Microglial activation and responses to vasculature that result from an acute LPS exposure

• Published in: Neurotoxicology (Park Forest South) 2020-03, Vol.77, p.181-192
• Main Authors: Bowyer, John F., Sarkar, Sumit, Burks, Susan M., Hess, Jade N., Tolani, Serena, O’Callaghan, James P., Hanig, Joseph P.
• Format: Article
• Language: English
• Subjects: Activation; Animals; Antibodies; Astrocytes; Astrocytes - drug effects; Astrocytes - immunology; Blood circulation; Blood-brain barrier; Brain; CD14 antigen; Cell adhesion; Cell adhesion & migration; Cell adhesion molecules; Cell walls; Cortex (parietal); Cytokines; Damage; Encephalitis - chemically induced; Encephalitis - immunology; Endothelial cells; Endotoxins; Exposure; Female; Forebrain; Glial fibrillary acidic protein; Hippocampus; Hippocampus - blood supply; Hippocampus - drug effects; Hippocampus - immunology; Immune response; Immune system; Immunoglobulin G; Inflammation; Innate immunity; Lipopolysaccharide; Lipopolysaccharides; Lipopolysaccharides - toxicity; Mice, Inbred BALB C; Microglia; Microglia - drug effects; Microglia - immunology; Neurodegeneration; Neuroinflammation; Neuronal-glial interactions; Neurotoxicity; Prefrontal Cortex - blood supply; Prefrontal Cortex - drug effects; Prefrontal Cortex - immunology; Thalamus; TLR4 protein; Toll-like receptors
• ISSN: 0161-813X; 1872-9711
• DOI: 10.1016/j.neuro.2020.01.014

•Subcutaneous injection of 2 mg/kg LPS resulted in the activation of 55 to 65% of the microglia present in the hippocampus and cortex within 24 h.•However, none of the microglia had “phagocytic type” morphology that is normally seen in areas of neurodegeneration, and no neurodegeneration was observed in brain via Fluorojade C labeling.•The activated microglial soma and proximal enlarged processes became elongated and more closely associated with brain vasculature.•We hypothesize that by forming a much closer associations with vasculature the activated microglia would be more effective in preventing bacterial entry into the brain. Bacterial cell wall endotoxins, i.e. lipopolysaccharides (LPS), are some of the original compounds shown to evoke the classic signs of systemic inflammation/innate immune response and neuroinflammation. The term neuroinflammation often is used to infer the elaboration of proinflammatory mediators by microglia elicited by neuronal targeted activity. However, it also is possible that the microglia are responding to vasculature through several signaling mechanisms. Microglial activation relative to the vasculature in the hippocampus and parietal cortex was determined after an acute exposure of a single subcutaneous injection of 2 mg/kg LPS. Antibodies to allograft inflammatory factor (Aif1, a.k.a. Iba1) were used to track and quantify morphological changes in microglia. Immunostaining of platelet/endothelial cell adhesion molecule 1 (Pecam1, a.k.a. Cd31) was used to visualize vasculature in the forebrain and glial acidic fibrillary protein (GFAP) to visualize astrocytes. Neuroinflammation and other aspects of neurotoxicity were evaluated histologically at 3 h, 6 h, 12 h, 24 h, 3 d and 14 d following LPS exposure. LPS did not cause neurodegeneration as determined by Fluoro Jade C labeling. Also, there were no signs of mouse IgG leakage from brain vasculature due to LPS. Some changes in microglia size occurred at 6 h, but by 12 h microglial activation had begun with the combined soma and proximal processes size increasing significantly (1.5-fold). At 24 h, almost all the microglia soma and proximal processes in the hippocampus, parietal cortex, and thalamus were closely associated with the vasculature and had increased almost 2.0-fold in size. In many areas where microglia were juxtaposed to vasculature, astrocytic endfeet appeared to be displaced. The microglial activation had subsided slightly by 3 d with microglial size 1.6-fold tha