Abstract
Purpose :
There is increasing evidence linking retinal mitochondria defects with diabetic retinopathy. However, a causal role has yet to be confirmed. We directly tested whether oxidative phosphorylation (OXPHOS) is defective in the retina prior to the development of hyperglycemia in type 2 diabetes.
Methods :
Male Nile grass rats (Arvicanthis niloticus) were fed standard chow, which was associated with hyperinsulinemia at 2 mo, followed by hyperglycemia by 6 mo. Controls were fed a high fiber low-calorie diet, which prevented hyperglycemia up to 18 mo. The functional status of specific mitochondrial electron chain complexes and outer membrane integrity were assessed with high-resolution respirometry (Oxygraph 2k; OROBOROS) using retinas isolated from individual Nile grass rats (n=6-11 per group). We used a multiple substrate-inhibitor protocol to detect mitochondrial respiration in the presence of substrates feeding electrons into complexes I, II, I&II and IV, as well as the integrity of the outer mitochondrial membrane. Respirometry parameter data were expressed as flux per mass and flux control ratios (FCR). Citrate synthase activity was used as a marker of mitochondrial content.
Results :
Mitochondrial functional changes were present in 2 mo retinas. The addition of exogenous cytochrome c allowed us to measure damage to the mitochondrial outer membrane associated with leakage of endogenous cytochrome c. Increased membrane damage was observed in the 2 mo hyperinsulinemic animals fed the standard chow compared to controls. The flux per mass for each respiratory complex did not vary between groups, at any time point. In order to detect quantitative changes in the OXPHOS system, the respiration was then expressed as FCR, normalized for the maximal OXPHOS capacity (coupled, with saturating ADP and providing electron entries into complexes I&II simultaneously). FCR showed an increased contribution of Complex I to the maximal OXPHOS capacity in the 2 mo hyperinsulinemic animals compared to the controls, which supports a compensatory effect.
Conclusions :
Hyperinsulinemia alone (without hyperglycemia) is associated with a breach in mitochondrial outer membrane integrity and oxidative phosphorylation compensatory changes. These results imply that metabolic syndrome might predispose to retinopathy.
This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.