15.8G: Prion Diseases - Biology

15.8G: Prion Diseases - Biology

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Prion diseases are transmissible from host to host of a single species and, sometimes, even from one species to another (such as a laboratory animal). They destroy brain tissue giving it a spongy appearance. For these reasons, prion diseases are also called transmissible spongiform encephalopathies or TSEs.

Some examples:

Creutzfeldt-Jakob DiseaseCJDhumans
variant Creutzfeldt-Jakob DiseasevCJDhumans; acquired from cattle with BSE
Bovine Spongiform EncephalopathyBSE"mad cow disease"
Kuruinfectious; in humans who practiced cannibalism in Papua New Guinea
Gerstmann-Sträussler-Scheinker diseaseGSSinherited disease of humans
Fatal Familial InsomniaFFIinherited disease of humans
Scrapieinfectious disease of sheep and goats
other animal TSEscats, mink, elk, mule deer

Before the victim dies of a TSE, the damage to the brain is reflected in such signs as loss of coordination and in humans dementia. Injections of ground-up brain tissue from an animal or human patient with a prion disease into another animal (of the appropriate species) transmits the disease. This suggests that the disease is caused by an infectious agent such as a virus. But viruses have a genome and despite intense efforts no evidence of a virus has ever been found in these brain extracts. In fact, treating the extracts with agents (e.g., ultraviolet light) that destroy DNA does not reduce their infectiousness.

To date, the evidence indicates that the infectious agent in the TSEs is a protein. Stanley Prusiner who pioneered in the study of these proteins and was awarded the Nobel Prize in 1997 for his efforts has named them prion proteins (designated PrP) or simply prions. It turns out that prions are molecules of a normal body protein that have changed their three-dimensional configuration.


The normal protein

  • is called PrPC (for cellular)
  • is a glycoprotein normally anchored to the surface of cells.
  • has its secondary structure dominated by alpha helices (probably 3 of them)
  • is easily soluble
  • is easily digested by proteases
  • is encoded by a gene designated (in humans) PRNP located on our chromosome 20.


The abnormal, disease-producing protein

  • is called PrPSc (for scrapie)
  • has the same amino acid sequence as the normal protein; that is, their primary structures are identical but
  • its secondary structure is dominated by beta conformation
  • is insoluble in all but the strongest solvents
  • is highly resistant to digestion by proteases
  • When PrPSc comes in contact with PrPC, it converts the PrPC into more of itself (even in the test tube).
  • These molecules bind to each other forming aggregates.
  • It is not yet clear if these aggregates are themselves the cause of the cell damage or are simply a side effect of the underlying disease process.

Inherited Prion Diseases

Creutzfeldt-Jakob Disease (CJD)

10–15% of the cases of CJD are inherited; that is, the patient comes from a family in which the disease has appeared before. The disease is inherited as an autosomal dominant. The patients have inherited at least one copy of a mutated PRNP gene. Some of the most common mutations are:

  • a change in codon 200 converting glutamic acid (E) at that position to lysine (K) (thus designated "E200K")
  • a change from aspartic acid (D) at position 178 in the protein to asparagine (D178N) when it is accompanied by a polymorphism in both PRNP genes that encodes valine at position 129. When the polymorphism at codon 129 is Met on both genes, the D178N mutation produces Fatal Familial Insomnia instead.
  • a change from valine (V) at position at position 210 to isoleucine (V210I)

Extracts of autopsied brain tissue from these patients can transmit the disease to

  • apes (whose PRNP gene is probably almost identical to that of humans).
  • transgenic mice who have been given a Prnp gene that contains part of the human sequence.

These results lead to the important realization that prion diseases can only be transmitted to animals that already carry a PRNP gene with a sequence that is at least similar to the one that encoded the PrPSc. In fact, knockout mice with no Prnp genes at all cannot be infected by PrPSc.

Gerstmann-Sträussler-Scheinker disease (GSS)

This prion disease is caused by the inheritance of a PRNP gene with a mutations encoding most commonly

  • leucine instead of proline at position 102 (P102L) or
  • valine instead of alanine at position 117 (A117V)

Again, the disease is also strongly associated with homozygosity for a polymorphism at position 129 (both residues being methionine).

Brain extracts from patients with GSS can transmit the disease to

  • monkeys and apes
  • transgenic mice containing a portion of the human PRNP gene.

Transgenic mice expressing the P102L gene develop the disease spontaneously.

