Nigrostriatal overabundance of α-synuclein leads to decreased vesicle density and deficits in dopamine release that correlate with reduced motor activity

α-Synuclein (α-syn) is a presynaptic protein present at most nerve terminals, but its function remains largely unknown. The familial forms of Parkinson's disease associated with multiplications of the α-syn gene locus indicate that overabundance of this protein might have a detrimental effect on dopaminergic transmission. To investigate this hypothesis, we use adeno-associated viral (AAV) vectors to overexpress human α-syn in the rat substantia nigra. Moderate overexpression of either wild-type (WT) or A30P α-syn differs in the motor phenotypes induced, with only the WT form generating hemiparkinsonian impairments. Wild-type α-syn causes a reduction of dopamine release in the striatum that exceeds the loss of dopaminergic neurons, axonal fibers, and the reduction in total dopamine. At the ultrastructural level, the reduced dopamine release corresponds to a decreased density of dopaminergic vesicles and synaptic contacts in striatal terminals. Interestingly, the membrane-binding-deficient A30P mutant does neither notably reduce dopamine release nor it cause ultrastructural changes in dopaminergic axons, showing that α-syn's membrane-binding properties are critically involved in the presynaptic defects. To further determine if the affinity of the protein for membranes determines the extent of motor defects, we compare three forms of α-syn in conditions leading to pronounced degeneration. While membrane-binding α-syns (wild-type and A53T) induce severe motor impairments, an N-terminal deleted form with attenuated affinity for membranes is inefficient in inducing motor defects. Overall, these results demonstrate that α-syn overabundance is detrimental to dopamine neurotransmission at early stages of the degeneration of nigrostriatal dopaminergic axon

Nigrostriatal overabundance of alpha-synuclein leads to decreased vesicle density and deficits in dopamine release that correlate with reduced motor activity

alpha-Synuclein (alpha-syn) is a presynaptic protein present at most nerve terminals, but its function remains largely unknown. The familial forms of Parkinson's disease associated with multiplications of the alpha-syn gene locus indicate that overabundance of this protein might have a detrimental effect on dopaminergic transmission. To investigate this hypothesis, we use adeno-associated viral (AAV) vectors to overexpress human alpha-syn in the rat substantia nigra. Moderate overexpression of either wild-type (WT) or A30P alpha-syn differs in the motor phenotypes induced, with only the WT form generating hemiparkinsonian impairments. Wild-type alpha-syn causes a reduction of dopamine release in the striatum that exceeds the loss of dopaminergic neurons, axonal fibers, and the reduction in total dopamine. At the ultrastructural level, the reduced dopamine release corresponds to a decreased density of dopaminergic vesicles and synaptic contacts in striatal terminals. Interestingly, the membrane-binding-deficient A30P mutant does neither notably reduce dopamine release nor it cause ultrastructural changes in dopaminergic axons, showing that alpha-syn's membrane-binding properties are critically involved in the presynaptic defects. To further determine if the affinity of the protein for membranes determines the extent of motor defects, we compare three forms of alpha-syn in conditions leading to pronounced degeneration. While membrane-binding alpha-syns (wild-type and A53T) induce severe motor impairments, an N-terminal deleted form with attenuated affinity for membranes is inefficient in inducing motor defects. Overall, these results demonstrate that alpha-syn overabundance is detrimental to dopamine neurotransmission at early stages of the degeneration of nigrostriatal dopaminergic axons

Motor Deficits upon Alpha-Synuclein Expression in the Nigrostriatal System Are Due to a Loss of Dopaminergic Vesicles and Reduced Dopamine Release at an early Stage of Neurodegeneration

Alpha-synuclein is linked to both sporadic and familial forms of Parkinson's disease. The protein represents the major component of Lewy bodies – one of the hallmarks of the disease. Additionally, several point mutations and locus multiplications in the gene for α-synuclein lead to familial parkinsonism. Interestingly, in conditions of a hereditary excess of wild-type protein, the difference in clinical severity between duplication and triplication carriers seems to suggest a direct relation to gene dosage. In the adult brain, where the protein is highly expressed, α-synuclein is thought to participate in regulating multiple crucial cellular functions related to neurotransmission. While the clinical motor symtoms of Parkinson's disease are usually attributed to a massive cell loss in the substantia nigra, we investigate how a pathogenic overabundance of wild-type α-synuclein can lead to parkinsonian symptoms as a direct result of a perturbation in its normal endogenous functions, and before an overt lesion. We show that mild overexpression of wild-type α-synuclein in the rat substantia nigra induces motor behavior attributable to disturbances in dopamine transmission, concomitant with early neurodegenerative events. Importantly, these changes are tied to the membrane association of α-synuclein, as they fail to appear upon overexpression of the binding-deficient A30P mutant. Moreover, we correlate behavioral deficits with a reduction of dopaminergic vesicles in nigrostriatal axons – showing ultrastructurally how overexpressed α-synuclein leads to deficits in neurotransmission. We demonstrate that impaired dopaminergic function can lead to Parkinson-related symtoms at an early stage of degeneration, and might indeed represent a primary step towards the demise of nigral neurons