An organelle with many jobs: Mitochondrial specialization in the retina and RPE

Abstract

The eye is a complex organ with many tissue and cell types working together to facilitate vision. Photoreceptor cells in the retina initiate vision, converting light into a neuronal signal that is integrated and used to form an image in the brain. These polarized cells have high energy demands that fluctuate with light exposure, and they employ a unique metabolic program to balance energy production and essential anabolic pathways. Neighboring retinal pigment epithelial (RPE) cells and Müller glia cells support photoreceptors with complementary and distinct metabolic programs of their own. Breakdown of these metabolic relationships, and consequent photoreceptor death, is thought to underlie many types of inherited and age-related retinal disease. We studied metabolic and mitochondrial adaptations in RPE and photoreceptor cells. Using a combination of animal imaging studies and cultured RPE cells, we demonstrated that RPE cells are capable of transporting large amounts of glucose for uptake by the retina. Photoreceptors extract energy from the glucose by converting it to lactate, then they release lactate from the cell. Additionally, we showed that RPE cells can use lactate to fuel energy production in their mitochondria, which reduces their use of glucose destined for the retina. These findings in the context of recent literature point to distinct metabolic schemes in the RPE and retina. RPE cells have active mitochondria capable of making energy from a wide range of fuels and don’t always need to oxidize glucose. Photoreceptors rely heavily on glucose for both immediate energy production and anabolism, but little is known about how these processes are affected by light or subsequently Ca2+. We performed imaging experiments with transgenic zebrafish expressing a fluorescent Ca2+ indicator in cones to explore the effects of Ca2+ uptake into cone mitochondria. We found that cytosolic Ca2+ in cones is separated into two distinct pools on either side of a large dense cluster of mitochondria. In addition to being a potential physical barrier to diffusion of free Ca2+, we found that cone mitochondria take up Ca2+ and that this uptake is required to keep the cytosolic pools separate. Ca2+ uptake into mitochondria occurs through a uniporter, and could have far-reaching effects on metabolism and other processes necessary for photoreceptor function. Collectively this work highlights some of the mitochondrial adaptations RPE and photoreceptors might employ in a healthy retina. Additionally, a live tissue slice preparation was developed for fluorescent imaging experiments with retinas of zebrafish and mice. This adaptable tool can help answer future questions about metabolism, mitochondrial dynamics, and signaling in the retina.