Abstract
The retinal pigment epithelium (RPE) is a monolayer of pigmented cells between the choroid and the retina. RPE dysfunction underlies many retinal degenerative diseases, including age-related macular degeneration, the leading cause of age-related blindness. To perform its various functions in nutrient transport, phagocytosis of the outer segment, and cytokine secretion, the RPE relies on an active energy metabolism. We previously reported that human RPE cells prefer proline as a nutrient and transport proline-derived metabolites to the apical, or retinal, side. In this study, we investigated how RPE utilizes proline in vivo and why proline is a preferred substrate. By using [13C]proline labeling both ex vivo and in vivo, we found that the retina rarely uses proline directly, whereas the RPE utilizes it at a high rate, exporting proline-derived mitochondrial intermediates for use by the retina. We observed that in primary human RPE cell culture, proline is the only amino acid whose uptake increases with cellular maturity. In human RPE, proline was sufficient to stimulate de novo serine synthesis, increase reductive carboxylation, and protect against oxidative damage. Blocking proline catabolism in RPE impaired glucose metabolism and GSH production. Notably, in an acute model of RPE-induced retinal degeneration, dietary proline improved visual function. In conclusion, proline is an important nutrient that supports RPE metabolism and the metabolic demand of the retina.
Highlights
The retinal pigment epithelium (RPE) is a monolayer of pigmented cells between the choroid and the retina
We previously reported that human fetal RPE in culture consumes more proline than other amino acids [2]
We found that the RPE/choroid complex consumed proline Ͼ100 times faster than the retina (Fig. 1B)
Summary
Lobular loss of the RPE/choroid and progressive retinal degeneration (14 –16). OAT deficiency results in accumulation of more than 10-fold levels of ornithine in the plasma, and ornithine can inhibit P5CS in vitro [17]. In RPE cell lines, supplementation of proline has been shown to rescue ornithine cytotoxicity [18, 19]. A recent large-scale genome-wide association study reports that the locus of this proline transporter is significantly associated with AMD [22, 23]. Previous findings implicate proline metabolism as a potentially essential component of RPE health and function. We investigate how proline is utilized by the RPE and retina as well as the functional roles of proline consumption. By using 13C tracing, we found that proline fuels RPE mitochondrial metabolism. The RPE exports proline-derived intermediates to the retina ex vivo and in vivo. As RPE cells mature in vitro, they become more dependent on proline as a nutrient substrate. Proline supplementation confers resistance against oxidative damage and improves visual function
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
