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Michelle Giarmarco, Mark Kanow, Abbi Engel, Jianhai Du, Jennifer R Chao, James Hurley; Fuel exchange between photoreceptors and RPE underlies a retinal metabolic ecosystem. Invest. Ophthalmol. Vis. Sci. 2017;58(8):3018.
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The choroid and retinal pigment epithelium (RPE) supply glucose to the outer retina as a vital component of retinal function and health. Photoreceptors (PRs) are highly glycolytic, converting ~90% of glucose they receive to lactate. The RPE is also metabolically active, raising questions about how it can maintain glucose export. We propose that a “metabolic ecosystem” exists in the outer retina, whereby RPE uses PR lactate as fuel, enabling net flow of glucose towards PRs. To test this hypothesis, we examined glucose transporter (GLUT) distribution in mouse retina, assayed retinal glucose uptake in vivo, and measured glucose transport across cultured human fetal RPE (hfRPE) cells using mass spectrometry.
Immunohistochemistry: Adult mouse eyecups were fixed in 4% paraformaldehyde and cut into 20-µm cryosections. Sections were incubated with primary antibodies against glutamine synthetase, arrestin and GLUT1, followed by incubation with fluorescent-tagged secondary antibodies. Sections were imaged via confocal microscopy.In vivo Glucose Uptake: A fluorescent glucose analog, 2-NBDG, was administered orally to mice or zebrafish expressing red fluorescent protein in PRs or Müller glia. Following incubation period eyes were collected, and fresh retinal slices were prepared and imaged via confocal microscopy.hfRPE Glucose Transport: hfRPE cells were grown in transwell filter plates until transepithelial resistance was greater than 200 Ω cm2. 13C-glucose was added to the basolateral compartment, while increasing concentrations of different metabolites were added to the apical chamber. Transport of 13C-glucose into the apical chamber was quantified using liquid chromatography mass spectroscopy.
GLUT1 is abundant in mouse PRs, and PRs but not Müller glia accumulate 2-NBDG in vivo. Together these results indicate that PRs take up outer retinal glucose whereas Müller glia do not. We also found that lactate enhances glucose transport across RPE, suggesting that lactate made by PRs and released into the interphotoreceptor matrix promotes translocation of glucose across the RPE.
Evidence of glucose uptake into PRs, paired with a reliance of RPE on lactate for basal-to-apical glucose transport, supports the existence of a novel metabolic relationship between RPE and PRs. Breakdown of PR or RPE cells’ metabolic specificity could contribute to retinal dysfunction in aging or disease.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.
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