Search

Tau seed amplification assays (SAAs) directly measure the seeding activity of tau and would therefore be ideal biomarkers for clinical trials targeting seeding-competent tau in Alzheimer's disease (AD). However, the precise relationship between tau seeding measured by SAA and the levels of pathological forms of tau in the AD brain remains unknown. We developed a new tau SAA based on full-length 0N3R tau with sensitivity in the low fg/ml range and used it to characterize 103 brain samples from three independent cohorts. Tau seeding clearly discriminated between AD and control brain samples. Interestingly, seeding was absent in Progressive Supranuclear Palsy (PSP) putamen, suggesting that our tau SAA did not amplify 4R tau aggregates from PSP brain. The specificity of our tau SAA for AD brain was further supported by analysis of matched hippocampus and cerebellum samples. While seeding was detected in hippocampus from Braak stages I-II, no seeding was present in AD cerebellum that is devoid of tau inclusions. Analysis of 40 middle frontal gyrus samples encompassing all Braak stages showed that tau SAA seeding activity gradually increased with Braak stage. This relationship between seeding activity and the presence of tau inclusions in AD brain was further supported by robust correlations between tau SAA results and the levels of phosphorylated tau212/214, phosphorylated tau181, aggregated tau, and sarkosyl-insoluble tau. Strikingly, we detected tau seeding in the middle frontal gyrus already at Braak stage II-III, suggesting that tau SAA can detect tau pathology earlier than conventional immunohistochemical staining. In conclusion, our data suggest a quantitative relationship between tau seeding activity and pathological forms of tau in the human brain and provides an important basis for further development of tau SAA for accessible human samples. [ABSTRACT FROM AUTHOR]

Tau seed amplification assays (SAAs) directly measure the seeding activity of tau and would therefore be ideal biomarkers for clinical trials targeting seeding-competent tau in Alzheimer's disease (AD). However, the precise relationship between tau seeding measured by SAA and the levels of pathological forms of tau in the AD brain remains unknown. We developed a new tau SAA based on full-length 0N3R tau with sensitivity in the low fg/ml range and used it to characterize 103 brain samples from three independent cohorts. Tau seeding clearly discriminated between AD and control brain samples. Interestingly, seeding was absent in Progressive Supranuclear Palsy (PSP) putamen, suggesting that our tau SAA did not amplify 4R tau aggregates from PSP brain. The specificity of our tau SAA for AD brain was further supported by analysis of matched hippocampus and cerebellum samples. While seeding was detected in hippocampus from Braak stages I-II, no seeding was present in AD cerebellum that is devoid of tau inclusions. Analysis of 40 middle frontal gyrus samples encompassing all Braak stages showed that tau SAA seeding activity gradually increased with Braak stage. This relationship between seeding activity and the presence of tau inclusions in AD brain was further supported by robust correlations between tau SAA results and the levels of phosphorylated tau212/214, phosphorylated tau181, aggregated tau, and sarkosyl-insoluble tau. Strikingly, we detected tau seeding in the middle frontal gyrus already at Braak stage II-III, suggesting that tau SAA can detect tau pathology earlier than conventional immunohistochemical staining. In conclusion, our data suggest a quantitative relationship between tau seeding activity and pathological forms of tau in the human brain and provides an important basis for further development of tau SAA for accessible human samples. [ABSTRACT FROM AUTHOR]

