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Molecular imaging of mGluR5 and P2X7 in preclinical models of Parkinson's disease

Book - Dissertation

With an ageing population, the search continues for curative or at least improved symptomatic treatment of neurodegenerative disorders such as Parkinson's disease (PD). Current treatments are primarily aimed at restoring basal ganglia signaling through targeting of the striatal dopaminergic synapse, most often with the dopamine precursor levodopa. However, these interventions do not slow down the disease or prevent the occurrence of detrimental side effects such as levodopa-induced dyskinesia (LID) and progressive non-motor symptoms. Functional imaging approaches, including positron emission tomography (PET) can be utilized in vivo to identify and evaluate novel candidate targets and provide valuable information on the involved neuropathological mechanisms in patients and rodent models. This way, PET may enable the non-invasive longitudinal quantification of biomarkers for disease identification, progression, prognosis and monitoring. A triad of aberrant protein deposition, excitotoxicity and neuroinflammation characterizes the pathophysiology of neurodegenerative disorders. As no radioligands for α-synuclein have been developed yet, it was our goal in this manuscript to investigate two PD-related targets, namely the metabotropic glutamate receptor type 5 (mGluR5) and the purinergic P2X7 receptor using a multi-parametric in vivo and in vitro approach. In chapter 2 & 3, we aimed to investigate the role of the metabotropic glutamate receptor type 5 (mGluR5) in toxin-induced models. The mGlu5 receptor regulates motor and cognitive neurocircuits that are affected by PD and may be a target for therapeutic interventions. However, conflicting results have been described regarding the involvement of mGluR5 in PD and LID. Using in vivo [18F]FPEB imaging, we evaluated whether differences in regional [18F]FPEB binding were present between diseased and healthy animals. In both chapters, PET imaging was combined with behavioral testing and immunohistochemistry to cross-validate in vivo findings. In the acute 6-OHDA PD model, we found decreased mGluR5 availability in different anatomical regions associated with motor control, including the striatum, somatosensory cortex, motor cortex, and parietal association cortex, and most pronounced in the ipsilateral striatum. By contrast, following LID onset, increased relative [18F]FPEB binding was present in the motor and somatosensory cortex and correlated positively to abnormal involuntary movement scores, a rating scale for LID severity. Although striatal glutamine levels were significantly elevated in levodopa- and saline-treated rats, we did not find altered glutamate levels in dyskinetic or naïve PD rats. We showed decreased regional mGluR5 availability in Parkinsonian animals with a limited mGluR5 increase in dyskinetic animals, linked to LID severity, emphasizing the potential of treatment with mGluR5 negative allosteric modulators in PD-LID. In chapter 3, we utilized the quinolinic acid (QA) model, which causes localized excitotoxicity by overactivation of NMDA receptors, a common neuropathological pathway implied in neurodegenerative disorders, including PD and HD. In a longitudinal PET study, we assessed the effect of QA lesioning on regional mGluR5 availability until 7 weeks after lesioning. In contrast to 6-OHDA, which causes degeneration of presynaptic dopaminergic terminals, a striatal injection of QA causes postsynaptic dysfunction of medium spiny neurons. Rats depicted subtle motor deficits but no cognitive impairment. QA lesioning decreased mGluR5 availability in the area of striatal degeneration, which correlated with a worsening of motor symptoms. Interestingly, we observed that mGluR5 immunoreactivity in the lesion area co-localized predominantly with an astrocytic marker. Although toxin-induced models are characterized by robust and reproducible neurodegeneration and behavioral symptoms, these models do not reproduce the cardinal pathophysiological hallmarks that are underlying the neurodegenerative process in PD patients. Therefore, we included a model that mimics both the behavioral symptoms and α-synucleinopathy characteristic to clinical PD, and investigated disease-related deficits to the brain glucose metabolism over time using [18F]FDG PET (chapter 4). We observed a dynamic pattern of changes in regional glucose metabolism that was localized in cortical and basal ganglia structures throughout the neurodegenerative process. Both in the striatum and SN, these changes were correlated to disease-associated motor symptoms, whereas elevated glucose metabolism in the SN coincided with focal TSPO overexpression, suggesting an ongoing neuroinflammatory process. Even though the TSPO protein has been the central target for PET inflammation imaging, its use has several disadvantages. Therefore, the purinergic P2X7 receptor has been proposed as a novel marker for neuroinflammation imaging. In chapter 5, we evaluated P2X7 availability within the rAAV2/7 α-synuclein and 6-OHDA model using in vitro autoradiography and compared those changes to the TSPO binding pattern. TSPO overexpression was observed in the SN of α-SYN rats (chapter 4), but was not paralleled by alterations in [11C]JNJ-717 binding. In contrast, the acute 6-OHDA model did show a focal increase of [11C]JNJ-717 binding that largely coincided with elevated [18F]DPA-714 uptake. Although these results are promising for acute inflammatory disorders, the dynamics and pro/anti-inflammatory aspects of P2X7 imaging need further clarification before its use can be advocated to monitor neuroinflammation in vivo. Overall, our work demonstrates the value of non-invasive PET imaging to assess the potential of therapeutic targets in combination with additional ex vivo or in vitro readouts. The implications and limitations of this work are considered in each respective research chapter, including a general discussion where also suggestions for future research are made (chapter 6).
Publication year:2019
Accessibility:Open