The role of microglia and Toll-like Receptor-4 in neuronal apoptosis in a subarachnoid hemorrhage model
LeBlanc III, Robert H.
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BACKGROUND A subarachnoid hemorrhage (SAH) is a bleed into the subarachnoid space surrounding the brain. This disease affects roughly 30,000 Americans each year and approximately one in six affected individuals die at the time of the ictal event. Individuals that do survive suffer from many complications including delayed cerebral vasospasm (DCV), cerebral edema, fever, and increased intracranial pressure (ICP) amongst others. These patients often suffer from brain damage due to neuronal apoptosis as a consequence of excess neuroinflammation. Microglia, the resident macrophage of the central nervous system, and Toll-like Receptor-4 (TLR4), a pro-inflammatory transmembrane receptor, have both been shown to play a role in the neuroinflammation seen in SAH. RBC components have been shown to activate microglial TLR4, and this event is suggested to trigger downstream mechanisms leading to neuronal apoptosis. The presented research takes a closer look at the role of microglial TLR4 in early neuronal apoptosis seen in an SAH model. METHODS All mice used were 10- to 12-week-old males on a C57BL/6 background: TLR4−/−, MyD88−/−, TRIF−/− and wild type (WT). To induce an SAH, a total of 60 ul of arterial blood from a donor WT mouse was injected for over 30 seconds into another mouse. For in vitro experiments, either primary microglia (PMG) or murine microglial BV2 cells were used. Microglia were separated from murine neuronal HT22 cells by 3um cell culture inserts or transwells, before being stimulated with lipopolysaccharide (LPS), red blood cells (RBCs), or RBC components including hemin (structurally similar to heme) and hemoglobin. In vivo samples were studied using either immunohistochemistry (IHC) or Fluorescence Activated Cell Sorting (FACS), and in vitro cells were studied using IHC and Light Microscopy. Neuronal cell death was measured using TUNEL and/or FloroJade C (FJC) assays. Cognitive function after SAH was measured using the Barnes Maze protocol. RESULTS In a 24-hour time course, more death occurred in the HT22 cells associated with BV2s treated with RBCs for 12-hour and 24-hour incubation time points as compared to 1-hour and 3-hour time points. Similar results were seen in the WT PMGs, as HT22 apoptosis increased in the RBC treated WT groups as the incubation time points increased. The WT PMG and MyD88−/− RBC treated PMGs showed significant death as compared to a WT untreated control (p<0.05) using a FJC assay, and both showed more death in a TUNEL assay as compared to an untreated control. WT mice treated with whole blood and hemoglobin had significantly more apoptosis as compared with a normal saline (NS)-treated control mouse (p<0.05). WT PMGs treated with whole blood and hemoglobin had more apoptosis as compared with an untreated control. MyD88-/- treated with RBC, hemoglobin, and hemin had more HT22 cell death compared with other genotypes treated with the same component. For the Barnes Maze, TLR4−/− mice performed significantly less total errors than WT mice on POD5 and 6 (p<0.01), and took significantly less time to reach the goal chamber on POD4, POD5 (p<0.05), and POD6 (p<0.01). CONCLUSION Our experimental results suggest that a microglial TLR4-dependent, MyD88-independent pathway is involved in neuronal apoptosis very early in an SAH model via RBC and hemoglobin activation, and that neuronal cell apoptosis due to TLR4 expression may be related to SAH-related cognitive and behavioral deficits. Our results suggest that TRIF may be the intracellular adaptor that is involved in this mechanism, but further experiments are needed to confirm this.