The protein alpha-synuclein (α-synuclein) accumulates in the brain in disorders such as Parkinson’s disease (PD) and dementia with Lewy bodies (DLB). It is believed that the monomeric form of α-synuclein can adopt a partially folded structure and start to aggregate and form intermediately sized oligomers or protofibrils. The aggregation process can continue with the formation of insoluble fibrils, which are deposited as Lewy bodies. The oligomers/protofibrils have been shown to be toxic to neurons and are therefore believed to be involved in the pathogenesis of the actual diseases.       The overall aims of this thesis were to investigate the properties of α-synuclein oligomers and to generate and characterize antibodies against these species. In addition, the potential for immunotherapy of the α-synuclein oligomer-selective antibodies were evaluated in a transgenic mouse model with α-synuclein pathology. Stable, β-sheet rich α-synuclein oligomers were induced by incubation with either one of the reactive aldehydes 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE). The oligomers exhibited distinct morphological properties, although both types were toxic when added to a neuroblastoma cell line. The seeding effects of ONE-induced oligomers were studied in vitro and in vivo. The oligomers induced seeding of monomeric α-synuclein in a fibrillization assay but not in a cell model or when injected intracerebrally in transgenic mice. It seemed, however, as if the oligomers affected α-synuclein turnover in the cell model. By immunizing mice with HNE-induced oligomers antibody producing hybridomas were generated. Three monoclonal antibodies were found to have strong selectivity for α-synuclein oligomers. These antibodies recognized Lewy body pathology in brains from patients with PD and DLB as well as inclusions in the brain from young α-synuclein transgenic mice, but did not bind to other amyloidogenic proteins. Finally, immunotherapy with one of the oligomer/protofibril selective antibodies resulted in lower levels of such α-synuclein species in the spinal cord of α-synuclein transgenic mice. To conclude, this thesis has focused on characterizing properties of α-synuclein oligomers. In particular, antibodies selectively targeting such neurotoxic forms were generated and evaluated for passive immunization in a transgenic mouse model. Such immunotherapy may represent a future treatment strategy against Lewy body disorders.

Parkinson’s disease, dementia with Lewy bodies and multiple system atrophy are disorders featuring accumulation of Lewy bodies in brain. The main component of these large insoluble intracellular inclusions is the presynaptic protein alpha-synuclein (α-synuclein). It is generally believed that α-synuclein monomers adopt an abnormal conformation that favors the formation of soluble oligomers or protofibrils and, eventually, insoluble fibrils depositing as Lewy bodies. Notably, the intermediately sized oligomers/protofibrils seem to have particular neurotoxic effects. Several factors may influence the formation of α-synuclein oligomers/protofibrils, e.g. the reactive aldehydes 4-hydroxy-2-nonenal (HNE) and 4-oxo-2-nonenal (ONE) formed during oxidative stress. The overall aims of this thesis were to investigate biophysical and biochemical properties of in vitro generated α-synuclein oligomers, characterize their functional effects on cell and animal disease models as well as to explore whether their formation could be prevented in a cell culture model for oligomerization.  Here, it was found that α-synuclein rapidly formed oligomers after incubation with both ONE and HNE. The resulting oligomers were stable and did not continue to form insoluble fibrils. By comparing HNE- and ONE induced α-synuclein oligomers biochemically they were both found to exhibit extensive β-beta sheet structure and had a molecular size of ~2000 kDa. However, they differed in morphology; the ONE induced α-synuclein oligomers described round amorphous species whereas the HNE induced α-synuclein oligomers appeared as elongated protofibril-like structures. Both these oligomers were cell internalized to varying degrees and induced toxicity in neuroblastoma cells. In addition, the ONE induced α-synuclein oligomers seemed to initiate aggregation of monomeric α-synuclein in vitro, but failed to do so in vivo. Finally, treatment of α-synuclein overexpressing cells with monoclonal antibodies specific for α-synuclein significantly reduced aggregation and lowered levels of the protein, suggesting increased turnover in these cells.  To conclude, this thesis has characterized different oligomeric α-synuclein species, which may have properties similar to soluble species central to the pathogenesis of Parkinson’s disease and other disorders with α-synuclein pathology. For therapeutic strategies it is important to selectively target such harmful protein species and avoid interaction with other forms of α-synuclein, which may have vital physiological cellular functions.