The phases of the cell cycle, particularly that of mitosis, were taught in college as part of my studies in biology. The cell cycle is a fundamental process for all organisms and constantly happens within our bodies. While cells generally spend most of the time in interphase, many scientists focus on what happens as the cells prepare to divide from one cell into two. Mitosis, the part of the cell cycle that refers to cell division, is subject to extensive regulation to ensure all materials and genetic information are sound and ready to segregate correctly. If there is damage to the chromosomes, the cells will halt cell division.
In an article published August 25 in PLoS ONE, Baker et al. examined how chromosomal damage affects cells in mitosis at the stage where the chromosomes have been separated and the cell is preparing to divide into two cells. This was a rather simple setup for the experiment: some cells, a laser and imaging equipment (a camera and a microscope). This laser was not used to threaten your city and demand moderately large sums of money, but to see how targeted chromosomal damage would affect cells in anaphase, a later part of mitosis. The targets were three different locations: chromosome arms, the cytoplasm and the tips of the chromosomes, the presumptive location of telomeres. Telomeres have been heavily implicated in aging and cancer as they protect the ends of the chromosome from degrading and losing genetic information. In this article, the start of anaphase was defined as the point when the chromosomes visibly begin to separate. The cells used in this study were PtK2 kidney epithelial cells from the long-nosed potoroo. These cells have been used for similar studies because of the large chromosome size and small number of chromosomes in the cell. The laser was used for 30ms on an area 0.17µm2 to damage the cytoplasm and chromosomes.
Chromosomal damage, laser-induced or otherwise, affects the genetic material in a potential daughter cell. Earlier in the cell cycle, DNA damage would halt all progression while the damage is repaired. However, once the cell is in mitosis and preparing for the physical division, how does the cell react to damage?
Interestingly, damaging or removing a chromosomal arm, as seen with a phase change or a visible chromosome fragment under the microscope, did not affect timing of cell progression through anaphase. The cell divided normally without any delay when the cytoplasm was targeted by the laser. However, targeting the chromosome ends delayed progression of a majority of cells through anaphase. Specifically, no cleavage furrow formed (created when the mother cell is preparing to split into two daughter cells), the furrow took more time to form, or the furrow regressed after beginning cytokinesis.
How much different is the timing? Cytokinesis, the division of the cytoplasm, was usually completed 30 minutes after anaphase began. For cells where the laser affected chromosome tips, 68% did not make it through cytokinesis within 30 minutes. While some of the cells did complete mitosis without delay, about half were unable to progress at all; approximately a quarter did not even show a cleavage furrow.
These results hinted that targeting telomeric regions caused a cellular response to this type of chromosomal damage. To confirm that the effects on the cleavage furrow were from the chromosomal ends and not due to some other cellular machinery, the researchers targeted different portions of the anaphase cell, including the spindle. Aside from some moderate mitosis delay when the spindle was targeted, there was little effect on cell cycle timing and no effect on the cleavage furrow at all.
The researchers also wondered what effect damaging multiple ends would have on the cell. They saw an increase in the percentage of cells lacking cleavage furrow formation (approximately 50% of all cells targeted) and a greater bias toward furrow regression in the remaining cells (24% for multiple chromosomal tips damaged versus 14% for a single damaged end). However, for those cells that did progress to cytokinesis, the time required did not change whether one or more ends were targeted.
This study was limited by the resolution of the imaging technology. While the images that accompanied the text seemed to demonstrate a real effect, much of the laser damage was indicated by a change in the refractive index. The authors acknowledged that the visual change was no guarantee that the targeted area was damaged as they intended.
While I prefer not to have lasers targeting my chromosomes, ends or otherwise, this is an interesting study on a late phase of mitosis that had not been examined for cycle arrest. This is especially fascinating as the effect seen by the authors occurs when the chromosomal ends are targeted, not another portion of the chromosome or even other parts of the cell. This article opens a new area to explore, one where it seems damaging telomeric regions arrest cleavage furrow formation and may indicate a putative new signaling pathway for DNA damage.
Baker, N., Zeitlin, S., Shi, L., Shah, J. and Berns, M. (2010) Chromosome Tips Damaged in Anaphase Inhibit Cytokinesis PLoS ONE, 5 (8) DOI: 10.1371/journal.pone.0012398