Ionic changes in the mitotic apparatus at the metaphase/anaphase transition

Time-dependent expression of ryanodine receptors in sea urchin eggs, zygotes and early embryos

In this work, the presence of calcium-dependent calcium channels and their receptors (RyR) has been investigated in Paracentrotus lividus eggs and early embryos, from unfertilized egg to four-blastomere stages. Electrophysiological recordings of RyR single-channel current fluctuations showed that RyRs are functional during the first developmental events with a maximum at zygote stage, c. 40 min after fertilization, corresponding to the first cleavage. The nature of vertebrate-like RyRs active at this stage was established by specific activation/blockade experiments.

Ca(2+) signaling, genes and the cell cycle

Changes in the concentration and spatial distribution of Ca(2+) ions in the cytoplasm constitute a ubiquitous intracellular signaling module in cellular physiology. With the advent of Ca(2+) dyes that allow direct visualization of Ca(2+) transients, combined with powerful experimental tools such as electrophysiological recordings, intracellular Ca(2+) transients have been implicated in practically every aspect of cellular physiology, including cellular proliferation. Ca(2+) signals are associated with different phases of the cell cycle and interfering with Ca(2+) signaling or downstream pathways often disrupts progression of the cell cycle. Although there exists a dependence between Ca(2+) signals and the cell cycle the mechanisms involved are not well defined and given the cross-talk between Ca(2+) and other signaling modules, it is difficult to assess the exact role of Ca(2+) signals in cell cycle progression. Two exceptions however, include fertilization and T-cell activation, where well-defined roles for Ca(2+) signals in mediating progression through specific stages of the cell cycle have been clearly established. In the case of T-cell activation Ca(2+) regulates entry into the cell cycle through the induction of gene transcription.

Fluctuation of the Ca-sequestering activity of permeabilized sea urchin embryos during the cell cycle

We have followed the sequestration of Ca(2+) by intracellular compartments in sea urchin embryos through the first cell cycles. To gain biochemical access to these compartments, the embryos were permeabilized by brief exposure to an intense electric field. Sequestration was determined as the retention of tracer, (45)Ca, after filtration of aliquots on Millipore filters. The permeabilized cells sequester Ca(2+) at a constant rate for at least 20 min, with the following characteristics: (i) ATP is required. (ii) Sequestration occurs at Ca(2+) levels corresponding to those estimated in vivo. (iii) The Ca(2+) concentration dependence of sequestration and its insensitivity to mitochondrial poisons imply that the activity derives from a single, nonmitochondrial transport system. The Ca(2+)-sequestering activities of embryos that are permeabiized at successive stages of the first cell cycle (one-cell stage) progressively increase to 5 times the initial level. The rate of sequestration is maximal during telophase and, in some populations of zygotes, is nearly as great throughout prophase. Over the course of the second cell cycle (two-cell stage), the activity undergoes a 2-fold oscillation that bears the same temporal relationship to mitosis as the previous fluctuation.

Visualization of the Ca-transport system of the mitotic apparatus of sea urchin eggs with a monoclonal antibody

Monoclonal antibodies have been obtained to components of Ca(2+)-sequestering vesicles from the endoplasmic reticulum of HeLa cells by isolating hybridomas that were generated by the in vitro immunization of lymphocytes followed by fusion with plasmocytoma cells. One of these monoclonal antibodies specifically labels punctate structures which appear in the mitotic apparatus of sea urchin eggs at the beginning of prophase and disappear upon the completion of cytokinesis. The antibody inhibits the Ca(2+) uptake of the membrane system in vitro. It reacts with one 46-kDa protein out of the complex protein mixture from the membrane fraction. We take all this as evidence that in fact a specific Ca(2+)-transport system is part of the mitotic apparatus, that such a system is very conserved, and that it is most probably derived from the endoplasmic reticulum.

Quantitative imaging of subcellular calcium stores in mammalian LLC-PK1 epithelial cells undergoing mitosis by SIMS ion microscopy

Quantitative 3-D total calcium gradients, representing subcellular stored calcium, were imaged with a CAMECA IMS-3f SIMS ion microscope in cryogenically prepared frozen freeze-dried LLC-PK1 cells captured in interphase and various stages of mitosis. 39K and 23Na concentrations were also measured in the same cells. Correlative optical (or SEM) and SIMS analysis of cells revealed a redistribution of the interphase Golgi calcium store in prophase and prometaphase cells. In metaphase cells, simultaneous SIMS imaging of total calcium in both the spindle and the non-spindle cytoplasm of individual cells revealed a gradual and dynamic alignment of calcium stores in both half-spindles prior to the onset of anaphase. The anaphase cells revealed the highest local total calcium concentrations in the spindle regions behind the daughter chromosomes and the lowest in the central spindle region. The pericentriolar material in telophase cells contained calcium stores. Quantitatively, a typical metaphase cell with well-aligned calcium stores in the spindle region contained 1.1 mM total calcium in each half-spindle, 0.8 mM total calcium in the non-spindle cytoplasm, and 0.5mM total calcium in the chromosomes. At the submicron scale, the distribution of total calcium was heterogeneous in the chromosomes, metaphase spindle, and non-spindle cytoplasm. An increased binding of calcium to chromosomes is not a physiological requirement for chromosomal condensation in mitosis, since interphase nuclei and mitotic chromosomes contained comparable total calcium concentrations measured per unit volume. A significant reduction of total calcium in the non-spindle cytoplasm was observed in the metaphase, anaphase, and telophase cells, which is indicative of the limited storage of the releasable calcium pool in these specific stages of mitosis. Direct total calcium measurements in subcellular regions confirmed that both the spindle and the non-spindle cytoplasm of metaphase cells contained inositol 1,4,5-trisphosphate (IP3)-sensitive calcium stores sensitive to arginine vasopressin, thapsigargin, and calcium ionophore A23187. The dynamic alignment of calcium stores in both half-spindles may be an integral part of the time-dependent process of a cell's overall preparation for exiting the metaphase stage in mammalian LLC-PK1 cells.

