[edit]

Discovery

The radiation biologist Tikvah Alper and the mathematician John Stanley Griffith developed the hypothesis during the 1960s that sometransmissible spongiform encephalopathies are caused by an infectious agent consisting solely of proteins.^[7][8] This theory was developed to explain the discovery that the mysterious infectious agent causing the diseases scrapie and Creutzfeldt-Jakob Diseaseresisted ultraviolet radiation (UV radiation damages nucleic acids). Francis Crick recognized the potential importance of the Griffith protein-only hypothesis for scrapie propagation in the second edition of his famous "Central dogma of molecular biology".^[9] While asserting that the flow of sequence information from protein to protein, or from protein to RNA and DNA was "precluded" by this dogma, he noted that Griffith's hypothesis was a potential contradiction to this dogma (although it was not so promoted by Griffith). Since the revised "dogma" was formulated, in part, to accommodate the then-recent discovery of reverse transcription by Howard Temin andDavid Baltimore (who won the Nobel Prize in 1975), proof of the protein-only hypothesis might be seen as a "sure bet" for a future Nobel Prize.

Stanley B. Prusiner of the University of California, San Francisco announced in 1982 that his team had purified the hypothetical infectious prion, and that the infectious agent consisted mainly of a specific protein – though they did not manage to satisfactorily isolate the protein until two years after Prusiner's announcement.^[10] Prusiner coined the word "prion" as a name for the infectious agent. While the infectious agent was named a prion, the specific protein that the prion was composed of is also known as the PrionProtein (PrP), though this protein may occur both in infectious and non-infectious forms. Prusiner was awarded the Nobel Prize in Physiology or Medicine in 1997 for his research into prions.^[11]

Structure

See also: PrP structure

Isoforms

The protein that prions are made of (PrP) is found throughout the body, even in healthy people and animals. However, PrP found in infectious material has a different structure and is resistant to proteases, the enzymes in the body that can normally break down proteins. The normal form of the protein is called PrP^C, while the infectious form is called PrP^Sc — the C refers to 'cellular' or 'common' PrP, while the Sc refers to 'scrapie', a prion disease occurring in sheep.^[12] While PrP^C is structurally well-defined, PrP^Sc is certainlypolydisperse and defined at a relatively poor level. PrP can be induced to fold into other more-or-less well-defined isoforms in vitro, and their relationship to the form(s) that are pathogenic in vivo is not yet clear.

PrP^C

PrP^C is a normal protein found on the membranes of cells. It has 209 amino acids (in humans), one disulfide bond, a molecular weight of 35-36 kDa and a mainly alpha-helical structure. Several topological forms exist; one cell surface form anchored via glycolipid and twotransmembrane forms.^[13] Its function has not been fully resolved. PrP^C binds copper (II) ions with high affinity.^[14] The significance of this is not clear, but it presumably relates to PrP structure or function. PrP^C is readily digested by proteinase K and can be liberated from the cell surface in vitro by the enzyme phosphoinositide phospholipase C (PI-PLC), which cleaves the glycophosphatidylinositol(GPI) glycolipid anchor.^[15] PrP may function in cell-cell adhesion of neural cells, and/or be involved in cell-cell signaling in the brain.^[16]

PrP^Sc

The infectious isoform of PrP, known as PrP^Sc, is able to convert normal PrP^C proteins into the infectious isoform by changing theirconformation, or shape; this, in turn, alters the way the proteins interconnect. Although the exact 3D structure of PrP^Sc is not known, it has a higher proportion of β-sheet structure in place of the normal α-helix structure.^[17] Aggregations of these abnormal isoforms form highly structured amyloid fibers, which accumulate to form plaques. The end of each fiber acts as a template onto which free protein molecules may attach, allowing the fiber to grow. Only PrP molecules with an identical amino acid sequence to the infectious PrP^Scare incorporated into the growing fiber.^{[citation needed]}

Function

The function of the prion protein remains controversial, but there is evidence that it serves as a copper-dependent antioxidant.^[18]

PrP and long-term memory

There is evidence that PrP may have a normal function in maintenance of long term memory.^[19] Maglio and colleagues have shown that mice without the genes for normal cellular PrP protein have altered hippocampal long-term potentiation.^[20]

