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Positron emission tomography (PET) is a non-invasive medical imaging technique that allows visualisation and quantification of biochemical, physiological and pharmacological processes in living subjects. This is achieved through application of radiotracers – compounds labelled with positron emitting radionuclides. Neurodegeneration is the progressive loss of neurons resulting in impairment of brain function leading to cognitive decline and can affect movement. The underlying pathology of many neurodegenerative diseases is misfolding of proteins such as α-synuclein, the key pathological hallmark of Parkinson’s disease. Also implicated in the processes of neurodegeneration is neuroinflammation, which is observed by the activation of microglia – the immune cells of the brain. Activation of microglia is associated with the upregulation of the 18 kDa mitochondrial translocator protein (TSPO). This work has involved the synthesis and characterisation of novel compounds that have the potential for being applied as radiotracers for imaging α-synuclein fibrils (project 1), or TSPO (project 2) via PET. Over the course of project 1 a library of compounds was synthesised based upon structural modifications of a lead structure identified from the literature. These compounds then underwent screening via biophysical methodologies in order to determine their affinity to α-synuclein fibrils. This stage of the work involved the development of a novel biophysical technique – microscale thermophoresis (MST). A general automated radiosynthetic method to afford the [18F]fluoro-derivatives of these compounds has also been developed, and preliminary in vitro autoradiography studies and an in vivo microPET scan has been performed. For project 2, an automated radiosynthetic method was developed to produce [18F]GE387, a lead compound identified through collaboration with GE Healthcare. This radiotracer has then been applied to preliminary in vitro autoradiography and an in vivo microPET study using rats with induced neuroinflammation alongside control rats.

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The molecular mechanisms underlying the association between mutations in GBA1 and risk of developing the ‘synucleinopathy’ disorders Parkinson’s disease (PD) and dementia with Lewy bodies (DLB) remain elusive. To better understand the precise molecular cascade that connects GBA1 mutations with α-synuclein dysregulation, a modified lipid extraction and HPTLC protocol was optimized to detect changes in levels of neutral sphingolipids (SLs) from neural cells and tissue expressing wild-type (WT) GBA1, mutant GBA1, or both. We demonstrate that mutant GBA1 does not confer a dominant-negative effect on WT GBA1-mediated activity; however, bona fide loss-of-enzymatic function mutation events led to the accumulation of lipid substrates in neural cells and tissue, and enhance α- synuclein/ubiquitin reactivity in brain tissue of mutant gba1 mice. Our HPLC-MS/MS data are consistent with other studies demonstrating that heterozygous GBA1 mutations do not lead to lipid accumulation, but may alter α-synuclein degradation through a yet-to-be defined novel gain-of-toxic function event.

Parkinson’s disease is a movement disorder characterized by nigrostriatal dopamine pathway degeneration and neuronal α-synuclein accumulation. Pathogenesis is associated with mutations in α-synuclein and Gba1 encoding alleles. Animal models created to date do not recapitulate the spectrum of clinical disease features. This thesis characterizes the bi-genic Synergy mouse, hypothesized to demonstrate motor behavioural and histological abnormalities downstream of α-synuclein overexpression and mutated Gba1. Synergy and SNCA mice (overexpressed α-synuclein with wild-type Gba1) have early onset deficits in motor coordination, muscle strength and nest building. Both exhibit increased α-synuclein concentration in the brain and cerebellar inclusions positive for two markers of pathological α-synuclein processing. Overall mutant Gba1 expression within Synergy mice does not worsen the behaviour or the histopathological findings associated with overexpression of human α-synuclein in SNCA mice. Future studies will determine whether mutant Gba1 expression alters cognitive behaviour and/or lipid homeostasis in this new bi-genic model of Parkinson’s disease.

Parkinson’s disease is a neurodegenerative disease resulting primarily from loss of dopaminergic innervation in the striatum subsequent to cell loss in the substantia nigra pars compacta. The abnormal accumulation of the normal pre-synaptic protein α-synuclein (αsyn) forms intraneuronal inclusions known as Lewy neurites and Lewy bodies. The origins of central Lewy pathology have been suggested to lie in the enteric nervous system, ascending through the vagus nerve to the dorsal motor nucleus of the vagus. To ascertain gastrointestinal regions most likely to be the source of central Lewy pathology, αsyn expression was evaluated in the neural elements of gastrointestinal regions receiving the densest vagal innervation. The vermiform appendix was found to have the densest αsyn-immunoreactive innervation in all layers of the gut wall. In addition, macrophages in the appendiceal mucosa were laden with αsyn within lysosomes, consistent with attempts to prevent the spread of disease or to correct synaptic dysfunction.

