Investigating the effect of Parkinson's disease-causing alpha-synuclein mutations on alpha-synuclein aggregation

Parkinson's disease (PD) is the second most common age-related neurodegenerative disorder and it is clinically characterized by tremor of the hands, arms, and legs, rigidity of the limbs and trunk, and slowness of movement. Pathologically, PD is characterized by the selective loss of dopaminergic neurons in the substantia nigra (SNpc) of the brain and the formation of Lewy bodies (LBs) in the neuronal cells. LBs are abnormal cytoplasmic protein aggregates and the primary structural component of LBs is alpha-synuclein (αSyn) which deposits as β-sheet rich fibrils. αSyn is a natively unfolded presynaptic protein consisted of 140 amino acids and is believed to play a role in synaptic vesicle recycling, storage and compartmentalization of neurotransmitters by associating with vesicular and membranous structures. Studies have shown that αSyn aggregation is involved in the pathogenesis of PD. Recently, a histidine-to-glutamine substitution at residue 50 of αSyn was identified as a new familial PD mutation. In this research, we found that H50Q substitution leads to an increase of aggregation propensity of αSyn in in vitro aggregation assay. Addtionally, nuclear magnetic resonance (NMR) studies revealed that the H50Q substitution causes an increase of the C-terminal region flexibility. This finding provides direct evidence that this PD-causing mutant can mediate long-range effects on the sampling of αSyn conformations. In vitro aggregation assays showed that substitution of His50 with Gln(Q), Asp(D) or Ala(A) promotes αSyn aggregation, whereas substitution with the positively-charged Arg(R) suppresses αSyn aggregation. At physiological pH, histidine residue is partially positive-charged, thus our result suggests that the positively-charged His50 may play a role in protecting αSyn from aggregation under physiological conditions.