PGC-1 alpha activity in nigral dopamine neurons determines vulnerability to alpha-synuclein

Introduction: Mitochondrial dysfunction and oxidative stress are critical factors in the pathogenesis of age-dependent neurodegenerative diseases. PGC-1 alpha, a master regulator of mitochondrial biogenesis and cellular antioxidant defense, has emerged as a possible therapeutic target for Parkinson's disease, with important roles in the function and survival of dopaminergic neurons in the substantia nigra. The objective of this study is to determine if the loss of PGC-1 alpha activity contributes to alpha-synuclein-induced degeneration. Results: We explore the vulnerability of PGC-1 alpha null mice to the accumulation of human alpha-synuclein in nigral neurons, and assess the neuroprotective effect of AAV-mediated PGC-1 alpha expression in this experimental model. Using neuronal cultures derived from these mice, mitochondrial respiration and production of reactive oxygen species are assessed in conditions of human alpha-synuclein overexpression. We find ultrastructural evidence for abnormal mitochondria and fragmented endoplasmic reticulum in the nigral dopaminergic neurons of PGC-1 alpha null mice. Furthermore, PGC-1 alpha null nigral neurons are more prone to degenerate following overexpression of human alpha-synuclein, an effect more apparent in male mice. PGC-1 alpha overexpression restores mitochondrial morphology, oxidative stress detoxification and basal respiration, which is consistent with the observed neuroprotection against alpha-synuclein toxicity in male PGC-1 alpha null mice. Conclusions: Altogether, our results highlight an important role for PGC-1 alpha in controlling the mitochondrial function of nigral neurons accumulating alpha-synuclein, which may be critical for gender-dependent vulnerability to Parkinson's disease.

FOXO3 determines the accumulation of alpha-synuclein and controls the fate of dopaminergic neurons in the substantia nigra

Ryoji Amamoto, Graham Knott, Emilda Pino, Juan-Carlos Sarria, Bernard Schneider, Lu Zheng

Parkinson's disease (PD) is characterized by the selective degeneration of neuronal populations presumably due to pathogenic interactions between aging and predisposing factors such as increased levels of a-synuclein. Here, we genetically modulate the activity of the transcription factor Forkhead box protein O-3 (FOXO3) in adult nigral dopaminergic neurons using viral vectors and explore how this determinant of longevity impacts on neuronal fate in normal and diseased conditions. We find that dopaminergic neurons are particularly vulnerable to changes in FOXO3 activity in the substantia nigra. While constitutive activation has proapoptotic effects leading to neuronal loss, inhibition of FOXO-mediated transcription by a dominant-negative competitor causes oxidative damage and is detrimental at high vector dose. To address the role of FOXO3 in PD, we modulate its activity in dopaminergic neurons overexpressing human a-synuclein. In this pathogenic condition, we find that FOXO inhibition has protective effects, suggesting that this transcription factor ultimately contributes to neuronal cell death. Nevertheless, mild FOXO3 activity also protects nigral neurons against the accumulation of human a-synuclein, albeit to a lesser extent. FOXO3 reduces the amount of a-synuclein present in the soluble protein fraction and promotes the coalescence of dense proteinase K-resistant aggregates, with an accumulation of autophagic vacuoles containing lipofuscin. Consistent with these in vivo observations, we find that FOXO3 controls autophagic flux in neuronal cells. Altogether, these results point to FOXO3 as an important determinant of neuronal survival in the substantia nigra, which may oppose a-synuclein accumulation and proteotoxicity.

Oxford University Press2014

Conditional expression of Parkinson's disease-related R1441C LRRK2 in midbrain dopaminergic neurons of mice causes nuclear abnormalities without neurodegeneration

Shi-Yan Caroline Foo, Liliane Glauser, Meghna Kannan, Graham Knott, Darren Moore, Elpida Tsika

