Dec. 16, 2003 STANFORD, Calif. - Doctors regularly inject stem cells into patients whose bone marrow has been destroyed by chemotherapy or radiation, but they haven't known where these cells go after being injected. Research at the Stanford University School of Medicine has yielded an unexpected answer: when injected into mice, these cells may set up camp in one tissue early on but then move to another location or disappear entirely.
Published in the Dec. 15 online edition of the Proceedings of the National Academies of Science, the work upsets current thinking that transplanted stem cells find a habitable niche, settle in for the long haul and begin producing new blood cells. Instead, the newly transplanted cells drift throughout the body, nestling in one of a few homes where their populations subsequently wax and wane until some finally flourish.
Researchers said the procedure used to follow the injected cells' movements could one day help scientists hone their techniques for transplanting bone-marrow stem cells in humans and optimize therapies for cancer and immunodeficiencies. Developing these types of new stem cell-based treatments for cancer is among the primary goals of Stanford's Institute for Cancer/Stem Cell Biology and Medicine.
Yu-An Cao, PhD, a research associate and first author of the paper, said that until now injecting bone-marrow stem cells into a patient was like injecting them into a black box. "We didn't know where those cells were going," he said. Watching the fates of these cells after transplantation had raised more questions than it answered. He said in testing a new protocol, they now can watch to see whether the cells proliferate more quickly or if the patterns of inhabitation are altered.
"We are really curious about what is happening," Cao said. "We want to know why the process is so dynamic with unpredictable fates for the initial stem cell foci. There's no obvious reason for the stem cells to leave what appears to be a perfectly good place to homestead and proliferate."
Eventually, the work also could help guide transplantation procedures using other types of stem cells. Cao said an upcoming experiment will use the same technique to monitor transplanted neuronal stem cells. "We can monitor the fate of those stem cells and help evaluate transplantation protocols," he said. This type of approach could speed the development of stem cell transplantation therapies for disorders such as Parkinson's disease.
Cao and Christopher Contag, PhD, assistant professor of pediatrics, radiology, microbiology and immunology, and lead author of the paper, were able to follow the transplanted cells' travels because they all made a firefly protein called luciferase. This protein produces a dim light when it comes in contact with another molecule called luciferin. Unlike fireflies, mice don't normally make luciferin, but the recipient mice received doses of the molecule throughout the experiment. Once injected into the recipient mice - whose bone marrow had been destroyed by radiation - the luciferase-producing transplanted cells produced a faint glow. Like a campfire at a new settlement, this dim light pinpointed the cells' location.
Although the light from luciferase isn't bright enough to see by eye, an ultrasensitive video camera originally developed by Contag can detect the faint light and show researchers where the glowing cells have settled. The experiment highlighted a handful of stem cell resting places, including the spleen and the bone marrow in the vertebrae, thighbone, shinbone, skull, ribs and sternum, where stem cells were already known to produce new blood cells.
Of all the locations, the spleen and the vertebrae were the two most likely sites for the new cells to settle. These are also the two roomiest compartments, according to Contag. "Where the cells go initially seems to relate to the size of the compartment and its openness," he said. If that location contained existing stem cells, the transplanted stem cell would detect signals indicating, "this compartment is full, we don't want you here," he added. An empty compartment probably lacks these unwelcoming signals. "The cell knows there's an empty seat to jump into, and now we can watch them play musical chairs - we just don't hear the music yet."
What surprised the researchers is how much the pattern varied. In many cases one location would initially house a healthy population of glowing stem cells, only to have that population fade over time while daughter cells set up camp at a distant location. In other mice, locations that initially contained a languishing population of cells would suddenly flourish. When the researchers took stem cells from sites within one transplanted animal and put them into a second mouse lacking bone marrow, those stem cells once again seemed to take a random path to new niches and started the game of musical chairs over again. "This shows that the niche preferences aren't programmed into the cells," Contag said.
Other Stanford researchers who contributed the work include postdoctoral scholars Amy Wagers, PhD, and Andreas Beilhack, PhD; technician Joan Dusich; research associate Michael Bachmann, MD, DSc; Robert Negrin, MD, associate professor of medicine; and Irving Weissman, MD, the Karel and Avice Beekhuis Professor of Cancer Biology and director of Stanford's Institute for Cancer/Stem Cell Biology and Medicine.
Contag is one of the founders of Xenogen, which makes the sensitive video camera used in this study.
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