Nucleo-cytoplasmic interactions in the acheivement of nuclear synchrony in DNA synthesis and mitosis in multinucleate cells

Asynchronous nuclear cycles in multinucleated Plasmodium falciparum facilitate rapid proliferation

Malaria-causing parasites proliferate within erythrocytes through schizogony, forming multinucleated stages before cellularization. Nuclear multiplication does not follow a strict geometric 2n progression, and each proliferative cycle produces a variable number of progeny. Here, by tracking nuclei and DNA replication, we show that individual nuclei replicate their DNA at different times, despite residing in a shared cytoplasm. Extrapolating from experimental data using mathematical modeling, we provide strong indication that a limiting factor exists, which slows down the nuclear multiplication rate. Consistent with this prediction, our data show that temporally overlapping DNA replication events were significantly slower than partially overlapping or nonoverlapping events. Our findings suggest the existence of evolutionary pressure that selects for asynchronous DNA replication, balancing available resources with rapid pathogen proliferation.

Cell Cycle Progression and Synchronization: An Overview

The cell cycle is the series of events that take place in a cell that drives it to divide and produce two new daughter cells. Through more than 100 years of efforts by scientists, we now have a much clearer picture of cell cycle progression and its regulation. The typical cell cycle in eukaryotes is composed of the G1, S, G2, and M phases. The M phase is further divided into prophase, prometaphase, metaphase, anaphase, telophase, and cytokinesis. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that controls the activity of various Cdk-cyclin complexes. Most cellular events, including DNA duplication, gene transcription, protein translation, and post-translational modification of proteins, occur in a cell-cycle-dependent manner. To understand these cellular events and their underlying molecular mechanisms, it is desirable to have a population of cells that are traversing the cell cycle synchronously. This can be achieved through a process called cell synchronization. Many methods have been developed to synchronize cells to the various phases of the cell cycle. These methods could be classified into two groups: synchronization methods using chemical inhibitors and synchronization methods without using chemical inhibitors. All these methods have their own merits and shortcomings.

Regulation of Cell Cycle Progression by Growth Factor-Induced Cell Signaling

The cell cycle is the series of events that take place in a cell, which drives it to divide and produce two new daughter cells. The typical cell cycle in eukaryotes is composed of the following phases: G1, S, G2, and M phase. Cell cycle progression is mediated by cyclin-dependent kinases (Cdks) and their regulatory cyclin subunits. However, the driving force of cell cycle progression is growth factor-initiated signaling pathways that control the activity of various Cdk–cyclin complexes. While the mechanism underlying the role of growth factor signaling in G1 phase of cell cycle progression has been largely revealed due to early extensive research, little is known regarding the function and mechanism of growth factor signaling in regulating other phases of the cell cycle, including S, G2, and M phase. In this review, we briefly discuss the process of cell cycle progression through various phases, and we focus on the role of signaling pathways activated by growth factors and their receptor (mostly receptor tyrosine kinases) in regulating cell cycle progression through various phases.

Analysis of localization of cell-cycle regulators in Neurospora crassa

Most fungi are multinucleated organisms. In some fungi, they have asynchronous nuclei in the same cytoplasm. We analyzed a cell-cycle regulation mechanism using a model fungus Neurospora crassa, which can make heterokaryon cells. G1/S cyclin CLN-1 and cyclin-dependent kinase CDC-2 were tagged with different fluorescence in different strains and expressed. By forming a heterokaryon strain of these, two different fluorescence-tagged proteins were expressed in the same cytoplasm. CDC-2 was localized in all nuclei, whereas CLN-1 was not detected in most of the nuclei and was dispersed in the cytoplasm with small granular clusters. This indicates that in multinucleated fungi, cell-cycle regulators, similar to other proteins, are shared around the nuclei regardless of different cell-cycle stages. Moreover, each nucleus can select and use a special cell-cycle regulator only when it is necessary. Fungal nuclei may have a novel pickup mechanism of necessary proteins from their cytoplasm at the point of use.

Clustered nuclei maintain autonomy and nucleocytoplasmic ratio control in a syncytium

Nuclei in syncytia found in fungi, muscles, and tumors can behave independently despite cytoplasmic translation and the homogenizing potential of diffusion. We use a dynactin mutant strain of the multinucleate fungus Ashbya gossypii with highly clustered nuclei to assess the relative contributions of nucleus and cytoplasm to nuclear autonomy. Remarkably, clustered nuclei maintain cell cycle and transcriptional autonomy; therefore some sources of nuclear independence function even with minimal cytosol insulating nuclei. In both nuclear clusters and among evenly spaced nuclei, a nucleus' transcriptional activity dictates local cytoplasmic contents, as assessed by the localization of several cyclin mRNAs. Thus nuclear activity is a central determinant of the local cytoplasm in syncytia. Of note, we found that the number of nuclei per unit cytoplasm was identical in the mutant to that in wild-type cells, despite clustered nuclei. This work demonstrates that nuclei maintain autonomy at a submicrometer scale and simultaneously maintain a normal nucleocytoplasmic ratio across a syncytium up to the centimeter scale.

