Department of Biology, CB#3280, University of North Carolina at Chapel Hill, Chapel Hill, NC 27599, USA
* Author for correspondence (e-mail: cimini{at}email.unc.edu)
Accepted 19 June 2003
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Summary |
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Key words: Merotelic orientation, Lagging chromosomes, Mitotic checkpoint, Chromosome segregation
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Introduction |
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It is likely that merotelic kinetochore orientation is mainly an error inherent to the stochastic nature of the `search and capture' mechanism of kinetochore microtubule formation that depends on microtubule dynamic instability (Desai and Mitchison, 1997; Inoue and Salmon, 1995
; Rieder and Salmon, 1998
; Wittmann et al., 2001
), although other mechanisms are also possible (Khodjakov et al., 2003
). Mammalian kinetochores have multiple microtubule plus end attachment sites, typically 15-25, within the outer plate (McEwen et al., 1997
). Attachment to one of these sites stabilizes a plus end and forms a kinetochore microtubule that mechanically links the kinetochore to the site of microtubule nucleation (Rieder and Salmon, 1998
). Normally, sister kinetochores face opposite directions so that when one sister captures microtubules from one pole the subsequent chromosome movement toward that pole causes the other sister to face away from the pole. This mono-orientation prevents both sister kinetochores from capturing microtubule plus ends from the same pole (syntelic orientation) and promotes the correct attachment of sisters to opposite spindle poles (amphytelic orientation, or bi-orientation) (Nicklas, 1997
; Rieder and Salmon, 1998
).
Little is known about the initial incidence of merotelic kinetochore orientation and the correction mechanisms that may operate to prevent lagging chromosomes in normal mitotic tissue cells (Ault and Rieder, 1992; Roos, 1973
; Roos, 1976
). In previous studies merotelic kinetochore orientation in mitotic cells has been studied in fairly abnormal situations, such as impaired sister kinetochore structure (Khodjakov et al., 1997
; Wise and Brinkley, 1997
; Yu and Dawe, 2000
), and multipolar mitoses (Heneen, 1975
; Sluder et al., 1997
). Studies of normal meiosis in invertebrates have shown that attachment errors, in particular syntelic orientation (both sister kinetochores attached to the same pole), are initially frequent and correction mechanisms exist (Bajer, 1972; Nicklas, 1971
; Nicklas, 1997
). Recently, correction mechanisms for syntelic kinetochore orientation both in meiosis and mitosis have been identified in budding yeast (Biggins and Murray, 2001
; Shannon and Salmon, 2002
; Tanaka et al., 2002
) and there is evidence that a similar molecular mechanism may function to correct syntelic and merotelic errors during mitosis in higher eucaryotes (Shannon and Salmon, 2002
).
A major question is whether merotelic kinetochore orientation in cells with bipolar spindles and unperturbed chromosomes normally occurs in prometaphase at frequencies higher than the frequencies of lagging chromosomes exhibited in anaphase. Another major question is why transient spindle disassembly is such a potent promoter of lagging chromosomes (Cimini et al., 2002; Cimini et al., 2001
). Upon spindle reassembly and progression into anaphase the frequency of cells with merotelically oriented lagging chromosomes is substantially increased to 8-30% (Cimini et al., 2002
; Cimini et al., 2001
; Ladrach and LaFountain, 1986
).
To test in mammalian mitosis for the initial incidence and subsequent correction of merotelic kinetochore orientation, we performed several assays using PtK1 cells. To test for the incidence of merotelic attachments in early prometaphase, we induced prometaphase cells to enter anaphase precociously and compared the frequency of lagging chromosomes in these cells and in cells proceeding to anaphase normally. To examine why recovery from spindle depolymerization produces a high incidence of merotelic kinetochore orientation, we analyzed spindle and kinetochore structure in both nocodazole-treated and untreated PtK1 cells. To determine if time before anaphase onset is an important factor in eliminating merotelic kinetochore orientation, we delayed cells in metaphase by a reversible inhibitor of the proteasome. And finally, to see if merotelic orientations are corrected by activities at or after anaphase onset, we used high resolution confocal fluorescence microscopy and 3D image deconvolution to compare the frequency of merotelic kinetochores at late prometaphase/metaphase with the frequency of merotelic orientation in `delayed' metaphase, and the frequency of lagging chromosomes in anaphase. Finally, we tested whether the ratio of microtubules to opposite poles is an important factor in producing lagging chromosomes by quantification of fiber fluorescence for merotelic kinetochores in normal and delayed metaphases.
