Atypical chemokine receptor 3 regulates synaptic removal in disease astrocytes

Astrocytes participate in the clearance of obsolete or unwanted neuronal synapses. However, the molecular machinery involved in recognizing these synapses remains unclear, particularly under pathological conditions. Here, we investigated the phagocytic process of astrocytes through individual gene silencing with a druggable gene library. Our study demonstrates that astrocyte- mediated synapse engulfment is regulated by the Atypical chemokine receptor 3 (Ackr3). Mechanistically, we showed that Ackr3 recognizes phosphatidylethanolamine (PE)-bound C-X-C motif chemokine 12 (CXCL12) at synaptic terminals both in vitro and in vivo, thus serving as a novel marker of synaptic dysfunction. Notably, both the receptor and its ligand are upregulated in post- mortem human Alzheimer’s disease (AD) brains, and AD mouse models. Downregulation of the Ackr3 in AD mice significantly diminishes astrocyte-mediated synaptic elimination, and rescues pathological phenotypes, including synapse loss and cognitive impairment. Overall, this work unveils a novel, possibly targetable mechanism of astrocyte-mediated synaptic engulfment implicated in neurodegenerative disease.