Vol 5. Issue 19 / June 6, 2005
Scripps Research Scientists Convert Mad-Cow-Like Prion Disease into Something Similar to Alzheimer's
By Jason Socrates Bardi
A group of researchers led by scientists at The Scripps Research Institute have done something unusual with prion proteins, which are the underlying cause of mad cow disease, scrapie in sheep, chronic wasting disease in deer, and variant Creutzfeldt-Jakob Disease in humans. Prion proteins cause these diseases as a misfolded form of the protein accumulates in the brain and interferes with nervous system functions.
In the latest issue of the journal Science, the researchers describe the effect of removing a stretch of amino acids at the COOH end of the protein—called the glycophosphoinositol (GPI) anchor. This GPI anchor is essential for anchoring the prion protein into the membranes of cells, where it is believed this host prion protein interacts with the abnormal disease-producing isoform to yield more and more of the disease associated prion protein. Suspecting that this anchor may also be essential to the pathogenesis of prion diseases, the scientists removed it and looked at the effect of the removal on prion disease pathogenesis.
By taking off this anchor, the researchers showed that the prion protein still folded but was no longer able to attach in normal amounts onto the surface of cells. They then looked at the effect of the anchorless prions on the disease in vivo, and they found evidence that the GPI anchor plays a role in prion disease pathogenesis. Transgenic mice that express a form of prion protein without the GPI anchor no longer show the normal characteristics of clinical prion disease when they are infected with infectious prions. That is, they do not develop a progressive neurodegenerative disease fatal by 160 to 170 days after infection. Unexpectedly, these mice lived past 600 days with minimal symptoms.
They found that the anchorless prions instead induced a disease that mimicked Alzheimer's—deposits of amyloid fibrils associated with dystrophic neurons were observed. However unlike Alzheimer's, in which a different protein called "Ab" is deposited, there were heavy deposits of the disease-associated prion protein.
"You can convert the normal form of the prion protein and not get classical [prion] disease," says Michael B. A. Oldstone, a professor in the Departments of Neuropharmacology and Infectology at The Scripps Research Institute. "The protein doesn't cause disease, but it is converted into an amyloid form that gets deposited in a manner similar to Alzheimer's."
What is the significance of converting prion plaques into amyloid plaques? The results have implications for understanding both diseases, says Oldstone.
"The association with the abnormal prion protein raises the possibility that a similar mutation in human prion gene may lead, in certain instances, to a corresponding amyloid-like disease in humans," Oldstone adds. He and his colleagues are vigorously evaluating this possibility with tissues from a variety of human diseases.
Oldstone was one of the senior authors on the study, which he conducted with his postdoctoral fellow Matthew Trifilo, Ph.D., Eliezer Masliah, M.D., a neuropathologist at the University of California San Diego and Bruce Chesebro, M.D., and associates in the Laboratory of Persistent Viral Diseases at the Rocky Mountain Laboratories, which is part of the National Institutes of Allergy and Infectious Diseases at the National Institutes of Health.
Recently, as a result of this and other work, Trifilo received the prestigious Young Scientist Award from Chiron in recognition of his work on "Extra Neural Disease Manifestations of Prion Infection" presented at the second International Symposium on The New Prion Biology: Basic Science, Diagnosis and Therapy last month in Venice, Italy.
Prions and Prion Diseases
Prion diseases, which are also known as transmissible spongiform encephalopathies, are known to affect a number of mammals, including humans, sheep, rodents, cows, elk, and deer. These diseases are strange because unlike other infectious diseases, prion diseases appear not to be associated with nucleic acids. Infectivity is transmitted by a protein.
Thus, they are protein misfolding diseases, and the infectious material is simply a misfolded form of a normal cellular protein, the prion, various forms of which are found in mammalian brains and other tissues. To date, no virus, bacterium, or other pathogen to which a mammal is exposed and through which an infection is established has been found. There need not even be any exposure at all, since some prion diseases occur spontaneously.
These prion proteins are expressed in a wide variety of tissues throughout the body, where they sit anchored onto the surfaces of cells—particularly on cells in neuronal tissue. They are something of an enigma because scientists don't know what the prions do on the surface of these cells. But if the normal function of prions is mysterious, their abnormal malfunction is notorious.
