Abstract
Purpose :
Despite chronic stress and high metabolic demands, photoreceptors remain viable to perform their essential function throughout a lifetime. Maintaining healthy pools of mitochondria is crucial in photoreceptor health and identifying mitochondrial quality control and turnover mechanisms can yield insight into photoreceptor robustness. Photoreceptors may have specialized adaptations in response to mitochondrial stress.
Methods :
We generated zebrafish lines that expressed mitochondrially-targeted fluorophores specifically in cone photoreceptors: Tg(gnat2:Su9-mKate2) and Tg(gnat2:Su9-TagBFP). To visualize Müller glia cells, we used the Tg(GFAP:TdTomato) zebrafish line and cone cell bodies were visualized with the Tg(gnat2:eGFP) zebrafish line. Zebrafish larvae were imaged on a Leica SP8 confocal microscope at 6dpf following 48 hours of being maintained at 16°C (cold stress) or 28°C (normal temperature). To assess if mitochondria were within acidic compartments, zebrafish larvae were incubated in 10µM Lysotracker Green for 2 hours prior to imaging. Results are reported as mean ± SEM.
Results :
Mitochondria in zebrafish cone photoreceptors are found in a dense cluster in the ellipsoid of the cell. Rarely, photoreceptor mitochondria can move away from the mitochondrial cluster towards the synapse, and this is observed much more frequently following cold stress. These mislocalized photoreceptor mitochondria are typically observed in the photoreceptor layer but can also be observed in other areas of the retina. We found that 23.3% ± 5.1% (n = 7 zebrafish) of photoreceptor mislocalized mitochondria colocalized with Müller glia. Mislocalized mitochondria are more likely to colocalize with LysoTracker Green (8.4% ± 0.6% of ellipsoid mitochondria vs 28.3% ± 4.2% of mislocalized mitochondria (n = 7 zebrafish)) suggesting increased mitochondrial turnover upon movement away from the ellipsoid.
Conclusions :
Zebrafish photoreceptors can traffic some mitochondria away from the ellipsoid in response to stress, and these mislocalized mitochondria are more likely to be acidic, which suggests they may be in lysosomes. This may be a protective mechanism in response to mitochondrial stress that aids photoreceptor health and longevity.
This is a 2021 ARVO Annual Meeting abstract.