Fibril growth and seeding are key to Parkinson's disease

In Parkinson’s disease, neurons that regulate movement fill up with large clumps called “Lewy bodies”. These contain the protein alpha-synuclein, which aggregates through at least four “species”: from single molecules (monomers), to oligomers, to fibrils (stringy knots), which accumulate in Lewy bodies. It is unclear at which stage of this process alpha-synuclein begins to destroy neurons. EPFL scientists now show that this may be linked to the actual process of polymerization and fibril formation rather than a single aggregation species. The study, published in Cell Death & Differentiation, can lead to new treatments for Parkinson’s disease.

For the past two decades, scientists have been searching for the toxic form of alpha-synuclein with the hope of developing strategies for early detection and treatment of Parkinson’s disease. Nonetheless, important pieces of the puzzle are still missing: at which point along its aggregation does alpha-synuclein become dangerous to the cells – and how?

A deadly process

The lab of Hilal Lashuel at EPFL discovered that the answer lies in the ability of extracellular alpha-synuclein to grow and/or catalyze the formation of new fibrils that are formed inside the cells but also at the cell plasma membrane level. The research was led by Anne-Laure Mahul-Mellier and Filip Vercruysse.

The researchers tested the toxicity of different aggregation species for alpha-synuclein by utilizing their world-renowned expertise in synthesizing and producing custom aggregation species of the protein. They fed alpha-synuclein monomers, oligomers and fibrils to neurons in culture.

What they found was that none of the individual species killed the cells. But when they gave cells a mixture of pre-formed alpha-synuclein fibrils and monomers, neurons began to die. This showed that the toxicity of alpha-synuclein is not linked specifically to fibrils or oligomers, but to the actual process of fibril growth.

The drug connection

The research team also investigated the impact of drugs that were previously shown to stop alpha-synuclein from forming fibrils and/or block further growth or elongation of preformed fibrils. “Proteins like alpha-synuclein are very difficult to target as monomers,” says Hilal Lashuel. “Because they are unstructured, it is difficult to develop small molecules that can bind them, stabilize them, and prevent their aggregation. On the other hand, fibrils are highly structured, so it is much easier to target the ends of their structured regions.”

The team tested the Parkinson’s drug tolcapone on cultured neurons. In addition, they also gave them a different, non-toxic member of the synuclein family instead of alpha-synuclein.

With this two-step approach, the scientists were able to prevent the neurons from dying altogether. This further confirmed that it is the ongoing fibrilization of alpha-synuclein that destroys cells. The question, of course, is how.

Protein attachments

Using bioimaging, Lashuel’s team found evidence that exogenously added pre-fibrillar forms of alpha-synuclein anchor on their plasma membranes. Once attached, the fibrils not only grow, but also promote the growth of even more alpha-synuclein aggregation on their surfaces. These are generally known as “nucleation sites”.

The researchers propose a model: nucleation sites promote the growth of alpha-synuclein fibrils on membranes of neurons. These in turn disrupt the integrity of the membrane and that facilitate the entry of the lethal alpha-synuclein mixture. Through their actions on the plasma membranes and inside the neuron, the mixture triggers cell-death molecular pathways.

Based on their findings, the authors suggest that novel courses of treatment must aim to block extracellular alpha-synuclein from forming fibrils, as well as its ability to grow and seed the formation of new fibrils. This strategy provides opportunities to develop drugs that interfere with the disease progression even at later stages of the disease.

“This is a very effective therapeutic strategy,” says Hilal Lashuel. “Such drugs can inhibit the protein’s toxicity and prevent further spreading of Parkinson’s, while allowing the brain’s natural clearance mechanisms to detoxify it by removing aggregates.”

This work was funded by EPFL, the ERC, ERA-NET, and the Swiss National Science Foundation.

Reference

Mahul-Mellier A-L, Vercruysse F, Maco B, Ait-Bouziad N, De Roo M, Muller D, Lashuel HA. Fibril growth and seeding capacity play key roles in α-synuclein-mediated apoptotic cell death. Cell Death & Differentiation 03 July 2015. DOI: 10.1038/cdd.2015.79

Related references

Eleuteri S, Di Giovanni S, Rockenstein E, Mante M, Adame A, Trejo M, Wrasidlo W, Wu F, Fraering PC, Masliah E, Lashuel HA. Novel therapeutic strategy for neurodegeneration by blocking Aβ seeding mediated aggregation in models of Alzheimer's disease. Neurobiol Dis. 74:144-57. DOI: 10.1016/j.nbd.2014.08.017.

Jan A, Adolfsson O, Allaman I, Buccarello AL, Magistretti PJ, Pfeifer A, Muhs A, Lashuel HA. Abeta42 neurotoxicity is mediated by ongoing nucleated polymerization process rather than by discrete Abeta42 species. J Biol Chem. 286(10):8585-96; Mar 11, 2011. DOI: 10.1074/jbc.M110.172411.