Background: Microglial polarization is a dynamic response to acute brain hypoxia induced by stroke and traumatic brain injury (TBI). However, studies on the polarization of microglia in chronic cerebral circulation insufficiency (CCCI) are limited. Our objective was to investigate the effect of CCCI on microglial polarization after chronic brain hypoperfusion (CBH) and explore the underlying molecular mechanisms.
Methods: CBH model was established by bilateral common carotid artery occlusion (2-vessel occlusion, 2VO) in rats. Using the stereotaxic injection technique, lenti-pre-miR-195 and anti-miR-195 oligonucleotide fragments (lenti-pre-AMO-miR-195) were injeted into the CA1 region of the hippocampus to construct animal models with high or low expression of miR-195. Immunofluorescence staining and flow cytometry were conducted to examine the status of microglial polarization. In vitro, Transwell co-culture system was taken to investigate the role of miR-195 on neuronal-microglial communication through CX3CL1-CX3CR1 signaling. Quantitative real-time PCR was used to detect the level of miR-195 and inflammatory factors. The protein levels of CX3CL1 and CX3CR1 were evaluated by both western blot and immunofluorescence staining.
Results: CBH induced by 2VO initiated microglial/macrophage activation in the rat hippocampus from 1 week to 8 weeks, as evaluated by increased ratio of (CD68+ and CD206+)/Iba-1 immunofluorescence. And the microglial/macrophage polarization was shifted towards the M1 phenotype at 8 weeks following CBH. The expression of CX3CL1 and CX3CR1 was increased in the hippocampus of 2VO rats at 8 weeks. An in vitro study in a Transwell co-culture system demonstrated that transfection of either primary-cultured neonatal rat neurons (NRNs) or microglial BV2 cells with AMO-195-induced M1 polarization of BV2 cells and increased CX3CL1 and CX3CR1 expression and that these effects were reversed by miR-195 mimics. Furthermore, the upregulation of miR-195 induced by lenti-pre-miR-195 injection prevented microglial/macrophage polarization to M1 phenotype triggered by hippocampal injection of lenti-pre-AMO-miR-195 and 2VO surgery.
Conclusions: Our findings conclude that downregulation of miR-195 in the hippocampus is involved in CBH-induced microglial/macrophage polarization towards M1 phenotype by governing communication between neurons and microglia through the regulation of CX3CL1 and CX3CR1 signaling. This indicates that miR-195 may provide a new strategy for clinical prevention and treatment of CBH.
Keywords: Chronic brain hypoperfusion; MicroRNA-195; Microglial/macrophage polarization; Neuroinflammation; Neuronal-microglial cross-talk.