that the upregulation of specific genes through aneuploidy may provide a survival and growth advantage to strains with poor chromosome segregation fidelity.

AUTHOR SUMMARY

Accurate chromosome segregation is crucial for the proper development of all living organisms. Errors in chromosome segregation can lead to aneuploidy, characterized by an abnormal number of chromosomes, which generally impairs cell survival and growth. However, under certain stress conditions, such as in various cancers, cells with specific mutations and extra copies of advantageous chromosomes exhibit improved survival and proliferation. In our study, we discovered that cells lacking the spindle checkpoint protein Bub3 became aneuploid, retaining specific chromosomes. This finding was unexpected because although bub3 Δ cells have a higher rate of chromosome mis-segregation, they were not thought to maintain an aneuploid karyotype. We investigated whether the increased copy number of specific genes on these acquired chromosomes offered a benefit to Bub3-deficient cells. Our results revealed that several genes involved in chromosome segregation and cell cycle regulation prevented the gain of chromosomes upon Bub3-depletion, suggesting that these genes confer a survival advantage. Overall, our study demonstrates that cells lacking Bub3 selectively retain specific chromosomes to increase the copy number of genes that promote proper chromosome segregation.

The Molecular Mechanism of Aurora-B Regulating Kinetochore-Microtubule Attachment in Mitosis and Oocyte Meiosis

BACKGROUND

Aurora kinase B (Aurora-B), a member of the chromosomal passenger complex, is involved in correcting kinetochore-microtubule (KT-MT) attachment errors and regulating sister chromatid condensation and cytoplasmic division during mitosis.

SUMMARY

However, few reviews have discussed its mechanism in oocyte meiosis and the differences between its role in mitosis and meiosis. Therefore, in this review, we summarize the localization, recruitment, activation, and functions of Aurora-B in mitosis and oocyte meiosis. The accurate regulation of Aurora-B is essential for ensuring accurate chromosomal segregation and correct KT-MT attachments. Aurora-B regulates the stability of KT-MT attachments by competing with cyclin-dependent kinase 1 to control the phosphorylation of the SILK and RVSF motifs on kinetochore scaffold 1 and by competing with protein phosphatase 1 to influence the phosphorylation of NDC80 which is the substrate of Aurora-B. In addition, Aurora-B regulates the spindle assembly checkpoint by promoting the recruitment and activation of mitotic arrest deficient 2.

KEY MESSAGES

This review provides a theoretical foundation for elucidating the mechanism of cell division and understanding oocyte chromosomal aneuploidy.

Insights into the Clinical, Biological and Therapeutic Impact of Copy Number Alteration in Cancer

Copy number alterations (CNAs), resulting from the gain or loss of genetic material from as little as 50 base pairs or as big as entire chromosome(s), have been associated with many congenital diseases, de novo syndromes and cancer. It is established that CNAs disturb the dosage of genomic regions including enhancers/promoters, long non-coding RNA and gene(s) among others, ultimately leading to an altered balance of key cellular functions. In cancer, CNAs have been associated with almost all steps of the disease: predisposition, initiation, development, maintenance, response to treatment, resistance, and relapse. Therefore, understanding how specific CNAs contribute to tumourigenesis may provide prognostic insight and ultimately lead to the development of new therapeutic approaches to improve patient outcomes. In this review, we provide a snapshot of what is currently known about CNAs and cancer, incorporating topics regarding their detection, clinical impact, origin, and nature, and discuss the integration of innovative genetic engineering strategies, to highlight the potential for targeting CNAs using novel, dosage-sensitive and less toxic therapies for CNA-driven cancer.

Chromosome segregation: Mps1 and Dam1 battle to bind a shared interaction site at the kinetochore

The attachment of kinetochores to spindle microtubules is highly regulated to ensure proper chromosome segregation. Three new studies identify an interaction hub at the kinetochore that integrates kinetochore attachment state with spindle checkpoint activity and kinetochore assembly.

