The various aggregation states of beta-amyloid 1-42 mediate different effects on oxidative stress, neurodegeneration, and BACE-1 expression

Evidence for natural antisense transcript-mediated inhibition of microRNA function

BACKGROUND

MicroRNAs (miRNAs) have the potential to regulate diverse sets of mRNA targets. In addition, mammalian genomes contain numerous natural antisense transcripts, most of which appear to be non-protein-coding RNAs (ncRNAs). We have recently identified and characterized a highly conserved non-coding antisense transcript for beta-secretase-1 (BACE1), a critical enzyme in Alzheimer's disease pathophysiology. The BACE1-antisense transcript is markedly up-regulated in brain samples from Alzheimer's disease patients and promotes the stability of the (sense) BACE1 transcript.

RESULTS

We report here that BACE1-antisense prevents miRNA-induced repression of BACE1 mRNA by masking the binding site for miR-485-5p. Indeed, miR-485-5p and BACE1-antisense compete for binding within the same region in the open reading frame of the BACE1 mRNA. We observed opposing effects of BACE1-antisense and miR-485-5p on BACE1 protein in vitro and showed that Locked Nucleic Acid-antimiR mediated knockdown of miR-485-5p as well as BACE1-antisense over-expression can prevent the miRNA-induced BACE1 suppression. We found that the expression of BACE1-antisense as well as miR-485-5p are dysregulated in RNA samples from Alzheimer's disease subjects compared to control individuals.

CONCLUSIONS

Our data demonstrate an interface between two distinct groups of regulatory RNAs in the computation of BACE1 gene expression. Moreover, bioinformatics analyses revealed a theoretical basis for many other potential interactions between natural antisense transcripts and miRNAs at the binding sites of the latter.

Spinal levels of nonprotein thiols are related to nociception in mice

Oxidative stress markers are thought to be related to nociception. Because thiolic compounds are important antioxidants, we investigated the relationship between thiols, endogenous or exogenous, and nociception. Systemic or spinal, but not peripheral, administration of the exogenous thiolic compound N-acetyl-L-cysteine (NAC) reduced nociception induced by intraplantar capsaicin injection. Moreover, we detected an increase in lipid peroxidation and 3-nitrotyrosine and a decrease in nonprotein thiolic levels in the lumbar spinal cord of capsaicin-injected animals. All these effects were prevented by NAC treatment (i.p. and i.t.). Our findings confirm a role for the spinal cord in NAC actions because systemic NAC administration also reduced the nociception trigged by intrathecal injection of capsaicin. Moreover, adjuvant-induced arthritis, but not paw incision, also -decreases nonprotein thiol levels in the spinal cord. Similarly, NAC produced antinociception in adjuvant-treated animals, but not in paw-incised animals. Finally, we investigated the role of endogenous thiol compounds in the nociceptive process administrating buthionine-suphoxamine (BSO), an inhibitor of glutathione-synthesis. Intrathecal BSO treatment decreased nonprotein thiol levels in the spinal cord, as well as induced mechanical allodynia and chemical and thermal hyperalgesia. In conclusion, our results indicate a critical role for nonprotein thiols in nociception at the level of the spinal cord.

PERSPECTIVE

The results presented here indicate that the loss of nonprotein thiols in the spinal cord is involved in pain development. Therefore, the administration of thiolic compounds or other strategies allow thiol levels to be maintained and could be a beneficial action in the therapy of painful conditions.

Induction of amyloid precursor protein by the neurotoxic peptide, amyloid-beta 25-35, causes retinal ganglion cell death

Patients with Alzheimer's disease (AD) show a significantly increased incidence of glaucoma. AD is also associated with the occurrence of the neurotoxic peptide amyloid beta (Abeta). Therefore, we investigated whether Abeta is associated with retinal cell death in a retinal ganglion cell line (RGC-5). Treatment with Abeta(25-35), a neurotoxic fragment of Abeta, induced cell death in RGC-5 in both a concentration- and time-dependent manner. The amount of amyloid precursor protein was increased by treatment of RGC-5 and primary culture of mouse cortical neurons with fibril Abeta(25-35) and Abeta(1-42), which is a putative physiological neurotoxic fragment of Abeta present in AD. Amyloid precursor protein knockdown inhibited the cell death induced by Abeta(25-35). Treatment with Abeta(25-35) increased the amount of intracellular Abeta(1-40) and Abeta(1-42), while beta- and gamma-secretase inhibitors reduced cell death. Thus, the regulation of Abeta can be viewed as a new therapeutic target for glaucoma, especially in patients with coincident AD.

Alzheimer's disease: synaptic dysfunction and Abeta

Synapse loss is an early and invariant feature of Alzheimer's disease (AD) and there is a strong correlation between the extent of synapse loss and the severity of dementia. Accordingly, it has been proposed that synapse loss underlies the memory impairment evident in the early phase of AD and that since plasticity is important for neuronal viability, persistent disruption of plasticity may account for the frank cell loss typical of later phases of the disease. Extensive multi-disciplinary research has implicated the amyloid β-protein (Aβ) in the aetiology of AD and here we review the evidence that non-fibrillar soluble forms of Aβ are mediators of synaptic compromise. We also discuss the possible mechanisms of Aβ synaptotoxicity and potential targets for therapeutic intervention.

Biomarkers of oxidative and nitrosative damage in Alzheimer's disease and mild cognitive impairment

Alzheimer's disease (AD) is the most common type of dementia in the elderly. Products of oxidative and nitrosative stress (OS and NS, respectively) accumulate with aging, which is the main risk factor for AD. This provides the basis for the involvement of OS and NS in AD pathogenesis. OS and NS occur in biological systems due to the dysregulation of the redox balance, caused by a deficiency of antioxidants and/or the overproduction of free radicals. Free radical attack against lipids, proteins, sugars and nucleic acids leads to the formation of bioproducts whose detection in fluids and tissues represents the currently available method for assessing oxidative/nitrosative damage. Post-mortem and in-vivo studies have demonstrated an accumulation of products of free radical damage in the central nervous system and in the peripheral tissues of subjects with AD or mild cognitive impairment (MCI). In addition to their individual role, biomarkers for OS and NS in AD are associated with altered bioenergetics and amyloid-beta (Abeta) metabolism. In this review we discuss the main results obtained in the field of biomarkers of oxidative/nitrosative stress in AD and MCI in humans, in addition to their potential role as a tool for diagnosis, prognosis and treatment efficacy in AD.

