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E. N. Soniat, K. G. Sheets, E. Knott, W. C. Gordon, D. R. Bergsma, N. G. Bazan; Müller Cell Response During Photoreceptor Oxidative Stress. Invest. Ophthalmol. Vis. Sci. 2007;48(13):5058.
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Light-induced retinal damage initiates events that lead to photoreceptor apoptosis. We have shown that as oxidative stress intensifies (e.g., increased 8-oxoguanine [8-OG]), mDNA is damaged, and mDNA repair enzymes (8-oxoguanine-DNA-glycosylase [OGG-1] and DNA polymerase γ) are-up-regulated. If these responses are successful, photoreceptors survive. If not, further corrective mechanisms are initiated. Pro-apoptotic molecules are activated and nDNA fragmentation is followed by up-regulation of nDNA repair enzymes (e.g., DNA polymerase ß). Then, if successful, photoreceptors survive. Otherwise, cell death occurs. The purpose of this study was to determine if Müller cells undergo oxidative stress initiated by light-damaged photoreceptors.
Rats were dark-adapted, stimulated with bright light and examined immunohistochemically for up to 12 hours and after two weeks following light onset. Antibodies to vimentin and glutamine synthetase were ocalized; oxidative stress was indicated by the presence of 8-OG, and the Müller cell-specific glutamate transporter GLAST-1 was monitored. OGG-1 was also localized. These observations were compared to dark controls and corroborated by Western blots.
An indicator of oxidative stress (8-OG) was localized within Müller cells and showed up-regulation within 5 hours following onset of light, then rapidly increased. OGG-1 was also up-regulated. However, GLAST-1 was rapidly down-regulated within the distal retinas, followed by loss of glutamine synthetase. In regions where photoreceptors survived, these activities returned. Similar results were obtained by immunohistochemistry and Western blotting.
Increases in 8-OG within Müller cells indicates a secondary response conditions within and/or around stressed photoreceptors. Cytoplasmic OGG-1 (not nuclear) increased, suggesting Müller cell mDNA damage. Finally, glutamate uptake was attenuated or halted and glutamine synthetase was greatly diminished, signifying manipulation of Müller cell regulatory activity. Müller cells undergo oxidative stress in response to that of photoreceptors and stop the process of glutamate uptake and conversion to glutamine. Because these changes return to normal if photoreceptors survive oxidative insult, Müller cells must be active participants in the retina’s response to damaging stress, and the ability of the Müller cell to manipulate its intra- and extra-cellular environment may be critical for photoreceptor repair and survival.
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