J. Neurochem. (2011) 116, 588–605. Abstract Accumulation of misfolded proteins in the endoplasmic reticulum (ER) is the main event leading to the induction of the ER stress‐related unfolded protein response (UPR). Recent postmortem evaluation, showing that the UPR pathway is activated in nigral dopaminergic neurons bearing α‐synuclein inclusions in the brain of Parkinson’s disease (PD) patients, suggests that the activation of the UPR may be induced by the accumulation of α‐synuclein. In this study, we show that the misfolded protein‐sensor/UPR activator glucose‐regulated protein 78/immunoglobulin heavy chain‐binding protein was bound to α‐synuclein and was increased in ‘in vitro’ and ‘in vivo’ models showing aggregated α‐synuclein accumulation. Moreover, α‐synuclein accumulation induced the expression of the UPR‐related activating transcription factor 4/cAMP‐responsive element‐2. These findings indicate that activation of the UPR pathway in the PD brain is associated with α‐synuclein accumulation occurring in part within the ER.

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.

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, a-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 a-synuclein aggregation decreases the expression and membrane trafficking of the DAT as the DAT is retained into a-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 asynuclein are missing. By using the proximity ligation assay, a technique which allows the ''in situ'' visualization of proteinprotein interactions, we studied the occurrence of alterations in the distribution of DAT/a-synuclein complexes in the SYN120 transgenic mouse model, showing insoluble a-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/asynuclein 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, a-synuclein acts as a fine modulator of the dopaminergic synapse by regulating the subcellular distribution of key proteins such as the DAT.