A stable microtubule bundle formed through an orchestrated multistep process controls quiescence exit

Cells fine-tune microtubule assembly in both space and time to give rise to distinct edifices with specific cellular functions. In proliferating cells, microtubules are highly dynamics, and proliferation cessation often leads to their stabilization. One of the most stable microtubule structures identified to date is the nuclear bundle assembled in quiescent yeast. In this article, we characterize the original multistep process driving the assembly of this structure. This Aurora B-dependent mechanism follows a precise temporality that relies on the sequential actions of kinesin-14, kinesin-5, and involves both microtubule-kinetochore and kinetochore-kinetochore interactions. Upon quiescence exit, the microtubule bundle is disassembled via a cooperative process involving kinesin-8 and its full disassembly is required prior to cells re-entry into proliferation. Overall, our study provides the first description, at the molecular scale, of the entire life cycle of a stable microtubule structure in vivo and sheds light on its physiological function.

Aurora B kinase erases monopolar microtubule-kinetochore arrays at the meiosis I-II transition

During meiosis, faithful chromosome segregation requires monopolar spindle microtubule-kinetochore arrays in MI to segregate homologous chromosomes, but bipolar in MII to segregate sister chromatids. Using fission yeasts, we found that the universal Aurora B kinase localizes to kinetochores in metaphase I and in the mid-spindle during anaphase I, as in mitosis; but in the absence of an intervening S phase, the importin α Imp1 propitiates its release from the spindle midzone to re-localize at kinetochores during meiotic interkinesis. We show that "error-correction" activity of kinetochore re-localized Aurora B becomes essential to erase monopolar arrangements from anaphase I, a prerequisite to satisfy the spindle assembly checkpoint (SAC) and to generate proper bipolar arrays at the onset of MII. This microtubule-kinetochore resetting activity of Aurora B at the MI-MII transition is required to prevent chromosome missegregation in meiosis II, a type of error often associated with birth defects and infertility in humans.

Identification of Novel Diagnostic and Prognostic Gene Signature Biomarkers for Breast Cancer Using Artificial Intelligence and Machine Learning Assisted Transcriptomics Analysis

BACKGROUND

Breast cancer (BC) is one of the most common female cancers. Clinical and histopathological information is collectively used for diagnosis, but is often not precise. We applied machine learning (ML) methods to identify the valuable gene signature model based on differentially expressed genes (DEGs) for BC diagnosis and prognosis.

METHODS

A cohort of 701 samples from 11 GEO BC microarray datasets was used for the identification of significant DEGs. Seven ML methods, including RFECV-LR, RFECV-SVM, LR-L1, SVC-L1, RF, and Extra-Trees were applied for gene reduction and the construction of a diagnostic model for cancer classification. Kaplan-Meier survival analysis was performed for prognostic signature construction. The potential biomarkers were confirmed via qRT-PCR and validated by another set of ML methods including GBDT, XGBoost, AdaBoost, KNN, and MLP.

RESULTS

We identified 355 DEGs and predicted BC-associated pathways, including kinetochore metaphase signaling, PTEN, senescence, and phagosome-formation pathways. A hub of 28 DEGs and a novel diagnostic nine-gene signature (COL10A, S100P, ADAMTS5, WISP1, COMP, CXCL10, LYVE1, COL11A1, and INHBA) were identified using stringent filter conditions. Similarly, a novel prognostic model consisting of eight-gene signatures (CCNE2, NUSAP1, TPX2, S100P, ITM2A, LIFR, TNXA, and ZBTB16) was also identified using disease-free survival and overall survival analysis. Gene signatures were validated by another set of ML methods. Finally, qRT-PCR results confirmed the expression of the identified gene signatures in BC.

CONCLUSION

The ML approach helped construct novel diagnostic and prognostic models based on the expression profiling of BC. The identified nine-gene signature and eight-gene signatures showed excellent potential in BC diagnosis and prognosis, respectively.

