Editing Spindle checkpoint (section)

== Discovery of the spindle assembly checkpoint (SAC) ==
[[File:Metaphase anaphase.png|thumb|350px|Microscopy image showing two cells with their [[chromosome]]s stained with DAPI, one at anaphase (left) and the other in metaphase (right), with most of its chromosomes in the metaphase plate and some chromosomes still not aligned.]]
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.<ref name="Zirkle1970">{{cite journal | vauthors = Zirkle RE | title = Ultraviolet-microbeam irradiation of newt-cell cytoplasm: spindle destruction, false anaphase, and delay of true anaphase | journal = Radiation Research | volume = 41 | issue = 3 | pages = 516–37 | date = March 1970 | pmid = 5438206 | doi = 10.2307/3572841 | jstor = 3572841 | bibcode = 1970RadR...41..516Z }}</ref> This observation, together with similar ones, suggested that a control mechanism exists 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<ref name="Rieder1992">{{cite journal | vauthors = Rieder CL, Palazzo RE | title = Colcemid and the mitotic cycle | journal = Journal of Cell Science | volume = 102 ( Pt 3) | issue = 3 | pages = 387–92 | date = July 1992 | doi = 10.1242/jcs.102.3.387 | pmid = 1506421 }}</ref>), the putative control mechanism was named '''Spindle Assembly Checkpoint''' (SAC). This regulatory mechanism has been intensively studied since.<ref name="Burke2008">{{cite journal | vauthors = Burke DJ, Stukenberg PT | title = Linking kinetochore-microtubule binding to the spindle checkpoint | journal = Developmental Cell | volume = 14 | issue = 4 | pages = 474–9 | date = April 2008 | pmid = 18410725 | pmc = 2696048 | doi = 10.1016/j.devcel.2008.03.015 }}</ref>

Using different types of genetic studies, it has been established that diverse kinds of defects are able to activate the SAC: spindle depolymerization,<ref name="Li1991">{{cite journal | vauthors = Li R, Murray AW | title = Feedback control of mitosis in budding yeast | journal = Cell | volume = 66 | issue = 3 | pages = 519–31 | date = August 1991 | pmid = 1651172 | doi = 10.1016/0092-8674(81)90015-5 | s2cid = 11306198 }}</ref><ref name="Hoyt1991">{{cite journal | vauthors = Hoyt MA, Totis L, Roberts BT | title = S. cerevisiae genes required for cell cycle arrest in response to loss of microtubule function | journal = Cell | volume = 66 | issue = 3 | pages = 507–17 | date = August 1991 | pmid = 1651171 | doi = 10.1016/0092-8674(81)90014-3 | s2cid = 10832842 | doi-access = free }}</ref> the presence of dicentric chromosomes (with two centromeres),<ref name="Neff1992">{{cite journal | vauthors = Neff MW, Burke DJ | title = A delay in the Saccharomyces cerevisiae cell cycle that is induced by a dicentric chromosome and dependent upon mitotic checkpoints | journal = Molecular and Cellular Biology | volume = 12 | issue = 9 | pages = 3857–64 | date = September 1992 | pmid = 1324407 | pmc = 360258 | doi = 10.1128/MCB.12.9.3857 }}</ref> centromeres segregating in an aberrant way,<ref name="Wells1996">{{cite journal | vauthors = Wells WA, Murray AW | title = Aberrantly segregating centromeres activate the spindle assembly checkpoint in budding yeast | journal = The Journal of Cell Biology | volume = 133 | issue = 1 | pages = 75–84 | date = April 1996 | pmid = 8601615 | pmc = 2120768 | doi = 10.1083/jcb.133.1.75 }}</ref> defects in the spindle pole bodies in ''S. cerevisiae'',<ref name="Hardwick1996">{{cite journal | vauthors = Hardwick KG, Weiss E, Luca FC, Winey M, Murray AW | title = Activation of the budding yeast spindle assembly checkpoint without mitotic spindle disruption | journal = Science | volume = 273 | issue = 5277 | pages = 953–6 | date = August 1996 | pmid = 8688079 | doi = 10.1126/science.273.5277.953 | bibcode = 1996Sci...273..953H | s2cid = 37404757 }}</ref> defects in the kinetochore proteins,<ref name="Wang1995">{{cite journal | vauthors = Wang Y, Burke DJ | title = Checkpoint genes required to delay cell division in response to nocodazole respond to impaired kinetochore function in the yeast Saccharomyces cerevisiae | journal = Molecular and Cellular Biology | volume = 15 | issue = 12 | pages = 6838–44 | date = December 1995 | pmid = 8524250 | pmc = 230938 | doi = 10.1128/MCB.15.12.6838 }}</ref> mutations in the centromeric DNA<ref name="Spencer1992">{{cite journal | vauthors = Spencer F, Hieter P | title = Centromere DNA mutations induce a mitotic delay in Saccharomyces cerevisiae | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 89 | issue = 19 | pages = 8908–12 | date = October 1992 | pmid = 1409584 | pmc = 50033 | doi = 10.1073/pnas.89.19.8908 | jstor = 2360300 | bibcode = 1992PNAS...89.8908S | doi-access = free }}</ref> or defects in the [[molecular motors]] active during mitosis.<ref name="Li1991" /> A summary of these observations can be found in the article from Hardwick and collaborators in 1999.<ref name="Hardwick1999">{{cite journal | vauthors = Hardwick KG, Li R, Mistrot C, Chen RH, Dann P, Rudner A, Murray AW | title = Lesions in many different spindle components activate the spindle checkpoint in the budding yeast Saccharomyces cerevisiae | journal = Genetics | volume = 152 | issue = 2 | pages = 509–18 | date = June 1999 | doi = 10.1093/genetics/152.2.509 | pmid = 10353895 | pmc = 1460633 }}</ref>

