Glutamate excitotoxicity attenuates insulin-like growth factor-I prosurvival signaling

Reduced Insulin Signaling Targeted to Serotonergic Neurons but Not Other Neuronal Subtypes Extends Lifespan in Drosophila melanogaster

Reduced Insulin/IGF-like signaling (IIS) plays an evolutionarily conserved role in improving longevity and some measures of health-span in model organisms. Recent studies, however, have found a disconnection between lifespan extension and behavioral health-span. We have previously shown that reduction of IIS in Drosophila neurons extends female lifespan but does not improve negative geotaxis senescence and has a detrimental effect on exploratory walking senescence in both sexes. We hypothesize that individual neuronal subtypes respond differently to IIS changes, thus the behavioral outcomes of pan-neuronal IIS reduction are the balance of positive, negative and neutral functional effects. In order to further understand how reduced IIS in neurons independently modulates lifespan and locomotor behavioral senescence we expressed a dominant negative Insulin receptor transgene selectively in individual neuronal subtypes and measured the effects on lifespan and two measures of locomotor senescence, negative geotaxis and exploratory walking. IIS reduction in cholinergic, GABAergic, dopaminergic, glutamatergic, and octopaminergic neurons was found to have either no affect or a detrimental effect on lifespan and locomotor senescence. However, reduction of IIS selectively in serotonergic neurons resulted in extension of lifespan in females with no effect on locomotor senescence. These data indicate that individual neuronal subtypes respond differently to IIS changes in the modulation of lifespan and locomotor senescence, and identify a specific role for the insulin receptor in serotonergic neurons in the modulation of lifespan.

Interactions of Glutamate and Gamma Amino Butyric Acid with the Insulin-like growth factor system in Traumatic Brain Injury (TBI) and/or Cardiovascular Accidents (CVA or stroke): A systematic review

The brain maintains homeostasis of neural excitation in part through the receptor-mediated signaling of Glutamate (Glu) and Gamma Amino Butyric Acid (GABA), but localized injuries cause cellular release of excess Glu leading to neurotoxicity. The literature strongly supports the role of Insulin-like growth factor-1 (IGF-1) in adult brain neuroprotection and repair, and research supporting the existence of molecular interactions between Glu, GABA, and IGF-1 in vitro and in normal animals raises the question of whether and/or how the Glu/GABA system interacts with IGF-1 post-injury. This systematic review was undertaken to explore works addressing this question among adults with a history of traumatic brain injury (TBI) and/or cerebrovascular accident (CVA; stroke). The literature was searched for human and animal studies and only four animal papers met inclusion criteria. The SYRCLE criteria was used to evaluate risk of bias; results varied between categories and papers. All the included studies, one on TBI and three on stroke, supported the molecular relationship between the excitatory and IGF-1 systems; two studies provided direct, detailed molecular evidence. The results point to the importance of research on the role of this protective system in pathological brain injury; a hypothetical proposal for future studies is presented.

Research Progress on Neuroprotection of Insulin-like Growth Factor-1 towards Glutamate-Induced Neurotoxicity

Insulin-like growth factor-1 (IGF-1) and its binding proteins and receptors are widely expressed in the central nervous system (CNS), proposing IGF-1-induced neurotrophic actions in normal growth, development, and maintenance. However, while there is convincing evidence that the IGF-1 system has specific endocrine roles in the CNS, the concept is emerging that IGF-I might be also important in disorders such as ischemic stroke, brain trauma, Alzheimer’s disease, epilepsy, etc., by inducing neuroprotective effects towards glutamate-mediated excitotoxic signaling pathways. Research in rodent models has demonstrated rescue of pathophysiological and behavioral abnormalities when IGF-1 was administered by different routes, and several clinical studies have shown safety and promise of efficacy in neurological disorders of the CNS. Focusing on the relationship between IGF-1-induced neuroprotection and glutamate-induced excitatory neurotoxicity, this review addresses the research progress in the field, intending to provide a rationale for using IGF-I clinically to confer neuroprotective therapy towards neurological diseases with glutamate excitotoxicity as a common pathological pathway.

Akt / GSK3β / Nrf2 / HO-1 pathway activation by flurbiprofen protects the hippocampal neurons in a rat model of glutamate excitotoxicity