Fatal Familial Insomnia (FFI)

People with this rare disorder have inherited

  • a PRNP gene with asparagine instead of aspartic acid encoded at position 178 (D178N)
  • the susceptibility polymorphism of methionine at position 129 of the PRNP genes.

Extracts from autopsied brains of FFI victims can transmit the disease to transgenic mice.

Infectious Prion Diseases


Kuru was once found among the Fore tribe in Papua New Guinea whose rituals included eating the brain tissue of recently deceased members of the tribe. Since this practice was halted, the disease has disappeared. Before then, the disease was studied by transmitting it to chimpanzees using injections of autopsied brain tissue from human victims.


This disease of sheep (and goats) was the first TSE to be studied. It seems to be transmitted from animal to animal in feed contaminated with nerve tissue. It can also be transmitted by injection of brain tissue.

Bovine Spongiform Encephalopathy (BSE) or "Mad Cow Disease"

An epidemic of this disease began in Great Britain in 1985 and before it was controlled, some 800,000 cattle were sickened by it. Its origin appears to have been cattle feed that contained brain tissue from sheep infected with scrapie and had been treated in a new way that no longer destroyed the infectiousness of the scrapie prions.

The use of such food was banned in 1988 and after peaking in 1992, the epidemic declined quickly.

Creutzfeldt-Jakob Disease (CJD)

A number of humans have acquired CJD through accidental exposure to material contaminated with CJD prions.

  • Grafts of dura mater taken from patients with inherited CJD have transmitted the disease to 228 recipients.
  • Corneal transplants have also inadvertently transmitted CJD.
  • Instruments used in brain surgery on patients with CJD have transmitted the disease to other patients. Two years after their supposed sterilization, these instruments remained infectious.
  • 226 people have acquired CJD from injections of human growth hormone (HGH) or human gonadotropins prepared from pooled pituitary glands that inadvertently included glands taken from humans with CJD.

Now that both HGH and human gonadotropins are available through recombinant DNA technology, such disastrous accidents need never recur.

Variant Creutzfeldt-Jakob Disease (vCJD)

This human disorder appeared some years after the epidemic of BSE (Mad Cow Disease) swept through the cattle herds in Great Britain. Even though the cow and human PRNP genes differ at 30 codons, the sequence of their prions suggests that these patients (155 by 2005) acquired the disease from eating contaminated beef.

All the patients are homozygous for the susceptibility polymorphism of methionine at position 129. The BSE epidemic has waned, and slaughter techniques that allow cattle nervous tissue in beef for human consumption have been banned since 1989. Now we must wait to see whether more cases of vCJD are going to emerge or whether the danger is over.

Miscellaneous Infectious Prion Diseases

A number of TSEs have been found in other animals. Cats are susceptible to Feline Spongiform Encephalopathy (FSE). Mink are also susceptible to a TSE. Even though mad cow disease has not been seen in North America, a similar disease is found in elk and mule deer in the Rocky Mountains of the U.S.

Sporadic Prion Diseases

CJD and FFI occasionally occur in people who have no family history of the disease and no known exposure to infectious prions. The cause of their disease is uncertain.

  • Perhaps a spontaneous somatic mutation has occurred in one of the PRNP genes in a cell.
  • Perhaps their normal PrPC protein has spontaneously converted into the PrPSc form.
  • Or perhaps the victims were simply unknowingly exposed to infectious prions, and sporadic prion diseases do not exist!

Whatever the answer, all the cases are found in people with a susceptibility polymorphism in their PRNP genes.

Prions in Yeast

Two proteins in yeast (Saccharomyces cerevisiae)

  • the Sup35 protein ("Sup35p") and
  • the Ure2 protein (Ure2p)

are able to form prions; that is, they can exist either

  • in a PrPC-like form that is functional or
  • in a PrPSc-like form that is not.

The greater ease with which yeast can be studied has proved that only protein is involved in prion formation and provided insight into the need for PrPSc to find PrPC molecules of a similar primary structure in order to be able to convert them into the PrPSc form.

Evidence that prions are a "protein-only" phenomenon

  • A few molecules of a PrPSc form of the Sup35 protein, when introduced into yeast cells, convert the yeast cell's own Sup35 protein into prion aggregates. The resulting "disease" phenotype is then passed on to the cell's daughters.

    The introduced protein was synthesized in bacteria making it unlikely that it could be contaminated by any gene-containing infectious agent of yeast.

