Aurora B controls the association of condensin I but not condensin II with mitotic chromosomes

Regulation of the mitotic chromosome folding machines

Over the last several years enormous progress has been made in identifying the molecular machines, including condensins and topoisomerases that fold mitotic chromosomes. The discovery that condensins generate chromatin loops through loop extrusion has revolutionized, and energized, the field of chromosome folding. To understand how these machines fold chromosomes with the appropriate dimensions, while disentangling sister chromatids, it needs to be determined how they are regulated and deployed. Here, we outline the current understanding of how these machines and factors are regulated through cell cycle dependent expression, chromatin localization, activation and inactivation through post-translational modifications, and through associations with each other, with other factors and with the chromatin template itself. There are still many open questions about how condensins and topoisomerases are regulated but given the pace of progress in the chromosome folding field, it seems likely that many of these will be answered in the years ahead.

Dephosphorylation in nuclear reassembly after mitosis

In most animal cell types, the interphase nucleus is largely disassembled during mitotic entry. The nuclear envelope breaks down and chromosomes are compacted into separated masses. Chromatin organization is also mostly lost and kinetochores assemble on centromeres. Mitotic protein kinases play several roles in inducing these transformations by phosphorylating multiple effector proteins. In many of these events, the mechanistic consequences of phosphorylation have been characterized. In comparison, how the nucleus reassembles at the end of mitosis is less well understood in mechanistic terms. In recent years, much progress has been made in deciphering how dephosphorylation of several effector proteins promotes nuclear envelope reassembly, chromosome decondensation, kinetochore disassembly and interphase chromatin organization. The precise roles of protein phosphatases in this process, in particular of the PP1 and PP2A groups, are emerging. Moreover, how these enzymes are temporally and spatially regulated to ensure that nuclear reassembly progresses in a coordinated manner has been partly uncovered. This review provides a global view of nuclear reassembly with a focus on the roles of dephosphorylation events. It also identifies important open questions and proposes hypotheses.

Transcription factor Sp1 regulates mitotic chromosome assembly and segregation

Aneuploidy is a pervasive feature of cancer cells that results from chromosome missegregation. Several transcription factors have been associated with aneuploidy; however, no studies to date have demonstrated that mammalian transcription factors directly regulate chromosome segregation during mitosis. Here, we demonstrate that the ubiquitously expressed transcription factor specificity protein 1 (Sp1), which we have previously linked to aneuploidy, has a mitosis-specific role regulating chromosome segregation. We find that Sp1 localizes to mitotic centromeres and auxin-induced rapid Sp1 degradation at mitotic onset results in chromosome segregation errors and aberrant mitotic progression. Furthermore, rapid Sp1 degradation results in anomalous mitotic chromosome assembly characterized by loss of condensin complex I localization to mitotic chromosomes and chromosome condensation defects. Consistent with these defects, Sp1 degradation results in reduced chromosome passenger complex activity and histone H3 serine 10 phosphorylation during mitosis, which is essential for condensin complex I recruitment and chromosome condensation. Together, these data provide the first evidence of a mammalian transcription factor acting specifically during mitosis to regulate chromosome segregation.

The roles of transcription, chromatin organisation and chromosomal processes in holocentromere establishment and maintenance

The centromere is a unique functional region on each eukaryotic chromosome where the kinetochore assembles and orchestrates microtubule attachment and chromosome segregation. Unlike monocentromeres that occupy a specific region on the chromosome, holocentromeres are diffused along the length of the chromosome. Despite being less common, holocentromeres have been verified in almost 800 nematode, insect, and plant species. Understanding of the molecular and epigenetic regulation of holocentromeres is lagging that of monocentromeres. Here we review how permissive locations for holocentromeres are determined across the genome, potentially by chromatin organisation, transcription, and non-coding RNAs, specifically in the nematode C. elegans. In addition, we discuss how holocentric CENP-A or CENP-T-containing nucleosomes are recruited and deposited, through the help of histone chaperones, licensing factors, and condensin complexes, both during de novo holocentromere establishment, and in each mitotic cell cycle. The process of resolving sister centromeres after DNA replication in holocentric organisms is also mentioned. Conservation and diversity between holocentric and monocentric organisms are highlighted, and outstanding questions are proposed.

CCAR2 controls mitotic progression through spatiotemporal regulation of Aurora B

CCAR2 (cell cycle and apoptosis regulator 2) is a multifaceted protein involved in cell survival and death following cytotoxic stress. However, little is known about the physiological functions of CCAR2 in regulating cell proliferation in the absence of external stimuli. The present study shows that CCAR2-deficient cells possess multilobulated nuclei, suggesting a defect in cell division. In particular, the duration of mitotic phase was perturbed. This disturbance of mitotic progression resulted from premature loss of cohesion with the centromere, and inactivation of the spindle assembly checkpoint during prometaphase and metaphase. It resulted in the formation of lagging chromosomes during anaphase, leading ultimately to the activation of the abscission checkpoint to halt cytokinesis. The CCAR2-dependent mitotic progression was related to spatiotemporal regulation of active Aurora B. In conclusion, the results suggest that CCAR2 governs mitotic events, including proper chromosome segregation and cytokinetic division, to maintain chromosomal stability.