Nuclear autonomy in multinucleate fungi

Within many fungal syncytia, nuclei behave independently despite sharing a common cytoplasm. Creation of independent nuclear zones of control in one cell is paradoxical considering random protein synthesis sites, predicted rapid diffusion rates, and well-mixed cytosol. In studying the surprising fungal nuclear autonomy, new principles of cellular organization are emerging. We discuss the current understanding of nuclear autonomy, focusing on asynchronous cell cycle progression where most work has been directed. Mechanisms underlying nuclear autonomy are diverse including mRNA localization, ploidy variability, and nuclear spacing control. With the challenges fungal syncytia face due to cytoplasmic size and shape, they serve as powerful models for uncovering new subcellular organization modes, variability sources among isogenic uninucleate cells, and the evolution of multicellularity.

Life as a moving fluid: fate of cytoplasmic macromolecules in dynamic fungal syncytia

In fungal syncytia dozens, or even millions of nuclei may coexist in a single connected cytoplasm. Recent discoveries have exposed some of the adaptations that enable fungi to marshall these nuclei to produce complex coordinated behaviors, including cell growth, nuclear division, secretion and communication. In addition to shedding light on the principles by which syncytia (including embryos and osteoplasts) are organized, fungal adaptations for dealing with internal genetic diversity and physically dynamic cytoplasm may provide mechanistic insights into how cells generally are carved into different functional compartments. In this review we focus on enumerating the physical constraints associated with maintaining macromolecular distributions within a fluctuating and often flowing cytoplasmic interior.

Minimization of extracellular space as a driving force in prokaryote association and the origin of eukaryotes

BACKGROUND

Internalization-based hypotheses of eukaryotic origin require close physical association of host and symbiont. Prior hypotheses of how these associations arose include chance, specific metabolic couplings between partners, and prey-predator/parasite interactions. Since these hypotheses were proposed, it has become apparent that mixed-species, close-association assemblages (biofilms) are widespread and predominant components of prokaryotic ecology. Which forces drove prokaryotes to evolve the ability to form these assemblages are uncertain. Bacteria and archaea have also been found to form membrane-lined interconnections (nanotubes) through which proteins and RNA pass. These observations, combined with the structure of the nuclear envelope and an energetic benefit of close association (see below), lead us to propose a novel hypothesis of the driving force underlying prokaryotic close association and the origin of eukaryotes.

RESULTS

Respiratory proton transport does not alter external pH when external volume is effectively infinite. Close physical association decreases external volume. For small external volumes, proton transport decreases external pH, resulting in each transported proton increasing proton motor force to a greater extent. We calculate here that in biofilms this effect could substantially decrease how many protons need to be transported to achieve a given proton motor force. Based as it is solely on geometry, this energetic benefit would occur for all prokaryotes using proton-based respiration.

CONCLUSIONS

This benefit may be a driving force in biofilm formation. Under this hypothesis a very wide range of prokaryotic species combinations could serve as eukaryotic progenitors. We use this observation and the discovery of prokaryotic nanotubes to propose that eukaryotes arose from physically distinct, functionally specialized (energy factory, protein factory, DNA repository/RNA factory), obligatorily symbiotic prokaryotes in which the protein factory and DNA repository/RNA factory cells were coupled by nanotubes and the protein factory ultimately internalized the other two. This hypothesis naturally explains many aspects of eukaryotic physiology, including the nuclear envelope being a folded single membrane repeatedly pierced by membrane-bound tubules (the nuclear pores), suggests that species analogous or homologous to eukaryotic progenitors are likely unculturable as monocultures, and makes a large number of testable predictions.

REVIEWERS

This article was reviewed by Purificación López-García and Toni Gabaldón.

Nuclear repulsion enables division autonomy in a single cytoplasm

BACKGROUND

Current models of cell-cycle control, based on classic studies of fused cells, predict that nuclei in a shared cytoplasm respond to the same CDK activities to undergo synchronous cycling. However, synchrony is rarely observed in naturally occurring syncytia, such as the multinucleate fungus Ashbya gossypii. In this system, nuclei divide asynchronously, raising the question of how nuclear timing differences are maintained despite sharing a common milieu.

RESULTS

We observe that neighboring nuclei are highly variable in division-cycle duration and that neighbors repel one another to space apart and demarcate their own cytoplasmic territories. The size of these territories increases as a nucleus approaches mitosis and can influence cycling rates. This nonrandom nuclear spacing is regulated by microtubules and is required for nuclear asynchrony, as nuclei that transiently come in very close proximity will partially synchronize. Sister nuclei born of the same mitosis are generally not persistent neighbors over their lifetimes yet remarkably retain similar division cycle times. This indicates that nuclei carry a memory of their birth state that influences their division timing and supports that nuclei subdivide a common cytosol into functionally distinct yet mobile compartments.

