A study on role and mechanism of TLR4/NF-κB pathway in cognitive impairment induced by cerebral small vascular disease

To investigate the role and potential mechanism of Toll-like receptor 4 (TLR4)/nuclear factor-kappa B (NF-κB) signaling pathway in cognitive impairment induced by cerebral small vascular disease (CSVD), so as to provide a reference for the clinical treatment of CSVD-induced cognitive impairment.Mice with TLR4 gene knockout (n = 20) and those with wild-type TLR4 gene (n = 40) aged 8-10 weeks old were divided into blank control group (Control group, n = 20), wild-type + CSVD group (WT + CSVD group, n = 20) and TLR4 gene knockout + CSVD group (TLR4 KO + CSVD group, n = 20). Allogeneic thrombosis (particle diameter: 50-70 mm) was injected to the mouse's external carotid artery to create a model of learning and memory dysfunction. Step-down test and Y-type maze test were utilized to examine the learning and memory abilities of the mice. Reverse transcription-polymerase chain reaction (RT-PCR) and immunoblotting techniques were adopted to measure the levels of apoptosis-related genes [B-cell lymphoma/leukemia-2 (Bcl-2), Bcl-2-associated X protein (Bax), C-caspase-3 and T-caspase-3] in the brain tissues of mice. Terminal dexynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL) method was applied to detect the apoptosis of neuronal cells in the brain tissues. Meanwhile, the levels of oxidative stress markers, including superoxide dismutase (SOD), gp91 and malondialdehyde (MDA), were measured. Finally, the expression level of TLR4/NF-κB pathway was detected.The latency in the step-down test in the WT + CSVD group was remarkably longer than that in the Control group, and the number of errors was evidently larger than that in the Control group (p < 0.05). At the same time, in the WT + CSVD group, the expression levels of pro-apoptotic genes Bax and C-caspase-3 were up-regulated markedly, while the expression level of anti-apoptotic gene Bcl-2 declined notably (p < 0.05). TUNEL results showed that the number of apoptotic cells in the brain tissues in the WT + CSVD group was about 12 times that in the Control group (p < 0.05). Meanwhile, the SOD expression level was lowered, and the MDA expression level was elevated in the brain tissues in the WT + CSVD group. In addition, the TLR4/NF-κB pathway was prominently activated in the mice in the WT + CSVD group (p < 0.05). After TLR4 gene knockout, the cognitive functions of the mice were improved markedly, and the apoptosis of neuronal cells and oxidative stress in the brain tissues were suppressed significantly in the meantime. Moreover, the activation of the TLR4/NF-κB signaling pathway was also inhibited.The TLR4/NF-κB pathway is involved in the occurrence and development of CSVD-induced cognitive impairment through regulating oxidative stress and cell apoptosis.