High glucose-induced expression of inflammatory cytokines and reactive oxygen species in cultured astrocytes

Modulation of endoplasmic reticulum chaperone GRP78 by high glucose in hippocampus of streptozotocin-induced diabetic mice and C6 astrocytic cells

Diabetes mellitus is known to increase the risk of neurodegeneration, and both diseases are reported to be linked to dysfunction of endoplasmic reticulum (ER). Astrocytes are important in the defense mechanism of central nervous system (CNS), with great ability of tolerating accumulation of toxic substances and sensitivity in Ca(2+) homeostasis which are two key functions of ER. Here, we investigated the modulation of the glucose-regulated protein 78 (GRP78) in streptozotocin (STZ)-induced diabetic mice and C6 cells cultured in high glucose condition. Our results showed that more reactive astrocytes were presented in the hippocampus of STZ-induced diabetic mice. Simultaneously, decrease of GRP78 expression was found in the astrocytes of diabetic mice hippocampus. In in vitro study, C6 cells were treated with high glucose to investigate the role of high glucose in GRP78 modulation in astrocytic cells. GRP78 as well as other chaperones like GRP94, calreticulin and calnexin, transcription levels were down-regulated after high glucose treatment. Also C6 cells challenged with 48h high glucose were activated, as indicated by increased level of glial fibrillary acidic protein (GFAP). Activated C6 cells simultaneously exhibited significant decrease of GRP78 level and was followed by reduced phosphorylation of Akt. Moreover, unfolded protein response was induced as an early event, which was marked by the induction of CHOP with high glucose treatment, followed by the reduction of GRP78 after 48h. Finally, the upsurge of ROS production was found in high glucose treated C6 cells and chelation of ROS could partially restore the GRP78 expression. Taken together, these data provide evidences that high glucose induced astrocytic activation in both in vivo and in vitro diabetic models, in which modulation of GRP78 would be an important event in this activation.

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

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

Mesencephalic astrocyte-derived neurotrophic factor inhibits oxygen-glucose deprivation-induced cell damage and inflammation by suppressing endoplasmic reticulum stress in rat primary astrocytes

Astrocyte inflammation plays important roles both in physiological and pathological processes in the central nervous system (CNS). Ischemic injury in the CNS causes damage to astrocytes and the release of proinflammatory cytokines, such as tumor necrosis factor-α, interleukin-1β, and interleukin-6. This current study investigates whether mesencephalic astrocyte-derived neurotrophic factor (MANF) inhibits oxygen-glucose deprivation (OGD)-induced cell damage and inflammatory cytokine secretion by suppressing endoplasmic reticulum stress in rat primary astrocytes. We found that MANF alleviated OGD-induced astrocyte damage and rescued the cell viability, and the upregulation of GRP78 (endoplasmic reticulum (ER) stress marker) and NF-κB p65 (one of the central mediators of proinflammatory pathways) induced by OGD were significantly reduced by preincubation of MANF. In addition, the increases of secretion and mRNA expression levels of the proinflammatory cytokines IL-1β, IL-6, and TNF-α in astrocytes induced by OGD were significantly suppressed by MANF. These findings demonstrate that MANF shows the potential to alleviate cell damage and inflammation in rat primary astrocytes by suppressing ER stress, indicating that MANF plays an important role in astrocyte inflammation and functioning and may suggest a promising strategy for neuroprotection in the CNS.

