Acceleration of oligomerization, not fibrillization, is a shared property of both alpha-synuclein mutations linked to early-onset Parkinson's disease: Implications for pathogenesis and therapy

The Parkinson's disease (PD) substantia nigra is characterized by the prese nce of Lewy bodies containing fibrillar alpha-synuclein. Early-onset Po has been linked to two point mutations in the gene that encodes alpha-synuclei n, suggesting that disease may arise from accelerated fibrillization. Howev er, the identity of the pathogenic species and its relationship to the alph a-synuclein fibril has not been elucidated. In this in vitro study, the rat es of disappearance of monomeric alpha-synuclein and appearance of fibrilla r alpha-synuclein were compared for the wild-type (WT) and two mutant prote ins, as well as equimolar mixtures that may model the heterozygous PD patie nts. Whereas one of the mutant proteins (A53T) and an equimolar mixture of A53T and WT fibrillized more rapidly than WT alpha-synuclein, the other (A3 0P) and the corresponding equimolar mixture with WT fibrillized more slowly . However, under conditions that ultimately produced fibrils, the A30P mono mer was consumed at a comparable rate or slightly more rapidly than the WT monomer, whereas A53T was consumed even more rapidly. The difference betwee n these trends suggested the existence of nonfibrillar alpha-synuclein olig omers, some of which were separated from fibrillar and monomeric alpha-synu clein by sedimentation followed by gel-filtration chromatography. Spheres ( range of heights: 2-6 nm), chains of spheres (protofibrils), and rings rese mbling circularized protofibrils (height: ca. 4 nm) were distinguished from fibrils (height: ca. 8 nm) by atomic force microscopy. Importantly, drug c andidates that inhibit alpha-synuclein fibrillization but do not block its oligomerization could mimic the A30P mutation and thus may accelerate disea se progression.