Meiotic cells escape prolonged spindle checkpoint activity through kinetochore silencing and slippage

To prevent chromosome mis-segregation, a surveillance mechanism known as the spindle checkpoint delays the cell cycle if kinetochores are not attached to spindle microtubules, allowing the cell additional time to correct improper attachments. During spindle checkpoint activation, checkpoint proteins bind the unattached kinetochore and send a diffusible signal to inhibit the anaphase promoting complex/cyclosome (APC/C). Previous work has shown that mitotic cells with depolymerized microtubules can escape prolonged spindle checkpoint activation in a process called mitotic slippage. During slippage, spindle checkpoint proteins bind unattached kinetochores, but the cells cannot maintain the checkpoint arrest. We asked if meiotic cells had as robust of a spindle checkpoint response as mitotic cells and whether they also undergo slippage after prolonged spindle checkpoint activity. We performed a direct comparison between mitotic and meiotic budding yeast cells that signal the spindle checkpoint through two different assays. We find that the spindle checkpoint delay is shorter in meiosis I or meiosis II compared to mitosis, overcoming a checkpoint arrest approximately 150 minutes earlier in meiosis than in mitosis. In addition, cells in meiosis I escape spindle checkpoint signaling using two mechanisms, silencing the checkpoint at the kinetochore and through slippage. We propose that meiotic cells undertake developmentally-regulated mechanisms to prevent persistent spindle checkpoint activity to ensure the production of gametes.

Variation of Female Pronucleus Reveals Oocyte or Embryo Chromosomal Copy Number Variations

The characteristics of the human pronuclei (PNs), which exist 16-22 h after fertilization, appear to serve as good indicators to evaluate the quality of human oocyte and embryo, and may reflect the status of female and male chromosome composition. Here, a quantitative PN measurement method that is generated by applying expert experience combined with deep learning from large annotated datasets is reported. After mathematic reconstruction of PNs, significant differences are obtained in chromosome-normal rate and chromosomal small errors such as copy number variants by comparing the size of the reconstructive female PN. After integrating the whole procedure of PN dynamics and adjusting for errors that occur during PN identification, the results are robust. Notably, all positive prediction results are obtained from the female propositus population. Thus, the size of female PNs may mirror the internal quality of the chromosomal integrity of the oocyte. Embryos that develop from zygotes with larger female PNs may have a reduced risk of copy number variations.

Distinct Aurora B pools at the inner centromere and kinetochore have different contributions to meiotic and mitotic chromosome segregation

Proper chromosome segregation depends on establishment of bioriented kinetochore-microtubule attachments, which often requires multiple rounds of release and reattachment. Aurora B and C kinases phosphorylate kinetochore proteins to release tensionless attachments. Multiple pathways recruit Aurora B/C to the centromere and kinetochore. We studied how these pathways contribute to anaphase onset timing and correction of kinetochore-microtubule attachments in budding yeast meiosis and mitosis. We find that the pool localized by the Bub1/Bub3 pathway sets the normal duration of meiosis and mitosis, in differing ways. Our meiosis data suggests that disruption of this pathway leads to PP1 kinetochore localization, which dephosphorylates Cdc20 for premature anaphase onset. For error correction, the Bub1/Bub3 and COMA pathways are individually important in meiosis but compensatory in mitosis. Finally, we find that the haspin and Bub1/3 pathways function together to ensure error correction in mouse oogenesis. Our results suggest that each recruitment pathway localizes spatially distinct kinetochore-localized Aurora B/C pools that function differently between meiosis and mitosis.

HN1 Is Enriched in the S-Phase, Phosphorylated in Mitosis, and Contributes to Cyclin B1 Degradation in Prostate Cancer Cells

HN1 has previously been shown as overexpressed in various cancers. In Prostate cancer, it regulates AR signaling and centrosome-related functions. Previously, in two different studies, HN1 expression has been observed as inversely correlated with Cyclin B1. However, HN1 interacting partners and the role of HN1 interactions in cell cycle pathways have not been completely elucidated. Therefore, we used Prostate cancer cell lines again and utilized both transient and stable inducible overexpression systems to delineate the role of HN1 in the cell cycle. HN1 characterization was performed using treatments of kinase inhibitors, western blotting, flow cytometry, immunofluorescence, cellular fractionation, and immunoprecipitation approaches. Our findings suggest that HN1 overexpression before mitosis (post-G2), using both transient and stable expression systems, leads to S-phase accumulation and causes early mitotic exit after post-G2 overexpression. Mechanistically, HN1 interacted with Cyclin B1 and increased its degradation via ubiquitination through stabilized Cdh1, which is a co-factor of the APC/C complex. Stably HN1-expressing cells exhibited a reduced Cdt1 loading onto chromatin, demonstrating an exit from a G1 to S phenotype. We found HN1 and Cdh1 interaction as a new regulator of the Cyclin B1/CDK1 axis in mitotic regulation which can be explored further to dissect the roles of HN1 in the cell cycle.

