Embryonic stem (ES) cells are versatile stem cells capable of generating every cell type found in an organism. These cells divide quickly and were previously believed to experience significant levels of replication stress. However, a recent study published in EMBO Reports by Kurashima et al. calls this belief into question by offering an in-depth molecular analysis of replication dynamics within these cells.
Under the leadership of Dr. Tomomi Tsubouchi at the National Institute for Basic Biology (NIBB) in Japan, the research team examined how replication forks behave, focusing on factors like replication fork speed, frequency of pauses, and origin firing density across various stages of the S phase. Their results highlight previously unnoticed features of replication, not just in ES cells but also in various non-pluripotent cells.
Dr. Kiminori Kurashima, the lead author of the study, states, “By dividing cells according to substages of the S phase and conducting DNA fiber assays on these sorted groups, we found that mammalian pluripotent stem cells consistently maintain a slow fork speed and a high density of active origins throughout the S phase, with very little fork pausing.” This finding contrasts with non-pluripotent cells, which start the S phase with slower fork speeds but later accelerate. In non-pluripotent cells, forks pause during the early S phase, likely activating checkpoint mechanisms to promote fork acceleration and diminish pausing.
The study also indicates that when mouse ES cells differentiate, they adopt replication patterns akin to those of non-pluripotent cells. Additionally, various replication characteristics seen in mouse ES cells are also present in human iPS cells, implying that the combination of slow replication forks and high origin density may be a defining feature of pluripotency. Interestingly, attempts to speed up replication forks resulted in a misalignment between genome replication completion and the progression of the cell cycle.
Dr. Tsubouchi concludes, “We suggest that slow replication forks are not signs of replication challenges but are instead a fundamental aspect of DNA replication in ES cells. Our research highlights the intricate regulation of DNA replication and shows how different cell types utilize distinct mechanisms.”
