Alpha-Synuclein Effects at the ER-to-Golgi Level and Potential Biomarkers in Rat Models of the Early Phase of Parkinson's Disease

While Parkinson's disease has been described nearly 200 years ago, the mechanisms leading to the degeneration of selectively vulnerable populations of neurons, such as dopaminergic neurons in the substantia nigra, remain mostly unknown. Our poor understanding of the disease etiology has dramatically hampered the rational development of therapies interfering with the processes underlying neuronal degeneration. Current therapeutic approaches provide symptomatic relief, but fail to slow down the course of Parkinson's disease. In addition, the diagnosis of Parkinson's disease relies primarily on the clinical assessment of motor symptoms that become detectable only when a large part of the nigral dopaminergic neurons have already degenerated. There is therefore a strong need to better understand the pathological processes underlying early stages of Parkinson's disease to improve the tools for diagnosis as well as to develop effective disease-modifying therapies. Towards these goals, it is important to develop, analyze and validate animal models, which faithfully replicate the human disease. In particular, the induced degeneration of nigral dopaminergic neurons in the adult mammalian brain provides an experimental tool to evaluate in vivo how the disease process leads to motor symptoms. Although neurotoxins such as 6-hydroxydopamine produce a selective degeneration of nigral dopaminergic neurons, their mode of action is clearly different from what takes places in the human pathology. During the last decade, the identification of genes linked to familial inheritance of Parkinson's disease led to the development of novel animal models based on the actual genetic cause of these rare forms of the disease. Alpha-synuclein, an abundant brain protein encoded by a gene involved in autosomal dominant forms of Parkinson's disease, is now considered a primary actor in the pathogenesis of this neurodegenerative disorder. Our laboratory has developed a rat model of Parkinson's disease based on the over-expression of human α-synuclein in the rat substantia nigra using adeno-associated viral vectors. In this model, α-synuclein over-expression progressively leads to a moderate loss of nigral dopaminergic neurons. Remarkably, animals develop motor deficits that are partly due to impaired neurotransmitter release in the remaining dopaminergic axons. This model system, where some nigral dopaminergic neurons survive in presence of overabundant human α-synuclein, can be used to explore early pathologic mechanisms that undermine the nigrostriatal system prior to outright neurodegeneration. The first part of this thesis investigates the detrimental effects of α-synuclein accumulation on the secretory pathway at the level of the endoplasmic reticulum (ER) and Golgi apparatus in the rat substantia nigra. We found that a significant proportion of the nigral neurons expressing α-synuclein display a pathological fragmentation of the Golgi apparatus. This effect has been explored