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Evaluation of the Dopaminergic System Using In Vivo PET Imaging and Post-Mortem Analyses in the Context of Mild Neurodegenerative and Neuroinflammatory PD Models

Pauline Roost 1 
1 LMN - Laboratoire des Maladies Neurodégénératives - UMR 9199
MIRCEN - Service MIRCEN : DRF/JACOB, CNRS - Centre National de la Recherche Scientifique : UMR 9199
Abstract : The cerebral pathology of Parkinson’s disease is characterized by a progressive loss of dopaminergic neurons of the substantia nigra, and an accumulation of α-synuclein aggregates. neuroinflammation and genetic predisposition contribute to PD as the main confounding factors. In this PhD thesis, I aimed to evaluate, in vivo and post-mortem, the effects of three factors on the dopaminergic system: 1) α-synuclein overexpression, 2) α-synuclein and LRRK2 co-expression, and 3) mild neuroinflammation, on the dopaminergic system and dopaminergic neuronal cell loss.To this end, positron emission tomography (PET) imaging and behavioural studies have been selected as the main in vivo tools. I have used [18F]LBT999 and [18F]FMT PET imaging to evaluate striatal levels of dopamine transporter (DAT) and the dopamine-synthesising AADC enzyme, respectively. To assess neuroinflammation, I have used [18F]DPA714 to evaluate 18kDa TSPO binding. The in vivo data were validated by post-mortem techniques evaluating the expression of genes (qPCR) and proteins (immunohistochemistry).The results of my work show that overexpression of human WT-α-synuclein in the substantia nigra through viral vectors (AAV2/6-PGK-WT-α-synuclein) does not generate detectable neuronal loss in the substantia nigra, nor does it generate in vivo motor deficits or changes in the dopaminergic system as seen by in vivo PET imaging. On the other hand, I have demonstrated here that overexpression of A53T-α-synuclein in the substantia nigra, using an AAV2/6-PGK-A53T-α-synuclein viral vector approach, resulted in significant α-synuclein aggregation in the substantia nigra as soon as 8wpi, but not in the striatum. Quantitative microscopic analyses show that A53T-α-synuclein aggregation induced a mild but progressive degeneration of dopaminergic neurons in the substantia nigra. The loss of dopaminergic fibres in the striatum as detected by immunohistochemistry of tyrosine hydroxylase (TH) remains, however, moderate. DAT-PET imaging, but not AADC-PET imaging, was able to measure the progressive neuronal loss. Taken together, our in vivo and post-mortem data suggest that DAT-PET does not only reflect neuronal loss induced by α-synuclein accumulation, but also functional compensation mechanisms of the dopaminergic synapse. A reduction in DAT levels, combined with normal TH and AADC levels, could normalise the synaptic dopamine concentrations in the striatum. In addition, this dopaminergic neuronal loss coincided with in initially moderate, followed by a more pronounced microglial response. Finally, in a viral vector model of co-overexpression of AAV2/6-PGK-A53T-α-synuclein and AAV2/6-PGK-G2019S-LRRK2, we did not observe added neurotoxicity of G2019S mutated LRRK2 to A53T-α-synuclein toxicity.In an acute neuroinflammatory model following LPS injection in the striatum, post-mortem analysis revealed the absence of dopaminergic neuronal loss in the substantia nigra and synaptic loss in the striatum. Nevertheless, I observed a significant inverse correlation between inflammation markers (TSPO-PET and IBA1 expression) and markers for dopamine production (TH) and storage (VMAT2). These data support the hypothesis that neuroinflammation may impair the functionality of the dopaminergic system, regardless of the presence dopaminergic neuron loss.In summary, my thesis results confirm the interest of PET in demonstrating functional damage in vivo, which cannot be demonstrated post-mortem in animal models of PD.
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Submitted on : Thursday, June 23, 2022 - 1:01:40 AM
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Pauline Roost. Evaluation of the Dopaminergic System Using In Vivo PET Imaging and Post-Mortem Analyses in the Context of Mild Neurodegenerative and Neuroinflammatory PD Models. Neurobiology. Université Paris-Saclay, 2020. English. ⟨NNT : 2020UPASS086⟩. ⟨tel-03702252⟩

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