Mechanisms of Chromosome Congression during Mitosis

TH588 and Low-Dose Nocodazole Impair Chromosome Congression by Suppressing Microtubule Turnover within the Mitotic Spindle

Simple Summary

A promising anti-cancer compound TH588 has been recently identified as a microtubule-targeting agent that inhibits tubulin polymerization in vitro and interferes with microtubule dynamics in interphase cells. Although it was shown to arrest cells in mitosis, its effect on microtubule dynamics in dividing cells remained unknown. By analyzing microtubule dynamics in living cells treated with either TH588 or low-dose nocodazole, we revealed that both of these drugs stabilize microtubules within the mitotic spindle, leading to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes. This causes mitotic arrest, ultimately resulting in cell death or cell division with uncongressed chromosomes. Both of these cell fates could contribute to the selective effect associated with the activity of TH588 in cancer cells.

Abstract

Microtubule-targeting agents (MTAs) have been used for decades to treat different hematologic and solid cancers. The mode of action of these drugs mainly relies on their ability to bind tubulin subunits and/or microtubules and interfere with microtubule dynamics. In addition to its MTH1-inhibiting activity, TH588 has been recently identified as an MTA, whose anticancer properties were shown to largely depend on its microtubule-targeting ability. Although TH588 inhibited tubulin polymerization in vitro and reduced microtubule plus-end mobility in interphase cells, its effect on microtubule dynamics within the mitotic spindle of dividing cells remained unknown. Here, we performed an in-depth analysis of the impact of TH588 on spindle-associated microtubules and compared it to the effect of low-dose nocodazole. We show that both treatments reduce microtubule turnover within the mitotic spindle. This microtubule-stabilizing effect leads to premature formation of kinetochore-microtubule end-on attachments on uncongressed chromosomes, which consequently cannot be transported to the cell equator, thereby delaying cell division and leading to cell death or division with uncongressed chromosomes.

Mechanics of microtubule organizing center clustering and spindle positioning in budding yeast Cryptococcus neoformans

The dynamic process of mitotic spindle assembly depends on multitudes of inter-dependent interactions involving kinetochores (KTs), microtubules (MTs), spindle pole bodies (SPBs), and molecular motors. Before forming the mitotic spindle, multiple visible microtubule organizing centers (MTOCs) coalesce into a single focus to serve as an SPB in the pathogenic budding yeast, Cryptococcus neoformans. To explain this unusual phenomenon in the fungal kingdom, we propose a "search and capture" model, in which cytoplasmic MTs (cMTs) nucleated by MTOCs grow and capture each other to promote MTOC clustering. Our quantitative modeling identifies multiple redundant mechanisms mediated by a combination of cMT-cell cortex interactions and inter-cMT coupling to facilitate MTOC clustering within the physiological time limit as determined by time-lapse live-cell microscopy. Besides, we screen various possible mechanisms by computational modeling and propose optimal conditions that favor proper spindle positioning-a critical determinant for timely chromosome segregation. These analyses also reveal that a combined effect of MT buckling, dynein pull, and cortical push maintains spatiotemporal spindle localization.

Attenuated Chromosome Oscillation as a Cause of Chromosomal Instability in Cancer Cells

Simple Summary

Chromosomal instability (CIN), a condition in which chromosome missegregation occurs at high rates, is widely seen in cancer cells. Causes of CIN in cancer cells are not fully understood. A recent report suggests that chromosome oscillation, an iterative chromosome motion typically seen in metaphase around the spindle equator, is attenuated in cancer cells, and is associated with CIN. Chromosome oscillation promotes the correction of erroneous kinetochore-microtubule attachments through phosphorylation of Hec1, a kinetochore protein that binds to microtubules, by Aurora A kinase residing on the spindle. In this review, we focused on this unappreciated link between chromosome oscillation and CIN.

Abstract

Chromosomal instability (CIN) is commonly seen in cancer cells, and related to tumor progression and poor prognosis. Among the causes of CIN, insufficient correction of erroneous kinetochore (KT)-microtubule (MT) attachments plays pivotal roles in various situations. In this review, we focused on the previously unappreciated role of chromosome oscillation in the correction of erroneous KT-MT attachments, and its relevance to the etiology of CIN. First, we provided an overview of the error correction mechanisms for KT-MT attachments, especially the role of Aurora kinases in error correction by phosphorylating Hec1, which connects MT to KT. Next, we explained chromosome oscillation and its underlying mechanisms. Then we introduced how chromosome oscillation is involved in the error correction of KT-MT attachments, based on recent findings. Chromosome oscillation has been shown to promote Hec1 phosphorylation by Aurora A which localizes to the spindle. Finally, we discussed the link between attenuated chromosome oscillation and CIN in cancer cells. This link underscores the role of chromosome dynamics in mitotic fidelity, and the mutual relationship between defective chromosome dynamics and CIN in cancer cells that can be a target for cancer therapy.

Anaphase B: Long-standing models meet new concepts

Mitotic cell divisions ensure stable transmission of genetic information from a mother to daughter cells in a series of generations. To ensure this crucial task is accomplished, the cell forms a bipolar structure called the mitotic spindle that divides sister chromatids to the opposite sides of the dividing mother cell. After successful establishment of stable attachments of microtubules to chromosomes and inspection of connections between them, at the heart of mitosis, the cell starts the process of segregation. This spectacular moment in the life of a cell is termed anaphase, and it involves two distinct processes: depolymerization of microtubules bound to chromosomes, which is also known as anaphase A, and elongation of the spindle or anaphase B. Both processes ensure physical separation of disjointed sister chromatids. In this chapter, we review the mechanisms of anaphase B spindle elongation primarily in mammalian systems, combining different pioneering ideas and concepts with more recent findings that shed new light on the force generation and regulation of biochemical modules operating during spindle elongation. Finally, we present a comprehensive model of spindle elongation that includes structural, biophysical, and molecular aspects of anaphase B.

Wnt signaling recruits KIF2A to the spindle to ensure chromosome congression and alignment during mitosis

Canonical Wnt signaling plays critical roles in development and tissue renewal by regulating β-catenin target genes. Recent evidence showed that β-catenin-independent Wnt signaling is also required for faithful execution of mitosis. However, the targets and specific functions of mitotic Wnt signaling still remain uncharacterized. Using phosphoproteomics, we identified that Wnt signaling regulates the microtubule depolymerase KIF2A during mitosis. We found that Dishevelled recruits KIF2A via its N-terminal and motor domains, which is further promoted upon LRP6 signalosome formation during cell division. We show that Wnt signaling modulates KIF2A interaction with PLK1, which is critical for KIF2A localization at the spindle. Accordingly, inhibition of basal Wnt signaling leads to chromosome misalignment in somatic cells and pluripotent stem cells. We propose that Wnt signaling monitors KIF2A activity at the spindle poles during mitosis to ensure timely chromosome alignment. Our findings highlight a function of Wnt signaling during cell division, which could have important implications for genome maintenance, notably in stem cells.