CONCLUSIONS

These findings support that nuclei use cytoplasmic microtubules to establish "cells within cells." Individual compartments appear to push against one another to compete for cytoplasmic territory and insulate the division cycle. This provides a mechanism by which syncytial nuclei can spatially organize cell-cycle signaling and suggests size control can act in a system without physical boundaries.

Protein aggregation behavior regulates cyclin transcript localization and cell-cycle control

Little is known about the active positioning of transcripts outside of embryogenesis or highly polarized cells. We show here that a specific G1 cyclin transcript is highly clustered in the cytoplasm of large multinucleate cells. This heterogeneous cyclin transcript localization results from aggregation of an RNA-binding protein, and deletion of a polyglutamine stretch in this protein results in random transcript localization. These multinucleate cells are remarkable in that nuclei cycle asynchronously despite sharing a common cytoplasm. Notably, randomization of cyclin transcript localization significantly diminishes nucleus-to-nucleus differences in the number of mRNAs and synchronizes cell-cycle timing. Thus, nonrandom cyclin transcript localization is important for cell-cycle timing control and arises due to polyQ-dependent behavior of an RNA-binding protein. There is a widespread association between polyQ expansions and RNA-binding motifs, suggesting that this is a broadly exploited mechanism to produce spatially variable transcripts and heterogeneous cell behaviors. PAPERCLIP:

A Monte Carlo simulation of plasmid replication during the bacterial division cycle

Plasmids have cell cycle replication patterns that need to be considered in models of their replication dynamics. To compare current theories for control of plasmid replication with experimental data for timing of plasmid replication with the cell cycle, a Monte Carlo simulation of plasmid replication and partition was developed. High-copy plasmid replication was simulated by incorporating equations previously developed from the known molecular biology of ColE1-type plasmids into the cell-cycle simulation. Two types of molecular mechanisms for low-copy plasmid replication were tested: accumulation of an initiator protein in proportion to cell mass and binding of the plasmid origin to the cell membrane. The low-copy plasmids were partitioned actively, with a specific mechanism to mediate the transfer from mother to daughter cells, whereas the high-copy plasmids were partitioned passively with cell mass.The simulation results and experimental data demonstrate cell-cycle-specific replication for the low-copy F plasmid and cell-cycle-independent replication for the high-copy pBR322, ColBM, and R6K plasmids. The simulation results indicate that synchronous replication at multiple plasmid origins is critical for the cell-cycle-specific pattern observed in rapidly growing cells. Variability in the synchrony of initiation of multiple plasmid origins give rise to a cell-cycle-independent pattern and is offered as a plausible explanation for the controversy surrounding the replication pattern of the low-copy plasmids. A comparison of experimental data and simulation results for the low-copy F plasmid at several growth rates indicates that either initiation mechanism would be sufficient to explain the timing of replication with the cell cycle. The simulation results also demonstrate that, although cell-cycle-specific and cell-cycle independent replication patterns give rise to very different gene-expression patterns during short induction periods in age-selected populations, long-term expression of genes encoded on low-copy and high-copy plasmids in exponentially growing cells have nearly the same patterns. These results may be important for the future use of low-copy plasmids as expression vectors and validate the use of simpler models for high-copy plasmids that do not consider cell-cycle phenomena. (c) 1996 John Wiley & Sons, Inc.

Centriole duplication: analogue control in a digital age

In preparation for mitosis, the centrosome doubles once and only once to provide the two poles of the mitotic spindle. The presence of more than two centrosomes increases the chances that mitosis will be multipolar, and chromosomes will be distributed unequally. Since the number of mother-daughter centriole pairs determines the number of centrosomes, it is important that only one daughter centriole is assembled at, but slightly separated from, the proximal end of each mother centriole. This numerical and spatial specificity has led to the belief that a 'template' on the mother centriole provides a unique site for procentriole assembly. We review observations that are leading to the demise of this intuitively attractive idea. In its place, we are left with the notion that pericentriolar material at the wall of the mother centriole provides a local environment that promotes the assembly of a macromolecular complex that seeds the daughter centriole. Even though the system normally behaves in a digital fashion to go from zero to just one daughter centriole per mother, this behaviour appears to be based in the precise analogue control of multiple proteins, their activities, and the structure provided by the mother centriole.