Molecular characterization of p62, a mitotic apparatus protein required for mitotic progression

A 62-kDa (p62) mitotic apparatus-associated protein is important for the proper progression of mitosis in sea urchin embryos (Dinsmore, J. H., and Sloboda, R. D. (1989) Cell 53, 769-780). We have isolated and characterized a full-length p62 cDNA of 3374 base pairs which encodes an extremely acidic polypeptide of 411 amino acids having a calculated Mr of 46,388 and a pI of 4.01; p62 is a unique protein with no significant identity to any known proteins. Southern and Northern blot analyses demonstrate that the gene for p62 is present once in the sea urchin genome and the corresponding mRNA is present in unfertilized eggs and in early embryos through and up to the gastrula stage. Sequence analysis suggests certain regions may participate in chromatin association and microtubule binding, an observation that is consistent with previous immunological data (Ye, X., and Sloboda, R. D. (1995) Cell Motil. Cytoskeleton 30, 310-323) as well as data reported herein. Confocal microscopy reveals that during interphase the protein binds to chromatin in the nuclei of sea urchin eggs. In the germinal vesicles of clam oocytes at prophase of meiosis I, p62 binds to the condensed chromosomes. Currently, truncated clones of p62 are being used to identify the tubulin and chromatin binding domains.

Calcium, BOBs, QEDs, microdomains and a cellular decision: control of mitotic cell division in sand dollar blastomeres

The role of Ca2+ in controlling cell processes (e.g. mitosis) presents an enigma in its ubiquity and selectivity. Intracellular free Ca2+ (Ca2+i) is an essential regulator of specific biochemical and physiological aspects of mitosis (e.g. nuclear envelope breakdown (NEB)). Changes in Ca2+i concentrations during mitosis in second cell-cycle sand dollar (Echinaracnius parma) blastomeres were imaged as Ca(2+)-dependent luminescence of the photoprotein aequorin with multi-spectral analytical video microscopy. Photons of this luminescence were seen as bright observable blobs (BOBs). Spatiotemporal patterns of BOBs were followed through one or more cell cycles to detect directly changes in Ca2+i, and were seen to change in a characteristic fashion prior to NEB, the onset of anaphase chromosome movement, and during cytokinesis. These patterns were observed from one cell cycle to the next in a single cell, from cell to cell, and from egg batch to egg batch. In both mitosis and synaptic transmission increases in Ca2+i concentration occurs in discrete, short-lived, highly localized pulses we name quantum emission domains (QEDs) within regions we named microdomains. Signal and statistical optical analyses of spatiotemporal BOB patterns show that many BOBs are linked by constant displacements in space-time (velocity). Linked BOBs are thus nonrandom and are classified as QEDS. Analyses of QED patterns demonstrated that the calcium signals required for NEB are nonrandom, and are evoked by an agent(s) generated proximal to a Ca2+i-QED; models of waves, diffusible agonists and Ca(2+)-activated Ca2+ release do not fit pre-NEB cell data. Spatial and temporal resolution of this multispectral approach significantly exceeds that reported for other methods, and avoids the perturbations associated with many fluorescent Ca2+ reporters that interfere with cells being studied (Ca(2+)-buffering, UV toxicity, etc.). Spatiotemporal patterns of Ca2+i-QED can control so many different processes, i.e. specific frequencies used to control particular processes. Predictive and structured patterns of calcium signals (e.g. a language expressed in Ca2+) may selectively regulate specific Ca(2+)-dependent cellular processes.

Intracellular ionic changes induced by bullous pemphigoid IgG subclasses

To ascertain whether membrane signal transduction is induced by bullous pemphigoid (BP) antibody and whether cell lysis is induced by its complement activation, we assessed the intracellular Ca2+ concentration ([Ca2+]i), intracellular pH, membrane potential and morphology of living cells by following the time course of fluorescence intensity of Fluo-3/AM, Snaff-1/AM, Dioc-5 and Luciffer yellow, respectively. A transient increase of Fluo-3 fluorescence intensity in DJM-1 cells (a squamous cell carcinoma line) was revealed when the cells were incubated with 2 of five IgG1 BP antibodies. However, no transient increase of Fluo-3 fluorescence intensity was revealed when the cells were incubated with IgG2 and IgG4 BP antibodies. A transient increase of Fluo-3 fluorescence intensity was revealed in DJM-1 cells incubated with 3 of seven IgG1 and 1 of four IgG2 BP antibodies in an EGTA-containing low-Ca2+ medium. On the other hand, the Dioc-5 fluorescence intensity did not change significantly, though the increase of Fluo-3 fluorescence intensity was observed. The increase of Snarf-1 fluorescence intensity was revealed in DJM-1 cells incubated with 2 of five IgG1 BP antibodies, but was not revealed in the cells incubated with IgG2 or IgG4 of BP antibodies. Study of complement activation by BP IgG1 showed a transient increase of Fluo-3 fluorescence intensity of with 3 of five IgG1 BP antibodies when DJM-1 cells were incubated with complement-supplemented normal-Ca2+ medium. At the same time, however, endocytosis and cell lysis were not observed with 2 IgG1 BP antibodies which did induce an increase of Fluo-3 fluorescence intensity when Lucifer-yellow-loaded DJM-1 cells were incubated with complement-supplemented normal-Ca2+ medium. We examined next whether anti-180 kD BP antigen monoclonal antibodies (mAbs R-223 and 233) induce an increase of Fluo-3 fluorescence intensity. MAb R-223 did not induce any increase of Fluo-3 fluorescence intensity in DJM-1 cells, when incubated with normal- and low-Ca2+ media However, mAb R-223 induced a transient increase of Fluo-3 fluorescence intensity in DJM-1 cells when incubated with complement-supplemented normal-Ca2+ medium. MAb 233 did not induced an increase of Fluo-3 fluorescence intensity in DJM-1 cells when incubated with normal- and low-Ca2+ media. These results suggest that the BP IgG1 induces Ca2+ release from intracellular storage sites, however, the complement activated by BP IgG1 does not induce cell lysis. It could not be confirmed that anti-180 kD BP antigen antibody induced Ca2+ release from intracellular storage sites.

