Adult cells can only give rise to the same cell type. For example, a skin cell cannot give rise to a muscle cell but to skin cells only. This limits the potential use of adult cells for therapy.

During early development, however, the cells in the embryo have the capacity to generate all cell types of our body, including stem cells. This totipotency has served as an inspiration for researchers to find new ways to recapitulate totipotency through cellular reprogramming in the lab.

Totipotent cells have many properties, but we do not know all of them yet. Researchers at Helmholtz Munich have now made a new discovery, which is described in a paper (“DNA replication fork speed underlies cell fate changes and promotes reprogramming”) in Nature Genetics.

“Totipotency emerges in early embryogenesis, but its molecular underpinnings remain poorly characterized. In the present study, we employed DNA fiber analysis to investigate how pluripotent stem cells are reprogrammed into totipotent-like 2-cell-like cells (2CLCs),” write the investigators.

“We show that totipotent cells of the early mouse embryo have slow DNA replication fork speed and that 2CLCs recapitulate this feature, suggesting that fork speed underlies the transition to a totipotent-like state. 2CLCs emerge concomitant with DNA replication and display changes in replication timing (RT), particularly during the early S-phase.

“RT changes occur prior to 2CLC emergence, suggesting that RT may predispose to gene expression changes and consequent reprogramming of cell fate. Slowing down replication fork speed experimentally induces 2CLCs. In vivo, slowing fork speed improves the reprogramming efficiency of somatic cell nuclear transfer.

“Our data suggest that fork speed regulates cellular plasticity and that remodeling of replication features leads to changes in cell fate and reprogramming.”

“We found out that in totipotent cells, the mother cells of stem cells, DNA replication occurs at a different pace compared to other more differentiated cells. It is much slower than in any other cell type we studied,” says Tsunetoshi Nakatani, PhD, first-author of the new study.

The researchers discovered that the speed of DNA replication is also low in totipotent-like cells, which scientists can culture in a petri dish. According to Nakatani, “This led us to the question: If we manage to change the speed at which DNA replicates, can we improve the reprogramming of cells into totipotent cells?”

Less speed, improved cellular reprogramming

In their experimental effort, the researchers observed indeed that slowing down the DNA replication speed, for example, by limiting the substrate that the cells use for DNA synthesis, increases reprogramming efficiency (the rate at which cells can convert to another cell type).

“This is amazing,” says Maria-Elena Torres-Padilla, PhD, the leader of the study and head of both the Stem Cell Center at Helmholtz Munich and the Institute for Epigenetics and Stem Cells. She is also professor for stem cell biology at Ludwig-Maximilans-Universität München.

“Over the years, we have been studying totipotent cells in order to learn how nature has made them so incredibly capable of generating all cell types of our bodies. This is a fundamental strategy of our research towards regenerative medicine approaches. This new concept is very simple, yet extremely important and we believe that it is a huge advance for stem cell therapy.”

Nakatani is a postdoc in Torres-Padilla’s group at Helmholtz Munich.