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S. Joly, V. Pernet, C. Grimm; Differential Localization of Pax6-Positive Mueller Cell Nuclei After Light-Induced Photoreceptor Degeneration. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2251.
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Pax6 is a transcription factor expressed by multipotent progenitors in the retina during development. It is also a stem cell marker and its expression can be re-initiated in adult Mueller cells, for example after NMDA excitotoxicity. The aim of the present study is to follow the pattern of expression of Pax6 in a mouse model of light-induced photoreceptor degeneration and to determine if Mueller cells can proliferate and dedifferentiate after light-induced photoreceptor cell death.
Adult 129S6/SvEvTac mice were exposed to 13,000 lux of white light for 2 hours to induce photoreceptor degeneration. Intraperitoneal injections of bromodeoxyuridine (BrdU; 50 µg/g) were performed daily for 4 consecutive days, starting immediately after the offset of the light exposure. For immunohistochemical analysis, mice were perfused at different time points after light exposure (N=3 per group). Pax6 protein levels were evaluated by Western-blotting.
By immunohistochemistry, Pax6 was detected in amacrine, horizontal and Mueller cells of adult, unexposed mice. Starting at one day after light exposure, Pax6-positive Mueller cell nuclei appeared in the upper half of the inner nuclear layer (INL), close to the outer nuclear layer (ONL). Total retinal Pax6 protein levels were slightly increased after light exposure. Although light exposure led to Mueller cell gliosis and to the expression of the mitosis marker phospho-histone 3 (PH3) in some Mueller cells, BrdU was only incorporated into nuclear DNA of photoreceptors. BrdU colocalized with ligase-4 positive nuclei suggesting that BrdU incorporation reflected DNA repair rather than DNA duplication. Although intravitreal injections of fibroblast growth factor 2(FGF2)/insulin slightly increased the appearance of Pax6 positive Muller cell nuclei in the upper INL after light exposure, the effect did not differ from PBS/BSA control injections.
Light-induced retinal degeneration caused the appearance of Pax6-positive Mueller cell nuclei in the upper INL/lower ONL of the mouse retina. Interestingly, a similar localization of Pax6-positive nuclei was also detected in the rd10 mouse, a model for inherited photoreceptor degeneration. The expression of PH3 and the lack of BrdU incorporation suggest that Mueller cells attempt but fail to enter the cell cycle in the degenerating retina. Further experiments are ongoing to clarify the role of the differential localization of Pax6-expressing Mueller cell nuclei in retinas undergoing photoreceptor degeneration.
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