Müller glial dysfunction during diabetic retinopathy in rats is linked to accumulation of advanced glycation end-products and advanced lipoxidation end-products

In vivo glycation-interplay between oxidant and carbonyl stress in bone

Metabolic syndromes (eg, obesity, type 2 diabetes (T2D), atherosclerosis, and neurodegenerative diseases) and aging, they all have a strong component of carbonyl and reductive-oxidative (redox) stress. Reactive carbonyl (RCS) and oxidant (ROS) stress species are commonly generated as products or byproducts of cellular metabolism or are derived from the environment. RCS and ROS can play a dual role in living organisms. Some RCS and ROS function as signaling molecules, which control cellular defenses against biological and environmental assaults. However, due to their high reactivity, RCS and ROS inadvertently interact with different cellular and extracellular components, which can lead to the formation of undesired posttranslational modifications of bone matrix proteins. These are advanced glycation (AGEs) and glycoxidation (AGOEs) end products generated in vivo by non-enzymatic amino-carbonyl reactions. In this review, metabolic processes involved in generation of AGEs and AGOEs within and on protein surfaces including extracellular bone matrix are discussed from the perspective of cellular metabolism and biochemistry of certain metabolic syndromes. The impact of AGEs and AGOEs on some characteristics of mineral is also discussed. Different therapeutic approaches with the potential to prevent the formation of RCS, ROS, and the resulting formation of AGEs and AGOEs driven by these chemicals are also briefly reviewed. These are antioxidants, scavenging agents of reactive species, and newly emerging technologies for the development of synthetic detoxifying systems. Further research in the area of in vivo glycation and glycoxidation should lead to the development of diverse new strategies for halting the progression of metabolic complications before irreversible damage to body tissues materializes.

TGF-β Signaling Pathways in the Development of Diabetic Retinopathy

Diabetic retinopathy (DR), a prevalent complication of diabetes mellitus affecting a significant portion of the global population, has long been viewed primarily as a microvascular disorder. However, emerging evidence suggests that it should be redefined as a neurovascular disease with multifaceted pathogenesis rooted in oxidative stress and advanced glycation end products. The transforming growth factor-β (TGF-β) signaling family has emerged as a major contributor to DR pathogenesis due to its pivotal role in retinal vascular homeostasis, endothelial cell barrier function, and pericyte differentiation. However, the precise roles of TGF-β signaling in DR remain incompletely understood, with conflicting reports on its impact in different stages of the disease. Additionally, the BMP subfamily within the TGF-β superfamily introduces further complexity, with BMPs exhibiting both pro- and anti-angiogenic properties. Furthermore, TGF-β signaling extends beyond the vascular realm, encompassing immune regulation, neuronal survival, and maintenance. The intricate interactions between TGF-β and reactive oxygen species (ROS), non-coding RNAs, and inflammatory mediators have been implicated in the pathogenesis of DR. This review delves into the complex web of signaling pathways orchestrated by the TGF-β superfamily and their involvement in DR. A comprehensive understanding of these pathways may hold the key to developing targeted therapies to halt or mitigate the progression of DR and its devastating consequences.

Protective Effect of Pemafibrate Treatment against Diabetic Retinopathy in Spontaneously Diabetic Torii Fatty Rats

Diabetic retinopathy (DR) can cause visual impairment and blindness, and the increasing global prevalence of diabetes underscores the need for effective therapies to prevent and treat DR. Therefore, this study aimed to evaluate the protective effect of pemafibrate treatment against DR, using a Spontaneously Diabetic Torii (SDT) fatty rat model of obese type 2 diabetes. SDT fatty rats were fed either a diet supplemented with pemafibrate (0.3 mg/kg/day) for 16 weeks, starting at 8 weeks of age (Pf SDT fatty: study group), or normal chow (SDT fatty: controls). Normal chow was provided to Sprague-Dawley (SD) rats (SD: normal controls). Electroretinography (ERG) was performed at 8 and 24 weeks of age to evaluate the retinal neural function. After sacrifice, retinal thickness, number of retinal folds, and choroidal thickness were evaluated, and immunostaining was performed for aquaporin-4 (AQP4). No significant differences were noted in food consumption, body weight, or blood glucose level after pemafibrate administration. Triglyceride levels were reduced, and high-density lipoprotein cholesterol levels were increased. Extension of oscillatory potential (OP)1 and OP3 waves on ERG was suppressed in the Pf SDT fatty group. Retinal thickness at 1,500 microns from the optic disc improved in the Pf SDT fatty group. No significant improvements were noted in choroidal thickness or number of retinal folds. Quantitative analyses showed that AQP4-positive regions in the retinas were significantly larger in the Pf SDT fatty group than in the SDT fatty group. The findings suggest that pemafibrate treatment can exert protective effects against DR.

Hu-Zhang Qing-Mai Formulation anti-oxidative stress alleviates diabetic retinopathy: Network pharmacology analysis and in vitro experiment

BACKGROUND

In this study, the potential mechanism of the Hu-Zhang Qing-Mai Formulation (HZQMF) on diabetic retinopathy (DR) in inhibiting oxidative stress was explored through network pharmacology analysis and in vitro experiments.

METHODS

The Traditional Chinese Medicine Systematic Pharmacology Analysis Platform was used to retrieve the active pharmaceutical ingredients and targets of HZQMF. DR-related genes and oxidative stress-related genes were obtained from PharmGKB, TTD, OMIM, GeneCards, and Drugbank. STRING was used to construct a protein-protein interaction network to screen core targets. Gene ontology and Kyoto encyclopedia of genes and genomes enrichment analyses were performed using R 4.0.3. Network topology analysis was carried out using Cytoscape 3.8.2. Finally, we looked into how well the main API protected human retinal pigment epithelial cells from damage brought on by hydrogen peroxide (H2O2).

RESULTS

Quercetin (Que) was identified as the primary API of HZQMF through network pharmacology analysis, while JUN, MAPK1, and STAT3 were identified as the primary hub genes. Kyoto encyclopedia of genes and genomes enrichment analysis showed that the AGE-RAGE signaling pathway may be crucial to the therapeutic process. In vitro experiments confirmed that Que increased cell vitality and inhibited apoptosis.

CONCLUSION

Que might significantly reduce H2O2-induced ARPE-19 cell injury by inhibiting apoptosis-related genes of the AGE-RAGE pathway (JUN, MAPK1, STAT3). This study lays the foundation for further research on HZQMF in treating DR.

Revisiting Retinal Degeneration Hallmarks: Insights from Molecular Markers and Therapy Perspectives

Visual impairment and blindness are a growing public health problem as they reduce the life quality of millions of people. The management and treatment of these diseases represent scientific and therapeutic challenges because different cellular and molecular actors involved in the pathophysiology are still being identified. Visual system components, particularly retinal cells, are extremely sensitive to genetic or metabolic alterations, and immune responses activated by local insults contribute to biological events, culminating in vision loss and irreversible blindness. Several ocular diseases are linked to retinal cell loss, and some of them, such as retinitis pigmentosa, age-related macular degeneration, glaucoma, and diabetic retinopathy, are characterized by pathophysiological hallmarks that represent possibilities to study and develop novel treatments for retinal cell degeneration. Here, we present a compilation of revisited information on retinal degeneration, including pathophysiological and molecular features and biochemical hallmarks, and possible research directions for novel treatments to assist as a guide for innovative research. The knowledge expansion upon the mechanistic bases of the pathobiology of eye diseases, including information on complex interactions of genetic predisposition, chronic inflammation, and environmental and aging-related factors, will prompt the identification of new therapeutic strategies.

Oxidative Stress and Its Regulation in Diabetic Retinopathy

Diabetic retinopathy is the retinal disease associated with hyperglycemia in patients who suffer from type 1 or type 2 diabetes. It includes maculopathy, involving the central retina and characterized by ischemia and/or edema, and peripheral retinopathy that progresses to a proliferative stage with neovascularization. Approximately 10% of the global population is estimated to suffer from diabetes, and around one in 5 of these individuals have diabetic retinopathy. One of the major effects of hyperglycemia is oxidative stress, the pathological state in which elevated production of reactive oxygen species damages tissues, cells, and macromolecules. The retina is relatively prone to oxidative stress due to its high metabolic activity. This review provides a summary of the role of oxidative stress in diabetic retinopathy, including a description of the retinal cell players and the molecular mechanisms. It discusses pathological processes, including the formation and effects of advanced glycation end-products, the impact of metabolic memory, and involvements of non-coding RNA. The opportunities for the therapeutic blockade of oxidative stress in diabetic retinopathy are also considered.

Aldehyde Dehydrogenase and Aldo-Keto Reductase Enzymes: Basic Concepts and Emerging Roles in Diabetic Retinopathy

Diabetic retinopathy (DR) is a complication of diabetes mellitus that can lead to vision loss and blindness. It is driven by various biochemical processes and molecular mechanisms, including lipid peroxidation and disrupted aldehyde metabolism, which contributes to retinal tissue damage and the progression of the disease. The elimination and processing of aldehydes in the retina rely on the crucial role played by aldehyde dehydrogenase (ALDH) and aldo-keto reductase (AKR) enzymes. This review article investigates the impact of oxidative stress, lipid-derived aldehydes, and advanced lipoxidation end products (ALEs) on the advancement of DR. It also provides an overview of the ALDH and AKR enzymes expressed in the retina, emphasizing their growing importance in DR. Understanding the relationship between aldehyde metabolism and DR could guide innovative therapeutic strategies to protect the retina and preserve vision in diabetic patients. This review, therefore, also explores various approaches, such as gene therapy and pharmacological compounds that have the potential to augment the expression and activity of ALDH and AKR enzymes, underscoring their potential as effective treatment options for DR.

