Excitotoxic and excitoprotective mechanisms: abundant targets for the prevention and treatment of neurodegenerative disorders

Neuroprotective actions of 2,4-dinitrophenol: Friend or foe?

2,4-dinitrophenol (DNP) has long been known to be toxic at high concentrations, an effect related to uncoupling of mitochondrial oxidative phosphorylation. Five years ago, however, we reported that low concentrations of DNP protect neurons against the toxicity of the amyloid-β peptide. Since then, a number of other studies have provided evidence of beneficial actions of DNP (at low concentrations), including neuroprotection against different types of insult, blockade of amyloid aggregation, stimulation of neurite outgrowth and neuronal differentiation, and even extension of lifespan in certain organisms. Some of these effects appear due to mild mitochondrial uncoupling and prevention of oxidative stress, whereas other actions are related to activation of additional intracellular signaling pathways. This study discusses the evidence supporting beneficial neuroprotective actions of DNP. DNP and other compounds with similar biological activities may be of interest in the development of novel therapeutic approaches for neurodegenerative diseases and other neurological disorders.

Leptin-mediated cell survival signaling in hippocampal neurons mediated by JAK STAT3 and mitochondrial stabilization

Leptin plays a pivotal role in the regulation of energy homeostasis and metabolism, primarily by acting on neurons in the hypothalamus that control food intake. However, leptin receptors are more widely expressed in the brain suggesting additional, as yet unknown, functions of leptin. Here we show that both embryonic and adult hippocampal neurons express leptin receptors coupled to activation of STAT3 and phosphatidylinositol 3-kinase-Akt signaling pathways. Leptin protects hippocampal neurons against cell death induced by neurotrophic factor withdrawal and excitotoxic and oxidative insults. The neuroprotective effect of leptin is antagonized by the JAK2-STAT3 inhibitor AG-490, STAT3 decoy DNA, and phosphatidylinositol 3-kinase/Akt inhibitors but not by an inhibitor of MAPK. Leptin induces the production of manganese superoxide dismutase and the anti-apoptotic protein Bcl-xL, and stabilizes mitochondrial membrane potential and lessens mitochondrial oxidative stress. Leptin receptor-deficient mice (db/db mice) are more vulnerable to seizure-induced hippocampal damage, and intraventricular administration of leptin protects neurons against seizures. By enhancing mitochondrial resistance to apoptosis and excitotoxicity, our findings suggest that leptin signaling serves a neurotrophic function in the developing and adult hippocampus.

Riluzole in Huntington's disease: a 3-year, randomized controlled study

OBJECTIVE

We conducted a randomized double-blind trial of riluzole in Huntington's disease to investigate the efficacy of this antiexcitotoxic drug in slowing disease progression.

METHODS

The study included 537 adult patients with a clinical diagnosis of Huntington's disease confirmed by genotyping. Patients were randomized (2:1) to treatment with riluzole (50mg twice daily) or placebo for 3 years. Concomitant use of antichoreic medication was forbidden, and introduction of such medication was a predefined end point. The primary outcome measure was change in a combined score derived from the motor and total functional capacity subscores of the Unified Huntington's Disease Rating Scale. Safety was also evaluated.

RESULTS

A total of 379 patients completed the study (mean age, 47 [standard deviation, 9.5] years; 50% female patients). The principal reason for discontinuation was introduction of antichoreic medication. The median change from baseline in the combined score (primary outcome) for the "per protocol" population was 13.7 (95% confidence interval, 11.1-17.2) in the placebo group and 14.3 (95% confidence interval, 11.7-16.6) in the riluzole group. No intergroup difference in outcome could thus be demonstrated (p = 0.93, Mann-Whitney U test). No differences in secondary efficacy outcome variables were observed except for more frequent recourse to antichoreic medication in the placebo group. No unexpected adverse events were reported, and tolerability was acceptable.

INTERPRETATION

No neuroprotective or beneficial symptomatic effects of riluzole in Huntington's disease were demonstrated.

The hemo-neural hypothesis: on the role of blood flow in information processing

Brain vasculature is a complex and interconnected network under tight regulatory control that exists in intimate communication with neurons and glia. Typically, hemodynamics are considered to exclusively serve as a metabolic support system. In contrast to this canonical view, we propose that hemodynamics also play a role in information processing through modulation of neural activity. Functional hyperemia, the basis of the functional MRI (fMRI) BOLD signal, is a localized influx of blood correlated with neural activity levels. Functional hyperemia is considered by many to be excessive from a metabolic standpoint, but may be appropriate if interpreted as having an activity-dependent neuro-modulatory function. Hemodynamics may impact neural activity through direct and indirect mechanisms. Direct mechanisms include delivery of diffusible blood-borne messengers and mechanical and thermal modulation of neural activity. Indirect mechanisms are proposed to act through hemodynamic modulation of astrocytes, which can in turn regulate neural activity. These hemo-neural mechanisms should alter the information processing capacity of active local neural networks. Here, we focus on analysis of neocortical sensory processing. We predict that hemodynamics alter the gain of local cortical circuits, modulating the detection and discrimination of sensory stimuli. This novel view of information processing-that includes hemodynamics as an active and significant participant-has implications for understanding neural representation and the construction of accurate brain models. There are also potential medical benefits of an improved understanding of the role of hemodynamics in neural processing, as it directly bears on interpretation of and potential treatment for stroke, dementia, and epilepsy.

Selective overexpression of gamma1 laminin in astrocytes in amyotrophic lateral sclerosis indicates an involvement in ALS pathology

Our earlier studies indicate that the KDI tripeptide of gamma1 laminin reverts paralysis and protects adult rat CNS from excitotoxicity of glutamate and from oxidative stress. Here we show that gamma1 laminin is selectively overexpressed in reactive astrocytes of the amyotrophic lateral sclerosis (ALS) spinal cord, with both gray and white matter astrocytes overexpressing gamma1 laminin. Intensely gamma1 laminin-positive, aggressive-looking reactive astrocytes of the lateral columns of both cervical and thoracic spinal cord surround the lateral ventral horns and roots and extend into the area of the lateral corticospinal tract. In the cervical ALS spinal cord, large numbers of strongly gamma1 laminin-immunoreactive astrocytes are also present in the dorsal columns of the ascending sensory pathways. No other laminin or any other ALS-associated protein localizes in this manner. This unique distribution of gamma1 laminin-immunoreactive astrocytes in the ALS white matter together with our recent results on the efficacy of the KDI domain as a neuronal protector strongly suggest that gamma1 laminin may be expressed by astrocytes of the ALS spinal cord as a protective measure intended to aid neuronal survival. Further comparative studies on ALS spinal cord tissues and those of the animal models of ALS are needed to clarify the specific role of gamma1 laminin and its KDI domain in ALS and its putative interactions with the additional ALS-associated factors, such as excitotoxicity, oxidative stress, and neurofilament accumulation. Most importantly, further studies are urgently needed to test the potential of the KDI tripeptide as a therapeutic treatment for ALS.

Dextromethorphan as a Potential Neuroprotective Agent With Unique Mechanisms of Action

BACKGROUND

Dextromethorphan (DM) is a widely-used antitussive. DM's complex central nervous system (CNS) pharmacology became of interest when it was discovered to be neuroprotective due to its low-affinity, uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonism.

REVIEW SUMMARY

Mounting preclinical evidence has proven that DM has important neuroprotective properties in various CNS injury models, including focal and global ischemia, seizure, and traumatic brain injury paradigms. Many of these protective actions seem functionally related to its inhibitory effects on glutamate-induced neurotoxicity via NMDA receptor antagonist, sigma-1 receptor agonist, and voltage-gated calcium channel antagonist actions. DM's protection of dopamine neurons in parkinsonian models may be due to inhibition of neurodegenerative inflammatory responses. Clinical findings are limited, with preliminary evidence indicating that DM protects against neuronal damage. Negative findings seem to relate to attainment of inadequate DM brain concentrations. Small studies have shown some promise for treatment of perioperative brain injury, amyotrophic lateral sclerosis, and symptoms of methotrexate neurotoxicity. DM safety/tolerability trials in stroke, neurosurgery, and amyotrophic lateral sclerosis patients demonstrated a favorable safety profile. DM's limited clinical benefit is proposed to be associated with its rapid metabolism to dextrorphan, which restricts its central bioavailability and therapeutic utility. Systemic concentrations of DM can be increased via coadministration of low-dose quinidine (Q), which reversibly inhibits its first-pass elimination. Potential drug interactions with DM/Q are discussed.

