Early retinal pigment epithelium dysfunction is concomitant with hyperglycemia in mouse models of type 1 and type 2 diabetes

Resilience to diabetic retinopathy

Chronic elevation of blood glucose at first causes relatively minor changes to the neural and vascular components of the retina. As the duration of hyperglycemia persists, the nature and extent of damage increases and becomes readily detectable. While this second, overt manifestation of diabetic retinopathy (DR) has been studied extensively, what prevents maximal damage from the very start of hyperglycemia remains largely unexplored. Recent studies indicate that diabetes (DM) engages mitochondria-based defense during the retinopathy-resistant phase, and thereby enables the retina to remain healthy in the face of hyperglycemia. Such resilience is transient, and its deterioration results in progressive accumulation of retinal damage. The concepts that co-emerge with these discoveries set the stage for novel intellectual and therapeutic opportunities within the DR field. Identification of biomarkers and mediators of protection from DM-mediated damage will enable development of resilience-based therapies that will indefinitely delay the onset of DR.

Manifestation of Pathology in Animal Models of Diabetic Retinopathy Is Delayed from the Onset of Diabetes

Diabetic retinopathy (DR) is the most common complication that develops in patients with diabetes mellitus (DM) and is the leading cause of blindness worldwide. Fortunately, sight-threatening forms of DR develop only after several decades of DM. This well-documented resilience to DR suggests that the retina is capable of protecting itself from DM-related damage and also that accumulation of such damage occurs only after deterioration of this resilience. Despite the enormous translational significance of this phenomenon, very little is known regarding the nature of resilience to DR. Rodent models of DR have been used extensively to study the nature of the DM-induced damage, i.e., cardinal features of DR. Many of these same animal models can be used to investigate resilience because DR is delayed from the onset of DM by several weeks or months. The purpose of this review is to provide a comprehensive overview of the literature describing the use of rodent models of DR in type-1 and type-2 diabetic animals, which most clearly document the delay between the onset of DM and the appearance of DR. These readily available experimental settings can be used to advance our current understanding of resilience to DR and thereby identify biomarkers and targets for novel, prevention-based approaches to manage patients at risk for developing DR.

The glucocorticoid receptor as a master regulator of the Müller cell response to diabetic conditions in mice

Diabetic retinopathy (DR) is considered a primarily microvascular complication of diabetes. Müller glia cells are at the centre of the retinal neurovascular unit and play a critical role in DR. We therefore investigated Müller cell-specific signalling pathways that are altered in DR to identify novel targets for gene therapy. Using a multi-omics approach on purified Müller cells from diabetic db/db mice, we found the mRNA and protein expression of the glucocorticoid receptor (GR) to be significantly decreased, while its target gene cluster was down-regulated. Further, oPOSSUM TF analysis and ATAC- sequencing identified the GR as a master regulator of Müller cell response to diabetic conditions. Cortisol not only increased GR phosphorylation. It also induced changes in the expression of known GR target genes in retinal explants. Finally, retinal functionality was improved by AAV-mediated overexpression of GR in Müller cells. Our study demonstrates an important role of the glial GR in DR and implies that therapeutic approaches targeting this signalling pathway should be aimed at increasing GR expression rather than the addition of more ligand.

Local modulation of thyroid hormone signaling in the retina affects the development of diabetic retinopathy

Thyroid hormone (TH) dyshomeostasis is associated with poor prognosis in acute and prolonged illness, but its role in diabetic retinopathy (DR) has never been investigated. Here, we characterized the TH system in the retinas of db/db mice and highlighted regulatory processes in MIO-M1 cells. In the db/db retinas, typical functional traits and molecular signatures of DR were paralleled by a tissue-restricted reduction of TH levels. A local condition of low T3 (LT3S) was also demonstrated, which was likely to be induced by deiodinase 3 (DIO3) upregulation, and by decreased expression of DIO2 and of TH receptors. Concurrently, T3-responsive genes, including mitochondrial markers and microRNAs (miR-133-3p, 338-3p and 29c-3p), were downregulated. In MIO-M1 cells, a feedback regulatory circuit was evidenced whereby miR-133-3p triggered the post-transcriptional repression of DIO3 in a T3-dependent manner, while high glucose (HG) led to DIO3 upregulation through a nuclear factor erythroid 2-related factor 2-hypoxia-inducible factor-1 pathway. Finally, an in vitro simulated condition of early LT3S and hyperglycemia correlated with reduced markers of both mitochondrial function and stress response, which was reverted by T3 replacement. Together, the data suggest that, in the early phases of DR, a DIO3-driven LT3S may be protective against retinal stress, while, in the chronic phase, it not only fails to limit HG-induced damage, but also increases cell vulnerability likely due to persistent mitochondrial dysfunction.

Diabetes-Induced Changes of the Rat ERG in Relation to Hyperglycemia and Acidosis

PURPOSE

To understand the mechanism of changes in the c-wave of the electroretinogram (ERG) in diabetic rats, and to explore how glucose manipulations affect the c-wave.

METHODS

Vitreal ERGs were recorded in control and diabetic Long-Evans rats, 3-60 weeks after IP vehicle or streptozotocin. A few experiments were performed on Brown Norway rats. Voltage responses to current pulses were used to measure the transepithelial resistance of the retinal pigment epithelium (RPE).

RESULTS

During development of diabetes the b-wave amplitude progressively decreased to about half of the initial amplitude after a year. In contrast, the c-wave was strongly affected from the very beginning (3 weeks) of diabetes. In control rats, the c-wave was cornea-positive at lower illuminations but was cornea-negative at higher (photopic) illumination. In diabetics, the whole amplitude-intensity curve was shifted toward negativity. The magnitude of this shift was markedly affected by acute glucose manipulations in diabetics but not in controls. Increased blood glucose made the c-wave more negative, and decreased blood glucose with insulin had the opposite effect. Experimentally induced acidification of the retina had a small effect that was different from diabetes, shifting the c-wave toward positivity, slightly in controls and more noticeably in diabetics. One reason for the significant negativity of the diabetic ERG was a decrease of the cornea-positive response of the RPE due to a decrease of the transepithelial resistance.

CONCLUSIONS

The ERG c-wave is more negative in diabetics than in control animals, and is far more sensitive to changes in blood glucose. The increased negativity is largely if not entirely due to changes in the transepithelial resistance of the RPE, an electrical analog of the breakdown of the blood-retinal barrier observed in other studies. The sensitivity of the c-wave to glucose in diabetics may also be due to changes in transepithelial resistance.

Bioinformatics and in vitro study reveal the roles of microRNA-346 in high glucose-induced human retinal pigment epithelial cell damage

AIM

To study microRNAs (miRNAs) and their potential effects in high glucose-induced human retinal pigment epithelial cell damage.

METHODS

We screened the GSE52233 miRNA expression dataset for differentially expressed miRNAs (DEMs). The target genes of the top 10 DEMs were predicted using miRWalk 2.0 database, followed by function enrichment and protein-protein interaction analysis. miRNA expression was determined in the human retinal pigment epithelial cell line ARPE-19 treated with high glucose (HG) by quantitative reverse transcription-polymerase chain reaction (qRT-PCR). Cell proliferation was determined using cell counting kit (CCK)-8 assay. Cell cycle, apoptosis, and reactive oxygen species (ROS) levels were determined by flow cytometry. The direct interaction between miRNA and targets was validated using dual-luciferase reporter assay.

RESULTS

Thirty-nine DEMs were screened, and we predicted 125 miRNA-mRNA pairs for the top 10 DEMs, including 119 target genes of seven DEMs such as miR-346, which was upregulated in diabetic retinopathy (DR). miR-346 target genes were substantially enriched in the regulation of intracellular transport and retinoic acid-inducible gene I (RIG-I)-like receptor signaling pathway. Expression of three upregulated and downregulated miRNAs were verified by qRT-PCR in HG-treated ARPE-19 cells. Expression of miR-346 was elevated in HG treated ARPE-19 cells in a dose-dependent manner. HG inhibited cell proliferation and induced apoptosis, which were partly reversed by transfecting an miR-346 inhibitor, which even decreased the ROS levels elevated due to HG. Argonaute 2 (AGO2) was a target of miR-346.

CONCLUSION

miR-346 is a key miRNA and plays an important role in HG-induced damage in human retinal pigment epithelial cells.

