A Combination of Essential Fatty Acids, Panax Ginseng Extract, and Green Tea Catechins Modifies Brain fMRI Signals in Healthy Older Adults

OBJECTIVES

To assess the effects of a combination of omega 3 essential fatty acids, green tea catechins, and ginsenosides on cognition and brain functioning in healthy older adults.

DESIGN

Double-blind, placebo-controlled, crossover design randomized controlled trial with 26-day intervention phases and a 30-day washout period.

SETTING

The Institute for Dementia Research and Prevention at the Pennington Biomedical Research Center.

PARTICIPANTS

Ten independently-living, cognitively-healthy older adults (mean age: 67.3 + 2.01 years).

INTERVENTION

Daily consumption of an investigational product (trade name "Cerbella TM") consisting of an emulsified liquid combination of standardized fish oil, panax ginseng extract, and green tea catechins in a flavored base of lecithin phospholipids optimized to maximize bioavailability of the active ingredients.

MEASUREMENTS

Before and after supplementation with the investigational product or placebo, participants completed cognitive tests including the Mini Mental State Exam (MMSE), Stroop test, Digit Symbol Substitution Test (DSST), and Immediate and Delayed Recall tests, as well as functional magnetic resonance imaging (fMRI) during a standard cognitive task switching paradigm.

RESULTS

Performance on the MMSE, Stroop test, and DSST increased significantly over one month of supplementation with the investigational product (one-sample t tests, p<.05) although differences between these changes and corresponding changes during supplementation with placebo were not significant (two-sample t tests, p>.05). During supplementation with the investigational product, brain activation during task performance increased significantly more than during supplementation with placebo in brain regions known to be activated by this task (anterior and posterior cingulate cortex). Functional connectivity during task execution between task regions (middle frontal gyrus and anterior cingulate cortex) increased significantly during supplementation with the investigational product, relative to placebo. Functional connectivity during rest between task regions (precentral gyrus and middle frontal gyrus) and default mode network regions (medial frontal gyrus and precuneus) decreased during supplementation with the investigational product relative to placebo, suggesting greater segregation of task and rest related brain activity.

CONCLUSION

One-month supplementation with a combination of omega 3 essential fatty acids, green tea catechins, and ginsenosides was associated with suggestive changes in cognitive functioning as well as modification of brain activation and brain functional connectivity in cognitively healthy older adults.

Evidence of increased oxidative damage in subjects with mild cognitive impairment

OBJECTIVE

To determine if increased levels of oxidative damage are present in the brains of persons with mild cognitive impairment (MCI), a condition that often precedes Alzheimer disease (AD).

METHODS

The authors assessed the amount of protein carbonyls, thiobarbituric acid-reactive substances (TBARS), and malondialdehyde in the superior and middle temporal gyri (SMTG) and cerebellum of short postmortem interval and longitudinally evaluated normal subjects and those with MCI and early AD.

RESULTS

Elevated levels of protein carbonyls (approximately 25%), malondialdehyde (approximately 60%), and TBARS (approximately 210%) were observed in the SMTG of individuals with MCI and early AD vs normal control subjects. The elevation in TBARS was associated with the numbers of neuritic but not diffuse plaques. Levels of protein carbonyls increased as delayed verbal memory performance declined.

CONCLUSION

Oxidative damage occurs in the brain of subjects with mild cognitive impairment, suggesting that oxidative damage may be one of the earliest events in the onset and progression of Alzheimer disease.

Proteasomes and proteasome inhibition in the central nervous system

Although the proteasome is responsible for the majority of intracellular protein degradation, and has been demonstrated to play a pivotal role in a diverse array of cellular activities, the role of the proteasome in the central nervous system is only beginning to be elucidated. Recent studies have demonstrated that proteasome inhibition occurs in numerous neurodegenerative conditions, and that proteasome inhibition is sufficient to induce neuron death, elevate intracellular levels of protein oxidation, and increase neural vulnerability to subsequent injury. The focus of this review is to describe what is currently known about proteasome biology in the central nervous system and to discuss the possible role of proteasome inhibition in the neurodegenerative process.

Pro-inflammatory and pro-oxidant properties of the HIV protein Tat in a microglial cell line: attenuation by 17 beta-estradiol

Microglia are activated in humans following infection with human immunodeficiency virus (HIV), and brain inflammation is thought to be involved in neuronal injury and dysfunction during HIV infection. Numerous studies indicate a role for the HIV regulatory protein Tat in HIV-related inflammatory and neurodegenerative processes, although the specific effects of Tat on microglial activation, and the signal transduction mechanisms thereof, have not been elucidated. In the present study, we document the effects of Tat on microglial activation and characterize the signal transduction pathways responsible for Tat's pro-inflammatory effects. Application of Tat to N9 microglial cells increased multiple parameters of microglial activation, including superoxide production, phagocytosis, nitric oxide release and TNF alpha release. Tat also caused activation of both p42/p44 mitogen activated protein kinase (MAPK) and NF kappa B pathways. Inhibitor studies revealed that Tat-induced NF kappa B activation was responsible for increased nitrite release, while MAPK activation mediated superoxide release, TNF alpha release, and phagocytosis. Lastly, pre-treatment of microglial cells with physiological concentrations of 17 beta-estradiol suppressed Tat-mediated microglial activation by interfering with Tat-induced MAPK activation. Together, these data elucidate specific components of the microglial response to Tat and suggest that Tat could contribute to the neuropathology associated with HIV infection through microglial promulgation of oxidative stress.

The glial glutamate transporter, GLT-1, is oxidatively modified by 4-hydroxy-2-nonenal in the Alzheimer's disease brain: the role of Abeta1-42

Glutamate transporters are involved in the maintenance of synaptic glutamate concentrations. Because of its potential neurotoxicity, clearance of glutamate from the synaptic cleft may be critical for neuronal survival. Inhibition of glutamate uptake from the synapse has been implicated in several neurodegenerative disorders. In particular, glutamate uptake is inhibited in Alzheimer's disease (AD); however, the mechanism of decreased transporter activity is unknown. Oxidative damage in brain is implicated in models of neurodegeneration, as well as in AD. Glutamate transporters are inhibited by oxidative damage from reactive oxygen species and lipid peroxidation products such as 4-hydroxy-2-nonenal (HNE). Therefore, we have investigated a possible connection between the oxidative damage and the decreased glutamate uptake known to occur in AD brain. Western blots of immunoprecipitated HNE-immunoreactive proteins from the inferior parietal lobule of AD and control brains suggest that HNE is conjugated to GLT-1 to a greater extent in the AD brain. A similar analysis of beta amyloid (Abeta)-treated synaptosomes shows for the first time that Abeta1-42 also increases HNE conjugation to the glutamate transporter. Together, our data provide a possible link between the oxidative damage and neurodegeneration in AD, and supports the role of excitotoxicity in the pathogenesis of this disorder. Furthermore, our data suggests that Abeta may be a possible causative agent in this cascade.

Proteasome inhibition in oxidative stress neurotoxicity: implications for heat shock proteins

Recent studies have demonstrated that inhibition of the proteasome, an enzyme responsible for the majority of intracellular proteolysis, may contribute to the toxicity associated with oxidative stress. In the present study we demonstrate that exposure to oxidative injury (paraquat, H(2)O(2), FeSO(4)) induces a rapid increase in reactive oxygen species (ROS), loss of mitochondrial membrane potential, inhibition of proteasome activity, and induction of cell death in neural SH-SY5Y cells. Application of proteasome inhibitors (MG115, epoxomycin) mimicked the effects of oxidative stressors on mitochondrial membrane potential and cell viability, and increased vulnerability to oxidative injury. Neural SH-SY5Y cells stably transfected with human HDJ-1, a member of the heat shock protein family, were more resistant to the cytotoxicity associated with oxidative stressors. Cells expressing increased levels of HDJ-1 displayed similar degrees of ROS formation following oxidative stressors, but demonstrated a greater preservation of mitochondrial function and proteasomal activity following oxidative injury. Cells transfected with HDJ-1 were also more resistant to the toxicity associated with proteasome inhibitor application. These data support a possible role for proteasome inhibition in the toxicity of oxidative stress, and suggest heat shock proteins may confer resistance to oxidative stress, by preserving proteasome function and attenuating the toxicity of proteasome inhibition.

