Cohesion fatigue

A non-canonical role of the inner kinetochore in regulating sister-chromatid cohesion at centromeres

The 16-subunit Constitutive Centromere-associated Network (CCAN)-based inner kinetochore is well-known for connecting centromeric chromatin to the spindle-binding outer kinetochore. Here, we report a non-canonical role for the inner kinetochore in directly regulating sister-chromatid cohesion at centromeres. We provide biochemical, X-ray crystal structure, and intracellular ectopic localization evidence that the inner kinetochore directly binds cohesin, a ring-shaped multi-subunit complex that holds sister chromatids together from S-phase until anaphase onset. This interaction is mediated by binding of the 5-subunit CENP-OPQUR sub-complex of CCAN to the Scc1-SA2 sub-complex of cohesin. Mutation in the CENP-U subunit of the CENP-OPQUR complex that abolishes its binding to the composite interface between Scc1 and SA2 weakens centromeric cohesion, leading to premature separation of sister chromatids during delayed metaphase. We further show that CENP-U competes with the cohesin release factor Wapl for binding the interface of Scc1-SA2, and that the cohesion-protecting role for CENP-U can be bypassed by depleting Wapl. Taken together, this study reveals an inner kinetochore-bound pool of cohesin, which strengthens centromeric sister-chromatid cohesion to resist metaphase spindle pulling forces.

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.

Tetraploidy-linked sensitization to CENP-E inhibition in human cells

Tetraploidy is a hallmark of cancer cells, and tetraploidy-selective cell growth suppression is a potential strategy for targeted cancer therapy. However, how tetraploid cells differ from normal diploids in their sensitivity to anti-proliferative treatments remains largely unknown. In this study, we found that tetraploid cells are significantly more susceptible to inhibitors of a mitotic kinesin (CENP-E) than are diploids. Treatment with a CENP-E inhibitor preferentially diminished the tetraploid cell population in a diploid-tetraploid co-culture at optimum conditions. Live imaging revealed that a tetraploidy-linked increase in unsolvable chromosome misalignment caused substantially longer mitotic delay in tetraploids than in diploids upon moderate CENP-E inhibition. This time gap of mitotic arrest resulted in cohesion fatigue and subsequent cell death, specifically in tetraploids, leading to tetraploidy-selective cell growth suppression. In contrast, the microtubule-stabilizing compound paclitaxel caused tetraploidy-selective suppression through the aggravation of spindle multipolarization. We also found that treatment with a CENP-E inhibitor had superior generality to paclitaxel in its tetraploidy selectivity across a broader spectrum of cell lines. Our results highlight the unique properties of CENP-E inhibitors in tetraploidy-selective suppression and their potential use in the development of tetraploidy-targeting interventions in cancer.

Absence of a robust mitotic timer mechanism in early preimplantation mouse embryos leads to chromosome instability

Preimplantation embryos often consist of a combination of euploid and aneuploid cells, suggesting that safeguards preventing the generation and propagation of aneuploid cells in somatic cells might be deficient in embryos. In somatic cells, a mitotic timer mechanism has been described, in which even a small increase in the duration of M phase can cause a cell cycle arrest in the subsequent interphase, preventing further propagation of cells that have undergone a potentially hazardously long M phase. Here, we report that cell divisions in the mouse embryo and embryonic development continue even after a mitotic prolongation of several hours. However, similar M-phase extensions caused cohesion fatigue, resulting in prematurely separated sister chromatids and the production of micronuclei. Only extreme prolongation of M phase caused a subsequent interphase arrest, through a mechanism involving DNA damage. Our data suggest that the simultaneous absence of a robust mitotic timer and susceptibility of the embryo to cohesion fatigue could contribute to chromosome instability in mammalian embryos.

This article has an associated ‘The people behind the papers’ interview.

Spindle assembly checkpoint activation and silencing at kinetochores

The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.

