Expression of antiapoptotic and proapoptotic molecules in diabetic retinas

Mechanistic insights into the alterations and regulation of the AKT signaling pathway in diabetic retinopathy

In the early stages of diabetic retinopathy (DR), diabetes-related hyperglycemia directly inhibits the AKT signaling pathway by increasing oxidative stress or inhibiting growth factor expression, which leads to retinal cell apoptosis, nerve proliferation and fundus microvascular disease. However, due to compensatory vascular hyperplasia in the late stage of DR, the vascular endothelial growth factor (VEGF)/phosphatidylinositol 3 kinase (PI3K)/AKT cascade is activated, resulting in opposite levels of AKT regulation compared with the early stage. Studies have shown that many factors, including insulin, insulin-like growth factor-1 (IGF-1), VEGF and others, can regulate the AKT pathway. Disruption of the insulin pathway decreases AKT activation. IGF-1 downregulation decreases the activation of AKT in DR, which abrogates the neuroprotective effect, upregulates VEGF expression and thus induces neovascularization. Although inhibiting VEGF is the main treatment for neovascularization in DR, excessive inhibition may lead to apoptosis in inner retinal neurons. AKT pathway substrates, including mammalian target of rapamycin (mTOR), forkhead box O (FOXO), glycogen synthase kinase-3 (GSK-3)/nuclear factor erythroid 2-related factor 2 (Nrf2), and nuclear factor kappa-B (NF-κB), are a research focus. mTOR inhibitors can delay or prevent retinal microangiopathy, whereas low mTOR activity can decrease retinal protein synthesis. Inactivated AKT fails to inhibit FOXO and thus causes apoptosis. The GSK-3/Nrf2 cascade regulates oxidation and inflammation in DR. NF-κB is activated in diabetic retinas and is involved in inflammation and apoptosis. Many pathways or vital activities, such as the Janus kinase (JAK)/signal transducer and activator of transcription (STAT) and mitogen-activated protein kinase (MAPK) signaling pathways, interact with the AKT pathway to influence DR development. Numerous regulatory methods can simultaneously impact the AKT pathway and other pathways, and it is essential to consider both the connections and interactions between these pathways. In this review, we summarize changes in the AKT signaling pathway in DR and targeted drugs based on these potential sites.

Corneal endothelial cell morphology and optical coherence tomography findings in children with type 1 diabetes mellitus

Purpose

To evaluate central macular thickness, retinal nerve fibre layer thickness, corneal endothelial cell density and central corneal thickness in children with Type 1 Diabetes Mellitus (DM).

Methods

Thirty children with Type 1 DM and 30 age-matched children as controls were examined. Central macular thickness (CMT) and four quadrants of retinal nerve fibre layer thickness (RNFLT) were measured by optical coherence tomography (OCT), while endothelial cell density (ECD), coefficient of variation in cell size (CV), hexagonality (HEX) and central corneal thickness (CCT) were measured by noncontact specular microscopy.

Results

The mean ECD was 2810.77  ±  273.47 cells/mm2 and the mean hexagonality ratio was 49.77  ±  13.2 both were significantly lower (p < 0.001, p  =  0.037 respectively) in the diabetic group compared to the control group. The mean CV was 37.6  ±  7.27, it was significantly higher (p  =  0.024) in the diabetic group than the control group. The mean corneal thickness was greater in the DM group than the controls, but the difference was not significant (p  =  0.176). There were no significant differences in CMT or RNFLT between the two groups. There was a negative correlation between the HbA1c levels and the thickness of the superior quadrants of RNFL (r  =  -0.406, p  =  0.026). The duration of diabetes had no significant correlations with the corneal and retinal parameters.

Conclusion

The diabetic children had changes in corneal endothelial morphology and there was a negative correlation between HbA1c levels and superior quadrant of RNFLT.

Diabetes mellitus associated neurovascular lesions in the retina and brain: A review

Diabetes mellitus (DM) is now recognized as a system-wide, autoimmune, inflammatory, microvascular disorder, which, in the retina and brain results in severe multifocal injury now recognized as a leading cause, world-wide, of progressive vision loss and dementia. To address this problem, resulting primarily from variations in glycemia in the prediabetic and overt diabetic states, it must be realized that, although some of the injury processes associated with diabetes may be system wide, there are varying responses, effector, and repair mechanisms that differ from organ to organ or within varying cell structures. Specifically, within the retina, and similarly within the brain cortex, lesions occur of the "neurovascular unit", comprised of focal microvascular occlusions, inflammatory endothelial and pericyte injury, with small vessel leakage resulting in injury to astrocytes, Müller cells, and microglia, all of which occur with progressive neuronal apoptosis. Such lesions are now recognized to occur before the first microaneurysms are visible to imaging by fundus cameras or before they result in detectable symptoms or signs recognizable to the patient or clinician. Treatments, therefore, which currently are not initiated within the retina until edema develops or there is progression of vascular lesions that define the current staging of retinopathy, and in the brain only after severe signs of cognitive failure. Treatments, therefore are applied relatively late with some reduction in progressive cellular injury but with resultant minimal vision or cognitive improvement. This review article will summarize the multiple inflammatory and remediation processes currently understood to occur in patients with diabetes as well as pre-diabetes and summarize as well the current limitations of methods for assessing the structural and functional alterations within the retina and brain. The goal is to attempt to define future screening, monitoring, and treatment directions that hopefully will prevent progressive injury as well as enable improved repair and attendant function.

Effect of retinol and α-tocopherol supplementation on photoreceptor and retinal ganglion cell apoptosis in diabetic rats model

BACKGROUND

Diabetic retinopathy (DR) is the most common microvascular complication of diabetes. Retinol and α-tocopherol of diabetic models prevent the damage of photoreceptor and retinal ganglion cells (RGC) caused by hyperglycemia.

