The influence of diabetes on glutamate metabolism in retinas

Branched-Chain Amino Acids Metabolism and Their Roles in Retinopathy: From Relevance to Mechanism

Retinopathy is one of the leading causes of irreversible blindness and vision loss worldwide. Imbalanced nutrients play important roles in the pathogenesis and pathophysiology of retinal diseases. Branched-Chain Amino Acids (BCAAs), as essential amino acids, perform a variety of biological functions, including protein synthesis, glucose metabolism, lipid metabolism, inflammation, and oxidative stress in metabolic tissues of diabetes and aging-related diseases. Recently, it has been shown that BCAAs are highly related to neuroprotection, oxidative stress, inflammatory and glutamate toxicity in the retina of retinopathy. Therefore, this review summarizes the alterations of BCAA levels in retinopathy, especially diabetic retinopathy and aging-related macular disease, and the genetics, functions, and mechanisms of BCAAs in the retina as well as other metabolic tissues for reference. All of these efforts aim to provide fundamental knowledge of BCAAs for further discoveries and research on retina health based on the sensing and signaling of essential amino acids.

Metabolic and proteomic indications of diabetes progression in human aqueous humor

Diabetes mellitus is a multiorgan systemic disease impacting numerous ocular structures that results in significant ocular morbidity and often results in more frequent corneal and glaucoma surgeries for affected individuals. We hypothesize that the systemic metabolic and proteomic derangement observed in the progression of diabetes influences the composition of the aqueous humor (AH), which ultimately impacts the anterior segment health of the eye. To identify changes associated with diabetes progression, we mapped the metabolite profile and proteome of AH samples from patients with varying severities of type II diabetes (T2DM). Patients were classified as nondiabetic (ND or control), non-insulin-dependent diabetic without advanced features of disease (NAD-ni), insulin-dependent diabetic without advanced features (NAD-i), or diabetic with advanced features (AD). AH samples collected from the anterior chamber during elective ophthalmic surgery were evaluated for metabolite and protein expression changes associated with diabetic severity via gas chromatography/mass spectrometry and ultra-high performance liquid chromatography tandem mass spectrometry, respectively. Metabolic and proteomic pathway analyses were conducted utilizing MetaboAnalyst 4.0 and Ingenuity Pathway Analysis. A total of 14 control, 12 NAD-ni, 4 NAD-I, and 14 AD samples were included for analysis. Elevated levels of several branched amino acids (e.g., valine, leucine, isoleucine), and lipid metabolites (e.g., palmitate) were found only with increasing diabetic severity (i.e., the AD group). Similar proteomic trends were noted in amino acid and fatty acid metabolism and the unfolded protein/stress response. These results represent the first report of both metabolomic and proteomic evaluation of aqueous humor. Diabetes results in metabolic and proteomic perturbations detectable in the AH, and unique changes become manifest as T2DM severity worsens. Changes in AH composition may serve as an indicator of disease severity, risk assessment of anterior segment cells and structures, and potential future therapies.

Proinflammatory Cytokines Trigger the Onset of Retinal Abnormalities and Metabolic Dysregulation in a Hyperglycemic Mouse Model

Purpose

Recent evidence has shown that retinal inflammation is a key player in diabetic retinopathy (DR) pathogenesis. To further understand and validate the metabolic biomarkers of DR, we investigated the effect of intravitreal proinflammatory cytokines on the retinal structure, function, and metabolism in an in vivo hyperglycemic mouse model.

Methods

C57Bl/6 mice were rendered hyperglycemic within one week of administration of a single high-dose intraperitoneal injection of streptozotocin, while control mice received vehicle injection. After confirming hyperglycemia, the mice received an intravitreal injection of either proinflammatory cytokines (TNF-α and IL-1β) or vehicle. Similarly, control mice received an intravitreal injection of either proinflammatory cytokines or vehicle. The retinal structure was evaluated using fundus imaging and optical coherence tomography, and retinal function was assessed using a focal electroretinogram (ERG), two days after cytokine injection. Retinas were collected for biochemical analysis to determine key metabolite levels and enzymatic activities.

Results

Hyperglycemic mice intraocularly injected with cytokines developed visible retinal vascular damage and intravitreal and intraretinal hyper-reflective spots two days after the cytokines injection. These mice also developed a significant functional deficit with reduced a-wave and b-wave amplitudes of the ERG at high light intensities compared to control mice. Furthermore, metabolic disruption was evident in these mice, with significantly higher retinal glucose, lactate, ATP, and glutamine levels and a significant reduction in glutamate levels compared with control mice. Minimal or no metabolic changes were observed in hyperglycemic mice without intraocular cytokines or in control mice with intraocular cytokines at 2 days post hyperglycemia.

Conclusions

Proinflammatory cytokines accelerated the development of vascular damage in the eyes of hyperglycemic mice. Significant changes were observed in retinal structure, function, and metabolic homeostasis. These findings support the idea that with the onset of inflammation in DR, there is a deficit in metabolism. Therefore, early intervention to prevent inflammation-induced retinal changes in diabetic patients may improve the disease outcome.

Are we overlooking the neurodegeneration in the early stage of Type 1 Diabetes Mellitus without Visual Impairment or Diabetic Retinopathy: is it probably occurred before retinal vasculature dysfunction?

