Mitosis in Barbulanympha. II. Dynamics of a two-stage anaphase, nuclear morphogenesis, and cytokinesis

Anaphase in Barbulanympha proceeds in two discrete steps. In anaphase- A, chromosomal spindle fibers shorten and chromosomes move to the stationary centrosomes. In anaphase-B, the central spindle elongates and ("telophasic") bouquets of chromosomes, with kinetochores still connected by the shortened chromosomal fibers to the centrosomes, are moved far apart. The length, width, and birefringence of the central spindle remain unchanged throughout anaphase-A. In anaphase-B, the central spindle elongates up to fivefold. During elongation, the peripheral fibers of the central spindle splay, first anteriorly and then laterally. The remaining central spindle progressively becomes thinner and the retardation decreases; however, the coefficient of birefringence stays approximately constant. The nuclear envelope persists throughout mitosis in Barbulanympha and the nucleus undergoes an intricate morphological change. In prophase, the nucleus engulfs the spindle; in early anaphase-A, the nuclear envelope forms a seam anterior to the spindle, the nucleus thus transforms into a complete sleeve surrounding the central spindle. In late anaphase-A, the middle of the seam opens up in a cleft as the lips part; in anaphase-B, the cleft expands posteriorly, progressively exposing the central spindle. Finally, the cleft partitions the nucleus into two. The nuclear envelope shows an apparent elasticity and two-dimensional fluidity. Localized, transient deformations of the nuclear envelope indicate poleward and counter-poleward forces acting on the kinetochores embedded in the envelope. These forces appear responsible for nuclear morphogenesis as well as anaphase chromosome movement. At the end of anaphase-B, the two rostrate Barbulanympha may swim apart of be poked apart into two daughter cells by another organism cohabiting the host's hindgut.     

Centromere and kinetochore structure.

Recent studies have begun to yield some insight into the structural and regulatory components of centromeres, and new assays have been developed that promise to be of use in advancing our understanding of centromere structure and function. In the budding yeast Saccharomyces cerevisiae new proteins that are required for centromere function have been identified and an in vitro microtubule-binding assay that should assist in dissecting the process of centromere microtubule attachment has been developed. The centromere-specific DNA sequences in the fission yeast Schizosaccharomyces pombe have been identified and partially characterized. In addition, several mammalian centromere proteins have been further characterized, and localization and inhibition studies suggest roles for these proteins in the regulation and assembly of a functional kinetochore.     

Spindle and kinetochore morphology of Dictyostelium discoideum

The metaphase spindle of haploid Dictyostelium discoideum (n = 7) is 2 mum long. It consists of some 20 microtubules which seem continuous between the spindle pole bodies and there are about 20 chromosomal microtubules at each end of the spindle. During anaphase the central spindle elongates and the chromosomal microtubules shorten. The spindle length and structure at this stage suggests that lengthening is caused by elongation as well as parallel sliding of the nonchromosomal microtubules. The nuclear envelope remains mostly intact during mitosis, and nuclear separation through medial constriction takes place when the spindle is 6 mum long. Cytokinesis occurs when the spindle is 10 mum long. At that time the kinetochores double in size. During interphase, the spindle pole body separates from the nucleus to a distance of 0.7 mum, and it returns at the onset of the next prophase when it becomes functionally double, thereby starting the formation of a central spindle. When comparing mitosis in the cellular slime molds Polysphondylium violaceum and D. discoideum, several similarities and some differences are apparent.     

Regulation of Mitosis in Living Cells. I. Mitosis under Controlled Conditions

A quantitative method has been devised to study mitosis in vitro by phase contrast and polarization microscopy. Mitosis in cell-wall-free endosperm cells of Haemanthus kathrinar Baker (the African blood lily) has been divided into 18 arbitrary stages or events. The time course for the various stapes, as well as the percentage of cells that proceed from one stage to another during a four hour observation period, are presented. Cells that were in prophase when selected for study proceeded from nuclear membrane breakdown to melaphase in 60 minutes and remained in melaphase for 30 minutes. Only 13 minutes was required to proceed from onset of anaphase to mid-anaphasc. Mid-anaphase provides a clear and precise baseline for determining the time required for succeeding stages to appear. The cell plate made its appearance 40 minutes after mid-anaphase and was completely formed 20 minutes later. The nuclear membranes also became evident at this latter time and nucleoli were visible 30 minutes later. Thus, the average time for a cell observed initially in prophase to proceed from nuclear membrane breakdown to formation of two daughter cells was just over three hours. A high percentage of cells that were in late prophase or later stages of mitosis at the time of initial observation completed mitosis during the observation period. The effect of the length of time a cell is subjected to experimental conditions upon its subsequent behaviour is assessed. These results form the basis for future studies of the effects of chemicals, particularly herbicides, upon cells in mitosis as observed in vitro by phase contrast and polarization microscopy.     

