SC IENCE & TECHNOLOGY
AL ZHEIMER’S PRION CONNECTION Scientists debate PUBLIC HEALTH IMPACT of the prionlike behavior of neurodegenerative proteins LAUREN K. WOLF, C&EN WASHINGTON
ASK A SCIENCE-SAVVY person what
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comes to mind when faced with the term “prion,” and the answer will likely be “mad cow disease.” The malady, known in scientific circles as bovine spongiform encephalopathy, famously broke out in the U.K. during the 1980s, infecting at least 180,000 cattle and leading to the slaughter of millions of others because of its infectivity. Unlike many transmissible diseases, mad cow is not caused by a bacterium or a virus.
So when a scientist goes on record saying that the signature protein of Alzheimer’s disease, amyloid-β, behaves like a prion and therefore has the potential to be infectious, people take notice—and get a little worried. That’s exactly what happened late last year after Claudio Soto, a neurologist at the University of Texas Medical School, in Houston, published a study demonstrating that “seed” particles of misfolded amyloid-β from humans can cause damage
BAD INFLUENCE Like prions, proteins associated with other neurodegenerative diseases can misfold and then recruit similar proteins to do the same, eventually forming harmful aggregates in the brain such as plaques (~50 µm in diameter), tangles (~10–40 µm), and Lewy bodies (~5–10 µm).
Rather, the infectious agent is a prion, a misfolded protein that can wreak havoc in the brain. After appearing inside the body, a prion slowly recruits similar proteins to misfold, clump, and clog the nerve-cell network. Prior to the 1980s, scientists found that prions could pass from person to person under unusual circumstances, such as implantation with infected tissue. But when researchers later discovered that the proteins could cause variant Creutzfeldt-Jakob disease in humans who ate infected beef, prions cemented their reputation as bad actors.
in the brains of mice (Mol. Psychiatry, DOI: 10.1038/mp.2011.120). When Soto and his group took brain tissue from a deceased Alzheimer’s patient and injected it into the rodents’ brains, they observed amyloid clumps accumulate and spread. After the paper’s publication, media outlets such as Fox News featured stories with sensational headlines like, “Can You ‘Catch’ Alzheimer’s Disease?” Soto’s findings caused a stir, even though they were not the first to show similarities between conventional misfolded neuro
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degenerative proteins and prions. Over the past decade, laboratory evidence has been mounting that both amyloid-β and tau protein, another macromolecule that misfolds and tangles up in the brains of Alzheimer’s patients, behave like prions. Researchers have also seen similar mechanisms of seeding, recruitment, and spreading for α-synuclein, the main protein player involved in Parkinson’s disease. BUT THE QUESTION IS, if amyloid-β, tau,
and α-synuclein act like prions in the lab, can these misfolded proteins pass from person to person, too? All of the neuroscientists C&EN interviewed agree that these neurodegenerative proteins are transmissible in the laboratory. But injecting animals with brain tissue is a far cry from what goes on in the real world. No firm evidence exists that the diseases caused by misfolded amyloid-β and its kin are infectious outside the lab, experts say. On the other hand, few are willing to rule out the possibility that Alzheimer’s might be transmitted from person to person. Like prion diseases, neuroscientists say, Alzheimer’s and other neurodegenerative diseases would not be spread via casual contact, but perhaps via other routes such as blood transfusion or neurosurgery. Still, some in the research community object to labeling amyloid-β and other misfoldedneurodegenerative proteins as prions, and thereby scaring the public, until more is known. The idea that misfolded proteins involved in neurodegeneration, such as amyloid-β and tau, might be transmissible was born in the 1970s from studies of prions. But it was famously revisited in the early 1990s, says Virginia M.-Y. Lee, a pathologist at the University of Pennsylvania Perelman School of Medicine. That’s when Heiko Braak, an anatomist now at the University of Ulm, in Germany, studied the pattern of protein aggregates distributed in the brains of Alzheimer’s patients. Looking at a multitude of autopsied brains, Braak observed that both amyloid plaques and tau tangles moved into specific regions of the brain during different stages
