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      Dystrophic (senescent) rather than activated microglial cells are associated with tau pathology and likely precede neurodegeneration in Alzheimer’s disease

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          Abstract

          The role of microglial cells in the pathogenesis of Alzheimer’s disease (AD) neurodegeneration is unknown. Although several works suggest that chronic neuroinflammation caused by activated microglia contributes to neurofibrillary degeneration, anti-inflammatory drugs do not prevent or reverse neuronal tau pathology. This raises the question if indeed microglial activation occurs in the human brain at sites of neurofibrillary degeneration. In view of the recent work demonstrating presence of dystrophic (senescent) microglia in aged human brain, the purpose of this study was to investigate microglial cells in situ and at high resolution in the immediate vicinity of tau-positive structures in order to determine conclusively whether degenerating neuronal structures are associated with activated or with dystrophic microglia. We used a newly optimized immunohistochemical method for visualizing microglial cells in human archival brain together with Braak staging of neurofibrillary pathology to ascertain the morphology of microglia in the vicinity of tau-positive structures. We now report histopathological findings from 19 humans covering the spectrum from none to severe AD pathology, including patients with Down’s syndrome, showing that degenerating neuronal structures positive for tau (neuropil threads, neurofibrillary tangles, neuritic plaques) are invariably colocalized with severely dystrophic (fragmented) rather than with activated microglial cells. Using Braak staging of Alzheimer neuropathology we demonstrate that microglial dystrophy precedes the spread of tau pathology. Deposits of amyloid-beta protein (Aβ) devoid of tau-positive structures were found to be colocalized with non-activated, ramified microglia, suggesting that Aβ does not trigger microglial activation. Our findings also indicate that when microglial activation does occur in the absence of an identifiable acute central nervous system insult, it is likely to be the result of systemic infectious disease. The findings reported here strongly argue against the hypothesis that neuroinflammatory changes contribute to AD dementia. Instead, they offer an alternative hypothesis of AD pathogenesis that takes into consideration: (1) the notion that microglia are neuron-supporting cells and neuroprotective; (2) the fact that development of non-familial, sporadic AD is inextricably linked to aging. They support the idea that progressive, aging-related microglial degeneration and loss of microglial neuroprotection rather than induction of microglial activation contributes to the onset of sporadic Alzheimer’s disease. The results have far-reaching implications in terms of reevaluating current treatment approaches towards AD.

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          Most cited references38

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          Systemic infections and inflammation affect chronic neurodegeneration.

          It is well known that systemic infections cause flare-ups of disease in individuals with asthma and rheumatoid arthritis, and that relapses in multiple sclerosis can often be associated with upper respiratory-tract infections. Here we review evidence to support our hypothesis that in chronic neurodegenerative diseases such as Alzheimer's disease, with an ongoing innate immune response in the brain, systemic infections and inflammation can cause acute exacerbations of symptoms and drive the progression of neurodegeneration.
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            Dystrophic microglia in the aging human brain.

            We have studied microglial morphology in the human cerebral cortex of two nondemented subjects using high-resolution LN-3 immunohistochemistry. Several abnormalities in microglial cytoplasmic structure, including deramification, spheroid formation, gnarling, and fragmentation of processes, were identified. These changes were determined to be different from the morphological changes that occur during microglial activation and they were designated collectively as microglial dystrophy. Quantitative evaluation of dystrophic changes in microglia revealed that these were much more prevalent in the older subject (68-year-old) than in the younger one (38-year-old). Thus, we conclude that microglial dystrophy is a sign of microglial cell senescence. We hypothesize that microglial senescence could be important for understanding age-related declines in cognitive function. Copyright 2003 Wiley-Liss, Inc.
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              Activation of microglial cells by beta-amyloid protein and interferon-gamma.

              Alzheimer's disease is the most common cause of progressive intellectual failure. The lesions that develop, called senile plaques, are extracellular deposits principally composed of insoluble aggregates of beta-amyloid protein (A beta), infiltrated by reactive microglia and astrocytes. Although A beta, and a portion of it, the fragment 25-35 (A beta (25-35)), have been shown to exert a direct toxic effect on neurons, the role of microglia in such neuronal injury remains unclear. Here we report a synergistic effect between A beta and interferon-gamma (IFN-gamma) in triggering the production of reactive nitrogen intermediates and tumour-necrosis factor-alpha (TNF-alpha) from microglia. Furthermore, using co-culture experiments, we show that activation of microglia with IFN-gamma and A beta leads to neuronal cell injury in vitro. These findings suggest that A beta and IFN-gamma activate microglia to produce reactive nitrogen intermediates and TNF-alpha, and this may have a role in the pathogenesis of neuronal degeneration observed in ageing and Alzheimer's disease.
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                Author and article information

                Contributors
                +1-352-3923910 , +1-352-3928347 , streit@mbi.ufl.edu
                Journal
                Acta Neuropathol
                Acta Neuropathologica
                Springer-Verlag (Berlin/Heidelberg )
                0001-6322
                1432-0533
                10 June 2009
                October 2009
                : 118
                : 4
                : 475-485
                Affiliations
                [1 ]Department of Neuroscience, McKnight Brain Institute, University of Florida College of Medicine, PO Box 100244, Bldg. 59, Gainesville, FL 32610-0244 USA
                [2 ]Institute for Clinical Neuroanatomy, J.W. Goethe University Clinic, Frankfurt am Main, Germany
                [3 ]Clinical Neuroanatomy, Department of Neurology, University of Ulm, Ulm, Germany
                Article
                556
                10.1007/s00401-009-0556-6
                2737117
                19513731
                7f5eeea4-a97b-4004-bf50-28678e9ba220
                © The Author(s) 2009
                History
                : 7 May 2009
                : 2 June 2009
                : 2 June 2009
                Categories
                Original Paper
                Custom metadata
                © Springer-Verlag 2009

                Neurology
                microglia,dementia,alzheimer’s disease,down’s syndrome,aging
                Neurology
                microglia, dementia, alzheimer’s disease, down’s syndrome, aging

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