Antiparallel microtubule interactions: spindle formation and anaphase B

Measuring microtubule polarity in spindles with second-harmonic generation

The spatial organization of microtubule polarity, and the interplay between microtubule polarity and protein localization, is thought to be crucial for spindle assembly, anaphase, and cytokinesis, but these phenomena remain poorly understood, in part due to the difficulty of measuring microtubule polarity in spindles. We develop and implement a method to nonperturbatively and quantitatively measure microtubule polarity throughout spindles using a combination of second-harmonic generation and two-photon fluorescence. We validate this method using computer simulations and by comparison to structural data on spindles obtained from electron tomography and laser ablation. This method should provide a powerful tool for studying spindle organization and function, and may be applicable for investigating microtubule polarity in other systems.

Motor proteins in cell division

The movements of eukaryotic cell division depend upon the conversion of chemical energy into mechanical work, which in turn involves the actions of motor proteins, molecular transducers that generate force and motion relative cytoskeletal elements. In animal cells, microtubule-based motor proteins of the mitotic apparatus are involved in segregating chromosomes and perhaps in organizing the mitotic apparatus itself, while microfilament-based motors in the contractile ring generate the forces that separate daughter cells during cytokinesis. This review outlines recent advances in our understanding of the roles of molecular motors in mitosis and cytokinesis.

Behavior of sex chromosomes, autosomes, and the spindle during nonrandom segregation in a flea beetle

We have analyzed autosome, sex chromosome, and spindle behavior in spermatocytes of the flea beetle, Alagoasa bicolor. In this species, males have very large X and Y chromosomes, which, although they are never physically connected, always segregate to opposite spindle poles at anaphase I, thus preserving the sex ratio in the next generation. We find that the sex chromosomes are partitioned to a peripheral spindle domain early in prometaphase I and that their segregation can be accounted for mainly by their reorientation from the parallel to the linear configuration, and little by chromosome-to-pole movement. Further, the behavior of the autosomes and that of the sex chromosomes seem to have little to do with each other. Spindle elongation is minimal; barely segregating the large sex chromosomes into the daughter cells at telophase I.Key words: nonrandom segregation, sex chromosomes, kinetochores.

Spindle assembly and mitosis without centrosomes in parthenogenetic Sciara embryos

In Sciara, unfertilized embryos initiate parthenogenetic development without centrosomes. By comparing these embryos with normal fertilized embryos, spindle assembly and other microtubule-based events can be examined in the presence and absence of centrosomes. In both cases, functional mitotic spindles are formed that successfully proceed through anaphase and telophase, forming two daughter nuclei separated by a midbody. The spindles assembled without centrosomes are anastral, and it is likely that their microtubules are nucleated at or near the chromosomes. These spindles undergo anaphase B and successfully segregate sister chromosomes. However, without centrosomes the distance between the daughter nuclei in the next interphase is greatly reduced. This suggests that centrosomes are required to maintain nuclear spacing during the telophase to interphase transition. As in Drosophila, the initial embryonic divisions of Sciara are synchronous and syncytial. The nuclei in fertilized centrosome-bearing embryos maintain an even distribution as they divide and migrate to the cortex. In contrast, as division proceeds in embryos lacking centrosomes, nuclei collide and form large irregularly shaped nuclear clusters. These nuclei are not evenly distributed and never successfully migrate to the cortex. This phenotype is probably a direct result of a failure to form astral microtubules in parthenogenetic embryos lacking centrosomes. These results indicate that the primary function of centrosomes is to provide astral microtubules for proper nuclear spacing and migration during the syncytial divisions. Fertilized Sciara embryos produce a large population of centrosomes not associated with nuclei. These free centrosomes do not form spindles or migrate to the cortex and replicate at a significantly reduced rate. This suggests that the centrosome must maintain a proper association with the nucleus for migration and normal replication to occur.

