Endoplasmic reticulum stress and diabetic retinopathy

Docosahexaenoic acid eliminates endoplasmic reticulum stress and inflammatory pathways in diabetic rat keratopathy

Diabetic keratopathy, characterized by corneal structural changes, is a common complication of diabetes mellitus (DM). Docosahexaenoic acid (DHA), an omega-3 fatty acid, has shown potential therapeutic benefits in various diabetic complications. This study aimed to investigate the protective effect of DHA on corneal tissue in streptozotocin (STZ)-induced type 2 DM in rats. Forty male Sprague-Dawley rats were randomly assigned to four groups (n = 10 per group): Control, DHA, DM, and DM + DHA. The DHA group received DHA by oral gavage at a dose of 100 mg/kg daily for 10 days. In the DM group, diabetes was induced by a single intraperitoneal injection of STZ at 50 mg/kg. Confirmation of diabetes induction was based on monitoring fasting blood glucose levels on the third day post-injection. The DM + DHA group underwent the same diabetes induction protocol with STZ and received DHA at 100 mg/kg daily via oral gavage for 10 consecutive days. Corneal tissue samples were collected at the end of the study period for histopathological, immunohistochemical, qRT-PCR, and ELISA analyses. Histopathological analysis showed significant edema, angiogenesis, and degeneration in the DM group compared to the control (p < 0.001). DHA treatment significantly mitigated these changes, approaching control levels (p < 0.01). Immunohistochemistry showed increased VEGFR2 and iNOS expression in the DM group, which was significantly reduced in the DM + DHA group (p < 0.01). qRT-PCR results indicated a significant decrease in Bcl-2 expression (p < 0.001) and an increase in ATF-6, IRE1, NF-κB, TNF-α, IL-1β, NLRP3, Bax, and Caspase-3 expressions in the DM group (p < 0.001). ELISA analyses revealed significantly elevated levels of inflammatory markers NF-κB, TNF-α, IL-1β, and IL-6 in the DM group compared to the control (p < 0.001). DHA treatment significantly upregulated Bcl-2 and downregulated apoptotic and inflammatory markers (p < 0.01). DHA demonstrated significant protective effects against STZ-induced corneal damage in diabetic rats by modulating apoptotic and inflammatory pathways. These findings suggest that DHA may be a promising therapeutic agent for preventing diabetic keratopathy.

The ghrelin-GHSR-1a pathway inhibits high glucose-induced retinal angiogenesis in vitro by alleviating endoplasmic reticulum stress

Background

To investigate the effect of ghrelin, a brain-gut peptide hormone, on high glucose-induced retinal angiogenesis in vitro and explore its association with endoplasmic reticulum (ER) stress.

Methods

Human retinal microvascular endothelial cells (HRMECs) were first divided into control and high-glucose groups, and the mRNA and protein expression levels of the receptor for ghrelin [growth hormone secretin receptor 1a, (GHSR-1a)] in cells were determined. HRMECs were then treated with high glucose alone or in combination with ghrelin or siGHSR-1a, and cell viability, migration, tube formation and the expression of the ER stress-related proteins PERK, ATF4 and CHOP were detected. Finally, to clarify whether the effects of ghrelin are related to ER stress, tunicamycin, an inducer of ER stress, was used to treat HRMECs, and cell viability, cell migration, and tube formation were evaluated.

Results

GHSR-1a expression in HRMECs at both the mRNA and protein levels was inhibited by high-glucose treatment. Under high-glucose conditions, ghrelin promoted cell viability and inhibited migration and tube formation, which were blocked by siGHSR-1a treatment. Ghrelin inhibited the increases in the protein levels of p-PERK, ATF4 and CHOP induced by high-glucose treatment, and combination treatment with siGHSR-1a reversed this effect of ghrelin. When tunicamycin was added, the effects of ghrelin on cell viability, migration and tube formation were all weakened.

Conclusions

This study experimentally revealed that ghrelin can inhibit high glucose-induced retinal angiogenesis in vitro through GHSR-1a, and alleviation of ER stress may be one of the mechanisms underlying this effect.

Hyperglycemia induces retinal ganglion cell endoplasmic reticulum stress to the involvement of glaucoma in diabetic mice

Glaucoma is a neurodegenerative disease leading to visual loss. Since glaucoma is associated with chronic renal diseases (RDs) their rate is higher in patients with RDs, and end-stage RDs (ESRDs) than in the general population and kidney transplant recipients.

OBJECTIVE

To explore the molecular mechanism of diabetic internal environment in regulating the endoplasmic reticulum stress of the retinal ganglion cells (RGCs).

METHODS

Thirty-six SPF grade type 2 diabetes models were divided into 3 groups: Diabetes mellitus (DM), DM + glaucoma and 4-phenylbutyric acid-DM (4-PBA-DM) + glaucoma group. C57BL6 mice of the same week age were taken as the negative control (NC) group. The morphology of RGCs and their axon in the 4 groups were labeled by fluorescent reactive dye Dil. The apoptosis situation of RGCs was determined by terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) assay. The protein expression values of RTN4IP1, Protein kinase R-like endoplasmic reticulum kinase (PERK), eukaryotic initiation factor 2A (eIF2a) and X-box-binding Protein 1 (XBP1) were determined by western blot. The relative mRNA levels of cyclophosphamide, doxorubicin, vincristine and prednisone (CHOP), Caspase12 and Bax were determined by quantitative real-time polymerase chain reaction (qRT-PCR).

RESULTS

Glaucoma promotes the apoptosis of RGCs. The protein expression values of RTN4IP1, PERK and XBP1 in DM mouse models with glaucoma were much higher compared to only DM mouse models. Further injection of endoplasmic reticulum stress inhibitor 4-PBA decreased the expression values. The relative mRNA levels of CHOP, Cysteine aspartic acid specific protease12 (Caspase12) and BCL2-associated X protein (Bax) in DM + glaucoma were significantly higher compared to those in DM group. Further injection of endoplasmic reticulum stress inhibitor 4-PBA decreased the mRNA levels.

CONCLUSION

Endoplasmic reticulum stress (ERS) is the underlying cause of glaucoma, which could promote the apoptosis of RGCs in diabetic mice.

Diabetic Complications: An Update on Pathobiology and Therapeutic Strategies

Despite the advent of novel therapies which manage and control diabetes well, the increased risk of morbidity and mortality in diabetic subjects is associated with the devastating secondary complications it produces. Long-standing diabetes majorly drives cellular and molecular alterations, which eventually damage both small and large blood vessels. The complications are prevalent both in type I and type II diabetic subjects. The microvascular complications include diabetic neuropathy, diabetic nephropathy, diabetic retinopathy, while the macrovascular complications include diabetic heart disease and stroke. The current therapeutic strategy alleviates the complications to some extent but does not cure or prevent them. Also, the recent clinical trial outcomes in this field are disappointing. Success in the drug discovery of diabetic complications may be achieved by a better understanding of the underlying pathophysiology and by recognising the crucial factors contributing to the development and progression of the disease. In this review, we discuss the well-studied cellular mechanisms leading to the development and progression of diabetic complications. In addition, we also highlight the various therapeutic paradigms currently in clinical practice.

Regulation of Endothelium-Reticulum-Stress-Mediated Apoptotic Cell Death by a Polymethoxylated Flavone, Nobiletin, Through the Inhibition of Nuclear Translocation of Glyceraldehyde 3-Phosphate Dehydrogenase in Retinal Müller Cells

In the early stages of diabetic retinopathy (DR), subtle biochemical and functional alterations occur in Müller cells, which are one of the components of the blood-retinal barrier (BRB). Müller cells are the principal glia of the retina and have shown a strong involvement in the maintenance of homeostasis and the development of retinal tissue. Their functional abnormalities and eventual loss have been correlated with a decrease in the tight junctions between endothelial cells and a consequent breakdown of the BRB, leading to the development of DR. We demonstrated that the endothelium reticulum (ER) triggers Müller cell death and that nuclear accumulation of glyceraldehyde 3-phosphate dehydrogenase is closely associated with ER-induced Müller cell death. In addition, induction of ER stress in Müller cells increased vascular endothelial growth factor expression but decreased pigment-epithelium-derived factor (PEDF) expression in Müller cells. We found that nobiletin, a polymethoxylated flavone from citrus explants, exerts protective action against ER-stress-induced Müller cell death. In addition, nobiletin was found to augment PEDF expression in Müller cells, which may lead to the protection of BRB integrity. These results suggest that nobiletin can be an attractive candidate for the protection of the BRB from breakdown in DR.