Vulnerability of synaptosomes from apoE knock-out mice to structural and oxidative modifications induced by A beta(1-40): implications for Alzheimer's disease

Apolipoprotein E (apoE) plays an important role in the response to central nervous system injury. The e4 allele of apoE and amyloid beta-peptide (Abeta) are associated with Alzheimer's disease (AD) and may be central to the pathogenesis of this disorder. Recent studies demonstrate evidence for neurodegeneration and increased lipid peroxidation in transgenic mice lacking apoE (KO). In the current study, synaptosomes were prepared from apoE KO mice to determine the role of apoE in synaptic membrane structure and to determine susceptibility to oxidative damage by Abeta(1-40). ApoE KO mice exhibited structural modifications to lipid and protein components of synaptosomal membranes as determined by electron paramagnetic resonance in conjunction with lipid- and protein- specific spin labels. Incubation with 5 microM Abeta(1-40) resulted in more severe oxidative modifications to proteins and lipids in apoE KO synaptosomes as measured by protein carbonyls, an index of protein oxidation, and TBARs and protein-bound 4-hydroxynonenal (HNE), markers of lipid oxidation. Together, these data support a role for apoE in the modulation of oxidative injury and in the maintenance of synaptic integrity and are discussed with reference to alterations in AD brain.

Dopamine induces proteasome inhibition in neural PC12 cell line

The autoxidation and enzymatic catabolism of dopamine results in the generation of reactive oxygen species (ROS), which may possibly contribute to oxidative stress in multiple neurodegenerative disorders. Recent studies indicate that proteasome inhibition occurs in numerous neurodegenerative conditions, possibly as the result of oxidative stress, although the effects of dopamine on proteasome activity have not been determined. In the present study we examined the effects of dopamine on proteasome activity in the neural PC12 cell line. Application of dopamine induced a dose- and time-dependent decrease in proteasome activity, which occurred prior to cell death. Application of an antioxidant (gluthathione monoethyl ester), monoamine oxidase inhibitors (deprenyl, clogyline, paragyline), or an inhibitor of dopamine uptake (nomifensine) attenuated dopamine toxicity and dopamine-induced proteasome impairment. Application of the proteasome inhibitor lactacystin increased the toxicity of dopamine and the levels of protein oxidation following administration of dopamine. Together, these data indicate that dopamine induces proteasome inhibition that is dependent, in part, on ROS and dopamine uptake, and suggest a possible role for proteasome inhibition in dopamine toxicity.

Antiinflammatory effects of estrogen on microglial activation

In the present study the effects of 17beta-estradiol on microglial activation are described. Estrogen replacement therapy has been associated with decreased severity of age-related neurodegenerative diseases such as Alzheimer's disease, and estrogens have potent immunosuppressive properties outside of the brain. To determine the role that microglial cells might play in estrogen-mediated neuroprotection, primary rat microglia and N9 microglial cell lines were treated with increasing doses of 17beta-estradiol before or during immunostimulation by lipopolysaccharide, phorbol ester, or interferon-gamma. Pretreatment with 17beta-estradiol, but not 17alpha-estradiol or progesterone, dose dependently attenuated microglial superoxide release and phagocytic activity. Additionally, 17beta-estradiol attenuated increases in inducible nitric oxide synthase protein expression, but did not alter nuclear factor-KB activation. The antiinflammatory effects of 17beta-estradiol were blocked by the antiestrogen ICI 182,780. Additionally, 17beta-estradiol induced rapid phosphorylation of the p42/p44 mitogen-activated protein kinase (MAP kinase), and the MAP kinase inhibitor PD 98059 blocked the antiinflammatory effects of 17beta-estradiol. Overall, these results suggest that estrogen receptor-dependent activation of MAP kinase is involved in estrogen-mediated antiinflammatory pathways in microglial cells. These results describe a novel mechanism by which estrogen may attenuate the progression of neurodegenerative disease and suggest new pathways for therapeutic intervention in clinical settings.

Oxidative stress-associated impairment of proteasome activity during ischemia-reperfusion injury

Numerous studies indicate a role for oxidative stress in the neuronal degeneration and cell death that occur during ischemia-reperfusion injury. Recent data suggest that inhibition of the proteasome may be a means by which oxidative stress mediates neuronal cell death. In the current study, the authors demonstrate that there is a time-dependent decrease in proteasome activity, which is not associated with decreased expression of proteasome subunits, after cerebral ischemia-reperfusion injury. To determine the role of oxidative stress in mediating proteasome inhibition, ischemia-reperfusion studies were conducted in mice that either overexpressed the antioxidant enzyme glutathione peroxidase [GPX 1(+)], or were devoid of glutathione peroxidase activity (GPX -/-). After ischemia-reperfusion, GPX 1(+) mice displayed decreased infarct size, attenuated neurologic impairment, and reduced levels of proteasome inhibition compared with either GPX -/- or wild type mice. In addition, GPX 1(+) mice displayed lower levels of 4-hydroxynonenal-modified proteasome subunits after ischemia-reperfusion injury. Together, these data indicate that proteasome inhibition occurs during cerebral ischemia-reperfusion injury and is mediated, at least in part, by oxidative stress.

Decreased levels of proteasome activity and proteasome expression in aging spinal cord

Neuron death and neuron degeneration occur in the CNS during the course of aging. Although multiple cellular alterations transpire during the aging process, those that mediate age-associated neuron death have not been identified. Recent evidence implicates oxidative stress as a possible means of neuron death and neuron degeneration during aging. In the present study, we demonstrate a marked decrease in multicatalytic proteasome activity in the spinal cord of Fisher 344 rats at 12, 24 and 28 months, compared with spinal cord tissue from 3-week- and 3-month-old animals. Application of oxidative injury (FeSO(4)) or the lipid peroxidation product 4-hydroxynonenal decreases multicatalytic proteasome activity in a time- and dose-dependent manner in a motor neuron cell line. Loss of multicatalytic proteasome activity occurs before the loss of multicatalytic proteasome immunoreactivity, with FeSO(4)- and 4-hydroxynonenal-mediated decreases ameliorated by the application of a cell permeable form of the antioxidant glutathione. Application of multicatalytic proteasome inhibitors, but not inhibitors of lysosomal proteases, induced neuron death that was attenuated by the caspase inhibitors benzyloxycarbonyl-Val-Ala-Asp-(O-methyl) fluoromethyl ketone or N-acetyl-Asp-Glu-Val-Asp-Cho (aldehyde). Together, these data suggest that multicatalytic proteasome inhibition occurs during aging of the spinal cord, possibly as the result of oxidative stress, and that multicatalytic proteasome inhibition may be causally related to neuron death.

Proteasome inhibition results in increased poly-ADP-ribosylation: implications for neuron death

This study demonstrates the ability of proteasome inhibitors (lactacystin, MG 115, MG 132) adenosine diphosphate to induce a time- and dose-dependent increase in poly-ADP-ribosylation (PAR) in the neural PC6 cell line, a subclone of PC12 cells. Elevated levels of PAR contribute to the toxicity associated with impaired proteasome activity, based on the ability of PAR inhibitors to ameliorate the toxicity associated with the application of lactacystin, MG 115, and MG 132. Proteasome inhibitors induced the accumulation of PAR and neuron death in primary hippocampal neuron cultures, which were both ameliorated by treatment with PAR inhibitors. Together, these data indicate a role for increased PAR in the toxicity associated with proteasome inhibition, and suggest that inhibitors of PAR may provide neuroprotection in conditions where proteasome inhibition occurs.

