Forebrain microglia from wild-type but not adult 5xFAD mice prevent amyloid-β plaque formation in organotypic hippocampal slice cultures

Scientific Reports, Sep 2015

The role of microglia in amyloid-β (Aβ) deposition is controversial. In the present study, an organotypic hippocampal slice culture (OHSC) system with an in vivo-like microglial-neuronal environment was used to investigate the potential contribution of microglia to Aβ plaque formation. We found that microglia ingested Aβ, thereby preventing plaque formation in OHSCs. Conversely, Aβ deposits formed rapidly in microglia-free wild-type slices. The capacity to prevent Aβ plaque formation was absent in forebrain microglia from young adult but not juvenile 5xFamilial Alzheimer’s disease (FAD) mice. Since no loss of Aβ clearance capacity was observed in both wild-type and cerebellar microglia from 5xFAD animals, the high Aβ1−42 burden in the forebrain of 5xFAD animals likely underlies the exhaustion of microglial Aβ clearance capacity. These data may therefore explain why Aβ plaque formation has never been described in wild-type mice, and point to a beneficial role of microglia in AD pathology. We also describe a new method to study Aβ plaque formation in a cell culture setting.

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Forebrain microglia from wild-type but not adult 5xFAD mice prevent amyloid-β plaque formation in organotypic hippocampal slice cultures

Abstract The role of microglia in amyloid-β (Aβ) deposition is controversial. In the present study, an organotypic hippocampal slice culture (OHSC) system with an in vivo-like microglial-neuronal environment was used to investigate the potential contribution of microglia to Aβ plaque formation. We found that microglia ingested Aβ, thereby preventing plaque formation in OHSCs. Conversely, Aβ deposits formed rapidly in microglia-free wild-type slices. The capacity to prevent Aβ plaque formation was absent in forebrain microglia from young adult but not juvenile 5xFamilial Alzheimer’s disease (FAD) mice. Since no loss of Aβ clearance capacity was observed in both wild-type and cerebellar microglia from 5xFAD animals, the high Aβ1−42 burden in the forebrain of 5xFAD animals likely underlies the exhaustion of microglial Aβ clearance capacity. These data may therefore explain why Aβ plaque formation has never been described in wild-type mice, and point to a beneficial role of microglia in AD pathology. We also describe a new method to study Aβ plaque formation in a cell culture setting. Introduction Extracellular deposition of amyloid-β (Aβ) in plaque form is one of the neuropathological hallmarks of Alzheimer´s disease (AD)1. The amyloid cascade hypothesis, which postulates that the deposition of the Aβ-peptide in the brain is a key event in AD pathology, has influenced AD research for several decades. However, a number of studies on therapeutics (i.e., anti-amyloid antibody treatment) intended to reduce Aβ production or aggregation have failed at various stages of development2, possibly because the mechanisms of Aβ plaque formation and maintenance remain poorly understood. Thus, a better understanding of this pathophysiological phenomenon may promote research into new treatment options for AD. Microglia are the resident brain phagocytes whose role in Aβ plaque formation is debated3,4. On one hand, previous studies have shown that microglia are able to take up Aβ in vitro5,6,7,8. On the other hand, microglial depletion in vivo does not affect Aβ plaque load, as demonstrated in two AD mouse models in which the amyloid precursor protein (APP) was overexpressed9. Two aspects may explain the difference in microglia Aβ uptake capacity in in vitro vs. in vivo conditions. Recent evidence has suggested that cultured microglia display mRNA expression profiles that more closely resemble those of peritoneal macrophages, making them poor models for in vivo microglia10,11,12,13,14. Furthermore, it was found that in vivo depletion of microglia is followed by rapid repopulation of microglia-like cells of unknown origin, making it difficult to establish permanent microglia-free conditions in vivo15,16. Thus, a comprehensive analysis of the role of microglia in Aβ plaque development requires an experimental system that allows reliable microglia depletion in vivo. Organotypic hippocampal slice cultures (OHSCs) serve as a powerful in vitro tool for studying cellular functions, since they maintain many structural and functional properties of the hippocampus in vivo. For example, OHSCs keep their in vivo capacity for supporting different types of neurons and glia, and also retain the complex three-dimensional (3D) organization of the hippocampus. Given that the hippocampus is a strategic region for memory encoding and exhibits early neurodegeneration in AD17,18, OHSCs have been used to study various aspects of AD pathology such as tangle formation, or neuronal loss as a marker of neurodegeneration19,20,21. Of note, Aβ plaque formation has hitherto not been observed or induced in OHSCs of wild-type mice. Thus far, transgenic mouse models of AD have been required to study cerebral Aβ plaque formation (for review see:22), as these do not form in wild-type mice and also cannot be induced23,24,25. However, the mechanistic basis for the lack of Aβ plaque formation in wild-type brain tissue has remained obscure. Remarkably, it is possible to deplete and replenish microglia in OHSCs26,27. Importantly, we have shown that after replenishment, microglia rapidly acquire an in vivo-like distribution and the typical ramified morphology26. Moreover, microglia precursor cells differentiate into ramified microglia when applied to microglia-free OHSC28. Taken together, our previous findings indicate that OHSCs provide an appropriate cellular environment which allows the investigation of microglia in an in vivo-like setting, making them a suitable model to explore the role of microglia in Aβ plaque formation. Against this background, the aims of the present study were two-fold: To investigate the effect of microglia on cerebral Aβ plaque formation in OHSCs derived from wild-type mice. To evaluate potential differences in amyloid-clearance capacity between microglia from wild-type and 5xFamilial Alzheimer’s disease (FAD) mice. Results Depletion of microglia induces the formation of amyloid-beta deposits in wild-type organotypic hippocam (...truncated)


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Sabine Hellwig, Annette Masuch, Sigrun Nestel, Natalie Katzmarski, Melanie Meyer-Luehmann, Knut Biber. Forebrain microglia from wild-type but not adult 5xFAD mice prevent amyloid-β plaque formation in organotypic hippocampal slice cultures, Scientific Reports, 2015, Issue: 5, DOI: 10.1038/srep14624