Neuroinflammatory processes are important in neurodegenerative diseases: an hypothesis to explain the increased formation of reactive oxygen and nitrogen species as major factors involved in neurodegenerative disease development

Nitrones, their value as therapeutics and probes to understand aging

The nitrone-based free radical traps have significant potential in the treatment of neurodegenerative diseases as well as in the prolongation of life span. The mass action free radical trapping activity of these compounds is the property, which first brought them to the attention of the scientific community. Nevertheless extensive research has demonstrated that these reactions are not responsible for their therapeutic mechanistic basis of activity. Rather the mechanism of action in the case of their neuroprotective activity appears to involve the inhibition of enhanced signal transduction processes that mediate the upregulation of genes, which produce neurotoxic products. The most widely used compound in this series, alpha-phenyl-tert-butyl-nitrone (PBN), has been shown to extend life span in three published studies, i.e. two mouse models and one rat model. Significant prolongation of life span was noted in all three studies. We report the summary of a recent study with a novel nitrone, CPI-1429, which demonstrated the ability to extend life span even though administration of the compound was begun in older animals. Despite these promising studies, much more rigorous research examining the anti-aging activity of the nitrones needs to be conducted. It is not known exactly why the nitrones possess anti-aging activity. They have been shown to quell enhanced signal transduction processes associated with enhanced pro-inflammatory cytokine mediated events. The nitrones interfere in some unknown steps preventing receptor triggered MAP kinase phosphorylation cascades. Stabilization of phosphorylation networks associated with checkpoint proteins could slow cell cycle processes and this could be the basis of the nitrones anti-senescent activity.

Nitrones as neuroprotectants and antiaging drugs

Specific nitrones have been used for more than 30 years in analytical chemistry and biochemistry to trap and stabilize free radicals for the purpose of their identification and characterization. PBN (alpha-phenyl-tert-butyl nitrone), one of the more widely used nitrones for this purpose, has been shown to have potent pharmacologic activities in models of a number of aging-related diseases, most notably the neurodegenerative diseases of stroke and Alzheimer's disease. Studies in cell and animal models strongly suggest that PBN has potent antiaging activity. A novel nitrone, CPI-1429, has been shown to extend the life span of mice when administration was started in older animals. It has also shown efficacy in the prevention of memory dysfunction associated with normal aging in a mouse model. Mechanistic studies have shown that the neuroprotective activity of nitrones is not due to mass-action free radical-trapping activity, but due to cessation of enhanced signal transduction processes associated with neuroinflammatory processes known to be enhanced in several neurodegenerative conditions. Enhanced neuroinflammatory processes produce higher levels of neurotoxins, which cause death or dysfunction of neurons. Therefore, quelling of these processes is considered to have a beneficial effect allowing proper neuronal functioning. The possible antiaging activity of nitrones may reside in their ability to quell enhanced production of reactive oxygen species associated with age-related conditions. On the basis of novel ideas about the action of secretory products formed by senescent cells on bystander cells, it is postulated that nitrones will mitigate these processes and that this may be the mechanism of their antiaging activity.

In utero bacterial endotoxin exposure causes loss of tyrosine hydroxylase neurons in the postnatal rat midbrain

We investigated whether in utero exposure to the Gram(-) bacteriotoxin lipopolysaccharide (LPS) induces dopamine (DA) neuron loss in rats. The proinflammatory cytokine tumor necrosis factor alpha (TNF-alpha) kills DA neurons and is elevated in the brains of patients with Parkinson's disease (PD). LPS is a potent inducer of TNF-alpha, and both are increased in the chorioamniotic environment of women who have bacterial vaginosis (BV) during pregnancy, suggesting that BV might interfere with the normal development of fetal DA neurons. Gravid female rats were injected intraperitoneally with either LPS or normal saline at embryonic day 10.5 and their pups were killed at postnatal day 21. The brains of the pups were assessed for DA and TNF-alpha levels and DA cell counts in the mesencephalon using tyrosine hydroxylase immunoreactive (THir) cells as a DA neuron marker. Prenatal LPS exposure significantly reduced striatal DA (29%) and increased DA activity (72%) as well as TNF-alpha (101%). Stereological cell counts in the mesencephalon were also significantly reduced (27%) by prenatal LPS exposure. Prenatal exposure to LPS, as might occur in humans with BV, produces a significant loss of THir cells in rats that is still present 33 days following a single injection of LPS. Since this cell loss is well past the normal phase of DA neuron apoptosis that occurs in early postnatal life, rats so exposed may have a permanent loss of DA neurons, suggesting that prenatal infections may represent risk factors for PD.