Mutations in the leucine-rich repeat kinase 2 (LRRK2) gene cause late-onset, autosomal dominant Parkinson's disease (PD). The clinical and neurochemical features of LRRK2-linked PD are similar to idiopathic disease although neuropathology is somewhat heterogeneous. Dominant mutations in LRRK2 precipitate neurodegeneration through a toxic gain-of-function mechanism which can be modeled in transgenic mice overexpressing human LRRK2 variants. A number of LRRK2 transgenic mouse models have been developed that display abnormalities in dopaminergic neurotransmission and alterations in tau metabolism yet without consistently inducing dopaminergic neurodegeneration. To directly explore the impact of mutant LRRK2 on the nigrostriatal dopaminergic pathway, we developed conditional transgenic mice that selectively express human R1441C LRRK2 in dopaminergic neurons from the endogenous murine ROSA26 promoter. The expression of R1441C LRRK2 does not induce the degeneration of substantia nigra dopaminergic neurons or striatal dopamine deficits in mice up to 2 years of age, and fails to precipitate abnormal protein inclusions containing alpha-synuclein, tau, ubiquitin or autophagy markers (LC3 and p62). Furthermore, mice expressing R1441C LRRK2 exhibit normal motor activity and olfactory function with increasing age. Intriguingly, the expression of R1441C LRRK2 induces age-dependent abnormalities of the nuclear envelope in nigral dopaminergic neurons including reduced nuclear circularity and increased invaginations of the nuclear envelope. In addition, R1441C LRRK2 mice display increased neurite complexity of cultured midbrain dopaminergic neurons. Collectively, these novel R1441C LRRK2 conditional transgenic mice reveal altered dopaminergic neuronal morphology with advancing age, and provide a useful tool for exploring the pathogenic mechanisms underlying the R1441C LRRK2 mutation in PD. (C) 2014 Elsevier Inc All rights reserved.

Elsevier2014

The effect of the functional interaction between PGC-1α and Parkin on mitochondrial quality control in Parkinson's disease

Lu Zheng

Although several genetic factors have been directly associated to Parkinsonâs disease (PD), no single pathway has emerged to explain the etiology of this neurodegenerative disease. To further understand the cause of Parkinsonâs disease (PD), it is therefore important to deter-mine if and which functional interactions exist between gene products linked to this complex brain disorder. Mutations in the gene encoding the E3 ubiquitin ligase Parkin are associated to autosomal recessive PD with early onset. Although the Parkin protein carries out various critical func-tions in the cell, its role in mitochondria quality control, by inducing the autophagic degrada-tion of damaged mitochondria, has attracted a lot of attention. Furthermore, Parkin has been found to control the transcription of PGC-1Î±, a master regulator of mitochondrial biogenesis. Based on the important role that mitochondria are considered to play in PD, and the evi-dence for the coordinated expression of Parkin and PGC-1Î±, we sought to explore in neurons how these two factors functionally interact to regulate mitochondria turnover and quality control. In cortical neuronal cultures, we found that Parkin and PGC-1Î± have synergistic effects on mitochondrial biogenesis, and that the co-expression of these two factors leads to a dramatic increase in the number of mitochondria per cell. We further explored the effects of these fac-tors on the mitochondrial function. Parkin was shown to enhance the maximal respiration rate of neurons exposed to a mitochondrial uncoupling agent only in the presence of PGC-1Î±. Consistent with this effect, the co-expression of Parkin and PGC-1Î± leads to rapid recovery of the mitochondrial membrane potential following mitochondrial uncoupling. Next, we explored the molecular changes, which may underlie the cooperation between Par-kin and PGC-1Î±. The expression of Tfam, an important mitochondrial transcription factor, was found to be controlled synergistically by these two factors. Furthermore, PGC-1Î± en-hances the turnover of Mfn2, a key ubiquitylation target of Parkin. This result suggests that part of the functional interaction between PGC-1Î± and Parkin may be mediated via the co-regulation of Mfn2, a critical factor in mitochondrial physiology which controls the dynamics of mitochondria as well as their association with the endoplasmic reticulum (ER). In the rat substantia nigra, Parkin activity has significant protective effects on the survival and function of nigral dopaminergic neurons in which the chronic expression of PGC-1Î± is in-duced. Ultrastructural analysis shows that the conjunction of these two factors controls the density of mitochondria and their interaction with the ER. In PGC-1Î±-expressing dopamin-ergic neurons, the co-expression of mutated variants of Parkin associated with familial PD leads to accelerated degeneration. Overall, this study shows that Parkin, in conjunction with the transcription co-regulator PGC-1Î±, may play an important role in modulating the biogenesis of mitochondria as well as the quality control of this organelle. In nigral dopaminergic neurons which may critically depend on the mitochondria for their function and survival, the functional interaction be-tween these two factors should be further explored in the context of PD.

EPFL2016