Chromosome movement in mitosis requires microtubule anchorage at spindle poles

DNA methylation of KIFC1 gene in determination of histological diagnosis, prognosis and metastasis of lung cancer

BACKGROUND

One of the main features of cancer, especially lung cancer (LC), is abnormal cell division. Abnormal expression of kinesin family member C1 (KIFC1/HSET), which is involved in mitotic cell division and ensures equatorial alignment of chromosomes during division, is observed in both premalignant and malignant lesions. There are no studies in the literature addressing the role of KIFC1 in the diagnosis and follow-up of LC. In this study, we investigated the epigenetic role of KIFC1 in the diagnosis, stage, and prognosis of various histological subtypes diagnosed with LC.

MATERIAL AND METHODS

The expression and methylation status of the KIFC1 gene were examined after DNA/RNA isolation in tumor, conjugate normal tissue, and blood samples from 39 patients diagnosed with LC and in blood samples from 39 healthy controls. Changes in KIFC1 gene expression were examined by the Quantitative Real Time-PCR (qRT-PCR) method after cDNA synthesis following RNA isolation. The Methylation-Specific PCR (MSP) method was used to determine the methylation status of the KIFC1 gene. In this study, the expression/methylation profiles of the KIFC1 gene and the clinical and pathological characteristics of the patients were analyzed by statistical methods.

RESULT

Hypomethylation was detected in 95.8% of the 62.1% of patients' tissues with increased KIFC1 gene expression. The expression level of the KIFC1 gene was found to be increased 3.2-fold in the tumor tissues of the patients compared with the conjugated normal tissues and 2.4-fold in the serum of the patients compared with the healthy serum. Statistical comparison of patients' clinical parameters and methylation and expression results revealed statistical significance between KIFC1 expression and metastasis, tumor stage and tumor grade.

CONCLUSION

In conclusion, the increase in the expression level of the KIFC1 gene is higher in patients diagnosed with LC than in the healthy population, and therefore, the increase in the expression level of the KIFC1 gene due to hypomethylation can be used as a screening biomarker in LC. It can also be considered that the methylation profile of the KIFC1 gene may be a potential biomarker for determining the subtype of squamous cell carcinoma in LC. The results of the study need to be analyzed and continued with a larger number of patients.

Quantifying yeast microtubules and spindles using the Toolkit for Automated Microtubule Tracking (TAMiT)

Fluorescently labeled proteins absorb and emit light, appearing as Gaussian spots in fluorescence imaging. When fluorescent tags are added to cytoskeletal polymers such as microtubules, a line of fluorescence and even non-linear structures results. While much progress has been made in techniques for imaging and microscopy, image analysis is less well developed. Current analysis of fluorescent microtubules uses either manual tools, such as kymographs, or automated software. As a result, our ability to quantify microtubule dynamics and organization from light microscopy remains limited. Despite development of automated microtubule analysis tools for in vitro studies, analysis of images from cells often depends heavily on manual analysis. One of the main reasons for this disparity is the low signal-to-noise ratio in cells, where background fluorescence is typically higher than in reconstituted systems. Here, we present the Toolkit for Automated Microtubule Tracking (TAMiT), which automatically detects, optimizes and tracks fluorescent microtubules in living yeast cells with sub-pixel accuracy. Using basic information about microtubule organization, TAMiT detects linear and curved polymers using a geometrical scanning technique. Images are fit via an optimization problem for the microtubule image parameters that is solved using non-linear least squares in Matlab. We benchmark our software using simulated images and show that it reliably detects microtubules, even at low signal-to-noise ratios. Then, we use TAMiT to measure monopolar spindle microtubule bundle number, length, and lifetime in a large dataset that includes several S. pombe mutants that affect microtubule dynamics and bundling. The results from the automated analysis are consistent with previous work, and suggest a direct role for CLASP/Cls1 in bundling spindle microtubules. We also illustrate automated tracking of single curved astral microtubules in S. cerevisiae , with measurement of dynamic instability parameters. The results obtained with our fully-automated software are similar to results using hand-tracked measurements. Therefore, TAMiT can facilitate automated analysis of spindle and microtubule dynamics in yeast cells.

NuMA deficiency causes micronuclei via checkpoint-insensitive k-fiber minus-end detachment from mitotic spindle poles

Micronuclei resulting from improper chromosome segregation foster chromosome rearrangements.^1,2 To prevent micronuclei formation in mitosis, the dynamic plus ends of bundled kinetochore microtubules (k-fibers) must establish bipolar attachment with all sister kinetochores on chromosomes,³ whereas k-fiber minus ends must be clustered at the two opposing spindle poles, which are normally connected with centrosomes.⁴ The establishment of chromosome biorientation via k-fiber plus ends is carefully monitored by the spindle assembly checkpoint (SAC).⁵ However, how k-fiber minus-end clustering near centrosomes is maintained and monitored remains poorly understood. Here, we show that degradation of NuMA by auxin-inducible degron technologies results in micronuclei formation through k-fiber minus-end detachment from spindle poles during metaphase in HCT116 colon cancer cells. Importantly, k-fiber minus-end detachment from one pole creates misaligned chromosomes that maintain chromosome biorientation and satisfy the SAC, resulting in abnormal chromosome segregation. NuMA depletion also causes minus-end clustering defects in non-transformed Rpe1 cells, but it additionally induces centrosome detachment from partially focused poles, resulting in highly disorganized anaphase. Moreover, we find that NuMA depletion causes centrosome clustering defects in tetraploid-like cells, leading to an increased frequency of multipolar divisions. Together, our data indicate that NuMA is required for faithful chromosome segregation in human mitotic cells, generally by maintaining k-fiber minus-end clustering but also by promoting spindle pole-centrosome or centrosome-centrosome connection in specific cell types or contexts. Similar to erroneous merotelic kinetochore attachments,⁶ detachment of k-fiber minus ends from spindle poles evades spindle checkpoint surveillance and may therefore be a source of genomic instability in dividing cells.

Mitosis: Augmin-based bridges keep kinetochores in line

A recent study highlights the indispensability of the augmin complex for the construction of mitotic spindle bridging fibers, which in turn support accurate chromosome attachment and segregation.

Exclusive modifications of NuMA in malignant epithelial cells: A potential therapeutic mechanism

NuMA (nuclear mitotic apparatus) protein is indispensable in the mitosis of human proliferating cells, both malignant and benign. The progression of mitosis requires stable spindles, which depend on the bipolar clustering of NuMA within the spindles. The phenanthridine PJ34 kills malignant epithelial cells during mitosis and targets NuMA. PJ34 exclusively blocks the post-translational modification of NuMA in a variety of malignant epithelial cells, but not in benign cells. This blockage of the post-translational modification of NuMA affects its protein-binding capacity and causes construction faults in the mitotic spindle poles of PJ34-treated cancer cells, leading to mitotic catastrophe cell death. PJ34 is a potent PARP1 inhibitor, so its cytotoxicity in human malignant cells is exclusively independent of PARP, challenging the currently accepted notion that inhibition of PARP1 halts cancer by preventing DNA repair. Certain molecules that act as PARP1 inhibitors target other proteins and vital mechanisms in human cancer cells.

