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Xue Cai, James McGinnis; ER stress involvement in photoreceptor death in tubby mice. Invest. Ophthalmol. Vis. Sci. 2013;54(15):1808.
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© ARVO (1962-2015); The Authors (2016-present)
The tubby mouse arose from a Tub gene mutation and exhibits retinal and cochlear degeneration. Previously we showed that oxidative stress is involved in tubby retinal degeneration. However, the mechanism underlying the photoreceptor death is unknown. Here we test our hypothesis that endoplasmic reticulum (ER) stress promotes photoreceptor cell death.
The retinas from tubby mutant and wild type (wt) mice were collected during retinal development at P4-P28. The expression and distribution of rod- and cone-opsins were examined by immunocytochemistry. The expression of ER stress markers, eukaryotic initiation factor 2α (eIF2α), inositol-requiring protein 1 (IRE1), glucose-regulated protein 78/immunoglobulin heavy chain binding protein (Grp78/Bip) and activating transcription factor 6 (ATF6), was evaluated by western blots. The alteration of the unfolded protein response (UPR) signaling was surveyed by PCR arrays at P7 and P28. The levels of caspase 12, CCAAT/enhancer-binding protein homologous protein (CHOP) and nuclear factor kappa B (NF-κB) were analyzed by western blots or quantitative real-time RT-PCR (qRT-PCR).
Rhodopsin is initially seen at P4 and M-opsin at P12 in wt retina, and both opsins are properly localized in the nascent outer segment (OS). They reach their highest expression level at P28. In tubby retina, rhodopsin is detectable at P7 and localizes in OS, outer nuclear layer (ONL) and outer plexiform layer (OPL). M-opsin appears at P12 and is mislocalized in a pattern similar to rhodopsin. The expression of activated ER stress markers (phospho-eIF2α, phospho-IRE1 and cleaved ATF6) in tubby retina is up regulated compared to age-matched wt in a time-dependent manner, e.g. increases in early time points and down regulated in later time points. PCR arrays of UPR support these observations. Grp78/Bip is elevated at P4 and P7, and decreases at P12 through P28 and this result was confirmed by immunocytochemistry. The dynamics of the expression in these markers concur with the time course of photoreceptor apoptosis in tubby mice. Expression of caspase 12, CHOP and NF-κB is also elevated during retina development suggesting they mediate photoreceptor cell death.
Our data demonstrate that the protein mislocalization in tubby retina is an early and primary event. ER stress is involved in tubby retinal degeneration. Photoreceptor death is promoted by elevation of caspase 12, CHOP and NF-κB expression.
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