Mechanisms of mitotic spindle assembly and function

Macroscopic simulations of microtubule dynamics predict two steady-state processes governing array morphology

Microtubule polymers typically function through their collective organization into a patterned array. The formation of the pattern, whether it is a relatively simple astral array or a highly complex mitotic spindle, relies on controlled microtubule nucleation and the basal dynamics parameters governing polymer growth and shortening. We have investigated the interaction between the microtubule nucleation and dynamics parameters, using macroscopic Monte Carlo simulations, to determine how these parameters contribute to the underlying microtubule array morphology (i.e. polymer density and length distribution). In addition to the well-characterized steady state achieved between free tubulin subunits and microtubule polymer, we propose that microtubule nucleation and extinction constitute a second, interdependent steady state process. Our simulation studies show that the magnitude of both nucleation and extinction additively impacts the final steady state free subunit concentration. We systematically varied individual microtubule dynamics parameters to survey the effects on array morphology and find specific sensitivity to perturbations of catastrophe frequency. Altering the cellular context for the microtubule array, we find that nucleation template number plays a defining role in shaping the microtubule length distribution and polymer density.

Novel functions of endocytic player clathrin in mitosis

Clathrin has been widely recognized as a pivotal player in endocytosis, in which several adaptors and accessory proteins are involved. Recent studies suggested that clathrin is also essential for cell division. Here this review mainly focuses on the clathrin-dependent mechanisms involved in spindle assembly and chromosome alignment. In mitosis, clathrin forms a complex with phosphorylated TACC3 to ensure spindle stability and proper chromosome alignment. The clathrin-regulated mechanism in mitosis requires the crosstalk among clathrin, spindle assembly factors (SAFs), Ran-GTP and mitotic kinases. Meanwhile, a coordinated mechanism is required for role transitions of clathrin during endocytosis and mitosis. Taken together, the findings of the multiple functions of clathrin besides endocytosis have expanded our understanding of the basic cellular activities.

Aurora A in cell division: kinase activity not required

Aurora A kinase is a key regulator of cell division, whose functions were attributed to its ability to phosphorylate diverse substrates. Aurora A is now shown to have a kinase-independent role in the regulation of chromatin-mediated microtubule assembly.

Aurora B regulates formin mDia3 in achieving metaphase chromosome alignment

Proper bipolar attachment of sister kinetochores to the mitotic spindle is critical for accurate chromosome segregation in mitosis. Here we show an essential role of the formin mDia3 in achieving metaphase chromosome alignment. This function is independent of mDia3 actin nucleation activity, but is attributable to EB1-binding by mDia3. Furthermore, the microtubule binding FH2 domain of mDia3 is phosphorylated by Aurora B kinase in vitro, and cells expressing the nonphosphorylatable mDia3 mutant cannot position chromosomes at the metaphase plate. Purified recombinant mDia3 phosphorylated by Aurora B exhibits reduced ability to bind microtubules and stabilize microtubules against cold-induced disassembly in vitro. Cells expressing the phosphomimetic mDia3 mutant do not form stable kinetochore microtubule fibers; despite they are able to congress chromosomes to the metaphase plate. These findings reveal a key role for mDia3 and its regulation by Aurora B phosphorylation in achieving proper stable kinetochore microtubule attachment.

Alzheimer Aβ disrupts the mitotic spindle and directly inhibits mitotic microtubule motors

Chromosome mis-segregation and aneuploidy are greatly induced in Alzheimer's disease and models thereof by mutant forms of the APP and PS proteins and by their product, the Ab peptide. Here we employ human somatic cells and Xenopus egg extracts to show that Aβ impairs the assembly and maintenance of the mitotic spindle. Mechanistically, these defects result from Aβ's inhibition of mitotic motor kinesins, including Eg5, KIF4A and MCAK. In vitro studies show that oligomeric Aβ directly inhibits recombinant MCAK by a noncompetitive mechanism. In contrast, inhibition of Eg5 and KIF4A is competitive with respect to both ATP and microtubules, indicating that Aβ interferes with their interactions with the microtubules of the mitotic spindle. Consistently, increased levels of polymerized microtubules or of the microtubule stabilizing protein Tau significantly decrease the inhibitory effect of Aβ on Eg5 and KIF4A. Together, these results indicate that by disrupting the interaction between specific kinesins and microtubules and by exerting a direct inhibitory effect on the motor activity, excess Ab deregulates the mechanical forces that govern the spindle and thereby leads to the generation of defective mitotic structures. The resulting defect in neurogenesis can account for the over 30% aneuploid/hyperploid, degeneration-prone neurons observed in Alzheimer disease brain. The finding of mitotic motors including Eg5 in mature post-mitotic neurons implies that their inhibition by Ab may also disrupt neuronal function and plasticity.

Anaphase B spindle dynamics in Drosophila S2 cells: Comparison with embryo spindles

Background

In the Drosophila melanogaster syncytial blastoderm stage embryo anaphase B is initiated by a cell cycle switch in which the suppression of microtubule minus end depolymerization and spatial reorganization of the plus ends of outwardly sliding interpolar microtubules triggers spindle elongation. RNA interference in Drosophila cultured S2 cells may present a useful tool for identifying novel components of this switch, but given the diversity of spindle design, it is important to first determine the extent of conservation of the mechanism of anaphase B in the two systems.

Results

The basic mechanism, involving an inverse correlation between poleward flux and spindle elongation is qualitatively similar in these systems, but quantitative differences exist. In S2 cells, poleward flux is only partially suppressed and the rate of anaphase B spindle elongation increases with the extent of suppression. Also, EB1-labelled microtubule plus ends redistribute away from the poles and towards the interpolar microtubule overlap zone, but this is less pronounced in S2 cells than in embryos. Finally, as in embryos, tubulin FRAP experiments revealed a reduction in the percentage recovery after photobleaching at regions proximal to the pole.

