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A special light microscopy system developed by Michael Shribak and colleagues at the Marine Biological Laboratory (MBL) has provided novel insight into chromosome condensation during cell division in living human cells. Their work was published last week in (PNAS).

“Chromosome condensation is an essential process for transmitting replicated chromosomes to two daughter cells during mitotic cell division,” said first author Shiori Iida of the Genome Dynamics Laboratory, National Institute of Genetics and the Graduate Institute for Advanced Studies, SOKENDAI, Japan.

“We investigated whether depletion attraction, a force that attracts large structures in crowded cell environments, is involved in this [condensation] process,” Iida said.

During chromosome condensation, chromatin strands (consisting of DNA, RNA and protein) are compacted into short chromosomes. This makes the chromosomes rigid to resist the pulling force from the mitotic spindle as the cell divides. While several proteins involved in the condensation process have been identified and extensively investigated, the physical bases of the process remained unclear.

To carry out the study, Shribak and team developed a microscopy system capable of imaging the molecular density of cellular environments. The system, orientation-independent-differential interference contrast (OI-DIC) microscopy combined with a confocal laser scanning microscope, can precisely map optical path differences and estimate molecular densities.

“With the OI-DIC system, we quantified the absolute density of the molecules around chromosomes during mitosis of living cells,” Shribak said.

Using this system, “we found that [macromolecular] crowding around chromosomes increases during cell division, leading to a rise in depletion attraction. Our results suggest that the rise in depletion attraction makes chromosomes more rigid, ensuring accurate chromosome transmission during cell division,” said lead author Kazuhiro Maeshima of the Genome Dynamics Laboratory, National Institute of Genetics and SOKENDAI, Japan.

Their analysis showed that the molecular density surrounding chromosomes increased with the progression from the prophase to anaphase stages of mitosis, concurring with chromosome condensation. However, the molecular density went down in telophase, when chromosome decondensation began.

“A transient rise in the molecular density appears to be induced by nuclear membrane breakdown during mitosis,” Maeshima said. “Upon nuclear membrane breakdown, the nuclear envelope, nuclear pore complexes, nuclear lamina, and nucleoli are disassembled into small pieces. As a result, cytoplasmic and nucleolar factors are exposed to the chromosomes and fully contribute to an increase in depletion attraction," he said.

"This provides us with a novel insight into the physical bases of mitotic chromosome condensation in living human cells,” said Maeshima.

The authors foresee other applications for this novel microscopy system for measuring molecular densities.

“Looking ahead, we aim to elucidate how physical properties of DNA, and the physical forces affecting their properties, contribute to DNA transactions, including transcription, DNA replication and repair,” said Iida.

Citation:

Iida, Shiori et al (2024) Orientation-independent-DIC imaging reveals that a transient rise in depletion attraction contributes to mitotic chromosome condensation. Proc. Natl. Acad. Sci., DOI:

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