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

An autonomous mathematical model for the mammalian cell cycle

A mathematical model for the mammalian cell cycle is developed as a system of 13 coupled nonlinear ordinary differential equations. The variables and interactions included in the model are based on detailed consideration of available experimental data. A novel feature of the model is inclusion of cycle tasks such as origin licensing and initiation, nuclear envelope breakdown and kinetochore attachment, and their interactions with controllers (molecular complexes involved in cycle control). Other key features are that the model is autonomous, except for a dependence on external growth factors; the variables are continuous in time, without instantaneous resets at phase boundaries; mechanisms to prevent rereplication are included; and cycle progression is independent of cell size. Eight variables represent cell cycle controllers: the Cyclin D1-Cdk4/6 complex, APC^Cdh1, SCF^βTrCP, Cdc25A, MPF, NuMA, the securin-separase complex, and separase. Five variables represent task completion, with four for the status of origins and one for kinetochore attachment. The model predicts distinct behaviors corresponding to the main phases of the cell cycle, showing that the principal features of the mammalian cell cycle, including restriction point behavior, can be accounted for in a quantitative mechanistic way based on known interactions among cycle controllers and their coupling to tasks. The model is robust to parameter changes, in that cycling is maintained over at least a five-fold range of each parameter when varied individually. The model is suitable for exploring how extracellular factors affect cell cycle progression, including responses to metabolic conditions and to anti-cancer therapies.

The genetics and cell biology of Wolbachia-host interactions

Wolbachia are gram-negative bacteria that are widespread in nature, carried by the majority of insect species as well as some mites, crustaceans, and filarial nematodes. Wolbachia can range from parasitic to symbiotic, depending upon the interaction with the host species. The success of Wolbachia is attributed to efficient maternal transmission and manipulations of host reproduction that favor infected females, such as sperm-egg cytoplasmic incompatibility (CI). Much remains unknown about the mechanistic basis for Wolbachia-host interactions. Here we summarize the current understanding of Wolbachia interaction with insect hosts, with a focus on Drosophila. The areas of discussion include Wolbachia transmission in oogenesis, Wolbachia distribution in spermatogenesis, induction and rescue of the CI phenotype, Wolbachia genomics, and Wolbachia-membrane interactions.

Grapes(Chk1) prevents nuclear CDK1 activation by delaying cyclin B nuclear accumulation

Entry into mitosis is characterized by a dramatic remodeling of nuclear and cytoplasmic compartments. These changes are driven by cyclin-dependent kinase 1 (CDK1) activity, yet how cytoplasmic and nuclear CDK1 activities are coordinated is unclear. We injected cyclin B (CycB) into Drosophila melanogaster embryos during interphase of syncytial cycles and monitored effects on cytoplasmic and nuclear mitotic events. In untreated embryos or embryos arrested in interphase with a protein synthesis inhibitor, injection of CycB accelerates nuclear envelope breakdown and mitotic remodeling of the cytoskeleton. Upon activation of the Grapes(checkpoint kinase 1) (Grp(Chk1))-dependent S-phase checkpoint, increased levels of CycB drives cytoplasmic but not nuclear mitotic events. Grp(Chk1) prevents nuclear CDK1 activation by delaying CycB nuclear accumulation through Wee1-dependent and independent mechanisms.

Cyclin E/Cdk2 is required for sperm maturation, but not DNA replication, in early sea urchin embryos

The cell cycle is driven by the activity of cyclin/cdk complexes. In somatic cells, cyclin E/cdk2 oscillates throughout the cell cycle and has been shown to promote S-phase entry and initiation of DNA replication. In contrast, cyclin E/cdk2 activity remains constant throughout the early embryonic development of the sea urchin and localizes to the sperm nucleus following fertilization. We now show that cyclin E localization to the sperm nucleus following fertilization is not unique to the sea urchin, but also occurs in the surf clam, and inhibition of cyclin E/cdk2 activity by roscovitine inhibits the morphological changes indicative of male pronuclear maturation in sea urchin zygotes. Finally, we show that inhibition of cyclin E/cdk2 activity does not block DNA replication in the early cleavage cycles of the sea urchin. We conclude that cyclin E/cdk2 activity is required for male pronuclear maturation, but not for initiation of DNA replication in early sea urchin development.