Alteration of Aquaporins 1 and 4 immunohistochemical and gene expression in the cerebellum of diabetic albino rat

Aquaporins (AQPs) are a family of transmembrane channel proteins. AQP1 and AQP4 are expressed in cerebellum amongst others. This study was designed to assess the effect of diabetes on AQP1 and AQP4 expression in cerebellum of rats. Diabetes was induced by a single intraperitoneal injection of Streptozotocin 45 mg/kg in 24 adult male Sprague Dawley rats. Six rats from control and diabetic groups were sacrificed at one, four, and eight weeks post diabetic confirmation. After eight weeks, measurement of malondialdehyde (MDA), reduced glutathione (GSH) concentrations, and cerebellar mRNA expression for AQP1 and AQP4 genes were performed. Immunohistochemical evaluation of AQP1, AQP4, and glial fibrillary acidic protein (GFAP) for cerebellar sections was performed for all groups. Diabetes caused degenerative changes in Purkinje cells with a significant increase in the cerebellar level of MDA and AQP1 immunoreactivity and a significant decrease in GSH level and AQP4 expression levels. However, the alteration in the AQP1 mRNA level was not statistically significant. GFAP immunoreactivity was increased in 8 W diabetic rats following its decrease in 1 W diabetic rats. Diabetes caused some alteration in the AQPs 1 and 4 expression in the cerebellum of diabetic rats which may contribute to diabetes-induced cerebellar complications.

Elucidating glial responses to products of diabetes-associated systemic dyshomeostasis

Diabetic retinopathy (DR) is a leading cause of blindness in working age adults. DR has non-proliferative stages, characterized in part by retinal neuroinflammation and ischemia, and proliferative stages, characterized by retinal angiogenesis. Several systemic factors, including poor glycemic control, hypertension, and hyperlipidemia, increase the risk of DR progression to vision-threatening stages. Identification of cellular or molecular targets in early DR events could allow more prompt interventions pre-empting DR progression to vision-threatening stages. Glia mediate homeostasis and repair. They contribute to immune surveillance and defense, cytokine and growth factor production and secretion, ion and neurotransmitter balance, neuroprotection, and, potentially, regeneration. Therefore, it is likely that glia orchestrate events throughout the development and progression of retinopathy. Understanding glial responses to products of diabetes-associated systemic dyshomeostasis may reveal novel insights into the pathophysiology of DR and guide the development of novel therapies for this potentially blinding condition. In this article, first, we review normal glial functions and their putative roles in the development of DR. We then describe glial transcriptome alterations in response to systemic circulating factors that are upregulated in patients with diabetes and diabetes-related comorbidities; namely glucose in hyperglycemia, angiotensin II in hypertension, and the free fatty acid palmitic acid in hyperlipidemia. Finally, we discuss potential benefits and challenges associated with studying glia as targets of DR therapeutic interventions. In vitro stimulation of glia with glucose, angiotensin II and palmitic acid suggests that: 1) astrocytes may be more responsive than other glia to these products of systemic dyshomeostasis; 2) the effects of hyperglycemia on glia are likely to be largely osmotic; 3) fatty acid accumulation may compound DR pathophysiology by promoting predominantly proinflammatory and proangiogenic transcriptional alterations of macro and microglia; and 4) cell-targeted therapies may offer safer and more effective avenues for DR treatment as they may circumvent the complication of pleiotropism in retinal cell responses. Although several molecules previously implicated in DR pathophysiology are validated in this review, some less explored molecules emerge as potential therapeutic targets. Whereas much is known regarding glial cell activation, future studies characterizing the role of glia in DR and how their activation is regulated and sustained (independently or as part of retinal cell networks) may help elucidate mechanisms of DR pathogenesis and identify novel drug targets for this blinding disease.

Orally Delivered Connexin43 Hemichannel Blocker, Tonabersat, Inhibits Vascular Breakdown and Inflammasome Activation in a Mouse Model of Diabetic Retinopathy

Diabetic retinopathy (DR), a microvascular complication of diabetes, is associated with pronounced inflammation arising from the activation of a nucleotide-binding and oligomerization domain-like receptor (NLR) protein 3 (NLRP3) inflammasome. Cell culture models have shown that a connexin43 hemichannel blocker can prevent inflammasome activation in DR. The aim of this study was to evaluate the ocular safety and efficacy of tonabersat, an orally bioavailable connexin43 hemichannel blocker, to protect against DR signs in an inflammatory non-obese diabetic (NOD) DR mouse model. For retina safety studies, tonabersat was applied to retinal pigment epithelial (ARPE-19) cells or given orally to control NOD mice in the absence of any other stimuli. For efficacy studies, either tonabersat or a vehicle was given orally to the inflammatory NOD mouse model two hours before an intravitreal injection of pro-inflammatory cytokines, interleukin-1 beta, and tumour necrosis factor-alpha. Fundus and optical coherence tomography images were acquired at the baseline as well as at 2- and 7-day timepoints to assess microvascular abnormalities and sub-retinal fluid accumulation. Retinal inflammation and inflammasome activation were also assessed using immunohistochemistry. Tonabersat did not have any effect on ARPE-19 cells or control NOD mouse retinas in the absence of other stimuli. However, the tonabersat treatment in the inflammatory NOD mice significantly reduced macrovascular abnormalities, hyperreflective foci, sub-retinal fluid accumulation, vascular leak, inflammation, and inflammasome activation. These findings suggest that tonabersat may be a safe and effective treatment for DR.

Differences in junction-associated gene expression changes in three rat models of diabetic retinopathy with similar neurovascular phenotype

Diabetic retinopathy, also defined as microvascular complication of diabetes mellitus, affects the entire neurovascular unit with specific aberrations in every compartment. Neurodegeneration, glial activation and vasoregression are observed consistently in models of diabetic retinopathy. However, the order and the severity of these aberrations varies in different models, which is also true in patients. In this study, we analysed rat models of diabetic retinopathy with similar phenotypes to identify key differences in the pathogenesis. For this, we focussed on intercellular junction-associated gene expression, which are important for the communication and homeostasis within the neurovascular unit. Streptozotocin-injected diabetic Wistar rats, methylglyoxal supplemented Wistar rats and polycystin-2 transgenic (PKD) rats were analysed for neuroretinal function, vasoregression and retinal expression of junction-associated proteins. In all three models, neuroretinal impairment and vasoregression were observed, but gene expression profiling of junction-associated proteins demonstrated nearly no overlap between the three models. However, the differently expressed genes were from the main classes of claudins, connexins and integrins in all models. Changes in Rcor1 expression in diabetic rats and Egr1 expression in PKD rats confirmed the differences in upstream transcription factor level between the models. In PKD rats, a possible role for miRNA regulation was observed, indicated by an upregulation of miR-26b-5p, miR-122-5p and miR-300-3p, which was not observed in the other models. In silico allocation of connexins revealed not only differences in regulated subtypes, but also in affected retinal cell types, as well as connexin specific upstream regulators Sox7 and miR-92a-3p. In this study, we demonstrate that, despite their similar phenotype, models for diabetic retinopathy exhibit significant differences in their pathogenic pathways and primarily affected cell types. These results underline the importance for more sensitive diagnostic tools to identify pathogenic clusters in patients as the next step towards a desperately needed personalized therapy.

Classification of diabetic retinopathy: Past, present and future

Diabetic retinopathy (DR) is a leading cause of visual impairment and blindness worldwide. Since DR was first recognized as an important complication of diabetes, there have been many attempts to accurately classify the severity and stages of disease. These historical classification systems evolved as understanding of disease pathophysiology improved, methods of imaging and assessing DR changed, and effective treatments were developed. Current DR classification systems are effective, and have been the basis of major research trials and clinical management guidelines for decades. However, with further new developments such as recognition of diabetic retinal neurodegeneration, new imaging platforms such as optical coherence tomography and ultra wide-field retinal imaging, artificial intelligence and new treatments, our current classification systems have significant limitations that need to be addressed. In this paper, we provide a historical review of different classification systems for DR, and discuss the limitations of our current classification systems in the context of new developments. We also review the implications of new developments in the field, to see how they might feature in a future, updated classification.

Oxidative Stress Implication in Retinal Diseases-A Review

Oxidative stress (OS) refers to an imbalance between free radicals (FRs), namely highly reactive molecules normally generated in our body by several pathways, and intrinsic antioxidant capacity. When FR levels overwhelm intrinsic antioxidant defenses, OS occurs, inducing a series of downstream chemical reactions. Both reactive oxygen species (ROS) and reactive nitrogen species (RNS) are produced by numerous chemical reactions that take place in tissues and organs and are then eliminated by antioxidant molecules. In particular, the scientific literature focuses more on ROS participation in the pathogenesis of diseases than on the role played by RNS. By its very nature, the eye is highly exposed to ultraviolet radiation (UVR), which is directly responsible for increased OS. In this review, we aimed to focus on the retinal damage caused by ROS/RNS and the related retinal pathologies. A deeper understanding of the role of oxidative and nitrosative stress in retinal damage is needed in order to develop targeted therapeutic interventions to slow these pathologies.

