General intro

Prion proteins are a common part of cell surface proteins in the mammalian nervous system, and they can, upon change of its conformation, become a highly infectious and pathogenic agent. The physiological form (PrPC) is encoded by the PRNP gene on chromosome 20 and when misfolded and aggregated, the pathological form (PrPSc) occurs, lacking any specific nucleic acid and its primary structure being determined by the PrPC form. When accumulated in the central nervous system (CNS) of mammals, PrPSc is known to be responsible for uprise of several untreatable progressive neurodegenerative diseases, generally called as transmissible spongiform encephalopathies (TSEs). These include kuru, fatal familial insomnia (FFI) and Creutzfeld-Jacob disease (CJD) in humans, bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep and goats, transmissible mink encephalopathy (TME) in mink, feline spongiform encephalopathy (FSE) in cat or chronic wasting disease (CWD) in deer and elk ^[1].

Protein structure

The PrPSc differs from PrPC solely in conformation and is its isoform. The mature PrPC consists of approx. 208 amino acids, arranged as a disordered N-terminus and a globular C-terminal domain consisting of three α-helices and a short, antiparallel β-pleated sheet ^[2][3]. There is a GPI membrane anchor at the C-terminus that tethers the protein to cell membranes and proteins that are secreted and lacking the anchor component has been proven to be unaffected by the infectious isoform ^[4]. In contrast to the natural form of prion protein with only about 3 % of β-sheet secondary structure, the PrPSc form has about 47 % of the secondary structure in β-sheets ^[5] that create a core of four-rung β-solenoid fold ^[6]. Accordingly, they also differ in their properties. PrPC is soluble, has a life-span between 2 and 4 hours, and is sensitive to proteolytic cleavage – when exposed to proteases, the protein is degraded completely ^[5]. The two most important cleavage events are the α cleavage which removes the unstructured N-terminal tail and leaves the globular domain attached to the cell membrane, and the cleavage on the C-terminal end (termed PrPC shedding) which releases PrPC into the extracellular space ^[7]. Under the same conditions, PrPSc is hydrolysed by proteases only partially by forming resistant core fragment PrP 27-30 ^[5]. In addition, it is insoluble in detergents and has a very long half-life, therefore accumulates in tissues easily. It has a tendency to form aggregates and fibrillar structures and is generally susceptible to oligomerization, whereas the PrPC form mainly exist as a monomer ^[8]. Monomeric PrPSc has never been isolated.

Misfolding

The fundamental event in propagation of the infectious form lies in the PrPSc template-directed misfolding of the natural form into the pathogenic, β-sheet-rich version of itself ^[5]. This process is now widely accepted as a current prion theory, and the most striking fact is that this action lacks any nucleic acid template ^[7]. However, the replication cycle of PrPSc does need the PRNP gene to direct PrPC synthesis. Also, the interaction between the pathogenic and physiological form must be quite specific to propagate the conversion. The replication process itself can be explained by stochastic fluctuations in the PrPC structure, that would create the intermediate form, PrP*. This partially unfolded monomer can then switch back to the natural conformation, adopt a PrPSc one, or be degraded. Normally, there is an equilibrium between the PrPC and PrP* states which favors the physiological one. Depending on the specific cause of the disease, the PrP* state can adopt a PrPSc conformation either upon contant with a dimer of this infectious form by forming a heteromultimer which is further converted into a homomultimer of PrPSc, or through encounter with another PrP* molecule. This PrP*/PrP* dimer can then form an infectious dimer and initiate the replication cycle ^{[9] [8]}. It has been found that certain host-specific RNAs can assist with the conversion into the pathogenic form ^[10].

Pathogenesis

In the case of infectious/iatrogenic diseases, pathogenic prion proteins enter the body alimentary via ingestion of affected neural tissues, or via infected materials and tissues such as during blood transfusions, corneal transplants or dura mater grafts. In this case, the exogenous PrPSc form would serve as a template to promote the conversion of PrP* and, due to its insolubility, make this exponential conversion process irreversible ^[9]. Genetic, or inherited cause comprise the familial TSEs and comes from DNA mutations in the PRNP gene, which then produces mutant, unstable forms of PrPC with higher tendency of folding into the PrP* form. This further increases the chance of PrPSc forming ^[8]. The mutations in PRNP are autosomal dominant, highly penetrant, and consist of missense mutations, insertions and deletions ^[7]. With higher probability of protein misfolding in the old age, they usually incite disease onset in the late decades of life and ultimately lead to accumulation of prion proteins and rapid development of neurodegenerative disease ^[11]. Concerning sporadic form of prion disease, the concentration of PrPSc may eventually reach a threshold level upon which a positive feedback loop would stimulate the formation of PrPSc. It requires solely a rare molecular event of formation of the PrP*/PrP* complex, or a somatic cell mutation followed by the mechanism of the initiation of inherited disease. Once formed, the replication cycle is primed for subsequent conversion ^{[9] [8]}. Ultimately, in all cases this leads to PrPSc polymerization, forming a rod-like structures and amyloid plaques.

Prion diseases

Up to this date, many different types of TSEs are known (see General intro), affecting many animal species as well as humans and showing various symptoms. As was mentioned in previous chapters, all prion diseases promote their negative effects through accumulation of PrPSc in the CNS. However, since most of the TSEs are transmitted by peripheral routes, either orally or transcutaneously, events critical for their pathogenesis take place at peripheral parts of the organism, especially in peripheral lymph organs (Aucouturier et al., 2000). In the following text, probably the two most important prion diseases and facts known about their mechanism of infection are described.

