Glycated tau protein in Alzheimer disease: a mechanism for induction of oxidant stress

Role of oxidative stress in neurodegeneration: recent developments in assay methods for oxidative stress and nutraceutical antioxidants

Reactive oxygen species (ROS) are produced in the course of normal metabolism and they serve important physiological functions. However, because of their high reactivity, accumulation of ROS beyond the immediate needs of the cell may affect cellular structure and functional integrity, by bringing about oxidative degradation of critical molecules, such as the DNA, proteins, and lipids. Although cells possess an intricate network of defense mechanisms to neutralize excess ROS and reduce oxidative stress, some tissues, especially the brain, are much more vulnerable to oxidative stress because of their elevated consumption of oxygen and the consequent generation of large amounts of ROS. For the same reason, the mitochondrial DNA (mtDNA) of brain cells is highly susceptible to structural alterations resulting in mitochondrial dysfunction. Several lines of evidence strongly suggest that these effects of ROS may be etiologically related to a number of neurodegenerative disorders. Nutraceutical antioxidants are dietary supplements that can exert positive pharmacological effects on specific human diseases by neutralizing the negative effects of ROS. The present communication concentrates on a review of recent concepts and methodological developments, some of them based on the results of work from our own laboratory, on the following aspects: (1) the complex interactions and complementary interrelationships between oxidative stress, mitochondrial dysfunction, and various forms of neural degeneration; (2) fractionation and isolation of substances with antioxidant properties from plant materials, which are extensively used in the human diet and, therefore, can be expected to be less toxic in any pharmacological intervention; (3) recent developments in methodologies that can be used for the assay of oxidative stress and determination of biological activities of exogenous and endogenous antioxidants; and (4) presentation of simple procedures based on polymerase chain reaction (PCR) and restriction fragment length polymorphism (RFLP) of the resulting amplicon for investigations of structural alterations in mtDNA.

Nonenzymatic Glycation at the N Terminus of Pathogenic Prion Protein in Transmissible Spongiform Encephalopathies

Transmissible spongiform encephalopathies (TSEs) are transmissible neurodegenerative diseases characterized by the accumulation of an abnormally folded prion protein, termed PrPSc, and the development of pathological features of astrogliosis, vacuolation, neuronal cell loss, and in some cases amyloid plaques. Although considerable structural characterization of prion protein has been reported, neither the method of conversion of cellular prion protein, PrPC, into the pathogenic isoform nor the post-translational modification processes involved is known. We report that in animal and human TSEs, one or more lysines at residues 23, 24, and 27 of PrPSc are covalently modified with advanced glycosylation end products (AGEs), which may be carboxymethyl-lysine (CML), one of the structural varieties of AGEs. The arginine residue at position 37 may also be modified with AGE, but not the arginine residue at position 25. This result suggests that nonenzymatic glycation is one of the post-translational modifications of PrP(Sc). Furthermore, immunostaining studies indicate that, at least in clinically affected hamsters, astrocytes are the first site of this glycation process.

Role of hydrogen peroxide in the aetiology of Alzheimer's disease: implications for treatment

Hydrogen peroxide (H(2)O(2)) is a stable, uncharged and freely diffusable reactive oxygen species (ROS) and second messenger. The generation of H(2)O(2) in the brain is relatively high because of the high oxygen consumption in the tissue. Alzheimer's disease is a neurodegenerative disorder characterised by the appearance of amyloid-beta (Abeta)-containing plaques and hyperphosphorylated tau-containing neurofibrillary tangles. The pathology of Alzheimer's disease is also associated with oxidative stress and H(2)O(2) is implicated in this and the neurotoxicity of the Abeta peptide. The ability for Abeta to generate H(2)O(2), and interactions of H(2)O(2) with iron and copper to generate highly toxic ROS, may provide a mechanism for the oxidative stress associated with Alzheimer's disease. The role of heavy metals in Alzheimer's disease pathology and the toxicity of the H(2)O(2) molecule may be closely linked. Drugs that prevent oxidative stress include antioxidants, modifiers of the enzymes involved in ROS generation and metabolism, metal chelating agents and agents that can remove the stimulus for ROS generation. In Alzheimer's disease the H(2)O(2) molecule must be considered a therapeutic target for treatment of the oxidative stress associated with the disease. The actions of H(2)O(2) include modifications of proteins, lipids and DNA, all of which are effects seen in the Alzheimer's disease brain and may contribute to the loss of synaptic function characteristic of the disease. The effectiveness of drugs to target this component of the disease pathology remains to be determined; however, metal chelators may provide an effective route and have the added bonus in the case of clioquinol of potentially reducing the Abeta load. Future research and development of agents that specifically target the H(2)O(2) molecule or enzymes involved in its metabolism may provide the future route to Alzheimer's disease therapy.

Histochemical accumulation of oxidative damage products is associated with Alzheimer-like pathology in the canine

An important lesion in Alzheimer's disease (AD) patient brains is the neurofibrillary tangle (NFT). Hyperphosphorylated tau is its major component. In a former paper we described some NFT in the canine brain. During aging, moreover, advanced glycation end products (AGE) might accumulate. Glycated tau induces lipid peroxidation in vivo and tau and AGE antigens have been mentioned to co-localize in NFT. This indicates that AGE may play an important role in Alzheimer disease (AD) by oxidation of tau. The aim of the present study was to investigate amyloid, neurofibrillary tangles, Abeta precursor protein, Abeta, tau, ubiquitin, advanced glycation end products, 4-hyroxynonenal protein and lipofuscin in a series of dogs of varying ages. The results showed a significant positive correlation between age and amyloid quantity (Congo red staining), HNE staining and lipofuscin (LF), and between amyloid quantity and HNE staining and LF. Staining for AbetaPP seemed to have a tendency to increase with age, whereas staining for tau, ubiquitin and AGE each only gave limited positive results in a proportion of the older dogs. Preliminary studies including loss of cognitive capabilities in the older dogs and chemical measurement of lipofuscin-like pigment (LFP) accumulation in brain extracts revealed an increase with old age and dementia. The Congo red, HNE and LF results suggest that deposition of amyloid with aging might be associated with formation of end products of lipid peroxidation. The finding of the limited positive signals for tau, ubiquitin and AGE in some old cases might indicate that the spontaneous brain pathology of the aged dog reveals similarities to early stages observed in AD in humans especially those with Down syndrome.

