Information

H2. Prions and Disease - Biology


The normal cellular form of this protein, PrPc is highly conserved in mammals, and is widely expressed in embryogenesis. Techniques exist to delete or make ineffectual genes in mice. When a mice knockout of the PrPc (i.e. the gene for the protein was deleted in all cells) was made, the mice appeared normal. More recent data suggests however, that these mice had altered circadian rhythms and sleep patterns, which suggest a possible link to Fatal Familial Insomnia. The PrPc is a normal membrane protein in neurons. It is anchored to the membrane through a glycosyl-phosphatidyl inositol link, with the protein chain on the outside of the neuronal plasma membrane. The PrPc (without the PI link) is water soluble, protease sensitive, and consists of 42% alpha helix and 3% beta sheet.

Jmol: Updated Prion Protein, Mad Cow Disease, and Mutations Jmol14 (Java) | JSMol (HTML5)

The problem in the transmissible spongiform encephalopathies (TSE's) is that amyloid-like protein aggregates form which appear to be neurotoxic. The protein found in the plaques (in cases other that those that are inherited) has the same primary sequence as the PrPc but a different secondary and presumably tertiary structure. The protein found in the plaques, called the PrPsc (the scrapie form of the the normal protein) is insoluble in aqueuous solution, protease resistant, and has a high beta sheet content (43%) and lower alpha helix content (30%) than the normal version of the protein PrPc.

Figure: Cartoon Models of PrPc and PrPsc

A genetic, inheritable form of disease also exists, in which a mutant form of the PrPc occurs, whose normal structure is destabilized by the mutation. The aggregates caused by the mutant form of the disease are understandable in light of the other diseases which we discussed above. The question is how does the normal PrPc form PrPsc . Evidence shows that if radiolabeled PrP*c from scrapie free cells is added to unlabeled PrPsc from scapie infected cells, the PrP*c is converted to PrP*sc! It appears that the PrPc protein has two forms not that much different in energy, one composed of mostly alpha helix and the other of beta sheet. A dimer of PrPc.PrPsc might form, which destabilizes the PrP*c causing a conformational shift to the PrPsc form, which would then aggregate. Exposure to the PrPsc form would then catalyze the conversion of normal PrPc to PrPsc . Hence, it would be transmissible by contact with just the PrPsc form of the protein. Likewise species specificity could be explained if only dimers of PrPc.PrPsc formed from proteins of the same species could occur. The inherited form of the disease would be explained since the mutant form of the normal protein would more easily form the beta structure found in the aggregate.

It has recently been found that the very same mutation in PrPc, Asp178Asn can cause two different diseases - CJD and FFI. Which disease you get depends on if you have 1 of two naturally occurring, nonharmful variants at amino acid 129 of the normal PrPc gene. If you have a Met at that position, and acquire the Asp178Asn mutation, you get CJD. If, on the other hand, you have a Val at amino acid 129 and acquire the Asp178Asn mutation, you get FFI. This disease was first observed in 1986 and has been reported in five families in the world. It occurs in the late 50's, equally in men and women. It is characterized by a progressive loss of the ability to sleep and disrupted circadian rhythms. The brain shows neuronal loss. It is known that amino acids 129 and 178 occur at the start of alpha helices, as predicted from propensity calculations. Chronic exposure to micromolar levels of synthetic fragment 106-126 of PrPc kills hippocampal neurons. This peptide also has the greatest tendency to aggregate synthetic PrPc peptides.

A series of recent studies have expanded on our knowledge of prion structure. Nelson et al. have obtained the crystal structure of a fibril aggregate made of a short peptide (7 amino acids) from the yeast prion protein Sup35. As presumably occurs in amyloid fibers, these crystals show beta-sheet structures stacked vertically to produce fibril structures. The unit of stacking appears to be pairs of beta-sheets, with the inner side amino acids of one member of the pair interacting with the inner amino acid side chains of the other member of the pair, in a process which excludes water. Similar studies by Ritter et al, using NMR and fluorescence, found pair of beta sheets to be the motif of the fibril. Using fluorescence, they identified two regions, each 15 amino acids, important in collapse to molten-globule like state for nucleation of fibril formation.