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Materials and Methods |
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Microinjection and time-lapse microcopy
Coverslips with untreated cells or with nocodazole-released cells were mounted into modified Rose chambers (Rieder and Hard, 1990) without the top coverslip, the chambers were filled with Hepes-buffered L-15 medium, and a thin layer of mineral oil was poured on the medium to avoid evaporation. Stage temperature was maintained at
35°C using an air curtain incubator (Nevtek, Model ASI 400, Burnsville, VA). Microinjection with GST-Mad1F10 and Mad2
C was performed as previously described (Canman et al., 2002a
). Control prometaphase cells were microinjected with either 2 mg/ml (needle concentration) purified GST in HEK buffer (20 mM Hepes, 100 mM KCl, and 1 mM DTT, pH 7.7 with KOH), HEK buffer alone, 2 mg/ml GST-Mad1F10, or 7 mg/ml Mad2
C in HEK buffer. Nocodazole-released cells were either time-lapsed uninjected or were microinjected with either GST-Mad1F10 or Mad2
C after about 15 minutes recovery from the mitotic block. Phase contrast time-lapse images were acquired every 30 seconds using a MetaView image acquisition system (Universal Imaging Corp., Downingtown, PA).
-tubulin-GFP-expressing PtK1 cells (PtK6-23) were time-lapsed by phase contrast and fluorescence microscopy using a Nikon TE300 inverted microscope equipped with a 100x (1.4 NA) Plan Apo phase 3 objective, a Chroma Hy-Q FITC filter set, and an Orca II CCD camera (Hamamatsu Photonics, Bridgewaters, NJ). The microscope was controlled by MetaMorph imaging software (Universal Imaging). Phase contrast and fluorescence images were acquired almost simultaneously every 2 minutes, using a 4x4 binning.
Immunostaining with CREST, anti--tubulin, anti-
-tubulin and anti-Mad2 antibodies
For CREST staining and combined CREST and -tubulin immunostaining, cells were fixed and processed as previously described (Cimini et al., 2001
). For combined Mad2/
-tubulin and Mad2/
-tubulin immunostaining the same procedure was used, except for the fixation, which was performed in a freshly prepared 4% formaldehyde solution at room temperature. We used the following primary antibodies: CREST SH serum (a generous gift from Dr B. R. Brinkley, Baylor College of Medicine, TX), diluted 1:600; mouse anti-
-tubulin antibody (Sigma), diluted 1:600; rabbit anti-Mad2 antibody (Waters et al., 1998
), diluted 1:100; mouse anti
-tubulin antibody (Sigma), diluted 1:5000. The following secondary antibodies were used in various combinations in different experiments: Rhodamine Red-X anti-human (Jackson ImmunoResearch, West Grove, PA), diluted 1:200; Alexa 488 anti-mouse antibody (Molecular Probes, Eugene, OR), diluted 1:1000; Alexa 488 anti-rabbit antibody (Molecular Probes), diluted 1:1000; Rhodamine Red-X anti-mouse (Jackson ImmunoResearch), diluted 1:200.
Fluorescence microscopy and image acquisition
Wide-field epifluorescence was used for analysis of anaphase lagging chromosomes by CREST staining. For each slide all the anaphase cells were analyzed and recorded as normal or as showing one or multiple lagging chromosomes. The data shown in the paper are the sum of three independent experiments. For Mad2/-tubulin and Mad2/
-tubulin immunostained cells z-series stacks of images at 0.2 µm intervals through each cell analyzed were collected. The images were obtained by projecting into a single image the maximal brightness at each pixel location. The microscope used was a Nikon TE300 inverted microscope equipped with a 100x (1.4 NA) Plan Apo phase 3 objective, Chroma filters, and an Orca II CCD camera (Hamamatsu Photonics). To analyze merotelic kinetochore orientation in late prometaphase and metaphase we acquired z-series stacks of confocal images at 0.2 µm intervals through each cell analyzed and processed the image stacks to obtain 3D projections after digital image deconvolution using a Delta Vision image processing workstation (Applied Precision; Seattle, WA). To improve the accuracy of the analysis, two different people performed the analysis blindly and independently. Images were recorded with a Nikon TE300 inverted microscope equipped with epifluorescence illumination by a Yokogawa CS10 spinning disk confocal attachment (Perkin Elmer Life Sciences Wallac; Gaithersburg, MD), containing filters and filter wheels for illumination at 488 nm or 568 nm from a 60 mW argon/krypton laser, and equipped with a 100X/1.4 NA plan-apo chromatic phase contrast objective lens. Digital images were obtained with an Orca ER cooled CCD camera (Hamamatsu Photonics). Image acquisition, microscope shutters and z-axis focus were all controlled by MetaMorph (Universal Imaging) software on PC computers. Adobe Photoshop software was used to process all digital images.
