Exploration of the mechanism of Toll like receptor 4 signaling pathway in the inflammatory reaction of microglia and astrocyte after brain injury

Abstract

Objective To investigate the role of Toll like receptor 4 (TLR4)/nuclear factor-κB (NF-κB) signaling pathway in the inflammatory response of microglia and astrocyte after brain injury. Methods C57BL/10ScNJ mice were used to create a traumatic brain injury (TBI) model in vivo. Primary cultured microglia and primary astrocytes were also used to create an in vitro TBI models by lipopolysaccharide (LPS) stimulation. Downstream signaling pathways of TLR4 were detected by immunofluorescence, Western blotting technique and real-time quantitative polymerase chain reaction (Real-time PCR) to show the expression of inflammatory cytokines. Results In vivo, the numbers of microglia and apoptotic neurons were increased in cortical lesions after TBI, accompanied by TLR4/MyD88/NF-κB signaling pathway related protein up-expression. In vitro, the TLR4/MyD88/NF-κB signaling pathway related protein were up-regulated both in microglia and astrocyte, besides, the mRNA levels of interleukins-1β (IL-1β) and inducible nitric oxide synthase (iNOS) in microglia were 577.03 and 888.70 times compared with control group respectively, which was much same as the astrocyte. What’s more, robust microglia proliferation also emerged. After administration of TAK242 and BAY117086, expression of TLR4, MyD88 and phosphorylated NF-κB decreased as well as IL-1β and iNOS were upregulated both in the microglia and astrocyte. Additionally, when using conditioned medium from microglia and astrocyte stimulated by LPS to activate astrocytes and microglia, expression of both IL-1β and iNOS was found to significantly increase in comparison with LPS stimulation alone. Such effect can be inhibited by TAK242 and BAY117086 respectively. Conclusion In the TBI model, TLR4/NF-κB signaling pathway were activated significantly near the injured brain area. LPS can mimic TBI in vitro and activate microglia and astrocytes to produce proinflammatory responses via the TLR4/NF-κB pathway, besides, microglia and astrocytes can enhance the pro-inflammation effects in astrocytes and microglia respectively through TLR4/NF-κB pathway, the pro-inflammatory factors of which can further promote inflammatory reaction. Key words: Microglia; Astrocyte; Lipopolysaccharide; Toll like receptor 4; Nuclear factor-κB