CONCLUSIONS

Given the compelling preclinical evidence for neuroprotective properties of DM, initial clinical neuroprotective findings, and clinical demonstrations that the DM/Q combination is well tolerated, this strategy may hold promise for the treatment of various acute and degenerative neurologic disorders.

Viewpoint: mechanisms of action and therapeutic potential of neurohormetic phytochemicals

The nervous system is of fundamental importance in the adaptive (hormesis) responses of organisms to all types of stress, including environmental "toxins". Phytochemicals present in vegetables and fruits are believed to reduce the risk of several major diseases including cardiovascular disease, cancers and neurodegenerative disorders. Although antioxidant properties have been suggested as the basis of health benefits of phytochemicals, emerging findings suggest a quite different mechanism of action. Many phytochemicals normally function as toxins that protect the plants against insects and other damaging organisms. However, at the relatively low doses consumed by humans and other mammals these same "toxic" phytochemicals activate adaptive cellular stress response pathways that can protect the cells against a variety of adverse conditions. Recent findings have elucidated hormetic mechanisms of action of phytochemicals (e.g., resveratrol, curcumin, sulforaphanes and catechins) using cell culture and animal models of neurological disorders. Examples of hormesis pathways activated by phytochemicals include the transcription factor Nrf-2 which activates genes controlled by the antioxidant response element, and histone deacetylases of the sirtuin family and FOXO transcription factors. Such hormetic pathways stimulate the production of antioxidant enzymes, protein chaperones and neurotrophic factors. In several cases neurohormetic phytochemicals have been shown to suppress the disease process in animal models relevant to neurodegenerative disorders such as Alzheimer's and Parkinson's diseases, and can also improve outcome following a stroke. We are currently screening a panel of biopesticides in order to establish hormetic doses, neuroprotective efficacy, mechanisms of action and therapeutic potential as dietary supplements.

The organotellurium compound ammonium trichloro(dioxoethylene-0,0') tellurate enhances neuronal survival and improves functional outcome in an ischemic stroke model in mice

Ammonium trichloro(dioxoethylene-0,0') tellurate (AS101) is a non-toxic organotellurium compound with pleiotropic activities. It was recently shown to induce production of the neurotrophic factor glial cell line-derived neurotrophic factor and to rescue neuronal-like PC-12 cells from neurotrophic factor deprivation-induced apoptosis. In this study, we show that AS101 improves functional outcome and reduces brain damage in a mouse model of focal ischemic stroke. Both pre-stroke and post-stroke intraperitoneal treatments with AS101 reduced infarct size and edema and improved the neurological function of the animals. AS101 treatments reduced both apoptotic and inflammatory caspase activities, and also inhibited protein tyrosine nitration suggesting that AS101 suppresses oxidative stress. Studies of cultured neurons showed that AS101 confers protection against apoptosis induced by either glucose deprivation or the lipid peroxidation product 4-hydroxynonenal. Moreover, AS101 treatment reduced glutamate-induced intracellular calcium elevation, a major contributor to neuronal death in stroke. As AS101 has an excellent safety profile in humans, our pre-clinical data suggest a potential therapeutic benefit of AS101 in patients suffering from stroke and other neurodegenerative conditions.

Changes in calcium dynamics following the reversal of the sodium-calcium exchanger have a key role in AMPA receptor-mediated neurodegeneration via calpain activation in hippocampal neurons

Proteolytic cleavage of the Na(+)/Ca(2+) exchanger (NCX) by calpains impairs calcium homeostasis, leading to a delayed calcium overload and excitotoxic cell death. However, it is not known whether reversal of the exchanger contributes to activate calpains and trigger neuronal death. We investigated the role of the reversal of the NCX in Ca(2+) dynamics, calpain activation and cell viability, in alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-stimulated hippocampal neurons. Selective overactivation of AMPA receptors caused the reversal of the NCX, which accounted for approximately 30% of the rise in intracellular free calcium concentration ([Ca(2+)](i)). The NCX reverse-mode inhibitor, 2-[2-[4-(4-nitrobenzyloxy)phenyl]ethyl]isothiourea (KB-R7943), partially inhibited the initial increase in [Ca(2+)](i), and prevented a delayed increase in [Ca(2+)](i). In parallel, overactivation of AMPA receptors strongly activated calpains and led to the proteolysis of NCX3. KB-R7943 prevented calpain activation, cleavage of NCX3 and was neuroprotective. Silencing of NCX3 reduced Ca(2+) uptake, calpain activation and was neuroprotective. Our data show for the first time that NCX reversal is an early event following AMPA receptor stimulation and is linked to the activation of calpains. Since calpain activation subsequently inactivates NCX, causing a secondary Ca(2+) entry, NCX may be viewed as a new suicide substrate operating in a Ca(2+)-dependent loop that triggers cell death and as a target for neuroprotection.

Raloxifene acutely reduces glutamate-induced intracellular calcium increase in cultured rat cortical neurons via inhibition of high-voltage-activated calcium current

There is increasing evidence indicating that estrogen replacement therapy produces neuroprotective actions but has undesirable side effects on the reproductive system. Raloxifene is a selective estrogen receptor modulator that exerts estrogen agonist action in the brain while acting as an estrogen antagonist in the reproductive system. In the present study, we investigated whether raloxifene affected the glutamate-induced calcium (Ca2+) overload in rat cultured cortical neurons. The bulk cytosolic intracellular Ca2+ level was measured by using confocal microscopy with fluorescent Ca2+ probe fluo3. Whole-cell recording technique was used to observe the effects of raloxifene on N-methyl-D-aspartate (NMDA)-evoked and voltage-activated Ca2+ currents in cultured cortical neurons. Pre-exposure of cortical neurons to raloxifene (0.5 microM-10 microM) for 3 min attenuated intracellular Ca2+ increase induced by application of glutamate (300 microM) for 1 min. The action of raloxifene was reversible after washout. ICI 182,780 and thapsigargin did not block the action of raloxifene. In whole-cell recording experiments, raloxifene (10 microM) significantly reduced the amplitude of the high-voltage-activated Ca2+ current but had no effect on NMDA-evoked Ca2+ current. The present study demonstrates that raloxifene acutely reduces glutamate-induced intracellular Ca2+ increase probably via inhibition of high-voltage-activated calcium channels.

Calcium and neurodegeneration

When properly controlled, Ca2+ fluxes across the plasma membrane and between intracellular compartments play critical roles in fundamental functions of neurons, including the regulation of neurite outgrowth and synaptogenesis, synaptic transmission and plasticity, and cell survival. During aging, and particularly in neurodegenerative disorders, cellular Ca2+-regulating systems are compromised resulting in synaptic dysfunction, impaired plasticity and neuronal degeneration. Oxidative stress, perturbed energy metabolism and aggregation of disease-related proteins (amyloid beta-peptide, alpha-synuclein, huntingtin, etc.) adversely affect Ca2+ homeostasis by mechanisms that have been elucidated recently. Alterations of Ca2+-regulating proteins in the plasma membrane (ligand- and voltage-gated Ca2+ channels, ion-motive ATPases, and glucose and glutamate transporters), endoplasmic reticulum (presenilin-1, Herp, and ryanodine and inositol triphosphate receptors), and mitochondria (electron transport chain proteins, Bcl-2 family members, and uncoupling proteins) are implicated in age-related neuronal dysfunction and disease. The adverse effects of aging on neuronal Ca2+ regulation are subject to modification by genetic (mutations in presenilins, alpha-synuclein, huntingtin, or Cu/Zn-superoxide dismutase; apolipoprotein E isotype, etc.) and environmental (dietary energy intake, exercise, exposure to toxins, etc.) factors that may cause or affect the risk of neurodegenerative disease. A better understanding of the cellular and molecular mechanisms that promote or prevent disturbances in cellular Ca2+ homeostasis during aging may lead to novel approaches for therapeutic intervention in neurological disorders such as Alzheimer's and Parkinson's diseases and stroke.

