C3 lowering in adult APP KI mice rescues synapses and spares cognitive decline

We previously reported that germline complement C3 deletion protected cognition and hippocampal synapses in aged APP/PS1dE9 mice despite increased amyloid plaques. To assess whether global C3 lowering in adult amyloid mice might be neuroprotective, we crossed our C3 inducible conditional mice with APPNL−G−F/NL−G−F knockin mice. C3fl/fl;Rosa26-Cre-ERT2 (C3iKO) mice were crossed with C3fl/fl;APPNL−G−F/NL−G−Fmice to generate APP;C3iKO mice, which were treated with tamoxifen (TAM, n = 16) or corn oil (CO, n = 16) for 5 consecutive days at 3.6 months of age. Serum was collected 30 days post-treatment and at study termination to measure C3 levels. Behavioral testing was conducted at 15 months, followed by euthanasia and brain tissue analysis via ELISA, immunofluorescence, qPCR, and RNAseq. Serum C3 levels were reduced by ~ 85% 30-days post-TAM treatment and ~ 70% at the end of the study. TAM-treated APP;C3iKO mice performed significantly better on cognitive tests compared to CO-treated mice. Complement C3 and C1q levels were significantly reduced in brain. No differences were observed in cerebral amyloid load. Iba-1 immunoreactivity of microglia was reduced in the hippocampal CA3 region of TAM-injected mice, while no differences were seen in GFAP labeling of astrocytes. However, hippocampal plaques were associated with fewer CD68- positive microglia and GFAP-positive astrocytes. Additionally, presynaptic markers (SYN and Bassoon) and postsynaptic markers (PSD95 and Homer1) were elevated in the CA3 and CA1 subregions of hippocampus. TAM treatment of APP;C3iKO mice led to reduced mRNA levels of C3, TNF-α, IL-10, CX3CR1, and IL-6. RNAseq identified 1071 differentially expressed genes (569 upregulated, 502 downregulated), with many related to synaptic signaling (e.g., Bassoon, Homer1, Syn2, SYNPO, and SNAP25). Global complement C3 lowering in adult APP-KI mice mitigated neuroinflammation, preserved synaptic integrity, and improved cognition despite having no effect on cerebral amyloid. These findings suggest that complement-targeted therapies, especially delivered to the brain, may protect synapses and slow cognitive decline.