Cytotoxicity and vitreous stability of chemically modified connexin43 mimetic peptides for the treatment of optic neuropathy

Optic neuropathy is associated with retinal ganglion cell (RGC) loss leading to optic nerve damage and visual impairment. Unregulated connexin (Cx) hemichannel opening plays a role in RGC loss. Thus, inhibition via Cx43-specific mimetic peptides (MP) may prevent further cell death. However, the highly hydrophilic character and poor stability of native peptides prevent their efficient delivery across biological membranes. The present study aimed to improve the stability of Cx43 MP by conjugation to C12-lipoamino acid (C12-Laa) or sugar groups. Unmodified and modified Cx43 MP were synthesized using solid-phase peptide synthesis. Their functionality was assessed by propidium iodide (PI) uptake into NT2 cells, a human testicular carcinoma progenitor cell line able to differentiate into astrocytes, whereas the stability in ocular vitreous was measured by reversed-phase high-performance liquid chromatography. PI uptake studies showed inhibition of hemichannel opening for unmodified and modified Cx43 MP. Stability measurements revealed improved stability of modified Cx43 MP, with two Laa groups increasing the peptide half-life in bovine vitreous more than twofold. Conjugation to C12 -Laa or sugar did not affect the functionality of Cx43 MP, but addition of two C12-Laa groups significantly improved peptide stability. Laa-modifications may therefore offer improved stability and retinal delivery of peptides in vivo.

Diabetes mellitus and inflammatory pulpal and periapical disease: a review

Diabetes mellitus (DM) is one of the most common metabolic disorders. DM is characterized by hyperglycaemia, resulting in wound healing difficulties and systemic and oral manifestations, which have a direct effect on dental pulp integrity. Experimental and clinical studies have demonstrated a higher prevalence of periapical lesions in patients with uncontrolled diabetes. The influence of DM on periapical bone resorption and its impact on dental intervention of such patients are reviewed, and its aetiology and pathogenesis are analysed at molecular level. Pulps from patients with diabetes have the tendency to present limited dental collateral circulation, impaired immune response, increased risk of acquiring pulp infection (especially anaerobic ones) or necrosis, besides toothache and occasional tendency towards pulp necrosis caused by ischaemia. In regard to molecular pathology, hyperglycaemia is a stimulus for bone resorption, inhibiting osteoblastic differentiation and reducing bone recovery. The relationship between poorly controlled diabetes and bone metabolism is not clearly understood. Molecular knowledge about pulp alterations in patients with diabetes could offer new therapeutic directions. Knowledge about how diabetes affects systemic and oral health has an enduring importance, because it may imply not only systemic complications but also a higher risk of oral diseases with a significant effect on pulp and periapical tissue.

Melatonin suppresses migration and invasion via inhibition of oxidative stress pathway in glioma cells

Melatonin, an indolamine produced and secreted predominately by the pineal gland, exhibits a variety of physiological functions, possesses antioxidant and antitumor properties. In this study, we have shown that pharmacologic concentration (1mm) of melatonin significantly reduced cell migration and invasion of T98G and U251 glioma cells after 24-hr treatment and inhibited expression of matrix metalloproteinase 2 (MMP 2) and MMP 9. The melatonin inhibition of cell migration and invasion was associated with its reduction of intracellular basal free radical generation. Melatonin at pharmacologic concentration also inhibited the constitutive activation of the reactive oxygen species downstream transcription factor nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB). Furthermore, pyrrolidine dithiocarbamate, a NF-κB-specific inhibitor, at 10μm displayed anti-migration and invasion effects and inhibition of MMP 2 and MMP 9 expression resembling that of melatonin. Taken together, it is concluded that inhibition of migration and invasion of glioma cells by melatonin is associated with the latter in its inhibition of oxidative stress pathway. This suggests a potential therapeutic application of melatonin in the treatment of glioma.

Primary cultures of astrocytes: their value in understanding astrocytes in health and disease

During the past few decades of astrocyte research it has become increasingly clear that astrocytes have taken a central position in all central nervous system activities. Much of our new understanding of astrocytes has been derived from studies conducted with primary cultures of astrocytes. Such cultures have been an invaluable tool for studying roles of astrocytes in physiological and pathological states. Many central astrocytic functions in metabolism, amino acid neurotransmission and calcium signaling were discovered using this tissue culture preparation and most of these observations were subsequently found in vivo. Nevertheless, primary cultures of astrocytes are an in vitro model that does not fully mimic the complex events occurring in vivo. Here we present an overview of the numerous contributions generated by the use of primary astrocyte cultures to uncover the diverse functions of astrocytes. Many of these discoveries would not have been possible to achieve without the use of astrocyte cultures. Additionally, we address and discuss the concerns that have been raised regarding the use of primary cultures of astrocytes as an experimental model system.

