Proteome analysis of the human mitotic spindle

The accurate distribution of sister chromatids during cell division is crucial for the generation of two cells with the same complement of genetic information. A highly dynamic microtubule-based structure, the mitotic spindle, carries out the physical separation of the chromosomes to opposite poles of the cells and, moreover, determines the cell division cleavage plane. In animal cells, the spindle comprises microtubules that radiate from the microtubule organizing centers, the centrosomes, and interact with kinetochores on the chromosomes. Malfunctioning of the spindle can lead to chromosome missegregation and hence result in aneuploidy, a hallmark of most human cancers. Despite major progress in deciphering the temporal and spatial regulation of the mitotic spindle, its composition and function are not fully understood. A more complete inventory of spindle components would therefore constitute an important advance. Here we describe the purification of human mitotic spindles and their analysis by MS/MS. We identified 151 proteins previously known to associate with the spindle apparatus, centrosomes, and/or kinetochores and 644 other proteins, including 154 uncharacterized components that did not show obvious homologies to known proteins and did not contain motifs indicative of a particular localization. Of these uncharacterized proteins, 17 were tagged and localized in transfected mitotic cells, resulting in the identification of six genuine spindle components (KIAA0008, CdcA8, KIAA1187, FLJ12649, FLJ90806, and C20Orf129). This study illustrates the strength of a proteomic approach for the analysis of isolated human spindles and identifies several novel spindle components for future functional studies.     

Three-dimensional ultrastructural analysis of the Saccharomyces cerevisiae mitotic spindle

The three dimensional organization of microtubules in mitotic spindles of the yeast Saccharomyces cerevisiae has been determined by computer- aided reconstruction from electron micrographs of serially cross- sectioned spindles. Fifteen spindles ranging in length from 0.6-9.4 microns have been analyzed. Ordered microtubule packing is absent in spindles up to 0.8 micron, but the total number of microtubules is sufficient to allow one microtubule per kinetochore with a few additional microtubules that may form an interpolar spindle. An obvious bundle of about eight interpolar microtubules was found in spindles 1.3- 1.6 microns long, and we suggest that the approximately 32 remaining microtubules act as kinetochore fibers. The relative lengths of the microtubules in these spindles suggest that they may be in an early stage of anaphase, even though these spindles are all situated in the mother cell, not in the isthmus between mother and bud. None of the reconstructed spindles exhibited the uniform populations of kinetochore microtubules characteristic of metaphase. Long spindles (2.7-9.4 microns), presumably in anaphase B, contained short remnants of a few presumed kinetochore microtubules clustered near the poles and a few long microtubules extending from each pole toward the spindle midplane, where they interdigitated with their counterparts from the other pole. Interpretation of these reconstructed spindles offers some insights into the mechanisms of mitosis in this yeast.     

UV-microbeam irradiations of the mitotic spindle: spindle forces and structural analysis of lesions

Mitotic PtK1 spindles were UV irradiated (285 nm) during metaphase and anaphase between the chromosomes and the pole. The irradiation, a rectangle measuring 1.4 x 5 microns parallel to the metaphase plate, severed between 90 and 100% of spindle microtubules (MTs) in the irradiated region. Changes in organization of MTs in the irradiated region were analyzed by EM serial section analysis coupled with 3-D computer reconstruction. Metaphase cells irradiated 2 to 4 microns below the spindle pole (imaged by polarization optics) lost birefringence in the irradiated region. Peripheral spindle fibers, previously curved to focus on the pole, immediately splayed outwards when severed. We demonstrate via serial section analysis that following irradiation the lesion was devoid of MTs. Within 30 s to 1 min, recovery in live cells commenced as the severed spindle pole moved toward the metaphase plate closing the lesion. This movement was concomitant with the recovery of spindle birefringence and some of the severed fibers becoming refocused at the pole. Ultrastructurally we confirmed that this movement coincided with bridging of the lesion by MTs presumably growing from the pole. The non-irradiated half spindle also lost some birefringence and shortened until it resembled the recovered half spindle. Anaphase cells similarly irradiated did not show recovery of birefringence, and the pole remained disconnected from the remaining mitotic apparatus. Reconstructions of spindle structure confirmed that there were no MTs in the lesion which bridged the severed spindle pole with the remaining mitotic apparatus. These results suggest the existence of chromosome-to-pole spindle forces are dependent upon the existence of a MT continuum, and to a lesser extent to the loss of MT initiation capacity of the centrosome at the metaphase/anaphase transition.     