Early social isolation decreases the expression of calbindin D-28k and dendritic branching in the medial prefrontal cortex of the rat

Social isolation in rodents is the most well characterized animal model for early stressful experiences and their neurobehavioral consequences. The present study analyzed the effects of early social isolation on the expression of the calcium binding protein calbindin-D28k (CAD) and dendritic arborization in the medial prefrontal cortex (mPFC) of the rat. Sprague-Dawley male rats were reared either under isolation or social conditions from 21 to 51 postnatal days. At the end of this period the animals were behaviorally evaluated in the open-field test, sacrificed, and mPFC serial sections were processed either for immunocytochemical labeling against CAD or Golgi-Cox-Sholl staining. Isolated-reared rats exhibited a dramatic decrease in the number of CAD immunoreactive neurons and a significant dendritic atrophy of layer II/III pyramidal cells in association with a reduced exploratory behavior.

Adult retinal neuronal cell culture

Despite a relatively long history, general knowledge is not widespread that adult neurons can be maintained in cell culture for fairly extended periods of time. Within the central nervous system, this capacity seems to be particularly well developed in the retina, although it is still not clear whether this property is due to physical reasons (spatial configuration, simple connections) or to more fundamental differences (molecular composition, physiological function). Irrespective of the reasons, in vitro model systems are useful for investigating physiological and pathological processes occurring in mature retina. The authors argue that the numerous molecular changes undergone during maturation (modifications in ion channels and receptors, apoptotic pathways and growth factor effects) should be taken into account when using in vitro approaches to study processes involved in photoreceptor and ganglion cell degeneration, and hence that more classical methods relying on embryonic or newborn tissue should be interpreted with caution. A number of examples are given where the use of adult retinal neuronal culture may be especially informative: neurite regeneration, neuroprotection assays and pathogenic mechanisms; and areas of further research that should be explored: cell transplantation.

Increased vulnerability of hippocampal neurons with age in culture: temporal association with increases in NMDA receptor current, NR2A subunit expression and recruitment of L-type calcium channels

Excessive glutamate (Glu) stimulation of the NMDA-R is a widely recognized trigger for Ca(2+)-mediated excitotoxicity. Primary neurons typically show a large increase in vulnerability to excitotoxicity with increasing days in vitro (DIV). This enhanced vulnerability has been associated with increased expression of the NR2B subunit or increased NMDA-R current, but the detailed age-courses of these variables in primary hippocampal neurons have not been compared in the same study. Further, it is not clear whether the NMDA-R is the only source of excess Ca(2+). Here, we used primary hippocampal neurons to examine the age dependence of the increase in excitotoxic vulnerability with changes in NMDA-R current, and subunit expression. We also tested whether L-type voltage-gated Ca(2+) channels (L-VGCCs) contribute to the enhanced vulnerability. The EC(50) for Glu toxicity decreased by approximately 10-fold between 8-9 and 14-15 DIV, changing little thereafter. Parallel experiments found that during the same period both amplitude and duration of NMDA-R current increased dramatically; this was associated with an increase in protein expression of the NR1 and NR2A subunits, but not of the NR2B subunit. Compared to MK-801, ifenprodil, a selective NR2B antagonist, was less effective in protecting older than younger neurons from Glu insult. Conversely, nimodipine, an L-VGCC antagonist, protected older but not younger neurons. Our results indicate that enhanced excitotoxic vulnerability with age in culture was associated with a substantial increase in NMDA-R current, concomitant increases in NR2A and NR1 but not NR2B subunit expression, and with apparent recruitment of L-VGCCs into the excitotoxic process.

Chronic membrane depolarization-induced morphological alteration of developing neurons

During development of CNS, young neurons experience various stimuli, and thereafter differentiate to mature neurons in an activity-dependent manner. Membrane depolarization acts as an inducer of excitability and various signals in the neurons, which can be used as a model of neuronal activity. However, the mechanisms of the influence of membrane depolarization on neuronal differentiation have not been fully understood. Therefore, we investigated the effect of membrane depolarization on morphology of spines and generation of valid electrical activity. Using rat hippocampal cultures treated from the plating day with or without high KCl (35 mM, termed HK), we directly observed living neurons transfected with green fluorescence protein-expressing plasmid through a two-photon laser scanning confocal microscope and electrophysiological recording using a patch-clamp technique. Compared with controls, the neurons cultured with HK for 3 days in vitro (DIV) showed marked filopodia-like protrusions as well as an increase in the number of spines, but those cultured with HK for 6 DIV profoundly lost these spines, resulting in a small number of fine filopodia-like protrusions proximally and on the cell body, and a smooth surface of distal dendrites. Electrophysiological recordings showed no spontaneous responses in 6 DIV HK-treated neurons. Moreover, addition of an N-methyl-D-aspartate receptor (NMDAR) antagonist to HK-treated neurons blocked the shrinkage and decrease in the number of filopodia-like protrusions significantly. These findings suggest that membrane depolarization of developing neurons induces synaptogenesis in the early stages of development but chronic treatment with HK causes pathological changes through NMDAR, and that there may be alternative mechanisms for the physiological differentiation of neurons in later developmental stages.

Oxidative stress mediates tau-induced neurodegeneration in Drosophila

Markers of oxidative damage have been detected in brain tissue from patients with Alzheimer disease (AD) and other neurodegenerative disorders. These findings implicate oxidative injury in the neurodegenerative process, but whether oxidative stress is a cause or a consequence of neurotoxicity remains unclear. We used a Drosophila model of human tauopathies to investigate the role of oxidative stress in neurodegeneration. Genetic and pharmacological manipulation of antioxidant defense mechanisms significantly modified neurodegeneration in our model, suggesting that oxidative stress plays a causal role in neurotoxicity. We demonstrate that the JNK signaling pathway is activated in our model, which is in agreement with previous findings in AD tissue. Furthermore, we show that the extent of JNK activation correlates with the degree of tau-induced neurodegeneration. Finally, our findings suggest that oxidative stress acts not to promote tau phosphorylation, but to enhance tau-induced cell cycle activation. In summary, our study identifies oxidative stress as a causal factor in tau-induced neurodegeneration in Drosophila.

A novel isoform of prostate apoptosis response 4 (PAR-4) that co-distributes with F-actin and prevents apoptosis in neural stem cells

The elevated expression of prostate apoptosis response-4 (PAR-4) induces apoptosis in differentiating mouse embryonic stem (ES) cells. In embryoid body (EB) cells and the E15.5 stage of embryonic mouse brain, PAR-4 is expressed as two isoforms (38 and 33 kDa). Using mouse EB-derived RNA as a template we have cloned and characterized a novel isoform of PAR-4 (PAR-4/p33) that lacks exon 3 and shows a bona fide splice junction of exons 2 and 4. The molecular mass for PAR-4/p33 is estimated to be 33 kDa, corresponding to the short form found in the EB cells and E15.5 mouse brain. The fluorescent fusion protein of PAR-4/p33 is mainly found in the cytosol and is co-distributed with F-actin filaments, while that of the 38 kDa full length PAR-4/p38 is predominantly translocated to the nucleus. In contrast to the full length PAR-4 (PAR-4/p38), ectopic expression of PAR-4/p33 does not result in the activation of caspase 3 and the induction of apoptosis. PAR-4/p33 forms a complex with PAR-4/p38, which inhibits its nuclear translocation and the induction of apoptosis. PAR-4/p33 is suggested to be a dominant negative isoform of PAR-4/p38 and may regulate PAR-4-dependent apoptosis.

Delayed neurodegeneration and early astrogliosis after excitotoxicity to the aged brain

Excitotoxicity is well recognised as a mechanism underlying neuronal cell death in several brain injuries. To investigate age-dependent differences in neurodegeneration, edema formation and astrogliosis, intrastriatal N-methyl-d-aspartate injections were performed in young (3 months) and aged (22-24 months) male Wistar rats. Animals were sacrificed at different times between 12h and 14 days post-lesion (DPL) and cryostat sections were processed for Toluidine blue, Fluoro-Jade B staining, NeuN and GFAP immunohistochemistry. Our results show that both size of tissue injury and edema were reduced in the old subjects only up to 1DPL, correlating with a slower progression of neurodegeneration with peak numbers of degenerating neurons at 3DPL in the aged, contrasting with maximum neurodegeneration at 1DPL in the young. However, old animals showed an earlier onset of astroglial response, seen at 1DPL, and a larger area of astrogliosis at all time-points studied, including a greater glial scar. In conclusion, after excitotoxic striatal damage, progression of neurodegeneration is delayed in the aged but the astroglial response is earlier and exacerbated. Our results emphasize the importance of using aged animals and several survival times for the study of acute age-related brain insults.

