Novel Approach to Visualize Microglia Death and Proliferation After Intracerebral Hemorrhage in Mice.

BACKGROUND: Microglia are important brain immune cells. However, it is difficult to differentiate microglia from monocyte-derived macrophages. To visualize microglia changes following intracerebral hemorrhage (ICH), we utilized a genetic knock-in mouse line, Tmem119 (transmembrane protein 119)-EGFP (enhanced green fluorescent protein), which expresses EGFP specifically in microglia. METHODS: There were 2 parts in this study. First, autologous blood was injected into the right basal ganglia to model ICH in Tmem119-EGFP mice. Mice were euthanized at 4 hours, days 1, 3, and 7 after ICH. Sham animals were used as controls. Second, Tmem119-EGFP mice were injected with iron or thrombin, factors involved in ICH-induced injury, and were euthanized at 4 hours. Naïve mice were controls. Brains were harvested for histology. RESULTS: The number of perihematomal microglia significantly decreased 1 day after ICH, but markedly increased by days 3 and 7. Microglia death was also induced by intracerebral iron injection while microglia proliferation was found with intracerebral thrombin injection. CONCLUSIONS: Perihematomal microglia death and proliferation after ICH are visualized in vivo with a Tmem119-EGFP transgenic mouse line. Iron and thrombin may contribute to ICH-induced microglia death and proliferation, respectively.