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 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

Rapid Eye Movement Sleep Behavior Disorder:Abnormal Cardiac Image and Progressive Abnormal Metabolic Brain Pattern

BACKGROUND: Isolated rapid eye movement sleep behavior disorder (iRBD) is prodromal for α-synucleinopathies. OBJECTIVE: The aim of this study was to determine whether pathological cardiac [123 I]meta-iodobenzylguanidine scintigraphy ([123 I]MIBG) is associated with progression of [18 F]fluorodeoxyglucose-positron emission tomography-based Parkinson's disease (PD)-related brain pattern (PDRP) expression in iRBD. METHODS: Seventeen subjects with iRBD underwent [18 F]fluorodeoxyglucose-positron emission tomography brain imaging twice ~3.6 years apart. In addition, [123 I]MIBG and [123 I]N-ω-fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)nortropane single-photon emission computed tomography ([123 I]FP-CIT-SPECT) at baseline were performed. Olfactory, cognitive, and motor functions were tested annually. RESULTS: Twelve of 17 subjects had pathological [123 I]MIBG. At baseline, 6 of 12 of these expressed the PDRP (suprathreshold PDRP z score). At follow-up, 12 of 17 subjects had suprathreshold PDRP z scores, associated with pathological [123 I]MIBG in 92% and with pathological [123 I]FP-CIT-SPECT in 75%. Subjects with pathological [123 I]MIBG had higher PDRP z score change per year (P = 0.027). Three subjects phenoconverted to PD; all had pathological [123 I]MIBG and [123 I]FP-CIT-SPECT, suprathreshold baseline PDRP z scores, and hyposmia. CONCLUSIONS: Pathological [123 I]MIBG was associated with progressive and suprathreshold PDRP z scores at follow-up. Abnormal [123 I]MIBG likely identifies iRBD as prodromal PD earlier than pathological [123 I]FP-CIT-SPECT. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society

Efficient characterization of multiple binding sites of small molecule imaging ligands on amyloid-beta, tau and alpha-synuclein

PURPOSE: There is an unmet need for compounds to detect fibrillar forms of alpha-synuclein (αSyn) and 4-repeat tau, which are critical in many neurodegenerative diseases. Here, we aim to develop an efficient surface plasmon resonance (SPR)-based assay to facilitate the characterization of small molecules that can bind these fibrils. METHODS: SPR measurements were conducted to characterize the binding properties of fluorescent ligands/compounds toward recombinant amyloid-beta (Aβ)$_{42}$, K18-tau, full-length 2N4R-tau and αSyn fibrils. In silico modeling was performed to examine the binding pockets of ligands on αSyn fibrils. Immunofluorescence staining of postmortem brain tissue slices from Parkinson's disease patients and mouse models was performed with fluorescence ligands and specific antibodies. RESULTS: We optimized the protocol for the immobilization of Aβ$_{42}$, K18-tau, full-length 2N4R-tau and αSyn fibrils in a controlled aggregation state on SPR-sensor chips and for assessing their binding to ligands. The SPR results from the analysis of binding kinetics suggested the presence of at least two binding sites for all fibrils, including luminescent conjugated oligothiophenes, benzothiazole derivatives, nonfluorescent methylene blue and lansoprazole. In silico modeling studies for αSyn (6H6B) revealed four binding sites with a preference for one site on the surface. Immunofluorescence staining validated the detection of pS129-αSyn positivity in the brains of Parkinson's disease patients and αSyn preformed-fibril injected mice, 6E10-positive Aβ in arcAβ mice, and AT-8/AT-100-positivity in pR5 mice. CONCLUSION: SPR measurements of small molecules binding to Aβ$_{42}$, K18/full-length 2N4R-tau and αSyn fibrils suggested the existence of multiple binding sites. This approach may provide efficient characterization of compounds for neurodegenerative disease-relevant proteinopathies

Enhanced firing of locus coeruleus neurons and SK channel dysfunction are conserved in distinct models of prodromal Parkinson's disease