Centrosomes promote timely mitotic entry in C. elegans embryos

Several mitotic regulators, including Cyclin B1/Cdk1, are present at centrosomes prior to mitosis onset, but it is unclear whether centrosomes promote mitotic entry in vivo. Here we developed a sensitive assay in C. elegans embryos for the temporal analysis of mitotic entry, in which the male and female pronuclei undergo asynchronous entry into mitosis when separated from one another. Using this assay, we found that centrosome integrity is necessary for timing mitotic entry. Centrosomes do not function in this instance through their ability to nucleate microtubules. Instead, centrosomes serve to focus the Aurora A kinase AIR-1, which is essential for timely mitotic entry. Furthermore, analysis of embryos in which centrosomes and pronuclei are detached from one another demonstrates that centrosomes are sufficient to promote mitosis onset. Together, our findings support a model in which centrosomes serve as integrative centers for mitotic regulators and thus trigger mitotic entry in a timely fashion.

The Drosophila calcipressin sarah is required for several aspects of egg activation

Activation of mature oocytes initiates development by releasing the prior arrest of female meiosis, degrading certain maternal mRNAs while initiating the translation of others, and modifying egg coverings. In vertebrates and marine invertebrates, the fertilizing sperm triggers activation events through a rise in free calcium within the egg. In insects, egg activation occurs independently of sperm and is instead triggered by passage of the egg through the female reproductive tract ; it is unknown whether calcium signaling is involved. We report here that mutations in sarah, which encodes an inhibitor of the calcium-dependent phosphatase calcineurin, disrupt several aspects of egg activation in Drosophila. Eggs laid by sarah mutant females arrest in anaphase of meiosis I and fail to fully polyadenylate and translate bicoid mRNA. Furthermore, sarah mutant eggs show elevated cyclin B levels, indicating a failure to inactivate M-phase promoting factor (MPF). Taken together, these results demonstrate that calcium signaling is involved in Drosophila egg activation and suggest a molecular mechanism for the sarah phenotype. We also find the conversion of the sperm nucleus into a functional male pronucleus is compromised in sarah mutant eggs, indicating that the Drosophila egg's competence to support male pronuclear maturation is acquired during activation.

Inactivation of MAPK in mature oocytes triggers progression into mitosis via a Ca2+-dependent pathway but without completion of S phase

Unfertilized sea urchin eggs that are arrested at G1 phase after completion of meiosis contain a highly phosphorylated mitogen-activated protein (MAP) kinase (MAPK), the ERK-like protein (ERK-LP). Several data including our previous results show that ERK-LP is inactivated after fertilization, which agrees with results obtained in other species including Xenopus, starfish and mammals. The question is to elucidate the function of a high MAPK activity in sea urchin eggs. We report here that dephosphorylation of ERK-LP with very low concentrations of two MEK inhibitors, PD98059 or U0126, triggers entry into mitosis. Under these conditions, recurrent oscillations of the phosphorylation of ERK-LP and of a tyrosine residue in Cdc2 occur, and the intracellular Ca2+ level (Ca2+i) progressively and slowly increases. Nuclear envelope breakdown and all mitotic events initiated after dephosphorylation of ERK-LP are inhibited when changes in Ca2+i are prevented; however, they are independent of the intracellular pH. These results suggest that inactivation of a MEK-ERK pathway, normally induced after fertilization of sea urchin eggs, triggers entry into mitosis by altering Ca2+i but cannot trigger full DNA replication. We discuss the hypothesis that neither inactivation nor activation of a MEK-ERK pathway is required for S phase completion in sea urchin egg.

A genetic test of the role of the maternal pronucleus in Wolbachia-induced cytoplasmic incompatibility in Drosophila melanogaster

Cytoplasmic incompatibility (CI) is a reproductive sterility found in arthropods that is caused by the endoparasitic bacteria Wolbachia. In CI, host progeny fail to develop during early embryogenesis if Wolbachia-infected males fertilize uninfected females. It is widely accepted that this lethality is caused by some unknown Wolbachia-induced modification of the paternal nuclear material in the host testes. However, the direct means by which this modification leads to early embryonic death are currently unresolved. Results from previous studies suggested that CI lethality occurs as a result of asynchrony in cell cycle timing between the paternal and maternal pronuclei. This hypothesis can be tested experimentally by the prediction that the Wolbachia-modified paternal pronucleus should support androgenetic development (i.e., from the paternal pronucleus only). Using specific mutations in Drosophila melanogaster that produce androgenetic progeny, we demonstrate that the Wolbachia-induced modification inhibits this type of development. This result suggests that CI occurs independently of the maternal pronucleus and argues against pronuclear asynchrony as the primary cause of CI lethality. We propose that CI occurs instead as the result of either a developmentally incompetent paternal pronucleus or asynchrony between the paternal pronucleus and the cell cycle of the egg cytoplasm.

