Early retinal pigment epithelium dysfunction is concomitant with hyperglycemia in mouse models of type 1 and type 2 diabetes

In the diabetic retina, cellular changes in the retinal pigment epithelium (RPE) and neurons occur before vision loss or diabetic retinopathy can be identified clinically. The precise etiologies of retinal pathology are poorly defined, and it remains unclear if the onset and progression of cellular dysfunction differ between type 1 and type 2 diabetes. Three mouse models were used to compare the time course of RPE involvement in type 1 and type 2 diabetes. C57BL/6J mice injected with streptozotocin (STZ mice) modeled type 1 diabetes, whereas Lepr(db/db) mice on both BKS and B6.BKS background strains modeled type 2 diabetes. Electroretinogram (ERG)-based techniques were used to measure light-evoked responses of the RPE (direct current-coupled ERG, dc-ERG) and the neural retina (a-wave, b-wave). Following onset of hyperglycemia, a-wave and b-wave amplitudes of STZ mice declined progressively and by equivalent degrees. Components of the dc-ERG were also altered, with the largest reduction seen in the c-wave. Lepr(db/db) mice on the BKS strain (BKS.Lepr) displayed sustained hyperglycemia and a small increase in insulin, whereas Lepr(db/db) mice on the B6.BKS background (B6.BKS.Lepr) were transiently hyperglycemic and displayed severe hyperinsulinemia. BKS.Lepr mice exhibited sustained reductions in the dc-ERG c-wave, fast oscillation, and off response that were not attributable to reduced photoreceptor activity; B6.BKS.Lepr mice displayed transient reductions in the c-wave and fast oscillation that correlated with hyperglycemia and magnitude of photoreceptor activity. In summary, all mouse models displayed altered RPE function concomitant with the onset of hyperglycemia. These results suggest that RPE function is directly reduced by elevated blood glucose levels. That RPE dysfunction was reversible and mitigated in hyperinsulinemic B6.BKS.Lepr mice provides insight into the underlying mechanism.