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HEMATOLOGY Lab Lifts the Brake on Stem Cell DivisionAccelerating Division and Differentiation Limits Cell Life Span A protein well known to researchers studying the control of cell division, p21, now has a new claim to fame: it appears to be the molecular brake that prevents hematopoietic stem cells from dividing. The discovery, reported in the March 10 Science, equips scientists with a new molecule to target when developing strategies to increase the supply of these valuable cells. Residing in various compartments throughout the adult body, stem cells have enormous medical potential. As precursors to many cell types, they may be able to seed ailing tissues, such as bone marrow or brain, with fresh healthy cells. They may even be used one day to assist in growing new organs for patients awaiting transplants. Despite their promise, stem cells will remain impractical for clinical use until scientists can figure out a way to increase their supply. One would imagine that removing cells from the body and then provoking them to divide should be fairly routine. A number of compounds, including growth-promoting cytokines, promote cell division. But stem cells stubbornly resist, remaining in a resting state referred to as the Go stage of the cell cycle. And when finally coaxed to divide with megadoses of cytokines, they also differentiate. This seals the fate of their lineage, not to mention their life span. Researchers in the lab of David Scadden at Massachusetts General Hospital decided to take up the challenge: how could they increase the number of stem cells without sacrificing their stem-ness? Instead of trying to figure out how they could cause the cells to divide, they decided to focus on what was blocking the cells from dividing in the first place. "In other words," says Scadden, HMS associate professor of medicine, "rather than stepping on the accelerator, we figured we'd take the foot off the brake." They were alerted to p21 as the potential brake when they noticed that it was highly expressed in human bone marrow stem cells. A well-known inhibitor of cell proliferation by virtue of its ability to block the action of proteins called cyclin-dependent kinases, p21 seemed to be a good candidate.
Survival of wild-type and p21 null mice given drug that kills dividing cells
Mice missing the p21 gene die earlier of hematopoietic exhaustion after weekly injections of a drug, antimetabolite 5-fluorouracil, that targets only cycling cells. The bone marrow stem cells of the mice, lacking the p21 brake on division, exit their quiescent state and begin cycling, rendering them susceptible to the drug. To prove that p21 keeps stem cells in Go, the group examined the cells in p21 knockout mice. At first glance, these mice seem to have a normal hematologic profile. Other researchers had found no change in the amount of white or red blood cells or in the number of platelets. But there was one hidden difference that was left for Scadden's group to discover—the mice had twice as many stem cells. To increase in number, the cells must have exited Go and begun cycling. Indeed, within the stem cell pool, many had more mitochondria and a higher RNA content—signs the cells had become metabolically active. If, in fact, the cells had begun cycling, they would be susceptible to drugs that kill only actively dividing cells. And, after exposure to such a drug, mice having a smaller original supply of Go cells would die early. Indeed, one month after a weekly injection of a drug that selectively kills cycling cells, 90 percent of p21 null mice died from hematopoietic failure (see graph). Only 30 percent of the wild-type mice died. Fingering the SwitchAs further proof of p21's protective effect on the stem cell compartment, cells from p21 null mice became easily exhausted in serial bone marrow transplants. Wild-type or p21 knockout cells were transplanted into a group of lethally irradiated mice. Later, cells from these mice were removed and transplanted into a second irradiated group, and so on. Bone marrow cells from p21 knockout mice soon became depleted, unable to repopulate the bone marrow of irradiated mice receiving the fifth transplant. The protein p21 appears to protect stem cells from exhaustion after cytokine stress. Animals produce high levels of these compounds after bone marrow transplantation.
Researchers in the lab of David Scadden (left), have identified the molecular brake that maintains the quiescence of hematopoietic stem cells. The study's lead author is Tao Cheng (right). The other contributors shown are David Dombkowski (back left) and Neil Rodrigues (back right). "Therefore," say the authors in their paper, "p21 is the molecular switch governing the entry of stem cells into the cell cycle, and in its absence, increased cell cycling leads to stem cell exhaustion. Under conditions of stress, restricted cell cycling is crucial to prevent premture stem cell deletion and hematopoietic death." Scadden hopes to exploit the knowledge gleaned from his group's work to enhance the population of stem cells outside the body so that more could be available to people needing bone marrow transplants. He says, "If you just have a release of the p21 brake, without high levels of cytokines, perhaps you can expand the population of true stem cells." Scadden gives high praise to the members of the research team, and particularly to lead author Tao Cheng, HMS instructor in medicine. Other contributors are Neil Rodrigues, David Dombkowski, Hongmei Shen, Yong-guang Yang, and Megan Sykes. The research was supported by the Defense Advanced Research Projects Agency, the Richard Saltonstall Charitable Foundation, and the National Institutes of Health. —Lorene Leiter |