Role for glyoxalase I in Alzheimer's disease

P301L mutant tau transgenic mice develop neurofibrillary tangles, a histopathologic hallmark of Alzheimer's disease and frontotemporal dementia (FTDP-17). To identify differentially expressed genes and to gain insight into pathogenic mechanisms, we performed a stringent analysis of the microarray dataset obtained with RNA from whole brains of P301L mutant mice and identified a single up-regulated gene, glyoxalase I. This enzyme plays a critical role in the detoxification of dicarbonyl compounds and thereby reduces the formation of advanced glycation end products. In situ hybridization analysis revealed expression of glyoxalase I in all brain areas analyzed, both in transgenic and control mice. However, levels of glyoxalase I protein were significantly elevated in P301L brains, as shown by Western blot analysis and immunohistochemistry. Moreover, a glyoxalase I-specific antiserum revealed many intensely stained flame-shaped neurons in Alzheimer's disease brain compared with brains from nondemented controls. In addition, we examined a single nucleotide polymorphism predicting a nonconservative amino acid substitution at position 111 (E111A) in ethnically independent populations. We identified significant and consistent deviations from Hardy-Weinberg equilibrium, which points to the presence of selection forces. The E111A single nucleotide polymorphism was not associated with the risk for Alzheimer's disease in the overall population. Together, our data demonstrate the potential of transcriptomics applied to animal models of human diseases. They suggest a previously unidentified role for glyoxalase I in neurodegenerative disease.

Role of tau protein in both physiological and pathological conditions

The morphology of a neuron is determined by its cytoskeletal scaffolding. Thus proteins that associate with the principal cytoskeletal components such as the microtubules have a strong influence on both the morphology and physiology of neurons. Tau is a microtubule-associated protein that stabilizes neuronal microtubules under normal physiological conditions. However, in certain pathological situations, tau protein may undergo modifications, mainly through phosphorylation, that can result in the generation of aberrant aggregates that are toxic to neurons. This process occurs in a number of neurological disorders collectively known as tauopathies, the most commonly recognized of which is Alzheimer's disease. The purpose of this review is to define the role of tau protein under normal physiological conditions and to highlight the role of the protein in different tauopathies.

Expression of nitric oxide system in clinically evaluated cases of Alzheimer's disease

The expression of neuronal nitric oxide (nNOS) and inducible nitric oxide (iNOS) as isoforms of the nitric oxide synthase (NOS) as well as nitrotyrosine as an end product of protein nitration was analyzed in sections of temporal cortex taken from postmortem brains of patients with Alzheimer's disease (AD). The patients were evaluated by the Clinical Dementia Rating scale (CDR0-CDR3) and studied in the Memory and Aging Project (MAP) of the Washington University Alzheimer Disease Research Center (ADCR). With the use of immunocytochemical procedures, neurons immunoreactive to nNOS were found to show large and small multipolar and pyramidal morphologies over the entire chronic AD evolution. The iNOS and nitrotyrosine immunoreactivities were also found in pyramidal-like cortical neurons and glial cells. Here, we speculate on the interaction among all specific neurodegenerative changes in AD and nitric oxide as an additional contribution to neuronal death in AD.

Will preventing protein aggregates live up to its promise as prophylaxis against neurodegenerative diseases?

Protein aggregation and misfolding characterize most age-related neurodegenerative diseases including Alzheimer, Parkinson and Huntington diseases. Protein aggregation has generally been assumed to be responsible for neurodegeneration in these disorders due to association and genetics. However, protein aggregation may, in fact, be an attempt to protect neurons from the stress resulting from the disease etiology. In this review, we weigh the evidence of whether removal of amyloids, aggregates and neuronal inclusions represent a reasonable strategy for protecting neurons.

The contribution of mitochondria to common disorders

Mitochondrial dysfunction secondary to mitochondrial and nuclear DNA mutations has been associated with energy deficiency in multiple organ systems and a variety of severe, often fatal, clinical syndromes. Although the production of energy is indeed the primary function of mitochondria, attention has also been directed toward their role producing reactive oxygen and nitrogen species and the subsequent widespread deleterious effects of these intermediates. The generation of toxic reactive intermediates has been implicated in a number of relatively common disorders, including neurodegenerative diseases, diabetes, and cancer. Understanding the role mitochondrial dysfunction plays in the pathogenesis of common disorders has provided unique insights into a number of diseases and offers hope for potential new therapies.

Hydroxynonenal, toxic carbonyls, and Alzheimer disease

Cytoskeletal disruption is one of the distinguishing characteristics of the vulnerable neurons in Alzheimer disease (AD). It has been suggested that these cytoskeletal changes occur secondarily to covalent modifications of the protein components. Despite the abundance and probable importance of these changes, there has been very little data regarding the identity of the modified proteins or the precise chemistry of the modifications. Here we review a specific type of modification, namely carbonylation of proteins, which has been shown to be a common result of cellular oxidative stress. Hopefully, the following discussion will help elucidate the relationship between oxidative stress, protein modification and the pathogenesis of AD.

Quantification of antioxidant activity in brain tissue homogenates using the 'total equivalent antioxidant capacity'

We demonstrate herein that the standard 'Trolox equivalent antioxidant capacity' (TEAC) assay, typically utilized to quantify total antioxidant levels within plasma, can also be utilized for tissue homogenates. Normal mice and transgenic mice lacking apolipoprotein E were subjected to a diet including iron as a generic pro-oxidant for 1 month (which has been shown to induce oxidative damage in our prior studies) and homogenates of brain tissue were subjected to the TEAC assay. Levels of the endogenous antioxidant glutathione levels were also monitored by HPLC. As described previously, ApoE-deficient mice expressed increased levels of glutathione; total antioxidant levels, as determined by TEAC, were increased to a similar extent. The increase in total antioxidant levels, as determined by TEAC, following dietary iron challenge paralleled the increase in glutathione levels, as determined by HPLC. These findings indicate that the TEAC assay may be useful for tissue homogenates. The rapid nature of this assay compared to HPLC, coupled with its lack of requirement for sophisticated equipment, makes it well suited for analyses of multiple tissue samples.

Neurotoxicity of acetaldehyde-derived advanced glycation end products for cultured cortical neurons

The Maillard reaction that leads to the formation of advanced glycation end products (AGEs) plays an important role in the pathogenesis of angiopathy in diabetic patients, in aging, and in neurodegenerative processes. We hypothesize that acetaldehyde (AA), one of the main metabolites of alcohol, may be involved in alcohol-induced neurotoxicity in vivo by formation of AA-derived AGEs (AA-AGEs) with brain proteins. Incubation of cortical neurons with AA-AGE produced a dose-dependent increase in neuronal cell-death, and the neurotoxicity of AA-AGE was neutralized by the addition of an anti-AA-AGE-specific antibody, but not by anti-N-ethyllysine (NEL) antibody. The AA-AGE epitope was detected in human brain of alcoholism. We propose that the structural epitope AA-AGE is an important toxic moiety for neuronal cells in alcoholism.

