Hyperglycemia is the major risk factor for development of diabetic retinopathy. Tight control of serum glucose levels is currently viewed as the primary means to slow progression to retinopathy, but the mechanisms by which glucose enters the retina and how they become perturbed in diabetes remains unclear. Therefore, understanding the mechanism of how hyperglycemia affects the outer retina (RPE and photoreceptors) and identifying methods to ameliorate glucose-induced outer retina dysfunction and early biomarkers of diabetic retinopathy is important. Recently, it has been shown that systemic reduction of the facilitative diffusion glucose transporter, GLUT1 will reduce glucose levels in the retina; however, these studies do not provide a mechanism or means to control glucose entry specifically to the retina via the blood retinal barrier (BRB). The first objective of this proposal is to determine if reductions in GLUT1 within the RPE leads to reduced retinal glucose accumulation within the RPE/outer retina, prevention of RPE dysfunction and early biomarkers of diabetic retinopathy in mouse models of diabetes. The second objective is to investigate mechanisms of GLUT1 modulation by insulin and to determine if abrogation of insulin signal transduction in the RPE can mitigate abnormal RPE function in mouse models of diabetes by regulation of GLUT1 expression and localization. We hypothesize that (1) hyperglycemia-induced increases in the expression and redistribution of GLUT1 in the RPE mediates diabetes-associated reductions in RPE/outer retina function and early signs of retinopathy, (2) that insulin also modulates glucose transport by GLUT1 in the RPE and (3) that blockade of insulin signaling in the RPE will exacerbate retinal pathophysiology found in a mouse model of diabetes. To test these hypotheses, a mouse model of type 1 diabetes will be employed to investigate GLUT1 expression and distribution within the apical and basolateral membranes of the RPE and determine if this is concomitant with an accumulation of intracellular glucose. To determine if reductions in GLUT1 expression prevents outer retina dysfunction, a haploinsufficient Glut1+/- mouse and mice in which the Glut1 gene is specifically inactivated only within the RPE will be utilized. To additionally assess how insulin affects GLUT1-mediated glucose entry and accumulation in the retina, conditional inactivation the insulin receptor (IR) will be employed to abrogate downstream signaling from this receptor. In vitro analysis of cultured RPE cells will also be used to determine if insulin treatment alters glucose transport through regulation of GLUT1 by affecting its association with a binding partner, GIPC and/or the E3 ubiquitin ligase, Nedd4-2. These experiments will determine the role that GLUT1 plays in outer retina dysfunction as a result of hyperglycemia, elucidate how insulin is involved in GLUT1 modulation, and identify a mechanism which can be targeted for therapeutic intervention.
Nearly twenty percent of veterans are affected by diabetes and are therefore at risk for diabetes-related loss of vision. In patients with diabetes, elevated glucose levels in the blood lead to increased glucose within the retina by transport through special glucose transporters on cells that form the blood-retina-barrier. Diabetic retinopathy is a vision-threatening disorder characterized by a constellation of problems that arise within the eye as a result of the elevated glucose levels in the retina. Because diabetic retinopathy is the leading cause of new cases of blindness among adults aged 20-74, it is important to examine how this disorder develops due to elevated glucose and identify new therapies that aid in the treatment of veterans with diabetes for the prevention of diabetic retinopathy and blindness.
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