Diabetic retinopathy (DR) is a vision-threatening microvascular complication of diabetes mellitus, and around 67% of patients have some degree of retinopathy after ten years of diabetes [1]. In spite of decades of investigations, the precise molecular mechanisms involved in the pathogenesis of DR have not been completely deciphered. The main aim of this dissertation is to provide an important molecular link connecting diabetic dyslipidemia with retinal vascular degeneration associated with diabetes. Our studies reveal a central pathway of sphingolipid metabolism involved in the development of DR, concurrently affecting function of bone marrow (BM)-derived inflammatory cells contributing to retinal inflammation and microvascular injury, and negatively affecting repair of retinal vasculature by BM-derived circulating angiogenic cells (CAC). Notably, we reveal that normalizing the pro-inflammatory and reparative functions of these BM-derived cells by modulating their lipid metabolism improves the outcomes of DR.First, we explored the link between bone marrow and DR using a mouse model of diabetes, stably engrafted with GFP+ bone marrow. We demonstrated that diabetes has a significant long-term effect on the BM-derived inflammatory monocytes as well as reparative circulating angiogenic cells (CAC) that circulate in the blood, localize to the retina and undergo further differentiation. Our findings indicate that BM-derived cells could play a central role in the development of DR. Secondly, we investigated the role of altered sphingolipid metabolism in DR. We addressed the hypothesis that perturbation of a specific sphingolipid pathway in BM-derived cells may contribute to inflammation and vascular damage in the diabetic retina. We demonstrated that activation of an essential enzyme of sphingolipid metabolism, acid sphingomyelinase (ASM) in BM-derived cells plays a crucial role in retinal microvascular deterioration associated with diabetes. Inhibition of ASM in the diabetic BM prevented activation of BM-derived microglia-like cells and normalized retinal levels of proinflammatory cytokines. Notably, ASM also caused accumulation of ceramide on cell membrane of BM-derived reparative CACs, thereby reducing membrane fluidity and impairing CAC migration. Inhibition of ASM in these reparative cells improved membrane fluidity and homing of these cells to damaged retinal vessels in a mouse model of DR.Finally, to demonstrate the effect of diabetes on the in vivo homing ability of CACs, we injected purified GFP-expressing CACs into the vitreous of healthy mice. We observed that increased numbers of diabetic CACs remained trapped in the retina without returning to their BM niche, implying impaired migration and homing efficiency of the diabetic CACs. However, inhibition of ASM in diabetic CACs improved their clearance from retina and homing into the BM niche, demonstrating that ASM upregulation in diabetes contributes to impaired migration and homing of reparative CACs.Collectively, these findings indicate that modulation of sphingolipid metabolism in dysfunctional BM-derived cell populations could normalize the reparative/pro-inflammatory cell balance and can be explored as a novel therapeutic strategy for treating DR.
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