Fluorescence measurements
Fluorescence intensity measurements of the two microtubule bundles attached to a merotelic kinetochore were performed on deconvolved images by measuring the fluorescence intensity of the microtubule bundle, as detected by -tubulin immunostaining, and subtracting the average background intensity. The fluorescence intensity of the microtubule bundle was determined in a 5x5 pixel square region on the microtubule side of the microtubulekinetochore interface. The background fluorescence intensity was measured in two 5x5 pixel regions on both sides of the same microtubule bundle, and then these values were averaged. The measurements were made using the data inspector tool of the Deltavision's Softworx Deconvolution package (Applied Precision), which is run on a Silicon Graphics platform.
Online supplemental material
QuickTime movies and a supplemental table (Table S1) summarizing both data `per cell' and `per kinetochore' in different experimental conditions are also available online (http://JCS.biologists.org/supplemental/).
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Results |
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When PtK1 prometaphase cells were microinjected with either GST-Mad1F10 (11 cells microinjected) or with Mad2C (11 cells microinjected), they entered anaphase precociously and 27.3% (for GST-Mad1F10) and 36.4% (for Mad2
C) of the cells showed lagging chromosomes at anaphase (Fig. 1B, Fig. 2). As a control, we microinjected 10 PtK1 prometaphase cells with GST-containing buffer and 10 PtK1 prometaphase cells with the Mad2
C dilution buffer. None of the 20 microinjected cells entered anaphase precociously and none showed anaphase lagging chromosomes (Fig. 1A, Fig. 2), as expected for normal untreated PtK1 cells, where the frequency of anaphase lagging chromosomes is about 1% (Cimini et al., 2001
). To test if lagging chromosomes were somehow induced by the injected proteins, we injected 19 metaphase PtK1 cells with Mad2
C and found that none of them displayed lagging chromosomes at anaphase.
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We next tested for the incidence of merotelic orientation in prometaphase of cells recovering from a nocodazole-induced mitotic block because of the higher incidence of anaphase lagging chromosomes compared to controls (Cimini et al., 2001; Cimini et al., 1999
). Cells were released from the mitotic block for about 15 minutes to allow spindle reassembly and then microinjected with either GSTMad1F10 (13 cells) or Mad2
C (11 cells) to induce precocious anaphase. The frequencies of cells showing lagging chromosomes in anaphase were 92.3% and 90.9% for GST-Mad1F10 and Mad2
C, respectively, indicating a very high incidence of merotelic kinetochore orientation (Fig. 1D, Fig. 2; Movie 1: http://JCS.biologists.org/supplemental/). As a control, we measured by live cell imaging the frequency of lagging chromosomes in 10 PtK1 cells recovering from nocodazole and progressing normally into anaphase. This frequency was 20% (Fig. 1C, Fig. 2), similar to the 16-18% obtained by fixed time-point assays (Cimini et al., 2001
).
Merotelic kinetochore orientation is responsible for the induction of lagging chromosomes in cells progressing normally into anaphase (Cimini et al., 2001). To confirm the merotelic orientation of lagging chromosomes in cells induced to enter anaphase precociously, some of the cells microinjected with GST-Mad1F10 were followed by time-lapse microscopy until mid-anaphase and then fixed and immunostained for kinetochores and microtubules. The immunostaining showed that lagging chromosomes had merotelically oriented kinetochores that were stretched laterally between bundles of microtubules from opposite spindle poles (Fig. 3).