Cell cycle-specific loading of condensin I is regulated by the N-terminal tail of its kleisin subunit

Condensin I is a pentameric protein complex that plays an essential role in mitotic chromosome assembly in eukaryotic cells. Although it has been shown that condensin I loading is mitosis specific, it remains poorly understood how the robust cell cycle regulation of condensin I is achieved. Here, we set up a panel of in vitro assays to demonstrate that cell cycle-specific loading of condensin I is regulated by the N-terminal tail (N-tail) of its kleisin subunit CAP-H. Deletion of the N-tail accelerates condensin I loading and chromosome assembly in Xenopus egg mitotic extracts. Phosphorylation-deficient and phosphorylation-mimetic mutations in the CAP-H N-tail decelerate and accelerate condensin I loading, respectively. Remarkably, deletion of the N-tail enables condensin I to assemble mitotic chromosome-like structures even in interphase extracts. Together with other extract-free functional assays in vitro, our results uncover one of the multilayered mechanisms that ensure cell cycle-specific loading of condensin I onto chromosomes.

Epigenetic-Mediated Regulation of Gene Expression for Biological Control and Cancer: Fidelity of Mechanisms Governing the Cell Cycle

The cell cycle is governed by stringent epigenetic mechanisms that, in response to intrinsic and extrinsic regulatory cues, support fidelity of DNA replication and cell division. We will focus on (1) the complex and interdependent processes that are obligatory for control of proliferation and compromised in cancer, (2) epigenetic and topological domains that are associated with distinct phases of the cell cycle that may be altered in cancer initiation and progression, and (3) the requirement for mitotic bookmarking to maintain intranuclear localization of transcriptional regulatory machinery to reinforce cell identity throughout the cell cycle to prevent malignant transformation.

Changing places: Chromosomal Passenger Complex relocation in early anaphase

The Chromosomal Passenger Complex (CPC) regulates a plethora of processes during multiple stages of nuclear and cytoplasmic division. Early during mitosis, the CPC is recruited to centromeres and kinetochores, and ensures that the duplicated chromosomes become properly connected to microtubules from opposite poles of the mitotic spindle. Progression into anaphase is accompanied by a striking relocation of the CPC from centromeres to the antiparallel microtubule overlaps of the anaphase spindle and to the equatorial cortex. This translocation requires direct interactions of the CPC with the kinesin-6 family member MKLP2/KIF20A, and the inactivation of cyclin B-cyclin-dependent kinase-1 (CDK1). Here, we review recent progress in the regulation of this relocation event. Furthermore, we discuss why the CPC must be relocated during early anaphase in light of recent advances in the functions of the CPC post metaphase.

Unraveling pathologies underlying chromosomal instability in cancers

Aneuploidy is a widespread feature of malignant tumors that arises through persistent chromosome mis‐segregation in mitosis associated with a pathological condition called chromosomal instability, or CIN. Since CIN is known to have a causal relationship with poor prognosis accompanying by multi‐drug resistance, tumor relapse, and metastasis, many research groups have endeavored to understand the mechanisms underlying CIN. In this review, we overview possible etiologies of CIN. The key processes to achieve faithful chromosome segregation include the regulation of sister chromatid cohesion, kinetochore‐microtubule attachment, bipolar spindle formation, spindle‐assembly checkpoint, and the activity of separase. Aberrant chromosome structures during DNA replication might also be a potential cause of CIN. Defective regulation in these processes can lead to chromosome mis‐segregation, manifested by lagging chromosomes, and DNA bridges in anaphase, leading to gross chromosome rearrangements. Investigation into the molecular etiologies of CIN should allow us to explore novel strategies to intervene in CIN to control cancers.

Aurora B kinase: a potential drug target for cancer therapy

BACKGROUND

Ensuring genetic integrity is essential during the cell cycle to avoid aneuploidy, one of the underlying causes of malignancies. Aurora kinases are serine/threonine kinase that play a vital role in maintaining the genomic integrity of the cells. There are three forms of aurora kinases in the mammalian cells, which are highly conserved and act together with several other proteins to control chromosome alignment and its equal distribution to daughter cells in mitosis and meiosis.

METHODS

We provide here a detailed analysis of Aurora B kinase (ABK) in terms of its expression, structure, function, disease association and potential therapeutic implications.

RESULTS

ABK plays an instrumental in mitotic entry, chromosome condensation, spindle assembly, cytokinesis, and abscission. Small-molecule inhibitors of ABK are designed and synthesized to control cancer progression. A detailed understanding of ABK pathophysiology in different cancers is of great significance in designing and developing effective therapeutic strategies.

CONCLUSION

In this review, we have discussed the physiological significance of ABK followed by its role in cancer progression. We further highlighted available small-molecule inhibitors to control the tumor proliferation and their mechanistic insights.

Common Fragile Sites Are Characterized by Faulty Condensin Loading after Replication Stress

Cells coordinate interphase-to-mitosis transition, but recurrent cytogenetic lesions appear at common fragile sites (CFSs), termed CFS expression, in a tissue-specific manner after replication stress, marking regions of instability in cancer. Despite such a distinct defect, no model fully provides a molecular explanation for CFSs. We show that CFSs are characterized by impaired chromatin folding, manifesting as disrupted mitotic structures visible with molecular fluorescence in situ hybridization (FISH) probes in the presence and absence of replication stress. Chromosome condensation assays reveal that compaction-resistant chromatin lesions persist at CFSs throughout the cell cycle and mitosis. Cytogenetic and molecular lesions are marked by faulty condensin loading at CFSs, a defect in condensin-I-mediated compaction, and are coincident with mitotic DNA synthesis (MIDAS). This model suggests that, in conditions of exogenous replication stress, aberrant condensin loading leads to molecular defects and CFS expression, concomitantly providing an environment for MIDAS, which, if not resolved, results in chromosome instability.