PURPOSE

Using electrophysiology (ERG) to investigate the early alterations of retinal function in diabetic children and adolescents without diabetic retinopathy (DR) or visual impairment (VI).

METHODS

We recorded and compared the data of full-field flicker ERGs between 59 normal subjects and 60 children and adolescents with type 1 diabetes mellitus (T1DM) from the Children's Hospital of Fudan University in Shanghai.

RESULTS

In both groups, patients with diabetes and healthy controls were matched for age, gender, weight, height, BMI, intraocular pressure (IOP), and best-corrected visual acuity (BCVA). Among the parameters of the outcomes of ERG, the implicit time in eyes in DM patients was significantly prolonged compared to normal eyes (p = .008, 16 Td-s; p = .000, 32 Td-s). In the case group, we found significantly positive correlation between implicit time and BMI (p < .05), as well as implicit time and axial length (AL).

CONCLUSIONS

The study reveals that the dysfunction of retina in DM children can be detected with ERGs. It also shows that hyperglycemia has an impact on the occurrence of neurodegeneration in the early stage of DM.

Implications of Diabetes-Induced Altered Metabolites on Retinal Neurodegeneration

Diabetic retinopathy (DR) is one of the major complications of diabetic eye diseases, causing vision loss and blindness worldwide. The concept of diabetic retinopathy has evolved from microvascular disease into more complex neurovascular disorders. Early in the disease progression of diabetes, the neuronal and glial cells are compromised before any microvascular abnormalities clinically detected by the ophthalmoscopic examination. This implies understanding the pathophysiological mechanisms at the early stage of disease progression especially due to diabetes-induced metabolic alterations to damage the neural retina so that early intervention and treatments options can be identified to prevent and inhibit the progression of DR. Hyperglycemia has been widely considered the major contributor to the progression of the retinal damage, even though tight control of glucose does not seem to have a bigger effect on the incidence or progression of retinal damage that leads to DR. Emerging evidence suggests that besides diabetes-induced hyperglycemia, dyslipidemia and amino acid defects might be a major contributor to the progression of early neurovascular retinal damage. In this review, we have discussed recent advances in the alterations of key metabolites of carbohydrate, lipid, and amino acids and their implications for neurovascular damage in DR.

Retinal Glutamate Neurotransmission: From Physiology to Pathophysiological Mechanisms of Retinal Ganglion Cell Degeneration

Glutamate neurotransmission and metabolism are finely modulated by the retinal network, where the efficient processing of visual information is shaped by the differential distribution and composition of glutamate receptors and transporters. However, disturbances in glutamate homeostasis can result in glutamate excitotoxicity, a major initiating factor of common neurodegenerative diseases. Within the retina, glutamate excitotoxicity can impair visual transmission by initiating degeneration of neuronal populations, including retinal ganglion cells (RGCs). The vulnerability of RGCs is observed not just as a result of retinal diseases but has also been ascribed to other common neurodegenerative and peripheral diseases. In this review, we describe the vulnerability of RGCs to glutamate excitotoxicity and the contribution of different glutamate receptors and transporters to this. In particular, we focus on the N-methyl-d-aspartate (NMDA) receptor as the major effector of glutamate-induced mechanisms of neurodegeneration, including impairment of calcium homeostasis, changes in gene expression and signalling, and mitochondrial dysfunction, as well as the role of endoplasmic reticular stress. Due to recent developments in the search for modulators of NMDA receptor signalling, novel neuroprotective strategies may be on the horizon.

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.

Glyceraldehyde-3-phosphate dehydrogenase and glutamine synthetase inhibition in the presence of pro-inflammatory cytokines contribute to the metabolic imbalance of diabetic retinopathy

Diabetic retinopathy (DR) is the leading cause of vision impairment in working age adults. In addition to hyperglycemia, retinal inflammation is an important driving factor for DR development. Although DR is clinically described as diabetes-induced damage to the retinal blood vessels, several studies have reported that metabolic dysregulation occurs in the retina prior to the development of microvascular damage. The two most commonly affected metabolic pathways in diabetic conditions are glycolysis and the glutamate pathway. We investigated the role of glyceraldehyde 3-phosphate dehydrogenase (GAPDH) and glutamine synthetase (GS) in an in-vitro model of DR incorporating high glucose and pro-inflammatory cytokines. We found that GAPDH and GS enzyme activity were not significantly affected in hyperglycemic conditions or after exposure to cytokines alone, but were significantly decreased in the DR model. This confirmed that pro-inflammatory cytokines IL-1β and TNFα enhance the hyperglycemic metabolic deficit. We further investigated metabolite and amino acid levels after specific pharmacological inhibition of GAPDH or GS in the absence/presence of pro-inflammatory cytokines. The results indicate that GAPDH inhibition increased glucose and addition of cytokines increased lactate and ATP levels and reduced glutamate levels. GS inhibition did not alter retinal metabolite levels but the addition of cytokines increased ATP levels and caused glutamate accumulation in Müller cells. We conclude that it is the action of pro-inflammatory cytokines concomitantly with the inhibition of the glycolytic or GS mediated glutamate recycling that contribute to metabolic dysregulation in DR. Therefore, in the absence of good glycemic control, therapeutic interventions aimed at regulating inflammation may prevent the onset of early metabolic imbalance in DR.

