Biophysical properties and cellular toxicity of covalent crosslinked oligomers of alpha-synuclein formed by photoinduced side-chain tyrosyl radicals
Alpha-synuclein (alpha S), a 140 amino acid presynaptic protein, is the major component of the fibrillar aggregates (Lewy bodies) observed in dopaminergic neurons of patients affected by Parkinson's disease. It is currently believed that noncovalent oligomeric forms of alpha S, arising as intermediates in its aggregation, may constitute the major neurotoxic species. However, attempts to isolate and characterize such oligomers in vitro, and even more so in living cells, have been hampered by their transient nature, low concentration, polymorphism, and inherent instability. In this work, we describe the preparation and characterization of low molecular weight covalently bound oligomeric species of alpha S obtained by crosslinking via tyrosyl radicals generated by blue-light photosensitization of the metal coordination complex ruthenium (II) tris-bipyridine in the presence of ammonium persulfate. Numerous analytical techniques were used to characterize the alpha S oligomers: biochemical (anion-exchange chromatography, SOS-PAGE, and Western blotting); spectroscopic (optical: UV/Vis absorption, steady state, dynamic fluorescence, and dynamic light scattering); mass spectrometry; and electrochemical. Light-controlled protein oligomerization was mediated by formation of Tyr-Tyr (dityrosine) dimers through -C-C- bonds acting as covalent bridges, with a predominant involvement of residue Y39. The diverse oligomeric species exhibited a direct effect on the in vitro aggregation behavior of wild-type monomeric alpha S, decreasing the total yield of amyloid fibrils in aggregation assays monitored by thioflavin T (ThioT) fluorescence and light scattering, and by atomic forte microscopy (AFM). Compared to the unmodified monomer, the photoinduced covalent oligomeric species demonstrated increased toxic effects on differentiated neuronal-like SH-SY5Y cells. The result's highlight the importance of protein modification induced by oxidative stress in the initial molecular events leading to Parkinson's disease. (C) 2012 Elsevier Inc. All rights reserved.