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

Bilirubin-induced inflammatory response, glutamate release, and cell death in rat cortical astrocytes are enhanced in younger cells

Unconjugated bilirubin (UCB) encephalopathy is a predominantly early life condition resulting from the impairment of several cellular functions in the brain of severely jaundiced infants. However, only few data exist on the age-dependent effects of UCB and their association with increased vulnerability of premature newborns, particularly in a sepsis condition. We investigated cell death, glutamate efflux, and inflammatory cytokine dynamics after exposure of astrocytes at different stages of differentiation to clinically relevant concentrations of UCB and/or lipopolysaccharide (LPS). Younger astrocytes were more prone to UCB-induced cell death, glutamate efflux, and inflammatory response than older ones. Furthermore, in immature cells, LPS exacerbated UCB effects, such as cell death by necrosis. These findings provide a basis for the increased susceptibility of premature newborns to UCB deleterious effects, namely when associated with sepsis, and underline how crucial the course of cell maturation can be to UCB encephalopathy during moderate to severe neonatal jaundice.

Molecular and cellular mechanisms of neuronal cell death in HIV dementia

The deaths of neurons, astrocytes and endothelial cells have been described in patients with HIV (human immunodeficiency virus) dementia. HIV-1 does not infect neurons; instead, neurotoxic substances shed by infected glia and macrophages can induce a form of programmed cell death called apoptosis in neurons. These neurotoxins include the HIV-1 proteins Tat and gp120, as well as pro-inflammatory cytokines, chemokines, excitotoxins and proteases. In this article we review the evidence for apoptosis of various cell types within the brain of HIV-infected patients, and describe in vitro and in vivo experimental studies that have elucidated the mechanisms by which HIV causes apoptosis of brain cells.

Antioxidant Treatment Inhibited Glutamate-Evoked NF-κB Activation in Primary Astroglial Cell Cultures

In glial cells, glutamate exposure causes alterations in cell redox status, mainly mediated by glutathione depletion and reactive oxygen species generation. These effects finally lead to astrocyte dysfunction which contributes to the pathogenesis of several neurological disorders. This study was aimed to investigate the involvement of the NF-kappaB pathway in oxidative stress induced by glutamate exposure in primary cultures of astrocytes. Further, we evaluated the power of the antioxidants genistein (0.1-10 microM) and IRFI 016 (20-80 microM), a synthetic tocopherol analogue, compared with glutathione ethyl ester (10-50 microM) and cysteamine-HCl (100-500 microM), to antagonize the effects elicited by glutamate (500 microM). Alterations of cell redox status were reduced, in a dose-dependent way, by antioxidants; in particular, 80 microM IRFI 016 and 10 microM genistein almost completely restored glutathione basal levels and significantly diminished ROS production, as well as 100 microM glutathione ethyl ester. These antioxidant effects were stronger than those caused by 500 microM cysteamine-HCl. Further, glutamate promoted the up-regulation of p50 and p65 NF-kappaB subunits and their nuclear translocation, as revealed by Western blot analysis and electrophoretic mobility shift assay of both subunits. The activation of p50 and p65 NF-kappaB subunits induced by glutamate exposure was significantly reduced by IRFI 016, acting in a dose-dependent manner. Altogether, these data confirm that the NF-kappaB pathway is involved in cell response to oxidative stress induced by glutamate injury in primary astrocyte cultures, and suggest that the use of antioxidants, such as IRFI 016, may be a helpful pharmacological strategy for neuroprotection.

The substituted aspartate analogue L-beta-threo-benzyl-aspartate preferentially inhibits the neuronal excitatory amino acid transporter EAAT3

The excitatory amino acid transporters (EAATs) play key roles in the regulation of CNS L-glutamate, especially related to synthesis, signal termination, synaptic spillover, and excitotoxic protection. Inhibitors available to delineate EAAT pharmacology and function are essentially limited to those that non-selectively block all EAATs or those that exhibit a substantial preference for EAAT2. Thus, it is difficult to selectively study the other subtypes, particularly EAAT1 and EAAT3. Structure activity studies on a series of beta-substituted aspartate analogues identify L-beta-benzyl-aspartate (L-beta-BA) as among the first blockers that potently and preferentially inhibits the neuronal EAAT3 subtype. Kinetic analysis of D-[(3)H]aspartate uptake into C17.2 cells expressing the hEAATs demonstrate that L-beta-threo-BA is the more potent diastereomer, acts competitively, and exhibits a 10-fold preference for EAAT3 compared to EAAT1 and EAAT2. Electrophysiological recordings of EAAT-mediated currents in Xenopus oocytes identify L-beta-BA as a non-substrate inhibitor. Analyzing L-beta-threo-BA within the context of a novel EAAT2 pharmacophore model suggests: (1) a highly conserved positioning of the electrostatic carboxyl and amino groups; (2) nearby regions that accommodate select structural modifications (cyclopropyl rings, methyl groups, oxygen atoms); and (3) a unique region L-beta-threo-BA occupied by the benzyl moieties of L-TBOA, L-beta-threo-BA and related analogues. It is plausible that the preference of L-beta-threo-BA and L-TBOA for EAAT3 and EAAT2, respectively, could reside in the latter two pharmacophore regions.

Mitochondrial inhibition and oxidative stress: reciprocating players in neurodegeneration

Although the etiology for many neurodegenerative diseases is unknown, the common findings of mitochondrial defects and oxidative damage posit these events as contributing factors. The temporal conundrum of whether mitochondrial defects lead to enhanced reactive oxygen species generation, or conversely, if oxidative stress is the underlying cause of the mitochondrial defects remains enigmatic. This review focuses on evidence to show that either event can lead to the evolution of the other with subsequent neuronal cell loss. Glutathione is a major antioxidant system used by cells and mitochondria for protection and is altered in a number of neurodegenerative and neuropathological conditions. This review also addresses the multiple roles for glutathione during mitochondrial inhibition or oxidative stress. Protein aggregation and inclusions are hallmarks of a number of neurodegenerative diseases. Recent evidence that links protein aggregation to oxidative stress and mitochondrial dysfunction will also be examined. Lastly, current therapies that target mitochondrial dysfunction or oxidative stress are discussed.