PPARα Modulation-Based Therapy in Central Nervous System Diseases

The burden of neurodegenerative diseases in the central nervous system (CNS) is increasing globally. There are various risk factors for the development and progression of CNS diseases, such as inflammatory responses and metabolic derangements. Thus, curing CNS diseases requires the modulation of damaging signaling pathways through a multitude of mechanisms. Peroxisome proliferator-activated receptors (PPARs) are a family of nuclear hormone receptors (PPARα, PPARβ/δ, and PPARγ), and they work as master sensors and modulators of cellular metabolism. In this regard, PPARs have recently been suggested as promising therapeutic targets for suppressing the development of CNS diseases and their progressions. While the therapeutic role of PPARγ modulation in CNS diseases has been well reviewed, the role of PPARα modulation in these diseases has not been comprehensively summarized. The current review focuses on the therapeutic roles of PPARα modulation in CNS diseases, including those affecting the brain, spinal cord, and eye, with recent advances. Our review will enable more comprehensive therapeutic approaches to modulate PPARα for the prevention of and protection from various CNS diseases.

Does Hyperglycemia Cause Oxidative Stress in the Diabetic Rat Retina?

Diabetes, being a metabolic disease dysregulates a large number of metabolites and factors. However, among those altered metabolites, hyperglycemia is considered as the major factor to cause an increase in oxidative stress that initiates the pathophysiology of retinal damage leading to diabetic retinopathy. Diabetes-induced oxidative stress in the diabetic retina and its damaging effects are well known, but still, the exact source and the mechanism of hyperglycemia-induced reactive oxygen species (ROS) generation especially through mitochondria remains uncertain. In this study, we analyzed precisely the generation of ROS and the antioxidant capacity of enzymes in a real-time situation under ex vivo and in vivo conditions in the control and streptozotocin-induced diabetic rat retinas. We also measured the rate of flux through the citric acid cycle by determining the oxidation of glucose to CO2 and glutamate, under ex vivo conditions in the control and diabetic retinas. Measurements of H2O2 clearance from the ex vivo control and diabetic retinas indicated that activities of mitochondrial antioxidant enzymes are intact in the diabetic retina. Short-term hyperglycemia seems to influence a decrease in ROS generation in the diabetic retina compared to controls, which is also correlated with a decreased oxidation rate of glucose in the diabetic retina. However, an increase in the formation of ROS was observed in the diabetic retinas compared to controls under in vivo conditions. Thus, our results suggest of diabetes/hyperglycemia-induced non-mitochondrial sources may serve as major sources of ROS generation in the diabetic retina as opposed to widely believed hyperglycemia-induced mitochondrial sources of excess ROS. Therefore, hyperglycemia per se may not cause an increase in oxidative stress, especially through mitochondria to damage the retina as in the case of diabetic retinopathy.

Pemafibrate prevents retinal neuronal cell death in NMDA-induced excitotoxicity via inhibition of p-c-Jun expression

Excitotoxicity is involved in the retinal neuronal cell death in diabetic retinopathy. Although fenofibrate has been shown to ameliorate the progression of diabetic retinopathy, the effect of pemafibrate, which is highly selective for peroxisome proliferator-activated receptor α on retinal neuronal cell death has not been documented. Here, we investigated whether pemafibrate exerts a beneficial effect against retinal ganglion cell (RGC) death induced by N-methyl-D-aspartate (NMDA) in rats. Experiments were performed on adult male Wistar rats that received an intravitreal injection of 20 nmol NMDA. Fluoro-Gold labeled RGC morphometry showed that oral intake of pemafibrate once a day for 7 days resulted in significant protection on RGC death induced by NMDA. Phosphorylated c-Jun protein, which is involved in apoptosis, was upregulated after NMDA exposure, and this increase was significantly lessened by the systemic pemafibrate treatment. Phosphorylated c-Jun immunopositive cells were colocalized with Thy-1 immunopositive cells, and the increased these cells were ameliorated by the pemafibrate treatment. An increase in TUNEL-positive cells was significantly suppressed by the pemafibrate treatment. Phosphorylated c-Jun immunopositive cells were colocalized with TUNEL-positive cells, and they were decreased by pemafibrate treatment. These results suggest that the RGC protection achieved with pemafibrate appears to be associated with inhibition of phosphorylated c-Jun and its anti-apoptotic effect.

Of neurons and pericytes: The neuro-vascular approach to diabetic retinopathy

Diabetic retinopathy (DR) is a frequent complication of diabetes mellitus and an increasingly common cause of visual impairment. Blood vessel damage occurs as the disease progresses, leading to ischemia, neovascularization, blood-retina barrier (BRB) failure and eventual blindness. Although detection and treatment strategies have improved considerably over the past years, there is room for a better understanding of the pathophysiology of the diabetic retina. Indeed, it has been increasingly realized that DR is in fact a disease of the retina's neurovascular unit (NVU), the multi-cellular framework underlying functional hyperemia, coupling neuronal computations to blood flow. The accumulating evidence reveals that both neurochemical (synapses) and electrical (gap junctions) means of communications between retinal cells are affected at the onset of hyperglycemia, warranting a global assessment of cellular interactions and their role in DR. This is further supported by the recent data showing down-regulation of connexin 43 gap junctions along the vascular relay from capillary to feeding arteriole as one of the earliest indicators of experimental DR, with rippling consequences to the anatomical and physiological integrity of the retina. Here, recent advancements in our knowledge of mechanisms controlling the retinal neurovascular unit will be assessed, along with their implications for future treatment and diagnosis of DR.

