Cell cycle checkpoint
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Cell cycle checkpoints are control mechanisms in eukaryotic cells which ensure proper division of the cell. Each checkpoint serves as a potential halting point along the cell cycle, during which the conditions of the cell are assessed, with progression through the various phases of the cell cycle occurring when favorable conditions are met. Currently, there are three known checkpoints: the G1 checkpoint, also known as the restriction or start checkpoint; the G2/M checkpoint; and the metaphase checkpoint, also known as the spindle checkpoint.
All living organisms are products of repeated rounds of cell growth and division. During this process, known as the cell cycle, a cell duplicates its contents and then divides in two. The purpose of the cell cycle is to accurately duplicate each organisms’ DNA and then divide the cell and its contents evenly between the two resulting cells. In eukaryotes, the cell cycle consists of four main stages: G1, during which a cell is metabolically active and continuously grows; S phase, during which DNA replication takes place; G2, during which the growth of cell continues and the cell synthesizes various proteins in preparation for division; and the M (mitosis) phase, during which the duplicated chromosomes (known as the sister chromatids) separate into two daughter nuclei, and the cell divides into two daughter cells, each with a full copy of DNA. Compared to the eukaryotic cell cycle, the prokaryotic cell cycle (known as binary fission) is relatively simple and quick: the chromosome replicates from the origin of replication, a new membrane is assembled, and the cell wall form a septum which divides the cell into two.
As the eukaryotic cell cycle is a complex process, eukaryotes have evolved a network of regulatory proteins, known as the cell cycle control system, which monitors and dictates the progression of the cell through the cell cycle. This system acts like a timer, or a clock, which sets a fixed amount of time for the cell to spend in each phase of the cell cycle, while at the same time it also responds to information received from the processes it controls. The cell cycle checkpoints play an important role in the control system by sensing defects that occur during essential processes such as DNA replication or chromosome segregation, and inducing a cell cycle arrest in response until the defects are repaired. The main mechanism of action of the cell cycle checkpoints is through the regulation of the activities of a family of protein kinases known as the cyclin-dependent kinases (CDKs), which bind to different classes of regulator proteins known as cyclins, with specific cyclin-CDK complexes being formed and activated at different phases of the cell cycle. Those complexes, in turn, activate different downstream targets to promote or prevent cell cycle progression.
G1 (Restriction) Checkpoint
The G1 checkpoint, also known as the restriction point in mammalian cells and the start point in yeast, is the point at which the cell becomes committed to entering the cell cycle. As the cell progresses through G1, depending on internal and external conditions, it can either delay G1, enter a quiescent state known as G0, or proceed past the restriction point. The decision to commit a new round of cell division occurs when the cell activates cyclin-CDK-dependent transcription which promotes entry into S phase.
During early G1, the transcriptional repressors Rb (retinoblastoma), p107 and p130, known as pocket proteins, bind to the E2F transcription factors to prevent G1-to-S transition. Rb binds and represses activator E2F transcription factors (E2F1-3), while p107 and p130 bind E2F4 and E2F5 to form complexes which repress transcription of G1-to-S promoting factors. Upon the decision to progress past the G1 checkpoint, cyclin D levels rise, and cyclin D forms a complex with CDK4 and CDK6, which in turn phosphorylate the pocket proteins. Phosphorylation of the pocket proteins causes the release of their bound targets, thereby relieving the repression of the E2F1-3 activators and translocating repressors E2F4 and E2F5 from the nucleus to the cytoplasm, resulting in the transcriptional activation of downstream targets which promote the G1-to-S transition, including another cyclin, known as cyclin E, which forms a complex with CDK2. The formation of the cyclin E-CDK2 complex then promotes a positive feedback loop which creates an “all or nothing” switch from which the cell can not return. Following entry to S-phase and initiation of DNA replication, S-phase cyclin A, a transcriptional target of E2F1-3, forms a complex with CDK2 which phosphorylates E2F1-3 and prevents its ability to bind to DNA, thus forming a negative feedback loop. In another negative feedback loop, E2F1-3 promote the transcription of E2F6-8, which in turn repress G1-S transcription.
