Cell cycle control and early embryogenesis: Xenopus laevis maturation and early embryonic cell cycles

ABSTRACT Understanding the mechanisms of embryonic cell cycles is a central goal of developmental biology, as the regulation of the cell cycle must be closely coordinated with other events during early embryogenesis. Quantitative imaging approaches have recently begun to reveal how the cell cycle oscillator is controlled in space and time, and how it is integrated with mechanical signals to drive morphogenesis. Here, we discuss how the Drosophila embryo has served as an excellent model for addressing the molecular and physical mechanisms of embryonic cell cycles, with comparisons to other model systems to highlight conserved and species-specific mechanisms. We describe how the rapid cleavage divisions characteristic of most metazoan embryos require chemical waves and cytoplasmic flows to coordinate morphogenesis across the large expanse of the embryo. We also outline how, in the late cleavage divisions, the cell cycle is inter-regulated with the activation of gene expression to ensure a reliable maternal-to-zygotic transition. Finally, we discuss how precise transcriptional regulation of the timing of mitosis ensures that tissue morphogenesis and cell proliferation are tightly controlled during gastrulation.

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Dissection of the XChk1 Signaling Pathway in Xenopus laevis Embryos

Molecular Biology of the Cell ◽

10.1091/mbc.11.9.3101 ◽

2000 ◽

Vol 11 (9) ◽

pp. 3101-3108 ◽

Cited By ~ 23

Author(s):

Nicholas C. Kappas ◽

Pamela Savage ◽

Katherine C. Chen ◽

Allan T. Walls ◽

Jill C. Sible

Keyword(s):

Cell Cycle ◽

Xenopus Laevis ◽

Signaling Pathway ◽

Embryonic Cell ◽

Midblastula Transition ◽

Cell Cycles ◽

Cyclin Dependent Kinases ◽

Xenopus Embryos ◽

Egg Extracts ◽

Sperm Nuclei

Checkpoint pathways inhibit cyclin-dependent kinases (Cdks) to arrest cell cycles when DNA is damaged or unreplicated. Early embryonic cell cycles of Xenopus laevis lack these checkpoints. Completion of 12 divisions marks the midblastula transition (MBT), when the cell cycle lengthens, acquiring gap phases and checkpoints of a somatic cell cycle. Although Xenopus embryos lack checkpoints prior to the MBT, checkpoints are observed in cell-free egg extracts supplemented with sperm nuclei. These checkpoints depend upon the Xenopus Chk1 (XChk1)-signaling pathway. To understand why Xenopus embryos lack checkpoints,xchk1 was cloned, and its expression was examined and manipulated in Xenopus embryos. Although XChk1 mRNA is degraded at the MBT, XChk1 protein persists throughout development, including pre-MBT cell cycles that lack checkpoints. However, when DNA replication is blocked, XChk1 is activated only after stage 7, two cell cycles prior to the MBT. Likewise, DNA damage activates XChk1 only after the MBT. Furthermore, overexpression of XChk1 inXenopus embryos creates a checkpoint in which cell division arrests, and both Cdc2 and Cdk2 are phosphorylated on tyrosine 15 and inhibited in catalytic activity. These data indicate that XChk1 signaling is intact but blocked upstream of XChk1 until the MBT.

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A Maternal Smad Protein Regulates Early Embryonic Apoptosis in Xenopus laevis

Molecular and Cellular Biology ◽

10.1128/mcb.22.5.1317-1328.2002 ◽

2002 ◽

Vol 22 (5) ◽

pp. 1317-1328 ◽

Cited By ~ 15

Author(s):

Yuko Miyanaga ◽

Ingrid Torregroza ◽

Todd Evans

Keyword(s):