OBJECTIVE

This study aims to examine the effect of retinol and α-tocopherol on photoreceptor and RGC densities and the expression of caspase-3 and -7 on the retinal layers of the diabetic rat model.

METHODS

Alloxan 150 mg/kg body weight single dose was used to develop animal models, which were separated into eight groups. These consist of one group without intervention (group 1), one positive control with only induced alloxan (group 2), and others receiving retinol (group 3 and 6), α-tocopherol (group 4 and 7), or their combination (group 5 and 8). Furthermore, histopathological examination was performed using Hematoxylin-Eosin staining to evaluate the photoreceptor and RGC densities, while immunohistochemistry staining evaluated the caspase-3 and -7 expressions.

RESULTS

In the treatment group, the highest and lowest densities were identified in diabetic rats given α-tocopherol (group 7) and retinol (group 3) respectively. The caspase-3 and -7 expression showed that the group given α-tocopherol (group 7) had the lowest value.

CONCLUSION

In diabetic rats, retinol and α-tocopherol compounds maintained densities and prevented photoreceptor and RGC death. However, α-tocopherol was more promising than retinol or combinations in the prevention of retinal cells apoptosis.

The Pathogenesis and Therapeutic Approaches of Diabetic Neuropathy in the Retina

Diabetic retinopathy is a major retinal disease and a leading cause of blindness in the world. Diabetic retinopathy is a neurovascular disease that is associated with disturbances of the interdependent relationship of cells composed of the neurovascular units, i.e., neurons, glial cells, and vascular cells. An impairment of these neurovascular units causes both neuronal and vascular abnormalities in diabetic retinopathy. More specifically, neuronal abnormalities including neuronal cell death and axon degeneration are irreversible changes that are directly related to the vision reduction in diabetic patients. Thus, establishment of neuroprotective and regenerative therapies for diabetic neuropathy in the retina is an emergent task for preventing the blindness of patients with diabetic retinopathy. This review focuses on the pathogenesis of the neuronal abnormalities in diabetic retina including glial abnormalities, neuronal cell death, and axon degeneration. The possible molecular cell death pathways and intrinsic survival and regenerative pathways are also described. In addition, therapeutic approaches for diabetic neuropathy in the retina both in vitro and in vivo are presented. This review should be helpful for providing clues to overcome the barriers for establishing neuroprotection and regeneration of diabetic neuropathy in the retina.

Prediabetes influences the structure of the macula: thinning of the macula in the Northern Finland Birth Cohort

BACKGROUND/AIM

The aim of this study was to evaluate the effect of prediabetes and diabetes on macular thickness and retinal vascular calibres in our population-based cohort (Northern Finland Birth Cohort).

METHODS

The population of 2005 individuals was divided into diabetes (n=57), prediabetes (n=1638) and normal glucose metabolism (NGM) groups (n=310). Total thickness of the macula was measured using Cirrus HD-OCT 4000. Central retinal arteriolar equivalent (CRAE) and central retinal venular equivalent (CRVE) calibres were measured from the fundus images. The diagnosis of diabetes and prediabetes was made according to WHO 2006 diagnostic standards.

RESULTS

Significant macular thinning was observed in subjects with prediabetes (-2.69 μm (95% CI -4.29 to -1.09), p<0.05 and -0.10 mm³ (95% CI -0.16 to -0.04), p<0.05 for macular cube average thickness and cube volume, respectively) and it was greatest in the pericentral area. Macular cube average thickness and macular cube volume decreased significantly by worsening glucose metabolism. Furthermore, CRAE was decreased by increases in 2-hour post-load glucose, glucose area under the curve and increase in Matsuda index (p<0.001, 0.019 and <0.001, respectively). In mediation analysis, macular thickness had significant average causal mediation effect (ACME) on CRVE and CRAE in subjects with prediabetes.

CONCLUSION

We detected significant thinning of the macula in subjects with prediabetes. The diameters of retinal arteries were decreased by impaired glucose metabolism. This study provides a new perspective since it revealed that the early and subtle changes caused by prediabetes as macular thinning had significant ACME on retinal vessels, therefore supporting the neurodegenerative theory of diabetes-induced changes in the retina.

Z-Ligustilide Ameliorates Diabetic Rat Retinal Dysfunction Through Anti-Apoptosis and an Antioxidation Pathway

Background

Diabetic retinopathy (DR) is one of the major causes of vision impairment. Z-ligustilide (3-butylidene-4,5-dihydrophthalide; Z-LIG) is an important volatile oil from the Chinese herb Angelica sinensis (Oliv.) Diels. It has been extensively studied and reportedly has anti-inflammatory, antioxidant, antitumor, analgesic, vasodilatory, and neuroprotective effects. Its effects on DR, however, remain obscure. In this study, we attempted to explore the protective effects of Z-LIG on retinal dysfunction and the potential underlying mechanisms.

Material/Methods

A diabetic rat model was constructed with streptozotocin injection. Three study groups were constituted: control (CON), diabetic model (DM), and DM+Z-LIG. The DM+Z-LIG group was injected intraperitoneally with 10 mg/kg of Z-LIG. The other groups received the same volume of 3% solution of polysorbate 80. After a 12-week intervention, a series of assessments were performed, including tests for retinal function, morphology, and molecular biology.

Results

Z-LIG treatment significantly elevated b-wave and OPs2-wave amplitude and thickened the inner layer of the nucleus of the retina, and the outer plexiform and nuclear layers (INL+OPL+ONL). Moreover, the rate of apoptosis and expression of bcl-2- associated X protein (BAX) and cleaved-Caspase-3 were clearly reduced, and the expression of bcl-2 was raised by Z-LIG in retinas of diabetic mice. In addition, the levels of retinal proinflammatory cytokines interleukin-1 and tumor necrosis factor-α were downregulated by Z-LIG. Furthermore, Z-LIG inhibited expression of vascular endothelial growth factor-α (VEGF-α) at the mRNA and protein levels.