High-resolution scans of immunohistochemical (IHC) stains of Alzheimer's disease (AD) brain slices and radioligand autoradiography both provide information about the distribution of Aβ plaques and Tau, the two common proteinopathies in AD. Accurate assessment of the amount and regional location of Aβ plaques and Tau is essential to understand the progression of AD pathology. Our goal was to develop a quantitative method for the analysis of IHC–autoradiography images. Postmortem anterior cingulate (AC) and corpus callosum (CC) from AD and control (CN) subjects were IHC stained with anti-Aβ for Aβ plaques and autoradiography with [18F]flotaza and [125I]IBETA for Aβ plaques. For Tau, [124I]IPPI, a new radiotracer, was synthesized and evaluated in the AD brain. For Tau imaging, brain slices were IHC stained with anti-Tau and autoradiography using [125I]IPPI and [124I]IPPI. Annotations for Aβ plaques and Tau using QuPath for training and pixel classifiers were generated to measure the percent of the area of Aβ plaques and Tau in each slice. The binding of [124I]IPPI was observed in all AD brains with an AC/CC ratio > 10. Selectivity to Tau was shown by blocking [124I]IPPI with MK-6240. Percent positivity for Aβ plaques was 4–15%, and for Tau, it was 1.3 to 35%. All IHC Aβ plaque-positive subjects showed [18F]flotaza and [125I]IBETA binding with a positive linear correlation (r2 > 0.45). Tau-positive subjects showed [124/125I]IPPI binding with a stronger positive linear correlation (r2 > 0.80). This quantitative IHC–autoradiography approach provides an accurate measurement of Aβ plaques and Tau within and across subjects. [ABSTRACT FROM AUTHOR]

Highlights: Imaging of the dopamine transporter (DAT) (in particular using 11C- or 18F-PE2I) most reliably and precisely measures neurodegeneration of the dopaminergic system in parkinsonian disorders and its progression in longitudinal studies. Combining DAT imaging and iron- and neuromelanin-sensitive MR imaging in vivo unveiled the sequential progression of neurodegeneration along the sensorimotor, associative, and limbic territories in the striatum followed by the substantia nigra in Parkinson's disease. DAT imaging is the most accurate marker for prodromal PD and predicts phenoconversion to clinical motor symptoms in patients with REM-sleep behavior disorders (RBD). Dopaminergic imaging can be used to investigate motor reserve in PD patients. Peripheral molecular imaging shows that autonomous denervation of the heart and gut in prodromal and de novo PD with RBD precedes striatal dopaminergic loss, thereby supporting peripheral starting points in those patients. Serotonergic denervation in the subcortical and cortical limbic regions is associated with apathy, depression, and anxiety in PD. Noradrenergic terminals loss plays a critical role in RBD and was shown to exceed locus coeruleus pathology in PD using 11C-MeNER PET. Cholinergic imaging is now complemented by the new PET tracer 18F-FEOBV, which binds to presynaptic transporters, particularly in subcortical structures. Dual-phase dynamic PET imaging may enable to inform both metabolic and molecular disease-related patterns with a single injection, including tracers for DAT or tau. Using 11C-UCB-J, prominent loss of synaptic density in the substantia nigra and brainstem was demonstrated in PD, even in the early stage, extending to the cortex in DLB, as well as PSP and CBD. Tau imaging using second-generation tracers with increased specificity greatly improved the diagnosis of tau aggregation in PSP and CBD. Despite recent advances and promising results in MSA patients using 18F-ACI12589 PET, no tracer is readily available to image α-synuclein aggregates in vivo in parkinsonian disorders. Computational modeling of tracer affinity in silico, coupled with cryogenic electron microscopy will improve efficiency and accelerate the development of novel tracers. Neurodegenerative parkinsonian disorders are characterized by a great diversity of clinical symptoms and underlying neuropathology, yet differential diagnosis during lifetime remains probabilistic. Molecular imaging is a powerful method to detect pathological changes in vivo on a cellular and molecular level with high specificity. Thereby, molecular imaging enables to investigate functional changes and pathological hallmarks in neurodegenerative disorders, thus allowing to better differentiate between different forms of degenerative parkinsonism, improve the accuracy of the clinical diagnosis and disentangle the pathophysiology of disease-related symptoms. The past decade led to significant progress in the field of molecular imaging, including the development of multiple new and promising radioactive tracers for single photon emission computed tomography (SPECT) and positron emission tomography (PET) as well as novel analytical methods. Here, we review the most recent advances in molecular imaging for the diagnosis, prognosis, and mechanistic understanding of parkinsonian disorders. First, advances in imaging of neurotransmission abnormalities, metabolism, synaptic density, inflammation, and pathological protein aggregation are reviewed, highlighting our renewed understanding regarding the multiplicity of neurodegenerative processes involved in parkinsonian disorders. Consequently, we review the role of molecular imaging in the context of disease-modifying interventions to follow neurodegeneration, ensure stratification, and target engagement in clinical trials. [ABSTRACT FROM AUTHOR]

