For cells that hold so much promise, stem cells' potential has so far gone largely untapped. But new research from Rockefeller University and Howard Hughes Medical Institute scientists now shows that adult stem cells taken from skin can be used to clone mice using a procedure called nuclear transfer. The findings are reported in the Feb. 12 online edition of the Proceedings of the National Academy of Sciences.
Embryonic stem cells have received the most press for their potential to generate healthy cells and tissues that could replace damaged or diseased organs. "Scientists are well-aware that tissue derived from someone else's embryonic stem cells would be recognized as foreign and rejected by the patient," says senior co-author Elaine Fuchs, the Rebecca Lancefield Professor at Rockefeller and a Howard Hughes Medical Institute investigator. "This is one of the reasons why scientists have focused so much attention toward using nuclear transfer, which would allow us to use adult stem cells from the same patient rather than those harvested from an unrelated embryo."
Fuchs and her colleagues tested the method in adult stem cells taken from the skin of mice.
Using purification methods developed in Fuchs' Laboratory of Mammalian Cell Biology and Development, postdocs Valentina Greco and Géraldine Guasch isolated stem cells from the mice's hair follicles. They gave these stem cells to Jinsong Li, a postdoc in Rockefeller's Laboratory of Developmental Biology and Neurogenetics, headed by senior co-author Peter Mombaerts. To execute the nuclear transfer procedure, Li took unfertilized mouse oocytes and replaced the nucleus of each oocyte with a nucleus from these adult skin stem cells.
A main hurdle in nuclear transfer with adult cells has been its efficiency -- out of a hundred attempts, only a handful may succeed -- with reported success rates never reaching into double digits. "The efficiency of nuclear transfer is very low," says Li. "Using purified adult skin stem cells as our source of nuclei, we have found that higher nuclear transfer efficiencies can be achieved."
Greco, Guasch and Li compared the cloning efficiency of adult skin stem cells with that of more differentiated skin cells and also with cumulus cells -- the cells that surround a developing oocyte and have traditionally been the preferred cell type for nuclear transfer. The stem cells gave the best efficiency, yielding 19 pups, nine of which grew up into normal, healthy, breeding adult mice.
This is not the first time scientists have tried to use adult stem cells to clone mice. Experiments using adult hematopoetic stem cells -- the cells in the bone marrow that all blood cells are derived from -- were reported last year. But their conclusions were confusing, says Mombaerts, and there are no reports on using adult stem cells for reproducible cloning of mice that survive until adulthood. By using cells from the same mouse and performing the experiments on the two successive days, the Rockefeller scientists could directly compare adult stem cells with other cell types.
Nuclear transfer can also be used to make embryonic stem cell lines, a process which can be done in a tissue culture dish and which is simpler and more efficient than generating a cloned mouse. Although this procedure has not yet successfully generated human embryonic stem cell lines, once technological hurdles are overcome, it may be possible in the future to use a patient's skin stem cells to tailor make embryonic stem cell lines, circumventing the problem of immune rejection.
Such stem cells might also be used to study a variety of different diseases, for which patient tissue is often hard to come by.
"There are many diseases, such as liver, pancreatic and neurodegenerative disorders where researchers are only able to obtain affected tissue from autopsies," says Fuchs.
If on the other hand, scientists are able to generate embryonic stem cells from the skin of a patient, for example an Alzheimer's patient, these embryonic stem cells might be used in the laboratory to enable scientists to generate neurons and study the neurodegenerative process.
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