Cdc48/p97-Ufd1-Npl4 antagonizes Aurora B during chromosome segregation in HeLa cells

During exit from mitosis in Xenopus laevis egg extracts, the AAA+ ATPase Cdc48/p97 (also known as VCP in vertebrates) and its adapter Ufd1-Npl4 remove the kinase Aurora B from chromatin to allow nucleus formation. Here, we show that in HeLa cells Ufd1-Npl4 already antagonizes Aurora B on chromosomes during earlier mitotic stages and that this is crucial for proper chromosome segregation. Depletion of Ufd1-Npl4 by small interfering RNA (siRNA) caused chromosome alignment and anaphase defects resulting in missegregated chromosomes and multi-lobed nuclei. Ufd1-Npl4 depletion also led to increased levels of Aurora B on prometaphase and metaphase chromosomes. This increase was associated with higher Aurora B activity, as evidenced by the partial resistance of CENP-A phosphorylation to the Aurora B inhibitor hesperadin. Furthermore, low concentrations of hesperadin partially rescued chromosome alignment in Ufd1-depleted cells, whereas, conversely, Ufd1-depletion partially restored congression in the presence of hesperadin. These data establish Cdc48/p97-Ufd1-Npl4 as a crucial negative regulator of Aurora B early in mitosis of human somatic cells and suggest that the activity of Aurora B on chromosomes needs to be restrained to ensure faithful chromosome segregation.

Mitotic control of kinetochore-associated dynein and spindle orientation by human Spindly

Mitotic spindle formation and chromosome segregation depend critically on kinetochore-microtubule (KT-MT) interactions. A new protein, termed Spindly in Drosophila and SPDL-1 in C. elegans, was recently shown to regulate KT localization of dynein, but depletion phenotypes revealed striking differences, suggesting evolutionarily diverse roles of mitotic dynein. By characterizing the function of Spindly in human cells, we identify specific functions for KT dynein. We show that localization of human Spindly (hSpindly) to KTs is controlled by the Rod/Zw10/Zwilch (RZZ) complex and Aurora B. hSpindly depletion results in reduced inter-KT tension, unstable KT fibers, an extensive prometaphase delay, and severe chromosome misalignment. Moreover, depletion of hSpindly induces a striking spindle rotation, which can be rescued by co-depletion of dynein. However, in contrast to Drosophila, hSpindly depletion does not abolish the removal of MAD2 and ZW10 from KTs. Collectively, our data reveal hSpindly-mediated dynein functions and highlight a critical role of KT dynein in spindle orientation.

Aurora B-mediated abscission checkpoint protects against tetraploidization

Genomic abnormalities are often seen in tumor cells, and tetraploidization, which results from failures during cytokinesis, is presumed to be an early step in cancer formation. Here, we report a cell division control mechanism that prevents tetraploidization in human cells with perturbed chromosome segregation. First, we found that Aurora B inactivation promotes completion of cytokinesis by abscission. Chromosome bridges sustained Aurora B activity to posttelophase stages and thereby delayed abscission at stabilized intercellular canals. This was essential to suppress tetraploidization by furrow regression in a pathway further involving the phosphorylation of mitotic kinesin-like protein 1 (Mklp1). We propose that Aurora B is part of a sensor that responds to unsegregated chromatin at the cleavage site. Our study provides evidence that in human cells abscission is coordinated with the completion of chromosome segregation to protect against tetraploidization by furrow regression.

Automated live microscopy to study mitotic gene function in fluorescent reporter cell lines

Fluorescence live microscopy is a powerful technique to study complex cellular dynamics such as cell division. The availability of fluorescent markers based on GFP fusion proteins for virtually any cellular structure allows efficient visualization of specific processes, and the combination of different fluorophores can be used to study their coordination. In this chapter, we present methods for automated live cell microscopy to study mitotic gene function systematically and in high throughput. In particular, we provide protocols for efficient generation of fluorescent reporter cell lines stably expressing combinations of cellular markers, and provide detailed guidelines for optimizing imaging protocols for automated long-term live microscopy.