Mitotic chromosomes

Our understanding of the structure and function of mitotic chromosomes has come a long way since these iconic objects were first recognized more than 140 years ago, though many details remain to be elucidated. In this chapter, we start with the early history of chromosome studies and then describe the path that led to our current understanding of the formation and structure of mitotic chromosomes. We also discuss some of the remaining questions. It is now well established that each mitotic chromatid consists of a central organizing region containing a so-called "chromosome scaffold" from which loops of DNA project radially. Only a few key non-histone proteins and protein complexes are required to form the chromosome: topoisomerase IIα, cohesin, condensin I and condensin II, and the chromokinesin KIF4A. These proteins are concentrated along the axis of the chromatid. Condensins I and II are primarily responsible for shaping the chromosome and the scaffold, and they produce the loops of DNA by an ATP-dependent process known as loop extrusion. Modelling of Hi-C data suggests that condensin II adopts a spiral staircase arrangement with an extruded loop extending out from each step in a roughly helical pattern. Condensin I then forms loops nested within these larger condensin II loops, thereby giving rise to the final compaction of the mitotic chromosome in a process that requires Topo IIα.

Cell division requires RNA eviction from condensing chromosomes

During mitosis, the genome is transformed from a decondensed, transcriptionally active state to a highly condensed, transcriptionally inactive state. Mitotic chromosome reorganization is marked by the general attenuation of transcription on chromosome arms, yet how the cell regulates nuclear and chromatin-associated RNAs after chromosome condensation and nuclear envelope breakdown is unknown. SAF-A/hnRNPU is an abundant nuclear protein with RNA-to-DNA tethering activity, coordinated by two spatially distinct nucleic acid–binding domains. Here we show that RNA is evicted from prophase chromosomes through Aurora-B–dependent phosphorylation of the SAF-A DNA-binding domain; failure to execute this pathway leads to accumulation of SAF-A–RNA complexes on mitotic chromosomes, defects in metaphase chromosome alignment, and elevated rates of chromosome missegregation in anaphase. This work reveals a role for Aurora-B in removing chromatin-associated RNAs during prophase and demonstrates that Aurora-B–dependent relocalization of SAF-A during cell division contributes to the fidelity of chromosome segregation.

From the cytoskeleton to the nucleus: An integrated view on early spindle assembly

Accurate chromosome segregation requires a complete restructuring of cellular organization. Microtubules remodel to assemble a mitotic spindle and the actin cytoskeleton rearranges to form a stiff actomyosin cortex. These cytoplasmic events must be spatially and temporally coordinated with mitotic chromosome condensation and nuclear envelope permeabilization, in order to ensure mitotic timing and fidelity. Here, we discuss the main cytoskeletal and nuclear events that occur during mitotic entry in proliferating animal cells, focusing on their coordinated contribution for early mitotic spindle assembly. We will also explore recent progress in understanding their regulatory biochemical and mechanical pathways.

Condensin complexes: understanding loop extrusion one conformational change at a time

Condensin and cohesin, both members of the structural maintenance of chromosome (SMC) family, contribute to the regulation and structure of chromatin. Recent work has shown both condensin and cohesin extrude DNA loops and most likely work via a conserved mechanism. This review focuses on condensin complexes, highlighting recent in vitro work characterising DNA loop formation and protein structure. We discuss similarities between condensin and cohesin complexes to derive a possible mechanistic model, as well as discuss differences that exist between the different condensin isoforms found in higher eukaryotes.

Aurora B and condensin are dispensable for chromosome arm and telomere separation during meiosis II

In mitosis, while the importance of kinetochore (KT)-microtubule (MT) attachment has been known for many years, increasing evidence suggests that telomere dysfunctions also perturb chromosome segregation by contributing to the formation of chromatin bridges at anaphase. Recent evidence suggests that Aurora B kinase ensures proper chromosome segregation during mitosis not only by controlling KT-MT attachment but also by regulating telomere and chromosome arm separation. However, whether and how Aurora B governs telomere separation during meiosis has remained unknown. Here, we show that fission yeast Aurora B localizes at telomeres during meiosis I and promotes telomere separation independently of the meiotic cohesin Rec8. In meiosis II, Aurora B controls KT-MT attachment but appears dispensable for telomere and chromosome arm separation. Likewise, condensin activity is nonessential in meiosis II for telomere and chromosome arm separation. Thus, in meiosis, the requirements for Aurora B are distinct at centromeres and telomeres, illustrating the critical differences in the control of chromosome segregation between mitosis and meiosis II.

PRMT6-mediated H3R2me2a guides Aurora B to chromosome arms for proper chromosome segregation

The kinase Aurora B forms the chromosomal passenger complex (CPC) together with Borealin, INCENP, and Survivin to mediate chromosome condensation, the correction of erroneous spindle-kinetochore attachments, and cytokinesis. Phosphorylation of histone H3 Thr3 by Haspin kinase and of histone H2A Thr120 by Bub1 concentrates the CPC at the centromere. However, how the CPC is recruited to chromosome arms upon mitotic entry is unknown. Here, we show that asymmetric dimethylation at Arg2 on histone H3 (H3R2me2a) by protein arginine methyltransferase 6 (PRMT6) recruits the CPC to chromosome arms and facilitates histone H3S10 phosphorylation by Aurora B for chromosome condensation. Furthermore, in vitro assays show that Aurora B preferentially binds to the H3 peptide containing H3R2me2a and phosphorylates H3S10. Our findings indicate that the long-awaited key histone mark for CPC recruitment onto mitotic chromosomes is H3R2me2a, which is indispensable for maintaining appropriate CPC levels in dynamic translocation throughout mitosis.

The proteins of the chromosomal passenger complex help chromosomes condense before cell division, but how this complex arrives at chromosomes was not known. Here the authors show that PRMT6 methylates histone H3 to recruit the chromosomal passenger complex.