Linking vascular disorders and Alzheimer's disease: potential involvement of BACE1

The etiology of Alzheimer's disease (AD) remains unknown. However, specific risk factors have been identified, and aging is the strongest AD risk factor. The majority of cardiovascular events occur in older people and a close relationship between vascular disorders and AD exists. Amyloid plaques, composed of the beta amyloid peptide (Abeta), are hallmark lesions in AD and evidence indicates that Abeta plays a central role in AD pathophysiology. The BACE1 enzyme is essential for Abeta generation, and BACE1 levels are elevated in AD brain. The cause(s) of this BACE1 elevation remains undetermined. Here we review the potential contribution of vascular disease to AD pathogenesis. We examine the putative vasoactive properties of Abeta and how the cellular changes associated with vascular disease may elevate BACE1 levels. Despite increasing evidence, the exact role(s) vascular disorders play in AD remains to be determined. However, given that vascular diseases can be addressed by lifestyle and pharmacologic interventions, the potential benefits of these therapies in delaying the clinical appearance and progression of AD may warrant investigation.

BACE and gamma-secretase characterization and their sorting as therapeutic targets to reduce amyloidogenesis

Secretases are named for enzymes processing amyloid precursor protein (APP), a prototypic type-1 membrane protein. This led directly to discovery of novel Aspartyl proteases (beta-secretases or BACE), a tetramer complex gamma-secretase (gamma-SC) containing presenilins, nicastrin, aph-1 and pen-2, and a new role for metalloprotease(s) of the ADAM family as a alpha-secretases. Recent advances in defining pathways that mediate endosomal-lysosomal-autophagic-exosomal trafficking now provide targets for new drugs to attenuate abnormal production of fibril forming products characteristic of AD. A key to success includes not only characterization of relevant secretases but mechanisms for sorting and transport of key metabolites to abnormal vesicles or sites for assembly of fibrils. New developments we highlight include an important role for an 'early recycling endosome' coated in retromer complex containing lipoprotein receptor LRP-II (SorLA) for switching APP to a non-amyloidogenic pathway for alpha-secretases processing, or to shuttle APP to a 'late endosome compartment' to form Abeta or AICD. LRP11 (SorLA) is of particular importance since it decreases in sporadic AD whose etiology otherwise is unknown.

Signaling effect of amyloid-beta(42) on the processing of AbetaPP

The effects of amyloid-beta are extremely complex. Current work in the field of Alzheimer disease is focusing on discerning the impact between the physiological signaling effects of soluble low molecular weight amyloid-beta species and the more global cellular damage that could derive from highly concentrated and/or aggregated amyloid. Being able to dissect the specific signaling events, to understand how soluble amyloid-beta induces its own production by up-regulating BACE1 expression, could lead to new tools to interrupt the distinctive feedback cycle with potential therapeutic consequences. Here we describe a positive loop that exists between the secretases that are responsible for the generation of the amyloid-beta component of Alzheimer disease. According to our hypothesis, in familial Alzheimer disease, the primary overproduction of amyloid-beta can induce BACE1 transcription and drive a further increase of amyloid-beta precursor protein processing and resultant amyloid-beta production. In sporadic Alzheimer disease, many factors, among them oxidative stress and inflammation, with consequent induction of presenilins and BACE1, would activate a loop and proceed with the generation of amyloid-beta and its signaling role onto BACE1 transcription. This concept of a signaling effect by and feedback on the amyloid-beta precursor protein will likely shed light on how amyloid-beta generation, oxidative stress, and secretase functions are intimately related in sporadic Alzheimer disease.

Generation of reactive oxygen species by beta amyloid fibrils and oligomers involves different intra/extracellular pathways

A neuropathological characteristic of Alzheimer's disease is the extracellular accumulation of amyloid beta peptide (Abeta) in neuritic plaques. Recent evidences suggested that soluble Abeta oligomers are the predominant neurotoxic species for neurons. Thus, considerable attention has been paid to discriminate the cytotoxic pathways of Abeta pre-fibrillar aggregates and mature fibrils. We showed that the mechanisms by which Abeta oligomers and fibrils generated reactive oxygen species differ in terms of site of production and kinetics, suggesting the involvement of different intra/extracellular pathways.

Galantamine protects against oxidative stress induced by amyloid-beta peptide in cortical neurons

Galantamine is currently used in the treatment of patients with mild-to-moderate Alzheimer's disease (AD). Although its action is mostly directed at the regulation of cholinergic transmission, galantamine can also afford neuroprotection against amyloid-beta peptide (Abeta), which is involved in AD pathogenesis. In this study, we used cultured rat cortical neurons treated with two forms of Abeta(1-40), fresh and previously aged (enriched in fibrils). First, we confirmed that galantamine prevented neurodegeneration induced by both peptide forms in a concentration-dependent manner. Moreover, we observed that when neurons were co-incubated with fresh Abeta(1-40) plus galantamine, the amount of amyloid aggregates was reduced. As oxidative conditions influence Abeta aggregation, we investigated whether galantamine prevents oxidative stress induced by this peptide. The data show that either fresh or aged Abeta(1-40) significantly increased the amount of reactive oxygen species and lipoperoxidation, these effects being prevented by galantamine. In Abeta(1-40)-treated neurons, the depletion of reduced glutathione (GSH) seems to be related to the decrease in glutathione peroxidase and glutathione reductase activities(.) These alterations in the GSH antioxidant system were prevented by galantamine. Overall, these results constitute the first evidence that galantamine can prevent the neuronal oxidative damage induced by Abeta, providing an in vitro basis for the beneficial actions of galantamine in the AD neurodegenerative process.