Concentrating on the mitotic spindle

In eukaryotes, the microtubule-based spindle drives chromosome segregation. In this issue, Schweizer et al. (2015; J. Cell Biol. http://dx.doi.org/10.1083/jcb.201506107) find that the spindle area is demarcated by a semipermeable organelle barrier. Molecular crowding, which is microtubule independent, causes the enrichment and/or retention of crucial factors in the spindle region. Their results add an important new feature to the models of how this structure assembles and is regulated.Mitosis is marked by the assembly of the mitotic spindle, a microtubule-based structure that facilitates accurate chromosome segregation. Many biochemical reactions are coupled to spindle assembly, from tubulin polymerization itself to the mitotic checkpoint, which inhibits chromosome disjunction until all the chromosomes are properly attached and aligned (Cleveland et al., 2003). Interestingly, these reactions are virtually unaltered over a broad morphological range; large spindles and small spindles follow roughly the same biochemical rules despite quite distinct geometries (Brown et al., 2007; Wühr et al., 2008). In this issue, Schweizer et al. report that the mitotic spindle area is delineated by membrane-bound organelles, generating a “spindle envelope” with unique molecular constituents compared with the surrounding cytoplasm. Spindle envelope–based molecular crowding provides an enticing hypothetical solution to the broad problem of confining mitotic biochemistry to a specific cellular space irrespective of cell size.Schweizer et al. (2015) used FRAP and fluorescence correlation spectroscopy (FCS) to measure the mobility of specific proteins. The authors found that tubulin and the Mad2 spindle assembly checkpoint protein were enriched in the spindle area in a microtubule polymer-independent manner. Given that the mobility of these proteins outside and within the spindle area was the same, changes in local concentration were likely a result of a barrier effect. In support of this hypothesis, modeling their FCS data with a fenestrated barrier separating the spindle area from the cytoplasm reproduced the measured FCS results. To test if a barrier surrounding the spindle area was important for spindle function, the authors disrupted the envelope area by laser microsurgery and found chromosome segregation errors consistent with defects in spindle assembly and kinetochore attachment monitoring. Thus, spindle envelope–based concentration of basal components in two critical spindle reactions, spindle assembly and mitotic checkpoint signaling, could mechanistically catalyze cell division.Molecular crowding can catalyze reactions and stabilize proteins by altering the local concentration of one or more rate-limiting components and is best characterized by membrane-bound organelles. Crowding by aggregation can greatly increase biochemical reactions without the need for a contiguous membrane (Weber and Brangwynne, 2012; Brangwynne, 2013) and this phenomenon can regulate cell cycle states (Lee et al., 2013). Here, Schweizer et al. (2015) propose that by creating a membrane-bound organelle exclusion zone, a spindle envelope could cause the molecular crowding of important spindle proteins and thereby their enrichment in the spindle area.The mitotic spindle is known to scale with cell size: smaller cells have smaller spindles (Levy and Heald, 2012). Spindle size scaling is prominent during development when repeated cell division without embryonic growth results in cells that can be several orders of magnitude smaller than that of the zygote. Recently, cytoplasm volume and tubulin concentration was shown to be an important factor in spindle size scaling; however, a curious exception to the size scaling rule is that there seems to be an upper limit to spindle size, resulting in stable spindle size when a threshold cell size is reached (Wühr et al., 2008; Good et al., 2013; Hazel et al., 2013). A spindle envelope would provide mechanisms to maintain increased local tubulin concentration independent of the absolute amount available in the cell. The net effect would be that spindle size scales in very large cells to the spindle envelope size rather than cell size in a manner analogous to chromosome size scaling to nuclear size independently of cell size (Fig. 1 A). Clearly this is a more complex problem and factors such as tubulin protein production and polymerization cofactors (such as the Tog family of proteins) clearly play an important role (Slep, 2009). However, spindle envelope–based molecular crowding could provide an elegant solution to a biochemical problem.Open in a separate windowFigure 1.Molecular crowding in the mitotic spindle. Schematic view of how a spindle envelope could mitigate spindle size scaling during development (A) or cell cycle control (B) in a common cytoplasm. (A) A spindle envelope (black dotted line) that excludes large membrane-bound organelles (yellow) could locally increase the concentration of spindle proteins (depicted as a red background) such as tubulin to control spindle size independent of cell size during early development. (B) Neighboring nuclei in a common cytoplasm (e.g., the syncytial mitotic gonad in C. elegans) could have differing mitotic states by restricting the diffusion of important regulatory proteins such as Mad2 (similar coloring as in A).A spindle envelope could also provide a cell biological solution to another developmental problem—independent cell cycle control of separate nuclei within a single cytoplasm (syncytia). For example, the mitotic region of the Caenorhabditis elegans germline contains germ cell precursors that divide independently of one another in a common cytoplasm. In some cases, two neighboring dividing nuclei can have different biochemistry, one arrested in metaphase because of a kinetochore microtubule attachment defect while the other is progressing into anaphase (Gerhold et al., 2015). By restricting the diffusive radius of signaling molecules like Mad2, a steep threshold of checkpoint activity can be maintained, allowing independent cell cycle control even in a common cytoplasm (Fig. 1 B).The mitotic spindle has long been known to exclude large membrane-bound organelles, even in the absence of microtubule polymer, leading to a hypothesized nontubulin-based “spindle matrix.” A spindle matrix would be an excellent candidate to underlie the spindle envelope. The molecular nature of a spindle matrix, however, has never been agreed upon with candidate mechanisms ranging from nonprotein macromolecules to actin (Pickett-Heaps et al., 1984; Chang et al., 2004). A convincing argument can be made that the Skeletor/Megator/Chromator proteins first identified in Drosophila melanogaster constitute a spindle matrix (Walker et al., 2000; Qi et al., 2004; Rath et al., 2004; Schweizer et al., 2014). These proteins are large and are found in the nucleus in interphase and as microtubule-independent fibrous structures in and around the spindle in mitosis. Depletion of these proteins results in mitotic errors; however, these may or may not be caused by a role as the spindle matrix.Schweizer et al. (2015) evaluated Megator (as a representative member of the complex) as a possible basis for generating the spindle envelope. FRAP and FCS showed that, like tubulin and Mad2, Megator is concentrated in the spindle envelope region independent of microtubules. However, Megator in the spindle region had slower diffusive properties compared with that around the cell periphery. Thus, unlike tubulin and Mad2, the mobility of Megator within the spindle was altered, indicating that Megator likely forms a high molecular weight complex with its binding partners Skeletor and Chromator in the spindle area, which may help form the spindle envelope.The sum of these results lead to a possible model whereby the Skeletor/Megator/Chromator proteins complex together and subsequently support a spindle envelope independent of microtubules. The spindle envelope excludes large membrane-bound organelles, leading to increased concentration of mitotic reaction constituents and thus ultimately catalyzing cell division. It will be exciting in the future to determine if the spindle area is indeed subject to molecular crowding in the purest of forms (solvent exclusion) and how this effect drives cell division.     

