Changes in the redox state in the retina and brain during the onset of diabetes in rats

Unlocking the role of lactate: metabolic pathways, signaling, and gene regulation in postmitotic retinal cells

The Warburg effect, which was first described a century ago, asserts that mitotic tumor cells generate higher quantities of lactate. Intriguingly, even in typical physiological circumstances, postmitotic retinal photoreceptor cells also produce elevated levels of lactate. Initially classified as metabolic waste, lactate has since gained recognition as a significant intracellular signaling mediator and extracellular ligand. This current review endeavors to provide a concise overview and discourse on the following topics: the localization of lactate-producing enzymes, the functional significance of these enzymes, the signaling functions of lactate, and its impact on the gene expression of photoreceptors in retinal cells.

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.

Anorexia disrupts glutamate-glutamine homeostasis associated with astroglia in the prefrontal cortex of young female rats

Anorexia nervosa (AN) is an eating disorder characterized by self-starvation and excessive weight loss with a notorious prevalence in young women. The neurobiology of AN is unknown but murine models, like dehydration induced anorexia (DIA), reproduce weight loss and avoidance of food despite its availability. Astrocytes are known to provide homeostatic support to neurons, but it is little explored if anorexia affects this function. In this study, we tested if DIA disrupts glutamate-glutamine homeostasis associated with astrocytes in the prefrontal cortex (PFC) of young female rats. Our results showed that anorexia reduced the redox state, as well as endogenous glutamate and glutamine. These effects correlated with a reduced expression of the glutamate transporters (GLT-1 and GLAST) and glutamine synthetase, all of them are preferentially expressed by astrocytes. Accordingly, the expression of GFAP was reduced. Anorexia reduced the astrocyte density, promoted a de-ramified morphology, and augmented the de-ramified/ramified astrocyte ratio in the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC), but not in the motor cortex (M2). The increase of a de-ramified phenotype correlated with increased expression of vimentin and nestin. Based on these results, we conclude that anorexia disrupts glutamate-glutamine homeostasis and the redox state associated with astrocyte dysfunction.

Crocin Improves Oxidative Stress in Testicular Tissues of Streptozotocin-Induced Diabetic Rats

Crocin has been shown to have potent antioxidant properties, but its potential antioxidative effects on testicular tissue during uncontrolled diabetes is unknown. Wistar rats were randomly divided into four separate groups; normal, normal-treated, diabetic and diabetic treated (n = 6 per group). Diabetes was induced by a single intravenous injection of streptozotocin (45 mg/kg). Two treated groups of animals (diabetic and non-diabetic) received Crocin daily for 56 days (40 mg/kg/intraperitoneally). At the end of the 56th day, animals were sacrificed and blood and testicular tissue obtained. The level of nitrate, malondialdehyde, glutathione, and the activities of superoxide dismutase and catalase enzymes were determined. Crocin therapy moderated the increased oxidative stress in testicular tissue induced by diabetes with a significant reduction in nitrate and malondialdehyde, whilst reducing superoxide dismutase and catalase enzyme activities in diabetes (p < 0.001), though glutathione was unaffected. Treatment by Crocin in normal rats also modestly improved parameters of oxidative stress (p < 0.05). Crocin has a protective effect on diabetes induced oxidative stress in testicular tissue in an animal model, though it is unclear if this is a direct antioxidant effect.

Plasma N-Acetylaspartate Is Related to Age, Obesity, and Glucose Metabolism: Effects of Antidiabetic Treatment and Bariatric Surgery

Background: N-acetylaspartate (NAA) is synthesized only by neurons and is involved in neuronal metabolism and axonal myelination. NAA is the strongest signal on brain magnetic resonance spectroscopy, and its concentration have been associated with cognitive dysfunction in neurodegenerative diseases, obesity, and type 2 diabetes (T2D). Materials and Methods: We explored the impact of obesity and T2D on circulating NAA as well as the impact of bariatric surgery and antidiabetic treatments. We developed an LC-MS method for the accurate measurements of fasting plasma NAA levels in 505 subjects (156 subjects with normal glucose tolerance, 24 subjects with impaired glucose tolerance, and 325 patients with T2D) to examine the associations of NAA with obesity and dysglycemia. To validate cross-sectional findings, plasma NAA was measured 6 months after Roux-en-Y Gastric Bypass (RYGB) in 55 morbidly obese subjects, and after 1 year of antidiabetic treatment (with dapagliflozin, exenatide, or dapagliflozin plus exenatide) in 192 T2D patients. Results: In the whole population, NAA was associated with age (r = 0.31, p <0.0001) and BMI (r = -0.20, p <0.0001). Independently of age and BMI, NAA was reciprocally related to HbA1c and fasting plasma glucose (partial r = -0.13, both p = 0.01). Surgically-induced weight loss raised NAA (by 18 nmol/L on average, p <0.02). Glucose lowering treatment increased NAA in proportion to the drop in HbA1c (r = 0.31, p <0.0001) regardless of the agent used. Conclusions: Circulating NAA concentrations are modulated by age, obesity, and glycemic control. Whether they may mark for the corresponding metabolic effects on brain function remains to be established by joint measurements of spectroscopic signal and cognitive function.

