Diabetic retinopathy (DR) is one of the most threatening microvascular complications of diabetes. Despite extensive research, the mechanisms responsible for DR are unknown. In this dissertation, we fill this critical gap in knowledge by providing a better understanding of the vasodegenerative stage of DR. Our study reveals the fundamental mechanism of DR where diabetes simultaneously induces endothelial damage in the retinal vasculature and negatively affects endothelial progenitor cells (EPC) function, thus preventing efficient vascular repair in the retina. More importantly, we have separated these two factors and revealed that either preventing initial vascular damage or normalizing EPC-mediated repair improves the outcome of DR. First, we studied the role of inflammation in DR. Although the exact mechanism leading to inflammatory conditions in the diabetic retina is not well resolved, it is likely to involve dyslipidemia and perturbation of fatty acid metabolism. We have demonstrated that reduced fatty acid remodeling results in the depletion of the n3 PUFA products and this translates into a pro-inflammatory state in the diabetic retina. This low-grade inflammatory state is likely to lead to premature death of endothelial cells in the retina.Secondly, we addressed the hypothesis that diabetic animals cannot effectively regenerate the damaged vascular endothelium due to reduced numbers and function of circulating EPCs. In a rat model of diabetes, we found an altered circadian pattern of EPC levels in the peripheral blood; likely due to their immobilization within the bone marrow (BM). Notably, the defect of EPC function was associated with BM neuropathy, thus allowing us to propose BM neuropathy as a mechanism for ineffective EPC release from the BM niche and ineffective vascular repair.Importantly, dietary supplementation rich in n3 PUFAs improved the outcome of DR by both preventing inflammation in the retina and correcting EPC function. Feeding diabetic animals with n3 PUFA rich diet prevented increases in inflammatory cytokine levels, stimulated EPC migration and inhibited the loss of retinal capillaries. Finally, to demonstrate a direct link between BM neuropathy and DR, we examined the effect of surgical BM denervation on the ability to repair retinal vasculature. The experiment was performed on non-diabetic rats, allowing us to study the direct effect of the denervation without other factors associated with diabetes. We did not find any signs of retinopathy in intact retinas of animals with surgically denervated BM. This finding suggests that a lack of regulated EPC release does not affect healthy vasculature. However, when coupled with retinal damage due to ischemia-reperfusion injury, animals with BM denervation developed more severe retinopathy than animals with healthy BM, demonstrating that the combination of endothelial damage and EPC dysfunction is required for the development of diabetic retinopathy. In summary, both endothelial damage and EPC dysfunction are critical for the development of the vasodegenerative phase of DR. Preventing the initial inflammatory injury to retinal vasculature, stimulating EPC-mediated repair, or combining both approaches would lead to an effective therapy for DR.
Read
