The properties of α-synuclein secondary nuclei are dominated by the solution conditions rather than the seed fibril strain

Background

Amyloid fibrils are a hallmark of various neurodegenerative diseases. In Parkinson’s disease, fibrils formed by the protein α-synuclein are found in the characteristic Lewy Bodies in dopaminergic neurons. Recent evidence suggests that α-synuclein pathology may spread through the brain in a prion-like fashion, whereby existing fibrils serve as templates for the formation of new fibrils. Different mechanisms can lead to an increase in fibril mass: 1) primary nucleation, the first step for monomeric protein molecules to convert to a fibrillar structure; 2) fibril elongation, in which monomers are added to the ends of the growing filament; 3) secondary nucleation, which entails the formation of new nuclei catalysed at the surface of previously formed fibrils. Whereas it is well established that the structural features of fibrils are preserved upon elongation of short fibril seeds, the properties of fibrils generated by secondary nucleation with respect to the ‘parent’ fibrils have not yet been investigated.

 

Results of the preprint

The authors of this preprint – recently posted on ChemRxiv – make use of the fact that α-synuclein adopts fibrils with different properties depending on the buffer conditions in vitro, most notably pH and ionic strength. They first characterise Thioflavin T aggregation kinetics and morphology under three different buffer conditions, leading to fibrils with a ribbon-like appearance, twisted fibrils, and short needle-like fibrils, respectively. They also show that the needle-like fibrils are more sensitive to proteinase K digestion than the other two polymorphs, providing a straightforward read-out to distinguish this fibril type.

 

 

The authors then tweak the conditions of their aggregation assay to favour amplification of the fibril mass by elongation versus secondary nucleation. When a high concentration of seeds is provided, elongation dominates, and as expected the authors find that the fibril types are preserved even if they switch to a buffer condition that would lead to the formation of a different fibril type de novo. However, when they perform the assay with only a small amount of seeds and at lower pH, favouring secondary nucleation, they find that adding seeds of the ribbon and twisted fibrils leads to the formation of the needle type that would spontaneously form at this pH. Thus, secondary nucleation generates fibrils that have the morphology dictated by the buffer conditions, as if they were formed by primary nucleation.

 

Why I chose this preprint

The spreading of α-synuclein pathology in a prion-like fashion throughout the brain, and even starting from the gut (1), has been studied extensively in recent years (see (2-4) for reviews). Many questions remain about the molecular processes that underlie this phenomenon, and in particular about the propagation of different α-synuclein fibril types, or ‘strains’ analogously to prions (5).

This preprint is the first to show that the properties of α-synuclein fibrils are not retained if buffer conditions are changed and secondary nucleation is the dominant mechanism of fibril amplification. These findings are important for the interpretation of α-synuclein spreading experiments. Furthermore, they reveal fundamental insights into the process of secondary nucleation, which also occurs for other disease-associated proteins such as Alzheimer’s amyloid-β peptide, and is thought to be a key mechanism that generates toxic oligomeric species.

 

Questions

If the structural features of the fibrils are not preserved during secondary nucleation, do they matter at all? Can fibrils formed by different proteins promote each other’s aggregation by secondary nucleation? Could any other surface with the right charge, hydrophobicity etc. perform this role?

Do these results suggest that secondary nucleation plays a minor role in the spread of α-synuclein pathology in vivo, compared to fibril fragmentation followed by elongation? Would you expect α-synuclein fibrils to encounter different solution conditions while travelling from cell to cell?

 

References

1. Kim, S. et al. Transneuronal Propagation of Pathologic α-Synuclein from the Gut to the Brain Models Parkinson’s Disease. Neuron 103, 627-641.e7 (2019).
2. Dehay, B., Vila, M., Bezard, E., Brundin, P. & Kordower, J. H. Alpha-synuclein propagation: New insights from animal models. Mov. Disord. 31, 161–168 (2016).
3. Goedert, M., Masuda-Suzukake, M. & Falcon, B. Like prions: the propagation of aggregated tau and α-synuclein in neurodegeneration. Brain 140, 266–278 (2016).
4. Karpowicz, R. J., Trojanowski, J. Q. & Lee, V. M.-Y. Transmission of α-synuclein seeds in neurodegenerative disease: recent developments. Lab. Investig. 99, 971–981 (2019).
5. Peelaerts, W. et al. α-Synuclein strains cause distinct synucleinopathies after local and systemic administration. Nature 522, 340 (2015).

Tags: amyloid fibrils, secondary nucleation, α-synuclein

doi: https://doi.org/10.1242/prelights.13997

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