PrP and stem cell renewal

A 2006 article from the Whitehead Institute for Biomedical Research indicates that PrP expression on stem cells is necessary for an organism's self-renewal of bone marrow. The study showed that all long-term hematopoietic stem cells expressed PrP on their cell membrane and that hematopoietic tissues with PrP-null stem cells exhibited increased sensitivity to cell depletion.^[21]

Prion disease

Main article: Transmissible spongiform encephalopathy

Prions cause neurodegenerative disease by aggregating extracellularly within the central nervous system to form plaques known as amyloid, which disrupt the normal tissue structure. This disruption is characterized by "holes" in the tissue with resultant spongy architecture due to thevacuole formation in the neurons.^[22] Other histological changes include astrogliosis and the absence of an inflammatory reaction.^[23] While the incubation period for prion diseases is generally quite long, once symptoms appear the disease progresses rapidly, leading to brain damage and death.^[24] Neurodegenerative symptoms can include convulsions, dementia, ataxia (balance and coordination dysfunction), and behavioural or personality changes.

All known prion diseases, collectively called transmissible spongiform encephalopathies (TSEs), are untreatable and fatal.^[25] However, a vaccine has been developed in mice that may provide insight into providing a vaccine in humans to resist prion infections.^[26] Additionally, in 2006 scientists announced that they had genetically engineered cattle lacking a necessary gene for prion production – thus theoretically making them immune to BSE,^[27] building on research indicating that mice lacking normally-occurring prion protein are resistant to infection by scrapie prion protein.^[28]

Many different mammalian species can be affected by prion diseases, as the prion protein (PrP) is very similar in all mammals.^[29] Due to small differences in PrP between different species, it is unusual for a prion disease to be transmitted from one species to another. However, the human prion disease variant Creutzfeldt-Jakob disease is believed to be caused by a prion which typically infects cattle, causing Bovine spongiform encephalopathy and is transmitted through infected meat.^[30]

The following diseases are caused by prions.

• In animals:
  • Scrapie in sheep and goats^[31]
  • Bovine spongiform encephalopathy (BSE) in cattle (known as mad cow disease)^[31]
  • Transmissible mink encephalopathy (TME) in mink^[31]
  • Chronic wasting disease (CWD) in white-tailed deer, elk, mule deer and moose^[31]
  • Feline spongiform encephalopathy (FSE) in cats^[31]
  • Exotic ungulate encephalopathy (EUE) in nyala, oryx and greater kudu^[31]
  • Spongiform encephalopathy of the ostrich^[32] Though this has not been shown to be transmissible.
• In humans:
  • Creutzfeldt-Jakob disease (CJD)^[31] and its varieties: iatrogenic Creutzfeldt-Jakob disease (iCJD), variant Creutzfeldt-Jakob disease (vCJD), familial Creutzfeldt-Jakob disease (fCJD), and sporadic Creutzfeldt-Jakob disease (sCJD)
  • Gerstmann-Sträussler-Scheinker syndrome (GSS)^[31]
  • Fatal familial insomnia (sFI)^[33]
  • Kuru^[31]

Transmission

Although the identity and general properties of prions are now well understood, the mechanism of prion infection and propagation remains mysterious. It is often assumed that the diseased form directly interacts with the normal form to make it rearrange its structure. One idea, the "Protein X" hypothesis, is that an as-yet unidentified cellular protein (Protein X) enables the conversion of PrP^C to PrP^Sc by bringing a molecule of each of the two together into a complex.^[34]

Current research suggests that the primary method of infection in animals is through ingestion. It is thought that prions may be deposited in the environment through the remains of dead animals and via urine, saliva, and other body fluids. They may then linger in the soil by binding to clay and other minerals.^[35]

Sterilization

Infectious particles possessing nucleic acid are dependent upon it to direct their continued replication. Prions however, are infectious by their effect on normal versions of the protein. Therefore, sterilizing prions involves the denaturation of the protein to a state where the molecule is no longer able to induce the abnormal folding of normal proteins. However, prions are generally quite resistant to proteases, heat,radiation, and formalin treatments,^[36] although their infectivity can be reduced by such treatments. Effective prion decontamination relies upon protein hydrolysis or reduction and/or destruction of protein tertiary structure. Examples include bleach, caustic soda, or strong acidic detergents such as LpH^[37]. 134 degrees Celsius (274 degrees Fahrenheit) for 18 minutes in a pressurised steamautoclave may not be enough to deactivate the agent of disease.^[38][39] Ozone sterilization is currently being studied as a potential method for prion denature and deactivation.^[40] Renaturation of a completely denatured prion to infectious status has not yet been achieved, however partially denatured prions can be renatured to an infective status under certain artificial conditions.^[41]

