FINE STRUCTURAL STUDIES ON THE INTERPHASE AND DIVIDING APICAL CELLS OF SPHACELARIA TRIBULOIDES (PHAEOPHYTA)1

The apical cells of Sphacelaria tribuloides Menegh. are larger than other thallus cells, contain more organelles and appear polarized. Their tip portion, where they grow, contains a well developed Golgi apparatus, abundant endoplasmic reticulum (ER) membranes, mitochondria, chloroplasts and a large number of small vacuoles. It seems likely that a continuous flow of membranous material from the ER membranes to the dictyosomes and from the latter to the plasmalemma of the extending tip portion takes place. In contrast, the basal pole possesses fewer organelles and is occupied mainly by large-sized, sometimes central vacuoles. The apical cells undergo two distinct types of highly asymmetrical differential divisions giving rise to cells of the thallus and hair initials. During the early stages of mitosis the nuclear envelope remains intact, except for fenestrated poles. Microtubules pass through the fenestrae into the nucleoplasm. During meta-phase, a typical chromosome plate is organized. The sites of attachment of spindle microtubules to the chromosomes are structurally different from the rest of the chromosomes. At late anaphase, the nuclear envelope breaks down completely. During telophase, a new membrane encloses the chromosomes which are decondensed and the nucleoli are reorganized. Cytokinesis proceeds long after mitosis at a stage in which the nuclei have increased in size and have moved farther apart. A membranous furrow develops centripetally, without the participation of microtubules. However, microtubules traverse the thin cytoplasmic strands which, in both interphase and cytokinetic cells, meander among the vacuoles of the basal pole of the cell and the internuclear space. Dictyosomes appear to be involved in the subsequent wall deposition.     

CHRONOCRISIS: When Cell Cycle Asynchrony Generates DNA Damage in Polyploid Cells

Polyploid cells contain multiple copies of all chromosomes. Polyploidization can be developmentally programmed to sustain tissue barrier function or to increase metabolic potential and cell size. Programmed polyploidy is normally associated with terminal differentiation and poor proliferation capacity. Conversely, non-programmed polyploidy can give rise to cells that retain the ability to proliferate. This can fuel rapid genome rearrangements and lead to diseases like cancer. Here, the mechanisms that generate polyploidy are reviewed and the possible challenges upon polyploid cell division are discussed. The discussion is framed around a recent study showing that asynchronous cell cycle progression (an event that is named “chronocrisis”) of different nuclei from a polyploid cell can generate DNA damage at mitotic entry. The potential mechanisms explaining how mitosis in non-programmed polyploid cells can generate abnormal karyotypes and genetic instability are highlighted.     

Dynamics of Arabidopsis SUN proteins during mitosis and their involvement in nuclear shaping

The nuclear envelope (NE) is a highly active structure with a specific set of nuclear envelope proteins acting in diverse cellular events. SUN proteins are conserved NE proteins among eukaryotes. Although they form nucleocytoplasmic linkage complexes in metazoan cells, their functions in the plant kingdom are unknown. To understand the function of plant SUN proteins, in this study we first investigated the dynamics of Arabidopsis SUN proteins during mitosis in Arabidopsis roots and cultured cells. For this purpose, we performed dual and triple visualization of these proteins, microtubules, chromosomes, and endoplasmic reticulum (ER) in cultured cells, and observed their dynamics during mitosis using a high-speed spinning disk confocal microscope. The localizations of SUN proteins changed dynamically during mitosis, tightly coupled with NE dynamics. Moreover, NE re-formation marked with SUN proteins is temporally and spatially coordinated with plant-specific microtubule structures such as phragmoplasts. Finally, the analysis with gene knockdowns of AtSUN1 and AtSUN2 indicated that they are necessary for the maintenance and/or formation of polarized nuclear shape in root hairs. These results suggest that Arabidopsis SUN proteins function in the maintenance or formation of nuclear shape as components of the nucleocytoskeletal complex.     

CELL DIVISION IN THE FILAMENTOUS PLEURASTRUM AND ITS COMPARISON WITH THE UNICELLULAR PLATYMONAS (CHLOROPHYCEAE)1

At prophase in Pleurastrum, extranuclear spindle microtubules develop from the region of centrioles, which lie lateral to the nucleus midway between the future sites of the metaphase spindle poles. The microtubules then move laterally to overarch the nucleus and finally become incorporated into the spindle. The centrioles do not migrate and therefore lie in the same plane as the chromosomes at metaphase. At telophase, 2, more different systems of microtubules develop from the vicinity of the centrioles—a phycoplast and extensive arrays of microtubules that ensheath the daughter nuclei. Cell division in the filamentous Pleurastrum is compared to that in the green flagellate, Platymonas. The similarities between cell division in the 2 algae are interpreted as evidence: (i) that rhizoplasts (which in Platymonas resemble myofibrils) are somehow homologous to microtubules; and, (ii) that cell division in Pleurastrum differs from cell division in other examined filamentous chlorophycean genera because Pleurastrum has an independent evolutionary origin from a monad with Platymonas-like characteristics.     

All together now

Maintenance of genomic stability during eukaryotic cell division relies on the Spindle Assembly Checkpoint (SAC), which has evolved as a surveillance mechanism that monitors kinetochore-microtubule attachment and prevents APC/C-mediated mitotic exit until all chromosomes are properly attached to the mitotic spindle. Reversible protein phosphorylation has long been accredited as a regulatory mechanism of the SAC. Nevertheless, knowledge of how several mitotic kinases act in concert within the signaling pathway to orchestrate SAC function is still emerging. In a recent study, we undertook a comprehensive dissection of the hierarchical framework controlling SAC function in Drosophila cells. We found that Polo lies at the top of the SAC pathway promoting the efficient recruitment of Mps1 to unattached kinetochores. This renders Mps1 fully active to control BubR1 phosphorylation that generates the 3F3/2 phosphoepitope at tensionless kinetochores. We have proposed that Polo is required for SAC function and that the molecular outcome of Mps1-dependent 3F3/2 formation is to promote the association of Cdc20 with BubR1 allowing proper kinetochore recruitment of Cdc20 and efficient assembly of the Mitotic Checkpoint Complex (MCC) required for a sustained SAC response.     

