Since the 1800s, scientists have noticed a configuration of centromeres, a special chromosomal region that is vital for cell division, in the nucleus. Until now, however, the determining mechanisms and biological significance of centromere distribution were poorly understood. A team led by researchers from the University of Tokyo and their collaborators recently proposed a two-step regulatory mechanism that shapes the distribution of centromeres. Their findings also suggest that centromeric configuration in the nucleus plays a role in maintaining genome integrity.
The results are published in Natural plants.
During the process of cell division, special chromosome domains called centromeres are pulled to opposite ends of the cell. After the completion of cell division and the construction of the cell nucleus, the centromeres are spatially distributed in the nucleus. If the distribution of centromeres drawn to both poles remains unchanged, the cell nucleus will have centromeres clustered on only one side of the nucleus. This uneven distribution of centromeres is called the Rabl configuration, after the 19th-century cytologist Karl Rabl. The nuclei of some species show instead a scattered distribution of centromeres, which is known as a non-Rabl configuration.
“The biological function and molecular mechanism of the Rabl or non-Rabl configuration has been a mystery for centuries,” said corresponding author Sachihiro Matsunaga, a professor at Tokyo University of Frontier Sciences. “We have successfully revealed the molecular mechanism for constructing a non-Rabl configuration.”
The researchers studied the plant Arabidopsis thaliana, also known as thale cress and a specimen known to have a non-Rabl configuration, and its mutant form which has a Rabl configuration. Through their work, they discovered that protein complexes known as condensin II (CII) and protein complexes known as linker of the nucleoskeleton and catoskeleton complex (LINC) work together to determine the distribution of centromeres during cell division.
“Centromere distribution for the non-Rabl configuration is independently regulated by the CII-LINC complex and a nuclear lamina protein known as CROWDED NUCLEI (CRWN),” Matsunaga said.
The first step in the two-step regulatory mechanism of centromere distribution that the researchers discovered was that the CII-LINC complex mediates the dispersion of centromeres from late anaphase to telophase, two phases at the end of cell division. The second step in the process is that CRWNs stabilize the scattered centromeres on the nuclear lamina in the nucleus.
Next, to investigate the biological significance, the researchers analyzed gene expression in A. thaliana and in its Rabl-structural mutant. Because the change in the spatial arrangement of centromeres also changes the spatial arrangement of genes, the researchers expected to find differences in gene expression, but this hypothesis turned out to be incorrect. However, when DNA damage stress was applied, the mutants grew organs at a slower rate than the normal plant.
“This suggests that precise control of centromere spatial arrangement is required for organ growth in response to DNA damage stress, and there is no difference in tolerance to DNA damage stress between non-Rabl and Rabl organisms,” Matsunaga said. “This suggests that the appropriate spatial arrangement of DNA in the nucleus, independent of Rabl configuration, is important for the stress response.”
According to Matsunaga, the next steps are to identify the energy source that changes the spatial arrangement of specific DNA regions and the mechanism that recognizes specific DNA.
“Such discoveries will lead to the development of technology to artificially arrange DNA in the cell nucleus in an appropriate spatial arrangement,” he said. “This technology is expected to make it possible to create stress-resistant organisms, as well as to impart new properties and functions by changing the spatial arrangement of DNA, rather than editing its nucleotide sequence.”
Materials provided by University of Tokyo. Note: Content may be edited for style and length.