Force by minus-end motors Dhc1 and Klp2 collapses the S. pombe spindle after laser ablation

A microtubule-based machine called the mitotic spindle segregates chromosomes when eukaryotic cells divide. In the fission yeast Schizosaccharomyces pombe, which undergoes closed mitosis, the spindle forms a single bundle of microtubules inside the nucleus. During elongation, the spindle extends via antiparallel microtubule sliding by molecular motors. These extensile forces from the spindle are thought to resist compressive forces from the nucleus. We probe the mechanism and maintenance of this force balance via laser ablation of spindles at various stages of mitosis. We find that spindle pole bodies collapse toward each other after ablation, but spindle geometry is often rescued, allowing spindles to resume elongation. Although this basic behavior has been previously observed, many questions remain about the phenomenon's dynamics, mechanics, and molecular requirements. In this work, we find that previously hypothesized viscoelastic relaxation of the nucleus cannot explain spindle shortening in response to laser ablation. Instead, spindle collapse requires microtubule dynamics and is powered by the minus-end-directed motor proteins dynein Dhc1 and kinesin-14 Klp2, but it does not require the minus-end-directed kinesin Pkl1.

WDR62 localizes katanin at spindle poles to ensure synchronous chromosome segregation

Mutations in the WDR62 gene cause primary microcephaly, a pathological condition often associated with defective cell division that results in severe brain developmental defects. The precise function and localization of WDR62 within the mitotic spindle is, however, still under debate, as it has been proposed to act either at centrosomes or on the mitotic spindle. Here we explored the cellular functions of WDR62 in human epithelial cell lines using both short-term siRNA protein depletions and long-term CRISPR/Cas9 gene knockouts. We demonstrate that WDR62 localizes at spindle poles, promoting the recruitment of the microtubule-severing enzyme katanin. Depletion or loss of WDR62 stabilizes spindle microtubules due to insufficient microtubule minus-end depolymerization but does not affect plus-end microtubule dynamics. During chromosome segregation, WDR62 and katanin promote efficient poleward microtubule flux and favor the synchronicity of poleward movements in anaphase to prevent lagging chromosomes. We speculate that these lagging chromosomes might be linked to developmental defects in primary microcephaly.

Mitotic checkpoint regulator RAE1 promotes tumor growth in colorectal cancer

Microtubules are among the most successful targets for anticancer therapy because they play important roles in cell proliferation as they constitute the mitotic spindle, which is critical for chromosome segregation during mitosis. Hence, identifying new therapeutic targets encoding proteins that regulate microtubule assembly and function specifically in cancer cells is critical. In the present study, we identified a candidate gene that promotes tumor progression, ribonucleic acid export 1 (RAE1), a mitotic checkpoint regulator, on chromosome 20q through a bioinformatics approach using datasets of colorectal cancer (CRC), including The Cancer Genome Atlas (TCGA). RAE1 was ubiquitously amplified and overexpressed in tumor cells. High expression of RAE1 in tumor tissues was positively associated with distant metastasis and was an independent poor prognostic factor in CRC. In vitro and in vivo analysis showed that RAE1 promoted tumor growth, inhibited apoptosis, and promoted cell cycle progression, possibly with a decreased proportion of multipolar spindle cells in CRC. Furthermore, RAE1 induced chemoresistance through its anti-apoptotic effect. In addition, overexpression of RAE1 and significant effects on survival were observed in various types of cancer, including CRC. In conclusion, we identified RAE1 as a novel gene that facilitates tumor growth in part by inhibiting apoptosis and promoting cell cycle progression through stabilizing spindle bipolarity and facilitating tumor growth. We suggest that it is a potential therapeutic target to overcome therapeutic resistance of CRC.

Mud binds the kinesin-14 Ncd in Drosophila

Maintenance of proper mitotic spindle structure is necessary for error-free chromosome segregation and cell division. Spindle assembly is controlled by force-generating kinesin motors that contribute to its geometry and bipolarity, and balancing motor-dependent forces between opposing kinesins is critical to the integrity of this process. Non-claret dysjunctional (Ncd), a Drosophila kinesin-14 member, crosslinks and slides microtubule minus-ends to focus spindle poles and sustain bipolarity. However, mechanisms that regulate Ncd activity during mitosis are underappreciated. Here, we identify Mushroom body defect (Mud), the fly ortholog of human NuMA, as a direct Ncd binding partner. We demonstrate this interaction involves a short coiled-coil domain within Mud (Mud^CC) binding the N-terminal, non-motor microtubule-binding domain of Ncd (Ncd^nMBD). We further show that the C-terminal ATPase motor domain of Ncd (Ncd^CTm) directly interacts with Ncd^nMBD as well. Mud binding competes against this self-association and also increases Ncd^nMBD microtubule binding in vitro. Our results describe an interaction between two spindle-associated proteins and suggest a potentially new mode of minus-end motor protein regulation at mitotic spindle poles.

A non-canonical function for Centromere-associated protein-E controls centrosome integrity and orientation of cell division

Centromere-associated protein-E (CENP-E) is a kinesin motor localizing at kinetochores. Although its mitotic functions have been well studied, it has been challenging to investigate direct consequences of CENP-E removal using conventional methods because CENP-E depletion resulted in mitotic arrest. In this study, we harnessed an auxin-inducible degron system to achieve acute degradation of CENP-E. We revealed a kinetochore-independent role for CENP-E that removes pericentriolar material 1 (PCM1) from centrosomes in late S/early G2 phase. After acute loss of CENP-E, centrosomal Polo-like kinase 1 (Plk1) localization is abrogated through accumulation of PCM1, resulting in aberrant phosphorylation and destabilization of centrosomes, which triggers shortened astral microtubules and oblique cell divisions. Furthermore, we also observed centrosome and cell division defects in cells from a microcephaly patient with mutations in CENPE. Orientation of cell division is deregulated in some microcephalic patients, and our unanticipated findings provide additional insights into how microcephaly can result from centrosomal defects.

Insight on [1,3]thiazolo[4,5-e]isoindoles as tubulin polymerization inhibitors

A series of [1,3]thiazolo[4,5-e]isoindoles has been synthesized through a versatile and high yielding multistep sequence. Evaluation of the antiproliferative activity of the new compounds on the full NCI human tumor cell line panel highlighted several compounds that are able to inhibit tumor cell proliferation at micromolar-submicromolar concentrations. The most active derivative 11g was found to cause cell cycle arrest at the G2/M phase and induce apoptosis in HeLa cells, following the mitochondrial pathway, making it a lead compound for the discovery of new antimitotic drugs.

The Modified Phenanthridine PJ34 Unveils an Exclusive Cell-Death Mechanism in Human Cancer Cells

This overview summarizes recent data disclosing the efficacy of the PARP inhibitor PJ34 in exclusive eradication of a variety of human cancer cells without impairing healthy proliferating cells. Its cytotoxic activity in cancer cells is attributed to the insertion of specific un-repairable anomalies in the structure of their mitotic spindle, leading to mitotic catastrophe cell death. This mechanism paves the way to a new concept of cancer therapy.

