Ska3 is required for spindle checkpoint silencing and the maintenance of chromosome cohesion in mitosis

Abstract

The mitotic spindle checkpoint monitors proper bipolar attachment of chromosomes to the mitotic spindle. Previously, depletion of the novel kinetochore complex Ska1/Ska2 was found to induce a metaphase delay. By using bioinformatics, we identified C13orf3, predicted to associate with kinetochores. Recently, three laboratories independently indentified C13orf3 as an additional Ska complex component, and therefore we adopted the name Ska3. We found that cells depleted of Ska3 by RNAi achieve metaphase alignment but fail to silence the spindle checkpoint or enter anaphase. After hours of metaphase arrest, chromatids separate but retain robust kinetochore-microtubule attachments. Ska3-depleted cells accumulate high levels of the checkpoint protein Bub1 at kinetochores. Ska3 protein accumulation at kinetochores in prometaphase is dependent on Sgo1 protein. Sgo1, which accumulates at the centromeres earlier, in prophase, is not dependent on Ska3. Sgo1-depleted cells show a stronger premature chromatid separation phenotype than those depleted of Ska3. We hypothesize that Ska3 functions to coordinate checkpoint signaling from the microtubule binding sites within a kinetochore by laterally linking the individual binding sites. We suggest that this network plays a major role in silencing the spindle checkpoint when chromosomes are aligned at metaphase to allow timely anaphase onset and mitotic exit.

Figure 1. Mitotic Defects in Cells Depleted of Ska3 and Kinetochore Localization of Ska3-GFP

(A) Fluorescence images of a Hela H2B-GFP cell 30 hours after transfection with shRNAi plasmid targeting Ska3 (red cytoplasm) adjacent to a non-transfected control cell in the same field. The transfected cell arrests at metaphase for approximately three hours after which chromosomes begin to scatter. The control cell proceeds normally through mitosis. Time is hours:minutes. (B) Ska3-GFP is diffusely distributed in prophase Hela cells with some concentration on centrosomes. After nuclear envelope breakdown, Ska3 concentrates strongly on kinetochores and to a lesser extent along spindles. The kinetochore concentration diminishes during anaphase and is lost in telophase. (C) Ska3 remains at the kinetochores of cells treated with the proteasome inhibitor, MG132, with the microtubule disruptor, Nocodazole, and with the microtubule stabilizer, Taxol. Bars, 5 µm.

Figure 2. Ska3 Depleted Cells Separate Chromatids but Retain Microtubule Attachment

(A) Cells arrested in mitosis by siRNA to Ska3 were hypotonically swollen, lysed and centrifuged onto coverslips before fixation and labeling. A mixture of cells showing single chromatids (arrow and inset 1) and cells with paired chromatids (inset 2) were found. Presumably the former derive from cells with scattered chromosomes arrested for longer periods with the latter derive from cells arrested at metaphase. (B) Chromosome spreads reveal loss of chromatid cohesion in Ska3-depleted cells. (C) Microtubule bundles extending to kinetochores are retained in Ska3-depleted cells after cold treatment to destabilize non-kinetochore microtubules. Total levels of cold stable spindle microtubules (averages ± SD) are increased somewhat the spindles of Ska3-depleted cells both at metaphase and after chromosome scattering in comparison with mock transfected cells at metaphase. (D) Mock treated and Ska3 siRNA-treated Hela cells were incubated with the proteasome inhibitor MG132 for 35 min, fixed, and labeled with human anti-kinetochore and anti-Ska3 antibodies. Five cells at metaphase in both the mock and RNAi-treated cultures were selected and inter-kinetochore distances were measured (>160 sister chromatid pairs for each). The inter-kinetochore distances were plotted indicating average separation for each cell ± SD. No significant differences in the inter-kinetochore distances between control and Ska3 siRNA-treated metaphase cells were detected. Bars, 5 µm.

Figure 3. Ska3 Depletion Activates the Spindle Checkpoint Inducing Strong Kinetochore Accumulation of Bub1

(A) Control cells and cells treated with Ska3 siRNA were fixed and labeled with anti-kinetochore serum and anti-Bub1 antibody. Bub1 labeling is increased at all stages in Ska3-depleted cells but most enhanced in the arrested cells showing scattered chromosomes. (B) Metaphase cells in mock treated and Ska3-depleted cultures show on average comparable levels of kinetochore-associated Mad2 although the range of Mad2 at kinetochores in Ska3-depleted cells is greater. Ska3-dpeleted cells with scattered chromosomes show a modest increase in the average kinetochore concentration of Mad2. In the graphs, the black horizontal bars indicate averages; the colored boxes indicate standard errors and the thin vertical “whiskers” indicate ranges. Bars, 5 µm.

Figure 4. Ska3 Accumulation at Kinetochores is Dependent upon Shugoshin and May Coordinate Checkpoint Signaling and Chromatid Separation

(A) Sgo1 remains associated with kinetochores in cells depleted of Ska3. (B) Depletion of Sgo1 blocks kinetochore but not spindle association of Ska3. (C) In the presence of intact microtubules both Ska3-depleted and Sgo1-depleted cells show high levels of chromatid separation in chromosome spreads. In Nocodazole alone, no chromatid separation occurs. The disruption of microtubules with Nocodazole nearly eliminates chromatid separation in Ska3-depleted cells but only partially decreases chromatid separation in Sgo1-depleted cells. Bars, 5 urn. (D) Ska3-depleted cells arrest at metaphase for substantially longer times than do Sgo1-depleted cells before undergoing chromosome scattering. For Ska3, n = 84; for Sgo1, n = 53. (E) Model for Ska3 in integration of spindle checkpoint signaling within the kinetochore. (Diagram I) At kinetochores where most end-on microtubule binding sites are unattached, checkpoint signaling is active. (Diagram II). At kinetochores where most end-on microtubule binding sites are attached checkpoint signaling is inactive. Ska3 and associated proteins provide lateral attachments to inhibit checkpoint signaling in the face of momentary loss of some end-on microtubule attachments (star) that occurs as a consequence of microtubule dynamics. (Diagram III). In the absence of the Ska3 meshwork, release of an end-on attachment immediately results in binding site collapse (star) and reactivation of checkpoint signaling.