Endothelial Progenitor Cell Transplantation Attenuated Synaptic Loss by Enhancing CR3 Dependent Microglial Phagocytosis in Mice

Abstract Background: Endothelial progenitor cell (EPC) transplantation has been shown to have therapeutic effects in cerebral ischemia. However, whether the therapeutic effect of EPCs is a result of the modulation of microglia activity remain elusive. Methods: Adult male mice (n=184) underwent 90 minute-middle cerebral artery occlusion and EPCs were transplanted into the peri-infarct region immediately after the surgery. Microglia migration and phagocytosis were evaluated in the ischemic brain in vivo and underwent oxygen-glucose-deprivation culture condition in vitro. Complement receptor 3 was examined in ischemic brain and cultured primary microglia. Complement receptor 3 agonist leukadherin-1 was intraperitoneally injected to mice immediately after ischemia to imitate the EPC effect. Expression of synapse remodeling related synaptophysin and PSD-95 proteins was detected in the EPC and leukadherin-1 treated mice, separately. Results: EPC transplantation increased the number of microglia in the peri-infarct region of the brain at 3 days after focal ischemia (p<0.05). The ability of phagocytizing apoptotic cells of microglia was higher in EPCs transplanted group at 3 days after ischemia compared to the controls (p<0.05). In vitro study showed that cultured microglia displayed a higher migration (p<0.05) and phagocytosis ability (p<0.05) under the stimulation of EPC conditioned medium or cultured EPCs compared to the controls. Complement receptor 3 expression in the ischemic mouse brain with EPC transplantation (p<0.05), and primary microglia treated by EPC conditioned medium or cultured EPCs was up-regulated (p<0.05). Leukadherin-1 reduced brain atrophy volume at 14 days (p<0.05) and ameliorated neurological deficiency during 14 days after cerebral ischemia (p<0.05). Both EPC transplantation and leukadherin-1 injection increased synaptophysin (p<0.05) and PSD-95 expressions (p<0.05) at 14 days after focal ischemia. Conclusion: We concluded that EPC transplantation promoted regulating complement receptor 3 mediated microglial phagocytosis at acute phase, and subsequently benefited for attenuating synaptic loss at the recovery phase of ischemic stroke, which provided a novel therapeutic mechanism of EPC for cerebral ischemia.