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A Wenzel, C Grimm, MS Obin, MI Simon, J Lem, CE Remé; Light Induced Photoreceptor Apoptosis: Is Phototransduction Involved? . Invest. Ophthalmol. Vis. Sci. 2002;43(13):3720.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Light induced apoptosis of rod-photoreceptors is mediated by rhodopsin and involves activation of the transcription factor AP-1. However, the signaling pathways leading from photon absorption by rhodopsin to the initiation of the death program and/or activation of AP-1 are unknown. Evidence from mice deficient for arrestin (Arr-/-) or rhodopsin kinase (Rhok-/-) - both involved in phototransduction shut off - suggested that undamped phototransduction increases a photoreceptor's sensitivity to light damage. Thus, phototransduction may be a way to mediate light damage. Here we analyze mice deficient for the transducin alpha-subunit (Gnat1-/-), which lack rod phototransduction, and Arr-/- / Rhok-/- doubly mutant mice to test for an involvement of phototransduction in white light induced apoptosis of photoreceptors. Methods: Mice were exposed to 3 different white light regimens: (1) 5000lux for 1-30 minutes (2) 1700lux for 1 week (3) 800lux 12:12h cyclic for 5 month. Light damage was analyzed by retinal morphology. Electromobility shift assays (EMSA) were employed to test for an involvement of AP-1. Results: With regimen (1), Arr-/- / Rhok-/- doubly mutant mice were extremely sensitive to light damage with apoptosis of photoreceptors occurring after 2 minutes of light exposure, thus being at least 10 times more sensitive than a corresponding wild type mouse. Gnat1-/- mice showed light damage after 20-30 minutes of light exposure, thus being indistinguishable from wild type mice. In Arr-/- / Rhok-/- and in Gnat1-/- mice light induced AP-1 DNA binding activity as in wild type mice. With regimen (2) and (3), Gnat1-/- and Gnat1+/+ mice displayed comparable patterns of photoreceptor cell loss. Conclusion: Death signals generated by rhodopsin upon absorption of a lethal dose of photons are not mediated via phototransduction. Alternative pathways must exist that lead from bleaching of rhodopsin via activation of AP-1 to cell death. Those pathways may be modulated by the interaction of rhodopsin with arrestin and / or rhodopsin kinase.
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