Intracellular free Ca2+ in the cell cycle in human fibroblasts: transitions between G1 and G0 and progression into S phase

Intracellular free calcium ([Ca2+]i) has been proposed to play an important part in the regulation of the cell cycle. Although a number of studies have shown that stimulation of quiescent cells with growth factors causes an immediate rise in [Ca2+]i (Rabinovitch et al., 1986; Vincentini and Villereal, 1986; Hesketh et al., 1988; Tucker et al., 1989, Wahl et al., 1990), a causal relationship between the [Ca2+]i transient and the ability of the cells to reenter the cell cycle has not been firmly established. We have found that blocking the mitogen-induced elevation of [Ca2+]i with the cytoplasmic [Ca2+]i buffer dimethyl BAPTA (dmBAPTA) also blocks subsequent entry of cells into S phase. The dose response curves for inhibition of serum stimulation of [Ca2+]i and DNA synthesis by dmBAPTA are virtually identical including an anomalous stimulation observed at low levels of dmBAPTA. Reversal of the [Ca2+]i buffering effect of dmBAPTA by transient exposure of the cells to the Ca2+ ionophore ionomycin also reverses the inhibition of DNA synthesis 20-24 h later. Ionomycin by itself does not stimulate DNA synthesis. These data are consistent with the conclusion that a transient increase in [Ca2+]i occurring shortly after serum stimulation of quiescent fibroblasts is necessary but not sufficient for subsequent entry of the cells into S phase. This study is the first to show a direct relationship between early serum stimulated Cai2+ increase and subsequent DNA synthesis in human cells. It also goes beyond recent studies on BALB/3T3 cells by providing dose response data and demonstrating reversibility, which are strong indications of a cause and effect relationship.

Mitosis-arresting effect of the calcium channel inhibitor SK&F 96365 on human leukemia cells

The effect of SK&F 96365 (1-(beta-[3-(4-methoxyphenyl)propoxyl]-4- methoxyphenethyl)-1H-imidazole hydrochloride), a recently synthesized inhibitor of receptor-mediated calcium entry, was investigated on human hematopoietic cell lines. We found that treatment of the T-cell leukemia line Jurkat with SK&F 96365 inhibited the Ca2+ influx triggered by antibodies against the CD3/TCR complex, while the inositol trisphosphate-dependent Ca2+ release from intracellular stores remained intact. A 50% inhibition of the Ca2+ influx was obtained with 5 microM SK&F 96365, while higher concentrations of the drug blocked the CD3-dependent Ca2+ influx completely. In addition to its blocking of the Ca2+ influx, treatment with SK&F 96365 was found to accumulate mitotic cells. The drug (5 microM) imposed a total cell cycle arrest in G2/M. The mitosis block could be reversed by removal of the inhibitor from the cultures, while elevation of intracellular or extracellular Ca2+ did not restore cell cycle progression. This suggests that the cell cycle block induced by SK&F 96365 is not directly related to its action as an inhibitor of receptor-mediated calcium entry. Our findings indicate that SK&F 96365, in addition to its ability to inhibit receptor-triggered Ca2+ influx, offers a new method for imposing a reversible mitosis arrest in hematopoietic cell lines.

A comparison of the effects of various spindle toxins on metaphase arrest and formation of micronuclei in cell-suspension cultures ofNicotiana plumbaginifolia

The effects of the spindle toxins colchicine, oryzalin and amiprophos-methyl (APM) on metaphase arrest, chromosome scattering, and on the induction and yield of micronuclei were compared in suspension cells ofNicotiana plumbaginifolia (kanamycin-resistant "Doba" line). The inhibition of spindle formation is stronger with oryzalin and APM than with colchicine, which resulted in a more efficient accumulation of meta-phases with well-scattered chromosomes, allowing the isolation of single chromosomes. Further, APM and oryzalin treatments resulted in a higher frequency of micro-nucleated cells and greater yield of micronuclei than after colchicine treatment. The different actions of the chemicals on the functioning of the spindle, development of nuclear membranes around the chromosomes, formation of micronuclei and fusion of micronuclei, resulting in restitution nuclei, are discussed.