The excitatory amino acid transporters: Pharmacological insights on substrate and inhibitor specificity of the EAAT subtypes

L-glutamate serves as the primary excitatory neurotransmitter in the mammalian CNS, where it can contribute to either neuronal communication or neuropathological damage through the activation of a wide variety of excitatory amino acid (EAA) receptors. By regulating the levels of extracellular L-glutamate that have access to these receptors, glutamate uptake systems hold the potential to effect both normal synaptic signaling and the abnormal over-activation of the receptors that can trigger excitotoxic pathology. Among the various membrane transporters that are capable of translocating this dicarboxylic amino acid, the majority of glutamate transport in the CNS, particularly as related to excitatory transmission, is mediated by the high-affinity, sodium-dependent, excitatory amino acid transporters (EAATs). At least 5 subtypes of EAATs have been identified, each of which exhibits a distinct distribution and pharmacology. Our growing appreciation for the functional significance of the EAATs is closely linked to our understanding of their pharmacology and the consequent development of inhibitors and substrates with which to delineate their activity. As was the case with EAA receptors, conformationally constrained glutamate mimics have been especially valuable in this effort. The success of these compounds is based upon the concept that restricting the spatial positions that can be occupied by required functional groups can serve to enhance both the potency and selectivity of the analogues. In the instance of the transporters, useful pharmacological probes have emerged through the introduction of additional functional groups (e.g., methyl, hydroxyl, benzyloxy) onto the acyclic backbone of glutamate and aspartate, as well as through the exploitation of novel ring systems (e.g., pyrrolidine-, cyclopropyl-, azole-, oxazole-, and oxazoline-based analogues) to conformationally lock the position of the amino and carboxyl groups. The focus of the present review is on the pharmacology of the EAATs and, in particular, the potential to identify those chemical properties that differentiate the processes of binding and translocation (i.e., substrates from non-substrate inhibitors), as well as strategies to develop glutamate analogues that act selectively among the various EAAT subtypes.

The galanin-R2 agonist AR-M1896 reduces glutamate toxicity in primary neural hippocampal cells

Galanin is a neuropeptide involved in a variety of biological functions, including having a strong anticonvulsant activity. To assess a possible role of galanin in modulation of glutamatergic synapses and excitotoxicity, we studied effects of a galanin receptor 2(3) agonist (AR-M1896) on several molecular events induced by glutamate administration in primary neural hippocampal cells. Exposure of cells, after 5 days in vitro, to glutamate 0.5 mM for 10 min caused morphological alterations, including disaggregation of beta-tubulin and MAP-2 cytoskeletal protein assembly, loss of neurites and cell shrinkage. When present in culture medium together with glutamate, 1 and 10 nM of AR-M1896 reduced these alterations. Moreover, AR-M1896 counteracted glutamate-induced c-fos mRNA and c-Fos protein up-regulation after 30-150 min, and 24 h, respectively. Massive nuclear alterations (Hoechst 33258 staining), observed 24 h after glutamate exposure, were also antagonized by AR-M1896 (0.1-100 nM) in a dose-dependent manner. These findings indicate that galanin, probably mainly through its type 2 receptor, interferes with events associated with glutamate toxicity.

Prophylactic activation of neuroprotective stress response pathways by dietary and behavioral manipulations

It is well established that when most types of cells, including neurons, are exposed to a mild stress they increase their ability to resist more severe stress. This "preconditioning" phenomenon involves up-regulation of genes that encode cytoprotective proteins such as heat-shock proteins and growth factors. We found that a similar beneficial cellular stress response can be induced in neurons throughout the brain by a "meal-skipping" dietary restriction (DR) regimen in rats and mice. DR is effective in protecting neurons and improving functional outcome in models of stroke, Alzheimer's, Parkinson's and Huntington's diseases. DR induces an increase in the levels of brain-derived neurotrophic factor (BDNF) and heat-shock proteins in neurons. DR also stimulates neurogenesis in the hippocampus, and BDNF plays a role in this effect of DR. Physical exercise and environmental enrichment are two other manipulations that have been shown to induce BDNF expression in the brain, presumably because it is a mild cellular stress. When taken together with epidemiological and clinical studies in humans, the data from animal studies suggest that it may be possible to reduce the risk for age-related neurodegenerative disorders through dietary and behavioral modifications that act by promoting neuronal plasticity and survival.

The excitoprotective effect of N-methyl-D-aspartate receptors is mediated by a brain-derived neurotrophic factor autocrine loop in cultured hippocampal neurons

The neuroprotective effect and molecular mechanisms underlying preconditioning with N-methyl-D-aspartate (NMDA) in cultured hippocampal neurons have not been described. Pre-incubation with subtoxic concentrations of the endogenous neurotransmitter glutamate protects vulnerable neurons against NMDA receptor-mediated excitotoxicity. As a result of physiological preconditioning, NMDA significantly antagonizes the neurotoxicity resulting from subsequent exposure to an excitotoxic concentration of glutamate. The protective effect of glutamate or NMDA is time- and concentration-dependent, suggesting that sufficient agonist and time are required to establish an intracellular neuroprotective state. In these cells, the TrkB ligand, brain-derived neurotrophic factor (BDNF) attenuates glutamate toxicity. Therefore, we tested the hypothesis that NMDA protects neurons via a BDNF-dependent mechanism. Exposure of hippocampal cultures to a neuroprotective concentration of NMDA (50 microM) evoked the release of BDNF within 2 min without attendant changes in BDNF protein or gene expression. The accumulated increase of BDNF in the medium is followed by an increase in the phosphorylation (activation) of TrkB receptors and a later increase in exon 4-specific BDNF mRNA. The neuroprotective effect of NMDA was attenuated by pre-incubation with a BDNF-blocking antibody and TrkB-IgG, a fusion protein known to inhibit the activity of extracellular BDNF, suggesting that BDNF plays a major role in NMDA-mediated survival. These results demonstrate that low level stimulation of NMDA receptors protect neurons against glutamate excitotoxicity via a BDNF autocrine loop in hippocampal neurons and suggest that activation of neurotrophin signaling pathways plays a key role in the neuroprotection of NMDA.

Iptakalim hydrochloride protects cells against neurotoxin-induced glutamate transporter dysfunction in in vitro and in vivo models

Iptakalim hydrochloride (Ipt), a novel antihypertensive drug, exhibits K(ATP) channel activation. Here, we report that Ipt remarkably protects cells against neurotoxin-induced glutamate transporter dysfunction in in vitro and in vivo models. Chronic exposure of cultured PC12 cells to neurotoxins, such as 6-OHDA, MPP+, or rotenone, decreased overall [3H]-glutamate uptake in a concentration-dependent manner. Pre-treatment using 10 microM Ipt significantly protected cells against neurotoxin-induced glutamate uptake diminishment, and this protection was abolished by the K(ATP) channel blocker glibenclamide (20 microM), suggesting that the protective mechanisms may involve the opening of K(ATP) channels. In 6-OHDA-treated rats (as an in vivo Parkinson's disease model), [3H]-glutamate uptake was significantly lower in synaptosomes isolated from the striatum and cerebral cortex, but not the hippocampus. Pre-conditioning using 10, 50, and 100 microM Ipt significantly restored glutamate uptake impairment and these protections were abolished by blockade of K(ATP) channels. It is concluded that Ipt exhibits substantial protection of cells against neurotoxicity in in vitro and in vivo models. The cellular mechanisms of this protective effect may involve the opening of K(ATP) channels. Collectively, Ipt may serve as a novel and effective drug for PD therapy.