In prion diseases, the normal form of the prion protein, which is water-soluble, is converted into an abnormal, misfolded, water-insoluble form. Infection occurs because these abnormal proteins have the ability to convert other proteins into the abnormal form. Like bad apples spoiling the lot, the infectious prions will multiply, and their insolubility will lead to their aggregation and the formation of plaques in the brain where they can interfere with the normal function of the neurons and other cells where they are deposited.
Probably the most notorious of these diseases is bovine spongiform encephalopathy (BSE), or mad cow disease, which has caused widespread public concern over the last two decades after it has appeared primarily in cattle in England but also in other countries in Europe, Canada, and the United States. Perhaps the number one reason why a disease that infects cows is of such concern to world governments is that scientists believe that the disease can be transmitted across species through the consumption of tainted meat from a diseased animal's central nervous system. The first major outbreak of mad cow disease in Britain in 1986 is believed to have originated with the now outlawed practice of feeding cattle meat and bone meal derived from other slaughtered animals.
Likewise, scientists know that humans who eat beef from BSE-infected cattle may be susceptible to a human transmissible spongiform encephalopathy disease called new variant Creutzfeldt-Jakob. There are now more than 140 such cases. This incurable disease is named after a similar condition called Creutzfeldt-Jakob disease, after the German neurologists Hans Gerhard Creutzfeldt and Alfons Maria Jakob, who first diagnosed it. Creutzfeldt-Jakob disease most commonly strikes older people, and it causes neurologic abnormalities, dementia, memory loss, hallucinations, seizures, and eventually death. New variant Creutzfeldt-Jakob is similar clinically, but can strike much younger people. According to the U.S. Centers for Disease Control and Prevention, the median age of death for Americans with Creutzfeldt-Jakob disease is 68, whereas the median age of death of people with new variant Creutzfeldt-Jakob in Great Britain, where most cases have occurred, is 28.
From Prion to Amyloid
A few years ago, Trifilo arrived as a postdoctoral fellow at Oldstone's La Jolla laboratory, and the two began looking at the pathogenesis of the disease and how this might be related to different parts of the prion protein itself, including 21 amino acid stretch at the protein's carboxy end.
This domain contains the GPI anchor, which is essential for anchoring the protein into the membrane and which Oldstone and Trifilo suspected also plays a role in disease pathogenesis. So they looked at the effect of removing it, and the results, reported in the latest issue of the journal Science, show that by removing the GPI anchor, they were able to have a dramatic effect on the disease.
The anchorless prion is still soluble, and Oldstone, Trifilo, and their colleagues showed that it still folds into a normal prion form. But the difference is that without the GPI motif, the protein cannot anchor onto the cell surface. So instead of having 95 percent of the prion proteins that are produced anchored to the cell surface, most of them wind up being secreted.
Significantly, the anchorless prion proteins had a dramic reduction in their ability to cause prion disease in vivo. Transgenic mice that express a form of prion protein without the GPI anchor no longer show the normal characteristics of prion disease when they are infected with infectious prions. In the mice with the GPI anchor removed, inoculation with tissue containing the misfolded "scrapie" form of prions failed to induce the usual clinical manifestations of prion disease, even after 600 days. By comparison, inoculation of normal mice with the same scrapie samples caused disease in 160 to 180 days.
Intriguingly, the anchorless prions still deposited in the brain, but in different locations as well as being converted into a different type of protein deposit—one that was more characteristic of the deposits seen in amyloid diseases like Alzheimer's. In fact, the brain tissue of the transgenic mice showed similarity to the brain tissue of mice that are used to model Alzheimer's disease. Interestingly the tissue was infectious on transfer into recipient mice.
However, the GPI-anchorless prion proteins didn't cause amyloid disease in a classical sense—they were merely converted from a non-amyloid to an amyloid form that became deposited in the brain. The mice showed minimal clinical manifestations.
The article, "Anchorless Prion Protein Results in Infectious Amyloid Disease without Clinical Scrapie" by Bruce Chesebro, Matthew Trifilo, Richard Race, Kimberly Meade-White, Chao Teng, Rachel LaCasse, Lynne Raymond, Cynthia Favara, Gerald Baron, Suzette Priola, Byron Caughey, Eliezer Masliah, and Michael Oldstone appears in the June 3, 2005 issue of the journal Science. See http://www.sciencemag.org.
This work was supported in part by the National Institutes of Health.
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"You can convert the normal form of the prion protein and not get classical [prion] disease."
—Michael B. A. Oldstone