Metabolic ecosystems exist between retina and RPE in vivo

Abstract

Purpose : Our lab has proposed a working model for a metabolic ecosystem between the retina and retinal pigment epithelium (RPE). In this model, lactate derived from photoreceptor glycolysis fuels energy metabolism and minimizes glycolysis in RPE. We based this model on findings from ex vivo retina tissue and cultured RPE cells. The tissue microenvironment is different in vivo and ex vivo. These differences could influence metabolism and affect the physiological relevance of our model. In this study we directly evaluate the metabolic ecosystem hypothesis in vivo using ¹³C-labeled glucose infused into the circulation of wild-type mice and mice lacking photoreceptors.

Methods : We infused a 100 mg/kg bolus of ¹³C₆-glucose though external jugular catheters in freely moving 3 month-old C57BL/6J or C57BL/6J; AIPL1^-/- mice. Photoreceptors degenerate by 1 month of age in AIPL1^-/- retinas. We euthanized mice 1-, 2-, 3-, or 5-minutes post-infusion and collected retina and eyecup (RPE, choroid, and sclera) tissue. We extracted and derivatized metabolites from both tissues then assessed metabolite abundance and ¹³C labeling with gas chromatography-mass spectrometry.

Results : Following the infusion, ¹³C labeling is saturated at the earliest time points on glycolytic intermediates in retina and eyecup tissue. In contrast, labeling of TCA cycle intermediates accumulates linearly over 5 minutes, suggesting this time period is optimal for assessing differences in ¹³C flux. We compared ¹³C labeling on C57BL6/J and AIPL1^-/- eyecup tissue 3 minutes following a ¹³C₆-glucose infusion. If photoreceptor-derived lactate is a metabolic intermediate in the eyecup, ¹³C labeling of downstream intermediates should decrease in AIPL1^-/- eyecups. In AIPL1^-/- eyecups, ¹³C enrichment is significantly higher in middle glycolytic intermediates and significantly lower on late glycolytic intermediates (lactate, pyruvate) and TCA cycle intermediates.

Conclusions : Our data illustrates that without photoreceptors, flux of glucose through glycolysis to phosphoenolpyruvate increases in eyecup tissue. In contrast, flux of ¹³C from glucose to lactate, pyruvate, and TCA cycle intermediates decreases. This disconnect in eyecup glucose metabolism in vivo can be explained by shuttling of photoreceptor-derived lactate to feed RPE metabolism. This is the first experimental evidence that a glucose/lactate metabolic ecosystem occurs in a mammalian eye in vivo.

This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.