Synapsin III gene silencing redeems alpha-synuclein transgenic mice from Parkinson's disease-like phenotype

Fibrillary aggregated alpha-synuclein (alpha-syn) deposition in Lewy bodies (LB) characterizes Parkinson's disease (PD) and is believed to trigger dopaminergic synaptic failure and a retrograde terminal-to-cell body neuronal degeneration. We described that the neuronal phosphoprotein synapsin III (Syn III) cooperates with alpha-syn to regulate dopamine (DA) release and can be found in the insoluble alpha-syn fibrils composing LB. Moreover, we showed that a-syn aggregates deposition, and the associated onset of synaptic deficits and neuronal degeneration occurring following adeno-associated viral vectors-mediated overexpression of human alpha-syn in the nigrostriatal system are hindered in Syn III knock out mice. This supports that Syn III facilitates alpha-syn aggregation. Here, in an interventional experimental design, we found that by inducing the gene silencing of Syn III in human alpha-syn transgenic mice at PD-like stage with advanced alpha-syn aggregation and overt striatal synaptic failure, we could lower alpha-syn aggregates and striatal fibers loss. In parallel, we observed recovery from synaptic vesicles clumping, DA release failure, and motor functions impairment. This supports that Syn III consolidates alpha-syn aggregates, while its downregulation enables their reduction and redeems the PD-like phenotype. Strategies targeting Syn III could thus constitute a therapeutic option for PD.