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Clemens Alt, Judith M. Runnels, Kin-Sang Cho, Charles P. Lin; In Vivo Imaging of Microglial Turnover in Response to Ionizing Radiation with a multi-color Confocal Scanning Laser Ophthalmoscope. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4998.
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To investigate cellular and vascular responses to total body irradiation (TBI) by longitudinal, non-invasive in vivo imaging of the murine retina as an optically accessible compartment of the central nervous system (CNS).
We have developed a multi-color confocal scanning laser ophthalmoscope (SLO) specifically for mouse retinal imaging. The instrument allows detection of three different colors simultaneously and enables detailed visualization of murine retinal microstructure, such as micro-vasculature and microglia. Mice were exposed to lethal and sublethal doses of gamma radiation; some mice were protected with a head-shield. The integrity of the blood retinal barrier was evaluated by quantifying fluorescein angiography contrast between retinal capillaries and surrounding parenchyma. To investigate microglia turnover, lethally irradiated CX3CR1-GFP mice that express GFP in microglial cells were rescued by bone marrow transplantation from universal DsRed donor mice. The populations of native (GFP) and bone marrow derived cells (DsRed) were tracked for four months after irradiation.
Increased vascular leakage was detected when the head was not shielded from the radiation; leakage occurred even after sublethal irradiation. Dose dependent decrease of the native GFP+ microglia population was observed. After sublethal irradiation, the native microglia population shrank by up to 20% after six weeks, but nearly recovered to baseline (pre-irradiation) density after three months. Lethal irradiation resulted in progressive loss of the native microglia population and concomitant but delayed engraftment of bone marrow derived cells (DsRed+). Time lapse imaging shows that leukocyte endothelial interaction is still present four months after irradiation.
Our results suggest that exposure to gamma radiation, even at sub-therapeutic doses, compromises the integrity of the blood neural barrier and affects microglial turnover in the CNS. Studies utilizing ionizing radiation to prime engraftment of BMDCs may not necessarily measure homeostatic microglial turnover, but instead observe response to radiation-induced injury.
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