Mutant presenilin 1 increases the expression and activity of BACE1

Mutations of the presenilin 1 (PS1) gene are the most common cause of early onset familial Alzheimer disease (FAD). PS1 mutations alter the activity of the gamma-secretase on the beta-amyloid precursor protein (APP), leading to selective overproduction of beta-amyloid (Abeta) 42 peptides, the species that forms oligomers that may exert toxic effects on neurons. Here we show that PS1 mutations, expressed both transiently and stably, in non-neuronal and neuronal cell lines increase the expression and the activity of the beta-secretase (BACE1), the rate-limiting step of Abeta production. Also, BACE1 expression and activity are elevated in brains of PS1 mutant knock-in mice compared with wild type littermates as well as in cerebral cortex of FAD cases bearing various PS1 mutations compared with in sporadic AD cases and controls. The up-regulation of BACE1 by PS1 mutations requires the gamma-secretase cleavage of APP and is proportional to the amount of secreted Abeta42. Abeta42, and not AICD (APP intracellular domain), is indeed the APP derivative that mediates the overexpression of BACE1. The effect of PS1 mutations on BACE1 may contribute to determine the wide clinical and pathological phenotype of early onset FAD.

Inhibition by KMI-574 leads to dislocalization of BACE1 from lipid rafts

BACE1 initiates processing of the amyloid precursor protein (APP) in the production of amyloid beta (Abeta) peptide. After beta-cleavage by BACE1, the C-terminal stub of the APP fragment is further processed by the gamma-secretase complex to produce Abeta. Because APP, Abeta, the gamma-secretase complex, and BACE1 are found in lipid raft membranes, Abeta production is widely accepted to occur in lipid rafts. However, whether BACE1 is activated within the rafts is unclear. To analyze the relationship between the activity and the localization of BACE1, we used a new BACE1 inhibitor, KMI-574, and separated raft membranes on sucrose density gradients. In the presence of KMI-574, the localization of BACE1 shifted from the rafts to nonraft membranes in HEK293 cells. We also analyzed the proteolytically inactive mutants, D93A, D289A, and D93A/D289A, of BACE1. These mutants also moved from rafts to nonrafts, and the D93A/D289A double-mutant localized exclusively to nonraft membranes. The mutants were defective in maturation by glynosylation and formed hyperoligomers, suggesting that the BACE1 oligomers could not exit from the ER and be transported to the Golgi apparatus. Our findings suggest that the activated conformation of BACE1 is important for protein transport and localization to lipid rafts.

Antioxidants have a rapid and long-lasting effect on neuritic abnormalities in APP:PS1 mice

Senile plaques are a major pathological hallmark of Alzheimer's disease (AD). Compelling evidence suggests that senile plaques lead to structural alterations of neuronal processes and that local toxicity may be mediated by increased oxidative stress. Anti-oxidant therapy can alleviate the neuronal abnormalities in APP mice, but the time-course of this beneficial effect is unknown. We used multiphoton microscopy to assess in vivo the characteristics of antioxidant treatment on senile plaques and neurites in AD model mice (APPswe/PS1dE9). We observed that α-phenyl-N-tert-butyl nitrone (PBN), Ginkgo biloba extract (EGb 761) and Trolox had no effect on the size of existing senile plaques. However, all anti-oxidants had a straightening effect on curved neurites. This effect was detected as soon as 4 days after commencing the treatment, and was maintained after 1 month of daily treatment, with no further increase in the effect. The straightening of neurites persisted 15 days after stopping the treatment. These data indicate that neuronal plasticity is fast and still active in adult animals, and suggest that amelioration of the neuritic distortions associated with senile plaques with antioxidants is both rapid and long lasting.

Anti-oligomeric Abeta single-chain variable domain antibody blocks Abeta-induced toxicity against human neuroblastoma cells

The Amyloid-beta (Abeta) peptide is a major component of the amyloid plaques associated with Alzheimer's disease (AD). Recent studies suggest that the most toxic forms of Abeta are small, soluble oligomeric aggregates. Here, we report the isolation and characterization of a single-chain variable domain (scFv) antibody isolated against oligomeric Abeta using a protocol developed in our laboratory that combines phage display technology and atomic force microscopy (AFM). Starting with a randomized, single framework phage display library, after three rounds of selection against oligomeric Abeta, we identified an scFv that bound oligomeric Abeta specifically, but not monomeric or fibrillar forms. The anti-oligomeric scFv inhibits Abeta aggregation and toxicity, and reduces the toxicity of preformed oligomeric Abeta towards human neuroblastoma cells. When used to probe samples of human brain tissue, the scFv reacted with AD tissue but not a healthy control or Parkinson's disease brain samples. The anti-oligomeric Abeta scFv therefore has potential therapeutic and diagnostic applications in specifically targeting or identifying the toxic morphologies of Abeta in AD brains.

Formation of toxic Abeta(1-40) fibrils on GM1 ganglioside-containing membranes mimicking lipid rafts: polymorphisms in Abeta(1-40) fibrils

The abnormal aggregation and deposition of amyloid beta protein (Abeta) on neuronal cells are critical to the onset of Alzheimer's disease. The entity (oligomers or fibrils) of toxic Abeta species responsible for the pathogenesis of the disease has been controversial. We have reported that the Abeta aggregates on ganglioside-rich domains of neuronal PC12 cells as well as in raft-like model membranes. Here, we identified toxic Abeta(1-40) aggregates formed with GM1-ganglioside-containing membranes. Abeta(1-40) was incubated with raft-like liposomes composed of GM1/cholesterol/sphingomyelin at 1:2:2 and 37 degrees C. After a lag period, toxic amyloid fibrils with a width of 12 nm were formed and subsequently laterally assembled with slight changes in their secondary structure as confirmed by viability assay, thioflavin-T fluorescence, circular dichroism, and transmission electron microscopy. In striking contrast, Abeta fibrils formed without membranes were thinner (6.7 nm) and much less toxic because of weaker binding to cell membranes and a smaller surface hydrophobicity. This study suggests that toxic Abeta(1-40) species formed on membranes are not soluble oligomers but amyloid fibrils and that Abeta(1-40) fibrils exhibit polymorphisms.