Diabetic retinopathy: a comprehensive update on in vivo, in vitro and ex vivo experimental models

Diabetic retinopathy (DR), one of the leading causes of visual impairment and blindness worldwide, is one of the major microvascular complications in diabetes mellitus (DM). Globally, DR prevalence among DM patients is 25%, and 6% have vision-threatening problems among them. With the higher incidence of DM globally, more DR cases are expected to be seen in the future. In order to comprehend the pathophysiological mechanism of DR in humans and discover potential novel substances for the treatment of DR, investigations are typically conducted using various experimental models. Among the experimental models, in vivo models have contributed significantly to understanding DR pathogenesis. There are several types of in vivo models for DR research, which include chemical-induced, surgical-induced, diet-induced, and genetic models. Similarly, for the in vitro models, there are several cell types that are utilised in DR research, such as retinal endothelial cells, Müller cells, and glial cells. With the advancement of DR research, it is essential to have a comprehensive update on the various experimental models utilised to mimic DR environment. This review provides the update on the in vitro, in vivo, and ex vivo models used in DR research, focusing on their features, advantages, and limitations.

Metabolic fingerprinting on retinal pigment epithelium thickness for individualized risk stratification of type 2 diabetes mellitus

The retina is an important target organ of diabetes mellitus, with increasing evidence from patients and animal models suggesting that retinal pigment epithelium (RPE) may serve as an early marker for diabetes-related damages. However, their longitudinal relationship and the biological underpinnings remain less well understood. Here, we demonstrate that reduced in vivo measurements of RPE thickness (RPET) represents a significant risk factor for future type 2 diabetes mellitus (T2DM) and its microvascular phenotypes. After performing systematic analyses of circulating plasma metabolites using two complementary approaches, we identify a wide range of RPET metabolic fingerprints that are independently associated with reduced RPET. These fingerprints hold their potential to improve predictability and clinical utility for stratifying future T2DM and related microvascular phenotypes beyond traditional clinical indicators, providing insights into the promising role of retinas as a window to systemic health.

Oxidative Stress-Involved Mitophagy of Retinal Pigment Epithelium and Retinal Degenerative Diseases

The retinal pigment epithelium (RPE) is a highly specialized and polarized epithelial cell layer that plays an important role in sustaining the structural and functional integrity of photoreceptors. However, the death of RPE is a common pathological feature in various retinal diseases, especially in age-related macular degeneration (AMD) and diabetic retinopathy (DR). Mitophagy, as a programmed self-degradation of dysfunctional mitochondria, is crucial for maintaining cellular homeostasis and cell survival under stress. RPE contains a high density of mitochondria necessary for it to meet energy demands, so severe stimuli can cause mitochondrial dysfunction and the excess generation of intracellular reactive oxygen species (ROS), which can further trigger oxidative stress-involved mitophagy. In this review, we summarize the classical pathways of oxidative stress-involved mitophagy in RPE and investigate its role in the progression of retinal diseases, aiming to provide a new therapeutic strategy for treating retinal degenerative diseases. The role of mitophagy in AMD and DR. In AMD, excessive ROS production promotes mitophagy in the RPE by activating the Nrf2/p62 pathway, while in DR, ROS may suppress mitophagy by the FOXO3-PINK1/parkin signaling pathway or the TXNIP-mitochondria-lysosome-mediated mitophagy.

Visual Dysfunction in Diabetes

Although diabetic retinopathy (DR) is clinically diagnosed as a vascular disease, many studies find retinal neuronal and visual dysfunction before the onset of vascular DR. This suggests that DR should be viewed as a neurovascular disease. Prior to the onset of DR, human patients have compromised electroretinograms that indicate a disruption of normal function, particularly in the inner retina. They also exhibit reduced contrast sensitivity. These early changes, especially those due to dysfunction in the inner retina, are also seen in rodent models of diabetes in the early stages of the disease. Rodent models of diabetes exhibit several neuronal mechanisms, such as reduced evoked GABA release, increased excitatory glutamate signaling, and reduced dopamine signaling, that suggest specific neuronal deficits. This suggests that understanding neuronal deficits may lead to early diabetes treatments to ameliorate neuronal dysfunction.

Systemic Reduction of Glut1 Normalizes Retinal Dysfunction, Inflammation, and Oxidative Stress in the Retina of Spontaneous Type 2 Diabetic Mice

Defects in the light-evoked responses of the retina occur early in the sequalae of diabetic retinopathy (DR). These defects, identified through the electroretinogram (ERG), represent dysfunction of retinal neurons and the retinal pigment epithelium and are commonly identifiable at the timing of, or almost immediately following, diabetes diagnosis. Recently, systemic reduction of the facilitated glucose transporter type 1, Glut1, in type 1 diabetic mice was shown to reduce retinal sorbitol accumulation, mitigate ERG defects, and prevent retinal oxidative stress and inflammation. Herein, the study investigated whether systemic reduction of Glut1 also diminished hallmarks of DR in type 2 diabetic mice. Transgenic nondiabetic Leprdb/+ and spontaneously diabetic Leprdb/db mice that expressed wild-type (Glut1+/+) or systemically reduced levels of Glut1 (Glut1+/-) were aged and subjected to standard strobe flash electroretinography and c-wave analysis before evaluation of inflammatory cytokines and oxidative stress molecules. Although Leprdb/dbGlut1+/- mice still displayed overt obesity and diabetes, no scotopic, photopic, or c-wave ERG defects were present through 16 weeks of age, and expression of inflammatory cytokines and oxidative stress molecules was also normalized. These findings suggest that systemic reduction of Glut1 is sufficient to prevent functional retinal pathophysiology in type 2 diabetes. Targeted, moderate reductions of Glut1 or inhibition of Glut1 activity in the retina of diabetic patients should be considered as a novel therapeutic strategy to prevent development and progression of DR.

The Slow Progression of Diabetic Retinopathy Is Associated with Transient Protection of Retinal Vessels from Death

The purpose of this study was to investigate the reason that diabetic retinopathy (DR) is delayed from the onset of diabetes (DM) in diabetic mice. To this end, we tested the hypothesis that the deleterious effects of DM are initially tolerated because endogenous antioxidative defense is elevated and thereby confers resistance to oxidative stress-induced death. We found that this was indeed the case in both type 1 DM (T1D) and type 2 DM (T2D) mouse models. The retinal expression of antioxidant defense genes was increased soon after the onset of DM. In addition, ischemia/oxidative stress caused less death in the retinal vasculature of DM versus non-DM mice. Further investigation with T1D mice revealed that protection was transient; it waned as the duration of DM was prolonged. Finally, a loss of protection was associated with the manifestation of both neural and vascular abnormalities that are diagnostic of DR in mice. These observations demonstrate that DM can transiently activate protection from oxidative stress, which is a plausible explanation for the delay in the development of DR from the onset of DM.

A closer association between blood urea nitrogen and the probability of diabetic retinopathy in patients with shorter type 2 diabetes duration

Blood urea nitrogen (BUN) is an indicator of renal function and catabolic status in human body. Diabetic retinopathy (DR) is a major microvascular complication of diabetes mellitus (DM) and a serious threat to the vision of diabetic patients. We included 426 type 2 diabetic patients who visited the endocrinology department of Guangdong Provincial People's Hospital and received an ophthalmology consultation from December 2017 to November 2018. The outcome was the probability of DR in participants. Multivariable logistics analysis was used to confirm the relationship between BUN and the probability of DR. And interaction tests were conducted to find the effects of DM duration on their association. A total of 167 of 426 patients with type 2 diabetes had DR, with a probability of 39.20%. After adjusting for potential confounders, a positive association between BUN and the probability of DR (OR = 1.12; 95% CI 1.03-1.21; P = 0.0107). And a test for interaction between DM duration and BUN on the probability of DR was significant (P = 0.0295). We suggested that in patients with type 2 diabetes, BUN was positively associated with the probability of DR and the association was influenced by DM duration.

