Successful search for the all-rounders among the embryonic stem cells

Embryonic stem cells are regarded as a beacon of hope in regenerative medicine, because in principle they are capable to develop into any cell type of the body. The hope is that one day it will be possible to replace damaged tissue and cure numerous diseases with them. One of the obstacles facing research is that actually only a fraction of the cells in culture dishes with human embryonic stem cells represent this early, truly pluripotent state. MDC Researchers together with collaborators from the University of Bath in England have found a method to identify these “naïve” embryonic stem cells. Their work was recently described in the journal Nature.

_Human stem cells (green) in a culture dish with human embryonic stem cells. Photo: Jichang Wang/ Copyright: MDC_

The cultivation of human embryonic stem cells and their differentiation into various cell types represent a major challenge. While mouse embryonic stem cells can remain in a truly pluripotent state in culture, meaning they still can give rise to any cell type of the body, for some unknown reason cells from human embryonic cell lines cannot. In human embryonic stem cell cultures the majority of cells are in a more advanced state and are, although still plastic, already pre-determined to some extent in their development. However, researchers led by Zsuzsanna Iszvák, principal investigator at the MDC and senior author of the Nature paper, and co-author Laurence Hurst from the University of Bath observed that completely pluripotent stem cells are generally present in human stem cell cultures. These “naïve” cells represent about five percent of the cells.

This constitutes a major obstacle for stem cell researchers. How can they spot the “naïve” cells in a culture dish? “There is a worldwide quest for the naïve human embryonic stem cells. Many researchers are looking for them”, explains Izsvák. However, there are currently no criteria researchers agree on that can define these cells.

The researchers led by Izsvák and Hurst now have found a method that makes it possible to identify the “naïve” cells in cultures of human embryonic stem cells. They searched for sequences in the genome that are active in the early embryo, the developmental stage where stem cells still remain in the “naïve” state. Surprisingly, they found activity in an ancient class of viruses, the so-called human endogenous retroviruses H (HERVH) that integrated into our DNA millions of years ago, but are not capable of retroviral activity anymore. Izsvák wondered whether they could play a role in the early embryo.

Many scientists would be skeptical to follow this line, because these ancient viral sequences in our genome are usually regarded as functionless “junk”. However, the researchers led by Izsvák found what they were looking for: They propose that, through the course of evolution, some of the HERVH-elements have acquired a cellular function in early embryonic stem cells. The researchers found evidence that they regulate the activity of genes that are known to be active in “naïve” stem cells.

One would expect that such an important mechanism is conserved through evolution in different species of mammals. However, the HERVH-circuit is missing from the mouse genome. “The viral elements apparently add a new regulatory layer to the circuitry in humans”, says Izsvák. This might explain why human embryonic stem cells differ from those of mice.

In addition, the researchers were able to identify a transcription factor named LBP9 as central to the switch that turns the HERVH-system on in early embryos. Based on their findings the researchers developed a method that allows them to spot the “naïve” cells. They managed to link the switch of HERVH in stem cell cultures with a fluorescent protein. As a result the cells glowed green when HERVH was active, which meant that the cells were in the naïve state. Thus the researchers were able to identify, isolate and enrich the “naïve” cells in culture.

This is an important step forward, however so far it is not known how the isolated “naïve” embryonic stem cells can be kept stably and grown in culture. Apparently, the HERVH-system inhibits differentiation of the stem cells, but is active only for a short time in the developing embryo. “We are confident that our method can now help us to find the best conditions to maintain the naïve human cells in culture”, says Izsvák. This would be a mile stone in stem cell research. Then it would finally be possible to use these valuable, truly pluripotent cells in regenerative medicine.

- Elke Binder

Highlight Reference

Wang, J., Xie, G., Singh, M. and Ghanbarian, A.T., Rasko, T., Szvetnik, A., Cai, H., Besser, D., Prigione, A., Fuchs, N.V., Schumann, G.G., Chen, W., Lorincz, M.C., Ivics, Z., Hurst, L.D. and Izsvak, Z. Primate-specific endogenous retrovirus-driven transcription defines naive-like stem cells Nature 516, 405–409

Link to MDC press release

Featured Image: Newly discovered naïve-like human stem cells (green) in a culture dish with human embryonic stem cells. Photo: Jichang Wang/ Copyright: MDC