Calcium and cellular clocks orchestrate cell division

The results of experiments recently reported from this and other laboratories provide firm support for Heilbrunn's thesis that mitotic events are initiated by transient elevation of intracellular Ca2+, derived from intracellular stores. The ATP-dependent MA Ca2(+)-pump working in concert with an endomembrane Ca2+ channel appears to share the responsibility for regulating these Ca2+ signals. Further results demonstrated a limited time window during which the cell is sensitive to agents that impose mitotic arrest by interfering with transient elevations in intracellular "free" Ca2+ concentration. From this it appears that a discrete, timed increase in cytosolic Ca2+ derived from endomembrane stores is a necessary signal for regulating the onset of NEB, AO, and mitosis. Results from the arrest and release experiments provide support for a model in which Ca2+ is used to coordinate the action of parallel independent and interdependent biochemical pathways whose interaction results in the cytologic events of mitosis. These pathways apparently are operating under the influence of a metabolic "clock" that continues to cycle, at least once, in the absence of a Ca2+ transient sufficient to initiate NEB or AO. The discrete and temporal regulation of this Ca2+ transient through the interaction of the endomembrane Ca2+ pump, an endomembrane Ca2+ channel, and intracellular Ca2(+)-dependent reaction pathways suggest a mechanism incorporating a negative feedback loop to limit the size and duration of the Ca2+ transient and prevent the release of excessive amounts of Ca2+. Deeper understanding of the regulatory mechanism that governs the onset of mitosis requires: (1) quantitative imaging of intracellular Ca2+, especially the Ca2+ signal throughout the cell cycle, with high spatial and temporal resolution; and (2) identifying the molecules responsible for regulating the expression and reception of the Ca2+ signal itself. It is clear that Ca2(+)-dependent pathways are necessary elements of the mitotic process. Molecular candidates for the regulators and regulatees have yet to be identified. The upstream controlling molecules of these transmembrane Ca2+ regulatory elements, as well as the initial mitotic "start" signal, await future identification. Downstream regulation is also clearly indicated, perhaps through regulation of cyclin expression, degradation, or both.

Mitochondrial mutations may increase oxidative stress: implications for carcinogenesis and aging?

The sensitivity of mitochondrial DNA to damage by mutagens predisposes mitochondria to injury on exposure of cells to genotoxins or oxidative stress. Damage to the mitochondrial genome causing mutations or loss of mitochondrial gene products, or to some nuclear genes encoding mitochondrial membrane proteins, may accelerate release of reactive species of oxygen. Such aberrant mitochondria may contribute to cellular aging and promotion of cancer.

Oxidative influence on development and differentiation: an overview of a free radical theory of development

Metabolic gradients exist in developing organisms and are believed to influence development. It has been postulated that the effects of these gradients on development result from differential oxygen supplies to tissues. Oxygen has been found to influence the course of development. Cells and tissues in various stages of differentiation exhibit discrete changes in their antioxidant defenses and in parameters of oxidation. Metabolically generated oxidants have been implicated as one factor that directs the initiation of certain developmental events. Also implicated as factors that modulate developmental processes are the cellular distribution of ions and the cytoskeleton both of which can be influenced by oxidants. The interaction of oxidants with ion balance and cytoskeleton is discussed.

Mechanism of action of antimitotic drugs: a new hypothesis based on the role of cellular calcium

The antimitotic drugs such as colchicine, podophyllotoxin, etc. are currently believed to exert their cytotoxic and antimitotic effects due to binding of the drug-tubulin complex to the growing ends of microtubules (MTs), leading to an "end-capping or poisoning" effect. However, to account for a number of apparently puzzling observations regarding antimitotic drugs (which cannot be readily explained by the current model) and the mitotic process, a new hypothesis regarding the mechanism of action of antimitotic drugs is proposed. The key observations in this context are as follows: (i) The antimitotic drugs bind specifically to free tubulin. (ii) Cell growth by these drugs is specifically blocked in metaphase, and interphase microtubules do not seem to play any role in the drugs' cytotoxic or antimitotic effects. (iii) Tubulin is specifically associated with a number of membranous organelles (viz. mitochondria, plasma membranes, endoplasmic reticulum) which are responsible for intracellular Ca+2 homeostasis. (iv) Fluorescent derivatives of antimitotic drugs also bind to the above membranous organelles and not to MTs. (v) Ca+2 plays a central role in the control of MT assembly/disassembly in vivo and a Ca+2 pulse is necessary for the metaphase to anaphase transition. (vi) Cellular mutants which exhibit specific resistance to various antimitotic drugs are altered in either tubulin(s) or mitochondrial matrix proteins. To account for these observations, it is suggested that free tubulin present in the above membranous organelles serves as the cellular receptor for these drugs and this binding interferes with the Ca+2 regulatory/signalling mechanism essential for anaphase chromosome movement. The effect of these drugs on interphase MTs appears to be a secondary consequence of this alteration in Ca+2 regulation. The observed changes in mitochondrial matrix proteins in many of the mutants resistant to antimitotic drugs further indicate that mitochondria should play an important role in Ca+2 homeostasis, as it relates to mitosis. The possible mechanisms by which these drugs may interfere with the Ca+2 regulation and some implications of this hypothesis are discussed.

Nuclear envelope breakdown and mitosis in sand dollar embryos is inhibited by microinjection of calcium buffers in a calcium-reversible fashion, and by antagonists of intracellular Ca2+ channels

Transient elevations in intracellular free Ca2+ are believed to signal the initiation of mitosis. This model predicts that mitosis might be arrested prior to nuclear envelope breakdown (NEB) or anaphase onset if intracellular Ca2+ concentration is buffered or dampened. Microinjection of a discrete dose of Ca2+ into the cell might then release the cell to resume mitotic cycling. Experimentally, one blastomere of two cell sand dollar (Echinaracnius parma) embryos was microinjected with Ca2+ buffers, Ca2+ solutions, or Ca2+ channel antagonists; the uninjected blastomere was the control. Cells were loaded with 10 pl doses of the Ca2+ buffer antipyrylazo III (ApIII) at specific times in the cell cycle to attempt a competitive inhibition of Ca2+-dependent steps in NEB and initiation of mitosis. Injection of 50 microM ApIII 6 min prior to NEB blocked NEB and further cell cycling. Injections of solutions between 0 and 30 microM ApIII were without observable effect. Control injections had no observable effect on the injected cell. Cells injected with 50 microM ApIII 2 min prior to the onset of anaphase in control cells were blocked in metaphase. Cells were sensitive to Ca2+ buffer injections 6 min prior to NEB (with a 40- to 45-sec duration), and 2 min prior to anaphase onset (with a 10- to 20-sec duration). Vital staining of these cells with H33342 demonstrated that they contained only one nucleus that had the same fluorescence intensity as seen prior to microinjection, and thus did not undergo DNA synthesis following the imposition of the Ca2+ buffer block to mitosis. Cells arrested in this fashion did not spontaneously resume mitotic cycling. This Ca2+ buffer-induced mitotic arrest was, however, experimentally reversible. Cells arrested with 50 microM ApIII 6 min prior to NEB could be returned to mitotic activity by injecting 300 microM CaCl2 5 min after the ApIII injection. The double injected cells resumed cycling, NEB, and mitosis after a delay of one cell cycle period, and remained one cell cycle out of phase with the sister (control) cell. Microinjection of antagonists of endomembrane Ca2+ channels inhibited NEB and anaphase onset in a concentration- and time-dependent fashion. The effective doses of compounds tested were 7 micrograms/ml ryanodine and 500 micrograms/ml TMB-8. These results indicate that a transient elevation of intracellular Ca2+ from endomembrane stores is required to initiate mitotic events, namely NEB and anaphase onset.(ABSTRACT TRUNCATED AT 400 WORDS)