Regulation of microtubule dynamics, mechanics and function through the growing tip

Microtubule dynamics and their control are essential for the normal function and division of all eukaryotic cells. This plethora of functions is, in large part, supported by dynamic microtubule tips, which can bind to various intracellular targets, generate mechanical forces and couple with actin microfilaments. Here, we review progress in the understanding of microtubule assembly and dynamics, focusing on new information about the structure of microtubule tips. First, we discuss evidence for the widely accepted GTP cap model of microtubule dynamics. Next, we address microtubule dynamic instability in the context of structural information about assembly intermediates at microtubule tips. Three currently discussed models of microtubule assembly and dynamics are reviewed. These are considered in the context of established facts and recent data, which suggest that some long-held views must be re-evaluated. Finally, we review structural observations about the tips of microtubules in cells and describe their implications for understanding the mechanisms of microtubule regulation by associated proteins, by mechanical forces and by microtubule-targeting drugs, prominently including cancer chemotherapeutics.

Prometaphase

The establishment of a metaphase plate in which all chromosomes are attached to mitotic spindle microtubules and aligned at the cell equator is required for faithful chromosome segregation in metazoans. The achievement of this configuration relies on the precise coordination between several concurrent mechanisms that start upon nuclear envelope breakdown, mediate chromosome capture at their kinetochores during mitotic spindle assembly and culminate with the congression of all chromosomes to the spindle equator. This period is called 'prometaphase'. Because the nature of chromosome capture by mitotic spindle microtubules is error prone, the cell is provided of error correction mechanisms that sense and correct most erroneous kinetochore-microtubule attachments before committing to separate sister chromatids in anaphase. In this review, aimed for newcomers in the field, more than providing an exhaustive mechanistic coverage of each and every concurrent mechanism taking place during prometaphase, we provide an integrative overview of these processes that ultimately promote the subsequent faithful segregation of chromosomes during mitosis.

Biomechanics of chromosome alignment at the spindle midplane

During metaphase, chromosomes are aligned in a lineup at the equatorial plane of the spindle to ensure synchronous poleward movement of chromatids in anaphase and proper nuclear reformation at the end of mitosis. Chromosome alignment relies on microtubules, several types of motor protein and numerous other microtubule-associated and regulatory proteins. Because of the multitude of players involved, the mechanisms of chromosome alignment are still under debate. Here, we discuss the current models of alignment based on poleward pulling forces exerted onto sister kinetochores by kinetochore microtubules, which show length-dependent dynamics and undergo poleward flux, and polar ejection forces that push the chromosome arms away from the pole. We link these models with the recent ideas based on mechanical coupling between bridging and kinetochore microtubules, where sliding of bridging microtubules promotes overlap length-dependent sliding of kinetochore fibers and thus the alignment of sister kinetochores at the spindle equator. Finally, we discuss theoretical models of forces acting on chromosomes during metaphase.

Pyrrolizidine alkaloids cause cell cycle and DNA damage repair defects as analyzed by transcriptomics in cytochrome P450 3A4-overexpressing HepG2 clone 9 cells

Pyrrolizidine alkaloids (PAs) are a large group of highly toxic chemical compounds, which are found as cross-contaminants in numerous food products (e.g., honey), dietary supplements, herbal teas, and pharmaceutical herbal medicines. PA contaminations are responsible for serious hepatotoxicity and hepatocarcinogenesis. Health authorities have to set legal limit values to guarantee the safe consumption of plant-based nutritional and medical products without harmful health. Toxicological and chemical analytical methods are conventionally applied to determine legally permitted limit values for PAs. In the present investigation, we applied a highly sensitive transcriptomic approach to investigate the effect of low concentrations of five PAs (lasiocarpine, riddelliine, lycopsamine, echimidine, and monocrotaline) on human cytochrome P450 3A4-overexpressing HepG2 clone 9 hepatocytes. The transcriptomic profiling of deregulated gene expression indicated that the PAs disrupted important signaling pathways related to cell cycle regulation and DNA damage repair in the transfected hepatocytes, which may explain the carcinogenic PA effects. As PAs affected the expression of genes that involved in cell cycle regulation, we applied flow cytometric cell cycle analyses to verify the transcriptomic data. Interestingly, PA treatment led to an arrest in the S phase of the cell cycle, and this effect was more pronounced with more toxic PAs (i.e., lasiocarpine and riddelliine) than with the less toxic monocrotaline. Using immunofluorescence, high fractions of cells were detected with chromosome congression defects upon PA treatment, indicating mitotic failure. In conclusion, the tested PAs revealed threshold concentrations, above which crucial signaling pathways were deregulated resulting in cell damage and carcinogenesis. Cell cycle arrest and DNA damage repair point to the mutagenicity of PAs. The disturbance of chromosome congression is a novel mechanism of Pas, which may also contribute to PA-mediated carcinogenesis. Transcriptomic, cell cycle, and immunofluorescence analyses should supplement the standard techniques in toxicology to unravel the biological effects of PA exposure in liver cells as the primary target during metabolization of PAs.

A quantitative and semiautomated method for determining misaligned and lagging chromosomes during mitosis

Accurate chromosome alignment at metaphase facilitates the equal segregation of sister chromatids to each of the nascent daughter cells. Lack of proper metaphase alignment is an indicator of defective chromosome congression and aberrant kinetochore-microtubule attachments which in turn promotes chromosome missegregation and aneuploidy, hallmarks of cancer. Tools to sensitively, accurately, and quantitatively measure chromosome alignment at metaphase will facilitate understanding of the contribution of chromosome segregation errors to the development of aneuploidy. In this work, we have developed and validated a method based on analytical geometry to measure several indicators of chromosome misalignment. We generated semiautomated and flexible ImageJ2/Fiji pipelines to quantify kinetochore misalignment at metaphase plates as well as lagging chromosomes at anaphase. These tools will ultimately allow sensitive and systematic quantitation of these chromosome segregation defects in cells undergoing mitosis.

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.

KINESIN-12E regulates metaphase spindle flux and helps control spindle size in Arabidopsis

The bipolar mitotic spindle is a highly conserved structure among eukaryotes that mediates chromosome alignment and segregation. Spindle assembly and size control are facilitated by force-generating microtubule-dependent motor proteins known as kinesins. In animals, kinesin-12 cooperates with kinesin-5 to produce outward-directed forces necessary for spindle assembly. In plants, the relevant molecular mechanisms for spindle formation are poorly defined. While an Arabidopsis thaliana kinesin-5 ortholog has been identified, the kinesin-12 ortholog in plants remains elusive. In this study, we provide experimental evidence for the function of Arabidopsis KINESIN-12E in spindle assembly. In kinesin-12e mutants, a delay in spindle assembly is accompanied by the reduction of spindle size, demonstrating that KINESIN-12E contributes to mitotic spindle architecture. Kinesin-12E localization is mitosis-stage specific, beginning with its perinuclear accumulation during prophase. Upon nuclear envelope breakdown, KINESIN-12E decorates subpopulations of microtubules in the spindle and becomes progressively enriched in the spindle midzone. Furthermore, during cytokinesis, KINESIN-12E shares its localization at the phragmoplast midzone with several functionally diversified Arabidopsis KINESIN-12 members. Changes in the kinetochore and in prophase and metaphase spindle dynamics occur in the absence of KINESIN-12E, suggest it might play an evolutionarily conserved role during spindle formation similar to its spindle-localized animal kinesin-12 orthologs.