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A delay in achieving spindle bipolarity promotes very high frequencies of merotelic orientations in nocodazole-recovering cells
When mitotic spindle assembly is inhibited by nocodazole treatment, centrosomes appear close to each other (Cassimeris et al., 1990) and kinetochores expand into curved crescents around the centromeres (Cimini et al., 2001
; Hoffman et al., 2001
). It has been proposed that the enlargement of kinetochores into a crescent shape around their centromeres might promote merotelic kinetochore orientation (Cimini et al., 2001
). However, a delay in establishing spindle bipolarity during recovery from nocodazole might also contribute to the very high frequency of merotelic kinetochore orientation seen during recovery from nocodazole treatment.
To test whether and how these two mechanisms contribute to the incidence of merotelic kinetochore orientation, we fixed cells after a 3 hour nocodazole treatment and after 1, 5 and 15 minutes recovery from the nocodazole-induced mitotic block. These cells were immunostained either for -tubulin (centrosomes) and Mad2 (kinetochore outer domain), or for
-tubulin (microtubules) and Mad2. As expected (Cimini et al., 2001
; Hoffman et al., 2001
), Mad2 accumulated at kinetochores in cells arrested in mitosis by a 3 hour nocodazole treatment and some of the kinetochores showed a clear crescent structure (Fig. 4A, Fig. 5A'). Furthermore, the centrosomes, as seen by
-tubulin staining, were unseparated (Fig. 4A). Surprisingly, 1 minute after nocodazole washout most of the Mad2 staining had disappeared, even though centrosomes had not separated, yet (Fig. 4B). Most of the cells showed clear centrosome separation 5 minutes after release from the mitotic block (Fig. 4C). At this recovery time, however, not all the chromosomes were aligned at the metaphase plate and some Mad2 was still present at kinetochores. Well separated centrosomes and chromosomes aligned at the metaphase plate were observed after 15 minutes recovery from the mitotic block (Fig. 4D).
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When cells recovering from the mitotic block were analyzed by -tubulin immunostaining, multiple sources of microtubules surrounding the chromosomes (typically about 4->8 microtubule organizing centers) were observed after 1 minute recovery (Fig. 5B). After 5 minutes recovery the number of
-tubulin-positive microtubule organizing centers ranged between 2 and 5, with most of the cells showing 3 or 4 (Fig. 5C). At 15 minutes recovery, many cells (50%) showed 2 major spindle poles and a clearly bipolar spindle (Fig. 5D), but a minor fraction (35%) of the population still showed 3 spindle poles. However, by anaphase, most of the cells reached bipolarity and only about 4% of the anaphases were tri- or tetrapolar. This frequency was not significantly higher than the 3% occurrence of multipolar anaphases observed in control PtK1 cells. These data suggest that the high incidence of merotelic kinetochore orientation observed during recovery from a nocodazole-induced mitotic block is primarily produced by single kinetochores becoming attached to microtubules coming from microtubule organizing centers that are initially close together and later separate into opposite spindle poles. The enlargement of kinetochores into crescents might favor their interaction with more than one microtubule organizing center.
Merotelic orientations in untreated cells are not caused by a delay in establishing spindle bipolarity
To test in untreated cells whether and how often centrosomes fail to separate to the opposite sides of the nucleus before nuclear envelope breakdown and spindle assembly, we used -tubulin-GFP-expressing PtK1 cells (PtK6-23; a generous gift from Dr Alexey Khodjakov, Wadsworth Center, Albany, NY) to follow spindle pole separation in living cells. As detected by CREST staining on fixed cells, only 1% of the anaphases (3/300) in PtK6-23 cells showed lagging chromosomes, indicating that the expression of the chimeric protein alone does not induce an increase in anaphase lagging chromosomes. PtK6-23 cells were recorded by time-lapse phase contrast and fluorescence microscopy from prophase to telophase. One out of 30 cells analyzed showed a lagging chromosome in anaphase. This frequency is not significantly different from the 1% frequency of anaphase lagging chromosomes in fixed PtK6-23 cells (
2 test, P=0.811). However, in all cells analyzed, including the one showing a lagging chromosome in anaphase, centrosomes were well separated, 29 out of 30 to opposite sides of the nucleus, when nuclear envelope breakdown occurred (see Movies 2 and 3: http://JCS.biologists.org/supplemental/). These data indicate that defects in centrosome separation are not responsible for the high incidence of merotelic kinetochore orientation in normal prometaphase.
