The Role Of TLR4 - Regulated Neuroinflammation In The Pathogenesis Of Postoperative Cognitive Dysfunction

Part I Prophylactic lithium alleviates splenectomy-induced cognitive dysfunction possibly by inhibiting hippocampal TLR4 activation in aged ratsThough the pathogenesis of postoperative cognitive dysfunction(POCD) remains unclear, evidence is accumulating for a pivotal role of neuroinflammation in the disease process. Advanced age and severe surgical trauma are two main risk factors for POCD. Lithium, a neuroprotective agent, can alleviate peripheral surgery-induced memory impairment in aged rats. The results of in vivo and in vitro experiments also showed that toll like receptor 4(TLR4) was associated with the occurrence and development of neuroinflammation and POCD. So we hypothesized that inhibition of TLR4 signaling in the hippocampus maybe involved in the protective effects of prophylactic lithium on the occurrence of inflammation and POCD. In the present study, we incubated BV-2 microglia with 1 μg/ml lipopolysaccharide(LPS) to mimic neuroinflammation in vitro. We found that pretreatment with 10 m M of lithium or 100 n M of TLR4 si RNA could inhibit the tumor necrosis factor(TNF)-α and TLR4 m RNA expression induced by LPS in BV-2 microglia. Furthermore, combination of prophylactic lithium and TLR4 si RNA even decreased their m RNA expression to the baseline levels, which showed that TLR4 signaling may be vital in protective effects of prophylactic lithium on neuroinflammation. So we further undergone the in vivo experiment. Then, we firstly demonstrated that prophylactic 2 m M/kg of lithium alleviated splenectomy-induced cognitive impairments, decreased splenectomy-associated systemic, central, and hippocampal TNF-α and interleukin(IL)-1β expression and reduced the increase of CD11b+ area in hippocampal CA1 region caused by the surgery. Then, we also found that splenectomy merely increased hippocampal TLR2 and TLR4 m RNA levels in aged rats. At last, we confirmed that prophylactic lithium reduced the increased levels of hippocampal TLR4/NF-κB induced by splenectomy. Taken together, these results demonstrate that TLR4 signaling inactivation may contribute to the protective effects of prophylactic lithium on the occurrence of POCD by inhibiting systemic inflammation and especially neuroinflammation.Part II S100A8 contributes to postoperative cognitive dysfunction in mice undergoing tibial fracture surgery by activating the TLR4/My D88 pathwayNeuro-inflammation plays a key role in the occurrence and development of postoperative cognitive dysfunction(POCD). Although S100A8 and Toll-like receptor 4(TLR4) have been increasingly recognized to contribute to neuro-inflammation, little is known about the interaction between S100A8 and TLR4/My D88 signaling in the process of systemic inflammation that leads to neuro-inflammation. Firstly, we demonstrated that C57BL/6 wide-type mice exhibit cognitive deficit 24 h after the tibial fracture surgery. Subsequently, increased S100A8 and S100A9 expression was found in the peripheral blood mononuclear cells(PBMCs), spleen, and hippocampus of C57BL/6 wide-type mice within 48 h after the surgery. Pre-operative administration of S100A8 antibody significantly inhibited hippocampal microgliosis and improved cognitive function 24 h after the surgery. Secondly, we also observed TLR4/My D88 activation in the PBMCs, spleen, and hippocampus after the surgery. Compared with those in their corresponding wide-type mice, TLR4-/- and My D88-/- mice showed lower immunoreactive area of microglia in the hippocampal CA3 region after operation. TLR4 deficiency also led to reduction of CD45hiCD11b+ cells in the brain and better performance in both Y maze and open field test after surgery, suggesting a new regulatory mechanism of TLR4-dependent POCD. At last, the co-location of S100A8 and TLR4 expression in spleen after operation suggested a close relationship between them. On the one hand, S100A8 could induce TLR4 activation of CD11b+ cells in the blood and hippocampus via intraperitoneal or intracerebroventricular injection. On the other hand, TLR4 deficiency conversely alleviated S100A8 protein-induced hippocampal microgliosis. Furthermore, the increased expression of S100A8 protein in the hippocampus induced by surgery sharply decreased in both TLR4 and My D88 genetically deficient mice. Taken together, these data suggest that S100A8 exerts pro-inflammatory effect on the occurrence and development of neuro-inflammation and POCD by activating TLR4/My D88 signaling in the early pathological process of the postoperative stage.