A conserved G₁ regulatory circuit promotes asynchronous behavior of nuclei sharing a common cytoplasm

Synthesis and accumulation of conserved cell cycle regulators such as cyclins are thought to promote G₁/S and G₂/M transitions in most eukaryotes. When cells at different stages of the cell cycle are fused to form heterokaryons, the shared complement of regulators in the cytoplasm induces the nuclei to become synchronized. However, multinucleate fungi often display asynchronous nuclear division cycles, even though the nuclei inhabit a shared cytoplasm. Similarly, checkpoints can induce nuclear asynchrony in multinucleate cells by arresting only the nucleus that receives damage. The cell biological basis for nuclear autonomy in a common cytoplasm is not known. Here we show that in the filamentous fungus Ashbya gossypii, sister nuclei born from one mitosis immediately lose synchrony in the subsequent G₁ interval. A conserved G₁ transcriptional regulatory circuit involving the Rb-analogue Whi5p promotes the asynchronous behavior yet Whi5 protein is uniformly distributed among nuclei throughout the cell cycle. The homologous Whi5p circuit in S. cerevisiae employs positive feedback to promote robust and coherent entry into the cell cycle. We propose that positive feedback in this same circuit generates timing variability in a multinucleate cell. These unexpected findings indicate that a regulatory program whose products (mRNA transcripts) are translated in a common cytoplasm can nevertheless promote variability in the individual behavior of sister nuclei.

Regularities and irregularities in the cell cycle of the fission yeast, Schizosaccharomyces pombe (a review)

In an exponentially growing wild-type fission yeast culture a size control mechanism ensures that mitosis is executed only if the cells have reached a critical size. However, there is some scattering both in cell length at birth (BL) and in cycle time (CT). By computational simulations we show here that this scattering cannot be explained solely by asymmetric cell division, therefore we assume that nuclear division is a stochastic, asymmetric process as well. We introduce an appropriate stochastic variable into a mathematical model and prove that this assumption is suitable to describe the CT vs. BL graph in a wild-type fission yeast population. In a double mutant of fission yeast (namely wee1-50 cdc25 delta) this CT vs. BL plot is even more curious: cycle time splits into three different values resulting in three clusters in this coordinate system. We show here that it is possible to describe these quantized cycles by choosing the appropriate values of the key parameters of mitotic entry and exit and even more the clustered behavior may be simulated by applying a further stochastic parameter.

From oocyte maturation to the in vitro cell cycle: the history of discoveries of Maturation-Promoting Factor (MPF) and Cytostatic Factor (CSF)

This article briefly reviews the classical cell cycle studies using oocytes and zygotes of mainly amphibians in the past century. The discussions are focused on the investigations into the cytoplasmic factors that regulate meiosis during oocyte maturation and the initiation of mitosis during fertilisation, which were carried out in the author's lab between 1967 and 1987. This chronicle traces the development of the problems and the direction in which their solutions were attempted in the course of these investigations. The author tries to answer the following questions: why he decided to study oocyte maturation, how he discovered progesterone as a maturation-inducing hormone, how he discovered and characterised the cytoplasmic regulators of the cell cycle, Maturation-Promoting Factor (MPF) and Cyto-Static Factor (CSF), and how he invented the method of observing cell cycle processes in a cytoplasmic extract in vitro.

A stochastic, molecular model of the fission yeast cell cycle: role of the nucleocytoplasmic ratio in cycle time regulation

We propose a stochastic version of a recently published, deterministic model of the molecular mechanism regulating the mitotic cell cycle of fission yeast, Schizosaccharomyces pombe. Stochasticity is introduced in two ways: (i) by considering the known asymmetry of cell division, which produces daughter cells of slightly different sizes; and (ii) by assuming that the nuclear volumes of the two newborn cells may also differ. In this model, the accumulation of cyclins in the nucleus is proportional to the ratio of cytoplasmic to nuclear volumes. We have simulated the cell-cycle statistics of populations of wild-type cells and of wee1(-) mutant cells. Our results are consistent with well known experimental observations.

Transition of the blastomere cell cycle from cell size-independent to size-dependent control at the midblastula stage in Xenopus laevis

Dissociated animal cap blastomeres of Xenopus laevis blastulae were cultured at a low Ca level (1 microM) from 9th to 18th cell cycle at 22 +/- 1 degrees C and observed by a time-lapse video recorder. Blastomeres cleaved unequally to increase variability in cell size as cell cycles progressed, but synchronously at a constant cell cycle time of about 30 min up to the 12th cleavage in diploid cells, and up to the 13th cleavage in haploid cells, regardless of their cell sizes. Thereafter, blastomeres cleaved asynchronously at varying cell cycle times in proportion to the inverse square of their radii. The transition from the cell size-independent to -dependent cell cycles occurred at the critical cell radius, 37.5 microm for the diploid and 27.9 microm for the haploid. While the protein synthesis inhibitor, cycloheximide (CHX) lengthened cell cycle times two- to six-fold, epidermal growth factor (EGF) had no significant effect on the cell cycle. CHX-treated blastomeres synchronously cleaved at a constant cell cycle time of 60 min up to the 12th cleavage. Thereafter, cell cycle times became variable in proportion to the inverse square of radii in the presence of CHX at 0.10-0.14 microg/ml, but to the inverse cube of radii at 0.18 microg/ml. The critical cell size of CHX-treated blastomeres for the transition from cell size-independent to -dependent cell cycles remained the same as that of untreated blastomeres. Frequency distributions of cell cycle times of synchronous cell cycles were monomodal with the peak at 30 min, except for CHX-treated blastomeres with the peak at 60 min. In contrast, frequency distributions of asynchronous cell cycles were polymodal with peaks at multiples of a unit time of 30-35 min. To explain these results, we propose that blastomere cytoplasm has 30-min cycles that repeatedly produce mitosis promoting factor (MPF) in a quantity proportional to the cell surface area. MPF is neutralized when it titrates a nuclear inhibitor present in a quantity proportional to the genome size, and sequestered in the nucleus. When the total amount of MPF produced exceeds the threshold required to titrate all of the inhibitor, mitosis is initiated.