CDK1 inactivation regulates anaphase spindle dynamics and cytokinesis in vivo

Through association with CDK1, cyclin B accumulation and destruction govern the G2/M/G1 transitions in eukaryotic cells. To identify CDK1 inactivation-dependent events during late mitosis, we expressed a nondestructible form of cyclin B (cyclin BDelta90) by microinjecting its mRNA into prometaphase normal rat kidney cells. The injection inhibited chromosome decondensation and nuclear envelope formation. Chromosome disjunction occurred normally, but anaphase-like movement persisted until the chromosomes reached the cell periphery, whereupon they often somersaulted and returned to the cell center. Injection of rhodamine-tubulin showed that this movement occurred in the absence of a central anaphase spindle. In 82% of cells cytokinesis was inhibited; the remainder split themselves into two parts in a process reminiscent of Dictyostelium cytofission. In all cells injected, F-actin and myosin II were diffusely localized with no detectable organization at the equator. Our results suggest that a primary effect of CDK1 inactivation is on spindle dynamics that regulate chromosome movement and cytokinesis. Prolonged CDK1 activity may prevent cytokinesis through inhibiting midzone microtubule formation, the behavior of proteins such as TD60, or through the phosphorylation of myosin II regulatory light chain.

Tubulin transport in neurons

A question of broad importance in cellular neurobiology has been, how is microtubule cytoskeleton of the axon organized? It is of particular interest because of the history of conflicting results concerning the form in which tubulin is transported in the axon. While many studies indicate a stationary nature of axonal microtubules, a recent series of experiments reports that microtubules are recruited into axons of neurons grown in the presence of a microtubule-inhibitor, vinblastine (Baas, P.W., and F.J. Ahmad. 1993.J. Cell Biol. 120:1427-1437: Ahmad F.J., and P.W. Baas. 1995. J. Cell Sci, 108:2761-2769; Sharp, D.J., W. Yu, and P.W. Baas. 1995. J. Cell Biol, 130:93-103; Yu, W., and P.W. Baas. 1995. J. Neurosci. 15:6827-6833.). Since vinblastine stabilizes bulk microtubule-dynamics in vitro, it was concluded that preformed microtubules moved into newly grown axons. By visualizing the polymerization of injected fluorescent tubulin, we show that substantial microtubule polymerization occurs in neurons grown at reported vinblastine concentrations. Vinblastine inhibits, in a concentration-dependent manner, both neurite outgrowth and microtubule assembly. More importantly, the neuron growth conditions of low vinblastine concentration allowed us to visualize the footprints of the tubulin wave as it polymerized and depolymerized during its slow axonal transport. In contrast, depolymerization resistant fluorescent microtubules did not move when injected in neurons. We show that tubulin subunits, not microtubules, are the primary form of tubulin transport in neurons.

Interpolar spindle microtubules in PTK cells

Spindle microtubules (MTs) in PtK1 cells, fixed at stages from metaphase to telophase, have been reconstructed using serial sections, electron microscopy, and computer image processing. We have studied the class of MTs that form an interdigitating system connecting the two spindle poles (interpolar MTs or ipMTs) and their relationship to the spindle MTs that attach to kinetochores (kMTs). Viewed in cross section, the ipMTs cluster with antiparallel near neighbors throughout mitosis; this bundling becomes much more pronounced as anaphase proceeds. While the minus ends of most kMTs are near the poles, those of the ipMTs are spread over half of the spindle length, with at least 50% lying > 1.5 microns from the poles. Longitudinal views of the ipMT bundles demonstrate a major rearrangement of their plus ends between mid- and late anaphase B. However, the minus ends of these MTs do not move appreciably farther from the spindle midplane, suggesting that sliding of these MTs contributes little to anaphase B. The minus ends of ipMTs are markedly clustered in the bundles of kMTs throughout anaphase A. These ends lie close to kMTs much more frequently than would be expected by chance, suggesting a specific interaction. As sister kinetochores separate and kMTs shorten, the minus ends of the kMTs remain associated with the spindle poles, but the minus ends of many ipMTs are released from the kMT bundles, allowing the spindle pole and the kMTs to move away from the ipMTs as the spindle elongates.