Borealin-nucleosome interaction secures chromosome association of the chromosomal passenger complex

Chromosome association of the chromosomal passenger complex (CPC; consisting of Borealin, Survivin, INCENP, and the Aurora B kinase) is essential to achieve error-free chromosome segregation during cell division. Hence, understanding the mechanisms driving the chromosome association of the CPC is of paramount importance. Here using a multifaceted approach, we show that the CPC binds nucleosomes through a multivalent interaction predominantly involving Borealin. Strikingly, Survivin, previously suggested to target the CPC to centromeres, failed to bind nucleosomes on its own and requires Borealin and INCENP for its binding. Disrupting Borealin-nucleosome interactions excluded the CPC from chromosomes and caused chromosome congression defects. We also show that Borealin-mediated chromosome association of the CPC is critical for Haspin- and Bub1-mediated centromere enrichment of the CPC and works upstream of the latter. Our work thus establishes Borealin as a master regulator determining the chromosome association and function of the CPC.

Interplay between Phosphatases and the Anaphase-Promoting Complex/Cyclosome in Mitosis

Accurate division of cells into two daughters is a process that is vital to propagation of life. Protein phosphorylation and selective degradation have emerged as two important mechanisms safeguarding the delicate choreography of mitosis. Protein phosphatases catalyze dephosphorylation of thousands of sites on proteins, steering the cells through establishment of the mitotic phase and exit from it. A large E3 ubiquitin ligase, the anaphase-promoting complex/cyclosome (APC/C) becomes active during latter stages of mitosis through G1 and marks hundreds of proteins for destruction. Recent studies have revealed the complex interregulation between these two classes of enzymes. In this review, we highlight the direct and indirect mechanisms by which phosphatases and the APC/C mutually influence each other to ensure accurate spatiotemporal and orderly progression through mitosis, with a particular focus on recent insights and conceptual advances.

Cells Lacking the RB1 Tumor Suppressor Gene Are Hyperdependent on Aurora B Kinase for Survival

Small cell lung cancer (SCLC) accounts for 15% of lung cancers and is almost always linked to inactivating RB1 and TP53 mutations. SCLC frequently responds, albeit briefly, to chemotherapy. The canonical function of the RB1 gene product RB1 is to repress the E2F transcription factor family. RB1 also plays both E2F-dependent and E2F-independent mitotic roles. We performed a synthetic lethal CRISPR/Cas9 screen in an RB1 -/- SCLC cell line that conditionally expresses RB1 to identify dependencies that are caused by RB1 loss and discovered that RB1 -/- SCLC cell lines are hyperdependent on multiple proteins linked to chromosomal segregation, including Aurora B kinase. Moreover, we show that an Aurora B kinase inhibitor is efficacious in multiple preclinical SCLC models at concentrations that are well tolerated in mice. These results suggest that RB1 loss is a predictive biomarker for sensitivity to Aurora B kinase inhibitors in SCLC and perhaps other RB1 -/- cancers. SIGNIFICANCE: SCLC is rarely associated with actionable protooncogene mutations. We did a CRISPR/Cas9-based screen that showed that RB1 -/- SCLC are hyperdependent on AURKB, likely because both genes control mitotic fidelity, and confirmed that Aurora B kinase inhibitors are efficacious against RB1 -/- SCLC tumors in mice at nontoxic doses.See related commentary by Dick and Li, p. 169.This article is highlighted in the In This Issue feature, p. 151.

Are SMC Complexes Loop Extruding Factors? Linking Theory With Fact

The extreme length of chromosomal DNA requires organizing mechanisms to both promote functional genetic interactions and ensure faithful chromosome segregation when cells divide. Microscopy and genome-wide contact frequency analyses indicate that intra-chromosomal looping of DNA is a primary pathway of chromosomal organization during all stages of the cell cycle. DNA loop extrusion has emerged as a unifying model for how chromosome loops are formed in cis in different genomic contexts and cell cycle stages. The highly conserved family of SMC complexes have been found to be required for DNA cis-looping and have been suggested to be the enzymatic core of loop extruding machines. Here, the current body of evidence available for the in vivo and in vitro action of SMC complexes is discussed and compared to the predictions made by the loop extrusion model. How SMC complexes may differentially act on chromatin to generate DNA loops and how they could work to generate the dynamic and functionally appropriate organization of DNA in cells is explored.

Protein phosphatases in chromatin structure and function

Chromatin structure and dynamics are highly controlled and regulated processes that play an essential role in many aspects of cell biology. The chromatin transition stages and the factors that control this process are regulated by post-translation modifications, including phosphorylation. While the role of protein kinases in chromatin dynamics has been quite well studied, the nature and regulation of the counteracting phosphatases represent an emerging field but are still at their infancy. In this review we summarize the current literature on phosphatases involved in the regulation of chromatin structure and dynamics, with emphases on the major knowledge gaps that should require attention and more investigation.

Subcellular scaling: does size matter for cell division?

Among different species or cell types, or during early embryonic cell divisions that occur in the absence of cell growth, the size of subcellular structures, including the nucleus, chromosomes, and mitotic spindle, scale with cell size. Maintaining correct subcellular scales is thought to be important for many cellular processes and, in particular, for mitosis. In this review, we provide an update on nuclear and chromosome scaling mechanisms and their significance in metazoans, with a focus on Caenorhabditis elegans, Xenopus and mammalian systems, for which a common role for the Ran (Ras-related nuclear protein)-dependent nuclear transport system has emerged.