Non-invasive multimodal imaging of Diabetic Retinopathy: A survey on treatment methods and Nanotheranostics

Diabetes Retinopathy (DR) is one of the most prominent microvascular complications of diabetes. It is one of the pre-eminent causes for vision impairment followed by blindness among the working-age population worldwide. The de facto cause for DR remains challenging, despite several efforts made to unveil the mechanism underlying the pathology of DR. There is quite less availability of the low cost pre-emptive theranostic imaging tools in terms of in-depth resolution, due to the multiple factors involved in the etiology of DR. This review work comprehensively explores the various reports and research works on all perspectives of diabetic retinopathy (DR), and its mechanism. It also discusses various advanced non-destructive imaging modalities, current, and future treatment approaches. Further, the application of various nanoparticle-based drug delivery strategies used for the treatment of DR are also discussed. In a nutshell, the present review work bolsters the pursuit of the development of an advanced non-invasive optical imaging modal with a nano-theranostic approach for the future diagnosis and treatment of DR and its associated ocular complications.

Implication of Hyperhomocysteinemia in Blood Retinal Barrier (BRB) Dysfunction

Elevated plasma homocysteine (Hcy) level, known as hyperhomocysteinemia (HHcy) has been linked to different systemic and neurological diseases, well-known as a risk factor for systemic atherosclerosis and cardiovascular disease (CVD) and has been identified as a risk factor for several ocular disorders, such as diabetic retinopathy (DR) and age-related macular degeneration (AMD). Different mechanisms have been proposed to explain HHcy-induced visual dysfunction, including oxidative stress, upregulation of inflammatory mediators, retinal ganglion cell apoptosis, and extracellular matrix remodeling. Our previous studies using in vivo and in vitro models of HHcy have demonstrated that Hcy impairs the function of both inner and outer blood retinal barrier (BRB). Dysfunction of BRB is a hallmark of vision loss in DR and AMD. Our findings highlighted oxidative stress, ER stress, inflammation, and epigenetic modifications as possible mechanisms of HHcy-induced BRB dysfunction. In addition, we recently reported HHcy-induced brain inflammation as a mechanism of blood–brain barrier (BBB) dysfunction and pathogenesis of Alzheimer’s disease (AD). Moreover, we are currently investigating the activation of glutamate receptor N-methyl-d-aspartate receptor (NMDAR) as the molecular mechanism for HHcy-induced BRB dysfunction. This review focuses on the studied effects of HHcy on BRB and the controversial role of HHcy in the pathogenesis of aging neurological diseases such as DR, AMD, and AD. We also highlight the possible mechanisms for such deleterious effects of HHcy.

Quercetin ameliorates diabetic encephalopathy through SIRT1/ER stress pathway in db/db mice

Studies have shown that diabetes is an important risk factor for cognitive dysfunction, also called diabetic encephalopathy (DE). Quercetin has been reported to be effective in improving cognitive dysfunction in DE. But its detailed mechanism is still ambiguous. In this study, we used db/db mice to investigate whether quercetin could activate SIRT1 and inhibit ER pathways to improve DE. Behavioral tests (Morris water maze and new objects) showed that quercetin (70 mg/kg) can effectively improve the learning and memory ability in db/db mice. OGTT and ITT tests indicated that quercetin could alleviate impaired glucose tolerance and insulin resistance in db/db mice. Western blot analysis and Nissl staining showed that quercetin can improve the expression of nerve and synapse-associated proteins (PSD93, PSD95, NGF and BDNF) and inhibit neurodegeneration. Meanwhile, quercetin up-regulates SIRT1 protein expression and inhibits the expression of ER signaling pathway-related proteins (PERK, IRE-1α, ATF6, eIF2α, BIP and PDI). In addition, oxidative stress levels were significantly reduced after quercetin treatment. In conclusion, current experimental results indicated that SIRT1/ER stress is a promising mechanism involved in quercetin-treated diabetic encephalopathy.

Tunicamycin-induced photoreceptor atrophy precedes degeneration of retinal capillaries with minimal effects on retinal ganglion and pigment epithelium cells

Endoplasmic reticulum (ER) stress is recognized as a contributing factor to various ocular neurovascular pathologies including retinitis pigmentosa, glaucoma, and diabetic retinopathy (DR). ER stress in particular is implicated in the development of DR, which is significantly influenced by inflammation driven retinal vascular degeneration and dysfunction. Ultimately, loss of vision occurs if left untreated. However, the identity of the target cells and their temporal involvement in diabetes-mediated dysfunction need further investigation. Early diabetes-induced stress in photoreceptor cells is proposed as the driver of inflammatory mediated neurovascular changes during diabetes. Although tunicamycin induced ER stress results in photoreceptor loss, its consequences for retinal vascular degeneration and retinal ganglion (RGC) and pigment epithelium (RPE) cell loss remains unclear. Here we show intravitreal delivery of tunicamycin primarily induced ER stress in photoreceptor cells resulting in their loss by apoptosis. This was concomitant with induced expression of the unfolded protein response marker CHOP in these cells. We also demonstrated significant degeneration of retinal capillaries following the loss of photoreceptor cells with minimal impact on loss of RGC and RPE cells. However, activation of retinal microglial and Muller cells were noticeable. Thus, our data support the notion that ER stress mediated dysfunction and/or loss of photoreceptor cells in response to inflammation and oxidative stress could precede retinal vascular and neuronal dysfunction and degeneration.

Loss of XBP1 Leads to Early-Onset Retinal Neurodegeneration in a Mouse Model of Type I Diabetes

Retinal neuronal injury and degeneration is one of the primary manifestations of diabetic retinopathy, a leading cause of vision loss in working age adults. In pathological conditions, including diabetes and some physiological conditions such as aging, protein homeostasis can become disrupted, leading to endoplasmic reticulum (ER) stress. Severe or unmitigated ER stress can lead to cell death, which in retinal neurons results in irreversible loss of visual function. X-box binding protein 1 (XBP1) is a major transcription factor responsible for the adaptive unfolded protein response (UPR) to maintain protein homeostasis in cells undergoing ER stress. The purpose of this study is to determine the role of XBP1-mediated UPR in retinal neuronal survival and function in a mouse model of type 1 diabetes. Using a conditional retina-specific XBP1 knockout mouse line, we demonstrate that depletion of XBP1 in retinal neurons results in early onset retinal function decline, loss of retinal ganglion cells and photoreceptors, disrupted photoreceptor ribbon synapses, and Müller cell activation after induction of diabetes. Our findings suggest an important role of XBP1-mediated adaptive UPR in retinal neuronal survival and function in diabetes.

Protective Effects of Leukemia Inhibitory Factor on Retinal Vasculature and Cells in Streptozotocin-induced Diabetic Mice

BACKGROUND

Leukemia inhibitory factor (LIF) has been reported to possess various pharmacological effects, including displaying vascular and neuroprotective properties, during retinal disease. The aim of this study was to investigate the vascular and structural changes in the retina of diabetic mice and to explore whether LIF prevents experimental diabetes-induced retinal injury in the early stages.

METHODS

Diabetes was induced in C57Bl/6J mice with streptozotocin (STZ) injections. Successful diabetic animal models were randomly separated into two groups: the diabetic group (n = 15) and the LIF-treated group (n = 15). Normal C57BL/6 mice served as the normal control group (n = 14). Recombinant human LIF was intravitreally injected 8 weeks after the diabetic model was successfully established. Retinas were collected and evaluated using histological and immunohistochemical techniques, and flat-mounted retinas and Western blotting were performed at 18 weeks after the induction of diabetes and 2 days after the intravitreal injection of LIF. The analysis of variance test were used.

RESULTS

Histological analysis showed that there were fewer retinal ganglion cells (RGCs) and the inner nuclear layer (INL) became thinner in the diabetic model group (RGC 21.8 ± 4.0 and INL 120.2 ± 4.6 μm) compared with the normal control group (RGC 29.0 ± 6.7, t = -3.02, P = 0.007; INL 150.7 ± 10.6 μm, t = -8.88, P < 0.001, respectively). After LIF treatment, the number of RGCs (26.9 ± 5.3) was significantly increased (t = 3.39, P = 0.030) and the INL (134.5 ± 14.2 μm) was thicker compared to the diabetic group (t = 2.75, P = 0.013). In the anti-Brn-3a-labeled retinas, the number of RGCs in the LIF-treated group (3926.0 ± 143.9) was obviously increased compared to the diabetic group (3507.7 ± 286.1, t = 2.38, P = 0.030), while no significance was found between the LIF-treated group and the control group (4188.3 ± 114.7, t = -2.47, P = 0.069). Flat-mounted retinas demonstrated that a disorganized, dense distribution of the vessel was prominent in the diabetic model group. Vessel distribution in the LIF-treated mouse group was typical and the thickness was uniform. The levels of phosphosignal transducer and activator of transcription 3 activation were obviously higher in the LIF-injected retinas than those in the diabetic control group (t = 3.85, P = 0.019) and the normal control (t = -3.20, P = 0.019).

CONCLUSION

The present study provides evidence that LIF treatment protects the integrity of the vasculature and prevents retinal injury in the early stages of diabetic retinopathy in STZ-induced diabetic models.

