Abstract
Purpose :
The retina and the RPE maintain a symbiotic metabolic relationship, disruption of which leads to wide ranging pathologies. Most metabolic analyses have focused on retinal metabolism while only few studies, using in vitro and ex vivo models, have investigated the metabolic interplay between the retina and RPE. Our goal is to identify if there are any differences in the steady state metabolite levels and the metabolic pathways functioning between the murine retina and RPE.
Methods :
Retinas and RPEs were isolated from 6-7 weeks old C57BL6/J mice at the middle of the light cycle (2PM). For high sensitivity, samples from 6 different animals were pooled for a single N value, with a final N=8 (retina) and N=9 (RPE). Lipophilic and hydrophilic metabolites were separately extracted and analyzed by UPLC-MS/MS for global metabolomics. Normalization, statistical and pathway analyses were done using Python 3.6.
Results :
A total of 650 metabolites were identified in both retina and RPE. ANOVA contrast yielded metabolites with significant differences (p≤0.05): 387 were higher in the RPE and 163 were higher in the retina. All glycolytic intermediates up to and including phosphoglycerate (PGA) were much higher (>3 fold, p<0.0001) in the RPE than the retina. In coherence, pentose phosphate pathway (PPP) and serine biosynthesis, the two pathways feeding off the metabolites upstream to PGA were upregulated several folds (>3, p=0.0054 and >2, p<0.001 respectively) in the RPE. Also phospholipid biosynthesis, especially phosphatidylserine is almost 10-fold higher (p<0.001) in the RPE. Selective upregulation of anaplerotic reactions in the TCA cycle (>2.5 fold, p<0.0001) in the RPE led to elevated proline (1.5 fold, p<0.0001) and glutamine (1.7 fold, p=0.0018) synthesis while a vastly enhanced β oxidation of fatty acid (2-5 fold, p<0.001) was both responsible for driving the elevated TCA cycle as well as being the chief ATP source of RPE. Phototransduction metabolites were higher (>3.5 fold, p<0.0001) in the retina while sphingolipid metabolism was elevated (>2 fold, p<0.001) in the RPE.
Conclusions :
Under normal conditions, the RPE metabolism was highly distinctive from the retina by selective upregulation of metabolic pathways such as PPP and lipid metabolism to facilitate free radical scavenging. Further studies are warranted on these new insights, especially in RPE associated retinopathies like LCA and AMD.
This abstract was presented at the 2019 ARVO Annual Meeting, held in Vancouver, Canada, April 28 - May 2, 2019.