Nuclear localization of platelet-activating factor receptor accounts for microglial phagocytosis in ischemic stroke

Abstract Platelet-activating factor receptor (PAFR), known as a G-protein-coupled receptor (GPCR), is extensively expressed in the plasma membrane of several cells. Several studies showed that nuclear PAFR performs distinct functions from cell surface. Our previous studies showed that inhibition of microglial PAFR by miR-98 overexpression can protect salvageable neurons from microglial phagocytosis in ischemic stroke; however, the underlying mechanism remains to be fully elucidated. Herein, we created PAFR knockout mice by Crispr/Cas9-mediated genome engineering and performed in vitro studies to determine whether nuclear PAFR is a crucial regulator of microglial phagocytosis in ischemic stroke. Our results demonstrated that nuclear PAFR recruited transcription factor Specificity protein 1 (Sp1) to trigger gene transcription of milk fat globule EGF factor 8 (MFG-E8), which binds to the exposed phosphatidylserine, mediates primary phagocytosis of viable neurons during inflammation, and, in turn, enhances microglial phagocytosis. Membrane-permeable PAFR antagonist Apafant had greater therapeutic effects than membrane-impermeable antagonist Ginkgolide B in ischemic stroke. Our findings provide an explanation for the nuclear PAFR-mediated microglial phagocytosis and have implications for the selection of PAFR antagonists for use in the treatment of ischemic stroke based on the nuclear receptors target.