Manipulating the Human Embryo: Cell Cycle Checkpoint Controls

Micromanipulation techniques are widely used in assisted human reproduction and it is logical to assume that successes with recent animal cloning will invariably raise the question of human cloning along with its related ethical problems. However, it is often overlooked that even in animals many complications are still associated with this technique. The purpose of our article is to highlight and discuss some of these problems in the context of the eventual use of nuclear and/or cytoplasmic transfer techniques in assisted human reproduction.

Checkpoint control of the G2/M phase transition during the first mitotic cycle in mammalian eggs

The high incidence of chromosomally abnormal human embryos is frequently assumed to be due to a lack of checkpoint controls operating during early embryogenesis. In our study we have analysed when these mechanisms first become functional. Mouse oocytes treated in late metaphase I with either of two different cyclin-dependent kinase inhibitors [butyrolactone 1 (BL1) or 6-dimethylaminopurine (6-DMAP)] form nuclei in the cytoplasm. BL1-treated eggs enter S-phase at 16-18 h post-treatment and, after completion of DNA synthesis, cleave to 2-cell stage embryos. 6-DMAP treatment results in the rapid initiation of DNA synthesis, its completion by 12 h and then arrest in the G2 phase. Thus, two different cell cycle stages can be obtained at the same time point after the initiation of treatment: G1- after BL1 and G2-staged nuclei after 6-DMAP treatment. That this approach greatly facilitates cell cycle studies has been shown by analysing checkpoint function during the first division. Whilst G2-staged eggs enter M phase within 2-3 h when 6-DMAP is washed out, the onset of M phase is delayed after their fusion to G1 (BL1) cells. Here M phase occurs only after the less advanced nucleus completes DNA replication. Our results indicate that checkpoints in mammalian eggs are functional during the first mitotic cycle.

Mitosis in vertebrate somatic cells with two spindles: implications for the metaphase/anaphase transition checkpoint and cleavage

During mitosis an inhibitory activity associated with unattached kinetochores prevents PtK1 cells from entering anaphase until all kinetochores become attached to the spindle. To gain a better understanding of how unattached kinetochores block the metaphase/anaphase transition we followed mitosis in PtK1 cells containing two independent spindles in a common cytoplasm. We found that unattached kinetochores on one spindle did not block anaphase onset in a neighboring mature metaphase spindle 20 microm away that lacked unattached kinetochores. As in cells containing a single spindle, anaphase onset occurred in the mature spindles x = 24 min after the last kinetochore attached regardless of whether the adjacent immature spindle contained one or more unattached kinetochores. These findings reveal that the inhibitory activity associated with an unattached kinetochore is functionally limited to the vicinity of the spindle containing the unattached kinetochore. We also found that once a mature spindle entered anaphase the neighboring spindle also entered anaphase x = 9 min later regardless of whether it contained monooriented chromosomes. Thus, anaphase onset in the mature spindle catalyzes a "start anaphase" reaction that spreads globally throughout the cytoplasm and overrides the inhibitory signal produced by unattached kinetochores in an adjacent spindle. Finally, we found that cleavage furrows often formed between the two independent spindles. This reveals that the presence of chromosomes and/or a spindle between two centrosomes is not a prerequisite for cleavage in vertebrate somatic cells.

Malaria and the cell cycle

The current model of cell cycle control features a succession of active cyclin-CDK (cyclin-dependent kinase) complexes, where accumulation of each successive cyclin leads to activation of its associated kinase. Cell fusion experiments have shown that nuclei sharing common cytoplasm progress through the cell cycle in synchrony. During schizogony of Plasmodium falciparum, nuclear division occurs asynchronously, and thus cannot be regulated by synthesis and accumulation of cyclins in the cytoplasm. We suggest that schizonts must have a ready pool of cyclins for activating all stages of the cycle, and that the cell cycle is regulated independently in each nucleus.