Basal Body Protein TbSAF1 Is Required for Microtubule Quartet Anchorage to the Basal Bodies in Trypanosoma brucei

Trypanosoma brucei contains a large array of single-copied organelles and structures. Through extensive interorganelle connections, these structures replicate and divide following a strict temporal and spatial order. A microtubule quartet (MtQ) originates from the basal bodies and extends toward the anterior end of the cell, stringing several cytoskeletal structures together along its path. In this study, we examined the interaction network of TbSpef1, the only protein specifically located to the MtQ. We identified an interaction between TbSpef1 and a basal body protein TbSAF1, which is required for MtQ anchorage to the basal bodies. This study thus provides the first molecular description of MtQ association with the basal bodies, since the discovery of this association ∼30 years ago. The results also reveal a general mechanism of the evolutionarily conserved Spef1/CLAMP, which achieves specific cellular functions via their conserved microtubule functions and their diverse molecular interaction networks.

Sperm flagellar protein 1 (Spef1, also known as CLAMP) is a microtubule-associated protein involved in various microtubule-related functions from ciliary motility to polarized cell movement and planar cell polarity. In Trypanosoma brucei, the causative agent of trypanosomiasis, a single Spef1 ortholog (TbSpef1) is associated with a microtubule quartet (MtQ), which is in close association with several single-copied organelles and is required for their coordinated biogenesis during the cell cycle. Here, we investigated the interaction network of TbSpef1 using BioID, a proximity-dependent protein-protein interaction screening method. Characterization of selected candidates provided a molecular description of TbSpef1-MtQ interactions with nearby cytoskeletal structures. Of particular interest, we identified a new basal body protein TbSAF1, which is required for TbSpef1-MtQ anchorage to the basal bodies. The results demonstrate that MtQ-basal body anchorage is critical for the spatial organization of cytoskeletal organelles, as well as the morphology of the membrane-bound flagellar pocket where endocytosis takes place in this parasite.

The phenanthrene derivative PJ34 exclusively eradicates human pancreatic cancer cells in xenografts

Recent reports demonstrate an exclusive eradication of a variety of human cancer cells by the modified phenanthridine PJ34. Their eradication during mitosis is attributed to PJ34 preventing NuMA clustering in the mitotic spindle poles of human malignant cells, which is crucial for their normal mitosis. Here, the effect of PJ34 is tested in cell cultures and xenografts of human pancreas ductal adenocarcinoma. Evidence is presented for a substantial reduction (80-90%) of PANC1 cancer cells in xenografts, measured 30 days after the treatment with PJ34 has been terminated. Benign cells infiltrated into the PANC1 tumors (stroma) were not affected. Growth, weight gain and behavior of the treated nude mice were not impaired during, and 30 days after the treatment with PJ34. The efficient eradication of malignant cells in human pancreas cancer xenografts presents a new model of pancreas cancer treatment.

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.

Maize VKS1 Regulates Mitosis and Cytokinesis During Early Endosperm Development

Cell number is a critical factor that determines kernel size in maize (Zea mays). Rapid mitotic divisions in early endosperm development produce most of the cells that make up the starchy endosperm; however, the mechanisms underlying early endosperm development remain largely unknown. We isolated a maize mutant that shows a varied-kernel-size phenotype (vks1). Vks1 encodes ZmKIN11, which belongs to the kinesin-14 subfamily and is predominantly expressed in early endosperm development. VKS1 dynamically localizes to the nucleus and microtubules and plays key roles in the migration of free nuclei in the coenocyte as well as in mitosis and cytokinesis in early mitotic divisions. Absence of VKS1 has relatively minor effects on plants but causes deformities in spindle assembly, sister chromatid separation, and phragmoplast formation in early endosperm development, thereby resulting in reduced cell proliferation. Severities of aberrant mitosis and cytokinesis within individual vks1 endosperms differ, thereby resulting in varied kernel sizes. Our discovery highlights VKS1 as a central regulator of mitosis in early maize endosperm development and provides a potential approach for future yield improvement.

Kinesin-14 motor protein KIFC1 participates in DNA synthesis and chromatin maintenance

The nuclear localization signal (NLS) in kinesin-14 KIFC1 is associated with nuclear importins and Ran gradient, but detailed mechanism remains unknown. In this study, we found that KIFC1 proteins have specific transport characteristics during cell cycle. In the absence of KIFC1, cell cycle kinetics decrease significantly with a prolonged S phase. After KIFC1 overexpression, the duration of S phase becomes shorten. KIFC1 may transport the recombinant/replicate-related proteins into the nucleus, meanwhile avoiding excessive KIFC1 in the cytoplasm, which results in aberrant microtubule bundling. Interestingly, the deletion of kifc1 in human cells results in a higher ratio of aberrant nuclear membrane, and the degradation of lamin B and lamin A/C. We also found that kifc1 deletion leads to defects in metaphase mitotic spindle assembly, and then results in chromosome structural abnormality. The kifc1^-/- cells finally form micronuclei in daughter cells, and results in aneuploidy and chromosome loss in cell cycle. In this study, we demonstrate that kinesin-14 KIFC1 proteins involve in regulating DNA synthesis in S phase, and chromatin maintenance in mitosis, and maintain cell growth in a nuclear transport-independent way.

p37/UBXN2B regulates spindle orientation by limiting cortical NuMA recruitment via PP1/Repo-Man

The p97 adapter p37 was known to regulate spindle orientation in human cells, but the mechanism was unknown. In this study, we show that it limits the cortical recruitment of NuMA in a PP1–Repo-Man–dependent manner. This study identifies a novel pathway controlling cortical NuMA localization.

Spindle orientation determines the axis of division and is crucial for cell fate, tissue morphogenesis, and the development of an organism. In animal cells, spindle orientation is regulated by the conserved Gαi–LGN–NuMA complex, which targets the force generator dynein–dynactin to the cortex. In this study, we show that p37/UBXN2B, a cofactor of the p97 AAA ATPase, regulates spindle orientation in mammalian cells by limiting the levels of cortical NuMA. p37 controls cortical NuMA levels via the phosphatase PP1 and its regulatory subunit Repo-Man, but it acts independently of Gαi, the kinase Aurora A, and the phosphatase PP2A. Our data show that in anaphase, when the spindle elongates, PP1/Repo-Man promotes the accumulation of NuMA at the cortex. In metaphase, p37 negatively regulates this function of PP1, resulting in lower cortical NuMA levels and correct spindle orientation.

Exclusive destruction of mitotic spindles in human cancer cells

We identified target proteins modified by phenanthrenes that cause exclusive eradication of human cancer cells. The cytotoxic activity of the phenanthrenes in a variety of human cancer cells is attributed by these findings to post translational modifications of NuMA and kinesins HSET/kifC1 and kif18A. Their activity prevented the binding of NuMA to α-tubulin and kinesins in human cancer cells, and caused aberrant spindles. The most efficient cytotoxic activity of the phenanthridine PJ34, caused significantly smaller aberrant spindles with disrupted spindle poles and scattered extra-centrosomes and chromosomes. Concomitantly, PJ34 induced tumor growth arrest of human malignant tumors developed in athymic nude mice, indicating the relevance of its activity for cancer therapy.

