Self-organization of microtubule asters induced in Xenopus egg extracts by GTP-bound Ran

Spatiotemporal Regulation of Nuclear Transport Machinery and Microtubule Organization

Spindle microtubules capture and segregate chromosomes and, therefore, their assembly is an essential event in mitosis. To carry out their mission, many key players for microtubule formation need to be strictly orchestrated. Particularly, proteins that assemble the spindle need to be translocated at appropriate sites during mitosis. A small GTPase (hydrolase enzyme of guanosine triphosphate), Ran, controls this translocation. Ran plays many roles in many cellular events: nucleocytoplasmic shuttling through the nuclear envelope, assembly of the mitotic spindle, and reorganization of the nuclear envelope at the mitotic exit. Although these events are seemingly distinct, recent studies demonstrate that the mechanisms underlying these phenomena are substantially the same as explained by molecular interplay of the master regulator Ran, the transport factor importin, and its cargo proteins. Our review focuses on how the transport machinery regulates mitotic progression of cells. We summarize translocation mechanisms governed by Ran and its regulatory proteins, and particularly focus on Ran-GTP targets in fission yeast that promote spindle formation. We also discuss the coordination of the spatial and temporal regulation of proteins from the viewpoint of transport machinery. We propose that the transport machinery is an essential key that couples the spatial and temporal events in cells.

GM130 Regulates Golgi-Derived Spindle Assembly by Activating TPX2 and Capturing Microtubules

Spindle assembly requires the coordinated action of multiple cellular structures to nucleate and organize microtubules in a precise spatiotemporal manner. Among them, the contributions of centrosomes, chromosomes, and microtubules have been well studied, yet the involvement of membrane-bound organelles remains largely elusive. Here, we provide mechanistic evidence for a membrane-based, Golgi-derived microtubule assembly pathway in mitosis. Upon mitotic entry, the Golgi matrix protein GM130 interacts with importin α via a classical nuclear localization signal that recruits importin α to the Golgi membranes. Sequestration of importin α by GM130 liberates the spindle assembly factor TPX2, which activates Aurora-A kinase and stimulates local microtubule nucleation. Upon filament assembly, nascent microtubules are further captured by GM130, thus linking Golgi membranes to the spindle. Our results reveal an active role for the Golgi in regulating spindle formation to ensure faithful organelle inheritance.

Emergent Properties of the Metaphase Spindle

A metaphase spindle is a complex structure consisting of microtubules and a myriad of different proteins that modulate microtubule dynamics together with chromatin and kinetochores. A decade ago, a full description of spindle formation and function seemed a lofty goal. Here, we describe how work in the last 10 years combining cataloging of spindle components, the characterization of their biochemical activities using single-molecule techniques, and theory have advanced our knowledge. Taken together, these advances suggest that a full understanding of spindle assembly and function may soon be possible.

The Ran-GTP gradient spatially regulates XCTK2 in the spindle

Ran is a small GTP binding protein that was originally identified as a regulator of nucleocytoplasmic transport [1] and subsequently found to be important for spindle formation [2-5]. In mitosis, a gradient of Ran-GTP emanates from chromatin and diminishes toward spindle poles [6]. Ran-GTP promotes spindle self-organization through the release of importin-bound spindle assembly factors (SAFs), which stimulate microtubule (MT) nucleation and organization and regulate MT dynamics [7-9]. Although many SAFs are non-motile MT-associated proteins, such as NuMA, TPX2, and HURP [7, 10-12], Ran also controls motor proteins, including Kid and HSET/XCTK2 [13, 14]. The Kinesin-14 XCKT2 is important for spindle assembly and pole organization [15-20], and Ran-GTP is proposed to promote XCKT2 MT crosslinking activity by releasing importin α/β from a bipartite nuclear localization signal (NLS) located in the tail domain [14]. Here, we show that the Ran-GTP gradient spatially regulates XCTK2 within the spindle. A flattened Ran-GTP gradient blocked the ability of excess XCTK2 to stimulate bipolar spindle assembly and resulted in XCTK2-mediated bundling of free MTs. These effects required the XCTK2 tail, which promoted the motility of XCTK2 within the spindle independent of the Ran-GTP gradient. In addition, the turnover kinetics of XCTK2 were spatially controlled: they were faster near the poles relative to the chromatin, but not with a mutant XCTK2 that cannot bind to importin α/β. Our results support a model in which the Ran-GTP gradient spatially coordinates motor localization with motility to ensure efficient spindle formation.

Chromatids segregate without centrosomes during Caenorhabditis elegans mitosis in a Ran- and CLASP-dependent manner

Laser ablation of centrosomes in one-cell Caenorhabditis elegans embryos shows that chromatids can segregate independently of centrosomes and also independently of the activity of kinetochore microtubules during mitosis. CLASP and RanGTP are required to generate this centrosome-independent force, whereas SPD-1 and BMK-1 act as brakes to oppose it.

During mitosis, chromosomes are connected to a microtubule-based spindle. Current models propose that displacement of the spindle poles and/or the activity of kinetochore microtubules generate mechanical forces that segregate sister chromatids. Using laser destruction of the centrosomes during Caenorhabditis elegans mitosis, we show that neither of these mechanisms is necessary to achieve proper chromatid segregation. Our results strongly suggest that an outward force generated by the spindle midzone, independently of centrosomes, is sufficient to segregate chromosomes in mitotic cells. Using mutant and RNAi analysis, we show that the microtubule-bundling protein SPD-1/MAP-65 and BMK-1/kinesin-5 act as a brake opposing the force generated by the spindle midzone. Conversely, we identify a novel role for two microtubule-growth and nucleation agents, Ran and CLASP, in the establishment of the centrosome-independent force during anaphase. Their involvement raises the interesting possibility that microtubule polymerization of midzone microtubules is continuously required to sustain chromosome segregation during mitosis.

