It was bound to happen, cell, the top tier journal when it comes to cell biology spun off a new journal dedicated to stem cells last week. It took a few days for the university to get access, but I was finally able to peruse it today. I had a little bit of a double take actually regarding the landmark paper published in this first edition. You may have heard that Myc, sox2, oct4 and klf4 are sufficient to reprogram fibroblasts into stem cells. In fact you may have heard of it a full year ago, as this experiment was done by a Japanese group (Takahashi K, Yamanaka S.) and published in Cell. This new paper seems to be almost the same experiments except it takes 12 American scientists to do the work of 2 Japanese :). And if that wasn't confusing enough, another team from the Whitehead Institute also published similar findings last week in nature. This latter group is a proponent of the bivalent histone code regulation of key stem-cell factor. The idea is that both repressive and active histone modifications mark the promoters of these factors making them easily inducible but also primed for repression may the cell wish to differentiate. As talked about previously on the bayblab, these bivalent promoters may be suceptible to dysregulation by epigenetic factors (trithorax/polycomb) over time and may be one of the mechanisms to transformation...While we have talked about sox2, klf4, c-myc and oct4 when the first paper came around there are a few things worth highlighting this time around... While the ectopic expression of these transcription factors is required for the reprogramming it is not really sufficient. There was a large lag period between the expression and the reprogramming, suggesting there is an additional stochastic event that needs to occur. Also, c-myc is the odd one of these transcription factor as it tends to regulate very large areas of chromatin rather than just specific genes. Perhaps the lag is due to chance remodelling event over large areas. For example the authors show that the inactive X chromosome is re-activated by these factors. So this begs the question: what happens to the chromatin, how is the histone code changed, what is the lag for, would expression of members of the trithorax/polycomb make the process more efficient?
More on that later....
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I'd just like to add one note about the importance of the Nature paper by Jaenisch's group. To me, the results of this paper are truly monumental. They built on the original results of the Japanese group by using homologous recombination to insert an antibiotic resistance cassette into the Nanog locus and selected for drug resistant colonies after transduction with retroviral vectors encoding the 4 transcription factors of interest.
The truly amazing result was that drug-resistant colonies were no longer expressing the transgenes; rather, the endogenous genes encoding the four transcription factors were activated in the host cell and the retroviral vector promoters driving expression of the transgenes was silenced via DNA methylation. This explains how the resultant "ES-like" cells were able to differentiate despite ectopic expression of factors that promote "stemness", even in terminally differentiated cells. Basically, expression of these four transcription factors resulted in a heterogeneous population of cells, some of which were truly ES-like; these ES-like cells were those that expressed nanog on their own after vector-transduction. Also, it is truly amazing that these cells were able contributed to the germ line of chimeric mice and resulted in the production of late-term lethal fetuses after injection into 4N blastocysts.
I wonder what is missing to get viable offspring from those stem cells. I guess even nuclear transfer doesn't work very efficiently. However I've heard that if you wait until cell division so that the nuclear content is released, the efficiency of the reprogramming is greater. Also, how long before these transgenes are tested in a human fibroblast....
Glad to see the original findings have been verified by several labs, what with all the scandal in the stem cell field these days. This should be huge for therapeutics if the results can be translated to human fibroblasts. I'm psyched!
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