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PubMed1999

Distinct parameters are involved in controlling the number of rounds of cell division in each tissue during ascidian embryogenesis.

We counted cell numbers during embryogenesis of the ascidian, Halocynthia roretzi, every hour. Cell numbers were determined by counting the numbers of nuclei in squashed embryos. The cell number of a larva just after hatching was approximately 3000. Our study addresses the question of what factors control the number of rounds of cell division during development. Three kinds of egg fragments were prepared by cutting unfertilized eggs to alter the volume of cytoplasm and the amount of DNA. After the egg fragments were fertilized, the cell numbers were estimated at the hatching stage. The cell numbers of the resulting larvae differed from those of normal larvae. Precursor blastomeres of various tissues were then isolated from normal and manipulated embryos, and cultured as partial embryos. The cell numbers of the resulting partial embryos were counted to estimate the number of cell divisions in each larval tissue. The results suggested that the number of cell divisions is controlled by a distinct mechanism in each tissue. We propose that the number of rounds of cell division during ascidian embryogenesis is controlled by three mechanisms: the first depending on the volume of cytoplasm; the second on the nucleo-cytoplasmic ratio; and the third depending on neither of these parameters. J. Exp. Zool. 284:379-391, 1999.

Animals↗
PubMed1984

Altered cell-division rates after laser-induced cell fusion in nematode embryos.

In embryos of Caenorhabditis elegans adjacent cells belonging to different cell lines have been fused by disrupting the cell membrane between them with a single laser pulse. This leads to cytoplasmic mixing. Synchronous mitoses of the two nuclei result in the direct formation of four cells. Subsequent alteration of the cell cycle rhythm in some specific--but not all--descendants of the fused cells is observed, leading to abnormal cell patterns and often embryonic arrest.

Animals↗
PubMed1983

Bombesin stimulation of DNA synthesis and cell division in cultures of Swiss 3T3 cells.

Bombesin is shown to be a potent mitogen for Swiss 3T3 cells. At nanomolar concentrations the peptide markedly enhances the ability of fresh serum to stimulate DNA synthesis in confluent and quiescent cultures of these cells. In the presence of a low concentration (3.5%) of serum, bombesin stimulates 3T3 cell proliferation. In serum-free medium, bombesin induces DNA synthesis in the absence of any other added growth factor; half-maximal effect is obtained at 1 nM. The mitogenic effect of bombesin is dependent on dose and time, is mimicked by litorin, and is markedly potentiated by insulin, colchicine, platelet-derived growth factor, and fibroblast-derived growth factor. These mitogens increase the maximal response elicited by bombesin and decrease the bombesin concentration required to produce half-maximal effect (from 1 nM to 0.3 nM). In contrast, vasopressin, phorbol esters, or cAMP increasing agents fail to enhance the maximal level of DNA synthesis induced by bombesin. Bombesin and litorin may provide useful model peptides for studies on the mechanism(s) by which extracellular ligands control cell proliferation.

Animals↗
PubMed2003

Dehydroascorbate uptake activity correlates with cell growth and cell division of tobacco bright yellow-2 cell cultures.

Recently, ascorbate (ASC) concentration and the activity of a number of enzymes from the ASC metabolism have been proven to correlate with differences in growth or cell cycle progression. Here, a possible correlation between growth and the activity of a plasma membrane dehydroascorbate (DHA) transporter was investigated. Protoplasts were isolated from a tobacco (Nicotiana tabacum) Bright Yellow-2 cell culture at different intervals after inoculation and the activity of DHA transport was tested with (14)C-labeled ASC. Ferricyanide (1 mM) or dithiothreitol (1 mM) was included in the test to keep the external (14)C-ASC in its oxidized respectively reduced form. Differential uptake activity was observed, correlating with growth phases of the cell culture. Uptake of DHA in cells showed a peak in exponential growth phase, whereas uptake in the presence of dithiothreitol did not. The enhanced DHA uptake was not due to higher endogenous ASC levels that are normally present in exponential phase because preloading of protoplasts of different ages did not affect DHA uptake. Preloading was achieved by incubating cells before protoplastation for 4 h in a medium supplemented with 1 mM DHA. In addition to testing cells at different growth phases, uptake of DHA into the cells was also followed during the cell cycle. An increase in uptake activity was observed during M phase and the M/G1 transition. These experiments are the first to show that DHA transport activity into plant cells differs with cell growth. The relevance of the data to the action of DHA and ASC in cell growth will be discussed.