Nuclear factor erythroid 2-related factor 2 (Nrf2) is a transcription factor that regulates redox homeostasis of the cell through regulation of the antioxidant response element genes transcription. Nrf2 also regulates the antiapoptotic Bcl-2 gene. Nrf2 degradation and nuclear translocation is regulated by upstream kinases Akt and GSK3β. Glutamate excitotoxicity is a process of neuronal cells death due to excessive activation of glutamate receptors. Glutamate excitotoxicity participates in the pathophysiology of several acute and chronic neurological conditions. In addition, glutamate excitotoxicity interrupts the PI3K/Akt prosurvival pathway so GSK3β remains active. Active GSK3β increases Nrf2 degradation, decreases Nrf2 nuclear translocation and increases Nrf2 nuclear export which decreases the ARE genes transcription such as, SOD, GSH synthesis enzyme and HO-1. Also, Bcl-2 transcription decreases. Flurbiprofen is a COX inhibitor. Previous studies showed that it has a neuroprotective effect in neurodegeneration and in focal cerebral ischemia/reperfusion model. In our research we aimed to test the hypothesis that flurbiprofen may have a neuroprotective effect in a rat model of glutamate-induced excitotoxicity and this neuroprotection may occur through modulation of (Akt/GSK3β/Nrf2/HO-1) pathway. Rats were divided into 4 groups; control, MSG (2.5 g/Kg, i.p), low dose FB (5 mg/kg, i.p) and high dose FB (10 mg/kg, i.p). We found that low and high doses FB decreased COX-2, PGE₂, NO and MDA and increased SOD and GSH in brain compared to MSG group. High dose was more effective than low dose. Western blotting analysis in hippocampus tissue showed that high dose FB increased p-Akt, p-GSK3β, nuclear Nrf2 and HO-1 and decreased cytosolic Nrf2 level in comparison with MSG group. Immunohistochemical analysis in hippocampus and cerebral cortex showed that high dose FB increased Bcl-2 and decreased Bax compared to MSG group. In addition, FB increased the number of intact neurons in hippocampus areas and cerebral cortex neurons and showed an anxiolytic-like action in OF and EPM tests. These findings suggest that FB has a neuroprotective effect in glutamate-induced excitotoxicity model through reduction of the glutamate excitotoxicity damage and activation of the survival pathway. These may occur due to modulation the survival pathway (Akt/GSK3β/Nrf2/HO-1) and inhibition of COX-2.

A Coordinated Action of Blood-Borne and Brain Insulin-Like Growth Factor I in the Response to Traumatic Brain Injury

In response to injury, the brain produces different neuroprotective molecules, such as insulin-like growth factor I (IGF-I). However, IGF-I is also taken up by the brain from the circulation in response to physiological stimuli. Herein, we analyzed in mice the relative contribution of circulating and locally produced IGF-I to increased brain IGF-I levels after insult. Traumatic brain injury (TBI) induced by a controlled impact resulted in increased IGF-I levels in the vicinity of the lesion, but mice with low serum IGF-I showed significantly lower increases. Indeed, in normal mice, peripheral IGF-I accumulated at the lesion site after injury, and at the same time serum IGF-I levels decreased. Collectively, these data suggest that serum IGF-I enter into the brain after TBI and contributes to increased brain IGF-I levels at the injury site. This connection between central and circulating IGF-I provides an amenable route for treatment, as subcutaneous administration of IGF-I to TBI mice led to functional recovery. These latter results add further support to the use of systemic IGF-I or its mimetics for treatment of brain injuries.

Deleting IGF-1 receptor from forebrain neurons confers neuroprotection during stroke and upregulates endocrine somatotropin

Insulin-like growth factors control numerous processes, namely somatic growth, metabolism and stress resistance, connecting this pathway to aging and age-related diseases. Insulin-like growth factor signaling also impacts on neurogenesis, neuronal survival and structural plasticity. Recent reports demonstrated that diminished insulin-like growth factor signaling confers increased stress resistance in brain and other tissues. To better understand the role of neuronal insulin-like growth factor signaling in neuroprotection, we inactivated insulin-like growth factor type-1-receptor in forebrain neurons using conditional Cre-LoxP-mediated gene targeting. We found that brain structure and function, including memory performance, were preserved in insulin-like growth factor receptor mutants, and that certain characteristics improved, notably synaptic transmission in hippocampal neurons. To reveal stress-related roles of insulin-like growth factor signaling, we challenged the brain using a stroke-like insult. Importantly, when charged with hypoxia-ischemia, mutant brains were broadly protected from cell damage, neuroinflammation and cerebral edema. We also found that in mice with insulin-like growth factor receptor knockout specifically in forebrain neurons, a substantial systemic upregulation of growth hormone and insulin-like growth factor-I occurred, which was associated with significant somatic overgrowth. Collectively, we found strong evidence that blocking neuronal insulin-like growth factor signaling increases peripheral somatotropic tone and simultaneously protects the brain against hypoxic-ischemic injury, findings that may contribute to developing new therapeutic concepts preventing the disabling consequences of stroke.

Evidence for synergistic action of transthyretin and IGF-I over the IGF-I receptor

Transthyretin (TTR) has a neuroprotective role in the central nervous system (CNS) in Alzheimer's disease (AD) and cerebral ischemia. Increased levels of TTR and activated insulin-like growth factor I receptor (IGF-IR) are associated with reduced neurodegeneration in an AD mouse model. In the present study, we found that TTR and IGF-I have a synergistic effect on activation of one of the IGF-IR signaling pathways. Hippocampus of TTR null mice present decreased levels of phosphorylated IGF-IR and Akt when compared with TTR wild type littermate animals. Cell studies reveal the synergistic effect of TTR and IGF-I in promoting IGF-IR signaling even under glutamate induced toxicity. TTR:IGF-IR complexes are identified and a bio-layer interferometry assay demonstrated an interaction between TTR and IGF-IR with a KD ranging from 99 to 744nM. In summary, our results point to a new TTR role through the IGF-I axis, mediated through TTR-IGF-IR interactions.

