Modulation of GLUT1 expression in the RPE impacts outer segment renewal and results in photoreceptor degeneration

Abstract

Purpose : The primary energy source of the retina is glucose which is transported into the retina by the facilitated glucose transporter GLUT1 expressed in the outer (retinal pigment epithelium (RPE)) and inner blood retinal barriers. We used transgenic mice to study the impact of reducing glucose transport across the RPE on photoreceptor outer segment renewal and light responses by deleting GLUT1 from the RPE.

Methods : GLUT1 was knocked out of RPE by crossing transgenic mice expressing Cre recombinase under control of the VMD2 promoter with Glut1^Flox mice to generate Tg-VMD2-Cre:Glut1^Flox/Flox mice (RPEΔGLUT1). Immunolabeling was used to quantify the expression and determine the subcellular localization of GLUT1 in the RPEΔGLUT1 mice. Optical coherence tomography (OCT) was used to monitor retinal morphology. Outer retina and RPE function was assessed by electroretinography (ERG). TUNEL, ATP and lactate were measured using commercially available kits.

Results : GLUT1 is expressed in both the apical and basal membranes of the RPE. Immunolabeling of the RPEΔGLUT1 mice displayed a patchy phenotype for expression of Cre which varied greatly between individual mice, even from the same litter. We focused our analyses on RPEΔGLUT1 mice with ~50% (RPEΔGLUT1₅₀) or ~95% (RPEΔGLUT1₉₅) recombination. RPEΔGLUT1₅₀ mice had normal retinal morphology on OCT imaging, normal amplitude ERGs and no change in retinal levels of ATP or lactate. In comparison, progressive apoptotic cell death of photoreceptors, outer segment shortening, reduced amplitude ERGs and reduced levels of retinal lactate were measured in RPEΔGLUT1₉₅ mice although ATP levels were not significantly different.

Conclusions : The severe phenotype seen in RPEΔGLUT₉₅ mice indicates that glucose transport via the RPE is required to maintain outer segment renewal and function. In contrast, the normal phenotype seen in RPEΔGLUT1₅₀ mice indicates the retina can tolerate at least a 50% reduction in GLUT1 delivery from the RPE.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.