Impaired proteasome function in Alzheimer's disease

Inhibition of proteasome activity is sufficient to induce neuron degeneration and death; however, altered proteasome activity in a neurodegenerative disorder has not been demonstrated. In the present study, we analyzed proteasome activity in short-postmortem-interval autopsied brains from 16 Alzheimer's disease (AD) and nine age- and sex-matched controls. A significant decrease in proteasome activity was observed in the hippocampus and parahippocampal gyrus (48%), superior and middle temporal gyri (38%), and inferior parietal lobule (28%) of AD patients compared with controls. In contrast, no significant decrease in proteasome activity was observed in either the occipital lobe or the cerebellum. The loss of proteasome activity was not associated with a decrease in proteasome expression, suggesting that the proteasome may become inhibited in AD by a posttranslational modification. Together, these data indicate a possible role for proteasome inhibition in the neurodegeneration associated with AD.

Lysophosphatidic acid induction of neuronal apoptosis and necrosis

Lysophosphatidic acid (LPA) elicits a unique response in primary hippocampal neurons and sympathetic neuron-like cells, PC12 cells differentiated with nerve growth factor; LPA is cytotoxic. Treatment of rat hippocampal neurons with 50 microM LPA resulted in necrosis, as determined morphologically and by release of lactate dehydrogenase. Lower concentrations of LPA, 0.1, and 1 microM, induced neuronal apoptosis, as assessed by chromatin condensation, annexin V binding, TUNEL staining, and the caspase sensitivity of these events. In addition, 10 and 25 microM LPA induced apoptosis of PC12 cells. In order to define intracellular events associated with this neuronal apoptosis, protective agents were identified. Neurons and PC12 cells were protected against LPA-induced apoptosis by pretreatment with the antioxidant, propyl gallate, or with nitric oxide synthase inhibitors. PC12 cells were protected by insulin and insulin-like growth-factor-1 treatment. There is also evidence for mitochondrial participation in LPA-mediated apoptosis, including cyclosporin A-mediated protection. Thus, LPA-induced neuronal apoptosis is associated with mitochondrial alterations, the generation of reactive oxygen species and nitric oxide, and protection by pretreatment with a serum constituent, insulin-like growth factor 1.

Amyloid beta-peptide effects on synaptosomes from apolipoprotein E-deficient mice

Apolipoprotein E (apoE) is present in the brain and may contribute to neurophysiologic or neuropathologic events, depending on environmental and genetic influences. Recent studies indicate a role for apoE in synaptic plasticity and maintenance of synaptic membrane symmetry, suggesting that apoE may be involved in regulating synaptic homeostasis. In the present study, cerebrocortical synaptosomes were prepared from transgenic mice lacking apoE (apoE KO) to analyze the possible contribution of apoE toward maintaining homeostasis in synaptosomes. Synaptosomal preparations from apoE KO and wild-type mice exhibited similar basal levels of reactive oxygen species, mitochondrial function, and caspase activity; however, following application of amyloid beta-peptide [Abeta(1-40)], apoE KO synaptosomes displayed increased levels of oxidative stress, mitochondrial dysfunction, and caspase activation compared with synaptosomes from wild-type mice. Synaptosomal membranes from apoE KO mice were more fluid than wild-type synaptosomes and contained higher levels of thiobarbituric acid-reactive substances, consistent with elevated levels of lipid peroxidation occurring in the synapses of apoE KO mice. Together, these data are consistent with a role for apoE in maintaining homeostasis by attenuating oxidative stress, caspase activation, and mitochondrial homeostasis in synapses.

Oxidized high-density lipoprotein induces neuron death

High-density lipoprotein (HDL) exists within the brain and is highly vulnerable to oxidative modifications. The focus of the present study was to determine the effect of HDL and oxidized HDL (oxHDL) upon neurons, astrocytes, and microglia. Administration of highly oxidized HDL, but not native, minimally, or moderately modified HDL resulted in a dose- and time-dependent increase in oxidative stress and death of cultured rat embryonic neurons. Astrocyte and microglia cultures treated with highly oxidized HDL displayed increased reactive oxygen species formation but no toxicity. Application of oxHDL exacerbated oxidative stress and neuron death induced by beta-amyloid peptide. Studies using pharmacological inhibitors implicate the involvement of calcium and reactive oxygen species in oxHDL-induced neuronal loss. Neural cells expressing increased levels of BCL-2 had decreased levels of oxidative stress and neuron death following exposure to oxHDL. Together, these data demonstrate that oxHDL increases oxidative stress in neurons, astrocytes, and microglia which ultimately culminate in neuron death.

Possible involvement of proteasome inhibition in aging: implications for oxidative stress

Oxidative stress may contribute to the cellular alterations, which occur as the result of aging, and the nervous system is particularly vulnerable to aging associated oxidative injury. The multicatalytic proteasome (MCP) is responsible for the majority of protein degradation and is sensitive to oxidative stress. To determine if MCP activity is altered during aging, studies were conducted in multiple tissues from aged Fisher 344 rats. Analysis of heart, lung, kidney, and liver revealed decreased MCP activity in 12, 24, and 28 month old rats, compared with 3 week or 3 month old animals. The spinal cord, hippocampus, and cerebral cortex demonstrated age dependent decreases in MCP activity, but at no timepoint was MCP activity decreased in either the brain stem or cerebellum. Oxidative injury and the lipid oxidation product 4-hydroxynonenal caused decreased MCP activity in neural PC6 cells, while application of MCP inhibitors was sufficient to induce cell death in neural PC6 cells. Together, these data indicate a role for MCP inhibition in cellular dysfunction associated with aging and oxidative injury.

4-hydroxynonenal increases neuronal susceptibility to oxidative stress

Increased levels of reactive oxygen species occur in neurodegenerative disorders and may promote neuron death. The lipid peroxidation product 4-hydroxynonenal (HNE) is increased in neurons following oxidative stress and promotes neuron death in vitro and in vivo. The present study examined the possibility that HNE can increase neuron vulnerability to oxidative stress. Application of low concentrations of HNE (50-500 nM) increased neuron death induced by beta-amyloid or glutamate when added within 3 hr of injury. In addition, treatment with HNE exacerbated mitochondrial reactive oxygen species formation and loss of mitochondrial membrane potential in response to beta-amyloid and glutamate. The ability to exacerbate oxidative stress, mitochondrial dysfunction, and neuron death appears to be specific to HNE, because application of other lipid peroxidation products had no effect. These data indicate a role for low levels of HNE in promoting reactive oxygen species accumulation and neuron degeneration by altering mitochondrial homeostasis. In addition, the present study indicates a possible mechanism for reactive oxygen species and lipid peroxidation toxicity in neurodegenerative conditions.