Astrocyte Resilience to Oxidative Stress Induced by Insulin-like Growth Factor I (IGF-I) Involves Preserved AKT (Protein Kinase B) Activity

Disruption of insulin-like growth factor I (IGF-I) signaling is a key step in the development of cancer or neurodegeneration. For example, interference of the prosurvival IGF-I/AKT/FOXO3 pathway by redox activation of the stress kinases p38 and JNK is instrumental in neuronal death by oxidative stress. However, in astrocytes, IGF-I retains its protective action against oxidative stress. The molecular mechanisms underlying this cell-specific protection remain obscure but may be relevant to unveil new ways to combat IGF-I/insulin resistance. Here, we describe that, in astrocytes exposed to oxidative stress by hydrogen peroxide (H2O2), p38 activation did not inhibit AKT (protein kinase B) activation by IGF-I, which is in contrast to our previous observations in neurons. Rather, stimulation of AKT by IGF-I was significantly higher and more sustained in astrocytes than in neurons either under normal or oxidative conditions. This may be explained by phosphorylation of the phosphatase PTEN at the plasma membrane in response to IGF-I, inducing its cytosolic translocation and preserving in this way AKT activity. Stimulation of AKT by IGF-I, mimicked also by a constitutively active AKT mutant, reduced oxidative stress levels and cell death in H2O2-exposed astrocytes, boosting their neuroprotective action in co-cultured neurons. These results indicate that armoring of AKT activation by IGF-I is crucial to preserve its cytoprotective effect in astrocytes and may form part of the brain defense mechanism against oxidative stress injury.

The Drosophila insulin receptor independently modulates lifespan and locomotor senescence

The Insulin/IGF-like signalling (IIS) pathway plays an evolutionarily conserved role in ageing. In model organisms reduced IIS extends lifespan and ameliorates some forms of functional senescence. However, little is known about IIS in nervous system ageing and behavioural senescence. To investigate this role in Drosophila melanogaster, we measured the effect of reduced IIS on senescence of two locomotor behaviours, negative geotaxis and exploratory walking. Two long-lived fly models with systemic IIS reductions (daGAL4/UAS-InR^DN (ubiquitous expression of a dominant negative insulin receptor) and d2GAL/UAS-rpr (ablation of insulin-like peptide producing cells)) showed an amelioration of negative geotaxis senescence similar to that previously reported for the long-lived IIS mutant chico. In contrast, exploratory walking in daGAL4/UAS-InR^DN and d2GAL/UAS-rpr flies declined with age similarly to controls. To determine the contribution of IIS in the nervous system to these altered senescence patterns and lifespan, the InR^DN was targeted to neurons (elavGAL4/UAS-InR^DN), which resulted in extension of lifespan in females, normal negative geotaxis senescence in males and females, and detrimental effects on age-specific exploratory walking behaviour in males and females. These data indicate that the Drosophila insulin receptor independently modulates lifespan and age-specific function of different types of locomotor behaviour. The data suggest that ameliorated negative geotaxis senescence of long-lived flies with systemic IIS reductions is due to ageing related effects of reduced IIS outside the nervous system. The lifespan extension and coincident detrimental or neutral effects on locomotor function with a neuron specific reduction (elavGAL4/UAS-InR^DN) indicates that reduced IIS is not beneficial to the neural circuitry underlying the behaviours despite increasing lifespan.

Blood brain barrier and neuroinflammation are critical targets of IGF-1-mediated neuroprotection in stroke for middle-aged female rats

Ischemia-induced cerebral infarction is more severe in older animals as compared to younger animals, and is associated with reduced availability of insulin-like growth factor (IGF)-1. This study determined the effect of post-stroke IGF-1 treatment, and used microRNA profiling to identify mechanisms underlying IGF-1’s neuroprotective actions. Post-stroke ICV administration of IGF-1 to middle-aged female rats reduced infarct volume by 39% when measured 24h later. MicroRNA analyses of ischemic tissue collected at the early post-stroke phase (4h) indicated that 8 out of 168 disease-related miRNA were significantly downregulated by IGF-1. KEGG pathway analysis implicated these miRNA in PI3K-Akt signaling, cell adhesion/ECM receptor pathways and T-and B-cell signaling. Specific components of these pathways were subsequently analyzed in vehicle and IGF-1 treated middle-aged females. Phospho-Akt was reduced by ischemia at 4h, but elevated by IGF-1 treatment at 24h. IGF-1 induced Akt activation was preceded by a reduction of blood brain barrier permeability at 4h post-stroke and global suppression of cytokines including IL-6, IL-10 and TNF-α. A subset of these cytokines including IL-6 was also suppressed by IGF-1 at 24h post-stroke. These data are the first to show that the temporal and mechanistic components of post-stroke IGF-1 treatment in older animals, and that cellular components of the blood brain barrier may serve as critical targets of IGF-1 in the aging brain.

Astrocytes require insulin-like growth factor I to protect neurons against oxidative injury

Oxidative stress is a proposed mechanism in brain aging, making the study of its regulatory processes an important aspect of current neurobiological research. In this regard, the role of the aging regulator insulin-like growth factor I (IGF-I) in brain responses to oxidative stress remains elusive as both beneficial and detrimental actions have been ascribed to this growth factor. Because astrocytes protect neurons against oxidative injury, we explored whether IGF-I participates in astrocyte neuroprotection and found that blockade of the IGF-I receptor in astrocytes abrogated their rescuing effect on neurons. We found that IGF-I directly protects astrocytes against oxidative stress (H 2O 2). Indeed, in astrocytes but not in neurons, IGF-I decreases the pro-oxidant protein thioredoxin-interacting protein 1 and normalizes the levels of reactive oxygen species. Furthermore, IGF-I cooperates with trophic signals produced by astrocytes in response to H 2O 2 such as stem cell factor (SCF) to protect neurons against oxidative insult. After stroke, a condition associated with brain aging where oxidative injury affects peri-infarcted regions, a simultaneous increase in SCF and IGF-I expression was found in the cortex, suggesting that a similar cooperative response takes place in vivo. Cell-specific modulation by IGF-I of brain responses to oxidative stress may contribute in clarifying the role of IGF-I in brain aging.