Shake It Off: The Elimination of Erroneous Kinetochore-Microtubule Attachments and Chromosome Oscillation

Cell proliferation and sexual reproduction require the faithful segregation of chromosomes. Chromosome segregation is driven by the interaction of chromosomes with the spindle, and the attachment of chromosomes to the proper spindle poles is essential. Initial attachments are frequently erroneous due to the random nature of the attachment process; however, erroneous attachments are selectively eliminated. Proper attachment generates greater tension at the kinetochore than erroneous attachments, and it is thought that attachment selection is dependent on this tension. However, studies of meiotic chromosome segregation suggest that attachment elimination cannot be solely attributed to tension, and the precise mechanism of selective elimination of erroneous attachments remains unclear. During attachment elimination, chromosomes oscillate between the spindle poles. A recent study on meiotic chromosome segregation in fission yeast has suggested that attachment elimination is coupled to chromosome oscillation. In this review, the possible contribution of chromosome oscillation in the elimination of erroneous attachment is discussed in light of the recent finding.

Leaving no-one behind: how CENP-E facilitates chromosome alignment

Chromosome alignment and biorientation is essential for mitotic progression and genomic stability. Most chromosomes align at the spindle equator in a motor-independent manner. However, a subset of polar kinetochores fail to bi-orient and require a microtubule motor-based transport mechanism to move to the cell equator. Centromere Protein E (CENP-E/KIF10) is a kinesin motor from the Kinesin-7 family, which localizes to unattached kinetochores during mitosis and utilizes plus-end directed microtubule motility to slide mono-oriented chromosomes to the spindle equator. Recent work has revealed how CENP-E cooperates with chromokinesins and dynein to mediate chromosome congression and highlighted its role at aligned chromosomes. Additionally, we have gained new mechanistic insights into the targeting and regulation of CENP-E motor activity at the kinetochore. Here, we will review the function of CENP-E in chromosome congression, the pathways that contribute to CENP-E loading at the kinetochore, and how CENP-E activity is regulated during mitosis.

Chiasmata and the kinetochore component Dam1 are crucial for elimination of erroneous chromosome attachments and centromere oscillation at meiosis I

Establishment of proper chromosome attachments to the spindle requires elimination of erroneous attachments, but the mechanism of this process is not fully understood. During meiosis I, sister chromatids attach to the same spindle pole (mono-oriented attachment), whereas homologous chromosomes attach to opposite poles (bi-oriented attachment), resulting in homologous chromosome segregation. Here, we show that chiasmata that link homologous chromosomes and kinetochore component Dam1 are crucial for elimination of erroneous attachments and oscillation of centromeres between the spindle poles at meiosis I in fission yeast. In chiasma-forming cells, Mad2 and Aurora B kinase, which provides time for attachment correction and destabilizes erroneous attachments, respectively, caused elimination of bi-oriented attachments of sister chromatids, whereas in chiasma-lacking cells, they caused elimination of mono-oriented attachments. In chiasma-forming cells, in addition, homologous centromere oscillation was coordinated. Furthermore, Dam1 contributed to attachment elimination in both chiasma-forming and chiasma-lacking cells, and drove centromere oscillation. These results demonstrate that chiasmata alter attachment correction patterns by enabling error correction factors to eliminate bi-oriented attachment of sister chromatids, and suggest that Dam1 induces elimination of erroneous attachments. The coincidental contribution of chiasmata and Dam1 to centromere oscillation also suggests a potential link between centromere oscillation and attachment elimination.

Mitotic spindle: lessons from theoretical modeling

Cell biology is immensely complex. To understand how cells work, we try to find patterns and suggest hypotheses to identify underlying mechanisms. However, it is not always easy to create a coherent picture from a huge amount of experimental data on biological systems, where the main players have multiple interactions or act in redundant pathways. In such situations, when a hypothesis does not lead to a conclusion in a direct way, theoretical modeling is a powerful tool because it allows us to formulate hypotheses in a quantitative manner and understand their consequences. A successful model should not only reproduce the basic features of the system but also provide exciting predictions, motivating new experiments. Much is learned when a model based on generally accepted knowledge cannot explain experiments of interest, as this indicates that the original hypothesis needs to be revised. In this Perspective, we discuss these points using our experiences in combining experiments with theory in the field of mitotic spindle mechanics.

Optogenetic control of PRC1 reveals its role in chromosome alignment on the spindle by overlap length-dependent forces

During metaphase, chromosome position at the spindle equator is regulated by the forces exerted by kinetochore microtubules and polar ejection forces. However, the role of forces arising from mechanical coupling of sister kinetochore fibers with bridging fibers in chromosome alignment is unknown. Here, we develop an optogenetic approach for acute removal of PRC1 to partially disassemble bridging fibers and show that they promote chromosome alignment. Tracking of the plus-end protein EB3 revealed longer antiparallel overlaps of bridging microtubules upon PRC1 removal, which was accompanied by misaligned and lagging kinetochores. Kif4A/kinesin-4 and Kif18A/kinesin-8 were found within the bridging fiber and largely lost upon PRC1 removal, suggesting that these proteins regulate the overlap length of bridging microtubules. We propose that PRC1-mediated crosslinking of bridging microtubules and recruitment of kinesins to the bridging fiber promote chromosome alignment by overlap length-dependent forces transmitted to the associated kinetochore fibers.

Before cells divide to create copies of themselves, they need to duplicate their genetic material. To help split their DNA evenly, they build a machine called the mitotic spindle. The mitotic spindle is made of fine, tube-like structures called microtubules, which catch the chromosomes containing the genetic information and line them up at the center of the spindle.

Microtubules push and pull the chromosomes by elongating or shortening their tips. But it remains unclear how the microtubules know when the chromosomes have reached center point. One way to find out is to remove proteins that accumulate in the middle of the spindle during division, such as the protein PRC1, which helps to assemble a subset of microtubules called bridging fibers, and the proteins Kif4A and Kif18A, which work like molecular rulers, shortening long microtubules. Usually, scientists would delete one of these proteins to see what impact this has. However, these experiments take days, giving the cell enough time to adapt and thus making it difficult to study the role of each of the proteins.

Here, Jagrić, Risteski, Martinčić et al. used light to manipulate proteins at the exact moment of chromosome alignment and to move PRC1 from the spindle to the cell membrane. Consequently, Kif4A and Kif18A were removed from the spindle center. This caused the bridging fibers, which overlap with the microtubules that connect to the chromosomes, to become thinner. Jagrić et al. discovered that without the molecular ruler proteins, the bridging fibers were also too long. This increased the overlap between the microtubules in the center of the spindle, causing the chromosomes to migrate away from the center. This suggests that the alignment of chromosomes in the middle of the spindle depends on the bridging microtubules, which need to be of a certain length to effectively move and keep the chromosomes at the center. Thus, forces that move the chromosomes are generated both at the tips of the microtubules and along the wall of microtubules.