  • Yeast can be "cured" of their prion "disease" by increasing the activity of their chaperones. Presumably the chaperone helps maintain the folded state (with alpha helices) of the protein.
  • When the gene for the glucocorticoid receptor is altered to include sequences coding for one part (domain) of the Sup35 protein, the resulting protein forms prions and produces an entirely new phenotype.

Possible basis of species specificity of prions

  • A particular PrPSc can only convert PrPC molecules of the same or at least similar primary structure.
  • This requirement of "like-with-like" resides in a short sequence at the N-terminal of the protein (rather like an antibody epitope).
  • Yeasts engineered to form two types of prion form two types of "pure" aggregates within the cell.
  • Even in the test tube, each type of prion finds and aggregates with others of its own type.

So the picture that emerges is that a molecule of PrPSc acts as a "seed" providing a template for converting PrPC to more PrPSc. These interact with each other to form small soluble aggregates. These interact with each other to form large insoluble deposits. Although only a small portion of the prion protein is responsible for its specificity, other parts of the molecule are needed for flipping the molecule from the alpha-helical to the beta conformation. All prion proteins contain tracts of repeated Gln-Asn residues which appear to be essential for the conversion process.

Other Pathogenic Prion-like Proteins

The deposits of PrPScin the brain are called amyloid. Amyloid deposits are also found in other diseases.


  • Alzheimer's disease is characterized by amyloid deposits of
    • the peptide amyloid-beta (Aβ)
    • the protein tau
    in the brain.
  • The brains of Parkinson's disease patients have deposits of α-synuclein.
  • Deposits of the protein huntingtin are found in the brains of victims of Huntington's disease.
  • Amyloid deposits of the protein transthyretin are found in peripheral nerves, the kidney, and other organs.

With all of these diseases there is evidence that their amyloid-forming proteins, like PrPSc, can act as a "seed" converting a correctly-folded protein into an incorrectly-folded one and have this effect spread from cell to cell. However, they do not seem to be able to be spread from person to person (unlike the TSEs). Perhaps this is because they are not so incredibly resistant to degradation as PrPSc is.

Most cells, including neurons in the brain, contain proteasomes that are responsible for degrading misfolded or aggregated proteins. In the various brain diseases characterized by a build-up of amyloid deposits, it appears that as the small insoluble amyloid precursors accumulate, they bind to proteasomes but cannot be degraded by them. Furthermore, this binding blocks the ability of the proteasomes to process other proteins that are normal candidates for destruction. Because of the critical role of proteasomes in many cell functions, such as mitosis, it is easy to see why this action leads to death of the cell.

Prion-like proteins not always harmful


  • Yeast are not harmed when Sup35p and Ure2p form prions.
  • The role of CPEB.

CPEB ("cytoplasmic polyadenylation element binding protein") is a protein that

  • is found in neurons of the central nervous system (as well as elsewhere)
  • stimulates messenger RNA (mRNA) translation
  • is needed for long-term facilitation (LTF)
  • accumulates at activated (by serotonin) synapses
  • has the ability to undergo a change in tertiary structure that
    • persists for long periods
    • induces the same conformational change in other molecules of CPEB forming prion-like aggregates

Perhaps the accumulation of these aggregates at a stimulated synapse causes a long-term change in its activity (memory).

Why there aren't any pandemic diseases by prions?

Learning biology in school, I became interested in the fact that there aren't any diseases by prions which are globally infectious (as far as I know), unlike diseases by viruses (ex. COVID-19, SARS etc.) or by bacteria (ex. plague, . ). Why is it?

cf. Here are some my hypothesis

  1. Prions are not efficient pathogens because they don't have DNAs.
  2. Such prions are not yet discovered or does not exist.
  3. There exists lots of people who are immune to such diseases.

Are there any answers to the questions? Or is there any answers?

Prions: The New Biology of Proteins

Prions are believed to be the causative agents of a group of rapidly progressive neurodegenerative diseases called transmissible spongiform encephalopathies, or prion diseases. They are infectious isoforms of a host-encoded cellular protein known as the prion protein. Prion diseases affect humans and animals and are uniformly fatal. The most common prion disease in humans is Creutzfeldt-Jakob disease (CJD), which occurs as a sporadic disease in most patients and as a familial or iatrogenic disease in some patients. Whether prions are infectious proteins that act alone to cause prion diseases remains a matter of scientific debate. However, mounting experimental evidence and lack of a plausible alternative explanation for the occurrence of prion diseases as both infectious and inherited has led to the widespread acceptance of the prion hypothesis.