Proteasome inhibition by paired helical filament-tau in brains of patients with Alzheimer's disease

Alzheimer's disease (AD) is characterized neuropathologically by intracellular neurofibrillary tangles (NFTs) formed of tau-based paired helical filaments (PHFs) and extracellular beta-amyloid plaques. The degree of Alzheimer dementia correlates with the severity of PHFs and NFTs. As an intraneuronal accumulation of oxidatively damaged proteins has been found in the brains of patients with AD, a dysfunction of the proteasomal system, which degrades damaged proteins, has been assumed to cause protein aggregation and therefore neurodegeneration in AD. In this study, we revealed that such proteasome dysfunction in AD brain results from the inhibitory binding of PHF-tau to proteasomes. We analysed the proteasome activity in brains from patients with AD and age-matched controls, and observed a significant decrease to 56% of the control level in the straight gyrus of patients with AD. This loss of activity was not associated with a decrease in the proteasome protein. PHF-tau co-precipitated during proteasome immunoprecipitation and proteasome subunits could be co-isolated during isolation of PHFs from AD brain. Furthermore, the proteasome activity in human brains strongly correlated with the amount of co-precipitated PHF-tau during immunoprecipitation of proteasome. Incubation of isolated proteasomes with PHF-tau isolated from AD brain, and with PHFs after in vitro assembly from human recombinant tau protein, resulted in a distinct inhibition of proteasome activity by PHF-tau. As this inhibition of proteasome activity was sufficient to induce neuronal degeneration and death, we suggest that PHF-tau is able directly to induce neuronal damage in the AD brain.

Is oxidative damage the fundamental pathogenic mechanism of Alzheimer's and other neurodegenerative diseases?

In less than a decade, beginning with the demonstration by Floyd, Stadtman, Markesbery et al. of increased reactive carbonyls in the brains of patients with Alzheimer's disease (AD), oxidative damage has been established as a feature of the disease. Here, we review the types of oxidative damage seen in AD, sites involved, possible origin, relationship to lesions, and compensatory changes, and we also consider other neurodegenerative diseases where oxidative stress has been implicated. Although much data remain to be collected, the broad spectrum of changes found in AD are only seen, albeit to a lesser extent, in normal aging with other neurodegenerative diseases showing distinct spectrums of change.

Differential expression of phosphorylated translation initiation factor 2 alpha in Alzheimer's disease and Creutzfeldt-Jakob's disease

Studies in vitro have shown that phosphorylated translation initiation factor 2 alpha (TIF 2 alpha) may have several functions, including regulation of protein synthesis, control of cell death and procurement of resistance to oxidative stress in nerve cells. These properties may have implications in certain human neurodegenerative diseases, such as Alzheimer's disease (AD) and Creutzfeldt-Jakob's disease (CJD), in which oxidative stress appears to be involved in the process of neurodegeneration and neurone death. Single and double-labelling immunohistochemistry to phosphorylated TIF 2 alpha, phosphorylated SAPK/JNK, phosphorylated p38, tau, Cu/Zn superoxide dismutase 1 (SOD 1) and cleaved caspase-3 (17 kDa), and in situ end-labelling of nuclear DNA fragmentation, was carried out in postmortem samples of 10 patients with AD (stages III and VI of Braak and Braak), seven patients with CJD (five cases with methionine/methionine and two cases with methionine/valine at the codon 129 of the PrP gene) and eight age-matched controls. No phosphorylated TIF 2 alpha immunoreactivity was found in control brains, but strong phosphorylated TIF 2 alpha expression was observed in subpopulations of neurones bearing neurofibrillary tangles (NFTs) or pretangles in the hippocampus, entorhinal cortex and isocortex in AD. Phosphorylated TIF 2 alpha is restricted to neurones with abnormal tau deposition, but only approximately 80% of neurones with NFTs in the hippocampus and 60% in the isocortex colocalize phosphorylated TIF 2 alpha, thus indicating that not all neurones with NFTs over-express phosphorylated TIF 2 alpha. Moreover, phosphorylated TIF 2 alpha immunoreactivity was found in a percentage of neurones expressing phosphorylated SAPK/JNK and p38, which, in turn, are involved in tau phosphorylation in AD. However, dystrophic neurites of senile plaques that contain abnormal tau and express SOD 1 are negative to antiphosphorylated TIF 2 alpha antibodies. Smooth muscle cells in blood vessels affected by amyloid angiopathy, which are putative targets of beta A 4 amyloid-derived oxidative stress, are not associated with phosphorylated TIF 2 alpha immunoreactivity. Double-staining with the method of in situ end-labelling of nuclear DNA fragmentation demonstrated no relationship between phosphorylated TIF 2 alpha expression and increased nuclear DNA vulnerability in individual cells. Moreover, no single caspase-3-immunoreactive cell in AD expressed phosphorylated TIF 2 alpha. Oxidative stress response, manifested as positive SOD 1 expression in Bergmann glia and in a few reactive astrocytes, has been demonstrated in CJD. No phosphorylated SAPK/JNK or phosphorylated p38 kinase immunoreactivity was observed in these cases. Moreover, neurones and glial cells do not over-express phosphorylated TIF 2 alpha in CJD. The present results demonstrate selective expression of phosphorylated TIF 2 alpha in subpopulations of nerve cells with abnormal tau deposition, and suggest that factors linked with tau deposition regulate protein synthesis throughout TIF 2 alpha phosphorylation in certain neurones sensitive to oxidative stress in AD.

Folate quenches oxidative damage in brains of apolipoprotein E-deficient mice: augmentation by vitamin E

We demonstrate that folate and vitamin E can compensate for the diminished oxidative buffering capacity of brains of apolipoprotein E-deficient mice. Normal and ApoE(tmlUne) homozygous 'knockout' mice were maintained for 1 month on a diet either lacking or supplemented with folate, vitamin E or iron as a pro-oxidant after which brain tissue was harvested and analyzed for for thiobarbituric acid-reactive substances (TBARs) as an index of oxidative damage. Normal mice exhibited no significant difference in TBARs following iron challenge in the presence or absence of vitamin E, folic acid or both. Similarly, ApoE knockout mice exhibited no significant differences following dietary iron challenge in the presence or absence of vitamin E. However, ApoE knockout mice accumulated significantly increased TBARs following iron challenge when folic acid was withheld, and accumulated even more TBARs when both folic acid and vitamin E were withheld. These findings demonstrate that ApoE knockout mice during vitamin deficiency are less capable of buffering the consequences of dietary iron challenge than are normal mice. Since the apolipoprotein E4 allele, which exhibits diminished oxidative buffering capacity, is linked to Alzheimer's disease (AD), these data underscore the possibility that critical nutritional deficiencies may modulate the impact of genetic compromise on neurodegeneration in AD.