Isolation and characterization of string-forming female germline stem cells from ovaries of neonatal mice

Germline stem cells are essential in the generation of both male and female gametes. In mammals, the male testis produces sperm throughout the entire lifetime, facilitated by testicular germline stem cells. Oocyte renewal ceases in postnatal or adult life in mammalian females, suggesting that germline stem cells are absent from the mammalian ovary. However, studies in mice, rats, and humans have recently provided evidence for ovarian female germline stem cells (FGSCs). A better understanding of the role of FGSCs in ovaries could help improve fertility treatments. Here, we developed a rapid and efficient method for isolating FGSCs from ovaries of neonatal mice. Notably, our FGSC isolation method could efficiently isolate on average 15 cell "strings" per ovary from mice at 1-3 days postpartum. FGSCs isolated from neonatal mice displayed the string-forming cell configuration at mitosis (i.e. a "stringing" FGSC (sFGSC) phenotype) and a disperse phenotype in postnatal mice. We also found that sFGSCs undergo vigorous mitosis especially at 1-3 days postpartum. After cell division, the sFGSC membranes tended to be connected to form sFGSCs. Moreover, F-actin filaments exhibited a cell-cortex distribution in sFGSCs, and E-cadherin converged in cell-cell connection regions, resulting in the string-forming morphology. Our new method provides a platform for isolating FGSCs from the neonatal ovary, and our findings indicate that FGCSs exhibit string-forming features in neonatal mice. The sFGSCs represent a valuable resource for analysis of ovary function and an in vitro model for future clinical use to address ovarian dysfunction.

Mapping Load-Bearing in the Mammalian Spindle Reveals Local Kinetochore Fiber Anchorage that Provides Mechanical Isolation and Redundancy

Active forces generated at kinetochores move chromosomes, and the dynamic spindle must robustly anchor kinetochore fibers (k-fibers) to bear this load. The mammalian spindle bears the load of chromosome movement far from poles, but we do not know where and how-physically and molecularly-this load distributes across the spindle. In part, this is because probing spindle mechanics in live cells is difficult. Yet answering this question is key to understanding how the spindle generates and responds to force and performs its diverse mechanical functions. Here, we map load-bearing across the mammalian spindle in space-time and dissect local anchorage mechanics and mechanism. To do so, we laser-ablate single k-fibers at different spindle locations and in different molecular backgrounds and quantify the immediate relaxation of chromosomes, k-fibers, and microtubule speckles. We find that load redistribution is locally confined in all directions: along the first 3-4 μm from kinetochores, scaling with k-fiber length, and laterally within ∼2 μm of k-fiber sides, without detectable load sharing between neighboring k-fibers. A phenomenological model suggests that dense, transient crosslinks to the spindle along k-fibers bear the load of chromosome movement but that these connections do not limit the timescale of spindle reorganization. The microtubule crosslinker NuMA is needed for the local load-bearing observed, whereas Eg5 and PRC1 are not detectably required, suggesting specialization in mechanical function. Together, the data and model suggest that NuMA-mediated crosslinks locally bear load, providing mechanical isolation and redundancy while allowing spindle fluidity. These features are well suited to support robust chromosome segregation.

Molecular mechanisms of kinesin-14 motors in spindle assembly and chromosome segregation

During eukaryote cell division, molecular motors are crucial regulators of microtubule organization, spindle assembly, chromosome segregation and intracellular transport. The kinesin-14 motors are evolutionarily conserved minus-end-directed kinesin motors that occur in diverse organisms from simple yeasts to higher eukaryotes. Members of the kinesin-14 motor family can bind to, crosslink or slide microtubules and, thus, regulate microtubule organization and spindle assembly. In this Commentary, we present the common subthemes that have emerged from studies of the molecular kinetics and mechanics of kinesin-14 motors, particularly with regard to their non-processive movement, their ability to crosslink microtubules and interact with the minus- and plus-ends of microtubules, and with microtubule-organizing center proteins. In particular, counteracting forces between minus-end-directed kinesin-14 and plus-end-directed kinesin-5 motors have recently been implicated in the regulation of microtubule nucleation. We also discuss recent progress in our current understanding of the multiple and fundamental functions that kinesin-14 motors family members have in important aspects of cell division, including the spindle pole, spindle organization and chromosome segregation.

HGF/Met Signaling in Cancer Invasion: The Impact on Cytoskeleton Remodeling

The invasion of cancer cells into surrounding tissue and the vasculature is essential for tumor metastasis. Increasing evidence indicates that hepatocyte growth factor (HGF) induces cancer cell migration and invasion. A broad spectrum of mechanisms underlies cancer cell migration and invasion. Cytoskeletal reorganization is of central importance in the development of the phenotype of cancer cells with invasive behavior. Through their roles in cell mechanics, intracellular trafficking, and signaling, cytoskeleton proteins participate in all essential events leading to cell migration. HGF has been involved in cytoskeleton assembly and reorganization, and its role in regulating cytoskeleton dynamics is still expanding. This review summarizes our current understanding of the role of HGF in regulating cytoskeleton remodeling, distribution, and interactions.

Mechanical properties of spindle poles are symmetrically balanced

The metaphase spindle is organized for accurate chromosome segregation. One of the fundamental features of the spindle across the species is its symmetrical shape; the spindle consists of two polar arrays of microtubules at both ends. Although it has been suggested that the formation of the bipolar shape requires force balance coordination by molecular motors, i.e., kinesins and dyneins, quantitative analysis for the pole mechanics has not been conducted. Here, we demonstrate that it is not only the shape but also the stiffness and microtubule density of the pairs of pole regions are symmetrically balanced in single spindles self-assembled in Xenopus egg extracts. We found that the inhibition of dynein functions dramatically reduced the stiffness and microtubule density in the pole region. By contrast, the inhibition of one of the kinesins, Eg5, which is the antagonistic motor protein of dynein, increased the value of these parameters. Moreover, the inhibition of both dynein and Eg5 recovered these parameter values to those of non-treated spindle poles. We also found that, when one pole structure was held widened with the use of two glass microneedles, the opposite pole structure spontaneously widened, resulting in the formation of the barrel-like shaped spindle. The values of stiffness and microtubule density in the manipulated pole region decreased, following the spontaneous decrement of those in the paired unmanipulated pole region. These results suggest that the spindle possesses a mechanism to dynamically maintain its symmetry in mechanical properties.

Metaphase Spindle Assembly

A microtubule-based bipolar spindle is required for error-free chromosome segregation during cell division. In this review I discuss the molecular mechanisms required for the assembly of this dynamic micrometer-scale structure in animal cells.

The mitotic kinesin-14 KlpA contains a context-dependent directionality switch

Kinesin-14s are commonly known as nonprocessive minus end-directed microtubule motors that function mainly for mitotic spindle assembly. Here we show using total internal reflection fluorescence microscopy that KlpA—a kinesin-14 from Aspergillus nidulans—is a context-dependent bidirectional motor. KlpA exhibits plus end-directed processive motility on single microtubules, but reverts to canonical minus end-directed motility when anchored on the surface in microtubule-gliding experiments or interacting with a pair of microtubules in microtubule-sliding experiments. Plus end-directed processive motility of KlpA on single microtubules depends on its N-terminal nonmotor microtubule-binding tail, as KlpA without the tail is nonprocessive and minus end-directed. We suggest that the tail is a de facto directionality switch for KlpA motility: when the tail binds to the same microtubule as the motor domain, KlpA is a plus end-directed processive motor; in contrast, when the tail detaches from the microtubule to which the motor domain binds, KlpA becomes minus end-directed.