Truncated apolipoprotein E (ApoE) causes increased intracellular calcium and may mediate ApoE neurotoxicity

Apolipoprotein E (apoE)-related synthetic peptides, the 22 kDa N-terminal thrombin-cleavage fragment of apoE (truncated apoE), and full-length apoE have all been shown to exhibit neurotoxic activity under certain culture conditions. In the present study, protease inhibitors reduced the neurotoxicity and proteolysis of full-length apoE but did not block the toxicity of truncated apoE or a synthetic apoE peptide, suggesting that fragments of apoE may account for its toxicity. Additional experiments demonstrated that both truncated apoE and the apoE peptide elicit an increase in intracellular calcium levels and subsequent death of embryonic rat hippocampal neurons in culture. Similar effects on calcium were found when the apoE peptide was applied to chick sympathetic neurons. The rise in intracellular calcium and the hippocampal cell death caused by the apoE peptide were significantly reduced by receptor-associated protein, removal of extracellular calcium, or administration of the specific NMDA glutamate receptor antagonist MK-801. These results suggest that apoE may be a source of both neurotoxicity and calcium influx that involves cell surface receptors. Such findings strengthen the hypothesis that apoE plays a direct role in the pathology of Alzheimer's disease.

Oxidized low-density lipoprotein induces neuronal death: implications for calcium, reactive oxygen species, and caspases

Low-density lipoprotein (LDL) exists within the brain and is highly vulnerable to oxidative modifications. Once formed, oxidized LDL (oxLDL) is capable of eliciting cytotoxicity, differentiation, and inflammation in nonneuronal cells. Although oxLDL has been studied primarily for its role in the development of atherosclerosis, recent studies have identified a possible role for it in neurological disorders associated with oxidative stress. In the present study application of oxLDL, but not LDL, resulted in a dose- and time-dependent death of cultured rat embryonic neurons. Studies using pharmacological inhibitors implicate the involvement of calcium, reactive oxygen species, and caspases in oxLDL-induced neuronal death. Coapplication of oxLDL with either amyloid beta-peptide or glutamate, agents that enhance oxidative stress, resulted in increased neuronal death. Taken together, these data demonstrate that oxLDL induces neuronal death and implicate a possible role for oxLDL in conditions associated with increased levels of reactive oxygen species, including Alzheimer's disease.

Oxidized lipoproteins increase reactive oxygen species formation in microglia and astrocyte cell lines

Lipoproteins exist in the central nervous system and surrounding vasculature possibly mediating effects upon cells in the brain during times of oxidative stress or compromised blood-brain barrier. The focus of the present study was to determine the effect of unmodified and oxidatively modified lipoproteins on astrocytes and microglia. Application of oxidized low-density lipoprotein resulted in an increase in DCF fluorescence, which was inhibited by pretreatment with antioxidants, consistent with the formation of reactive oxygen species (ROS). Low-density at concentrations below 20 microg/ml likewise increased ROS formation. Because ROS are associated with numerous astrocyte and microglia activities including proliferation, activation, and cytokine production it is possible that lipoproteins may mediate such effects on glial cells in the central nervous system.

Opposing actions of native and oxidized lipoprotein on motor neuron-like cells

Lipoproteins are present in the central nervous system and surrounding vasculature and possibly mediate effects relevant to neuronal physiology and pathology. To determine the effects of lipoproteins on motor neurons, native low density lipoproteins (LDL) and oxidized LDL (oxLDL) were applied to a motor neuron cell line. Oxidized LDL, but not native LDL, resulted in a dose- and time-dependent increase in reactive oxygen species and neuron death. Oxidized LDL-induced toxicity was attenuated by a calcium chelator, antioxidants, caspase inhibitors, and inhibitors of macromolecular synthesis. In addition to being nontoxic, application of native LDL attenuated reactive oxygen species formation and neuron loss following glucose deprivation injury. Together, these data demonstrate a possible neuroprotective role for unmodified lipoproteins and suggest oxidized lipoproteins may amplify oxidative stress and neuron loss.

Anti-death properties of TNF against metabolic poisoning: mitochondrial stabilization by MnSOD

The cytokine tumor necrosis factor (TNF) is toxic to some mitotic cells, but protects cultured neurons from a variety of insults by mechanisms that are unclear. Pretreatment of neurons or astrocytes with TNF caused significant increases in MnSOD activity, and also significantly attenuated 3-nitropropionic acid (3-NP) induced superoxide accumulation and loss of mitochondrial transmembrane potential. In oligodendrocytes, however, MnSOD activity was not increased, and 3-NP toxicity was unaffected by TNF. Genetically engineered PC6 cells that overexpress MnSOD also were resistant to 3-NP-induced damage. TNF pretreatment and MnSOD overexpression prevented 3-NP induced apoptosis, and shifted the mode of death from necrosis to apoptosis in response to high levels of 3-NP. Mitochondria isolated from either MnSOD overexpressing PC6 cells or TNF-treated neurons maintained resistance to 3-NP-induced loss of transmembrane potential and calcium homeostasis, and showed attenuated release of caspase activators. Overall, these results indicate that MnSOD activity directly stabilizes mitochondrial transmembrane potential and calcium buffering ability, thereby increasing the threshold for lethal injury. Additional studies showed that levels of oxidative stress and striatal lesion size following 3-NP administration in vivo are increased in mice lacking TNF receptors.

Cyclic nucleotides attenuate lipid peroxidation-mediated neuron toxicity

Recent studies suggest that increased lipid peroxidation and lipid peroxidation products, such as 4-hydroxynonenal (HNE), contribute to neuronal loss in conditions associated with oxidative stress. The focus of the present study was to determine possible neuroprotective effects of elevated cyclic nucleotide levels against lipid peroxidation and HNE-mediated neural toxicity. Application of 8-bromo derivative analogs of cAMP or cGMP resulted in attenuation of HNE-induced increases in mitochondrial calcium, reactive oxygen species, and neuron loss. Similar results were obtained when neural cells were pretreated with the phosphodiesterase inhibitors zaprinast or isobutylmethylxanthanine (IBMX). These data are consistent with a possible neuroprotective role for elevated cyclic nucleotide levels in disorders associated with increases in lipid peroxidation and HNE.

Protein modification by the lipid peroxidation product 4-hydroxynonenal in the spinal cords of amyotrophic lateral sclerosis patients

We report increased modification of proteins by 4-hydroxynonenal (HNE), a product of membrane lipid peroxidation, in the lumbar spinal cord of sporadic amyotrophic lateral sclerosis (ALS) patients versus that of neurologically normal controls. By immunohistochemistry, HNE-protein modification was detected in ventral horn motor neurons, and immunoprecipitation analysis revealed that one of the proteins modified by HNE was the astrocytic glutamate transporter EAAT2. Given that the function of proteins modified by HNE can be severely compromised as previously demonstrated for glutamate transporters in cortical synaptosome preparations, our findings suggest a scenario in which oxidative stress leads to the production of HNE, impairment of glutamate transport, and excitotoxic motor neuron degeneration in ALS.

Traumatic brain injury alters synaptic homeostasis: implications for impaired mitochondrial and transport function

This study utilized a unilateral controlled cortical impact model of traumatic brain injury to assess disruptions of synaptic homeostasis following trauma. Adult rats were subjected to a moderate (2 mm) cortical deformation and synaptosomes were prepared from the entire ipsilateral (injured) hemisphere or dissected into different regions (hippocampus, injured cortical area including penumbra, residual hemisphere) at various times postinjury (10 and 30 min, and 1, 6, and 24 h). Synaptosomes from the corresponding regions of the contralateral hemisphere were used as controls to assess alterations in synaptic ATP levels, lipid peroxidation, and glutamate and glucose transport. The results demonstrate significant time-dependent alterations in synaptic homeostasis, which included an immediate reduction in ATP levels, coupled with a significant increase in lipid peroxidation within 30 min postinjury. Lipid peroxidation demonstrated a biphasic response with elevations observed 24 h postinjury, a time at which decreases in glutamate and glucose transport occurred. These results suggest that disruption of synaptic homeostasis is an extremely early event following trauma that should be considered when designing pharmacological interventions.

