Mechanisms of sodium butyrate inhibition of microglia inflammatory activation in hippocampus via Toll-like receptor 4/nuclear factor-κB p65 pathway

Abstract

To investigate the effects of sodium butyrate (NaB) on long-term anxiety like behavior and inflammatory activation of microglia in the hippocampus of sepsis-associated encephalopathy (SAE) mice. (1) Animal experiment: fifty C57BL/6 mice aged 6-8 weeks were randomly divided into Sham group (only the cecum was found by laparotomy without perforation or ligation), and SAE model group caused by cecal ligation and puncture (CLP; SAE model group, the cecum was found by laparotomy and perforated after ligation. The open field test indicated that the ability of independent exploration decreased and showed anxiety like behavior, which proved that the SAE model was successfully replicated) and NaB pretreatment group was established (NaB was administered at a dose of 500 mg×kg-1×d-1 for 3 days before modeling, and the same dose once a day for 3 days after modeling). Open field test was used to detect the anxiety like behavior of mice at 7 days. The protein expressions and content changes of interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α) in hippocampus of mice at 1 day and 3 days after operation were detected by Western blotting and enzyme linked immunosorbent assay (ELISA). Immunofluorescence staining was used to observe microglia labeled protein ionized calcium bindingadaptor molecule-1 (Iba-1) and TNF-α protein co localization. (2) Cell experiment: mouse microglia cell line BV-2 microglia were divided into blank control group, lipopolysaccharide (LPS) group (cells were treated with 1 mg/L LPS), and NaB treatment group (cells were treated with 1 mg/L LPS+5 mmol/L NaB). The protein expressions of IL-1β, TNF-α, Toll-like receptor 4 (TLR4), phosphorylated nuclear factor-κB p65 (p-NF-κB p65), nuclear factor-κB p65 (NF-κB p65) and NF-κB inhibitor protein-α (IκB-α) were detected by Western blotting. The expressions of Iba-1 and TNF-α in each group were observed by immunofluorescence. (1) Animal experiment: compared with the Sham group, the distance and duration of movement in the central area, the total distance moved of mice decreased 7 days after the establishment of SAE model group were decreased [distance of movement in the central area (mm): 13.45±3.97 vs. 161.44±27.00, duration of movement in the central area (s): 1.82±0.58 vs. 13.45±2.17, the total distance moved (mm): 835.01±669.67 vs. 2 254.51±213.45, all P < 0.05]. In the hippocampus tissues of mice, a large number of nerve nuclei were pyknotic and deeply stained, and the arrangement of nerve cells was disordered. The cell bodies of microglia in mouse hippocampus increased significantly. The number of positive cells of Iba-1/TNF-α (Iba-1+/TNF-α+) increased significantly. The contents and protein expression of proinflammatory factors TNF-α, IL-1β in hippocampal homogenate supernatant 3 days after operation in SAE model group were significantly higher than those in Sham group [TNF-α (ng/L): 119.17±18.40 vs. 90.18±21.17, IL-1β (ng/L): 407.89±70.64 vs. 313.69±34.63; TNF-α/GAPDH: 1.42±0.50 vs. 0.80±0.08, IL-1β/GAPDH: 1.27±0.22 vs. 0.85±0.25, all P < 0.05]. After intragastric administration of NaB, the distance and duration of movement in the central area of mice were significantly higher than those in SAE model group [distance of movement in the central area (mm): 47.39±15.63 vs. 13.45±3.97, duration of movement in the central area (s): 6.12±1.87 vs. 1.82±0.58, all P < 0.05]. There was no significant change in the total distance moved (mm: 1 550.59±1 004.10 vs. 835.01±669.67, P > 0.05). The pyknosis and deep staining of nerve nuclei in mice were significantly less than those in SAE model group. The number of Iba-1+/TNF-α+ positive cells decreased significantly. The contents and protein expression levels of proinflammatory factors TNF-α, IL-1β in hippocampal homogenate supernatant 3 days after operation were significantly lower than those in SAE model group [TNF-α (ng/L): 64.95±9.10 vs. 119.17±18.40, IL-1β (ng/L): 311.94±69.92 vs. 407.89±70.64; TNF-α/GAPDH: 1.02±0.36 vs. 1.42±0.50, IL-1β/GAPDH: 0.86±0.20 vs. 1.27±0.22, all P < 0.05]. (2) Cell experiment: after LPS intervention, the fluorescence intensity of TNF-α in BV-2 cells was significantly enhanced, the protein expression levels of TNF-α, IL-1β, TLR4 and p-NF-κB p65 protein increased (TNF-α/GAPDH: 0.39±0.06 vs. 0.20±0.02, IL-1β/GAPDH: 0.27±0.03 vs. 0.19±0.01, TLR4/GAPDH: 0.55±0.12 vs. 0.33±0.09, p-NF-κB p65/NF-κB p65: 0.55±0.05 vs. 0.29±0.04, all P < 0.05), the expression level of IκB-α was lower than that in the control group(IκB-α/GAPDH: 0.54±0.06 vs. 0.81±0.03, P < 0.05). After NaB treatment, the fluorescence intensity of TNF-α in BV-2 cells was decreased. The protein expression levels of TNF-α, IL-1β, TLR4 and p-NF-κB p65 protein were significantly lower than that of LPS model group (TNF-α/GAPDH: 0.26±0.02 vs. 0.39±0.06, IL-1β/GAPDH: 0.11±0.04 vs. 0.27±0.03, TLR4/GAPDH: 0.28±0.14 vs. 0.55±0.12, p-NF-κB p65/NF-κB p65: 0.29±0.01 vs. 0.55±0.05, all P < 0.05), the protein expression level of IκB-α was significantly higher than that in the LPS group (IκB-α/GAPDH: 0.75±0.01 vs. 0.54±0.06, P < 0.05). NaB could antagonism the TLR4 activation induced by LPS, thus inhibiting p-NF-κB p65 nuclear transcription and IκB-α degradation. It can reduce microglia activation and secretion of inflammatory factors, and finally improve the inflammation in the hippocampus of septic mice and long-term anxiety like behavior.