Prion Propagation

Prions are unique infectious agents that consist only of a protein, and are not replicated via nucleic acid. The wild type protein PrP^C (Prion-related Protein) consisting of approximately 209 amino acids (split evenly between an N-terminal domain and a C-terminal domain) in primarily alpha-helix secondary structures with one disulfide bond, is expressed throughout the body, though at highest levels in the central nervous system. The function is unknown, although it is a cell surface protein held in place by a glycolipid anchor. However, due to a mutation in the PRNP gene sequence, the amino acid sequence, or error in posttranslational protein folding, the PrP^C protein is misfolded into an isoform PrP^Sc, which consists of mainly beta-sheet fibrillar secondary structures that tend to aggregate, also known as a form of amyloid. This alteration in secondary structure leads to accumulation of PrP^Sc, formation of plaques, and prion diseases such as Creutzfeldt-Jakob disease (CJD), Gerstmann-Straussler-Scheinker disease, kuru, Transmissible Spongiform Encephalopathy (TSE), Bovine Spongiform Encephalopathy (BSE or "mad cow" disease), scrapie, and Familial Fatal Insomnia (FFI). Since the aggregates are formed from native protein sequences, no immune response is raised, and since the plaques assume a non-standard conformation, they are semi-resistant to proteinase K (PK) degradation. Polysaccharides have also been shown to play a role in PrP^Sc structure, and sphingolipids and cholesterol have even been found in purified preparations of prion rods in scrapie (a prion disease found in sheep).

400 x 400px

The Prion Hypothesis

The prion hypothesis, which states that prions propagate themselves without nucleic acid involvement, is controversial because it contradicts the fundamental tenet of molecular biology: proteins are translated from RNA which is transcribed from DNA. However, while the precise mechanism of prion propagation is unclear, there is much evidence to support the theory that PrP^sc is the primary, if not sole agent responsible for propagation of more PrP^sc, not the least of which is a lack of identification of nucleic acid involvement after over two decades of research. Different hypotheses of this interaction exist, most of which involve the PrP^sc protein acting as a template to induce misfolding of PrP^c, or a mysterious as-yet unidentified “protein X”, which binds one unit of PrP^c and one unit of PrP^sc together in a complex. Three known classes of prion propagation have been identified as distinct, including sporadic (85% of cases), acquired (5%), and inherited (10%) PrP^sc accumulation.

Attempts by researchers to recreate PrP^Sc formation de novo have been conducted, though perhaps raise more questions than they answer. PrP^Sc formation has been shown to be induced by polyanionic structures such as RNA and polysaccharides in brain homogenates. Attempts to induce PrP^Sc from mutations in PrP^C via an increase in PK-resistant and beta-rich sequences resulted in a number of different isoforms, also indicating that PrP^Sc formation can depend on intracellular conditions such as solvent concentration and the presence of anionic cofactors. Some researchers remain convinced that a virus or virion particle is responsible for all classified prion diseases, despite a lack of evidence in support of this theory.

Sporadic Propagation: Creutzfeldt-Jakob disease

CJD is classified as a neuropathological disorder cause by sporadic prion propagation, although there is evidence for a new form, variant-CJD or vCJD, that is hereditary. CJD cannot be absolutely diagnosed until a post mortem autopsy, but symptoms may begin as late in life as 60, and spinal taps and EEGs can rule out other forms of treatable dementia. Symptoms include progressively worsening dementia, loss of memory and muscle functions, myoclonus (muscle jerks), and even blindness, leading into a comatose state which is often accompanied by a form of pneumonia. Patients usually die within a single year of onset of symptoms. There is no cure or treatment for the disease as of 2009.

