Interactive role of the toll‐like receptor 4 and reactive oxygen species in LPS‐induced microglia activation

Abstract

Microglia are activated by lipopolysaccharide (LPS) to produce neurotoxic pro-inflammatory factors and reactive oxygen species (ROS). While a multitude of LPS receptors and corresponding pathways have been identified, the detailed mechanisms mediating the microglial response to LPS are unclear. Using mice lacking a functional toll-like receptor 4 (TLR4), we demonstrate that TLR4 and ROS work in concert to mediate microglia activation, where the contribution from each pathway is dependent on the concentration of LPS. Immunocytochemical staining of microglia in neuron-glia cultures with antibodies against F4/80 revealed that while TLR4(+/+) microglia were activated the low concentration of 1 ng/ml of LPS, TLR4(-/-) microglia exhibit activated morphology in response to LPS only at higher concentrations (100-1,000 ng/ml). Additionally, tumor necrosis factor-alpha (TNF-alpha) was only produced from higher concentrations (100-1,000 ng/ml) of LPS in TLR4(-/-) enriched microglia cultures. Diphenylene iodonium (DPI), an inhibitor of NADPH oxidase, reduced TNF-alpha production from TLR4(-/-) microglia. The influence of TLR4 on LPS-induced superoxide production was tested in rat enriched microglia cultures, where the presence or absence of serum failed to show any effect on the superoxide production. Further, both TLR4(-/-) and TLR4(+/+) microglia showed a similar increase in extracellular superoxide production when exposed to LPS (1-1,000 ng/ml). These data indicate that LPS-induced superoxide production in microglia is independent of TLR4 and that ROS derived from the production of extracellular superoxide in microglia mediates the LPS-induced TNF-alpha response of both the TLR4-dependent and independent pathway.

Original article. Patterns of microglial innate immune responses elicited by amyloid β1–42and lipopolysaccharide: the similarities of the differences

“…These findings are consistent with constitutive expression of TLR4, SR-BI, and CD36 in these cells, as shown in Figure 1 and with many reports showing that stimulation of these receptors on microglia by either LPS or fAβ facilitates ROS and free radical production [24,39,50,52]. Here, we report that microglia produced H 2 O 2 and NO in response to nfAβ .…”

Reverse of Age-Dependent Memory Impairment and Mitochondrial DNA Damage in Microglia by an Overexpression of Human Mitochondrial Transcription Factor A in Mice

“…LPS is therefore suggested to activate microglia by a multitude of receptors and interacting pathways (Qin et al, 2005), with the production of ROS. Recently, mitochondria have been found to contribute to LPS-induced ROS signaling in macrophages (Woo et al, 2004; Emre et al, 2007).…”

“…Behavioral dysfunctions associated with aging are also postulated to be associated with a decreased activity of mitochondrial electron transfer complexes with aging (Navarro et al, 2004(Navarro et al, , 2005. Furthermore, increased intracellular reactive oxygen species (ROS) activate microglia, which are representative resident mononuclear phagocyte populations in the brain, to induce an increased production of inflammatory mediators (Pawate et al, 2004;Qin et al, 2005). ROS generated in close proximity and in large concentrations by the mitochondrial respiratory chain cause oxidation of unsaturated fatty acid, proteins, and DNA.…”

Peroxisome Proliferator-Activated Receptor-γ Agonists Suppress the Production of IL-12 Family Cytokines by Activated Glia

“…LPS signaling is principally mediated through TLR4. However, at high doses, LPS signaling can also occur through TLR4-independent mechanisms, the details of which have not been elucidated (58). Furthermore, TLR2 and TLR4 have been demonstrated to act cooperatively in responding to bacterial cell wall components (57).…”