Spindle Checkpoint - Discovery of The Spindle Assembly Checkpoint (SAC)

Discovery of The Spindle Assembly Checkpoint (SAC)

Zirkle (in 1970) was one of the first researchers to observe that, when just one chromosome is retarded to arrive at the metaphase plate, anaphase onset is postponed until some minutes after its arrival. This observation, together with similar ones, suggested that it exists a control mechanism at the metaphase-to-anaphase transition. Using drugs such as nocodazole and colchicine, the mitotic spindle disassembles and the cell cycle is blocked at the metaphase-to-anaphase transition. Using these drugs (see the review from Rieder and Palazzo in 1992), the putative control mechanism was named Spindle Assembly Checkpoint (SAC). This regulatory mechanism has been intensively studied from then (see the review from Burke and Stukenberg in 2008).

Using different types of genetic studies, it has been established that diverse kinds of defects are able to activate de SAC: spindle depolimerization, the presence of dicentric chromosomes (with two centromeres), centromeres segregating in an aberrant way, defects in the spindle pole bodies in S. cerevisiae, defects in the kinetochore proteins, mutations in the centromeric DNA or defects in the molecular motors active during mitosis. A summary of these observations can be found in the article from Hardwick and collaborators in 1999.

Using its own observations, Zirkle was the first to propose that "some (…) susbstance, necessary for the cell to proceed to anaphase, appears some minutes after C (moment of the arrival of the last chromosome to the metaphase plate), or after a drastic change in the cytoplasmic condition, just at C or immediately after C", suggesting that this function is located on kinetochores unattached to the mitotic spindle. McIntosh extended this proposal, suggesting that one enzyme sensitive to tension located at the centromeres produces an inhibitor to the anaphase onset when the two sister kinetochores are not under bipolar tension. Indeed, the available data suggested that the signal "wait to enter in anaphase" is produced mostly on or close to unattached kinetochores. However, the primary event associated to the kinetochore attachment to the spindle, which is able to inactivate the inhibitory signal and release the metaphase arrest, could be either the acquisition of microtubules by the kinetochore (as proposed by Rieder and collaborators in 1995), or the tension stabilizing the anchoring of microtubules to the kinetochores (as suggested by the experiments realized at Nicklas' lab). Subsequent studies in cells containing two independent mitotic spindles in a sole cytoplasm showed that the inhibitor of the metaphase-to-anaphase transition is generated by unattached kinetochores and is not freely diffusible in the cytoplasm. Yet in the same study it was shown that, once the transition from metaphase to anaphase is initiated in one part of the cell, this information is extended all along the cytoplasm, and can overcome the signal "wait to enter in anaphase" associated to a second spindle containing unattached kinetochores.