Kuru killed many members of the Fore tribe in New Guinea until the cannibalistic practice of eating dead relatives was stopped. Analysis of the genes for the prion protein in the Fore tribe and other ethnic groups in the world show two version differing by just one amino acid in all people (remember that a single gene is represented in both maternal and paternal chromosomes. That these two forms exist through the world suggest that they have been selected for by evolution and confer some biological advantage. People who have just one form of the protein are more susceptible to the development of prion diseases. Mead and Collinge have shown that about 75% of older Fore women (who had lived through cannibalistic practices) had two different prion genes, compared to about 15% of women from other ethnic groups. This high percentage suggests that these women were protected from the disease, leading through natural selection to a high percentage of heterozygotes in this defined population. The general presence of two forms of the prion gene (which probably offers protection from prion disease) suggests that cannibalism might have been widespread in our early ancestors.

There appears to be one main difference between the formation of amyloid fibers from prion proteins and others such as mutant lysozymes. If you add mutant lysozyme to normal lysozyme, the amyloid fibers contain only the mutant protein. However, if you incubate mutant prion proteins with normal prions, the normal proteins become pathological.

QED - Protein Aggregates are not just test tube artifacts, but rather matters of life and death.

CDC: Prion Disease

Jsmol: Protopedia - Prions


H2. Prions and Disease

  • Contributed by Henry Jakubowski
  • Professor (Chemistry) at College of St. Benedict/St. John's University

The normal cellular form of this protein, PrPc is highly conserved in mammals, and is widely expressed in embryogenesis. Techniques exist to delete or make ineffectual genes in mice. When a mice knockout of the PrPc (i.e. the gene for the protein was deleted in all cells) was made, the mice appeared normal. More recent data suggests however, that these mice had altered circadian rhythms and sleep patterns, which suggest a possible link to Fatal Familial Insomnia. The PrPc is a normal membrane protein in neurons. It is anchored to the membrane through a glycosyl-phosphatidyl inositol link, with the protein chain on the outside of the neuronal plasma membrane. The PrPc (without the PI link) is water soluble, protease sensitive, and consists of 42% alpha helix and 3% beta sheet.

Jmol : Updated Prion Protein, Mad Cow Disease, and Mutations Jmol14 (Java) | JSMol (HTML5)

The problem in the transmissible spongiform encephalopathies (TSE's) is that amyloid-like protein aggregates form which appear to be neurotoxic. The protein found in the plaques (in cases other that those that are inherited) has the same primary sequence as the PrPc but a different secondary and presumably tertiary structure. The protein found in the plaques, called the PrPsc (the scrapie form of the the normal protein) is insoluble in aqueuous solution, protease resistant, and has a high beta sheet content (43%) and lower alpha helix content (30%) than the normal version of the protein PrPc.

Figure: Cartoon Models of PrPc and PrPsc

A genetic, inheritable form of disease also exists, in which a mutant form of the PrPc occurs, whose normal structure is destabilized by the mutation. The aggregates caused by the mutant form of the disease are understandable in light of the other diseases which we discussed above. The question is how does the normal PrPc form PrPsc . Evidence shows that if radiolabeled PrP*c from scrapie free cells is added to unlabeled PrPsc from scapie infected cells, the PrP*c is converted to PrP*sc! It appears that the PrPc protein has two forms not that much different in energy, one composed of mostly alpha helix and the other of beta sheet. A dimer of PrP c .PrPsc might form, which destabilizes the PrP*c causing a conformational shift to the PrPsc form, which would then aggregate. Exposure to the PrPsc form would then catalyze the conversion of normal PrPc to PrPsc . Hence, it would be transmissible by contact with just the PrPsc form of the protein. Likewise species specificity could be explained if only dimers of PrP c .PrPsc formed from proteins of the same species could occur. The inherited form of the disease would be explained since the mutant form of the normal protein would more easily form the beta structure found in the aggregate.

It has recently been found that the very same mutation in PrPc, Asp178Asn can cause two different diseases - CJD and FFI. Which disease you get depends on if you have 1 of two naturally occurring, nonharmful variants at amino acid 129 of the normal PrPc gene. If you have a Met at that position, and acquire the Asp178Asn mutation, you get CJD. If, on the other hand, you have a Val at amino acid 129 and acquire the Asp178Asn mutation, you get FFI. This disease was first observed in 1986 and has been reported in five families in the world. It occurs in the late 50's, equally in men and women. It is characterized by a progressive loss of the ability to sleep and disrupted circadian rhythms. The brain shows neuronal loss. It is known that amino acids 129 and 178 occur at the start of alpha helices, as predicted from propensity calculations. Chronic exposure to micromolar levels of synthetic fragment 106-126 of PrPc kills hippocampal neurons. This peptide also has the greatest tendency to aggregate synthetic PrPc peptides.

