display | more...
The concept of an infectious protein, or "prion", was first proposed in relationship to scrapie in sheep (Prusiner, 1982) as a "protein-dependent protein synthesis" model for the cause of the disease. That is, it was thought possible that a unique protein mediated and, in theory, performed its own peptide synthesis, independent of DNA or RNA. In order to be considered infectious, the protein would have to be capable of both transmission of itself (release) and regeneration (Wickner et al., 2002); therefore, the protein must encode itself, enhance its own replication, or affect its post-synthetic structure. Griffith (1967), mindful of the fundamentals of molecular biology, proposed that since proteins do not encode themselves, the scrapie-causing protein must either enhance its own transcription, or have an altered conformation that forces reconformation of the normal cellular protein into the infectious form. Current hypotheses follow the "altered conformation" concept, specifically that the infectious prion protein can mediate reconformation of native protein into the infectious form (Prusiner, 1998).

The term "prion", while first used to describe the proteinaceous infectious agent causing scrapie, was later used in association with other mammalian transmissible spongiform encephalopathies (TSEs), such as Mad Cow Disease (bovine spongiform encephalopathy, or BSE), Creutzfeldt-Jakob disease (CJD), and kuru (Prusiner, 1998). Later studies identified and confirmed the presence of several prions of various yeasts, including [PSI+], [URE3], and [PIN+]** of Saccharomyces cerevisiae and [Het-s] of Podospora anserina (Wickner et al., 2002).


It is worth noting here that much more is known about yeast prions than about about mammalian prions. The reason for this is somewhat obvious -- the mammalian life cycle is rather long, whereas yeasts produce mitotic progeny in a matter of hours and sexual progeny in a matter of days. Therefore, yeasts are used as model organisms for a number of mammalian disorders to understand the molecular biology and biochemistry of various phenomena; the discovery of yeast prions has led to a greater understanding of the prion mode of action.

Yeast prion modes of action have been well-characterized (Wickner et al., 2002). For example, Ure2p** is a protein involved in the a pathway that signals presence of good nitrogen sources, thereby blocking the uptake of allantoic acid (a poor nitrogen source) and a similar molecule, ureidosuccinic acid (USA), by Dal5p. [URE3] is an altered form of native Ure2p; this altered protein is incapable of blocking the uptake of USA, even in the presence of good nitrogen sources. This phenotype is identical to the mutated form of the ure2 gene, but ure2** strains are incapable of forming the [URE3] prion protein.

By contrast, human and mammalian prion models have been rather poorly characterized, mostly due to lack of data surrounding the phenomenon. They were originally described (in 1954) as "slow viruses", given the lack of evidence for any bacterial transmission factor or genetic predisposition to the diseases. Inability to uncover a viral factor and advances in molecular biology later led to the posing of the prion hypothesis in the early 1980s, which is still in the process of building into a theory (covered later).

Much of the frustration found in exploration of mammalian prions stems from the lack of an effective model for most of the diseases. For obvious reasons, it is implausible to examine the steps involved in inducing prion formation in humans, at least in vivo; ethical considerations often slow progress in large animal research. The obvious answer to this conundrum would be a murine model system; after all, mice share approximately 80% or so homology with humans on a genetic basis, and mouse models are effective in studies of hundreds of other diseases, including cancers and genetic disorders. However, a mouse model strain has not yet been developed that can mimic more than small portions of prion disease progression. So far, those small pieces have presented an interesting puzzle; however, until more can be seen together, it seems there will still be questions.

Incidentally, possibly the most vexing piece of the prion puzzle is the simple fact that scientists have not yet found a function for the mammalian prion protein, or PrP, in its native form (PrPc). In fact, very little is known about the function of the gene encoding the protein, except that it is consitutively expressed in adults and heavily regulated during development. Therefore, exploration the mechanism of attack of PrPSc (the altered form) has been largely the result of guesswork.


Because of the nature of prions, Wickner et al. (2002) have outlined three criteria for the definition of a prion:
  1. Reversible curing: The prion phenomenon must respond to some form of curing; when the curing stimulus is removed, a measurable rate of reversion to prion state should occur. [URE3], [PSI+], and [PIN+] are eliminated by guanidine, and appear to depend on the presence of heat shock protein chaperones for their maintenance (reviewed in Wickner et al., 2002).
  2. Overproduction of the parent protein increases the chance of prion protein formation: Overproduction of Ure2p causes increased induction of [URE3], and overproduction of Sup35p increases incidence of [PSI+] (reviewed in Wickner et al., 2002).
  3. Prion-infected phenotype mimics the phenotype of gene mutant: Neither [URE3] nor ure2 strains block uptake of ureidosuccinic acid (USA); [PSI+] and sup35 strains both result in improper translation termination (reviewed in Wickner et al., 2002).
Prions have several other properties, including a tendency to aggregate into amyloid fibers in a heritable, self-propagating fashion (Wickner et al., 2000, 2002; Prusiner, 1998). These proteins that form these aggregates have a region of protease resistance in their amyloid forms; this region of protection is called the prion domain (Wickner et al., 2000, 2002; Prusiner, 1998). This domain occurs at residues 1-65 in [URE3]; 1-114 in [PSI+]; and residues 1-25 in [Het-s] (Wickner et al., 2002).
Stanley Prusiner (1998) compiled a similar list of characteristics of mammalian prions in the guise of a table of arguments for the proteinaceous nature of prions:

    (adapted from Prusiner, 1998)
  1. PrPSc and scrapie infectivity copurify.
  2. "The unusual properties of PrPSc mimic those of prions." Modifications of PrPSc disrupt prion activity.
  3. Levels of PrPSc are directly proportional to prion titers.
  4. No evidence for a viral or nucleic acid source for prion disease has been found.
  5. Accumulation of PrPSc results in prion disease pathology.
  6. PrP genetic mutations result in heritable prion disease and cause formation of PrPSc.
  7. Overexpression of PrPc increases incidence of PrPSc (See #2 of Wickner's list, above). Loss of the PrP gene by knockout results in an inability to produce PrPSc (see description of [URE3], above).
  8. Special variations in PrP result in lack of crossover between certain species in reference to prion disease. For example, scrapie's varied conformation relative to CJD results (possibly) in the inability of humans to get CJD from scrapie-infected sheep.
  9. PrPSc (mutant) preferentially binds PrPc (native), which yields more PrPSc (see definition of prion, above).
  10. Truncations of PrP and chimeric PrP genes alter special infectivity and result in novel prions.
  11. Diversity of prion disease is caused by various conformations of PrPSc. This variation can be generated by passage of PrPSc through different species. For example, BSE may be capable of being passed to humans and converted into vCJD, a more recently discovered version of CJD. (
  12. Strain propagation can be seen in instances of exposure of model mice to two different variants of PrPSc (the paper lists fCJD and FFI, see below), which results in different disease properties in the mice.
While some of these examples seem self-referential ("well, levels of prion protein correlate to prion titer"), the fundamental principle is sound: it is the alterations in the protein itself, not in the DNA or RNA intermediates, that results in the symptoms of prion disease.

Human prions have been shown numerous times to form amyloid fibers (reviewed in Prusiner, 1998). This fiber formation takes on an apparently random shape and size, distinguishing it from viral formation (which always follows a particular pattern); however, this pattern is comparable to such disorders as Alzheimer's disease. Amyloid aggregation is also thought to be the root cause of plaque formation in the brain, which is a major defining symptom of most prion diseases. Along this line, anti-prion drugs and anti-Alzheimer's drugs are being co-examined for cross-effectiveness.


On the conformational variation of PrPSc relative to PrPc:

The native form of PrP consists of approximately 40% alpha-helices and very few beta-sheet formations. Alpha helices are smaller, fairly flexible, and usually more susceptible to proteases (enzymes that cut other proteins) because of their relatively loose shape. The infectious form of PrP, however, consists of up to 45% beta-sheets and up to 30% alpha-helices. These beta-sheets are more solid, usually larger, and less susceptible to protease activity because of the inaccessibility of the peptide backbone. The region of amyloid formation, or "prion domain", usually falls in one of the larger regions of beta-sheet formation. (Prusiner (1998) remarks on the seeming impossibility of such a huge variation in proteins of the same sequence, but alpha-helices and beta-sheets are formed in the backbone, somewhat regardless of the sequence of the R-groups.) As reiterated exhaustively already, these conformational changes are the cause of the infectivity of PrP.


Having spent all this time on the genetics and molecular biology of prions, you might have thought I'd forgotten about the actual physical manifestations of mammalian prion disease. Well, you were wrong.

Mammalian prion diseases affect the brain and neural tissue; because of their small size, they are easily able to pass the blood-brain barrier (a feat not achieved by most bacteria and viruses). Therefore, prion infections directly result in various forms of neural degeneration. Most, but not all, humans affected with prion disease develop dementia, or impairment of brain function (usually associated with cognitive functions). Some develop ataxia, or the gradual loss of movement and coordination. Most, but not all, show evidence of spongiform degeneration upon autopsy; that is, their brains appear outwardly normal, but show necrotic pockets and a softer, porous, almost squishy texture as a result of loss of brain cells. Plaque formation can also occur as a result of amyloid aggregation; this will often occur along the axons of neurons, which may make them feel more solid than they already are.

Current therapies for prion disease are elusive, and consist largely of easing pain and alleviating minor demential symptoms. Quinacrine (FDA approved for treatment of malaria, a disease caused by a pathogen which also passes the blood-brain barrier), chlorpromazine (Thorazine, FDA approved for treatment of schizophrenia), and some of their derivatives have shown promising effects in vitro against the propagation of PrPSc (Korth et al., 2001). However, further investigation is pending.


Referential Information

For what it's worth, there are no known prions of other animals, plants, fungi, protists, or bacteria. Yet.

A list of known and suspected prions, their hosts, and methods of invasion (adapted partially from Prusiner, 1998):

** -- a crash course in genetic nomenclature:
-- all names of genetic elements, whether genomic or epigenetic, are italicized.
-- gene names are always lowercase letters, followed by a number. -- proteins are named as the gene name, with the first letter capitalized. The "p" following the number indicates "protein", and the name is not italicized. -- brackets indicate genetic elements that are not encoded by the nuclear genome (epigenetic elements).

Sources:

If you never read anything else on prions, read Stanley Prusiner's Nobel Prize speech (Prusiner, 1998). This man did much of the definitive work (such as it is), and is a brilliant scientist and orator. It's really heavy reading in parts, and even I didn't bother reading everything, but if ever you needed an overview, his 20-page abridged lecture, published by PNAS, is all you could possibly want.

Much of the heavy yeast content of this node came from a section of the Introduction to my master's thesis, 'Analysis of the [KIL-d] Phenomenon of Killer Virus of Saccharomyces cerevisiae (Rutgers University, October 2002).