Koprivec, I. | bioRxiv Science

bioRxiv2024

Microtubule pivoting driven by spindle elongation rescues polar chromosomes to ensure faithful mitosis

Polar chromosomes, which initially attach to the mitotic spindle behind the pole, are prone to missegregation and micronuclear entrapment, contributing to chromosomal instability in cancer. Yet, the mechanisms ensuring their faithful segregation remain unclear. Here, we show that polar chromosomes require a unique step involving spindle elongation, which repositions chromosome-bound astral microtubules by pivoting them around the centrosome toward the spindle surface. By modulating Eg5/KIF11 activity, we demonstrate that spindle elongation determines the direction and extent of pivoting, with microtubules from the opposite spindle half facilitating final movement. Kinetochores on polar chromosomes form mixed lateral and immature monotelic attachments, recruiting corona components and partially Mad2, but lacking Astrin. In several cancer cell lines, limited spindle elongation delays polar chromosome resolution, whereas enhanced elongation accelerates it. These findings highlight the role of spindle elongation in the timely rescue of polar chromosomes from the danger zone behind the pole, and provide mechanistic insight into how chromosome congression errors can arise in cancer.

cell biology↗

bioRxiv2020

Augmin regulates kinetochore tension and spatial arrangement of spindle microtubules by nucleating bridging fibers

The human mitotic spindle is made of microtubules nucleated at centrosomes, at kinetochores, and from pre-existing microtubules by the augmin complex. However, it is unknown how the augmin-mediated nucleation affects distinct microtubule classes and thereby mitotic fidelity. Here we use superresolution microscopy to analyze the previously indistinguishable microtubule arrangements within the crowded metaphase plate area and demonstrate that augmin is vital for the formation of uniformly arranged parallel units consisting of sister kinetochore fibers connected by a bridging fiber. This ordered geometry helps both prevent and resolve merotelic attachments. Whereas augmin-nucleated bridging fibers prevent merotelic attachments by creating a nearly parallel and highly bundled microtubule arrangement unfavorable for creating additional attachments, augmin-nucleated k-fibers produce robust force required to resolve errors during anaphase. STED microscopy revealed that bridging fibers were impaired twice as much as k-fibers following augmin depletion. The complete absence of bridging fibers from a significant portion of kinetochore pairs, especially in the inner part of the spindle, resulted in the specific reduction of the interkinetochore distance. Taken together, we propose a model where augmin promotes mitotic fidelity by generating assemblies consisting of bridging and kinetochore fibers that align sister kinetochores to face opposite poles, thereby preventing erroneous attachments.

cell biology↗