Fibrils are thought to act as reservoirs of these small pathological structures and as the natural end-point of the progressive growing of the aggregates 28. The minimum prion infectious particle has been described to be as small as a dimer/pentamer, whereas the most infectious is formed by 14–28 monomeric units 11. In prion diseases, oligomers have also been identified as highly toxic 20, 26, 27, as well as the most successful infectious entities 11, 12. Small Aβ aggregates such as dimers, trimers and the so called Aβ*56 have been described as highly toxic in vitro and in situ 19, 22, 23, 24, 25. In fact, neurodegeneration in the brain of AD patients has been suggested to better correlate with oligomeric amyloid-β (Aβ) than amyloid plaques 21. Although soluble and more sensitive to degradation than their fibrillar counterparts, misfolded oligomers have been placed as the main culprits for the toxicity observed in several PMDs 19, 20. However, several forms of prion diseases in humans or experimental animals lack noticeable fibrillar PrP deposits as analyzed by histological or biochemical techniques 17, 18. These fibrillar/dense aggregates were associated with cell toxicity, insolubility and resistance to biological clearance 16. For a long time, the presence of amyloid deposits has been used as the definitive ( post-mortem) diagnosis for several PMDs. Amyloid plaques (mostly composed of to fibrillar structures) were the first pathological entities described in other Protein Misfolding Disorders (PMDs) 16.
The profile of aggregation for prion polymers seems to depend on the specific conformation adopted by the constituent monomers 12, 15.ĭifferent biochemical and biological properties have been associated with misfolded protein aggregates of different sizes. As many natural and synthetic polymers, prion aggregates are thought to exist as a continuum of sizes, ranging from small oligomers to large fibrillar structures 11, 12, 13, 14. Misfolded prion polymers are thought to fragment, generating new “seeds” able to recruit monomeric PrP C units and resulting in an exponential generation of the infectious agent 2. The most important process related to prion infectivity involves the recruitment of the cellular prion (PrP C), a constitutively expressed protein, into the growing PrP Sc aggregate 1, 10. The mechanisms of prion virulence have been partially described 8, 9. The zoonotic potential of some animal prion diseases represents a persistent threat for public health 7. Despite its low incidence in humans, animal prionopathies can reach larger numbers in species such as deer, sheep and cattle 4, 5, 6. The accumulation of these particles in the brain is linked to neuronal death, synaptic alterations, brain inflammation and spongiform degenerationleading to various forms of prion diseases in humans and animals 1, 3. PrP Sc, the disease associated isoform of prions, is a polymer composed of many units of this protein arranged in an inter-molecular β-sheet conformation 1, 2. Our findings may contribute to understand the mechanisms of strain variation and the role of PrP Sc aggregates in prion-induced neurodegeneration. Therefore, the organization of PrP molecules in terms of the density of aggregates generated may determine some of the particular strain properties, whereas others are independent from it. Interestingly, no association was found between sensitivity to proteolytic degradation and aggregation profiles. The relative presence of PrP Sc in fractions of different sucrose densities was indicative of the protein deposits present in the brain as analyzed by histology. We observed a direct correlation between the density of the predominant PrP Sc aggregates and the incubation periods for the strains studied. We show that PrP Sc aggregates distribute in a wide range of arrangements and the relative proportion of each species depends on the prion strain. In this study, we used equilibrium sedimentation in sucrose density gradients to separate PrP Sc aggregates from three hamster prion strains (Hyper, Drowsy, SSLOW) subjected to minimal manipulations. An important question in the prion field has been to determine the identity of functional PrP Sc aggregates.
Prions are composed of the misfolded prion protein (PrP Sc) organized in a variety of aggregates.