Neuronal life-and-death signaling, apoptosis, and neurodegenerative disorders

When subjected to excessive oxidative stress, neurons may respond adaptively to overcome the stress, or they may activate a programmed cell death pathway called apoptosis. Apoptosis is characterized by alterations in mitochondria and the endoplasmic reticulum and activation of cysteine proteases called caspases. Increasing evidence suggests that apoptotic biochemical cascades are involved in the dysfunction and death of neurons in neurodegenerative disorders such as Alzheimer's, Parkinson, and Huntington's diseases. Studies of normal aging, of genetic mutations that cause disease, and of environmental factors that affect disease risk are revealing cellular and molecular alterations that may cause excessive oxidative stress and trigger neuronal apoptosis. Accumulation of self-aggregating proteins such as amyloid beta-peptide, tau, alpha-synuclein, and huntingtin may be involved in apoptosis both upstream and downstream of oxidative stress. Membrane-associated oxidative stress resulting in perturbed lipid metabolism and disruption of cellular calcium homeostasis may trigger apoptosis in several different neurodegenerative disorders. Counteracting neurodegenerative processes are an array of mechanisms including neurotrophic factor signaling, antioxidant enzymes, protein chaperones, antiapoptotic proteins, and ionostatic systems. Emerging findings suggest that the resistance of neurons to death during aging can be enhanced by modifications of diet and lifestyle.

KDI tripeptide of gamma1 laminin protects rat dopaminergic neurons from 6-OHDA induced toxicity

Our previous studies indicate that the KDI (Lys-Asp-Ile) tripeptide of gamma1 laminin protects central neurons from mechanical trauma and excitotoxicity. At least part of the neuroprotective effect of the KDI tripeptide may be mediated by its inhibitory function on ionotropic glutamate receptors. We studied the protective effect of the KDI tripeptide against 6-hydroxy-dopamine (6-OHDA) induced neurotoxicity in a rat experimental model of Parkinson's disease (PD). We found that a single unilateral injection of the KDI tripeptide into the substantia nigra before an injection of 6-OHDA protected the dopaminergic neurons from the neurotoxicity of 6-OHDA. Compared to rats treated with 6-OHDA alone, the KDI + 6-OHDA-treated substantia nigra was relatively intact with large numbers of dopaminergic neurons present at the injection side. In the rats treated with 6-OHDA alone, no dopaminergic neurons were detected, and the substantia nigra-area at the injection side was filled with blood-containing cavities. Quantification of the rescue effect of the KDI tripeptide indicated that, in animals receiving KDI before 6-OHDA, 33% of tyrosine hydroxylase-positive dopaminergic neurons of the substantia nigra were present as compared to the contralateral non-injected side. In animals receiving 6-OHDA alone, only 1.4% of the tyrosine hydroxylase expressing dopaminergic neurons could be verified. If this much protection were achieved in humans, it would be sufficient to diminish or greatly alleviate the clinical symptoms of PD. We propose that the KDI tripeptide or its derivatives might offer a neuroprotective biological alternative for treatment of PD.

Neurohormetic phytochemicals: Low-dose toxins that induce adaptive neuronal stress responses

Diets rich in vegetables and fruits are associated with reduced risk of several major diseases, including neurodegenerative disorders. Although some beneficial phytochemicals might function solely as antioxidants, it is becoming clear that many of the beneficial chemicals in vegetables and fruits evolved as toxins (to dissuade insects and other predators) that, at subtoxic doses, activate adaptive cellular stress-response pathways in a variety of cells including neurons. Examples of such 'preconditioning' or 'neurohormesis' pathways include those involving cell-survival signaling kinases, the transcription factors NRF2 and CREB, and histone deacetylases of the sirtuin family. In these ways, neurohormetic phytochemicals such as resveratrol, sulforaphanes and curcumin might protect neurons against injury and disease by stimulating the production of antioxidant enzymes, neurotrophic factors, protein chaperones and other proteins that help cells to withstand stress. Thus, as we discuss in this review, highly conserved longevity and survival pathways in neurons are the targets of many phytochemicals.

Novel neuroprotective, neuritogenic and anti-amyloidogenic properties of 2,4-dinitrophenol: the gentle face of Janus

In Roman mythology, Janus was the god of gates, doors, beginnings and endings. He was usually depicted with two faces looking in opposite directions. Janus was frequently used to symbolize change and transitions, such as the progression from past to future or from one viewpoint to another. 2,4-dinitrophenol (DNP) and other nitrophenols have long been known to be toxic at high concentrations (the 'bad' face of DNP), an effect that appears essentially related to interference with cellular energy metabolism due to uncoupling of mitochondrial oxidative phosphorylation. Five years ago, however, we published the first report showing that low concentrations of DNP protect neurons against the toxicity of the amyloid-beta peptide (De Felice et al. (2001) FASEB J. 15:1297 - 1299]. Since then, other studies have provided evidence of beneficial actions of DNP (at low concentrations), including neuroprotection against different types of insult, blockade of amyloid aggregation, stimulation of neurite outgrowth and neuronal differentiation, and even extension of lifespan in certain organisms. Some of these effects appear to be due to mild mitochondrial uncoupling and prevention of cellular oxidative stress, whereas other actions are related to activation of additional intracellular signaling pathways. Thus, a novel and 'gentle' face of DNP is emerging from such studies. In this review, we discuss both toxic and beneficial actions of DNP. The evidence available so far suggests that DNP and other compounds with similar biological activities may be of significant interest to the development of novel therapeutic approaches for neurodegenerative diseases and other neurological disorders.

17beta-estradiol is protective in spinal cord injury in post- and pre-menopausal rats

The neuroprotective effects of 17 beta -estradiol have been shown in models of central nervous system injury, including ischemia, brain injury, and more recently, spinal cord injury (SCI). Recent epidemiological trends suggest that SCIs in elderly women are increasing; however, the effects of menopause on estrogen-mediated neuroprotection are poorly understood. The objective of this study was to evaluate the effects of 17beta-estradiol and reproductive aging on motor function, neuronal death, and white matter sparing after SCI of post- and pre-menopausal rats. Two-month-old or 1- year-old female rats were ovariectomized and implanted with a silastic capsule containing 180 microg/mL of 17beta-estradiol or vehicle. Complete crush SCI at T8-9 was performed 1 week later. Additional animals of each age group were left ovary-intact but were spinal cord injured. The Basso, Beattie, Bresnahan (BBB) locomotor test was performed. Spinal cords were collected on post-SCI days 1, 7, and 21, and processed for histological markers. Administration of 17beta-estradiol to ovariectomized rats improved recovery of hind-limb locomotion, increased white matter sparing, and decreased apoptosis in both the post- and pre-menopausal rats. Also, ovary-intact 1-year-old rats did worse than ovary-intact 2-month-old rats, suggesting that endogenous estrogen confers neuroprotection in young rats, which is lost in older animals. Taken together, these data suggest that estrogen is neuroprotective in SCI and that the loss of endogenous estrogen-mediated neuroprotective seen in older rats can be attenuated with exogenous administration of 17beta-estradiol.

