Research ArticleAlzheimer’s Disease

Complement C3 deficiency protects against neurodegeneration in aged plaque-rich APP/PS1 mice

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Science Translational Medicine  31 May 2017:
Vol. 9, Issue 392, eaaf6295
DOI: 10.1126/scitranslmed.aaf6295

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Avoiding complements

Complement C3 is an immune molecule that protects against pathogens and plays a role in refinement of the developing visual system by removing weak nerve connections (that is, synapses). C3 is up-regulated in Alzheimer’s disease and, therefore, may contribute to the synapse loss that underlies cognitive decline. Shi et al. now report that an aged transgenic mouse model of Alzheimer’s disease that lacks C3 was protected against synapse loss and cognitive decline even in the presence of Aβ plaques, possibly by altering the glial response to Aβ deposition. Thus, modulation of complement signaling may have potential as a new therapeutic strategy for Alzheimer’s disease.

Abstract

The complement cascade not only is an innate immune response that enables removal of pathogens but also plays an important role in microglia-mediated synaptic refinement during brain development. Complement C3 is elevated in Alzheimer’s disease (AD), colocalizing with neuritic plaques, and appears to contribute to clearance of Aβ by microglia in the brain. Previously, we reported that C3-deficient C57BL/6 mice were protected against age-related and region-specific loss of hippocampal synapses and cognitive decline during normal aging. Furthermore, blocking complement and downstream iC3b/CR3 signaling rescued synapses from Aβ-induced loss in young AD mice before amyloid plaques had accumulated. We assessed the effects of C3 deficiency in aged, plaque-rich APPswe/PS1dE9 transgenic mice (APP/PS1;C3 KO). We examined the effects of C3 deficiency on cognition, Aβ plaque deposition, and plaque-related neuropathology at later AD stages in these mice. We found that 16-month-old APP/PS1;C3 KO mice performed better on a learning and memory task than did APP/PS1 mice, despite having more cerebral Aβ plaques. Aged APP/PS1;C3 KO mice also had fewer microglia and astrocytes localized within the center of hippocampal Aβ plaques compared to APP/PS1 mice. Several proinflammatory cytokines in the brain were reduced in APP/PS1;C3 KO mice, consistent with an altered microglial phenotype. C3 deficiency also protected APP/PS1 mice against age-dependent loss of synapses and neurons. Our study suggests that complement C3 or downstream complement activation fragments may play an important role in Aβ plaque pathology, glial responses to plaques, and neuronal dysfunction in the brains of APP/PS1 mice.

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