Glycation modulates alpha-synuclein fibrillization kinetics: A sweet spot for inhibition

Glycation is a nonenzymatic posttranslational modification (PTM) known to be increased in the brains of hyperglycemic patients. Alpha-synuclein (alpha SN), a central player in the etiology of Parkinson's disease, can be glycated at lysine residues, thereby reducing alpha SN fibril formation in vitro and modulating alpha SN aggregation in cells. However, the molecular basis for these effects is unclear. To elucidate this, we investigated the aggregation of alpha SN modified by eight glycating agents, namely the dicarbonyl compound methylglyoxal (MGO) and the sugars ribose, fructose, mannose, glucose, galactose, sucrose, and lactose. We found that MGO and ribose modify alpha SN to the greatest extent, and these glycation products are the most efficient inhibitors of fibril formation. We show glycation primarily inhibits elongation rather than nucleation of alpha SN and has only a modest effect on the level of oligomerization. Furthermore, glycated alpha SN is not significantly incorporated into fibrils. For both MGO and ribose, we discovered that a level of similar to 5 modifications per alpha SN is optimal for inhibition of elongation. The remaining sugars showed a weak but optimal inhibition at similar to 2 modifications per alpha SN. We propose that this optimal level balances the affinity for the growing ends of the fibril (which decreases with the extent of modification) with the ability to block incorporation of subsequent alpha SN subunits (which increases with modification). Our results are not only relevant for other alpha SN PTMs but also for understanding PTMs affecting other fibrillogenic proteins and may thus open novel avenues for therapeutic intervention in protein aggregation disorders.