Conclusions

Z-LIG can inhibit inflammatory response and cell apoptosis in retinas of diabetic rats by repressing the VEGF-α pathway. Therefore, it may serve as a potential therapeutic agent for DR.

Pharmacological Inhibition of Spermine Oxidase Reduces Neurodegeneration and Improves Retinal Function in Diabetic Mice

Diabetic retinopathy (DR) is a significant cause of blindness in working-age adults worldwide. Lack of effective strategies to prevent or reduce vision loss is a major problem. Since the degeneration of retinal neurons is an early event in the diabetic retina, studies to characterize the molecular mechanisms of diabetes-induced retinal neuronal damage and dysfunction are of high significance. We have demonstrated that spermine oxidase (SMOX), a mediator of polyamine oxidation is critically involved in causing neurovascular damage in the retina. The involvement of SMOX in diabetes-induced retinal neuronal damage is completely unknown. Utilizing the streptozotocin-induced mouse model of diabetes, the impact of the SMOX inhibitor, MDL 72527, on neuronal damage and dysfunction in the diabetic retina was investigated. Retinal function was assessed by electroretinography (ERG) and retinal architecture was evaluated using spectral domain-optical coherence tomography. Retinal cryosections were prepared for immunolabeling of inner retinal neurons and retinal lysates were used for Western blotting. We observed a marked decrease in retinal function in diabetic mice compared to the non-diabetic controls. Treatment with MDL 72527 significantly improved the ERG responses in diabetic retinas. Diabetes-induced retinal thinning was also inhibited by the MDL 72527 treatment. Our analysis further showed that diabetes-induced retinal ganglion cell damage and neurodegeneration were markedly attenuated by MDL 72527 treatment. These results strongly implicate SMOX in diabetes-induced retinal neurodegeneration and visual dysfunction.

Spermine oxidase: A promising therapeutic target for neurodegeneration in diabetic retinopathy

Diabetic Retinopathy (DR), is a significant public health issue and the leading cause of blindness in working-aged adults worldwide. The vision loss associated with DR affects patients' quality of life and has negative social and psychological effects. In the past, diabetic retinopathy was considered as a vascular disease; however, it is now recognized to be a neuro-vascular disease of the retina. Current therapies for DR, such as laser photocoagulation and anti-VEGF therapy, treat advanced stages of the disease, particularly the vasculopathy and have adverse side effects. Unavailability of effective treatments to prevent the incidence or progression of DR is a major clinical problem. There is a great need for therapeutic interventions capable of preventing retinal damage in DR patients. A growing body of evidence shows that neurodegeneration is an early event in DR pathogenesis. Therefore, studies of the underlying mechanisms that lead to neurodegeneration are essential for identifying new therapeutic targets in the early stages of DR. Deregulation of the polyamine metabolism is implicated in various neurodegenerative diseases, cancer, renal failure, and diabetes. Spermine Oxidase (SMOX) is a highly inducible enzyme, and its dysregulation can alter polyamine homeostasis. The oxidative products of polyamine metabolism are capable of inducing cell damage and death. The current review provides insight into the SMOX-regulated molecular mechanisms of cellular damage and dysfunction, and its potential as a therapeutic target for diabetic retinopathy. Structural and functional changes in the diabetic retina and the mechanisms leading to neuronal damage (excitotoxicity, loss of neurotrophic factors, oxidative stress, mitochondrial dysfunction etc.) are also summarized in this review. Furthermore, existing therapies and new approaches to neuroprotection are discussed.

Molecular Mechanisms Mediating Diabetic Retinal Neurodegeneration: Potential Research Avenues and Therapeutic Targets

Diabetic retinopathy (DR) is a devastating complication of diabetes with a prevalence rate of 35%, and no effective treatment options. Since the most visible clinical features of DR are microvascular irregularities, therapeutic interventions often attempt to reduce microvascular injury, but only after permanent retinal damage has ensued. However, recent data suggests that diabetes initially affects retinal neurons, leading to neurodegeneration as an early occurrence in DR, before onset of the more noticeable vascular abnormalities. In this review, we delineate the sequence of initiating events leading to retinal degeneration in DR, considering neuronal dysfunction as a primary event. Key molecular mechanisms and potential biomarkers associated with retinal neuronal degeneration in diabetes are discussed. In addition to glial reactivity and inflammation in the diabetic retina, the contribution of neurotrophic factors, cell adhesion molecules, apoptosis markers, and G protein signaling to neurodegenerative pathways warrants further investigation. These studies could complement recent developments in innovative treatment strategies for diabetic retinopathy, such as targeting retinal neuroprotection, promoting neuronal regeneration, and attempts to re-program other retinal cell types into functional neurons. Indeed, several ongoing clinical trials are currently attempting treatment of retinal neurodegeneration by means of such novel therapeutic avenues. The aim of this article is to highlight the crucial role of neurodegeneration in early retinopathy progression, and to review the molecular basis of neuronal dysfunction as a first step toward developing early therapeutic interventions that can prevent permanent retinal damage in diabetes. ClinicalTrials.gov: NCT02471651, NCT01492400.

Assessment of Global and Local Alterations in Retinal Layer Thickness in Ins2 (Akita) Diabetic Mice by Spectral Domain Optical Coherence Tomography

PURPOSE/AIM

The Ins2 (Akita) mouse is a spontaneous diabetic mouse model with a heterozygous mutation in the insulin 2 gene that results in sustained hyperglycemia. The purpose of the study was to assess global and local retinal layer thickness alterations in Akita mice by analysis of spectral domain optical coherence tomography (SD-OCT) images.