Diabetes Alters pH Control in Rat Retina

Purpose

The purpose of this study was to determine whether the ability of the rat retina to control its pH is affected by diabetes.

Methods

Double-barreled H+-selective microelectrodes were used to measure extracellular [H+] in the dark-adapted retina of intact control and diabetic Long-Evans rats 1 to 6 months after intraperitoneal injection of vehicle or streptozotocin, respectively. Two manipulations-increasing of blood glucose and intravenous injection of the carbonic anhydrase blocker dorzolamide (DZM)-were used to examine their effects on retinal pH regulation.

Results

An increase of retinal acidity was correlated with the diabetes-related increase in blood glucose, but only between 1 and 3 months of diabetes, not earlier or later. Adding intravenous glucose had no noticeable effect on the retinal acidity of control animals. In contrast, similar injections of glucose in diabetic rats significantly increased the acidity of the retina. Again, the largest increase of retinal acidity due to artificially elevated blood glucose was observed at 1 to 3 months of diabetes. Suppression of carbonic anhydrase by DZM dramatically increased the retinal acidity in both control and diabetic retinas to a similar degree. However, in controls, the strongest effect of DZM was recorded within 10 minutes after the injection, but in diabetics, the effect tended to increase with time and after 2 hours could be two to three times larger than at the beginning.

Conclusions

During development of diabetes in rats, the control over retinal pH is partly compromised so that conditions that perturb retinal pH lead to larger and/or more sustained changes than in control animals.

Hyperglycemia Alters Astrocyte Metabolism and Inhibits Astrocyte Proliferation

Diabetes milieu is a complex metabolic disease that has been known to associate with high risk of various neurological disorders. Hyperglycemia in diabetes could dramatically increase neuronal glucose levels which leads to neuronal damage, a phenomenon referred to as glucose neurotoxicity. On the other hand, the impact of hyperglycemia on astrocytes has been less explored. Astrocytes play important roles in brain energy metabolism through neuron-astrocyte coupling. As the component of blood brain barrier, glucose might be primarily transported into astrocytes, hence, impose direct impact on astrocyte metabolism and function. In the present study, we determined the effect of high glucose on the energy metabolism and function of primary astrocytes. Hyperglycemia level glucose (25 mM) induced cell cycle arrest and inhibited proliferation and migration of primary astrocytes. Consistently, high glucose decreased cyclin D1 and D3 expression. High glucose enhanced glycolytic metabolism, increased ATP and glycogen content in primary astrocytes. In addition, high glucose activated AMP-activated protein kinase (AMPK) signaling pathway in astrocytes. In summary, our in vitro study indicated that hyperglycemia might impact astrocyte energy metabolism and function phenotype. Our study provides a potential mechanism which may underlie the diabetic cerebral neuropathy and warrant further in vivo study to determine the effect of hyperglycemia on astrocyte metabolism and function.

Mitochondrial activity in different regions of the brain at the onset of streptozotocin-induced diabetes in rats

Diabetes affects a variety of tissues including the central nervous system; moreover, some evidence indicates that memory and learning processes are disrupted. Also, oxidative stress triggers alterations in different tissues including the brain. Recent studies indicate mitochondria dysfunction is a pivotal factor for neuron damage. Therefore, we studied mitochondrial activity in three brain regions at early type I-diabetes induction. Isolated mitochondria from normal hippocampus, cortex and cerebellum revealed different rates of oxygen consumption, but similar respiratory controls. Oxygen consumption in basal state 4 significantly increased in the mitochondria from all three brain regions from diabetic rats. No relevant differences were observed in the activity of respiratory complexes, but hippocampal mitochondrial membrane potential was reduced. However, ATP content, mitochondrial cytochrome c, and protein levels of β-tubulin III, synaptophysin, and glutamine synthase were similar in brain regions from normal and diabetic rats. In addition, no differences in total glutathione levels were observed between normal and diabetic rat brain regions. Our results indicated that different regions of the brain have specific metabolic responses. The changes in mitochondrial activity we observed at early diabetes induction did not appear to cause metabolic alterations, but they might appear at later stages. Longer-term streptozotocin treatment studies must be done to elucidate the impact of hyperglycemia in brain metabolism and the function of specific brain regions.

