Wednesday, September 21, 2011

Reviving stem cells may improve our life span

The body needs stem cells for the constant renewal of cells. It is said that certain organs can be completely renewed in a few months time, which means stem cells have replaced all of the old cells. This is because differentiated cells have a limited life span, while stem cells have the capacity of self-renewal. However, stem cells also reduce their functionality over time, and scientists hypothesize that the ageing of stem cells correspond to our own ageing process. Turning back the clock on stem cells could help us reach older age, and that is exactly what researchers from the Georgia Institute of Technology have done. A study with human stem cells showed that rejuvenation is possible, and that typical stem cell properties can be restored.

For their experiments, the researchers used adult stem cells. These cells are primitive, like the embryonic stem cells that can differentiate into all possible tissues, but have already committed to a certain cell line. Adult stem cells are found in almost all organs, where they are committed to generate new cells of the tissue they belong to. The scientists of the Georgia Institute of Technology poked around in the genome, to look for any changes that might explain why the stem cell function deteriorates with age.

Although stem cells have mechanisms built in to protect themselves against typical ageing processes found in differentiated cells, the researchers show that old stem cells have more DNA damage than young stem cells. They found the so called Retrotransposons to be responsible for 65 percent of the damage; genetic code derived from parts of the genome that was previously thought to be 'junk DNA'. These elements accumulate in the cell and account for the DNA damage found in old, but not in young stem cells. Apparently, young stem cells are able to suppress transcription of retrotransposons, but over time, these genetic elements accumulate and slowly induce DNA damage.

The scientists found a way to inhibit expression of retrotransposons, which restored the regenerative properties of old stem cells. Surprisingly, the adult stem cells weren't just reset to a young age, but actually returned to an earlier developmental stage: while adult stem cells are committed to a certain cell line, the stem cells that had their clock reset suddenly gained the possibility to differentiate into other lineages. These stem cells are called 'pluripotent' and have characteristics that more closely resemble embryonic stem cells, that have the greatest differentiating capacity. Inducing pluripotency is not new, however: nowadays scientists are able to make pluripotent stem cells with relative ease from terminally differentiated cells, such as fibroblasts that are found in skin.

If we find a way to keep adult stem cells in our body young and healthy, it might keep our organs functioning for longer, and delay the ageing process. Of course, much work still needs to be done, because no in vivo data is available. The next step would be to keep the stem cells young and fresh in animals. However, unraveling the mechanisms that induce ageing will probably help us developing methods that keep us healthy for a longer time.

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