Here we report multiple lines of evidence for a comprehensive model of energy metabolism in the vertebrate eye. Metabolic flux, locations of key enzymes, and our finding that glucose enters mouse and zebrafish retinas mostly through photoreceptors support a conceptually new model for retinal metabolism. In this model, glucose from the choroidal blood passes through the retinal pigment epithelium to the retina where photoreceptors convert it to lactate. Photoreceptors then export the lactate as fuel for the retinal pigment epithelium and for neighboring Müller glial cells. We used human retinal epithelial cells to show that lactate can suppress consumption of glucose by the retinal pigment epithelium. Suppression of glucose consumption in the retinal pigment epithelium can increase the amount of glucose that reaches the retina. This framework for understanding metabolic relationships in the vertebrate retina provides new insights into the underlying causes of retinal disease and age-related vision loss.

In Gram-negative methylotrophic bacteria, the first step in methylotrophic growth is the oxidation of methanol to formaldehyde in the periplasm by methanol dehydrogenase. In most organisms studied to date, this enzyme consists of the MxaF and MxaI proteins, which make up the large and small subunits of this heterotetrameric enzyme. The Methylobacterium extorquens AM1 genome contains two homologs of MxaF, XoxF1 and XoxF2, which are ϳ50% identical to MxaF and ϳ90% identical to each other. It was previously reported that xoxF is not required for methanol growth in M. extorquens AM1, but here we show that when both xoxF homologs are absent, strains are unable to grow in methanol medium and lack methanol dehydrogenase activity. We demonstrate that these defects result from the loss of gene expression from the mxa promoter and suggest that XoxF is part of a complex regulatory cascade involving the 2-component systems MxcQE and MxbDM, which are required for the expression of the methanol dehydrogenase genes.Methylobacterium extorquens AM1 is a methylotrophic bacterium ubiquitous in the environment and in particular on the undersides of leaves. This organism is able to metabolize both single-carbon compounds like methanol and multicarbon compounds like succinate and pyruvate (1,11,19). M. extorquens AM1 is a model organism for understanding the process of methylotrophic growth and has been studied biochemically and genetically for over 50 years (reviewed in reference 7). To utilize methanol as a sole source of carbon and energy, methanol is first oxidized to formaldehyde in the periplasm via methanol dehydrogenase, a soluble quinoprotein (2-4). This enzyme uses pyrroloquinoline quinone (PQQ) to sequentially transfer two electrons to cytochrome c L , which enters the electron transport chain, resulting in ϳ1 molecule of ATP per molecule of methanol oxidized (9). Methanol dehydrogenase is a heterotetrameric protein consisting of two 66-kDa large subunits (MxaF) and two small 8.5-kDa subunits (MxaI) (28, 37). The large subunit contains the active-site residues and the PQQ prosthetic group, which is coordinated to a calcium ion in the active site (37). The loss of this enzyme in M. extorquens

Production of energy in a cell must keep pace with demand.Photoreceptors use ATP to maintain ion gradients in darkness, whereas in light they use it to support phototransduction. Matching production with consumption can be accomplished by coupling production directly to consumption. Alternatively, production can be set by a signal that anticipates demand. In this report we investigate the hypothesis that signaling through phototransduction controls production of energy in mouse retinas. We found that respiration in mouse retinas is not coupled tightly to ATP consumption. By analyzing metabolic flux in mouse retinas, we also found that phototransduction slows metabolic flux through glycolysis and through intermediates of the citric acid cycle. We also evaluated the relative contributions of regulation of the activities of ␣-ketoglutarate dehydrogenase and the aspartate-glutamate carrier 1. In addition, a comprehensive analysis of the retinal metabolome showed that phototransduction also influences steady-state concentrations of 5-GMP, ribose-5-phosphate, ketone bodies, and purines. Warburg et al.(1) and Krebs (2) reported in the 1920s that tumors and retinas rely on aerobic glycolysis. Some of the biochemical mechanisms by which cancer cells adapt to aerobic glycolysis have been gleaned from investigations of specific metabolic adaptations of cancer cells either in culture or in a tumor (3, 4). Retinas offer distinct advantages for investigating aerobic glycolysis. They have high metabolic rates, a uniquely laminated structure, and the primary signaling pathway by which retinas respond to light is defined clearly.Retinas convert 80 -96% of glucose they consume into lactic acid (5-9), similar to the extent of aerobic glycolysis that fuels cancer cells (3). Aerobic glycolysis occurs primarily in photoreceptors (10) where the energy demands are very different in darkness than in light (5,7,(11)(12)(13). In darkness energy is consumed within the inner segments to support ion pumping (5, 13). In light energy is consumed by the outer segments (OS) 2 to support phototransduction and regeneration of visual pigments. A photoreceptor neuron also performs anabolic metabolism to replace the ϳ10% of its OS material that is lost each day to phagocytosis by the retinal pigmented epithelium (14, 15).Some of the carbons in glucose consumed by a retina reach the mitochondrial matrix where they are oxidized in biochemical reactions that reduce NAD ϩ to NADH. Transfer of electrons from NADH to O 2 then generates a proton gradient across the mitochondrial inner membrane. In some tissues dissipation of the proton gradient is coupled tightly to ATP demand. In others, proton leakage can dissipate the gradient even without ATP synthesis (16). In this report we show that mitochondria in retinas are more uncoupled than mitochondria in other tissues.The ability of a photoreceptor to respond to light, to release neurotransmitter, to regenerate visual pigment, to renew itself, and to remain viable requires that production of energy keeps pace...

Background: Pyruvate transport into mitochondria is a key step in energy metabolism. Zaprinast is a well known phosphodiesterase inhibitor.Results: Zaprinast has a strong influence on pyruvate transport into mitochondria. Conclusion: Inhibition of the mitochondrial pyruvate carrier by Zaprinast causes accumulation of aspartate at the expense of glutamate. Significance: Maintenance of normal amino acid levels in the retina relies on pyruvate transport into mitochondria.