When DNA damage occurs, or when the cell detects any defects which necessitate it to delay or halt the cell cycle in G1, arrest occurs through several mechanisms. The rapid response involves phosphorylation events that initiate with either kinase ATM (Ataxia telangiectasia mutated) or ATR (Ataxia Telangiectasia and Rad3 related), which act as the sensors, depending on the type of damage. These kinases phosphorylate and activate the effector kinases Chk2 and Chk1, respectively, which in turn phosphorylate the phosphatase Cdc25A, thus marking it for ubiquitination and degradation. As Cdc25A activates the previously mentioned cyclin E-CDK2 complex by removing inhibitory phosphates from CDK2, in the absence of Cdc25A, cyclin E-CDK2 remains inactive, and the cell remains in G1. To maintain the arrest, another response is initiated, by which Chk2 or Chk1 phosphorylate p53, a tumor suppressor, and this stabilizes p53 by preventing it from binding Mdm2, a ubiquitin ligase which inhibits p53 by targeting it for degradation. The stable p53 then acts a transcriptional activator of several target genes, including p21, an inhibitor of the G1-to-S promoting complex cyclin E-CDK21. In addition, another mechanism by which p21 is activated is through the accumulation of p16 in response to DNA damage. p16 disrupts cyclin D-CDK4 complexes, thus causing the release of p21 from the complexes, which leads to the dephosphorylation and activation of Rb, which allows Rb to bind and inhibit E2F1-3, thus keeping the cell from transitioning to S phase. Recently, some aspects of this model have been disputed.
Following the decision to enter the cell cycle and undergo division, the cell goes through S phase, in which it replicates its DNA, and, in most species, G2, in which it undergoes rapid growth and protein synthesis in preparation for mitosis, the process of cell division. The G2/M checkpoint, also known as the DNA damage checkpoint, ensures that the cell underwent all of the necessary changes during the S and G2 phases and is ready to divide.
The primary complex responsible for the transition from G2 to M is the Cyclin B-cdc2 (CDK1 homolog) complex. The activity of cdc2 is regulated directly by cyclins B and by the phosphatase cdc25. Prior entry to mitosis, cdc2 maintained in an inactive state by the kinases Wee1 and Myt1, which phosphorylate tyrosine residues on cdc2. As the cell progresses through G2 and reaches the G2/M transition, the kinase Plk1 phosphorylates Wee1, which targets Wee1 for degradation via the SCF ubiquitin ligase complex. An additional function of Plk1 is to activate Cdc25 through phosphorylation. The compounded effect of Wee1 degradation and Cdc25 activation is the net removal of inhibitory phosphorylation from cdc2, which activates cdc2. Plk1 is activated at the G2/M transition by the Aurora A and Bora, which accumulate during G2 and form an activation complex. The Plk1-Cdc2-cdc25 complex then initiates a positive feedback loop which serves to further activate Cdc2, and in conjunction with an increase in cyclin B levels during G2, the resulting cdc2-cyclin B complexes then activate downstream targets which promote entry to mitosis.
The mechanisms by which mitotic entry is prevented in response to DNA damage are similar to those in the G1/S checkpoint. DNA damage triggers the activation of the aforementioned ATM/ATR pathway, in which ATM/ATR phosphorylate and activate the Chk1/Chk2 checkpoint kinases. Chk1/2 phosphorylate cdc25 which, in addition to being inhibited, is also sequestered in the cytoplasm by the 14-3-3 proteins. 14-3-3 are upregulated by p53, which, as previously mentioned, is activated by Chk1 and ATM/ATR. p53 also transactivates p21, and both p21 and the 14-3-3 in turn inhibit cyclin B-cdc2 complexes through the phosphorylation and cytoplasmic sequestering of cdc2. In addition, the inactivation of cdc25 results in its inability to dephosphorylate and activate cdc2. Finally, another mechanism of damage response is through the negative regulation of Plk1 by ATM/ATR, which in turn results in the stabilization of Wee1 and Myt1, which can then phosphorylate and inhibit cdc2, thus keeping the cell arrested in G2 until the damage is fixed.
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. 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. Once this inhibitory protein is degraded via ubiquitination and subsequent proteolysis, separase then causes sister chromatid separation. After the cell has split into its two daughter cells, the cell enters G1.
Cell cycle checkpoints and cancer
DNA repair processes and cell cycle checkpoints have been intimately linked with cancer due to their functions regulating genome stability and cell progression, respectively. The precise molecular mechanisms that connect dysfunctions in these pathways to the onset of particular cancers are not well understood in most cases. The loss of ATM has been shown to precede lymphoma development presumably due to excessive homologous recombination, leading to high genomic instability. Disruption of Chk1 in mice led significant misregulation of cell cycle checkpoints, an acculumation of DNA damage, and an increased incidence of tumorigenesis. Perhaps most famously, single mutant inheritance of BRCA1 or BRCA2 predisposes women toward breast and ovarian cancers. BRCA1 is known to be required for S and G2/M transitions, and is involved in the cellular response to DNA damage. BRCA2 is believed to be involved in homologous recombination and regulating the S-phase checkpoint, and mutations of deficiencies in BRCA2 are strongly linked to tumorigenesis.
- Postreplication checkpoint
- Biochemical switches in the cell cycle
- Cell cycle analysis
- G2-M DNA damage checkpoint
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Prophase, metaphase, anaphase, telophase are the steps of mitosis.