Mechanisms ensuring proper chromosome segregation in mitosis

Human cells store their genetic information into 46 chromosomes. To maintain vital genetic information, a whole set of chromosomes must be inherited precisely by daughter cells when cells divide. Errors in this process would cause cell death and various human diseases, such as spontaneous miscarriage during pregnancy, genetic abnormalities and cancers. Our research aims to reveal fundamental mechanisms ensuring chromosome inheritance when cells divide, and thus to understand the mechanisms of relevant human diseases.

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Figure caption. The figure illustrates step-wise development of kinetochore–microtubule interactions in mitosis (prometaphase [steps 1-5] and metaphase [step 6]) (reviewed in Tanaka TU 2010).

 

To maintain genetic integrity, eukaryotic cells must segregate their duplicated chromosomes to their daughter cells with high fidelity during mitosis. The unravelling of the mechanisms ensuring proper chromosome segregation should improve our understanding of various human diseases such as cancers and congenital disorders, which are characterized by chromosome instability and aneuploidy. We study both budding yeast and human cells, taking advantages of both systems.

 

1) Kinetochore-microtubule interaction in mitosis

Kinetochores are large protein complexes formed at the centromere regions of chromosomes. For high-fidelity chromosome segregation, kinetochores must be properly caught on the mitotic spindle (reviewed in Tanaka TU 2010). We have found that kinetochores initially interact with the lateral surface of a single microtubule extending from a spindle pole (Figure, step 2); this process is often facilitated by microtubules generated at kinetochores (Figure, step 1; Kitamura et al, 2010). Subsequently kinetochores are tethered at the microtubule plus ends (Figure, step 3; Maure et al, 2011; Kalantzaki et al 2015). When this process fails, a failsafe mechanism prevents kinetochore detachment from a microtubule (Gandhi et al, 2011).
Following the initial interaction with microtubules, sister kinetochores must interact with microtubules extending from opposite spindle poles (sister kinetochore bi-orientation) before anaphase onset. If this interaction occurs with aberrant orientation, such errors must be corrected by turnover of the kinetochore–microtubule attachment (error correction). We have found that Aurora B/Ipl1 kinase and other factors have crucial roles in this process (Figure, step 4, 5; Kalantzaki et al 2015). Once bi-orientation is established and tension is applied across sister kinetochores, kinetochore–microtubule interaction is stabilized (Figure, step 6). Sister chromatid cohesion around centromeres plays important roles in this process (Natsume et al, 2013; Tanaka et al 2013). We are investigating these mechanisms in more detail in vivo in yeast and also by reconstituting kinetochore–microtubule interaction in vitro, and by extending our study from yeast to human cells.

 

2) Sister chromatid cohesion and chromosome compaction

Duplicated chromosomes (sister chromatids) are held together by sister chromatid cohesion until chromosome segregation takes place in anaphase. Without this cohesion, there would be no way to mark a pair of sister chromatids destined for segregation. In particular, robust sister chromatid cohesion around kinetochores is crucial for this purpose. We recently found that this robust cohesion is facilitated by Dbf4-dependent kinase recruited to kinetochores (Natsume et al, 2013). We are currently studying this mechanism in more detail.

For chromosome segregation, sister chromatid cohesion must be removed. Our recent study has revealed how removal of cohesion is completed during early anaphase in yeast (Renshaw et al, 2010). This process is coupled with chromosome recoiling/compaction. We assume similar regulation is present during prophase in human cells and are current testing this possibility.