Contribution of Müller Cells in the Diabetic Retinopathy Development: Focus on Oxidative Stress and Inflammation

Diabetic retinopathy is a neurovascular complication of diabetes and the main cause of vision loss in adults. Glial cells have a key role in maintenance of central nervous system homeostasis. In the retina, the predominant element is the Müller cell, a specialized cell with radial morphology that spans all retinal layers and influences the function of the entire retinal circuitry. Müller cells provide metabolic support, regulation of extracellular composition, synaptic activity control, structural organization of the blood-retina barrier, antioxidant activity, and trophic support, among other roles. Therefore, impairments of Müller actions lead to retinal malfunctions. Accordingly, increasing evidence indicates that Müller cells are affected in diabetic retinopathy and may contribute to the severity of the disease. Here, we will survey recently described alterations in Müller cell functions and cellular events that contribute to diabetic retinopathy, especially related to oxidative stress and inflammation. This review sheds light on Müller cells as potential therapeutic targets of this disease.

Beneficial Effect of Long-Term Administration of Supplement With Trapa Bispinosa Roxb. and Lutein on Retinal Neurovascular Coupling in Type 2 Diabetic Mice

Purpose

We investigated the effect of long-term administration of supplement with trapa bispinosa roxb. extract (TBE) and lutein on the susceptibility of retinal blood flow regulation in type 2 diabetic mice.

Methods

Six-week-old db/db mice were randomly divided into the untreated group (n = 6) and the treated group received the supplement with TBE and lutein (n = 6). The longitudinal changes in retinal blood flow responses to systemic hyperoxia and a flicker stimulation were evaluated every 2 weeks in diabetes db/db mice from age 8 to 14 weeks. The retinal blood flow was assessed using laser speckle flowgraphy. We also evaluated the expressions of glial fibrillary acid protein (GFAP) and vascular endothelial growth factor (VEGF) by immunofluorescence.

Results

The resting retinal blood flow was steady and comparable between two groups throughout the study. In db/db mice with supplement, both blood flow responses were restored from 8 to 14 weeks of age compared with diabetic mice treated with the placebo. Supplement prevented the activation of GFAP and decreased the expression of VEGF detected by immunofluorescence compared with the diabetic mice treated with placebo.

Conclusion

We found that the long-term administration of supplement with TBE and lutein improved the impaired regulation of retinal blood flow in response to systemic hyperoxia and flicker stimulation, suggesting that these supplements can prevent diabetic retinopathy by improving abnormal neurovascular coupling in type 2 diabetic mice.

Peperomia pellucida (L.) Kunth and eye diseases: A review on phytochemistry, pharmacology and toxicology

Peperomia pellucida (L.) Kunth is a medicinal plant used to manage inflammatory illnesses such as conjunctivitis, and gastrointestinal and respiratory tract disorders in tropical and subtropical regions. However, little is known about its pharmacological mechanism of action against eye diseases. This review aims to critically discuss the phytochemistry, pharmacology and toxicology of P. pellucida as well as its roles in the treatment of cataract, glaucoma and diabetic retinopathy. Recent developments in the uses of P. pellucida for healthcare and nutraceutical products by the pharmaceutical industry are also covered in this review. For this review, a literature search was performed with PubMed, ScienceDirect, SciFinder Scholar and Scopus databases, using relevant keywords. Among the various phytochemicals identified from P. pellucida, β-caryophyllene, carotol, dillapiole, ellagic acid, pellucidin A, phytol and vitexin exhibit strong pharmacological activities within the mitogen-activated protein kinase and nuclear factor-κB signalling pathways in inflammatory eye diseases. The antihypertensive, anti-inflammatory, antioxidant, antihyperglycemic and anti-angiogenic activities displayed by P. pellucida extracts in many in vitro, in vivo and clinical studies suggest its potential role in the management of inflammatory eye diseases. P. pellucida extract was non-toxic against normal cell lines but displayed mild toxicity in animal models. The growing public interest in P. pellucida has inspired the nutraceutical and pharmaceutical industries to process the plant into health products. Although the potential pharmacological mechanisms against eye diseases have been summarized, further studies of the interactions among constituent phytochemicals from P. pellucida within various signalling pathways shall support the use of the plant as an alternative therapeutic source.

Neutrophils and neutrophil extracellular trap components: Emerging biomarkers and therapeutic targets for age-related eye diseases

Age-related eye diseases, including dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy, represent a major global health issue based on their increasing prevalence and disabling action. Unraveling the molecular mechanisms underlying these diseases will provide novel opportunities to reduce the burden of age-related eye diseases and improve eye health, contributing to sustainable development goals achievement. The impairment of neutrophil extracellular traps formation/degradation processes seems to be one of these mechanisms. These traps formed by a meshwork of DNA and neutrophil cytosolic granule proteins may exacerbate the inflammatory response promoting chronic inflammation, a pivotal cause of age-related diseases. In this review, we describe current findings that suggest the role of neutrophils and their traps in the pathogenesis of the above-mentioned age-related eye diseases. Furthermore, we discuss why these cells and their constituents could be biomarkers and therapeutic targets for dry eye, glaucoma, age-related macular degeneration, and diabetic retinopathy. We also examine the therapeutic potential of some neutrophil function modulators and provide several recommendations for future research in age-related eye diseases.

Reaction dynamics and residue identification of haemoglobin modification by acrolein, a lipid-peroxidation by-product

BACKGROUND

Lipid hydroperoxides decompose to reactive aldehydes, such as acrolein. Measurement of oxidative stress markers in the clinic could improve risk stratification for patients.

METHODS

To aid the development of diagnostic oxidative stress markers, we defined the acrolein modifications of haemoglobin using mass spectrometry.

RESULTS

Acrolein modifications have little effect on the secondary structure of haemoglobin. They do not disrupt the quaternary structure, but instead promote crosslinked octamers. For acrolein modified haemoglobin the response to O2 binding is altered such that cooperativity is lost. Mass spectrometry experiments at a 1:1 acrolein:haemoglobin molar ratio demonstrate that the α-chain quickly forms an aza-Michael adduct (+56 Da), which then forms a more stable adduct, Nε-(3-methylpyridinium)lysine (MP-lysine, +76 Da) over 7 days. The β-chain remains relatively unchanged over the duration of the 7 days and the aza-Michael adduct is dominant. At 2:1 and 5:1 molar ratios the α-chain was consistently modified at K7, H20, H50, and the β-chain at C93 and H97 with the aza-Michael adduct. Beyond 5 h, an MP-adduct (+76 Da) was located predominantly at K7 of the α-chain, while an FDP-adduct (+94 Da) was observed at K95 of the β-chain.

CONCLUSIONS

We have generated qualitative evidence identifying the acrolein target sites on haemoglobin, a potential oxidative stress marker that is easily measured in circulation.

GENERAL SIGNIFICANCE

We provide data for the community to develop targeted mass spectrometric or immunometric assays for acrolein modified haemoglobin to further validate the potential of haemoglobin as an oxidative stress marker in patients .

Kir4.1 may represent a novel therapeutic target for diabetic retinopathy (Review)

As the major cause of irreversible loss of vision in adults, diabetic retinopathy (DR) is one of the most serious complications of diabetes. The imbalance of the retinal microenvironment and destruction of the blood-retinal barrier have a significant role in the progression of DR. Inward rectifying potassium channel 4.1 (Kir4.1) is located on Müller cells and is closely related to potassium homeostasis, water balance and glutamate clearance in the whole retina. The present review discusses the functions of Kir4.1 in regulating the retinal microenvironment and related biological mechanisms in DR. In the future, Kir4.1 may represent a novel alternative therapeutic target for DR through affecting the retinal microenvironment.

VEGF Mediates Retinal Müller Cell Viability and Neuroprotection through BDNF in Diabetes

To investigate the mechanism of vascular endothelial growth factor (VEGF) and brain-derived neurotrophic factor (BDNF) in Müller cell (MC) viability and neuroprotection in diabetic retinopathy (DR), we examined the role of VEGF in MC viability and BDNF production, and the effect of BDNF on MC viability under diabetic conditions. Mouse primary MCs and cells of a rat MC line, rMC1, were used in investigating MC viability and BDNF production under diabetic conditions. VEGF-stimulated BDNF production was confirmed in mice. The mechanism of BDNF-mediated MC viability was examined using siRNA knockdown. Under diabetic conditions, recombinant VEGF (rVEGF) stimulated MC viability and BDNF production in a dose-dependent manner. rBDNF also supported MC viability in a dose-dependent manner. Targeting BDNF receptor tropomyosin receptor kinase B (TRK-B) with siRNA knockdown substantially downregulated the activated (phosphorylated) form of serine/threonine-specific protein kinase (AKT) and extracellular signal-regulated kinase (ERK), classical survival and proliferation mediators. Finally, the loss of MC viability in TrkB siRNA transfected cells under diabetic conditions was rescued by rBDNF. Our results provide direct evidence that VEGF is a positive regulator for BDNF production in diabetes for the first time. This information is essential for developing BDNF-mediated neuroprotection in DR and hypoxic retinal diseases, and for improving anti-VEGF treatment for these blood-retina barrier disorders, in which VEGF is a major therapeutic target for vascular abnormalities.