MATERIALS AND METHODS

SD-OCT imaging was performed in Akita and wild-type mice at 12 and 24 weeks of age. Inner retinal thickness (IRT), outer retinal thickness (ORT), total retinal thickness (TRT), and photoreceptor outer segment length (OSL) were measured. Mean global thickness values were compared between Akita and wild-type mice. Local thickness variations in Akita mice were assessed based on normative values in wild-type mice.

RESULTS

Akita mice had higher blood glucose levels and lower body weights (p < 0.001). On average, IRT, ORT, and TRT were approximately 2% lower in Akita mice than in wild-type mice (p ≤ 0.02). In Akita mice, the percent difference between retinal areas with thickness below and above normative values for IRT, ORT, and TRT was 22%, 32%, and 38%, respectively.

CONCLUSIONS

These findings support the use of the Akita mouse model to study the retinal neurodegenerative effects of hyperglycemia.

Metabolic profile of visual cortex in diabetic rats measured with in vivo proton MRS

The purpose of the present study was to characterize the metabolic profile of the visual cortex in streptozotocin-induced Type 1 diabetic rats by means of in vivo proton MRS. Several metabolite concentration ratios in the visual cortex were calculated. In addition, postmortem histologic analyses for retinal ganglion cell (RGC) loss, optic nerve injury and visual cortex alterations were monitored. The results showed that diabetes induced several changes in visual cortex metabolites, such as reduced N-acetylaspartate, glutamate, γ-aminobutyric acid, taurine and choline-containing compound levels. Nevertheless, myo-inositol levels increased significantly as compared with controls. Remarkable RGC loss and optic nerve degeneration were observed by morphological analysis. Moreover, the results showed significant neuronal loss and glial activation in the visual cortex. These findings indicated that, besides vascular abnormalities, neuronal loss and degeneration in the visual pathway were induced due to disrupted glucose homeostasis in diabetes. Metabolic or functional abnormalities were induced in cerebral neurons of the visual cortex by diabetes.

Hyperglycemia Alters the Protein Levels of Prominin-1 and VEGFA in the Retina of Albino Rats

In this study, we addressed the potential relationship between prominin-1 (prom1) and vascular endothelial growth factor (VEGFA) in diabetes-induced retinopathy. In total, we examined 28 retinas from 14 rats with streptozotocin-induced diabetes and 30 retinas from 15 untreated control rats. ELISA was used to measure the level of prom1 and VEGFA in retinal tissue homogenates. Immunohistochemical techniques were used with antibodies directed against prom1, VEGFA, and CASP-3. After 180 days of diabetes induction, we performed light and electron microscopy studies on rat eyes to evaluate histopathological changes and to estimate the de novo metric "Diabetic Retinopathy Histopathological Index" (DRHI). These changes were then correlated to the tissue and immunoexpression levels of prom1 and VEGFA. The data showed a significant upregulation of the tissue levels and optical densities (ODs) of VEGFA and prom1 immunoreactivity in diabetic retinas compared with controls. Both the tissue levels and OD values of prom1 and VEGFA correlated significantly with each other and to the diabetic structural changes as calculated by DRHI. Taken together, these data provide new insight into the potential role of prom1 and VEGFA in the development of diabetic retinopathy.

Retinal Ganglion Cell Loss in Children With Type 1 Diabetes Mellitus Without Diabetic Retinopathy

BACKGROUND AND OBJECTIVE

Early diabetic retinal changes in children with type 1 diabetes mellitus (T1DM) without diabetic retinopathy (DR) were examined using spectral-domain optical coherence tomography (SD-OCT).

PATIENTS AND METHODS

Sixty children with T1DM without DR and 60 normal children were enrolled in the study. SD-OCT was used to measure the ganglion cell-inner plexiform layer (GC-IPL) and retinal nerve fiber (RNFL) thicknesses in all participants.

RESULTS

The GC-IPL thickness was significantly decreased in all quadrants except the superior-nasal quadrant in children with diabetes (P < .05). However, the RNFL thickness in all quadrants was not significantly different between the groups (P > .05).

CONCLUSIONS

There was a decreased GC-IPL thickness in children with T1DM without DR, suggesting that T1DM has an early neurodegenerative effect on retinal ganglion cells that occurs when the vascular component of DR is absent. SD-OCT may be more useful than ophthalmoscopic evaluation for detecting the earlier retinal structural changes of diabetes. [Ophthalmic Surg Lasers Imaging Retina. 2017;48:473-477.].

Neurodegeneration and Neuroinflammation in Diabetic Retinopathy: Potential Approaches to Delay Neuronal Loss

In spite of the extensive research the complex pathogenesis of diabetic retinopathy (DR) has not been fully elucidated. For many years it has been thought that diabetic retinopathy manifests only with microangiopathic lesions, which are totally responsible for the loss of vision in diabetic patients. In view of the current knowledge on the microangiopathic changes in the fundus of the eye, diabetic retinopathy is perceived as a neurodegenerative disease. Several clinical tools are available to detect neuronal dysfunction at early stages of diabetes. Many functional changes in the retina can be identified before vascular pathology develops, suggesting that they result from a direct effect of diabetes on the neural retina. In the course of diabetes there is a chronic loss of retinal neurons due to increased frequency of apoptosis. The neuronal apoptosis begins very early in the course of diabetes. This observation has led to suggestions that precautions against DR should be implemented immediately after diabetes is diagnosed. Neurodegeneration cannot be reversed; therefore treatments preventing neuronal cell loss in the retina need to be developed to protect diabetic patients. This review is an attempt to summarize what is currently known about the mechanisms of neuronal apoptosis in the context of diabetic retinopathy and vascular degeneration as well as about potential treatments of DR