The World Health Organization recommends any of the following three procedures for the sterilization of all heat-resistant surgical instruments to ensure that they are not contaminated with prions:

1. Immerse in a pan containing 1N NaOH and heat in a gravity-displacement autoclave at 121°C for 30 minutes; clean; rinse in water; and then perform routine sterilization processes.
2. Immerse in 1N NaOH or sodium hypochlorite (20,000 parts per million available chlorine) for 1 hour; transfer instruments to water; heat in a gravity-displacement autoclave at 121°C for 1 hour; clean; and then perform routine sterilization processes.
3. Immerse in 1N NaOH or sodium hypochlorite (20,000 parts per million available chlorine) for 1 hour; remove and rinse in water, then transfer to an open pan and heat in a gravity-displacement (121°C) or in a porous-load (134°C) autoclave for 1 hour; clean; and then perform routine sterilization processes.^[42]

Debate

Whether prions are the agent which causes disease or merely a symptom caused by a different agent is still debated by a minority of researchers. The following sections describe several contending hypotheses.

Genetics as a cause

A gene for the normal protein has been identified: the PRNP gene.^[43] In all inherited cases of prion disease, there is a mutation in thePRNP gene. Many different PRNP mutations have been identified and it is thought that the mutations somehow make PrP^C more likely to change spontaneously into the abnormal PrP^Sc form.^{[verification needed]} These mutations can occur throughout the gene. Some mutations involve expansion of the octapeptide repeat region at the N-terminal of PrP. Other mutations that have been identified as a cause of inherited prion disease occur at positions 102, 117 & 198 (GSS), 178, 200, 210 & 232 (CJD) and 178 (Fatal Familial Insomnia, FFI). The cause of prion disease can be sporadic, genetic, and infectious, or a combination of these factors.^{[verification needed]} For example, in order to have scrapie, both an infectious agent and a susceptible genotype need to be present.^[44]

Multi-component hypothesis

In 2007, biochemist Surachai Supattapone and his colleagues at Dartmouth College produced purified infectious prions de novo from defined components (PrP^C, co-purified lipids, and a synthetic polyanionic molecule).^[45] These researchers also showed that the polyanionic molecule required for prion formation was selectively incorporated into high-affinity complexes with PrP molecules, leading them to hypothesize that infectious prions may be composed of multiple host components, including PrP, lipid, and polyanionic molecules, rather than PrP^Sc alone.^[46]

Heavy metal poisoning hypothesis

Autoclavure destroys protein and genetic material, not the agent of disease.^[39] The protein that can become protease resistantamyloidosis may gain superoxide dismutase activity when bound to copper ions. ^[47] Mark Purdey has provided epidemiology to support the idea that low concentrations of copper and high concentrations of manganese in the environment or animal feed lead to disease.

Viral hypothesis

The protein-only hypothesis has been criticised by those who feel that the simplest explanation of the evidence to date^[48] is viral. For more than a decade, Yale University neuropathologist Laura Manuelidis has been proposing that prion diseases are caused instead by an unidentified "slow" virus. In January 2007, she and her colleagues published an article reporting to have found a virus in 10%, or less, of their scrapie-infected cells in culture.^[49][50]

The virion hypothesis states that TSEs are caused by a replicable informational molecule (which is likely to be a nucleic acid) bound to PrP. Many TSEs, including scrapie and BSE, show strains with specific and distinct biological properties, a feature which supporters of the virion hypothesis feel is not explained by prions.

Evidence in favor of a viral hypothesis includes:^[51]

• Differences in prion infectivity, incubation, symptomology and progression among species resembles the "strain variation" seen between viruses, especially RNA viruses
• The long incubation and rapid onset of symptoms resembles some viral infections, such as HIV-induced AIDS
• Viral-like particles that do not appear to be composed of PrP have been found in some of the cells of scrapie- or CJD-infected cell lines.^[50]

Protein-only hypothesis

Prior to the discovery of prions, it was thought that all pathogens used nucleic acids to direct their replication. The "protein-only hypothesis" states that a protein structure can replicate without the use of nucleic acid. This was initially controversial as it contradicts the so-called "central dogma of molecular biology," which describes nucleic acid as the central form of replicative information.