Hemin-induced apoptosis in PC12 and neuroblastoma cells: implications for local neuronal death associated with intracerebral hemorrhage

The exact pathogenesis of neuronal death following bleeding in brain parenchyma is still unknown. Hemoglobin (Hb) toxicity has been postulated to be one of the underlying mechanisms. The purpose of this study was to examine the possible contribution to neurotoxicity of each of the Hb compounds and to characterize the death pathway. Pheochromocytoma (PC12) and neuroblastoma (SH- SY5Y) cell lines were exposed to Hb, globin, hemin, protoporphyrin IX and iron for 1.5- 24 h. We found that Hb and hemin are highly toxic (LD(50) of 8 and 20 &mgr; mol/l, respectively) as compared to globin that was not toxic. In addition, protoporphyrin IX and iron, compounds of hemin, were less toxic than hemin itself (LD(50) of 962 and 2070 &mgr; mol/l respectively). We also demonstrated that non-specific protein digestion with proteinase-K, markedly increased Hb toxicity. Hemin-treated cells caused a typical apoptotic cell death pattern as indicated by DNA fragmentation, caspase activation and reduction in the mitochondrial membrane potential. Treatment with the antioxidant N-acetyl-L-cysteine or iron chelator, deferoxamine, diminished hemin-induced cell death, indicating a role of oxidative stress in this deleterious process. Thus, therapeutic strategies, based on antioxidant, iron chelator and anti-apoptotic agents may be effective in counteracting Hb neurotoxicity.

Antioxidants and Alzheimer's disease: from bench to bedside (and back again)

PURPOSE OF REVIEW

Accumulating evidence from both animal and human studies indicates a major role for oxidative damage in the pathogenesis of Alzheimer's disease, occurring even before symptoms arise and both beta-amyloid-containing plaques and neurofibrillary tangles are formed. This raises the possibility of preventing, or at least slowing down, the progression of Alzheimer's disease by the use of antioxidants. In this review, we present recent studies on the association between oxidative stress and Alzheimer's disease pathology, and on the efficacy of dietary, exogenous antioxidants to prevent or attenuate the progression of Alzheimer's disease.

RECENT FINDINGS

Recent prospective studies have indicated that dietary intake of several exogenous antioxidants is associated with a lower risk for Alzheimer's disease. This suggests that people at risk for developing Alzheimer's disease or being in the early phases of this disease may benefit from intervention with exogenous antioxidants. The clinical studies carried out so far, however, do not provide the final answer to whether antioxidants are truly protective against Alzheimer's disease.

SUMMARY

There is compelling evidence that oxidative stress is involved in Alzheimer's disease pathogenesis, and several lines of evidence indicate that administration of antioxidants may be useful in prevention and treatment of Alzheimer's disease. Further clinical studies, based on larger cohorts studied over a longer period of time, are needed, however, to test this hypothesis. Furthermore, for the future one might expect balanced upregulation of both exogenous and endogenous antioxidants as one of the best treatment strategies for preventing or at least slowing down the progression of Alzheimer's disease.

Antioxidants as treatment for neurodegenerative disorders

Oxidative stress is a ubiquitously observed hallmark of neurodegenerative disorders. Neuronal cell dysfunction and cell death due to oxidative stress may causally contribute to the pathogenesis of progressive neurodegenerative disorders, such as Alzheimer's disease and Parkinson's disease, as well as acute syndromes of neurodegeneration, such as ischaemic and haemorrhagic stroke. Neuroprotective antioxidants are considered a promising approach to slowing the progression and limiting the extent of neuronal cell loss in these disorders. The clinical evidence demonstrating that antioxidant compounds can act as protective drugs in neurodegenerative disease, however, is still relatively scarce. In the following review, the available data from clinical, animal and cell biological studies regarding the role of antioxidant neuroprotection in progressive neurodegenerative disease will be summarised, focussing particularly on Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis. The general complications in developing potent neuroprotective antioxidant drugs directed against these long-term degenerative conditions will also be discussed. The major challenges for drug development are the slow kinetics of disease progression, the unsolved mechanistic questions concerning the final causalities of cell death, the necessity to attain an effective permeation of the blood-brain barrier and the need to reduce the high concentrations currently required to evoke protective effects in cellular and animal model systems. Finally, an outlook as to which direction antioxidant drug development and clinical practice may be leading to in the near future will be provided.

Iron (III) induces aggregation of hyperphosphorylated tau and its reduction to iron (II) reverses the aggregation: implications in the formation of neurofibrillary tangles of Alzheimer's disease

Iron as well as aluminum is reported to accumulate in neurons with neurofibrillary tangles (NFTs) of Alzheimer's disease (AD) brain. Previously we demonstrated that aluminum (III) shows phosphate-dependent binding with hyperphosphorylated tau (PHFtau), the major constituent of NFTs, thereby inducing aggregation of PHFtau. Herein we report that iron (III) can also induce aggregation of soluble PHFtau. Importantly, for the aggregation of PHFtau to occur, iron in the oxidized state (III) is essential since iron in the reduced state (II) lacks such ability. Furthermore, iron (III)-induced aggregation is reversed by reducing iron (III) to iron (II). Thus the iron-participating aggregation is mediated not only by tau phosphorylation but also by the transition of iron between reduced (II) and oxidized (III) states. Further incubation of insoluble PHFtau aggregates isolated from AD brain with reducing agents produced liberation of solubilized PHFtau and iron (II), indicating that PHFtau in association with iron (III) constitutes the insoluble pool of PHFtau. These results indicate that iron might play a role in the aggregation of PHFtau leading to the formation of NFTs in AD brain.

Accumulation of imidazolone, pentosidine and N(epsilon)-(carboxymethyl)lysine in hippocampal CA4 pyramidal neurons of aged human brain

Previous studies from our laboratory demonstrated that N(epsilon)-(carboxymethyl)lysine (CML), one of the major advanced glycation end products (AGE), was accumulated in human pyramidal neurons in the hippocampus in an age-dependent manner. This suggests a potential link between AGE-accumulation and the aging process in neurons. The purpose of the present study was to examine whether this notion could be extended to other AGE structures, such as imidazolone and pentosidine. This was done using 19 human brains that were not affected by dementia. The immunohistochemical survey on distribution in brain tissues of imidazolone and pentosidine was carried out with monoclonal antibodies specific for imidazolone and pentosidine. A parallel control experiment was carried out with anti-CML antibody. The results showed that pentosidine and imidazolone were localized in neurons in different areas of human brain tissue, especially in neurons of CA4 in the hippocampus. The characteristic distribution of pentosidine and imidazolone is very similar to that of CML. Furthermore, when the accumulation of these AGE structures was compared with the age of individual brains it was found that accumulation of imidazolone, pentosidine and CML in the CA4 region increased with age. These findings taken together support the notion that the accumulation of AGE structures in the CA4 region might be closely related to the aging process in neurons.

The state versus amyloid-beta: the trial of the most wanted criminal in Alzheimer disease

Investigators studying the primary culprit responsible for Alzheimer disease have, for the past two decades, primarily focused on amyloid-beta (Abeta). Here, we put Abeta on trial and review evidence amassed by the prosecution that implicate Abeta and also consider arguments and evidence gathered by the defense team who are convinced of the innocence of their client. As in all trials, the arguments provided by the prosecution and defense revolve around the same evidence, with opposing interpretations. Below, we present a brief synopsis of the trial for you, the jury, to decide the verdict. Amyloid-beta: guilty or not-guilty?