Chronic back pain is associated with decreased prefrontal and thalamic gray matter density

The role of the brain in chronic pain conditions remains speculative. We compared brain morphology of 26 chronic back pain (CBP) patients to matched control subjects, using magnetic resonance imaging brain scan data and automated analysis techniques. CBP patients were divided into neuropathic, exhibiting pain because of sciatic nerve damage, and non-neuropathic groups. Pain-related characteristics were correlated to morphometric measures. Neocortical gray matter volume was compared after skull normalization. Patients with CBP showed 5-11% less neocortical gray matter volume than control subjects. The magnitude of this decrease is equivalent to the gray matter volume lost in 10-20 years of normal aging. The decreased volume was related to pain duration, indicating a 1.3 cm3 loss of gray matter for every year of chronic pain. Regional gray matter density in 17 CBP patients was compared with matched controls using voxel-based morphometry and nonparametric statistics. Gray matter density was reduced in bilateral dorsolateral prefrontal cortex and right thalamus and was strongly related to pain characteristics in a pattern distinct for neuropathic and non-neuropathic CBP. Our results imply that CBP is accompanied by brain atrophy and suggest that the pathophysiology of chronic pain includes thalamocortical processes.

Glutamate induces rapid loss of axonal neurofilament proteins from cortical neurons in vitro

One of the primary hallmarks of glutamate excitotoxicity is degradation of the neuronal cytoskeleton. Using a tissue culture approach, we have investigated the relationship between excitotoxicity and cytoskeletal degradation within axons, with particular reference to the axon specific neurofilament proteins. Neurofilaments were rapidly lost from axons over a 24-h period in response to excitotoxic insult (as observed by immunocytochemistry and western blotting), while other axonal cytoskeletal markers (such as betaIII-tubulin) remained intact. Treatment with kainic acid and NMDA, or complementary experiments using the pharmacological glutamate receptors blockers CNQX (kainate/AMPA receptor antagonist) and MK-801 (NMDA receptor antagonist), demonstrated that neurofilament degeneration was mediated primarily by NMDA receptor activity. This work suggests that excitotoxicity triggers a progressive pathway of cytoskeletal degeneration within axons, initially characterised by the loss of neurofilament proteins.

Glutamate promotes NF-κB pathway in primary astrocytes: protective effects of IRFI 016, a synthetic vitamin E analogue

Oxidative stress has been implicated in several neurodegenerative diseases affecting both neuronal and glial cells. The aim of this study was to investigate the involvement of reactive oxygen species in glutamate-evoked activation of NF-kappaB in primary astrocytes. A prolonged exposure to glutamate (24 h) caused a depletion of intracellular glutathione that, in astroglial cells, has been considered a biochemical change typical of early astrocyte dysfunction, leading to cell alterations occurring in the gliosis. These effects were initiated by AMPA/KA receptor activation and almost completely blocked by anti-oxidants. Indeed, we provide evidence that the incubation of primary astrocytes with a hydrophilic derivative of tocopherol, such as IRFI 016, was useful to reduce glutamate-induced oxidative effects. This agent also reduced in a dose-dependent manner the nuclear translocation of both p50 and p65 subunits of NF-kappaB. Altogether, these data confirm that GSH content plays a pivotal role to determine oxidative response to glutamate injury in primary astrocyte cultures and that NF-kappaB pathway is involved in this response. Furthermore, the positive effects obtained by IRFI 016 to prevent nuclear translocation of NF-kappaB may suggest new pharmacological strategies for antioxidant therapy and neuroprotection.

Protection of cortical cells by equine estrogens against glutamate-induced excitotoxicity is mediated through a calcium independent mechanism

Background

High concentrations of glutamate can accumulate in the brain and may be involved in the pathogenesis of neurodegenerative disorders such as Alzheimer's disease. This form of neurotoxicity involves changes in the regulation of cellular calcium (Ca²⁺) and generation of free radicals such as peroxynitrite (ONOO^-). Estrogen may protect against glutamate-induced cell death by reducing the excitotoxic Ca²⁺ influx associated with glutamate excitotoxicity. In this study, the inhibition of N-methyl-D-aspartate (NMDA) receptor and nitric oxide synthase (NOS) along with the effect of 17β-estradiol (17β-E₂) and a more potent antioxidant Δ⁸, 17β-estradiol (Δ⁸, 17β-E₂) on cell viability and intracellular Ca²⁺ ([Ca²⁺]_i), following treatment of rat cortical cells with glutamate, was investigated.

Results

Primary rat cortical cells were cultured for 7–12 days in Neurobasal medium containing B27 supplements. Addition of glutamate (200 μM) decreased cell viability to 51.3 ± 0.7% compared to control. Treatment with the noncompetitive NMDAR antagonist, MK-801, and the NOS inhibitor, L-NAME, completely prevented cell death. Pretreatment (24 hrs) with 17β-E₂ and Δ⁸, 17β-E₂ (0.01 to 10 μM) significantly reduced cell death. 17β-E₂ was more potent than Δ⁸, 17β-E₂. Glutamate caused a rapid 2.5 fold increase in [Ca²⁺]_i. Treatment with 0.001 to 10 μM MK-801 reduced the initial Ca²⁺ influx by 14–41% and increased cell viability significantly. Pretreatment with 17β-E₂ and Δ⁸, 17β-E₂ had no effect on Ca²⁺ influx but protected the cortical cells against glutamate-induced cell death.

Conclusion

Glutamate-induced cell death in cortical cultures can occur through NMDAR and NOS-linked mechanisms by increasing nitric oxide and ONOO^-. Equine estrogens: 17β-E₂ and Δ⁸, 17β-E₂, significantly protected cortical cells against glutamate-induced excitotoxicity by a mechanism that appears to be independent of Ca²⁺ influx. To our knowledge, this is a first such observation. Whether the decrease in NOS related products such as ONOO^-, is a mechanism by which estrogens protect against glutamate toxicity, remains to be investigated. Estrogen replacement therapy in healthy and young postmenopausal women may protect against neurodegenerative diseases by these mechanisms.

Misfolded proteins, endoplasmic reticulum stress and neurodegeneration

The accumulation of misfolded proteins (e.g. mutant or damaged proteins) triggers cellular stress responses that protect cells against the toxic buildup of such proteins. However, prolonged stress due to the buildup of these toxic proteins induces specific death pathways. Dissecting these pathways should be valuable in understanding the pathogenesis of, and ultimately in designing therapy for, neurodegenerative diseases that feature misfolded proteins.

