Abstract
Purpose :
To maintain vision throughout a lifetime, photoreceptors must remain viable despite chronic stress, including oxidative stress. Photoreceptors may have specialized stress adaptations, such as maintaining healthy pools of mitochondria, to remain robust and functional. We exposed zebrafish to systemic or cone-specific stressors and evaluated the impact on cone mitochondria.
Methods :
We generated transgenic zebrafish with fluorophores targeted to cone mitochondria (Tg(gnat2:Su9-mKate2); Tg(gnat2:Su9-TagBFP)). Zebrafish larvae were exposed to cold stress (16°C), cone-specific oxidative stress (using the transgenic line Tg(gnat2:Su9-KillerRed)), or 1.2 mM chloramphenicol starting at 3 or 4dpf. At 6dpf, larvae were imaged live using confocal microscopy.
To assess cone mitochondria localization, we used a triple transgenic fish expressing a cone mitochondrial marker, a Müller glia marker (Tg(GFAP:TdTomato)) and a cone cytosol marker (Tg(gnat2:EGFP)).
To assess mitophagy, we used larvae co-expressing the BFP mitochondria marker and an autophagosome marker fluorescently tagged under the Müller glia promotor (Tg(GFAP:EGFP-mCherry-LC3)). Results reported as mean ± SEM.
Results :
Cone mitochondria were rarely seen outside the ellipsoid region, but this was increased post all stressors. 23.6% ± 1.6% (n = 44 zebrafish) of mislocalized cone mitochondria co-localized with Müller glia after cold stress. This occurred to a lesser extent in unstressed conditions (14.0% ± 3.7%; n = 38 zebrafish). Of the mislocalized mitochondria in Müller glia, 4.5% ± 3.0% co-localized with unacidified LC3, 33.2% ± 5.7% co-localized with acidified LC3, and 48.4% ± 9.0% were proximal to LC3 post cold stress (n = 10 zebrafish).
Conclusions :
Photoreceptor mitochondria adapt to chronic stress by migrating away from the dense mitochondrial cluster in the ellipsoid region. These mitochondria can leave the cell and be taken up by Müller glia cells. Some cone mitochondria are likely undergoing mitophagy since they are associated with autophagosomes in Müller glia.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.