Mitochondrial Dysfunction and α-Synuclein Synaptic Pathology in Parkinson’s Disease: Who’s on First?

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. Its characteristic neuropathological features encompass the loss of dopaminergic neurons of the nigrostriatal system and the presence of Lewy bodies and Lewy neurites. These are intraneuronal and intraneuritic proteinaceous insoluble aggregates whose main constituent is the synaptic protein α-synuclein. Compelling lines of evidence indicate that mitochondrial dysfunction and α-synuclein synaptic deposition may play a primary role in the onset of this disorder. However, it is not yet clear which of these events may come first in the sequel of processes leading to neurodegeneration. Here, we reviewed data supporting either that α-synuclein synaptic deposition precedes and indirectly triggers mitochondrial damage or that mitochondrial deficits lead to neuronal dysfunction and α-synuclein synaptic accumulation. The present overview shows that it is still difficult to establish the exact temporal sequence and contribution of these events to PD

Mitochondrial Dysfunction and Alpha-Synuclein Synaptic Pathology in Parkinson’s Disease: Who’s on First?

Parkinson’s disease (PD) is the most common neurodegenerative movement disorder. Its characteristic neuropathological features encompass the loss of dopaminergic neurons of the nigrostriatal system and the presence of Lewy bodies and Lewy neurites. hese are intraneuronal and intraneuritic proteinaceous insoluble aggregates whose main constituent is the synaptic protein �-synuclein. Compelling lines of evidence indicate that mitochondrial dysfunction and�-synuclein synaptic deposition may play a primary role in the onset of this disorder. However, it is not yet clear which of these events may come irst in the sequel of processes leading to neurodegeneration. Here, we reviewed data supporting either that�-synuclein synaptic deposition precedes and indirectly triggers mitochondrial damage or that mitochondrial deicits lead to neuronal dysfunction and�-synuclein synaptic accumulation. he present overview shows that it is still diicult to establish the exact temporal sequence and contribution of these events to PD

From α-synuclein to synaptic dysfunctions: new insights into the pathophysiology of Parkinson's disease

Alpha-synuclein is a natively unfolded protein playing a key role in the regulation of several neuronal synaptic functions in physiological and pathological conditions. Many studies, over the past years, have shown that it is actively involved in PD pathophysiology. Alpha-synuclein is integrated in a complex network of neuronal processes through the interaction with cytosolic and synaptic proteins. Hence, it is not the sole α-synuclein pathology but its effects on diverse protein partners and specific cellular pathways in the membrane and/or cytosolic districts such as endoplasmic reticulum/Golgi, axonal and synaptic compartments of dopaminergic neurons, that may cause the onset of neuronal cell dysfunction and degeneration which are among the key pathological features of the PD brain. Here we summarize a series of experimental data supporting that α-synuclein aggregation may induce dysfunction and degeneration of synapses via these multiple mechanisms. Taken together, these data add new insights into the complex mechanisms underlying synaptic derangement in PD and other α-synucleinopathies. This article is part of a Special Issue entitled: Brain Integration

Alpha-synuclein modulates NR2B-containing NMDA receptors and decreases their levels after rotenone exposure

Alpha-synuclein (α-syn) is the main protein component of Lewy bodies (LBs), that together with nigrostriatal dopamine neuron loss constitute typical pathological hallmarks of Parkinson's disease (PD). Glutamate N-methyl-d-aspartate receptor (NMDAR) abnormalities, peculiarly involving NR2B-containing NMDAR, have been observed in the brain of PD patients and in several experimental models of the disease. Recent findings, indicating that α-syn can modulate NMDAR trafficking and function, suggest that this protein may be a pivotal regulator of NMDAR activity. Prompted by these evidences, we used fluorescence immunocytochemistry, western blotting and ratiometric Ca(2+) measurements to investigate whether wild type (wt) or C-terminally truncated α-syn can specifically modulate NR2B-containing NMDAR levels, subcellular trafficking and function. In addition, we evaluated whether the exposure of primary cortical neurons to increasing concentrations of rotenone could differentially regulate NR2B levels and cell viability in the presence or in the absence of α-syn. Our results indicate that both wt and C-terminally truncated α-syn negatively modulate NR2B-containing NMDAR levels, membrane translocation and function. Moreover, we found that absence of α-syn abolishes the rotenone-dependent decrease of NR2B levels and reduces neuronal vulnerability in primary cortical neurons. These findings suggest that α-syn can modulate neuronal resilience by regulating NR2B-containing NMDAR, whose specific alterations could connect α-syn pathology to neuronal degeneration in PD

Redistribution of DAT/α-synuclein complexes visualized by "in situ" proximity ligation assay in transgenic mice modelling early Parkinson's disease.

