Complement C3 Lowering in Adult Inducible Conditional Knockout Mice: Long‐Lasting Effects

Abstract

AbstractBackgroundComplement C3, a central component of the complement cascade, participates in synaptic elimination during brain development and is elevated in the brain with aging and Alzheimer’s disease (AD). Germline C3 knockout (KO) protects against age‐ and AD‐related hippocampal synapse loss and functional decline (Shi Q et al., J Neurosci 2015; Shi Q et al., Sci Transl Med 2017). We crossed C3fl/fl mice with Rosa26‐Cre‐ERT2 mice to generate global C3 inducible conditional KO mice (C3iKO). Previously, we reported that 5 daily TAM treatments in C3iKO mice at 4‐5 mo of age led to sustained C3 reductions of ∼95% in serum and ∼80% in brain, and protected mice against age‐related cognitive decline at 16‐17 months. In addition, TAM treatment at 3‐4 mo of age protected long‐term potentiation (LTP) in hippocampal slices exposed to toxic Aβ dimers (S26C) obtained at 7‐8 mo of age. Here, we extend our analyses to include changes in complement, cytokine and ApoE expression in these same C3iKO mice and provide confirmatory results in a second cohort of mice.MethodqRT‐PCR was used to quantify brain gene expression in the first cohort. In the second cohort, C3iKO, C3fl/fl and C3KO mice were treated with TAM or corn oil (CO) at 7‐8 mo of age. Cognitive testing commenced at 17‐18 mo and included the Spatial Novelty Y Maze (SPNY), Novel Object Recognition (NOR) and Displaced Object Recognition (DOR). Mice were euthanized and hippocampal synaptosomes isolated for Western blot analysis.ResultOne‐year post‐treatment, C1qa, C1qb, C1qc, C3, CD11b, CD18, IL‐1β, IFNα, IFNβ, and APOE mRNAs were reduced, and TGFβ and TGFβR1 mRNAs were elevated in brains of TAM‐treated C3iKO and C3KO mice compared to CO‐treated C3iKO mice. In the second cohort, C3 protein levels were reduced ∼92% in serum and ∼77% in hippocampal synaptosomes in TAM‐treated C3iKO mice. TAM‐treated C3iKO mice and both C3KO groups showed significantly better performance in SNYM, LOR and DOR tests compared to controls, consistent with our first study. Pre‐ and post‐synaptic markers were elevated in TAM‐treated vs. CO‐treated C3iKO hippocampal synaptosomes.ConclusionGlobal C3 lowering in adulthood has long‐lasting effects on immune signaling and hippocampal function. (NIH RF1 AG060057‐CAL)