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Astroglial tracer BU99008 detects multiple binding sites in Alzheimer's disease brain.

ABSTRACT: With reactive astrogliosis being established as one of the hallmarks of Alzheimer's disease (AD), there is high interest in developing novel positron emission tomography (PET) tracers to detect early astrocyte reactivity. BU99008, a novel astrocytic PET ligand targeting imidazoline-2 binding sites (I₂BS) on astrocytes, might be a suitable candidate. Here we demonstrate for the first time that BU99008 could visualise reactive astrogliosis in postmortem AD brains and propose a multiple binding site [Super-high-affinity (SH), High-affinity (HA) and Low-affinity (LA)] model for BU99008, I₂BS specific ligands (2-BFI and BU224) and deprenyl in AD and control (CN) brains. The proportion (%) and affinities of these sites varied significantly between the BU99008, 2-BFI, BU224 and deprenyl in AD and CN brains. Regional binding studies demonstrated significantly higher ³H-BU99008 binding in AD brain regions compared to CN. Comparative autoradiography studies reinforced these findings, showing higher specific binding for ³H-BU99008 than ³H-Deprenyl in sporadic AD brain compared to CN, implying that they might have different targets. The data clearly shows that BU99008 could detect I₂BS expressing reactive astrocytes with good selectivity and specificity and hence be a potential attractive clinical astrocytic PET tracer for gaining further insight into the role of reactive astrogliosis in AD.

SUBMITTER: Kumar A

PROVIDER: S-EPMC8758481 | biostudies-literature | 2021 Oct

REPOSITORIES: biostudies-literature

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Astroglial tracer BU99008 detects multiple binding sites in Alzheimer's disease brain.

Kumar Amit A Koistinen Niina A NA Malarte Mona-Lisa ML Nennesmo Inger I Ingelsson Martin M Ghetti Bernardino B Lemoine Laetitia L Nordberg Agneta A

Molecular psychiatry 20210423 10

With reactive astrogliosis being established as one of the hallmarks of Alzheimer's disease (AD), there is high interest in developing novel positron emission tomography (PET) tracers to detect early astrocyte reactivity. BU99008, a novel astrocytic PET ligand targeting imidazoline-2 binding sites (I<sub>2</sub>BS) on astrocytes, might be a suitable candidate. Here we demonstrate for the first time that BU99008 could visualise reactive astrogliosis in postmortem AD brains and propose a multiple ...[more]

PMID: 33888872

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Project description:BackgroundDespite significant astrocytic involvement in Parkinson's disease (PD), the knowledge regarding the role of reactive astrogliosis is still at the surface level; largely due to lack of specific biomarkers to track these processes. Novel astroglial PET-tracers BU99008 and Deprenyl, hold immense potential for visualizing reactive astrogliosis in PD. However, they have not been thoroughly investigated in PD.MethodsWe employed a multi-marker approach and performed in vitro radioligand binding and autoradiography studies with 3H-BU99008 and 3H-Deprenyl together with astrocytic immunofluorescence and morphometric analyses in the frontal cortex, temporal cortex, caudate and putamen brain regions of PD (n = 4) and control (n = 7) cases.Results and discussion3H-BU99008 and 3H-Deprenyl showed distinct binding behavior and displayed a diverse array of binding sites (single or multiple) in PD and control brains. Importantly, 3H-BU99008 and 3H-Deprenyl autoradiography studies captured pronounced reactive astrogliosis in PD brain regions, corroborated by marked changes in astrocytic markers, morphology, and cellular processes.HighlightsAstroglial tracers BU99008 and Deprenyl displayed a range of binding sites with different levels of affinity and proportions (%) in healthy control (CN) and Parkinson's disease (PD) brains. Astroglial tracers BU99008 and Deprenyl showed a highly specific (permanent) high-affinity (HA) binding site in the nanomolar range, which might be consistent across different pathologies. Astroglial tracers BU99008 and Deprenyl highlighted distinct tracer binding behavior, indicating that they might be targeting different subpopulations or specific states of astrocytes in CN and PD brains. Astroglial tracers BU99008 and Deprenyl captured prominent reactive astrogliosis at the advanced/end stages of PD, substantiated by a significant increase in intercellular adhesion molecule 1 (ICAM-1)-positive reactive astrocytes and marked changes in astrocytic morphology and processes.