Conclusions

The basic features of the anaphase B switch, involving the suppression of poleward flux and reorganization of growing microtubule plus ends, is conserved in these systems. Thus S2 cells may be useful for rapidly identifying novel components of this switch. The quantitative differences likely reflect the adaptation of embryonic spindles for rapid, streamlined mitoses.

Merotelic kinetochore attachment: causes and effects

Accurate chromosome segregation depends on the proper attachment of sister kinetochores to microtubules emanating from opposite spindle poles. Merotelic kinetochore orientation is an error in which a single kinetochore is attached to microtubules emanating from both spindle poles. Despite correction mechanisms, merotelically attached kinetochores can persist until anaphase, causing chromatids to lag on the mitotic spindle and hindering their timely segregation. Recent studies showing that merotelic kinetochore attachment represents a major mechanism of aneuploidy in mitotic cells and is the primary mechanism of chromosomal instability in cancer cells have underlined the importance of studying merotely. Here, we highlight recent progress in our understanding of how cells prevent and correct merotelic kinetochore attachments.

Phenotypic analysis of misato function reveals roles of noncentrosomal microtubules in Drosophila spindle formation

Mitotic spindle assembly in centrosome-containing cells relies on two main microtubule (MT) nucleation pathways, one based on centrosomes and the other on chromosomes. However, the relative role of these pathways is not well defined. In Drosophila, mutants without centrosomes can form functional anastral spindles and survive to adulthood. Here we show that mutations in the Drosophila misato (mst) gene inhibit kinetochore-driven MT growth, lead to the formation of monopolar spindles and cause larval lethality. In most prophase cells of mst mutant brains, asters are well separated, but collapse with progression of mitosis, suggesting that k-fibers are essential for maintenance of aster separation and spindle bipolarity. Analysis of mst; Sas-4 double mutants showed that mitotic cells lacking both the centrosomes and the mst function form polarized MT arrays that resemble monopolar spindles. MT regrowth experiments after cold exposure revealed that in mst; Sas-4 metaphase cells MTs regrow from several sites, which eventually coalesce to form a single polarized MT array. By contrast, in Sas-4 single mutants, chromosome-driven MT regrowth mostly produced robust bipolar spindles. Collectively, these results indicate that kinetochore-driven MT formation is an essential process for proper spindle assembly in Drosophila somatic cells.

The N-terminal coiled-coil of Ndel1 is a regulated scaffold that recruits LIS1 to dynein

Ndel1 has been implicated in a variety of dynein-related processes, but its specific function is unclear. Here we describe an experimental approach to evaluate a role of Ndel1 in dynein-dependent microtubule self-organization using Ran-mediated asters in meiotic Xenopus egg extracts. We demonstrate that extracts depleted of Ndel1 are unable to form asters and that this defect can be rescued by the addition of recombinant N-terminal coiled-coil domain of Ndel1. Ndel1-dependent microtubule self-organization requires an interaction between Ndel1 and dynein, which is mediated by the dimerization fragment of the coiled-coil. Full rescue by the coiled-coil domain requires LIS1 binding, and increasing LIS1 concentration partly rescues aster formation, suggesting that Ndel1 is a recruitment factor for LIS1. The interactions between Ndel1 and its binding partners are positively regulated by phosphorylation of the unstructured C terminus. Together, our results provide important insights into how Ndel1 acts as a regulated scaffold to temporally and spatially regulate dynein.

Towards a quantitative understanding of mitotic spindle assembly and mechanics

The 'simple' view of the mitotic spindle is that it self-assembles as a result of microtubules (MTs) randomly searching for chromosomes, after which the spindle length is maintained by a balance of outward tension exerted by molecular motors on the MTs connecting centrosomes and chromosomes, and compression generated by other motors on the MTs connecting the spindle poles. This picture is being challenged now by mounting evidence indicating that spindle assembly and maintenance rely on much more complex interconnected networks of microtubules, molecular motors, chromosomes and regulatory proteins. From an engineering point of view, three design principles of this molecular machine are especially important: the spindle assembles quickly, it assembles accurately, and it is mechanically robust--yet malleable. How is this design achieved with randomly interacting and impermanent molecular parts? Here, we review recent interdisciplinary studies that have started to shed light on this question. We discuss cooperative mechanisms of spindle self-assembly, error correction and maintenance of its mechanical properties, speculate on analogy between spindle and lamellipodial dynamics, and highlight the role of quantitative approaches in understanding the mitotic spindle design.

Nup98 regulates bipolar spindle assembly through association with microtubules and opposition of MCAK

During mitosis, the nuclear pore complex is disassembled and, increasingly, nucleoporins are proving to have mitotic functions when released from the pore. We find a contribution of the nucleoporin Nup98 to mitotic spindle assembly through regulation of microtubule dynamics. When added to Xenopus extract spindle assembly assays, the C-terminal domain of Nup98 stimulates uncontrolled growth of microtubules. Conversely, inhibition or depletion of Nup98 leads to formation of stable monopolar spindles. Spindle bipolarity is restored by addition of purified, recombinant Nup98 C-terminus. The minimal required region of Nup98 corresponds to a portion of the C-terminal domain lacking a previously characterized function. We show association between this region of the C-terminus of Nup98 and both Taxol-stabilized microtubules and the microtubule-depolymerizing mitotic centromere-associated kinesin (MCAK). Importantly, we demonstrate that this domain of Nup98 inhibits MCAK depolymerization activity in vitro. These data support a model in which Nup98 interacts with microtubules and antagonizes MCAK activity, thus promoting bipolar spindle assembly.

A computational model predicts Xenopus meiotic spindle organization

Spatially dispersed nucleation and minus end–directed transport of microtubule end disassembly activity can lead to bipolar spindle assembly.