Potential Role of Endoplasmic Reticulum Stress in Pathogenesis of Diabetic Retinopathy

Diabetes mellitus is a metabolic disease that leads to several complications which include retinopathy. Multiple biochemical abnormalities have been proposed to explain the development of retinopathy, including oxidative stress. Although the existence of oxidative stress has been established in the retina from long standing diabetic animals, pathogenesis and progression of retinopathy remain unclear. In order to gain insight into the pathogenesis of diabetic retinopathy, we analyzed the levels of different oxidative stress biomarkers in the retina at early stages during the progress of streptozotocin-induced diabetes. No significant changes in glutathione content, expression of NADPH-oxidase, levels of lipid peroxidation, nor production of free radicals were observed in the retina up to 45 days of diabetes induction. Likewise, a transient decrease in aconitase activity, parallel to an increase in the superoxide dismutase activity was observed at 20 days of hyperglycemia, suggesting a high capacity of retina to maintain its redox homeostasis, at least at early stages of diabetes. Nonetheless, we found an early and time-dependent increase in the levels of oxidized proteins, which was not affected by the administration of the antioxidant quercetin. Also, positive immunoreactivity to the reticulum stress protein CHOP was found in glial Müller cells of diabetic rat retinas. These findings suggest the occurrence of endoplasmic reticulum stress as a primary event in retina pathogenesis in diabetes.

In the Early Stages of Diabetes, Rat Retinal Mitochondria Undergo Mild Uncoupling due to UCP2 Activity

In order to maintain high transmembrane ionic gradients, retinal tissues require a large amount of energy probably provided by a high rate of both, glycolysis and oxidative phosphorylation. However, little information exists on retinal mitochondrial efficiency. We analyzed the retinal mitochondrial activity in ex vivo retinas and in isolated mitochondria from normal rat retina and from short-term streptozotocin-diabetic rats. In normal ex vivo retinas, increasing glucose concentrations from 5.6mM to 30mM caused a four-fold increase in glucose accumulation and CO₂ production. Retina from diabetic rats accumulated similar amounts of glucose. However, CO₂ production was not as high. Isolated mitochondria from normal rat retina exhibited a resting rate of oxygen consumption of 14.6 ± 1.1 natgO (min.mg prot)^-1 and a respiratory control of 4.0. Mitochondria from 7, 20 and 45 days diabetic rats increased the resting rate of oxygen consumption and the activity of the electron transport complexes; under these conditions the mitochondrial transmembrane potential decreased. In spite of this, the ATP synthesis was not modified. GDP, an UCP2 inhibitor, increased mitochondrial membrane potential and superoxide production in controls and at 45 days of diabetes. The role of UCP2 is discussed. The results suggest that at the early stage of diabetes we studied, retinal mitochondria undergo adaptations leading to maintain energetic requirements and prevent oxidative stress.

Retinal proteome alterations in a mouse model of type 2 diabetes

AIMS/HYPOTHESIS

Diabetic retinopathy is a major complication of type 2 diabetes and the leading cause of blindness in adults of working age. Neuronal defects are known to occur early in disease, but the source of this dysfunction is unknown. The aim of this study was to examine differences in the retinal membrane proteome among non-diabetic mice and mouse models of diabetes either with or without metformin treatment.

METHODS

Alterations in the retinal membrane proteome of 10-week-old diabetic db/db mice, diabetic db/db mice orally treated with the anti-hyperglycaemic metformin, and congenic wild-type littermates were examined using label-free mass spectrometry. Pathway enrichment analysis was completed with Genomatix and Ingenuity. Alterations in Slc17a7 mRNA and vesicular glutamate transporter 1 (VGLUT1) protein expression were evaluated using real-time quantitative PCR and IMMUNOFLUORESCENCE.

RESULTS

A total of 98 proteins were significantly differentially abundant between db/db and wild-type animals. Pathway enrichment analysis indicated decreases in levels of proteins related to synaptic transmission and cell signalling. Metformin treatment produced 63 differentially abundant proteins compared with untreated db/db mice, of which only 43 proteins were found to occur in both datasets, suggesting that treatment only partially normalises the alterations induced by diabetes. VGLUT1, which is responsible for loading glutamate into synaptic vesicles, was found to be differentially abundant in db/db mice and was not normalised by metformin. The decrease in Slc17a7/VGLUT1 was confirmed by transcriptomic and immunocytochemical analysis.

CONCLUSIONS/INTERPRETATION

These findings expand the knowledge of the protein changes in diabetic retinopathy and suggest that membrane-associated signalling proteins are susceptible to changes that are partially ameliorated by treatment

Role of hyperglycemia-mediated erythrocyte redox state alteration in the development of diabetic retinopathy

PURPOSE

To evaluate erythrocyte redox state and its surrogates in patients with different stages of diabetic retinopathy and their association with cellular metabolic derangement developed in retinal microvascular cells.