Towards a New Biomarker for Diabetic Retinopathy: Exploring RBP3 Structure and Retinoids Binding for Functional Imaging of Eyes In Vivo

Diabetic retinopathy (DR) is a severe disease with a growing number of afflicted patients, which places a heavy burden on society, both socially and financially. While there are treatments available, they are not always effective and are usually administered when the disease is already at a developed stage with visible clinical manifestation. However, homeostasis at a molecular level is disrupted before visible signs of the disease are evident. Thus, there has been a constant search for effective biomarkers that could signal the onset of DR. There is evidence that early detection and prompt disease control are effective in preventing or slowing DR progression. Here, we review some of the molecular changes that occur before clinical manifestations are observable. As a possible new biomarker, we focus on retinol binding protein 3 (RBP3). We argue that it displays unique features that make it a very good biomarker for non-invasive, early-stage DR detection. Linking chemistry to biological function and focusing on new developments in eye imaging and two-photon technology, we describe a new potential diagnostic tool that would allow rapid and effective quantification of RBP3 in the retina. Moreover, this tool would also be useful in the future to monitor therapeutic effectiveness if levels of RBP3 are elevated by DR treatments.

First Nation-Wide Study of the Incidence and Characteristics of Retinal Detachment in Poland during 2013-2019

AIMS

The present study aimed to analyze the incidence and characteristics of all types of retinal detachment (RD) in the overall population of Polish adults during 2013-2019.

METHODS

Data from all levels of healthcare services at public and private institutions recorded in the National Health Fund (NHF) database were evaluated. International Classification of Diseases codes (ICD-9 and ICD-10) and unique NHF codes were used to identify RD patients and RD treatment procedures.

RESULTS

In the period 2013-2019, 71,073 patients with RD were newly diagnosed in Poland. The average incidence was 32.64/100,000 person-years (95% CI: 31.28-33.99) and it increased with the age of patients, with the highest rate in the group of patients ≥70 years of age. The overall incidences of rhegmatogenous RD, traction RD, serous RD, other RD and unspecified RD were 13.72/100,000, 2.03/100,000, 1.02/100,000, 7.90/100,000 and 7.97/100,000 person-years, respectively. The most common surgical treatment for RD in Poland was PPV performed on average in 49.80% of RD patients. The risk factor analyses showed that rhegmatogenous RD was significantly associated with age (OR 1.026), male sex (OR 2.320), rural residence (OR 0.958), DM type 2 (OR 1.603), any DR (OR 2.109), myopia (OR 2.997), glaucoma (OR 2.169) and uveitis (OR 2.561). Traction RD was also significantly associated with age (OR 1.013) and male sex (OR 2.785) as well as with any DR (OR 2.493), myopia (OR 2.255), glaucoma (OR 1.904) and uveitis (OR 4.214). Serous RD was significantly associated with all analyzed risk factors except DM type 2.

CONCLUSIONS

The total incidence of retinal detachment in Poland was higher than found in previously published studies. Our study demonstrated that diabetes type 1 and diabetic retinopathy are risk factors of development of serous RD, which is presumably associated with the disruption of the blood-retinal barriers in these conditions.

Highly Retina-Permeating and Long-Acting Resveratrol/Metformin Nanotherapeutics for Enhanced Treatment of Macular Degeneration

The development of therapeutics for effective treatments of retinal diseases is significantly constrained by various biological barriers. We herein report a nanomedicine strategy to develop nanotherapeutics featured with not only high retinal permeability but also sustained bioactive delivery. Specifically, the nanotherapeutics are rationally designed via aminolysis of resveratrol-encapsulated polycaprolactone nanoparticles (R@PCL NPs), followed by the formation of amide linkages with carboxyl-terminated transacting activator of transcription cell penetrating peptide (T) and metformin (M). The R@PCL-T/M NP nanotherapeutics are demonstrated in vitro to possess persistent drug release profiles, good ocular biocompatibility, and potent bioactive activities for targeting prevailing risk factors associated with retinal diseases. In vivo studies indicate that single-dose intravitreal administration of the R@PCL-T/M NPs can effectively improve retinal permeability (∼15-fold increase), prevent loss of endogenous antioxidants, and suppress the growth of abnormal vessels in the retina with macular degeneration for 56 days. This high treatment efficacy can be ascribed to the enhanced retinal permeability of the nanotherapeutics in conjunction with the sustained pharmacological activity of the dual drugs (R and M) in the retinal pigment epithelial region. These findings show a great promise for the development of pharmacological nanoformulations capable of targeting the retina and thereby treating complex posterior segment diseases with improved efficacies.

Investigation of Retinal Metabolic Function in Type 1 Diabetic Akita Mice

Diabetic retinopathy (DR) is the leading cause of vision loss in working age adults. Understanding the retinal metabolic response to circulating high glucose levels in diabetic patients is critical for development of new therapeutics to treat DR. Measuring retinal metabolic function using the Seahorse analyzer is a promising technique to investigate the effect of hyperglycemia on retinal glycolysis and mitochondrial respiration. Here, we analyzed the retinal metabolic function in young and old diabetic and control mice. We also compared the expression of key glycolytic enzymes between the two groups. The Seahorse XF analyzer was used to measure the metabolic function of retina explants from young and old type 1 diabetic Akita (Ins2^Akita) mice and their control littermates. Rate-limiting glycolytic enzymes were analyzed in retina lysates from the two age groups by Western blotting. Retinas from young adult Akita mice showed a decreased glycolytic response as compared to control littermates. However, this was not observed in the older mice. Western blotting analysis showed decreased expression of the glycolytic enzyme PFKFB3 in the young Akita mice retinas. Measurement of the oxygen consumption rate showed no difference in retinal mitochondrial respiration between Akita and WT littermates under normal glucose conditions ex vivo despite mitochondrial fragmentation in the Akita retinas as examined by electron microscopy. However, Akita mice retinas showed decreased mitochondrial respiration under glucose-free conditions. In conclusion, diabetic retinas display a decreased glycolytic response during the early course of diabetes which is accompanied by a reduction in PFKFB3. Diabetic retinas exhibit decreased mitochondrial respiration under glucose deprivation.

Proteomics identifies new potential therapeutic targets of diabetic retinopathy

Retinal pigment epithelium (RPE) is an important component of the outer blood-retinal barrier and plays a critical role in maintaining retinal homeostasis. Alterations in RPE can be detected during the early stages of diabetic retinopathy (DR). However, the molecular mechanisms underlying these early changes remain unclear. We investigated the molecular changes induced in the RPE by high glucose concentrations by constructing a high glucose-induced ARPE-19 cell injury model simulating the DR environment in vitro. Proteomic analysis was conducted to measure differences in protein expression between cells treated with normal (5 mM) and high (25 mM) glucose concentrations, and bioinformatics techniques were used to analyze the mechanism of action. The results of the proteomic analyses were validated using western blotting. High glucose levels inhibited the proliferation of ARPE-19 cells. We identified 88 upregulated proteins and 114 downregulated proteins. Six of these proteins were selected for further validation. Changes in the proteome mainly affected the lysosome and cell cycle pathways. Proteomic differences between ARPE-19 cells treated with normal and high glucose concentrations indicate that damage to the RPE in DR may be caused by specific mechanisms. Our study verified protein changes in ARPE-19 cells in a high-glucose environment and may provide new strategies for understanding the molecular mechanisms underlying DR.

Oleuropein Protects Human Retinal Pigment Epithelium Cells from IL-1β-Induced Inflammation by Blocking MAPK/NF-κB Signaling Pathways

Proinflammatory mediators such as interleukin (IL)-1β cause retinal pigment epithelium (RPE) inflammation, which is related to visual deterioration, including age-related macular degeneration and diabetic retinopathy. Oleuropein is a polyphenol compound that shows potent anti-inflammatory, antioxidant, and anti-cancer activities, but its effects on IL-1β-induced inflammation have not been examined in the adult RPE cell line ARPE-19. Here, we assessed the ability of oleuropein to attenuate this inflammation in ARPE-19 cells. IL-1β induced secretion of the inflammatory cytokines IL-6, monocyte chemoattractant protein-1 (MCP)-1, and soluble intercellular adhesion molecule (sICAM)-1. As measured by enzyme-linked immunosorbent assay, oleuropein significantly inhibited levels of all three proteins and led to decreased monocyte adhesiveness to ARPE-19 cells. To clarify the underlying anti-inflammatory mechanisms, we used western blots to evaluate the effect of oleuropein on inactivation of the nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways. The results showed that oleuropein significantly decreased levels of the inflammatory mediator cyclooxygenase-2 and increased anti-inflammatory protein HO-1 expression. We next examined if the anti-inflammatory activity of oleuropein arises via inactivated NF-κB. We found that suppressing phosphorylation of the JNK1/2 and p38 MAPK signaling pathways inhibited IL-6, MCP-1, and sICAM-1 secretion, implicating these pathways and NF-κB suppression in the effects of oleuropein. These results indicate that oleuropein shows potential for the prevention and treatment of inflammatory diseases of the retina.