Proteolytic mechanisms in necrotic cell death and neurodegeneration

Programmed neuronal cell death is required during development to achieve the accurate wiring of the nervous system. However, genetic or accidental factors can lead to the premature, non-programmed death of neurons during adult life. Inappropriate death of cells in the nervous system is the cause of multiple neurodegenerative disorders. Pathological neuronal death can occur by apoptosis, by necrosis or by a combination of both. Necrotic cell death underlies the pathology of devastating neurological diseases such as neurodegenerative disorders, stroke or trauma. However, little is known about the molecular mechanisms that bring about necrotic cell death. Proteases play crucial roles in neuron degeneration by exerting both regulatory and catabolic functions. Elevated intracellular calcium is the most ubiquitous feature of neuronal death with the concomitant activation of cysteine calcium-dependent proteases, calpains. Calpains and lysosomal, catabolic aspartyl proteases, play key roles in the necrotic death of neurons. In this review, we survey the recent literature on the role of cysteine and aspartyl proteases in necrosis and neurodegeneration, aiming to delineate common proteolytic mechanisms mediating cellular destruction.

Calpain Mediates Excitotoxic DNA Fragmentation via Mitochondrial Pathways in Adult Brains

Calpain has been implicated in excitotoxic neurode-generation, but its mechanism of action particularly in adult brains remains unclear. We generated mutant mice lacking or overexpressing calpastatin, the only solely calpain-specific inhibitor ever identified or synthesized. Modulation of calpastatin expression caused no defect in the mice under normal conditions, indicating that calpastatin functions as a negative regulator of calpain only under pathological conditions. Kainate-evoked excitotoxicity in hippocampus resulted in proteolytic activation of a proapoptotic Bcl-2 subfamily member (Bid), nuclear translocation of mitochondria-derived DNA fragmentation factors (apoptosis-inducing factor and endonuclease G), DNA fragmentation, and nuclear condensation in pyramidal neurons. These apoptotic responses were significantly augmented by calpastatin deficiency. Consistently calpastatin overexpression suppressed them. No evidence of caspase-3 activation was detected. Our results demonstrated that calpain mediates excitotoxic signals through mobilization of proapoptotic factors in a caspase-independent manner. These mutant mice will serve as useful tools for investigating calpain involvement in various diseases.

Susceptibility to excitotoxic and metabolic striatal neurodegeneration in the mouse is genotype dependent

Previously, we had reported that hippocampal susceptibility to the neurotoxic effects of excitotoxin administration is strain dependent [Schauwecker and Steward, Proc. Natl. Acad. Sci. U.S.A. 94 (1997) 4103]. However, it has been unclear whether strain-related gene products may play a similar role in providing protection against drugs that produce striatal lesions. The present series of experiments sought to elucidate whether genetic background alters neuronal viability within the striatum following metabolic or excitotoxic injury. Thus, we have examined the effect of mouse strain on susceptibility to striatal injury using well-characterized animal models of Huntington's disease by examining whether C57BL/6 mice, previously identified as resistant to excitotoxin-induced hippocampal cell death, are resistant to quinolinate, malonate, and 3-nitropropionic acid (3-NP). Intrastriatal injection of either malonate or quinolinate and systemic administration of 3-NP resulted in significantly smaller striatal lesions in C57BL/6 mice as compared to FVB/N mice, previously identified as susceptible to hippocampal excitotoxic injury. The existence of an animal strain with decreased resistance to striatal lesions suggests that there are mediating factors involved in the preferential vulnerability of the striatum to neurotoxic lesioning. The identification of these factors could provide strategies for therapeutic intervention in Huntington's disease.

Effect of ebselen and organochalcogenides on excitotoxicity induced by glutamate in isolated chick retina

In this study, we evaluated the effects of three simple organochalcogenides (diphenyl diselenide, diphenyl ditelluride and diphenyl telluride) and ebselen on the glutamate-driven 45Ca2+ influx into chick embryonic retinal cells, as well as their effects on the excitotoxic injury in retina cells. None of the compounds tested interfered with basal 45Ca2+ uptake. Diphenyl diselenide and diphenyl ditelluride had no effects on glutamate-driven 45Ca2+ influx. Diphenyl telluride (100-400 microM) decreased and ebselen (100-400 microM) completely blocked the glutamate-driven 45Ca2+ influx (P < 0.01) into chick retinal explants. The assessment of neural injury was made spectrophotometrically by quantification of cellularly reduced MTT (3(4,5-dimethyl-thiazol-2-yl)-2,5-diphenyl tetrazolium bromide) 24 h after the beginning of glutamate exposure (8 h). Ebselen had no effects on retinal MTT reduction when co-incubated with glutamate for 8 h. However, when ebselen (100 and 400 microM) was co-incubated for 8 h with glutamate and remained in the incubation media until MTT evaluation (24 h after the beginning of incubation), it protected retinal cells against the decrease in MTT reduction induced by glutamate. These data indicate that besides its capacity of interacting with Ca2+ channels, other mechanisms are involved in the neuroprotection afforded by ebselen in this work, possibly its antioxidant properties.

Regeneration of adult rat spinal cord is promoted by the soluble KDI domain of gamma1 laminin

Regeneration in the central nervous system (CNS) of adult mammals is hampered by formation of a glial scar and by proteins released from the myelin sheaths of injured neuronal pathways. Our recent data indicate that the KDI (Lys-Asp-Ile) domain of gamma1 laminin neutralizes both glial- and myelin-derived inhibitory signals and promotes survival and neurite outgrowth of cultured human spinal cord neurons. We show that after complete transection of the adult rat spinal cord, animals receiving onsite infusion of the KDI domain via osmotic mini-pumps recover and are able to sustain their body weights and walk with their hindlimbs. Animals treated with placebo suffer from irreversible hindlimb paralysis. Microscopic and molecular analyses of the spinal cords indicate that the KDI domain reduces tissue damage at the lesion site and enables neurite outgrowth through the injured area to effect functional recovery of the initially paralyzed animals. That the KDI domain enhances regeneration of acute spinal cord injuries in the adult rat suggests that it may be used to promote regeneration of spinal cord injuries in humans.

The neuroprotective KDI domain of γ1-laminin is a universal and potent inhibitor of ionotropic glutamate receptors

Previous work from this laboratory indicates that the KDI (Lys-Asp-Ile) domain of gamma 1-laminin promotes functional regeneration of adult rat spinal cord injuries and protects adult rat hippocampal neurons against massive neuronal death induced by intracerebral injection of the glutamate analogue kainic acid. In the present study, we used patch clamp recordings on cultured human embryonic neocortical neurons and HEK 293 cells expressing recombinant glutamate receptor subunits to study a putative interaction of the KDI with the glutamate system. We show that the KDI domain of gamma 1-laminin is a universal and potent inhibitor of AMPA, kainate, and NMDA subclasses of glutamate receptors, with a noncompetitive action on the AMPA receptor channel activity. Glutamate neurotoxicity plays a key role in both CNS trauma and neurodegenerative disorders, so this unexpected, novel function of the gamma 1-laminin-derived tripeptide may prove clinically valuable in treatment of CNS trauma and/or disease.