An Update on Connexin Gap Junction and Hemichannels in Diabetic Retinopathy

Diabetic retinopathy (DR) is one of the main causes of vision loss in the working age population. It is characterized by a progressive deterioration of the retinal microvasculature, caused by long-term metabolic alterations inherent to diabetes, leading to a progressive loss of retinal integrity and function. The mammalian retina presents an orderly layered structure that executes initial but complex visual processing and analysis. Gap junction channels (GJC) forming electrical synapses are present in each retinal layer and contribute to the communication between different cell types. In addition, connexin hemichannels (HCs) have emerged as relevant players that influence diverse physiological and pathological processes in the retina. This article highlights the impact of diabetic conditions on GJC and HCs physiology and their involvement in DR pathogenesis. Microvascular damage and concomitant loss of endothelial cells and pericytes are related to alterations in gap junction intercellular communication (GJIC) and decreased connexin 43 (Cx43) expression. On the other hand, it has been shown that the expression and activity of HCs are upregulated in DR, becoming a key element in the establishment of proinflammatory conditions that emerge during hyperglycemia. Hence, novel connexin HCs blockers or drugs to enhance GJIC are promising tools for the development of pharmacological interventions for diabetic retinopathy, and initial in vitro and in vivo studies have shown favorable results in this regard.

CHANGES OF AQUEOUS HUMOR MÜLLER CELLS' BIOMARKERS IN HUMAN PATIENTS AFFECTED BY DIABETIC MACULAR EDEMA AFTER SUBTHRESHOLD MICROPULSE LASER TREATMENT

PURPOSE

To evaluate the changes in activity of biomarkers of Mu[Combining Diaeresis]ller cells (MC) in aqueous humor of patients with diabetic macular edema after subthreshold micropulse laser, over 1 year.

METHODS

Patients with untreated diabetic macular edema and central retinal thickness ≤ 400 μm were enrolled. Best-corrected visual acuity, full ophthalmic examination, and optical coherence tomography were performed. Subthreshold micropulse laser was applied every 3 months. Glial fibrillary acidic protein and inwardly rectifying potassium channel (Kir 4.1), MC activity markers, and vascular endothelial growth factor were quantified in the aqueous humor collected at baseline and at 1, 3, and 12 months after laser. Changes in the macular thickness and inner nuclear layer thickness, where MC bodies are located, were measured.

RESULTS

Ten eyes of 10 patients were included. Best-corrected visual acuity improved at 3 months (P = 0.047) and remained stable. Inner nuclear layer thickness significantly reduced at 12 months (P = 0.012). Glial fibrillary acidic protein, Kir 4.1, and vascular endothelial growth factor decreased at 1 and/or 3 and/or 12 months compared with baseline (P < 0.05).

CONCLUSION

Subthreshold micropulse laser improves visual function in diabetic macular edema. Kir 4.1 and glial fibrillary acidic protein decrease and inner nuclear layer thickness reduction demonstrate that subthreshold micropulse laser may restore MC function. Subthreshold micropulse laser also reduces vascular endothelial growth factor concentration. The effect of subthreshold micropulse laser in diabetic macular edema may in part be due to changes of MC metabolic activity.

Metformin Corrects Abnormal Circadian Rhythm and Kir4.1 Channels in Diabetes

Purpose

Diabetic retinopathy (DR) is a leading cause of visual impairment. Müller cells in DR are dysfunctional due to downregulation of the inwardly rectifying potassium channel Kir4.1. Metformin, a commonly used oral antidiabetic drug, is known to elicit its action through 5′ adenosine monophosphate-activated protein kinase (AMPK), a cellular metabolic regulator; however, its effect on Kir4.1 channels is unknown. For this study, we hypothesized that metformin treatment would correct circadian rhythm disruption and Kir4.1 channel dysfunction in db/db mice.

Methods

Metformin was given orally to db/db mice. Wheel-running activity, retinal levels of Kir4.1, and AMPK phosphorylation were determined at study termination. In parallel, rat retinal Müller cell line (rMC-1) cells were treated using metformin and 5-aminoimidazole-4-carboxamide ribonucleotide (AICAR) to assess the effect of AMPK activation on the Kir4.1 channel.

Results

The wheel-running activity of the db/db mice was improved following the metformin treatment. The Kir4.1 level in Müller cells was corrected after metformin treatment. Metformin treatment led to an upregulation of clock regulatory genes such as melanopsin (Opn4) and aralkylamine N-acetyltransferase (Aanat). In rMC-1 cells, AMPK activation via AICAR and metformin resulted in increased Kir4.1 and intermediate core clock component Bmal-1 protein expression. The silencing of Prkaa1 (gene for AMPKα1) led to decreased Kir4.1 and Bmal-1 protein expression.

Conclusions

Our findings demonstrate that metformin corrects abnormal circadian rhythm and Kir4.1 channels in db/db mouse a model of type 2 diabetes. Metformin could represent a critical pharmacological agent for preventing Müller cell dysfunction observed in human DR.

Acrolein: A Potential Mediator of Oxidative Damage in Diabetic Retinopathy

Diabetic retinopathy (DR) is the leading cause of vision loss among working-age adults. Extensive evidences have documented that oxidative stress mediates a critical role in the pathogenesis of DR. Acrolein, a product of polyamines oxidation and lipid peroxidation, has been demonstrated to be involved in the pathogenesis of various human diseases. Acrolein’s harmful effects are mediated through multiple mechanisms, including DNA damage, inflammation, ROS formation, protein adduction, membrane disruption, endoplasmic reticulum stress, and mitochondrial dysfunction. Recent investigations have reported the involvement of acrolein in the pathogenesis of DR. These studies have shown a detrimental effect of acrolein on the retinal neurovascular unit under diabetic conditions. The current review summarizes the existing literature on the sources of acrolein, the impact of acrolein in the generation of oxidative damage in the diabetic retina, and the mechanisms of acrolein action in the pathogenesis of DR. The possible therapeutic interventions such as the use of polyamine oxidase inhibitors, agents with antioxidant properties, and acrolein scavengers to reduce acrolein toxicity are also discussed.

The Relationship between Choroidal Thickness and Intracellular Oxidised-reduced Glutathione and Extracellular Thiol-disulfide Homeostasis at Different Stages of Diabetic Retinopathy

Purpose: To evaluate the relationship between diabetic retinopathy and oxidative damage by measuring intracellular and extracellular thiol levels, and to compare intracellular and extracellular thiol levels. Method: In this prospective, cross-sectional, and comparative study, 25 healthy control participants (group 1), a total of 25 diabetic macular edema (DME) patients with non-proliferative diabetic retinopathy (DRP) and without DME (group 2), and 25 DME patients with non-proliferative DRP and with DME (group 3) were included. Choroidal thickness (ChT) and central macular thickness (CMT) were measured by spectral domain optic coherence tomography. For the evaluation of antioxidant/oxidant balance, intracellular GSH (reduced glutathione) and GSSG (oxidized glutathione), extracellular SH (thiol) and SS (disulfide) levels were measured and recorded. Results: Comparing intracellular and extracellular thiol levels between groups, intracellular GSSG level and GSSG/GSH percent ratio, and extracellular disulfide and SS/SH percent ratio values were higher in diabetic patients than healthy participants. Choroidal thicknesses were significantly thinner in DRP groups compared to the healthy population. When the relationship between choroidal thicknesses and thiol levels was investigated, there were significant relationships between choroidal thicknesses and thiol levels in group 3. Conclusion: Oxidative stress and impaired intracellular GSH/GSSG and serum SH/SS balances were observed to have an effect on DRP and DME pathogenesis. In addition, in groups with and without DME, thinning in choroidal thicknesses and the relationship between these thicknesses and intra/extracellular oxidative stress indicators can also be explained.

Visual contrast sensitivity could be an early marker of diabetic retinopathy

The present study aimed to explore the early predictive marker of diabetic retinopathy (DR) and to elucidate the associated demographic, metabolic, and ocular factors. We enrolled 43 type 2 diabetic subjects with mild non-proliferative retinopathy (MNPDR), 30 diabetic subjects with no retinopathy (DNR), and 35 healthy controls (HC). The study groups showed no significant alteration in central macular thickness (CMT) and visual acuity (VA). The contrast sensitivity (CS) score was found to be significantly lower among DNR and MNPDR subjects compared to HCs (p < 0.0001). Between MNPDR and DNR subjects, the CS score was significantly lower in the former (p = 0.0036). CS score discriminated DNR subjects from HC, with 74% accuracy for the optimal threshold 0.71. The associated area under the ROC curve (AUC) is 0.82 (p < 0.0001) while the discrimination rule has 66% sensitivity and 80% specificity. The CS score also discriminated MNPDR subjects from DNR with 64% accuracy for the optimal threshold 0.53. The associated AUC is 0.65 (p < 0.023) and the rule has 86% sensitivity and 33% specificity. According to best subset regression analysis, not only glycaemic parameters but also lipid parameters [low-density lipoprotein cholesterol (LDL-C) (p = 0.045) and triglycerides (TG) (p = 0.0005)] were found to be significant predictors of CS. CMT (p = 0.058) was another marginally significant predictor of CS. CS may be used as an early predictive marker for DR. So, not only hyperglycemia, but also hyperlipidemia seems to significantly affect retinal CS function in diabetes.