Kinesin-14s are commonly considered to be minus end-directed microtubule motor proteins. Here the authors show that KlpA, a fungal kinesin-14 orthologue, relies on its N-terminal nonmotor microtubule-binding tail to achieve context-dependent bidirectional motility.

The overexpression of KIFC1 was associated with the proliferation and prognosis of non-small cell lung cancer

BACKGROUND

The kinesin family member C1 (KIFC1, also known as HSET) is a kinesin superfamily protein (KIFs). Although KIFC1 acts as a crucial role in the development of several human cancers, the KIFC1 expression profile and functional remain unclear in non-small cell lung cancer (NSCLC).

METHODS

We collected the fresh NSCLC samples and paired normal lung tissue in patients with lung cancer operation, and detected KIFC1 expression using quantitative reverse-transcription polymerase chain reaction (qRT-PCR) and Western blotting. To expand on previous smaller-scale studies, NSCLC tissue microarrays (TMA) were analyzed by IHC. Finally, cell lines were employed to further probe the potential mechanisms.

RESULTS

In this study, we described that KIFC1 was significantly upregulated in NSCLC tissues compared with the corresponding normal tissues. Moreover, KIFC1 overexpression was associated with the poor overall survival (OS) of NSCLC patients, and siRNA-mediated knockdown of KIFC1 significantly suppressed tumor cell proliferation in vitro. Further verification showed that inhibition of KIFC1 gene expression caused the upregulation of the cyclin-dependent kinases inhibitor p21 and downregulation of the cell cycle driver protein cdc2, which arrested cells in the G2-M phase.

CONCLUSIONS

we report that increased KIFC1 expression may promote cell proliferation and identified it as a biomarker of unfavorable prognosis in NSCLC patients.

KIFC1: a promising chemotherapy target for cancer treatment?

The kinesin motor KIFC1 has been suggested as a potential chemotherapy target due to its critical role in clustering of the multiple centrosomes found in cancer cells. In this regard, KIFC1 seems to be non-essential in normal somatic cells which usually possess only two centrosomes. Moreover, KIFC1 is also found to initiatively drive tumor malignancy and metastasis by stabilizing a certain degree of genetic instability, delaying cell cycle and protecting cancer cell surviving signals. However, that KIFC1 also plays roles in other specific cell types complicates the question of whether it is a promising chemotherapy target for cancer treatment. For example, KIFC1 is found functionally significant in vesicular and organelle trafficking, spermiogenesis, oocyte development, embryo gestation and double-strand DNA transportation. In this review we summarize a recent collection of information so as to provide a generalized picture of ideas and mechanisms against and in favor of KIFC1 as a chemotherapy target. And we also drew the conclusion that KIFC1 is a promising chemotherapy target for some types of cancers, because the side-effects of inhibiting KIFC1 mentioned in this review are theoretically easy to avoid, while KIFC1 is functionally indispensable during mitosis and malignancy of multi-centrosome cancer cells. Further investigations of how KIFC1 is regulated throughout the mitosis in cancer cells are needed for the understanding of the pathways where KIFC1 is involved and for further exploitation of indirect KIFC1 inhibitors.

Mitotic Spindle Disruption by Alternating Electric Fields Leads to Improper Chromosome Segregation and Mitotic Catastrophe in Cancer Cells

Tumor Treating Fields (TTFields) are low intensity, intermediate frequency, alternating electric fields. TTFields are a unique anti-mitotic treatment modality delivered in a continuous, noninvasive manner to the region of a tumor. It was previously postulated that by exerting directional forces on highly polar intracellular elements during mitosis, TTFields could disrupt the normal assembly of spindle microtubules. However there is limited evidence directly linking TTFields to an effect on microtubules. Here we report that TTFields decrease the ratio between polymerized and total tubulin, and prevent proper mitotic spindle assembly. The aberrant mitotic events induced by TTFields lead to abnormal chromosome segregation, cellular multinucleation, and caspase dependent apoptosis of daughter cells. The effect of TTFields on cell viability and clonogenic survival substantially depends upon the cell division rate. We show that by extending the duration of exposure to TTFields, slowly dividing cells can be affected to a similar extent as rapidly dividing cells.

Loss of kinesin-14 results in aneuploidy via kinesin-5-dependent microtubule protrusions leading to chromosome cut

Aneuploidy – chromosome instability leading to incorrect chromosome number in dividing cells – can arise from defects in centrosome duplication, bipolar spindle formation, kinetochore-microtubule attachment, chromatid cohesion, mitotic checkpoint monitoring, or cytokinesis. As most tumors show some degree of aneuploidy, mechanistic understanding of these pathways has been an intense area of research to provide potential therapeutics. Here, we present a mechanism for aneuploidy in fission yeast based on spindle pole microtubule defocusing by loss of kinesin-14 Pkl1, leading to kinesin-5 Cut7-dependent aberrant long spindle microtubule minus end protrusions that push the properly segregated chromosomes to the site of cell division, resulting in chromosome cut at cytokinesis. Pkl1 localization and function at the spindle pole is mutually dependent on spindle pole-associated protein Msd1. This mechanism of aneuploidy bypasses the known spindle assembly checkpoint that monitors chromosome segregation.

Turning the headlights on novel cancer biomarkers: Inspection of mechanics underlying intratumor heterogeneity

Although the existence of intratumoral heterogeneity (ITH) in the expression of common biomarkers has been described by pathologists since the late 1890s, we have only recently begun to fathom the staggering extent and near ubiquity of this phenomenon. From the tumor's perspective, ITH provides a stabilizing diversity that allows for the evolution of aggressive cancer phenotypes. As the weight of the evidence correlating ITH to poor prognosis burgeons, it has become increasingly important to determine the mechanisms by which a tumor acquires ITH, find clinically-adaptable means to quantify ITH and design strategies to deal with the numerous profound clinical ramifications that ITH forces upon us. Elucidation of the drivers of ITH could enable development of novel biomarkers whose interrogation might permit quantitative evaluation of the ITH inherent in a tumor in order to predict the poor prognosis risk associated with that tumor. This review proposes centrosome amplification (CA), aided and abetted by centrosome clustering mechanisms, as a critical driver of chromosomal instability (CIN) that makes a key contribution to ITH generation. Herein we also evaluate how a tumor's inherent mitotic propensity, which reflects the cell cycling kinetics within the tumor's proliferative cells, functions as the indispensable engine underpinning CIN, and determines the rate of CIN. We thus expound how the forces of centrosome amplification and mitotic propensity collaborate to sculpt the genetic landscape of a tumor and spawn extensive subclonal diversity. As such, centrosome amplification and mitotic propensity profiles could serve as clinically facile and powerful prognostic biomarkers that would enable more accurate risk segmentation of patients and design of individualized therapies.

