Modeling pathophysiological aspects of Parkinson’s disease: Manipulating DA handling and alpha-synuclein expression in the nigrostriatal pathway using viral vectors

Abstract: The pathological hallmark of Parkinson’s disease is dopaminergic neurodegeneration in the substantia nigra pars compacta neurons and accumulation of α-synuclein containing aggregates in the surviving neurons. The cause of cell death in Parkinson’s disease and the involvement of α-synuclein in the pathophysiology of the disease are unknown. Although other neuronal cell types exhibit α-synuclein positive aggregates, substantia nigra dopamine neurons display a selective vulnerability to α-synuclein mediated neurodegeneration. In this thesis work I have focused on the possible mechanisms underlying the vulnerability of dopamine producing neurons against α-synuclein induced neurotoxicity. To study the molecular interactions playing role in α-synuclein mediated dopaminergic neurodegeneration, we investigated putative mechanisms that has been implicated in α-synuclein toxicity. Interaction of the α-synuclein protein with other molecules has been suggested to enhance the aggregation. We studied the interaction between the full-length α-synculein protein and truncated α-synuclein in the rat substantia nigra. When the two forms are co-expressed the truncated form promotes full-length α-syn aggregation and enhance the pathology caused by the full-length protein. We next investigated the specific role of dopamine handling machinery in Parkinson’s disease pathophysiology and treatment related motor complications. To study the involvement of cytosolic DA and age related increase in the reactive DA species on α-syn toxicity, we utilized a transgenic mouse model carrying a hypomorphic VMAT2 mutation. The elevated cytosolic dopamine in these mice led to an increased vulnerability to α-synuclein overexpression. To show that this vulnerability was indeed dopamine dependent, we generated recombinant adeno-associated viral vectors to transfer short hairpin RNA sequences targeting the rate-limiting enzyme, tyrosine hydroxylase. Reducing dopamine production using the shRNA approach in these mice rescued the vulnerability against α-synuclein in the nigral dopamine neurons. Our results implicate the critical role of dopamine handling in Parkinson’s disease pathophysiology, thus suggest that regulating the specific pathways through which DA mediates its toxic effects can prevent the potential neurodegeneration.