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M. A. Scott, C. Zelinka, A. J. Fischer; Insulin-Like Growth Factor 1 (IGF1) Stimulates a Novel Type of Glia-Like Cell in the Retina and Renders Neurons Susceptible to Excitotoxic Damage. Invest. Ophthalmol. Vis. Sci. 2009;50(13):6255.
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© ARVO (1962-2015); The Authors (2016-present)
We have recently found that insulin influences Muller glia and glia-like cells in the retina, thereby rendering neurons more susceptible to excitotoxic damage. However, insulin receptors are not expressed in the retina. Thus, we assume that insulin may be acting at receptors for IGF1. The purpose of this study was to investigate how IGF1 influences cells in the chicken retina.
Postnatal day 5 chicks were injected intraocularly with 800 nanograms IGF1 for 2 consecutive days, with some birds received injections of 1 micromolar NMDA 24 hours after the final IGF1 injection. Tissues were harvested 24 hours after final injections. Using standard techniques, retinas were harvested, fixed, and labeled for known markers for glia, glial reactivity, and cellular proliferation.
IGF1 stimulated glia-like cells in undamaged retinas to up-regulate lysosomal membrane glycoprotein and the intermediate filament transitin. The glia-like cells were scattered across the inner layers of the retina, and these cells expressed Sox2 and Sox9, but were not positive for the microglial marker RCA1. IGF1 stimulated Müller glia to up-regulate p38 MAPK and GFAP, with irregular expression of cFos. By contrast, levels of pERK1/2, Egr1 and pCREB were not increased in retinal cells, neurons or glia, treated with IGF1. With Müller glia and glia-like cells stimulated by IGF1, we observed elevated levels of cell death in response to an excitotoxic insult. An increase in cell death was associated with retinal detachments and a loss of Müller glia in eyes that were treated with IGF1. Levels of cell death remained significantly elevated in detached retinal areas at 3 days after insult, whereas the detachments and "gaps" of Müller glia were repaired within 7 days after insult. By using RT-PCR and in situ hybridization, we found that the IGF1 receptor was expressed by cells in the vitread retinal layers. IGF2 had no effects upon retinal cells, and we failed to detect mRNA for the IGF2 receptor by using RT-PCR.
Intraocular injections of IGF1 have dramatic effects upon glia-like cells in the vitread retinal layer, but had subtle effects upon Müller glia. The identity of the glia-like cells remains uncertain; these cells may be a novel retinal cell type that has not previously been described. We propose that IGF1 influences both Müller glia and other glia-like cells, and this stimulation renders retinal neurons and Müller glia susceptible to an excitotoxic insult.
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