Subunits of human condensins are potential therapeutic targets for cancers

The main role of condensins is to regulate chromosome condensation and segregation during cell cycles. Recently, it has been suggested in the literatures that subunits of condensin I and condensin II are involved in some human cancers. This paper will first briefly discuss discoveries of human condensins, their components and structures, and their multiple cellular functions. This will be followed by reviews of most recent studies on subunits of human condensins and their dysregulations or mutations in human cancers. It can be concluded that many of these subunits have potentials to be novel targets for cancer therapies. However, hCAP-D2, a subunit of human condensin I, has not been directly documented to be associated with any human cancers to date. This review hypothesizes that hCAP-D2 can also be a potential therapeutic target for human cancers, and therefore that all subunits of human condensins are potential therapeutic targets for human cancers.

Cdc14 and Chromosome Condensation: Evaluation of the Recruitment of Condensin to Genomic Regions

Chromosome condensation is an essential morphological event required for successful DNA segregation during mitosis. The high level of genome compaction achieved during this process is attained by the evolutionary conserved condensin complex. Recently, several lines of evidences have demonstrated that the mitotic phosphatase Cdc14 is required to ensure condensin loading onto chromosomes. To date several approaches have been used in order to characterize condensin activity and regulation, however these techniques are time-consuming and require complex equipment. In this chapter we described an easy and reliable protocol to analyze Cdc14-dependent condensin loading onto specific genomic DNA regions by using a chromatin immunoprecipitation (ChIP) technique.

Interdependency and phosphorylation of KIF4 and condensin I are essential for organization of chromosome scaffold

Kinesin family member 4 (KIF4) and condensins I and II are essential chromosomal proteins for chromosome organization by locating primarily to the chromosome scaffold. However, the mechanism of how KIF4 and condensins localize to the chromosome scaffold is poorly understood. Here, we demonstrate a close relationship between the chromosome localization of KIF4 and condensin I, but not condensin II, and show that KIF4 and condensin I assist each other for stable scaffold formation by forming a stable complex. Moreover, phosphorylation of KIF4 and condensin I by Aurora B and polo-like kinase 1 (Plk1) is important for KIF4 and condensin I localization to the chromosome. Aurora B activity facilitates the targeting of KIF4 and condensin I to the chromosome, whereas Plk1 activity promotes the dissociation of these proteins from the chromosome. Thus, the interdependency between KIF4 and condensin I, and their phosphorylation states play important roles in chromosome scaffold organization during mitosis.

Phosphoproteomic analysis of human mitotic chromosomes identified a chromokinesin KIF4A

Protein phosphorylation is the prime post-translational modification to drive cell division. Identification of phosphorylated proteins and their related kinases has uncovered molecular networks underlying mitotic processes, including chromosome assembly. Here we aimed to identify phosphoproteins from a mitotic chromosome-enriched lysate biochemically using mass spectrometry. We employed the Polo-box domain (PBD) of Polo-like kinase to tether phosphorylated proteins in the lysate. Resulting candidates included a number of chromosomal proteins that would not be identified unless using chromosome-enriched fractions. Among them, we focused to a chromokinesin KIF4A which becomes concentrated along the longitudinal axis of mitotic condensed chromatids. We found that KIF4A is phosphorylated specifically during mitosis, depending on the activity of Cdk1 and Aurora B, which turned out to be required for KIF4A to interact with condensin I. The molecular link between KIF4A and condensin raises an interesting possibility that the interaction of two distantly related ATPases is triggered by mitotic phosphorylation.

Epigenetic countermarks in mitotic chromosome condensation

Mitosis in metazoans is characterized by abundant phosphorylation of histone H3 and involves the recruitment of condensin complexes to chromatin. The relationship between the 2 phenomena and their respective contributions to chromosome condensation in vivo remain poorly understood. Recent studies have shown that H3T3 phosphorylation decreases binding of histone readers to methylated H3K4 in vitro and is essential to displace the corresponding proteins from mitotic chromatin in vivo. Together with previous observations, these data provide further evidence for a role of mitotic histone H3 phosphorylation in blocking transcriptional programs or preserving the ‘memory’ PTMs. Mitotic protein exclusion can also have a role in depopulating the chromatin template for subsequent condensin loading. H3 phosphorylation thus serves as an integral step in the condensation of chromosome arms.

Condensin recruitment to chromatin is inhibited by Chk2 kinase in response to DNA damage

The DNA damage checkpoint, when activated in response to genotoxic damage during S phase, arrests cells in G2 phase of the cell cycle. ATM, ATR, Chk1 and Chk2 kinases are the main effectors of this checkpoint pathway. The checkpoint kinases prevent the onset of mitosis by eliciting well characterized inhibitory phosphorylation of Cdk1. Since Cdk1 is required for the recruitment of condensin, it is thought that upon DNA damage the checkpoint also indirectly blocks chromosome condensation via Cdk1 inhibition. Here we report that the G2 damage checkpoint prevents stable recruitment of the chromosome-packaging-machinery components condensin complex I and II onto the chromatin even in the presence of an active Cdk1. DNA damage-induced inhibition of condensin subunit recruitment is mediated specifically by the Chk2 kinase, implying that the condensin complexes are targeted by the checkpoint in response to DNA damage, independently of Cdk1 inactivation. Thus, the G2 checkpoint directly prevents stable recruitment of condensin complexes to actively prevent chromosome compaction during G2 arrest, presumably to ensure efficient repair of the genomic damage.

Condensin I-mediated mitotic chromosome assembly requires association with chromokinesin KIF4A

In this study, Takahashi et al. investigated the role of chromokinesin KIF4A in condensin complexes. They found that KIF4A associates with condensin I but not with condensin II during mitosis, providing new insight into the regulation of condensin I and mitotic chromsome organization.