Challenges for research on polyphenols from foods in Alzheimer's disease: bioavailability, metabolism, and cellular and molecular mechanisms

Polyphenols are the most abundant antioxidants in diet. Indeed, fruits, vegetables, beverages (tea, wine, juices), plants, and some herbs are loaded with powerful antioxidant polyphenols. Despite their wide distribution, research on human health benefits truly began in the mid-1990s (Scalbert, A.; Johnson, I. T.; Saltmarsh, M. Am. J. Clin. Nutr. 2005, 81, S15S-217S). Phenolic compounds have been receiving increasing interest from consumers and manufacturers because numerous epidemiological studies have suggested associations between consumption of polyphenol-rich foods or beverages and the prevention of certain chronic diseases such as cancers and cardiovascular diseases (Manach, C.; Mazur, A.; Scalbert, A. Curr. Opin. Lipidol. 2005, 16, 77-84; Duthie, S. J. Mol. Nutr. Food Res. 2007, 51, 665-674). Furthermore, in the past 10 years, research on the neuroprotective effects of dietary polyphenols has developed considerably. These compounds are able to protect neuronal cells in various in vivo and in vitro models through different intracellular targets (Ramassamy, C. Eur. J. Pharmacol. 2006, 545, 51-64). However, it is not at all clear whether these compounds reach the brain in sufficient concentrations and in a biologically active form to exert beneficial effects. On the other hand, it has become clear that the mechanisms of action of these polyphenols go beyond their antioxidant activity and the attenuation of oxidative stress. Therefore, there is a need for more research on their intracellular and molecular targets as special pathways underlying distinct polyphenol-induced neuroprotection. The focus of this review is aimed at presenting the role of some polyphenols from fruits, vegetables, and beverages in neuroprotection and particularly in Alzheimer's disease and the research challenges in this area.

Expression of a noncoding RNA is elevated in Alzheimer's disease and drives rapid feed-forward regulation of beta-secretase

Recent efforts have revealed that numerous protein-coding messenger RNAs have natural antisense transcript partners, most of which seem to be noncoding RNAs. Here we identify a conserved noncoding antisense transcript for beta-secretase-1 (BACE1), a crucial enzyme in Alzheimer's disease pathophysiology. The BACE1-antisense transcript (BACE1-AS) regulates BACE1 mRNA and subsequently BACE1 protein expression in vitro and in vivo. Upon exposure to various cell stressors including amyloid-beta 1-42 (Abeta 1-42), expression of BACE1-AS becomes elevated, increasing BACE1 mRNA stability and generating additional Abeta 1-42 through a post-transcriptional feed-forward mechanism. BACE1-AS concentrations were elevated in subjects with Alzheimer's disease and in amyloid precursor protein transgenic mice. These data show that BACE1 mRNA expression is under the control of a regulatory noncoding RNA that may drive Alzheimer's disease-associated pathophysiology. In summary, we report that a long noncoding RNA is directly implicated in the increased abundance of Abeta 1-42 in Alzheimer's disease.

Brain oxidative stress in a triple-transgenic mouse model of Alzheimer disease

Alzheimer disease (AD) is a neurodegenerative disease which is characterized by the presence of extracellular senile plaques mainly composed of amyloid-beta peptide (Abeta), intracellular neurofibrillary tangles, and selective synaptic and neuronal loss. AD brains revealed elevated levels of oxidative stress markers which have been implicated in Abeta-induced toxicity. In the present work we addressed the hypothesis that oxidative stress occurs early in the development of AD and evaluated the extension of the oxidative stress and the levels of antioxidants in an in vivo model of AD, the triple-transgenic mouse, which develops plaques, tangles, and cognitive impairments and thus mimics AD progression in humans. We have shown that in this model, levels of antioxidants, namely, reduced glutathione and vitamin E, are decreased and the extent of lipid peroxidation is increased. We have also observed increased activity of the antioxidant enzymes glutathione peroxidase and superoxide dismutase. These alterations are evident during the Abeta oligomerization period, before the appearance of Abeta plaques and neurofibrillary tangles, supporting the view that oxidative stress occurs early in the development of the disease.

DNA damage-inducing agents elicit gamma-secretase activation mediated by oxidative stress

According to the amyloid cascade hypothesis, Alzheimer's disease is the consequence of neuronal cell death induced by beta-amyloid (Abeta), which accumulates by abnormal clearance or production. On the other hand, recent studies have shown cell death-induced alteration in amyloid precursor protein (APP) processing, suggesting potential mutual interactions between APP processing and cell death. We have shown previously that the cell death caused by DNA damage-inducing agents (DDIAs) facilitated gamma-secretase activity and Abeta generation in a Bax/Bcl-2-dependent, but caspase-independent manner. Here, we attempted to elucidate the downstream mechanism that modulates gamma-secretase activity in DDIA-treated cells. N-acetyl cysteine, a potent antioxidant, attenuated DDIA-induced enhancement of gamma-secretase activity but failed to rescue cell death. Overexpression of heat shock protein 70, which blocks cytochrome c release from mitochondria, also reduced gamma-secretase activity. Moreover, glutathione depletion significantly facilitated gamma-secretase activity and Abeta generation by enhancing the formation of higher molecular weight gamma-secretase complex before signs of cell death developed. Finally, Abeta treatment, a known inducer of oxidative stress, also increased gamma-secretase activity. Taken together, these results indicate that DDIA-induced gamma-secretase activation is dependent on augmented oxidative stress, and that Abeta and gamma-secretase may activate each other. On the basis of these results, we propose a feed-back loop between oxidative stress and Abeta generation mediated by gamma-secretase activation.