Lysophosphatidic acid and apoptosis of nerve growth factor-differentiated PC12 cells

The lipid biomediator lysophosphatidic acid (LPA) elicits a unique response in hippocampal neurons, LPA induces neuronal apoptosis. This study explores the effects of LPA on cells with neuronal properties, nerve growth factor-differentiated PC6 cells, a clone of PC12 cells. LPA induced apoptosis in these cells as assessed by chromatin condensation, terminal dUTP nick end-labeling of DNA, protection against these nuclear alterations by a general caspase inhibitor and the lack of release of lactic dehydrogenase. LPA caused oxidative stress, namely a decreased reduction of MTT, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide. This oxidative stress appears to be of functional significance, since cells were protected by pretreatment with the antioxidant propyl gallate and by stable transfection with cDNA encoding the antioxidant enzyme, manganese superoxide dismutase. Mitochondrial and nitric oxide participation in LPA-induced apoptosis are suggested by the protection afforded by pretreatment with either cyclosporin A, an inhibitor of mitochondrial permeability transition, or nitric oxide synthase inhibitors. The nitric oxide synthase inhibitor findings are novel, since to our knowledge, LPA has not heretofore been associated with an increase in nitric oxide. In addition, as observed for many neurotoxic agents, insulin-like growth factor I protected against LPA-induced apoptosis of PC6 cells.

Cryopreservation of rat cortical synaptosomes and analysis of glucose and glutamate transporter activities, and mitochondrial function

Direct comparisons of synaptic functional parameters in brain tissues from different groups of experimental animals and different samples from post mortem human brain are often hindered by the inability to perform assays at the same time. To circumvent these difficulties we developed methods for cryopreservation and long-term storage of neocortical synaptosomes. The synaptosomes are suspended in a cryopreservation medium containing 10% dimethylsulfoxide and 10% fetal bovine serum, and are slowly cooled to -80 degreesC and then stored in liquid nitrogen. The function of plasma membrane glucose and glutamate transporters, and mitochondrial electron transport activity and membrane potential were measured in fresh, cryopreserved (CP), and non-cryopreserved freeze-thawed (NC) synaptosomes. Glucose and glutamate transporter activities, and mitochondrial functional parameters in CP synaptosomes were essentially identical to those in fresh unfrozen synaptosomes. Glucose and glutamate transport were severely compromised in NC synaptosomes, whereas mitochondrial function and cellular esterase activity were largely maintained. Electron paramagnetic resonance studies in conjunction with a protein-specific spin label indicated that cryopreservation did not alter the physical state of synaptosomal membrane proteins. These methods provide the opportunity to generate stocks of functional synaptosomes from different experiments or post mortem samples collected over large time intervals.

Evidence for synaptic apoptosis

Increasing evidence indicates that neurons die by apoptosis, an active form of cell death involving a relatively stereotyped series of biochemical changes that culminate in nuclear fragmentation, in many different developmental and pathophysiological settings. In contrast to most other cell types, neurons have elaborate morphologies with complex neuritic arbors that often extend great distances from the cell body. Neuronal death signals are likely to be activated at remote synaptic sites and, indeed, overactivation of glutamate receptors and underactivation of trophic factor receptors are implicated in neurodegenerative disorders. We now report that biochemical changes consistent with apoptosis are engaged locally in synapses. Exposure of cortical synaptosomes to staurosporine and Fe2+ resulted in loss of membrane phospholipid asymmetry, caspase activation, and mitochondrial alterations (membrane depolarization, calcium overload, and oxyradical accumulation) characteristic of apoptosis. The caspase inhibitor zVAD-fmk prevented mitochondrial membrane depolarization in synaptosomes. Studies of the effects of cytosolic extracts from synaptosomes exposed to apoptotic insults, on isolated nuclei, showed that signals capable of inducing nuclear apoptosis are generated locally in synapses. Exposure of cultured hippocampal neurons to staurosporine and glutamate resulted in caspase activation and mitochondrial membrane depolarization in dendrites, and zVAD-fmk prevented the membrane depolarization. Glutamate-induced increases in caspase activity were first observed in dendrites and later in the cell body, and focal application of glutamate to individual dendrites resulted in local activation of caspases. Collectively, the data demonstrate that apoptotic biochemical cascades can be activated locally in synapses and dendrites and suggest a role for such local apoptotic signals in synapse loss and neuronal death in neurodegenerative disorders that involve excessive activation of glutamate receptors.

Roles of lipid peroxidation in modulation of cellular signaling pathways, cell dysfunction, and death in the nervous system

Free radicals are known to occur as natural by-products under physiological conditions and have been implicated in the neuronal loss observed in a variety of neuropathological conditions including Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), and ischemia. Oxyradical-induced cytotoxicity arises from both chronic and acute increases in reactive oxygen species which give rise to subsequent lipid peroxidation (LP). By reacting with polyunsaturated fatty acids in the the various cellular membranes, oxyradicals such as hydroxyl (OH.) and peroxynitrite (ONOO) give rise to a variety of lipid peroxidation products (LPP), including 4-hydroxynonenal (HNE) and malondialdehyde (MD). Once formed, these peroxidation metabolites have been demonstrated to have relatively long half-lives within cells (minutes to hours), allowing for multiple interactions with cellular components. Emerging data suggest that LP and LPP may underlie the neuronal alterations and neurotoxicity observed in numerous neurodegenerative conditions. Data supporting this involvement include the detection of LP and formation of LPP in a variety of neuropathological conditions including AD, ALS, PD, and ischemia. Secondly, direct application of LPP, either in vivo or in vitro, has been shown to be cytotoxic and mimic neuronal alterations observed in neuropathological conditions. Furthermore, prevention of LP and subsequent LPP formation have been demonstrated to be neuroprotective in a variety of neurodegenerative paradigms. Additionally, LP and LPP have been implicated in the modulation of a wide array of activities within the central nervous system including long term potentiation, neurite outgrowth, and proliferation. Understanding the mechanism(s) and involvement of LP in these processes will greatly enhance the understanding of oxyradical and ion homeostasis in neurophysiological and neuropathological conditions. The focus of this review is to describe the process by which lipid peroxidation occurs and establish a framework for its involvement in the central nervous system.

Increased sensitivity to mitochondrial toxin-induced apoptosis in neural cells expressing mutant presenilin-1 is linked to perturbed calcium homeostasis and enhanced oxyradical production

Many cases of autosomal dominant early onset Alzheimer's disease (AD) result from mutations in the gene encoding presenilin-1 (PS-1). PS-1 is an integral membrane protein expressed ubiquitously in neurons throughout the brain in which it is located primarily in endoplasmic reticulum (ER). Although the pathogenic mechanism of PS-1 mutations is unknown, recent findings suggest that PS mutations render neurons vulnerable to apoptosis. Because increasing evidence indicates that mitochondrial alterations contribute to neuronal death in AD, we tested the hypothesis that PS-1 mutations sensitize neurons to mitochondrial failure. PC12 cell lines expressing a PS-1 mutation (L286V) exhibited increased sensitivity to apoptosis induced by 3-nitropropionic acid (3-NP) and malonate, inhibitors of succinate dehydrogenase, compared with control cell lines and lines overexpressing wild-type PS-1. The apoptosis-enhancing action of mutant PS-1 was prevented by antioxidants (propyl gallate and glutathione), zVAD-fmk, and cyclosporin A, indicating requirements of reactive oxygen species (ROS), caspases, and mitochondrial permeability transition in the cell death process. 3-NP induced a rapid elevation of [Ca2+]i, which was followed by caspase activation, accumulation of ROS, and decreases in mitochondrial reducing potential and transmembrane potential in cells expressing mutant PS-1. The calcium chelator BAPTA AM and agents that block calcium release from ER and influx through voltage-dependent channels prevented mitochondrial ROS accumulation and membrane depolarization and apoptosis. Our data suggest that by perturbing subcellular calcium homeostasis presenilin mutations sensitize neurons to mitochondria-based forms of apoptosis that involve oxidative stress.