Taurine Protects Retinal Cells and Improves Synaptic Connections in Early Diabetic Rats

Purpose: Taurine has long been thought to be involved in retinal protection from retinal degenerative diseases, but the underlying molecular mechanisms remain unclear. Retinal neurodegeneration is an early event in the pathogenesis of diabetic retinopathy (DR) that precedes and participates in the microcirculatory abnormalities that occur in DR. Our objective was to investigate the role and mechanisms of taurine in early diabetic retinas.Methods: Eight-week-old STZ-induced diabetic rats and control animals were randomly assigned to receive taurine or vehicle by intraperitoneal injection or by intragastric administration. The retinal function and retinal cell counts were evaluated using an electroretinography (ERG) and immunofluorescence microscopy. Plasma amino acids were measured by ion-exchange chromatography (IEC). The expression levels of retinal taurine transporter (Tau-T), mitochondria-dependent apoptosis-associated genes and reactive gliosis markers were studied by western blotting and immunofluorescence. Pre- and post-synaptic markers (PSD95 and mGluR6) in outer plexiform layer (OPL), and the bipolar cell marker protein kinase C alpha (PKCα) were localized by immunofluorescence. Levels of PSD95 and mGluR6 were determined by quantitative western blot.Results: Taurine significantly prevented the reduction of photopic b-wave amplitude and retinal cone cells and ganglion cells loss and maintained the Bcl-2/Bax ratio balance in diabetic rats. Taurine also prevented the upregulation of glial fibrillary acidic protein (GFAP) and reduced retinal reactive gliosis. Taurine reduced plasma glutamate and tyrosine levels, which were elevated in diabetic rats. Moreover, mGluR6 levels reduction detected by western blot and immunofluorescence in diabetic retinas was inhibited and the displacement of mGluR6 in OPL into the inner nuclear layer (INL) detected by immunofluorescence was reduced by Taurine treatment.Conclusion: Taurine may protect retinal cells from diabetic attacks by activating Tau-T, reducing retinal reactive gliosis, improving retinal synaptic connections and decreasing retinal cell apoptosis. Thus, taurine treatment may be a novel approach for early DR.

Library-assisted nonlinear blind separation and annotation of pure components from a single 1H nuclear magnetic resonance mixture spectra

Due to its capability for high-throughput screening ¹H nuclear magnetic resonance (NMR) spectroscopy is commonly used for metabolite research. The key problem in ¹H NMR spectroscopy of multicomponent mixtures is overlapping of component signals and that is increasing with the number of components, their complexity and structural similarity. It makes metabolic profiling, that is carried out through matching acquired spectra with metabolites from the library, a hard problem. Here, we propose a method for nonlinear blind separation of highly correlated components spectra from a single ¹H NMR mixture spectra. The method transforms a single nonlinear mixture into multiple high-dimensional reproducible kernel Hilbert Spaces (mRKHSs). Therein, highly correlated components are separated by sparseness constrained nonnegative matrix factorization in each induced RKHS. Afterwards, metabolites are identified through comparison of separated components with the library comprised of 160 pure components. Thereby, a significant number of them are expected to be related with diabetes type 2. Conceptually similar methodology for nonlinear blind separation of correlated components from two or more mixtures is presented in the Supplementary material. Single-mixture blind source separation is exemplified on: (i) annotation of five components spectra separated from one ¹H NMR model mixture spectra; (ii) annotation of fifty five metabolites separated from one ¹H NMR mixture spectra of urine of subjects with and without diabetes type 2. Arguably, it is for the first time a method for blind separation of a large number of components from a single nonlinear mixture has been proposed. Moreover, the proposed method pinpoints urinary creatine, glutamic acid and 5-hydroxyindoleacetic acid as the most prominent metabolites in samples from subjects with diabetes type 2, when compared to healthy controls.

The Diurnal Rhythm of Insulin Receptor Substrate-1 (IRS-1) and Kir4.1 in Diabetes: Implications for a Clock Gene Bmal1

Purpose

Diabetes leads to the downregulation of the retinal Kir4.1 channels and Müller cell dysfunction. The insulin receptor substrate-1 (IRS-1) is a critical regulator of insulin signaling in Müller cells. Circadian rhythms play an integral role in normal physiology; however, diabetes leads to a circadian dysrhythmia. We hypothesize that diabetes will result in a circadian dysrhythmia of IRS-1 and Kir4.1 and disturbed clock gene function will have a critical role in regulating Kir4.1 channels.

Methods

We assessed a diurnal rhythm of retinal IRS-1 and Kir4.1 in db/db mice. The Kir4.1 function was evaluated using a whole-cell recording of Müller cells. The rat Müller cells (rMC-1) were used to undertake in vitro studies using a siRNA.

Results

The IRS-1 exhibited a diurnal rhythm in control mice; however, with diabetes, this natural rhythm was lost. The Kir4.1 levels peaked and troughed at times similar to the IRS-1 rhythm. The IRS-1 silencing in the rMC-1 led to a decrease in Kir4.1 and BMAL1. The insulin treatment of retinal explants upregulated Kir4.1 possibly via upregulation of BMAL1 and phosphorylation of IRS-1 and Akt-1.