Ablation of gene expression of N-methyl-D-aspartate receptor one by antisense oligonucleotides in striatal neurons in culture

In the present study, a twenty-mer antisense oligonucleotide specific for N-methyl-D-aspartate receptor one (ANR1) was applied to striatal neurons in primary cell culture. The ANR1 was found to be specific and nontoxic. Significant reductions in expression of NR1 mRNA and proteins were resulted after a single dose of ANR1 transcripts. Interestingly, there were reductions in total NR1 proteins but two phosphorylated forms of NR1 proteins at serine 896 and 897 residues were not reduced. There was also no change in the pattern of distribution of NR1 immunoreactivity in the striatal neurons. In addition, significant reductions of NMDA-mediated peak inward current were found after application of a higher concentration of ANR1 (20-100 microM) by patch clamp recordings. The present results indicate that ANR1 is a useful agent in reducing NMDA receptor functions. The present data thus provide detailed cellular and molecular mechanisms to explain our previous findings of amelioration of motor symptoms in a rat model of Parkinson's disease. More importantly, application of ANR1 was also found to display neuroprotective effects of striatal neurons against NMDA-induced excitotoxic cell death. The findings have implications in development of new approach in prevention of cell death in neurodegenerative diseases and new treatments for these diseases.

Ampakines and the threefold path to cognitive enhancement

Ampakines are the first peripherally administered drugs that increase excitatory monosynaptic responses in the brain. Because of this effect, the compounds improve communication in complex networks, potently facilitate long-term potentiation (LTP) and induce the expression of neurotrophic factors. Ampakines are effective in animal models of neuropsychiatric disorders and have produced positive results in a small number of human studies. Neurobiological considerations and modeling studies suggest that the drugs, in addition to their effects on disturbed behavior, will alter the encoding and organization of information in normal brains. Results from physiological and behavioral studies accord with this prediction. Building on these findings, this article considers how the threefold effects of ampakines will modify, and enhance, cognition.

Neuroprotection by two polyphenols following excitotoxicity and experimental ischemia

Brain ischemia induces neuronal loss which is caused in part by excitotoxicity and free radical formation. Here, we report that mangiferin and morin, two antioxidant polyphenols, are neuroprotective in both in vitro and in vivo models of ischemia. Cell death caused by glutamate in neuronal cultures was decreased in the presence of submicromolar concentrations of mangiferin or morin which in turn attenuated receptor-mediated calcium influx, oxidative stress as well as apoptosis. In addition, both antioxidants diminished the generation of free radicals and neuronal loss in the hippocampal CA1 region due to transient forebrain ischemia in rats when administered after the insult. Importantly, neuroprotection by these antioxidants was functionally relevant since treated-ischemic rats performed significantly better in three hippocampal-dependent behavioral tests. Together, these results indicate that mangiferin and morin have potent neuroprotectant activity which may be of therapeutic value for the treatment of acute neuronal damage and disability.

Modifications of brain tissue volumes in facioscapulohumeral dystrophy

Facioscapulohumeral muscular dystrophy (FSHD), a pathology primarily characterized by involvement of the muscles in the face, shoulder and upper arm, can be associated to several CNS disorders, including sensorineural hearing deficits, schizophrenia, epilepsy and mental retardation. Aim of our study was to verify if brain tissue volumes, as measured by segmentation of MRI studies, are altered in FSHD. Volumes of gray matter (GM), white matter (WM), and cerebrospinal fluid (CSF) were compared, taking into account head size age and sex, both globally (by multiple regression analysis) and regionally (by optimized voxel-based morphometry-VBM) in thirty patients with FSHD and 39 normal subjects (NS). FSHD patients had significantly lower GM volumes and higher CSF volumes (P < 10(-4)). GM loss showed a borderline correlation with clinical severity (P < 0.05). Brain tissue volumes did not correlate with disease duration, size of the genetic deletion, age at onset and the presence at MRI of WM hyperintensities (detected in 4/22 patients). At VBM three clusters of GM loss were detected, in the left precentral cortex (Brodmann areas 6, 2 and 44, P < 10(-14) corrected for multiple comparisons at cluster level), in the anterior cingulate (Brodmann areas 33, 24 and 11, P < 10(-4)) and in the right fronto-polar region (Brodmann area 10, P < 5.10(-3)). To the best of our knowledge, this is the first study to demonstrate a reduction in GM volume in FSHD. We hypothesize that localized GM loss in FSHD is the consequence of a selective involvement of specific CNS structures.

Tapered progesterone withdrawal promotes long-term recovery following brain trauma

We previously demonstrated that after traumatic brain injury (TBI), acute progesterone withdrawal (AW) causes an increase in anxiety behaviors and cerebro-cellular inflammation compared to tapered progesterone withdrawal (TW). Our current study investigates the behavioral and cellular effects of AW two weeks after termination of treatments to determine the longer-term influence of withdrawal after injury. Adult, male Sprague-Dawley rats received either bilateral frontal cortex contusion (L) or sham (S) surgery. Rats were injected at 1 and 6 h post-injury, then every 24 h for six days. Vehicle (V)-treated rats were given 9 injections of 22.5% cyclodextrin, whereas AW rats received 9 injections of 16 mg/kg progesterone and TW rats received 7 injections of P at 16 mg/kg, followed by one at 8 mg/kg and one at 4 mg/kg. On day 8, sensory neglect and locomotor activity tests were initiated. Animals were killed 22 days post-TBI and the brains prepared for either molecular or histological analysis. Western blotting revealed increased brain-derived neurotrophic factor (BDNF) and heat shock protein 70 (HSP70) in TW vs. AW animals. P53 was increased in VL animals, whereas all progesterone-treated groups were equivalent to shams. TW animals had markedly decreased sensory neglect compared to AW animals and increased center time in locomotor activity assays. In addition, lesion reconstruction revealed a decreased lesion size for TWL over AWL over VL animals. Glial fibrillary acidic protein (GFAP) immunofluorescent staining followed this pattern as well. In conclusion, after TBI, AW affects select behaviors and molecular markers in the chronic recovery period.

Homeostatic regulation of glutamate release in response to depolarization

Proper nervous system function requires a balance between excitation and inhibition. Systems of homeostasis may have evolved in neurons to help maintain or restore balance between excitation and inhibition, presumably because excessive excitation can cause dysfunction and cell death. This article reviews evidence for homeostatic mechanisms within the hippocampus that lead to differential regulation of glutamate and gamma-aminobutyric acid release in response to conditions of excess depolarization. We recently found differential effects on glutamate release at the level of action potential coupling to transmitter release, vesicular release probability, and vesicle availability. Such differential regulation may help to prevent excitotoxicity and runaway excitation.

Interferon-gamma is up-regulated in the hippocampus in response to intermittent fasting and protects hippocampal neurons against excitotoxicity

Dietary restriction (DR) increases the life span of many different organisms, and recent findings have demonstrated neuroprotective effects of DR in rodent and nonhuman primate models of neurodegenerative disorders. The neuroprotective mechanism of action of DR is unknown, but it may result from a mild cellular stress response involving increased production of neurotrophic factors. Because several different cytokines are known to be up-regulated in brain cells in response to stress, we determined whether DR affected cytokine expression in the rat brain. Levels of expression of interferon-gamma (IFN-gamma) and its receptor were significantly increased in the hippocampus of rats that had been maintained on an intermittent fasting DR regimen compared with rats on the ad libitum control diet. Pretreatment of embryonic rat hippocampal cell cultures with IFN-gamma protected neurons against glutamate-induced death. IFN-gamma-mediated neuroprotection was associated with an enhanced recovery of intracellular Ca(2+) concentrations following exposure to glutamate. Our data show that intermittent fasting DR stimulates IFN-gamma-mediated neuroprotective signaling in the hippocampus, suggesting a role for this cytokine in the previously reported ability of DR to protect neurons in animal models of severe epileptic seizures, stroke, and neurodegenerative disorders.

