Press release

Pressrelease (In Japanese only)

Dr. Toyoaki Natsume and Prof. Masato Kanemaki at National Institute of Genetics, ROIS, together with the group led by Prof. Ian D. Hickson at University of Copenhagen, reported a new system to deal with failure in DNA replication. The finding was published in Genes & Development in advance of the print journal.

In cell proliferation, genomic DNA has to be precisely copied into two (DNA replication) before equal distribution to two daughter cells. For doing this, double-stranded DNA is unwound, the process of which is similar to open a zipper of your clothes (Figure 1A). Similar to pulling the ‘slider’ for opening a zipper, the replicative helicase known as MCM2–7 moves on DNA for unwinding double-stranded DNA. However, because human genomic DNA is very long (approx. 2 m per cell), it is challenging to entirely unwind the genomic DNA. Occasionally, the MCM2–7 helicase falls off from DNA when it encounters a roadblock (such as DNA damage). Because reloading of MCM2–7 is strictly inhibited during S phase (when cells carry out DNA replication), this might lead to incompletion of DNA replication, which causes the loss of genetic information from daughter cells unless the cells have a mechanism to deal with the problem.

In this study, the research groups observed how human cells responded to artificial removal of the MCM2–7 helicase by using the auxin-inducible degron (AID) technology that they had developed previously (the information about this technology is described here). They revealed that the MCM8–9 helicase, which is evolutionally related to the MCM2–7 helicase, promotes a non-canonical DNA synthesis as a backup system after removal of MCM2–7 (Figure 1B).

Many anticancer drugs kills cancer cells by inducing DNA lesions, which enhance removal of MCM2–7 from DNA during DNA replication. An inhibitor of the MCM8–9 helicase might enhance the effect of existing anticancer drugs by shutting off this backup system (Figure 2).

Figure 1. The process of unwinding double-stranded DNA during DNA replication is similar to that of opening a zipper of your clothes.
(A) Similar to the slider that opens a zipper, the MCM2–7 replicative helicase opens double-stranded DNA in cells.(B) When the replicative MCM2–7 helicase (a normal slider) encounters to a roadblock, it occasionally falls off from DNA. To continue DNA synthesis, cells recruit the MCM8–9 helicase as a backup slider.

Figure 2. An MCM8–9-dependent backup system against failure in DNA replication.
The MCM2–7 replicative helicase falls off when encountered to a roadblock on DNA, leading to generation of DNA breaks. In this study, the research groups found that the MCM8–9 helicase continues DNA synthesis on behalf of the MCM2–7 (top right). If MCM8–9 does not work, the accumulation of DNA breaks results in cell death (bottom right).

Biological Macromolecules Laboratory / Maeshima Group

In eukaryotic cells, highly condensed inactive/silenced chromatin has long been called “heterochromatin.” However, recent research suggests that such regions are in fact not fully transcriptionally silent, and that there exists only a moderate access barrier to heterochromatin. To further investigate this issue, it is critical to elucidate the physical properties of heterochromatin such as its total density in live cells. Here, using orientation-independent differential interference contrast (OI-DIC) microscopy, which is capable of mapping optical path differences, we investigated the density of the total materials in pericentric foci, a representative heterochromatin model, in live mouse NIH3T3 cells. We demonstrated that the total density of heterochromatin (208 mg/mL) was only 1.53-fold higher than that of the surrounding euchromatic regions (136 mg/mL) while the DNA density of heterochromatin was 5.5- to 7.5-fold higher. This surprisingly small difference may be due to that non-nucleosomal materials (proteins/RNAs)(∼120 mg/mL) are dominant in both chromatin regions. Monte Carlo simulation suggested that non-nucleosomal materials contribute to creating a moderate access barrier to heterochromatin, allowing minimal protein access to functional regions. Our OI-DIC imaging offers insight into the density of live cellular environments.

Typical images of the Optical Path Distance (OPD) map (reflecting density at each pixel) by OI-DIC, DNA staining, and MeCP2-EGFP (heterochromatin marker) signals in live NIH3T3 cells. Large foci seen in DNA staining and MeCP2-EGFP images (arrowheads) were assumed to be heterochromatin. Note that the OPD of the foci was similar or only slightly higher than that of the surrounding euchromatin. (Right) The analyzed total densities of pericentric heterochromatin foci (Hch, 208 mg/mL) and euchromatin (Ech, 136 mg/mL). The median density ratio between them was 1.53.

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Press release

Pressrelease (In Japanese only)

Social stress is one of the major causes of depression in humans. It is therefore important that the effects of social stress on behaviour and gene expression in the brain are studied. We performed experiments using male mice to analyse social hierarchy in groups of mice in cages and investigated emotional behaviour and hippocampal gene expression in the mice. We found significantly different emotional behaviour and expression of genes associated with the serotonergic system in dominant and subordinate mice. Treating the mice with a selective serotonin reuptake inhibitor antidepressant restored gene expression in subordinate mice and caused the emotional behaviour of the subordinate mice to be recovered, suggesting that alterations to the serotonergic system are key factors in phenotypic changes caused by the social ranks of subordinate mice. We believe that the results of this study are important because they improve our understanding of how social stress induces mental health problems in humans.