Abstract
Alpha-synuclein, the major component of Lewy bodies, is thought to play a central role in the onset of synaptic dysfunctions in Parkinson's disease (PD). In particular, α-synuclein may affect dopaminergic neuron function as it interacts with a key protein modulating dopamine (DA) content at the synapse: the DA transporter (DAT). Indeed, recent evidence from our “in vitro” studies showed that α-synuclein aggregation decreases the expression and membrane trafficking of the DAT as the DAT is retained into α-synuclein-immunopositive inclusions. This notwithstanding, “in vivo” studies on PD animal models investigating whether DAT distribution is altered by the pathological overexpression and aggregation of α-synuclein are missing. By using the proximity ligation assay, a technique which allows the “in situ” visualization of protein-protein interactions, we studied the occurrence of alterations in the distribution of DAT/α-synuclein complexes in the SYN120 transgenic mouse model, showing insoluble α-synuclein aggregates into dopaminergic neurons of the nigrostriatal system, reduced striatal DA levels and an altered distribution of synaptic proteins in the striatum. We found that DAT/α-synuclein complexes were markedly redistributed in the striatum and substantia nigra of SYN120 mice. These alterations were accompanied by a significant increase of DAT striatal levels in transgenic animals when compared to wild type littermates. Our data indicate that, in the early pathogenesis of PD, α-synuclein acts as a fine modulator of the dopaminergic synapse by regulating the subcellular distribution of key proteins such as the DAT.
Highlights
Parkinson’s disease (PD) is characterized by a progressive loss of dopamine (DA) neurons of the nigrostriatal system and by the presence of Lewy bodies (LB), proteinaceous inclusions mainly composed by filamentous a-synuclein aggregates [1,2,3].Alpha-synuclein is a natively unfolded protein which plays a central role in the control of dopaminergic neuronal functions [3;4] and which is thought to be critically implicated in PD pathophysiology
In order to assess whether DA transporter (DAT) and a-synuclein complexes are detectable by the in situ proximity ligation assay (PLA) we used an ‘‘in vitro’’ cell system which has been extensively characterized by our group: dopaminergic differentiated SH-SY5Y (SH-SY5Y+) cells subjected to glucose deprivation, where we previously demonstrated the occurrence of DAT interaction with both full length and truncated a-synuclein [14]
We found that in these cells, glucose deprivation (GD) stimulated a-synuclein aggregation and that this event decreased DAT membrane levels, as the DAT and a-synuclein are co-localized in intracytoplasmic inclusions following the Glucose deprivation (GD) insult [14]
Summary
Parkinson’s disease (PD) is characterized by a progressive loss of dopamine (DA) neurons of the nigrostriatal system and by the presence of Lewy bodies (LB), proteinaceous inclusions mainly composed by filamentous a-synuclein aggregates [1,2,3].Alpha-synuclein is a natively unfolded protein which plays a central role in the control of dopaminergic neuronal functions [3;4] and which is thought to be critically implicated in PD pathophysiology. Recent findings showed that decreased putaminal DA transporter (DAT) binding and DA deficits occur in patients bearing nigral a-synuclein burden [5] shading light upon the notion that, in the PD brain, asynuclein deposition in the substantia nigra inversely correlates with striatal DAT functions. This is a relevant observation, as the DAT acts as a key modulator of dopaminergic signalling by mediating rapid clearance of DA from the synaptic cleft [6;7].
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