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Marcel Victor Alavi, Wei-Chieh Chiang, Heike Kroeger, Douglas Yasumura, Michael T Matthes, Matthew M LaVail, Douglas B Gould, Jonathan H Lin; IRE1 Signaling and ER Stress Levels Increase With Age in Retina. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4565.
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© ARVO (1962-2015); The Authors (2016-present)
Endoplasmic reticulum (ER) stress causes different adaptive responses of the cell, among them the endoplasmic unfolded protein response (UPR). ER stress has been found in the pathogenesis of retinal degenerations associated with constant light and genetic mutations that cause protein misfolding. To date, however, the role of ER stress during normal physiologic aging of the retina is unclear. We have addressed this question by investigating the ER-stress activated indicator (ERAI) mouse line, which expresses GFP upon XBP1 mRNA splicing by IRE1 during ER stress. XBP1 encodes a transcription factor whose gene targets include ER protein-folding chaperones. Here, we report the levels of ER stress monitored in real-time in vivo in the eye of living wild-type mice as they age under standardized laboratory conditions from postnatal day (P) 4 to P270.
The ERAI mouse line has been described previously. Levels of ER stress were assessed in vivo by fluorescence ophthalmoscopy and confirmed by immunohistochemistry (IHC) and molecular biology. All studies and procedures were performed in accordance with the guidelines of the UCSF IACUC and in compliance with the ARVO Statement for the Use of Animals in Ophthalmic and Vision Research.
We confirmed that ERAI mice allow us to monitor ER stress in vivo by chemically and genetically inducing ER stress. We also tested for any corrupting effects of the ERAI transgene on the integrity of the retina by optical coherence tomography and found no signs of retinal degeneration in wild-type mice during the study period. Our in vivo analyses revealed an unexpected and significant age-dependent increase in fluorescence levels in wild-type mice, which is indicative of increased ER stress in the retina of aged animals. We confirmed activation of this reporter by IHC and immunoblotting. We also provide evidence for age-dependent increase in XBP1 splicing, upregulation of transcriptional targets of IRE1-XBP1 signaling, and other ER stress-induced genes in aging retinas.
Our findings show that the UPR and IRE1 pathway are significantly activated during normal physiologic aging of the retina under standard laboratory conditions, even in the absence of genetic mutations or environmental insults. We suggest that damaged proteins and lipids build up in retinal cells as they age leading to increased ER stress and UPR signaling.
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