GSK-3beta regulates proper mitotic spindle formation in cooperation with a component of the gamma-tubulin ring complex, GCP5

Glycogen synthase kinase-3beta (GSK-3beta) is known to play a role in the regulation of the dynamics of microtubule networks in cells. Here we show the role of GSK-3beta in the proper formation of the mitotic spindles through an interaction with GCP5, a component of the gamma-tubulin ring complex (gammaTuRC). GCP5 bound directly to GSK-3beta in vitro, and their interaction was also observed in intact cells at endogenous levels. Depletion of GCP5 dramatically reduced the GCP2 and gamma-tubulin in the gammaTuRC fraction of sucrose density gradients and disrupted gamma-tubulin localization to the spindle poles in mitotic cells. GCP5 appears to be required for the formation or stability of gammaTuRC and the recruitment of gamma-tubulin to the spindle poles. A GSK-3 inhibitor not only led to the accumulation of gamma-tubulin and GCP5 at the spindle poles but also enhanced microtubule nucleation activity at the spindle poles. Depletion of GCP5 rescued this disrupted organization of spindle poles observed in cells treated with the GSK-3 inhibitor. Furthermore, the inhibition of GSK-3 enhanced the binding of gammaTuRC to the centrosome isolated from mitotic cells in vitro. Our findings suggest that GSK-3beta regulates the localization of gammaTuRC, including GCP5, to the spindle poles, thereby controlling the formation of proper mitotic spindles.     