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Les protéines alpha-synucléine (α-syn) et leucine-rich repeat kinase 2 (LRRK2) sont deux protéines ayant un rôle majeur dans la physiopathologie de la maladie de Parkinson (MP) et interviennent aussi bien dans les formes dites sporadiques que dans les formes familiales. La mutation G2019S du gène codant pour LRRK2 est la mutation la plus fréquente. Cette mutation induit une augmentation de l’activité kinase de LRRK2 qui conduit à sa toxicité. Plusieurs hypothèses convergent vers l’idée que LRRK2 et l’α-syn interagiraient pour conduire à la dysfonction et/ou la mort des neurones dopaminergiques (DA) de la substance noire (SNc) dans la MP. Dans la première partie de cette étude, différentes formes sauvage (WT) ou mutée (G2019S) de LRRK2 ont été surexprimées spécifiquement dans les neurones de la SNc via l’utilisation de vecteurs lentiviraux (LV) et adéno-viraux associés (AAV). La question principale de cette étude était d’évaluer si l’expression spécifiquement neuronale de LRRK2 induisait la dégénérescence des neurones DA de la SNc. Nous avons généré des constructions comportant uniquement la partie C-terminale de LRRK2 (ΔLRRK2) en aval du domaine LRR. In vitro, le fragment ΔLRRK2G2019S présente une activité kinase supérieure au fragment ΔLRRK2WT avec une augmentation d’activité comparable à la forme entière de LRRK2. In vivo, six mois après l’injection (PI) de ΔLRRK2 WT ou G2019S dans la SNc, les mesures du nombre de neurones montrent que seul le fragment ΔLRRK2G2019S induit une mort neuronale significative (30%) comparé à la forme ΔLRRK2WT, uniquement lorsque l’expression est générée via des vecteurs AAV. Ces résultats suggèrent que l’expression purement neuronale d’un fragment contenant le domaine kinase de LRRK2 est suffisante pour induire une dégénérescence de la SN. Dans la seconde partie du projet, nous avons étudié l’hypothèse que ΔLRRK2G2019S via son activité kinase amplifiée, pourrait augmenter la toxicité le l’α-syn mutée A53T. Pour répondre à cette question, les vecteurs AAV codant pour ΔLRRK2 G2019S ou une forme inactive de la kinase (ΔLRRK2G2019S/D1994A), et celui codant pour l’α-syn A53T ont été co-injectés dans la SNc. Les analyses réalisées à 6 et 15 semaines PI montrent que ΔLRRK2G2019S augmente la mort neuronale induite par l’α-syn A53T d’une manière kinase dépendante. Tous ces résultats supportent l’hypothèse que l’existence d’une interaction fonctionnelle entre LRRK2 et l’α-syn pourrait jouer un rôle fondamental dans la physiopathologie de la MP offrant des possibilités de stratégie de neuroprotection ciblant l’interaction LRRK2/α-syn. / Alpha-synuclein (α-syn) and leucine-rich repeat kinase 2 (LRRK2) proteins are likely to play crucial roles both in sporadic and familial forms of Parkinson’s disease (PD). The most prevalent mutation in LRRK2 is the G2019S substitution which induces neurotoxicity through a marked increase of its kinase activity. A possible interplay between LRRK2 and α-syn may be involved in the dysfunction and/or in the death of dopaminergic (DA) neurons in the substantia nigra (SNc) in PD. In the first part of the study, we evaluated whether the overexpression of LRRK2G2019S using lentiviral vectors (LVs) and adeno-associated virus (AAV2/9), which can overexpress transgenes selectively in neurons could trigger neurodegeneration in the SNc, in other words, whether cell-autonomous mechanisms are sufficient to trigger the degeneration of DA neurons. We generated constructs corresponding to the C-terminal domain of LRRK2 (ΔLRRK2) containing the kinase domain. Results of assays performed in vitro indicated that ΔLRRK2 retains biochemical properties of full length LRRK2. Six months after the stereotaxic injection of LV-ΔLRRK2G2019S in the SNc, the number of DA neurons was unchanged, however, the infection of the SNc with AAV-ΔLRRK2G2019S but not with AAV-ΔLRRK2WT induced a significant ~30% loss of DA neurons. These results suggested that neuronal overexpression of the mutant kinase domain of LRRK2 was sufficient to trigger neurodegeneration in the SNc in the adult brain. In the second part of the study, we aimed at studying whether ΔLRRK2G2019S could increase the neurotoxicity of a mutant form of α-syn (A53T mutation) in vivo in DA neurons. We used a co-infection approach with AAV vectors encoding the α-synA53T, and ΔLRRK2 G2019S alone or with the D1994A mutation (ΔLRRK2G2019S/D1994A) that inactivates the kinase activity of LRRK2. AAVs were stereotaxically co-injected into the rat SNc and histological evaluation was performed at 6 and 15 weeks (early and late time points) post-infection. Results showed that ΔLRRK2G2019S increased the toxicity of α-synA53T in a kinase-dependent manner. Altogether, the present study supports the hypothesis that a functional interaction between LRRK2 and α-syn may play a key role in PD pathogenesis. The new “double hit” model we developed in rats may be of interest to test novel neuroprotective strategies targeting LRRK2/α-syn in vivo.

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α-Synuclein (AS) is a small intrinsically disordered protein expressed in neurons and abundantly present in synapses where it is involved in regulation of synaptic vesicle-mediated protein trafficking. Misfolding of AS into amyloid fibrils is a key process in progression of Parkinson's disease (PD), the second most common neurodegenerative disorder which has no cure to date. Inhibition of AS aggregation and blocking of cell-to-cell spreading of AS fibrils is a promising strategy for PD treatment. However, rational design of inhibitors of this type remains complicated due to the lack of thorough knowledge about the mechanisms of aggregation. Therefore, the aim of this thesis was to gain deeper knowledge about AS aggregation and to apply it for developing inhibitors of AS fibrillization. In my work on the mechanisms of AS aggregation, I first determined that the concentration of AS that enables the fibril growth is an order of magnitude lower than the concentration of AS required for initial fibril formation from monomers. I explored fibril disaggregation at AS concentrations below its Kd value, and characterized AS aggregation at low micromolar concentrations. I then investigated how different modifications of AS C-terminus (namely, extensions of various sizes and charges) affect fibril growth and...