METHODS

Sixty type 2 diabetic patients with nonproliferative diabetic retinopathy (NPDR), 85 patients with proliferative diabetic retinopathy (PDR), and 70 patients with diabetes but without retinopathy were considered as diabetic control (DC) for the study. In addition, 65 normal individuals without diabetes were enrolled as healthy control in this study. Erythrocyte oxidized nicotinamide adenine dinucleotide phosphate / reduced nicotinamide adenine dinucleotide phosphate (NADP / NADPH), oxidized nicotinamide adenine dinucleotide / reduced nicotinamide adenine dinucleotide (NAD / NADH) glutathione, plasma and vitreous lactate, and pyruvate levels were determined by enzymatic reaction-based spectrophotometric assay for the patients and individuals.

RESULT

Erythrocyte NADP+ to NADPH ratio to NADPH ratio was found to be significantly higher among NPDR and PDR patients compared with DC subjects (P < 0.0001). Erythrocyte-reduced glutathione was significantly decreased in patients of NPDR (P = 0.0004) and patients of PDR (P = 0.0157) compared to DC. Erythrocyte NAD to NADH ratio was also significantly decreased in patients of NPDR (P < 0.0001) and PDR (P < 0.0001) compared to DC subjects. Lactate to pyruvate ratio of plasma was elevated significantly in patients with NPDR compared with DC (P < 0.0001) and those having PDR (P = 0.0046). In the vitreous fluid, the lactate to pyruvate ratios were found to be significantly lower in normal individuals without diabetes compared with patients having PDR (P < 0.0001).

CONCLUSION

Hyperglycemia-mediated erythrocyte redox state alterations might be a potential risk factor for the development of NPDR in poorly controlled diabetic subjects.

Insulin stimulated-glucose transporter Glut 4 is expressed in the retina

The vertebrate retina is a very metabolically active tissue whose energy demands are normally met through the uptake of glucose and oxygen. Glucose metabolism in this tissue relies upon adequate glucose delivery from the systemic circulation. Therefore, glucose transport depends on the expression of glucose transporters. Here, we show retinal expression of the Glut 4 glucose transporter in frog and rat retinas. Immunohistochemistry and in situ hybridization studies showed Glut 4 expression in the three nuclear layers of the retina: the photoreceptor, inner nuclear and ganglionar cell layers. In the rat retina immunoprecipitation and Western blot analysis revealed a protein with an apparent molecular mass of 45 kDa. ¹⁴C-glucose accumulation by isolated rat retinas was significantly enhanced by physiological concentrations of insulin, an effect blocked by inhibitors of phosphatidyl-inositol 3-kinase (PI3K), a key enzyme in the insulin-signaling pathway in other tissues. Also, we observed an increase in ³H-cytochalasin binding sites in the presence of insulin, suggesting an increase in transporter recruitment at the cell surface. Besides, insulin induced phosphorylation of Akt, an effect also blocked by PI3K inhibition. Expression of Glut 4 was not modified in retinas of a type 1 diabetic rat model. To our knowledge, our results provide the first evidence of Glut4 expression in the retina, suggesting it as an insulin- responsive tissue.

Effects of acute and chronic hyperglycemia on the neurochemical profiles in the rat brain with streptozotocin-induced diabetes detected using in vivo ¹H MR spectroscopy at 9.4 T

Chronic hyperglycemia could lead to cerebral metabolic alterations and CNS injury. However, findings of metabolic alterations in poorly managed diabetes in humans and animal models are rather inconsistent. We have characterized the cerebral metabolic consequences of untreated hyperglycemia from the onset to the chronic stage in a streptozotocin-induced rat model of diabetes. In vivo ¹H magnetic resonance spectroscopy was used to measure over 20 neurochemicals longitudinally. Upon the onset of hyperglycemia (acute state), increases in brain glucose levels were accompanied by increases in osmolytes and ketone bodies, all of which remained consistently high through the chronic state of over 10 weeks of hyperglycemia. Only after over 4 weeks of hyperglycemia, the levels of other neurochemicals including N-acetylaspartate and glutathione were significantly reduced and these alterations persisted into the chronic stage. However, glucose transport was not altered in chronic hyperglycemia of over 10 weeks. When glucose levels were acutely restored to euglycemia, some neurochemical changes were irreversible, indicating the impact of prolonged uncontrolled hyperglycemia on the CNS. Furthermore, progressive changes in neurochemical levels from control to acute and chronic conditions demonstrated the utility of ¹H magnetic resonance spectroscopy as a non-invasive tool in monitoring the disease progression in diabetes.