Chrysin Ameliorates Malfunction of Retinoid Visual Cycle through Blocking Activation of AGE-RAGE-ER Stress in Glucose-Stimulated Retinal Pigment Epithelial Cells and Diabetic Eyes

Diabetes-associated visual cycle impairment has been implicated in diabetic retinopathy, and chronic hyperglycemia causes detrimental effects on visual function. Chrysin, a naturally occurring flavonoid found in various herbs, has anti-inflammatory, antioxidant, and neuroprotective properties. The goal of the current study was to identify the retinoprotective role of chrysin in maintaining robust retinoid visual cycle-related components. The in vitro study employed human retinal pigment epithelial (RPE) cells exposed to 33 mM of glucose or advanced glycation end products (AGEs) in the presence of 1–20 μM chrysin for three days. In the in vivo study, 10 mg/kg of chrysin was orally administrated to db/db mice. Treating chrysin reversed the glucose-induced production of vascular endothelial growth factor, insulin-like growth factor-1, and pigment epithelium-derived factor (PEDF) in RPE cells. The outer nuclear layer thickness of chrysin-exposed retina was enhanced. The oral gavage of chrysin augmented the levels of the visual cycle enzymes of RPE65, lecithin retinol acyltransferase (LRAT), retinol dehydrogenase 5 (RDH5), and rhodopsin diminished in db/db mouse retina. The diabetic tissue levels of the retinoid binding proteins and the receptor of the cellular retinol-binding protein, cellular retinaldehyde-binding protein-1, interphotoreceptor retinoid-binding protein and stimulated by retinoic acid 6 were restored to those of normal mouse retina. The presence of chrysin demoted AGE secretion and AGE receptor (RAGE) induction in glucose-exposed RPE cells and diabetic eyes. Chrysin inhibited the reduction of PEDF, RPE 65, LRAT, and RDH5 in 100 μg/mL of AGE-bovine serum albumin-exposed RPE cells. The treatment of RPE cells with chrysin reduced the activation of endoplasmic reticulum (ER) stress. Chrysin inhibited the impairment of the retinoid visual cycle through blocking ER stress via the AGE-RAGE activation in glucose-stimulated RPE cells and diabetic eyes. This is the first study demonstrating the protective effects of chrysin on the diabetes-associated malfunctioned visual cycle.

Reduction of Endoplasmic Reticulum Stress Improves Angiogenic Progenitor Cell function in a Mouse Model of Type 1 Diabetes

Persistent vascular injury and degeneration in diabetes are attributed in part to defective reparatory function of angiogenic cells. Our recent work implicates endoplasmic reticulum (ER) stress in high-glucose-induced bone marrow (BM) progenitor dysfunction. Herein, we investigated the in vivo role of ER stress in angiogenic abnormalities of streptozotocin-induced diabetic mice. Our data demonstrate that ER stress markers and inflammatory gene expression in BM mononuclear cells and hematopoietic progenitor cells increase dynamically with disease progression. Increased CHOP and cleaved caspase­ 3 levels were observed in BM­-derived early outgrowth cells (EOCs) after 3 months of diabetes. Inhibition of ER stress by ex vivo or in vivo chemical chaperone treatment significantly improved the generation and migration of diabetic EOCs while reducing apoptosis of these cells. Chemical chaperone treatment also increased the number of circulating angiogenic cells in peripheral blood, alleviated BM pathology, and enhanced retinal vascular repair following ischemia/reperfusion in diabetic mice. Mechanistically, knockdown of CHOP alleviated high-glucose-induced EOC dysfunction and mitigated apoptosis, suggesting a pivotal role of CHOP in mediating ER stress-associated angiogenic cell injury in diabetes. Together, our study suggests that targeting ER signaling may provide a promising and novel approach to enhancing angiogenic function in diabetes.

Elevated Activating Transcription Factor 4 and Glucose-Regulated 78 Kda Protein Levels Correlate with Inflammatory Cytokines in the Aqueous Humor and Vitreous of Proliferative Diabetic Retinopathy

PURPOSE

To determine concentrations of endoplasmic reticulum (ER) stress-related factors activating transcription factor 4 (ATF4) and glucose-regulated 78 kDa protein (GRP78) in vitreous and aqueous humor (AqH) of patients with proliferative diabetic retinopathy (PDR) and the correlation of ATF4, GRP78 and inflammatory cytokines interleukin-6(IL-6) and monocyte chemoattractant protein-1 (MCP-1).

MATERIALS AND METHODS

AqH and vitreous samples were collected from eyes of patients with PDR and idiopathic macular hole (IMH) which needed vitrectomy. Protein Levels of ATF4, GRP78, and IL-6, MCP-1 in samples were evaluated using enzyme-linked immunosorbent assay (ELISA).

RESULTS

ELISA analysis revealed significantly increased levels in both AqH and vitreous of ATF4 and GRP78 in eyes affected with PDR compared to the controls (all p < 0.001). The mean concentrations of IL-6, MCP-1 were also higher in both AqH and vitreous samples from patients with PDR compared to those of IMH (all p < 0.001). (Independent Student t-test, normality test followed with Skewness-Kurtosis Test). In addition, correlations of ATF4 and GRP78 with inflammatory factors IL-6 and MCP-1 in subjects of patients were analyzed. No significant correlation between the AqH concentrations of ATF4/IL-6 and ATF4/MCP-1 was detected in eyes of PDR patients (r = 0.346, p = 0.072 and r = 0.275, p = 0.157). Significant correlations were observed between AqH concentrations of GRP78/IL-6 (r = 0.724, p < 0.001), GRP78/MCP-1 (r = 0.654, p < 0.001) in PDR patients. Significant correlations were observed between vitreous concentrations of ATF4/IL-6 (r = 0.918, p < 0.001), ATF4/MCP-1 (r = 0.921, p < 0.001), GRP78/IL-6 (r = 0.978, p < 0.001), GRP78/MCP-1 (r = 0.979, p < 0.001) in PDR patients. No significant correlations was observed between AqH concentrations of ATF4/IL-6 (r = 0.187, p = 474), ATF4/MCP-1 (r = 0.240, p = 0.353), GRP78/IL-6 (r = 0.321, p = 0.209) and GRP78/MCP-1 (r = 0.169, p = 0.516) in eyes of IMH patients. And also no significant correlation was observed between vitreous concentrations of ATF4/IL-6 (r = 0.130, p = 0.563), ATF4/MCP-1(r = 0.029, p = 0.897), GRP78/IL-6 (r = 0.078, p = 0.717), GRP78/MCP-1 (r = 0.005, p = 0.982) in IMH patients. (Pearson correlation coefficient (two-tailed)).

CONCLUSIONS

Our results demonstrated that ATF4 and GRP78 may play an important role in the pathogenesis of PDR and work in concert with inflammatory cytokines IL-6 and MCP-1 in pathological process. ATF4 and GRP78 may be good diagnostic biomarkers and new therapeutic targets for PDR.

ABBREVIATIONS

ER stress, endoplasmic reticulum stress; ATF4, activating transcription factor 4; GRP78, glucose-regulated 78 kDa protein; AqH, aqueous humor; PDR, proliferative diabetic retinopathy; IL-6, interleukin-6; MCP-1, monocyte chemoattractant protein-1; IMH, idiopathic macular hole.

Ursodeoxycholic Acid Attenuates Endoplasmic Reticulum Stress-Related Retinal Pericyte Loss in Streptozotocin-Induced Diabetic Mice

Loss of pericytes, an early hallmark of diabetic retinopathy (DR), results in breakdown of the blood-retinal barrier. Endoplasmic reticulum (ER) stress may be involved in this process. The purpose of this study was to examine the effects of ursodeoxycholic acid (UDCA), a known ameliorator of ER stress, on pericyte loss in DR of streptozotocin- (STZ-) induced diabetic mice. To assess the extent of DR, the integrity of retinal vessels and density of retinal capillaries in STZ-induced diabetic mice were evaluated. Additionally, induction of ER stress and the unfolded protein response (UPR) were assessed in diabetic mice and human retinal pericytes exposed to advanced glycation end products (AGE) or modified low-density lipoprotein (mLDL). Fluorescein dye leakage during angiography and retinal capillary density were improved in UDCA-treated diabetic mice, compared to the nontreated diabetic group. Among the UPR markers, those involved in the protein kinase-like ER kinase (PERK) pathway were increased, while UDCA attenuated UPR in STZ-induced diabetic mice as well as AGE- or mLDL-exposed retinal pericytes in culture. Consequently, vascular integrity was improved and pericyte loss reduced in the retina of STZ-induced diabetic mice. Our findings suggest that UDCA might be effective in protecting against DR.

Retinal changes in diabetic patients without diabetic retinopathy

PURPOSE

The purpose of this study was to measure retinal vessel caliber and to examine early changes in macular thickness using optical coherence tomography (OCT). We evaluated to what extend vascular caliber and macular thickness differed between patients with type 2 diabetes mellitus without diabetic retinopathy compared with healthy individuals.