Bovine spongiform encephalopathy

Commonly known as the mad cow disease, bovine spongiform encephalopathy (BSE) is a type of prion disease that affects cattle. Among major symptoms observed in affected animals are abnormal behavior, anxiety, ataxia, hypersensitivity to touch and noise and poor body condition – from movement and posture problems all the way down up to paralysis. Onset symptoms usually emerge after 4-4.5 years from the infection ^[12]. From that point, the disease is very progressive in degeneration of animal’s nervous system and leads to its death, generally within the time horizon of weeks to months (Konold et al., 2004). Several types of the disease are distinguished: classic BSE (C-type BSE), L-type BSE and H-type BSE. Latter two types are considered to be sporadic, uncommon and classified as atypical since they arise spontaneously. H and L denotation has its origin in structural features of these two forms. The classic form, on the other hand, is classified as typical and arise most likely from ruminant-derived protein feed supplements (i.e. meat-and-bone meal) as epidemiological analyses of BSE-affected herds implied (Kimberlin and Wilesmith, 1994). After oral uptake of infected feed, it was found that PrPSc gather in some intestinal lymphatic tissues (mainly in Peyer’s patches of the distal ileum and also tonsils). Infectivity of BSE subsequently slowly spreads centripetally into the CNS, probably through the peripheral nervous system. However, it still is not clear, how the disease passes from intestinal mucosa to the lymphoid system of the cattle (Espinosa JC et. al., 2007).

References

1. ↑ Imran M, Mahmood S. An overview of animal prion diseases. Virol J. 2011 Nov 1;8:493. doi: 10.1186/1743-422X-8-493. PMID:22044871 doi:http://dx.doi.org/10.1186/1743-422X-8-493
2. ↑ Zahn R, Liu A, Luhrs T, Riek R, von Schroetter C, Lopez Garcia F, Billeter M, Calzolai L, Wider G, Wuthrich K. NMR solution structure of the human prion protein. Proc Natl Acad Sci U S A. 2000 Jan 4;97(1):145-50. PMID:10618385
3. ↑ Riek R, Hornemann S, Wider G, Billeter M, Glockshuber R, Wuthrich K. NMR structure of the mouse prion protein domain PrP(121-321). Nature. 1996 Jul 11;382(6587):180-2. PMID:8700211 doi:10.1038/382180a0
4. ↑ Chesebro B, Trifilo M, Race R, Meade-White K, Teng C, LaCasse R, Raymond L, Favara C, Baron G, Priola S, Caughey B, Masliah E, Oldstone M. Anchorless prion protein results in infectious amyloid disease without clinical scrapie. Science. 2005 Jun 3;308(5727):1435-9. doi: 10.1126/science.1110837. PMID:15933194 doi:http://dx.doi.org/10.1126/science.1110837
5. ↑ ^{5.0 5.1 5.2 5.3} Pan KM, Baldwin M, Nguyen J, Gasset M, Serban A, Groth D, Mehlhorn I, Huang Z, Fletterick RJ, Cohen FE, et al.. Conversion of alpha-helices into beta-sheets features in the formation of the scrapie prion proteins. Proc Natl Acad Sci U S A. 1993 Dec 1;90(23):10962-6. PMID:7902575
6. ↑ Wille H, Requena JR. The Structure of PrP(Sc) Prions. Pathogens. 2018 Feb 7;7(1). pii: pathogens7010020. doi: 10.3390/pathogens7010020. PMID:29414853 doi:http://dx.doi.org/10.3390/pathogens7010020
7. ↑ ^{7.0 7.1 7.2} Wille H, Requena JR. The Structure of PrP(Sc) Prions. Pathogens. 2018 Feb 7;7(1). pii: pathogens7010020. doi: 10.3390/pathogens7010020. PMID:29414853 doi:http://dx.doi.org/10.3390/pathogens7010020
8. ↑ ^{8.0 8.1 8.2 8.3} Cohen FE, Prusiner SB. Pathologic conformations of prion proteins. Annu Rev Biochem. 1998;67:793-819. doi: 10.1146/annurev.biochem.67.1.793. PMID:9759504 doi:http://dx.doi.org/10.1146/annurev.biochem.67.1.793
9. ↑ ^{9.0 9.1 9.2} doi: https://dx.doi.org/10.1126/science.7909169
10. ↑ Deleault NR, Lucassen RW, Supattapone S. RNA molecules stimulate prion protein conversion. Nature. 2003 Oct 16;425(6959):717-20. doi: 10.1038/nature01979. PMID:14562104 doi:http://dx.doi.org/10.1038/nature01979
11. ↑ Khanam H, Ali A, Asif M, Shamsuzzaman. Neurodegenerative diseases linked to misfolded proteins and their therapeutic approaches: A review. Eur J Med Chem. 2016 Nov 29;124:1121-1141. doi: 10.1016/j.ejmech.2016.08.006., Epub 2016 Aug 6. PMID:27597727 doi:http://dx.doi.org/10.1016/j.ejmech.2016.08.006
12. ↑ Casalone C, Hope J. Atypical and classic bovine spongiform encephalopathy. Handb Clin Neurol. 2018;153:121-134. doi: 10.1016/B978-0-444-63945-5.00007-6. PMID:29887132 doi:http://dx.doi.org/10.1016/B978-0-444-63945-5.00007-6

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