Interplay between the cell cycle control machinery and the microtubule network in mouse oocytes

The mouse oocyte provides a system in which it is possible to follow the behaviour and activity of the major components of the cell cycle control machinery and their principle cellular targets the chromosomes and the microtubules. In this article, we summarize our present knowledge of the interplay between the cell cycle control machinery and the microtubule network during the meiotic maturation and after activation of the mouse oocytes.     

From growth to cell cycle control.

How does a quiescent cell decide to re-enter the cell cycle and start replicating its DNA? What controls cell proliferation? These are fundamental questions that have to be solved in order to understand the mechanisms of oncogenesis. Some recent data have provided clues about how signal transduction pathways may be connected to the cell cycle. A protein kinase cascade starting from the membrane growth factor receptor is thought to be involved in transducing extracellular stimuli to the master switches of the cell cycle control machinery. The recently identified extracellular-signal regulated kinases (ERKs) appear to play an important role in this pathway. Expression of cyclins, which are regulatory subunits of the universal cell cycle oscillator cdc2, may also be controlled through this kinase cascade. The products of tumor suppressor genes Rb and p53 also play an important role in regulating cell proliferation by interfering with the cell cycle pathway. Here, I will review and discuss the importance of these different new results.     

Regulation of the female mouse germ cell cycle during entry into meiosis

During mouse embryonic development germ cells proliferate extensively until they commit to the male or female pathway and arrest in mitosis or meiosis respectively. Whilst the transition of female germ cells exiting the mitotic cell cycle and entering meiosis is well defined histologically, the essential cell cycle proteins involved in this process have remained unresolved. Using flow cytometry we have examined the entry of female germ cells into meiosis, their termination of DNA synthesis and entry into prophase I. Analysis of key G₂/M cell cycle proteins revealed that entry into meiosis and cell cycle exit at G₂/M involves repression of G₂/M promoting Cyclin B1, coincident upregulation of G₂/M repressing Cyclin B3 and robust establishment of the ATM/CHK2 pathway. By contrast we show that the ATR/CHK1 pathway is activated in male and female germ cells. This data indicates that an important G₂/M surveillance mechanism operates during germ cell proliferation and that passage into meiotic G₂/M involves the combined repression of G₂/M through Cyclin B3 and activation of the key G₂/M checkpoint regulatory network modulated through ATM and CHK2. This work shows that the core regulatory machinery that controls G₂/M progression in mitotic cells is activated in female mouse germ cells as they enter meiosis.     

Four-dimensional control of the cell cycle

The cell-division cycle has to be regulated in both time and space. In the time dimension, the cell ensures that mitosis does not begin until DNA replication is completed and any damaged DNA is repaired, and that DNA replication normally follows mitosis. This is achieved by the synthesis and destruction of specific cell-cycle regulators at the right time in the cell cycle. In the spatial dimension, the cell coordinates dramatic reorganizations of the subcellular architecture at the entrance to and exit from mitosis, largely through the actions of protein kinases and phosphatases that are often localized to specific subcellular structures. Evidence is now accumulating to suggest that the spatial organization of cell-cycle regulators is also important in the temporal control of the cell cycle. Here I will focus on how the locations of the main components of the cell-cycle machinery are regulated as part of the mechanism by which the cell controls when and how it replicates and divides.     

Expression of Cell Cycle Genes in Shoot Apical Meristems

This article reviews cell proliferation in the shoot apical meristem. The morphology and function of the meristem depends on the positional control of cell growth and division. The review describes the historical framework of research in this area and then discusses the regulatory pathways that might link developmental controls to the core cell cycle machinery.     