Using its own observations, Zirkle<ref name="Zirkle1970" /> was the first to propose that "some (…) substance, 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 [[cytoplasm]]ic 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.<ref name="McIntosh1991">{{cite journal | vauthors = McIntosh JR | title = Structural and mechanical control of mitotic progression | journal = Cold Spring Harbor Symposia on Quantitative Biology | volume = 56 | pages = 613–9 | year = 1991 | pmid = 1819511 | doi = 10.1101/sqb.1991.056.01.070 }}</ref> Indeed, the available data suggested that the signal "wait to enter in anaphase" is produced mostly on or close to unattached kinetochores.<ref name="Rieder1995">{{cite journal | vauthors = Rieder CL, Cole RW, Khodjakov A, Sluder G | title = The checkpoint delaying anaphase in response to chromosome monoorientation is mediated by an inhibitory signal produced by unattached kinetochores | journal = The Journal of Cell Biology | volume = 130 | issue = 4 | pages = 941–8 | date = August 1995 | pmid = 7642709 | pmc = 2199954 | doi = 10.1083/jcb.130.4.941 }}</ref> 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<ref name="Rieder1995" />), or the tension stabilizing the anchoring of microtubules to the kinetochores (as suggested by the experiments realized at Nicklas' lab<ref name="Li1997">{{cite journal | vauthors = Li X, Nicklas RB | title = Tension-sensitive kinetochore phosphorylation and the chromosome distribution checkpoint in praying mantid spermatocytes | journal = Journal of Cell Science | volume = 110 ( Pt 5) | issue = 5 | pages = 537–45 | date = March 1997 | doi = 10.1242/jcs.110.5.537 | pmid = 9092936 | doi-access = free }}</ref>). 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.<ref name="Rieder1997">{{cite journal | vauthors = Rieder CL, Khodjakov A, Paliulis LV, Fortier TM, Cole RW, Sluder G | title = Mitosis in vertebrate somatic cells with two spindles: implications for the metaphase/anaphase transition checkpoint and cleavage | journal = Proceedings of the National Academy of Sciences of the United States of America | volume = 94 | issue = 10 | pages = 5107–12 | date = May 1997 | pmid = 9144198 | pmc = 24639 | doi = 10.1073/pnas.94.10.5107 | bibcode = 1997PNAS...94.5107R | doi-access = free }}</ref> 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.