Insight into the kinetic of amyloid beta (1-42) peptide self-aggregation: elucidation of inhibitors' mechanism of action

The initial transition of amyloid beta (1-42) (Abeta42) soluble monomers/small oligomers from unordered/alpha-helix to a beta-sheet-rich conformation represents a suitable target to design new potent inhibitors and to obtain effective therapeutics for Alzheimer's disease. Under optimized conditions, this reliable and reproducible CD kinetic study showed a three-step sigmoid profile that was characterized by a lag phase (prevailing unordered/alpha-helix conformation), an exponential growth phase (increasing beta-sheet secondary structure) and a plateau phase (prevailing beta-sheet secondary structure). This kinetic analysis brought insight into the inhibitors' mechanism of action. In fact, an increase in the duration of the lag phase can be related to the formation of an inhibitor-Abeta complex, in which the non-amyloidogenic conformation is stabilized. When the exponential rate is affected exclusively, such as in the case of Congo red and tetracycline, then the inhibitor affinity might be higher for the pleated beta-sheet structure. Finally, by adding the inhibitor at the end of the exponential phase, the soluble protofibrils can be disrupted and the Abeta amyloidogenic structure can revert into monomers/small oligomers. Congo red and tetracycline preferentially bind to amyloid in the beta-sheet conformation because both decreased the slope of the exponential growth, even if to a different extent, whereas no effect was observed for tacrine and galantamine. Some very preliminary indications can be derived about the structural requirements for binding to nonamyloidogenic or beta-sheet amyloid secondary structure for the development of potent antiaggregating agents. On these premises, memoquin, a multifunctional molecule that was designed to become a drug candidate for the treatment of Alzheimer's disease, was investigated under the reported circular dichroism assay and its anti-amyloidogenic mechanism of action was elucidated.

Neurosecretases provide strategies to treat sporadic and familial Alzheimer disorders

Recent discoveries on neurosecretases and their trafficking to release fibril-forming neuropeptides or other products, are of interest to pathology, cell signaling and drug discovery. Nomenclature arose from the use of amyloid precursor protein (APP) as a prototypic type-1 substrate leading to the isolation of beta-secretase (BACE), multimeric complexes (gamma-secretase, gamma-SC) for intramembranal cleavage, and attributing a new function to well-characterized metalloproteases of the ADAM family (alpha-secretase) for normal APP turnover. While purified alpha/beta-secretases facilitate drug discovery, gamma-SC presents greater challenges for characterization and mechanisms of catalysis. The review comments on links between mutation or polymorphisms in relation to enzyme mechanisms and disease. The association between lipoprotein receptor LRP11 variants and sporadic Alzheimer's disease (SAD) offers scope to integrate components of pre- and post-Golgi membranes, or brain clathrin-coated vesicles within pathways for trafficking as targets for intervention. The presence of APP and metabolites in brain clathrin-coated vesicles as significant cargo with lipoproteins and adaptors focuses attention as targets for therapeutic intervention. This overview emphasizes the importance to develop new therapies targeting neurosecretases to treat a major neurological disorder that has vast economic and social implications.

The Alzheimer's disease beta-secretase enzyme, BACE1

The pathogenesis of Alzheimer's disease is highly complex. While several pathologies characterize this disease, amyloid plaques, composed of the β-amyloid peptide are hallmark neuropathological lesions in Alzheimer's disease brain. Indeed, a wealth of evidence suggests that β-amyloid is central to the pathophysiology of AD and is likely to play an early role in this intractable neurodegenerative disorder. The BACE1 enzyme is essential for the generation of β-amyloid. BACE1 knockout mice do not produce β-amyloid and are free from Alzheimer's associated pathologies including neuronal loss and certain memory deficits. The fact that BACE1 initiates the formation of β-amyloid, and the observation that BACE1 levels are elevated in this disease provide direct and compelling reasons to develop therapies directed at BACE1 inhibition thus reducing β-amyloid and its associated toxicities. However, new data indicates that complete abolishment of BACE1 may be associated with specific behavioral and physiological alterations. Recently a number of non-APP BACE1 substrates have been identified. It is plausible that failure to process certain BACE1 substrates may underlie some of the reported abnormalities in the BACE1-deficient mice. Here we review BACE1 biology, covering aspects ranging from the initial identification and characterization of this enzyme to recent data detailing the apparent dysregulation of BACE1 in Alzheimer's disease. We pay special attention to the putative function of BACE1 during healthy conditions and discuss in detail the relationship that exists between key risk factors for AD, such as vascular disease (and downstream cellular consequences), and the pathogenic alterations in BACE1 that are observed in the diseased state.

Oxidative stress activates a positive feedback between the gamma- and beta-secretase cleavages of the beta-amyloid precursor protein

Sequential cleavages of the beta-amyloid precursor protein cleaving enzyme 1 (BACE1) by beta-secretase and gamma-secretase generate the amyloid beta-peptides, believed to be responsible of synaptic dysfunction and neuronal cell death in Alzheimer's disease (AD). Levels of BACE1 are increased in vulnerable regions of the AD brain, but the underlying mechanism is unknown. Here we show that oxidative stress (OS) stimulates BACE1 expression by a mechanism requiring gamma-secretase activity involving the c-jun N-terminal kinase (JNK)/c-jun pathway. BACE1 levels are increased in response to OS in normal cells, but not in cells lacking presenilins or amyloid precursor protein. Moreover, BACE1 is induced in association with OS in the brains of mice subjected to cerebral ischaemia/reperfusion. The OS-induced BACE1 expression correlates with an activation of JNK and c-jun, but is absent in cultured cells or mice lacking JNK. Our findings suggest a mechanism by which OS induces BACE1 transcription, thereby promoting production of pathological levels of amyloid beta in AD.