A series of recent studies have expanded on our knowledge of prion structure. Nelson et al. have obtained the crystal structure of a fibril aggregate made of a short peptide (7 amino acids) from the yeast prion protein Sup35. As presumably occurs in amyloid fibers, these crystals show beta-sheet structures stacked vertically to produce fibril structures. The unit of stacking appears to be pairs of beta-sheets, with the inner side amino acids of one member of the pair interacting with the inner amino acid side chains of the other member of the pair, in a process which excludes water. Similar studies by Ritter et al, using NMR and fluorescence, found pair of beta sheets to be the motif of the fibril. Using fluorescence, they identified two regions, each 15 amino acids, important in collapse to molten-globule like state for nucleation of fibril formation.

Kuru killed many members of the Fore tribe in New Guinea until the cannibalistic practice of eating dead relatives was stopped. Analysis of the genes for the prion protein in the Fore tribe and other ethnic groups in the world show two version differing by just one amino acid in all people (remember that a single gene is represented in both maternal and paternal chromosomes. That these two forms exist through the world suggest that they have been selected for by evolution and confer some biological advantage. People who have just one form of the protein are more susceptible to the development of prion diseases. Mead and Collinge have shown that about 75% of older Fore women (who had lived through cannibalistic practices) had two different prion genes, compared to about 15% of women from other ethnic groups. This high percentage suggests that these women were protected from the disease, leading through natural selection to a high percentage of heterozygotes in this defined population. The general presence of two forms of the prion gene (which probably offers protection from prion disease) suggests that cannibalism might have been widespread in our early ancestors.

There appears to be one main difference between the formation of amyloid fibers from prion proteins and others such as mutant lysozymes. If you add mutant lysozyme to normal lysozyme, the amyloid fibers contain only the mutant protein. However, if you incubate mutant prion proteins with normal prions, the normal proteins become pathological.

QED - Protein Aggregates are not just test tube artifacts, but rather matters of life and death.


H2. Prions and Disease - Biology

“Because of the increasing importance of prions to public health, and the burgeoning rate of discovery in prion biology, this is a timely occasion for a second edition of the book Prion Biology and Diseases. As with the first edition (published in 1999), the extensively revised and updated second edition is edited and substantially written by Stanley Prusiner.

The book is comprehensive, authoritative, accessible and, for the most part, exciting to read. It will serve admirably as a standard of the new science of “prionology” for scientists, physicians and students.”
Nature Cell Biology

“The Editor hoped this book would stimulate and tempt young investigators to prion research. It certainly informs the novice that there is more to prions than mad cows and cannibals! And for the established researcher? This book should make a useful reference text as data from an extensive number of studies are contained within the same volume.”
Microbiology Today


Properly cleaning and sterilizing medical equipment may prevent the spread of the disease. If you have or may have CJD, do not donate organs or tissue, including corneal tissue.
Newer regulations that govern the handling and feeding of cows may help prevent the spread of prion diseases.

As prion diseases progress, people with these diseases generally need help taking care of themselves. In some cases they may be able to stay in their homes, but they eventually may need to move to a care facility.


At the Intersection of Math and Biology, Sindi Lab Sees a Breakthrough in Prion Disease

A UC Merced researcher and her lab have unlocked one of the mysteries that could lead to treatments — or even cures — for prion diseases in mammals.

Prion diseases are a family of rare, progressive neurodegenerative disorders that affect both humans — such as with Creutzfeldt-Jakob disease or fatal familial insomnia — and animals, such as mad-cow disease. These disorders are usually rapidly progressive and always fatal, according to the Centers for Disease Control.

Prions are abnormal, pathogenic, transmissible agents that induce abnormal folding of specific normal cellular proteins called prion proteins, which are found most abundantly in the brain.

But Department of Applied Mathematics Professor Suzanne Sindi and her students have discovered a structural difference between two strains of prions that no one thought existed.

“We’re able to explain previously inconsistent research results through these differences,” Sindi said. “We showed them in yeasts, but then we analyzed the data on human prions, and these differences provide a plausible explanation there, too.”

Sindi’s work is detailed in a new paper in the prestigious journal Nature Structural Molecular Biology.

“Professor Sindi’s discovery is a testament to her creativity and the power of working at the intersections of disciplines, which are fertile ground for novel approaches to solving important problems,” School of Natural Sciences Dean Betsy Dumont said.