Divide and differentiate

The elegant choreography of metazoan development demands exquisite regulation of cell-division timing, orientation, and asymmetry. In this review, we discuss studies in Drosophila and C. elegans that reveal how the cell cycle machinery, comprised of cyclin-dependent kinase (CDK) and cyclins functions as a master regulator of development. We provide examples of how CDK/cyclins: (1) regulate the asymmetric localization and timely destruction of cell fate determinants; (2) couple signaling to the control of cell division orientation; and (3) maintain mitotic zones for stem cell proliferation. These studies illustrate how the core cell cycle machinery should be viewed not merely as an engine that drives the cell cycle forward, but rather as a dynamic regulator that integrates the cell-division cycle with cellular differentiation, ensuring the coherent and faithful execution of developmental programs.     

Cell cycle control and plant morphogenesis: is there an essential link?

Plant morphogenesis has some interesting features that may have consequences for the regulation of cell division. In particular, the immobility of plant cells implies the necessity for highly accurate controls, in contrast with the flexibility of many developmental processes in animals. An important question in plant development concerns the status of the relationship between plant morphogenesis and cell division. In this review, we discuss the current knowledge of the molecular mechanisms controlling the plant cell cycle and how this could be differentially regulated during plant morphogenesis. The plant genes involved are homologous to those of other higher eukaryotes, suggesting a similar cell cycle machinery. A variety of mechanisms control these genes, reflecting the complexity of internal and environmental signals to which plants should respond. This intricate network requires an upstream control mechanism to function as a failsafe system and to govern cell division and growth to produce the correct plant shape. BioEssays 21:29–37, 1999. © 1999 John Wiley & Sons, Inc.     

Engagement of DYRK2 in proper control for cell division

Dysregulation of cell cycle machinery causes abnormal cell division, leading to cancer development. To drive cell cycle properly, expression levels of cell cycle regulators are tightly regulated through the cell cycle. Dual specificity tyrosine phosphorylation-regulated kinase 2 (DYRK2) is a Ser/Thr kinase, and its intracellular functions had not been elucidated for decades. Recent studies have shown that DYRK2 down-regulates key molecules on cell cycle control. This review mainly highlights the DYRK2 function during cell division. In addition, we summarize tumor suppressive role of DYRK2 in cancer cells and discuss future research directions for DYRK2 toward the novel cancer therapies.     

Cell cycle and cell fate interactions in neural development

Mechanisms coupling cell cycle and cell fate operate at different steps during neural development. Intrinsic factors control the cell proliferation of distinct brain regions and changes of cell fate competence, whereas components of the cell cycle machinery could play a major role in setting the appropriate timing of the generation of different cell types.     

Regulation of cell proliferation by intermediate-conductance Ca2+-activated potassium and volume-sensitive chloride channels in mouse mesenchymal stem cells

Bone marrow mesenchymal stem cells (MSCs) are a promising cell source for regenerative medicine; however, their cellular physiology is not fully understood. The present study aimed at exploring the potential roles of the two dominant functional ion channels, intermediate-conductance Ca(2+)-activated potassium (IK(Ca)) and volume-sensitive chloride (I(Cl.vol)) channels, in regulating proliferation of mouse MSCs. We found that inhibition of IK(Ca) with clotrimazole and I(Cl.vol) with 5-nitro-1-(3-phenylpropylamino) benzoic acid (NPPB) reduced cell proliferation in a concentration-dependent manner. Knockdown of KCa3.1 or Clcn3 with specific short interference (si)RNAs significantly reduced IK(Ca) or I(Cl.vol) density and channel protein and produced a remarkable suppression of cell proliferation (by 24.4 +/- 9.6% and 29.5 +/- 7.2%, respectively, P < 0.05 vs. controls). Flow cytometry analysis showed that mouse MSCs retained at G(0)/G(1) phase (control: 51.65 +/- 3.43%) by inhibiting IK(Ca) or I(Cl.vol) using clotrimazole (2 microM: 64.45 +/- 2.20%, P < 0.05) or NPPB (200 microM: 82.89 +/- 2.49%, P < 0.05) or the specific siRNAs, meanwhile distribution of cells in S phase was decreased. Western blot analysis revealed a reduced expression of the cell cycle regulatory proteins cyclin D1 and cyclin E. Collectively, our results have demonstrated that IK(Ca) and I(Cl.vol) channels regulate cell cycle progression and proliferation of mouse MSCs by modulating cyclin D1 and cyclin E expression.     