Functional Analysis ◽

Cell Survival ◽

Caspase 3 ◽

Embryonic Cell ◽

Early Embryogenesis ◽

Cell Cycles ◽

Smad Proteins ◽

Smad Protein ◽

Smad Signaling Pathway ◽

A Cell

ABSTRACT We identified cDNAs encoding the Xenopus Smad proteins most closely related to mammalian Smad8, and we present a functional analysis of this activity (also referred to recently as xSmad11). Misexpression experiments indicate that xSmad8(11) regulates pathways distinct from those regulated by the closely related xSmad1. Embryos that develop from eggs depleted of xSmad8(11) mRNA fail to gastrulate; instead, at the time of gastrulation, they initiate a widespread program of apoptosis, via a CPP32/caspase 3 pathway. Embryos that avoid this fate display gastrulation defects. Activation of apoptosis is rescued by expression of xSmad8(11) but not xSmad1. Our results demonstrate an embryonic requirement for Smad8(11) activity and show that a maternally derived Smad signaling pathway is required for gastrulation and for mediating a cell survival program during early embryogenesis. We suggest that xSmad8(11) functions as part of a maternally derived mechanism shown previously by others to monitor Xenopus early embryonic cell cycles.

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Expression of the cell cycle control gene, cdc25, is constitutive in the segmental founder cells but is cell-cycle-regulated in the micromeres of leech embryos

Development ◽

10.1242/dev.121.9.3035 ◽

1995 ◽

Vol 121 (9) ◽

pp. 3035-3043 ◽

Cited By ~ 2

Author(s):

S.T. Bissen

Keyword(s):

Cell Cycle ◽

Cell Division ◽

Cell Cycle Control ◽

Control Gene ◽

Cdc25 Phosphatase ◽

Cell Cycles ◽

Cell Divisions ◽

Zygotic Transcription ◽

Founder Cells ◽

Drosophila Embryos

The identifiable cells of leech embryos exhibit characteristic differences in the timing of cell division. To elucidate the mechanisms underlying these cell-specific differences in cell cycle timing, the leech cdc25 gene was isolated because Cdc25 phosphatase regulates the asynchronous cell divisions of postblastoderm Drosophila embryos. Examination of the distribution of cdc25 RNA and the zygotic expression of cdc25 in identified cells of leech embryos revealed lineage-dependent mechanisms of regulation. The early blastomeres, macromeres and teloblasts have steady levels of maternal cdc25 RNA throughout their cell cycles. The levels of cdc25 RNA remain constant throughout the cell cycles of the segmental founder cells, but the majority of these transcripts are zygotically produced. Cdc25 RNA levels fluctuate during the cell cycles of the micromeres. The levels peak during early G2, due to a burst of zygotic transcription, and then decline as the cell cycles progress. These data suggest that cells of different lineages employ different strategies of cell cycle control.

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The NIMA-related kinase X-Nek2B is required for efficient assembly of the zygotic centrosome in Xenopus laevis

Journal of Cell Science ◽

10.1242/jcs.113.11.1973 ◽

2000 ◽

Vol 113 (11) ◽

pp. 1973-1984 ◽

Cited By ~ 7

Author(s):

A.M. Fry ◽

P. Descombes ◽

C. Twomey ◽

R. Bacchieri ◽

E.A. Nigg

Keyword(s):

Cell Cycle ◽

Xenopus Laevis ◽

Basal Body ◽

Chromatin Condensation ◽

Embryonic Cells ◽

Serine Threonine Kinase ◽

Cell Cycles ◽

Functional Studies ◽

Mammalian Cell Cycle ◽

Egg Extracts

Nek2 is a mammalian cell cycle-regulated serine/threonine kinase that belongs to the family of proteins related to NIMA of Aspergillus nidulans. Functional studies in diverse species have implicated NIMA-related kinases in G(2)/M progression, chromatin condensation and centrosome regulation. To directly address the requirements for vertebrate Nek2 kinases in these cell cycle processes, we have turned to the biochemically-tractable system provided by Xenopus laevis egg extracts. Following isolation of a Xenopus homologue of Nek2, called X-Nek2B, we found that X-Nek2B abundance and activity remained constant through the first mitotic cycle implying a fundamental difference in Nek2 regulation between embryonic and somatic cell cycles. Removal of X-Nek2B from extracts did not disturb either entry into mitosis or the accompanying condensation of chromosomes providing no support for a requirement for Nek2 in these processes at least in embryonic cells. In contrast, X-Nek2B localized to centrosomes of adult Xenopus cells and was rapidly recruited to the basal body of Xenopus sperm following incubation in egg extracts. Recruitment led to phosphorylation of the X-Nek2B kinase. Most importantly, depletion of X-Nek2B from extracts significantly delayed both the assembly of microtubule asters and the recruitment of gamma-tubulin to the basal body. Hence, these studies demonstrate that X-Nek2B is required for efficient assembly of a functional zygotic centrosome and highlight the possibility of multiple roles for vertebrate Nek2 kinases in the centrosome cycle.