Effects of Enucleation and Caffeine on Maturation-Promoting Factor (MPF) and Mitogen-Activated Protein Kinase (MAPK) Activities in Ovine Oocytes Used as Recipient Cytoplasts for Nuclear Transfer1

In general, oocytes arrested at metaphase of the second meiotic division (MII) are used as recipient cytoplasts for nuclear transfer (NT) procedures. MII oocytes contain high levels of maturation-promoting factor (MPF) and mitogen-activated protein kinase (MAPK), which cause nuclear envelope breakdown (NEBD) and premature chromosome condensation (PCC) in the transferred nucleus and have been implicated in nuclear reprogramming. However, the occurrence of NEBD and the extent of PCC are variable between individual oocytes and species and are dependent on donor cell type and cell cycle stage. Enucleation, which removes oocyte cytoplasm, may reduce MPF and MAPK activities and reduce reprogramming; conversely, increasing kinase activities may increase reprogramming. We compared the effects of enucleation of ovine oocytes at anaphase/telophase of the first meiotic division (AI-TI) and at MII. MPF and MAPK activities were maximal at MII; blind enucleation at AI-TI was more efficient than at MII and removed a smaller volume of cytoplasm. Neither protocol significantly affected the activity of either kinase and the fate of the donor nucleus; however, enucleation per se significantly reduced the occurrence of NEBD in NT embryos. Treatment with 10 mM caffeine significantly increased the activities of both kinases and the occurrence of NEBD but did not affect the frequency of development to the blastocyst stage; however, a significant increase in total cell numbers was observed. The results show that caffeine can increase MPF and MAPK activities in ovine oocytes and that this may contribute to an increased reprogramming in NT embryos.

Inhibition of cysteine protease activity disturbs DNA replication and prevents mitosis in the early mitotic cell cycles of sea urchin embryos

Recent findings suggested that the role of cysteine proteases would not be limited to protein degradation in lysosomes but would also play regulatory functions in more specific cell mechanisms. We analyzed here the role of these enzymes in the control of cell cycle during embryogenesis. The addition of the potent cysteine protease inhibitor E64d to newly fertilized sea urchin eggs disrupted cell cycle progression, affecting nuclear as well as cytoplasmic characteristic events. Monitoring BrdU incorporation in E64d treated eggs demonstrated that DNA replication is severely disturbed. Moreover, this drug treatment inhibited male histones degradation, a step that is necessary for sperm chromatin remodeling and precedes the initiation of DNA replication in control eggs. This inhibition likely explains the DNA replication disturbance and suggests that S phase initiation requires cysteine protease activity. In turn, activation of the DNA replication checkpoint could be responsible for the consecutive block of nuclear envelope breakdown (NEB). However, in sea urchin early embryos this checkpoint doesn't control the mitotic cytoplasmic events that are not tightly coupled with NEB. Thus the fact that microtubule spindle is not assembled and cyclin B-cdk1 not activated under E64d treatment more likely rely on a distinct mechanism. Immunofluorescence experiments indicated that centrosome organization was deficient in absence of cysteine protease activity. This potentially accounts for mitotic spindle disruption and for cyclin B mis-localization in E64d treated eggs. We conclude that cysteine proteases are essential to trigger S phase and to promote M phase entry in newly fertilized sea urchin eggs.

The maternal-effect gene futile cycle is essential for pronuclear congression and mitotic spindle assembly in the zebrafish zygote

Embryos have been successfully used for the general study of the cell cycle. Although there are significant differences between the early embryonic and the somatic cell cycle in vertebrates, the existence of specialised factors that play a role during the early cell cycles has remained elusive. We analysed a lethal recessive maternal-effect mutant, futile cycle (fue), isolated in a maternal-effect screen for nuclear division defects in the zebrafish (Danio rerio). The pronuclei fail to congress in zygotes derived from homozygous fue mothers. In addition, a defect in the formation of chromosomal microtubules prevents mitotic spindle assembly and thus chromosome segregation in fue zygotes. However, centrosomal functions do not appear to be affected in fue embryos, suggesting this mutant blocks a subset of microtubule functions. Cleavage occurs normally for several divisions resulting in many anucleate cells, thus showing that nuclear- and cell division can be uncoupled genetically. Therefore, we propose that in mitotic spindle assembly chromosome-dependent microtubule nucleation is essential for the coupling of nuclear and cell division.