Microvascular impairment as a biomarker of diabetic retinopathy progression in the long-term follow up in type 1 diabetes

This study aimed to explore differences in vascular and structural parameters using optical coherence tomography angiography in patients with type 1 diabetes (DM1) with mild signs of diabetic retinopathy (DR) over a two-year follow-up period. Parafoveal vessel density (PVD) and foveal avascular zone (FAZ) area were analyzed. The thickness of three predefined retinal slabs was measured, including the inner limiting membrane (ILM)–inner plexiform layer (IPL), IPL–inner nuclear layer (INL), and the IPL–outer nuclear layer (ONL). Twenty-two patients with DM1 and 21 controls were included. There was no significant difference in the FAZ area, perimeter and acircularity index between cohorts over time. Baseline superficial capillary plexus PVD was approximately 10% lower in patients with diabetes than in controls (p = 0.001), and was 12% lower at 2 years (p = 0.002). There was no difference in the annual linear trend between the groups (− 0.5% in diabetics vs. controls, p = 0.736). Baseline deep capillary plexus (DCP) PVD was slightly lower in diabetics than in controls (− 4.4%, p = 0.047) and the difference increased at 2 years (− 12.6%, p < 0.001). The annual linear trend was − 2.7% in diabetic patients compared to controls (p = 0.009)_. In addition, the PVD of the DCP and the intermediate capillary plexus (ICP) were evaluated separately. Regarding the DCP PVD, no statistically significant difference at any time points in diabetic patients compared to controls and no statistically significant difference in the linear trend was found (p > 0.1). Conversely, no difference was recorded for parafoveal ICP density at individual time points (p > 0.1), but a statistically significant difference in the linear trend over time in diabetic patients compared to controls was recoded (− 3.2% per year, p = 0.001). Despite the apparent intergroup differences at baseline in structural OCT parameters, the differences including ILM–IPL (p = 0.273), IPL–INL (p = 0.708), and IPL–ONL (p = 0.054) were modest and not statistically significant with time. Therefore, the microvascular change of the deeper vessels might be a robust biomarker to evaluate the clinical progression of DR in DM1.

Pathological Role of Unsaturated Aldehyde Acrolein in Diabetic Retinopathy

With increasing prevalence of diabetes and a progressively aging society, diabetic retinopathy is emerging as one of the global leading causes of blindness. Recent studies have shown that vascular endothelial growth factor (VEGF) plays a central role in the pathogenesis of diabetic retinopathy and anti-VEGF agents have become the first-line therapy for the vision-threatening disease. However, recent studies have also demonstrated that diabetic retinopathy is a multifactorial disease and that VEGF-independent mechanism(s) also underlie much of the pathological changes in diabetic retinopathy. Acrolein is a highly reactive unsaturated aldehyde and is implicated in protein dysfunction. As acrolein is common in air pollutants, previous studies have focused on it as an exogenous causative factor, for instance, in the development of respiratory diseases. However, it has been discovered that acrolein is also endogenously produced and induces cell toxicity and oxidative stress in the body. In addition, accumulating evidence suggests that acrolein and/or acrolein-conjugated proteins are associated with the molecular mechanisms in diabetic retinopathy. This review summarizes the pathological roles and mechanisms of endogenous acrolein production in the pathogenesis of diabetic retinopathy.

Elevated 4-hydroxynonenal induces hyperglycaemia via Aldh3a1 loss in zebrafish and associates with diabetes progression in humans

Increased methylglyoxal (MG) formation is associated with diabetes and its complications. In zebrafish, knockout of the main MG detoxifying system Glyoxalase 1, led to limited MG elevation but significantly elevated aldehyde dehydrogenases (ALDH) activity and aldh3a1 expression, suggesting the compensatory role of Aldh3a1 in diabetes. To evaluate the function of Aldh3a1 in glucose homeostasis and diabetes, aldh3a1^−/− zebrafish mutants were generated using CRISPR-Cas9. Vasculature and pancreas morphology were analysed by zebrafish transgenic reporter lines. Corresponding reactive carbonyl species (RCS), glucose, transcriptome and metabolomics screenings were performed and ALDH activity was measured for further verification. Aldh3a1^−/− zebrafish larvae displayed retinal vasodilatory alterations, impaired glucose homeostasis, which can be aggravated via pdx1 silencing induced hyperglycaemia. Unexpectedly, MG was not altered, but 4-hydroxynonenal (4-HNE), another prominent lipid peroxidation RCS exhibited high affinity with Aldh3a1, was increased in aldh3a1 mutants. 4-HNE was responsible for the retinal phenotype via pancreas disruption induced hyperglycaemia and can be rescued via l-Carnosine treatment. Furthermore, in type 2 diabetic patients, serum 4-HNE was increased and correlated with disease progression. Thus, our data suggest impaired 4-HNE detoxification and elevated 4-HNE concentration as biomarkers but also the possible inducers for diabetes, from genetic susceptibility to the pathological progression.

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Aldh3a1 mutant was generated using CRISPR/Cas9 and displayed impaired glucose homeostasis.

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Elevated 4-Hydroxynonenal (4-HNE) was responsible for hyperglycaemia in aldh3a1 mutants and was rescued by Carnosine.

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Patient serum 4-HNE level was correlated with HbA1c and fasting glucose.

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Impaired 4-HNE detoxification acts as possible inducers for diabetes, from genetic susceptibility to pathological progress.

An innovative method for screening and evaluating the degree of diabetic retinopathy and drug treatment based on artificial intelligence algorithms

Current methods of evaluating the degree of diabetic retinopathy are highly subjective and have no quantitative standard. To objectively evaluate the slight changes in tissue structure during the early stage of retinal diseases, a subjective interpretation and qualitative analysis of the pathological sections of retinal HE in diabetic animals is required for screening and evaluating the degree of diabetic retinopathy and drug efficacy. To develop an innovative method for screening and evaluating the degree of diabetic retinopathy and drug treatment based on artificial intelligence algorithms. Based on the change law of the early nerve fiber layer and the ganglion cells, we get disparate characteristics of the microscopic image of diabetes animal retina HE slices. Using image recognition and deep learning methods on these HE slices, we can identify the changes in the ganglion cells and nerve fiber layer for diagnosing early retinopathy and evaluated the therapeutic effect of the potential drugs. We conduct quantitative calculation per unit length of the nerve fiber layer and total area of the nerve fiber layer to identify biology significance of edema. Additionally, we also perform quantitative calculation with the number of unit area ganglion cells to identify the section in biology cell hyperplasia. Finally, we get the significance of quantitative calculation on the unit cell area to identify ganglion cell shriveling in biology. In addition to the evaluation of the disease degree and changes, we also obtained retinal HE sections after different drug interventions and evaluated the therapeutic effect of the drugs. This study presents a novel quantitative method for screening and evaluating of diabetic retinopathy and drug efficacy.

Citicoline and Vitamin B12 Eye Drops in Type 1 Diabetes: Results of a 3-year Pilot Study Evaluating Morpho-Functional Retinal Changes

INTRODUCTION

This study aimed to evaluate the effect of treatment with eye drops containing citicoline and vitamin B12 on changes in function of the inner retina, morphology of the inner and outer retina, and microvascular condition in patients with type 1 diabetes (DM1) with mild signs of non-proliferative diabetic retinopathy (NPDR) during 3 years of follow-up.

METHODS

A pilot study with prospective, randomized, and double-masked design was conducted to address the aims. Twenty patients with DM1 were enrolled and randomly divided into two groups: the DC group comprising patients treated with citicoline and vitamin B12 eye drops (10 patients; mean age ± standard deviation, 46.86 ± 8.78 years) and the DP group comprising those treated with placebo (10 patients; mean age ± standard deviation, 47.89 ± 7.74 years). In the DC group, one eye of each patient was treated with citicoline and vitamin B12 eye drops (OMK2®, Omikron Italia srl, Italy, 3 drops/day), while in the DP group, it was treated with placebo (eye drops containing hypromellose 0.3%, 3 drops/day) for a 3-year period. In both groups, Humphrey Matrix frequency doubling technology (FDT), spectral domain optical coherence tomography (SD-OCT) and OCT angiography (OCTA), and adaptive optics (AO) were applied at baseline and 12, 24, and 36 months of the follow-up period.

RESULTS

In the results of follow-up evaluation, the DC and DP groups were significantly different: Significant reduction in function in terms of 10-2 FDT mean sensitivity and in morphology reflected by an increase in inner nuclear layer thickness and decrease in other plexiform layer thickness and foveal vessel density were observed in the DP group, while no such significant changes were observed in the DC group in the long term.

CONCLUSIONS

This pilot study indicated that patients with DM1 with mild signs of diabetic retinopathy (DR) who underwent treatment with citicoline and vitamin B12 eye drops for a 3-year duration achieved stabilization or decreased rate of functional impairment, neuroretinal degeneration, and microvascular damage.

TRIAL REGISTRATION

ClinicalTrials.gov identifier, NCT04009980.

Unsaturated Aldehyde Acrolein Promotes Retinal Glial Cell Migration

Purpose

To investigate the effect of the unsaturated aldehyde acrolein on retinal glial cell migration.

Methods

Müller glial cell markers expression in TR-MUL5 were confirmed by RT-PCR and immunostaining. Cell viability and migration rate of TR-MUL5 cells were assessed after the stimulation with acrolein. DNA microarray analysis was performed to analyze changes in the expression levels of migration-related genes in Müller glial cells stimulated with acrolein. Real-time PCR and ELISA were performed to validate DNA microarray analysis results. Inhibitors of C-X-C motif chemokine ligand 1 (CXCL1), one of the genes highly upregulated after the exposure to acrolein, and blockers of its receptor, CXCR2, were used to investigate the role of the CXCL1-CXCR2 axis on glial cell migration. CXCL1 concentration was measured in vitreous fluid samples obtained from proliferative diabetic retinopathy (PDR) and nondiabetic control eyes. CXCL1 and CXCR2 expression in glial cells of fibrovascular tissues obtained from PDR patients was examined by immunostaining.

