DURHAM, N.C. - Medical researchers have been searching for a reliable method to separate rare and primitive stem cells from human blood because these cells can regenerate a blood supply and immune system damaged by disease or medical treatment.
Now researchers from the Duke Comprehensive Cancer Center at Duke University Medical Center have developed a new method to identify and isolate stem cells from samples of umbilical cord blood based on an enzyme in the cells. The enzyme, which is more abundant in stem cells than other blood cells, changes a fluorescent tag the researchers developed to a form that can't escape the cell. The brightest cells - the stem cells - are then selected automatically.
Current separation methods, primarily focused on detecting proteins on the surface of stem cells, are complex and expensive and are complicated by the possibility that not all stem cells express these proteins, the researchers say.
The advance, reported in the Aug. 3 issue of the Proceedings of the National Academy of Sciences (PNAS), has immediate implications for laboratory research involving stem cells. There is also the potential for clinical applications if further experiments show that the selected stem cells can mature into needed blood cells in humans, says the study's principal investigator, Dr. Clay Smith, an associate professor of hematology and oncology.
In the shorter run, the technique could help researchers isolate enough of the elusive cells to investigate many fundamental questions regarding stem cells - from how they maintain their primitive nature, to how they differentiate into specific kinds of cells, to even what tissues might contain them.
To date, several types of stem cell have been identified. Embryonic stem cells, found in developing fetuses, can mature into any of the body's cell types. Hematopoietic stem cells, those isolated by Smith and his colleagues, are found in bone marrow and blood, including umbilical cord blood, and can only mature into blood cells. Recently, neuronal stem cells have been discovered in the brain.
The Duke researchers say their technique might be a good way to isolate as-yet-undiscovered stem cells from other tissues. "Our method represents a new and different way to get a purified population of stem cells," says study co-author Dr. Michael Colvin, director of Duke Comprehensive Cancer Center.
While researchers have coaxed isolated stem cells to mature into almost every type of blood cell in the lab, experiments in mice and humans are still required for formal proof that the cells can solve clinical problems by repopulating a real-life blood supply, Smith says. Even then, he says, more work will be needed before the method can be applied to human therapies.
If the isolated cells can, in fact, regrow a person's blood supply, the technique could possibly improve success rates and reduce complications of stem cell transplants. Certain diseases, such as leukemia, or medical treatments, like high-dose chemotherapy, can harm blood cells. As a result, new blood cells - such as white cells (the body's immune system), red cells (the body's oxygen transport system), and platelets (the body's clotting factor) - all need to be produced from scratch.
But while stem cells may work wonders, mature blood cells that can't be excluded from the transplants frequently wreak havoc. Without a reliable method to separate stem cells from the other cells, patients' "immunity fingerprints" must be closely matched to their bone marrow donors. If not, either the patient can reject the foreign cells or the mature foreign blood cells can actually attack the patient's tissues.
While these problems are less common with an umbilical cord blood transplant since its regular blood cells are not fully matured, they are still a concern. If a patient's own bone marrow or blood is used for the transplant, immune matching is not required, but it's possible that unwanted diseased cells could be returned to the patient. The new isolation technique could potentially eliminate such problems by making it possible to collect and deliver pure stem cells.
If the isolated stem cells work in living systems, the method might also provide accurate counts of the stem cells contained in transplants. This is an important potential ability, since scientific studies have linked the number of stem cells transplanted with the eventual success of the graft.
In designing their new isolation method, the researchers took advantage of the fact that stem cells, more than any other blood cell, contain a great deal of an enzyme known as aldehyde dehydrogenase. Led by Colvin, researchers developed a fluorescent tag that would be altered by the enzyme in a way that would trap it inside the cell. Their fluorescing molecule is called BAAA for BODIPY aminoacetaldehyde. (BODIPY is the part of the molecule that absorbs light of one wavelength and then fluoresces, releasing light of a different wavelength.)
Aldehyde dehydrogenase changes BAAA into BAA, or BODIPY amino acetate, which becomes "stuck" inside a cell because it is negatively charged. The researchers proved this in experiments with special non-stem cells. However, despite the fact that hematopoietic stem cells have lots of the enzyme, they quickly expelled BAAA before it could be transformed to BAA - in the same way that multidrug resistant cancer cells quickly pump out chemotherapy drugs.
But the researchers had a solution. They knew that a drug called "verapamil" can stop cancer cells from pumping out chemotherapy drugs. So, the study's first author, Robert Storms, a research associate in the Center for Genetic and Cellular Therapies at Duke, treated the cord blood samples with verapamil to prevent stem cells from expelling BAAA. It was a success. With ample time to work, the aldehyde dehydrogenase in the stem cells changed BAAA into BAA and the fluorescent tag built up inside the desired cells.
After being treated with both BAAA and verapamil, the cord blood samples are processed by a computerized cell sorter, which shines a laser on one cell at a time and monitors the cell's fluorescence. The machine gives the brightest cells - the cells with lots of aldehyde dehydrogenase and hence lots of BAA trapped inside - an electric charge to separate them from the others. Those charged cells then drip one by one into a collection vessel while all the other cells are discarded. The sorter can analyze 3,000 cells per second.
Aldehyde dehydrogenase's normal role in stem cells isn't known, but it might play an important role in embryo development and hence cell differentiation, Colvin says. Exactly how it works awaits further investigation.
The researchers used funding from the National Institutes of Health and a Public Health Service Grant from the National Cancer Institute to develop the technique.
In addition to Storms, Colvin, and Smith, co-authors on the article are Aliana Trujillo and Lisa Shah of the Center for Genetic and Cellular Therapies in the Division of Experimental Surgery at Duke Comprehensive Cancer Center, and James Springer and Susan Ludeman of the division of hematology and medical oncology in the department of medicine at Duke University Medical Center.
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