Kinesin-like molecules involved in spindle formation

To study the possible involvement of kinesin-like molecules in mitosis a polyclonal antibody against the head domain of Drosophila kinesin heavy chain (HD antibody) was microinjected into PtK1 cells at the prophase-prometaphase transition. Progress of the cell through mitosis was recorded for subsequent detailed analysis. Cells injected with pre-immune IgG progressed through mitosis at rates similar to those for noninjected cells. After HD antibody injections, chromosomes failed to congress to an equatorial plane and cells failed to form a bipolar spindle. Rather, the spindle poles came together, resulting in a monopolar-like configuration with chromosomes arranged about the poles in a rosette. Sometimes the monopolar array moved to the margin of the cell in a way similar to anaphase B movement in normal cells. Antibody-injected cells progressed into the next cell cycle as evidenced by chromosome decondensation and nuclear envelope reformation. Anti-tubulin immunofluorescence confirmed the presence of a radial monopolar array of microtubules in injected cells. HD antibody stained in a punctate pattern in interphase and the spindle region in mitotic PtK1 cells. The antibody also reacted with spindle fibers of isolated mitotic CHO spindles and with kinetochores of isolated CHO chromosomes. Immunoblotting indicated that the major component recognized by the antibody is the 120 kDa kinesin heavy chain. At higher protein loads the antibody recognized also a 34 kDa polypeptide in PtK1 cell extracts, a 135 kDa polypeptide in a preparation of CHO spindles and a 300 kDa polypeptide in a preparation of CHO mitotic chromosomes. We conclude that a kinesin-like molecule is important for the formation and/or maintenance of the structure of mitotic spindle.

Inhibition of anaphase spindle elongation in vitro by a peptide antibody that recognizes kinesin motor domain

Isolated central spindles or spindles in detergent-permeabilized cells from the diatom Cylindrotheca fusiformis can undergo ATP-dependent reactivation of spindle elongation in vitro. We have used a peptide antibody raised against a 10-amino acid portion common to the kinesin superfamily motor domain to look for kinesin-like motor activity during anaphase B of mitosis. The peptide antibody localizes to central spindles. Upon ATP reactivation of spindle elongation, antigens recognized by the antibody are associated exclusively with the central spindle midzone where antiparallel microtubules of each half-spindle overlap. The antibody recognizes several polypeptides by immunoblot using isolated spindle extracts. One of these polypeptides behaves like kinesin with respect to nucleotide-specific binding to and release from taxol-stabilized microtubules. Preincubation of the spindle model with the peptide antibody inhibits subsequent ATP reactivation of spindle elongation. Coincubation of the peptide antibody with peptide antigen rescues spindle function. These results support a role for kinesin-related protein(s) in spindle elongation (anaphase B) of mitosis and suggest that one or several polypeptides that we have identified in spindle extracts may fulfill this function.

The force-producing mechanism for centrosome separation during spindle formation in vertebrates is intrinsic to each aster

A popular hypothesis for centrosome separation during spindle formation and anaphase is that pushing forces are generated between interacting microtubules (MTs) of opposite polarity, derived from opposing centrosomes. However, this mechanism is not consistent with the observation that centrosomes in vertebrate cells continue to separate during prometaphase when their MT arrays no longer overlap (i.e., during anaphase-like prometaphase). To evaluate whether centrosome separation during prophase/prometaphase, anaphase-like prometaphase and anaphase is mediated by a common mechanism we compared their behavior in vivo at a high spatial and temporal resolution. We found that the two centrosomes possess a considerable degree of independence throughout all stages of separation, i.e., the direction and migration rate of one centrosome does not impart a predictable behavior to the other, and both exhibit frequent and rapid (4-6 microns/min) displacements toward random points within the cell including the other centrosome. The kinetic behavior of individual centrosomes as they separate to form the spindle is the same whether or not their MT arrays overlap. The characteristics examined include, e.g., total displacement per minute, the vectorial rate of motion toward and away from the other centrosome, the frequency of toward and away motion as well as motion not contributing to separation, and the rate contributed by each centrosome to the separation process. By contrast, when compared with prometaphase, anaphase centrosomes separated at significantly faster rates even though the average vectorial rate of motion away from the other centrosome was the same as in prophase/prometaphase. The difference in separation rates arises because anaphase centrosomes spend less time moving toward one another than in prophase/prometaphase, and at a significantly slower rate. From our data we conclude that the force for centrosome separation during vertebrate spindle formation is not produced by MT-MT interactions between opposing asters, i.e., that the mechanism is intrinsic to each aster. Our results also strongly support the contention that forces generated independently by each aster also contribute substantially to centrosome separation during anaphase, but that the process is modified by interactions between opposing astral MTs in the interzone.