The sporadic mutation of the PRNP gene results in about 1 case of CJD per million individuals, consistent with random mutation statistical analysis. This prion disease, caused by rare genetic event or somatic mutation, seems to be focused on amino acid 129. Several different classifications of the PrP^Sc have been primarily identified by differential cleavage patterns by proteinase K. These types have been additionally separated based on amino acid sequence with respect to a valine or methionine at position 129 in the PrP^Sc protein, and then by location and formation of plaques in different regions of the brain. Type 1 PrP^Sc is characterized by homozygous methionine (MM) at codon 129, and synaptic plaque formation primarily in the cerebral cortex and occipital lobe. Type 2 PrP^Sc can be homozygous methionine, valine (VV), or heterozygous (MV), and causes mainly perivacuolar plaque formation in the basal ganglia and cerebellum in addition to the cerebral cortex. Type 3 PrP^Sc is characterized by MV or VV at codon 129, and show linear plaque formation similar to that of the kuru prion, located mainly in the basal ganglia and cerebellum. Type 4 PrP^Sc (MM)was associated mainly with the vCJD prion, which evidence points toward hereditary transmission of the isoform mutation. Incubation periods of different prion strains also differ, with Types 1 and 3 Prp^Sc having the shortest disease length (around 3-10 months). Homozygosity seems to correspond to shorter duration of illness, but the age of onset of symptoms is unrelated.

File:Types of PrPSc.jpg

400 x 400px

Prion strains 1-4 can also be identified by differential glycosylation patterns upon partial digestion by proteinase K. Types 1 and 2 showed similar percentages of unglycosylated, monoglycosylated, and diglycosylated substrate, while types 3 MM, 3 MV, 3 VV and type 4 MM showed very distinct patterns. Additionally, the ability of the N-terminal region of PrP^Sc to bind copper ions may play a role in differential classifications between types 1 and 2 PrP^Sc.

Acquired Propagation: Kuru

Kuru is an acquired prion disease symptomatically divergent from all forms of CJD. In documented kuru cases, severe dementia did not become apparent until much later after onset of the disease. Instead, it is characterized by radical behavioral changes (alternating bouts of depression and inappropriate laughter), joint pain, and loss of muscle function. Kuru is prevalent in the Fore linguistic group of the Eastern Highlands in Papua New Guinea. Members of these societies classically participated in ceremonies in which practically the entire body of deceased relatives would be feasted upon as a sign of honor and respect. Records of these ceremonies indicate that while men took choice cuts of the human flesh, mainly women and children ate the internal organs such as the brain, which is consistent with a statistically higher proportion of the 2700 documented cases of kuru being found in women and grown children. Although prion disease cannot be transmitted ordinarily through contact with skin or even blood, contact with brain tissue or spinal fluid does effectively transmit the disease. Dura mater grafts, corneal transplants, or surgical implantation of non-sterile electrodes in the brain can also transmit prion disease. It is not known how much abnormal Prp^Sc it is necessary to have ingested or injected to cause kuru or other acquired prion disease.

Once the source of the kuru disease was discovered in 1959 after anthropologically raised questions, ritual endocannabalism ceased, and incidences of kuru have all but disappeared. Remarkably, the incubation of the prion Prp^Sc lasted from a minimum of 4 years to well over 40 years, the onset seeming to be more age-dependent than linked to any other factor.

Another

Inherited Propagation: Fatal Familial Insomnia

Fatal Familial Insomnia is an autosomal dominant disease caused by mutation of the PRNP gene. Early symptoms include insomnia, bizarre phobias, headaches, panic attacks, and hallucinations, followed by recurrence of fetal reflexes, weight loss, apathy, breathing disorders, myoclonus, and eventually coma and death. Studies of the Prp^Sc found in FFI patients show a methionine at codon 129, as well as an asparagine mutation at codon 178.

Conclusion

What importance does prion propagation have to the field of microbiology?

Microbes are defined as living organisms that require a microscope to be seen; however, the jury is still out on whether even viruses are classified as living organisms, despite their classification as microbes. Other microbes include microscopic eukaryotes, bacteria, and archaea. Can prions, viroid-like particles with no other molecular parts, protective coats, or nucleic acids be classified as microbes? They represent an even smaller, simpler form of pathogenic material, that can propagate itself, perhaps without the aid of any other proteins, enzymes, or cofactors.

References

Andrew F. Hill, Susan Joiner, Jonathan D. F. Wadsworth, Katie C. L. Sidle, Jeanne E. Bell, Herbert Budka, James W. Ironside and John Collinge, "Molecular classification of sporadic Creutzfeldt-Jakob disease". Brain. 2003. Volume 126. p. 1333-1346.

Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.