Conclusions

Our studies highlight that IRS-1, by regulating BMAL1, is an important regulator of Kir4.1 in Müller cells and the dysfunctional signaling mediated by IRS-1 may be detrimental to Kir4.1.

New insights into the mechanisms of diabetic complications: role of lipids and lipid metabolism

Diabetes adversely affects multiple organs, including the kidney, eye and nerve, leading to diabetic kidney disease, diabetic retinopathy and diabetic neuropathy, respectively. In both type 1 and type 2 diabetes, tissue damage is organ specific and is secondary to a combination of multiple metabolic insults. Hyperglycaemia, dyslipidaemia and hypertension combine with the duration and type of diabetes to define the distinct pathophysiology underlying diabetic kidney disease, diabetic retinopathy and diabetic neuropathy. Only recently have the commonalities and differences in the metabolic basis of these tissue-specific complications, particularly those involving local and systemic lipids, been systematically examined. This review focuses on recent progress made using preclinical models and human-based approaches towards understanding how bioenergetics and metabolomic profiles contribute to diabetic kidney disease, diabetic retinopathy and diabetic neuropathy. This new understanding of the biology of complication-prone tissues highlights the need for organ-specific interventions in the treatment of diabetic complications.

NMR-Based Metabolomic Approach Tracks Potential Serum Biomarkers of Disease Progression in Patients with Type 2 Diabetes Mellitus

Type 2 diabetes mellitus (T2DM) is a metabolic disorder characterized by chronic hyperglycemia associated with alterations in carbohydrate, lipid, and protein metabolism. The prognosis of T2DM patients is highly dependent on the development of complications, and therefore the identification of biomarkers of T2DM progression, with minimally invasive techniques, is a huge need. In the present study, we applied a ¹H-Nuclear Magnetic Resonance (¹H-NMR)-based metabolomic approach coupled with multivariate data analysis to identify serum metabolite profiles associated with T2DM development and progression. To perform this, we compared the serum metabolome of non-diabetic subjects, treatment-naïve non-complicated T2DM patients, and T2DM patients with complications in insulin monotherapy. Our analysis revealed a significant reduction of alanine, glutamine, glutamate, leucine, lysine, methionine, tyrosine, and phenylalanine in T2DM patients with respect to non-diabetic subjects. Moreover, isoleucine, leucine, lysine, tyrosine, and valine levels distinguished complicated patients from patients without complications. Overall, the metabolic pathway analysis suggested that branched-chain amino acid (BCAA) metabolism is significantly compromised in T2DM patients with complications, while perturbation in the metabolism of gluconeogenic amino acids other than BCAAs characterizes both early and advanced T2DM stages. In conclusion, we identified a metabolic serum signature associated with T2DM stages. These data could be integrated with clinical characteristics to build a composite T2DM/complications risk score to be validated in a prospective cohort.

Human vitreous in proliferative diabetic retinopathy: Characterization and translational implications

Diabetic retinopathy (DR) is one of the leading causes of visual impairment in the working-age population. DR is a progressive eye disease caused by long-term accumulation of hyperglycaemia-mediated pathological alterations in the retina of diabetic patients. DR begins with asymptomatic retinal abnormalities and may progress to advanced-stage proliferative diabetic retinopathy (PDR), characterized by neovascularization or preretinal/vitreous haemorrhages. The vitreous, a transparent gel that fills the posterior cavity of the eye, plays a vital role in maintaining ocular function. Structural and molecular alterations of the vitreous, observed during DR progression, are consequences of metabolic and functional modifications of the retinal tissue. Thus, vitreal alterations reflect the pathological events occurring at the vitreoretinal interface. These events are caused by hypoxic, oxidative, inflammatory, neurodegenerative, and leukostatic conditions that occur during diabetes. Conversely, PDR vitreous can exert pathological effects on the diabetic retina, resulting in activation of a vicious cycle that contributes to disease progression. In this review, we recapitulate the major pathological features of DR/PDR, and focus on the structural and molecular changes that characterize the vitreal structure and composition during DR and progression to PDR. In PDR, vitreous represents a reservoir of pathological signalling molecules. Therefore, in this review we discuss how studying the biological activity of the vitreous in different in vitro, ex vivo, and in vivo experimental models can provide insights into the pathogenesis of PDR. In addition, the vitreous from PDR patients can represent a novel tool to obtain preclinical experimental evidences for the development and characterization of new therapeutic drug candidates for PDR therapy.

Gabapentin Attenuates Oxidative Stress and Apoptosis in the Diabetic Rat Retina

Neurodegeneration in diabetic retina has been widely considered as initiating factor that may lead to vascular damage, the classical hallmark of diabetic retinopathy. Diabetes induced altered glutamate metabolism in the retina, especially through glutamate excitotoxicity might play a major role in the neurodegeneration. Increased level of branched chain amino acids (BCAAs) measured in diabetic retina might cause an increase in the neurotoxic level of glutamate by transamination of citric acid cycle intermediates. In order to analyze the transamination of BCAAs and their influence on neurodegenerative factors, we treated streptozotocin-induced diabetic rats with gabapentin, a leucine analogue and an inhibitor of branched chain amino transferase (BCATc). Interestingly, gabapentin lowered the retinal level of BCAAs in diabetic rats. Furthermore, gabapentin treatments ameliorated the reduced antioxidant glutathione level and increased malondialdehyde (MDA), the marker of lipid peroxidation in diabetic rat retinas. In addition, gabapentin also reduced the expression of proapoptotic caspase-3, a marker of apoptosis and increased anti-apoptotic marker Bcl-2 in diabetic retinas. Thus, these results suggest that gabapentin stimulates glutamate disposal, and ameliorates apoptosis and oxidative stress in diabetic rat retina. The influence of gabapentin may be due to its capacity to increase the ratio of BCKA to BCAA which in turn would reduce glutamate excitotoxicity in diabetic retina.