Photoreceptor cells and RPE contribute to the development of diabetic retinopathy

Diabetic retinopathy (DR) is a leading cause of blindness. It has long been regarded as vascular disease, but work in the past years has shown abnormalities also in the neural retina. Unfortunately, research on the vascular and neural abnormalities have remained largely separate, instead of being integrated into a comprehensive view of DR that includes both the neural and vascular components. Recent evidence suggests that the most predominant neural cell in the retina (photoreceptors) and the adjacent retinal pigment epithelium (RPE) play an important role in the development of vascular lesions characteristic of DR. This review summarizes evidence that the outer retina is altered in diabetes, and that photoreceptors and RPE contribute to retinal vascular alterations in the early stages of the retinopathy. The possible molecular mechanisms by which cells of the outer retina might contribute to retinal vascular damage in diabetes also are discussed. Diabetes-induced alterations in the outer retina represent a novel therapeutic target to inhibit DR.

Circ_0000615 promotes high glucose-induced human retinal pigment epithelium cell apoptosis, inflammation and oxidative stress via miR-646/YAP1 axis in diabetic retinopathy

BACKGROUND

Diabetic retinopathy (DR), a common complication of diabetes mellitus, is the major cause of visual impairment and blindness. Circ_0000615 was found to be elevated in retina samples of diabetic patients. Hence, the detailed effects and molecular mechanisms of circ_0000615 in DN progression were explored.

METHODS

The levels of circ_0000615, microRNA (miR)-646 and YAP1 (yes-associated protein 1) were detected using quantitative real-time polymerase chain reaction and Western blot assays. Cell viability, apoptosis, inflammation and reactive oxygen species (ROS) generation were determined using cell counting kit-8 assay, flow cytometry, caspase3 activity analysis, Western blot, enzyme-linked immunosorbent assay (ELISA) and Dichlorofluorescein diacetate (DCFH-DA) assay, respectively. The binding interaction between miR-646 and circ_0000615 or YAP1 was determined using dual-luciferase reporter, RNA immunoprecipitation (RIP), and RNA pull-down assays.

RESULTS

Circ_0000615 was elevated in high glucose (HG)-induced human retinal pigment epithelium (HRPE) cells. Knockdown of circ_0000615 attenuated HG-triggered HRPE cell apoptosis, inflammation, and ROS generation. Mechanistically, miR-646 was confirmed to be a target of circ_0000615, inhibition of miR-646 reversed the protective effects of circ_0000615 knockdown on HG-evoked HRPE cell dysfunction. MiR-646 was verified to target YAP1, overexpression of YAP1 abolished the impairment induced by miR-646 on HG-induced HRPE cell damage. Besides that, we confirmed that circ_0000615 could regulate YAP1 expression via miR-646.

CONCLUSION

Circ_0000615 contributed to HG-induced HRPE cell dysfunction via miR-646/YAP1 axis, suggesting a novel insight into the pathogenesis of DR and a potential candidate for DR treatment.

The Predictive Role of Thyroid Hormone Levels for Early Diabetic Retinal Changes in Experimental Rat and Human Diabetes

Purpose

In diabetic subjects, early visual functional alterations such as color vision deficiencies (CVDs) are known to precede clinically apparent diabetic retinopathy. Prominent photoreceptor outer segment degeneration and an increase in the number of retinal dual cones (co-expressing S- and M-opsins simultaneously) have been described in diabetic rat models, suggesting a connection with the development of CVDs. As cone opsin expression is controlled by thyroid hormones, we investigated the diabetic retina in association with thyroid hormone alterations.

Methods

In rat models of type 1 and 2 diabetes, dual cones were labeled by immunohistochemistry, and their numbers were analyzed in relation to free triiodothyronine (fT3) and free thyroxine (fT4) levels. Quantification of dual cones was also performed in human postmortem retinas. Additionally, a cross-sectional case–control study was performed where thyroid hormone levels were measured and color vision was assessed with Lanthony desaturated D15 discs.

Results

A higher number of dual cones was detectable in diabetic rats, correlating with fT4 levels. Dual cones were also present in postmortem human retinas, with higher numbers in the three diabetic retinas. As expected, age was strongly associated with CVDs in human patients, and the presence of diabetes also increased the risk. However, the current study failed to detect any effect of thyroid hormones on the development of CVDs.

Conclusions

Our results point toward the involvement of thyroid homeostasis in the opsin expression changes in diabetic rats and human samples. The evaluation of the possible clinical consequences warrants further research.

Silenced SNHG1 Inhibited Epithelial-Mesenchymal Transition and Inflammatory Response of ARPE-19 Cells Induced by High Glucose

PURPOSE

The lncRNA small nucleolar RNA host gene 1 (SNHG1) is a cerebral infarction-associated gene, its biological role and mechanism in diabetic retinopathy remain to be illuminated. The present study was designed to investigate the role of SNHG1 in high glucose induced human retinal pigment epithelial cells (ARPE-19).

METHODS

ARPE-19 cells were cultured and exposed to 60 mM high glucose for 48h, and 5.5mM glucose-exposed ARPE-19 cells were used as the control. The levels of the epithelial-mesenchymal transition (EMT) markers E-cadherin, ZO-1, vimentin and α-SMA were measured, and the Cell inflammatory response was evaluated by detecting IL-6 and IL-1β levels. Then, cell migration, proliferation and apoptosis were detected. The expression of the lncRNA SNHG1 in ARPE-19 cells was detected by quantitative real-time PCR. SNHG1 was knocked down by small interfering RNA (siRNA) transfection. The effects of SNHG1 inhibition on inflammation, EMT, migration, proliferation and apoptosis were observed.

RESULTS

The results showed that the expression of SNHG1 was significantly increased in ARPE-19 cells exposed to high glucose. Silencing SNHG1 reduced the expression of vimentin, α-SMA, and the expression of inflammatory chemokines IL-6 and IL-1β, inhibited migration and proliferation, elevated the expression of E-cadherin and ZO-1, and promoted apoptosis in ARPE-19 cells.

CONCLUSION

The lncRNA SNHG1 is involved in hyperglycemia-induced EMT and the inflammatory response of ARPE-19 cells and provides a new understanding of the pathogenesis of DR.

Reduction of Glut1 in the Neural Retina But Not the RPE Alleviates Polyol Accumulation and Normalizes Early Characteristics of Diabetic Retinopathy

Hyperglycemia is a key determinant for development of diabetic retinopathy (DR). Inadequate glycemic control exacerbates retinopathy, while normalization of glucose levels delays its progression. In hyperglycemia, hexokinase is saturated and excess glucose is metabolized to sorbitol by aldose reductase via the polyol pathway. Therapies to reduce retinal polyol accumulation for the prevention of DR have been elusive because of low sorbitol dehydrogenase levels in the retina and inadequate inhibition of aldose reductase. Using systemic and conditional genetic inactivation, we targeted the primary facilitative glucose transporter in the retina, Glut1, as a preventative therapeutic in diabetic male and female mice. Unlike WT diabetics, diabetic Glut1 ^+/- mice did not display elevated Glut1 levels in the retina. Furthermore, diabetic Glut1 ^+/- mice exhibited ameliorated ERG defects, inflammation, and oxidative stress, which was correlated with a significant reduction in retinal sorbitol accumulation. Retinal pigment epithelium-specific reduction of Glut1 did not prevent an increase in retinal sorbitol content or early hallmarks of DR. However, like diabetic Glut1 ^+/- mice, reduction of Glut1 specifically in the retina mitigated polyol accumulation and diminished retinal dysfunction and the elevation of markers for oxidative stress and inflammation associated with diabetes. These results suggest that modulation of retinal polyol accumulation via Glut1 in photoreceptors can circumvent the difficulties in regulating systemic glucose metabolism and be exploited to prevent DR.SIGNIFICANCE STATEMENT Diabetic retinopathy affects one-third of diabetic patients and is the primary cause of vision loss in adults 20-74 years of age. While anti-VEGF and photocoagulation treatments for the late-stage vision threatening complications can prevent vision loss, a significant proportion of patients do not respond to anti-VEGF therapies, and mechanisms to stop progression of early-stage symptoms remain elusive. Glut1 is the primary facilitative glucose transporter for the retina. We determined that a moderate reduction in Glut1 levels, specifically in the retina, but not the retinal pigment epithelium, was sufficient to prevent retinal polyol accumulation and the earliest functional defects to be identified in the diabetic retina. Our study defines modulation of Glut1 in retinal neurons as a targetable molecule for prevention of diabetic retinopathy.