Ascorbic Acid↗
PubMed1980

Effect of adenosine and adenosine 5'-monophosphate on cell division of cultured mastocytoma P-815 cells.

The growth of mouse mastocytoma P-815 cells in culture (37 degrees, 42 hr) was inhibited by exogenous adenosine (0.2 to 1.0 mM) and more effectively by AMP (0.01 to 0.1 mM), but not by adenine. The inhibited growth (a 25% inhibition by 0.5 mM adenosine and a 80% inhibition by 0.25 mM AMP) was restored to a near control level by the addition of uridine (0.5 mM) to the medium. The pretreatment (37 degrees, 3 hr) of the cells with adenosine or AMP caused a 60% inhibition of incorporation (37 degrees, 2 hr) of [U-14C]aspartate into uracil nucleotides, accumulating 14C-orotate and orotidine. Both dipyridamole, an inhibitor of adenosine uptake, and exogenous adenosine deaminase suppressed the growth inhibition induced by not only adenosine but also AMP. 2-Chloroadenosine, which is resistant to the action of adenosine deaminase, was a more potent growth inhibitor, while 3'AMP and 2'-AMP, which are not hydrolyzed to adenosine by membrane 5'-nucleotidase, were ineffective. Adenosine 5'-sulfate and other 5'-substituted adenosines were also ineffective. These observations indicate that AMP inhibits the growth of mastocytoma P-815 cells as a result of its continuous conversion to adenosine and a constant exposure of the cells to a low concentration of adenosine which readily permeates the cell membrane. In addition, adenosine, AMP and their agarose-linked forms rapidly (37 degrees, 20 min) elevated cellular levels of cAMP. This effect was not suppressed by dipyridamole. Apparently adenosine and AMP also act extracellularly for growth inhibition by regulating cAMP levels.

Adenine Nucleotides↗
PubMed1997

G1 cyclins and control of the cell division cycle in normal and transformed cells.

The G1 cyclins are clearly important factors that control progression through the eukaryotic cell cycle. The expression and activity of these factors are regulated at many different levels and in response to a large number of signals. Such complicated, multilevel controls on expression and activation of cyclin/cdk complexes permit exquisite and necessary coordination of the stages of the cell cycle. Any of the large number of pathways involved in the regulation of cyclin activity also can be disrupted, leading to inappropriate expression and/or activity of complexes containing cyclins D1, D2, D3, and E. Characterization of these regulatory mechanisms and their synergistic effects on the G1 cyclins and cell cycle progression is a major area of investigation. While little evidence exists indicating that dysregulation of cyclin activity is an initiating event leading to malignant transformation, many studies indicate that disruption of the normal expression and/or activity of the G1 cyclins contributes to the transformed phenotype, potentially by overcoming negative proliferative signals in G1.

Animals↗
PubMed2004

Transient exposure to ethylene stimulates cell division and alters the fate and polarity of hypocotyl epidermal cells.

After transient exposure to the gaseous hormone ethylene, dark-grown cucumber (Cucumis sativus) hypocotyls developed unusual features. Upon ethylene's removal, the developing epidermis showed significant increases in cell division rates, producing an abundance of guard cells and trichomes. These responses to ethylene depended on the stage of development at the time of ethylene exposure. In the upper region of the hypocotyl, where cells were least differentiated at the onset of ethylene treatment, complex, multicellular protuberances formed. Further down the hypocotyl, where stomata and trichomes were beginning to develop at the onset of ethylene exposure, an increase in the number of stomata and trichomes was observed. Stomatal complexes developing after the ethylene treatment had a significant increase in the number of stomatal subsidiary cells and the number of cells per trichome increased. Analysis of division patterns in stomatal complexes indicated that exposure to ethylene either suspended or altered cell fate. Ethylene also altered cell division polarity, resulting in aberrant stomatal complexes and branched trichomes. To our knowledge, the results of this study demonstrate for the first time that transient treatment with physiological concentrations of ethylene can alter cell fate and increase the propensity of cells to divide.

