Athough it has been established for over 100 years that Lewy bodies (LBs) represent the major pathological hallmark of Parkinson's disease (PD), we still do not know why these fibrillar intra-neuronal inclusions of the α-Synuclein (α-Syn) protein form, and how they contribute to disease progression. One of the major causes underlying this gap in knowledge is the unavailability of rodent animal models that truly reproduce the de novo formation of LBs. Although doubling α-Syn expression is sufficient to cause PD in humans, hα-Syn over-expression in rodents does not lead to the formation of authentic LB-like structures. This caveat has significantly hindered efforts towards understanding the mechanism underlying LB formation, their link to neurology, and the screening for drugs and pathways that block their formation. In the first chapter of this thesis, we show that the lack of hα-Syn fibrillization in rodent models can be attributed to interactions between hα-Syn and its endogenously expressed mouse α-Syn homologue. As such, hα-Syn over-expression on mouse α-Syn KO backgrounds provokes the formation of inclusions that reproduce several histological and biochemical features of LBs in cultured primary neurons, as well as in mouse brains in vivo. To unequivocally determine the authenticity of the formed inclusions, we performed correlative light/electron microscopy which established the fibrillar ultrastructure of the inclusions, and real-time FRAP/photo-conversion experiments which clearly revealed the incorporation of soluble protein into inclusions within living neurons. Moreover, we showed that the deletion of another endogenous α-Syn homologue (β-Syn) also provokes hα-Syn fibrillization, thereby proposing a role for these two homologous proteins as natural inhibitors of abnormal aggregation. These findings provide novel well-characterized primary neuronal and in vivo models which recapitulate some of the main molecular features of PD, including de novo α-Syn fibrillization. We believe that these models pave the way towards a systematic understanding the mechanisms underlying hα-Syn intra-neuronal fibrillization, as well as the contribution of this process to the pathogenesis of PD. These models also present powerful tools for screening pharmacologic and genetic modulators of fibrillization, monomer incorporation and clearance, all of which could help in generating novel therapeutic strategies for PD. In the second chapter of this thesis, we investigate how α-Syn PD-linked mutations cause pathology, independent from affecting α-Syn aggregation potential per se. Specifically, we explored how a novel α-Syn mutation (G51D) that has been recently identified in patients showing features of PD and multiple system atrophy, affects the biophysical and cellular properties of α-Syn. Indeed, our results suggest that unlike all other PD-linked mutations which have been shown to enhance α-Syn oligomerization and/or fibril formation, the G51D mutant aggregates significantly slower than WT α-Syn. In addition, the G51D mutant exhibits impaired membrane binding in vitro and in yeast, is secreted more rapidly by mammalian cells and is enriched in the nuclear compartment; an effect that is concomitant with enhanced S129 phosphorylation and exacerbated mitochondrial fragmentation. These results intriguingly suggest that PD-linked mutations may cause neurodegeneration via different mechanisms, some of which may be actually independent of α-Syn aggregation