Current understanding of the molecular and cellular pathology of diabetic retinopathy

Diabetes mellitus has profound effects on multiple organ systems; however, the loss of vision caused by diabetic retinopathy might be one of the most impactful in a patient's life. The retina is a highly metabolically active tissue that requires a complex interaction of cells, spanning light sensing photoreceptors to neurons that transfer the electrochemical signal to the brain with support by glia and vascular tissue. Neuronal function depends on a complex inter-dependency of retinal cells that includes the formation of a blood-retinal barrier. This dynamic system is negatively affected by diabetes mellitus, which alters normal cell-cell interactions and leads to profound vascular abnormalities, loss of the blood-retinal barrier and impaired neuronal function. Understanding the normal cell signalling interactions and how they are altered by diabetes mellitus has already led to novel therapies that have improved visual outcomes in many patients. Research highlighted in this Review has led to a new understanding of retinal pathophysiology during diabetes mellitus and has uncovered potential new therapeutic avenues to treat this debilitating disease.

Combination of pathological and spectroscopic characterization to promote diagnosis of retinal pigment epithelium-Bruch's membrane complex in a diabetic rat model

Diabetic retinopathy (DR) is a common condition of diabetes, and approaches to detecting early DR using the unique characteristics of the retinal pigment epithelium-Bruch's membrane complex (RBC) have increasingly attracted attention. A diabetic model was established in Sprague-Dawley rats via streptozocin (STZ) injection for 1 (DM1) and 6 months (DM6), confirmed by weekly blood glucose measurement. Serum and retinal tissue-based advanced glycation endproducts (AGE) levels significantly elevated in diabetic rats, and RBC was evaluated by transmission electron microscopy and Raman spectroscopy. The results showed that whole Raman spectra and all marked band intensities could respectively achieve almost equal and accurate discrimination of all animal groups, along with the determination of important molecules from the band data. Further quantitative analyses indicated series of metabolic disturbance due to hyperglycemia were involved while the body self-regulation mechanism still played a role with different effects during the disease progression. Given this, Raman spectroscopy can reliably distinguish the early characterization of DR in addition to providing intrinsic key molecules that is sensitive to identify the early disease progression.

MicroRNA-191-5p ameliorates amyloid-β1-40 -mediated retinal pigment epithelium cell injury by suppressing the NLRP3 inflammasome pathway

Amyloid β (Aβ) is a crucial component of drusen, the hallmark of the early stage of age-related macular degeneration (AMD), and can cause retinal pigment epithelium (RPE) cell damage through activation of the inflammatory response. MicroRNAs play a critical role in inflammation. However, the mechanism underlying the effect of microRNAs on the NLRP3 inflammasome induced by Aβ remains poorly understood. In the present study, we demonstrated that Aβ_1-40 -mediated RPE damage by inducing a decrease in endogenous miR-191-5p expression. This led to the upregulation of its target gene, C/EBPβ. C/EBPβ acts as a transcription factor for NLRP3, promotes its transcription, and upregulates the downstream inflammatory factors Caspase-1 and IL-1β. Correspondingly, overexpression of miR-191-5p alleviated RPE cell injury by suppressing inflammation. The present study elucidates a novel transcriptional regulatory mechanism of the NLRP3 inflammasome. Our findings suggest an anti-inflammatory effect of miR-191-5p in Aβ_1-40 -induced RPE impairment, shedding light on novel preventive or therapeutic approaches for AMD-associated RPE impairment.

Vitamin A and Daucus carota root extract mitigate STZ-induced diabetic retinal degeneration in Wistar albino rats by modulating neurotransmission and downregulation of apoptotic pathways

The objective of our study was to explore the deleterious effects of diabetes on the visual functions of the retina and to address whether the administration of vitamin A and carrot root extract (CE) confer retinal protection in hyperglycemic rats via modulation of oxidative stress, biochemical alternations, and retinal neurotransmission. Fifty male Wistar albino rats weighing 180 ± 12.41 g were randomized into five groups (n = 10): controls, diabetic group (injected with 40 mg/kg dissolved in 0.1 sodium citrate buffer), diabetic group treated with vitamin A (2,500 IU/kg, low dose), diabetic group treated with vitamin (5,000 IU/kg, high dose), and diabetic groups administered CE (200 mg/kg/every other day). Our findings showed that, compared to controls, diabetic rats showed a significant decrease in their retinal thickness, increased apoptotic ganglion cells, and a noticeable degeneration of their synaptic layers. The inner retina displayed increased activity of neovascularization; however, the outer retina exhibited vacuolar degeneration of the photoreceptor cell layer. Our biochemical assessments showed reduced levels of CAT, SOD, and GST along with increased lipid peroxidation. Concurrently, cellular angiogenic and stress markers were significantly elevated associated with increased apoptotic activities as evidenced by increased expressions of annexin-V and PARP. Furthermore, the neurotransmitter content of the retina was altered in diabetic rats compared to controls and diabetic-treated groups. Paradoxically, vitamin A and CE supplementation attenuate these retinal insults in diabetic animals and normalized aforementioned assayed parameters; evidencing that both treatments exerted ameliorative impacts and restored visual functions by diminishing oxidative stress and neuronal degeneration. PRACTICAL APPLICATIONS: Diabetes is a complex disease that involves various physiological perturbations especially visual functions. In our study, we showed that vitamin A and carrot root extract (CE) confer remarkable protection against retinal degeneration in STZ-induced diabetic rats. Our findings showed that the chemical and phytochemical ingredients of the vitamin A and CE substantially attenuated the histopathological changes, oxidative stress, inflammatory reactions, and cellular death in diabetic rats. These favorable changes are attributable to the high content of retinoic acid, carotenoids, and phenolic compounds that effectively regulates the production of visual pigments, increases the antioxidant defense system, and diminishes the pro-inflammatory and apoptotic pathways. Thus, the nutritional values of vitamin A and CE represent promising therapeutic choices to mitigate the retinal-induced diabetic insults.

Targeted pharmacotherapy against neurodegeneration and neuroinflammation in early diabetic retinopathy

Diabetic retinopathy (DR), the most frequent complication of diabetes, is one of the leading causes of irreversible blindness in working-age adults and has traditionally been regarded as a microvascular disease. However, increasing evidence has revealed that synaptic neurodegeneration of retinal ganglion cells (RGCs) and activation of glial cells may represent some of the earliest events in the pathogenesis of DR. Upon diabetes-induced metabolic stress, abnormal glycogen synthase kinase-3β (GSK-3β) activation drives tau hyperphosphorylation and β-catenin downregulation, leading to mitochondrial impairment and synaptic neurodegeneration prior to RGC apoptosis. Moreover, glial cell activation triggers enhanced inflammation and oxidative stress, which may accelerate the deterioration of diabetic RGCs neurodegeneration. These findings have opened up opportunities for therapies, such as inhibition of GSK-3β, glial cell activation, glutamate excitotoxicity and the use of neuroprotective drugs targeting early neurodegenerative processes in the retina and halting the progression of DR before the manifestation of microvascular abnormalities. Such interventions could potentially remedy early neurodegeneration and help prevent vision loss in people suffering from DR.

