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细胞周期检查点（Cell cycle checkpoint）

热度 1已有 7737 次阅读 2014-2-11 16:59 |关键词:important normal

细胞周期检查点是细胞周期中的一套保证DNA复制和染色体分配质量的检查机制，为负反馈调节机制。当细胞周期进程中出现异常时被激活，及时地中断细胞周期的运行。待细胞修复或排除故障后，细胞周期才能恢复运转。

　　细胞周期检查点（checkpoint）是细胞周期（cell cycle）中的一套保证DNA复制和染色体（chromosome）分配质量的检查机制。是一类负反馈调节机制。当细胞周期进程中出现异常事件，如DNA损伤或DNA复制受阻时，这类调节机制就被激活，及时地中断细胞周期的运行。待细胞修复或排除故障后，细胞周期才能恢复运转。

　　根据在细胞周期（cell cycle）中的时间顺序，可将checkpoint分为三类

　　1：G1期 (Restriction) Checkpoint

　　2：G2 Checkpoint

　　3：Metaphase Checkpoint （细胞分裂期，M期）

　　根据调控内容，可分为三类

　　1: DNA损伤检查点(DNAdamage checkpoint) 负责查看DNA有无损伤；

　　2：DNA复制检查点(DNAreplication checkpoint) 负责DNA复制的进度；

　　3：纺锤体组装检查点(spindleassembly checkpoint) 管理染色体的正确分配已否，因为染色体的分配主要依赖于纺锤体的作用。

　　从检查点的工作方式来看，又可分为三个部分

　　1：探测器(sensor)，负责检查质量问题；

　　2：传感器(signaltransducer)，负责信号传递；

　　3：效应器(effector)，由效应器去中断细胞周期进程并开动修复机制。

　　关于检查点工作的分子机制还不很清楚，但随着研究工作的深入，人们现在已能描绘出一个初步轮廓。

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http://www.baike.com/wiki/细胞周期检查点

The Cell Cycle and Checkpoint controls

: Toward understanding of genome maintenance mechanisms

Eishi Noguchi

Department of Biochemistry and Molecular Biology

Drexel University College of Medicine

What is the Cell Cycle?

The cell cycle is required for cell growth and cell division into two daughter cells. A eukaryotic cell cannot divide unless it replicates its genome (DNA) and then separates the duplicated genome. To achieve these tasks cells must perform DNA synthesis and mitosis. The cell cycle is an ordered set of events. The G1 phase stands for “GAP-1” and is required for cell growth and preparation of DNA synthesis. The S-phase stands for “Synthesis” and replicates the genome. The G2 phase is “GAP-2” and needed for cell growth and preparation for mitosis. The last phase is M and it stands for “Mitosis” in which cells segregate duplicated chromosomes.

What is the Checkpoint?

The cell cycle proceeds by a defined sequence of events where late events depend upon completion of early events 1. The aim of the dependency of events is to distribute complete and accurate replicas of the genome to daughter cells 2. To monitor this dependency, cells are equipped with the checkpoints that are set at various stages of the cell cycle. When cells have DNA damages that have to be repaired, cells activate DNA damage checkpoint that arrests cell cycle. According to the cell cycle stages, DNA damage checkpoints are classified into at least 3 checkpoints: G1/S (G1) checkpoint, intra-S phase checkpoint, and G2/M checkpoint. Upon perturbation of DNA replication by drugs that interfere with DNA synthesis, DNA lesions, or obstacles on DNA, cells activate DNA replication checkpoint that arrests cell cycle at G2/M transition until DNA replication is complete. There are more checkpoints such as Spindle checkpoint and Morphogenesis checkpoint. The spindle checkpoint arrests cell cycle at M phase until all chromosomes are aligned on spindle. This checkpoint is very important for equal distribution of chromosomes. Morphogenesis checkpoint detects abnormality in cytoskeleton and arrests cell cycle at G2/M transition.

What happens if you lose one of checkpoints?

DNA replication and chromosome distribution are indispensable events in the cell cycle control. Cells must accurately copy their chromosomes, and through the process of mitosis, segregate them to daughter cells. The checkpoints are surveillance mechanism and quality control of the genome to maintain genomic integrity. Checkpoint failure often causes mutations and genomic arrangements resulting in genetic instability. Genetic instability is a major factor of birth defects and in the development of many diseases, most notably cancer. Therefore, checkpoint studies are very important for understanding mechanisms of genome maintenance as they have direct impact on the ontogeny of birth defects and the cancer biology.