Calcium dysregulation in Alzheimer's disease

Alzheimer disease (AD) is the most common form of adult dementia. Its pathological hallmarks are synaptic degeneration, deposition of amyloid plaques and neurofibrillary tangles, leading to neuronal loss. A few hypotheses have been proposed to explain AD pathogenesis. The beta-amyloid (Abeta) and hyperphosphorylated tau hypotheses suggest that these proteins are the main players in AD development. Another hypothesis proposes that the dysregulation of calcium homeostasis may be a key factor in accelerating other pathological changes. Although Abeta and tau have been extensively studied, recently published data provide a growing body of evidence supporting the critical role of calcium signalling in AD. For example, presenilins, which are mutated in familial cases of AD, were demonstrated to form low conductance calcium channels in the ER and elevated cytosolic calcium concentration increases amyloid generation. Moreover, memantine, an antagonist of the NMDA-calcium channel receptor, has been found to have a beneficial effect for AD patients offering novel possibilities for a calcium signalling targeted therapy of AD. This review underscores the growing importance of calcium ions in AD development and focuses on the relevant aspects of calcium homeostasis.

Alzheimer’s disease, oxidative stress and B-vitamin depletion

There is an association between cognitive function and vitamin B12 and folate status. Both vitamins participate in recycling the potentially toxic amino acid homocysteine to methionine and, ultimately, to the methyl donor S-adenosylmethionine (SAM). Consequently, B12 and folate indirectly influence glutathione synthesis – a major intracellular antioxidant. Neuroinflammation and oxidative stress are early features of Alzheimer’s disease (AD). Such stress impairs homocysteine recycling, degrades folate and decreases its cellular retention, resulting in limited SAM availability and increased homocysteine levels. Oxidized homocysteine derivatives, such as homocysteic acid, can initiate a vicious cycle by promoting free-radical formation. Decreased SAM also fosters development of characteristic AD neuropathologies – neurofibrillary tangles and amyloid plaques. The latter generate additional free radicals in a further feed-forward cascade. Future therapies should simultaneously halt neuroinflammation, restore redox homeostasis and replace depleted intracellular B vitamins. Developing early markers for these harmful processes will allow targeting of such therapy before irreversible cellular damage ensues.

Formation of Toxic Fibrils of Alzheimer’s Amyloid β-Protein-(1–40) by Monosialoganglioside GM1, a Neuronal Membrane Component

A pathological hallmark of Alzheimer's disease (AD) is the deposition of amyloid beta-protein (Abeta) in fibrillar form on neuronal cells. However, the role of Abeta fibrils in neuronal dysfunction is highly controversial. This study demonstrates that monosialoganglioside GM1 (GM1) released from damaged neurons catalyzes the formation of Abeta fibrils, the toxicity and the cell affinity of which are much stronger than those of Abeta fibrils formed in phosphate-buffered saline. Abeta-(1-40) was incubated with equimolar GM1 at 37 degrees C. After a lag period of 6-12 h, amyloid fibrils were formed, as confirmed by circular dichroism, thioflavin-T fluorescence, size-exclusion chromatography, and transmission electron microscopy. The fibrils showed significant cytotoxicity against PC12 cells differentiated with nerve growth factor. Trisialoganglioside GT1b also facilitated the fibrillization, although the effect was weaker than that of GM1. Our study suggests an exacerbation mechanism of AD and an importance of polymorphisms in Abeta fibrils during the pathogenesis of the disease.

Cellular therapy using microglial cells

Recent insights into the function and dysfunction of microglia may inform future therapies to combat neurodegeneration. We hypothesise how different aspects of microglial activity including migration, activation, oxidative response, phagocytosis, proteolysis, and replenishment could be targeted by novel therapeutic approaches. A combined approach is suggested, encompassing opsonization and anti-inflammatory strategies in conjunction with an engineering of microglial precursors. Xenoproteases for bioremediation could be used to enhance intracellular and extracellular proteolytic capacity. The capacity of microglial precursors to cross the blood-brain barrier and to home in on sites of neural damage and inflammation might prove to be particularly useful for future therapeutic strategies.

Soluble amyloid beta1-42 reduces dopamine levels in rat prefrontal cortex: relationship to nitric oxide

Several studies suggest a pivotal role of amyloid beta (Abeta)(1-42) and nitric oxide (NO) in the pathogenesis of Alzheimer's disease. NO also possess central neuromodulatory properties. To study the soluble Abeta(1-42) effects on dopamine concentrations in rat prefrontal cortex, microdialysis technique was used. We showed that i.c.v. injection or retrodialysis Abeta(1-42) administration reduced basal and K(+)-stimulated dopamine levels, measured 2 and 48 h after peptide administration. Immunofluorescent experiments revealed that after 48 h from i.c.v. injection Abeta(1-42) was no longer detectable in the ventricular space. We then evaluated the role of NO on Abeta(1-42)-induced reduction in dopamine concentrations. Subchronic L-arginine administration decreased basal dopamine levels, measured either 2 h after i.c.v. Abeta(1-42) or on day 2 post-injection, whereas subchronic 7-nitroindazole administration increased basal dopamine concentrations, measured 2 h after i.c.v. Abeta(1-42) injection, and decreased them when measured on day 2 post-Abeta(1-42)-injection. No dopaminergic response activity was observed after K(+) stimulation in all groups. These results suggest that the dopaminergic system seems to be acutely vulnerable to soluble Abeta(1-42) effects. Finally, the opposite role of NO occurring at different phases might be regarded as a possible link between Abeta(1-42)-induced effects and dopaminergic dysfunction.