Neuroprotective effect of insulin-like growth factor-1: effects on tyrosine kinase receptor (Trk) expression in dorsal root ganglion neurons with glutamate-induced excitotoxicity in vitro

Insulin-like growth factor-1 (IGF-1) may play an important role in regulating the expression of distinct tyrosine kinase receptor (Trk) in primary sensory dorsal root ganglion (DRG) neurons. Glutamate (Glu) is the main excitatory neurotransmitter and induces neuronal excitotoxicity for primary sensory neurons. It is not known whether IGF-1 influences expression of TrkA, TrkB, and TrkC in DRG neurons with excitotoxicity induced by Glu. In the present study, primary cultured DRG neurons with Glu-induced excitotoxicity were used to determine the effects of IGF-1 on TrkA, TrkB, and TrkC expression. The results showed that IGF-1 increased the expression of TrkA and TrkB and their mRNAs, but not TrkC and its mRNA, in primary cultured DRG neurons with excitotoxicity induced by Glu. Interestingly, neither the extracellular signal-regulated protein kinase (ERK1/2) inhibitor PD98059 nor the phosphatidylinositol 3-kinase (PI3K) inhibitor LY294002 blocked the effect of IGF-1, but both inhibitors together were effective. IGF-1 may play an important role in regulating different Trk receptor expression in DRG neurons through ERK1/2 and PI3K/Akt signaling pathways. The contribution of distinct Trk receptors might be one of the mechanisms that IGF-1 rescues dying neurons from Glu excitotoxic injury. These data imply that IGF-1 signaling might be a potential target on modifying distinct Trk receptor-mediated biological effects of primary sensory neurons with excitotoxicity.

Hydrogen peroxide attenuates the prosurvival signaling of insulin-like growth factor-1 through two pathways

Although it has been well established that oxidative stress triggering a variety of signaling pathways leads to cell death, little attention has been paid to how these pathways affect prosurvival factors such as insulin-like growth factor-1 (IGF-1). In this study, we found that the prosurvival signaling of IGF-1 was attenuated by H₂O₂. To study the mechanism underlying this phenomenon, cells pretreated with Trolox or various glutamate receptor antagonists [i.e. N-methyl-D-aspartate (NMDA) receptor antagonist dizocilpine maleate (MK-801), non-NMDA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX), metabolic glutamate receptor antagonists LY341495 and CPCCOEt] were exposed to H₂O₂, and then stimulated by IGF-1. The phosphorylation statuses of IGF-1 receptors, Akt and ERK, were determined by western blotting, and cell viability was analyzed by an MTT assay. IGF-1 exerted a potent neuroprotective effect against B27 deprivation, and this effect was abolished by 100 μM H₂O₂. Meanwhile, the phosphorylation of IGF-1 receptors, Akt and ERK, was attenuated. Moreover, the phosphorylation of Akt was more susceptible to H₂O₂ insult than IGF-1 receptors. MK-801 increased the phosphorylation of IGF-1 receptors and its downstream target Akt, and thereby promoted cell survival, whereas the other glutamate receptor antagonists exerted no effect. Antioxidant Trolox did not restore IGF-1 signaling, but it increased Akt phosphorylation and also increased cell viability. These results showed that H₂O₂ impaired IGF-1 prosurvival signaling through two pathways. One pathway disrupted the autophosphorylation of IGF-1 receptors through NMDA receptors and the other directly dephosphorylated Akt.

Insulin-like growth factor-1 and central neurodegenerative diseases

The previously undisputed neuroprotective role of insulin-like growth factor 1 (IGF-1) has been challenged by recent observations in IGF-1 receptor (IGF-1R) defective mutants. As new ligand-dependent and ligand-independent roles for IGF-1R are now emerging, new insights into the biologic role of brain IGF-1R and its connection with serum and brain IGF-1 function are urgently required. In the meantime, treatment of specific neurodegenerative diseases with IGF-1 may still be explored using adequate preclinical procedures.

Glutamate signaling in bone

Mechanical loading plays a key role in the physiology of bone, allowing bone to functionally adapt to its environment, however characterization of the signaling events linking load to bone formation is incomplete. A screen for genes associated with mechanical load-induced bone formation identified the glutamate transporter GLAST, implicating the excitatory amino acid, glutamate, in the mechanoresponse. When an osteogenic load (10 N, 10 Hz) was externally applied to the rat ulna, GLAST (EAAT1) mRNA, was significantly down-regulated in osteocytes in the loaded limb. Functional components from each stage of the glutamate signaling pathway have since been identified within bone, including proteins necessary for calcium-mediated glutamate exocytosis, receptors, transporters, and signal propagation. Activation of ionotropic glutamate receptors has been shown to regulate the phenotype of osteoblasts and osteoclasts in vitro and bone mass in vivo. Furthermore, glutamatergic nerves have been identified in the vicinity of bone cells expressing glutamate receptors in vivo. However, it is not yet known how a glutamate signaling event is initiated in bone or its physiological significance. This review will examine the role of the glutamate signaling pathway in bone, with emphasis on the functions of glutamate transporters in osteoblasts.