We then tested whether ectopic microtubule nucleation centers are present in early prometaphase of untreated cells, as seen by -tubulin staining in nocodazole-recovering cells. Almost all the cells showed normal bipolar spindles with two well separated spindle poles, and no ectopic microtubule nucleation centers were observed in cells fixed immediately after nuclear envelope breakdown (Fig. 6A,B) or later in prometaphase (Fig. 6C). A small number of cells (2%) showed tripolar or tetrapolar spindles, but this represents a multipolar subpopulation always observed in cultured PtK1 cells, at any mitotic stage.
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In control PtK1 cells immediately after nuclear envelope breakdown, there was great variability seen between different chromosomes in the orientation of the centromere axis through sister kinetochores relative to the interpolar spindle axis (Fig. 6A,B). Some sister pairs were aligned nearly parallel to the spindle axis (yellow arrowhead in Fig. 6A), while others were nearly perpendicular (white arrows in Fig. 6B). This perpendicular orientation of sister kinetochores may promote merotelic attachment as discussed below.
Prolonging metaphase significantly reduces lagging chromosomes in anaphase
Our microinjection experiments indicate that correction mechanisms must function to reduce the number of merotelic orientations and prevent anaphase lagging chromosomes. The next obvious investigation was to determine whether lengthening the time before anaphase enhances error correction. Both untreated cells and cells recovering from a nocodazole-induced mitotic block were treated for 2 hours with 5 µM MG-132 to inhibit the proteasome and prolong metaphase (Hoffman et al., 2001; Rock et al., 1994
; Topper et al., 2001
). MG-132 induced a metaphase arrest, but did not affect spindle assembly. Chromosomes aligned at the metaphase plate, showing normal chromosome oscillations back and forth in the equatorial region (data not shown). At the end of the MG-132 treatment, the drug was washed out and cells were fixed 75 minutes later for analysis of anaphase lagging chromosomes. The delay in metaphase produced by MG-132 reduced the incidence of lagging chromosomes in anaphase to 0.4% for untreated and 2.6% for nocodazole-recovering cells, representing 2.7- and 6.2-fold reduction, respectively, compared to cells progressing to anaphase without MG-132 delay (Fig. 7A). Fig. 7B shows that in nocodazole-released cells both single and multiple lagging chromosomes were substantially decreased. Taken together, these results indicate that time before anaphase is an important factor for the mechanisms correcting chromosome segregation errors produced by merotelic kinetochore orientation.
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The frequency of merotelic kinetochores seen at late prometaphase/metaphase or after a metaphase delay is much higher than the frequency of lagging chromosomes in anaphase
In a previous study, we showed that merotelically oriented kinetochores can be seen in late prometaphase/metaphase cells by high resolution 3D confocal and deconvolution microscopy of fixed cells immunofluorescently stained for kinetochores and kinetochore microtubules (Cimini et al., 2001). Here we used the same 3D imaging technology (Fig. 8) to evaluate the actual frequency of merotelic orientation in prometaphase/metaphase cells and to test how the frequency of cells with merotelically oriented kinetochores before anaphase onset compares with the frequency of cells with lagging chromosomes in anaphase under our various experimental conditions.
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Typically, a merotelically oriented kinetochore was shifted closer to the equator than normally oriented kinetochores of bioriented chromosomes at the metaphase plate, making the detection easier (Fig. 8B-E). Furthermore, the resolution in our images in combination with 3-D image rotation to check for parallax problems (Cimini et al., 2001) allowed for accurate detection of kinetochores that had attached bundles of fluorescent microtubules extending toward both poles.