Direct-Coupled Electroretinogram (DC-ERG) for Recording the Light-Evoked Electrical Responses of the Mouse Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is a specialized monolayer of cells strategically located between the retina and the choriocapillaris that maintain the overall health and structural integrity of the photoreceptors. The RPE is polarized, exhibiting apically and basally located receptors or channels, and performs vectoral transport of water, ions, metabolites, and secretes several cytokines. In vivo noninvasive measurements of RPE function can be made using direct-coupled ERGs (DC-ERGs). The methodology behind the DC-ERG was pioneered by Marmorstein, Peachey, and colleagues using a custom-built stimulation recording system and later demonstrated using a commercially available system. The DC-ERG technique uses glass capillaries filled with Hank's buffered salt solution (HBSS) to measure the slower electrical responses of the RPE elicited from light-evoked concentration changes in the subretinal space due to photoreceptor activity. The prolonged light stimulus and length of the DC-ERG recording make it vulnerable to drift and noise resulting in a low yield of useable recordings. Here, we present a fast, reliable method for improving the stability of the recordings while reducing noise by using vacuum pressure to reduce/eliminate bubbles that result from outgassing of the HBSS and electrode holder. Additionally, power line artifacts are attenuated using a voltage regulator/power conditioner. We include the necessary light stimulation protocols for a commercially available ERG system as well as scripts for analysis of the DC-ERG components: c-wave, fast oscillation, light peak, and off response. Due to the improved ease of recordings and rapid analysis workflow, this simplified protocol is particularly useful in measuring age-related changes in RPE function, disease progression, and in the assessment of pharmacological intervention.

Preventing diabetic retinopathy by mitigating subretinal space oxidative stress in vivo

Patients with diabetes continue to suffer from impaired visual performance before the appearance of overt damage to the retinal microvasculature and later sight-threatening complications. This diabetic retinopathy (DR) has long been thought to start with endothelial cell oxidative stress. Yet newer data surprisingly finds that the avascular outer retina is the primary site of oxidative stress before microvascular histopathology in experimental DR. Importantly, correcting this early oxidative stress is sufficient to restore vision and mitigate the histopathology in diabetic models. However, translating these promising results into the clinic has been stymied by an absence of methods that can measure and optimize anti-oxidant treatment efficacy in vivo. Here, we review imaging approaches that address this problem. In particular, diabetes-induced oxidative stress impairs dark-light regulation of subretinal space hydration, which regulates the distribution of interphotoreceptor binding protein (IRBP). IRBP is a vision-critical, anti-oxidant, lipid transporter, and pro-survival factor. We show how optical coherence tomography can measure subretinal space oxidative stress thus setting the stage for personalizing anti-oxidant treatment and prevention of impactful declines and loss of vision in patients with diabetes.

Metabolic and Non-metabolic Roles of Pyruvate Kinase M2 Isoform in Diabetic Retinopathy

The main therapeutic goal for diabetic retinopathy (DR) is to prevent vision loss in patients with diabetes mellitus. Identifying the visual complications at a preclinical juncture will offer an early therapeutic window for diagnosis and intervention. Very recently, we found that pyruvate kinase M2 isoform (PKM2) regulates visual function through regulation of a key enzyme, phosphodiesterase 6β (Pde6β), involved in modulating photoreceptor functions. A recent study showed that the activation of PKM2 protects mitochondrial integrity in diabetic nephropathy. In the present study, we examined the role of PKM2 in DR in a mouse model that has both phenotypes of obesity and type II diabetes. In DR, we found decreased expression of PKM2 and Pde6β expression, but not PKM1. Consistent with decreased Pde6β expression, the db/db mice had reduced rod photoreceptor function. We found increased pyruvate kinase activity and a decreased ratio of reduced/oxidized redox in db/db mouse retina compared with control retinas. There was no significant difference in the levels of lactate between db/db and control mouse retina. Our findings suggest that reduced expression of PKM2 with unchanged PKM1 expression might be responsible for higher pyruvate kinase activity in db/db mouse retina. Our studies suggest that PKM2 has a role in DR. The results support that PKM2 may serve as a therapeutic target in the treatment of DR.

Elucidating the mechanism of action of alpha-1-antitrypsin using retinal pigment epithelium cells exposed to high glucose. Potential use in diabetic retinopathy

BACKGROUND

Alpha-1-antitrypsin is a protein involved in avoidance of different processes that are seen in diabetic retinopathy pathogenesis. These processes include apoptosis, extracellular matrix remodeling and damage of vessel walls and capillaries. Furthermore, because of its anti-inflammatory effects, alpha-1-antitrypsin has been proposed as a possible therapeutic approach for diabetic retinopathy. Our group tested alpha-1-antitrypsin in a type 1 diabetes mouse model and observed a reduction of inflammation and retinal neurodegeneration. Thus, shedding light on the mechanism of action of alpha-1-antitrypsin at molecular level may explain how it works in the diabetic retinopathy context and show its potential for use in other retinal diseases.

METHODS

In this work, we evaluated alpha-1-antitrypsin in an ARPE-19 human cell line exposed to high glucose. We explored the expression of different mediators on signaling pathways related to pro-inflammatory cytokines production, glucose metabolism, epithelial-mesenchymal transition and other proteins involved in the normal function of retinal pigment epithelium by RT-qPCR and Western Blot.

RESULTS

We obtained different expression patterns for evaluated mediators altered with high glucose exposure and corrected with the use of alpha-1-antitrypsin.

CONCLUSIONS

The expression profile obtained in vitro for the evaluated proteins and mRNA allowed us to explain our previous results obtained on mouse models and to hypothesize how alpha-1-antitrypsin hinder diabetic retinopathy progression on a complex network between different signaling pathways.

GENERAL SIGNIFICANCE

This network helps to understand the way alpha-1-antitrypsin works in diabetic retinopathy and its scope of action.

Lutein reverses hyperglycemia-mediated blockage of Nrf2 translocation by modulating the activation of intracellular protein kinases in retinal pigment epithelial (ARPE-19) cells

Diabetic retinopathy (DR) is a major cause of acquired blindness among working adults. The retinal pigment epithelium (RPE), constitutes an outer blood-retinal barrier, is vastly affected in diabetic humans and animals. Lower levels of lutein in the serum and retina of diabetic population, and beneficial effects of carotenoids supplementation in diabetic retinopathy patients created an interest to examine the protective effect of lutein on hyperglycemia-mediated changes in oxidative stress and antioxidant defense system in ARPE-19 cells. The WST-1 assay was performed to analyze the impact of glucose, and lutein on the viability of ARPE-19. The intracellular oxidative stress was measured by a DCF (dichlorofluorescein) assay, mitochondrial membrane potential (MMP) was monitored using a JC-10 MMP assay kit and GSH level was examined using GSH/GSSG ratio detection kit. The oxidative stress markers, protein carbonyl and malondialdehyde were spectrophotometrically measured using 2,4-dinitrophenylhydrazine and 2-thiobarbituric acid, respectively. The expression of endogenous antioxidant enzymes and regulatory proteins in ARPE-19 was quantified by western blotting. The localization of Nrf2 protein was examined by immunofluorescent staining. The results show that lutein (up to 1.0 μM) did not affect the viability of ARPE-19 grown in both normal and high-glucose conditions. Lutein treatment blocked high glucose-mediated elevation of intracellular ROS, protein carbonyl and malondialdehyde content in ARPE-19 cells. The decreased MMP and GSH levels observed in ARPE-19 grown under high-glucose condition were rescued by lutein treatment. Further, lutein protected high glucose-mediated down-regulation of a redox-sensitive transcription factor, Nrf2, and antioxidant enzymes, SOD2, HO-1, and catalase. This protective effect of lutein was linked with activated nuclear translocation of Nrf2, which was associated with increased activation of regulatory proteins such as Erk and AKT. Our study indicates that improving the concentration of lutein in the retina could protect RPE from diabetes-associated damage.

Retinal proteomic evaluation of rats following streptozotocin‑injection using shotgun proteomics

It is important to elucidate how retinal stimulation leads to retinal protection and dysfunction. The current study aimed to identify factors that are up‑ and downregulated in the retinas of streptozotocin (STZ)‑induced diabetic rats with acute retinal dysfunction. Retinal function was measured and changes in protein expressions were determined using electroretinograms (ERGs) and liquid chromatography/mass spectroscopy‑based shotgun proteomics, respectively. The results revealed that the plasma glucose levels of STZ rats were markedly higher when compared with normal rats. Furthermore, levels of a‑waves, b‑waves and oscillatory potential amplitudes on ERGs in STZ rats were decreased compared with healthy animals. With use of shotgun proteomics, 391 proteins were identified in the retinas of normal rats and 541 proteins were found in the retinas of STZ rats. Of the 560 proteins identified in rat retinas, 372 (66.4%) were present in both normal and STZ rats. Of these, 19 (3.39%) were unique to normal rats and 169 (30.1%) were unique to STZ rats. Gene Ontology analysis was performed on the candidate proteins that were differentially regulated in the retinas of STZ rats and focused on those classified as 'protein binding', which serve important roles in retinal neurodegeneration. The results revealed an excessive expression of retinol‑binding protein 1 (RBP1) and a negative expression of rod outer segment membrane protein 1 (Rom-1) in the retinas of STZ rats. Therefore, retinal function may be decreased with STZ‑induced injury, and expressions of Rom‑1 and RBP1 may be altered in the retinas of STZ rats.