Nicotinic receptor activation by epibatidine induces heme oxygenase-1 and protects chromaffin cells against oxidative stress

Activation of neuronal nicotinic acetylcholine receptors (nAChR) provides neuroprotection against different toxic stimuli that often lead to overproduction of reactive oxygen species (ROS) and cell death. ROS production has been related with disease progression in several neurodegenerative pathologies such as Alzheimer's or Parkinson's diseases. In this context, we investigated here if the exposure of bovine chromaffin cells to the potent nAChR agonist epibatidine protected against rotenone (30 micromol/L) plus oligomycin (10 micromol/L) (rot/oligo) toxicity, an in vitro model of mitochondrial ROS production. Epibatidine induced a concentration- and time-dependent protection, which was maximal at 3 mumol/L after 24 h. Pre-incubation with dantrolene (100 micromol/L) (a blocker of the ryanodine receptor channel), chelerythrine (1 micromol/L) (a protein kinase C inhibitor), or PD98059 (50 micromol/L) (a MEK inhibitor), aborted epibatidine-elicited cytoprotection. Mitochondrial depolarization, ROS, and caspase 3 active produced by rot/oligo were also prevented by epibatidine. Epibatidine doubled the amount of heme oxygenase-1 (HO-1), a critical cell defence enzyme against oxidative stress. Furthermore, the HO-1 inhibitor Sn(IV) protoporphyrin IX dichloride reversed the epibatidine protecting effects and HO-1 inducer Co (III) protoporphyrin IX dichloride exhibited neuroprotective effects by itself. The results of this study point to HO-1 as the cytoprotective target of nAChR activation through the following pathway: endoplasmic reticulum Ca(2+)-induced Ca(2+)-release activates the protein kinase C/extracellular regulated kinase/HO-1 axis to mitigate mitochondrial depolarization and ROS production. This study provides a mechanistic insight on how nAChR activation translates into an antioxidant and antiapoptotic signal through up-regulation of HO-1.

Up-regulation and increased activity of KV3.4 channels and their accessory subunit MinK-related peptide 2 induced by amyloid peptide are involved in apoptotic neuronal death

The aim of the present study was to investigate whether K(V)3.4 channel subunits are involved in neuronal death induced by neurotoxic beta-amyloid peptides (Abeta). In particular, to test this hypothesis, three main questions were addressed: 1) whether the Abeta peptide can up-regulate both the transcription/translation and activity of K(V)3.4 channel subunit and its accessory subunit, MinK-related peptide 2 (MIRP2); 2) whether the increase in K(V)3.4 expression and activity can be mediated by the nuclear factor-kappaB (NF-kappaB) family of transcriptional factors; and 3) whether the specific inhibition of K(V)3.4 channel subunit reverts the Abeta peptide-induced neurodegeneration in hippocampal neurons and nerve growth factor (NGF)-differentiated PC-12 cells. We found that Abeta(1-42) treatment induced an increase in K(V)3.4 and MIRP2 transcripts and proteins, detected by reverse transcription-polymerase chain reaction and Western blot analysis, respectively, in NGF-differentiated PC-12 cells and hippocampal neurons. Patch-clamp experiments performed in whole-cell configuration revealed that the Abeta peptide caused an increase in I(A) current amplitude carried by K(V)3.4 channel subunits, as revealed by their specific blockade with blood depressing substance-I (BDS-I) in both hippocampal neurons and NGF-differentiated PC-12 cells. The inhibition of NF-kappaB nuclear translocation with the cell membrane-permeable peptide SN-50 prevented the increase in K(V)3.4 protein and transcript expression. In addition, the SN-50 peptide was able to block Abeta(1-42)-induced increase in K(V)3.4 K(+) currents and to prevent cell death caused by Abeta(1-42) exposure. Finally, BDS-I produced a similar neuroprotective effect by inhibiting the increase in K(V)3.4 expression. As a whole, our data indicate that K(V)3.4 channels could be a novel target for Alzheimer's disease pharmacological therapy.

Mitochondrial respiratory inhibition and oxidative stress elevate beta-secretase (BACE1) proteins and activity in vivo in the rat retina

Cerebral hypometabolism, oxidative stress and beta-amyloid peptide (Abeta) accumulation are key pathological events in Alzheimer's disease (AD). Beta-secretase (BACE, i.e., BACE1), a prerequisite for Abeta genesis, is elevated in sporadic AD. Recent studies show BACE upregulation in experimental conditions likely associated with energy insufficiency and/or oxidative stress. We investigated the effect of sublethal doses of mitochondrial respiratory inhibitors and potential endogenous oxidative substances on BACE expression in vivo using the retina as a model. Retinas were analyzed biochemically and anatomically 48 h following intraocular applications of mitochondrial complex I, II and IV inhibitors including rotenone, 3-nitropropionic acid and sodium azide, and plaque-containing oxidants including Fe(3+) and Abeta42 fibrils (Abeta42f). All agents caused elevations of BACE proteins and beta-site amyloid precursor protein (APP) cleavage product, beta-CTF, in retinal lysates in a dose-dependant manner. BACE activity and Abeta40 levels were also increased in agent-treated retinas relative to vehicle controls. BACE immunoreactivity in normal adult rat retina was present mostly in the plexiform layers, indicating a localization of the enzyme to synaptic terminals. No apparent change in laminar or cellular distribution of BACE labeling was detected in the experimental retinas. However, signs of neuronal stress including glial activation were observed in agent-treated retinas especially in high dosage groups. Our data suggest that mitochondrial respiratory inhibition and oxidative stress facilitate BACE expression in vivo. In addition, plaque constituents such as Fe(3+) and Abeta42f may participate in a self-enforcing cycle of amyloidogenesis via BACE upregulation.

