University of California San Francisco (UCSF) scientists are reporting what they say is compelling evidence that the infectious agent known as prion is composed solely of protein. Their findings promise to create new tools for early diagnosis of prions causing bovine spongiform encephalopathy (BSE), or "mad cow" disease, in cattle and Creutzfeldt-Jakob disease in people, they say.
The researchers believe that their work may also help advance investigations of more common neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis.
The finding is reported in todayâ€™s (July 30) issue of Science.
In the study, the researchers created a large fragment of the normal prion protein -- a harmless protein found in all mammals examined. They then folded this fragment into the abnormal shape that they suspected would give it the infectious properties of the prion. Next, they injected the folded protein fragment into the brains of mice genetically engineered to over express the same fragment, but with the shape of the normal prion protein. After a year, the mice developed prion disease and brain tissue from the inoculated mice was injected into wild-type mice that subsequently developed prion disease in about half a year.
"Our study demonstrates that misfolding a particular segment of the normal prion protein is sufficient to transform the protein into infectious prions," says the lead author of the study, Giuseppe Legname, assistant adjunct professor of neurology. He works in the laboratory of the senior author, Stanley B. Prusiner, professor of neurology and director of the Institute for Neurodegenerative Diseases. He won the Nobel Prize in 1997 for his discovery of Prions.
"A great deal of evidence indicates that prions are composed only of protein, but this is the first time that this has been directly shown in mammals. The challenge in the last few years has been to figure out exactly how to demonstrate that prions are made entirely of protein."
The discovery that a small change in the condition of a cell can cause the development of a prion offers an explanation, says Prusiner, for the sporadic form of Creutzfeldt Jakob disease (CJD). CJD is responsible for 85% of cases of prion disease in humans (occurring in 1 or 2 people per million) and is believed to develop spontaneously. It also supports his belief, he says, that sporadic forms of prion disease are caused by prion strains that are different from the one causing BSE in cattle in Britain. He says he thinks that sporadic BSE will be found in one to five cattle per million and predicts such numbers will be found with increased testing for BSE.
"The finding represents a renaissance in prion biology," says Prusiner. "For the first time, we can create prions in the test tube, which will change the way scientists do experiments in the field. We now have a tool for exploring the mechanism by which a protein can spontaneously fold into a shape that causes disease."
More broadly, he says, the advance may lead to similar changes in the way studies are conducted for other neurodegenerative diseases that involve protein misprocessing, including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Each disease involves a particular protein that undergoes some form of misprocessing, in terms of a shape change, metabolism or degradation, or proteolysis. At this point, it is not clear which of these forms of misprocessing occurs in each disease, says Prusiner. However, as in prion diseases, the misprocessing involves a profound conformational change that most often occurs spontaneously.
"The insights that scientists have made into the spontaneous misprocessing of prion proteins have already aided progress in studies of other neurodegenerative diseases," says Prusiner. "But we hope that our new findings with synthetic prions will help scientists investigating other neurodegenerative diseases to move one step further in understanding how misprocessing is spontaneously initiated, and how it progresses."
The production of synthetic prions is the latest milestone in the 30-year effort by UCSF scientists to move in on the biochemical composition of the elusive agent. The agent causes a variety of similar rare, fatal, brain-destroying diseases, including sporadic CJD and variant CJD, in humans, BSE, or "mad cow" disease, in cattle, scrapie in sheep, and like illnesses in deer, elk and mink.
The researchers have long maintained that a prion does not contain nucleic acid, the genetic material of life (DNA or RNA). Viruses, which have a nucleic acid core, replicate by high jacking the machinery of a cell and using it to synthesize more nucleic acid. In contrast, prions are an aberrant form of a normal protein (thus composed of amino acids) that form when a particular segment of normal prion protein in the brain's nerve cells, or neurons, loses its corkscrew-shape structure (known as an alpha helix) and flattens into so-called beta sheets. They suspect that individual normal prion proteins occasionally misform in all people and relevant animals, but are routinely "cleared," or removed, from brain cells. However, in rare cases, they suspect, the abnormal protein, or prion, is not cleared.
Once conversion occurs, they hypothesize, the prion moves on to other normal prion proteins, pinning and flattening their spirals, initiating a process that occurs repeatedly, akin to a deadly "Virginia reel" in the brain. The accumulation and aggregation of the flattened beta sheets leads to structural damage of the nerve cells, causing cell degradation that generally leads to death in less than a year.