Glucose Neurotoxicity Research Articles

Effects of a Diabetic Microenvironment on Neurodegeneration: Special Focus on Neurological Cells.

Diabetes is a chronic metabolic condition associated with high levels of blood glucose which leads to serious damage to the heart, kidney, eyes, and nerves. Elevated blood glucose levels damage brain function and cognitive abilities. They also lead to various neurological and neuropsychiatric disorders, including chronic neurodegeneration and cognitive decline. High neuronal glucose levels can cause drastic neuronal damage due to glucose neurotoxicity. Astrocytes, a type of glial cell, play a vital role in maintaining brain glucose levels through neuron-astrocyte coupling. Hyperglycemia leads to progressive decline in neuronal networks and cognitive impairment, contributing to neuronal dysfunction and fostering a neurodegenerative environment. In this review, we summarize the various connections, functions, and impairments of glial cells due to metabolic dysfunction in the diabetic brain. We also summarize the effects of hyperglycemia on various neuronal functions in the diabetic brain.

Physiological assessment of high glucose neurotoxicity in mouse and rat retinal explants.

One of the tissues of the central nervous system most affected by diabetes is the retina. Despite a growing understanding of the biochemical processes involved in glucose toxicity, little is known about the physiological consequences of chronic high glucose (HG) on individual neurons and neuronal circuits. Electroretinogram recordings suggest that retinal bipolar cells (BCs), which filter and transmit photoreceptor output to the inner retina, are among the first cells affected by diabetic conditions, and may therefore serve as sensitive early biomarkers for incipient neuronal damage caused in diabetes. Here, we comparatively assessed retinal integrity, calcium responses, and the electrophysiological profiles of specific BC types of mouse and rat organotypic retinal explants after 1 to 3 weeks in tissue culture, under moderate glucose (MG) and HG conditions. While the retinal layers of both rodent species displayed a progressively reduced thickness in culture, BCs retained their electrophysiological profiles and remained morphologically identifiable for up to 2 weeks. Responses to glutamate and endogenous inhibitory responses were routinely observed, indicating that the retinal circuitry remained intact during this period. Significant physiological differences between MG and HG conditions were evident in calcium signals and in the time course of responses to glutamate, but the voltage-gated current profiles of BCs displayed only minor variations. Overall, rat retina appeared slightly more sensitive to HG levels compared with mouse. In conclusion, electrophysiological analysis of neuronal function in rodent retinal explants is useful for the study of early damage due to HG neurotoxicity.

Photobiomodulation increases cell viability via AKT activation in an in vitro model of diabetes induced by glucose neurotoxicity

Peripheral neuropathy (PN) is a serious complication of diabetes mellitus (DM) and is known to be resistant to conventional treatment. Photobiomodulation (PBM) is demonstrated to be effective in treating PN and in protecting nerve fiber damage. To better understand the mechanisms underlying the regenerative effects of PBM on diabetic neuropathy, we conducted a study in an in vitro model of diabetes induced by glucose neurotoxicity. Neuro 2A cells (1 × 104 cells/ well; N2A) were cultured in Minimum Essential Medium (MEM) supplemented with high glucose concentrations (100mM) for 48h and after the incubation period were submitted to either one or three consecutive applications of PBM, once a day (low-level InGaAlP, continuous wave mode, 660nm, 30mW, 1.6J/cm2, 15s, per well). Cell viability was measured by MTT method, neurotoxicity by LDH release, neurite outgrowth was evaluated through morphometric analysis, and AKT/ERK protein expression levels were assessed by western blotting. Results demonstrate that PBM increased N2A viability as well as induced neurogenesis observed by the increase in neurite outgrowth being this effect modulated by AKT activation. Data obtained herein reinforce the regenerative potential of PBM in the treatment of PN and strongly suggests that phototherapy should be considered adjuvant in the treatment of diabetes.