Control of glycogen content in retina: allosteric regulation of glycogen synthase

Retinal tissue is exceptional because it shows a high level of energy metabolism. Glycogen content represents the only energy reserve in retina, but its levels are limited. Therefore, elucidation of the mechanisms controlling glycogen content in retina will allow us to understand retina response under local energy demands that can occur under normal and pathological conditions. Thus, we studied retina glycogen levels under different experimental conditions and correlated them with glucose-6-phosphate (G-6-P) content and glycogen synthase (GS) activity. Glycogen and G-6-P content were studied in ex vivo retinas from normal, fasted, streptozotocin-treated, and insulin-induced hypoglycemic rats. Expression levels of GS and its phosphorylated form were also analyzed. Ex vivo retina from normal rats showed low G-6-P (14±2 pmol/mg protein) and glycogen levels (43±3 nmol glycosyl residues/mg protein), which were increased 6 and 3 times, respectively, in streptozotocin diabetic rats. While no changes in phosphorylated GS levels were observed in any condition tested, a positive correlation was found between G-6-P levels with GS activity and glycogen content. The results indicated that in vivo, retina glycogen may act as an immediately accessible energy reserve and that its content was controlled primarily by G-6-P allosteric activation of GS. Therefore, under hypoglycemic situations retina energy supply is strongly compromised and could lead to the alterations observed in type 1 diabetes.

Early neural and vascular dysfunctions in diabetic rats are largely sequelae of increased sorbitol oxidation

These experiments were undertaken to assess the importance of cytoplasmic (c) sorbitol oxidation versus mitochondrial (m) pyruvate oxidation in mediating neural and vascular dysfunction attributable to hyperglycemia in diabetic rats. Increased oxidation of sorbitol is coupled to enzymatic reduction of free oxidized NAD(+)c to reduced NADHc, manifested by an increased ratio of NADH to NAD(+)c. Likewise, increased oxidation of pyruvate is coupled to reduction of NAD(+)m to NADHm, which increases the NADH/NAD(+)m ratio. Specific inhibitors of sorbitol production or sorbitol oxidation normalized: increased diabetic nerve NADH/NAD(+)c, impaired nerve-conduction velocity, and vascular dysfunction in sciatic nerve, retina, and aorta; however, they had little or no impact on increased NADH/NAD(+)m. These observations provide, for the first time, strong in vivo evidence for the primacy of sorbitol oxidation versus. pyruvate oxidation in mediating the metabolic imbalances, impaired nerve conduction, and vascular dysfunction evoked by diabetes. These findings are consistent with (a) the fact that oxidation of sorbitol produces "prooxidant" NADHc uncoupled from subsequent production of "antioxidant" pyruvate required for reoxidation of NADHc to NAD(+)c by lactate dehydrogenase, and (b) the hypothesis that neural and vascular dysfunction in early diabetes are caused primarily by increased NADHc, which fuels superoxide production by NADH-driven oxidases.

1-methylnicotinamide (MNA) in prevention of diabetes-associated brain disorders

The present study has been designed to establish the potential benefits from 1-methylnicotinamide (MNA) treatment on brain disorders associated with type 1 diabetes. All experiments were carried out after 6 weeks of streptozotocin-induced diabetes (60 mg/kg of body weight, i.p.) in male Wistar rats treated for 5 weeks with or without MNA (100 mg/kg of body weight, per os in drinking water) after 1 week of diabetes induction. Diabetes was shown to reduce monoamine neurotransmitter serotonin transporters activity, as assessed by significant inhibition of [2-(14)C]serotonin uptake, that was accompanied by elevation of spontaneous mediator release in rat brain synaptosomes. Treatment with MNA slightly attenuated diabetes-induced changes in brain serotoninergic system. The precise mechanism underlying MNA action on central serotonin neurotransmission is not known, but appears to be linked to metabolic and signalling pathways involved in controlling synaptic function rather than being associated with direct modulation of serotonin transporters. In particular, MNA action was associated with its partial normalizing effects on such biochemical indices of neuropathy development as decrease in synaptosomal Na(+),K(+)-ATPase activity and plasma membrane depolarization of synaptic endings. Elevated sorbitol formation in brain and NAD(+) deficits resulted from diabetes as major metabolic imbalances were remarkably countered by MNA treatment. However, diabetes-induced decrease in cytosolic NAD(+) to NADH ratio in brain remained unchanged. Notably, MNA supplementation to diabetic rats caused a slight lowering effect on blood glucose level. Accordingly, our findings indicate that neuroprotective properties of MNA are linked to modulation of synaptic activity through multiple mechanisms. In conclusion, we suggest that 1-methylnicotinamide might be a useful agent for treating brain failures related to diabetes.

