Insulin-induced oxidative neuronal injury in cortical culture: mediation by induced N-methyl-D-aspartate receptors

Assessment of the prevalence and risk factors of ophthalmoplegia among diabetic patients in a large national diabetes registry cohort

BACKGROUND

There are limited data on the epidemiology and risk factors of ophthalmoplegia among diabetic patients. This study aims to determine the prevalence and important risk factors related to ophthalmoplegia among diabetic patients.

METHODS

This is an observational registry-based study using the Saudi National Diabetes Registry (SNDR) database to select diabetic patients regardless of their diabetes type. A total of 64,351 Saudi diabetic patients aged more than 18 years and registered in SNDR between January 2000 and December 2010 were analyzed to identify ophthalmoplegic cases. Demographic, clinical, and biochemical parameters were studied and STROBE guidelines were used to design and report the results of this study.

RESULTS

The overall prevalence of ophthalmoplegia cases was 0.32 %, further distributed into: 53.11 %, 36.36 %, and 2.8 % for cranial nerves VI, III, IV palsies respectively. Ophthalmoplegic cases were predominantly type 2 diabetic males with older age and longer diabetes duration. The most important and significant risk factors were age ≥ 45 years, diabetes duration ≥ 10 years, male gender and presence of retinopathy and nephropathy.

CONCLUSIONS

Ophthalmoplegia is a rare entity associated mainly with type 2 diabetes. Clinicians have to consider its risk factors when screening or planning for prevention of this condition.

Insulin neuroprotection and the mechanisms

Objective:

To analyze the mechanism of neuroprotection of insulin and which blood glucose range was benefit for insulin exerting neuroprotective action.

Data Sources:

The study is based on the data from PubMed.

Study Selection:

Articles were selected with the search terms “insulin”, “blood glucose”, “neuroprotection”, “brain”, “glycogen”, “cerebral ischemia”, “neuronal necrosis”, “glutamate”, “γ-aminobutyric acid”.

Results:

Insulin has neuroprotection. The mechanisms include the regulation of neurotransmitter, promoting glycogen synthesis, and inhibition of neuronal necrosis and apoptosis. Insulin could play its role in neuroprotection by avoiding hypoglycemia and hyperglycemia.

Conclusions:

Intermittent and long-term infusion insulin may be a benefit for patients with ischemic brain damage at blood glucose 6–9 mmol/L.

Elevated risk of type 2 diabetes for development of Alzheimer disease: a key role for oxidative stress in brain

Alzheimer disease (AD) is the most common form of dementia among the elderly and is characterized by progressive loss of memory and cognition. Epidemiological data show that the incidence of AD increases with age and doubles every 5 years after 65 years of age. From a neuropathological point of view, amyloid-β-peptide (Aβ) leads to senile plaques, which, together with hyperphosphorylated tau-based neurofibrillary tangles and synapse loss, are the principal pathological hallmarks of AD. Aβ is associated with the formation of reactive oxygen (ROS) and nitrogen (RNS) species, and induces calcium-dependent excitotoxicity, impairment of cellular respiration, and alteration of synaptic functions associated with learning and memory. Oxidative stress was found to be associated with type 2 diabetes mellitus (T2DM), which (i) represents another prevalent disease associated with obesity and often aging, and (ii) is considered to be a risk factor for AD development. T2DM is characterized by high blood glucose levels resulting from increased hepatic glucose production, impaired insulin production and peripheral insulin resistance, which close resemble to the brain insulin resistance observed in AD patients. Furthermore, growing evidence suggests that oxidative stress plays a pivotal role in the development of insulin resistance and vice versa. This review article provides molecular aspects and the pharmacological approaches from both preclinical and clinical data interpreted from the point of view of oxidative stress with the aim of highlighting progresses in this field.