Cell Differentiation↗
PubMed2000

Roles for polarity and nuclear determinants in specifying daughter cell fates after an asymmetric cell division in the maize leaf.

Asymmetric cell divisions occur repeatedly during plant development, but the mechanisms by which daughter cells are directed to adopt different fates are not well understood [1,2]. Previous studies have demonstrated roles for positional information in specification of daughter cell fates following asymmetric divisions in the embryo [3] and root [4]. Unequally inherited cytoplasmic determinants have also been proposed to specify daughter cell fates after some asymmetric cell divisions in plants [1,2,5], but direct evidence is lacking. Here we investigate the requirements for specification of stomatal subsidiary cell fate in the maize leaf by analyzing four mutants disrupting the asymmetric divisions of subsidiary mother cells (SMCs). We show that subsidiary cell fate does not depend on proper localization of the new cell wall during the SMC division, and is not specified by positional information acting on daughter cells after completion of the division. Instead, our data suggest that specification of subsidiary cell fate depends on polarization of SMCs and on inheritance of the appropriate daughter nucleus. We thus provide evidence of a role for unequal inheritance of an intracellular determinant in specification of cell fate after an asymmetric plant cell division.

Cell Division↗
PubMed2006

The bifunctional FtsK protein mediates chromosome partitioning and cell division in Caulobacter.

Bacterial chromosome partitioning and cell division are tightly connected cellular processes. We show here that the Caulobacter crescentus FtsK protein localizes to the division plane, where it mediates multiple functions involved in chromosome segregation and cytokinesis. The first 258 amino acids of the N terminus are necessary and sufficient for targeting the protein to the division plane. Furthermore, the FtsK N terminus is required to either assemble or maintain FtsZ rings at the division plane. The FtsK C terminus is essential in Caulobacter and is involved in maintaining accurate chromosome partitioning. In addition, the C-terminal region of FtsK is required for the localization of the topoisomerase IV ParC subunit to the replisome to facilitate chromosomal decatenation prior to cell division. These results suggest that the interdependence between chromosome partitioning and cell division in Caulobacter is mediated, in part, by the FtsK protein.

Bacterial Proteins↗
PubMed2006

Time-lapse tracing of mitotic cell divisions in the early Xenopus embryo using microscopic MRI.

Mitotic cell division is a highly regulated cellular event in all organisms, but its direct visualization in the vertebrates is limited to animals with transparent embryos. Here, we report on the use of microscopic magnetic resonance imaging (mMRI) to noninvasively observe mitotic cell division of early blastomeres in the optically opaque Xenopus laevis embryo. Due to intrinsic tissue contrast, cell nuclei can be directly visualized without the need for contrast enhancing labeling. By taking two-dimensional in vivo time-lapse image sequences, the karyokinesis of a blastomere is followed. Timing and orientation of the cleavages can be traced for five cell divisions to establish a cell lineage tree, including orientation and timing of the mitosis. This work demonstrates for the first time the use of MRI for the visualization of cell divisions and expands the experimental scope of the Xenopus embryo.

Animals↗
PubMed2003

Kinetics of the initial rounds of cell division of Toxoplasma gondii.