METHODS

26 diabetic patients without diabetic retinopathy and 26 normal participants without any retinal and optic nerve diseases underwent ophthalmic examination, fundus photography, and OCT imaging. Temporal inferior retinal vessel diameters were measured using OCT. Also, we measured macular thickness in nine ETDRS subfields using Cirrus OCT.

RESULTS

The mean age in the diabetic group was 61.5 years and in the control group, 55.5 years. Wider retinal arterioles and venules were found in patients with diabetes compared with healthy subjects (120 µm versus 96 µm, p<0.005 and 137 µm versus 120.5 µm, p value <0.001, respectively). In patients with type 2 diabetes mellitus, central macular thickness was significantly thinner than that of control eyes (243.5 µm versus 269.9 µm, p value <0.001).

CONCLUSIONS

Our results support the hypothesis that the association between vascular damage and structural changes of the neuroretina is an early indicator of retinal impairment in patients with diabetes without diabetic retinopathy.

Crude Saponins of Panax notoginseng Have Neuroprotective Effects To Inhibit Palmitate-Triggered Endoplasmic Reticulum Stress-Associated Apoptosis and Loss of Postsynaptic Proteins in Staurosporine Differentiated RGC-5 Retinal Ganglion Cells

Increased apoptosis of retinal ganglion cells (RGCs) contributes to the gradual loss of retinal neurons at the early phase of diabetic retinopathy (DR). There is an urgent need to search for drugs with neuroprotective effects against apoptosis of RGCs for the early treatment of DR. This study aimed to investigate the neuroprotective effects of saponins extracted from Panax notoginseng, a traditional Chinese medicine, on apoptosis of RGCs stimulated by palmitate, a metabolic factor for the development of diabetes and its complications, and to explore the potential molecular mechanism. We showed that crude saponins of P. notoginseng (CSPN) inhibited the increased apoptosis and loss of postsynaptic protein PSD-95 by palmitate in staurosporine-differentiated RGC-5 cells. Moreover, CSPN suppressed palmitate-induced reactive oxygen species generation and endoplasmic reticulum stress-associated eIF2α/ATF4/CHOP and caspase 12 pathways. Thus, our findings address the potential therapeutic significance of CSPN for the early stage of DR.

Diabetic Retinopathy: Animal Models, Therapies, and Perspectives

Diabetic retinopathy (DR) is one of the major complications of diabetes. Although great efforts have been made to uncover the mechanisms underlying the pathology of DR, the exact causes of DR remain largely unknown. Because of multifactor involvement in DR etiology, currently no effective therapeutic treatments for DR are available. In this paper, we review the pathology of DR, commonly used animal models, and novel therapeutic approaches. Perspectives and future directions for DR treatment are discussed.

Experimental study of the protective effects of SYVN1 against diabetic retinopathy

Genetic factors play an important role in the pathogenesis of diabetic retinopathy (DR). While many studies have focused on genes that increase susceptibility to DR, herein, we aimed to explore genes that confer DR resistance. Previously, we identified Hmg CoA reductase degradation protein 1 (SYVN1) as a putative DR protective gene via gene expression analysis. Transgenic mice overexpressing SYVN1 and wild-type (WT) mice with streptozotocin-induced diabetes were used in this experiment. Retinal damage and vascular leakage were investigated 6 months after induction of diabetes by histopathological and retinal cell apoptosis analyses and by retinal perfusion of fluorescein isothiocyanate-conjugated dextran. Compared with diabetic WT mice, diabetic SYVN1 mice had significantly more cells and reduced apoptosis in the retinal ganglion layer. Retinal vascular leakage was significantly lower in diabetic SYVN1 mice than in diabetic WT mice. The expression levels of endoplasmic reticulum (ER) stress-related, pro-inflammatory, and pro-angiogenic genes were also analyzed. Lower expression levels were observed in diabetic SYVN1 mice than in WT controls, suggesting that SYVN1 may play an important role in inhibiting ER stress, chronic inflammation, and vascular overgrowth associated with DR. Thus, these results strongly supported our hypothesis that SYVN1 confers DR resistance.

Increased O-GlcNAcylation of NF-κB Enhances Retinal Ganglion Cell Death in Streptozotocin-induced Diabetic Retinopathy

PURPOSE

Hyperglycemia results in increased flux through the hexoxamine biosynthetic pathway. We examined whether hyperglycemia increases O-GlcNAcylation in the diabetic retina and whether elevated O-GlcNAcylation of nuclear factor (NF)-κB increases apoptosis of retinal ganglion cells (RGCs) in diabetic retinopathy (DR).

MATERIALS AND METHODS

Diabetes was induced in C57BL/6 mice by five consecutive intraperitoneal injections of 55 mg/kg streptozotocin. All mice were killed 2 months after injections and expression levels of O-GlcNAcylated proteins, O-linked N-acetylglucosamine transferase (OGT), β-d-N-acetylglucosaminidase and NF-κB, and the extent of RGC death were examined. Immunoprecipitations were performed to investigate whether O-GlcNAcylation of NF-κB led to its activation and RGC death in DR.

RESULTS

The expression levels of O-GlcNAcylated proteins and OGT were markedly higher in diabetic retinas than in control retinas. OGT colocalized with NeuN, a RGC-specific marker, and terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling-positive cells in the ganglion cell layer of diabetic retinas. The p65 subunit of NF-κB was O-GlcNAcylated and the level of O-GlcNAcylated p65 was higher in diabetic retinas than in control retinas.

CONCLUSION

The present data suggest that hyperglycemia increases O-GlcNAcylation in DR and that O-GlcNAcylation of the p65 subunit of NF-κB is involved in hyperglycemia-induced NF-κB activation and RGC death in DR.

Identification of p58IPK as a novel neuroprotective factor for retinal neurons

PURPOSE

Endoplasmic reticulum (ER)-resident chaperone protein p58(IPK) plays a vital role in regulation of protein folding and biosynthesis. The goal of this study was to examine the role of p58(IPK) in retinal neuronal cells under normal and stressed conditions.

METHODS

Retinal expression of p58(IPK), retinal morphology, apoptosis, ER stress, and apoptotic gene expression were examined in p58(IPK) knockout (KO) and/or wild-type (WT) mice with or without intravitreal injection of N-methyl-D-aspartic acid (NMDA). In in vitro experiments, differentiated R28 retinal neuronal cells transduced with adenovirus encoding p58(IPK) (Ad-p58(IPK)) or control virus (Ad-LacZ) were exposed to tunicamycin (TM) or hydrogen peroxide (H2O2). Levels of ER stress, apoptosis, and cell survival were evaluated.

RESULTS

Chaperone protein p58(IPK) is expressed predominantly in retinal ganglion cells (RGC), inner retinal neurons, and the photoreceptor inner segments. Mice lacking p58(IPK) exhibited increased CHOP expression and loss of RGCs with aging (8-10 months). Intravitreal injection of NMDA induced retinal ER stress and increased p58(IPK) expression in WT mice; this resulted in greater ER stress and enhanced RGC apoptosis in p58(IPK) KO mice. In cultured R28 cells, overexpression of p58(IPK) significantly reduced eIF2α phosphorylation, decreased CHOP expression, and alleviated the activation of caspase-3 and PARP. Overexpression of p58(IPK) also protected against oxidative and ER stress-induced cell apoptosis. Furthermore, p58(IPK) downregulated the proapoptotic gene Bax and upregulated the antiapoptotic gene Bcl-2 expression in stressed R28 cells.

CONCLUSIONS

Our study has demonstrated a protective role of p58(IPK) in retinal neurons, which may act in part through a mechanism involving modulation of ER homeostasis and apoptosis, particularly under conditions of cellular stresses.

Tonicity response element binding protein associated with neuronal cell death in the experimental diabetic retinopathy

AIM

To study the contribution of tonicity response element binding protein (TonEBP) in retinal ganglion cell (RGC) death of diabetic retinopathy (DR).

METHODS

Diabetes was induced in C57BL/6 mice by five consecutive intraperitoneal injections of 55 mg/kg streptozotocin (STZ). Control mice received vehicle (phosphate-buffered saline). All mice were killed 2mo after injections, and the extent of cell death and the protein expression levels of TonEBP and aldose reductase (AR) were examined.

RESULTS

The TonEBP and AR protein levels and the death of RGC were significantly increased in the retinas of diabetic mice compared with controls 2mo after the induction of diabetes. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick end labeling (TUNEL)-positive signals co-localized with TonEBP immunoreactive RGC. These changes were increased in the diabetic retinas compared with controls.

CONCLUSION

The present data show that AR and TonEBP are upregulated in the DR and TonEBP may contribute to apoptosis of RGC in the DR.

Stress in the kidney is the road to pERdition: is endoplasmic reticulum stress a pathogenic mediator of diabetic nephropathy?