The Nuclear Apparatus of the Ditransversal Ciliate Homalozoon Vermiculare (Ciliophora, Rhabdophora) During Interphase and Division Ii. the Micronuclei and Some Observations On Conjugation

ABSTRACT. The nuclear apparatus of H. vermiculare consists of a single moniliform macronucleus and about 25 micronuclei. the micronuclei are about 3 μm in diameter and characterized by a meshwork of thick condensed chromatin. Mitosis is intranuclear and acentric as in all other ciliates. In metaphase, interpolar and chromosomal microtubules are abundant and the length of the micronuclei increases to about 5 μm. In late anaphase, interzonal microtubules become prominent and the spindle elongates to about 50 μ. In meta- and anaphase, the microtubules of the spindle are attached to the polar vesicles, and in anaphase, chromosomes become attached to it. In contrast to most other eukaryotes, micronuclear mitosis is not strictly bound to cell division in H. vermiculare. While most of the micronuclei divide prior to cytokinesis, others retain their interphasic shape or degenerate. In addition, some micronuclei divide in the interdivision period, i.e. between two successive divisions of the cell and macronucleus. Mating cells of H. vermiculare become joined to each other in the cilia-free region covering the cytostome. In the course of conjugation, the cell membranes and the underlying oral filamentous sheaths of both cells fuse, thus uniting the endoplasm of both cells in the mouth region. Synaptonemal complexes in the meiotic chromosomes are more distinct in H. vermiculare than in most other dilates. the micrographs presented here depict dearly the central filament, transverse elements, and other substructures.     

Poly(ADP-ribosyl)ation is recognized by ECT2 during mitosis

Poly(ADP-ribosyl)ation is an unique posttranslational modification and required for spindle assembly and function during mitosis. However, the molecular mechanism of poly(ADP-ribose) (PAR) in mitosis remains elusive. Here, we show the evidence that PAR is recognized by ECT2, a key guanine nucleotide exchange factor in mitosis. The BRCT domain of ECT2 directly binds to PAR both in vitro and in vivo. We further found that α-tubulin is PARylated during mitosis. PARylation of α-tubulin is recognized by ECT2 and recruits ECT2 to mitotic spindle for completing mitosis. Taken together, our study reveals a novel mechanism by which PAR regulates mitosis.     

Synergistic interactions between transforming growth factor beta and fibroblast growth factor regulate Schwann cell mitosis

Cultured Schwann cells divide in response to a limited repertoire of mitogens. In addition to cyclic AMP analogs and reagents that raise intracellular cyclic AMP, the only purified mitogens for Schwann cells are transforming growth factor beta (TGFβ), acidic (a) and basic (b) fibroblast growth factor (FGF), and the BB and AB dimers of platelet-derived growth factor (PDGF). Although individually each one of these growth factors is only weakly mitogenic, it is shown here that when TGFβ and bFGF are added to Schwann cell cultures together, they interact to produce a mitogenic response that is much greater than that produced by either growth factor alone. Both the absolute concentration of each protein and the molar ratio of TGFβ to bFGF determines the magnitude of the Schwann cell response.     

The proliferating cell marker monoclonal antibody Ki-67 recognizes specific antigens associated with the nuclear envelope of the early Drosophila embryo

Summary— Immunofluorescence and immunoelectron microscopy indicated that the antibody raised against the nuclear antigen Ki-67 of mammalian cells recognized antigenic determinants of early Drosophila embryos, localized on the outside of the nuclear envelope. Hence, the nuclear envelope of Drosophila appears to share a similar epitope with the chromosome scaffold of mitotic mammalian cells. With the progression of mitosis the antigen persisted around the mitotic spindle region and was also found in the pole regions at metaphase and anaphase. The antibody also stained the equatorial regions of the spindles from anaphase to late telophase. The antibody may therefore be used as a biochemical marker of the nuclear envelope for studying nuclear membrane biogenesis and behavior during the mitotic divisions of the Drosophila embryo.     

Characterization of vertebrate cohesin complexes and their regulation in prophase

In eukaryotes, sister chromatids remain connected from the time of their synthesis until they are separated in anaphase. This cohesion depends on a complex of proteins called cohesins. In budding yeast, the anaphase-promoting complex (APC) pathway initiates anaphase by removing cohesins from chromosomes. In vertebrates, cohesins dissociate from chromosomes already in prophase. To study their mitotic regulation we have purified two 14S cohesin complexes from human cells. Both complexes contain SMC1, SMC3, SCC1, and either one of the yeast Scc3p orthologs SA1 and SA2. SA1 is also a subunit of 14S cohesin in Xenopus. These complexes interact with PDS5, a protein whose fungal orthologs have been implicated in chromosome cohesion, condensation, and recombination. The bulk of SA1- and SA2-containing complexes and PDS5 are chromatin-associated until they become soluble from prophase to telophase. Reconstitution of this process in mitotic Xenopus extracts shows that cohesin dissociation does neither depend on cyclin B proteolysis nor on the presence of the APC. Cohesins can also dissociate from chromatin in the absence of cyclin-dependent kinase 1 activity. These results suggest that vertebrate cohesins are regulated by a novel prophase pathway which is distinct from the APC pathway that controls cohesins in yeast.