Saturday, April 26, 2008

Induced Pluripotent Stem Cells II

In this article from the April 4 Science, which I mentioned here, several research reports dealing with induced pluripotent stem cells were discussed. One of these I covered in the post I just noted.

Another just as important report apparently has not yet been formally published, but is (at least temporarily) available online since February 14 at Science Express:

Generation of Pluripotent Stem Cells from Adult Mouse Liver and Stomach Cells
Induced pluripotent stem (iPS) cells have been generated from mouse and human fibroblasts by the retroviral transduction of four transcription factors. However, the cell origins and molecular mechanisms of iPS cell induction remain elusive. This report describes the generation of iPS cells from adult mouse hepatocytes and gastric epithelial cells. These iPS cell clones appear to be equivalent to ES cells in gene expression and are competent to generate germ-line chimeras.

It's not surprising that this is significant research, as it's from the same team of Shinya Yamanaka that was the first to report successful creation of induced pluripotent stem cells. (See here.)

So what is this research about? Well, the investigators used the same four transcription factors (Oct3/4, Sox2, Klf4, and c-Myc) as employed in the majority of previous iPS studies. However, instead of applying the transcription factors to fibroblast cells, they were applied to two types of epithelial cells instead.

Fibroblasts are part of a body's connective tissue. They are involved in structure and support for other tissues and contain large amounts of the protein collagen. They do not divide for the most part, and so it is especially significant that it was possible to reprogram them into a stemcell-like state at all.

Epithelial cells, on the other hand, line the inner and outer surfaces of various body structures, including skin and the gastrointestinal tract. Such cells divide more frequently. They have to, in order to replace other cells of the same kind that are exposed to hostile environments. Epithelial cells also tend to be more adherent to other cells, because they more highly express an adherence protein called E-cadherin.

In some sense, then, epithelial cells are a little more like stem cells to begin with, so one might expect better results when attempting to reprogram them.

This expectation seems to have been met. One of the key differences the researchers found is that reprogrammed epithelial cells had less tendency to form cancerous tumors in mice into which they were included. Certainly not an inconsiderable advantage. This characteristic may be related to the finding that c-Myc seems to play a less essential role in reprogramming epithelial cells.

Specifically, reprogramming of epithelial cells was almost as efficient when c-Myc was not used as when it was included with the other three transcription factors. Yet it was not possible to accomplish reprogramming if any of the other three factors was omitted. In contrast, the efficiency of reprogramming fibroblasts dropped by 90% when c-Myc was omitted.

Another intriguing difference was that reprogrammed epithelial cells contained higher levels of expression of β-catenin than reprogrammed fibroblasts did. (You may recall – see here – that β-catenin is an important part of the Wnt signaling pathway.) In this regard, the reprogrammed epithelial cells are more like true embryonic stem cells than reprogrammed fibroblasts are. It's probably not a coincidence that expression of Nanog is stimulated by β-catenin, (see here), since Nanog is considered important for maintaining stem cell pluripotency.

A further advantage of the use of epithelial cells is that many fewer retroviral "integration sites" were needed to include the transcription factor genes into the cell genome, in comparison with fibroblasts. This is another way the risk of cancer is reduced.

Further reading:

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