Membrane-mediated amyloidogenesis and the promotion of oxidative lipid damage by amyloid beta proteins

Evidence of oxidative stress and the accumulation of fibrillar amyloid beta proteins (Abeta) in senile plaques throughout the cerebral cortex are consistent features in the pathology of Alzheimer disease. To define a mechanistic link between these two processes, various aspects of the relationship between oxidative lipid membrane damage and amyloidogenesis were characterized by chemical and physical techniques. Earlier studies of this relationship demonstrated that oxidatively damaged synthetic lipid membranes promoted amyloidogenesis. The studies reported herein specify that 4-hydroxy-2-nonenal (HNE) is produced in both synthetic lipids and human brain lipid extracts by oxidative lipid damage and that it can account for accelerated amyloidogenesis. Abeta promotes the copper-mediated generation of HNE from polyunsaturated lipids, and in turn, HNE covalently modifies the histidine side chains of Abeta. HNE-modified Abeta have an increased affinity for lipid membranes and an increased tendency to aggregate into amyloid fibrils. Thus, the prooxidant activity of Abeta leads to its own covalent modification and to accelerated amyloidogenesis. These results illustrate how lipid membranes may be involved in templating the pathological misfolding of Abeta, and they suggest a possible chemical mechanism linking oxidative stress with amyloid formation.

Physicochemical interactions of amyloid beta-peptide with lipid bilayers

The aggregation and deposition onto neuronal cells of amyloid beta-peptide (Abeta) is central to the pathogenesis of Alzheimer's disease. Accumulating evidence suggests that membranes play a catalytic role in the aggregation of Abeta. This article summarizes the structures and properties of Abeta in solution and the physicochemical interaction of Abeta with lipid bilayers of various compositions. Reasons for discrepancies between results by different research groups are discussed. The importance of ganglioside clusters in the aggregation of Abeta is emphasized. Finally, a hypothetical physicochemical cascade in the pathogenesis of the disease is proposed.

A beta oligomers - a decade of discovery

Converging lines of evidence suggest that progressive accumulation of the amyloid beta-protein (A beta) plays a central role in the genesis of Alzheimer's disease, but it was long assumed that A beta had to be assembled into extracellular amyloid fibrils to exert its cytotoxic effects. Over the past decade, data have emerged from the use of synthetic A beta peptides, cell culture models, beta-amyloid precursor protein transgenic mice and human brain to suggest that pre-fibrillar, diffusible assemblies of A beta are also deleterious. Although the precise molecular identity of these soluble toxins remains unsettled, accumulating evidence suggests that soluble forms of A beta are indeed the proximate effectors of synapse loss and neuronal injury. Here we review recent progress in understanding the role of soluble oligomers in Alzheimer's disease.

Acute impairment of mitochondrial trafficking by beta-amyloid peptides in hippocampal neurons

Defects in axonal transport are often associated with a wide variety of neurological diseases including Alzheimer's disease (AD). Beta-amyloid (Abeta) is a major component of neuritic plaques associated with pathological conditions of AD brains. Here, we report that a brief exposure of cultured hippocampal neurons to Abeta molecules resulted in rapid and severe impairment of mitochondrial transport without inducing apparent cell death and significant morphological changes. Such acute inhibition of mitochondrial transport was not associated with a disruption of mitochondria potential nor involved aberrant cytoskeletal changes. Abeta also did not elicit significant Ca2+ signaling to affect mitochondrial trafficking. However, stimulation of protein kinase A (PKA) by forskolin, cAMP analogs, or neuropeptides effectively alleviated the impairment. We also show that Abeta inhibited mitochondrial transport by acting through glycogen synthase kinase 3beta (GSK3beta). Given that mitochondria are crucial organelles for many cellular functions and survival, our findings thus identify an important acute action of Abeta molecules on nerve cells that could potentially contribute to various abnormalities of neuronal functions under AD conditions. Manipulation of GSK3beta and PKA activities may represent a key approach for preventing and alleviating Abeta cytotoxicity and AD pathological conditions.

Free radicals and antioxidants in normal physiological functions and human disease

Reactive oxygen species (ROS) and reactive nitrogen species (RNS, e.g. nitric oxide, NO(*)) are well recognised for playing a dual role as both deleterious and beneficial species. ROS and RNS are normally generated by tightly regulated enzymes, such as NO synthase (NOS) and NAD(P)H oxidase isoforms, respectively. Overproduction of ROS (arising either from mitochondrial electron-transport chain or excessive stimulation of NAD(P)H) results in oxidative stress, a deleterious process that can be an important mediator of damage to cell structures, including lipids and membranes, proteins, and DNA. In contrast, beneficial effects of ROS/RNS (e.g. superoxide radical and nitric oxide) occur at low/moderate concentrations and involve physiological roles in cellular responses to noxia, as for example in defence against infectious agents, in the function of a number of cellular signalling pathways, and the induction of a mitogenic response. Ironically, various ROS-mediated actions in fact protect cells against ROS-induced oxidative stress and re-establish or maintain "redox balance" termed also "redox homeostasis". The "two-faced" character of ROS is clearly substantiated. For example, a growing body of evidence shows that ROS within cells act as secondary messengers in intracellular signalling cascades which induce and maintain the oncogenic phenotype of cancer cells, however, ROS can also induce cellular senescence and apoptosis and can therefore function as anti-tumourigenic species. This review will describe the: (i) chemistry and biochemistry of ROS/RNS and sources of free radical generation; (ii) damage to DNA, to proteins, and to lipids by free radicals; (iii) role of antioxidants (e.g. glutathione) in the maintenance of cellular "redox homeostasis"; (iv) overview of ROS-induced signaling pathways; (v) role of ROS in redox regulation of normal physiological functions, as well as (vi) role of ROS in pathophysiological implications of altered redox regulation (human diseases and ageing). Attention is focussed on the ROS/RNS-linked pathogenesis of cancer, cardiovascular disease, atherosclerosis, hypertension, ischemia/reperfusion injury, diabetes mellitus, neurodegenerative diseases (Alzheimer's disease and Parkinson's disease), rheumatoid arthritis, and ageing. Topics of current debate are also reviewed such as the question whether excessive formation of free radicals is a primary cause or a downstream consequence of tissue injury.