Photodynamic effects of Photofrin II on cell division in human NHIK 3025 cells

Human cervix carcinoma cells of the line NHIK 3025 were exposed to light after 18 h incubation with Photofrin II. After this photodynamic treatment cells in the interphase were retarded with respect to entry into mitosis for a period which increased with increasing light dose. Following the prolonged interphase, an increase in the mitotic index was observed, giving rise to a 3-fold higher level of mitotic cells compared to the control level. Staining of methanol-fixed cells with the DNA-specific dye mithramycin indicated that the increase in mitotic index was due to a prolongation of the metaphase. For all the light doses studied most of the metaphase cells could be characterized as three-group metaphases or c-metaphase-like structures for the first 8 h after treatment. An approximately 10-fold increase above the control level in the number of tripolar mitoses was also observed. A 2h incubation in a Photofrin II-free medium after the 18 h incubation with Photofrin II and before light exposure reduced the fluorescence of the cells by 30 per cent. However, this wash-out period had no effect on the increase in mitotic index after light exposure. A light dose corresponding to 80 per cent survival (as assayed on asynchronous cells) was given to cells in mitosis after Photofrin II incubation. This treatment delayed more than 90 per cent of the metaphase cells from entering the anaphase for at least 1 h. Cells photodynamically treated in the anaphase and telophase entered the interphase at a similar rate as control cells. These observations indicate a temporary block in the initiation of the anaphase and a prolongation of the metaphase. A microscopic study of cells immunologically stained for beta-tubulin 1 h after photodynamic treatment indicated that the organization of the spindle apparatus was disturbed by the photodynamic treatment. Such perturbations are suggested to be the cause of the observed accumulation of cells in mitosis.

Intracellular free calcium rise triggers nuclear envelope breakdown in the sea urchin embryo

Cytosolic free calcium has recently been implicated in the regulation of mitosis in plant and animal cells. We have previously found correlations between increases in the levels of intracellular free calcium [Ca2+]i and visible transitions of structure at nuclear envelope breakdown (NEBD) and the onset of anaphase during mitosis in sea urchin embryos and tissue culture cells. To go beyond correlations it is necessary to manipulate [Ca2+]i, and in sea urchin embryos this requires the injection of calcium-chelator buffer solutions as the changes in free calcium in the cell cycle are dependent on intracellular stores. We report here that blocking the increase in [Ca2+]i which just precedes NEBD prevents this from taking place and halts mitosis. Subsequent injections which momentarily increase [Ca2+]i, or a natural recovery of the higher calcium levels, result in NEBD and the successful continuation of mitosis. Similarly, artificially increasing calcium by early injections results in early NEBD. We conclude that the increase in [Ca2+]i preceding NEBD is an essential regulatory step required for entry into mitosis.

Dynamics of a fluorescent calmodulin analog in the mammalian mitotic spindle at metaphase

We have compared the exchange kinetics of fluorescein-labeled calmodulin and tubulin in the spindles of living mitotic cells at metaphase. Cultured mammalian cells in early stages of mitosis were microinjected with labeled calmodulin or tubulin and returned to an incubator to allow equilibration of the fluorescent protein with the endogenous protein pools. Calmodulin becomes concentrated in the mitotic spindle, and treatments with inhibitors of tubulin assembly show that this concentration is dependent on the presence of microtubules. The steady-state exchange rates of both tubulin and calmodulin were measured by an analysis of fluorescence redistribution after photobleaching (FRAP), using cells pre-equilibrated to either 26 +/- 2 degrees C or 36 +/- 2 degrees C. A pulse of laser light focused to a 5-microns diameter column was used to destroy the fluorescence at one pole of a metaphase mitotic spindle. Ratios of fluorescence intensity from the two half-spindles and from the two polar regions were calculated for each image in a post-bleach time series to determine the rates and extents of FRAP. For tubulin, we confirm earlier observations concerning the temperature dependence of the extent of FRAP, but our data do not show a significant temperature dependence for the rate of FRAP. We hypothesize that the reduced extent of tubulin FRAP at the lower temperatures is a result of microtubules that are stable to depolymerization at 26 degrees C and are thus less likely to exchange subunits. Calmodulin's FRAP, however, does not exhibit any of the temperature dependence observed with fluorescent tubulin. At 26 +/- 2 degrees C calmodulin exchanges rapidly with the relatively stable population of microtubules, suggesting that calmodulin is bound, either directly or indirectly, to microtubule walls.

Microscopic imaging of cells

The microworld was revealed to investigators through a glass bead or a hanging water droplet long before optics was understood. The cellular structure of plants was well resolved by such simple magnifying glasses, van Leeuwenhoek, the Dutch merchant and amateur microscopist, was the first to report to the English Royal Society his observations of bacteria with his single-lens microscope in 1665.

