Correcting Errors microtubule fibers

To devise a strategy to address the question of how attachment errors were corrected, several issues needed to be addressed. Eirst, Aurora kinases have been implicated in multiple processes in mitosis (44). Ideally, kinase inhibition temporally would be controlled to isolate experimentally the error correction process. Second, the microtubules fibers that attach chromosomes to the spindle are highly dynamic, and the error correction likely involves some regulation of these dynamics. Live imaging would permit the analysis of microtubule dynamics with high temporal and spatial resolution. Finally, analysis of microtubule dynamics is difficult if individual fibers are obscured by other microtubules in the spindle. By creating conditions in which the improperly attached chromosomes are positioned away from the spindle body, individual fibers could be observed clearly. 

Figure 4 Correction of improper chromosome attachments by activation of Aurora kinase (45). (a) Assay schematic, (i) Treatment with the Eg5 inhibitor monastrol arrests cells in mitosis with monopolar spindles, in which sister chromosomes often are both attached to the single spindle pole, (ii) Hesperadin, an Aurora kinase inhibitor, is added as monastrol is removed. As the spindle bipolarizes with Aurora kinase inhibited, attachment errors fail to correct so that some sister chromosomes are still attached to the same pole of the bipolar spindle, (iii) Removal of hesperadin activates Aurora kinase. Incorrect attachments are destabilized by disassembling the microtubule fibers, which pulls the chromosomes to the pole, whereas correct attachments are stable, (iv) Chromosomes move from the pole to the center of the spindle as correct attachments form, (b) Structures of the Eg5 inhibitor monastrol and two Aurora kinase inhibitors, hesperadin and AKI-1. (c) Spindles were fixed after bipolarization either in the absence (i) or presence (ii) of an Aurora kinase inhibitor. Arrows indicate sister chromosomes that are both attached to the same spindle pole. Projections of multiple image planes are shown, with optical sections of boxed regions (1 and 2) to highlight attachment errors. Scale bars 5 xm. (d) After the removal of hesperadin, GFP-tubulin (top) and chromosomes (bottom) were imaged live by three-dimensional confocal fluorescence microcopy and DIC, respectively. Arrow and arrowhead show two chromosomes that move to the spindle pole (marked by circle in DIC images) as the associated kinetochore-microtubule fibers shorten and that then move to the center of the spindle. Time (minutes seconds) after the removal of hesperadin. Scale bar 5 (cm.

Accurate chromosome segregation in mitosis requires not only feedback control of anaphase onset but also regulation of chromosome attachment to the spindle. Each pair of replicated chromosomes must achieve a particular orientation in which microtubule fibers attach sister chromosomes to opposite poles of the spindle. Experiments in yeast showed that inhibition of the Ipll/Aurora family of kinases stabilized improper attachments [45, 46], but how the active kinase corrected attachment errors was not known. Because attachment errors are rarely observed in the presence of active Aurora kinase, this problem was particularly difficult to address. Inhibition of Aurora kinase, through experimental approaches such as genetic mutation, could be used to accumulate attachment errors, but not to examine error correction by the active kinase. Reversible small molecule Aurora kinase inhibitors present a... 

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