Ascorbate uptake in normal and diabetic rat retina and retinal pigment epithelium

Oxidative stress is an important causative factor in the pathogenesis of diabetic retinopathy. Therefore, it becomes important to understand the mechanisms that help maintain appropriate levels of a small molecule antioxidant such as ascorbate in the retina. The outer blood-barrier which results from the tight junctions between the retinal pigment epithelial cells (RPE) restricts the flow of nutrients reaching the retina. In this study, we characterized the transport properties of carboxyl-(14)C ascorbate (AA) in normal rat retina and RPE, and compared them with those in streptozotocin-diabetic rats. Retina and RPE accumulated AA by a temperature-sensitive and energy-dependent kinetic mechanism with an apparent K(M) of 380 and 420 microM, respectively. Accumulation of AA was significantly reduced in a sodium-free medium. Although high glucose concentrations reduced AA uptake by 40%, this was not affected by cytochalasin B. The RPE and retina of diabetic rats presented lower levels of AA accumulation. These findings suggest the presence of the specific vitamin C transporter SVCT in retina and RPE, which may be involved in the manifestation of diabetic retinopathy.

Reexamining the hyperglycemic pseudohypoxia hypothesis of diabetic oculopathy

PURPOSE

To test the hypothesis that diabetes alters retinal NAD+-to-NADH ratios early in the course of the disease (e.g., the hyperglycemic pseudohypoxia hypothesis).

METHODS

In freshly excised age-matched control and diabetic rat retinas, measurements were made of the NAD+ and NADH content as well as a surrogate marker of NAD+-to-NADH ratios obtained from lactate and pyruvate levels. In addition, the effect of various hyperglycemic levels was assessed from measurements of retinal lactate and pyruvate concentrations and the rate of lactic acid production in vitro (isolated rat retinas, monolayer cultures of human retinal pigment epithelial cells, and rabbit lens epithelial cells).

RESULTS

No significant differences (P>0.05) were found between control and diabetic tissues in their amount of total NAD+ and NADH/retina, and the ratio of NAD+ to NADH, or in their content of lactate, pyruvate, and adenosine triphosphate (ATP) or in the ratio of lactate to pyruvate. The content of lactate and pyruvate in retinas incubated for 2 hours in media containing 10 or 30 mM glucose was the same as found in fresh tissues, but the levels of these metabolites in retinas incubated in media containing 5 mM glucose declined in comparison to the fresh values. There were no significant differences in lactate content in cultured retinal and lens cells that were exposed to 5 or 30 mM glucose-containing media.

DISCUSSION

The present results do not support the hyperglycemic pseudohypoxia hypothesis of diabetic retinopathy.

Analysis of glucose metabolism in diabetic rat retinas

This study was conceived in an effort to understand cause and effect relationships between hyperglycemia and diabetic retinopathy. Numerous studies show that hyperglycemia leads to oxidative stress in the diabetic retinas, but the mechanisms that generate oxidative stress have not been resolved. Increased electron pressure on the mitochondrial electron transfer chain, increased generation of cytosolic NADH, and decreases in cellular NADPH have all been cited as possible sources of reactive oxygen species and nitrous oxide. In the present study, excised retinas from control and diabetic rats were exposed to euglycemic and hyperglycemic conditions. Using a microwave irradiation quenching technique to study retinas of diabetic rats in vivo, glucose, glucose-derived metabolites, and NADH oxidation/reduction status were measured. Studying excised retinas in vitro, glycolytic flux, lactate production, and tricarboxylic acid cycle flux were evaluated. Enzymatically assayed glucose 6-phosphate and fructose 6-phosphate were only slightly elevated by hyperglycemia and/or diabetes, but polyols were increased dramatically. Cytosolic NADH-to-NAD ratios were not elevated by hyperglycemia nor by diabetes in vivo or in vitro. Tricarboxylic acid cycle flux was not increased by the diabetic state nor by hyperglycemia. On the other hand, small increases in glycolytic flux were observed with hyperglycemia, but glycolytic flux was always lower in diabetic compared with control animals. An observed decrease in activity of glyceraldehyde-3-phosphate dehydrogenase may be partially responsible for slow glycolytic flux for retinas of diabetic rats. Therefore, it is concluded that glucose metabolism, downstream of hexokinase, is not elevated by hyperglycemia or diabetes. Metabolites upstream of glucose such as the sorbitol pathway (which decreases NADPH) and polyol synthesis are increased.

Glucose Metabolism in Rat Retinal Pigment Epithelium

The retinal pigment epithelium (RPE) is the major transport pathway for exchange of metabolites and ions between choroidal blood supply and the neural retina. To gain insight into the mechanisms controlling glucose metabolism in RPE and its possible relationship to retinopathy, we studied the influence of different glucose concentrations on glycogen and lactate levels and CO(2) production in RPE from normal and streptozotocin-treated diabetic rats. Incubation of normal RPE in the absence of glucose caused a decrease in lactate production and glycogen content. In normal RPE, increasing glucose concentrations from 5.6 mM to 30 mM caused a four-fold increase in glucose accumulation and CO(2) yield, as well as reduction in lactate and glycogen production. In RPE from diabetic rats glucose accumulation did not increase in the presence of high glucose substrate, but it showed a four- and a seven-fold increase in CO(2) production through the mitochondrial and pentose phosphate pathways, respectively. We found high glycogen levels in RPE which can be used as an energy reserve for RPE itself and/or neural retina. Findings further show that the RPE possesses a high oxidative capacity. The large increase in glucose shunting to the pentose phosphate pathway in diabetic retina exposed to high glucose suggests a need for reducing capacity, consistent with increased oxidative stress.