Metallothionein reduces central nervous system inflammation, neurodegeneration, and cell death following kainic acid-induced epileptic seizures

We examined metallothionein (MT)-induced neuroprotection during kainic acid (KA)-induced excitotoxicity by studying transgenic mice with MT-I overexpression (TgMT mice). KA induces epileptic seizures and hippocampal excitotoxicity, followed by inflammation and delayed brain damage. We show for the first time that even though TgMT mice were more susceptible to KA, the cerebral MT-I overexpression decreases the hippocampal inflammation and delayed neuronal degeneration and cell death as measured 3 days after KA administration. Hence, the proinflammatory responses of microglia/macrophages and lymphocytes and their expression of interleukin (IL)-1, IL-6, IL-12, tumor necrosis factor-alpha and matrix metalloproteinases (MMP-3, MMP-9) were significantly reduced in hippocampi of TgMT mice relative to wild-type mice. Also by 3 days after KA, the TgMT mice showed significantly less delayed damage, such as oxidative stress (formation of nitrotyrosine, malondialdehyde, and 8-oxoguanine), neurodegeneration (neuronal accumulation of abnormal proteins), and apoptotic cell death (judged by TUNEL and activated caspase-3). This reduced bystander damage in TgMT mice could be due to antiinflammatory and antioxidant actions of MT-I but also to direct MT-I effects on the neurons, in that significant extracellular MT presence was detected. Furthermore, MT-I overexpression stimulated astroglia and increased immunostaining of antiinflammatory IL-10, growth factors, and neurotrophins (basic fibroblastic growth factor, transforming growth factor-beta, nerve growth factor, brain-derived neurotrophic factor, glial-derived neurotrophic factor) in hippocampus. Accordingly, MT-I has different functions that likely contribute to the increased neuron survival and improved CNS condition of TgMT mice. The data presented here add new insight into MT-induced neuroprotection and indicate that MT-I therapy could be used against neurological disorders.

New functions for an old enzyme: nonhemostatic roles for tissue-type plasminogen activator in the central nervous system

Tissue-type plasminogen activator (tPA) is a highly specific serine proteinase that activates the zymogen plasminogen to the broad-specificity proteinase plasmin. Tissue-type plasminogen activator is found not only in the blood, where its primary function is as a thrombolytic enzyme, but also in the central nervous system (CNS), where it promotes events associated with synaptic plasticity and acts as a regulator of the permeability of the neurovascular unit. Tissue-type plasminogen activator has also been associated with pathological events in the CNS such as cerebral ischemia and seizures. Neuroserpin is an inhibitory serpin that reacts preferentially with tPA and is located in regions of the brain where either tPA message or tPA protein are also found, indicating that neuroserpin is the selective inhibitor of tPA in the CNS. There is a growing body of evidence demonstrating the participation of tPA in a number of physiological and pathological events in the CNS, as well as the role of neuroserpin as the natural regulator of tPA's activity in these processes. This review will focus on nonhemostatic roles of tPA in the CNS with emphasis on its newly described function as a regulator of permeability of the neurovascular unit and on the regulatory role of neuroserpin in these events.

The Lathyrus excitotoxin beta-N-oxalyl-L-alpha,beta-diaminopropionic acid is a substrate of the L-cystine/L-glutamate exchanger system xc-

Beta-N-oxalyl-L-alpha-beta-diaminopropionic acid (beta-L-ODAP) is an unusual amino acid present in seeds of plants from the Lathyrus genus that is generally accepted as the causative agent underlying the motor neuron degeneration and spastic paraparesis in human neurolathyrism. Much of the neuropathology produced by beta-L-ODAP appears to be a direct consequence of its structural similarities to the excitatory neurotransmitter L-glutamate and its ability to induce excitotoxicity as an agonist of non-NMDA receptors. Its actions within the CNS are, however, not limited to non-NMDA receptors, raising the likely possibility that the anatomical and cellular specificity of the neuronal damage observed in neurolathyrism may result from the cumulative activity of beta-L-ODAP at multiple sites. Accumulating evidence suggests that system xc-, a transporter that mediates the exchange of L-cystine and L-glutamate, is one such site. In the present work, two distinct approaches were used to define the interactions of beta-L-ODAP with system xc-: Traditional radiolabel-uptake assays were employed to quantify inhibitory activity, while fluorometrically coupled assays that follow the exchange-induced efflux of L-glutamate were used to assess substrate activity. In addition to confirming that beta-L-ODAP is an effective competitive inhibitor of system xc-, we report that the compound exhibits a substrate activity comparable to that of the endogenous substrate L-cystine. The ability of system xc- to transport and accumulate beta-L-ODAP identifies additional variables that could influence its toxicity within the CNS, including the ability to limit its access to EAA receptors by clearing the excitotoxin from the extracellular synaptic environment, as well as serving as a point of entry through which beta-L-ODAP could have increased access to intracellular targets.

Intracellular Abeta42 activates p53 promoter: a pathway to neurodegeneration in Alzheimer's disease

The amyloid beta-protein (Abeta) ending at 42 plays a pivotal role in Alzheimer's disease (AD). We have reported previously that intracellular Abeta42 is associated with neuronal apoptosis in vitro and in vivo. Here, we show that intracellular Abeta42 directly activated the p53 promoter, resulting in p53-dependent apoptosis, and that intracellular Abeta40 had a similar but lesser effect. Moreover, oxidative DNA damage induced nuclear localization of Abeta42 with p53 mRNA elevation in guinea-pig primary neurons. Also, p53 expression was elevated in brain of sporadic AD and transgenic mice carrying mutant familial AD genes. Remarkably, accumulation of both Abeta42 and p53 was found in some degenerating-shape neurons in both transgenic mice and human AD cases. Thus, the intracellular Abeta42/p53 pathway may be directly relevant to neuronal loss in AD. Although neurotoxicity of extracellular Abeta is well known and synaptic/mitochondrial dysfunction by intracellular Abeta42 has recently been suggested, intracellular Abeta42 may cause p53-dependent neuronal apoptosis through activation of the p53 promoter; thus demonstrating an alternative pathogenesis in AD.