Identification of Wolbachia--host interacting factors through cytological analysis

Manipulation of host reproduction and efficient maternal transmission have facilitated the global spread of Wolbachia through millions of insect species. Cytological studies of the most common Wolbachia-induced phenotype, cytoplasmic incompatibility (CI), demonstrate that Wolbachia induce CI by altering host cell cycle timing. Cytological analyses also suggest that microtubules and motor proteins may play a role in the maternal and somatic transmission of Wolbachia.

Anti-mitotic activity towards sea urchin embryos in extracts from the marine haptophycean Phaeocystis pouchetii (Hariot) Lagerheim collected along the coast of northern Norway

The marine bloom-forming alga Phaeocystis pouchetii is suspected to produce some toxic compound responsible for reduced growth, fecundity and survival of other marine organisms. Sea urchin early development was used as a model to investigate the degree and nature of toxicity. Colonial cells of P. pouchetii were collected during its spring-bloom along the coast of northern Norway and maintained in culture for a short period of time in order to evaluate the concentration of toxic compounds present inside the cells or excreted to the surrounding seawater medium. Cells were harvested by filtration and toxins were extracted separately from the collected cells and the filtrate using organic solvents. We found that extracts from the filtered seawater at a concentration corresponding to 9.0 x 10(5) cells ml(-1) completely blocked cell divisions in embryos of the sea urchin Sphaerechinus granularis, whereas extracts from intact algal cells were only mildly cytotoxic. When the extracts from seawater culture medium were purified by RP-HPLC, cytotoxic activity towards S. granularis embryos was recovered in three consecutive fractions. Moreover, unfertilised eggs incubated in the active HPLC fractions became unproductive, whereas incubation of sperm gave a reduced fertilisation rate. This anti-proliferative effect was further characterized by immunofluorescence staining of sea urchin embryos. DNA labelling revealed that incubating sea urchin embryos in the purified algal extracts inhibited both pronuclei migration and fusion. Incorporation and detection of the DNA-base analogue 5-bromo-2-deoxyuridine showed that DNA-replication was blocked. Furthermore, staining of alpha-tubulin subunits demonstrated that embryonic tubulin organisation was altered. We conclude that P. pouchetii produce some anti-mitotic compound, and that senescent colonial cells to a great extent excrete this compound to their surroundings.

Presence of a nucleus or nucleus-deriving factors is indispensable for the formation of the spindle, the diastema and the cleavage furrow in the blastomere of the Xenopus embryo

The present study examines the indispensability of a nucleus or nucleus-deriving factors in the induction of cleavage in Xenopus eggs by testing cleavage in Xenopus eggs fertilized with ultraviolet (UV)-damaged sperm and deprived of the female nucleus. These eggs, which contain only one UV-damaged nucleus with one set of centrioles, undergo unique cleavages. Cleavage takes place in only one of the two blastomeres formed by the immediately preceding cleavage. Histologically, only one nucleus, which does not appear to be organized into typical chromosomes, is found in one of the two blastomeres formed by the immediately preceding cleavage. The typical bipolar spindle and the diastema, or a slit of astral rays, are formed in the blastomere that contains the nucleus. By contrast, only asters lacking the spindle and the diastema are formed in the remaining blastomeres, which do not contain a nucleus. The same results are obtained in eggs that contain two UV-damaged nuclei with one set of centrioles. In these eggs, cleavage appears to occur in one or two blastomeres that contain either or both of the nuclei and one bipolar spindle. In eggs that contain one intact and one UV-damaged nuclei, cleavage takes place quite normally with each blastomere containing one nucleus or one set of chromosomes as well as one bipolar spindle. Thus, there is a very close correlation between the presence of a nucleus and the formation of the mitotic spindle, the diastema and the cleavage furrow in the blastomeres of Xenopus embryos. We conclude that the presence of a nucleus or nucleus-deriving factors is indispensable for the formation of the bipolar spindle, the diastema and the cleavage furrow in the blastomeres of the Xenopus embryos.

p34(Cdc2) kinase activity is excluded from the nucleus during the radiation-induced G(2) arrest in HeLa cells