Redox-regulating role of insulin: the essence of insulin effect

It is well-known that insulin acts as an important hormone, controlling energy metabolism, cellular proliferation and biosynthesis of functional molecules to maintain a biological homeostasis. Over the past few years, intensive insulin therapy has been believed to be benefit for the outcome of diabetic patients, in which the suppression of oxidative stress plays a role. Moreover, insulin is accepted as a key component of glucose-insulin-potassium, a treatment which has been believed to exert significant cardiovascular protective effect via the reduction of oxidative stress. Furthermore, accumulating evidence has suggested that insulin exerts important redox-regulating actions in various insulin-sensitive target organs, implying the systematic antioxidative role of insulin as a hormone. It is time for us to revisit insulin effects, through summarizing and evaluating the novel functions of insulin and their mechanisms. This review focuses on the antioxidative effect of insulin and highlights insulin-induced regulation of various antioxidant enzymes via insulin signaling pathways and the cross talk between key transcription factors, including nuclear factor erythroid 2-related factor 2 (Nrf2) and nuclear factor κB (NF-κB) which are responsible for the transcription of antioxidant enzymes, leading to reduced generation of reactive oxygen species (ROS) and the enhancement of the elimination of ROS.

Insulin in central nervous system: more than just a peripheral hormone

Insulin signaling in central nervous system (CNS) has emerged as a novel field of research since decreased brain insulin levels and/or signaling were associated to impaired learning, memory, and age-related neurodegenerative diseases. Thus, besides its well-known role in longevity, insulin may constitute a promising therapy against diabetes- and age-related neurodegenerative disorders. More interestingly, insulin has been also faced as the potential missing link between diabetes and aging in CNS, with Alzheimer's disease (AD) considered as the "brain-type diabetes." In fact, brain insulin has been shown to regulate both peripheral and central glucose metabolism, neurotransmission, learning, and memory and to be neuroprotective. And a future challenge will be to unravel the complex interactions between aging and diabetes, which, we believe, will allow the development of efficient preventive and therapeutic strategies to overcome age-related diseases and to prolong human "healthy" longevity. Herewith, we aim to integrate the metabolic, neuromodulatory, and neuroprotective roles of insulin in two age-related pathologies: diabetes and AD, both in terms of intracellular signaling and potential therapeutic approach.

Postnatal age influences hypoglycemia-induced neuronal injury in the rat brain

Acute hypoglycemia is associated with neuronal injury in the mature human and rodent brains. Even though hypoglycemia is a common metabolic problem during development, its effects on the developing brain are not well understood. To characterize the severity of regional brain injury, postnatal day (P) 7, P14, P28 (N=20-30/age) and adult rats (N=8-12) were subjected to acute hypoglycemia of equivalent severity and duration (mean blood glucose concentration: 30.0+/-0.1 mg/dL for 210 min). Neuronal injury in the cerebral cortex, striatum, hippocampus and hypothalamus was assessed 24 h, 72 h and 1 wk later by determining the number of degenerating cells positive for Fluoro-Jade B (FJB+) in the region. Compared with age-matched control, greater number of FJB+ cells was present per brain section of P14, P28 and adult hypoglycemia groups (p<0.005, each). The cerebral cortex was more vulnerable than hippocampus and striatum at all three ages (p<0.01). Compared with P28 (131+/-21) and adult (171+/-21) rats, fewer FJB+ cells (39+/-6) per brain section were present in P14 hypoglycemic rats (p<0.01, each). Hypoglycemia was not associated with cell injury in P7 rats. FJB+ cells were absent in the hypothalamus in all four ages. Similar results were present 24 h post-hypoglycemia, whereas analysis at 1 wk demonstrated efficient clearing of FJB+ cells in the brain regions of developing rats. Varying the duration of fasting did not alter the severity of regional cell injury. These results suggest that postnatal age influences the regional vulnerability to hypoglycemia-induced neuronal death in the rat brain.