PtK1 cells were fixed untreated or after a 3 hour nocodazole block plus 15-30 minutes recovery. Late prometaphase and metaphase cells (prometaphases/metaphases) with bipolar spindles were analyzed for the presence of merotelically oriented kinetochores. The percentage of prometaphase/metaphase cells with at least one merotelically oriented kinetochore was 16.3% for untreated cells (Fig. 8B-B''; Fig. 9A, -MG-132) and 45.8% for cells recovering from the nocodazole-induced mitotic block (Fig. 8C-C''; Fig. 9A, -MG- 132). Nocodazole-treated cells also exhibited high frequencies of cells possessing more than one merotelic kinetochore (Fig. 9B, -MG-132, diagonally striped bar). These results show that the frequency of merotelic kinetochore orientations remains high in late prometaphase/metaphase, much higher than seen for the frequency of lagging chromosomes in control or nocodazole-released cells that progressed normally into anaphase (16.3% vs. 1.1% for controls, and 45.8% vs. 16% for nocodazole-released cells; compare Figures 7 and 9).
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To test whether spending longer times in metaphase before anaphase onset induces a reduction in the frequency of merotelic kinetochores, we assayed the frequency for cells delayed in metaphase for 2 hours. PtK1 cells were fixed after two hours in 5 µM MG-132. Alternatively, PtK1 cells were arrested in mitosis by a 3 hour nocodazole treatment and then fixed after 25 minutes recovery from nocodazole plus 2 hours in MG-132. At the end of a 2 hour MG-132 treatment the `delayed' metaphases analyzed showed only a slight decrease in merotelic kinetochore orientations compared to `non-delayed' metaphases (12.2% vs. 16.3% in untreated cells; 32.6% vs. 45.8% in cells recovering from nocodazole; Fig. 9A). The frequency of merotelic orientations observed in these `delayed' metaphases were still much higher than the frequency of anaphase lagging chromosomes (see Fig. 7A for comparison). The analysis of merotelic orientations in nocodazole-released cells also showed that the delay in anaphase onset produced a similar reduction of merotelic orientations for cells possessing single and multiple merotelic kinetochores (Fig. 9B).
Taken together, the above results show that a second mechanism of error correction functions at or after the onset of anaphase to reduce the number of lagging chromosomes produced by kinetochores that are still merotelically oriented at the metaphase-to-anaphase transition. We do not know how this mechanism functions, but one possible factor is the ratio of microtubules attached to the kinetochore from opposite poles, with a ratio of 1 giving the highest probability and higher ratios showing lower probabilities of lagging chromosomes. To test this possibility, we measured the size of the two microtubule bundles connected to a merotelic kinetochore by measuring the fluorescence intensity of -tubulin staining (see Materials and methods for details). For each merotelic kinetochore we calculated the ratio between the highest and the lowest intensity. In non-delayed metaphases the average ratio was smaller than 2 for both untreated cells and cells recovering from a nocodazole block (Table 1). After an MG-132-induced delay, the ratio increased to about 2.4 (Table 1). This increase was observed in both control cells (t-test, P<0.05) and cells recovering from a nocodazole-induced mitotic block (t-test, P=0.1) indicating that the ratio of microtubules from opposite poles is a critical factor in determining whether a merotelic kinetochore produces an anaphase lagging chromosome.
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Discussion |
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This high incidence of merotelic orientations during prometaphase suggests that the back-to-back arrangement of sister kinetochores on opposite sides of the centromere does not guarantee their interaction with opposite spindle poles. However, the back-to-back geometry of sister kinetochores probably plays a relevant role in preventing even higher percentages of merotelic kinetochore orientation, since perturbations of the centromeric region that produce single kinetochores associated to single chromatids or chromatid fragments, result in curled kinetochores that become merotelically oriented (Khodjakov et al., 1997; Wise and Brinkley, 1997
; Yu and Dawe, 2000
).
