Aggregation and toxicity of alpha-synuclein

Parkinson Disease is one of the most frequent neurodegenerative disorders worldwide. The aetiology of this disease is associated with the accumulation of a protein called alpha-synuclein in dopaminergic neurons of the substantia nigra. Alpha-synuclein is an intrinsically unstructured protein with the propensity to form amyloid fibrils. These originate by the assembly of soluble prefibrillar oligomeric aggregates, which are supposed to be the neurotoxic agents of the disease. We have previously demonstrated that the molecular chaperone Heat shock protein 90 (Hsp90) is able to modulate alpha-synuclein oligomerisation and fibril formation in an ATP-dependent mode. Such an active modulation of the alpha-synuclein assembly was previously unreported. Now, we intend to unveil the mechanistic and functional tie between Hsp90, the assembly of alpha-synuclein, and the occurrence of toxic alpha-synuclein species. To this end, we will adopt a biophysical and cell biological approach to analyse a) the role of naturally occurring alpha- synuclein mutations for the Hsp90 interaction, b) the influence of Hsp90 co-regulators, and c) the cytotoxicity of Hsp90-stabilised alpha-synuclein oligomers. This will substantially contribute to pinpoint the mechanisms that govern the formation of toxic alpha-synuclein folding species, which is indispensible for curing Parkinson Disease.

This research project examined cellular mechanisms that influence the process of amyloid aggregation of the protein alpha-synuclein. Amyloid protein fibres usually form consequently to a pathological change in the original three-dimensional protein structure, coming along with a pathogenic functional loss. Fibrous alpha-synuclein deposits accumulate mainly in motor neurons of the central nervous system and are linked to Parkinson disease. In detail, the effects of three different molecules were analysed, namely of the proteins Hsp90 and MOAG-4/SERF, and of the neurotransmitter serotonin. Based on in vitro studies, we could demonstrate that each of these molecules affects the polymerization of alpha- synuclein by a separate mechanism: A) Hsp90, a protein that usually stabilises protein folding intermediates, modulates the aggregation of alpha-synuclein under consumption of ATP. This process is affected by mutated, pathogenic forms of alpha-synuclein. Furthermore, Hsp90 acts independently of traditional partner proteins, which usually fine-tune the consumption of ATP. This suggests that Hsp90 and its partner proteins regulate this process decoupled from each other. B) MOAG-4/SERF, a recently discovered class of proteins of unknown function, is able to control the amyloid aggregation of alpha-synuclein by releasing intramolecular interactions of alpha-synuclein, thereby promotes its aggregation. C) serotonin, a neurotransmitter that plays a major role in non-motor symptoms of Parkinson disease, is able to binds to aggregating alpha-synuclein and interrupts its further polymerisation. The next question now is to determine how these different mechanisms affect the toxicity of the resulting aggregates, and how they might relate to the pathogenesis of Parkinson disease.