Aims/hypothesis Microglial activation in diabetic retinopathy and the protective effect of erythropoietin (EPO) have been extensively studied. However, the regulation of microglia in the retina and its relationship to inner blood-retinal barrier (iBRB) maintenance have not been fully characterised. In this study, we investigated the role of microglia in iBRB breakdown in diabetic retinopathy and the protective effects of EPO in this context. Methods Male Sprague Dawley rats were injected intraperitoneally with streptozotocin (STZ) to establish the experimental model of diabetes. At 2 h after STZ injection, the right and left eyes were injected intravitreally with EPO (16 mU/eye, 2 μl) and an equivalent volume of normal saline (NaCl 154 mmol/l), respectively. The rats were killed at 2 or 8 weeks after diabetes onset. Microglia activation was detected by ionised calcium binding adaptor molecule (IBA)-1 immunolabelling. Leakage of the iBRB was evaluated by albumin staining and FITC-dextran permeability assay. BV 2 cells and primary rat microglia under hypoxic conditions were used to model microglial activation in diabetic retinopathy. Phagocytosis was examined by confocal microscopy in flat-mounted retina preparations and in microglia and endothelial cell cocultures. Protein levels of IBA-1, CD11b, complement component 1r (C1r), and Src/Akt/cofilin signalling pathway components were assessed by western blotting. Results In diabetic rat retinas, phagocytosis of endothelial cells by activated microglia was observed at 8 weeks, resulting in an increased number of acellular capillaries (increased by 426.5%) and albumin leakage. Under hypoxic conditions, activated microglia transmigrated to the opposite membrane of the transwell, where they disrupted the endothelial cell monolayer by engulfing endothelial cells. The activation and phagocytic activity of microglia was blocked by intravitreal injection of EPO. In vitro, IBA-1, CD11b and C1r protein levels were increased by 50.9%, 170.0% and 135.5%, respectively, by hypoxia, whereas Jingfa Zhang and Guo-Tong Xu contributed equally to this study.

PurposeTo explore the mechanisms of erythropoietin (EPO) in maintaining outer blood‐retinal barrier (BRB) in diabetic rats.MethodsSprague‐Dawley rats were rendered diabetic with intraperitoneal injection of streptozotocin, and then followed by intravitreal injection of EPO. Two and four weeks later, the permeability of outer BRB was examined with FITC‐dextran leakage assay, following a method to demarcate the inner and outer retina based on retinal blood supply. The glyoxal‐treated ARPE‐19 cells, incubated with EPO, soluble EPO receptor (sEPOR), Gö6976, or digoxin, were studied for cell viability and barrier function. The expressions of ZO‐1, occludin, VEGFR2, HIF‐1α, MAPKs, and AKT were examined with Western blot and immunofluorescence.ResultsThe major Leakage of FITC‐dextran was detected in the outer nuclear layer in both 2‐ and 4‐week diabetic rats. The leakage was largely ameliorated in EPO‐treated diabetic rats. The protein expressions of ZO‐1 and occludin in the RPE‐Bruch's membrane choriocapillaris complex were significantly decreased, whereas HIF‐1α and JNK pathways were activated, in 4‐week diabetic rats. These changes were prevented by EPO treatment. The in vitro study with ARPE‐19 cells confirmed these changes, and the protective effect of EPO was abolished by sEPOR. Gö6976 and digoxin rescued the tight junction and barrier function in glyoxal‐treated ARPE‐19 cells.ConclusionsIn early diabetic rats, the outer BRB might be more severely damaged and its breakdown is the major factor for retinal oedema. EPO maintains the outer BRB integrity through down‐regulation of HIF‐1α and JNK signallings, and thus up‐regulating ZO‐1 and occludin expressions in RPE cells.

The pathophysiology of diabetic retinopathy (DR) was complex. Under hyperglycemic conditions, the release of proinflammatory cytokines and the adhesion of leukocytes to retinal capillaries contribute to endothelial damage and the subsequent increase in vascular permeability resulting in macular edema. Melatonin, produced in the retina to regulate redox reactions and dopamine metabolism, plays protective roles against inflammation and oxidative stress. Considering its anti‐inflammatory and antioxidative properties, melatonin was speculated to exert beneficial effects in DR. In this study, we characterized the protective effects of melatonin on the inner blood–retinal barrier (iBRB), as well as the possible mechanisms in experimental DR. Results showed that in diabetic rat retinas, the leakage of iBRB and the expression of inflammatory factors (VEGF, TNF‐α, IL‐1β, ICAM‐1, and MMP9) increased dramatically, while the expression of tight junction proteins (ZO‐1, occludin, JAM‐A, and claudin‐5) decreased significantly. The above changes were largely ameliorated by melatonin. The in vivo data were confirmed in vitro. In addition, the protein expressions of p38 MAPK, NF‐κB, and TXNIP were upregulated significantly in diabetes and were downregulated following melatonin treatment. Melatonin could maintain the iBRB integrity by upregulating the expression of tight junction proteins via inhibiting p38/TXNIP/NF‐κB pathway, thus decreasing the production of inflammatory factors. This study may shed light on the development of melatonin‐based DR therapy.

Background Although vascular endothelial growth factor A (VEGF-A) is known to play a key role in causing retinal edema, whether and how VEGF-A induces intracellular edema in the retina still remains unclear. Methods Sprague-Dawley rats were rendered diabetic with intraperitoneal injection of streptozotocin. Intravitreal injection of ranibizumab was performed 8 weeks after diabetes onset. rMC-1 cells (rat Müller cell line) were treated with glyoxal for 24 h with or without ranibizumab. The expression levels of inwardly rectifying K+ channel 4.1 (Kir4.1), aquaporin 4 (AQP4), Dystrophin 71 (Dp71), VEGF-A, glutamine synthetase (GS) and sodium-potassium-ATPase (Na+-K+-ATPase) were examined using Western blot. VEGF-A in the supernatant of the cell culture was detected with ELISA. The intracellular potassium and sodium levels were detected with specific indicators. Results Compared with normal control, protein expressions of Kir4.1 and AQP4 were down-regulated significantly in diabetic rat retinas, which were prevented by ranibizumab. The above changes were recapitulated in vitro. Similarly, the intracellular potassium level in glyoxal-treated rMC-1 cells was increased, while the intracellular sodium level and Na+-K+-ATPase protein level remained unchanged, compared with control. However, ranibizumab treatment decreased intracellular sodium, but not potassium. Conclusion Ranibizumab protected Müller cells from diabetic intracellular edema through the up-regulation of Kir4.1 and AQP4 by directly binding VEGF-A. It also caused a reduction in intracellular osmotic pressure.