The endoplasmic reticulum (ER) is an organelle that primarily functions to synthesise new proteins and degrade old proteins. Owing to the continual and variable nature of protein turnover, protein synthesis is inherently an error-prone process and is therefore tightly regulated. Fortunately, if this balance between synthesis and degradation is perturbed, an intrinsic response, the unfolded protein response (UPR) is activated to restore ER homoeostasis through the action of inositol-requiring protein 1, activating transcription factor 6 and PKR-like ER kinase transmembrane sensors. However, if the UPR is oversaturated and misfolded proteins accumulate, the ER can shift into a cytotoxic response, a physiological phenomenon known as ER stress. The mechanistic pathways of the UPR have been extensively explored; however, the role of this process in such a synthetic organ as the kidney requires further clarification. This review will focus on these aspects and will discuss the role of ER stress in specific resident kidney cells and how this may be integral in the pathogenesis and progression of diabetic nephropathy (DN). Given that diabetes is a perturbed state of protein turnover in most tissues, it is important to understand if ER stress is a secondary or tertiary response to other changes within the diabetic milieu or if it is an independent accelerator of kidney disease. Modulators of ER stress could provide a valuable tool for the treatment of DN and are under active investigation in other contexts.

Increased ER stress as a mechanism of retinal neurovasculopathy in mice with severe hyperhomocysteinemia

Hyperhomocysteinemia is implicated in retinal neurovascular diseases including arterial occlusive disease, venous occlusive disease and pseudoexfoliation glaucoma. The mechanism for these diseases is not known. Here we used hyperhomocysteinemic mice lacking the gene encoding cystathionine-beta-synthase (cbs^-/-) to examine whether ER stress could be a mechanism for the retinal neurovasculopathy reported in these mice. Retinas of cbs^+/+ and cbs^-/- mice (age: 3-5 wks) were used to investigate the expression of ER stress genes (BiP/GRP78, Perk, Atf6, Atf4, Ire1α, Chop) and the proteins they encode. The levels of poly(ADP-ribose) polymerase (PARP) and cleaved cysteine-aspartic proteases-3 (caspase-3), proteins known to be involved in apoptosis, were also examined. Quantitative reverse transcription polymerase chain reaction and western blotting revealed an increase in BiP/GRP78 and PERK in retinas of cbs^-/- mice compared with cbs^+/+ mice. There was an elevation of CCAAT-enhancer-binding protein homologous protein (CHOP) in retinal cryosections of cbs^-/- mice indicating apoptosis, which was confirmed by increased levels of PARP and cleaved caspase-3. The data suggest that the genes and proteins that are major players in the ER stress pathway, particularly the PERK pathway, are upregulated in retinas of cbs^-/- mice. The data support a role for ER stress in the pathophysiology associated with the hyperhomocysteinemia-linked retinal disease.

The unfolded protein response and diabetic retinopathy

Diabetic retinopathy, a common complication of diabetes, is the leading cause of blindness in adults. Diabetes chronically damages retinal blood vessels and neurons likely through multiple pathogenic pathways such as oxidative stress, inflammation, and endoplasmic reticulum (ER) stress. To relieve ER stress, the cell activates an adaptive mechanism known as the unfolded protein response (UPR). The UPR coordinates the processes of protein synthesis, protein folding, and degradation to ensure proteostasis, which is vital for cell survival and activity. Emerging evidence suggests that diabetes can activate all three UPR branches in retinal cells, among which the PERK/ATF4 pathway is the most extensively studied in the development of diabetic retinopathy. X-box binding protein 1 (XBP1) is a major transcription factor in the core UPR pathway and also regulates a variety of genes involved in cellular metabolism, redox state, autophagy, inflammation, cell survival, and vascular function. The exact function and implication of XBP1 in the pathogenesis of diabetic retinopathy remain elusive. Focusing on this less studied pathway, we summarize recent progress in studies of the UPR pertaining to diabetic changes in retinal vasculature and neurons, highlighting the perspective of XBP1 as a potential therapeutic target in diabetic retinopathy.

Increased expression of phosphorylated c-Jun and phosphorylated c-Jun N-terminal kinase associated with neuronal cell death in diabetic and high glucose exposed rat retinas

The aim of this study is to examine whether the increased expression of phosphorylated c-Jun (p-c-Jun) and phosphorylated c-Jun N-terminal kinase (p-JNK) are significantly associated with neuronal cell death in diabetic rat retinas and retinas exposed to high glucose. Retinas isolated from six adult male Sprague-Dawley rats and six streptozotocin-induced diabetic rats (DM) were cultured in serum-free medium. The explants from non-diabetic controls were cultured in normal-glucose (N) or high-glucose (HG) medium. Furthermore, neurotrophin-4 (NT-4) and Taurine-conjugated ursodeoxycholic acid (TUDCA) were incubated in HG medium. After 7 days, the numbers of regenerating neurites were counted per explant. After counting, the explants were fixed, cryosectioned, and stained by TUNEL, and also immunostained for p-c-Jun and p-JNK. The numbers of TUNEL-positive, p-c-Jun- and p-JNK-immunopositive cells in the GCL were significantly higher and the numbers of regenerating neurites were significantly lower in the HG and the DM groups than in the N groups. In the HG groups supplemented with NT-4 and TUDCA, the numbers of TUNEL-positive, p-c-Jun- and p-JNK-immunopositive cells were significantly lower and the numbers of neurites were significantly higher than in the HG group without NT-4 and TUDCA. Increased expression of p-c-Jun and p-JNK is associated with neuronal cell death in diabetic rat retinas and retinas exposed to high glucose. Neuroprotective effect of TUDCA and NT-4 is correlated with the suppression of p-c-Jun and p-JNK expression. These results provide a better understanding of the neurodegenerative process underlying DR.

Vascular effects of advanced glycation endproducts: Clinical effects and molecular mechanisms

The enhanced generation and accumulation of advanced glycation endproducts (AGEs) have been linked to increased risk for macrovascular and microvascular complications associated with diabetes mellitus. AGEs result from the nonenzymatic reaction of reducing sugars with proteins, lipids, and nucleic acids, potentially altering their function by disrupting molecular conformation, promoting cross-linking, altering enzyme activity, reducing their clearance, and impairing receptor recognition. AGEs may also activate specific receptors, like the receptor for AGEs (RAGE), which is present on the surface of all cells relevant to atherosclerotic processes, triggering oxidative stress, inflammation and apoptosis. Understanding the pathogenic mechanisms of AGEs is paramount to develop strategies against diabetic and cardiovascular complications.

Effects of modified LDL and HDL on retinal pigment epithelial cells: a role in diabetic retinopathy?

AIMS/HYPOTHESIS

Blood-retina barrier leakage in diabetes results in extravasation of plasma lipoproteins. Intra-retinal modified LDLs have been implicated in diabetic retinopathy (DR), but their effects on retinal pigment epithelial (RPE) cells and the added effects of extravasated modified HDLs are unknown.

METHODS

In human retinas from individuals with and without diabetes and DR, immunohistochemistry was used to detect ApoB, ApoA1 and endoplasmic reticulum (ER) stress markers. In cell culture, human RPE cells were treated with native LDL (N-LDL) or heavily-oxidised glycated LDL (HOG-LDL) with or without pretreatment with native HDL (N-HDL) or heavily-oxidised glycated HDL (HOG-HDL). Cell viability, oxidative stress, ER stress, apoptosis and autophagy were assessed by Cell Counting Kit-8 assay, dichlorofluorescein assay, western blotting, immunofluorescence and TUNEL assay. In separate experiments, RPE cells were treated with lipid oxidation products, 7-ketocholesterol (7-KC, 5-40 μmol/l) or 4-hydroxynonenal (4-HNE, 5-80 μmol/l), with or without pretreatment with N-HDL or HOG-HDL.

RESULTS

ApoB, ApoA1 staining and RPE ER stress were increased in the presence of DR. HOG-LDL but not N-LDL significantly decreased RPE cell viability and increased reactive oxygen species generation, ER stress, apoptosis and autophagy. Similarly, 4-HNE and 7-KC decreased viability and induced ER stress. Pretreatment with N-HDL mitigated these effects, whereas HOG-HDL was less effective by most, but not all, measures.

CONCLUSIONS/INTERPRETATION

In DR, extravascular modified LDL may promote RPE injury through oxidative stress, ER stress, autophagy and apoptosis. N-HDL has protective effects, but HOG-HDL is less effective. Extravasation and modification of HDL may modulate the injurious effects of extravasated modified LDL on the retinal pigment epithelium.

Modulation of angiogenesis by genetic manipulation of ATF4 in mouse model of oxygen-induced retinopathy [corrected]

PURPOSE

The activation of the unfolded protein response (UPR) and an increase in activating transcription factor 4 (ATF4) has been previously reported in the diabetic retina. Despite this, a direct link between ATF4 and the degree of proliferative retinopathy has not been demonstrated to date. Therefore, the objective of this study was to determine whether ATF4 deficiency could reduce neovascularization in mice with oxygen-induced retinopathy (OIR).