These results might inspire other researchers to reassess the role of bridging fibers in cell division. The optogenetic technique described here could also help to determine the parts other proteins have to play. Ultimately, this might allow researchers to identify all the proteins needed to align the chromosomes.

chTOG is a conserved mitotic error correction factor

Accurate chromosome segregation requires kinetochores on duplicated chromatids to biorient by attaching to dynamic microtubules from opposite spindle poles, which exerts forces to bring kinetochores under tension. However, kinetochores initially bind to microtubules indiscriminately, resulting in errors that must be corrected. While the Aurora B protein kinase destabilizes low-tension attachments by phosphorylating kinetochores, low-tension attachments are intrinsically less stable than those under higher tension in vitro independent of Aurora activity. Intrinsic tension-sensitive behavior requires the microtubule regulator Stu2 (budding yeast Dis1/XMAP215 ortholog), which we demonstrate here is likely a conserved function for the TOG protein family. The human TOG protein, chTOG, localizes to kinetochores independent of microtubules by interacting with Hec1. We identify a chTOG mutant that regulates microtubule dynamics but accumulates erroneous kinetochore-microtubule attachments that are not destabilized by Aurora B. Thus, TOG proteins confer a unique, intrinsic error correction activity to kinetochores that ensures accurate chromosome segregation.

Mechanobiology of the Mitotic Spindle

The mitotic spindle is a microtubule-based assembly that separates the chromosomes during cell division. As the spindle is basically a mechanical micro machine, the understanding of its functioning is constantly motivating the development of experimental approaches based on mechanical perturbations, which are complementary to and work together with the classical genetics and biochemistry methods. Recent data emerging from these approaches in combination with theoretical modeling led to novel ideas and significant revisions of the basic concepts in the field. In this Perspective, we discuss the advances in the understanding of spindle mechanics, focusing on microtubule forces that control chromosome movements.

Nuclear Isoforms of Neurofibromin Are Required for Proper Spindle Organization and Chromosome Segregation

Mitotic spindles are highly organized, microtubule (MT)-based, transient structures that serve the fundamental function of unerring chromosome segregation during cell division and thus of genomic stability during tissue morphogenesis and homeostasis. Hence, a multitude of MT-associated proteins (MAPs) regulates the dynamic assembly of MTs in preparation for mitosis. Some tumor suppressors, normally functioning to prevent tumor development, have now emerged as significant MAPs. Among those, neurofibromin, the product of the Neurofibromatosis-1 gene (NF1), a major Ras GTPase activating protein (RasGAP) in neural cells, controls also the critical function of chromosome congression in astrocytic cellular contexts. Cell type- and development-regulated splicings may lead to the inclusion or exclusion of NF1exon51, which bears a nuclear localization sequence (NLS) for nuclear import at G2; yet the functions of the produced NLS and ΔNLS neurofibromin isoforms have not been previously addressed. By using a lentiviral shRNA system, we have generated glioblastoma SF268 cell lines with conditional knockdown of NLS or ΔNLS transcripts. In dissecting the roles of NLS or ΔNLS neurofibromins, we found that NLS-neurofibromin knockdown led to increased density of cytosolic MTs but loss of MT intersections, anastral spindles featuring large hollows and abnormal chromosome positioning, and finally abnormal chromosome segregation and increased micronuclei frequency. Therefore, we propose that NLS neurofibromin isoforms exert prominent mitotic functions.

Genetic Material Manipulation and Modification by Optical Trapping and Nanosurgery-A Perspective

Light can be employed as a tool to alter and manipulate matter in many ways. An example has been the implementation of optical trapping, the so called optical tweezers, in which light can hold and move small objects with 3D control. Of interest for the Life Sciences and Biotechnology is the fact that biological objects in the size range from tens of nanometers to hundreds of microns can be precisely manipulated through this technology. In particular, it has been shown possible to optically trap and move genetic material (DNA and chromatin) using optical tweezers. Also, these biological entities can be severed, rearranged and reconstructed by the combined use of laser scissors and optical tweezers. In this review, the background, current state and future possibilities of optical tweezers and laser scissors to manipulate, rearrange and alter genetic material (DNA, chromatin and chromosomes) will be presented. Sources of undesirable effects by the optical procedure and measures to avoid them will be discussed. In addition, first tentative approaches at cellular-level genetic and organelle surgery, in which genetic material or DNA-carrying organelles are extracted out or introduced into cells, will be presented.

Clathrin's adaptor interaction sites are repurposed to stabilize microtubules during mitosis

Clathrin ensures mitotic spindle stability and efficient chromosome alignment, independently of its vesicle trafficking function. Although clathrin localizes to the mitotic spindle and kinetochore fiber microtubule bundles, the mechanisms by which clathrin stabilizes microtubules are unclear. We show that clathrin adaptor interaction sites on clathrin heavy chain (CHC) are repurposed during mitosis to directly recruit the microtubule-stabilizing protein GTSE1 to the spindle. Structural analyses reveal that these sites interact directly with clathrin-box motifs on GTSE1. Disruption of this interaction releases GTSE1 from spindles, causing defects in chromosome alignment. Surprisingly, this disruption destabilizes astral microtubules, but not kinetochore-microtubule attachments, and chromosome alignment defects are due to a failure of chromosome congression independent of kinetochore-microtubule attachment stability. GTSE1 recruited to the spindle by clathrin stabilizes microtubules by inhibiting the microtubule depolymerase MCAK. This work uncovers a novel role of clathrin adaptor-type interactions to stabilize nonkinetochore fiber microtubules to support chromosome congression, defining for the first time a repurposing of this endocytic interaction mechanism during mitosis.

The C-terminal helix of BubR1 is essential for CENP-E-dependent chromosome alignment

During cell division, misaligned chromosomes are captured and aligned by motors before their segregation. The CENP-E motor is recruited to polar unattached kinetochores to facilitate chromosome alignment. The spindle checkpoint protein BubR1 (also known as BUB1B) has been reported as a CENP-E interacting partner, but the extent to which BubR1 contributes to CENP-E localization at kinetochores has remained controversial. Here we define the molecular determinants that specify the interaction between BubR1 and CENP-E. The basic C-terminal helix of BubR1 is necessary but not sufficient for CENP-E interaction, and a minimal key acidic patch on the kinetochore-targeting domain of CENP-E is also essential. We then demonstrate that BubR1 is required for the recruitment of CENP-E to kinetochores to facilitate chromosome alignment. This BubR1-CENP-E axis is critical for alignment of chromosomes that have failed to congress through other pathways and recapitulates the major known function of CENP-E. Overall, our studies define the molecular basis and the function for CENP-E recruitment to BubR1 at kinetochores during mammalian mitosis.This article has an associated First Person interview with the first author of the paper.

Two Antagonistic Microtubule Targeting Drugs Act Synergistically to Kill Cancer Cells

Paclitaxel is a microtubule stabilizing agent and a successful drug for cancer chemotherapy inducing, however, adverse effects. To reduce the effective dose of paclitaxel, we searched for pharmaceutics which could potentiate its therapeutic effect. We screened a chemical library and selected Carba1, a carbazole, which exerts synergistic cytotoxic effects on tumor cells grown in vitro, when co-administrated with a low dose of paclitaxel. Carba1 targets the colchicine binding-site of tubulin and is a microtubule-destabilizing agent. Catastrophe induction by Carba1 promotes paclitaxel binding to microtubule ends, providing a mechanistic explanation of the observed synergy. The synergistic effect of Carba1 with paclitaxel on tumor cell viability was also observed in vivo in xenografted mice. Thus, a new mechanism favoring paclitaxel binding to dynamic microtubules can be transposed to in vivo mouse cancer treatments, paving the way for new therapeutic strategies combining low doses of microtubule targeting agents with opposite mechanisms of action.

