Chromosome, a substance to pass our genome from mother to daughter cells, is highly organized structure that is changed dynamically under the regulation of cell cycle. To divide the genome successfully into two daughters, chromosomes, in the first place, must establish sister-chromatid cohesion during DNA replication, otherwise they are randomly separated and an equal distribution of the genome would not be possible. Next, in mitosis, chromosomes must be packed into a micrometer-sized spindle apparatus, which captures chromosomes in a bipolar fashion. How does chromosome achieve those events towards successful segregation? The aim of our group is to understand molecular mechanisms that organize chromosome structure toward proper chromosome segregation. Specifically, we are focusing on the questions: 1) how cohesin organizes chromosome structure, and 2) how genome is packed into mitotic chromosome and spindle apparatus.

How does cohesin organize chromosome structure?

Cohesin, a ring-shaped protein complex (Fig. 1), is essential for sister chromatid cohesion. This proteinaceous ring changes the fashion of its association with DNA in a spatio-temporal manner. In vertebrates, cohesin loosely binds to chromatin in G1 phase, the cohesin is stabilized onto chromatin during DNA replication in S phase, most of them are dissociated from chromosome arms in prophase, and the remaining 10% of the cohesin stays at centromere and keeps cohesion until its cleavage in anaphase for chromosome segregation (Fig. 2). The every step of association, stabilization, dissociation, and cleavage is essential for timely chromosome segregation. We are aiming to understand how the dynamics of cohesin is spatiotemporally regulated both in vertebrates and invertebrates. Furthermore, we are interested in the mechanisms of cohesin-dependent higher-order chromatin structure. Recent studies suggest that cohesin may contribute to higher-order chromosome structure. It has been hypothesized that cohesin participated in chromatin looping, which then regulated gene expression by modulating promoter-enhancer interaction. We are trying to understand both in vitro and in vivo system how cohesin organizes chromatin structure.

How is genome packed into chromosome and spindle apparatus?

Another important aspect of chromosome dynamics is chromosome compaction. When cohesin is dissociated from chromosome arms, chromosome is coincidentally compacted. Although condensin and topoisomerase are well-established factors required for chromosome condensation, it has been implied that these factors are not sufficient for the chromosome condensation. We are focusing on other potential condensation factors and trying to understand the network of these factors in the system of chromosome condensation.

We are conducting these projects through in vitro reconstitution approach with combination of Xenopus egg extracts, which recapitulate cell cycle events, and quantitative microscopic analysis, and in vivo cell biological approach exploiting mammalian somatic cells, fly cells, and Xenopus eggs/oocytes.