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Daniela M. Verra, Maria A. Contin, David Hicks, Mario E. Guido; Melanopsin X Undergoes A Radical Temporo-spatial Switch In Cellular Expression During Chick Retinal Development. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6001.
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Two orthologs of melanopsin (Opn4) have been identified, Opn4m and Opn4x. We examined expression patterns of photodetection molecules and assessed the histological localization of both Opn4 isoforms in chicken retina during development.
Retinas from embryonic (E) day 4 to 19 or post-hatch day 7 (P7) chicks were used for western blotting (WB), immunohistochemistry (IHC) or RT-PCR. WB used anti-Opn4m/x antibodies, and RT-PCR was done for the transcription factors Pax6, Brn3 and Crx, the G transducin proteins Gq and α-transducin, and opsins Opn4m, Opn4x, rhodopsin and red opsin. IHC was done on E8-19 and P7 retina using antibodies against Opn4m/x and cell-specific markers.
We observed two onsets of transcription, one at E4 with expression of Pax6, Brn3, Gq, and Opn4m and the second at E7-12 with Crx, α-transducin, rhodopsin and red opsin. Strikingly, Opn4x did not appear till E7-E8. WB confirmed mRNA data for Opn4m and x. Immunolocalization of both Opn4 showed that Opn4m was restricted to the GC layer at all ages. In stark contrast, Opn4x was also limited to the GC layer and optic nerve at E8, but by E15 its expression was mainly confined to cells in the outer retina, presumably horizontal cells; Opn4x remained expressed in the optic nerve of the mature retina.
The initial onset of expression of non-visual photocascade molecules could confer early photosensitivity to RGCs. Opn4x switches expression to horizontal cells concomitantly with their birth. These cells play roles in receptive field construction, and the data suggest the existence of a new type of photosensitive retinal cell in birds.
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