RanBP1 governs spindle assembly by defining mitotic Ran-GTP production

Accurate control of the Ras-related nuclear protein (Ran) GTPase cycle depends on the regulated activity of regulator of chromosome condensation 1 (RCC1), Ran's nucleotide exchange factor. RanBP1 has been characterized as a coactivator of the Ran GTPase-activating protein RanGAP1. RanBP1 can also form a stable complex with Ran and RCC1, although the dynamics and function of this complex remain poorly understood. Here, we show that formation of the heterotrimeric RCC1/Ran/RanBP1 complex in M phase Xenopus egg extracts controls both RCC1's enzymatic activity and partitioning between the chromatin-bound and soluble pools of RCC1. This mechanism is critical for spatial control of Ran-guanosine triphosphate (GTP) gradients that guide mitotic spindle assembly. Moreover, phosphorylation of RanBP1 drives changes in the dynamics of chromatin-bound RCC1 pools at the metaphase-anaphase transition. Our findings reveal an important mitotic role for RanBP1, controlling the spatial distribution and magnitude of mitotic Ran-GTP production and thereby ensuring accurate execution of Ran-dependent mitotic events.

TPX2 levels modulate meiotic spindle size and architecture in Xenopus egg extracts

TPX2 levels modulate spindle architecture through Eg5, partitioning microtubules between a tiled, antiparallel array that promotes spindle expansion and a cross-linked, parallel architecture that concentrates microtubules at spindle poles.

The spindle segregates chromosomes in dividing eukaryotic cells, and its assembly pathway and morphology vary across organisms and cell types. We investigated mechanisms underlying differences between meiotic spindles formed in egg extracts of two frog species. Small Xenopus tropicalis spindles resisted inhibition of two factors essential for assembly of the larger Xenopus laevis spindles: RanGTP, which functions in chromatin-driven spindle assembly, and the kinesin-5 motor Eg5, which drives antiparallel microtubule (MT) sliding. This suggested a role for the MT-associated protein TPX2 (targeting factor for Xenopus kinesin-like protein 2), which is regulated by Ran and binds Eg5. Indeed, TPX2 was threefold more abundant in X. tropicalis extracts, and elevated TPX2 levels in X. laevis extracts reduced spindle length and sensitivity to Ran and Eg5 inhibition. Higher TPX2 levels recruited Eg5 to the poles, where MT density increased. We propose that TPX2 levels modulate spindle architecture through Eg5, partitioning MTs between a tiled, antiparallel array that promotes spindle expansion and a cross-linked, parallel architecture that concentrates MTs at spindle poles.

CDK-dependent phosphorylation of Alp7-Alp14 (TACC-TOG) promotes its nuclear accumulation and spindle microtubule assembly

As cells transition from interphase to mitosis, the microtubule cytoskeleton is reorganized to form the mitotic spindle. In the closed mitosis of fission yeast, a microtubule-associated protein complex, Alp7-Alp14 (transforming acidic coiled-coil-tumor overexpressed gene), enters the nucleus upon mitotic entry and promotes spindle formation. However, how the complex is controlled to accumulate in the nucleus only during mitosis remains elusive. Here we demonstrate that Alp7-Alp14 is excluded from the nucleus during interphase using the nuclear export signal in Alp14 but is accumulated in the nucleus during mitosis through phosphorylation of Alp7 by the cyclin-dependent kinase (CDK). Five phosphorylation sites reside around the nuclear localization signal of Alp7, and the phosphodeficient alp7-5A mutant fails to accumulate in the nucleus during mitosis and exhibits partial spindle defects. Thus our results reveal one way that CDK regulates spindle assembly at mitotic entry: CDK phosphorylates the Alp7-Alp14 complex to localize it to the nucleus.

Swinging a sword: how microtubules search for their targets

The cell interior is in constant movement, which is to a large extent determined by microtubules, thin and long filaments that permeate the cytoplasm. To move large objects, microtubules need to connect them to the site of their destination. For example, during cell division, microtubules connect chromosomes with the spindle poles via kinetochores, protein complexes on the chromosomes. A general question is how microtubules, while being bound to one structure, find the target that needs to be connected to this structure. Here we review the mechanisms of how microtubules search for kinetochores, with emphasis on the recently discovered microtubule feature to explore space by pivoting around the spindle pole. In addition to accelerating the search for kinetochores, pivoting helps the microtubules to search for cortical anchors, as well as to self-organize into parallel arrays and asters to target specific regions of the cell. Thus, microtubule pivoting constitutes a mechanism by which they locate targets in different cellular contexts.

Transportin acts to regulate mitotic assembly events by target binding rather than Ran sequestration

Transportin-specific molecular tools are used to show that the mitotic cell contains importin β and transportin “global positioning system” pathways that are mechanistically parallel. Transportin works to control where the spindle, nuclear membrane, and nuclear pores are formed by directly affecting assembly factor function.

The nuclear import receptors importin β and transportin play a different role in mitosis: both act phenotypically as spatial regulators to ensure that mitotic spindle, nuclear membrane, and nuclear pore assembly occur exclusively around chromatin. Importin β is known to act by repressing assembly factors in regions distant from chromatin, whereas RanGTP produced on chromatin frees factors from importin β for localized assembly. The mechanism of transportin regulation was unknown. Diametrically opposed models for transportin action are as follows: 1) indirect action by RanGTP sequestration, thus down-regulating release of assembly factors from importin β, and 2) direct action by transportin binding and inhibiting assembly factors. Experiments in Xenopus assembly extracts with M9M, a superaffinity nuclear localization sequence that displaces cargoes bound by transportin, or TLB, a mutant transportin that can bind cargo and RanGTP simultaneously, support direct inhibition. Consistently, simple addition of M9M to mitotic cytosol induces microtubule aster assembly. ELYS and the nucleoporin 107–160 complex, components of mitotic kinetochores and nuclear pores, are blocked from binding to kinetochores in vitro by transportin, a block reversible by M9M. In vivo, 30% of M9M-transfected cells have spindle/cytokinesis defects. We conclude that the cell contains importin β and transportin “global positioning system”or “GPS” pathways that are mechanistically parallel.