of the disease, increasing in density with Alzheimer’s progression. These “tantalizing” results, Lee says, “suggested that there is a stereotypical manner in which Alzheimer’s pathology spreads” in the brain. To uncover the mechanism of this patterned spread, though, scientists would first need proper tools—engineered nerve cells and animals—to reproduce and then study the process, Lee says. These tools became available over time. In 2000, a team led by Lary C. Walker, a neuroscientist now at Emory University, possessed one of the first mice genetically engineered to produce and accumulate in their brains the amyloid-β plaques associated with human Alzheimer’s. “Because the idea had been floating around that Alzheimer’s and other diseases might have some similarities to prion diseases, we decided to test the theory out,” Walker says. The team injected brain tissue from Alzheimer’s patients into the brains of three-month-old engineered mice. Typically, these mice would not begin developing amyloid plaques until nine months of age. Walker’s team showed that the injections accelerate disease progression: Five months afterward, the rodents already carried the brain plaques (J. Neurosci. 2000, 20, 3606). FAST-FORWARD TO 2011, when Soto
carried out similar experiments with a different type of mouse. Soto’s rodents were also engineered to express the protein building blocks necessary to make human amyloid-β, but the mice were designed not to accumulate plaques during their normal two- to three-year life spans. About 280 days after injecting Alzheimer’s brain tissue, however, the research team observed the protein aggregates in the rodents’ brains. About 580 days afterward, the brains of nearly 100% of the mice were riddled with amyloid plaques. Walker’s studies were seminal, Soto says. “But we wanted to start with an animal that is ‘normal,’ one that would never develop the disease unless infected,” he says. That’s more akin to what happens in prion diseases, he adds. Similar experiments have also been car-
ried out for tau protein and α-synuclein. In fact, Penn’s Lee has been instrumental in demonstrating that α-synuclein behaves like a prion with respect to its transmissibility. Her group added synthetic α-synuclein fibrils to mouse nerve cells carrying a normal level of native α-synuclein. The researchers observed those fibril seeds gain access to the cells—although the entrance mechanism is not yet known, she says—and then spread through the neurons. As the fibrils spread, they formed aggregates usually found in the brains of Parkinson’s patients, called Lewy bodies (Neuron, DOI: 10.1016/j.neuron.2011.08.033). “Once you have a seed of misfolded species inside the cell,” Lee says, “it can corrupt the native protein inside, causing it to adopt the bad conformation.” Lee and her group have also shown that this process can happen in a mouse’s brain seeded with synthetic α-synuclein fibrils. About 100 days after the researchers injected the brains of two- to three-monthold mice with misfolded synthetic fibrils, Lewy bodies started developing (J. Exp. Med., DOI: 10.1084/jem.20112457). The genetically engineered mice they used typically accumulate aggregates no earlier than eight months of age. Demonstrating protein transmission with synthetic seeds is important, Lee says, because when the fibrils are derived from brain tissue “you can never be certain that there aren’t other cofactors in there that work together to cause spreading.” In other words, “the best way to prove that the fibrils are wholly responsible for the spreading pathology,” she adds, “is to inject synthetic versions by themselves.”
Researchers hadn’t been able to get experiments with synthetic amyloid-β to work, and thus prove its seed status in Alzheimer’s, until two weeks ago. That’s when a team led by Kurt Giles and Stanley B. Prusiner of the University of California, San Francisco, published an article in Proceedings of the National Academy of Sciences USA (DOI: 10.1073/pnas.1206555109) that did just that: The researchers successfully accelerated the formation of amyloid plaques in the brains of mice by injecting the rodents’ brains with misfolded syn-
“Should we be scaring people with a possibility that may not be a problem at all?” WWW.CEN-ONLIN E .ORG
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A NN. NEUROL.