[Diabetic retinopathy]

Diabetic retinopathy is a frequently observed complication in both type 1 and type 2 diabetes, specially in patients with long term disease and poor glicemic control. Irreversible visual loss appears at the final stages of diabetic retinopathy and it is considered one of the most tragic of diabetic complications. It is also considered an important factor of morbidity and has a high economical impact once it is the leading cause of blindness. The pathophysiology of the retinal microvascular alterations is related to the chronic hyperglycemia that leads to the following circulatory disturbances: loss of vascular tonus, increase in vascular permeability, edema and exudation, with vascular obstruction and ischemia that stimulates neovascularization, which may lead to fibrous retraction and vitreous hemorrhages with retinal detachment. Recent studies have indicated that the strict glicemic and blood pressure controls are effective in reducing or blocking the progression of retinopathy. Up to now no pharmacological agents have shown to be effective in preventing or reducing neovascularization and visual loss. Presently, the most effective available treatment for proliferative retinopathy is laser photocoagulation. Further studies are needed to obtain new products and technologies that could effectively prevent or block retinopathy progression.

Distinct pathways of insulin-regulated versus diabetes-regulated gene expression: an in vivo analysis in MIRKO mice

Diabetes mellitus is a complex metabolic disorder accompanied by alterations in cellular physiology, metabolism, and gene expression. These alterations can be primary (due to loss of direct insulin action) or secondary (due to the metabolic perturbations associated with the disease). To dissect and quantitate these two separate effects, we compared the skeletal muscle gene-expression profiles of muscle insulin receptor knockout (MIRKO) mice and their Lox controls in the basal, streptozotocin-induced diabetic, and insulin-treated diabetic states. Pure deficiency of insulin action as present in the MIRKO mouse results in regulation of 130 genes, with down-regulation of NSF (N-ethylmaleimide-sensitive fusion protein) and VAMP-2 (vesicle-associated membrane protein 2), stearoyl CoA desaturase 1, and cAMP-specific phosphodiesterase 4B, as well as up-regulation of some signaling-related genes, such as Akt2, and the fatty-acid transporter CD36. In diabetes, additional transcriptional mechanisms are activated, resulting in alterations in expression of approximately 500 genes, including a highly coordinated down-regulation of genes of the mitochondrial electron-transport chain and one of the mammalian homologues of the histone deacetylase Sir2, which has been implicated in the link between nutrition and longevity. These distinct pathways of direct and indirect regulation of gene expression provide insights into the complex mechanisms of transcriptional control in diabetes and areas of potential therapeutic targeting.

Interactions between hyperglycemia and hypoxia: implications for diabetic retinopathy

The primary aim of these experiments was to assess in vitro effects of hyperglycemia (30 mmol/l glucose) and hypoxia (Po(2) = 36 torr) of 2-h duration, separately and in combination, on cytosolic and mitochondrial free NADH (NADHc and NADHm, respectively) in retinas from normal rats. NADH is the major carrier of electrons and protons that fuel ATP synthesis and several metabolic pathways linked to diabetic complications. Hyperglycemia and hypoxia increase free NADHc by different mechanisms that are additive. Hyperglycemia increases transfer of electrons and protons from sorbitol to NAD(+)c, reducing it to NADHc, but does not increase NADHm. Hypoxia increases NADHm by inhibiting its oxidation. Electrons and protons accumulating in NADHm restrain transfer of electrons and protons from NADHc to NAD(+)m via the malate-aspartate electron shuttle. Hyperglycemia and hypoxia also increase glycolysis by different mechanisms that are additive, and hyperglycemia increases ATP levels in hypoxic and in aerobic retinas. The additive effects of hyperglycemia and hypoxia on accumulation of electrons and protons in a common pool of free NADHc confirm the test hypothesis and the potential of a combination of these two risk factors to accelerate the onset and progression of diabetic retinopathy (and other complications of diabetes) by augmenting metabolic pathways fueled by free NADHc.

Glycogen metabolism in the rat retina

It has been reported that glycogen levels in retina vary with retinal vascularization. However, the electrical activity of isolated retina depends on glucose supply, suggesting that it does not contain energetic reserves. We determined glycogen levels and pyruvate and lactate production under various conditions in isolated retina. Ex vivo retinas from light- and dark-adapted rats showed values of 44 +/- 0.3 and 19.5 +/- 0.4 nmol glucosyl residues/mg protein, respectively. The glycogen content of retinas from light-adapted animals was reduced by 50% when they were transferred to darkness. Glycogen levels were low in retinas incubated in glucose-free media and increased in the presence of glucose. The highest glycogen values were found in media containing 20 mm of glucose. A rapid increase in lactate production was observed in the presence of glucose. Surprisingly, glycogen levels were the lowest and lactate production was also very low in the presence of 30 mm glucose. Our results suggest that glycogen can be used as an immediate accessible energy reserve in retina. We speculate on the possibility that gluconeogenesis may play a protective role by removal of lactic acid.