Interest in prion disease research dramatically increased after the identification in the 1980s of a large international outbreak of bovine spongiform encephalopathy (BSE, also known as mad cow disease) in cattle and after accumulating scientific evidence indicated the zoonotic transmission of BSE to humans causing variant CJD. In recent years, secondary bloodborne transmission of variant CJD has been reported in the United Kingdom.

Prions: The New Biology of Proteins describes the current state of knowledge about the enigmatic world of prion diseases. The book is organized into 12 mostly brief chapters, which nicely summarize the various types of prion diseases and the challenges associated with their diagnosis and treatment. These sections review the biology of prions, the underlying hypotheses for prion replication, and the biochemical basis for strain diversity. Chapters 2 through 5 describe the various characteristic features of prions, including the historical evolution of the prion hypothesis, a detailed description of the possible mechanisms by which the normal prion protein is converted into the pathogenic form, and the cellular biology and putative functions of the normal prion protein. The author’s lucid descriptions of the various topics are supported by diagrams and key references. Subsequent chapters describe prion disease laboratory diagnostic tools that are available or under development. Chapter 9 succinctly summarizes the most likely target sites, from the formation of the infectious agent to its effects on neurodegeneration, which can be exploited for likely therapeutic development. The same chapter describes the various antiprion compounds that have been or are being tested as therapeutic interventions for prion diseases.

The book is unusual because its entire content was exclusively authored by 1 person, resulting in a paucity of in-depth information in some areas, which may have been provided by multiple authors. However, all things considered, the book can be a valuable resource for scientists beginning to understand the world of prion diseases, the underlying biochemical mechanism of disease occurrence, and the challenges associated with the diagnosis and treatment of prion diseases.

Scientists Identify Locations of Early Prion Protein Deposition in Retina

(left panel) Early in prion infection, a prion protein aggregate (magenta) blocks the entrance to a cilium (green) in a retinal photoreceptor. (lower right) In prion-infected retina, prion protein (magenta) accumulates under the horseshoe-shaped ribbon synapses (green) found in photoreceptor terminals.

(left panel) Early in prion infection, a prion protein aggregate (magenta) blocks the entrance to a cilium (green) in a retinal photoreceptor. (lower right) In prion-infected retina, prion protein (magenta) accumulates under the horseshoe-shaped ribbon synapses (green) found in photoreceptor terminals.

The earliest eye damage from prion disease takes place in the cone photoreceptor cells, specifically in the cilia and the ribbon synapses, according to a new study of prion protein accumulation in the eye by National Institutes of Health scientists. Prion diseases originate when normally harmless prion protein molecules become abnormal and gather in clusters and filaments in the human body and brain.

Understanding how prion diseases develop, particularly in the eye because of its diagnostic accessibility to clinicians, can help scientists identify ways to slow the spread of prion diseases. The scientists say their findings, published in the journal Acta Neuropathologica Communications, may help inform research on human retinitis pigmentosa, an inherited disease with similar photoreceptor degeneration leading to blindness.

Prion diseases are slow, degenerative and usually fatal diseases of the central nervous system that occur in people and some other mammals. Prion diseases primarily involve the brain, but also can affect the eyes and other organs. Within the eye, the main cells infected by prions are the light-detecting photoreceptors known as cones and rods, both located in the retina.

In their study, the scientists, from NIH’s National Institute of Allergy and Infectious Diseases at Rocky Mountain Laboratories in Hamilton, Montana, used laboratory mice infected with scrapie, a prion disease common to sheep and goats. Scrapie is closely related to human prion diseases, such as variant, familial and sporadic Creutzfeldt-Jakob disease (CJD). The most common form, sporadic CJD, affects an estimated one in one million people annually worldwide. Other prion diseases include chronic wasting disease in deer, elk and moose, and bovine spongiform encephalopathy in cattle.

Using confocal microscopy that can identify prion protein and various retinal proteins at the same time, the scientists found the earliest deposits of aggregated prion protein in cone photoreceptors next to the cilia, tube-like structures required for transporting molecules between cellular compartments. Their work suggests that by interfering with transport through cilia, these aggregates may provide an important early mechanism by which prion infection selectively destroys photoreceptors. At a later study timepoint, they observed similar findings in rods.

Prion protein also was deposited in cones and rods adjacent to ribbon synapses just before the destruction of these structures and death of photoreceptors. Ribbon synapses are specialized neuron connections found in ocular and auditory neural pathways, and their health is critical to the function of retinal photoreceptors in the eye, as well as hair cells in the ear.