At the heart of the budding yeast cell cycle

It might now seem obvious that the mechanisms regulating cell division would be found to be a highly conserved feature of eukaryotic cells. This was less clear 20 years ago when the pioneering genetic studies of the cell cycle were initiated. This article presents one view as to what lies at the heart of the budding yeast cell cycle. It is written on the premise that most of the key players, such as cyclin-dependent kinases, the anaphase-promoting complex, the origin recognition complex, Cdc6p and Mcm proteins, were performing similar functions in the common ancestor of yeast and man. Ideas about the budding yeast cell cycle might, therefore, have universal significance for other eukaryotic cells.

Response of avian nuclei to mammalian maturation promoting factor (MPF)

Chicken blastodermal cells (stage X) were fused to mouse enucleated oocytes with either no or high maturation promoting factor (MPF) activity. High MPF levels always induced premature chromosome condensation (PCC) irrespective of the number of nuclei fused to a single oocyte. When a single blastodermal cell was fused to a single oocyte without MPF activity the nucleus remained intact for up to 3 h and thereafter underwent PCC. A quite different situation was observed after multiple fusion of several blastodermal cells to a single oocyte without MPF activity. Here, the transplanted nuclei remained intact even after prolonged culture but underwent extensive swelling. DNA synthesis was detected in almost all unfused blastodermal cells. However, after the fusion of several blastodermal cells to a single oocyte no DNA synthesis could be detected. These results provide further evidence that MPF is the universal cell-cycle regulator in the animal kingdom. Its activity is blocked (or neutralised) after fusion to several S-phase cells. Interestingly, our results further suggest that DNA synthesis is suppressed in meiotic cytoplasm even in the presence of an intact nuclear envelope.

S-phase-promoting cyclin-dependent kinases prevent re-replication by inhibiting the transition of replication origins to a pre-replicative state

BACKGROUND

DNA replication and mitosis are triggered by activation of kinase complexes, each made up of a cyclin and a cyclin-dependent kinase (Cdk). It had seemed possible that the association of Cdks with different classes of cyclins specifies whether S phase (replication) or M phase (mitosis) will occur. The recent finding that individual B-type cyclins (encoded by the genes CLB1-CLB6) can have functions in both processes in the budding yeast Saccharomyces cerevisiae casts doubt on this notion.

RESULTS

S. cerevisiae strains lacking C1b1-C1b4 undergo DNA replication once but fail to enter mitosis. We have isolated mutations in two genes, SIM1 and SIM2 (SIM2 is identical to SEC72), which allow such cells to undergo an extra round of DNA replication without mitosis. The Clb5 kinase, which promotes S phase, remains active during the G2-phase arrest of cells of the parental strain, but its activity declines rapidly in sim mutants. Increased expression of the CLB5 gene prevents re-replication. Thus, a cyclin B-kinase that promotes DNA replication in G1-phase cells can prevent re-replication in G2-phase cells. Inactivation of C1b kinases by expression of the specific C1b-Cdk1 inhibitor p40SIC1 is sufficient to induce a prereplicative state at origins of replication in cells blocked in G2/M phase by nocodazole. Re-activation of C1b-Cdk1 kinases induces a second round of DNA replication.

CONCLUSIONS

We propose that S-phase-promoting cyclin B--Cdk complexes prevent re-replication during S, G2 and M phases by inhibiting the transition of replication origins to a pre-replicative state. This model can explain both why origins 'fire' only once per S phase and why S phase is dependent on completion of the preceding M phase.

Nuclear envelope breakdown is under nuclear not cytoplasmic control in sea urchin zygotes

Nuclear envelope breakdown (NEB) and entry into mitosis are though to be driven by the activation of the p34cdc2-cyclin B kinase complex or mitosis promoting factor (MPF). Checkpoint control mechanisms that monitor essential preparatory events for mitosis, such as DNA replication, are thought to prevent entry into mitosis by downregulating MPF activation until these events are completed. Thus, we were surprised to find that when pronuclear fusion in sea urchin zygotes is blocked with Colcemid, the female pronucleus consistently breaks down before the male pronucleus. This is not due to regional differences in the time of MPF activation, because pronuclei touching each other break down asynchronously to the same extent. To test whether NEB is controlled at the nuclear or cytoplasmic level, we activated the checkpoint for the completion of DNA synthesis separately in female and male pronuclei by treating either eggs or sperm before fertilization with psoralen to covalently cross-link base-paired strands of DNA. When only the maternal DNA is cross-linked, the male pronucleus breaks down first. When the sperm DNA is cross-linked, male pronuclear breakdown is substantially delayed relative to female pronuclear breakdown and sometimes does not occur. Inactivation of the Colcemid after female NEB in such zygotes with touching pronuclei yields a functional spindle composed of maternal chromosomes and paternal centrosomes. The intact male pronucleus remains located at one aster throughout mitosis. In other experiments, when psoralen-treated sperm nuclei, over 90% of the zygote nuclei do not break down for at least 2 h after the controls even though H1 histone kinase activity gradually rises close to, or higher than, control mitotic levels. The same is true for normal zygotes treated with aphidicolin to block DNA synthesis. From these results, we conclude that NEB in sea urchin zygotes is controlled at the nuclear, not cytoplasmic, level, and that mitotic levels of cytoplasmic MPF activity are not sufficient to drive NEB for a nucleus that is under checkpoint control. Our results also demonstrate that the checkpoint for the completion of DNA synthesis inhibits NEB by acting primarily within the nucleus, not by downregulating the activity of cytoplasmic MPF.