Project description:Tauopathies are a subclass of neurodegenerative diseases characterized by an accumulation of microtubule binding tau fibrils in brain regions. Diseases such as Alzheimer's (AD), chronic traumatic encephalopathy (CTE), Pick's disease (PiD), and corticobasal degeneration (CBD) belong to this subclass. Development of tracers which can visualize and discriminate between different tauopathies is of clinical importance in the diagnosis of various tauopathies. Currently, several tau tracers are available for in vivo imaging using a positron emission tomography (PET) technique. Among these tracers, PBB3 is reported to bind to various types of tau fibrils with comparable binding affinities. In contrast, tau tracer AV-1451 is reported to bind to specific types of tau fibrils (in particular to AD-associated and CTE) with higher binding affinity and only show nonspecific or weaker binding toward tau fibrils dominant with 3R isoforms (associated with PiD). The tau fibrils associated with different tauopathies can adopt different microstructures with different binding site microenvironments. By using detailed studies of the binding profiles of tau tracers for different types of tau fibrils, it may be possible to design tracers with high selectivity toward a specific tauopathy. The microstructures for the tau fibrils from patients with AD, PiD, and CTE have recently been demonstrated by cryogenic electron microscopy (cryo-EM) measurements allowing structure-based in silico simulations. In the present study, we have performed a multiscale computational study involving molecular docking, molecular dynamics, free energy calculations, and QM fragmentation calculations to understand the binding profiles of tau tracer AV-1451 and its potential use for diagnosis of AD, CTE, and PiD tauopathies. Our computational study reveals that different affinity binding sites exist for AV-1451 in the tau fibrils associated with different tauopathies. The binding affinity of this tracer toward different tau fibrils goes in this order: PiD > AD > CTE. The interaction energies for different tau fibril-tracer complexes using the QM fragmentation scheme also showed the same trend. However, by carrying out molecular dynamics simulations for the AD-derived tau fibrils in organic solvents, we found additional high affinity binding sites for AV-1451. The AV-1451 binding profile in these cryptic sites correctly explains the preferential binding of this tracer toward the AD fibrils when compared with the PiD fibrils. This study clearly demonstrates having a cryo-EM structure is still not sufficient for the structure-based tracer discovery for certain targets, as they may have "potential but hidden" high affinity binding sites, and we need additional strategies to identify them.

Project description:PurposeMK6240 is a second-generation tau PET tracer designed to detect the neurofibrillary tangles in the brains of patients with Alzheimer's disease (AD). The aim of the study was to characterize 3H-MK6240 in AD and control brain tissue and to compare its binding properties with those of first-generation tau PET tracers.MethodsSaturation binding assays with 3H-MK6240 were carried out in the temporal and parietal cortices of AD brains to determine the maximum number of binding sites (Bmax) and the dissociation constants (Kd) at these sites. Competitive binding assays were carried out between 3H-MK6240 and unlabelled MK6240, AV-1451 (aka T807, flortaucipir) and THK5117, and between 3H-THK5351 and unlabelled MK6240. Regional binding studies with 3H-MK6240 were carried out in homogenates from six AD and seven control brains and, using autoradiography, on large frozen sections from two AD brains and one control brain.ResultsThe saturation binding assays gave Bmax and Kd values of 59.2 fmol/mg and 0.32 nM in the temporal cortex and 154.7 fmol/mg and 0.15 nM in the parietal cortex. The competitive binding assays revealed two binding sites with affinities in the picomolar and nanomolar range shared by 3H-MK6240 and all the tested unlabelled compounds. There were no binding sites in common between 3H-THK5351 and unlabelled MK6240. Regional binding of 3H-MK6240 was significantly higher in AD brain tissue than in controls. Binding in brain tissue from AD patients with early-onset AD was significantly higher than in brain tissue from patients with late-onset AD. Binding of 3H-MK6240 was not observed in off-target regions. Autoradiography showed high regional cortical binding in the two AD brains and very low binding in the control brain.Conclusions3H-MK6240 has a high binding affinity for tau deposits in AD brain tissue but also has different binding characteristics from those of the first-generation tau tracers. This confirms the complexity of tau tracer binding on tau deposits with different binding affinities for different binding sites.

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Astroglial tracer BU99008 detects multiple binding sites in Alzheimer's disease brain.

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Astroglial tracer BU99008 detects multiple binding sites in Alzheimer's disease brain.

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