The spindle protein CHICA mediates localization of the chromokinesin Kid to the mitotic spindle

Microtubule-based motor proteins provide essential forces for bipolar organization of spindle microtubules and chromosome movement, prerequisites of chromosome segregation during the cell cycle. Here, we describe the functional characterization of a novel spindle protein, termed "CHICA," that was originally identified in a proteomic survey of the human spindle apparatus [1]. We show that CHICA localizes to the mitotic spindle and is both upregulated and phosphorylated during mitosis. CHICA-depleted cells form shorter spindles and fail to organize a proper metaphase plate, highly reminiscent of the phenotype observed upon depletion of the chromokinesin Kid, a key mediator of polar ejection forces [2-6]. We further show that CHICA coimmunoprecipitates with Kid and is required for the spindle localization of Kid without affecting its chromosome association. Moreover, upon depletion of either CHICA or Kid (or both proteins simultaneously), chromosomes collapse onto the poles of monastrol-induced monopolar spindles. We conclude that CHICA represents a novel interaction partner of the chromokinesin Kid that is required for the generation of polar ejection forces and chromosome congression.     

Live-cell analysis of mitotic spindle formation in taxol-treated cells

Taxol functions to suppress the dynamic behavior of individual microtubules, and induces multipolar mitotic spindles. However, little is known about the mechanisms by which taxol disrupts normal bipolar spindle assembly in vivo. Using live imaging of GFP-alpha tubulin expressing cells, we examined spindle assembly after taxol treatment. We find that as taxol-treated cells enter mitosis, there is a dramatic re-distribution of the microtubule network from the centrosomes to the cell cortex. As they align there, the cortical microtubules recruit NuMA to their embedded ends, followed by the kinesin motor HSET. These cortical microtubules then bud off to form cytasters, which fuse into multipolar spindles. Cytoplasmic dynein and dynactin do not re-localize to cortical microtubules, and disruption of dynein/dynactin interactions by over-expression of p50 "dynamitin" does not prevent cytaster formation. Taxol added well before spindle poles begin to form induces multipolarity, but taxol added after nascent spindle poles are visible-but before NEB is complete-results in bipolar spindles. Our results suggest that taxol prevents rapid transport of key components, such as NuMA, to the nascent spindle poles. The net result is loss of mitotic spindle pole cohesion, microtubule re-distribution, and cytaster formation.     

PRC1 is a microtubule binding and bundling protein essential to maintain the mitotic spindle midzone

Midzone microtubules of mammalian cells play an essential role in the induction of cell cleavage, serving as a platform for a number of proteins that play a part in cytokinesis. We demonstrate that PRC1, a mitotic spindle-associated Cdk substrate that is essential to cell cleavage, is a microtubule binding and bundling protein both in vivo and in vitro. Overexpression of PRC1 extensively bundles interphase microtubules, but does not affect early mitotic spindle organization. PRC1 contains two Cdk phosphorylation motifs, and phosphorylation is possibly important to mitotic suppression of bundling, as a Cdk phosphorylation-null mutant causes extensive bundling of the prometaphase spindle. Complete suppression of PRC1 by siRNA causes failure of microtubule interdigitation between half spindles and the absence of a spindle midzone. Truncation mutants demonstrate that the NH2-terminal region of PRC1, rich in alpha-helical sequence, is important for localization to the cleavage furrow and to the center of the midbody, whereas the central region, with the highest sequence homology between species, is required for microtubule binding and bundling activity. We conclude that PRC1 is a microtubule-associated protein required to maintain the spindle midzone, and that distinct functions are associated with modular elements of the primary sequence.     