Indomethacin for the treatment of Alzheimer's disease patients

BACKGROUND

Inflammatory processes involving cytokines, prostaglandins, free radicals and glial cells have been implicated in the pathogenesis of Alzheimer's disease. Non-steroidal anti-inflammatory drugs such as indomethacin attenuate inflammatory reactions. Hence, there may be a role for some of these drugs in the treatment of Alzheimer's disease.

OBJECTIVES

To examine the efficacy of indomethacin in the treatment of patients suffering from Alzheimer's disease.

SEARCH STRATEGY

The trials were identified from a search of the Specialized Register of the Cochrane Dementia and Cognitive Improvement Group (which contains records from many different medical and trials databases) on 14 June 2001 using the terms indomethacin and indome* and NSAIDS. In addition two independent reviewers systematically searched relevant computerized databases and Internet sites. This was supplemented by hand searching and additional references sought from selected papers.

SELECTION CRITERIA

Single or multi-centre placebo-controlled randomized trials examining the efficacy of indomethacin in patients diagnosed with Alzheimer's disease were eligible for selection for this review. Using a standard extraction form, inclusion/exclusion criteria were set to ensure design quality and lack of bias of all trials included.

DATA COLLECTION AND ANALYSIS

Data were collected independently by two reviewers and any discrepancies were subject to discussion. Corresponding authors were contacted for any missing data needed for statistical analysis.

MAIN RESULTS

Only one study was selected for this review (~~Rogers 1993~~). We detected no statistically significant difference between indomethacin treatment and placebo for the individual cognitive tests, Mini Mental State Examination (MMSE), Alzheimer's Disease Assessment Scale ( ADAS), Boston Naming Test (BNT) and Token Test (TK). Dropouts and death rate were the only reported results that were amenable to evaluation. The dropout rate was higher in the indomethacin group (10/24) than in the control group (6/20). Gastrointestinal adverse events were more prevalent in the treatment group (5/24 compared with 1/20 in control group). There was no statistically significant difference in death rate between the two groups (p=0.9).

REVIEWER'S CONCLUSIONS

On the basis of this one trial and subsequent analysis of data as reported by the authors, indomethacin cannot be recommended for the treatment of mild to moderate severity Alzheimer's disease. At doses of 100-150 mg daily, serious side effects will limit its use.

Ischemic injury and faulty gene transcripts in the brain

The brain has the highest metabolic rate of all organs and depends predominantly on oxidative metabolism as a source of energy. Oxidative metabolism generates reactive oxygen species, which can damage all cellular components, including protein, lipids and nucleic acids. The processes of DNA repair normally remove spontaneous gene damage with few errors. However, cerebral ischemia followed by reperfusion leads to elevated oxidative stress and damage to genes in brain tissue despite a functional mechanism of DNA repair. These critical events occur at the same time as the expression of immediate early genes, the products of which trans-activate late effector genes that are important for sustaining neuronal viability. These findings open the possibility of applying genetic tools to identify molecular mechanisms of gene repair and to derive new therapies for stroke and brain injury.