Neuroprotection as a Therapeutic Target for Diabetic Retinopathy

Diabetic retinopathy (DR) is a multifactorial progressive disease of the retina and a leading cause of vision loss. DR has long been regarded as a vascular disorder, although neuronal death and visual impairment appear before vascular lesions, suggesting an important role played by neurodegeneration in DR and the appropriateness of neuroprotective strategies. Upregulation of vascular endothelial growth factor (VEGF), the main target of current therapies, is likely to be one of the first responses to retinal hyperglycemic stress and VEGF may represent an important survival factor in early phases of DR. Of central importance for clinical trials is the detection of retinal neurodegeneration in the clinical setting, and spectral domain optical coherence tomography seems the most indicated technique. Many substances have been tested in animal studies for their neuroprotective properties and for possible use in humans. Perhaps, the most intriguing perspective is the use of endogenous neuroprotective substances or nutraceuticals. Together, the data point to the central role of neurodegeneration in the pathogenesis of DR and indicate neuroprotection as an effective strategy for treating this disease. However, clinical trials to determine not only the effectiveness and safety but also the compliance of a noninvasive route of drug administration are needed.

Early Neurodegeneration of the Inner Retinal Layers in Type 1 Diabetes Mellitus

Purpose: This study explores retinal structural changes in type 1 diabetes without clinically diagnosed diabetic retinopathy (DR). Methods: Peripapillary retinal nerve fiber layer (RNFL) thickness, macular ganglion cell complex (GCC) thickness, and macular thickness (MT) were measured in 90 type 1 diabetic patients by using spectral domain optical coherence tomography. The values were compared with 100 sex- and age-matched healthy controls. The independent t test was used to assess differences in the mean age, mean diabetic and ocular parameters, and the thickness values between the diabetic and control groups. Multiple linear regression analysis was performed to investigate the correlation between the thickness values and diabetic and ocular parameters. Results: Whole-RNFL, the superior and inferior quadrants, and the superior half of the peripapillary RNFL thicknesses were significantly thinner in diabetic patients compared with controls (p < 0.05). GCC thicknesses in the average macular, outer temporal superior and outer temporal inferior sectors were significantly thinner in diabetic patients (p < 0.05). Central and average MTs were similar in both groups (p > 0.05). There were significant negative correlations of the duration of type 1 diabetes with the inner nasal MT, inner temporal superior GCC thickness, inner nasal inferior GCC thickness, and outer nasal superior GCC thickness (p < 0.05). Similarly, there were significant negative correlations of the level of HbA1c with the whole-RNFL thickness, superior-half-RNFL thickness, and superior-quadrant-RNFL thickness (p < 0.05). Conclusions: Type 1 diabetic patients without clinically diagnosed DR had neurodegeneration in the inner retinal layers compared with healthy controls.

Diabetes induces changes in neuroretina before retinal vessels: a spectral-domain optical coherence tomography study

Purpose

To investigate retinal changes prior to vascular signs in patients with type 2 diabetes without diabetic retinopathy or with mild non proliferative diabetic retinopathy.

Methods

A cross-sectional study was performed in three groups: patients without diabetes, patients with type 2 diabetes without diabetic retinopathy, and patients with diabetes with mild diabetic retinopathy. Analysis of retinal layers was performed objectively with the Cirrus Review Software 6.0 (Carl Zeiss Meditec, Dublin, CA, USA). Macular cube scans were analyzed with regard to: the ganglion cell layer + inner plexiform layer analysis, retinal nerve fiber layer thickness, central subfoveal retinal thickness and average macular thickness.

Results

In total, 102 patients were included in this study, of which 28 (27.4%) were classified into control group, 46 (45.0%) classified as diabetic patients with no diabetic retinopathy and 28 (27.4%) classified as mild diabetic retinopathy. Quantitative analysis with the Cirrus software showed that the mean ganglion cell layer and mean retinal nerve fiber layer were thinner in diabetes without diabetic retinopathy group when compared to controls. ANOVA with Bonferroni post test indicated a statistically significant reduction in average retinal thickness in mild diabetic retinopathy group (P = 0.032) compared to control and reduction in ganglion cell layer in diabetes with no diabetic retinopathy (P = 0.039) and mild diabetic retinopathy (P = 0.003). Also indicated reduction in retinal nerve fiber layer in diabetic without diabetic retinopathy and eyes with mild diabetic retinopathy (P < 0.001), compared to controls.

Conclusions

Our study found reduction in thickness of ganglion cell layer and retinal nerve fiber layer in patients with diabetes without diabetic retinopathy, which suggests neuroretinal changes before vascular signs of diabetic retinopathy.

Diabetic retinopathy--biomolecules and multiple pathophysiology

One of the major complications in patients with diabetes is diabetic retinopathy (DR), a leading cause of blindness worldwide. It causes visual impairment and finally blindness, a result of long-term accumulated damage to the small blood vessels in the retina. It takes several years before any clinical symptoms of retinopathy appear in diabetic patients. Consequently, glycemic control, blood pressure and lipid-lowering therapy have all shown benefits in reducing the incidence and progression of DR. A number of hyperglycemia-induced metabolic stresses have been implicated in the pathophysiology of DR. The microvasculature of the retina responds to hyperglycemia through a number of biochemical changes, including the activation of protein kinase C (PKC), increased advanced glycation end-products (AGEs) formation, polyol pathway and oxidative stress. There is an accumulating body of evidence indicating that inflammation and neurodegeneration play an important role in the pathogenesis of DR.