Cellular Origin of [18F]FDG-PET Imaging Signals During Ceftriaxone-Stimulated Glutamate Uptake: Astrocytes and Neurons

Ceftriaxone stimulates astrocytic uptake of the excitatory neurotransmitter glutamate, and it is used to treat glutamatergic excitotoxicity that becomes manifest during many brain diseases. Ceftriaxone-stimulated glutamate transport was reported to drive signals underlying [¹⁸F]fluorodeoxyglucose-positron emission tomographic ([¹⁸F]FDG-PET) metabolic images of brain glucose utilization and interpreted as supportive of the notion of lactate shuttling from astrocytes to neurons. This study draws attention to critical roles of astrocytes in the energetics and imaging of brain activity, but the results are provocative because (1) the method does not have cellular resolution or provide information about downstream pathways of glucose metabolism, (2) neuronal and astrocytic [¹⁸F]FDG uptake were not separately measured, and (3) strong evidence against lactate shuttling was not discussed. Evaluation of potential metabolic responses to ceftriaxone suggests lack of astrocytic specificity and significant contributions by pre- and postsynaptic neuronal compartments. Indeed, astrocytic glycolysis may not make a strong contribution to the [¹⁸F]FDG-PET signal because partial or complete oxidation of one glutamate molecule on its uptake generates enough ATP to fuel uptake of 3 to 10 more glutamate molecules, diminishing reliance on glycolysis. The influence of ceftriaxone on energetics of glutamate-glutamine cycling must be determined in astrocytes and neurons to elucidate its roles in excitotoxicity treatment.

Metabolomics in diabetic complications

With a global prevalence of 9%, diabetes is the direct cause of millions of deaths each year and is quickly becoming a health crisis. Major long-term complications of diabetes arise from persistent oxidative stress and dysfunction in multiple metabolic pathways. The most serious complications involve vascular damage and include cardiovascular disease as well as microvascular disorders such as nephropathy, neuropathy, and retinopathy. Current clinical analyses like glycated hemoglobin and plasma glucose measurements hold some value as prognostic indicators of the severity of complications, but investigations into the underlying pathophysiology are still lacking. Advancements in biotechnology hold the key to uncovering new pathways and establishing therapeutic targets. Metabolomics, the study of small endogenous molecules, is a powerful toolset for studying pathophysiological processes and has been used to elucidate metabolic signatures of diabetes in various biological systems. Current challenges in the field involve correlating these biomarkers to specific complications to provide a better prediction of future risk and disease progression. This review will highlight the progress that has been made in the field of metabolomics including technological advancements, the identification of potential biomarkers, and metabolic pathways relevant to macro- and microvascular diabetic complications.

Early changes in system [Formula: see text] and glutathione in the retina of diabetic rats

Diabetic retinopathy (DR), the main cause of blindness among diabetic patients, affects both neuronal and vascular cells of the retina. Studies show that neuronal cell death begins after 4 weeks of diabetes and could be related with an increase in oxidative stress. System [Formula: see text] is a glutamate/cystine exchanger, formed by a catalytic subunit called xCT and a regulatory subunit 4F2hc, whose activity is crucial to the synthesis of glutathione, which is a key antioxidant molecule for cells. Although some studies have shown that glutamate transport mediated by excitatory amino acid transporters (EAATs) in diabetic rats is downregulated, there are no studies investigating system [Formula: see text] in this context. To evaluate whether system [Formula: see text] is modified by early onset of diabetes, primary retinal cell culture exposed to high glucose and retinas of rats 3 weeks after streptozotocin injection were used. We observed that xCT subunit protein expression both in cultures and in vivo were diminished. Furthermore, system [Formula: see text] activity and GSH levels were also decreased whereas oxidative stress was increased in retinas of diabetic animals. Therefore, this study raises the possibility that alterations in system [Formula: see text] expression and activity could occur during early onset of diabetes. In that way, system [Formula: see text] modifications could be related to increased ROS in diabetic retinopathy.

Diabetic hyperglycemia reduces Ca2+ permeability of extrasynaptic AMPA receptors in AII amacrine cells