METHODS

We induced OIR in C57BL/6, ATF4(+/-), and endoplasmic reticulum stress-activated indicator (ERAI) mice and used quantitative RT-PCR and Western blot analysis to evaluate relative gene and protein expression. Histology and microscopy were used to calculate the extent of neovascularization in flat-mounted retinas.

RESULTS

Experimental data revealed Xbp1 splicing in the retinal ganglia cells, outer plexiform layer, inner nuclear layer, and outer nuclear layer and in pericytes of postdevelopment day 17 ERAI OIR mice, confirming the activation of IRE1 UPR signaling. In naive ATF4-deficient mice, we also observed an elevation in UPR-associated and vascular-associated gene expression (Bip, Atf6, Hif1a, Pik3/Akt, Flt1/Vegfa, and Tgfb1), which may have contributed to the alleviation of hypoxia-driven neovascularization in experimental ATF4(+/-) retinas. The OIR ATF4(+/-) retinas demonstrated reprogramming of the UPR seen at both the mRNA (Atf6 and Bip) and protein (pATF6 and peIf2α) levels, as well as a reduction in vascularization-associated gene expression (Flt1, Vegf1, Hif1, and Tgb1). These changes corresponded to the decline in the rate of neovascularization.

CONCLUSIONS

Our study validates ATF4 as a prospective therapeutic target to inhibit neovascularization in proliferative retinopathy.

TCF7L2 variation and proliferative diabetic retinopathy

Proliferative diabetic retinopathy (PDR) is the most severe vision-threatening complication of diabetes. For investigation of genetic association between TCF7L2 and PDR in Caucasian type 2 diabetes mellitus (T2DM) and its functional consequences, 383 T2DM patients with PDR (T2DM-PDR) and 756 T2DM patients without diabetic retinopathy (T2DM-no DR) were genotyped with rs7903146 in TCF7L2. We found that risk allele (T) frequency of rs7903146 was significantly higher in T2DM-PDR patients (allelic P = 2.52E-04). In lymphoblastoid cells induced to undergo endoplasmic reticulum (ER) stress by treatment of tunicamycin, higher fold change of TCF7L2 and VEGFA mRNA levels were observed in rs7903146-TT cells than in rs7903146-CC cells (P = 0.02 for TCF7L2; P = 0.004 for VEGFA), suggesting that ER stress plays a role in PDR pathogenesis. Silencing TCF7L2 resulted in decreased mRNA levels of both TCF7L2 and VEGFA (P < 0.001). Retinas of oxygen-induced retinopathy mice (a model for PDR) had higher TCF7L2 and VEGFA mRNA levels than those of controls (P = 2.9E-04 for TCF7L2; P = 1.9E-07 for VEGFA). Together, data from our study show that TCF7L2-rs7903146 is associated with PDR in Caucasian T2DM and suggest that TCF7L2 promotes pathological retinal neovascularization via ER stress-dependent upregulation of VEGFA.

The unfolded protein response to endoplasmic reticulum stress in cultured astrocytes and rat brain during experimental diabetes

Oxidative-nitrosative stress and inflammatory responses are associated with endoplasmic reticulum (ER) stress in diabetic retinopathy, raising the possibility that disturbances in ER protein processing may contribute to CNS dysfunction in diabetics. Upregulation of the unfolded protein response (UPR) is a homeostatic response to accumulation of abnormal proteins in the ER, and the present study tested the hypothesis that the UPR is upregulated in two models for diabetes, cultured astrocytes grown in 25mmol/L glucose for up to 4weeks and brain of streptozotocin (STZ)-treated rats with diabetes for 1-7months. Markers associated with translational blockade (phospho-eIF2α and apoptosis (CHOP), inflammatory response (inducible nitric oxide synthase, iNOS), and nitrosative stress (nuclear translocation of glyceraldehyde-3-phosphate dehydrogenase, GAPDH) were not detected in either model. Nrf2 was present in nuclei of low- and high-glucose cultures, consistent with oxidative stress. Astrocytic ATF4 expression was not altered by culture glucose concentration, whereas phospho-IRE and ATF6 levels were higher in low- compared with high-glucose cultures. The glucose-regulated chaperones, GRP78 and GRP94, were also expressed at higher levels in low- than high-glucose cultures, probably due to recurrent glucose depletion between feeding cycles. In STZ-rat cerebral cortex, ATF4 level was transiently reduced at 4months, and p-IRE levels were transiently elevated at 3months. However, GRP78 and GRP94 expression was not upregulated, and iNOS, amyloid-β, and nuclear accumulation of GAPDH were not evident in STZ-diabetic brain. High-glucose cultured astrocytes and STZ-diabetic brain are relatively resistant to diabetes-induced ER stress, in sharp contrast with cultured retinal Müller cells and diabetic rodent retina.

Thioredoxin Interacting Protein (TXNIP) and Pathogenesis of Diabetic Retinopathy

Chronic hyperglycemia (HG)-associated reactive oxygen/nitrogen species (ROS/RNS) stress and low grade inflammation are considered to play critical roles in the development of diabetic retinopathy (DR). Excess glucose metabolic flux through the aldose reductase/polyol pathway, advanced glycation end product (AGE) formation, elevated hexosamine biosynthesis pathway (HBP), diacyl glycerol/PKC activation, and mitochondrial ROS generation are all implicated in DR. In addition, endoplasmic reticulum stress/unfolded protein response (er-UPR) and deregulation of mitochondrial quality control by autophagy/mitophagy are observed causing cellular bioenergetic deficiency and injury. Recently, a pro-oxidant and pro-apoptotic thioredoxin interacting protein (TXNIP) was shown to be highly upregulated in DR and by HG in retinal cells in culture. TXNIP binds to thioredoxin (Trx) inhibiting its oxidant scavenging and thiolreducing capacity. Hence, prolonged overexpression of TXNIP causes ROS/RNS stress, mitochondrial dysfunction, inflammation and premature cell death in DR. Initially, DR was considered as microvascular complications of endothelial dysfunction and pericyte loss characterized by capillary basement membrane thickening, pericyte ghost, blood retinal barrier leakage, acellular capillary and neovascularization. However, it is currently acknowledged that neuro-glia are also affected by HG in diabetes and that neuronal injury, glial activation, innate immunity/sterile inflammation, and ganglion apoptosis occur early in DR. In addition, retinal pigment epithelium (RPE) becomes dysfunctional in DR. Since TXNIP is induced by HG in most cells, its effects are not restricted to a particular cell type in DR. However, depending on the metabolic activity and anti-oxidant capacity, some cells may be affected earlier by TXNIP than others. Identification of TXNIP sensitive cells and elucidating the underlying mechanism(s) will be critical for preventing pre-mature cell death and progression of DR.

Modified Lipoproteins in Diabetic Retinopathy: A Local Action in the Retina

Clinical epidemiological studies have revealed relatively weak, yet statistically significant, associations between dyslipidemia/dyslipoproteinemia and diabetic retinopathy (DR). Recent large interventional studies, however, demonstrated an unexpectedly robust efficacy of fenofibrate on the development of DR, possibly independent of plasma lipids. To unify the apparent discrepancies, we hypothesize that plasma lipoproteins play an indirect but important role in DR, contingent on the integrity of the blood-retina-barrier (BRB). In retinas with an intact BRB, plasma lipoproteins may be largely irrelevant; however, important effects become operative after the BRB is impaired in diabetes, leading to lipoprotein extravasation and subsequent modification, hence toxicity to the neighbouring retinal cells. In this hypothesis, BRB leakage is the key, plasma lipoprotein concentrations mainly modulate its consequences, and fenofibrate has intra-retinal actions. This review summarizes our current knowledge of the direct effects and mechanisms of modified lipoproteins on retinal cells and their potential contribution to the pathogenesis of DR.

Oxidative and endoplasmic reticulum stresses mediate apoptosis induced by modified LDL in human retinal Müller cells

PURPOSE

We previously showed that extravasated, modified LDL is implicated in pericyte loss in diabetic retinopathy (DR). Here, we investigate whether modified LDL induces apoptosis in retinal Müller glial cells.

METHODS

Cultured human retinal Müller cells (MIO-M1) were treated with highly oxidized glycated LDL (HOG-LDL, 200 mg protein/L) or native LDL (N-LDL, 200 mg protein/L) for up to 24 hours with or without pretreatment with N-acetyl-cysteine (NAC, a blocker of oxidative stress) and 4-phenylbutyrate (4-PBA, a blocker of endoplasmic reticulum [ER] stress). Effects of HOG-LDL on cell viability, apoptosis, oxidative stress, and ER stress were assessed by cell viability, TUNEL, and Western blot assays. In separate experiments, Müller cells were treated with 7-ketocholesterol (7-KC, 5-20 μM) or 4-hydroxynonenal (4-HNE, 5-40 μM) for up to 24 hours. The same markers were measured.