Free insulin-like growth factor (IGF)-1 and IGF-binding proteins-2 and -3 in serum and cerebrospinal fluid of amyotrophic lateral sclerosis patients

BACKGROUND

The insulin-like growth factor-1 (IGF-1) signaling system is regulated by many factors which interact in regulating the bioavailability of IGF-I. In this context, little information is available on free IGF-1, the bioactive form of IGF-1, in amyotrophic lateral sclerosis (ALS).

METHODS

We investigated the endogenous expression of IGF-1, and two related binding proteins (IGF-binding proteins, IGFBP-2 and BP-3) in serum and cerebrospinal fluid (CSF) of 54 sporadic ALS (sALS) patients. Twenty-five healthy individuals and 25 with other neurological diseases (OND) were used as controls. Total and free IGF-1, and IGFBP-3 levels were detected by immunoradiometric assay (IRMA); IGFBP-2 levels were determined by radioimmunoassay (RIA).

RESULTS

Total and free IGF-1, IGFBP-2 and BP-3 serum levels were not significantly different between patients and controls, although in sALS patients free IGF-1 was negatively correlated with ALS-Functional Rating Scale-revised (ALS-FRS-R) score (r = -0.4; P = 0.046) and forced vital capacity (FVC) (r = -0.55; P < 0.04). In CSF, free IGF-1 was significantly increased in sALS patients compared with OND (P < 0.0001).

CONCLUSIONS

Though in the serum we did not find significant differences amongst the three groups, IGF-1 bioavailability, represented by the free IGF-1 levels, correlated with disease severity. In the CSF, the significant increment of the free fraction of IGF-1 suggests an up-regulation of the IGF-1 system in the intrathecal compartment of sALS patients. Since IGF-1 is a trophic factor for different tissues, we speculate that high levels of the free IGF-1 in sALS might reflect a physiological defensive mechanism promoted in response to neural degeneration and/or muscle atrophy.

Abnormalities of IGF-I signaling in the pathogenesis of diseases of the bone, brain, and fetoplacental unit in humans

IGF-I action is essential for the regulation of tissue formation and remodeling, bone growth, prenatal growth, brain development, and muscle metabolism. Cellular effects of IGF-I are mediated through the IGF-I receptor, a transmembrane tyrosine kinase that phosphorylates intracellular substrates, resulting in the activation of multiple intracellular signaling cascades. Dysregulation of IGF-I actions due to impairment in the postreceptor signaling machinery may contribute to multiple diseases in humans. This article will review current information on IGF-I signaling and illustrate recent results demonstrating how impaired IGF-I signaling and action may contribute to the pathogenesis of human diseases, including osteoporosis, neurodegenerative disorders, and reduced fetal growth in utero.

Neuronal death by oxidative stress involves activation of FOXO3 through a two-arm pathway that activates stress kinases and attenuates insulin-like growth factor I signaling

Oxidative stress kills neurons by stimulating FOXO3, a transcription factor whose activity is inhibited by insulin-like growth factor I (IGF-I), a wide-spectrum neurotrophic signal. Because recent evidence has shown that oxidative stress blocks neuroprotection by IGF-I, we examined whether attenuation of IGF-I signaling is linked to neuronal death by oxidative stress, as both events may contribute to neurodegeneration. We observed that in neurons, activation of FOXO3 by a burst of oxidative stress elicited by 50 muM hydrogen peroxide (H(2)O(2)) recruited a two-pronged pathway. A first, rapid arm attenuated AKT inhibition of FOXO3 through p38 MAPK-mediated blockade of IGF-I stimulation of AKT. A second delayed arm involved activation of FOXO3 by Jun-kinase 2 (JNK2). Notably, blockade of IGF-I signaling through p38 MAPK was necessary for JNK2 to activate FOXO3, unveiling a competitive regulatory interplay between the two arms onto FOXO3 activity. Therefore, an abrupt rise in oxidative stress activates p38 MAPK to tilt the balance in a competitive AKT/JNK2 regulation of FOXO3 toward its activation, eventually leading to neuronal death. In view of previous observations linking attenuation of IGF-I signaling to other causes of neuronal death, these findings suggest that blockade of trophic input is a common step in neuronal death.

Circulating insulin-like growth factors and related binding proteins are selectively altered in amyotrophic lateral sclerosis and multiple sclerosis

OBJECTIVE

To provide a detailed profile of the peripheral IGF system in the neurological conditions; amyotrophic lateral sclerosis (ALS), post polio syndrome (PPS) and multiple sclerosis (MS). To determine whether subsets of patients within the disease groups could be identified in whom one or more components of the IGF regulatory system are altered compared to healthy control subjects matched for age, sex and BMI.

DESIGN

Three cohorts of patients were recruited, 28 with ALS, 18 with PPS and 23 with MS. Patients were individually matched to a healthy control based on sex, age (+/-3 yr), and BMI (+/-2.5 kg m(-2)). The concentration (ng/ml) of serum IGF-I, IGF-II, IGFBP-1, IGFBP-2 and IGFBP-3 and acid-labile subunit (microg/ml) was determined by IRMA.