The 18 kDa translocator protein (TSPO) is increasingly recognized as an interesting target for the imaging of glioblastoma (GBM). Here, we investigated TSPO PET imaging and autoradiography in the frequently used GL261 glioblastoma mouse model and aimed to generate insights into the temporal evolution of TSPO radioligand uptake in glioblastoma in a preclinical setting. We performed a longitudinal [18F]GE-180 PET imaging study from day 4 to 14 post inoculation in the orthotopic syngeneic GL261 GBM mouse model (n = 21 GBM mice, n = 3 sham mice). Contrast-enhanced computed tomography (CT) was performed at the day of the final PET scan (±1 day). [18F]GE-180 autoradiography was performed on day 7, 11 and 14 (ex vivo: n = 13 GBM mice, n = 1 sham mouse; in vitro: n = 21 GBM mice; n = 2 sham mice). Brain sections were also used for hematoxylin and eosin (H&E) staining and TSPO immunohistochemistry. [18F]GE-180 uptake in PET was elevated at the site of inoculation in GBM mice as compared to sham mice at day 11 and later (at day 14, TBRmax +27% compared to sham mice, p = 0.001). In GBM mice, [18F]GE-180 uptake continuously increased over time, e.g., at day 11, mean TBRmax +16% compared to day 4, p = 0.011. [18F]GE-180 uptake as depicted by PET was in all mice co-localized with contrast-enhancement in CT and tissue-based findings. [18F]GE-180 ex vivo and in vitro autoradiography showed highly congruent tracer distribution (r = 0.99, n = 13, p < 0.001). In conclusion, [18F]GE-180 PET imaging facilitates non-invasive in vivo monitoring of TSPO expression in the GL261 GBM mouse model. [18F]GE-180 in vitro autoradiography is a convenient surrogate for ex vivo autoradiography, allowing for straightforward identification of suitable models and scan time-points on previously generated tissue sections. [ABSTRACT FROM AUTHOR]

Objectives: Autoradiography on brain tissue is used to validate binding targets of newly discovered radiotracers. The purpose of this study was to correlate quantification of autoradiography signal using the novel next-generation tau positron emission tomography (PET) radiotracer [18F]PI-2620 with immunohistochemically determined tau-protein load in both formalin-fixed paraffin-embedded (FFPE) and frozen tissue samples of patients with Alzheimer's disease (AD) and Progressive Supranuclear Palsy (PSP). Methods: We applied [18F]PI-2620 autoradiography to postmortem cortical brain samples of six patients with AD, five patients with PSP and five healthy controls, respectively. Binding intensity was compared between both tissue types and different disease entities. Autoradiography signal quantification (CWMR = cortex to white matter ratio) was correlated with the immunohistochemically assessed tau load (AT8-staining, %-area) for FFPE and frozen tissue samples in the different disease entities. Results: In AD tissue, relative cortical tracer binding was higher in frozen samples when compared to FFPE samples (CWMRfrozen vs. CWMRFFPE: 2.5-fold, p < 0.001), whereas the opposite was observed in PSP tissue (CWMRfrozen vs. CWMRFFPE: 0.8-fold, p = 0.004). In FFPE samples, [18F]PI-2620 autoradiography tracer binding and immunohistochemical tau load correlated significantly for both PSP (R = 0.641, p < 0.001) and AD tissue (R = 0.435, p = 0.016), indicating a high agreement of relative tracer binding with underlying pathology. In frozen tissue, the correlation between autoradiography and immunohistochemistry was only present in AD (R = 0.417, p = 0.014) but not in PSP tissue (R = −0.115, p = n.s.). Conclusion: Our head-to-head comparison indicates that FFPE samples show superiority over frozen samples for autoradiography assessment of PSP tau pathology by [18F]PI-2620. The [18F]PI-2620 autoradiography signal in FFPE samples reflects AT8 positive tau in samples of both PSP and AD patients. [ABSTRACT FROM AUTHOR]