Mutations in amyloid precursor protein and presenilin-1 genes increase the basal oxidative stress in murine neuronal cells and lead to increased sensitivity to oxidative stress mediated by amyloid beta-peptide (1-42), HO and kainic acid: implications for Alzheimer's disease

Oxidative stress is observed in Alzheimer's disease (AD) brain, including protein oxidation and lipid peroxidation. One of the major pathological hallmarks of AD is the brain deposition of amyloid beta-peptide (Abeta). This 42-mer peptide is derived from the beta-amyloid precursor protein (APP) and is associated with oxidative stress in vitro and in vivo. Mutations in the PS-1 and APP genes, which increase production of the highly amyloidogenic amyloid beta-peptide (Abeta42), are the major causes of early onset familial AD. Several lines of evidence suggest that enhanced oxidative stress, inflammation, and apoptosis play important roles in the pathogenesis of AD. In the present study, primary neuronal cultures from knock-in mice expressing mutant human PS-1 and APP were compared with those from wild-type mice, in the presence or absence of various oxidizing agents, viz, Abeta(1-42), H2O2 and kainic acid (KA). APP/PS-1 double mutant neurons displayed a significant basal increase in oxidative stress as measured by protein oxidation, lipid peroxidation, and 3-nitrotyrosine when compared with the wild-type neurons (p < 0.0005). Elevated levels of human APP, PS-1 and Abeta(1-42) were found in APP/PS-1 cultures compared with wild-type neurons. APP/PS-1 double mutant neuron cultures exhibited increased vulnerability to oxidative stress, mitochondrial dysfunction and apoptosis induced by Abeta(1-42), H2O2 and KA compared with wild-type neuronal cultures. The results are consonant with the hypothesis that Abeta(1-42)-associated oxidative stress and increased vulnerability to oxidative stress may contribute significantly to neuronal apoptosis and death in familial early onset AD.

The Alzheimer's disease neuroimaging initiative

With increasing life expectancy in developed countries, the incidence of Alzheimer's disease (AD) and its socioeconomic impact are growing. Increasing knowledge of the mechanisms of AD facilitates the development of treatment strategies aimed at slowing down or preventing neuronal death. AD treatment trials using clinical outcome measures require long observation times and large patient samples. There is increasing evidence that neuroimaging and cerebrospinal fluid and blood biomarkers may provide information that may reduce sample sizes and observation periods. The Alzheimer's Disease Neuroimaging Initiative will help identify clinical, neuroimaging, and biomarker outcome measures that provide the highest power for measurement of longitudinal changes and for prediction of transitions.

Methylene blue prevents neurodegeneration caused by rotenone in the retina

An experimental optic neuropathy model was used to test the hypothesis that methylene blue may protect the retinal ganglion cell layer from neurodegeneration caused by rotenone. Rotenone is a widely used pesticide that inhibits complex I, the first enzyme of the mitochondrial respiratory chain. Complex I dysfunction is linked to the degeneration of retinal ganglion cells in Leber's optic neuropathy. Methylene blue is a reduction-oxidation agent that can act as a powerful antioxidant and also as an enhancer of the electron transport chain, preventing formation of mitochondrial oxygen free radicals and promoting oxygen consumption. The neurodegeneration of the retina was studied in mice with intravitreal microinjection of rotenone alone, or in combination with increasing doses of methylene blue, in one eye, and the vehicle in the contralateral control eye. The effect of rotenone and rotenone plus methylene blue was investigated using two histological stains, complex I and Nissl, and two measurements, morphometric layer thickness and non-biased stereological cell counts. Rotenone induced neurodegeneration in the retinal ganglion cell layer 24 h after injection, as indicated by significant reductions in both the thickness and cell numbers of the retinal ganglion cell layer of eyes microinjected with rotenone as compared to the control eyes. This neurodegeneration was prevented in a dose dependent manner by the injection of methylene blue along with rotenone. It was concluded that rotenone-induced degeneration in the ganglion cell layer can be prevented by intravitreal injection of methylene blue. In vitro experiments showed that methylene blue is both a powerful antioxidant as well as an enhancer of cellular oxygen consumption and is able to reverse the oxidative stress and decrease in oxygen consumption induced by rotenone in brain homogenates. The findings suggest that methylene blue may be a promising neuroprotective agent in optic neuropathy and perhaps other neurodegenerative diseases caused by mitochondrial dysfunction.

Nuclear factor kappaB deficiency is associated with auditory nerve degeneration and increased noise-induced hearing loss

Degeneration of the spiral ganglion neurons (SGNs) of the auditory nerve occurs with age and in response to acoustic injury. Histopathological observations suggest that the neural degeneration often begins with an excitotoxic process affecting the afferent dendrites under the inner hair cells (IHCs), however, little is known about the sequence of cellular or molecular events mediating this excitotoxicity. Nuclear factor kappaB (NFkappaB) is a transcription factor involved in regulating inflammatory responses and apoptosis in many cell types. NFkappaB is also associated with intracellular calcium regulation, an important factor in neuronal excitotoxicity. Here, we provide evidence that NFkappaB can play a central role in the degeneration of SGNs. Mice lacking the p50 subunit of NFkappaB (p50(-/-) mice) showed an accelerated hearing loss with age that was highly associated with an exacerbated excitotoxic-like damage in afferent dendrites under IHCs and an accelerated loss of SGNs. Also, as evidenced by immunostaining intensity, calcium-buffering proteins were significantly elevated in SGNs of the p50(-/-) mice. Finally, the knock-out mice exhibited an increased sensitivity to low-level noise exposure. The accelerated hearing loss and neural degeneration with age in the p50(-/-) mice occurred in the absence of concomitant hair cell loss and decline of the endocochlear potential. These results indicate that NFkappaB activity plays an important role in protecting the primary auditory neurons from excitotoxic damage and age-related degeneration. A possible mechanism underlying this protection is that the NFkappaB activity may help to maintain calcium homeostasis in SGNs.

Neuroprotective effects of 17beta-estradiol and nonfeminizing estrogens against H2O2 toxicity in human neuroblastoma SK-N-SH cells

Neuroprotective effects of estrogens have been shown in various in vitro and in vivo models, but the mechanisms underlying protection by estrogen are not clear. Mounting evidence suggests antioxidant effects contribute to the neuroprotective effects of estrogens. In the present study, we assessed the protective effects of estrogens against H2O2-induced toxicity in human neuroblastoma cells and the potential mechanisms involved in this protection. We demonstrate that 17beta-estradiol (17beta-E2) increases cell survival against H2O2 toxicity in human neuroblastoma cells. 17beta-E2 effectively reduced lipid peroxidation induced by 5-min H2O2 exposure. Furthermore, 17beta-E2 exerts the protective effects by maintaining intracellular Ca2+ homeostasis, attenuating ATP depletion, ablating mitochondrial calcium overloading, and preserving mitochondrial membrane potential. Two nonfeminizing estrogens, 17alpha- and ent-estradiol, were as effective as 17beta-E2 in increasing cell survival, alleviating lipid peroxidation, preserving mitochondrial function, and maintaining intracellular glutathione levels and Ca2+ homeostasis against H2O2 insult. Moreover, the estrogen receptor antagonist fulvestrant (ICI 182,780) did not block effects of 17beta-E2, but increased cell survival and blunted intracellular Ca2+ increases. However, these estrogens failed to reduce cytosolic reactive oxygen species, even at concentrations as high as 10 microM. In conclusion, estrogens exert protective effects against oxidative stress by inhibiting lipid peroxidation and subsequently preserving Ca2+ homeostasis, mitochondrial membrane potential, and ATP levels.

Aminoglutethimide prevents excitotoxic and ischemic injuries in cortical neurons

Aminoglutethimide is a clinically available drug that suppresses steroid biosynthesis by inhibiting enzymes such as cytochrome P450scc and aromatase. Because several members of neurosteroids regulate glutamate receptors, we investigated the effect of aminoglutethimide on cell death induced by overactivation of glutamate receptors in CNS neurons. Long-term pretreatment of organotypic cerebrocortical slice cultures with aminoglutethimide (100-1000 microM) for 6 days or over resulted in concentration-dependent suppression of neuronal cell death induced by NMDA. Aminoglutethimide (1000 microM) also inhibited neurotoxicity of AMPA and kainate, but not of ionomycin or staurosporine. The protective effect of aminoglutethimide against NMDA cytotoxicity was not mimicked by other steroid synthesis inhibitors including trilostane and exemestane, and was not reversed by concurrent application of steroids such as pregnenolone, estrone, 17beta-estradiol and estriol. In dissociated rat cerebrocortical cell cultures, long-term treatment with aminoglutethimide (10-1000 microM) attenuated NMDA receptor-mediated glutamate cytotoxicity but produced no significant effect on glutamate-induced increases in intracellular Ca2+. Brief as well as long-term pretreatment with aminoglutethimide (30-1000 microM) prevented NMDA receptor-dependent ischemic neuronal injury in organotypic cerebrocortical slice cultures, which was associated with suppression of glutamate release during the ischemic insult. These results indicate that aminoglutethimide, irrelevant to its actions on neurosteroid synthesis, protects CNS neurons from excitotoxic and ischemic injuries. Development of aminoglutethimide analogs possessing neuroprotective properties may be of therapeutic value.