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.     

Epigenetic regulation of centromere formation and kinetochore function.

In the midst of an increasingly detailed understanding of the molecular basis of genome regulation, we still only vaguely understand the relationship between molecular biochemistry and the structure of the chromatin inside of cells. The centromere is a structurally and functionally unique region of each chromosome and provides an example in which the molecular understanding far exceeds the understanding of the structure and function relationships that emerge on the chromosomal scale. The centromere is located at the primary constriction of the chromosome. During entry into mitosis, the centromere specifies the assembly site of the kinetochore, the structure that binds to microtubules to enable transport of the chromosomes into daughter cells. The epigenetic contributions to the molecular organization and function of the centromere are reviewed in the context of structural mechanisms of chromatin function.     

Mitosis puts sisters in a strained relationship: force generation at the kinetochore

During mitosis, kinetochores couple chromosomes to the dynamic tips of spindle microtubules. These attachments convert chemical energy stored in the microtubule lattice into mechanical energy, generating force to move chromosomes. In addition to mediating robust microtubule attachments, kinetochores also integrate and respond to regulatory signals that ensure the accuracy of chromosome segregation during each cell division. Signals for corrective detachment act specifically on kinetochore-microtubule attachments that fail to generate normal levels of tension, although it is unclear how tension is sensed and how the attachments are released. In this review, we discuss the mechanisms by which kinetochore-microtubule attachments generate force during chromosome biorientation, and the pathways of maturation and regulation that lead to the formation of correct attachments.     

Epigenetics regulate centromere formation and kinetochore function

The eukaryote centromere was initially defined cytologically as the primary constriction on vertebrate chromosomes and functionally as a chromosomal feature with a relatively low recombination frequency. Structurally, the centromere is the foundation for sister chromatid cohesion and kinetochore formation. Together these provide the basis for interaction between chromosomes and the mitotic spindle, allowing the efficient segregation of sister chromatids during cell division. Although centromeric (CEN) DNA is highly variable between species, in all cases the functional centromere forms in a chromatin domain defined by the substitution of histone H3 with the centromere specific H3 variant centromere protein A (CENP-A), also known as CENH3. Kinetochore formation and function are dependent on a variety of regional epigenetic modifications that appear to result in a loop chromatin conformation providing exterior CENH3 domains for kinetochore construction, and interior heterochromatin domains essential for sister chromatid cohesion. In addition pericentric heterochromatin provides a structural element required for spindle assembly checkpoint function. Advances in our understanding of CENH3 biology have resulted in a model where kinetochore location is specified by the epigenetic mark left after dilution of CENH3 to daughter DNA strands during S phase. This results in a self-renewing and self-reinforcing epigenetic state favorable to reliably mark centromere location, as well as to provide the optimal chromatin configuration for kinetochore formation and function.     

Spindle microtubules generate tension-dependent changes in the distribution of inner kinetochore proteins

The kinetochore forms a dynamic interface with microtubules from the mitotic spindle. Live-cell light microscopy-based observations on the dynamic structural changes within the kinetochore suggest that molecular rearrangements within the kinetochore occur upon microtubule interaction. However, the source of these rearrangements is still unclear. In this paper, we analyze vertebrate kinetochore ultrastructure by immunoelectron microscopy (EM) in the presence or absence of tension from spindle microtubules. We found that the inner kinetochore region defined by CENP-A, CENP-C, CENP-R, and the C-terminal domain of CENP-T is deformed in the presence of tension, whereas the outer kinetochore region defined by Ndc80, Mis12, and CENP-E is not stretched even under tension. Importantly, based on EM, fluorescence microscopy, and in vitro analyses, we demonstrated that the N and C termini of CENP-T undergo a tension-dependent separation, suggesting that CENP-T elongation is at least partly responsible for changes in the shape of the inner kinetochore.     