NETs and cell cycle regulation

There are many ways that the nuclear envelope can influence the cell cycle. In addition to roles of lamins in regulating the master cell cycle regulator pRb and nuclear envelope breakdown in mitosis, many other nuclear envelope proteins influence the cell cycle through regulatory or structural functions. Of particular note among these are the nuclear envelope transmembrane proteins (NETs) that appear to influence cell cycle regulation through multiple separate mechanisms. Some NETs and other nuclear envelope proteins accumulate on the mitotic spindle, suggesting functional or structural roles in the cell cycle. In interphase exogenous overexpression of some NETs promotes an increase in G1 populations, while others promote an increase in G2/M populations, sometimes associated with the induction of senescence. Intriguingly, most of the NETs linked to the cell cycle are highly restricted in their tissue expression; thus, their misregulation in cancer could contribute to the many tissue-specific types of cancer.

Ran GTPase in nuclear envelope formation and cancer metastasis

Ran is a small ras-related GTPase that controls the nucleocytoplasmic exchange of macromolecules across the nuclear envelope. It binds to chromatin early during nuclear formation and has important roles during the eukaryotic cell cycle, where it regulates mitotic spindle assembly, nuclear envelope formation and cell cycle checkpoint control. Like other GTPases, Ran relies on the cycling between GTP-bound and GDP-bound conformations to interact with effector proteins and regulate these processes. In nucleocytoplasmic transport, Ran shuttles across the nuclear envelope through nuclear pores. It is concentrated in the nucleus by an active import mechanism where it generates a high concentration of RanGTP by nucleotide exchange. It controls the assembly and disassembly of a range of complexes that are formed between Ran-binding proteins and cellular cargo to maintain rapid nuclear transport. Ran also has been identified as an essential protein in nuclear envelope formation in eukaryotes. This mechanism is dependent on importin-β, which regulates the assembly of further complexes important in this process, such as Nup107-Nup160. A strong body of evidence is emerging implicating Ran as a key protein in the metastatic progression of cancer. Ran is overexpressed in a range of tumors, such as breast and renal, and these perturbed levels are associated with local invasion, metastasis and reduced patient survival. Furthermore, tumors with oncogenic KRAS or PIK3CA mutations are addicted to Ran expression, which yields exciting future therapeutic opportunities.

New mitotic regulators released from chromatin

Faithful action of the mitotic spindle segregates duplicated chromosomes into daughter cells. Perturbations of this process result in chromosome mis-segregation, leading to chromosomal instability and cancer development. Chromosomes are not simply passengers segregated by spindle microtubules but rather play a major active role in spindle assembly. The GTP bound form of the Ran GTPase (RanGTP), produced around chromosomes, locally activates spindle assembly factors. Recent studies have uncovered that chromosomes organize mitosis beyond spindle formation. They distinctly regulate other mitotic events, such as spindle maintenance in anaphase, which is essential for chromosome segregation. Furthermore, the direct function of chromosomes is not only to produce RanGTP but, in addition, to release key mitotic regulators from chromatin. Chromatin-remodeling factors and nuclear pore complex proteins, which have established functions on chromatin in interphase, dissociate from mitotic chromatin and function in spindle assembly or maintenance. Thus, chromosomes actively organize their own segregation using chromatin-releasing mitotic regulators as well as RanGTP.

CHD4 is a RanGTP-dependent MAP that stabilizes microtubules and regulates bipolar spindle formation

BACKGROUND

Production of the GTP-bound form of the Ran GTPase (RanGTP) around chromosomes induces spindle assembly by activating nuclear localization signal (NLS)-containing proteins. Several NLS proteins have been identified as spindle assembly factors, but the complexity of the process led us to search for additional proteins with distinct roles in spindle assembly.

RESULTS

We identify a chromatin-remodeling ATPase, CHD4, as a RanGTP-dependent microtubule (MT)-associated protein (MAP). MT binding occurs via the region containing an NLS and chromatin-binding domains. In Xenopus egg extracts and cultured cells, CHD4 largely dissociates from mitotic chromosomes and partially localizes to the spindle. Immunodepletion of CHD4 from egg extracts significantly reduces the quantity of MTs produced around chromatin and prevents spindle assembly. CHD4 RNAi in both HeLa and Drosophila S2 cells induces defects in spindle assembly and chromosome alignment in early mitosis, leading to chromosome missegregation. Further analysis in egg extracts and in HeLa cells reveals that CHD4 is a RanGTP-dependent MT stabilizer. Moreover, the CHD4-containing NuRD complex promotes organization of MTs into bipolar spindles in egg extracts. Importantly, this function of CHD4 is independent of chromatin remodeling.

CONCLUSIONS

Our results uncover a new role for CHD4 as a MAP required for MT stabilization and involved in generating spindle bipolarity.

Tissue specificity in the nuclear envelope supports its functional complexity

Nuclear envelope links to inherited disease gave the conundrum of how mutations in near-ubiquitous proteins can yield many distinct pathologies, each focused in different tissues. One conundrum-resolving hypothesis is that tissue-specific partner proteins mediate these pathologies. Such partner proteins may have now been identified with recent proteome studies determining nuclear envelope composition in different tissues. These studies revealed that the majority of the total nuclear envelope proteins are tissue restricted in their expression. Moreover, functions have been found for a number these tissue-restricted nuclear envelope proteins that fit with mechanisms proposed to explain how the nuclear envelope could mediate disease, including defects in mechanical stability, cell cycle regulation, signaling, genome organization, gene expression, nucleocytoplasmic transport, and differentiation. The wide range of functions to which these proteins contribute is consistent with not only their involvement in tissue-specific nuclear envelope disease pathologies, but also tissue evolution.