Mismatch between GLUTs and glucose levels causes neuronal damage during glucose fluctuations

Abnormal glucose levels damage the central nervous system, especially in case of rapid fluctuations. Even a single episode of glucose reperfusion can result in overt impairment of neurons. Oxidative stress plays an important role in this process, sharing properties with the pathophysiologic changes of glucose neurotoxicity. Glucose transporters (GLUTs) located in the brain are involved in direct glucose uptake by neurons. Instead of being insulin-sensitive, these transporters are regulated by glucose levels in the extracellular fluid, increasing their expression while glucose levels fall, to absorb more glucose. Therefore, we hypothesized that mismatch between altered GLUTs and sudden glucose level changes is responsible for neuronal damage during glucose fluctuations. Modulating hypoglycemia by increasing blood glucose slowly may improve the neurological outcomes of hypoglycemia.

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.

Oxidative stress-dependent MMP-13 activity underlies glucose neurotoxicity

BackgroundA complication of diabetes is neuropathy, a condition of sensory axon degeneration that originates in the epidermis. The mechanisms remain unknown but reactive oxygen species (ROS) have been implicated in this condition. In this study, we assessed the role of ROS and a candidate downstream target, MMP-13 in glucose-induced sensory axon degeneration in zebrafish and mice. MethodsThe effects of glucose on metabolism and sensory axon degeneration were assessed using qPCR and live imaging. ROS were analyzed using pentafluorobenzene-sulfonyl fluorescein and activation of the NF-κB stress response was determined using Tg(NF-κB:GFP) zebrafish. The role of MMP-13 and ROS in glucose-dependent axon degeneration was determined in zebrafish following treatment with the antioxidant, N-acetylcysteine and the MMP-13 inhibitor, DB04760. Neuropathic mice fed on a high-fat/high-sugar diet were treated with the MMP-13 inhibitor, CL-82198 to assess sensory recovery. ResultsGlucose treatment of zebrafish induced metabolic changes that resemble diabetes. Sensory axon degeneration was mediated by ROS-induced MMP-13 and prevented upon antioxidant treatment or MMP-13 inhibition. MMP-13 inhibition also reversed neuropathy in diabetic mice. ConclusionWe demonstrate that zebrafish are suitable to study glucose-induced neurotoxicity. Given the effects in zebrafish and mice, MMP-13 inhibition may be beneficial in the treatment of human diabetic neuropathy.

Cytoprotective Effects of Citicoline and Homotaurine against Glutamate and High Glucose Neurotoxicity in Primary Cultured Retinal Cells.

Citicoline and homotaurine are renowned compounds that exhibit potent neuroprotective activities through distinct molecular mechanisms. The present study was undertaken to demonstrate whether cotreatment with citicoline and homotaurine affects cell survival in primary retinal cultures under experimental conditions simulating retinal neurodegeneration. Primary cultures were obtained from the retina of fetal rats and exposed to citicoline plus homotaurine (100 μM). Subsequently, neurotoxicity was induced using excitotoxic levels of glutamate and high glucose concentrations. The effects on retinal cultures were assessed by cell viability and immunodetection of apoptotic oligonucleosomes. The results showed that a combination of citicoline and homotaurine synergistically decreases proapoptotic effects associated with glutamate- and high glucose-treated retinal cultures. This study provides an insight into the potential application of citicoline and homotaurine as a valuable tool to exert neuroprotective effects against retinal damage.

Berberine, a natural antidiabetes drug, attenuates glucose neurotoxicity and promotes Nrf2-related neurite outgrowth