The metaphase spindle is a dynamic bipolar structure crucial for proper chromosome segregation, but how microtubules (MTs) are organized within the bipolar architecture remains controversial. To explore MT organization along the pole-to-pole axis, we simulated meiotic spindle assembly in two dimensions using dynamic MTs, a MT cross-linking force, and a kinesin-5–like motor. The bipolar structures that form consist of antiparallel fluxing MTs, but spindle pole formation requires the addition of a NuMA-like minus-end cross-linker and directed transport of MT depolymerization activity toward minus ends. Dynamic instability and minus-end depolymerization generate realistic MT lifetimes and a truncated exponential MT length distribution. Keeping the number of MTs in the simulation constant, we explored the influence of two different MT nucleation pathways on spindle organization. When nucleation occurs throughout the spindle, the simulation quantitatively reproduces features of meiotic spindles assembled in Xenopus egg extracts.

Kinesin-12, a mitotic microtubule-associated motor protein, impacts axonal growth, navigation, and branching

Kinesin-12 (also called Kif15) is a mitotic motor protein that continues to be expressed in developing neurons. Depletion of kinesin-12 causes axons to grow faster, more than doubles the frequency of microtubule transport in both directions in the axon, prevents growth cones from turning properly, and enhances the invasion of microtubules into filopodia. These results are remarkably similar to those obtained in previous studies in which neurons were depleted of kinesin-5 (also called Eg5 or Kif11), another mitotic motor protein that continues to be expressed in developing neurons. However, there are also notable differences in the phenotypes obtained with depleting each of these motors. Depleting kinesin-12 decreases axonal branching and growth cone size, whereas inhibiting kinesin-5 increases these parameters. In addition, depleting kinesin-12 diminishes the appearance of growth-cone-like waves along the length of the axon, an effect not observed with depletion of kinesin-5. Finally, depletion of kinesin-12 abolishes the "waggling" behavior of microtubules that occurs as they assemble along actin bundles within filopodia, whereas inhibition of kinesin-5 does not. Interestingly, and perhaps relevant to these differences in phenotype, in biochemical studies, kinesin-12 coimmunoprecipitates with actin but kinesin-5 does not. Collectively, these findings support a scenario whereby kinesin-12 shares functions with kinesin-5 related to microtubule-microtubule interactions, but kinesin-12 has other functions not shared by kinesin-5 that are related to the ability of kinesin-12 to interact with actin.

Nakiterpiosin targets tubulin and triggers mitotic catastrophe in human cancer cells

Agents that interfere with mitotic progression by perturbing microtubule dynamics are commonly used for cancer chemotherapy. Here, we identify nakiterpiosin as a novel antimitotic drug that targets microtubules. Nakiterpiosin induces mitotic arrest and triggers mitotic catastrophe in human cancer cells by impairing bipolar spindle assembly. At higher concentration, it alters the interphase microtubule network and suppresses microtubule dynamics. In the presence of nakiterpiosin, microtubules are no longer arranged in a centrosomal array and centrosome-mediated microtubule regrowth after cold depolymerization is inhibited. However, centrosome organization, the ultrastructure of Golgi stacks, and protein secretion are not affected, suggesting that the drug has minimal toxicity toward other cellular functions. Nakiterpiosin interacts directly with tubulin, inhibits microtubule polymerization in vitro, and decreases polymer mass in cells. Furthermore, it enhances tubulin acetylation and reduces viability of paclitaxel-resistant cancer cells. In conclusion, nakiterpiosin exerts antiproliferative activity by perturbing microtubule dynamics during mitosis that activates the spindle assembly checkpoint and triggers cell death. These findings suggest the potential use of nakiterpiosin as a chemotherapeutic agent.

Caenorhabditis elegans cyclin B3 is required for multiple mitotic processes including alleviation of a spindle checkpoint-dependent block in anaphase chromosome segregation

The master regulators of the cell cycle are cyclin-dependent kinases (Cdks), which influence the function of a myriad of proteins via phosphorylation. Mitotic Cdk1 is activated by A-type, as well as B1- and B2-type, cyclins. However, the role of a third, conserved cyclin B family member, cyclin B3, is less well defined. Here, we show that Caenorhabditis elegans CYB-3 has essential and distinct functions from cyclin B1 and B2 in the early embryo. CYB-3 is required for the timely execution of a number of cell cycle events including completion of the MII meiotic division of the oocyte nucleus, pronuclear migration, centrosome maturation, mitotic chromosome condensation and congression, and, most strikingly, progression through the metaphase-to-anaphase transition. Our experiments reveal that the extended metaphase delay in CYB-3–depleted embryos is dependent on an intact spindle assembly checkpoint (SAC) and results in salient defects in the architecture of holocentric metaphase chromosomes. Furthermore, genetically increasing or decreasing dynein activity results in the respective suppression or enhancement of CYB-3–dependent defects in cell cycle progression. Altogether, these data reveal that CYB-3 plays a unique, essential role in the cell cycle including promoting mitotic dynein functionality and alleviation of a SAC–dependent block in anaphase chromosome segregation.

Every time a cell divides in two, the genetic material, DNA, is copied; each copied chromosome is referred to as a pair of sister chromatids. Each chromatid must be cleanly separated from its sister so that each daughter cell inherits the same DNA complement as the starting cell. The mitotic spindle is a cellular machine that physically separates the sister chromatids from one another. The chromatids are attached to the spindle at kinetochores, which are structures built at specific sites (centromeres) on each chromatid. The cell monitors the attachment of each chromatid and blocks their separation until they are all properly attached. This process is called the spindle assembly checkpoint (SAC). Here we report that loss of an evolutionarily conserved cell cycle regulator, Cyclin B3/CYB-3, results in an unusual and strikingly persistent SAC–dependent delay in sister chromatid separation. Furthermore, CYB-3 promotes the activity of a cellular motor, dynein, in this and other mitotic processes. Altogether, our results indicate that Cyclin B3 genetically interacts with mitotic dynein and is absolutely required to satisfy a SAC–dependent inhibition in sister chromatid separation.