Mechanisms of microtubule dynamics and force generation examined with computational modeling and electron cryotomography

Microtubules are dynamic tubulin polymers responsible for many cellular processes, including the capture and segregation of chromosomes during mitosis. In contrast to textbook models of tubulin self-assembly, we have recently demonstrated that microtubules elongate by addition of bent guanosine triphosphate tubulin to the tips of curving protofilaments. Here we explore this mechanism of microtubule growth using Brownian dynamics modeling and electron cryotomography. The previously described flaring shapes of growing microtubule tips are remarkably consistent under various assembly conditions, including different tubulin concentrations, the presence or absence of a polymerization catalyst or tubulin-binding drugs. Simulations indicate that development of substantial forces during microtubule growth and shortening requires a high activation energy barrier in lateral tubulin-tubulin interactions. Modeling offers a mechanism to explain kinetochore coupling to growing microtubule tips under assisting force, and it predicts a load-dependent acceleration of microtubule assembly, providing a role for the flared morphology of growing microtubule ends.

Mechanisms driving acentric chromosome transmission

The kinetochore-microtubule association is a core, conserved event that drives chromosome transmission during mitosis. Failure to establish this association on even a single chromosome results in aneuploidy leading to cell death or the development of cancer. However, although many chromosomes lacking centromeres, termed acentrics, fail to segregate, studies in a number of systems reveal robust alternative mechanisms that can drive segregation and successful poleward transport of acentrics. In contrast to the canonical mechanism that relies on end-on microtubule attachments to kinetochores, mechanisms of acentric transmission largely fall into three categories: direct attachments to other chromosomes, kinetochore-independent lateral attachments to microtubules, and long-range tether-based attachments. Here, we review these "non-canonical" methods of acentric chromosome transmission. Just as the discovery and exploration of cell cycle checkpoints provided insight into both the origins of cancer and new therapies, identifying mechanisms and structures specifically involved in acentric segregation may have a significant impact on basic and applied cancer research.

Mechanics of Multicentrosomal Clustering in Bipolar Mitotic Spindles

To segregate chromosomes in mitosis, cells assemble a mitotic spindle, a molecular machine with centrosomes at two opposing cell poles and chromosomes at the equator. Microtubules and molecular motors connect the poles to kinetochores, specialized protein assemblies on the centromere regions of the chromosomes. Bipolarity of the spindle is crucial for the proper cell division, and two centrosomes in animal cells naturally become two spindle poles. Cancer cells are often multicentrosomal, yet they are able to assemble bipolar spindles by clustering centrosomes into two spindle poles. Mechanisms of this clustering are debated. In this study, we computationally screen effective forces between 1) centrosomes, 2) centrosomes and kinetochores, 3) centrosomes and chromosome arms, and 4) centrosomes and cell cortex to understand mechanics that determines three-dimensional spindle architecture. To do this, we use the stochastic Monte Carlo search for stable mechanical equilibria in the effective energy landscape of the spindle. We find that the following conditions have to be met to robustly assemble the bipolar spindle in a multicentrosomal cell: 1) the strengths of centrosomes' attraction to each other and to the cell cortex have to be proportional to each other and 2) the strengths of centrosomes' attraction to kinetochores and repulsion from the chromosome arms have to be proportional to each other. We also find that three other spindle configurations emerge if these conditions are not met: 1) collapsed, 2) monopolar, and 3) multipolar spindles, and the computational screen reveals mechanical conditions for these abnormal spindles.

RZZ-SPINDLY-DYNEIN: you got to keep 'em separated

To maintain genome stability, chromosomes must be equally distributed among daughter cells at the end of mitosis. The accuracy of chromosome segregation requires sister-kinetochores to stably attach to microtubules emanating from opposite spindle poles. However, initial kinetochore-microtubule interactions are able to turnover so that defective attachment configurations that typically arise during early mitosis may be corrected. Growing evidence supports a role for the RZZ complex in preventing the stabilization of erroneous kinetochore-microtubule attachments. This inhibitory function of RZZ toward end-on attachments is relieved by DYNEIN-mediated transport of the complex as chromosomes congress and appropriate interactions with microtubules are established. However, it remains unclear how DYNEIN is antagonized to prevent premature RZZ removal. We recently described a new mechanism that sheds new light on this matter. We found that POLO kinase phosphorylates the DYNEIN adaptor SPINDLY to promote the uncoupling between RZZ and DYNEIN. Elevated POLO activity during prometaphase ensures that RZZ is retained at kinetochores to allow the dynamic turnover of kinetochore-microtubule interactions and prevent the stabilization of erroneous attachments. Here, we discuss additional interpretations to explain a model for POLO-dependent regulation of the RZZ-SPINDLY-DYNEIN module during mitosis.

Listeria monocytogenes Interferes with Host Cell Mitosis through Its Virulence Factors InlC and ActA

Listeria monocytogenes is among the best-characterized intracellular pathogens. Its virulence factors, and the way they interfere with host cells to hijack host functions and promote the establishment and dissemination of the infection, have been the focus of multiple studies over the last 30 years. During cellular infection, L. monocytogenes was shown to induce host DNA damage and delay the host cell cycle to its own benefit. However, whether the cell cycle stage would interfere with the capacity of Listeria to infect human cultured cell lines was never assessed. We found here that L. monocytogenes preferentially infects cultured cells in G2/M phases. Inside G2/M cells, the bacteria lead to an increase in the overall mitosis duration by delaying the mitotic exit. We showed that L. monocytogenes infection causes a sustained activation of the spindle assembly checkpoint, which we correlated with the increase in the percentage of misaligned chromosomes detected in infected cells. Moreover, we demonstrated that chromosome misalignment in Listeria-infected cells required the function of two Listeria virulence factors, ActA and InlC. Our findings show the pleiotropic role of Listeria virulence factors and their cooperative action in successfully establishing the cellular infection.

The centromere comes into focus: from CENP-A nucleosomes to kinetochore connections with the spindle

Eukaryotic chromosome segregation relies upon specific connections from DNA to the microtubule-based spindle that forms at cell division. The chromosomal locus that directs this process is the centromere, where a structure called the kinetochore forms upon entry into mitosis. Recent crystallography and single-particle electron microscopy have provided unprecedented high-resolution views of the molecular complexes involved in this process. The centromere is epigenetically specified by nucleosomes harbouring a histone H3 variant, CENP-A, and we review recent progress on how it differentiates centromeric chromatin from the rest of the chromosome, the biochemical pathway that mediates its assembly and how two non-histone components of the centromere specifically recognize CENP-A nucleosomes. The core centromeric nucleosome complex (CCNC) is required to recruit a 16-subunit complex termed the constitutive centromere associated network (CCAN), and we highlight recent structures reported of the budding yeast CCAN. Finally, the structures of multiple modular sub-complexes of the kinetochore have been solved at near-atomic resolution, providing insight into how connections are made to the CCAN on one end and to the spindle microtubules on the other. One can now build molecular models from the DNA through to the physical connections to microtubules.