A link between hyperbilirubinemia, oxidative stress and injury to neocortical synaptosomes

Cytotoxicity by unconjugated bilirubin involves disturbances of membrane structure, excitotoxicity and cell death. These events were reported to trigger elevated free radicals production and impairment of calcium homeostasis, and to result in loss of cell membrane integrity. Therefore, this study was designed to investigate whether interaction of clinically relevant concentrations of free unconjugated bilirubin with synaptosomal membrane vesicles could be linked to oxidative stress, cytosolic calcium accumulation and perturbation of membrane function. Synaptosomal vesicles were prepared from gerbil cortical brain tissue and incubated with purified bilirubin (<or=1 microM), for 4 h at 37 degrees C. Intracellular concentrations of reactive oxygen species (ROS) and calcium were determined by dichlorofluorescin and BAPTA fluorescent probes, respectively. Membrane protein and lipid oxidation were evaluated by immunocytochemistry and phosphatidylserine exposure by annexin V binding. Levels of reduced and oxidized glutathione (GSH and GSSG, respectively), as well as activities of Mg(2+)-ATPase aminophospholipid translocase (flippase) and Na(+),K(+)-ATPase, were also measured. Our results showed that bilirubin induced oxidative stress, due to a rise in lipid (>or=10%, P<0.05) and protein oxidation (>or=20%, P<0.01), ROS content (approximately 17%, P<0.01), and a decrease in GSH/GSSG ratio (>30%, P<0.01). In addition, synaptosomes exposed to bilirubin exhibited increased externalization of phosphatidylserine (approximately 10%, P<0.05), together with decreased flippase and NA(+),K(+)-ATPase (>or=15%, P<0.05) activities, events that were accompanied by enhanced intracellular calcium levels ( approximately 20%, P<0.01). The data obtained point out that interaction of unconjugated bilirubin with synaptosomal membrane vesicles leads to oxidative injury, loss of membrane asymmetry and functionality, and calcium intrusion, thus potentially contributing to the pathogenesis of encephalopathy by hyperbilirubinemia.

Protective effect of melatonin on 3-nitropropionic acid-induced oxidative stress in synaptosomes in an animal model of Huntington's disease

The effect of melatonin (1 mg/kg BW i.p./day) on the oxidative changes produced by 3-nitropropionic acid (20 mg/kg BW/day for 4 days) in rat striatal and cortical synaptosomes was investigated. The effects of 3-nitropropionic acid were evaluated as changes in the quantity of lipid peroxidation products, protein carbonyl groups and superoxide dismutase and succinate dehydrogenase activities. 3-Nitropropionic acid caused a rise in lipid peroxidation levels and protein carbonyls content whereas it induced a reduction in the activity of succinate dehydrogenase and triggered an enhancement in superoxide dismutase activity. These changes were prevented by previous administration of melatonin. Our results reveal: (i) 3-nitropropionic acid induces a status of oxidative stress in some brain regions of the Wistar rat; (ii) melatonin prevents the deleterious effects induced by the acid. In conclusion, the results show the ability of melatonin to modify the neural response to 3-nitropropionic acid with the protective mechanism likely involving the antioxidative processes of melatonin.

Molecular basis of bilirubin-induced neurotoxicity

Unconjugated bilirubin (UCB), at slightly elevated unbound concentrations, is toxic to astrocytes and neurons, damaging mitochondria (causing impaired energy metabolism and apoptosis) and plasma membranes (causing oxidative damage and disrupting transport of neurotransmitters). Accumulation of UCB in the CSF and CNS is limited by its active export, probably mediated by MRP1/Mrp1 present in choroid plexus epithelia, capillary endothelia, astrocytes and neurons. Upregulation of MRP1/Mrp1 protein levels by UCB might represent an important adaptive mechanism that protects the CNS from UCB toxicity. These concepts could explain the varied susceptibility of newborns to bilirubin neurotoxicity and the occurrence of neurological damage at plasma UCB concentrations well below therapeutic guidelines, and are relevant to the increasing prevalence of bilirubin encephalopathy in newborns.

Soluble KDI domain of γ1 laminin protects adult hippocampus from excitotoxicity of kainic acid

Recent data indicate that the soluble KDI domain of gamma1 laminin promotes survival and neurite outgrowth of human central neurons in vitro (Liebkind et al.[2003] J Neurosci Res 73:637-643), and seems to neutralize both glia- and myelin-derived signals that hamper regeneration in the central nervous system (CNS) of adult mammals. We show that damage of adult rat neocortical and hippocampal areas by a stereotaxic injection of kainic acid (KA) is prevented by a preceding injection of the soluble KDI domain. In the presence of the KDI domain, both neocortical and hippocampal areas show extensive gliosis but have viable neurons and glial cells, which are absent and the areas fully destroyed after injection of KA alone. This result indicates that the KDI domain of the gamma1 laminin protects the CNS against excitotoxic insults and promotes survival of both neurons and glial cells. The KDI domain may thus be a potential drug to prevent CNS damage induced by neurodegenerative disorders, mechanical injury, or ischemia.

Cytokine production, glutamate release and cell death in rat cultured astrocytes treated with unconjugated bilirubin and LPS

In hyperbilirubinemic newborns, sepsis is considered a risk factor for kernicterus. Evidence shows that injury to astrocytes triggers cytokine release. We examined the effects of unconjugated bilirubin (UCB) alone, or in combination with LPS, on the release of glutamate and cytokines from astrocytes in conditions inducing less than 10% of cell death. UCB leads to an increase of extracellular glutamate and highly enhances the release of TNF-alpha and IL-1beta, while inhibiting the production of IL-6. LPS potentiates immunostimulatory properties of UCB. These results point out the role of cytokines and provide a basis for the significance of sepsis in UCB encephalopathy.

Activated Microglia Initiate Motor Neuron Injury by a Nitric Oxide and Glutamate-Mediated Mechanism

Recent studies suggest that motor neuron (MN) death may be non-cell autonomous, with cell injury mediated by interactions involving non-neuronal cells, such as microglia and astrocytes. To help define these interactions, we used primary MN cultures to investigate the effects of microglia activated by lipopolysaccharide or IgG immune complexes from patients with amyotrophic lateral sclerosis. Following activation, microglia induced MN injury, which was prevented by a microglial iNOS inhibitor as well as by catalase or glutathione. Glutamate was also required since inhibition of the MN AMPA/kainate receptor by CNQX prevented the toxic effects of activated microglia. Peroxynitrite and glutamate were synergistic in producing MN injury. Their toxic effects were also blocked by CNQX and prevented by calcium removal from the media. The addition of astrocytes to cocultures of MN and activated microglia prevented MN injury by removing glutamate from the media. The protective effects could be reversed by inhibiting astrocytic glutamate transport with dihydrokainic acid or pretreating astrocytes with H2O2. Astrocytic glutamate uptake was also decreased by activated microglia or by added peroxynitrite. These data suggest that free radicals released from activated microglia may initiate MN injury by increasing the susceptibility of the MN AMPA/kainate receptor to the toxic effects of glutamate.

Symptomatic and Disease-Modifying Therapy for the Progressive Ataxias

BACKGROUND

The progressive ataxias are a diverse group of neurologic diseases that share features of degeneration of the cerebellum and its inflow/outflow pathways but differ in etiology, course, and associated noncerebellar system involvement. Some will have treatable causes, but for most, the pathophysiology is incompletely known.

REVIEW SUMMARY

Treatment strategies will include (1) definitive therapy when available, (2) symptomatic treatment and prevention of complications, and (3) rehabilitation and support resources. The physician will have to decide whether to introduce or approve the use of therapies based on as yet-unproven mechanisms or the use of complementary medicine approaches.

