In vivo evaluation of ocular inflammatory responses in experimental diabetes

The Effects of Metformin on Obesity-Induced Dysfunctional Retinas

Purpose

The purpose of this study was to determine the effects of metformin on dysfunctional retinas in obesity-induced type 2 diabetic mice.

Methods

A high-fat diet (HFD)-induced diabetic mouse model (C57BL/6J) was used in this study. After 2 months of the HFD regimen, HFD mice were given daily metformin through oral gavage. Body weights, glucose tolerance, and retinal light responses were monitored regularly. Fluorescein angiography (FA) was used to assess changes in retinal vasculature. Ocular tissues (retina, vitreous, and lens) were harvested and analyzed for molecular changes as determined by immunofluorescent staining, Western blot analysis, and cytokine profiling.

Results

Starting 1 month after the diet regimen, mice fed the HFD had mildly compromised retinal light responses as measured by electroretinography (ERG), which worsened over time compared to that in the control. In HFD mice treated with metformin, systemic glucose levels reverted back to normal, and their weight gain slowed. Metformin reversed HFD-induced changes in phosphorylated protein kinase B (pAKT), extracellular signal-regulated kinase (pERK), and 5′AMP-activated protein kinase (pAMPK) in the retina. However, metformin treatments for 3 months did not restore the retinal light responses nor lessen the HFD-induced retinal neovascularization, even though it did reduce intraocular inflammation.

Conclusions

Although metformin was able to reverse systemic changes induced by HFD, it was not able to restore HFD-caused retinal light responses or deter neovascularization.

Influence of endotoxin-mediated retinal inflammation on phenotype of diabetic retinopathy in Ins2 Akita mice

AIMS

To evaluate the impact of systemic exposure to bacterial lipopolysaccharide (LPS) on a rodent model of background diabetic retinopathy.

METHODS

Toll-like receptor 4 (TLR4)-mediated systemic inflammation was induced in Ins2(Akita) heterozygotes and age-matched C57BL6/J-Ins2(+) littermates by single or repeated intraperitoneal injections of the TLR4 ligand LPS (9 µg/g body weight). 24 hours after a single injection in 7-week-old mice retinal Il1b, Tnfa and Vegf transcripts were measured with real-time PCR. Vascular endothelial growth factor (VEGF) protein levels were evaluated with bead-based immunoassay. Leukostasis and endothelial injury were assessed in retinal wholemounts following perfusion with rhodamine or FITC conjugated concanavalin A to label leukocytes and propidium iodide to label dead or injured cells. In mice which had received three fortnightly injections between 10 and 16 weeks of age, retinal thicknesses and vascular structure were evaluated at 17-18 weeks of age using optical coherence tomography (OCT) and fluorescein angiography. Retinal architecture was assesed using resin-based histology.

RESULTS

Compared with normoglycaemic controls, systemic LPS exposure in Ins2(Akita) mice was associated with a 3.5-fold increase in endothelial cell injury and attenuated leukostasis in the retinal vasculature. Hyperglycaemia or acute LPS inflammation did not increase retinal VEGF content. Thinning (10-13 µm) of posterior retina was detected with OCT 2 weeks after repeated exposure to LPS in Ins2(Akita) mice but not in normoglycaemic controls. Capillary networks and retinal morphology were unaffected by recurrent LPS inflammation in Ins2(Akita) and control mice.

CONCLUSIONS

In hyperglycaemic mice, exposure to systemic LPS was associated with two hallmark pathologies of early background diabetic retinopathy, namely, the injury of capillary endothelium and in vivo thinning of the retina.

Inflammation in diabetic retinopathy

Diabetes causes a number of metabolic and physiologic abnormalities in the retina, but which of these abnormalities contribute to recognized features of diabetic retinopathy (DR) is less clear. Many of the molecular and physiologic abnormalities that have been found to develop in the retina in diabetes are consistent with inflammation. Moreover, a number of anti-inflammatory therapies have been found to significantly inhibit development of different aspects of DR in animal models. Herein, we review the inflammatory mediators and their relationship to early and late DR, and discuss the potential of anti-inflammatory approaches to inhibit development of different stages of the retinopathy. We focus primarily on information derived from in vivo studies, supplementing with information from in vitro studies were important.

Lack of inducible nitric oxide synthases attenuates leukocyte-endothelial cell interactions in retinal microcirculation

Aim:

To investigate the effect of inducible nitric oxide synthases (iNOS) on inflammatory reactions during endotoxin-induced uveitis (EIU) in mice by studying leukocyte–endothelial cell interactions.

Methods:

EIU was produced in immunosuppressed iNOS^−/− mice and C57BL/6 (normal) mice by footpad injection of lipopolysaccharide. Leukocytes were labelled with acridine orange. Leukocyte rolling in the retinal microcirculation was evaluated in vivo with acridine orange digital fluorography. The number of migrated leukocytes was counted in flat-mounted retina.

Results:

Both leukocyte rolling and migration peaked at 48 h after lipopolysaccharide injection. The maximal numbers of rolling leukocytes in the immunosuppressed iNOS^−/− mouse retina decreased by 98.2% (p<0.001) compared with that in the normal mouse retina at 48 h after lipopolysaccharide injection. In addition, the maximal numbers of migrated leukocytes in the immunosuppressed iNOS^−/− mouse retina decreased by 74.0% (p<0.001) compared with that in the normal mouse retina at 24 h after lipopolysaccharide injection. Furthermore, the diameters of major retinal veins of the immunosuppressed iNOS^−/− group were smaller at both 24 and 48 h after lipopolysaccharide injection than were those of the normal group (p<0.001, respectively).

Conclusions:

A lack of iNOS suppresses leukocyte–endothelial cell interactions in the retinas of mice with EIU. This suggests that iNOS may play a role in the management of patients with uveitis and other inflammatory conditions.