Kinetochore-microtubule interactions: steps towards bi-orientation

Eukaryotic cells segregate their chromosomes accurately to opposite poles during mitosis, which is necessary for maintenance of their genetic integrity. This process mainly relies on the forces generated by kinetochore-microtubule (KT-MT) attachment. During prometaphase, the KT initially interacts with a single MT extending from a spindle pole and then moves towards a spindle pole. Subsequently, MTs from the other spindle pole also interact with the KT. Eventually, one sister KT becomes attached to MTs from one pole while the other sister to those from the other pole (sister KT bi-orientation). If sister KTs interact with MTs with aberrant orientation, this must be corrected to attain proper bi-orientation (error correction) before the anaphase is initiated. Here, I discuss how KTs initially interact with MTs and how this interaction develops into bi-orientation; both processes are fundamentally crucial for proper chromosome segregation in the subsequent anaphase.

A minimal midzone protein module controls formation and length of antiparallel microtubule overlaps

During cell division, microtubules are arranged in a large bipolar structure, the mitotic spindle, to segregate the duplicated chromosomes. Antiparallel microtubule overlaps in the spindle center are essential for establishing bipolarity and maintaining spindle stability throughout mitosis. In anaphase, this antiparallel microtubule array is tightly bundled forming the midzone, which serves as a hub for the recruitment of proteins essential for late mitotic events. The molecular mechanism of midzone formation and the control of its size are not understood. Using an in vitro reconstitution approach, we show here that PRC1 autonomously bundles antiparallel microtubules and recruits Xklp1, a kinesin-4, selectively to overlapping antiparallel microtubules. The processive motor Xklp1 controls overlap size by overlap length-dependent microtubule growth inhibition. Our results mechanistically explain how the two conserved, essential midzone proteins PRC1 and Xklp1 cooperate to constitute a minimal protein module capable of dynamically organizing the core structure of the central anaphase spindle.

Changes in the nuclear envelope environment affect spindle pole body duplication in Saccharomyces cerevisiae

The Saccharomyces cerevisiae nuclear membrane is part of a complex nuclear envelope environment also containing chromatin, integral and peripheral membrane proteins, and large structures such as nuclear pore complexes (NPCs) and the spindle pole body. To study how properties of the nuclear membrane affect nuclear envelope processes, we altered the nuclear membrane by deleting the SPO7 gene. We found that spo7Δ cells were sickened by the mutation of genes coding for spindle pole body components and that spo7Δ was synthetically lethal with mutations in the SUN domain gene MPS3. Mps3p is required for spindle pole body duplication and for a variety of other nuclear envelope processes. In spo7Δ cells, the spindle pole body defect of mps3 mutants was exacerbated, suggesting that nuclear membrane composition affects spindle pole body function. The synthetic lethality between spo7Δ and mps3 mutants was suppressed by deletion of specific nucleoporin genes. In fact, these gene deletions bypassed the requirement for Mps3p entirely, suggesting that under certain conditions spindle pole body duplication can occur via an Mps3p-independent pathway. These data point to an antagonistic relationship between nuclear pore complexes and the spindle pole body. We propose a model whereby nuclear pore complexes either compete with the spindle pole body for insertion into the nuclear membrane or affect spindle pole body duplication by altering the nuclear envelope environment.

The chromosome peripheral proteins play an active role in chromosome dynamics

The chromosome periphery is a chromosomal structure that covers the surface of mitotic chromosomes. The structure and function of the chromosome periphery has been poorly understood since its first description in 1882. It has, however, been proposed to be an insulator or barrier to protect chromosomes from subcellular substances and to act as a carrier of nuclear and nucleolar components to direct their equal distribution to daughter cells because most chromosome peripheral proteins (CPPs) are derived from the nucleolus or nucleus. Until now, more than 30 CPPs were identified in mammalians. Recent immunostaining analyses of CPPs have revealed that the chromosome periphery covers the centromeric region of mitotic chromosomes in addition to telomeres and regions between two sister chromatids. Knockdown analyses of CPPs using RNAi have revealed functions in chromosome dynamics, including cohesion of sister chromatids, kinetochore-microtubule attachments, spindle assembly and chromosome segregation. Because most CPPs are involved in various subcellular events in the nucleolus or nuclear at interphase, a temporal and spatial-specific knockdown method of CPPs in the chromosome periphery will be useful to understand the function of chromosome periphery in cell division.

Curcumin suppresses the dynamic instability of microtubules, activates the mitotic checkpoint and induces apoptosis in MCF-7 cells

In this study, curcumin, a potential anticancer agent, was found to dampen the dynamic instability of individual microtubules in living MCF-7 cells. It strongly reduced the rate and extent of shortening states, and modestly reduced the rate and extent of growing states. In addition, curcumin decreased the fraction of time microtubules spent in the growing state and strongly increased the time microtubules spent in the pause state. Brief treatment with curcumin depolymerized mitotic microtubules, perturbed microtubule-kinetochore attachment and disturbed the mitotic spindle structure. Curcumin also perturbed the localization of the kinesin protein Eg5 and induced monopolar spindle formation. Further, curcumin increased the accumulation of Mad2 and BubR1 at the kinetochores, indicating that it activated the mitotic checkpoint. In addition, curcumin treatment increased the metaphase/anaphase ratio, indicating that it can delay mitotic progression from the metaphase to anaphase. We provide evidence suggesting that the affected cells underwent apoptosis via the p53-dependent apoptotic pathway. The results support the idea that kinetic stabilization of microtubule dynamics assists in the nuclear translocation of p53. Curcumin exerted additive effects when combined with vinblastine, a microtubule depolymerizing drug, whereas the combination of curcumin with paclitaxel, a microtubule-stabilizing drug, produced an antagonistic effect on the inhibition of MCF-7 cell proliferation. The results together suggested that curcumin inhibited MCF-7 cell proliferation by inhibiting the assembly dynamics of microtubules.

