"For years we've wondered why evolution has preserved this protein, what positive role it could possibly be playing," says Whitehead Member Susan Lindquist. Along with Whitehead Member Harvey Lodish, Lindquist is a coauthor on the paper which will published online in Proceedings of the National Academy of Sciences during the week of January 30. "With these findings, we have our first answer," she says.
For over ten years, researchers have known that a protein called PrP causes mad cow disease and its human equivalent, Creutzfeld-Jakob disease. PrP is a prion, a class of proteins that has the unusual ability to recruit other proteins to change their shape (PrP is shorthand for "prion protein."). This is significant, because a protein's form determines its function. When a prion changes shape, or "misfolds," it creates a cascade where neighboring proteins all assume that particular conformation. In some organisms, such as yeast cells, this process can be harmless, even beneficial. But in mammals, it can lead to the fatal brain lesions that characterize diseases such as Creutzfeld-Jakob.
Curiously, however, PrP can be found throughout healthy human bodies, particularly in the brain where it's highly abundant. In fact, it's found in many mammalian species, and only on the rarest occasions does it result in disease. Clearly, scientists have reasoned, such a widely conserved protein also must play a positive role.
In 1993, scientists created a line of mice in which the gene that codes for PrP was knocked out, preventing the mice from expressing the prion in any tissues. Surprisingly, the mice appeared fine, showing no sign of any ill effect. The only difference between these mice and the control mice was that the knock-out animals were incapable of contracting prion-related neurodegenerative disease when infected. Researchers knew then that PrP was necessary for mad-cow type diseases; any other kind of normal function remained unknown. (There is, however, some weak data suggesting that in certain cultured cells PrP may help prevent cell death.)
Chengcheng Zhang, a postdoctoral researcher in the lab of Harvey Lodish, was studying hematopoietic (blood forming) stem cells in mouse fetal tissue when he discovered that PrP was expressed abundantly on the surfaces of these stem cells. "I found that while not all blood cells with PrP on their surface were stem cells, any cell that lacked PrP was definitely not a stem cell," says Zhang.
Zhang teamed up with the Lindquist lab's graduate student Andrew Steele, an expert in prions, to discover what role PrP might play in stem cell biology. Zhang and Steele took bone marrow from mice in which PrP had been knocked out, and transferred that marrow into normal mice whose blood and immune systems had been irradiated. The new bone marrow took hold, and these mice flourished, although all their blood cells lacked PrP. Zhang and Steele continued the experiment, this time taking bone marrow from the newly reconstituted mice, and transplanting it into another group of mice. They repeated this process again and again--transplanting bone marrow from one group of mice to another like passing a baton.
Soon they noticed that with each subsequent transplant, the stem cells began to lose their ability to reconstitute. Eventually, the scientists ended up with mice whose hematopoietic stem cells completely lacked the ability to generate new cells. However, in the control group, where they mimicked the experiment with bone marrow abundant with PrP, each transplant was as good as the next, and at no point down the line did stem cells lose their efficacy.
"Clearly, PrP is important for maintaining stem cells," says Lodish. "We're not sure yet how it does this, but the correlation is obvious."
"PrP is a real black box," adds Lindquist. "This is the first clear indication we have of beneficial role for it in a living animal. Now we need to discover its molecular mechanism."
This research was funded by the National Science Foundation, the National Institutes of Health, the Ellison Medical Research Foundation and the Leukemia and Lymphoma Society.
Written by David Cameron
Proceedings of the National Academy of Sciences