Interactions between N-terminal Modules in MPS1 Enable Spindle Checkpoint Silencing

Faithful chromosome segregation relies on the ability of the spindle assembly checkpoint (SAC) to delay anaphase onset until chromosomes are attached to the mitotic spindle via their kinetochores. MPS1 kinase is recruited to kinetochores to initiate SAC signaling and is removed from kinetochores once stable microtubule attachments have been formed to allow normal mitotic progression. Here, we show that a helical fragment within the kinetochore-targeting N-terminal extension (NTE) module of MPS1 is required for interactions with kinetochores and forms intramolecular interactions with its adjacent tetratricopeptide repeat (TPR) domain. Bypassing this NTE-TPR interaction results in high MPS1 levels at kinetochores due to loss of regulatory input into MPS1 localization, inefficient MPS1 delocalization upon microtubule attachment, and SAC silencing defects. These results show that SAC responsiveness to attachments relies on regulated intramolecular interactions in MPS1 and highlight the sensitivity of mitosis to perturbations in the dynamics of the MPS1-NDC80-C interactions.

Measurement of Microtubule Half-Life and Poleward Flux in the Mitotic Spindle by Photoactivation of Fluorescent Tubulin

The study of microtubule dynamics is of utmost importance for the understanding of the mechanisms underlying mitotic fidelity. During mitosis, the microtubular cytoskeleton reorganizes to assemble a mitotic spindle necessary for chromosome segregation. Several methods, such as controlled exposure to cold, high pressure, high calcium concentration, or microtubule depolymerizing drugs, have been widely used to evaluate the dynamic properties of specific spindle microtubule populations. However, while these methods offer a qualitative approach that is sufficient to discern differences among specific spindle microtubule populations, they fall short in providing a robust quantitative picture that is sensitive enough to highlight minor differences, for example when comparing spindle microtubule dynamics in different genetic backgrounds. In this chapter we describe a detailed methodology to measure spindle microtubule dynamics using photoactivation of fluorescently tagged tubulin in living cells. This methodology allows the quantitative discrimination of the turnover of specific microtubule populations (e.g., kinetochore vs. non-kinetochore microtubules), as well as determination of microtubule poleward flux rates. These two conspicuous features of metazoan spindles must be tightly regulated to allow, on the one hand, efficient error correction, and on the other hand the satisfaction of the spindle assembly checkpoint that controls mitotic fidelity.

Sister centromere fusion during meiosis I depends on maintaining cohesins and destabilizing microtubule attachments

Sister centromere fusion is a process unique to meiosis that promotes co-orientation of the sister kinetochores, ensuring they attach to microtubules from the same pole during metaphase I. We have found that the kinetochore protein SPC105R/KNL1 and Protein Phosphatase 1 (PP1-87B) regulate sister centromere fusion in Drosophila oocytes. The analysis of these two proteins, however, has shown that two independent mechanisms maintain sister centromere fusion. Maintenance of sister centromere fusion by SPC105R depends on Separase, suggesting cohesin proteins must be maintained at the core centromeres. In contrast, maintenance of sister centromere fusion by PP1-87B does not depend on either Separase or WAPL. Instead, PP1-87B maintains sister centromeres fusion by regulating microtubule dynamics. We demonstrate that this regulation is through antagonizing Polo kinase and BubR1, two proteins known to promote stability of kinetochore-microtubule (KT-MT) attachments, suggesting that PP1-87B maintains sister centromere fusion by inhibiting stable KT-MT attachments. Surprisingly, C(3)G, the transverse element of the synaptonemal complex (SC), is also required for centromere separation in Pp1-87B RNAi oocytes. This is evidence for a functional role of centromeric SC in the meiotic divisions, that might involve regulating microtubule dynamics. Together, we propose two mechanisms maintain co-orientation in Drosophila oocytes: one involves SPC105R to protect cohesins at sister centromeres and another involves PP1-87B to regulate spindle forces at end-on attachments.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

A positive feedback mechanism ensures proper assembly of the functional inner centromere during mitosis in human cells

