Melatonin Maintains Inner Blood–Retinal Barrier by Regulating Microglia via Inhibition of PI3K/Akt/Stat3/NF-κB Signaling Pathways in Experimental Diabetic Retinopathy

Abstract

Microglial activation and melatonin protection have been reported in diabetic retinopathy (DR). Whether melatonin could regulate microglia to protect the inner blood–retinal barrier (iBRB) remains unknown. In this study, the role of microglia in iBRB breakdown and the mechanisms of melatonin’s regulation on microglia were explored. In diabetic rat retinas, activated microglia proliferated and migrated from the inner retina to the outer retina, accompanied by the obvious morphological changes. Meanwhile, significant leakage of albumin was evidenced at the site of close interaction between activated microglia and the damaged pericytes and endothelial cells. In vitro, inflammation-related cytokines, such as tumor necrosis factor-α (TNF-α), inducible nitric oxide synthase (iNOS), interleukin (IL)-1β, and arginase-1 (Arg-1), were increased significantly in CoCl2-treated BV2 cells. The supernatant derived from CoCl2-treated BV2 cells significantly decreased the cell viability and disrupted the junctional proteins in both pericytes and endothelial cells, resulting in severe leakage. Melatonin suppressed the microglial overactivation, i.e., decreasing the cell number and promoting its anti-inflammatory properties in diabetic rat retinas. Moreover, the leakage of iBRB was alleviated and the pericyte coverage was restored after melatonin treatment. In vitro, when treated with melatonin in CoCl2-treated BV2 cells, the inflammatory factors were decreased, while the anti-inflammatory factors were increased, further reducing the pericyte loss and increasing the tight junctions. Melatonin deactivated microglia via inhibition of PI3K/Akt/Stat3/NF-κB signaling pathways, thus maintaining the integrity of iBRB. The present data support a causal role for activated microglia in iBRB breakdown and highlight the therapeutic potential of melatonin in the treatment of DR by regulating microglia.

Microglia: The breakthrough to treat neovascularization and repair blood-retinal barrier in retinopathy

“…CD5-2, as a novel oligonucleotide-based drug, enlarges the VE-cadherin transcription which is silenced by miR-27a in endothelial cells and defends the integrity of BRB and preserves the coverage of pericytes; the activation state of microglia is suppressed after treatment with CD5-2 ( Ting et al, 2019 ). Melatonin can inhibit microglia activation through the PI3K/Akt/Stat3/NF-κB signal axis and protect pericytes from apoptosis ( Tang L. et al, 2022 ). And MicroRNA-93-5p can restrain M1 microglia activation by silencing STAT3 ( Wang et al, 2021 ).…”

Diabetic retinopathy: Involved cells, biomarkers, and treatments

“…In the retina, retinal neurons [photoreceptors: cones and rods, horizontal and bipolar cells, amacrine cells, and retinal ganglion cells (RGCs)], glial cells (Müller cells, astrocytes, and microglia), and blood cells (endothelial cells [ECs] and pericytes, which cooperatively form the inner BRB [iBRB]) are linked to form a vital structure called the retinal neurovascular unit, and ECs and retinal pigment epithelial (RPE) cells constitute the outer blood-retina barrier (oBRB) ( Yang et al, 2020 ; Meng et al, 2021 ; Tang et al, 2022 ). DR is caused by retinal neurovascular unit dysfunction, including loss of cells, inflammatory factor production, neovascularization, and BRB impairment ( Wang and Lo, 2018 ), among which inflammatory process is one of the most important features ( Öhman et al, 2018 ).…”

SRY-box transcription factor 9 modulates Müller cell gliosis in diabetic retinopathy by upregulating TXNIP transcription

“…In our study, the increased levels of the inflammatory factors (NLRP3 and cleaved caspase-1) were regulated by SOX9 in Müller cells under HG conditions. Furthermore, inflammatory reaction induces dysfunction of Müller cells, destroying the blood-retinal barrier and aggravating retinal dysfunction with diabetes [49].…”