The progression of cells from G(2) into mitosis is blocked by exposure to DNA-damaging agents such as ionizing radiation. This G(2) delay is associated with reduced cyclin B1-specific associated histone H1 kinase activity, increased inhibitory phosphorylation of p34(Cdc2), and depressed cyclin B1 levels in HeLa cells. Induction of cyclin B1 or expression of Cdc2AF, a mutant p34(Cdc2) that lacks the sites of inhibitory phosphorylation, only partially reverses the radiation-associated G(2) delay, although both maneuvers rapidly result in increased histone H1 kinase activity. To account for the persistent G(2) delay in the face of active p34(Cdc2) kinase, we determined the location of the kinase activity. Although p34(Cdc2) was active in the cytoplasm, the nuclear p34(Cdc2) was inactive. Irradiation led to nuclear accumulation of the inactive tyrosine-phosphorylated form of p34(Cdc2), whereas the active form was seen in the cytoplasm. At later times when cells had resumed cell cycle progression, nuclear kinase activity was detectable. These results give evidence of segregation of cytoplasmic and nuclear kinase activity after DNA damage that has the effect of enhancing checkpoint control. Shielding the nucleus from the potentially deleterious effects of kinase activity after DNA damage may help irradiated human cancer cells respond to irradiation.

Nucleo-cytoplasmic interactions that control nuclear envelope breakdown and entry into mitosis in the sea urchin zygote

In sea urchin zygotes and mammalian cells nuclear envelope breakdown (NEB) is not driven simply by a rise in cytoplasmic cyclin dependent kinase 1-cyclin B (Cdk1-B) activity; the checkpoint monitoring DNA synthesis can prevent NEB in the face of mitotic levels of Cdk1-B. Using sea urchin zygotes we investigated whether this checkpoint prevents NEB by restricting import of regulatory proteins into the nucleus. We find that cyclin B1-GFP accumulates in nuclei that cannot complete DNA synthesis and do not break down. Thus, this checkpoint limits NEB downstream of both the cytoplasmic activation and nuclear accumulation of Cdk1-B1. In separate experiments we fertilize sea urchin eggs with sperm whose DNA has been covalently cross-linked to inhibit replication. When the pronuclei fuse, the resulting zygote nucleus does not break down for >180 minutes (equivalent to three cell cycles), even though Cdk1-B activity rises to greater than mitotic levels. If pronuclear fusion is prevented, then the female pronucleus breaks down at the normal time (average 68 minutes) and the male pronucleus with cross-linked DNA breaks down 16 minutes later. This male pronucleus has a functional checkpoint because it does not break down for >120 minutes if the female pronucleus is removed just prior to NEB. These results reveal the existence of an activity released by the female pronucleus upon its breakdown, that overrides the checkpoint in the male pronucleus and induces NEB. Microinjecting wheat germ agglutinin into binucleate zygotes reveals that this activity involves molecules that must be actively translocated into the male pronucleus.

Dissection of cell division processes in the one cell stage Caenorhabditis elegans embryo by mutational analysis

To identify novel components required for cell division processes in complex eukaryotes, we have undertaken an extensive mutational analysis in the one cell stage Caenorhabditis elegans embryo. The large size and optical properties of this cell permit observation of cell division processes with great detail in live specimens by simple differential interference contrast (DIC) microscopy. We have screened an extensive collection of maternal-effect embryonic lethal mutations on chromosome III with time-lapse DIC video microscopy. Using this assay, we have identified 48 mutations in 34 loci which are required for specific cell division processes in the one cell stage embryo. We show that mutations fall into distinct phenotypic classes which correspond, among others, to the processes of pronuclear migration, rotation of centrosomes and associated pronuclei, spindle assembly, chromosome segregation, anaphase spindle positioning, and cytokinesis. We have further analyzed pronuclear migration mutants by indirect immunofluorescence microscopy using antibodies against tubulin and ZYG-9, a centrosomal marker. This analysis revealed that two pronuclear migration loci are required for generating normal microtubule arrays and four for centrosome separation. All 34 loci have been mapped by deficiencies to distinct regions of chromosome III, thus paving the way for their rapid molecular characterization. Our work contributes to establishing the one cell stage C. elegans embryo as a powerful metazoan model system for dissecting cell division processes.

cut11(+): A gene required for cell cycle-dependent spindle pole body anchoring in the nuclear envelope and bipolar spindle formation in Schizosaccharomyces pombe