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Chromosome Association of Minichromosome Maintenance Proteins in Drosophila Mitotic Cycles

The Journal of Cell Biology ◽

10.1083/jcb.139.1.13 ◽

1997 ◽

Vol 139 (1) ◽

pp. 13-21 ◽

Cited By ~ 31

Author(s):

Tin Tin Su ◽

Patrick H. O'Farrell

Keyword(s):

Cell Cycle ◽

Cyclin E ◽

Embryonic Cell ◽

Cyclin B ◽

Chromatin Interaction ◽

G1 Phase ◽

Minichromosome Maintenance ◽

Cell Cycles ◽

Mcm Proteins ◽

Replication Factors

Minichromosome maintenance (MCM) proteins are essential DNA replication factors conserved among eukaryotes. MCMs cycle between chromatin bound and dissociated states during each cell cycle. Their absence on chromatin is thought to contribute to the inability of a G2 nucleus to replicate DNA. Passage through mitosis restores the ability of MCMs to bind chromatin and the ability to replicate DNA. In Drosophila early embryonic cell cycles, which lack a G1 phase, MCMs reassociate with condensed chromosomes toward the end of mitosis. To explore the coupling between mitosis and MCM–chromatin interaction, we tested whether this reassociation requires mitotic degradation of cyclins. Arrest of mitosis by induced expression of nondegradable forms of cyclins A and/or B showed that reassociation of MCMs to chromatin requires cyclin A destruction but not cyclin B destruction. In contrast to the earlier mitoses, mitosis 16 (M16) is followed by G1, and MCMs do not reassociate with chromatin at the end of M16. dacapo mutant embryos lack an inhibitor of cyclin E, do not enter G1 quiescence after M16, and show mitotic reassociation of MCM proteins. We propose that cyclin E, inhibited by Dacapo in M16, promotes chromosome binding of MCMs. We suggest that cyclins have both positive and negative roles in controlling MCM–chromatin association.

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Excess histone H3 is a Chk1 inhibitor that controls embryonic cell cycle progression

10.1101/2020.06.09.142414 ◽

2020 ◽

Author(s):

Yuki Shindo ◽

Amanda A. Amodeo

Keyword(s):

Cell Cycle ◽

Cell Fate ◽

Cell Cycle Progression ◽

Histone H3 ◽

Embryonic Cell ◽

Cycle Progression ◽

Cell Cycles ◽

Embryonic Cell Cycle

AbstractThe early embryos of many species undergo a switch from rapid, reductive cleavage divisions to slower, cell fate-specific division patterns at the Mid-Blastula Transition (MBT). The maternally loaded histone pool is used to measure the increasing ratio of nuclei to cytoplasm (N/C ratio) to control MBT onset, but the molecular mechanism of how histones regulate the cell cycle has remained elusive. Here, we show that excess histone H3 inhibits the DNA damage checkpoint kinase Chk1 to promote cell cycle progression in the Drosophila embryo. We find that excess H3-tail that cannot be incorporated into chromatin is sufficient to shorten the embryonic cell cycle and reduce the activity of Chk1 in vitro and in vivo. Removal of the Chk1 phosphosite in H3 abolishes its ability to regulate the cell cycle. Mathematical modeling quantitatively supports a mechanism where changes in H3 nuclear concentrations over the final cell cycles leading up to the MBT regulate Chk1-dependent cell cycle slowing. We provide a novel mechanism for Chk1 regulation by H3, which is crucial for proper cell cycle remodeling during early embryogenesis.

