In a series of extensive cell culture and animal studies, U-M scientists discovered that a gene called Bmi-1 was required for self-renewal in two types of adult stem cells - neural stem cells from the central nervous system and neural crest stem cells from the peripheral nervous system. In a previous study, other U-M scientists found that Bmi-1 also was necessary for continued self-renewal in a third variety of blood-forming or hematopoietic stem cells.
"So far, we and our colleagues have studied three important types of adult stem cells and Bmi-1 appears to work similarly in every case," says Sean Morrison, Ph.D., an assistant professor of internal medicine in the U-M Medical School and a Howard Hughes Medical Institute investigator. "This raises the intriguing possibility that Bmi-1 could be a universal regulator controlling self-renewal in all adult stem cells."
The U-M study of Bmi-1's role in central nervous system (CNS) stem cells and neural crest stem cells from the peripheral nervous system (PNS) will be published Oct. 22 in Nature's advance online edition.
Co-first authors are Anna Molofsky, a student in the M.D./Ph.D. program at the U-M Medical School, and Ricardo Pardal, Ph.D., a U-M research fellow. Previous U-M research on Bmi-1 and hematopoietic stem cells was conducted by In-Kyung Park, Ph.D., research investigator, and Michael F. Clarke, M.D., professor of internal medicine. Results from that study were published in Nature on April 20.
Unlike embryonic stem cells, which exist for a just few days in the early embryo, various types of adult stem cells remain in many tissues throughout life. When adult stem cells divide, they give rise to more stem cells, in addition to mature cells that replace dead or damaged cells in the body. So, the ability of adult stem cells to divide throughout life is necessary for the maintenance of adult tissues.
Most cells in the body are programmed to stop dividing after a limited number of cell divisions, but adult stem cells and cancer cells have the ability to continue making identical copies of themselves for long periods of time, if not indefinitely. Exactly how they do this has remained a mystery - one that scientists all over the world are trying to solve.
"This paper defines one of the mechanisms that make stem cells special," Morrison says. "We now know that Bmi-1 is an important part of the mechanism used by stem cells to persist through adult life. Certainly there are other genes involved and we need much more research to fully understand the process, but Bmi-1 is a major key to unlocking this important mechanism of self-renewal."
Since cancer cells share the secret of self-renewal with adult stem cells, Morrison says his research "raises the possibility that inappropriate activation or over-expression of Bmi-1 in stem cells could lead to uncontrolled growth and cancer."
Morrison and his research team cultured central nervous system stem cells and neural crest stem cells removed from the brain and gut, respectively, of mice that lacked the Bmi-1 gene. They compared the results with cultured stem cells removed from the same locations of normal mice with the Bmi-1 gene.
"Neural stem cells form structures called neurospheres when grown in culture, but Bmi-1-negative mice formed fewer and smaller neurospheres, which produced fewer daughter neurospheres on subcloning," says Molofsky. "This suggested there were fewer stem cells in tissue from the Bmi-1-deficient mice, and that these stem cells were less able to self-renew."
When U-M scientists compared colonies of central nervous system and peripheral nervous system stem cells taken from Bmi-1-negative and Bmi-1-postive mice immediately after birth and 30 days after birth, they found that the effect of Bmi-1 deficiency on stem cells increased over time. By the time they were one-month-old, it was difficult to detech any neural stem cells in either the central or peripheral nervous systems of the Bmi-1-deficient mice.
"This failure of neural stem cells to persist into adulthood closely paralleled the failure of Bmi-1-deficient hematopoietic stem cells to persist into adulthood, as observed by Park and Clarke," Morrison says. "This suggests that Bmi-1 is consistently required for a variety of adult stem cells to persist into adulthood. Bmi-1-deficient mice are smaller than normal mice, and were previously shown by Maarten van Lohuizen at the Netherlands Cancer Institute to die of hematopoietic and neurological abnormalities between one and two months of age."
"Lack of Bmi-1 doesn't appear to lead to the death of neural stem cells," says Pardal. "Instead it interferes with the cell's ability to copy itself. So their numbers continue to decline gradually after birth. The differences become more apparent in adulthood. Compared to normal adult mice, Bmi-1-deficient mice have very few neural stem cells left."
Although it had a major impact on the ability of neural stem cells to self-renew, lack of Bmi-1 had no effect on restricted progenitor cells, which are formed by neural stem cells and give rise only to neurons or glia in the central and peripheral nervous systems, respectively.
"Proliferation of these restricted neural progenitor cells appears to be regulated differently in ways that make them independent from Bmi-1," Morrison says. "This is important, because it suggests that, while Bmi-1 is consistently required for the proliferation of many types of stem cells, it is not required for the proliferation of many types of other cells."
When U-M scientists analyzed alterations in gene expression within Bmi-1-deficient neural stem cells, they found that one of the genes, which was consistently expressed at higher levels, was p16(Ink4a) - a gene known to inhibit cell proliferation.
In additional experiments designed to discover the relationship between Bmi-1 and p16(Ink4a), Morrison's team found that Bmi-1's ability to suppress expression of p16(Ink4a) in adult stem cells was critical to preserving their ability to self-renew.
"Deleting p16 from the stem cells only partially rescued the ability of neural stem cells to self-renew," Morrison. "This indicates that Bmi-1 likely regulates multiple different pathways that are important for stem cell self-renewal."
The research was funded by the National Institutes of Health, the Searle Scholars Program, the Howard Hughes Medical Institute, the U-M Medical Scholars' Training Program and the Spanish Ministry of Science and Technology.
In addition to Clarke and Park, Toshihide Iwashita, M.D., Ph.D., a U-M research fellow, also collaborated on the study.