RESULTS

In ALS patients, there was an increase of 11% in [IGF(TOTAL)] (p=0.042) ([IGF(TOTAL)]=[IGF-I]+[IGF-II]) and [IGFBP-1] was decreased by 34% (p=0.050) compared to matched controls. In "surviving" ALS patients, defined as those ALS patients with long disease duration (+2 SD from the mean survival time for Irish patients post diagnosis), there was an increase in [IGF-I] 36% (p=0.032) and a large decrease in [IGFBP-1] -58% (p=0.020) compared to controls. These differences were not evident in pre-agonal ALS patients. The concentration of serum IGF-I was 38% (p=0.018), acid-labile subunit 17% (p=0.044) and IGFBP-2 43% (p=0.035) higher in MS patients compared to controls. When stratified for interferon-beta (IFN-beta) use, we observed an increase in serum [IGF-I] 52% (p=0.013) and [IGF(TOTAL)] 19% (p=0.043) in MS patients undergoing IFN-beta treatment, but MS patients not undergoing IFN-beta treatment had similar IGF and IGFBP concentration to controls. Serum [IGFBP-3] 18% (p=0.033), [IGFBP-2] 86% (p=0.015) and (acid-labile subunit) 33% (p=0.012) was also higher in IFN-beta patients compared to controls. Stratified by stage of disease the most significant increase in components of the peripheral IGF system was attributed to relapsing-remitting MS patients treated with IFN-beta. All components of the peripheral IGF system in PPS patients were similar to controls.

CONCLUSIONS

The increase in circulating IGF-I and a reduction in regulatory binding protein IGFBP-1 in ALS patients with a "stable" disease profile suggest a potential change in peripheral IGF bioavailability in these subjects. In MS, we report a change in a number of components of the peripheral IGF system, the observed increase in IGF-I in patients treated with IFN-beta being of most significance as a potential therapeutic biomarker.

Modulation by insulin-like growth factor I of the phosphatase PTEN in astrocytes

Characterization of intracellular pathways underlying the pleiotropic actions of insulin-like growth factor-I (IGF-I) on brain cells is incomplete. We analyzed IGF-I signalling on astrocytes through the canonical phosphatidylinositol 3-kinase (PI3K)/Akt pathway and focused on possible changes in PTEN, a phosphatase that modulates IGF-I signalling by inhibiting Akt activation and, in turn is positively regulated by PI3K. After exposure of astrocytes to IGF-I, PTEN mRNA and protein levels were reduced and its phosphatase activity diminished. Inhibition of PTEN involved activation of a PI3K/protein kinase C (PKC) pathway that decreased in a proteasome-dependent step the levels of the transcription factor Egr-1, a key regulator of PTEN levels in astrocytes, causing decreased binding of Egr-1 to the PTEN promoter. Enhanced mitogenesis in PTEN siRNA-transduced astrocytes after IGF-I suggested that reduced PTEN may be a permissive factor for the mitogenic activity of IGF-I. Subsequent recovery of reduced PTEN required also activation by IGF-I of PI3K to recruit in this case protein kinase A (PKA) which stimulated Egr-1 levels and, consequently PTEN synthesis. Because basal levels of PTEN in astrocytes are also governed by PI3K, IGF-I appears to modulate PTEN in astrocytes by redirecting its homeostasic control through PI3K in a timed fashion.

Interleukin-1beta interferes with signal transduction induced by neurotrophin-3 in cortical neurons

It was previously observed that IL-1beta interferes with BDNF-induced TrkB-mediated signal transduction and protection of cortical neurons from apoptosis evoked by deprivation from trophic support [Tong L., Balazs R., Soiampornkul R., Thangnipon W., Cotman C.W., 2007. Interleukin-1beta impairs brain derived neurotrophic factor-induced signal transduction. Neurobiol. Aging]. Here we investigated whether the effect of the cytokine on neurotrophin signaling is more general. The influence of IL-1beta on NT-3 signaling was therefore studied under conditions when NT-3 primarily activated the TrkC receptor. The cytokine reduced NT-3-induced activation of MAPK/ERK and Akt, but did not interfere with Trk receptor autophosphorylation. IL-1beta reduced tyrosine phosphorylation of the docking proteins, IRS-1 and Shc, which convey receptor activation to the downstream protein kinase cascades. These are the steps that are also inhibited by IL-1beta in BDNF-induced signal transduction. The functional consequences of the effect of IL-1beta on NT-3 signaling were severe, as NT-3 protection of the trophic support-deprived cortical neurons was abrogated. In view of the role in the maintenance and plasticity of neurons of ERK, Akt and CREB, which are activated by neurotrophins, elevated IL-1beta levels in the brain in Alzheimer's disease and other neurodegenerative diseases might contribute to the decline in cognitive functions before the pathological signs of the disease develop.

Interleukin-1 beta impairs brain derived neurotrophic factor-induced signal transduction

The expression of IL-1 is elevated in the CNS in diverse neurodegenerative disorders, including Alzheimer's disease. The hypothesis was tested that IL-1 beta renders neurons vulnerable to degeneration by interfering with BDNF-induced neuroprotection. In trophic support-deprived neurons, IL-1 beta compromised the PI3-K/Akt pathway-mediated protection by BDNF and suppressed Akt activation. The effect was specific as in addition to Akt, the activation of MAPK/ERK, but not PLC gamma, was decreased. Activation of CREB, a target of these signaling pathways, was severely depressed by IL-1 beta. As the cytokine did not influence TrkB receptor and PLC gamma activation, IL-1 beta might have interfered with BDNF signaling at the docking step conveying activation to the PI3-K/Akt and Ras/MAPK pathways. Indeed, IL-1 beta suppressed the activation of the respective scaffolding proteins IRS-1 and Shc; this effect might involve ceramide generation. IL-1-induced interference with BDNF neuroprotection and signal transduction was corrected, in part, by ceramide production inhibitors and mimicked by the cell-permeable C2-ceramide. These results suggest that IL-1 beta places neurons at risk by interfering with BDNF signaling involving a ceramide-associated mechanism.