Pentoxifylline and insulin-like growth factor-I (IGF-I) abrogate kainic acid-induced cognitive impairment in mice

Hippocampal insults involving neuroimmune mechanisms can impair learning and memory in a variety of tasks. The present study was designed to assess the effect of pentoxifylline, an inhibitor of tumor necrosis factor alpha (TNFalpha), and insulin-like growth factor-I (IGF-I) on kainate (KA)-induced impairment in spatial memory. Male mice received a subcutaneous injection of a dose of KA (15 mg/kg) that had no cytotoxic effect on hippocampal neurons as confirmed by Fluorojade B staining. This dose resulted in an impairment of spatial memory in a two-trial recognition task 11 days later. Intraperitoneal administration of pentoxifylline (200 mg/kg) abrogated this effect. Repeated intracerebroventricular injection of IGF-I (2 microg/mouse on day 1 followed by 1 microg/mouse on days 2-5) abrogated KA-induced deficits in spatial memory whereas acute IGF-I (2 microg/mouse on day 1 only) had mixed effects. These findings indicate that endogenous TNFalpha is probably involved in the detrimental effects of kainate on cognition and that exogenous IGF-I can oppose these effects, probably by antagonizing TNFalpha-induced neurotoxicity.

Insulin decreases isoflurane minimum alveolar anesthetic concentration in rats independently of an effect on the spinal cord

The observation that insulin supplies an element of analgesia suggests that insulin administration might decrease the concentration of inhaled anesthetic required to produce MAC (the minimum alveolar anesthetic concentration required to eliminate movement in response to noxious stimulation in 50% of subjects). We hypothesized that insulin decreases MAC by directly affecting the nervous system, by decreasing blood glucose, or both. To test these hypotheses, we infused increasing doses of insulin either intrathecally or IV in rats anesthetized with isoflurane and determined the resulting MAC change (assessing forelimb and hindlimb movement separately). Infusion of insulin produced a dose-related decrease in MAC that did not differ among groups. That is, the IV and intrathecal infusions caused similar decreases in MAC at a given infusion rate. Blood glucose concentrations were larger in the rats given insulin with 5% dextrose. However, the percentage change in MAC determined from forelimb versus hindlimb movement did not differ. For a given insulin infusion rate, MAC changes and glucose levels did not correlate with each other, except, possibly, for the most rapid infusion rate, for which smaller glucose concentrations were associated with a marginally larger decrease in MAC. Intrathecal infusions of insulin did not produce spinal cord injury. In summary, we found that insulin decreases isoflurane MAC in a dose-related manner independently of its effects on the blood concentration of glucose. The sites at which insulin acts to decrease MAC appear to be supraspinal rather than spinal. The effect may be due to a capacity of insulin to produce analgesia through an action on one or more neurotransmitter receptors.

IMPLICATIONS

Intrathecal and IV insulin administration equally decrease isoflurane MAC in rats, regardless of the concentration of blood sugar. These findings indicate that although insulin decreases MAC, the decrease is not mediated by actions on the spinal cord.

NR2A induction and NMDA receptor-dependent neuronal death by neurotrophin-4/5 in cortical cell culture

We have previously shown that prolonged exposure to neurotrophins induces oxidative neuronal death. In the present study, we further examined the cascades involved in neurotrophin-4/5 (NT-4/5)-induced neuronal death. Exposure of mature cortical cultures for 48 h to NT-4/5 induced neuronal death through TrkB activation. The NT-4/5-induced neuronal death was largely attenuated by addition of MK-801, indicating a critical role for NMDA receptors. Western blots revealed the induction of NR2A by NT-4/5. In addition, levels of phospho-NR2A and 2B increased, suggesting the upregulation of the NMDA receptor function. Whereas glutamate levels in the media changed little, levels of D-serine and L-glycine, co-agonists at NMDA receptors, increased significantly following NT-4/5 treatment. Exposure to NT-4/5 resulted in the activation of Src and extracellular signal-regulated kinase-1/2 (Erk-1/2). Their inhibitors blocked NR2A induction and phosphorylation as well as neuronal death induced by NT-4/5. In addition, Egr-1 was induced in an Src- and Erk-1/2-dependent manner. Anti-sense oligodeoxynucleotides to egr-1 attenuated NR2A induction as well as neuronal death. Although induction of NADPH oxidase and neuronal nitric oxide synthase (nNOS) contributes to NT-4/5-induced neuronal death, inhibition of their activity did not reduce NR2A induction. Conversely, blockade of NMDA receptors did not attenuate induction of NADPH oxidase or nNOS. These results indicate that two events are largely independent of each other. Our results demonstrate that the signaling cascade of TrkB leads to increase in NMDA receptor activity. Whereas this cascade may play an important role in the modulation of NMDA receptors in physiologic conditions, in the context of TrkB overactivation, it may contribute to neuronal death.

