Microglial Pruning of Excitatory Synapses in the Hippocampus Is Complement C3-Independent in Physiological and...

A key feature of brain development is the refinement of exuberant synapses, known as synapse pruning. This phenomenon is executed by microglia, the resident immune cells of the brain. Evidence that microglia prune retinal inputs via activation of the complement cascade in the dorsolateral geniculate nucleus (dLGN) has led to the widely held view that complement-dependent synapse pruning occurs ubiquitously in the brain. However, evidence for developmental complement-dependent pruning in regions outside of the dLGN is largely absent. Here, we tested for complement-dependent synapse pruning by microglia in the hippocampus, a central region for learning and memory. We hypothesized that complement-dependent pruning by microglia occurs at VGLUT2⁺ synapses, where complement proteins are enriched in the hippocampus. We analyzed mice genetically lacking the central complement component C3 (C3^-/-) that have broad inhibition of complement cascade activity. We found that microglia-mediated VGLUT2 engulfment was not significantly impaired in C3^-/- compared to wildtype (WT) mice in the hippocampus in both late postnatal development and early adulthood. Further, functional synapse properties were not significantly altered in C3^-/- compared to WT mice. Finally, we tested whether a complement-dependent pruning mechanism could be induced in the hippocampus via peripheral inflammation. Lipopolysaccharide (LPS) injections induced a neuroinflammatory phenotype associated with loss of VGLUT2⁺ synapses. However, in an LPS-induced neuroinflammatory state, VGLUT2⁺ synapse pruning by microglia was not affected in C3^-/- mice. These findings uncover that C3 is not ubiquitously required for synapse engulfment in physiological and neuroinflammatory conditions in the hippocampus.