PRC1 Cooperates with CLASP1 to Organize Central Spindle Plasticity in Mitosis

During cell division, chromosome segregation is governed by the interaction of spindle microtubules with the kinetochore. A dramatic remodeling of interpolar microtubules into an organized central spindle between the separating chromatids is required for the initiation and execution of cytokinesis. Central spindle organization requires mitotic kinesins, microtubule-bundling protein PRC1, and Aurora B kinase complex. However, the precise role of PRC1 in central spindle organization has remained elusive. Here we show that PRC1 recruits CLASP1 to the central spindle at early anaphase onset. CLASP1 belongs to a conserved microtubule-binding protein family that mediates the stabilization of overlapping microtubules of the central spindle. PRC1 physically interacts with CLASP1 and specifies its localization to the central spindle. Repression of CLASP1 leads to sister-chromatid bridges and depolymerization of spindle midzone microtubules. Disruption of PRC1-CLASP1 interaction by a membrane-permeable peptide abrogates accurate chromosome segregation, resulting in sister chromatid bridges. These findings reveal a key role for the PRC1-CLASP1 interaction in achieving a stable anti-parallel microtubule organization essential for faithful chromosome segregation. We propose that PRC1 forms a link between stabilization of CLASP1 association with central spindle microtubules and anti-parallel microtubule elongation.To ensure that each daughter cell receives the full complement of the genome in each cell division, chromosomes move poleward, and non-kinetochore fibers become bundled at the onset of anaphase, initiating assembly of the central spindle, a set of anti-parallel microtubules that serves to concentrate key regulators of cytokinesis (1–3). Chromosomal passengers are a group of evolutionarily conserved proteins that orchestrates chromosome segregation and central spindle plasticity (4, 5). This protein complex containing Aurora B, Survivin, INCENP, and Borealin is relocated from the kinetochore to the central spindle upon anaphase onset (5–9). Perturbation of their function results in defects in metaphase chromosome alignment, chromosome segregation, and cytokinesis (10).Among the central spindle maintenance components, only two have been reported to mediate the microtubule bundling in the central spindle. One is centralspindlin, a heterotetramer containing CeMKLP1/ZEN-4 and RhoGAP/CYK-4 (11), and the other one is an evolutionarily conserved protein, PRC1 (also named Feo in fruit fry, Ase1 in yeast, and MAP65 in plant cells). PRC1 is a non-motor microtubule-binding and -bundling protein in human cells originally identified as a Cdc2 substrate essential for cytokinesis (12, 13). Similar microtubule regulatory activities have been reported in yeast, fruit fly, and plant cells. It is well known that overexpression of wild type PRC1 in HeLa cells can result in thick microtubule bundles in cells at interphase (13). Bundling activity of PRC1, as well as centralspindlin, is required for the organization of the central spindle as well as for the successful progression of cytokinesis. PRC1 molecules accumulate on the midline of a central spindle with the cell cycle progression to anaphase. As a non-motor microtubule-binding protein, transportation of PRC1 to the midline is promoted by its association to kinesin, KIF4A, and timing of this progression is controlled by the dephosphorylation of Thr-481 on PRC1 when the cell exits metaphase by phosphatase Cdc14 (14). Our recent study shows that prevention of the phosphorylation of PRC1 at Thr-470 causes an inhibition in PRC1 oligomerization in vitro and an aberrant organization of central spindle in vivo, suggesting that this phosphorylation-dependent PRC1 oligomerization ensures that central spindle assembly occurs at the appropriate time in the cell cycle (15).Spatiotemporal regulation of microtubule organization and dynamics is responsible for the mitotic apparatus such as the central spindle. However, it has remained elusive as to how the central spindle microtubule organization and dynamics are regulated. There are large varieties of microtubule-associated proteins responsible for regulation of the dynamic behavior of microtubules and microtubule-mediated transport. Among these, proteins that associate with the tips of microtubules are called +TIPs, for “plus-end tracking proteins.” These proteins have been shown to be important in different organisms and cellular systems (16). Using yeast two-hybrid assay, CLASPs were identified as interacting partners of the CLIPs and characterized as new +TIP proteins (17).The microtubule-binding protein CLASP is emerging as an important microtubule regulator in the formation of the mitotic apparatus (18–22). CLASP is required for promoting plus-end growth of spindle microtubules in prometaphase (23). Although the molecular mechanisms underlying its regulation of microtubule dynamics remain elusive, it is generally believed that CLASP orchestrates microtubule dynamics via its physical interacting with EB1, CLIP170, and microtubules (17, 24).To delineate the molecular function of PRC1 in central spindle organization and spatiotemporal regulation, we carried out a new search for PRC1-interacting proteins. Our studies show that PRC1 physically interacts with CLASP1, and the two proteins cooperate in the organization of the central spindle. Our studies provide a novel regulatory mechanism in which the PRC1 complex operates central spindle organization in mitosis.     