Branching microtubule nucleation in Xenopus egg extracts mediated by augmin and TPX2

The microtubules that comprise mitotic spindles in animal cells are nucleated at centrosomes and by spindle assembly factors that are activated in the vicinity of chromatin. Indirect evidence has suggested that microtubules also might be nucleated from pre-existing microtubules throughout the spindle, but this process has not been observed directly. Here, we demonstrate microtubule nucleation from the sides of existing microtubules in meiotic Xenopus egg extracts. Daughter microtubules grow at a low branch angle and with the same polarity as mother filaments. Branching microtubule nucleation requires γ-tubulin and augmin and is stimulated by factors previously implicated in chromatin-stimulated nucleation, guanosine triphosphate(GTP)-bound Ran and its effector, TPX2. Because of the rapid amplification of microtubule numbers and the preservation of microtubule polarity, microtubule-dependent microtubule nucleation is well suited for spindle assembly and maintenance.

The MRN-CtIP pathway is required for metaphase chromosome alignment

Faithful duplication of the genome in S phase followed by its accurate segregation in mitosis is essential to maintain genomic integrity. Recent studies have suggested that proteins involved in DNA transactions are also required for whole-chromosome stability. Here we demonstrate that the MRN (Mre11, Rad50, and Nbs1) complex and CtIP are required for accurate chromosome segregation. Depletion of Mre11 or CtIP, antibody-mediated inhibition of Mre11, or small-molecule inhibition of MRN using mirin results in metaphase chromosome alignment defects in Xenopus egg extracts. Similarly, loss of MRN function adversely affects spindle assembly around DNA-coated beads in egg extracts. Inhibition of MRN function in mammalian cells triggers a metaphase delay and disrupts the RCC1-dependent RanGTP gradient. Addition of the Mre11 inhibitor mirin to egg extracts and mammalian cells reduces RCC1 association with mitotic chromosomes. Thus, the MRN-CtIP pathway contributes to Ran-dependent mitotic spindle assembly by modulating RCC1 chromosome association.

New insights into the mechanism for chromosome alignment in metaphase

During mitosis, duplicated sister chromatids are properly aligned at the metaphase plate of the mitotic spindle before being segregated into two daughter cells. This requires a complex process to ensure proper interactions between chromosomes and spindle microtubules. The kinetochore, the proteinaceous complex assembled at the centromere region on each chromosome, serves as the microtubule attachment site and powers chromosome movement in mitosis. Numerous proteins/protein complexes have been implicated in the connection between kinetochores and dynamic microtubules. Recent studies have advanced our understanding on the nature of the interface between kinetochores and microtubule plus ends in promoting and maintaining their stable attachment. These efforts have demonstrated the importance of this process to ensure accurate chromosome segregation, an issue which has great significance for understanding and controlling abnormal chromosome segregation (aneuploidy) in human genetic diseases and in cancer progression.

Rules of engagement: centrosome-nuclear connections in a closed mitotic system

The assembly of a functional mitotic spindle is essential for cell reproduction and requires a precise coordination between the nuclear cycle and the centrosome. This coordination is particularly prominent in organisms that undergo closed mitosis where centrosomes must not only respond to temporal signals, but also to spatial considerations, e.g. switching from the production of cytoplasmic microtubule arrays to the generation of dynamic intra-nuclear microtubules required for spindle assembly. We utilize a gene knockout of Kif9, a Dictyostelium discoideum Kin-I kinesin, to destabilize the physical association between centrosomes and the nuclear envelope. This approach presents a unique opportunity to reveal temporal and spatial components in the regulation of centrosomal activities in a closed-mitosis organism. Here we report that centrosome–nuclear engagement is not required for the entry into mitosis. Although detached centrosomes can duplicate in the cytoplasm, neither they nor nuclei alone can produce spindle-like microtubule arrays. However, the physical association of centrosomes and the nuclear envelope is required to progress through mitosis beyond prometaphase.

The transition from meiotic to mitotic spindle assembly is gradual during early mammalian development

The transition from a meiotic-like spindle formation characterized by lack of centrioles to a typical mitotic spindle occurs gradually in embryos during the preimplantation stage.

The transition from meiosis to mitosis, classically defined by fertilization, is a fundamental process in development. However, its mechanism remains largely unexplored. In this paper, we report a surprising gradual transition from meiosis to mitosis over the first eight divisions of the mouse embryo. The first cleavages still largely share the mechanism of spindle formation with meiosis, during which the spindle is self-assembled from randomly distributed microtubule-organizing centers (MTOCs) without centrioles, because of the concerted activity of dynein and kinesin-5. During preimplantation development, the number of cellular MTOCs progressively decreased, the spindle pole gradually became more focused, and spindle length progressively scaled down with cell size. The typical mitotic spindle with centrin-, odf2-, kinesin-12–, and CP110-positive centrosomes was established only in the blastocyst. Overall, the transition from meiosis to mitosis progresses gradually throughout the preimplantation stage in the mouse embryo, thus providing a unique system to study the mechanism of centrosome biogenesis in vivo.

Microtubule assembly during mitosis - from distinct origins to distinct functions?

The mitotic spindle is structurally and functionally defined by its main component, the microtubules (MTs). The MTs making up the spindle have various functions, organization and dynamics: astral MTs emanate from the centrosome and reach the cell cortex, and thus have a major role in spindle positioning; interpolar MTs are the main constituent of the spindle and are key for the establishment of spindle bipolarity, chromosome congression and central spindle assembly; and kinetochore-fibers are MT bundles that connect the kinetochores with the spindle poles and segregate the sister chromatids during anaphase. The duplicated centrosomes were long thought to be the origin of all of these MTs. However, in the last decade, a number of studies have contributed to the identification of non-centrosomal pathways that drive MT assembly in dividing cells. These pathways are now known to be essential for successful spindle assembly and to participate in various processes such as K-fiber formation and central spindle assembly. In this Commentary, we review the recent advances in the field and discuss how different MT assembly pathways might cooperate to successfully form the mitotic spindle.