Initiation of DNA synthesis by nuclei from scrape-ruptured quiescent mammalian cells in high-speed supernatants of Xenopus egg extracts

Demembranated sperm heads, detergent-isolated somatic nuclei and even naked DNA are efficiently replicated in cytoplasmic extracts of activated amphibian eggs, but only after nuclear assembly and the formation of an intact nuclear envelope. DNA synthesis has not previously been shown to be initiated in high-speed (200,000 g) supernatants of egg cytoplasm because they are depleted of the vesicular material required to support nuclear envelope formation. Here we show that mammalian nuclei prepared by scrape-rupture are able to initiate DNA replication in such high-speed supernatants. These nuclei begin DNA synthesis asynchronously. This asynchrony cannot be attributed to differences in the time taken for nuclear assembly. Instead, we suggest that the asynchrony reflects intrinsic differences between nuclei and that these differences are a major cause of cell cycle variability. Our demonstration of initiation in high-speed supernatants now enables the initiation of eukaryotic DNA synthesis to be studied independently of nuclear assembly.

The cell cycle in plant development

The aim of this review is to discuss the molecular controls of the cell cycle in relation to higher plant development. An analysis is made of the current models of the cell cycle based on the biochemistry and genetics of the budding yeast, Saccharomyces cerevisiae and the fission yeast, Schizosaccharomyces pombe. What emerges are universal mechanisms observed in a wide range of taxonomic groups involving a group of protein kinases which regulate the transition from both post-synthetic interphase (G2) to mitosis and from pre-synthetic interphase (G1) to DNA synthetic-(S) phase. The data are consistent in showing the activity of protein kinase complexes operating in conjunction with at least one dephosphorylating enzyme. The natural substrate(s) for the key cell division cycle gene product, p34^{cdc 2} , has yet to be resolved although the nuclear lamins and microtubular apparatus are strong candidates. These models serve as a basis for assessing the cell cycle in higher plants. Mitosis and various stages of nuclear DNA replication are considered in relation to the presumed initiation and termination factors that regulate these events. In order to make a link between the cell cycle and plant development special consideration has been given to plant meristems. In particular, the activity of the cell cycle in cells that have the capacity to regenerate whole tissue systems ('stem cells') within the meristem is discussed. In the root meristem, the quiescent centre cells conform to a stem cell population; a non-cycling stem cell may be immune to the morphogenetic signals that cause cycling cells to arrest and differentiate. The pericycle may act as a vestigial stem cell population. The shoot apex is also discussed in relation to both vegetative and floral growth. Although gradients of cell division exist in shoot meristems it is far less obvious where 'stem', or founder, cells reside in the apex. The way in which the cell cycle shortens on transition to floral growth is considered critical for identifying when the meristem becomes florally determined. Temperature and toxic metals are given special attention where it is emphasized that G1 phase becomes protracted when plants are stressed. Species that can tolerate stressful environments may have meristems in which a greater number of cells are competent for division. Finally, the cell cycle in vitro is discussed in relation to rapid changes in gene expression which are linked to the transition from G1 to S phase. The latter emerges as a key cell cycle transition for plant meristems both in vivo and in vitro. CONTENTS Summary 1 I. Introduction 2 II. The cell division cycle (cdc) genes 2 III. The plant cell cycle 5 IV. Meristems 9 V. Effects of external stress on the cell cycle in plant meristems 14 VI. The plant cell cycle in vitro 15 VII. Conclusions 15 Acknowledgements 16 References 16.

Gamete interactions and the fate of sperm organelles in fertilized echinoderm eggs

Investigations of gamete fusion, sperm entry and the fate of the sperm nucleus, plasma membrane, mitochondrion, and axonemal complex in fertilized echinoderm eggs are reviewed. The timing of gamete fusion with respect to the onset of electrical activity characteristic of the activated egg and the affects of fixation conditions on the stability of fusing membranes are discussed. Observations from investigations using cationized ferritin labeled gametes and immunogold cytochemistry to demonstrate the mixing of sperm plasma membrane components within the egg plasma membrane, in particular along the surface of the fertilization cone, are compared with results from studies in somatic cells. Transformations of the sperm nucleus into a male pronucleus, consisting of sperm nuclear envelope breakdown, chromatin dispersion, and formation of a pronuclear envelope, are correlated with recent biochemical observation of similar processes in other cellular systems. Fates of the sperm mitochondrion and axonemal complex are examined.