DNA maintenance checkpoint

Accurate duplication of eukaryotic genome is a challenging task, given that environment of cell growth and division is rarely ideal. Cells are constantly under the stress of intrinsic and extrinsic agents that cause DNA damage or interference with DNA replication. To cope with these assaults, cells are equipped with DNA maintenance checkpoints 3 to arrest cell cycle and facilitate DNA repair pathways. DNA maintenance checkpoints include (a) the DNA damage checkpoints that recognize and respond to DNA damage, and (b) the DNA replication checkpoint that monitors the fidelity of copying DNA 3.

(a) DNA damage checkpoint

DNA damage checkpoints ensure the fidelity of genetic information both by arresting cell cycle progression and facilitating DNA repair pathways. Studies on many different species have uncovered a network of proteins that form the DNA damage checkpoints. Central to this network are protein kinases of ATM/ATR family known as Tel1/Mec1 in budding yeast and Tel1/Rad3 in fission yeast 4. These kinases sense DNA damages and phosphorylate number of proteins that regulate cell cycle progression and DNA repair pathways 3.

(b) DNA replication checkpoint

Accurate replication of the millions or billions of DNA base pairs in a eukaryotic genome is a remarkable achievement. This accomplishment is even more astonishing when one considers for DNA synthesis are rarely ideal. Damaged template, protein complexes bound to DNA, and poor supply of dNTPs are among the many obstacles that must be overcome to replicate genome. All of these situations can stall replication forks. Stalled forks pose grave threats to genome integrity because they can rearrange, break, or collapse through disassembly of the replication complex 5. The pathways that respond to replication stress are signal transduction pathways that are conserved across evolution 6 7. Atop the pathways are also ATM/ATR family kinases. These kinases together with a trimeric checkpoint clamp (termed 9-1-1 complex) and five-subunit checkpoint clamp loader (Rad17-RFC2-RFC3-RFC4-RFC5) senses stalled replication forks and transmit a checkpoint signal 3. One of major functions of replication checkpoint is to prevent the onset of mitosis by regulating mitotic control proteins such as Cdc25. But perhaps the most important activity of replication checkpoint is to stabilize and protect replication forks 8. The protein kinase Cds1 (human Chk2 homolog; in human, Chk1 is a functional Cds1 homolog) is a critical effector of the replication checkpoint in the fission yeast Schizosaccharomyces pombe 9, 10. Cds1 is required to prevent stabilization of replication fork in cells treated with hydroxyurea (HU), a ribonucleotide reductase inhibitor that stalls replication by depleting dNTPs 11. In the budding yeast Saccharomyces cerevisiae, a failure to activate Rad53 (Chk2 homolog) is associated with collapse and regression of replication forks and gross chromosomal rearrangements in cells treated with HU 12-15.

Replication fork protection complex (FPC)

The DNA replication checkpoint stabilizes replication forks that have stalled at DNA adducts and other lesions that block DNA polymerases. In the absence of DNA replication checkpoint, stalled forks are thought to collapse, creating strand break that threatens genome stability and cell viability 5. Therefore, discovering how cells cope with aberrant replication forks is essential for understanding mechanisms of genome maintenance. The Chk1 and Chk2/Cds1 checkpoint kinases, which are key mediators of DNA damage and DNA replication checkpoints, are thought to be involved in cancer development 16. We found the Swi1 protein is required for survival of replication fork arrest and effective activation of Chk2 kinase in fission yeast. Swi1 forms tight complex with Swi3 protein and moves with replication forks. Swi1-Swi3 complex is also important for proficient DNA replication even in the absence of agents that cause genotoxic stress, creating single-strand DNA gaps at replication forks 11, 17. These results led us to propose Swi1-Swi3 define a replication fork protection complex (FPC) that stabilizes replication forks in a configuration that is recognized by replication checkpoint sensors11, 17. Interestingly, Tof1 protein (Budding yeast Swi1 homolog) has been reported to have similar functions. Tof1 is also involved in Rad53 (Chk2 homolog) activation and travels with replication fork 18, 19. Tof1 is needed to restrain fork progression when DNA synthesis is inhibited by HU indicating that Tof1 is required for coordination of DNA synthesis and replisome (replication machinery) movement 19.

FPC may be conserved across evolution

Swi1 and Tof1 belong to a large protein family that was first defined by metazoan Tim1 (Timeless) 11, 17, 20, 21. Drosophila melanogaster and mammalian Tim1s are implicated in circadian rhythmic oscillation 22, whereas the Caenorhabditis elegans Tim1 is required for proper chromosome cohesion and segregation. All species listed above have Swi3 homolgs in their genomes 20 suggesting that Swi1-Swi3 complex may be conserved amongst eukaryotes 17. It will be interesting to determine whether these conserved complexes are involved in DNA replication and maintenance of genome integrity.