Nuclear envelope influences on cell-cycle progression

The nuclear envelope is a complex double membrane system that serves as a dynamic interface between the nuclear and cytoplasmic compartments. Among its many roles is to provide an anchor for gene regulatory proteins on its nucleoplasmic surface and for the cytoskeleton on its cytoplasmic surface. Both sets of anchors are proteins called NETs (nuclear envelope transmembrane proteins), embedded respectively in the inner or outer nuclear membranes. Several lines of evidence indicate that the nuclear envelope contributes to cell-cycle regulation. These contributions come from both inner and outer nuclear membrane NETs and appear to operate through several distinct mechanisms ranging from sequestration of gene-regulatory proteins to activating kinase cascades.

Centrosomes can initiate a polarity axis from any position within one-cell C. elegans embryos

The stereotyped asymmetry of one-cell C. elegans embryos has proven to be an important model for identifying molecular determinants of cell polarity. How polarity is initiated is less well understood. Polarity establishment depends on centrosomes, which use two molecularly distinct pathways to break symmetry. In both, the centrosome's position adjacent to the cell cortex is thought to determine where polarization starts. Defects in centrosome-cortex juxtaposition correlate with defects in polarity establishment in several mutants, suggesting that these processes may be linked, but there is no direct test of this. Here we assess how centrosome position relative to the cortex affects polarity establishment. We find that centrosomes can initiate polarity from any position within the embryo volume, but centrosome-cortex proximity decreases the time required to initiate polarity. Polarization itself brings about close centrosome-cortex proximity. Prior to polarization, cytoplasmic microtubules constrain centrosome movement near the cortex, expanding the controversial role of microtubules during polarity establishment. The ability of centrosomes to induce a single polarity axis from any position within the egg emphasizes the flexible, self-organizing properties of polarization in C. elegans embryos and contrasts the common view of C. elegans development as invariant.

Localized accumulation of tubulin during semi-open mitosis in the Caenorhabditis elegans embryo

The assembly of microtubules inside the cell is controlled both spatially and temporally. During mitosis, microtubule assembly must be activated locally at the nascent spindle region for mitotic spindle assembly to occur efficiently. In this paper, we report that mitotic spindle components, such as free tubulin subunits, accumulated in the nascent spindle region, independent of spindle formation in the Caenorhabditis elegans embryo. This accumulation coincided with nuclear envelope permeabilization, suggesting that permeabilization might trigger the accumulation. When permeabilization was induced earlier by knockdown of lamin, tubulin also accumulated earlier. The boundaries of the region of accumulation coincided with the remnant nuclear envelope, which remains after nuclear envelope breakdown in cells that undergo semi-open mitosis, such as those of C. elegans. Ran, a small GTPase protein, was required for tubulin accumulation. Fluorescence recovery after photobleaching analysis revealed that the accumulation was accompanied by an increase in the immobile fraction of free tubulin inside the remnant nuclear envelope. We propose that this newly identified mechanism of accumulation of free tubulin-and probably of other molecules-at the nascent spindle region contributes to efficient assembly of the mitotic spindle in the C. elegans embryo.

Cancer biology and the nuclear envelope: a convoluted relationship

Although its properties have long been used for both typing and prognosis of various tumors, the nuclear envelope (NE) itself and its potential roles in tumorigenesis are only beginning to be understood. Historically viewed as merely a protective barrier, the nuclear envelope is now linked to a wide range of functions. Nuclear membrane proteins connect the nucleus to the cytoskeleton on one side and to chromatin on the other. Several newly identified nuclear envelope functions associated with these connections intersect with cancer pathways. For example, the nuclear envelope could affect genome stability by tethering chromatin. Some nuclear envelope proteins affect cell cycle regulation by directly binding to the master regulator pRb, others by interacting with TGF-ß and Smad signaling cascades, and others by affecting the mitotic spindle. Finally, the NE directly affects cytoskeletal organization and can also influence cell migration in metastasis. In this review we discuss the link between the nuclear envelope and cellular defects that are common in cancer cells, and we show that NE proteins are often aberrantly expressed in tumors. The NE represents a potential reservoir of diagnostic and prognostic markers in cancer.

Mitotic spindle assembly around RCC1-coated beads in Xenopus egg extracts

Beads coated with the guanine nucleotide exchange factor RCC1 and a kinesin motor protein are sufficient to induce mitotic spindle assembly in Xenopus egg cytoplasm.

During cell division the genetic material on chromosomes is distributed to daughter cells by a dynamic microtubule structure called the mitotic spindle. Here we establish a reconstitution system to assess the contribution of individual chromosome proteins to mitotic spindle formation around single 10 µm diameter porous glass beads in Xenopus egg extracts. We find that Regulator of Chromosome Condensation 1 (RCC1), the Guanine Nucleotide Exchange Factor (GEF) for the small GTPase Ran, can induce bipolar spindle formation. Remarkably, RCC1 beads oscillate within spindles from pole to pole, a behavior that could be converted to a more typical, stable association by the addition of a kinesin together with RCC1. These results identify two activities sufficient to mimic chromatin-mediated spindle assembly, and establish a foundation for future experiments to reconstitute spindle assembly entirely from purified components.

The mitotic spindle is a bipolar structure that is responsible for separating the two sets of duplicated chromosomes in a dividing cell, thereby delivering one set to each of the two daughter cells. It is built from dynamic filaments called microtubules, as well as hundreds of other components that contribute to the organization and dynamics of the microtubules and to chromosome movement. To understand which proteins are essential for spindle formation and function, we would like to be able to build it from purified components. As a step towards this goal, we coupled individual proteins to inert glass beads (as a substitute for chromosomes), to determine what combination of proteins can induce spindle assembly in a complex cytoplasm derived from frog eggs. We found that a single enzyme called RCC1 is sufficient to activate a pathway that stabilizes and organizes microtubules into a bipolar structure around the bead, but that this bead then oscillated back and forth between the poles of the spindle. By coupling a microtubule-based motor protein together with RCC1 on the bead, we were able to balance the bead in the center of the spindle. Thus, two proteins immobilized on a bead can substitute for a chromosome and induce stable spindle formation.

