Glycated α-Synuclein Renders Glial Cell Activation and Induces Degeneration of Dopaminergic Neurons: A Potential Implication for the Development of Parkinson's Disease

Accumulation of misfolded alpha-synuclein (alpha-Syn) leads to the formation of Lewy bodies and is a major hallmark of Parkinson's disease (PD). The accumulation of alpha-Syn involves several post-translational modifications. Recently, though, glycation of alpha-Syn (advanced glycation end products) and activation of the receptor for advanced glycation end products (RAGE) have been linked to neuroinflammation, which leads to oxidative stress and accumulation of alpha-Syn. The present study aims to detect the effect of glycated alpha-Syn (gly-alpha-Syn)-induced synucleinopathy and loss of dopaminergic (DAergic) neurons in the development of PD. We isolated, purified, and prepared glycated recombinant human alpha-Syn using d-ribose. Gly-alpha-Syn was characterized by SDS-PAGE, intact mass analysis, and bottom-up peptide sequence through LC-HRMS/MS. The aggregation propensity of gly-alpha-Syn has been verified by morphological and shape analysis through Bio-AFM. The gly-alpha-Syn (2 mu g/mu L) was injected stereotaxically in the substantia nigra (SN) of ICR mice (3-4 months) and compared with the normal alpha-Syn, d ribose, and Tris-HCl/artificial CSF groups. 56 days postsurgery (DPS), an immunohistochemical examination was conducted to investigate gly-alpha-Syn-induced alpha-Syn accumulation, neuroinflammation, and neurodegeneration. The glycation of alpha-Syn led to the expression of transglutaminase 2 (TGM2), an enzyme that cross-linked with AGEs and may have caused the accumulation of alpha-Syn. Significant RAGE activation was also observed in gly-alpha-Syn, which might have induced glial cell activation, resulting in oxidative stress and, ultimately, apoptosis of dopaminergic neurons. It is important to note that TGM2, phosphorylated alpha-Syn, RAGE expression, and glial cell activation were only found in the gly-alpha-Syn group and not in the other groups. This suggests that gly-alpha-Syn plays a major role in synucleinopathy, neuroinflammation, and neurodegeneration. Overall, the present study demonstrated glycation of alpha-Syn as one of the important age-associated post-translational modifications that are involved in the degeneration of dopaminergic neurons, at least in a subset of the diabetic patients susceptible to developing PD.