Conversion of Amadori products of the Maillard reaction to N(epsilon)-(carboxymethyl)lysine by short-term heating: possible detection of artifacts by immunohistochemistry

Accumulation of advanced glycation end products (AGE) of the Maillard reaction increases by aging and in age-enhanced diseases such as atherosclerosis and diabetic complications. Immunohistochemical analysis has been used to demonstrate AGE in vivo. In immunochemistry, the heat-induced epitope retrieval technique is extensively used with formalin-fixed, paraffin-embedded tissue sections. Here we examined whether AGE could be formed artificially through the heating process. Normal rat skin and liver samples were divided into two groups, one rapidly frozen, the other formalin-fixed, paraffin-embedded and submitted to heat-induced epitope retrieval treatment. In heat-treated sections, the cytoplasm of rat epidermal cells and hepatocytes were strongly stained by monoclonal antibody against N(epsilon)-(carboxymethyl)lysine (CML), while the staining was negligible in either frozen sections or in paraffin-embedded but heat-untreated sections. To clarify the mechanism, we conducted heat treatment to glycated human serum albumin (HSA), a model Amadori protein, and generation of CML was determined by immunochemical and HPLC analysis. CML was generated from glycated HSA by heat treatment (above 80 degrees C) and increased in a time-dependent manner. In contrast, generation of CML from glycated HSA was significantly inhibited in the presence of NaBH4, a reducing agent, diethylenetriamine pentaacetic acid, a chelator of transition metal ion, or aminoguanidine, a trapping reagent for alpha-oxoaldehydes. Furthermore, heat-induced CML formation in rat liver samples determined by HPLC was markedly reduced by pretreatment with NaBH4. Reactive intermediates such as glucosone, 3-deoxyglucosone, methylglyoxal, and glyoxal were formed upon heat treatment of glycated HSA at 100 degrees C, indicating that these aldehydes generated from Amadori products by oxidative cleavage can contribute to further CML formation. CML generated by heating, directly from Amadori products or via these aldehydes, might serve as an artifact upon immunohistochemistry.

Tau function and dysfunction in neurons: its role in neurodegenerative disorders

Alzheimer's disease (AD) is the most usual neurodegenerative disorder leading to dementia in the aged human population. It is characterized by the presence of two main brain pathological hallmarks: senile plaques and neurofibrillary tangles (NFTs). NFTs are composed of fibrillar polymers of the abnormally phosphorylated cytoskeletal protein tau.

Viewing molecular mechanisms of ageing through a lens

Many late-life diseases are conformational diseases in tissues where there are unfolded or misfolded proteins which can form aggregates. These diseases have other common features in their aetiology. Cataract is one such disease and post-translational modifications of proteins in the lens during cataract formation are described as a possible guide to the changes in other age-related conditions. Delineation of common pathways in these diseases could lead to common treatment regimes, and in this respect, there are promising results for aspirin-like drugs in Alzheimer's disease, cataract, myocardial infarction, stroke and various cancers.

Sulfo-glycosaminoglycan content affects PHF-tau solubility and allows the identification of different types of PHFs

Sulfo-glycosaminoglycans (sGAGs) are involved in the assembly of tau in at least a subpopulation of paired helical filaments (PHFs) in Alzheimer's disease (AD). To further understand the role of sGAG molecules in the structure of PHFs, we isolated PHFs from patients with AD and treated them with heparinase. Immunoelectron microscopy and Western blotting (WB) were used later on to analyze the changes obtained. The heparinase treatment abolished Tau14 and AT8 immunodecoration (two N-terminal tau antibodies) and increased PHF-1 labeling (a C-terminal antibody). In addition, heparinase-treated filaments are more labile than control ones as demonstrated by sodium dodecyl sulfate-extraction and subsequent WB. In summary, our results demonstrate that sGAG content affects PHF conformation as well as PHF-tau solubilization.

Antioxidant strategies for Alzheimer's disease

Oxidative damage is present within the brains of patients with Alzheimer's disease (AD), and is observed within every class of biomolecule, including nucleic acids, proteins, lipids and carbohydrates. Oxidative injury may develop secondary to excessive oxidative stress resulting from beta-amyloid-induced free radicals, mitochondrial abnormalities, inadequate energy supply, inflammation or altered antioxidant defences. Treatment with antioxidants is a promising approach for slowing disease progression to the extent that oxidative damage may be responsible for the cognitive and functional decline observed in AD. Although not a uniformly consistent observation, a number of epidemiological studies have found a link between antioxidant intake and a reduced incidence of dementia, AD and cognitive decline in elderly populations. In AD clinical trials molecules with antioxidant properties such as vitamin E and Ginkgo biloba extract have shown modest benefit. A clinical trial with vitamin E is currently ongoing to determine if it can delay progression to AD in individuals with mild cognitive impairment. Combinations of antioxidants might be of even greater potential benefit for AD, especially if the agents worked in different cellular compartments or had complementary activity (e.g. vitamins E, C and ubiquinone). Naturally-occurring compounds with antioxidant capacity are available and widely marketed (e.g. vitamin C, ubiquinone, lipoic acid, beta-carotene, creatine, melatonin, curcumin) and synthetic compounds are under development by industry. Nevertheless, the clinical value of these agents for AD prevention and treatment is ambiguous, and will remain so until properly designed human trials have been performed.

Involvement of Maillard reactions in Alzheimer disease

Maillard reactions have been explored by food chemists for many years. It is only recently that the advanced glycation end products (AGEs), the end products of the Maillard reaction, have been detected in a wide variety of diseases such as diabetes, atherosclerosis, cataractogenesis, Parkinson disease and Alzheimer disease (AD). In this review, we discuss the chemistry and biochemistry of AGE-related crosslinks such as pyrraline, pentosidine, carboxymethyllysine (CML), crosslines, imidazolidinones, and dilysine crosslinks (GOLD and MOLD), as well as their possible involvement in neurodegenerative conditions. Pentosidine and CML are found in elevated amounts in the major lesions of the AD brain. Glycation is also implicated in the formation of the paired helical filaments (PHF), a component of the neurofibrillary tangles (NFTs). Amyloid-beta peptide and proteins of the cerebrospinal fluid are also glycated in patients with AD. In order to ameliorate the effects of AGEs on AD pathology, various inhibitors of AGEs have been increasingly explored. It is hoped that understanding of the mechanism of the AGEs formation and their role in the neurodegeneration will result in novel therapeutics for neuroprotection.