Results

At a high concentration, acrolein (100 μM) significantly decreased cell viability. However, in moderate, sublethal concentrations (25-50 μM), acrolein induced cell migration and substantially increased the production of CXCL1 in TR-MUL5 cells. CXCL1 concentration was significantly elevated in vitreous fluids of PDR patients, and CXCL1 and CXCR2 were present in glial cells in fibrovascular tissues of PDR patients. CXCL1 stimulation increased glial cell migration in a dose-dependent manner, which was abrogated by the neutralization of the CXCL1-CXCR2 axis.

Conclusions

Our data demonstrate that acrolein promotes retinal Müller glial cell migration by enhancing CXCL1 production.

The Role of Lipoxidation in the Pathogenesis of Diabetic Retinopathy

Lipids can undergo modification as a result of interaction with reactive oxygen species (ROS). For example, lipid peroxidation results in the production of a wide variety of highly reactive aldehyde species which can drive a range of disease-relevant responses in cells and tissues. Such lipid aldehydes react with nucleophilic groups on macromolecules including phospholipids, nucleic acids, and proteins which, in turn, leads to the formation of reversible or irreversible adducts known as advanced lipoxidation end products (ALEs). In the setting of diabetes, lipid peroxidation and ALE formation has been implicated in the pathogenesis of macro- and microvascular complications. As the most common diabetic complication, retinopathy is one of the leading causes of vision loss and blindness worldwide. Herein, we discuss diabetic retinopathy (DR) as a disease entity and review the current knowledge and experimental data supporting a role for lipid peroxidation and ALE formation in the onset and development of this condition. Potential therapeutic approaches to prevent lipid peroxidation and lipoxidation reactions in the diabetic retina are also considered, including the use of antioxidants, lipid aldehyde scavenging agents and pharmacological and gene therapy approaches for boosting endogenous aldehyde detoxification systems. It is concluded that further research in this area could lead to new strategies to halt the progression of DR before irreversible retinal damage and sight-threatening complications occur.

Age-related changes of the human retinal vessels: Possible involvement of lipid peroxidation

BACKGROUND

Aging of the human retina is accompanied by oxidative stress that exerts profound changes in the retinal neurons. It is unknown if oxidative stress influences the cellular components of the retinal vessels in some ways.

METHODS

We examined changes in retinal vessels in human donor eyes (age: 35-94 years; N=18) by light and transmission electron microscopy, TUNEL and immunohistochemistry for biomarkers of vascular smooth muscle cells (SMC; actin), oxidative stress (4-hydroxy 2-nonenal [HNE] and nitrotyrosine), microglia (Iba-1) and vessels (isolectin B₄).

RESULTS

The earliest changes in the endothelium and pericytes of capillaries are apparent from the seventh decade. With aging, there is clear loss of organelles and cytoplasmic filaments, and a progressive thickening of the endothelial and pericyte basal lamina. Loss of filaments, accumulation of lipofuscin and autophagic vacuoles are significant events in aging pericytes and SMC. Actin immunolabelling reveals discontinuity in arterial SMC layers during eighth decade, indicating partial degeneration of SMC. This is followed by hyalinization, with degeneration of the endothelium and SMC in arteries and arterioles of the nerve fibre layer (NFL) and ganglion cell layer in ninth decade. Iba-1 positive microglia were in close contact with the damaged vessels in inner retina, and their cytoplasm was rich in lysosomes. HNE immunoreactivity, but not of nitrotyrosine, was detected in aged vessels from seventh decade onwards, suggesting that lipid peroxidation is a major problem of aged vessels. However, TUNEL positivity seen during this period was limited to few arteries and venules of NFL.

CONCLUSION

This study shows prominent age-related alterations of the pericytes and SMC of retinal vessels. These changes may limit the energy supply to the neurons and be responsible for age-related loss of neurons of the inner retina.

The combination of loss of glyoxalase1 and obesity results in hyperglycemia

The increased formation of methylglyoxal (MG) under hyperglycemia is associated with the development of microvascular complications in patients with diabetes mellitus; however, the effects of elevated MG levels in vivo are poorly understood. In zebrafish, a transient knockdown of glyoxalase 1, the main MG detoxifying system, led to the elevation of endogenous MG levels and blood vessel alterations. To evaluate effects of a permanent knockout of glyoxalase 1 in vivo, glo1-/- zebrafish mutants were generated using CRISPR/Cas9. In addition, a diet-induced-obesity zebrafish model was used to analyze glo1-/- zebrafish under high nutrient intake. Glo1-/- zebrafish survived until adulthood without growth deficit and showed increased tissue MG concentrations. Impaired glucose tolerance developed in adult glo1-/- zebrafish and was indicated by increased postprandial blood glucose levels and postprandial S6 kinase activation. Challenged by an overfeeding period, fasting blood glucose levels in glo1-/- zebrafish were increased which translated into retinal blood vessel alterations. Thus, the data have identified a defective MG detoxification as a metabolic prerequisite and glyoxalase 1 alterations as a genetic susceptibility to the development of type 2 diabetes mellitus under high nutrition intake.

Pyridoxamine Treatment of HK-2 Human Proximal Tubular Epithelial Cells Reduces Oxidative Stress and the Inhibition of Autophagy Induced by High Glucose Levels

Background

Diabetic nephropathy is a predominant cause of renal failure, which is an important chronic complication of diabetes. Pyridoxamine (PM) has been reported to protect renal tubular epithelial cells against oxidative damage and delay or inhibit the development and generation of glucose-induced renal insufficiency at the early stage of disease. In this study, we attempted to explore the protection mechanism of PM on human proximal tubular epithelial cells (HK-2 cells) induced by high glucose.

Material/Methods

HK-2 cells were cultivated by high glucose medium in the absence or presence of PM. Cell Counting Kit-8 was used to investigate the most appropriate drug concentration of PM by detecting the cell viability of HK-2 cells. The expression of autophagy-related protein Beclin-1, LC-3II, and p62 was measured by western blot analysis, reverse transcription-quantitative polymerase chain reaction (RT-qPCR), and immunofluorescence. The expression and localization of Beclin-1 and p62 were also detected via immunofluorescence. The intracellular reactive oxygen species generation was detected using the reactive oxygen species assay kit. The effects of PM on antioxidant defenses were evaluated with glutathione peroxidase (GPx), manganese superoxide dismutase (MnSOD) activity, and glutathione/glutathione disulfide (GSH/GSSG) ratio.

Results

High glucose levels were able to upregulate the expression of oxidative stress associated protein and inhibit autophagy-associated changes verified by western blotting, RT-qPCR and immunofluorescence. Administration of PM reversed the high glucose-induced low-expressed Beclin-1 and LC-3II, and overexpressed p62 and intracellular reactive oxygen species levels. Furthermore, non-enzymatic antioxidant defenses and enzymatic antioxidant defenses were turned on by the application of PM.

Conclusions

Treatment with PM could reverse high glucose-induced inhibition of autophagy and oxidative stress.

Too sweet: Problems of protein glycation in the eye

Laboratory and epidemiological data indicate that high blood sugar levels and/or consuming high glycemia diets are linked to multiple age-related diseases, including age-related macular degeneration, cataract, Parkinson's disease, Alzheimer's disease, diabetic retinopathy, and, apparently glaucoma. High concentrations of blood sugar and perturbations of the systems that regulate blood sugar lead to the accumulation of advanced-glycation end products (AGEs). AGEs are toxic compounds that are formed from the combination of sugars and their metabolites with biomolecules in a non-enzymatic biochemical reaction called glycation. In vitro and in vivo data indicate that high sugar consumption is associated with accumulation of AGEs in a variety of human tissues. Hyperglycemia, along with an oxidative environment and limited cell proliferation in many ocular tissues, encourages formation and precludes dilution of AGEs and associated damage by cell division. These circumstances make many eye tissues vulnerable to glycation-derived damage. Here, we summarize research regarding glycation-induced ocular tissue dysfunction and its contribution to the onset and development of eye disorders. We also discuss how management of carbohydrate nutrition may provide a low-cost way to ameliorate the progression of AGEs-related diseases, including age related macular degeneration and some cataracts, as they do for cardiovascular disease and diabetes.

Müller glial dysfunction during diabetic retinopathy in rats is reduced by the acrolein-scavenging drug, 2-hydrazino-4,6-dimethylpyrimidine

AIMS/HYPOTHESIS

Recent studies suggest that abnormal function in Müller glial cells plays an important role in the pathogenesis of diabetic retinopathy. This is associated with the selective accumulation of the acrolein-derived advanced lipoxidation end-product, N^ε-(3-formyl-3,4-dehydropiperidino)lysine (FDP-lysine), on Müller cell proteins. The aim of the current study was to identify more efficacious acrolein-scavenging drugs and determine the effects of the most potent on Müller cell FDP-lysine accumulation and neuroretinal dysfunction during diabetes.

METHODS

An ELISA-based in vitro assay was optimised to compare the acrolein-scavenging abilities of a range of drugs. This identified 2-hydrazino-4,6-dimethylpyrimidine (2-HDP) as a new and potent acrolein scavenger. The ability of this agent to modify the development of diabetic retinopathy was tested in vivo. Male Sprague Dawley rats were divided into three groups: (1) non-diabetic; (2) streptozotocin-induced diabetic; and (3) diabetic treated with 2-HDP in their drinking water for the duration of diabetes. Liquid chromatography high-resolution mass spectrometry was used to detect 2-HDP reaction products in the retina. Immunohistochemistry, real-time quantitative (q)RT-PCR and electroretinography were used to assess retinal changes 3 months after diabetes induction.

