Glia-to-glia serotonin signaling directs MMP-dependent infiltration for experience-dependent synapse pruning

Synapse connectivity is optimized in response to environmental input in critical periods, characterized by experience-dependent, temporally-restricted, and transiently-reversible synapse pruning by glial phagocytes. This precise, targeted synaptic elimination process requires glial serotonergic intercellular signaling. We discover glia-to-glia communication between different glial classes is essential for experience-dependent synaptic pruning in a well-defined Drosophila juvenile brain olfactory critical period. We find ensheathing glia infiltrate specific target synaptic glomeruli in response to guiding odorant experience via 5-HT2A receptor (5-HT2AR) signaling. Using cell-targeted tryptophan hydroxylase (Trhn) RNAi to block serotonin production, we discover serotonin signaling from ensheathing glia is required for experience-dependent synapse pruning. Using cell-targeted 5-HT2AR RNAi, we find the serotonin receptor is required exclusively in astrocyte-like glia (ALG). Using cell-targeted 5-HT2AR rescue in 5-HT2AR null mutants, we discover the serotonin receptor mediates experience-dependent synapse pruning. Thus, glia-to-glia serotonin signaling between different glial classes mediated by 5-HT2A receptors is necessary and sufficient for synapse elimination. We discover that ALG-targeted conditional 5-HT2AR in mature adults induces experience-dependent synapse pruning indistinguishable from the critical period mechanism. Thus, astrocyte 5-HT2AR signaling is sufficient to 're-open' this characteristic critical period remodeling capability at maturity. We find astrocytic matrix metalloproteinase-1 (MMP-1) induced by critical period odorant experience is required for experience-dependent synapse pruning downstream of 5HT2AR activation. We discover that ALG-targeted MMP-1 induction restores synapse pruning in the absence of 5HT2AR signaling. Taken together, we conclude that glia-to-glia serotonergic 5HT2AR signaling drives MMP-1 for experience-dependent infiltration phagocytosis synapse pruning, and can rekindle this remodeling capacity at adult maturity.