The inner centromere region of a mitotic chromosome critically regulates sister chromatid cohesion and kinetochore-microtubule attachments. However, the molecular mechanism underlying inner centromere assembly remains elusive. Here, using CRISPR/Cas9-based gene editing in HeLa cells, we disrupted the interaction of Shugoshin 1 (Sgo1) with histone H2A phosphorylated on Thr-120 (H2ApT120) to selectively release Sgo1 from mitotic centromeres. Interestingly, cells expressing the H2ApT120-binding defective mutant of Sgo1 have an elevated rate of chromosome missegregation accompanied by weakened centromeric cohesion and decreased centromere accumulation of the chromosomal passenger complex (CPC), an integral part of the inner centromere and a key player in the correction of erroneous kinetochore-microtubule attachments. When artificially tethered to centromeres, a Sgo1 mutant defective in binding protein phosphatase 2A (PP2A) is not able to support proper centromeric cohesion and CPC accumulation, indicating that the Sgo1-PP2A interaction is essential for the integrity of mitotic centromeres. We further provide evidence indicating that Sgo1 protects centromeric cohesin to create a binding site for the histone H3-associated protein kinase Haspin, which not only inhibits the cohesin release factor Wapl and thereby strengthens centromeric cohesion but also phosphorylates histone H3 at Thr-3 to position CPC at inner centromeres. Taken together, our findings reveal a positive feedback-based mechanism that ensures proper assembly of the functional inner centromere during mitosis. They further suggest a causal link between centromeric cohesion defects and chromosomal instability in cancer cells.

Mapping Mitotic Death: Functional Integration of Mitochondria, Spindle Assembly Checkpoint and Apoptosis

Targeting the mitotic pathways of rapidly proliferating tumor cells has been an effective strategy in traditional cancer therapy. Chemotherapeutics such as taxanes and vinca alkaloids, which disrupt microtubule function, have enjoyed clinical success; however, the accompanying side effects, toxicity and multi drug resistance remain as serious concerns. The emerging classes of inhibitors targeting mitotic kinases and proteasome face their own set of challenges. It is hoped that elucidation of the regulatory interface between mitotic checkpoints, mitochondria and mitotic death will aid the development of more efficacious anti-mitotic agents and improved treatment protocols. The links between the spindle assembly checkpoint (SAC) and mitochondrial dynamics that control the progression of anti-mitotic agent-induced apoptosis have been under investigation for several years and the functional integration of these various signaling networks is now beginning to emerge. In this review, we highlight current research on the regulation of SAC, the death pathway and mitochondria with particular focus on their regulatory interconnections.

Cytokinesis defects and cancer

Whole-genome and centrosome duplication as a consequence of cytokinesis failure can drive tumorigenesis in experimental model systems. However, whether cytokinesis failure is in fact an important cause of human cancers has remained unclear. In this Review, we summarize evidence that whole-genome-doubling events are frequently observed in human cancers and discuss the contribution that cytokinesis defects can make to tumorigenesis. We provide an overview of the potential causes of cytokinesis failure and discuss how tetraploid cells that are generated through cytokinesis defects are used in cancer as a transitory state on the route to aneuploidy. Finally, we discuss how cytokinesis defects can facilitate genetic diversification within the tumour to promote cancer development and could constitute the path of least resistance in tumour evolution.

Activation of the apoptotic pathway during prolonged prometaphase blocks daughter cell proliferation

When untransformed human cells spend >1.5 h in prometaphase under standard culture conditions, all daughters arrest in G1 despite normal division of their mothers. We investigate what happens during prolonged prometaphase that leads to daughter cell arrest in the absence of DNA damage. We find that progressive loss of anti-apoptotic MCL-1 activity and oxidative stress act in concert to partially activate the apoptosis pathway, resulting in the delayed death of some daughters and senescence for the rest. At physiological oxygen levels, longer prometaphase durations are needed for all daughters to arrest. Partial activation of apoptosis during prolonged prometaphase leads to persistent caspase activity, which activates the kinase cascade mediating the post–mitotic activation of p38. This in turn activates p53, and the consequent expression of p21stops the cell cycle. This mechanism can prevent cells suffering intractable mitotic defects, which modestly prolong mitosis but allow its completion without DNA damage, from producing future cell generations that are susceptible to the evolution of a transformed phenotype.

The nonmotor adaptor HMMR dampens Eg5-mediated forces to preserve the kinetics and integrity of chromosome segregation

The nonmotor adaptor protein HMMR maintains the kinetics and integrity of chromosome segregation by promoting TPX2-Eg5 complexes that dampen Eg5-mediated forces and support K-fiber stability, kinetochore–microtubule attachments, and inter-kinetochore tension. HMMR is needed to prevent the generation of aneuploid progeny cells.