The researchers say such detailed identification of disease-associated prion protein, and the correlation with retinal damage, has not been seen previously and is likely to occur in all prion-susceptible species, including people.

Next the researchers are hoping to study whether similar findings occur in retinas of people with other degenerative diseases characterized by misfolded host proteins, such as Alzheimer’s and Parkinson’s diseases.

J. Striebel et al. Prion-induced photoreceptor degeneration begins with misfolded prion protein
accumulation in cones at two distinct sites: cilia and ribbon synapses. Acta Neuropathologica Communications DOI: 10.1186/s40478-021-01120-x (2021).

J Striebel et al. Microglia are not required for prion-induced retinal photoreceptor degeneration. Acta Neuropathologica Communications DOI: 10.1186/s40478-019-0702-x (2019).

J Carroll et al. Microglia are critical in host defense against prion disease. Journal of Virology DOI: 10.1128/JVI.00549-18 (2018).

  • the Sup35 protein ("Sup35p") and
  • the Ure2 protein (Ure2p)
  • in a PrP C-like form that is functional or
  • in a PrP Sc-like form that is not.
  • proved that only protein is involved in prion formation and
  • provided insight into the need for PrP Sc to find PrP C molecules of a similar primary structure in order to be able to convert them into the PrP Sc form.

Evidence that prions are a "protein-only" phenomenon

  • A few molecules of a PrP Sc form of the Sup35 protein, when introduced into yeast cells, convert the yeast cell's own Sup35 protein into prion aggregates. The resulting "disease" phenotype is then passed on to the cell's daughters.

The introduced protein was synthesized in bacteria making it unlikely that it could be contaminated by any gene-containing infectious agent of yeast.

Possible basis of species specificity of prions

  • A particular PrP Sc can only convert PrP C molecules of the same &mdash or at least similar &mdash primary structure.
  • This requirement of "like-with-like" resides in a short sequence at the N-terminal of the protein (rather like an antibody epitope).
  • Yeasts engineered to form two types of prion form two types of "pure" aggregates within the cell.
  • Even in the test tube, each type of prion finds and aggregates with others of its own type.
  • acts as a "seed" providing a template for converting PrP C to more PrP Sc.
  • These interact with each other to form small soluble aggregates.
  • These interact with each other to form large insoluble deposits.

Although only a small portion of the prion protein is responsible for its specificity, other parts of the molecule are needed for flipping the molecule from the alpha-helical to the beta conformation. All prion proteins contain tracts of repeated Gln-Asn residues which appear to be essential for the conversion process.

NIH Researchers Discover How Prion Protein Damages Brain Cells

Findings Could Advance Understanding of Mad Cow Disease, Related Disorders.

Scientists at the National Institutes of Health have gained a major insight into how the rogue protein responsible for mad cow disease and related neurological illnesses destroys healthy brain tissue.

"This advance sets the stage for future efforts to develop potential treatments for prion diseases or perhaps to prevent them from occurring." said Duane Alexander, M.D., Director of NIH’s Eunice Kennedy ShriverNational Institute of Child Health and Human Development (NICHD), where the study was conducted.

The researchers discovered that the protein responsible for these disorders, known as prion protein (PrP), can sometimes wind up in the wrong part of a cell. When this happens, PrP binds to Mahogunin, a protein believed to be essential to the survival of some brain cells. This binding deprives cells in parts of the brain of functional Mahogunin, causing them to die eventually. The scientists believe this sequence of events is an important contributor to the characteristic neurodegeneration of these diseases.

The findings were published in the current issue of the journal Cell. The study was conducted by Oishee Chakrabarti, Ph.D. and Ramanujan S. Hegde, M.D., Ph.D., of the NICHD Cell Biology and Metabolism Program.

Central to prion diseases like mad cow disease and to many other diseases is the phenomenon known as protein misfolding, Dr. Hegde explained. Proteins are made up of long chains of molecules known as amino acids. When proteins are created, they must be carefully folded into distinct configurations. The process of protein folding is analogous to origami, where a sheet of paper is folded into intricate shapes. Upon correct folding, proteins are transported to specific locations within cells where they can perform their various functions. However, the protein chains sometimes misfold. When this happens, the incorrectly folded protein takes the wrong shape, cannot function properly, and as a consequence, is sometimes relegated to a different part of the cell.

In the case of prion diseases, the culprit protein that misfolds and causes brain cell damage is PrP. Normally, PrP is found on the surface of many cells in the body, including in the brain. However, the normal folding and distribution of PrP can go wrong. If a rogue misfolded version of PrP enters the body, it can sometimes bind to the normal PrP and "convert" it into the misfolded form.

