Scientists have shown for the first time that embryonic stem (ES) cells are able to self-renew without the natural chemicals that scientists have so far used to maintain them and grow stem cell lines. This discovery contradicts previously held views and could have wide-ranging implications for stem cell research. It is hoped the findings, from the Cambridge team lead by Medical Research Council Professor Austin Smith and published in Nature, will lead to a better biological understanding of ES cells and more straightforward translation to the human system of detailed work done only in mouse ES cells to date.
ES cells grown in inhibitor conditions
In the three decades since pluripotent mouse ES cells were first described they have been derived and grown using various combinations of feeder cells, conditioned media, cytokines, growth factors, hormones, foetal calf serum, and serum extracts. Their reliance on signals from their culture environment was supported by the observation that the LIF (leukaemia inhibitory factor) and BMP4 (bone morphogenetic protein 4) must be provided to support self-renewal. The thinking was that these provided the signals and instructions to stem cells to maintain them in an undifferentiated state and that without these the cells would specialise to become whatever type of cell they were prompted to become.
The MRC team were able to show that ES cells produce their own signalling molecules and that these signals are the key driving force behind differentiation - the process by which a stem cell gives rise to more specialised cell types. This means that while they still need to be grown in a culture giving them the sugars and proteins they need to stay alive, if these signalling molecules are eliminated or blocked, the ES cells can remain in their pluripotent state indefinitely.
To show this, the Cambridge team collaborated with Sir Philip Cohen, Director of the MRC Protein Phosphorylation Unit, to obtain small molecule inhibitors which would block the action of the ES cell’s own signalling molecule, FGF4. When these signals were blocked, the cells remained in their undifferentiated state. The team then looked at ES cells where FGF4 had been disabled through genetic modification and these also remained pluripotent without external instructions.
Jason Wray, one of the authors on the paper, said: “That ES cells can self-replicate without external instruction implies that they are intrinsically programmed to self-renew. The new culture conditions will help us to understand the nature of the pluripotent state and how it might be manipulated to produce specialised cells in the laboratory.”
A better understanding of what controls differentiation in mouse ES cells should also allow the work that has so far taken place only with mouse ES to be replicated in human ES cells.
Jason Wray added: “ES cells with similar characteristics to mouse ES cells have yet to be derived from any other species. Last year it was shown that established human "ES" cells are more similar to mouse “Epi-stem cells”, which correspond to a later stage of development. We have reason to believe that what we have observed in mice will read across to other mammals and that the new culture conditions we have established may allow us to transfer our experience and techniques from animal models to human.”
The work was funded by the Medical Research Council, the Biotechnology and Biological Sciences Research Council, the Canadian Institutes of Health Research, and the European Commission Framework VI project EuroStemCell. It took place at the Wellcome Trust Centre for Stem Cell Research, University of Cambridge.