Selective cellular vulnerability and pathology progression patterns in two mouse models of Parkinson’s disease
Parkinson's disease is a highly debilitating disorder classically characterized by the degeneration of dopaminergic midbrain neurons of the substantia nigra. The resulting nigrostriatal dopamine deficiency is thought to be responsible for the onset of the cardinal Parkinson's motor symptom...
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|Summary:||Parkinson's disease is a highly debilitating disorder classically characterized by the degeneration of dopaminergic midbrain neurons of the substantia nigra. The resulting nigrostriatal dopamine deficiency is thought to be responsible for the onset of the cardinal Parkinson's motor symptomtomatology; bradykinesia, rigidity, and resting tremor. However, recent studies show that Parkinson's disease is a multisystem disorder. Thus, it comes not only to degeneration in the nigrostriatal system, but also to pronounced cell loss in many other brain regions. Histopathologically, Parkinson's disease is characterized by the presence of so-called Lewy bodies or neurites. These are intracytoplasmic proteinaceous inclusions consisting mainly of aggregated α-synuclein. Two neuronal structures that both have pronounced Lewy pathology in Parkinson's disease and prominent neurodegeneration are the noradrenergic locus coeruleus and the neurochemically heterogeneous pedunculopontine nucleus. Remarkably, in the pedunculopontine nucleus Lewy pathology and neurodegeneration are predominantly restricted to the cholinergic cell population, while the GABAergic and glutamatergic cell groups exhibit only minor Lewy pathology and are largely spared of neurodegeneration.
The present dissertation pursued two main goals. On the one hand, we investigated whether the selective vulnerability pattern of the cholinergic subpopulation of the pedunculopontine nucleus could be reproduced in a mouse model based on the intracerebral injection of preformed α-synuclein fibrils. Second, the brain-spreading pattern of two focal-induced α-synucleinopathy mouse models were compared with respect to the methodology used to initiate the aggregation process (vector-mediated overexpression vs. α-synuclein fibril model).
In the first part of the study, we used a targeted intracerebral injection of preformed α-synuclein fibrils to induce a focal α-synucleinopathy in the pedunculopontine nucleus. Our data show that the injection of α-synuclein fibrils resulted in the recruitment and misfolding of endogenous α-synuclein leading to formation of Lewy body-like aggregates in neuronal perikarya and axons. Interestingly, the observed inclusion bodies were immunoreactive for S129-phosphorylated α-synuclein, p62 positive and resistant to proteinase K digestion. We thereby showed that the experimentally induced α-synuclein pathology possessed several key features of human Lewy pathology. Remarkably, the major burden of Lewy-like pathology and quantified cell loss was limited to the cholinergic subpopulation of the pedunculopontine nucleus, while the non-cholinergic neurons were largely spared of Lewy pathology and degeneration at any investigated time-point. Interestingly, in both fibril and monomer-α-synuclein (control) injected animals, induction of reactive microgliosis occurred, although no α-synuclein pathology was observed in the control group. Our analysis also showed that the formation of α-synuclein pathology was not limited to the immediate vicinity of the site of injection, but propageted over considerable distances to other interconnected brain regions. Since α-synuclein positive aggregates were found in neuronal cell bodies of distant brain regions, which lay all within the neuronal network of the pedunculopontine nucleus, it can be concluded that the α-synucleinopathy spread only within the neural network of the pedunculopontine nucleus.
In the second part of the thesis, focal α-synucleinopathy was induced in the locus coeruleus by intracerebral injection of adeno-associated viral vectors containing the gene for human mutant A53T-α-synuclein or luciferase (control protein). The obtained data showed that local overexpression of human α-synuclein led to widespread propagation of the protein consistent with anterograde axonal transport. Analysis of the α-synuclein propagation pattern demonstrated that the brain-wide α-synucleinopathy was confined to the output regions of the noradrenergic locus coeruleus. Furthermore, there was no evidence of cell-to-cell transmission of human α-synuclein. Based on these findings we concluded that the induced Lewy-like pathology did not leave the noradrenergic locus coeruleus system in the studied time frame of 9 weeks. In addition, unbiased stereological quantification of the dopaminergic substantia nigra revealed no significant cell loss at the relatively short time-frame of 9 weeks. In conclusion, the studies presented in this dissertation show that cholinergic pedunculopontine neurons are significantly more vulnerable to α-synuclein fibril-induced α-synucleinopathy than non-cholinergic neurons. In addition, we were able to show that the brain-wide progression pattern of Lewy-like pathology is significantly different between the two studied α-synucleinopathy models. While in the fibril model the α-synucleinopathy pattern was consistent with cell-to-cell transmission of pathological α-synuclein species, we only observed axonal transport of α-synuclein but not cell-to-cell transmission in the overexpression-based model. The studies carried out within this dissertation therefore provide a valuable starting point for the further investigation of cellular vulnerability factors and mechanisms of disease progression.|
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