The blocked pinocytic activity of mitotic cells is restored in mitotic-interphase hybrids

During mitosis there is an abrupt inhibition of a wide range of membrane functions, including fluid-phase and adsorptive pinocytosis. We have used cell hybrids formed between mitotic and interphase cells to approach the mechanism of this inhibition. We report that fluid pinocytosis is reactivated in the mitotic partner of hybrids formed between mitotic and interphase Chinese hamster ovary (CHO) cells. It thus appears that the interphase cell provides some necessary element(s) for membrane activity during mitosis. This dominance of interphase membrane properties stands in contrast with earlier evidence that mitotic nuclear properties dominate in similar mitotic-interphase hybrids.

Is nuclear envelope re-formation an inducible event?

In large multinucleate cells the nuclei enter mitosis and reach metaphase almost synchronously by interaction of the different parts of the cell, but some degrees of postmetaphase asynchrony still persist. Apart from chromosome movements, the important postmetaphase events are re-formation of the nuclear envelope, chromosome decondensation, and back-formation of the spindle. From ultrastructural studies of multinucleate cells showing asynchronous mitotic progression beyond metaphase, we observed that nuclear envelope re-formation takes place nearly synchronously in all chromosome groups as soon as one group has reached telophase and while others are still in earlier mitotic stages. This indicates that nuclear envelope re-formation is an inducible event independent of the degree of condensation or decondensation of the chromatin and may depend on a factor(s) opposite in behavior to the maturation-promoting factor.

The role of the phosphatidylinositol cycle in mitosis in sea urchin zygotes. Lithium inhibition is overcome by myo-inositol but not by other cyclitols or sugars

We have investigated the role of the phosphatidylinositol (PI) cycle in cellular events between fertilization and first cleavage in zygotes of the sea urchin Lytechinus pictus. The effects of lithium were studied: The lithium-induced changes due to effects on the PI cycle were reversed by myo-inositol, the next step in the cycle after the lithium block, but were not reversed by scyllo-inositol or other cyclitols or sugars. In this way we implicated the PI cycle in the formation of streak birefringence, in nuclear membrane breakdown, in onset of anaphase, and in cytokinesis. With respect to karyokinesis, mitotic apparatus (MA) structure often was altered when the PI cycle was blocked, and anaphase was blocked when the PI cycle was blocked. For all stages, the effects of 400 mM lithium were overcome by 10-100 microM myo-inositol. Excess myo-inositol potentiated the effect of lithium on MA structure (and on cytokinesis), suggesting that there is a negative feedback loop in the control of the PI cycle.

Mechanistic aspects on chemical induction of spindle disturbances and abnormal chromosome numbers

Work on the chemical induction of spindle disturbances and abnormal chromosome numbers, and work on the composition and biochemistry of the spindle are reviewed. Some early investigations have shown that there is an unspecific mechanism for chemical induction of spindle disturbances. This mechanism is based on the interaction of compounds with cellular hydrophobic compartments. Some compounds act differently and are more active than predicted from their lipophilic character. Selected compounds of that kind and their possible mechanisms of action are discussed. Changes in sulfhydryl and ATP levels, oxidative damage of membranes and impaired control of cytoplasmic Ca2+ levels are discussed in this context.

Transition from metaphase to anaphase is accompanied by local changes in cytoplasmic free calcium in Pt K2 kidney epithelial cells

We have used a Ca2+-sensitive dye, fura-2, to investigate the role of Ca2+ during mitosis in Pt K2 epithelial cells. The concentration of cytoplasmic free calcium, [Ca2+]i, increased 2-fold between metaphase and anaphase. Digital image analysis revealed two patterns of [Ca2+]i localization during anaphase. In half of the anaphase cells, the increase in [Ca2+]i was greatest in the region near the spindle poles and decreased radially. In the other anaphase cells, there was a ring of high [Ca2+]i in the cytoplasm, surrounding an area of low [Ca2+]i in the spindle midzone. Although the reason for the different patterns is not known, peak [Ca2+]i in both cases was sufficient to maintain a 2- to 6-fold gradient in [Ca2+]i from the polar region to the midzone. [Ca2+]i gradients may thus regulate spindle microtubule equilibria and directed chromosome movement during mitosis.

Microtubule dynamics in the spindle. Theoretical aspects of assembly/disassembly reactions in vivo

The mitotic spindle contains several classes of microtubules (MTs) whose lengths change independently during mitosis. Precise control over MT polymerization and depolymerization during spindle formation, anaphase chromosome movements, and spindle breakdown is necessary for successful cell division. This model proposes the site of addition and removal of MT subunits in each of four classes of spindle MTs at different stages of mitosis, and suggests how this addition and removal is controlled. We propose that spindle poles and kinetochores significantly alter the assembly-disassembly kinetics of associated MT ends. Control of MT length is further modulated by localized forces affecting assembly and disassembly kinetics of individual sets of MTs.