Diabetic retinopathy: loss of neuroretinal adaptation to the diabetic metabolic environment

Diabetic retinopathy (DR) impairs vision of patients with type 1 and type 2 diabetes, associated with vascular dysfunction and occlusion, retinal edema, hemorrhage, and inappropriate growth of new blood vessels. The recent success of biologic treatments targeting vascular endothelial growth factor (VEGF) demonstrates that treating the vascular aspects in the later stages of the disease can preserve vision in many patients. It would also be highly desirable to prevent the onset of the disease or arrest its progression at a stage preceding the appearance of overt microvascular pathologies. The progression of DR is not necessarily linear but may follow a series of steps that evolve over the course of multiple years. Abundant data suggest that diabetes affects the entire neurovascular unit of the retina, with an early loss of neurovascular coupling, gradual neurodegeneration, gliosis, and neuroinflammation occurring before observable vascular pathologies. In this article, we consider the pathology of DR from the point of view that diabetes causes measurable dysfunctions in the complex integral network of cell types that produce and maintain human vision.

Dopamine deficiency contributes to early visual dysfunction in a rodent model of type 1 diabetes

Dopamine (DA) functions as an essential neuromodulator in the brain and retina such that disruptions in the dopaminergic system are associated with common neurologic disorders such as Parkinson's disease. Although a reduction in DA content has been observed in diabetes, its effects in the development of diabetes-induced neuropathy remains unknown. Because the retina is rich in DA and has a well known diabetes-induced pathology (diabetic retinopathy or DR), this study was designed to examine the role of retinal DA deficiency in early visual defects in DR. Using rodent models of type 1 diabetes mellitus, we investigated whether diabetes caused a reduction in retinal DA content in both rats and mice and determined whether restoring DA levels or activating specific DA receptor pathways could improve visual function (evaluated with optokinetic tracking response) of diabetic mice, potentially via improvement of retinal function (assessed with electroretinography). We found that diabetes significantly reduced DA levels by 4 weeks in rats and by 5 weeks in mice, coincident with the initial detection of visual deficits. Treatment with l-DOPA, a DA precursor, improved overall retinal and visual functions in diabetic mice and acute treatment with DA D1 or D4 receptor agonists improved spatial frequency threshold or contrast sensitivity, respectively. Together, our results indicate that retinal DA deficiency is an underlying mechanism for early, diabetes-induced visual dysfunction and suggest that therapies targeting the retinal dopaminergic system may be beneficial in early-stage DR.

Comparison of the nerve fiber layer of type 2 diabetic patients without glaucoma with normal subjects of the same age and sex

BACKGROUND

The retinal nerve fiber layer (RNFL) thickness in patients with diabetes mellitus type 2 was compared to normal subjects of similar age and sex, having first excluded any risk factors for glaucoma. The correlation between the RNFL thickness and the severity of diabetic retinopathy was investigated at its primary stages and with other ocular and diabetic parameters.

METHODS

A prospective, case series study was carried out. Twenty-seven diabetic patients without diabetic retinopathy, 24 diabetic patients with mild retinopathy, and 25 normal, age-matched subjects underwent a complete ophthalmological examination and imaging with scanning laser polarimetry for the evaluation of the RNFL. Multivariate analysis was applied in order to investigate the correlation between RNFL and diabetic parameters, such as age, duration of diabetes, insulin therapy, levels of glycosylated hemoglobin; and ocular parameters, such as cup to disc ratio, levels of normal intraocular pressure, and central corneal thickness.

RESULTS

The mean inferior average of RNFL and the temporal-superior-nasal-inferior-temporal standard deviation were statistically significantly lower in both diabetic groups, and the nerve fiber index was higher (P=0.04) compared to the normal group. There was no statistically significant difference between the diabetic groups. The factor analysis showed no significant correlation between the RNFL and the previously mentioned diabetic and ocular parameters.

CONCLUSION

The existence of diabetes should be seriously considered in evaluating the results of scanning laser polarimetry. Multivariate analysis for RNFL was used for the first time.

Neuropeptides, trophic factors, and other substances providing morphofunctional and metabolic protection in experimental models of diabetic retinopathy

Vision is the most important sensory modality for many species, including humans. Damage to the retina results in vision loss or even blindness. One of the most serious complications of diabetes, a disease that has seen a worldwide increase in prevalence, is diabetic retinopathy. This condition stems from consequences of pathological metabolism and develops in 75% of patients with type 1 and 50% with type 2 diabetes. The development of novel protective drugs is essential. In this review we provide a description of the disease and conclude that type 1 diabetes and type 2 diabetes lead to the same retinopathy. We evaluate existing experimental models and recent developments in finding effective compounds against this disorder. In our opinion, the best models are the long-term streptozotocin-induced diabetes and Otsuka Long-Evans Tokushima Fatty and spontaneously diabetic Torii rats, while the most promising substances are topically administered somatostatin and pigment epithelium-derived factor analogs, antivasculogenic substances, and systemic antioxidants. Future drug development should focus on these.

Neural constraints on visual acuity in proliferative diabetic retinopathy

PURPOSE

Visual acuity (VA) in normally sighted individuals is highly correlated with equivalent intrinsic blur, a measure of the amount of blur within the visual system that is generated by optical and neural sources. This study assessed the extent to which VA, equivalent intrinsic blur, optical blur, and neural blur are abnormal in subjects with proliferative diabetic retinopathy (PDR) and characterized the relationships among these parameters.

METHODS

Best-corrected VA of 10 subjects with PDR (ages 25 to 68) and 10 normally sighted individuals (ages 46 to 63) was measured for tumbling E optotypes. The Es were either unblurred or blurred through convolution with Gaussian functions of different widths. Values of equivalent intrinsic blur (σ(int)) and unblurred VA (MAR0) were derived using a standard model. Optical blur (σ(opt)), a measure of blur generated by higher-order aberrations, was quantified using Shack-Hartmann aberrometry. An index of neural blur (η) was defined as 1--σ(opt)/σ(int), which represents the remaining blur once the contributions of σ(opt) to σ(int) have been accounted for.