RH strain Toxoplasma gondii tachyzoites that had naturally lysed their host cells were allowed to infect new host cells for a limited amount of time; subsequent parasite cell divisions were observed closely. On the basis of 4 independent trials, the estimated time to first cell division was 9.8 hr postinfection (PI) and was quite variable (95% confidence interval [CI]: 3.1-16.5 hr PI). The estimated time to second cell division was 14.9 hr PI and was less variable (95% CI: 12.1-17.7 hr PI). Few parasites divided before 6 hr PI in these 4 trials. When tachyzoites were derived by forced lysis (scraping an infected host cell culture and passing it through 27-gauge needles), the first parasite cell division occurred much more rapidly than had been observed in any of the trials with parasites derived by natural lysis. When parasites derived by forced lysis were held away from host cells for 3 hr PI, the first cell division was delayed in a manner similar to that seen in parasites derived by natural lysis. No differences were observed in the timing of the second cell division of parasites derived by forced lysis whether or not they had been held away from cells. These studies demonstrate that the conditions to which tachyzoites are exposed during transit from one host cell to another can affect the kinetics of parasite cell division in the new host cell.

Analysis of Variance↗
PubMed2004

Murein (peptidoglycan) binding property of the essential cell division protein FtsN from Escherichia coli.

The binding of the essential cell division protein FtsN of Escherichia coli to the murein (peptidoglycan) sacculus was studied. Soluble truncated variants of FtsN, including the complete periplasmic part of the protein as well as a variant containing only the C-terminal 77 amino acids, did bind to purified murein sacculi isolated from wild-type cells. FtsN variants lacking this C-terminal region showed reduced or no binding to murein. Binding of FtsN was severely reduced when tested against sacculi isolated either from filamentous cells with blocked cell division or from chain-forming cells of a triple amidase mutant. Binding experiments with radioactively labeled murein digestion products revealed that the longer murein glycan strands (>25 disaccharide units) showed a specific affinity to FtsN, but neither muropeptides, peptides, nor short glycan fragments bound to FtsN. In vivo FtsN could be cross-linked to murein with the soluble disulfide bridge containing cross-linker DTSSP. Less FtsN, but similar amounts of OmpA, was cross-linked to murein of filamentous or of chain-forming cells compared to levels in wild-type cells. Expression of truncated FtsN variants in cells depleted in full-length FtsN revealed that the presence of the C-terminal murein-binding domain was not required for cell division under laboratory conditions. FtsN was present in 3,000 to 6,000 copies per cell in exponentially growing wild-type E. coli MC1061. We discuss the possibilities that the binding of FtsN to murein during cell division might either stabilize the septal region or might have a function unrelated to cell division.

Bacterial Proteins↗
PubMed1988

Identification of two new cell division genes that affect a high-molecular-weight penicillin-binding protein in Caulobacter crescentus.

Penicillin-binding proteins (PBPs) are membrane proteins associated with the synthesis of the bacterial cell wall. We report the characterization of 14 PBPs in Caulobacter crescentus, using in vivo and in vitro penicillin-binding assays and experiments to determine their possible role in cell division. New conditional cell cycle mutants were isolated by selecting cephalosporin-C-resistant mutants of the beta-lactamase strain SC1107 at 30 degrees C that are also defective in cell division at 37 degrees C. They fall into two classes, represented by strains PC8002 and PC8003. Strain PC8002 produced short cells arrested at all stages of cell division at 37 degrees C and was found to contain a high-molecular-weight PBP 1B which was temperature sensitive when assayed in vivo and in vitro. Strain PC8003 was blocked at an early stage of cell division and formed tightly coiled, unpinched filaments. This cephalosporin-C-resistant strain was also defective in PBP 1B, but only when assayed in vivo. PBP 1B behaved like a high-affinity PBP, and in competition assays, beta-lactams that induced filamentation bound preferentially to PBP 1B. These results and the phenotype of mutant PC8002 suggest that PBP 1B is required for cell division, as well as for cell growth, in C. crescentus. The behavior of strain PC8003 suggests that it contains a conditionally defective gene product that interacts in some way with PBP 1B at an early stage of cell division. None of the mutants showed an allele-specific PBP pattern when assayed in vitro at the nonpermissive temperature, but all of them displayed temperature-sensitive PBP 1C (102 kilodaltons) activity. Thus, it appears that PBP 1C is inhibited at 37 degree C as a consequence of filamentous growth.

Anti-Bacterial Agents↗