CONCLUSIONS

There are as yet no drugs that have been approved by the Food and Drug Administration for the treatment of the progressive ataxias and relatively few disease-modifying therapies, but symptomatic and rehabilitation interventions can greatly improve the quality of life of individuals with these disabling neurodegenerative disorders.

Caspase-3 and caspase-9 mediate developmental apoptosis in the mouse olfactory system

Neuronal apoptosis is a key component in the sculpting of tissues during embryonic and postnatal development and is driven largely by the action of caspases. In the mouse olfactory system, caspase-3 and -9 are expressed in olfactory receptor neurons (ORNs) of adult mice, and their selective retrograde activation drives ORN apoptosis following ablation of their target, the olfactory bulb (OB). Here, we show that both of these caspases are expressed at the earliest stages of ORN embryonic development, and their expression is concentrated in outgrowing ORN axons. The retention, in null mice for both caspases, of a population of ORNs that would normally undergo developmental apoptosis beginning at E13 of development, results in a permanently expanded population of ORNs. In turn, in some caspase-3 null mice, the ORN target organ, the OB, also develops abnormally, resulting in the formation of secondary, apparently functional, extracranial ectopic OBs.

Taurine prevents the neurotoxicity of beta-amyloid and glutamate receptor agonists: activation of GABA receptors and possible implications for Alzheimer's disease and other neurological disorders

Alzheimer's disease (AD) and several other neurological disorders have been linked to the overactivation of glutamatergic transmission and excitotoxicity as a common pathway of neuronal injury. The beta-amyloid peptide (Abeta) is centrally related to the pathogenesis of AD, and previous reports have demonstrated that the blockade of glutamate receptors prevents Abeta-induced neuronal death. We show that taurine, a beta-amino acid found at high concentrations in the brain, protects chick retinal neurons in culture against the neurotoxicity of Abeta and glutamate receptor agonists. The protective effect of taurine is not mediated by interaction with glutamate receptors, as demonstrated by binding studies using radiolabeled glutamate receptor ligands. The neuroprotective action of taurine is blocked by picrotoxin, an antagonist of GABA(A) receptors. GABA and the GABA(A) receptor agonists phenobarbital and melatonin also protect neurons against Abeta-induced neurotoxicity. These results suggest that activation of GABA receptors decreases neuronal vulnerability to excitotoxic damage and that pharmacological manipulation of the excitatory and inhibitory neurotransmitter tonus may protect neurons against a variety of insults. GABAergic transmission may represent a promising target for the treatment of AD and other neurological disorders in which excitotoxicity plays a relevant role.

Neurodegenerative diseases and oxidative stress

Oxidative stress has been implicated in the progression of Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. Oxygen is vital for life but is also potentially dangerous, and a complex system of checks and balances exists for utilizing this essential element. Oxidative stress is the result of an imbalance in pro-oxidant/antioxidant homeostasis that leads to the generation of toxic reactive oxygen species. The systems in place to cope with the biochemistry of oxygen are complex, and many questions about the mechanisms of oxygen regulation remain unanswered. However, this same complexity provides a number of therapeutic targets, and different strategies, including novel metal-protein attenuating compounds, aimed at a variety of targets have shown promise in clinical studies.

Caspase inhibitors prevent endothelial apoptosis and cerebral vasospasm in dog model of experimental subarachnoid hemorrhage

Apoptosis in the endothelium of major cerebral arteries may play a role in the initiation and maintenance of cerebral vasospasm after subarachnoid hemorrhage (SAH). We tested the therapeutic effect of caspase inhibitors on endothelial apoptosis and on cerebral vasospasm in an established dog double-hemorrhage model. Thirty-one mongrel dogs were divided into five groups: control; SAH; SAH treated with vehicle [DMSO]; SAH treated with Ac-DEVD-CHO [a specific caspase-3 inhibitor]; and SAH treated with Z-VAD-FMK [a broad caspase inhibitor]. The inhibitors (100 microM) were injected into the cisterna magna daily from Day 0 through Day 3. Angiography was performed on Day 0 and Day 7. Histology, TUNEL staining, and immunohistochemistry were conducted on basilar arteries collected on Day 7 after SAH. Positive staining of TUNEL, poly(ADP)-ribose polymerase (PARP), caspase-3, and caspase-8 was observed in the endothelial cells of the spastic arteries. Double fluorescence labeling demonstrated co-localization of TUNEL with caspase-3 and TNFalpha receptor-1 (TNFR1). Ac-DEVD-CHO and Z-VAD-FMK prevented endothelial apoptosis and reduced angiographic vasospasm. The mechanism of apoptosis in endothelial cells involves TNFR1 and the caspase-8 and caspase-3 pathways. Caspase inhibitors may have potential in the treatment of cerebral vasospasm.

Effect of glucocorticoids on 3-nitropropionic acid-induced oxidative stress in synaptosomes

The present study with rat striatal and cortical synaptosomes evaluated the effect of dexamethasone (300 microg/kg i.p./day) with and without simultaneous adrenalectomy on the oxidative stress induced by 3-nitropropionic acid (20 mg/kg/day for 4 days). Adrenalectomy enhanced the oxidative stress induced by 3-nitropropionic acid. These changes were prevented by previous and simultaneous administration of dexamethasone. Adrenalectomy alone induced oxidative stress with decreases in succinate dehydrogenase activity. Our results revealed that adrenal glucocorticoids, and especially dexamethasone (synthetic glucocorticoid), have a protective effect against oxidative stress induced by 3-nitropropionic acid in some brain regions of the Wistar rat.

AATF Inhibits Aberrant Production of Amyloid β Peptide 1-42 by Interacting Directly with Par-4

Aggregation of the neurotoxic amyloid beta peptide 1-42 (Abeta-(1-42)) in the brain is considered to be an early event in the pathogenesis of Alzheimer's disease (AD). Par-4 (prostate apoptosis response-4) is a leucine zipper protein that is pro-apoptotic and associated with neuronal degeneration in AD. Overexpression of Par-4 significantly increased production of Abeta-(1-42) after initiation of apoptotic cascades, indicating factors regulating apoptotic pathways may also affect processing of beta-amyloid precursor protein (APP). AATF (apoptosis-antagonizing transcription factor) was recently identified as an interaction partner of DAP-like kinase (Dlk), a member of the DAP (death-associated protein) kinase family. AATF antagonizes apoptosis induced by Par-4, suggesting that AATF might directly or indirectly participate in regulation of Par-4 activity. We now report that AATF colocalizes with Par-4 in both cytoplasmic and nuclear compartments, and it interacts directly and selectively with Par-4 via the leucine zipper domain in neural cells. Par-4 induced an aberrant production and secretion of Abeta in neuroblastoma IMR-32 cells after apoptotic cascades are initiated. Co-expression of AATF completely blocked aberrant production and secretion of Abeta-(1-42) induced by Par-4, and AATF/Par-4 complex formation was essential for the inhibitory effect of AATF on aberrant Abeta secretion. These results indicate that AATF is an endogenous antagonist of Par-4 activity and an effective inhibitor of aberrant Abeta production and secretion under apoptotic conditions.