This conversion process is what causes mad cow disease, also known as bovine spongiform encephalopathy. Feed prepared from cattle tissue containing an abnormally folded form of PrP can infect cows. In very rare instances, people eating meat from infected cows are thought to have contracted a similar illness called variant Creutzfeld Jacob disease (vCJD). In other human disorders, genetic errors cause other abnormal forms of PrP to be produced.

"The protein conversion process has been well studied," Dr. Hegde said. "But the focus of our laboratory has been on how — and why — abnormal forms of PrP cause cellular damage."

To investigate this problem, Dr. Hegde’s team has been studying exactly how, when, and where the cell produces abnormal forms of PrP. They had found that many of the abnormal forms of PrP were located in the wrong part of the cell. Rather than being on the cell’s surface, some PrP is exposed to the cytoplasm, the gelatinous interior of the cell. Moreover, several studies from Dr. Hegde’s group and others showed that when too much of a cell’s PrP is exposed to the cytoplasm in laboratory mice, they develop brain deterioration.

"The sum of these discoveries provided us with a key insight," Dr. Hegde said. "We realized that in at least some cases, PrP might be inflicting its damage by interfering with something in the cytoplasm."

In the current study, Drs. Chakrabarti and Hegde sought to determine what went wrong when PrP was inappropriately exposed to the cytoplasm. Their next clue came from a strain of mice with dark mahogany-colored fur. Although these mice develop normally at first, parts of their nervous systems deteriorate with age. Upon autopsy, their brains are riddled with tiny holes, and have the same spongy appearance as the brains of people and animals that died of prion diseases. The gene that is defective in this strain of mice is named Mahogunin.

"The similarity in brain pathology between the Mahogunin mutant mice and that seen in prion diseases suggested to us that there might be a connection," Dr. Hegde said.

To investigate this possible connection, the researchers first analyzed PrP and Mahogunin in cells growing in a laboratory dish. When the researchers introduced altered forms of PrP into the cytoplasm of cells, they saw that Mahogunin molecules in the cytoplasm bound to the PrP, forming clusters. This clustering led to damage in the cell that was very similar to the damage occurring when cells are deprived of Mahogunin.

The researchers found that this damage did not occur in the cell cultures if PrP was confined to the surface of the cell, if the cells were provided with additional Mahogunin, or if PrP was prevented from binding to Mahogunin.

The researchers then studied mice with a laboratory induced version of a human hereditary prion disorder called GSS, or Gerstmann-Straussler-Scheinker Syndrome. This extremely rare disease causes progressive neurological deterioration, typically leading to death between age 40 to 60. Dr. Hegde explained that some GSS mutations result in a form of PrP that comes in direct contact with the cytoplasm. In mice that contain one of these mutations, the researchers discovered that cells in parts of the brain were depleted of Mahogunin. The researchers did not see this depletion if PrP was engineered to avoid the cytoplasm.

The findings, Dr. Hedge said, strongly suggest that altered forms of PrP interfere with Mahogunin to cause some of the neurologic damage that occurs in prion diseases.

"PrP probably interferes with other proteins too," Dr. Hegde said. "But our findings strongly suggest that the loss of Mahogunin is an important factor."

An understanding of how PrP interacts with Mahogunin sets the stage for additional studies that may find ways to prevent PrP from entering the cytoplasm, or to replace Mahogunin that has been depleted.

These diseases are usually diagnosed clinically and confirmed by histopathological examination of brain tissue obtained during a biopsy or after death. The characteristic histopathological and immunohistochemical features produced by these diseases helps in definite identification and confirmation. Obtaining brain biopsy tissue in living humans or animals is risky and is usually not performed routinely for diagnosis of prion diseases.

It is also important to rule out other diseases with similar symptoms. Prion diseases should however always be considered in people with rapidly progressive dementia.

The following battery of tests can be helpful in supporting a clinical diagnosis

  • Magnetic resonance imaging (MRI) scans of the brain
  • Electroencephalogram (EEG) which analyses brain waves
  • Neurological and visual examinations to check for nerve damage and vision loss
  • Cerebrospinal fluid protein tests
  • Genetic tests
  • Tonsil biopsy

The only reliable molecular marker for prion diseases is PrP Sc , the pathological prion protein that accumulates in the central nervous system and lymphoreticular tissues. For BSE, several commercial diagnostic kits based on the post-mortem immunochemical detection of PrP Sc in brain tissue are available. These rapid screening tests have been used in active surveillance of BSE and have greatly improved the detection of infected cattle before their entry into the human food chain.

