Adult stem cells may be free of the ethical concerns that hamper embryonic stem cell research, but they still pose formidable scientific challenges. Chief among these is the doggedness with which adult stem cells differentiate into mature tissue the moment they're isolated from the body. This makes it nearly impossible for researchers to multiply them in the laboratory. And because adult stem cells are so rare, that makes it difficult to use them for treating disease.
Now, researchers in the lab of Whitehead Institute Member and MIT professor of biology Harvey Lodish have discovered a way to multiply an adult stem cell 30-fold, an expansion that offers tremendous promise for treatments such as bone marrow transplants and perhaps even gene therapy.
"A 30-fold increase is ten times higher than anyone's achieved before," says Lodish, senior author on the paper, which will be published January 22 online in Nature Medicine.
Unlike embryonic stem cells, adult stem cells are generally tissue-specific, each one destined to develop into several kinds of cells. Chengcheng Zhang, a postdoctoral researcher in the Lodish lab, was determined to develop a way to multiply adult stem cells once they've been isolated from tissue. Achieving this goal required some intricate laboratory sleuthing.
Zhang began by studying adult hematopoietic--blood cell forming--stem cells. Offspring of some of these cells develop into all of the red and white blood cells, while others form the immune system. Using fetal tissue from mice as the source of these cells, Zhang discovered a population of cells that were not stem cells, yet appeared to interact with stem cells, preserving and allowing them to multiply in the fetal environment. When he isolated the stem cells in the lab and cultured them in a dish by themselves, they died. When he mixed them with these newly discovered cells, they thrived. But how did these new cells manage to sustain the stem cells so dramatically?
Zhang used a microarray platform to search for genes that were active in these newly discovered cells, but not active in similar neighboring cells. Some such genes, he reasoned, might encode secreted proteins that sustained stem cells. Eventually, he located a number of such genes.
In the fall of 2003 and early 2005, Zhang reported in the journal Blood how one of these genes codes for a growth factor protein called IGF-2. When Zhang purified IGF-2 and added it in a solution to hematopoietic stem cells that he had isolated, the stem cells increased eight-fold in number.
Zhang then discovered that two more growth factor proteins, Angiopoietin-like 2 and –3, abbreviated as angpt12 and angpt13, were also abundantly expressed in these stem-cell supporting cells. When Zhang combined these two proteins with IGF-2 and added them to hematopoietic stem cells, the result was a 30-fold increase.
"People have been culturing and working with these cells for years, and never before have we seen such an increase," says Zhang.
A 30-fold expansion, if replicated in human cells, could open up a number of doors for researchers working on adult stem cells. Currently, patients with certain blood diseases are treated with stem cells. These stem cells can be acquired either from a donor's bone marrow, or even from cord blood (donated cord blood, or the patient's own). Still, in both these cases, the actual number of stem cells from a donor often falls short of the number needed to adequately treat the patient. This technique could directly address this problem.
Gene therapy is another area where these findings can be of immediate value, Lodish says.
With gene therapy, a genetic defect is corrected by administering a healthy version of the gene into a patient. For example, a physician isolates hematopoietic stem cells from a patient, introduces a harmless virus into them that expresses a correct version of the mutated gene, and then re-administers the stem cells back into the patients. While many clinical trials have succeeded, some ended tragically when the virus ended up activating a cancer-causing gene. Because of this, the Food and Drug Administration is not currently approving any gene-therapy clinical trials.
"If, before the stem cells have been re-introduced into the patients, the physicians could first multiply them in the lab, they could then run assays determining if the virus has landed in any undesirable places," says Lodish. "They could then discard those bad cells, and only administer the good ones to the patients."
But most importantly, these findings aid basic research. "We want to know all sorts of things, like what genes are active in this stem cell, or how this stem cell decides to develop into one kind of cell as opposed to another," says Lodish.
Lodish and his colleagues are collaborating with researchers at Lund University in Sweden to repeat these results with human cord blood.
This research was funded by the National Institutes of Health and the National Science Foundation.
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