Perturbed signal transduction in neurodegenerative disorders involving aberrant protein aggregation

Aggregation of abnormal proteins, both inside and outside of cells, is a prominent feature of major neurodegenerative disorders, including Alzheimer's, Parkinson's, polyglutamine expansion, and prion diseases. Other articles in this special issue of NeuroMolecular Medicine describe the genetic and molecular factors that promote aberrant protein aggregation. In the present article, we consider how it is that pathogenic aggregation-prone proteins compromise signal transduction pathways that regulate neuronal plasticity and survival. In some cases the protein in question may have widespread and relatively nonspecific effects on signaling. For example, amyloid beta-peptide induces membrane-associated oxidative stress, which impairs the function of various receptors, ion channels and transporters, as well as downstream kinases and transcription factors. Other proteins, such as polyglutamine repeat proteins, may affect specific protein -protein interactions, including those involved in signaling pathways activated by neurotransmitters, neurotrophins, and steroid hormones. Synapses are particularly sensitive to abnormal protein aggregation and impaired synaptic signaling may trigger apoptosis and related cell death cascades. Impairment of signal transduction in protein aggregation disorders may be amenable to therapy as demonstrated by a recent study showing that dietary restriction can preserve synaptic function and protect neurons in a mouse model of Huntington's disease. Finally, emerging findings are revealing how activation of certain signaling pathways can suppress protein aggregation and/or the cytotoxicity resulting from the abnormal protein aggregation. A better understanding of how abnormal protein aggregation occurs and how it affects and is affected by specific signal transduction pathways, is leading to novel approaches for preventing and treating neurodegenerative disorders.

Infectious agents and age-related neurodegenerative disorders

chlamdAs with other organ systems, the vulnerability of the nervous system to infectious agents increases with aging. Several different infectious agents can cause neurodegenerative conditions, with prominent examples being human immunodeficiency virus (HIV-1) dementia and prion disorders. Such infections of the central nervous system (CNS) typically have a relatively long incubation period and a chronic progressive course, and are therefore increasing in frequency as more people live longer. Infectious agents may enter the central nervous system in infected migratory macrophages, by transcytosis across blood-brain barrier cells or by intraneuronal transfer from peripheral nerves. Synapses and lipid rafts are important sites at which infectious agents may enter neurons and/or exert their cytotoxic effects. Recent findings suggest the possibility that infectious agents may increase the risk of common age-related neurodegenerative disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS) and stroke. While scenarios can be envisioned whereby viruses such as Chlamydia pneumoniae, herpes simplex and influenza promote damage to neurons during aging, there is no conclusive evidence for a major role of these pathogens in neurodegenerative disorders. In the case of stroke, blood vessels may be adversely affected by bacteria or viruses resulting in atherosclerosis.

RNA Interference in Biology and Medicine

First discovered in plants the nematode Caenorhabditis elegans, the production of small interfering RNAs (siRNAs) that bind to and induce the degradation of specific endogenous mRNAs is now recognized as a mechanism that is widely employed by eukaryotic cells to inhibit protein production at a post-transcriptional level. The endogenous siRNAs are typically 19- to 23-base double-stranded RNA oligonucleotides, produced from much larger RNAs that upon binding to target mRNAs recruit RNases to a protein complex that degrades the targeted mRNA. Methods for expressing siRNAs in cells in culture and in vivo using viral vectors, and for transfecting cells with synthetic siRNAs, have been developed and are being used to establish the functions of specific proteins in various cell types and organisms. RNA interference methods provide several major advantages over prior methods (antisense DNA or antibody-based techniques) for suppressing gene expression. Recent preclinical studies suggest that RNA interference technology holds promise for the treatment of various diseases. Pharmacologists have long dreamed of the ability to selectively antagonize or eliminate the function of individual proteins--RNAi technology may eventually make that dream a reality.

Inhibition of Cell Cycle Pathway by Flavopiridol Promotes Survival of Cerebellar Granule Cells after an Excitotoxic Treatment

Kainic acid (KA)-induced neuronal damage and the protective effects of flavopiridol were studied in primary cultures of rat cerebellar granule cells (CGNs). When neurons were treated with 500 microM KA, the percentage of cells with condensed nuclei measured by nuclear counting increased by up to 55%. After flavopiridol treatment, an antitumoral drug that is a broad inhibitor of cyclin-dependent kinases, the percentage of condensed nuclei decreased by up to 26%. Furthermore, this KA-mediated cell death was only partially dependent on the activation of the initiator caspase-9 and the effector caspases-3 and -6. This argues for a minor role of caspases in the intracellular pathway leading to KA-induced programmed cell death in CGNs. We examined the possible implication of cell cycle proteins in KA-induced neurotoxicity. We found an increase in the expression of proliferating cell nuclear antigen and E2F-1, two proteins implicated in S-phase, by Western blot. KA increased bromodeoxyuridine incorporation in CGNs, a marker of cell proliferation, and flavopiridol attenuated this effect. These results indicated that flavopiridol decreased the expression of cell cycle markers in CGNs after KA treatment. Flavopiridol might thus be used as a preventive agent against neurodegenerative diseases associated with cell cycle activation.

Glucagon-like peptide 1 modulates calcium responses to glutamate and membrane depolarization in hippocampal neurons

Glucagon-like peptide 1 (GLP-1) activates receptors coupled to cAMP production and calcium influx in pancreatic cells, resulting in enhanced glucose sensitivity and insulin secretion. Despite evidence that the GLP-1 receptor is present and active in neurons, little is known of the roles of GLP-1 in neuronal physiology. As GLP-1 modulates calcium homeostasis in pancreatic beta cells, and because calcium plays important roles in neuronal plasticity and neurodegenerative processes, we examined the effects of GLP-1 on calcium regulation in cultured rat hippocampal neurons. When neurons were pre-treated with GLP-1, calcium responses to glutamate and membrane depolarization were attenuated. Whole-cell patch clamp analyses showed that glutamate-induced currents and currents through voltage-dependent calcium channels were significantly decreased in neurons pre-treated with GLP-1. Pre-treatment of neurons with GLP-1 significantly decreased their vulnerability to death induced by glutamate. Acute application of GLP-1 resulted in a transient elevation of intracellular calcium levels, consistent with the established effects of GLP-1 on cAMP production and activation of cAMP response element-binding protein. Collectively, our findings suggest that, by modulating calcium responses to glutamate and membrane depolarization, GLP-1 may play important roles in regulating neuronal plasticity and cell survival.