RESULTS

Log MAR0 and log σ(int) were correlated significantly (r = 0.98, p < 0.05) for the PDR subjects and the values of these parameters ranged from normal to more than a factor of 2 above the upper limit of normal. In comparison, log MAR measured for the most blurred E was elevated by a relatively small amount for all PDR subjects and was not correlated significantly with log σ(int) (r = 0.40, p = 0.25). MAR0, σ(int), and η differed significantly between the PDR subjects and the controls (all p < 0.05) but σ(opt) did not (p = 0.50).

CONCLUSIONS

Subjects with PDR and VA loss had higher than normal equivalent intrinsic blur that resulted primarily from neural blur elevations, suggesting that neural blur is an important factor that limits VA in these patients.

Tauroursodeoxycholic acid protects retinal neural cells from cell death induced by prolonged exposure to elevated glucose

Diabetic retinopathy is one of the most frequent causes of blindness in adults in the Western countries. Although diabetic retinopathy is considered a vascular disease, several reports demonstrate that retinal neurons are also affected, leading to vision loss. Tauroursodeoxycholic acid (TUDCA), an endogenous bile acid, has proven to be neuroprotective in several models of neurodegenerative diseases, including models of retinal degeneration. Since hyperglycemia is considered to play a central role in retinal cell dysfunction and degeneration, underlying the progression of diabetic retinopathy, the purpose of this study was to investigate the neuroprotective effects of TUDCA in rat retinal neurons exposed to elevated glucose concentration. We found that TUDCA markedly decreased cell death in cultured retinal neural cells induced by exposure to elevated glucose concentration. In addition, TUDCA partially prevented the release of apoptosis-inducing factor (AIF) from the mitochondria, as well as the subsequent accumulation of AIF in the nucleus. Biomarkers of oxidative stress, such as protein carbonyl groups and reactive oxygen species production, were markedly decreased after TUDCA treatment as compared to cells exposed to elevated glucose concentration alone. In conclusion, TUDCA protected retinal neural cell cultures from cell death induced by elevated glucose concentration, decreasing mito-nuclear translocation of AIF. The antioxidant properties of TUDCA might explain its cytoprotection. These findings may have relevance in the treatment of diabetic retinopathy patients.

BDNF protects retinal neurons from hyperglycemia through the TrkB/ERK/MAPK pathway

Diabetic retinopathy (DR) is one of the most common diabetic eye diseases and a leading cause of blindness. It is characterized by changes in the blood vessels of the retina. The pathogenesis of DR is complex and to date, the precise mechanisms involved remain unclear. Previous studies have reported that DR is associated with neurodegeneration and that apoptosis may occur in diabetic retinas. In the present study, retinal neurons under conditions of hyperglycemia were used as a model to study apoptosis in diabetic retinas. Retinal neurons exposed to hyperglycemia exhibited high levels of apoptosis. Brain‑derived neurotrophic factor (BDNF), a member of the neurotrophin family, was effective in protecting retinal neurons from hyperglycemia in vitro. BDNF promoted neuronal cell survival in a concentration‑dependent manner. In addition, BDNF was demonstrated to promote the expression of tropomyosin‑related kinase B (TrkB) and elevate the phosphorylation levels of TrkB and ERK in retinal neurons exposed to hyperglycemia. The results of the present study demonstrated that BDNF may protect retinal neurons from hyperglycemia via the TrkB/ERK/MAPK pathway and provides novel insights into the pathogenesis of DR.

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.

Cyclo-oxygenase-2 expression in human idiopathic epiretinal membrane

PURPOSE

The purpose of this study was to examine the expression of cyclo-oxygenase (COX)-2 in the idiopathic epiretinal membrane (IERM), inner limiting membrane (ILM), and proliferative diabetic retinopathy membrane.

METHODS

Twenty membranes, consisting of eight IERMs, four ILMs, and eight proliferative diabetic retinopathy membranes, were surgically removed. Formalin-fixed, paraffin-embedded tissue sections were processed for immunohistochemistry using anti-COX-2 antibody. The nuclear density showing the density of cells situated in IERM and ILM specimens was calculated under high-power fields using a light microscope.

RESULTS

The IERM comprised flattened cells with oval nuclei constituting a monolayer. The ILM contained a few cells with abundant collagenous tissues. Neither endothelial nor inflammatory cells were observed in the IERM and ILM. COX-2 immunoreactivity was markedly detected in cells located in the IERM. In contrast, COX-2 immunoreactivity was faintly detected in the ILM. The COX-2-positive rate was 65.4 +/- 15.5% and 34.3 +/- 20.3% in the IERM and ILM, respectively, being significantly higher in the former (P = 0.046). The nuclear density was 39.3 +/- 10.3 and 8.6 +/- 7.2 in the IERM and ILM, respectively, being significantly higher in the former (P = 0.0003). The proliferative diabetic retinopathy membranes consisted of many vascular endothelial and stromal cells. Cytoplasmic immunoreactivity for COX-2 was detected in endothelial and stromal cells in the proliferative diabetic retinopathy membranes.

CONCLUSION

These results suggest that COX-2 plays a potential role in the formation of avascular and vascularized epiretinal membranes if an epiphenomenon of COX-2 expression within these epiretinal membranes has been ruled out in future studies.