Nuclear transport and the mitotic apparatus: an evolving relationship

The trafficking of macromolecules between the cytoplasm and the nucleus is controlled by the nuclear pore complexes (NPCs) and various transport factors that facilitate the movement of cargos through the NPCs and their accumulation in the target compartment. While their functions in transport are well established, an ever-growing number of observations have also linked components of the nuclear transport machinery to processes that control chromosome segregation during mitosis, including spindle assembly, kinetochore function, and the spindle assembly checkpoint. In this review, we will discuss this evolving area of study and emerging hypotheses that propose key roles for components of the nuclear transport apparatus in mitotic progression.

Microtubule organization by the antagonistic mitotic motors kinesin-5 and kinesin-14

During cell division, different molecular motors act synergistically to rearrange microtubules. Minus end-directed motors are thought to have a dual role: focusing microtubule ends to poles and establishing together with plus end-directed motors a balance of force between antiparallel microtubules in the spindle. We study here the competing action of Xenopus laevis kinesin-14 and -5 in vitro in situations in which these motors with opposite directionality cross-link and slide microtubules. We find that full-length kinesin-14 can form microtubule asters without additional factors, whereas kinesin-5 does not, likely reflecting an adaptation to mitotic function. A stable balance of force is not established between two antiparallel microtubules with these motors. Instead, directional instability is generated, promoting efficient motor and microtubule sorting. A nonmotor microtubule cross-linker can suppress directional instability but also impedes microtubule sorting, illustrating a conflict between stability and dynamicity of organization. These results establish the basic organizational properties of these antagonistic mitotic motors and a microtubule bundler.

A membranous spindle matrix orchestrates cell division

Eukaryotic cell division uses morphologically different forms of mitosis, referred to as open, partially open and closed mitosis, for accurate chromosome segregation and proper partitioning of other cellular components such as endomembranes and cell fate determinants. Recent studies suggest that the spindle matrix provides a conserved strategy to coordinate the segregation of genetic material and the partitioning of the rest of the cellular contents in all three forms of mitosis.

Spindly/CCDC99 is required for efficient chromosome congression and mitotic checkpoint regulation

Human Spindly is required for kinetochore localization of cytoplasmic dynein, which is essential for poleward movement of chromosomes and for kinetochore protein streaming. In addition, Spindly controls the activity and kinetochore abundance of the RZZ complex, which contributes to microtubule attachment and mitotic checkpoint activity.

Spindly recruits a fraction of cytoplasmic dynein to kinetochores for poleward movement of chromosomes and control of mitotic checkpoint signaling. Here we show that human Spindly is a cell cycle–regulated mitotic phosphoprotein that interacts with the Rod/ZW10/Zwilch (RZZ) complex. The kinetochore levels of Spindly are regulated by microtubule attachment and biorientation induced tension. Deletion mutants lacking the N-terminal half of the protein (NΔ253), or the conserved Spindly box (ΔSB), strongly localized to kinetochores and failed to respond to attachment or tension. In addition, these mutants prevented the removal of the RZZ complex and that of MAD2 from bioriented chromosomes and caused cells to arrest at metaphase, showing that RZZ-Spindly has to be removed from kinetochores to terminate mitotic checkpoint signaling. Depletion of Spindly by RNAi, however, caused cells to arrest in prometaphase because of a delay in microtubule attachment. Surprisingly, this defect was alleviated by codepletion of ZW10. Thus, Spindly is not only required for kinetochore localization of dynein but is a functional component of a mechanism that couples dynein-dependent poleward movement of chromosomes to their efficient attachment to microtubules.

Spindle assembly: more than just microtubules

Do actin dynamics play an active role in mitotic spindle assembly? A new study demonstrates that cortical actin polymerization assists with the earliest phase of spindle pole migration.

Kinetochores generate microtubules with distal plus ends: their roles and limited lifetime in mitosis

In early mitosis, microtubules can be generated at kinetochores as well as at spindle poles. However, the role and regulation of kinetochore-derived microtubules have been unclear. In general, metaphase spindle microtubules are oriented such that their plus ends bind to kinetochores. However, we now have evidence that, during early mitosis in budding yeast, microtubules are generated at kinetochores with distal plus ends. These kinetochore-derived microtubules interact along their length with microtubules that extend from a spindle pole, facilitating kinetochore loading onto the lateral surface of spindle pole microtubules. Once kinetochores are loaded, microtubules are no longer generated at kinetochores, and those that remain disappear rapidly and do not contribute to the metaphase spindle. Stu2 (the ortholog of vertebrate XMAP215/ch-TOG) localizes to kinetochores and plays a central role in regulating kinetochore-derived microtubules. Our work provides insight into microtubule generation at kinetochores and the mechanisms that facilitate initial kinetochore interaction with spindle pole microtubules.

Directly probing the mechanical properties of the spindle and its matrix

Several recent models for spindle length regulation propose an elastic pole to pole spindle matrix that is sufficiently strong to bear or antagonize forces generated by microtubules and microtubule motors. We tested this hypothesis using microneedles to skewer metaphase spindles in Xenopus laevis egg extracts. Microneedle tips inserted into a spindle just outside the metaphase plate resulted in spindle movement along the interpolar axis at a velocity slightly slower than microtubule poleward flux, bringing the nearest pole toward the needle. Spindle velocity decreased near the pole, which often split apart slowly, eventually letting the spindle move completely off the needle. When two needles were inserted on either side of the metaphase plate and rapidly moved apart, there was minimal spindle deformation until they reached the poles. In contrast, needle separation in the equatorial direction rapidly increased spindle width as constant length spindle fibers pulled the poles together. These observations indicate that an isotropic spindle matrix does not make a significant mechanical contribution to metaphase spindle length determination.