Hydrogen peroxide attenuates insulin-like growth factor-1 neuroprotective effect, prevented by minocycline

Oxidative stress-induced neuronal death due to hydrogen peroxide overload plays a critical role in the pathogenesis of numerous neurological diseases. Insulin-like growth factor-1 (IGF-1) is important in maintaining neuronal survival, proliferation, and differentiation in the central nervous system. We now report that sublethal doses of hydrogen peroxide attenuated IGF-1 neuroprotective activity on cultured cerebellar granule neurons under potassium and serum deprivation. Interestingly, this attenuation can be prevented by minocycline, an antibiotic that has been shown to have neuroprotective activity in animal models of neuronal injury. Furthermore, hydrogen peroxide also blocked IGF-1's neuroprotection for cortical neurons deprived of neurotrophic factors (B27), which was prevented by minocycline. Our data suggest that inhibition of IGF-1 signaling by hydrogen peroxide may constitute an additional pathway contributing to its neurotoxicity. More importantly, combining minocycline and IGF-1 could be an effective treatment in neurological diseases associated with both oxidative stress and deficiency of IGF-1.

Kainic acid triggers oligodendrocyte precursor cell proliferation and neuronal differentiation from striatal neural stem cells

Glutamate is an excitatory amino acid that serves important functions in mammalian brain development through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/ kainate receptor stimulation. Neural stem cells with self-renewal and multilineage potential are a useful tool to study the signals involved in the regulation of brain development. We have investigated the role played by AMPA/kainate receptors during the differentiation of neural stem cells derived from fetal rat striatum. The application of 1 and 10 microM kainic acid increased significantly the phosphorylation of the cyclic AMP response element binding protein (CREB), raised bromodeoxyuridine incorporation in O4-positive oligodendrocyte precursors, and increased the number of O1-positive cells in the cultures. Increased CREB phosphorylation and proliferation were prevented by the AMPA receptor antagonist 4-4(4-aminophenyl)-1,2-dihydro-1-methyl-2-propylcarbamoyl-6,7-methylenedioxyphthalazine (SYM 2206) and by protein kinase A and protein kinase C inhibitors. Cultures treated with 100 microM kainic acid showed decreased proliferation, a lower proportion of O1-positive cells, and apoptosis of O4-positive cells. None of these effects were prevented by SYM 2206, suggesting that kainate receptors take part in these events. We conclude that AMPA receptor stimulation by kainic acid promotes the proliferation of oligodendrocyte precursors derived from neural stem cells through a mechanism that requires the activation of CREB by protein kinase A and C. In the neurons derived from these cells, either AMPA or kainate receptor stimulation produces neuritic growth and larger cell bodies.

Cross-talk between estrogen receptors and insulin-like growth factor-I receptor in the brain: cellular and molecular mechanisms

Accumulating evidence suggests that insulin-like growth factor-I (IGF-I) and estradiol interact to regulate neural function. In this review, we focus on the cellular and molecular mechanisms involved in this interaction. The expression of estrogen receptors (ERs) and IGF-I receptor is cross-regulated in the central nervous system and many neurons and astrocytes coexpress both receptors. Furthermore, estradiol activates IGF-I receptor and its intracellular signaling. This effect may involve classical ERs since recent findings suggest that ERalpha may affect IGF-I actions in the brain by a direct interaction with some of the components of IGF-I signaling. In turn, IGF-I may regulate ER transcriptional activity in neuronal cells. In conclusion, ERs appear to be part of the signaling mechanism of IGF-I, and IGF-I receptor part of the mechanism of estradiol signaling in the nervous system.

Growth hormone releasing peptide-6 acts as a survival factor in glutamate-induced excitotoxicity

Chronic systemic treatment given to adult male rats with growth hormone releasing peptide-6, an agonist of the ghrelin receptor, increases insulin-like growth factor I levels in various brain regions, including the hypothalamus and cerebellum. Furthermore, intracellular signalling cascades normally associated with anti-apoptotic actions are activated in the same areas and are coincident with decreased basal cell death. Because abnormally high concentrations of glutamate can lead to overexcitation of neurones leading to cell damage and/or death, we investigated whether administration of growth hormone releasing peptide-6 attenuates monosodium glutamate-induced apoptosis in the rat hypothalamus and cerebellum. Glutamate increased activation of caspase 9 followed by cleavage of caspase 7, which in turn fragmented poly(ADP-ribose) polymerase, terminating in cell death in both the hypothalamus and cerebellum. Growth hormone releasing peptide-6 reversed glutamate-induced cell death by decreasing activation of caspases 9 and 7 and poly(ADP-ribose) polymerase fragmentation. These results provide a better understanding of the neuroprotective role of growth hormone secretagogues and the mechanisms involved.