Neuronal and glial coexpression of argininosuccinate synthetase and inducible nitric oxide synthase in Alzheimer disease

The enzyme argininosuccinate synthetase (ASS) is the rate limiting enzyme in the metabolic pathway leading from L-citrulline to L-arginine, the physiological substrate of all isoforms of nitric oxide synthases (NOS). ASS and inducible NOS (iNOS) expression in neurons and glia was investigated by immunohistochemistry in brains of Alzheimer disease (AD) patients and nondemented, age-matched controls. In 3 areas examined (hippocampus, frontal, and entorhinal cortex), a marked increase in neuronal ASS and iNOS expression was observed in AD brains. GFAP-positive astrocytes expressing ASS were not increased in AD brains versus controls, whereas the number of iNOS expressing GFAP-positive astrocytes was significantly higher in AD brains. Density measurements revealed that ASS expression levels were significantly higher in glial cells of AD brains. Colocalization of ASS and iNOS immunoreactivity was detectable in neurons and glia. Occasionally, both ASS-and iNOS expression was detectable in CD 68-positive activated microglia cells in close proximity to senile plaques. These results suggest that neurons and astrocytes express ASS in human brain constitutively, whereas neuronal and glial ASS expression increases parallel to iNOS expression in AD. Because an adequate supply of L-arginine is indispensable for prolonged NO generation, coinduction of ASS enables cells to sustain NO generation during AD by replenishing necessary supply of L-arginine.

Roles of the metallothionein family of proteins in the central nervous system

Metallothioneins (MTs) constitute a family of proteins characterized by a high heavy metal [Zn(II), Cu(I)] content and also by an unusual cysteine abundance. Mammalian MTs are comprised of four major isoforms designated MT-1 trough MT-4. MT-1 and MT-2 are expressed in most tissues including the brain, whereas MT-3 (also called growth inhibitory factor) and MT-4 are expressed predominantly in the central nervous system and in keratinizing epithelia, respectively. All MT isoforms have been implicated in disparate physiological functions, such as zinc and copper metabolism, protection against reactive oxygen species, or adaptation to stress. In the case of MT-3, an additional involvement of this isoform in neuromodulatory events and in the pathogenesis of Alzheimer's disease has also been suggested. It is essential to gain insight into how MTs are regulated in the brain in order to characterize MT functions, both in normal brain physiology, as well as in pathophysiological states. The focus of this review concerns the biology of the MT family in the context of their expression and functional roles in the central nervous system.

Hypoxia-ischemia in fetal rabbit brain increases reactive nitrogen species production: quantitative estimation of nitrotyrosine

Reactive nitrogen species (RNS) cause nitration of protein-bound tyrosine that is used as biomarker for detection. We hypothesized that RNS are formed in fetal rabbit brain following acute placental insufficiency. Near-term pregnant rabbits were randomized to either repetitive uterine ischemia or no ischemia, and fetal brains obtained. Only one electrochemical HPLC method (of three tested) was successful in detecting brain nitrotyrosine. Protein nitrotyrosine was significantly increased following cumulative 40 min ischemia and 20 min reperfusion compared to controls. Repetitive hypoxia-ischemia results in the increased formation of RNS in near-term fetal brains.

Oxidative biochemical markers; clues to understanding aging in long-lived species

Clues as to why long-lived species live so much longer than short-lived species may reside in the amount of reactive oxygen species (ROS) produced and their effect on damaging cell components (especially proteins) and alterations of crucial cellular processes. Rigorous evaluation of these concepts required critical comparisons of oxidative damage markers and/or parameters with assess difference in ROS flux and the critical age-modifying processes they influence. The limited experimental comparative results available implicate that ROS production per unit weight of total oxygen consumed is much less in the longer-lived species than in shorter-lived species. Mitochondria are the major site of ROS production. They are also the functional nexus for intracellular signaling thus modulating stress and growth factor mediated cellular survival, proliferation and apoptotic processes. Mitochondrial DNA mutations, perhaps caused by ROS, increase with age. Mutant mitochondria possess comparative replicative advantage, which leads to age-specific intracellular swarms. General inflammatory stress tends to increase with age. Disruption in coordinated cell-to-cell signaling triggered by alterations in intracellular signaling may be the basis of the age-related increases in tissue inflammation, which may explain some of the differences between long-lived species and short-lived species.