How hyperglycemia promotes atherosclerosis: molecular mechanisms

Both type I and type II diabetes are powerful and independent risk factors for coronary artery disease (CAD), stroke, and peripheral arterial disease. Atherosclerosis accounts for virtually 80% of all deaths among diabetic patients. Prolonged exposure to hyperglycemia is now recognized a major factor in the pathogenesis of atherosclerosis in diabetes. Hyperglycemia induces a large number of alterations at the cellular level of vascular tissue that potentially accelerate the atherosclerotic process. Animal and human studies have elucidated three major mechanisms that encompass most of the pathological alterations observed in the diabetic vasculature: 1) Nonenzymatic glycosylation of proteins and lipids which can interfere with their normal function by disrupting molecular conformation, alter enzymatic activity, reduce degradative capacity, and interfere with receptor recognition. In addition, glycosylated proteins interact with a specific receptor present on all cells relevant to the atherosclerotic process, including monocyte-derived macrophages, endothelial cells, and smooth muscle cells. The interaction of glycosylated proteins with their receptor results in the induction of oxidative stress and proinflammatory responses 2) oxidative stress 3) protein kinase C (PKC) activation with subsequent alteration in growth factor expression. Importantly, these mechanisms may be interrelated. For example, hyperglycemia-induced oxidative stress promotes both the formation of advanced glycosylation end products and PKC activation.

Formation of aberrant phosphotau fibrillar polymers in neural cultured cells

Here we show, for the first time, the in vitro formation of filamentous aggregates of phosphorylated tau protein in SH-SY5Y human neuroblastoma cells. The formation of such aberrant aggregates, similar to those occurring in vivo in Alzheimer's disease and other tauopathies, requires okadaic acid, a phosphatase inhibitor, to increase the level of phosphorylated tau, and hydroxynonenal, a product of oxidative stress that selectively adducts and modifies phosphorylated tau. Our findings suggest that both phosphorylation and oxidative modification are required for tau filament formation. Importantly, the in vitro formation of intracellular tau aggregates could be used as a model of tau polymerization and facilitate the development of novel therapeutic approaches.

Protein glycation: creation of catalytic sites for free radical generation

In a glycation reaction, alpha-dicarbonyl compounds such as deoxyglucosone, methylglyoxal, and glyoxal are more reactive than the parent sugars with respect to their ability to react with amino groups of proteins to form inter- and intramolecular cross-links of proteins, stable end products called advanced Maillard products or advanced end products (AGEs). The AGEs, which are irreversibly formed, accumulate with aging, atherosclerosis, and diabetes mellitus, and are especially associated with long-lived proteins such as collagens, lens crystallins, and nerve proteins. It was suggested that the formation of AGEs not only modifies protein properites but also induces biological damage in vivo. In this report, we summerize results obtained from our studies for (1) identifying the structure of the cross-linked radical species formed in the model system-the reaction between alpha-dicarbonyl methylglyoxal with amino acids, and (2) the reactivity of the radical center of the protein created by the similar reaction. These results indicate that glycation of protein generates active centers for catalyzing one-electron oxidation-reduction reactions. This active center, which exhibits enzyme-like character, is suggested to be the cross-linked Schiff-based radical cation of the protein. It mimics the characteristics of the metal-catalyzed oxidation system. These results together indicate that glycated proteins accumulated in vivo provide stable active sites for catalyzing the formation of free redicals.

Advanced glycation end products activate endothelium through signal-transduction receptor RAGE: a mechanism for amplification of inflammatory responses

BACKGROUND

The products of nonenzymatic glycation and oxidation of proteins, the advanced glycation end products (AGEs), form under diverse circumstances such as aging, diabetes, and kidney failure. Recent studies suggested that AGEs may form in inflamed foci, driven by oxidation or the myeloperoxidase pathway. A principal means by which AGEs alter cellular properties is through interaction with their signal-transduction receptor RAGE. We tested the hypothesis that interaction of AGEs with RAGE on endothelial cells enhances vascular activation.

METHODS AND RESULTS

AGEs, RAGE, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, and E-selectin are expressed in an overlapping manner in human inflamed rheumatoid synovia, especially within the endothelium. In primary cultures of human saphenous vein endothelial cells, engagement of RAGE by heterogeneous AGEs or Nepsilon(carboxymethyl)lysine-modified adducts enhanced levels of mRNA and antigen for vascular cell adhesion molecule-1, intercellular adhesion molecule-1, and E-selectin. AGEs increased adhesion of polymorphonuclear leukocytes to stimulated endothelial cells in a manner reduced on blockade of RAGE.

CONCLUSIONS

AGEs, through RAGE, may prime proinflammatory mechanisms in endothelial cells, thereby amplifying proinflammatory mechanisms in atherogenesis and chronic inflammatory disorders.

Potential clinical applications of poly(ADP-ribose) polymerase (PARP) inhibitors

Poly(ADP-ribose) polymerases (PARPs) are defined as cell signaling enzymes that catalyze the transfer of ADP-ribose units from NAD(+)to a number of acceptor proteins. PARP-1, the best characterized member of the PARP family, that presently includes six members, is an abundant nuclear enzyme implicated in cellular responses to DNA injury provoked by genotoxic stress (oxygen radicals, ionizing radiations and monofunctional alkylating agents). Due to its involvement either in DNA repair or in cell death, PARP-1 is regarded as a double-edged regulator of cellular functions. In fact, when the DNA damage is moderate, PARP-1 participates in the DNA repair process. Conversely, in the case of massive DNA injury, elevated PARP-1 activation leads to rapid NAD(+)/ATP consumption and cell death by necrosis. Excessive PARP-1 activity has been implicated in the pathogenesis of numerous clinical conditions such as stroke, myocardial infarction, shock, diabetes and neurodegenerative disorders. PARP-1 could therefore be considered as a potential target for the development of pharmacological strategies to enhance the antitumor efficacy of radio- and chemotherapy or to treat a number of clinical conditions characterized by oxidative or NO-induced stress and consequent PARP-1 activation. Moreover, the discovery of novel functions for the multiple members of the PARP family might lead in the future to additional clinical indications for PARP inhibitors.

Non-enzymatic covalent modifications of proteins: mechanisms, physiological consequences and clinical applications

Given the complexity of the biosynthetic machinery and the delicate chemical composition of proteins, it is remarkable that cells manage to produce and maintain normally functioning proteins under most conditions. However, it is now well known that proteins are susceptible to various non-enzymatic covalent modifications (NECM) under physiological conditions. Such modifications can be of no or little importance to the protein or they can be absolutely detrimental. Often NECM are difficult to study due to the complex and technically demanding methods required to identify many of these modifications. Thus, the role of NECM has not yet been adequately resolved but recent research has allowed a better understanding of such modifications. The present review outlines the various forms of NECM that involve covalent modifications of proteins, and discusses their relevance, biological impact and potential applications in the study of protein turnover and diagnosis of disease.