Properties of the kinetochore in vitro. I. Microtubule nucleation and tubulin binding

We have isolated chromosomes from Chinese hamster ovary cells arrested in mitosis with vinblastine and examined the interactions of their kinetochores with purified tubulin in vitro. The kinetochores nucleate microtubule (MT) growth with complex kinetics. After an initial lag phase, MTs are continuously nucleated with both plus and minus ends distally localized. This mixed polarity seems inconsistent with the formation of an ordered, homopolar kinetochore fiber in vivo. As isolated from vinblastine-arrested cells, kinetochores contain no bound tubulin. The kinetochores of chromosomes isolated from colcemid-arrested cells or of chromosomes incubated with tubulin in vitro are brightly stained after anti-tubulin immunofluorescence. This bound tubulin is probably not in the form of MTs. It is localized to the corona region by immunoelectron microscopy, where it may play a role in MT nucleation in vitro.     

Mitosis and autospore formation in the green algaKirchneriella lunaris

Summary Asexual reproduction inKirchneriella lunaris involves autospore formation. After an initial mitosis, the curved cell cleaves to a variable extent, and then the nuclei divide again; finally the cytoplasm is partitioned into four around each nucleus. Rudimentary centrioles appear prior to the first mitosis; centriole complexes then become associated with a developing sheath of extranuclear microtubules at prophase; fenestrae appear at the poles through which both microtubules and centrioles migrate, preceding intranuclear spindle formation. The nucleus meanwhile is enveloped by a perinuclear layer of endoplasmic reticulum which is also interposed between the golgi body and nuclear envelope. Chromosome separation is accompanied by considerable spindle elongation. Finally the reforming nuclear envelope excludes both centriole complex and interzonal spindle apparatus from daughter nuclei. Cleavage is preceded by i) nuclear movement to the cell center, ii) movement of centriole complexes around daughter nuclei until they are opposite one another, and iii) the concurrent formation of a system of transverse microtubules extending across the cell. Other microtubules encircle the cell predicting the cleavage plane. A septum then appears amongst these cytokinetic microtubules, possibly derived from the plasmalemma; it extends across the cell too, through the cleaving peripheral chloroplast. Secondary mitoses follow (as above) during which this septum may be partially resorbed. Finally this septum is reformed, if necessary, and two other septa appear (as above) to quadripartition the cell. Mitotic and cytokinetic structures in this algae are briefly compared with some others.     

Cell Division without Chromatin in Trichonympha and Barbulanympha

SUMMARY. Oxygen concentrations of 70–80 per cent of an atmosphere destroy all chromosomes of the flagellate Trichonympha provided the oxygen treatment is carried out during the early stages of gametogenesis at which time the chromosomes are in the process of duplicating themselves. This treatment does no damage to the cytoplasm and its organelles. Following the loss of chromosomes, the centrioles function in the production of the achromatic figure, the flagella, and'parabasal bodies. Then the cytoplasm divides, thus producing two anucleate gametes which make some progress in the cytoplasmic differentiations characteristic of normal male and female gametes of Trichonympha.
It is also possible, with somewhat higher concentrations of oxygen, with temperatures slightly above the freezing point and a longer period of treatment, to destroy the chromosomes of resting asexual nuclei in several genera of the flagellates that live in the roach Cryptocercus. So far as one can determine by observing organisms so treated, their cytoplasm and organelles are not injured.     