Meiotic Cells Escape Prolonged Spindle Checkpoint Activity Through Premature Silencing and Slippage

To prevent chromosome mis-segregation, a surveillance mechanism known as the spindle checkpoint delays the cell cycle if kinetochores are not attached to spindle microtubules, allowing the cell additional time to correct improper attachments. During spindle checkpoint activation, checkpoint proteins bind the unattached kinetochore and send a diffusible signal to inhibit the anaphase promoting complex/cyclosome (APC/C). Previous work has shown that mitotic cells with depolymerized microtubules can escape prolonged spindle checkpoint activation in a process called mitotic slippage. During slippage, spindle checkpoint proteins bind unattached kinetochores, but the cells cannot maintain the checkpoint arrest. We asked if meiotic cells had as robust of a spindle checkpoint response as mitotic cells and whether they also undergo slippage after prolonged spindle checkpoint activity. We performed a direct comparison between mitotic and meiotic budding yeast cells that signal the spindle checkpoint due to a lack of either kinetochore-microtubule attachments or due to a loss of tension-bearing attachments. We find that the spindle checkpoint is not as robust in meiosis I or meiosis II compared to mitosis, overcoming a checkpoint arrest approximately 150 minutes earlier in meiosis. In addition, cells in meiosis I escape spindle checkpoint signaling using two mechanisms, silencing the checkpoint at the kinetochore and through slippage. We propose that meiotic cells undertake developmentally-regulated mechanisms to prevent persistent spindle checkpoint activity to ensure the production of gametes.

AUTHOR SUMMARY

Mitosis and meiosis are the two major types of cell divisions. Mitosis gives rise to genetically identical daughter cells, while meiosis is a reductional division that gives rise to gametes. Cell cycle checkpoints are highly regulated surveillance mechanisms that prevent cell cycle progression when circumstances are unfavorable. The spindle checkpoint promotes faithful chromosome segregation to safeguard against aneuploidy, in which cells have too many or too few chromosomes. The spindle checkpoint is activated at the kinetochore and then diffuses to inhibit cell cycle progression. Although the checkpoint is active in both mitosis and meiosis, most studies involving checkpoint regulation have been performed in mitosis. By activating the spindle checkpoint in both mitosis and meiosis in budding yeast, we show that cells in meiosis elicit a less persistent checkpoint signal compared to cells in mitosis. Further, we show that cells use distinct mechanisms to escape the checkpoint in mitosis and meiosis I. While cells in mitosis and meiosis II undergo anaphase onset while retaining checkpoint proteins at the kinetochore, cells in meiosis I prematurely lose checkpoint protein localization at the kinetochore. If the mechanism to remove the checkpoint components from the kinetochore is disrupted, meiosis I cells can still escape checkpoint activity. Together, these results highlight that cell cycle checkpoints are differentially regulated during meiosis to avoid long delays and to allow gametogenesis.

Checkpoint control in meiotic prophase: Idiosyncratic demands require unique characteristics

Chromosomal transactions such as replication, recombination and segregation are monitored by cell cycle checkpoint cascades. These checkpoints ensure the proper execution of processes that are needed for faithful genome inheritance from one cell to the next, and across generations. In meiotic prophase, a specialized checkpoint monitors defining events of meiosis: programmed DNA break formation, followed by dedicated repair through recombination based on interhomolog (IH) crossovers. This checkpoint shares molecular characteristics with canonical DNA damage checkpoints active during somatic cell cycles. However, idiosyncratic requirements of meiotic prophase have introduced unique features in this signaling cascade. In this review, we discuss the unique features of the meiotic prophase checkpoint. While being related to canonical DNA damage checkpoint cascades, the meiotic prophase checkpoint also shows similarities with the spindle assembly checkpoint (SAC) that guards chromosome segregation. We highlight these emerging similarities in the signaling logic of the checkpoints that govern meiotic prophase and chromosome segregation, and how thinking of these similarities can help us better understand meiotic prophase control. We also discuss work showing that, when aberrantly expressed, components of the meiotic prophase checkpoint might alter DNA repair fidelity and chromosome segregation in cancer cells. Considering checkpoint function in light of demands imposed by the special characteristics of meiotic prophase helps us understand checkpoint integration into the meiotic cell cycle machinery.