Elevated levels of oxidative DNA damage in lymphocytes from patients with Alzheimer's disease

Previous studies have provided evidence of the involvement of oxidative damage in the pathogenesis of Alzheimer's disease (AD). Although the role of oxidative stress in the aetiology of the disease is still not clear, the detection of an increased damage status in the cells of patients could have important therapeutic implications. The level of oxidative damage and repair capacity in peripheral lymphocytes of AD patients and of age-matched controls was determined by the Comet assay applied to freshly isolated blood samples with oxidative lesion-specific DNA repair endonucleases. This is less prone to errors arising from oxidative artifacts than chemical analytical methods; and is therefore a relatively reliable, as well as rapid method for assay of oxidative DNA damage in cells. Statistically significant elevations (P < 0.05) of oxidized purines were observed in nuclear DNA of peripheral lymphocytes from AD patients, compared to age matched control subjects, both at basal level and after oxidative stress induced by H(2)O(2.) AD patients also showed a diminished repair of H(2)O(2) -induced oxidized purines.

Transglutaminase bonds in neurofibrillary tangles and paired helical filament tau early in Alzheimer's disease

Transglutaminase-catalyzed epsilon(gamma-glutamyl)lysine cross-links exist in Alzheimer's disease (AD) paired helical filament (PHF) tau protein but not normal soluble tau. To test the hypothesis that these cross-links could play a role in the formation of neurofibrillary tangles (NFT), we used single- and double-label immunofluorescence confocal microscopy and immunoaffinity purification and immunoblotting to examine epsilon(gamma-glutamyl)lysine cross-links in AD and control brains. The number of neurons that are immunoreactive with an antibody directed at the epsilon-(gamma-glutamyl)lysine bond was significantly higher in AD cortex compared with age-matched controls and schizophrenics. PHF tau-directed antibodies AT8, MC-1 and PHF-1 co-localized with epsilon(gamma-glutamyl)lysine immunolabeling in AD NFT. Immunoaffinity purification and immunoblotting experiments demonstrated that PHF tau contains epsilon(gamma-glutamyl)lysine bonds in parietal and frontal cortex in AD. In control cases with NFT present in the entorhinal cortex and hippocampus, indicative of Braak and Braak stage II, epsilon(gamma-glutamyl)lysine bonds were present in PHF tau in parietal and frontal cortex, despite the lack of microscopically detectable NFT or senile plaques in these cortical regions. The presence of PHF tau with epsilon(gamma-glutamyl)lysine bonds in brain regions devoid of NFT in stage II (but regions, which would be expected to contain NFT in stage III) suggests that these bonds occur early in the formation of NFT.

Interaction of tau isoforms with Alzheimer's disease abnormally hyperphosphorylated tau and in vitro phosphorylation into the disease-like protein

The microtubule-associated protein tau is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in neurons undergoing neurodegeneration in the brains of patients with Alzheimer disease (AD). We investigated the isoform-specific interaction of normal tau with AD hyperphosphorylated tau (AD P-tau). We found that the binding of AD P-tau to normal human recombinant tau was tau4L > tau4S > tau4 and tau3L > tau3S > tau3, and that its binding to tau4L was greater than to tau3L. AD P-tau also inhibited the assembly of microtubules promoted by each tau isoform and caused disassembly when added to preassembled microtubules. This inhibition and depolymerization of microtubules by the AD P-tau corresponded directly to the degree of its interaction with the different tau isoforms. In vitro hyperphosphorylation of recombinant tau (P-tau) conferred AD P-tau-like characteristics. Like AD P-tau, P-tau interacted with and sequestered normal tau and inhibited microtubule assembly. These studies suggest that the AD P-tau interacts preferentially with the tau isoforms that have the amino-terminal inserts and four microtubule binding domain repeats and that hyperphosphorylation of tau appears to be sufficient to acquire AD P-tau characteristics. Thus, lack of amino-terminal inserts and extra microtubule binding domain repeat in fetal human brain might be protective from Alzheimer's neurofibrillary degeneration.

Activation of MKK6, an upstream activator of p38, in Alzheimer's disease

Mitogen-activated protein kinase (MAPK) p38 has been implicated in the pathogenesis of Alzheimer's disease, but the upstream cascade leading to p38 activation has not been elucidated in the disease. In the present study, we focused on mitogen-activated protein kinase kinase 6 (MKK6), one of the upstream activators of p38 MAPK. We found that MKK6 was not only increased but also specifically associated with granular structures in the susceptible neurons in the hippocampus and cortex of Alzheimer's disease patients, but was only weakly diffuse in the cytoplasm in neurons in control cases. Immunoblot analysis demonstrated a significant increase of MKK6 level in Alzheimer's disease compared with age-matched controls. In this regard, in hippocampal and cortical regions of individuals with Alzheimer's disease, the activated phospho-MKK6 was localized exclusively in association with pathological alterations including neurofibrillary tangles, senile plaques, neuropil threads and granular structures, overlapping with activated p38 MAPK suggesting both a functional and mechanic link. By immunoblot analysis, phospho-MKK6 is also significantly increased in AD compared with control cases. Together, these findings lend further credence to the notion that the p38 MAPK pathway is dysregulated in Alzheimer's disease and also indicates an active role for this pathway in disease pathogenesis.

Celastrol, a potent antioxidant and anti-inflammatory drug, as a possible treatment for Alzheimer's disease

In the brains of patients with Alzheimer's disease (AD) signs of neuronal degeneration are accompanied by markers of microglial activation, inflammation, and oxidant damage. The presence of nitrotyrosine in the cell bodies of neurons in AD suggests that peroxynitrite contributes to the pathogenesis of the disease. A drug with antioxidant and anti-inflammatory activity may prevent neuronal degeneration in AD. Celastrol, a plant-derived triterpene, has these effects. In low nanomolar concentrations celastrol was found to suppress the production by human monocytes and macrophages of the pro-inflammatory cytokines TNF-alpha and IL-1beta. Celastrol also decreased the induced expression of class II MHC molecules by microglia. In macrophage lineage cells and endothelial cells celastrol decreased induced but not constitutive NO production. Celastrol suppressed adjuvant arthritis in the rat, demonstrating in vivo anti-inflammatory activity. Low doses of celastrol administered to rats significantly improved their performance in memory, learning and psychomotor activity tests. The potent antioxidant and anti-inflammatory activities of celastrol, and its effects on cognitive functions, suggest that the drug may be useful to treat neurodegenerative diseases accompanied by inflammation, such as AD.