RanGTP is required for meiotic spindle organization and the initiation of embryonic development in Drosophila

RanGTP is important for chromosome-dependent spindle assembly in Xenopus extracts. Here we report on experiments to determine the role of the Ran pathway on microtubule dynamics in Drosophila oocytes and embryos. Females expressing a dominant-negative form of Ran have fertility defects, suggesting that RanGTP is required for normal fertility. This is not, however, because of a defect in acentrosomal meiotic spindle assembly. Therefore, RanGTP does not appear to be essential or sufficient for the formation of the acentrosomal spindle. Instead, the most important function of the Ran pathway in spindle assembly appears to be in the tapering of microtubules at the spindle poles, which might be through regulation of proteins such as TACC and the HURP homolog, Mars. One consequence of this spindle organization defect is an increase in the nondisjunction of achiasmate chromosomes. However, the meiotic defects are not severe enough to cause the decreased fertility. Reductions in fertility occur because RanGTP has a role in microtubule assembly that is not directly nucleated by the chromosomes. This includes microtubules nucleated from the sperm aster, which are required for pronuclear fusion. We propose that following nuclear envelope breakdown, RanGTP is released from the nucleus and creates a cytoplasm that is activated for assembling microtubules, which is important for processes such as pronuclear fusion. Around the chromosomes, however, RanGTP might be redundant with other factors such as the chromosome passenger complex.

Molecular analysis of a ras-like nuclear (Ran) gene from Penaeus monodon and its expression at the different ovarian stages of development

In the present study, a ras-like nuclear (Ran) gene was obtained from the ovary and neurosecretory organ in eyestalk cDNA library of black tiger prawn (Penaeus monodon). The full-length black tiger prawn Ran (PmRan) cDNA consisted of 1140 nucleotides including an open reading frame (ORF) 648 bp, a 5' untranslated region (5'UTR) of 117 bp and a 3'UTR of 375 bp with a polyadenylation signal sequence "aataaa" and a poly (A) tail. The ORF encoded a peptide of 215 amino acids with molecular mass 24.6 kDa and a theoretical isoelectric point of 7.39. ScanProsite analysis indicated that PmRan protein sequence contained a small GTPase Ran family motif. Homology analysis of the deduced amino acid sequence of the PmRan with other known Ran sequences by MatGAT software revealed that the PmRan show very high homology with the sequences of other animals (92.1-98.6% similarity, 85.6-98.1% identity). Analysis of the tissue expression pattern of the PmRan gene showed that the PmRan mRNA was expressed in all tested tissues, including hepatopancreas, ovary, muscle, intestine, neurosecretory organ in eyestalk, neurosecretory organ in brain, stomach, and heart, with the highest levels in ovary. Furthermore, the PmRan expression was found to be high level in the six ovarian stages of development. The results indicated PmRan might play an important role in ovarian development.

Variations on theme: spindle assembly in diverse cells

The mitotic spindle faithfully separates the genetic material, and has been reverently observed for well over a century. Across eukaryotes, while the mechanisms for moving chromosomes seem quite conserved, mechanisms for assembling the spindle often seem distinct. Two major pathways for spindle assembly are known, one based on centrosomes and the other based on chromatin, and these pathways are usually considered to be fundamentally different. We review observations of spindle assembly in animals, fungi, and plants, and argue that microtubule assembly at a particular location, centrosomes, or chromatin, reflects contingent, cell-type specific factors, rather than reflecting a fundamental distinction in the process of spindle building. We hypothesize that the essential process for spindle assembly is the motor-driven organization of microtubules that accumulate in the form of dense bundles at or near the chromosomes.

A first-passage-time theory for search and capture of chromosomes by microtubules in mitosis

The mitotic spindle is an important intermediate structure in eukaryotic cell division, in which each of a pair of duplicated chromosomes is attached through microtubules to centrosomal bodies located close to the two poles of the dividing cell. Several mechanisms are at work toward the formation of the spindle, one of which is the 'capture' of chromosome pairs, held together by kinetochores, by randomly searching microtubules. Although the entire cell cycle can be up to 24 hours long, the mitotic phase typically takes only less than an hour. How does the cell keep the duration of mitosis within this limit? Previous theoretical studies have suggested that the chromosome search and capture is optimized by tuning the microtubule dynamic parameters to minimize the search time. In this paper, we examine this conjecture. We compute the mean search time for a single target by microtubules from a single nucleating site, using a systematic and rigorous theoretical approach, for arbitrary kinetic parameters. The result is extended to multiple targets and nucleating sites by physical arguments. Estimates of mitotic time scales are then obtained for different cells using experimental data. In yeast and mammalian cells, the observed changes in microtubule kinetics between interphase and mitosis are beneficial in reducing the search time. In Xenopus extracts, by contrast, the opposite effect is observed, in agreement with the current understanding that large cells use additional mechanisms to regulate the duration of the mitotic phase.

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.

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.

The role of RanGTP gradient in vertebrate oocyte maturation

The maturation of vertebrate oocyte into haploid gamete, the egg, consists of two specialized asymmetric cell divisions with no intervening S-phase. Ran GTPase has an essential role in relaying the active role of chromosomes in their own segregation by the meiotic process. In addition to its conserved role as a key regulator of macromolecular transport between nucleus and cytoplasm, Ran has important functions during cell division, including in mitotic spindle assembly and in the assembly of nuclear envelope at the exit from mitosis. The cellular functions of Ran are mediated by RanGTP interactions with nuclear transport receptors (NTRs) related to importin β and depend on the existence of chromosome-centered RanGTP gradient. Live imaging with FRET biosensors indeed revealed the existence of RanGTP gradient throughout mouse oocyte maturation. NTR-dependent transport of cell cycle regulators including cyclin B1, Wee2, and Cdc25B between the oocyte cytoplasm and germinal vesicle (GV) is required for normal resumption of meiosis. After GVBD in mouse oocytes, RanGTP gradient is required for timely meiosis I (MI) spindle assembly and provides long-range signal directing egg cortex differentiation. However, RanGTP gradient is not required for MI spindle migration and may be dispensable for MI spindle function in chromosome segregation. In contrast, MII spindle assembly and function in maturing mouse and Xenopus laevis eggs depend on RanGTP gradient, similar to X. laevis MII-derived egg extracts.

