Cell cycle spindle checkpoint

Whether the DNA replication checkpoint directly affects the Plxl activation pathway for Cdc25C has not yet been established. It is possible that the replication checkpoint arrests the cell cycle prior to initiation of the Plxl kinase cascade. Further characterization of upstream components of the cascade should reveal whether it is directly regulated by replication checkpoint activation. Such a characterization will also have importance for other M phase events, inasmuch as Plxl also regulates bipolar spindle formation, APC activation and cytokinesis (Qian et al 1998, 1999). These multiple functions of Plxl are associated with changes in localization of Plxl, and are most likely mediated by protein—protein interaction with the polo box motif in the non-catalytic C-terminal half of Plxl. 

Eehner The spindle checkpoint works well and the DNA replication checkpoint is compromised but present in early Drosophila embryos. In addition, Sullivan has proposed that there is a back-up checkpoint mechanism during the syncytial stages (Fogarty et al 1997). If nuclei continue to go through the cell cycle in the presence of unreplicated DNA, they get disposed. The nuclei lose the connection with the centrosome, they fall into the interior of the egg and fail to become cellularized. 

Other important cell cycle transitions are entry into S phase and the G2/M transition. At the G2/M transition, it is registered whether S phase has been completely executed, and the integrity of the DNA is examined at a DNA damage checkpoint. There are other important cell cycle transitions in M phase between metaphase and anaphase. At this point, an important and irreversible decision is made for progress of mitosis if the spindle apparatus is correctly formed and the sister chromatids are correctly aligned, the cell cycle may proceed. 

The cell cycle contains built-in control mechanisms that register defects in the course of the cell cycle and bring about a halt in the cell cycle to enable the fault to be repaired or to lead the cell to programmed cell death. These control mechanisms are also known as checkpoints. These are biochemical pathways that are activated when a fault occurs and can influence other critical steps of the cell cycle. Of particular importance for the cell cycle is the DNA damage checkpoint. Another important checkpoint is the spindle assembly checkpoint, which is not well biochemically characterized, however. 

Activation of p53 can bring about a halt in the cell cycle at the important cell cycle transitions. Thus, the p53 protein is involved in the control of the Gl/S transition, the mitotic spindle checkpoint and the G2/M transition. There is increasing experimental evidence that the halt is irreversible and that the cell can survive for a very long time in this resting state. 

Chabalier C, Lamare C, Racca C et al. BRCAl down-regulation leads to premature inactivation of spindle checkpoint and confers paclitaxel resistance. Cell Cycle 2006 5 1001-1007. 

Another class of substrates comprises the anaphase inhibitors, e. g., the protein securin. Securin is an inhibitor of a protease named separase, which cleaves proteins responsible for sister chromatid cohesion in metaphase. Destruction of these inhibitors is necessary for triggering of sister chromatid separation and progression into anaphase. Because of its central function, the APC is part of several cell cycle checkpoints, e. g., a DNA damage checkpoint and the mitotic spindle checkpoint. 

Cells with abnormal numbers of chromosomes form when certain cell-cycle checkpoints are nonfunctional. As discussed in Chapter 21, the unreplicated-DNA checkpoint normally prevents entry into mitosis unless all chromosomes have completely replicated their DNA the spindle-assembly checkpoint prevents entry into anaphase unless all the replicated chromosomes attach properly to the metaphase mitotic apparatus and the chromosome-segregation checkpoint prevents exit from mitosis and cytokinesis if the chromosomes segregate improperly (see Figure 21-32, steps [H- 3D. As advances are made in Identifying the proteins that detect these abnormalities and mediate cell-cycle arrest, the molecular basis for the functional defects leading to aneuploidy in tumor cells will become clearer. 

Most human tumor cells are aneuplold, containing an abnormal number of chromosomes (usually too many). Failure of cell-cycle checkpoints that normally detect unrepll-cated DNA, improper spindle assembly, or mis-segregatlon of chromosomes permits aneuplold cells to arise. 

EXAMPLE 8.9 There are a number of checkpoints in the cell cycle. Gj has a checkpoint that prevents progression into M phase unless replication of all chromosomal DNA is complete. The M phase checkpoint ensures that the chromosomes have been correctly attached to the mitotic spindle. Checkpoints in the G, and S phases lead to arrest of the cell cycle if damaged DNA is detected by the relevant protein complex. 

The analysis of the distribution of both enzymes further in the cell cycle indicates a brief association of PARP-2 with the outer kinetochore at centromeres (inset panel G). In contrast to PARP-1 that stays on condensed chromatin during the next stages, i.e., metaphase and early anaphase (panels C and D), PARP-2 relocates to the spindle (panel H) and to the spindle midzone (panel 1) and finally to the midbody (panel J) during cytokinesis. The dynamic association of PARP-2 with centromeres is therefore more akin to the previously described checkpoint proteins involved in spindle assembly. In line with this idea, the enhanced binding of PARP-2 to centromeres is observed when loss of spindle tension is induced by colcemid or taxol. ... 

Halappanavar SS, Shah GM. Defective control of mitotic and post-mitotic checkpoints in poly(ADP-tibose) polymerase-l(-/-)fibroblasts after mitotic spindle disruption. Cell cycle 2004 3(3) 335-342. 

BubRl is a spindle assembly checkpoint protein regulating meiotic cell cycle progression of mouse oocyte. Cell Cycle 9(6) 1112-1121. 

Cell biologists consider the cell cycle to be divided into five phases Gl, S, G2, M and GO. There are three principal checkpoints at which the cell cycle is stopped if defects are detected the Gl checkpoint, the G2 checkpoint and the spindle checkpoint during the mitosis phase. The tumour suppressors p53, the guardian angel protein , and Rh protein regulate progression from the Gl phase to the S phase. Mutations of p53 and Rb that fail to suppress progression result in uncontrolled cell division, i.e. tumour development. 

Malfunction of critical organelles or structures (e.g., faulty mitotic spindle) and/or DNA damage prompts checkpoints to activate cell cycle arrest and trigger apoptotic cell death cascades if the damage is not repcured. 

---