At present, no diagnostic test exists for the detection of prion diseases in live animals or humans. New diagnostic techniques aimed at increasing sensitivity and specificity of PrP Sc detection in body fluids and at identifying novel surrogate markers are under development.

Prion diagnostics

The ability to secure early diagnosis is vital for therapeutic interventions to be of real value. With respect to animals destined for the human food chain, there is the additional demand to determine presence of the prion agent in tissues in asymptomatic organisms, well before the appearance of any clinical symptoms. This applies equally to the detection of prions in humans, who may participate in tissue donation programs.

Prions were transmitted via blood transfusion in sheep using blood obtained from infected animals prior to the onset of clinical symptoms (48, 49). If the same route applies to humans, this could represent a nightmare scenario for the blood transfusion services (50). A transfusion recipient received blood from an individual harboring the vCJD agent 3.5 years prior to the development of any clinical signs of prion disease in the donor. The unfortunate recipient developed disease 6.5 years after the transfusion.

Connection found in pathogenesis of neurological diseases, HIV

A new study by George Washington University (GW) researcher Michael Bukrinsky, M.D., Ph.D., shows similarities in the pathogenesis of prion disease -- misfolded proteins that can lead to neurological diseases -- and the HIV virus.

The research, published in the Journal of Biological Chemistry, looks at the relationship between cholesterol metabolism and prion infection as a follow-up to previous research on the relationship between cholesterol metabolism and HIV. Bukrinsky, a professor of microbiology, immunology, and tropical medicine at the GW School of Medicine and Health Sciences, and his research team found a striking relationship between impairment of cellular cholesterol transporter ABCA1 and the conversion of prion into the pathological form, which occurs in lipid rafts -- the membrane domains of neuronal cells.

"The effect of prions on ABCA1 and lipid rafts is very similar to what we found with HIV before, suggesting that while prions and viruses are very different, they seem to target the same cellular mechanism of cholesterol metabolism," said Bukrinsky. "This mechanism may be key to controlling many different diseases. It may be that drugs that stimulate ABCA1 can help not only to target prions and HIV, but also a number of other pathogens."

Under normal circumstances, an abundance of ABCA1 limits the number of lipid rafts -- and vice versa. With prions, the opposite effect takes place. During the conversion of prions into a pathogenic form, an abundance of ABCA1 in cells increases, but so does the amount of lipid rafts. The reason for this paradox is that ABCA1 in prion-infected cells is non-functional. The researchers found that ABCA1 was displaced from the plasma membrane and from lipid rafts by prions and was internalized, inhibiting its function. Stimulation of ABCA1 with drugs inhibited conversion of prions from non-pathogenic to pathogenic form, reducing the number of lipid rafts in the cell, and opening the possibility of treating prion disease with these drugs.

Bukrinsky and his research team also found that when cells are loaded with cholesterol, it likewise counteracts this effect of prions on ABCA1 and lipid metabolism in a cell. While in most circumstances having lots of lipids and fats in one's diet is not recommended, this finding suggests that being loaded with fat actually stops the conversion of prions from the non-pathogenic to pathogenic form. Neuronal cells loaded with lipids are actually less prone to becoming susceptible to prion disease. "This isn't a recommendation as we are talking about a very specific cell type and under special circumstances," said Bukrinsky, "but it's an interesting possibility."

Prion-mediated phenotypic diversity in fungi

[PSI + ]/Sup35: regulating the decoding of stop codons and more

The [PSI + ] prion has been the most widely studied yeast prion and much of our knowledge of prion formation, propagation, and transmission has come from studying its behavior in vivo and in vitro ( Tuite et al., 2015 , review). [PSI + ] was originally identified by Cox (1965) as a extrachromosomal modifier of nonsense suppression mediated by mutant tRNAs. Such a phenotype can arise through a defect in recognition of the offending stop codon by the eRF1/eRF3 release factor (RF) complex ( Stansfield et al., 1995 ). This shifts the competition for the stop codon in favor of the nonsense suppressor tRNA and hence one gets translation of the stop codon as a sense codon. Given that the PFP that gives rise to the [PSI + ] prion is eRF3 (more usually referred to by its historical name of Sup35), the [PSI + ]-linked phenotype is entirely consistent with a loss of function of eRF3 possibly interfering with its key functional interaction with eRF1.

