Spindle-Length-Dependent HURP Localization Allows Centrosomes to Control Kinetochore-Fiber Plus-End Dynamics.

During mitosis, centrosomes affect the length of kinetochore fibers (k-fibers) and the stability of kinetochore-microtubule attachments, implying that they regulate k-fiber dynamics. However, the exact cellular and molecular mechanisms of this regulation remain unknown. Here, we created human cells with only one centrosome to investigate these mechanisms. Such cells formed asymmetric bipolar spindles that resulted in asymmetric cell divisions. K-fibers in the acentrosomal half-spindles were shorter, more stable, and had a reduced poleward microtubule flux at minus ends and more frequent pausing events at their plus ends. This indicates that centrosomes regulate k-fiber dynamics both locally at minus ends and far away at plus ends. At the molecular level, we find that the microtubule-stabilizing protein HURP is enriched on the k-fiber plus ends in the acentrosomal half-spindles of cells with only one centrosome. HURP depletion rebalances k-fiber stability and plus-end dynamics in such cells and improves spindle and cell division symmetry. Our data from 3 different cell lines indicate that HURP accumulates on k-fibers inversely proportionally to half-spindle length. We therefore propose that centrosomes regulate k-fiber plus ends indirectly via length-dependent accumulation of HURP.

Mild replication stress causes chromosome mis-segregation via premature centriole disengagement.

Replication stress, a hallmark of cancerous and pre-cancerous lesions, is linked to structural chromosomal aberrations. Recent studies demonstrated that it could also lead to numerical chromosomal instability (CIN). The mechanism, however, remains elusive. Here, we show that inducing replication stress in non-cancerous cells stabilizes spindle microtubules and favours premature centriole disengagement, causing transient multipolar spindles that lead to lagging chromosomes and micronuclei. Premature centriole disengagement depends on the G2 activity of the Cdk, Plk1 and ATR kinases, implying a DNA-damage induced deregulation of the centrosome cycle. Premature centriole disengagement also occurs spontaneously in some CIN+ cancer cell lines and can be suppressed by attenuating replication stress. Finally, we show that replication stress potentiates the effect of the chemotherapeutic agent taxol, by increasing the incidence of multipolar cell divisions. We postulate that replication stress in cancer cells induces numerical CIN via transient multipolar spindles caused by premature centriole disengagement.