CENP-W plays a role in maintaining bipolar spindle structure

The CENP-W/T complex was previously reported to be required for mitosis. HeLa cells depleted of CENP-W displayed profound mitotic defects, with mitotic timing delay, disorganized prometaphases and multipolar spindles as major phenotypic consequences. In this study, we examined the process of multipolar spindle formation induced by CENP-W depletion. Depletion of CENP-W in HeLa cells labeled with histone H2B and tubulin fluorescent proteins induced rapid fragmentation of originally bipolar spindles in a high proportion of cells. CENP-W depletion was associated with depletion of Hec1 at kinetochores. The possibility of promiscuous centrosomal duplication was ruled out by immunofluorescent examination of centrioles. However, centrioles were frequently observed to be abnormally split. In addition, a large proportion of the supernumerary poles lacked centrioles, but were positively stained with different centrosomal markers. These observations suggested that perturbation in spindle force distribution caused by defective kinetochores could contribute to a mechanical mechanism for spindle pole disruption. ‘Spindle free’ nocodazole arrested cells did not exhibit pole fragmentation after CENP-W depletion, showing that pole fragmentation is microtubule dependent. Inhibition of centrosome separation by monastrol reduced the incidence of spindle pole fragmentation, indicating that Eg5 plays a role in spindle pole disruption. Surprisingly, CENP-W depletion rescued the monopolar spindle phenotype of monastrol treatment, with an increased frequency of bipolar spindles observed after CENP-W RNAi. We overexpressed the microtubule cross-linking protein TPX2 to create spindle poles stabilized by the microtubule cross-linking activity of TPX2. Spindle pole fragmentation was suppressed in a TPX2-dependent fashion. We propose that CENP-W, by influencing proper kinetochore assembly, particularly microtubule docking sites, can confer spindle pole resistance to traction forces exerted by motor proteins during chromosome congression. Taken together, our findings are consistent with a model in which centrosome integrity is controlled by the pathways regulating kinetochore-microtubule attachment stability.

Force on spindle microtubule minus ends moves chromosomes

After the loss of continuous spindle microtubule attachment to the spindle pole, a previously undescribed mechanism of chromosome transport, powered by dynein pulling on minus ends of severed microtubules, repairs spindle architecture and integrity.

The spindle is a dynamic self-assembling machine that coordinates mitosis. The spindle’s function depends on its ability to organize microtubules into poles and maintain pole structure despite mechanical challenges and component turnover. Although we know that dynein and NuMA mediate pole formation, our understanding of the forces dynamically maintaining poles is limited: we do not know where and how quickly they act or their strength and structural impact. Using laser ablation to cut spindle microtubules, we identify a force that rapidly and robustly pulls severed microtubules and chromosomes poleward, overpowering opposing forces and repairing spindle architecture. Molecular imaging and biophysical analysis suggest that transport is powered by dynein pulling on minus ends of severed microtubules. NuMA and dynein/dynactin are specifically enriched at new minus ends within seconds, reanchoring minus ends to the spindle and delivering them to poles. This force on minus ends represents a newly uncovered chromosome transport mechanism that is independent of plus end forces at kinetochores and is well suited to robustly maintain spindle mechanical integrity.

EGFR controls IQGAP basolateral membrane localization and mitotic spindle orientation during epithelial morphogenesis

Establishing the correct orientation of the mitotic spindle is an essential step in epithelial cell division in order to ensure that epithelial tubules form correctly during organ development and regeneration. While recent findings have identified some of the molecular mechanisms that underlie spindle orientation, many aspects of this process remain poorly understood. Here, we have used the 3D-MDCK model system to demonstrate a key role for a newly identified protein complex formed by IQGAP1 and the epithelial growth factor receptor (EGFR) in controlling the orientation of the mitotic spindle. IQGAP1 is a scaffolding protein that regulates many cellular pathways, from cell-cell adhesion to microtubule organization, and its localization in the basolateral membrane ensures correct spindle orientation. Through its IQ motifs, IQGAP1 binds to EGFR, which is responsible for maintaining IQGAP1 in the basolateral membrane domain. Silencing IQGAP1, or disrupting the basolateral localization of either IQGAP1 or EGFR, results in a non-polarized distribution of NuMA, mitotic spindle misorientation and defects in single lumen formation.

Design, synthesis, and biological evaluation of an allosteric inhibitor of HSET that targets cancer cells with supernumerary centrosomes

Centrosomes associate with spindle poles; thus, the presence of two centrosomes promotes bipolar spindle assembly in normal cells. Cancer cells often contain supernumerary centrosomes, and to avoid multipolar mitosis and cell death, these are clustered into two poles by the microtubule motor protein HSET. We report the discovery of an allosteric inhibitor of HSET, CW069, which we designed using a methodology on an interface of chemistry and biology. Using this approach, we explored millions of compounds in silico and utilized convergent syntheses. Only compound CW069 showed marked activity against HSET in vitro. The inhibitor induced multipolar mitoses only in cells containing supernumerary centrosomes. CW069 therefore constitutes a valuable tool for probing HSET function and, by reducing the growth of cells containing supernumerary centrosomes, paves the way for new cancer therapeutics.

RanGTP and CLASP1 cooperate to position the mitotic spindle

Accurate positioning of the mitotic spindle is critical to ensure proper distribution of chromosomes during cell division. The small GTPase Ran, which regulates a variety of processes throughout the cell cycle, including interphase nucleocytoplasmic transport and mitotic spindle assembly, was recently shown to also control spindle alignment. Ran is required for the correct cortical localization of LGN and nuclear-mitotic apparatus protein (NuMA), proteins that generate pulling forces on astral microtubules (MTs) through cytoplasmic dynein. Here we use importazole, a small-molecule inhibitor of RanGTP/importin-β function, to study the role of Ran in spindle positioning in human cells. We find that importazole treatment results in defects in astral MT dynamics, as well as in mislocalization of LGN and NuMA, leading to misoriented spindles. Of interest, importazole-induced spindle-centering defects can be rescued by nocodazole treatment, which depolymerizes astral MTs, or by overexpression of CLASP1, which does not restore proper LGN and NuMA localization but stabilizes astral MT interactions with the cortex. Together our data suggest a model for mitotic spindle positioning in which RanGTP and CLASP1 cooperate to align the spindle along the long axis of the dividing cell.

Poly-ADP ribosylation of Miki by tankyrase-1 promotes centrosome maturation

During prometaphase, dense microtubule nucleation sites at centrosomes form robust spindles that align chromosomes promptly. Failure of centrosome maturation leaves chromosomes scattered, as seen routinely in cancer cells, including myelodysplastic syndrome (MDS). We previously reported that the Miki (LOC253012) gene is frequently deleted in MDS patients, and that low levels of Miki are associated with abnormal mitosis. Here we demonstrate that Miki localizes to the Golgi apparatus and is poly(ADP-ribosyl)ated by tankyrase-1 during late G2 and prophase. PARsylated Miki then translocates to mitotic centrosomes and anchors CG-NAP, a large scaffold protein of the γ-tubulin ring complex. Due to impairment of microtubule aster formation, cells in which tankyrase-1, Miki, or CG-NAP expression is downregulated all show prometaphase disturbances, including scattered and lagging chromosomes. Our data suggest that PARsylation of Miki by tankyrase-1 is a key initial event promoting prometaphase.