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References

1. Hartwell, L. H. & Weinert, T. A. Checkpoints: controls that ensure the order of cell cycle events. Science 246, 629-34 (1989).

2. Russell, P. Checkpoints on the road to mitosis. Trends Biochem Sci 23, 399-402 (1998).

3. Nyberg, K. A., Michelson, R. J., Putnam, C. W. & Weinert, T. A. TOWARD MAINTAINING THE GENOME: DNA Damage and Replication Checkpoints. Annu Rev Genet 36, 617-56 (2002).

4. McGowan, C. H. & Russell, P. The DNA damage response: sensing and signaling. Curr Opin Cell Biol 16, 629-33 (2004).

5. McGlynn, P. & Lloyd, R. G. Recombinational repair and restart of damaged replication forks. Nat Rev Mol Cell Biol 3, 859-70 (2002).

6. Zhou, B. B. & Elledge, S. J. The DNA damage response: putting checkpoints in perspective. Nature 408, 433-9 (2000).

7. Osborn, A. J., Elledge, S. J. & Zou, L. Checking on the fork: the DNA-replication stress-response pathway. Trends Cell Biol 12, 509-16 (2002).

8. Boddy, M. N. & Russell, P. DNA replication checkpoint. Curr. Biol. 11, R953-R956 (2001).

9. Boddy, M. N., Furnari, B., Mondesert, O. & Russell, P. Replication checkpoint enforced by kinases Cds1 and Chk1. Science 280, 909-912 (1998).

10. Lindsay, H. D. et al. S-phase-specific activation of Cds1 kinase defines a subpathway of the checkpoint response in Schizosaccharomyces pombe. Genes Dev 12, 382-95 (1998).

11. Noguchi, E., Noguchi, C., Du, L. L. & Russell, P. Swi1 prevents replication fork collapse and controls checkpoint kinase Cds1. Mol Cell Biol 23, 7861-74 (2003).

12. Kolodner, R. D., Putnam, C. D. & Myung, K. Maintenance of genome stability in Saccharomyces cerevisiae. Science 297, 552-7 (2002).

13. Lopes, M. et al. The DNA replication checkpoint response stabilizes stalled replication forks. Nature 412, 557-61 (2001).

14. Sogo, J. M., Lopes, M. & Foiani, M. Fork reversal and ssDNA accumulation at stalled replication forks owing to checkpoint defects. Science 297, 599-602 (2002).

15. Tercero, J. A. & Diffley, J. F. Regulation of DNA replication fork progression through damaged DNA by the Mec1/Rad53 checkpoint. Nature 412, 553-7 (2001).

16. Bartek, J. & Lukas, J. Chk1 and Chk2 kinases in checkpoint control and cancer. Cancer Cell 3, 421-9 (2003).

17. Noguchi, E., Noguchi, C., McDonald, W. H., Yates, J. R., 3rd & Russell, P. Swi1 and Swi3 are components of a replication fork protection complex in fission yeast. Mol Cell Biol 24, 8342-55 (2004).

18. Foss, E. J. Tof1p regulates DNA damage responses during S phase in Saccharomyces cerevisiae. Genetics 157, 567-77 (2001).

19. Katou, Y. et al. S-phase checkpoint proteins Tof1 and Mrc1 form a stable replication-pausing complex. Nature 424, 1078-83 (2003).

20. Chan, R. C. et al. Chromosome cohesion is regulated by a clock gene paralogue TIM-1. Nature 424, 1002-9 (2003).

21. Dalgaard, J. Z. & Klar, A. J. swi1 and swi3 perform imprinting, pausing, and termination of DNA replication in S. pombe. Cell 102, 745-51 (2000).

22. Barnes, J. W. et al. Requirement of mammalian Timeless for circadian rhythmicity. Science 302, 439-42 (2003).

http://eishinoguchi.com/checkpoint.htm

Cell cycle checkpoint

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Cell cycle checkpoints are control mechanisms that ensure the fidelity of cell division in eukaryotic cells. These checkpoints verify whether the processes at each phase of the cell cycle have been accurately completed before progression into the next phase. Multiple checkpoints have been identified, though some of them are less understood than others

Function

An important function of many checkpoints is to assess DNA damage, which is detected by sensor mechanisms. When damage is found, the checkpoint uses a signal mechanism either to stall the cell cycle until repairs are made or, if repairs cannot be made, to target the cell for destruction via apoptosis (effector mechanism). All the checkpoints that assess DNA damage appear to utilize the same sensor-signal-effector mechanism.