Necrosis: a specific form of programmed cell death?

For a long time necrosis was considered as an alternative to programmed cell death, apoptosis. Indeed, necrosis has distinct morphological features and it is accompanied by rapid permeabilization of plasma membrane. However, recent data indicate that, in contrast to necrosis caused by very extreme conditions, there are many examples when this form of cell death may be a normal physiological and regulated (programmed) event. Various stimuli (e.g., cytokines, ischemia, heat, irradiation, pathogens) can cause both apoptosis and necrosis in the same cell population. Furthermore, signaling pathways, such as death receptors, kinase cascades, and mitochondria, participate in both processes, and by modulating these pathways, it is possible to switch between apoptosis and necrosis. Moreover, antiapoptotic mechanisms (e.g., Bcl-2/Bcl-x proteins, heat shock proteins) are equally effective in protection against apoptosis and necrosis. Therefore, necrosis, along with apoptosis, appears to be a specific form of execution phase of programmed cell death, and there are several examples of necrosis during embryogenesis, a normal tissue renewal, and immune response. However, the consequences of necrotic and apoptotic cell death for a whole organism are quite different. In the case of necrosis, cytosolic constituents that spill into extracellular space through damaged plasma membrane may provoke inflammatory response; during apoptosis these products are safely isolated by membranes and then are consumed by macrophages. The inflammatory response caused by necrosis, however, may have obvious adaptive significance (i.e., emergence of a strong immune response) under some pathological conditions (such as cancer and infection). On the other hand, disturbance of a fine balance between necrosis and apoptosis may be a key element in development of some diseases.

Modification of glutamate binding sites in newborn brain during hypoglycemia

We have shown that acute insulin-induced hypoglycemia leads to specific changes in the cerebral NMDA receptor-associated ion channel in the newborn piglet. The present study tests the hypothesis that exposure to acute hypoglycemia in the newborn will alter the glutamate binding site of both NMDA and kainate receptors. Studies were performed in 3-6 days-old piglets randomized to control (n=6) or hypoglycemic (n=6) groups. Hypoglycemia was maintained for 120 min using insulin infusion. Saturation binding assays were performed in cerebral cell membranes using (3)H-glutamate or (3)H-kainate to determine the characteristics of the glutamate binding sites of the NMDA and kainate receptors, respectively. The concentration of glucose in cerebral cortex was 10-fold less in hypoglycemic piglets than in controls (P<0.05). Brain ATP was not significantly decreased during hypoglycemia, but phosphocreatine decreased from control of 6.6 +/- 1.3 micromoles/g brain to 3.2 +/- 1.9 micromoles/g brain in hypoglycemic piglets. The B(max) for NMDA-displaceable (3)H-glutamate binding was 992 +/- 64 fmol/mg protein in hypoglycemic animals, significantly higher than the control value of 746 +/- 42 fmol/mg protein. However, the dissociation constant for glutamate was unchanged during hypoglycemia. The (3)H-kainate binding studies demonstrated no change in B(max) of high-affinity kainate receptors during hypoglycemia. In contrast, the affinity of the kainate receptor glutamate binding site significantly increased compared to control. Thus, acute hypoglycemia in the newborn piglet had specific effects on the glutamate binding sites of the NMDA and kainate receptors that could be due to alterations in cell membrane lipids or modification of receptor proteins.