Mitotic spindle assembly and organization require forces generated by motor proteins. The activity of these motors is regulated by nonmotor adaptor proteins. However, there are limited studies reporting the functional importance of adaptors on the balance of motor forces and the promotion of faithful and timely cell division. Here we show that genomic deletion or small interfering RNA silencing of the nonmotor adaptor Hmmr/HMMR disturbs spindle microtubule organization and bipolar chromosome–kinetochore attachments with a consequent elevated occurrence of aneuploidy. Rescue experiments show a conserved motif in HMMR is required to generate interkinetochore tension and promote anaphase entry. This motif bears high homology with the kinesin Kif15 and is known to interact with TPX2, a spindle assembly factor. We find that HMMR is required to dampen kinesin Eg5-mediated forces through localizing TPX2 and promoting the formation of inhibitory TPX2-Eg5 complexes. In HMMR-silenced cells, K-fiber stability is reduced while the frequency of unattached chromosomes and the time needed for chromosome segregation are both increased. These defects can be alleviated in HMMR-silenced cells with chemical inhibition of Eg5 but not through the silencing of Kif15. Together, our findings indicate that HMMR balances Eg5-­mediated forces to preserve the kinetics and integrity of chromosome segregation.

HP1 links centromeric heterochromatin to centromere cohesion in mammals

Heterochromatin protein-1 (HP1) is a key component of heterochromatin. Reminiscent of the cohesin complex which mediates sister-chromatid cohesion, most HP1 proteins in mammalian cells are displaced from chromosome arms during mitotic entry, whereas a pool remains at the heterochromatic centromere region. The function of HP1 at mitotic centromeres remains largely elusive. Here, we show that double knockout (DKO) of HP1α and HP1γ causes defective mitosis progression and weakened centromeric cohesion. While mutating the chromoshadow domain (CSD) prevents HP1α from protecting sister-chromatid cohesion, centromeric targeting of HP1α CSD alone is sufficient to rescue the cohesion defects in HP1 DKO cells. Interestingly, HP1-dependent cohesion protection requires Haspin, an antagonist of the cohesin-releasing factor Wapl. Moreover, HP1α CSD directly binds the N-terminal region of Haspin and facilitates its centromeric localization. The need for HP1 in cohesion protection can be bypassed by centromeric targeting of Haspin or inhibiting Wapl activity. Taken together, these results reveal a redundant role for HP1α and HP1γ in the protection of centromeric cohesion through promoting Haspin localization at mitotic centromeres in mammalian cells.

A guide to classifying mitotic stages and mitotic defects in fixed cells

Cell division is fundamental to life and its perturbation can disrupt organismal development, alter tissue homeostasis, and cause disease. Analysis of mitotic abnormalities provides insight into how certain perturbations affect the fidelity of cell division and how specific cellular structures, molecules, and enzymatic activities contribute to the accuracy of this process. However, accurate classification of mitotic defects is instrumental for correct interpretation of data and formulation of new hypotheses. In this article, we provide guidelines for identifying specific mitotic stages and for classifying normal and deviant mitotic phenotypes. We hope this will clarify confusion about how certain defects are classified and help investigators avoid misnomers, misclassification, and/or misinterpretation, thus leading to a unified and standardized system to classify mitotic defects.

A kinase-dependent role for Haspin in antagonizing Wapl and protecting mitotic centromere cohesion

Sister-chromatid cohesion mediated by the cohesin complex is fundamental for precise chromosome segregation in mitosis. Through binding the cohesin subunit Pds5, Wapl releases the bulk of cohesin from chromosome arms in prophase, whereas centromeric cohesin is protected from Wapl until anaphase onset. Strong centromere cohesion requires centromeric localization of the mitotic histone kinase Haspin, which is dependent on the interaction of its non-catalytic N-terminus with Pds5B. It remains unclear how Haspin fully blocks the Wapl-Pds5B interaction at centromeres. Here, we show that the C-terminal kinase domain of Haspin (Haspin-KD) binds and phosphorylates the YSR motif of Wapl (Wapl-YSR), thereby directly inhibiting the YSR motif-dependent interaction of Wapl with Pds5B. Cells expressing a Wapl-binding-deficient mutant of Haspin or treated with Haspin inhibitors show centromeric cohesion defects. Phospho-mimetic mutation in Wapl-YSR prevents Wapl from binding Pds5B and releasing cohesin. Forced targeting Haspin-KD to centromeres partly bypasses the need for Haspin-Pds5B interaction in cohesion protection. Taken together, these results indicate a kinase-dependent role for Haspin in antagonizing Wapl and protecting centromeric cohesion in mitosis.