The chromokinesin KIF4A has been implicated in shaping mitotic chromosomes, but its functional relationship to condensin complexes remains controversial. Here, we found that, in mitosis, KIF4A associates with condensin I but not with condensin II. Mutational analyses indicated that the enrichment of condensin I to chromosomal axes depends on its association with KIF4A in a way that likely involves its motor activity. Remarkably, this interaction is required for condensin I to confer physiological properties to chromosomes. These observations provide an insight into how condensin I is enriched at chromosomal axes and underscore the significance of axial structure in organizing mitotic chromosomes.

The Epigenetic Pathways to Ribosomal DNA Silencing

Heterochromatin is the transcriptionally repressed portion of eukaryotic chromatin that maintains a condensed appearance throughout the cell cycle. At sites of ribosomal DNA (rDNA) heterochromatin, epigenetic states contribute to gene silencing and genome stability, which are required for proper chromosome segregation and a normal life span. Here, we focus on recent advances in the epigenetic regulation of rDNA silencing in Saccharomyces cerevisiae and in mammals, including regulation by several histone modifications and several protein components associated with the inner nuclear membrane within the nucleolus. Finally, we discuss the perturbations of rDNA epigenetic pathways in regulating cellular aging and in causing various types of diseases.

A mathematical model of Aurora B activity in prophase and metaphase

Aurora B kinase is a protein that controls several processes in mitosis when it is found associated with INCENP, Survivin and Borealin in a complex known as the Chromosomal Passenger Complex. Aurora B in complex with INCENP is phosphorylated on three sites, resulting in the full activation of Aurora B. In prophase and metaphase, Aurora B is activated at centromeres, the region of chromatin linking sister chromatids, due to an autophosphorylation mechanism, and it has been hypothesised that Aurora B is activated throughout the cytoplasm due to its concentration at centromeres. In this article, we first develop a time-dependent model of Aurora B activation that does not incorporate spatial variation. This model is used to demonstrate the various qualitative behaviours that the activation of Aurora B is capable of displaying for different model parameters. Next, we develop a spatio-temporal model of Aurora B activation that includes diffusion of soluble Aurora B and binding of Aurora B to immobile centromeric binding sites. This model describes the activation of Aurora B throughout the cytoplasm due to its concentration-dependent activation at centromeres. The models demonstrate the effects that a soluble phosphatase concentration, multisite phosphorylation and diffusion have on the activation of Aurora B.

Conditional mutation of Smc5 in mouse embryonic stem cells perturbs condensin localization and mitotic progression

Correct duplication of stem cell genetic material and its appropriate segregation into daughter cells are requisites for tissue, organ and organism homeostasis. Disruption of stem cell genomic integrity can lead to developmental abnormalities and cancer. Roles of the Smc5/6 structural maintenance of chromosomes complex in pluripotent stem cell genome maintenance have not been investigated, despite its important roles in DNA synthesis, DNA repair and chromosome segregation as evaluated in other model systems. Using mouse embryonic stem cells (mESCs) with a conditional knockout allele of Smc5, we showed that Smc5 protein depletion resulted in destabilization of the Smc5/6 complex, accumulation of cells in G2 phase of the cell cycle and apoptosis. Detailed assessment of mitotic mESCs revealed abnormal condensin distribution and perturbed chromosome segregation, accompanied by irregular spindle morphology, lagging chromosomes and DNA bridges. Mutation of Smc5 resulted in retention of Aurora B kinase and enrichment of condensin on chromosome arms. Furthermore, we observed reduced levels of Polo-like kinase 1 at kinetochores during mitosis. Our study reveals crucial requirements of the Smc5/6 complex during cell cycle progression and for stem cell genome maintenance.

Unconventional Functions of Mitotic Kinases in Kidney Tumorigenesis

Human tumors exhibit a variety of genetic alterations, including point mutations, translocations, gene amplifications and deletions, as well as aneuploid chromosome numbers. For carcinomas, aneuploidy is associated with poor patient outcome for a large variety of tumor types, including breast, colon, and renal cell carcinoma. The Renal cell carcinoma (RCC) is a heterogeneous carcinoma consisting of different histologic types. The clear renal cell carcinoma (ccRCC) is the most common subtype and represents 85% of the RCC. Central to the biology of the ccRCC is the loss of function of the Von Hippel–Lindau gene, but is also associated with genetic instability that could be caused by abrogation of the cell cycle mitotic spindle checkpoint and may involve the Aurora kinases, which regulate centrosome maturation. Aneuploidy can also result from the loss of cell–cell adhesion and apical–basal cell polarity that also may be regulated by the mitotic kinases (polo-like kinase 1, casein kinase 2, doublecortin-like kinase 1, and Aurora kinases). In this review, we describe the “non-mitotic” unconventional functions of these kinases in renal tumorigenesis.

Quantitative phosphoproteomics reveals new roles for the protein phosphatase PP6 in mitotic cells

Protein phosphorylation is an important regulatory mechanism controlling mitotic progression. Protein phosphatase 6 (PP6) is an essential enzyme with conserved roles in chromosome segregation and spindle assembly from yeast to humans. We applied a baculovirus-mediated gene silencing approach to deplete HeLa cells of the catalytic subunit of PP6 (PP6c) and analyzed changes in the phosphoproteome and proteome in mitotic cells by quantitative mass spectrometry-based proteomics. We identified 408 phosphopeptides on 272 proteins that increased and 298 phosphopeptides on 220 proteins that decreased in phosphorylation upon PP6c depletion in mitotic cells. Motif analysis of the phosphorylated sites combined with bioinformatics pathway analysis revealed previously unknown PP6c-dependent regulatory pathways. Biochemical assays demonstrated that PP6c opposed casein kinase 2-dependent phosphorylation of the condensin I subunit NCAP-G, and cellular analysis showed that depletion of PP6c resulted in defects in chromosome condensation and segregation in anaphase, consistent with dysregulation of condensin I function in the absence of PP6 activity.

