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T. J. Bailey, S. L. Fossum, J. E. Montgomery, D. R. Hyde; Müller Glia and Phagocytosis of Constant Intense Light-Damaged Photoreceptors. Invest. Ophthalmol. Vis. Sci. 2010;51(13):2260.
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Intense constant light treatment of dark-adapted albino zebrafish results in photoreceptor apoptosis. Subsequently, many, but not all Müller glia in the light-damaged retina dedifferentiate and proliferate to produce retinal progenitor cells, which continue to proliferate, migrate from the inner nuclear layer to the outer nuclear layer and replace the lost photoreceptors. Although a robust regeneration response is seen in zebrafish, only meager cell proliferation is found in damaged mammalian retinas. The mechanism by which Müller glia are triggered to respond to damage is not known. We tested whether engulfment of apoptotic cell bodies are required to initiate the Müller glial proliferative response after injury.
Dark-adapted albino zebrafish were intravitreally injected with control saline, that either contained or lacked a small molecule inhibitor of phagocytosis, L-serine-O-phosphate (L-SOP) and were placed in constant intense light. Treated fish retinas were analyzed for cell death, by TUNEL, and proliferation, by immunohistochemistry. The ability of L-SOP to inhibit Müller glial cell proliferation was also compared for potential side effects with metabotropic glutamate receptors using the agonist L-2-amino-4-phosphonobutyrate (L-AP4), or antagonist L-2-amino-3-phophonopropanoic acid (L-AP3).
Müller glia endocytosed both TUNEL label and a rod-specific marker in light damaged retinas. Engulfment of apoptotic photoreceptor cell bodies correlated with Proliferating cell nuclear antigen (Pcna) expression in Müller glial cells. Injection of L-SOP, L-AP4, or L-AP3 failed to neuroprotect the photoreceptors in light-damaged retinas. L-SOP, but not L-AP4 or L-AP3, however, showed reduced numbers of Pcna-positive Müller glia in the light-damaged retina.
No evidence was found for the involvement of metabotropic glutamate receptor signaling in retinal regeneration. The Müller glia proliferative response to retinal damage is due, at least in part, to the engulfment of apoptotic cellular debris.
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