Mitosis: spindle assembly and chromosome motion

New studies on mitosis demonstrate the complexity of interactions that contribute to chromosome motion and spindle assembly. Genetic and immunological approaches reveal the requirement for kinesin-related proteins during cell division in diverse cells. Observations of the dynamic behavior of microtubules demonstrate that their disassembly can produce sufficient force to move chromosomes in vitro, that their poleward movement, or flux, contributes to anaphase motion, and that the direction of anaphase motion can be reversed by induction of kinetochore microtubule elongation.

Requirement for ESP1 in the nuclear division of Saccharomyces cerevisiae

Mutations in the ESP1 gene of Saccharomyces cerevisiae disrupt normal cell-cycle control and cause many cells in a mutant population to accumulate extra spindle pole bodies. To determine the stage at which the esp1 gene product becomes essential for normal cell-cycle progression, synchronous cultures of ESP1 mutant cells were exposed to the nonpermissive temperature for various periods of time. The mutant cells retained viability until the onset of mitosis, when their viability dropped markedly. Examination of these cells by fluorescence and electron microscopy showed the first detectable defect to be a structural failure in the spindle. Additionally, flow cytometric analysis of DNA content demonstrated that massive chromosome missegregation accompanied this failure of spindle function. Cytokinesis occurred despite the aberrant nuclear division, which often resulted in segregation of both spindle poles to the same cell. At later times, the missegregated spindle pole bodies entered a new cycle of duplication, thereby leading to the accumulation of extra spindle pole bodies within a single nucleus. The DNA sequence predicts a protein product similar to those of two other genes that are also required for nuclear division: the cut1 gene of Schizosaccharomyces pombe and the bimB gene of Aspergillus nidulans.

Physiological evidence for involvement of a kinesin-related protein during anaphase spindle elongation in diatom central spindles

We have developed a new model system for studying spindle elongation in vitro using the pennate, marine diatom Cylindrotheca fusiformis. C. fusiformis can be grown in bulk to high densities while in log phase growth and synchronized by a simple light/dark regime. Isolated spindles can be attained in quantities sufficient for biochemical analysis and spindle tubulin is approximately 5% of the total protein present. The spindle isolation procedure results in a 10-fold enrichment of diatom tubulin and a calculated 40-fold increase in spindle protein. Isolated spindles or spindles in permeabilized cells can elongate in vitro by the same mechanism and with the same pharmacological sensitivities as described for other anaphase B models (Cande and McDonald, 1986; Masuda et al., 1990). Using this model, in vitro spindle elongation rate profiles were developed for a battery of nucleotide triphosphates and ATP analogs. The relative rates of spindle elongation produced by various nucleotide triphosphates parallel relative rates seen for kinesin-based motility in microtubule gliding assays. Likewise ATP analogs that allow discrimination between myosin-, dynein-, and kinesin-mediated motility produce relative spindle elongation rates characteristic of kinesin motility. Also, isolated spindle fractions are enriched for a kinesin related protein as identified by a peptide antibody against a conserved region of the kinesin superfamily. These data suggest that kinesin-like motility contributes to spindle elongation during anaphase B of mitosis.

Astral microtubules are not required for anaphase B in Saccharomyces cerevisiae

tub2-401 is a cold-sensitive allele of TUB2, the sole gene encoding beta-tubulin in the yeast, Saccharomyces cerevisiae. At 18 degrees C, tub2-401 cells are able to assemble spindle microtubules but lack astral microtubules. Under these conditions, movement of the spindle to the bud neck is blocked. However, spindle elongation and chromosome separation are unimpeded and occur entirely within the mother cell. Subsequent cytokinesis produces one cell with two nuclei and one cell without a nucleus. The anucleate daughter can not bud. The binucleate daughter proceeds through another cell cycle to produce a cell with four nuclei and another anucleate cell. With additional time in the cold, the number of nuclei in the nucleated cells continues to increase and the percentage of anucleate cells in the population rises. The results indicate that astral microtubules are needed to position the spindle in the bud neck but are not required for spindle elongation at anaphase B. In addition, cell cycle progression does not depend on the location or orientation of the spindle.