Repression of proinflammatory cytokine and inducible nitric oxide synthase (NOS2) gene expression in activated microglia by N-acetyl-O-methyldopamine: protein kinase A-dependent mechanism

Excessive proinflammatory cytokine and NO production by activated microglia play a role in neurodegenerative disorders. To investigate whether the neuroprotectant N-acetyl-O-methyldopamine (NAMDA) downregulates genes associated with microglial activation, we measured gene expression of TNF-alpha, IL-1beta, inducible nitric oxide synthase (NOS2), and an associated cofactor synthesis gene, GTP cyclohydrolase I (GTPCH) in LPS-stimulated microglia cells in the presence or absence of NAMDA. The temporal pattern of cytokine gene expression showed that LPS (0.2 microg/ml) increased TNF-alpha and IL-1beta gene expression at 1 and 3 h, which was repressed by cotreatment of NAMDA. Similarly, LPS also induced GTPCH and NOS2 gene expression at 3 and 6 h, and cotreatment of NAMDA repressed the induction with parallel reduction of nitrite, an oxidative metabolite of nitric oxide. Since transcription factor NF-kappaB is involved in regulating expression of these genes, the effects of NAMDA on NF-kappaB nuclear translocation and DNA binding in immunostimulated microglia were investigated. We found that neither LPS-induced NF-kappaB translocation nor DNA binding activity was affected by cotreatment with NAMDA in BV-2 microglia. On the other hand, NAMDA increased intracellular cAMP levels and potentiated LPS-induced phosphorylated cAMP-responsive element binding protein (pCREB) expression. Treatment with adenosine 3'5'-cyclic monophosphothioate, a specific inhibitor of cAMP-dependent protein kinase (PKA), reversed not only NAMDA-induced pCREB upregulation but also NAMDA-induced repression of TNF-alpha and IL-1beta gene transcription. The data demonstrate that NAMDA represses LPS-induced proinflammatory cytokines gene expression via a cAMP-dependent protein kinase pathway. Thus, repressing proinflammatory cytokines and NOS2 gene expression in activated microglia by NAMDA may provide new therapeutic strategies for ischemic cerebral disease as well as other neurodegenerative diseases.

The mitochondrial spiral. An adequate cause of dementia in the Alzheimer's syndrome

A variety of chronic, relatively low-grade injuries to the brain occur in Alzheimer's disease (AD). The extent to which each of these contributes to the clinical syndrome is unclear. Several of the abnormalities that occur in AD brain can cause dementia by themselves, even in people who do not have the neuropathological hallmarks of AD. Prominent among these abnormalities is a deleterious "mitochondrial spiral," which consists of reduced brain metabolism, oxidative stress, and calcium dysregulation. The hypothesis presented in this paper is that the mitochondrial spiral contributes to dementia in AD and presents a reasonable target for the development of new approaches to the treatment of this syndrome.

Association between polymorphism in regulatory region of gene encoding tumour necrosis factor alpha and risk of Alzheimer's disease and vascular dementia: a case-control study

BACKGROUND

Deposition of beta-amyloid in the brains of patients with Alzheimer's disease is thought to precede a chain of events that leads to an inflammatory response by the brain. We postulated that genetic variation in the regulatory region of the gene for the proinflammatory cytokine tumour necrosis factor alpha (TNF-alpha) leads to increased risk of Alzheimer's disease and vascular dementia.

METHODS

A polymorphism in the regulatory region of the TNF-alpha gene was analysed in a case-control study. The polymorphism (C-850T) was typed in 242 patients with sporadic Alzheimer's disease, 81 patients with vascular dementia, 61 stroke patients without dementia, and 235 normal controls. These groups of individuals were also genotyped for the apolipoprotein E polymorphism, and the vascular dementia and stroke groups were typed at the HLA-DR locus.