Aldose reductase inhibition prevents glucose-induced apoptosis in cultured bovine retinal microvascular pericytes

The pathogenesis of pericyte loss, an initial deficit in the early stage of diabetic retinopathy, remains unclear. Polyol pathway hyperactivity has been implicated in the pathogenesis of diabetic retinopathy, and recent studies have suggested that apoptosis may be involved in pericyte loss. The present study was conducted to investigate whether high glucose induces apoptosis in cultured bovine retinal pericytes. The effect of an aldose reductase inhibitor, SNK-860, was also examined. After a 5 day incubation with various concentrations of glucose (5.5-40 m M) in the presence or absence of SNK-860, the cell viability and the percentages of dead cells were measured, and staining with the TUNEL method and Hoechst 33342, and DNA electrophoresis were performed. High glucose reduced the viability and increased the percentages of dead cells. TUNEL-positive cells were observed in pericytes under high glucose, but not in those under 5.5 m M glucose. In the staining of nuclei with Hoechst 33342, the percentage of apoptotic cells in total cells counted under high glucose was higher than that under 5.5 m M glucose. DNA electrophoresis of pericytes cultured with high glucose demonstrated a 'ladder pattern'. Hyperosmolarity also induced apoptosis in pericytes, but less than that by high glucose. SNK-860 inhibited the glucose-induced apoptosis in pericytes. These observations suggest that the pericyte loss in diabetic retinopathy involves an apoptotic process, and that the polyol pathway hyperactivity plays an important role in inducing apoptosis in pericytes by high glucose.

Increased cerebral glucose utilization and decreased glucose transporter Glut1 during chronic hyperglycemia in rat brain

Whereas acute hyperglycemia has been shown to result in an unchanged local cerebral glucose utilization (LCGU) the changes of LCGU during chronic hyperglycemia are a matter of dispute. The present study had three aims: (1) To compare the effects of acute and chronic hyperglycemia on LCGU and to investigate in vivo the lactate level as a potential indicator of glycolytic flux. (2) To investigate local changes in brain Glut1 and/or Glut3 glucose transporter densities during chronic hyperglycemia. (3) To analyze the relationship between LCGU and local Glut densities during chronic hyperglycemia. To induce chronic hyperglycemia in rats steptozotocin was given i.p. and experiments were performed 3 weeks later. LCGU was measured by the 2-[14C]deoxyglucose method and intraparenchymal lactate concentration by MR-spectroscopy. Local densities of the glucose transport proteins were determined by immunoautoradiographic methods. During chronic hyperglycemia weighted average of LCGU increased by 13.9% whereas it remained unchanged during acute hyperglycemia. The cerebral lactate/choline ratio was increased by 143% during chronic hyperglycemia. The average density of glucose transporters Glut1 decreased by 7.5%. Local densities of Glut1 were decreased in 12 of 28 brain structures. Glut3 remained unchanged. Positive correlations were found between LCGU and local Glut densities during control conditions and during chronic hyperglycemia. It was concluded that (1) Chronic, but not acute hyperglycemia is followed by an increased LCGU. (2) The capacity to transport glucose is decreased during chronic hyperglycemia. (3) Increased LCGU and decreased densities of Glut1 are matched on a local level.

A review of current research on the effect of diabetes mellitus on the eye

It is estimated that almost one million Australians will have diabetes by the year 2000. Of those with diabetes a significant proportion will have eye-related conditions, the most debilitating being diabetic retinopathy. Appropriate identification and treatment can result in prevention of visual loss and blindness. The importance of diabetes as a cause of blindness in our community is realised by the commencement of a national program by the National Health and Medical Research Council to develop clinical practice guidelines for the management of diabetic retinopathy. The development of these guidelines was based on available evidence following an extensive review of the literature up to May 1996. This review is a summary of our advances in research on the effect of diabetes on various aspects of the eye and vision over the past two years. This review is a compilation of articles of research on the effect of diabetes on various aspects of the eye and vision. As a result of the enormous amount of effort and work by scientists and clinicians around the world, as well as space restrictions, the review covers the past two years only. Although every effort has been made to include as many research articles as possible, not all articles of research are covered. It is intended that this review provide an overview of the latest trends in research, particularly relating to new techniques and methods in the study of diabetes in ocular tissue as well as the new theories in the development of ocular damage to each of the tissue.