Kinesin-related proteins required for structural integrity of the mitotic spindle

For S. cerevisiae cells, the assembly of a bipolar mitotic spindle requires the action of either Cin8p or Kip1p, gene products related to the mechanochemical enzyme kinesin. In this paper we demonstrate that the activity of either one of these proteins is also required following spindle assembly. When their function was eliminated, preanaphase bipolar spindles rapidly collapsed, with previously separated poles being drawn together. In contrast, anaphase spindles were apparently resistant to collapse. Deletion of kinesin-related KAR3 partially suppressed the phenotypes associated with loss of Cin8p/Kip1p function. Our findings suggest that the structure of the preanaphase bipolar spindle is maintained by counteracting forces produced by kinesin-related proteins.

Kinetochore microtubules in PTK cells

We have analyzed the fine structure of 10 chromosomal fibers from mitotic spindles of PtK1 cells in metaphase and anaphase, using electron microscopy of serial thin sections and computer image processing to follow the trajectories of the component microtubules (MTs) in three dimensions. Most of the kinetochore MTs ran from their kinetochore to the vicinity of the pole, retaining a clustered arrangement over their entire length. This MT bundle was invaded by large numbers of other MTs that were not associated with kinetochores. The invading MTs frequently came close to the kinetochore MTs, but a two-dimensional analysis of neighbor density failed to identify any characteristic spacing between the two MT classes. Unlike the results from neighbor density analyses of interzone MTs, the distributions of spacings between kinetochore MTs and other spindle MTs revealed no evidence for strong MT-MT interactions. A three-dimensional analysis of distances of closest approach between kinetochore MTs and other spindle MTs has, however, shown that the most common distances of closest approach were 30-50 nm, suggesting a weak interaction between kinetochore MTs and their neighbors. The data support the ideas that kinetochore MTs form a mechanical connection between the kinetochore and the pericentriolar material that defines the pole, but that the mechanical interactions between kinetochore MTs and other spindle MTs are weak.

Two Saccharomyces cerevisiae kinesin-related gene products required for mitotic spindle assembly

Two Saccharomyces cerevisiae genes, CIN8 and KIP1 (a.k.a. CIN9), were identified by their requirement for normal chromosome segregation. Both genes encode polypeptides related to the heavy chain of the microtubule-based force-generating enzyme kinesin. Cin8p was found to be required for pole separation during mitotic spindle assembly at 37 degrees C, although overproduced Kip1p could substitute. At lower temperatures, the activity of at least one of these proteins was required for cell viability, indicating that they perform an essential but redundant function. Cin8p was observed to be a component of the mitotic spindle, colocalizing with the microtubules that lie between the poles. Taken together, these findings suggest that these proteins interact with spindle microtubules to produce an outwardly directed force acting upon the poles.

Poleward microtubule flux mitotic spindles assembled in vitro

In the preceding paper we described pathways of mitotic spindle assembly in cell-free extracts prepared from eggs of Xenopus laevis. Here we demonstrate the poleward flux of microtubules in spindles assembled in vitro, using a photoactivatable fluorescein covalently coupled to tubulin and multi-channel fluorescence videomicroscopy. After local photoactivation of fluorescence by UV microbeam, we observed poleward movement of fluorescein-marked microtubules at a rate of 3 microns/min, similar to rates of chromosome movement and spindle elongation during prometaphase and anaphase. This movement could be blocked by the addition of millimolar AMP-PNP but was not affected by concentrations of vanadate up to 150 microM, suggesting that poleward flux may be driven by a microtubule motor similar to kinesin. In contrast to previous results obtained in vivo (Mitchison, T. J. 1989. J. Cell Biol. 109:637-652), poleward flux in vitro appears to occur independently of kinetochores or kinetochore microtubules, and therefore may be a general property of relatively stable microtubules within the spindle. We find that microtubules moving towards poles are dynamic structures, and we have estimated the average half-life of fluxing microtubules in vitro to be between approximately 75 and 100 s. We discuss these results with regard to the function of poleward flux in spindle movements in anaphase and prometaphase.