The mito::mKate2 mouse: A far‐red fluorescent reporter mouse line for tracking mitochondrial dynamics in vivo

Mitochondria are incredibly dynamic organelles that undergo continuous fission and fusion events to control morphology, which profoundly impacts cell physiology including cell cycle progression. This is highlighted by the fact that most major human neurodegenerative diseases are due to specific disruptions in mitochondrial fission or fusion machinery and null alleles of these genes result in embryonic lethality. To gain a better understanding of the pathophysiology of such disorders, tools for the in vivo assessment of mitochondrial dynamics are required. It would be particularly advantageous to simultaneously image mitochondrial fission‐fusion coincident with cell cycle progression. To that end, we have generated a new transgenic reporter mouse, called mito::mKate2 that ubiquitously expresses a mitochondria localized far‐red mKate2 fluorescent protein. Here we show that mito::mKate2 mice are viable and fertile and that mKate2 fluorescence can be spectrally separated from the previously developed Fucci cell cycle reporters. By crossing mito::mKate2 mice to the ROSA26R‐mTmG dual fluorescent Cre reporter line, we also demonstrate the potential utility of mito::mKate2 for genetic mosaic analysis of mitochondrial phenotypes.     

Cyclin dependent protein kinases and stress responses in plants

Plants have to adjust, grow and establish themselves in various changing environmental conditions. Additionally, the sessile life-style of plants requires the development of response mechanisms for their adaptation in such environmental cues. Under biotic and abiotic stress, plant growth is negatively affected mainly as a result of cell cycle inhibition. The perception of stress involves the activation of signaling cascades that result in a prolonged S-phase and delayed entry into mitosis. Although the molecular interactions that link the cell cycle machinery to perception of stress are not fully understood, recent studies indicated the involvement of Cyclin Dependent Kinases (CDKs) in the plant response machinery. CDKs are core cell cycle regulators but their activity has been implicated in additional diverse cellular processes. Here we review the impact of different types of abiotic stress on plant cell cycle progression and CDK activity, and discuss the contribution of CDK function in the signaling control of stress tolerance.Key words: abiotic stress, cell cycle, CDK, cyclin     

Function of cyclins in regulating the mitotic and meiotic cell cycles in male germ cells

The unique cell cycles that characterize various aspects of the differentiation of germ cells provide a unique opportunity to understand heretofore elusive aspects of the in vivo function of cell cycle regulators. Key components of the cell cycle machinery are the regulatory sub-units, the cyclins, and their catalytic partners, the cyclin-dependent kinases. Some of the cyclins exhibit unique patterns of expression of germ cells that suggest possible concomitant distinct functions, predictions that are being explored by targeted mutagenesis in mouse models. A novel, meiosis-specific function has been shown for one of the A-type cyclins, cyclin A1. Embryonic lethality has obviated understanding of the germline functions of cyclin A2 and cyclin B1, while yet other cyclins, although expressed at specific stages of germ cell development, may have less essential function in the male germline.     

Beyond proliferation--cell cycle control of neuronal survival and differentiation in the developing mammalian brain

Cell cycle proteins are critical regulators of proliferation in dividing cells. Paradoxically, accumulating evidence supports the view that core components of the cell cycle also play key roles in the development of terminally differentiated postmitotic neurons. Distinct cell cycle proteins including cell cycle-dependent kinases may contribute to naturally occurring programmed neuronal cell death in the developing mammalian brain. In addition, recent studies have uncovered a novel role for the cell cycle-associated ubiquitination machinery in the control of axonal growth and patterning in the developing brain. The underlying molecular mechanisms regulating these distinct cell cycle-based developmental events in neurons are just beginning to be understood.