RESULTS

HOG-LDL induced apoptosis (decreased cell viability, increased TUNEL staining, increased expression of cleaved PARP, cleaved caspase-3, and BAX; decreased Bcl-2), oxidative stress (increased NOX4 and antioxidant enzymes, catalase, and superoxide dismutase 2), and ER stress (increased phospho-eIF2α, KDEL, ATF6, and CHOP). Pretreatment with NAC or 4-PBA partially attenuated apoptosis. In addition. NAC attenuated activation of ER stress. Similar to HOG-LDL, 7KC, and 4HNE also induced apoptosis, oxidative stress, and ER stress.

CONCLUSIONS

Our data suggest that extravasated, modified lipoproteins may be implicated in apoptotic Müller cell death, acting at least partially via enhanced levels of oxidative and ER stresses. They support our main hypothesis that, in addition to hyperglycemia, extravasated and oxidized LDL is an important insult to the diabetic retina.

Decrease in retinal neuronal cells in streptozotocin-induced diabetic mice

PURPOSE

Little is known about retinal neuronal loss in the retinas of diabetic mice. The purpose of this study was the quantitative assessment of retinal neural cell number in diabetic mice.

METHODS

Five-week-old C57BL/6 mice were used as a diabetic model with streptozotocin. Mice were studied over the course of 6 and 12 weeks after the onset of diabetes. Intraocular pressure (IOP) was measured with a noninvasive TonoLab tonometer. The retinal ganglion cells (RGCs) were counted at two different time points after the induction of diabetes and examined using the immunofluorescence technique and quantitative analysis.

RESULTS

The diabetic mice had significantly elevated IOP levels at 6 and 12 weeks after the onset of diabetes compared with the age-matched control mice (p<0.01 and p<0.001, respectively). The temporal course of Brn3a+ RGC and Neuronal Nuclei+RGC (NeuN+ RGC) loss induced by intraperitoneal injection of streptozotocin followed a similar trend. At 6 and 12 weeks after the onset of diabetes, the number of Brn3a+ RGCs (p<0.05 at 6 weeks; p<0.001 at 12 weeks) and NeuN+ RGCs (p<0.05 at 6 weeks; p<0.001 at 12 weeks) was significantly lower in diabetic mice than age-matched control mice. In the retinal flatmounts, the number of Brn3a+ RGCs (p<0.05 at 6 weeks, p<0.01 at 12 weeks) was also significantly lower in diabetic mice than control mice. The IOP in diabetic mice was negatively related with RGCs in cross sections. The cut-off value of IOP was 14.2 mmHg for diabetes.

CONCLUSIONS

This is a specific quantitative study of neural cell loss in the retina during diabetes. These data suggest that retinal neural cell reduction occurs in diabetic mice. It indicates that RGC loss may be an important component of diabetic retinopathy.

Endoplasmic reticulum stress-related factors protect against diabetic retinopathy

The endoplasmic reticulum (ER) is a principal mediator of signal transduction in the cell, and disruption of its normal function (a mechanism known as ER stress) has been associated with the pathogenesis of several diseases. ER stress has been demonstrated to contribute to onset and progression of diabetic retinopathy (DR) by induction of multiple inflammatory signaling pathways. Recent studies have begun to describe the gene expression profile of ER stress-related genes in DR; moreover, genes that play a protective role against DR have been identified. P58^IPK was determined to be able to reduce retinal vascular leakage under high glucose conditions, thus protecting retinal cells. It has also been found by our lab that ER-associated protein degradation factors exhibit significantly different expression patterns in rat retinas under sustained high glucose conditions. Future research based upon these collective genomic findings will contribute to our overall understanding of DR pathogenesis as well as identify potential therapeutic targets.

Expression of endoplasmic reticulum stress-related factors in the retinas of diabetic rats

Recent reports show that ER stress plays an important role in diabetic retinopathy (DR), but ER stress is a complicated process involving a network of signaling pathways and hundreds of factors, What factors involved in DR are not yet understood. We selected 89 ER stress factors from more than 200, A rat diabetes model was established by intraperitoneal injection of streptozotocin (STZ). The expression of 89 ER stress-related factors was found in the retinas of diabetic rats, at both 1- and 3-months after development of diabetes, by quantitative real-time polymerase chain reaction arrays. There were significant changes in expression levels of 13 and 12 ER stress-related factors in the diabetic rat retinas in the first and third month after the development of diabetes, Based on the array results, homocysteine- inducible, endoplasmic reticulum stress-inducible, ubiquitin-like domain member 1(HERP), and synoviolin(HRD1) were studied further by immunofluorescence and Western blot. Immunofluorescence and Western blot analyses showed that the expression of HERP was reduced in the retinas of diabetic rats in first and third month. The expression of Hrd1 did not change significantly in the retinas of diabetic rats in the first month but was reduced in the third month.

Increased expression of IRE1alpha and stress-related signal transduction proteins in ischemia-reperfusion injured retina

The purpose of this study was to determine whether the expression of ER stress-related factors IRE1alpha, apoptosis signal-regulating kinase 1 (ASK1), SAPK/ERK kinase 1 (SEK1) and c-Jun N-terminal kinase (JNK) is associated with the damaged retinal neurons induced by ischemia-reperfusion injury. After 60 minutes of ischemia, the rat retinas were reperfused, and retinas were isolated and fixed after 6, 9, 12, 18, and 24 hours, and 2, 5, and 9 days of reperfusion. Cryosections were immunostained with Fluoro-Jade B, a degenerating neuron marker to label degenerating neurons. Semi-quantitative analysis of the expression of IRE1alpha, ASK1, SEK1, and JNK were performed in both control and ischemic retinas. In ischemic retinas, the intensities of IRE1alpha immunoreactivity in the ganglion cell layer (GCL) were significantly higher than in the control retinas. In ischemic retinas, the numbers of SEK1-, ASK1-, and JNK-positive cells were significantly increased in the GCL compared to those in the control retinas. In addition, the cells that were positive for SEK1-, ASK1-, and JNK were also positive for Fluoro-Jade B-positive cells. These results indicate that the increased expression of ER stress-related factors was, in part, associated with the retinal neuronal abnormalities after ischemia-reperfusion injury in rat retinas.

Dietary wolfberry ameliorates retinal structure abnormalities in db/db mice at the early stage of diabetes

Hyperglycemia-linked oxidative stress and/or consequent endoplasmic reticulum (ER) stress are the causative factors of pathogenesis of diabetic retinopathy. Dietary bioactive components which mitigate oxidative stress may serve as potential chemopreventive agents to prevent or slow down the disease progression. Wolfberry is a traditional Asian fruit consumed for years to prevent aging eye diseases in Asian countries. Here we report that dietary wolfberry ameliorated mouse retinal abnormality at the early stage of type 2 diabetes in db/db mice. Male mice at six weeks of age were fed the control diet with or without 1% (kcal) wolfberry for eight weeks. Dietary wolfberry restored the thickness of the whole retina, in particular the inner nuclear layer and photoreceptor layer, and the integrity of the retinal pigment epithelia (RPE), and the ganglion cell number in db/db mice. Western blotting of whole retinal cell lysates revealed that addition of wolfberry lowered expression of ER stress biomarkers binding immunoglobulin protein (BiP), protein kinase RNA-like ER kinase (PERK), activating transcription factor 6 (ATF6) and caspase-12, and restored AMP-activated protein kinase (AMPK), thioredoxin, Mn superoxide dismutase (Mn SOD) and forkhead O transcription factor 3 α (FOXO3α) activities. To determine if our observations were due to the high contents of zeaxanthin and lutein in wolfberry, additional studies using these carotenoids were conducted. Using the human adult diploid RPE cell line ARPE-19, we demonstrated that both zeaxanthin and lutein could mimic the wolfberry preventive effect on activation of AMPK, thioredoxin, Mn SOD, FOXO3α activities, normalize cellular reactive oxygen species and attenuate ER stress in ARPE-19 cells exposed to a high glucose challenge. The zeaxanthin preventive effect was abolished by small interfering RNA knockdown of AMPKα. These results suggested that AMPK activation appeared to play a key role in upregulated expression of thioredoxin and Mn SOD, and mitigation of cellular oxidative stress and/or ER stress by wolfberry and zeaxanthin and/or lutein. Taken together, dietary wolfberry on retinal protection in diabetic mice is, at least partially, due to zeaxanthin and/or lutein.

Effect of neurotrophin-4 on endoplasmic reticulum stress-related neuronal apoptosis in diabetic and high glucose exposed rat retinas

The purpose of this study was to investigate the effect of NT-4 on the endoplasmic reticulum (ER) stress-related apoptosis of retinal neurons of isolated retinas. The retinas were isolated from normal and diabetic rats, and the normal retinas were exposed to high glucose (HG). Our results showed that the number of TUNEL-positive, and PERK- and CHOP-positive cells was significantly higher in diabetic and HG exposed retinas than in normal retinas. In diabetic and HG exposed retinas supplemented with NT-4, the number of TUNEL-positive, and PERK- and CHOP-positive cells was significantly lower than in retinas without NT-4. The neuroprotective effect of NT-4 on retinas cultured under diabetic stress was correlated with the suppression in the expression of PERK and CHOP, ER stress-related factors.