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thetic amyloid-β particles. after the transfusions,” he Asked how he and Prusinsays, “then we might get a er, a 1997 Nobel Prize winner hint that these diseases are for his research on prions, transmissible.” succeeded where others Another reason there had failed, Giles says others might be a lack of evidence might not have used properly that these neurodegeneramisfolded amyloid-β seeds tive proteins are infectious, or might not have waited Emory’s Walker says, is that long enough for plaques to their misfolded forms just start forming before looking aren’t as sturdy as prions. at rodents’ brains. “Prions are remarkably STAGES OF DISEASE Misfolded protein clumps such as To address the latter resistant to destruction,” amyloid plaques (top) and tau tangles (bottom) spread through the problem, Giles and Prusiner he says. “It’s likely that a lot brains of Alzheimer’s patients in a specific pattern (left to right). used a technique called bioof these other proteopathic Darker shading indicates increased clump density over time. luminescence imaging. They seeds are much less so and genetically engineered mice are perhaps more easily so that the rodents’ brains fluoresce when the wire and, to a lesser extent, elsewhere degraded by enzymes in the body.”That’s an amyloid plaques start to build up. Giles in the rodents’ brains. Heating the wires to idea that will need to be investigated, he adds. explains that, with the imaging technique, 95 °C before implantation did not prevent Until researchers know the answers to the team can see the fluorescence through the misfolded proteins from spreading, but these questions, though,“should we be scarthe rodents’ skulls. “Then we can say, ‘Right, plasma sterilization did (Proc. Natl. Acad. ing people with a possibility that may not be we know that we have disease progression, Sci. USA, DOI: 10.1073/pnas.0903200106). a problem at all?” Walker asks. “That’s the so now’s the time to take the brains out and UT Houston’s Soto has also been makessence of the controversy going on in the analyze them,’ ” he adds. ing some noise recently about unpublished field right now.” On the other hand, he adds, work from his group showing that amyloid “it would be a mistake to ignore something TAKEN TOGETHER, these results point to plaque formation can be triggered in the that is a potential health risk.” amyloid-β and other neurodegenerative brains of mice through blood transfusion. In the meantime, some in the field are proteins as behaving like prions, says Neil Cases of prion diseases transmitted to othcalling for researchers to be careful about R. Cashman, a neurologist at the University ers by infected blood donors are known, how they discuss their work with the media of British Columbia, in Vancouver. “It’s Soto says, “so we wanted to look at whether and public. In a recent article, John Hardy becoming a widely accepted idea,”he adds. this process is applicable to neurodegeneraand Tamas Revesz of University College “But it’s also opening a Pandora’s box.” tive diseases as well.” Although he won’t yet London called on fellow scientists to avoid Only one in a million people in the U.S. discuss the details, he says “the results seem using the term prion to describe amyloid-β die of a prion disease each year, according to be positive” and suggest that amyloid-β and similar proteins associated with neuroto the Centers for Disease Control & Preseeds can cross the blood-brain barrier. degenerative disease (N. Engl. J. Med., DOI: vention. Those diseases are never transKnowing that amyloid-β and similar 10.1056/NEJMcibr1202401). mitted from person to person via casual, proteins act like prions, researchers are left “We’re in danger of devaluing the word or even intimate, contact, Cashman says. wondering why no one has recorded a case ‘ prion, ’ ” Hardy says. Prions can be highly But people have contracted prion diseases of the proteins passing from person to perinfectious, particularly among animals. For via tissue implants, tainted neurosurgical son. On the basis of laboratory results, “we instance, studies have shown that prions instruments, or blood transfusions. all think they should be infectious,” Jucker are so robust, they stick around in soil years So far, there’s been no evidence of infecsays, “but there’s no evidence.” He and othafter flocks of infected sheep roamed on tion via any of those routes for neurodeers are awaiting Soto’s blood transfusion top of it, Hardy says. That situation is hardgenerative proteins such as amyloid-β, but results to be published before further conly comparable with the one for amyloid-β, it’s something to keep an eye on, Cashman sidering the possibility. he argues. Prions and these other proteins adds. “We have all been of the opinion that Soto and Cashman say evidence for may spread by similar mechanisms, Hardy there is no public health risk” in the spreadinfectivity may be lacking because proper says, but nothing like the devastating mad ing of these misfolded proteins, he says, but epidemiological studies have just not been cow disease transmission of the 1980s has “the recent experimental data make it a valperformed yet. “Neurodegenerative disever been observed for Alzheimer’s. “It’s irid concern that deserves further research.” ease symptoms take a long time to show responsible to stir up that fear,” he says. Some infectivity research has already up,” Soto says. “A person might be exposed Despite the concerns this current wave been carried out. In 2009, Mathias Jucker one day to a misfolded protein but show of research has dredged up, Penn’s Lee of Tübingen University, in Germany, along signs of the disease several decades afterand others see a silver lining. What these with Emory’s Walker, implanted stainless ward. It would be difficult to track.” results provide the research community, steel wires coated with amyloid-β seed According to Cashman, one test of the Lee says, are potential new drug targets at particles into the brains of mice. The wires infectivity hypothesis might be to examine which to block progression of these devaswere meant to simulate the use of steel inblood transfusions from elderly donors tating neurodegenerative diseases. “This struments in neurosurgery. to young recipients. “If the incidence of really creates new opportunities,” she says, Plaques formed in the tissue surrounding nongenetic Alzheimer’s increased 30 years “and we’re very excited about it.” ◾ WWW.CEN-ONLIN E .ORG
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