Aurora B prevents chromosome arm separation defects by promoting telomere dispersion and disjunction

The segregation of centromeres and telomeres at mitosis is coordinated at multiple levels to prevent the formation of aneuploid cells, a phenotype frequently observed in cancer. Mitotic instability arises from chromosome segregation defects, giving rise to chromatin bridges at anaphase. Most of these defects are corrected before anaphase onset by a mechanism involving Aurora B kinase, a key regulator of mitosis in a wide range of organisms. Here, we describe a new role for Aurora B in telomere dispersion and disjunction during fission yeast mitosis. Telomere dispersion initiates in metaphase, whereas disjunction takes place in anaphase. Dispersion is promoted by the dissociation of Swi6/HP1 and cohesin Rad21 from telomeres, whereas disjunction occurs at anaphase after the phosphorylation of condensin subunit Cnd2. Strikingly, we demonstrate that deletion of Ccq1, a telomeric shelterin component, rescued cell death after Aurora inhibition by promoting the loading of condensin on chromosome arms. Our findings reveal an essential role for telomeres in chromosome arm segregation.

Shaping mitotic chromosomes: From classical concepts to molecular mechanisms

How eukaryotic genomes are packaged into compact cylindrical chromosomes in preparation for cell divisions has remained one of the major unsolved questions of cell biology. Novel approaches to study the topology of DNA helices inside the nuclei of intact cells, paired with computational modeling and precise biomechanical measurements of isolated chromosomes, have advanced our understanding of mitotic chromosome architecture. In this Review Essay, we discuss - in light of these recent insights - the role of chromatin architecture and the functions and possible mechanisms of SMC protein complexes and other molecular machines in the formation of mitotic chromosomes. Based on the information available, we propose a stepwise model of mitotic chromosome condensation that envisions the sequential generation of intra-chromosomal linkages by condensin complexes in the context of cohesin-mediated inter-chromosomal linkages, assisted by topoisomerase II. The described scenario results in rod-shaped metaphase chromosomes ready for their segregation to the cell poles.

The chromosomal passenger complex (CPC) as a key orchestrator of orderly mitotic exit and cytokinesis

Understanding the molecular network of orderly mitotic exit to re-establish a functional interphase nucleus is critical because disordered mitotic exit inevitably leads to genomic instability. In contrast to the mechanisms of the entrance to mitosis, however, little is known about what controls the orderly exit from mitosis, particularly in mammalian cells. The chromosomal passenger complex (CPC), which is composed of Aurora B, INCENP, Borealin and Survivin, is one of the most widely studied and highly conserved hetero-tetrameric complexes. The CPC orchestrates proper chromosome segregation with cytokinesis by targeting to specific locations at different stages of mitosis. Recent studies reveal that controlling CPC localization and Aurora B kinase activity also serves as a key surveillance mechanism for the orderly mitotic exit. This ensures the reformation of a functional interphase nucleus from condensed mitotic chromosomes by delaying mitotic exit and cytokinetic processes in response to defects in chromosome segregation. In this review, we will summarize the latest insight into the molecular mechanisms that regulate CPC localization during mitotic exit and discuss how targeting Aurora B activity to different locations at different times impacts executing multiple mitotic exit events in order and recently proposed surveillance mechanisms. Finally, we briefly discuss the potential implication of deregulated Aurora B in inducing genomic damage and tumorigenesis with current efforts in targeting Aurora B activity for anti-cancer therapy.

Chromosome Dynamics during Mitosis

The primary goal of mitosis is to partition duplicated chromosomes into daughter cells. Eukaryotic chromosomes are equipped with two distinct classes of intrinsic machineries, cohesin and condensins, that ensure their faithful segregation during mitosis. Cohesin holds sister chromatids together immediately after their synthesis during S phase until the establishment of bipolar attachments to the mitotic spindle in metaphase. Condensins, on the other hand, attempt to "resolve" sister chromatids by counteracting cohesin. The products of the balancing acts of cohesin and condensins are metaphase chromosomes, in which two rod-shaped chromatids are connected primarily at the centromere. In anaphase, this connection is released by the action of separase that proteolytically cleaves the remaining population of cohesin. Recent studies uncover how this series of events might be mechanistically coupled with each other and intricately regulated by a number of regulatory factors.

MicroRNA let-7b regulates genomic balance by targeting Aurora B kinase

The let-7 microRNA (miRNA) family has been implicated in the regulation of diverse cellular processes and disease pathogenesis. In cancer, loss-of-function of let-7 miRNAs has been linked to tumorigenesis via increased expression of target oncogenes. Excessive proliferation rate of tumor cells is often associated with deregulation of mitotic proteins. Here, we show that let-7b contributes to the maintenance of genomic balance via targeting Aurora B kinase, a key regulator of the spindle assembly checkpoint (SAC). Our results indicate that let-7b binds to Aurora B kinase 3'UTR reducing mRNA and protein expression of the kinase. In cells, excess let-7b induced mitotic defects characteristic to Aurora B perturbation including increased rate of polyploidy and multipolarity, and premature SAC inactivation that leads to forced exit from chemically induced mitotic arrest. Moreover, the frequency of aneuploid HCT-116 cells was significantly increased upon let-7b overexpression compared to controls. Interestingly, together with a chemical Aurora B inhibitor, let-7b had an additive effect on polyploidy induction in HeLa cells. In breast cancer patients, reduced let-7b expression was found to be associated with increased Aurora B expression in grade 3 tumors. Furthermore, let-7b was found downregulated in the most aggressive forms of breast cancer determined by clinicopathological parameters. Together, our findings suggest that let-7b contributes to the fidelity of cell division via regulation of Aurora B. Moreover, the loss of let-7b in aggressive tumors may drive tumorigenesis by up-regulation of Aurora B and other targets of the miRNA, which further supports the role of let-7b in tumor suppression.