FINDINGS

The distribution of TNF-alpha genotypes in the vascular dementia group differed significantly from that in the stroke and normal control groups, giving an odds ratio of 2.51 (95% CI 1.49-4.21) for the development of vascular dementia for individuals with a CT or TT genotype. Logistic regression analysis indicated that the possession of the T allele significantly increased the risk of Alzheimer's disease associated with carriage of the apolipoprotein E epsilon4 allele (odds ratio 2.73 [1.68-4.44] for those with apolipoprotein E epsilon4 but no TNF-alpha T, vs 4.62 [2.38-8.96] for those with apolipoprotein E epsilon4 and TNF-alpha T; p=0.03).

INTERPRETATION

Possession of the TNF-alpha T allele significantly increases the risk of vascular dementia, and increases the risk of Alzheimer's disease associated with apolipoprotein E. Although further research is needed, these findings suggest a potential role for anti-inflammatory therapy in vascular dementia and Alzheimer's disease, and perhaps especially in patients who have had a stroke.

Nitrone inhibition of age-associated oxidative damage

The mechanistic basis of the neuroprotective activity of the nitrone-based free radical trap PBN (alpha-phenyl-N-tert-butyl nitrone) has been investigated extensively. Key observations exclude its simple mass action spin trapping of free radicals activity as the key mechanism of action. These include: A) the fact that it protects in experimental stroke even if administered several hours after the event and B) the fact that its chronic low-level administration to old experimental animals reverses their age-enhanced susceptibility to stroke even several days after the last dosage. PBN was found to inhibit gene induction in several models including stroke and an LPS-mediated septic shock model. Stoke causes inducible nitric oxide synthase (iNOS) to be expressed. High levels of nitric oxide and peroxynitrite (formed from nitric oxide), produced by iNOS, is particularly neurotoxic. PBN inhibits iNOS induction. Therefore, it seems that prevention of the formation of neurotoxic products is a rational mechanism of action of PBN in the stroke model. There is strong rationale to consider that there is an enhanced propensity for a "smoldering" neuro-inflammatory state in the old brain. Reversal of this state by PBN may explain its action in preventing age-enhanced stroke susceptibility in old experimental animals. Significant new findings underscore the importance of neuro-inflammatory processes in neuronal death or dysfunction in Alzheimer's disease. Neuro-inflammatory processes implicate enhanced signal transduction processes. Strong evidence for this is the enhanced p38 kinase activation in neurons near plaques and tangles of the Alzheimer's brain in contrast to normal aged-matched control brain which did not show p38 activation. In rat primary astrocytes p38 activation by the pro-inflammatory cytokine IL-1 beta, as well as by H2O2, was significantly suppressed by PBN. Mechanistically it was shown that PBN suppresses the amount of reactive oxygen species (ROS) produced in mitochondrial respiration. Much evidence indicates that ROS are signaling molecules and that they also are involved to maintaining brain phosphatases in an inactive state. We argue that finding a specific high affinity site mechanism for the neuroprotective action of PBN is unlikely based on the complexity of the system reflecting ROS generation and signal transduction processes that have apparently evolved to maintain adaptive responses. The promising pharmacological activity of molecules like PBN is not diminished by this however, for only excessive amounts of ROS is considered detrimental. The action of PBN in suppressing signal transduction processes, most likely by suppressing ROS production in mitochondrial respiration, effectively controls excessive oxidative damage and prevents induction of genes that form neurotoxic products.