RESULTS

2-HDP was the most potent of six acrolein-scavenging agents tested in vitro (p < 0.05). In vivo, administration of 2-HDP reduced Müller cell accumulation of FDP-lysine at 3 months in rats rendered diabetic with streptozotocin (p < 0.001). A 2-HDP adduct was identified in the retinas of diabetic animals treated with this compound. 2-HDP supplementation was associated with reduced Müller cell gliosis (p < 0.05), reduced expression of the oxidative stress marker haem oxygenase-1 (p < 0.001) and partial normalisation of inwardly rectifying K⁺ channel 4.1 (Kir4.1) expression (p < 0.001 for staining in perivascular regions and the innermost region of the ganglion cell layer). Diabetes-induced retinal expression of inflammatory markers, inflammatory signalling compounds and activation of retinal microglial cells were all reduced in 2-HDP-treated animals. Retinal neurophysiological defects in diabetic animals, as indicated by changes in the electroretinogram 7 weeks after induction of diabetes, were also reduced by 2-HDP (p < 0.05-0.01 for b-wave amplitudes at flash intensities from -10 to +10 dB; p < 0.01 for time to peak of summed oscillatory potentials at +10 dB).

CONCLUSIONS/INTERPRETATION

These findings support the hypothesis that Müller cell accumulation of FDP-lysine plays an important role in the development of diabetic retinopathy. Our results also suggest that 2-HDP may have therapeutic potential for delaying or treating this sight-threatening complication.

Methylglyoxal induces retinopathy-type lesions in the absence of hyperglycemia: studies in a rat model

The aim of this study was to evaluate whether damage to the neurovascular unit in diabetes depends on reactive metabolites such as methylglyoxal (MG), and to assess its impact on retinal gene expression. Male Wistar rats were supplied with MG (50 mM) by drinking water and compared with age-matched streptozotocin-diabetic animals and untreated controls. Retinal damage was evaluated for the accumulation of MG-derived advanced glycation end products, changes in hexosamine and PKC pathway activation, microglial activation, vascular alterations (pericyte loss and vasoregression), neuroretinal function assessed by electroretinogram, and neurodegeneration. Retinal gene regulation was studied by microarray analysis, and transcription factor involvement was identified by upstream regulator analysis. Systemic application of MG by drinking water increased retinal MG to levels comparable with diabetic animals. Elevated retinal MG resulted in MG-derived hydroimidazolone modifications in the ganglion cell layer, inner nuclear layer, and outer nuclear layer, a moderate activation of the hexosamine pathway, a pan-retinal activation of microglia, loss of pericytes, increased formation of acellular capillaries, decreased function of bipolar cells, and increased expression of the crystallin gene family. MG mimics important aspects of diabetic retinopathy and plays a pathogenic role in microglial activation, vascular damage, and neuroretinal dysfunction. In response to MG, the retina induces expression of neuroprotective crystallins.-Schlotterer, A., Kolibabka, M., Lin, J., Acunman, K., Dietrich, N., Sticht, C., Fleming, T., Nawroth, P., Hammes, H.-P. Methylglyoxal induces retinopathy-type lesions in the absence of hyperglycemia: studies in a rat model.

Glycation Damage: A Possible Hub for Major Pathophysiological Disorders and Aging

Glycation is both a physiological and pathological process which mainly affects proteins, nucleic acids and lipids. Exogenous and endogenous glycation produces deleterious reactions that take place principally in the extracellular matrix environment or within the cell cytosol and organelles. Advanced glycation end product (AGE) formation begins by the non-enzymatic glycation of free amino groups by sugars and aldehydes which leads to a succession of rearrangements of intermediate compounds and ultimately to irreversibly bound products known as AGEs. Epigenetic factors, oxidative stress, UV and nutrition are important causes of the accumulation of chemically and structurally different AGEs with various biological reactivities. Cross-linked proteins, deriving from the glycation process, present both an altered structure and function. Nucleotides and lipids are particularly vulnerable targets which can in turn favor DNA mutation or a decrease in cell membrane integrity and associated biological pathways respectively. In mitochondria, the consequences of glycation can alter bioenergy production. Under physiological conditions, anti-glycation defenses are sufficient, with proteasomes preventing accumulation of glycated proteins, while lipid turnover clears glycated products and nucleotide excision repair removes glycated nucleotides. If this does not occur, glycation damage accumulates, and pathologies may develop. Glycation-induced biological products are known to be mainly associated with aging, neurodegenerative disorders, diabetes and its complications, atherosclerosis, renal failure, immunological changes, retinopathy, skin photoaging, osteoporosis, and progression of some tumors.

Impact of Non-Enzymatic Glycation in Neurodegenerative Diseases: Role of Natural Products in Prevention

Non-enzymatic protein glycosylation is the addition of free carbonyls to the free amino groups of proteins, amino acids, lipoproteins and nucleic acids resulting in the formation of early glycation products. The early glycation products are also known as Maillard reaction which undergoes dehydration, cyclization and rearrangement to form advanced glycation end-products (AGEs). By and large the researchers in the past have also established that glycation and the AGEs are responsible for most type of metabolic disorders, including diabetes mellitus, cancer, neurological disorders and aging. The amassing of AGEs in the tissues of neurodegenerative diseases shows its involvement in diseases. Therefore, it is likely that inhibition of glycation reaction may extend the lifespan of an individual. The hunt for inhibitors of glycation, mainly using in vitro models, has identified natural compounds able to prevent glycation, especially polyphenols and other natural antioxidants. Extrapolation of results of in vitro studies on the in vivo situation is not straightforward due to differences in the conditions and mechanism of glycation, and bioavailability problems. Nevertheless, existing data allow postulating that enrichment of diet in natural anti-glycating agents may attenuate glycation and, in consequence may halt the aging and neurological problems.

The effect of fenofibrate on early retinal nerve fiber layer loss in type 2 diabetic patients: a case-control study

BACKGROUND

Previous studies suggested that use of fenofibrate could significantly reduce the rate of progression into diabetic retinopathy (DR), and that retinal nerve fiber layer (RNFL) loss, which has been considered an important indicator for retinal neurodegeneration, might precede microvascular changes. The aim of this study was to assess the effect(s) of fenofibrate on RNFL thickness at early stage of DR in patients with type 2 diabetes mellitus (DM).

METHODS

In this retrospective matched case-control study we included a cohort of 89 patients with type 2 DM, aged 40 or above, between Jan 1, 2017 and March 31, 2017. Among the subjects, 48 patients received fenofibrate therapy and the other 41 patients did not receive fenofibrate treatment. We defined use of fenofibrate as the presence of any prescription for fenofibrate within 1 year before or any time after the diagnosis of DM, and all the patients had either no DR or non-proliferative diabetic retinopathy (NPDR). The fibrate users were well matched with non-fenofibrate users for gender, age and axial length. The RNFL thickness in all quadrants of both eyes was examined with spectral domain optical coherence tomography (SD-OCT). The multiple linear regression analysis was used to assess the association of RNFL thickness with potential risk factors of DR other than fenofibrate use.

RESULTS

The non-fenofibrate users had significantly reduced RNFL thickness of the superior quadrant of the right eye compared to the fenofibrate users (t = 2.384, P = 0.019). On the contrary, BMI (p = 0.034) and ACR (p = 0.024) were both negatively correlated to the RNFL thickness of the right eye.

CONCLUSION

Oral administration of fenofibrate was suggestively associated with thicker RNFL in superior quadrant of the right eye of patents with early DR.

Advanced glycation end (AGE) product modification of laminin downregulates Kir4.1 in retinal Müller cells

Diabetic retinopathy (DR) is a major cause of adult blindness. Retinal Müller cells maintain water homeostasis and potassium concentration via inwardly rectifying Kir4.1 channels. Accumulation of advanced glycation end products (AGEs) is a major pathologic event in DR. While diabetes leads to a decrease in the Kir4.1 channels, it remains unknown whether AGEs-linked to the basement membrane (BM) affect normal Kir4.1 channels. For this study, we hypothesized that AGE-modification of laminin is detrimental to Kir4.1 channels, therefore, disrupting Müller cell function. The AGE-modified laminin-coated substrates were prepared by incubating Petri-dishes with laminin and methylglyoxal for seven days. The rat Müller cells (rMC-1) were propagated on AGE-modified laminin, and Kir4.1 expression and function were evaluated. Quantification of AGEs using ELISA revealed a dose-dependent increase in methylglyoxal-hydro-imidazolone adducts. The rMC-1 propagated on AGE-modified laminin demonstrated a decrease in Kir4.1 levels in immunofluorescence and western blot studies and a decrease in the Kir4.1 channel function. Kir4.1 decrease on AGE-modified laminin resulted in a disorganization of an actin cytoskeleton and disruption of α-dystroglycan-syntrophin-dystrophin complexes. Our studies suggest that AGE-modification of laminin is detrimental to Kir4.1 channels. By studying the role of AGEs in Kir4.1 channels we have identified a novel mechanism of Müller cell dysfunction and its subsequent involvement in DR.

