Generation of G51D and 3D mice reveals decreased -synuclein tetramer-monomer ratios promote Parkinson's disease phenotypes

Mutations in the alpha-Synuclein (alpha S) gene promote alpha S monomer aggregation that causes neurodegeneration in familial Parkinson's disease (fPD). However, most mouse models expressing single-mutant alpha S transgenes develop neuronal aggregates very slowly, and few have dopaminergic cell loss, both key characteristics of PD. To accelerate neurotoxic aggregation, we previously generated fPD alpha S E46K mutant mice with rationally designed triple mutations based on the alpha-helical repeat motif structure of alpha S (fPD E46K -> 3 K). The 3 K variant increased alpha S membrane association and decreased the physiological tetramer:monomer ratio, causing lipid- and vesicle-rich inclusions and robust tremor-predominant, L-DOPA responsive PD-like phenotypes. Here, we applied an analogous approach to the G51D fPD mutation and its rational amplification (G51D -> 3D) to generate mutant mice. In contrast to 3 K mice, G51D and 3D mice accumulate monomers almost exclusively in the cytosol while also showing decreased alpha S tetramer:monomer ratios. Both 1D and 3D mutant mice gradually accumulate insoluble, higher-molecular weight alpha S oligomers. Round alpha S neuronal deposits at 12 mos immunolabel for ubiquitin and pSer129 alpha S, with limited proteinase K resistance. Both 1D and 3D mice undergo loss of striatal TH+ fibers and midbrain dopaminergic neurons by 12 mos and a bradykinesia responsive to L-DOPA. The 3D alpha S mice have decreased tetramer:monomer equilibria and recapitulate major features of PD. These fPD G51D and 3D mutant mice should be useful models to study neuronal alpha S-toxicity associated with bradykinetic motor phenotypes.