IGF-1 and bFGF reduce glutaric acid and 3-hydroxyglutaric acid toxicity in striatal cultures

Glutaric acid (GA) and 3-hydroxyglutaric acid (3GA) are thought to contribute to the degeneration of the caudate and putamen that is seen in some children with glutaric acidaemia type I, a metabolic disorder caused by a glutaryl-CoA dehydrogenase deficiency. This study assessed the neurotoxicity of GA and 3GA (0-50 mmol/L) compared to quinolinic acid (QUIN) in striatal and cortical cultures. All three acids were neurotoxic in a dose-dependent manner; however, GA and 3GA were both more toxic than QUIN. The neurotoxic effects of low concentrations of GA or 3GA were additive to QUIN toxicity. A series of hormones and growth factors were tested for protection against GA and 3GA toxicity. Insulin (5-500 microU /ml), basic fibroblast growth factor (bFGF; 10 ng/ml), insulin-like growth factor (IGF-1; 50 ng/ml), brain-derived neurotrophic factor (BDNF; 10 ng/ml), glial-derived neurotrophic factor (GDNF; 10 ng/ml), and two glutamate antagonists were evaluated in brain cultures to which 7 mmol/L GA or 3GA were added. GA and 3GA neurotoxicities were prevented by bFGF. Attenuation of 3GA-induced neurotoxicity was seen with insulin (5 microU/ml) and IGF-1. BDNF and GDNF had no effects on neuronal survival. Glutamate antagonists MK801 (10 micromol/L) and NBQX (10 micromol/L) failed to prevent GA or 3GA neurotoxicity. We conclude that GA and 3GA are neurotoxic in cultures of embryonic rat striatum and cortex. Striatal neurons were rescued from death by bFGF and IGF-1 but not by glutamate antagonist, suggesting that toxicity in this embryonic system is not necessarily mediated by glutamate receptors.

A heady message for lifespan regulation

Mutant Caenorhabditis elegans in which the age-1 and daf-2 genes (involved in insulin-receptor-like signalling) are expressed at low levels exhibit extended lifespan. Wolkow and colleagues recently showed that restricted re-expression of age-1 and daf-2 genes in neurons of these mutants rescues wild-type lifespan as effectively as ubiquitous re-expression. Low levels of insulin-like signalling in neurons might control longevity by enhancing protection against free radical damage. However, in mammalian cells (including neurons) reduced insulin-like signalling is generally thought to be deleterious to antioxidant defence and to neuron survival. Here we discuss the new work and several hypotheses to explain this apparent contradiction.

Epidermal growth factor induces oxidative neuronal injury in cortical culture

Recently, we have demonstrated that certain neurotrophic factors can induce oxidative neuronal necrosis by acting at the cognate tyrosine kinase-linked receptors. Epidermal growth factor (EGF) has neurotrophic effects via the tyrosine kinase-linked EGF receptor (EGFR), but its neurotoxic potential has not been studied. Here, we examined this possibility in mouse cortical culture. Exposure of cortical cultures to 1-100 ng/ml EGF induced gradually developing neuronal death, which was complete in 48-72 h; no injury to astrocytes was noted. Electron microscopic findings of EGF-induced neuronal death were consistent with necrosis; severe mitochondrial swelling and disruption of cytoplasmic membrane occurred, whereas nuclei appeared relatively intact. The EGF-induced neuronal death was accompanied by increased free radical generation and blocked by the anti-oxidant Trolox. Suggesting mediation by the EGFR, an EGFR tyrosine kinase-specific inhibitor, C56, attenuated EGF-induced neuronal death. In addition, inhibitors of extracellular signal-regulated protein kinase 1/2 (Erk-1/2) (PD98056), protein kinase A (H89), and protein kinase C (GF109203X) blocked EGF-induced neuronal death. A p38 mitogen-activated protein kinase inhibitor (SB203580) or glutamate antagonists (MK-801 and 6-cyano-7-nitroquinoxaline-2,3-dione) showed no protective effect. The present results suggest that prolonged activation of the EGFR may trigger oxidative neuronal injury in central neurons.