The potential benefit of decoding a premature (i.e., mutant) stop codon as sense is evident since this would restore synthesis to some level of full-length protein. The potential benefits of decoding authentic stop codons at the end of each reading frame as “sense” codons are less evident. In fact such “termination readthrough” events should be detrimental to the cell since they extend the protein sequence beyond its native C-terminus. Alternatively, the ability to extend the C-terminus of a protein may give rise to a form of that protein with an altered potentially beneficial function although one example of this in S. cerevisiae has so far been uncovered ( Namy et al., 2002 ). However, translation termination is rarely 100% efficient even at authentic stop codons with the efficiency being modulated by the choice of stop codon, the nucleotide context in which the stop codon is placed, and the presence of endogenous tRNAs able to decode a stop codon albeit inefficiently ( von der Haar and Tuite, 2007 Dabrowski et al., 2015 reviews).

Yeast cells have therefore adapted to a low level of stop codon readthrough as is evident by the neutral impact of most form of the [PSI + ] prion on cell growth ( Byrne et al., 2009 ) even though some strains of S. cerevisiae contain a significant number of inactivating stop codon mutations in their genome ( Fitzpatrick et al., 2011 ). Analysis of the yeast translatome, that is, ribosome-associated mRNAs, using ribosome profiling has also revealed that over 100 proteins show a detectable level of C-terminal extension in a [PSI + ] strain compared to the isogenic [psi − ] strain ( Baudin-Baillieu et al., 2014 ). However, if one introduces [PSI + ] into a strain expressing an efficient nonsense suppressor tRNA such as the tyrosine-inserting SUP4 suppressor tRNA ( Cox, 1971 ) or carries a mutation in the SUP35 gene that impairs Sup35/eRF3 function ( Cox, 1977 ), the cells die presumably because of the unviable levels of stop codon readthrough.

Analysis of a wide range of phenotypes plus the differences in the transcriptome, proteome and translatome of [PSI + ] strains compared to an otherwise isogenic [psi − ] strain paints a much more complex picture of the impact the [PSI + ] prion has on its host. Most strikingly, the ability of a yeast cell to successfully reduce the disadvantageous impact of a variety of physical and chemical stresses ( Eaglestone et al., 1999 True and Lindquist, 2000 True et al., 2004 ) is evident although some detrimental phenotypes are exacerbated by the prion. Some of the phenotypic differences can be explained by the finding that [PSI + ] cells show a significant increase in a +1 frameshift event during the translation of the OAZ1 mRNA thereby increasing the levels of the encoded protein, antizyme ( Namy et al., 2008 ). Antizyme is a negative regulator of polyamine synthesis and the changes in levels of polyamines in the cell triggered by the action of the [PSI + ] prion can explain many—but not all—of the phenotypic differences between [PSI + ] and [PSI − ] cells first reported by True and Lindquist ( True and Lindquist, 2000 True et al., 2004 ).

The +1 frameshift event in the OAZ1 gene promoted by the [PSI + ] prion indicates that the effects of the prion form of eRF3 on translation might not be restricted to just the consequences of stimulating stop codon readthrough. Mutations in the SUP35 gene can also act as suppressors of certain +1 frameshift mutations ( Wilson and Culbertson, 1988 ) although whether [PSI + ] has a similar suppressor activity has not been reported. Comparisons of the transcriptome and translatome also revealed both effects of [PSI + ] on the transcription of subset of some 75 genes and a significant level of errors in selecting the correct reading frame in certain genes ( Baudin-Baillieu et al., 2014 ). This raises the possibility that the phenotypic impact of [PSI + ] may be via several different mechanisms not all of which are linked to a defect in translation termination. This understanding is further complicated by a report that the aggregated prion form of Sup35 actually remains active as a translation termination factor ( Pezza et al., 2014 ).

The most likely explanation for the phenotypes linked to [PSI + ], but not accountable for by a translation termination defect is that the Sup35 amyloid aggregates sequester other functionally unrelated proteins ( Baudin-Baillieu et al., 2014 ). This is supported by a mass spectrometric analysis of Sup35 aggregates which revealed

40 different proteins associated with these aggregates including translation factors, molecular chaperones, and proteins involved in the oxidative stress response ( Nevzglyadova et al., 2009 ). There is also evidence that [PSI + ] amyloid aggregates sequester the native binding partner of Sup35, namely eRF1 and this latter interaction can account for the observation that overexpression of Sup35 in a [PSI + ] cells is toxic ( Vishveshwara et al., 2009 ).