Abstract
Purpose :
Photoreceptors consume large amounts of energy, yet their metabolism is poorly understood. Alterations in energy production likely underlie photoreceptor degenerative diseases. Seminal studies by Otto Warburg in the 1920s suggested that the retina rely mainly on inefficient aerobic glycolysis to satisfy its energy demand. However, photoreceptors – rods and cones – harbor large numbers of mitochondria, suggesting an extensive use of oxidative and Krebs-cycle metabolism.
Methods :
We manipulated retinal energy metabolism using organotypic retinal explants cultured under entirely controlled, serum-free conditions. Retinal pigment epithelium (RPE) metabolism was investigated by culturing retina with or without RPE. To alter energy metabolism, we used highly selective drugs targeting glucose transport, glycolysis, and mitochondrial proton gradients. Analysis included 1H-NMR metabolomics, immunostaining, and cell viability. Data analysis employed unsupervised hierarchical cluster analysis by Ward’s linkage, pattern hunter correlation, and the KEGG pathway database.
Results :
When compared to control retina, disruption of oxidative phosphorylation with FCCP led to a 20.7-fold increase in rod cell death (±6.9, n=5), while cones were unaffected. Conversely, inhibition of glucose transport caused an almost complete cone loss (-91%±4.9, n=5), with only minor effects on rods (-6.7%±2.1, n=5). Remarkably, culturing of retina without RPE increased glucose consumption and lactate production 4.6-fold (%±1.6, n=5). Metabolomic analysis suggested that photoreceptors uncouple glycolysis and Krebs-cycle via three pathways: 1) an energy-efficient mini-Krebs cycle, fueled by glutamine and branched chain amino acids, generating N-acetylaspartate (NAA); 2) the Cahill cycle, fueled by glutamate, generating alanine; 3) the Cori cycle, fueled by glucose, generating lactate. The latter pathway (Warburg effect) may be artefactual, occurring only when photoreceptors and RPE are separated.
Conclusions :
Our study stresses the importance of oxidative metabolism for rods and glycolysis for cones. In response to rapidly changing demand photoreceptors flexibly uncouple glycolysis from mitochondrial respiration, producing the characteristic signature metabolites lactate, alanine, and NAA, which reveal energy status and may serve as readout and guide for novel therapies for retinal diseases.
This abstract was presented at the 2024 ARVO Annual Meeting, held in Seattle, WA, May 5-9, 2024.