Novel binding of the mitotic regulator TPX2 (target protein for Xenopus kinesin-like protein 2) to importin-alpha

Several aspects of mitotic spindle assembly are orchestrated by the Ran GTPase through its modulation of the interaction between spindle assembly factors and importin-α. One such factor is TPX2 that promotes microtubule assembly in the vicinity of chromosomes. TPX2 is inhibited when bound to importin-α, which occurs when the latter is bound to importin-β. The importin-α:β interaction is disrupted by the high RanGTP concentration near the chromosomes, releasing TPX2. In more distal regions, where Ran is predominantly GDP-bound, TPX2 remains bound to importin-α and so is inhibited. Here we use a combination of structural and biochemical methods to define the basis for TPX2 binding to importin-α. A 2.2 Å resolution crystal structure shows that the primary nuclear localization signal (²⁸⁴KRKH²⁸⁷) of TPX2, which has been shown to be crucial for inhibition, binds to the minor NLS-binding site on importin-α. This atypical interaction pattern was confirmed using complementary binding studies that employed importin-α variants in which binding to either the major or minor NLS-binding site was impaired, together with competition assays using the SV40 monopartite NLS that binds primarily to the major site. The different way in which TPX2 binds to importin-α could account for much of the selectivity necessary during mitosis because this would reduce the competition for binding to importin-α from other NLS-containing proteins.

Cyclin G-associated kinase promotes microtubule outgrowth from chromosomes during spindle assembly

During mitosis, all chromosomes must attach to microtubules of the mitotic spindle to ensure correct chromosome segregation. Microtubule attachment occurs at specialized structures at the centromeric region of chromosomes, called kinetochores. These kinetochores can generate microtubule attachments through capture of centrosome-derived microtubules, but in addition, they can generate microtubules themselves, which are subsequently integrated with centrosome-derived microtubules to form the mitotic spindle. Here, we have performed a large scale RNAi screen and identify cyclin G-associated kinase (GAK) as a novel regulator of microtubule generation at kinetochores/chromatin. This function of GAK requires its C-terminal J-domain, which is essential for clathrin recycling from endocytic vesicles. Consistently, cells lacking GAK show strongly reduced levels of clathrin on the mitotic spindle, and reduction of clathrin levels also inhibits microtubule generation at kinetochores/chromosomes. Finally, we present evidence that association of clathrin with the spindle is promoted by a signal coming from the chromosomes. These results identify a role for GAK and clathrin in microtubule outgrowth from kinetochores/chromosomes and suggest that GAK acts through clathrin to control microtubule outgrowth around chromosomes.

Finding the middle ground: how kinetochores power chromosome congression

Genomic stability requires error-free chromosome segregation during mitosis. Chromosome congression to the spindle equator precedes chromosome segregation in anaphase and is a hallmark of metazoan mitosis. Here we review the current knowledge and concepts on the processes that underlie chromosome congression, including initial attachment to spindle microtubules, biorientation, and movements, from the perspective of the kinetochore.

ESCRT & Co

Components of the ESCRT (endosomal sorting complex required for transport) machinery mediate endosomal sorting of ubiquitinated membrane proteins. They are key regulators of biological processes important for cell growth and survival, such as growth-factor-mediated signalling and cytokinesis. In addition, enveloped viruses, such as HIV-1, hijack and utilize the ESCRTs for budding during virus release and infection. Obviously, the ESCRT-facilitated pathways require tight regulation, which is partly mediated by a group of interacting proteins, for which our knowledge is growing. In this review we discuss the different ESCRT-modulating proteins and how they influence ESCRT-dependent processes, for example, by acting as positive or negative regulators or by providing temporal and spatial control. A number of the interactors influence the classical ESCRT-mediated process of endosomal cargo sorting, for example, by modulating the interaction between ubiquitinated cargo and the ESCRTs. Certain accessory proteins have been implicated in regulating the activity or steady-state expression levels of the ESCRT components, whereas other interactors control the cellular localization of the ESCRTs, for example, by inducing shuttling between cytosol and nucleus or endosomes. In conclusion, the discovery of novel interactors has and will extend our knowledge of the biological roles of ESCRTs.

Learning about cancer from frogs: analysis of mitotic spindles in Xenopus egg extracts

The mitotic spindle is responsible for correctly segregating chromosomes during cellular division. Disruption of this process leads to genomic instability in the form of aneuploidy, which can contribute to the development of cancer. Therefore, identification and characterization of factors that are responsible for the assembly and regulation of the spindle are crucial. Not only are these factors often altered in cancer, but they also serve as potential therapeutic targets. Xenopus egg extract is a powerful tool for studying spindle assembly and other cell cycle-related events owing, in large part, to the ease with which protein function can be manipulated in the extract. Importantly, the spindle factors that have been characterized in egg extract are conserved in human spindle assembly. In this review, we explain how the extract is prepared and manipulated to study the function of individual factors in spindle assembly and the spindle checkpoint. Furthermore, we provide examples of several spindle factors that have been defined functionally using the extract system and discuss how these factors are altered in human cancer.

Towards building a chromosome segregation machine

All organisms, from bacteria to humans, face the daunting task of replicating, packaging and segregating up to two metres (about 6 x 10(9) base pairs) of DNA when each cell divides. This task is carried out up to a trillion times during the development of a human from a single fertilized cell. The strategy by which DNA is replicated is now well understood. But when it comes to packaging and segregating a genome, the mechanisms are only beginning to be understood and are often as variable as the organisms in which they are studied.