Reactive oxygen intermediates production and apoptotic damage induced by high glucose are major causes of neuronal damage in diabetic neuropathy. Berberine (BBR), a natural antidiabetes drug with PI3K-activating activity, holds promise for diabetes because of its dual antioxidant and anti-apoptotic activities. We have previously reported that BBR attenuated H2O2 neurotoxicity via activating the PI3K/Akt/Nrf2-dependent pathway. In this study, we further explored the novel protective mechanism of BBR on high glucose-induced apoptotic death and neurite damage of SH-SY5Y cells. Results indicated BBR (0.1–10nM) significantly attenuated reactive oxygen species (ROS) production, nucleus condensation, and apoptotic death in high glucose-treated cells. However, AG1024, an inhibitor of insulin growth factor-1 (IGF-1) receptor, significantly abolished BBR protection against high glucose-induced neuronal death. BBR also increased Bcl-2 expression and decreased cytochrome c release. High glucose down-regulated IGF-1 receptor and phosphorylation of Akt and GSK-3β, the effects of which were attenuated by BBR treatment. BBR also activated nuclear erythroid 2-related factor 2 (Nrf2), the key antioxidative transcription factor, which is accompanied with up-regulation of hemeoxygenase-1 (HO-1). Furthermore, BBR markedly enhanced nerve growth factor (NGF) expression and promoted neurite outgrowth in high glucose-treated cells. To further determine the role of the Nrf2 in BBR neuroprotection, RNA interference directed against Nrf2 was used. Results indicated Nrf2 siRNA abolished BBR-induced HO-1, NGF, neurite outgrowth and ROS decrease. In conclusion, BBR attenuated high glucose-induced neurotoxicity, and we are the first to reveal this novel mechanism of BBR as an Nrf2 activator against glucose neurotoxicity, providing another potential therapeutic use of BBR on the treatment of diabetic complications.

Is cognitive impairment the fourth diabetic microvascular complication?

Is cognitive impairment the fourth diabetic microvascular complication?

Insulin receptor substrate-1 and -2 mediate resistance to glucose-induced caspase-3 activation in human neuroblastoma cells

Hyperglycemia in patients with type 2 diabetes causes multiple neuronal complications, e.g., diabetic polyneuropathy, cognitive decline, and embryonic neural crest defects due to increased apoptosis. Possible mechanisms of neuronal response to increased glucose burden are still a matter of debate. Insulin and insulin-like growth factor-1 (IGF-1) receptor signaling inhibits glucose-induced caspase-3 activation and apoptotic cell death. The insulin receptor substrates (IRS) are intracellular adapter proteins mediating insulin's and IGF-1's intracellular effects. Even though all IRS proteins have similar function and structure, recent data suggest different actions of IRS-1 and IRS-2 in mediating their anti-apoptotic effects in glucose neurotoxicity. We therefore investigated the role of IRS-1/-2 in glucose-induced caspase-3 activation using human neuroblastoma cells. Overexpression of IRS-1 or IRS-2 caused complete resistance to glucose-induced caspase-3 cleavage. Inhibition of PI3-kinase reversed this protective effect of IRS-1 or IRS-2. However, MAP-kinases inhibition had only minor impact. IRS overexpression increased MnSOD abundance as well as BAD phosphorylation while Bim and BAX levels remained unchanged. Since Akt promotes cell survival at least partially via phosphorylation and inhibition of downstream forkhead box-O (FoxO) transcription factors, we generated neuroblastoma cells stably overexpressing a dominant negative mutant of FoxO1 mimicking activation of the insulin/IGF-1 pathway on FoxO-mediated transcription. Using these cells we showed that FoxO1 is not involved in neuronal protection mediated by increased IRS-1/-2 expression. Thus, overexpression of both IRS-1 and IRS-2 induces complete resistance to glucose-induced caspase-3 activation via PI3-kinase mediated BAD phosphorylation and MnSOD expression independent of FoxO1.

Protective effect of saffron extract and crocin on reactive oxygen species-mediated high glucose-induced toxicity in PC12 cells.

Diabetic neuropathy is one of the most frequent complications of diabetes. Despite some studies, the exact mechanism of glucose neurotoxicity has not been fully elucidated. Increased reactive oxygen species (ROS) has proposed as a possible mechanism. Crocus sativus L. (saffron) has been known as a source of antioxidants. Therefore, neuroprotective effect of saffron extract, its active component crocin and gamma-glutamylcysteinylglycine (GSH) was studied in glucose-induced neurotoxicity, using PC12 cells as a suitable in vitro model of diabetic neuropathy. Cell viability was quantitated by MTT assay. ROS was measured using DCF-DA by flow cytometry analysis. The result showed that glucose (13.5 and 27 mg/ml) reduced the cell viability of PC12 cells after 4 days. Saffron extract (5 and 25 mg/ml), crocin (10 and 50 muM) and GSH (10 muM) could decrease this toxicity. Glucose toxicity was consistent with increased ROS production which reduced by saffron, crocin and GSH pretreatment. These results suggest saffron and its carotenoid crocin could be potentially useful in diabetic neuropathy treatment.