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A Link between Deoxyribonucleotide Metabolites and Embryonic Cell-Cycle Control

Current Biology ◽

10.1016/j.cub.2019.02.021 ◽

2019 ◽

Vol 29 (7) ◽

pp. 1187-1192.e3 ◽

Cited By ~ 10

Author(s):

Boyang Liu ◽

Franziska Winkler ◽

Marco Herde ◽

Claus-Peter Witte ◽

Jörg Großhans

Keyword(s):

Cell Cycle ◽

Cell Cycle Control ◽

Embryonic Cell ◽

Embryonic Cell Cycle

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Systems-level feedback in cell-cycle control

Biochemical Society Transactions ◽

10.1042/bst0381242 ◽

2010 ◽

Vol 38 (5) ◽

pp. 1242-1246 ◽

Cited By ~ 8

Author(s):

Béla Novák ◽

P.K. Vinod ◽

Paula Freire ◽

Orsolya Kapuy

Keyword(s):

Cell Cycle ◽

Feedback Loop ◽

Cell Cycle Control ◽

Double Negative ◽

Cyclin Dependent Kinase ◽

Successful Completion ◽

Cell Cycles ◽

Underlying Network ◽

Oscillatory Mechanism ◽

Negative Feedbacks

Alternation of chromosome replication and segregation is essential for successful completion of the cell cycle and it requires an oscillation of Cdk1 (cyclin-dependent kinase 1)–CycB (cyclin B) activity. In the present review, we illustrate the essential features of checkpoint controlled and uncontrolled cell-cycle oscillations by using mechanical metaphors. Despite variations in the molecular details of the oscillatory mechanism, the underlying network motifs responsible for the oscillations are always well-conserved. The checkpoint-controlled cell cycles are always driven by a negative-feedback loop amplified by double-negative feedbacks (antagonism).

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Aurora Kinase Inhibitor ZM447439 Blocks Chromosome-induced Spindle Assembly, the Completion of Chromosome Condensation, and the Establishment of the Spindle Integrity Checkpoint inXenopusEgg Extracts

Molecular Biology of the Cell ◽

10.1091/mbc.e04-10-0891 ◽

2005 ◽

Vol 16 (3) ◽

pp. 1305-1318 ◽

Cited By ~ 91

Author(s):

Bedrick B. Gadea ◽

Joan V. Ruderman

Keyword(s):

Cell Cycle ◽

Mitotic Spindle ◽

Kinase Inhibitor ◽

Spindle Assembly ◽

Aurora Kinase ◽

Chromosome Morphology ◽

Chromosome Condensation ◽

Embryonic Cell ◽

Cell Cycle Regulators ◽

Cell Cycles

The Aurora family kinases contribute to accurate progression through several mitotic events. ZM447439 (“ZM”), the first Aurora family kinase inhibitor to be developed and characterized, was previously found to interfere with the mitotic spindle integrity checkpoint and chromosome segregation. Here, we have used extracts of Xenopus eggs, which normally proceed through the early embryonic cell cycles in the absence of functional checkpoints, to distinguish between ZM's effects on the basic cell cycle machinery and its effects on checkpoints. ZM clearly had no effect on either the kinetics or amplitude in the oscillations of activity of several key cell cycle regulators. It did, however, have striking effects on chromosome morphology. In the presence of ZM, chromosome condensation began on schedule but then failed to progress properly; instead, the chromosomes underwent premature decondensation during mid-mitosis. ZM strongly interfered with mitotic spindle assembly by inhibiting the formation of microtubules that are nucleated/stabilized by chromatin. By contrast, ZM had little effect on the assembly of microtubules by centrosomes at the spindle poles. Finally, under conditions where the spindle integrity checkpoint was experimentally induced, ZM blocked the establishment, but not the maintenance, of the checkpoint, at a point upstream of the checkpoint protein Mad2. These results show that Aurora kinase activity is required to ensure the maintenance of condensed chromosomes, the generation of chromosome-induced spindle microtubules, and activation of the spindle integrity checkpoint.

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