Evidence in favor of a protein-only hypothesis includes:^[51]

• No virus particles, bacteria, or fungi have been conclusively associated with prion diseases
• No nucleic acid has been conclusively associated with infectivity; agent is resistant to degradation by nucleases
• No immune response to infection
• PrP^Sc experimentally transmitted between one species and another results in PrP^Sc with the amino-acid sequence of the recipient species, suggesting that replication of the donor agent does not occur
• PrP^Sc and PrP^C do not differ in amino-acid sequence, therefore a PrP^Sc-specific nucleic acid is a redundant concept
• Familial prion disease occurs in families with a mutation in the PrP gene, and mice with PrP mutations develop prion disease despite controlled conditions where transmission is prevented
• Animals lacking PrP^C do not contract prion disease.

In yeast and other fungi

Prion proteins were discovered in the yeast Saccharomyces cerevisiae by Reed Wickner in the early 1990s. Subsequently, a prion has also been found in the fungus Podospora anserina. These prions behave similarly to PrP, but are generally non-toxic to their hosts.Susan Lindquist's group at the Whitehead Institute has argued that some of the fungal prions are not associated with any disease state, but may have a useful role; however, researchers at the NIH have also provided strong arguments demonstrating that fungal prions should be considered a diseased state^{[verification needed]}.

Research into fungal prions has given strong support to the protein-only hypothesis for mammalian prions, since it has been demonstrated that purified protein extracted from cells with the prion state can convert the normal form of the protein into the infectious form in vitro, and in the process, preserve the information corresponding to different strains of the prion state. It has also shed some light on prion domains, which are regions in a protein that promote the conversion into a prion. Fungal prions have helped to suggest mechanisms of conversion that may apply to all prions, though mammalian prions may operate by an independent mechanism.

Potential treatments

Advancements in computer modeling have allowed for scientists to identify compounds which can serve as a treatment for prion caused diseases, such as one compound found to bind a cavity in the PrP^C and stabilize the conformation, reducing the amount of harmful PrP^Sc.^[52]

See also

• Protein folding
• Protein Misfolding Cyclic Amplification
• Proteopathy
• Tertiary structure

Further reading

Deadly Feasts: The "Prion" Controversy and the Public's Health^[53], by Richard Rhodes offers a history of research into Kuru, CJD, Mad Cow Disease, Scrapie and related disorders through 1998. The Touchstone paperback edition includes an Afterword that reviews the viral and virion hypotheses. Deadly Feasts extensively covers public policy debates on food safety standards.

The Pathological Protein: Mad Cow, Chronic Wasting, and Other Deadly Prion Diseases covers the science of TSE diseases in greater depth than Deadly Feasts but is not so thorough on policy issues.^[54] The Family That Couldn't Sleep by D. T. Max provides a history of prion diseases for a popular audience.

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External links

• Prion Animation (Flash required)
• CDC – USA Centers for Disease Control and Prevention – information on prion diseases
• World Health Organisation – WHO information on prion diseases
• The UK BSE Inquiry – Report of the UK public inquiry into BSE and variant CJD
• UK Spongiform Encephalopathy Advisory Committee (SEAC)
  • Mammalian prion classification International Committee on Taxonomy of Viruses –ICTVdb
  • Online Mendelian Inheritance in Man: Prion protein – PrP, inherited prion disease and transgenic animal models.
  • The Surprising World of Prion Biology--A New Mechanism of Inheritance on-line lecture by Susan Lindquist
  • UCSF Memory and Aging Center – medical center for diagnosis and care of people with prion disease and research into origin and treatment of prion diseases
    • UCSF Memory and Aging Center YouTube channel – films about CJD
  • Institute for Neurodegenerative Diseases – labs studying prion diseases, run by Stanley B. Prusiner, MD

[edit]

Proposed mechanism of prion propagation

Microscopic "holes" are characteristic in prion-affected tissue sections, causing the tissue to develop a "spongy" architecture