Parkinson’s disease (PD) is clinically defined by the presence of the cardinal motor symptoms, which are associated with a loss of dopaminergic nigrostriatal neurons in the substantia nigra pars compacta (SNpc). While SNpc neurons serve as the prototypical cell-type to study cellular vulnerability in PD, there is an unmet need to extent our efforts to other neurons at risk. The noradrenergic locus coeruleus (LC) represents one of the first brain structures affected in Parkinson’s disease (PD) and plays not only a crucial role for the evolving non-motor symptomatology, but it is also believed to contribute to disease progression by efferent noradrenergic deficiency. Therefore, we sought to characterize the electrophysiological properties of LC neurons in two distinct PD models: (1) in an in vivo mouse model of focal α-synuclein overexpression; and (2) in an in vitro rotenone-induced PD model. Despite the fundamental differences of these two PD models, α-synuclein overexpression as well as rotenone exposure led to an accelerated autonomous pacemaker frequency of LC neurons, accompanied by severe alterations of the afterhyperpolarization amplitude. On the mechanistic side, we suggest that Ca(2+)-activated K(+) (SK) channels are mediators of the increased LC neuronal excitability, as pharmacological activation of these channels is sufficient to prevent increased LC pacemaking and subsequent neuronal loss in the LC following in vitro rotenone exposure. These findings suggest a role of SK channels in PD by linking α-synuclein- and rotenone-induced changes in LC firing rate to SK channel dysfunction

Membrane potential and delta pH dependency of reverse electron transport-associated hydrogen peroxide production in brain and heart mitochondria

Succinate-driven reverse electron transport (RET) is one of the main sources of mitochondrial reactive oxygen species (mtROS) in ischemia-reperfusion injury. RET is dependent on mitochondrial membrane potential (Δψm) and transmembrane pH difference (ΔpH), components of the proton motive force (pmf); a decrease in Δψm and/or ΔpH inhibits RET. In this study we aimed to determine which component of the pmf displays the more dominant effect on RET-provoked ROS generation in isolated guinea pig brain and heart mitochondria respiring on succinate or α-glycerophosphate (α-GP). Δψm was detected via safranin fluorescence and a TPP+ electrode, the rate of H2O2 formation was measured by Amplex UltraRed, the intramitochondrial pH (pHin) was assessed via BCECF fluorescence. Ionophores were used to dissect the effects of the two components of pmf. The K+/H+ exchanger, nigericin lowered pHin and ΔpH, followed by a compensatory increase in Δψm that led to an augmented H2O2 production. Valinomycin, a K+ ionophore, at low [K+] increased ΔpH and pHin, decreased Δψm, which resulted in a decline in H2O2 formation. It was concluded that Δψm is dominant over ∆pH in modulating the succinate- and α-GP-evoked RET. The elevation of extramitochondrial pH was accompanied by an enhanced H2O2 release and a decreased ∆pH. This phenomenon reveals that from the pH component not ∆pH, but rather absolute value of pH has higher impact on the rate of mtROS formation. Minor decrease of Δψm might be applied as a therapeutic strategy to attenuate RET-driven ROS generation in ischemia-reperfusion injury

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 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

A53T-α-synuclein overexpression in murine locus coeruleus induces Parkinson’s disease-like pathology in neurons and glia

Abstract Degeneration of noradrenergic locus coeruleus neurons occurs during the prodromal phase of Parkinson’s disease and contributes to a variety of non-motor symptoms, e.g. depression, anxiety and REM sleep behavior disorder. This study was designed to establish the first locus coeruleus α-synucleinopathy mouse model, which should provide sufficient information about the time-course of noradrenergic neurodegeneration, replicate cardinal histopathological features of the human Parkinson’s disease neuropathology and finally lead to robust histological markers, which are sufficient to assess the pathological changes in a quantitative and qualitative way. We show that targeted viral vector-mediated overexpression of human mutant A53T-α-synuclein in vivo in locus coeruleus neurons of wild-type mice resulted in progressive noradrenergic neurodegeneration over a time frame of 9 weeks. Observed neuronal cell loss was accompanied by progressive α-synuclein phosphorylation, formation of proteinase K-resistant α-synuclein-aggregates, accumulation of Ubi-1- and p62-positive inclusions in microglia and induction of progressive micro- and astrogliosis. Apart from this local pathology, abundant α-synuclein-positive axons were found in locus coeruleus output regions, indicating rapid anterograde axonal transport of A53T-α-synuclein. Taken together, we present the first model of α-synucleinopathy in the murine locus coeruleus, replicating essential morphological features of human Parkinson’s disease pathology. This new model may contribute to the research on prodromal Parkinson’s disease, in respect to pathophysiology and the development of disease-modifying therapy