The transcriptional repressor Kaiso localizes at the mitotic spindle and is a constituent of the pericentriolar material

Kaiso is a BTB/POZ zinc finger protein known as a transcriptional repressor. It was originally identified through its in vitro association with the Armadillo protein p120ctn. Subcellular localization of Kaiso in cell lines and in normal and cancerous human tissues revealed that its expression is not restricted to the nucleus. In the present study we monitored Kaiso's subcellular localization during the cell cycle and found the following: (1) during interphase, Kaiso is located not only in the nucleus, but also on microtubular structures, including the centrosome; (2) at metaphase, it is present at the centrosomes and on the spindle microtubules; (3) during telophase, it accumulates at the midbody. We found that Kaiso is a genuine PCM component that belongs to a pericentrin molecular complex. We analyzed the functions of different domains of Kaiso by visualizing the subcellular distribution of GFP-tagged Kaiso fragments throughout the cell cycle. Our results indicate that two domains are responsible for targeting Kaiso to the centrosomes and microtubules. The first domain, designated SA1 for spindle-associated domain 1, is located in the center of the Kaiso protein and localizes at the spindle microtubules and centrosomes; the second domain, SA2, is an evolutionarily conserved domain situated just before the zinc finger domain and might be responsible for localizing Kaiso towards the centrosomal region. Constructs containing both SA domains and Kaiso's aminoterminal BTB/POZ domain triggered the formation of abnormal centrosomes. We also observed that overexpression of longer or full-length Kaiso constructs led to mitotic cell arrest and frequent cell death. Knockdown of Kaiso accelerated cell proliferation. Our data reveal a new target for Kaiso at the centrosomes and spindle microtubules during mitosis. They also strongly imply that Kaiso's function as a transcriptional regulator might be linked to the control of the cell cycle and to cell proliferation in cancer.

Robust transport by multiple motors with nonlinear force-velocity relations and stochastic load sharing

Transport by processive molecular motors plays an important role in many cell biological phenomena. In many cases, motors work together to transport cargos in the cell, so it is important to understand the mechanics of the multiple motors. Based on earlier modeling efforts, here we study effects of nonlinear force-velocity relations and stochastic load sharing on multiple motor transport. We find that when two or three motors transport the cargo, then the nonlinear and stochastic effects compensate so that the mechanical properties of the transport are robust. Similarly, the transport is insensitive to compliance of the cargo-motor links. Furthermore, the rate of movement against moderate loads is not improved by increasing the small number of motors. When the motor number is greater than 4, correlations between the motors become negligible, and the earlier analytical mean-field theory of the multiple motor transport holds. We predict that the effective diffusion of the cargo driven by the multiple motors under load increases by an order of magnitude compared to that for the single motor. Finally, our simulations predict that the stochastic effects are responsible for a significant dispersion of velocities generated by the 'tug-of-war' of the multiple opposing motors.

Prometaphase spindle maintenance by an antagonistic motor-dependent force balance made robust by a disassembling lamin-B envelope

We tested the classical hypothesis that astral, prometaphase bipolar mitotic spindles are maintained by balanced outward and inward forces exerted on spindle poles by kinesin-5 and -14 using modeling of in vitro and in vivo data from Drosophila melanogaster embryos. Throughout prometaphase, puncta of both motors aligned on interpolar microtubules (MTs [ipMTs]), and motor perturbation changed spindle length, as predicted. Competitive motility of purified kinesin-5 and -14 was well described by a stochastic, opposing power stroke model incorporating motor kinetics and load-dependent detachment. Motor parameters from this model were applied to a new stochastic force-balance model for prometaphase spindles, providing a good fit to data from embryos. Maintenance of virtual spindles required dynamic ipMTs and a narrow range of kinesin-5 to kinesin-14 ratios matching that found in embryos. Functional perturbation and modeling suggest that this range can be extended significantly by a disassembling lamin-B envelope that surrounds the prometaphase spindle and augments the finely tuned, antagonistic kinesin force balance to maintain robust prometaphase spindles as MTs assemble and chromosomes are pushed to the equator.

Self-organization of intracellular gradients during mitosis

Gradients are used in a number of biological systems to transmit spatial information over a range of distances. The best studied are morphogen gradients where information is transmitted over many cell lengths. Smaller mitotic gradients reflect the need to organize several distinct events along the length of the mitotic spindle. The intracellular gradients that characterize mitosis are emerging as important regulatory paradigms. Intracellular gradients utilize intrinsic auto-regulatory feedback loops and diffusion to establish stable regions of activity within the mitotic cytosol. We review three recently described intracellular mitotic gradients. The Ran GTP gradient with its elaborate cascade of nuclear transport receptors and cargoes is the best characterized, yet the dynamics underlying the robust gradient of Ran-GTP have received little attention. Gradients of phosphorylation have been observed on Aurora B kinase substrates both before and after anaphase onset. In both instances the phosphorylation gradient appears to result from a soluble gradient of Aurora B kinase activity. Regulatory properties that support gradient formation are highlighted. Intracellular activity gradients that regulate localized mitotic events bare several hallmarks of self-organizing biologic systems that designate spatial information during pattern formation. Intracellular pattern formation represents a new paradigm in mitotic regulation.

Microtubule motors in eukaryotic spindle assembly and maintenance

The spindle is a microtubule-based structure that facilitates chromosome segregation during mitosis and meiosis. Spindle assembly from dynamic microtubule building blocks is a major challenge for the dividing cell and a process that critically requires microtubule motors. In this review we focus on the mechanisms by which microtubule motors shape the spindle. Specifically, we address how motors are thought to move and arrange microtubules to form the characteristic bipolar morphology shared by all eukaryotic spindles as well as motor-dependent mechanisms of microtubule length regulation.

Purification and assay of mitotic motors

To understand how mitotic kinesins contribute to the assembly and function of the mitotic spindle, we need to purify these motors and analyze their biochemical and ultrastructural properties. Here we briefly review our use of microtubule (MT) affinity and biochemical fractionation to obtain information about the oligomeric state of native mitotic kinesin holoenzymes from eggs and early embryos. We then detail the methods we use to purify full length recombinant Drosophila embryo mitotic kinesins, using the baculovirus expression system, in sufficient yields for detailed in vitro assays. These two approaches provide complementary biochemical information on the basic properties of these key mitotic proteins, and permit assays of critical motor activities, such as MT-MT crosslinking and sliding, that are not revealed by assaying motor domain subfragments.