Establishment of a mitotic model system by transient expression of the D-type cyclin in differentiated leaf cells of tobacco (Nicotiana benthamiana)

Investigations of plant cell division would greatly benefit from a fast, inducible system. Therefore, we aimed to establish a mitotic model by transiently expressing D-type cyclins in tobacco leaf cells. Two different D-type cyclins, CYCD3;1 and CYCD4;2 from Arabidopsis thaliana, were expressed by agrobacterial infiltration in the cells of expanded leaves in tobacco (Nicotiana benthamiana). Leaf pavement cells were examined after cyclin expression while target and reference (histone or tubulin) proteins were marked by fluorescent protein-tagging. Ectopic expression of the D-type cyclin induced pavement cells to re-enter cell division by establishing mitotic microtubule arrays. The induced leaf cells expressed M phase-specific genes of Arabidopsis encoding the mitotic kinase AtAurora 1, the microtubule-associated proteins AtEDE1 and AtMAP65-4, and the vesicle fusion protein AtKNOLLE by recognizing their genomic elements. Their distinct localizations at spindle poles (AtAurora1), spindle microtubules (AtEDE1), phragmoplast microtubules (AtMAP65-4) and the cell plate (AtKNOLLE) were indistinguishable from those in their native Arabidopsis cells. The dividing cells also revealed two rice (Oryza sativa) microtubule-associated proteins in the phragmoplast and uncovered a novel spindle-associated microtubule motor protein. Hence, this cell division-enabled leaf system predicts hypothesized cell cycle-dependent functions of heterologous genes by reporting the dynamics of encoded proteins.

ALK Inhibitors-Induced M Phase Delay Contributes to the Suppression of Cell Proliferation

Anaplastic lymphoma kinase (ALK), a receptor-type tyrosine kinase, is involved in the pathogenesis of several cancers. ALK has been targeted with small molecule inhibitors for the treatment of different cancers, but absolute success remains elusive. In the present study, the effects of ALK inhibitors on M phase progression were evaluated. Crizotinib, ceritinib, and TAE684 suppressed proliferation of neuroblastoma SH-SY5Y cells in a concentration-dependent manner. At approximate IC₅₀ concentrations, these inhibitors caused misorientation of spindles, misalignment of chromosomes and reduction in autophosphorylation. Similarly, knockdown of ALK caused M phase delay, which was rescued by re-expression of ALK. Time-lapse imaging revealed that anaphase onset was delayed. The monopolar spindle 1 (MPS1) inhibitor, AZ3146, and MAD2 knockdown led to a release from inhibitor-induced M phase delay, suggesting that spindle assembly checkpoint may be activated in ALK-inhibited cells. H2228 human lung carcinoma cells that express EML4-ALK fusion showed M phase delay in the presence of TAE684 at about IC₅₀ concentrations. These results suggest that ALK plays a role in M phase regulation and ALK inhibition may contribute to the suppression of cell proliferation in ALK-expressing cancer cells.

Poly(ADP-ribose) polymerase enzymes and the maintenance of genome integrity

DNA damage response (DDR) relies on swift and accurate signaling to rapidly identify DNA lesions and initiate repair. A critical DDR signaling and regulatory molecule is the posttranslational modification poly(ADP-ribose) (PAR). PAR is synthesized by a family of structurally and functionally diverse proteins called poly(ADP-ribose) polymerases (PARPs). Although PARPs share a conserved catalytic domain, unique regulatory domains of individual family members endow PARPs with unique properties and cellular functions. Family members PARP-1, PARP-2, and PARP-3 (DDR-PARPs) are catalytically activated in the presence of damaged DNA and act as damage sensors. Family members tankyrase-1 and closely related tankyrase-2 possess SAM and ankyrin repeat domains that regulate their diverse cellular functions. Recent studies have shown that the tankyrases share some overlapping functions with the DDR-PARPs, and even perform novel functions that help preserve genomic integrity. In this review, we briefly touch on DDR-PARP functions, and focus on the emerging roles of tankyrases in genome maintenance. Preservation of genomic integrity thus appears to be a common function of several PARP family members, depicting PAR as a multifaceted guardian of the genome.

Development and application of an optimized drop-slide technique for metaphase chromosome spreads in maize

Chromosome spreads are important for complex molecular cytogenetic studies. An adequate chromosome spreading method for identification and isolation of the maize B chromosome, however, has not been reported. We used the maize inbred lines, B73 and Mo17, the hybrid YD08 line and the landrace DP76 to develop an optimized chromosome spreading method. We investigated the effects of four treatment factors on the quality of metaphase chromosome spreads using a factorial analysis of variance. Optimal conditions for metaphase spreading were identified using regression analysis based on multifactor orthogonal design of four treatment factors with five levels for each factor. We developed optimal conditions for metaphase spreading as follows: nitrous oxide treatment for 2 h, glacial acetic acid fixation for 2 h, enzyme hydrolysis for 6.0 h, and a drop height of 35 cm for cell suspension. We obtained high quality metaphase chromosome spreads with large metaphase areas, large numbers of chromosomes, few chromosome overlaps and high frequency of intact metaphases. Our optimized drop-slide procedure was markedly better than the traditional flame smear technique. We identified 487 B chromosomes in three forms from maize landraces from Southwest China. We found no relation between the C-band number and B chromosome. Single B chromosomes also were isolated directly from a metaphase chromosome drop-slide using a micromanipulator. Our optimized method provides a simple, efficient and reproducible procedure for preparing high quality plant chromosome spreads.

Chromokinesins

During the cell cycle it is critical that the duplicated DNA faithfully segregates to give rise to two genetically identical daughter cells. An even distribution of the genome during mitosis is mediated by mitotic spindle microtubules, assisted by, among others, motor proteins of the kinesin superfamily. Chromokinesins are members of the kinesin superfamily that harbour a specific DNA-binding domain. The best characterized chromokinesins belong to the kinesin-4/Kif4 and kinesin-10/Kif22 families, respectively. Functional analysis of chromokinesins in several model systems revealed their involvement in chromosome arm orientation and oscillations. This is consistent with their originally proposed role in the generation of polar ejection forces that assist chromosome congression to the spindle equator. Kinesin-12/Kif15 members comprise a third family of chromokinesins, but their role remains less understood. Noteworthy, all chromokinesins exhibit chromosome-independent localization on spindle microtubules, and recent works have significantly extended the portfolio of mitotic processes in which chromokinesins play a role, from error correction and DNA compaction, to the regulation of spindle microtubule dynamics.

