Neuronal exosomes facilitate synaptic pruning by up-regulating complement factors in microglia

Selective elimination of synaptic connections is a common phenomenon which occurs during both developmental and pathological conditions. Glial cells have a central role in the pruning of synapses by specifically engulfing the degenerating neurites of inappropriate connections, but its regulatory mechanisms have been largely unknown. To identify mediators of this process, we established an in vitro cell culture assay for the synapse elimination. Neuronal differentiation and synapse formation of PC12 cells were induced by culturing the cells with nerve growth factor (NGF) in a serum-free medium. To trigger synapse elimination, the NGF-containing medium was replaced with DMEM containing 10% FBS, and the neurites of PC12 cells degenerated within two days. Co-culturing with MG6 cells, a mouse microglial cell line, accelerated the removal of degenerating neurites of PC12 cells by phagocytosis. When MG6 cells were pre-incubated with exosomes secreted from the differentiated PC12 cells after depolarization, the removal was further accelerated by increasing the expression levels of complement component 3 in the MG6 cells. These results define a role for exosomes as a regulator of synapse elimination and clarify a novel mechanism whereby active synapses promote the pruning of inactive ones by stimulating microglial phagocytosis with exosomes.