4-hydroxynonenal, a lipid peroxidation product, impairs glutamate transport in cortical astrocytes

Astrocytes possess plasma membrane glutamate transporters that rapidly remove glutamate from the extracellular milieu and thereby prevent excitotoxic injury to neurons. Cellular oxidative stress is increased in neural tissues in a variety of acute and chronic neurodegenerative conditions. Recent findings suggest that oxidative stress increases neuronal vulnerability to excitotoxicity and that membrane lipid peroxidation plays a key role in this process. We now report that 4-hydroxynonenal (HNE), an aldehydic product of membrane lipid peroxidation, impairs glutamate transport in cultured cortical astrocytes. Impairment of glutamate transport occurred within 1-3 h of exposure to HNE; FeSO4, an inducer of membrane lipid peroxidation, also impaired glutamate transport. Vitamin E prevented impairment of glutamate transport induced by FeSO4, but not that induced by HNE, consistent with HNE acting as an effector of lipid peroxidation-induced impairment of glutamate transport. Glutathione, which binds and thereby detoxifies HNE, prevented HNE from impairing glutamate transport. Western blot, immunoprecipitation, and immunocytochemical analyses using an antibody against HNE-protein conjugates provided evidence that HNE covalently binds to many different astrocytic proteins including the glutamate transporter GLT-1. Data further suggest that HNE promotes intermolecular cross-linking of GLT-1 monomers to form dimers. HNE also induced mitochondrial dysfunction and accumulation of peroxides in astrocytes. Impairment of glutamate transport and mitochondrial function occurred with sublethal concentrations of HNE, concentrations known to be generated in cells exposed to various oxidative insults. Collectively, our data suggest that HNE may be an important mediator of oxidative stress-induced impairment of astrocytic glutamate transport and may thereby play a role in promoting neuronal excitotoxicity.

Mitochondrial manganese superoxide dismutase prevents neural apoptosis and reduces ischemic brain injury: suppression of peroxynitrite production, lipid peroxidation, and mitochondrial dysfunction

Oxidative stress is implicated in neuronal apoptosis that occurs in physiological settings and in neurodegenerative disorders. Superoxide anion radical, produced during mitochondrial respiration, is involved in the generation of several potentially damaging reactive oxygen species including peroxynitrite. To examine directly the role of superoxide and peroxynitrite in neuronal apoptosis, we generated neural cell lines and transgenic mice that overexpress human mitochondrial manganese superoxide dismutase (MnSOD). In cultured pheochromocytoma PC6 cells, overexpression of mitochondria-localized MnSOD prevented apoptosis induced by Fe2+, amyloid beta-peptide (Abeta), and nitric oxide-generating agents. Accumulations of peroxynitrite, nitrated proteins, and the membrane lipid peroxidation product 4-hydroxynonenal (HNE) after exposure to the apoptotic insults were markedly attenuated in cells expressing MnSOD. Glutathione peroxidase activity levels were increased in cells overexpressing MnSOD, suggesting a compensatory response to increased H2O2 levels. The peroxynitrite scavenger uric acid and the antioxidants propyl gallate and glutathione prevented apoptosis induced by each apoptotic insult, suggesting central roles for peroxynitrite and membrane lipid peroxidation in oxidative stress-induced apoptosis. Apoptotic insults decreased mitochondrial transmembrane potential and energy charge in control cells but not in cells overexpressing MnSOD, and cyclosporin A and caspase inhibitors protected cells against apoptosis, demonstrating roles for mitochondrial alterations and caspase activation in the apoptotic process. Membrane lipid peroxidation, protein nitration, and neuronal death after focal cerebral ischemia were significantly reduced in transgenic mice overexpressing human MnSOD. The data suggest that mitochondrial superoxide accumulation and consequent peroxynitrite production and mitochondrial dysfunction play pivotal roles in neuronal apoptosis induced by diverse insults in cell culture and in vivo.

Lysophosphatidic acid induces necrosis and apoptosis in hippocampal neurons

A diverse body of evidence indicates a role for the lipid biomediator lysophosphatidic acid (LPA) in the CNS. This study identifies and characterizes the induction of neuronal death by LPA. Treatment of cultured hippocampal neurons from embryonic rat brains with 50 microM LPA resulted in neuronal necrosis, as determined morphologically and by the release of lactate dehydrogenase. A concentration of LPA as low as 10 microM led to the release of lactate dehydrogenase. In contrast, treatment of neurons with 0.1 or 1.0 microM LPA resulted in apoptosis, as determined by chromatin condensation. In addition, neuronal death induced by 1 microM LPA was characterized as apoptotic on the basis of terminal dUTP nick end-labeling (TUNEL) staining, externalization of phosphatidylserine, and protection against chromatin condensation, TUNEL staining, and phosphatidylserine externalization by treatment with N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone, a broad-spectrum inhibitor of caspases, i.e., members of the interleukin-1beta converting enzyme family. Studies with antagonists of ionotropic glutamate receptors did not indicate a significant role for these receptors in apoptosis induced by 1 microM LPA. LPA (1 microM) also induced a decrease in mitochondrial membrane potential. Moreover, pretreatment of neurons with cyclosporin A protected against the LPA-induced decrease in mitochondrial membrane potential and neuronal apoptosis. Thus, LPA, at pathophysiological levels, can induce neuronal apoptosis and could thereby participate in neurodegenerative disorders.

17Beta-estradiol attenuates oxidative impairment of synaptic Na+/K+-ATPase activity, glucose transport, and glutamate transport induced by amyloid beta-peptide and iron

Synapse loss, deposits of amyloid beta-peptide (Abeta), impaired energy metabolism, and cognitive deficits are defining features of Alzheimer's disease (AD). Estrogen replacement therapy reduces the risk of developing AD in postmenopausal women. Because synapses are likely sites for initiation of neurodegenerative cascades in AD, we tested the hypothesis that estrogens act directly on synapses to suppress oxidative impairment of membrane transport systems. Exposure of rat cortical synaptosomes to Abeta25-35 (Abeta) and FeSO4 induced membrane lipid peroxidation and impaired the function of the plasma membrane Na+/K+-ATPase, glutamate transporter, and glucose transporter. Pretreatment of synaptosomes with 17beta-estradiol or estriol largely prevented impairment of Na+/K+-ATPase activity, glutamate transport, and glucose transport; other steroids were relatively ineffective. 17Beta-estradiol suppressed membrane lipid peroxidation induced by Abeta and FeSO4, but did not prevent impairment of membrane transport systems by 4-hydroxynonenal (a toxic lipid peroxidation product), suggesting that an antioxidant property of 17beta-estradiol was responsible for its protective effects. By suppressing membrane lipid peroxidation in synaptic membranes, estrogens may prevent impairment of transport systems that maintain ion homeostasis and energy metabolism, and thereby forestall excitotoxic synaptic degeneration and neuronal loss in disorders such as AD and ischemic stroke.