We also found that prevention of merotelic kinetochore orientation depends critically on establishing two and only two well-separated spindle poles before nuclear envelope breakdown and spindle assembly. In cells recovering from a nocodazole block, our minimum estimate for the percentage of merotelic kinetochores in early prometaphase is 8.2%, about threefold higher than the minimum estimate for untreated cells (Fig. 10A; Table S1). Several factors may contribute to this enhancement, including the enlargement of the kinetochore outer plate in nocodazole (Cimini et al., 2001; Hoffman et al., 2001
), a delay in centrosome separation, and the formation of multiple non-centrosomal centers around the chromosomes after nocodazole wash-out. The presence of multiple microtubule-organizing centers during recovery from a mitotic block had been previously described in different cell lines (De Brabander et al., 1981
; Tousson et al., 1991
), and a role of such ectopic centers in growing functional microtubules had been suggested by their association with the nuclear protein NuMA (Tousson et al., 1991
), a protein that concentrates at functional spindle poles (Compton and Cleveland, 1994
). However, the dynamics of mitotic spindle reassembly after nocodazole washout had never been directly related to kinetochore misorientation and chromosome missegregation. In our study the presence of multiple microtubule organizing centers during the initial stages of spindle reassembly provides an explanation for the fast disappearance of kinetochore crescents and Mad2 by interactions between kinetochores and growing microtubules (Hoffman et al., 2001
; Waters et al., 1998
). It also suggests that during recovery from the nocodazole-induced mitotic block, single kinetochores can interact with microtubules coming from two microtubule organizing centers that are initially close together and subsequently move apart into opposite spindle poles. This would strongly contribute to the higher incidence of merotelic orientations observed in nocodazole-recovering cells. In support of this conclusion, we have found that 19% of PtK1 cells exhibit one or more lagging chromosomes in anaphase (D. Cimini and E. D. Salmon, unpublished observation) when they are initially treated with the drug monastrol to produce monopolar spindles in prometaphase and then washed free of monastrol to allow spindle bipolarization and subsequent anaphase (Mayer et al., 1999
).
It is important to note that our data show that neither a delay in centrosome separation, nor the presence of multiple microtubule organizing centers provides an explanation for merotelic kinetochore orientations in control cells. In normal prometaphase cells, unattached kinetochores can be wider and appear more curved, when seen by conventional electron microscopy, than kinetochores at the spindle equator with a full complement of microtubules (Rieder, 1982; Salmon et al., 1998
). We also found that the orientation of the centromere axis through sister kinetochores was highly variable between different chromosomes relative to the spindle interpolar axis in early prometaphase. As shown in Fig. 11A, a curved kinetochore of a chromosome oriented with its intercentromere axis perpendicular to the interpolar spindle axis will be more likely to capture microtubules from opposite poles, as previously proposed for Drosophila meiotic cells (Church and Lin, 1982
). Given available information, this mechanism seems the most likely explanation for the high frequency of merotelic kinetochore formation in early prometaphase in control cells. However, another mechanism may also contribute to merotelic attachment. Kinetochore microtubules are infrequently assembled from kinetochores and their minus-ends recruited to the spindle poles (Khodjakov et al., 2003
). Merotelic orientation might occur if some minus ends are recruited to the wrong pole.
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Correction mechanisms substantially reduce the frequency of lagging chromosomes produced by merotelic kinetochore orientation
Our results also identify the existence of at least two mechanisms that prevent anaphase lagging chromosomes during normal mitosis: one that functions before anaphase and one or more that function at or after anaphase onset. To see better the dynamics of these correction mechanisms, we normalized the percentage merotelic kinetochore data (Fig. 10A) for both untreated and nocodazole treated cells by values measured in early prometaphase (Fig. 10B). The first information that we obtain from this normalization is that in general the major effect of nocodazole is the promotion of merotelic kinetochore orientation, and not the suppression of correction, this is particularly true for correction mechanisms operating before anaphase onset. The second significant information is that with or without nocodazole treatment, delaying metaphase before anaphase onset did not substantially decrease the percentage merotelic kinetochores, but did decrease substantially the incidence of lagging chromosomes in anaphase. This suggests that the correction mechanism operating before anaphase onset not only reduces the number of detectable merotelic kinetochores, but also reduces the ability of those persisting until anaphase to produce lagging chromosomes.