Several novel nuclear envelope transmembrane proteins identified in skeletal muscle have cytoskeletal associations

Nuclear envelopes from liver and a neuroblastoma cell line have previously been analyzed by proteomics; however, most diseases associated with the nuclear envelope affect muscle. To determine whether muscle has unique nuclear envelope proteins, rat skeletal muscle nuclear envelopes were prepared and analyzed by multidimensional protein identification technology. Many novel muscle-specific proteins were identified that did not appear in previous nuclear envelope data sets. Nuclear envelope residence was confirmed for 11 of these by expression of fusion proteins and by antibody staining of muscle tissue cryosections. Moreover, transcript levels for several of the newly identified nuclear envelope transmembrane proteins increased during muscle differentiation using mouse and human in vitro model systems. Some of these proteins tracked with microtubules at the nuclear surface in interphase cells and accumulated at the base of the microtubule spindle in mitotic cells, suggesting they may associate with complexes that connect the nucleus to the cytoskeleton. The finding of tissue-specific proteins in the skeletal muscle nuclear envelope proteome argues the importance of analyzing nuclear envelopes from all tissues linked to disease and suggests that general investigation of tissue differences in organellar proteomes might yield critical insights.

Subgroup II PAK-mediated phosphorylation regulates Ran activity during mitosis

Ran is an essential GTPase that controls nucleocytoplasmic transport, mitosis, and nuclear envelope formation. These functions are regulated by interaction of Ran with different partners, and by formation of a Ran-GTP gradient emanating from chromatin. Here, we identify a novel level of Ran regulation. We show that Ran is a substrate for p21-activated kinase 4 (PAK4) and that its phosphorylation on serine-135 increases during mitosis. The endogenous phosphorylated Ran and active PAK4 dynamically associate with different components of the microtubule spindle during mitotic progression. A GDP-bound Ran phosphomimetic mutant cannot undergo RCC1-mediated GDP/GTP exchange and cannot induce microtubule asters in mitotic Xenopus egg extracts. Conversely, phosphorylation of GTP-bound Ran facilitates aster nucleation. Finally, phosphorylation of Ran on serine-135 impedes its binding to RCC1 and RanGAP1. Our study suggests that PAK4-mediated phosphorylation of GDP- or GTP-bound Ran regulates the assembly of Ran-dependent complexes on the mitotic spindle.

NuMA after 30 years: the matrix revisited

The large nuclear mitotic apparatus (NuMA) protein is an abundant component of interphase nuclei and an essential player in mitotic spindle assembly and maintenance. With its partner, cytoplasmic dynein, NuMA uses its cross-linking properties to tether microtubules to spindle poles. NuMA and its invertebrate homologs play a similar tethering role at the cell cortex, thereby mediating essential asymmetric divisions during development. Despite its maintenance as a nuclear component for decades after the final mitosis of many cell types (including neurons), an interphase role for NuMA remains to be established, although its structural properties implicate it as a component of a nuclear scaffold, perhaps as a central constituent of the proposed nuclear matrix.

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.

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.

Space shuttling in the cell: nucleocytoplasmic transport and microtubule organization during the cell cycle

Microtubules form a multifunctional filamentous structure essential for the cell. In interphase, microtubules form networks in the cytoplasm and play pivotal roles in cell polarity and intracellular transport of various biomolecules. In mitosis, microtubules dramatically change their morphology to assemble the mitotic spindle, thereby pulling the chromosomes toward the spindle poles. One long-standing question is how microtubules are reorganized upon mitotic entry. Yeast cells undergo closed mitosis, in which the nuclear envelope persists, whereas higher eukaryotes undergo open mitosis, in which the nuclear envelope breaks down. Microtubule reorganization must be controlled by selective localization of microtubule-assembly factors. Recent findings in fission yeast indicate that several microtubule-associated proteins (MAPs) shuttle between the cytoplasm and the nucleus through regulation by Ran GTPase, the universal organizer of nucleocytoplasmic transport. Furthermore, the synergistic interplay of Ran and cyclin-dependent kinase (CDK) induces the critical spatiotemporal shift of modes in microtubule assembly from cytoplasmic arrays to nuclear spindles. A MAP complex Alp7/TACC-Alp14/TOG undergoes nucleocytoplasmic shuttling in interphase, whereas it is retained in the mitotic nucleus through a decrease of its nuclear export by CDK. Our understanding of how microtubules are reorganized during the cell cycle is beginning to emerge.

Structure-specific recognition protein 1 facilitates microtubule growth and bundling required for mitosis

Tight regulation of microtubule (MT) dynamics is essential for proper chromosome movement during mitosis. Here we show, using mammalian cells, that structure-specific recognition protein 1 (SSRP1) is a novel regulator of MT dynamics. SSRP1 colocalizes with the spindle and midbody MTs, and associates with MTs both in vitro and in vivo. Purified SSRP1 facilitates tubulin polymerization and MT bundling in vitro. Knockdown of SSRP1 inhibits the growth of MTs and leads to disorganized spindle structures, reduction of K-fibers and midbody fibers, disrupted chromosome movement, and attenuated cytokinesis in vivo. These results demonstrate that SSRP1 is crucial for MT growth and spindle assembly during mitosis.