Sld5 Ensures Centrosomal Resistance to Congression Forces by Preserving Centriolar Satellites

The migration of chromosomes during mitosis is mediated primarily by kinesins that bind to the chromosomes and move along the microtubules, exerting pulling and pushing forces on the centrosomes. We report that a DNA replication protein, Sld5, localizes to the centrosomes, resisting the microtubular pulling forces experienced during chromosome congression. In the absence of Sld5, centriolar satellites, which normally cluster around the centrosomes, are dissipated throughout the cytoplasm, resulting in the loss of their known function of recruiting the centrosomal protein, pericentrin. We observed that Sld5-deficient centrosomes lacking pericentrin were unable to endure the CENP-E- and Kid-mediated microtubular forces that converge on the centrosomes during chromosome congression, resulting in monocentriolar and acentriolar spindle poles. The minus-end-directed kinesin-14 motor protein, HSET, sustains the traction forces that mediate centrosomal fragmentation in Sld5-depleted cells. Thus, we report that a DNA replication protein has an as yet unknown function of ensuring spindle pole resistance to traction forces exerted during chromosome congression.

Precocious centriole disengagement and centrosome fragmentation induced by mitotic delay

The spindle assembly checkpoint (SAC) delays mitotic progression until all sister chromatid pairs achieve bi-orientation, and while the SAC can maintain mitotic arrest for extended periods, moderate delays in mitotic progression have significant effects on the resulting daughter cells. Here we show that when retinal-pigmented epithelial (RPE1) cells experience mitotic delay, there is a time-dependent increase in centrosome fragmentation and centriole disengagement. While most cells with disengaged centrioles maintain spindle bipolarity, clustering of disengaged centrioles requires the kinesin-14, HSET. Centrosome fragmentation and precocious centriole disengagement depend on separase and anaphase-promoting complex/cyclosome (APC/C) activity, which also triggers the acquisition of distal appendage markers on daughter centrioles and the loss of procentriolar markers. Together, these results suggest that moderate delays in mitotic progression trigger the initiation of centriole licensing through centriole disengagement, at which point the ability to maintain spindle bipolarity becomes a function of HSET-mediated spindle pole clustering.

The spindle assembly checkpoint delays mitotic progression until sister chromatids are bi-oriented. Here the authors show that moderate delays in mitotic progression induce centrosome fragmentation and centriole disengagement and that spindle bipolarity is ensured by HSET-mediated spindle pole clustering.

The N-Terminal Non-Kinase-Domain-Mediated Binding of Haspin to Pds5B Protects Centromeric Cohesion in Mitosis

Sister-chromatid cohesion, mediated by the multi-subunit cohesin complex, must be precisely regulated to prevent chromosome mis-segregation. In prophase and prometaphase, whereas the bulk of cohesin on chromosome arms is removed by its antagonist Wapl, cohesin at centromeres is retained to ensure chromosome biorientation until anaphase onset. It remains incompletely understood how centromeric cohesin is protected against Wapl in mitosis. Here we show that the mitotic histone kinase Haspin binds to the cohesin regulatory subunit Pds5B through a conserved YGA/R motif in its non-catalytic N terminus, which is similar to the recently reported YSR-motif-dependent binding of Wapl to Pds5B. Knockout of Haspin or disruption of Haspin-Pds5B interaction causes weakened centromeric cohesion and premature chromatid separation, which can be reverted by centromeric targeting of a N-terminal short fragment of Haspin containing the Pds5B-binding motif or by prevention of Wapl-dependent cohesin removal. Conversely, excessive Haspin capable of binding Pds5B displaces Wapl from Pds5B and suppresses Wapl activity, and it largely bypasses the Wapl antagonist Sgo1 for cohesion protection. Taken together, these data indicate that the Haspin-Pds5B interaction is required to ensure proper sister-chromatid cohesion, most likely through antagonizing Wapl-mediated cohesin release from mitotic centromeres.