The obvious hypothesis for the mechanism operating before anaphase onset is that it depletes merotelic kinetochores of microtubules from the incorrect pole while enhancing attachment to microtubules from the correct pole. When a kinetochore is merotelically oriented and aligned at the metaphase plate, part of it is pulled in the same direction as its sister (towards the incorrect pole) and part of it is pulled in the opposite direction (towards the correct pole) (Fig. 11B). This orientation might in some cases resolve itself simply through the instability of kinetochore microtubules, which have been shown to have a half-life of about 5 minutes in metaphase (Cassimeris et al., 1990; Shelden and Wadsworth, 1996
; Zhai et al., 1995
). If the kinetochore loses attachment to microtubules to the incorrect pole, that part of the kinetochore will have a tendency to face the correct pole and so attachments to the correct pole will be favored. Furthermore, the chromatin might shield the kinetochore, thus preventing re-attachment to microtubules from the incorrect pole. However, the correction of merotelic orientations might depend on other factors, rather than simply on the stochastic detachment and re-attachment process that characterizes kinetochore microtubules in prometaphase/metaphase. For example, the tension at each attachment site might play an important role. Work on grasshopper spermatocytes has shown that elevated tension at kinetochores stabilizes microtubule attachment (King and Nicklas, 2000
; Nicklas and Ward, 1994
). The force on the correctly attached side of a merotelic kinetochore is greater than at its sister kinetochore because of the force produced by the part of the merotelic kinetochore attached to the wrong pole. This makes microtubules attached to the correct pole more stable and consequently less likely to detach than microtubules to the incorrect pole. Finally, the Aurora BINCENP complex may also have a role in promoting loss of incorrectly attached microtubules. This complex has been proposed to promote microtubule destabilization at kinetochores not under tension (Stern, 2002
; Tanaka et al., 2002
) and depletion of either Aurora B or INCENP have been found to produce lagging chromosomes both in C. elegans (Kaitna et al., 2000
) and Drosophila embryos (Adams et al., 2001
). Merotelic kinetochore orientation may reduce centromere stretch, as suggested by a shift of merotelic kinetochores closer to the equator than normal (Fig. 8). Furthermore, the part of the merotelic kinetochore incorrectly attached is pulled back toward the centromeric region and closer to the inner centromere where the Aurora B-INCENP complex is localized (Adams et al., 2001
; Canman et al., 2002b
), whereas the part attached to the correct pole is pulled away from the centromeric region (Fig. 11B). In this way, the Aurora B-INCENP complex could destabilize attachment of microtubules from the incorrect pole. In fact, there is evidence in both yeast and higher organisms that Aurora B functions to destabilize microtubule attachment and promote re-orientation (Shannon and Salmon, 2002
).
We found that anaphase correction plays a major role in preventing chromosome missegregation, since more than 90% of merotelic orientations present in late metaphase control cells do not produce a lagging chromosome in anaphase (Fig. 10B, Table S1). Previous studies of merotelic kinetochores on damaged or unreplicated chromosomes showed that only a fraction remained near the spindle equator (Khodjakov et al., 1997; Wise, 1999
; Wise and Brinkley, 1997
; Yu and Dawe, 2000
). Our results are the first evidence that uncompromised kinetochores exhibit similar behavior. Microtubule attachment to kinetochores is more stable after anaphase onset than before anaphase (McEwen et al., 1997
; Nicklas, 1983
; Shelden and Wadsworth, 1996
; Zhai et al., 1995
). The Aurora B-INCENP complex leaves the centromere at anaphase onset (Adams et al., 2001
; Canman et al., 2002b
), so it is likely that the `anaphase' correction mechanism is Aurora B-INCENP-independent.
Our data suggest that the potential of a merotelic kinetochore to produce a lagging chromosome in anaphase decreases as the ratio of correct to incorrect microtubules increases. This hypothesis is supported by our findings that prolonging metaphase did not substantially decrease the number of merotelic kinetochores at metaphase, but increased the ratio of microtubules to opposite poles and substantially reduced the frequency of anaphase lagging chromosomes (Fig. 10, Table 1). A higher ratio of correct to incorrect microtubules might produce segregation of a merotelic chromosome to the pole in two ways. One is kinetochore breakage (Fig. 11C, bottom), because of the high degree of lateral stretching (Cimini et al., 2001), which, beyond a certain point, might break the kinetochore. The other is that the chromosome could move to the pole with the greater number of microtubules although remaining attached to both poles (Fig. 11C, middle). Although poleward movement of merotelic kinetochores does not occur before anaphase (Cimini et al., 2002
), after anaphase onset the direction of chromosome movement might become strongly dependent on the ratio of microtubules going to opposite poles. This might depend on changes in kinetochore function, sister chromatid separation, loss of polar ejection forces on chromosome arms (Funabiki and Murray, 2000
; Levesque and Compton, 2001
) and strong pulling forces on kinetochore microtubules produced by spindle elongation (anaphase B).
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