Active glycation in neurofibrillary pathology of Alzheimer disease: N(epsilon)-(carboxymethyl) lysine and hexitol-lysine

Advanced glycation end products are a diverse class of posttranslational modifications, stemming from reactive aldehyde reactions, that have been implicated in the pathogenesis of a number of degenerative diseases. Because advanced glycation end products are accelerated by, and result in formation of, oxygen-derived free radicals, they represent an important component of the oxidative stress hypothesis of Alzheimer disease (AD). In this study, we used in situ techniques to assess N(epsilon)-(Carboxymethyl)lysine (CML), the predominant advanced glycation end product that accumulates in vivo, along with its glycation-specific precursor hexitol-lysine, in patients with AD as well as in young and aged-matched control cases. Both CML and hexitol-lysine were increased in neurons, especially those containing intracellular neurofibrillary pathology in cases of AD. The increase in hexitol-lysine and CML in AD suggests that glycation is an early event in disease pathogenesis. In addition, because CML can result from either lipid peroxidation or advanced glycation, while hexitol-lysine is solely a product of glycation, this study, together with studies demonstrating the presence of 4-hydroxy-2-nonenal adducts and pentosidine, provides evidence of two distinct oxidative processes acting in concert in AD neuropathology. Our findings support the notion that aldehyde-mediated modifications, together with oxyradical-mediated modifications, are critical pathogenic factors in AD.

Microglial interaction with beta-amyloid: implications for the pathogenesis of Alzheimer's disease

The etiology of Alzheimer's disease (AD) involves a significant inflammatory component as evidenced by the presence of elevated levels of a diverse range of proinflammatory molecules in the AD brain. These inflammatory molecules are produced principally by activated microglia, which are found to be clustered within and adjacent to the senile plaque. Moreover, long-term treatment of patients with non-steroidal anti-inflammatory drugs has been shown to reduce risk and incidence of AD and delay disease progression. The microglia respond to beta-amyloid (Abeta) deposition in the brain through the interaction of fibrillar forms of amyloid with cell surface receptors, leading to the activation of intracellular signal transduction cascades. The activation of multiple independent signaling pathways ultimately leads to the induction of proinflammatory gene expression and production of reactive oxygen and nitrogen species. These microglial inflammatory products act in concert to produce neuronal toxicity and death. Therapeutic approaches focused on inhibition of the microglial-mediated local inflammatory response in the AD brain offer new opportunities to intervene in the disease.

Hyperphosphorylation induces self-assembly of tau into tangles of paired helical filaments/straight filaments

The microtubule-associated protein tau is a family of six isoforms that becomes abnormally hyperphosphorylated and accumulates in the form of paired helical filaments (PHF) in the brains of patients with Alzheimer's disease (AD) and patients with several other tauopathies. Here, we show that the abnormally hyperphosphorylated tau from AD brain cytosol (AD P-tau) self-aggregates into PHF-like structures on incubation at pH 6.9 under reducing conditions at 35 degrees C during 90 min. In vitro dephosphorylation, but not deglycosylation, of AD P-tau inhibits its self-association into PHF. Furthermore, hyperphosphorylation induces self-assembly of each of the six tau isoforms into tangles of PHF and straight filaments, and the microtubule binding domains/repeats region in the absence of the rest of the molecule can also self-assemble into PHF. Thus, it appears that tau self-assembles by association of the microtubule binding domains/repeats and that the abnormal hyperphosphorylation promotes the self-assembly of tau into tangles of PHF and straight filaments by neutralizing the inhibitory basic charges of the flanking regions.

O-Glycosylation in sprouting neurons in Alzheimer disease, indicating reactive plasticity

Reactive plasticity, including axonal and dendritic sprouting and reactive synaptogenesis, has been proposed to contribute to the pathogenesis of several neurological disorders. This work was aimed at identifying the possible role of protein glycosylation in the brain from patients with Alzheimer disease (AD), using lectin histochemistry, as determinants of reactive plasticity. Results indicate an increase in the production of cryptic O-glycosidically linked proteins (NeuAcalpha2,6 Galbeta1,3GalNAcalpha1,0 Ser/Thr or sialyl-T-antigen) in neuritic sprouting in AD brains as determined by positive labeling with Amaranthus leucocarpus (ALL, T-antigen-specific) and Macrobrachium rosenbergii (MRL, specific for NeuAc5,9Ac2) lectins. Immunohistochemistry indicated that lectin staining was specific for the synaptic sprouting process (meganeurites) in AD. These results were confirmed using anti-synaptophysin and anti-GAP 43 antibodies, which recognized meganeurites and dystrophic neurites around amyloid-beta deposits. In normal control brains, labeling with the aforementioned lectins was restricted to microvessels. Control experiments with neuraminidase-treated brain samples revealed positivity to the lectin from Arachis hypogaea (PNA), which is specific for galactose. Our results suggest specific O-glycosylation patterns of proteins closely related to neuronal plasticity in AD.

The cerebrocortical areas in normal brain aging and in Alzheimer's disease: noticeable differences in the lipid peroxidation level and in antioxidant defense

The markers of oxidative stress were measured in four cerebrocortical regions of Alzheimer's disease (AD) and age-matched control brains. In controls the levels of diene conjugates (DC) and lipid peroxides (LOOH) were significantly higher in the sensory postcentral and occipital primary cortex than in the temporal inferior or frontal inferior cortex. The antioxidant capacity (AOC) was highest in the temporal, and GSH in the frontal inferior cortex. The highest activity of superoxide dismutase (SOD) and catalase (CAT) was found in the occipital primary cortex. Compared with controls, significantly higher level of DC and LOOH and attenuated AOC were evident in AD temporal inferior cortex. In AD frontal inferior cortex moderate increase in LOOH was associated with positive correlation between SOD activity and counts of senile plaques. Our data suggest that in AD cerebral cortex, the oxidative stress is expressed in the reducing sequence: temporal inferior cortex > frontal inferior cortex > sensory postcentral cortex approximately = occipital primary cortex, corresponding to the histopathological spreading of AD from the associative to primary cortical areas.

Acrolein is increased in Alzheimer's disease brain and is toxic to primary hippocampal cultures

Accumulating evidence implicates oxidative stress in the pathogenesis of several neurodegenerative diseases including Alzheimer's disease (AD). Increased lipid peroxidation, decreased levels of polyunsaturated fatty acids, and increased levels of 4-hydroxynonenal (HNE), F(2)-isoprostanes, and F(4)-neuroprostanes are present in the brain in AD. Acrolein, an alpha,beta-unsaturated aldehydic product of lipid peroxidation, is approximately 100 times more reactive than HNE and recently was demonstrated in neurofibrillary tangles in the brain in AD. In three brain regions of 10 AD patients compared with 8 age-matched control subjects, we found increased mean extractable acrolein, with the increases reaching statistical significance in the amygdala and hippocampus/parahippocampal gyrus. In hippocampal neuron cultures, acrolein was neurotoxic in a time- and concentration-dependent manner and more toxic than HNE at 5 microM concentrations of each. Acrolein exposure led to a significant concentration-dependent increase in intracellular calcium concentrations. Collectively, these data show that acrolein is increased in the brain in AD and demonstrate neurotoxicity mechanisms that might be important in the pathogenesis of neuron degeneration in AD.