Mechanisms of Chromosome Congression during Mitosis

Chromosome congression during prometaphase culminates with the establishment of a metaphase plate, a hallmark of mitosis in metazoans. Classical views resulting from more than 100 years of research on this topic have attempted to explain chromosome congression based on the balance between opposing pulling and/or pushing forces that reach an equilibrium near the spindle equator. However, in mammalian cells, chromosome bi-orientation and force balance at kinetochores are not required for chromosome congression, whereas the mechanisms of chromosome congression are not necessarily involved in the maintenance of chromosome alignment after congression. Thus, chromosome congression and maintenance of alignment are determined by different principles. Moreover, it is now clear that not all chromosomes use the same mechanism for congressing to the spindle equator. Those chromosomes that are favorably positioned between both poles when the nuclear envelope breaks down use the so-called "direct congression" pathway in which chromosomes align after bi-orientation and the establishment of end-on kinetochore-microtubule attachments. This favors the balanced action of kinetochore pulling forces and polar ejection forces along chromosome arms that drive chromosome oscillatory movements during and after congression. The other pathway, which we call "peripheral congression", is independent of end-on kinetochore microtubule-attachments and relies on the dominant and coordinated action of the kinetochore motors Dynein and Centromere Protein E (CENP-E) that mediate the lateral transport of peripheral chromosomes along microtubules, first towards the poles and subsequently towards the equator. How the opposite polarities of kinetochore motors are regulated in space and time to drive congression of peripheral chromosomes only now starts to be understood. This appears to be regulated by position-dependent phosphorylation of both Dynein and CENP-E and by spindle microtubule diversity by means of tubulin post-translational modifications. This so-called "tubulin code" might work as a navigation system that selectively guides kinetochore motors with opposite polarities along specific spindle microtubule populations, ultimately leading to the congression of peripheral chromosomes. We propose an integrated model of chromosome congression in mammalian cells that depends essentially on the following parameters: (1) chromosome position relative to the spindle poles after nuclear envelope breakdown; (2) establishment of stable end-on kinetochore-microtubule attachments and bi-orientation; (3) coordination between kinetochore- and arm-associated motors; and (4) spatial signatures associated with post-translational modifications of specific spindle microtubule populations. The physiological consequences of abnormal chromosome congression, as well as the therapeutic potential of inhibiting chromosome congression are also discussed.

A Cell Biological Perspective on Past, Present and Future Investigations of the Spindle Assembly Checkpoint

The spindle assembly checkpoint (SAC) is a quality control mechanism that ensures accurate chromosome segregation during cell division. It consists of a mechanochemical signal transduction mechanism that senses the attachment of chromosomes to the spindle, and a signaling cascade that inhibits cell division if one or more chromosomes are not attached. Extensive investigations of both these component systems of the SAC have synthesized a comprehensive understanding of the underlying molecular mechanisms. This review recounts the milestone results that elucidated the SAC, compiles a simple model of the complex molecular machinery underlying the SAC, and highlights poorly understood facets of the biochemical design and cell biological operation of the SAC that will drive research forward in the near future.

Observing Mitotic Division and Dynamics in a Live Zebrafish Embryo

Mitosis is critical for organismal growth and differentiation. The process is highly dynamic and requires ordered events to accomplish proper chromatin condensation, microtubule-kinetochore attachment, chromosome segregation, and cytokinesis in a small time frame. Errors in the delicate process can result in human disease, including birth defects and cancer. Traditional approaches investigating human mitotic disease states often rely on cell culture systems, which lack the natural physiology and developmental/tissue-specific context advantageous when studying human disease. This protocol overcomes many obstacles by providing a way to visualize, with high resolution, chromosome dynamics in a vertebrate system, the zebrafish. This protocol will detail an approach that can be used to obtain dynamic images of dividing cells, which include: in vitro transcription, zebrafish breeding/collecting, embryo embedding, and time-lapse imaging. Optimization and modifications of this protocol are also explored. Using H2A.F/Z-EGFP (labels chromatin) and mCherry-CAAX (labels cell membrane) mRNA-injected embryos, mitosis in AB wild-type, auroraB(hi1045) (,) and esco2(hi2865) mutant zebrafish is visualized. High resolution live imaging in zebrafish allows one to observe multiple mitoses to statistically quantify mitotic defects and timing of mitotic progression. In addition, observation of qualitative aspects that define improper mitotic processes (i.e., congression defects, missegregation of chromosomes, etc.) and improper chromosomal outcomes (i.e., aneuploidy, polyploidy, micronuclei, etc.) are observed. This assay can be applied to the observation of tissue differentiation/development and is amenable to the use of mutant zebrafish and pharmacological agents. Visualization of how defects in mitosis lead to cancer and developmental disorders will greatly enhance understanding of the pathogenesis of disease.