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

Depletion of Progranulin Reduces GluN2B-Containing NMDA Receptor Density, Tau Phosphorylation, and Dendritic Arborization in Mouse Primary Cortical Neurons

Loss-of-function mutations in the progranulin (PGRN) gene are a common cause of familial frontotemporal lobar degeneration (FTLD). This age-related neurodegenerative disorder, characterized by brain atrophy in the frontal and temporal lobes and such typical symptoms as cognitive and memory impairment, profound behavioral abnormalities, and personality changes is thought to be related to connectome dysfunctions. Recently, PGRN reduction has been found to induce a behavioral phenotype reminiscent of FTLD symptoms in mice by affecting neuron spine density and morphology, suggesting that the protein can influence neuronal structural plasticity. Here, we evaluated whether a partial haploinsufficiency-like PGRN depletion, achieved by using RNA interference in primary mouse cortical neurons, could modulate GluN2B-containing N-methyl-d-aspartate (NMDA) receptors and tau phosphorylation, which are crucially involved in the regulation of the structural plasticity of these cells. In addition, we studied the effect of PGRN decrease on neuronal cell arborization both in the presence and absence of GluN2B-containing NMDA receptor stimulation. We found that PGRN decline diminished GluN2B-containing NMDA receptor levels and density as well as NMDA-dependent tau phosphorylation. These alterations were accompanied by a marked drop in neuronal arborization that was prevented by an acute GluN2B-containing NMDA receptor stimulation. Our findings support that PGRN decrease, resulting from pathogenic mutations, might compromise the trophism of cortical neurons by affecting GluN2B-contaning NMDA receptors. These mechanisms might be implicated in the pathogenesis of FTLD

Synapsin III alterations in Parkinson's disease

Objectives: The main neuropathological hallmarks of Parkinson's disease (PD) are loss of nigro-striatal dopamine neurons and intraneuronal Lewy Bodies (LB), proteinaceous inclusions mainly composed by a-synuclein. Recently, we found that a-synuclein interacts and cooperates with a specific member of the synapsin phosphoprotein family: synapsin III, in the regulation of dopaminergic neuron function. The aim of this study was to investigate whether synapsin III alterations may be related to a-synuclein pathology in PD. We thus investigated the occurrence of alterations of synapsin III expression and distribution in “the brain of patients affected by PD and DLB (Human brain samples were kindly provided by the UK Parkinson's disease Brain Bank). Methods: We used molecular biology and immunohistochemical techniques as well as the “in situ” proximity ligation assay (PLA) to investigate the expression levels and the distribution of synapsin III in the PD brain. Results: By immunohistochemistry, we showed a marked accumulation of synapsin III in the caudate/putamen of patients affected by PD when compared to age matched controls. In addition, many LB-like structures in the substantia nigra of PD patients were found to be immunoreactive for synapsin III, that by confocal fluorescence microscopy resulted to be accumulated in the core of LB. By co-immunoprecipitation and “in situ” PLA we found that synapsin III directly interacted with a-synuclein and that these two proteins were co-redistributed in mice and human brains. Finally, western blot analysis showed significant alterations of synapsin III levels that correlated with alteration of a-synuclein in the caudate/putamen and substantia nigra of PD patients. Conclusions: Altogether, our data support a critical involvement of synapsin III in PD pathophysiolog

Synapsin III is a key component of α-synuclein fibrils in Lewy bodies of PD brains

Lewy bodies (LB) and Lewy neurites (LN), which are primarily composed of α-synuclein (α-syn), are neuropathological hallmarks of Parkinson's disease (PD) and dementia with Lewy bodies (DLB). We recently found that the neuronal phosphoprotein synapsin III (syn III) controls dopamine release via cooperation with α-syn and modulates α-syn aggregation. Here, we observed that LB and LN, in the substantia nigra of PD patients and hippocampus of one subject with DLB, displayed a marked immunopositivity for syn III. The in situ proximity ligation assay revealed the accumulation of numerous proteinase K-resistant neuropathological inclusions that contained both α-syn and syn III in tight association in the brain of affected subjects. Most strikingly, syn III was identified as a component of α-syn-positive fibrils in LB-enriched protein extracts from PD brains. Finally, a positive correlation between syn III and α-syn levels was detected in the caudate putamen of PD subjects. Collectively, these findings indicate that syn III is a crucial α-syn interactant and a key component of LB fibrils in the brain of patients affected by P

α-synuclein and synapsin III cooperatively regulate synaptic function in dopamine neurons

The main neuropathological features of Parkinson's disease are dopaminergic nigrostriatal neuron degeneration, and intraneuronal and intraneuritic proteinaceous inclusions named Lewy bodies and Lewy neurites, respectively, which mainly contain alpha-synuclein (alpha-syn, also known as SNCA). The neuronal phosphoprotein synapsin III (also known as SYN3), is a pivotal regulator of dopamine neuron synaptic function. Here, we show that alpha-syn interacts with and modulates synapsin III. The absence of alpha-syn causes a selective increase and redistribution of synapsin III, and changes the organization of synaptic vesicle pools in dopamine neurons. In alpha-syn-null mice, the alterations of synapsin III induce an increased locomotor response to the stimulation of synapsin-dependent dopamine overflow, despite this, these mice show decreased basal and depolarization-dependent striatal dopamine release. Of note, synapsin III seems to be involved in alpha-syn aggregation, which also coaxes its increase and redistribution. Furthermore, synapsin III accumulates in the caudate and putamen of individuals with Parkinson's disease. These findings support a reciprocal modulatory interaction of alpha-syn and synapsin III in the regulation of dopamine neuron synaptic function