S phase of the cell cycle

In each cell cycle the complex structure of the chromosome must be replicated accurately. In the last few years there have been major advances in understanding eukaryotic chromosome replication. Patterns of replication origins have been mapped accurately in yeast chromosomes. Cellular replication proteins have been identified by fractionating cell extracts that replicate viral DNA templates in vitro. Cell-free systems that initiate eukaryotic DNA replication in vitro have demonstrated the importance of complex nuclear architecture in the control of DNA replication. Although the events of S phase were relatively neglected for many years, knowledge of DNA replication is now advancing rapidly in step with other phases of the cell cycle.

Induction of premature chromosome condensation in Drosophila melanogaster-rat heterokaryons

After fusion mediated by polyethylene glycol (PEG) plus dimethyl sulfoxide (DMSO) between rat and Drosophila melanogaster cells cultured in vitro, the phenomenon of premature chromosome condensation (PCC) induced by mitotic rat chromosomes was detected in Drosophila nuclei. Exceptionally PCC was induced in rat nuclei but only in the presence of a very high ploidy level of Drosophila mitotic chromosomes. This provides further evidence of the lack of species specificity and of the effect of dosage of the PCC inducing factors, even among very distant species.

Roles of cytosol and cytoplasmic particles in nuclear envelope assembly and sperm pronuclear formation in cell-free preparations from amphibian eggs

A cell-free cytoplasmic preparation from activated Rana pipiens eggs could induce in demembranated Xenopus laevis sperm nuclei morphological changes similar to those seen during pronuclear formation in intact eggs. The condensed sperm chromatin underwent an initial rapid, but limited, dispersion. A nuclear envelope formed around the dispersed chromatin and the nuclei enlarged. The subcellular distribution of the components required for these changes was examined by separating the preparations into soluble (cytosol) and particulate fractions by centrifugation at 150,000 g for 2 h. Sperm chromatin was incubated with the cytosol or with the particulate material after it had been resuspended in either the cytosol, heat-treated (60 or 100 degrees C) cytosol or buffer. We found that the limited dispersion of chromatin occurred in each of these ooplasmic fractions, but not in the buffer alone. Nuclear envelope assembly required the presence of both untreated cytosol and particulate material. Ultrastructural examination of the sperm chromatin during incubation in the preparations showed that membrane vesicles of approximately 200 nm in diameter, found in the particulate fraction, flattened and fused together to contribute the membranous components of the nuclear envelope. The enlargement of the sperm nuclei occurred only after the nuclear envelope formed. The pronuclei formed in the cell-free preparations were able to incorporate [3H]dTTP into DNA. This incorporation was inhibited by aphidicolin, suggesting that the DNA synthesis by the pronuclei was dependent on DNA polymerase-alpha. When sperm chromatin was incubated greater than 3 h, the chromatin of the pronuclei often recondensed to form structures resembling mitotic chromosomes within the nuclear envelope. Therefore, it appeared that these ooplasmic preparations could induce, in vitro, nuclear changes resembling those seen during the first cell cycle in the zygote.

Synchronous DNA synthesis and mitosis in multinucleate cells with one chromosome in each nucleus

Multinucleate cells were induced by colcemid treatment in PtK1 cells in culture. DNA synthesis and mitotic behavior of those cells in which each nucleus contained a single chromosome were studied. More than 80% of such cells showed synchronous DNA synthesis and mitosis in all nuclei. As these genetically different nuclei respond identically to the molecules that initiate DNA synthesis and mitosis, intranuclear control of initiation of DNA synthesis and induction of mitosis by genes on individual chromosomes can be excluded. The occasional occurrence of asynchronous division in multinucleate cells is assumed to result from unequal availability of the inducer molecules to the individual nuclei.

Control mechanisms of the cell cycle: role of the spatial arrangement of spindle components in the timing of mitotic events

To characterize the control mechanisms for mitosis, we studied the relationship between the spatial organization of microtubules in the mitotic spindle and the timing of mitotic events. Spindles of altered geometry were produced in sea urchin eggs by two methods: (a) early prometaphase spindles were cut into half spindles by micromanipulation or (b) mercaptoethanol was used to indirectly induce the formation of spindles with only one pole. Cells with monopolar spindles produced by either method required an average of 3 X longer than control cells to traverse mitosis. By the time the control cells started their next mitosis, the experimental cells were usually just finishing the original mitosis. In all cases, only the time from nuclear envelope breakdown to the start of telophase was prolonged. Once the cells entered telophase, events leading to the next mitosis proceeded with normal timing. Once prolonged, the cell cycle never resynchronized with the controls. Several types of control experiments showed that were not an artifact of the experimental techniques. These results show that the spatial arrangement of spindle components plays an important role in the mechanisms that control the timing of mitotic events and the timing of the cell cycle as a whole.