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Mad cow protein aids creation of brain cells

February 14, 2006

Few conditions are more detrimental to human brains than the one popularly referred to as mad cow disease. But now there's reason to suspect that the protein which, when malformed, causes bovine spongiform encephalopathy in cows and Creutzfeldt-Jakob disease in people, might also be necessary for healthy brain function. Researchers from Whitehead Institute for Biomedical Research and Harvard Medical School/Massachusetts General Hospital have discovered that the normal form of this detrimental protein may actually help the brain create neurons, those electricity-conducting cells that make cognition possible.

"It's been difficult to understand why this prion protein, which when malformed subjects us to this horrible disease, is so abundant in our brains in the first place," says Whitehead Member Susan Lindquist, who is also a professor of biology at MIT. Along with Jeffrey Macklis of Harvard Medical School and Massachusetts General Hospital, she is co-senior author on this Proceedings of the National Academy of Sciences paper, scheduled to be published the week of February 13. "We've known for years what happens when this protein goes wrong. Now we're starting to see what its normal form does right."

For over ten years, researchers have known that a protein called PrP causes mad cow disease and its human equivalent, Creutzfeld-Jakob disease, when it forms incorrectly. 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 or 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. In fact, it's found in many mammalian species, and only on the rarest occasions does it misfold and cause disease. Clearly, scientists have reasoned, such a widely conserved protein also must play a beneficial 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 showed 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.

Recently, researchers from the labs of Lindquist and Whitehead Member Harvey Lodish discovered that PrP helps preserve stem cells in the blood. Because of this, Lindquist teamed up with Macklis to see if there might also be a similar connection between PrP and cells in the brain, where the prion protein is far more abundant.

Andrew Steele, a graduate student from the Lindquist lab, teamed up with Jason Emsley and Hande Ozdinler, postdoctoral researchers in the Macklis lab, to investigate the effects PrP might have on neurogenesis. (Neurogenesis is the process by which the brain creates new neurons in the developing embryonic brain and, to a limited extent, even in the adult brain.) To do this they studied embryonic brain tissue from three kinds of mice: those in which the PrP gene was permanently disabled, or knocked out; those in which the gene was over-expressed, producing an unusually large amount of PrP; and normal control mice.

Steele and Emsley isolated neural precursor cells-early stage cells that give rise to mature neurons and so-called glial support cells. (These precursor cells are often referred to as neural stem cells, though they lack certain properties that are characteristic of broader stem cells.) After placing these embryonic precursor cells under culture conditions that enabled them to grow and differentiate, they noticed striking differences. Cells from the knock-out mouse remained in the precursor stage for a long time, compared to the control mice. But cells in which PrP was over-expressed began forming mature neurons almost immediately.

"The more PrP you have, the faster you become a neuron. The less you have, the longer you'll stay in a precursor state," says Steele.

In addition, the researchers discovered that in adult mouse brains, PrP is only expressed in neurons, but not in the glial cells, cells that form the brain's connective tissue. They also found that while the amount of PrP does affect the speed with which neurons were produced in the adult brain, ultimately the different mice ended up with the same number of neurons. In order to further investigate these findings, the researchers are currently placing these different groups of mice in stimulation-rich environments that will require the quick production of new neurons. The idea is to observe the mice and see if there are any significant differences in how they perform and behave.

"We now see that the normal form of this prion protein is one of many key players in the fascinating and important process of neurogenesis," says Macklis, who is also a member of the Harvard Stem Cell Institute.

Whitehead Institute for Biomedical Research