Computational model demonstrates that Ndc80-associated proteins strengthen kinetochore-microtubule attachments in metaphase

Chromosome segregation is mediated by spindle microtubules that attach to the kinetochore via dynamic protein complexes, such as Ndc80, Ska, Cdt1 and ch-TOG during mitotic metaphase. While experimental studies have previously shown that these proteins and protein complexes are all essential for maintaining a stable kinetochore-microtubule (kMT) interface, their exact roles in the mitotic metaphase remains elusive. In this study, we employed experimental and computational methods in order to characterize how these proteins can strengthen kMT attachments in both nonload-bearing and load-bearing conditions, typical of prometaphase and metaphase, respectively. Immunofluorescence staining of HeLa cells showed that the levels of Ska and Cdt1 significantly increased from prometaphase to metaphase, while levels of the Ndc80 complex remained unchanged. Our new computational model showed that by incorporating binding and unbinding of each protein complex coupled with a biased diffusion mechanism, the displacement of a possible complex formed by Ndc80-Ska-Cdt1 is significantly higher than that of Ndc80 alone or Ndc80-Ska. In addition, when we incorporate Ndc80/ch-TOG in the model, rupture force and time of attachment of the kMT interface increases. These results support the hypothesis that Ndc80-associated proteins strengthen kMT attachments, and that the interplay between kMT protein complexes in metaphase ensures stable attachments.

Force-generating mechanisms of anaphase in human cells

What forces drive chromosome segregation remains one of the most challenging questions in cell division. Even though the duration of anaphase is short, it is of utmost importance for genome fidelity that no mistakes are made. Seminal studies in model organisms have revealed different mechanisms operating during chromosome segregation in anaphase, but the translation of these mechanisms to human cells is not straightforward. Recent work has shown that kinetochore fiber depolymerization during anaphase A is largely motor independent, whereas spindle elongation during anaphase B is coupled to sliding of interpolar microtubules in human cells. In this Review, we discuss the current knowledge on the mechanisms of force generation by kinetochore, interpolar and astral microtubules. By combining results from numerous studies, we propose a comprehensive picture of the role of individual force-producing and -regulating proteins. Finally, by linking key concepts of anaphase to most recent data, we summarize the contribution of all proposed mechanisms to chromosome segregation and argue that sliding of interpolar microtubules and depolymerization at the kinetochore are the main drivers of chromosome segregation during early anaphase in human cells.

Mapping the kinetochore MAP functions required for stabilizing microtubule attachments to chromosomes during metaphase

In mitosis, faithful chromosome segregation is orchestrated by the dynamic interactions between the spindle microtubules (MTs) emanating from the opposite poles and the kinetochores of the chromosomes. However, the precise mechanism that coordinates the coupling of the kinetochore components to dynamic MTs has been a long-standing question. Microtubule-associated proteins (MAPs) regulate MT nucleation and dynamics, MT-mediated transport and MT cross-linking in cells. During mitosis, MAPs play an essential role not only in determining spindle length, position, and orientation but also in facilitating robust kinetochore-microtubule (kMT) attachments by linking the kinetochores to spindle MTs efficiently. The stability of MTs imparted by the MAPs is critical to ensure accurate chromosome segregation. This review primarily focuses on the specific function of nonmotor kinetochore MAPs, their recruitment to kinetochores and their MT-binding properties. We also attempt to synthesize and strengthen our understanding of how these MAPs work in coordination with the kinetochore-bound Ndc80 complex (the key component at the MT-binding interface in metaphase and anaphase) to establish stable kMT attachments and control accurate chromosome segregation during mitosis.

Contractile acto-myosin network on nuclear envelope remnants positions human chromosomes for mitosis

To ensure proper segregation during mitosis, chromosomes must be efficiently captured by spindle microtubules and subsequently aligned on the mitotic spindle. The efficacy of chromosome interaction with the spindle can be influenced by how widely chromosomes are scattered in space. Here, we quantify chromosome-scattering volume (CSV) and find that it is reduced soon after nuclear envelope breakdown (NEBD) in human cells. The CSV reduction occurs primarily independently of microtubules and is therefore not an outcome of interactions between chromosomes and the spindle. We find that, prior to NEBD, an acto-myosin network is assembled in a LINC complex-dependent manner on the cytoplasmic surface of the nuclear envelope. This acto-myosin network remains on nuclear envelope remnants soon after NEBD, and its myosin-II-mediated contraction reduces CSV and facilitates timely chromosome congression and correct segregation. Thus, we find a novel mechanism that positions chromosomes in early mitosis to ensure efficient and correct chromosome-spindle interactions.

Cell Division: NuMA Bears the Load in the Spindle

The mitotic spindle bears the load of chromosomes during mitosis, but how this load is distributed across the spindle is unclear. A new study shows that load distribution in the spindle is confined and requires the microtubule cross-linking protein NuMA.

Microtubule end conversion mediated by motors and diffusing proteins with no intrinsic microtubule end-binding activity

Accurate chromosome segregation relies on microtubule end conversion, the ill-understood ability of kinetochores to transit from lateral microtubule attachment to durable association with dynamic microtubule plus-ends. The molecular requirements for this conversion and the underlying biophysical mechanisms are elusive. We reconstituted end conversion in vitro using two kinetochore components: the plus end-directed kinesin CENP-E and microtubule-binding Ndc80 complex, combined on the surface of a microbead. The primary role of CENP-E is to ensure close proximity between Ndc80 complexes and the microtubule plus-end, whereas Ndc80 complexes provide lasting microtubule association by diffusing on the microtubule wall near its tip. Together, these proteins mediate robust plus-end coupling during several rounds of microtubule dynamics, in the absence of any specialized tip-binding or regulatory proteins. Using a Brownian dynamics model, we show that end conversion is an emergent property of multimolecular ensembles of microtubule wall-binding proteins with finely tuned force-dependent motility characteristics.

Mitotic chromosome alignment ensures mitotic fidelity by promoting interchromosomal compaction during anaphase

Chromosome alignment at the equator of the mitotic spindle is a highly conserved step during cell division; however, its importance to genomic stability and cellular fitness is not understood. Normal mammalian somatic cells lacking KIF18A function complete cell division without aligning chromosomes. These alignment-deficient cells display normal chromosome copy numbers in vitro and in vivo, suggesting that chromosome alignment is largely dispensable for maintenance of euploidy. However, we find that loss of chromosome alignment leads to interchromosomal compaction defects during anaphase, abnormal organization of chromosomes into a single nucleus at mitotic exit, and the formation of micronuclei in vitro and in vivo. These defects slow cell proliferation and are associated with impaired postnatal growth and survival in mice. Our studies support a model in which the alignment of mitotic chromosomes promotes proper organization of chromosomes into a single nucleus and continued proliferation by ensuring that chromosomes segregate as a compact mass during anaphase.

Helical Twist and Rotational Forces in the Mitotic Spindle

The mitotic spindle segregates chromosomes into two daughter cells during cell division. This process relies on the precise regulation of forces acting on chromosomes as the cell progresses through mitosis. The forces in the spindle are difficult to directly measure using the available experimental techniques. Here, we review the ideas and recent advances of how forces can be determined from the spindle shape. By using these approaches, it has been shown that tension and compression coexist along a single kinetochore fiber, which are balanced by a bridging fiber between sister kinetochore fibers. An extension of this approach to three dimensions revealed that microtubule bundles have rich shapes, and extend not simply like meridians on the Earth’s surface but, rather, twisted in a helical manner. Such complex shapes are due to rotational forces, which, in addition to linear forces, act in the spindle and may be generated by motor proteins such as kinesin-5. These findings open new questions for future studies, to understand the mechanisms of rotational forces and reveal their biological roles in cells.