gamma-tubulin plays an essential role in the coordination of mitotic events

Recent data from multiple organisms indicate that gamma-tubulin has essential, but incompletely defined, functions in addition to nucleating microtubule assembly. To investigate these functions, we examined the phenotype of mipAD159, a cold-sensitive allele of the gamma-tubulin gene of Aspergillus nidulans. Immunofluorescence microscopy of synchronized material revealed that at a restrictive temperature mipAD159 does not inhibit mitotic spindle formation. Anaphase A was inhibited in many nuclei, however, and after a slight delay in mitosis (approximately 6% of the cell cycle period), most nuclei reentered interphase without dividing. In vivo observations of chromosomes at a restrictive temperature revealed that mipAD159 caused a failure of the coordination of late mitotic events (anaphase A, anaphase B, and chromosomal disjunction) and nuclei reentered interphase quickly even though mitosis was not completed successfully. Time-lapse microscopy also revealed that transient mitotic spindle abnormalities, in particular bent spindles, were more prevalent in mipAD159 strains than in controls. In experiments in which microtubules were depolymerized with benomyl, mipAD159 nuclei exited mitosis significantly more quickly (as judged by chromosomal condensation) than nuclei in a control strain. These data reveal that gamma-tubulin has an essential role in the coordination of late mitotic events, and a microtubule-independent function in mitotic checkpoint control.     

Bi-orienting chromosomes on the mitotic spindle

For the proper segregation of sister chromatids before cell division, each sister kinetochore must attach to microtubules that extend to opposite spindle poles. This process is called bipolar microtubule attachment or chromosome bi-orientation. The mechanism for chromosome bi-orientation lies at the heart of chromosome segregation, but is still poorly understood. Recent studies suggest that cells can promote bi-orientation by re-orienting kinetochore-spindle pole connections.     

Chromosome bi-orientation on the mitotic spindle

For proper chromosome segregation, sister kinetochores must attach to microtubules extending from opposite spindle poles prior to anaphase onset. This state is called sister kinetochore bi-orientation or chromosome bi-orientation. The mechanism ensuring chromosome bi-orientation lies at the heart of chromosome segregation, but is still poorly understood. Recent evidence suggests that mal-oriented kinetochore-to-pole connections are corrected in a tension-dependent mechanism. The cohesin complex and the Ipl1/Aurora B protein kinase seem to be key regulators for this correction. In this article, I discuss how cells ensure sister kinetochore bi-orientation for all chromosomes, mainly focusing on our recent findings in budding yeast.     

Polarity of microtubules of the mitotic spindle

Microtubules are polar structures that grow preferentially at one end. Measurement of their rate of directional growth can be used as a polarity indicator to determine their orientation with respect to a nucleation site. The results are interpreted to signify that the microtubules originating from the centrosomes and chromosomes of the mitotic spindle are antiparallel to each other.     

Mechanisms and molecules of the mitotic spindle

In all eukaryotes, morphogenesis of the microtubule cytoskeleton into a bipolar spindle is required for the faithful transmission of the genome to the two daughter cells during division. This process is facilitated by the intrinsic polarity and dynamic properties of microtubules and involves many proteins that modulate microtubule organization and stability. Recent work has begun to uncover the molecular mechanisms behind these dynamic events. Here we describe current models and discuss some of the complex repertoire of factors required for spindle assembly and chromosome segregation.     

Molecular components of the mitotic spindle.

Mitotic spindles constitute the machinery responsible for equidistribution of the genetic material into each daughter cell during cell division. They are transient and hence quite labile structures, changing their morphology even while performing their function. Biochemical, immunological and genetic analyses of mitotic cells have allowed us to identify a variety of molecules that are recruited to form the spindle at the onset of mitosis. Evaluation of the roles of these molecules in both the formation and in the dynamics of spindle microtubules should be important for understanding the molecular basis of mitosis and its regulation. We have recently identified a novel mitosis-specific microtubule-associated protein (MAP) using a monoclonal antibody probe raised against the mitotic spindles isolated from cultured mammalian cells. This 95/105 kDa antigen represents a unique component of the spindle distinct from any of the other MAPs reported so far. Antibody microinjection resulted in mitotic inhibition in a stage-specific and dose-dependent manner, indicating that the protein is an essential spindle component.     

Mitotic membrane helps to focus and stabilize the mitotic spindle

During mitosis, microtubules (MTs), aided by motors and associated proteins, assemble into a mitotic spindle. Recent evidence supports the notion that a membranous spindle matrix aids spindle formation; however, the mechanisms by which the matrix may contribute to spindle assembly are unknown. To search for a mechanism by which the presence of a mitotic membrane might help spindle morphology, we built a computational model that explores the interactions between these components. We show that an elastic membrane around the mitotic apparatus helps to focus MT minus ends and provides a resistive force that acts antagonistically to plus-end-directed MT motors such as Eg5.