Dysbindin-1 is a synaptic and microtubular protein that binds brain snapin

Variations in the gene encoding the novel protein dysbindin-1 (DTNBP1) are among the most commonly reported genetic variations associated with schizophrenia. Recent studies show that those variations are also associated with cognitive functioning in carriers with and without psychiatric diagnoses, suggesting a general role for dysbindin-1 in cognition. Such a role could stem from the protein's known ability to affect neuronal glutamate release. How dysbindin-1 might affect glutamate release nevertheless remains unknown without the discovery of the protein's neuronal binding partners and its subcellular locus of action. We demonstrate here that snapin is a binding partner of dysbindin-1 in vitro and in the brain. Tissue fractionation of whole mouse brains and human hippocampal formations revealed that both dysbindin-1 and snapin are concentrated in tissue enriched in synaptic vesicle membranes and less commonly in postsynaptic densities. It is not detected in presynaptic tissue fractions lacking synaptic vesicles. Consistent with that finding, immunoelectron microscopy showed that dysbindin-1 is located in (i) synaptic vesicles of axospinous terminals in the dentate gyrus inner molecular layer and CA1 stratum radiatum and in (ii) postsynaptic densities and microtubules of dentate hilus neurons and CA1 pyramidal cells. The labeled synapses are often asymmetric with thick postsynaptic densities suggestive of glutamatergic synapses, which are likely to be derived from dentate mossy cells and CA3 pyramidal cells. The function of dysbindin-1 in presynaptic, postsynaptic and microtubule locations may all be related to known functions of snapin.

Inhibition of calpain and caspase-3 prevented apoptosis and preserved electrophysiological properties of voltage-gated and ligand-gated ion channels in rat primary cortical neurons exposed to glutamate

Glutamate toxicity in traumatic brain injury, ischemia, and Huntington's disease causes cortical neuron death and dysfunction. We tested the efficacy of calpain and caspase-3 inhibitors alone and in combination to prevent neuronal death and preserve electrophysiological functions in rat primary cortical neurons following glutamate exposure. Cortical neurons exposed to 0.5 microM glutamate for 24 h committed mostly apoptotic death as determined by Wright staining and ApopTag assay. Levels of expression, formation of active forms, and activities of calpain and caspase-3 were increased following glutamate exposure. Also, in situ double labeling identified conformationally active caspase-3-p20 fragment and chromatin condensation in apoptotic neurons. Pretreatment of cortical neurons with 0.2 microM N-benzyloxylcarbonyl-Leu-Nle-aldehyde (calpain-specific inhibitor) and 100 microM N-benzyloxylcarbonyl-Asp(OCH3)-Glu(OCH3)-Val-Asp(OCH3)-fluoromethyl ketone (caspase-3-specific inhibitor) provided strong neuroprotection. Standard patch-clamp techniques were used to measure the whole-cell currents associated with Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors. The lack of a change in capacitance indicated that neurons treated with inhibitor(s) plus glutamate did not undergo apoptotic shrinkage and maintained the same size as the control neurons. Whole-cell currents associated with Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors were similar in amplitude and activation/inactivation kinetics for cells untreated and treated with inhibitor(s) and glutamate. Spontaneous synaptic activity as observed by miniature end-plate currents was also similar. Prevention of glutamate-induced apoptosis by calpain and caspase-3 inhibitors preserved normal activities of crucial ion channels such as Na+ channels, N-methyl-D-aspartate receptors, and kainate receptors in neurons. Our studies strongly imply that calpain and caspase-3 inhibitors may also provide functional neuroprotection in the animal models of traumatic brain injury and neurodegenerative diseases.

Down-regulation of Bcl-2 in rat substantia nigra after focal cerebral ischemia

After occlusion of the middle cerebral artery in rats, a robust neuronal loss occurs in the ipsilateral substantia nigra reticulata. In this study we have assessed whether degeneration of the substantia nigra is accompanied by changes in the expression of the anti-apoptotic protein Bcl-2. Neuronal loss was assessed by neuronal nuclei (NeuN) immunoreactivity. A significant decrease of Bcl-2 expression was observed in the substantia nigra 12, 24 and 72 h after middle cerebral artery occlusion. These results suggest that the secondary neuronal loss in the substantia nigra could be related with the modification of proteins regulating programmed cell death. Exo-focal cell death may explain the appearance of neuropsychiatric symptoms that are not correlated with the primary site of lesion.

Role of insulin-like growth factor I signaling in neurodegenerative diseases

Disturbed trophic support to neurons has long been considered a potential mechanism in neurodegeneration. Recent evidence indicates that intracellular trophic signaling may be compromised in several neurodegenerative diseases. Changes in the levels of insulin-like growth factor I (IGF-I), a trophic hormone with multiple neuroprotective actions, have recently been observed in several human neurodegenerative illnesses. Therefore analysis of IGF-I pathways could help provide greater insight into trophic disturbances to neurons. However, neurodegenerative diseases with similar clinical manifestations show either high or low levels of circulating IGF-I. This apparently puzzling observation can be explained if we consider that IGF-I input to target neurons is disrupted by either lower IGF-I availability or by reduced cell sensitivity to IGF-I. The latter disturbance may be associated with high IGF-I levels. We hypothesize that in the majority of neurodegenerative diseases compromised IGF-I support to neurons emerges as part of the pathological cascade during the degenerative process and contributes to neuronal demise. In addition, loss of IGF-I input to specific neuronal populations might be the cause of a small group of neurodegenerative diseases.