The role of Drosophila CID in kinetochore formation, cell-cycle progression and heterochromatin interactions

Centromere function requires the coordination of many processes including kinetochore assembly, sister chromatid cohesion, spindle attachment and chromosome movement. Here we show that CID, the Drosophila homologue of the CENP-A centromere-specific H3-like proteins, colocalizes with molecular-genetically defined functional centromeres in minichromosomes. Injection of CID antibodies into early embryos, as well as RNA interference in tissue-culture cells, showed that CID is required for several mitotic processes. Deconvolution fluorescence microscopy showed that CID chromatin is physically separate from proteins involved in sister cohesion (MEI-S332), centric condensation (PROD), kinetochore function (ROD, ZW10 and BUB1) and heterochromatin structure (HP1). CID localization is unaffected by mutations in mei-S332, Su(var)2-5 (HP1), prod or polo. Furthermore, the localization of POLO, CENP-meta, ROD, BUB1 and MEI-S332, but not PROD or HP1, depends on the presence of functional CID. We conclude that the centromere and flanking heterochromatin are physically and functionally separable protein domains that are required for different inheritance functions, and that CID is required for normal kinetochore formation and function, as well as cell-cycle progression.     

Spindle checkpoint signaling requires the mis6 kinetochore subcomplex, which interacts with mad2 and mitotic spindles

The spindle checkpoint coordinates cell cycle progression and chromosome segregation by inhibiting anaphase promoting complex/cyclosome until all kinetochores interact with the spindle properly. During early mitosis, the spindle checkpoint proteins, such as Mad2 and Bub1, accumulate at kinetochores that do not associate with the spindle. Here, we assess the requirement of various kinetochore components for the accumulation of Mad2 and Bub1 on the kinetochore in fission yeast and show that the necessity of the Mis6-complex and the Nuf2-complex is an evolutionarily conserved feature in the loading of Mad2 onto the kinetochore. Furthermore, we demonstrated that Nuf2 is required for maintaining the Mis6-complex on the kinetochore during mitosis. The Mis6-complex physically interacts with Mad2 under the condition that the Mad2-dependent checkpoint is activated. Ectopically expressed N-terminal fragments of Mis6 localize along the mitotic spindle, highlighting the potential binding ability of Mis6 not only to the centromeric chromatin but also to the spindle microtubules. We propose that the Mis6-complex, in collaboration with the Nuf2-complex, monitors the spindle-kinetochore attachment state and acts as a platform for Mad2 to accumulate at unattached kinetochores.     

Abnormal kinetochore structure activates the spindle assembly checkpoint in budding yeast.

Saccharomyces cerevisiae cells containing one or more abnormal kinetochores delay anaphase entry. The delay can be produced by using centromere DNA mutations present in single-copy or kinetochore protein mutations. This observation is strikingly similar to the preanaphase delay or arrest exhibited in animal cells that experience spontaneous or induced failures in bipolar attachment of one or more chromosomes and may reveal the existence of a conserved surveillance pathway that monitors the state of chromosome attachment to the spindle before anaphase. We find that three genes (MAD2, BUB1, and BUB2) that are required for the spindle assembly checkpoint in budding yeast (defined by antimicrotubule drug-induced arrest or delay) are also required in the establishment and/or maintenance of kinetochore-induced delays. This was tested in strains in which the delays were generated by limited function of a mutant kinetochore protein (ctf13-30) or by the presence of a single-copy centromere DNA mutation (CDEII delta 31). Whereas the MAD2 and BUB1 genes were absolutely required for delay, loss of BUB2 function resulted in a partial delay defect, and we suggest that BUB2 is required for delay maintenance. The inability of mad2-1 and bub1 delta mutants to execute kinetochore-induced delay is correlated with striking increases in chromosome missegregation, indicating that the delay does indeed have a role in chromosome transmission fidelity. Our results also indicated that the yeast RAD9 gene, necessary for DNA damage-induced arrest, had no role in the kinetochore-induced delays. We conclude that abnormal kinetochore structures induce preanaphase delay by activating the same functions that have defined the spindle assembly checkpoint in budding yeast.