Pathogenic effects of alpha-synuclein: modifications and toxicity

Accumulation of intraneuronal inclusions, containing mainly a protein called alpha-synuclein (asyn), is thought to play a critical role in neurodegenerative disorders such as Parkinson's disease (PD). In these so called Lewy bodies, the majority of asyn is phosphorylated at serine 129 (pS129) suggesting that post-translational modifications may play a critical role in the process of aggregate formation. To date, the precise pathological aspects and molecular mechanisms underlying asyn aggregation remain unclear. Previous studies have aimed to elucidate which region of human asyn (h-asyn) is influencing its aggregation properties and therefore possibly its toxicity towards neuronal cells. By investigating truncated h-asyn as well as naturally occurring asyn variants, it has been shown that synuclein lacking the central hydrophobic region is less prone to aggregate in vitro, although not all experimental studies support this view.To address this critical issue, we directly compared several human and rodent synuclein variants in a dose-depended manner to determine their toxic effects in vivo. Interestingly, we found that all human variants followed the same toxicity profile while rat asyn (r-asyn) did not induce neurodegeneration. Whereas phosphorylation levels of h-asyn was greater than that of r-asyn, increasing the amount of endogenous pS129 r-asyn did not result in neurotoxicity. Additionally, using our newly established duplexing assay to measure total and pS129 asyn simultaneously, we showed that endogenous pS129 h-asyn levels are naturally higher than that of r-asyn. This could point to phosphorylation being a side effect of h-asyn accumulation.Interestingly, both rat and mouse asyn are 95% homologous to h-asyn but show no accumulation or neurotoxicity over time in vivo. Furthermore, rodent asyn naturally contains a threonine at position 53 (T53), which in humans causes familial PD. To investigate why rodent asyn is less toxic especially when containing a T53 residue, we altered two C-terminally located amino acids of h-asynA53T to the mouse asyn (m-asyn) sequence and could demonstrate that dopaminergic neurodegeneration was reduced in the rat nigra. Moreover, mutating the C-terminal region of m-asyn to the human amino acids results in an increase of toxicity in vivo. These changes in the neurotoxicity profile could be due to the alterations we observed in the membrane-induced aggregation and vesicle disruption property of asyn.Taken together, these findings not only have an impact on our understanding of the formation and toxicity of asyn aggregates but could also lead to the development of novel therapeutic strategies targeting specifically the C-terminal region of h-asyn in PD