Glucose neurotoxicity

Neurons have a constantly high glucose demand, and unlike muscle cells they cannot accommodate episodic glucose uptake under the influence of insulin. Neuronal glucose uptake depends on the extracellular concentration of glucose, and cellular damage can ensue after persistent episodes of hyperglycaemia--a phenomenon referred to as glucose neurotoxicity. This article reviews the pathophysiological manifestation of raised glucose in neurons and how this can explain the major components of diabetic neuropathy.

Involvement of nitric oxide in glucose toxicity on differentiated PC12 cells: prevention of glucose toxicity by tetrahydrobiopterin, a cofactor for nitric oxide synthase.

Effects of high concentrations of glucose on cell survival of differentiated PC12 cells were examined. Seven day-culture with D-glucose (9.0-27.0 mg/ml as 2-6-fold of the optimal level) induced cell death in a dose-related manner but 3-day culture with high concentrations of glucose had no effect on cell viability. L-glucose had no effect on viability of PC12 cells, suggesting that D-glucose toxicity was independent of its osmolarity effect. Seven-day culture with D-glucose (13.5 mg/ml as 3-fold of the optimal level) increased nitric oxide metabolites (NOx) in the culture medium. Glucose-induced increase in NOx was eliminated by 0.1 mM L-nitro-arginine methylester (L-NAME), a nitric oxide synthase (NOS) inhibitor. Intracellular Ca(2+) concentration was increased by D-glucose in a dose-related manner, suggesting that D-glucose activated NOS by increasing intracellular Ca(2+) concentration in PC12 cells. Glucose-induced cell death was blunted by 0.1 mM L-NAME, showing that nitric oxide (NO) was involved in the glucose toxicity to PC12 cells. Tetrahydrobiopterin (BH(4)), a cofactor for NOS, attenuated both glucose-induced cell death and NOx production at 1 microM but not at 10 microM. The effects of BH(4) on glucose-induced cell death and NOx production were not mimicked by reducing agents such as ascorbate and cysteine. These results taken together suggest that high concentrations of glucose induced cell death via NO production and that low concentration of BH(4) had a protective effect against glucose neurotoxicity in differentiated PC12 cells.

Genetic influences on glucose neurotoxicity, aging, and diabetes: a possible role for glucose hysteresis.

Glucose may drive some age-correlated impairments and may mediate some effects of dietary restriction on senescence. The hypothesis that cumulative deleterious effects of glucose may impair hypothalamic neurons during aging, leading to hyperinsulinemia and other age-correlated pathologies, is examined in the context of genetic influences. Susceptibility to toxic effects of gold-thio-glucose (GTG) is correlated with longevity across several mouse strains. GTG and chronic hyperglycemia induce specific impairments in the ventromedial hypothalamus similar to impairments which occur during aging. GTG and a high-calorie diet both induce chronic hyperinsulinemia, leading initially to hypoglycemia, followed by the development of insulin resistance and hyperglycemia. Aging in humans and rodents appears to entail a similar pattern of hyperinsulinemia followed by insulin resistance. In humans, genetic susceptibility to high-calorie diet-induced impairments in glucose metabolism is extremely common in many indigenous populations, possibly due to the selection of the 'thrifty genotype'. It is suggested that the 'thrifty genotype' may entail enhanced sensitivity to the neurotoxic effects of glucose, and may represent an example of antagonistic pleiotropy in human evolution. These data are consistent with the hypothesis that genetic susceptibility of hypothalamic neurons to the cumulative toxic effects of glucose (glucose neurohumoral hysteresis) may correlate with genetic influences on longevity.