Montelukast Prevents Early Diabetic Retinopathy in Mice

Chronic inflammation and oxidative stress are critical components in the pathogenic cascade of early diabetic retinopathy, characterized by neuronal and vascular degeneration. We investigated pharmacologic inhibition of the proinflammatory leukotriene cascade for therapeutic benefit in early diabetic retinopathy. Using the streptozotocin-induced diabetes mouse model, we administered montelukast, a leukotriene receptor antagonist, and diabetes-related retinal pathology was assessed. Early biochemical and cellular function measures were evaluated at 3 months' diabetes duration and included vascular permeability, superoxide production, leukotriene generation, leukocyte-induced microvascular endothelial cell death, and retinal function by electroretinography. Histopathology assessments at 9 months' diabetes duration included capillary degeneration and retinal ganglion cell loss. Leukotriene receptor antagonism resulted in a significant reduction of early, diabetes-induced retinal capillary leakage, superoxide generation, leukocyte adherence, and leukotriene generation. After 9 months of diabetes, the retinal microvasculature from untreated diabetic mice demonstrated a nearly threefold increase in capillary degeneration compared with nondiabetic mice. Montelukast inhibited the diabetes-induced capillary and neuronal degeneration, whether administered as a prevention strategy, immediately after induction of diabetes, or as an intervention strategy starting at 4.5 months after confirmation of diabetes. Pharmacologic blockade of the leukotriene pathway holds potential as a novel therapy to prevent or slow the development of diabetic retinopathy.

Erythropoietin protects outer blood-retinal barrier in experimental diabetic retinopathy by up-regulating ZO-1 and occludin

PURPOSE

To explore the mechanisms of erythropoietin (EPO) in maintaining outer blood-retinal barrier (BRB) in diabetic rats.

METHODS

Sprague-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.

RESULTS

The 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.

CONCLUSIONS

In 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.

Use of Direct Current Electroretinography for Analysis of Retinal Pigment Epithelium Function in Mouse Models

A monolayer of pigmented epithelial cells, the retinal pigment epithelium (RPE), supports photoreceptor function in many ways. Consistent with these roles, RPE dysfunction underlies a number of hereditary retinal disorders. To monitor RPE function in vivo models for these conditions, we adapted an electroretinographic (ERG) technique based on direct current amplification (DC-ERG). This chapter describes the main features of this approach and its application to mouse models involving the RPE.

Endogenous insulin signaling in the RPE contributes to the maintenance of rod photoreceptor function in diabetes

In diabetes, there are two major physiological aberrations: (i) Loss of insulin signaling due to absence of insulin (type 1 diabetes) or insulin resistance (type 2 diabetes) and (ii) increased blood glucose levels. The retina has a high proclivity to damage following diabetes, and much of the pathology seen in diabetic retinopathy has been ascribed to hyperglycemia and downstream cascades activated by increased blood glucose. However, less attention has been focused on the direct role of insulin on retinal physiology, likely due to the fact that uptake of glucose in retinal cells is not insulin-dependent. The retinal pigment epithelium (RPE) is instrumental in maintaining the structural and functional integrity of the retina. Recent studies have suggested that RPE dysfunction is a precursor of, and contributes to, the development of diabetic retinopathy. To evaluate the role of insulin on RPE cell function directly, we generated a RPE specific insulin receptor (IR) knockout (RPEIRKO) mouse using the Cre-loxP system. Using this mouse, we sought to determine the impact of insulin-mediated signaling in the RPE on retinal function under physiological control conditions as well as in streptozotocin (STZ)-induced diabetes. We demonstrate that loss of RPE-specific IR expression resulted in lower a- and b-wave electroretinogram amplitudes in diabetic mice as compared to diabetic mice that expressed IR on the RPE. Interestingly, RPEIRKO mice did not exhibit significant differences in the amplitude of the RPE-dependent electroretinogram c-wave as compared to diabetic controls. However, loss of IR-mediated signaling in the RPE reduced levels of reactive oxygen species and the expression of pro-inflammatory cytokines in the retina of diabetic mice. These results imply that IR-mediated signaling in the RPE regulates photoreceptor function and may play a role in the generation of oxidative stress and inflammation in the retina in diabetes.

Interaction between microglia and retinal pigment epithelial cells determines the integrity of outer blood-retinal barrier in diabetic retinopathy

Inner and outer blood-retinal barriers (BRBs), mainly composed of retinal endothelial cells and retinal pigment epithelial (RPE) cells, respectively, maintain the integrity of the retinal tissues. In this study, we aimed to investigate the mechanisms of the outer BRB disruption regarding the interaction between RPE and microglia. In mice with high-fat diet-induced obesity and streptozotocin-induced hyperglycemia, microglia accumulated on the RPE layer, as in those after intravitreal injection of interleukin (IL)-6, which is elevated in ocular fluids of patients with diabetic retinopathy. Although IL-6 did not directly affect the levels of zonula occludens (ZO)-1 and occludin in RPE cells, IL-6 increased VEGFA mRNA in RPE cells to recruit microglial cells. In microglial cells, IL-6 upregulated the mRNA levels of MCP1, MIP1A, and MIP1B, to amplify the recruitment of microglial cells. In this manner, IL-6 modulated RPE and microglial cells to attract microglial cells on RPE cells. Furthermore, IL-6-treated microglial cells produced and secreted tumor necrosis factor (TNF)-α, which activated NF-κB and decreased the levels of ZO-1 in RPE cells. As STAT3 inhibition reversed the effects of IL-6-treated microglial cells on the RPE monolayer in vitro, it reduced the recruitment of microglial cells and the production of TNF-α in RPE tissues in streptozotocin-treated mice. Taken together, IL-6-treated RPE and microglial cells amplified the recruitment of microglial cells and IL-6-treated microglial cells produced TNF-α to disrupt the outer BRB in diabetic retinopathy.

Polarized retinal pigment epithelium generates electrical signals that diminish with age and regulate retinal pathology

The transepithelial potential difference (TEP) across the retinal pigment epithelial (RPE) is dependent on ionic pumps and tight junction "seals" between epithelial cells. RPE cells release neurotrophic growth factors such as pigment epithelial derived factor (PEDF), which is reduced in age-related macular degeneration (AMD). The mechanisms that control the secretion of PEDF from RPE cells are not well understood. Using the CCL2/CX3CR1 double knockout mouse model (DKO), which demonstrates RPE damage and retinal degeneration, we uncovered an interaction between PEDF and the TEP which is likely to play an important role in retinal ageing and in the pathogenesis of AMD. We found that: (a) the expression of ATP1B1 (the Na+ /K+ -ATPase β1 subunit) was reduced significantly in RPE from aged mice, in patients with CNV (Choroidal Neovascularization) and in DKO mice; (b) the expression of PEDF also was decreased in aged persons and in DKO mice; (c) the TEP across RPE was reduced markedly in RPE cells from DKO mice and (d) an applied electric field (EF) of 50-100 mV/mm, used to mimic the natural TEP, increased the expression and secretion of PEDF in primary RPE cells. In conclusion, the TEP across the RPE depends on the expression of ATP1B1 and this regulates the secretion of PEDF by RPE cells and so may regulate the onset of retinal disease. Increasing the expression of PEDF using an applied EF to replenish a disease or age-reduced TEP may offer a new way of preventing or reversing retinal dysfunction.