The "cut" mutants of Schizosaccharomyces pombe are defective in spindle formation and/or chromosome segregation, but they proceed through the cell cycle, resulting in lethality. Analysis of temperature-sensitive alleles of cut11(+) suggests that this gene is required for the formation of a functional bipolar spindle. Defective spindle structure was revealed with fluorescent probes for tubulin and DNA. Three-dimensional reconstruction of mutant spindles by serial sectioning and electron microscopy showed that the spindle pole bodies (SPBs) either failed to complete normal duplication or were free floating in the nucleoplasm. Localization of Cut11p tagged with the green fluorescent protein showed punctate nuclear envelope staining throughout the cell cycle and SPBs staining from early prophase to mid anaphase. This SPB localization correlates with the time in the cell cycle when SPBs are inserted into the nuclear envelope. Immunoelectron microscopy confirmed the localization of Cut11p to mitotic SPBs and nuclear pore complexes. Cloning and sequencing showed that cut11(+) encodes a novel protein with seven putative membrane-spanning domains and homology to the Saccharomyces cerevisiae gene NDC1. These data suggest that Cut11p associates with nuclear pore complexes and mitotic SPBs as an anchor in the nuclear envelope; this role is essential for mitosis.

Entry into mitosis in vertebrate somatic cells is guarded by a chromosome damage checkpoint that reverses the cell cycle when triggered during early but not late prophase

When vertebrate somatic cells are selectively irradiated in the nucleus during late prophase (<30 min before nuclear envelope breakdown) they progress normally through mitosis even if they contain broken chromosomes. However, if early prophase nuclei are similarly irradiated, chromosome condensation is reversed and the cells return to interphase. Thus, the G2 checkpoint that prevents entry into mitosis in response to nuclear damage ceases to function in late prophase. If one nucleus in a cell containing two early prophase nuclei is selectively irradiated, both return to interphase, and prophase cells that have been induced to returned to interphase retain a normal cytoplasmic microtubule complex. Thus, damage to an early prophase nucleus is converted into a signal that not only reverses the nuclear events of prophase, but this signal also enters the cytoplasm where it inhibits e.g., centrosome maturation and the formation of asters. Immunofluorescent analyses reveal that the irradiation-induced reversion of prophase is correlated with the dephosphorylation of histone H1, histone H3, and the MPM2 epitopes. Together, these data reveal that a checkpoint control exists in early but not late prophase in vertebrate cells that, when triggered, reverses the cell cycle by apparently downregulating existing cyclin-dependent kinase (CDK1) activity.

The paternal effect gene ms(3)sneaky is required for sperm activation and the initiation of embryogenesis in Drosophila melanogaster

Although a large number of maternal factors are known to be essential for fertilization or the earliest stages of embryogenesis in Drosophila melanogaster, the role of paternally supplied products is not clearly understood. Paternal effect mutations provide a means to identify factors specifically required by the sperm after its entry into the egg. Here we describe the third strict paternal effect gene to be identified in Drosophila ms(3)sneaky(snky), which defines the earliest developmental arrest phenotype so far described. Characterization of two independently isolated snky mutations showed that they affected male fertility, but not viability or female fertility. Cytological analyses showed that spermatogenesis proceeded normally in snky males. However, the snky defect was evident after sperm entry into the egg; snky sperm did not undergo nuclear decondensation, form a functional male pronucleus, or initiate mitotic divisions in the egg. Immunolocalization of tubulin and Drosophila Centrosomin, a known centrosomal component, showed that snky-inseminated eggs failed to reconstitute a microtubule-organizing center. In addition, snky sperm chromatin retained the histochemical properties of mature sperm chromatin for several hours after sperm entry, showed reduced staining with membrane-impermeant nuclear dyes, and failed to replicate. We conclude that the snky+ product is required for the initial response of the sperm to cytoplasmic cues in the egg and for the subsequent initiation of embryogenesis in Drosophila. We suggest that all of the snky defects can be explained by the failure of the sperm plasma membrane to break down after entry into the egg.

