Reduction of Glut1 in the Neural Retina But Not the RPE Alleviates Polyol Accumulation and Normalizes Early Characteristics of Diabetic Retinopathy

Abstract

Hyperglycemia is a key determinant for development of diabetic retinopathy (DR). Inadequate glycemic control exacerbates retinopathy, while normalization of glucose levels delays its progression. In hyperglycemia, hexokinase is saturated and excess glucose is metabolized to sorbitol by aldose reductase via the polyol pathway. Therapies to reduce retinal polyol accumulation for the prevention of DR have been elusive due to low sorbitol dehydrogenase levels in the retina and inadequate inhibition of aldose reductase. Using systemic and conditional genetic inactivation, we targeted the primary facilitative glucose transporter in the retina, Glut1, as a preventative therapeutic in diabetic male and female mice.Unlike wildtype diabetics, diabetic Glut1 +/mice did not display elevated Glut1 levels in the retina. Furthermore, diabetic Glut1 +/mice exhibited ameliorated ERG defects, inflammation and oxidative stress, which was correlated with a significant reduction in retinal sorbitol accumulation. RPE-specific reduction of Glut1 did not prevent an increase in retinal sorbitol content or early hallmarks of DR. However, like diabetic Glut1 +/mice, reduction of Glut1 specifically in retinal neurons mitigated polyol accumulation and completely prevented retinal dysfunction and the elevation of markers for oxidative stress and inflammation associated with diabetes. These results suggest that modulation of retinal polyol accumulation via Glut1 in photoreceptors can circumvent the difficulties in regulating systemic glucose metabolism and be exploited to prevent DR.Significance: Diabetic retinopathy (DR) affects one third of diabetic patients and is the primary cause of vision loss in adults aged 20-74. While anti-VEGF and photocoagulation treatments for the late-stage vision threatening complications can prevent vision loss, a significant proportion of patients do not respond to anti-VEGF therapies and mechanisms to stop progression of early-stage symptoms remain elusive. Glut1 is the primary facilitative glucose transporter for the retina. We determined that a moderate reduction in Glut1 levels, specifically in retinal neurons, but not the RPE, was sufficient to prevent retinal

Glucose uptake by GLUT1 in photoreceptors is essential for outer segment renewal and rod photoreceptor survival

“…Localization of GLUT1 protein was examined by immunolabeling retinal cryosections from control and RetΔGlut1 mice (Figure 2B). In control mice, the brightest labeling was observed in the basolateral and apical membranes of the RPE, retinal capillaries, and MGC somas and their processes in the INL and ONL (Figure 2B) as described previously 7,26,41,42 . Faint GLUT1 labeling was also detected in the IS, consistent with its distribution in photoreceptors 7,26,41 .…”

“…In control mice, the brightest labeling was observed in the basolateral and apical membranes of the RPE, retinal capillaries, and MGC somas and their processes in the INL and ONL (Figure 2B) as described previously 7,26,41,42 . Faint GLUT1 labeling was also detected in the IS, consistent with its distribution in photoreceptors 7,26,41 . In comparison, GLUT1 immunolabeling was mostly absent from the retinal layers of Ret∆Glut1 mice but was detected in the RPE and inner blood vessels.…”

“…7,26,41,42 Faint GLUT1 labeling was also detected in the IS, consistent with its distribution in photoreceptors. 7,26,41 In comparison, GLUT1 immunolabeling was mostly absent from the retinal layers of Ret∆Glut1 mice but was detected in the RPE and inner blood vessels. Consistent with the in situ data, GLUT1 staining was detected only sporadically in MGCs or photoreceptors in the outer retina.…”

Cellular stress signaling and the unfolded protein response in retinal degeneration: mechanisms and therapeutic implications

“…The retina has high metabolic demands to support its function in generating and transmitting visual signals and maintain the normal structure of photoreceptors. In diabetes, retinal metabolism is disrupted due to elevated glucose levels, correlated with enhanced glycolysis and sorbitol oxidation, which has been implicated in the pathogenesis of DR [ 209 – 211 ]. Diverting upstream metabolites from glycolysis into other pathways, such as the hexosamine, diacylglycerol (DAG)/PKC, and AGE pathways, leads to endothelial injury in diabetes [ 212 ].…”

“…Diverting upstream metabolites from glycolysis into other pathways, such as the hexosamine, diacylglycerol (DAG)/PKC, and AGE pathways, leads to endothelial injury in diabetes [ 212 ]. Systemic reduction of GLUT1 or deletion of GLUT1 in retinal neurons prevents polyol accumulation and improves retinal function in diabetic animals, suggesting a role of metabolic dysregulation in neurodegeneration in DR [ 209 ]. In addition, mitochondrial dysfunction and damage leads to reduced mitochondrial respiratory activity further contributing to the imbalance between glycolysis and oxidative phosphorylation in diabetic retinal cells [reviewed in [ 213 ].…”