One month of hyperglycemia alters spectral responses of the zebrafish photopic electroretinogram

Prolonged hyperglycemia can alter retinal function, ultimately resulting in blindness. Adult zebrafish adults exposed to alternating conditions of 2% glucose/0% glucose display a 3× increase in blood sugar levels. After 4 weeks of treatment, electroretinograms (ERGs) were recorded from isolated, perfused, in vitro eyecups. Control animals were exposed to alternating 2% mannitol/0% mannitol (osmotic control) or to alternating water (0% glucose/0% glucose; handling control). Two types of ERGs were recorded: (1) native ERGs measured using white-light stimuli and medium without synaptic blockers; and (2) spectral ERGs measured with an AMPA/kainate receptor antagonist, isolating photoreceptor-to-ON-bipolar-cell synapses, and a spectral protocol that separated red (R), green (G), blue (B) and UV cone signals. Retinas were evaluated for changes in layer thickness and for the inflammatory markers GFAP and Nf-κB (RelA or p65). In native ERGs, hyperglycemic b- and d-waves were lower in amplitude than the b- and d-waves of mannitol controls. Alteration of waveshape became severe, with b-waves becoming more transient and ERG responses showing more PIII-like (a-wave) characteristics. For spectral ERGs, waveshape appeared similar in all treatment groups. However, a1- and b2-wave implicit times were significantly longer, and amplitudes were significantly reduced, in response to hyperglycemic treatment, owing to the functional reduction in signals from R, G and B cones. Nf-κB increased significantly in hyperglycemic retinas, but the increase in GFAP was not significant and retinal layer thickness was unaffected. Thus, prolonged hyperglycemia triggers an inflammatory response and functional deficits localized to specific cone types, indicating the rapid onset of neural complications in the zebrafish model of diabetic retinopathy.

Summary: Zebrafish can be used to examine diabetic complications, including vision loss. Here, in zebrafish, we show that prolonged (4 week) hyperglycemia causes an inflammatory response associated with functional deficits localized to specific cone types.

Evidence for an Association between Macular Degeneration and Thyroid Cancer in the Aged Population

Direct evidence of whether thyroid cancer patients have a higher risk of age-related macular degeneration (AMD) has yet to be investigated. Patients older than 50 years-old and newly diagnosed with thyroid cancer between 2000 and 2008 were identified from the national health insurance research database (NHIRD). We applied time-varying Cox proportional hazard models to assess the association between thyroid cancer and AMD. The multivariable models included conventional cardiovascular risk factors, myopia, vitreous floaters, hypothyroidism, hyperthyroidism, and treatment modality of thyroid cancer. The analysis process was stratified by age, gender, and comorbidity. In this study, 5253 patients were included in a thyroid cancer cohort (men 24.5%; median age 59.1 years (53.7–67.4 years), and 21,012 matched controls were included in a non-thyroid cancer cohort. The AMD incidence was 40.7 per 10,000 person/year in the thyroid cancer cohort. The thyroid cancer cohort had a higher risk (adjusted hazard ratio (aHR) = 1.38, 95% confidence interval, CI = 1.09–1.75) of AMD than the non-thyroid cohort. Thyroid cancer patients had a higher risk of AMD, especially the male patients (aHR = 1.92, 95% CI = 1.38–3.14) and the patients with comorbidities (aHR = 1.38, 95% CI = 1.09–1.74). In conclusion, thyroid cancer patients older than 50 years-old have increased risk of AMD.

Loss of CD40 attenuates experimental diabetes-induced retinal inflammation but does not protect mice from electroretinogram defects

Chronic low grade inflammation is considered to contribute to the development of experimental diabetic retinopathy (DR). We recently demonstrated that lack of CD40 in mice ameliorates the upregulation of inflammatory molecules in the diabetic retina and prevented capillary degeneration, a hallmark of experimental diabetic retinopathy. Herein, we investigated the contribution of CD40 to diabetes-induced reductions in retinal function via the electroretinogram (ERG) to determine if inflammation plays a role in the development of ERG defects associated with diabetes. We demonstrate that diabetic CD40-/- mice are not protected from reduction to the ERG b-wave despite failing to upregulate inflammatory molecules in the retina. Our data therefore supports the hypothesis that retinal dysfunction found in diabetics occurs independent of the induction of inflammatory processes.

Nepetin inhibits IL-1β induced inflammation via NF-κB and MAPKs signaling pathways in ARPE-19 cells

BACKGROUNDS

Chronic inflammation in retinal pigment epithelial (RPE) cells is related to the pathogenesis of retinal inflammatory blind causing diseases such as age-related macular degeneration (AMD) and diabetic retinopathy (DR). Nepetin, a natural flavonoid compound, has shown potent anti-inflammatory activities but has not been studied on ocular resident cells yet. Here, we assess the ability of Nepetin to alleviate the inflammatory responses of ARPE-19 cells induced by interleukin (IL)-1β.

METHODS

The secretion and mRNA expression of inflammatory cytokines IL-6, IL-8 and monocyte chemoattractant protein-1 (MCP-1) induced by IL-1β are measured by enzyme-linked immunosorbent assay (ELISA) and real-time polymerase chain reaction (RT-PCR) respectively. To clarify the underlying action mechanism, we examine the effect of Nepetin on activation of nuclear factor of kappa B (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways using Western blot.

RESULTS

Nepetin can significantly decrease the three inflammatory mediators at both protein and mRNA level in a dose-dependent manner. Western blot results show that Nepetin can decrease the nuclear translocation of p65 through suppressing phosphorylation of inhibitor of nuclear factor kappa B (IκB) and IκB kinase (IKK). Also, Nepetin can decrease the phosphorylation of extracellular signal-regulated kinases (ERK) 1/2, c-Jun N-terminal kinase (JNK) and p38 MAPK.

CONCLUSIONS

Taken together, Nepetin abolishes IL-1β-induced IL-6, IL-8 and MCP-1 secretion and mRNA expression by repressing the activation of NF-κB and MAPKs. These results indicate that Nepetin shows potential to be used for prevention and treatment of inflammatory retinal diseases or as a lead compound.

Evaluation of Retinal Function in Streptozotocin-induced Diabetic Rats by Using the Electroretinography and Immunohistochemistry Methods

Streptozotocin-induced diabetic rat (STZ rat) was used in many studies for the diabetic mellitus. In this study, we demonstrated whether the electroretinograms (ERG) was changed in the retina of STZ rats. In addition, we investigated the histopathological alteration in the retina of STZ rats by using the immunological method. The 100 mg/kg of STZ was injected continuously for 2 d (100 mg/kg×2). The insulin level was decreased, and the glucose level was enhanced 14 d after the injection of STZ. Moreover, the levels of a-wave, b-wave and OP amplitude were decreased in the rat at 14 d after the injection of STZ. Although, the damage and apoptosis was not observed in the retinal ganglion cell of STZ rats by the immunological experiment using the phospho-H2A.X and cleaved caspase-3, the distance between cell and cell was increased in both of outer- and inner- nuclear (granule) layer in retina of STZ rats. In conclusion, we showed that the enhanced thickening in retina was caused by the injection of excessive STZ. The thickening in retina of STZ rats may lead to the dysfunction of retina, resulting in the decrease in ERG. These findings provide significant information that can be used in the design of a model of diabetic retinopathy.

NAMPT-Mediated NAD(+) Biosynthesis Is Essential for Vision In Mice

Photoreceptor death is the endpoint of many blinding diseases. Identifying unifying pathogenic mechanisms in these diseases may offer global approaches for facilitating photoreceptor survival. We found that rod or cone photoreceptor-specific deletion of nicotinamide phosphoribosyltransferase (Nampt), the rate-limiting enzyme in the major NAD(+) biosynthetic pathway beginning with nicotinamide, caused retinal degeneration. In both cases, we could rescue vision with nicotinamide mononucleotide (NMN). Significantly, retinal NAD(+) deficiency was an early feature of multiple mouse models of retinal dysfunction, including light-induced degeneration, streptozotocin-induced diabetic retinopathy, and age-associated dysfunction. Mechanistically, NAD(+) deficiency caused metabolic dysfunction and consequent photoreceptor death. We further demonstrate that the NAD(+)-dependent mitochondrial deacylases SIRT3 and SIRT5 play important roles in retinal homeostasis and that NAD(+) deficiency causes SIRT3 dysfunction. These findings demonstrate that NAD(+) biosynthesis is essential for vision, provide a foundation for future work to further clarify the mechanisms involved, and identify a unifying therapeutic target for diverse blinding diseases.