Transportin regulates major mitotic assembly events: from spindle to nuclear pore assembly

Mitosis in higher eukaryotes is marked by the sequential assembly of two massive structures: the mitotic spindle and the nucleus. Nuclear assembly itself requires the precise formation of both nuclear membranes and nuclear pore complexes. Previously, importin alpha/beta and RanGTP were shown to act as dueling regulators to ensure that these assembly processes occur only in the vicinity of the mitotic chromosomes. We now find that the distantly related karyopherin, transportin, negatively regulates nuclear envelope fusion and nuclear pore assembly in Xenopus egg extracts. We show that transportin-and importin beta-initiate their regulation as early as the first known step of nuclear pore assembly: recruitment of the critical pore-targeting nucleoporin ELYS/MEL-28 to chromatin. Indeed, each karyopherin can interact directly with ELYS. We further define the nucleoporin subunit targets for transportin and importin beta and find them to be largely the same: ELYS, the Nup107/160 complex, Nup53, and the FG nucleoporins. Equally importantly, we find that transportin negatively regulates mitotic spindle assembly. These negative regulatory events are counteracted by RanGTP. We conclude that the interplay of the two negative regulators, transportin and importin beta, along with the positive regulator RanGTP, allows precise choreography of multiple cell cycle assembly events.

Dynamic release of nuclear RanGTP triggers TPX2-dependent microtubule assembly during the apoptotic execution phase

During apoptosis, the interphase microtubule network is dismantled then later replaced by a novel, non-centrosomal microtubule array. These microtubules assist in the peripheral redistribution of nuclear fragments in the apoptotic cell; however, the regulation of apoptotic microtubule assembly is not understood. Here, we demonstrate that microtubule assembly depends upon the release of nuclear RanGTP into the apoptotic cytoplasm because this process is blocked in apoptotic cells overexpressing dominant-negative GDP-locked Ran (T24N). Actin-myosin-II contractility provides the impetus for Ran release and, consequently, microtubule assembly is blocked in blebbistatin- and Y27632-treated apoptotic cells. Importantly, the spindle-assembly factor TPX2 (targeting protein for Xklp2), colocalises with apoptotic microtubules, and siRNA silencing of TPX2, but not of the microtubule motors Mklp1 and Kid, abrogates apoptotic microtubule assembly. These data provide a molecular explanation for the assembly of the apoptotic microtubule network, and suggest important similarities with the process of RanGTP- and TPX2-mediated mitotic spindle formation.

Comparative proteomic analyses of the nuclear envelope and pore complex suggests a wide range of heretofore unexpected functions

Since the discovery of several inherited diseases linked to the nuclear envelope the number of functions ascribed to this subcellular organelle has skyrocketed. However the molecular pathways underlying these functions are not clear in most cases, perhaps because of missing components. Several recent proteomic analyses of the nuclear envelope and nuclear pore complex proteomes have yielded not only enough missing components to potentially elucidate these pathways, but suggest an exponentially greater number of functions at the nuclear periphery than ever imagined. Many of these functions appear to derive from recapitulation of pathways utilized at the plasma membrane and from other membrane systems. Additionally, many proteins identified in the comparative nuclear envelope studies have sequence characteristics suggesting that they might also contribute to nuclear pore complex functions. In particular, the striking enrichment for proteins in the nuclear envelope fractions that carry phenylalanine-glycine (FG) repeats may be significant for the mechanism of nuclear transport. In retrospect, these findings are only surprising in context of the notion held for many years that the nuclear envelope was only a barrier protecting the genome. In fact, it is arguably the most complex membrane organelle in the cell.

Hepatoma up-regulated protein is required for chromatin-induced microtubule assembly independently of TPX2

The production of RanGTP around chromosomes is crucial for spindle microtubule assembly in mitosis. Previous work has shown that hepatoma up-regulated protein (HURP) is a Ran target, required for microtubule stabilization and spindle organization. Here we report a detailed analysis of HURP function in Xenopus laevis mitotic egg extracts. HURP depletion severely impairs bipolar spindle assembly around chromosomes: the few spindles that do form show a significant decrease in microtubule density at the spindle midzone. HURP depletion does not interfere with microtubule growth from purified centrosomes, but completely abolishes microtubule assembly induced by chromatin beads or RanGTP. Simultaneous depletion of the microtubule destabilizer MCAK with HURP does not rescue the phenotype, demonstrating that the effect of HURP is not to antagonize the destabilization activity of MCAK. Although the phenotype of HURP depletion closely resembles that reported for TPX2 depletion, we find no evidence that TPX2 and HURP physically interact or that they influence each other in their effects on spindle microtubules. Our data indicate that HURP and TPX2 have nonredundant functions essential for chromatin-induced microtubule assembly.

The spindle function of CDCA4

In an attempt to discover novel proteins functioning in both interphase nucleus and mitotic spindle as NuMA does, we carried out cDNA library screening with pooled autoimmune antibodies. Among positive clones we found a recently identified transcription regulatory protein (CDCA4) with the distinctive nuclear-mitotic apparatus distribution. CDCA4 localizes at metaphase spindle poles and the midzone in later stages. Additionally, an intensive CDCA4 accumulation parallel to spindle was observed in half of metaphase cells but not in later stages, implying a transient form of CDCA4 binding to midzone from anaphase. Mitotic arrest dissolved CDCA4 from centrosomes but during the spindle recovery, CDCA4 invariably colocalized with the microtubule nucleation foci as a component of microtubule organization center. RNA interference of CDCA4 resulted in significant increase of multinuclei and multipolar spindles, suggesting impaired function in chromosome segregation or cytokinesis. However, the spindle checkpoint and the centrosome cycle appeared not to be affected by such interference. Furthermore, CDCA4 depletion resulted in accelerated cell proliferation, perhaps due to the disruption of CDCA4 nuclear function as a transcription suppressor. Interphase CDCA4 is localized in nucleoli by immunofluorescence, although GFP-CDCA4 expressed in the nucleoplasm. An N-terminal KRKC domain appears to be the nuclear localization signal as identified by sequence alignment and the expression of truncated mutants. Taken together, our results suggested that as a novel nuclearmitotic apparatus protein, CDCA4 is involved in spindle organization from prometaphase. When anaphase begins, CDCA4 may play a different role as a midzone factor involved in chromosome segregation or cytokinesis.