Haspin participates in AURKB recruitment to centromeres and contributes to chromosome congression in male mouse meiosis

Chromosome segregation requires that centromeres properly attach to spindle microtubules. This essential step regulates the accuracy of cell division and therefore must be precisely regulated. One of the main centromeric regulatory signaling pathways is the Haspin-H3T3ph-chromosomal passenger complex (CPC) cascade, which is responsible for the recruitment of the CPC to the centromeres. In mitosis, Haspin kinase phosphorylates histone H3 at threonine 3 (H3T3ph), an essential epigenetic mark that recruits the CPC, whose catalytic component is Aurora B kinase. However, the centromeric Haspin-H3T3ph-CPC pathway remains largely uncharacterized in mammalian male meiosis. We have analyzed Haspin functions by either its chemical inhibition in cultured spermatocytes using LDN-192960, or the ablation of Haspin gene in Haspin-/-. Our studies suggest that Haspin kinase activity is required for proper chromosome congression during both meiotic divisions and for the recruitment of Aurora B and kinesin MCAK to meiotic centromeres. However, the absence of H3T3ph histone mark does not alter Borealin and SGO2 centromeric localization. These results add new and relevant information regarding the regulation of the Haspin-H3T3ph-CPC pathway and centromere function during meiosis.

On the Regulation of Mitosis by the Kinetochore, a Macromolecular Complex and Organising Hub of Eukaryotic Organisms

The kinetochore is the multiprotein complex of eukaryotic organisms that is assembled on mitotic or meiotic centromeres to connect centromeric DNA with microtubules. Its function involves the coordinated action of more than 100 different proteins. The kinetochore acts as an organiser hub that establishes physical connections with microtubules and centromere-associated proteins and recruits central protein components of the spindle assembly checkpoint (SAC), an evolutionarily conserved surveillance mechanism of eukaryotic organisms that detects unattached kinetochores and destabilises incorrect kinetochore-microtubule attachments. The molecular communication between the kinetochore and the SAC is highly dynamic and tightly regulated to ensure that cells can progress towards anaphase until each chromosome is properly bi-oriented on the mitotic spindle. This is achieved through an interplay of highly cooperative interactions and concerted phosphorylation/dephosphorylation events that are organised in time and space.This contribution discusses our current understanding of the function, structure and regulation of the kinetochore, in particular, how its communication with the SAC results in the amplification of specific signals to exquisitely control the eukaryotic cell cycle. This contribution also addresses recent advances in machine learning approaches, cell imaging and proteomics techniques that have enhanced our understanding of the molecular mechanisms that ensure the high fidelity and timely segregation of the genetic material every time a cell divides as well as the current challenges in the study of this fascinating molecular machine.

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.

How to Switch from Mitosis to Meiosis: Regulation of Germline Entry in Plants

One of the major cell fate transitions in eukaryotes is entry into meiosis. While in single-celled yeast this decision is triggered by nutrient starvation, in multicellular eukaryotes, such as plants, it is under developmental control. In contrast to animals, plants have only a short germline and instruct cells to become meiocytes in reproductive organs late in development. This situation argues for a fundamentally different mechanism of how plants recruit meiocytes, and consistently, none of the regulators known to control meiotic entry in yeast and animals are present in plants. In recent years, several factors involved in meiotic entry have been identified, especially in the model plant Arabidopsis, and pieces of a regulatory network of germline control in plants are emerging. However, the corresponding studies also show that the mechanisms of meiotic entry control are diversified in flowering plants, calling for further analyses in different plant species. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.