The cell cycle, according to Temple and Raff (1986),[1] was expected to function as a clock; but, if this were the case, it would be expected that the stages of the cell cycle must function according to some sort of internal clock, which would determine how long a phase should last. However, the cell cycle is now depicted as falling dominoes: The preceding phase has to "fall" before the next phase can begin. The cell cycle checkpoints are, therefore, made up of composites of protein kinases and adaptor proteins that all play salient roles in the maintenance of the cell division's integrity.

The DNA damage checkpoint is always active. Nonetheless, most human cells, for example, are terminally differentiated and must exit the cell cycle. There is a phase late in G1 phase called the restriction point (RP, or the restriction checkpoint); cells that should cease division exit the cell cycle and enter G0. Cells that continually divide in the adult human include hematopoietic stem cells and gut epithelial cells. Therefore, the re-entrant into the cell cycle is possible only by overcoming the RP. This is achieved by growth factor-induced expression of cyclin D proteins. These then overcome the G0 barrier and are able to enter the cell cycle.

The main checkpoints that control the cell division cycle in eukaryotes include:

G1 (Restriction) Checkpoint

Main article: restriction point

The first checkpoint is located at the end of the cell cycle's G1 phase, just before entry into S phase, making the key decision of whether the cell should divide, delay division, or enter a resting stage. Many cells stop at this stage and enter a resting state called G0. Liver cells, for instance, enter mitosis only around twice a year.[citation needed] The G1 checkpoint is where eukaryotes typically arrest the cell cycle if environmental conditions make cell division impossible or if the cell passes into G0 for an extended period. In animal cells, the G1 phase checkpoint is called the restriction point, and in yeast cells it is called the Start point.

The restriction point is controlled mainly by action of the CKI p16 (CDK inhibitor p16). This protein inhibits CDK4/6 and ensures that it can no longer interact with cyclin D1 to cause cell cycle progression. In growth-induced or oncogenic-induced cyclin D expression, this checkpoint is overcome because the increased expression of cyclin D allows its interaction with CDK4/6 by competing for binding. Once active CDK4/6-cyclin D complexes form, they phosphorylate the tumor suppressor retinoblastoma protein (Rb), which relieves the inhibition of the transcription factor E2F. E2F is then able to cause expression of cyclin E, which then interacts with CDK2 to allow for G1-S phase transition. This brings the cell to the end of the first checkpoint, signaling the G0-G1-S-phase transition.

In simpler terms, the CDK inhibitor p16 inhibits another CDK from binding to its cyclin (D). When growth is induced, the expression of this cyclin is so high that they do bind. The new CDK/cyclin complex now phosphorylates retinoblastoma (a tumor suppressor). Un-phosphorylated retinoblastoma releases the inhibition of a transcription factor. This factor then brings about the G1-S phase transition.

G2 Checkpoint

The second checkpoint is located at the end of G2 phase, triggering the start of the M phase (mitotic phase). In order for this checkpoint to be passed, the cell has to check a number of factors[examples needed] to ensure the cell is ready for mitosis. If this checkpoint is passed, the cell initiates the many molecular processes that signal the beginning of mitosis. The CDKs associated with this checkpoint are activated by phosphorylation of the CDK by the action of a "Maturation promoting factor" (Mitosis Promoting Factor, MPF).

The molecular nature of this checkpoint involves an activating phosphatase, known as Cdc25, which under favorable conditions removes the inhibitory phosphates present within the MPF (term for the cyclin B/CDK1 complex). However, DNA is frequently damaged prior to mitosis, and, to prevent transmission of this damage to daughter cells, the cell cycle is arrested via inactivation of the Cdc25 phosphatase. This is done by the ATM kinase protein which phosphorylates Cdc25 which leads to its ubiquitinylation and destruction.

Metaphase Checkpoint

Main article: Spindle checkpoint

The mitotic spindle checkpoint occurs at the point in metaphase where all the chromosomes should/have aligned at the mitotic plate and be under bipolar tension. The tension created by this bipolar attachment is what is sensed, which initiates the anaphase entry. To do this, the sensing mechanism ensures that the anaphase-promoting complex (APC/C) is no longer inhibited, which is now free to degrade cyclin B, which harbors a D-box (destruction box), and to break down securin.[2] The latter is a protein whose function is to inhibit separase, which in turn cuts the cohesins, the protein composite responsible for cohesion of sister chromatids.[3] Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separase then causes sister chromatid separation.[4] After the cell has split into its two daughter cells, the cell enters G1.