Increasing Ca2+ in photoreceptor mitochondria alters metabolites, accelerates photoresponse recovery, and reveals adaptations to mitochondrial stress

Abstract

Photoreceptors are specialized neurons that rely on Ca 2+ to regulate phototransduction and neurotransmission. Photoreceptor dysfunction and degeneration occur when intracellular Ca 2+ homeostasis is disrupted. Ca 2+ homeostasis is maintained partly by mitochondrial Ca 2+ uptake through the mitochondrial Ca 2+ uniporter (MCU), which can influence cytosolic Ca 2+ signals, stimulate energy production, and trigger apoptosis. Here we discovered that zebrafish cone photoreceptors express unusually low levels of MCU. We expected that this would be important to prevent mitochondrial Ca 2+ overload and consequent cone degeneration. To test this hypothesis, we generated a cone-specific model of MCU overexpression. Surprisingly, we found that cones tolerate MCU overexpression, surviving elevated mitochondrial Ca 2+ and disruptions to mitochondrial ultrastructure until late adulthood. We exploited the survival of MCU overexpressing cones to additionally demonstrate that mitochondrial Ca 2+ uptake alters the distributions of citric acid cycle intermediates and accelerates recovery kinetics of the cone response to light. Cones adapt to mitochondrial Ca 2+ stress by decreasing MICU3, an enhancer of MCU-mediated Ca 2+ uptake, and selectively transporting damaged mitochondria away from the ellipsoid toward the synapse. Our findings demonstrate how mitochondrial Ca 2+ can influence physiological and metabolic processes in cones and highlight the remarkable ability of cone photoreceptors to adapt to mitochondrial stress.

Mitochondrial Calcium Uniporter (MCU) deficiency reveals an alternate path for Ca2+ uptake in photoreceptor mitochondria

“…If this Ca 2+ were to enter mitochondria, it could enhance ATP production by stimulating TCA cycle dehydrogenases 3 . In line with this hypothesis, overexpressing MCU in zebrafish cones alters the distribution of TCA cycle metabolites in a way which is consistent with enhanced activity of Ca 2+ -sensitive dehydrogenases 12 .…”

“…These observations suggested to us that mitochondrial Ca 2+ uptake could play an important role in modulating photoreceptor function. Given this, it is surprising that zebrafish cone photoreceptors express extremely low levels of MCU 12 . MCU has been widely thought to be the sole route of Ca 2+ entry into mitochondria in eukaryotes, as loss of MCU completely inhibits mitochondrial Ca 2+ uptake in skeletal muscle, liver, heart, brown adipose tissue, and a wide variety of cell lines 15 – 19 .…”

“…Mitochondria from zebrafish cones can prevent Ca 2+ in the outer segment from reaching the rest of the cell and conversely, they can prevent Ca 2+ in the synapse from reaching the outer segment 11 . Increasing mitochondrial Ca 2+ uptake in zebrafish cones by overexpressing MCU also accelerates cytosolic Ca 2+ clearance, which causes cones to recover from light exposure faster 12 .…”

“…(F) Normalized response amplitude data for experiments shown in (C) and (D) indicate that sensitivity is not changed in global mcu -/cones (Bars indicate standard error). 12,29 . Our cyto-GCaMP3 measurements indicate that mcu -/cones clear cytoplasmic Ca 2+ slower compared to WT cones.…”

Daily mitochondrial dynamics in cone photoreceptors

“…The fate of degraded mitochondrial material varies by cell type; canonically it enters the endolysosomal pathway (Strappazzon et al 2015), but in neurons it can translocate and leave the cell (Melentijevic et al 2017;C.-H. O. Davis et al 2014). Mitochondria mislocalize toward the nucleus in degenerating human cones and (Litts et al 2015) and in a zebrafish cone model of mitochondrial calcium overload (Hutto et al 2020). 4 h before light onset (05:00, ZT20) we observed mislocalized cone mitochondria using IHC with gnat2:mito-cpYFP transgenic zebrafish and 3 different markers of the mitochondrial respiratory chain ( Figure 4G, yellow arrowheads).…”