Alzheimer's disease: A hypothesis on pathogenesis

Alzheimer's disease (AD) is the major cause of dementia. It is a systemic disorder whose major manifestations are in the brain. AD cases can be categorized into two groups on the basis of the age of onset-before or after about age 60. The majority of cases, 90-95 percent, are in the late onset category. Early onset cases are largely, if not all, familial (FAD). These are caused by mutations in the genes for the amyloid precursor protein (APP), presenilin 1 (PS1), and presenilin 2 (PS2). In contrast late onset cases are mainly sporadic. The disorder is characterized by intraneuronal fibrillary tangles, plaques, and cell loss. The brain lesions in both early and late-onset AD are the same, and in the same distribution pattern, as those seen in individuals with Down's syndrome (DS) and in smaller numbers in normal older individuals. Extensive studies of AD have yet to result in a generally accepted hypothesis on the pathogenesis of the disorder. Major emphasis has been placed on the role of amyloid, the neurotoxin formed by the action of free radicals on preamyloid. The observation that AD lesions are frequently present in normal older individuals prompted the hypothesis that AD is the result of faster than normal aging of the neurons associated with it. This hypothesis provides plausible explanations for FAD and AD. FAD is associated with mutations in APP, PS1, and PS2. These substances, along with their normal counterparts, undergo proteolytic processing in the endoplasmic reticulum (ER). The mutated compounds, aside from increasing the ratio of βA42 to βA40, may down-regulate the calcium buffering activity of the ER in a manner akin to one or more of the many compounds known to do so. Decreases in the ER calcium pool would cause compensatory increases in other calcium pools, particularly in mitochondria. Increases in mitochondrial calcium levels are associated with enhanced formation of superoxide radical formation, and hence of the rate of aging. SAD may be caused by nuclear and/or mitochondrial DNA mutations beginning early in life that enhance mitochondrial superoxide radical formation in the neurons associated with the disorder. The above explanations for FAD and AD are suggestive of measures to prevent and for treatment.

Peroxynitrite-induced secondary oxidative lesions at guanine nucleobases: chemical stability and recognition by the Fpg DNA repair enzyme

Synthetic oligodeoxynucleotides containing secondary oxidative lesions at guanine nucleobases have been prepared by the site-specific oxidation by ONOO(-) of oligomers containing 8-oxoguanine (8-oxo-G). The oligomers have been tested for their stability to the standard hot piperidine treatment that is commonly used to uncover oxidized DNA lesions. While DNA containing oxaluric acid and oxazolone was cleaved at the site of modification under hot piperidine conditions, the corresponding cyanuric acid and 8-oxo-G lesions were resistant to piperidine. The recognition of the oxidative lesions by formamidopyrimidine glycosylase (Fpg enzyme) was examined in double-stranded versions of the synthetic oligodeoxynucleotides. Fpg efficiently excised 8-oxo-G and oxaluric acid and to some extent oxazolone, but not cyanuric acid. These data suggest that some DNA lesions formed via ONOO(-) exposures (cyanuric acid) are not repaired by Fpg and are not uncovered by assays based on piperidine cleavage at the site of lesion. Our results indicate that cryptic secondary and tertiary oxidation products arising from 8-oxo-G may contribute to the overall mutational spectra arising from oxidative stress.

Evidence for enhanced neuro-inflammatory processes in neurodegenerative diseases and the action of nitrones as potential therapeutics

A brief review is presented on observations leading to the current notions regarding neuro-inflammatory processes. The greatest focus is on Alzheimer's disease (AD) since this is where the most convincing data has been obtained. A brief summary of observations on the neuroprotective action of alpha-phenyl-tert-butyl-nitrone (PBN) as well as results of research designed to understand its mechanism of action is presented. We hypothesize that the mechanism of action of PBN involves inhibition of signal transduction processes, which are involved in the upregulation of genes mediated by pro-inflammatory cytokines and H2O2 that cause formation of toxic gene products. Results from recent experiments on Kainic acid (KA) mediated brain damage are provided to suggest the validity of the in vivo action of PBN to inhibit neuro-inflammatory processes. The accumulating scientific facts are helping to provide concepts that may become the basis for novel therapeutic approaches to the treatment of several neurodegenerative diseases.