Chemically induced aneuploidy in mammalian cells: mechanisms and biological significance in cancer

A growing body of evidence from human and animal cancer cytogenetics indicates that aneuploidy is an important chromosome change in carcinogenesis. Aneuploidy may be associated with a primary event of carcinogenesis in some cancers and a later change in other tumors. Evidence from in vitro cell transformation studies supports the idea that aneuploidy has a direct effect on the conversion of a normal cell to a preneoplastic or malignant cell. Induction of an aneuploid state in a preneoplastic or neoplastic cell could have any of the following four biological effects: a change in gene dosage, a change in gene balance, expression of a recessive mutation, or a change in genetic instability (which could secondarily lead to neoplasia). To understand the role of aneuploidy in carcinogenesis, cellular and molecular studies coupled with the cytogenetic studies will be required. There are a number of possible mechanisms by which chemicals might induce aneuploidy, including effects on microtubules, damage to essential elements for chromosome function (ie, centromeres, origins of replication, and telomeres), reduction in chromosome condensation or pairing, induction of chromosome interchanges, unresolved recombination structures, increased chromosome stickiness, damage to centrioles, impairment of chromosome alignment, ionic alterations during mitosis, damage to the nuclear membrane, and a physical disruption of chromosome segregation. Therefore, a number of different targets exist for chemically induced aneuploidy. Because the ability of certain chemicals to induce aneuploidy differs between mammalian cells and lower eukaryotic cells, it is important to study the mechanisms of aneuploidy induction in mammalian cells and to use mammalian cells in assays for potential aneuploidogens (chemicals that induce aneuploidy). Despite the wide use of mammalian cells for studying chemically induced mutagenesis and chromosome breakage, aneuploidy studies with mammalian cells are limited. The lack of a genetic assay with mammalian cells for aneuploidy is a serious limitation in these studies.

A model for chromosome movement based on lateral interaction of spindle microtubules

Quantitative morphological studies of meiotic spindles in the crane fly Pales ferruginea (Fuge, 1980, 1984, 1985) were the basis for the development of a model explaining anaphase chromosome transport in higher eukaryotes. Two main features of chromosome fibres were important for the model: (1) the existence of microtubules oriented obliquely with respect to kinetochore microtubules, and (2) a higher degree of disorder in fibres exerting a pulling force. It is postulated that microtubules of the same polarity being inclined to each other at a certain angle are able to slide past each other by means of mechano-chemically active side-arms working in alternate succession. Sliding is suggested to lead to a displacement of microtubules and chromosomes in direction towards the poles. Furthermore, it is suggested that the chromosome fibre in anaphase becomes progressively disintegrated by fragmentation and disassembly of microtubules. Fragmentation may be induced by bending stress within the dynamic system.

Characterization and immunocytochemical distribution of calmodulin in higher plant endosperm cells: localization in the mitotic apparatus

In this study we have examined the immunocytochemical distribution of calmodulin during mitosis of higher plant endosperm cells. Spindle development in these cells occurs without centrioles. Instead, asterlike microtubule converging centers appear to be involved in establishing spindle polarity. By indirect immunofluorescence and immunogold staining methods with anti-calmodulin antibodies, we found endosperm calmodulin to be associated with the mitotic apparatus, particularly with asterlike and/or polar microtubule converging centers and kinetochore microtubules, in an immunocytochemical pattern distinct from that of tubulin. In addition, endosperm calmodulin and calcium showed analogous distribution profiles during mitosis. Previous reports have demonstrated that calmodulin is associated with the mitotic apparatus in animal cells. The present observation that calmodulin is also associated with the mitotic apparatus in acentriolar, higher plant endosperm cells suggests that some of the regulatory mechanisms involved in spindle formation, microtubule disassembly, and chromosome movement in plant cells may be similar to those in animal cells. However, unlike animal cell calmodulin, endosperm calmodulin appears to associate with kinetochore microtubules throughout mitosis, which suggests a specialized role for higher plant calmodulin in the regulation of kinetochore microtubule dynamics.

Changes of free calcium levels with stages of the cell division cycle

Although the regulation of events in the cell division cycle by calcium or other cations has been the subject of much interest and speculation, experimental studies have been hampered by the difficulty of measuring submicromolar intracellular free calcium concentrations ([Ca2+]i) over an entire cell cycle. We now describe experiments using a new fluorescent calcium chelator, fura-2 (see Fig. 1c for structure), for continuous measurement of [Ca2+]i from fertilization through the first cleavage of individual eggs of the sea urchin Lytechinus pictus. We also show for comparison the results of parthenogenetic activation by ammonia. In addition to the known transient rise of [Ca2+]i at fertilization, further peaks are now revealed during pronuclear migration, nuclear envelope breakdown, the metaphase/anaphase transition and cleavage. Parthenogenetic activation by ammonia also elicits a sustained rise starting at nuclear envelope breakdown.

Calcium restriction prolongs metaphase in dividing Tradescantia stamen hair cells

Agents that lower extracellular calcium concentration (EGTA) or modulate calcium transport (lanthanum or D600) have been applied to dividing stamen hair cells of Tradescantia and analyzed for their ability to change the following: (a) the time required to progress from nuclear envelope breakdown to the onset of anaphase (metaphase transit time), (b) the time required to progress from anaphase to the initiation of the cell plate, and (c) the rate of chromosome motion in anaphase. Control cells complete metaphase in 32 min, initiate a cell plate in 19 min, and display a chromosome motion rate of 1.45 micron/min. If cells are treated with a calcium-EGTA buffer (pCa 8) for 4 h, the metaphase transit time is increased to 53 min without any change in the time of cell plate formation or the rate of chromosome motion. Lanthanum and D600, under conditions in which their access to the plasmalemma has been facilitated by pretreating the cells with cutinase, also markedly extend metaphase and in several instances permanently arrest cells. Lanthanum, however, produce little or no change in cell plate initiation or the rate of chromosome motion. Microscopic observations of the mitotic apparatus in calcium-stressed cells reveal normal chromatin condensation and metaphase progression. Chromosomes partly untwine but remain attached at their kinetochores. It is suggested that a flux of calcium, derived from the extracellular compartment, may cause the final splitting of sister chromosomes and trigger the onset of anaphase. However, once anaphase has begun, chromosome motion and cell plate initiation proceed normally even under conditions of extracellular calcium restriction.

