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

BACKGROUND: 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 APP NL-G-F/NL-G-F knockin mice. METHODS: C3fl/fl;Rosa26-Cre-ERT2 (C3iKO) mice were crossed with C3fl/fl;APP NL-G-F/NL-G-F mice 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. RESULTS: 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). CONCLUSION: 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.