There is increasing evidence that diabetic retinopathy is a primary neuropathological disorder that precedes the microvascular pathology associated with later stages of the disease. Recently, we found evidence for altered functional properties of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in A17, but not AII, amacrine cells in the mammalian retina, and the observed changes were consistent with an upregulation of the GluA2 subunit, a key determinant of functional properties of AMPA receptors, including Ca(2+) permeability and current-voltage (I-V) rectification properties. Here, we have investigated functional changes of extrasynaptic AMPA receptors in AII amacrine cells evoked by diabetes. With patch-clamp recording of nucleated patches from retinal slices, we measured Ca(2+) permeability and I-V rectification in rats with ∼3 wk of streptozotocin-induced diabetes and age-matched, noninjected controls. Under bi-ionic conditions (extracellular Ca(2+) concentration = 30 mM, intracellular Cs(+) concentration = 171 mM), the reversal potential (Erev) of AMPA-evoked currents indicated a significant reduction of Ca(2+) permeability in diabetic animals [Erev = -17.7 mV, relative permeability of Ca(2+) compared with Cs(+) (PCa/PCs) = 1.39] compared with normal animals (Erev = -7.7 mV, PCa/PCs = 2.35). Insulin treatment prevented the reduction of Ca(2+) permeability. I-V rectification was examined by calculating a rectification index (RI) as the ratio of the AMPA-evoked conductance at +40 and -60 mV. The degree of inward rectification in patches from diabetic animals (RI = 0.48) was significantly reduced compared with that in normal animals (RI = 0.30). These results suggest that diabetes evokes a change in the functional properties of extrasynaptic AMPA receptors of AII amacrine cells. These changes could be representative for extrasynaptic AMPA receptors elsewhere in AII amacrine cells and suggest that synaptic and extrasynaptic AMPA receptors are differentially regulated.

Disruption of a neural microcircuit in the rod pathway of the mammalian retina by diabetes mellitus

Diabetes leads to dysfunction of the neural retina before and independent of classical microvascular diabetic retinopathy, but previous studies have failed to demonstrate which neurons and circuits are affected at the earliest stages. Here, using patch-clamp recording and two-photon Ca(2+) imaging in rat retinal slices, we investigated diabetes-evoked changes in a microcircuit consisting of rod bipolar cells and their dyad postsynaptic targets, AII and A17 amacrine cells, which play an essential role in processing scotopic visual signals. AII amacrines forward their signals to ON- and OFF-cone bipolar cells and A17 amacrines provide GABAergic feedback inhibition to rod bipolar cells. Whereas Ca(2+)-permeable AMPA receptors mediate input from rod bipolar cells to both AII and A17 amacrines, diabetes changes the synaptic receptors on A17, but not AII amacrine cells. This was expressed as a change in pharmacological properties and single-channel conductance of the synaptic receptors, consistent with an upregulation of the AMPA receptor GluA2 subunit and reduced Ca(2+) permeability. In addition, two-photon imaging revealed reduced agonist-evoked influx of Ca(2+) in dendritic varicosities of A17 amacrine cells from diabetic compared with normal animals. Because Ca(2+)-permeable receptors in A17 amacrine cells mediate synaptic release of GABA, the reduced Ca(2+) permeability of these receptors in diabetic animals leads to reduced release of GABA, followed by disinhibition and increased release of glutamate from rod bipolar cells. This perturbation of neuron and microcircuit dynamics can explain the decreased dynamic range and sensitivity of scotopic vision that has been observed in diabetes.

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.

Neurodegeneration in diabetic retina and its potential drug targets

Diabetic retinopathy (DR) is one of the major complications of diabetes causing vision loss and blindness worldwide. DR is widely recognized as a neurodegenerative disease as evidenced from early changes at cellular and molecular levels in the neuronal component of the diabetic retina, which is further supported by various retinal functional tests indicating functional deficits in the retina soon after diabetes progression. Diabetes alters the level of a number of neurodegenerative metabolites, which increases influx through several metabolic pathways which in turn induce an increase in oxidative stress and a decrease in neurotrophic factors, thereby damage retinal neurons. Loss of neurons may implicate in vascular pathology, a clinical signs of DR observed at later stages of the disease. Here, we discuss diabetes-induced potential metabolites known to be detrimental to neuronal damage and their mechanism of action. In addition, we highlight important neurotrophic factors, whose level have been found to be dysregulated in diabetic retina and may damage neurons. Furthermore, we discuss potential drugs and strategies based on targeting diabetes-induced metabolites, metabolic pathways, oxidative stress, and neurotrophins to protect retinal neurons, which may ameliorate vision loss and vascular damage in 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.

Phenotypes and biomarkers of diabetic retinopathy

Diabetic retinopathy (DR) remains a major cause of blindness as the prevalence of diabetes is expected to approximately double globally between 2000 and 2030. DR progresses over time at different rates in different individuals with only a limited number developing significant vision loss due to the two major vision-threatening complications, clinically significant macular edema and proliferative retinopathy. Good metabolic control is important to prevent and delay progression, but whereas some patients escape vision loss even with poor control, others develop vision loss despite good metabolic control. Our research group has been able to identify three different DR phenotypes characterized by different dominant retinal alterations and different risks of progression to vision-threatening complications. Microaneurysm turnover has been validated as a prognostic biomarker of development of clinically significant macular edema, whereas subclinical macular edema identified by OCT and mfERG appear to be also good candidates as organ-specific biomarkers of DR. Hemoglobin A1c remains the only confirmed systemic prognostic biomarker of DR progression. The availability of biomarkers of DR progression and the identification of different phenotypes of DR with different risks for development of vision-threatening complications offers new perspectives for understanding DR and for its personalized management.

Astrocytic energetics during excitatory neurotransmission: What are contributions of glutamate oxidation and glycolysis?