4-Hydroxynonenal, an aldehydic product of membrane lipid peroxidation, impairs glutamate transport and mitochondrial function in synaptosomes

Removal of extracellular glutamate at synapses, by specific high-affinity glutamate transporters, is critical to prevent excitotoxic injury to neurons. Oxidative stress has been implicated in the pathogenesis of an array of prominent neurodegenerative conditions that involve degeneration of synapses and neurons in glutamatergic pathways including stroke, and Alzheimer's, Parkinson's and Huntington's diseases. Although cell culture data indicate that oxidative insults can impair key membrane regulatory systems including ion-motive ATPases and amino acid transport systems, the effects of oxidative stress on synapses, and the mechanisms that mediate such effects, are largely unknown. This study provides evidence that 4-hydroxynonenal, an aldehydic product of lipid peroxidation, mediates oxidation-induced impairment of glutamate transport and mitochondrial function in synapses. Exposure of rat cortical synaptosomes to 4-hydroxynonenal resulted in concentration- and time-dependent decreases in [3H]glutamate uptake, and mitochondrial function [assessed with the dye 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT)]. Other related aldehydes including malondialdehyde and hexanal had little or no effect on glutamate uptake or mitochondrial function. Exposure of synaptosomes to insults known to induce lipid peroxidation (FeSO4 and amyloid beta-peptide) also impaired glutamate uptake and mitochondrial function. The antioxidants propyl gallate and glutathione prevented impairment of glutamate uptake and MTT reduction induced by FeSO4 and amyloid beta-peptide, but not that induced by 4-hydroxynonenal. Western blot analyses using an antibody to 4-hydroxynonenal-conjugated proteins showed that 4-hydroxynonenal bound to multiple cell proteins including GLT-1, a glial glutamate transporter present at high levels in synaptosomes. 4-Hydroxynonenal itself induced lipid peroxidation suggesting that, in addition to binding directly to membrane regulatory proteins, 4-hydroxynonenal potentiates oxidative cascades. Collectively, these findings suggest that 4-hydroxynonenal plays important roles in oxidative impairment of synaptic functions that would be expected to promote excitotoxic cascades.

The lipid peroxidation product, 4-hydroxy-2-trans-nonenal, alters the conformation of cortical synaptosomal membrane proteins

Alzheimer's disease (AD) is widely held to be a disorder associated with oxidative stress due, in part, to the membrane action of amyloid beta-peptide (A beta). A beta-associated free radicals cause lipid peroxidation, a major product of which is 4-hydroxy-2-trans-nonenal (HNE). We determined whether HNE would alter the conformation of synaptosomal membrane proteins, which might be related to the known neurotoxicity of A beta and HNE. Electron paramagnetic resonance spectroscopy, using a protein-specific spin label, MAL-6 (2,2,6,6-tetramethyl-4-maleimidopiperidin-1-oxyl), was used to probe conformational changes in gerbil cortical synaptosomal membrane proteins, and a lipid-specific stearic acid label, 5-nitroxide stearate, was used to probe for HNE-induced alterations in the fluidity of the bilayer domain of these membranes. Synaptosomal membranes, incubated with low concentrations of HNE, exhibited changes in protein conformation and bilayer order and motion (fluidity). The changes in protein conformation were found to be concentration- and time-dependent. Significant protein conformational changes were observed at physiologically relevant concentrations of 1-10 microM HNE, reminiscent of similar changes in synaptosomal membrane proteins from senile plaque- and A beta-rich AD hippocampal and inferior parietal brain regions. HNE-induced modifications in the physical state of gerbil synaptosomal membrane proteins were prevented completely by using excess glutathione ethyl ester, known to protect neurons from HNE-caused neurotoxicity. Membrane fluidity was found to increase at higher concentrations of HNE (50 microM). The results obtained are discussed with relevance to the hypothesis of A beta-induced free radical-mediated lipid peroxidation, leading to subsequent HNE-induced alterations in the structure and function of key membrane proteins with consequent neurotoxicity in AD brain.

Lysophosphatidic acid-induced proliferation-related signals in astrocytes

Lysophosphatidic acid (LPA) is a potent lipid biomediator that is likely to have diverse roles in the brain. Thus, LPA-induced events in astrocytes were defined. As little as 1 nM LPA induced a rapid increase in the concentration of intracellular free calcium ([Ca2+]i) in astrocytes from neonatal rat brains. This increase was followed by a slow return to the basal level. Intracellular calcium stores were important for the initial rise in [Ca2+]i, whereas the influx of extracellular calcium contributed significantly to the extended elevation of [Ca2+]i. LPA treatment also resulted in increases in lipid peroxidation and DNA synthesis. These increases in [Ca2+]i, lipid peroxidation, and DNA synthesis were inhibited by pretreatment of cells with pertussis toxin or H7, a serine/threonine protein kinase inhibitor. Moreover, the LPA-induced increase in [Ca2+]i was inhibited by a protein kinase C inhibitor, Ro 31-8220, and a calcium-dependent protein kinase C inhibitor, Gö 6976. The increase in [Ca2+]i was important for the LPA-induced increase in lipid peroxidation, whereas the antioxidant, propyl gallate, inhibited the LPA-stimulated increases in lipid peroxidation and DNA synthesis. In contrast, pertussis toxin, H7, and propyl gallate had no effect on LPA-induced inhibition of glutamate uptake. Thus, LPA appears to signal via at least two distinctive mechanisms in astrocytes. One is a novel pathway, namely, activation of a pertussis toxin-sensitive G protein and participation of a protein kinase, leading to sequential increases in [Ca2+]i, lipid peroxidation, and DNA synthesis.

Secreted form of beta-amyloid precursor protein shifts the frequency dependency for induction of LTD, and enhances LTP in hippocampal slices

The secreted form of beta-amyloid precursor protein (sAPP alpha) is released from neurons in an activity-dependent manner, and has been reported to modulate neuronal excitability in dissociated hippocampal neurons. We now report that sAPP alpha shifts the frequency dependence for induction of long-term depression of synaptic transmission (LTD) in hippocampal slices from adult rats. Whereas low frequency stimulation (1 Hz) of Schaffer collateral axons induced LTD of the post-synaptic response of CA1 neurons in control slices, it did not induce LTD in slices pretreated with sAPP alpha. On the other hand, whereas a 10 Hz stimulation normally induced neither LTD or LTP, it did induce LTD in slices pretreated with sAPP alpha. sAPP alpha potentiated LTP induced by high frequency stimulation. sAPP alpha induced cGMP production in hippocampal slices, and pretreatment of slices with 8-bromo-cyclic GMP mimicked the effect of sAPP alpha on LTD suggesting a role for cyclic GMP in modulation of LTD. The data suggest an important role for sAPP alpha in modulation of synaptic plasticity in the hippocampus.

Impairment of glucose and glutamate transport and induction of mitochondrial oxidative stress and dysfunction in synaptosomes by amyloid beta-peptide: role of the lipid peroxidation product 4-hydroxynonenal

Deposits of amyloid beta-peptide (A beta), reduced glucose uptake into brain cells, oxidative damage to cellular proteins and lipids, and excitotoxic mechanisms have all been suggested to play roles in the neurodegenerative process in Alzheimer's disease. Synapse loss is closely correlated with cognitive impairments in Alzheimer's disease, suggesting that the synapse may be the site at which degenerative mechanisms are initiated and propagated. We report that A beta causes oxyradical-mediated impairment of glucose transport, glutamate transport, and mitochondrial function in rat neocortical synaptosomes. A beta induced membrane lipid peroxidation in synaptosomes that occurred within 1 h of exposure; significant decreases in glucose transport occurred within 1 h of exposure to A beta and decreased further with time. The lipid peroxidation product 4-hydroxynonenal conjugated to synaptosomal proteins and impaired glucose transport; several antioxidants prevented A beta-induced impairment of glucose transport, indicating that lipid peroxidation was causally linked to this adverse action of A beta. FeSO4 (an initiator of lipid peroxidation), A beta, and 4-hydroxynonenal each induced accumulation of mitochondrial reactive oxygen species, caused concentration-dependent decreases in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide reduction, and reduced cellular ATP levels significantly. A beta also impaired glutamate transport, an effect blocked by antioxidants. These data suggest that A beta induces membrane lipid peroxidation, which results in impairment of the function of membrane glucose and glutamate transporters, altered mitochondrial function, and a deficit in ATP levels; 4-hydroxynonenal appears to be a mediator of these actions of A beta. These data suggest that oxidative stress occurring at synapses may contribute to the reduced glucose uptake and synaptic degeneration that occurs in Alzheimer's disease patients. They further suggest a sequence of events whereby oxidative stress promotes excitotoxic synaptic degeneration and neuronal cell death in a variety of different neurodegenerative disorders.