High glucose and oxidative/nitrosative stress conditions induce apoptosis in retinal endothelial cells by a caspase-independent pathway

Diabetic retinopathy (DR) is a leading cause of vision loss among working-age adults. Retinal endothelial cell apoptosis is an early event in DR, and oxidative stress is known to play an important role in this pathology. Recently, we found that high glucose induces apoptosis in retinal neural cells by a caspase-independent mechanism. Here, we investigated the mechanisms underlying retinal endothelial cell apoptosis induced by high glucose and oxidative/nitrosative stress conditions. Endothelial cells (TR-iBRB2 rat retinal endothelial cell line) were exposed to high glucose (long-term exposure, 7 days), or to NOC-18 (nitric oxide donor; 250microM) or H(2)O(2) (100microM) for 24h. Cell viability was assessed by the MTT assay and cell proliferation by [methyl-(3)H]-thymidine incorporation into DNA. Apoptotic cells were detected with Hoechst or Annexin V staining. Active caspases were detected by an apoptosis detection kit. Active caspase-3 and apoptosis-inducing factor (AIF) protein levels were assessed by Western blot or immunohistochemistry. High glucose, NOC-18 and H(2)O(2) increased apoptosis in retinal endothelial cells. High glucose and mannitol decreased cell proliferation, but mannitol did not induce apoptosis. Caspase activation did not increase in high glucose- or NOC-18-treated cells, but it increased in cells exposed to H(2)O(2). However, the protein levels of AIF decreased in mitochondrial fractions and increased in nuclear fractions, in all conditions. These results are the first demonstrating that retinal endothelial cell apoptosis induced by high glucose is independent of caspase activation, and is correlated with AIF translocation to the nucleus. NOC-18 and H(2)O(2) also activate a caspase-independent apoptotic pathway, although H(2)O(2) can also induce caspase-mediated apoptosis.

Novel potential mechanisms for diabetic macular edema: leveraging new investigational approaches

This article evaluates the current knowledge of the molecular mechanisms by which diabetes ocular and systemic inflammation induce breakdown of the blood-retinal barrier resulting in macular edema. We also summarize the relationship between molecular targets and the use of therapeutic inhibitors in preclinical studies and clinical trials. Further studies are needed to understand the regulation of normal blood-retinal barrier physiology and the relationship between events in animal models of diabetic retinopathy and humans with diabetes.

Retinal ganglion cells in diabetes

Diabetic retinopathy has long been recognized as a vascular disease that develops in most patients, and it was believed that the visual dysfunction that develops in some diabetics was due to the vascular lesions used to characterize the disease. It is becoming increasingly clear that neuronal cells of the retina also are affected by diabetes, resulting in dysfunction and even degeneration of some neuronal cells. Retinal ganglion cells (RGCs) are the best studied of the retinal neurons with respect to the effect of diabetes. Although investigations are providing new information about RGCs in diabetes, including therapies to inhibit the neurodegeneration, critical information about the function, anatomy and response properties of these cells is yet needed to understand the relationship between RGC changes and visual dysfunction in diabetes.

Diabetes changes ionotropic glutamate receptor subunit expression level in the human retina

Early diabetic retinopathy is characterized by changes in subtle visual functions such as contrast sensitivity and dark adaptation. The outcome of several studies suggests that glutamate is involved in retinal neurodegeneration during diabetes. We hypothesized that the protein levels of ionotropic glutamate receptor subunits are altered in the retina during diabetes. Therefore, we investigated whether human diabetic patients have altered immunoreactivity of ionotropic glutamate receptor subunits in the retina. In total, 12 donor eyes from subjects with diabetes mellitus were examined and compared to 6 eyes from non-diabetic subjects without known ocular disease, serving as controls. Immunohistochemical analysis was performed using specific antibodies directed against the ionotropic alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate (AMPA) receptor subunits GluR1, GluR2, GluR4, and against the N-methyl-d-aspartate glutamate receptor subunit NR1. In the inner plexiform and outer plexiform layers the immunoreactivity of GluR2 and NR1 subunits was significantly increased in subjects with diabetes when compared to the levels found in controls. No significant changes in GluR1 and GluR4 subunit expression were observed. These results suggest that early visual dysfunction in diabetic patients may be due, at least partially, to changes in glutamate receptor subunit expression or distribution.

Therapeutic potential of AIF-mediated caspase-independent programmed cell death

Resistance to anticancer drugs is often related to deficient cell death execution pathways in cancer cells. Apoptosis, which denotes a form of cell death executed by caspases, was traditionally considered as the only physiological and programmed form of cell death. However, recent evidence indicates that programmed cell death (PCD) can occur in complete absence of caspase activation. Indeed, a large number of caspase-independent models are now defined and a key protein implicated in this type of PCD, apoptosis-inducing factor (AIF), has been identified. AIF is a mitochondrial protein with two faces looking in opposite life/death directions. Recently, the identification of five different isoforms allowed a better characterization of AIFs life/mitochondrial versus death/nuclear functions, as well as definition of its pro-apoptotic region and some of its nuclear partners. Importantly, much work on caspase-independent PCD has revealed that AIF participates in more PCD systems than initially thought. A wider molecular knowledge of AIF, and of the caspase-independent PCDs in which it is involved, are key to provide new insights into the role of PCD. There is no doubt that these insights will lead to the development of more selective and efficient drugs against cancer, degenerative diseases, and other pathological disorders implicating AIF.

High glucose induces caspase-independent cell death in retinal neural cells

Diabetic retinopathy is a leading cause of blindness among adults in the western countries. It has been reported that neurodegeneration may occur in diabetic retinas, but the mechanisms underlying retinal cell death are poorly understood. We found that high glucose increased the number of cells with condensed nuclei and the number of TUNEL-positive cells, and caused an increase in the translocation of phosphatidylserine to the outer leaflet of the plasma membrane, indicating that high glucose induces apoptosis in cultured retinal neural cells. The activity of caspases did not increase in high glucose-treated cells, but apoptosis-inducing factor (AIF) levels decreased in the mitochondria and increased in the nucleus, indicating a translocation to the nucleus where it may cause DNA fragmentation. These results demonstrate that elevated glucose induces apoptosis in cultured retinal neural cells. The increase in apoptosis is not dependent on caspase activation, but is mediated through AIF release from the mitochondria.