Mesenchymal marker expression is elevated in Müller cells exposed to high glucose and in animal models of diabetic retinopathy

Müller cells are retinal glial cells and exhibit a fibroblast-like phenotype and ability to migrate in diabetic retinopathy (DR). However, expression of mesenchymal markers, which promote fibrosis in various organs, has not been characterized in the diabetic retina. We examined changes in the expression of these markers in Müller cells exposed to high glucose and in animal models of diabetic retinopathy. High glucose conditions increased mesenchymal maker expression and migration in Müller cells. Snail, N-cadherin, Vimentin, β-catenin, and α-smooth muscle actin (α-SMA) levels were all dramatically increased in retinas from humans with diabetic retinopathy (DR) and from DR mouse models. In addition, Snail overexpression increased the expression of connective tissue growth factor (CTGF) and fibronectin, while Snail knockdown attenuated high glucose-induced increases in fibronectin and CTGF expression. These results demonstrate for the first time that mesenchymal markers are upregulated in retinas from a diabetic mouse model, and that Snail and N-cadherin levels are also increased in Müller cells exposed to high glucose. This suggests mesenchymal proteins may play a crucial role in the development of DR.

The Role of Microglia in Diabetic Retinopathy: Inflammation, Microvasculature Defects and Neurodegeneration

Diabetic retinopathy is a common complication of diabetes mellitus, which appears in one third of all diabetic patients and is a prominent cause of vision loss. First discovered as a microvascular disease, intensive research in the field identified inflammation and neurodegeneration to be part of diabetic retinopathy. Microglia, the resident monocytes of the retina, are activated due to a complex interplay between the different cell types of the retina and diverse pathological pathways. The trigger for developing diabetic retinopathy is diabetes-induced hyperglycemia, accompanied by leukostasis and vascular leakages. Transcriptional changes in activated microglia, mediated via the nuclear factor kappa-light-chain-enhancer of activated B cells (NFκB) and extracellular signal–regulated kinase (ERK) signaling pathways, results in release of various pro-inflammatory mediators, including cytokines, chemokines, caspases and glutamate. Activated microglia additionally increased proliferation and migration. Among other consequences, these changes in microglia severely affected retinal neurons, causing increased apoptosis and subsequent thinning of the nerve fiber layer, resulting in visual loss. New potential therapeutics need to interfere with these diabetic complications even before changes in the retina are diagnosed, to prevent neuronal apoptosis and blindness in patients.

Mechanisms of macular edema: Beyond the surface

Macular edema consists of intra- or subretinal fluid accumulation in the macular region. It occurs during the course of numerous retinal disorders and can cause severe impairment of central vision. Major causes of macular edema include diabetes, branch and central retinal vein occlusion, choroidal neovascularization, posterior uveitis, postoperative inflammation and central serous chorioretinopathy. The healthy retina is maintained in a relatively dehydrated, transparent state compatible with optimal light transmission by multiple active and passive systems. Fluid accumulation results from an imbalance between processes governing fluid entry and exit, and is driven by Starling equation when inner or outer blood-retinal barriers are disrupted. The multiple and intricate mechanisms involved in retinal hydro-ionic homeostasis, their molecular and cellular basis, and how their deregulation lead to retinal edema, are addressed in this review. Analyzing the distribution of junction proteins and water channels in the human macula, several hypotheses are raised to explain why edema forms specifically in the macular region. "Pure" clinical phenotypes of macular edema, that result presumably from a single causative mechanism, are detailed. Finally, diabetic macular edema is investigated, as a complex multifactorial pathogenic example. This comprehensive review on the current understanding of macular edema and its mechanisms opens perspectives to identify new preventive and therapeutic strategies for this sight-threatening condition.

Müller cells and diabetic retinopathy

Müller cells are one of the primary glial cell types found in the retina and play a significant role in maintaining retinal function and health. Since Müller cells are the only cell type to span the entire width of the retina and have contact to almost every cell type in the retina they are uniquely positioned to perform a wide variety of functions necessary to maintaining retinal homeostasis. In the healthy retina, Müller cells recycle neurotransmitters, prevent glutamate toxicity, redistribute ions by spatial buffering, participate in the retinoid cycle, and regulate nutrient supplies by multiple mechanisms. Any disturbance to the retinal environment is going to influence proper Müller cell function and well being which in turn will affect the entire retina. This is evident in a disease like diabetic retinopathy where Müller cells contribute to neuronal dysfunction, the production of pro-angiogenic factors leading to neovascularization, the set up of a chronic inflammatory retinal environment, and eventual cell death. In this review, we highlight the importance of Müller cells in maintaining a healthy and functioning retina and discuss various pathological events of diabetic retinopathy in which Müller cells seem to play a crucial role. The beneficial and detrimental effects of cytokine and growth factor production by Müller cells on the microvasculature and retinal neuronal tissue will be outlined. Understanding Müller cell functions within the retina and restoring such function in diabetic retinopathy should become a cornerstone for developing effective therapies to treat diabetic retinopathy.

Soluble Vascular Adhesion Protein-1 Mediates Spermine Oxidation as Semicarbazide-Sensitive Amine Oxidase: Possible Role in Proliferative Diabetic Retinopathy

Purpose/Aim of the study: To explore the possible role of vascular adhesion protein-1 (VAP-1) via its enzymatic function as a semicarbazide-sensitive amine oxidase (SSAO) in the pathogenesis of proliferative diabetic retinopathy (PDR).

MATERIALS AND METHODS

The levels of soluble VAP-1/SSAO and the unsaturated aldehyde acrolein (ACR)-conjugated protein, N^ε-(3-formyl-3, 4-dehydropiperidino) lysine adduct (FDP-Lys), were measured in vitreous fluid samples of PDR and non-diabetic patients using ELISA. Recombinant human VAP-1/SSAO (rhVAP-1/SSAO) was incubated with spermine, with or without semicarbazide or RTU-1096 (a specific inhibitor for VAP-1/SSAO). Immunofluorescence assays were performed to assess the localization of VAP-1/SSAO and FDP-Lys in fibrovascular tissues from patients with PDR. The impact of ACR on cultured retinal capillary endothelial cells was assessed using a cell viability assay and total glutathione (GSH) measurements.

RESULTS

The levels of sVAP-1/SSAO and FDP-Lys were elevated in the vitreous fluid of patients with PDR. Incubation of rhVAP-1 with spermine resulted in the generation of hydrogen peroxide and FDP-Lys and the production was inhibited by semicarbazide and RTU-1096. In fibrovascular tissues, FDP-Lys and VAP-1/SSAO were present in endothelial cells. ACR stimulation reduced GSH levels in the cultured endothelial cells in a dose-dependent manner and caused cellular toxicity.

CONCLUSIONS

Our results indicate the pathological role of sVAP-1/SSAO to generate hydrogen peroxide and toxic aldehyde ACR, both of which are associated with oxidative stress, as a consequence of spermine oxidation in eyes with PDR.

Early Functional and Morphologic Abnormalities in the Diabetic Nyxnob Mouse Retina

Purpose

The electroretinogram c-wave is generated by the summation of the positive polarity hyperpolarization of the apical RPE membrane and a negative polarity slow PIII response of Müller glia cells. Therefore, the c-wave reduction noted in prior studies of mouse models of diabetes could reflect a reduction in the RPE component or an increase in slow PIII. The present study used a genetic approach to distinguish between these two alternatives.

Methods

Nyx^nob mice lack the ERG b-wave, revealing the early phase of slow PIII. To visualize changes in slow PIII due to diabetes, Nyx^nob mice were given streptozotocin (STZ) injections to induce diabetes or received vehicle as a control. After 1, 2, and 4 weeks of sustained hyperglycemia (>250 mg/dL), standard strobe flash ERG and dc-ERG testing were conducted. Histological analysis of the retina was performed.

Results

A reduced c-wave was noted at the 1 week time point, and persisted at later time points. In comparison, slow PIII amplitudes were unaffected after 1 week of hyperglycemia, but were significantly reduced in STZ mice at the 2-week time point. The decrease in amplitude occurred before any identifiable decrease to the a-wave. At the later time point, the a-wave became involved, although the slow PIII reductions were more pronounced. Morphological abnormalities in the RPE, including increased thickness and altered melanosome distribution, were identified in diabetic animals.

Conclusions

Because the c-wave and slow PIII were both reduced, these results demonstrated that diabetes-induced reductions to the c-wave cannot be attributed to an early increase in the Müller glia-derived potassium conductance. Furthermore, because the a-wave, slow PIII and c-wave reductions were not equivalent, and varied in their onset, the reductions cannot reflect the same mechanism, such as a change in membrane resistance. The presence of small changes to RPE architecture indicate that the c-wave reductions present in diabetic mice likely represents a primary change in the RPE induced by hyperglycemia.

The pathology associated with diabetic retinopathy

This review summarizes the pathological features of diabetic retinopathy. The lesions occurring in the diabetic retina have been described over many decades using descriptive and experimental approaches based on clinical studies on patients, human post-mortem material, animal models and various in vitro systems. We have also accumulated a wealth of knowledge about basic molecular mechanisms and key pathogenic processes that drive these abnormalities in diabetic retina. Despite these advances, there are still limited therapeutic options for diabetic retinopathy with those currently available only addressing late-stage disease. With a particular focus on the earlier stages of diabetes, there is growing appreciation the complex neuronal, glial and microvascular abnormalities which progressively disrupt retinal function. This is especially true from the perspective of the neurovascular unit during health and disease. Based on a strong appreciation of cellular and molecular pathology that underpins diabetic retinopathy, further advances are anticipated as we drive towards development of efficacious therapeutic options that can address all stages of disease.