Lysophosphatidic acid induces a sustained elevation of neuronal intracellular calcium

Lysophosphatidic acid (LPA) is a lipid biomediator enriched in the brain. A novel LPA-induced response in rat hippocampal neurons is described herein, namely, a rapid and sustained elevation in the concentration of free intracellular calcium ([Ca2+]i). This increase is specific, in that the related lipids phosphatidic acid and lysophosphatidylcholine did not induce an alteration in [Ca2+]i. Moreover, consistent with a receptor-mediated process, there was no further increase in [Ca2+]i after a second addition of LPA. The LPA-induced increase in [Ca2+]i required extracellular calcium. However, studies with Cd2+, Ni2+, and nifedipine and nystatin-perforated patch clamp analyses did not indicate involvement of voltage-gated calcium channels in the LPA-induced response. In contrast, glutamate appears to have a significant role in the LPA-induced increase in [Ca2+]i, because this increase was inhibited by NMDA receptor antagonists and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA)/kainate receptor antagonists. Thus, LPA treatment may result in an increased extracellular glutamate concentration that could stimulate AMPA/kainate receptors and thereby alleviate the Mg2+ block of the NMDA receptors and lead to glutamate stimulation of an influx of calcium via NMDA receptors.

Basic FGF attenuates amyloid beta-peptide-induced oxidative stress, mitochondrial dysfunction, and impairment of Na+/K+-ATPase activity in hippocampal neurons

Basic fibroblast growth factor (bFGF) exhibits trophic activity for many populations of neurons in the brain, and can protect those neurons against excitotoxic, metabolic and oxidative insults. In Alzheimer's disease (AD), amyloid beta-peptide (A beta) fibrils accumulate in plaques which are associated with degenerating neurons. A beta can be neurotoxic by a mechanism that appears to involve induction of oxidative stress and disruption of calcium homeostasis. Plaques in AD brain contain high levels of bFGF suggesting a possible modulatory role for bFGF in the neurodegenerative process. We now report that bFGF can protect cultured hippocampal neurons against A beta25-35 toxicity by a mechanism that involves suppression of reactive oxygen species (ROS) accumulation and maintenance of Na+/K+-ATPase activity. A beta25-35 induced lipid peroxidation, accumulation of H2O2, mitochondrial ROS accumulation, and a decrease in mitochondrial transmembrane potential; each of these effects of A beta25-35 was abrogated in cultures pre-treated with bFGF. Na+/K+-ATPase activity was significantly reduced following exposure to A beta25-35 in control cultures, but not in cultures pre-treated with bFGF. bFGF did not protect neurons from death induced by ouabain (a specific inhibitor of the Na+/K+-ATPase) or 4-hydroxynonenal (an aldehydic product of lipid peroxidation) consistent with a site of action of bFGF prior to induction of oxidative stress and impairment of ion-motive ATPases. By suppressing accumulation of oxyradicals, bFGF may slow A beta-induced neurodegenerative cascades.

Elevated phosphocholine and phosphatidylcholine following rat entorhinal cortex lesions

At early stages of Alzheimer's disease, phosphomonoesters (PMEs) including phosphocholine (P-choline) are present at elevated levels. PMEs also are elevated in the developing brain during the period of neurite extension. To determine if the elevation of PMEs in AD could reflect neuritic sprouting, 31P-NMR was used to examine phospholipid metabolites and membrane phospholipids at various times following unilateral lesions of the entorhinal cortex, a well-defined model of neuritic sprouting. Two to 7 days postlesion, P-choline levels were elevated 48% in the hippocampus ipsilateral to the entorhinal cortex lesion, but not in the contralateral hippocampus or cerebral cortex. P-choline levels declined by day 15, and reached control levels 45 days following the lesion. The lesion-induced elevation in P-choline could result from increased P-choline synthesis via choline kinase, decreased activity of CTP:phosphocholine cytidylyltransferase, or breakdown of phosphatidylcholine (PC). To distinguish between these possibilities, the membrane phospholipids PC and phosphatidylethanolamine (PE) were measured. Both phospholipids were maintained at or above control levels at each of the postlesion time points, arguing against membrane breakdown or decreased PC synthesis contributing to the elevation of P-choline levels. Other alterations included a widespread elevation in inositol phosphate 2 days postlesion, but not at later time points. The alterations in phospholipid metabolites observed in the rat hippocampus following entorhinal cortex lesions closely resemble those observed in the human brain in the early stages of AD.

Lysophosphatidic acid decreases glutamate and glucose uptake by astrocytes

The brain is a rich source of the lipid biomediator lysophosphatidic acid, and lysophosphatidic acid levels can significantly increase following brain trauma. Responses of primary rat brain astrocytes to this novel lipid are defined in the current study. Treatment of cells with lysophosphatidic acid resulted in a time- and dose-dependent inhibition of glutamate uptake. Inhibition of glutamate uptake was specific because the related phospholipids, phosphatidic acid, lysophosphatidylcholine, and lysophosphatidylglycerol, did not inhibit this uptake under comparable conditions, i.e., treatment with 10 microM lipid for 30 min. Lysophosphatidic acid treatment of cells resulted in an increase in lipid peroxidation, as measured by the thiobarbituric acid assay. This increase in content of thiobarbituric acid-reactive substances was largely inhibited by treatment with dithiothreitol or propyl gallate; however, such treatment did not affect the lysophosphatidic acid-induced inhibition of glutamate uptake. Lysophosphatidic acid also inhibited glucose uptake with a dose-response curve that paralleled the inhibition of glutamate uptake. By impairing uptake of glutamate by astrocytes, lysophosphatidic acid may exacerbate excitotoxic processes in various neurodegenerative conditions.

Effects of intrahippocampal colchicine administration on the levels and localization of microtubule-associated proteins, tau and MAP2

Colchicine, a microtubule disrupting agent, has been used to model several aspects of Alzheimer's disease-related neuropathology. The formation of neurofibrillary tangles, one of the pathological hallmarks of Alzheimer's disease, involves the loss of tau (a low mol. wt. microtubule-associated protein) from axons and accumulation of abnormally phosphorylated tau in somatodendritic compartments. Other cytoskeletal proteins, such as microtubule-associated protein 2 (MAP2), disappear as tau accumulates. The present study was directed at evaluating the effects of colchicine on tau and MAP2, to determine if changes in their levels or distribution might be similar to those which precede the formation of neurofibrillary tangles in Alzheimer's disease. Six hours following intrahippocampal colchicine injection (3.5 micrograms injected into two rostro-caudal locations) tau-1 immunostaining was enhanced in CA1 s. radiatum and decreased in the outer molecular layer of the dentate gyrus. In addition, a shift in the relative abundance of tau isoforms was observed in Western blots. Both the immunocytochemical and immunoblot results are consistent with a dephosphorylation of tau. Loss of MAP2 was evident 3 days postinjection which coincided with a loss of Cresyl violet staining in granule cell, CA3, subicular and entorhinal neurons. Accumulation of tau or MAP2 in neuronal perikarya was not observed at any postinjection time points. Thus, intrahippocampal colchicine administration does not model the shift in tau localization, excessive tau phosphorylation, or other cytoskeletal alterations that are suggested to precede or accompany the formation of neurofibrillary pathology in Alzheimer's disease.