No chromosome left behind: The importance of metaphase alignment for mitotic fidelity

Chromosome alignment is a hallmark of mitosis in metazoans, but the physiological relevance of this orderly behavior has remained unclear. In this issue, Fonseca et al. (2019. J. Cell Biol. https://doi.org/10.1083/jcb.201807228) show that chromosome alignment ensures mitotic fidelity by promoting interchromosomal compaction during anaphase.

Chromosome alignment maintenance requires the MAP RECQL4, mutated in the Rothmund-Thomson syndrome

RECQL4, which is mutated in the Rothmund–Thomson syndrome characterized by premature aging and cancer susceptibility, is a microtubule-associated protein required for mitotic chromosome alignment.

RecQ-like helicase 4 (RECQL4) is mutated in patients suffering from the Rothmund–Thomson syndrome, a genetic disease characterized by premature aging, skeletal malformations, and high cancer susceptibility. Known roles of RECQL4 in DNA replication and repair provide a possible explanation of chromosome instability observed in patient cells. Here, we demonstrate that RECQL4 is a microtubule-associated protein (MAP) localizing to the mitotic spindle. RECQL4 depletion in M-phase–arrested frog egg extracts does not affect spindle assembly per se, but interferes with maintaining chromosome alignment at the metaphase plate. Low doses of nocodazole depolymerize RECQL4-depleted spindles more easily, suggesting abnormal microtubule–kinetochore interaction. Surprisingly, inter-kinetochore distance of sister chromatids is larger in depleted extracts and patient fibroblasts. Consistent with a role to maintain stable chromosome alignment, RECQL4 down-regulation in HeLa cells causes chromosome misalignment and delays mitotic progression. Importantly, these chromosome alignment defects are independent from RECQL4’s reported roles in DNA replication and damage repair. Our data elucidate a novel function of RECQL4 in mitosis, and defects in mitotic chromosome alignment might be a contributing factor for the Rothmund–Thomson syndrome.

Impaired CENP-E Function Renders Large Chromosomes More Vulnerable to Congression Failure

It has recently emerged that human chromosomes vary between one another in terms of features that impact their behaviour during impaired chromosome segregation, leading to non-random aneuploidy in the daughter cell population. During the process of chromosome congression to the metaphase plate, chromosome movement is guided by kinesin-like proteins, among which centromere-associated protein E (CENP-E) is important to transport chromosomes along the microtubules of the mitotic spindle. It is known that the inhibition of CENP-E notably impairs alignment for a subset of chromosomes, particularly those positioned close to the centrosome at nuclear envelope breakdown (‘polar chromosomes’); it is, however, not clear whether chromosome identity could influence this process. Since a popular strategy to model aneuploidy is to induce congression defects (for example combining CENP-E inhibitors with mitotic checkpoint abrogation), variance in congression efficiency between chromosomes might influence the landscape of aneuploidy and subsequent cell fates. By combining immunofluorescence, live cell imaging and fluorescence in situ hybridisation, we investigated the behaviour of polar chromosomes and their dependency upon CENP-E-mediated congression in human cells. We observed a bias in congression efficiency related to chromosome size, with larger chromosomes more sensitive to CENP-E inhibition. This bias is likely due to two contributing factors; an initial propensity of larger chromosomes to be peripheral and thus rely more upon CENP-E function to migrate to the metaphase plate, and additionally a bias between specific chromosomes’ ability to congress from a polar state. These findings may help to explain the persistence of a subset of chromosomes at the centrosome following CENP-E disruption, and also have implications for the spectrum of aneuploidy generated following treatments to manipulate CENP-E function.

Delayed Chromosome Alignment to the Spindle Equator Increases the Rate of Chromosome Missegregation in Cancer Cell Lines

For appropriate chromosome segregation, kinetochores on sister chromatids have to attach to microtubules from opposite spindle poles (bi-orientation). Chromosome alignment at the spindle equator, referred to as congression, can occur through the attachment of kinetochores to the lateral surface of spindle microtubules, facilitating bi-orientation establishment. However, the contribution of this phenomenon to mitotic fidelity has not been clarified yet. Here, we addressed whether delayed chromosome alignment to the spindle equator increases the rate of chromosome missegregation. Cancer cell lines depleted of Kid, a chromokinesin involved in chromosome congression, showed chromosome alignment with a slight delay, and increased frequency of lagging chromosomes. Delayed chromosome alignment concomitant with an increased rate of lagging chromosomes was also seen in cells depleted of kinesin family member 4A (KIF4A), another chromokinesin. Cells that underwent chromosome missegregation took relatively longer time to align chromosomes in both control and Kid/KIF4A-depleted cells. Tracking of late-aligning chromosomes showed that they exhibit a higher rate of lagging chromosomes. Intriguingly, the metaphase of cells that underwent chromosome missegregation was shortened, and delaying anaphase onset ameliorated the increased chromosome missegregation. These data suggest that late-aligning chromosomes do not have sufficient time to establish bi-orientation, leading to chromosome missegregation. Our data imply that delayed chromosome alignment is not only a consequence, but also a cause of defective bi-orientation establishment, which can lead to chromosomal instability in cells without severe mitotic defects.

BUB-1 promotes amphitelic chromosome biorientation via multiple activities at the kinetochore

Accurate chromosome segregation relies on bioriented amphitelic attachments of chromosomes to microtubules of the mitotic spindle, in which sister chromatids are connected to opposite spindle poles. BUB-1 is a protein of the Spindle Assembly Checkpoint (SAC) that coordinates chromosome attachment with anaphase onset. BUB-1 is also required for accurate sister chromatid segregation independently of its SAC function, but the underlying mechanism remains unclear. Here we show that, in Caenorhabditis elegans embryos, BUB-1 accelerates the establishment of non-merotelic end-on kinetochore-microtubule attachments by recruiting the RZZ complex and its downstream partner dynein-dynactin at the kinetochore. In parallel, BUB-1 limits attachment maturation by the SKA complex. This activity opposes kinetochore-microtubule attachment stabilisation promoted by CLS-2^CLASP-dependent kinetochore-microtubule assembly. BUB-1 is therefore a SAC component that coordinates the function of multiple downstream kinetochore-associated proteins to ensure accurate chromosome segregation.

The Spindle: Integrating Architecture and Mechanics across Scales

The spindle segregates chromosomes at cell division, and its task is a mechanical one. While we have a nearly complete list of spindle components, how their molecular-scale mechanics give rise to cellular-scale spindle architecture, mechanics, and function is not yet clear. Recent in vitro and in vivo measurements bring new levels of molecular and physical control and shed light on this question. Highlighting recent findings and open questions, we introduce the molecular force generators of the spindle, and discuss how they organize microtubules into diverse architectural modules and give rise to the emergent mechanics of the mammalian spindle. Throughout, we emphasize the breadth of space and time scales at play, and the feedback between spindle architecture, dynamics, and mechanics that drives robust function.