A model for feedback control of spindle checkpoint activity
Feedback control of spindle checkpoint activity. (Left) Prior to metaphase, chromosomes that are incorrectly aligned on the mitotic spindle generate a signal that activates the spindle checkpoint. The checkpoint is functional and the APC is repressed, arresting the cell cycle at metaphase. (Right) When all chromosomes have aligned correctly on the mitotic spindle the inhibition of the APC is relieved. MPS1, an essential checkpoint component, is targeted for destruction by the APC, which maintains the checkpoint in an inactive state. Credit: P. Huey/Science
Cell cycle checkpoints are surveillance mechanisms that ensure the ordered progression of events during the cell division cycle. Checkpoints prevent progression to the next stage of the cycle until the upstream event has been completed. Genomic instability results when the strict order of cell cycle events breaks down, and inactive checkpoints are often associated with aneuploidy, a common feature of cancer cells.
It is important that checkpoints function at the correct stage of the cell cycle. For example, during mitosis, the spindle checkpoint monitors chromosome alignment on the mitotic spindle. If errors in chromosome alignment are detected, spindle checkpoint activity delays the cell cycle, which allows time for the defects to be corrected. When chromosomes segregate into daughter nuclei it is too late for chromosome alignment to be corrected, and a cell cycle delay caused by spindle checkpoint activation would be disadvantageous.
The process of chromosome separation generates chromosomes attached to a single spindle pole, one of the signals that would normally activate the spindle checkpoint. This presents a regulatory problem for the cell: switching the checkpoint to an inactive state after it has performed its function.
In this study (Palframan, W. J., Meehl, J. B., Jaspersen, S. L., Winey, M., and Murray, A. W. Anaphase inactivation of the spindle checkpoint. Science 2006) we have looked at how the budding yeast spindle checkpoint is regulated as cells leave metaphase. MPS1, an essential component of the spindle checkpoint, was found to be a cell cycle regulated protein. MPS1 protein abundance decreases after metaphase, at the same time the checkpoint is inactivated, implicating MPS1 stability in the regulation of checkpoint activity. Overexpressing the protein after metaphase reactivated the spindle checkpoint, suggesting that decreased MPS1 levels have a role in checkpoint inactivation.
Repressing the target of the spindle checkpoint, the multisubunit ubiquitin ligase, the anaphase promoting complex (APC) stabilized MPS1 and allowed the checkpoint to be reactivated. The identification of APC recognition sites in MPS1 supports the conclusion that MPS1 is a direct target of the APC. Taken together, these results suggest that the target of the spindle checkpoint, the APC, feeds back on at least one checkpoint component to regulate checkpoint activity at the metaphase to anaphase transition. Prior to metaphase, MPS1 is stable, the checkpoint is functional, and the APC is repressed. After metaphase, the APC is active, MPS1 is unstable, and the checkpoint is inactive. We show that experimentally manipulating this feedback loop can effectively switch the spindle checkpoint between active and inactive states.
In summary, the opposing activities of the checkpoint and the APC let cells switch rapidly between metaphase, when they can sensitively monitor chromosome alignment, and anaphase, when they are irreversibly committed to entering the next cell cycle. It is likely that other factors play a role in spindle checkpoint inactivation; the feedback circuit provides an example of a simple mechanism the cell can use to help solve this regulatory problem.