Researchers do not know a lot about prion diseases, or even about prions themselves. Even the functions of normal prion proteins are not completely understood, according to the CDC. What is known is that prion diseases affect the nervous system in humans and animals, and in people, impair brain function, and cause rapidly developing dementia, difficulty walking, hallucinations, muscle stiffness, confusion, fatigue and difficulty speaking.

Prion diseases are sometimes spread to humans by infected meat products, though some can also be inherited.

Sindi, a member of the Health Sciences Research Institute, uses yeasts to learn how to take advantage of normal biological systems to reverse the proteins’ aggregate misfolding folding processes.

“If we can understand them in yeasts, it could be incredibly helpful in finding treatments for the human prion diseases,” she said.

Her work is being supported through a Research Project (RO1) Grant from the National Institutes of Health — the original and oldest grant program through the federal agency. It’s rare for an applied math researcher to receive such a grant, as it is still unusual for applied math researchers to turn their focus to health-related matters.

“Getting this award speaks to both the exceptional quality of Suzanne’s research program as well as her effectiveness in working across disciplinary boundaries, bringing advanced mathematical methods to help to solve important problems in biology,” said Professor Mayya Tokman, applied math department chair. “Our department has fostered interdisciplinary research since its beginning and it is good to see that such efforts are paying off by bringing such prestigious awards to applied math faculty.”


What Causes Prion Diseases?

Prion diseases may develop in three situations.

  • The disease may appear spontaneously due to the formation of prions in the body without a known stimulus.
  • Prions may enter the body from another organism, causing illness.
  • Prions may be made in our bodies due to altered genes. Genes contain instructions for making proteins. Some inherited genes may contain a mutation (a change in a gene) that alters the structure of the gene and causes it to code for a misfolded prion protein, or prion.

Finding out what makes a prion protein go bad may help control these brain-eating pathogens.

Prions are believed to be the cause of an array of rare but horrifying neurological diseases, such as Variant Creutzfeldt-Jakob disease (known, in cattle, as mad cow disease).

These misfolded proteins essentially eat microscopic moth holes into the brain. They are untreatable and always fatal.

Researchers at Imperial College London and the University of Zurich have now identified a critical step in the misfolding that creates a prion.

They were also able to halt the process, in a Petri dish, using antibodies — paving the way to possible treatments.

"Prion diseases are aggressive and devastating, and currently there is no cure," Imperial's Alfonso De Simone, the study's lead researcher, said in a release.

"Discovering the mechanism by which prions become pathogenic is a crucial step in one day tackling these diseases, as it allows us to search for new drugs. Now we know what we're targeting, we know what features drugs need to have to stop prions becoming pathogenic."

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A Nasty Twist: Proteins are the essential unit of life — complex molecules folded up with more turns, curls, and hairpins than a Hot Wheels track. Those folds determine the proteins' jobs and functions, which are . well, everything. Proteins are the physical stuff that makes up tissues, cells, signaling chemicals, enzymes, you name it.

Prions are proteins gone rogue, twisted like a comic book villain into pathogenic shapes. Even worse, a prion can twist other proteins it meets into its misshapen image.

As single proteins, prions are unimaginably tiny — tinier than cells, bacteria, and even viruses — but they are capable of causing devastating diseases. Usually, prion diseases occur spontaneously or from inherited genetic mutations, but rarely, they can be transmitted through contaminated food, blood, or surgical instruments.

These are called transmissible spongiform encephalopathies, the most famous of which is bovine spongiform encephalopathy — aka mad cow.

That "spongiform" part is a funny word for a very unfunny condition. The prions cause a cascade of misfolding that basically turns your brain into a sponge, riddled with tiny holes.

Luckily, human prion diseases are quite rare Johns Hopkins pegs the number of cases at around 300 a year in the U.S. These diseases include kuru — originally identified among populations that practiced ritual cannibalism, and now all but eliminated — and the most common form in humans, Creutzfeldt-Jakob disease (CJD).

Human prion protein naturally occurs in the body, but it is "usually well behaved," says Valerie Sim, an associate professor at the University of Alberta's Centre for Prions and Protein Folding Disease, who is not affiliated with the Imperial College/Zurich study.