Cell biology. Reversible centriole depletion with an inhibitor of Polo-like kinase 4

Centrioles are ancient organelles that build centrosomes, the major microtubule-organizing centers of animal cells. Extra centrosomes are a common feature of cancer cells. To investigate the importance of centrosomes in the proliferation of normal and cancer cells, we developed centrinone, a reversible inhibitor of Polo-like kinase 4 (Plk4), a serine-threonine protein kinase that initiates centriole assembly. Centrinone treatment caused centrosome depletion in human and other vertebrate cells. Centrosome loss irreversibly arrested normal cells in a senescence-like G1 state by a p53-dependent mechanism that was independent of DNA damage, stress, Hippo signaling, extended mitotic duration, or segregation errors. In contrast, cancer cell lines with normal or amplified centrosome numbers could proliferate indefinitely after centrosome loss. Upon centrinone washout, each cancer cell line returned to an intrinsic centrosome number "set point." Thus, cells with cancer-associated mutations fundamentally differ from normal cells in their response to centrosome loss.

PUL21a-Cyclin A2 interaction is required to protect human cytomegalovirus-infected cells from the deleterious consequences of mitotic entry

Entry into mitosis is accompanied by dramatic changes in cellular architecture, metabolism and gene expression. Many viruses have evolved cell cycle arrest strategies to prevent mitotic entry, presumably to ensure sustained, uninterrupted viral replication. Here we show for human cytomegalovirus (HCMV) what happens if the viral cell cycle arrest mechanism is disabled and cells engaged in viral replication enter into unscheduled mitosis. We made use of an HCMV mutant that, due to a defective Cyclin A2 binding motif in its UL21a gene product (pUL21a), has lost its ability to down-regulate Cyclin A2 and, therefore, to arrest cells at the G1/S transition. Cyclin A2 up-regulation in infected cells not only triggered the onset of cellular DNA synthesis, but also promoted the accumulation and nuclear translocation of Cyclin B1-CDK1, premature chromatin condensation and mitotic entry. The infected cells were able to enter metaphase as shown by nuclear lamina disassembly and, often irregular, metaphase spindle formation. However, anaphase onset was blocked by the still intact anaphase promoting complex/cyclosome (APC/C) inhibitory function of pUL21a. Remarkably, the essential viral IE2, but not the related chromosome-associated IE1 protein, disappeared upon mitotic entry, suggesting an inherent instability of IE2 under mitotic conditions. Viral DNA synthesis was impaired in mitosis, as demonstrated by the abnormal morphology and strongly reduced BrdU incorporation rates of viral replication compartments. The prolonged metaphase arrest in infected cells coincided with precocious sister chromatid separation and progressive fragmentation of the chromosomal material. We conclude that the Cyclin A2-binding function of pUL21a contributes to the maintenance of a cell cycle state conducive for the completion of the HCMV replication cycle. Unscheduled mitotic entry during the course of the HCMV replication has fatal consequences, leading to abortive infection and cell death.

Cyclin A2 is a key regulator of the cell division cycle. Interactors of Cyclin A2 typically contain short sequence elements (RXL/Cy motifs) that bind with high affinity to a hydrophobic patch in the Cyclin A2 protein. Two types of RXL/Cy-containing factors are known: i) cyclin-dependent kinase (CDK) substrates, which are processed by the CDK subunit that complexes to Cyclin A2, and ii) CDK inhibitors, which stably associate to Cyclin A2-CDK due to the lack of CDK phosphorylation sites. Human cytomegalovirus (HCMV) has evolved a novel type of RXL/Cy-containing protein. Its UL21a gene product, a small and highly unstable protein, binds to Cyclin A2 via an RXL/Cy motif in its N-terminus, leading to efficient degradation of Cyclin A2 by the proteasome. Here, we show that this mechanism is not only essential for viral inhibition of cellular DNA synthesis, but also to prevent entry of infected cells into mitosis. Unscheduled mitotic entry is followed by aberrant spindle formation, metaphase arrest, precocious separation of sister chromatids, chromosomal fragmentation and cell death. Viral DNA replication and expression of the essential viral IE2 protein are abrogated in mitosis. Thus, pUL21a-Cyclin A2 interaction protects HCMV from a collapse of viral and cellular functions in mitosis.