Ageing and neuronal vulnerability

Everyone ages, but only some will develop a neurodegenerative disorder in the process. Disease might occur when cells fail to respond adaptively to age-related increases in oxidative, metabolic and ionic stress, thereby resulting in the accumulation of damaged proteins, DNA and membranes. Determinants of neuronal vulnerability might include cell size and location, metabolism of disease-specific proteins and a repertoire of signal transduction pathways and stress resistance mechanisms. Emerging evidence on protein interaction networks that monitor and respond to the normal ageing process suggests that successful neural ageing is possible for most people, but also cautions that cures for neurodegenerative disorders are unlikely in the near future.

Brain mitochondrial defects amplify intracellular [Ca2+] rise and neurodegeneration but not Ca2+entry during NMDA receptor activation

According to the "indirect" excitotoxicity hypothesis, mitochondrial defects increase Ca2+ entry into neurons by rendering NMDA-R hypersensitive to glutamate. We tested this hypothesis by investigating in the rat striatum and cultured striatal cells how partial mitochondrial complex II inhibition produced by 3-nitropropionic acid (3NP) modifies the toxicity of the NMDA-R agonist quinolinate (QA). We showed that nontoxic 3NP treatment, leading to partial inhibition of complex II activity, greatly exacerbated striatal degeneration produced by slightly toxic QA treatment through an "all-or-nothing" process. The potentiation of QA-induced cell death by 3NP was associated with increased calpain activity and massive calpain-mediated cleavage of several postsynaptic proteins, suggesting major neuronal Ca2+ deregulation in the striatum. However, Ca2+ anomalies probably do not result from NMDA-R hypersensitivity. Indeed, brain imaging experiments using [(18)F]fluorodeoxyglucose indirectly showed that 3NP did not increase QA-induced ionic perturbations at the striatal glutamatergic synapses in vivo. Consistent with this, the exacerbation of QA toxicity by 3NP was not related to an increase in the QA-induced entry of 45Ca2+ into striatal neurons. The present results demonstrate that the potentiation of NMDA-R-mediated excitotoxicity by mitochondrial defects involves primarily intracellular Ca2+ deregulation, in the absence of NMDA-R hypersensitivity.

Macrophage migration inhibitory factor induces cell death and decreases neuronal nitric oxide expression in spinal cord neurons

Macrophage migration inhibitory factor is a potent proinflammatory cytokine; however, its role in spinal cord injury is poorly understood. Therefore, the aim of the present study was to investigate the effects of macrophage migration inhibitory factor on spinal cord neuron survival and viability. Due to the importance of nitric oxide metabolism in these events, part of our study was also focused on the influence of recombinant macrophage migration inhibitory factor on neuronal nitric oxide expression. Exposure of cultured mouse spinal cord neurons to macrophage migration inhibitory factor markedly increased cellular oxidative stress as measured by 2',7'-dichlorofluorescein fluorescence and intracellular calcium levels. In addition, an antagonist of the inositol 1,4,5-triphosphate receptor, 8-(diethylamino)octyl 3,4,5-trimethoxybenzoate, completely blocked the macrophage migration inhibitory factor-induced increase in intracellular calcium levels. Macrophage migration inhibitory factor treatment also decreased cell viability, increased cellular lactate dehydrogenase release, and induced chromatin condensation and aggregation in cultured spinal cord neurons. Finally, exposure to macrophage migration inhibitory factor markedly decreased expression and activity of neuronal nitric oxide, accompanied by a decrease in cellular guanosine 3'5'-cyclic monophosphate levels. The present results indicate that macrophage migration inhibitory factor can induce dysfunction of spinal cord neurons, leading to cell death through oxidative stress and intracellular calcium-dependent pathways.

Ontogeny of AMPA and NMDA receptor gene expression in the developing sheep white matter and cerebral cortex

This study examined the hypothesis that the high prevalence of white matter injury in premature infants is associated with increased expression of calcium-permeable forms of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) subtype of glutamate receptors in pre-myelinating white matter. We characterized expression of subunits of the AMPA, and for reference, the N-methyl-d-aspartate (NMDA), glutamate receptors at 0.5, 0.65, 0.85, and term gestation in the ovine fetal white matter and cerebral cortex. There was a low expression of the critical calcium-impermeable AMPA receptor GluR2 subunit in subcortical white matter both absolutely and relative to other AMPA subunits throughout gestation. In contrast, GluR2 subunit mRNA expression fell in the cerebral cortex with increasing gestation whereas protein expression increased. These findings suggest a vulnerability of subcortical white matter to AMPA receptor-mediated calcium toxicity throughout the second half of gestation. Thus, the hypothesis that AMPA receptor-mediated glutamate toxicity contributes to brain damage in premature infants needs to be revised.

Ischaemic preconditioning: therapeutic implications for stroke?

Ischaemic preconditioning (IPC), also known as ischaemic tolerance (IT), is a phenomenon whereby tissue is exposed to a brief, sublethal period of ischaemia, which activates endogenous protective mechanisms, thereby reducing cellular injury that may be caused by subsequent lethal ischaemic events. The first description of this phenomenon was in the heart, which was reported by Murry and co-workers in 1986. Subsequent studies demonstrated IPC in lung, kidney and liver tissue, whereas more recent studies have concentrated on the brain. The cellular mechanisms underlying the beneficial effects of IPC remain largely unknown. This phenomenon, which has been demonstrated by using various injury paradigms in both cultured neurons and animal brain tissue, may be utilised to identify and characterise therapeutic targets for small-molecule, antibody, or protein intervention. This review will examine the experimental evidence demonstrating the phenomenon termed IPC in models of cerebral ischaemia, the cellular mechanisms that may be involved and the therapeutic implications of these findings.

KEL-8 is a substrate receptor for CUL3-dependent ubiquitin ligase that regulates synaptic glutamate receptor turnover

The regulated localization of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-type glutamate receptors (AMPARs) to synapses is an important component of synaptic signaling and plasticity. Regulated ubiquitination and endocytosis determine the synaptic levels of AMPARs, but it is unclear which factors conduct these processes. To identify genes that regulate AMPAR synaptic abundance, we screened for mutants that accumulate high synaptic levels of the AMPAR subunit GLR-1 in Caenorhabditis elegans. GLR-1 is localized to postsynaptic clusters, and mutants for the BTB-Kelch protein KEL-8 have increased GLR-1 levels at clusters, whereas the levels and localization of other synaptic proteins seem normal. KEL-8 is a neuronal protein and is localized to sites adjacent to GLR-1 postsynaptic clusters along the ventral cord neurites. KEL-8 is required for the ubiquitin-mediated turnover of GLR-1 subunits, and kel-8 mutants show an increased frequency of spontaneous reversals in locomotion, suggesting increased levels of GLR-1 are present at synapses. KEL-8 binds to CUL-3, a Cullin 3 ubiquitin ligase subunit that we also find mediates GLR-1 turnover. Our findings indicate that KEL-8 is a substrate receptor for Cullin 3 ubiquitin ligases that is required for the proteolysis of GLR-1 receptors and suggest a novel postmitotic role in neurons for Kelch/CUL3 ubiquitin ligases.

Possible role of excitotoxicity in the pathogenesis of glaucoma

Excitotoxicity describes the process of neuronal injury by excess stimulation of amino acid receptors. This form of insult was first described in the retina, and subsequently has been shown to be an important component of the pathogenesis of ischaemic and traumatic injury in the central nervous system. Furthermore, there is increasing evidence that excitotoxicity is involved in several chronic neurological conditions, and anti-excitotoxic treatment has already been approved for some of these conditions. A large-scale trial is currently underway that will determine the efficacy of an anti-excitotoxic drug (memantine) in the management of glaucoma. This review provides an overview of neurotransmission and the mechanisms of excitotoxicity. The evidence for excitotoxicity as a component of certain neurological diseases, including glaucoma, is discussed.