Expression of ER quality control-related genes in response to changes in BiP1 levels in developing rice endosperm

Binding protein (BiP) is the key chaperone involved in folding of secretory proteins such as seed storage proteins in the ER lumen. To obtain functional information about BiP1, a gene that is predominantly expressed during rice seed maturation, we generated several transgenic rice plants in which various levels of BiP1 protein accumulated in an endosperm-specific manner. Severe suppression (BiP1 KD) or significant over-expression (BiP1 OEmax) of BiP1 not only altered seed phenotype and the intracellular structure of endosperm cells, but also reduced seed storage protein content, starch accumulation and grain weight. Microarray and RT-PCR analyses indicated that expression of many chaperone and co-chaperone genes was induced in transgenic plants, with more prominent expression in the BiP1 KD line than in the BiP1 OEmax line. Transcriptional induction of most chaperones was observed in calli treated with dithiothreitol or tunicamycin, treatments that trigger ER stress, indicating that induction of the chaperone genes in transgenic rice was caused by an ER stress response. In transient assays using rice protoplasts, the ortholog (Os06g0622700) of the AtbZIP60 transcription factor was shown to be involved in activation of some chaperone genes. Slight increases in the BiP1 level compared with wild-type, accompanied by increased levels of calnexin and protein disulfide isomerase-like proteins, resulted in significant enhancement of seed storage protein content, without any change in intracellular structure or seed phenotype. Judicious modification of BiP1 levels in transgenic rice can provide suitable conditions for the production of secretory proteins by alleviating ER stress.

Functional characterization of 58-kilodalton inhibitor of protein kinase in protecting against diabetic retinopathy via the endoplasmic reticulum stress pathway

OBJECTIVE

58-kilodalton inhibitor of protein kinase (P58(IPK)) plays an important role in preventing endoplasmic reticulum (ER) stress. It is an interferon-induced kinase that targets the eukaryotic translation initiation factor eukaryotic initiation factor 2 alpha. The aim of this study was to determine the roles of P58(IPK) in protecting against diabetic retinopathy (DR) by inhibiting ER stress-signaling mediators.

METHODS

A rat diabetic model was established by intraperitoneal injection of streptozotocin. Overexpression of P58(IPK) was achieved by intravitreal injection of purified recombinant adeno-associated virus vector (rAAV2)-P58(IPK) or transfection into rat retinal capillary endothelial cells. Retinal vascular permeability was determined by assessing the Evans Blue retinal leakage. To downregulate the P58(IPK) level in cultured rat retinal capillary endothelial cells, pGIPZ-P58(IPK) RNA interference (P58(IPK)RNAi) was introduced in these cells. Real time reverse transcription (RT)-PCR and western blot analyses were performed to evaluate the mRNA and protein levels of Core/emopamil binding protein (C/EBP) homologous protein (CHOP), vascular endothelial growth factor (VEGF), and tumor necrosis factor-α (TNF-α).

RESULTS

Retinal blood vessel leakage was significantly decreased in rAAV2-P58(IPK)-transfected diabetic rats compared with the control diabetic rats. Both mRNA and protein levels of CHOP, TNF-α, and VEGF in the retina of diabetic rats were remarkably reduced in P58(IPK)-transfected rats. In vitro study further demonstrated that overexpression of P58(IPK) downregulated the expression of CHOP, TNF-α, and VEGF under high glucose conditions, whereas introduction of P58(IPK)RNAi enhanced the expression of CHOP, TNF-α, and VEGF.

CONCLUSIONS

These results revealed the protecting role of P58(IPK) against ER stress-mediated DR in diabetic rats, suggesting that P58(IPK) may act as a DR-resistant gene during diabetes.

Sigma receptor 1 modulates endoplasmic reticulum stress in retinal neurons

PURPOSE

To investigate the mechanism of σ receptor 1 (σR1) neuroprotection in retinal neurons.

METHODS

Oxidative stress, which is implicated in diabetic retinopathy, was induced in mouse primary ganglion cells (GCs) and RGC-5 cells, and the effect of the σR1 ligand (+)-pentazocine on pro- and anti-apoptotic and endoplasmic reticulum (ER) stress gene expression was examined. Binding of σR1 to BiP, an ER chaperone protein, and σR1 phosphorylation status were examined by immunoprecipitation. Retinas were harvested from Ins2Akita/+ diabetic mice treated with (+)-pentazocine, and the expression of ER stress genes and of the retinal transcriptome was evaluated.

RESULTS

Oxidative stress induced the death of primary GCs and RGC-5 cells. The effect was decreased by the application of (+)-pentazocine. Stress increased σR1 binding to BiP and enhanced σR1 phosphorylation in RGC-5 cells. BiP binding was prevented, and σR1 phosphorylation decreased in the presence of (+)-pentazocine. The ER stress proteins PERK, ATF4, ATF6, IRE1α, and CHOP were upregulated in RGC-5 cells during oxidative stress, but decreased in the presence of (+)-pentazocine. A similar phenomenon was observed in retinas of Ins2Akita/+ diabetic mice. Retinal transcriptome analysis of Ins2Akita/+ mice compared with wild-type revealed differential expression of the genes critically involved in oxidative stress, differentiation, and cell death. The expression profile of those genes was reversed when the Ins2Akita/+ mice were treated with (+)-pentazocine.

CONCLUSIONS

In retinal neurons, the molecular chaperone σR1 binds BiP under stressful conditions; (+)-pentazocine may exert its effects by dissociating σR1 from BiP. As stress in retinal cells increases, phosphorylation of σR1 is increased, which is attenuated when agonists bind to the receptor.

Resveratrol inhibits neuronal apoptosis and elevated Ca2+/calmodulin-dependent protein kinase II activity in diabetic mouse retina

OBJECTIVE

This study investigated the effects of resveratrol, a natural polyphenol with neuroprotective properties, on retinal neuronal cell death mediated by diabetes-induced activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII).

RESEARCH DESIGN AND METHODS

Diabetes was induced in C57BL/6 mice by five consecutive intraperitoneal injections of 55 mg/kg streptozotocin (STZ). Control mice received buffer. All mice were killed 2 months after the injections, and the extent of neuronal cell death, CaMKII, and phospho-CaMKII protein expression levels and CaMKII kinase activity were examined in the retinas. To assess the role of CaMKII in the death of retinal neurons, a small-interfering RNA (siRNA) or specific inhibitor of CaMKII was injected into the right vitreous humor, and vehicle only was injected into the left vitreous humor, 2 days before death. Resveratrol (20 mg/kg) was administered by oral gavage daily for 4 weeks, beginning 1 month after the fifth injection of either STZ or buffer.

RESULTS

The death of retinal ganglion cells (RGCs), CaMKII, phospho-CaMKII protein levels, and CaMKII activity were all greatly increased in the retinas of diabetic mice compared with controls, 2 months after induction of diabetes. Terminal deoxynucleotidyl transferase (TdT)-mediated dUTP nick-end labeling (TUNEL)-positive signals co-localized with CaMKII- and phospho-CaMKII immunoreactive RGCs. However, in addition to CaMKII knockdown and inhibition by siRNA or a specific inhibitor, respectively, resveratrol provided complete protection from diabetes-induced retinal cell death.

CONCLUSIONS

In the present study, resveratrol prevented diabetes-induced RGC death via CaMKII downregulation, implying that resveratrol may have potential therapeutic applications for prevention of diabetes-induced visual dysfunction.

Prediction of diabetic retinopathy: role of oxidative stress and relevance of apoptotic biomarkers

Diabetic retinopathy (DR) is the foremost cause of blindness in working-aged worldwide; it is characterized by vascular and neuronal degeneration. Features of DR include leukocyte adhesion, increased vascular permeability, neovascularization and neuronal cell death. Early diagnosis and intervention are important to prevent or at least ameliorate the development of DR. Recent reports indicate that pathophysiological mechanisms leading to diabetic retinopathy include oxidative stress and retinal cell death cascades. Circulating biomarkers of oxidative stress such as malondialdehyde (MDA), thiobarbituric acid reacting substances (TBARS), conjugated diene (CD), advanced oxidation protein products (AOPP), protein carbonyl, 8-hydroxydeoxyguanosin (8-OHdG), nitrotyrosine, and F(2) isoprostanes and pro-apoptosis molecules (caspase-3, Fas, and Bax) are associated with increased susceptibility to develop DR in diabetic subjects. Thus, identification of oxidative stress and cell death biomarkers in diabetic patients could be in favor of predicting, diagnosis, and prevention of DR, and to target for novel therapeutic interventions.