Let's huddle to prevent a muddle: centrosome declustering as an attractive anticancer strategy

Nearly a century ago, cell biologists postulated that the chromosomal aberrations blighting cancer cells might be caused by a mysterious organelle-the centrosome-that had only just been discovered. For years, however, this enigmatic structure was neglected in oncologic investigations and has only recently reemerged as a key suspect in tumorigenesis. A majority of cancer cells, unlike healthy cells, possess an amplified centrosome complement, which they manage to coalesce neatly at two spindle poles during mitosis. This clustering mechanism permits the cell to form a pseudo-bipolar mitotic spindle for segregation of sister chromatids. On rare occasions this mechanism fails, resulting in declustered centrosomes and the assembly of a multipolar spindle. Spindle multipolarity consigns the cell to an almost certain fate of mitotic arrest or death. The catastrophic nature of multipolarity has attracted efforts to develop drugs that can induce declustering in cancer cells. Such chemotherapeutics would theoretically spare healthy cells, whose normal centrosome complement should preclude multipolar spindle formation. In search of the 'Holy Grail' of nontoxic, cancer cell-selective, and superiorly efficacious chemotherapy, research is underway to elucidate the underpinnings of centrosome clustering mechanisms. Here, we detail the progress made towards that end, highlighting seminal work and suggesting directions for future research, aimed at demystifying this riddling cellular tactic and exploiting it for chemotherapeutic purposes. We also propose a model to highlight the integral role of microtubule dynamicity and the delicate balance of forces on which cancer cells rely for effective centrosome clustering. Finally, we provide insights regarding how perturbation of this balance may pave an inroad for inducing lethal centrosome dispersal and death selectively in cancer cells.

CLASPs prevent irreversible multipolarity by ensuring spindle-pole resistance to traction forces during chromosome alignment

Loss of spindle-pole integrity during mitosis leads to multipolarity independent of centrosome amplification. Multipolar-spindle conformation favours incorrect kinetochore-microtubule attachments, compromising faithful chromosome segregation and daughter-cell viability. Spindle-pole organization influences and is influenced by kinetochore activity, but the molecular nature behind this critical force balance is unknown. CLASPs are microtubule-, kinetochore- and centrosome-associated proteins whose functional perturbation leads to three main spindle abnormalities: monopolarity, short spindles and multipolarity. The first two reflect a role at the kinetochore-microtubule interface through interaction with specific kinetochore partners, but how CLASPs prevent spindle multipolarity remains unclear. Here we found that human CLASPs ensure spindle-pole integrity after bipolarization in response to CENP-E- and Kid-mediated forces from misaligned chromosomes. This function is independent of end-on kinetochore-microtubule attachments and involves the recruitment of ninein to residual pericentriolar satellites. Distinctively, multipolarity arising through this mechanism often persists through anaphase. We propose that CLASPs and ninein confer spindle-pole resistance to traction forces exerted during chromosome congression, thereby preventing irreversible spindle multipolarity and aneuploidy.

Spindle pole mechanics studied in mitotic asters: dynamic distribution of spindle forces through compliant linkages

During cell division, chromosomes must faithfully segregate to maintain genome integrity, and this dynamic mechanical process is driven by the macromolecular machinery of the mitotic spindle. However, little is known about spindle mechanics. For example, spindle microtubules are organized by numerous cross-linking proteins yet the mechanical properties of those cross-links remain unexplored. To examine the mechanical properties of microtubule cross-links we applied optical trapping to mitotic asters that form in mammalian mitotic extracts. These asters are foci of microtubules, motors, and microtubule-associated proteins that reflect many of the functional properties of spindle poles and represent centrosome-independent spindle-pole analogs. We observed bidirectional motor-driven microtubule movements, showing that microtubule linkages within asters are remarkably compliant (mean stiffness 0.025 pN/nm) and mediated by only a handful of cross-links. Depleting the motor Eg5 reduced this stiffness, indicating that Eg5 contributes to the mechanical properties of microtubule asters in a manner consistent with its localization to spindle poles in cells. We propose that compliant linkages among microtubules provide a mechanical architecture capable of accommodating microtubule movements and distributing force among microtubules without loss of pole integrity-a mechanical paradigm that may be important throughout the spindle.

Dynamic distribution of NuMA and microtubules in human fetal fibroblasts, developing oocytes and somatic cell nuclear transferred embryos

BACKGROUND

The nuclear mitotic apparatus (NuMA) plays a central role in the assembly and maintenance of spindle poles. Somatic cell nuclear transfer (SCNT) studies on non-human primates have shown that meiotic spindle removal during enucleation causes depletion of NuMA and the minus-end-directed motor protein (HSET) from the ooplasm, and this in turn leads to failure of embryo development. To determine whether NuMA from somatic cells could compensate for NuMA loss during enucleation, the distribution of NuMA and microtubule organization were investigated in human fibroblasts, developing oocytes and SCNT embryos.

METHODS

Human fetal fibroblasts, oocytes at various maturation stages and human embryos reconstructed by different SCNT methods were analyzed for NuMA and α-tubulin using immunofluorescent confocal microscopy.

RESULTS

NuMA was detected in interphase nuclei of fibroblasts and oocytes. During mitosis and meiosis, NuMA relocated to the domain surrounding the two spindle poles. During the enucleation process, NuMA was removed along with the meiotic spindle. At 2 h after injection into a donor cell, transitory bipolar spindles were organized and NuMA was detected in the reformed poles. NuMA could be detected spreading uniformly across the nucleoplasm of one pseudo-pronucleus in SCNT embryos but was excluded from the nucleolus. Regardless of the method used for SCNT (enucleation-injection or injection-pronuclei enucleation), NuMA aggregated and relocated to the reformed spindle poles at metaphase of the first mitotic event. At interphase, NuMA relocated throughout the nucleus in developmentally arrested SCNT embryos.

CONCLUSIONS

Our results show that donor cell nuclei contain NuMA, which might contribute to the maintenance of spindle morphology in SCNT embryos. Normal spindle and NuMA expression were found in human SCNT embryos at different developmental stages.

The pericentriolar satellite protein CEP90 is crucial for integrity of the mitotic spindle pole

Pericentriolar satellites are electron-dense granules that are concentrated around the centrosome. They are involved in the recruitment of centrosomal proteins and microtubule organization in interphase cells, but their mitotic functions are largely unknown. In this study, we characterize CEP90 as a component of pericentriolar satellites. CEP90 is present both in the centrosome and in the cytoplasm, but is transiently concentrated at the centrosome once cells enter mitosis. Depletion of CEP90 caused mitotic arrest with misaligned chromosomes. Spindle pole fragmentation was the most characteristic phenotype in CEP90-depleted cells. Spindle poles were fragmented as soon as the spindles attached, suggesting that the mechanical forces of spindle microtubules physically stress the structure of CEP90-depleted spindle poles. Based on these results, we propose that CEP90 is crucial for maintaining the integrity of spindle poles during mitosis.

