From Seeds to Fibrils and Back: Fragmentation as an Overlooked Step in the Propagation of Prions and Prion-Like Proteins

Abstract

Many devastating neurodegenerative diseases are driven by the misfolding of normal proteins into a pathogenic abnormal conformation. Examples of such protein misfolding diseases include Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, amyotrophic lateral sclerosis, and prion diseases. The misfolded proteins involved in these diseases form self-templating oligomeric assemblies that recruit further correctly folded protein and induce their conversion. Over time, this leads to the formation of high molecular and mostly fibrillar aggregates that are increasingly inefficient at converting normal protein. Evidence from a multitude of in vitro models suggests that fibrils are fragmented to form new seeds, which can convert further normal protein and also spread to neighboring cells as observed in vivo. While fragmentation and seed generation were suggested as crucial steps in aggregate formation decades ago, the biological pathways involved remain largely unknown. Here, we show that mechanisms of aggregate clearance—namely the mammalian Hsp70–Hsp40–Hsp110 tri-chaperone system, macro-autophagy, and the proteasome system—may not only be protective, but also play a role in fragmentation. We further review the challenges that exist in determining the precise contribution of these mechanisms to protein misfolding diseases and suggest future directions to resolve these issues.

Unzipping the Secrets of Amyloid Disassembly by the Human Disaggregase

“…Despite the key function of the proteasome in the protein quality control network, under severe aggregation protein stress, it could play an opposite role by fragmenting and producing more seeds in cells [120]. Autophagy has also been shown to switch roles between seeding and aggregate clearance depending on the level of autophagic flux [121,122].…”

“…Despite the key function of the proteasome in the protein quality control network, under severe aggregation protein stress, it could play an opposite role by fragmenting and producing more seeds in cells [120]. Autophagy has also been shown to switch roles between seeding and aggregate clearance depending on the level of autophagic flux [121,122]. In a healthy cell state (low aggregate:disaggregase ratio; green arrow), the human disaggregase rapidly disassemble seed-competent intermediates into the native state, avoiding their toxic effect.…”

Size-dependent secondary nucleation and amplification of α-synuclein amyloid fibrils

“…It has been established that fibril fragmentation is critical for the propagation of misfolded prion proteins (Pezza and Serio, 2007, Kundel et al, 2018, Scheckel and Aguzzi, 2018, Marrero-Winkens et al, 2020. To explore, how the fragmentation-dependent seed length of α-Syn fibrils determines the amyloid amplification mechanism and disease-associated cellular activities, we prepared α-Syn fibrils by incubation under physiological conditions and rationally generated different-sized fibril fragments by controlled time-dependent sonication (Figures S1).…”

“…Intriguingly, the mechanism of α-Syn aggregation is a nucleation-dependent polymerization phenomenon (Wood et al, 1999), wherein the natively unstructured α-Syn monomers undergo self-assembly and lead to the formation of thermodynamically stable amyloid aggregates (Chiti and Dobson, 2006) through various microscopic processes (Cohen et al, 2012. The rate of formation of these assemblies can be regulated by dominant secondary processes including fibril fragmentation and seeded-nucleation events (Xue et al, 2008, Knowles et al, 2009, Arosio et al, 2015, Marrero-Winkens et al, 2020. For instance; in the seeded aggregation model, the assembly of amyloid fibrils occurs either through templated addition of monomers on the growth competent ends of preformed fibrils (PFF) via fibril elongation (Collins et al, 2004, Ferrone, 1999, Xue, 2015, Pinotsi et al, 2014, or interaction/binding of monomers on the catalytic surface of fibril species by surface-mediated secondary nucleation (Zimmermann et al, 2021, Gaspar et al, 2017, Kumari et al, 2021.…”

Amyloid conformation-dependent disaggregation revealed by a reconstituted yeast prion system

“…However, disaggregation could result in production of new amyloid fragments (seeds), which can enhance successive accumulation of amyloid and thereby exacerbate disease pathophysiology. In fact, disaggregation-mediated production of seeds may contribute to cell-to-cell transmission of amyloid, which is universal and at least partly account for disease mechanisms in many neurodegenerative diseases 3,4 . These notions highlight the importance of our better understanding of amyloid disaggregation process.…”