Human brain tissue from a patient with variant Creutzfeldt-Jakob disease showing the spongification of brain tissue and loss of neurons that are the hallmarks of prion disease. Credit: Sherif Zaki MD, PhD Wun-Ju Shieh MD, PhD, MPH / CDC

But sometimes (hence the name), prion protein will twist the wrong way, triggering a domino effect that ends in disease and death.

It's that moment, when the prion breaks bad, that the researchers were looking to figure out.

Into the Fold: For their study, published in PNAS, the researchers compared normal human prion protein with a mutated, pathogenic version.

The mutant prion protein was chosen for its aggressive, contagious nature — more prions equals more opportunities to try and catch them in the act. To do so, the researchers used advanced imaging techniques backed up with computer analysis.

The researchers found a specific spot on the prion where it began to fold into the pathogenic form. The University of Zurich team then produced antibodies that took aim at that exact spot. When the mutant prions were exposed to the antibodies in a test tube, they did not fold into their pathogenic shape.

"That supported their hypothesis that this spot on the mutant protein is the driver of misfolding," Sim says.

The antibody's success also serves as a proof-of-concept, of sorts, for potential prion therapies that take aim at folding sites.

"Now (researchers) are saying 'we think it's this specific spot on the prion protein that's the trouble spot,'" Sim says. If a molecule could be found that blocks or stabilizes that spot, it could finally provide a prion treatment.

"How you do that is a whole other level of investigation," Sim says.

Besides being performed in vitro, the study comes with a few other caveats. While the site the researchers identified may be the starting point for the mutated prion protein's bad fold, it's not necessarily the same place it happens in the regular human prion protein without the same mutation.

As single proteins, prions are unimaginably tiny — tinier than cells, bacteria, and even viruses — but they are capable of causing devastating diseases.

Also, while the team's mutated prion protein is known to cause disease in humans, the researchers did not infect an animal model with their proteins, so they didn't prove the ability of these specific prions to cause disease. Unlike a virus, this can't be determined just by looking at it: it's actually pretty tricky business growing a pathogenic prion, Sim says — usually, you need to use slurried infected brain matter.

Still, the finding may prove an important insight into the unusual pathogens.

"The intermediate stage of prion pathogenesis is so transient it's like a ghost — almost impossible to image," study lead author Máximo Sanz-Hernández says in the release.

"But now we have a picture of what we're dealing with, we can design more specific interventions that can one day potentially control these devastating diseases."

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5 We Don&rsquot Know How Many Types There Are


Because of being rare conditions, prion diseases remain poorly studied. Growing research in recent times has shed some light on the mysterious diseases, though we still don&rsquot quite understand the underlying mechanisms that cause them. Not just that, we&rsquore not even aware of all the types of prions there can be.

Many diseases that were previously considered to be related to something else have recently been found to be caused by prions, as our definition of what prions are is consistently expanding. We still have little idea about all of their varieties in existence, as even the ones we know of are still not properly understood. [6]


Identified Prion Diseases

Listed below are the prion diseases identified to date. CDC does not currently offer information on every prion disease listed below.

Classic CJD is a human prion disease. It is a neurodegenerative disorder with characteristic clinical and diagnostic features.

vCJD has a different clinical and pathologic characteristics from classic CJD. Each disease also has a particular genetic profile of the prion protein gene.

BSE also known as Mad Cow Disease is a progressive neurological disorder of cattle that results from infection by an unusual transmissible agent called a prion.

CWD is a prion disease that affects deer, elk and moose in some areas of North America, South Korea and Norway. In North America, it has been found in both free-ranging and captive deer populations.


Molecular biology and pathogenesis of prion diseases

Prions cause a group of human and animal neurodegenerative diseases, which are now classified together because their etiology and pathogenesis, involve modification of the prion protein (PrP) 1 . Prion diseases are manifest as infectious, genetic and sporadic disorders. These diseases can be transmitted among mammals by the infectious particle designated ‘prion’ 2 . Despite intensive searches over the past three decades, no nucleic acid has been found within prions 3,4 yet a modified isoform of the host-encoded PrP designated PrP Sc is essential for infectivity 1,5–8 . In fact, considerable experimental data argue that prions are composed exclusively of PrP Sc . Earlier terms used to describe the prion diseases include transmissible encephalopathies, spongiform encephalopathies and slow virus diseases 9 . The human prion disorders include kuru, Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS) and fatal familial insomnia (FFI).

S. B. Prusiner is at the Departments of Neurology and of Biochemistry and Biophysics, University of California, San Francisco, CA 94143-0518, USA.