Influence of centriole number on mitotic spindle length and symmetry

The functional role of centrioles or basal bodies in mitotic spindle assembly and function is currently unclear. Although supernumerary centrioles have been associated with multipolar spindles in cancer cells, suggesting centriole number might dictate spindle polarity, bipolar spindles are able to assemble in the complete absence of centrioles, suggesting a level of centriole-independence in the spindle assembly pathway. In this report we perturb centriole number using mutations in Chlamydomonas reinhardtii, and measure the response of the mitotic spindle to these perturbations in centriole number. Although altered centriole number increased the frequency of monopolar and multipolar spindles, the majority of spindles remained bipolar regardless of the centriole number. But even when spindles were bipolar, abnormal centriole numbers led to asymmetries in tubulin distribution, half-spindle length and spindle pole focus. Half spindle length correlated directly with number of centrioles at a pole, such that an imbalance in centriole number between the two poles of a bipolar spindle correlated with increased asymmetry between half spindle lengths. These results are consistent with centrioles playing an active role in regulating mitotic spindle length. Mutants with centriole number alteration also show increased cytokinesis defects, but these do not correlate with centriole number in the dividing cell and may therefore reflect downstream consequences of defects in preceding cell divisions.

CDK5RAP2 functions in centrosome to spindle pole attachment and DNA damage response

Two domains of centrosomal protein CDK5RAP2, CNN1 and CNN2, link centrosomes to mitotic spindle poles. CNN1 lacking centrosomes are unable to recruit pericentriolar matrix components that mediate attachment to spindle poles.

The centrosomal protein, CDK5RAP2, is mutated in primary microcephaly, a neurodevelopmental disorder characterized by reduced brain size. The Drosophila melanogaster homologue of CDK5RAP2, centrosomin (Cnn), maintains the pericentriolar matrix (PCM) around centrioles during mitosis. In this study, we demonstrate a similar role for CDK5RAP2 in vertebrate cells. By disrupting two evolutionarily conserved domains of CDK5RAP2, CNN1 and CNN2, in the avian B cell line DT40, we find that both domains are essential for linking centrosomes to mitotic spindle poles. Although structurally intact, centrosomes lacking the CNN1 domain fail to recruit specific PCM components that mediate attachment to spindle poles. Furthermore, we show that the CNN1 domain enforces cohesion between parental centrioles during interphase and promotes efficient DNA damage–induced G2 cell cycle arrest. Because mitotic spindle positioning, asymmetric centrosome inheritance, and DNA damage signaling have all been implicated in cell fate determination during neurogenesis, our findings provide novel insight into how impaired CDK5RAP2 function could cause premature depletion of neural stem cells and thereby microcephaly.

LGN regulates mitotic spindle orientation during epithelial morphogenesis

Coordinated cell polarization and mitotic spindle orientation are thought to be important for epithelial morphogenesis. Whether spindle orientation is indeed linked to epithelial morphogenesis and how it is controlled at the molecular level is still unknown. Here, we show that the NuMA- and Galpha-binding protein LGN is required for directing spindle orientation during cystogenesis of MDCK cells. LGN localizes to the lateral cell cortex, and is excluded from the apical cell cortex of dividing cells. Depleting LGN, preventing its cortical localization, or disrupting its interaction with endogenous NuMA or Galpha proteins all lead to spindle misorientation and abnormal cystogenesis. Moreover, artificial mistargeting of endogenous LGN to the apical membrane results in a near 90 degrees rotation of the spindle axis and profound cystogenesis defects that are dependent on cell division. The normal apical exclusion of LGN during mitosis appears to be mediated by atypical PKC. Thus, cell polarization-mediated spatial restriction of spindle orientation determinants is critical for epithelial morphogenesis.

Mechanisms of chromosome behaviour during mitosis

For over a century, scientists have strived to understand the mechanisms that govern the accurate segregation of chromosomes during mitosis. The most intriguing feature of this process, which is particularly prominent in higher eukaryotes, is the complex behaviour exhibited by the chromosomes. This behaviour is based on specific and highly regulated interactions between the chromosomes and spindle microtubules. Recent discoveries, enabled by high-resolution imaging combined with the various genetic, molecular, cell biological and chemical tools, support the idea that establishing and controlling the dynamic interaction between chromosomes and microtubules is a major factor in genomic fidelity.

TOGp regulates microtubule assembly and density during mitosis and contributes to chromosome directional instability

TOGp, a member of the XMAP215 MAP family, is required for bipolar mitotic spindle assembly. To understand how TOGp contributes to spindle assembly, we examined microtubule dynamics after depleting TOGp by siRNA. Fluorescence recovery after photobleaching of GFP-tubulin demonstrated that spindle microtubule turnover is slowed two-fold in the absence of TOGp. Consistent with photobleaching results, microtubule regrowth after washout of the microtubule depolymerizing drug nocodazole was slower at the centrosomes and in the vicinity of mitotic chromatin in cells depleted of TOGp. The slower microtubule turnover is likely due to either nucleation or the transitions of dynamic instability because TOGp depletion did not effect the rate of plus end growth, measured by tracking EB1-GFP at microtubule ends. In contrast, microtubule regrowth after nocodazole washout was unaffected by prior depletion of TACC3, a centrosomal protein that interacts with TOGp. Kinetochore fibers in both untreated and TOGp-depleted cells were stable to incubation at 4 degrees C or lysis in buffer containing calcium indicating that stable kinetochore-microtubule attachments are formed in the absence of TOGp. Depletion of TOGp, but not TACC3, reduced kinetochore oscillations during prometaphase/metaphase. Defects in oscillations are not due simply to multipolarity or loss of centrosome focus in the TOGp-depleted cells, since kinetochore oscillations appear normal in cells treated with the proteosome inhibitor MG132, which also results in multipolar spindles and centrosome fragmentation. We hypothesize that TOGp is required for chromosome motility as a downstream consequence of reduced microtubule dynamics and/or density. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Compression regulates mitotic spindle length by a mechanochemical switch at the poles

BACKGROUND

Although the molecules involved in mitosis are becoming better characterized, we still lack an understanding of the emergent mechanical properties of the mitotic spindle. For example, we cannot explain how spindle length is determined. To gain insight into how forces are generated and responded to in the spindle, we developed a method to apply controlled mechanical compression to metaphase mitotic spindles in living mammalian cells while monitoring microtubules and kinetochores by fluorescence microscopy.

RESULTS

Compression caused reversible spindle widening and lengthening to a new steady state. Widening was a passive mechanical response, and lengthening was an active mechanochemical process requiring microtubule polymerization but not kinesin-5 activity. Spindle morphology during lengthening and drug perturbations suggested that kinetochore fibers are pushed outward by pole-directed forces generated within the spindle. Lengthening of kinetochore fibers occurred by inhibition of microtubule depolymerization at poles, with no change in sliding velocity, interkinetochore stretching, or kinetochore dynamics.

CONCLUSIONS

We propose that spindle length is controlled by a mechanochemical switch at the poles that regulates the depolymerization rate of kinetochore fibers in response to compression and discuss models for how this switch is controlled. Poleward force appears to be exerted along kinetochore fibers by some mechanism other than kinesin-5 activity, and we speculate that it may arise from polymerization pressure from growing plus ends of interpolar microtubules whose minus ends are anchored in the fiber. These insights provide a framework for conceptualizing mechanical integration within the spindle.