Unconjugated bilirubin activates and damages microglia

Microglia are the resident immune cells of the brain and are the principal source of cytokines produced during central nervous system inflammation. We have previously shown that increased levels of unconjugated bilirubin (UCB), which can be detrimental to the central nervous system during neonatal life, induce the secretion of inflammatory cytokines and glutamate by astrocytes. Nevertheless, the effect of UCB on microglia has never been investigated. Hence, the main goal of the present study was to evaluate whether UCB leads to microglial activation and to the release of the cytokines tumor necrosis factor (TNF)-alpha, interleukin (IL)-1beta, and IL-6. Additionally, we investigated the effects of UCB on glutamate efflux and cell death. The results showed that UCB induces morphological changes characteristic of activated microglia and the release of high levels of TNF-alpha, IL-1beta, and IL-6 in a concentration-dependent manner. In addition, UCB triggered extracellular accumulation of glutamate and an increased cell death by apoptosis and necrosis. These results demonstrate, for the first time, that UCB is toxic to microglial cells and point to microglia as an important target of UCB in the central nervous system. Moreover, they suggest that UCB-induced cytokine production, by mediating cell injury, can further contribute to exacerbate neurototoxicity. Interestingly, microglia cells are much more responsive to UCB than astrocytes. Collectively, these data indicate that microglia may play an important role in the pathogenesis of encephalopathy during severe hyperbilirubinemia.

Down-regulation of microglial activation may represent a practical strategy for combating neurodegenerative disorders

Chronic neurodegenerative disorders are characterized by activation of microglia in the affected neural pathways. Peroxynitrite, prostanoids, and cytokines generated by these microglia can potentiate the excitotoxicity that contributes to neuronal death and dysfunction in these disorders--both by direct effects on neurons, and by impairing the capacity of astrocytes to sequester and metabolize glutamate. This suggests a vicious cycle in which the death of neurons leads to microglial activation, which in turn potentiates neuronal damage. If this model is correct, measures which down-regulate microglial activation may have a favorable effect on the induction and progression of neurodegenerative disease, independent of the particular trigger or target involved in a given disorder. Consistent with this possibility, the antibiotic minocycline, which inhibits microglial activation, shows broad utility in rodent models of neurodegeneration. Other agents which may have potential in this regard include PPARgamma agonists, genistein, vitamin D, COX-2 inhibitors, statins (and possibly policosanol), caffeine, cannabinoids, and sesamin; some of these agents could also be expected to be directly protective to neurons threatened with excitotoxicity. To achieve optimal clinical outcomes, regimens which down-regulate microglial activation could be used in conjunction with complementary measures which address other aspects of excitotoxicity.

Nuclear factor-kappab activation is associated with glutamate-evoked tissue transglutaminase up-regulation in primary astrocyte cultures

We have previously demonstrated that alterations of cell redox state, evoked by glutamate, are associated with tissue transglutaminase increases in primary astrocyte cultures. Furthermore, glutamate exposure activated the nuclear factor (NF)-kappaB pathway, and its effects were significantly reduced by antioxidants. Here, we investigated the possible involvement of activated NF-kappaB pathway in glutamate-evoked tissue transglutaminase up-regulation in primary astrocytes. The presence of DNA binding activity by NF-kappaB in nuclear extracts of astrocytes, treated for 24 hr with glutamate (500 microM) or untreated, was assessed by EMSA, using an oligonucleotide probe containing the NF-kappaB consensus sequence present in the tissue transglutaminase promoter. Supershifting with monoclonal antibodies revealed that activated NF-kappaB dimer complexes were composed of p50 and p65 subunits. Interestingly, the specific NF-kappaB inhibitor SN50 (but not its inactive analogue SN50M), when added to cell cultures 30 min prior to glutamate treatment, was able gradually to reduce glutamate-induced NF-kappaB activation. Western blot analysis confirmed the reduction of the p50 amount in nuclear extracts. Notably, the preincubation with SN50 also diminished glutamate-increased tissue transglutaminase expression, as showed by both RT-PCR and Western blotting. Competition experiments, carried out with an excess of a probe containing the NF-kappaB consensus sequence present in the kappa-light-chain promoter, demonstrated a preferential binding of the tissue transglutaminase specific NF-kappaB probe in the nuclear extracts of glutamate-treated astrocytes compared with untreated astrocytes. These preliminary data suggest that NF-kappaB activation, which has been demonstrated to be involved in astrocyte response to glutamate, could also be associated with the molecular pathway leading to glutamate-evoked tissue transglutaminase up-regulation.

Class II G protein-coupled receptors and their ligands in neuronal function and protection

G protein-coupled receptors (GPCRs) play pivotal roles in regulating the function and plasticity of neuronal circuits in the nervous system. Among the myriad of GPCRs expressed in neural cells, class II GPCRs which couples predominantly to the Gs-adenylate cyclase-cAMP signaling pathway, have recently received considerable attention for their involvement in regulating neuronal survival. Neuropeptides that activate class II GPCRs include secretin, glucagon-like peptides (GLP-1 and GLP-2), growth hormone-releasing hormone (GHRH), pituitary adenylate cyclase activating peptide (PACAP), corticotropin-releasing hormone (CRH), vasoactive intestinal peptide (VIP), parathyroid hormone (PTH), and calcitonin-related peptides. Studies of patients and animal and cell culture models, have revealed possible roles for class II GPCRs signaling in the pathogenesis of several prominent neurodegenerative conditions including stroke, Alzheimer's, Parkinson's, and Huntington's diseases. Many of the peptides that activate class II GPCRs promote neuron survival by increasing the resistance of the cells to oxidative, metabolic, and excitotoxic injury. A better understanding of the cellular and molecular mechanisms by which class II GPCRs signaling modulates neuronal survival and plasticity will likely lead to novel therapeutic interventions for neurodegenerative disorders.

NMDA neuroprotection against a phosphatidylinositol-3 kinase inhibitor, LY294002 by NR2B-mediated suppression of glycogen synthase kinase-3beta-induced apoptosis

To identify the intracellular signaling pathways that mediate the pro-survival activity of NMDA receptors (NMDARs), we studied effects of exogenous NMDA on cultured rat cortical and hippocampal neurons that were treated with a phosphatidylinositol-3-kinase (PI3K) inhibitor, LY294002. NMDA at 5 or 10 microm protected against LY294002-induced apoptosis, suggesting NMDAR-mediated activation of a survival signaling pathway that is PI3K-independent. NR2B-specific NMDAR blockers antagonized anti-apoptotic effects of NMDA, indicating a critical role of NR2B NMDARs in the neuroprotection. NMDA at 10 microm suppressed LY294002-induced activation of a pro-apoptotic kinase, glycogen synthase kinase 3beta (GSK3beta). GSK3beta activation by LY294002 was associated with decreased levels of inhibitory GSK3beta phosphorylation at the Ser9 residue. However, NMDA did not prevent the LY294002-mediated decline of phospho-Ser9 levels. In addition, NMDA inhibited cortical neuron apoptosis induced by the overexpression of either wild type (wt) or Ser9Ala mutant form of GSK3beta, suggesting that NMDA suppressed GSK3beta in a Ser9-independent manner. Finally, inhibition of NR2B NMDARs reduced the NMDA protection against overexpression of GSK3betawt. These data indicate that moderate stimulation of NR2B NMDAR protects against inhibition of PI3K by a Ser9-independent inhibition of the pro-apoptotic activity of GSK3beta. Hence, the activation of NR2B and the Ser9-independent inhibition of GSK3beta are two newly identified elements of the signaling network that mediates the pro-survival effects of NMDA.

Targeting antioxidants to mitochondria: a new therapeutic direction

Mitochondria play an important role in controlling the life and death of a cell. Consequently, mitochondrial dysfunction leads to a range of human diseases such as ischemia-reperfusion injury, sepsis, and diabetes. Although the molecular mechanisms responsible for mitochondria-mediated disease processes are not fully elucidated yet, the oxidative stress appears to be critical. Accordingly, strategies are being developed for the targeted delivery of antioxidants to mitochondria. In this review, we shall briefly discuss cellular reactive oxygen species metabolism and its role in pathophysiology; the currently existing antioxidants and possible reasons why they are not effective in ameliorating oxidative stress-mediated diseases; and recent developments in mitochondrially targeted antioxidants and their future promise for disease treatment.