Alterations in metaphase durations in cells derived from human tumours

With the use of time-lapse cinemicrography, we previously found that metaphase durations were significantly prolonged in SV40-transformed human fibroblasts when compared to untransformed controls. This was consistent with some earlier reports and suggested that prolonged metaphases could account for high metaphase/prophase ratios and possibly, in part, for increased mitotic indices seen in advanced tumours. However, there are inconsistencies in the literature and no comparable data available from malignant carcinomas. Presented in this paper are data from two cervical dysplasias, two cases of carcinoma in situ, nine malignant carcinomas and several other types of human cells. The results show that mean metaphase durations were prolonged in cells derived from most of the carcinomas but not from the other cell types. On the other hand, cytokinesis appears to progress more rapidly than normal in most of the tumour-derived cells. These and other findings indicate that the changes are a result of some metabolic alteration common to many but not all tumour cells. For reasons presented, we suggest as a working hypothesis that the alterations may be due to changes in calcium regulation, possibly resulting from alterations in mitochondrial metabolism.

Intracellular free calcium levels are reduced in mitotic Pt K2 epithelial cells

Using a fluorescence ratio method, we have studied the intracellular free calcium levels in individual quin-2-loaded mitotic cells under the microscope. We have found that intracellular free calcium concentrations in Pt K2 epithelial cells drop by approximately 50% as they pass through mitosis. Calcium levels in interphase cells were 53 +/- 7 nM. During prophase, free cytoplasmic calcium begins to decrease, reaching 28 +/- 3 nM in prometaphase. Calcium levels remain low until the nuclear envelope is re-formed in late telophase, when they increase again to interphase levels. This decrease in overall free calcium in mitosis suggests that the mitotic cell has mechanisms for the general sequestration, and perhaps local release, of calcium ions.

Striated flagellar roots: isolation and partial characterization of a calcium-modulated contractile organelle

We report the isolation of striated flagellar roots from the Prasinophycean green alga Tetraselmis striata using sedimentation in gradients of sucrose and flotation on gradients of colloidal silica. PAGE in the presence of 0.1% SDS demonstrates that striated flagellar roots are composed of a number of polypeptides, the most predominant one being a protein of 20,000 Mr. The 20,000 Mr protein band represents approximately 63% of the Coomassie Brilliant Blue staining of gels of isolated flagellar roots. Two-dimensional gel electrophoresis (isoelectric focusing and SDS PAGE) resolves the major 20,000 Mr flagellar root protein into two components of nearly identical Mr, but of differing isoelectric points (i.e., pl's of 4.9 and 4.8), which we have designated 20,000-Mr-alpha and 20,000-Mr-beta, respectively. Densitometric scans of two-dimensional gels of cell extracts indicate that the 20,000-Mr-alpha and -beta polypeptides vary, in their stoichiometry, between 2:1 and 1:1. This variability appears to be related to the state of contraction or extension of the striated flagellar roots at the time of cell lysis. Incubation of cells with 32PO4 followed by analysis of cell extracts by two-dimensional gel electrophoresis and autoradiography reveals that the more acidic 20,000-Mr-beta component is phosphorylated and the 20,000-Mr-alpha component contains no detectable label. These results suggest that the 20,000-Mr-alpha component is converted to the more acidic 20,000-Mr-beta form by phosphorylation. Both the 20,000-Mr-alpha and -beta flagellar root components exhibit a calcium-induced reduction in relative electrophoretic mobilities in two-dimensional alkaline urea gels. Antiserum raised in rabbits against the 20,000-Mr protein binds to both the 20,000-Mr-alpha and 20,000-Mr-beta forms of the flagellar root protein when analyzed by electrophoretic immunoblot techniques. Indirect immunofluorescence on vegetative or interphase cells demonstrate that the antibodies bind to two cyclindrical organelles located in the anterior region of the cell. Immunocytochemical investigations at ultrastructural resolution using this antiserum and a colloidal gold-conjugated antirabbit-IgG reveals immunospecific labeling of striated flagellar roots and their extensions. We conclude that striated flagellar roots are simple ion-sensitive contractile organelles composed predominantly of a 20,000 Mr calcium-binding phosphoprotein, and that this protein is largely responsible for the motile behavior of these organelles.

A calcium-rich intraspindle membrane system in spermatocytes of wolf spiders

The meiotic spindle of spermatocytes of two wolf spiders contains a highly organized system of ER-like membranes. In cells observed ultrastructurally at early prometaphase, these membranes completely invest each bivalent and are present in the periphery of the spindle in association with the centrosomes. By metaphase each bivalent and its kinetochore fibers are completely encased in a tube of this membrane. We have treated living spermatocytes with the permeant, fluorescent-chelate probe, chlorotetracycline (CTC) to determine whether or not the intraspindle membrane system is rich in associated Ca2+. Spider testes were dissected into PIPES-buffered saline containing 200 microM CTC and were kept in this solution for 10 min. Autofluorescence controls were prepared by incubation in saline without CTC, and nonspecific effects of CTC were assessed by incubation for 10 min in 200 microM oxytetracycline (OTC). Neither unstained nor OTC-treated spermatocytes emit significant fluorescence. In contrast, CTC treatment yields bright, punctate fluorescence, which coincides with the distribution of the mitochondria. The plasma membrane is only weakly fluorescent, while the nuclear envelope exhibits prominent fluorescence. The chromosomes are not fluorescent during prophase, but after nuclear envelope breakdown, they become outlined by dim, but distinct fluorescence. As spindle formation commences, the CTC signal from the intraspindle membrane system becomes strong. In some cells, thin lines of CTC fluorescence are apparent in the metaphase half spindle; this fluorescence pattern mimics the distribution of the intraspindle membrane system and suggests that it is rich in associated Ca2+. We suggest that the intraspindle membrane system functions in the regulation of cytosolic Ca2+ during meiosis through sequestration of the cation.