Astrocytic energetics of excitatory neurotransmission is controversial due to discrepant findings in different experimental systems in vitro and in vivo. The energy requirements of glutamate uptake are believed by some researchers to be satisfied by glycolysis coupled with shuttling of lactate to neurons for oxidation. However, astrocytes increase glycogenolysis and oxidative metabolism during sensory stimulation in vivo, indicating that other sources of energy are used by astrocytes during brain activation. Furthermore, glutamate uptake into cultured astrocytes stimulates glutamate oxidation and oxygen consumption, and glutamate maintains respiration as well as glucose. The neurotransmitter pool of glutamate is associated with the faster component of total glutamate turnover in vivo, and use of neurotransmitter glutamate to fuel its own uptake by oxidation-competent perisynaptic processes has two advantages, substrate is supplied concomitant with demand, and glutamate spares glucose for use by neurons and astrocytes. Some, but not all, perisynaptic processes of astrocytes in adult rodent brain contain mitochondria, and oxidation of only a small fraction of the neurotransmitter glutamate taken up into these structures would be sufficient to supply the ATP required for sodium extrusion and conversion of glutamate to glutamine. Glycolysis would, however, be required in perisynaptic processes lacking oxidative capacity. Three lines of evidence indicate that critical cornerstones of the astrocyte-to-neuron lactate shuttle model are not established and normal brain does not need lactate as supplemental fuel: (i) rapid onset of hemodynamic responses to activation delivers oxygen and glucose in excess of demand, (ii) total glucose utilization greatly exceeds glucose oxidation in awake rodents during activation, indicating that the lactate generated is released, not locally oxidized, and (iii) glutamate-induced glycolysis is not a robust phenotype of all astrocyte cultures. Various metabolic pathways, including glutamate oxidation and glycolysis with lactate release, contribute to cellular energy demands of excitatory neurotransmission.

GABA and Glutamate Uptake and Metabolism in Retinal Glial (Müller) Cells

Müller cells, the principal glial cells of the retina, support the synaptic activity by the uptake and metabolization of extracellular neurotransmitters. Müller cells express uptake and exchange systems for various neurotransmitters including glutamate and γ-aminobutyric acid (GABA). Müller cells remove the bulk of extracellular glutamate in the inner retina and contribute to the glutamate clearance around photoreceptor terminals. By the uptake of glutamate, Müller cells are involved in the shaping and termination of the synaptic activity, particularly in the inner retina. Reactive Müller cells are neuroprotective, e.g., by the clearance of excess extracellular glutamate, but may also contribute to neuronal degeneration by a malfunctioning or even reversal of glial glutamate transporters, or by a downregulation of the key enzyme, glutamine synthetase. This review summarizes the present knowledge about the role of Müller cells in the clearance and metabolization of extracellular glutamate and GABA. Some major pathways of GABA and glutamate metabolism in Müller cells are described; these pathways are involved in the glutamate-glutamine cycle of the retina, in the defense against oxidative stress via the production of glutathione, and in the production of substrates for the neuronal energy metabolism.

Recent advances in understanding the biochemical and molecular mechanism of diabetic retinopathy

One of the major complications in patients with diabetes is diabetic retinopathy (DR), a leading cause of blindness worldwide. It takes several years before any clinical signs of retinopathy appear in diabetic patients, which gives an ample opportunity for scientists to uncover biochemical and molecular mechanism implicated early in the development and progression of the disease. During the past few decades, research progress has been made in investigating the pathophysiology of the disease; however, due to nonavailability of human retinal samples at different stages of the disease and also due to lack of a proper animal model of DR, the exact molecular mechanism has not been elucidated, making therapeutic a difficult task. In this review article, we have discussed a number of diabetes-induced metabolites such as glucose, lipids, amino acids, and other related factors and molecules that are implicated in the pathophysiology of the DR. Furthermore, we have highlighted neurodegeneration and regulation of neurotrophic factors, being recognized as early events that may be involved in the pathology of the disease in the course of DR. An understanding of the biochemical and molecular changes especially early in the diabetic retina may lead to new and effective therapies towards prevention and amelioration of DR, which is important for the millions of individuals who already have or are likely to develop the disease before a cure becomes available.

Regulation of glutamate metabolism by hydrocortisone and branched chain keto acids in cultured rat retinal Müller cells (TR-MUL)

Glutamate released from retinal neurons during neurotransmission is taken up by retinal Müller cells, where much of the amino acid is subsequently amidated to glutamine or transaminated to α-ketoglutarate for oxidation. Müller cell glutamate levels may have to be carefully maintained at fairly low concentrations to avoid excesses of glutamate in extracellular spaces of the retina that would otherwise cause excitotoxicity. We employed a cultured rat retinal Müller cell line in order to study the metabolism and the role of Müller cell specific enzymes on the glutamate disposal pathways. We found that the TR-MUL cells express the glial specific enzymes, glutamine synthetase, the mitochondrial isoform of branched chain aminotransferase (BCATm) and pyruvate carboxylase, all of which are involved in glutamate metabolism and homeostasis in the retina. Hydrocortisone treatment of TR-MUL cells increased glutamine synthetase expression and the rate of glutamate amidation to glutamine. Addition of branched chain keto acids (BCKAs) increased lactate and aspartate formation from glutamate and also oxidation of glutamate to CO(2) and H(2)O. The two glutamate disposal pathways (amidation and oxidation) did not influence each other. When glutamate levels were independently depleted within TR-MUL cells, the uptake of glutamate from the extracellular fluid increased compared to uptake from control (undepleted) cells suggesting that the level of intracellular glutamate may influence clearing of extracellular glutamate.