Disruption of a conserved CAP-D3 threonine alters condensin loading on mitotic chromosomes leading to chromosome hypercondensation

The condensin complex plays a key role in organizing mitotic chromosomes. In vertebrates, there are two condensin complexes that have independent and cooperative roles in folding mitotic chromosomes. In this study, we dissect the role of a putative Cdk1 site on the condensin II subunit CAP-D3 in chicken DT40 cells. This conserved site has been shown to activate condensin II during prophase in human cells, and facilitate further phosphorylation by polo-like kinase I. We examined the functional significance of this phosphorylation mark by mutating the orthologous site of CAP-D3 (CAP-D3(T1403A)) in chicken DT40 cells. We show that this mutation is a gain of function mutant in chicken cells; it disrupts prophase, results in a dramatic shortening of the mitotic chromosome axis, and leads to abnormal INCENP localization. Our results imply phosphorylation of CAP-D3 acts to limit condensin II binding onto mitotic chromosomes. We present the first in vivo example that alters the ratio of condensin I:II on mitotic chromosomes. Our results demonstrate this ratio is a critical determinant in shaping mitotic chromosomes.

The maintenance of chromosome structure: positioning and functioning of SMC complexes

Structural maintenance of chromosomes (SMC) complexes, which in eukaryotic cells include cohesin, condensin and the Smc5/6 complex, are central regulators of chromosome dynamics and control sister chromatid cohesion, chromosome condensation, DNA replication, DNA repair and transcription. Even though the molecular mechanisms that lead to this large range of functions are still unclear, it has been established that the complexes execute their functions through their association with chromosomal DNA. A large set of data also indicates that SMC complexes work as intermolecular and intramolecular linkers of DNA. When combining these insights with results from ongoing analyses of their chromosomal binding, and how this interaction influences the structure and dynamics of chromosomes, a picture of how SMC complexes carry out their many functions starts to emerge.

Phosphorylation of threonine 3 on histone H3 by haspin kinase is required for meiosis I in mouse oocytes

Meiosis I (MI), the division that generates haploids, is prone to errors that lead to aneuploidy in females. Haspin is a kinase that phosphorylates histone H3 on threonine 3, thereby recruiting Aurora kinase B (AURKB) and the chromosomal passenger complex (CPC) to kinetochores to regulate mitosis. Haspin and AURKC, an AURKB homolog, are enriched in germ cells, yet their significance in regulating MI is not fully understood. Using inhibitors and overexpression approaches, we show a role for haspin during MI in mouse oocytes. Haspin-perturbed oocytes display abnormalities in chromosome morphology and alignment, improper kinetochore-microtubule attachments at metaphase I and aneuploidy at metaphase II. Unlike in mitosis, kinetochore localization remained intact, whereas the distribution of the CPC along chromosomes was absent. The meiotic defects following haspin inhibition were similar to those observed in oocytes where AURKC was inhibited, suggesting that the correction of microtubule attachments during MI requires AURKC along chromosome arms rather than at kinetochores. Our data implicate haspin as a regulator of the CPC and chromosome segregation during MI, while highlighting important differences in how chromosome segregation is regulated between MI and mitosis.

Mps1 phosphorylation of condensin II controls chromosome condensation at the onset of mitosis

Mps1 is necessary for proper condensin II loading onto chromatin and subsequent chromosome condensation during mitosis.

During mitosis, genomic DNA is condensed into chromosomes to promote its equal segregation into daughter cells. Chromosome condensation occurs during cell cycle progression from G2 phase to mitosis. Failure of chromosome compaction at prophase leads to subsequent misregulation of chromosomes. However, the molecular mechanism that controls the early phase of mitotic chromosome condensation is largely unknown. Here, we show that Mps1 regulates initial chromosome condensation during mitosis. We identify condensin II as a novel Mps1-associated protein. Mps1 phosphorylates one of the condensin II subunits, CAP-H2, at Ser492 during mitosis, and this phosphorylation event is required for the proper loading of condensin II on chromatin. Depletion of Mps1 inhibits chromosomal targeting of condensin II and accurate chromosome condensation during prophase. These findings demonstrate that Mps1 governs chromosomal organization during the early stage of mitosis to facilitate proper chromosome segregation.

Feedback control of chromosome separation by a midzone Aurora B gradient

Accurate chromosome segregation during mitosis requires the physical separation of sister chromatids before nuclear envelope reassembly (NER). However, how these two processes are coordinated remains unknown. Here, we identified a conserved feedback control mechanism that delays chromosome decondensation and NER in response to incomplete chromosome separation during anaphase. A midzone-associated Aurora B gradient was found to monitor chromosome position along the division axis and to prevent premature chromosome decondensation by retaining Condensin I. PP1/PP2A phosphatases counteracted this gradient and promoted chromosome decondensation and NER. Thus, an Aurora B gradient appears to mediate a surveillance mechanism that prevents chromosome decondensation and NER until effective separation of sister chromatids is achieved. This allows the correction and reintegration of lagging chromosomes in the main nuclei before completion of NER.