Fueling and imaging brain activation

Metabolic signals are used for imaging and spectroscopic studies of brain function and disease and to elucidate the cellular basis of neuroenergetics. The major fuel for activated neurons and the models for neuron–astrocyte interactions have been controversial because discordant results are obtained in different experimental systems, some of which do not correspond to adult brain. In rats, the infrastructure to support the high energetic demands of adult brain is acquired during postnatal development and matures after weaning. The brain's capacity to supply and metabolize glucose and oxygen exceeds demand over a wide range of rates, and the hyperaemic response to functional activation is rapid. Oxidative metabolism provides most ATP, but glycolysis is frequently preferentially up-regulated during activation. Underestimation of glucose utilization rates with labelled glucose arises from increased lactate production, lactate diffusion via transporters and astrocytic gap junctions, and lactate release to blood and perivascular drainage. Increased pentose shunt pathway flux also causes label loss from C1 of glucose. Glucose analogues are used to assay cellular activities, but interpretation of results is uncertain due to insufficient characterization of transport and phosphorylation kinetics. Brain activation in subjects with low blood-lactate levels causes a brain-to-blood lactate gradient, with rapid lactate release. In contrast, lactate flooding of brain during physical activity or infusion provides an opportunistic, supplemental fuel. Available evidence indicates that lactate shuttling coupled to its local oxidation during activation is a small fraction of glucose oxidation. Developmental, experimental, and physiological context is critical for interpretation of metabolic studies in terms of theoretical models.

Endothelial Cells Derived from the Blood-Brain Barrier and Islets of Langerhans Differ in their Response to the Effects of Bilirubin on Oxidative Stress Under Hyperglycemic Conditions

Unconjugated bilirubin (UCB) is a neurotoxic degradation product of heme. Its toxic effects include induction of apoptosis, and ultimately neuronal cell death. However, at low concentrations, UCB is a potent antioxidant that may protect cells and tissues against oxidative stress by neutralizing toxic metabolites such as reactive oxygen species (ROS). High glucose levels (hyperglycemia) generate reactive metabolites. Endothelial cell dysfunction, an early vascular complication in diabetes, has been associated with hyperglycemia-induced oxidative stress. Both glucose and UCB are substrates for transport proteins in microvascular endothelial cells of the blood-brain barrier (BBB). In the current study we show that UCB (1–40 μM) induces apoptosis and reduces survival of bEnd3 cells, a mouse brain endothelial cell line which serves as an in vitro model of the BBB. These deleterious effects of UCB were enhanced in the presence of high glucose (25 mM) levels. Interestingly, the bEnd3 cells exhibited an increased sensitivity to the apoptotic effects of UCB when compared to the MS1 microcapillary endothelial cell line. MS1 cells originate from murine pancreatic islets of Langerhans, and are devoid of the barrier characteristics of BBB-derived endothelial cells. ROS production was increased in both bEnd3 and MS1 cells exposed to high glucose, as compared with cells exposed to normal (5.5 mM) glucose levels. While UCB (0.1–40 μM) did not alter ROS production in cells exposed to normal glucose, relatively low (“physiological”) UCB concentrations (0.1–5 μM) attenuated ROS generation in both cell lines exposed to high glucose levels. Most strikingly, higher UCB concentrations (20–40 μM) increased ROS generation in bEnd3 cells exposed to high glucose, but not in similarly treated MS1 cells. These results may be of critical importance for understanding the vulnerability of the BBB endothelium upon exposure to increasing UCB levels under hyperglycemic conditions.