The coordination of centrosome reproduction with nuclear events of the cell cycle in the sea urchin zygote

Centrosomes repeatedly reproduce in sea urchin zygotes arrested in S phase, whether cyclin-dependent kinase 1-cyclin B (Cdk1-B) activity remains at prefertilization levels or rises to mitotic values. In contrast, when zygotes are arrested in mitosis using cyclin B Delta-90, anaphase occurs at the normal time, yet centrosomes do not reproduce. Together, these results reveal the cell cycle stage specificity for centrosome reproduction and demonstrate that neither the level nor the cycling of Cdk1-B activity coordinate centrosome reproduction with nuclear events. In addition, the proteolytic events of the metaphase-anaphase transition do not control when centrosomes duplicate. When we block protein synthesis at first prophase, the zygotes divide and arrest before second S phase. Both blastomeres contain just two complete centrosomes, which indicates that the cytoplasmic conditions between mitosis and S phase support centrosome reproduction. However, the fact that these daughter centrosomes do not reproduce again under such supportive conditions suggests that they are lacking a component required for reproduction. The repeated reproduction of centrosomes during S phase arrest points to the existence of a necessary "licensing" event that restores this component to daughter centrosomes during S phase, preparing them to reproduce in the next cell cycle.

Nuclear envelope dynamics during male pronuclear development

Upon fertilization, the sperm nucleus undergoes reactivation. The poreless sperm nuclear envelope is replaced by a functional male pronuclear envelope and the highly compact male chromatin decondenses. Here some recent evidence is examined: that disassembly of the sperm lamina is required for chromatin decondensation, that remnant portions of the sperm nuclear envelope target the binding of egg membrane vesicles that form the male pronuclear envelope, that functional male pronuclear envelopes containing lamin B receptor assemble prior to lamin import and lamina formation, and that lamina assembly drives male pronuclear swelling. Several unresolved issues are discussed.

Caffeine overrides the S-phase cell cycle block in sea urchin embryos

During the early mitotic cell cycles of the sea urchin embryo, the cell oscillates between S-phase and M-phase. In the presence of aphidicolin, a DNA synthesis inhibitor, a checkpoint control blocks the activation of the p34cdc2 protein kinase, by keeping it in the inactive, tyrosine phosphorylated form, and the embryos do not enter mitosis. Caffeine has been shown to bypass the G2/M-phase checkpoint in mammalian cells and in cycling Xenopus extracts and to induce mitosis despite the presence of damaged or unreplicated DNA. In this study we show that caffeine also induces mitosis and cell division in sea urchin embryos, in the presence of unreplicated DNA, by stimulating the tyrosine dephosphorylation of p34cdc2 and switching on its protein kinase activity. We also show that the caffeine-induced activation of the p34cdc2 protein kinase is not mediated by either of the two second messengers, calcium and cAMP, or by inhibition of the p34cdc2 tyrosine kinase. Thus, none of the mechanisms proposed for caffeine's action can explain how it overrides the S-phase checkpoint in the early cell cycles of the sea urchin embryo.

The checkpoint control for anaphase onset does not monitor excess numbers of spindle poles or bipolar spindle symmetry

Exit from mitosis in animal cells is substantially delayed when spindle assembly is inhibited, spindle bipolarity is disrupted, or when a monopolar spindle is formed. These observations have led to the proposal that animal cells have a ‘spindle assembly’ checkpoint for the metaphase-anaphase transition that monitors bipolar spindle organization. However, the existence of such a checkpoint is uncertain because perturbations in spindle organization can produce unattached kinetochores, which by themselves are known to delay anaphase onset. In this study we have tested if cells monitor bipolar spindle organization, independent of kinetochore attachment, by analyzing the duration of mitosis in sea urchin zygotes and vertebrate somatic cells containing multipolar spindles in which all kinetochores are attached to spindle poles. We found that sea urchin zygotes containing tripolar or tetrapolar spindles progressed from nuclear envelope breakdown to anaphase onset with normal timing. We also found that the presence of supernumerary, unpaired spindle poles did not greatly prolong mitosis. Observation of untreated PtK1 cells that formed tripolar or tetrapolar spindles revealed that they progressed through mitosis, on average, at the normal rate. More importantly, the interval between the bipolar attachment of the last monooriented chromosome and anaphase onset was normal. Thus, neither of these cell types can detect the presence of gross aberrations in spindle architecture that inevitably lead to aneuploidy. We conclude that animal cells do not have a checkpoint for the metaphase-anaphase transition that monitors defects in spindle architecture independent of the checkpoint that monitors kinetochore attachment to the spindle. For dividing cells in which spindle microtubule assembly is not experimentally compromised, we propose that the completion of kinetochore attachment is the event which limits the time of the metaphase-anaphase transition.