Improved kymography tools and its applications to mitosis

Although applicability of kymographs is limited to nearly one-dimensional (1D) processes, they have been instrumental in the analysis and interpretation of a wide range of dynamic biological processes. We focus here on some applications of kymography in the study of one among the range of 'nearly-1D' processes -mitosis. Using this biological context, we suggest generalized procedures in kymograph assembly that allow a partial retrieval of spatial information which is typically lost or distorted in conventional kymography. These kymograph variations, namely guided-kymography and chromo-kymography, are helpful in the determination of actual velocities and discrimination of structures when using thick regions of interest (ROIs). The method used to generate chromo-kymographs is generalized to other (non-kymograph) projection techniques, which include time-stack and z-stack projections.

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.

Assessment of mitotic spindle phenotypes in Drosophila S2 cells

The Drosophila S2 cell line is popularly used to study mitosis. In this cell line, multiple genes can be easily and efficiently knocked down by RNA interference (RNAi), and the associated mitotic phenotypes can be assessed with high-resolution microscopy after immunofluorescence or in a living cell. However, compared to untransformed cells in wild-type organisms such as yeasts or worms, mitosis in the S2 cell line is more variable and often looks abnormal even in RNAi-untreated cells. Therefore, in order to judge whether a phenotype is derived from RNAi of the target gene or is simply a variation of control cells, it is critical to prepare proper control samples and perform objective imaging and image analysis. Here, we discuss how bona fide mitotic phenotypes associated with RNAi can be identified, avoiding selecting false positives, in S2 cells.

Kinetochore-microtubule dynamics and attachment stability

Mitosis is the process by which a cell divides its genetic material equally into two daughter cells. Successful division requires that the two identical sister chromatids of a mitotic chromosome attach to the plus-ends of spindle microtubules (MTs) via their kinetochores, which are large protein structures built on centromeric DNA. Attachments between kinetochores and MTs must be persistent so that forces can be generated for chromosome movements, but at the same time they must be compliant, because attached MT plus-ends continuously polymerize and depolymerize to provide force for chromosome congression to the spindle equator. Both the attachment stability of kinetochore-MTs and the degree of dynamic instability exhibited by kinetochore-MTs must be precisely controlled to avoid errors in chromosome segregation. This chapter provides an overview of techniques used in cultured mammalian cells that measure stability and polymerization/depolymerization dynamics of kinetochore-MTs during mitosis.

Traffic control: regulation of kinesin motors

Kinesins are a family of molecular motors that use the energy of ATP hydrolysis to move along the surface of, or destabilize, microtubule filaments. Much progress has been made in understanding the mechanics and functions of the kinesin motors that play important parts in cell division, cell motility, intracellular trafficking and ciliary function. How kinesins are regulated in cells to ensure the temporal and spatial fidelity of their microtubule-based activities is less well understood. Recent work has revealed molecular mechanisms that control kinesin autoinhibition and subsequent activation, binding to cargos and microtubule tracks, and localization at specific sites of action.

Kinesin-5 in Drosophila embryo mitosis: sliding filament or spindle matrix mechanism?

The Drosophila syncytial embryo uses multiple astral mitotic spindles that are specialized for rapid mitosis. The homotetrameric kinesin-5, KLP61F contributes to various aspects of mitosis in this system, all of which are consistent with it exerting outward forces on spindle poles. In principle, kinesin-5 could accomplish this by (i) sliding microtubules (MTs), minus end leading, relative to a static spindle matrix or (ii) crosslinking and sliding apart adjacent pairs of antiparallel interpolar (ip) MTs. Here, I critically review data on the biochemistry of purified KLP61F, its localization and dynamic properties within spindles, and quantitative modeling of KLP61F function. While a matrix-based mechanism may operate in some systems, the work tends to support the latter "sliding filament" mechanism for KLP61F action in Drosophila embryo spindles. Cell Motil. Cytoskeleton 2009. (c) 2009 Wiley-Liss, Inc.

Predominant regulators of tubulin monomer-polymer partitioning and their implication for cell polarization

The microtubule-system organizes the cytoplasm during interphase and segregates condensed chromosomes during mitosis. Four unrelated conserved proteins, XMAP215/Dis1/TOGp, MCAK, MAP4 and Op18/stathmin, have all been implicated as predominant regulators of tubulin monomer-polymer partitioning in animal cells. However, while studies employing the Xenopus egg extract model system indicate that the partitioning is largely governed by the counteractive activities of XMAP215 and MCAK, studies of human cell lines indicate that MAP4 and Op18 are the predominant regulators of the interphase microtubule-array. Here, we review functional interplay of these proteins during interphase and mitosis in various cell model systems. We also review the evidence that MAP4 and Op18 have interphase-specific, counteractive and phosphorylation-inactivated activities that govern tubulin subunit partitioning in many mammalian cell types. Finally, we discuss evidence indicating that partitioning regulation by MAP4 and Op18 may be of significance to establish cell polarity.

CLIP-170-dependent capture of membrane organelles by microtubules initiates minus-end directed transport

Cytoplasmic microtubules (MTs) continuously grow and shorten at free plus ends. During mitosis, this dynamic behavior allows MTs to capture chromosomes to initiate their movement to the spindle poles; however, the role of MT dynamics in capturing organelles for transport in interphase cells has not been demonstrated. Here we use Xenopus melanophores to test the hypothesis that MT dynamics significantly contribute to the efficiency of MT minus-end directed transport of membrane organelles. We demonstrate that initiation of transport of membrane-bounded melanosomes (pigment granules) to the cell center involves their capture by MT plus ends, and that inhibition of MT dynamics or loss of the MT plus-end tracking protein CLIP-170 from MT tips dramatically inhibits pigment aggregation. We conclude that MT dynamics are required for the initiation of MT transport of membrane organelles in interphase cells, and that +TIPs such as CLIP-170 play an important role in this process.