Astroglial Responses to Amyloid-Beta Progression in a Mouse Model of Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by amyloid-beta (A beta) deposition, hyperphosphorylation of tau, and neuroinflammation. Astrocytes, the most abundant glial cell type in the nervous system, respond to neurodegenerative disorders through astrogliosis, i.e., converting to a reactive inflammatory state. The aim of this study was to investigate how in vivo quantification of astrogliosis using positron emission tomography (PET) radioligand deuterium-l-[C-11]deprenyl ([C-11]DED), binding to enzyme monoamine oxidase-B (MAO-B) which is overexpressed in reactive astrocytes during AD, corresponds to expression of glial fibrillary acidic protein (GFAP) and vimentin, i.e., two well-established markers of astrogliosis, during A beta pathology progression. APP(ArcSwe) mice (n = 37) and wild-type (WT) control mice (n = 23), 2-16-month old, were used to investigate biomarkers of astrogliosis. The radioligand, [C-11]DED, was used as an in vivo marker while GFAP, vimentin, and MAO-B were used to investigate astrogliosis and macrophage-associated lectin (Mac-2) to investigate microglia/macrophage activation by immunohistochemistry of the mouse brain. A beta and GFAP levels were also measured with ELISA in brain homogenates. The intrabrain levels of aggregated A beta and reactive astrocytes were found to be elevated in APP(ArcSwe) compared with WT mice. GFAP and vimentin expression increased with age, i.e., with A beta pathology, in the APP(ArcSwe) mice. This was not the case for in vivo marker [C-11]DED that showed elevated binding of the same magnitude in APP(ArcSwe) mice compared with WT mice at both 8 and 16 months. Further, immunohistochemistry indicated that there was limited co-expression of MAO-B and GFAP. MAO-B levels are increased early in A beta pathology progression, while GFAP and vimentin appear to increase later, most likely as a consequence of abundant A beta plaque formation. Thus, [C-11]DED is a useful PET radioligand for the detection of changes in MAO-B at an early stage of AD progression but does not measure the total extent of astrogliosis at advanced stages of A beta pathology.